BiVO4 /Bi4Ti3O12 heterojunction enabling efficient photocatalytic reduction of CO2 with H2O to CH3OH and CO
Graphical abstract
Introduction
High carbon dioxide (CO2) emissions from the overuse of fossil energy have broken the carbon cycle in nature, the resulting greenhouse effect and frequent extreme weather has attracted increasing attention [1,2]. How to eliminate CO2, especially transform it to valuable organics has been one of hotspot research topics. The photocatalytic reduction of CO2 with H2O is one of the most tempting routes [[3], [4], [5], [6]]. However, the reaction of CO2 with H2O is both thermodynamically and dynamically unfavorable process. All photocatalysts reported so far showed very low efficiency, because the reactions are multi-electron reduction processes to any organic compound. Such reactions require photocatalyst to have long-life photogenerated electron and fast separation efficiency of photogenerated electrons and holes [7,8]. In addition, photocatalyst must have both an effective conduction band and an effective valence band. The conduction band must have more negative potential than reduction potential of CO2 to organics (such as CH4, HCOOH, CH3OH, etc.). The valance band has more positive than oxidation potential of H2O to O2 [[9], [10], [11]]. Obviously, anyone single semiconductor, such as ZnO, TiO2, SiC, BiOCl, and WO3, is difficult to meet these requirements at the same time [[12], [13], [14], [15]]. Coupling two or more semiconductors into a heterojunction hybrid has been considered as a highly promising method to obtain a photocatalyst with highly photocatalytic activity [[16], [17], [18]].
Based on band alignment of the involved semiconductor components, there are three types of heterojunction structure modes to be classed, including a straddling gap (type-I), a staggered gap (type-II), and a broken gap (type-III) [19,20]. Thereinto, the type-II heterojunction photocatalysts show wider light-absorption range, especially the build-in electric field at the heterojunction interface can induce and accelerate directional migration and therefore increase separation efficiency and lifetime of photogenerated electron–hole [21,22]. Additionally, when the semiconductor component responsible for the reduction half-reaction possess appropriate adsorption capacity to CO2 molecule, and the semiconductor component for the oxidation half-reaction has good adsorption capacity to H2O molecule, it can be expected that the type-II heterojunction photocatalysts could achieve excellent activity for the photocatalytic reduction of CO2 with H2O.
BiVO4 has been documented to be an excellent visible light photocatalyst for various reactions, the monoclinic scheelite phase (m–s) of which shows the best performance owning to its unique layered structure and strong ability capturing solar energy [[23], [24], [25], [26], [27]]. As a photocatalyst, BiVO4 has been showed to be excellent for the oxidation of water into oxygen because of its relative positive valence band [28,29]. Nevertheless, it can be difficult to use for the reduction of CO2 into C1 compound (CO, CH4, CH3OH, HCOOH, HCHO) since its conduction band position is slightly more positive than that required. In addition, short photoelectron lifetime and fast recombination rate of photogenerated electron-hole pairs also make it inefficient to reduce CO2 with pure water. However, a recent report showed that the specially prepared BiVO4 is able to photocatalyze the reduction of CO2 to methanol [30]. Combining BiVO4 with other suitable semiconductors into a heterojunction material has proved to be a feasible strategy to overcome these drawbacks of BiVO4 by hybridization of the electron structures [[31], [32], [33]]. Titanate-based semiconductors have also been considered as promising photocatalysts from photocatalytic activity, and they are eco-friendly and abundance [[34], [35], [36]]. Typically such as Bi4Ti3O12, it possesses many other unique properties such as piezoelectric effects, ferroelectric phenomena, and photoelectric phenomena beneficial to photocatalysis. But, large band gap (Eg =2.95 eV) and fast recombination rate of photo-generated electron-hole pairs limit its application as a photocatalyst [37,38]. Fortunately, BiVO4 and Bi4Ti3O12 meet the conditions to form the type-II heterojunction [39,40]. Liu et al. prepared Bi4Ti3O12/BiVO4 heterostructure via a simple electrospinning technique and solvothermal method [35], but its photocatalytic activity was not researched. Different from the Liu’s work, herein, a novel BiVO4/Bi4Ti3O12 heterojunction photocatalyst was constructed by a simple in situ hydrothermal treatment. The material system show excellent activity for photocatalytic reduction of CO2 to CH3OH and CO in the gas-solid phase. The BiVO4/Bi4Ti3O12 composite is an original photocatalyst of reducing CO2 to CH3OH in the gas-solid phase [12,41].
Section snippets
Materials and synthesis
All the chemical reagents were of analytical grade and purchased from Aladdin Industrial Corporation and used without further purification. Deionized water was used throughout the work.
Preparation of Bi4Ti3O12 nanosheets
The molten salt synthesis method was used to prepare Bi4Ti3O12, where Bi2O3 and TiO2 (anatase) were used as raw materials. Since mole ratio of Bi2O3 to TiO2 is 2:3 in Bi4Ti3O12, Bi2O3 and TiO2 were mixed by the stoichiometric amounts. Then, NaCl, KCl and Bi4Ti3O12 were mixed at a mole ratio of 60:60:1 and
Crystal phase of photocatalyst
Fig. 1a shows XRD pattern of samples at 2θ = 5 ∼ 80°. The diffraction peaks of the nude Bi4Ti3O12 sample match with that of tetrahedron phase (JCPDS No. 72-1019). Diffraction peaks of the nude BiVO4 sample could be indexed to the monoclinic scheelite phase (JCPDS No. 75-1867), and no impurity peaks were observed. These indicate that both the as-prepared Bi4Ti3O12 and BiVO4 are highly pure and high-crystallized. When Bi4Ti3O12 was composited with BiVO4, the resulting composites show the
Conclusions
A novel BiVO4/Bi4Ti3O12 heterojunction composite was successfully designed and synthesized via a facile in situ hydrothermal approach. The composite photocatalyst showed superior activity for photocatalytic CO2 reduction with pure H2O. Thereinto, the BiVO4/10 % Bi4Ti3O12 composite showed much higher photocatalytic activity than the composites with other content of Bi4Ti3O12 as well as the nude BiVO4 or the nude Bi4Ti3O12. The reaction products are methanol and carbon monoxide, the yield of them
Credit author statement
Yingshu Wang, Huaxiang Lin and Meichao Gao conceived the idea and designed this study. Xianying, Wang, Yingshu Wang and Meichao Gao contributed equally to this work, and synthesized the samples and conducted characterizations. Xianying Wang performed photocatalytic reactions. Xipeng Pu tested and analyzed the SEM tests. Xianying Wang, Yingshu Wang, Meichao Gao, Zizhong Zhang, Jinni Shen and Xuxu Wang wrote the paper. All authors discussed the results and edited the paper.
Declaration of Competing Interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work is financially supported by the National Natural Science Foundation of China (Grants No. 21673043, 51702053, 21673042), the Natural Science Foundation of Fujian Province of PR China (2017J01411), and the Technology Project of Education Office of Fujian Province of PR China (JAT160045).
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