Ooi Kiang Tan

Sensing Mechanisms for Carbon Nanotube Based NH3 Gas Detection

There has been an argument on carbon nanotube (CNT) based gas detectors with a field-effect transistor (FET) geometry: do the response signals result from charge transfer between adsorbed gas molecules and the CNT channel and/or from the gas species induced Schottky barrier modulation at the CNT/metal contacts? To differentiate the sensing mechanisms, we employed three CNTFET structures, i.e., (1) the entire CNT channel and CNT/electrode contacts are accessible to NH(3) gas; (2) the CNT/electrode contacts are passivated with a Si(3)N(4) thin film, leaving the CNT channel open to the gas and, in contrast, (3) the CNT channel is covered with the film, while the contacts are open to the gas. We suggest that the Schottky barrier modulation at the contacts is the dominant mechanism from room temperature to 150 degrees C. At higher temperatures, the charge transfer process contributes to the response signals. There is a clear evidence that the adsorption of NH(3) on the CNT channel is facilitated by environmental oxygen.

Preparation and characterization of Pb(Zr0.52Ti0.48)O3 ceramics from high-energy ball milling powders

Abstract Ultra-fine Pb(Zr0.52Ti0.48)O3 powder has been synthesized from the commercial PbO, TiO2 and ZrO2 powders using the high-energy ball milling technique in air at room temperature without any post-annealing. The synthesized powders milling for different hours are characterized using XRD, SEM, TEM and Raman spectra. XRD patterns and Raman spectra show that the perovskite phase of PZT can be formed from the mixture of the starting materials after milling for 20 h. The grain sizes of the powders have been estimated from the XRD patterns and TEM images to be ∼10 nm. The relationship between the microstructure of the PZT powders and the milling hours has been discussed. The PZT ceramics are derived for the synthesized powders by sintering the green pellets at 1100°C for 1 h. The dielectric properties of the PZT samples are comparable with those of the same composition PZT prepared by other methods, but the high-energy ball milling process is much easier than other reported methods. The high-energy ball milling method offers a very attractive advantage of avoiding the loss of volatile elements, such as Pb, Bi, in the procedure of synthesis of nano-powders with their uniform compositions.

Size effect and gas sensing characteristics of nanocrystalline xSnO2-(1−x)α-Fe2O3 ethanol sensors

Abstract Non-equilibrium nanocrystalline x SnO 2 -(1− x )α-Fe 2 O 3 powders have been prepared using the mechanical alloying technique. The thick film screen printing technology is then employed to fabricate these ethanol gas sensors. Their particle size and structural properties are systematically characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The gas sensing characteristics are also measured. Based on the experimental results, we have observed that particle size of the powders is drastically milled down to about 10 nm after 24 h of high-energy milling. A very high gas sensitivity value of 845 for 1000 ppm of ethanol gas in air has been obtained. Our proposed new structural model for these non-equilibrium nanocrystalline x SnO 2 -(1− x )α-Fe 2 O 3 materials explains both the lattice expansion of these high energy mechanically alloyed powders as well as the charge neutrality in terms of additional oxygen dangling bonds at the nano-sized particle surfaces. It is those enormous oxygen-dangling bonds at the particle surfaces that give rise to the high gas sensitivity. The sensors are found to be 32.5 times more selective to the ethanol gas compared to CO and H 2 gases.

Preparation of Bi4Ti3O12 ceramics via a high-energy ball milling process

Abstract Nano-sized bismuth titanate (Bi4Ti3O12) powders were prepared by a high-energy ball milling process directly from their oxide mixture of Bi2O3 and TiO2. Bi4Ti3O12 phase can be formed from the oxide mixture after milling for 9 h. Almost single phase of Bi4Ti3O12 is obtained after milling for 15 h. With increasing milling time, the particle size of the mixture is gradually reduced. Bi4Ti3O12 ceramics were obtained by sintering the synthesized powders at temperatures ranging from 750°C to 950°C. The Bi4Ti3O12 ceramics sintered at 850°C for 1 h exhibited a density of 7.91 g/cm3, a dielectric of 243 and a dielectric loss of 0.017, and a remnant polarization of 24 μC/cm2 and a coercive field of 11 kV/cm, respectively. Piezoelectric parameters of the Bi4Ti3O12 ceramics are k33=56%, k31=58%. The Bi4Ti3O12 ceramics also possessed good pyroelectric properties. These results indicate that high-energy ball milling process is a promising way to prepare Bi4Ti3O12 ceramics.

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.

High sensitivity SnO2 single-nanorod sensors for the detection of H2 gas at low temperature

Uniform SnO(2) nanorods were grown by inductively coupled plasma-enhanced chemical vapor deposition without catalysts and additional heating. The SnO(2) nanorods were aligned on a pair of Au/Ti electrodes by the dielectrophoresis method. SnO(2) single-nanorod gas sensors were fabricated by connecting individual SnO(2) nanorods to a pair of Au/Ti electrodes with Pt stripes deposited by a focused ion beam. The sensing properties of the SnO(2) single-nanorod sensor were studied. The SnO(2) single-nanorod sensor could detect 100 ppm H(2) at room temperature with repeated response and showed a large change of resistance, fast response time and good reversibility at an elevated operating temperature of 200 degrees C. The optimal sensing performance of the sensor is achieved at the operating temperature of around 250 degrees C.

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.

Microstructure, dielectric properties and hydrogen gas sensitivity of sputtered amorphous Ba0.67Sr0.33TiO3 thin films

Abstract A novel metal-ferroelectric hydrogen gas sensing device was fabricated on platinum-coated silicon wafer with an amorphous ferroelectric Ba0.67Sr0.33TiO3 layer using the RF magnetron co-sputtering process and was characterized by X-ray diffraction, transmission electron microscopy, dielectric characterization and gas sensing measurement. Experimental results show that the microstructure and the dielectric properties are closely correlated with the deposition parameters. The studies on the dielectric properties indicate that the non-stoichiometric defects in the amorphous films are largely reduced by depositing in 50% oxygen content just below the crystallization temperature of the films. J–E performances exhibit the typical Schottky behavior, both in air and in hydrogen gas and a voltage shift 0.6 V has been observed in 1042 ppm hydrogen diluted in air. Compared to the sol-gel case, it is believed that the electronic defects, both in bulk and at interface, cause the degradation of the hydrogen gas sensitivity and weaken the induced H2 potential built-up across the space charge layer at the interface. The gas sensing mechanism based on the proton induced Pd/BST interfacial polarization potential is also discussed. Related to the MIS hydrogen sensor device, it is believed that the high permittivity of the amorphous ferroelectric thin films enhances the proton polarization at the Pd/BST interface and, in turn, greatly improves the built-up interfacial potential induced by the hydrogen.

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.