Metal Oxide Networks Research Articles

Hybrid cluster precursors of the LaZrO insulator for transistors: lowering the processing temperature

Solution processing of ternary and multinary amorphous metal oxide insulators at processing temperatures below 250 °C remains challenging. Here, we report that the synthesis of a hybrid cluster structure, where the metal oxide core is coordinated by ligands and the different metal elements are incorporated into one core, is an effective strategy for the low-temperature processing of the ternary LaZrO insulator. Solvothermal treatment at 160–180 °C facilitated the development of a cluster structure. From the cluster precursor, high-performance insulating LaZrO films were obtained at 200 °C under the irradiation of ultraviolet light. The analysis data indicate that the solvothermal treatment led to structural unification of the metal oxide network and facilitated stabilization of the residual organic ingredients in UV annealing, which both contributed to the improved insulating properties of LaZrO. Together with a solution-processed channel, we have been able to fabricate LaZrO-based transistors at 200 °C. Though the channel material has not been optimized, the transistor have showed a low gate leakage current around 10 pA at an operating voltage of 15 V, an on/off ratio of near 106, a field-effect saturation mobility of 0.37 cm2 V−1 s−1, a subthreshold swing factor of 0.61 V decade−1.

Graph Theoretical Representation, Analysis and Synthesis of Amorphous Metal Oxide Networks

ABSTRACTWith the advent of amorphous oxide semiconductors (AOS) like amorphous indium gallium zinc oxide (a-IGZO), the analysis and prediction of amorphous structures has regained importance, more so, since first principles based studies are being increasingly employed to explain device behavior. Negative bias illumination stress in a-IGZO thin film transistors is one such example. However, the amorphous atomic structure is complex and defect or dopant studies require each site to be modeled independently and this leads to significant computational costs. Therefore, a simplification in the representation of the amorphous oxide network is effected so that it may lead to identifying similar atomic sites. The amorphous network is visualized as a network of polyhedra. The polyhedra has at its center a cation with the bonded oxygen atoms at the vertices and it comprises the short range interactions characterized by bond lengths and bond angles. Based on a first principles study of 10 a-IGZO models containing 36 cations each, it was found that the 360 polyhedra of the a-IGZO models can actually be described with only ten polyhedral motifs. These polyhedra are then connected to each other via a shared vertex or an edge; face-sharing was not observed in these models. Graph theory is used to map this network using either a graph of cationic polyhedra as the nodes or a bipartite graph (composed of cations and anions as individual nodes), each of which is described using the respective adjacency matrix. The second nearest interactions are characterized by the degree of each vertex and each atomic site is now characterized by a polyhedron and network metrics; and hence, can be compared with same-element sites. The changes in network itself, are quantified as the composition changes, when varying the ratio of In:Ga:Zn in a-IGZO. For example, the average vertex connectivity of a pair of indium sites reflects on the continuity of overlap between the In-5s orbitals which compose the conduction band minimum in a-IGZO, which, in turn, affects the transport properties of the semiconductor. Thus, the long range interactions of the physical amorphous network are described by the graph metrics. Moreover, evolutionary algorithm in conjunction with this graph theoretic representation can be used to generate new amorphous models. Two parent graph are chosen and then spliced and then bred. The new graph is then reverse-engineered to form an amorphous model which then undergoes ionic and volume relaxation in the framework of density functional theory. The resulting graph is the child and the new amorphous model, with the energy as the fitness criteria.

Low-Temperature Chemical Transformations for High-Performance Solution-Processed Oxide Transistors

The challenges associated with low-temperature solution-processed metal oxide network formation have hindered the realization of high-performance solution-based electronic circuitry at temperatures lower than 200 °C. Here, UV irradiation is embarked upon as a route to effectively transform the chemical precursors to semiconducting metal oxides with high electrical quality. High-performance UV-irradiated indium oxide (In2O3) and indium zinc oxide (IZO) thin film transistors with mobility greater than 30 cm2/(V s) have been obtained from nitrate-based precursors. The chemical transformation has been monitored by detailed spectroscopic studies, physical characterization, and temperature-dependent electrical transport measurements. In comparison to thermal annealing, UV annealing seems to result in higher M–O–M network formation (depicted by M–O bonds in XPS), better removal of chemical impurities (depicted by FTIR and XPS), and structural relaxation driven electron doping, transforming the oxygen vacancies t...

Binary Oxides with Defined Hierarchy of Pores in the Esterification of Glycerol

Various porous binary oxides with elevated textural properties were obtained in this work. The as-synthesized solids were calcined or modified by reflux and extraction processes. Characterizations through SEM, nitrogen physisorption and TEM techniques demonstrated the formation of porous metal oxide networks over all solids. XRD, thermal analyses and FTIR measurements showed the existence of nanosized rutile TiO2, tetragonal ZrO2, SiO2 and γ-Al2O3 phases on the solids. The structure and texture of the as-synthesized SiAl sol-gel derived solid resulted in the formation of well-dispersed nanoparticles on the support. The removal of the organic compounds by ethanol extraction or reflux from SiAl resulted in the presence of structures with defined hierarchy of pores. Among the solids studied, the catalytic results in the esterification of glycerol with acetic acid indicated that best performances were obtained over SiAl sample when submitted to extraction and reflux treatments. This was due to the creation of accessible pores, which facilitated the reaction occurrence at glycerol to acetic acid molar ratio = 1:3 and T = 80 °C for 20 h using 75 mg of catalyst. The solids can be reused three times without complete loss of their catalytic performance.

Mesoporous tertiary oxides via a novel amphiphilic approach

We report a facile biomimetic sol-gel synthesis using the sponge phase formed by the lipid monoolein as a structure-directing template, resulting in high phase purity, mesoporous dysprosium- and gadolinium titanates. The stability of monoolein in a 1,4-butanediol and water mixture complements the use of a simple sol-gel metal oxide synthesis route. By judicious control of the lipid/solvent concentration, the sponge phase of monoolein can be directly realised in the pyrochlore material, leading to a porous metal oxide network with an average pore diameter of 10 nm.

Synthesis of hierarchically porous metal oxides and Au/TiO2 nanohybrids for photodegradation of organic dye and catalytic reduction of 4-nitrophenol

Hierarchically porous metal oxide materials including titanium dioxide (TiO2), ferric oxide (Fe2O3), zinc oxide (ZnO), and ceria (CeO2) were prepared on a large scale by an improved sol–gel method assisted with evaporation-induced surfactant self-assembly. Hierarchical metal oxide networks with well-defined macroporous windows and mesoporous walls were obtained after the surfactant molecules were removed by calcination. The factors influencing the morphology, pore structure and size of these porous metal oxides were studied. Moreover, this method can also be used for one-pot synthesis of hierarchical nanohybrids loaded with noble-metal nanoparticles. The synthesis and catalytic properties of Au/TiO2 nanohybrids with different Au loading (2, 5, and 10wt.%) were typically demonstrated. Benefiting from the uniform dispersion of Au nanoparticles, the Au/TiO2 nanohybrids show favorable catalytic activity in both the photooxidation of rhodamine B (RhB) under visible light and the catalytic reduction of 4-nitrophenol with sodium borohydride to 4-aminophenol. The photooxidation mechanism of RhB in aqueous solution over Au/TiO2 nanohybrid was verified experimentally. This work provides a general method for the scalable preparation of diverse hierarchically porous metal oxides and noble-metal/oxide nanohybrids, which are expected to have more applications in catalysis and energy conversion.

Guided in Situ Polymerization of MEH-PPV in Mesoporous Titania Photoanodes

Incorporation of conjugated polymers into porous metal oxide networks is a challenging task, which is being pursued via many different approaches. We have developed the guided in situ polymerization of poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) in porous titania films by means of surface functionalization. The controlled polymerization via the Gilch route was induced by an alkoxide base and by increasing the temperature. The selected and specially designed surface-functionalizing linker molecules mimic the monomer or its activated form, respectively. In this way, we drastically enhanced the amount of MEH-PPV incorporated into the porous titania phase compared to nonfunctionalized samples by a factor of 6. Additionally, photovoltaic measurements were performed. The devices show shunting or series resistance limitations, depending on the surface functionalization prior to in situ polymerization of MEH-PPV. We suggest that the reason for this behavior can be found in the orientation of the grown polymer chains with respect to the titania surface. Therefore, the geometry of the anchoring via the linker molecules is relevant for exploiting the full electronic potential of the conjugated polymer in the resulting hybrid composite. This observation will help to design future synthesis methods for new hybrid materials from conjugated polymers and n-type semiconductors to take full advantage of favorable electronic interactions between the two phases.

Effects of the Covalent Bonding Entrapment of Tetrapyrrole Macrocycles Inside Translucent Monolithic ZrO<sub>2</sub> Xerogels

While searching for adequate sol-gel methodologies for successfully trapping in monomeric and stable form either porphyrins or phthalocyanines, inside translucent monolithic silica xerogels, it was discovered that the interactions of these trapped tetrapyrrole macrocycles with Si-OH surface groups inhibit or spoil the efficient display of physicochemical, especially optical, properties of the confined species. Consequently, we have developed strategies to keep the inserted macrocycle species as far as possible from these interferences by substituting the surface-OH groups foralkylorarylgroups or trapping these species inside alternative metal oxide networks, such as ZrO2, TiO2, and Al2O3. In the present manuscript, we present, for the first time to our knowledge, a methodology for preserving the spectroscopic characteristics of metal tetrasulfophthalocyanines and cobalt tetraphenylporphyrins trapped inside the pores of ZrO2xerogels. The results obtained are contrasting with analogous silica systems and demonstrate that, in ZrO2networks, the macrocyclic species remain trapped in stable and monomeric form while keeping their original spectroscopic characteristics in a better way than when captured inside silica systems. This outcome imply a lower hydrophilic character linked to the existence of a smaller amount of surface hydroxyl groups in ZrO2networks, if compared to analogous SiO2xerogel systems. The development and study of the possibility of trapping or fixing synthetic or natural tetrapyrrole macrocycles inside inorganic networks suggest the possibility of synthesizing hybrid solid systems suitable for important applications in technological areas such as optics, catalysis, sensoring and medicine

Nanostructured cerium oxide thin films by nebulized spray pyrolysis (NSP) technique: Impact of surfactants on the structural, optical and compositional properties

Cerium oxide thin films are deposited on glass substrates with different surfactants by nebulized spray pyrolysis technique. Their crystallographic structures, surface morphology, optical properties and I–V characteristics are analyzed as a function of surfactants. All films exhibit cubic fluorite structure with preferred orientations along (111) and (200) direction depending upon preparation conditions with an average grain size 37nm. Fourier Transform-Infra Red analysis confirmed the presence of terminal stretching (Ce=O) and phonon band (Ce–O) of the metal oxide network. The intensity of carbonation bands is diminished with the addition of amine groups. The films exhibit dense, smooth and crack-free tiny particles composed of closed chains, golf-ball, hollow spherical and slap shaped nanostructures with an average size 180nm. It also exhibits nicely separated well-outlined cones and spheres with rounded apex having an average roughness 121nm. Optical spectra reveal that the films are transparent (70%) in the visible region. Optical parameters like refractive index, optical conductivity and band gap are calculated for different surfactants. Photo Luminescence analysis indicates the presence of blue and green emission peaks in the visible region centered around ~425 and ~467nm attributed to the persistence of electron–phonon interaction between Ce4f and O2p bands. X-ray Photoelectron Spectroscopy analysis reveals the existence of Ce, O, Na and C atoms in the prepared films. It also brings out the dominant occurrence of Ce4+ rather than Ce3+ in the films. It is ensured that the rare earth compounds having unpaired electrons can give raise an extra satellite peak (923.5eV) due to photoelectron energy gain or loss assigned to finger print of CeO2 phase. The electrical conductivities are in the range 7.20×10−7 to 1.21×10−4S/cm for different surfactants.

On Tuning the Fluorescence Emission of Porphyrin Free Bases Bonded to the Pore Walls of Organo-Modified Silica

A sol-gel methodology has been duly developed in order to perform a controlled covalent coupling of tetrapyrrole macrocycles (e.g., porphyrins, phthalocyanines, naphthalocyanines, chlorophyll, etc.) to the pores of metal oxide networks. The resulting absorption and emission spectra intensities in the UV-VIS-NIR range have been found to depend on the polarity existing inside the pores of the network; in turn, this polarization can be tuned through the attachment of organic substituents to the tetrapyrrrole macrocycles before bonding them to the pore network. The paper shows clear evidence of the real possibility of maximizing fluorescence emissions from metal-free bases of substituted tetraphenylporphyrins, especially when these molecules are bonded to the walls of functionalized silica surfaces via the attachment of alkyl or aryl groups arising from the addition of organo-modified alkoxides.

The role of oxidizing agents in the structural and morphological properties of CeO2 nanoparticles

Cerium oxide (CeO2) nanoparticles with good crystallinity and smooth surface are prepared by chemical precipitation method with different bases (NH3, NaOH and KOH) using cerium nitrate as a source material. The effect of precipitating agents on the growth of cerium oxide nanoparticles are investigated by Photoluminescence (PL), X-ray diffraction (XRD), Fourier transform-infra red spectroscopy (FTIR), thermo gravimetric–differential thermal analysis (TG-DTA), Scanning electron microscope (SEM), Transmission electron microscope (TEM), and X-ray Photoelectron Spectroscopy (XPS). Cubic fluorite crystallites are detected by XRD pattern with preferred orientation along (111) direction. PL spectra revealed the presence of a strong and broad emission band at425nm due to the blue shift in the visible region. The broad band below 700cm−1 is due to the envelope of the phonon band of metal oxide (Ce–O) network as revealed by the IR spectra. The TG-DTA curves revealed that the total weight loss of the samples is 19.67% when the samples are heated upto 800°C. SEM images exhibits weakly agglomerated spheroid crystallites are obtained with the typical size in the range 10–50nm. TEM images display that the particles are nearly spherical and square in shape with diameter 8–12nm. XPS spectrum confirms the existence of Ce4+ oxidation states in CeO2samples.

A quantitative approach to tune metal oxide network morphology based on grazing-incidence small-angle X-ray scattering investigations

Based on a diblock copolymer-assisted sol–gel route, zinc oxide (ZnO) films with a foam-like network morphology are synthesized using zinc acetate dihydrate as a precursor. Poly(styrene-block-ethylene oxide) diblock copolymers with two different molecular weights are examined to compare the influence on the resulting morphologies. The variation in the porosity and in the pore size of the ZnO foam as a function of increasing amount of ZnO precursor are probed with grazing-incidence small-angle X-ray scattering (GISAXS). The GISAXS data are complemented with a surface topography investigation based on scanning electron microscopy. It is observed that the block copolymer template with the higher molecular weight yields a smaller average pore size and a lower average porosity of about 21%.

Synthesis of nano-scale fast ion conducting cubic Li7La3Zr2O12

A solution-based process was investigated for synthesizing cubic Li7La3Zr2O12 (LLZO), which is known to exhibit the unprecedented combination of fast ionic conductivity, and stability in air and against Li. Sol–gel chemistry was developed to prepare solid metal–oxide networks consisting of 10 nm cross-links that formed the cubic LLZO phase at 600 ° C. Sol–gel LLZO powders were sintered into 96% dense pellets using an induction hot press that applied pressure while heating. After sintering, the average LLZO grain size was 260 nm, which is 13 times smaller compared to LLZO prepared using a solid-state technique. The total ionic conductivity was 0.4 mS cm−1 at 298 K, which is the same as solid-state synthesized LLZO. Interestingly, despite the same room temperature conductivity, the sol–gel LLZO total activation energy is 0.41 eV, which 1.6 times higher than that observed in solid-state LLZO (0.26 eV). We believe the nano-scale grain boundaries give rise to unique transport phenomena that are more sensitive to temperature when compared to the conventional solid-state LLZO.

Large-scale, three-dimensional, free-standing, and mesoporous metal oxide networks for high-performance photocatalysis.

Mesoporous nanostructures represent a unique class of photocatalysts with many applications, including splitting of water, degradation of organic contaminants, and reduction of carbon dioxide. In this work, we report a general Lewis acid catalytic template route for the high–yield producing single– and multi–component large–scale three–dimensional (3D) mesoporous metal oxide networks. The large-scale 3D mesoporous metal oxide networks possess large macroscopic scale (millimeter–sized) and mesoporous nanostructure with huge pore volume and large surface exposure area. This method also can be used for the synthesis of large–scale 3D macro/mesoporous hierarchical porous materials and noble metal nanoparticles loaded 3D mesoporous networks. Photocatalytic degradation of Azo dyes demonstrated that the large–scale 3D mesoporous metal oxide networks enable high photocatalytic activity. The present synthetic method can serve as the new design concept for functional 3D mesoporous nanomaterials.

Highly Porous Metal Oxide Networks of Interconnected Nanotubes by Atomic Layer Deposition

Mesoporous metal oxide networks composed of interconnected nanotubes with ultrathin tube walls down to 3 nm and high porosity up to 90% were fabricated by atomic layer deposition (ALD) of alumina or titania onto templates of swelling-induced porous block copolymers. The nanotube networks possessed dual sets of interconnected pores separated by the tube wall whose thickness could be finely tuned by altering ALD cycles. Because of the excellent pore interconnectivity and high porosity, the alumina nanotube networks showed superior humidity-sensing performances.

NbVO5 Mesoporous Thin Films by Evaporation Induced Micelles Packing: Pore Size Dependence of the Mechanical Stability upon Thermal Treatment and Li Insertion/Extraction

Mesoporous thin films (MTFs) appear as an interesting architecture for positive electrodes in Li-ion energy storage systems because they offer high specific area and interconnected porosity presenting homogeneous pore size and wall thickness. However, it must be ascertained that the mesostructure survives template removal or/and crystallization and is retained on electrochemical cycling. In order to investigate the potentialities and limits of the soft-templating approach in the case of complex transition metal oxide networks, we deliberately selected a “difficult” compound: NbVO5 was chosen because it combines a challenging synthesis with reported severe structural distortions during the first lithium insertion in the bulk material. In this work, NbVO5 MTFs with different pore sizes were synthesized using the evaporation induced micelles packing (EIMP) method. PS-b-PEO diblock copolymers of different molar weights were used as structure directing agent in order to obtain wormlike porous networks with pore size and wall thickness ranging from 15 to 100 nm. Thermal ellipsometry analysis, used to track surfactant removal and crystallization of the layer, reveals that partial crystallization is possible while retaining the mesoporous architecture. Electron tomography complements result from environmental ellipsometric porosimetry, atomic force microscopy, and transmission electron microscopy to provide a comprehensive description of the structure. A multilayer process is also proposed to build crack-free thick mesoporous films. The mechanical stability of MTFs presenting three different pore sizes is tested by inserting Li+ in amorphous NbVO5 MTFs using cyclic voltammetry. Capacity retention data show that the mechanical stresses associated with Li+ insertion are better accommodated by MTFs compared to nonporous films, and this ability is enhanced as the pore size decreases.

Monolithic Zinc Oxide Aerogels from Organometallic Sol−Gel Precursors

Aerogels belong to the large class of porous solids. They are characterized by a network of a mechanically stable solid, most likely inorganic in nature comprising a large gas volume in comparison to the volume of the solid material. Although a large variety of aerogels with silicate networks already exist, examples for materials with transition metal oxide networks are rare. One particularly interesting target is zinc oxide because of its semiconducting and multifunctional character. A sol−gel process facilitating an organometallic precursor system is established. The mechanism of gelation has been studied in detail. Unlike most other sol−gel processes, at first a large number of nanocrystalline ZnO particles are formed that rapidly agglomerate to secondary aggregates instead of forming a network directly. Thus, these secondary aggregates determine the textural properties of the pore walls as they assemble into the final, highly cross-linked network. A monolithic ZnO aerogel with porosities greater than ...

Production of Ti–W Alloys from Mixed Oxide Precursors via the FFC Cambridge Process

W alloys were produced via the electrochemical deoxidation of mixed sintered precursors in a molten electrolyte at . Fully metallic samples were retrieved after of reduction. This reduction time was longer than that observed for metallization of sintered precursors. This was believed to occur as a result of significant differences in the reduction pathway, despite tungsten and molybdenum possessing similar interactions with titanium. It was found that the reduction initiated with the rapid reduction of to a fine W–Ti particulate. then proceeded to reduce sequentially through the lower oxides, with concurrent formation of . Between 1 and of reduction the sample is believed to be composed of and TiO. A comproportionation reaction between these two phases is then observed, resulting in the formation of W–Ti and . However homogenization between the product titanium and W–Ti does not take place until the titanium is sufficiently deoxidized; thus, forms late in the reduction process. It is believed that the late formation of in the reduction process, coupled with the lack of a conductive metal oxide network, accounts for the relatively slow reduction time.

Effect of Mixing of Metal Cations on the Topology of Metal Oxide Networks

Das Durchfahren des Co/Ni-Verhältnisses im Hydrothermalsystem CoCl2/NiCl2/Bernsteinsäure/KOH/H2O führte zu vier diskreten Phasen. Die strukturellen Beziehungen zwischen den bekannten organisch-anorganischen Hybridmaterialen 1, 2 und 4 wurden aufgeklärt, und das hybride Metalloxid [M7(OH)2(H2O)2(C4H4O4)6]⋅3.5 H2O (3) mit einem seltenen dreidimensionalen Metall-Sauerstoff-Metall-Gerüst wurde entdeckt. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2007/z700247_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Effect of Mixing of Metal Cations on the Topology of Metal Oxide Networks

Four distinct phases were isolated by adjusting the Co/Ni ratio in the hydrothermal system CoCl2/NiCl2/succinic acid/KOH/H2O. In addition to clarifying the relationships between known hybrid inorganic–organic materials 1, 2, and 4, [M7(OH)2(H2O)2(C4H4O4)6]⋅3.5 H2O (3), a hybrid metal oxide with a rare three-dimensional metal–oxygen–metal framework, was discovered.