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Copper-modified immobilized nanoporous TiO2 for photocatalytic degradation of Imidacloprid

Environmental pollution and absence of cost- effective and renewable energy resources are well- acknowledged problems nowadays. As a consequence, development of a low-cost, clean, energy-efficient and sustainable technologies is required. One of the possibilities is photocatalysis because titanium dioxide is environmentally friendly, chemically stable, commercially available, non- toxic and efficient photocatalyst, widely used for water and air purification. Titanium dioxide is a powerful photocatalyst that is chemically stable, non-toxic, and extremely reactive. It has been used for water purification, pesticide removal, degradation of air pollutants, water splitting, etc. [1] Another benefit is the flexibility to tune TiO2’s size and shape. Nanostructured TiO2 has a higher surface-to-volume ratio, different chemical and physical characteristics than bulk TiO2, and longer diffusion lengths and lifetimes of photogenerated charge carriers (electrons and holes), all of which contribute to improved photocatalytic efficiency. Electrochemical anodization of titanium foil is a low-cost, simple, fast, and scalable method for producing TiO2 nanotube arrays with the ability to manipulate the material's morphology [2]. Synthesized immobilized nanoporous TiO2 photocatalysts, prepared by electrochemical anodization of titanium foils, were modified by wet–impregnation by immersing samples for 5 hours in five different solutions containing various Cu concentrations (0.2 – 1 M). The objective of this research was to investigate how modification with Cu compounds can improve the photocatalytic activity of immobilized nanoporous TiO2 under the UV/Vis light irradiation. Furthermore, correlation of photocatalytic activity and different Cu concentrations was studied. In order to identify an optimal Cu concentration needed for the most efficient photocatalytic degradation of imidacloprid, photocatalytic activity of the unmodified reference (0M_Cu_TiO2) and five Cu- modified (0.2, 0.4, 0.6, 0.8 and 1M) samples was evaluated by monitoring degradation of imidacloprid using SPC and a full-spectrum compact fluorescent bulb with high UVB intensity. The photodegradation process was modelled using a pseudo-first-order kinetics (Eq. 1): ln (C/C0) = - kt (1) In order to resolve the Cu valence state and local symmetry in samples, XANES spectra were analysed with IFEFFIT program package Athena using standard procedure [3]. The best results determining the average valence state of the Cu atom in the samples, obtained by fitting XANES spectra of proper reference compounds with known valence state of the element, similar symmetry, and same type of neighbour atoms in the nearest coordination shells that are arranged in the same local structure, are shown in Fig. 2. XANES results showed that Cu species are in divalent form and have distorted octahedral structure, where Cu(II) cations are located in the centre of Jahn–Teller tetragonally distorted octahedron of six oxygen atoms. With the aim of determining the local structure around the Cu(II) ions, Cu-K edge EXAFS analysis was done. Fourier transforms (FT) of the k2 weighted EXAFS spectra of the photocatalyst samples are shown in Fig. 3. Peaks observed in the FT spectra in a range of 1 to 3.8 Å represent a contribution of photoelectron backscattering on the nearest shells of Cu neighbouring atoms. For the quantitative analysis, a FEFF [4] model was constructed. The results show that Cu atoms are coordinated to six oxygen atoms in a distorted octahedron. Four O atoms are at shorter Cu–O distance (~1.95 Å) and two O atoms at longer distance (2.3 – 2.6 Å) on axial positions. The second shell of neighbours is comprised of Ti atoms at ~2.95 Å and Cu and O atoms distributed from 3.1 Å to 3.7 Å. The Cu(II) forms Ti–O–Cu and Cu–O–Cu connections which indicates that a part of the Cu(II) is attached to the surface of TiO2 and in the form of CuO nanoparticles.

titanium dioxide ; Cu modification ; photocatalytic activity ; XANES ; EXAFS

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