Design, Fabrication, and Modification of Cost-Effective Nanostructured TiO for Solar Energy Applications - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 35

2.1.5 Other Methods
Vapor deposition describes any process in which materials in a vapor state are
condensed to form a solid phase material, usually conducted within a vacuum
chamber. If no chemical reaction occurs, this process is called physical vapor
deposition (PVD); otherwise it is called chemical vapor deposition (CVD) [ 20 , 35 ].
In PVD, materials are evaporated followed by condensation to form a solid
material. The primary PVD methods include thermal deposition, ion plating, ion
implantation, sputtering, laser vaporization, and laser surface alloying [ 20 , 35 ].
PVD is considered an effective method to fabricate uniform, high-quality semi-
conductors, especially composite and doped semiconductors. Moreover, PVD
enables the formation of uniform nanostructured thin films, in which the size and
shape can be precisely controlled [ 242 ]. Two versatile PVD methods, oblique angle
deposition (OAD) and glancing angle deposition (GLAD), are based on the geo-
metric shadowing effect, and are widely applied to prepare well-aligned nanorod
arrays [ 243 ]. In OAD, the incident vapor flux is directed onto a substrate at a
nonzero angle h with respect to the substrate normal. When the vapor incident angle
h is large (i.e., h [ 70), a well-defined nanorod array tilted toward the direction of
the vapor flux can be obtained [ 244 ]. The GLAD method is similar to OAD in that it
uses a large incident vapor angle, but the substrate is rotated azimuthally at a
constant speed during the deposition. The result is vertically aligned nanorod arrays
[ 245 ]. By changing the deposition parameters in these PVD methods, one can easily
fabricate specific nanostructured porous array films such as cylinders, helices,
spheres, and zigzags all with controllable surface areas [ 242 , 243 , 246 , 247 ].
In addition, femtosecond and nanosecond pulsed laser deposition (PLD) at
different wavelengths is also widely used to construct nanoparticle-assembled TiO 2
films. This technique is useful for controlling the dimensions and the crystalline
phase of nanoparticles by varying the laser parameters and the deposition condi-
tions. It is also suitable for depositing TiO 2 films at a high deposition rate and low
cost [ 248 - 250 ]. Recently, a novel forest-like architecture consisting of hierarchical
assemblies of tree-like nanocrystalline particles of anatase TiO 2 , were grown on
FTO substrates via pulsed laser deposition (PLD) at room temperature by ablation
of a Ti target in a background O 2 atmosphere. The resulting architecture was
proposed to be beneficial to reduce the electron recombination and also control
mass transport in the mesopores, and thus achieved a 4.9 % conversion efficiency in
a DSSC [ 251 ].
However, in CVD processes, thermal energy heats the gases in the coating
chamber to induce the deposition reaction. Typical CVD approaches include
electrostatic spray hydrolysis, diffusion flame pyrolysis, thermal plasma pyrolysis,
ultrasonic spray pyrolysis, laser-induced pyrolysis, and atmospheric pressure and
ultronsic-assisted hydrolysis [ 20 , 35 , 252 , 253 ]. Several TiO 2 nanostructures
prepared though CVD have been reported. For example, TiO 2 nanoparticles with
sizes below 10 nm were prepared by pyrolysis of TTIP via CVD in a mixed
helium/oxygen atmosphere [ 254 ]. TiO 2 nanorods were grown on a Si substrate
using TTIP as the precursor by metal organic CVD (MOCVD) [ 64 ].