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Enhanced photocatalytic hydrogen production from water–methanol mixture using cerium and nonmetals (B/C/N/S) co-doped titanium dioxide

Enhanced photocatalytic hydrogen production from water–methanol mixture using cerium and... Mater Renew Sustain Energy (2014) 3:25 DOI 10.1007/s40243-014-0025-6 OR IGINAL PAPER Enhanced photocatalytic hydrogen production from water–methanol mixture using cerium and nonmetals (B/C/N/S) co-doped titanium dioxide N. Vinothkumar Mahuya De Received: 23 November 2013 / Accepted: 24 February 2014 / Published online: 18 March 2014 The Author(s) 2014. This article is published with open access at Springerlink.com Abstract In the present study, photocatalytic hydrogen Introduction production from water/methanol solution was investigated over cerium and nonmetal (B/C/N/S) co-doped titanium The energy economy of the world mainly depends on the dioxide catalyst under visible light irradiation. The cerium fossil fuels such as coal, natural gas, and petroleum pro- and nonmetal co-doped titania photocatalysts were pre- ducts. Gradual depletion of fossil fuels and fast-growing pared by co-precipitation and characterized by surface area energy demand has necessitated to develop alternative and pore size analysis, X-ray diffraction analysis, diffuse fuels, which should also be pollution-free, storable, and reflectance UV–Vis spectroscopy analysis, and photolu- economical [1]. Hydrogen is considered as a promising minescence analysis. The UV–Visible spectra showed that alternative fuel for the future. It has higher energy content incorporation of cerium and nonmetals to TiO resulted in and high heating value compared to other fuels such as narrow band gap and improved absorption of visible light. methane, methanol, gasoline, and diesel [2]. At present, The band gap energy of co-doped samples depended on the hydrogen is mainly produced by steam reforming of properties of nonmetals. Photoluminescence studies methane or naphtha. Various alternative processes such as showed that the radiative recombination rates of photo- electrolysis, thermolysis, thermochemical reactions, pho- generated electron–hole pairs were effectively suppressed tolysis, photoelectro-chemical, photocatalytic, and bio- by the addition of cerium and nonmetals and contributed to chemical processes are being studied for the production of higher activity. The highest hydrogen production of hydrogen. But only few of these methods are efficient and 206 lmol/h was obtained for Ce–N–TiO sample, which economically viable [3]. The production of hydrogen from can be attributed to the higher surface area, higher water using solar radiation is one of the potential routes to absorption of visible light, and higher separation efficiency achieve clean, low-cost, and eco-friendly fuel. Several of electron–hole pairs in Ce–N–TiO . oxide materials, such as TiO ,Fe O , ZnO, ZrO , 2 2 2 3 2 K Nb O ,K La Ti O ,BaTi O , and Ta O , have been 4 6 17 2 2 3 10 4 9 2 5 Keywords Photocatalysts  Hydrogen  Visible light  studied for the photodecomposition of water [4–8]. TiO Cerium  Nonmetals has been reported to be an excellent photocatalyst for water-splitting reaction. Advantages of TiO include chemical inertness, photostability, and low cost. However, the activity of TiO is limited in UV region because of its wide band gap (3.2 eV). The visible light (k [ 400 nm) constitutes the major fraction of solar spectrum reaching Electronic supplementary material The online version of this the earth surface. For better utilization of the sunlight, it is article (doi:10.1007/s40243-014-0025-6) contains supplementary material, which is available to authorized users. essential to develop stable photocatalytic systems with suitable band gap to absorb most of the solar spectrum. In N. Vinothkumar  M. De (&) order to develop an efficient titania-based visible light Department of Chemical Engineering, Indian Institute active photocatalyst, various methods have been reported of Technology, Guwahati 781 039, Assam, India such as doping with metals and nonmetals, dye e-mail: mahuya@iitg.ernet.in 123 Page 2 of 10 Mater Renew Sustain Energy (2014) 3:25 sensitization as well as coupling with other semiconductors 2.5 and 1 mol% of titanium, respectively. The loadings of [9]. The dopant impurities create additional energy levels Ce (x) and nonmetals (y) are defined as within the band gap of titania acting either as donor or CeðmolÞ x ¼  100 ¼ 2:5 mol% and acceptor level. An extensive review has been done on TiðmolÞ visible light activity of titania by Pelaez et al. [10]. Pho- NMðmolÞ y ¼  100 ¼ 1 mol%: tocatalytic activities of TiO doped with rare earth metals TiðmolÞ and nonmetals are mostly reported for the degradation of Aqueous ammonia (25 wt%) and solution B were pollutants. Xu et al. [11] investigated various rare earth- simultaneously added dropwise to the solution A with doped TiO samples for the degradation of nitrite and continuous stirring. Addition of ammonia was continued observed an enhanced photoabsorption in the visible light until complete precipitation occurred. The resultant region. Ce-doped TiO materials were also reported to mixture was stirred for 2 h at room temperature and was show high activity under visible light irradiation for pho- placed in a water bath at 363 K for overnight. The tocatalytic degradation of dye and phenol derivatives [12– precipitate was filtered and dried in an oven at 373 K for 14]. The N–TiO [15], and S–TiO [16] were studied for 2 2 24 h. The as-prepared samples were calcined at 723 K for the photodegradations of methylene blue under UV/Visible 3 h. In text, nonmetal co-doped samples are referred as Ce– light irradiation, while C–TiO [17] was reported to be NM–TiO (NM = B, C, N, S). For reference, cerium- active for the degradation of phenol. The titania co-doped doped titania were prepared by dissolving required amount with rare earth metals and nitrogen are also reported to of cerium nitrate hexahydrate in deionized water (solution show an enhanced activity for the decomposition of pol- lutants [18, 19]. Though doped TiO catalysts were B). The amount of Ce was 2.5 mol% of Ti, and sample is represented as Ce–TiO in text. The solution B was added extensively investigated for the degradation of pollutants, to solution A along with aqueous ammonia as described comparatively fewer studies have been reported for pho- earlier to obtain cerium-doped titania samples. Similar tocatalytic hydrogen production [20–22]. In this study, the aging condition and calcination temperature were photocatalytic activities of TiO co-doped with cerium and maintained for the preparation of Ce–TiO sample. nonmetals were studied for hydrogen evolution from 2 Undoped titania was also prepared for reference by water–methanol mixture under visible light irradiation. The similar procedure without the addition of cerium or effect of various nonmetals such as B, C, N, and S on nonmetals and represented by TiO in text. hydrogen production was studied. The physical and optical properties of the photocatalysts affecting the activity were Characterization of photocatalyst investigated using various characterization techniques such as surface and pore analysis, XRD, UV–Vis, and photo- The surface areas of the samples were determined from N luminescence spectroscopy. isotherms collected at 77 K using a Beckman Coulter TM SA 3100 analyzer. Prior to the experiments, the samples were degassed at 423 K for a period of 2 h. The pore size Experimental distribution of the photocatalysts was determined by Bar- rett–Joyner–Halenda (BJH) method. The X-ray diffraction Preparation of photocatalyst (XRD) patterns were recorded on a Bruker D2 phaser X-ray diffractometer using Ni-filtered Cu K as radiation Cerium and nonmetal co-doped TiO samples were pre- source (k = 1.5406 A) in the 2h range from 20 to 80 at a pared by co-precipitation method. Titanium tetra isoprop- scanning rate of 1/min. A beam voltage and beam current oxide (TTIP) and cerium nitrate hexahydrate were used as of 40 kV and 40 mA were used, respectively. The diffuse precursors for titania and ceria, respectively, and were reflectance UV–vis spectra of the photocatalysts were obtained from Sigma-Aldrich. The boric acid, glucose, measured on a PerkinElmer Lambda 750 instrument with a urea, and thiourea were used as sources for boron, carbon, 60-mm labsphere specular reflectance accessory at room nitrogen, and sulfur, respectively, and were procured from temperature. The baseline correction was done using a Merck. The isopropanol and ammonia solution were also calibrated sample of barium sulfate as reference. The scan procured from Merck. parameter was set with slit size 2 nm and scan in the Requisite amount of TTIP was mixed with isopropanol spectral range of 250–650 nm. The photoluminescence under continuous stirring condition for 10 min to form (PL) measurements were taken in a Thermo Spectronic solution A. Solution B was prepared by dissolving the (Aminco Bowman Series 2) instrument with a Xe lamp as required amount of cerium nitrate hexahydrate and the excitation source at room temperature. The powders respective precursors of nonmetals in deionized water. The were dispersed in ethanol, and the emission spectra were loadings in co-doped samples for Ce and nonmetals were 123 Mater Renew Sustain Energy (2014) 3:25 Page 3 of 10 collected at an excitation wavelength of 325 nm. The same supplementary information. The temperature of the reac- quantity was used for recording the PL spectra of all the tion mixture was maintained at 307 K using a water cir- samples. The entrance and exit slit widths were fixed as culation bath, which also acted as an IR filter. The evolved same for all the measurements. Field emission scanning gas was collected in an inverted burette by water dis- electron microscopic analysis was done using instrument of placement method. The gas mixture was analyzed using a ZEISS-Sigma. For FESEM analysis, the samples were gas chromatograph (Varian, CP 3800) equipped with car- dispersed in a solvent and deposited in an aluminum foil bosieve SII column and thermal conductivity detector. that was mounted on a sample holder for gold coating. The After completion of the photocatalytic reaction, the liquid composition of the samples was determined by EDS ana- solution was analyzed for the detection of intermediates lysis (energy-dispersive X-ray spectroscopy) equipped with (formaldehyde and formic acid) by HPLC (SHIMADZU, SEM instrument (LEO 1430 VP). C18 column). To confirm the photocatalytic activity of the catalysts, experiments were conducted first in the absence Photocatalytic activity studies of light with catalysts and next in the presence of light without catalyst. In both cases, no hydrogen evolution was The activities of the photocatalysts were examined using observed. The hydrogen evolution was observed only when the experimental setup shown in Fig. 1. The reaction was the reaction was carried out in the presence of both light typically carried out by adding 0.2 g catalyst to solution of and catalyst. In all cases, same feed mixture was used. This water (25 ml) and methanol (1 ml) under stirred condition. confirmed the photocatalytic activity of the catalysts. Prior to irradiation, the reaction mixture was de-aerated with N gas (50 ml/min) for 30 min to completely remove the dissolved oxygen. Then, the reaction mixture was Results and discussion irradiated with a 500-W tungsten halogen lamp (Halonix, India), placed approximately 15 cm away from the reactor, Characterization of prepared catalysts as source of visible light. The emission spectrum of the lamp was measured in front of the reactor using Ocean The actual composition of the prepared photocatalysts was optics USB4000 spectrometer as shown in Figure S1 in the determined by EDS, and the results are shown in Table S1 Fig. 1 Schematic representation of experimental setup for photocatalytic water splitting 3 9 1. N gas 7.Thermocouple 2. Flowmeter 8.Product gas outlet 3. Halogen lamp 9.Water bath 4. Magnetic stirrer 10. Gas collector 5. Reactor 11.Cooling water inlet 6. Reactant mixture 12. Gas Chromatograph 123 Page 4 of 10 Mater Renew Sustain Energy (2014) 3:25 Table1 Physicochemical and crystalline properties of undoped TiO , 0.008 (a) Ce–TiO , and Ce–NM–TiO (NM = B, C, N, S) samples Ce-TiO 2 2 0.007 2 0.006 Samples BET Pore Lattice Lattice d spacing ˚ 0.005 surface volume parameters distortion (A) 150 0.004 area (cc/g) Anatase (m /g) 0.003 ˚ ˚ a (A) c (A) 0.002 (101) (004) 0.001 0.000 TiO 84 0.45 3.7435 9.4217 0.378 3.4790 0 20406080 100 120 140 160 Ce–TiO 70 0.36 3.7451 9.4599 0.268 3.4822 Pore diameter (nm) Ce–B–TiO 87 0.57 3.7907 9.5197 0.271 3.5128 Ce–C–TiO 53 0.67 3.7659 9.5080 0.387 3.5013 Ce–N–TiO 158 0.06 3.7936 9.7181 0.301 3.2240 Ce–S–TiO 39 0.18 3.7727 9.7014 1.391 3.5174 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P ) of supplementary information. The actual cerium content in samples varied in the range of 2.39–2.49 mol%, and nonmetal content varied in the range of 0.8–0.97 mol% Ce-N-TiO (b) 2 0.12 Ce-C-TiO (Table S2 of supplementary information). Thus, the actual Ce-S-TiO 0.10 composition agreed well with the intended composition of Ce-B-TiO 0.08 the photocatalysts within experimental error. 0.06 The surface area and pore volumes of undoped TiO , 0.04 Ce–TiO , Ce–NM–TiO (NM = B, C, N, S) samples are 2 2 0.02 shown in Table 1. Surface area and pore volume of titania 0.00 decreased with the addition of ceria. Structure modification 0 20 40 60 80 100 120 140 160 Pore diameter (nm) and partial pore blockage in the presence of cerium may be responsible for decrease in surface area and pore volume in cerium-doped samples. The isotherm of Ce–TiO and corresponding pore size distribution are shown in Fig. 2a. 0.0 0.2 0.4 0.6 0.8 1.0 The Ce–TiO sample exhibited a type IV isotherm with a H1-type hysteresis loop associated with open-ended Relative Pressure (P/P ) cylindrical pores. The pore size distribution of Ce–TiO Fig. 2 N adsorption–desorption isotherms of Ce–TiO and Ce–NM– 2 2 showed the presence of both mesopores and macropores TiO (NM = B, C, N, S) samples (inset pore size distribution) with pore diameter in the range of 3–148 nm. Co-doping of titania with nonmetals along with cerium resulted in increase in surface area as shown in Table 1. The co-doped The X-ray diffraction patterns of Ce–TiO and Ce–NM– Ce–N–TiO , Ce–C–TiO , and Ce–B–TiO exhibited Type TiO are shown in Fig. 3. All the samples, except Ce–N– 2 2 2 II isotherm with H3 hysteresis loop corresponding to slit- TiO , were dominated by anatase phase (2h = 25.5,37, shaped pores (Fig. 2b). For sulfur co-doped sample, H1 48, 53.8,55.1,64, JCPDS 21-1272). For Ce–N–TiO , hysteresis loop was observed, which indicated the presence significant amount of rutile phase (2h = 28, 36.1, 41.5, of open cylindrical pores. Figure 2b shows that the pore JCPDS 21-1276) was observed. In general, the phase size distribution varied significantly with nonmetals. For transformation of anatase to rutile occurs at temperature Ce–N–TiO and Ce–B–TiO , narrow pore distribution was above 873 K and anatase phase is expected to be dominant 2 2 obtained, with 39 and 47 % pores being below 10 nm, in samples calcined at lower temperature of 723 K, used in this study. Accordingly, anatase phase was dominant in respectively. The pore size distributions of Ce–C–TiO and Ce–S–TiO co-doped samples were much broader. The XRD profiles of all samples except Ce–N–TiO . For Ce– 2 2 presence of significant amount of pores in the range of N–TiO sample, the presence of significant amount of 20–50 nm may have contributed to their lower surface rutile phase suggests that nitrogen can catalyzes the ana- area. The surface area and pore size distribution results tase–rutile phase transformation significantly at lower showed that the presence of nonmetal had significant effect temperature [23]. In addition, Ce–S–TiO co-doped sample on the stabilization of porous network. The maximum was more amorphous in nature as observed from broad effect was observed for Ce–N–TiO with highest surface XRD peaks of low intensity compared to that of other area. nonmetal-doped samples. No characteristic peaks of CeO Volume adsorbed (cc/g STP) Volume Adsorbed (cc/g STP) Pore volume (cc/g) Pore volume (cc/g) Mater Renew Sustain Energy (2014) 3:25 Page 5 of 10 definite change in morphology of the samples when cerium A - Anatase (f) Ce-C-TiO and nitrogen were added to titania suggesting that the do- R - Rutile pants played a prominent role in the development of A A material structure during co-precipitation. This resulted in A different physical properties as was observed in Table 1. (e) Ce-B-TiO Particles of irregular shape and size were observed for A A undoped titania, whereas ceria-doped titania, prepared by co-precipitation, consisted of particles of regular size and (d) Ce-S-TiO shape. The particles were spherical in the range of A A A A 100–200 nm. The morphology again changed when titania was co-doped with cerium and nonmetals. All the co-doped 200 0 (c) Ce-N-TiO Ce–NM–TiO samples were agglomerated in nature. The UV–Visible absorption spectra of TiO , Ce–TiO , 100 2 2 R A and Ce–NM–TiO are shown in Fig. 5. The Ce–TiO 2 2 sample showed intense absorption bands in the visible light (b) Ce-TiO region ranging from 400 to 500 nm. The band gap energies 100 R R R A of the samples were determined by extrapolating the rising part of the onset of the absorption edge to the x-axis (k, (a)TiO nm). The values of k was then used in the Planck’s Einstein equation to calculate the band gap, E = hc/k, where E is g g A A R A R A band gap energy in eV, h is Planck’s constant in eV. s, c is the speed of light in m/s, and k is the absorption wave- 20 30 40 50 60 70 80 length in nm [26]. The calculated band gap energy values for undoped TiO and Ce–TiO were 3.11, and 2.81, 2 2 respectively. The decreased band gap energy values of Fig. 3 XRD patterns of TiO , Ce–TiO , and Ce–NM–TiO 2 2 2 cerium-doped sample indicated the formation of new (NM = B, C, N, S) samples energy levels within the TiO band gap and resulted in phases were observed in the XRD profiles, which suggest redshift of the absorption edge. This shift can be attributed that the ceria was well dispersed in TiO matrix or were to the incorporation of Ce 4f levels into the TiO crystal 2 2 below detection limit. In addition, no peaks due to any Ce– structure just below the conduction band of TiO and Ti mixed oxides were observed in any of the samples. thereby reducing the effective band gap [27]. Addition of On addition of Ce to TiO , cerium ions can substitute nonmetal to cerium-doped TiO samples resulted in further 2 2 4? Ti sites or can occupy the interstitial sites with Ce–O–Ti shift in the band gap energy to longer wavelength. Sig- linkages [24]. But this is expected to be associated with nificant reductions in the optical band gap energies from 4? distortion in titania lattice due to larger size of Ce 3.11 eV for undoped TiO to about 2.88 eV for Ce–B– 3? (IR = 0.101 nm) and Ce (IR = 0.111 nm) compared to TiO , 2.66 eV for Ce–C–TiO , 2.29 eV for Ce–N–TiO , 2 2 2 4? Ti (IR = 0.068 nm) ions. The lattice distortion values are and 2.24 eV for Ce–S–TiO were observed for co-doped shown in Table 1 and correspond to maximum strain samples. For the co-doped catalysts, the band gap energy observed in crystal lattice due to distortion by incorporation was affected by the formation of additional impurity states of dopant metals. The lattice distortion was calculated using within titania matrix by the interaction of p-orbitals of the the formula defined by Stokes and Wilson as e = b/(4 tan h), nonmetal dopants (B, C, N, S) and 2p-orbitals of oxygen where b is the full width at half maximum of diffracted peaks [28–31]. The relative position of the new states with and h is the Bragg angle of the [hkl] reflection [25]. Max- respect to valence band of titania depends on the properties imum value of lattice distortion was obtained, as expected of dopants and its position within the matrix. Whether the for Ce–S–TiO sample, since sulfur has largest ionic radii nonmetal dopant would occupy anionic or cationic substi- among the nonmetals used. The corresponding change in tutional sites or interstitial site depends upon its size, lattice parameters for tetragonal titania lattice is also inclu- chemical valence and electronegativity compared that of ded in Table 1. With the addition of Ce in TiO , due to the the host TiO . The probability that a dopant will substitute 2 2 lattice distortion, the lattice parameters increased compared oxygen depends on its electronegativity [32]. As difference to that of undoped titania. Co-addition of nonmetals resulted in electronegativity values of dopant and oxygen becomes in further increase in lattice parameter values. smaller, the probability that dopant will substitute oxygen The FESEM images of undoped TiO , Ce–TiO , Ce– becomes higher. Substitutional dopants form the new band 2 2 NM–TiO are shown in Fig. 4. The images showed a states at higher energy level compared to interstitial dopant Intensity (a.u) Page 6 of 10 Mater Renew Sustain Energy (2014) 3:25 Fig. 4 FESEM images of samples a TiO , b Ce–TiO , c Ce–N–TiO , d Ce–S–TiO , e Ce–B–TiO , and f Ce–C–TiO 2 2 2 2 2 2 123 Mater Renew Sustain Energy (2014) 3:25 Page 7 of 10 1.4 (a) TiO (a) Ce-C-TiO 2.0 (b) Ce-TiO (b) Ce-B-TiO 1.2 (c) Ce-N-TiO (c) Ce-N-TiO (d) Ce-S-TiO 1.0 (d) Ce-S-TiO (e) Ce-B-TiO 1.5 (e) Ce-TiO (f) Ce-C-TiO (a) 0.8 (f) TiO (b) (e) 1.0 (d) 0.6 (f) (c) (c) (d) 0.4 (a) 0.5 (b) 0.2 (e) (f) 0.0 0.0 350 400 450 500 550 600 250 300 350 400 450 500 550 600 650 Wavelength (nm) Wavelength (nm) Fig. 6 Photoluminescence spectra of TiO , Ce–TiO , and Ce–NM– 2 2 Fig. 5 UV–Visible diffuse reflectance spectra of TiO , Ce–TiO , and 2 2 TiO (NM = B, C, N, S) samples Ce–NM–TiO (NM = B, C, N, S) samples [33]. For boron co-doped catalyst, it is difficult to replace 360 nm can be ascribed to the latter as particle is much 4? 3? 3? Ti sites by B due to the smaller ionic radius of B larger than 10 nm. The emission signal at 396 nm 4? (IR = 0.023 nm) compared to Ti (IR = 0.068 nm). (3.13 eV) corresponds to the band gap transition of the Therefore, boron may replace either an oxygen atom or anatase [38]. This agrees well with the band gap energy of incorporate in the interstitial position in TiO matrix. But titania calculated from UV–Vis spectra. The excitonic due to the lower electronegativity of boron, its probability emissions in titania result due to the presence of defects of occupying the interstitial sites is higher. Similarly, car- and oxygen vacancies. The bands at 450 and 468 nm can bon can also substitute oxygen or occupy an interstitial site. be attributed to free excitons, and emission peaks at longer In nitrogen co-doped sample, the substitution of oxygen wavelength of 481 and 492 nm can be attributed to the sites is most probable due to their closer electronegativity bound excitons [40, 41]. The bound excitons result due to values [34]. Substitutions of S in both anionic and cationic the presence of oxygen vacancies [40, 42, 43]. It can be sites of TiO have been reported in the literature [35, 36]. observed from the Fig. 6 that Ce–TiO had lower PL 2 2 2- However, anionic sulfur substitution by S intensity compared to TiO . In Ce–TiO , cerium dopant- 2 2 (IR = 0.17 nm) might be difficult because of its larger induced energy level in TiO band structure, as discussed 2- ionic radius than O (IR = 0.122 nm). But the cationic earlier, serves as an electron trap as represented by Eq. (1) 4? 6? substitution of Ti (IR = 0.068 nm) by S resulting in non-radiative recombinations. (IR = 0.029 nm) would be more favorable due to smaller 4þ  3þ Ce þ e ! Ce ð1Þ size of latter. When TiO is doped with sulfur, either the mixing of S 3p states with valence band O 2p states or the This reduces the rate of radiative recombination of photogenerated electrons with holes in valence band of formation of isolated p-orbitals of S above the valence band maximum of TiO contributes toward narrowing the titania as shown in Fig. 6. The PL emission intensities of the cerium and nonmetal co-doped samples decreased in band gap [37]. The radiative recombination of electron–hole pairs in the order of TiO [ Ce–TiO [ Ce–B–TiO [ Ce–C– 2 2 2 photocatalysts can be studied by the photoluminescence TiO [ Ce–N–TiO [ Ce–S–TiO . In the cerium and 2 2 2 (PL) emission spectra. Figure 6 shows the room-tempera- nonmetal co-doped samples, the oxygen vacancies ture photoluminescence spectra of the TiO , Ce–TiO , and generated by substituting nonmetal dopants can act as 2 2 Ce–NM–TiO samples in the range of 350–600 nm. For all additional effective traps for photo-induced electrons, the samples, six emission peaks appeared at 360, 396, 450, thereby reducing the radiative recombination rate of charge carriers further. The photogenerated electrons are 468, 481, and 492 nm wavelengths and the corresponding transition energy are 3.44, 3.13, 2.76, 2.65, 2.8, and trapped by oxygen vacancies by non-radiative combinations [42]. The decreasing emission intensity 2.52 eV, respectively. In the literature, the peak in the range of 360–370 nm has been assigned to either anatase order can be attributed to increased oxygen vacancies resulted by increasing substitution effects from B to N as particle \10 nm [38] or to self-trapped excitons localized in TiO octahedral [39]. In this study, the emission peak at discussed in earlier section. In sulfur co-doped samples, the Absorbance (a.u) Intensity (a.u) Page 8 of 10 Mater Renew Sustain Energy (2014) 3:25 Fig. 8 Schematic representation of hydrogen production over cerium TiO Ce-TiO Ce-B-TiO Ce-C-TiO Ce-N-TiO Ce-S-TiO 2 2 2 2 2 2 and nonmetal co-doped TiO photocatalysts Catalyst Type Fig. 7 Hydrogen evolution over TiO , Ce–TiO , and Ce–NM–TiO 2 2 2 (NM = B, C, N, S) samples charge separation. The lower activity of Ce–S–TiO can be attributed to its very low surface area. Moreover, the S may 6? presence of isolated cationic S dopant state may further be in isolated state and that may lead to lower mobility of act as surface trap states for photogenerated electrons, the holes and limits the number of charge carriers reaching thereby resulting in a significant decrease in the the catalyst surface. This may have also resulted in lower recombination rate and lowering the PL emission. photocatalytic activity in S-doped samples. The apparent quantum efficiency of the photocatalysts Photocatalytic activity studies was calculated using the method followed by Sasikala et al. [44]. The apparent quantum efficiency values of TiO , Ce– The photocatalytic hydrogen production from water– TiO , Ce–B–TiO , Ce–C–TiO , Ce–N–TiO , and Ce–S– 2 2 2 2 TiO were 1.6, 6.7, 10.2, 9.2, 17.2, and 2.2 %, respectively. methanol mixture for TiO , Ce–TiO , and Ce–NM–TiO 2 2 2 2 (NM = B, C, N, S) catalysts is shown in Fig. 7. The The calculated values agreed with the reported apparent activity of undoped TiO was very low but Ce–TiO quantum efficiency range of 2–22 % for different photo- 2 2 showed significant photocatalytic activity and high hydro- catalysts [45]. For comparison, the photocatalytic activity gen evolution. The high activity of cerium-doped TiO can of N–TiO was also measured. The apparent quantum 2 2 be attributed to their ability of absorption of light in visible efficiency of N–TiO (8.6 %) was slightly higher than that light region as shown in Fig. 5. Addition of nonmetals such Ce–TiO (6.7 %) but the efficiency increased significantly for co-doped Ce–N–TiO to 17.2, suggesting that both as B, C, and N to Ce–TiO samples further enhanced 2 2 hydrogen evolution activity. This may be attributed to doping components were contributory to photocatalytic activity, as discussed earlier, giving higher hydrogen pro- higher surface area, enhanced visible light absorption as well as lower recombination rate of electron–hole pairs in duction. The results are shown in Supplementary Figure S2. The durability test of Ce–N–TiO for photocatalytic H co-doped samples. The highest activity of Ce–N–TiO 2 2 2 among co-doped samples may be attributed to its higher production from water–methanol mixture was performed surface area, higher visible light absorption, and lower for 10 h irradiation period with hydrogen evolution being recombination rate of electron–hole pairs compared to recorded at every 1 h as shown in Figure S3 of supple- other nonmetals, B and C, doped samples. The activity of mentary information. The total hydrogen evolution of Ce–C–TiO should be higher than Ce–B–TiO due to 737 lmol was obtained after 10 h of irradiation time. The 2 2 higher absorption of visible light and lower recombination observed results showed that the Ce–N–TiO had high H 2 2 evolution rate as well as desired durability. of photogenerated electron–hole pairs for former as shown by absorption spectra (Fig. 5) and PL spectra (Fig. 6), Based on the observed band gap energies of co-doped samples, the position of the impurity states and probable respectively. But slightly higher hydrogen evolution observed for Ce–B–TiO compared to that of Ce–C–TiO transitions of electrons are schematically shown in Fig. 8. 2 2 In Ce–TiO , the photogenerated electrons are excited from may be attributed to higher surface area of former pro- viding more active sites. Much lower hydrogen production the valence band to extended conduction band where 4? 3? was observed for Ce–S–TiO sample, although it has electrons are trapped effectively by Ce /Ce couple. enhanced absorption in the visible light region and efficient This results in the accumulation of electrons in extended µ Mater Renew Sustain Energy (2014) 3:25 Page 9 of 10 conduction band, which can serve as hydrogen formation References site. On addition of nonmetals to cerium-doped titania, the 1. Ashokkumar, M.: An overview on semiconductor particulate excitation of electrons can also takes place from nonmetal- systems for photoproduction of hydrogen. Int. J. Hydrog. 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Enhanced photocatalytic hydrogen production from water–methanol mixture using cerium and nonmetals (B/C/N/S) co-doped titanium dioxide

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Material Science; Materials Science, general; Renewable and Green Energy; Renewable and Green Energy
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10.1007/s40243-014-0025-6
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Mater Renew Sustain Energy (2014) 3:25 DOI 10.1007/s40243-014-0025-6 OR IGINAL PAPER Enhanced photocatalytic hydrogen production from water–methanol mixture using cerium and nonmetals (B/C/N/S) co-doped titanium dioxide N. Vinothkumar Mahuya De Received: 23 November 2013 / Accepted: 24 February 2014 / Published online: 18 March 2014 The Author(s) 2014. This article is published with open access at Springerlink.com Abstract In the present study, photocatalytic hydrogen Introduction production from water/methanol solution was investigated over cerium and nonmetal (B/C/N/S) co-doped titanium The energy economy of the world mainly depends on the dioxide catalyst under visible light irradiation. The cerium fossil fuels such as coal, natural gas, and petroleum pro- and nonmetal co-doped titania photocatalysts were pre- ducts. Gradual depletion of fossil fuels and fast-growing pared by co-precipitation and characterized by surface area energy demand has necessitated to develop alternative and pore size analysis, X-ray diffraction analysis, diffuse fuels, which should also be pollution-free, storable, and reflectance UV–Vis spectroscopy analysis, and photolu- economical [1]. Hydrogen is considered as a promising minescence analysis. The UV–Visible spectra showed that alternative fuel for the future. It has higher energy content incorporation of cerium and nonmetals to TiO resulted in and high heating value compared to other fuels such as narrow band gap and improved absorption of visible light. methane, methanol, gasoline, and diesel [2]. At present, The band gap energy of co-doped samples depended on the hydrogen is mainly produced by steam reforming of properties of nonmetals. Photoluminescence studies methane or naphtha. Various alternative processes such as showed that the radiative recombination rates of photo- electrolysis, thermolysis, thermochemical reactions, pho- generated electron–hole pairs were effectively suppressed tolysis, photoelectro-chemical, photocatalytic, and bio- by the addition of cerium and nonmetals and contributed to chemical processes are being studied for the production of higher activity. The highest hydrogen production of hydrogen. But only few of these methods are efficient and 206 lmol/h was obtained for Ce–N–TiO sample, which economically viable [3]. The production of hydrogen from can be attributed to the higher surface area, higher water using solar radiation is one of the potential routes to absorption of visible light, and higher separation efficiency achieve clean, low-cost, and eco-friendly fuel. Several of electron–hole pairs in Ce–N–TiO . oxide materials, such as TiO ,Fe O , ZnO, ZrO , 2 2 2 3 2 K Nb O ,K La Ti O ,BaTi O , and Ta O , have been 4 6 17 2 2 3 10 4 9 2 5 Keywords Photocatalysts  Hydrogen  Visible light  studied for the photodecomposition of water [4–8]. TiO Cerium  Nonmetals has been reported to be an excellent photocatalyst for water-splitting reaction. Advantages of TiO include chemical inertness, photostability, and low cost. However, the activity of TiO is limited in UV region because of its wide band gap (3.2 eV). The visible light (k [ 400 nm) constitutes the major fraction of solar spectrum reaching Electronic supplementary material The online version of this the earth surface. For better utilization of the sunlight, it is article (doi:10.1007/s40243-014-0025-6) contains supplementary material, which is available to authorized users. essential to develop stable photocatalytic systems with suitable band gap to absorb most of the solar spectrum. In N. Vinothkumar  M. De (&) order to develop an efficient titania-based visible light Department of Chemical Engineering, Indian Institute active photocatalyst, various methods have been reported of Technology, Guwahati 781 039, Assam, India such as doping with metals and nonmetals, dye e-mail: mahuya@iitg.ernet.in 123 Page 2 of 10 Mater Renew Sustain Energy (2014) 3:25 sensitization as well as coupling with other semiconductors 2.5 and 1 mol% of titanium, respectively. The loadings of [9]. The dopant impurities create additional energy levels Ce (x) and nonmetals (y) are defined as within the band gap of titania acting either as donor or CeðmolÞ x ¼  100 ¼ 2:5 mol% and acceptor level. An extensive review has been done on TiðmolÞ visible light activity of titania by Pelaez et al. [10]. Pho- NMðmolÞ y ¼  100 ¼ 1 mol%: tocatalytic activities of TiO doped with rare earth metals TiðmolÞ and nonmetals are mostly reported for the degradation of Aqueous ammonia (25 wt%) and solution B were pollutants. Xu et al. [11] investigated various rare earth- simultaneously added dropwise to the solution A with doped TiO samples for the degradation of nitrite and continuous stirring. Addition of ammonia was continued observed an enhanced photoabsorption in the visible light until complete precipitation occurred. The resultant region. Ce-doped TiO materials were also reported to mixture was stirred for 2 h at room temperature and was show high activity under visible light irradiation for pho- placed in a water bath at 363 K for overnight. The tocatalytic degradation of dye and phenol derivatives [12– precipitate was filtered and dried in an oven at 373 K for 14]. The N–TiO [15], and S–TiO [16] were studied for 2 2 24 h. The as-prepared samples were calcined at 723 K for the photodegradations of methylene blue under UV/Visible 3 h. In text, nonmetal co-doped samples are referred as Ce– light irradiation, while C–TiO [17] was reported to be NM–TiO (NM = B, C, N, S). For reference, cerium- active for the degradation of phenol. The titania co-doped doped titania were prepared by dissolving required amount with rare earth metals and nitrogen are also reported to of cerium nitrate hexahydrate in deionized water (solution show an enhanced activity for the decomposition of pol- lutants [18, 19]. Though doped TiO catalysts were B). The amount of Ce was 2.5 mol% of Ti, and sample is represented as Ce–TiO in text. The solution B was added extensively investigated for the degradation of pollutants, to solution A along with aqueous ammonia as described comparatively fewer studies have been reported for pho- earlier to obtain cerium-doped titania samples. Similar tocatalytic hydrogen production [20–22]. In this study, the aging condition and calcination temperature were photocatalytic activities of TiO co-doped with cerium and maintained for the preparation of Ce–TiO sample. nonmetals were studied for hydrogen evolution from 2 Undoped titania was also prepared for reference by water–methanol mixture under visible light irradiation. The similar procedure without the addition of cerium or effect of various nonmetals such as B, C, N, and S on nonmetals and represented by TiO in text. hydrogen production was studied. The physical and optical properties of the photocatalysts affecting the activity were Characterization of photocatalyst investigated using various characterization techniques such as surface and pore analysis, XRD, UV–Vis, and photo- The surface areas of the samples were determined from N luminescence spectroscopy. isotherms collected at 77 K using a Beckman Coulter TM SA 3100 analyzer. Prior to the experiments, the samples were degassed at 423 K for a period of 2 h. The pore size Experimental distribution of the photocatalysts was determined by Bar- rett–Joyner–Halenda (BJH) method. The X-ray diffraction Preparation of photocatalyst (XRD) patterns were recorded on a Bruker D2 phaser X-ray diffractometer using Ni-filtered Cu K as radiation Cerium and nonmetal co-doped TiO samples were pre- source (k = 1.5406 A) in the 2h range from 20 to 80 at a pared by co-precipitation method. Titanium tetra isoprop- scanning rate of 1/min. A beam voltage and beam current oxide (TTIP) and cerium nitrate hexahydrate were used as of 40 kV and 40 mA were used, respectively. The diffuse precursors for titania and ceria, respectively, and were reflectance UV–vis spectra of the photocatalysts were obtained from Sigma-Aldrich. The boric acid, glucose, measured on a PerkinElmer Lambda 750 instrument with a urea, and thiourea were used as sources for boron, carbon, 60-mm labsphere specular reflectance accessory at room nitrogen, and sulfur, respectively, and were procured from temperature. The baseline correction was done using a Merck. The isopropanol and ammonia solution were also calibrated sample of barium sulfate as reference. The scan procured from Merck. parameter was set with slit size 2 nm and scan in the Requisite amount of TTIP was mixed with isopropanol spectral range of 250–650 nm. The photoluminescence under continuous stirring condition for 10 min to form (PL) measurements were taken in a Thermo Spectronic solution A. Solution B was prepared by dissolving the (Aminco Bowman Series 2) instrument with a Xe lamp as required amount of cerium nitrate hexahydrate and the excitation source at room temperature. The powders respective precursors of nonmetals in deionized water. The were dispersed in ethanol, and the emission spectra were loadings in co-doped samples for Ce and nonmetals were 123 Mater Renew Sustain Energy (2014) 3:25 Page 3 of 10 collected at an excitation wavelength of 325 nm. The same supplementary information. The temperature of the reac- quantity was used for recording the PL spectra of all the tion mixture was maintained at 307 K using a water cir- samples. The entrance and exit slit widths were fixed as culation bath, which also acted as an IR filter. The evolved same for all the measurements. Field emission scanning gas was collected in an inverted burette by water dis- electron microscopic analysis was done using instrument of placement method. The gas mixture was analyzed using a ZEISS-Sigma. For FESEM analysis, the samples were gas chromatograph (Varian, CP 3800) equipped with car- dispersed in a solvent and deposited in an aluminum foil bosieve SII column and thermal conductivity detector. that was mounted on a sample holder for gold coating. The After completion of the photocatalytic reaction, the liquid composition of the samples was determined by EDS ana- solution was analyzed for the detection of intermediates lysis (energy-dispersive X-ray spectroscopy) equipped with (formaldehyde and formic acid) by HPLC (SHIMADZU, SEM instrument (LEO 1430 VP). C18 column). To confirm the photocatalytic activity of the catalysts, experiments were conducted first in the absence Photocatalytic activity studies of light with catalysts and next in the presence of light without catalyst. In both cases, no hydrogen evolution was The activities of the photocatalysts were examined using observed. The hydrogen evolution was observed only when the experimental setup shown in Fig. 1. The reaction was the reaction was carried out in the presence of both light typically carried out by adding 0.2 g catalyst to solution of and catalyst. In all cases, same feed mixture was used. This water (25 ml) and methanol (1 ml) under stirred condition. confirmed the photocatalytic activity of the catalysts. Prior to irradiation, the reaction mixture was de-aerated with N gas (50 ml/min) for 30 min to completely remove the dissolved oxygen. Then, the reaction mixture was Results and discussion irradiated with a 500-W tungsten halogen lamp (Halonix, India), placed approximately 15 cm away from the reactor, Characterization of prepared catalysts as source of visible light. The emission spectrum of the lamp was measured in front of the reactor using Ocean The actual composition of the prepared photocatalysts was optics USB4000 spectrometer as shown in Figure S1 in the determined by EDS, and the results are shown in Table S1 Fig. 1 Schematic representation of experimental setup for photocatalytic water splitting 3 9 1. N gas 7.Thermocouple 2. Flowmeter 8.Product gas outlet 3. Halogen lamp 9.Water bath 4. Magnetic stirrer 10. Gas collector 5. Reactor 11.Cooling water inlet 6. Reactant mixture 12. Gas Chromatograph 123 Page 4 of 10 Mater Renew Sustain Energy (2014) 3:25 Table1 Physicochemical and crystalline properties of undoped TiO , 0.008 (a) Ce–TiO , and Ce–NM–TiO (NM = B, C, N, S) samples Ce-TiO 2 2 0.007 2 0.006 Samples BET Pore Lattice Lattice d spacing ˚ 0.005 surface volume parameters distortion (A) 150 0.004 area (cc/g) Anatase (m /g) 0.003 ˚ ˚ a (A) c (A) 0.002 (101) (004) 0.001 0.000 TiO 84 0.45 3.7435 9.4217 0.378 3.4790 0 20406080 100 120 140 160 Ce–TiO 70 0.36 3.7451 9.4599 0.268 3.4822 Pore diameter (nm) Ce–B–TiO 87 0.57 3.7907 9.5197 0.271 3.5128 Ce–C–TiO 53 0.67 3.7659 9.5080 0.387 3.5013 Ce–N–TiO 158 0.06 3.7936 9.7181 0.301 3.2240 Ce–S–TiO 39 0.18 3.7727 9.7014 1.391 3.5174 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P ) of supplementary information. The actual cerium content in samples varied in the range of 2.39–2.49 mol%, and nonmetal content varied in the range of 0.8–0.97 mol% Ce-N-TiO (b) 2 0.12 Ce-C-TiO (Table S2 of supplementary information). Thus, the actual Ce-S-TiO 0.10 composition agreed well with the intended composition of Ce-B-TiO 0.08 the photocatalysts within experimental error. 0.06 The surface area and pore volumes of undoped TiO , 0.04 Ce–TiO , Ce–NM–TiO (NM = B, C, N, S) samples are 2 2 0.02 shown in Table 1. Surface area and pore volume of titania 0.00 decreased with the addition of ceria. Structure modification 0 20 40 60 80 100 120 140 160 Pore diameter (nm) and partial pore blockage in the presence of cerium may be responsible for decrease in surface area and pore volume in cerium-doped samples. The isotherm of Ce–TiO and corresponding pore size distribution are shown in Fig. 2a. 0.0 0.2 0.4 0.6 0.8 1.0 The Ce–TiO sample exhibited a type IV isotherm with a H1-type hysteresis loop associated with open-ended Relative Pressure (P/P ) cylindrical pores. The pore size distribution of Ce–TiO Fig. 2 N adsorption–desorption isotherms of Ce–TiO and Ce–NM– 2 2 showed the presence of both mesopores and macropores TiO (NM = B, C, N, S) samples (inset pore size distribution) with pore diameter in the range of 3–148 nm. Co-doping of titania with nonmetals along with cerium resulted in increase in surface area as shown in Table 1. The co-doped The X-ray diffraction patterns of Ce–TiO and Ce–NM– Ce–N–TiO , Ce–C–TiO , and Ce–B–TiO exhibited Type TiO are shown in Fig. 3. All the samples, except Ce–N– 2 2 2 II isotherm with H3 hysteresis loop corresponding to slit- TiO , were dominated by anatase phase (2h = 25.5,37, shaped pores (Fig. 2b). For sulfur co-doped sample, H1 48, 53.8,55.1,64, JCPDS 21-1272). For Ce–N–TiO , hysteresis loop was observed, which indicated the presence significant amount of rutile phase (2h = 28, 36.1, 41.5, of open cylindrical pores. Figure 2b shows that the pore JCPDS 21-1276) was observed. In general, the phase size distribution varied significantly with nonmetals. For transformation of anatase to rutile occurs at temperature Ce–N–TiO and Ce–B–TiO , narrow pore distribution was above 873 K and anatase phase is expected to be dominant 2 2 obtained, with 39 and 47 % pores being below 10 nm, in samples calcined at lower temperature of 723 K, used in this study. Accordingly, anatase phase was dominant in respectively. The pore size distributions of Ce–C–TiO and Ce–S–TiO co-doped samples were much broader. The XRD profiles of all samples except Ce–N–TiO . For Ce– 2 2 presence of significant amount of pores in the range of N–TiO sample, the presence of significant amount of 20–50 nm may have contributed to their lower surface rutile phase suggests that nitrogen can catalyzes the ana- area. The surface area and pore size distribution results tase–rutile phase transformation significantly at lower showed that the presence of nonmetal had significant effect temperature [23]. In addition, Ce–S–TiO co-doped sample on the stabilization of porous network. The maximum was more amorphous in nature as observed from broad effect was observed for Ce–N–TiO with highest surface XRD peaks of low intensity compared to that of other area. nonmetal-doped samples. No characteristic peaks of CeO Volume adsorbed (cc/g STP) Volume Adsorbed (cc/g STP) Pore volume (cc/g) Pore volume (cc/g) Mater Renew Sustain Energy (2014) 3:25 Page 5 of 10 definite change in morphology of the samples when cerium A - Anatase (f) Ce-C-TiO and nitrogen were added to titania suggesting that the do- R - Rutile pants played a prominent role in the development of A A material structure during co-precipitation. This resulted in A different physical properties as was observed in Table 1. (e) Ce-B-TiO Particles of irregular shape and size were observed for A A undoped titania, whereas ceria-doped titania, prepared by co-precipitation, consisted of particles of regular size and (d) Ce-S-TiO shape. The particles were spherical in the range of A A A A 100–200 nm. The morphology again changed when titania was co-doped with cerium and nonmetals. All the co-doped 200 0 (c) Ce-N-TiO Ce–NM–TiO samples were agglomerated in nature. The UV–Visible absorption spectra of TiO , Ce–TiO , 100 2 2 R A and Ce–NM–TiO are shown in Fig. 5. The Ce–TiO 2 2 sample showed intense absorption bands in the visible light (b) Ce-TiO region ranging from 400 to 500 nm. The band gap energies 100 R R R A of the samples were determined by extrapolating the rising part of the onset of the absorption edge to the x-axis (k, (a)TiO nm). The values of k was then used in the Planck’s Einstein equation to calculate the band gap, E = hc/k, where E is g g A A R A R A band gap energy in eV, h is Planck’s constant in eV. s, c is the speed of light in m/s, and k is the absorption wave- 20 30 40 50 60 70 80 length in nm [26]. The calculated band gap energy values for undoped TiO and Ce–TiO were 3.11, and 2.81, 2 2 respectively. The decreased band gap energy values of Fig. 3 XRD patterns of TiO , Ce–TiO , and Ce–NM–TiO 2 2 2 cerium-doped sample indicated the formation of new (NM = B, C, N, S) samples energy levels within the TiO band gap and resulted in phases were observed in the XRD profiles, which suggest redshift of the absorption edge. This shift can be attributed that the ceria was well dispersed in TiO matrix or were to the incorporation of Ce 4f levels into the TiO crystal 2 2 below detection limit. In addition, no peaks due to any Ce– structure just below the conduction band of TiO and Ti mixed oxides were observed in any of the samples. thereby reducing the effective band gap [27]. Addition of On addition of Ce to TiO , cerium ions can substitute nonmetal to cerium-doped TiO samples resulted in further 2 2 4? Ti sites or can occupy the interstitial sites with Ce–O–Ti shift in the band gap energy to longer wavelength. Sig- linkages [24]. But this is expected to be associated with nificant reductions in the optical band gap energies from 4? distortion in titania lattice due to larger size of Ce 3.11 eV for undoped TiO to about 2.88 eV for Ce–B– 3? (IR = 0.101 nm) and Ce (IR = 0.111 nm) compared to TiO , 2.66 eV for Ce–C–TiO , 2.29 eV for Ce–N–TiO , 2 2 2 4? Ti (IR = 0.068 nm) ions. The lattice distortion values are and 2.24 eV for Ce–S–TiO were observed for co-doped shown in Table 1 and correspond to maximum strain samples. For the co-doped catalysts, the band gap energy observed in crystal lattice due to distortion by incorporation was affected by the formation of additional impurity states of dopant metals. The lattice distortion was calculated using within titania matrix by the interaction of p-orbitals of the the formula defined by Stokes and Wilson as e = b/(4 tan h), nonmetal dopants (B, C, N, S) and 2p-orbitals of oxygen where b is the full width at half maximum of diffracted peaks [28–31]. The relative position of the new states with and h is the Bragg angle of the [hkl] reflection [25]. Max- respect to valence band of titania depends on the properties imum value of lattice distortion was obtained, as expected of dopants and its position within the matrix. Whether the for Ce–S–TiO sample, since sulfur has largest ionic radii nonmetal dopant would occupy anionic or cationic substi- among the nonmetals used. The corresponding change in tutional sites or interstitial site depends upon its size, lattice parameters for tetragonal titania lattice is also inclu- chemical valence and electronegativity compared that of ded in Table 1. With the addition of Ce in TiO , due to the the host TiO . The probability that a dopant will substitute 2 2 lattice distortion, the lattice parameters increased compared oxygen depends on its electronegativity [32]. As difference to that of undoped titania. Co-addition of nonmetals resulted in electronegativity values of dopant and oxygen becomes in further increase in lattice parameter values. smaller, the probability that dopant will substitute oxygen The FESEM images of undoped TiO , Ce–TiO , Ce– becomes higher. Substitutional dopants form the new band 2 2 NM–TiO are shown in Fig. 4. The images showed a states at higher energy level compared to interstitial dopant Intensity (a.u) Page 6 of 10 Mater Renew Sustain Energy (2014) 3:25 Fig. 4 FESEM images of samples a TiO , b Ce–TiO , c Ce–N–TiO , d Ce–S–TiO , e Ce–B–TiO , and f Ce–C–TiO 2 2 2 2 2 2 123 Mater Renew Sustain Energy (2014) 3:25 Page 7 of 10 1.4 (a) TiO (a) Ce-C-TiO 2.0 (b) Ce-TiO (b) Ce-B-TiO 1.2 (c) Ce-N-TiO (c) Ce-N-TiO (d) Ce-S-TiO 1.0 (d) Ce-S-TiO (e) Ce-B-TiO 1.5 (e) Ce-TiO (f) Ce-C-TiO (a) 0.8 (f) TiO (b) (e) 1.0 (d) 0.6 (f) (c) (c) (d) 0.4 (a) 0.5 (b) 0.2 (e) (f) 0.0 0.0 350 400 450 500 550 600 250 300 350 400 450 500 550 600 650 Wavelength (nm) Wavelength (nm) Fig. 6 Photoluminescence spectra of TiO , Ce–TiO , and Ce–NM– 2 2 Fig. 5 UV–Visible diffuse reflectance spectra of TiO , Ce–TiO , and 2 2 TiO (NM = B, C, N, S) samples Ce–NM–TiO (NM = B, C, N, S) samples [33]. For boron co-doped catalyst, it is difficult to replace 360 nm can be ascribed to the latter as particle is much 4? 3? 3? Ti sites by B due to the smaller ionic radius of B larger than 10 nm. The emission signal at 396 nm 4? (IR = 0.023 nm) compared to Ti (IR = 0.068 nm). (3.13 eV) corresponds to the band gap transition of the Therefore, boron may replace either an oxygen atom or anatase [38]. This agrees well with the band gap energy of incorporate in the interstitial position in TiO matrix. But titania calculated from UV–Vis spectra. The excitonic due to the lower electronegativity of boron, its probability emissions in titania result due to the presence of defects of occupying the interstitial sites is higher. Similarly, car- and oxygen vacancies. The bands at 450 and 468 nm can bon can also substitute oxygen or occupy an interstitial site. be attributed to free excitons, and emission peaks at longer In nitrogen co-doped sample, the substitution of oxygen wavelength of 481 and 492 nm can be attributed to the sites is most probable due to their closer electronegativity bound excitons [40, 41]. The bound excitons result due to values [34]. Substitutions of S in both anionic and cationic the presence of oxygen vacancies [40, 42, 43]. It can be sites of TiO have been reported in the literature [35, 36]. observed from the Fig. 6 that Ce–TiO had lower PL 2 2 2- However, anionic sulfur substitution by S intensity compared to TiO . In Ce–TiO , cerium dopant- 2 2 (IR = 0.17 nm) might be difficult because of its larger induced energy level in TiO band structure, as discussed 2- ionic radius than O (IR = 0.122 nm). But the cationic earlier, serves as an electron trap as represented by Eq. (1) 4? 6? substitution of Ti (IR = 0.068 nm) by S resulting in non-radiative recombinations. (IR = 0.029 nm) would be more favorable due to smaller 4þ  3þ Ce þ e ! Ce ð1Þ size of latter. When TiO is doped with sulfur, either the mixing of S 3p states with valence band O 2p states or the This reduces the rate of radiative recombination of photogenerated electrons with holes in valence band of formation of isolated p-orbitals of S above the valence band maximum of TiO contributes toward narrowing the titania as shown in Fig. 6. The PL emission intensities of the cerium and nonmetal co-doped samples decreased in band gap [37]. The radiative recombination of electron–hole pairs in the order of TiO [ Ce–TiO [ Ce–B–TiO [ Ce–C– 2 2 2 photocatalysts can be studied by the photoluminescence TiO [ Ce–N–TiO [ Ce–S–TiO . In the cerium and 2 2 2 (PL) emission spectra. Figure 6 shows the room-tempera- nonmetal co-doped samples, the oxygen vacancies ture photoluminescence spectra of the TiO , Ce–TiO , and generated by substituting nonmetal dopants can act as 2 2 Ce–NM–TiO samples in the range of 350–600 nm. For all additional effective traps for photo-induced electrons, the samples, six emission peaks appeared at 360, 396, 450, thereby reducing the radiative recombination rate of charge carriers further. The photogenerated electrons are 468, 481, and 492 nm wavelengths and the corresponding transition energy are 3.44, 3.13, 2.76, 2.65, 2.8, and trapped by oxygen vacancies by non-radiative combinations [42]. The decreasing emission intensity 2.52 eV, respectively. In the literature, the peak in the range of 360–370 nm has been assigned to either anatase order can be attributed to increased oxygen vacancies resulted by increasing substitution effects from B to N as particle \10 nm [38] or to self-trapped excitons localized in TiO octahedral [39]. In this study, the emission peak at discussed in earlier section. In sulfur co-doped samples, the Absorbance (a.u) Intensity (a.u) Page 8 of 10 Mater Renew Sustain Energy (2014) 3:25 Fig. 8 Schematic representation of hydrogen production over cerium TiO Ce-TiO Ce-B-TiO Ce-C-TiO Ce-N-TiO Ce-S-TiO 2 2 2 2 2 2 and nonmetal co-doped TiO photocatalysts Catalyst Type Fig. 7 Hydrogen evolution over TiO , Ce–TiO , and Ce–NM–TiO 2 2 2 (NM = B, C, N, S) samples charge separation. The lower activity of Ce–S–TiO can be attributed to its very low surface area. Moreover, the S may 6? presence of isolated cationic S dopant state may further be in isolated state and that may lead to lower mobility of act as surface trap states for photogenerated electrons, the holes and limits the number of charge carriers reaching thereby resulting in a significant decrease in the the catalyst surface. This may have also resulted in lower recombination rate and lowering the PL emission. photocatalytic activity in S-doped samples. The apparent quantum efficiency of the photocatalysts Photocatalytic activity studies was calculated using the method followed by Sasikala et al. [44]. The apparent quantum efficiency values of TiO , Ce– The photocatalytic hydrogen production from water– TiO , Ce–B–TiO , Ce–C–TiO , Ce–N–TiO , and Ce–S– 2 2 2 2 TiO were 1.6, 6.7, 10.2, 9.2, 17.2, and 2.2 %, respectively. methanol mixture for TiO , Ce–TiO , and Ce–NM–TiO 2 2 2 2 (NM = B, C, N, S) catalysts is shown in Fig. 7. The The calculated values agreed with the reported apparent activity of undoped TiO was very low but Ce–TiO quantum efficiency range of 2–22 % for different photo- 2 2 showed significant photocatalytic activity and high hydro- catalysts [45]. For comparison, the photocatalytic activity gen evolution. The high activity of cerium-doped TiO can of N–TiO was also measured. The apparent quantum 2 2 be attributed to their ability of absorption of light in visible efficiency of N–TiO (8.6 %) was slightly higher than that light region as shown in Fig. 5. Addition of nonmetals such Ce–TiO (6.7 %) but the efficiency increased significantly for co-doped Ce–N–TiO to 17.2, suggesting that both as B, C, and N to Ce–TiO samples further enhanced 2 2 hydrogen evolution activity. This may be attributed to doping components were contributory to photocatalytic activity, as discussed earlier, giving higher hydrogen pro- higher surface area, enhanced visible light absorption as well as lower recombination rate of electron–hole pairs in duction. The results are shown in Supplementary Figure S2. The durability test of Ce–N–TiO for photocatalytic H co-doped samples. The highest activity of Ce–N–TiO 2 2 2 among co-doped samples may be attributed to its higher production from water–methanol mixture was performed surface area, higher visible light absorption, and lower for 10 h irradiation period with hydrogen evolution being recombination rate of electron–hole pairs compared to recorded at every 1 h as shown in Figure S3 of supple- other nonmetals, B and C, doped samples. The activity of mentary information. The total hydrogen evolution of Ce–C–TiO should be higher than Ce–B–TiO due to 737 lmol was obtained after 10 h of irradiation time. The 2 2 higher absorption of visible light and lower recombination observed results showed that the Ce–N–TiO had high H 2 2 evolution rate as well as desired durability. of photogenerated electron–hole pairs for former as shown by absorption spectra (Fig. 5) and PL spectra (Fig. 6), Based on the observed band gap energies of co-doped samples, the position of the impurity states and probable respectively. But slightly higher hydrogen evolution observed for Ce–B–TiO compared to that of Ce–C–TiO transitions of electrons are schematically shown in Fig. 8. 2 2 In Ce–TiO , the photogenerated electrons are excited from may be attributed to higher surface area of former pro- viding more active sites. Much lower hydrogen production the valence band to extended conduction band where 4? 3? was observed for Ce–S–TiO sample, although it has electrons are trapped effectively by Ce /Ce couple. enhanced absorption in the visible light region and efficient This results in the accumulation of electrons in extended µ Mater Renew Sustain Energy (2014) 3:25 Page 9 of 10 conduction band, which can serve as hydrogen formation References site. On addition of nonmetals to cerium-doped titania, the 1. Ashokkumar, M.: An overview on semiconductor particulate excitation of electrons can also takes place from nonmetal- systems for photoproduction of hydrogen. Int. J. Hydrog. 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