High efficient photocatalytic selective oxidation of benzyl alcohol to benzaldehyde by solvothermal-synthesized ZnIn2S4 microspheres under visible light irradiation

Journal of Solid State Chemistry 205 (2013) 134–141

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High efficient photocatalytic selective oxidation of benzyl alcohol to benzaldehyde by solvothermal-synthesized ZnIn2S4 microspheres under visible light irradiation Zhixin Chen a,b,n, Jingjing Xu a, Zhuyun Ren a, Yunhui He a,b, Guangcan Xiao a,b a b

State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, P.R. China Instrumental Measurement & Analysis Center, Fuzhou University, Fuzhou 350002, P.R. China

art ic l e i nf o

a b s t r a c t

Article history: Received 20 April 2013 Received in revised form 5 July 2013 Accepted 14 July 2013 Available online 22 July 2013

Hexagonal ZnIn2S4 samples have been synthesized by a solvothermal method. Their properties have been determined by X-ray diffraction, ultraviolet–visible-light diffuse reflectance spectra, field emission scanning electron microscopy, nitrogen adsorption–desorption and X-ray photoelectron spectra. These results demonstrate that ethanol solvent has significant influence on the morphology, optical and electronic nature for such marigold-like ZnIn2S4 microspheres. The visible light photocatalytic activities of the ZnIn2S4 have been evaluated by selective oxidation of benzyl alcohol to benzaldehyde using molecular oxygen as oxidant. The results show that 100% conversion along with 499% selectivity are reached over ZnIn2S4 prepared in ethanol solvent under visible light irradiation (λ 4420 nm) of 2 h, but only 58% conversion and 57% yield are reached over ZnIn2S4 prepared in aqueous solvent. A possible mechanism of the high photocatalytic activity for selective oxidation of benzyl alcohol over ZnIn2S4 is proposed and discussed. & 2013 Elsevier Inc. All rights reserved.

Keywords: ZnIn2S4 Solvothermal Photocatalyst Selective oxidation Marigold-like

1. Introduction Selective oxidation of alcohols to carbonyls is a fundamental and significant transformation for the synthesis of fine chemicals, because carbonyl compounds such as ketone and aldehyde derivatives are widely utilized in the confectionary, fragrance and pharmaceutical industries [1–6]. However, conventional organic synthesis for the oxidation of alcohols into corresponding aldehydes or ketones not only involves environmentally, toxic, corrosive stoichiometric oxidants (such as chromate, hypochlorite or permanganate etc.) and harsh condition (such as high temperature and pressure), but also produces a large quantity of hazardous wastes [7–10]. Therefore, increasing efforts have been made to develop some clean processes to solve these problems [5–13]. Introducing the utility of sunlight into selective oxidation of organic chemistry, i.e. photocatalytic selective oxidation, is a green technique for organic synthesis. The required mild conditions and the possibility to decrease the undesired environmental pollutions highlight its potential as a promising route for organic synthesis. Thus, enormous attention has been paid to the photocatalytic selective oxidation of alcohols [1–3,12–15].

n Corresponding author at: State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, P.R. China. Fax: +86 591 8789 2447. E-mail address: [email protected] (Z. Chen).

0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.07.015

In the last few years, most of the reports on photocatalysis regarding selective oxidation of alcohols are focused on TiO2 or TiO2-based materials [2–4,15–18]. For instance, Zhao et al. have synthesized an ingenious coupled system consisting of dyesensitized TiO2 and 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) for visible-light-induced aerobic oxidation of alcohols to corresponding aldehydes, for benzyl alcohol, the 80% conversion is reached under visible light irradiation of 18 h [3]. Xu et al. have synthesized the nanocomposites of TiO2-5%GR which exhibits visible light photocatalytic performance toward selective oxidation of benzyl alcohol to benzaldehyde, yet, only 30% conversion is reached under the irradiation of 4 h [18]. The similar phenomenon is also observed in nontitania-based photocatalysts [19,20], for example, the coenocytic Pd@CdS synthesized by Xu et al. which exhibits enhanced photocatalytic activity toward selective oxidation of benzyl alcohol to benzaldehyde as compared to blank-CdS, under visible light irradiation of 4 h, the conversion for benzyl alcohol and the yield for benzaldehyde are about 31% and 30% over the coenocytic Pd@CdS nanocomposite, respectively [19]. Overall, the utility of sunlight and the transformation efficiency of alcohols to corresponding aldehydes for these photocatalysts are relative low. In order to improve the efficiency of this photocatalytic process, one of the challenges as well as opportunities faced by the researchers is to develop other novel visible-light-driven photocatalysts used for selective organic transformation. ZnIn2S4, an important semiconducting material of ternary chalcogenides, has been extensively studied as a potential eco-

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friendly photocatalyst, because it has a band gap corresponding to the visible light absorption with considerable chemical stability [21–36]. However, a literature survey leads us to the finding that most of the reports of ZnIn2S4 are focused on water splitting [22–29] and nonselective degradation of volatile organic compounds [30–36]. It still remains unclear whether ZnIn2S4 can be applied to photocatalytic selective organic transformations. Herein, we synthesized ZnIn2S4 samples via a facile solvothermal method and utilized them for photocatalytic selective oxidation of benzyl alcohol under ambient conditions. To the best of our knowledge, it is the first time to apply ZnIn2S4 prepared in ethanol solvent for photocatalytic selective oxidation. Furthermore, the ZnIn2S4 microspheres show remarkably enhanced photoactivity compared with ZnIn2S4 sample prepared in aqueous solvent under the same conditions. A possible mechanism of the high photocatalytic activity is proposed and discussed.

2. Experimental section 2.1. Materials Zinc chloride (ZnCl2), indium chloride (InCl3
4H2O), thiacetamide (TAA), ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All materials were used as received without further purification. Deionized water that was used in the synthesis was obtained from local sources. 2.2. Photocatalyst preparation In the typical reaction, ZnCl2 (1 mmol) and InCl3
4H2O (2 mmol) were dispersed in 27 ml of ethanol by stirring 0.5 h to obtain the homogeneous dispersion. Then, excessive TAA (8 mmol) was added into the above mixture solution. The mixture solution was aged with vigorous stirring for 0.5 h. Then, it was transferred to a 100 ml Teflon-lined stainless steel autoclave and maintained at 433 K for 24 h. The resulting product was cooled at room temperature and recovered by filtration, washed with deionized water and absolute ethanol several times. The final sample was fully dried at 333 K in a vacuum for characterization and phtocatalytic reaction. For comparison, the sample was synthesized using the similar approach except that aqueous instead of ethanol was used as the solvent. In this paper, ZnIn2S4 prepared in aqueous-, ethanol-mediated conditions were labeled as ZIS-H2O, ZIS-EtOH, respectively. 2.3. Characterization Crystal phase properties of the samples were analyzed with a Bruker D8 Advance X-ray diffractometer (XRD) using Ni-filtered Cu Kα radiation at 40 kV and 40 mA in the 2θ range from 151 to 801 with a scan rate of 0.021 per second. The optical properties of the samples were characterized by UV–vis diffuse reflectance spectroscopy (DRS) using a UV–vis spectrophotometer (Cary500, Varian Co.), in which BaSO4 was used as the internal reflectance standard. The morphology of the samples was determined by a field emission scanning electron microscopy (FESEM) on a FEI Nova NANOSEM 230 instrument. Nitrogen adsorption–desorption isotherms and the Brunauer–Emmett–Teller (BET) surface areas were collected at 77 K on a Micrometritics ASAP2020 analyzer. The photoluminescence spectra (PL) for solid samples were investigated on an Edinburgh FL/FS900 spectrophotometer. The irradiation source (λ4 420 nm) was a 300 W Xe arc lamp system, which was the light source for our photocatalytic selective oxidation of alcohols. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a ESCALAB 250 photoelectron spectroscope

135

(Thermo Fisher Scientific) at 3.0  10  10 mbar with monochromatic Al Kα radiation (E ¼1486.2 eV). Photoelectrochemical measurements were performed in a homemade three electrode quartz cells with a PAR VMP3Multi potentiotat apparatus. Pt plate was used as the counter, and Ag/AgCl electrode used as the reference electrodes, while the working electrode was prepared on fluoride– tin oxide (FTO) conductor glass. The sample powder (10 mg) was ultrasonicated in 0.5 mL of anhydrous ethanol to disperse it evenly to get slurry. The slurry was spread onto a FTO glass whose side part was previously protected using Scotch tape. The working electrode was dried overnight under ambient conditions. A copper wire was connected to the side part of the working electrode using a conductive tape. Uncoated parts of the electrode were isolated with epoxy resin. The electrolyte was 0.2 M of aqueous Na2SO4 solution without additive. The visible light irradiation source was a 300 W Xe arc lamp system equipped with a UV cutoff filter (λ 4420 nm). 2.4. Photocatalytic activity The photocatalytic selective oxidation of benzyl alcohol was performed as follows. A mixture of benzyl alcohol (BA, 0.1 mmol) and 8 mg of catalyst was dissolved in the solvent of benzotrifluoride (BTF, 1.5 mL), which was saturated with pure molecular oxygen. The above mixture was transferred into a 10 mL Pyrex glass bottle filled with molecular oxygen at a pressure of 0.1 MPa and stirred for half an hour to make the catalyst blend evenly in the solution. The suspensions were irradiated by a 300 W Xe arc lamp (PLS-SXE 300, Beijing Trusttech Co. Ltd.) with a UV-CUT filter to cut off light of wavelength o420 nm. After the reaction, the mixture was centrifuged at 12,000 rmp for 20 min to completely remove the catalyst particles. The remaining solution was analyzed with an Aglient Gas Chromatograph (GC-7820). The catalytic activity with higher concentration of benzyl alcohol (0.2 mmol and 0.3 mmol) over ZIS-EtOH was labeled as ZIS-EtOH-2BA and ZISEtOH-3BA, respectively. Conversion of benzyl alcohol, yield of benzaldehyde, and selectivity for benzaldehyde were defined as follows: Conversion (%) ¼[(C0  Cbenzyl alcohol)/C0]  100 Yield (%)¼ Cbenzaldehyde/C0  100 Selectivity (%) ¼[Cbenzaldehyde/(C0  Cbenzyl alcohol)]  100 where C0 is the initial concentration of benzyl alcohol and Cbenzyl and Cbenzaldehyde are the concentration of the substrate benzyl alcohol and the corresponding benzaldehyde, respectively, at a certain time after the photocatalytic reaction.

alcohol

3. Results and discussion 3.1. Properties of ZnIn2S4 photocatalyst Fig. 1 shows the XRD patterns of the ZIS-H2O and ZIS-EtOH samples, the peaks at 2θ values of 21.6, 27.7, 39.8, 47.8, 52.5, 55.6 and 75.6 can be indexed to (006), (102), (108), (112), (1012), (202) and (213) crystal lanes of hexagonal phase of ZnIn2S4 (JCPDS no. 65-2023), respectively. No other impurities, such as binary sulfides, oxides or organic compounds related to reactants, are detected. As displayed in Fig. 1, XRD patterns of ZIS-EtOH and ZISH2O show almost the same profiles, although their reaction solvents are different. To further confirm the composition and chemical state of ZnIn2S4 samples prepared in different solvents, XPS measurements are carried out. As shown in Fig. 2, all peaks are calibrated using C1s (284.6 eV) as the reference. A survey spectrum shown in

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(213)

(202)

(108)

(1012)

(006)

ZIS-EtOH

ZIS-H2O JCPDS No. 65-2023

30

40

50

60

70

80

2 Theta (degree) Fig. 1. XRD patterns of ZIS-H2O and ZIS-EtOH.

(Fig. 3b), the morphology of as-formed ZnIn2S4 microspheres is significantly different from the ZIS-H2O sample, the marigold-like spherical superstructures are obtained with the average diameter around 8 mm (in the inset of Fig. 3b) and the microspheres are composed of numerous nanosheets. The FESEM result indicates that the solvent has a significant influence on the morphology of samples, that is, ethanol can improve the formation of such novel marigold-like microspheres of ZnIn2S4 under analogous synthetic conditions as compared to aqueous. DRS are used to determine the optical properties of the samples. It can be seen in Fig. 4a that ZnIn2S4 samples exhibit a steep absorption edge in the visible range, which suggests that the relevant band gap is due to the intrinsic transition of the nanomaterials rather than transition from impurity levels. As indicated in Fig. 4a, a qualitative shift to shorter wavelength is observed in the absorption edge of ZIS-EtOH microspheres, namely, the extent of visible light which can be used is enlarged for ZIS-EtOH compared with ZIS-H2O, thus, it is anticipated that more visible light can be used by ZIS-EtOH to photoexcite electron-hole charge carriers. A plot obtained via the transformation based on the Kubelka–Munk function versus the energy of light is shown in the inset of Fig. 4a. The estimated band gap values of the samples are 2.67 and 2.56 eV, corresponding to ZISEtOH, ZIS-H2O samples, respectively. This result indicates that the synthesis solvent has a significant effect on the optical property of light absorption for the ZnIn2S4 samples. The surface area and porosity of ZIS-H2O and ZIS-EtOH are investigated. As shown in Fig. 4b, the nitrogen adsorption–desorption isotherms of two samples exhibit type IV isotherm with typical hysteresis loops characteristic of mesoporous structure [39,40], and the pore size distribution plots (inset) indicate that the pore diameter of samples is 2–4 nm. It is well known that the typical hysteresis loop is observed with aggregates of plate like particles giving rise to slit-shaped pores [41]. That is, both of the two samples are all consisting of a large number of ZnIn2S4

Zn2p Counts (a.u.)

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(102)

(112)

Fig. 2a indicates the presence of Zn, In, S, C and O in which C and O may come from the absorbed gaseous molecules. High-resolution spectra of the samples are also characterized for Zn2p, In3d and S2p to determine the valency state and atomic ratio. As shown in Fig. 2b–d, the data are consistent with the values of literatures [35–38]. Meanwhile, it is clearly seen that the binding energies of Zn2p, In3d and S2p for ZIS-EtOH and ZIS-H2O samples are nearly the same not only in the full survey spectrum but also in the highresolution spectra. Overall, XRD and XPS results indicate that the pure hexagonal ZnIn2S4 could be easily obtained under the current synthetic conditions. FESEM is taken to directly analyze the morphology of the samples. As shown in Fig. 3a, with regard to ZIS-H2O, there are few marigold-like microspheres consisting of nanosheets, most nanosheets on the surface of microspheres are seriously collapsed, which is consistent with our previous report [34]. On the contrary, when ethanol is used as solvent during the synthesis of ZnIn2S4

ZIS-EtOH

ZIS-H2O ZIS-H2O

445

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Binding Energy (eV)

460

158

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Binding Energy (eV)

Fig. 2. XPS spectra of ZIS-H2O and ZIS-EtOH: (a) survey XPS spectrum and (b–d) high-resolution spectra of Zn2p, In3d and S2p.

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Fig. 3. FESEM images of the as-prepared samples: (a) ZIS-H2O and (b) ZIS-EtOH.

90 ZIS-H O

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Fig. 4. DRS spectra (a) and N2 adsorption-desorption isotherms (b) of ZIS-H2O and ZIS-EtOH.

nanosheets, which is consistent with the FESEM result. The BET surface area of ZIS-EtOH and ZIS-H2O is measured to be ca. 63 m2 g  1 and 98 m2 g  1, respectively. According to our previous study, the surface area have an important influence on the photocatalytic activity, namely, larger surface area can endow the ZnIn2S4 sample with better activity toward visible light photocatalytic degradation of dyes [34]. The photocatalytic activity of the ZnIn2S4 samples for selective oxidation has been of interest to us. Herein, we use selective oxidation of benzyl alcohol as a probe reaction to study photocatalytic activity of the sample under ambient conditions. 3.2. Photocatalytic activity and stability Fig. 5 lists the photocatalytic performance of selective oxidation of benzyl alcohol over ZIS-EtOH and ZIS-H2O under visible light irradiation. As can be clearly seen in Fig. 5a and b, ZIS-EtOH exhibits higher photoactivity than ZIS-H2O in the same photocatalytic reaction system. The conversion of benzyl alcohol increases gradually along the evolution of the reaction. The yield of corresponding benzaldehyde also rises with the reaction time progressing. After irradiation for 2 h, 100% conversion of benzyl alcohol and 99% yield of benzaldehyde are reached over the ZIS-EtOH, but only 58% conversion and 57% yield are reached over ZIS-H2O. In addition, the selectivity to benzaldehyde is 499% over both ZIS-EtOH and ZIS-H2O. Furthermore, the catalytic activity with higher concentration of benzyl alcohol and the constant quantity of catalyst has been performed. As shown in Fig. 5c, the conversion of benzyl alcohol and the yield of benzaldehyde increase gradually along the evolution of the reaction when the benzyl alcohol concentration increases to two times (0.2 mmol), both the conversion of benzyl alcohol and the yield of benzaldehyde are nearly 100% after irradiation of 5 h. When the benzyl alcohol concentration increases to three times (0.3 mmol, as

shown in Fig. 5d), similarly, the benzyl alcohol is almost transformed to benzaldehyde after irradiation of 10 h. Namely, the high concentration benzyl alcohol can also be selective oxidation to benzaldehyde over constant quantity of catalyst (8mg) as long as extend the time of irradiation. Importantly, the conversion and yield is much higher than many other photocatalysts. For example, Zhang et al. report that CdS-5%GR nanocomposite exhibits only about 42% conversion and 41% yield for selective oxidation of benzyl alcohol to benzaldehyde under same condition except the reaction time is 4 h [41]. As well, core–shell Pd@CeO2 nanocomposite has been designed by Zhang et al., with visible light irradiation for 20 h; the complete 100% selectivity for the target product of benzaldehyde can be obtained over the multi-Pd@CeO2 shell nanocomposite along with a ca. 28% yield [42]. These results indicate that the ZIS-EtOH sample can serve as a promising photocatalyst for selective oxidation of benzyl alcohol to corresponding benzaldehyde. It is known that the photocorrosion or photodissolution of catalysts might occur on the photocatalyst surface in the photocatalytic reaction. Herein, the ZIS-EtOH photocatalysts before and after selective oxidation of benzyl alcohol are compared by XRD and XPS investigation. Fig. 6a shows the XRD patterns of ZIS-EtOH before and after reaction. The position and ratio of peaks are nearly the same and no new peaks are created. Furthermore, XPS is applied to test the surface chemical states of the photocatalysts. Fig. 6b indicates that the main peaks are owned by elements Zn, In and S. The binding energies of Zn2p, In3d and S2p before and after reaction are nearly the same. Furthermore, the four runs lifetime test has been done and the results are shown in Fig. 7. During the experiments repeated four times, the selectivity is nearly constant (100%) for each run of recycled test, and there is no significant loss of the conversion and the yield during four successive recycling tests for selective oxidation of benzyl alcohol to benzaldehyde over ZIS-EtOH under visible light irradiation. These results all demonstrate that the as-prepared ZIS-EtOH sample is a relatively stable

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Counts (a.u.)

Intensity (a.u.)

Fig. 5. Time-online photocatalytic selective oxidation of benzyl alcohol (BA) to benzaldehyde under visible light irradiation (λ4420 nm) over (a) ZIS-EtOH/ 0.1 mmol BA, (b) ZIS-H2O/ 0.1 mmol BA, (c) ZIS-EtOH/ 0.2 mmol BA and (d) ZIS-EtOH/ 0.3 mmol BA.

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Fig. 6. XRD (a) and XPS (b) patterns of ZIS-EtOH before and after photocatalytic reaction.

Conversion Yield Selectivity

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Percentage (%)

100 80 60 40 20 0 1

2

3

4

Recycled Times Fig. 7. Cycling photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over ZIS-EtOH under visible light irradiation (λ 4420 nm) for 2 h.

visible-light-driven photocatalyst for the selective oxidation of benzyl alcohol in the reaction medium of BTF solvent under ambient conditions.

Fig. 8. Remaining fraction of benzyl alcohol after the dark adsorption–desorption equilibrium achieved over ZnIn2S4 samples.

3.3. Discussion of mechanism To study the influence of synthesis solvents on the photocatalytic activity over the ZnIn2S4 samples, many efforts have been

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paid to reveal the essence of the higher photocatalytic activity for ZIS-EtOH. To begin with, adsorption experiments in the dark for benzyl alcohol have been performed. As seen in Fig. 8, there is no obvious difference in terms of adsorptivity between ZIS-EtOH and ZIS-H2O, and both of them exhibit almost similar adsorptivity for benzyl alcohol in the dark. The BET surface area and dark adsorption results strongly suggest that the enhanced photocatalytic activity of ZIS-EtOH compared with ZIS-H2O cannot be attributed to the change in surface area [41]. To further understand the significant role of ethanol solvent on enhancing the photocatalytic activity, photoelectronchemical experiments are performed. Fig. 9a shows the photocurrent transient response for ZIS-EtOH and ZIS-H2O electrodes under visible light irradiation. It is easy to observe that the significant photocurrent decay appears with the increased switch-on and -off cycles, indicating that the obvious recombination process of photogenerated electron–hole pairs is occurring. Furthermore, it should be noted that the ethanol solvent in the synthesis procedure is beneficial for improving the lifetime of photogenerated charge carriers, which can be reflected by the higher photocurrent density of ZIS-EtOH than ZIS-H2O. Moreover, the photocurrent is quite stable, i.e., no obvious photocurrent decay is observed. This suggests that the lifetime of photogenerated charge carriers is able to be remarkably boosted by the solvothermal strategy and the transport of photogenerated electrons is markedly effective. Therefore, the inhibition degree of photogenerated electron–hole pairs' recombination is better than ZIS-H2O. This result is also strongly evidenced by the following results of PL spectra. The PL spectra are often employed to study surface processes involving the electron–hole fate of the semiconductor TiO2

[43–45], which results from emitted photons with the recombination of electron–hole pairs after a photocatalyst is irradiated. As displayed in Fig. 9b, the PL intensity obtained over ZIS-EtOH is much weaker than that of ZIS-H2O. This might result from the suppressed recombination of photoexcited electrons and holes via band-to-band emission transition, and this phenomenon might be beneficial for the photocatalytic reaction [23]. The result of the PL further verifies that the ZIS-EtOH can be beneficial to the separation of electron–hole pairs under irradiation, which is benefit to photocatalytic activity toward selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation [14,23,41]. In addition, the electrochemical impedance spectroscopy (EIS) Nyquist plots and Mott–Schottky plots have been collected to further determine the advantage of ZIS-EtOH over ZIS-H2O and ascertain the detail of energy band. As shown in Fig. 10a, the Nyquist plots of ZIS-EtOH and ZIS-H2O electrode materials cycled in 0.2 M Na2SO4 electrolyte solution both show semicycles at high frequencies. In electrochemical spectra, the high-frequency arc corresponds to the charge transfer limiting process and can be attributed to the double-layer capacitance in parallel with the charge transfer resistance at the contact interface between electrode and electrolyte solution [46,47]. It can be clearly seen from Fig. 10a that the decrease of the arc when use the ZIS-EtOH as the electrode material in comparison to ZIS-H2O, which means ZISEtOH can facilitate the interfacial charge transfer [41]. Fig. 10b exhibits the Mott–Schottky plots for ZIS-EtOH and ZIS-H2O samples. The positive slope of the C  2–E plots suggests the expected n-type semiconductor of ZnIn2S4 in the nanomaterials [48]. The flatband potentials of ZIS-EtOH and ZIS-H2O obtained by extrapolation of the Mott–Schottky plots approximately equals 0.41 V and -0.82 V respectively [49], which are more negative than the

ZIS-EtOH ZIS-H2O

ON OFF

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Fig. 9. Transient photocurrent response in 0.2 M Na2SO4 aqueous solution (pH¼ 6.8) without bias versus Ag/AgCl (a) and PL spectra (b) of ZIS-EtOH and ZIS-H2O.

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Fig. 10. Nyquist impedance plots (a) and Mott–Schottky plots (b) for the ZIS-EtOH and ZIS-H2O samples in 0.2 M Na2SO4 aqueous solution (pH ¼6.8).

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for the utilization of semiconducting materials of ternary chalcogenides as visible light photocatalyst for selective organic transformation.

Acknowledgments This work is financially supported by the Natural Science Foundation of Fujian, China (2011J05024). References

Fig. 11. Illustration of the proposed reaction mechanism for selective oxidation of benzyl alcohol to benzaldehyde over the ZIS-EtOH under the visible light irradiation.

standard reduction potential of O2/O2  (  0.15 V vs NHE) [3]. Therefore, they are thermodynamically permissible for the transformation of photogenerated electrons to the absorbed O2 for produce the superoxide radicals (
O2  ). Based on the above discussion, a tentative reaction mechanism for photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over ZIS-EtOH is proposed, as illustrated in Fig. 11. Under visible light irradiation, the electron is excited from valence band (VB) of ZnIn2S4 microspheres to its conduction band (CB), leaving the hole in the VB. The photoinduced electrons transfer to the surface of ZnIn2S4, and molecular oxygen in the reaction system can be activated by accepting the photogenerated electrons, for example, the formation of superoxide radicals (
O2  ) [20]. Then, the substrate benzyl alcohol adsorbed on the surface of ZnIn2S4 can be oxidized by activated oxygen (
O2  ) and holes (h+) to give the corresponding benzaldehyde. For ZIS-EtOH, its higher photocatalytic activity compared with ZIS-H2O may be due to more visible light absorption and the prolonged lifetime of photogenerated electron–hole pairs.

4. Conclusions In summary, we have synthesized the ternary chalcogenides ZnIn2S4 photocatalysts with hexagonal crystal phase and marigold-like microspheres morphology via a facile one-pot approach in ethanol-mediated condition. The results demonstrate that ZIS-EtOH exhibits a higher visible light photocatalytic activity than ZIS-H2O toward selective oxidation of benzyl alcohol to benzaldehyde using oxygen as oxidant under ambient conditions. It is found that ethanol solvent has a significant effect on the properties of the samples, including morphology, optical, electronic nature and photocatalytic activity as compared with aqueous solvent. The improved photocatalytic activity of ZIS-EtOH samples can be ascribed to synergetic effect of visible light absorption and the prolonged lifetime of photogenerated electron–hole pairs. Importantly, our work is the first time to utilized ZnIn2S4 microspheres synthesized by solvothermal method for selective organic transformation. The results suggest that the as-obtained ZIS-EtOH microspheres are able to serve as a promising visiblelight-driven photocatalyst for selective oxidation of benzyl alcohol to corresponding benzaldehyde under ambient conditions. Of course, we will study whether ZIS-EtOH can exhibit well photocatalytic activity for others alcohols such as p-methybenzyl alcohol, p-methoxbenzyl alcohol, p-nitrobenzyl alcohol, cinnamyl alcohol etc., detailed investigation is still undergoing in our laboratory. It is hoped that our current work could widen the application of ZnIn2S4 and open promising prospects

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