**2.1. Syntheses of 2D titania nanomaterials**

preparations are vivid [1]. Researchers have been focused on the morphology control, such as synthesis of nanotubes [2], nanowires [3, 4], nanoribbons [5], diskettes [6], nanobelts, nanosaws, nanowalls, nanomultipods, nanorings, nanocages, nanohelixes, nanopropellers, and many others [7–9]. While one-dimensional nanomaterials such as nanowires and nanorods have been extensively studied, 2D nanostructured materials have attracted relatively less attention. However, 2D nanomaterials show strong potential as chemical and biological sensors, nanoelectronic devices, and catalysts with high surface areas and large pore volumes.

morphologies have been already synthesized successfully [10–12]. For example, Ye et al. synthesized thinner ZnO nanoplates by restricting the crystal growth along (0001) plane with complexation between Zn2+ and citrate ions. The 50 nm thick hexagonal nanoplates with uniform 1 μm diameter presented the highest photocatalytic activity over other morphologies including nanorods, microrods, and dumbell-shaped microrods [13]. Liu et al. developed a combustion CVD method to deposit 50 nm to 1 μm thick ZnO flakes on Si substrate for ethanol vapor sensing [14]. Duan et al. developed an integrated autoclaving and pyrolysis process to obtain porous hexagonal ZnO micro-flakes [15]. Amano et al. autoclaved the mix-

(20–25 nm) with shifted absorption edge to the longer wavelength at 440 nm. The tungstate nanoplates completely oxidize gaseous acetaldehyde under visible light illumination resulting from slow recombination and a long lifetime of generated carriers in the time-resolved

reaction of titanium sulfate and lanthanum nitrate under the hydrothermal condition [16]. The thin lanthanum titanate flakes demonstrated significantly higher photoactivity to pro-

tion material due to higher surface area and light absorption. Highly porous CoOOH flakes were prepared by the deposition of layered cobalt acetate hydroxide solution on a nickel foil in 1.0 M KOH solution. The cyclic voltammetry (CV) measurements indicated high rate capacitance with good cycle ability of CoOOH flakes [17]. A novel pigment derived from silica flakes coated with titania from a web coating process were developed in a range of 50–1000 nm thickness [18]. The special color variation effect is achieved by the combination of silica flakes and titania coating resulting in strong light interference and angle-dependent behaviors. Nanosheets with a spacing of 0.6 nm were precipitated on a glass substrate from a

gel solution under vibration in a 90°C hot water bath [19]. The good hydrophilicity

and antifogging capability without any light exposure for 2000 h received from the hydrated nanosheets with unique physicochemical properties such as roughness and surface chemistry. Although the above materials possess unique properties on optical, gas sensing, electrochemical, and cleaning applications, the relatively expensive precursors or processes hinder

Titanium dioxide is a versatile and low cost material for many industrial applications; many scientific works have been focused on particle size control down to the order of tens of nanometers. Nanostructured titanium dioxide were continuously developed in the field of environmental purification, solar energy conversion, pigment, optics, gas sensing, and energy

WO6

Ti2 O7

from water under UV irradiation over conventional solid-state reac-

nanoparticles for water treatment is limited because of

WO6

with flake-like or plate-like

crystalline platelets

) flakes were prepared by the

Many small band gap materials such as ZnO, ZnS, and Bi<sup>2</sup>

42 Titanium Dioxide - Material for a Sustainable Environment

ture of bismuth nitrate and sodium tungstate to precipitate thin Bi<sup>2</sup>

infrared absorption spectra [11]. Lanthanum titanate (La<sup>2</sup>

duce water splitting H<sup>2</sup>

SiO<sup>2</sup>


practical applications.

storage [20–22]. However, using TiO<sup>2</sup>

The most common shape of the fine titanium dioxide particles is spherical in many syntheses and applications. Thin films or fibers have been fabricated by being supported on a substrate or in the interstices in some three-dimension network [27, 28]. Sasaki fabricated thin titania flakes through exfoliation of a layered titanate precursor [25]. Although the specific surface area of the flakes is about 110 m<sup>2</sup> /g, the photocatalytic activity is still less than commercial product, Degussa P25 (49 m<sup>2</sup> /g). The freeze-dried nanosheets were also adopted as the anode material in a liquid electrolyte lithium ion battery [29]. The promising electrochemical performance of titania nanosheets were exploring in the charge-discharge characterizations as discharging with smaller slope and lower average voltage than titanium dioxide and lithium titanate. Li et al. in 2007 synthesized Brookite phase titania nanoplates by using titanium trichloride (TiCl<sup>3</sup> ) precursor through hydrothermal processes [30]. Under the same surface area of loaded TiO<sup>2</sup> , the brookite nanoplates exhibit the highest efficiency in the beaching of methyl orange solution under UV irradiation. The lamellar titania were synthesized within the lamellar micelle of non-ionic surfactant in cyclohexane [31]. The resembling nanostructures consisted of 40 nm titania flakes with flat, homogenous surface and low defects. Wu et al. utilized the micro-arc oxidation process associated with alkali treatment at pure titanium substrate to develop titania flakes [32]. However, most synthesis methods for these types of particles require multiple, complicated procedures and are typically nonconductive to be scaled up manufacturing as yields are typically milligram or less quantities of material. Examples include template, chemical vapor deposition, hydrothermal, electrochemical anodization, etc. [33–36].
