Boron-Doped Boron Nitride Photocatalyst for Visible Light-Driven H2 Evolution and CO2 Photoreduction

Abstract

<p>Developing robust, multifunctional photocatalysts that can facilitate both hydrogen evolution <i>via</i> photoreforming of water and gas phase CO<sub>2</sub> photoreduction is highly desirable with the long-term vision of integrated photocatalytic setups. Here, we present a step-change in the family of boron oxynitride materials by introducing the first example of a B-doped boron oxynitride (B-BNO). This material resolves an on-going bottleneck associated with BN-based materials, i.e. the lack of photoactivity under visible light. Detailed EPR studies revealed distinct hyperfine interactions between the free oxygen radicals and 3 neighbouring boron nuclei. This confirmed isolated OB<sub>3 </sub>sites, which contribute to band gap narrowing, as the radical species and origin of paramagnetism in BNO materials. We show that B-BNO can facilitate both liquid phase H<sub>2 </sub>evolution and gas phase CO<sub>2</sub> photoreduction, using UV-Vis and deep visible irradiation (λ > 550 nm), without any co-catalysts. The evolution rates, quantum efficiencies, and selectivities observed for both reactions with B-BNO exceed those of its porous BNO counterpart, P25 TiO<sub>2</sub> and bulk g-C<sub>3</sub>N<sub>4</sub>.</p>

A Response Surface Model to Predict and Experimentally Tune the Chemical, Magnetic and Optoelectronic Properties of Oxygen‐Doped Boron Nitride**

“…The high resolution O 1s core level spectrum and atomic composition for the representative BNO sample is shown in Figures 2b and 2c, respectively. The peak centred at 533.1 eV in Figure 2b is attributed to boron oxynitride (B−O x −N 3‐x ) species, which stems from the in‐plane substitution of oxygen atoms in the BN lattice, as we have described in a prior studies [9,10] . The analysis indicates a significant proportion of oxygen (10.6±0.6 at.…”

“…The presence of the boron oxynitride (B−O x −N 3‐x ) species observed in Figure 2b can give rise to the formation of isolated OB 3 sites. In a previous study, [9] we showed that these isolated OB 3 sites induce a paramagnetic radical signal through the introduction of an unpaired electron from the incoming oxygen atom to the system, which contributes to band gap narrowing. The paramagnetic signature of the BNO species, measured using X‐band electron paramagnetic resonance (EPR) spectroscopy under ambient conditions, is shown in Figure 3a (inset).…”

“…Computational and experimental work has shown that oxygen doping can introduce semiconducting and magnetic properties in BN materials [18,19,26,31] . This is particularly relevant given the recent interest in employing BN for photocatalysis [9–11] …”

“…Chemical modification, via atomistic doping, has been utilised to tailor the photochemistry of semiconductors [4–8] . Indeed, this has been true for hexagonal boron nitride (BN), the isoelectronic analogue of graphene, which has been recently employed for the applications listed above [9–14] . A handful of elements have been used to dope both amorphous and crystalline forms of BN, such as carbon, [5,15–17] oxygen, [18,19] boron, [9] sulphur, [20,21] and silicon, [22] as well as metals, such as molybdenum, [23] nickel [24] and copper [25] .…”

“…Indeed, this has been true for hexagonal boron nitride (BN), the isoelectronic analogue of graphene, which has been recently employed for the applications listed above [9–14] . A handful of elements have been used to dope both amorphous and crystalline forms of BN, such as carbon, [5,15–17] oxygen, [18,19] boron, [9] sulphur, [20,21] and silicon, [22] as well as metals, such as molybdenum, [23] nickel [24] and copper [25] . In this study, we focus our attention on oxygen doping in BN and particularly porous boron BN, a combination of hexagonal (hBN), turbostratic (tBN) and amorphous BN (aBN).…”