Photocatalytic H2 evolution under visible light irradiation on a novel CdxCuyZn1−x−yS catalyst
Introduction
Photocatalytic water splitting using solar energy has been regarded as an attractive solution to resolve the global energy and environmental problems, but most of the photocatalysts only work in ultraviolet region due to their wide band gap [1], [2]. In order to use the solar energy more effectively, visible-light-driven photocatalysts must be developed. However, there are just a few of photocatalysts that can split water under visible light irradiation until now [3], [4].
It is necessary for photocatalysts with visible light response to have an appropriate band gap. To control the band structure, both doping of foreign elements into active photocatalysts with wide band gap and making solid solutions between photocatalysts with wide and narrow band gaps are often used. TiO2 codoped with Ni and Ta [5], ZnS doped with Cu or Ni [6], [7] have been reported as active photocatalysts under visible-light irradiation. In recent years, designing photocatalysts by making solid solutions has made a great progress, such as ZnS–CdS [8], [9], [10], AgInS2–ZnS [11], CuInS2–ZnS [12], AgInS2–CuInS2–ZnS [13] and GaN–ZnO [14].
The conduction band level of ZnS is high enough to reduce H2O to H2, so it has an excellent ability to produce H2 from aqueous solution containing sacrificial reagents even without Pt cocatalyst under UV light irradiation [15]. Therefore, it is a good host material for the development of visible-light-driven photocatalysts by doping of foreign elements or making solid solutions with other photocatalysts with narrow band gaps [6], [7], [8], [9], [10], [11], [12], [13]. CdxZn1−xS has been extensively studied as a sulfide solid solution photocatalyst. It is well-known that CdS can produce H2 under visible light because of its small band gap, but the ability is weak without Pt cocatalyst [8], [16]. As mentioned above, ZnS is incapable to produce H2 under visible light irradiation because of its wide band gap. The CdxZn1−xS solid solution can solve these problems to some extent, nevertheless there is still a shortcoming, that is, when the value of x is low, the band gap of the solid solution is wide, and when the value is high, the level of conduction band is low which leads to weak photoactivity without Pt cocatalyst [8]. It is reported that energy levels around valence bands formed by Cu doped in ZnS result in a visible light response of the Zn1−xCuxS photocatalyst [6]. If Cu is doped in CdxZn1−xS where the value of x is low to adjust the valence band, a photocatalyst with a narrow band gap and a high conduction band level may be obtained.
In this article, it is reported that the preparation and characterization of the new photocatalyst CdxCuyZn1−x−yS. Photocatalytic H2 evolution under visible light (λ ⩾ 430) from aqueous solutions containing Na2S and Na2SO3 was examined.
Section snippets
Preparation of photocatalysts
All chemicals were of analytical grade and used without further purification. The samples of CdxCuyZn1−x−yS were prepared by adding dropwise an aqueous Na2S solution to an aqueous solution mixture of Zn(NO3)2, Cu(NO3)2 and Cd(NO3)2 in a stoichiometric molar ratio under vigorous stirring. The amount of Na2S was in excess for complete reaction. The mixed solution was stirred for an additional 12 h at the room temperature. The resulting precipitates were filtered and washed by distilled water
Crystal structure and morphology
Fig. 1 shows X-ray diffraction patterns of CdxCuyZn1−x−yS. In Fig. 1c, the diffraction peaks at 2θ values of 28.6, 47.6 and 56.3°, matched perfectly with the (1 1 1), (2 2 0) and (3 1 1) crystalline planes of cubic ZnS. When Cd2+ existed in the photocatalyst, the diffraction peaks were shifted to the lower-angel side. The reason of the shift is that, the radius of Cd2+ (0.97 Å) is larger than Zn2+ (0.74 Å). It also indicates that Cd2+ entered into the frame of ZnS. Because the radius of Cu2+ (0.72 Å) is
Conclusion
CdxCuyZn1−x−yS were prepared by coprecipitation method and studied as photocatalysts for H2 evolution under visible-light irradiation. The results of diffuse reflectance spectra showed that the existence of Cd and Cu simultaneity altered the band structure of the host material ZnS, and made the new compound possess a narrow band gap. Compared with Cd0.1Zn0.9S and Cu0.01Zn0.99S, Cd0.1Cu0.01Zn0.89S had the smallest band gap and the highest activity for H2 evolution. It is noteworthy that the high
Acknowledgments
The authors gratefully acknowledge the financial support of the National Basic Research Program of China (Grant No. 2003CB214500) and the National Natural Science Foundation of China (Grant No. 90610022, 50521604).
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