Continuous microwave flow synthesis (CMFS) of nanosized titania: Structural, optical and photocatalytic properties
Introduction
Ceramic based semiconductor photocatalytic materials are regarded as low cost, effective and environmental friendly materials for air and water purification [1]. Among them Titania (TiO2) is considered a good photocatalytic material due to its non-toxic behavior, cost-effectiveness and high redox efficiency [2], [3]. High photocatalytic efficiency of TiO2 depends on its morphology, crystallinity, particle size, surface area and electronic band structure [4], [5]. TiO2 is a wide bandgap (3.2 eV) semiconductor material, which is virtually inactive in visible light applications like solar energy, photoelectrode material and electrochemically enhanced catalysts [6], [7], [8].
A wide number of synthetic methods like hydrothermal [9], sol–gel [10] and sonochemical [11] have been used to prepare TiO2 but due to their multistep procedures, toxic solvents and annealing at high temperature to increase the degree of crystallinity have made them unsuitable for the large scale production of nanosized TiO2. The use of microwave (Mw) heating is becoming popular due to its superior mode of heating [12], [13]. However, Mw radiations can penetrate only up to few centimeters into the absorbing materials [14], which makes it ineffective to heat large volume of reactants [15]. Therefore, development of continuous flow methods has received considerable attention over the last few years because they have the ability to rapidly synthesize nanomaterials at large scale [16], [17]. To utilize the full potential of diverse and multifunctional applications of TiO2 CMFS is highly desirable. This technique is relatively simple and requires no toxic solvents for synthesis of TiO2 on large/industrial scale.
In this study, we have successfully used an economical, pressure less, green and template free but efficient CMFS technique to synthesize phase pure visible light active anatase TiO2. We report here the effect of MwP on the physicochemical properties, energy bandgap and optical properties of TiO2. This technique may open new avenues of research to fabricate other metal oxides on large scale using green synthesis route.
Section snippets
Continuous microwave flow synthesis of titania
TiO2 synthesis was carried out using 0.37 mmol of titanium-n-butaoxide (Ti(OC4H9)4) in ethanol and 0.015 mmol of HCl in double distilled water. All the reactions were performed utilizing the in-house developed microwave accelerated reaction system (Samsung, Mw 71 B) operated at 450, 600 and 800 W equipped with two peristaltic pumps (Longer Pumps Model BT-300-2J) (Fig. S1). All samples were named according to the following nomenclature: TiO2-micowave power in Watts-retention time in minutes. Other
Results and discussions
XRD and FTIR patterns of TiO2-450-05, TiO2-600-05 and TiO2-800-05 are shown in Fig. 1(a) and (b), respectively. XRD peaks at 25.38°, 37.85°, 48.18°, 55.17° and 62.81° were assigned to the (101), (004), (200), (211) and (204) planes of the anatase phase of TiO2 respectively (JCPDS no. 21-1272). XRD pattern of TiO2 prepared at 450 W contained very broad and low intensity peaks, indicating the formation of poorly crystalline TiO2, however increase in MwP to 800 W showed a substantial increase in the
Conclusion
Phase pure, visible light active anatase TiO2 was synthesized by CMFS method which allowed rapid synthesis of anatase TiO2 at relatively low temperature and pressure. The XRD analysis confirmed an increase in degree of crystallinity from 21.65% to 67.81% and particle size from 4.35 to 6.98 nm on increasing the MwP. The TEM results showed the formation of spheroidal particles. Eg studies showed a red-shift of UV–vis spectrum and reduction in the surface defects on enhancing MwP, where
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