Nano-Ag particles doped TiO₂ for efficient photodegradation of Direct azo dyes

Abstract

Silver nanoparticles doped TiO₂ has been prepared and characterised by surface analytical methods such as BET surface area, scanning electron micrographs (SEM), X-ray diffraction (XRD), energy dispersive X-ray micro analysis (EDX), electron spin resonance (ESR) and diffuse reflectance spectroscopy (DRS). We have investigated the photocatalytic degradation of two Direct diazo dyes, Direct red 23 (DR 23) and Direct blue 53 (DB 53) in the aqueous suspensions of TiO₂ and Ag deposited TiO₂ nanoparticles under UV-A light irradiation in order to evaluate the various effects of silver deposition on the photocatalytic activity of TiO₂. The presence of silver in TiO₂ was found to enhance the photodegradation of DR 23 and DB 53. The higher activity of silver doped TiO₂ is due to the enhancement of electron–hole separation by the electron trapping of silver particles.

Introduction

Heterogeneous photocatalysis is an alternative economical and harmless technology of advanced oxidation processes (AOP) for removal of organic impurities. During the process, illuminated semiconductor absorbs light and generates active species which leads to complete oxidation of organic components in waste water. A distinct advantage of the photocatalysis lies in its ability to utilise solar energy in the production of active species OH. Photocatalysis on semiconductors has been studied in many fields. For example: (i) fuel production: hydrogen from water photolysis [1], [2], [3], (ii) removal/recovery of heavy metal ions [4], [5], (iii) water detoxification: removal of toxic, harmful or hazardous pollutants [6], [7], [8], [9], [10], [11].

A great many photocatalysts have been examined for the degradation of organic pollutants in waste water. Among various semiconductor metal oxides, TiO₂ has been the focus of photocatalysts under UV irradiation because of its physical and chemical stability, low cost, ease of availability, non-toxicity and electronic and optical properties. Despite the positive attributes of TiO₂, there are a few drawbacks associated with its use; it has a high bandgap (E_g > 3.2 eV) and it is excited only by UV light (λ < 388 nm) to inject electrons into the conduction band and to leave holes into the valence band [12]. Thus, this practically limits the use of sunlight or visible light as an irradiation source in photocatalytic reactions on TiO₂ [13]. In addition, the high rate of electron–hole recombination on TiO₂ particles results in a low efficiency of photocatalysis [14]. For the purpose of overcoming these limitations of TiO₂ as a photocatalyst, numerous studies have been recently performed to enhance electron–hole separation and to extend the absorption range of TiO₂ into the visible range. These studies include doping metal ions into the TiO₂ lattice [15], [16], dye photosensitisation on the TiO₂ surface [17], [18], [19], [20], [21], addition of inert support [22] and deposition of noble metals [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34].

In particular, noble metal-modified semiconductor nanoparticles become of current importance for maximising the efficiency of photocatalytic reactions. The noble metals such as Pt [23], [24] and Au [25], [26] deposited or doped on TiO₂ have the high Schottky barriers among the metals and thus act as electron traps, facilitating electron–hole separation and promotes the interfacial electron transfer process [5], [35], [36], [37]. Most studies of noble metal-modified TiO₂ photocatalysts have focused on the details of the photoinduced electron transfer from the conduction band of UV-irradiated TiO₂ to noble metals for improving the photocatalytic activity of TiO₂ under UV irradiation.

Noble metals doped or deposited on TiO₂ are expected to show various effects on the photocatalytic activity of TiO₂ by different mechanisms. These noble metals act separately or simultaneously depending on the photoreaction conditions. They may (i) enhance the electron–hole separation by acting as electron traps, (ii) extend the light absorption into the visible range and enhance surface electron excitation by plasmon resonances excited by visible light and (iii) modify the surface properties of photocatalysts.

In the recent years, silver ions have attracted the interests of several researchers [38], [39], [40], because of both their novel effects on the improvement of photoactivity of semiconductor photocatalysis nanocrystallites [39], [40] and their effects on antibacterial activity [38]. These properties can be applied to a tremendous range of applications, for instance, environment, textiles, engineering materials and so on. However, the studies on silver doped photocatalyst nanocrystallites are still limited in the literature [41], [42], [43], [44], [45], [46], [47], [48], [49].

The aim of the present work is to prepare Ag doped TiO₂ (anatase) by a photodeposition method and to compare the activity of the photocatalyst before and after surface modification with metallic silver for the degradation of two azo dyes DB 53 and DR 23.