Effect of different solvent on the photocatalytic activity of ZnIn2S4 for selective oxidation of aromatic alcohols to aromatic aldehydes under visible light irradiation

Applied Surface Science 384 (2016) 161–174

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Effect of different solvent on the photocatalytic activity of ZnIn2 S4 for selective oxidation of aromatic alcohols to aromatic aldehydes under visible light irradiation Li Su a , Xiangju Ye b,∗ , Sugang Meng a,∗ , Xianliang Fu a , Shifu Chen a,b,∗ a b

College of Chemistry and Materials Science, Huaibei Normal University, Anhui Huaibei 235000, People’s Republic of China College of Chemistry and Materials Engineering, Anhui Science and Technology University, Anhui, Fengyang 233100, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 28 February 2016 Received in revised form 9 April 2016 Accepted 12 April 2016 Available online 14 April 2016 Keywords: ZnIn2 S4 Aromatic alcohol Selective oxidation Visible light Photocatalysis

a b s t r a c t A series of ternary chalcogenides, zinc indium sulphide (ZnIn2 S4 ), were synthesized by a simple solvothermal method with different solvents. The structure, textural, and optical properties of the resulting materials were thoroughly characterized by several techniques. The as-prepared ZnIn2 S4 samples could all be employed as excellent photocatalysts to activate O2 for selective oxidation of aromatic alcohols to aromatic aldehydes under visible light illumination. The results showed that ZnIn2 S4 prepared in ethanol solvent (ZIS-EtOH) exhibited the highest photocatalytic activity among the screened samples. The differences of photocatalytic performance for ZnIn2 S4 samples prepared in different media were mainly attributed to the different levels of exposed {0001} special facets caused by the exposure extent of the basic crystal plane. In addition, • O2 − and positive holes were proved to be the main active species during the photocatalytic process. Combined with the previous reports, a possible photocatalytic mechanism for the selective oxidation of benzyl alcohol to benzaldehyde over ZnIn2 S4 sample was proposed. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Photocatalytic technology based on the semiconductor materials had already become an important research field because it can be applied to the treatment of pollutants, hydrogen production from H2 O, selective conversion of organics and reduction of CO2 using sunlight [1–8]. Recently, developing a green, efficient, sustainable synthetic method to partially oxidize aromatic alcohols to corresponding aldehydes has attracted widespread interest, because aromatic aldehydes and their derivatives are paramount intermediates in the synthesis of a range of pharmaceuticals, perfumes (e.g., mandelic acid), and other fine chemicals [9–12]. Traditional industrial methods for the synthesis of related products involve the use of chromate, permanganate, or Br2 /acetic acid, etc. as a stoichiometric oxygen donor [12,13]. Unfortunately, these are all toxic and release considerable amounts of by-products [14]. Therefore, it is urgent and necessary to establish a cost-efficient and environmentally friendly method for selective oxidation of aromatic alcohols to corresponding aldehydes under mild conditions.

∗ Corresponding authors at: College of Chemistry and Materials Science, Huaibei Normal University, Anhui Huaibei, 235000, People’s Republic of China. E-mail address: [email protected] (S. Chen). http://dx.doi.org/10.1016/j.apsusc.2016.04.084 0169-4332/© 2016 Elsevier B.V. All rights reserved.

In recent years, selective oxidation of aromatic alcohols to corresponding aldehydes with semiconductor materials as photocatalysts has aroused extensive attention of scholars both at home and abroad. It is known that TiO2 has been used for the selective oxidation of benzyl alcohol to benzaldehyde [15]. The modified TiO2 , such as M/TiO2 (M = Au, Pd, Pt) and Pd@CeO2 etc. as the photocatalysts are proved to have excellent catalytic performance for the selective oxidation of aromatic alcohols to corresponding aldehydes [16,17]. Although the reported catalysts have made a great progress in the conversions and selectivities of organic synthesis, it is still far from what we expected. This is mainly due to the fact that the reaction system strongly depends on the presence of UV light when the broad band gap semiconductor materials are used as photocatalysts. Furthermore, they have more positive values of the valence band (for example, the valence band value of TiO2 is 2.7 eV), which thus makes the catalysts have strong oxidation abilities and leads to selectivities obviously decreased. It is a key problem that how to design and synthesize a photocatalyst with high photocatalytic properties and selective oxidation ability without side effects. Recently, the narrow band gap semiconductor materials with higher negative conduction band position and lower valence band position, such as g-C3 N4 [18], CdS [13], and In2 S3 [19] etc., have been used for the selective oxidation of aromatic alcohols to corresponding aldehydes. The results showed that the semicon-

162

L. Su et al. / Applied Surface Science 384 (2016) 161–174

ductor materials exhibit excellent photocatalytic performance. It is suggested that the narrow band gap semiconductor materials may be more appropriate for the selective oxidation of aromatic alcohols to corresponding aldehydes. As is known, upon visible light irradiation, the transfer process of photo-induced electrons and holes is important to the photocatalytic performance of a semiconductor material. It is widely recognized that the superior catalytic performance of a catalyst should confer the ability to optimize the migration of electrons and holes to the surface of a photocatalyst for catalysing redox reactions [20–24]. For instance, Zhang et al. [25] synthesized BiOCl single-crystalline nanosheets for efficient charge separation and transfer along the {001} direction in nanosheets induced by internal electric fields. Results revealed that the excellent photocatalytic performance is closely dependent on the varying levels of exposed {001} facets. It is proposed that the transfer process of photogenerated electron-hole pairs manipulated by an internal electric field has been widely involved in specific layered compounds [26–28]. Recently, a ternary chalcogenide, ZnIn2 S4 nanosheets, has been widely used as a potential catalyst in many important fields [29,30], such as photocatalytic hydrogen evolution [31], photodegradation of organic pollutants [30,32], and organic photosynthesis [33]. For example, Chen et al. reported that ZnIn2 S4 microspheres were synthesized successfully by a hydrothermal method at low temperature [24]. The results indicated that the as-synthesized catalyst has an excellent performance toward the selective oxidation of benzyl alcohol to benzaldehyde. In addition, it has been reported that hierarchical ZnIn2 S4 microspheres with exposed {0001} facets lead to a high photodegradation activity of RhB under visible light irradiation [30,34]. It is suggested that negative-charge {0001} can accumulate the photo-generated holes, thus suppressing the recombination of photo-generated electron-hole pairs and finally improving the photocatalytic performance. However, the effects of different solvothermal conditions on the structure and the photocatalytic performance of ZnIn2 S4 , and the reaction mechanism of the photocatalytic conversion of benzyl alcohol to benzaldehyde have not been studied thoroughly. In this paper, ternary chalcogenides, ZnIn2 S4 compounds, were fabricated by a simple solvothermal method using indium chloride tetrahydrate, zine sulphate heptahydrate, and thiacetamide as precursors in different solvents (water, ethanol, methanol, and ethylene glycol). The photocatalytic activities of the resulting samples were evaluated by selective oxidation of aromatic alcohols to corresponding aldehydes in an oxygen atmosphere (0.1 MPa) under visible light illumination. Effect of different solvothermal synthesis on the photocatalytic performance of ZnIn2 S4 for selective oxidation of aromatic alcohols was investigated. It is inferred that the superior photocatalytic performance of the ZnIn2 S4 microspheres is mainly attributed to the crystallization, special surface area, the peak intensity ratio of (112)/(102) and the exposed level of {0001} facets as the special crystal faces. • O2 − and h+ as significant active species in the photocatalytic reactions are generated. Based on the experiment results, a possible reaction mechanism for the photocatalytic oxidation of benzyl alcohol to benzaldehyde over ZnIn2 S4 is proposed

2. Experimental 2.1. Materials Indium chloride tetrahydrate (InCl3 ·4H2 O), zine sulphate heptahydrate (ZnSO4 ·7H2 O), thiacetamide (CH3 CSNH2 ), benzotrifluoride (BTF), ethanol (EtOH), methanol (MeOH), ethylene glycol (EG) and other chemicals used in the experiments were of analytical grade and were purchased from Sinopharm Chemical Reagent Co.,

Ltd. (Shanghai, China). All of the materials were used as received without further purification. Deionised water was used throughout the experiments. 2.2. Synthesis of ZnIn2 S4 In a typical procedure, ZnSO4 ·7H2 O (1.0 mmol), InCl3 ·4H2 O (2.0 mmol), and CH3 CSNH2 (8.0 mmol) were dissolved in 30 ml of ethanol in a Teflon-liner under vigorous stirring for 30 min to obtain a homogeneous solution. Then, the Teflon-liner was transferred into a stainless steel autoclave and maintained at 160 ◦ C for 12 h. After the reaction, the autoclave was cooled to room temperature. The obtained product was collected by means of centrifugation, washed with deionised water and absolute ethanol several times, and dried in air at 60 ◦ C overnight. For comparison, the other samples were synthesized using a similar approach except that water (H2 O), methanol (MeOH), and ethylene glycol (EG) were used as the solvent. Here, ZnIn2 S4 prepared in ethanol-, aqueous-, methanol, and ethylene glycol-mediated media were denoted as ZIS-EtOH, ZIS-H2 O, ZIS-MeOH, and ZIS-EG, respectively. 2.3. Characterization Powder X-ray diffraction (XRD) data were collected at room temperature using a Bruker D8 advance X-ray powder diffractometer with Cu K␣ radiation ( = 1.5406 Å) and a scanning speed of 3/min. The accelerating voltage and emission current were 40 kV and 40 mA, respectively. X-ray photoelectron spectroscopy (XPS) was preformed on a Thermo Scientific K-Alpha at a residual gas pressure below 1 × 10−8 Pa, and all of the binding energies were calibrated by the C 1s peak at 284.6 eV. UV–vis diffuse reflectance spectroscopy (UV–vis DRS) measurements were carried out using a Hitachi UV-365 spectrophotometer equipped with an integrating sphere attachment. The analysis range was from 200 to 800 nm and BaSO4 was used as a reference sample. The specific surface area and pore size distribution of samples were determined at 77 K by a Micromeritics ASAP 2020 surface area and porosity analyser. Before analysis, the samples were degassed in vacuum at 150 ◦ C for 5 h. Scanning electron microscopy (SEM) was performed using a JEOL JSM-6610LV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) images were collected using a JEOL-2010 transmission electron microscope with an accelerating voltage of 200 kV. Electrochemical impedance spectroscopy (EIS) measurements were recorded with a KL 2006A electrochemical work-station using a conventional three-electrode configuration. Among these, a Pt electrode was used as the counter electrode, an Ag/AgCl (saturated KCl) electrode was the reference electrode, and a glassy carbon electrode was the working electrode. The electrolyte system consists of 5.0 mM K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] and 1.0 M KCl and an applied AC voltage of 150 MV. The Mott-Schottky (M-S) experiments of the prepared sample were analyzed by the electrochemical work station (CHI600E, shanghai, Inc.). The potential range was −1.0 to 0.2 V with the amplitude of 0.01 at a constant frequency of 500 Hz. The electrolyte system is composed by 0.2 M Na2 SO4 , and a Pt electrode as the counter electrode, an Ag/AgCl electrode as the reference electrode, a glassy carbon electrode as the working electrode, respectively. The amount of H2 O2 was measured by N,N-deithyl phenylenediamine (DPD) in the presence of peroxidase (POD). The spectra of spectrophotometric analysis were recorded on a UT-1901 and the wavelength range was 450–600 nm. 2.4. Evaluation of photocatalytic activity The photocatalytic selective oxidation of aromatic alcohols to corresponding aldehydes by ZnIn2 S4 was performed in a photore-

L. Su et al. / Applied Surface Science 384 (2016) 161–174

163

Fig. 1. The schematic diagram of photocatalytic reaction: apparatus (A); reactor (B); and light source (C).

action apparatus. The detail procedure was performed as follows: 80 mg catalyst was dispersed in an inert solvent (BTF, 15 ml) containing aromatic alcohol (0.5 mmol). The mixture was transferred into a 100 ml Teflon-lined stainless steel autoclave filled with pure oxygen (0.1 MPa). Prior to the reaction, the suspension was stirred for 20 min to establish an adsorption- desorption equilibrium. The photocatalytic reactions were carried out using a 300 W Xe lamp (PLS-SXE 300C, Beijing Perfect Light) with a 420 nm cut-off filter as a visible light source. The reaction temperature was controlled at about 70 ◦ C. The reaction apparatus is shown in Fig. 1. After the reaction, the suspension was centrifuged at 4000 rmp for 30 min to remove the catalyst. The obtained reaction solutions were analyzed by gas chromatograph (FuLi-GC9790) equipped with an SE-30 capillary column (30 m, 0.53 mm, Lanzhou Atech Technologies Co., Ltd.). Conversion of alcohols, yield of aldehydes, and selectivity for aldehydes were defined as follows: Conversion(%) = [(C0 − Calcohol )/C0 ] × 100% Yield(%) = Caldehyde /C0 × 100% Selectivity(%) = [Caldehyde /(C0 − Calcohol )] × 100% Where C0 is the initial concentration of alcohols; Calcohol is the concentration of alcohols at a certain time after the photocatalytic reaction; Caldehyde is the concentration of aldehydes at a certain time after the photocatalytic reaction. 3. Results and discussion 3.1. Structural characterization 3.1.1. XRD analysis The phase and crystallographic structure of the as-obtained samples can be described by powder X-ray diffraction. Fig. 2 shows the XRD patterns of the ZnIn2 S4 samples prepared in ethanol-,

methanol-, aqueous-, and ethylene glycol-mediated media, respectively. As can be seen, although the reaction media are different, the XRD patterns of the samples present similar profiles. The major characteristic peaks appear at 28.5◦ and 48.1◦ that are well indexed as (102) and (112) diffraction of the hexagonal phase of ZnIn2 S4 crystalline structure (JCPDS No. 65-2023), respectively, which is consistent with previous reports [20]. And no diffraction peaks originating from other impurities, such as ZnS, In2 S3 , oxides, or organic compounds related to reactants have been observed, indicating that a high-purity phase of ZnIn2 S4 compound had been synthesized. Meanwhile, this result can be validated by EDS analysis. From the inset, it can be seen that the product is comprised of only zinc, indium, and sulphur. Moreover, the atomic percentages of Zn, In, and S are 14.5%, 26.28%, and 59.22%, respectively, which suggests that their molar ratio is about 1:2:4. It is worth noting that the intensity of the corresponding typical (112) peaks for ZnIn2 S4 samples is enhanced gradually when ethylene glycol, water, methanol and ethanol are used as solvents, respectively, and the sequences of the intensity ratio values of (112)/(102) follow the orders: ZIS-EtOH> ZIS-MeOH > ZISH2 O > ZIS-EG (the intensity ratio values of (112)/(102) show in Table 1, the calculation method shows in Fig. S1). X-ray diffraction, which also reflects an overall impression of the sample and its orientation properties, is applied to characterize the relevant surface facets of microcrystal [11,35,36]. Herein, the intensity ratio of the major characteristic diffraction peaks can be used as an index for the crystal facets of powder samples. It indicates that synthetic catalysts used different solvents lead to the different crystallinity of the catalysts, and further makes the different exposure levels of crystal face. 3.1.2. XPS analysis XPS analyses are introduced to investigate the surface chemical composition and electronic state of ZIS-EtOH samples (Fig. 3). The survey spectrum is shown in Fig. 3A. It is clear that no peaks of other elements except Zn, In, S, C and O are observed. The carbon and oxygen peaks are attributed to the graphite conductive adhesive or

164

L. Su et al. / Applied Surface Science 384 (2016) 161–174

Fig. 2. XRD patterns of ZnIn2 S4 microspheres synthesized with different solvents. The inset shows the EDS pattern of the ZnIn2 S4 sample. Table 1 Textural parameters, ratio values of (112)/(102) of ZnIn2 S4 samples, and yield of benzaldehyde. Sample

Intensity ratio of (112)/(102)

Area ratio of (112)/(102)

BET surface area (m2 g−1 )

BJH desorption cumulative volume of pores (cm3 g−1 )

Yield (%)

ZIS-EtOH ZIS-MeOH ZIS-H2 O ZIS-EG

1.50 1.51 1.34 1.28

0.63 0.61 0.56 0.53

86.2 76.2 63.5 49.5

0.221 0.209 0.166 0.150

60.1 48.2 44.6 25.2

the absorbed gaseous molecules. As shown in Fig. 3B–D, the highresolution spectra of samples are characterized as Zn 2p, In 3d and S 2p to determine the valence state and atomic ratio. Fig. 3B shows the high resolution spectrum of Zn 2p, in which the binding energies are 1021.8 eV (Zn 2P3/2 ) and 1044.8 eV (Zn 2P1/2 ), respectively. The regional spectra of In 3d (Fig. 3C) shows two peaks centered at the binding energy (BE) of 443.8 (In 3d5/2 ) and 451.4 eV (In 3d3/2 ). The spectrum of S 2p (Fig. 3D) indicates that the peak can be deconvoluted into two peaks. One is located at ca. 161.8 eV (S 2p1/2 ) and the other at ca. 162.1 eV (S 2p3/2 ). The BE values are mainly consistent to the reported results [34,37], indicating that the chemical valence states of Zn, In and S are +2, +3, and −2, respectively. Importantly, the atomic percentages of Zn, In, and S are 12.41%, 26.58%, and 49.96%, respectively, which suggests that the atomic ratio of the sample is about 1:2:4. The result is well coincided with the EDS.

3.1.3. N2 -sorption analysis The structural and textural properties of ZnIn2 S4 samples were evaluated by N2 -sorption measurements [37,38]. As shown in Fig. 4, it is clear that the N2 adsorption-desorption isotherms of the samples prepared in different solvents are similar. According to the IUPAC classification, they all exhibit type IV isotherms with a typical H3 hysteresis loop [39], suggesting the existence of a mesoporous structure and slit-like pores. The specific surface area values of ZIS-EtOH, ZIS-MeOH, ZIS-H2 O and ZIS-EG are 86.2, 76.2, 63.5 and 49.5 m2 g−1 , respectively. It is clear that the ZIS-EtOH sample

exhibits the highest surface area among the samples, and the difference of specific surface area may due to different solvents used to prepared catalysts. In addition, it can be seen that the yield of benzaldehyde over ZIS-EtOH sample is also the highest (the value is shown in Table 1). Furthermore, the sequence of specific surface area is in accordance with the order of the photocatalytic performance (e.g., ZIS-EtOH > ZIS-MeOH> ZIS-H2 O> ZIS-EG). It may be attributed to the fact that the higher the surface area it has, the more active crystal facets may exposes, which will obviously promote the reaction probability between the catalyst and the substrate, which improves its photocatalytic activity.

3.1.4. SEM analysis The morphologies of the as-prepared ZnIn2 S4 samples were investigated by SEM. As shown in Fig. 5, the morphologies of the ZnIn2 S4 samples prepared in different solvents are quite different. It is clear that when the solvent is pure ethanol or methanol, the products are composed of microspheres with a fluffy structure and the size of microspheres is about 3–5 ␮m. It is attention that many granules with tiny fluffy structure are formed around the microspores prepared by the ethanol solvent, and many irregular microstructures are smaller broken microspheres scattered around the microspheres when prepared in methanol (show in Fig. 5A and B). When water was used as the solvent, the morphology of the as-prepared ZnIn2 S4 is relatively uniform. It is composed of microspheres with diameters in the range of 2–6 ␮m and also has

L. Su et al. / Applied Surface Science 384 (2016) 161–174

165

Fig. 3. XPS survey spectrum(A) and high-resolution spectra of Zn 2p (B), In 3d (C), S 2p (D).

Fig. 4. N2 adsorption-desorption isotherms of ZnIn2 S4 samples synthesized with different solvents.

more obvious the fluffy structure. When ethylene glycol was used as the solvent, irregular microspheres were formed with diameters of about 0.5–1 ␮m. Clearly, the microspheres have indistinct fluffy structure. It may be due to ethylene glycol as solvent in the synthetic process of samples has the capping effect [40]. The SEM results indicate that the different solvents could determine the solubility, reactivity, and diffusion behavior of the reagents and the intermediates [41–43], so that the solvent has a significant effect on the morphology of the final products. The samples morphology combining with the BTE test results well explain why ZIS-EtOH sample with larger specific surface area than others. It is further indicated that the crystal plane of the catalyst with fluffy structure will be more fully exposed.

3.1.5. TEM and HR-TEM analysis In order to further investigate the microscopic structure of the resulting ZnIn2 S4 samples, TEM and HR-TEM analysis were carried out. Fig. 6 shows the TEM images of representative ZnIn2 S4 microspheres. It is clear that the ZIS-EtOH sample shows a typical fluffy structure, and the shape of microspheres is like ball plumage which is composed of ultrathin and vertically oriented mixed nanosheets. The Fig. 6B shows the TEM image of ZIS-MeOH, which also shows the fluffy microsphere structure with composed by the ultrathin nanosheets. The ZIS-H2 O samples show surface shaggy structure and the middle part of samples are dense accumulation of matter (shows Fig. 6C). It can be clearly seen that the microscopic structure of ZIS-EtOH, ZIS-MeOH and ZIS-H2 O samples are in consistent with the SEM images. The irregular solid microspheres with no gauzelike ultrathin nanosheet are shown in Fig. 6 D when the ethylene glycol as solvent. Based on the results, it is clear that the TEM results is consistent with the SEM results, which the samples with fluffy structure will have higher specific surface area. Fig. 7 shows the high-resolution TEM (HR-TEM) images of ZnIn2 S4 synthesized with different solvents. It can be seen that the lattice fringe spacings of ZIS-EtOH and ZIS-H2 O are 0.32 nm, respectively. It is corresponding to the (102) crystal planes of hexagonal ZnIn2 S4 (Fig. 7A and C). The lattice fringe spacing of ZIS-MeOH is estimated to be approximately 0.27 nm, which corresponds to the gap of (009) lattice planes. And the lattice fringe spacings of ZIS-EG are 0.29 and 0.32 nm, which are assigned to the (104) and (102) crystal facets of the ZnIn2 S4 , respectively. In addition, Fig. 7A shows the existence of ultrathin nanosheets with a twodimensional structure of the ZIS-EtOH, whose lattice fringe spacing d is approximately 0.33 nm and is in accordance with the (100) spacing of hexagonal ZnIn2 S4 . Moreover, the SAED pattern of ZISEtOH (the inset) shows two diffraction rings indexed to (102) and

166

L. Su et al. / Applied Surface Science 384 (2016) 161–174

Fig. 5. SEM images of ZnIn2 S4 microspheres synthesized with different solvents.

Fig. 6. TEM images of ZnIn2 S4 microspheres synthesized with different solvents. The insets are their magnified parts.

L. Su et al. / Applied Surface Science 384 (2016) 161–174

167

Fig. 7. High-resolution TEM (HR-TEM) images of ZnIn2 S4 microspheres synthesized with different solvents. The inset of (A) is the SAED pattern of ZIS-EtOH.

Fig. 8. UV–vis diffuse reflectance spectra of ZnIn2 S4 synthesized with different solvents. The inset shows the corresponding (␣h)2 -hv curve.

(112) crystal planes, which is consistent with both the HR-TEM image and XRD analysis. 3.1.6. Optical characteristics Besides the structural properties, the different solvothermal synthesis also significantly affected the optical properties of ZnIn2 S4 samples. The UV–vis diffuse reflectance spectra of ZnIn2 S4 prepared with different solvents are shown in Fig. 8. It can be seen

that all of the products reveal almost the same absorption profile with a steep absorption edge in the visible light range, suggesting that the absorption relevant to the band gap is due to the intrinsic transition of the materials rather than a transition from impurity levels [44,45]. The optical band gap of ZnIn2 S4 can be calculated from (˛hv)2 = A(hv-Eg ) [45] (where ˛,, A, and Eg are the absorption coefficient, light frequency, proportionality constant, and band gap values of ZnIn2 S4 , respectively) and a plot of (˛hv)2 versus hv based on the direct forbidden transition is shown in the inset of Fig. 8 It is clear that the band gap energy value of ZIS-H2 O, ZIS-MeOH, ZIS-EtOH, and ZIS-EG are 2.37, 2.41, 2.52, and 2.56 eV, respectively. Meanwhile, the band edge potentials of semiconductors are estimated by an equation relevant to their Mulliken electro-negativity. The valence band potential of pure ZnIn2 S4 at the point of zero charge (pHZPC ) can be predicted based on the empirical equation: [46] EVB = X − Ee + 0.5Eg 0·5Eg (EVB is the valence band edge potential, X (XZnIn2S4 ≈ 4.82 eV) is the electro-negativity of a semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), and Eg is the band gap energy of the semiconductor) [35]. The values of EVB and ECB for ZnIn2 S4 are shown in Fig. 9A. It is clear that all of the values of ECB for pure ZnIn2 S4 are more negative than O2 /• O− 2 (−0.33 V). Thus, the electrons in the conduction band can reduce the oxygen molecule to • O2 − . However, the valence band values are less than that both of the reduction potential of benzyl alcohol/benzaldehyde (BA/BAD) and BAD/oxidized BAD (2.5 eV) [11]. So the holes in the valence band are inadequate with regards their ability to directly oxidize BA to BAD or BAD to oxidized BAD. Furthermore, the electrochemical analysis of the as-synthesized samples was also carried out. Fig. 9B shows

168

L. Su et al. / Applied Surface Science 384 (2016) 161–174

Fig. 9. Band gap structure (A) and Mott-Schottky plots (B) of ZnIn2 S4 synthesized with different solvents.

Fig. 10. EIS Nyquist plots of ZnIn2 S4 microspheres synthesized with different solvents in 1.0 M KCl solution containing 5.0 mM K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ].

Fig. 11. The photocatalytic performance of samples for selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation for 2 h.

the Mott-Schottky plots in the dark. It can be seen that the linear slopes of the M-S plots for the ZnIn2 S4 synthesized with different solvents are positive, suggesting that the samples belong to typical n-type semiconductors. The test values of conduction band are about −0.96 V, −0.83 V, −0.93 V, and −0.86 V vs NHE (the difference value of NHE relative to the Ag/AgCl reference electrode is 0.2 V) for ZIS-EtOH, ZIS-MeOH, ZIS-H2 O and ZIS- EG samples, respectively. It is clear that the test values are close to the result of the theoretical calculation.

ZnIn2 S4 samples prepared in different solvents had a similar semicircular form in the dark, but the ZnIn2 S4 (ZIS-EtOH) prepared in an ethanol solvent had a significantly decreased semicircle radius compared to that prepared in methanol, aqueous, or ethylene glycol media. It means that ZIS-EtOH has the highest electronic conductivity in the non-photoexcited state [36]. That is, it promotes the effective transfer of the electron from bulk to surface of the ZnIn2 S4 sample. It is suggested that the ZIS-EtOH sample will exhibit high photocatalytic efficiency.

3.1.7. Electrochemical characteristics The electrochemical properties of the as-synthesized ZnIn2 S4 samples were estimated by EIS. According to the electrolytic theory, the charge transfer limiting process is depended on the highfrequency arc. This can be imputed to the double- layer capacity in parallel with the charge transfer resistance at the contact interface between electrode and electrolyte solution [47,48]. The semicircle in the Nyquist plots delivers information on charge transfer process with the diameter of the semicircle corresponding to the charge transfer resistance [36]. As shown in Fig. 10, Nyquist plots of

3.2. Evaluation of photocatalytic activity The selective oxidation of benzyl alcohol to benzaldehyde under mild conditions (T = 70 ◦ C and 0.1 atm O2) was selected as a model reaction to estimate the photocatalytic performance of ZnIn2 S4 samples prepared in different media. Fig. 11 shows the photocatalytic performance of ZIS-EtOH, ZIS-MeOH, ZIS-H2O and ZIS-EG for the selective transformation of benzyl alcohol to benzaldehyde under visible light irradiation for 2 h, respectively. The experimental results show that the samples prepared in different solvents

L. Su et al. / Applied Surface Science 384 (2016) 161–174

169

Table 2 Photocatalytic performance of ZnIn2 S4 samples for selective oxidation of various substrates with different substitutions. Entry Substrate

Product

ZIS-EtOH

ZIS-MeOH

ZIS-H2 O

ZIS-EG

Yie. (%) Conv. (%) Sel. (%) Yie. (%) Conv. (%) Sel. (%) Yie. (%) Conv. (%) Sel. (%) Yie. (%) Conv. (%) Sel. (%)

1

60.1

69.8

86.1

48.2

58.6

82.3

44.6

53.8

82.9

25.4

33.8

75.0

2

53.5

55.9

95.7

44.7

48.5

92.2

35.7

48.2

74.1

10.3

34.1

30.3

3

51.0

52.7

96.7

36.9

38.7

95.3

31.7

35.0

90.5

8.2

35.8

22.9

4

40.0

45.9

87.1

35.7

43.8

81.5

29.9

42.0

71.4

8.0

25.0

32.0

5

72.5

80.7

89.9

55.6

63.3

87.8

56.6

63.4

89.2

28.2

30.5

92.4

6

45.6

53.6

85.0

41.0

44.3

92.5

38.3

45.8

83.7

12.9

20.1

64.1

7

41.1

42.8

96.1

28.0

40.0

70.0

27.0

31.0

87.0

10.2

12.9

79.4

all exhibit excellent photocatalytic performance at 2 h and the ZIS-EtOH sample shows the highest photocatalytic activity among the samples, which the conversion, yield and selectivity values of selective oxidation of benzyl alcohol to benzaldehyde are 69.8%, 60.1% and 86.1%, respectively. In addition, ZIS-MeOH and ZISH2O exhibit the same photocatalytic performances, and the ZIS-EG shows the lowest the photocatalytic performance, which the conversion of benzyl alcohol and the yield of benzaldehyde are only 33.8% and 25.4%, respectively. The photocatalytic performance of the samples gradually decreases as follows: ZIS-EtOH > ZISMeOH > ZIS-H2 O > ZIS-EG

To further study of the general applicability of the ZnIn2 S4 synthesized by different solvents, the selective transformation of a series of aromatic alcohols with different substituted groups were carried out and the results are summarized in Table 2. It can be seen that the aromatic alcohols with electron-donating (−CH3 , −OCH3 ) and electron-withdrawing groups (−F, −Cl) all show an excellent conversion and high selectivity over ZIS-EtOH, ZIS-MeOH and ZIS-H2 O except ZIS-EG sample. The photocatalytic performance sequence of the samples is as follows: ZIS-EtOH > ZIS-MeOH > ZISH2 O > ZIS-EG. From Table 2, it is clear that compared with benzyl alcohol, the formation rates of aromatic aldehydes were retarded by electron- withdrawing substituents (−F, −Cl). And it was

170

L. Su et al. / Applied Surface Science 384 (2016) 161–174

Fig. 12. The relationship between the crystal facets ratio of (112)/(102) and the photocatalytic activity of the catalysts (the yield of benzaldehyde was detected under visible light for 2 h).

Fig. 13. The crystal structure of hexagonal ZnIn2 S4 and {0001} facets.

increased significantly by electron-donating one (para-positionOCH3 ). However, when the para-position-OCH3 transforms to the meta-position and ortho-position, the conversion and selectively is decreased. It is due to the position change of the substituents lead to an enhanced space steric effect [49]. Meanwhile, when the paraposition is −CH3 , the aromatic alcohol exhibits a low conversion and selectivity (Table 2, entry 4). The reason for this was ongoing at time of writing and study. In order to determine the stability of ZnIn2 S4 samples, the cyclic tests of the as-prepared samples were carried out under the aforementioned conditions (Fig. S3). The results indicate that the photocatalytic activity of ZIS-EtOH sample has no obvious decrease for selective transformation of benzyl alcohol to benzaldehyde. Fur-

thermore, from the XRD pattern of the sample used 5 times (Fig. S4), the position and intensity of the characteristic diffraction peaks are nearly the same to those of the fresh sample. Therefore, it is considered that ZIS-EtOH is a stable photocatalyst for the selective oxidation of aromatic alcohols to benzaldehyde in this reaction system. 3.3. Differential analysis of photocatalytic performance The relative linear relationship between the peak intensity or the peak area ratio of (112)/(102) and the photocatalytic activities of the samples are shown in Fig. 12. (The ratio values and yield values can be obtained from Table 1). It is obvious that the higher ratio

L. Su et al. / Applied Surface Science 384 (2016) 161–174

Fig. 14. Controlled experiments using a series of radical scavengers on the selective oxidation of benzyl alcohol over ZnIn2 S4 microspheres.

171

the ab plane, which is perpendicular to the [0001]. Meanwhile, hexagonal ZnIn2 S4 has a layered structure, in which the stacking of atoms repeated the sequence: S-Zn-S-In-S-In-S along its c-axis [51,52], and the schematic diagram is displayed in Fig. 13. It has been reported that the {0001} facets of the ZnIn2 S4 can accumulate the photogenerated holes, thus suppressing the recombination of photo-generated electron-hole pairs [29,34]. The {0001} facets belong to the basic crystal plane, and combined with the analysis result of Fig. 12, we can further speculate that more exposed {0001} facets will increase the interactions and extents of adsorption with the substrates. Therefore, the expose level of the {0001} facets as the special crystal faces is the main factor for the difference of the catalytic performance. The ZIS-EtOH microspheres with a fluffy structure have the largest specific surface area and hence the catalyst has the greatest expose level of the {0001} facets. This is why ZIS-EtOH catalyst exhibits the highest photocatalytic performance. 3.4. Reaction mechanisms

values of peak intensity and area of (112)/(102) it has, the higher the photocatalytic activity of the samples. It suggests that the ratio values are consistent with the order of the catalyst performance. Because of the intrinsic anisotropy by itself driven of the ZnIn2 S4 hexagonal crystal structure, the crystal nucleus strongly tend to reunite together and self-assemble into hierarchically nanosheets structure [29,50]. Combining with the above analysis, and on account of the consistent sequence between the specific surface area and the catalyst activity, we infer that it is reasonable to relate the difference of basic crystal plane exposure extent to different microstructure caused by different solvent. As a result, it is proposed that the difference of crystal plane exposed level is an important factor causing the different catalytic performances. From Fig. 13, it can be seen that the HR-TEM image of ZIS-EtOH shows the lattice fringe spacing d is approximately 0.33 nm, which corresponds to the {0001} crystal face of hexagonal ZnIn2 S4 . The {0001} faceted hexagonal crystal structure of ZnIn2 S4 shows that the intersection angle of the arbitrary two sides of hexagon is 120◦ . It is demonstrated that the nanoplates of ZnIn2 S4 grow along

To understand the possible reaction mechanism for the photocatalytic oxidation of benzyl alcohol over ZnIn2 S4 photocatalysts, a series of controlled experiments using different radical scavengers was carried out and the results are shown in Fig. 14. Different radical scavengers were added to a system for removing the corresponding active species, such as: p-benzoquinone (BQ) for quenching superoxide radicals (• O2−), isopropyl alcohol (IPA) for removing • OH, and oxalic acid (OA) for capturing holes. When scavengers of BQ and OA were added to the four reaction systems, respectively, the conversion of benzyl alcohol and the yield of benzaldehyde decreased rapidly. While, when IPA was added to the reaction system, the photocatalytic activity decreased slightly. Based on the above results, it is clear that the selective oxidation of benzyl alcohol to benzaldehyde is mainly triggered by superoxide radicals and positive holes From Fig. 9, it is clear that the valence band position of ZnIn2 S4 sample will not be appropriate for the direct generation of • OH radicals (2.4 eV vs. NHE, pH = 7). However, a small amount of • OH radicals were produced in the reaction process on the basis of

Fig. 15. Detection of H2 O2 in water dispersions of ZnIn2 S4 in the presence of DPD and POD under visible light irradiation. Curves were obtained by addition of DPD (20 ␮l) and POD (0.5 ml) to the dispersions (5 ml) after irradiation with visible light.

172

L. Su et al. / Applied Surface Science 384 (2016) 161–174

Fig. 16. Proposed mechanism for photocatalytic oxidation of benzyl alcohol to benzaldehyde over ZnIn2 S4 under visible light irradiation.

scavenger experiment. It is concluded that the potential route for • OH radical generation in this ZnIn S photocatalytic system is the 2 4 interaction of photoexcited electrons, • O2 − , and H2 O2 . e− + O2 → • O2 −

(1)

(2)• O2 − + e− + 2H+ → H2 O2 (3)• O2 − + H2 O2 → • OH + OH− + O2 H2 O2 → 2• OH

oxidation potential, oxidizes the alkoxide to yield benzaldehyde and release H+ . Another possible step is that hydroxyl radicals (• OH) attack benzyl alcohol, two hydrogen radicals were withdrawn from the alcohol and a C O bond can be formed. It was suggested that the catalytic oxidation process, with a positive hole, was a key step. The catalytic process with hydroxyl radicals underwent the secondary step, in which the H2 O2 presumably rapidly rebounded causing the formation of a more stable structure (most likely state for water molecules, but nevertheless an active state) [18] (Fig. 16).

(4)

In order to determine the formation of H2 O2 in the photocatalytic reaction system, the spectrophotometric analysis with DPD in the presence of POD was carried out [53,54]. Fig. 15 shows the curves of the generated hydrogen peroxide in water dispersion of ZnIn2 S4 prepared in different solvents for 30 min, and ZIS-EtOH for different times. Fig. 15A shows that the characteristic peaks of the oxidized DPD appeared at 512 and 552 nm in the photoexcited ZnIn2 S4 system. ZIS-EtOH exhibits the strongest absorption peak intensity among the samples examined. In addition, under dark conditions in the presence of the catalyst, or light conditions without the catalyst, no obvious peaks were observed. Fig. 15B shows the absorption peak intensity of ZIS-EtOH reflected the quantity of generated H2 O2 after irradiation for different times. The highest absorption peak is occurred when illumination for 60 min. A wide peak appeared at 512 nm with 15 min irradiation, which may have been due to the low concentration of generated H2 O2 . In conclusion, as an intermediate, H2 O2 can be generated in the ZnIn2 S4 photocatalytic process. H2 O2 is an inevitable source of • OH species and simultaneously it verifies the existence of • O2 − species again. It has been reported that the photoexcited electrons and holes migrate in two sub-layers of ZnIn2 S4 nanosheets, respectively (in the ab plane) [45,50]. The superoxide radical (• O2 − ) was formed by molecular oxygen capture photoexcited e− in CB of ZnIn2 S4 . Based on the results, it is proposed that the reaction for the selective oxidation of aromatic alcohols to corresponding aromatic aldehydes may contain two processes. The major process is that • O2 − attacks a substrate (i.e. benzyl alcohol) in the system, followed by the formation of an alkoxide anion. The positive hole, with its moderate

4. Conclusion A series of ZnIn2 S4 samples were synthesized by a simple solvothermal method with different solvents. The ZnIn2 S4 samples exhibited a superior photocatalytic performance for the selective oxidation of aromatic alcohols to corresponding aromatic aldehydes under mild conditions. ZIS-EtOH showed the highest photocatalytic activity among those samples examined. The enhanced photocatalytic performance was mainly attributed to the more exposed {0001} crystal facets than other samples and higher specific surface area. • O2 − and positive holes (h+ ) were important active species for the selective oxidation of aromatic alcohols to corresponding aromatic aldehydes. This research may provide a simple and mild method to fabricated efficient, visible-light-driven photocatalysts for the selective transformation of organics under mild conditions.

Acknowledgement This study was supported by the Natural Science Foundation of China (NSFC, grant Nos. 51472005, 51172086, 21103060, and 51272081). This work was financially supported by the Natural Science Foundation of China (Grant Nos. 21473066) the Natural Science Foundation of Anhui Province, China (Grant Nos. 1408085QB38), and the High Education Revitalization Plan of Anhui province, China.

L. Su et al. / Applied Surface Science 384 (2016) 161–174

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.04. 084.

References [1] Q. Li, C. Cui, H. Meng, J.G. Yu, Visible-light photocatalytic hydrogen production activity of ZnIn2 S4 microspheres using carbon quantum dots and platinum as dual co-catalysts, Chem. Asian J. 9 (2014) 1766–1770. [2] W.L. Jiang, X.H. Yin, F. Xin, Y.D. Bi, Y. Liu, X. Li, Preparation of CdIn2 S4 microspheres and application for photocatalytic reduction of carbon dioxide, Appl. Surf. Sci. 288 (2014) 138–142. [3] Y. Peng, L. Shang, Y.T. Cao, Q. Wang, Y.F. Zhao, C. Zhou, T. Bian, L.Z. Wu, C.H. Tung, T.R. Zhang, Effects of surfactants on visible-light-driven photocatalytic hydrogen evolution activities of AgInZn7 S9 nanorods, Appl. Surf. Sci. 358 (2015) 485–490. [4] I.B. Assaker, M. Gannouni, J.B. Naceur, M.A. Almessiere, A.L. Al-Otaibi, T. Ghrib, S.W. Shen, R. Chtourou, Electrodeposited ZnIn2 S4 onto TiO2 thin films for semiconductor-sensitized photocatalytic and photoelectrochemical applications, Appl. Surf. Sci. 351 (2015) 927–934. [5] S.F. Chen, Y.F. Hu, L. Ji, X.L. Jiang, X.L. Fu, Preparation and characterization of direct Z-scheme photocatalyst Bi2 O3 /NaNbO3 and its reaction mechanism, Appl. Surf. Sci. 292 (2014) 357–366. [6] S.G. Meng, Z.S. Cao, X.L. Fu, S.F. Chen, Fabrication of hydrophilic S/In2 O3 core-shell nanocomposite for enhancement of photocatalytic performance under visible light irradiation, Appl. Surf. Sci. 324 (2015) 188–197. [7] C.C. Zhou, Y.J. Zhao, J. Cao, H.L. Lin, S.F. Chen, Partial oxidation controlled activity regeneration of used Ag3 PO4 photocatalyst via removing the in situ surface metallic silver, Appl. Surf. Sci. 351 (2015) 33–39. [8] Q. Li, X. Li, S. Wageh, A.A. Al-Ghamdi, J.G. Yu, CdS/Graphene nanocomposite photocatalysts, Adv. Energy Mater. 1500010 (2015). [9] T. Mallat, A. Baiker, Oxidation of alcohols with molecular oxygen on solid catalysts, Chem. Rev. 104 (2004) 3037–3058. [10] J.A. Mueller, C.P. Goller, M.S. Sigman, Elucidating the significance of ␤-hydride elimination and the dynamic role of acid/base chemistry in a palladiumcatalyzed aerobic oxidation of alcohol, J. Am. Chem. Soc. 126 (2004) 9724–9734. [11] X. Xiao, J. Jiang, L. Zhang, Selective oxidation of benzyl alcohol into benzaldehyde over semiconductors under visible light: the case of Bi12 O17 Cl2 nanobelts, Appl. Catal. B: Environ. 142–143 (2013) 487–493. [12] R.M. Leithall, V.N. Shetti, S. Maurelli, M. Chiesa, E. Gianotti, R. Raja, Toward understanding the catalytic synergy in the design of bimetallic molecular sieves for selective aerobic oxidations, J. Am. Chem. Soc. 135 (2013) 2915–2918. [13] X. Dai, M.L. Xie, S.G. Meng, X.L. Fu, S.F. Chen, Coupled systems for selective oxidation of aromatic alcohols to aldehydes and reduction of nitrobenzene into aniline using CdS/g-C3 N4 photocatalyst under visible light irradiation, Appl. Catal. B: Environ. 158–159 (2014) 382–390. [14] Y.M. Liu, H. Tsunoyama, T. Akita, T. Tsukuda, Preparation of ∼1 nm gold clusters confined within mesoporous silica and microwave-assisted catalytic application for alcohol oxidation, J. Phys. Chem. C 113 (2009) 13457–13461. [15] M.A. Zhang, C.C. Chen, W.H. Ma, J.C. Zhao, Visible-light-induced aerobic oxidation of alcohols in a coupled photocatalytic system of dye-sensitized TiO2 and TEMPO, Angew. Chem. Int. Ed. 47 (2008) 9730–9733. [16] W.Y. Zhai, S.J. Xue, A.W. Zhu, Y.P. Luo, Y. Tian, Plasmon-driven selective oxidation of aromatic alcohols to aldehydes in water with recyclable Pt/TiO2 nanocomposites, ChemCatChem 3 (2011) 127–130. [17] N. Zhang, S.Q. Liu, X.Z. Fu, Y.J. Xu, A simple strategy for fabrication of Plum-Pudding type Pd@CeO2 semiconductor nanocomposite as a visible-light -driven photocatalyst for selective oxidation, J. Phys. Chem. C 115 (2011) 22901–22909. [18] F.Z. Su, S.C. Mathew, G. Lipner, X.Z. Fu, M. Antonietti, S. Blechert, X.C. Wang, mpg-C3 N4 -catalyzed selective oxidation of alcohols using O2 and visible light, J. Am. Chem. Soc. 132 (2010) 16299–16301. [19] M.L. Xie, X. Dai, S.G. Meng, X.L. Fu, S.F. Chen, Selective oxidation of aromatic alcohols to corresponding aromatic aldehydes using In2 S3 microsphere catalyst under visible light irradiation, Chem. Eng. J. 245 (2014) 107–116. [20] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [21] X.B. Chen, S.H. Shen, L.J. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [22] J. Zhang, J.G. Yu, Y.M. Zhang, Q. Li, J.R. Gong, Visible light photocatalytic H2 -production activity of CuS/ZnS porous nanosheets based on photoinduced interfacial charge transfer, Nano Lett. 11 (2011) 4774–4779. [23] J.G. Yu, B. Yang, B. Cheng, Noble-metal-free carbon nanotube-Cd0.1 Zn0.9 S composites for high visible-light photocatalytic H2 -production performance, Nanoscale 4 (2012) 2670–2677. [24] Z.X. Chen, J.J. Xu, Z.Y. Ren, Y.H. He, G.C. Xiao, Low temperature synthesis of ZnIn2 S4 microspheres as a visible light photocatalyst for selective oxidation, Catal. Commun. 41 (2013) 83–86.

173

[25] J. Jiang, K. Zhao, X.Y. Xiao, L.Z. Zhang, Synthesis and facet-dependent photoreactivity of BiOCl single-crystalline nanosheets, J. Am. Chem. Soc. 134 (2012) 4473–4476. [26] U. Sivan, P.M. Solomon, H. Shtrikman, Coupled electron-hole transport, Phys. Rev. Lett. 68 (1992) 1196–1199. [27] X.X. Xu, C. Randorn, P. Efstathiou, J.T.S. Irvine, A red metallic oxide photocatalyst, Nat. Mater. 11 (2012) 595–598. [28] Z. Zhen, J.T. Yates, Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces, Chem. Rev. 112 (2012) 5520–5551. [29] L. Wei, Y.J. Chen, Y.P. Lin, H.S. Wu, R.S. Yuan, Z.H. Li, MoS2 as non-noble-metal co-catalyst for photocatalytic hydrogen evolution over hexagonal ZnIn2 S4 under visible light irradiations, Appl. Catal. B: Environ. 144 (2014) 521–527. [30] Y.J. Chen, R.K. Huang, D.Q. Chen, Y.S. Wang, W.J. Liu, X.N. Li, Z.H. Li, Exploring the different photocatalytic performance for dye degradations over hexagonal ZnIn2 S4 microspheres and cubic ZnIn2 S4 nanoparticles, Appl. Mater. Interfaces 4 (2012) 2273–2279. [31] B. Chai, T. Peng, P. Zeng, X. Zhang, X. Liu, Template-free hydrothermal synthesis of ZnIn2 S4 floriated microsphere as an efficient photocatalyst for H2 production under visible-light irradiation, J. Phys. Chem. C 115 (2011) 6149–6155. [32] Q. Liang, H. Zhao, L. Ning, J. Wang, C. Zhang, L. Wang, A. Wei, Q. Zhao, H. Yang, S. liu, InOCl nanosheets with exposed {001} facets: synthesis electronic. structure and surprisingly high photocatalytic activity, Appl. Catal. B: Environ. 152–153 (2014) 390–396. [33] C. Parmeggiani, F. Cardona, Transition metal based catalysts in the aerobic oxidation of alcohols, Green Chem. 14 (2012) 547–564. [34] C.W. Tan, G.Q. Zhu, M. Hojamberdiev, K.S. Lokesh, X.C. Luo, L. Jin, J.P. Zhou, Adsorption and enhanced photocatalytic activity of the {0001} faceted Smdoped ZnIn2 S4 microspheres, J. Hazard. Mater. 278 (2014) 572–583. [35] B. Chai, T. Peng, P. Zeng, X. Zhang, Preparation of a MWCNTs/ZnIn2 S4 composite and its enhanced photocatalytic hydrogen production under visible- light irradiation, Dalton Trans. 41 (2012) 1179–1186. [36] N. Zhang, Y. Zhang, X. Pan, X. Fu, S. Liu, Y.J. Xu, Assembly of CdS nanoparticles on the two-dimensional graphene scaffold as visible-light-driven photocatalyst for selective organic transformation under ambient conditions, J. Phys. Chem. C 115 (2011) 23501–23511. [37] L. Yuan, M.Q. Yang, Y.J. Xu, A low-temperature and one-step method for fabricating ZnIn2 S4 -GR nanocomposites with enhanced visible light photoactivity, J. Mater. Chem. A 2 (2014) 14401–14412. [38] D.V. Bavykin, V.N. Parmon, A.A. Lapkin, F.C. Walsh, The effect of hydrothermal conditions on the mesoporous structure of TiO2 nanotubes, J. Mater. Chem. 14 (2004) 3370–3377. [39] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure. Appl. Chem. 57 (1985) 603–619. [40] S. Senapati, S.K. Srivastava, S.B. Singh, K. Biswas, Capping agent assisted and Ag-catalyzed growth of Ni nanoflowers, Cryst. Growth Des. 10 (2010) 4068–4075. [41] Y. Zhu, T. Mei, Y. Wang, Y. Qian, Formation and morphology control of nanoparticles via solution routes in an autoclave, J. Mater. Chem. 21 (2011) 11457–11463. [42] R.C. Jin, G. Chen, Q. Wang, J.X. Sun, Y. Wang, A facile solvothermal synthesis of hierarchical Sb2 Se3 nanostructures with high electrochemical hydrogen storage ability, J. Mater. Chem. 21 (2011) 6628–6635. [43] D. Chen, G. Chen, Q. Wang, R. Jin, Y. Wang, J. Pei, H. Xu, X. Shi, Facile hydrothermal synthesis of AgPb10 LaTe12 materials: controlled synthesis, growth mechanism and shape-dependent electrical transportation properties, Cryst. Eng. Comm. 14 (2012) 7771–7779. [44] J. Tang, J. Ye, Correlation of crystal structures and electronic structures and photocatalytic properties of the W-containing oxides, J. Mater. Chem. 15 (2005) 4246–4251. [45] G. Wang, G. Chen, Y. Yu, X. Zhou, Y. Teng, Mixed solvothermal synthesis of hierarchical ZnIn2 S4 spheres: specific facet-induced photocatalytic activity enhancement and a DFT elucidation, RSC Adv. 3 (2013) 18579–18586. [46] X. Lin, J. Xing, W. Wang, Z. Shan, F. Xu, F. Huang, Photocatalytic activities of heterojunction semiconductors Bi2 O3 /BaTiO3 : a atrategy for the design of efficient combined photocatalysts, J. Phys. Chem. C 111 (2007) 18288–18293. [47] T. Lu, Y. Zhang, H. Li, L. Pan, Y. Li, Z. Sun, Electrochemical behaviors of graphene-ZnO and graphene-SnO2 composite films for supercapacitors, Electrochim. Acta 55 (2010) 4170–4173. [48] H.L. Guo, X.F. Wang, Q.Y. Qian, F.B. Wang, X.H. Xia, A green approach to the synthesis of graphene nanosheets, ACS Nano 3 (2009) 2653–2659. [49] X.H. Li, J.S. Chen, X. Wang, J. Sun, M. Antonietti, Metal-free activation of dioxygen by graphene/g-C3 N4 nanocomposites: functional dyads for selective oxidation of saturated hydrocarbons, J. Am. Chem. Soc. 133 (2011) 8074–8077. [50] X.L. Hu, J.C. Yu, J.M. Gong, Q. Li, Rapid mass production of hierarchically porous ZnIn2 S4 submicrospheres via a microwave-solvothermal process, Cryst. Growth Des. 7 (2007) 2444–2448. [51] X.L. Gou, F.Y. Cheng, Y.H. Shi, L. Zhang, S.J. Peng, J. Chen, P.W. Shen, Shape-controlled synthesis of ternary chalcogenide ZnIn2 S4 and CuIn(S, Se)2 nano-/microstructures via facile solution route, J. Am. Chem. Soc. 128 (2006) 7222–7229. [52] F. Donika, S. Radautsan, G. Kiosse, S. Semiletov, T. Donika, I. Mustya, Crystal structure of double-pack polytpye ZnIn2 S4 (II) A, and more careful

174

L. Su et al. / Applied Surface Science 384 (2016) 161–174

cetermination of structure of triple-pack polytype ZnIn2 S4 (III), Sov. Phys. Crystallogr. 16 (1971) 190. [53] Y. Cui, J. Huang, X. Fu, X. Wang, Metal-free photocatalytic degradation of 4-chlorophenol in water by mesoporous carbon nitride semiconductors, Catal. Sci. Technol. 2 (2012) 1396–1402.

[54] H. Bader, V. Sturzenegger, J. Hoigné, Photometric method for the determination of low concentrations of hydrogen peroxide by the peroxidase catalyzed oxidation of N,N-diethyl-p-phenylenediamine (DPD), Water Res. 22 (1988) 1109–1115.