Elsevier

Solid State Sciences

Volume 100, February 2020, 106098
Solid State Sciences

One-step synthesis of Bi4Ti3O12/Bi2O3/Bi12TiO20 spherical ternary heterojunctions with enhanced photocatalytic properties via sol-gel method

https://doi.org/10.1016/j.solidstatesciences.2019.106098Get rights and content

Highlights

  • Bi4Ti3O12/Bi2O3/Bi12TiO20 (BTO) ternary heterojunction materials have been synthesized for the first time.

  • It has excellent photocatalytic properties under visible light irradiation for remove of 17β-Estradiol (E2).

  • The Z-type degradation mechanism was proposed.

Abstract

In this paper, the Bi4Ti3O12/Bi2O3/Bi12TiO20 (BTO) spherical ternary heterojunction composite was successfully prepared by a simple sol-gel method. The phase, morphology and optical properties of the catalyst were studied by XRD, SEM, FT-IR, UV–vis DRS and PL. The catalyst exhibits excellent catalytic performance under visible light irradiation for remove of 17β-Estradiol (E2). The degradation efficiency of E2 could approach 100% using Bi4Ti3O12/Bi2O3/Bi12TiO20 (the molar ratio of Bi: Ti = 6:1) as photocatalyst under visible light irradiation for 100 min. The removal rate of 6-BTO catalyst was significantly better than P25 (38.9%) and pure Bi2O3 (83.7%). The active species and band gaps of photocatalysts were studied by the capture experiments of active substances and UV–Vis diffuse reflectance spectra, and the possible photocatalytic mechanism was speculated.

Graphical abstract

Two proposed photocatalytic mechanism of the 6- BTO photocatalyst: (a) Ⅱ type, (b) Z type.

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Introduction

In recent years, water pollution has become a growing concern worldwide. The harm to the organisms in the presence of environmental endocrine disruptors (EDCs) in water is also valued by scientists in various fields. Bisphenol A (BPA), 17β-estradiol (E2), and 17α-ethynylestradiol (EE2) are the most representative three hormones in EDCs [1,2]. When these EDCs pass through certain pathways into the body, such as contaminated water, food or absorbed through the skin, it could interfere endocrine hormones with the synthesis, release, transport, binding to receptors, metabolism thereby affecting the function of the endocrine system and destroy the coordination and stability of the internal environment of the organism. Therefore, many scientists have begun to study environmentally friendly and cheap methods of degrading EDCs.

Among the numerous research findings, photocatalysts have stood out in various methods for degrading pollutants due to their environmentally friendly and inexpensive features. In most of the photocatalytic degradation studies, TiO2 is the most used photocatalyst owing to its nontoxicity, good stability and excellent photocatalytic activity [3,4]. However, TiO2 absorbs only ultraviolet light which cannot make full use of sunlight, only 4–5% of which is visible solar radiation. Meanwhile the rapid recombination of photogenerated electron–hole pairs limits the practical application of TiO2 [[5], [6], [7]]. Therefore, developing visible-light-driven photocatalysts has been a hot topic. Bismuth-based materials have a narrow band gaps and good photocatalytic activity under visible light,so many scientists used Bismuth-based materials as the first choice for photocatalytic degradation. J. Tian et al. [8] prepared a new hybrid photocatalyst Bi2WO6/TiO2 heterojunction with high photocatalytic activity. The photocatalytic activity of Bi2WO6/TiO2 heterojunction is excellent under full spectrum sunlight in the range of UV, visible light and near infrared. The formation of Bi2WO6/TiO2 heterojunction improves the utilization of light and makes up for the defect of low light utilization of titanium dioxide.

Besides, Bi2O3 have attracted much interest due to its narrow band gap energy, thermal stability, nontoxicity and corrosion resistance. However, it is found that pure Bi2O3 has the disadvantage of easy recombination of photo-generated electrons and holes, result in poor photocatalytic efficiency [[9], [10], [11]]. Many researchers have improved the catalytic performance by element doping [[12], [13], [14]], compounding with semiconductors to form heterojunctions [[15], [16], [17]], and changing morphology [[18], [19], [20]] and other modification methods. Among them, it was the most common and effective method to improve the photocatalytic performance by forming a heterojunction with other semiconductors. Therefore, we have studied the formation of heterojunctions of pure Bi2O3 with other semiconductors to further improve photocatalytic performance.

Titanium complex has aroused the interest of researchers in studying its catalytic properties due to its unique electronic and crystal structure. CaO/SrTiO3 and TiO2/SrTiO3 heterojunctions were successfully synthesized by U. Coleto Junior et al. [21,22] and the effects of interfacial tension defects on photocatalytic properties were analyzed. And the experimental data analysis shows that the mono and double ionized vacancies affect the performance of the photocatalyst in the degradation of micropollutants, further explaining the mechanism of photocatalytic degradation. Weng et al. [23] successfully prepared a novel hierarchical photocatalyst of Bi2O3/Bi4Ti3O12 p–n junction nanoribbons (NRs) on carbon fibers. The catalytic performance of the composites was 14.9 and 11.9 times that of pure phase Bi2O3 and Bi4Ti3O12, respectively. In addition, Shi et al. [24] prepared a Cr-doped Bi4Ti3O12/Bi2Ti2O7 (BTO) heterojunction composite photocatalyst by one-step method. The results show that the doping of Cr can effectively promote the separation of photogenerated electron-holes and improve the photocatalytic performance.

In this study, series of Bi4Ti3O12/Bi2O3/Bi12TiO20 spherical ternary composite heterojunctions were prepared via one step sol-gel method with different Bi/Ti molar ratios. The photocatalytic degradation efficiency was assessed by removal of 17β-Estradiol (E2) under visible light irradiation. In addition, the factors affecting the degradation efficiency of Bi4Ti3O12/Bi2O3/Bi12TiO20 and possible photoreaction mechanisms are also discussed in detail.

Section snippets

One-step synthesis of the Bi4Ti3O12/Bi2O3/Bi12TiO20 composites

All chemical reagents were of analytical grade and used without further purification. The Bi4Ti3O12/Bi2O3/Bi12TiO20 composites was prepared by sol-gel method. The specific process was as follows: 8 mmol of Bi(NO3)3·5H2O was dissolved in 25 mL of mixture solvent of glacial acetic acid and ethylene glycol with a volume ratio of 1:4. Then, 1.33 mmol butyl titanate was added drop by drop after complete dissolution. The above solution was stirred in a water bath at 80 °C for 2 h to obtain a clear

XRD analysis

The composition and crystal form of the photocatalyst were characterized by XRD, as shown in Fig. 1. When Bi:Ti = 1:0, the diffraction peaks of pure Bi2O3 at 2θ values of 24.56°, 25.7°,26.9°,27.4°,27.9°,33.0°, 35.0°, 37.6°, 42.4°, 45.1°, 46.3°, 48.6° and 52.4° were observed, which matched well with the (−102), (002),(-112),(-121),(012),(-122), (−212), (−113), (−123), (023), (041), (−104) and (−322) crystal planes of Bi2O3 (JCPDS 71–2274), respectively. When Bi:Ti = 2:1, Bi2O3/Bi4Ti3O12

Conclusion

In this work, Bi4Ti3O12/Bi2O3/Bi12TiO20 ternary heterojunction materials were successfully prepared by sol-gel method. The existence of two heterojunctions of p-n and p-p type significantly improved the separation efficiency of photo-generated carriers. The electrons accumulated after transfer can reduce the oxygen adsorbed on the catalyst surface to form O2, and while holes can directly participate in the oxidation of organic pollutants, thereby the catalytic performance of the catalyst was

Acknowledgments

Those works were supported by National Natural Science Foundation of China (No. 21763032, Nos. 21261026 and 21767030) and Major Project of Kunming Science and Technology Bureau (No. 2017-1-S-12305).

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