The initial stages of multicomponent particle formation during the gas phase combustion synthesis of mixed SiO2/TiO2

The ability to properly scale the synthesis of advanced materials through combustion synthesis routes is limited by our lack of knowledge regarding the initial stages of particle formation. In flame aerosol reactors, the high temperatures, fast reaction rates, and flame chemistry can all play a critical role in determining the properties of the resulting nanomaterials. In particular, multicomponent systems pose a unique challenge as most studies rely on empirical approaches toward designing advanced composite materials. The lack of predictive capabilities can be attributed to a lack of data on particle inception and growth below 2 nm. Measurements for the initial stages of particle formation during the combustion synthesis of SiO₂ and composite SiO₂/TiO₂ using an atmospheric pressure inlet time-of-flight mass spectrometer are presented. Both positively and negatively charged clusters can be measured and results show the presence of silicic acid species which grow through dehydration, hydrogen abstraction, and interactions with hydroxyl radicals. In the case of composite SiO₂/TiO₂ particle formation, new molecular species containing Ti atoms emerge. Tandem differential mobility analysis-mass spectrometry (DMA-MS) provided further insight into the size-resolved chemistry of particle formation to reveal that at each cluster size, further hydroxyl-driven reactions take place. From this we can conclude that previous assumptions on collisional growth from simple monomer species of SiO₂ and TiO₂ do not sufficiently describe the collisional growth mechanisms for particle growth below 2 nm.

Copyright © 2018 American Association for Aerosol Research     

Structural and luminescence properties of undoped,Nd3+ and Er3+ doped TiO2 nanoparticles synthesized by flame spray pyrolysis method

Bulk and thin film forms of titanium dioxide (TiO₂) have been studied many times due to its very promising optical properties. In this study, low-cost flame spray pyrolysis (FSP) synthesis of Nd³⁺/Er³⁺doped TiO₂ nanoparticles has been reported for the first time. The produced particles were post-annealed after FSP process at 550 °C in order to obtain crystalline structure. The phase and elemental analysis of the produced materials were performed by X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS), respectively. The surface morphology, accurate size and specific surface area of the primary particles were identified using scanning electron microscopy (SEM) and particle size analyser. Luminescent properties of the produced nanoparticles were investigated by steady state and time resolved fluorescence spectra. Doping of TiO₂ nanoparticles with the rare earths of Nd³⁺and Er³⁺resulted in visible and near-infrared light emission when excited at 364 nm. The utilized nanoparticles yielded bi-and tri-exponential decay curves. Additionally, they exhibited typical upconversion luminescence when radiated by 810 nm.     

Preparation and testing of Nafion/titanium dioxide nanocomposite membrane electrode assembly by ultrasonic coating technique

Membrane electrode assemblies with Nafion/nanosize titanium dioxide (TiO₂) composite membranes were manufactured with a novel ultrasonic‐spray technique (UST) and tested in proton exchange membrane fuel cell (PEMFC). The structures of the membranes were investigated by scanning electron microscopy (SEM), X‐ray diffraction (XRD), and thermogravimetric analysis. The composite membranes gained good thermal resistance with insertion of TiO₂. The SEM and XRD techniques have proved the uniform and homogeneous distribution of TiO₂ and the consequent enhancement of crystalline character of these membranes. The existence of nanometer size TiO₂ has improved the thermal resistance, water uptake, and proton conductivity of composite membranes. Gas diffusion electrodes were fabricated by UST. Catalyst loading was 0.4 (mg Pt) cm^?2 for both anode and cathode sides. The membranes were tested in a single cell with a 5 cm² active area operating at the temperature range of 70°C to 110°C and in humidified under 50% relative humidity (RH) conditions. Single PEMFC tests performed at different operating temperatures indicated that Nafion/TiO₂ composite membrane is more stable and also performed better than Nafion membranes. The results show that Nafion/TiO₂ is a promising membrane material for possible use in PEMFC at higher temperature. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40541.     

Generation of TiO2 Aerosols from Liquid Suspensions: Influence of Colloid Characteristics

The influence of the colloidal characteristics of aqueous TiO₂ nanoparticle suspensions and of the operating conditions on the total particle concentration and the particle size distribution of aerosols generated by nebulization has been studied. A commercial nebulization unit coupled to a diffusion dryer was used to generate aerosols using two different sources of titanium dioxide nanoparticles. Stable, concentration-tunable aerosols could be obtained for both types of nanoparticle suspensions. The effect of operating conditions during nebulization (air flow rate, purity of water source, nanoparticle concentration, and pH of the precursor suspension) was studied. The results obtained indicate that the degree of agglomeration in the liquid phase previous to aerosol formation has a direct influence both on the total nanoparticle count and on the particle size distribution of the generated aerosols.

Copyright 2013 American Association for Aerosol Research     

Mesoporous TiO<Subscript>2</Subscript> microspheres synthesized via a facile hydrothermal method for dye sensitized solar cell applications

Mesoporous TiO₂ microspheres were successfully synthesized by a facile hydrothermal process and the obtained product was sintered at 450 °C. The sintered TiO₂ powder was characterised by powder X-ray diffraction pattern and the result shows pure anatase phase with good crystalline nature. The morphological image of field emission scanning electron microscopy and high resolution transmission electron microscopy shows spherical shape and size of the particles is around 100 to 300 nm. The Brunauer–Emmett–Teller surface area of synthesized TiO₂ material was 56.32 m² g^?1 and average pore width of synthesized materials was 7.1 and 9.3 nm. Bimodal pore structure of TiO₂ microspheres has been very effective for electrolyte diffusion into photoanode in dye sensitized solar cells. The synthesized anatase TiO₂ microsphere based dye sensitized solar cells have high surface area with light scattering effect to enhance the photocurrent and conversion efficiency than the commercial P25 photoanode material. The power conversion efficiency of synthesized mesoporous TiO₂ microspheres and commercial P25 material is 4.2 and 2.7 % respectively. Therefore bimodal mesoporous anatase TiO₂ microsphere appears to be a promising and potential candidate for dye sensitized solar cells (DSSC) application.     

Cluster formation mechanisms of titanium dioxide during combustion synthesis: Observation with an APi-TOF

Few studies reported the formation of Ti-containing clusters in the initial stages of TiO₂ flame synthesis. The conversion from synthesis precursor to TiO₂ monomers was commonly assumed to take place through global reaction such as thermal decomposition and/or hydrolysis at high temperatures. More recent studies have been able to identify stable intermediates of Ti-containing monomers, most commonly Ti(OH)₄, as the final step before the formation of TiO₂. However, no larger Ti-containing cluster formation mechanisms or interactions between these monomers have been tracked. To investigate cluster formation pathways of TiO₂ during flame synthesis, Charged clusters were measured in an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer. TiO₂ nanoparticles were synthesized by adding titanium tetraisopropoxide (TTIP) precursor to a premixed CH₄/O₂/N₂ flat flame aerosol reactor. Pure TiO₂ clusters were not detected by the APi-TOF. Results from measured mass spectra and mass defect plots show that for positively charged clusters, the abstraction of CH₂ groups occurs simultaneously with the clustering of larger intermediate organometallic species. For negatively charged clusters, NO_x formation pathways in the flame may play a role during the initial stages of TiO₂ formation, since a lot of Ti-containing clusters were attached with nitrate-related species. These research findings provide insights on quantum dot synthesis and molecular doping where rapid dilution of the flame synthesized nanoparticles is needed to better control the particle size and chemical composition. The possible influences of and potential artifacts brought by the dilution system on observing the incipient particle formation in flames were also discussed.

© 2017 American Association for Aerosol Research     

A New Observation on the Phase Transformation of TiO2 Nanoparticles Produced by a CVD Method

Titania (TiO₂) nanoparticles with primary diameters of less than 30 nm were produced by the thermal decomposition of TTIP and by the oxidation of TiCl₄ in a cylindrical furnace reactor at 1200°C. Particle size, crystalline phase, and phase transformation were investigated as a function of precursor concentration and total flow rate by TEM, a DMA/CNC system, XRD, and TG-DTA. The results show that both particle size and number concentration were increased with increasing precursor concentration, and that the primary size could be controlled by changing the operating conditions. An anatase-to-rutile phase transformation occurred at TTIP concentrations above 7.68 × 10^?6 mol/l and this was enhanced with increasing precursor concentration. It is noteworthy that the transformation is independent of grain size but appears to be related to the presence of carbon impurities in the nanoparticles.     

Single-step synthesis of N-doped TiO2 by flame aerosol method and the effect of synthesis parameters

We have established a novel route for the synthesis of N-doped TiO₂ by adopting flame aerosol (FSP) technique and investigated the effect of water content on the physico-chemical properties of the as-synthesized nanoparticles. The key characteristics of the developed method are to modify the precursor solution in order to incorporate nitrogen atoms into the TiO₂ lattice without altering the FSP set-up. The reduction of the flame enthalpy resulting in N-incorporation into the TiO₂ and the N-doping can be greatly enhanced further by the addition of secondary N-source (urea). Our XRD results reveal a shift of the (101) plane anatase diffraction peak to lower angles in our N-doped TiO₂ compared to undoped TiO_2, which suggest the distortion and strain in the crystal lattice prompted by the incorporation of the nitrogen atoms. The growth or expansion of crystal lattice can be attributed to the larger atomic radius of respective nitrogen atoms (r?=?1.7 Å) compared to oxygen (r?=?1.40 Å). Our XPS and EDX spectroscopy results elucidate that the nitrogen was effectively doped into the crystal lattice of TiO₂ in our as-synthesized N-TiO₂ catalysts predominantly in the form of interstitial nitrogen (Ti?O?N). The nitrogen atoms incorporation into the crystal lattice of titania modifies the electronic band structure of TiO₂, resulting in a new mid-gap energy state N 2p band formed above O 2p valence band. This occurrence narrows the band gap of TiO₂ (from 3.12 to ～2.51?eV) in our N-doped TiO₂ and shifts the optical absorption to the visible region.

Copyright © 2018 American Association for Aerosol Research     

Silica-coated nanocrystalline TiO2 with improved thermal stability

Deposition of a SiO₂ coating on anatase TiO₂ nanocrystals is shown to improve their thermal stability. As low as 0.5% Si was shown to preserve the small anatase crystallite size after calcination at 600?°C. Such treatment led to considerable sintering of TiO₂ nanocrystals without the silica with the average particle size growth from 9 to 50?nm, surface area decrease from 135 to 22?m²/g and partial anatase conversion to rutile. The phase composition, crystallite size, and surface area of 5%Si-TiO₂ samples were largely preserved till the temperatures as high as 800?°C whereas the anatase phase was mostly stably even after calcination at 1000?°C. The phase transformation from anatase to rutile in xerogel TiO₂ and TiO₂@SiO₂ samples apparently did not occur until the crystallites grew larger than the critical size about 50?nm. Electron-acceptor sites capable of ionizing perylene to its radical cations were observed on all samples with anatase crystalline structure. So, the silica shell deposition improves the TiO₂ thermal stability without limiting access to the surface active sites.     

Fractal scaling of soot packing density across five size decades

We experimentally map the scaling laws for packing density (θ_f, solid component volume fraction) of soot aggregates across five orders of magnitude of size (R_g/a, normalized radius of gyration by monomer radius). The θ_f ? R_g/a scaling relationship evolves through three successive regimes with distinct power-law exponents of ?1.20 ± 0.01, ?0.58 ± 0.06, and ?1.31 ± 0.14. The first cross-over agrees with the classical aerosol-to-gel transition theory. This agreement, however, breaks down at the second cross-over point, where a late-stage cluster-cluster aggregation of aerosol gels takes over.

© 2017 American Association for Aerosol Research     

Design of aerosol particle coating: Thickness,texture and efficiency

Core–shell particles preserve the performance (e.g. magnetic, plasmonic or opacifying) of a core material, while modifying its surface with a shell that facilitates (e.g. by blocking its reactivity) their incorporation into a host liquid or polymer matrix. Here coating of titania (core) aerosol particles with thin silica shells (films or layers) is investigated at non-isothermal conditions by a trimodal aerosol dynamics model, accounting for SiO₂ generation by gas phase and surface oxidation of hexamethyldisiloxane (HMDSO) vapor, coagulation and sintering. After TiO₂ particles have reached their final primary particle size (e.g. upon completion of sintering during their flame synthesis), coating starts by uniformly mixing them with HMDSO vapor that is oxidized either in the gas phase or on the particles’ surface resulting in SiO₂ aerosols or deposits, respectively. Sintering of SiO₂ deposited onto the core TiO₂ particles takes place transforming rough into smooth coating shells depending on the process conditions. The core–shell characteristics (thickness, texture and efficiency) are calculated for two limiting cases of coating shells: perfectly smooth (e.g. hermetic) and fractal-like. At constant TiO₂ core particle production rate, the influence of coating weight fraction, surface oxidation and core particle size on coating shell characteristics is investigated and compared to pertinent experimental data through coating diagrams. With an optimal temperature profile for complete precursor conversion, the TiO₂ aerosol and SiO₂-precursor (HMDSO) vapor concentrations have the strongest influence on product coating shell characteristics.     

Synthesis of non-aggregated titania nanoparticles in atmospheric pressure diffusion flames

Flame aerosol synthesis has been employed to synthesize nanoscale titania (TiO₂) particles by oxidation of titanium tetraisopropoxide (TTIP) vapor. The influence of reactant mixing and flow rates on particle morphology, size and phase composition has been studied for two different diffusion flame configurations using transmission electron microscopy, X-ray diffraction and photon correlation spectroscopy. Spherical, loosely agglomerated powders with a minimum secondary particle size of 90 nm and a rutile content of up to 35 w % were obtained at low oxygen flow rates in the double diffusion flame, while large anatase-rich aggregates formed at high oxygen flow rates. It is shown that the degree of aggregation of the as-synthesized particles is represented better by the ratio of d_PCS³/d_TEM³, than by the ratio of d_BET³/d_XRD³ commonly used in literature. The differences observed in particle morphology and phase composition can be explained by considering their time-temperature history as a function of flame configuration and gas flow rates.     

Synthesis of 3D Silver-Graphene-Titanium Dioxide Composite via Aerosol Spray Pyrolysis for Sensitive Glucose Biosensor

A sensitive glucose biosensor was developed based on the adsorption of glucose oxidase by a three-dimensional silver-graphene-titanium dioxide (3D Ag-GR-TiO₂) composite electrode. Aerosol spray pyrolysis was employed to synthesize the 3D Ag-GR-TiO₂ composite using a colloidal mixture of a silver acetate precursor (C₂H₃AgO₂), graphene oxide, and TiO₂ nanoparticles. The effects of the operating temperature, gas flowrate, and TiO₂ concentration on the particle properties were investigated. The particle morphology of all 3D Ag-GR-TiO₂ composites was spherical in shape. The average sizes of composites could be controlled from 0.45 to 0.64 μm with the variation of process variables. Ag nanoparticles less than 10 nm in diameter were deposited on the surfaces of the TiO₂ nanoparticles and GR after a reduction process. The characteristics of the glucose biosensor fabricated with the as-prepared 3D Ag-GR-TiO₂ composite were assessed through cyclic voltammetry measurements. The biosensor exhibited a high current flow as well as clear redox peaks, resulting in a superior ability of the catalyst in terms of the electrochemical reactions. The highest sensitivity of glucose biosensor was obtained by 3D Ag-GR-TiO₂ composite, which was 12.2 μA/mM·cm², among 3D Ag-GR-TiO₂, 3D Ag-GR, and 3D GR-TiO₂ composites.

Copyright 2015 American Association for Aerosol Research     

Low-temperature Ti-containing 3:2 and 2:1 mullite nanocrystals from single-phase gels

TiO₂-containig single-phase gels with (Al₂O₃ + TiO₂)/(SiO₂) molar ratios 3/2 and 2/1 were prepared by gelling mixtures of aluminium nitrate, tetraethylorthosilicate and titanium isopropoxide. Gels were fast heated at several temperatures up to 1100 °C. Dried and heated gels were characterized by differential thermal analysis (DTA), magic angle spinning nuclear magnetic resonance (MAS-NMR), X-ray powder diffraction (XRD), and scanning and transmission electron microscopies (SEM and TEM). Coupled DTA and XRD results of gels fast heated at 900 °C showed the crystallisation of two mullites as well as a small amount of alumina-silica spinel. ²⁷Al NMR spectra showed the formation of pentacoordinated aluminium before mullite crystallization. The increase of lattice parameters of single-phase mullites heated at 1100 °C indicated that the amount of TiO₂ incorporated into the mullite structure increased on raising the amount of nominal TiO₂ in both series. SEM and TEM images of heated gels at 1100 °C displayed the formation of well-shaped parallelepiped of titanium-doped mullite nanocrystals with crystalline anisotropy.     

Atomically dispersed Pd on nanostructured TiO2 for NO removal by solar light

Reducing the particle size of noble metals on ceramic supports can maximize noble metal performance and minimize its use. Here Pd clusters onto nanostructured TiO₂ particles are prepared in one step by scalable flame aerosol technology while controlling the Pd cluster size from a few nanometers to that of single atoms. Annealing such materials at appropriate temperatures leads to solar photocatalytic NO_x removal in a standard ISO reactor up to 10 times faster than that of commercial TiO₂ (P25, Evonik). Such superior performance can be attained by only 0.1 wt.% Pd loading on TiO₂. Annealing these flame‐made powders in air up to 600 °C decreases the amorphous TiO₂ fraction and increases its crystal and particle sizes as observed by x‐ray diffraction (XRD) and N₂ adsorption. The growth of single Pd atoms to Pd clusters on TiO₂ prepared at different Pd loading and annealing conditions was investigated by scanning transmission electron microscopy and XRD. The single Pd atoms and clusters on TiO₂ are stable up to, at least, 600 °C for 2 h in air but at 800 °C they grow into PdO nanoparticles whose fraction is comparable with the nominal Pd loading. Hence, most of Pd atoms are on the TiO₂ surface where at 800 °C they diffuse and coalesce. Diffuse reflectance infrared Fourier transform spectroscopy reveals NO adsorption on single, double, three and fourfold coordinated Pd atoms depending on their synthesis and annealing conditions. The peak intensity of NO adsorption sites involving multiple Pd atoms is substantially lower in TiO₂ containing 0.1 wt.% than 1 wt.% Pd but that intensity from single Pd atoms is comparable. This indicates the dominance of isolated Pd atoms compared to clusters in Pd/TiO₂ containing 0.1 wt.% Pd that match or exceed the photocatalytic NO_x removal of Pd/TiO₂ of higher Pd contents. © 2016 American Institute of Chemical Engineers AIChE J, 63: 139–146, 2017     

Population Balance-Monte Carlo Simulation for Gas-to-Particle Synthesis of Nanoparticles

The process simulation of nanoparticle synthesis via the gas-phase method is essential to understanding the detailed dynamic evolution of nanoparticles within a very short time period under high temperature. The task is, however, very challengeable up to now as the conversion of the gaseous precursor to the end-use nanoparticle is a complex physicochemical process involving nucleation of the particulate phase, agglomeration between particles and sintering under industrial production conditions. In this article, we extended the differentially weighted Monte Carlo method for population balance to simulate the dynamic evolution of titania (TiO₂) nanoparticles synthesized by gas-to-particle conversion in a single aerosol reactor, considering simultaneous nucleation, agglomeration, and sintering. The simulated size distribution of TiO₂ agglomerate and primary particles produced by the thermal decomposition of titanium tetraisoproxide agreed well with the experimental data. In the simulation, the fast population balance-Monte Carlo method was utilized to accelerate the process simulation on a desktop PC. Results were obtained up to 178 times faster than that of a normal Monte Carlo method. The inhomogeneous internal structure of primary particles was considered through solving population balance of polydisperse primary particles within agglomerate. It was found the polydisperse model could predict the primary particle size distribution better. Simulation results revealed a complex competition relation among nucleation, agglomeration and sintering.

Copyright 2013 American Association for Aerosol Research     

Preparation,characterization and the antimicrobial properties of metal ion-doped TiO2 nano-powders

TiO₂ samples doped with lithium, sodium, magnesium, iron or cobalt were prepared by high-energy ball milling for different periods of time. The crystalline phase, chemical composition, crystalline size and photo-absorption were characterized by X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS) and Ultraviolet visible diffuse reflectance spectroscopy (UV - Vis - DRS), Fourier transform infrared spectroscopy (FT - IR) and scanning electron microscopy (SEM). The antimicrobial properties of the modified TiO₂ samples were evaluated with E. coli and S.aureus assays. The results of the XRD show that the TiOSO₄, Ti₃O₅, Li₂TiO₃ and NaTi₂O₄ phases appear along with Li, Na and Mg doped TiO₂. However, XPS spectra indicated that Ti exists as both Ti³⁺ and Ti⁴⁺ in Na-doped TiO₂ samples. Ti³⁺, due to its narrow band gap, is highly active in promoting visible light-induced photocatalytic activity. SEM images showed that the crystalline size of TiO₂ is reduced and has a common-round and hexagonal plate morphology after milling. The modified TiO₂ samples had the best antimicrobial activities after 3 h of milling. In particular, the antimicrobial rate of TiO₂ 5% doped with transition metals (Co, Fe) reached 100% against E. coli, but the antibacterial rate against S. aureus for Co and Fe dopants was 98.4% and 98.2%, respectively.     

Aerosol Gelation: Synthesis of a Novel,Lightweight, High Specific Surface Area Material

We demonstrate that an aerosol can gel. This gelation is then used for a one-step method to produce an ultralow density porous carbon material. This material is named an aerosol gel because it is made via gelation of particles in the aerosol phase. The carbon aerosol gels have high specific surface area (200–350 m ² /g), an extremely low density (2.5–5.0 mg/cc) and a high electrical conductivity, properties similar to conventional aerogels. The primary particles of the carbon aerosol gels are highly crystalline with a narrow (002) graphitic X-ray diffraction peak. Key aspects to form a gel from an aerosol are large volume fraction, ca. 10 ^?4 or greater, and small primary particle size, 50 nm or smaller, so that the gel time is fast compared to other characteristic times.     

Morphology and kinetic mechanism of facile one-step preparing TiO2/polyacrylate/TiO2 multilayer core–shell hybrid emulsion via miniemulsion polymerization

In this paper, we report a facile method for the preparation of TiO₂/polyacrylate/TiO₂ multilayer core–shell hybrid emulsion through polymerization. The chemical compositions of the copolymer were studied with Fourier transform infrared. TEM images reveal that nanocomposites show different core–shell structures with different TiO₂ contents. As the weight percentage of TiO₂ is 2 wt% (based on monomer, same below), there are no TiO₂ cores in some nanocomposites. When TiO₂ increases to 3 wt%, the TiO₂/polymer/TiO₂ multilayer core–shell composite particles are prepared. But the TiO₂ shells disappeared when the TiO₂ content kept increasing. TGA shows that the TiO₂ dispersed in latex films uniformly and the thermal stabilization improved with increasing TiO₂ contents. The effect of operating variables such as polymerization temperature and the concentrations of polymerizable emulsifier, initiator, extremely hydrophilic monomer, modified TiO₂ and HD on the kinetic behaviors was investigated. The formation mechanism of TiO₂/polymer/TiO₂ multilayer core–shell structure was inferred.