The preparation of averrhoa bilimbi-derived carbon-titania composite and its structure-function relationship in photocatalytic and catalytic reactions

Abstract

Nowadays, much interest has been shown in the synthesis of carbon-based titania due to its improved electronic properties and its efficiency in the photocatalytic and catalytic activities. In the meantime, cellulose has emerged as one of the promising sources of carbon. Therefore, in this research, a new approach in the preparation of cellulose-derived carbon/titania composite is introduced by using natural cellulosic material, namely Averrhoa bilimbi fruits or bilimbi, as the carbon source. The purpose of using bilimbi is to utilize its interconnected porous structures and hydrophilic properties, in order to obtain a good interfacial interaction between carbon and titania. The bilimbi was first freeze-dried before being impregnated with titanium isopropoxide as the titania precursor. Bilimbi/TiO2 composite was then calcined at 200, 500 and 800°C in order to change the cellulosic material into carbon and subsequently formed cellulose-derived carbon (BDC)/TiO2 composites. The interfacial interactions between carbon and titania were comprehensively studied through the changes in the physical and electronic properties. The composites were characterized using X-ray photoelectron spectrometer (XPS), X-ray diffraction (XRD) spectrometer, nitrogen adsorption-desorption analyser, Fourier transform infrared (FTIR) spectrometer, thermogravimetric analyser, photoluminescence spectrometer and UV-Visible diffuse reflectance (UV-Vis DR) spectrometer. The strong interfacial interaction between bilimbi and titania resulted in the changes on the surface area and the porosity. This suggested that the interconnected porous structures and the hydrophilicity of the freeze-dried bilimbi led to the good attachment and well distribution of titania particles on the bilimbi’s surface. As the calcination temperature was increased, carbon was located at different locations. At calcination temperatures of 200 and 500°C, titania was at the interstitial titania lattice. However, as the calcination temperature was increased to 800°C, carbon substituted the oxygen atoms in the titania lattice as proved by the XPS analysis. This affected the phase transformation of titania from anatase (calcination at 500°C) to rutile (calcination at 800°C) and formed a mixture of anatase and rutile phases. Besides that, the band gap energies of the composites decreased from 3.2 to 2.9 eV with the increase of calcination temperature. Such changes did not occur in the synthesis of titania without bilimbi. The changes in the physical and electronic properties of the composites were then correlated to the photocatalytic and catalytic activities. The photodegradation of phenol under the irradiation of ultraviolet and visible lights was significantly improved by bilimbi/TiO2 and BDC/TiO2 composites. The formation of the mixture of anatase, rutile phases and the defect sites, as analysed by photoluminescence spectroscopy, reduced the rate of the electrons and holes recombination which consequently increased the photocatalytic activities of the composites. Meanwhile, the catalytic activity of the composites for the catalytic oxidation of styrene was not affected by the presence of carbon since the carbon did not change the titanium catalytic active sites. In conclusion, the amount and location of the carbon in bilimbi/TiO2 composite, whether on the surface, interstitial, and substitution positions, were changed with the increase of calcination temperature and these changes affect the physical and electronic properties of the composites, and enhanced the photocatalytic activity of the composites.