Relationship between the Photocatalytic Hydrogen Ion Reduction and Charge Carrier Dynamics of Pt/Cd1- xNixS Catalysts

The performance of a semiconductor photocatalyst is affected by bulk and surface doping because of changes in the charge carrier dynamics. Finding relationships between these changes upon doping and catalytic performance offers needed information for the fundamental understanding of the overall catalytic reaction. In this work, the electron transfer for the hydrogen ion reduction reaction over a prototype Pt/CdS catalyst doped with Ni2+ cations (Cd0.99Ni0.01S) was studied using fs-pump probe transient absorption spectroscopy (TAS) under photocatalytic reaction conditions. Cd0.99Ni0.01S is composed of both hexagonal and cubic phases (X-ray diffraction) with average particles of 7 nm in size (transmission electron microscopy). TAS of Cd0.99Ni0.01S in the presence and absence of a hole scavenger (benzyl alcohol) and in the presence and absence of Pt particles helped to further probe into the origin of the two most pronounced transient signals in the 400-800 nm range. These are the ground-state bleaching at ca. 480 nm and the photoinduced absorption signal at ca. 600 nm. The first is largely linked to electron de-excitation lifetime and the latter to hole lifetime. From the decay kinetics under catalytic reaction conditions, it was possible to compute for the charge transfer yields (φ) from the semiconductor to the Pt metal particles (electron transfer) and from the benzyl alcohol to the semiconductor (hole trapping). The rate of the photocatalytic hydrogen production shows a positive relationship with the decay kinetics obtained by fs-pump probe measurements. While the photocatalytic reaction rates were found to be constant for successive runs, X-ray photoelectron spectroscopy Pt 4f and Ni 2p lines of the used catalyst showed a decrease in their content. Thus, care needs to be taken when relying solely on reaction rate measurements to test for catalytic stability.