INVESTIGATING THE SYNTHESIS OF TRANSITION METAL OXIDE NANOSTRUCTURES IN A COUNTER-FLOW FLAME

In this dissertation, the synthesis and growth mechanisms of various transition metal oxide (TMO) nanostructures inside a counter-flow flame medium were investigated. Transition metal oxide nanostructures with distinctive properties have broad applications in microelectronic devices, gas sensors, lithium-ion batteries, and catalysts. A flame is an exothermic chemical reaction that provides required energy for inexpensive synthesis of TMO nanostructures. The counter-flow flame is characterized by high temperature and chemical species gradients, one-dimensional axial temperature variation, and an oxygen-rich zone suitable for growth of metal oxide nanomaterials. Transition metal oxide nanostructures can be grown and collected through both solid-support and gas-phase synthesis methods inside a counter-flow flame. Herein, grown TMO nanostructures include Nb2O5, ZnO, W-doped MoO3, and WOx nanostructures. The insertion of a high purity Nb probe in the flame resulted in an instantaneous formation of a material layer coating the surface of the probe. The results show that the material layer is composed of Nb2O5 nanorods. Size and growth rate of the Nb2O5 nanorods depend on the insertion positions of the Nb probe (source) inside the flame volume. Content of oxygen in the oxidizer stream plays an important role in the growth rate of Nb2O5 nanorods; higher oxygen content leads to higher growth rates. Low electron mobility of Nb leads to the basal growth mechanism for the synthesis of Nb2O5 inside the flame. On the other hand, transition metals with high electron mobility (such as Mo, W, and Zn) mainly form through the vapor-phase growth mechanism inside the flame. The vapor-phase growth mechanism was observed during the growth of ZnO nanostructures through the solid-support synthesis. It was found that the morphology of the grown ZnO nanocrystals strongly depends on the insertion position inside the flame. Structural variations of the synthesized ZnO nanostructures include nanorods and microprisms with a large number of facets, and microprisms with a protruding nanorod. Grown nanorods are less than 100 nm in diameter and less than 1µm in length. W-doped MoO3 nanocubes were synthesized through the gas-phase synthesis by introducing high purity Mo and W probes inside the counter-flow flame volume. Energy dispersive X-ray spectroscopy elemental mapping shows evenly distributed W, Mo, and O2 in the nanocubes. The measured lattice spacing of the nanocubes showed expanded lattice spacing which was attributed to an intercalation of tungsten atoms in the MoO3 layers. Collected samples of fully grown W-doped MoO3 nanocubes in the upper region of the flame volume show that the nanocubes have widths of less than 100 nm and well-defined edges like their base structures of MoO3. By inserting a high purity W probe inside the flame volume, fragments of tungsten oxide material formed over the oxidizer side of the W probe. We found that this material can be converted to 1-D tungsten oxide nanorods with lower oxidation state (WOx) as exposed to the electron beam (EB) of a TEM. In this process, tungsten oxide nanorods reached ~90% of their final length within approximately one second of EB irradiation. The EB irradiation led to evaporation of a part of the fragment and subsequent growth of lower state tungsten oxide (WOx) nanorods in the vicinity of the irradiation spot. It revealed that grown WOx nanorods follow the vapor-phase growth mechanism. The evaporated material particles coalesced and deposited on the TEM grid to form seeds for further growth. These early seeds were the building blocks for the formation of fully grown structures. Further influx of tungsten particle deposition on the surface of the seeds caused growth of the seeds in the preferred direction and formation of the nanorods. The smooth surface was evidence of total diffusion of deposited particles into the surface of the early formed nanorods. The length of WOx nanorods was an exponential function of their distance to the irradiated spot. Longer nanorods were observed closer to the irradiated spot. This finding gives another unique characteristic of the flame to synthesize TMOs

WO3–based photocatalysts: A review on synthesis, performance enhancement and photocatalytic memory for environmental applications

A significant drawback of the traditional photocatalysts such as titanium dioxide (TiO2) is their inability to absorb visible light from the solar spectrum due to their wide band gap energy. They are only photoactive in the ultraviolet (UV) region which is just a little fraction of the solar spectrum and could be harmful with much exposure to it. Due to its abundance in the solar spectrum, visible light needs to be harnessed for environmental applications. However, we lack visible light driven photocatalysts with long-lasting energy storage capacity for “round-the-clock photocatalytic” (RTCP) applications. For this reason, there is a growing need to find new photocatalysts that can mitigate these bottlenecks. It is evident from some carefully selected published articles (1976–2021) that tungsten oxide (WO3) and its composites have attracted popularity in recent years because of its outstanding properties and particularly its smaller band gap energy of 2.8 eV. However, pristine WO3 is limited due to relatively low energy density and smaller specific surface area. These drawbacks can be addressed by developing various WO3 – based materials to improve their performance. This paper reviews and discusses their recent development in surface advancement, morphology control, modification of nanostructured WO3 and its composites, and their RTCP energy storage for photocatalytic activities in visible light and the dark for environmental applications. Specific aspects focused on its nature, structure, properties, synthesis, coatings, deposition, approaches at modifying and enhancing its visible light photoactivity for enhanced performance and energy storage potential

Current Research in Pulsed Laser Deposition

Despite its limitation in terms of surface covered area, the PLD technique still gathers interest among researchers by offering endless possibilities for tuning thin film composition and enhancing their properties of interest due to: (i) the easiness of a stoichiometric transfer even for very complex target materials, (ii) high adherence of the deposited structures to the substrate, (iii) controlled degree of phase, crystallinity, and thickness of deposited coatings, (iv) versatility of the experimental set-up which allows for simultaneous ablation of multiple targets resulting in combinatorial maps or consecutive ablation of multiple targets producing multi-layered structures, and (v) adjustment of the number of laser pulses, resulting in either a spread of nanoparticles, islands of materials or a complete covering of a surface. Moreover, a variation of PLD, known as Matrix Assisted Pulsed Laser Evaporation, allows for deposition of organic materials, ranging from polymers to proteins and even living cells, otherwise difficult to transfer unaltered in the form of thin films by other techniques. Furthermore, the use of laser light as transfer agent ensures purity of films and pulse-to-pulse deposition allows for an unprecedented control of film thickness at the nm level. This Special Issue is a collection of state-of-the art research papers and reviews in which the topics of interest are devoted to thin film synthesis by PLD and MAPLE, for numerous research and industry field applications, such as bio-active coatings for medical implants and hard, protective coatings for cutting and drilling tools withstanding high friction and elevated temperatures, sensors, solar cells, lithography, magnetic devices, energy-storage and conversion devices, controlled drug delivery and in situ microstructuring for boosting of surface properties

Synthesis and gas sensing properties of inorganic semiconducting, p-n heterojunction nanomaterials

En aquesta tesis utilitzant principalment Aerosol Assited Chemical Vapor Deposition, AACVD, com a metodologia de síntesis d'òxid de tungstè nanoestructurat s'han fabricat diferents sensors de gasos. Per tal d'estudiar la millora en la selectivitat i la sensibilitat dels sensors de gasos basats en òxid de tungstè aquest s'han decorat, via AACVD, amb nanopartícules d'altres òxids metàl·lics per a crear heterojuncions per tal d'obtenir un increment en la sensibilitat electrònica, les propietats químiques del material o bé ambdues. En particular, s'han treballat en diferents sensors de nanofils d'òxid de tungstè decorats amb nanopartícules d'òxid de níquel, òxid de cobalt i òxid d'iridi resultant en sensors amb un gran increment de resposta i selectivitat cap al sulfur d'hidrogen, per a l'amoníac i per a l'òxid de nitrogen respectivament a concentracions traça. A més a més, s'han estudiat els mecanismes de reacció que tenen lloc entre les espècies d'oxigen adsorbides a la superfície del sensor quan interactua amb un gas. I també s'ha treballat en intentar controlar el potencial de superfície de les capes nanoestructurades per tal de controlar la deriva en la senyal al llarg del temps, quan el sensor està operant, a través d'un control de temperatura.En esta tesis utilizando principalmente Aerosol Assited Chemical Vapor Deposition, AACVD, como metodología de síntesis de óxido de tungsteno nanoestructurado se han fabricado diferentes sensores de gases. Para estudiar la mejora en la selectividad y la sensibilidad de los sensores de gases basados en óxido de tungsteno estos se han decorado, vía AACVD, con nanopartículas de otros óxidos metálicos para crear heterouniones para obtener un incremento en la sensibilidad electrónica, las propiedades químicas del material o bien ambas. En particular, se han trabajado en diferentes sensores de nanohilos de óxido de tungsteno decorados con nanopartículas de óxido de níquel, óxido de cobalto y óxido de iridio resultante en sensores con un gran incremento de respuesta y selectividad hacia el sulfuro de hidrógeno, para el amoníaco y para el óxido de nitrógeno respectivamente a concentraciones traza. Además, se han estudiado los mecanismos de reacción que tienen lugar entre las especies de oxígeno adsorbidas en la superficie del sensor cuando interactúa con un gas. Y también se ha trabajado en intentar controlar el potencial de superficie de las capas nanoestructuradas para controlar la deriva en la señal a lo largo del tiempo, cuando el sensor está trabajando, a través de un control de temperatura.In this thesis, using mainly Aerosol Assited Chemical Vapor Deposition, AACVD, as a synthesis methodology for nanostructured tungsten oxide, different gas sensors have been manufactured. To study the improvement in the selectivity and sensitivity of gas sensors based on tungsten oxide, they have been decorated, via AACVD, with nanoparticles of other metal oxides to create heterojunctions to obtain an increase in electronic sensitivity, in the chemical properties of the material or at the same time in both. Particularly, we have worked on different tungsten oxide nanowire sensors decorated with nanoparticles of nickel oxide, cobalt oxide and iridium oxide resulting in sensors with a large increase in response and selectivity towards hydrogen sulfide, for ammonia. and for nitrogen oxide respectively at trace concentrations. In addition, the reaction mechanisms that take place between oxygen species adsorbed on the sensor surface when it interacts with a gas have been also studied. Furthermore, efforts have been put on trying to control the surface potential of the nanostructured layers to control the drift in the signal over time, when operating the sensors, through temperature control

Electronic Nanodevices

The start of high-volume production of field-effect transistors with a feature size below 100 nm at the end of the 20th century signaled the transition from microelectronics to nanoelectronics. Since then, downscaling in the semiconductor industry has continued until the recent development of sub-10 nm technologies. The new phenomena and issues as well as the technological challenges of the fabrication and manipulation at the nanoscale have spurred an intense theoretical and experimental research activity. New device structures, operating principles, materials, and measurement techniques have emerged, and new approaches to electronic transport and device modeling have become necessary. Examples are the introduction of vertical MOSFETs in addition to the planar ones to enable the multi-gate approach as well as the development of new tunneling, high-electron mobility, and single-electron devices. The search for new materials such as nanowires, nanotubes, and 2D materials for the transistor channel, dielectrics, and interconnects has been part of the process. New electronic devices, often consisting of nanoscale heterojunctions, have been developed for light emission, transmission, and detection in optoelectronic and photonic systems, as well for new chemical, biological, and environmental sensors. This Special Issue focuses on the design, fabrication, modeling, and demonstration of nanodevices for electronic, optoelectronic, and sensing applications

Non-covalent interactions in organotin(IV) derivatives of 5,7-ditertbutyl- and 5,7-diphenyl-1,2,4-triazolo[1,5-a]pyrimidine as recognition motifs in crystalline self- assembly and their in vitro antistaphylococcal activity

Non-covalent interactions are known to play a key role in biological compounds due to their stabilization of the tertiary and quaternary structure of proteins [1]. Ligands similar to purine rings, such as triazolo pyrimidine ones, are very versatile in their interactions with metals and can act as model systems for natural bio-inorganic compounds [2]. A considerable series (twelve novel compounds are reported) of 5,7-ditertbutyl-1,2,4-triazolo[1,5-a]pyrimidine (dbtp) and 5,7-diphenyl- 1,2,4-triazolo[1,5-a]pyrimidine (dptp) were synthesized and investigated by FT-IR and 119Sn M\uf6ssbauer in the solid state and by 1H and 13C NMR spectroscopy, in solution [3]. The X-ray crystal and molecular structures of Et2SnCl2(dbtp)2 and Ph2SnCl2(EtOH)2(dptp)2 were described, in this latter pyrimidine molecules are not directly bound to the metal center but strictly H-bonded, through N(3), to the -OH group of the ethanol moieties. The network of hydrogen bonding and aromatic interactions involving pyrimidine and phenyl rings in both complexes drives their self-assembly. Noncovalent interactions involving aromatic rings are key processes in both chemical and biological recognition, contributing to overall complex stability and forming recognition motifs. It is noteworthy that in Ph2SnCl2(EtOH)2(dptp)2 \u3c0\u2013\u3c0 stacking interactions between pairs of antiparallel triazolopyrimidine rings mimick basepair interactions physiologically occurring in DNA (Fig.1). M\uf6ssbauer spectra suggest for Et2SnCl2(dbtp)2 a distorted octahedral structure, with C-Sn-C bond angles lower than 180\ub0. The estimated angle for Et2SnCl2(dbtp)2 is virtually identical to that determined by X-ray diffraction. Ph2SnCl2(EtOH)2(dptp)2 is characterized by an essentially linear C-Sn-C fragment according to the X-ray all-trans structure. The compounds were screened for their in vitro antibacterial activity on a group of reference staphylococcal strains susceptible or resistant to methicillin and against two reference Gramnegative pathogens [4] . We tested the biological activity of all the specimen against a group of staphylococcal reference strains (S. aureus ATCC 25923, S. aureus ATCC 29213, methicillin resistant S. aureus 43866 and S. epidermidis RP62A) along with Gram-negative pathogens (P. aeruginosa ATCC9027 and E. coli ATCC25922). Ph2SnCl2(EtOH)2(dptp)2 showed good antibacterial activity with a MIC value of 5 \u3bcg mL-1 against S. aureus ATCC29213 and also resulted active against methicillin resistant S. epidermidis RP62A