Family of Single-Micelle-Templated Organosilica Hollow Nanospheres and Nanotubes Synthesized through Adjustment of Organosilica/Surfactant Ratio

A family of hollow organosilica nanospheres and nanotubes was synthesized at appropriately low organosilica-precursor/block-copolymer-surfactant ratios. In Pluronic F127 (EO₁₀₆PO₇₀EO₁₀₆) block copolymer templated synthesis of ethylene-bridged organosilicas in the presence of a swelling agent, the lowering of the organosilica-precursor/surfactant ratio led to a change from highly ordered face-centered cubic structure of spherical mesopores to individual hollow spherical nanoparticles. It was hypothesized that at low ratios of organosilica precursor to PEO-PPO-PEO, the framework precursor is solubilized in the micelles and its concentration on their surface is not sufficient to induce appreciable cross-linking between the resulting nanoobjects and the consolidation into larger particles. The inner pore size of the nanospheres was adjusted by varying the micelle expander, allowing us to obtain pore diameters up to ∼20 nm. By employing low precursor/surfactant ratios, hollow spheres of methylene-, ethenylene-, and phenylene-bridged organosilicas were synthesized. Hollow silica spheres were also obtained through judicious choice of block copolymer. The synthesis strategy involving the adjustment of the framework-precursor/surfactant ratio was further extended on organosilica nanotubes synthesized using Pluronic P123 surfactant and cyclohexane as a swelling agent. One can envision a large number of framework compositions for which hollow nanospheres and nanotubes can be obtained using our synthesis approach

Surfactant-Templated Synthesis of Ordered Silicas with Closed Cylindrical Mesopores

Ordered mesoporous silicas with 2-dimensional hexagonal arrays of closed cylindrical pores were synthesized via templating with block copolymer surfactant followed by calcination at appropriately high temperatures. Precursors to closed-pore silicas, including SBA-15 silicas and organosilicas, were selected based on the existence of narrow passages to the mesopores. The increase in calcination temperature to 800–950 °C led to a dramatic decrease in nitrogen uptake by the materials, indicating the loss of accessible mesopores, whereas small-angle X-ray scattering (SAXS) indicated no major structural changes other than the framework shrinkage. Since SAXS patterns for ordered mesoporous materials are related to periodic arrays of mesopores, the existence of closed mesopores was evident, as additionally confirmed by TEM. The formation of closed-pore silicas was demonstrated for ultralarge-pore SBA-15 and large-pore phenylene-bridged periodic mesoporous organosilicas. The increase in the amount of tetraethyl orthosilicate in standard SBA-15 synthesis also allowed us to observe the thermally induced pore closing. It is hypothesized that the presence of porous plugs in the cylindrical mesopores and/or caps at their ends was responsible for the propensity to the pore closing at sufficiently high temperatures. The observed behavior is likely to be relevant to a variety of silicas and organosilicas with cylindrical mesopores

Investigation of High-Pressure and Temperature Behavior of Surfactant-Containing Periodic Mesostructured Silicas

Surfactant-containing periodic mesostructured silica materials, namely SBA-16 and FDU-12, were studied under pressures between 1 and 4 GPa and temperatures between 100 and 400 °C. At 4 GPa crystallization of coesite can be achieved already at 200 °C. The mild transition of amorphous to crystalline silica is believed to be accomplished by the inbuilt hydroxyl groups present in the starting material. At 2 GPa the crystallization of quartz is accomplished at a temperature of 400 °C. Both quartz and coesite are obtained in nanocrystalline form

Face-Centered-Cubic Large-Pore Periodic Mesoporous Organosilicas with Unsaturated and Aromatic Bridging Groups

Large-pore ethenylene-bridged (−CHCH−) and phenylene-bridged (−C₆H₄−) periodic mesoporous organosilicas (PMOs) with face-centered-cubic structure (<i>Fm</i>3<i>m</i> symmetry) of spherical mesopores were synthesized at 7 °C at low acid concentration (0.1 M HCl) using Pluronic F127 triblock copolymer surfactant in the presence of aromatic swelling agents (1,3,5-trimethylbenzene, xylenes–isomer mixture, and toluene). In particular, this work reports an unprecedented block-copolymer-templated well-ordered ethenylene-bridged PMO with cubic structure of spherical mesopores and an unprecedented block-copolymer-templated face-centered cubic phenylene-bridged PMO, which also has an exceptionally large unit-cell size and pore diameter. The unit-cell parameters of 30 and 25 nm and the mesopore diameters of 14 and 11 nm (nominal BJH-KJS pore diameters of 12–13 and 9 nm) were obtained for ethenylene-bridged and phenylene-bridged PMOs, respectively. Under the considered reaction conditions, the unit-cell parameters and pore diameters were found to be similar when the three different methyl-substituted benzene swelling agents were employed, although the degree of structural ordering appeared to improve for phenylene-bridged PMOs in the sequence of decreased number of methyl groups on the benzene ring

Low-Temperature Synthesis of Magic-Sized CdSe Nanoclusters: Influence of Ligands on Nanocluster Growth and Photophysical Properties

We present a low-temperature (68–70 °C) synthesis of green light-emitting, trioctylphosphine oxide-capped magic-sized CdSe nanoclusters from the reaction of trioctylphosphine oxide–cadmium acetate precursors with trioctylphosphine selenide. We observed continuous growth of these magic-sized nanoclusters, which displayed a first absorption peak at 422 nm and broad luminescence covering the entire visible region. The diameter of the nanoclusters determined by transmission electron microscopic measurement was ∼1.8 nm. Powder X-ray diffraction analysis showed a sharp peak at low angle (2θ = 5.3°), confirming the formation of ultrasmall, magic-sized nanoclusters. The nanocluster formation was also studied using different purities of trioctylphosphine oxide. The synthetic protocol was extended to the preparation of oleylamine-, ethylphosphonic acid-, lauric acid-, and trioctylamine-stabilized magic-sized CdSe nanoclusters. Importantly, the investigation showed that the nature of the cadmium precursors plays a crucial role in the nanocluster growth mechanism. The applicability of the trioctylphosphine oxide-capped nanoclusters was further investigated through a ligand exchange reaction with oleylamine, which displayed an extremely narrow absorption peak at 415 nm (full width at half-maximum of 14 nm) and a band edge emission peak at 456 nm with a shoulder at 438 nm

Isolation of Bright Blue Light-Emitting CdSe Nanocrystals with 6.5 kDa Core in Gram Scale: High Photoluminescence Efficiency Controlled by Surface Ligand Chemistry

Alkylamine-capped blue light-emitting (CdSe)₃₄ nanocrystals were synthesized via a phosphine-free method and isolated in gram-scale quantity. The exclusive formation of 6.5 kDa core mass was confirmed by combined optical spectroscopy and high resolution mass spectrometry studies. Variable power laser desorption ionization-mass spectrometry further confirmed the formation of the (CdSe)₃₄ core. The surface ligand chemistry was found to be extremely important in enhancing the photoluminescence properties. The nanocrystals were highly stable during the postsynthetic ligand treatment with triphenylphosphine, which increased their fluorescent quantum yield up to 23.6% without compromising the core composition as determined by mass spectrometry. Examination of their ³¹P and ¹H NMR spectra demonstrated the presence of amine and phosphine on the surface of the nanocrystals where phosphines were selectively attached to surface selenium sites that stabilized the nonradiative trap states and increased the fluorescence quantum yield. The gram-scale synthesis and high quantum yield of single-sized nanocrystals should greatly facilitate new and improved semiconductor nanocrystal applications in the field of nanoscience and nanotechnology, resulting in more rapid and less expensive production of future advanced electrochromic and light-emitting devices