Etching the road to a hydrogen economy using plasma jets
Scientists improve light-driven water-splitting to produce hydrogen by
etching the reaction catalyst with plasma jets in solution

Date:
June 3, 2020
Source:
Tokyo University of Science
Summary:
Hydrogen is a clean energy source that can be produced by splitting
water molecules with light. However, it is currently impossible to
achieve this on a large scale. In a recent breakthrough, scientists
developed a novel method that uses plasma discharge in solution to
improve the performance of the photocatalyst in the water-splitting
reaction. This opens doors to exploring a number of photocatalysts
that can help scale-up this reaction.



FULL STORY ========================================================================== Hydrogen is a clean energy source that can be produced by splitting water molecules with light. However, it is currently impossible to achieve this
on a large scale. In a recent breakthrough, scientists developed a novel
method that uses plasma discharge in solution to improve the performance
of the photocatalyst in the water-splitting reaction. This opens doors to exploring a number of photocatalysts that can help scale-up this reaction.


==========================================================================
The ever-worsening global environmental crisis, coupled with the
depletion of fossil fuels, has motivated scientists to look for clean
energy sources.

Hydrogen (H2) can serve as an eco-friendly fuel, and hydrogen
generation has become a hot research topic. While no one has yet found
an energy-efficient and affordable way to produce hydrogen on a large
scale, progress in this field is steady and various techniques have
been proposed.

One such technique involves using light and catalysts (materials that
speed up reactions) to split water (H2O) into hydrogen and oxygen. The catalysts have crystalline structures and the ability to separate
charges at the interfaces between some of their sides. When light hits
the crystal at certain angles, the energy from the light is absorbed
into the crystal, causing certain electrons to become free from their
original orbits around atoms in the material. As an electron leaves its original place in the crystal, a positively charged vacancy, known as
a hole, is created in the structure. Generally, these "excited" states
do not last long, and free electrons and holes eventually recombine.

This is the case with bismuth vanadate (BiVO4) crystal catalysts as
well. BiVO4 has been recently explored for water-splitting reactions,
given its promise as a material in which charge-separation can occur
upon excitation with visible light. The quick recombination of pairs of
charged entities ("carriers") is a disadvantage because carriers must separately partake in reactions that break up water.

In a recent study published in Chemical Engineering Journal,
scientists from the Photocatalysis International Research Center
at Tokyo University of Science, Japan, together with scientists from
Northeast Normal University in China, developed a novel method to improve
the charge-separation characteristics of decahedral (ten-sided) BiVO4
crystal catalysts. Prof Terashima, lead scientist in the study, explains, "Recent studies have shown that carriers can be generated and separated
at the interfaces between the different faces of certain crystals. In
the case of BiVO4, however, the forces that separate carriers are too
weak for electron-hole pairs that are generated slightly away from the interfaces. Therefore, carrier separation in BiVO4 decahedrons called
for further improvements, which motivated us to carry out this study."
In the technique they propose, BiVO4 nanocrystals are exposed to what
is called "solution plasma discharge," a highly charged jet of energetic
matter that is produced by applying high voltages between two terminals submerged in water.

The plasma discharge removes some vanadium (V) atoms from the surface
of specific faces of the crystals, leaving vanadium vacancies. These
vacancies act as "electron traps" that facilitate the increased separation
of carriers.

Because these vacancies are in greater number on the eight side faces
of the decahedron, electrons are trapped on these faces while holes
accumulate on the top and bottom faces. This increased charge separation results in better catalytic performance of the BiVO4 nanocrystals,
thereby improving its water splitting performance.

This study represents a novel use of solution plasma discharge to
enhance the properties of crystals. Prof Akira Fujishima, co-author of
the paper, says, "Our work has inspired us to reconsider other crystals
that are apparently ineffective for water splitting. It provides a
promising strategy using solution plasma to 'activate' them." The use
of solution-plasma discharge has many advantages over using conventional gaseous plasma that make it far more attractive from both technical and economic standpoints. Prof Xintong Zhang from Northeast Normal University, China, remarks, "Unlike gaseous plasma, which has to be generated in
closed chambers, solution plasma can be generated in an open reactor at
room temperature and in a normal air atmosphere. In addition, by working
with crystal powders in a solution, it becomes more convenient to change
the parameters of the process, and it is also easier to scale up."
This study hopefully takes us one step closer to an efficient way of
producing hydrogen so that we can finally do without fossil fuels and
other energy sources that are harmful to our planet. Further commenting
on the promise of this study, Prof Terashima says, "If efficient hydrogen energy can be produced using sunlight and water, two of the most abundant resources on earth, a dream clean society could be realized."

========================================================================== Story Source: Materials provided by Tokyo_University_of_Science. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Guangshun Che, Dandan Wang, Changhua Wang, Fei Yu, Dashuai Li,
Norihiro
Suzuki, Chiaki Terashima, Akira Fujishima, Yichun Liu, Xintong
Zhang.

Solution plasma boosts facet-dependent photoactivity of decahedral
BiVO4.

Chemical Engineering Journal, 2020; 397: 125381 DOI: 10.1016/
j.cej.2020.125381 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200603120549.htm https://www.sciencedaily.com/releases/2020/06/200603120549.htm

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