Optical studies of cobalt implanted rutile TiO2 (110) surfaces
Introduction
Studies of metal oxide semiconductors like TiO2, at nanoscales, have been of great interest for many years due to their numerous technological applications in photo-catalysis, solar cells, photovoltaics, magnetic storage media, waste water management, etc. [1], [2], [3]. TiO2 is a wide band gap semiconductor and under UV irradiation can produce hydroxyl radical (OH−) which act as a powerful oxidizing agent to disintegrate many organic pollutants dissolved in water [4]. However, due to its large band gap (3.0 eV for rutile TiO2) it absorbs visible light poorly, and thus has been nearly ineffective for visible light photo-catalysis. Various methods have been used to improve the photocatalytic activity of rutile TiO2, e.g. through dye sensitization, synthesis as thin films, formation of nanocrystals, incorporation of dopants by chemical methods, etc. [5], [6], [7]. The enhancement of photocatalytic activity of rutile TiO2 by most of the above methods is however somehow limited, as organic dyes can become unstable at high temperature and the diffusion of dopants depend on the temperature and complexity of the chemical methods [8], [9], [10].
Many dopants, like metallic Ni, Pt, Cu, Ag [11], [12], [13], [14] and non metallic N, C, S [15], [16], [17], [18], have been used to improve the optical properties of TiO2 thin films. Cobalt exhibits remarkable combination of good optical characteristics [19], [20], [21] conjugated with room temperature ferromagnetism [22], [23] which make it an attractive candidate as dopant for enhancing photo-absorption in TiO2. Although such studies on single crystals can provide unambiguous results, without any discrepancy due to preparation methods, as demonstrated by Co induced magnetism in TiO2 [24], [25], such studies on photo-absorbance are absent.
Ion implantation is an important technique for introducing dopants in the host lattice. Implantation sometimes leads to the development of nanostructures on the surfaces. Such fabrication of self organized nano-patterns have been observed on a variety of semiconductor and metal surfaces [26]. These patterns develop due to the competition between the curvature dependent sputtering process, which erode the surface atoms, and various relaxation mechanisms [27]. During implantation of bi-atomic surfaces, preferential sputtering of low mass atoms can also take place which may lead to the development of vacancies as well as metal rich centers [28].
The present study investigates the photoabsorption properties of rutile TiO2 (110) surfaces after their implantation with Co ions. The results show formation of nanostructured patterns as well as creation of oxygen vacancies, or Ti3+ states, on the ion implanted surfaces. For these ion implanted surfaces an enhanced photoabsorption in UV as well as visible regimes is observed. The presented results demonstrate that development of self organized nanostructures, creation of vacancy states, formation of Co nano-clusters and modification in band gap, upon Co ion implantation, leads to the observed increase in photoabsorption in TiO2 (110).
Section snippets
Experimental details
Commercially available rutile single crystal TiO2 (110) substrates were irradiated with 200 keV cobalt ions, at room temperature, using 15 MV Pelletron Accelerator. The Co ions, incident along the substrate normal, were implanted in TiO2 at two fluences; 3 × 1016 and 8 × 1016 ions/cm2. The flux of the ion source was 1.6 × 1013 ions/cm2 s and the ion current was 1.5 μA. The range of Co atoms in TiO2 has been estimated to be 98 nm by SRIM [29].
Rutherford backscattering spectroscopy (RBS) experiments were
Results and discussion
Fig. 1 shows RBS spectra from pristine as well as 200 keV cobalt ion implanted TiO2 (110) samples. These spectra have also been simulated using SIMNRA code [30] and the results are presented in Fig. 1. Simulation results indicate cobalt concentration in the implanted samples to be as expected, i.e. 3 × 1016 and 8 × 1016 atoms/cm2, respectively. The fitting of RBS data, using SIMNRA, indicates diffusion of cobalt towards the surface.
Morphological evolution of rutile TiO2 surfaces is shown in Fig. 2.
Conclusion
In conclusion, we have investigated the photoabsorption properties of TiO2 surfaces after cobalt implantation with 3 × 1016 and 8 × 1016 ions/cm2, respectively. Implantation leads to the fabrication of nanostructures as well as anisotropic ripple patterns. Development of oxygen vacancy states (Ti3+) and cobalt nano-clusters has also been observed. The implanted samples show an enhanced photo-absorption, nearly twice and five times, in UV and visible regimes, respectively. Formation of
Acknowledgments
We would like to acknowledge the help of Ramesh Chandra (IIT, Roorkee) for XRD measurements. Authors would also like to acknowledge the help of Devrani Devi and Sunil Ojha during Implantation and RBS measurements at IUAC, respectively, Priyadarshini Dash during UV–vis, and Santosh Kumar Choudhury for XPS experiments.
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