Hydrothermal Routes

Because of the highly controlled diffusivity in a strong solvent medium and in a closed system, hydrothermal/solvothermal techniques offer interesting advantages in the processing or preparation of nanomaterials. 

Knowledge of the fundamental aspects that drive hydrothermal/solvothermal processes has increased in recent years, and a solid understanding of changes in solvent parameters (e.g. structure at critical, supercritical and subcritical conditions, dielectric constant, pH, viscosity, density, etc.) under hydrothermal conditions, as well as on changing pressure and temperature, is considered a key aspect for programming experimental approaches, as they influence the solubility and transport behavior of the precursors involved in liquid-solid NP synthesis. 

Spherical [241] or platelet-shaped [242] Cu NPs could also be prepared. In the former case, glucose was used as reducing agent and CuO particles as precursor, whereas in the latter case hypophosphite and SDS were used to hydrothermally convert Cu ions into elongated nanoplatelets. 

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Even though Ti02-based materials have been far more investigated than any other photocatalyst and this chapter is dedicated to these systems, it is important at least to mention here some of the promising alternative materials studied for wastewater treatment. CdS hollow nanospheres were prepared in a single-step hydrothermal route by Li et al. [109] using the ionic liquid l-butyl-3-methylimidazolium... 

Diffuse reflectance UV-spectra showed the framework incorporation of titanium and niobium in the all sample studied. The detailed characterisation of the materials will be presented in the subsequent paper. The catalytic properties are studied by the epoxidation of a-pinene, using H20, as an oxidizing agent. The framework 2.0wt % Ti-MCM-41 and 2.0wt % Ti-MCM-48 mesoporous molecular sieves samples synthesized by direct hydrothermal route,... 

Ambient aqueous routes and hydrothermal routes The synthesis of rare earth orthophosphate NCs could be carried out in aqueous solutions by the precipitation of rare earth cations by phosphate anions, due to the low aqueous solubility of RPO4. A typical synthesis begins with soluble rare earth salts in aqueous solution, such as nitrates or chlorides, and a phosphate source like phosphate M H3 P04, triphosphate M5P3O10 (M = Na, K, or NH4), or PI3PO4. The two solutions are mixed at room temperature or elevated temperature to form gel-like precipitation as precursor. Then the mixture of precipitation and mother liquor is... 

FIGURE 26 TEM (upper part) and HRTEM (lower part) micrographs of LaP04 Eu NCs synthesized by a hydrothermal route, which is prepared at pHstart = 1-7 (tight) and at pHstart = 12.5 (left), respectively. Reprinted with permission from Meyssamy et al. (1999). Copyright 1999 Wiley-VCH. 

FIGURE 28 TEM images of hexagonal LnP04- nH20 products synthesized by a hydrothermal route (A) La, (B) Ce, (C) Eu, (D) Gd. Reprinted with permission from Yan et al. (2004). Copyright 2004 Elsevier. 

In addition to the orthophosphates, other rare earth phosphates could also be obtained through hydrothermal routes. Tang et al. reported the synthesis of so-called CeP (with a composition of Ce(FIP04)2" H2O) nanotubes (Tang et al., 2005c). Concentrated phosphoric acid solution of 6 M was heated to a temperature ranging from 50 to 110 °C to form a condensed lirrear polyphosphate (P 03 +i) +2 Then, the (NH4)2Ce(N03)e solution is... 

Hydrothermal routes Under ambient conditions, the low reaction temperature and fast precipitation rate have deleterious effect on the crystallization and optical performance of rare earth vanadate nanomaterials. Referring to traditional solid-state reactions, bulk YV04 Eu phosphors require a calcinations temperature above 1300 K, but it is too high for the preparation of nanomaterials. Alternatively, hydrothermal routes could provide the adequate energy for solution phase reactions, which have been widely described in preparation of ceramic powders. The high pressure and temperature largely promote the dissolution-reprecipitation process, so as to decrease the lattice defects of NCs. With fine modulation, this method is also efficient to produce nano-sized crystals. 

Li and coworkers operated the hydrothermal route under extremely acidic conditions and observed single crystalline t-YVO with nanobelt morphology. Compared with other growth directions, the crystal growth along the [010] direction seemed to be less perturbed by hydrated proton at pH value of 1. Below this acidity, polyhedron microcrystals were the primary product, which agree with previous reports (Li et al., 2008f). 

Sheet-like, flower-like, and bimdle-like YB03 Eu " NCs are synthesized via a hydrothermal route. The time-resolved emission spectra... 

Tang, X. L., Jiang, P., Ge, G. L., Tsuji, M., Xie, S. S. and Guo, Y. J. (2008). Poly(N-vinyl-2-pyrrolidone) (PVP)-capped dendritic gold nanoparticles by a one-step hydrothermal route and their high SERS effect. Langmuir 24 1763-1768. 

A survey of recent literature on zeolite membrane preparation reveals that synthesis processes, even for well-known zeolite structures (i.e., MFl, LTA), are still carried out batchwise, using a hydrothermal route to produce a thin layer from hydrogels or sols containing the corresponding nutrients. As a general rule, the reactant mixture in contact with the support changes in composition with time provoking a reduction of the membrane quality. 

Ardizzone, S. et al.. Hydrothermal route to pure phase ZiDj. Interfacial reactivity by XPS and electrochemical determinations. Colloids Surf. A, 90,45, 1994. 

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