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

Influence of oxygen pressure on the fs laserinduced oxidation of molybdenum thin films

We present a study of femtosecond (1028 nm, 230 fs, 54.7 MHz) laser processing on molybdenum (Mo) thin films. Irradiations were done under ambient air as well as pure oxygen (O2) at various gauge pressures (4, 8, 12 and 16 psi). Our results indicate that the high heating rates associated with laser processing allow the production of different molybdenum oxides. Raman spectroscopy and scanning electron microscopy are used to characterize the molybdenum oxidation for the different irradiation and oxygen pressures parameters chosen showing a high correlation between well-defined oxidation zones and the oxygen pressure surrounding the samples during the irradiation of the Mo thin films

Electrochromic Properties of Sol-Gel Prepared Hybrid Transition Metal Oxides A Short Review

This short review paper revisits the progress achieved in the last 10-15 years in the field of hybrid electrochromic materials, synthesized through sol-gel methods.During the recent decade, new avenues have been opened, exploring new concepts and particularly interesting applications of electrochromism. In this paper, we will discusssome of the new research directions in the field of electrochromism, together with novel applications of many electrochromic hybrid oxides. The most important incentive for enhanced properties of traditional materials has been the advent of nanotechnology. The discoveries in the field of synthesis of nanomaterials enabled to expand the materials and connect the morphological features of nanoparticles to the electrochromic properties at the macro level. This was possible because of the emergence of the new and more elaborate characterization methods, enabling to unveil hitherto unknown structural and morphological properties of electrochromic materials.It is important to mention the development of novel hybrid materials with significantly improved EC properties, where tungsten oxide is associated with carbonaceous materials such as MWCNT or graphene. These hybrid materials with enhanced EC properties, compared to the inorganic hybrids, will be remarkable in the future,for a series of novel applications.Retracing briefly the history of EC hybrid materials and summarizing the principal achievements will be useful not only for researchers in the field but for a wider readership as well

Tungsten Oxide Nanorods Array and Nanobundle Prepared by Using Chemical Vapor Deposition Technique

Tungsten oxide (WO3) nanorods array prepared using chemical vapor deposition techniques was studied. The influence of oxygen gas concentration on the nanoscale tungsten oxide structure was observed; it was responsible for the stoichiometric and morphology variation from nanoscale particle to nanorods array. Experimental results also indicated that the deposition temperature was highly related to the morphology; the chemical structure, however, was stable. The evolution of the crystalline structure and surface morphology was analyzed by scanning electron microscopy, Raman spectra and X-ray diffraction approaches. The stoichiometric variation was indicated by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy

噴霧法によるセシウムタングステンブロンズナノ粒子の合成と光学特性

広島大学(Hiroshima University)博士(工学)Doctor of Engineeringdoctora

Electric field driven manipulation of nanostructured metal oxide thin films: applications in chromic devices

Chromism by definition is the process that induces coloration change in a material and is generally favourable for many applications when it is reversible. Many modern applications such as optical modulators, smart windows and optical displays are based on the chromic effect. This chromic effect is always aided with a compatible stimulus. In semiconductors, particularly those made of thin films, an induced coloration is often initiated by the intercalation of positive ions such as Li + or H + into the exposed active sites of the material’s structure. Nanostructure synthesis of semiconducting crystals continues to expand and evolve. Each synthesis method offers unique prospects that affect morphology, stoichiometry, crystallinity, dopant behaviour and eventually performance of the semiconducting crystals. Electric field driven methods such as anodization and electrodeposition are especially applicable since they are often carried out under ambient conditions with non-toxic electrolytes. In addition, manipulation of anodization and electrodeposition parameters is simple, depending upon facile changes of parameters such as voltage, current and electrolytes while delivering astounding results. In this PhD thesis, the author focuses on the electric field deposited and manipulated transition metal oxides (TMOs). The target TMOs are molybdenum trioxide (MoO 3 ), titanium dioxide (TiO 2 ) and niobium pentoxide (Nb 2 O 5 ) which have suitable band energy diagrams and crystal structure for chromic devices. The author shows that these TMOs can be synthesised into high surface area, highly crystalline and homogenous nanostructures using the two aforementioned techniques. However, these potential candidates also experience inherent limitations that restrict their chromic performances. Therefore the author of this thesis intended to seek out solutions in overcoming these limitations.  In order to achieve the goals of this PhD research program, the author comprehensively investigated chromic properties of nanostructured MoO 3 , TiO 2 and Nb 2 O 5 and assessed their chromic performance and potential strategies to enhance them. Based on the strategies and investigations by the author, the PhD project was conducted in four distinct stages that each resulted in novel outcomes. In the first stage, the author demonstrated gasochromic devices based on MoO 3 . MoO 3 was implemented due to its well-known chromic capabilities. The author developed a novel method to fabricate chromic devices based on selective electrodeposition of α- and β-MoO 3 . In the second stage, the author carried out a novel combination of anodised TiO 2 ordered nanotubular template and electrodeposited α-MoO 3 chromic interface as complimentary binary EC semiconducting materials in order to overcome the chromic limitations of each of these individual TMO. In the third stage, the author demonstrated EC devices based on ordered anodized Nb 2 O 5 , where a coloration efficiency (CE) value of 47.0 cm 2 C −1 was calculated. The calculated CE value was the highest in comparison to all other Nb 2 O 5 based EC devices at the time. In the final stage, to overcome the limitations Nb 2 O 5 , the author applied the concept of binary complimentary TMO system incorporating MoO 3 as the chromic layer while the ordered Nb 2 O 5 nanostructure functions as the template. In summary, the author believes that the outcomes of this PhD research provide an in-depth analysis of chromic devices based on TMOs including MoO 3 , TiO 2 , Nb 2 O 5 and their selected binary systems synthesised using electric field driven techniques. The author also believes that this study has contributed significantly towards improving TMO capabilities for chromic applications

Nanochromics: old materials, new structures and architectures for high performance devices

Due to the development of nanoscience, the interest in electrochromism has increased and new assemblies of electrochromic materials at nanoscale leading to higher efficiencies and chromatic contrasts, low switching times and the possibility of color tuning have been developed. These advantages are reached due to the extensive surface area found in nanomaterials and the large amount of organic electrochromic molecules that can be easily attached onto inorganic nanoparticles, as TiO2 or SiO2. Moreover, the direct contact between electrolyte and nanomaterials produces high ionic transfer rates, leading to fast charge compensation, which is essential for high performance electrochromic electrodes. Recently, the layer-by-layer technique was presented as an interesting way to produce different architectures by the combination of both electrochromic nanoparticles and polymers. The present paper shows some of the newest insights into nanochromic science.Com o desenvolvimento da nanociência, o estudo do fenômeno do eletrocromismo desperta forte e continuado interesse devido à possibilidade de obter maiores eficiências eletrocrômicas, contrastes cromáticos, sintonização de cores e baixos tempos de resposta por meio da montagem de nanomateriais. Estas vantagens são possíveis devido à alta área superficial que os nanomateriais possuem e a enorme quantidade de moléculas orgânicas eletrocrômicas que podem ser facilmente ligadas a nanopartículas inorgânicas como TiO2 ou SiO2. Somado a isto, o contato direto entre o eletrólito e os nanomateriais produz altas velocidades de transferência iônica, com a concomitante compensação de carga rápida, o que é essencial para preparar eletrodos eletrocrômicos de alto desempenho. Recentemente a técnica de deposição eletrostática camada por camada foi apresentada como uma maneira interessante de preparar diferentes arquiteturas combinando nanopartículas e polímeros. O presente trabalho mostra alguns dos últimos avanços em nanocromismo.FAPESPCNPqInstituto do Milênio de Materiais Complexo

Structurally and mechanically tunable molybdenum oxide films and patterned submicrometer structures by electrodeposition

Altres ajuts: M. Guerrero acknowledges the support of the Secretary for Universities and Research of the Government of Catalonia and the COFUND Programme of the Marie Curie Actions of the 7th R&D Framework Programme of the European Union for the 'Beatriu de Pinos' contract (2013 BP-B 00077). M. D. Baró acknowledges partial financial support from an ICREA-Academia Award.1.5 μm-thick molybdenum oxide films have been electrodeposited potentiostatically from 0.2 M Na₂MoO₄ electrolyte onto indium tin oxide (ITO)/glass substrates at pH = 1, 6 and 9. The influence of cetyltrimethylammonium bromide (CTAB) surfactant on films' adhesion, morphology, degree of porosity, molybdenum speciation, and crystallographic structure has been systematically investigated. The addition of CTAB (0.01 M) to the bath clearly improves film adherence to the substrate, reduces cracking, and increases crystallinity. This has an impact on the physical properties of the films. In particular, both hardness (H) and Young's modulus (E) increase, as determined from nanoindentation tests. The growth of ordered arrays of molybdenum oxide submicrometer structures, including pillars and stripes, by electrodeposition onto e-beam lithographed Au/Ti/Si substrates is also reported