ar9b00353_si_001.pdf (442.39 kB)
Hybrid Microwave Annealing Synthesizes Highly Crystalline Nanostructures for (Photo)electrocatalytic Water Splitting
journal contribution
posted on 2019-10-11, 15:37 authored by Hemin Zhang, Jae Sung LeeConspectusHydrogen is regarded as an ideal
energy carrier for the hydrogen economy that could
replace the current hydrocarbon
economy in order to achieve global energy security and mitigate climate
change. For this purpose, H2 has to be produced from renewable
sources (e.g., solar and wind) without producing global-warming CO2.(Photo)electrolysis of water into H2 and
O2 is one of the most promising technologies for the production
of
renewable H2, which requires (photo)electrocatalysts of
high efficiency, chemical robustness, and scalability. An essential
attribute required for high-efficiency (photo)electrodes is high crystallinity
with few defects to facilitate charge transfer without recombination.
To this end, fabrication of photoelectrodes is usually completed with
high temperature thermal annealing in a furnace. However, conventional
thermal annealing (CTA) always results in undesirable crystal sintering,
which reduces the surface area, and damage to the transparent conducting
oxide (TCO) substrate. An emerging alternative method, hybrid microwave
annealing (HMA), offers the beneficial effect of the high-temperature
annealing (crystallinity) while minimizing its negative effects of
sintering and TCO damage, enabling the fabrication of efficient (photo)electrodes
for water splitting.HMA combines direct microwave heating with
additional heating from
an effective microwave absorber (called a susceptor), thereby avoiding
a nonuniform temperature distribution between the interior and exterior
of the synthesized material. More importantly, an extremely high temperature
of the entire sample can be reached in only a few minutes. Compared
with CTA, HMA has several advantages in the preparation of (photo)electrodes:
(i) formation of a high-purity phase; (ii) high crystallinity with
fewer defects; (iii) preservation of the original nanostructure; (iv)
less damage to the TCO substrate for photoelectrodes; (v) smaller
nanocrystals and uniform dispersion of catalyst particles. Overall,
HMA is a convenient, ultrafast, and energy-economical technology for
the synthesis of efficient (photo)electrodes.In this Account,
we discuss recent progress made in our laboratory
on HMA for preparing photoanodes (Fe2O3, BiVO4, ZnFe2O4, and Fe2TiO5), photocathodes (Cu2O and CuFeO2),
and a graphene-based electrocatalyst (MoS2/graphene composite),
which exhibit distinctive behavior and efficient performance in (photo)electrocatalytic
water splitting. In particular, we have advanced the HMA technique
further to synthesize hematite-based photoanodes with core–shell
heterojunction nanorods (Nb,Sn:Fe2O3@FeNbO4 and Ta,Sn:Fe2O3@FeTaO4)
by solid–solid interface reaction, which simultaneously achieves
multiple doping effects (Nb or Ta, Sn) to improve the photoelectrocatalysis
of water splitting. Thus, this Account focuses on the synthetic aspects
of HMA, which may offer new research opportunities for the synthesis
of other metal oxide (photo)electrode materials and hybrid electrocatalysts
in the fields of solar energy conversion and storage, secondary batteries,
and H2 fuel production.