Surface properties, modifications, and application of low-dimensional titanates

Adam Donald Walter

doi:10.17918/00011029

In 2022, the Barsoum group reported on a simple, scalable method to synthesize a new form of titanate nanomaterial; one-dimensional lepidocrocite titanate (LT) nanofilaments (1DLs). With only one dimension measurable on the nanoscale, and a base unit cross-section of 5x7 Å² or 2x2 TiO₆ octahedra -- 1DLs are truly 1D. As suspected, such extreme dimensionality is accompanied with unique confinement and surface area effects that endow 1DLs with remarkable activity and versatility. Somewhat surprisingly, making them is quite simple: cheap and abundant Ti-containing precursors (borides, carbides, oxysulfate, etc.) are reacted with tetramethylammonium hydroxide (TMAOH) under ambient pressure and at temperatures below water's boiling point for tens of hours to convert them to 1DLs. However, the potential does not stop at the synthetic route, since 1DLs are essentially all surface, they easily support applications from adsorption, catalysis, and as battery electrode materials. This dissertation furthers the development of 1DLs from a surface science perspective. Herein, the 1DL surface fundamentals, aggregation, and select surface-driven applications (adsorption and photocatalysis) are discussed. The surfaces of 1DLs are negatively charged due to an oxygen to titanium atomic ratio > 2. This and their layered morphology allow for facile ion exchange and high colloidal stability demonstrated by zeta-potentials ([zeta]) of ~ - 85 mV at their unadjusted pH of ~ 10.4. This is nearly maintained across a 20 to 70 °C temperature range, with only a slight decrease in stability. The acid resistance of 1DL solids (no dissolution until pH 1) is demonstrated through inductively coupled plasma mass spectrometry. The Fourier transform infrared (FTIR) spectra of the dried 1DLs are also discussed. From a fundamental charge perspective, these materials offer an ion exchange capacity of ~ 1.8 mmol/g, nearly twice that of highly charged clays or Nafion. As a Brønsted-Lowry base, they readily adsorb protons onto their heterogenous surfaces, as illustrated by an isothermal adherence to the Freundlich model. 1DLs have two pK_a values, one at pH ~ 10.9 the other at ~ 3.2, and can be protonated to their point of zero charge (PZC) of ~ pH 6.5 before they destabilize out of suspension. With the understanding of the acid/base properties of 1DLs, cation-stabilized, hydrogel-like solids were formed using H⁺, Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, and Fe³⁺. A gelation mechanism is proposed which relies on cation exchange being the driving force for water removal from between adjacent 1DLs. The rheological properties of the soft H₃O⁺-crosslinked gel-like solids show a 1000-fold increase in the viscosity of the 1DL colloidal suspension before gelation. Using solid-state UV-Vis spectroscopy - that reducing the concentration of aqueous 1DL colloidal suspensions from 40 g/L to 0.01 g/L, increases the band gap energy (E_g), and light absorption onset, of dried filtered films from ~ 3.5 eV to ~ 4.5 eV. This range is ascribed to quantum confinement as the system transitions from 2D into 1D with dilution. It is only after the colloidal suspensions are dried and the 1DLs start to self-assemble into ribbons and sheets that the E_g values change. This self-assembly is manifested in the X-ray diffraction (XRD) patterns and the emergence of a Raman band characteristic of 2D LT. In colloidal form, 1DLs exhibit a lyotropic liquid crystal (LC) phase with a critical concentration of between 1 and 10 g/L. Additionally, the Beer-Lambert law applies with a mass absorbance coefficient of 2 ± 0.4 Lg⁻¹cm⁻¹. The experimental findings are theoretically supported by density functional theory (DFT) calculations of 1D and 2D LT atomic structures. The calculations include band structures and the entire set of Raman vibration modes of 1D and 2D LT. The self-assembly of 1DLs in 10 different water miscible organic solvents was investigated. The nanofilament snippets, with minimal cross sections of ~ 5 x 7 Å² and lengths around 30 nm, begin as an aqueous colloidal suspension. Upon addition, and brief mixing, of the colloidal suspension into a given water miscible organic solvent, a multitude of morphologies -- seemingly based on the hydrophilicity and polarity of the solvent -- emerge. These morphologies vary between sheets, highly networked webs, and discrete fibers, all with no apparent change in the lepidocrocite structure. On the micro- and nanoscales, the morphologies are reminiscent of biological, rather than inorganic, materials. The results of this work yield insight into the self-assembly of 1DLs and offer new pathways for novel macrostructures/morphologies assembled from these highly adsorbent and catalytically active low-dimensional materials. A method to produce fluorine (F-1DL), sulfate (S-1DL), and phosphate (P-1DL) modified 1DLs was developed. Each method will be discussed independently, but the logic remains the same: at low pH, Ti-OH₂⁺ surface groups predominate allowing for the dehydration of the Ti surface site and attachment of the more nucleophilic anion (F⁻, SO₄²⁻, PO₄³⁻). The resulting modified 1DLs include high concentrations of surface groups, more specifically atomic ratios of F/Ti ~ 12%, S/Ti ~ 40%, and P/Ti ~ 125%, for F-, S-, and P-1DL, respectively, analyzed with an X-ray photoelectron spectroscope (XPS). Using an inductively coupled plasma triple quadrupole mass spectrometer (ICP-QQQ), we were able to study the S- and P-1DL samples and their interactions with water and acid. It is apparent that some of the SO₄²⁻ and PO₄³⁻ groups can be removed by water, thus the chemically bound S/Ti and P/Ti atomic ratios are ~ 15% and 77%, respectively. XPS analysis shows that in F-1DL, the surface binding mode can be modulated (O₅-Ti-F to O_(6-x)-Ti-F_x) simply by controlling the pH of the synthesis procedure. Using FTIR the binding modes of each oxyanion was determined to be bidentate. 1DLs were evaluated for their ability to decolorize and degrade two common dye pollutants, viz. rhodamine 6G (Rh6G) and crystal violet (CV), under a full simulated solar spectrum as well as ultraviolet (UV) and visible (Vis) light spectra, individually. The materials were characterized by XRD as well as scanning and transmission electron microscopes (SEM and TEM). The dye decolorization was monitored via UV-Vis spectroscopy. Mineralization was quantified by chemical oxygen demand (COD). As a colloidal nanomaterial, 1DLs present exceptional maximum uptake for both Rh6G and CV, at 1,850 mmol·kg⁻¹ and 1,930 mmol·kg⁻¹, respectively. They also become dye sensitized and can decolorize Rh6G and CV using Vis light only, by an average of 90% and 64%, respectively, in 30 minutes, when the starting catalyst to dye mass ratio was 1 to 1 (concentrations of 10 mg/L). 1DLs were explored in the adsorption and photocatalytic degradation of aqueous malachite green (MG), a toxic polluting dye. Decolorization is monitored by UV-Vis spectroscopy and mineralization is confirmed by total organic carbon analysis (TOC). The 1DL/MG flocs are characterized by SEM and XRD. 1DLs exhibit flocculating behavior while demonstrating high affinity for MG, with a maximum uptake of >680 mg/g rapidly via ion-exchange. Additionally, 1DLs decolorize MG under visible light only, unlike most available titania products, via a self-sensitization effect. MG is decolorized by 1DLs by >70% in 30 min under 1 sun exposure of visible light. Counterintuitively, dye adsorption increases as the normalized concentration by mass of 1DL decreases. Demonstrating high adsorption capacity and dye mineralization, supports the use of 1DLs in water treatment and self-sensitization for photoelectrochemical devices, like solar cells.

Surface properties, modifications, and application of low-dimensional titanates

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Surface properties, modifications, and application of low-dimensional titanates

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