Synthesis and structure tuning of quantum-confined one-dimensional lepidocrocite titania-based nanofilaments

Mohamed Ahmed Ibrahim Ibrahim

doi:10.17918/00011468

Quantum-confined, one-dimensional lepidocrocite (1DL) titania-based nanofilaments (NFs) are a recently discovered family of titanate nanostructures whose minimal 5 x 7 Å² cross-section places every atom at, or one atom away from, a surface. Made by a simple, one-pot, scalable reaction of a titanium precursor with tetramethylammonium hydroxide (TMAH) at ambient pressure and 80 °C, 1DLs combine an extreme surface-to-volume ratio with a wide, quantum-confinement-derived band gap energy, E_g, of 4 eV and exceptional performance in photocatalysis, adsorption, and electrocatalysis. Until recently, however, their synthesis relied almost exclusively on TMAH as the growth-directing and charge-balancing species, and most reactions were run under a narrow window of conditions, typically 80 °C for ~ 4 d. It therefore remained unclear how broadly the 1DL platform could be modified. This thesis shows that both the synthesis and composition of 1DLs can be deliberately tuned along four distinct axes: the chemistry of the organic growth-directing species, the physical synthesis conditions, including reaction temperature and time, the composition of the 1DL backbone, and the composition of the interlayer gallery. First, the chemical processing space is expanded. A broad family of less-toxic quaternary ammonium and phosphonium hydroxides--tetraethyl-, tetrapropyl-, and tetrabutylammonium hydroxide, biocompatible choline hydroxide, and tetrabutylphosphonium hydroxide--all convert titanium precursors into structurally, electronically, and photocatalytically similar 1DL NFs. Despite large differences in interlayer cation size, and interlayer distances (11.4 to 17 Å) the products retain high, interlayer-insensitive E_g of ~ 3.8-3.9 eV, confirming that their electronic structure is governed by quantum confinement arising from the ultranarrow cross-section rather than by the identity of the interlayer cation. Flocculation behavior, however, differs; less polar cations and washing solvents favor porous mesostructured particles (PMPs), whereas more polar conditions favor stable colloidal suspensions. TPA-1DL colloidal suspension can be dispersed in dimethyl sulfoxide, methanol, ethanol, acetonitrile, isopropyl alcohol, butanol, acetone, and water, opening many applications where non-aqueous suspensions are required, such as coatings, inks, and catalysis. Second, the physical processing space is expanded through a facile, scalable, room-temperature (RT) synthesis of 1DL NFs from titanium oxysulfate, a cheap and ubiquitous precursor, replacing routes that required 80 °C and days of reaction. By varying the RT reaction time from 4 to 12 h, the NF dimensions are tailored. Small-angle X-ray scattering showed that ribbons reacted for 4 h are ~ 33 nm long and ~ 2.7 nm wide, whereas those reacted for 12 h exceed 100 nm in length and are ~ 3.1 nm in width. This represents the first diffraction-based evidence that the ribbons lengthen with time. PMPs formed at RT are more reminiscent of classic nanoparticle agglomerates and do not show the 1D filamentous texture observed in TiB₂-derived PMPs made at 80 °C. Powders made this way, however, can be dried and redispersed in water to regenerate colloids. The latter adsorbs more rhodamine 6G (R6G) than any previous 1DL suspension tested. The Eg remains at ~ 4.0 eV and is only weakly time-dependent. Together these results establish a simple, inexpensive, and highly scalable route to 1DL NFs, with reaction time serving as a practical knob for length, redispersibility, and dye uptake. The thesis then turns from how the filaments are made to what they are made of. A scalable, bottom-up approach dopes the 1DL backbone with transition-metal (TM) cations--Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺--by introducing metal salts during synthesis while preserving the NF structure and morphology. All TM-doped NFs exhibit lower E_g than the undoped material (~ 3.89 eV); the largest decrease, ~ 0.8 eV, occurs for Mn²⁺ at 1 mol.%, extending absorption into the visible. This is accompanied by enhanced function: 1 mol.% TM-doped NFs degrade up to 95% of R6G under visible light within 30 min, versus ~ 65% for the undoped material. In electrocatalysis, Ni-doped NFs give the best oxygen evolution reaction (OER) performance, with an overpotential of 319 mV at 10 mA cm⁻² (versus 383 mV for undoped 1DLs), faster kinetics (Tafel slopes, 143-145 mV dec⁻¹ versus 204 mV dec⁻¹), and stable operation for > 50 h. Finally, the compositional argument is completed by intercalating TM cations--Fe, Cu, Ni, Zn, Co, Mn, and co-intercalated Ni/Fe--into the interlayer spacing by a simple, RT aqueous cation exchange with TMA⁺. X-ray diffraction confirms that all samples retain the characteristic 1DL reflections and ABAB stacking, while systematic shifts of the low-angle 020 peak track the change in interfilamentous spacing as bulky TMA⁺ is replaced by smaller hydrated TM cations; the PMP morphology is fully preserved. Thermogravimetric analysis (TGA) and Fourier-transform infrared spectroscopy (FTIR) independently confirm complete TMA⁺ replacement, and combining inductively coupled plasma (ICP) spectroscopy, X-ray photoelectron spectroscopy (XPS), and TGA yields the full chemical formula, with ion-exchange capacities clustering near ~ 2 meq g⁻¹. All intercalated samples show reduced E_g's from 3.6 eV for TMA-1DL to as low as 2.7 eV for the Co-intercalated material.

Synthesis and structure tuning of quantum-confined one-dimensional lepidocrocite titania-based nanofilaments

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Synthesis and structure tuning of quantum-confined one-dimensional lepidocrocite titania-based nanofilaments

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