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.