Guadalupe Valverde-Aguilar

Silver Core - Silver Oxide Shell Nanoparticles Embedded on Mesostructured Silica Films

Metallic silver particles in the nanometer size range were obtained in SiO2 matrix by the reduction of AgNO3 with the non-ionic diblock copolymer (Brij 58). Hexagonal mesostructured sol-gel films were synthesized by dip-coating method using the surfactant Brij58 to produce channels into the film, which house the silver nanoparticles. Optical properties of the metallic nanoparticles were studied by UV-Vis spectroscopy, TEM and HRTEM images. The experimental absorption spectrum of the metallic silver nanoparticles exhibits an absorption band located at 438 nm and a shoulder at longer wavelength. The TEM images show randomly distributed silver nanoparticles (Type I) along with some oriented as long line (Type II). Both distributions exhibit a silver oxide shell around of them. The second shell covering the silver core - silver oxide shell system is related to the surfactant. The optical absorption spectrum was modelled using the Gans theory. The fit shows two main contributions related to metallic silver nanoparticles with different axial ratios, and surrounding of a dielectric medium with high refractive index. Presence of the high refractive index silver oxide shell was confirmed by X-ray diffraction technique. The contributions of silver core and silver oxide shell play important roles in the optical properties of the films.

Tuning Aryl, Hydrazine Radical Cation Electronic Interactions Using Substitutent Effects

Absorption spectra for 2,3-diaryl-2,3-diazabicyclo[2.2.2]octane radical cations (2(X)(*+)) and for their monoaryl analogues 2-tert-butyl-3-aryl-2,3-diazabicyclo[2.2.2]octane radical cations (1(X)(*+)) having para chloro, bromo, iodo, cyano, phenyl, and nitro substituents are reported and compared with those for the previously reported 1- and 2(H)(*+) and 1- and 2(OMe)(*+). The calculated geometries and optical absorption spectra for 2(Cl)(*+) demonstrate that p-C6H4Cl lies between p-C6H4OMe and C6H5 in its ability to stabilize the lowest energy optical transition of the radical cation, which involves electron donation from the aryl groups toward the pi*(NN)(+)-centered singly occupied molecular orbital of 2(X)(*+). Resonance Raman spectral determination of the reorganization energy for their lowest energy transitions (lambda(v)(sym)) increase in the same order, having values of 1420, 5300, and 6000 cm(-1) for X = H, Cl, and OMe, respectively. A neighboring orbital analysis using Koopmans-based calculations of relative orbital energies indicates that the diabatic aryl pi-centered molecular orbital that interacts with the dinitrogen pi system lies closest in energy to the bonding pi(NN)-centered orbital and has an electronic coupling with it of about 9200 +/- 600 cm(-1), which does not vary regularly with electron donating power of the X substituent.

Photoinduced electron transfer in 2-tert-butyl-3-(anthracen-9-yl)-2,3-diazabicyclo[2.2.2]octane

Intramolecular photoinduced electron transfer from a hydrazine unit to an aromatic group is studied by resonance Raman spectroscopy and electronic absorption spectroscopy. Substituted hydrazine functional groups have played an important role in studies of electron-transfer reactions, photoinduced intramolecular electron transfer, and of mixed valence. A prototypical compound, 2-tert-butyl-3-(anthracen-9-yl)-2,3-diazabicyclo[2.2.2]octane, that has the hydrazine-to-anthracene charge-transfer band in a region of the visible spectrum suitable for detailed resonance Raman spectroscopy is studied in detail. Excitation profiles are obtained, calculated quantitatively by using time-dependent theoretical methods, and interpreted with the assistance of molecular orbital calculations. Excited-state distortions are calculated. The largest distortions occur on the hydrazine unit; the normal mode showing the largest distortion (659 cm(-1), calculated at 665 cm(-1)) involves an out-of-plane C-N-N-C bend consistent with removing an electron from the N-N pi antibonding orbital. Anthracene ring-centered C-C stretches also are enhanced, consistent with populating an antibonding pi orbital centered on the ring. Excellent fits to all of the excitation profiles and to the absorption band are obtained using one set of excited-state potential surfaces.

Synthesis, optical properties and modeling of silver core-silver oxide shell nanostructures in silica films

Sol-gel silica films containing silver ions were annealed in a rich-hydrogen atmosphere and subsequently in a rich-oxygen atmosphere. High resolution transmission electronic microscopy measurements shown unusual core-shell structures of silver-silver oxide nanoparticles in these films. The optical properties of these nanostructures exhibited red shift and damping in the UV-vis spectra. In order to calculate the optical properties were used the Mie theory using two approaches. The first one, corresponding to the core-shell model that includes a refractive index of the shell (ns) different to the one of the host matrix (nm). While in the second approach, we replaced in the Mie theory the refractive index of the environment (silica) by a local refractive index (nl) depending on the thickness of the silver oxide shell. Both calculations given the same results because they are equivalents.

Photoconductivity and Stabilization of Dopamine embedded in sol-gel TiO2 matrix

This work reports the synthesis of amorphous TiO2 matrix by sol-gel method at atmospheric conditions. DA was encapsulated in a TiO2 matrix to reduce its chemical instability. To TiO2/DA sample was added the 15C5 to diminish the oxidation process. The stabilization process was followed by absorption spectra, colour change and infrared spectroscopy. Oxidation processes of the DA were identified by the presence of DA-quinone and DA-chrome. The TiO2/DA complex retarded the oxidation process for 30 days, while the TiO2/DA/15C5 complex this period was extended for 47 days. Photoconductivity studies were performed on both kinds of samples to analyze their charge transports. The experimental data were fitted with straight lines at darkness and under illumination at 320 nm, 400 nm, and 515 nm. This indicates an ohmic behavior. Transport parameters were calculated. The conductive effect is stronger under darkness than under illumination at 320 nm because the oxidation process in the darkness is less intense than under illumination.

Eu3+ as optical probe of the structure in amorphous and nanocrystalline ZnO sol-gel films

Amorphous and nanocrystalline Eu³⁺-doped ZnO thin films were synthesized by the sol–gel process. The films were spin-coated on glass substrates. The samples were annealed at 500°C for 1 hour to produce a nanocrystalline ZnO/Eu³⁺ films. The samples were characterized using UV-Vis absorption and infrared spectroscopy. A crystalline phase, wurtzite, was detected by X-ray diffraction. A spectroscopic study of the Eu³⁺ impurity in function of the heat treatment provided to the ZnO matrix was done. Results of emission and excitation studies at room temperature of Eu³⁺ inserted in the ZnO matrix are presented. For amorphous and nanocrystalline samples, the relative ratio of the ⁷F₂/⁷F₁ transitions was calculated. The evolution of this ratio was interpreted in terms of the Eu³⁺ symmetry site change when the nanocrystalline ZnO films. This fact was confirmed measuring the Eu³⁺ lifetimes.

Photoconductivity studies on amorphous and crystalline TiO2 and TiO2:Eu3+ thin films synthesized by sol-gel

Amorphous and crystalline TiO2 and TiO2:Eu3+ thin films were synthesized by the sol–gel process at room temperature. The films were spin-coated on glass substrates. The samples were calcined at 400°C, 500°C for 2h to produce crystalline films. The films were characterized using X-ray diffraction, infrared spectroscopy, scanning electron microscopy and transmission electronic microscopy and UV-Vis absorption spectroscopy. An anatase phase was determined in the samples calcined at 500°C. Film thickness was calculated by SEM. Absorption peaks were located between 293-298 nm which are due to the titania host. It can be observed that this peak position depends of the calcination temperature. Band gap was calculated, and it indicates a non-linear behavior of the samples. Photoconductivity studies were performed on amorphous and crystalline films. The experimental data were fitted with curve lines to order square at darkness and under illumination (310 nm and 515 nm). This indicates a non-ohmic behavior. Transport parameters, photovoltaic and photoconductive; were calculated for both samples.

Optical and morphological characterization of TiO2 films doped with silver nanoparticles

Silver-doped titanium dioxide thin films were deposited on glass substrates by the sol-gel process. Undoped films and films doped with 1 mol% AgNO3 were annealed at 100°C for 30 minutes and sintered at 520°C for 1 hour. The optical and morphological properties of the films were analyzed by optical absorption spectroscopy, X-ray diffraction and scanning electron microscopy. Two crystalline phases, anatase and rutile were identified in the films by X-ray diffraction. An absorption band was located at 415 nm which is associated to the surface plasmon resonance of the silver nanoparticles. This spectrum was modelled using the Gans theory considering small silver nanoparticles with diameters of 3 nm and a refractive index lower than that coming from host matrix.

Solvent and ligand effects on the optical properties of silver nanoparticles in silica sol-gel films

Silver nanoparticles in sol-gel silica films have been synthesized by heat treatment in air atmosphere. We find that the surface plasmon resonance exhibits a principal peak at 534 nm, longer wavelength than that corresponding to the spherical silver nanoparticles in silica (400 nm). The anisotropy in the geometry of the metallic nanoparticles explains this noticeably red shift of the silver nanoparticles. The effect of solvents (ethanol, cyclohexane and toluene) and ligand (pyridine) on the optical properties of these nanoparticles are measured by UV-vis spectroscopy. The position of the surface plasmon resonance varies from 534 nm up to 573 nm depending on the refractive index or the concentration of the solvents. On the contrary, the surface plasmon resonance is gradually shifted to the blue from 534 nm up to 462 nm when the films were immersed in pyridine due to complexing on the surface of silver nanoparticles. These results show highest sensitivity of the surface plasmon to variations in the local environment of the nanoparticles and they suggest that the films can be used as colorimetric sensors.

Synthesis, Characterization and Photoconductivity studies on nanocrystalline TiO2 films doped with gold nanoparticles

Nanocrystalline TiO2 films doped with gold nanoparticles were synthesized by the sol-gel process at room temperature. The TiO2 films were synthesized by using tetrabutyl orthotitanate as the inorganic precursor. The films were spin-coated on glass wafers. The samples were annealed at 100°C for 30 minutes and sintered at 520°C for 1 hour to generated anatase and rutile phases. The films were characterized using UV-Vis absorption spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. An absorption peak located at around 651 nm is due to the surface plasmon resonance of the gold nanoparticles. Optical absorption spectrum was fitted by Gans model by using a high refractive index (nlocal = 2.6). This high index is related to the high content of anatase nanoparticles embedded in the film. Photoconductivity studies were performed on nanocrystalline (anatase phase) films. The experimental data were fitted with straight lines at darkness and under illumination at 515 nm and 645 nm. This indicates an ohmic behavior. Transport parameters were calculated. Results are discussed.