Sunlight-Driven Hydrogen Generation: Acceleration of Synergism between Cu–Ag Cocatalysts on a CdS System

The high cost and limited sources of fossil fuels led researchers to discover alternative sources of energy. The aim of this work is to explore stable and efficient catalysts for hydrogen evolution under sunlight. From this perspective, various catalysts including CdS, Ag@CdS, Cu@CdS, and Cu/Ag@CdS have been designed and synthesized via the hydrothermal method. To enhance the surface stability and activity of CdS, copper and silver metals were selected as cocatalysts. Cocatalyst contents were in situ deposited over the surfaces of CdS using a chemical reduction approach. The optical and structural properties of the catalysts were investigated via UV–vis/diffuse reflectance spectroscopy (DRS), Raman, photoluminescence (PL), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM) techniques. Their chemical compositions and surface characteristics were confirmed by energy-dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) studies. Thermal stability was assured by thermogravimetric analysis (TGA), whereas magnetic properties were determined by electron paramagnetic analysis (EPR). Photoreactions were done in a quartz photoreactor (150 mL/Velp Scienific) and hydrogen generation activities were monitored using gas chromatographs, i.e., GC-TCD (Shimadzu-2010/Japan). The results depict that catalysts with 2% Cu and 1% Ag on CdS exhibit the highest activity (i.e., 18.93 mmol g–1 h–1 with 45.04% quantum efficiencies). The higher activities were attributed to the synergism generated between the Cu and Ag cocatalysts during the photoreaction. It has been predicted that cocatalysts enhance the electron populations on CdS surfaces by the surface plasmon resonance (SPR) of Cu/Ag cocatalysts. Schottky junctions formed by Ag cocatalysts on CdS restrict the charge recombination (i.e., back reactions). Additionally, various factors like pH, temperature, photocatalyst dosage, and intensity of light have been evaluated and discussed to optimize the ideal conditions. The results of this study hold promise for an eventual transition to replace the costly and conventional catalysts used for sustainable hydrogen generation.