ZnS hexagonal plates with enhanced hydrogen evolution activity under visible light irradiation

Powder Technology 288 (2016) 103–108

Contents lists available at ScienceDirect

Powder Technology journal homepage: www.elsevier.com/locate/powtec

CuS/ZnS hexagonal plates with enhanced hydrogen evolution activity under visible light irradiation Longhao Wang, Hua Chen, Liang Xiao, Jianhua Huang ⁎ Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China

a r t i c l e

i n f o

Article history: Received 22 August 2015 Received in revised form 23 October 2015 Accepted 26 October 2015 Available online 26 October 2015

a b s t r a c t CuS/ZnS hexagonal plates were prepared through a solvothermal and subsequent cation-exchange reaction between the as-prepared precursor and Cu(NO3)2 aqueous solution. The as-prepared CuS/ZnS composite photocatalysts were characterized by X-ray diffraction (XRD), inductively coupled plasma atomic emission spectrometry (ICP-AES), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), UV–Vis diffuse reflection spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. The CuS/ZnS hexagonal plates exhibited efficient photocatalytic H2 evolution activity from an aqueous Na2S and Na2SO3 solution without noble metal loading. The composite photocatalyst with 6% CuS content displayed the highest photocatalytic activity (1233.5 μmol h−1 g−1), which was almost 77 times higher than that of the mechanical mixture of CuS and ZnS. The enhanced visible lightinduced photocatalytic activity is likely a result of the interfacial charge transfer of the CuS/ZnS heterojunction. Our proposed photocatalytic mechanism is supported by XPS and PL results. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Visible light photocatalysis, or the conversion of solar energy into chemical energy, is an attractive yet very challenging process. Recently, the photocatalytic H2 evolution from water under visible light irradiation by using semiconductors has received considerable attention [1–3]. This is because developing cheap photocatalysts which are efficiently responsive to visible light, rather than UV light, is important for practical applications [4–7]. Transition-metal chalcogenides have recently been extensively studied owing to their excellent luminescent and photochemical properties. In particular, zinc sulfide has been successfully applied in photocatalysis, sensors and optoelectronics [8]. However, with a wide bandgap of 3.71 eV for cubic zinc blend and 3.78 eV for hexagonal wurtzite [9], ZnS is only responsive to UV light. This severely limits its use in practical application. In contrast, CuS catalyst has acted as an efficient photocatalyst for dye degradation under visible light [10,11]. In recent years, CuS/ZnS composites have also proven to be efficient photocatalysts for the generation of hydrogen and the degradation of organic dyes under visible light irradiation. For example, CuS/ZnS hollow spheres [12], CuS/ZnS core/shell nanocrystals [13], and ZnS–CuS microspheres [14,15] all exhibited enhanced visible light photocatalytic activity in the degradation of rhodamine B and methylene blue. Chen and coworkers fabricated reduced graphene oxide (rGO) loaded-ZnS/ CuS hetero-nanostructures, which exhibited efficient visible light⁎ Corresponding author. E-mail address: [email protected] (J. Huang).

http://dx.doi.org/10.1016/j.powtec.2015.10.042 0032-5910/© 2015 Elsevier B.V. All rights reserved.

driven photocatalytic activity for methyl orange dye degradation [16]. CuS/ZnS porous nanosheets [17] and ZnS–In2 S3 –CuS nanospheres [18] also show high visible light (λ N 400 nm) photocatalytic activity of H2 production without Pt cocatalyst. Kim et al. prepared a heterostructured ZnS–CuS–CdS photocatalyst for water splitting H2 production under standard solar irradiation [19]. These reports illustrate that a combination of ZnS and CuS provide an extensive absorption range. The heterostructures of the composites are also useful to separate the photogenerated electrons and holes to produce highly active photocatalysts. In this work, we report the first synthesis of CuS/ZnS hexagonal plates through a solvothermal reaction and a subsequent hydrothermal cation-exchange reaction using a ZnS(en)0.5 precursor. The photocatalytic activities of the CuS/ZnS hexagonal plates were tested under visible light irradiation for H2 generation from an aqueous solution containing sacrificial reagents Na2SO3 and Na2S. The effect of the CuS content in the CuS/ZnS plates on the H2 generation rate was studied and a possible photocatalytic mechanism is proposed. 2. Experimental 2.1. Preparation of CuS/ZnS hexagonal plates All reagents were analytical grade and used without further purification. Ultrapure water (N17 MΩ cm) from a Milli-Q water system was used throughout the experiments. CuS/ZnS hexagonal plates involved the production of ZnS precursor and subsequent reaction with Cu(NO3)2. The ZnS precursor

104

L. Wang et al. / Powder Technology 288 (2016) 103–108

ZnS(H2NCH2CH2NH2)0.5 (ZnS(en)0.5) was synthesized by a solvothermal reaction. In a typical reaction, 3 mmol zinc chloride and 6 mmol sulfur powders were dissolved in an ethanol/ethylenediamine solution (15 mL/15 mL). After magnetic stirring for 30 min at room temperature, the mixture was transferred to a Teflon-lined autoclave and heated at 150 °C for 12 h. After cooling to room temperature, the white precipitates were collected by centrifugation, washed thoroughly with ultrapure water and ethanol, and dried at 60 °C for 10 h. The as-prepared ZnS(en)0.5 precursor was reacted with an aqueous solution of Cu(NO3)2 through a cation-exchange reaction. In a typical experiment, 1.54 mmol ZnS(en)0.5 was dispersed ultrasonically in 10 mL of ultrapure water. A 20 mL solution of Cu(NO3)2 with the desired Cu2+ concentration was slowly added to the ZnS(en)0.5 suspension and stirred for 30 min. The mixture was then transferred to a Teflon-lined autoclave and heated at 140 °C for 10 h. The resulted product was rinsed two times with water and ethanol and then dried at 60 °C. The molar ratio of Cu2+ to Zn2+ varied from 0, 1, 3, 5, 7, to 9 (mol %), and the obtained samples were labeled as CZS0, CZS1, CZS3, CZS5, CZS7, and CZS9, respectively. 2.2. Characterization X-ray diffraction (XRD) patterns of the photocatalysts were recorded on a DX-2700 diffractometer (Dandong Fangyuan Instrument Co. Ltd., China) using Cu Kα radiation (1.5418 Å). Scanning electron microscope (SEM) images were recorded using a Hitachi S-4800 equipped with an energy dispersive X-ray spectrophotometer (EDS). Transmission electron microscope (TEM) and high resolution-TEM (HRTEM) images were taken using a JEOL JEM-2100 transmission electron microscopy. The Cu2+ content in the CuS/ZnS samples was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) using an IRIS Intrepid II XSP. Diffuse reflectance spectra were recorded on a Shimadzu 2450 UV–Vis spectrometer with an integrating sphere using BaSO4 as the reference. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXIS Ultra DLD system. The photoluminescence (PL) spectra of photocatalysts were obtained on a Hitachi Fluorescence spectrophotometer F-7000 with excitation wavelength of 350 nm. 2.3. Photocatalytic activity The photocatalytic hydrogen evolution experiments were carried out in a commercial water splitting system (Labsolar-H2, Beijing Perfect Light Technology Co., Ltd. China). The system included a Xenon light source, a Pyrex reaction cell, a gas circulation and evacuation system, and a sampling device. Online sampling and analysis was realized by connecting the system to a gas chromatograph (GC9800, Shanghai Kechuang Chromatograph Instruments Co., Ltd. China). In a typical photocatalytic experiment, in a Pyrex reaction cell (250 mL), the CuS/ZnS photocatalyst (50 mg) was dispersed by stirring in an aqueous solution containing 100 mL of 0.35 M Na2S and 0.25 M Na2SO3. The reaction cell was then sealed with a top window and evacuated. Under visible light irradiation using a cutoff filter (λ N 400 nm) from a 300 W PLS-SXE300 Xe lamp, the hydrogen evolved was periodically analyzed by online gas chromatograph. To avoid evaporation of the solution during irradiation, the temperature of the reaction cell was maintained at 8 ± 3 °C by flowing cold water passed the cell.

Fig. 1. The XRD pattern of ZnS(en)0.5 precursor inset with a SEM image of the ZnS(en)0.5 precursor.

indicates the formation of pure ZnS(en)0.5. The strong and sharp diffraction peak implies the formation of highly crystalline ZnS(en)0.5 precursor. ZnS(en)0.5 usually has a lamellar structure [20–22]. Interestingly, hexagonal platelet ZnS(en)0.5 was obtained for the first time in our experiment (Fig. 1). The side length of a hexagonal plate is ~ 2 μm and the thickness is ~200 nm. The surface of the plates is smooth. Comparing our experiment to literature preparations, we find three differences: solvent, reaction temperature and time. In the literature preparations, ethylenediamine was used as solvent [20–22], whereas our experiment was carried out in a mixed solvent of ethanol and ethylenediamine with a volume ratio of 1:1. Our experiment was also performed at lower temperatures and for a shorter time compared with the literature [20,21]. The results show that the morphology of ZnS(en)0.5 strongly depends on experimental conditions. After the cation-exchange reaction between ZnS(en)0.5 and the Cu(NO3)2 aqueous solution, the diffraction peaks of ZnS(en)0.5 disappeared. All samples with various molar ratios of Cu2+/Zn2+ show similar XRD patterns (Fig. 2). All diffraction peaks can be readily indexed to wurtzite ZnS (JCPDS NO. 36-1450). Meanwhile, we observe a weak peak at 33.1° when magnifying five times, which is attributed to the (200) crystalline plane of cubic ZnS (JCPDS NO. 65-0309). The fact that CuS phase is not detected in all samples may be due to the weak crystallization and high dispersion of CuS particles deposited on the surface of ZnS [17]. Zhang et al. [23] also reported a similar phenomenon that no CuS phase was observed in the XRD pattern of CuS/CdS composite. All CZSx samples show similar morphology. A typical SEM image of CZS7 is shown in Fig. 3(a). The hexagonal platelet morphology is remained. However, the plate surface becomes rough. The chemical composition of CZS7 was analyzed with an energy-dispersive X-ray

3. Results and discussion 3.1. Characterization of ZnS(en)0.5 precursor and CuS/ZnS photocatalysts Fig. 1 shows the XRD pattern of the solvothermally prepared precursor. A refinement has been carried out by using Jade program. All diffraction peaks are in good agreement with literature reports of ZnS(en)0.5 [20–22]. No other diffraction peaks were observed which

Fig. 2. XRD patterns of CZS0, CZS1, CZS3, CZS5, CZS7, CZS9 samples.

L. Wang et al. / Powder Technology 288 (2016) 103–108

105

Fig. 3. (a) SEM image, (b) EDS spectrum, (c) TEM and (d) HRTEM image of CZS7.

spectrophotometer (EDS). The EDS spectrum (Fig. 3(b)) indicates that CZS7 consists of copper, zinc and sulfur elements. The silicon peak comes from the Si substrate. The Cu2+ content in CZS7 is calculated to be ~5.8 mol%. A typical TEM image in Fig. 3(c) shows the porous structure which indicates that the plate is composed of numerous nanoparticles. In the HRTEM image in Fig. 3(d), the lattice fringes can be clearly observed which confirms the well-defined crystal structure. The fringe with lattice spacing of ca. 0.31 nm corresponds to the (002) plane of wurtzite ZnS or the (111) plane of cubic ZnS. And the lattice spacing of ca. 0.28 nm corresponds to the (103) plane of CuS (JCPDS NO. 060464) [17]. These results imply that ZnS and CuS could form a heterojunction. With a heterojunction, the transfer of photogenerated electrons and holes between ZnS and CuS will be increased which would enhance the photocatalytic activity [12]. The Cu2 + content in CZS7 was further measured by ICP-AES. The CuS content in CZS7 is 6.1 mol% which indicates that the Cu2 + added in the reaction was completely converted to CuS. During the ion-exchange reaction, there is an obvious gradual sample color change from a white to a dark gray solution with increasing initial concentration of Cu2+. Fig. 4 shows the UV–Vis diffuse reflection spectra (DRS) of the CZSx samples and the mechanical mixture of 7 mol% CuS and ZnS. The absorption edge of CZS0 is ~371 nm and the bandgap energy is estimated as 3.34 eV using the Kubelka–Munk method. Other CZSx samples have two additional absorption bands (~350– 500 nm and 700–800 nm) and the absorption is enhanced by increasing the Cu2+ content. The absorption band at ~350–500 nm is generally ascribed to the direct interfacial charge transfer (IFCT) from the VB of ZnS to CuS [24] and the absorption band at 700–800 nm is assigned to the d– d transition of Cu2+ [25]. However, for the mechanical mixture of ZnS and CuS (7 mol%), the absorption band at ~350–500 nm is not observed.

These results further support a heterojunction formation between CuS and ZnS in the CuS/ZnS hexagonal plates. 3.2. Photocatalytic hydrogen evolution activity The photocatalytic activity of the prepared CuS/ZnS hexagonal plates (without noble metal cocatalyst loading) was determined by H2 evolution from an aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3 under visible light irradiation ( λ N400 nm). Fig. 5(a) shows the H2 evolution curves over CZSx photocatalysts. The amount of H2 increases nearly linearly with irradiation time. The evolution rate of H2 was calculated from these data (Fig. 5(b)). It is clear that the evolution

Fig. 4. UV–Vis diffuse reflectance spectra of CZSx (x = 0, 1, 3, 5, 7, 9 mol%), CuS, and the mechanical mixture of ZnS and CuS (with 7 mol% CuS).

106

L. Wang et al. / Powder Technology 288 (2016) 103–108

Fig. 5. (a) Time course of H2 evolution and (b) comparison of the rates of H2 evolution for different photocatalysts. Reaction condition: 50 mg catalyst, 100 mL aqueous Na2S (0.35 M) and Na2SO3 (0.25 M) solution, 300 W Xe lamp (λ N 400 nm).

rate of H2 strongly depends on the initial concentration of Cu2+ in the ion-exchange reaction. No H2 was formed when CZS0 was used as the photocatalyst. Generally, the H2 evolution rate increased with increasing Cu2 + content. The highest H2 evolution rate was reached when using CZS7 (rate = 1233.5 μmol h−1 g−1). Increasing the Cu2+ content above 7 mol%, however, led to decrease in H2 evolution activity. This may be because excessive CuS covering the surface of ZnS may shield the incident light [17], thereby reducing the active sites on the surface of ZnS and acting as charge recombination centers [26]. The mechanical mixture of 7 mol% CuS and ZnS showed negligible H2 evolution activity compared with CZS7. Therefore, we conclude that the heterojunction between ZnS and CuS promotes electron transfer and improves photocatalytic activity [17,27]. As the most efficient photocatalyst, the photocatalytic stability of CZS7 was further investigated by performing cycling photocatalytic experiments. After three cycling runs, no significant decrease in the H2 evolution rate was observed (Fig. 6). The slight decline in the third cycle may be due to the consumption of the sacrificial reagents. These results indicate that CuS/ZnS hexagonal plates have good stability for photocatalytic H2 evolution. 3.3. Photocatalytic mechanism The proposed mechanism for photocatalytic H2 evolution on CuS/ ZnS hexagonal plates under visible light irradiation is shown in Fig. 7. The positions of the conduction band and the valence band of ZnS can be determined with the equation ECB = χ − Ee − 0.5Eg, where χ is the electronegativity of the semiconductor (5.26 eV) [28], Ee is the energy of free electrons on the hydrogen scale ca. 4.5 eV [17], and Eg is the bandgap of ZnS (calculated as 3.34 eV from UV–Vis DRS results). The

Fig. 6. The time course of photocatalytic H2 evolution over CZS7. The reaction system was evacuated every four hours to remove generated H2.

calculated conduction and valence band positions of the as-prepared ZnS are − 0.91 and 2.43 eV, respectively. These values indicated that ZnS alone cannot be excited upon visible light irradiation. It is known that CuS shows no visible light photocatalytic activity for H2 generation. However, the as-prepared CuS/ZnS hexagonal plates show high H2 production rate under visible light irradiation. This is likely because the visible light initiates the IFCT as theoretically predicted by Creutz et al. [29]. Under light irradiation, electrons are photoexcited from the valence band of ZnS directly to CuS by IFCT. Similar IFCT phenomenon between ZnS and CuS components have been reported [17,24,30,31]. The transferred electrons cause a partial reduction of CuS to Cu2S. The potential of CuS/Cu2S is about −0.5 V (vs SHE, pH = 0) [17]. Therefore, the transferred electrons in CuS clusters provide enough energy to produce H2. Meanwhile, photogenerated electron–hole recombination events can be suppressed by transferring electrons to CuS/Cu2S clusters. The above factors account for the enhanced photocatalytic activity of the as-prepared CuS/ZnS hexagonal plates. To identify the chemical status of Cu in the CuS/ZnS hexagonal plates, X-ray photoelectron spectroscopy (XPS) was performed on photocatalyst CZS7 before and after photocatalytic cycling. Fig. 8(a) shows the XPS survey spectrum of sample CZS7. This spectrum clearly shows the existence of C, O, S, Zn and Cu elements in the sample. A trace amount of C and O may come from O2, H2O and CO2 adsorbed on the surface of sample [32]. Fig. 8(b) shows the high-resolution XPS spectra of Cu in the 2p region for CZS7 before and after three cycling runs, respectively. The sample before light irradiation has binding energies of Cu 2p3/2 and Cu 2p1/2 peaks located at 932.9 and 952.9 eV, respectively; these are typical values for Cu2+ in CuS [12,33]. After 12 h of visible light

Fig. 7. A schematic illustration of the band energy levels and charge transfer for the as-prepared CuS/ZnS hexagonal plates.

L. Wang et al. / Powder Technology 288 (2016) 103–108

107

Fig. 8. (a) The XPS survey spectrum of CZS7, (b) the Cu 2p region of the high-resolution XPS spectra of CZS7 before and after three cycling runs over 12 h under visible light irradiation. The inset in Fig. 8(b) is the Auger Cu LMM spectrum of CZS7 after three cycling runs.

irradiation, these binding energies shift to lower regions (932.5 eV for Cu 2p3/2 and 952.5 eV for Cu 2p1/2). This indicates the presence of Cu+ [17,19, 31]. Furthermore, the Auger Cu LMM spectrum (inset in Fig. 8(b)) shows the peak of Cu LMM at 917.6 eV which suggests that a small amount of Cu2+ in CuS is reduced to Cu2S after the light irradiation [34]. The reduced amount of electron–hole recombination events in the as-prepared CuS/ZnS hexagonal plates was proven by photoluminescence (PL) spectroscopy. Fig. 9 presents the room PL spectra of pure ZnS and CZS7 with an excitation wavelength of 350 nm. Both samples exhibit a broad band between 385 and 425 nm. However, the peak intensity of CZS7 is clearly diminished. This implies that the electron– hole pair in the excited CZS7 is efficiently separated due to IFCT between ZnS and CuS [35,36]. This shows that the electron–hole recombination is suppressed in the as-prepared CuS/ZnS hexagonal plate.

4. Conclusion CuS/ZnS hexagonal plates were synthesized via a solvothermal reaction followed by a hydrothermal cation-exchange reaction between ZnS(en)0.5 and aqueous solution of Cu(NO3)2. The prepared CuS/ZnS hexagonal plates showed high visible light photocatalytic H2 evolution activity without the need of a noble metal cocatalyst. The H2 evolution rate showed a strong dependence on the CuS content in the CuS/ZnS composite. The highest H2 evolution rate reached 1233.5 μmol h−1 g−which is almost 77 times higher than the rate obtained from the mechanical mixture of ZnS and CuS. This work demonstrates that the heterojunction between ZnS and CuS facilitates the interfacial charge transfer, prevents hole–electron pair recombination, and, ultimately, enhances the visible light-driven photocatalytic activity.

Fig. 9. The photoluminescence spectra of ZnS and CZS7.

Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 21171145). References [1] X.B. Chen, S.H. Shen, L.J. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [2] A. Iwase, Y.H. Ng, Y. Ishiguro, A. Kudo, R. Amal, Reduced graphene oxide as a solidstate electron mediator in Z-scheme photocatalytic water splitting under visible light, J. Am. Chem. Soc. 133 (2011) 11054–11057. [3] S.X. Ouyang, J.H. Ye, β-AgAl1 − xGaxO2 solid-solution photocatalysts: continuous modulation of electronic structure toward high-performance visible-light photoactivity, J. Am. Chem. Soc. 133 (2011) 7757–7763. [4] J. Lv, T. Kako, Z.S. Li, Z.G. Zou, J.H. Ye, Synthesis and photocatalytic activities of NaNbO3 rods modified by In2O3 nanoparticles, J. Phys. Chem. C 114 (2010) 6157–6162. [5] X. Zong, G.P. Wu, H.J. Yan, G.J. Ma, J.Y. Shi, F.Y. Wen, L. Wang, C. Li, Photocatalytic H2 evolution on MoS2/CdS catalysts under visible light irradiation, J. Phys. Chem. C 114 (2010) 1963–1968. [6] D.N. Ke, S.L. Liu, K. Dai, J.P. Zhou, L.N. Zhang, T.Y. Peng, CdS/regenerated cellulose nanocomposite films for highly efficient photocatalytic H2 production under visible light irradiation, J. Phys. Chem. C 113 (2009) 16021–16026. [7] F. Zuo, L. Wang, T. Wu, Z.Y. Zhang, D. Borchardt, P.Y. Feng, Self-doped Ti3+ enhanced photocatalyst for hydrogen production under visible light, J. Am. Chem. Soc. 132 (2010) 11856–11857. [8] X.S. Fang, T.Y. Zhai, U.K. Gautam, L. Li, L.M. Wu, Y. Bando, D. Golberg, ZnS nanostructures: from synthesis to applications, Prog. Mater. Sci. 56 (2011) 175–287. [9] T.K. Tran, W. Park, W. Tong, M.M. Kyi, B.K. Wagner, C.J. Summers, Photoluminescence properties of ZnS epilayers, J. Appl. Phys. 81 (1997) 2803–2809. [10] M. Saranya, C. Santhosh, R. Ramachandran, P. Kollu, P. Saravanan, M. Vinoba, S.K. Jeong, A.N. Grace, Hydrothermal growth of CuS nanostructures and its photocatalytic properties, Powder Technol. 252 (2014) 25–32. [11] M. Saranya, R. Ramachandran, E.J.J. Samuel, S.K. Jeong, A.N. Grace, Enhanced visible light photocatalytic reduction of organic pollutant and electrochemical properties of CuS catalyst, Powder Technol. 279 (2015) 209–220. [12] J.G. Yu, J. Zhang, S.W. Liu, Ion-exchange synthesis and enhanced visible-light photoactivity of CuS/ZnS nanocomposite hollow spheres, J. Phys. Chem. C 114 (2010) 13642–13649. [13] U.T.D. Thuy, N.Q. Liem, C.M.A. Parlett, G.M. Lalev, K. Wilson, Synthesis of CuS and CuS/ZnS core/shell nanocrystals for photocatalytic degradation of dyes under visible light, Catal. Commun. 44 (2014) 62–67. [14] X.W. Wang, Y.A. Li, M.R. Wang, W.J. Li, M.F. Chen, Y. Zhao, Synthesis of tunable ZnS– CuS microspheres and visible-light photoactivity for rhodamine B, New J. Chem. 38 (2014) 4182–4189. [15] X.H. Guan, P. Qu, X. Guan, G.S. Wang, Hydrothermal synthesis of hierarchical CuS/ ZnS nanocomposites and their photocatalytic and microwave absorption properties, RSC Adv. 4 (2014) 15579–15585. [16] B. Zeng, X.H. Chen, C.S. Chen, X.T. Ning, W.N. Deng, Reduced graphene oxides loaded-ZnS/CuS heteronanostructures as high-activity visible-light-driven photocatalysts, J. Alloys Compd. 582 (2014) 774–779. [17] J. Zhang, J.G. Yu, Y.M. Zhang, Q. Li, J.R. Gong, Visible light photocatalytic H2production activity of CuS/ZnS porous nanosheets based on photoinduced interfacial charge transfer, Nano Lett. 11 (2011) 4774–4779. [18] Y.X. Li, G. Chen, Q. Wang, X. Wang, A.K. Zhou, Z.Y. Shen, Hierarchical ZnS–In2S3–CuS nanospheres with nanoporous structure: facile synthesis, growth mechanism, and excellent photocatalytic activity, Adv. Funct. Mater. 20 (2010) 3390–3398. [19] E. Hong, D. Kim, J.H. Kim, Heterostructured metal sulfide (ZnS–CuS–CdS) photocatalyst for high electron utilization in hydrogen production from solar water splitting, J. Ind. Eng. Chem. 20 (2014) 3869–3874.

108

L. Wang et al. / Powder Technology 288 (2016) 103–108

[20] Z.X. Deng, C. Wang, X.M. Sun, Y.D. Li, Structure-directing coordination template effect of ethylenediamine in formations of ZnS and ZnSe nanocrystallites via solvothermal route, Inorg. Chem. 41 (2002) 869–873. [21] Y.H. Ni, X.F. Cao, G.Z. Hu, Z.S. Yang, X.W. Wei, Y.H. Chen, J. Xu, Preparation, conversion, and comparison of the photocatalytic and electrochemical properties of ZnS(en)0.5, ZnS, and ZnO, Cryst. Growth Des. 7 (2007) 280–285. [22] S.H. Yu, M. Yoshimura, Shape and phase control of ZnS nanocrystals: template fabrication of wurtzite ZnS single-crystal nanosheets and ZnO flake-like dendrites from a lamellar molecular precursor ZnS(NH2CH2CH2NH2)0.5, Adv. Mater. 14 (2002) 296–300. [23] L.J. Zhang, T.F. Xie, D.J. Wang, S. Li, L.L. Wang, L.P. Chen, Y.C. Lu, Noble-metal-free CuS/CdS composites for photocatalytic H2 evolution and its photogenerated charge transfer properties, Int. J. Hydrog. Energy 38 (2013) 11811–11817. [24] H. Irie, K. Kamiya, T. Shibanuma, S. Miura, D.A. Tryk, T. Yokoyama, K. Hashimoto, Visible light-sensitive Cu(II)-grafted TiO2 photocatalysts: activities and X-ray absorption fine structure analyses, J. Phys. Chem. C 113 (2009) 10761–10766. [25] M. Liu, X.Q. Qiu, M. Miyauchi, K. Hashimoto, Cu(II) oxide amorphous nanoclusters grafted Ti3+ self-doped TiO2: an efficient visible light photocatalyst, Chem. Mater. 23 (2011) 5282–5286. [26] X. Zong, J.F. Han, G.J. Ma, H.J. Yan, G.P. Wu, C. Li, Photocatalytic H2 evolution on CdS loaded with WS2 as cocatalyst under visible light irradiation, J. Phys. Chem. C 115 (2011) 12202–12208. [27] X. Zong, H.J. Yan, G.P. Wu, G.J. Ma, F.Y. Wen, L. Wang, C. Li, Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as cocatalyst under visible light irradiation, J. Am. Chem. Soc. 130 (2008) 7176–7177.

[28] Y. Xu, M.A.A. Schoonen, The absolute energy positions of conduction and valence bands of selected semiconducting minerals, Am. Mineral. 85 (2000) 543–556. [29] C. Creutz, B.S. Brunschwig, N. Sutin, Interfacial charge-transfer absorption: 3. Application to semiconductor-molecule assemblies, J. Phys. Chem. B 110 (2006) 25181–25190. [30] X.Q. Qiu, M. Miyauchi, H.G. Yu, H. Irie, K. Hashimoto, Visible-light-driven Cu(II)(Sr1 − yNay)(Ti1 − xMox)O3 photocatalysts based on conduction band control and surface ion modification, J. Am. Chem. Soc. 132 (2010) 15259–15267. [31] M. Lee, K. Yong, Highly efficient visible light photocatalysis of novel CuS/ZnO heterostructure nanowire arrays, Nanotechnology 23 (2012) 194014. [32] J.G. Yu, Y. Hai, B. Cheng, Enhanced photocatalytic H2-production activity of TiO2 by Ni(OH)2 cluster modification, J. Phys. Chem. C 115 (2011) 4953–4958. [33] J. Ghijsen, L.H. Tjeng, J. van Elp, H. Eskes, J. Westerink, G.A. Sawatzky, M.T. Czyzyk, Electronic structure of Cu2O and CuO, Phys. Rev. B 38 (1988) 11322–11330. [34] J. Shan, P. Pulkkinen, U. Vainio, J. Maijala, J. Merta, H. Jiang, R. Serimaa, E. Kauppinen, H. Tenhu, Synthesis and characterization of copper sulfide nanocrystallites with low sintering temperatures, J. Mater. Chem. 18 (2008) 3200–3208. [35] L. Ge, C.C. Han, X.L. Xiao, L.L. Guo, Synthesis and characterization of composite visible light active photocatalysts MoS2–g-C3N4 with enhanced hydrogen evolution activity, Int. J. Hydrog. Energy 38 (2013) 6960–6969. [36] K. Zhang, D.W. Jing, Q.Y. Chen, L.J. Guo, Influence of Sr-doping on the photocatalytic activities of CdS–ZnS solid solution photocatalysts, Int. J. Hydrog. Energy 35 (2010) 2048–2057.