3.1 Metal Oxides/Sulfides
35
Understanding the evolution and formation of crystalline and amorphous phases
by atomic-layer deposition is necessary to create high-quality, multifunctional dielectric coatings/films, and to predict the surface functionalization (Shi et al. 2013). Integrated, atomistic electron-microscopy research of TiO 2 structures at designed growth
cycles during atomic-layer deposition has elucidated transformation sequences and
different atom arrangement processes during TiO 2 atomic-layer deposition growth.
The evolution of TiO 2 structures during atomic-layer deposition was revealed by
following the transformation from amorphous layers to amorphous particles to
metastable crystallites and finally to stable crystalline forms. These changes were
attributed to the Ostwald-Lussac law, which controlled the ratio and sequence of
disparate phases of the TiO 2 nanostructures during the relatively high-temperature
atomic-layer deposition. Moreover, the crystalline and amorphous mixture enabled
specific anisotropic crystal growth at a relatively high temperature, forming TiO 2
nanorods by a vapor-phase oriented attachment mechanism.
3.1.1.5 Vapor–Liquid–Solid
Vapor–liquid–solid refers to a growth mechanism in which the material is directly
absorbed via liquid catalysts during crystal growth. In the vapor–liquid–solid
method, the 1D nanostructure is developed by solid precipitation from disperse and
supersaturated catalyst droplets.
Titanium dioxide is a very promising material for various applications, including,
for example, supercapacitors, lithium-ion batteries, dye-sensitized solar cells and
water splitting systems, and there are various methods to synthesize TiO 2 nanostructures. However, intentionally controlling the spatial location and range of the
length/diameter seems to be unfeasible with other common methods. Therefore, a
position- and size-controlled vapor–liquid–solid method for synthesizing nanowires
is a solution to overcome the above issues. Nevertheless, creating a TiO 2 singlecrystal nanowire by the vapor–liquid–solid mechanism has been a challenging issue
because of the difficulty in understanding and controlling complex nanomaterial
transport events across three phases. Experiments have demonstrated that the vapor–
liquid–solid growth of 1D TiO 2 structures can emerge consistently only within a
quite narrow range of material flux (Zhuge et al. 2012). This phenomenon distinctly
contrasts that of typical vapor–liquid–solid metal oxides, such as ZnO, In 2 O 3 , SnO 2,
and MgO, whose metal oxide nanowires are easily grown by the vapor–liquid–
solid mechanism with much wider ranges of material flux. Moreover, experimental
results have shown that rutile TiO 2 nanowires primarily grow along the (0 0 1)
direction, which differs from the usual (0 0 1)-oriented growth of TiO 2 nanowires
formed by vapor-phase processes. This method, based on the control of material
flux, is a reasonable strategy to tailor TiO 2 nanowires using the vapor–liquid–solid
mechanism.
The growth of 1D metal oxides can be achieved through electron-beam evaporation processes (Yu and Lee 2014). Most metal oxides are easily resolved by the
condensed electron-beam method. As seen in Fig. 3.1a, b the metal oxide source
35
Understanding the evolution and formation of crystalline and amorphous phases
by atomic-layer deposition is necessary to create high-quality, multifunctional dielectric coatings/films, and to predict the surface functionalization (Shi et al. 2013). Integrated, atomistic electron-microscopy research of TiO 2 structures at designed growth
cycles during atomic-layer deposition has elucidated transformation sequences and
different atom arrangement processes during TiO 2 atomic-layer deposition growth.
The evolution of TiO 2 structures during atomic-layer deposition was revealed by
following the transformation from amorphous layers to amorphous particles to
metastable crystallites and finally to stable crystalline forms. These changes were
attributed to the Ostwald-Lussac law, which controlled the ratio and sequence of
disparate phases of the TiO 2 nanostructures during the relatively high-temperature
atomic-layer deposition. Moreover, the crystalline and amorphous mixture enabled
specific anisotropic crystal growth at a relatively high temperature, forming TiO 2
nanorods by a vapor-phase oriented attachment mechanism.
3.1.1.5 Vapor–Liquid–Solid
Vapor–liquid–solid refers to a growth mechanism in which the material is directly
absorbed via liquid catalysts during crystal growth. In the vapor–liquid–solid
method, the 1D nanostructure is developed by solid precipitation from disperse and
supersaturated catalyst droplets.
Titanium dioxide is a very promising material for various applications, including,
for example, supercapacitors, lithium-ion batteries, dye-sensitized solar cells and
water splitting systems, and there are various methods to synthesize TiO 2 nanostructures. However, intentionally controlling the spatial location and range of the
length/diameter seems to be unfeasible with other common methods. Therefore, a
position- and size-controlled vapor–liquid–solid method for synthesizing nanowires
is a solution to overcome the above issues. Nevertheless, creating a TiO 2 singlecrystal nanowire by the vapor–liquid–solid mechanism has been a challenging issue
because of the difficulty in understanding and controlling complex nanomaterial
transport events across three phases. Experiments have demonstrated that the vapor–
liquid–solid growth of 1D TiO 2 structures can emerge consistently only within a
quite narrow range of material flux (Zhuge et al. 2012). This phenomenon distinctly
contrasts that of typical vapor–liquid–solid metal oxides, such as ZnO, In 2 O 3 , SnO 2,
and MgO, whose metal oxide nanowires are easily grown by the vapor–liquid–
solid mechanism with much wider ranges of material flux. Moreover, experimental
results have shown that rutile TiO 2 nanowires primarily grow along the (0 0 1)
direction, which differs from the usual (0 0 1)-oriented growth of TiO 2 nanowires
formed by vapor-phase processes. This method, based on the control of material
flux, is a reasonable strategy to tailor TiO 2 nanowires using the vapor–liquid–solid
mechanism.
The growth of 1D metal oxides can be achieved through electron-beam evaporation processes (Yu and Lee 2014). Most metal oxides are easily resolved by the
condensed electron-beam method. As seen in Fig. 3.1a, b the metal oxide source