Metal Oxide-Based Materials

Any metal oxide can, in principle, be prepared by sol-gel processing. There are two important differences between silicon (as a semimetal) and typical main group or transition metals that are highly relevant for sol-gel chemistry [3]  

As in silicate sol-gel processes, inorganic or metal-oiganic (alkoxide) precursors can be used. Many metal salts are hydrolytically unstable that is, they form oxide/hydroxide precipitates from aqueous solutions upon pH changes. This is due to the fact that water molecules coordinated to metal ions are more acidic than those in the noncoordinated state due to charge transfer from the oxygen to the metal atom. The series of equilibria shown in Eq. (1.5) is more easily shilled to the right than in water itself when the pH is increased (Le., if a base is added). 

As in the case of silica-based sol-gel processes, M—OH groups can also be created by hydrolysis of M—OR groups, that is, by addition of water to metal alkoxides. As outlined above, metal alkoxides are stronger Lewis acids than silicon alkoxides, and the formation of higher coordinated species is easier. Nucleophilic attack at the metal is thus facilitated, and the hydrolysis rates are strongly increased. For example, the hydrolysis rate of Ti(OR)4 is about 10 times faster than that of Si(OR)4 with the same alkoxide substituents. The reactivity of some tetravalent isopropoxides in hydrolysis reactions increases in the following order [4]  

The reactivity of many metal alkoxides toward water is so high that precipitates are often formed spontaneously upon addition of water. While the reactivity of alkoxysilanes has to be promoted by catalysts, the reaction rates of metal alkoxides must be moderated to obtain gels instead of precipitates (see below). 

In principle, the same parameters influence the rates of the hydrolysis and condensation reactions of metal alkoxides as those already discussed for silicon alkoxides (apart from the feet that no catalyst is needed). An additional fector is the degree of oligomerization of the alkoxide precursors. Silicon alkoxides are always monomeric, while metal alkoxides can associate through pa- or Ps-OR bridges (the subscript denotes the number of metal atoms coordinated to the bridging ligand). The reason for the association is, as discussed above, that the 

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The synthesis of nanostructured carbon using aliphatic alcohols as selfassembling molecules has demonstrated that this strategy can be extended beyond metal oxide-based materials [38]. Recently, we have reported the synthesis of a novel carbon material with tunable porosity by using a liquid-crystalline precursor containing a surfactant and a carbon-yielding chemical, furfuryl alcohol. The carbonization of the cured self-assembled carbon precursor produces a new carbon material with both controlled porosity and electrical conductivity. The unique combination of both features is advantageous for many relevant applications. For example, when tested as a supercapacitor electrode, specific capacitances over 120 F/g were obtained without the need to use binders, additives, or activation to increase surface area [38]. The proposed synthesis method is versatile and economically attractive, and allows for the precise control of the structure. 

Metal oxide-based materials are widely employed as catalysts for a wide number of applications, particularly in processes such as dehydrogenation and oxidation, where redox chemistry is important The structure of metal oxides facilitates these reactions through the transfer of oxygen, or the removal of hydrogen. In order to fully understand the structural dependence of these processes, and hence to refine existing catalysts and catalytic processes and to develop new active materials, it is... 

The coexisting CO and C02 in the reformate might also influence the desulfurization performance of metal oxide-based materials. Novochinskii et al. found that the coexisting C02 reduces the kinetic capacity of ZnO.177,178 They contributed it to competitive adsorption between H2S and C02 in the presence of steam. Another... 

Wu HB, Chen JS, Hng HH, Wen Lou X (2012) Nanostmctured metal oxide-based materials as advanced anodes for lithium-ion batteries. Nanoscale 4 2526-2542. doi 10.1039/ c2nrll966h... 

Metal oxide-based materials, carbon materials, and conducting polymers for electrochemical supercapacitor electrodes have been reviewed in detail (6). Two important future research directions have been summarized The development of composite and nano-structured electrochemical supercapacitor materials to overcome the problem of low energy density of electrochemical supercapacitors. 

Although all the different solution-processed metal oxide based materials exhibit great potential for performance optimization and commercialization, there are still some limitations regarding optical, electrical, chemical and mechanical properties for each solution-process method and materials. Therefore additional research is required in the field of CTL development to improve the performance of the OSCs and OLEDs further. 

During the past several years we have been working on an Anderson-type heteropolyanion, [Al(OH)6Mo60i8] and exploiting its linking propensity with metal ions-metal complex cations to obtain extended structures of new metal oxide based materials. We have demonstrated a chain-like extended structure based on an Anderson-type polyanion and a lanthanide cation linker in the compound [La(H20)7Al(OH)6Mo60i8] 4 H20." The structure of this... 

Metal/metal oxides are the materials of choice for construction of all-solid-state pH microelectrodes. A further understanding of pH sensing mechanisms for metal/metal oxide electrodes will have a significant impact on sensor development. This will help in understanding which factors control Nemstian responses and how to reduce interference of the potentiometric detection of pH by redox reactions at the metal-metal oxide interface. While glass pH electrodes will remain as a gold standard for many applications, all-solid-state pH sensors, especially those that are metal/metal oxide-based microelectrodes, will continue to make potentiometric in-vivo pH determination an attractive analytical method in the future. 

Microhotplates, however, are not only used for metal-oxide-based gas sensor applications. In all cases, in which elevated temperatures are required, or thermal decoupling from the bulk substrate is necessary, microhotplate-like structures can be used with various materials and detector configurations [25]. Examples include polymer-based capacitive sensors [26], pellistors [27-29], GasFETs [30,31], sensors based on changes in thermal conductivity [32], or devices that rely on metal films [33,34]. Only microhotplates for chemoresistive metal-oxide materials will be further detailed here. The relevant design considerations will be addressed. 

Since 2004 [183], graphene research has evolved from a heavily theoretical and fundamental field into a variety of research areas [301]. Its electrical, magnetic, physical-mechanical, and chemical properties position it as the most promising material for molecular electronic and optoelectronic applications, possibly replacing the currently used silicon and metal oxide based devices. Nonetheless, further research is essential in order to control easily such properties and construct devices with specific and novel architectures to explore in depth all of these exciting properties, as well as to achieve the synthesis of large-scale, size- and layer-count controlled graphene. 

Typical n-alkyl ligand densities that can be achieved with -alkylchlorosi lanes are within the range of 2.5-3.2pmolm-2, whilst with disilazanes, ligand densities approaching the limited values can be reached under optimized conditions, i.e. between 3.50 and 4.20 pmolm-2. Surface-modified zirconia or other metal oxide based RPC sorbents can be similarly prepared by either of the above two strategies. Compared to n-alkylsilicas, these ceramic RPC sorbents show different selectivities with synthetic peptides, as well as different chemical stability profiles. Consequently, RPC sorbents based on these types on surface-modified, porous metal oxide materials fulfill useful and complementary roles, but at this point in time, have achieved a more limited range of applications for the resolution of synthetic peptides due to their limited availability. 

Since the first synthesis of mesoporous materials MCM-41 at Mobile Coporation,1 most work carried out in this area has focused on the preparation, characterization and applications of silica-based compounds. Recently, the synthesis of metal oxide-based mesostructured materials has attracted research attention due to their catalytic, electric, magnetic and optical properties.2 5 Although metal sulfides have found widespread applications as semiconductors, electro-optical materials and catalysts, to just name a few, only a few attempts have been reported on the synthesis of metal sulfide-based mesostructured materials. Thus far, mesostructured tin sulfides have proven to be most synthetically accessible in aqueous solution at ambient temperatures.6-7 Physical property studies showed that such materials may have potential to be used as semiconducting liquid crystals in electro-optical displays and chemical sensing applications. In addition, mesostructured thiogermanates8-10 and zinc sulfide with textured mesoporosity after surfactant removal11 have been prepared under hydrothermal conditions. 

Despite their solid appearance, within the gel the liquid component is mobile and is only held by capillary forces. The solid network can be either a covalent polymer or a supramolecular assembly of small molecules. The latter class of compound, termed low molecular weight gelators (LMWG) of which 14.11 14.15 are examples, is perhaps of most interest to supramolecular chemists. Perhaps the most well known gels are metal oxide based polymeric materials produced by the sol-gel process. The sol-gel process involves the hydrolysis and polycondensation of monomeric metal salts such as early transition... 

To ensure a sufficient electronic and thermal conductivity of the positive electrode during the charge and discharge process, conductive additives are required in the positive, transition metal oxide-based electrode.3133 Compared to metal powders as potential conductivity enhancer, carbon materials combine high electronic and thermal conductivity with low weight, low costs, relatively high chemical inertness, and nontoxicity. Conductive carbons optimize the electrical resistivity of the positive electrode mass but are not involved in the electrochemical redox process which delivers the... 

Lanthanide alkoxide complexes are of fundamental interest because of the associated synthetic challenges and the great variety of their structures. They are also an important class of materials, either as catalysts to promote useful but otherwise difficult chemical transformations or as molecular precursors for the realization of high-quality metal oxide-based advanced materials. Selected examples illustrating these applications are discussed below. 

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