Hydrothermal crystallization

Key words Point defects, photoluminescence, photo-EPR, single crystals, hydrothermal method, exciton, donor, acceptor, thermoluminescence... 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

Hydrothermal precipitation— hydrodiermal crystallization Hydrothermal decomposition... 

Ba(OH)2 and FeClj were used as starting materials. The precipitate was crystallized hydrothermally in an apparatus (Figure 1.14) combined with an attritor and... 

Materials in addition to quartz that can be crystallized hydrothermally include carbonates, germanates, and silicates (see Table 1). ... 

Table 1 summarizes the surface properties of the various mesoporous titanosilicate samples. The BET surface areas of these titanosilicates (850-1250 mg ) was typical for that shown by mesoporous materials. The BET surface area and BJH average pore diameter is found to decrease after Ti grafting onto Ti-MCM-41 sample. The postsynthesis grafted Ti is expected to react with the surface silanol groups of the walls in a random fashion in the most accessible sites near the pore mouth and wider pores. As a consequence there is clear decrease in pore size after titanium grafting. The pore size distribution is also found to become narrower around the average pore diameter. The BET surface areas of disordered mesoporous materials is found to be lower than that of the ordered MCM-41 type materials. BJH average pore diameter of mesoporous titanosilicates TiO-SiO(l) and TiO-SiO(2) prepared by modified sol-gel method is much lower than that of disorderd mesoporous Ti-Meso and ordered Ti-MCM-41 samples crystallized hydrothermally. But all these samples show a narrow pore size distribution. All the samples exhibit isotherms of type IV, typical of mesoporous materials, with a H2 hysteresis loop (Fig. not shown). 

Ordered mesoporous carbons, prepared by nanocasting, have also been used successfully as templates for the synthesis of hierarchical zeolites [159, 160). This is the case of CMK-3, an ordered mesoporous carbon attained by nanoreplication of pure silica SBA-15. The hierarchical zeolites obtained by employing CMK-3 as a template mainly present supermicropores or small mesopores with a size around 2 nm. The textural properties of the hierarchical zeohtes can be tuned by changing the type of CMK-3 carbon used. A modification of this method consists of directly impregnating the composite SBA-15-carbon or MCM-41-carbon with TPAOH and leaving the mixture crystallizing hydrothermally under steam. After calcination, a mesoporous ZSM-5 is formed with mesopore sizes around 3.5 and... 

Considerable interest in preparation of oriented films such as in the review by Brinker is apparent [11]. Much of the work involves gas phase preparation from methods like chemical vapor deposition (CVD). However, solution phase sol-gel methods were also recently used in many studies. Heterogeneous nucleation and growth on preorganized modified substrates takes away the need to use single crystals. Hydrothermal alteration routes were also summarized [11]. 

Hydrothermal synthesis involves the hydrothermal treatment of an aqueous suspension of two metal oxides in a pressurized vessel at a high temperature for a few days [7]. With this method, the synthesis of LDHs is obtained leading to the conversion of small LDH crystallites to larger and well-defined crystals. Hydrothermal synthesis method, also known as hydrothermal crystallization, is used when precise LDH structural properties are required because it enables the transformation of the amorphous precipitates to the crystalline LDH form. According to hterature, with this synthesis method the crystallization of an amorphous trivalent metal oxide (M2 "Oj) precursor in the presence of a suitable divalent metal oxide (M"0) is achieved [7]. 

---