Formation of Anisotropic Nanostructures on Rutile TiO 2 (110) Surfaces and Their Photo-Absorption Properties
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NTRODUCTION
TITANIUM dioxide expresses a variety of intriguing and fascinating properties. In addition to the distinct appeal of this material as a model oxide system for fundamental investigations, its applications as photovoltaic, photocatalyst, gas sensor and biocompatible materials are very attractive.[1–4] Photo-induced surface reactivity and splitting of water by photo-activated TiO2 have spurred several stimulating activities.[1–3] Interest in TiO2 as a photo-reactive material has however been limited owing to its wide bandgap (3.2 eV for anatase and 3.0 eV for rutile phase), restricting its applications primarily to UV regimes.[4] Enhancing the photo-absorption response, by utilizing UV as well as visible regions of the solar spectrum, is an important research area with exciting technological implications.[5,6] Nanostructures and surfaces of TiO2 have been actively investigated in this direction[7,8] through a variety of routes such as doping and dye sensitization.[9,10] However, substitutional incorporation of dopants usually requires high temperature annealing treatments,
VANARAJ SOLANKI is with the Institute of Physics, Sachivalaya Marg, Bhubaneswar 751005, India and also with the Materials Research Centre, Indian Institute of Science, Bangalore 560012, India. SHALIK RAM JOSHI, INDRANI MISHRA and SHIKHA VARMA are with the Institute of Physics. Contact e-mail: [email protected] D. KANJILAL is with the Inter University Accelerator Center, New Delhi 110067, India. Manuscript submitted 10 November 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
whereas organic dye can be unstable, causing material modification. In this respect, nanostructuring of surfaces through ion beams has received enormous attention because of the technologic simplicity it presents for achieving self-organized structures through mostly single-step processes.[11–14] The morphology of nanostructures on surfaces manifests intriguing control in several domains having implications in optics, electronics, magnetism, etc..[15–17] Dissociation and catalytic properties reveal several orders of modification on ion beam-patterned metals[18] where specifically tuned parameters have the capability to produce a distinct nano-dimensional organization. On oxide surfaces, such as TiO2, ion beam patterning also produces surface vacancies that hold promise for becoming photo-active sites.[19] This article investigates the evolution of the nanostructures and ripple patterns fabricated on rutile TiO2(110) surfaces by ion beams. Independent of the ion beam direction, the nanostructures are always observed to be anisotropic in nature. They are constrained in [110] crystallographic direction but elongated along the [001] direction of the crystal. Anisotropic kinetic effects caused by the diffusion of vacancy states as well as itinerant Ti on the surfaces, created during irradiation, predominantly contribute to the preferential growth of these nanostructures along the [001] crystallographic direction. Surprisingly, the photo-absorption intensity from anisotropi
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