Jinlian Hu

Violet photoluminescence from shell layer of Zn/ZnO core-shell nanoparticles induced by laser ablation

A strong violet photoluminescence (PL) band at 425nm (2.92eV) was observed from the ZnO shell layer of the Zn∕ZnO core-shell nanoparticles prepared by laser ablation in liquid media. Such violet PL decreases with increase of the shell thickness or annealing temperature, showing good controllability. Based on the electron paramagnetic resonance measurements, the violet emission is attributed to the electronic transition from the defect level, corresponding to high-concentration zinc interstitials, to the valence band. This study is in favor to clarify the defect-related emissions and to extend the optical and electronic applications of nanostructured ZnO.

Room temperature synthesized rutile TiO2 nanoparticles induced by laser ablation in liquid and their photocatalytic activity

TiO(2) nanoparticles were prepared by one-step pulsed laser ablation of a titanium target immersed in a poly-(vinylpyrrolidone) solution at room temperature. The products were systematically characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy and x-ray photoelectron spectroscopy (XPS). The results indicated that the rutile TiO(2) nanocrystalline particles were one-step synthesized at room temperature and the mean size in diameter is about 50 nm with a narrow size distribution. A probable formation process was proposed on the basis of the microstructure and the instantaneous plasma plume induced by the laser. Photocatalytic activity was monitored by degradation of a methylene blue solution. The as-prepared rutile TiO(2) nanoparticles demonstrate a good photocatalytic performance. This work shows that pulsed laser ablation in liquid media is a good method to synthesize some nanosized materials which are difficult to produce by other conventional methods.

Surface optical phonon Raman scattering in Zn∕ZnO core-shell structured nanoparticles

Surface optical (SO) phonon vibration mode predominant Raman scattering spectra were observed in the range of 545–565cm−1 in metal-semiconductor Zn∕ZnO core-shell structured nanoparticles, prepared by laser ablation of a zinc target in a surfactant aqueous solution. The SO phonon mode exhibits significant size confinement effect. Such SO dominant Raman scattering is attributed to the existence of a large number of disorderly arranged areas among ultrafine ZnO grains formed under the extreme condition of laser ablation. This study demonstrates that the properties of surface phonons can availably reflect some important physical information.

In situ x-ray diffraction study of the thermal expansion of silver nanoparticles in ambient air and vacuum

The thermal expansion behavior of silver nanoparticles in ambient air and vacuum was studied by in situ x-ray diffraction (XRD) measurement of the particles dispersed within mesoporous silica in the temperature range of 25–700 °C. It has been shown that thermal expansion coefficient of Ag nanoparticles in vacuum is about 0.6×10−5∕°C, only near one fourth that of bulk silver (2.2×10−5∕°C). However, the coefficient in air is about 1.7×10−5∕°C, about 3 times as high as that in vacuum and close to the value of bulk Ag. These were explained in terms of Ag particles’ surface energy, oxygen surface adsorption, and dissolution into lattice. This study is of importance in architectonics of future nanodevices.

Evolution of the optical spectra of an Ag/mesoporous SiO2 nanostructure heat-treated in air and H2 atmospheres

Evolution of the various optical spectra of Ag nanoparticles dispersed in mesoporous SiO2 after heating alternately in H2 and air atmospheres has been investigated. It has been observed that, in accordance with alternate heat treatment in H2 and air, surface plasmon resonance (SPR) absorption of the Ag/mesoporous SiO2 nanostructure alternately appeared and disappeared and its Raman scattering spectra reversibly increased and decreased. In contrast, its photoluminescence (PL) spectra alternately disappeared and appeared, whereas the infrared (IR) spectra did not change. Such novel optical sensitivities of silver nanoparticles are discussed on the basis of the changes in chemical states of silver induced by the interaction of silver nanoparticles with the atmospheres and the hosting medium. It is suggested that the chemical state of silver in mesoporous SiO2 changes reversibly between Ag and the Ag+ ion by the redox process.

Substrate dependent surface plasmon resonance evolution of Ag nanoparticles treated in atmospheres

The evolution of surface plasmon resonance (SPR) with alternate air and H2 treatment was investigated for silver nanoparticles on different SiO2-based substrates (soda-lime glass, quartz glass and mesoporous SiO2). It has been shown that evolution behaviours on the three substrates are very different. For silver nanoparticles on the mesoporous SiO2 substrate, alternate air and H2 heat treatment led to the alternate disappearance and appearance of the SPR, showing the reversibility of optical changes. For a soda-lime glass substrate, the SPR absorption also disappeared after the first run heat treatment in air and re-appeared after subsequent H2 heat treatment, but the second run heat treatment in air could not induce the re-disappearance of the SPR. After alternate heating of the sample in air and H2 for four or more cycles, the SPR stayed almost unchanged. For a quartz glass substrate, however, the SPR absorption became negligible after alternate air and H2 heat treatment. Such effects of the substrate on the SPR are explained in terms of the subtle difference of the three SiO2-based substrates in micro-structure or chemical composition, and the redox of silver in the treatment atmospheres. This study could be important for the design and architectonics of new optical devices based on SiO2 materials and could also promote the understanding of the interaction of silver nanoparticles with the environment.

Oxygen-induced enhancement of surface plasmon resonance of silver nanoparticles for silver-coated soda-lime glass

The optical properties of a silver film on soda-lime glass substrate were studied after treatment in different atmospheres at different temperatures. It has been shown that pre-treatment in air (at about 500 °C for 1 h) can induce the enhancement of the surface plasmon resonance (SPR) of silver nanoparticles after subsequent treatment in H2. This enhancement effect decreases with decrease in the pre-treatment temperature in air up to 500 °C. Alternately heat-treating the as-prepared sample in air and then H2 results in a continuous increase of the SPR. If the treatment temperature is at or above 600 °C, the SPR absorption can drastically be increased only by treatment in air without subsequent treatment in H2. We can thus control the SPR intensity of the sample. A further experiment has revealed that the SPR is from the silver nanoparticles formed in the sublayer of the glass substrate. Pre-treatment in air is crucial to the formation of silver nanoparticles in the sublayer after subsequent treatment in H2, which enhances the SPR.

Design and fabrication of ZnO/Ni heterogeneous binary arrays with selective control of structure, size and distance via stepwise colloidal lithography

A strategy is proposed for the synthesis of heterogeneous/homogeneous binary arrays via stepwise colloidal lithography, using ZnO and Ni as model materials. A series of controllable and specific binary arrays like pore/pillar, pore/pore, ring/triangle nanoparticle and ring/pore were designed and then fabricated. The periodicity of the binary array can be tuned by the size of the polystyrene spheres, the size and structure of each unit in the array can be selectively controlled by proper heat treatment, and the distance between the adjacent units of two arrays can be manipulated by the incidence angle of metal vapor flow. This study gives an effective guide to designing desired patterns of heterogeneous/homogeneous binary arrays and accurately determining their size and structure. Furthermore, this strategy allows two materials with different properties, such as magnetic, photonic, catalytic and semiconducting, to be assembled in one array with tunable arrangements, with potential applications in multi-function nanodevices.