Research paperAir plasma treated TiO2/MWCNT composite for enhanced photocatalytic activity
Graphical abstract
Introduction
The rapidly increasing industrial production is unfortunately accompanied by environment pollution; releasing waste products into water [1]. The waste water can be cleaned by several physical, chemical, and biological processes, but some contaminants cannot be removed by these conventional methods [2]. Photocatalysis is one of the challenging methods which are effective for the organic pollutant degradation and water splitting [3].
Titanium dioxide (TiO2) has gained vast attention due to its high availability, low cost, good optoelectronic properties, chemical stability, and environmental friendliness among the group of photocatalysts [4]. However, the use of TiO2 for photocatalysis has suffered a setback due to the fast recombination of the photogenerated charge pairs [5], [6]. In addition, its wide bandgap limits its use under the visible light which is the most intense solar spectrum. Therefore, elongation of the recombination time of charge carriers and modulation of the energy bandgap of TiO2 had been investigated [7], [8].
Carbon nanotube (CNT) have been widely coupled with TiO2 for the enhanced photodegradation of pollutants because of its good electronic transport and electron accepting property with large electroactive surface area for the longer recombination time and smaller energy bandgap [9], [10]. It has been reported that the loading of CNT on TiO2 can cause a redshift in the intrinsic absorption edge of pristine TiO2 and consequently reduce its band gap energy by about 24%, thereby improving its visible light absorbance [11], [12]. However, hydrophobicity of CNT hinders the TiO2/CNT composite preparation [13]. Hence there is a need to functionalize the hydrophobic surface of CNT.
Plasma treatment has been used to improve the hydrophilicity of CNT as well as interfacial adhesion with metallic and non-metallic matrices [14], [15]. The plasma treatment also increased active sites, oxygen vacancies and Ti3+ states, on TiO2, resulting in the enhancement of photocatalytic activities [16], [17]. The oxygen vacancies and Ti3+ states in TiO2 have been reported to serve as trapping centers for the photogenerated charge pairs [17]. Therefore, the plasma treatment can be expected to provide the surface functionalization of TiO2.
In this study, the hydrothermal synthesis for the TiO2/CNT composite was carried out, and then the composite was further treated by air plasma for improved photodegradation of methyl blue (MB-1) and methylene blue (MB-2) under UV and visible irradiation. The introduction of CNT into TiO2 increased the surface area and the lifetime of the photogenerated charges as well as reduced the energy bandgap. The ambient plasma treatment also increased the surface area with active sites, the oxygen vacancies and Ti3+ states. Infusion of CNT into the TiO2 matrix after the plasma treatment, consequently exhibited improved photocatalytic performance.
Section snippets
Synthesis of catalysts
The TiO2/CNT composites were synthesized via the hydrothermal method using TiO2 (99.8%, 25 nm, Sigma-Aldrich) and multi-walled CNT (MWCNT, CM-150, 87–93% purity, Hanwha Chemical Limited) without further purification. 5 mg of the CNT were dispersed in 10 mL (0.5 g/L) of deionized water and sonicated for 2 h to get a homogenized solution. At the same time, 1 g of TiO2 was dispersed in 40 mL of deionized water and vigorously stirred with a magnetic bar for 2 h. The CNT suspension was dropped into
Physicochemical properties
The surface morphology of TiO2, CNT, TC, and PTC was shown in Fig. 2. Pristine TiO2 appeared to be tightly bounded three dimensional spherical particles (Fig. 2a), while CNT was observed to have entangled one dimensional thin structure (Fig. 2b). After loading of CNT on TiO2, TC (Fig. 2c) appeared to have both physical appearances of pristine TiO2 and CNT, retaining a moderately bounded rough three dimensional structure. However, the three dimensional structure gave way to a finer and flatter
Conclusions
The TiO2/CNT composites have been prepared successfully by the hydrothermal method, and the surface modification and functionalization were achieved by using the ambient plasma. The electroactive surface area of the composite increased by twice compared with that of pristine TiO2. The crystallite size of TiO2 in the composite was also reduced by about 20% compared to the pristine TiO2, and oxygen vacancies and Ti3+ created in TiO2 increased further by the plasma treatment. However, the most
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.
Acknowledgement
This research was supported by the Daegu University Research Grant, 2019.
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