Geoscience Reference
In-Depth Information
10.6 Hydrothermal Processing of Selected
Advanced Materials
The term advanced materials refers to a chemical substance whether organic or inor-
ganic or biological or mixed in composition possessing desired physical or chemical
or biological properties. In this chapter, the processing of selected advanced materi-
als using hydrothermal, solvothermal, and supercritical processes is dealt with.
Luis Brus et al. [45] first explained that hydrothermally prepared nanoparticles of
cadmium sulfide, CdS, in an aqueous suspension, had a blue shift in the visible
absorption and emission spectra compared with bulk CdS. Particles whose radius is
less than the exciton Bohr radius exhibit discrete energy levels similar to single
atoms. Unlike the band energies observed in bulk materials, every unique crystal
diameter on the nanoscale corresponds to a discrete energy. Materials that exhibit
this characteristic are called “artificial atoms” or QDs. Recent reviews [46
48] elu-
cidate the degree to which solvothermal synthetic techniques are now an essential
technique for controlling the size of the II
V semiconductor materials.
Synthesis of the QDs typically requires a cation source material that is soluble in the
chosen solvent and a surfactant that caps or stabilizes the QD, arresting its growth.
VI and III
10.6.1 Processing of Native Metals
In recent years, noble metal particles (like Au, Ag, and Pt), magnetic metals (Co,
Ni, and Fe), metal alloys (like FePt and CoPt), and multilayers (like Cu/Co and Co/
Pt) have attracted the attention of researchers owing to their new interesting funda-
mental properties and potential applications as advanced materials with electronic,
magnetic, optical, thermal, and catalytic properties
[49
52]
. Although metallic par-
ticles can also exhibit the QD behavior, their exciton Bohr radius is much smaller
than the semiconductors, resulting in significant synthetic challenges. However,
metallic nanoparticle synthesis is of current interest for applications in nanocircuits
and devices. The size, shape, and type of material desired depend on the applica-
tion. For example, the desire for higher density magnetic recording devices initiated
the development of a new nanosized ferromagnetic material based on the 3D self-
assembly of Fe
58
Pt
42
(4 nm) into a superlattice colloidal crystal [53].
The intrinsic properties of noble metal nanoparticles strongly depend upon their
morphology and structure. The synthesis and study of these metals have implica-
tions for the fundamental study of the crystal growth process and shape control.
Majority of the nanostructures of these metals alloys and multilayers form under
far-from-equilibrium conditions
[54]
. Among these metals, alloys, and multilayers,
shape anisotropy exhibits interesting properties. Both hydrothermal and hydrother-
mal supercritical water techniques have been extensively used in the preparation of
these nanoparticles.
Zhu et al. (2003) have reported the synthesis of silver dendrite nanostructures
using anisotropic nickel nanotubes
[50]
via mild hydrothermal reactions. The nickel
nanotubes acted as a reducing agent. The crystal morphologies which changed
