XXXVI Meeting of the Italian Section of the Combustion Institute NANO-TiO 2 PARTICLES PRODUCED IN A VAPOUR-FED AEROSOL FLAME REACTOR

Nanoparticles of titanium dioxide were produced in a vapour-fed aerosol flame reactor by feeding ethanol solutions of titanium tetr aisopropoxide (TTIP) in a premixed laminar flame of ethylene and air. The synthe sized samples was characterized by Differential Mobility Analizer (DMA), Raman scattering, UV-Visible absorption and Photoluminescence (PL) spectroscopy. A s-prepared nanoparticles appeared to be pure anatase crystallite of 1.7-3.1 nm in diameter, depending on TTIP concentration in the precursor solution. Optical ba nd gap value of TiO2 particles was calculated to be 3.15 eV, and photoluminescence in the ultra-violet region, extending also to the visible spectral region, was ob erved. Introduction Titanium dioxide (titania, TiO2) can be considered as one of the most fascinating and useful materials of the modern era. In the bulk phase, it is primarily used as a white pigment in paints, coating and plastics, whil e, as a nanomaterial, it is a common additive in personal care products and food [1] . TiO2 nanoparticles of 10 nm or less have recently received great attention due to higher surface area and altered properties with respect to the bulk material [2]. O wing to their large band-gap semiconductor behavior and their response to UV light excitation, TiO2 nanostructures have been successfully tested as high-activity cata lyst in photocatalytic devices and as the major component of dye-sensitized solar cell s and organic photovoltaics [3]. The most common polymorphic forms in which titania particles crystallize are anatase and rutile. Crystal phase as well as particle dimension strongly affect properties and characteristics of synthesized nanomateria ls. Compared to other nanotitania synthesis methods, such as wet-chemical rou tes and chemical vapor deposition, flame synthesis offers a continuous and low-c st process with fine control over cristallinity and phase purity, avoiding any p ost-processing treatments [4]. Flame reactors configurations are usually classifie d according to the state and the enthalpy content of the supplied precursors [5], an d divided into Vapor-fed Aerosol Flame Synthesis (VAFS), Flame Spray Pyrolysis (FSP) , and Flame-Assisted Spray Pyrolysis (FASP). XXXVI Meeting of the Italian Section of the Combust ion Institute In this work, single phase TiO 2 nanoparticles of few nanometers in diameter and high purity were synthesized in a VAFS reactor. Pro duced particles were analyzed online with a Differential Mobility Analyzer (DMA) to obtain their size distributions. Raman spectroscopy was used to investigate p ar icle crystal structure, and an optical characterization was carried out by means o f UV-vis absorption and photoluminescence spectroscopy. Experimental The aerosol flame synthesis set-up consists of the combination of a Berglund Liu type Vibrating Orifice Aerosol Generator (VOAG, TSI 3450) and a honeycomb burner. The burner is composed by an 18 mm inside d iameter (ID) stainless steel tube, with 26 mm long Mullite Zirconia Honeycomb (4 00 CPSI) placed on its top, used to stabilize the flame. Burner walls are equip ped with electrical resistances controlled by a proportional-integrated controller. The precursor solution was prepared by dissolving titanium tetraisopropoxide (TTI P, Aldrich, 97%) into ethanol (Aldrich, >97%). The flame reactor was a fuel-lean atmospheric laminar premixed flame of C2H4 and air (equivalent ratio Φ=0.60, cold gas velocity=100 cm/s), homogenously doped with monodisperse droplets (105 μm) of precursor solution, generated by VOAG using a 50 μm orifice. The burner walls were heated to 450 K to ensure compete droplets evaporation. Resulting l uminous flame height was 18 cm, corresponding to a particle residence time of 3 3 ms. Characterization of the flame temperatures was carried out using an R-type hermocouple (Pt/13%Rh vs. Pt). On-line measurement of particle size distribution w as performed using a TapCon 3/150 Wien Type DMA; the sampling probe, placed at 18 cm above the burner, was made of a stainless steel tube (8 mm in inside d ameter and 1 mm in wall thickness), with a 0.3 mm side pinhole through whic h flame gases were sampled and diluted in nitrogen. Raman analysis was made with a Horiba XploRA Raman Microscope System on particles collected on Teflon filters with the aid of a vacuum pump, placing right above the flame a funnel-shaped glass collector con nected to the filter holder. For optical characterization, particles collected o n filter were dispersed in pure ethanol and sonicated to ensure uniformity of the disp er ion; UV-vis absorption measurements were made using a Agilent 8453 spectro hot meter, while photoluminescence spectra were measured on a Perkin-Elmer LS-50 spectrofluorimeter. Results and discussion Flame temperature profile, reported in Fig. 1, was obtained measuring temperature every centimetre above the burner down to 10 cm, wh ere temperature reached a value (2000 K) which is above the maximum operating level of the thermocouple; therefore, temperature of the flame was modelled fr om 10 cm down to 1 cm using heat balances, assuming a linear decrease. 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