Sunlight-harnessing and storing heterojunction TiO2/Al2O3/WO3 electrodes for night-time applications

Abstract

Heterojunction TiO₂/Al₂O₃/WO₃ (TAW) photoelectrodes were fabricated to harvest sunlight and simultaneously store the photogenerated electrons for prolonged periods. The insertion of an Al₂O₃ (A) blocking layer between the TiO₂ and WO₃ layers enhances the open circuit potentials (OCPs) of TiO₂/WO₃ (TW) in 0.1 M Na₂SO₄ upon light-on (photo-charging). Surprisingly, Al₂O₃ increases the residence time of the OCPs at sufficiently negative potentials (<−0.3 V_SCE) by 4–16 times depending on the Al₂O₃ content, even after AM 1.5 G light-off (discharging). Such prolonged discharging periods are further verified by a comparison of the electron lifetimes, which shows that the photogenerated electrons at TAW survive 6 times longer than those at TW. When Cr⁶⁺ is spiked into the solution during the discharging period, the change in OCP is accelerated, and simultaneously Cr⁶⁺ is reduced to Cr³⁺. The number of electrons stored and utilized for Cr⁶⁺ reduction is estimated to be around 1.3 × 10¹⁷, which is almost tripled by the insertion of Al₂O₃. The Al₂O₃ effect is similarly found in Ag⁺ reduction with 4 times more electrons, but is less pronounced for the reduction of methylene blue and polyoxometalates due to both their reduction potentials being so close to the WO₃ conduction band level and re-oxidation pathway by dissolved oxygen. Al₂O₃ appears to effectively block the transfer of trapped electrons at WO₃ to TiO₂, making the electrons more available for the reduction of substrates. Various surface analyses (SEM, EDX, XPS, and UV-Vis) were completed to examine and compare single, binary, and ternary component electrodes (T, W, TA, TAW, etc.). On the basis of the surface and electrochemical study, the effects of Al₂O₃ on the charging and discharging processes are discussed in detail.