Surface plasmon resonance effect of a Pt-nano-particles-modified TiO₂ nanoball overlayer enables a significant enhancement in efficiency to 3.5% for a Cu₂ZnSnS₄-based thin film photocathode used for solar water splitting

TiO₂ nanoballs modified with Pt nano particles (Pt_np/TiO₂-NB) were first applied with a Cu₂ZnSnS₄-based photocathode for solar water splitting. It was found that the loaded amount of Pt_np and the thickness of the Pt_np/TiO₂-NB layer significantly impact the photoelectrochemical (PEC) properties of the Cu₂ZnSnS₄-based photocathode. Compared with the TiO₂ layer achieved by atomic layer deposition (ALD), the nanoball-structured TiO₂ exhibits many superiorities, such as a high specific surface area with significant hydrophilia, low preparation cost and controllable size. Pt_nps were observed to be easily and homogeneously dispersed on the outer surface of TiO₂-NBs. This combination not only greatly increased the number of active points and enhanced the solar water splitting efficiency but also induced the Pt catalyst points to be uniformly located on the 3D-structured TiO₂ layer of the resultant photoelectrode. In addition, the hydrophilicity of TiO₂-NB and Pt_np's adsorption of H⁺ ions allow the surface area of the 3D layer composed of Pt_np/TiO₂-NB spheres to be fully utilized for hydrogen production. Interestingly, the surface SPR effect exists at the Pt_np/TiO₂-NB layer and can influence the charge transfer efficiency at the photocathode/solution interface. Furthermore, the SPR-induced enhanced electrical field also affected the inner potential of the Pt_np/TiO₂-NB/CdS/CZTS photocathode and therefore impacted the carrier transfer within the photocathode. As a result, the best V_OC of 0.7 V_RHE, a J₀ of 17.2 mA/cm² and the highest HC-STH efficiency of over 3.5% were achieved by the Pt_np/TiO₂-NB/CdS/CZTS photocathode.