Controllable synthesis of TiO2 nanoparticles and their photocatalytic activity in dye degradation
Graphical abstract
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.
Section snippets
Materials
All the chemicals obtained commercially were of analytical grade and were used without further purification. Titanium Tetrachloride (TiCl4) and concentrated hydrochloric acid (HCl) were purchased from Sigma Aldrich. Methylene blue and Congo red were purchased from Alfa Aesar. Millipore water (deionized water, DI) with a resistivity of >15 MΩ·cm was used throughout the experiments. All glassware used in the research was washed using aqua regia and rinsed thoroughly with DI water before use.
Preparation of the TiO2 colloidal suspensions
In a
TiO2 characterization
The TEM image in Fig. 2(A) shows two kinds of particle populations - short and long particles. The size-distribution histogram, Fig. 2(B), shows that the average size of shorter particles is 6.6 nm, and that of the longer particles is 8.3 nm. The selected area electron diffraction (SAED) image represented in Fig. 3(B) shows the crystallinity of the TiO2 NPs, which have a polygonal geometry. The titania crystalline planes resulting from SAED correspond to the Anatase and Brookite phases and
Conclusions
TiO2 NPs with prominent anatase and brookite phases were synthesized by controlling the violent hydrolysis of the titanium tetrachloride (TiCl4) precursor, with a homemade glass apparatus. This simplifies the elimination of water vapors and controls the addition rate in order to obtain the desired particle size. The degradation of two dyes (cationic and anionic dyes) was demonstrated for determining the photoactivity of the synthesized TiO2. The photocatalytic degradation of MB and CR, which
CRediT authorship contribution statement
Krishnamoorthy Sathiyan: Conceptualization, Formal analysis, Investigation, Writing - original draft, Visualization. Ronen Bar-Ziv: Conceptualization, Validation, Writing - review & editing, Funding acquisition. Orit Mendelson: Methodology, Formal analysis. Tomer Zidki: Conceptualization, Methodology, Validation, Resources, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was partially supported by the Israeli Atomic Energy Commission – Prof. A. Pazy Joint Foundation under Grant number 273/16. S.K. is thankful to Ariel University for a Ph.D. fellowship. We gratefully thank Dr. Avi Rave for the XPS analysis and Rotem Ind for performing the BET measurements.
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