Hydrothermal Coating Deposition

Some work has been reported on deposition of hydroxyapatite under hydrothermal conditions, that is much above 100 °C. This includes a study by Liu, Savino and Yates (2011) who coated hydroxyapatite on titanium, stainless steel, aluminium and copper substrates by a seeded hydrothermal deposition method. The deposition strategy included an electrochemical reaction to form quickly a thin layer of HAp seed crystals. Subsequent hydrothermal crystal growth from the seed layer resulted in dense and durable HAp films. In a typical hydrothermal synthesis, a solution of Na2EDTA (0.20 M) and Ca(NOs)2 (0.20 M) was prepared in 15 ml water and a solution of (NH4)2HP04 (0.12 M) in 15 ml water was prepared in a second container. The two source solutions were mixed together after the pH of each solution was raised to 10.0 with ammonium hydroxide. The resulting combined solution was stirred at room temperature for about 20 min and then transferred to a Teflon-lined stainless steel pressure vessel of 40 ml internal volume. 

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If substantial arsenic is present in a sandstone or conglomerate, it may occur in hematite or other iron (oxy)(hydr)oxide cements or coatings on mineral grains. Arsenic may also be enriched in sandstones, conglomerates, and other clastic sedimentary rocks if hydrothermal or other secondary sulfide minerals are present (e.g. St. Peter Sandstone, (Gotkowitz et al., 2001) arsenian pyrite cement in the Marshall Sandstone of the Michigan Basin, USA, (Kolker et al., 2000 Szramek, Walter and McCall, 2004)). Sections of the St. Peter Sandstone in eastern Wisconsin, USA, are enriched in sulfide minerals and contain about 500 mg kg-1 of arsenic. In contrast, unmineralized portions typically have <10 mg kg-1 (Gotkowitz et al., 2001). If present in sedimentary rocks, hydrothermal sulfide deposits often tend to concentrate in veins and faults more than the rock matrices. 

Trace metals are introduced to the ocean by atmospheric feUout, river runoff, and hydrothermal activity. The latter two are sources of soluble metals, which are primarily reduced species. Upon introduction into seawater, these metals react with O2 and are converted to insoluble oxides. Some of these precipitates settle to the seafloor to become part of the sediments others adsorb onto surfaces of sinking and sedimentary particles to form crusts, nodules, and thin coatings. Since reaction rates are slow, the metals can be transported considerable distances before becoming part of the sediments. In the case of the metals carried into the ocean by river runoff, a significant fraction is deposited on the outer continental shelf and slope. Hydrothermal emissions constitute most of the somce of the metals in the hydrogenous precipitates that form in the open ocean. 

Coats, R. 1940. Propylitization and related types of alteration on the Comstock Lode. Economic Geology, 35, 1-16 Creasey, S. C. 1959. Some phase relations in hydrothermally altered rocks of porphyry copper deposits. Economic Geology, 54, 351-373. 

B.A. McCool, N. Hill, J. DiCarlo and W.J. DeSisto, Synthesis and characterization of mesoporous silica membranes via dip-coating and hydrothermal deposition techniques, J. Memb. Sci., 218, 55-67 (2003). 

Liu, D., Savino, K., and Yates, M.Z. (2011) Coating of hydroxyapatite films on metal substrates by seeded hydrothermal deposition. Surf. Coat. Technol., 205, 3975-3986. 

Dozens of methods to synthesize nanotubes, nanowires, and nanorods have been reported that can be found in the references included in Table 1. In addition to the most well known ones, such as hot plasmas, laser ablation, chemical vapor deposition, high temperature solid state and hydrothermal synthesis, fill-ing/coating of carbon nanotubes and similar types of materials, three methods have been developed that enable the synthesis of a wealth of new anisotropic nanoparticles. 

Nanocomposites are materials in which nanoparticles (in this case, nanorods) are dispersed in a continuous matrix. The matrix may be a polymer, nanorods, or other nanoparticles. Nanorod composites find applications in diverse areas such as efficient charge storage, removal of contaminants (e.g. surfactant) from water, emissivity control devices, and metallodielectrics, and so on. A number of methods such as electroless deposition, the sol-gel method, the hydrothermal method, solution casting, carbother-mal reduction, the template-based method, the sonochemical method, and electrospinning can be used to prepare composite nanorods. Nanorod composites are different from core-shell nanorods. In core-shell nanorods, the coating is uniform, whereas in the nanorod composite (consisting of a nanorod and a nanoparticle on a surface), fine nanoparticles are dispersed on the surface of the nanorods. Some specific examples of the preparation of nanocomposites consisting of nanorods are described below. 

Kashima T, Matsuda Y, Fujiyama H (1991) Development of the quadrupole plasma chemical vapour deposition method for low temperature, high speed coating on an optical fibre. Mater Sci Eng A 139 79-84 Kaya C, He JY, Gu X, Butler EG (2002) Nanostructured ceramic powders by hydrothermal synthesis and their applications. Microporous Mesoporous Mater 54 37-49... 

In contrast to water splitting, only one example has been reported for CO2 photoconversion. Although CO2 photoconversion has a similar mechanism, it requires two to eight electrous to reduce one CO2 molecule to the desired product. In other words, more free electrons are required in the photocatalyst, which is often accompanied by a dramatic increase in the recombination rate. Xia et al. studied the reduction of CO2 with H2O using M WCNTs-supported Ti02 that were prepared by both sol-gel and hydrothermal methods. In using the sol-gel method, the MWCNTs were coated with anatase nanoparticles, and by the hydrothermal method, rutile nauorods were uniformly deposited on the MWCNTs. The selectivity of the product depended on the method used in material preparation formic acid was obtained from hydrothermal synthesis and ethanol was produced from sol-gel synthesis. 

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