X-ray photoelectron spectroscopy and magnetic studies on the effect of pore size, wall thickness, and wall composition on superparamagnetic cobaltocene mesoporous Nb, Ta, and Ti composites

2005 Pure or Applied Inorganic Chemistry Award Lecture — Host–guest inclusion chemistry of electroactive, mesoporous transition metal oxides oxidation and 1-D confinement in one step and why amorphous is better

The flexible oxidation states of mesoporous Nb, Ta, and Ti oxides make them unique amongst porous materials allowing reaction pathways and cascades that are not possible for mesoporous silica or microporous materials such as zeolites. This electronic activity coupled with the 20–30 Å pores and the amorphous wall structure, which provides greater bandwidth (W) and hence an even greater range of redox potentials, leads to a rich variety of host–guest inclusion chemistry, which serves as an unprecedented 1-D analogue to layered 2-D host–guest inclusion reactions studied for decades. In this paper we survey a series of reactions between these mesoporous hosts and a wide variety of organic and organometallic guest species including alkali fullerides, cobaltocene, and other organometallic sandwhich species, and discuss the electronic and magnetic properties of the resulting composites.Key words: mesoporous materials, semiconductors, fullerides, superconductors, oxides, nanomaterials, metallocenes.

Solid-State 23Na and 7Li NMR Investigations of Sodium- and Lithium-Reduced Mesoporous Titanium Oxides

Mesoporous titanium oxide synthesized using a dodecylamine template was treated with 0.2, 0.6, and 1.0 equiv of Li- or Na-naphthalene. The composite materials were characterized by nitrogen adsorption, powder X-ray diffraction, X-ray photoelectron spectroscopy, elemental analysis, thermogravimetric analysis, and solid-state 23Na and 7Li NMR spectroscopy. In all cases the wormhole mesoporosity was retained as evidenced by BET surface areas from 400 to 700 m(2)/g, Horvath-Kawazoe pore sizes in the 20 Angstroms range, and a lack of hysteresis in the nitrogen adsorption isotherms. Variable-temperature conductivity studies show that the Li-reduced materials are semiconductors, with conductivity values 3 orders of magnitude higher than those of the Na-reduced materials. Electrochemical measurements demonstrate reversible intercalation/deintercalation of Li+ ions into pristine mesoporous Ti oxides with good cycling capacity. Solid-state 23Na NMR reveals two distinct Na environments: one corresponding to sodium ions in the mesoporous channels and the other corresponding to sodium ions intercalated into the metal framework. 23Na NMR spectra also indicate that the relative population of the framework site increases with increased reduction levels. Solid-state 7Li NMR spectra display a single broad resonance, which increases in breadth with increased reduction levels, though individual resonances inferring the presence of channel and framework Li species are not resolved. Comparisons of the lithium chemical shifts with published values suggests an "anatase-like structure" with no long-range order in the least-reduced samples but a "lithium titanate-like structure" with no long-range order in the higher reduced materials.

Effects of template and precursor chemistry on structure and properties of mesoporous TiO2 thin films

Mesoporous TiO2 thin films were synthesized by sol-gel processing using an aqueous-based, inexpensive, and environmentally friendly precursor and cationic surfactants as templates under mild reaction conditions. The films were prepared by spin-coating on glass substrates followed by calcination to remove the surfactant. N2 sorption, X-ray diffraction, and transmission electron microscopy were used to characterize the porosity, pore size, and pore structure before and after calcination. Films were found to have wormlike pore structures after calcination and surface areas on the order of 200 m2/g. These results show that the mesostructure and porosity of the thin films can be controlled by the surfactant template chemistry such as surfactant/Ti ratio, pH, and rate of solvent evaporation.

Compositional and 2H NMR studies of bis(benzene)chromium composites of mesoporous vanadium-niobium mixed oxides

New mesoporous niobium oxides with 5, 10, and 15 mol% vanadium(V) doped into the walls of the structure were synthesized by the ligand-assisted templating method with an octadecylamine template. These materials were characterized by XRD, XPS, EPR, elemental analysis, and nitrogen adsorption before being treated with excess bis(benzene)chromium to give new composites with an organometallic phase in the walls. All materials were also characterized by EPR, XRD, nitrogen adsorption, XPS, SQUID magnetometry, and elemental analysis. The materials with higher percentages of vanadium absorbed more bis(benzene)chromium, because this process depends largely on the electron transfer between the organometallic and the walls of the mesostructure and vanadium(V) is a stronger oxidant than niobium(V). Conductivity studies on these materials revealed that the ratio of Cr(0) to Cr(l) in the pores was more important than the absolute Cr loading level in governing electron transport properties but that increasing the V content led to more insulating behavior regardless of the Cr concentration. Solid-state 2H NMR studies on perdeuteriobenzene analogues of these composites showed the presence of the neutral and cationic Cr species in different ratios depending on the loading. Tumbling of these species was also slow on the NMR time scale, indicating that the charge-carrying Cr species are not rapidly moving through the pore channels of the mesostructure. This suggests that the walls of the structure may play a key role in charge transfer in these composites, contrary to what was previously believed.

Synthesis and electronic properties of low-dimensional bis(benzene) vanadium reduced mesoporous niobium oxide composites

The first bis(benzene) vanadium mesoporous niobium oxide composite was synthesized and characterized. The XRD pattern of this material shows a peak (100) centered at 45 A, identical to that of starting material. The nitrogen adsorption and desorption analyses of this material exhibit a slight decrease of BET surface area from 580 m(2) g(-1) to 467 m(2) g(-1) and a concomitant decrease in pore size and volume from 28 A and 0.500 cm(3) g(-1) to 25 A and 0.363 cm(3) g(-1), respectively. The powder EPR spectrum shows eight lines which can be assigned to (51)V from bis(benzene) vanadium as well as other resonances that can be assigned to the corresponding cation. The presence of two or more organometallic vanadium species was further confirmed by UV, (1)H-MAS NMR, and XPS methods. Broadened and shifted Nb 3/2, 5/2 peaks were also observed, providing further evidence that the mesoporous transition metal oxide framework was reduced by the organometallic. SQUID magnetometer measurements on this material show paramagnetic behavior with a small contribution of spin glass behavior. The conductivity of this material was 10(-4) ohm(-1) cm(-1), significantly greater than that measured for the analogous bis(benzene) chromium composite previously studied. Since alkali-metal reduced mesoporous niobium oxide materials are insulating, this conductivity was attributed to the low-dimensional bis-arene vanadium phase in the pores and rationalized according to the balance between the Hubbard potential and the bandwidth estimated for the relevant organometallic species. A bis-1,3,5-tri-tert-butylbenzene yttrium composite was also synthesized; however, complete loss of ligand upon reduction of the mesostructure was observed, indicating that this dopant behaves more like an alkali naphthalene reagent in its reactions with mesoporous niobium oxide than other bis(arene) complexes investigated by our group.