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 (Ptnp/TiO2-NB) were first applied with a Cu2ZnSnS4-based photocathode for solar water splitting. It was found that the loaded amount of Ptnp and the thickness of the Ptnp/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. Ptnps 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 Ptnp's adsorption of H+ ions allow the surface area of the 3D layer composed of Ptnp/TiO2-NB spheres to be fully utilized for hydrogen production. Interestingly, the surface SPR effect exists at the Ptnp/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 Ptnp/TiO2-NB/CdS/CZTS photocathode and therefore impacted the carrier transfer within the photocathode. As a result, the best VOC of 0.7 VRHE, a J0 of 17.2 mA/cm2 and the highest HC-STH efficiency of over 3.5% were achieved by the Ptnp/TiO2-NB/CdS/CZTS photocathode.