Nanocasting Synthesis of Ordered Mesoporous Silicon Nitrides with a High Nitrogen Content

Ordered mesoporous silicon nitrides with a high nitrogen content (32 wt %) were synthesized by using polycarbosilane (PCS) as a ceramic precursor and mesoporous carbon CMK-8 as a hard template via nanocasting synthesis. Small-angle X-ray scattering, TEM, and nitrogen sorption analyses showed that the mesoporous silicon nitride products have a 3-D bicontinuous cubic mesostructure (Ia3̄d) similar to KIT-6, a specific BET surface area of 384 m2 g-1, a large pore volume of 0.71 cm3 g-1, and a narrow pore size distribution at the mean value of 5.7 nm. The PCS precursor can be transformed into silicon nitride by reactive pyrolysis under ammonia atmosphere. The nitrogen protected 1400 °C crystallization process is a key step for the synthesis of ordered mesoporous silicon nitrides. The secondary impregnation−pyrolysis cycle can reduce the structural shrinkage and improve the mesostructural regularity

Ordered Mesostructured Rare-Earth Fluoride Nanowire Arrays with Upconversion Fluorescence

Ordered mesostructured LaF3 nanoarrays have been, for the first time, synthesized via a one-step nanocasting process using La(CF3COO)3 as a precursor and mesoporous silica as a hard template. The characterization of SAXS and XRD patterns and TEM and SEM images shows that the LaF3 nanowire arrays have long-range regularity of hexagonal mesostructure (p6mm) and single-crystalline feature. N2-sorption isotherms reveal that ordered mesoporous LaF3 products have high BET specific surface area (∼75 m2/g), large pore volume (0.15 cm3/g), and narrow pore-size distribution (the mean value of 4.3 nm). The upconversion fluorescence has been realized in the Yb3+/Er3+ (red/green) and Yb3+/Tm3+ (blue) codoped LaF3 nanoarrays by upconversion (UC) excitation in the near-infrared region. The UC emission population for 4F9/2, 2H11/2, 4S3/2, and 2H9/2 levels in the Yb3+/Er3+ codoped LaF3 nanowire array matrixes depends on Er3+ concentration, the excitation density, and the specific surface areas

Synthesis of Highly Ordered Mesoporous Crystalline WS₂ and MoS₂ <i>via</i> a High-Temperature Reductive Sulfuration Route

A high-temperature reductive sulfuration method is demonstrated to synthesize highly ordered mesoporous metal sulfide crystallites by using mesoporous silica as hard templates. H2S gas is utilized as a sulfuration agent to in situ convert phosphotungstic acid H3PW12O40·6H2O to hexagonal WS2 crystallites in the silica nanochannels at 600 °C. Upon etching silica, mesoporous, layered WS2 nanocrystal arrays are produced with a yield as high as 96 wt %. XRD, nitrogen sorption, SEM, and TEM results reveal that the WS2 products replicated from the mesoporous silica SBA-15 hard template possess highly ordered hexagonal mesostructure (space group, p6mm) and rodlike morphology, analogous to the mother template. The S−W−S trilayers of the WS2 nanocrystals are partially oriented, parallel to the mesochannels of the SBA-15 template. This orientation is related with the reduction of the high-energy layer edges in layered metal dichalcogenides and the confinement in anisotropic nanochannels. The mesostructure can be 3-D cubic bicontinuous if KIT-6 (Ia3̄d) is used as a hard template. Mesoporous WS2 replicas have large surface areas (105−120 m2/g), pore volumes (∼0.20 cm3/g), and narrow pore size distributions (∼4.8 nm). By one-step nanocasting with the H3PMo12O40·6H2O (PMA) precursor into the mesochannels of SBA-15 or KIT-6 hard template, highly ordered mesoporous MoS2 layered crystallites with the 2-D hexagonal (p6mm) and 3-D bicontinuous cubic (Ia3̄d) structures can also be prepared via this high-temperature reductive sulfuration route. When the loading amount of PMA precursor is low, multiwalled MoS2 nanotubes with 5−7 nm in diameter can be obtained. The high-temperature reductive sulfuration method is a general strategy and can be extended to synthesize mesoporous CdS crystals and other metal sulfides

Formation of Hollow Upconversion Rare-Earth Fluoride Nanospheres: Nanoscale Kirkendall Effect During Ion Exchange

In this work, we report a facile solution-phase synthesis of hollow cubic phase α-NaYF4 nanoparticles by a controlled ion exchange process from cubic phase Y2O3 nanospheres. We demonstrate that hollow nanoparticles with controlled size are formed owing to the nanoscale Kirkendall effect. The formation mechanism was studied with the XRD, STEM, and EDS line scanning characterization. The crystal structure similarity between the parent and the final product is essential for framework and morphology preservation. Although the sphere particles are polycrystalline and composed of the nanocrystals, the cubic structure of α-NaYF4 nanocrystals displays a noticeable structure similarity with Y2O3, which we believe facilitates ion exchange between the primary nanocrystals and preservation of the secondary sphere morphology. The multicolor upconversion (UC) fluorescence (PL) was successfully realized in the Yb3+/Er3+(Tm3+) codoped α-NaYF4 hollow nanospheres by excitation in the near-infrared (NIR) region. The various UC emission ratios of the samples were investigated as a function of hydrothermal reaction time to research the UC properties of the products and to further demonstrate the thermodynamic driving solution ion-exchange process

Ordered Mesoporous Crystalline Mo-Doped WO₂ Materials with High Tap Density as Anode Material for Lithium Ion Batteries

Highly ordered mesoporous crystalline Mo-doped WO₂ (Mo_<i>x</i>W_1–<i>x</i>O₂: 1 > <i>x</i> > 0.08) materials with different molybdenum contents were synthesized via a nanocasting strategy using mesoporous silica KIT-6 as a hard template. The presence of molybdenum significantly increased the rate of reduction of tungsten trioxide to tungsten dioxide using hydrogen gas as the reducing agent, and it also prevented the dioxide product from being further reduced to zerovalent metal tungsten. This molybdenum doping strategy provides a new solution for the synthesis of WO₂-based materials with well-defined nanostructures. The obtained mesoporous Mo_0.14W_0.86O2 material possessed a metallic conductivity (0.8 Ω cm, 300 K) and a high tap density of 3.6 g cm^–3. This material exhibits a high and reversible lithium storage capacity of 635 mAh g^–1 and is stable up to at least 70 cycles without noticeable fading

Magnetic Permanently Confined Micelle Arrays for Treating Hydrophobic Organic Compound Contamination

Magnetic permanently confined micelle arrays (Mag-PCMAs) have been successfully synthesized as sorbents for hydrophobic organic compound (HOC) removal from contaminated media. The synthesis of Mag-PCMAs involves coating a silica/surfactant mesostructured hybrid layer on the negatively charged Fe3O4 microparticles to create a core/shell structure. The surfactant, 3-(trimethoxysily)propyl-octadecyldimethyl-ammonium chloride (TPODAC), has a reactive endgroup -Si(OCH3)3 on its hydrophilic groups, which allows the surfactant micelles to permanently anchor on the silica framework through covalent bonding. This unique structural property avoids surfactant loss during application and allows for sorbent regeneration. The isotherms and kinetics of four representative HOCs (atrazine, diuron, naphthalene, and biphenyl) onto Mag-PCMAs were determined, and the regeneration and reusability of Mag-PCMAs for diuron removal was also investigated. As a proof of principle for application of Mag-PCMAs for soil-washing, the use of Mag-PCMAs for removal of diuron from a contaminated soil was also demonstrated. All of the results showed that Mag-PCMAs are reusable sorbents for fast, convenient, and highly efficient removal of HOCs from contaminated media

2D Single Crystal WSe₂ and MoSe₂ Nanomeshes with Quantifiable High Exposure of Layer Edges from 3D Mesoporous Silica Template

The design and fabrication of layered transition metal chalcogenides with high exposure of crystal layer edges is one of the key paths to achieve distinctive performances in their catalysis and electrochemistry applications. Two-dimensional WSe2 and MoSe2 nanomeshes with orderly arranged nanoholes were synthesized by using a mesoporous silica material KIT-6 with three-dimensional mesoporous structure as a hard template via a nanocasting strategy. Each piece of the nanomesh is a single crystal, and its c axis is always perpendicular to the nanomesh plane. The highly porous structure brings these nanomeshes extremely high exposure of layer edges, and the well-defined nanostructure provides an opportunity to quantitatively estimate the specific length of the crystal layer edges for the WSe2 and MoSe2 nanomeshes synthesized in this work, which are estimated to be 3.8 × 1010 and 6.0 × 1010 m g–1, respectively. The formation of a 2D sheet-like nanomesh structure inside a 3D confined pore space should be attributed to the synergistic effect from the crystal self-limitation growth that is caused by their layered crystal structures and the space-limitation effect coming from the unique pore structure of the KIT-6 template. The catalytic activities of the nanomeshes in an electrocatalytic hydrogen evolution reaction were also investigated

Controlled Synthesis of Ordered Mesoporous C−TiO₂ Nanocomposites with Crystalline Titania Frameworks from Organic−Inorganic−Amphiphilic Coassembly

Highly ordered mesoporous carbon−titania nanocomposites with nanocrystal-glass frameworks have been synthesized via the organic−inorganic−amphiphilic coassembly followed by the in situ crystallization technology. A soluble resol polymer was used as a carbon precursor, prehydrolyzed TiCl4 as an inorganic precursor, and triblock copolymer F127 as a template. The carbon−titania nanocomposites with controllable texture properties and composition can be obtained in a wide range from 20 to 80 wt% TiO2 by adjusting the initial mass ratios. The C−TiO2 nanocomposites with “bricked-mortar”frameworks exhibit highly ordered 2D hexagonal mesostructure and high thermal stability up to 700 °C. The nanocomposites have high surface area (465 m2 g−1) and large pore size (∼4.1 nm). Additionally, the nanocomposites show good performance in degradation of Rhodamine B due to the photocatalytic activity of the titania nanocrystals and the strong adsorptive capacity of the glasslike carbon

2D Single Crystal WSe₂ and MoSe₂ Nanomeshes with Quantifiable High Exposure of Layer Edges from 3D Mesoporous Silica Template

The design and fabrication of layered transition metal chalcogenides with high exposure of crystal layer edges is one of the key paths to achieve distinctive performances in their catalysis and electrochemistry applications. Two-dimensional WSe2 and MoSe2 nanomeshes with orderly arranged nanoholes were synthesized by using a mesoporous silica material KIT-6 with three-dimensional mesoporous structure as a hard template via a nanocasting strategy. Each piece of the nanomesh is a single crystal, and its c axis is always perpendicular to the nanomesh plane. The highly porous structure brings these nanomeshes extremely high exposure of layer edges, and the well-defined nanostructure provides an opportunity to quantitatively estimate the specific length of the crystal layer edges for the WSe2 and MoSe2 nanomeshes synthesized in this work, which are estimated to be 3.8 × 1010 and 6.0 × 1010 m g–1, respectively. The formation of a 2D sheet-like nanomesh structure inside a 3D confined pore space should be attributed to the synergistic effect from the crystal self-limitation growth that is caused by their layered crystal structures and the space-limitation effect coming from the unique pore structure of the KIT-6 template. The catalytic activities of the nanomeshes in an electrocatalytic hydrogen evolution reaction were also investigated

Ordered Mesoporous SiOC and SiCN Ceramics from Atmosphere-Assisted in Situ Transformation

An atmosphere-assisted heating process was utilized to transform mesostructured SiC−C nanocomposites in situ into ordered mesoporous SiOC and SiCN ceramics with relatively small structural shrinkage. The mesostructured SiC−C nanocomposites were fabricated using commercially available polycarbosilane (PCS) as a ceramic precursor and mesoporous carbon CMK-3 as a hard template that itself was prepared by a nanocasting procedure from mesoporous silica SBA-15. Reactive gases including air and ammonia were employed to simultaneously incorporate O or N into SiC ceramics and oxidize or reduce the carbon template and excess carbon deposits. The procedure was carried out at 500 °C for 10 h and at 1000 °C for 10 h for air- and ammonia-assisted in situ transformations, respectively. SAXS, XRD, N2 sorption, and TEM analyses revealed that the mesoporous SiOC and SiCN ceramics exhibit open, continuous frameworks similar to that of the primary template ordered mesoporous SBA-15. The ordered mesoporous SiOC and SiCN ceramics have high surface areas (200−400 m2 g-1), large pore volumes (0.4−0.8 cm3 g-1), and narrow pore size distributions (4.9−10.3 nm). The structural shrinkage from mesostructured SiC−C composites to mesoporous SiC-based ceramics decreased with increasing initial pyrolysis temperature for SiC−C nanocomposites owing to the improvement of structural rigidity. This shrinkage was found to be as low as 2.6% from mesostructured SiC−C pyrolysis at 1400 °C to mesoporous SiCN-1400 via ammonia-assisted in situ transformation