We studied the growth of SnO2 nanocrystals with a tailored structure via surface capping assisted hydrothermal approach with tetramethyl ammonium hydroxide (N(CH3)4OH; TMAH). KOH and NaOH were also used instead of TMAH for comparison. The nanocrystals with a size ranging from 3.2 ± 0.9 to 74 ± 20 nm were grown at 150 °C for 24 h depending on the pH. N(CH3)4 + capped the surface of SnO2 and improved the dispersion of the nanocrystals in basic aqueous solutions. The capping provided nanocrystals finer than those grown with KOH and NaOH because of suppressed Ostwald ripening via a reduction in surface energy. High-resolution transmission electron microscopy revealed that the nanocrystals grown in strong basic solutions with TMAH had cubic morphology terminated by the (001) and (110) faces. This strongly suggests that N(CH3)4 + preferentially caps the (001) face with the highest surface energy and decreases its surface energy to be comparable to that of the (110) face with the lowest surface energy. Anisotropic capping promotes the formation of the cubic superstructure via the directed self-assembly of primary cubic nanocrystals in very strong basic solutions. Raman spectroscopy suggested that the SnO2 nanocubes grown in strong basic solutions with TMAH have less of a surface hydration layer as well as bulk combined water.

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46) C6H6 2,4,6-C13C6H2 C(CN)Me, (47) 2,4,6-Cl3C6H2 CF,CF, (48) C6F5 C(CN)Me, a (49) C6H6 C6Fii C(CN)Ph, (50) C6H6 Generated from azobisisobutyronitrile. f C6H6-CH2C12 (1 : 1).b Generated from lauroyl peroxide.P Perfluoro(methylcyc1ohexane).8 Generated by the photolyscs of perfluoro-6 Generated by the photolysis of * The six y-fluorines are not completely equivalent. alkyl iodides. di-t-butyl peroxide.1 Generated by the oxidation of diphenylacetonitrile with nickel peroxide.

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A facile method to synthesize WO 3 ·H 2 O square nanoplates via a mild aging (50°C) of ion-exchanged precursor was developed. The ion-exchanged precursor was prepared by passing the sodium tungstate solution (Na 2 WO 4 ) through a protonated cationexchange resin, and used as impurity-free acidified solution (H 2 WO 4 ) for synthesizing WO 3 ·H 2 O nanoplates. No shape-directing additive was employed. It was observed that the yellow particles were precipitated under aging at 50°C. After aging for 8 h, the precipitated particles were characterized as the WO 3 ·H 2 O nanoplates, and their morphological evolution to square platelet proceeded with an increase of aging time. After aging for 24 h, the WO 3 ·H 2 O square nanoplates were predominantly synthesized. The square nanoplates consisted of a few or several stacked thin layers (thickness, ³10 nm/layer), and provided the well-defined {010} facet for two dominantly exposed surfaces and {101} side facets. Their lateral dimension reached several hundreds of nanometers. It is thus demonstrated that the mild aging (50°C) of the ion-exchanged precursor is a simple and impurity-free synthetic route for WO 3 ·H 2 O square nanoplates. In addition, the monoclinic WO 3 nanoplates were successfully obtained by dehydration-induced topochemical transformation of the WO 3 ·H 2 O nanoplates.