Photosensitized and photocatalyzed degradation of azo dye using Lnⁿ⁺-TiO₂ sol in aqueous solution under visible light irradiation

Abstract

With attempts to improve the photocatalytic activity of titanium dioxide (TiO₂) catalysts and also extend the light absorption towards the visible light region, three types of the lanthanide ion-modified titanium dioxide (Lnⁿ⁺-TiO₂) sol catalysts were prepared by a chemical method of coprecipitation–peptization. The microstructure and morphology of Lnⁿ⁺-TiO₂ sol samples were characterized by atom force microscope, particle size distribution, and X-ray diffraction measurements. The analytical results showed that these sol catalysts had better particles distribution and interfacial adsorption ability than the powder catalysts in suspension. The photocatalytic degradation of azo dye (X-3B) in Lnⁿ⁺-TiO₂ hydrosol reaction system was studied to determine photocatalytic activity of the crystallized Lnⁿ⁺-TiO₂ sol catalysts. Both TiO₂ and Lnⁿ⁺-TiO₂ sol catalysts demonstrated higher photocatalytic reactivity than Deggusa P25 TO₂ powder catalyst significantly. The experiments also confirmed that the modification of TiO₂ with lanthanide ions doping can improve the efficiency of interfacial adsorption and photocatalytic reactivity with azo dye. The photocurrent response of catalysts under visible light irradiation showed that the Lnⁿ⁺-TiO₂ sol catalysts had significant absorption to visible light. Since this hydrosol reaction system using the Lnⁿ⁺-TiO₂ sol catalyst has several advantages over most conventional powder reaction systems, it may provide a new approach for further development of photocatalytic reaction systems in the future.

Introduction

Titanium dioxide (TiO₂) has proven to be the most effective and suitable catalyst for photocatalytic reaction due to its economical, chemically stable, and insoluble properties [1], [2], [3]. So far, a variety of physical and chemical approaches have succeeded to synthesize anatase and rutile TiO₂ catalysts, including sputtering synthesis [4], flame pyrolysis [5], electrochemical deposition [6], chemical vapor deposition [7], precipitation [8], and sol–gel methods [9]. However, the physico-chemical properties of the synthesized TiO₂ particles are significantly affected by the precursor used and the preparation procedure applied. For example, in the sol–gel methods, the pretreatment of sol particles and thermal treatment of powder particles have been observed to affect the phase formation and morphology of product TiO₂ particles [10]. In these methods, high temperature above 450 °C in calcination is usually required to form regular crystal structure. However, in the meantime, the high temperature treatment can decline the surface area and also lose some surface hydroxyl groups of TiO₂ catalysts. Alternatively, a new method of chemical coprecipitation–peptization to synthesize the crystallized TiO₂ sol at low temperature of <100 °C became attractive to further improve the photocatalytic activity of TiO₂ catalysts. Compared with most TiO₂ powders, theses TiO₂ sol catalysts have several advantages of: (1) finer particle size with more uniform distribution and better dispersion in water; (2) stronger interfacial adsorption ability; and (3) easy coating on different supporting materials including those substrates with a poor character of thermal resistance such as some polymers, optical fibers, plastics, wood, and papers. However, it is generally believed that most sol without high temperature treatment has an amorphous structure, which contains non-bridging oxygen in the bulk TiO₂ and a lot of Ti–O atomic arrangement defects acting as centers for recombination of photogenerated electron–hole pairs. Therefore, a regular crystal structure is the prerequisite for TiO₂ semiconductor acting as an effective photocatalyst [11]. Additionally, the crystal phase of TiO₂ is also a critical factor. The anatase phase usually showed a better photocatalytic activity than the rutile phase [12], [13].

On the other hand, many studies have succeeded in addition of either metals or metallic oxides into TiO₂ structure to extend the light absorption toward the visible light range and also to eliminate the recombination of holes (h⁺) and electrons (e⁻). Some recent studies have been focused on the doping with lanthanide ions/oxides with 4f electron configuration because lanthanide ions could form complexes with various Lewis bases including organic acids, amines, aldehydes, alcohols, and thiols in the interaction of the functional groups with their f-orbital [14], [15], [16]. Xu et al. [14] reported that doping with La³⁺, Ce³⁺, Er³⁺, Pr³⁺, Gd³⁺, Nd³⁺, or Sm³⁺ was beneficial to NO₂⁻ adsorption. Ranjit et al. [15], [16] reported that doping with Eu³⁺, Pr³⁺, or Yb³⁺ increased the adsorption capacity and also adsorption rate of TiO₂ catalysts simultaneously in aqueous salicylic acid, t-cinnamic acid, and p-chlorophenoxy-acetic acid solutions. However, the effect of lanthanide oxides on the separation of electron–hole pairs and the photoresponse had not been extensively investigated so far. For all lanthanide ions, they have special electronic structure of 4f^x5d^y which would lead to different optical properties and dissimilar catalytic properties, and also have a redox couple of Lnⁿ⁺/Ln⁽ⁿ⁺¹⁾⁺ which would be able to form the labile oxygen vacancies (OV) with the relatively high mobility of bulk oxygen species [17].

In this study, three kinds of lanthanide ion-doped TiO₂ (Lnⁿ⁺-TiO₂) sol catalysts including Nd³⁺-TiO₂, Eu³⁺-TiO₂, and Ce⁴⁺-TiO₂, and also a pure TiO₂ sol catalyst were prepared using a chemical method of coprecipitation–peptization at low temperature and ambient pressure. The photocatalytic activity of these sol samples was evaluated in photocatalytic degradation of azo dye (reactive brilliant red, X-3B) in aqueous solution under visible light irradiation.