Piezo-Photocatalysis – a Promising Strategy for Energy Conversion
Authors: Jiang Yue , Mofarah Sajjad Seifi , Wang Danyang , Koshy Pramod , Sorrell Charles 
Published: December 2024

Piezocatalysis has gained considerable attention since it can deliver appreciable efficiencies by coupling the piezotronic and electrochemical effects. The piezopotentials generated by external stress are efficient in inducing charge carrier separation, thereby enabling energy conversion from mechanical energy (ultrasound, wind, and tidal) to chemical energy (hydrogen). However, the piezocatalytic hydrogen evolution reaction (HER) efficiencies currently remain lower than those from electrolysis or photocatalysis. An alternative approach to improve the HER performance is to activate the material using combined ultrasound and light (viz., piezo-photocatalysis). During the piezo-photocatalysis, the synergistic effect in charge carrier generation and separation from the creation of piezoelectric potential and photoactivation will lead to an enhanced HER efficiency. These phenomena indicate the synergy of the two processes, where the initial activation of the material by piezocatalysis overcomes the limitation of photocatalysis alone, thereby facilitating subsequent charge carrier generation and separation by combined piezocatalysis and photocatalysis.
Our present work reveals the potential of piezo-photocatalytic in energy conversion,especially in splitting of water and seawater, using lead-free perovskites, including BaTiO3-x (BTO) [1] and Na0.5Bi0.5TiO3 (NBT) [2] nanoparticles. The BTO was purchased commercially, and further heat treated in H2 to create defects. This annealing conditions were observed to impact the extent of Ti4+ → Ti(4-x)+ reductions and subsequent defect formation, leading to structural alterations which affected the stability of piezoelectric tetragonal BTO domains. Furthermore, reduction also altered the energy band structure. An efficient H2 evolution rate of 132.4 μmol/g/h was achieved from DI water using piezo-photocatalysis. On the other hand, the NBT was synthesized hydrothermally and then its defect structures were modified by varying the NaOH concentration. This engineering facilitates the substitution of Bi3+ by Na+ in the NBT solid solution, which also enables the regulation of oxygen vacancy and modification of the band structure. Consequently, the NBT exhibited an efficient HER rate of 140 μmol/g/h from DI water through piezo-photocatalysis. More significantly, the heterojunction also causes notable seawater splitting capability, with the HER rates of 68 μmol/g/h from simulated seawater and 58 μmol/g/h from natural seawater.
This approach of piezo-photocatalysis represents a new strategy for commercial H2 production, potentially using large-scale green technologies, including ultrasound vibrations deriving from wind energy and/or tidal energy (piezocatalysis) and sunlight (photocatalysis). It has the potential to provide the bases for novel systems aimed at a net zero emissions future while driving economic growth.
[1] Y. Jiang, C. Y. Toe, S. S. Mofarah, C. Cazorla, S. L. Chang, Y. Yin, Q. Zhang, P. Koshy, & C. C. Sorrell, ACS Sustainable Chemistry Engineering 11(8), 3370-3389 (2023).
[2] Y. Jiang, Zhou. S, S. S. Mofarah, P. Koshy, D. Wang & C. C. Sorrell, Nano Energy, 116, 108830 (2023).

Figure 1