Pt Particle Size Affects Both the Charge Separation and Water Reduction Efficiencies of CdS–Pt Nanorod Photocatalysts for Light Driven H2 Generation

Decreasing the metal catalyst size into nanoclusters or even single atom is an emerging direction of developing more efficient and cost-effective photocatalytic systems. Because the catalyst particle size affects both the catalyst activity and light driven charge separation efficiency, their effects on the overall photocatalytic efficiency are still poorly understood. Herein, using a well-defined semiconductor-metal heterostructure with Pt nanoparticle catalysts selectively grown on the apexes of CdS nanorods (NRs), we study the effect of the Pt catalyst size on light driven H₂ generation quantum efficiency (QE_H2). With the increase of the Pt catalyst size from 0.7 ± 0.3 to 3.0 ± 0.8 nm, the QE_H2 of CdS-Pt increases from 0.5 ± 0.2% to 38.3 ± 5.1%, by nearly 2 orders of magnitude. Transient absorption spectroscopy measurement reveals that the electron transfer rate from the CdS NR to the Pt tip increases with the Pt diameter following a scaling law of d^5.6, giving rise to the increase of electron transfer efficiency at larger Pt sizes. The observed trend can be understood by a simplified kinetic model that assumes the overall efficiency is the product of the quantum efficiencies of charge separation (including hole transfer, electron transfer, and hole scavenging) and water reduction steps, and for CdS-Pt NRs, the quantum efficiencies of electron transfer and water reduction steps increase with the Pt sizes. Our findings suggest the importance of improving the quantum efficiencies of both charge separation and catalysis in designing efficient semiconductor-metal hybrid photocatalysts, especially in the regime of small metal particle sizes.