Direct hydro-alcohol thermal synthesis of special core–shell structured Fe-doped titania microspheres with extended visible light response and enhanced photoactivity

Abstract

Fe-doped titanium dioxide (TiO₂) microspheres with special core–shell structure were prepared by a simple hydro-alcohol thermal method. The morphology and microstructural characteristics of Fe-doped titania microspheres with different Fe³⁺ doping concentrations were characterized by means of BET, TEM, SEM, XPS, UV–vis DRS, PLS and XRD. The Fe³⁺ doped TiO₂ samples showed the best photocatalytic activity, which were much superior to P25 under both visible and ultraviolet light irradiations. The concentration of Fe³⁺ was found playing a key role in the photocatalytic degradation of phenol, moreover, 0.5 mol% Fe³⁺ doping was an optimal amount. The probable mechanism was proposed: it was presumed that doping Fe³⁺ ions into TiO₂ structure may overlap the conduction band of TiO₂ and the d orbital of Fe³⁺, which leads to the marked narrowing of the band gap and the extension of visible light response. Meanwhile, since the Fe³⁺/Fe²⁺ energy level was just lower than the conduction band of TiO₂ while the Fe³⁺/Fe⁴⁺ energy level was slightly above the valence band of TiO₂, the Fe³⁺ dopant can not only play as a temporary trapping sites of photo-induced electrons but can also act as shallow capturing sites of photo-induced holes, which will efficiently separate the photoexcited electrons and holes, prolong the lifetime and at last improve the photocatalytic activity. The superior activity of Fe³⁺–TiO₂ photocatalysts can also be ascribed to the special core–shell structure with high surface area, mesoporous pore and well-crystallized anatase phase.

Introduction

Titanium dioxide (TiO₂) with anatase crystal structure is used as a photocatalyst in a wide variety of applications because of its superior material and economic properties [1], [2], [3], [4]. Since the structure and the crystalline state strongly affect the photocatalytic activity and selectivity, proper control of the morphology of nanostructured TiO₂ materials plays an important role [5], [6], [7]. A considerable variety of nanostructured TiO₂ materials with different morphologies have been successfully synthesized, including nanowires, nanorods, nanotubes, nanowhiskers, microspheres, foams and films. Among various morphologies, the design and fabrication of spherical materials with hollow interiors have attracted considerable attention recently because of their potential applications as low density capsules for controlled release of drugs, dyes and inks; development of artificial cells, protection of proteins, enzymes, and especially as supports for catalysts [8], [9], [10], [11], [12].

However, the main problem preventing the wide application of TiO₂ photocatalyst is its lower visible light absorbance. As we know, because of the wide band gap of pure TiO₂, only a small UV fraction of solar light (3–5%) can be utilized [13], [14], [15], [16], [17], [18], [19]. So in many years even though TiO₂ is a promising photocatalyst for waste water purification, exploitation for practical circumstances has not been achieved as expected. In the last decade, research efforts have been directed to enhance the activity of the photocatalysts using various methods such as surface modification, metal or nonmetal ion doping, combination with other semiconductors, and so forth. These methods have been widely adopted to prepare TiO₂ photocatalysts sensitive to visible light. Among these strategies, doping TiO₂ with Fe³⁺ has been considered as one of the most promising ways [20], [21], [22], [23]. For example, Praveen et al. [20] recently reported that impregnation of Fe³⁺ salts on Degussa P25 TiO₂ with different anions has been found to enhance the activity of the photocatalyst for the degradation of acetophenone, and the final degradation percentage increases continuously with an increase in Fe³⁺ ion concentration. Zhu et al. [21] used a sol–gel method to prepare Fe–TiO₂ and studied the effect of Fe³⁺ doping concentration on the photoactivity of yellow XRG dye. Cong et al. [22] studied the co-doping effect of N and Fe to TiO₂ which improved the activity under both visible and UV lights. It was commonly reported that the improvement of photocatalysts was attributed to the Fe³⁺ doping which can help the separation of photogenerated electrons and holes, and also can absorb and utilize the visible light to photocatalyze the degradation of pollutants.

In our previous work, mesoporous core–shell structured TiO₂ microspheres with high thermal stability and high surface area have been synthesized by a novel hydro-alcohol thermal route [13]. These TiO₂ microspheres with high specific surface areas exhibit considerably high activity in photocatalytic degradation of phenol under UV light irradiation. The interspace between the core and shell can act as a micro-reactor, which enables higher relative consistency in this area than in the rest. Both the excellent photocatalytic ability of these microsphere photocatalysts and its much easier separation from the reaction mixture make them a promising candidate in further fundamental study and industrial application. Nevertheless, the photocatalytic ability of these core–shell structured photocatalysts under visible light irradiation was not satisfying. Considering that the structure and visible light adsorption are two major factors affecting the activity and the wide application of the photocatalysts, it seems therefore worthwhile to search for a modified titania microsphere exhibiting a red-shifted adsorption. In this work, detailed study was performed on the effect of the doped Fe³⁺ salts for the extension of the microspheres TiO₂ with visible light response. Herein a similar simple one-pot method was used to prepare Fe-doped TiO₂ microspheres with core–shell structure, which showed not only higher activity in the UV light region but also an extended visible light response than the blank one and the commercial Degussa P25. The photodegradation of phenol was chosen as a probe reaction to evaluate the photocatalytic activity of all the as-prepared samples. The factors influencing the photocatalytic activity and the probable mechanism of Fe-doped TiO₂ microspheres are also investigated.