NSF Nanoscale Science and Engineering Grantees Conference, Dec 3-6, 2007
Grant # : 0608896
NIRT: Active Nanoparticles in Nanostructured Materials Enabling Advances
in Renewable Energy and Environmental Remediation
NSF NIRT Grant CTS-0608896
David A. Dixon, James L. Gole, Andrei Federov, Clemens Burda, Greg Szulczewski
The University of Alabama, Georgia Institute of Technology, & Case Western University
During the first year of our NIRT, our research efforts have employed a combined experimental
and computational effort to better understand the behavior of active nanostructures. A primary
focus has been to study porous silicon (PS) interfaces and titanium dioxide nanostructures. A
goal of our overall effort is to produce nanostructure modified porous silicon based
microreactors leading to efficient photocatalysis. We have developed techniques to produce TiO2
and TiO2-xNx thin and thick films which are photocatalytic. We are assessing the nature of the
transformation of the films from the anatase to the rutile phase and evaluating their
characteristics by different spectroscopic techniques.[1]
A significant portion of our work has focused on the characterization and processing of titanium
dioxide (TiO2) thin films beginning at the synthesis stage, with special attention paid to
microstructure phase transformation, and alternative doping methods. These two areas are of
critical importance to the formation of practical TiO2-based systems. For example, the phase of
TiO2 is intrinsic to its surface-adsorbate and bulk carrier transport characteristics, whereas
dopant incorporation is critical to lowering the band gap energy to enhance quantum efficiency
and solar photon conversion, especially for photochemical water splitting.
We have continued to develop a general approach to PS nanostructure coating techniques that
extrapolates from our previous studies using gold and tin/tin-oxide nanostructured coatings to
greatly improve the sensitivity of the PS micro/nanostructured surface and its rejuvenation.
Within the framework of PS microreactors, we are close to completing the first phase of
micro/nanostructure filter development. Micro/nanostructured filters can then be seeded with
active nanostructures to form a flow-thru reactor configuration that will compliment the
photoluminescence and electroluminescence-based microreactors we are developing.
We have developed novel TiO2 nitrogen doping (TiO2-xNx) and metal ion seeding techniques
leading to the discovery of novel room temperature anatase to rutile phase transformations in
contrast to the usual high temperatures needed for the transformation, magnetic ion (FeII, CoII,
NiII) enhanced Raman and stimulated infrared spectroscopy of the oxynitride, and magnetic ion
enhanced nitrogen doping. The nitrogen doped TiO2 has an absorption wavelength shifted
towards the visible from the ultraviolet so that one can make better use of the solar spectrum.
Porous silicon (PS) films have broad applications in sensors, microreactors, opto- and quantum
electronics, and photovoltaics. If procedures can be developed so that a PS layer can be removed
in a controlled manner from an etched silicon surface, porous silicon-based films of distinct pore
size can be generated similar to the anodization and subsequent removal of alumina films. We
have developed the first single step etch-liftoff procedure (one step separation) based on the
formation and removal of macroporous silicon (PS) layers. With silicon wafers with resistivities
in the range from 14 to 22 ohm-cm, films with pore diameters ranging from one to two microns
NSF Nanoscale Science and Engineering Grantees Conference, Dec 3-6, 2007
Grant # : 0608896
and thickness ranging from 3 to 70 micrometers have been prepared. These silicon-based films
(filters), which carry a polarizing negative charge, represent an alternative to porous alumina
(films) filters (pore diameter not yet in the one to two micron range) in terms of their size range
and their potential interaction-reaction at elevated temperatures. Cu and Pt can be introduced to
these filters using electroless solutions to create an effective reductive surface. Using these and
alternate material combinations, interactive-reactive filters in the one micron size range which
operate at temperatures well in excess of porous polymer films can be realized with potential
applications as microreactors for chemical transformations.[2]
Figure 1. Mechanical Lift-off of PS filter
using the ability to readily remove the
silicon oxyfluoride polymer that forms on
the PS surface.
Figure 2. Direct Lift-off of PS-based
film- filter (~ 20 microns) from the
surface of an etched PS film. Pore
diameter is approximately one micron.
Figure 3. SEM micrographs of a porous silicon filter. From the left, the tips of the pores can be
seen before and after they begin to spread out in their face directions. The next two images show
the front and back of a filter. Note that pore spreading is significant in the filter back.
Recent studies in our laboratories have suggested the importance of structural porosity as it
influences and enhances the rate and efficiency of the main group nitrogen doping process in
porous TiO2 nanocolloids to form improved visible light absorbing oxynitride photocatalysts at
the nanoscale. The seeding of the high spin transition metal ions Co (II) and Ni (II) into porous
nanoscale nitrogen doped titanium oxynitride, TiO2-xNx structures leads to a significant
enhancement of the infrared spectrum due to adventitious water and minor contaminants
associated with the oxynitride synthesis from a porous TiO2 nanocolloid sample. There are a
number of contributions to the infrared enhancement (1) the formation of protonated amines near
NSF Nanoscale Science and Engineering Grantees Conference, Dec 3-6, 2007
Grant # : 0608896
the oxynitride surface; (2) modification of
the anatase ionic crystal dipole moment due
to the displacement of charged ions by the
electric field associated with the high spin
Co (II) and Ni (II) ions; and (3) the
incorporation of a spinel-like structure into
the TiO2 lattice as demonstrated by Raman
spectra. The presence of partial proton
transfer from the amines to the surface can
lead to enhanced intensities. The higher the
proton affinity of the amine, the less proton
transfer occurs leading to a higher N-H
stretch frequency with less enhancement.[3]
Figure 4. Note the enhancement of the infrared
intensity in the region between 2000 and 3000 cm-1.
The surprising room temperature phase conversion of anatase to rutile TiO2, a process that
normally requires temperatures well in excess of 700 C, is facilitated through the use of
transition metal ions with unpaired electron spin. Raman spectroscopy has been used to
demonstrate this unexpected process, which is facilitated by sseding high spin cobalt (CoII ) and
nickel (NiII) transition metal ions into a TiO2 or TiO2-xNx nanocolloid lattice. The low
temperature transition is not observed upon seeding with low spin ions including CuII. The
Raman spectra demonstrate the concomitant incorporation of a spinel-like structure into the
titania lattice.[4]
442
606
240
10000
Raman Intensity
690
TiO2- .0125 Co
TiO2- .025 Co
TiO2- .05 Co
TiO2- 0.1 Co
with Co(Cl)2
244
Raman Intensity
430
606
380
TiO2-(NO3) 0.025
TiO2-(NO3) 0.05
TiO2-(NO3) 0.2
688
1000
100
1000
200
400
600
800
1000
-1
Wavenumber (cm )
1200
200
400
600
800
1000
1200
-1
Wavenumber (cm )
Figure 4. Raman spectra for a TiO2 nanocolloid (<10 nm) prepared with various concentrations of
CoII using a) CoCl2 and b) Co(NO3)2. The Raman signal was obtained using a 1 μm spot size and a
power of 25 mW or less.
Publications
[1] For further information about this project email: dadixon@bama.ua.edu or jim.gole@physics.gatech.edu.
[2] “Development of Porous-Silicon-based Active Microfilters,” J.Campbell, J.A. Corno, Ni.Larsen, and J.L. Gole,
Georgia Institute of Technology, J. Electrochem . Soc. Accepted, Oct. 2007.
[3] “Evidence for High Spin Transition Metal Ion Induced Infrared Spectral Enhancement,” J. L. Gole, S. M.
Prokes, M. G. White, T.-H. Wang, R. Craciun, and D. A. Dixon, J. Phys. Chem., 2007, WEB ASAP, Oct. 24.
[4] “Efficient Room Temperature Conversion of Anatase to Rutile TiO 2 Induced by High Spin Ion Doping,” J.L.
Gole, S. M. Prokes, M. G. White, J. Phys. Chem. B, submitted.
[5] “Experimental and Theoretical Studies of the Photoreduction of Copper(II)-Dendrimer Complexes,” H. Wan, S.
Li, T. A. Konovalova, S. Shuler, D. A. Dixon, and S. C. Street, J. Phys. Chem., accepted, Oct. 2007.