Duangmanee Wongratanaphisan

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

We present here the results of the Raman study of lattice dynamics and local compositional fluctuations in cubic spinel Zn2TiO4 nanostructures. The observed Raman modes of the spinel structure are identified and attributed to local vibrational modes. Additionally, the analysis of the nonpolar E2(H) (∼437 cm−1, 54.2 meV) and A 1g (∼716 cm−1, 88.7 meV) modes are made and contributed to lattice deformation associated with the distinct coordination preferences of Zn and Ti.

Zn2TiO4 Nanostructures Prepared by Thermal Oxidation Method

Surface modification of porous ZnO photoelectrode using one- and two-step etching process is investigated for enhancing power conversion efficiency of ZnO dye-sensitized solar cells. ZnO films are modified by the diluted NH4OH solutions for one-step etching process and used as photoelectrode of dye-sensitized solar cells. Rough porous films are observed after one-step etching process. The fabricated cells based on the optimized one-step etched films show a significant increase in short-circuit current density. The short-circuit current density is directly changed with amount of dye adsorption, which is related to specific surface area. The etched films exhibit higher specific surface area over two times than nonetched films. Thus, the large specific surface area is the key success for increasing amount of dye adsorption. Internal electrochemical property of fabricated cells is also improved, indicating that chemical surface of ZnO films is modified in the same time. The DSSCs fabricated on two-step etched films with NH4OH and mixed acid HCl : HNO3 show the maximum power conversion efficiency of 2.26%. Moreover, fill factor is also increased due to better redox process because of the formation of fine porous structure during the etching process. Therefore, these results implied that the roles of etching processes are improving specific surface area and fine porous formation which can provide better dye adsorption and redox process for dye-sensitized solar cell application.

Raman Scattering of Internal Dynamics in Spinel Zn2TiO4 Nanostructures

ZnO and ZnO:Al nanostructure were synthesized and fabricated as ethanol gas sensors. For FE-SEM images, the diameter and length measured at the middle of the wire-like structure were in range of 100-500 nm and several micrometers, respectively. From TEM analysis, it was suggested that the ZnO:Al nanostructure grew along (112̅0) direction on [0001] plane. The Raman spectra of ZnO and ZnO:Al nanostructures can confirm existence of defect effects due to oxygen vacancies and Zn interstitials of ZnO. Besides, it also suggested that the ZnO:Al nanostructures had (112̅0) direction perpendicular to the surface. The ethanol sensors based on ZnO:Al nanostructure sensors can be improved when compare with pure ZnO nanostructure sensor at the ethanol concentrations of 50-1000 ppm. The highest sensitivity of 32 was obtained in ZnO:Al nanostructure sensors with Al 1% by mol compared to 14 of pure ZnO nanostructure sensor at optimum temperature of 300°C. The sensitivity improvement of ZnO:Al sensors can be explained by an increase of oxygen vacancy-related defects which increase the surface depletion layer width as described in sensitivity equation. The larger surface depletion layer width results in higher the potential barrier height at the contacts and finally, sensitivity improvement.

Surface Modification of Porous Photoelectrode Using Etching Process for Efficiency Enhancement of ZnO Dye-Sensitized Solar Cells

ABSTRACT Influence of surface modification with D205 dye on charge dynamics of hybrid solar cells based on ZnO nanorods and poly(3-hexylthiophene) or P3HT was investigated. Electrochemical impedance spectroscopy analysis reveals that ZnO nanorods/D205/P3HT device shows longer effective lifetime (less charge recombination) in comparison with ZnO nanorods/P3HT one. Moreover, transit time of electrons travelling to the electrode for the D205-modified device is shorter than that of the unmodified one. These results indicate that the adsorption of D205 dye promotes efficient charge collection, leading to the improvement in fill factor. The surface modification efficiently enhances charge dynamics of hybrid ZnO/P3HT devices.

Ethanol sensing characteristics of sensors based on ZnO:Al nanostructures prepared by thermal oxidation

For applications in the field of optoelectronics, zinc oxide (ZnO) based transparent conducting oxide thin films such as aluminium-doped zinc oxide (AZO) have recently received much attention. It is one of the most promising alternative materials to the widely used indium tin oxide (ITO or tin-doped indium oxide) and fluorine-doped tin oxide (FTO). In this work, the ZnO and AZO base thin films were prepared by radio frequency (rf) magnetron sputtering technique using ZnO and AZO (1at%Al) ceramic target. The Ga interlayer films, added in between base thin films (ZnO and AZO), were deposited by evaporation technique. Both ZnO/Ga/ZnO and AZO/Ga/AZO multilayer thin film structures were grown on glass substrates. Then the obtained multilayer structures were annealed in argon (Ar) ambient at 400°C for 1 hr. The morphology, qualitative and quantitative elemental analysis, crystal structure, electrical properties, and optical properties of the multilayer thin films were characterized by field emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), a four-point probe method and UV-Vis spectroscopy. The results revealed that after annealing treatment, the average transmittance of the both multilayer thin films in visible region was improved due to a decrease in surface roughness resulting in an increase in crystalline surface. Furthermore, the results revealed lower sheet resistance in the AZO/Ga/AZO relative to pure ZnO multilayer base thin films. The significance of the electrical properties of the AZO/Ga/AZO films can be achieved due to local structure of Al site in ZnO and partial diffusion of Ga atoms into the base thin film layers. This indicates that the Ga interlayer in both ZnO/Ga/ZnO and AZO/Ga/AZO multilayer thin films can be used to further improve electrical conductivity without degrading optical transmission.

Influence of surface modification with D205 dye on charge dynamics of hybrid ZnO nanorods/polymer solar cells

ZnO dye-sensitized solar cells (ZnO DSSCs) with different thickness of NiO thin films coated in photo-electrode and counter-electrode were investigated. NiO thin films were prepared by thermal evaporation of NiO onto FTO glass substrate. The films were characterized by FE-SEM. For the photo-electrode, NiO thin films were coated on ZnO with 0.2, 0.6, 1.1 and 2.2 mg to form a barrier layer. For the counter-electrode, NiO thin films were coated on FTO glass with 5.4, 10.8, 16.2 and 21.6 mg in order to increase a surface-to-volume ratio. The photoconversion efficiency of ZnO DSSCs was measured under illumination of stimulated sunlight obtained from solar simulator with the radiant power of 100 mW/cm2. It was found that ZnO DSSCs coated with 0.6 mg NiO in photo-electrode and 10.8 mg in counter-electrode exhibited the highest photoconversion efficiency of 1.00% and 0.92%, respectively. The enhancement of photoconversion efficiency with NiO coating maybe explained by decreasing of charge recombination in photo-electrode and increasing of active surface area in counter-electrode.

Effect of Gallium Interlayer in ZnO and Al-doped ZnO Thin Films

Zinc titanate nanostructures were prepared by oxidation reaction technique. Here Zn mixed with 0, 10, 20 and 30 mol% of TiO2 powder was screened on alumina and FTO substrate, and then sintered at 400–600°C for 12 h under normal atmosphere. Through a detailed field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD) indicated that the nanostructures exhibited belt-like shapes of Zn2TiO4 phase. Moreover, the belt-like nanostructures of the synthesized Zn2TiO4 were studied in terms of optical properties by the UV-vis spectroscopy (UV-vis) to obtain band gap energy (Eg). The results showed that the Eg gap energy ranged from 3.57 eV to 3.63 eV as the mol% of TiO2 increased. In addition to the characterization of the nanobelts, the synthesized Zn2TiO4 nanostructures were applied as a bilayer semiconductor electrode in ZnO-based dye-sensitized solar cell (DSSC). It was shown that the use of Zn2TiO4 nanostructure with 10 mol% TiO2 in the ZnO/Zn2TiO4 electrode components of the DSSC revealed the highest obtainable efficiency.

Effect of Nickel Oxide Thin Films on Photoconversion Efficiency in Zinc Oxide Dye-Sensitized Solar Cells

Zinc oxide tetrapods (T-ZnO) were synthesized using thermal oxidation technique from Zn powders mixed with hydrogen per oxide (H2O2). Through a detailed field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD) showed that the T-ZnO exhibited single crystalline hexagonal wurtzite structure. The leg tip of the T-ZnO was about 8.17±1.17 µm in length and 47.80 nm in diameter. The ethanol sensors, based on the T-ZnO and the T-ZnO doped with Au nanoparticles (Au/T-ZnO), were fabricated and investigated for the ethanol sensing properties. The ethanol sensor response of the T-ZnO and the Au/T-ZnO sensors was tested at the operating temperature of 260-360°C with the ethanol concentration of 50, 100, 500, and 1000 ppm. The results showed that the Au/T-ZnO sensors exhibited exceptionally higher sensitivity than the pure T-ZnO sensors for entire ethanol concentration with optimum temperature of 340°C and 320°C, respectively. This enhancement can be explained in terms of the electron concentration of sensor in air, n0 and the reaction rate constant, kEth between the adsorbed oxygen species and the ethanol vapor due to the increase of effective surface for adsorption of ethanol on the surface. With an excellent catalytic ability, the Au nanoparticles doping on the T-ZnO sensors would result in higher reaction rate constant than the undoped T-ZnO sensors.