Synthesis of Ti3+ self-doped mesoporous TiO2 cube with enhanced visible-light photoactivity by a simple reduction method
Graphical abstract
Introduction
Titanium dioxide (TiO2) is a widely used inorganic substance because of its superior chemical stability, low-toxicity, cost effectiveness and environmental friendliness [[1], [2], [3]]. It is widely used in the conventional areas such as coating, plastics, printing ink, and more recently, in photocatalysis, water splitting, lithium ion batteries, dye-sensitized solar cells, gas sensors and drug conveyor [4,5]. In general, TiO2 is viewed as a potential material for energy generation and environmental protection. However, the wide band gap (3.0–3.2 eV) and fast electron-hole recombination rate of TiO2 limit the practical application of TiO2 in environmental protection [6]. Significant effort has been dedicated to improve its visible light utilization and inhibit photogenerated electron-hole recombination. The doping of various transition-metal cations (such as Nd, V, Ag, Co, Au, Fe, Ni) [[7], [8], [9], [10]] and anions (such as N, C, S and B) [6,11,12] into TiO2 has successfully been used to narrow the band gap and enhance the visible light utilization. The introduction of defects on the surface or in the bulk of TiO2 could promote charge separation and therefore lead to enhance the photocatalytic activity [[13], [14], [15]].
An effective and environmentally friendly solution to introduce defects is to form Ti3+ species and counterpart-oxygen vacancies as this method does not damage the structure and morphology of TiO2. Considerable effort has been devoted to develop facile and effective approaches for the fabrication of Ti3+ self-doped TiO2. The preparation methods include reduction [[16], [17], [18], [19]], doping [[20], [21], [22]], thermal treatment [[23], [24], [25]] and hydrogen plasmas [26,27]. For instance, Zuo Fan et al. [15] reported a facile one-step combustion method to induce Ti3+ into the bulk of TiO2, which exhibits high visible-light photocatalytic activity and stability. The Ti3+ doped TiO2 was used to split hydrogen gas from water. Zhang Dainan et al. [10] synthesized C and N co-doped TiO2 hollow spheres with Ti3+ and oxygen vacancies by hydrothermal strategy in H2O–C2H5OH–HF–H2O2 mixed solution with TiCN as a precursor. The sample showed extremely high visible-light photocatalytic activity for hydrogen production. A notable strategy was recently developed by Cao Yan et al. [28] via an evaporation-induced self-assembly method to synthesize mesoporous Ti3+/N–TiO2 spheres. One advantage of this strategy is that the prepared sample has a narrow band gap (2.11 eV) and high surface area (100 m2/g), and can enhance visible light utilization. Wei Shunhang at al [13] developed a solvothermal method to prepare bulk Ti3+ self-doped TiO2. In their paper, they found that the distribution and concentration of oxygen vacancies affect the visible light absorption.
Although the synthesis and photocatalytic mechanism of Ti3+ self-doping TiO2 have been investigated extensively in recent decade, there are still some problems which limit its large-scale commercial applications. In general, Ti3+ defects introduced on the surface of TiO2 by reducing methods are unstable, and are easily oxidized by air or dissolving oxygen in solution. In addition, these methods to induce Ti3+ require high energy and use unstable or expensive raw materials (such as Ti [13], TiCN [10], TiH2 [29,30] and TiOF2 [31]). Therefore, it still remains a great challenge to synthesize a stable Ti3+ self-doped TiO2 photocatalyst by a simple and economic method.
In this work, we report the synthesis of the Ti3+ self-doped mesoporous TiO2 cube using low-cost Ti(SO4)2 as a precursor and the oxalic acid as reducing agent by a simple hydrothermal strategy. The participation of oxalic acid during the preparation process not only allows controlling the concentration of Ti3+ species and oxygen vacancies but is also critical for controlling the porous microstructures. A possible mechanism for the formation of the Ti3+ self-doped porous TiO2 is proposed. Under visible light irradiation, the Ti3+ self-doped mesoporous TiO2 has exhibited enhanced degradation efficiency of methyl orange (MO) in aqueous solution owing to Ti3+ self-doping and its mesoporous structure.
Section snippets
Materials
Titanium sulfate (Ti(SO4)2, ≥96.0%), oxalic acid (H2C2O4, ≥99.0%), methyl orange (A.R., ≥99.0%), sodium sulfate (Na2SO4, ≥99.0%) were all obtained from Sinopharm Chemical Regent Co., Ltd. (Shanghai, China). Nano-TiO2 (P25) was obtained from Degussa (75% anatase and 25% rutile). They were all used directly after purchase without any further treatment.
Photocatalyst preparation
Ti3+ self-doped TiO2 was synthesized by a hydrothermal reaction of Ti(SO4)2 aqueous solution and oxalic acid. In a typical synthesis procedure,
Characterization of Ti3+ self-doped TiO2
The crystal structures and crystallite size of the samples were characterized using X-ray diffraction. Fig. 1 and Fig. S1 (see Supporting Information) shows the XRD patterns of Ti3+ self-doped TiO2 samples with different amount of oxalic acid, comparing with the PDF cards JCPDS 21–1276 (Rutile) and JCPDS 21–1272 (Anatase). In the XRD patterns, TiO2-0, TiO2-3.70 display the typical diffraction peaks at 25.3°, 37.7°, 48.0°, 53.9° and 55.02°, corresponding to the (101), (004), (200), (105) and
Conclusions
The Ti3+ self-doped TiO2 samples with mesoporous structure have been prepared in the Ti(SO4)2–H2C2O4–H2O mixed solution at 200 °C for 24 h. The as-prepared TiO2 has a cubic structure with exposure of (001) reactive facets, and has uniform length of about 45–65 nm. In the high-temperature hydrothermal reaction, the gas generated by the thermal decomposition of oxalic acid acts not only as a soft template for forming a mesoporous structure, but also as a reducing agent for reducing Ti4+ to
CRediT authorship contribution statement
Li Li: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Xinhong Chen: Validation, Formal analysis, Software. Liang Wang: Resources, Writing - review & editing, Supervision, Data curation. Changyuan Tao: Writing - review & editing. Xiaoping Wu: Resources, Writing - review & editing, Supervision, Data curation. Jun Du: Writing - review & editing. Zuohua Liu: Writing - review & editing.
Declaration of competing interest
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.
Acknowledgements
This work was supported by National Key Research and Development Plan of China (2017YFB0603105).
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