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DOI: 10.1039/C6RA16076J (Communication) RSC Adv., 2016, 6, 76269-76272

A solid-state approach to fabricate a CdS/CuS nano-heterojunction with promoted visible-light photocatalytic H2-evolution activity†

Feiyue Cheng and Quanjun Xiang*
College of Resources and Environment, Huazhong Agricultural University, Wuhan, 430070, PR China. E-mail: xiangqj@mail.hzau.edu.cn

Received 21st June 2016 , Accepted 7th August 2016

First published on 8th August 2016

Abstract

Room-temperature solid-state synthesis has been shown to be a facile, energy conservation and solvent-free chemosynthetic method, and it has great potential in fabricating metal chalcogenides-based semiconductor photocatalysts. CdS/CuS nano-heterojunction composite photocatalysts were prepared via a one-pot low-temperature solid-state strategy. The as-prepared CdS/CuS composites revealed enhanced visible-light photocatalytic H2 evolution activity due to the promoted charge carrier separation/transfer efficiency and the improved photo-stability.

The conversion of solar energy into renewable and environmentally friendly energy stored in H2 is a potential approach for convenient utilization of solar energy, and photocatalytic H2 production is regarded as a promising strategy to achieve the aforementioned conversion.1–3 In order to improve the conversion efficiency of solar energy, more and more researchers are paying attention to designing and synthesizing semiconductor materials with improved visible-light photocatalytic H2-production activity.4,5 Among a variety of semiconductor materials, CdS is extensively explored for visible-light photocatalytic H2 evolution because of the moderate band-gap and the sufficiently negative flat-band potential.6,7 Unfortunately, pristine CdS usually displayed relatively low visible-light photocatalytic H2 production activity due to the high charge carrier recombination rate and serious photo-corrosion.8 Heterostructure in heterogeneous semiconductors can effectively improve charge carrier separation, photo-stability and then enhance the photocatalytic H2 production activity of the nanocomposites.9–12 Therefore, an efficient method for the formation of heterojunctions is a pressing need for the fabrication of CdS-based nanocomposites with enhanced photocatalytic H2 production activity. Unfortunately, the most commonly used synthetic strategy for the fabrication of hetero-structured semiconductors is multi-step, low productivity and generally strict for high temperature and high pressure.13,14 However, the one-pot low-temperature solid-state method as a facile, high-yield, energy saving chemosynthetic method has scarcely got enough attention in fabricating semiconductor heterojunction materials.15–18 Recently, the p-type semiconductor CuS with a narrow bandgap become increasingly concerned due to the potential application value as a co-catalyst.19–22 It was reported that the CuS-based heterojunctions with ZnS, ZnO, TiO2 and BiOCl were formed, and these heterojunctions result in markedly enhanced photoelectric or the photocatalytic efficiency of the materials.23–26 However, the inherent wide bandgap of these semiconductors limits the further improvement of visible-response. It is promising to project CdS/CuS nanocomposite heterojunction via one-pot low-temperature solid-state method to enhance the visible-light photocatalytic H2 production activity.

Here, CdS/CuS heterojunction materials were fabricated through a one-pot room-temperature solid-state method with the metal acetates and thioacetamide (TAA) as precursor, and no solvent was used in the synthetic process. The optimal CdS/CuS sample with 2.5 at% of CuS shows a high visible-light photocatalytic H2 production rate of 68.34 μmol h−1, which is more than 5 times that of pure CdS synthesized by the same method. The CdS/CuS samples were proved to have optimized energy band structure, a much lower electron–hole recombination rate and ameliorative photo-stability. The enhanced visible-light photocatalytic H2 evolution activity of CdS/CuS samples mainly ascribed to the promoted charge carrier separation/transfer property and improved photo-stability.

The samples of CdS, CuS and CdS/CuSx (where x = 1, 2.5, 5, 10, 20, 50 represent the theoretical atomic percentage of Cu vs. Cd, and the corresponding sample was labeled as CCx) were synthesized through a facile procedure of mix and grind of Cd(Ac)2·2H2O, Cu(Ac)2·H2O and thioacetamide (TAA) under room-temperature (experimental details are given in ESI†).15,17,27 The XRD pattern of the as-prepared CdS in Fig. 1A indicates the coexistence of the principal phase of cubic CdS with minor amount of hexagonal CdS. Herein, cubic CdS that is stable at low temperature appears as a main phase in CdS semiconductor on the account of the low-temperature solid-state reaction environment.15 All the diffraction peaks of pristine cupper sulfide in Fig. 1A belongs to hexagonal CuS (JCPDS card file no. 06-0464). The XRD pattern of CC50 indicates coexistence of cubic CdS, hexagonal CdS and hexagonal CuS, and the cubic CdS is the main phase (Fig. 1A). On the other hand, the main diffraction peaks of CCx (Fig. S1 in ESI†) belong to cubic CdS, and the weak peaks located at 36.8° and 47.8° were belong to hexagonal CdS.28 The diffraction peaks of CuS can hardly detected even through the atomic percentage of Cu/Cd reach 10% owning to the poor crystallinity and the high dispersion crystallinity degree.29–31 However, the diffraction peaks at 32.0° (103) and 48.1° (110) become stronger when further increase the ratio of CuS. No diffraction peak of impurity was detected in the XRD patterns. All the aforementioned samples show relatively low crystallinity owning to the low reaction temperature and short reaction time.12,14,32 Furthermore, the X-ray photoelectron spectroscopy (XPS) spectrum of the sample CC10 (Fig. 1B) displays the peaks of Cd, Cu, S, C and O and the metal/S atomic ratio approximately equals to 1/1, which match well with the atomic composition of MS. In addition, the atomic percentage of Cu/Cd in CC10 is calculated about 9.3% from the quantitative analysis, which corresponds with the molar ratio of Cu/Cd in CC10 sample. Furtherly, the high-resolution XPS spectra (Fig. S2 in ESI†) of Cu 2p shows the peaks of Cu 2p3/2 and Cu 2p1/2 located at the binding energy of 932.8 eV and 952.7 eV, respectively.33 The results of XPS demonstrate the coexistence of CuS and CdS in the CCx samples, which accords with the results of XRD patterns.


image file: c6ra16076j-f1.tif
Fig. 1 (A) XRD patterns of (i) pristine CdS, (ii) CdS/CuS50 composite and (iii) pristine CuS sample. (B) XPS survey spectrum of CdS/CuS10 sample. (C) TEM, (D) high-resolution TEM images of CdS/CuS10 sample.

To observe the grain size and the morphology of the prepared photocatalysts, the transmission electron microscopy (TEM) and the high-resolution TEM (HRTEM) images of the selected samples were obtained. The TEM and HRTEM images of CC10 in Fig. 1C and D indicate the uniform distribution and the intimate contact of the nanoparticles, respectively. According to the distinctions in the respect of nanocrystal morphology and lattice fringes, it is obvious that CC10 sample consists of CdS and CuS nanocrystals. The TEM and HRTEM images of pristine CdS (Fig. S3 in ESI†) indicate that CdS nanocrystals with a grain size about 5–10 nm have obvious agglomeration. The lattice fringes of 0.34 nm and 0.21 nm were assigned to (111), (311) crystal plane of cubic CdS, respectively. The TEM image of pristine CuS nanocrystal (Fig. S3 in ESI†) synthesized through the one-pot room-temperature solid-state strategy exhibit that CuS has oval and/or short-nanorod-like structure.

The UV-vis diffuse reflectance spectra (UV-vis DRS) of pristine CdS, CuS and CCx composites are revealed in Fig. 2 for comparing the optical absorption property of the materials. Pristine CdS sample has a simple and steeper UV-vis DRS absorption edge which located at about the wavelength of 525 nm. However, the as-prepared pristine hexagonal CuS reveals strong absorbance on the whole UV-vis waveband, especially on the waveband of 500–800 nm. Compared with pure CdS, CC1 and CC2.5 hybrid materials possess similar red-shifted visible-light absorption edges, but shows a new absorption shoulder at 500–650 nm. When further increase the ratio of Cu of the hybrid material, the absorption shoulder tend to be stronger. In addition, the background absorption between the wavelength of 650 nm and 800 nm were much enhanced with the increase of CuS content. The newly red-shift absorption edge of CCx were attributed to the donor level adjacent to the conduct band of CdS formed by Cu2+ on the surface of CdS. The high background absorption in wavelength of 650–800 nm is due to the increase of the ratio of CuS crystalline grain which has a relative high absorption in this area.34 The results of UV-vis DRS absorption demonstrate the optical absorption property of the CdS/CuS composites were signally affected by content of CuS, which mediately indicate the energy band structures of the CdS/CuS composites were change accordingly.


image file: c6ra16076j-f2.tif
Fig. 2 UV-visible diffuse absorption spectra of as prepared (a) CdS, (b)–(g) CdS/CuSx composites and (h) CuS sample. The inset is the photograph of CdS, CC2.5 and CuS.

The visible-light photocatalytic H2 production activity of the synthesized materials were evaluated by using Na2S/Na2SO3 aqueous solution as sacrificial agents with a Xe lamp provide visible light (λ ≥ 420 nm). Fig. 3 shows the comparison of visible-light H2 production activity of CdS, CuS and CCx. Pristine CdS shows a relative low H2 production rate of 12.50 μmol h−1 owning to the high charge carrier recombination rate, while pure CuS almost has no visible light H2 production activity due to the narrow bandgap and improper band position. However, it should be noted that a certain amount of CuS loading on CdS can significantly promote the visible-light photocatalytic H2 evolution activity of the hybrid materials. When the content of CuS reaches 2.5 at% in CCx hybrid material, the visible-light photocatalytic H2 production rate reaches 68.34 μmol h−1, which is almost 5.5 times that of CdS synthesized by the same method. Regularly, the photocatalytic H2 evolution activity of a hybrid photocatalyst will significantly decrease when further increasing the content of CuS.34 Herein, the visible-light photocatalytic H2 production activity of CCx indeed reduce with further increase the content of CuS because that the CuS nanoparticles block the accessibility of CdS to visible light. The results of photocatalytic H2 production activity indicate CuS with scarcely photocatalytic H2 production activity can significantly enhance the visible-light photocatalytic H2 generation activity of the CdS/CuS composites. The heterojunctions and the induced special energy band structure of CdS/CuS composites are considered as the main factor for the enhanced visible-light photocatalytic activity. On the other hand, the photo-stability of the optimized CdS/CuS2.5 photocatalyst was evaluated by 4 cyclics of photocatalytic H2 production reaction under 3 hours of visible-light irradiation. It is obvious that the visible-light photocatalytic H2 evolution rate of CdS/CuS2.5 markedly increased after the first hour, and the visible-light photocatalytic H2 production activity maintained very stable in the following cycles. The results indicate that the CdS/CuS composite material has favorable stability for visible-light photocatalytic H2 production. In addition, the photocorrosion of CdS is ascribed to the oxidation reaction of S2− driven by the excessive holes.35,36 Thus the enhanced stability of CdS/CuS is owning to the transfer of hole from CdS to CuS, which further suppressed the reduction of CuS into metallic Cu.


image file: c6ra16076j-f3.tif
Fig. 3 (A) Comparison of the average photocatalytic H2 production rate of CdS, CdS/CuSx composites and CuS sample obtained from 0.35 M Na2S/0.25 M Na2SO3 mixed aqueous solution under visible-light (λ ≥ 420 nm) irradiation, (B) cyclic time courses of photocatalytic H2 production on CdS/CuS2.5 under visible-light (λ ≥ 420 nm) irradiation.

The aforementioned results indicate the amount of CuS have a great impact on the bandgap structure and visible-light H2 evolution activity of the CCx composite materials. In order to clarify the mechanism for enhanced visible light photocatalytic H2 evolution on the CdS/CuS heterojunction materials, a schematic mechanism based on the p–n junction after the equilibrium of Fermi levels is proposed in Fig. 4.37,38 Actually, p–n junctions will fabricate at the interfaces when the p-type semiconductor CuS intimates contact with n-type semiconductor CdS. The relative bandgap positions of CdS and CuS semiconductors were adjusted by the equilibrium of the Fermi level.9,39,40 Electron and hole generated on semiconductors under visible-light illumination occupied the conduct band (CB) and the valence band (VB) under visible-light illumination, respectively. Then the charge carriers were efficient separated by the p–n junction, electrons on the CB of CuS transferred to CB of CdS and holes on VB of CdS transferred to the VB of CuS. Consequently, H2 generated on CdS from the reduction of water by photo-generated electron, and the consumption of holes main occurred on CuS.23 Owning to the efficient charge separation caused by the p–n junctions between CdS and CuS semiconductor, the visible-light H2 production activity of CdS/CuS2.5 is 5.5 times of pristine CdS. When further increase the ratio of CuS, the black CuS shelter the accessibility to visible light of CdS while CuS can still utilize the vis-near infrared light and provide electrons for the photocatalytic reaction.41 As a result, the visible-light H2 production activity of the composite photocatalysts did not slack so rapid even though the ratio of CuS/CdS reaches 50%. Therefore, the enhanced visible light photocatalytic H2 evolution activity of the CdS/CuS hybrid materials in this study is attributed to the p–n junctions between the interfaces of CdS and CuS. The results of transient photocurrent test in the air (Fig. S5 in ESI†) shows the positive correlation between the transient photocurrent density and the visible-light H2 production activity. It demonstrated that the improved charge separation and transfer properties lead to the enhanced visible-light H2 production activity of CdS/CuS composite photocatalyst.42 Moreover, the transient photocurrent further proved the rationality of the mechanism based on p–n junction between the interfaces of CdS and CuS.


image file: c6ra16076j-f4.tif
Fig. 4 Inferential schematic mechanism proposed for the photocatalytic H2 production over CdS/CuS photocatalyst under visible-light irradiation.

In conclusion, a high-yield and facile low-temperature solid-state synthetic method was developed for fabrication of CdS/CuS heterojunction hybrid material. The visible-light photocatalytic H2 evolution activity of the CdS/CuS hybrid materials were noticeable enhanced compared with pristine CdS. The optimal CdS/CuS hybrid sample with 2.5 at% of CuS showed the highest visible-light photocatalytic H2 evolution rate of 68.34 μmol h−1. Remarkably, the hybrid CdS/CuS materials maintain a considerable visible-light photocatalytic H2 evolution activity even though the molar ratio of CuS is as high as 50%. The results of UV-vis DRS and the transient photocurrent indicate CdS/CuS hybrid materials have the optimized bandgap structure, improved charge carrier separation/transfer properties and favorable stability. Consequently, the enhancement of the photocatalytic H2 evolution activity is ascribed to the p–n junction at the interface of CdS and CuS. This study presented a facile and effective one-pot low-temperature solid-state strategy for the fabrication of metal chalcogenides based heterojunction semiconductor materials with promoted visible light photocatalytic H2 evolution activity.

Acknowledgements

This work was supported by NSFC (21403079) and Fundamental Research Funds for the Central Universities (2662015PY039 and 2662015PY210).

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthesis, characterization procedures, XRD, XPS, TEM, photoelectrochemical date. See DOI: 10.1039/c6ra16076j
This journal is © The Royal Society of Chemistry 2016
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