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1211-R08-33

Nanoparticle Layers Transformed from Ordered TiO2 Nanotube Arrays and Dye-Sensitized Solar Cells Yahya Alivov1, Xuan Pan1, Mahesh Pandikunta1, Vladimir Kuryatkov1, Sergey Nikishin1, Mark Holtz2, and Zhaoyang Fan1 1 Nano Tech Center and Department of Electrical and Computer Engineering 2 Nano Tech Center and Department of Physics Texas Tech University, P. O. Box 43102, Lubbock, TX, 79409-3102, USA ABSTRACT Transformation of titanium dioxide (TiO2) nanotubes (NTs) to truncated bipyramidal shape nanoparticles (NPs) with a large fraction of photo-catalytically active {001} facet surface was observed after thermal annealing TiO2 ordered nanotube arrays in fluorine ambient. Size of the formed nanoparticles depended on fluorine concentration and can be controlled from 20 nm to 350 nm. The crystal and optical properties of nanoparticle layers are superior to those of nanotube arrays, which are also annealed but without geometrical transformation. Using nanoparticle layers formed by this method we have fabricated dye-sensitized solar cells (DSSCs) with different size NPs in the range 35-350 nm. The dependence of solar cell performance on NP size is discussed. INTRODUCTION The size, surface, geometry, and crystal phase of nanostructures are important parameters for control of their chemical, optical, and electrical properties. To this end, nanostructured titanium dioxide (TiO2), including both nanoparticles (NPs) and nanotubes (NTs), has attracted great attention for use in dye-sensitized solar cells (DSSC), water splitting for hydrogen generation, photocatalysis for purification of air and water, and biological and chemical sensors.1-5 TiO2 based DSSCs have received considerable attention. A typical DSSC consists of a dye-sensitized n-type wide band gap semiconductor layer on a semitransparent photoanode, liquid electrolyte for hole transport, and a platinum-covered counter electrode as a cathode. As the heart of the device, the mesoporous semiconductor oxide layer is composed of nanometersized particles sintered together for electron transport, and anatase TiO2 is frequently chosen due to its suitable conduction band energy, low cost, wide availability, and non-toxicity. Attached to the surface of the mesoporous film is a monolayer of sensitizer dye for light absorption. For DSSC, an overall energy conversion efficiency exceeding 11.0% has been achieved under AM 1.5 sun light.6 In this paper, we report on a unique method for fabrication of truncated pyramid shaped TiO2 NP layers by transformation of TiO2 NTs through thermal annealing7, and the latter was grown by electrochemical anodization of Ti foil in ethylene glycol.8 Unlike the typically available TiO2 NPs used for DSSCs which are dominated by photo-catalytic inert {101} facets, the transformed TiO2 NPs reported here have a large fraction of photo-catalytically active {001} facets. The higher percentage of {001} facets of TiO2 particles is advantageous, since it has been shown9,10 that the {001} surface is more photo catalytically active than the {101} sur