Full Length ArticleA generic strategy for preparation of TiO2/BixMyOz (M = W, Mo) heterojunctions with enhanced photocatalytic activities
Graphical abstract
Introduction
Semiconductor photocatalysts are capable of directly absorbing sunlight to generate highly reactive electrons/holes without causing any unfavorable changes to the environment. These photo-generated charge carriers can be used to produce clean chemical fuels [1], degrade harmful pollutants [2], [3] and induce organic synthesis [4]. In this regard, photocatalysis is deemed to be one of the most promising technologies for the utilization of solar energy [5]. In a typical process of semiconductor photocatalysis, the electrons are excited to the conduction band by a given wavelength with the reciprocal generation of holes in the valence band. The photo-generated free electrons/holes can migrate to the surface and subsequently be consumed by surface redox reactions. On the other hand, the surface reaction often compete against the recombination of free electrons/holes [6]. For instance, recombination of electrons/holes in TiO2 takes picoseconds to nanoseconds, whereas the electrons/holes need several hundred nanoseconds to allow the completion of reaction with surface reducing or oxidizing agents [7]. Obviously, the photocatalytic performance of semiconductor materials is determined by the number of photo-generated electrons/holes that can be transferred into the surface redox reactions. Heterojunction is broadly defined as the interface between two semiconductors with dissimilar crystalline structures, band gap energy and physicochemical properties [8], which can be engineered to spatially separate photo-generated electron-hole pairs, prolong the lifetime of photo-generated carriers [9] and expand the responsive spectrum of photocatalysts to visible light [10].
TiO2/BixMyOz (M = W, Mo) heterojunctions are one of the most prominent photocatalysts for efficient utilization of solar energy and show great potential and versatility in disinfection of bacteria [11], degradation of organic pollutants [12], [13] and splitting water [14]. However, it is quite challenging to synthesize high-performance and stable TiO2/BixMyOz heterojunctions, because of the complex heterostructures and large difference in physiochemical properties between TiO2 and BixMyOz. In the traditional synthesis routes, either BixMyOz directly precipitated from saturated solution and deposited on the surface of TiO2 at random [15], [16], [17], or TiO2 deposited on the surface of BixMyOz as a secondary phase [18]. Despite the great advances in controllable synthesis of TiO2/BixMyOz heterojunctions [19], [20], it is still very difficult to tailor the growth site and size of BixMyOz on the surface of TiO2.
We were inspired by the fact that the surface energy of a crystal will proportionally increase with the increase of the number of broken bonds [21]. The surface atoms in the vicinity of a defect, which are weakly bonded to the adjacent atoms, have higher free energies than those in a defect-free lattice. Moreover, the more weakly the surface atom is bonded to surrounding atoms, the stronger its ability to attract small adsorbates [22]. Taking these into consideration, we propose to utilize the surface defects of TiO2 nanobelts to adsorb hydrolysis-produced [Bi6O6(OH)3−x](NO3)3+x, which is commonly used as bismuth precursor to synthesize bismuth-containing oxides such as Bi2WO6 [23], BiOX [24] and Bi2MoO6 [25]. Subsequently, OH− and NO3− groups of the precursors are replaced by MO42− in the process of hydrothermal reaction to allow in situ heterogeneous nucleation of BixMyOz at surface defect sites of TiO2 nanobelts. The BixMyOz crystal nuclei will grow to form stable TiO2/BixMyOz heterojunctions. Both Bi2WO6 and Bi2MoO6 are representatives of Aurivillius semiconductive oxides with visible-light-driven photocatalytic activity [26], [27]. Cubic phase Bi3.64Mo0.36O6.55, which is a visible-light responsive photocatalyst, is often present in the hydrothermal preparation process of Bi2MoO6 as a by-product [28]. In view of that, we employed WO42− and MoO42− to synthesize TiO2/Bi2MoO6, TiO2/Bi2WO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions in a controllable way for crosschecking our strategy. Our results show that the defects can be deliberately used to induce the hetero-growth of Bi2WO6, Bi2MoO6 and Bi3.64Mo0.36O6.55 on the surface of TiO2 nanobelts, and tailor the size of Bi2WO6 nanosheets in TiO2/Bi2WO6 heterojunctions. The photocatalytic properties of our TiO2/BixMyOz (M = W, Mo) heterojunctions were investigated under visible light irradiation. The outstanding visible-light-driven photocatalytic activities are attributed to the higher carrier separation efficiency and longer lifetime of the photo-generated carriers.
Section snippets
Experimental
Analytical grade reagents were used for the hydrothermal synthesis without further purification. TiO2 nanobelts with intentional defects were prepared by a two-step method. The detailed preparation process of TiO2 nanobelts was given in the Supporting Information.
In a typical synthesis of TiO2/[Bi6O6(OH)3−x](NO3)3+x precursor, 0.25 mmol of Bi(NO3)3·5H2O was first dispersed in 15 ml of deionized water, and the intrinsic pH value of the suspension was 1.1. After being stirred for 45 min, 0.08 g
Characterization of TiO2/Bi2WO6 heterojunctions.
Fig. 2 shows the XRD patterns of TiO2/Bi2WO6 heterojunctions and TiO2 nanobelts (growth template). The XRD pattern of TiO2 nanobelts shows an anatase phase having tetragonal structure (JCPDS card No. 65-5714) without any detectable peaks from impurities or secondary phases. In the XRD pattern of TiO2/Bi2WO6 heterojunctions, the characteristic diffraction peaks of anatase TiO2 phase i.e. (1 0 1), (0 0 4), (2 0 0) and (1 0 5) are still visible, while the other diffraction peaks can be indexed to
Conclusions
A generic two-step hydrothermal synthesis method, consisting of an inducing adsorption and an in situ anion exchange, is reported to selectively prepare TiO2/Bi2WO6, TiO2/Bi2MoO6 and TiO2/Bi3.64Mo0.36O6.55 heterojunctions. The defects on the surface of TiO2 play a key role in promoting the adsorption and growth of secondary phase. The morphology of TiO2/BixMyOz heterojunctions can be tailored by controlling the density of TiO2 surface defects. The photocatalytic activities of TiO2/Bi2WO6 and TiO
Acknowledgment
G. Fang and J. Liu are contributed equally to this work and should be considered co-first authors. Financial supports from the Natural Science Foundation of NingXia Province (No. NZ17108), National Natural Science Foundation of China (No. 51262001), and China Scholarship Council are acknowledged. This work was also partially supported by the Australian Research Council Discovery Project (Grant No. DP170103514). We also thank Prof. Guozhong Cao, University of Washington, for his valuable
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