Cooperation of oxygen vacancies and 2D ultrathin structure promoting CO₂ photoreduction performance of Bi₄Ti₃O₁₂

Abstract

Reduction of CO₂ to solar fuels by artificial photosynthesis technology has attracted considerable attention. However, insufficient separation of charge carriers and weak CO₂ adsorption hamper the photocatalytic CO₂ reduction activity. Herein, we tackle these challenges by introducing oxygen vacancies (OVs) on the two-dimensional Bi₄Ti₃O₁₂ ultrathin nanosheets via a combined hydrothermal and post-reduction process. Selective photodeposition experiment of Pt over Bi₄Ti₃O₁₂ discloses that the ultrathin structure shortens the migration distance of photo-induced electrons from bulk to the surface, benefiting the fast participation in the CO₂ reduction reaction. The introduction of OVs on ultrathin Bi₄Ti₃O₁₂ nanosheets leads to enormous amelioration on surface state and electronic structure, thereby resulting in enhanced CO₂ adsorption, photoabsorption and charge separation efficiency. The photocatalytic experiments uncover that ultrathin Bi₄Ti₃O₁₂ nanosheets with OVs reveal a largely enhanced CO₂ photoreduction activity for producing CO with a rate of 11.7 μmol g⁻¹ h⁻¹ in the gas–solid system, ~3.2 times higher than that of bulk Bi₄Ti₃O₁₂. This work not only yields efficient ultrathin photocatalysts with OVs, but also furthers our understanding on enhancing CO₂ reduction via cooperative tactics.

Introduction

With the rapid development of industry and consumption of fossil energy, environmental pollution and energy shortage are becoming increasingly prominent. Meanwhile, the concentration of carbon dioxide (CO₂) has risen sharply in the atmosphere [1], [2]. Fortunately, photocatalytic CO₂ reduction to chemical fuels is considered to be an effective and environmental-friendly way to solve the above problems using solar energy [3], [4], [5].

Amounts of semiconductors have the ability of converting CO₂ because of the appropriate band structure. Bismuth-based semiconductors have triggered much interest because of the unique crystal structure, optical and electronic properties, which show excellent performances in photocatalytic CO₂ reduction [6], [7], [8]. Among them, layered bismuth-based photocatalysts show large exposed surface and low dimension in layer stacking direction, which have lately received tremendous attentions, such as Bi₂MO₆ (M = W, Mo), BiOX (X = Cl, Br, I), etc. [9], [10], [11], [12], [13], [14]. Bismuth titanate (Bi₄Ti₃O₁₂, BTO) which consists of [Bi₂O₂] slices and perovskite-type TiO₆ octahedral layers demonstrates benign photocatalytic CO₂ reduction performance because of its special layered structure and appropriate band structure [15], [16]. However, the poor photoabsorption in visible light region, high recombination of electron-hole pairs and weak CO₂ adsorption are the primary challenges for further improving the photocatalytic CO₂ reduction activity of BTO. It was reported that fabrication of two-dimensional (2D) ultrathin structure can facilitate the charge separation by shortening the charge migration pathway and promote the adsorption of CO₂ [17]. On the other hand, defect engineering of semiconductors is regarded as a promising strategy to solve the above issues. Introducing oxygen vacancies (OVs) on the surface of semiconductors provides more reactive sites to boost the adsorption and activation of CO₂ by lowering the energies in these processes [18]. For example, Bi₂₄O₃₁Cl₁₀ with OVs shows better photocatalytic performance for CO₂ reduction with a CO generation rate of 0.9 μmol g⁻¹ h⁻¹, which is about 4 times than that of Bi₂₄O₃₁Cl₁₀ [19]. The introduction of OVs leads to a new defect level, which accelerates the photogenerated electrons transfer and increases charge carriers density. Constructing surface OVs on Sr₂Bi₂Nb₂TiO₁₂ not only boosts the photoresponse to cover the whole visible region, but also tremendously enhances separation of electron-hole pairs and the adsorption and activation of CO₂ molecules, thus extremely promoting the CO production [20]. Therefore, fabrication of 2D BTO with OVs should be a desirable way to boost the photocatalytic CO₂ reduction activity of BTO. However, the preparation of BTO always requires a high-temperature (>180 °C) hydrothermal process assisted by mineralizers, and the commonly-used structural control reagents for fabrication of thin-layered bismuth-based materials (e.g., cetyltrimethylammonium bromide, sodium dodecyl benzene sulfonate, polyvinylpyrrolidone) and reductants (e.g., ethylene glycol) for introduction of OVs always unavoidably cause carbonization of these organics [21], [22], [23]. Thus, construction of 2D BTO with OVs is challenging and has not been achieved so far to the best of our knowledge.

Herein, we first fabricated Bi₄Ti₃O₁₂ ultrathin nanosheets (BTO-U) by introducing sodium oleate in the hydrothermal process, and employed a low-temperature hydrothermal post-treatment process with glyoxal as the reductant to construct OVs in BTO. 2D ultrathin structure shortens the migration distance of photoinduced charge carriers, which is beneficial to the separation of electron-hole pairs in bulk material. The introduction of OVs on the surface of BTO broadens the photoabsorption, improves the charge separation efficiency and promotes the CO₂ adsorption as surface active sites. The above benefits largely promote the photocatalytic CO₂ reduction activity of BTO, and the CO production rate is about 3.2 times that of pristine BTO. Besides, theoretical calculations were conducted to explain the experimental results.