Flame synthesis of (W, Mo)-doped titanium dioxide nanoparticles using solid metal wire mesh as precursor dopant and ignition spectra of Al/WOx nanowire thermites
Creators: name: Zhang, Yuqian
Publisher: name: No Publisher Supplied
Year: 2021

description: Transition metal (tungsten and molybdenum) and non-metal (nitrogen) doped titanium dioxide nanoparticles are produced using flame synthesis. A novel burner configuration is used where titanium isopropoxide (TTIP) flows through a separate center tube to create TiO2 nanoparticles, while solid metal wire/mesh is situated above the surrounding multiple over-ventilated diffusion flames to generate volatile metal-oxide vapor as precursor for the dopant. Metal-doping can enhance the performance of titanium dioxide as a photocatalyst by the dispersion of metal ions into the TiO2 matrix to promote the transport of electrons from valence band to conduction band. Utilization of a metal wire mesh as precursor for doping is especially advantageous for commonly low-vapor-pressure precursors. The entire nanoparticle synthesis process is still gas-phase based. In a different way, nitrogen doping is accomplished using vaporized nitrogen-containing precursor solutions.The effect of varying the tungsten loading rate is studied for synthesizing doped titanium dioxide with different tungsten loadings at a fixed metal source temperature. The results show that a high loading rate of tungsten can trigger homogenous nucleation of WO3 prior to the dopant vapor reaching the TTIP precursor loaded region, resulting in mainly mixed TiO2 and WO3 particles. Conversely, a low loading rate of tungsten produces tungsten-doped TiO2 particles. Heat treatment of W-doped TiO2 at 973 K in argon atmosphere moves some of the tungsten out of the TiO2 molecular structure, thus making a new WOx-TiO2 solid solution, where tungsten ions are reduced to lower oxidation states. Moreover, the annealing process also increases the unit cell volume of W-doped TiO2, moving the value closer to that of the un-doped TiO2, by releasing some of the tungsten out of the TiO2 molecular structure. XRD, SEM, TEM, and XPS characterize crystallinity, morphology, and valence state. UV-Vis spectroscopy shows that tungsten doping and heat treatment improve the absorption of titanium dioxide in the visible light wavelength range significantly.The metal-source temperature effect on the molybdenum dopant loading rate is investigated. With a fixed dopant mass, lower temperature of the source metal (corresponding to lower dopant loading rate) favors heterogeneous nucleation, resulting in pristine Mo6+ doped TiO2 nanoparticles. On the other hand, higher source dopant temperature (corresponding to higher dopant loading rate) results in homogeneous nucleation of MoO3 and production of MoO3 nanoparticles along with TiO2. Phase-pure crystalline anatase Mo-TiO2 is successfully synthesized at low characteristic flow-field temperatures with a low hydrogen flow rate through the center tube carrying TTIP. The creation of N-TiO2 by solution pyrolysis further highlights the vast potential of using the multiple diffusion flame burner for non-metal doping.Synthesis of co-doped (i.e., tungsten and molybdenum) TiO2 nanoparticles are researched using fixed dopant loading rate with different precursor carrier conditions. Although neither major WO3 nor MoO3 peaks are observed by XRD, except for the low characteristic temperature case, XPS analyses confirm that the core binding levels are W6+ and Mo6+ within the as-synthesized particles, which suggests doping of W and Mo into the TiO2 lattice. The temperature profile of the center precursor jet flame is controlled by adjusting the H2/N2 ratio in the carrier gas. The rutile content ratio of TiO2 increases continuously with temperature until the ratio peaks at ~800oC (50%), where the synergistic effect between sintering and phase transformation yields the highest rutile ratio. With rising characteristic flame temperature, although the transformation process is supposed to become faster, the collision rate turns out to be so high that rutile nuclei do not form around anatase clusters. Meanwhile, the sintering between anatase nanoparticles becomes rapid and dominates, producing larger anatase particles instead of rutile ones.In a related project involving synthesis of metal-oxide nanowires from base metal wire, WO2.9/Al coaxial nanowires are fabricated by a sequential process involving flame synthesis and ionic-liquid electrodeposition. Different than the natural thick passivation oxide layer formed surrounding Al nanoparticles, a distinct interface is established between tungsten-oxide and Al, with only a possible atomic layer of Al2O3 present. Spectral analyses of emissions from single-pulse laser ignition of different samples reveal different mechanisms. For the thermite from micropowder mixture of Al and WO3 particles, the laser pulse needs to break down both the Al2O3 layer and the WO3 oxidizer so that the melted Al core can diffuse to the surface in order to establish a solid-gas phase reaction. For above-mentioned fabricated nanowire thermite, without the passivation Al2O3 layer between the coating Al and the WO2.9 core, ignition can be triggered at a lower pulse energy such that heated Al atoms can directly migrate inwards to the WO2.9 nanowire core (through any built-up Al2O3 product layer) in solid-to-solid phase to react self-sustained.