Enhanced photocatalytic nitrogen fixation performance via in situ constructing BiO2ex/NaNbO3 heterojunction

Abstract

The fabrication of heterojunction catalysts is an effective strategy to enhance charge separation efficiency, thereby boosting the performance of photocatalysts. In this study, BiO2ex nanosheets were synthesized through a hydrothermal process and loaded onto NaNbO3 microcube to construct a series of BiO2ex/NaNbO3 heterojunctions for photocatalytic N2 fixation. Results indicated that 2.5% BiO2ex/NaNbO3 had the highest photocatalytic performance. The NH3 production rate under simulated solar light reached 406.4 mmol·L−1·g−1·h−1, which reaches 2.6 and 3.8 times that of NaNbO3 and BiO2ex, respectively. BiO2ex nanosheets primarily act as electron trappers to enhance the separation efficiency of charge carriers. The strong interaction between BiO2ex and NaNbO3 facilitates the electron migration between them. Meanwhile, the abundant oxygen vacancies in BiO2ex nanosheets may facilitate the adsorption and activation of N2, which may be another possible reason of the high photocatalytic activity of the BiO2ex/NaNbO3. This study may offer new insights for the development of semiconductor materials in photocatalytic nitrogen fixation.

1. Introduction

Global warming and energy crisis have become important issues related to human survival and development. Therefore, the development and utilization of new energy sources has received extensive attention. Ammonia, as an ideal hydrogen storage fuel, plays a very important role in industry and agriculture. In present, the N2 to NH3 conversion in industry is achieved by Bosh-Haber process, which consumes a huge sum of fossil energy and has a serious environment impact. Photocatalysis can achieve the N2 to NH3 conversion driven by solar energy, which has the advantages of being clean and pollution-free. Hence, photocatalytic nitrogen fixation (PNF) has been widely followed by scientists since the pioneering work of Schrauzer in 1977 [1]. Given that efficient photocatalysts are the key to the practical implementation of PNF technology, a large amount of work has focused on the preparation of novel high-efficiency photocatalytic materials and their promotion mechanisms. Some high-efficiency catalysts including CdS [2], g-C3N4 [3,4], ZnO [5,6], KTN [7,8], CeO2 [9], and BiOX (X = Cl, Br, I) [10,11] have been gradually reported. Nevertheless, more efforts are still needed to achieve these goals.

In recent years, perovskite materials such as SrTiO3 [12], KNbO3 [13], BaTiO3 [14], and CeFeO3 [15] have gained significant attention and exhibited promising prospects in the field of PNF. Among them, NaNbO3, a representative perovskite material, has found wide applications in photocatalytic degradation, hydrogen production, and CO2 reduction. Nevertheless, its application in photocatalytic NH3 synthesis is restricted. Our research group recently investigated the synthesis of Pt-loaded NaNbO3 with oxygen vacancies for NH3 synthesis [16]. The PNF rate of the synthesized composite exhibits a remarkable enhancement compared to pure NaNbO3. In the Pt/Ov-NaNbO3 system, the introduction of Pt enhances both carrier separation and N2 adsorption and activation. Nevertheless, Pt concurrently promotes the photocatalytic hydrogen production reaction, which attenuates its overall promoting effect on PNF. Interestingly, besides precious metals, bismuth oxide has been recognized as an active phase for N2 adsorption activation owing to its facile formation of oxygen vacancies [17]. As a result, various Bi-based catalysts, such as BiOX (X = Cl, Br, I) [10,11,18], Bi2WO6 [19,20], Bi2MoO6 [21], Bi2O2CO3 [22,23], and Bi2O3 [24,25], have been documented to demonstrate outstanding properties in PNF. BiO2ex, a bismuth oxide compound, has received much attention owing to its narrow band gap, abundant defect sites, and distinctive layered structure. With a band gap ranging from 1.5 to 1.8 eV, BiO2ex exhibits broad-spectrum responsiveness. However, this characteristic also facilitates the rapid recombination of photogenerated electron-hole pairs, limiting its photocatalytic efficiency. To address this, constructing heterojunctions with suitable semiconductors has been proven effective in promoting charge separation. In this study, we report the in situ construction of BiO2ex/NaNbO3 heterojunctions via a hydrothermal method, aiming to enhance photocatalytic nitrogen fixation performance.