Cobalt-doped zinc oxide thin films as model Fischer-Tropsch nano-catalysts grown by pulsed electron beam ablation

The production of materials in thin film form with unique properties is of growing scientific and technological interest. Zinc oxide is a low cost, and environmentally benign wide band gap semiconductor, which makes it an excellent supporting material for nanoparticles with a plethora of potential applications. Upon doping with Co and other transition metals, ZnO exhibits room temperature ferromagnetic properties with enhanced performance and new functionalities when used in thin film devices. Zinc oxide-supported cobalt nano-composites are promising materials with desirable catalytic properties making it an interesting material for use as an efficient nano-catalyst in many important reactive processes such as Fischer-Tropsch synthesis (FTS), photocatalysis, hydrogen production and steam reforming. Pulsed electron beam ablation (PEBA) has recently emerged as a potential technique for the fabrication of superior quality thin films. The production of well controlled nano-sized particulates is a characteristic feature of PEBA, which has a strong bearing on the surface morphology of the deposited films. In the current work, the potential of PEBA in the deposition of Co-doped ZnO thin films has been assessed and the critical process conditions that affect the growth of the thin films on different substrates have been thoroughly investigated. The main objective of the current work is to deposit Co-doped ZnO thin films via PEBA, and assess the potential of the deposited films as model nano-structured catalysts for the synthesis of green liquid fuels from syngas. PEBA has several advantages including modest requirements for vacuum, control of film thickness, easy set-up, low capital cost, reduced operation and maintenance costs, small footprint, enhanced efficiency, and relative safety (no toxic gases as in pulsed laser ablation or potential noxious by-products as in solvo-thermal routes) over other film preparation techniques. In this project, Co:ZnO thin films have been synthesized from a single target on various substrates. Numerous process parameters have been assessed such as substrate material, deposition temperature, electron beam voltage, beam pulse frequency. Targets of varying cobalt loads viz., 5 w%, 10 w%, and 20 w% have been investigated as well. The effects of pre and post annealing on the physico-chemical properties of the thin films have also been studied. The deposited films have been characterized using complementary analytical techniques such as xray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive x-ray (EDX), visible reflectance spectroscopy (VRS), and atomic force microscopy (AFM). Such comprehensive analyses have helped in assessing the quality of the films and in guiding the experimental strategy in the quest to find the optimal process conditions. Finally, the films have been evaluated for their potential as model nano-catalysts for FischerTropsch synthesis in a 3-phase continuously-stirred tank slurry reactor (3-φ-CSTSR) using a Robinson-Mahoney stationary basket (RMSB). The results have been described in terms of activity and selectivity of the thin film nano-catalysts.Doctor of Philosophy (PhD) in Natural Resources Engineerin

Dilute magnetic semiconductor nanowires

Semiconductor materials form the basis of modern electronics, communication, data storage and computing technologies. One of today’s challenges for the development of future technologies is the realization of devices that control not only the electron charge, as in present electronics, but also its spin, setting the basis for future spintronics. Spintronics represents the concept of the synergetic and multifunctional use of charge and spin dynamics of electrons, aiming to go beyond the traditional dichotomy of semiconductor electronics and magnetic storage technology. The most direct method to induce spin-polarized electrons into a semiconductor is by introducing appropriate transition-metal or rare-earth dopants producing a dilute magnetic semiconductor (DMS). At the same time the seamless integration of future spintronic devices into nanodevices would require the fabrication of one-dimensional DMS nanostructures in well-defined architectures. In this review we focus on recent advances in the synthesis of DMS nanowires as well discussing the structural, optical and magnetic properties of these materials

Tuning of defects in ZnO nanorod arrays used in bulk heterojunction solar cells.

With particular focus on bulk heterojunction solar cells incorporating ZnO nanorods, we study how different annealing environments (air or Zn environment) and temperatures impact on the photoluminescence response. Our work gives new insight into the complex defect landscape in ZnO, and it also shows how the different defect types can be manipulated. We have determined the emission wavelengths for the two main defects which make up the visible band, the oxygen vacancy emission wavelength at approximately 530 nm and the zinc vacancy emission wavelength at approximately 630 nm. The precise nature of the defect landscape in the bulk of the nanorods is found to be unimportant to photovoltaic cell performance although the surface structure is more critical. Annealing of the nanorods is optimum at 300°C as this is a sufficiently high temperature to decompose Zn(OH)2 formed at the surface of the nanorods during electrodeposition and sufficiently low to prevent ITO degradation.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Morphological, structural and optical properties of Mg-doped ZnO nanocrystals synthesized using polyol process

У цій роботі, леговані ZnO нанокристали, та легірований Mg (0,5–20,0 ат.%) були синтезовані з використанням поліольного процесу. Також досліджені їх морфологічні, структурні, оптичні властивості, а також їх хімічний склад. Рентгенівська дифракція, передавальна та скануюча електронна мікроскопія, енергодисперсна рентгенографія, Раман та УФ-спектроскопії були використані для ідентифікації ефективного включення атомів Mg у решітку ZnO без утворення вторинних фаз при Mg до 5 ат.%. При більш високому рівні допінгу Mg були виявлені сліди фази Mg(OH)2. Результати виявили зменшення розмірів та погіршення якості кристалів нанокристалів ZnO із збільшенням легування Mg. Нанокристали ZnO втрачають сферичну форму, утворюючи стрижнеподібні та аморфні наноструктури при Mg ≥ 5 ат.%. Раманові спектри підтвердили режими Е2 (високий), Е2 (низький), Е2 (високий) - Е2 (низький), режим А1 (ТО) для недозволених та режим Eu (ТО) для нанокристалів ZnO, легованих Mg. Оптичний зазор знайдено в діапазоні 3,40–3,80 еВ.В этой работе, легированные ZnO нанокристаллы, и легированные Mg (0,5-20,0 ат.%) Были синтезированы с использованием полиольного процесса. Также исследованы их морфологические, структурные, оптические свойства, а также их химический состав. Рентгеновская дифракция, передающая и сканирующая электронная микроскопия, енергодисперсна рентгенография, Раман и УФ-спектроскопии были использованы для идентификации эффективного включения атомов Mg в решетку ZnO без образования вторичных фаз при Mg до 5 ат.%. При более высоком уровне допинга Mg были обнаружены следы фазы Mg (OH) 2. Результаты выявили уменьшение размеров и ухудшение качества кристаллов нанокристаллов ZnO с увеличением легирования Mg. Нанокристаллы ZnO теряют сферическую форму, образуя стрижнеподибни и аморфные наноструктуры при Mg ≥ 5 ат.%. Раман спектры подтвердили режима Е2 (высокий), Е2 (низкий), Е2 (высокий) - Е2 (низкий), режим А1 (ТО) для неразрешенных и режим Eu (ТО) для нанокристаллов ZnO, легированных Mg. Оптический зазор найдено в диапазоне 3,40-3,80 эВ.In this work, the undoped and Mg (0.5–20.0 at.%) doped ZnO nanocrystals have been synthesized using the polyol process and their morphological, structural, optical properties as well as chemical composition have been investigated. X-ray diffraction, transmission and scanning electron microscopy, energy dispersive X-ray, Raman and UV-vis spectroscopies were used to identify effective incorporation of Mg atoms into ZnO lattice without the formation of secondary phases at Mg up to 5 at.%. At higher Mg doping level, Mg(OH)2 phase traces were evidenced. The results have revealed the reduction of sizes and worsening the crystal quality of ZnO nanocrystals with increase of Mg doping. ZnO nanocrystals lose spherical shape forming the rod-like and amorphous nanostructures at Mg ≥ 5 at.%. Raman spectra have confirmed E2 (high), E2 (low), E2 (high) - E2 (low), A1(TO) modes for undoped, and Eu(TO) mode for Mg-doped ZnO nanocrystals. The optical band gap has been found in the range of 3.40–3.80 eV

Synthesis, self-assembly and properties of ZnO-related nanostructures and nanocomposites

Ph.DDOCTOR OF PHILOSOPH

In Search of Magnetic Properties of Samarium Cobalt (Sm2Co17) within a Low-Temperature Sintering Process

Samarium cobalt is known as super high density magnetic material with large magnetic anisotropy energy. Samarium–cobalt exhibits manipulative magnetic properties as a rare-earth material which has different properties in a low sintering temperature. It is therefore of paramount importance to investigate samarium cobalt (Sm2Co17) magnetic properties in the low temperature sintering condition. Sm2Co17, which is utilized in this research, is synthesized via the sol–gel process at sintering temperatures of 400, 500, and 600 °C. Subsequently, the crystallites indicate the formation of a single-phase Sm2Co17 on all the samples in all temperature variations. Moreover, the peaks in the X-ray diffraction analysis of crystallite sizes calculated using the Scherrer equation are 17.730, 15.197, and 13.296 nm at 400, 500, and 600 °C. Through scanning electron microscopy, the particles are found to be relatively large and agglomerated, with average sizes of 143.65, 168.78, and 237.26 nm. The functional groups are also analyzed via Fourier-transform infrared spectroscopy, which results in the appearance of several bonds in the samples, for example, alkyl halides, alkanes, and esters with aromatic functional groups on the fingerprint area and alkynes, alkyl halides, and alcohol functional groups at a wavelength of above 1500 cm. The test results of the magnetic properties using vibrating-sample magnetometer (VSM) revealed high coercivity and retentivity in the samples sintered at 400 °C. However, the highest saturation occurs in the samples sintered at 600 ℃. At a low sintering temperature (below 1000 °C), samarium cobalt shows as the soft magnetic material. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Review—Influence of Processing Parameters to Control Morphology and Optical Properties of Sol-Gel Synthesized ZnO Nanoparticles

ZnO has several potential applications into its credit. This review article focuses on the influence of processing parameters involved during the synthesis of ZnO nanoparticles by sol-gel method. During the sol-gel synthesis technique, the processing parameters/experimental conditions can affect the properties of the synthesized material. Processing parameters are the operating conditions that are to be kept under consideration during the synthesis process of nanoparticles so that various properties exhibited by the resulting nanoparticles can be tailored according to the desired applications. Effect of parameters like pH of the sol, additives used (like capping agent, surfactant), the effect of annealing temperature and calcination on the morphology and the optical properties of ZnO nanoparticles prepared via sol-gel technique is analyzed in this study. In this study, we tried to brief the experimental investigations done by various researchers to analyze the influence of processing parameters on ZnO nanoparticles. This study will provide a platform to understand and establish a correlation between the experimental conditions and properties of ZnO nanoparticles prepared through sol-gel route which will be helpful in meeting the desired needs in various application areas

A comprehensive review of ZnO materials and devices

The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev.142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys.6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. Lett.16, 439 (1970)]. In terms of devices, Au Schottky barriers in 1965 by Mead [Phys. Lett.18, 218 (1965)], demonstration of light-emitting diodes (1967) by Drapak [Semiconductors 2, 624 (1968)], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by Minami et al. [Jpn. J. Appl. Phys.13, 1475 (1974)], ZnO∕ZnSe n-p junctions (1975) by Tsurkan et al. [Semiconductors 6, 1183 (1975)], and Al∕Au Ohmic contacts by Brillson [J. Vac. Sci. Technol.15, 1378 (1978)] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by Look and Claflin [Phys. Status Solidi B241, 624 (2004)]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures