Elsevier

Materials Research Bulletin

Volume 109, January 2019, Pages 227-232
Materials Research Bulletin

One-pot wet chemical synthesis of fluorine-containing TiO2 nanoparticles with enhanced photocatalytic activity

https://doi.org/10.1016/j.materresbull.2018.09.027Get rights and content

Highlights

  • F-TiO2 nanoparticles were synthesized using simple wet chemical route.

  • The produced F-TiO2 nanoparticles with tens of nanometers had a hierarchical structure.

  • Photocatalytic activity of F-TiO2 showed the improved behavior compare with H-TiO2.

  • The possible mechanism for improved photocatalytic activity due to surface fluorine was discussed.

Abstract

Two types of TiO2 nanoparticles: i) fluorine-containing TiO2 (F-TiO2) and ii) fluorine-free TiO2 (H-TiO2) nanoparticles, were prepared through a simple, scalable wet-chemical synthesis process, and a comparative study of their photocatalytic properties was conducted. This facile process begins with a one-pot precipitation process at 90 °C, followed by heat treatment at 600 °C for 1 h in air, resulting in hierarchical, sphere-like, mesoporous structures composed of primary nanoparticles. The microstructural features and crystallographic structures of both types of nanoparticles were systematically investigated by X-ray diffraction, thermogravimetric/differential thermal analysis, X-ray photoelectron spectroscopy, N2 physical adsorption-desorption, field emission scanning electron microscopy, and transmission electron microscopy analyses. The photocatalytic activities were also evaluated by measuring the degradation of MB. The F-TiO2 nanoparticles showed enhanced photocatalytic activity compared to H-TiO2, which can be attributed to adsorbed fluorine on the surface which leads to various positive effects on the photocatalytic degradation reactions.

Graphical abstract

F-containing TiO2 nanoparticles prepared by one-pot wet chemical methods show improved photocatalytic activity.

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Introduction

Semiconductor photocatalysis has been attracting immense attention, particularly for its applications in the purification of water and air pollution by photodegradation of organic pollutants [[1], [2], [3]]. Because photocatalytic reactions generally occur on the surface of the catalyst, photocatalytic reaction efficiencies and their related mechanisms may be strongly dependent on surface properties. In recent years, many kinds of semiconductor photocatalysts have been studied, such as ZnO [4], SnO2 [5], Cu2O [6], ZnS [7], WO3 [8], AgCl [9], Fe2O3 [10], CdS [11], and many others. Among the various materials explored for photocatalysis, titanium dioxide (TiO2) is one of the most important, due to its many promising features such as its self-cleaning and air-cleaning abilities, low cost, non-toxicity, and long-term photochemical stability [12]. However, the photocatalytic reaction efficiencies that have been achieved with TiO2-based materials thus far are still low; further improvement of TiO2 photoactivity is needed and may be achieved by optimizing or controlling various parameters.

Recently, the surface modification of TiO2 with fluorine has been studied in relation to doping (TiO2-xFx) [13,14] or surface fluorination (F-TiO2) [[15], [16], [17]] to better understand the photocatalytic mechanism. Many researchers have reported that the photocatalytic efficiency of TiO2 can be improved using fluorine doping. It is an effective method for improving the crystallinity, photocatalytic activity, and stability of anatase as compared to undoped TiO2. F-TiO2 has also been investigated as a method to understand the detailed formation mechanism of ≡Ti-F bonds by the ligand exchange reaction between surface hydroxyl groups of TiO2 and fluorine (Eq. (1)). F-TiO2, with its fluorine modified surface, favors the generation of OH radicals and shows enhanced photocatalytic activity [18,19].Ti-OH+F-Ti-F+OH-

Over the past few decades, various synthesis methods have been developed to fabricate F-TiO2 nanopowders, such as vapor-phase fluorination [12], precipitation [20], and hydrothermal [21] routes. Yo et al. successfully fabricated F-TiO2 powder by a one-step low-temperature hydrothermal approach. Murcia et al. [22] also prepared F-TiO2 powder by adding 10 mM NaF to pure TiO2 via an aqueous chemical reaction, followed by calcination at 650 °C for 2 h. Kim et al. [23] synthesized F-TiO2 nanoparticles using direct F2 gas while varying conditions such as temperature, fluorine pressure, and reaction time. However, there is still a need for an effective method to fabricate nanoparticles with improved properties for various applications.

Herein, we report the fabrication of F-TiO2 and H-TiO2 nanoparticles by one-pot wet chemical precipitation and subsequent heat treatment. The presence of fluorine on the surface of F-TiO2 nanoparticles was analyzed and studied. The F-TiO2 nanoparticles showed improved photocatalytic performance compared to H-TiO2.

Section snippets

Materials

Ammonium hexafluorotitanate (AHFT; (NH4)2TiF6, 99.6%, China) and boric acid (H3BO3, 99.5%, DAEJUNG, Korea) were purchased and used as starting materials without further purification.

Synthesis of F-containing TiO2 nanoparticles (F-TiO2)

Following typical procedures, AHFT (21 g/L) and H3BO3 (19 g/L) were dissolved in deionized water and heated to 90 °C under constant stirring. Subsequently, the clear boric acid solution was poured into the AHFT solution. The mixture was kept at 90 °C for 1 h without stirring. Thereafter, the white precipitates were

Results and discussion

The morphology and crystallographic structures of the obtained F-TiO2 and H-TiO2 nanoparticles were investigated by FESEM with XRD. The FESEM image of F-TiO2 in Fig. 2a shows spherical aggregates of nanoparticles. After calcination, the H-TiO2 sample (Fig. 2b) showed no change in morphology, but the particle size was slightly greater than that of F-TiO2 due to the heat-treatment. Fig. 2c shows XRD patterns of the F-TiO2 and H-TiO2 samples. All peaks are well-matched to the anatase phase of TiO2

Conclusion

F-TiO2 was successfully synthesized by a wet-chemical technique, and H-TiO2 was synthesized by the calcination of F-TiO2. The produced F-TiO2 nanoparticles with tens of nanometers had a hierarchical structure. Surface fluorination had a positive effect on the photocatalytic activity as determined from MB degradation. The synthesized F-TiO2 sample exhibited superb photocatalytic properties compared to H-TiO2; this is attributed to the increased generation of hydroxyl radicals, enhanced MB

Acknowledgements

This study was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which is funded by the Ministry of Trade, Industry and Energy, Republic of Korea (No. 20152510101950).

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