Metal Cd can serve as an analogue of cocatalysts loaded on CdS to separate electrons and holes to significantly enhance the efficiency of CdS photocatalytic hydrogen production. A series of Cd/CdS photocatalysts were synthesized at various molar ratios of CdSO4 and Na2S2O3·5H2O in the presence of simulated solar irradiation. Superior photocatalytic activities relative to that of pure CdS were observed on the Cd/CdS photocatalysts. When the optimal molar ratio R of Na2S2O3·5H2O to cadmium salt is 7, it results in a high average H2-production rate of 1753 μmol h−1 . Possible mechanisms for both the formation and the enhanced photocatalytic activity of Cd/CdS were proposed on the basis of theoretical speculation and experimental observations. Most of all, this work highlights that (i) Cd/CdS leads to an improvement in the photocatalytic activity for H2 generation; and (ii) adding Na2S2O3·5H2O into the solution containing cadmium ions to prepare Cd/CdS can effectively remove cadmiumions.

Experimental 

2.1. Preparation of Cd/CdS photocatalysts

 All starting materials and reagents were commercially available and were used without further purification. In a typical synthesis, a 40 mL solution containing 0.15 M CdSO4·8/3H2O and 1.2 M Na2S2O3·5H2O in a beaker was used as a precursor under vigorous stirring for 20 min and by sonication for 30 min. The beaker is then placed under a simulated solar lamp (Aulight, CEL-HXF300) and kept stirring. When the lamp is turned on, a photochemical reaction takes place. A gray yellow precipitate is slowly generated with the reaction time in the presence of simulated solar irradiation at room temperature. After 24 h, the gray-yellow precipitate composed of Cd and CdS had formed over the whole solution. We varied the molar ratio of Na2S2O3·5H2O to cadmium salt, represented as R, from 5 to 10 while keeping the CdSO4 concentration fixed. When Cd/CdS were put into 0.1 M nitric acid, pure CdS formed after several minutes. Finally, the resulting Cd/CdS photocatalysts were centrifuged and subsequently washed with deionized water and ethanol, and then dried under vacuum at 60 °C for 8 h. 2.2. Photocatalytic activity About 0.15 g of Cd/CdS photocatalyst powders was dispersed in 100 mL of aqueous solution containing 0.5 M Na2S and 0.5 M Na2SO3 as the sacrificial reagents in the reactor under the vertical irradiation of a 300 W Xe lamp. The suspension was then thoroughly degassed and irradiated by a 300 W Xe lamp (Aulight, CEL-HXF300) which is equipped with an optical filter (0.1 M NaNO2 aqueous solution) to cut off the light in the ultraviolet region. The amounts of H2 evolution were measured by using gas chromatography (QC-9101, 5 Å column) with a thermal conductivity detector (TCD) and Ar as the carrier gas.

Conclusions 

A series of Cd/CdS photocatalysts with various molar ratios of Na2S2O3·5H2O to CdSO4 were successfully synthesized. Cd as an analogue of cocatalysts introduced in the preparation of CdS was demonstrated to be an efficient visible light (λ > 400 nm) responsive photocatalyst for hydrogen evolution in photocatalytic water splitting reactions. The experimental results showed that most of the Cd/CdS photocatalysts had higher photocatalytic activity than pure CdS because of effective photoexcited electron–hole pair separation. In addition, the reaction time and stability of the composite photocatalyst were enhanced a lot. Through this research we found that Cd has a great influence on the photocatalytic activity of CdS, which could effectively prevent the recombination of photogenerated electrons and holes and plays an important role in the highly active photocatalysts for solar energy conversion. In summary, our experimental results demonstrated that a certain amount of Na2S2O3·5H2O solution added to a solution containing cadmium ions to form Cd/CdS photocatalysts could not only effectively remove cadmium ions but also produce hydrogen efficiently under visible light irradiation. Furthermore, this approach is a green and effective way to control heavy metal pollution. We put forward a new method to reclaim valuable raw materials from industrial wastewater, and it will have good application prospects. 

Acknowledgements 

This work was financially supported by the National Key Basic Research and Development Program (2009CB220000), the National Natural Science Foundation of China (51262028, 21065010), the Program for Changjiang Scholars and Innovative Research Team in University (IRT1177), Fundamental Research Funds for the Gansu Universities, the Natural Science Foundation of Gansu Province (1107RJZA194), the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (LZJ1206) and the Young Teacher Research Foundation of Northwest Normal University (NWNU-LKQN-11-17)