SrTi1−yFeyO3 samples obtained by hydrothermal method: The effect of the amount of Fe on structural and photocatalytic properties

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Abstract

This manuscript describes the synthesis and the photocatalytic efficiency of SrTiO3 (STO) and SrTi1−yFeyO3 (STFO, with 2.5, 5, 10, 25 and 40 mol% of Fe) materials evaluated by monitoring the photodegradation of methylene blue (MB) dye under visible and UVC irradiation. Samples synthesized by conventional hydrothermal method were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption isotherms, UV–Vis diffuse reflectance spectroscopy (DRS), X-ray absorption spectroscopy (XAS) and Mössbauer spectroscopy. The results show that a controlled introduction of Fe into Ti sites caused changes in morphology, local order, band gap, textural properties and photoactivity of the STO compound. By increasing the Fe content, the surface roughness was also increased when compared with the STO sample. Furthermore, the highest amount of Fe caused significant changes in the STO lattice, especially in relation to the local order around Ti atoms. The Mössbauer analysis confirmed that Ti4+ atoms were replaced by Fe3+. The sample with the lower amount of Fe (2.5 mol%) presented photoactivity similar to that of pure STO. The samples containing intermediate amounts of Fe (5 and 10 mol%) presented good performances on the methylene blue (MB) dye photodegradation, while others with higher amounts of Fe (25 and 40 mol%) led to a large distortion of the crystalline structure, which disfavored the photocatalytic performance. The enhanced photoactivity of the some STFO samples was related to three combined effects: an improvement of the textural property; a decrease in the rate of electron-hole recombination; and a broadening of the photocatalyst optical absorption band, which is important for the photodegradation under visible irradiation. These positive characteristics were predominant for both 5 mol% and 10 mol% STFO samples. The one with 5 mol% has presented the best performance for MB photodegradation, which was mainly attributed to its best textural properties.

Introduction

Materials with perovskite structure generally show notable properties, including a large group of compounds that can be represented by the ABO3 chemical formula. Among such materials, strontium titanate (SrTiO3 or just STO) has been widely studied for different applications, such as gas sensors and photocatalysts [1], [2], [3], [4]. The STO is considered an important n-type semiconductor with band gap of approximately 3.2 eV [1]. In the perovskite STO structure, Sr2+ ions occupy site ‘A’, while Ti4+ ions are located in site ‘B,’ although the STO lattice can accommodate several atoms and ions with different oxidation states and ionic radius [5]. These characteristics bring special attention to this material, once they allow the controlled modification of the STO structure at atomic level, targeting different characteristics and improving their properties. For instance, Fe can be introduced into the STO lattice [6], causing important modifications in its structure, and consequently in its properties [5], [7], including the photocatalytic effectiveness promoting the degradation of common water contaminants, such as organic molecules, or water splitting.

In our research group, we have already carried out some studies about the synthesis of SrTi1−yFeyO3 (STFO) using microwave-assisted hydrothermal method [6]. Although STO has been synthesized by conventional hydrothermal method [4], to the best of our knowledge, STFO compounds synthesized by this method and the effect of Fe addition on the photoactivity of SrTiO3 were not reported in the literature. Therefore, this paper reports the synthesis of SrTiO3 and SrTi1−yFeyO3 (STFO) by a conventional hydrothermal method, aiming to evaluate the effects of the substitution of Ti by Fe atoms, especially on the photoactivity towards the photodegradation of methylene blue (MB) dye under visible and UVC radiation. The properties of samples containing different amounts of Fe (2.5 mol%, 5 mol%, 10 mol%, 25 mol% and 45 mol%) were evaluated by X-ray Diffraction (XRD), X-ray absorption spectroscopy (XAS) and Mössbauer techniques.

Section snippets

Hydrothermal syntheses of SrTiO3 (STO) and SrTi1−yFeyO3 (STFO) samples

All the samples were synthesized by conventional hydrothermal method. For the synthesis of the STO sample, 1.07 g of TiOSO4·yH2SO4·yH2O (99%, Sigma-Aldrich) was added to 50 mL of distilled water at around 50 °C under vigorous stirring. Then, 0.270 g of SrCl2·2H2O (99%, Sigma-Aldrich) was also added to this solution in order to obtain a 1:1 (Ti: Sr) molar proportion, corresponding to the stoichiometry of SrTiO3. Finally, 50 mL of an aqueous KOH solution (KOH, 85%, Merck) of approximately 5.0 mol L−1

Material characterization

The XRD patterns of the STO and some of STFO samples (5–40 mol%) presented in Fig. 1A show that all samples are fully crystallized, and the XRD patterns can be assigned to the SrTiO3 cubic perovskite structure (JCPDS 35-0734, Fig. 1B). It is worth emphasizing that no peak associated with secondary phases containing elemental iron (Fe) can be observed in Fig. 1. However, STFO samples present diffraction peaks of lower intensity than the STO sample. This effect is clear for the STFO 40% sample and

Conclusions

This study presented a hydrothermal synthesis of SrTiO3 (STO) and SrTi1−yFeyO3 (STFO) with different molar proportions of Fe, and the evaluation of their photoactivities under visible and UVC irradiation. The introduction of Fe3+ into Ti4+ sites changed the structural, optical, morphological, textural and photocatalytic properties of the STO compound. It was observed that STFO 5% and STFO 10% exhibited the better photocatalytic performance on MB dye photodegradation under UVC irradiation,

Acknowledgements

The authors would like to acknowledge CNPq (503272/2011-6 and 454438/2014-1) and FAPESP (2012/11246-8, 2013/17639-4 and 2013/13888-0) for the financial support provided for this research, as well as the LNLS (Campinas, SP, Brazil) laboratories for XAS analyses (XAFS1-14284 and XAFS2-12470). We also would like to thank Professor José Domingos Fabris, member of a research group at UFVJM (Materials Science and Technology Group), for the Mössbauer analysis.

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