Year Two Activities:
This proposal deals with a systematic study of the synthesis, characterization, and
application of TiO2 nanoparticles as photocatalyst. Specifically, the multifold objectives
of this proposal are:
• To utilize a unique physical vapor deposition process to obtain TiO2 nanoparticles
with particle sizes ranging between 1 and 20 nm, and to reproducibly dope the
nanoparticles with various dopants.
• To characterize the nanoparticles for structural, chemical and optoelectronic
properties.
• To utilize first-principles calculations to acquire an atomistic understanding of
nanoparticle properties.
• To develop an understanding of the chemical and photochemical properties of
pure and modified TiO2 nanoparticles. Modification involves the selective
decoration of nanoparticle surfaces with noble metal particles including Ag, Au,
Pt, etc.
Theory:
1. The band structure and density of states of pure TiO2 (anatase and rutile) have
been calculated using density functional theory.
2. The relative free energies and electronic properties of Nd-doped TiO2 were
calculated for substitutional and interstitial dopants. The states introduced by the
more-stable substitutional Nd dopant are located at the bottom edge of the
conduction band, and thus effectively narrow the band gap. However, the states
introduced by the interstitial Nd dopant are mostly inserted into the original
conduction band of TiO2, having little effect on the band gap.
3. Properties of the two most commonly exposed surfaces, (100) and (110), were
calculated using a nine layers slab. The band gaps of the thin films are narrowed
due to contributions from the surface atoms. This may explain why particles on
the 10-nanometer scale (small enough to have a large fraction of surface atoms,
but too large for quantum confinement effects) are observed to have a smaller
band gap than the bulk material.
Synthesis and Characterization:
1. Polycrystalline TiO2 nanoparticles prepared with MOCVD method were
characterized by various techniques, including x-ray diffraction (XRD), x-ray
photoelectron spectroscopy (XPS), energy dispersive x-ray (EDX), secondary
electron microscopy SEM), BET specific surface area, and UV-vis
spectrophotometry.
2. Particles of sizes 12, 17, and 25 nm, have been obtained. The surface areas
measured by BET were 125, 96, 63 m2/gm.
3. The ultraviolet-visible (UV-VIS) light absorption experiments on synthesized
samples were conducted. A red shift was observed as a function of particle size
down to 12 nm. Samples obtained from other sources with particle size below 12
nm show blue shift, indicative of quantum confinement effect.
4. In order to study the surface charge of the TiO2 particles in the liquid phase, zeta
potential measurements were conducted. The point of zero charge (PZC) in all
cases were located between pH of 5 and 6. Thus, TiO2 particles in our system will
be negatively charged at the pH of 9.5. Electrostatically adsorbed OH- ions can
readily trap the photo-generated holes on TiO2 surface under the UV irradiation.
Another reason to maintain pH at 9.5 is to maintain 2CP ionized and to prevent
the evaporation of 2CP during experiments since the pKa of 2CP is 8.52.
Decoration of Nanoparticles:
1. We are developing photodeposition of metals as 1. a novel method for quantifying
the photoactivity of semiconducting oxide nanoparticles and 2. a synthetic route
for preparing supported metal nanoparticle catalysts with unprecedented control
of particle size distributions.
2. Both TiO2 anatase particles (Aldrich Co.) and nanoparticles (obtained via
MOCVD) with silver deposited under different irradiation conditions were
prepared and characterized by Transmission Electron Microscopy (TEM), High
Angle Annual Dark Field (HAADF) and Elemental Dispersion Analysis using Xrays (EDAX), yielding information about chemical composition and metal
particle-size distributions.
3. The TiO2 nanoparticles prepared by MOCVD have a higher photoactivity in both
the UV and visible regions of the spectrum than do conventional anatase particles.
4. The photoactive sites on TiO2 nanoparticles are more abundant and more
homogeneously distributed over the surface than are those on crystalline anatase
samples.
5. Metal photodeposition on TiO2 nanoparticles gives rise to highly uniform metal
nanoparticles in the 1-2 nm size range. These appear to be much more uniform
than Haruta-type catalysts prepared by conventional methods, and we plan to
investigate them for low temperature oxidation processes.
These results are described in details the contribution sections.
Presentations:
1. S. Ismat Shah, Nanocatalysts for Environment, SAE workshop on
Nanotechnology for Automobiles, Carnegie Melon University, Pittsburgh, PA.
May, 2004.
2. S. Ismat Shah, M. Barteau, C-P. Huang, D. Doren, J.G. Chen, Visible Light
Photocatalysis with Nanostructured Metal Oxide Semiconductors, NSF NIRT
Grantee’s Workshop, December 2003.
3. S. Ismat Shah, 3 lectures on Nanostructured Materials, in Nanotechnology
Workshop, Quaid-e-Azam University, Islamabad, Paksitan, April 2004.
4. W. Li, H. Lin, C. Ni, S. I. Shah, C.-P. Huang, Size Dependency of Structural,
Optical, and Photocatalytical Properties of TiO2 Nanoparticles, American
Vacuum Society National Symposium, Baltimore, MD, October 2003.
5. S. Ismat Shah, W. Li, H. Lin, C. Ni, S. I. Shah, C.-P. Huang, M. Barteau, Y.
Wang, D. Doren, S. Rayko, J.G. Chen, Band Gap Tailoring of Nd3+ Doped TiO 2
Nanoparticles, MRS Fall Meeting, Boston, MA 2003.
6. S. Ismat Shah, Series of 3 lectures on Nanostructured Synthesis Processes,
Physics for Contemporary Needs, Nathiagali Summer College, Pakistan, JuneJuly 2003.
7. S. Ismat Shah, Synthesis and Applications of Compound Nanocatalysts for
Environmental Technology, ACS Annual Meeting, New Orleans, LA, March
2003.
8. S. Ismat Shah, Nanostructures in Electronics, Presented at the Colloquium of the
Department of Electrical and Computer Engineering, University of Delaware,
March (2003)
9. S. Ismat Shah, Environmental Nanocatalysts, University of Bremen, Bremen,
Germany, January (2003).
10. S. Ismat Shah, Environmental Impact of Nanotechnology, Institute of
Environmental Economics, Berlin, Germany, January 2003.