A physico-chemical insight into the benzaldehyde reduction mechanism catalysed by CdS/TiO2 composites activated by visible light

CdS/TiO 2 1/n ( n = 4, 8 and 32) composite materials are synthesized maintaining constant the amount of CdS and varying the amount of TiO 2 . XRD, HRTEM, FESEM/EDS analysis point out that both CdS and TiO 2 maintain their original crystalline structure but are in intimate interconnection. BET and porosity analysis evidence that SSA are similar in all cases, with CdS/TiO 2 1/32 showing also mesoporosity in addition to microporosity. Microcalorimetric analysis demonstrates that all composite materials are hydrophilic in nature, but CdS/TiO 2 1/8 and CdS/TiO 2 1/32 show a larger number of sites interacting with water molecules . Photoelectrochemical investigation showed that the intimate contact between CdS and TiO 2 , allows electrons injection from CdS to TiO 2 increasing the lifetimes of separated charges. Cyclic voltammetry experiments demonstrate that both CdS and TiO 2 can reduce benzaldehyde with electrons residing in their conduction bands, whereas the surface features of the composite favor the photocatalyst/substrate interactions. The understanding of how to combine energetics, heterojunctions, morphology and surface characteristics of the photoactive materials opens the way towards an efficient design of CdS/TiO 2 1/n composite systems, which show significant advances over the use of the two separate components: i) the presence of TiO 2 modifies the characteristics of the CdS surface, favouring the approach of organic molecules, such as benzaldehyde and its reaction intermediates; ii) CdS, activated by irradiation with visible light, can inject electrons onto TiO 2 which is in turn activated without using UV light. This paves the way for the application of more sustainable experimental conditions to achieve reductive transformations in the presence of TiO 2 . • In the prepared CdS/TiO 2 1/n composites, only the amount of TiO 2 is varied. • Effect of heterojunction extension on the photocatalytic properties is investigated. • Favourable band energetics is not enough to efficient photocatalysis. • Photoactivity depends on surface properties. • Aldehyde to alcohol photoreduction is related to the hydrophilicity of the surface.