Synthesis of solar light responsive Fe, N co-doped TiO₂ photocatalyst by sonochemical method

Abstract

Fe, N co-doped TiO₂ photocatalyst has been synthesized by sonochemical method. The as-prepared samples were characterized by XRD, TEM, UV–vis-DRS, XPS, FT-IR and physisorption of N₂. Experimental results show that the as-prepared TiO₂ photocatalyst has the anatase TiO₂ crystalline phase. The band-gap energy absorption edge of Fe, N co-doped TiO₂ shifted to longer wavelength as compared to commercial TiO₂–P25 and N–TiO₂. The specific surface area of the TiO₂ was increased to 75 m² g⁻¹ with the doping of Fe and N ions into the TiO₂ framework. The photocatalytic activity of Fe, N co-doped TiO₂ for degradation of indigo carmine dye (ICD) under solar simulator was enhanced as compared to TiO₂–P25 and N–TiO₂.

Introduction

Since the discovery of the photosplitting of water on TiO₂ electrode by Honda and Fujishima in 1972 [1], TiO₂ photocatalyst has been widely applied for the removal of toxic agents in air and water [2], [3], [4]. However, TiO₂ photocatalysts operate fundamentally under UV light of wavelengths shorter than 400 nm, which means only 3–5% of solar light can activate these wide band-gap materials [5]. It is, therefore, required to develop a photocatalytic system, which can work even under visible light irradiation. Recently, many studies have been devoted to the modification of TiO₂ photocatalysts by the substitutional doping of metals or nonmetals in order to extend their absorption edge into the visible light region and to improve their photocatalytic activity [6], [7], [8], [9], [10], [11]. Among them, metals or nonmetals doping is a practical approach because the properties of the material are largely determined by chemical nature of the atoms or ions and of the bonds between them. The most popular dopants for modification of the optical and photoelectrochemical properties of TiO₂ are nonmetal such as N [8], [12], F [13], C [14], S [9], [15] and Cl [16]. On the other hand, doping with metal ions is also a typical approach to extend the spectral response of TiO₂ to the visible light region by providing defect states in the band gap [17], [18], [19]. Transition metal ions such as Fe, Cr, Mo, La, V and W have been reported as effective in enhancing visible light photocatalytic activity [20], [21], [22], [23]. Asahi et al. [8], [24] reported that the absorption edge of N-doped TiO₂ could be red-shifted to 404 nm and that the N-doped TiO₂ had good photocatalytic activity under visible light irradiation. However, due to strongly localized N 2p states at the top of valence band, the photocatalytic efficiency of N-doped TiO₂ decreases because isolated empty states tend to trap photogenerated electrons, thereby reducing the photogenerated current [25]. Recently, co-doped TiO₂ with two different elements, especially nonmetal and metal elements co-doping has become a rapidly growing field of interest for computational studies. Gai et al. [26] proposed using passivated co-doping of nonmetal and metal elements to extend the TiO₂ absorption edge to the visible light range; because the defect bands are passivated, they will not be active as carrier recombination centers [27]. Because TiO₂ doped with Fe³⁺ forms electron or hole traps to reduce the recombination probability of photon generated electron–hole pairs [28].

Sonochemistry has proven to be an excellent method in the preparation of amorphous and crystalline nanosized materials [29], [30]. The collapse of bubbles generates localized hot spots with transient temperatures of above 5000 K, pressure of about 20 MPa, and heating and cooling rates greater than 10⁹ K s⁻¹ [31]. These extreme conditions can obviously accelerate the hydrolysis or condensation reaction.

In the present study, the sonochemical method was used to prepare Fe–N–TiO₂ photocatalysts with the assistance of high power ultrasound. The photocatalytic activity of the Fe–N–TiO₂ was tested by degradation of indigo carmine dye (ICD) in aqueous model solution under solar light irradiation.