Facile Cd - Thiourea complex thermolysis synthesis of phase-controlled CdS nanocrystals for photocatalytic hydrogen production under visible light

We describe a simple cadmium - thiourea complex thermolysis route for the formation of CdS nanocrystals with controlled dispersity, crystalline phase, composition, average grain size, and band gap. Visible-light-driven photocatalytic activities for hydrogen production over the different CdS products have been compared. Phase structure and composition of the obtained CdS nanocrystals has been optimized either by changing the ratio of thiourea to Cd or by changing the annealing temperature. Over a broad annealing temperature range of 150-500°C, either cubic, a mixture of cubic and hexagonal, or hexagonal CdS nanocrystals are obtained at thiourea/Cd molar ratios of <1.0, 1.5-2.5, and 3.0-4.5, respectively. Nanocrystalline cubic CdS is stable at temperatures as high as 500°C for 0.5 h, and is converted to hexagonal CdS for annealing time longer man 1 h. The phase transition from cubic to hexagonal CdS occurs at temperatures of 200-300°C, and pure hexagonal CdS is formed at annealing temperatures higher than 600°C. The dispersity, crystallinity, and average grain size of the CdS nanocrystals has been determined as a function of annealing temperature and time. Increased photocatalytic activity is observed from the mixture of cubic and hexagonal CdS as compared to pure cubic or hexagonal CdS. Nearly monodisperse hexagonal CdS with good crystallinity and very fine particle size is expected to offer the highest photocatalytic activity for hydrogen production under visible light.