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Nano casting method of synthesis of ordered mesoporous high - temperature ceramic materials and metal sulfides , nitrides materials

Author: ShiYiFeng
Tutor: ZhaoDongYuan
School: Fudan University
Course: Physical and chemical
Keywords: Mesoporous materials Non - oxide high - temperature ceramic materials Metal nitride materials Metal sulfide materials Nano - casting method Inorganic polymer ceramic precursor Gas - solid state reaction
CLC: TB383.4
Type: PhD thesis
Year: 2007
Downloads: 680
Quote: 1
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Abstract


Ordered mesoporous materials possess quite unique properties, such as high surface area, regularly arranged mesopores with various topologies, tunable pore size, large pore volume and abundant morphologies, which make them show enormous potential applications in catalysis, sorption, separation, sensor, drug delivery, and so on. The research, including synthesis, characterization and application of mesoporous materials, attracted a lot of interests since the first discover of ordered mesoporous silica and become a special field.Till now, the pore structure, mesoscale arrangement and morphology of ordered mesoporous materials can be tailored and pre-designed in some ways. However, except metals and carbon, most of them are oxides or oxysalt, which show poor thermal stability and functionality and mostly only act as containers more than functional materials. On the other hand, non-oxide high temperature ceramics, transition metal sulfides and nitrides with ordered mesoporous structures have seldom been successfully synthesized in despite of their high thermal stability and excellent functionality in electricity, optics and magnetics.In this thesis, we focus on the synthesis of these non-oxide materials in order to develop thermal stable and principal functional mesoporous materials. By introducing a polycarbosilane (PCS) as ceramic precursor, several kinds of non-oxide ceramics with high thermal stability can be obtained. And by introducing the gas-solid reaction into the nanocasting synthesis, transition metal sulfides (WS2, MoS2 and CdS) and nitrides (CoN and CrN) can be prepared. The details of each chapter are list below:Chapter 2: Highly ordered mesoporous SiC ceramics have been successfully synthesized by using PCS as precursors and mesoporous silica as hard templates via a one-step nanocasting process. Mesoporous SiC nanowires with two-dimensional (2-D) hexagonal or 3-D bicontinuous cubic arrays can be easily replicated from mesoporous silica SBA-15 and KIT-6 templates, respectively. The majority constituent of the products is SiC with 12% excess carbon and 14% oxygen measured by the elemental analysis. These ordered mesoporous SiC ceramics have high BET specific surface areas (720 m2 g-1), large pore volumes (0.8 cm3 g-1), narrow pore size distributions (3.7 nm) and ultra-high thermal stability (1400℃).Chapter3: Atmosphere-assisted heating process is utilized to in situ transform mesostructured SiC-C nanocomposites to ordered mesoporous SiOC and SiCN ceramics. The mesostmctured SiC-C nanocomposites are fabricated by using PCS as a ceramic precursor and mesoporous carbon CMK-3 as a hard template. Reactive gases including air and ammonia are involved to simultaneously incorporate O or N into SiC ceramics and oxidize or reduce the carbon template and excess carbon deposition. The procedure is carried out at 500℃for 10 h and 1000℃for 10 h in the case of air- and ammonia-assisted in situ transformation, respectively. The obtained mesoporous SiOC and SiCN ceramics possess similar open, continuous frameworks to 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 (4.9 - 10.3 nm) distributions. The structural shrinkage from mesostructured SiC-C composites to mesoporous SiC-based ceramics reduces with the increase of the initial pyrolysis temperature for SiC-C nanocomposites owing to the improvement of structural rigidity.Chapter 4: Ordered mesoporous silicon nitride materials with 31 wt% nitrogen content were synthesized by reactive pyrolysis of PCS in ammonia atmosphere and mesoporous carbon CMK-8 as hard template via nanocasting strategy. The obtained materials has a Ia3d symmetry, a specific surface areas of 384 m2 g-1, a large pore volumes of 0.71 cm3 g-1 and a narrow pore size distributions at the mean values of 5.7 nm. The nitrogen protected 1400℃-crystalization :process is a key fact to the synthesis of ordered mesoporous silicon nitride due to the increase of Si-N connection. The second impregnation-pyrolysis cycle can improve the mesostructure regularity and decrease the structure shrinkage.Chapter 5: Highly ordered mesoporous WS2 crystallites are synthesized by using mesoporous silica (SBA-15 and KIT-6) as hard templates and phosphotungstic acid H3PW12O40·xH2O (PTA) as a precursor via nanocasting strategy. H2S reduction sulfuration at 600℃high temperature is utilized to in Situ transform PTA precursor to WS2 layered crystallites in the confined nanochannels of the hard templates. The obtained products possess similar particle morphologies in macroscale with their mother templates and highly ordered arrangement structures in mesoscale. The HRTEM images and selected-area electron diffraction patterns both confirm that the S-W-S trilayers of WS2 crystallites are mostly parallel to the oriented nanochannels of the mesoporous silica template. This crystal orientation come from the natural tendency of layered transition metal dichalcogenide to reduce their high energy layer edges and the confinement effect of the anisotropic nanochannels. The obtained mesoporous WS2 has large surfaces areas (105 m2/g) and narrow pore size distributions (~4.8 nm). Furthermore, this method can be analogously extended to synthesize mesoporous MoS2, CdS and other metal sulfides.Chapter 6: Self-supported mesoporous metal nitrides (CON, CrN) nanowires with regular long-range arrangement were synthesis from nanocasting-synthesized mesostructureed metal oxides (Co3O4, Cr2O3) by dirrect ammonia nitridation using mesoporous silica SBA-15 and KIT-6 as hard tempelates. Mesostructured metal oxides-silica nanocomposites were synthesized firstly by incorporating the metal nitrate and the subsequently thermal pyrolysis. Ammonia treatment nitridation were subsequently carried out to transform the metal oxides to metal nitrides under specifically reaction temperature corresponding to their stability and reaction temperature. For CoN, nitridation was carried on 280℃after removing the silica template. For CrN, nitridation was Carried on 900℃with the support of silica template and the silica temperate was removed after the nitridation.

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