Co2+ Addition Effect on the Photocatalytic Activity of TiO2 Nanorod Thin Films Özge Kerkez,1* İsmail Boz 2

1Chemical Engineering Department, Beykent University, İstanbul 34396, Turkey 2Chemical Engineering Department, İstanbul University, İstanbul 34320, Turkey

Abstract— TiO2 nanorod array thin films were synthesized by a hydrothermal method and then modified with an ultrasonic-assisted sequential cation adsorption method with Co2+. The samples were characterised by XRD, UV–vis DRS, SEM, ICP-MS analysis. Photocatalytic activities were tested by methylene blue degradation under visible light. Methylene blue degradation efficiency under visible light was increased 29% with respect to the efficiency of the pure TiO2/FTO sample.

TiO2 is the most suitable photocatalyst in terms of stability, resistance, being harmless to the environment. TiO2 has a disadvantage that it only can be excited under UV illumination. Various studies have been reported such as noble metal/transition metal/anions doping to increase its absorption in the visible light range to utilize the sunlight. On the other hand, it was determined that the 1D nanostructures of TiO2 have shown superior performance than conventional thin films in photocatalytic processes [1,2]. In this context, the aim of this study to increase the absorbance in the visible light by Co2+ adding and the activity by synthesizing TiO2 in nanorod shape.

TiO2 Nanorod thin film (TiO2/FTO) was prepared by the hydrothermal method [3]. Co2+ adding was performed by ultrasonic assisted sequential cation adsorption method [4]. Five different concentrations (0.01; 0.025; 0.050; 0.075; 0.100M) of aqueous Co(NO3)2∙6H2O solutions were used to see the effect of Co2+ amount. The samples were named as 10Co2+-TiO2/FTO according to the initial metal concentration. Methylene blue photodegradation reactions were done in a home made reaction chamber with a 105-W energy saver white fluorescent (main emission wavelength 530 nm). The experiments were carried out in a quartz cell (1cm x 1cm x 4cm) filled with 3 ml, 5 ppm methylene blue solution.

Figure-1: SEM photo of Figure-2: XRD pattern of TiO2/FTO sample Co2+-TiO2/FTO samples Figure 1 represents the SEM photo of TiO2/FTO sample. The image reveal that the surface of the substrate uniformly covered with TiO2 nanorods. Figure 2 represents the XRD pattern of the Co2+ modified TiO2 nanorod thin films. It is supported the rutile phase of TiO2 (JCPDS No. 88-1175) is synthesized successfully in nanorod shape understood from the (002) (2h = 62.77°) diffraction peak. No separate Co/Co2+ phases were observed because of low concentration. Figure 3 represents the UV-vis absorption spectra of the samples. The UV-vis spectra of the pure TiO2/FTO show very low absorption in the visible light region, while Co2+–TiO2/FTO films exhibit higher absorption in the visible light. The direct band gap energy of the samples were calculated and given in Table I. Visible light absorption has increased and band gap

energy has decreased as the Co2+ amount increased on the film. That indicates the photocatalyst film has became more sensitive to the visible light. ICP-MS analysis was conducted to determine the Co2+ amount on the film; results are given in Table I. Figure 4 represents the kinetic behaviour of MB photodegradation. Langmuir–Hinshelwood rate expression has been fitted well with the relationship between the initial degradation rate and the concentration of the organic dye for heterogeneous photocatalytic processes occurring at the solid–liquid interface [5,6]. Degradation% values and apparent reaction rate constants (k) are summarized in Table I. TiO2/FTO obtained 47.2% conversion of methylene blue by the self-sensitised degradation. Methylene blue degradation was increased under visible light through Co2+ addition. The maximum dye degradation efficiency was achieved with 50Co2+-TiO2/FTO. Degradation efficiency under visible light was increased 29% with respect to the efficiency of the pure TiO2/FTO sample. The maximum reaction rate constant was found as 3.8x10-3 min-1 with 50Co2+-TiO2/FTO.

Figure-3: UV-vis absorption spectra Figure-4: Reaction kinetics of MB of the samples degradation Table-I Some properties of photocatalyst samples, MB degradation% and reaction rate constants

*Corresponding author: okerkez@gmail.com [1] J.-H. Yoon et al., Journal of Photochemistry and Photobiology A: Chemistry 180, 184 (2006). [2] X.D. Li et al., Materials Chemistry and Physics 124, 179 (2010). [3] B. Liu et al., Journal of American Chemical Society 131, 3985 (2009). [4] Ö. Kerkez et al., Journal of Physics and Chemistry of Solids 75, 611 (2014). [5] Y.-H.Xu et al., Materials Research Bulletin 43, 3474 (2008). [6] K. Naeem et al., Physica B 405, 221 (2010).