Flame-synthesized Nanoparticles Research Articles

Antimicrobial Activity of TiO2 Coatings Prepared by Direct Thermophoretic Deposition of Flame-Synthesized Nanoparticles

ABSTRACTThis study reports the development of a one-step method for the production of antimicrobial protective coatings for aluminum surfaces with titania nanoparticles. An aerosol flame synthesis system was used to produce monodisperse, ultra-fine TiO2 nanoparticles, which were directly deposited by thermophoresis onto plates of aluminum alloy by means of a rotating disc. Fuel-lean reactor conditions were used to synthesize pure anatase nanoparticles of 3.5 nm in diameter. Substrates were mounted onto the rotating disc that repetitively passes through the flame. Convection due to the rotational motion cooled the substrates, on which particles were deposited as films by thermophoresis. Such a system allowed to obtain submicron coatings of different thickness, by varying the total time of deposition. The antimicrobial activity of TiO2 coatings was tested against the Gram positive bacterium Staphylococcus aureus. To determine the inhibition of biofilms formation, microbes were plated on TiO2 coatings and a semi-quantitative colorimetric assay was performed using crystal violet. The tests showed that the TiO2 coating obtained with tdes=10 s inhibits up to 70-80% Staphylococcus aureus biofilm formation, and the inhibition of biofilms formation was confirmed by means of Scanning Electron Microscopy observation. Also, the antimicrobial properties of the coatings was enhanced by irradiating the samples in the UV region. The results of the present work are promising for using titania films as protective coatings for applications where an antimicrobial activity is required.

Gas phase synthesis of encapsulated iron oxide–titanium dioxide composite nanoparticles by spray pyrolysis

Composite nanoparticles are of a great interest due to the possibility of combining properties of several different materials. In this study, a spray pyrolysis process utilizing flame-synthesized nanopowder was used to generate iron oxide–titanium dioxide (γ-Fe2O3–TiO2) composite nanoparticles. Iron oxide and titanium dioxide were selected because they are known to have magnetic and photocatalytic properties, respectively. First, dry γ-Fe2O3 nanopowder was prepared with the liquid flame spray technique. After that, the flame-synthesized nanoparticles were mixed with liquid titanium(IV) isopropoxide (TTIP) and isopropyl alcohol. This mixed-phase precursor was sprayed into a tube furnace where TTIP thermally decomposed to form solid TiO2, encapsulating the γ-Fe2O3 powder particles. The synthesized nanoparticles were characterized with aerosol measurements, transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The size distributions of the composite nanoparticles were broad, and the mode diameters were 80–130 nm. The particles consisted of a γ-Fe2O3 agglomerate that was either partially or fully encapsulated by a spherical TiO2 particle, depending on the concentration of TTIP in the mixed-phase precursor. The flame-synthesized iron oxide powder was crystalline maghemite, whereas, spray pyrolyzed titanium dioxide had amorphous phase. The as-synthesized composite nanoparticles were calcinated to attain crystalline anatase TiO2. The crystal structures as well as demonstrated photoactivity and magnetic response suggest that the composite nanoparticles could find use as magnetically separable photocatalyst.

화염법으로 제조된 감마-Al2O3나노입자의 화염조건에 따른 입자특성 연구

본 논문은 수소를 연료로 하는 확산화염을 이용하여 알루미나 나노입자를 합성할 때, 합성되는 알루미나 나노입자의 특성에 미치는 화염온도의 영향을 조사하였다. 합성된 나노입자의 특성을 전자현미경 이미지, 결정 구조 분석, 비표면적과 기공의 크기 분석, 화염온도 측정 등의 여러 특성분석 방법으로 조사하였다. 사용된 화염의 중심축 최고온도는 산화제의 산소농도가 19, 21, 30, 47%인 각각의 실험조건에서 1507.8K, 1593.8K, 1753.1K, 1998.7K으로 측정되었다. SEM 이미지 분석 및 BET 비표면적 측정을 통해서는 47% 산소농도인 경우 에는 50 nm 수준의 독립적인 구형입자가 생성되었음을 확인할 수 있었으며, 19%와 21%의 경우에는 응집된 상태의 20-30 nm 수준의 입자를 볼 수 있었다. XRD 결과에서는 감마(γ)-알루미나가 주를 이루는 것으로 판단되었다. 이상의 결과를 바탕으로 촉매 담체로 사용하기 위한 알루미나 나노입자를 연소합성 하기 위한 가장 적절한 조건으로 실험했던 네 경우 중에서는 산화제의 산소농도가 21%인 두 번째 경우를 선택할 수 있었다.

In situ measurement of the mass concentration of flame-synthesized nanoparticles using quartz-crystal microbalance

A novel in situ method for measurement of mass concentration of nanoparticles (NPs) formed in flames is proposed. In this method, the deposition rate of NPs collected by a molecular beam sampling system is measured by quartz-crystal microbalance (QCM). It is the only existing method which allows direct measurement of NP mass concentration profiles in flames. The feasibility of the method was demonstrated by studying iron oxide NP formation in low-pressure methane/oxygen/nitrogen flames doped with iron pentacarbonyl. The system was tested under fuel-lean and fuel-rich flame conditions. Good agreement between measured QCM deposition rates and their estimations obtained by the transmission electron microscopy analysis of samples collected from the molecular beam has been demonstrated. The sensitivity of the method is comparable to that of particle mass spectrometry (PMS). Combination of the QCM technique with PMS and/or optical measurements can provide new qualitative information which is important for elucidation of the mechanisms governing the NP flame synthesis.

Effect of synthesis condition and annealing on the sensitivity and stability of gas sensors made of Zn-doped γ-Fe2O3 particles

Gas sensors made of flame-synthesized Zn-doped γ-Fe2O3 nanoparticles were found to have high sensitivity and high aging resistance. Zinc-doped γ-Fe2O3 nanoparticles and microparticles were synthesized by flame spray py- rolysis (FSP). Gas sensors were fabricated with as-synthesized particles, and with particles that had been annealed. The sensors' response to acetone vapor and H2 was measured as fabricated, and measured again after the sensors were aged for three days. The sensors made from as-synthesized particles showed a gas sensing sensitivity 20 times higher than the literature value. However, sensors made of microparticles lost their sensing ability after three days of aging; sensors made of nanoparticles retained their gas sensing capability after aging. Sensors made of annealed particles did not have significant gas sensing capabilities. Analysis using the William and Hall method showed that the microstrains decreased significantly in both H2/O2 and H2/Air flame synthesized particles after annealing. The results showed that sensors made of flame-synthesized particles have much higher sensitivity than sensors made of particles previously reported. Especially, sensors made of flame-synthesized nanoparticles are resistant towards aging. This aging resistance may be attributed to the particles' ability to retain their microstrains.

Thermal stability of heat-treated flame-synthesized anatase TiO2 nanoparticles

In this article, TiO2 nanoparticles were synthesized by using O2-enriched coflow, hydrogen, diffusion flames. We investigated the thermal stability of the flame-synthesized TiO2 nanoparticles by examining the crystalline structures of the nanoparticles and by analyzing the photocatalytic degradations of methylene blue solutions. Also, the results were compared with those of commercial P-25 nanoparticles. The maximum centerline temperature of the flame was measured to be 1,743 °C. Under this synthesis condition, TiO2 nanoparticles, which were spherical with diameters approximately ranging from 30 to 60 nm, were synthesized. From the XRD analyses, about 96 wt.% of the synthesized nanoparticles were anatase-phase. After the heat-treatment at 800 °C for 30 min, the synthesized TiO2 nanoparticles showed no significant changes of their shapes and crystalline phases. On the other hand, most of the commercial particles sintered with each other and changed to the rutile-phase. Whereas the photocatalytic ability of heat-treated commercial particles deteriorated, that of the flame-synthesized particles improved. On the basis of the improved result of photocatalytic degradation of methylene blue by using the heat-treated flame-synthesized nanoparticles, it is believed that the flame-synthesized TiO2 nanoparticles have higher thermal stability at 800 °C than the commercial particles.

Improved Rate Laws and Population Balance Simulation Methods; CRE Applications, Including the Combustion Synthesis of Valuable Nano-Particles

Using ‘flame-synthesized’ nanoparticles (nps) as one prototypical application, we illustrate our recent progress in two broad areas of current CRE-interest, viz., the development of: 1. Improved rate laws/transport coefficients for next-generation Eulerian, multi-(state) variable population-balance formulations, and 2. Quadrature-based multi-variate moment methods (hereafter QMOM) suitable for articulation with evolving Eulerian CFD simulation methods Admittedly, in previous work much insight was obtained by introducing deliberately (over-) simplified rate laws (for nucleation, Brownian coagulation, vapor growth/evaporation, sintering, thermophoresis, ) into the generally nonlinear integro-partial differential equation called the ‘population balance’ equation (PBE). However, despite the complexity of this equation, and the need to satisfy it along with many other local PDE-balance principles in multi-dimensional CRE environments, in our view current requirements for reactor design, as well as the frequent need to infer meaningful physico-chemical parameters based on laboratory measurements on populations rather than individual ‘particles’, make the introduction of more accurate rate/transport laws essential for next-generation particle synthesis reactor models. Our present examples are motivated both by measurements/calculations of the structure of laminar counterflow flames synthesizing Al2O3 nps and/or the predicted performance of well-mixed steady-flow devices in which sintering or sublimation occurs. Corresponding illustrative results, which focus on the rate laws for sphere dissolution or aggregate Brownian coagulation support our contentions that: i) systematic introduction of more accurate rate laws (including nucleation, sintering, growth, )/transport coefficients will be essential to meet the quantitative demands of next-generation PBE-based CRE-simulation models for high-value particulate synthesis equipment, and, ii) QMOM is able to incorporate realistic rate laws and faithfully generate their effects on important ‘moments’ characterizing the product joint distribution functions.

In situ light-scattering measurements of morphologically evolving flame-synthesized oxide nanoaggregates

Nonspherical Al2O3 aggregates produced in a laminar counterflow nonpremixed methane flame were investigated with an in situ laser light-scattering (LLS) technique in combination with a thermophoretic sampling-transmission electron microscope (TS-TEM) method. These flame-synthesized nanoparticles clearly underwent morphological changes following their formation (from precursor trimethylaluminum hydrolysis), mainly as a result of aggregation and sintering processes in the approximately 3.3 x 10(4) K/s heating environment. To characterize this particulate morphological evolution conveniently we made multiangular absolute LLS measurements and interpreted them based on the Rayleigh-Debye-Gans scattering theory for fractal aggregates. Optically determined fractal dimension D(f), mean radius of gyration, aggregate size distribution, and local particle volume fraction phi(p) were found to be consistent with our independent ex situ TS-TEM experiments. D(f) (optically inferred) increased from 1.60 to 1.84 with axial position, confirming the morphological evolution of alumina aggregates owing to finite-rate, spatially resolved high-temperature sintering. An extension of our TS-TEM method was successfully applied, for the first time to our knowledge, to inorganic particles. Phi(p) inferred by means of this ex situ technique generally agreed with that from the in situ LLS technique, supporting our interpretation of both measurements. Moreover, an optically inferred net sintering rate of alumina aggregates approaching the flame was estimated to be consistent with the available TEM data. The LLS methods and results presented here are expected to permit more comprehensive mechanistic analyses of nanoaggregate sintering and coagulation kinetics in such flame environments, ultimately improving the modeling of more-complex (e.g., turbulent, high-pressure) combustion systems involving nanoparticle formation and evolution.