Colloidal Processing and Photocatalytic Properties of Titanate-Niobate Nanosheets

Abstract

Nanosheets are a class of materials defined by extreme anisotropy, typically with widths and thicknesses on the order of micrometers and nanometers, respectively. These materials are often synthesized from a bulk ceramic using soft-chemical processing to exfoliate individual sheets or clusters of sheets. Nanosheets are advantageous to applications dominated by surface chemistry, for example photocatalysis and electrochemistry. Titanate and niobate nanosheets have been extensively studied for these applications. The layered titanate-niobate nanosheets defined by the chemistries TiNbO5, H3Ti5NbO14 and HTi2NbO7 demonstrate photocatalytic behavior and interesting colloidal properties due to the liquid crystalline nature of nanosheet colloids. In this work, colloidal suspensions of single-layer titanate-niobate nanosheets were synthesized by exfoliation of the parent oxides. AFM showed single layer sheets to be between 1 and 2 nm thick, while XRD measurements showed a mean restacking distance between 2 and 2.5 nm. This restacking distance was shown to vary approximately 6 Å, presumably due to displacement of H2O as suggested by FTIR. Nanosheet colloids displayed liquid crystalline behavior which could be eliminated in favor of new mesophases by reducing pH or by adding various salts such as Na2SO4 in a process called reassembly. Adding acid resulted in gelled colloids defined by a sharp increase in viscosity below a critical pH of approximately 6. Rheological and diffraction measurements support a gelation mechanism resulting from the excluded volume effect and electrostatic repulsion between sheets. Adding a salt, however, resulted in flocculated nanosheets without an accompanying viscosity increase. Flocculation proceeds by some face-to-face restacking of neighboring nanosheets into tactoids which are themselves assembled into an edge-to-face 3D network. XRD patterns suggest tactoid thicknesses of 4-6 sheets. Powders prepared from assembled gels or floccules possessed unique mesostructures dependent on their colloidal processing as shown by BET and SEM. Assembled gels were found to be mostly mesoporous and dominated by small pores formed between restacked sheets, as indicated by average pore widths between 6-15 nm and large N2 adsorption hysteresis. Assembled floccules were significantly less mesoporous with essentially no adsorption hysteresis and larger pores averaging between 20-27 nm formed between linked tactoids. UV-visible adsorption spectra showed band gaps to be reduced from 4.1 eV for the parent phase to 3.6 eV for the exfoliated nanosheets. Reassembly with various cations raised the band gaps to 3.9-4.1 eV. Each chemistry demonstrated photocatalytic hydrogen production with a co-catalyst when exfoliated or reassembled. Exfoliated H3Ti5NbO14 producing the most hydrogen at a rate of 426 μmol H2/hr/g, whereas HTiNbO5 produced the least hydrogen. Assemblies of commercial TiO2 photocatalyst and exfoliated HTiNbO5 nanosheets were found to be highly photochemically active without an additional co-catalyst, producing 412 μmol H2/hr/g. Exfoliated nanosheets were also found to photocatalytically decompose methylene blue with exfoliated H3Ti5NbO14 demonstrating the highest decomposition rate. Photocatalytic behavior was therefore found to vary with nanosheet chemistry but not crystallographic motif.