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CdIn S (CIS) has attracted widespread attention due to its structural stability and photoelectric properties, however, 2 4 it is difficult to recycle when after usage. Carbon nanofibers (CNFs) as a suitable electron acceptor due to its stable physicochemical properties enhanced the mechanical properties and easily to recycle. There are also few reports on applying CIS/CNFs composite as photocatalyst in removing volatile organic compounds (VOCs). In this study, a novel CIS/CNFs composite was synthesized via a simple hydrothermal method. Various characterizations, such as X- ray diffraction, Scanning Electron Microscope, X-ray Photoelectron Spectroscopy and Transmission Electron Microscopy proved the successful synthesis of CIS/CNFs composite and revealed that CNFs grow on the surfaces of CIS connected with three-dimensional (3D) conductive network. Under visible light irradiation, degradation of toluene reached the optimal level of 86% as the CIS doped with 3% CNFs. Furthermore, 95% removal efficiency was achieved as 200 ppm ozone was added into the system and mineralization rate is also improved. The 3D network of CNFs can facilitate the effective separation and transfer of the photogenerated electron-hole pairs, protect CIS core from photo-corrosion and easily be recycled. Ultimately, plausible of ozone-enhanced photocatalytic mechanisms were proposed. Hence, this study presents a new photocatalyst with visible-light driven ozone- enhanced photocatalysis process toward VOCs. Keywords: CdIn S /CNFs, Photocatalyst, Toluene, Ozone-enhanced photocatalytic oxidation 2 4 1 Introduction enhanced photocatalytic oxidation(O -PCO) is one of the Along with the continuous progress of industrialization, ex- promising toluene control technologies [6]. The recent cessive amounts of volatile organic compounds (VOCs) studies found that introduction of ozone not only improves have been used and released from indoor decoration, the performance of photocatalytic oxidation, but also chemical material production and coal-fired boilers [1]. facilitates catalyst regeneration. Therefore, it is essen- Long-termexposuretotoluene mayexbibit adversehealth tial to develop stable structure and excellent proper- effects, such as allergic reactions and even cancer [2]. Ther- ties for photocatalyst. mal catalytic oxidation [3] and non-thermal plasma [4]have CdIn S (CIS) as a ternary n-type chalcogenide with 2 4 been proven to be effective for VOC removal. However, superior thermal stability and unique photoelectric prop- low mineralization rate, high energy consumption and de- erties; for instance, narrow band gap (2.0 eV) and large activation of catalyst are the major disadvantages. To solve specific surface area which benefit the rapid excitation of the bottleneck, combination of above techniques to treat charge carriers due to effective absorption of visible toluene has recently gained much attention [5]. Ozone- light. CIS exhibit more photo-corrosion resistance than 3+ cadmium sulfide (CdS), due to the presence of In ions [7]. However, powdered materials are difficult to recycle * Correspondence: mbchang@ncuen.ncu.edu.tw Graduate Institute of Environmental Engineering, National Central University, Taoyuan 32051, Taiwan © The Author(s). 2022 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Liu et al. Sustainable Environment Research (2022) 32:10 Page 2 of 13 after dispersion, which limits their application in the real For the CIS/CNFs preparation, 50 mg CNFs were dis- environment [8]. solved in 40 mL deionized water under magnetic stirring, Carbon nanofibers (CNFs) as a suitable electron ac- and then mixed with 601 mg In (NO ) ·4.5H O, 308 mg 3 3 2 ceptor can inhibit the secondary. Cd (NO ) ·4H O and 1441 mg Na S·9H O, and the sub- 3 2 2 2 2 recombination of electron holes due to its superior sequently preparation steps were similar to CdIn S as 2 4 electron transport behavior, stable physicochemical stated above. The samples are named as CIS/CNFs-1, properties and high adsorption capacity [9]. Zhang et al. CIS/CNFs-2, CIS/CNFs-3 and CIS/CNFs-4 based on the [10] adopted network structure of CNFs enhanced deg- loading of CNFs wt%, respectively. radation efficiency of Rhodamine B, and improved resist- ance of photo-corrosion. Wang et al. [11] synthesized 2.2 Photocatalytic tests Bi WO /CNFs composite material, the heterostructure Catalytic degradation of toluene was tested in a self- 2 6 and 3D network structure enhanced photocatalytic and designed reactor at room temperature. The experimental electron transmission activity. Therefore, it could expect setup is shown in Fig. 1. The volume of dark organic- that coupling CIS with CNFs would inhibit the elec- glass reactor is 0.5 L. prior to each experiment, 50 mg tron–hole pair recombination to enhanced photoactivity photocatalyst powder was uniformly dispersed in 20 mL and regeneration of catalyst. ethanol solution and then coated on a round glass plate In this study, the CIS/CNFs photocatalytic material with a diameter of 5 cm, placing the sample-coated was synthesized by hydrothermal method. The physical dishes in the bottom of reactor with a glass slide cover. and chemical structures of photocatalysts were investi- After that, the standard gas toluene about 60 ppm was gated by scanning electron microscopy (SEM), transmis- passed into the reactor. The reactor was kept in the dark sion electron microscopy (TEM), X-ray diffraction condition for 1 h to achieve an equilibrium of adsorption (XRD) and X-ray photoelectron spectroscopy (XPS). and desorption. The initial toluene concentration Photoluminescence (PL) was used to investigate the remained at 60 ppm after adsorption equilibrium. The photoelectric properties of the samples. The catalytic glass slide cover on the petri dish was then removed to ability of the CIS/CNFs materials for the degradation of begin the catalytic oxidation of toluene. Xenon lamp toluene was evaluated under visible light, and O -PCO (MAX-350, Nmerry Technology Co., Beijing) occluded was applied to improve the removal efficiency. Through by a fixed wavelength filter produces a visible light the analysis of the experimental data, a viable catalytic source. Reaction in light condition for 3.5 h. The sample pathway for the degradation of toluene by the materials was collected and then injected by a sampling probe into prepared was explored and discussed. This study pro- the gas chromatograph (GC) for measurement. The ana- vides a new high-performance catalyst for degrading lysis of CO , CO, and water vapor was conducted online toluene. with a Photoacoustic IR Multi-gas Monitor (Model 1412, INNOVA air Tech Instruments, Denmark). As for the ozone-enhanced photocatalysis experiment, ozone 2 Materials and methods with a concentration of 200 ppm was fed into the reactor 2.1 Synthesis of CIS and CIS-CNFs composites and the above experimental steps were repeated. The All chemicals for synthesis and analysis are analytically ozone concentration was monitored by an ozone de- grade. The CIS and CIS/CNFs were synthesized by sim- tector (Model 106-L, 2B Technologies, USA). The deg- ple hydrothermal method. For the CIS preparation, 601 radation tests were repeated for three times and low mg In (NO ) ·4.5H O (Sinopharm Chemical Reagent deviations (less than 10%) were observed. The equations 3 3 2 Co., Shanghai), 308 mg Cd (NO ) ·4H O(Sinopharm for calculating toluene removal efficiency, ozone 3 2 2 Chemical Reagent Co., Shanghai) and 1441 mg utilization efficiency and toluene mineralization effi- Na S·9H O (Sinopharm Chemical Reagent Co., Shanghai) ciency are shown in supporting information. 2 2 were dissolved in deionized water under ultrasonication to obtain homogeneous solution. After dissolution, place the 2.3 Characterization solution in a beaker with a magnetic stirrer, during the The specific surface area and pore volume of catalysts stirring process, In (NO ) ·4.5H O and Cd (NO ) ·4H O were determined by nitrogen adsorption (Micromeritics 3 3 2 3 2 2 are slowly dropped into the Na S·9H O until formed uni- ASAP 2020, USA). The crystal structure of the as- 2 2 form pale-yellow sol. Finally, the mixed solution was kept synthesized CIS/CNFs was determined by Bruker XRD at 100 °C for 18 h in a Teflon-lined autoclave for continu- (D8-Advance, Germany) using Cu-Kα radiation. Data − 1 ous hydrothermal reaction, and then cooled to room was collected at scan rate of 0.02° s and from 10 to 80° temperature. The slurry was recovered by filtration, (2θ, diffraction angle). XPS analyses were performed on a washed with deionized water and dried at 120 °C Thermo Fisher Scientific ESCALAB 250 photoelectron overnight. spectrometers. High-resolution transmission electron Liu et al. Sustainable Environment Research (2022) 32:10 Page 3 of 13 Fig. 1 Schematic of experimental set-up for ozone-enhanced photocatalysis oxidation of toluene: (1) glass reactor, (2) glass petri dish coated with catalyst, (3) fan, (4) injection port, (5) sampling port, (6) GC-MS, (7) Photoacoustic IR multi-gas monitor, (8) ozone detector, (9) Xenon lamp light source microscopy (HRTEM) micrographs were obtained with a As shown in Fig. 2a, all samples reach adsorption- Tecnai G2 F20 S-TWIN microscope and operated at 200 desorption equilibrium of toluene under dark conditions kV. SEM images of the catalysts were obtained on a Nova after 30 min. The pure CNF shows strong adsorption NanoSEM 450 microscope operated at 200 kV. Photocur- ability possibly attributing to the large specific surface rent responses was observed by electrochemical worksta- area of the 3D network structure. As the CNFs content tion (CHI 660B). PL spectra was carried out on a increases, CIS/CNFs adsorption capacity increases and it spectrometer with an excitation wavelength of 325 nm will combine with toluene molecules more closely. (Varian Cary Eclipse, USA). As shown in Fig. 2b, the pure CIS removal percentages were only 65%. After doping 3% CNFs, the removal per- centages significantly increased to 86%, but when doping 2.4 GC-MS heating program rate increases to 4%, photocatalytic efficiency is reduced Determination of intermediates and products produced to 72%. When too much CNFs are added, it may reduce after toluene degradation reaction by GC coupled with light absorption and utilization. In addition, the result of mass spectrometer (GC–MS) (Agilent Technologies, dark conditions confirmed that adsorption effect of CIS/ USA). Data was collected at temperature program on CNFs was negligible and photocatalysis effect was re- 35 °C for 10 min and rose to 125 °C at a rate of 5 °C sponsible for toluene removal at irradiation stage. − 1 min under column HP-5MS (30 m × 0.250 mm). As shown in Fig. S1, the degradation data of toluene could be fitting pseudo-first-order kinetics, which can be 3 Results and discussion manifested as Eq. (1). 3.1 Photocatalytic performance C0 In order to determine the optimal dose of CIS/CNFs, ex- ln ¼ kapp t ð1Þ periments with various materials including CNFs doped Ct with different percentage CIS are conducted. In the pre- liminary experiment, it is found that the degradation ef- where the t is the reaction time, C and C are the tolu- 0 t ficiency of toluene achieved with CIS/CNFs was lower ene concentration of 0 and t, k is the apparent first- αpp − 1 than pure CIS materials when the CNFs content is ≥5%. order rate constant (h ). The CIS/CNFs composite dis- − 1 We speculate that when excessive CNF amount was plays the highest rate constant (0.56 h ) for toluene, − 1 doped, some pores and active sites of CIS might be nearly 2 times higher than pure CIS (0.29 h ). It follows blocked due to the high dispersability of CNFs. There- that CNFs can effectively improve the photocatalytic ac- fore, 4% is chosen as the maximum doping ratio of tivity of CIS, great improvement of the separation effi- CNFs. All degradation tests were conducted for three ciency of electron hole pairs. The catalytic activity of times and low deviations (less than 10%) were observed. CIS/CNFs-3 is compared with other studies catalysts Liu et al. Sustainable Environment Research (2022) 32:10 Page 4 of 13 Fig. 2 (a) Toluene adsorption performance; (b) toluene degradation performance by prepared catalysts under the same experimental (Table 1). It is observed that the removal efficiency of CIS/CNFs-3 is one of the highest in the list. 3.2 Ozone-enhanced photocatalysis tests Ozone is a powerful oxidizing agent which has been ap- Table 1 Comparison of photocatalytic decomposition of plied in a wide range of photocatalytic oxidation (PCO) toluene obtained in this study and literature to improve the performance. In this study, 200 ppm of Catalyst C (ppm) Removal efficiency (%) Ref. in ozone was introduced into the system to conduct O - CIS/CNFs-3 60 86 This study PCO tests of toluene with CIS, CNFs, and CIS/CNFs, re- ZnO 50 70 [12] spectively, as shown in Fig. 3. TiO /WO 50 74 [13] 2 3 The experiment is divided into three processes, i.e., BiVO /g-C N 25 68.2 [14] 4 3 4 CIS/CNFs + Vis, CIS/CNFs + O and CIS/CNFs + O + 3 3 30%-In S /g-C N 60 80 [15] Vis. Toluene degradation efficiencies achieved with 2 3 3 4 CIS + Vis and CIS/CNFs + O reach 86 and 76%, C : inlet concentration of toluene in 3 Liu et al. Sustainable Environment Research (2022) 32:10 Page 5 of 13 Fig. 3 Degradation of toluene achieved with performance different processes respectively. After ozone is injected, the efficiency of improves the photocatalytic performance, but also in- PCO system increases significantly to 95%. The deacti- creases mineralization efficiency. vated of photocatalyst, which may be caused by the ad- hesion of intermediates produced of toluene blocking of 3.3 Characterizations of photocatalytic materials the surface active sites and oxygen vacancy. However, 3.3.1 Crystalline structures and surface areas of catalysts the situation is greatly improved for O -PCO, the intro- The crystal structure and phase properties of CNFs, CIS, duction of ozone can improve the durability of catalysts. and CIS/CNFs samples were investigated by XRD pat- Furthermore, the introduction of ozone significantly tern and the results shown in Fig. 4. The six peaks at improves the mineralization efficiency. As shown in Fig. 2θ = 23.18, 27.25, 33.00, 43.32 and 47.41° can be consist- S2, the mineralization efficiency of CIS/CNFs + O is ent with the (220), (311), (400), (511) and (440) planes of about 85%; while that of CIS/CNFs + O + Vis is up to the cubic crystal lattice with a spinel structure of CIS 87%. Meanwhile, in CIS/CNFs + O + Vis, the ozone (JCPDS 270060), respectively. As for CNFs, the first wide utilization efficiency is over 80%. It indicates that O - peak at 2θ = 25.0° is corresponding to (002) crystal plane. PCO has the optimal mineralization efficiency and ozone As CIS nanoparticles were grown on the carbon fiber consumption efficiency, and toluene mineralization effi- surface, the XRD patterns of all CIS/CNFs composites ciency is positively correlated with ozone consumption were similar to pure CIS materials. The peak in the efficiency. Future research should be devoted to improve (311) plane of CIS/CNFs composite material was com- the ozone utilization. pared with that of pure CIS as shown in Fig. 4b, the peak Water vapor is a double-edged sword, water molecules obvious shift to lower 2θ with the increase of CNFs dop- could be competitively adsorbed on active sites which ing content become more intensive, which may be causes deactivated in catalyst; therefore, the test was car- caused by the strong interaction between CIS and CNFs. ried out in relative humidity (RH) = 20, 40 and 80%, re- When 3% carbon fiber was doped, the diffraction peaks spectively, with CIS/CNFs-3 as catalyst for tests (Fig. were broadened and weakened in intensity compared to S3). However, CIS/CNFs-3 exhibits significant humidity those of CIS, which indicates that with the CNFs doping resistance, with increasing RH. However, water vapor degree of crystallinity declined. The poor crystallinity generates more hydroxyl radicals in the photocatalytic generates more surface defects in the catalyst structure, reaction, thus improving the efficiency. In brief, the re- which is conducive to the adsorption and decomposition sult confirms that introduction of ozone not only of toluene and ozone molecules [16]. The average grain Liu et al. Sustainable Environment Research (2022) 32:10 Page 6 of 13 Fig. 4 (a) XRD patterns of CIS, CNFs, and CIS/CNFs heterojunctions (CIS/CNFs-1, 2, 3 and 4); (b) the magnified XRD patterns of CIS and CIS/ CNFs-x composites size (D) of the catalysts can be calculated from the line at half-maximum and θ is the angle at position of peak broadening of XRD peaks using Scherrer’s formula, as maximum. The as-calculated grain size of CIS is 27.5 shown in Eq. (2)[17]. nm, while the grain sizes of CIS/CNFs composites are 25.6 nm. In general, particle of smaller grain size has a K λ larger specific surface area [18]. D ¼ ð2Þ β cosθ The N sorption isotherms of CIS and CIS/CNFs-3 re- sembled typical type III curves (Fig. S4). Compared with where K is a constant (0.89); λ is the wavelength of the the CIS, the introduction of CNFs increased the specific 2 − 1 X-ray radiation (Cu Kɑ = 0.1541 nm); β is the full width surface area, from 39 to 43 m g . Thus, we can Liu et al. Sustainable Environment Research (2022) 32:10 Page 7 of 13 conclude that the network structure of CNFs enhances CIS, respectively, indicating that the hydrothermal syn- the dispersion of the CIS. This result was also supported thesis does not destroy the microscopic morphology of by the data obtained by XRD analysis. CIS crystal. In conclusion, CIS and CNFs were success- fully coupled to form composite material and CIS was 3.3.2 Morphology of CIS/CNFs photocatalyst not destroyed during the growth process of CNFs. The morphology of the sample is analyzed by SEM and TEM, as shown in Fig. 5. The diameter of CNFs is about 3.3.3 Structure and composition of CIS/CNFs photocatalyst 300 nm and the lengths are up to several microns, which The surface element states of the constituent elements has a high aspect ratio, and the surface is smooth and in the CIS and CIS/CNFs composite samples were inves- closely connected to form a 3D conductive network. It is tigated by XPS spectra as shown in Fig. 6. The Cd 3d beneficial to the transmission of electrons generated dur- spectra could be deconvoluted into two peaks, the peaks ing light irradiation. The CIS particles have 3D octahedral at 405.5 and 412.6 eV were attributed to Cd 3d and 5/2 2+ shape and crystal plane, as shown in Fig. 5b. In addition, 3d microstates states Cd in sulfide environment 3/2 CIS particles grow evenly on the CNFs, and no obvious (Fig. 6a) [19]. In the In 3d spectra, the peaks at 444.6 aggregation phenomenon (Fig. 5c). TEM images in Fig. 5d and 452.1 eV were respectively assigned to the 3d and 5/2 3+ reveal that octahedral CIS has a clear interface, with the 3d states when the In ion is coordination with the 3/2 particle size of about 100 nm. Also, it can be found in Fig. sulfide ion (Fig. 6b) [20]. The S 2p spectra were decon- 5e that the crystal structure of CIS nanomaterial remains voluted into two peaks at 161.3 and 162.5 eV, which 2− intact and tightly bound to carbon fiber. were assigned to 2p and 2p levels of S , respect- 3/2 1/2 To further understand the microstructures of CIS/ ively (Fig. 6c) [21]. In addition, the spin orbital splits of CNFs, the HRTEM was performed with the results Cd 3d, In 3d, and S 2p in CIS and CIS/CNFs samples shown in Fig. 5f. The CIS/CNFs have a distinct visible were 6.76, 7.56, and 1.18 eV, respectively, indicating that lattice fringe of 0.338 nm, which corresponds to the the valence states of Cd, In, and S in CdIn S and CIS/ 2 4 2+ 3+ 2− (002) plane of CNFs. The lattice fringes of 0.626 and CNFs samples are Cd ,In , and S , respectively. 0.327 nm corresponding to (111) and (311) planes of Compared with the pure CIS sample, the spin orbit of S Fig. 5 SEM images of (a) pure CNFs, (b) pure CIS and (c) CIS/CNFs-3; TEM images of (d) pure CNFs and (e) CIS/CNFs-3; (f) HRTEM images of CIS/CNFs-3 Liu et al. Sustainable Environment Research (2022) 32:10 Page 8 of 13 Fig. 6 XPS analysis of as-prepared CIS and CIS/CNFs-3 (a) Cd 3d; (b) In 3d; (c) S 2p; (d)C1 s 2p in the CIS/CNFs sample deviated and moved towards photocurrent improvement of CIS/CNFs composite can the direction of low binding energy, possibly due to strong be ascribed to enhanced rapid transfer of interfacial charge interaction between two materials. The spectra of C 1 s in CNFs 3D structure, which suppressed recombination could be deconvoluted into three peaks centered at 284.6, rate of the photogenerated electron-hole pairs, thus im- 285.7 and 286.6 eV, respectively (Fig. 6d). The peak at proved photocatalyst performance. 284.6 eV is attributed to C-C bond derived from amorph- To investigate the recombination of photogenerated ous carbon phase or indefinite carbon. Peaks at 285.7 eV electron-hole in depth, the PL spectra were measured. As it was characteristic of C-O groups, and the weak peak at is well known, the recombination rate of electron-hole is dir- 286.6 eV was ascribed to carboxyl carbon (OC = O) [22]. ectly proportional to the PL intensity [24]. In Fig. 7b, all Therefore, the XPS results further demonstrate that samples are of corresponding characteristic emission peaks CIS and CNFs coexist in CIS/CNFs-3 composites, and it at approximately 445 nm. The PL intensity of CIS/CNF-3 turns out that CNFs have been successfully doped on composites exhibits much lower emission than that of pure the surface of CIS. This result is also consistent with the CIS, which is consistent with the results of electron-hole re- results of XRD patterns. combination. The quenching of PL emission spectra of the CIS/CNFs can be attributed to efficient electron transfer be- 3.4 Photoelectrochemical performance tween CIS and CNFs. Due to the narrow band gap of CIS, Photocurrent had been considered as an efficient method electron–hole pairs are easy to recombine. The addition of to estimate recombination rate of electron-hole pairs, as CNFs, due to the conductive 3D structure as a suitable elec- shown in Fig. 7a[23]. CIS and CIS/CNFs-3 show a repeat- tron acceptor, can effectively inhibit recombination of the able photocurrent response under visible light irradiation, electron–hole pairs and promote the efficient separation of indicating that the samples prepared by hydrothermal photogenerated carriers due to rapid transfer of interfacial method is stable. Owing to the poor absorption of visible charge, thus improving the photocatalytic performance. light, CIS shows a low anodic photocurrent 1.61 × − 3 − 2 10 μAcm . Compared with pure CIS, the CIS/CNFs-3 3.5 Photocatalytic recycling and stability of CIS/CNFs composite exhibits higher anodic photocurrent of 1.63 × The stability of catalysts is of great importance in prac- − 3 − 2 10 μAcm , which is 1.2% more than pure CIS. The tical application. To test the stability of ozone-enhanced Liu et al. Sustainable Environment Research (2022) 32:10 Page 9 of 13 Fig. 7 (a) Transient photocurrent response of CIS and CIS/CNFs-3 under irradiation of visible light [Na SO = 0.5 M]; (b) PL emission spectra of CIS 2 4 and CIS/CNFs-3 composites photocatalysis for CIS/CNFs-3 composite, we reused the fresh photocatalyst. The results proves that the micro- catalyst 10 times, and each experiment was carried out morphology of the photocatalyst almost unchanged after under the consistent condition for three tests, as shown the 10 times tests, it fully proves that the addition of CNFs in Fig. 8a. After ten cycles of the experiment, CIS/CNFs- enhances the photo-corrosion resistance to catalyst. 3 composite material of photocatalytic activity decreases slightly from 95 to 91%. 3.6 Possible degradation mechanisms Figure 8b shows the XRD pattern of fresh and used CIS/ 3.6.1 Photocatalytic degradation mechanism CNFs-3 composite. After 10 times reactions, the peak pos- The possible pathways of toluene degradation were pro- ition and areas of the used one were nearly the same as posed as shown in Fig. 9. With the irradiation of visible Liu et al. Sustainable Environment Research (2022) 32:10 Page 10 of 13 Fig. 8 (a) Degradation effect of recombination experiment; (b) XRD pattern of fresh and used catalyst light, CIS is excited to produce photogenic electrons and electrons on CNFs combine with O to produce super- •− holes. Due to phenomenal conductivity of CNFs, the oxide radical (O ) with strong redox and further react conduction band of CIS is excited to generate photogen- with water molecules to generate hydroxyl radical erated electrons and transfer to CNFs. Moreover, the (•OH). According to the above results, the transport and close combination of CIS and CNFs provides a good degradation mechanism of the photo-generated carrier transmission platform for photogenerated carriers. of CIS/CNFs composite photocatalyst can be described Therefore, the photogenerated electron-hole pairs can by the following equations: be separated effectively. In the process of toluene deg- radation, the holes in the CIS valency band oxidize − þ CIS=CNFs þ Vis→e þ h ð3Þ − + H O(g) molecules to OH and H , then photogenerated 2 Liu et al. Sustainable Environment Research (2022) 32:10 Page 11 of 13 Fig. 9 Visible light photocatalysis mechanism of CIS/CNFs composite þ − þ trons react with ozone to generate O , which combines H O þ h →OH þ H ð4Þ 2 3 + • with H to generate HO and further decomposes to − þ OH þ h →•OH ð5Þ •OH. Subsequently, ozone reacts with the OH gener- •− ated in photocatalysis to form O and hydrogen peroxyl O2 þ e−→O ð6Þ radical (HO ), which further produces the highly oxidiz- •− H2O2 þ e−→•OH þ OH− ð7Þ ing superoxide radical (O ). Finally, repetitionary steps proceed to finish ozone-enhanced photocatalysis cycle. •− 2O þ H2O→H2O2 þ 2OH− ð8Þ O3 þ e−→O ð12Þ According to the stoichiometry, one oxygen molecule H2O þ h þ→OH− þ Hþð13Þ reacts with three electron holes and two water molecules − • O þ H þ→HO →O2 þ •OH ð14Þ to produce four •OH [25]. Toluene is then oxidized to 3 3 carbon dioxide and water [26]. O3 þ OH−→O2 þ HO ð15Þ þ − 3h þ 3e þ 2H O þ O →4•OH ð9Þ 2 2 − • − O3 þ HO →HO þ O ð16Þ 2 2 3 • •− HO →O þ Hþð17Þ 2 2 3.6.2 Possible mechanism for ozone-enhanced photocatalysis The effect of humidity on toluene removal with O -PCO Mainstream view believes that O• and •OH play major indicates that water molecules play an important role in the roles in O -PCO system [6]. In the CIS/CNFs + O 3 3 initial stage of O -PCO chain reaction. However, in the case process, oxygen molecules react with active sites (⁎)of of high humidity, ozone molecules and water molecules the photocatalyst, O• reacts with water vapor to pro- compete for adsorption sites on the catalyst surface, which duces •OH. Ozone, as a high electron affinity gas which is not conducive for toluene removal [27]. is easier to capture the electrons generated by light, thus We monitored some intermediates by GC-MS, including reduces the electron-hole pair recombination rate and benzaldehyde, phenol, benzoic acid. Therefore, according to accelerates the formation of •OH. the intermediates, we summarized reactions leading to tolu- ene degradation in O -PCO system, as shown in Fig. 10. CIS=CNFs 3 O þ → O• þ O ð10Þ 3 2 (1) When •OH is the main oxidant, •OH was H- abstraction from methyl resulting in the production of O• þ H2O→2•OH ð11Þ benzyl alcohol and benzaldehyde, which is further In the CIS/CNFs + Vis + O process, under the irradi- attacked by •OH oxidant and then the aromatic ring ation of visible light, CIS/CNFs are excited to produce opens, gradually forming CO and H O. In this pathway 2 2 photogenic electrons and holes, and at the same time, (Fig. 10a), byproducts such as formic acid and benzalde- •− − + O ,OH , and H are generated. The photogenic elec- hyde are abundant [28]. 2 Liu et al. Sustainable Environment Research (2022) 32:10 Page 12 of 13 Fig. 10 Possible mechanism for ozone-enhanced photocatalysis of toluene: (a) � OH degradation process; (b)O� degradation process (2) The primary pathway of toluene oxidation by O• is and TEM results showed that CNFs (about 300 nm in via abstraction of two H atoms from methyl to directly diameter) were well connected with CIS to form 3D con- produce benzaldehyde which is then opened after being ductive network and the CISs with 100 nm in average par- continuously attacked by O•, and subsequent reaction is ticle size were uniformly grown onto the surface of CNFs. similar to •OH [6, 29]. In this path, the intermediate Through photocurrent and PL spectrum analysis, it is product is only benzaldehyde (Fig. 10b). The reaction found that the addition of CNFs could delay the process steps with O• generate fewer intermediates, and the re- of electron-hole recombination. Based on the intermediate action is faster. In summary, O -PCO process produces by-products measured by GC-MS, the mechanism of more oxidants and less byproducts. ozone catalytic oxidation of toluene was proposed. The re- sults show that ozone can generate more hydroxyl and 4 Conclusions oxygen radicals, thereby further reduce the recombination In this study, CdIn S /CNFs were synthesized by a simple of electron-hole pairs. This study provides a new strategy 2 4 hydrothermal method, the CIS/CNFs composite materials to prepare the CIS/CNFs composites with high photo- were irradiation by visible light to degrade toluene, with catalytic activity and excellent recyclable performance. O addition for enhancing degradation efficiency. Under Due to research budget limitation, the ozone- visible light irradiation, the degradation efficiency of tolu- enhanced photocatalytic oxidation developed in this ene achieved 86% with CIS doped with 3% CNFs. To fur- study is conducted in a batch reactor which may not ther improve the efficiency, with the introduction of 200 have direct practical applications. Several bottlenecks ppm ozone, the toluene removal efficiency is increased to including long residence time, low ozone utilization 95%. Ozone catalytic oxidation has significantly improved efficiency and lack of kinetic interpretation are identi- toluene removal efficiency and mineralization efficiency. fied in this study. Thus, a continuous flow reactor Meanwhile, after ten times recycle, the photocatalytic ac- will be designed and optimization of operating param- tivity decreased by only 3.4%. On this basis, we conclude eters including space velocity, catalyst amount and that the addition of CNFs enhances CIS photocatalytic ac- light intensity will be conducted in our future study. tivity, recycle performance and photo-corrosion resist- ance. The XRD and XPS results indicate that CIS/CNFs 5 Supplementary Information The online version contains supplementary material available at https://doi. composite material is successfully synthesized and CIS is org/10.1186/s42834-022-00117-y. not destroyed during the process of CNFs growth. After CNFs doping, the specific surface area is increased, allow- Additional file 1. Supplementary materials. ing CIS/CNFs to have significant adsorption. The SEM Liu et al. Sustainable Environment Research (2022) 32:10 Page 13 of 13 Acknowledgments 15. Zhang M, Liu XZ, Zeng X, Wang MF, Shen JY, Liu RY. Photocatalytic Not applicable. degradation of toluene by In S /g-C N heterojunctions. Chem Phys Lett 2 3 3 4 2020;738:100049. 16. Li XT, Ma JZ, Yang L, He GZ, Zhang CB, Zhang RD, et al. Oxygen vacancies Authors’ contributions induced by transition metal doping in γ-MnO for highly efficient ozone Run Yu Liu provided and analyzed the test data, and wrote the manuscript. decomposition. 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Sustainable Environment Research – Springer Journals
Published: Jan 24, 2022
Keywords: CdIn2S4/CNFs; Photocatalyst; Toluene; Ozone-enhanced photocatalytic oxidation
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