Chapter 8 - Energy landscapes of pure and doped ZnO: from bulk crystals to nanostructures

Boron doped zinc oxide (ZnO) nanotubes have successfully been grown using seed-mediated hydrothermal method at various concentrations of precursor-surfactant solution. The growth of ZnO nanotubes was carried out at a temperature of 90°C for 8 hours and a drop in temperature to 50°C for 16 hours. In this study, the effect of concentration ratio of precursor-surfactant was evaluated. Samples were characterized using UV-Vis Spectroscopy, X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray (EDX). UV-Vis spectra of the samples showed that the ZnO nanotubes were grown on the Flourine Tin Oxide (FTO) and the strong absorption occurred in the wavelength range of 300-400 nm, which is typical spectrum for hexagonal-nanostructure. XRD patterns showed five diffraction peaks at 2θ = 32.05°, 34.5°, 36.35°, 47.75° and 56,9°. The analysis of EVA Diffrac Plus confirmed the peaks were represented to the lattice of (100), (002), (101), (102), and (110) respectively. FESEM images of the samples showed the hexagonal-shaped ZnO nanotubes with an average diameter of 50-340 nm and an average thickness of 1.2 - 4.4 μm observed for all samples. Uniformity the size and shape of ZnO nanotubes become higher as concentrations of ZNH decreased. The EDX spectra of the samples showed the percentage of weight of Zn, O, and B was 69.35%, 21.60% and 4.76% respectively, while the percentage of their atoms was 33, 06%, 42.08% and 13.71% respectively. The B-doped ZnO nanotubes solar cells were fabricated by arranging a sandwich structure, consisting of the FTO, ZnO nanotubes, dye, electrolyte and platinum thin film. I-V characteristics of cell were carried out under irradiation of 100 mWcm<sup>-2</sup> halogen lamp. The I-V curves produced the highest efficiency from the cells utilizing the B-doped ZnO nanotubes with their concentration ratio of precursor-surfactant of 0.1 M : 0.04 M as the active material, which was 0.352%. This value is much higher than that of pure ZnO nanotube based DSSC of 0.05%.

Zinc oxide (ZnO) nanostructures were successfully prepared by a simple, highly efficient, and low-cost using the hydrothermal method. A superhydrophobic surface with a static water contact angle (CA) >150° has been synthesized by modifying ZnO nanostructures with 100°C at 1 h stable oleic acid (OA) as coupling agents, in order to modify their surface properties and make them more hydrophobic. Surface modification of ZnO nanostructures has been performed, and the effect of the modification on the structure and morphological properties were investigated. The resulting nanostructures were characterized by XRD, FESEM, UV-VIS spectroscopy. XRD pattern revealed that ZnO nanostructures prepared by hydrothermal method (crystallite size ~30 nm) have hexagonal wurtzite structure with a good crystalline quality. FESEM images of ZnO nanostructures prepared by hydrothermal showed hexagonal nanorods assembled in flower-like shape, there was much change in the surface morphology of modified samples after surface modification such as (nanorods, nanoflowers, and nanotube). Results show the water CA of ZnO superhydrophobic surfaces increased steadily from 147±2° to 154±2° when the OA weight percentage increased from 2mg to 10mg. The optical measurements for ZnO nanostructures showed that all samples the absorption band in the ultraviolet region. The band gap of pure ZnO nanostructures 3.5 eV and after modification ZnO surface 3.6 eV. All samples of ZnO were maintained at room temperature for 1 hour to 5 months to test the stability of the surface. The water CAs were measured for each condition, and very little change was observed in the CAs. In addition, the ZnO surface remained superhydrophobic without any contamination observed after water was sprayed on it.

Numerous zinc oxide (ZnO) nanomaterials, with unique physical and chemical properties, have recently been synthesised by various methods. In this study, a hydrothermal method was developed to synthesise ZnO nanomaterials with different morphologies on indium doped tin oxide (ITO), and the growth mechanism was discussed. The structure and morphologies of the synthesised samples were characterised by X-ray diffraction and SEM. The photoluminescence (PL) and field emission characteristics of ZnO nanomaterials with different morphologies were measured. X-ray diffraction and SEM images indicate that ZnO with morphologies of nanocones, nanorods and nanotubes is grown along c axis to the ITO substrates. The room temperature PL spectra reveal a strong and sharp ultraviolet emission band at 386 nm and a weak blue emission band at 470 nm. The field emission measurements show that the turn-on field of ZnO nanocones, nanorods and nanotubes at emission current density of 10 μA cm−2 is approximately 2·62, 4·31 and 3·92 V μm−1. Moreover, the emission image of ZnO nanocones is more homogeneous than that of ZnO nanorods and nanotubes at the electric field of 4·5 V μm−1. The experimental results indicate that ZnO nanomaterials with different morphologies have good photoelectric properties. As cold cathode materials, ZnO has a great number of potential applications for the field emission display in the future.

In this study, zinc oxide (ZnO) nanotubes were simulated and calculated to get their bandgap and density of states to predict their performance as anode materials for lithium-ion batteries. With the insight of reducing the band gap by nanomaterialisation to improve the performance, ZnO nanotubes were fabricated with hydrothermal reaction. We tested its performance after 100 cycles and confirmed that nanotubes are better than bulk ZnO in many regards, including electron conductivity, bandgap, coulomb efficiency, cyclic stability. An excellent reversible capacity of 861 mAh g-1 was achieved after 100 cycles at 0.5 mA g-1. Compared with ZnO bulk, nanotubes show better stability and higher coulomb efficiency.

In this work, pure and manganese (Mn) doped zinc oxide (ZnO) nanocrystallites are synthesized using a sol-gel technique. 0.25 M solution of zinc nitrate hexahydrate is prepared in 50 ml of DI water with stirring condition. An equimolar citric acid (0.25 M) solution is added slowly into the above solution and stirred for 2 hrs. at 70oC. The obtained gel is dried for 3 hrs in hot air oven at 120°C. Further, the nanoparticles are annealed at 400°C and the samples are characterized by X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, photoluminescence spectroscopy (PL) and photo catalytic studies. XRD analysis deciphered the polycrystalline hexagonal of the samples and the crystallites sizes are observed to be 18 nm and 42 nm for the pure and Mn doped ZnO particles, respectively. FE-SEM studies demonstrate that the crystallites are spherical in shape with agglomeration. PL studies reveal the emission bands at 490 nm for pure ZnO and 530 nm for Mn doped ZnO. The photocatalytic studies determine the photocatalytic performance of pure ZnO NPs and Mn doped ZnO NPs under the UV light irradiation (365 nm and 125 W) in which, the pure ZnO degrades MB dye more efficiently than Mn doped ZnO.

The nonlinear optical (NLO) properties of pure and doped zinc oxide (ZnO) thin films, including those doped with tin (TZO), aluminum (AZO), and both aluminum and tin (ATZO), were studied using the Z-scan technique. These films were prepared using the spray pyrolysis technique. The results demonstrate that doping ZnO with aluminum (Al), tin (Sn), or both significantly influences the material's nonlinear absorption coefficient and nonlinear refractive index. Specifically, the nonlinear absorption coefficient (βeff) decreased for doped films from (5.294 cm/W) for pure ZnO film to (4.794 cm/W) for ATZO. Additionally, the nonlinear refractive index (n₂) was found to be negative for all films, were (n2) increased from (-2.237 x 10-9 cm2/W) for pure ZnO film to (-1.995 x 10-9 cm2/W) for ATZO. The real, the imaginary parts of the nonlinear susceptibility and the third-order (NLO) susceptibility was also found. These results highlight how doping could improve ZnO thin film nonlinear optical performance.

Fabrication of 1D ZnO nanostructures on MEMS cantilever for VOC sensor application

Hydrothermal route and solution reaction method are adopted for the synthesis of zinc oxide (ZnO) nanopowders. Depending on precursors and preparation conditions, the obtained products possessed one of four following different morphologies: nanoparticle, microrod, nanoplate or nanotube. In this article, we report our investigation results of ZnO nanostructures preparation using a template-free aqueous solution based simple chemical route. Zinc nitrate hexahydrate Zn(NO3)2.6H2O was used as precursor for ZnO nanostructures. ZnO microrods and nanoplates were synthesized by a hydrothermal approach using Zn(NO3)2 and KOH as reaction chemicals. ZnO nanotubes were obtained by a chemical reaction of Zn(NO3)2 and NH4OH. And ZnO nanoparticles were prepared by precipitation method from zinc nitrate and ammonium carbonate (NH4)2CO3 in aqueous solution. The structures, morphology, and element components of these ZnO products fabricated by the above-mentioned methods were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). These experimental results demonstrated that the as-prepared ZnO nanoparticles have average diameter of 30-60 nm; rod-like ZnO has average diameter of about 350 nm and the length of 3.5 mm; plate-like ZnO has average thickness of about 40 nm and lateral size of 200´400 nm; ZnO nanotubes have outer diameter of about 400 nm and inner diameter of about 300 nm, the length of about 4 mm. The XRD results indicated that all four morphologies of ZnO are all wurtzite structure. It is found that the wet chemical technique is very promising for fabricating ZnO nanocrystallines with various morphologies. Our investigation makes useful contribution to the ZnO nanostructures studies, and can be used to explore potential applications in gas sensor, photocatalysts, surface enhanced Raman scattering (SERS) substrate and optoelectronic devices.

Influence of Sn doping on ZnO nanostructures from nanoparticles to spindle shape and their photoelectrochemical properties for dye sensitized solar cells

In this work, we propose zinc oxide (ZnO) surface functionalization with plasmonic silver nanoparticles (AgNP) of different sizes and shapes (spheres, prisms, and rods) creating ZnO/AgNP nanohybrids. These were characterized by UV-Vis spectroscopy, X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, diffuse reflectance spectroscopy, and photoluminescence spectroscopy. Surface functionalization with AgNP improved photocatalyst electronic properties, its visible light absorption, and slow electron/hole recombination on the ZnO surface. Photocatalysis assays performed with a polychromatic Hg lamp degraded methyl orange, a model of persistent organic pollutant in water. A systematic study showed that the photodegradation kinetics of the nanohybrids are significantly more efficient than pure ZnO (up to 18 times) and that AgNP size and especially its shape are important in dye degradation. Mechanistic studies revealed that degradation occurred by direct dye reduction on the ZnO surface holes, ZnO electron transfer to Ag followed by •O2- formation, and direct injection of AgNP hot electrons in the ZnO conduction band. The last effect was stronger for anisotropic AgNP, which explains their high kinetic degradation rates. Therefore, the rational design in ZnO/AgNP nanohybrid engineering and a systematic approach used in this manuscript allowed a detailed description of photodegradation process that occur at ZnO/AgNP interface. Our results are not conclusive about AgNP size; on the other hand, it clearly demonstrates that anisotropic nanoparticles (as Ag rods and prims) present superior photodegradation efficiency and are promising particles for further large-scale use of solar-irradiated nanohybrids.

We report on the room temperature ferromagnetism of various highly crystalline zinc oxide (ZnO) nanostructures, such as hexagonally shaped nanorods, nanocups, nanosamoosas, nanoplatelets, and hierarchical nano "flower-like" structures. These materials were synthesized in a shape-selective manner using simple microwave assisted hydrothermal synthesis. Thermogravimetric analyses demonstrated the as-synthesized ZnO nanostructures to be stable and of high purity. Structural analyses showed that the ZnO nanostructures are polycrystalline and wurtzite in structure, without any secondary phases. Combination of electron paramagnetic resonance, photoluminescence, and X-ray photoelectron spectroscopy studies revealed that the zinc vacancies (VZn) and singly ionized oxygen vacancies (VO(+)) located mainly on the ZnO surface are the primary defects in ZnO structures. A direct link between ferromagnetism and the relative occupancy of the VZn and VO(+) was established, suggesting that both VZn and VO(+) on the ZnO surface plays a vital role in modulating ferromagnetic behavior. An intense structure- and shape-dependent ferromagnetic signal with an effective g-value of >2.0 and a sextet hyperfine structure was shown. Moreover, a novel low field microwave absorption signal was observed and found to increase with an increase in microwave power and temperature.

본 논문에서는 zinc oxide (ZnO)와 gallium이 도핑 된 zinc oxide (GZO)를 이용하여 radio frequency (RF) magnetron sputtering 방법에 의해 상온에서 제작된 bottom-gate 박막 트랜지스터의 특성을 평가하고 분석하였다. 게이트 절연층 물질로서 새로운 물질을 사용하지 않고 열적 성장된 <TEX>$SiO_2$</TEX>를 사용하여 게이트 누설 전류를 수 pA 수준까지 줄일 수 있었다. ZnO와 GZO 박막의 표면 제곱평균제곱근은 각각 1.07 nm, 1.65 nm로 측정되었다. 그리고 ZnO 박막은 80% 이상, GZO 박막은 75% 이상의 투과도를 가지고 있었고, 박막의 두께에 따라 투과도가 달라졌다. 또한 두 시료 모두 (002) 방위로 잘 정렬된 wurtzite 구조를 가지고 있었다. 제작된 ZnO 박막 트랜지스터는 2.5 V의 문턱 전압, <TEX>$0.027\;cm^2/(V{\cdot}s)$</TEX>의 전계효과 이동도, 104의 on/off ratio, 1.7 V/decade의 gate voltage swing 값들을 가지고 있었고, enhancement 모드 특성을 가지고 있었다. 반면에 GZO 박막 트랜지스터의 경우에는 -3.4 V의 문턱 전압, <TEX>$0.023\;cm^2/(V{\cdot}s)$</TEX>의 전계효과 이동도, <TEX>$2{\times}10^4$</TEX>의 on/off ratio, 3.3 V/decade의 gate voltage swing 값들을 가지고 있었고, depletion 모드 특성을 가지고 있었다. 우리는 기존의 ZnO와 1wt%의 Ga이 도핑된 ZnO를 이용하여 두 가지 모드의 트랜지스터 특성을 보이는 박막 트랜지스터를 성공적으로 제작하고 분석하였다. In this paper we present a bottom-gate type of zinc oxide (ZnO) and Gallium (Ga) doped zinc oxide (GZO) based thin film transistors (TFTs) through applying a radio frequency (RF) magnetron sputtering method at room temperature. The gate leakage current can be reduced up to several ph by applying <TEX>$SiO_2$</TEX> thermally grown instead of using new gate oxide materials. The root mean square (RMS) values of the ZnO and GZO film surface were measured as 1.07 nm and 1.65 nm, respectively. Also, the transmittances of the ZnO and GZO film were more than 80% and 75%, respectively, and they were changed as their film thickness. The ZnO and GZO film had a wurtzite structure that was arranged well as a (002) orientation. The ZnO TFT had a threshold voltage of 2.5 V, a field effect mobility of <TEX>$0.027\;cm^2/(V{\cdot}s)$</TEX>, a on/off ratio of <TEX>$10^4$</TEX>, a gate voltage swing of 17 V/decade and it operated in a enhancement mode. In case of the GZO TFT, it operated in a depletion mode with a threshold voltage of -3.4 V, a field effect mobility of <TEX>$0.023\;cm^2/(V{\cdot}s)$</TEX>, a on/off ratio of <TEX>$2{\times}10^4$</TEX> and a gate voltage swing of 3.3 V/decade. We successfully demonstrated that the TFTs with the enhancement and depletion mode type can be fabricated by using pure ZnO and 1wt% Ga-doped ZnO.

Synthesis and application of manganese-doped zinc oxide as a potential adsorbent for removal of Congo red dye from wastewater

The II-VI semiconductors are well known for their tunable band gap. The tunning of band gap of these semiconductor materials is very significant in respect to their enhanced performance during their applications in optoelectronics as well as in photocatalysts. Zinc-oxide (ZnO) is one of those materials having a wide tunable band gap (~3.37eV). The band gap can be easily manipulated with suitable amount of external ion impurity. Researchers have found that the trivalent rare-earth ions are very much suitable candidates for doping in ZnO in order to have an influence on its band-gap. Though there are several works describing this band-gap manipulation, most of them are studied on bulk ZnO and quite a few works are there on nanocrystalline ZnO. Especially, in case of nanorods, there is hardly any work related to tunning of band gap. Here, the band gaps of pure and rare-earth doped ZnO nanorods are studied which are synthesized by a very low cost chemical co-precipitation method. The concentration of rare-earth ions in ZnO is varied from 0 to 3 wt%. The effect of Li+ ion incorporation is also studied as a charge compensator. This is to find the effect of charge compensation on band gap value when divalent Zn2+ are replaced by trivalent Eu3+, Tb3+. To find the bandgaps, diffuse reflectance studies of ZnO:RE3+ (RE=Eu, Tb) and ZnO:RE3+, Li+ were carried out. Diffuse reflectance of Eu3+/Tb3+ co-doped ZnO was also measured. Band gaps of the samples were calculated using the Kubelka-Munk relation. The band gap energy calculated for the RE3+ (RE=Eu, Tb) doped ZnO nano rods were found to be in the range 3.13eV~3.17eV. The band gap values obtained in our studies are much lower than that of bulk ZnO. The band gap values for ZnO:RE3+ are found to be increased when compared with that of the undoped sample. The increase in band gap value may be attributed to the ‘Burstein–Moss effect’, according to which the increase in the Fermi energy level of a degenerate semiconductor leads to energy band-widening. For Li+ co-doped ZnO:RE3+ samples, the band gap values are found to be decreased when compared with that of undoped ZnO, which is very much consistent with the absorption edge position obtained in diffuse reflectance data. In case of oxides, there are O 2p orbits as the valence band. The occupation of Zn2+ sites by the Li+ sites gives rise to quite a large number of oxygen vacancies which are able to change the structure of the energy band with the enhancement of the deformation degree of O 2p orbits. It causes the shifting of the absorption edge towards higher wavelength and decrease in the bandgap value for Li+ co-doped ZnO:RE3+ samples. Co-doping of rare-earth ions affects the size of the particles which affects the band gap too. Thus, controlling the band gap is very important because it determines the upper wavelength limit of light absorption in a material. So, for example, more efficient photovoltaic cells can be fabricated that have stronger solar light absorption.

Boron doped zinc oxide (ZnO) nanotubes have successfully been grown using seed-mediated hydrothermal method at various concentrations of precursor-surfactant solution. The growth of ZnO nanotubes was carried out at a temperature of 90°C for 8 hours and a drop in temperature to 50°C for 16 hours. In this study, the effect of concentration ratio of precursor-surfactant was evaluated. Samples were characterized using UV-Vis Spectroscopy, X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray (EDX). UV-Vis spectra of the samples showed that the ZnO nanotubes were grown on the Flourine Tin Oxide (FTO) and the strong absorption occurred in the wavelength range of 300-400 nm, which is typical spectrum for hexagonal-nanostructure. XRD patterns showed five diffraction peaks at 2θ = 32.05°, 34.5°, 36.35°, 47.75° and 56,9°. The analysis of EVA Diffrac Plus confirmed the peaks were represented to the lattice of (100), (002), (101), (102), and (110) respectively. FESEM images of the samples showed the hexagonal-shaped ZnO nanotubes with an average diameter of 50-340 nm and an average thickness of 1.2 - 4.4 μm observed for all samples. Uniformity the size and shape of ZnO nanotubes become higher as concentrations of ZNH decreased. The EDX spectra of the samples showed the percentage of weight of Zn, O, and B was 69.35%, 21.60% and 4.76% respectively, while the percentage of their atoms was 33, 06%, 42.08% and 13.71% respectively. The B-doped ZnO nanotubes solar cells were fabricated by arranging a sandwich structure, consisting of the FTO, ZnO nanotubes, dye, electrolyte and platinum thin film. I-V characteristics of cell were carried out under irradiation of 100 mWcm&lt;sup&gt;-2&lt;/sup&gt; halogen lamp. The I-V curves produced the highest efficiency from the cells utilizing the B-doped ZnO nanotubes with their concentration ratio of precursor-surfactant of 0.1 M : 0.04 M as the active material, which was 0.352%. This value is much higher than that of pure ZnO nanotube based DSSC of 0.05%.