Hydroxide-derived Nanostructures
Authors:   , Badr Hussein O. , Barsoum Michel W. 
Published: October 2023

Titanium oxide nanostructured materials have been, and remain, of significant commercial and research interest due to their potential use in a wide range of fields including paints, catalysis, photocatalysis, among many more. In this thesis, we report on the large-scale synthesis of TiO2-based one-dimensional (1D) nanofilaments (NFs) using a facile, bottom-up, one pot, solution-precipitation synthesis protocol. Our method entails mixing commercial, and earth abundant titanium-containing powders such as binary carbides, nitrides, borides, oxides among many more with tetramethylammonium hydroxide (TMAH) in plastic bottles at 25°C - 95°C for a few days under ambient pressure. The resulting 1D, lepidocrocite titania NFs, with cross sections of ~ 5x7 Å2 self-assemble in a plethora of nanostructures, e.g. pseudo-two-dimensional, 2D, flakes, nanobundles or mesoporous particles, when dispersed in different solvents. The mesoparticles are free-flowing and are more or less spherical with diameters in the ~ 5 to 30 [mu]m range. The morphologies of the mesoparticles are quite unique and can be best described as spheres comprised of entangled 1D lepidocrocite titania NFs. The latter self-assemble into bundles at the sub- and micrometer scale that then entangle to form porous spheres. The formation of the NFs occurs from the precursors' surfaces inwards, allowing for the formation of core-shell configurations. We readily replace the TMA+ cations between the 1D NFs with H3O+, Li⁺, Na⁺, Mg²⁺, Mn2+, Fe2+, Co2+, Ni2+, Zn2+ and U4+ cations. When dispersed in water, the resulting materials form quite stable colloids in water with zeta-potentials values of ~ - 50 mV. Electrodes made from these 1DL NFs performed well in lithium-ion and lithium-sulphur systems as examples for energy storage applications. In terms of photocatalytic activity, these 1DL NFs were found to photochemically generate H2 from 80:20 v/v water/methanol mixtures, when illuminated by simulated sunlight. The NFs were stable in the mixtures for times > 4,300 h, 300 h of which were under irradiation. Apparent quantum yields as high as 11.7% were obtained. Based on deuterated water results we conclude that water is the H2 source. Further, no CO₂ due to photocatalytic degradation of methanol was detected. We expect these NFs will lead to new lines for developing cheap and ultra-stable materials to produce H2 photochemically for a long time. Turning to other potential applications, these NFs were also found to reduce the viability of cancer cells thus showing potential in biomedical applications. To ascertain the general validity of our method, we describe a similar solution processing pathway to convert - again through a bottom-up approach - 5 different water-insoluble Mn-bearing precursors, viz. Mn3O4, Mn2O3, MnB, Mn5SiB2, and Mn2AlB2, into MnO2 birnessite-based 2D flakes that, in some cases, are quite crystalline. The precursor powders are immersed in 25 wt % TMAH aqueous solutions at 50 °C to 80 °C for 2 to 4 days. The structures, compositions, oxidation states, and morphologies of the synthesized flakes are determined using a battery of characterization techniques. The synthesized 2D sheets demonstrate reversible O2 electrocatalysis with activities comparable with those of a commercial Pt/C catalyst. This discovery is paradigm shifting and will undoubtedly open new and exciting avenues of research and applications of nanomaterials of different structures, compositions, and morphologies. This is especially true of the TiO2 lepidocrocite 1D nanofilaments.