Efficient Cocatalyst-Free Piezo-Photocatalytic Hydrogen Evolution of Defective BaTiO3–x Nanoparticles from Seawater

Hydrogen is a promising fossil-fuel alternative fuel owing to its environmentally neutral emissions and high energy density. However, the need for purified water and external power are critical hindrances to the implementation of hydrogen production. The present work demonstrates the potential to overcome these shortcomings through piezo-photocatalysis of seawater using defective BaTiO3–x (BTO) nanoparticles. This material was made piezoelectrically active by a straightforward annealing process under different atmospheres, including O2, N2, Ar, or H2, the latter of which caused Ti4+ → Ti(4–x)+ multiple reductions and structural distortions that stabilize piezoelectric tetragonal domains. A suite of experimental techniques was employed to reveal the effects of reduction on the energy band structure. A substantial piezoelectric effect and the presence of self-polarization were confirmed by piezoresponse force microscopy, while simulation work clarified the role of vibrations on band bending deriving from the self-polarization. The hydrogen evolution through photocatalysis, piezocatalysis, and piezo-photocatalysis over the defective BaTiO3–x nanoparticles was characterized with deionized (DI) water, simulated seawater, and natural seawater. A promising HER with a rate of 132.4 μmol/g/h was achieved using DI water through piezo-photocatalysis without a cocatalyst. In contrast, a substantial HER rate of 48.7 μmol/g/h was obtained for natural seawater, despite the deleterious impact of dissolved ions. The present work offers new perspectives for large-scale green H2 production using abundant natural resources with a conventional piezoelectric material that is readily available but still affected by the ions dissolved in seawater.