Introduction

Dye-sensitized solar cells (DSSCs) based on mesoporous nanocrystalline TiO₂ film have attracted considerable attention due to their potentially low cost, relatively high energy conversion efficiency, safety, and non-pollution.^1-3 Common DSSCs consist of three main parts including a sensitized TiO2 or ZnO photoelectrode, a Pt counter electrode, and a redox electrolyte. ^{4, 5} The TiO₂ photoelectrode is the most important part that strongly affects the DSSCs performances. The TiO2 photoelectrode is fabricated by a coating conducting glass plate with a mesoporous TiO₂ nanoparticle film, and adsorption of sensitizer dye molecules on the nanoparticles surfaces.

Therefore, a lot of effort has been made on the synthesis of TiO₂ nanoparticles to develop the high performance TiO2 photoelectrodes.^6-10 It is commonly considered that anatase-type TiO₂ nanoparticles with small particle size about 20 nm and high crystallinity exhibit high performance.¹¹ Usually spherical nanocrystals are used to fabricate the TiO₂ photoelectrode, while addition fibrous and rod-like particles into the spherical particles can increase the conductivity of TiO2 film, which improves cell performance.^12-14 Since dye adsorption reaction occurs on crystal surfaces, it is easy to be understood that the adsorption reaction would be strongly dependent on the crystal

facet on the particle surface. However, to study the effect of crystal facet on the DSSCs performances, it is necessary to prepare the TiO2 nanoparticles preferentially exposing a specific crystal facet on the surface but it is not easy.

Wen et al. have reported a pioneering study on the hydrothermal soft chemical synthesis of anatase TiO₂ nanocrystals with specific crystal facet on the surface, and found that {010}-faceted anatase TiO2 nanocrystals exhibit higher photocatalytic activity than the normal spherical nanocrystals without specific facet on the surface for the first time.^{15, 16} The results spur researchers to study the synthesis of TiO2

nanocrystals with specific crystal facet, and also the surface energy and photocatalytic activity of the crystal facets.^17-23 Wu et al. have reported syntheses of {001}- and {010}-faceted anatase nanocrystals and their photocatalytic performances in degradation of methyl orange, and found that the {010}-faceted nanocrystals exhibit higher photocatalytic activity than the {001}-faceted nanocrystals and commercial P25 sample.²⁴ Han et al. have reported that the {001}-faceted anatase nanosheets exhibit the higher photocatalytic activity than the P25 sample.²⁵ The Wulff construction and theoretically calculated surface energy predicts that the surface energy of anatase increases in an order of {101} facet (0.44 J/m²) < {010} facet (0.53 J/m²) < {001} facet (0.90 J/m²) < {111} facet (1.61 J/m²).^{26, 27} The higher photocatalytic activity of the {010} facet than that of {001} facet and {101} facet is owing to the superior electronic band structure of {010} facet for the photocatalytic reactions.²⁸

However, up to now, most studies on the TiO2 facets have focused on their synthesis, surface energy, and photocatalytic activity. Recently, Wen et al. have reported studies on the effects of the crystal facet on DSSC performance and sensitizer dye adsorption behavior for the first time.^{29, 30} The results suggested that the dye adsorption behavior is strongly affected by the crystal surface properties of the

TiO₂ nanocrystals and the dye molecules are strongly adsorbed on {010} facet which results enhancement of short-circuit current density. After that, other studies have also confirmed that the crystal facets on the surfaces affect the DSSCs performances.^{7, 28} Very recently, it has been reported that {001}-faceted anatase TiO2 nanocrystals exhibit a high performance for DSSCs due to the enhancement in light scattering effect and suppressed electron recombination,³¹ and also the strong adsorption of dye molecules on the {001} facet facilitates the electron transport from dye molecules to the conduction band of TiO2.^{32, 33} Unfortunately, only limited numbers of studies have been reported on the facet effect on the DSSCs performance, maybe due to the synthesis processes are difficult to provide enough amounts of nanocrystals with the specific facet for the DSSCs characterizations, and/or the nanocrystals sizes are too large for high performance DSSCs.

In this chapter, we describe synthesis of morphology controllable {010}-faceted anatase TiO2 nanocrystals from exfoliated layered titanate nanosheets with lepidocrocite-like structure. Regular rhombic, tetragonal, and leaflike anatase nanocrystals, and platelike anatase mesocrystals constructed from oriented anatase nanocrystals can be achieved using this process. In this process, n-propylamine (PA) is chose as the exfoliating agent because it is much cheaper than other exfoliating agents, such as tetrabutylammonium hydroxide and tetramethylammonium hydroxide, therefore, the mass production of the TiO2 nanoparticles is possible using PA exfoliating agents. The formation reaction mechanism of the anatase nanocrystals and mesocrystals is also investigated by a nanostructural analysis. The reaction mechanism is significant to understand the phase conversion and particle morphology evolution in the formation process of the {010}-faceted TiO2 nanoparticles from layered titanate nanosheets, which would give a guide for controlling particle morphology of the TiO2 nanoparticles with specific facet on the surface. Furthermore,

we characterized the DSSCs performances of the {010}-faceted anatase nanocrystals in detail, and compare their performances with which of commercial P25 nanocrystals containing [111]-faceted (facet vertical to [111]-direction) anatase nanocrystals and spherical anatase nanocrystals without specific facet on the surface. The {010}-faceted anatase nanocrystals exhibit the highest short-circuit current density that results the highest energy conversion efficiency. The results reveal the significant effect of the crystal facet on the DSSC performance.