Field emission from ZnO nanostructures prepared by electrodeposition

Abstract

ZnO nanostructures were synthesised on nickel metallic wires using electrodeposited method in a mixed solution of zinc nitrate [Zn(NO₃)₂] and sodium hydroxide (NaOH). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used for characterisation. The XRD patterns revealed the formation of a wurtzite phase of ZnO only. The SEM images showed that the formation of ZnO nanostructures could be adjusted by increasing the electrolyte concentration. Field emission measurements demonstrated that ZnO nanopillars with befitting density exhibited the lowest turn on field and the highest current density. The ZnO nanostructures with field emission performance may have potentials in the future of vacuum electronic devices.     

Electrochemical synthesize and characterization of ZnO/ZnS nanostructures for hydrogen production

In present study, zinc oxide/zinc sulfide (ZnO/ZnS) nanostructures were fabricated by anodization of Zn. The morphological, structural and compositional properties of ZnO/ZnS were studied by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). From FESEM results, well-defined and smooth spherical-like ZnO/ZnS nanostructures were obtained with tube-like ZnO nanostructures underneath the compact layer. XRD patterns revealed that achieved materials offer high crystallinity hexagonal ZnO as dominant with hexagonal close-packed polycrystalline Zn. Moreover, measure of water contact angle was realized to learn about wetting property of the electrodes. Electrochemical impedance spectroscopy (EIS) was recorded to examine electrocatalytic performance of electrodes against hydrogen evolution reaction (HER) in 1 M KOH solution. The values of energy consumption and energy efficiency were calculated as 571.9 kJ mol⁻¹ and 49.5% at current density of 50 mA cm⁻² for the HER on 40-ZnO/ZnS/Zn electrode at 25°C.     

Preparations of different ZnO nanostructures on TiO2 nanotube via electrochemical method and its application in hydrogen production

In the present study, zinc oxide doped titanium dioxide nanotubes (ZnO/TiO₂-NTs) were designed by using electrochemical deposition method. Titanium dioxide nanotubes (TiO₂-NTs) were fabricated by anodization method. Nanostructured ZnO was deposited with various deposition times on TiO₂-NTs. The morphological, structural, optoelectronic properties of ZnO/TiO₂-NTs were examined in detail. The morphological and structural characterization of obtained electrodes was investigated with help of field emission scanning electron microscopy and X-ray diffraction. ZnO nanostructures with three different morphologies were obtained from nanotowers to nanoleafs. XRD results depicted that ZnO nanostructures have the high crystallinity with hexagonal wurtzite structure. The measurements of the contact angle were utilized to determine the wetting behavior of the obtained surface of materials. Electrochemical impedance spectroscopy measurement was used in 1 M KOH to investigate electrocatalytic behavior of the obtained materials towards hydrogen evolution reaction. Flat band potentials, as well as charge carrier densities, were determined by using Mott-Schottky analysis. The charge carrier densities were calculated as 1.06 × 10¹⁹ and 1.66 × 10²⁰ cm⁻³ for TiO₂-NTs and 30-ZnO/TiO₂-NTs, respectively. The energy consumption and energy efficiency were determined for hydrogen evolution on ZnO/TiO₂-NTs electrodes.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

Simultaneous bio-hydrogen and reducing sugar formation by the aqueous phase biomass reforming

A hydrothermal–chemical process for the simultaneous biomass hydrolysis and bio-hydrogen formation was proposed in this study. Current results revealed that a maximum reducing sugar concentration of 26.01 g/L and the yield of bio-hydrogen of 0.9098 mmol/g-cellulose were attained with the designated experimental parameters. Consequently, this study provided a potential route to produce bio-hydrogen and reducing sugar simultaneously in a one-step reaction process by using the energy crops or the agriculture wastes as the substrate.     

Synthesis and characterisation of three-dimensional dendritic ZnO nanorods

Abstract

Intriguing ZnO three-dimensional (3D) dendritic nanorods on silicon substrates have been successfully synthesised by thermal evaporation of pure zinc powder at a relative low temperature of 478°C without any metal catalyst. ZnO dendritic nanostructure exhibits unique shape and it is composed of stems and nanorod branches. It is found that the nanorods are single crystalline wurtzite structures, and each nanorod grows along the [0001] direction. At different growth temperatures, the shapes of ZnO nanostructures can be altered. System analysis reveals that the formation and morphology of ZnO dendritic nanostructures are sensitive to the growth temperature. Finally, room temperature photoluminescence spectrum is also investigated, revealing that the ZnO dendritic nanostructure could find application in UV optoelectronic devices; the nanostructure implies some potential applications for nanoscale functional devices.     

Photocatalytic hydrogen production performance of 1-D ZnO nanostructures: Role of structural properties

Synthesis of zinc oxide (ZnO) nanowires (NWs) grown via vapor-liquid-solid (VLS) process using Gold (Au) as a catalyst metal on aluminum-doped zinc oxide (AZO) seed layer is reported in the present work. During the growth procedure, the nucleation process helps us to obtain ZnO nanowires with Au on the tip, confirming the VLS growth mechanism. Different morphologies were obtained after the variation in the growth parameters in the VLS process, and further, their role in the photocatalytic performance was studied. Changes in the structural properties of nanowires allowed us to modify the aspect ratio and surface area of the nanostructures. X-ray diffraction (XRD) showed that the principal orientation of the nanowires was (002) in the present case. Scanning electron microscopy (SEM) showed the structural properties of 1-D nanostructures (nanowires), and statistical analysis revealed that the average diameter in the present case was found to be varied from 57 to 85 nm. Scanning transmission electron microscopy (STEM) technique revealed the different elements present on the surface of ZnO NWs. Further, the compositional profile of nanostructures was cross-verified using Energy dispersive Spectroscopy (EDS). Photoluminescence (PL) and UV Visible studies were employed to study the optical properties of nanowires. UV–Vis measurements showed the role of different structural properties of nanowires on the absorption spectra, especially in the visible region. The ZnO nanowires were tested as photocatalysts for hydrogen production from water splitting reaction, and it was found in particular nanowires with random orientation with optimal diameter distribution show the stable and highest photocatalytic performance.     

Thermal decomposition behavior of tobacco stem Part I: TGA–FTIR–MS analysis

In the present study, the thermal degradation behavior of tobacco stem was examined by means of a thermogravimetric analyzer (TGA) under nitrogen atmosphere at temperature range of 25–1,000°C. The TG curve indicated that the pyrolysis process of tobacco included three zones, and main pyrolysis occurred in the second zone by means of the decomposition of hemicellulose, cellulose, and lignin in a temperature range of 180–540°C. Furthermore, the gases evolved during the degradation were analyzed simultaneously via TGA coupled with a Fourier transform infrared spectrometer (FTIR) and mass spectrometer (MS). Carbon dioxide, methane, water, formaldehyde, and propanal were the main volatiles detected via MS and confirmed by FTIR.     

Structural and hydrogenation studies of ZnO and Mg doped ZnO nanowires

In this work, Mg doped zinc oxide (Mg_xZn_1−xO, x = 5, 10 and 20 at. %) nanowires were successfully prepared by two step process. Initially, ZnO nanowires were grown by thermal evaporation of Zn powder under oxygen atmosphere. Mg powder was doped in as grown ZnO through solid state diffusion at low temperature. Energy dispersive x-ray spectroscopy (EDAX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV–Visible absorption spectra analysis reveals that the Mg doping on ZnO nanowires induces lattice strain in ZnO. Rietveld analysis of XRD data confirms the wurtzite structure and a continuous compaction of the lattice (in particular, the c-axis parameter) as x increases. The hydrogenation properties of ZnO nanowires and Mg doped ZnO (Mg_xZn_1−xO, x = 0, 5, 10 and 20 at. %) nanowires were studied. The hydrogenated samples were further investigated through XRD and Fourier transform infrared spectroscopy (FTIR). The hydrogen storage capacity of as grown ZnO nanowires has been estimated to be 0.57 wt. % H₂ at room temperature. However, the hydrogen storage capacity gets increased to ∼1 wt. % upon doping ZnO with 10 at. % Mg. Further increase in Mg concentration decreases the hydrogen storage capacity of ZnO nanowires. Thus for 20 at. % Mg doped ZnO; the hydrogen absorption capacity gets decreased from ∼1 wt. % to 0.74 wt. %. The mechanism of hydrogen storage in ZnO nanowires and Mg doped samples of ZnO has been discussed.     

Preparation and characterization of nano‐enhanced myristic acid using metal oxide nanoparticles for thermal energy storage

Fatty acids have been broadly used as phase change materials (PCMs) for thermal energy storage. However, low thermal conductivity limits their performances. This paper investigates the influence of metal oxide nanoparticle addition on myristic acid (MA) as nano‐enhanced PCM (NEPCM). Stability, chemical, and thermal properties were considered. Four types of nanoaprticles, TiO₂, CuO, Al₂O₃, and ZnO, were dispersed in MA at 0.1, 0.5, 1, and 2 wt%. Stability and dispersion were checked by sediment photograph capturing and scanning electron microscopy/energy‐dispersive spectroscopy. The Fourier‐transformed infrared (FTIR) and X‐ray diffraction analysis confirmed no chemical interaction between the nanoparticles and MA. The results revealed a ratio of thermal conductivity of 1.50, 1.49, 1.45, and 1.37, respectively, for 2 wt% of ZnO, Al₂O₃, CuO, and TiO₂. The T‐history method confirmed this enhancement. The latent heat thermal energy storage (LHTES) properties of the nano‐enhanced MA were evaluated using differential scanning calorimetry. The latent heat capacities of nano‐enhanced MA samples have dropped between 9.64 and 5.01 % compared with pure MA, and phase change temperature range was not affected significantly. The NEPCM was subjected to 500 thermal cycling, it showed a good thermal reliability as LHTES properties remained unchanged, while FTIR analysis showed similar characteristics compared with uncycled samples, indicating a good chemical stability. Based on the results regarding with the LHTES properties, cycling thermal reliability, and higher thermal conductivity improvement, it can be achieved that the MA/Al₂O₃ (2.0 wt%) and MA/ZnO (2.0 wt%) composites could be better PCMs for solar TES applications.     

Structure and optical properties of ZnO nanowires coated with silica

Abstract

Abstract

It is essential to passivate one-dimensional nanostructures with insulating materials to protect them from contamination and oxidation as well as to avoid cross-talking between the building blocks of complex nanoscale circuits. The ZnO nanowires synthesised by the thermal evaporation of ZnO powders were coated with SiO₂ by the sputtering technique. Transmission electron microscopy and X-ray diffraction analyses revealed that the cores and shells of the ZnO core–SiO₂ shell nanowires were single crystal wurtzite type ZnO and amorphous SiO₂ respectively. Photoluminescence measurements at room temperature showed that the passivation of the ZnO nanowires was successfully achieved with SiO₂ without nearly degrading the near band edge emission from the wires. However, subsequent thermal annealing treatment was found to be undesirable owing to the degradation of the near band edge emission in intensity.     

Solar thermochemical ZnO/ZnSO4 water splitting cycle for hydrogen production

In this paper, solar reactor efficiency analysis of the solar thermochemical two-step zinc oxide–zinc sulfate (ZnO–ZnSO₄) water splitting cycle. In step-1, the ZnSO₄ is thermally decomposed into ZnO, SO₂, and O₂ using solar energy input. In step-2, the ZnO is re-oxidized into ZnSO₄ via water splitting reaction producing H₂. The ZnSO₄ is recycled back to the solar reactor and hence can be re-used in multiple cycles. The equilibrium compositions associated with the thermal reduction and water-splitting steps are identified by performing HSC simulations. The effect of Ar towards decreasing the required thermal reduction temperature is also explored. The total solar energy input and the re-radiation losses from the ZnO–ZnSO₄ water splitting cycle are estimated. Likewise, the amount of heat energy released by different coolers and water splitting reactor is also determined. Thermodynamic calculations indicate that the cycle (η_cycle) and solar-to-fuel energy conversion efficiency (η_{solar-to-fuel}) of the ZnO–ZnSO₄ water splitting cycle are equal to 40.6% and 48.9% (without heat recuperation). These efficiency values are higher than previously investigated thermochemical water splitting cycles and can be increased further by employing heat recuperation.     

An efficient nanostructured ZnO for dye sensitized degradation of Reactive Red 120 dye under solar light

Nanostructured ZnO has been synthesized by thermal decomposition of zinc oxalate without using any additives or solvents and characterized by XRD, TGA, BET, FE-SEM and HR-TEM techniques. XRD analysis showed that the synthesized catalyst has a hexagonal wurtzite-type polycrystalline structure. HR-TEM analysis revealed the formation of ZnO nanostructures and the increase in size of ZnO nanoparticle from 40 to 100 nm during the formation. Nano-ZnO is found to be more efficient than commercial ZnO for mineralization of Reactive Red 120 under solar irradiation. Addition of oxone influences the photodegradation efficiency. The recyclability of ZnO nanocrystals is found to be higher than the commercial ZnO. The promoting effects of ZnO nanocatalyst for the sensitized degradation of colorants in aqueous phase have been investigated. Mineralization of RR 120 was confirmed by gas chromatography and COD values.     

Defects-free single-crystalline zinc oxide nanostructures for efficient photoelectrochemical solar hydrogen generation

The article reports a novel and highly efficient methodology for the development of surface defects-free zinc oxide (ZnO) nanostructures, which are highly useful for various optoelectronic and electronic devices. Using this approach, we have developed high-quality ZnO nanostructures with comparable physical and chemical properties to high-temperature grown ones. Initially, ZnO nanostructures were developed by low-temperature chemical bath deposition, and the surface defects passivated structures were obtained by atomic layer deposition of homo-molecular clusters, i.e., Zn and O atomic layers. The surface passivated ZnO nanostructures exhibited excellent chemical stoichiometry between their constituents with enhanced crystalline quality. These nanostructures also showed improved light transmittance in the wavelengths range of 450–1000 nm, and light emission in the ultraviolet region. Further, the surface passivated nanostructures exhibited remarkable device performance as photoanodes with a greatly improved photocurrent density, more than 3 times, and reduced cathodic current of 6.17 × 10⁻⁷ A@-0.4 V. Significantly, the light-to-dark current ratio of the PEC devices fabricated with passivated ZnO nanostructures is found to be 1761.     

基于热红联用分析的木质素热裂解动力学研究

利用热重红外联用系统对生物质的主要组分木质素进行了热裂解动力学研究.在用红外固体压片法研究木质素结构的基础上得到不同升温速率下木质素热裂解的热重曲线.实验结果表明,随着升温速率的增加,各个阶段的起始和终止温度向高温侧轻微移动,主反应区间增加;计算得到的木质素两阶段活化能分别为58.41 kJ/mol和119.98 kJ/mol.与纤维素热解气的联机红外分析谱图相比可知木质素热解过程中气体析出机理复杂,主要生成CO、CH4和呋喃等产物.     

Evidencing enhanced oxygen and hydrogen evolution reactions using In–Zn–Co ternary transition metal oxide nanostructures: A novel bifunctional electrocatalyst

Ternary transition metal oxides are gaining popularity for cost effective bifunctional electrocatalytic activities and to realization of novel water splitting devices. In this regard, In₂O₃/ZnO/Co₃O₄ based ternary oxide nanostructures were investigated in detail for their oxygen/hydrogen evolution reaction (OER/HER) in alkaline environment. The ternary oxides were at first processed through a simple chemical route involving hydrothermal treatment. The prepared nanostructures were then investigated by using high-resolution transmission electron microscopy (TEM/HRTEM) to ascertain their morphological traits. X-ray diffraction, Raman signals and photoluminescence data demonstrated the In₂O₃ phase to be prevalent in the ternary mixture on par with that of ZnO and Co₃O₄. The valence state of various metal ions and the In–O, Zn–O and Co–O bonding was verified using XPS. The ternary oxide coated electrodes exhibited excellent overall water splitting activity. Overpotential values of 398 and 510 mV were registered for OER and HER experiments under a current density of ±10 mA cm⁻², demonstrating the material to be an ideal OER/HER electrocatalyst at room temperature. The exceptional long-term stability in ternary oxides and their Tafel slope (88 mV/dec for OER and 60 mV/dec for HER) further affirmed their unique anodic/cathodic characteristics for water splitting applications.     

Effects of pretreatment methods on solubilization of beet-pulp and bio-hydrogen production yield

Sugar processing wastewater and beet-pulp are two major waste streams of sugar-beet processing plants. Contrary to wastewater, beet-pulp is generally used as animal feed in cattle-raising industry. However, it can serve as a substrate for bio-hydrogen production which corresponds to a higher valorization of beet-pulp. Moreover, pretreatment of lignocellulosic materials like beet-pulp is needed in order to improve overall energy efficiency and enable economic feasibility of bio-hydrogen production. Therefore, the effect pretreatment methods (alkaline, thermal, microwave, thermal-alkaline and microwave-alkaline) on bio-hydrogen production from sugar beet-pulp through dark fermentation were investigated in this study. Reactors pretreated with alkaline, microwave-alkaline and thermal-alkaline methods yielded significant solubilization of beet-pulp compared to others. Therefore, in the second phase of the study, they were used to pretreat the beet-pulp which was then subjected to dark fermentation for bio-hydrogen production. Maximum bio-hydrogen production yield of 115.6 mL H₂/g COD was observed in reactor which contained alkaline pretreated beet-pulp.     

Using pomegranate peel powders as a new capping agent for synthesis of CuO/ZnO/Al2O3 nanostructures; enhancement of visible light photocatalytic activity

CuO/ZnO/Al₂O₃ (CZA) nanoclusters were successfully synthesized via a facial and green method in the presence of pomegranate. The structural analysis of the samples confirmed the formation of CZO nanostructures in the range of 14–17 nm. The morphological studies of the samples indicated that, the shape and the particle size of the CZO nanoclusters depend on the sources, ratio of the cationic sources, time and heating temperature. The photocatalytic properties of the CZO nanostructures were obtained using photooxidation of azo dyes; Methyl orange, Methylene blue and Methyl red. The photocatalytic activity of the sample shows about 85% of azo dyes degradation after 75 min of visible light irradiations. The results were clearly demonstrated that the pure CZA nanoparticles can be used as a potential photocatalyst under visible lights for removal of contaminants.     

Fractionating lignocellulose by formic acid: Characterization of major components

Treatment of corn (Zea mays L.) cob under mild reaction conditions (60 °C and atmospheric pressure) in 88% formic acid was an effective method for separating cellulose from hemicellulose and lignin components in lignocellulose. Most of the hemicellulose degradation and lignin removal occurred within the first 90 min. After 6 h treatment, the decomposition of hemicellulose and the recovery of lignin were over 85% and 70%, respectively. Multi-level structures of lignin and solid residues were further characterized by FTIR, XRD, TG/DTG, SEM and SEC. Peaks attributable to lignin or hemicellulose disappeared in FTIR spectra, indicating complete removal of these two components. The remaining solid residues had a higher crystalline index. The major pyrolysis temperature of corncob was increased after formic acid treatment; the molecular weight (MW) of cellulose in solid residues was higher than that in intact cobs, whereas the hemicellulose remaining in the pulp had a lower MW than the original. Lignin was extracted in an esterified form designated as formic acid lignin (FAL). FAL had two thermal decomposition temperatures (T_d) at 277 °C and 385 °C. The MW of lignin increased following formic acid treatment, which may make it a better starting material for chemical syntheses.     

Tailoring morphological characteristics of zinc oxide using a one-step hydrothermal method for photoelectrochemical water splitting application

In the present paper describe the zinc oxide (ZnO) with various morphologies have been synthesized using the one-step hydrothermal method, in which the growth of ZnO nanostructures are significantly tailored by adjusting the pH level between 9 and 12 using 0.1 M Sodium hydroxide (NaOH). Significant results reveal the morphological properties of ZnO nanostructures varied with different pH values with the formation of ZnO nanostructures have different morphological such as a baton, star, flower, and rod-like structures. The present results show the rod-like structure of ZnO nanostructures exhibits the highest photocurrent density of 746.61 μAcm⁻² (at 1.23 V vs RHE) under simulated solar AM 1.5G illumination in Potassium hydroxide (KOH) medium, also the other morphologies. The dependent of the photoelectrochemical (PEC) water splitting properties on the different morphological of ZnO nanostructures are studied. Achieving the morphological evolution mechanism has become one of the method to obtain the production of the hydrogen growth regime used for solar energy conversion and their applied storage potentials. The application of the ZnO nanostructures for PEC water splitting was proposed with the adoption of screen-printed carbon electrodes (SPCEs). These are to quantify the best degree of the highest photocurrent density with one-step tailoring with an ideal modeling system to enhance PEC water splitting performances. Thus, the screen-printed carbon electrodes (SPEs) has been used as an alternative method for fabrication and photoelectrodes testings.