ArticleCooperation of oxygen vacancies and 2D ultrathin structure promoting CO2 photoreduction performance of Bi4Ti3O12
Graphical abstract
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.
Section snippets
Preparation of BTO
All chemicals are analytical grade without further purification. 0.84 g of tetrabutyl titanate, 6.00 g of NaOH and 1.60 g of bismuth nitrate pentahydrate were dissolved into 30 mL of deionized water under ultrasonic treatment for 5 min, and then stirred for 30 min (named as solution A). The above suspension was added into a 50 mL Teflon-lined stainless steel autoclave for the hydrothermal treatment at 180 °C for 20 h. Afterwards, the autoclave was cooled down to room temperature. The samples
Catalyst characterizations
BTO has a typical Aurivillius-type crystal structure built up by alternatively arranged [Bi2O2] layers and three layers [TiO6] octahedrons along the c axis direction, as shown in Fig. 1b. It crystalizes in an orthorhombic space group Aba2 with lattice parameters of a = 5.41 Å, b = 5.4480 Å and c = 32.84 Å [24]. Fig. S2 (online) shows the XRD patterns of BTO, BTO-U and BTO-UOV. The characteristic diffraction peaks of all the samples are corresponding to the standard peaks of BTO (PDF#35-0795)
Conclusions
In summary, BTO-UOV was synthesized via a combined hydrothermal and reduction reaction by using sodium oleate as structural control reagent and glyoxal as reductant. In comparison with bulk BTO, BTO-UOV demonstrated a 3.2 times enhancement on CO2 reduction with a CO evolution rate of 11.7 μmol g−1 h−1. It was unfolded by Pt selective photodeposition experiment that the migration distance of photo-induced electrons from bulk to the surface was shortened by fabricating ultrathin nanosheets, and
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
This work was jointly supported by the National Natural Science Foundation of China (51972288 and 51672258), and the Fundamental Research Funds for the Central Universities (2652018290).
Author contributions
Lizhen Liu and Hongwei Huang conceived the research; Lizhen Liu implemented the research; Hongjian Yu participated in part of experiments; Fang Chen and Na Tian preformed the DFT calculations; Lizhen Liu, Hongwei Huang and Hongjian Yu analyzed data and charted the figures and table; Lizhen Liu, Hongwei Huang, Yihe Zhang and Tierui Zhang wrote and edited the manuscript with contributions from all authors. All authors discussed the results and prepared the manuscript.
Lizhen Liu is currently a Ph.D. candidate at the School of Materials Science and Engineering, China University of Geosciences (Beijing). She obtained her bachelor degree in Faculty of Chemistry, Biology and Materials Science, East China University of Technology in 2018. Her research interests focus on the design and synthesis of layered bismuth-based photocatalysts for energy and environment.
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Lizhen Liu is currently a Ph.D. candidate at the School of Materials Science and Engineering, China University of Geosciences (Beijing). She obtained her bachelor degree in Faculty of Chemistry, Biology and Materials Science, East China University of Technology in 2018. Her research interests focus on the design and synthesis of layered bismuth-based photocatalysts for energy and environment.
Hongwei Huang is a Professor at Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, School of Materials Science and Technology, China University of Geosciences (Beijing). He received his Ph.D. degree in 2012 from Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, and worked as a visiting scholar in the lab of Prof. Thomas Mallouk in The Pennsylvania State University (2016–2017). His current research mainly focuses on the crystal structural design and charge regulation of layered photocatalytic nanomaterials and their applications for environment and energy.
Tierui Zhang is a full professor at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. He obtained his Ph.D. degree in Chemistry in 2003 at Jilin University, China. He then worked as a postdoctoral researcher in the labs of Prof. Markus Antonietti, Prof. Charl F. J. Faul, Prof. Hicham Fenniri, Prof. Z. Ryan Tian, Prof. Yadong Yin and Prof. Yushan Yan. His current scientific interests focus on catalyst nanomaterials for energy conversion.