  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 implementation
of hydrogen production. The present work reveals the potential to overcome
these shortcomings through piezo-photocatalysis of seawater using BaTiO3-x
(BTO) nanoparticles. This material was made piezoelectrically active by
annealing under different atmospheres, including O2, N2, Ar, and H2, the latter
of which caused Ti4+ to Ti(4-x)+ multiple reductions and structural expansions
that stabilized piezoelectric tetragonal BTO domains. The resultant defect
equilibria combine ionic and electron effects, including Ti redox reactions,
charge-compensating surface oxygen vacancy formation, and color centre
alterations. Further, variety of experimental techniques revealed the effects
of reduction on the energy band structure. A strong piezoelectric effect and
the presence of self-polarization were confirmed by piezoresponse force
microscopy, while simulation work clarified the role of vibration on band
bending deriving from the former. The performance data contrasted H2 evolution
using deionized (DI) water, simulated seawater, and natural seawater subjected
to photocatalysis, piezocatalysis, and piezo-photocatalysis. An efficient H2
evolution rate of 132.4 micromol/g/h was achieved from DI water using
piezo-photocatalysis for 5 h. In contrast, piezocatalysis for 2 h followed by
piezo-photocatalysis for 3 h resulted in H2 evolution rates of 100.7
micromol/g/h for DI water, 63.4 micromol/g/h for simulated seawater, and 48.7
micromol/g/h for natural seawater. This work provides potential new strategies
for large-scale green H2 production using abundant natural resources with
conventional piezoelectric material while leveraging the effects of ions
dissolved in seawater.
