An Efficient Noble-Metal-Free Photocatalyst for Visible-Light-Driven

Nov 14, 2016 - On the other hand, in the field of photocatalytic H2 evolution, noble metals such as Pt(25) and .... The amounts of hydrogen produced w...
0 downloads 0 Views 3MB Size
Download PDF
Research Article pubs.acs.org/journal/ascecg

An Efficient Noble-Metal-Free Photocatalyst for Visible-Light-Driven H2 Evolution: Cu/Ni-Codoped Cd0.5Zn0.5S Nanoplates Yanjie Hao, Shi-Zhao Kang,* Xing Liu, Xiangqing Li, Lixia Qin, and Jin Mu* Center of Graphene Research, School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, China

Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 15, 2018 at 13:25:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: In the present work, Cu/Ni-codoped Cd05Zn0.5S nanoplates were synthesized through a one-step hydrothermal process. Meanwhile, photocatalytic H2 evolution from water over the as-prepared Cu/Nicodoped Cd0.5Zn0.5S nanoplates was investigated under visible irradiation using Na2S and Na2SO3 as sacrificial reagents. The results indicate that the as-prepared Cu/Ni-codoped Cd0.5Zn0.5S nanoplates are an efficient photocatalyst for visible-light-driven H2 evolution from water, and there exists obvious synergy effect between Cu and Ni dopants. The photocatalytic activity of the Cd0.5Zn0.5S nanoplates can be significantly enhanced due to the introduction of Cu and Ni. Under optimal conditions, the H2 evolution rate over the Cu/Ni-codoped Cd0.5Zn0.5S nanoplates is up to 58.33 mmol h−1 g−1, which is 3.5 times higher than that over the pure Cd0.5Zn0.5S nanoplates. Finally, the photocatalytic mechanism was preliminarily discussed. KEYWORDS: Cd0.5Zn0.5S nanoplates, Cu, Ni, doping, photocatalytic hydrogen evolution

■

composite,19 Ag-doped Cd0.1Zn0.9S,20 were reported. However, the H2 evolution efficiency over the CdxZn1−xS-based photocatalysts need be further enhanced before their commercialization becomes possible. As a result, it is still desirable to prepare efficient CdxZn1−xS-based photocatalysts. The doping of Cd1−xZnxS with metal ions is one of the most common strategies that is used to enhance the photocatalytic activity of Cd1−xZnxS. The photocatalytic activity of Cd1−xZnxS can be obviously enhanced using Cu or Ni dopants.21−23 However, most of these Cu- and Ni-doped Cd1−xZnxS photocatalysts are prepared through doping of Cu or Ni species alone. There are limited papers on the photocatalytic activity of Cu/Ni-codoped Cd1−xZnxS for H2 evolution from water. The results reported previously indicate that the activity of photocatalysts can be further improved through multielements-doping due to the synergistic effect between dopants.24 Therefore, it is possible that the Cu/Ni-codoped Cd1−xZnxS possesses higher photocatalytic activity in comparison with the Cu-doped Cd1−xZnxS or the Ni-doped Cd1−xZnxS. On the other hand, in the field of photocatalytic H2 evolution, noble metals such as Pt25 and Pd26 are usually used as a cocatalyst to reduce the recombination of the photogenerated electrons and holes, as well as lower the activation potential for H2 evolution. Although the H2 evolution rate can be obviously accelerated due to the introduction of noble metals, the high cost makes their

INTRODUCTION For decades, photocatalytic hydrogen production from water splitting has been a research hotspot because it was regarded as a potential strategy for conversion of solar energy to chemical energy.1,2 A lot of efforts were devoted to the development of highly active photocatalysts. And numerous efficient photocatalysts were prepared, such as Au-TiO2 nanohybrids,3 core/ shell CdS/g-C3N4 nanowires,4 Sn3O4,5 graphene/TiO2−x nanocomposites,6 CdS loaded with MoS2,7 Ba5Nb4O15 nanosheets,8 MoS2 nanosheet/TiO2 nanowire hybrid nanostructures,9 nanoporous CdS nanostructures,10 and so on. Among these photocatalysts reported previously, metal sulfides, especially CdS, were referred to as a promising candidate due to their appropriate band gap and conduction band edge position.11,12 Nevertheless, the stability of binary metal sulfides is unsatisfied due to serious photocorrosion, which restricts their wide application. Compared with binary metal sulfides, ternary chalcogenides, such as CuGaS2 and CdxZn1−xS, possess higher photocatalytic activity as well as stability to photocorrosion.13,14 In particular, the band gap width and the band position of CdxZn1−xS can be easily tuned by adjusting the component ratio of cadmium to zinc.15 It is well-known that there is a strong relationship between the catalytic activity of photocatalysts and their band gap width and conduction band position. Therefore, recently, the application of CdxZn1−xS attracted more and more attention in the field of photocatalytic hydrogen production. A series of interested CdxZn1−xS-based photocatalysts, including Cd0.8Zn0.2S nanowires,16 CdxZn1−xS nanoparticles loaded with MoS2,17 Zn0.5Cd0.5S nanorods on reduced graphene oxide,18 TiO2 nanotube/Cd0.65Zn0.35S nano© 2016 American Chemical Society

Received: October 15, 2016 Revised: November 1, 2016 Published: November 14, 2016 1165

DOI: 10.1021/acssuschemeng.6b02499 ACS Sustainable Chem. Eng. 2017, 5, 1165−1172

Research Article

ACS Sustainable Chemistry & Engineering

Technologies Varian 710-ES inductively coupled plasma atomic emission spectrometer (U.S.A.). Photocatalytic Activity Test. The photocatalytic reaction was carried out in a gas-closed system with a reactor made of quartz. A 300 W xenon lamp equipped with an optical filter (λ > 420 nm) was used as a light source. The distance between the lamp and the reactor is 1 cm. In a typical experiment, 1% Cu/Ni (2:3)-Cd0.5Zn0.5S (6 mg) was added in the aqueous solution (60 mL) containing Na2S (0.3 mol L−1) and Na2SO3 (0.3 mol L−1). The mixture was sonicated for 5 min to form a homogeneous suspension. Then, N2 was bubbled in the suspension for 30 min. After the gas-closed system was vacuumed, the suspension was irradiated. The amounts of hydrogen produced were measured with a gas chromatograph (GC-7900, China, molecular sieve 5A, TCD) using N2 as a carrier gas after a certain period of irradiation. Using monochromatic light (450 nm, 51 mW cm−2) as a light source, the average hydrogen evolution rate was measured after irradiated for 4 h. Afterward, the quantum yield of hydrogen (QE) was calculated according to the equation below:

practical application become very difficult. Thus, it is very necessary to find a low-cost alternative cocatalyst. Recently, it has been found that Cu2S and NiS are efficient cocatalysts for the metal sulfide-based photocatalysts.27−30 In the photocatalytic process, Cu2S serves as an efficient water reduction cocatalyst while NiS is an efficient water oxidation cocatalyst.27,31 Moreover, the previous experimental results show that the combination of CuS and NiS can provide us with a highly efficient, noble metal-free photocatalyst for H2 evolution from water.32 On the basis of the results above, we can speculate that it is possible to obtain an efficient Cd1−xZnxS-based photocatalyst if the Cu2S clusters and the NiS clusters are incorporated simultaneously into the Cd1−xZnxS matrix. Unfortunately, to our best knowledge, it is still unclear so far whether the Cd1−xZnxS incorporated with Cu2S clusters and NiS cluster can possess high photocatalytic activity for H2 evolution from water or not. Therefore, it is meaningful to prepare the Cu/Ni-codoped Cd1−xZnxS photocatalyst and explore its photocatalytic activity for H2 evolution from water. In the present work, Cu/Ni-codoped Cd05Zn0.5S nanoplates were synthesized through a one-step hydrothermal process. Meanwhile, the photocatalytic activity of the Cu/Ni-codoped Cd0.5Zn0.5S nanoplates was evaluated using Na2S and Na2SO3 as sacrificial reagents. Finally, the photocatalytic mechanism was preliminarily discussed.

■

QE = number of reacted electrons/number of incident photons × 100% = 2 × number of evolved H 2 molecules/number of incident photons × 100% = 2vtNA /(IStλ(1/hc)) × 100%

(1) where v is the average hydrogen evolution rate (mol s−1), t is the irradiation time (s), NA is Avogadro’s constant 6.022 × 1023 (mol−1), I is the light density of incident light (W cm−2), S is the irradiation area (cm2), λ is the wavelength of incident light (nm), h is Planck constant 6.626 × 10−34 (J s), and c is the speed of light 3 × 108 m s−1.

EXPERIMENTAL SECTION

■

Materials. All materials purchased from Sinopharm Chemical Reagent Co. Ltd. (China) were of analytical grade and used without further purification. Ultrapure water with a resistivity of 18.2 MΩ·cm, produced with a Milli-Q Integral Water Purification System (Millipore), was used throughout the experiments. Synthesis of Cu/Ni-Codoped Cd05Zn0.5S Nanoplates. Cu/Nicodoped Cd0.5Zn0.5S nanoplates were prepared using a facile hydrothermal method. Typically, Cd(CH3COO)2·2H2O (2.5 mmol), Zn(CH3COO)2·2H2O (2.5 mmol), Cu(CH3COO)2·H2O (0.02 mmol), and Ni(CH3COO)2·4H 2O (0.03 mmol) were dissolved in 15 mL of ultrapure water to obtain solution A. Then the Na2S aqueous solution (1 mol·L−1, 10 mL) was dropped in to solution A. After it was stirred for 30 min at room temperature, the suspension obtained was transferred into a Teflon-lined stainless steel autoclave with a capacity of 40 mL and maintained at 180 °C for 12 h. Next, the autoclave was naturally cooled to room temperature. The precipitate was collected by centrifugation and washed with deionize water and ethanol several times, respectively. Finally, the resulting product was dried in vacuum oven at 60 °C for 10 h. The preliminary experimental results indicate that the as-prepared sample exhibits the highest photocatalytic activity for H2 evolution from water when the molar percent of Cu and Ni codopant and the molar ratio of Cu to Ni are 1% and 2:3. Thus, this composition was reported in this paper. For convenience, this sample is marked as 1% Cu/Ni (2:3)-Cd0.5Zn0.5S. Characterization. The X-ray diffraction (XRD) patterns were measured with a PANalytical Xpert Pro MRD X-ray diffractometer (Netherlands). The UV−vis spectra were recorded on a Hitachi U3900 UV−vis spectrophotometer (Japan). The fluorescence spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer (Japan). The X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Thermo Fisher ESCALAB 250Xi X-ray photoelectron spectrometer equipped with a monochromatic Al Kα source (U.S.A.). The scanning electron microscope (SEM) image was measured using a Hitachi S-4800 scanning electron microscope (Japan). The transmission electron microscope (TEM) images were taken with a JEOL JEM-2100 transmission electron microscope (Japan). The N2 adsorption and desorption isotherms were measured on a Micromeritics ASAP-2020 nitrogen adsorption apparatus (U.S.A.). The atomic emission spectra were recorded on an Agilent

RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of CdS, ZnS, Cd0.5Zn0.5S and 1% Cu/Ni(2:3)-Cd0.5Zn0.5S. From Figure 1, it can be observed

Figure 1. XRD patterns of (a) CdS, (b) Cd0.5Zn0.5S, (c) 1% Cu/ Ni(2:3)-Cd0.5Zn0.5S, and (d) ZnS.

that the diffraction peaks of the as-prepared CdS sample are in good agreement with those of cubic zinc blende CdS (JCPDS file no. 10-0454) except the peak at 2θ = 24.80° ascribed to the (100) plane of hexagonal wurtzite CdS (JCPDS file no. 411049).33 This indicates that the sample obtained is a mixture of cubic zinc blende CdS and hexagonal wurtzite CdS. Similarly, the diffraction peaks of the as-prepared ZnS sample are well indexed as cubic zinc blende ZnS (JCPDS file no. 05-0566).34 Furthermore, it can be also found that the Cd0.5Zn0.5S sample exhibits no additional diffraction peaks in comparison with CdS and ZnS. Additionally, the relevant diffraction peaks of the 1166

DOI: 10.1021/acssuschemeng.6b02499 ACS Sustainable Chem. Eng. 2017, 5, 1165−1172

Research Article

ACS Sustainable Chemistry & Engineering Cd0.5Zn0.5S sample shift to lower angle relative to those of ZnS and shift to higher angle relative to those of CdS. One possible explanation is that the radii of Zn2+ (0.074 nm) are smaller than those of Cd2+ (0.097 nm).15 When Zn2+ inserts into the CdS lattice, the lattice constants would decrease, which causes the diffraction peaks to shift to higher angle. This phenomenon indicates that the sample obtained is not a mixture of ZnS and CdS but a CdxZn1−xS solid solution. Moreover, the XRD patterns shown in Supporting Information Figure S1A indicate that the diffraction peak ascribed to hexagonal wurtzite CdS is observably weakened and almost disappears in the XRD pattern of the Cd0.5Zn0.5S sample, suggesting that our product obtained may be Cd0.5Zn0.5S adopting the cubic crystal structure. On the basis of these experimental results, it can be concluded that cubic Cd0.5Zn0.5S with fairly high phase-purity can be prepared in the process described above. On the other hand, Figure 1 shows that the XRD pattern of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S is similar to that of Cd0.5Zn0.5S. The corresponding diffraction peaks of 1% Cu/Ni(2:3)Cd0.5Zn0.5S do not shift in comparison with those of Cd0.5Zn0.5S, implying that neither Cu dopant nor Ni dopant is introduced into the lattice of Cd0.5Zn0.5S. Similarly, we can observe the same phenomenon from the XRD patterns of the Cu-doped Cd0.5Zn0.5S (1% Cu−Cd0.5Zn0.5S) and the Ni-doped Cd0.5Zn0.5S (1% Ni−Cd0.5Zn0.5S) (Figure S1B). Therefore, we speculate that Cu and Ni elements are segregated in the forms of small Cu sulfide clusters and Ni sulfide clusters, respectively. The experimental results below can confirm this assumption. However, no characteristic diffraction peaks of these sulfides can be observed from Figure 1. One possible explanation is that these sulfide clusters are too small to give well-defined diffraction peaks.35 As a result, it can be deduced that the asprepared sample is a CdxZn1−xS solid solution incorporated with Cu sulfide and Ni sulfide clusters. Figure 2 shows the XPS spectra of 1% Cu/Ni(2:3)Cd0.5Zn0.5S. From the survey spectrum (Figure 2A), it can be observed that there exist some XPS peaks ascribed to Cu, Ni, Cd, Zn, S, O, and C elements, indicating that the as-prepared sample is composed of the elements Cu, Ni, Cd, Zn, and S. The Cd/Zn ratio is close to the stoichiometric composition of Cd0.5Zn0.5S (Cu/In = 1:1) within experimental error. In combination with the results of XRD, the component of the as-synthesized sample can be defined as Cd0.5Zn0.5S, Cu sulfide and Ni sulfide. The Cu 2p high-resolution XPS spectrum (Figure 2B) shows two peaks at 932 and 951.7 eV, respectively. They should be assigned as the Cu 2p3/2 and Cu 2p1/2 peaks of Cu+ in Cu2S.28,36 Moreover, the “‘shakeup’” satellite peaks are absent, which further confirms that the valence state of Cu is Cu+. Thus, it can be deduced that the Cu sulfide clusters incorporated in Cd0.5Zn0.5S are Cu2S clusters. Moreover, it can be found that there exists one peak at 855.6 eV in the Ni 2p high-resolution XPS spectrum (Figure 2C), corresponding to the Ni 2p3/2 peak of Ni2+ in NiS.37 This indicates that the Ni sulfide clusters incorporated in Cd0.5Zn0.5S are NiS clusters. Besides, the molar percent of Cu and Ni codopant and the Cu/ Ni ratio are 1% and 2:3, respectively. Therefore, it can be concluded that the as-prepared sample is Cd 0.5 Zn 0.5 S incorporated with Cu2S and NiS. Figure 3 shows the TEM image and the HRTEM image of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S. From Figure 3A, we clearly observe the formation of the nanoplates which adopt an irregular morphology with an average size of approximate 20

Figure 2. (A) XPS survey spectrum of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S; (B) high-resolution XPS spectrum (solid line) of Cu 2p and curvefitting analysis (dot lines) of states of Cu (B); (C) high-resolution XPS spectrum (solid line) of Ni 2p and curve-fitting analysis (dot lines) of states of Ni (C).

nm. Although there exists aggregation of nanoplates, the individual nanoplates can be still observed. Moreover, some nanoplates are almost transparent, suggesting that the asprepared nanoplates are very thin. Likewise, the SEM image of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S (Figure S2) also confirms the formation of the nanoplates. Besides, the HRTEM image of a nanoplate (Figure 3B) also shows some lattice fringes, implying the good crystallinity of the as-prepared nanoplates. The lattice fringes with the interlayer spacing of 0.312 nm should be indexed to the (111) plane Cd0.5Zn0.5S.38 The lattice fringes with the interlayer spacing of 0.32 nm should correspond to the (111) plane of Cu2S.28 The lattice fringes with the interlayer spacing of 0.198 nm should be assigned to the (102) plane of NiS.39 These results further confirm that the Cu2S clusters and the NiS clusters are incorporated into the Cd0.5Zn0.5S nanoplates. Therefore, we can affirm that the sample obtained is the Cd0.5Zn0.5S nanoplates incorporated with the Cu2S clusters and the NiS clusters. On the other hand, Figure S3 shows that the morphologies of 1% Cu−Cd0.5Zn0.5S and 1% Ni−Cd0.5Zn0.5S are almost same as that of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S, suggesting that the introduction of Cu and Ni can hardly influence the morphology of sample. Figure 4A shows the UV−vis diffuse reflection spectra of the as-prepared Cd 0.5 Zn 0.5 S sample and 1% Cu/Ni(2:3)Cd0.5Zn0.5S. As can be seen from Figure 4A, the Cd0.5Zn0.5S 1167

DOI: 10.1021/acssuschemeng.6b02499 ACS Sustainable Chem. Eng. 2017, 5, 1165−1172

Research Article

ACS Sustainable Chemistry & Engineering

Figure 3. (A) TEM image and (B) HRTEM image of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S.

Figure 4. (A) UV−vis spectra of (a) Cd0.5Zn0.5S and (b) 1% Cu/Ni(2:3)-Cd0.5Zn0.5S; (B) photoluminescent spectra of (a) Cd0.5Zn0.5S, (b) 1% Cu− Cd0.5Zn0.5S, (c) 1% Ni−Cd0.5Zn0.5S, and (d) 1% Cu/Ni(2:3)-Cd0.5Zn0.5S, the excitation wavelength 350 nm.

Figure 5A shows the kinetic curves of H2 production over Cd0.5Zn0.5S, 0.4% Cu−Cd0.5Zn0.5S, 0.6% Ni−Cd0.5Zn0.5S, and 1% Cu/Ni(2:3)-Cd0.5Zn0.5S, respectively. As can be seen from Figure 5A, 1% Cu/Ni(2:3)-Cd0.5Zn0.5S possesses high photocatalytic activity for H2 evolution from water under visible irradiation. During the first 5 h irradiation, the mean rate of H2 evolution is up to 58.33 mmol h−1 g−1. The hydrogen production rate is almost constant in the first 8 h, and then it decreases slightly. However, the rate of 47.14 mmol h−1g−1 can be still achieved after irradiated for 15 h. This slight decrease should be ascribed to the consumption of Na2S and Na2SO3 rather than inactivation of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S. Hence, we can infer that 1% Cu/Ni(2:3)-Cd0.5Zn0.5S can keep its activity for a period of time, implying that it is a potential photocatalyst. Moreover, the quantum yield (QE) of 1% Cu/ Ni(2:3)-Cd0.5Zn0.5S was measured at 450 nm to further evaluate the photocatalytic activity of Cu/Ni(2:3)-Cd0.5Zn0.5S. The result shows that QE of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S is about 11.9% at 450 nm. As a result, it can be concluded that 1% Cu/Ni(2:3)-Cd0.5Zn0.5S is an efficient photocatalyst for hydrogen evolution under visible light irradiation. From Figure 5A, it can be also found that the photocatalytic activity of Cd0.5Zn0.5S is obviously enhanced when the Cu2S clusters and the NiS clusters are simultaneously introduced in

sample possesses an obvious absorption band with an edge at approximately 530 nm, which may be ascribed to the band gap transition. Similarly, 1% Cu/Ni(2:3)-Cd0.5Zn0.5S also possesses an absorption band whose edge is almost same as that of the pure Cd0.5Zn0.5S solid solution. This phenomenon further demonstrates that neither Cu dopant nor Ni dopant is incorporated into the lattice of Cd0.5Zn0.5S. However, compared with the pure Cd0.5Zn0.5S solid solution, 1% Cu/ Ni(2:3)-Cd0.5Zn0.5S exhibits higher absorption in the range of 500 to 800 nm. This phenomenon may be ascribed to enhancement of the surface electric charge of Cd0.5Zn0.5S due to introduction of Cu and Ni,40 implying that there exist some electronic interactions among the Cd0.5Zn0.5S matrix, the Cu2S clusters, and the NiS clusters. Therefore, it can be expected that the recombination of the photogenerated electrons and holes would be suppressed due to the introduction of the Cu2S clusters and the NiS clusters. Moreover, the Brunauer− Emmett−Teller (BET) measurement (Figure S4) indicates that 1% Cu/Ni(2:3)-Cd0.5Zn0.5S possesses relatively large surface area. The specific surface area of 1% Cu/Ni(2:3)Cd0.5Zn0.5S is 27 m2 g−1. These results imply that 1% Cu/ Ni(2:3)-Cd0.5Zn0.5S might be an efficient photocatalyst for H2 evolution from water under visible irradiation. 1168

DOI: 10.1021/acssuschemeng.6b02499 ACS Sustainable Chem. Eng. 2017, 5, 1165−1172

Research Article

ACS Sustainable Chemistry & Engineering

Figure 5. (A) Time-courses of H2 evolution over (a) Cd0.5Zn0.5S, (b) 0.4% Cu−Cd0.5Zn0.5S, (c) 0.6% Ni−Cd0.5Zn0.5S, and (d) 1% Cu/Ni(2:3)Cd0.5Zn0.5S (sacrificial agent 0.3 mol L−1 Na2S and 0.3 mol L−1 Na2SO3 aqueous solution, 60 mL; dosage of photocatalyst 0.1 mg mL−1); (B) H2 evolution rates over Cd0.5Zn0.5S, 1% Cu−Cd0.5Zn0.5S, 1% Ni−Cd0.5Zn0.5S, and 1% Cu/Ni(2:3)-Cd0.5Zn0.5S (sacrificial agent 0.1 mol L−1 Na2S and 0.1 mol L−1 Na2SO3 aqueous solution, 60 mL; dosage of photocatalyst 1 mg mL−1).

Cd0.5Zn0.5S. Under the same conditions, the hydrogen production rate over 1% Cu/Ni(2:3)-Cd0.5Zn0.5S is about 3.5 times more than that over pure Cd0.5Zn0.5S (12.92 mmol h−1 g−1). The BET measurement indicates that the specific surface area of Cd0.5Zn0.5S (33 m2 g−1) is comparable with that of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S. Therefore, it can be concluded that Cu2S and NiS are efficient cocatalysts for Cd0.5Zn0.5S. Moreover, the experimental results (Figure 5B) indicate that the photocatalytic activity of Cd0.5Zn0.5S can be enhanced more efficiently using the Cu2S/NiS cocatalyst in comparison with using the Cu2S cocatalyst or the NiS cocatalyst alone. This phenomenon demonstrates that there may exist some synergistic effect between Cu2S and NiS. This assumption can be confirmed by the experimental results shown in Figure 5A. The hydrogen production rates over 0.6% Ni−Cd0.5Zn0.5S and 0.4% Cu−Cd0.5Zn0.5S are 16.25 mmol h−1 g−1 and 14.91 mmol h−1 g−1, respectively. The hydrogen production rate over 1% Cu/Ni(2:3)-Cd0.5Zn0.5S is about 1.8 times the sum of the rates over 0.6% Ni−Cd0.5Zn0.5S and 0.4% Cu−Cd0.5Zn0.5S. As a result, it can be deduced that the high photocatalytic activity of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S might be ascribed to the synergistic effect between Cu2S and NiS. On the basis of the experimental results above, a possible mechanism is suggested as follows (Scheme 1). At first, when the Cd0.5Zn0.5S nanoplates are under irradiation, the electrons are excited from the valence band to the conduction band. Next, the excited electrons transfer to the conduction band of the Cu2S cluster because the conduction band level of Cd0.5Zn0.5S (−0.369 V vs NHE) is more negative than that of Cu2S (−0.06 V vs NHE).15,41 Subsequently, water molecules are reduced into H2 by the photogenerated electrons on the Cu2S clusters. Meanwhile, the holes transfer to the NiS clusters due to the inner electric field between the p-type NiS clusters and the n-type Cd0.5Zn0.5S matrix.31 At last, the holes are consumed irreversibly by the sacrificial reagents on the NiS

Scheme 1. Schematic Illustration of the Proposed Mechanism for Photocatalytic H2 Production over Cu/Ni− Cd0.5Zn0.5S

clusters. Here, the Cu2S clusters work as charge transferring sites and/or active sites; meanwhile, the NiS clusters serve as a water oxidation cocatalyst. The synergetic effect between the Cu2S clusters and the NiS clusters may be as follows. (1) The migration of the photogenerated electrons and holes can be accelerated simultaneously due to the introduction of both the Cu2S clusters and the NiS clusters. As a result, the recombination of the photogenerated electrons and holes would be greatly suppressed, and their lifetime can be efficiently prolonged.31 (2) The photogenerated electrons would be enriched in Cd0.5Zn0.5S driven by the p−n junctions between the NiS clusters and the Cd0.5Zn0.5S matrix, which is favorable to the electron transfer between the Cu2S clusters and the Cd0.5Zn0.5S matrix. (3) The photocatalytic centers would 1169

DOI: 10.1021/acssuschemeng.6b02499 ACS Sustainable Chem. Eng. 2017, 5, 1165−1172

Research Article

ACS Sustainable Chemistry & Engineering

the forms of small Cu2S clusters and NiS clusters, respectively. Furthermore, 1% Cu/Ni(2:3)-Cd0.5Zn0.5S is an efficient and stable photocatalyst for hydrogen evolution under visible light irradiation. In this photocatalytic process, the Cu2S clusters work as charge transferring sites and/or active sites, whereas the NiS clusters serve as a water oxidation cocatalyst. Additionally, there exists strong synergistic effect between Cu2S and NiS, which in turn results in the dramatic photocatalytic activity of Cu/Ni−Cd 0.5Zn0.5S for H2 evolution from water. This work provides a new strategy for designing an efficient noble-metalfree visible photocatalyst for hydrogen production.

increase due to the introduction of both the Cu2S clusters and the NiS clusters. Therein, the high photocatalytic activity of 1% Cu/Ni(2:3)-Cd0.5Zn0.5S may be mainly attributed to the more efficient separation of the photogenerated electrons and holes caused by Cu2S and NiS. In order to understand the above mechanism more deeply, the photoluminescent spectra of Cd0.5Zn0.5S, 1% Cu− Cd 0.5 Zn 0.5 S, 1% Ni−Cd 0.5 Zn 0.5 S, and 1% Cu/Ni(2:3)Cd0.5Zn0.5S were measured (Figure 4B). As can be seen from Figure 4B, Cd0.5Zn0.5S shows a broad peak in the region of 480−580 nm, which may be assigned to the emission from the defect levels/surface states of Cd0.5Zn0.5S.42 When both Cu2S and NiS are introduced, this emission of Cd0.5Zn0.5S is obviously quenched, suggesting that the recombination of the photogenerated electrons and holes are efficiently suppressed. Moreover, it can be also found that the quenching effect induced by the Cu2S/NiS cocatalyst is obviously stronger than that induced by the Cu2S cocatalyst or the NiS cocatalyst alone. These phenomena further confirm that the separation of the photogenerated electrons and holes would become more efficiently when Cu2S and NiS are simultaneously introduced in Cd0.5Zn0.5S, which in turn results in higher photocatalytic activity for H2 evolution from water. Furthermore, the XPS spectra of the used 1% Cu/Ni(2:3)Cd0.5Zn0.5S were measured. The Cu 2p high-resolution XPS spectrum (Figure S5A) shows that there exists a broad peak, which can be fitted by two peaks at 931.9 and 932.4 eV. The peak at binding energy of 932.4 eV should be assigned to the Cu 2p3/2 peak of Cu0.43 One possible explanation is that some Cu 2 S clusters might be reduced to Cu 0 due to the photogenerated electrons transfer. Similarly, the O 1s highresolution XPS spectrum (Figure S5B) shows a broad peak at 530.9 eV. This result implies that there exists NiO in the sample,44 which may be ascribed to oxidation of the NiS clusters due to the holes transfer. Thus, it can be deduced that the photogenerated electrons might transfer to the Cu2S clusters meanwhile the holes transfer to the NiS clusters in the photocatalytic process. The repeatability of photocatalyst is one of the key parameters for its commercialization. In order to test the repeatability of Cu/Ni−Cd0.5Zn0.5S, the stability of 1% Cu/ Ni(2:3)-Cd 0.5 Zn 0.5 S was examined by repeated the H 2 evolution for 4 cycles in the same conditions. The results are shown in Figure S6. From Figure S6, we can find that 1% Cu/ Ni(2:3)-Cd0.5Zn0.5S possesses considerable durability when it is used repeatedly. Although the rate of H2 evolution decreases with the number of cycle increasing, the rate is still up to 37.5 mmol g−1 h−1 in the fourth cycle. The inactivation of 1% Cu/ Ni(2:3)-Cd0.5Zn0.5S may be ascribed to leakage of Cu/Ni in the photocatalytic process. The atomic emission spectra of the used Na2S/Na2SO3 solution can confirm this assumption. The experimental results show that there exist Cu ions (