Uniform carbon-coated CdS core-shell nanostructures: synthesis, ultrafast charge carrier dynamics, and photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using solar energy has received widespread attention, and strong performance photocatalysts are highly desired. In this work, uniform carbon-coated CdS nanostructures have been fabricated using ascorbic acid as the carbon source by a facile hydrothermal method and characterized using transmission electron microscopy (TEM). The thickness of the carbon layer can be well controlled by the amount of ascorbic acid added during the reaction. Compared to pristine CdS, carbon-coated CdS nanostructures exhibit stronger light absorption and more efficient electron transfer as determined by absorption and photoluminescence (PL) spectroscopy. Ultrafast charge carrier dynamics in the composite CdS/C structures were studied using femtosecond transient absorption (TA) spectroscopy, which revealed direct evidence of effective charge transfer from CdS to the carbon layer. In addition, the CdS/C composites were employed as photoanodes for PEC hydrogen generation, which showed significant improvement in photoactivity over pristine CdS nanospheres. The photocurrent density (-1.0 V vs. Ag/AgCl) of one of the composite structures, CdS/7-C, exhibited similar to 20 times enhancement compared with that of pristine CdS. The enhanced PEC property can be attributed to increased light scattering and consequently the light harvesting throughout the whole spectral wavelength, and the effective electron transfer from CdS to the carbon layer. Such carbon-coated semiconductor composites based on a simple and low-cost synthesis method should be useful in PEC as well as other applications such as photovoltaics, detectors and sensors.