Anatase TiO 2 Nanowires, Thin Films, and Surfaces: Ab initio Studies of Electronic Properties and Non-adiabatic Excited
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Anatase TiO2 Nanowires, Thin Films, and Surfaces: Ab initio Studies of Electronic Properties and Non-adiabatic Excited State Dynamics Shuping Huang and Dmitri S. Kilin* Department of Chemistry, University of South Dakota, Vermillion, U.S.A. *Corresponding author. Email: [email protected] ABSTRACT We analyze and compare optoelectronic properties and hot carrier relaxation dynamics in different forms of TiO2 anatase materials: nanowires and thin films. The models are chosen in such way that the same crystallographic surfaces are exposed and any difference in properties is attributed to the change of the dimensionality of the nanostructure. Specifically, we give a brief review of the electronic properties and non-adiabatic excited state dynamics of anatase TiO2 nanowire as well as (100) and (001) anatase TiO2 surfaces. The calculated band gap of nanowire is larger than the ones of surfaces. The hole relaxation rate is higher than the electron relaxation rate for both the surfaces and nanowire, and the electron and hole relaxation rates of surfaces are larger than the ones of nanowire. INTRODUCTION Titanium dioxide (TiO2) has been extensively investigated due to its wide applications in both energy and environmental fields, such as photocatalyst, self-cleaning material, and electrode in dye-sensitized solar cells (DSSCs) [1-3]. The difference in the crystal phase, surface, size, and shape can cause significant difference in properties. TiO2 has three major structure forms: anatase, rutile, and brookite. Anatase phase has generally shown high photo-activity in many cases, including catalysis, photocatalysis, and especially, dye-sensitized solar cells [4]. Surface properties are of major importance in these applications. Among the low-index stoichiometric anatase surfaces, the (101) surface is the most stable, and the (001) surface is the second most stable surface. The (001) surface is more photo-active than (101) surface. The (100) surface of anatase TiO2 is not found in natural crystal form and therefore not studied often; however, the surface can be found in powder form and is predicted to be very stable [5]. Nanosized structures offer added property values that are greatly influenced by their diameters, crystallographic orientations, surface passivation, presence of dopants, and so forth. Compared with the zero-dimensional nanoparticles and two-dimensional (2D) thin films, one dimensional (1D) structures such as nanowires (NWs) and carbon nanotubes (CNTs) provide additional benefits in two aspects: (i) visible light scattering and absorption are much enhanced in NWs due to their high length to diameter ratio and a total length reaching hundreds of micrometers [6]; (ii) the 1D geometry facilitates rapid, diffusion-free electron transport to the electrodes [7-8]. Both effects have been demonstrated in experiments [9-10]. Knowledge about the carrier relaxation is important in evaluating the mechanism in photo-catalysis. Multilevel Redfield theory is considered as a useful and practical approach for carrier relaxation
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