Synthesis and characterisation of Au/TiO2 composites for plasmon-enhanced visible light driven photocatalytic degradation of reactive orange 16 dye

Abstract

Photocatalysis is one of the methods that have gained popularity for degradation of organic pollutants in water. Metal oxides, such as ZnO, Fe2O3, and TiO2 are considered to be good and efficient photocatalysts. TiO2, in particular, has been the most investigated because it is naturally abundant, non-toxic and stable. However, the wide band gap of TiO2 (3.2 eV), make TiO2 only to absorb UV light. For this reason, plasmon enhanced-photocatalysis has emerged as one of the appealing processes to achieve visible light utilization by TiO2. This process exploits the Localized Surface Plasmon Resonance (LSPR) of the metal nanoparticles such as gold to harvest the visible light and bring about photocatalytic process. LSPR is the effect of the oscillation of electrons in noble metals when they are in contact with light. Due to the LSPR phenomena, noble metals are able to increase the lifetime of the charge carriers and increase electron/hole generation semiconductors photocatalysts under visible light. In this study, TiO2 was coupled with gold nanoparticles in order to facilitate visible light absorption and to improve the photocatalytic performance. Gold nanoparticles (nanospheres and nanorods) were synthesised using the Turkivich and seed mediated methods. These were characterised by UV-visible spectrophotometry, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) for optical properties, size and morphology. The concentration of the as prepared gold samples was measured using the inductively coupled plasma optical emission spectrometry (ICP-OES). Gold nanospheres and gold nanorods were loaded into TiO2 using the nucleation and growth method to obtain the Au/TiO2 plasmonic composites. To investigate to effect of the gold size, two AuNRs samples with different aspect ratios (1.9 and 3.4) were prepared and used to form the nanocomposites with TiO2. Another Au/TiO2 composite sample was prepared by loading AuNS to compare the behaviour of the two shapes. The characterisation results of these samples from the transmission electron microscopy TEM and SEM confirmed the expected shapes (spheres and rods) and the formation of the nanocomposites. The energy dispersive spectroscopy (EDS) results showed the presence of the all the expected elements in the composites materials, further confirming the successful synthesis of the Au/TiO2 composites. The absorption spectra of the prepared nanocomposites showed the plasmonic peaks of the gold nanoparticles in the visible region, which also confirmed the formation of the composite materials. The photocatalytic performance of the photocatalysts was investigated for the degradation of reactive orange 16. From the results obtained in this study, it was found that the photocatalysts loaded with AuNRs gave higher photodegradation efficiencies compared to the one loaded with AuNS. The photocatalytic efficiency was found to increase with an increase on the aspect ratio of the AuNRs. For AuNRs (1.9) the achieved efficiency was 84.56 % and 86.65 % for AuNRs (3.4). Meanwhile, direct photolysis did not have an effect on the photodegradation of Reactive Orange 16 (RO 16). The combined effect of AuNRs and AuNS showed a drastic improvement on the photocatalytic efficiency and the rates of the process which was attributed to the synergistic effects of the transverse and the longitudinal plasmons peaks of both nanospheres and nanorods. The photocatalyst prepared with the mixed nanospheres and nanorods gave an efficiency of up to 90.15 % for the 1:1 ratio at 60 min reaction time. A number of reaction parameters were investigated for their effect on the photodegradation efficiency including: pH, Au content, and temperature. The photocatalytic degradation of RO 16 was very slow in very acidic (pH 2.5) and very basic conditions (pH 11.5). The highest degradation efficiency was achieved at pH of about 6.7. Furthermore, the rate of degradation also increased with an increase in temperature from 15 oC to 30 oC due to the reduction of the activation energy. The increase in Au loading from 0.1 wt % to 0.2 wt % increased the photocatalytic performance of the catalyst from 56.29 % to 86.65 %. However, further increase in gold loading blocked the light penetration and hence, caused a decrease on the efficiency to 66.35 %.