One-step synthesis of highly efficient three-dimensional Cd1-xZn x S photocatalysts for visible light photocatalytic water splitting
© Xiong et al.; licensee Springer. 2013
Received: 27 May 2013
Accepted: 9 July 2013
Published: 24 July 2013
Visible light accounts for about 43% of the solar spectrum, and developing highly efficient visible-light-driven photocatalyst is of special significance. In this work, highly efficient three-dimensional (3D) Cd1−xZn x S photocatalysts for hydrogen generation under the irradiation of visible light were synthesized via one-step solvothermal pathway. Scanning electron microscope, X-ray diffractometer, Raman spectrometer, and X-ray photoelectron spectrometer were utilized to characterize the morphology, crystal structure, vibrational states, and surface composition of the obtained 3D Cd1−xZn x S. UV-Vis spectra indicated that the as-synthesized Cd1−xZn x S had appropriate bandgap and position of the conduction band that is beneficial for visible light absorption and photo-generated electron-hole pair separation. Moreover, the 3D structure offers a larger surface area thus supplying more surface reaction sites and better charge transport environment, and therefore, the efficiency of water splitting was improved further.
KeywordsVisible light photocatalytic Water splitting Cd1−xZn x S Solvothermal pathway Solid solutions
The efficient conversion of solar energy into fuel via photochemical reactions is of great importance for the next-generation energy source for its cleanable, renewable, and abundant properties [1, 2]. Solar-hydrogen, the conversion of solar energy into hydrogen as chemical energy carrier, has been regarded as one of the most desirable ways in considering energy consumption, resource sustainability, and environmental issues [3, 4].
Since the pioneering work of Fujishima and Honda in 1972 , tremendous research on semiconductor-based photocatalysis and photoelectrolysis has yielded a better understanding of the mechanisms involved in photocatalytic and photoelectrochemical water splitting [6–9]. However, most of semiconductor photocatalysts can only absorb ultraviolet light due to their wide gap. As it is well known, ultraviolet light occupies only 3% ~ 5% of the solar spectrum; so, the energy conversion efficiency is usually very low [10–12]. Thus, exploiting of highly active visible-light-responsive photocatalysts to make the best use of solar energy in visible light region, which accounts for about 43% of the solar spectrum, is particularly important [13, 14]. In the past, developing and understanding of semicondutor electrodes or photocatalysts for photoelectrochemical or photocatalytic water splitting were mainly performed on simple binary systems (e.g., binary oxides [15, 16] and chalcogenides [17, 18]) and their composite structure . Recently, the ternary system as potentially excellent photoelectrode or photocatalyst material has attracted more and more attention [20–22] because ternary system can offer more possibilities for bandgap and band position tuning.
Cadmium sulfide is an important visible-light response photocatalytic material, in which sulfide ions serve as electron donors. However, the sulfide ion is readily oxidized to sulfate by the photo-generated holes, with Cd2+ ions escaping into the solution. A feasible way for enhancing the photocatalytic activity and stability of cadmium sulfide is to develop CdS-based composite materials. Zinc sulfide has the similar crystal structure as cadmium sulfide. It is a good host material for the development of a visible-light-driven photocatalyst without adding noble metals by forming Cd1−xZn x S solid solutions with a narrow bandgap semiconductor, CdS [22, 23]. The bandgap of the solid solutions formed between ZnS and CdS can be regulated by changing the compositions and therefore the photocatalytic properties can be varied [24, 25].
In this article, we reported a highly efficient three-dimensional (3D) visible-light-active Cd1−xZn x S photocatalysts synthesized via one-step solvothermal pathway. The obtained photocatalysts had good crystallinity and ordered structure and showed excellent photocatalytic activity under the irradiation of visible light.
Synthesis of photocatalyst
Three-dimensional Cd1−xZn x S nanowires were synthesized in a Teflon-lined stainless steel cylindrical closed chamber with a 100-mL capacity. All the chemicals were of analytical grade. Ethylenediamine (en; 60 ml) and H2O (20 ml) were used as solvent. Thiourea [NH2CSNH2] (15 mmol) was added into the solvent as sulfur source, then 5-mmol mixture of cadmium acetate [(CH3COO)2Cd·2H2O] and zinc acetate [(CH3COO)2Zn·2H2O] was added into the mixed solution. After stirring for a few minutes, the closed chamber was placed inside a preheated oven at 160°C for 10 h and then cooled to room temperature. The obtained precipitates were filtered off and washed several times with water and ethanol, respectively. The final products were dried in vacuum at 45°C for a few hours.
The morphology of the as-synthesized powder products were observed by field-emission scanning electron microscopy (Philips Sirion 200, Philips, Netherlands). The crystallographic structure was determined by X-ray diffraction (XRD, D8 DISCOVER X-ray diffractometer, Bruker, Karlsruhe, Germany) with Cu Kα radiation (1.54 Å). Surface composition of the sample was analyzed by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, Kratos, Japan). The Raman spectrum was measured by the Jobin Yvon LabRam HR 800 UV system (Horiba, Kyoto, Japan) at room temperature. A laser wavelength of 514.5 nm was used as the excitation sources. Reflectance spectra of the obtained were collected using a UV/vis spectrometer (Lambda 20, Perkin Elmer, Inc., USA).
Photocatalytic hydrogen evolution
The photocatalytic performance of the synthesized 3D Cd1−xZn x S photocatalysts were investigated in a gas-closed circulation system (Labsolar-III, Beijing Perfactlight Technology Co. Ltd., Beijing, China) with a top-window Pyrex cell. A 300-W Xe lamp (SOLAREDGE700, Beijing Perfactlight Technology Co. Ltd., Beijing, China) was used as the light source, and UV light was removed by a cut-off filter (λ > 420 nm). Luminous power of the light source is about 40 W. The amount of H2 evolved was analyzed by an online gas chromatography (GC7900, Techcomp Ltd., Beijing, China) equipped with a thermal conductivity detector, MS-5A column, and N2 was used as carrier. In all experiments, 100 mL deionized water containing the mixed sacrificial agent which composed of 0.25 M Na2SO3 and 0.35 M Na2S were added into the reaction cell. Then, these photocatalysts were directly placed into the electrolyte solution. The whole system was vacuumized with a vacuum pump before reaction to remove the dissolved air. The temperature for all photocatalytic reactions was kept at about 20°C.
Results and discussions
We reported highly efficient three-dimensional Cd1−xZn x S photocatalysts synthesized via one-step solvothermal pathway for photocatalytic H2 evolution under the irradiation of visible light. The Raman spectrum implied the obtained Cd1−xZn x S had good crystallinity and ordered structure. The XPS demonstrated that sulfur existed as a sulfur ion, while Cd and Zn are in 3d and 2p state, respectively. The bandgap of the synthesized Cd1−xZn x S varied from 2.37 to 2.86 eV, which were suitable for the absorption of visible light. The photocatalytic activity of the obtained Cd1−xZn x S photocatalysts were improved markedly compared with that of the sole CdS. This can be attributed to their appropriate bandgap and position of the conduction band that is beneficial for visible light absorption and photo-generated electron-hole pair separation, as well as 3D structure that offered a larger surface area, thus supplying more surface reaction sites and better charge transport environment.
X-ray photoelectron spectrometer.
This work was supported by the National Major Basic Research Project of 2012CB934302, National 863 Program 2011AA050518, the Natural Science Foundation of China (grant nos.11174197 and 61234005).
- Marban G, Valdes-Solis T: Towards the hydrogen economy? Int J Hydrogen Energy 2007, 12: 1625–1637.View Article
- Winter CJ: Hydrogen energy-abundant, efficient, clean: a debate over the energy-system-of change. Int. J Hydrogen Energy 2009, 34: S1-S52. 10.1016/j.ijhydene.2009.05.063View Article
- Lewis NS: Toward cost-effective solar energy issue. Science 2007, 315: 798–801. 10.1126/science.1137014View Article
- Andrews J, Shabani B: Re-envisioning the role of hydrogen in a sustainable energy economy. In.t J Hydrogen Energy 2012, 37: 1184–1203. 10.1016/j.ijhydene.2011.09.137View Article
- Fujishima A, Honda K: Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238: 37–38.View Article
- Bolton JR, Strickler SJ, Connolly JS: Limiting and realizable efficiencies of solar photolysis of water. Nature 1985, 316: 495–500. 10.1038/316495a0View Article
- Rajeshwar K: Hydrogen generation at irradiated oxide semiconductor-solution interfaces. J Appl Electrochem 2007, 37: 765–787. 10.1007/s10800-007-9333-1View Article
- Ohtani B: Photocatalysis A to Z-what we know and what we do not know in a scientific sense. J Photochem. and Photobio. C: Photochem Rev 2010, 11: 157–178. 10.1016/j.jphotochemrev.2011.02.001View Article
- Foley JM, Price MJ, Feldblyum JI, Maldonado S: Analysis of operation of thin nanowire photoelectrodes for solar energy conversion. Energy Environ Sci 2012, 5: 5203–5220. 10.1039/c1ee02518jView Article
- Kim HG, Hwang DW, Kim J, Kim YG, Lee JS: Highly donor-doped (110) layered perovskite materials as novel photocatalysts for overall water splitting. Chem Commun 1999, 1077–1078. 10.1039/A902892G
- Kim HG, Hwang DW, Bae SW, Jung JH, Lee JS: Photocatalytic water splitting over La2Ti2O7 synthesized by the polymerizable complex method. Catal Lett 2003, 91: 193–198.View Article
- Kato H, Asakura K, Kudo A: Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J Am Chem Soc 2003, 125: 3082–3089. 10.1021/ja027751gView Article
- Silva LA, Ryu SY, Choi J, Choi W, Hoffmann MR: Photocatalytic hydrogen production with visible light over Pt-interlinked hybrid composites of cubic-phase and hexagonal-phase CdS. J Phys Chem C 2008, 112: 12069–12073. 10.1021/jp8037279View Article
- Kudo A: Development of photocatalyst materials for water splitting. Int. J Hydrogen Energy 2006, 31: 197–202. 10.1016/j.ijhydene.2005.04.050View Article
- Chen X, Shen S, Guo L, Mao S: Semiconductor-based photocatalytic hydrogen generation. Chem Rev 2010, 110: 6503–6570. 10.1021/cr1001645View Article
- Masaaki K, Michikazu H: Heterogeneous photocatalytic cleavage of water. J Mater Chem 2010, 20: 627–641. 10.1039/b910180bView Article
- Lan X, Jiang Y, Su H, Li S, Wu D, Liu X, Han T, Han L, Qin K, Zhong H, Meng X: Magnificent CdS three-dimensional nanostructure arrays: the synthesis of a novel nanostructure family for nanotechnology. Cryst Eng Comm 2011, 13: 145–152. 10.1039/c0ce00093kView Article
- Zong X, Yan H, Wu G, Ma G, Wen F, Wang L, Li C: Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as cocatalyst under visible light irradiation. J Am Chem Soc 2008, 130: 7176–7177. 10.1021/ja8007825View Article
- Li YX, Chen G, Zhou C, Sun JX: A simple template-free synthesis of nanoporous ZnS–In2S3–Ag2S solid solutions for highly efficient photocatalytic H2 evolution under visible light. Chem Commun 2009, 2020–2022. 10.1039/B819300B
- Osterloh FE, Parkinson BA: Recent developments in solar water-splitting photocatalysis. MRS Bull 2011, 36: 17–22. 10.1557/mrs.2010.5View Article
- Berglund SP, Flaherty DW, Hahn NT, Bard AJ, Mullins CB: Photoelectrochemical oxidation of water using nanostructured BiVO4 films. J Phys Chem C 2011, 115: 3794–3802. 10.1021/jp1109459View Article
- Xing C, Zhang Y, Yan W, Guo L: Band structure-controlled solid solution of Cd1-xZnxS photocatalyst for hydrogen production by water splitting. Int. J. Hydrogen Energy 2006, 31: 2018–2024. 10.1016/j.ijhydene.2006.02.003View Article
- Zhang W, Xu R: Surface engineered active photocatalysts without noble metals: CuS–ZnxCd1−xS nanospheres by one-step synthesis. Int. J. Hydrogen Energy 2009, 34: 8495–8503. 10.1016/j.ijhydene.2009.08.041View Article
- Wang L, Wang W, Shang M, Yin W, Sun S, Zhang L: Enhanced photocatalytic hydrogen evolution under visible light over Cd1−xZnxS solid solution with cubic zinc blend phase. Int. J. Hydrogen Energy 2010, 35: 19–25. 10.1016/j.ijhydene.2009.10.084View Article
- Wang DH, Wang L, Xu AW: Room-temperature synthesis of Zn0.80Cd0.20S solid solution with a high visible-light photocatalytic activity for hydrogen evolution. Nanoscale 2012, 4: 2046–2053. 10.1039/c2nr11972bView Article
- Tsuji I, Kato H, Kobayashi H, Kudo A: Photocatalytic H2 evolution reaction from aqueous solutions over band structure-controlled (AgIn) x Zn2(1−x)S2 solid solution photocatalysts with visible-light response and their surface nanostructures. J Am Chem Soc 2004, 126: 13406–13413. 10.1021/ja048296mView Article
- Zeiri L, Patla I, Acharya S, Golan Y, Efrima S: Raman spectroscopy of ultranarrow CdS nanostructures. J Phys Chem C 2007, 111: 11843.View Article
- Zhang YC, Chen W, Hu XY: Controllable synthesis and optical properties of Zn-Doped CdS nanorods from single-source molecular precursors. Crystal Growth & Des 2007, 7: 581–586.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.