Direct observation of chemical reactions in silica‐coated gold and silver nanoparticles

Abstract

Communications reversible cis-trans isomerization of azobenzene occurs in A1P04-5, ZSM-5,andsilicalite-1. Itwas foundthat thesecomposites change their birefringence up to An' = 0.027 after irradiation. One can expect nanocompositesof organicmolecules andinorganic hostswithsuchsignificantphotosensitiverefractiveindices to be ofconsidcrableinterestforapplicationasadvancedphotoresponsivematerials.llxi ExperimentalLarge crystals of molecular sieve5 of the lUPAC structure types AFI and MFI have bccn uscd as host materials. The straight pore system of the AFI structure (AIP04-5) extends parallel to the crystal length axis. The MFI structures (ZSM-5, silicalite-1) form an anisotropic three-dimensional and bimodal channel network; straight channels arc cross-linked by sinnsoidal channels [19]. The organic guest molecule azobenzene (Kodak) was introduced into the molecular sieve pores by a gas-phase adsorption process. After calcination (AIP04-5 at 600°C. 6 h; ZSM-5 and silicalite-1 at 550"C, 4.5 h) and evacuation (< 10-' ton, 400°C). the activated molecular sieve crystals wcre exposed to the organic guest molecules under autogenous pressure at a temperature of 100 "C tor 4 h. This procedure leads to loading lcvels hctwecn 3 and 10 wt.-%, depending on the molecular sieves used.The spectra of the microcrystals were recorded with a microscope spectral photometer UMSP 80 (Carl Zeiss, Oberkochen, Germany) with quartz optics. The measurement Fields were chosen down to 5 km, depending on crystal size. The mcasuremcnts were performed in the region from 250 to 900 nm using a 75-W xenon lamp dispersed by a grating monochromator (A2 = 5 nm) and polarized by a Glan polarizer. The same source, restricted by a diaphragm according to the crystal size, was used for the illumination (with AA = 25 nm) of the microcrystals in the photochemical investigations. For the measurements of the birefringence of the composites, the length axis of molecular sieve crystals was placed at 45" to the linearly polarized incident light beam.The calculation of birefringence from the transmittance data in Figure 4 was done under the assumption that thc crystal acts as a zero-order retarder plate for 1. > 1. , , , , , .where E. , , , , , , is the wavelength of maximum transmittance occurring between 450 nni and 600 nm. Assumption of a higher order would lead to birefringence curves which increase monotonically with wavelength. This is very unlikely, since one expects normal dispersion outside the absorbance range. In addition, the zero-order assumption is supported by the measurement of the refractive index by the microcrystal prism method [20]. The crystals have the refractive indices n, = 1.538 ?c 0.006 and n,, = 1.485 & 0.005for 1 = 632 nm hcfore irradiation. This coincides with the n' values from the transmittance data under the zero-order assumption. Any other assumption leads to contradictory results.

Colloidal Strategies for Preparing Oxide‐Based Hybrid Nanocrystals

“…[29,30] In addition, these systems reveal shell-tuneable surface plasmon absorption and interesting nonlinear optical properties with a high laser damage threshold. [25,28] Another class of core@shell structures that have been obtained by this scheme is represented by oxide@metal HNCs, which normally combine ZnO or TiO 2 with Au, Ag, Pt or Cu. Typical preparations start from an alcohol suspension of weakly organic-passivated oxide NCs on which either chemical [31][32][33][34] or oxide-photocatalyzed reduction [35][36] of the desired metal precursors is accomplished.…”

“…Except for the case in which a harsh hydrothermal treatment is applied during oxide formation, all the aforemen-tioned routes typically yield amorphous coatings, which often exhibit discontinuities, nanoporosity and/or even inner cavities. Profiting from the chemical accessibility of their inner cores, these metal@oxide HNCs have been exploited as catalytically active nanoreactors, [29] as efficient electronic capacitors, [28] as well as sacrificial templates for the creation of hollow oxide capsules upon selective metal oxidation. [29,30] In addition, these systems reveal shell-tuneable surface plasmon absorption and interesting nonlinear optical properties with a high laser damage threshold.…”

Controllable Cobalt Oxide/Au Hierarchically Nanostructured Electrode for Nonenzymatic Glucose Sensing

“…Early hollow structures were typically made from amorphous materials like ceramics or polymeric substances where the sacrificial core was dissolved out chemically [87][88][89]. However, dissolvable template mediated approaches typically produce structures that are larger than 100 nm, making them undesirable for most biological applications where particles should be in the 20-100 nm size regime [72,74,80].…”

Synthesis and Applications of Hollow Particles

“…This kind of methods were characterized with the preparation of hollow particles by coating a template (such as polymer bead, emulsion and inorganic colloids) with a thin layer of the precursor through solgel process or layer-by-layer (LBL) self-assembly technique 22) and subsequent removal of the template via a thermal decomposition or a chemical dissolution 23) .…”