Enhanced photocatalytic activity over Cd0.5Zn0.5S with stacking fault structure combined with Cu2+ modified carbon nanotubes

Applied Surface Science 365 (2016) 280–290

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Enhanced photocatalytic activity over Cd0.5 Zn0.5 S with stacking fault structure combined with Cu2+ modified carbon nanotubes Beini Gong a,b,1 , Yonghong Lu a,b,1 , Pingxiao Wu a,b,c,∗ , Zhujian Huang a,b , Yajie Zhu a,b , Zhi Dang a,b , Nengwu Zhu a,b,c , Guining Lu a,b , Junyi Huang a,b a

School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangzhou 510006, PR China c Guangdong Provincial Engineering and Technology Research Centre for Environment Risk Prevention and Emergency Disposal, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China b

a r t i c l e

i n f o

Article history: Received 11 October 2015 Received in revised form 31 December 2015 Accepted 31 December 2015 Available online 4 January 2016 Keywords: CdZnS Photocatalytic activity Hydrogen production Stacking fault Carbon nanotube

a b s t r a c t For enhanced photocatalytic performance of visible light responsive CdZnS, a series of Cd0.5 Zn0.5 S solid solutions were fabricated by different methods. It was found that the semiconductor obtained through the precipitation-hydrothermal method (CZS-PH) exhibited the highest photocatalytic hydrogen production rate of 2154 ␮mol h−1 g−1 . The enhanced photocatalytic hydrogen production of CZS-PH was probably due to stacking fault formation as well as narrow bandgap, a large surface area and a small crystallite size. Based on this, carbon nanotubes modified with Cu2+ (CNTs (Cu)) were used as a cocatalyst for CZS-PH. The addition of CNTs (Cu) enhanced notably the absorption of the composites for visible light. The highest photocatalytic hydrogen production rate of the Cd0.5 Zn0.5 S-CNTs (Cu) composite was 2995 ␮mol h−1 g−1 with 1.0 wt.% of CNTs (Cu). The improvement of the photocatalytic activity by loading of CNTs (Cu) was not due to alteration of bandgap energy or surface area, and was probably attributed to suppression of the electron-hole recombination by the CNTs, with Cu2+ anchored in the interface optimizing the photogenerated electron transfer pathway between the semiconductor and CNTs. We report here the successful combination of homojunction and heterojunction in CdZnS semiconductor, which resulted in promotion of charge separation and enhanced photocatalytic activity. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Since the presentation of the concept of photoelectrochemically splitting water into H2 and O2 by Honda and Fujishima, photocatalytic hydrogen production under solar light irradiation has been attracting vast attention, being considered as one of the most promising ways to solve the energy crisis and environmental pollution simultaneously [1,2]. In the past decades, numerous semiconductor photocatalysts have been designed and developed for the photocatalytic hydrogen generation reaction, among which metal sulfides are regarded as the good ones because of their suitable band gap and catalytic function [3–5]. The Cdx Zn1−x S solid solution with a visible-light response is widely applied in photocatalytic reaction, the bandgap energy and absorption edge of which can be tuned by changing the molar ratio of ZnS and CdS [6,7]. In the past years, many methods, including solvothermal,

∗ Corresponding author. Tel.: +86 20 39380538; fax: +86 20 39383725. E-mail address: [email protected] (P. Wu). 1 These authors share first authorship. http://dx.doi.org/10.1016/j.apsusc.2015.12.239 0169-4332/© 2016 Elsevier B.V. All rights reserved.

thermolysis, hydrothermal, precipitation-hydrothermal methods, ultrasonic- and microwave-assisted methods have been taken to prepare Cdx Zn1-x S solid solutions with different electrical properties and photocatalytic activities [6,8–13]. The separation and transport of charge carriers play a key role in determining the photocatalytic activity of semiconductors. Various strategies have been investigated to prevent the recombination of photo-generated electron–hole pairs and to facilitate the transport of the charge carriers to the reaction surface for the catalysts’ optimal activity and higher photostability, including homojunction and heterojunction formation [11,14–16]. Homojunction refers to special crystal structures such as nano-twin structures or stacking faults [17], while heterojunction generally refers to coupling of two different materials to form hybrid photocatalysts, such as combining semiconductors with other types of semiconductors, noble metals or graphene [12,18,19], which have been proved to be successful in the separation of charge carriers and enhancement of photocatalytic conversion. In recent years, visible light-driven semiconductors combining carbon nanotubes (CNTs) as the cocatalysts have attracted considerable attention due to the high work functions (4.3–5.1 eV) and the large electron-storage capacity (one electron

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for every 32 carbon atoms) of CNTs to act as the electron acceptors so as to retard the charge recombination, what’s more, CNTs can act as photosensitizer to enhance the visible light absorption of the semiconductors. Even though it has been reported that excessive amount of CNTs added into the composite may lower the photocatalytic activity of the composite due to the “shielding effect” [20–23], the introduction of a small amount of metal ions into the interfacial layer matrix of graphene and the semiconductor as a “mediator” has been shown to be able to counteract the negative “shielding effect” and significantly improve the selective photoredox reaction [24]. The aim of this study is to synthesize the visible light responsive Cd0.5 Zn0.5 S with enhanced photocatalytic activity. Cd0.5 Zn0.5 S photocatalyst was prepared by four different methods, the properties (morphology, structure) and photocatalytic hydrogen production under visible light irradiation of the composites were analyzed. The method producing the composite with stacking faults and the highest photocatalytic activity was chosen to synthesize Cd0.5 Zn0.5 S loaded with Cu2+ modified CNTs, the photocatalytic hydrogen production of which was further enhanced. 2. Material and methods 2.1. Materials Cd(CH3 COO)2 ·2H2 O, Zn(CH3 COO)2 ·2H2 O, Cu(CH3 COO)2 ·2H2 O and thiourea (CH4 N2 S) were obtained from Aladdin Reagent Database Inc (Shanghai, PRC). NaOH and DMSO (C2 H6 OS) were purchased from Guangzhou Chemical Reagent Factory (Guangzhou, PRC). Carboxylic multiwalled carbon nanotubes were purchased from XFNANO Material Technologic Co. Ltd (Nanjing, PRC). 2.2. Synthesis of C0.5 Zn0.5 S (1) Preparation by the solvothermal method: Cd(CH3 COO)2 ·2H2 O (4.0 mmol) and Zn(CH3 COO)2 ·2H2 O (4.0 mmol) were dispersed in 50 mL of DMSO by ultrasonication. The mixture was transferred to a 100 mL Teflon-lined autoclave and treated at 180 ◦ C for 12 h. After cooling, the product was centrifugated, washed with deionized water for 5 times and vacuum dried at 80 ◦ C overnight. The composite obtained was denoted as CZS-Sol. (2) Preparation by the thermolysis method: Cd(CH3 COO)2 ·2H2 O (1.25 mmol) and Zn(CH3 COO)2 ·2H2 O (1.25 mmol) were dissolved in 30 mL of ethanol at room temperature, while 10 mmol of thiourea was dissolved separately in 30 mL of ethanol under constant stirring at 50 ◦ C. The two solutions were then mixed thoroughly and transferred to a quartz boat and dried at 80◦ C, the powder obtained was calcined at 300 ◦ C for 2 h in nitrogen atmosphere. The composite obtained was denoted as CZS-Therm. (3) Preparation by hydrothermal method: Cd(CH3 COO)2 ·2H2 O (10.0 mmol), Zn(CH3 COO)2 ·2H2 O (10.0 mmol) and thiourea (25.0 mmol) were dissolved in 130 mL of deionized water. The solution was further stirred for 60 min and then sealed in a 250 mL Teflon-lined autoclave and heated to 160 ◦ C for 8 h. The solution was then cooled to room temperature, washed by deionized water for 5 times through centrifugation and vacuum dried at 80 ◦ C overnight. The product obtained was denoted as CZS-H. (4) Preparation by the precipitation-hydrothermal method: 10 mL of NaOH solution (1.0 mol L−1 ) was added dropwise into an aqueous solution (40 mL) containing Cd(CH3 COO)2 ·2H2 O (10.0 mmol), Zn(CH3 COO)2 ·2H2 O (10.0 mmol) and thiourea (25.0 mmol) under magnetic stirring. The volume of the mixed solution was adjusted to 130 mL by deionized water, then the mixture was stirred for 60 min, sealed in a 250 mL Teflon-lined autoclave and heated at 160 ◦ C for 8 h. The product was then

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cooled and washed by deionized water for 5 times and vacuum dried at 80 ◦ C overnight. The obtained product was denoted as CZS-PH.

2.3. Preparation of CNTs (Cu) 0.1 g of carboxylic multiwalled carbon nanotubes was ultrasonically dispersed in 150 mL of copper acetate aqueous solution (0.1 mol L−1 ) and magnetically stirred overnight. The reaction mixture was washed with deionized water for 5 times, the solid was collected by suction filtration, and then dried at 120 ◦ C for 24 h. 2.4. Preparation of CZS-PH loaded with CNTs(Cu) CZS-PH loaded with CNTs (Cu) was prepared with the precipitation-hydrothermal method in the presence of CNTs (Cu). Briefly, 2.195 g of Zn(CH3 COO)2 ·2H2 O (10 mmol), 2.665 g of Cd(CH3 COO)2 ·2H2 O (10 mmol), 1.903 g of thiourea (25 mmol) and certain amount of CNTs (Cu) was dispersed in 40 mL deionized water under magnetic stirring. Afterwards the mixture was added with 10 mL of NaOH solution (1.0 mol L−1 ). The total volume of the mixture was adjusted to 130 mL, transferred to a Teflon-lined autoclave, heated and dried as described in Section 2.2. The weight percentages of CNTs (Cu) in the composites were 0.5, 1.0 and 1.5, and the corresponding composites were denoted as CNTs (Cu)0.5 wt.%, CNTs (Cu)-1.0 wt.% and CNTs (Cu)-1.5 wt.%. 2.5. Characterization The X-ray diffraction (XRD) patterns of the composites were obtained on a X-ray diffractometer (Bruker D8-ADVANCE) using Cu K␣ radiation ( = 0.154 nm) with a scan range between 5 and 90◦ and a step size of 0.02◦ . The morphology of the prepared composites were observed by a high resolution transmission electron microscopy (HR-TEM) (JEOL JEM-3010). The photoluminescence spectra of the composites were obtained on an Edinburgh luminescence spectrophotometer (FLS920). The X-ray photoelectron spectra (XPS) were measured with an Axis Ultra DLD X-ray photoelectron spectrometer and C1s (284.6 eV) was chosen as the reference line. The pass energy was 40 eV and conventional Al K␣ (1486.6 eV) anode radiation source was used as the excitation source. Specific surface area (BET surface area) was determined by N2 adsorption-desorption of nitrogen at 77 K using a Micromeritics ASAP 2020 surface area and porosity analyzer. Solid-state UV–vis diffuse reflectance spectra (UV–vis DRS) were acquired at room temperature in air using a SHIMADZU UV-2450 spectrophotometer equipped with an integrating spheric attachment using BaSO4

Fig. 1. The hydrogen production rates of Cd0.5 Zn0.5 S prepared by different methods.

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Fig. 2. XRD patterns of (a) Cd0.5 Zn0.5 S prepared by different methods; (b) CZS-PH, CNTs (Cu)-0.5 wt.%, CNTs (Cu)-1.0 wt.% and CNTs (Cu)-1.5 wt.%.

as background. The Raman spectra were obtained at room temperature using a micro-Raman spectrometer (Renishaw InVia) in a backscattering configuration with a 632 nm He–Ne laser as the excitation source. 2.6. Photoelctrochemical measurement The working electrodes were prepared as follows [25]: photocatalyst (20 mg) was dispersed in Nafion perfluorinated resin solution (10 ␮L) and ethanol (1 mL) to make a slurry. The slurry was then coated onto a 2 cm × 1.2 cm ITO glass electrode by the doctor blade method. The electrodes were dried in an oven for 8 h.

Photocurrents were measured using an electrochemical analyzer (CHI 660D Instrument) with a standard three-electrode system using the obtained samples as the working electrodes with an active area about 1.2 cm2 , a Pt wire as the counter electrode and Ag/AgCl as a reference electrode. The light was produced by a 300 W Xe arc lamp, with 0.2 M Na2 S + 0.04 M Na2 SO3 mixed aqueous solution as the electrolyte. 2.7. Electrochemical impedance spectroscopy (EIS) measurement The electrochemical impedance spectroscopy (EIS) experiments were conducted on a CHI660E workstation (Shanghai Chenhua

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Table 1 Physicochemical properties and photocatalytic activities of the as-synthesized composites. Samples

Crystallite size (nm)

SBET (m2 g−1 )

Total pore volume (cm3 g−1 )

Average pore size (nm)

Bandgap energy (eV)

Rate of H2 evolution (␮mol g−1 h−1 )

CZS-Sol CZS-Therm CZS-H CZS-PH CNTs (Cu)-0.5 wt.% CNTs (Cu)-1.0 wt.% CNTs (Cu)-1.5 wt.%

7.79 9.42 25.95 8.28 10.45 7.96 10.28

47.01 2.63 0.44 38.99 37.62 34.91 34.77

0.05 0.03 – 0.22 0.17 0.13 0.13

3.7 15.5 – 13.9 9.8 9.1 9.2

2.41 2.32 2.23 2.26 2.27 2.23 2.23

824 1132 72 2154 2856 2995 2106

Fig. 3. UV–-vis diffuse reflectance spectra of (a) Cd0.5 Zn0.5 S prepared by different methods and (b) Cd0.5 Zn0.5 S prepared by the precipitation-hydrothermal method with different amount of CNTs (Cu) loaded.

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Fig. 4. TEM images of (a) CZS-Sol; (b) CZS-Therm; (c) CZS-H and (d) CZS-PH. .

instrument, China) in the electrolyte of 0.1 M KCl aqueous solution containing 5 mM K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (1:1) under open circuit potential conditions. 2.8. Photocatalytic tests Photocatalytic hydrogen production experiments were carried out in an air free 350 mL cylindrical reaction cell made of quartz and the opening was sealed by a quartz cover with a silicone rubber washer. A Xe arc lamp (PLS-SXE) of 300 W equipped with a UV-cutoff filter ( ≥ 420 nm) was positioned 15 cm away from the reaction mixture as visible light source. The photocatalyst (75 mg) was dispersed in an aqueous solution (150 mL) with 0.35 M Na2 S and 0.25 M Na2 SO3 . Oxygen was removed by bubbling with nitrogen through the cell prior to irradiation. The amount of H2 generated was determined by a gas chromatograph (NaX zeolite column, TCD detector, N2 as carrier gas). 3. Results and discussion 3.1. Hydrogen production and characterization of Cd0.5 Zn0.5 S prepared by various methods The photocatalytic hydrogen production activity of the catalysts synthesized by different methods was examined under visible light irradiation in an aqueous solution with 0.35 M Na2 S and 0.25 M Na2 SO3 as sacrificial reagents to consume the photogenerated holes. Control experiments showed that no hydrogen

was detected in the absence of either visible light illumination or the photocatalyst (data not shown). It was found that CZSPH exhibited the highest photocatalytic activity with a hydrogen production rate of 2154 ␮mol h−1 g−1 compared with that of CZSSol (824 ␮mol h−1 g−1 ), CZS-Therm (1132 ␮mol h−1 g−1 ) and CZS-H (72 ␮mol h−1 g−1 ) (Fig. 1). The crystal structures of the prepared composites were investigated with XRD. It is observed in Fig. 2a that XRD peaks of the prepared composites didn’t agree with the patterns of pure CdS (PDF # 41-1049) or ZnS (PDF # 05-0566), indicating solid solution formation. The three main diffraction peaks of CZS-Sol which were located at 2␪ = 26.7◦ , 44.3◦ and 51.9◦ corresponded to the (1 1 1), (2 0 0) and (3 1 1) plane, respectively, suggesting the cubic sphalerite structure of CZS-Sol [13,26]. In contrast to CZS-Sol, the patterns of CZS-Therm, CZS-H and CZS-PH implied the mixture of cubic and hexagonal structures, and the proportion of hexagonal wurtzite structure is higher in CZS-PH in comparison with CZS-H. According to the Scherrer equation, the average crystallite sizes of CZS-Sol, CZS-Therm, CZS-H and CZS-PH were estimated to be 7.79, 9.42, 25.95 and 8.28 nm, respectively (Table 1). The UV-vis diffuse reflectance spectra (DRS) of CZS-Sol, CZSTherm, CZS-H and CZS-PH were displayed in Fig. 3a. It was observed that the absorption edge of both CZS-H and CZS-PH showed a very slight red shift compared with that of CZS-Sol and CZS-Therm. According to the Kubelka–Munk function, the bandgaps of CZS-Sol, CZS-Therm, CZS-H and CZS-PH were estimated to be 2.41, 2.32, 2.23 and 2.26 eV (Table 1). The smaller bandgap of CZS-PH presumably contributed to the higher photocatalytic hydrogen production rate

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Fig. 5. HRTEM images of (a) CZS-Sol; (b) CZS-Therm; (c) CZS-H and (d) CZS-PH.

of CZS-PH. However, as was shown in Fig. 1, the hydrogen production rate of CZS-H with the narrowest bandgap was the lowest among the four types of Cd0.5 Zn0.5 S prepared by different methods, which was probably due to the heavy aggregation (Fig. 4c) and the small surface area of CZS-H (Table 1). TEM observations revealed the existence of heterogeneous nanocrystals of CZS-Sol with the smallest size (Fig. 4a), which is in agreement with the values calculated by Scherrer equation (Table 1). The nanoparticles of CZS-Therm and CZS-H were heavily aggregated (Fig. 4b and c). The total pore volume and average pore size could not be detected for CZS-H due to severe aggregation. As is observed in Fig. 4d (inset), the solid nanospheres of CZS-PH were composed of many small crystallites with different orientations and sizes ranging from 8 to 10 nm. The HRTEM image showed the lattice fringes of CZS-PH with a d spacing of 0.31 nm which can be assigned to the (1 1 1) plane of the zinc-blende lattice (Fig. 5d). A lot of non-uniformed zigzag structures were found in the HRTEM image of CZS-PH, indicating the existence of twin-like structure or stacking faults, while no such structure was observed in CZS-Sol, CZS-Therm, and CZS-H (Fig. 5a, b and c). According to the literatures, a hexagonal phase is constituted by stacking of ABABAB layers, while a cubic phase is formed by stacking of ABCABC layers [27,28]. Therefore, the transformation in structure from cubic to hexagonal and vice versa is induced by stacking faults which can change the stacking sequence. The formation of stacking faults and especially the twins inside the photocatalyst can not only suppress the random scattering of photo-generated carriers, but also prevent the recombination of the electrons and holes, which may be crucial for CZS-PH

to show higher activity in photocatalytic hydrogen production [11,13,14,17]. The photoluminescence (PL) spectra of Cd0.5 Zn0.5 S prepared by different methods (Fig. 6) showed that CZS-PH exhibited the highest PL intensity quenching degree, confirming the efficient separation of electron-hole pairs [24,29]. It has been proposed that the twin plane with highly coordinated atoms has lower formation energy compared with the normal

Fig. 6. Photoluminescence spectra of Cd0.5 Zn0.5 S prepared by different methods.

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crystal facets, and ligands such as OH− or EN (ethylenediamine) are crucial in the hydrothermal formation of twinning structure of C0.5 Zn0.5 S [14]. In this study, OH− can act as the coordinating agent for Cd2+ and Zn2+ thus favored the formation of twin-like structure or stacking faults. We have also demonstrated in a previous report that the amount of OH− is a critical factor determining the formation of stacking faults in Cd0.5 Zn0.5 S and the photocatalytic hydrogen production activity [30], which also implied the positive role of stacking faults in enhanced photocatalytic activity. Apart from the stacking fault structure, the narrow bandgap, small crystallite size and large surface area of CZS-PH (Table 1) may also contribute to the higher photocatalytic performance of the composite. These properties are beneficial for the photo-excitation of electrons, migration of conduction band electrons to the reaction surface and more reaction sites on the surface. 3.2. Fabrication of composite photocatalyst by loading CNTs (Cu) The above results showed that C0.5 Zn0.5 S prepared by the precipitation-hydrothermal method exhibited the highest photocatalytic activity for hydrogen production due to stacking faults formation, we then developed a composite based on the precipitation-hydrothermal method by using Cu2+ modified carbon nanotubes as the cocatalyst to further improve the photocatalytic activity of the composites. Carboxylic multiwalled carbon nanotubes was modified with Cu2+ (CNTs (Cu)) and the high resolution XPS spectrum of Cu in CNTs (Cu) showed that the binding energies of C u 2p3/2 and Cu 2p1/2 peaks are located at 932.7 and 952.8 eV, respectively (Fig. 7a), indicating that Cu2+ was loaded on the surface of CNTs by electrostatic interaction with the carboxyl group. The C 1s spectra of CNTs (Cu) were shown in Fig. 7b. The spectra were deconvoluted into two peaks at 284.5 and 285.8 eV that were assigned to the graphite carbon (C C) and the hydrocarbons (C H) in CNTs [31]. The peaks corresponding to C O and C O were missing probably because the content of carboxyl groups in the CNTs used was low, which was about 3 wt.%. Fig. 2b showed the XRD patterns of CZS-PH and CNTs (Cu)r wt.% (r = 0.5, 1, 1.5). Hardly any difference of the crystal structure between the composites was detected, indicating the loading of CNTs (Cu) does not have a significant impact on phase structure and crystallinity of the solid solution which was possibly due to the low loading amount of CNTs (Cu) or partial overlapping of the diffraction peaks for CNTs (Cu) and Cd0.5 Zn0.5 S [22,23,32,33]. According to the Scherrer equation, the average crystallite sizes of CNTs (Cu)r wt.% (r = 0.5, 1, 1.5) were estimated to be 10.45, 7.96 and 10. 28 nm, respectively (Table 1). Raman peaks of CNTs (Cu)-1.0 wt.% at 1343 cm−1 and 1578 cm−1 in Fig. 8 corresponded to the D-band (Disordered carbon band) and G-band (Graphite carbon band) of the carboxylic multiwalled carbon nanotubes [34], suggesting that CNTs were successfully loaded onto Cd0.5 Zn0.5 S. The most intense Raman band at 308 cm−1 and a less intense band at 610 cm−1 were assigned to the longitudinal optical phonon (1 LO and 2 LO, respectively) peaks of Cd0.5 Zn0.5 S [22]. TEM image of CNTs(Cu)-1.0 wt.% showed that the CNTs (Cu) (red arrows) were wrapped around by the nanocrystals of CZS-PH (Fig. 9a) [35]. The intimate attachment between the CNTs (Cu) and the catalyst CZS-PH indicated that electric communication between them was possible. The stacking fault structure was observed in the HRTEM image of CNTs(Cu)-1.0 wt.% in Fig. 9b, indicated by white arrows. The UV-vis diffuse reflectance spectra clearly demonstrated that with increased amount of CNTs (Cu) loaded, the absorption capacity of the composites for visible light ( > 550 nm) gradually increased (Fig. 3b), which was attributed to the light absorption by CNTs. According to the Kubelka–Munk function, the bandgaps of CZS-PH,

Fig. 7. High-resolution XPS spectrum of (a) Cu 2p and (b) C 1s in CNT (Cu).

Fig. 8. Raman spectra of CZS-PH and CNTs (Cu)-1.0 wt.%.

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Fig. 9. (a) TEM and (b) HRTEM images of CNTs (Cu)-1.0 wt.%.

CNTs (Cu)-0.5 wt.%, CNTs (Cu)-1.0 wt.% and CNTs (Cu)-1.5 wt.% were estimated to be 2.26, 2.27, 2.23 and 2.23 eV (Table 1), suggesting that CNTs (Cu) was not incorporated into the lattice of CZS-PH. The surface area and pore structure of the samples were analyzed by N2 adsorption–desorption measurements in Fig. 10, it can be observed that the N2 adsorption–desorption isotherms of all the as-prepared composites were of type IV with H3 hysteresis loops, implying the existence of mesopores (2–100 nm), which can be further confirmed by the corresponding pore size distribution curves in the insets. Furthermore, as can be seen in Table 1, the surface areas, total pore volumes and average pore sizes of Cd0.5 Zn0.5 S-CNTs (Cu) composites showed slight reduction compared with CZS-PH.

3.3. Enhanced hydrogen generation of the photocatalyst by loading of CNTs (Cu) Time profiles of hydrogen generation by the synthesized composites under visible light irradiation were shown in Fig. 11. It can be observed that the photocatalysts loaded with CNTs (Cu) as a cocatalyst exhibited higher activity for hydrogen production than CZS-PH without CNTs (Cu) loading, and the amounts and rates of hydrogen production increased at first with increasing CNTs (Cu) content, achieving a maximum rate of 2995 ␮mol g−1 h−1 (Table 1) at 1.0 wt.% of CNTs (Cu) content, which was 1.4 times the rate of CZSPH (2154 ␮mol g−1 h−1 ). A further increase of the CNTs (Cu) content to 1.5 wt.% caused a significant decrease in hydrogen production,

Fig. 10. N2 adsorption and desorption isotherms and the corresponding pore-size distribution curves (inset) of (a) CZS-PH, (b) CNTs (Cu)-0.5 wt.%, (c) CNTs (Cu)-1.0 wt.% and (d) CNTs (Cu)-1.5 wt.%.

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Fig. 13. Electrochemical impedance spectroscopy (EIS) Nyquist diagrams of CZS-PH and CNTs (Cu)-1.0 wt.%. Fig. 11. The hydrogen production ability of the synthesized composites under visible light irradiation.

which was reduced to a level comparable to that of CZS-PH without CNTs (Cu) loading. The reduction is probably due to the excessive amount of opaque CNTs (Cu) shielding the light [18,23]. The photocatalytic performance of CZS-PH loaded with 1.0 wt.% of CNTs without the addition of Cu2+ hardly displayed any enhancement of hydrogen production compared with CZS-PH, confirming that Cu2+ played an vital role in improving the photocatalytic activity of CZSPH loaded with CNTs (Cu). For all the catalysts, the rate of hydrogen production increased obviously after 1 h of photoreaction due to the “activation effect” [13]. The enhancement of the photocatalytic activity by loading of CNTs (Cu) was not attributed to differences in the surface area or porosity of the synthesized composites as was shown in Table 1 that the surface area and pore volume of the composites loaded with CNTs (Cu) was somewhat reduced, indicating that the interfacial contact between Cd0.5 Zn0.5 S and CNTs may be of great importance for the enhanced photocatalytic activity. In order to determine the charge separation and transfer efficiency in the composites, transient photocurrent (TPC) responses of the as-synthesized composites were recorded over several visible light irradiation on and off cycles at a bias potential of 0.5 V. As can be seen in Fig. 12, both CZS-PH and CNTs (Cu)-1.0 wt.%

Fig. 12. Transient photocurrent responses of CZS-PH and CNTs (Cu)-1.0 wt.% in 0.2 M Na2 S + 0.04 M Na2 SO3 mixed aqueous solution under visible light irradiation.

showed instant and reproducible photocurrent responses when the light was turned on, suggesting that the photo-excited electrons were transported to the back contact across the catalyst to produce photocurrent under visible light irradiation [36]. In comparison with CZS-PH, a relatively slower response can be observed in CNTs(Cu)-1.0 wt.% when the light was switched on and off which was probably due to the transfer and trapping of conduction band electrons onto CNTs (Cu). Upon visible light irradiation, a part of conduction band electrons may first fill the traps on CNTs (Cu) with a lower Fermi level instead of being immediately transported to the back contact of the electrode, resulting in a relatively slow increase of photocurrent; and the charge carriers may be slowly released from the traps when the light was turned off, leading to a gradual decrease of the photocurrent [18,25]. The photocurrent value of CNTs (Cu)-1.0 wt.% is higher than that of CZS-PH, indicating that CNTs (Cu) in the composite can store the electrons and suppress the recombination of electron-hole pairs, thus enhancing the photocurrent response [37]. The EIS measurements were performed on CZS-PH and CNTs (Cu)-1.0 wt.% to investigate the charge carrier migration of the prepared composites (Fig. 13). The arc radius of CNTs (Cu)-1.0 wt.% was smaller than that of CZS-PH, which evidenced that CNTs (Cu)1.0 wt.% has lower interfacial charge-transfer resistance and more efficient interfacial transfer of charge carriers. The results further confirmed the crucial role of CNTs (Cu) in the process of charge carrier separation and transfer [33,38]. The mechanism of the composite prepared by precipitationhydrothermal method in the presence of CNTs (Cu) with optimized photocatalytic activity was illustrated in Fig. 14. Under visible light irradiation, electrons are excited from the valence band (VB) to the conduction band (CB), with holes created in VB at the same time. The stacking faults in the nanocrystals of Cd0.5 Zn0.5 S formed during preparation promoted the separation of photo-generated electrons and holes, thus enhancing the photocatalytic hydrogen production of Cd0.5 Zn0.5 S. The impurity level of copper in the +2 oxidation state can act as holes to effectively capture the electrons of the host semiconductor at conduction band and help optimize the transfer pathway of photogenerated electrons across the interface between Cd0.5 Zn0.5 S and CNTs. Instead of recombining with holes rapidly, the photogenerated electrons may be captured by copper ions and transferred to the surface of CNTs, thus improving the electronhole separation and prolonging the lifetime of the charge carriers. Furthermore, the high electrical conductivity of CNTs can accelerate the movement of the electrons which in turn enhance the photocatalytic hydrogen production. Therefore, the stacking fault

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possessed the optimal photocatalytic activity with a hydrogen production rate of 2154 ␮mol h−1 g−1 . Carboxylic multiwalled carbon nanotubes modified with Cu2+ was loaded on the solid solution to further increase the rate of photocatalytic hydrogen production to 2995 ␮mol h−1 g−1 . Compared with CZS-PH, CZS-PH loaded with CNTs (Cu) has higher absorption capacity for visible light and reduced level of electron-hole recombination which was attributed to the presence of CNTs for enhanced light absorption and improved photogenerated electron transportation across the interface between Cd0.5 Zn0.5 S and CNTs by anchoring Cu2+ . Acknowledgements The authors are grateful for financial support from the National Science Foundation of China (Grant No. 41472038, 41273122, 41073058), the Science and Technology Plan of Guangdong Province, China (No. 2014A020216002) and the Fundamental Research Funds for the Central Universities, SCUT(No. 2015ZP007).

Fig. 14. Schematic illustration of the enhanced photocatalytic H2 production by the Cd0.5 Zn0.5 S-CNTs (Cu) composite.

Fig. 15. Repeated time courses of the photocatalytic hydrogen production over CZSPH and CNTs (Cu)-1.0 wt.%.

structure together with the introduction of a tiny amount of copper ions into the interface between Cd0.5 Zn0.5 S and CNTs led to efficient suppression of the electron-hole pairs recombination and significant improvement of the photocatalytic performance of the composite. 3.4. Stability test The photocatalytic stability of CZS-PH and CNTs (Cu)-1.0 wt.% under visible light irradiation was tested. As was displayed in Fig. 15, after three cycles of photoreaction process, no decrease in the photocatalytic performance of the catalysts was observed, suggesting that neither CZS-PH nor CNTs (Cu)-1.0 wt.% was photocorroded. 4. Conclusions A series of Cd0.5 Zn0.5 S solid solution photocatalysts with different crystal sizes, phases, and morphologies were obtained by four methods, among which CZS-PH prepared by the precipitationhydrothermal method with stacking faults in its crystal structure

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