Adam Donald Walter

A solution-based approach has recently been developed to synthesize sub-nanometer-thick, one-dimensional (1D) lepidocrocite (1DL) titanate nanofilaments (NFs). Upon transitioning from colloidal to solid state, these 1DL NFs spontaneously organize into two-dimensional (2D) layered structures. We recently showed that a moderate-molecular-weight (MW) branched polyethyleneimine (bPEI) can wrap 1DL surfaces and arrest the 1D-2D structural transition. In this work, we systematically investigated the effects of polymer MW and chain architecture (branched vs. linear) on polymer wrapping of 1DL NFs. X-ray diffraction revealed the formation of hybrid liquid-crystalline (LC) phases, with a strong dependence on the polymer volume fraction. Three distinct phases— smectic-like layered, columnar hexagonal (Colhex), and isotropic—are identified for high MW bPEI, while low MW bPEI failed to arrest the transition to 2D. Linear PEI-wrapped 1DL NFs also form Colhex phases but with reduced structural order. These results demonstrate that both MW and chain architecture critically influence the nanoscale organization of the hybrid LC phases, providing design guidelines for polymer-directed assembly of 1D nanofilaments.

Direct ink writing (DIW) of nanomaterial dispersions enables the production of structures and devices that combine the benefits of the nanomaterial properties with use-specific manufacturing designs. However, printing nanomaterial inks requires the ability to produce stable dispersions at concentrations high enough to meet the rheological criteria for DIW. Herein, we report on DIW of one-dimensional lepidocrocite (1DL) nanofilament, NF, dispersions at concentrations of 150 g/L, which are much greater than previously achieved concentrations. Moreover, the resulting birefringent, binder-free nematic gel can be printed into standard test patterns with outstanding dimensional accuracy, structural integrity, and shape fidelity. Both the ability to directly produce higher concentrations and the ability to print them open new opportunities for the production of functional 1DL materials and devices.

Metal oxide corrosion severely limits anodic electrocatalysis, particularly at high potentials in acidic environments, where degradation pathways remain poorly defined. This study establishes explicit connections between corrosion and electrocatalysis on tin oxide during water oxidation by examining the roles of lattice defects, reactive oxygen species, interfacial pH variations, and speciation of corroded tin in acid. We first demonstrate the presence of structural defects such as oxygen vacancies and substoichiometric Sn­(II) species by integrating electron paramagnetic resonance spectroscopy, ultraviolet photoelectron spectroscopy, and Mott–Schottky analysis. Kohn–Sham density functional theory calculations reveal that explicit water structures thermodynamically stabilize reaction intermediates and lower reaction overpotentials. Moreover, we propose that water dissociation leads to hydrogen-bonding networks formed by H* and OH* intermediates, which may span the entire catalyst surface and decrease the interfacial pH to drive corrosion. In contrast, the electrochemical generation of reactive oxygen species is shown to play a minor role in catalyst corrosion during water oxidation using inductively coupled plasma mass spectrometry coupled with selective chemical scavengers. Square-wave voltammetry combined with rotating ring-disk electrodes is used to reveal that under open-circuit conditions, only Sn­(IV) cations chemically dissolve from tin oxide, while both Sn­(IV) and Sn­(II) species electrochemically corrode during water oxidation. Our results unveil a dynamic and complicated interplay between corrosive and catalytic pathways on metal oxide electrocatalysts: a decrease in interfacial pH due to water oxidation exacerbates Sn­(II)/Sn­(IV) corrosion. Subsequently, the electrochemical corrosion of Sn­(II)/Sn­(IV) facilitates product formation from lattice oxygen, while the redeposition of corroded Sn­(II) as Sn­(IV) can enable oxygen exchange with water. By elucidating the roles of defects and interfacial chemistry, this work provides a roadmap for engineering improved electrocatalysts that balance activity and stability, a critical step toward scalable and durable energy technologies.

The surfaces of 1D layered lepidocrocite-structured titanates (1DLs) are negatively charged due to an oxygen-to-titanium atomic ratio >2. This, and their layered structure, allow for facile ion exchange and high colloidal stability, demonstrated by ζ-potentials 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 (little dissolution until pH 1) is demonstrated through inductively coupled plasma mass spectrometry. The Fourier transform infrared 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 heterogeneous surfaces, as illustrated by an isothermal adherence to the Freundlich model. 1DLs have two pK_a values, one at pH ≈10.9 and the other at ≈3.2, and can be protonated to their point of zero charge (≈pH 6.8) before they destabilize. With the understanding of the acid/base properties of 1DLs, cation-stabilized hydrogel-like solids were formed using H⁺, Li⁺, Na^+, K⁺, Mg^2+, Ca²⁺, Ba²⁺, and Fe³⁺. A gelation mechanism is proposed that relies on cation exchange being the driving force for water removal from between adjacent 1DLs. The rheological properties of the soft H₃O⁺-cross-linked gel-like solids show a more than 1000-fold increase in the viscosity of the 1DL colloidal suspensions compared to before gelation.

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 Ų 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 Ų 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...

Highly crystalline two-dimensional (2D) flakes of birnessite, a polymorph of manganese oxide with a MnO₂ chemistry, were synthesized by reacting manganese oxide, Mn₃O₄, at 80 °C with aqueous solutions of tetramethylammonium hydroxide (TMAH) for tens of hours. Their colloidal stability, aggregation, and sedimentation were studied as a function of ionic strengths of Na⁺ and Li⁺ cations. After reaction, a water-based stable colloidal suspension (ζ-potential ∼ -31 ± 1 mV) was obtained. Mixing the colloidal suspension with a LiCl or NaCl aqueous solution resulted in the sedimentation of crumpled flakes, as evidenced by electron microscopy (transmission and scanning). Concomitant with the sedimentation, the TMA⁺ cations present after synthesis are exchanged by the alkali ions, as evidenced by a decrease in the d-spacings between the 2D sheets illustrated by X-ray diffraction (XRD). Both Na and Li uptakes were quantified by elemental analysis via inductively coupled plasma tandem mass spectrometry, giving Li_0.17Mn_0.96O₂ and Na_0.16Mn_0.96O₂. Rhodamine 6G dye was also studied as a sedimentation agent, resulting in a maximum uptake of 550 mg (1.15 mmol) of dye per g of birnessite. To explore the immune response of the Li⁺-intercalated crumpled flakes, the activation of antigen-presenting cells by the flakes was investigated. It was found that the immune cells were slightly activated in a dose-dependent manner, indicating that the materials may have good biocompatibility and thus possibe applications in healthcare.

Recently, we reported on the simple, scalable synthesis of quantum-confined one-dimensional (1D) lepidocrocite titanate nanofilaments (1DLs). Herein, we show, using solid-state UV-vis spectroscopy, that reducing the concentration of aqueous 1DL colloidal suspensions from 40 to 0.01 g/L increases the band gap energy and light absorption onset of dried filtered films from ≈3.5 to ≈4.5 eV. This range is ascribed to quantum confinement as the system transitions from two-dimensional (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 band gap values change. This self-assembly is manifested in the X-ray diffraction patterns and the emergence of a Raman band characteristic of 2D lepidocrocite titanates. In colloidal form, 1DLs exhibit a lyotropic liquid crystal phase with a critical concentration of between 10 and 1 g/L. Additionally, the Beer-Lambert law applies with a mass absorbance coefficient of 2 ± 0.4 Lg^-1 cm^-1. The optical absorbance edges of the colloidal suspensions are not a function of concentration. The experimental findings are theoretically supported by density functional theory calculations of the Raman vibrational modes and electronic band structures of the 1D and 2D lepidocrocite titanate atomic structures.

Herein, we discuss the conversion of one-dimensional lepidocrocite (1DL) titanate nanofilaments to anatase. Upon heating at temperatures >400 °C, the hierarchical 1DLs porous mesostructured particles transform to anatase, while retaining their morphology. These assemblies are characterized via X-ray diffraction, scanning and transmission electron microscopy, Fourier transform infrared spectroscopy, and solid-state ultraviolet absorbance. The assemblies were tested in the photodegradation of a water-soluble, endocrine-disrupting organic compound, bisphenol A (BPA). Using ultraviolet-visible spectroscopy, we show that 95% of BPA is degraded in 1 h under 1 sun of the simulated solar spectrum. Under the same conditions, the total organic carbon of the solution was reduced by 70%.

When a few drops of acid (hydrochloric, acrylic, propionic, acetic, or formic) are added to a colloid comprised of 1D lepidocrocite titanate nanofilaments (1DLs)–2 × 2 TiO ₆ octahedra in cross‐section–a hydrogel forms, in many cases, within seconds. The 1DL synthesis process requires the reaction between titanium diboride with tetramethylammonium (TMA ⁺ ), hydroxide. Using quantitative nuclear magnetic resonance (qNMR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), the mass percent of TMA ⁺ after synthesis is determined to be ≈ 13.1 ± 0.1%. The TMA ⁺ is completely removed from the gels after 2 water soak cycles, resulting in the first completely inorganic, TiO ₂ ‐based hydrogels. Ion exchanging the TMA ⁺ with hydronium results in gels with relatively strong hydrogen bonds. The hydrogels' compression strengths increased linearly with 1DL colloid concentration. At a 1DL concentration of 45 g L ⁻¹ , the compressive strength, at 80% deformation when acrylic acid is used, is ≈325 kPa. The strengths are ≈ 50% greater after the TMA ⁺ is removed. The removal of all residual organic components in the hydrogels, including TMA ⁺ , is confirmed by qNMR, Fourier‐transformed infrared spectroscopy (FTIR), and TGA/DSC. The 1DL phase is retained after gelation, TMA ⁺ removal, and 80% compression.