SrTiO3 Based Two Dimensional Nanoplatelets for Low-Cost Solar Hydrogen Generation: Materials Beyond the State-of-the-Art

Abstract

SrTiO3 is an important multifunctional perovskite, that can possess versatile characteristics such as photocatalytic behaviour, ferroelectricity, high electronic conductivity and others. In the context of solar hydrogen (H2) generation, Domen’s group has investigated SrTiO3 for almost two decades and finally have been able to achieve quantum efficiencies approaching unity [1] and large-scale demonstrations [2]. However, the achieved solar to hydrogen (STH) efficiencies remain well under 1%. One of the key bottlenecks limiting the photocatalytic efficiencies of SrTiO3 is its wide bandgap that limits its visible light absorption capability. Several strategies have been used to reduce the bandgap of SrTiO3 to improve visible light absorption but it can only be improved up to a certain limit. Nevertheless, there are other challenges as well in the form of higher recombination rates and an alternate way to further improve the photocatalytic rates is to employ strategies that can inhibit the recombination of photogenerated charges. Our group has been investigating the effect of Al doping on the morphological as well as chemical properties of SrTiO3 to gain fundamental understanding behind the improvement in photocatalytic rates. Our recent report explains that Al ions in SrTiO3 lattice act as hole trapping sites, which helps in suppressing charge recombination [3].Furthermore, to go beyond the current state-of-the-art, our group has been working on forming 2D SrTiO3 heterostructures. The 2D platelets facilitate efficient charge separation as their low thickness enables fast charge transport to the surface, where they are utilized for redox reactions. 2D area also offers large number of active sites and the possibility to form 2D/2D heterostructure with maximum face contact between two semiconductors. However, SrTiO3 does not have a thermodynamic tendency to grow in 2D morphology. Our group has designed hydrothermal conditions to form 2D SrTiO3-based platelets (Fig. 1) through topochemical transformation reaction at a much lower temperature of 200⁰C [4]. Without any support from noble-metal doping or cocatalysts, the epitaxially grown SrTiO3 heterostructural platelets showed stable and 15 times higher photocatalytic H2 production than the commercial SrTiO3 nanopowders. Our recent study [5] elucidates the role of supersaturation conditions on controlling the morphology of these 2D SrTiO3 nano-platelets and formulates their precise growth mechanism. By optimizing the synthetic procedure, the photocatalytic performance can be tuned to achieve much higher STH efficiencies. The presented synthesis strategy provides guiding principles and ideas for designing other defined 2D semiconductors under hydrothermal conditions, eventually going beyond the current state-of-the-art.