We report high-pressure synthesis of single-crystal boron suboxide (B6O), one of the superhard materials, using a reaction route between B and H3BO3 at 3–5.5 GPa and 600–2200 °C. At optimized pressure and temperature conditions of 5.5 GPa and 2100 °C, large B6O single crystals can be obtained with a crystallite size of more than 2 mm. Boric acid (H3BO3) was used as a starting material because it is easily decomposed into B2O3 and H2O at a lower temperature. The B2O3 produced by the decomposition should have a small particle size and thus have high reactivity, and the reaction byproduct H2O provides a good hydrostatic environment for the growth of B6O. Therefore, it is easier to synthesize and grow B6O by using a B-H3BO3 system as reaction raw materials. This work discussed the B6O growth mechanism in detail. In addition, photoluminescence experiments and first-principles calculations show that B6O is a direct band gap semiconductor material with good orange-red emission characteristics (the band gap is 1.83 eV). The results of this study indicate a method to grow large B6O single crystals that have great potential use in industrial applications.
Yang Qiang,Liu Jiali,Yan Dejian,Liu Jikai,Luo He’an
Abstract Boron-rich alloy boron suboxide (B 6 O) as a novel metal-free photocatalyst was recently explored for photocatalytic CO 2 reduction and water splitting. However, to broaden the potential applications of B 6 O in the field, further improving its activity and understanding its fundamental chemical properties are essential. Herein, transition metal doping (Cr or Co) is designed to engineer the band structure of B 6 O via a solid state reaction. The valence band edge of Cr or Co doped B 6 O is proposed to be more positive than that of pristine B 6 O while the original conduction band of B 6 O is not much influenced by doping, contributing to the better light harvesting ability and lower charge recombination rate of doped B 6 O. The photocatalytic and photoelectrochemical (PEC) performances of doped B 6 O were therefore remarkably enhanced. The apparent quantum yields (AQY) of 20 wt% Cr or Co doped B 6 O are much higher than that of pristine B 6 O. In addition, the 20 wt% Cr doped B 6 O exhibits the best electrical conductivity and photocatalytic performance compared with the Co doped B 6 O and pristine B 6 O. The present work demonstrates a facile and effective strategy to engineer the band structure and reduce the charge migration resistance of B 6 O. Graphic Abstract
We obtained a series of B 6 O, B 4 C, ZrB 2 crystals at a pressure of 6 GPa and temperature of 1673–1973 K. These crystals were characterized by XRD and SEM. At a pressure of 6 GPa, the minimum synthesis temperature for B 6 O is 1673 K. The grain size of B 6 O in all the samples remained consistent at around 30–50 nm, with a uniform distribution in each sample. The bonding force between B 6 O grains is weak, and the B 6 O grain size remains relatively stable, showing minimal change with increasing temperature and time. In the sample synthesized at 1973 K, B 6 O icosahedra (multiply twinned particles, MTPs) with a diameter of 12 μm, B 4 C particles with a diameter of 25 μm, and flaky ZrB 2 were observed. We conducted theoretical calculations on B 6 O and B 4 C using density functional theory (DFT).
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