Sunlight-assisted photocatalytic degradation of textile effluent and Rhodamine B by using iodine doped TiO₂ nanoparticles

Highlights

• I@TiO₂ nanoparticles were synthesized by the simple sol-gel method at room temperature.

• I@TiO₂ nanoparticles showed red shift in visible light absorption, increased surface hydroxylation with increasing iodine content.

• Efficient photodegradation of textile effluent and Rhodamine B using I@TiO₂ under sunlight irradiation directly.

Abstract

Iodine-doped TiO₂ nanoparticles (I@TiO₂ NPs), in different mol percentage (mol%) of iodine with anatase phase only, have been prepared by using simple sol-gel method. Samples were characterized by various physico-chemical techniques. Physico-chemical characterization revealed that crystallite size, surface hydroxylation, and tuning of optical band gap towards visible region of composites increases with increase in dopant concentration upto 7.0 mol% in TiO₂ host lattice. Thereafter, the samples were used to photocatalytic degradation of textile effluent (TE) as well as Rhodamine B (RhB) under sunlight irradiation directly. The progress of photodegradation reaction was monitored by UV–visible spectroscopy. The chemical oxygen demand (COD) of TE samples was evaluated before and after photocatalytic studies. During photocatalytic reaction, the effect of pH and amount of catalyst on the rate of degradation reaction was also studied. It is clearly shown that I@TiO₂ particles result in the enhancement of the degradation ability of RhB as well as TE in sunlight.

Introduction

Modified TiO₂ photocatalysts are promising materials for various applications due to their tunable physico-chemical properties, environment friendly nature, absorbing visible region of electromagnetic spectrum, capable of performing at room temperature, and effective use at extremely low concentrations [1], [2], [3], [4], [5], [6]. In literature, various strategies, such as coupling with a narrow band gap semiconductor [7], structural modification [8], quantum dot sensitization [9], and doping with metal or non-metal [10], [11], [12], have been reported for modifying the properties of TiO₂. Among these different techniques, doping is one of the superior methods to modify the opto-electrical properties of TiO₂ by introducing new energy levels from dopant between valence and conduction band [13]. For the maximum utilization of solar irradiation for photocatalytic experiments, several research groups recently successfully extended the absorption range of TiO₂ to the visible light region by adopting doping method. These doped TiO₂ are also widely employed for the effective treatment of all kinds of organic contaminants in the visible light than that of its bare form [14], [15]. Doping with metal, non-metal has been proved as an way to tune the material properties by changing the chemical or structural composition of TiO₂ with a degree of substitution, results broaden the photo response and improve the efficiency of TiO₂ [16], [17]. However, metal doping may cause the thermal instability and metal also can acts as an electron or hole traps, which can reduce the photocatalytic efficiency. Also, for the transition metal doping, it requires expensive facilities like ion-implantation or other physical methods [18], [19], [20].

Particularly, non-metal doped TiO₂ exhibited stronger absorption in the visible light range with a red shift in the band gap transition [21]. Non-metal doped TiO₂ nanomaterials have revealed additional electronic states above the valence band edge of pure TiO₂. These additional electronic states are responsible for the red-shifted absorption of non-metal doped TiO₂ photocatalysts and which also lower its oxidation potentials [22]. The different non-metal dopants, such as nitrogen [23], [24], carbon, [25], phosphorus [26], sulfur [27], boron [28], iodine [29], fluorine [30], etc. have been doped into TiO₂ lattice and these doped materials provide appropriate impurity levels to tune the optical band gap in the visible region with enhancement in the photocatalytic efficiency. Among them, iodine doping can induce oxygen vacancies and oxygen sub-stoichiometry, which is responsible for lowering the band gap of TiO₂ photocatalysts [31]. Many research groups explained that iodine doping results in extension of absorption edge up to 700 nm; which is the characteristic properties needed for the effective photocatalysts of TiO₂ [21], [29], [32], [33]. In addition, due to its variable oxidation states, it can change the surface charge and hence the overall composition of doped TiO₂ catalysts with shifting of photo-response in the visible region for inhibiting the recombination of electron-hole pair [34]. The much lower energy photon excitation pathways from the occupied states of I–O–Ti structure just above the valence band to the unoccupied states of I–O–I structure below the conduction band, can also be responsible for lowering the band gap and this is the cause of the ultra-long light response of iodine doped anatase TiO₂ [35]. Due to the high response in visible light, researchers have been effectively used this photocatalysts for photocatalytic degradation of various pollutants such as Rhodamine B, 2, 4-dichlorophenol [34], methylene blue, bovine serum albumin [15], orange II [36], methyl orange [37], etc.

Wenyue Su et al. reported that electronic structure of different oxidation states of iodine doped into TiO₂ host lattice. These doped particles were used for decomposition of gaseous acetone in the visible-light irradiation [38]. However, the substitution of O sites of TiO₂ by I⁻ ions is not efficient for photochemical transformations; which might be due to the multivalency nature of dopant drastically changes with the structural or chemical composition of TiO₂ [39]. Bicrystalline framework structure of hydrothermally prepared iodine doped TiO₂ was reported as a visible photocatalyst for the photodegradation of methylene blue [39]. However, the hydrothermal template route was not environmentally benign due to the use of high temperature and also this protocol was resulting mesoporous framework of host materials; which would have less surface area and more grain boundaries as compared to other common morphology of particles. For the synthesis of non-metal doped TiO₂, sol–gel is the most versatile and common technique which requires relatively simple equipment and provides the fine control on the structural or compositional properties of the host material [40].

In this study, we report the synthesis of I@TiO₂ NPs at room temperature by employing a simple and effective sol–gel method. The prepared NPs have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry analysis (TGA), field-emission scanning electron microscopy with energy dispersive atomic X-ray spectroscopy (FESEM-EDAX), Fourier-transform infra-red (FTIR), UV–visible spectroscopy. Most of the investigators reported, commercially available dyes used as a model pollutant for checking properties and photoactivity of the nanomaterials. Here, properties and activity of these photocatalytic materials firstly have been extensively studied for degradation of RhB and then we used the catalysts for the treatment of textile effluents in presence of sunlight. Also, the effect of pH of solution and the composition of catalyst on the degradation activity of effulent or dye has been studied.