.
Angewandte
Communications
DOI: 10.1002/anie.201302238
Elemental Photocatalysts
Visible-Light-Responsive b-Rhombohedral Boron Photocatalysts**
Gang Liu, Li-Chang Yin, Ping Niu, Wei Jiao, and Hui-Ming Cheng*
Photocatalytic solar-energy conversion has been attracting
worldwide attention owing to its great significance in the
provision of renewable energy and protection of the environ-
[
1–3]
ment.
As important as the tailoring of well-known photo-
[
4–7]
catalysts, such as TiO , for high photocatalytic efficiency
is
the investigation of unknown semiconductor photocata-
2
[
8–16]
lysts.
So far, hundreds of photocatalysts have been
[
3,17,18]
examined, most of which have been compounds.
Recently, elemental semiconductors (Si, Se, P, S) have
emerged as an attractive class of photocatalysts owing to
their visible-light response and suitable band edges for
[
19–24]
targeted photocatalysis reactions.
It is logical to also
anticipate the use of elemental boron in photocatalysis
because of its semiconducting properties. When we inves-
tigated b-rhombohedral boron crystals with and without an
amorphous oxide layer on their surface, we discovered that
the crystals were indeed photocatalytically active under
visible light, and that the existence of a surface amorphous
oxide layer substantially impaired their photocatalytic activ-
ity. The findings in this study may open a door to the
development boron-based photocatalysts.
Figure 1. Structure characterization: a) XRD pattern of sub-microme-
ter-sized b-rhombohedral boron. b) High-resolution TEM image
recorded at the edge of a sub-micrometer-sized b-rhombohedral boron
Boron has aroused wide interest owing to its fascinating
properties (light weight, high strength, high hardness, high
melting point, high chemical resistance, typical semiconduc-
tivity, and superconductivity at high pressure), although
particle. The inset in (a) shows the structure of an icosahedral B12
cluster.
[
25–27]
a pure phase was not obtained until 1909.
It has at least
a consequence, the theoretically predicted metallic property
does not agree with the experimentally derived p-type
semiconducting behavior of b-rhombohedral boron with
1
7 polymorphs (or more precisely, boron-rich compounds) as
[28]
a result of electron-deficient bonding.
All polymorphs
[
32]
contain B12 icosahedral clusters as a basic building block (see
the inset in Figure 1a). Among these polymorphs, a-tetrag-
onal, a-rhombohedral, b-tetragonal, and b-rhombohedral
boron are the four main forms under ambient conditions.
Experimentally, b-rhombohedral boron is the most thermo-
dynamically stable form, although its superior stability was
a proposed bandgap of 1.5–1.6 eV.
Improved crystallo-
graphic studies and optimized theoretical structure models
have reduced the gap between experiment and theory to some
extent and provided a strong background for the further
investigation of structure–property relationships and the
exploration of new uses for b-rhombohedral boron.
[29–31]
not supported by theoretical investigations until 2007.
The bandgap of 1.5–1.6 eV indicates that b-rhombohedral
boron should respond to visible light over a wide range of
wavelengths. It was also found in quasi-four-electrode meas-
urements that b-rhombohedral boron exhibited strong photo-
conductivity under illumination by a halogen lamp or argon-
This long-term discrepancy stems from the difference
between the idealized structural model of 105 boron atoms
(
B105) that is used to describe the b-rhombohedral form and
the real structure, which contains partially occupied sites. As
[
26]
ion laser (at 488 nm).
Encouraged by these favorable
properties, we investigated the photocatalytic activity of two
kinds of commercially available b-rhombohedral boron
crystals (sub-micrometer-sized and micrometer-sized) by
structural characterization, measurement of their optical
properties, and theoretical calculation of their electronic
structures.
[
*] Dr. G. Liu, Dr. L.-C. Yin, P. Niu, W. Jiao, Prof. H.-M. Cheng
Shenyang National Laboratory for Materials Science
Institute of Metal Research, Chinese Academy of Sciences
72 Wenhua Road, Shenyang 110016 (China)
E-mail: cheng@imr.ac.cn
[
**] We thank the Major Basic Research Program, Ministry of Science
and Technology of China (No. 2009CB220001), the NSFC (No.
Figure 1a shows the X-ray diffraction (XRD) pattern of
sub-micrometer-sized boron powder. All diffraction peaks
can be assigned to b-rhombohedral boron (JCPDS: 11-0618;
50921004, 51002160, 21090343, 51172243, 51202255), and the
Chinese Academy of Sciences (KJCX2-YW-H21-01) for financial
support. L.-C.Y. acknowledges support from Shenyang Supercom-
puting Center, CAS.
ꢀ
space group: R3m (166); a = 10.952 ꢀ, c = 23.824 ꢀ). Besides
the sharp peaks, there are several diffuse peaks in the
background, which indicate the existence of amorphous
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 6242 –6245