Photocatalytic Hydrogen Evolution Using Ni–Pd/TiO2: Correlation of Light Absorption, Charge-Carrier Dynamics, and Quantum Efficiency

TiO2 surface modification with bimetallic nanoparticles (NPs) has demonstrated to be a strategy to enhance the hydrogen generation via photocatalysis and to minimize the use of expensive noble metals. A better understanding of the role of bimetallic NPs is of crucial importance to design efficient photocatalysts. Here, we show a systematic study of surface modification of commercial TiO2 (P25) with mono- and bimetallic (Ni, Pd, and Ni–Pd) NPs synthesized by radiolysis. The photocatalysts were characterized by High Resolution Transmission Microscopy (HRTEM), Scanning Transmission Electron Microscope (STEM), X-ray Diffraction (XRD), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), and UV–vis Diffuse Reflectance Spectroscopy (DRS). The charge-carrier dynamics was studied by Time Resolved Microwave Conductivity (TRMC). The photocatalytic activity was evaluated for hydrogen generation under UV–vis irradiation using polychromatic and monochromatic lights (action spectra analysis of apparent quantum efficiency). TiO2 modified with Pd–Ni bimetallic NPs exhibits a high activity for H2 generation, and a synergetic effect of the two metals was obtained. The study of light absorption, charge-carrier dynamics, and photocatalytic activity revealed that the main role of the metal NPs is to act as catalytic sites for recombination of atomic hydrogen.