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In this work we demonstrated that uniform polycrystalline SnO2 nanotubes, in either array or free-standing form, can be fabricated by an infiltration technique using SnO2 nanoparticles as starting building units. With this method, the diameter, length, thickness, and texture of the nanotubes can be further controlled. The tubular SnO2 shows a significant improvement of electrochemical performance over the unorganized nanoparticles. The specific capacity of the Sn-based nanotube electrode was 525 mAh/g after 80 cycles. In principle, the tubular structure of active component can also be extended to other metal oxides and Li−metal alloy systems. In view of their unique features, the prepared SnO2 nanotubes may also find interesting applications in other fields such as gas sensing.

At present there are two basic types of self-assembly of inorganic nanomaterials:  surfactant-assisted organization and patterned-substrate deposition. In this paper, we report a novel assembled scheme for fabrication of hexagonal superlattice arrays of anatase TiO2 nanospheres on the (010) crystal plane of α-MoO3 without the assistance of organic surfactants or substrate prefabrication. The growth and self-assembly of TiO2 nanospheres take place simultaneously under hydrothermal conditions at 150−200 °C according to a continuous self-aligned process in which the localized enrichment of hydrolysis product HF (TiF4 is the precursor for TiO2) is believed to play a crucial role in initiating the self-alignment. This “growth-cum-assembly” mechanism has also been validated with our various growth experiments and materials characterization. Key synthetic parameters have been identified and future explorations have been suggested for this ordered scheme.

Recently we investigated a nitrate-salt-mediated formation route of Co3O4 spinel nanocubes with a uniform size of ca. 47 nm in aqueous solution at 95 °C (J. Phys. Chem. B 2003, 107 (4), 926−930). In the present work, we examined various synthetic parameters of this new method. By varying reaction temperature and time, the size control of Co3O4 nanocubes has been achieved, which produces the spinel nanocubes with a controllable edge length in the regime of 10−100 nm. A linear relationship between the cube size and reaction time has been revealed. Furthermore, a process has been developed to separate (or to purify) the growing Co3O4 nanocubes from remaining solid products at different synthetic stages. With a large amount of nitrate salt added, we have been able to observe some irregular crystalline intermediates, although the majority of the Co3O4 nanocrystals have highly symmetrical cubelike morphology. On the basis of the observed crystallite morphologies, we have elucidated in this work a mechanism &sbd; “surface wrapping” &sbd; to describe the size-controlled growth of Co3O4 nanocubes.

Self-redox decompositions of two cobalt−hydroxide−nitrates [Co^II0.80Co^III0.20 (OH)2.00(NO3)0.14(CO3)0.03·0.77H2O and Co^II(OH)1.50(NO3)0.40(CO3)0.05·0.05H2O] have been investigated with various analytical methods under an oxygen-free atmosphere. The investigated compounds all have a lamellar structure in which an oxidant layer (nitrate) and a reductant layer (Co²⁺) are stacked in an alternate manner. In addition to the “molecular” level mixing of reagents, these compounds are purposely prepared with variations of reactant concentration and anion configuration in the interlayer space. It is found that redox decompositions within the compounds take place according to the following sequence:  (i) decomposition of carbonate anions, (ii) dehydroxylation of brucite-like layers, and (iii) decomposition of nitrate anions, although actual reaction temperatures and gas-evolving patterns are different in the two cases. The nitrate decomposition can be ascribed to an attachment of nitrate anion to divalent cobalt of forming cobalt−oxides after the dehydroxylation. More complete decomposition has been observed for the compound that “stores” more redox reagents with direct contact in its lamellar structure, although the final decomposed products of both compounds are all in nanophase Co3O4 (12−15 nm; formed at 400 °C). Surface compositions of the studied samples have also been examined for the decompositions.

Individual sheets of MoO3 in MoO3(4,4‘-bipyridyl)0.5 (basal spacing d002 = 1.12 nm) can be considered as two-dimensional (2D) inorganic “macromolecules” while the interlayer organic ligands can be viewed as “pillars” to support or as “threads” to suspend these 2D host planes. This work concerns a sulfidation investigation of single molecular sheets of MoO3 in the single crystals of the above compound under a H2S/H2 stream with various characterization methods. It is found that at a temperature as low as 100 °C, formation of surface HxMoO3(4,4‘-bipyridyl)0.5 commences. A surface product of MoO2-xSx is also observed throughout 100−200 °C. At 250 °C, all these intermediate phases are transformed completely to a final phase MoS2(4,4‘-bipyridyl)0.2 (basal spacing d001‘ = 1.02 nm) accompanying a partial deintercalation (from 0.5 down to 0.2) of 4,4‘-bipyridine molecules from the interlayer space. In this connection, 4,4‘-bipyridine molecules change from a standing configuration to a lying one after this sulfidation process. Although this new phase is thermally stable at its formation temperature (250 °C), inter-slab condensation of MoS2 (basal spacing d002 = 0.62 nm) can also be observed in the topmost surface region. A mechanism based on these findings is also proposed. As demonstrated in this work, sulfurizing single molecular sheets of MoO3 in existing MoO3-containing layered compounds would also serve as a new means for synthesis of lamellar organic−inorganic nanohybrids comprising building units of transition metal dichalcogenides.

One-dimensional nanostructures of orthorhombic molybdenum trioxide (α-MoO3) have been synthesized in the forms of ribbons or rods via acidification under hydrothermal conditions at 140−200 °C. The reaction path has been revealed with our kinetic investigations, which shows the following sequence:  (i) from the starting compound (NH4)6Mo7O24·4H2O to (ii) formation of intermediate compound ((NH4)2O)0.0866·MoO3·0.231H2O, and then to (iii) final α-MoO3 nanoribbons or nanorods in 100% phase purity. The optimal growth temperature is in the range of 170−180 °C under the current experimental settings. At higher reaction temperatures, this transformation can be accelerated, but with poorer crystal morphological homogeneity. It has been found that the dimensions of these rectangular nanorods are about 50 nm in thickness, 150−300 nm (mean value at 200 nm) in width, and a few tens of micrometers in length. The crystal morphology can be further altered with inorganic salts such as NaNO3, KNO3, Mg(NO3)2, and Al(NO3)3. Using an H2S/H2 stream, the above-prepared α-MoO3 nanorods can be converted completely to 2H−MoS2 nanorods at 600 °C. The original rodlike morphology is well-retained, although the aspect ratio of the oxide template is reduced upon the sulfidation treatment.

Using AFM/XRD/XPS methods, in this paper, we investigate protonation and deprotonation processes in single-crystal samples of orthorhombic molybdenum trioxide (α-MoO3). At low temperatures, a small part of α-MoO3 is changed to needlelike HxMoO3 (x ≈ 0.33) along 〈203〉 directions in a H2S/H2 gas stream. When these elongated crystallites assemble into a maze structure, the growth of HxMoO3 is gradually ceased due to closing entrance for hydrogen. At higher temperatures, the needlelike HxMoO3 crystallites turn to a growth perpendicular to 〈203〉, which leads to the formation of HxMoO3 blocks. It is observed that the basal plane of α-MoO3 is severely buckled upon the protonation. Surface sulfidation is also observed. The formed HxMoO3 or surface MoS2, however, can be readily converted back to their original-phase α-MoO3 in air at 350−400 °C. This oxidation process gives rise to a flattened (010) topography (i.e., debuckling) on which shallowly divided α-MoO3 surface blocks bounded with {101} planes are formed. When an α-MoO3 (010) plane embedded with nanocrystallites is used to create surface stress or nucleation sites, the insertion mode of hydrogen along 〈001〉 is further reconfirmed in this work. A correlation of surface/bulk phases upon various chemical reactions is addressed, and a model to summarize these changes is also proposed.

Nanocomposite materials of Co^IICo^III2-xAlxO4−Al2O3 (0 ≤ x < 1.63) have been prepared from biphasic xerogels that contain the cobalt hydrotalcite-like compound Co^II0.73Co^III0.27(OH)2.00(NO3)0.23(CO3)0.02·0.5H2O and sol−gel-derived Al2O3. With XRD/FTIR/XPS/DSC/TGA methods, an investigation has been carried out to understand the formation processes of the composites and chemical reactivity of the embedded cobalt hydrotalcite-like compound with alumina gel matrixes at various heating temperatures. It has been found that cubic spinels of Co3O4 and Co^IICo^III2-xAlxO4 are generated sequentially inside alumina matrixes upon decomposition of the hydrotalcite-like compound in static air. The average spinel crystallite size is in the range of 9−13 nm at 350 °C to 31−38 nm at 800 °C, whereas the specific surface area of the composites is reduced gradually from 277−363 m² g^-1 at 350 °C to 184−224 m² g^-1 at 800 °C. On one hand, the catalytic effect of cobalt on combustion reactions of organics trapped within the amorphous alumina gel matrixes is elucidated by varying the cobalt content and synthesis conditions. On the other hand, the retardation effect of γ-Al2O3 matrixes on the growth of the crystallite is also revealed. It has been indicated that the content of aluminum (x) in Co^IICo^III2-xAlxO4−Al2O3 is indeed a function of the heating temperature. After thermal decomposition of the cobalt hydrotalcite-like compound, the Co3O4 phase starts to form at a temperature as low as 200 °C and is fully developed at 350 °C. At 500−800 °C, the formed Co3O4 is converted to the Co^IICo^III2-xAlxO4 phase due to the reaction between the included phase and alumina matrixes.

Nanosize (5−7 nm) supported Co^IIAl-hydrotalcite-like compounds (Co^IIAl-HTlcs) on γ-Al2O3 have been synthesized with a coprecipitation method in which the γ-Al2O3 acts as a source of aluminum for coprecipitation and a support for the compound formation. The synthesis parameters identified are initial concentrations of metal and base, aging time, and more importantly, the addition sequence of the metal or base solution. Using multiple characterization methods, the supported Co^IIAl-HTlcs have been investigated for their structural phase and chemical composition. The chemical formula determined for these samples is CoAlx(OH)2+2x(CO3)y(NO3)x-2y·nH2O/γ-Al2O3, where x = 0.25−0.40, y = 0.02−0.07, and n = 1.3−1.7, and the distance of inter-brucite-like sheets (d003) is in the range of 7.65−7.79 Å. Because of the formation of nanosize crystallites, decomposition of the supported Co^IIAl-HTlcs occurs at ≈211−217 °C, which is markedly lower than that of unsupported pure Co^IIAl-HTlcs. After calcination in air, the supported Co^IIAl-HTlcs are transformed into cubic spinel oxides Co^IICo2-x^IIIAlxO4/γ-Al2O3 and high catalytic activity for the N2O decomposition reaction is observed for these supported spinel catalysts.

In-situ generation of trivalent cobalt cations has been investigated for the hydrotalcite-like compounds MgxCo^II1-x-yCo^IIIy(OH)2(NO3)y·nH2O at 25−40 °C under oxygen-containing atmospheres. It is noted that with more involvement of Mg²⁺ in the compounds, less Co²⁺ cations are needed to maintain the hydrotalcite-like structure. Because of the presence of the Mg²⁺, more Co²⁺ can be oxidized under the current experimental conditions and the highest mole ratios of Co³⁺ to total cobalt cations (Co³⁺:Co) observed in this work is 57%. The mole ratio of Co³⁺ to total metal cations (Co³⁺:(Mg+Co)) achieved in this work is 31% after 4 days of oxidation reaction, which is close to its upper charge limit to produce a single-phase hydrotalcite-like structure (33%). Two major thermal events are observed when the compounds are heated. The first one at 113−134 °C is attributed to the removal of interlayer water molecules while the second at 274−333 °C to the dehydroxylation and decomposition of intercalated anions. Higher catalytic activity for nitrous oxide (N2O) decomposition is observed for the Mg−Co oxides with the same cobalt content but lower mole ratios of Co³⁺:Co in their hydrotalcite-like precursors. The reason for this activity variation has also been addressed.

Low-temperature synthesis of spinels MgxCo1-xCo2O4 (x = 0.0−0.9) has been investigated with two important synthetic parameters and XRD/FTIR/DSC/BET/ICP/CHN methods. It is found that with an increase in hydrothermal treatment temperature, precursor compounds change from the hydrotalcite-like to brucite-like and then to cubic spinel phase when the initial metal concentration ratio [Mg²⁺]:[Co²⁺] ≥ 1, while from the turbostratic to hydrotalcite-like and then to cubic spinel phase when the [Mg²⁺]:[Co²⁺] < 1. The MgxCo1-xCo2O4 (x ≤ ¹/3) spinels were formed at temperatures as low as 50−100 °C. In the postsynthesis thermal treatment, various thermal events have been observed, and in all cases the spinel phase can be prepared at temperatures below 300 °C. On the basis of FTIR results, a topotactic mechanism for the formation of the spinel phase has been confirmed. Specific surface area as high as 210 m²/g has been attained in 400 °C calcined MgxCo1-xCo2O4 with x = 0.9. Causes for the formation of the spinel phase at low temperatures have been addressed. Excellent catalytic activity for N2O decomposition (30.6−38.9 mmol (N2O)/g·h at GHSV = 21200 h^-1 and 380−400 °C; N2O = 10 mol % balanced with He) has been observed and discussed for these Mg−Co spinels.

Formation and interconversion of cobalt hydroxides have been investigated with XRD, FTIR, and compositional analysis. Effects of addition time, aging time, and preparative atmosphere on phase transformation and materials synthesis in ammoniacal solution have been systematically examined. A new precursor phase with hydrotalcite-like characteristics has been identified. While α-Co(OH)2 is prepared with short-aging in nitrogen atmosphere, this divalent metal (Co²⁺) hydrotalcite-like phase transforms into brucite-like β-Co(OH)2 under prolonged aging in nitrogen. The β-Co(OH)2 can be formed readily in air but later is converted into a new hydrotalcite-like phase which is stable and contains both Co²⁺ and Co³⁺ due to redox reactions. Interconversions among the four structural phases have been investigated and correlated.