As one of the most promising wide-band gap semiconductor materials, titanium dioxide (TiO2) has attracted a lot of interest for its potential applications, such as photocatalysts for water splitting and organic contaminator decomposition,1-3 self-cleaning surface,4 dye sensitized solar cells (DSSCs)5-7 as well as perovskite solar cells (PSCs).8-10 The structure, surface area, crystallinity, and crystal-facet exposed on the crystal surface of TiO2 strongly affect its photocatalytic and photovoltaic performances.
11-17 The anatase-type TiO2 is the most important TiO2 material due to its high photocatalytic activity and excellent photovoltaic performances.11 Recently, the crystal-facet effect attracts enormous attention because different crystal facets exhibit different surface energies and surface electronic band structures, which will affect the physicochemical properties on the surface.16-35 The surface energies of anatase TiO2 are 1.61, 0.90, 0.53, 0.44 J/m2 for {111}, {001}, {010} and {101}-facet, respectively.15,24 The {101}-facet with the lowest surface energy is the most thermodynamically stable and easily appears
on the crystal surface, while other crystal-facets with higher surface energy are not easy to appear on the anatase nanocrystal surface.15 Since Wen et al.16,31 reported the first challenge on {010}-faceted anatase TiO2 nanocrystals which exhibit a much higher photocatalytic performance than that of the normal anatase nanocrystals in 2006, many studies on the anatase nanocrystals with high-surface-energy facets have been carried out, especially, on the {010}-faceted and {001}-faceted anatase.18-23, 25-35
The {010}-faceted anatase nanocrystals exhibit a higher photocatalytic activity in the degradation of organic compounds than that of the {101}- and [111]-faceted anatase nanocrystals and also commercial Degussa P25 TiO2 nanocrystals.26-30 This is because the {010}-facet has a favorable surface crystal structure, that is, 100% under-coordinated Ti5c
atoms on the surface which can be acted as active reaction sites, 15, 36-37 and also a favorable surface electronic band structure that results in the higher potential reductive electrons on the conduction band.21,26 The higher potential photogenerated electrons can be transferred though the surface Ti5c atoms to the reactants, which give a synergism of surface crystal structure and surface electronic band structure on the photocatalytic activity. The {001}-faceted anatase nanocrystals exhibit a higher photocatalytic activity in the degradation of organic compounds and water splitting than those of the {101}-faceted anatase nanocrystals and commercial Degussa P25 nanocrystals.21,32,38-39 The excellent photocatalytic activity of the {001}-faceted surface can be explained by the 100% under-coordinated Ti5c atoms and very large Ti-O-Ti bond angles on the surface.15 Furthermore, water molecules are adsorbed on the {001}-faceted surface by a dissociative configuration, which can promote the interfacial charge transfer and the formation of reactive radicals to improve the photocatalytic performance.37,40 In addition, the combination of high-surface-energy {001}-facet and the low-surface-energy (101)-facet
can promote the charge separation and improve the photocatalytic performance.23,43,45-46 Some studies have been reported on the comparison of photocatalytic activity of different faceted anatase nanocrystals. In general, it is considered that the high-surface-energy facet exhibits a high photocatalytic performance because of its high potential reductive photoelectrons due to the large bandgap. However, some different results have been reported. Pan et al. have reported that the facet-dependent photocatalytic activity increases in an order of {001} < {101} < {010}, and explained that the reason for the low photocatalytic activity of {001}-facet is due to its low bandgap value.22 Ye et al. have also reported that the facet-dependent photocatalytic activity for liquid-phase photocatalytic reduction and oxidation increases in an order of {010} < {101} < {001} after normalizing specific surface area effect.35 Recently, Li et al. have reported that the photocatalytic activity increases in an order of {101} < {010} < {001} for anatase nanocrystals with a similar bandgap energy of 3.28 eV, which corresponds to the increasing order of surface energy.39
On the basis of these results, we think that after normalizing specific surface area effect, the surface photocatalytic activity is mainly dependent on the surface crystal structure that is dependent on the crystal structure and the facet, the surface electronic band structure including the bandgap, the band energy level, the surface energy, and the charge separation effect by two different facets. The bandgap is affected by the surface energy and the crystal size, namely a blue shift. Therefore, it is necessary to give a quantitative relationship between the bandgap, the surface energy, and the crystal size to explain the photocatalytic activity order for the different faceted nanocrystals.
Wen et al.47 have also reported the DSSC performance of the {010}-faceted anatase nanocrystals and found that the {010}-faceted anatase nanocrystals exhibit a high DSSC
performance owing to its large dye adsorption constant, thus, the strong adsorption of dye molecules on the {010}-facet. This result has been confirmed by other studies on the {010}-faceted nanocube,26-28 nanorod,26-28, 48 nanosheet,34 and nanobelt.49 A computational simulation has concluded that {010}-facet provides the most efficient and direct pathway for interfacial electron transfer in DSSCs.39 A study on the {001}-faceted anatase nanocrystals has suggested that the DSSCs performance increases with the increasing percentage of the {001}-faceted surface, and the {001} facet is not only favorable for the dye adsorption but also retards the charge recombination effectively.50 Recently, a study on comparison of {010}-, {001}-, and {101}-faceted anatase nanocrystals has revealed that the {010}-faceted anatase nanocrystals show the highest DSSC performance from these anatase nanocrystals, because the injection efficiency of MK-2 dye on the {010}-faceted surface is higher than that of the {001} and {101}-faceted surfaces.39
Up to now, only a few of studies have been reported on [111]-faceted anatase nanocrystals and these anatase nanocrystals show (101) and (011) planes in the HR-TEM, which reveals the exposed facet is vertical to [111] crystal zone axis which is referred as [111]-facet.27-28 The [111]-facet is different from the {111}-facet because the anatase belongs to tetragonal system but not cubic system, seen in Figure 2.1. The [111]-faceted anatase TiO2 nanocrystals exhibit an excellent photocatalysis on the H2 evolution,24 and a better selectivity than that of the {001}-faceted nanocrystals in the photocatalytic degradation of MB.30 The [111]-faceted anatase TiO2 nanocubes also exhibit a much higher DSSC power conversion efficiency than that of the {101}-faceted anatase TiO2
nanocrystals and P25 nanocrystals.51 However, these studies have made a mistake on the facet assignment, namely they treated the [111]-facet as the {111}-facet and explained the
superiority of the {111}-facet using the experiment results of the [111]-facet.
Figure 2.1. Schematic illustrations of (a) [111]-faceted cubic nanocrystal and (b) crystallographic
models of [111]-facet and {111}-facet in anatase TiO2 structure.
For the synthesis of anatase TiO2 nanocrystals with specific facet on the surface, including [111]-faceted anatase nanocrystals, surface fluorination route in hydrofluoric acid (HF) solutions to reduce the surface energy of the high-energy surface by adsorption F- on the surface is an effective and normal method.19-21,23-24,32-33,40-41,50-51 However, HF has a strong toxicity and corrosivity. We have reported an environment-friendly route for synthesis of (010)- and [111]-faceted anatase nanocrystals via a topotactic transformation reaction from exfoliated layered titanate nanosheets.16,25-29,31 However, this process includes three steps reactions, synthesis of layered titanate, exfoliation of layered titanate, and topotactic transformation of the nanosheets to anatase nanocrystals, which takes a long time.
This Chapter describes an environment-friendly and facile one-pot synthesis process for the [111]-faceted anatase nanocrystals with controllable crystal size (7-50 nm) from titanium isopropoxide (TTIP) and tetramethylammonium hydroxide (TMAOH). By using the size-controlled anatase nanocrystals, we systematically studied the influences of
specific surface area, crystal size and different facets on the surface electronic band structure, and also studied the photocatalytic and DSSC performances of the nanocrystals with different crystal facets. The crystal facet affects the bandgap energy of anatase nanocrystals and also the bandgap blue shift based on crystal size, which increase in an order of non-facet < [111]-facet < (010)-facet. The increasing order of photocatalytic activity corresponds to the increasing order of bandgap. These results give a strong evidence for that the crystal facet affects the surface electronic band structure. The DSSC performance enhances in an order of non-facet < [111]-facet, which corresponds to the increasing order of dye adsorption constant Kad, that is, the increasing order of binding energy of dye molecule on TiO2 surface. The strong binding of dye molecule on TiO2
surface can improve the injection rate of photo-generated electrons from dye molecules into the conduction band of TiO2.