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Syngas produced by biomass and waste gasification processes must be ade-quately clean of tar compounds before being utilized in value-added applica-tions. Syngas cleaning by tar cracking at high temperatures is a promisingtechnique that can utilize different kinds of catalysts. However, their use islimited by the deposition of coke layers, which induces a masking phenome-non on the active surface, and, consequently, the rapid deactivation of the cat-alyst. This study addresses how the temperature (750 and 800 C) and thesteam concentration (0% and 7.5%) can affect the extent of water–gas andreforming reactions between steam and coke deposits. Two catalysts wereused: a market-available activated carbon and an iron-based alumina catalyst.The tests showed better performance of the Fe/γ-Al2O3catalyst. A massincrease of the bed was measured in tests with both the catalysts, which con-firms the deposition of the coke layer produced by tar dehydrogenation andcarbonization. Scanning electronic microscopy-energy-dispersive X-ray analy-sis (SEM-EDX) and Raman spectroscopy were utilized to investigate the natureof coke layers over the catalyst surface, with the aim of acquiring informationabout their reactivity towards the water gas reaction. SEM-EDX observationsindicate that the thickness of these carbon layers is less than 2μm. Ramanspectra suggest a negligible effect of the reaction temperature in the testedrange and, in particular, that the amorphous nature of coke layers deposited inthe presence of steam is relatively more graphitic than that obtained withoutsteam.

Soot nucleation is one of the most complex and debated steps of the soot formation process in combustion. In this work, we used scanning tunneling microscopy (STM) and spectroscopy (STS) to probe morphological and electronic properties of incipient soot particles formed right behind the flame front of a lightly sooting laminar premixed flame of ethylene and air. Particles were thermophoretically sampled on an atomically flat gold film on a mica substrate. High-resolution STM images of incipient soot particles were obtained for the first time showing the morphology of sub-5 nm incipient soot particles. High-resolution single-particle spectroscopic properties were measured confirming the semiconductor behavior of incipient soot particles with an electronic band gap ranging from 1.5 to 2 eV, consistent with earlier optical and spectroscopic observations.

44th Meeting of The Italian Section of The Combustion Institute - Combustion for Sustainability - Book of Abstract

Elemental and organic carbon content of flame-generated soot nanoparticles produced under different operating conditions of equivalence ratio and particle residence time is determined by thermo-optical-transmission measurements, while particle carbon structure is characterized by Raman spectroscopy analysis. The sampled soot nanoparticles are produced in slightly-rich, almost bluish, flames and comprise small 2–4 nm nascent soot nanoparticles, often neglected in the primary carbonaceous emissions from combustion systems, and larger 4–15 nm soot particles. The application of the NIOSH-like protocol for the thermo-optical-transmission analysis allows the separation of the total carbon (TC) into fractions of organic carbon (OC), pyrolytic carbon (PC), and elemental carbon (EC). The lowest amount of elemental carbon is measured in young soot nanoparticles and it is progressively larger in mature soot particles. The organic carbon has the opposite trend and shows that it is the “pyrolyzable” carbon fraction of the young soot nanoparticles that transforms into elemental carbon in the primary soot. Raman spectroscopy showed consistent results as compared to the thermo-optical-transmission analysis indicating a analogous trend in the graphitization order of the different soot particles. Most importantly, the logarithm of the intensity of the measured photoluminescence background shows a linear dependence with the OC/EC ratio. Mass absorption coefficients (MAC) were retrieved by the thermo-optical-transmission analysis for the four soot samples; the estimated MAC coefficient ranged from 2 to 6 m2/g, dependently from the relative percentage of EC component. The data are in excellent agreement with currently available literature data and extend the determination of MACs to carbonaceous particles with a larger OC/TC content.

In this work, an experimental investigation of the nanomechanical properties of flame-formed carbonaceous particle layers has been performed for the first time by means of Atomic Force Microscopy (AFM). To this aim, carbon nanoparticles with different properties and nanostructures were produced in ethylene/air laminar premixed flames at different residence times. Particles were collected on mica substrates by means of a thermophoretic sampling system and then analyzed by AFM. An experimental procedure based on the combination between semi-contact AFM topography imaging, contact AFM topography imaging and AFM force spectroscopy has been implemented. More specifically, a preliminary topological characterization of the samples was first performed operating AFM in semi-contact mode and then tip-sample interaction forces were measured in contact spectroscopy mode. Finally, semi-contact mode was used to image the indented surface of the samples and to retrieve the projected area of indents. The hardness of investigated samples was obtained from the force–distance curves measured in spectroscopy mode and the images of intends acquired in semi-contact mode. Moreover, the Young’s modulus was measured by fitting the linear part of the retraction force curves using a model based on the Hertz theory. The extreme force sensitivity of this technique (down to nNewton) in addition to the small size of the probe makes it extremely suitable for performing investigation of mechanical properties of materials at the nanoscale. The experimental procedure was successfully tested on reference materials characterized by different plastic behavior, e.g., polyethylene naphthalate and highly oriented pyrolytic graphite. Both hardness and Young’s modulus values obtained from AFM measurements for different soot particle films were discussed.

Nanostructured films of carbon and TiO2 nanoparticles have been produced by means of a simple two-step procedure based on flame synthesis and thermophoretic deposition. At first, a granular carbon film is produced on silicon substrates by the self-assembling of thermophoretically sampled carbon nanoparticles (CNPs) with diameters of the order of 15 nm. Then, the composite film is obtained by the subsequent thermophoretic deposition of smaller TiO2 nanoparticles (diameters of the order of 2.5 nm), which deposit on the surface and intercalate between the carbon grains by diffusion within the pores. A bipolar resistive switching behavior is observed in the composite film of CNP-TiO2. A pinched hysteresis loop is measured with SET and RESET between low resistance and high resistance states occurring for the electric field of 1.35 × 104 V/cm and 1.5 × 104 V/cm, respectively. CNP-TiO2 film produced by flame synthesis is initially in the low resistive state and it does not require an electroforming step. The resistance switching phenomenon is attributed to the formation/rupture of conductive filaments through space charge mechanism in the TiO2 nanoparticles, which facilitate/hinder the electrical conduction between carbon grains. Our findings demonstrate that films made of flame-formed CNP-TiO2 nanoparticles are promising candidates for resistive switching components.

In this study, flame-formed carbon nanoparticles of different nanostructures have been produced by changing the flame temperature. Raman spectroscopy has been used for the characterization of the carbon nanoparticles, while the particle size has been obtained by online measurements made by electrical mobility analysis. The results show that, in agreement with recent literature data, a large variety of carbon nanoparticles, with a different degree of graphitization, can be produced by changing the flame temperature. This methodology allows for the synthesis of very small carbon nanoparticles with a size of about 3–4 nm and with different graphitic orders. Under the perspective of the material synthesis process, the variable-temperature flame-synthesis of carbon nanoparticles appears as an attractive procedure for a cost-effective and easily scalable production of highly tunable carbon nanoparticles.

TiO2 in the form of nanoparticles is characterized by high photocatalytic activity and high resistance to oxidation, making it an excellent candidate to realize coatings for improving the corrosion resistance of aluminium surfaces. Different coating technologies have been proposed over the years, which often involve the use of toxic compounds and very high temperatures. In this work, an alternative and novel one-step method for the coating of aluminium alloy surfaces with titania nanoparticles is presented. The method is based on the combination of aerosol flame synthesis and direct thermophoretic deposition and allows to produce nanostructured thin coating layers of titania with different features. Specifically, 3.5 nm anatase nanoparticles were synthesized and deposited onto aluminium alloy AA2024 samples. The thickness of the coating was changed by modifying the total deposition time. A thermal annealing treatment was developed to improve the adhesion of nano-titania on the substrates, and the morphology and structures of the coatings were characterized using (ultra violet) UV-vis absorption, scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The corrosion resistance behavior of the coatings was evaluated by means of electrochemical polarization measurements, coupled with a numerical analysis using COMSOL software. Both the experimental and numerical electrochemical polarization curves showed a significant increase in the corrosion potential of coated substrates with respect to the bare aluminium and a decrease in the current density. The coatings obtained with higher deposition time and greater thickness showed the best performances in terms of the resistance of the aluminium surfaces to corrosion.

In this work, we present particle size distribution (PSD), electron paramagnetic resonance (EPR) and Raman spectroscopy measurements of soot particles, collected across the inception region, in two ethylene-air premixed laminar flames and two flames obtained by substituting 30% of the ethylene carbon with benzene. The experimental results are used to investigate the effect of aromatic fuel on the nature and role of radicals in particle nucleation and growth. For all the particles sampled, both in pure ethylene flames and in the ethylene/benzene ones, an EPR signal typical of persistent carbon-centered aromatic radicals is measured. The results point out an involvement of π-radicals during particle nucleation and initial growth process. Under the conditions investigated, our results show that the electron spin density of particles produced by different fuels is mainly related to the amount of H-atoms available in the particles. Moreover, for all the investigated flames, the spin density presents an inverse dependence with the particle size suggesting that particle surface plays an important role in the early steps of the soot formation process.

TiO2 in the form of nanoparticles is characterized by high photocatalytic activity and high resistance to oxidation, making it an excellent candidate to realize coatings for improving the corrosion resistance of aluminium surfaces. Different coating technologies have been proposed over the years, which often involve the use of toxic compounds and very high temperatures. In this work, an alternative and novel one-step method for the coating of aluminium alloy surfaces with titania nanoparticles is presented. The method is based on the combination of aerosol flame synthesis and direct thermophoretic deposition and allows to produce nanostructured thin coating layers of titania with different features. Specifically, 3.5 nm anatase nanoparticles were synthesized and deposited onto aluminium alloy AA2024 samples. The thickness of the coating was changed by modifying the total deposition time. A thermal annealing treatment was developed to improve the adhesion of nano-titania on the substrates, and the morphology and structures of the coatings were characterized using (ultra violet) UV-vis absorption, scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The corrosion resistance behavior of the coatings was evaluated by means of electrochemical polarization measurements, coupled with a numerical analysis using COMSOL software. Both the experimental and numerical electrochemical polarization curves showed a significant increase in the corrosion potential of coated substrates with respect to the bare aluminium and a decrease in the current density. The coatings obtained with higher deposition time and greater thickness showed the best performances in terms of the resistance of the aluminium surfaces to corrosion.

Soot emitted from incomplete combustion of hydrocarbon fuels contributes to global warming and causes human disease. The mechanism by which soot nanoparticles form within hydrocarbon flames is still an unsolved problem in combustion science. Mechanisms proposed to date involving purely chemical growth are limited by slow reaction rates, whereas mechanisms relying on solely physical interactions between molecules are limited by weak intermolecular interactions that are unstable at flame temperatures. Here, we show evidence for a reactive π-diradical aromatic soot precursor imaged using non-contact atomic force microscopy. Localization of π-electrons on non-hexagonal rings was found to allow for Kekulé aromatic soot precursors to possess a triplet diradical ground state. Barrierless chain reactions are shown between these reactive sites, which provide thermally stable aromatic rim-linked hydrocarbons under flame conditions. Quantum molecular dynamics simulations demonstrate physical condensation of aromatics that survive for tens of picoseconds. Bound internal rotors then enable the reactive sites to find each other and become chemically cross-linked before dissociation. These species provide a rapid, thermally stable chain reaction toward soot nanoparticle formation and could provide molecular targets for limiting the emission of these toxic combustion products.

The electronic band gap of flame-formed carbon nanoparticles (CNPs) is determined for the first time using scanning tunneling spectroscopy (STS). Flame-formed CNPs are sampled from a laminar, burner-stabilized stagnation ethylene-air flame through in-situ insertion of gold-coated mica substrates, on which CNPs are collected as thin films. Optical band gap measurements using UV-vis absorption spectroscopy are also performed to compare with the STS measurements. The suitability of STS in measuring electronic band gaps is first demonstrated through measurements using reference materials: the electronic band gaps of naphthalene, pyrene, and coronene are found to be slightly larger than the respective optical band gaps, as expected, because of the over potential in the presence of a current. Flame-formed CNPs of three different particle size distributions were sampled and tested. The electronic band gap of each CNP shows a clear size-dependency that supports the earlier conclusion (Liu et al. Proc. Natl. Acad. Sci. U.S.A. 116 (2019) 12692-12697) that flame-formed CNPs exhibit apparent quantum confinement or quantum-dot behaviors. Furthermore, the measured electronic band gaps matches the respective optical results within the experimental uncertainty of 0.1-0.2 eV. The over potential observed in PAH samples is not present in CNP sampls, which may indicate stronger intermolecular interactions between the PAHs constituting the CNPs than those of crystalline PAHs.

Activated carbons are recognised as inexpensive, easily available, and efficient catalysts for tar cracking reactions at high temperatures. Their use in hot syngas cleaning is limited by the rapid deactivation resulting from the coke deposition, and the consequent masking phenomenon over the activated carbon surface. This study contributes to a better understanding of the problem and sheds light on possible solutions, by investigating how the temperature (750 °C–850 °C) and the steam concentration (0%–10%) can affect the extent of the water-gas and reforming reactions occurring between steam and the coke covering the catalyst surface. The activated carbons have been fully characterised before and after the tests utilizing naphthalene as a tar model compound, to study the evolution of their structure after long duration tests. Steam positively affects the naphthalene conversion efficiency, preserving the characteristics of the material. For example, at a temperature of 800 °C and a steam concentration of 7.5%, a naphthalene conversion of over 90% is achieved even after 7 h of testing. XRD and Raman spectroscopy analyses suggest that naphthalene cracking produces a coke layer that is more amorphous and, above all, more reactive towards the water gas reaction than the original activated carbon structure.

The role of polycyclic aromatic hydrocarbons (PAHs) in the formation of nascent soot particles in flames is well established and yet the detailed mechanisms are still not fully understood. Here we provide experimental evidence of the occurrence of dimerization of PAHs in the gas phase before soot formation in a laminar diffusion methane flame, supporting the hypothesis of stabilization of dimers through the formation of covalent bonds. The main findings of this work derive from the comparative chemical analysis of samples extracted from the gas to soot transition region of a laminar diffusion methane flame, and highlight two different groups of hydrocarbons that coexist in the same mass range, but show distinctly different behavior when processed with statistical analysis. In particular, the identified hydrocarbons are small-to-moderate size PAHs (first group) and their homo- and heterodimers stabilized by the formation of covalent bonds (second group). A comprehensive understanding of soot nucleation in flame combustion is still lacking. Here the authors identify homo- and heterodimers of small-to-moderate size polycyclic aromatic hydrocarbons in the gas to soot transition region of a laminar diffusion methane flame as viable intermediates in the soot nucleation process.

Recent advances in the soot studies have shown experimental evidences of π-radicals and cross-linked structures among the molecular constituents of just-nucleated soot particles. π-radicals could have an important role in particle nucleation by increasing the binding energy between polycyclic aromatic hydrocarbons with respect to pure van der Waals interactions. In this work we use density functional theory by Grimme D3 dispersion correction (DFT-D3) with hybrid functional and localized Gaussian basis set (B3LYP/6-31G**) to analyze and classify the clustering behaviors of two aromatic radicals visualized experimentally by atomic force microscopy (Commodo et al. Combust. Flame 205: 154–164, 2019). These aromatic radicals have different topological structures and delocalization of the unpaired electron. The binding energy and energy bandgap characteristics of the clusters are calculated. The theoretical results show a different clustering behavior for the two aromatic radicals. The one with a partial localization of the unpaired electron tends to form a σ-dimer; conversely, the radical with a greater delocalization of the unpaired electron leads to π-stacking formation with a slight overbinding of few kcal mol−1 with respect to pure van der Waals interactions and a marked lowering of the energy bandgap. The formation of π-stacking induced by delocalized π-radicals could in part explain some spectroscopic evidences observed during soot nucleation. © 2020 Elsevier Ltd

Although the majority of existing combustion devices operate at high pressure conditions, most of our understanding of the soot formation process and soot physicochemical properties rely on studies performed at atmospheric pressure. Pressure is known to have nonlinear effects on combustion processes and a significant influence on soot formation; soot loading increases with increasing combustion pressure. Soot characteristics directly affect soot oxidation and optical/radiative properties, and it is desirable to have a better insight into them under high-pressure conditions. Due to scarcity of information on soot primary particles, aggregate morphology, and soot nanostructure relevant to high-pressure combustion, there are challenges in predicting soot oxidation and radiation, particularly at engine-relevant conditions. In this study, we perform Raman spectroscopy measurements on soot sampled from a set of laminar diffusion flames of ethylene at various pressures, ranging from atmospheric pressure to 12 bar. Our results show an increase in soot maturity as the pressure increases within the range of investigated pressures. In the examined co-flow flames, pressure seems to have an indirect influence on soot nanostructure through an earlier inception of soot, resulting in longer residence times of the carbon soot particles in the hot and reactive flame environment. It is found that soot maturity, tracked through the size of graphitic domains, La, increases linearly with residence times. The longer residence time of soot in high-pressure flames could be the main cause of the higher degree of graphitization observed, which suggests a greater resistance to oxidation with increasing pressure.

Many studies have shown the potential of using flame-formed CNPs for several advanced technologies including: fluorescent nanoparticles, active material for new solar cells generation or as supercapacitor material; (in addition to the classical carbon black uses); Flame-formed CNPs can have different features: particles size, morphology, degree of carbonization, optical properties; In this poster we report some results achieved, in our lab, and in collaboration with other groups, by scanning probe microscopies on CNPs.

Coupling of traditional combustion technologies with solar thermal energy is fundamental to enlarge the field of applicability and to lower the costs of renewable energy sources. A possible solution is represented by direct irradiation of a flame with concentrated solar energy, so as to increase its temperature thanks to the high absorption coefficient of combustion by-products such as soot particles. In this study, we use a detailed model of soot formation and oxidation to simulate the behaviour of a coflowing diffusion ethylene flame with and without the exposure to a concentrated solar radiation. Modelling data are compared with experimental data reported in the literature. Results confirm that our model is able to reproduce the effect of radiation both matching the temperature and soot volume fraction increase. Successively, a modified version of the solar-combustion hybrid system has been setup to test the effect of solar light orientation on the combustion process. A simulator of concentrated solar energy has been used to directly irradiate the flame from the top. Thermocouple-based temperature measurements highlighted that the flame effectively absorbed solar radiation.

In this work, the physicochemical transformation of soot particles during the initial stages of formation and growth was investigated in a laminar premixed flame of ethylene and air. The selected flame condition allowed producing two classes of carbon nanoparticles, namely incipient and primary soot particles, on the basis of their size distribution. The two classes of particles have been selectively collected in well distinct flame zones for further chemico-physical characterization. The optical band gap decreases as particle size grows up. Particle medium range order and structure at molecular level have been investigated by Raman and fluorescence spectroscopy. Slight changes in particle composition were observed for incipient and primary soot. Both kinds of particles are made of a similar ensemble of fluorescence centers, which produce excitation dependent emission irrespective of particle sizes. In agreement with the Raman spectra, the molecular constituents of the particles are polyaromatic compounds with an average size of the order of ovalene molecules that only slightly increase from incipient to primary soot particles. These results suggest that the observed decrease of the optical band gap, as particle size increases, typical of a quantum-dot behavior, is not due to changes in particle composition at molecular level. Further investigations are needed to investigate possible evolution in supramolecular organization of the particles as they grow in flame and its effect on the optical band gap.

The problem relating to the formation of solid particles enabled by hypersonic re-entry in methane-containing atmospheres (such as that of Titan) has been tackled in the framework of a combined experimental&ndash, numerical approach implemented via a three-level analysis hierarchy. First experimental tests have been conducted using a wind tunnel driven by an industrial arc-heated facility operating with nitrogen as working gas (the SPES, i.e., the Small Planetary Entry Simulator). The formation of solid phases as a result of the complex chemical reactions established in such conditions has been detected and quantitatively measured with high accuracy. In a second stage of the study, insights into the related formation process have been obtained by using multispecies models relying on the NASA CEA code and the Direct Simulation Monte Carlo (DSMC) method. Through this approach the range of flow enthalpies in which carbonaceous deposits can be formed has been identified, obtaining good agreement with the experimental findings. Finally, the deposited substance has been analyzed by means of a set of complementary diagnostic techniques, i.e., SEM, spectroscopy (Raman, FTIR, UV&ndash, visible absorption and fluorescence), GC&ndash, MS and TGA. It has been found that carbon produced by the interaction of the simulated Titan atmosphere with a solid probe at very high temperatures can be separated into two chemically different fractions, which also include &ldquo, tholins&rdquo

Chemiluminescence and laser-induced breakdown spectroscopy (LIBS) measurements have been carried out in laminar ethylene/air partially premixed flames for real-time local equivalence ratio monitoring. OH* and CH* chemiluminescence measurements have been performed in the region where combustion processes mainly occur. Using LIBS measurements, the hydrogen to oxygen emission signal ratio has been obtained as an indicator of the fuel to air ratio and consequently of the mixing processes. Through a calibration procedure performed in nitrogen-diluted ethylene/air premixed flames of known equivalence ratio, the local values of the equivalence ratio have been derived in the flames under analysis. Target flames, proposed in the framework of the International Sooting Flame (ISF) workshop, have been investigated along the radius at different heights above the burner. Correspondingly, chemical kinetic modeling analysis has been performed and the results were compared with the experimental data in order to cross-check the reliability of both experimental techniques and modeling approach.

In this work, the physicochemical properties of flame-formed soot particles are investigated in a set of ethylene/air premixed flames as function of the equivalent ratio. The objective of this study is to highlight the peculiar characteristics of the particles produced under quasi-clean combustion conditions in comparison with soot formed in richer flames. The size distribution of flame-formed soot nanoparticles was measured by differential mobility analysis; particle composition by Raman and UV–vis light absorption spectroscopy and morphology by atomic force microscopy. Our results evidence that the particle size distribution is strongly affected by the flame equivalence ratio. Slightly rich flames produce mono-modal PSD with very small particles of only few nanometers. Bi-modal size distribution are found in richer flames when larger particles, i.e., 5–20 nm, are formed. Change from a mono-modal to a bi-modal particle size distribution can be related to changes in particle graphitization as evidenced by a slight increase of the in plane average size of the polyaromatic units within the particles, La, and a change in particle optical properties.

The effect of pressure on soot particle nanostructure has been investigated by Raman spectroscopy. For the first time soot samples produced in a set of coflow methane-air laminar diffusion flames stabilized in a high-pressure combustion chamber, and collected by thermophoresis, have been analyzed to obtain chemical/structural information. The first-order Raman spectra of soot particles collected at several elevated pressures have been analyzed and compared. As a result, within the investigated range of pressures, i.e., from 10 to 20 bar, our measurements show that soot nanostructure does not depend on the pressure conditions. Changes in soot nanostructure were only observed as a function of flame residence time. The results herein presented are particularly relevant by considering the importance of nanostructure on the soot reactivity toward oxidation and on the fact that soot from practical combustion devices is mostly produced at high pressure.

Carbon nanoparticles, with diameter d ∼10 nm, were produced in a premixed ethylene-air flame and used to grow self-assembled thin films by thermophoretic deposition on Si++/SiO2/Gold multilayer substrates inserted in flame. The particles’ size was measured by a Scanning Mobility Particle Sizer. UV–vis light absorption, Raman spectroscopy and Atomic Force Microscopy were used for the chemico-physical and morphological investigation of the carbon nanoparticle thin films. The electrical characterization was then performed in different environmental conditions with the basic aim to assess the potentiality of these films to be employed as electrically-active components in electronic and sensing applications. The films are composed by the aggregation of nanoscale grains whose size increases with the deposition time and are consequently highly porous with the electrical conduction properties showing a dependence on thickness suggesting a percolation-like behavior. Raman spectrum indicates that the chemical/structural composition of the film does not change with deposition time and consists in small graphitic crystallites with constant size. The electrical measurements show also the ohmic behavior of the IV curves. Moreover, an ambipolar response of the CNPs channels, when investigated in the field-effect configuration, was observed and, in parallel with conductivity, charge carrier mobility values were found to increase with thickness.

In spite of advances in recent years, the current understanding of the nucleation and initial growth of soot in flames is still rather limited. The chemico-physical properties of just-nucleated particles and particles at the early stage of growth are here investigated by X-ray and ultraviolet photoemission spectroscopy, UV–visible and Raman spectroscopy. Both classes of particles are produced and selectively analyzed by opportunely selecting both flame and sampling conditions. The complementarity of the methods and the consistency of the results allowed us to evidence the differences in the carbon structure of the two particles. Both kind of particles are predominantly composed by sp 2 carbon and contain a few percent of oxygen mainly as ether-like bonds. A few percent of sp 3 carbon has been observed in grown particles, and it coexists with a more developed graphitic structure with moderately larger aromatic islands, a lower band gap and higher density of states. The results of this work help to clarify the physicochemical transformation experienced by the just nucleated particles with sizes of 2–3 nm during the early steps of the growth process, which is fundamental for the improvement of current models of soot inception and growth.

TiO2 in the form of nanocrystals possesses photocatalyst properties leading to excellent capability of degrading a number of environmental contaminants such as organics, bacteria, and viruses. However, the antimicrobial activity of nano-TiO2 coatings against different microorganisms is not yet well established. Candida albicans, Aspergillus niger, Staphylococcus aureus and Streptococcus mutans are common nosocomial and environmental pathogens responsible for biofilm-associated infections especially in indoor environments. In this study, we produced thin coatings of TiO2 nanoparticles on aluminum substrates and assessed whether they may prevent fungi and bacteria biofilm formation and whether this may be a synergistic effect of TiO2 nanoparticles with UV irradiation, which is more representative of the natural conditions for passive devices. Coatings of flame-synthesized nanoparticles were produced by direct thermophoretic deposition on aluminum substrates through a rotating disk inserted in the flame. Particle and coating characterization was performed by means of differential mobility analysis, Raman and X-ray diffraction spectroscopy. Microbial biofilm formation on substrates with different TiO2 thicknesses were evaluated by the colorimetric crystal violet method and quantified by spectrophotometry in four different fungi and bacteria strains; the results were corroborated by scanning electron microscopy analysis. Substrates with minimal TiO2 nanoparticle layers demonstrated a strong anti-biofilm effect against both fungi and bacteria, due to the well-known cellular component photo-oxidation and to the enhanced nanomaterial surface area. Interestingly, UV irradiation of the substrates at a low intensity for a short time demonstrated a significant enhancement of the titania antimicrobial activity. TiO2 nanoparticle coatings may be proposed as self-cleaning and self-disinfecting materials able to reduce microbial infections, especially in indoor environments where hygiene is needed.

Spacecraft entry into Titan's atmosphere has been investigated using a dedicated (Small Planetary Entry Simulator) facility (SPES). While in earlier works much attention was paid to the joint numerical-experimental simulation of typical entry physical parameters (namely, heat flux and total enthalpy); in the present analysis we focus on some unexpected results recently obtained at the University of Naples, in collaboration with CNR, in the framework of a new test campaign dedicated to various planetary atmospheres (including Titan itself). Such findings concern the presence of a carbon-like substance on the surface of the measuring probes used for the experiments, which seem to align with the results yielded by other authors with other strategies (an inductive plasma torch). We have confirmed the carbonaceous nature of such particulate matter via various diagnostic techniques such as SEM, Raman, FT-IR, UV-visible absorption and fluorescence spectroscopy, GC-MS and TGA. The present work is devoted to the presentation of such results together with a critical discussion of the novelty relating to the present experimental approach (arc plasma versus inductive torch) and an analysis of the chemical-physical differences pertaining to the carbon obtained with the two different methods.

The production of thin coatings of TiO2 nanoparticles by aerosol flame synthesis and direct thermophoretic deposition is presented. Three different flame reactor configurations were designed in order to study the effect of particle size and film morphology on the performances of the coatings. Particle dimension, crystal phase and surface morphology were characterized using differential mobility analysis, Raman spectroscopy and atomic force microscopy, respectively. The wetting behavior was investigated by water contact angle analysis, showing that titania coatings are characterized by a high photoinduced hydrophilicity. Measurements of the inhibition of Staphylococcus aureus biofilm formation revealed a high antibacterial activity from TiO2 nanoparticle films. Finally, both the hydrophilic character and the bactericidal effects are found to be mainly dependent on the size of primary particles composing the coatings. The optimal synthesis conditions were identified in order to produce a self-cleaning and self-disinfecting coatings material, with a nearly superhydrophilicity and a high antibacterial activity, both activated by ordinary light radiation in standard room illumination conditions.

Combustion-generated soot particles can have significant impact on climate, environment and human health. Thus, understanding the processes governing the formation of soot particles in combustion is a topic of ongoing research. In this study, high-resolution atomic force microscopy (AFM) was used for direct imaging of the building blocks forming the particles in the early stages of soot formation. Incipient soot particles were collected right after the particle nucleation zone of a slightly sooting ethylene/air laminar premixed flame at atmospheric pressure and analyzed by AFM after a rapid sublimation procedure. Our data shed light on one of the most complex and still debated aspect on soot formation, i.e., the nucleation process. The molecular constituents of the initial particles have been individually analyzed in detail in their chemical/structural characteristics. Our data demonstrate the large complexity/variety of the aromatic compounds which are the building blocks of the initial soot particles. Nevertheless, some fundamental and specific characteristics have been clearly ascertained. These include a significant presence of penta-rings as opposed to the purely benzenoid aromatic compounds and the noticeable presence of aliphatic side-chains. In addition, there were indications for the presence of persistent π radicals. Incipient soot was also investigated by Raman spectroscopy, the results of which agreed in terms of chemical and structural composition of the particles with those obtained by AFM.

The early stages of soot formation, namely inception and growth, are highly debated and central to many ongoing studies in combustion research. Here, we provide new insights into these processes from studying different soot samples by atomic force microscopy (AFM). Soot has been extracted from a slightly sooting, premixed ethylene/air flame both at the onset of the nucleation process, where the particle size is of the order of 2–4 nm, and at the initial stage of particle growth, where slightly larger particles are present. Subsequently, the molecular constituents from both stages of soot formation were investigated using high-resolution AFM with CO-functionalized tips. In addition, we studied a model compound to confirm the atomic contrast and AFM-based unambiguous identification of aliphatic pentagonal rings, which were frequently observed on the periphery of the aromatic soot molecules. We show that the removal of hydrogen from such moieties could be a pathway to resonantly stabilized π-radicals, which were detected in both investigated stages of the soot formation process. Such π-radicals could be highly important in particle nucleation, as they provide a rational explanation for the binding forces among aromatic molecules.

We examine the quantum confinement in the photoemission ionization energy in air and optical band gap of carbon nanoparticles (CNPs). Premixed, stretched-stabilized ethylene flames are used to generate the CNPs reproducibly over the range of 4–23 nm in volume median diameter. The results reveal that flame-formed CNPs behave like an indirect band gap material, and that the existence of the optical band gap is attributed to the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap in the polycyclic aromatic hydrocarbons comprising the CNPs. Both the ionization energy and optical band gap are found to follow closely the quantum confinement effect. The optical band gaps, measured both in situ and ex situ on the CNPs prepared in several additional flames, are consistent with the theory and the baseline data of CNPs from stretched-stabilized ethylene flames, thus indicating the observed effect to be general and that the particle size is the single most important factor governing the variation of the band gap of the CNPs studied. Cyclic voltammetry measurements and density functional theory calculations provide additional support for the quantum dot behavior observed.