Synthesis of titania, doped-TiO<sub>2</sub>, and boron nitride nanoparticles by pulsed-laser decomposition in liquid precursor solutions
Creators: name: Mozael, Mustafa Mohsin
Publisher: name: No Publisher Supplied
Year: 2019

description: Titanium dioxide (TiO2) belongs to the transition-metal oxide family and crystallographically can form several polymorphs, including anatase (tetragonal), brookite (orthorhombic), and rutile (tetragonal). In comparison with other polymorphs, anatase-TiO2 is preferable for diverse applications because of its high electron mobility, low dielectric constant, and low density.In this research, amorphous and far-from-stoichiometric titania nanoparticles and films are produced by a novel method of pulsed-laser decomposition of liquid titanium tetra-isopropoxide (TTIP) precursor. Nanoparticles are formed innately, whereby the submerged laser-induced plasma generates vaporized species from the TTIP solution and are then rapidly quenched by the same surrounding (chilled) liquid precursor TTIP. Relatively-dense films are formed on glass substrates placed above the surface of the liquid TTIP by vapor transport and condensation of pyrolyzed species. Upon post-annealing at 400ºC (~ 0.3 TM) in ambient air, transformation of the amorphous non-stoichiometric titania nanoparticles into novel anatase-TiO2 morphologies, such as layered nanotubes and onion-like nanospheres, occurs. Chemical analysis of the nanostructured particles/films show that they are rich in oxygen and carbon relative to stoichiometric TiO2. Evidently, the amorphous-to-anatase phase transformation during heat treatment in air starts at the surfaces of the nanoparticles, irrespective of their morphologies, and propagates into the interior. In-situ reaction of trapped-in species likely yields gaseous products (e.g., CO, CH4, H2O) that diffuse out of the particles, leaving sufficient Ti and O to enable crystallization of anatase-TiO2. Further annealing at 800ºC in air transforms anatatase-TiO2 to rutile-TiO2. Catalytic activity is examined by heterogeneous hydrogen generation from water reduction with methanol as a sacrificial agent. Decreased band-gap energy of the nanopowder, as well as increased absorbance in both the high-energy (i.e., 200 nm – 250 nm) and low energy (i.e., 340 nm – 440 nm) UV regions, when compared to that of commercial anatase, indicates that the nanopowder is more active under UV illumination because of increased ability to harness the photons for photocatalysis. Carbon content, along with the new nanocrystalline layered morphology, likely play the main roles of shifting the band gap.To extend the light absorption edge, TiO2 can be doped with metal (e.g., W, Mo) and non-metal (e.g., N, C, F, S) elements to augment the photo-response and visible-light photoactivity. In this research, different ions such as tungsten, molybdenum, and vanadium are used as doping ions into the TiO2 nanostructure. The initial results show that the doped TiO2 nanostructure is more photocatalytically active than that of undoped TiO2 nanopowder. Nanostructured powders of tungsten-doped TiO2 are synthesized by pulsed-laser ablation of a tungsten foil immersed in liquid TTIP. Interaction between the focused laser beam and the W substrate generates a submerged-plasma, where ablation of the W substrate along with decomposition of the adjacent liquid precursor combine to produce W-doped TiO2 nanoparticles upon quenching by the surrounding un-reacted liquid precursor TTIP. The as-synthesized nanoparticles display various morphologies, including nano-sphere and nano-fiber, and occur in discrete agglomerated and aggregated forms. Whatever their morphologies, all nanoparticles have non-crystalline or amorphous structures, primarily because of rapid condensation and quenching of vaporized species from the plasma-reaction zone. Interestingly, upon subsequent heat treatment in air or oxygen, starting at ~400 ºC, transformation to the more stable anatase-TiO2 phase occurs, but now doped with tungsten. X-ray diffraction (XRD) identifies crystallinity and phase conversion of the photocatalyst. The phase transformation with increasing temperature from anatase to rutile TiO2 in the doped sample can be deferred in comparison to that of non-doped TiO2. In addition, the average crystallite size of TiO2 (about 13 nm) becomes slightly reduced by doping with W (10 nm). Preliminary results show that W-doped anatase TiO2 exhibits a higher UV and visible photochemical activity than un-doped anatase-TiO2. Using the same synthesis method, molybdenum-doped and vanadium-doped-TiO2 nanostructures are also produced. Initial results show that Mo6+ ions are doped into an anatase TiO2 lattice. As the ionic radius of molybdenum (0.62 nm) and titanium (0.68 nm) are quite similar, it is much easier for Mo to occupy a lattice position of Ti instead of an interstitial position. Mo-doping in TiO2 narrows the band gap (from 3.04 eV of TiO2 to 2.8 eV), shifting the optical absorption more into the visible range. Interestingly, the doped nanopowder exhibits higher UV and visible photochemical activity than does un-doped anatase-TiO2. For vanadium-doped TiO2 samples, the results show that the unit cell volume and parameters a and c decrease in comparison to those of un-doped samples. Based on the fact that the ionic radius of V5+ (0.054 nm) is smaller than that of Ti4+ (0.068 nm), the vanadium ions can replace titanium ions in the TiO2 lattice.As an extension of the processing method, boron nitride (BN) nanoparticles are generated through pulsed laser ablation of boron bulk immersed in an ammonia solution. The unique conditions of high-temperature plasma reaction with rapid subsequent quenching enable metastable phase formation. Short-range ordered BN structure is produced. Upon heat-treatment at 1000ºC in an ammonia atmosphere for 2 hrs, the powder transforms to the more thermodynamically-stable BN. XRD indicates the presence of hexagonal BN (h-BN), with some cubic BN (c-BN) as well.