3 research outputs found
Role of Vacancy Condensation in the Formation of Voids in Rutile TiO<sub>2</sub> Nanowires
Titanium dioxide
nanowire (NW) arrays are incorporated in many devices for energy conversion,
energy storage, and catalysis. A common approach to fabricate these
NWs is based on hydrothermal synthesis strategies. A drawback of this
low-temperature method is that the NWs have a high density of defects,
such as stacking faults, dislocations, and oxygen vacancies. These
defects compromise the performance of devices. Here, we report a postgrowth
thermal annealing procedure to remove these lattice defects and propose
a mechanism to explain the underlying changes in the structure of
the NWs. A detailed transmission electron microscopy study including
in situ observation at elevated temperatures reveals a two-stage process.
Additional spectroscopic analyses and X-ray diffraction experiments
clarify the underlying mechanisms. In an early, low-temperature stage,
the as-grown mesocrystalline NW converts to a single crystal by the
dehydration of surface-bound OH groups. At temperatures above 500 °C,
condensation of oxygen vacancies takes place, which leads to the fabrication
of NWs with internal voids. These voids are faceted and covered with
Ti<sup>3+</sup>-rich amorphous TiO<sub><i>x</i></sub>
Activation and Fluoride-Assisted Phosphating of Aluminum-Silicon-Coated Steel
Phosphating
is a crucial process in the corrosion protection of metals. Here,
activation and fluoride-assisted tricationic phosphating is investigated
on aluminum–silicon (AS) coated steel surfaces. Dynamic light
scattering results from the activation bath show a bimodal size distribution,
with hydrodynamic radii of ∼400 nm and ∼10 μm.
For the smaller particle fraction, static light scattering results
are consistent with the interpretation of disklike particles as scattering
objects. Particles of the larger fraction sediment with time. In the
presence of electrolyte, the scattering intensity from the larger
particle fraction increases. Coagulation with time is suggested to
be related to the decrease in activity of the activation bath. Scanning
Auger microscopy (SAM) shows a higher phosphorus concentration after
titanium phosphate activation in the Al-rich areas compared to the
Si-rich areas of the AS coatings. There is no correlation between
the size of the species in the activation bath, and the size of the
phosphate-containing regions on the activated surface. Phosphating
was performed in the presence of hexafluorosilicic acid, H<sub>2</sub>SiF<sub>6</sub>, ammonium hydrogen difluoride, NH<sub>4</sub>HF<sub>2</sub>, and both, at an initial pH of 2.5. The absence of crystals
after phosphating with H<sub>2</sub>SiF<sub>6</sub> is an indication
that SiF<sub>6</sub><sup>2–</sup> is the final product of the oxide dissolution in the presence of
fluoride. In the presence of NH<sub>4</sub>HF<sub>2</sub>, the Si-rich
regions of the surface are phosphated before the Si-poor (Al-rich)
regions. Hence, the phosphate distribution after activation and after
phosphating are opposite. These results show that a high surface concentration
of phosphate after activation is not sufficient for a high coverage
with phosphate crystals after phosphating
Low-Copy Number Protein Detection by Electrode Nanogap-Enabled Dielectrophoretic Trapping for Surface-Enhanced Raman Spectroscopy and Electronic Measurements
We
report a versatile analysis platform, based on a set of nanogap
electrodes, for the manipulation and sensing of biomolecules, as demonstrated
here for low-copy number protein detection. An array of Ti nanogap
electrode with sub-10 nm gap size function as templates for alternating
current dielectrophoresis-based molecular trapping, hot spots for
surface-enhanced Raman spectroscopy as well as electronic measurements,
and fluorescence imaging. During molecular trapping, recorded Raman
spectra, conductance measurements across the nanogaps, and fluorescence
imaging show unambiguously the presence and characteristics of the
trapped proteins. Our platform opens up a simple way for multifunctional
low-concentration heterogeneous sample analysis without the need for
target preconcentration