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The Preparation and Characterization of Immobilized TiO2/PEG by Using DSAT as a Support Binder

The Preparation and Characterization of Immobilized TiO2/PEG by Using DSAT as a Support Binder applied sciences Article The Preparation and Characterization of Immobilized TiO /PEG by Using DSAT as a Support Binder 1 , 1 , 2 1 2 Wan Izhan Nawawi *, Raihan Zaharudin , Mohd Azlan Mohd Ishak , Khudzir Ismail and Ahmad Zuliahani Faculty of Applied Sciences, Universiti Teknologi MARA, Perlis, 02600 Arau, Perlis, Malaysia; nurraihanzaharudin@gmail.com (R.Z.); azlanishak@perlis.uitm.edu.my (M.A.M.I.); zuliahani@perlis.uitm.edu.my (A.Z.) Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia; khudzir@salam.uitm.edu.my * Correspondence: wi_nawawi@perlis.uitm.edu.my; Tel.: +60-4-9882-570; Fax: +60-4-9882-026 Academic Editor: Giorgio Biasiol Received: 30 September 2016; Accepted: 13 December 2016; Published: 23 December 2016 Abstract: Immobilized TiO was prepared by adding a small composition of polyethylene glycol (PEG) as a binder, and this paper reported for the very first time the formation of C=O from oxidized PEG, which acted as an electron injector in enhancing photoactivity. Water-based TiO with PEG formulation was deposited by using a brush technique onto double-sided adhesive tape (DSAT) as a support binder to increase the adhesiveness of immobilized TiO . The photocatalytic activity of immobilized TiO -PEG was measured by photodegradation of 12 mgL methylene blue (MB) dye. The optimum condition of immobilized TiO -PEG was observed at TiO /PEG-2 (TP2) with 10:0.1 2 2 for the TiO /PEG ratio, which resulted in a 1.8-times higher photodegradation rate as compared to the suspension mode of pristine TiO . The high photodegradation rate was due to the formation of the active C=O bond from oxidized PEG binder in immobilized TiO -PEG as observed by Fourier transform infrared and X-ray photoelectron spectroscopy analyses. The presence of C=O has escalated the photoactivity by forming an electron injector to a conduction band of TiO as proven by higher photoluminescence intensity obtained for TP2 as compared to pristine TiO . The TP2 sample has also increased its adhesiveness when DSAT is applied as a support binder where it can be recycled up to eight times and comparable to recent photocatalysis cycle developments. Keywords: immobilization; titanium dioxide; oxidized PEG; support binder; methylene blue 1. Introduction Titanium oxide (TiO ) is a semiconductor that is widely known as a photocatalyst for the photodegradation of organic pollutants. According to Karimi et al. [1], when TiO is illuminated by a light with energy higher than its band gap energy, electron–hole pairs diffuse out, creating negative electrons and oxygen that combine to become O , while the positive electric holes and water generate hydroxyl radicals. This highly active oxygen species can then oxidize organic pollutants. For over three decades, modifications on TiO have improved two main issues. The first is to increase catalytic reaction performance where photocatalytic improvement of TiO has been studied by many researchers. This is important in order to make the photocatalyst become active in the wide solar spectrum since the TiO semiconductor is only active under high energy ultraviolet (UV) light [2]. Basically, there are two common types of modifications used in preparing the visible light TiO , which are the modification of bulk and the surface of TiO . Bulk modification often resulted in the narrowing of band gap energy, whereas surface modification does not change the band gap energy. However, surface modification successfully activates the photocatalyst under visible light by accepting Appl. Sci. 2017, 7, 24; doi:10.3390/app7010024 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 24 2 of 16 the electron from the sensitizing agent. The second is a recovery process of this photocatalyst after being treated with organic pollutants where the photodegradation of organic pollutants using TiO is basically applied under suspension mode, which provides a high surface to volume ratio [3]. However, the recovery process of this suspension of TiO powder with treated wastewater requires filtration. Separation of this nano-sized TiO will clog the filter membrane and eventually penetrate through the porous filter, making the recovery process less effective [4]. Immobilization of TiO has gained great interest since it can easily separate TiO with treated wastewater [5]. Many papers with different types of polymers in immobilized TiO have reported these findings after the first technique was discovered by Tennakone et al. [6]. Polymer is normally added as a binder in immobilized TiO to improve its durability, temperature resisting ability and absorbance affinity towards pollutants [7–9], and some polymers are also able to increase TiO photoactivity [10–25]. In most cases, prepared immobilized TiO used solvent-based polymers, such as polythene sheets [10], thin polythene films [11], polystyrene (PS) beads [12], expanded polystyrene (EPS) beads [13], cellulose microspheres [14], fluoro polymer resins [15], polyethylene terephthalate (PET) bottles [16], polypropylene (PP) granules [17], cellulose fibers [18], polypropylene fabric (PPF) [19], polyvinyl chloride (PVC) [20], polycarbonate (PC), poly(methyl methacrylate) (PMMA) [21], polyvinyl acetate (PVAc) [22], poly(styrene)-co-poly(4-vinylpyridine) (PSP4VP) [23], rubber latex (an elastic hydrocarbon polymer) [24], parylene and tedlar [25]. Recently, water-based polymer binders have shown a vast potential of usage in immobilized TiO for commercialization because they are environmentally friendly and economic. Water-based polymers, such as polyvinyl alcohol (PVA), polyaniline (PANI), polyvinyl pyrolydone (PVP) and polyethylene glycol (PEG), have performed greatly in water treatment separation, which also resulted in a significant photosensitivity to the visible light region [26–28]. For instance, Lei et al. [29] decorated TiO /PVA particles through a heat treatment method and obtained high photoactivity since the Ti–O–C bond generated led the immobilized sample to become fully in contact with the organic pollutant. TiO /PVA developed by Yang et al. [30] showed notable photoactivity under visible light irradiation due to the presence of conjugated polymer. They found that the conjugated molecules adsorbed on the TiO surface and excited the electrons, thus injecting the electrons into the conduction band using visible light. PANI in immobilized TiO studied by Nawi et al. [31] has shown a significant photocatalytic improvement by acting as a hole scavenger in TiO under photodegradation of reactive red 4 (RR4) dye. PEG is a polymer that is able to form a uniform surface, smooth coating and well-defined size particles [32]. PEG is a hydrophilic polymer, which is suitable for coating, as it reduces the formation of a cracked surface in the immobilized system [33]. Most of the reports on immobilized TiO with PEG were identified to enhance photoactivity due to the effect of porosity and larger specific surface area [34,35]. Trapalis et al. [36] found that the porosity increased with PEG amount introduced in the film. However, no detailed study was conducted on surface chemical interaction between PEG and TiO photocatalyst since the oxidation will only take place in the photocatalysis process. Technically, TiO and polymer mixed ratios are a very critical factor. Excessive binder makes immobilized film strong, but reduces its photoactivity; however, a low amount of polymer makes TiO leach out easily. This explained the lack of data discussed by other researchers on the surface chemical interaction of PEG in immobilized TiO . According to Wang et al. [37], visible light photocatalytic activity could also be enhanced by carbon-sensitized TiO , in which the carbon could be originated from titanium alkoxide and mainly existed as the C–C bonds (carbonaceous species), C=O and O=C–O bonds (carbonate species). Since visible light active TiO using surface modification cannot be judged by using UV–Vis diffuse reflectance spectra (DRS), the easiest way to detect the characteristic of the visible light activity of this modified TiO is through photoluminescence (PL) analysis where the highest PL intensity represented the highest photoactivity of the sample. Previously, we have discovered the ability of double-sided adhesive tape (DSAT) to become a support binder in immobilized TiO -only [38]. DSAT has greatly improved immobilized TiO -only in 2 2 Appl. Sci. 2017, 7, 24 3 of 16 its strength and recyclability. Photocatalytic activity of this immobilized TiO -only, however, is slightly lower as compared to TiO in suspension mode [39]. In this study, immobilized TiO mixed with a 2 2 small amount of PEG 6000 was prepared using DSAT as a support binder to observe the interaction between TiO and PEG during photocatalysis process. The photoactivity of this immobilized TiO -PEG 2 2 was observed by photodegradation of methylene blue, applied with specific parameters. 2. Results and Discussion 2.1. Characterization of Immobilized TiO A method for immobilizing TiO using DSAT was thoroughly described in the previous article [40]. In this work, a similar approach was taken where TiO was coated onto DSAT as a support binder, but in the presence of a specific amount of PEG. Table 1 shows the experimental condition and pseudo first order rate constant of immobilized TiO and the pristine TiO sample in degrading methylene 2 2 blue (MB) dye. As can be seen in Table 1, the increased amount of PEG in the formulation solutions has shown a significant increase of PEG in immobilized TiO -PEG samples detected by FTIR analysis. The 1160- and 2900-nm peaks of C–O and C–H stretching, respectively, as shown in Figure 1, corresponded to the PEG peaks based on similar peaks presented on the FTIR spectrum for pure PEG studied by Hyma et al. [41]. Photocatalytic activity of washed immobilized TiO -PEG was increased by increasing the amount of PEG from 0.05 to 0.1 g based on its photodegradation rate values. Photocatalytic activity starts to decrease beyond 0.1 g of PEG due to the agglomeration of PEG in TiO particles, which makes it harder for a large amount of light to penetrate through the surface particles of TiO . Mukherjee et al. [39] have reported same behavior on immobilized TiO with 2 2 PVA. The catalyst loading effect of immobilized TiO with 0.1 g of PEG samples has shown a different photocatalytic activity. Increasing the amount of catalyst loading from 0.05, 0.1, 0.2 and 0.3 grams will significantly increase the photodegradation rates from 0.039, 0.048, 0.048 and 0.087 min , respectively. The high photodegradation rate at 0.3 g was due to the high adsorption capacity of the photocatalyst thin film composite that enhanced the decolorization process of MB dye, and eventually, this dye pollutant becomes degraded by the photocatalysis process. However, too much adsorption capacity beyond 0.3 g of catalyst loading for the immobilized TiO -PEG sample makes the photodegradation rate become 2.5-times slower. This observation was due to the excessive adsorbed MB dye on the surface of immobilized TiO -PEG, thus forming a thick layer coating on the TiO surface and reducing 2 2 the penetration of light for photocatalysis. It can be deduced that the immobilized TiO -PEG ratio of 10:0.1 at a 0.3-g catalyst loading is the optimum immobilized TiO -PEG with a photodegradation rate 1.8-times faster compared to pristine TiO ; an optimum sample named as TiO /PEG-2 (TP2) (Table 1). 2 2 Table 1. The experimental condition and pseudo first order rate constant of immobilized TiO and the control sample in degrading methylene blue (MB) dye. Rate Constant k (min ) TiO Loading Mode Amount of Ratio BET a b 2 1 Sample (g) (S or I ) PEG (wt %) (TiO /PEG) (m g ) 2 Washed Unwashed Pristine TiO 0.3 S 0.00 10:0 50.00 0.048 - TP0 0.3 I 0.00 10:0 49.25 0.054 0.048 TP1 0.3 I 0.05 10:0.05 - 0.080 0.041 TP2 0.3 I 0.10 10:0.1 88.30 0.087 0.017 TP3 0.3 I 0.15 10:0.15 - 0.081 0.024 TP4 0.3 I 0.20 10:0.2 - 0.040 0.030 TP5 0.05 I 0.10 10:0.1 - 0.039 0.039 TP6 0.1 I 0.10 10:0.1 - 0.048 0.048 TP7 0.2 I 0.10 10:0.1 - 0.048 0.028 TP8 0.4 I 0.10 10:0.1 - 0.030 0.015 a b Suspension; immobilize. S : surface area; TP0 to TP8, TiO /PEG-0 to TiO -PEG-8. BET 2 2 Appl. Sci. 2017, 7, 24 4 of 16 Appl. Sci. 2016, 6, 451 4 of 16 Appl. Sci. 2016, 6, 451 4 of 16 TP1 (0.5% PEG) TP1 (0.5% PEG) C-O TP2 (1.0% PEG) C-O TP2 (1.0% PEG) C-H TP3 (1.5% PEG) C-H TP3 (1.5% PEG) TP4 (2.0% PEG) TP4 (2.0% PEG) 3000 2000 1000 4000 3000 2000 1000 3000 2000 1000 4000 3000 2000 1000 -1 Wavelength (cm ) -1 Wavelength (cm ) Figure 1. FTIR spectra of different percentages of polyethylene glycol (PEG) in immobilized Figure Figure 1. 1. FTIR FTspectra IR spectra of dif of ferdifferent perc ent percentages entag of e polyethylene s of polyethy glycol lene g (PEG) lycol (P in immobilized EG) in immo Tbilized iO -PEG. TiO2-PEG. TiO2-PEG. 2.2. XRD Analysis 2.2. XRD Analysis 2.2. XRD Analysis Figure 2 shows the XRD patterns for pristine TiO , TP0 (immobilized TiO DSAT) and TP2 2 2 Figure 2 shows the XRD patterns for pristine TiO2, TP0 (immobilized TiO2 DSAT) and TP2 Figure 2 shows the XRD patterns for pristine TiO2, TP0 (immobilized TiO2 DSAT) and TP2 (immobilized TiO /PEG DSAT) samples. All peaks were detected as anatase and rutile phases. (immobilized TiO2/PEG DSAT) samples. All peaks were detected as anatase and rutile phases. There (immobilized TiO2/PEG DSAT) samples. All peaks were detected as anatase and rutile phases. There There is no phase transformation occurring in TP0 and TP2, since the immobilization processes is no phase transformation occurring in TP0 and TP2, since the immobilization processes of TP0 and is no phase transformation occurring in TP0 and TP2, since the immobilization processes of TP0 and of TP0 and TP2 were prepared under low temperature (120 C). Phase transformation in the TP2 were prepared under low temperature (120 °C). Phase transformation in the presence of organic TP2 were prepared under low temperature (120 °C). Phase transformation in the presence of organic presence of organic binder was observed when the calcining temperature happened at 900 C [42]. binder was observed when the calcining temperature happened at 900 °C [42]. Wang et al. [43] also binder was observed when the calcining temperature happened at 900 °C [42]. Wang et al. [43] also Wang found tha et al. [43 t the pha ] also found se transf thatorm theaphase tion fotransformation r TiO2 calcined witho for TiOut organ calcined ic bin without der occur organic red at a binder found that the phase transformation for TiO2 calcined without organic binder occurred at a temperature of 700 °C onwards. All peaks for all samples correspond to the characteristic peak of the occurred at a temperature of 700 C onwards. All peaks for all samples correspond to the characteristic temperature of 700 °C onwards. All peaks for all samples correspond to the characteristic peak of the anatase and rutile phases of TiO2 nanoparticles detected at 2θ from 15° to 65° by using low angle peak of the anatase and rutile phases of TiO nanoparticles detected at 2 from 15 to 65 by using anatase and rutile phases of TiO2 nanoparticles detected at 2θ from 15° to 65° by using low angle XRD. The XRD pattern in all samples shows different crystallinity as the sharp diffraction peaks low angle XRD. The XRD pattern in all samples shows different crystallinity as the sharp diffraction XRD. The XRD pattern in all samples shows different crystallinity as the sharp diffraction peaks displayed the good crystallinity of the prepared nanoparticles [44]. Pristine TiO2 has the highest peaks displayed the good crystallinity of the prepared nanoparticles [44]. Pristine TiO has the highest displayed the good crystallinity of the prepared nanoparticles [44]. Pristine TiO2 has the 2 highest crystallinity as compared to TP0 and TP2, where TP2 is the lowest. This is due to the presence of crystallinity as compared to TP0 and TP2, where TP2 is the lowest. This is due to the presence of DSAT crystallinity as compared to TP0 and TP2, where TP2 is the lowest. This is due to the presence of DSAT and DSAT + PEG in TP0 and TP2, respectively. The increased porosity in both immobilized and DSAT an DSAT d DSAT + + PEG in TP0 PEGand in TP0 an TP2, respectively d TP2, resp.ect The ively. The increased incre por ased p osityoin rosit both y in b immobilized oth immobili samples zed samples is the main factor for low crystallinity and broad peaks, as reported by Kim [45]. In short, no samples is the main factor for low crystallinity and broad peaks, as reported by Kim [45]. In short, no is the main factor for low crystallinity and broad peaks, as reported by Kim [45]. In short, no phase phase transformations occurred for any of the samples prepared under low heat temperature. phase transformations occurred for any of the samples prepared under low heat temperature. transformations occurred for any of the samples prepared under low heat temperature. R R A R R A A A R R A R R A A A TP2 TP2 4000 4000 TP0 TP0 2000 2000 A – anatase A – anatase R – rutile R – rutile Pristine TiO Pristine TiO 15 30 45 60 15 30 45 60 15 30 45 60 45 60 15 30 2 theta 2 theta Figure 2. XRD patterns of pristine TiO2, TP0 and TP2 samples. Figure 2. XRD patterns of pristine TiO2, TP0 and TP2 samples. Figure 2. XRD patterns of pristine TiO , TP0 and TP2 samples. Intensity (a.u) Intensity (a.u) Transmittance % Transmittance % Appl. Sci. 2017, 7, 24 5 of 16 Appl. Sci. 2016, 6, 451 5 of 16 2.3. SEM Images 2.3. SEM Images Figure 3a,b shows the illustrative SEM images of the TP0 and TP2 surfaces after the photocatalytic Figure 3a,b shows the illustrative SEM images of the TP0 and TP2 surfaces after the degradation process. Few pores were observed in TP0, while for TP2, a variety of porous structures photocatalytic degradation process. Few pores were observed in TP0, while for TP2, a variety of evolved on the surface. As shown in Figure 3b, the pore depth is larger around the PEG concentration porous structures evolved on the surface. As shown in Figure 3b, the pore depth is larger around the of 8.0 g/100 mL as compared to TP0. It can be concluded that the porous structure of TP2 thin films is PEG concentration of 8.0 g/100 mL as compared to TP0. It can be concluded that the porous structure related to the molecular weight of PEG. The increased concentration of PEG has granted the formation of TP2 thin films is related to the molecular weight of PEG. The increased concentration of PEG has of big pores. It is obvious that the catalyst morphologies shown in Figure 3b are highly presumed to granted the formation of big pores. It is obvious that the catalyst morphologies shown in Figure 3b perform an optimum photocatalytic activity. In this regard, Bing et al. [46] stated that the increased are highly presumed to perform an optimum photocatalytic activity. In this regard, Bing et al. [46] porosity in the immobilized TiO /PEG film is accountable for the increased photodegradation of the stated that the increased porosity in the immobilized TiO2/PEG film is accountable for the increased methyl orange model pollutant dye. photodegradation of the methyl orange model pollutant dye. Figure 3. Scanning electron micrograph of surface morphologies for the (a) TP0 and Figure 3. Scanning electron micrograph of surface morphologies for the (a) TP0 and (b) TP2 samples. (b) TP2 samples. EHT, extra high tension. EHT, extra high tension. 2.4. N2 Adsorption-Desorption 2.4. N Adsorption-Desorption The porous structure of TP2 is displayed by N2 adsorption-desorption measurement as The porous structure of TP2 is displayed by N adsorption-desorption measurement as presented. presented. It was found that the TP2 sample in Figure 4 exhibited the type IV nitrogen isotherm It was found that the TP2 sample in Figure 4 exhibited the type IV nitrogen isotherm according to according to the International Union of Pure and Applied Chemists (IUPAC) classification; thus, this the International Union of Pure and Applied Chemists (IUPAC) classification; thus, this indicated indicated that the TP2 sample is a mesoporous structure. According to Li et al. [47], a large surface that the TP2 sample is a mesoporous structure. According to Li et al. [47], a large surface area with a area with a mesoporous structure is favorable to obtain a high photocatalytic activity, as it promotes mesoporous structure is favorable to obtain a high photocatalytic activity, as it promotes adsorption, adsorption, desorption and diffusion of reactants and products. The BET surface area of TP2 was desorption and diffusion of reactants and products. The BET surface area of TP2 was enhanced 2 −1 2 −1 enhanced to 88.3 m ·g as compared to pristine TiO2, which was circa 50 m ·g , where a 43.4% 2 1 2 1 to 88.3 m g as compared to pristine TiO , which was circa 50 m g , where a 43.4% increment increment occurred due to the increased porosity and number of pores. By comparing TP0 with TP2, occurred due to the increased porosity and number of pores. By comparing TP0 with TP2, it is clear that the BET surface area for TP2 was increased up to 44% as compared to TP0, which was it is clear that the BET surface area for TP2 was increased up to 44% as compared to TP0, 2 −1 circa 49.25 m ·g . The results of TP2 surface area measurements are consistent with the results of 2 1 which was circa 49.25 m g . The results of TP2 surface area measurements are consistent photocatalytic activity where the TP2 sample gave a higher photocatalytic degradation rate of 0.087 with the results of photocatalytic activity where the TP2 sample gave a higher photocatalytic −1 −1 min as compared to pristine TiO2, which is 0.048 and 0.054 min for TP0, respectively. A higher 1 1 degradation rate of 0.087 min as compared to pristine TiO , which is 0.048 and 0.054 min for TP0, BET surface area is vital for the adsorption and desorption of dyes and catalysts, since it encourages respectively. A higher BET surface area is vital for the adsorption and desorption of dyes and catalysts, higher photocatalytic performance. In addition, PEG had successfully enhanced the possibility of the since it encourages higher photocatalytic performance. In addition, PEG had successfully enhanced pollutant being trapped within the pores and showed better catalytic activity by providing the possibility of the pollutant being trapped within the pores and showed better catalytic activity by additional surface active groups [48]. providing additional surface active groups [48]. Appl. Sci. 2016, 6, 451 6 of 16 Appl. Sci. 2017, 7, 24 6 of 16 Appl. Sci. 2016, 6, 451 6 of 16 desorption desorption adsorption 15 adsorption 0 0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P ) Relative Pressure (P/P ) Figure 4. N2 adsorption–desorption isotherms of the washed immobilized TiO2/PEG (TP2). Figure 4. N adsorption–desorption isotherms of the washed immobilized TiO /PEG (TP2). Figure 4. N2 adsorption–desorption isotherms of the washed immobilized TiO2/PEG (TP2). 2 2 2.5. FTIR 2.5. FTIR 2.5. FTIR FTIR was used to acquire advanced information of the chemical bonding in TP0, washed TP2 FTIR was used to acquire advanced information of the chemical bonding in TP0, washed TP2 and unwashed TP2 films, as shown in Figure 5. The spectra showed significant differences, which FTIR was used to acquire advanced information of the chemical bonding in TP0, washed TP2 and −1 and unwashed TP2 films, as shown in Figure 5. The spectra showed significant differences, which unwashed were due to TP2 the ri films, sing bands as shown in from the a Figure 5. The dditi spectra on of PEG a showed t 338 significant 9.77 and dif33 fer 75 ences, .15 cm which . Alwer l speectra due 1 −1 were due to the rising bands from the addition of PEG at 3389.77 and 3375.15 cm . All spectra showed broad bands, which indicated the presence of O–H stretching vibrations of water absorbed to the rising bands from the addition of PEG at 3389.77 and 3375.15 cm . All spectra showed broad showed broad bands, which indicated the presence of O–H stretching vibrations of water absorbed bands, with the hydroxyl group which indicated the on the presencephotocatalyst surface. The O of O–H stretching vibrations – of H bo water nd was also noticed absorbed with the hydr in both oxyl −1 −1 with the hydroxyl group on the photocatalyst surface. The O–H bond was also noticed in both spectra at 1637.32 and 1637.45 cm . The peak at 2900 cm was detected in all spectra corresponding group on the photocatalyst surface. The O–H bond was also noticed in both spectra at 1637.32 and −1 −1 1 1 spectra at 1637.32 and 1637.45 cm . The peak at 2900 cm was detected in all spectra corresponding 1637.45 to C–H bonds. cm . The peak at 2900 cm was detected in all spectra corresponding to C–H bonds. to C–H bonds. C-H TP0 C-H TP2 C-C TP0 Unwashed TP2 TP2 C-C DSAT Unwashed TP2 40 C-O C=O DSAT C-O C=O 3000 2000 4000 3000 2000 1000 3000 2000 4000 3000 2000 1000 -1 Wavelength (cm ) -1 Wavelength (cm ) Figure 5. FTIR spectra of TP0, TP2 (washed TP2), unwashed TP2 and double-sided adhesive tape Figure 5. FTIR spectra of TP0, TP2 (washed TP2), unwashed TP2 and double-sided adhesive tape (DSAT) samples. (DSAT) sam Figure 5. FTIR spe ples. ctra of TP0, TP2 (washed TP2), unwashed TP2 and double-sided adhesive tape (DSAT) samples. Tra Tr ns am ns im tta in ttc ae n c % e % -1 -1 QuQu anta it n y t Ad ity Ad sors b o er d b ( e m d ( m m om l g ol) g ) Appl. Sci. 2017, 7, 24 7 of 16 Appl. Sci. 2016, 6, 451 7 of 16 −1 Interestingly, washed TP2 spectra showed a strong absorption in the region of 1705.02 cm , Interestingly, washed TP2 spectra showed a strong absorption in the region of 1705.02 cm , which justified that the carbonate species C=O existed. On the other hand, a new peak detected at which justified that the carbonate species C=O existed. On the other hand, a new peak detected −1 1160.06 cm , which is assigned to C–O species, was also present in the spectra. The presence of C=O at 1160.06 cm , which is assigned to C–O species, was also present in the spectra. The presence affirmed that TiO2 has reacted strongly with PEG, which could potentially lead to the high of C=O affirmed that TiO has reacted strongly with PEG, which could potentially lead to the high photoactivity of TP2. The C=O bonds detected were due to the oxidation of PEG in the irradiated photoactivity of TP2. The C=O bonds detected were due to the oxidation of PEG in the irradiated TP2 TP2 film. As shown in Figure 5, the carbonaceous species C–C bond was detected in all samples film. As shown in Figure 5, the carbonaceous species C–C bond was detected in all samples except TP0. except TP0. Generally, the IR spectrum for DSAT also produced the C=O bonds, but this factor can Generally, the IR spectrum for DSAT also produced the C=O bonds, but this factor can be discarded be discarded because during FTIR analysis of all samples, each sample’s powder was carefully because during FTIR analysis of all samples, each sample’s powder was carefully scratched and taken scratched and taken for analysis, excluding the DSAT. This is further supported by the IR spectrum for analysis, excluding the DSAT. This is further supported by the IR spectrum of unwashed TP2, of unwashed TP2, where the formation of C=O bonds was absent in Figure 5. In brief, even though where the formation of C=O bonds was absent in Figure 5. In brief, even though the composition the composition and structure of the investigated TP2 film do not greatly change, the photoactivity and structure of the investigated TP2 film do not greatly change, the photoactivity efficiency of TP2 efficiency of TP2 in Table 1 was indirectly attributed to the formation of C=O bond that promotes in Table 1 was indirectly attributed to the formation of C=O bond that promotes more reactions to more reactions to take place. take place. 2.6. 2.6. X-ray P X-ray Photoelectr hotoelectroon n Spec Spectr trosco oscopy py (XP (XPS) S) XPS XPS is used is used to determin to determine e tthe he bind binding ing e ener nergy gy of of e elements lements detect detected ed in unwashed and in unwashed and washed washed immobil immobilized ized TP2 TP2 s samples amples in o in or rder der t to o e examine xamine t the he e efffect fect of of w washing. ashing. Thi This s ef effe fect ct had had succe successfully ssfully produced produced an an oxidized oxidized PE PEG. G. The O1s The O1s and C1s spect and C1s spectra ra o of f TP TP2 2 un unwashed washed are shown in F are shown in Figur igure 6a,b, e 6a,b, while while Figure Figure 6c,d 6c,d represen represents ts the O1s the O1s and and C1s spectr C1s spectra a for w for washed ashed T TP2 P2 sample. sample. Figure 6. Figure 6. The TheO1s and O1s and C1s C1s X-ray photoelect X-ray photoelectr ron spectros on spectrco oscopy py (XPS) (XPS) spect spectr rums of immobilize ums of immobilized d for (for a,b( ) unwashe a,b) unwashed d TP2TP2 and and (c,d( )c washed ,d) washed TP2TP2. . CPS, CPS, cycle cycles s per se per cond. second. All samples demonstrated the chemical composition information where three elements existed: All samples demonstrated the chemical composition information where three elements existed: Ti, C and O. Figure 6a represents the O1s characteristic peaks of Ti–O and C–O at 529.9 and 531.6 eV, Ti, C and O. Figure 6a represents the O1s characteristic peaks of Ti–O and C–O at 529.9 and 531.6 eV, respectively, while C–O and C–C bonds are given by the values of 288.2 and 284.8 eV, respectively, respectively, while C–O and C–C bonds are given by the values of 288.2 and 284.8 eV, respectively, in Figure 6b. The O1s spectra in Figure 6c of washed TP2 revealed peaks at 529.4, 531.0 and 532.9 eV attributed to Ti–O, C–O and C=O bonds, respectively. Figure 6d shows the spectrum of washed TP2 Appl. Sci. 2017, 7, 24 8 of 16 in Figure 6b. The O1s spectra in Figure 6c of washed TP2 revealed peaks at 529.4, 531.0 and 532.9 eV attributed to Ti–O, C–O and C=O bonds, respectively. Figure 6d shows the spectrum of washed TP2 in the C1s spectra deconvoluted into three distinct curves, which are represented by three forms of Appl. Sci. 2016, 6, 451 8 of 16 carbon as C–C, C–O and C=O at 284.8, 286.7 and 288.7 eV, respectively. As such, the formation of C=O in the C1s spectra deconvoluted into three distinct curves, which are represented by three forms of bond can only be found in washed TP2. Yet, no peak for C=O bond was presented in unwashed TP2. carbon as C–C, C–O and C=O at 284.8, 286.7 and 288.7 eV, respectively. As such, the formation of This occurrence of C=O bond could be due to the oxidation process of hydroxyl radicals with PEG, C=O bond can only be found in washed TP2. Yet, no peak for C=O bond was presented in unwashed introduced by the washed sample of TP2 in Figure 6c,d. TP2. This occurrence of C=O bond could be due to the oxidation process of hydroxyl radicals with PEG, introduced by the washed sample of TP2 in Figure 6c,d. 2.7. UV–Vis DRS and Visible Light Photodegradation Studies 2.7. UV–Vis DRS and Visible Light Photodegradation Studies UV–Vis diffuse reflectance spectra (UV–Vis DRS) of pristine TiO , unwashed and washed TP2 are shown in Figure 7a. All samples have a bit of difference in the pattern in UV–Vis diffuse reflectance UV–Vis diffuse reflectance spectra (UV–Vis DRS) of pristine TiO2, unwashed and washed TP2 spectra. It is obvious that pristine TiO has the highest absorbance in UV–Vis spectra followed by are shown in Figure 7a. All samples have a bit of difference in the pattern in UV–Vis diffuse reflectance spectra. It is obvious that pristine TiO2 has the highest absorbance in UV–Vis spectra washed TP2 and unwashed TP2 samples. Even though the modified TiO or TP2 has lower absorbance followed by washed TP2 and unwashed TP2 samples. Even though the modified TiO2 or TP2 has as compared to unmodified pristine TiO , this result is expected because there are no bulk property lower absorbance as compared to unmodified pristine TiO2, this result is expected because there are modifications other than on the TP2 surface. Therefore, the chemical properties for TP2 and its band no bulk property modifications other than on the TP2 surface. Therefore, the chemical properties for gap energy do not notably change even after surface alteration. According to Marcela et al. [49], TP2 and its band gap energy do not notably change even after surface alteration. According to from the solid state band theory, the absorption coefficient can be described as a function of incident Marcela et al. [49], from the solid state band theory, the absorption coefficient can be described as a 2 1 photon energy; ( h) = A(h E ), where is the absorption coefficient (cm ), A is a constant, g 2 −1 function of incident photon energy; (αhν) = A(hν − Eg), where α is the absorption coefficient (cm ), h (eV) is the energy of excitation and E is the band gap energy. The band gap energy can be A is a constant, hν (eV) is the energy of excitation and Eg is the band gap energy. The band gap performed by plotting ( h) vs. h. The Tauc 2 plot on the photon energy-axis gives the value of the energy can be performed by plotting (αhν) vs. hν. The Tauc plot on the photon energy-axis gives the direct value o band fgap the d ener irect ban gy ofdsemiconductors gap energy of semi [50 condu ]. ctors [50]. a) b) TiO TiO 2 2 0.60.6 TP2- Washed TP2- Washed TP2- Unwashed TP2- Unwashed 1.5 1.5 0.40.4 0.20.2 0.50.5 3 5 300 400 500 300 400 500 3 5 hv (eV) Wavelength (nm) c) d)100 TP0 TP0 TP2 TP2 TP2 unwashed TP2 unwashed 0 204060 80 0 1530456075 Time (min) Time (min) Figure 7. Graph plots for: (a) UV–Vis diffuse reflectance spectra (DRS); (b) Tauc’s equation; Figure 7. Graph plots for: (a) UV–Vis diffuse reflectance spectra (DRS); (b) Tauc’s equation; (c) percentage remaining of MB dye under visible light irradiation; and (d) adsorption study. Abs, (c) percentage remaining of MB dye under visible light irradiation; and (d) adsorption study. absorption. Abs, absorption. Figure 7b shows a graph of (αhν) vs. hν, named as Tauc’s graph plot. Theoretically, it seems Figure 7b shows a graph of ( h) vs. h, named as Tauc’s graph plot. Theoretically, it seems that the band gap energy for all samples did not change and stayed approximately at 3.1 eV. As that the band gap energy for all samples did not change and stayed approximately at 3.1 eV. As such, such, the band gap energy of TiO2 photocatalyst in TP0 did not change to a visible light active −1 response, and this was similarly observed in Figure 7c where no photodegradation of 12 mg·L MB Abs % Remaining (αhν) % Remaining Appl. Sci. 2017, 7, 24 9 of 16 the band gap energy of TiO photocatalyst in TP0 did not change to a visible light active response, and this was similarly observed in Figure 7c where no photodegradation of 12 mgL MB was Appl. Sci. 2016, 6, 451 9 of 16 observed for TP0 under visible light irradiation. Only decolorization of MB was observed as reported on the adsorption site of the porous surface of TP0 as proven by the adsorption graph in Figure 7d was observed for TP0 under visible light irradiation. Only decolorization of MB was observed as and discussed in our previous study [51]. Unwashed TP2 showed a low photocatalytic degradation of reported on the adsorption site of the porous surface of TP0 as proven by the adsorption graph in MB dye due to the C–C bond in PEG that makes TP2 become slightly active under the visible light Figure 7d and discussed in our previous study [51]. Unwashed TP2 showed a low photocatalytic condition. Surprisingly, TP2 shows an active photodegradation under visible light where a complete degradation of MB dye due to the C–C bond in PEG that makes TP2 become slightly active under the decolorization visible light of condit 12 mgion. L Su MB rpris was ingly achieved , TP2 show at s 75an min. actiAlthough ve photodegradat there ision no und shifting er visible light occur in band −1 where a complete decolorization of 12 mg·L MB was achieved at 75 min. Although there is no gap energy of TP2 due to its sole-surface modifications, it is believed that the visible light active ability shifting occur in band gap energy of TP2 due to its sole-surface modifications, it is believed that the in TP2 is due to the formation of C=O resulting from the oxidation of C–O bond in PEG detected by visible light active ability in TP2 is due to the formation of C=O resulting from the oxidation of C–O XPS and FTIR spectrums, which have been discussed earlier in Figures 5 and 6. bond in PEG detected by XPS and FTIR spectrums, which have been discussed earlier in Figures 5 According to Wang et al. [37], C=O bond can act as an electron injector that initiated the and 6. formation of hydroxyl radical, thus eventually degraded the MB dye pollutant. TP2 had undergone According to Wang et al. [37], C=O bond can act as an electron injector that initiated the the photoluminescence (PL) analysis at low activation energy irradiation to observe the effect of C=O formation of hydroxyl radical, thus eventually degraded the MB dye pollutant. TP2 had undergone activation under visible light irradiation. The PL emission spectra can be used to reveal the efficiency the photoluminescence (PL) analysis at low activation energy irradiation to observe the effect of C=O of charge carrier trapping, immigration and transfer and to understand the fate of photo-induced activation under visible light irradiation. The PL emission spectra can be used to reveal the efficiency electrof ch ons arge and car holes rier t inrap a semiconductor ping, immigration [52 and ,53 t ].rans It is fer a known nd to understa that the nd the fa PL spectr te of um photo-i is the rnesult duced of the electrons and holes in a semiconductor [52,53]. It is known that the PL spectrum is the result of the recombination of excited electrons and holes where the lower PL intensity means a lower recombination recombination of excited electrons and holes where the lower PL intensity means a lower rate of electron–hole pairs under light irradiation [54]. recombination rate of electron–hole pairs under light irradiation [54]. Figure 8 shows the PL spectra of pristine TiO , TP2 and unwashed samples of TP2 using the Figure 8 shows the PL spectra of pristine TiO2, TP2 and unwashed samples of TP2 using the excitation wavelength of 500 nm. The photoluminescence spectrum in this study that was conducted excitation wavelength of 500 nm. The photoluminescence spectrum in this study that was conducted under low excitation energy (500 nm) was meant to confirm that the excitation of electrons happened under low excitation energy (500 nm) was meant to confirm that the excitation of electrons under a low energy level (visible light spectrum). It is shown that increased PL intensity leads to happened under a low energy level (visible light spectrum). It is shown that increased PL intensity increased absorption of low excitation energy, resulting in higher photocatalytic activity [55]. It can be leads to increased absorption of low excitation energy, resulting in higher photocatalytic activity seen that all samples exhibited an obvious PL signal with a similarly-shaped curve at the wavelength [55]. It can be seen that all samples exhibited an obvious PL signal with a similarly-shaped curve at range the wa fromvelength ra 458 to 468nge f nm with rom 45 the 8 to 468 nm wi TP2 sample gi th th ving e TP2 sa the highe mple gi st PL ving the intensihi tyghest followed PL intensi by TP0 ty and followed by TP0 and pristine TiO2. However, the PL energy is smaller than the band gap energy of pristine TiO . However, the PL energy is smaller than the band gap energy of TiO . According to 2 2 TiO2. According to Jing et al., 2006 [56], they observed that some of the lower PL intensities of Jing et al., 2006 [56], they observed that some of the lower PL intensities of semiconductor materials semiconductor materials are due to the presence of oxygen vacancy that acted as an electron are due to the presence of oxygen vacancy that acted as an electron scavenger, thus making electron scavenger, thus making electron recombination jump to the sub-band of TiO2. Moreover, the energy recombination jump to the sub-band of TiO . Moreover, the energy at the 458 to 468-nm wavelength at the 458 to 468-nm wavelength released from PL spectra is too high as compared to the excitation released from PL spectra is too high as compared to the excitation energy. This result is due to the energy. This result is due to the presence of the sensitizer, which allowed for the absorption of low presence of the sensitizer, which allowed for the absorption of low excitation energy to occur and excitation energy to occur and formed an electron-hole pair. This electron is then subsequently formed an electron-hole pair. This electron is then subsequently jumped to the conduction band of jumped to the conduction band of TiO2 and eventually recombined with the hole by releasing the TiO and eventually recombined with the hole by releasing the high energy wavelength. 2 high energy wavelength. 0.8 TP2 0.6 0.4 TP2-Unwashed 0.2 Pristine TiO 455 460 465 470 Wavelength (nm) Figure 8. Photoluminescence spectra of pristine TiO2, unwashed TP2 and washed TP2 samples. PL, Figure 8. Photoluminescence spectra of pristine TiO , unwashed TP2 and washed TP2 samples. photoluminescence. PL, photoluminescence. PL Intensity (a.u) Appl. Sci. 2017, 7, 24 10 of 16 Appl. Sci. 2016, 6, 451 10 of 16 As can be seen in Figure 8, all TP2 samples (washed and unwashed) have shown higher PL As can be seen in Figure 8, all TP2 samples (washed and unwashed) have shown higher PL intensity intensity a as s c compar ompared w ed with ith prist pristine ine T TiO iO2.. The The intensity intensity from the from the unwashed unwashed TP TP2 2 sample sample might might be be due due to the presence of C–C bond that allowed for the electron to recombine under low excitation energy. to the presence of C–C bond that allowed for the electron to recombine under low excitation energy. The The TP2 sam TP2 sample ple (w (washed) ashed) has has rrecorde ecorded d the the highe highest st PL PL intensity intensity a ass compa comparred ed to to others. others. Based Based on on the the XPS and FTIR spectra, this TP2 sample has a C=O bond as an extra species where it is not found in XPS and FTIR spectra, this TP2 sample has a C=O bond as an extra species where it is not found in pristine pristine and and unwashed unwashed TP2 TP2 samples. samples. Hence, it Hence, it can can be be conclude concluded d that the highest intensit that the highest intensity y of T of TP2 P2 showed showed a a s significant ignificant pre presence sence of C of C=O =O b bond, ond, which which a acted cted as as an an ele electr ctron inject on injector or in TP in TP2 2 p photocatalyst. hotocatalyst. The The electrons that we electrons that werre e in inj jected ected to the to the conduction conduction band of TiO band of TiO 2 cr created eated a a ser series ies of chain of chain re reactions, actions, −1 which which help help t too achieve a com achieve a complete plete 100% decolorization of 12 m 100% decolorization of 12 mggL ·L MB dye at MB dye at 75 m 75 min, in, as s as shown hown in in Figur Figure 7c. e 7c. 2.8. Recyclability Study 2.8. Recyclability Study For the stability study of the photocatalyst, the photocatalytic activity of TP2 was carried out For the stability study of the photocatalyst, the photocatalytic activity of TP2 was carried out by by eight cycles of photodegradation of 12 mg −1 L MB dye with 15-min intervals for 60 min in every eight cycles of photodegradation of 12 mg·L MB dye with 15-min intervals for 60 min in every cycle. cycle. Figure 9 shows the photodegradation cycle of MB dye using TP2. It was observed that each Figure 9 shows the photodegradation cycle of MB dye using TP2. It was observed that each recycled recycled application produced 100% removal of MB; indicating a sustainable photocatalytic efficiency application produced 100% removal of MB; indicating a sustainable photocatalytic efficiency characteristic. In other words, a strong interaction of TiO with PEG occurred due to its strong characteristic. In other words, a strong interaction of TiO2 2 with PEG occurred due to its strong chemisorption on the surface of TiO , where it was not easily leached out, even through up to eight chemisorption on the surface of TiO2, where it was not easily leached out, even through up to eight times of repeated usage. times of repeated usage. Figure 9. The recyclability graphs of TP2 under the photodegradation of MB dye. Figure 9. The recyclability graphs of TP2 under the photodegradation of MB dye. 2.9. Chemical Oxygen Demand Analysis 2.9. Chemical Oxygen Demand Analysis Decolorization of dye does not mean that there is complete removal of the organic carbons from Decolorization of dye does not mean that there is complete removal of the organic carbons the water samples. Mineralization, which is defined as the complete decomposition of organic from the water samples. Mineralization, which is defined as the complete decomposition of organic compounds into CO2 and H2O, should be the target of any photocatalytic processes. One of the compounds into CO and H O, should be the target of any photocatalytic processes. One of the results 2 2 results of mineralization is the lowering of the chemical oxygen demand (COD) values of the treated of mineralization is the lowering of the chemical oxygen demand (COD) values of the treated samples. samples. In this study, the presence of organic substances or intermediates can be detected by using In this study, the presence of organic substances or intermediates can be detected by using a COD a COD test. The COD test is attributed to the degradation of MB dye, as well as its by-products test. The COD test is attributed to the degradation of MB dye, as well as its by-products during the during the photocatalytic reaction using the TP2 sample. There is a possible contamination from photocatalytic reaction using the TP2 sample. There is a possible contamination from DSAT, and these DSAT, and these contaminations were completely cleaned up during the washing process prior to contaminations were completely cleaned up during the washing process prior to the photodegradation the photodegradation of MB dye. Figure 10 presents the detected COD values for the mineralization of MB dye. Figure 10 presents the detected COD values for the mineralization of MB dye versus −1 of MB dye versus irradiation time. The COD values (mg·L ) detected were 0.81, 0.75, 0.60, 0.45, 0.30, 0.25, 0.10 and 0.05 and kept decreasing with time at 60, 120, 180, 240, 300, 360, 420 and 480 min, Appl. Sci. 2017, 7, 24 11 of 16 Appl. Sci. 2016, 6, 451 11 of 16 irradiation time. The COD values (mgL ) detected were 0.81, 0.75, 0.60, 0.45, 0.30, 0.25, 0.10 and 0.05 respectively. Hence, the complete mineralization of MB dye through the COD test was greatly Appl. Sci. 2016, 6, 451 11 of 16 and kept decreasing with time at 60, 120, 180, 240, 300, 360, 420 and 480 min, respectively. Hence, the caused by the improved diffusion of dye into photocatalyst layers, stemming from the porous complete mineralization of MB dye through the COD test was greatly caused by the improved diffusion surface of the TP2 photocatalyst sample. respectively. Hence, the complete mineralization of MB dye through the COD test was greatly of dye into photocatalyst layers, stemming from the porous surface of the TP2 photocatalyst sample. caused by the improved diffusion of dye into photocatalyst layers, stemming from the porous surface of the TP2 photocatalyst sample. 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0 100 200 300 400 500 0 100 200 300 400 500 Time (hr) Time (hr) Figure 10. The chemical oxygen demand (COD) analysis of TP2 under photodegradation of MB dye. Figure 10. The chemical oxygen demand (COD) analysis of TP2 under photodegradation of MB dye. Figure 10. The chemical oxygen demand (COD) analysis of TP2 under photodegradation of MB dye. 3 3. . Ex Experimental perimental Se Section ction 3. Experimental Section 3.1. Preparation of Immobilized TiO -PEG 3.1. Preparatio 3.1. Preparatio n of Immobilized TiO n of Immobilized TiO 2-PE 2-PE GG The sample solution was prepared by mixing 6.5 g of titanium dioxide (TiO ) Degussa P25 powder The sample solution was prepared by mixing 6.5 g of titanium dioxide (TiO 2 2) Degussa P25 The sample solution was prepared by mixing 6.5 g of titanium dioxide (TiO2) Degussa P25 (20%powder (20% rutile, rutile, 80% anatase) 80% an in 50 matase) in 50 mL L distilled water dadded istilled w with ater 1 mL added of with 8% (w/v 1 mL ) of of 8% polyet ( hylene w/v) of glycol powder (20% rutile, 80% anatase) in 50 mL distilled water added with 1 mL of 8% (w/v) of polyethylene glycol (Merck, Kenilworth, NJ, USA, MW = 6000). The sample solution was sonicated (Merck, Kenilworth, NJ, USA, MW = 6000). The sample solution was sonicated under an ultrasonic polyethylene glycol (Merck, Kenilworth, NJ, USA, MW = 6000). The sample solution was sonicated under an ultrasonic vibrator for 30 min to make it homogenized. The immobilized sample was vibrator for 30 min to make it homogenized. The immobilized sample was prepared by using a under an ultrasonic vibrator for 30 min to make it homogenized. The immobilized sample was prepared by using a brush-coating method applied onto a clean glass plate prior to taping with brush-coating method applied onto a clean glass plate prior to taping with double-sided adhesive prepared by using a brush-coating method applied onto a clean glass plate prior to taping with double-sided adhesive tape (DSAT). The wet TiO2-PEG coated on glass was then dried using an tape (DSAT). The wet TiO -PEG coated on glass was then dried using an 850-W hot blower with a double-sided adhesive tape (DSAT). The wet TiO2-PEG coated on glass was then dried using an 850-W hot blower with a temperature of about 120 °C until dry. The process was continued by temperature of about 120 C until dry. The process was continued by repeating the process of coating 850-W hot blower with a temperature of about 120 °C until dry. The process was continued by repeating the process of coating onto dried TiO2-PEG until the desired loading of immobilized onto dried TiO -PEG until the desired loading of immobilized TiO -PEG was achieved. Figure 11 2 2 repeating the process of coating onto dried TiO2-PEG until the desired loading of immobilized TiO2-PEG was achieved. Figure 11 shows the coated TiO2 on DSAT attached to the glass plate with shows the coated TiO on DSAT attached to the glass plate with and without PEG binder. TiO2-PEG w and without as achieved. F PEG binder. igure 11 shows the coated TiO2 on DSAT attached to the glass plate with and without PEG binder. Figure 11. Picture of immobilized TiO2 with and without PEG binder and the molecular structure Figure 11. Picture of immobilized TiO with and without PEG binder and the molecular structure of PEG. of PEG. Figure 11. Picture of immobilized TiO2 with and without PEG binder and the molecular structure of PEG. -1 COD Concentration (mg L ) -1 COD Concentration (mg L ) Appl. Sci. 2016, 6, 451 12 of 16 Appl. Sci. 2017, 7, 24 12 of 16 3.2. Characterization Tests of Immobilized TiO2/PEG DSAT X-ray diffraction (XRD) spectra were obtained using a Rigakuminiflex II, X-ray diffractometer 3.2. Characterization Tests of Immobilized TiO /PEG DSAT (Rigaku, Tokyo, Japan). Structural information of the films was obtained in the range of 2θ angles X-ray diffraction (XRD) spectra were obtained using a Rigakuminiflex II, X-ray diffractometer from 3° to 80° with a step size increment of 1.00 s/step. FTIR spectra of powder samples were (Rigaku, Tokyo, Japan). Structural information of the films was obtained in the range of 2 angles from recorded on Perkin Elmer Spectrum Version equipped with an attenuated total reflectance device 3 to 80 with a step size increment of 1.00 s/step. FTIR spectra of powder samples were recorded on (Perkin Elmer, Waltham, MA, USA) with a diamond crystal. Spectra were collected in a frequency Perkin Elmer Spectrum Version equipped with an attenuated total reflectance device (Perkin Elmer, −1 −1 range of 600 to 4000 cm with 4 scans and a spectral resolution of 4 cm . The morphology of the Waltham, MA, USA) with a diamond crystal. Spectra were collected in a frequency range of 600 to samples was observed with field-emitting scanning electron microscopy (FE-SEM, JSM-6700F, 1 1 4000 cm with 4 scans and a spectral resolution of 4 cm . The morphology of the samples was Akishima, Tokyo, Japan) with an accelerating voltage of 10 kV. The surface area of the immobilized observed with field-emitting scanning electron microscopy (FE-SEM, JSM-6700F, Akishima, Tokyo, TiO2 film powders was measured by nitrogen adsorption using the BET equation at 77 K Japan) with an accelerating voltage of 10 kV. The surface area of the immobilized TiO film powders (Micrometrics ASAP 2020M + C, Norcross, GA, USA). A UV–Vis spectrophotometer UV-2550, was measured by nitrogen adsorption using the BET equation at 77 K (Micrometrics ASAP 2020M + C, Shimadzu was used to obtain the UV–Vis reflectance spectrum of the powder sample. X-ray Norcross, GA, USA). A UV–Vis spectrophotometer UV-2550, Shimadzu was used to obtain the UV–Vis photoelectron spectroscopy (XPS) with a Thermo ESCALAB 250 spectrometer using a radiation reflectance spectrum of the powder sample. X-ray photoelectron spectroscopy (XPS) with a Thermo source of monochromatic Al Kα with the energy of 1486.6 eV, 200 W and a photoluminescence ESCALAB 250 spectrometer using a radiation source of monochromatic Al K with the energy analyzer (JovinYvon, Chiyoda-ku, Tokyo, Japan) was used to determine the photoluminescence of 1486.6 eV, 200 W and a photoluminescence analyzer (JovinYvon, Chiyoda-ku, Tokyo, Japan) was intensity of the samples. used to determine the photoluminescence intensity of the samples. 3.3. Washing Process of Immobilized Samples 3.3. Washing Process of Immobilized Samples The washing process was conducted to oxidize PEG and also to clean all unwanted The washing process was conducted to oxidize PEG and also to clean all unwanted contaminants contaminants from immobilized TiO2-PEG samples. The process was done by irradiating the from immobilized TiO -PEG samples. The process was done by irradiating the immobilized samples immobilized samples in distilled water inside a glass cell of 150 mm × 10 mm × 80 mm (length × in distilled water inside a glass cell of 150 mm  10 mm  80 mm (length  width  height). width × height). An aquarium pump model NS 7200 (Minjiang, Jiangmen, China) was used as an An aquarium pump model NS 7200 (Minjiang, Jiangmen, China) was used as an aeration source and aeration source and irradiated with a 55-W fluorescent lamp for 1 h. The washing process was irradiated with a 55-W fluorescent lamp for 1 h. The washing process was repeated once again by repeated once again by replacing the distilled water with a new amount of distilled water irradiated replacing the distilled water with a new amount of distilled water irradiated for another 30 min to for another 30 min to affirm that zero contamination is achieved. This contamination was measured affirm that zero contamination is achieved. This contamination was measured by using chemical by using chemical oxygen demand analysis (COD) to detect any presence of organic compounds in oxygen demand analysis (COD) to detect any presence of organic compounds in washed distilled washed distilled water. This process was done prior to the photodegradation of MB dye. water. This process was done prior to the photodegradation of MB dye. 3.4. Photodegradation of MB Dye 3.4. Photodegradation of MB Dye The activity of the catalyst was tested by the degradation of methylene blue (MB), Fluka The activity of the catalyst was tested by the degradation of methylene blue (MB), Fluka Analytical, Analytical, with a chemical formula: C12H15O6; and the molecular structure of MB is shown in with a chemical formula: C H O ; and the molecular structure of MB is shown in Figure 12. 12 15 6 Figure 12. The experimental procedure was the same method from our previous report [57]. The The experimental procedure was the same method from our previous report [57]. The immobilized −1 immobilized TiO2-PEG was immersed into 20 mL of 12 mg·L MB dye placed inside a glass cell TiO -PEG was immersed into 20 mL of 12 mgL MB dye placed inside a glass cell under an aeration under an aeration source. Light was then irradiated using a 55-W fluorescent lamp, Model Ecotone, source. Light was then irradiated using a 55-W fluorescent lamp, Model Ecotone, with visible light −2 with visible light intensity measured for about 461 and 6.7 W·m of UV light detected as UV leakage. intensity measured for about 461 and 6.7 Wm of UV light detected as UV leakage. A 4-mL aliquot A 4-mL aliquot of treated MB dye was then taken out from the glass cell at 15-min intervals until it of treated MB dye was then taken out from the glass cell at 15-min intervals until it turned colorless by turned colorless by measuring its concentration using UV spectrophotometer Model HACH DR 1900 measuring its concentration using UV spectrophotometer Model HACH DR 1900 at a 661-nm  max at a 661-nm λ max detector (Hach, Loveland, CO, USA). The experimental procedure was repeated detector (Hach, Loveland, CO, USA). The experimental procedure was repeated by applying the same by applying the same steps for different catalysts loading and different TiO2/PEG ratios. steps for different catalysts loading and different TiO /PEG ratios. Figure 12. The molecular structure for methylene blue. Figure 12. The molecular structure for methylene blue. Appl. Sci. 2017, 7, 24 13 of 16 3.5. COD Analysis Initially, the immobilized TiO /PEG DSAT (TP2) film was immersed in 20 mL of distilled water inside a glass cell under the irradiation of a 55-W compact fluorescent lamp. After 1 h of irradiation, the water sample was withdrawn and replaced with another set of distilled water using the same immobilized TiO /PEG film until 8 h of irradiation. The withdrawn water samples were then subjected to the COD test. It can be observed that MB dye and its generated by-products had undergone almost 100% complete mineralization after 8 h of irradiation using an immobilized TiO /PEG film. 3.6. Recyclability Study The recyclability study was carried out to see the effect of immobilized TiO -PEG towards photodegradation stability. The experiment was conducted initially through the photodegradation method. Immobilized TiO -PEG was then subjected to the washing process using distilled water and irradiated for 30 min. Both the photodegradation and washing procedures for the immobilized the TiO -PEG sample were then repeated until eight cycles. The photodegradation percentage of MB in every cycle was recorded at every 15-min interval until MB became colorless. 4. Conclusions An immobilized active TiO photocatalyst was successfully prepared via adding a small amount of PEG as a binder onto a support binder of double-sided adhesive tape (DSAT). It was observed that utilization of 10:0.1 of a TiO /PEG ratio at 0.3 g of catalyst loading produced an immobilized TiO 2 2 with excellent photocatalytic activity. The preparation process did not produce any significant phase transformation, except for the typical TiO phase. From the XPS and FTIR spectra, both observed that washed TiO -PEG (TP2) produced C=O bond that was confirmed to initiate the photocatalytic activity of the sample and to be 1.8-times higher than pristine TiO under suspension mode in degrading 12 mgL MB dye. High PL intensity with low activation energy under immobilized TiO -PEG (TP2) proved that the presence of C=O increased the injected electron into the conduction band that eventually produced the hydroxyl radical agent used for the degradation of MB dye under visible light irradiation. Finally, TP2 or immobilized TiO -PEG was very stable and possessed excellent sustainable photocatalytic activity up to eight-times of reusability and comparable to recent photocatalysis cycles. As shown by the COD analysis, TP2 or immobilized TiO -PEG with DSAT leaves no organic pollutants during photodegradation cycles, which brings about a significant improvement in water quality. Acknowledgments: We would like to thank the Ministry of Education (MOE), Malaysia, for providing generous financial support under the Research Acculturation Grant Scheme (RAGS) grants (600-RMI/RAGS 5/3 (35/2014)) in conducting this study and Universiti Teknologi MARA (UiTM) for providing all of the needed facilities. Author Contributions: The experimental work and drafting of the manuscript were carried out by Raihan Zaharudin and assisted by Mohd Azlan Mohd Ishak, Khudzir Ismail and Ahmad Zuliahani participated in the interpretation of the scientific results and the preparation of the manuscript. Wan Izhan Nawawi supported the work and cooperation between UiTM Perlis and UiTM Shah Alam, supervised the experimental work, commented and approved the manuscript. The manuscript was written through comments and contributions of all authors. All authors have given approval to the final version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. References 1. Karimi, L.; Yazdanshenas, M.E.; Khajavi, R.; Rashidi, A.; Mirjalili, M. Optimizing the photocatalytic properties and the synergistic effects of graphene and nano titanium dioxide immobilized on cotton fabric. Appl. Surf. Sci. 2015, 332, 665–673. [CrossRef] 2. Hashimoto, K.; Irie, H.; Fujishima, A. 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The Preparation and Characterization of Immobilized TiO2/PEG by Using DSAT as a Support Binder

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applied sciences Article The Preparation and Characterization of Immobilized TiO /PEG by Using DSAT as a Support Binder 1 , 1 , 2 1 2 Wan Izhan Nawawi *, Raihan Zaharudin , Mohd Azlan Mohd Ishak , Khudzir Ismail and Ahmad Zuliahani Faculty of Applied Sciences, Universiti Teknologi MARA, Perlis, 02600 Arau, Perlis, Malaysia; nurraihanzaharudin@gmail.com (R.Z.); azlanishak@perlis.uitm.edu.my (M.A.M.I.); zuliahani@perlis.uitm.edu.my (A.Z.) Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia; khudzir@salam.uitm.edu.my * Correspondence: wi_nawawi@perlis.uitm.edu.my; Tel.: +60-4-9882-570; Fax: +60-4-9882-026 Academic Editor: Giorgio Biasiol Received: 30 September 2016; Accepted: 13 December 2016; Published: 23 December 2016 Abstract: Immobilized TiO was prepared by adding a small composition of polyethylene glycol (PEG) as a binder, and this paper reported for the very first time the formation of C=O from oxidized PEG, which acted as an electron injector in enhancing photoactivity. Water-based TiO with PEG formulation was deposited by using a brush technique onto double-sided adhesive tape (DSAT) as a support binder to increase the adhesiveness of immobilized TiO . The photocatalytic activity of immobilized TiO -PEG was measured by photodegradation of 12 mgL methylene blue (MB) dye. The optimum condition of immobilized TiO -PEG was observed at TiO /PEG-2 (TP2) with 10:0.1 2 2 for the TiO /PEG ratio, which resulted in a 1.8-times higher photodegradation rate as compared to the suspension mode of pristine TiO . The high photodegradation rate was due to the formation of the active C=O bond from oxidized PEG binder in immobilized TiO -PEG as observed by Fourier transform infrared and X-ray photoelectron spectroscopy analyses. The presence of C=O has escalated the photoactivity by forming an electron injector to a conduction band of TiO as proven by higher photoluminescence intensity obtained for TP2 as compared to pristine TiO . The TP2 sample has also increased its adhesiveness when DSAT is applied as a support binder where it can be recycled up to eight times and comparable to recent photocatalysis cycle developments. Keywords: immobilization; titanium dioxide; oxidized PEG; support binder; methylene blue 1. Introduction Titanium oxide (TiO ) is a semiconductor that is widely known as a photocatalyst for the photodegradation of organic pollutants. According to Karimi et al. [1], when TiO is illuminated by a light with energy higher than its band gap energy, electron–hole pairs diffuse out, creating negative electrons and oxygen that combine to become O , while the positive electric holes and water generate hydroxyl radicals. This highly active oxygen species can then oxidize organic pollutants. For over three decades, modifications on TiO have improved two main issues. The first is to increase catalytic reaction performance where photocatalytic improvement of TiO has been studied by many researchers. This is important in order to make the photocatalyst become active in the wide solar spectrum since the TiO semiconductor is only active under high energy ultraviolet (UV) light [2]. Basically, there are two common types of modifications used in preparing the visible light TiO , which are the modification of bulk and the surface of TiO . Bulk modification often resulted in the narrowing of band gap energy, whereas surface modification does not change the band gap energy. However, surface modification successfully activates the photocatalyst under visible light by accepting Appl. Sci. 2017, 7, 24; doi:10.3390/app7010024 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 24 2 of 16 the electron from the sensitizing agent. The second is a recovery process of this photocatalyst after being treated with organic pollutants where the photodegradation of organic pollutants using TiO is basically applied under suspension mode, which provides a high surface to volume ratio [3]. However, the recovery process of this suspension of TiO powder with treated wastewater requires filtration. Separation of this nano-sized TiO will clog the filter membrane and eventually penetrate through the porous filter, making the recovery process less effective [4]. Immobilization of TiO has gained great interest since it can easily separate TiO with treated wastewater [5]. Many papers with different types of polymers in immobilized TiO have reported these findings after the first technique was discovered by Tennakone et al. [6]. Polymer is normally added as a binder in immobilized TiO to improve its durability, temperature resisting ability and absorbance affinity towards pollutants [7–9], and some polymers are also able to increase TiO photoactivity [10–25]. In most cases, prepared immobilized TiO used solvent-based polymers, such as polythene sheets [10], thin polythene films [11], polystyrene (PS) beads [12], expanded polystyrene (EPS) beads [13], cellulose microspheres [14], fluoro polymer resins [15], polyethylene terephthalate (PET) bottles [16], polypropylene (PP) granules [17], cellulose fibers [18], polypropylene fabric (PPF) [19], polyvinyl chloride (PVC) [20], polycarbonate (PC), poly(methyl methacrylate) (PMMA) [21], polyvinyl acetate (PVAc) [22], poly(styrene)-co-poly(4-vinylpyridine) (PSP4VP) [23], rubber latex (an elastic hydrocarbon polymer) [24], parylene and tedlar [25]. Recently, water-based polymer binders have shown a vast potential of usage in immobilized TiO for commercialization because they are environmentally friendly and economic. Water-based polymers, such as polyvinyl alcohol (PVA), polyaniline (PANI), polyvinyl pyrolydone (PVP) and polyethylene glycol (PEG), have performed greatly in water treatment separation, which also resulted in a significant photosensitivity to the visible light region [26–28]. For instance, Lei et al. [29] decorated TiO /PVA particles through a heat treatment method and obtained high photoactivity since the Ti–O–C bond generated led the immobilized sample to become fully in contact with the organic pollutant. TiO /PVA developed by Yang et al. [30] showed notable photoactivity under visible light irradiation due to the presence of conjugated polymer. They found that the conjugated molecules adsorbed on the TiO surface and excited the electrons, thus injecting the electrons into the conduction band using visible light. PANI in immobilized TiO studied by Nawi et al. [31] has shown a significant photocatalytic improvement by acting as a hole scavenger in TiO under photodegradation of reactive red 4 (RR4) dye. PEG is a polymer that is able to form a uniform surface, smooth coating and well-defined size particles [32]. PEG is a hydrophilic polymer, which is suitable for coating, as it reduces the formation of a cracked surface in the immobilized system [33]. Most of the reports on immobilized TiO with PEG were identified to enhance photoactivity due to the effect of porosity and larger specific surface area [34,35]. Trapalis et al. [36] found that the porosity increased with PEG amount introduced in the film. However, no detailed study was conducted on surface chemical interaction between PEG and TiO photocatalyst since the oxidation will only take place in the photocatalysis process. Technically, TiO and polymer mixed ratios are a very critical factor. Excessive binder makes immobilized film strong, but reduces its photoactivity; however, a low amount of polymer makes TiO leach out easily. This explained the lack of data discussed by other researchers on the surface chemical interaction of PEG in immobilized TiO . According to Wang et al. [37], visible light photocatalytic activity could also be enhanced by carbon-sensitized TiO , in which the carbon could be originated from titanium alkoxide and mainly existed as the C–C bonds (carbonaceous species), C=O and O=C–O bonds (carbonate species). Since visible light active TiO using surface modification cannot be judged by using UV–Vis diffuse reflectance spectra (DRS), the easiest way to detect the characteristic of the visible light activity of this modified TiO is through photoluminescence (PL) analysis where the highest PL intensity represented the highest photoactivity of the sample. Previously, we have discovered the ability of double-sided adhesive tape (DSAT) to become a support binder in immobilized TiO -only [38]. DSAT has greatly improved immobilized TiO -only in 2 2 Appl. Sci. 2017, 7, 24 3 of 16 its strength and recyclability. Photocatalytic activity of this immobilized TiO -only, however, is slightly lower as compared to TiO in suspension mode [39]. In this study, immobilized TiO mixed with a 2 2 small amount of PEG 6000 was prepared using DSAT as a support binder to observe the interaction between TiO and PEG during photocatalysis process. The photoactivity of this immobilized TiO -PEG 2 2 was observed by photodegradation of methylene blue, applied with specific parameters. 2. Results and Discussion 2.1. Characterization of Immobilized TiO A method for immobilizing TiO using DSAT was thoroughly described in the previous article [40]. In this work, a similar approach was taken where TiO was coated onto DSAT as a support binder, but in the presence of a specific amount of PEG. Table 1 shows the experimental condition and pseudo first order rate constant of immobilized TiO and the pristine TiO sample in degrading methylene 2 2 blue (MB) dye. As can be seen in Table 1, the increased amount of PEG in the formulation solutions has shown a significant increase of PEG in immobilized TiO -PEG samples detected by FTIR analysis. The 1160- and 2900-nm peaks of C–O and C–H stretching, respectively, as shown in Figure 1, corresponded to the PEG peaks based on similar peaks presented on the FTIR spectrum for pure PEG studied by Hyma et al. [41]. Photocatalytic activity of washed immobilized TiO -PEG was increased by increasing the amount of PEG from 0.05 to 0.1 g based on its photodegradation rate values. Photocatalytic activity starts to decrease beyond 0.1 g of PEG due to the agglomeration of PEG in TiO particles, which makes it harder for a large amount of light to penetrate through the surface particles of TiO . Mukherjee et al. [39] have reported same behavior on immobilized TiO with 2 2 PVA. The catalyst loading effect of immobilized TiO with 0.1 g of PEG samples has shown a different photocatalytic activity. Increasing the amount of catalyst loading from 0.05, 0.1, 0.2 and 0.3 grams will significantly increase the photodegradation rates from 0.039, 0.048, 0.048 and 0.087 min , respectively. The high photodegradation rate at 0.3 g was due to the high adsorption capacity of the photocatalyst thin film composite that enhanced the decolorization process of MB dye, and eventually, this dye pollutant becomes degraded by the photocatalysis process. However, too much adsorption capacity beyond 0.3 g of catalyst loading for the immobilized TiO -PEG sample makes the photodegradation rate become 2.5-times slower. This observation was due to the excessive adsorbed MB dye on the surface of immobilized TiO -PEG, thus forming a thick layer coating on the TiO surface and reducing 2 2 the penetration of light for photocatalysis. It can be deduced that the immobilized TiO -PEG ratio of 10:0.1 at a 0.3-g catalyst loading is the optimum immobilized TiO -PEG with a photodegradation rate 1.8-times faster compared to pristine TiO ; an optimum sample named as TiO /PEG-2 (TP2) (Table 1). 2 2 Table 1. The experimental condition and pseudo first order rate constant of immobilized TiO and the control sample in degrading methylene blue (MB) dye. Rate Constant k (min ) TiO Loading Mode Amount of Ratio BET a b 2 1 Sample (g) (S or I ) PEG (wt %) (TiO /PEG) (m g ) 2 Washed Unwashed Pristine TiO 0.3 S 0.00 10:0 50.00 0.048 - TP0 0.3 I 0.00 10:0 49.25 0.054 0.048 TP1 0.3 I 0.05 10:0.05 - 0.080 0.041 TP2 0.3 I 0.10 10:0.1 88.30 0.087 0.017 TP3 0.3 I 0.15 10:0.15 - 0.081 0.024 TP4 0.3 I 0.20 10:0.2 - 0.040 0.030 TP5 0.05 I 0.10 10:0.1 - 0.039 0.039 TP6 0.1 I 0.10 10:0.1 - 0.048 0.048 TP7 0.2 I 0.10 10:0.1 - 0.048 0.028 TP8 0.4 I 0.10 10:0.1 - 0.030 0.015 a b Suspension; immobilize. S : surface area; TP0 to TP8, TiO /PEG-0 to TiO -PEG-8. BET 2 2 Appl. Sci. 2017, 7, 24 4 of 16 Appl. Sci. 2016, 6, 451 4 of 16 Appl. Sci. 2016, 6, 451 4 of 16 TP1 (0.5% PEG) TP1 (0.5% PEG) C-O TP2 (1.0% PEG) C-O TP2 (1.0% PEG) C-H TP3 (1.5% PEG) C-H TP3 (1.5% PEG) TP4 (2.0% PEG) TP4 (2.0% PEG) 3000 2000 1000 4000 3000 2000 1000 3000 2000 1000 4000 3000 2000 1000 -1 Wavelength (cm ) -1 Wavelength (cm ) Figure 1. FTIR spectra of different percentages of polyethylene glycol (PEG) in immobilized Figure Figure 1. 1. FTIR FTspectra IR spectra of dif of ferdifferent perc ent percentages entag of e polyethylene s of polyethy glycol lene g (PEG) lycol (P in immobilized EG) in immo Tbilized iO -PEG. TiO2-PEG. TiO2-PEG. 2.2. XRD Analysis 2.2. XRD Analysis 2.2. XRD Analysis Figure 2 shows the XRD patterns for pristine TiO , TP0 (immobilized TiO DSAT) and TP2 2 2 Figure 2 shows the XRD patterns for pristine TiO2, TP0 (immobilized TiO2 DSAT) and TP2 Figure 2 shows the XRD patterns for pristine TiO2, TP0 (immobilized TiO2 DSAT) and TP2 (immobilized TiO /PEG DSAT) samples. All peaks were detected as anatase and rutile phases. (immobilized TiO2/PEG DSAT) samples. All peaks were detected as anatase and rutile phases. There (immobilized TiO2/PEG DSAT) samples. All peaks were detected as anatase and rutile phases. There There is no phase transformation occurring in TP0 and TP2, since the immobilization processes is no phase transformation occurring in TP0 and TP2, since the immobilization processes of TP0 and is no phase transformation occurring in TP0 and TP2, since the immobilization processes of TP0 and of TP0 and TP2 were prepared under low temperature (120 C). Phase transformation in the TP2 were prepared under low temperature (120 °C). Phase transformation in the presence of organic TP2 were prepared under low temperature (120 °C). Phase transformation in the presence of organic presence of organic binder was observed when the calcining temperature happened at 900 C [42]. binder was observed when the calcining temperature happened at 900 °C [42]. Wang et al. [43] also binder was observed when the calcining temperature happened at 900 °C [42]. Wang et al. [43] also Wang found tha et al. [43 t the pha ] also found se transf thatorm theaphase tion fotransformation r TiO2 calcined witho for TiOut organ calcined ic bin without der occur organic red at a binder found that the phase transformation for TiO2 calcined without organic binder occurred at a temperature of 700 °C onwards. All peaks for all samples correspond to the characteristic peak of the occurred at a temperature of 700 C onwards. All peaks for all samples correspond to the characteristic temperature of 700 °C onwards. All peaks for all samples correspond to the characteristic peak of the anatase and rutile phases of TiO2 nanoparticles detected at 2θ from 15° to 65° by using low angle peak of the anatase and rutile phases of TiO nanoparticles detected at 2 from 15 to 65 by using anatase and rutile phases of TiO2 nanoparticles detected at 2θ from 15° to 65° by using low angle XRD. The XRD pattern in all samples shows different crystallinity as the sharp diffraction peaks low angle XRD. The XRD pattern in all samples shows different crystallinity as the sharp diffraction XRD. The XRD pattern in all samples shows different crystallinity as the sharp diffraction peaks displayed the good crystallinity of the prepared nanoparticles [44]. Pristine TiO2 has the highest peaks displayed the good crystallinity of the prepared nanoparticles [44]. Pristine TiO has the highest displayed the good crystallinity of the prepared nanoparticles [44]. Pristine TiO2 has the 2 highest crystallinity as compared to TP0 and TP2, where TP2 is the lowest. This is due to the presence of crystallinity as compared to TP0 and TP2, where TP2 is the lowest. This is due to the presence of DSAT crystallinity as compared to TP0 and TP2, where TP2 is the lowest. This is due to the presence of DSAT and DSAT + PEG in TP0 and TP2, respectively. The increased porosity in both immobilized and DSAT an DSAT d DSAT + + PEG in TP0 PEGand in TP0 an TP2, respectively d TP2, resp.ect The ively. The increased incre por ased p osityoin rosit both y in b immobilized oth immobili samples zed samples is the main factor for low crystallinity and broad peaks, as reported by Kim [45]. In short, no samples is the main factor for low crystallinity and broad peaks, as reported by Kim [45]. In short, no is the main factor for low crystallinity and broad peaks, as reported by Kim [45]. In short, no phase phase transformations occurred for any of the samples prepared under low heat temperature. phase transformations occurred for any of the samples prepared under low heat temperature. transformations occurred for any of the samples prepared under low heat temperature. R R A R R A A A R R A R R A A A TP2 TP2 4000 4000 TP0 TP0 2000 2000 A – anatase A – anatase R – rutile R – rutile Pristine TiO Pristine TiO 15 30 45 60 15 30 45 60 15 30 45 60 45 60 15 30 2 theta 2 theta Figure 2. XRD patterns of pristine TiO2, TP0 and TP2 samples. Figure 2. XRD patterns of pristine TiO2, TP0 and TP2 samples. Figure 2. XRD patterns of pristine TiO , TP0 and TP2 samples. Intensity (a.u) Intensity (a.u) Transmittance % Transmittance % Appl. Sci. 2017, 7, 24 5 of 16 Appl. Sci. 2016, 6, 451 5 of 16 2.3. SEM Images 2.3. SEM Images Figure 3a,b shows the illustrative SEM images of the TP0 and TP2 surfaces after the photocatalytic Figure 3a,b shows the illustrative SEM images of the TP0 and TP2 surfaces after the degradation process. Few pores were observed in TP0, while for TP2, a variety of porous structures photocatalytic degradation process. Few pores were observed in TP0, while for TP2, a variety of evolved on the surface. As shown in Figure 3b, the pore depth is larger around the PEG concentration porous structures evolved on the surface. As shown in Figure 3b, the pore depth is larger around the of 8.0 g/100 mL as compared to TP0. It can be concluded that the porous structure of TP2 thin films is PEG concentration of 8.0 g/100 mL as compared to TP0. It can be concluded that the porous structure related to the molecular weight of PEG. The increased concentration of PEG has granted the formation of TP2 thin films is related to the molecular weight of PEG. The increased concentration of PEG has of big pores. It is obvious that the catalyst morphologies shown in Figure 3b are highly presumed to granted the formation of big pores. It is obvious that the catalyst morphologies shown in Figure 3b perform an optimum photocatalytic activity. In this regard, Bing et al. [46] stated that the increased are highly presumed to perform an optimum photocatalytic activity. In this regard, Bing et al. [46] porosity in the immobilized TiO /PEG film is accountable for the increased photodegradation of the stated that the increased porosity in the immobilized TiO2/PEG film is accountable for the increased methyl orange model pollutant dye. photodegradation of the methyl orange model pollutant dye. Figure 3. Scanning electron micrograph of surface morphologies for the (a) TP0 and Figure 3. Scanning electron micrograph of surface morphologies for the (a) TP0 and (b) TP2 samples. (b) TP2 samples. EHT, extra high tension. EHT, extra high tension. 2.4. N2 Adsorption-Desorption 2.4. N Adsorption-Desorption The porous structure of TP2 is displayed by N2 adsorption-desorption measurement as The porous structure of TP2 is displayed by N adsorption-desorption measurement as presented. presented. It was found that the TP2 sample in Figure 4 exhibited the type IV nitrogen isotherm It was found that the TP2 sample in Figure 4 exhibited the type IV nitrogen isotherm according to according to the International Union of Pure and Applied Chemists (IUPAC) classification; thus, this the International Union of Pure and Applied Chemists (IUPAC) classification; thus, this indicated indicated that the TP2 sample is a mesoporous structure. According to Li et al. [47], a large surface that the TP2 sample is a mesoporous structure. According to Li et al. [47], a large surface area with a area with a mesoporous structure is favorable to obtain a high photocatalytic activity, as it promotes mesoporous structure is favorable to obtain a high photocatalytic activity, as it promotes adsorption, adsorption, desorption and diffusion of reactants and products. The BET surface area of TP2 was desorption and diffusion of reactants and products. The BET surface area of TP2 was enhanced 2 −1 2 −1 enhanced to 88.3 m ·g as compared to pristine TiO2, which was circa 50 m ·g , where a 43.4% 2 1 2 1 to 88.3 m g as compared to pristine TiO , which was circa 50 m g , where a 43.4% increment increment occurred due to the increased porosity and number of pores. By comparing TP0 with TP2, occurred due to the increased porosity and number of pores. By comparing TP0 with TP2, it is clear that the BET surface area for TP2 was increased up to 44% as compared to TP0, which was it is clear that the BET surface area for TP2 was increased up to 44% as compared to TP0, 2 −1 circa 49.25 m ·g . The results of TP2 surface area measurements are consistent with the results of 2 1 which was circa 49.25 m g . The results of TP2 surface area measurements are consistent photocatalytic activity where the TP2 sample gave a higher photocatalytic degradation rate of 0.087 with the results of photocatalytic activity where the TP2 sample gave a higher photocatalytic −1 −1 min as compared to pristine TiO2, which is 0.048 and 0.054 min for TP0, respectively. A higher 1 1 degradation rate of 0.087 min as compared to pristine TiO , which is 0.048 and 0.054 min for TP0, BET surface area is vital for the adsorption and desorption of dyes and catalysts, since it encourages respectively. A higher BET surface area is vital for the adsorption and desorption of dyes and catalysts, higher photocatalytic performance. In addition, PEG had successfully enhanced the possibility of the since it encourages higher photocatalytic performance. In addition, PEG had successfully enhanced pollutant being trapped within the pores and showed better catalytic activity by providing the possibility of the pollutant being trapped within the pores and showed better catalytic activity by additional surface active groups [48]. providing additional surface active groups [48]. Appl. Sci. 2016, 6, 451 6 of 16 Appl. Sci. 2017, 7, 24 6 of 16 Appl. Sci. 2016, 6, 451 6 of 16 desorption desorption adsorption 15 adsorption 0 0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P ) Relative Pressure (P/P ) Figure 4. N2 adsorption–desorption isotherms of the washed immobilized TiO2/PEG (TP2). Figure 4. N adsorption–desorption isotherms of the washed immobilized TiO /PEG (TP2). Figure 4. N2 adsorption–desorption isotherms of the washed immobilized TiO2/PEG (TP2). 2 2 2.5. FTIR 2.5. FTIR 2.5. FTIR FTIR was used to acquire advanced information of the chemical bonding in TP0, washed TP2 FTIR was used to acquire advanced information of the chemical bonding in TP0, washed TP2 and unwashed TP2 films, as shown in Figure 5. The spectra showed significant differences, which FTIR was used to acquire advanced information of the chemical bonding in TP0, washed TP2 and −1 and unwashed TP2 films, as shown in Figure 5. The spectra showed significant differences, which unwashed were due to TP2 the ri films, sing bands as shown in from the a Figure 5. The dditi spectra on of PEG a showed t 338 significant 9.77 and dif33 fer 75 ences, .15 cm which . Alwer l speectra due 1 −1 were due to the rising bands from the addition of PEG at 3389.77 and 3375.15 cm . All spectra showed broad bands, which indicated the presence of O–H stretching vibrations of water absorbed to the rising bands from the addition of PEG at 3389.77 and 3375.15 cm . All spectra showed broad showed broad bands, which indicated the presence of O–H stretching vibrations of water absorbed bands, with the hydroxyl group which indicated the on the presencephotocatalyst surface. The O of O–H stretching vibrations – of H bo water nd was also noticed absorbed with the hydr in both oxyl −1 −1 with the hydroxyl group on the photocatalyst surface. The O–H bond was also noticed in both spectra at 1637.32 and 1637.45 cm . The peak at 2900 cm was detected in all spectra corresponding group on the photocatalyst surface. The O–H bond was also noticed in both spectra at 1637.32 and −1 −1 1 1 spectra at 1637.32 and 1637.45 cm . The peak at 2900 cm was detected in all spectra corresponding 1637.45 to C–H bonds. cm . The peak at 2900 cm was detected in all spectra corresponding to C–H bonds. to C–H bonds. C-H TP0 C-H TP2 C-C TP0 Unwashed TP2 TP2 C-C DSAT Unwashed TP2 40 C-O C=O DSAT C-O C=O 3000 2000 4000 3000 2000 1000 3000 2000 4000 3000 2000 1000 -1 Wavelength (cm ) -1 Wavelength (cm ) Figure 5. FTIR spectra of TP0, TP2 (washed TP2), unwashed TP2 and double-sided adhesive tape Figure 5. FTIR spectra of TP0, TP2 (washed TP2), unwashed TP2 and double-sided adhesive tape (DSAT) samples. (DSAT) sam Figure 5. FTIR spe ples. ctra of TP0, TP2 (washed TP2), unwashed TP2 and double-sided adhesive tape (DSAT) samples. Tra Tr ns am ns im tta in ttc ae n c % e % -1 -1 QuQu anta it n y t Ad ity Ad sors b o er d b ( e m d ( m m om l g ol) g ) Appl. Sci. 2017, 7, 24 7 of 16 Appl. Sci. 2016, 6, 451 7 of 16 −1 Interestingly, washed TP2 spectra showed a strong absorption in the region of 1705.02 cm , Interestingly, washed TP2 spectra showed a strong absorption in the region of 1705.02 cm , which justified that the carbonate species C=O existed. On the other hand, a new peak detected at which justified that the carbonate species C=O existed. On the other hand, a new peak detected −1 1160.06 cm , which is assigned to C–O species, was also present in the spectra. The presence of C=O at 1160.06 cm , which is assigned to C–O species, was also present in the spectra. The presence affirmed that TiO2 has reacted strongly with PEG, which could potentially lead to the high of C=O affirmed that TiO has reacted strongly with PEG, which could potentially lead to the high photoactivity of TP2. The C=O bonds detected were due to the oxidation of PEG in the irradiated photoactivity of TP2. The C=O bonds detected were due to the oxidation of PEG in the irradiated TP2 TP2 film. As shown in Figure 5, the carbonaceous species C–C bond was detected in all samples film. As shown in Figure 5, the carbonaceous species C–C bond was detected in all samples except TP0. except TP0. Generally, the IR spectrum for DSAT also produced the C=O bonds, but this factor can Generally, the IR spectrum for DSAT also produced the C=O bonds, but this factor can be discarded be discarded because during FTIR analysis of all samples, each sample’s powder was carefully because during FTIR analysis of all samples, each sample’s powder was carefully scratched and taken scratched and taken for analysis, excluding the DSAT. This is further supported by the IR spectrum for analysis, excluding the DSAT. This is further supported by the IR spectrum of unwashed TP2, of unwashed TP2, where the formation of C=O bonds was absent in Figure 5. In brief, even though where the formation of C=O bonds was absent in Figure 5. In brief, even though the composition the composition and structure of the investigated TP2 film do not greatly change, the photoactivity and structure of the investigated TP2 film do not greatly change, the photoactivity efficiency of TP2 efficiency of TP2 in Table 1 was indirectly attributed to the formation of C=O bond that promotes in Table 1 was indirectly attributed to the formation of C=O bond that promotes more reactions to more reactions to take place. take place. 2.6. 2.6. X-ray P X-ray Photoelectr hotoelectroon n Spec Spectr trosco oscopy py (XP (XPS) S) XPS XPS is used is used to determin to determine e tthe he bind binding ing e ener nergy gy of of e elements lements detect detected ed in unwashed and in unwashed and washed washed immobil immobilized ized TP2 TP2 s samples amples in o in or rder der t to o e examine xamine t the he e efffect fect of of w washing. ashing. Thi This s ef effe fect ct had had succe successfully ssfully produced produced an an oxidized oxidized PE PEG. G. The O1s The O1s and C1s spect and C1s spectra ra o of f TP TP2 2 un unwashed washed are shown in F are shown in Figur igure 6a,b, e 6a,b, while while Figure Figure 6c,d 6c,d represen represents ts the O1s the O1s and and C1s spectr C1s spectra a for w for washed ashed T TP2 P2 sample. sample. Figure 6. Figure 6. The TheO1s and O1s and C1s C1s X-ray photoelect X-ray photoelectr ron spectros on spectrco oscopy py (XPS) (XPS) spect spectr rums of immobilize ums of immobilized d for (for a,b( ) unwashe a,b) unwashed d TP2TP2 and and (c,d( )c washed ,d) washed TP2TP2. . CPS, CPS, cycle cycles s per se per cond. second. All samples demonstrated the chemical composition information where three elements existed: All samples demonstrated the chemical composition information where three elements existed: Ti, C and O. Figure 6a represents the O1s characteristic peaks of Ti–O and C–O at 529.9 and 531.6 eV, Ti, C and O. Figure 6a represents the O1s characteristic peaks of Ti–O and C–O at 529.9 and 531.6 eV, respectively, while C–O and C–C bonds are given by the values of 288.2 and 284.8 eV, respectively, respectively, while C–O and C–C bonds are given by the values of 288.2 and 284.8 eV, respectively, in Figure 6b. The O1s spectra in Figure 6c of washed TP2 revealed peaks at 529.4, 531.0 and 532.9 eV attributed to Ti–O, C–O and C=O bonds, respectively. Figure 6d shows the spectrum of washed TP2 Appl. Sci. 2017, 7, 24 8 of 16 in Figure 6b. The O1s spectra in Figure 6c of washed TP2 revealed peaks at 529.4, 531.0 and 532.9 eV attributed to Ti–O, C–O and C=O bonds, respectively. Figure 6d shows the spectrum of washed TP2 in the C1s spectra deconvoluted into three distinct curves, which are represented by three forms of Appl. Sci. 2016, 6, 451 8 of 16 carbon as C–C, C–O and C=O at 284.8, 286.7 and 288.7 eV, respectively. As such, the formation of C=O in the C1s spectra deconvoluted into three distinct curves, which are represented by three forms of bond can only be found in washed TP2. Yet, no peak for C=O bond was presented in unwashed TP2. carbon as C–C, C–O and C=O at 284.8, 286.7 and 288.7 eV, respectively. As such, the formation of This occurrence of C=O bond could be due to the oxidation process of hydroxyl radicals with PEG, C=O bond can only be found in washed TP2. Yet, no peak for C=O bond was presented in unwashed introduced by the washed sample of TP2 in Figure 6c,d. TP2. This occurrence of C=O bond could be due to the oxidation process of hydroxyl radicals with PEG, introduced by the washed sample of TP2 in Figure 6c,d. 2.7. UV–Vis DRS and Visible Light Photodegradation Studies 2.7. UV–Vis DRS and Visible Light Photodegradation Studies UV–Vis diffuse reflectance spectra (UV–Vis DRS) of pristine TiO , unwashed and washed TP2 are shown in Figure 7a. All samples have a bit of difference in the pattern in UV–Vis diffuse reflectance UV–Vis diffuse reflectance spectra (UV–Vis DRS) of pristine TiO2, unwashed and washed TP2 spectra. It is obvious that pristine TiO has the highest absorbance in UV–Vis spectra followed by are shown in Figure 7a. All samples have a bit of difference in the pattern in UV–Vis diffuse reflectance spectra. It is obvious that pristine TiO2 has the highest absorbance in UV–Vis spectra washed TP2 and unwashed TP2 samples. Even though the modified TiO or TP2 has lower absorbance followed by washed TP2 and unwashed TP2 samples. Even though the modified TiO2 or TP2 has as compared to unmodified pristine TiO , this result is expected because there are no bulk property lower absorbance as compared to unmodified pristine TiO2, this result is expected because there are modifications other than on the TP2 surface. Therefore, the chemical properties for TP2 and its band no bulk property modifications other than on the TP2 surface. Therefore, the chemical properties for gap energy do not notably change even after surface alteration. According to Marcela et al. [49], TP2 and its band gap energy do not notably change even after surface alteration. According to from the solid state band theory, the absorption coefficient can be described as a function of incident Marcela et al. [49], from the solid state band theory, the absorption coefficient can be described as a 2 1 photon energy; ( h) = A(h E ), where is the absorption coefficient (cm ), A is a constant, g 2 −1 function of incident photon energy; (αhν) = A(hν − Eg), where α is the absorption coefficient (cm ), h (eV) is the energy of excitation and E is the band gap energy. The band gap energy can be A is a constant, hν (eV) is the energy of excitation and Eg is the band gap energy. The band gap performed by plotting ( h) vs. h. The Tauc 2 plot on the photon energy-axis gives the value of the energy can be performed by plotting (αhν) vs. hν. The Tauc plot on the photon energy-axis gives the direct value o band fgap the d ener irect ban gy ofdsemiconductors gap energy of semi [50 condu ]. ctors [50]. a) b) TiO TiO 2 2 0.60.6 TP2- Washed TP2- Washed TP2- Unwashed TP2- Unwashed 1.5 1.5 0.40.4 0.20.2 0.50.5 3 5 300 400 500 300 400 500 3 5 hv (eV) Wavelength (nm) c) d)100 TP0 TP0 TP2 TP2 TP2 unwashed TP2 unwashed 0 204060 80 0 1530456075 Time (min) Time (min) Figure 7. Graph plots for: (a) UV–Vis diffuse reflectance spectra (DRS); (b) Tauc’s equation; Figure 7. Graph plots for: (a) UV–Vis diffuse reflectance spectra (DRS); (b) Tauc’s equation; (c) percentage remaining of MB dye under visible light irradiation; and (d) adsorption study. Abs, (c) percentage remaining of MB dye under visible light irradiation; and (d) adsorption study. absorption. Abs, absorption. Figure 7b shows a graph of (αhν) vs. hν, named as Tauc’s graph plot. Theoretically, it seems Figure 7b shows a graph of ( h) vs. h, named as Tauc’s graph plot. Theoretically, it seems that the band gap energy for all samples did not change and stayed approximately at 3.1 eV. As that the band gap energy for all samples did not change and stayed approximately at 3.1 eV. As such, such, the band gap energy of TiO2 photocatalyst in TP0 did not change to a visible light active −1 response, and this was similarly observed in Figure 7c where no photodegradation of 12 mg·L MB Abs % Remaining (αhν) % Remaining Appl. Sci. 2017, 7, 24 9 of 16 the band gap energy of TiO photocatalyst in TP0 did not change to a visible light active response, and this was similarly observed in Figure 7c where no photodegradation of 12 mgL MB was Appl. Sci. 2016, 6, 451 9 of 16 observed for TP0 under visible light irradiation. Only decolorization of MB was observed as reported on the adsorption site of the porous surface of TP0 as proven by the adsorption graph in Figure 7d was observed for TP0 under visible light irradiation. Only decolorization of MB was observed as and discussed in our previous study [51]. Unwashed TP2 showed a low photocatalytic degradation of reported on the adsorption site of the porous surface of TP0 as proven by the adsorption graph in MB dye due to the C–C bond in PEG that makes TP2 become slightly active under the visible light Figure 7d and discussed in our previous study [51]. Unwashed TP2 showed a low photocatalytic condition. Surprisingly, TP2 shows an active photodegradation under visible light where a complete degradation of MB dye due to the C–C bond in PEG that makes TP2 become slightly active under the decolorization visible light of condit 12 mgion. L Su MB rpris was ingly achieved , TP2 show at s 75an min. actiAlthough ve photodegradat there ision no und shifting er visible light occur in band −1 where a complete decolorization of 12 mg·L MB was achieved at 75 min. Although there is no gap energy of TP2 due to its sole-surface modifications, it is believed that the visible light active ability shifting occur in band gap energy of TP2 due to its sole-surface modifications, it is believed that the in TP2 is due to the formation of C=O resulting from the oxidation of C–O bond in PEG detected by visible light active ability in TP2 is due to the formation of C=O resulting from the oxidation of C–O XPS and FTIR spectrums, which have been discussed earlier in Figures 5 and 6. bond in PEG detected by XPS and FTIR spectrums, which have been discussed earlier in Figures 5 According to Wang et al. [37], C=O bond can act as an electron injector that initiated the and 6. formation of hydroxyl radical, thus eventually degraded the MB dye pollutant. TP2 had undergone According to Wang et al. [37], C=O bond can act as an electron injector that initiated the the photoluminescence (PL) analysis at low activation energy irradiation to observe the effect of C=O formation of hydroxyl radical, thus eventually degraded the MB dye pollutant. TP2 had undergone activation under visible light irradiation. The PL emission spectra can be used to reveal the efficiency the photoluminescence (PL) analysis at low activation energy irradiation to observe the effect of C=O of charge carrier trapping, immigration and transfer and to understand the fate of photo-induced activation under visible light irradiation. The PL emission spectra can be used to reveal the efficiency electrof ch ons arge and car holes rier t inrap a semiconductor ping, immigration [52 and ,53 t ].rans It is fer a known nd to understa that the nd the fa PL spectr te of um photo-i is the rnesult duced of the electrons and holes in a semiconductor [52,53]. It is known that the PL spectrum is the result of the recombination of excited electrons and holes where the lower PL intensity means a lower recombination recombination of excited electrons and holes where the lower PL intensity means a lower rate of electron–hole pairs under light irradiation [54]. recombination rate of electron–hole pairs under light irradiation [54]. Figure 8 shows the PL spectra of pristine TiO , TP2 and unwashed samples of TP2 using the Figure 8 shows the PL spectra of pristine TiO2, TP2 and unwashed samples of TP2 using the excitation wavelength of 500 nm. The photoluminescence spectrum in this study that was conducted excitation wavelength of 500 nm. The photoluminescence spectrum in this study that was conducted under low excitation energy (500 nm) was meant to confirm that the excitation of electrons happened under low excitation energy (500 nm) was meant to confirm that the excitation of electrons under a low energy level (visible light spectrum). It is shown that increased PL intensity leads to happened under a low energy level (visible light spectrum). It is shown that increased PL intensity increased absorption of low excitation energy, resulting in higher photocatalytic activity [55]. It can be leads to increased absorption of low excitation energy, resulting in higher photocatalytic activity seen that all samples exhibited an obvious PL signal with a similarly-shaped curve at the wavelength [55]. It can be seen that all samples exhibited an obvious PL signal with a similarly-shaped curve at range the wa fromvelength ra 458 to 468nge f nm with rom 45 the 8 to 468 nm wi TP2 sample gi th th ving e TP2 sa the highe mple gi st PL ving the intensihi tyghest followed PL intensi by TP0 ty and followed by TP0 and pristine TiO2. However, the PL energy is smaller than the band gap energy of pristine TiO . However, the PL energy is smaller than the band gap energy of TiO . According to 2 2 TiO2. According to Jing et al., 2006 [56], they observed that some of the lower PL intensities of Jing et al., 2006 [56], they observed that some of the lower PL intensities of semiconductor materials semiconductor materials are due to the presence of oxygen vacancy that acted as an electron are due to the presence of oxygen vacancy that acted as an electron scavenger, thus making electron scavenger, thus making electron recombination jump to the sub-band of TiO2. Moreover, the energy recombination jump to the sub-band of TiO . Moreover, the energy at the 458 to 468-nm wavelength at the 458 to 468-nm wavelength released from PL spectra is too high as compared to the excitation released from PL spectra is too high as compared to the excitation energy. This result is due to the energy. This result is due to the presence of the sensitizer, which allowed for the absorption of low presence of the sensitizer, which allowed for the absorption of low excitation energy to occur and excitation energy to occur and formed an electron-hole pair. This electron is then subsequently formed an electron-hole pair. This electron is then subsequently jumped to the conduction band of jumped to the conduction band of TiO2 and eventually recombined with the hole by releasing the TiO and eventually recombined with the hole by releasing the high energy wavelength. 2 high energy wavelength. 0.8 TP2 0.6 0.4 TP2-Unwashed 0.2 Pristine TiO 455 460 465 470 Wavelength (nm) Figure 8. Photoluminescence spectra of pristine TiO2, unwashed TP2 and washed TP2 samples. PL, Figure 8. Photoluminescence spectra of pristine TiO , unwashed TP2 and washed TP2 samples. photoluminescence. PL, photoluminescence. PL Intensity (a.u) Appl. Sci. 2017, 7, 24 10 of 16 Appl. Sci. 2016, 6, 451 10 of 16 As can be seen in Figure 8, all TP2 samples (washed and unwashed) have shown higher PL As can be seen in Figure 8, all TP2 samples (washed and unwashed) have shown higher PL intensity intensity a as s c compar ompared w ed with ith prist pristine ine T TiO iO2.. The The intensity intensity from the from the unwashed unwashed TP TP2 2 sample sample might might be be due due to the presence of C–C bond that allowed for the electron to recombine under low excitation energy. to the presence of C–C bond that allowed for the electron to recombine under low excitation energy. The The TP2 sam TP2 sample ple (w (washed) ashed) has has rrecorde ecorded d the the highe highest st PL PL intensity intensity a ass compa comparred ed to to others. others. Based Based on on the the XPS and FTIR spectra, this TP2 sample has a C=O bond as an extra species where it is not found in XPS and FTIR spectra, this TP2 sample has a C=O bond as an extra species where it is not found in pristine pristine and and unwashed unwashed TP2 TP2 samples. samples. Hence, it Hence, it can can be be conclude concluded d that the highest intensit that the highest intensity y of T of TP2 P2 showed showed a a s significant ignificant pre presence sence of C of C=O =O b bond, ond, which which a acted cted as as an an ele electr ctron inject on injector or in TP in TP2 2 p photocatalyst. hotocatalyst. The The electrons that we electrons that werre e in inj jected ected to the to the conduction conduction band of TiO band of TiO 2 cr created eated a a ser series ies of chain of chain re reactions, actions, −1 which which help help t too achieve a com achieve a complete plete 100% decolorization of 12 m 100% decolorization of 12 mggL ·L MB dye at MB dye at 75 m 75 min, in, as s as shown hown in in Figur Figure 7c. e 7c. 2.8. Recyclability Study 2.8. Recyclability Study For the stability study of the photocatalyst, the photocatalytic activity of TP2 was carried out For the stability study of the photocatalyst, the photocatalytic activity of TP2 was carried out by by eight cycles of photodegradation of 12 mg −1 L MB dye with 15-min intervals for 60 min in every eight cycles of photodegradation of 12 mg·L MB dye with 15-min intervals for 60 min in every cycle. cycle. Figure 9 shows the photodegradation cycle of MB dye using TP2. It was observed that each Figure 9 shows the photodegradation cycle of MB dye using TP2. It was observed that each recycled recycled application produced 100% removal of MB; indicating a sustainable photocatalytic efficiency application produced 100% removal of MB; indicating a sustainable photocatalytic efficiency characteristic. In other words, a strong interaction of TiO with PEG occurred due to its strong characteristic. In other words, a strong interaction of TiO2 2 with PEG occurred due to its strong chemisorption on the surface of TiO , where it was not easily leached out, even through up to eight chemisorption on the surface of TiO2, where it was not easily leached out, even through up to eight times of repeated usage. times of repeated usage. Figure 9. The recyclability graphs of TP2 under the photodegradation of MB dye. Figure 9. The recyclability graphs of TP2 under the photodegradation of MB dye. 2.9. Chemical Oxygen Demand Analysis 2.9. Chemical Oxygen Demand Analysis Decolorization of dye does not mean that there is complete removal of the organic carbons from Decolorization of dye does not mean that there is complete removal of the organic carbons the water samples. Mineralization, which is defined as the complete decomposition of organic from the water samples. Mineralization, which is defined as the complete decomposition of organic compounds into CO2 and H2O, should be the target of any photocatalytic processes. One of the compounds into CO and H O, should be the target of any photocatalytic processes. One of the results 2 2 results of mineralization is the lowering of the chemical oxygen demand (COD) values of the treated of mineralization is the lowering of the chemical oxygen demand (COD) values of the treated samples. samples. In this study, the presence of organic substances or intermediates can be detected by using In this study, the presence of organic substances or intermediates can be detected by using a COD a COD test. The COD test is attributed to the degradation of MB dye, as well as its by-products test. The COD test is attributed to the degradation of MB dye, as well as its by-products during the during the photocatalytic reaction using the TP2 sample. There is a possible contamination from photocatalytic reaction using the TP2 sample. There is a possible contamination from DSAT, and these DSAT, and these contaminations were completely cleaned up during the washing process prior to contaminations were completely cleaned up during the washing process prior to the photodegradation the photodegradation of MB dye. Figure 10 presents the detected COD values for the mineralization of MB dye. Figure 10 presents the detected COD values for the mineralization of MB dye versus −1 of MB dye versus irradiation time. The COD values (mg·L ) detected were 0.81, 0.75, 0.60, 0.45, 0.30, 0.25, 0.10 and 0.05 and kept decreasing with time at 60, 120, 180, 240, 300, 360, 420 and 480 min, Appl. Sci. 2017, 7, 24 11 of 16 Appl. Sci. 2016, 6, 451 11 of 16 irradiation time. The COD values (mgL ) detected were 0.81, 0.75, 0.60, 0.45, 0.30, 0.25, 0.10 and 0.05 respectively. Hence, the complete mineralization of MB dye through the COD test was greatly Appl. Sci. 2016, 6, 451 11 of 16 and kept decreasing with time at 60, 120, 180, 240, 300, 360, 420 and 480 min, respectively. Hence, the caused by the improved diffusion of dye into photocatalyst layers, stemming from the porous complete mineralization of MB dye through the COD test was greatly caused by the improved diffusion surface of the TP2 photocatalyst sample. respectively. Hence, the complete mineralization of MB dye through the COD test was greatly of dye into photocatalyst layers, stemming from the porous surface of the TP2 photocatalyst sample. caused by the improved diffusion of dye into photocatalyst layers, stemming from the porous surface of the TP2 photocatalyst sample. 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0 100 200 300 400 500 0 100 200 300 400 500 Time (hr) Time (hr) Figure 10. The chemical oxygen demand (COD) analysis of TP2 under photodegradation of MB dye. Figure 10. The chemical oxygen demand (COD) analysis of TP2 under photodegradation of MB dye. Figure 10. The chemical oxygen demand (COD) analysis of TP2 under photodegradation of MB dye. 3 3. . Ex Experimental perimental Se Section ction 3. Experimental Section 3.1. Preparation of Immobilized TiO -PEG 3.1. Preparatio 3.1. Preparatio n of Immobilized TiO n of Immobilized TiO 2-PE 2-PE GG The sample solution was prepared by mixing 6.5 g of titanium dioxide (TiO ) Degussa P25 powder The sample solution was prepared by mixing 6.5 g of titanium dioxide (TiO 2 2) Degussa P25 The sample solution was prepared by mixing 6.5 g of titanium dioxide (TiO2) Degussa P25 (20%powder (20% rutile, rutile, 80% anatase) 80% an in 50 matase) in 50 mL L distilled water dadded istilled w with ater 1 mL added of with 8% (w/v 1 mL ) of of 8% polyet ( hylene w/v) of glycol powder (20% rutile, 80% anatase) in 50 mL distilled water added with 1 mL of 8% (w/v) of polyethylene glycol (Merck, Kenilworth, NJ, USA, MW = 6000). The sample solution was sonicated (Merck, Kenilworth, NJ, USA, MW = 6000). The sample solution was sonicated under an ultrasonic polyethylene glycol (Merck, Kenilworth, NJ, USA, MW = 6000). The sample solution was sonicated under an ultrasonic vibrator for 30 min to make it homogenized. The immobilized sample was vibrator for 30 min to make it homogenized. The immobilized sample was prepared by using a under an ultrasonic vibrator for 30 min to make it homogenized. The immobilized sample was prepared by using a brush-coating method applied onto a clean glass plate prior to taping with brush-coating method applied onto a clean glass plate prior to taping with double-sided adhesive prepared by using a brush-coating method applied onto a clean glass plate prior to taping with double-sided adhesive tape (DSAT). The wet TiO2-PEG coated on glass was then dried using an tape (DSAT). The wet TiO -PEG coated on glass was then dried using an 850-W hot blower with a double-sided adhesive tape (DSAT). The wet TiO2-PEG coated on glass was then dried using an 850-W hot blower with a temperature of about 120 °C until dry. The process was continued by temperature of about 120 C until dry. The process was continued by repeating the process of coating 850-W hot blower with a temperature of about 120 °C until dry. The process was continued by repeating the process of coating onto dried TiO2-PEG until the desired loading of immobilized onto dried TiO -PEG until the desired loading of immobilized TiO -PEG was achieved. Figure 11 2 2 repeating the process of coating onto dried TiO2-PEG until the desired loading of immobilized TiO2-PEG was achieved. Figure 11 shows the coated TiO2 on DSAT attached to the glass plate with shows the coated TiO on DSAT attached to the glass plate with and without PEG binder. TiO2-PEG w and without as achieved. F PEG binder. igure 11 shows the coated TiO2 on DSAT attached to the glass plate with and without PEG binder. Figure 11. Picture of immobilized TiO2 with and without PEG binder and the molecular structure Figure 11. Picture of immobilized TiO with and without PEG binder and the molecular structure of PEG. of PEG. Figure 11. Picture of immobilized TiO2 with and without PEG binder and the molecular structure of PEG. -1 COD Concentration (mg L ) -1 COD Concentration (mg L ) Appl. Sci. 2016, 6, 451 12 of 16 Appl. Sci. 2017, 7, 24 12 of 16 3.2. Characterization Tests of Immobilized TiO2/PEG DSAT X-ray diffraction (XRD) spectra were obtained using a Rigakuminiflex II, X-ray diffractometer 3.2. Characterization Tests of Immobilized TiO /PEG DSAT (Rigaku, Tokyo, Japan). Structural information of the films was obtained in the range of 2θ angles X-ray diffraction (XRD) spectra were obtained using a Rigakuminiflex II, X-ray diffractometer from 3° to 80° with a step size increment of 1.00 s/step. FTIR spectra of powder samples were (Rigaku, Tokyo, Japan). Structural information of the films was obtained in the range of 2 angles from recorded on Perkin Elmer Spectrum Version equipped with an attenuated total reflectance device 3 to 80 with a step size increment of 1.00 s/step. FTIR spectra of powder samples were recorded on (Perkin Elmer, Waltham, MA, USA) with a diamond crystal. Spectra were collected in a frequency Perkin Elmer Spectrum Version equipped with an attenuated total reflectance device (Perkin Elmer, −1 −1 range of 600 to 4000 cm with 4 scans and a spectral resolution of 4 cm . The morphology of the Waltham, MA, USA) with a diamond crystal. Spectra were collected in a frequency range of 600 to samples was observed with field-emitting scanning electron microscopy (FE-SEM, JSM-6700F, 1 1 4000 cm with 4 scans and a spectral resolution of 4 cm . The morphology of the samples was Akishima, Tokyo, Japan) with an accelerating voltage of 10 kV. The surface area of the immobilized observed with field-emitting scanning electron microscopy (FE-SEM, JSM-6700F, Akishima, Tokyo, TiO2 film powders was measured by nitrogen adsorption using the BET equation at 77 K Japan) with an accelerating voltage of 10 kV. The surface area of the immobilized TiO film powders (Micrometrics ASAP 2020M + C, Norcross, GA, USA). A UV–Vis spectrophotometer UV-2550, was measured by nitrogen adsorption using the BET equation at 77 K (Micrometrics ASAP 2020M + C, Shimadzu was used to obtain the UV–Vis reflectance spectrum of the powder sample. X-ray Norcross, GA, USA). A UV–Vis spectrophotometer UV-2550, Shimadzu was used to obtain the UV–Vis photoelectron spectroscopy (XPS) with a Thermo ESCALAB 250 spectrometer using a radiation reflectance spectrum of the powder sample. X-ray photoelectron spectroscopy (XPS) with a Thermo source of monochromatic Al Kα with the energy of 1486.6 eV, 200 W and a photoluminescence ESCALAB 250 spectrometer using a radiation source of monochromatic Al K with the energy analyzer (JovinYvon, Chiyoda-ku, Tokyo, Japan) was used to determine the photoluminescence of 1486.6 eV, 200 W and a photoluminescence analyzer (JovinYvon, Chiyoda-ku, Tokyo, Japan) was intensity of the samples. used to determine the photoluminescence intensity of the samples. 3.3. Washing Process of Immobilized Samples 3.3. Washing Process of Immobilized Samples The washing process was conducted to oxidize PEG and also to clean all unwanted The washing process was conducted to oxidize PEG and also to clean all unwanted contaminants contaminants from immobilized TiO2-PEG samples. The process was done by irradiating the from immobilized TiO -PEG samples. The process was done by irradiating the immobilized samples immobilized samples in distilled water inside a glass cell of 150 mm × 10 mm × 80 mm (length × in distilled water inside a glass cell of 150 mm  10 mm  80 mm (length  width  height). width × height). An aquarium pump model NS 7200 (Minjiang, Jiangmen, China) was used as an An aquarium pump model NS 7200 (Minjiang, Jiangmen, China) was used as an aeration source and aeration source and irradiated with a 55-W fluorescent lamp for 1 h. The washing process was irradiated with a 55-W fluorescent lamp for 1 h. The washing process was repeated once again by repeated once again by replacing the distilled water with a new amount of distilled water irradiated replacing the distilled water with a new amount of distilled water irradiated for another 30 min to for another 30 min to affirm that zero contamination is achieved. This contamination was measured affirm that zero contamination is achieved. This contamination was measured by using chemical by using chemical oxygen demand analysis (COD) to detect any presence of organic compounds in oxygen demand analysis (COD) to detect any presence of organic compounds in washed distilled washed distilled water. This process was done prior to the photodegradation of MB dye. water. This process was done prior to the photodegradation of MB dye. 3.4. Photodegradation of MB Dye 3.4. Photodegradation of MB Dye The activity of the catalyst was tested by the degradation of methylene blue (MB), Fluka The activity of the catalyst was tested by the degradation of methylene blue (MB), Fluka Analytical, Analytical, with a chemical formula: C12H15O6; and the molecular structure of MB is shown in with a chemical formula: C H O ; and the molecular structure of MB is shown in Figure 12. 12 15 6 Figure 12. The experimental procedure was the same method from our previous report [57]. The The experimental procedure was the same method from our previous report [57]. The immobilized −1 immobilized TiO2-PEG was immersed into 20 mL of 12 mg·L MB dye placed inside a glass cell TiO -PEG was immersed into 20 mL of 12 mgL MB dye placed inside a glass cell under an aeration under an aeration source. Light was then irradiated using a 55-W fluorescent lamp, Model Ecotone, source. Light was then irradiated using a 55-W fluorescent lamp, Model Ecotone, with visible light −2 with visible light intensity measured for about 461 and 6.7 W·m of UV light detected as UV leakage. intensity measured for about 461 and 6.7 Wm of UV light detected as UV leakage. A 4-mL aliquot A 4-mL aliquot of treated MB dye was then taken out from the glass cell at 15-min intervals until it of treated MB dye was then taken out from the glass cell at 15-min intervals until it turned colorless by turned colorless by measuring its concentration using UV spectrophotometer Model HACH DR 1900 measuring its concentration using UV spectrophotometer Model HACH DR 1900 at a 661-nm  max at a 661-nm λ max detector (Hach, Loveland, CO, USA). The experimental procedure was repeated detector (Hach, Loveland, CO, USA). The experimental procedure was repeated by applying the same by applying the same steps for different catalysts loading and different TiO2/PEG ratios. steps for different catalysts loading and different TiO /PEG ratios. Figure 12. The molecular structure for methylene blue. Figure 12. The molecular structure for methylene blue. Appl. Sci. 2017, 7, 24 13 of 16 3.5. COD Analysis Initially, the immobilized TiO /PEG DSAT (TP2) film was immersed in 20 mL of distilled water inside a glass cell under the irradiation of a 55-W compact fluorescent lamp. After 1 h of irradiation, the water sample was withdrawn and replaced with another set of distilled water using the same immobilized TiO /PEG film until 8 h of irradiation. The withdrawn water samples were then subjected to the COD test. It can be observed that MB dye and its generated by-products had undergone almost 100% complete mineralization after 8 h of irradiation using an immobilized TiO /PEG film. 3.6. Recyclability Study The recyclability study was carried out to see the effect of immobilized TiO -PEG towards photodegradation stability. The experiment was conducted initially through the photodegradation method. Immobilized TiO -PEG was then subjected to the washing process using distilled water and irradiated for 30 min. Both the photodegradation and washing procedures for the immobilized the TiO -PEG sample were then repeated until eight cycles. The photodegradation percentage of MB in every cycle was recorded at every 15-min interval until MB became colorless. 4. Conclusions An immobilized active TiO photocatalyst was successfully prepared via adding a small amount of PEG as a binder onto a support binder of double-sided adhesive tape (DSAT). It was observed that utilization of 10:0.1 of a TiO /PEG ratio at 0.3 g of catalyst loading produced an immobilized TiO 2 2 with excellent photocatalytic activity. The preparation process did not produce any significant phase transformation, except for the typical TiO phase. From the XPS and FTIR spectra, both observed that washed TiO -PEG (TP2) produced C=O bond that was confirmed to initiate the photocatalytic activity of the sample and to be 1.8-times higher than pristine TiO under suspension mode in degrading 12 mgL MB dye. High PL intensity with low activation energy under immobilized TiO -PEG (TP2) proved that the presence of C=O increased the injected electron into the conduction band that eventually produced the hydroxyl radical agent used for the degradation of MB dye under visible light irradiation. Finally, TP2 or immobilized TiO -PEG was very stable and possessed excellent sustainable photocatalytic activity up to eight-times of reusability and comparable to recent photocatalysis cycles. As shown by the COD analysis, TP2 or immobilized TiO -PEG with DSAT leaves no organic pollutants during photodegradation cycles, which brings about a significant improvement in water quality. Acknowledgments: We would like to thank the Ministry of Education (MOE), Malaysia, for providing generous financial support under the Research Acculturation Grant Scheme (RAGS) grants (600-RMI/RAGS 5/3 (35/2014)) in conducting this study and Universiti Teknologi MARA (UiTM) for providing all of the needed facilities. 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Published: Dec 23, 2016

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