Green synthesis of boron and nitrogen co-doped TiO₂ with rich B-N motifs as Lewis acid-base couples for the effective artificial CO₂ photoreduction under simulated sunlight

Abstract

Boron and nitrogen co-doped Titanium dioxide (TiO₂) nanosheets (BNT) with high surface area of 136.5 m² g⁻¹ were synthesized using ammonia borane as the green and triple-functional regent, which avoids the harmful and explosive reducing regents commonly used to create surface defects on TiO₂. The decomposition of ammonia borane could incorporate reactive Lewis acid-base (B, N) pairs, together with the as-generated H₂ to create mesoporous structure and rich oxygen vacancies in pristine TiO₂. The BNTs prepared from various ammonia borane loading are evaluated in photoreduction of carbon dioxide (CO₂) with steam under simulated sunlight, achieving about 3.5 times higher carbon monoxide (CO) production than pristine TiO₂ under the same conditions. Steady state and transient optical measurements indicated BNT with reduced band gap, rich defect states and elevated conduction band position could enhance the light harvesting efficiency and promote the charge transfer at the catalyst/CO₂ interface. Density functional theory simulation and in situ FTIR suggest that the Lewis acid-base (B, N) pairs on BNT may very substantially increase the activation of inert CO₂ which facilitates their photoreduction with the hydrogen from the water splitting at the surface defects on TiO₂. Finally, a reaction mechanism of Lewis acid-base assisted CO₂ photoreduction leading to substantially improved performance is proposed.

Introduction

As it is predicted by the International Panel on Climate Change (IPCC), the CO₂ released to the atmosphere may increase to 590 ppm by the year of 2100. CO₂, mainly derived from the combustion of fossil fuel, represents the chief greenhouse gas (GHG) in the atmosphere, which inevitably contributes to global temperature evaluation and climate change, causing the foremost environmental concern in the human society [1]. Photocatalysis is an effective strategy to resolve the globalized environmental and energy problems by harvesting the clean and abundant solar energy [2], [3], [4], [5]. The increasing CO₂ emissions greatly contribute to the green-house effect, and the photocatalytic reduction of CO₂ which can yield value-add chemicals and reduce the CO₂ emission has attracted worldwide attention. However, the CO₂ photo-conversion efficiency is still rather low for the commercial application [6], [7], [8]. First of all, CO₂ is at the fully oxidized state in the environmental carbon cycling with inherited thermodynamic stability, which requires high reducing over-potential to achieve the conversion. In addition, the nonpolar CO₂ with symmetrical liner architecture is hard to be adsorbed and activated on the catalyst surface, leading to the low reaction kinetics [8]. Furthermore, the photocatalytic reduction of CO₂ involves multi-step electron transfer steps and various intermediate products, which attributes to the poor catalytic selectivity [5], [6].

TiO₂ has attracted wide attention due to its versatile physicochemical properties of high reduction potential, abundance, chemical stability, low cost and resistance toward corrosion, which endows it a promising candidate applied in the wide range of solar energy assisted processes such as environmental remediation, solar water splitting, CO₂ photo-reduction and photovoltaic cells [9], [10], [11], [12]. However, as catalysts for CO₂ photo-reduction, pristine TiO₂ with large band gap of ~3.2 eV could only utilize UV light (merely taking up 3–5% of the solar light) and exhibit fast recombination of photo-generation carriers in catalytic process, which greatly inhibits the photoelectric conversion efficiency and limits its practical application. To compensate these problems, many strategies have been employed to improve the light utilization ability and electron-hole separation efficiency of TiO₂ including structure control [13], [14], impurity doping [15], [16], metal nanoparticle deposition [17], [18], heterojunction construction [19], [20], [21], [22], [23], hybridization with carbon-based materials [24], [25], [26] and so on. In the past decade, heteroatom doping (metal or non-metal atoms) is a widely used strategy to extend the light absorption range by reducing the band gap of TiO₂ or introducing impurity energy levels. Although metal atom doping is generally employed to enhance the photo-conversion performances, the metal impurity centers are very susceptible to experience photo-corrosion and trigger mass charge recombination [27], [28].

Compared with metal doping, non-metal doping owns higher photo-stability, low cost as well as low secondary environmental contamination. Recently, non-metal elements such as B, N, C, S, and F have been adopted (single or co-doped) to narrow the band gap and modify the pristine TiO₂ with rich surface states in order to enhance the photocatalytic properties. Among these doping strategies, B, N co-doped TiO₂ were confirmed as effective photocatalysts from both experimental and theoretical aspects. [29], [30], [31], [32]. For example, Liu et al.’s pioneer works suggest that the B, N doping can not only increase the visible light absorption, but reduce the recombination of photogenerated electrons and holes and improve the transmission efficiency of electrons by form the Ti-O-B-N moiety, which rise to the photodegradation performance under visible light irradiation [31], [32]. They also proposed the interstitial Boron formed in the TiO₂ lattice can effectively weaken nearby Ti-O bonds, leading to the easier N substitute for the lattice O, which could elevate the visible light absorption and increases the chemical stability of the doped TiO₂ [33]. In addition, various fabricating strategies have been adopted to incorporate B and N doping sites in the TiO₂ lattices for different application aims from photodegradation of organic pollutions in water, hydrogen production to lithium battery [34], [35], [36], [37], [38], [39], [40]. For examples, M. Abdelraheem et al. employed borane tert-butylamine complex as precursor to introduce B, N impurities in the TiO₂, which significantly enhance the optical absorption properties of the doped TiO₂ and the photocatalytic activities on the destruction of bisphenol [38]. A. Li et al. prepared B, N co-doped black TiO₂ through a facile sol-gel method combined with magnesiothermic reduction, which gives rise to greater light absorption properties as well as higher photocatalytic hydrogen production [39]. Chen et al. employed BmimBF4 ionic liquid as precursor to yield N/B co-doped TiO₂(B)/anatase nanotube assemblies, and the unique assembly exhibits exceptionally high rate capability and good durability when used as anode materials in lithium battery [40]. Based on these researches, two step doping processes employing adopting two types of precursors containing B and N respectively were usually employed to incorporate B, N atoms, and many of these precursors are expensive and toxic, which inevitably lead to the tedious procedure as well as the high cost and poor producing safety.

In addition, B, N co-doped TiO₂ was seldom, so far as we know, employed as photocatalysts to achieve the CO₂ photo-conversion. Considering B is an electron deficient element, the B doping sites could serve as Lewis acid centers. Meanwhile, the electron pairs in the N show prominent electron donating behavior. Therefore, the doping of B, N couples in the lattice of TiO₂ could serves as strong polarization centers (Lewis acid and base centers) to attract and then polarize the inert CO₂, which could be expected to benefit the CO₂ photo-conversion kinetics. Herein, we reported a facile and environmental benign method to yield B, N co-doped TiO₂ nanosheets (BNT) using NH₃BH₃ as B and N sources. During the doping process, the as-generated H₂ environment could create defects on the TiO₂, leading to the rich surface oxygen vacancy, porous structure and high surface area of the B, N co-doped TiO₂ (Scheme 1). Moreover, the B, N atoms were derived from the single source which facilitate the formation of the adjacent doping of the B-N couples in the TiO₂ matrix. According to the optical measurements, the visible light utilization of the BNT is enhanced due to the narrowed band gap and the introduced impurity energy levels. In addition, the abundant mesoporous characteristics and high surface area increases the CO₂ adsorption sites and facilitates the mass diffusion. Furthermore, the electrochemical tests demonstrate that the optimized sample shows higher photocurrent density and lower charge-transfer resistance compared with that of pristine TiO₂, indicting the faster charge-transfer rate at the catalyst and CO₂ interface. Moreover, the DFT simulation suggest that the B, N co-doped TiO₂ with and show higher CO₂ adsorption energy as well as moderate CO desorption energy, which improves the CO₂ surface activation and facilitates the regeneration of the active site due to the fast removal of the CO from the binding sites. Therefore, the optimized BNT demonstrates a high CO production of 37.1 μmol g⁻¹ under simulated sunlight and 18.8 μmol g⁻¹ under visible light, which are substantially enhanced compared with that of pristine TiO₂.