Nonhydrolytic Synthesis

Nonhydrolytic systems refer to those using solvents other than water in the synthesis of Pt nanoparticles. While all the control principles for aqueous systems should be applicable to nonhydrolytic systems, the possible choices for solvents, Pt precursors, surfactants and reducing reagents are broaden for the latter 

Precursor Sur ctant Solvent Reductanf Additive Shape Reference(s) 

4 Other Solvents Recently, toluene has been used as solvent to prepare Pt cubes and other nanostmctures [51, 52]. Pt nanospheres, nanowires and dendrites have also been obtained in tetrahydrofuran (THE) [139, 146, 147]. Both, supercritical methanol [148] and supercritical CO2 [149, 150] have also been used as solvents in the creahon of Pt nanoparticles. 

Solubility, redox potential and thermal stability are important parameters to be considered when designing experiments, and organometallics are commonly used 

Diols are a group of reducing agents that have attracted regular use in nonhydrolytic systems to prepare Pt nanostructures two such examples are HDD [11, 40, 43, 137] and DDD [40, 137]. These compounds have two neighboring hydroxyl groups, and function as mild reductants at elevated temperatures in the reactions this in turn leads to relatively slow reaction kinetics that can be controlled. 

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The nonhydrolytic M—O—M bond formation via ester elimination between metal alkoxides and carboxylic acids is a well-known approach in sol-gel chemistry. In this direction, titanium -butoxide and acetic acid were used for the nonhydrolytic synthesis of anatase Ti02 nanopartides at 100 °C [92]. Moreover, spindle-shaped nanoporous anatase Ti02 mesocrystals with a single-crystal-like structure and tunable sizes were synthesized in the tetrabutyl titanate and acetic acid system without any additives imder solvothermal conditions [93]. A complex mesoscale assembly process, involving oriented aggregation of tiny anatase nanocrystals and entrapment of in situ produced butyl acetate as a porogen, was proposed for the formation of the mesocrystals. They exhibited a good performance as anode material for lithium ion batteries [93]. 

Nonhydrolytic sol-gel synthesis is a more recent variation on the sol-gel method. A metal halide is reacted with an oxygen derivative of the metal (Eq. 2-234). The reaction is catalyzed by Lewis acids such as FeCl3 and alkyl halide is a by-product [Hay and Raval, 2001], The reaction has been used to produce alumina (A1203) and titania (Ti02) as well as silica [Corriu and Leclercq, 1996]. 

Kriesel JW, Sander MS, et al (2001) Block copolymer-assisted synthesis of mesoporous, multicomponent oxides by nonhydrolytic, thermolytic decomposition of molecular precursors in nonpolar media. Chemistry of Materials 13(10), 3554-3563... 

Trentier T J ef a/1999 Synthesis of TI02 nanoorystals by nonhydrolytic solution-based reactions J. Am. Chem. 

The synthesis of metal oxide nanocrystals by wet chemical processes can be divided basically into two major groups (a) chemical synthesis method based on the hydrolysis of metal alkoxides or metal halides (b) chemical synthesis based on the nonhydrolytic method. Examples of these methods are described below. 

N. L. Wu, S. Y. Wang, and I. A. Rusakova. Inhibition of crystallite growth in the sol-gel synthesis of nanocrystalline metal oxides. Science, 285 1375-1377, 1999 A. Vioux. Nonhydrolytic sol-gel routes to oxides. Chem. Mater., 9 2292-2299, 1997 J. Rockenberger, E. C. Scher, and A. P. Alivisatos. A new nonhydrolytic single-precursor approach to surfactant-capped nanocrystals of transition metal oxides. J. Am. Chem. Soc., 121 11595-11596, 1999... 

Jun YW, Lee JH, Choi JS, Cheon J (2005) Symmetry-controlled colloidal nanocrystals nonhydrolytic chemical synthesis and shape determining parameters. J Phys Chem B 109 14795-14806 Graham DL, Ferreira HA, Freitas PP (2004) -Magnetoresistive-based biosensors and biochips. Trends Biotechnol 22 455-462... 

Trentler TJ, Denier TE, Bertone JF, Agrawal A, Colvin VL. Synthesis of Ti02, nanocrystals via nonhydrolytic solution-based reactions. J Am Chem Soc 1999 121 1613-4. 

Chojnowski, J. Rubinzstajn, S. Fortuniak, W. Kurjata, J., Synthesis of Highly Branched Alkoxysilane-Dimethylsiloxane Copol)miers by Nonhydrolytic Dehydrocarbon Polycondensation Catalyzed by Tris(pentafluorophenyl)borane. 

Alternative routes of nonhydrolytic conversion of chlorosilanes into siloxanes are also known. The most common one uses dimethyl sulfoxide (DMSO) as the oxygen source. Another useful method of the siloxane bond formation is the reaction of dichlorodiorganylsilanes with some metal oxides, such as ZnO (eqn [3]). The reaction is particularly useful for the synthesis of cydotrisiloxanes, which are rather difficult to prepare in any other way. By this method they can be obtained with the yield 30-60%. The mechanism of this transformation was recently studied by theoretical methods indicating that the reaction involves insertion of -Zn-O- into the Si-Cl bond. An alternative pathway involving transient silanone was shown to be much less thermodynamically feasible. 

Figure 8.6 Zirconia nanoparticles obtained via the nonhydrolytic sol-gel synthesis, (a) Overview TEM image ...

Joo, J., Kwon, S.G., Yu, T., Cho, M., Lee, J., Yoon, J. and Hyeon, T. (2005) Large-scale synthesis of Ti02 nanorods via nonhydrolytic sol-gel ester elimination reaction and their application to photocatalytic inactivation of E. coli. The Journal of Physical Chemistry E, 109, 15297-302. 

Yu, T.Y., Joo,)., Park, Y.I. and Hyeon, T. (2005) Large-scale nonhydrolytic sol-gel synthesis of uniform-sized ceria nanocrystals with spherical, wire, and tadpole shapes. Angewandte Chemie-Intemational Edition, 44, 7411-14. 

The aim of this chapter is to highlight the contribution of the nonaqueous sol-gel approach not only toward nanoparticle synthesis, but also for the mechanistic understanding of the organic and inorganic reactions involved in nanoparticle formation. In addition, we provide selected examples on the assembly of such nonhydrolytically prepared nanoparticles into one-, two-, three-dimensional structures and their application in selected fields of technology. 

The nonhydrolytic ether route involving the reaction of metal chloride precursors with diisopropyl ether was developed for the synthesis of mesoporous mixed... 

After several decades of intense research on the development of synthesis routes, it is nowadays possible to prepare nanoparticles with amazing structural, compositional, and morphological sophistication. But in comparison to the highly advanced synthesis know-how, knowledge about mechanistic aspects of nanoparticle formation has not yet reached the same leveL The reasons for this are not lack of interest or research efforts, but may be foimd in both the complexity of the processes involved in nanoparticle formation and the difficulty to monitor the reactions from the dissolution of the precursor to the formation of the final nanocrystalline product However, in the case of nonhydrolytic sol-gel processes, chemical reaction mechanisms are relatively well established. The reaction rates are typically slower than those in aqueous systems due to the moderate reactivity of the C—O bond (which plays the major role in nonhydrolytic reactions in contrast to the O—H bond in aqueous systems), and it is relatively straightforward to investigate the organic reactions that are correlated to nanoparticle formation and thus provide information about possible formation mechanisms [5]. 

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