Natacha Krins

Assembly of ligand-stripped nanocrystals into precisely controlled mesoporous architectures.

The properties of mesoporous materials hinge on control of their composition, pore dimensions, wall thickness, and the size and shape of the crystallite building units. We create ordered mesoporous materials in which all of these parameters are independently controlled. Different sizes (from 4.5 to 8 nm) and shapes (spheres and rods) of ligand-stripped nanocrystals are assembled using the same structure-directing block copolymers, which contain a tethering domain designed to adsorb to their naked surfaces. Material compositions range from metal oxides (Sn-doped In(2)O(3) or ITO, CeO(2), TiO(2)) to metal fluorides (Yb,Er-doped NaYF(4)) and metals (FePt). The incorporation of new types of nanocrystals into mesoporous architectures can lead to enhanced performance. For example, TiO(2) nanorod-based materials withstand >1000 electrochemical cycles without significant degradation.

Ti Substitution Mechanisms in Phlogopites from the Suwalki Massif-type Anorthosite, NE Poland

Intercumulus titanian phlogopite occurs in leuco- and gabbro-noritic cumulates from the Suwalki anorthosite massif, NE Poland. The degree of Ti enrichment in the micas ranges from 2.59 to 9.41 wt.% TiO2. The chemical composition is highly variable for several other elements: Al 2O3 (13.07-16.75 wt.%), K2O (7.90-10.16 wt.%), FeOtot (6.92-16.69 wt.%), Fe2O3 (0.82-2.95 wt.%), and MgO (9.86-19.54 wt.%), with a Mg/(Fe + Mg) ratio ranging from 0.47 to 0.83. Substitution mechanisms for Ti are proposed, which suggest the presence of exchange vectors involving octahedral and tetrahedral cations. In samples characterized by low Ti contents (0.147-0.239 Ti a.p.f.u.), the Ti incorporation mechanism is: [6]Ti4+ + [6]□ = 2( [6]Mg2+, [6]Fe2+, [6]Mn2+), where [6]□ corresponds to a vacancy in octahedral coordination (Ti-vacancy). In the two groups with intermediate (0.164-0.326 Ti a.p.f.u.) and high Ti contents (0.477-0.532 Ti a.p.f.u.), the Ti substitution mechanism corresponds to the reaction: [6]Ti4+ + 2([4]A13+, [4]Fe3+) = ([6]Mg2+, [6]Fe2+, [6]Mn2+) + 2 [4]Si4+ (Ti-Tschermak). The Mossbauer spectral investigation shows that 0.046-0.167 a.p.f.u. Fe3+ occur on the octahedral sites of the structure. The substitution mechanism responsible for the incorporation of Fe3+ in phlogopites from Suwalki is 3( [6]Mg2+, [6]Fe2+) = 2( [6]Al3+, [6]Fe3+) + [6](M3+-vacancy). The use of the Ti content of phlogopite as geothermometer reveals crystallization temperatures from 729 ± 15 to 874 ± 15 °C for the phlogopites.

Li4Ti5O12 powders by spray-drying: influence of the solution concentration and particle size on the electrochemical properties

The lithium battery electrode compound Li4Ti5O12 was synthesized by calcination of precursor powders obtained through spray-drying of solutions prepared with titanium isopropoxide and lithium nitrate. X-ray diffraction and thermal analysis coupled to mass spectrometry show that single phase crystalline Li4Ti5O12 particles can be obtained after calcination at 800 °C for 2 hours. Decreasing the solution concentration leads to smaller particle sizes but also to an unexpected decrease of the electrochemical capacity, probably related to the presence of residual Li2TiO3. On the contrary, the capacity of the Li4Ti5O12 powder prepared with the high concentration solution can be increased from 150 mAh/g to 165 mAh/g (C/4 rate) by grinding. These results highlight the fact that smaller particles do not systematically display better performances for Li+ intercalation/desintercalation and confirm the need for a comprehensive approach including parameters such as crystallinity, phase purity or agglomeration.

Effect of freeze-drying and self-ignition process on the microstructural and electrochemical properties of Li{sub 4}Ti{sub 5}O{sub 12}

Graphical abstract: - Highlights: • Li{sub 4}Ti{sub 5}O{sub 12} is prepared by a method involving self-ignition of a freeze-dried gel. • Addition of NH{sub 4}NO{sub 3} modifies the self-ignition propagation mode. • Well-crystallized Li{sub 4}Ti{sub 5}O{sub 12} phase is obtained after only 2 h at 800 °C. • Li{sub 4}Ti{sub 5}O{sub 12} powder has 161 mAh g{sup −1} capacity and good retention at C/4 rate. - Abstract: Crystalline Li{sub 4}Ti{sub 5}O{sub 12} is synthesized by a method involving the freeze-drying and self-ignition of a gel prepared from titanium isopropoxide, lithium nitrate and hydroxypropylmethylcellulose (HPMC). This synthesis route yields crystalline Li{sub 4}Ti{sub 5}O{sub 12} particles after calcination at 800 °C for 2 h. In an alternative route, addition of ammonium nitrate shifts the self-ignition mode from wave-like propagation to simultaneous. Powders with different microstructures are thereby obtained. Electrochemical characterization shows that the best results for Li{sup +} intercalation/desintercalation are obtained for the powder prepared without ammonium nitrate addition. These results highlight the necessity for a control of the self-ignition mode to obtain adequate properties.