Visible-light driven FexOy/TiO2/Au photocatalyst – synthesis, characterization and application for methyl orange photodegradation

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Abstract

A visible-light driven ternary photocatalyst of magnetic properties was prepared by a simple three-step synthesis. The Fe3O4 nanoparticles obtained by solvothermal method were covered with a layer of TiO2 by sol-gel procedure and amount of TiO2 in the shell around the magnetic core was increased by repetition of sol-gel runs. After calcination the composite was decorated with Au nanoparticles by photodeposition using a [AuClx(OH)4−x] complex as a gold precursor. The obtained binary (FexOy/TiO2) and ternary (FexOy/TiO2/Au) systems were characterized with the use of X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray (EDX) spectroscopic mapping. The photocatalytic activity of the systems with and without Au nanoparticles was studied in the reaction of methyl orange (MO) photodegradation under irradiation with visible light lamp and monochromatic diode (523 nm) to determine the role of TiO2 and Au nanoparticles in the photocatalytic process. The main species involved in the photocatalytic degradation of MO were determined with the use of reactive radical scavengers. The alternative core/shell composite FexOy/SiO2/TiO2/Au was prepared to determine the role of FexOy/TiO2 interface in the photocatalytic process.

Introduction

Artificial photosynthesis in terms of water splitting and other ways of solar light conversion into chemical energy was defined in 1995 as one of so-called “Holy Grails” of chemistry [1,2]. Two decades later, in 2017, when a list of “Holy Grails” was redefined, the solar energy conversion still occupied an important position [3]. Although many various semiconductors have been tested as the photocatalysts, TiO2 still remains a benchmark owing to high oxidation and reduction power of photogenerated holes and electrons, respectively, as well as low cost, high chemical stability and non-toxicity [4]. However, there are two major drawbacks of TiO2: a rapid charge recombination of the electron-hole pairs, suppressing the quantum efficiency of the photocatalytic process, and a wide band gap (about 3.2 eV for anatase and 3.0 eV for the rutile). The latter one restricts the light absorption to ultraviolet region (which accounts for <5% of solar radiation) which is limiting for practical application of TiO2-based photocatalysts for solar light harvesting. Therefore, many strategies to enhance TiO2 photocatalytic properties under visible light irradiation have been proposed, such as construction of heterojunctions with other semiconductors or carbon materials, decoration with noble metal nanoparticles, reduction of TiO2 to TiO2-x, doping with metals or non-metals or dye-sensitization [[5], [6], [7]].

The TiO2 photocatalyst is most often used in the form of powder dispersed in the pollutant solution. On the one hand, this provides a large active surface area of the nanoparticles, but on the other, the removal of the material in this form from the solution after photocatalytic process needs additional operation, such as centrifugation. It is easy for laboratory scale but separation of dispersed nanoparticles from a large volume of water is time-consuming and generates additional costs. In order to overcome this problem TiO2 may be synthesized on a solid substrate in the form of 1D or 3D nanostructures [8,9] or as a shell around magnetic Fe3O4 core particles which can be easily separated by application of external magnetic field [10]. However, these two semiconductors form a heterojunction of type 1, in which the conduction band (CB) and the valence band (VB) of Fe3O4 are located respectively below the CB and above the VB of TiO2 [11]. This means that electrons and holes photogenerated in TiO2 may be injected respectively to CB and VB of Fe3O4 and undergo recombination. According to the literature, this problem may be overcome by separation of the core and shell materials with a thin SiO2 layer [[12], [13], [14]]. An alternative approach may be a combination of Fe3O4/TiO2 composite with an electron acceptor such as carbon nanomaterials or noble metal (Au, Pt) nanoparticles, since their work function (~5 eV for CNTs [15], 5.1 eV – 5.47 eV for Au [16]) is greater than that of TiO2 (~4.7 eV) [17]. Moreover, a decoration of TiO2 with Au nanoparticles allows obtaining the visible-light driven photocatalyst owing to the plasmon resonance effect [18,19].

The goal of this paper is the synthesis of a ternary material by combination of TiO2 both with Au and Fe3O4 nanoparticles to obtain the photocatalyst efficient in visible light and easily removable from the solution. This kind of the systems has been recently proposed in the literature [[20], [21], [22], [23], [24], [25], [26]] but the reported synthesis paths were rather complicated and needed a multistep preparation of each component. For example, the Fe3O4 nanoparticles were additionally functionalized with citrates [20] or cationic polymer, while Au nanoparticles were obtained by colloidal method [21] or by impregnation of the Fe3O4/TiO2 hollow microspheres with AuCl4 ions, followed by reduction with NaBH4 [25]. In this work, the Fe3O4 nanoparticles obtained by solvothermal synthesis are covered with a TiO2 layer by means of a simple sol-gel method. The relative amount of TiO2 to Fe3O4 may be changed by repetition of sol-gel cycles. The novelty of the approach consists also in decoration of Fe3O4/TiO2 composite dispersed in the solution with Au nanoparticles by photodeposition. The photocatalytic activity of the synthesized ternary systems is tested in methyl orange degradation under irradiation with visible light or monochromatic illumination with a diode of the wavelength 523 nm to utilize the plasmon resonance of Au nanoparticles. The mechanism of the photocatalytic process is proposed on the base of experiments with the use of various scavengers.

Section snippets

Materials

All reagents were of analytical grade and used without further purification. Ferric chloride (FeCl3⋅6 H2O, 97%), polyvinylpyrrolidone (PVP), titanium(IV) butoxide (C16H36O4Ti, 99.0%), gold(III) chloride hydrate (HAuCl4⋅H2O), tetramethyl orthosilicate (TMOS, Si(OCH3)4), methyl orange (MO, C14H14N3NaO3S, 85% dye content) and benzoquinone (BQ, C6H4O2) were purchased from Sigma-Aldrich; ethylene glycol (ethane-1,2-diol, 99.0%), isopropanol (2-propanol, 99.7%), methanol and potassium iodide (KI)

Characterization of Fe3O4 nanoparticles

The SEM images of Fe3O4 powder obtained by solvothermal synthesis presented in Fig. 1 indicate accumulation of the nanoparticles into aggregates of the size 50–100 nm as a result of their magnetic properties.

The crystalline structure of Fe3O4 is confirmed by the presence of the diffraction peaks at 2θ: 30.3°, 35.8°, 43.4°, 53.8°, 57.1°, 62.8° and 74.6° in the XRD pattern (Fig. 2), which correspond to (220), (311), (400), (422), (511), (440) and (533) facets of magnetite (JCPDS No 19-0629).

Conclusions

In this paper, a new, easy route of synthesis of core-shell systems FexOy/TiO2/Au was presented. The Fe3O4 nanoparticles were obtained using solvothermal synthesis and covered with TiO2 by means of sol-gel method.

It has been shown that the relative amount of TiO2 to Fe3O4 may be increased by repetition of sol-gel deposition cycles. Annealing at 450 °C which is necessary to transform amorphous TiO2 into anatase phase leads to partial oxidation of magnetite Fe3O4 to maghemite γ-Fe2O3 leading to

CRediT authorship contribution statement

Dariusz Boczar: Methodology, Investigation, Visualization, Writing - original draft. Tomasz Łęcki: Methodology, Investigation, Validation, Visualization. Magdalena Skompska: Conceptualization, Supervision, 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

The authors thank dr. Kamila Zarebska (Faculty of Chemistry) for SEM imaging, Kamil Sobczak (Biological and Chemical Research Centre, University of Warsaw) and dr. Jacek Ratajczak (Institute of Electron Technology, Warsaw) for HR-TEM imaging and EDX mapping. The authors thank also dr. R. Minikayev (Institute of Physics, Polish Academy of Sciences, Warsaw) for XRD measurements of FexOy/TiO2/Au composites.

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