Enhanced Stability, Photoluminescence and Charge Transport Properties of TiO₂-Coated CsPbBr₃ Quantum Dots for Photoelectrochemical Solar Fuel Production

All-inorganic cesium lead bromide perovskite quantum dots (CsPbBr₃ QDs) exhibit excellent optical and electrical properties such as strong light absorption, tunable band gap, long diffusion lengths, ambipolar charge transport, and high charge carrier mobility. Therefore, they have been widely used for light-energy conversion including photovoltaic (PV) and photoelectrochemical (PEC) applications. However, CsPbBr₃ QDs have suffered from instability against the environmental factors in broad applications due to the inherent perovskite properties. This work thus aimed for two purposes. The first is to enhance the stability of CsPbBr₃ QDs, and the second is to use CsPbBr₃ QDs as a light-harvesting material incorporated with TiO₂ electrode for PEC oxidation of methanol. For the first aspect, TiO₂ coating on CsPbBr₃ QDs by ex-situ and in-situ methods was performed to prevent the agglomeration of the nanocrystals and to improve the stability of CsPbBr₃ QDs without heat treatment at high temperatures. The stability test revealed that TiO₂-coated CsPbBr₃ QDs prepared from the in-situ method exhibited a significant stability improvement against toluene, ultrasonication treatment in water, and light illumination. Furthermore, the results demonstrated the remarkable enhancement of photocurrent generation due to a suitable alignment of energy levels of TiO₂ and CsPbBr₃ and a stable structure of QDs, which plays an important factor in improving the PEC performance. Therefore, it was proved to be used as a good light-harvesting and electrode material in various photoelectrochemical applications. For the second aspect, the photocurrent generation via PEC oxidation of methanol was studied using surface-modified TiO₂-coated fluorine-doped tin oxide (FTO) as a photoanode in combination with CsPbBr₃ QDs dispersed in an electrolyte solution. Detailed studies revealed that surface modification of the TiO₂ layer was crucial for good interfacial adhesion of CsPbBr₃ QDs, which were surrounded by hydrophobic ligands, with the TiO₂ surface. Self-assembled monolayers of octadecylphosphonic acid (ODPA) were applied on the TiO₂ surface, resulting in the change of hydrophilic nature to a superhydrophobic surface. The photoluminescence measurement of the ODPA-modified TiO₂/FTO photoanode demonstrated a significantly higher photoluminescence intensity than that of the unmodified one, indicating that CsPbBr₃ QDs were well adsorbed on the TiO₂ surface. The photocurrent was generated via methanol oxidation by holes in CsPbBr₃ QDs. The current-voltage measurements revealed that in the presence of methanol, the current density was increased from 1.2 mA/cm² (without methanol) to the maximum of 1.6 mA/cm² under visible light irradiation, indicating that methanol was a sacrificial hole scavenger. As a consequence, the multicomponent ODPA-modified TiO₂/FTO photoanode in combination with CsPbBr₃ QDs together with the efficient hole scavenger of methanol in the system has been shown to promote the PEC oxidation performance, which can be applied in any PEC solar fuel production.