Man Siu Tse

Semiconductor gas sensor based on tin oxide nanorods prepared by plasma-enhanced chemical vapor deposition with postplasma treatment

SnO2 thin films were deposited by radio-frequency inductively coupled plasma-enhanced chemical vapor deposition. Postplasma treatments were used to modify the microstructure of the as-deposited SnO2 thin films. Uniform nanorods with dimension of ∅7×100nm were observed in the plasma-treated films. After plasma treatments, the optimal operating temperature of the plasma-treated SnO2 thin films decreased by 80 °C, while the gas sensitivity increased eightfold. The enhanced gas sensing properties of the plasma-treated SnO2 thin film were believed to result from the large surface-to-volume ratio of the nanorods’ tiny grain size in the scale comparable to the space-charge length and its unique microstructure of SnO2 nanorods rooted in SnO2 thin films.

Facile fabrication and characterization of multi-type carbon-doped TiO2 for visible light-activated photocatalytic mineralization of gaseous toluene

Carbon-doped titanium dioxide (C-TiO2) nanoparticles were synthesized by conventional mild oxidation of precursor titanium carbide (TiC) at 350 °C for 2 to 50 hours and more aggressive oxidation at the higher temperature of 400 to 600 °C for 2 hours in air. XRD and TEM studies indicated the formation of nano-sized C-TiO2 with mixed anatase–rutile phases. With prolonged oxidation time or increase in oxidation temperature, an initial decrease in crystallite size was unveiled due to cracking of TiC grains, renucleation of TiO2 and diffusion of carbon atoms. Raman, FTIR and XPS measurements revealed the presence of graphite-like carbon and the coexistence of substitutional and interstitial carbon in the TiO2 lattice. This multi-type carbon doping either served as a photosensitizer or resulted in additional electronic states above the valence band of the TiO2 lattice, directly responsible for the red shift of the absorption edge in the UV-vis absorption spectrum. The band structure and possible visible light photocatalytic mechanism of the C-TiO2 were thus elucidated. The synthesized C-TiO2 nanoparticles demonstrated improved photocatalytic performance for the mineralization of gaseous toluene in comparison to commercial P25 TiO2 under visible light irradiation. The C-TiO2 nanoparticles prepared at higher oxidation temperature with shorter time exhibited a more pronounced enhancement than those prepared by the mild oxidation process, providing a facile method for large-scale production of C-TiO2 suitable for indoor photocatalytic applications.

Hydrothermal Growth of TiO2 Nanorod Arrays and In Situ Conversion to Nanotube Arrays for Highly Efficient Quantum Dot-Sensitized Solar Cells

TiO2 nanorod (NR) and nanotube (NT) arrays grown on transparent conductive substrates are attractive electrode for solar cells. In this paper, TiO2 NR arrays are hydrothermally grown on FTO substrate, and are in situ converted into NT arrays by hydrothermally etching. The TiO2 NR arrays are reported as single crystalline, but the TiO2 NR arrays are demonstrated to be polycrystalline with a bundle of 2-5 nm single crystalline nanocolumns grown along [001] throughout the whole NR from bottom to top. TiO2 NRs can be converted to NTs by hydrothermal selective etching of the (001) core and remaining the inert sidewall of (110) face. A growth mechanism of the NR and NT arrays is proposed. Quantum dot-sensitized solar cells (QDSCs) are fabricated by coating CdSe QDs on to the TiO2 arrays. After conversion from NRs to NTs, more QDs can be filled in the NTs and the energy conversion efficiency of the QDSCs almost double.

Pt surface modification of SnO2 nanorod arrays for CO and H2 sensors

Uniform SnO(2) nanorod arrays were deposited on a 4 inch SiO(2)/Si wafer by plasma-enhanced chemical vapor deposition (PEVCD) at low deposition temperature of around 300 degrees C. The SnO(2) nanorods were connected at the roots, thus the nanorod sensors could be fabricated by a feasible way compatible with microelectronic processes. The surface of the sensors was modified by Pt nanoparticles deposited by dip coating and sputtering, respectively. The sensing properties of the Pt-modified SnO(2) nanorod sensors to CO and H(2) gases were comparatively studied. After surface modification of Pt, the sensing response to CO and H(2) gases increased dramatically. The 2 nm Pt-modified SnO(2) nanorod sensors by sputtering showed the best sensing performance. By increasing Pt thickness from 2 nm up to 20 nm, the optimal working temperature decreased by 30 degrees C while the sensing response also decreased by about 4 times. Comparing these two Pt modification approaches by dip coating and sputtering, both could achieve comparable promotion effect if the Pt thickness can be controlled around its optimal value. The deposition technique of SnO(2) nanorod arrays by PECVD has good potential for scale-up and the fabrication process of nanorod sensors possesses simplicity and good compatibility with contemporary microelectronics-based technology.

Visualizing charge transport in silicon nanocrystals embedded in SiO2 films with electrostatic force microscopy

In this work, we report a mapping of charge transport in silicon nanocrystals (nc-Si) embedded in SiO2 dielectric films with electrostatic force microscopy. The charge diffusion from chargednc-Si to neighboring uncharged nc-Si in the SiO2 matrix is found to be the dominant mechanism for the decay of the trapped charge in the nc-Si. The trapped charge and the charge decay have been determined quantitatively from the electrical force measurement. An increase in the area of the charge cloud due to the charge diffusion has been observed clearly. In addition, the blockage and acceleration of charge diffusion by the neighboring charges with the same and opposite charge signs (i.e., positive or negative), respectively, have been observed.

TiO2 rutile–anatase core–shell nanorod and nanotube arrays for photocatalytic applications

Rutile and anatase mixed TiO2 is a promising material for photocatalytic applications. In this work, rutile–anatase core–shell nanotube (NT) and nanorod (NR) structures were fabricated directly on a fluorine doped SnO2 (FTO) glass substrate using a hydrothermal growing/etching and TiCl4 post-treatment method. A pure rutile phase NT/NR core was coated with a roughened anatase shell. The nanostructures were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV-vis reflectance and transmittance spectra. The photocatalytic capability was measured by investigating the speed of methyl blue degradation UV. The core–shell NT structure showed superior photocatalytic properties due to larger light absorbing areas, higher light trapping capability and enhanced charge separation efficiency.

m-BiVO4@γ-Bi2O3 core–shell p–n heterogeneous nanostructure for enhanced visible-light photocatalytic performance

In this paper, γ-Bi2O3, considered as the best photocatalyst among all Bi2O3 polymorphs, was successfully prepared on the surface of m-BiVO4 octahedral crystals through an alkaline “etching” process. Extensive XRD, SEM and TEM characterization revealed the formation of a p–n junction in the form of m-BiVO4@γ-Bi2O3 core–shell heterostructure. In addition, the alkaline concentration and reaction time during the etching process were studied and found to be critical parameters in the formation and yield of the Bi2O3 phase. The visible-light photocatalytic activities of these heterogeneous samples with different γ-Bi2O3/m-BiVO4 phase ratios were evaluated for the degradation of Rhodamine B (RhB). The results indicated that with an optimum amount of γ-Bi2O3 on the m-BiVO4 surface, the powders showed superior photocatalytic performance over pure m-BiVO4 octahedral crystals. The enhancement mechanisms were discussed based on the specific surface area and γ-Bi2O3 shell thickness, as well as the influences of improved charge carrier transfer on the p–n heterostructure.

Charging Effect on Electrical Characteristics of MOS Structures with Si Nanocrystal Distribution in Gate Oxide

We report a study of influence of charging and discharging in Si nanocrystals (nc-Si), which are embedded throughout the gate oxide in metal-oxide-semiconductor (MOS) structures, on the current-voltage and capacitance-voltage characteristics of the MOS structures. Very large current and capacitance are observed for the as-fabricated structures. However, charge trapping in the nanocrystals can reduce both the current and the capacitance dramatically. The trapped charges can also tunnel out from the nc-Si, leading to the recovery of both the current and the capacitance. The current reduction is attributed to the breaking of the nc-Si tunneling paths due to charge trapping in the nc-Si, while capacitance reduction is explained by an equivalent circuit in terms of the change of the nc-Si capacitance as a result of the charge trapping.

Dark ambient degradation of Bisphenol A and Acid Orange 8 as organic pollutants by perovskite SrFeO3−δ metal oxide

Current advanced oxidation processes (AOPs) are chemically and energetically intensive processes, which are undesirable for cost-effective and large-scale system water treatment and wastewater recycling. This study explored the Strontium Ferrite (SFO) metal oxide on the degradation of highly concentrated organic pollutants under dark ambient condition without any external stimulants. The SFO particles with single perovskite structure were successfully synthesized with a combined high temperature and high-energy ball milling process. An endocrine disruptor, Bisphenol A (BPA) and an azo dye, Acid Orange 8 (AO8) were used as probe organic pollutants. BPA was completely degraded with 83% of mineralization in 24 h while rapid decoloration of AO8 was achieved in 60 min and complete breakdown into primary intermediates and aliphatic acids occurred in 24 h under the treatment of dispersed SFO metal oxide in water. Such efficient degradation could be attributed to the enhanced adsorption of these anionic pollutants on positively charged ball-milled SFO metal oxide surface, resulted in higher degradation activity. Preliminary degradation mechanisms of BPA and AO8 under the action of SFO metal oxide were proposed. These results showed that the SFO metal oxide could be an efficient alternative material as novel advanced oxidation technology for low cost water treatment.

Snapback behavior of the postbreakdown I–V characteristics in ultrathin SiO2 films

With the I–V measurement technique that forced a current to an ultrathin gate oxide and measured the voltage drop, a snapback phenomenon, i.e., the gate oxide was switched from a higher-impedance state to a lower-impedance state suddenly, was observed during the postbreakdown I–V measurement. The snapback could be triggered at a very low measurement current. Single or multiple snapbacks have been observed, and it was found that the occurrence of snapback was a random event. The snapback is explained in terms of the formation of an additional percolation path due to the neutralization of negatively charged traps or the generation of neutral electron traps at certain strategic positions during the measurement.