Theerapong Santhaveesuk

Enhancement of Ethanol Sensing Properties by Alloying

The Ti_xZn_1-xO tetrapods were synthesized using simple thermal oxidation method from Zn and TiO₂ mixture. The tetrapods exhibited single crystalline hexagonal wurtzite structure with the prefer growth direction of [0002] along the legs. The sensors based on Ti_xZn_1-xO tetrapods were fabricated and investigated the ethanol sensing properties. The FE-SEM, HRTEM, SAED, XRD, and RS results suggested that Ti_xZn_1-xO alloy was formed with a slightly decrease of c-axis lattice parameter. The decrease of sensor resistance under ethanol atmosphere was observed and suggested that the Ti_xZn_1-xO tetrapods possessed n-type property of semiconductor similar to ZnO. The Ti_xZn_1-xO tetrapod sensors exhibited higher sensitivity than that of pure ZnO tetrapod sensors for entire ethanol concentration with optimum operating temperature of 300°C. Thus, the enhancement of sensitivity due to alloying TiO₂ with the ZnO tetrapods was observed and maybe explained by an increase of adsorbed oxygen ions due to substitution of Ti atom into Zn atom. Also, the slope value of the plot between log(S-1) and log C suggested that adsorbed oxygen ion species at the surface of the Ti_xZn_1-xO tetrapods was O^2- which was same as pure ZnO tetrapods. Finally, these results have an important implication for a development of ethanol sensors based on metal oxide semiconductors for alcohol breath analyzer.

{\rm TiO}_{2}

Zn2TiO4 nanostructures were synthesized by the thermal oxidation method. Zn with 0, 10, 20, and 30 mol% TiO2 mixed powder were blended in polyvinyl alcohol and coated on an alumina substrate to form thick films. The thick films were heated at temperature of 600, 700, and 800°C under normal atmosphere for 24 hrs. FE-SEM images showed belt-liked nanostructures with the length of 0.3-30 µm, the width of 30-1800 µm, and the thickness ranging in the order of nm. Ti was incorporated into the nanostructures with ZnO to form Zinc titanate compound, indicated by EDS. Raman spectra and XRD results suggested that phase of Zinc titanate is cubic Zn2TiO4. The oxidation temperature and TiO2 content are critical to the phase quality of the nanostructures.

With ZnO Tetrapods

ZnO nanoparticles were successfully synthesized via the co-precipitation method using zinc nitrate and sodium hydroxide as raw materials. Size and shape of ZnO nanoparticles were well controlled by varying the ratio of sodium hydroxide solutions (0.5-0.9 mole) and the synthesized temperatures (65, 75 and 85 °C). ZnO nanoparticles exhibited a high degree crystallinity with wurtzite hexagonal structure for all conditions carried out using SEM, XRD, EDS and Raman. It was clearly observed that both sodium hydroxide solution and synthesized temperatures strongly affected on the size and shape of ZnO nanoparticles. The smallest ZnO nanoparticle was observed to be 47 nm with 0.7 mole of sodium hydroxide solution at 75 °C. Uniformed ZnO nanoparticles were obtained at synthesized temperatures above 65 °C. Optical properties of ZnO nanoparticles were also studied and carried out as absorbance spectra. In addition, optical energy band gap of ZnO nanoparticles was in the range of 3.24-3.35 eV.

Zn2TiO4 Nanostructures Prepared by Thermal Oxidation Method

ZnO nanoparticles were successfully synthesized by a low cost co-precipitation method using zinc nitrate and sodium hydroxide as the raw materials. It was observed that the synthesized temperatures greatly effect on the size of ZnO nanoparticles. The lower synthesized temperatures resulted in the smaller nanoparticles. By adjusting the mole ratio of sodium hydroxide, the size of ZnO nanoparticles was also changed. The smallest ZnO particles was 47 nm obtained with 0.7 mole of sodium hydroxide. The smallest ZnO nanoparticles from each synthesized temperatures were fabricated as humidity sensor, showing an impressive performance under different relative humidity (17-94% RH). It should be noticed that the ZnO nanoparticles humidity sensor synthesized at 75 °C exhibited high response for 2 times higher than that of synthesized at 95 °C. This is attributed to the higher surface area of ZnO nanoparticles for absorbed water molecule.

Size and Shape Tailoring of ZnO Nanoparticles

Zn 2 TiO 2 thick films were prepared by using thermal oxidation method at 1000°C for 24 h under normal atmosphere, for 0, 10, 20 and 30 mol% of TiO 2 . The thick films were characterized using XRD, FE-SEM, EDS, and RS. The results indicated that the thick films consisted of hexagonal wurtzite ZnO and face-center cubic Zn 2 TiO 4 phases, for 10–30 mol% of TiO 2 . The Zn 2 TiO 4 phase was increased as the increasing of the mol% of TiO 2 and became a major phase when TiO 2 reached 30 mol%, in distinction with ZnO. The optical energy band gap of thick films was measured by the help of reflection spectra. The optical energy band gap were ranging from about 3.30–3.58 eV, as the mol% of TiO 2 increased from 0–30.

High Performance Humidity Sensor Based on ZnO Nanoparticles Synthesized by Co-Precipitation Method

Spinel Zn<inf>2</inf>TiO<inf>4</inf> nanostructures were obtained by the thermal oxidation method. Raman spectroscopy was used to investigate the local compositions of the cubic spinel Zn<inf>2</inf>TiO<inf>4</inf> nanostructures. The Raman mode intensities of the spinel structure are present and correlate linearly with the TiO<inf>2</inf> concentration. These modes are interpreted as local vibrational modes. In addition, the analysis of the E<inf>2</inf>(H) and A<inf>1g</inf> modes are made and discussed in terms of lattice deformation associated with the distinct coordination preferences of Zn and Ti.