Controllable synthesis of TiO₂ nanoparticles and their photocatalytic activity in dye degradation

Highlights

• A homemade apparatus for controlling the TiO₂ nanoparticles synthesis from TiCl₄ is presented.

• The synthesized TiO₂ particles show good crystallinity with the photoactive anatase as the major phase.

• The TiO₂ nanoparticles have demonstrated photocatalytic degradation of cationic and anionic dyes.

Abstract

Titanium dioxide nanoparticles were synthesized by hydrolyzing titanium tetrachloride (TiCl₄) in a controlled manner using a homemade glass apparatus. The structural properties and chemical composition were studied using X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, Brunauer-Emmett-Teller surface area analysis, selected area electron diffraction, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. The X-ray diffraction pattern and the high-resolution transmission electron microscope analyses confirmed that the TiO₂ nanoparticles are composed of photo-active anatase and brookite phases. The photocatalytic activity of the TiO₂ nanoparticles was demonstrated by photo-degradation of methylene blue and congo red under UV–Vis light illumination. Studying how pH affects the degradation of methylene blue revealed that its photocatalytic degradation on TiO₂ nanoparticles is more efficient under alkaline conditions, whereas the congo red degradation is more efficient under acidic conditions.

Introduction

The lack of clean and substantial natural energy, as well as the pollution and contamination of the environment, are some of the severe problems currently faced globally. Fast-growing industries have led to the generation of wastewater containing heavy metals and various organic dyes. These industrial wastewaters are highly toxic and could be carcinogenic. In addition, these industrial contaminants are responsible for water pollution, which has a severe ill-effect on aquatic life as well as human beings [1,2].

Consequently, the development of useful techniques that can convert harmful, toxic pollutants into harmless compounds is urgently needed. Semiconductor photocatalysis is considered to be one of the best strategies to tackle the energy crisis and environmental pollution issues [3,4]. When compared with various photocatalysts, TiO₂ NPs are thought to be one of the most promising dye treatment photocatalysts owing to their non-toxicity, strong oxidizing power, and high stability [5].

Titanium dioxide is a semiconductor with a wide bandgap (3.0–3.7 eV), and when illuminated with light of wavelengths <380 nm, the photogenerated conduction-band electrons, and valence-band holes subsequently initiate redox reactions [6]. Many organic compounds in aerated aqueous solutions can be oxidized by reactive oxygen species (ROS) produced by TiO₂ illumination. These oxidation reactions may lead to complete degradation (“mineralization”) of the organic contaminants to carbon dioxide in the presence of illuminated TiO₂ with near-UV light [7,8]. Photodynamic-therapy utilizes ROS produced by TiO₂ for treating various diseases such as vitiligo, psoriasis, and several types of cancer, including melanoma [9]. In addition, Fleminger et al. used Ag-doped TiO₂ nanoparticles for detoxifying phosphorous-organic pesticides and chemical warfare agents such as the extremely toxic VX [10].

TiO₂ has three crystallographic phases: anatase, rutile, and brookite [11]. Of these phases, anatase is regarded as the most active, due to its high photocatalytic efficiency and adsorption affinity for organic compounds [12,13]. Recent photocatalytic studies have demonstrated that the photoactivity of anatase TiO₂ is strongly size-dependent [14,15]. The synthesis of TiO₂ nanoparticles has attracted much attention; however, mediating the anatase phase is challenging. The sol-gel process is a common and cheap method for synthesizing TiO₂ NPs. However, the TiO₂ produced in this process is amorphous and requires a temperature higher than 623 K for transitioning from the amorphous titania to its anatase phase [16]. This increase in temperature will lead to an increase in particle size, thereby decreasing the surface area [17,18]. Moreover, heating the anatase phase results in a gradual phase transformation to rutile; therefore, depending on the method of preparation, mixed phase anatase-rutile is mostly formed [19]. In some previously reported articles, it was found that the anatase-brookite mixed phase of TiO₂ exhibits better activity than does the pure anatase phase [[20], [21], [22]]. Addamo et al. reported various methods for preparing nanostructured TiO₂ via the hydrolysis of titanium tetrachloride or titanium isopropoxide as the precursors [23]. The TiO₂ NPs prepared from TiCl₄ are of much interest, since the product constitutes the photoactive anatase phase without any additional calcination and filtration processes. Moreover, the final product obtained from TiCl₄ is free of organic residuals compared with the product obtained from organic precursors [24]. However, the hydrolysis is hard to control; thus, non-uniform and relatively large NPs are formed. L. Ren et al. in their recent work synthesized TiO₂ hollow nanoparticles with tuneable cavity size by microwave assisted hydrolysis method. In their study, by adjusting the amount of aqueous HF solution, they could able to control the cavity size of TiO₂ hollow nanoparticles [25]. Rabani et al. [26] reported a synthetic procedure that produces particles exhibiting a good crystallinity and having anatase as the major phase, as well as NPs with sizes that can be varied under different conditions. However, this procedure for TiO₂ NPs preparation from the TiCl₄ precursor is complex and hard to reproduce, since the precursor itself can react with moisture, leading to a non-uniform size distribution. Rabani et al. injected the precursor through a plastic tube, along with an argon flow. In this way, it is difficult to control the reaction, and the product often precipitates in the tube. Using the glass setup that we developed, the flow rate of the dropwise addition of the precursor can be easily controlled. Herein, we report a modified synthetic method to better control the synthesis of TiO₂ NPs from hydrolysis of TiCl₄ to produce small, reproducible, organics-free, and stable TiO₂ NPs.

Additionally, the photocatalytic degradation efficiency under different as-synthesized TiO₂ NP conditions was tested for conventional cationic and anionic dyes, namely, methylene blue (MB) and congo red (CR), respectively. The relationship between structure and performance is explained.