Visible‐Light‐Responsive β‐Rhombohedral Boron Photocatalysts

Abstract

Photocatalytic solar-energy conversion has been attracting worldwide attention owing to its great significance in the provision of renewable energy and protection of the environment. As important as the tailoring of well-known photocatalysts, such as TiO2, for high photocatalytic efficiency [4–7] is the investigation of unknown semiconductor photocatalysts. So far, hundreds of photocatalysts have been 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 targeted photocatalysis reactions. It is logical to also anticipate the use of elemental boron in photocatalysis because of its semiconducting properties. When we investigated 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 activity. The findings in this study may open a door to the development boron-based photocatalysts. Boron has aroused wide interest owing to its fascinating properties (light weight, high strength, high hardness, high melting point, high chemical resistance, typical semiconductivity, and superconductivity at high pressure), although a pure phase was not obtained until 1909. It has at least 17 polymorphs (or more precisely, boron-rich compounds) as a result of electron-deficient bonding. All polymorphs contain B12 icosahedral clusters as a basic building block (see the inset in Figure 1a). Among these polymorphs, a-tetragonal, a-rhombohedral, b-tetragonal, and b-rhombohedral boron are the four main forms under ambient conditions. Experimentally, b-rhombohedral boron is the most thermodynamically stable form, although its superior stability was not supported by theoretical investigations until 2007. 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 a consequence, the theoretically predicted metallic property does not agree with the experimentally derived p-type semiconducting behavior of b-rhombohedral boron with a proposed bandgap of 1.5–1.6 eV. Improved crystallographic 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. 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 measurements that b-rhombohedral boron exhibited strong photoconductivity under illumination by a halogen lamp or argonion 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. 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; space group: R 3m (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 Figure 1. Structure characterization: a) XRD pattern of sub-micrometer-sized b-rhombohedral boron. b) High-resolution TEM image recorded at the edge of a sub-micrometer-sized b-rhombohedral boron particle. The inset in (a) shows the structure of an icosahedral B12 cluster.