Weiping Cai

Photoluminescence of indium–oxide nanoparticles dispersed within pores of mesoporous silica

Photoluminescence study was performed on indium–oxide (INO) nanoparticles dispersed within pores of mesoporous silica, annealed at different temperatures (from 500 to 850 °C). It was found that, for the dispersed INO nanoparticles, there exists a broad luminescence band, consisting of three peaks at about 430, 480, and 520 nm, spanning the whole visible region, in contrast to the aggregates of INO nanoparticles which exhibit no luminescence. This band increases with rise of annealing temperature up to 650 °C, at which the band reaches maximum. When annealing temperature is 700 °C, the whole band redshifts and peaks at about 545 nm. Annealing at higher temperatures leads to additional redshift and decrease of the luminescence band. It has been shown that the luminescence in this study is associated with the size and the structure of the INO particles within pores of porous silica. The peaks at 430, 480, and 520 nm originate from amorphous INO particles about 2, 4, and 6 nm in diameter. The 545 nm peak is a...

Composition modulation of optical absorption in AgxAu1−x alloy nanocrystals in situ formed within pores of mesoporous silica

Nanoscaled AgxAu1−x (0.2<x<0.8) alloy particles were synthesized by putting gold and silver particles into pores of monolithic mesoporous silica by soaking and annealing method. It has been found that in optical absorption spectrum, the surface plasmon resonance (SPR) maximum of AgxAu1−x alloy particles can shift from about 524 nm (SPR maximum of Au nanoparticles) to about 400 nm (SPR maximum of Ag nanoparticles) with increasing concentration of Ag in AgxAu1−x alloy particle. The simulation of optical absorption spectrum of AgxAu1−x alloy particles was conducted based on Mie theory. The calculated values are found to be consistent with the experimental results.

Semiconducting optical properties of silver/silica mesoporous composite

The optical absorption properties were investigated for mesoporous silica loaded with different amounts (up to 5% in weight) of silver (Ag) nanoparticles (less than 4 nm in diameter) into its pores. In contrast to the previously reported glasses containing Ag particles, in this Ag particle-loaded silica no apparent surface plasmon resonance peak of Ag particle, other than an absorption edge, was observed. This composite exhibits the optical features of a semiconductor with direct band gap. The absorption edge shifts in the range from about 200 to about 700 nm with an increasing Ag loading amount up to 5% in weight, and hence, is controllable. Adsorption of oxygen in air on the surface of Ag particles within the pores, and scattering inside the porous structure were used to explain these results.

Synthesis and structure of indium oxide nanoparticles dispersed within pores of mesoporous silica

Abstract Nanosized indium oxide (In 2 O 3 ) particles, dispersed within pores of mesoporous silica, were synthesized by soaking and thermal decomposition of indium sulfate. The particles were characterized by transmission electron microscopy (TEM) and nitrogen sorption isotherms. Photoluminescence spectra of the samples annealed at different temperatures were measured. It was shown that the In 2 O 3 nanoparticles were isolated from each other and highly uniformly dispersed inside the pores of silica host, which were less than 8 nm in diameter. New photoluminescences were observed for the samples annealed at different temperatures.

Preparation and optical absorption of gold nanoparticles within pores of mesoporous silica

Abstract Nanoscale gold particles dispersed within pores of mesoporous silica were synthesized by soaking and thermal reduction of chloroauric acid (HAuCl 4 ) in H 2 atmosphere at 973 K for 1 h. This new material was characterized by X-ray diffraction, transmission electron microscopy, and Brunauer–Emmett–Teller (BET) techniques. It is shown that gold nanoparticles are isolated from each other and uniformly dispersed inside the pores of silica, which were less than 4 nm in diameter. It is found that in optical absorption spectra, the Mie resonance frequency of gold particles shows a significant red-shift with decreasing Au particle size. This red-shift phenomenon is qualitatively explained in terms of the interface interaction between the pore walls of porous silica and Au particles within the pores.

Controllable optical properties of Au/SiO2 nanocomposite induced by ultrasonic irradiation and thermal annealing

Nanostructured gold/monolithic mesoporous silica assembly was synthesized by a sonochemical method. The optical absorption maximum of the as-prepared nanocomposite varies from the visible to near-infrared region, depending on the reduction rate of AuCl4− ions in solution. There exists a critical reduction rate of AuCl4− ions in solution. When the reduction rate is lower than the critical value, the absorption maximum redshifts from 560 up to 1130 nm with increase of the reduction rate; otherwise, it blueshifts. The subsequent step thermal annealing of the as-prepared nanocomposite with a maximum in the near-infrared results in a blueshift in a large region down to 550 nm. We can thus realize control of the optical absorption in a large region by ultrasonic irradiation and subsequent step thermal annealing. Further experiments reveal that the position of the absorption maximum depends on the aggregate degree of Au nanoparticles supported in the surface layer of silica and that subsequent step annealing ind...

REVERSIBLE TRANSITION BETWEEN TRANSPARENCY AND OPACITY FOR THE POROUS SILICA HOST DISPERSED WITH SILVER NANOMETER PARTICLES WITHIN ITS PORES

The porous silica dispersed with silver nanometer particles (about 3 nm) within its pores (cages) has been prepared by a new method. In the process of alternative exposure to the ambient air and annealing, this material assumes reversible transition between transparency and opacity. This can be explained by oxidation of the outer part of the Ag particles during exposure to the ambient air and dissociation of the oxide during annealing in air.

Annealing-induced reversible change in optical absorption of Ag nanoparticles

Optical absorption of silver nanoparticles dispersed within pores of monolithic mesoporous silica was investigated after annealing in different atmospheres. It has been found that treatment in H2 or N2 induces a strong surface plasmon resonance (SPR) as usually expected, but air annealing results in the disappearance of the SPR, which is contrary to the fact that bulk metal silver is stable and not oxidized in air at above 200 °C. The absence and appearance of the SPR absorption can be controlled through alternate annealing in air and H2 (or N2). Thermodynamic analysis for oxidation of Ag nanoparticles, by additionally considering the surface and curvature effects of a particle, shows the existence of critical particle size. Only when a Ag particle size is smaller than the critical value is the oxidation possible in air at above 200 °C. The optical changes are attributed to the occurrence of the redox of the Ag particles in different atmospheres at a high temperature.

Reversible emission enhancement for Ce3+-doped silica nanoparticles dispersed within pores of mesoporous silica

Ce3+-doped silica nanoparticles dispersed within pores of mesoporous silica were synthesized by soaking and sol-gel technique. Liquid-like absorption and fluorescence have been observed. What is important is that the emission intensity increases with the exposure time to humid air and reaches saturation (which is about four times of the original value) at about 5 h. Subsequent drying leads to a decrease of the emission intensity to the original value. Reversible emission enhancement is obtained by alternately exposing and drying the sample. This is attributed to the formation of Ce3+ ion aqueous solution within pores of the sample during its exposure to air, due to the nonparticipation of Ce3+ ions in the silica network.

Optical absorption of ZnS nanocrystals inside pores of silica

The optical absorption of ZnS semiconductor nanocrystals inside pores of silica has been investigated. These ZnS nanoparticles were synthesized through sol–gel processing, followed by firing. X-ray diffraction and nitrogen adsorption/desorption results show that the ZnS nanocrystals are highly defective. Based on the analysis of x-ray and nitrogen adsorption–desorption results, the surface layers of ZnS nanocrystals can be regarded as being amorphous. It was found that, compared with that of the bulk ZnS, the optical absorption edge of ZnS nanoparticles shifts to a longer wavelength. We believe that the amorphous surface layers of ZnS nanocrystals can mainly be responsible for the observed redshift of the optical absorption edge.