Synthesis and characterization of hybrid silica and carbon on silica nanocomposites

• Main Author: Dube, Sibongile Mary-Anne
• Format: Others
• Language: en
• Published: 2010
• Online Access: http://hdl.handle.net/10539/7799

As-synthesized and surfactant extracted periodic mesoporous organosilica (PMO) materials were synthesized by the sol-gel method under acidic and basic conditions. Five different silica sources were used: tetraethylorthosilicate (TEOS), 1,2-bis(trimethoxysilyl)ethane (BTME), 1,4- bis(triethoxysilyl)benzene (BTEB), 4,4-bis(triethoxysilyl)-1,1-biphenyl (BTEBP) and bis[3- trimethoxysilyl)propyl]-amine (BTMSPA). Two structure directing agents were used, triblock copolymer (Pluronic P123) and cetyltrimethylammonium bromide (CTAB). As-synthesized PMO materials were used as templates for the synthesis of silica on carbon nanocomposites with various nanostructures (nanotubes, bamboo nanotubes, spheres and beads). FTIR spectroscopy and thermogravimetric analysis confirmed the formation of organosilica materials and show that the surfactants Pluronic P123 and CTAB were removed by solvent extraction. The use of the surfactant Pluronic P123 produced periodic mesoporous organosilica materials which had surface areas which followed a systematic trend. It was seen that the size of the organic group had an effect on the surface area obtained on the periodic mesoporous organosilica materials produced. From the results of the surface areas of solvent extracted samples, it was observed that the smaller the chain length of the organosilica precursor, the higher the surface area obtained. It can therefore be concluded that small organic groups (ethane) are more favorable for synthesizing PMOs with high surface areas followed by rigid organic groups (benzene and biphenyl) and lastly flexible organic groups (bis(propyl)amine). The surface areas are obtained for SE-TS-P123, SE-BMS-P123, SE-PS-P123, SE-BS-P123 and SE-BPS-P123 are 839.4, 802.3 and 344.3 m2/g, 527.1 and 386.2 m2/g. The use of the surfactant CTAB yielded periodic mesoporous organosilica materials which followed a systematic trend, with the exception of SE-BS-CTAB. In the surfactant extracted samples the BET surface areas decrease as the chain length of the organic groups is increased. SE-TS-CTAB, SE-BMS-CTAB and SE-PS-CTAB have surface areas of 947.4, 901.1 and 331.6 iv m2/g respectively. TEOS has no organic groups, BTME has two CH2 chains and bis(propyl)amine has six CH2 chains attached to a NH2 group making it very flexible hence yielding the lower BET surface area. It is evident from BET that the materials are porous but the structural periodicity must have been confirmed by low angle XRD. Low angle XRD was not done for all the surfactant extracted periodic mesoporous organosilica samples formed due to inadequate resources. PMOs synthesized using Pluronic P123 as the structure directing agent have lower surface areas than those obtained using CTAB. However SE-BS-CTAB had a higher surface area than SEBMS- CTAB. This was higher than what was anticipated. It shows that 1,4-Bis(triethoxysilyl) benzene forms PMOs with better structural properties when CTAB as the structure directing agent is used. It can therefore be concluded that CTAB is a better structure directing agent for synthesizing PMOs with better structural properties than CTAB. The carbonization of as-synthesized and surfactant extracted mesoporous materials in quartz tubes under an inert atmosphere resulted in a diverse range of carbon on silica nanostructures. AS-T-CS 1000 oC/5 h produced carbon on silica bamboo nanotubes with varying shapes and some amorphous material. The sample is thermally stable up to 581 oC in an air atmosphere. The presence of silica in the material was confirmed by TGA. AS-P-CS 1000 oC/5 h produced carbon on silica nanotubes with very thick internal diameters of between 160 – 170 nm, carbon spheres with average diameters of 400 nm and amorphous carbon. The sample is thermally stable up to 416 oC in an air atmosphere. EDX confirmed the presence of silica, carbon and oxygen in the various nanostructures obtained for this sample. AS-B-CS 1000 oC/5 h produced carbon on silica nanotubes with varying small internal diameters and amorphous carbon material. The sample was the most thermally stable of the three materials up to 650 oC in an air atmosphere. Raman spectroscopy revealed that all samples had a low degree of graphitization and are therefore amorphous. The ID/IG ratios of AS-T-CS 1000 oC/5 h, AS-B-CS 1000 oC/5 h and AS-P-CS 1000 oC/5 h samples are 0.81, 1.87 and 0.96. The disorder of the samples does not follow any systematic trend. v AS-BS-P123 samples heated at different carbonization temperatures produced different carbon on silica nanostructures which included spheres and nanotubes. Amorphous material (approximately 60%) was also produced. The presence of silicon and carbon in the carbon on silica nanocomposites produced was further confirmed by the use of scanning X-ray photoelectron spectroscopy (SXPS) (AS-B-CS 1000 oC/5 h sample). Raman spectral data showed that as the carbonization temperature is increased, the degree of disorder of the materials also increased. The temperature at which major loss changes occurred due to the oxidation of carbon increased as the carbonization temperature increased up to 1000oC. Low surface areas and pore volumes were obtained for all mesoporous phenyl-bridged organosilica/surfactant mesophases heated at different temperatures. This may be due to the high carbonization efficiency at temperatures from 700 oC to 1100 oC that there is rapid formation of carbon deposits which blocks some of the mesopores. Also at these high pyrolysis temperatures, the carbonization may be so fast that the surfactant molecules are directly converted to carbon which blocks the pores rather than being burnt off. Longer pyrolysis times yield various shaped carbon on silica nanocomposites (nanotubes, spheres, beads and amorphous material) and shorter pyrolysis times yield only amorphous material and beads. AS-P-CS 1000 oC/5 h produced carbon on silica nanocomposites of nanotubes, spheres, beads and amorphous material. Both AS-P-CS 1000 oC/1 h and AS-P-CS 1000 oC/0.5 h carbon on silica nanocomposites produced amorphous material and beads. Most carbon on silica beads of AS-P-CS 1000 oC/1 h and AS-P-CS 1000 oC/0.5 h have sizes which range from 1.8 – 2.4 and 1.2 – 2.0 μm, respectively. Shorter pyrolysis times of 0.5 h produced carbon on silica beads with smaller diameters which ranged from 1.2 – 2.0 μm and pyrolysis times of 1 h produced carbon on silica beads with larger diameters of 1.8 – 2.4 μm. The Raman data showed that as the carbonization time is increased, the degree of disorder in the carbon on silica nanocomposites also increases. The thermal stability of the carbon on silica nanocomposite samples decreased as the carbonization time was increased.