Enhanced photocatalytic hydrogen evolution under visible light over Cd1−xZnxS solid solution with cubic zinc blend phase
Introduction
As a potential answer to the global energy crisis and environmental pollution, the application of hydrogen energy has attracted great attention. Extensive work has been devoted to the efficient production of hydrogen at low cost. Since Fujishima and Honda firstly reported the decomposition of water on illuminated TiO2 electrodes in 1972 [1], photocatalytic water splitting has been considered as a promising strategy of converting solar energy into hydrogen energy. Because of the advantages of photocatalytic H2 production, such as pollution-free and low energy consumption, researchers have made enormous efforts to improve the efficiency of H2 production by modifying TiO2 nanostructures [2], [3], as well as developing new photocatalysts [4], [5], [6]. However, most of the photocatalysts could only respond to UV irradiation that takes up ∼4% of solar energy, which limits the application of photocatalysts to a great extent.
Due to the suitable energy band corresponding to visible-light absorption, chalcogenides are regarded as good candidates for photocatalysts. Among them, CdS is the most widely used sulfide owing to its superior physical properties compared with other sulfide semiconductors [7], [8], [9], [10], [11], [12], [13]. The band gap of CdS is relatively narrow and the absorption edge reaches 510 nm. Thus, CdS exhibits wide absorption of visible light. Meanwhile, CdS is favorable for reducing water due to the sufficiently high flat-band potential [14]. In spite of the tremendous advantages of CdS used as a photocatalyst, there are still some shortcomings prohibiting the wide utilization of this semiconductor, i.e. the indispensable deposition of expensive noble metal on the surface of CdS to split water efficiently [15], [16]. Moreover, the photocorrsion of CdS, involving the transformation of CdS into Cd2+ and S under prolonged irradiation [11], has not yet been figured out. On the other hand, ZnS is another metal sulfide that has been extensively studied for water splitting. The band gap of ZnS is 3.66 eV, which is too large for visible light response [17], [18], [19]. Therefore, metal ions (i.e. Ni2+,Pb2+ and Cu2+) were doped into ZnS in order to shift the absorption edge of ZnS into visible light region [17], [19], [20]. However, the photocatalytic efficiency of the doped ZnS under visible light has not been satisfying yet.
As the solid solution of CdS and ZnS, Cd1−xZnxS possesses tunable composition as well as band gap [21]. The newly formed energy band in Cd1−xZnxS could respond to visible light while not act as recombination centers. Furthermore, the more negative reduction potential of the conduction band of Cd1−xZnxS could lead to more efficient hydrogen generation than that of cadmium sulfide. Though photocatalytic H2 production over Cd1−xZnxS was firstly reported as early as in 1986 [22], not until recent years has this solid solution been widely studied. In 2006, Guo et al. prepared Cd1−xZnxS by a simple coprecipitation method and optimized its performance by tuning the band gap energy [21]. They also improved the photocatalytic activity of Cd1−xZnxS by thermal sulfuration [23]. Lately, Cd1−xZnxS doped with Cu2+ and Ni2+ were studied, too [24], [25]. These previous results from different groups indicated that the solid solutions with hexagonal phase exhibited the highest activity with the composition of Cd0.8Zn0.2S [21], [23], [26].
Herein, a series of Cd1−xZnxS solid solutions with cubic zinc blend phase were successfully synthesized with the assistance of sodium dodecylsulfate (SDS) as the surfactant and thiacetamide (TAA) as the S source. The average crystallite sizes of as-obtained photocatalysts are in the range of 3–6 nm. The optimum composition is different from that of the solid solution with hexagonal phase. In particular, even without a co-catalyst, the photocatalytic H2 evolution rate, as high as 2640 μmol h−1 g−1, was obtained from Cd0.44Zn0.56S suspension containing SO32− and S2− as sacrificial reagents. The possible reason for the optimum performance was also discussed.
Section snippets
Synthesis of the solid solutions
All the reagents were of analytical grade and used without further purification. In a typical synthesis, different molar ratios of Zn(CH3COOH)2 and Cd(CH3COOH)2 and appropriate amount of SDS were dissolved in 100 ml of deionized water. The solution was constantly stirred for 20 min at room temperature. Afterwards, excessive TAA was added. The final solution was heated at 80 °C for 5 h. The as-produced precipitate was washed with deionized water for several times, and then dried at 80 °C overnight
Characterization of photocatalysts
The XRD patterns of the solid solutions with different compositions and the standard diffraction patterns of cubic ZnS and CdS are depicted in Fig. 1A. Three diffraction peaks are detected in all samples, corresponding to the (111), (220), (311) planes of zinc-blende phase. The progressive broadening of all three reflection peaks with increasing amount of ZnS is observed, suggesting the systematic decrease in particle size. Since for true solid solutions there holds the well-known Vegard's law,
Conclusion
A series of Cd1−xZnxS solid solutions with high visible-light-induced photocatalytic activity of water splitting were synthesized via a simple SDS-assisted method. The solid solutions exhibited cubic zinc blend structure, and obvious peak shifts have been found in the XRD patterns. The photocatalytic H2 evolution from aqueous solutions of sulfide and sulfite were investigated and the solid solutions of Cd0.44Zn0.56S showed the highest photocatalytic activity. The high H2 evolution rate of this
Acknowledgement
We acknowledge the financial support from the National Natural Science Foundation of China (50672117, 50732004), National Basic Research Program of China (973 Program, 2007CB613305), and Solar Energy Project of Chinese Academy of Sciences.
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