Complex χ(2) spectra of buried silica/isotopically
diluted water (HOD-D2O) interfaces were measured using
multiplex heterodyne-detected vibrational sum frequency generation
spectroscopy to elucidate the hydrogen bond structure and up/down
orientation of water at the silica/water interface at different pHs.
The data show that vibrational coupling (inter- and/or intramolecular
coupling) plays a significant role in determining the χ(2) spectral feature of silica/H2O interfaces and
indicate that the doublet feature in the H2O spectra does
not represent two distinct water structures (i.e., the ice- and liquid-like
structures) at the silica/water interface. The observed pH dependence
of the imaginary χ(2) spectra is explained by (1)
H-up oriented water donating a hydrogen bond to the oxygen atom of
silanolate, which is accompanied by H-up water oriented by the electric
field created by the negative charge of silanolate, (2) H-up oriented
water which donates a hydrogen bond to the neutral silanol oxygen,
and (3) H-down oriented water which accepts hydrogen bonds from the
neutral silanol and donates hydrogen bonds to bulk water molecules.
The broad continuum of the OH stretch band of HOD-D2O and
a long tail in the low frequency region represent a wide distribution
of strong hydrogen bonds at the silica/water interface, particularly
at the low pH.
Despite recent significant advances in interface-selective nonlinear spectroscopy, the topmost water structure at a charged silica surface is still not clearly understood. This is because, for charged interfaces, not only interfacial molecules at the topmost layer but also a large number of molecules in the electric double layer are probed even with second-order nonlinear spectroscopy. In the present study, we studied water structure at the negatively charged silica/aqueous interface at pH 12 using heterodyne-detected vibrational sum frequency generation spectroscopy, and demonstrated that the spectral component of the topmost water can be extracted by examining the ionic strength dependence of the Imχ spectrum. The obtained Imχ spectrum indicates that the dominant water species in the topmost layer is hydrogen-bonded to the negatively charged silanolate at the silica surface with one OH group. There also exists minor water species that weakly interacts with the oxygen atom of a siloxane bridge or the remaining silanol at the silica surface, using one OH group. The ionic strength dependence of the Imχ spectrum indicates that this water structure of the topmost layer is unchanged in a wide ionic strength range from 0.01 to 2 M.
Small Bi nanoparticles prepared upon reduction of Bi‐III compounds catalyze the growth of thick CdSe nanowires (see image). The control of Bi particle and CdSe nanowire growth rate allows adjustment of the dimensions of the nanowires, which is important for the preparation of various types of nanowires in solution.
The diameter dependence of the optical band gap of single CdSe nanowires (NWs) is investigated by a combination of atomic force microscopy, scanning fluorescence microscopy, and transmission electron microscopy. We find a good congruence of the experimental data to calculations within the effective mass approximation taking into account quantization, exciton Coulomb interaction, and dielectric mismatch. The experimental data are furthermore compared to different theoretical approaches. We discuss the influence of alternating wurtzite and zinc blende segments along the NWs on their optical properties.
Confocal optical microscopy was employed to study the effect of surface-enhanced Raman scattering on individual single-walled carbon nanotubes covered with isolated gold particles. The gold particles with diameters between 10 and 120 nm were deposited in low densities on the tubes' sidewalls by an electrochemical method. In the spectra, Raman peaks associated with the nanotubes were found to be superimposed on a broad luminescence background originating from the metal particles. With increasing particle size, both the luminescence intensity as well as the Raman enhancement increased at longer wavelengths. This finding is consistent with a size-dependent broadening of the gold plasmon frequency and a corresponding extension of the energetic range for local field enhancement on the particle surface. In addition, wavelength-dependent experiments revealed a maximum Raman intensity when both nanotube and metal particle were in optical resonance.
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