Synthesis and Characterization of Nano-Aerogels

2.2. Supercritical and Subcritical Water or Alcohol

The high temperature SCF processing of inorganic materials is attractive for materials engineering, as: (1) a variety of morphologies are obtained that are difficult to be synthesized with alternative methods; (2) the high surface area and the microstructures are maintained due to the low interfacial tension of the SCF; (3) the crystalline phases are obtained immediately after the high temperature SCF process, and there is no need to recover the solid from solution and post-annealing; and (4) the process is easily adaptable to continuous production.^{94, 117}

The experimental setup of the SCF continuous system for synthesizing oxide nanoparticles includes: a high pressure pump for pumping the fluid into the reactor, a regulating valve for maintaining the pressure, and the high-pressure reactor.^118

The first direct sol-gel process of alkoxides in the high-temperature SCF was studied by Pommier et al. in 1992. After that, many crystalline nanoparticles have been synthesized. As an example, 20 ~ 60 nm TiO₂ anatase nanoparticles were prepared from titanium isopropoxide (TIP).^117 By using IR analysis, a kinetics study was conducted. The reaction was assumed to be a hydrolytic decomposition, rather than a pure thermal decomposition, followed by a polymerization-polycondensation and precipitation of the solid. Interestingly, it was found that the temperature and pressure had no significant effect on the particle size of TiO₂.^118 In other research, anatase TiO₂ was deposited on the substrate surface and inside the pores of commercial -Al₂O₃ by hydrolytic decomposition of titanium isopropoxide in supercritical isopropanol. Various experimental parameters, i.e. temperatures, precursor concentration, water-to-alkoxide ratio, solution flow rate, and deposition time, were studied by Brasseur-Tilmant.^119

Using the same technique, Znaidi et al. prepared submicron MgO powders with a surface area of 210 ~ 280 m²/g from magnesium isopropoxide or magnesium acetate at approximately 200 C in liquid alcohol or supercritical alcohol-CO₂ mixtures.^{120, 121} The mechanism of the sol-gel process in the SCF comprises hydrolysis and polycondensation:

-Mg-OR + H₂O ⇌ -Mg-OH + ROH (2.4)

-Mg-OH + RO-Mg- ⇌ -Mg-O-Mg- + ROH (2.5)

-Mg-OH + HO-Mg- ⇌ -Mg-O-Mg- + H₂O (2.6)

While the sol-gel reactions of magnesium alkoxide in the SCF were the same as those in the conventional solvent, Znaidi et al. also proposed the scheme of formation of the polycondensates from MgL₂ (L= acac or acetate):

MgL₂ + ROH ⇌ [MgL(OR)(ROH)]_n + nLH (2.7)

[MgL(OR)(ROH)]_n + ROH ⇌ [MgL(OR)_x(ROH)_4-x]_n + nLH (2.8)

The resulting polycondensates were thermally decomposed into cubic MgO with a diameter less than 100 nm at a higher temperature of 450 C.^121

Znaidi et al. also synthesized BaTiO₃ powders with a particle size of ca. 10 nm in a semi-continuous process under subcritical and supercritical isopropanol. There were two steps involved: (1) the hydrolysis of the alkoxide BaTi(O-iC₃H₇)₆ in isopropanol at temperatures 100  200 C in a tubular flow reactor; and (2) a thermal treatment of the formed solids at a higher temperature than 235 C.^122

In 1997, Hu et al. prepared hydrous zirconia nanoparticles via hydrolysis and controlled hydrothermal condensation of zirconyl inorganic salts, ZrOCl₂ and ZrO(NO₃)₂ in aqueous solution. The hydrous zirconium oxide particles were porous, cube-shaped aggregates of small crystallites (< 5 nm). The nucleation and nanoparticle growth were monitored with a dynamic laser light-scattering spectrophotometer.^123

From 1999 to 2000, Zeng et al. prepared Pb-zirconate-titanate (PZT), Pb(Zr_0.52Ti_0.48)O₃, and barium titanate (BaTiO₃) thin films on Si substrates using a sol-gel-hydrothermal technique which involved a combination of a conventional sol-gel process and a hydrothermal method. At a low processing temperature of 100 200 C, the dense and pinhole-free PZT films in single perovskite phase and polycrystalline BaTiO₃ were observed using atomic force microscopy (AFM).^124-126

In 2000, Shen et al. prepared ZrO₂ particles with a diameter of ca. 15 nm by hydrothermal processing of zirconium oxychloride octahydrate (ZrOCl₂8H₂O) at 150 C. The clear solution containing ZrO₂ sol was spin-coated onto a silicon wafer or glass substrate to form a ZrO₂ thin film. The multilayer films were characterized with FTIR, AFM and a laser damage test.^{127, 128} In the same year, Zhao et al. prepared nanocrystalline zirconia sols (4 nm) with a metastable cubic phase from diglycol-modified zirconium propoxide via the alcohol thermal process. The chelating bonding of diglycol with zirconium was found to be important in obtaining stable zirconia sols, and the formation of cubic ZrO₂ crystals was found due to the template of diglycol and the high alignment of solute oligomers through condensation. The ZrO₂ gel generated from the sols exhibited a mesoporous structure with a pore size of 2.5-3.8 nm and Brunauer-Emmett-Teller surface area of 130-266 m²/g.^129

In 2002, Hayashi et al. prepared TiO₂ powders with mesoporous structure under various subcritical and supercritical water conditions using titanium isopropoxide (TIP) as a starting material. The synthesized TiO₂ powders were used for photocatalytic hydrogen evolution in an aqueous methanol suspension. Activities per unit surface area were found to be higher from the TiO₂ synthesized under supercritical water conditions than those synthesized under subcritical water conditions. This phenomenon was explained by the higher crystallinity of the TiO₂ powders under supercritical water conditions.^130 In the same year, Yang et al. prepared anatase and rutile TiO₂ nanoparticles by means of a hydrothermal treatment of tetrabutylammonium hydroxide-peptized and HNO₃-peptized sols at 240°C.^131

In 2003, Parvulescu et al. prepared mixed vanadia-titania-silica catalysts (3 or 6 wt% V₂O₅, and 16-34 wt% TiO₂) in one pot by both sol-gel and hydrothermal methods in the presence of different surfactants, i.e., cetyltrimethylammonium bromide (CTMABr) or octadecyl-trimethylammonium bromide (ODCABr). Tetraethylorthosilicate (TEOS) was used as the precursor for silica, titanium isopropoxide (TIP) for titania, and vanadyl sulfate for vanadia. The resulting materials were characterized by N₂ adsorption and desorption curves at 77 K, Raman spectroscopy, XRD, XPS, TEM, NH₃-DRIFTS, and H₂-TPR.^132

In 2004, Tai et al. prepared ZrO₂ nanospherical particles using thermal hydrolysis of zirconyl nitrate solution at a temperature at 95 °C. Hydrolysis of ZrO₂ salt was confined in the droplets of the water/CTAB/hexanol reverse microemulsion. The particle shape and size distribution varied greatly with both the temperature and composition. However, calcination at 600 °C was required to obtain the tetragonal phase, and the size distribution of the particles was rather large.^133

In 2005, Corradi et al. prepared TiO₂ nanospheres with a diameter of ca.10 nm and crystal grains (100 by 50 nm) using microwave hydrothermal treatment of TiOCl₂ solution. An enhanced powder crystallinity and decreased processing time was achieved.^134 Kang et al. synthesized TiO₂ and Bi/TiO₂ particles with particle size below 25 nm using a solvothermal method, in which isopropoxide was mixed with ethanol in an autoclave and heated to 200 °C for 10 hours. The resulting materials were characterized with XRD, SEM, UV-visible spectrometry, and the plasma-photocatalytic activity for acetaldehyde was tested.^135