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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.

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.

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.

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

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.

Some aspects of soot formation from gas phase molecules at high temperatures are still unclear. Aromatic π-radicals may be key elements for a change in the model of carbon clustering and particle formation. However, experimental investigations are still needed on the radical nature of molecules and particles in flames and on their roles in the transition from molecules to incipient molecular clusters and their further evolution to mature soot. In this paper, we present electron paramagnetic resonance (EPR) and Raman spectroscopy measurements of particles collected in an ethylene-rich premixed flame at various residence times during nucleation. The experimental results, combined with measurement of the particle size distribution by a differential mobility analyzer, are used to investigate the role of radicals in particle nucleation and rearrangement of aromatic molecules in just-nucleated particles. For all the sampled particles, an EPR signal typical of persistent carbon-centered aromatic radicals is measured. An abrupt change in the EPR signal intensity, which is characteristic of stronger supramolecular interactions, is observed when the size distribution changes from monomodal to bimodal, as confirmed by Raman spectroscopy. Our experimental results indicate strong involvement of π-radicals during particle nucleation and growth. The paramagnetic/radical nature of the sampled particles is discussed on the basis of recent studies on the role of resonantly stabilized radicals in soot nucleation. The presence of localized π-electrons on the edge of aromatic soot constituents and their role in carbon clustering are also discussed.

Incipient carbon nanoparticles, CNP, formed by incomplete hydrocarbon combustion are composed of polycyclic aromatic hydrocarbons [1,2] and exhibit a quantum dot behavior [3], which make them ideal for a wide range of applications. Flame synthesis offers a continuous and low-cost process for the production of inorganic nanoparticles, such as TiO2, with fine control of cristallinity and phase. Likewise, carbon nanoparticles, CNPs, with tailored size and optical and electronic properties can be produced by optimizing the flame process. The assembly of nanoparticles into a uniform thin film with precise control over chemical and physical properties poses a significant challenge. Thermophoretic sampling relies on the thermophoretic forces driving the particles in the hot flame towards a cold substrate inserted in the flame and can be used to produce nanostructured, self-assembled films that can lead to novel applications. In this work, CNP and CNP/TiO2 nanoparticles have been produced in premixed ethylene-air flames and thin films have been realized by thermophoretic deposition on silicon substrates with gold interdigitated electrodes. Scanning mobility particle sizer furnished the particle size distribution and Raman and optical spectroscopy chemico-physical and structural information. The electrical characterization of the films was conducted by measuring the IV characteristics which shows a good linearity in the interval [-5 V, 5 V] whereas non-linear phenomena appear for wider voltage ranges and hysteresis phenomena are also observed in carbon/TiO2 films. Impedance spectroscopy was used to investigate the films with respect to their microstructure. This method is indeed particularly useful for nanostructured films with inter-grain and grain-edge impedances and capacities. Finally, noise spectral density at low and very low frequency was measured to investigate the electron transport. The main source of noise is of thermal type for the carbon material, whereas carbon/TiO2 film shows a Fliker type, 1/f noise, consistent to a larger amount of localized states responsible for trapping/detrapping processes of charge carriers and charge fluctuations during the conduction process. [1] Schulz, F., Commodo, M., Kaiser, K., De Falco, G., Minutolo, P., Meyer, G., D`Anna, A., Gross, L., "Insights into incipient soot formation by atomic force microscopy" Proc. Combust. Inst. 37: 885-892 (2019). [2] Commodo, M., Kaiser, K., De Falco, G., Minutolo, P., Schulz, F., D`Anna, A., Gross, L., "On the early stages of soot formation: molecular structure elucidation by high-resolution atomic force microscopy" Combust. Flame 205, pp. 154-164. [3] Liu, C, Singh, A.V., Saggese, C., Tang, Q., Chen, D., Wan, K., Vinciguerra, M., Commodo, M., De Falco, G., Minutolo, P., D'Anna, A., Wang H. , Flame-formed carbon nanoparticles exhibit quantum dot behaviors, 2019, DOI:10.1073/pnas.1900205116

Carbon-based nanomaterials (CNPs) have attracted considerable interest because of their unique electronic, optical and mechanical properties. Among them, CNPs have become increasingly popular mainly due to their photoluminescence (PL) properties which make them ideal for bio-medical imaging or drug-delivery in addition to light-emitting devices because of their low toxicity, environmental friendliness, low cost and relatively simple synthesis processes. There are many routes for the synthesis of CNPs divided into top-down nano-cutting methods and bottom-up organic approaches. The first one includes laser ablation, arc discharge, chemical and electrochemical oxidation of large carbon structures; recently, starting from the pioneering work of Liu et al. [1], also the so called "candle soot" turned out to be an excellent precursor for the synthesis of CNPs for a variety of applications [2]. A wide set of studies indicates that the constituents of CNPs in flames are mainly PAHs of moderately size [3, 4]. Furthermore, there is a large body of evidence indicating a change in the electronic and optical properties of CNPs during their early stages of nucleation and growth [5] and recently, Liu et al. [6] demonstrated quantum dots behavior of the CNPs, in the particle range 4-23 nm, directly produced and collected from flames. Therefore, carbon particle nucleation and growth in flames can be exploited as a bottom up process for CNPs through direct sampling CNPs from a well-controlled flame reactor. In this work particle size has been measured on-line in aerosol phase by a differential mobility analyzer. Particles with different average sizes have been sampled and characterized for the first time by scanning tunneling microscopy/spectroscopy (STM/STS). The electronic properties, i.e., density of states and energy band gap, of flame-formed nanoparticles measured by STS have been correlated to nanostructure determined by Raman spectroscopy and the UV-visible absorption spectra and optical band gap determined by Tauc analysis. [1] H. Liu et al., Fluorescent Carbon Nanoparticles Derived from Candle Soot, Angew. Chem. Int. Ed. 46 (2007) 6473 -6475. [2] M.R. Mulay et al., Candle soot: Journey from a pollutant to a functional material, Carbon 144 (2019) 684-712. [3] F. Schulzet al., Insights into incipient soot formation by atomic force microscopy, Proc. Combust. Inst. 37, 2019, 885-892. [4] M. Commodo et al., On the early stages of soot formation: Molecular structure elucidation by high-resolution atomic force microscopy, Combust. Flame 205 (2019) 154-164. [5] M. Commodo et al., Physicochemical evolution of nascent soot particles in a laminar premixed flame: from nucleation to early growth, Combust. Flame 162 (2015) 3854-3863. [6] C. Liu et al., Flame-Formed Carbon Nanoparticles Exhibit Quantum, Proc. Natl. Acad. Sci. U.S.A. 2019, DOI: 10.1073/pnas.1900205116.

Recent advances in the literature have shown experimental evidences of a consistent amount of ?-radicals and cross-linked structures among the molecular constituents of just-nucleated soot particles. The inclusion of such experimental observations in the mechanism of soot particle formation will improve the comprehension of the physics and chemistry involved during the evolution of soot particles in flames. In this regards ?-radical could have a fundamental role, particularly at the onset of soot formation, when gas-phase molecules convert to a condensed-phase material. According to the literature of highly delocalized ?-radicals, two type of dimerization process can be taken into account. These are: a ?-stacking mechanism that occurs by forming multi-electron/multi-center delocalized bonds, thus covalent-like, with shorter intermolecular distances and stronger interactions with respect to van der Waal interactions; a conventional ?-bond formation, with longer distance and weaker strength than regular C-C bond. In this work we use density functional theory (DFT) with hybrid functional and localized Gaussian basis set (B3LYP/6-31G**) to analyze and classify two radicals having different topological structures in term of localization/delocalization of the unpaired electron, and in turn binding energy and optical band gap characteristics of the clustering of two different molecular radicals. The ambition of this study can be represented by the investigation of the two polycyclic aromatic, labeled as radical 1 (R1), and radical 2 (R2), experimentally determined by Atomic Force Microscopy (AFM) studies and calculated by ab-initio quantum investigations. As results the different clustering of the two polycyclic aromatic hydrocarbons with a couple of unpaired electron delocalized, in different spin multiplicity (Sz=1, 3), leads to a different reductions of the optical gap.

L'attività di ricerca condotta dall'unità IRC-CNR per il PAR 2016-2017 ha riguardato la messa a punto di procedure di sintesi e di deposizione di nanoparticelle di TiO2 e compositi TiO2/C ottenute per sintesi in fiamma su elettrodi commerciali. In particolare, metodi di sintesi di nanoparticelle in fiamma sono stati utilizzati allo scopo di modificare le proprietà superficiali degli elettrodi. Sono state prodotte nanoparticelle di TiO2 igroscopico e compositi TiO2/C depositate direttamente su elettrodi mediante deposizione temoforetica in modo da migliorarne i parametri elettrochimici.

PREMESSA: L'attività di ricerca condotta dall'unità IRC-CNR per il PAR 2016-2017 ha riguardato la caratterizzazione di elettrodi ricoperti di nanoparticelle di TiO2, TiO2/C e di sole particelle carboniose. Pe le due classi di materiali, F-CNP e F-CNP/TiO2, si è effettuata la caratterizzazione chimico-fisica mediante spettroscopia Raman, XRD, sessile drop.

In spite of the advances in laser diagnostics in combustion, the effect of rapid laser irradiation on the physical/chemical properties of soot particles is far from being comprehensively understood. Optical properties, particle nanostructure and aggregate size of laser-irradiated soot particles are analyzed in this paper. Carbonaceous particles sampled from nitrogen-quenched diffusion flames of ethylene and methane are irradiated on-line by a 1064-nm short laser pulse. Wavelength-resolved extinction measurements in the visible range are used to follow their transformation by varying the laser energy density. A variation of the extinction coefficient of the irradiated particles compared to the extinction coefficient of the pristine ones is observed, especially for ethylene soot. The particle nanostructures are analyzed by Raman spectroscopy and the effect of laser irradiation on aggregate structure is evaluated by measuring particle size distributions. The results indicate that both soot nanostructure and optical properties are strongly dependent on the laser energy density when irradiated by a laser source. This is significant for ethylene soot, while for methane soot the degree of variation of such properties is less pronounced, at least in the investigated heating conditions. © 2020 Elsevier Inc.

Report tecnico inerente le procedure di sintesi in fiamma di aerosol di nanoparticelle carboniose, metodo di deposizione degli aerosol carboniosi per la produzione di film carboniosi nanostrutturati e le procedure di "annealing" termico per la ristrutturazione di materiali carboniosi (PAR 2015 deliverable).

In this work, a set of four partially premixed ethylene–air laminar flames, characterized by different equivalent ratios, was studied. The aim was to investigate the effect of partial premixing on soot formation, which is a relevant parameter for real combustion devices. Quantitative and qualitative information on soot particles were obtained using a modified version of the thermophoretic particle densitometry (TPD) method. Particle thermal emissivity values were measured from the transient thermocouple temperature response. Results show that the emissivity of flame-formed carbon nanoparticles ranges from about 0.5 for soot precursor particles up to 0.95, i.e., the typical emissivity value attributed to mature soot particles. Particle volume fraction was then accurately evaluated by means of the TPD method, using the values of particle emissivity previously measured. The changing of particle features throughout the flame, evidenced by the variation in thermal emissivity, was further investigated using Raman spectroscopy. The analysis of Raman spectra showed that the increase in thermal emissivity follows a reduction in the hydrogen-to-carbon ratio of the investigated soot nanoparticles. Measurements performed with the TPD method also evidence that both particle emissivity and volume fraction are strongly affected by oxidation, which prevents to accurately measure soot concentration in oxidative regions of the flames. An attempt to include soot oxidation in the TPD method for a correct evaluation of volume fraction procedure is also illustrated. OH concentration profiles, obtained from a modeling analysis and validated by radical chemiluminescence measurements, were used in the TPD method to correct particle concentration due to oxidation.