Enhanced Solar Water Oxidation Performance of TiO₂ via Band Edge Engineering: A Tale of Sulfur Doping and Earth-Abundant CZTS Nanoparticles Sensitization

We report the rational design and fabrication of earth-abundant, visible-light-absorbing Cu₂ZnSnS₄ (CZTS) nanoparticle (NP) in situ sensitized S doped TiO₂ nanoarchitectures for high-efficiency solar water splitting. Our systematic studies reveal that these nanoarchitectures significantly enhance the visible-light photoactivity in comparison to that of TiO₂, S doped TiO₂, and CZTS NP sensitized TiO₂. Detailed photoelectrochemical (PEC) studies demonstrate an unprecedented enhancement in the photocurrent density and incident photon to electron conversion efficiency (IPCE). This enhancement is attributed to the significantly improved visible-light absorption and more efficient charge separation and transfer/transport, resulting from the synergistic influence of CZTS NP sensitization and S doping, which were confirmed by electrochemical impedance spectroscopy (EIS). Moreover, density functional theory (DFT) calculations supported by the experimental evidence revealed that the gradient S dopant concentration along the depth direction of TiO₂ nanorods led to the band gap grading from ∼2.3 to 2.7 eV. This S gradient doping introduced a terraced band structure via upshift of the valence band (VB), which provides channels for easy hole transport from the VB of S-doped TiO₂ to the VB of CZTS and thereby enhances the charge transport properties of the CZTS/S-TNR photoanode. This work demonstrates the rational design and fabrication of nanoarchitectures via band edge engineering to improve the PEC performance using simultaneous earth-abundant CZTS NP sensitization and S doping. This work also provides useful insight into the further development of different nanoarchitectures using similar combinations for energy-harvesting-related applications.