Conspectus
Green hydrogen as a clean energy
carrier plays a critical role
in tackling climate change. Solar-driven water splitting is regarded
as one of the most promising strategies for green hydrogen production,
but the solar-to-hydrogen (STH) conversion efficiency is still far
from the industrial requirement. Defect engineering is an effective
strategy to enhance the performance of photocatalysts and photoelectrodes.
In a crystal, defects range from micro- to macro-levels, such as atomic-scale
vacancies and dopants, distortion along the crystal, and defective
surface, which all influence the photoresponse, electrical conductivity,
and surface properties of semiconductor materials. The methodologies
of creating defects have been vigorously explored, while accurate
identification and characterization of defects have been focused on
much less. Moreover, the widely reported benefits of defects in the
photocatalytic system may not necessarily cancel out the negative
spillover effect of charge recombination. As a Janus-role structure
with different influences, the defect is an old, yet complex, notion,
and in-depth understanding of defects is vital toward efficient solar-driven
hydrogen generation.
In this Account, we provide an overview
of the engineering of defects
with some typical examples, including atomic vacancies of anion and
metal, long-range lattice distortion, and macro-range surface defects.
We start with our effort to explore the controllable generation of
vacancies, including oxygen vacancies, nitrogen vacancies, and metal
vacancies via post-treatment and in situ synthesis. A comprehensive
understanding of anion vacancies, especially the oxygen vacancies,
has been discussed. The identification of metal vacancies and their
role in the photoelectrochemical (PEC) process are further discussed.
We then move on to the long-range defects of lattice distortion to
understand their formation mechanism and contributions to PEC water
splitting. Finally, the defective surface is discussed based on the
case studies of two-dimensional (2D) mesoporous crystalline photocatalysts
and photoelectrodes.
Although significant advances have been
achieved in solar-driven
hydrogen production, there is still a long way ahead before reaching
the industrial production target. To inspire innovative defect engineering
for further improving the STH conversion efficiency, we provide the
following perspectives. First, precise synthesis strategies should
be developed to control the type and concentration of defects. Second,
reliable techniques and measurements should be established to make
accurate identification and characterization of defects. Last but
not least, the interaction of different types of defects should be
further explored to guide the rational design of material systems,
which may lead to unique synergistic mechanisms in further promoting
solar energy conversion efficiency.
