Emeline Charon
email: emeline.charon@cea.fr

Mathieu Pinault

Martine Mayne-L'hermite

Cécile Reynaud

One-step synthesis of highly pure and well-crystallized vertically aligned carbon nanotubes

Keywords: vertically aligned carbon nanotubes, one-step catalytic chemical vapour deposition, iron content, purity, crystallinity, oxidation resistance

The one-step aerosol-assisted catalytic chemical vapor deposition (CCVD) process, when operated in a H2-based carrier gas, is shown to be effective at an extremely low ferrocene content (0.1wt.% in toluene), which enables to efficiently prepare forests of vertically aligned multiwalled carbon nanotubes (VACNTs) with high purity and good crystalline level. The resulting iron content in VACNT sample is only 0.5wt.% corresponding to a high catalytic yield of 200 while the steady growth rate (20 µm/min) is maintained at a significant value. The iron content in the sample is found proportional to the ferrocene concentration in the precursor, and when it decreases, a noticeable reduction in the iron encapsulation frequency in the nanotube channel is plainly observed. Lowering the ferrocene concentration generates a reduction in the VACNT number density while the growth rate increases and CNT diameters and crystalline structure remain similar. Such significant purity and crystalline levels involves a high oxidation resistance of the VACNT forest. These results, together with the in-situ continuous growth feature of the one-step aerosol assisted CCVD process, open up towards a new VACNT range made of pure and well-crystallized multi-walled carbon nanotubes, which is of interest for industrial production and commercial applications.

Introduction

Vertically Aligned Carbon NanoTubes (VACNT) arrays have demonstrated outstanding potentialities as a novel material to prepare nanostructured composite or multifunctional materials. The carbon nanotube (CNT) alignment allows to take advantage of their anisotropic properties. This is useful for many applications of high societal interest such as heat transfer [START_REF] Marconnet | Thermal Conduction in Aligned Carbon Nanotube-Polymer Nanocomposites with High Packing Density[END_REF], energy storage [START_REF] Lagoutte | Poly(3-methylthiophene)/Vertically Aligned Multi-walled Carbon Nanotubes: Electrochemical Synthesis, Characterizations and Electrochemical Storage Properties in Ionic Liquids[END_REF][START_REF] Evanoff | Towards ultrathick battery electrodes: Aligned carbon nanotube-enabled architecture[END_REF], or high performance structural composites [START_REF] Cebeci | Effect of nanofiber proximity on the mechanical behavior of high volume fraction aligned carbon nanotube arrays[END_REF][START_REF] Raney | Rate-independent dissipation and loading direction effects in compressed carbon nanotube arrays[END_REF]. Efforts are currently underway to develop efficient synthesis processes for the manufacture of VACNTs in order to achieve mass production while reducing costs [START_REF] Kumar | Chemical Vapor Deposition of Carbon Nanotubes: A Review on Growth Mechanism and Mass Production[END_REF][START_REF] Guzmán De Villoria | High-yield growth of vertically aligned carbon nanotubes on a continuously moving substrate[END_REF]. In particular, the process must be as simple as possible and must be operated at atmospheric pressure. The most effective method for synthesizing VACNT is catalytic chemical vapor deposition (CCVD) [START_REF] Jourdain | Current understanding of the growth of carbon nanotubes in catalytic chemical vapour deposition[END_REF] which can be operated at atmospheric pressure. However, it often involved several preliminary steps at low pressure to prepare the layer of catalytic nanoparticles from which the nanotubes will then grow. It is why the aerosol-assisted version of the CCVD process has been developed [START_REF] Andrews | Continuous production of aligned carbon nanotubes: a step closer to commercial realization[END_REF][START_REF] Mayne | Pyrolytic production of aligned carbon nanotubes from homogeneously dispersed benzene-based aerosols[END_REF][START_REF] Zhang | Rapid growth of well-aligned carbon nanotube arrays[END_REF][START_REF] Afre | Growth of vertically aligned carbon nanotubes on silicon and quartz substrate by spray pyrolysis of a natural precursor: Turpentine oil[END_REF][START_REF] Singh | Production of controlled architectures of aligned carbon nanotubes by an injection chemical vapour deposition method[END_REF][START_REF] Boulanger | Towards large scale aligned carbon nanotube composites: an industrial safe-by-design and sustainable approach[END_REF], where catalyst nanoparticles are produced in situ and continuously refreshed due to the simultaneous injection of both metal and carbon precursors [START_REF] Castro | Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition[END_REF]. This process involves a base-growth mechanism [START_REF] Pinault | Growth of multiwalled carbon nanotubes during the initial stages of aerosol-assisted CCVD[END_REF] enabling the continuous growth of VACNTs at steady growth rate over very long synthesis duration, which allows VACNTs greater than 1mm to be achieved. Therefore, aerosol-assisted CCVD is a versatile, cost-effective and scalable one-step process operated at atmospheric pressure potentially applicable for a roll to roll production of multi-walled VACNTs. The continuous supply of ferrocene, which is essential to sustain the growth through the refreshment of the catalyst nanoparticles [START_REF] Pinault | Growth of multiwalled carbon nanotubes during the initial stages of aerosol-assisted CCVD[END_REF], induces the encapsulation of short nanowires in the inner channel of the nanotubes [START_REF] Xiang | Encapsulation, compensation, and substitution of catalyst particles during continuous growth of carbon nanotubes[END_REF][START_REF] Landois | In situ time resolved wide angle X-ray diffraction study of nanotube carpet growth: Nature of catalyst particles and progressive nanotube alignment[END_REF]. As a consequence, the iron content of the pristine VACNTs (Table 1) is typically around a few wt.% [START_REF] Castro | Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition[END_REF][START_REF] Mckee | Dimensional control of multi-walled carbon nanotubes in floating-catalyst CVD synthesis[END_REF][START_REF] Cho | The influence of reagent stoichiometry on the yield and aspect ratio of acid-oxidised injection CVD-grown multi-walled carbon nanotubes[END_REF][START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study[END_REF]. Such metallic by-products can alter the intrinsic nanotube properties and be detrimental for several applications [START_REF] Hou | Purification of carbon nanotubes[END_REF]. In addition, it is also recognized that they can catalyze the oxidation of nanotubes and thus affect their stability in air at moderate temperatures [START_REF] Chiang | Purification and Characterization of Single-Wall Carbon Nanotubes[END_REF][START_REF] Leino | Low-temperature catalytic oxidation of multi-walled carbon nanotubes[END_REF][START_REF] Wang | Tailored cutting of carbon nanotubes and controlled dispersion of metal nanoparticles inside their channels[END_REF]. Therefore, these CNTs need to be purified and several methods have been developed to remove metal by-products [START_REF] Hou | Purification of carbon nanotubes[END_REF], either by acid treatment [START_REF] Edwards | Evaluation of residual iron in carbon nanotubes purified by acid treatments[END_REF], but their complete removal is difficult to achieve without altering the nanotube structure [START_REF] Suryanto | From the inside-out: Leached metal impurities in multiwall carbon nanotubes for purification or electrocatalysis[END_REF][START_REF] Yang | Electro-purification of carbon nanotube networks without damaging the assembly structure and crystallinity[END_REF], or by high temperature annealing which is the most effective method for their removal while improving the nanotube crystallinity [START_REF] Hou | Purification of carbon nanotubes[END_REF][START_REF] Pinault | Carbon nanotubes produced by aerosol pyrolysis: growth mechanisms and postannealing effects[END_REF][START_REF] Huang | 9% Purity Multi-Walled Carbon Nanotubes By Vacuum High-Temperature Annealing[END_REF]. However, such treatments complicate the CNT production process and are not always compatible with the targeted application.

Therefore, efforts must be done directly on the aerosol-assisted CCVD production process in order to reduce drastically the iron content in the pristine VACNTs. Recently and in the meantime of our study, this issue was addressed by T. Kinoshita et al. [START_REF] Kinoshita | Two step floating catalyst chemical vapor deposition including in situ fabrication of catalyst nanoparticles and carbon nanotube forest growth with low impurity level[END_REF] who reported a modified floating catalyst CCVD method (similar to the aerosol-assisted one) enabling the growth of CNT forests containing very low concentration of impurity (0.8 wt%). However, the changes made to the process have turned it into a two-step process involving in the first step the in-situ formation of the catalyst particles, then in the second step the CNT growth with carbon precursors only. Such experimental conditions do not allow the refreshment of the catalyst particles so that the lifetime of their process is limited. Moreover the second step was performed at low pressure which limits the industrial interest of the method.

In this context, the aim of our work is to reduce the Fe content of pristine VACNTs while preserving their crystallinity, alignment and growth rate by increasing the catalytic yield of the one-step aerosol-assisted CCVD process operated at atmospheric pressure. In the literature (Table 1), few studies [START_REF] Castro | Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition[END_REF][START_REF] Mckee | Dimensional control of multi-walled carbon nanotubes in floating-catalyst CVD synthesis[END_REF][START_REF] Cho | The influence of reagent stoichiometry on the yield and aspect ratio of acid-oxidised injection CVD-grown multi-walled carbon nanotubes[END_REF][START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study[END_REF] report the measurement of iron content of VACNTs formed by the one-step aerosol-assisted CCVD, but it is generally found that iron content increases with ferrocene concentration in the precursor solution [START_REF] Mckee | Dimensional control of multi-walled carbon nanotubes in floating-catalyst CVD synthesis[END_REF][START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study[END_REF]. Cho et al. [START_REF] Cho | The influence of reagent stoichiometry on the yield and aspect ratio of acid-oxidised injection CVD-grown multi-walled carbon nanotubes[END_REF] studied the [0.2-9] wt.% ferrocene range at 760°C and obtained 1.9wt.% as the lowest iron content in CNT samples for 1.5wt.% of ferrocene in toluene. However, they did not observe any growth of VACNT forests at 0.2wt.% of ferrocene, in agreement with the result of Singh et al. [START_REF] Singh | Production of controlled architectures of aligned carbon nanotubes by an injection chemical vapour deposition method[END_REF].

Similarly, Castro et al. [START_REF] Castro | Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition[END_REF] observed no growth of VACNT at 0.1 and 0.5wt.% when studying the [0.1-5] wt.% range of ferrocene at 850°C and the lowest iron content obtained is 3wt.% at 2.5wt% ferrocene with a higher growth rate due to a higher synthesis temperature. To our knowledge, for one-step aerosol-assisted CCVD process, the lowest concentration of ferrocene responsible for the growth of VACNTs is 0.1wt.% [START_REF] Ionescu | Hydrogen-free spray pyrolysis chemical vapor deposition method for the carbon nanotube growth: Parametric studies[END_REF] but the growth rate at 750°C is particularly low (0.4 µm/min) and the iron content in the resulting sample is not reported. The highest growth rate obtained with a low ferrocene concentration (0.58wt.%) is 13.6 µm/min at 780°C [START_REF] Bai | Effects of temperature and catalyst concentration on the growth of aligned carbon nanotubes[END_REF] but the iron content is not indicated. The latter result was obtained with 15% H2 added to Ar as carrier gas. Despite the wide disparity in the experimental conditions, the results of the literature highlight the important role played by the synthesis temperature and the hydrogen presence in the carrier gas to maintain a realistic growth rate while decreasing the ferrocene concentration in the aerosol. In a previous paper [START_REF] Castro | The role of hydrogen in the aerosol-assisted chemical vapor deposition process in producing thin and densely packed vertically aligned carbon nanotubes[END_REF], we studied the effect of adding hydrogen to Ar carrier gas and showed that it has a major impact on the aerosol-assisted CCVD process. Increasing the hydrogen content strongly shifts the CNT growth to the reactor inlet and reduces the CNT diameters, while the CNT number density increases. Indeed, hydrogen demonstrably affects the gas phase phenomena by lowering the decomposition temperature of ferrocene, promoting the formation of a larger number of smaller iron nanoparticles in the gas phase and increasing the nucleation site density of the catalytic nanoparticles. In addition, the optimal synthesis temperature in terms of yield and homogeneity of growth is reduced to 800°C under hydrogen (Table 1) [START_REF] Castro | The role of hydrogen in the aerosol-assisted chemical vapor deposition process in producing thin and densely packed vertically aligned carbon nanotubes[END_REF]. In these previous experiments, the concentration of ferrocene in toluene was set at 2.5wt.%, the optimal concentration determined at 850°C without H2 [START_REF] Castro | Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition[END_REF]. It should be reminded that without H2, no growth of VACNTs is obtained below 1.25wt.% of ferrocene, neither at 850°C nor at 800°C. However, the situation may change in the presence of H2 due to the significant improvement in the ferrocene decomposition it induces. Therefore, our objective is to determine, in the presence of H2, the lowest concentration of ferrocene in toluene allowing the growth of a VACNT forest with low impurity level while maintaining a high growth rate and a good structural quality.

In this paper, we investigate the role of ferrocene concentration in the precursor solution (toluene/ferrocene) on the VACNT growth and characteristics (iron content, length, diameter, density, structure, oxidation resistance as well as on growth efficiency (yields and growth rate).

As hydrogen was reported to be a determining chemical player in the decomposition rate of ferrocene [START_REF] Castro | The role of hydrogen in the aerosol-assisted chemical vapor deposition process in producing thin and densely packed vertically aligned carbon nanotubes[END_REF], all the synthesis of this study are performed by the one-step aerosol assisted CCVD process with hydrogen diluted in argon as carrier gas. The approach combines global analysis by Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA) and Raman Spectrometry, with local analysis by Transmission Electron Microscopy (TEM). Since in this one-step aerosol assisted CCVD process a continuous supply of ferrocene is essential to sustain growth [START_REF] Pinault | Evidence of sequential lift in growth of aligned multiwalled carbon nanotube multilayers[END_REF], our first objective is to determine the ferrocene concentration threshold above which VACNT growth is maintained once the catalytic particles are formed. To this end, we first carry out double injection experiments, by designing a specific set-up allowing to use a solution containing 2.5wt.% ferrocene in a first short injection sequence to form the catalyst particles, and then to use a second solution with a lower ferrocene concentration in a second long injection sequence. These experiments demonstrate that a very low ferrocene concentration (0.01wt.%) used in the second sequence can sustain the growth of VACNT forest, which is considerably lower than the ones reported in the literature for this one-step aerosol assisted CCVD process. Subsequently, by operating this process in a single injection mode with a ferrocene concentration decreasing from 2.5 to 0.01wt.%, we show that it is possible to determine conditions to synthesize a pristine VACNT carpet with unprecedented reported low iron content (0.5 wt%) and high oxidation resistance.

Experimental

Synthesis

The set-up (Fig. 1) and standard procedure used to synthesize VACNTs by the one-step aerosolassisted CCVD process at atmospheric pressure are similar to those described by Castro et al. [START_REF] Castro | The role of hydrogen in the aerosol-assisted chemical vapor deposition process in producing thin and densely packed vertically aligned carbon nanotubes[END_REF]. Note that syntheses in the presence of H2 cannot be done under the same conditions as in its absence because the addition of H2 considerably shifts the VACNT growth towards the reactor inlet. The synthesis temperature must then be lowered to restore growth in the reactor center. This is why all syntheses here are made in the presence of H2 and at the optimal temperature of 800°C [START_REF] Castro | The role of hydrogen in the aerosol-assisted chemical vapor deposition process in producing thin and densely packed vertically aligned carbon nanotubes[END_REF]. The furnace exhibits an isothermal area larger than the previous one. It is a 45 cm long tubular furnace equipped with a quartz reactor (16 mm inner diameter).

At 800°C, the isothermal area (±10°C), is extended from 10 to 35 cm from the furnace inlet (Fig. 1). A flat quartz substrate of 1.5x0.75 cm 2 is placed in the isothermal area at 14 cm from the reactor inlet. The VACNTs grown on this substrate are analyzed together with the VACNTs grown on the reactor walls. The VACNTs grown on the reactor walls are collected with a stainless scraper, a procedure which keeps intact their morphology and structure. They are weighed according to the location in the reactor, divided into 5 cm long sections with a surface area of 25 cm 2 each, in order to assess the distribution of the VACNT deposit. All the characterizations will be carried out on the VACNTs coming from the substrate of zone 2 and the VACNT powder coming from zone 3. These zones were chosen because the yield is better there. To perform double injection experiments, the present set-up is equipped with two liquid precursor tanks. Solutions of ferrocene dissolved in toluene at different concentrations are poured into these tanks connected to an injection system and an evaporator where they are nebulized and carried to the reactor by a gas flow of 1 L/min at a rate about 0.8 g/min. The gas flow is a mixture of Ar and 30 vol.% of H2. A solution with 2.5wt.% of ferrocene is poured into tank A (Fig. 1) while in tank B, the solution has variable concentrations. The solution in tank A is injected for 1 min 40 s and then the solution in tank B for 13 min 20 s. The switch from tank A to tank B is instantaneous, so that the process remains a one-step continuous process. In single injection experiments, only the solution in tank B is injected for 15 min, with ferrocene concentrations ranging from 0.01 to 2.5wt.%.

Characterizations

Morphology and thickness of VACNT carpets grown on the substrate and on the walls in the center of the reactor are investigated by SEM (Carl Zeiss Ultra 55). CNT structure and statistical analysis of external and internal diameters are obtained from TEM (Philips CM12, 120 kV) after ultrasonic dispersion of CNTs in ethanol and deposition on standard TEM holder (copper grids). For high resolution TEM, we use a JEOL 2011 operating at 200 kV, equipped with a CCD camera (GATAN system ORIUS SC100, 4008×2672 pixels, pixel size: 0.014 nm).

TGA under air of products collected in the area in the center of the reactor is performed with a 92-16.18 SETARAM apparatus, at a temperature rate of 10°C/min in the 20-1000°C range, which leads to the complete gasification of carbon. The rusty residue is used to determine the iron content of the sample (weight percentage of iron in the collected products), as well as the catalytic yield (ratio between the carbon and iron masses). This iron content is calculated knowing that the residue consists of Fe2O3 as shown by X-ray diffraction [START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: I. Mapping the reactor[END_REF]. In the unlikely event that FeO contributes to the residue, the error would be small since the mass of iron in Fe2O3 is 70%, while it is 78% in FeO. We will see in the following that this error would be negligible given the magnitude of the variation in iron content in our experiments.

Raman spectra are acquired from the substrate samples in ambient conditions using a Renishaw INVIA microspectrometer equipped with an Ar laser (514.5 nm wavelength), focused through a Leica microscope. A very low incident power (2 mW) is used to avoid heating effects. Spectra are recorded in the middle of the carpet cross-section in the 1000-3500 cm -1 range. Pertinent quantitative Raman parameters are obtained by conventional fitting procedures [START_REF] Charon | Graphitization at low temperatures (600-1200°C) in the presence of iron implications in planetology[END_REF] for the classical G, D, D' and 2D bands [START_REF] Pimenta | Studying disorder in graphite-based systems by Raman spectroscopy[END_REF] and the corresponding position, full-width at half maximum (FWHM) and intensity (band height) relative to the G band are reported in Table S1 (Supplementary data).

Results

Thickness and growth rate of the VACNT forests

Double injection experiments

Double injection syntheses are performed with a first injection sequence with 2.5wt.% ferrocene in toluene during 1 min 40 s immediately followed by a long injection sequence of 13 min 20 s with a ferrocene concentration varying between 0.01 and 1.5wt.%. In preliminary experiments, we checked that the 1 min 40 s duration is long enough to initiate a VACNT carpet representative of a VACNT growth with a ferrocene concentration of 2.5wt.% (See Fig. S1 in supplementary data). SEM images of the products grown on quartz substrates placed in the oven center (Fig. 2a) reveal VACNT forests of the same good quality as usually reported [START_REF] Castro | The role of hydrogen in the aerosol-assisted chemical vapor deposition process in producing thin and densely packed vertically aligned carbon nanotubes[END_REF].

The shortest carpet (170 µm) is obtained for 0.01wt.% of ferrocene in the second injection sequence, but then, the thickness increases sharply to 330 µm for 0.05wt.% and up to 380 µm for 0.1wt.%, after which it remains around 270 µm for higher concentrations. The corresponding growth rate (Fig. 2b) exceeds 25 µm/min at 0.1wt.% of ferrocene and stabilizes between 15 and 20 µm/min at higher concentrations. The values on the substrate are slightly higher than those obtained on samples collected on the reactor walls. (Fig. 2b). It is because the substrate is placed upstream along the reactor of the zone where the samples used for the SEM analysis are collected and that, in aerosol-assisted CCVD, the growth yield decreases along the reactor due to the dynamics of the catalytic nanoparticles deposition as observed previously by us [START_REF] Castro | Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition[END_REF][START_REF] Castro | The role of hydrogen in the aerosol-assisted chemical vapor deposition process in producing thin and densely packed vertically aligned carbon nanotubes[END_REF] and others [START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: I. Mapping the reactor[END_REF]. This effect is confirmed by the measurement of the mass distribution along the reactor displayed in Fig. S2.

As detailed in Supplementary Data, Fig. S2 shows that, whatever the ferrocene concentration, the yield is optimal in zone 2 where the substrate is placed while the collected zone considered in Fig. 2 is zone 3 where the yield is slightly lower.

The most striking result of these double injection experiments is that, in the presence of H2, a ferrocene concentration as low as 0.01wt.% in the second injection sequence is sufficient to sustain the growth, and a rate higher than 15 µm/min is obtained for a concentration from 0.05wt.%.

Single injection experiments

Considering the very low ferrocene concentration (0.01 wt%) able to sustain the growth in double injection synthesis, single injection synthesis with ferrocene concentration from 0.01 to 2.5wt.% are performed (Fig. 3a). For 0.01 and 0.05wt.% ferrocene, only a thin layer of entangled nanotubes is observed. However, as soon as the ferrocene concentration reaches 0.1wt.%, the growth of a well-defined VACNT forest becomes effective with a growth rate as high as 20 µm/min, close to its maximum of 22 µm/min obtained with 0.25wt.% (Fig. 3b). For higher concentrations, the growth rate decreases to values in the 10-15 µm/min range. The trends are the same on the substrate located at the center of the furnace and on the reactor walls, with still mean values slightly lower for samples collected on the walls, due to variation along the reactor (Fig. S2). The VACNT carpet thickness increases linearly with the synthesis duration in the tested duration range up to 420 min, with a growth rate of 11.4 µm/min at 1.25wt.% and 2.5wt.% ferrocene (Fig. S3 in supplementary data). Therefore for this one-step aerosol assisted CCVD process operated at 800°C with hydrogen in the gas phase, the ferrocene concentration threshold allowing an efficient growth of VACNTs is as low as 0.1wt.%, while no growth at all was observed without hydrogen at 850°C for ferrocene concentration equal to 0.1 or 0.5wt% [START_REF] Castro | Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition[END_REF]. More generally, the present results represent a great improvement as compared to the state of the art related to the one-step CCVD process (Table 1) in terms of growth rate at very low ferrocene concentration.

Purity, diameter and structure of carbon nanotubes

The main characteristics of the VACNTs are similar regardless of the injection mode and depend only on the ferrocene concentration. Results obtained for the single injection mode will be discussed here and the ones regarding the double injection reported in Supplementary data.

Let's focus on single injection samples and compare SEM and TEM images of the formed nanotubes with 0.1wt.% and 2.5wt.% of ferrocene respectively (Fig. 4). The striking difference between both samples is in terms of cleanliness. At 0.1wt.%, the forest is very clean and no parasitic particles can be observed outside the nanotubes at the resolutions used for the observations. There are very few iron-based nanowires inside the nanotubes (Fig. 4a and4c).

The number of embedded Fe nanoparticles has been drastically reduced as compared to our previous results. Indeed, TEM micrographs on figure 4c) clearly indicate a high purity of CNT at low ferrocene concentration. On the contrary, at 2.5wt.% numerous iron-based particles exhibiting a nanowire shape are located inside the nanotubes (see also Fig. S4). In addition, a considerable amount of nanoparticles are located on CNT surface (Fig. 4b and4d), as observed by others when the ferrocene concentration is high [START_REF] Mckee | Dimensional control of multi-walled carbon nanotubes in floating-catalyst CVD synthesis[END_REF][START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study[END_REF][START_REF] Bai | Effects of temperature and catalyst concentration on the growth of aligned carbon nanotubes[END_REF]. They are of varying sizes, some of them being encapsulated into carbon shells and some others giving rise to the growth of nanotubes of very small diameter which represent a low fraction of the sample mass and are not considered in the diameter distributions reported below. Such iron nanoparticles located on the CNT surface and the thin nanotubes observed on samples prepared with 2.5wt.% of ferrocene (Fig. 4d) cannot be detected in the sample obtained with 0.1wt.% of ferrocene (Fig. 4c).

Therefore, lowering the ferrocene concentration induces a drastically reduction of the iron impurities inside the VACNT samples.

A detailed HRTEM analysis of a few individual nanotubes over their length (see Fig. S4) is used to characterize the iron-based nanowires found in the inner channel of the tubes. Their average length and number density decrease significantly as the ferrocene concentration decreases. This detailed TEM analysis at higher magnification (Fig S4 ) shows also a highly reduced frequency of incorporation of nanowires into the tubes as well as a much lower mean size of these nanoparticles when the ferrocene concentration is reduced. Therefore, the amount of embedded nanoparticles is really low, and it is a great improvement for a one-step CCVD process. For 0.1wt.% ferrocene, only 2% of the length of the nanotube inner channel is occupied by iron nanowires, while it is 24% for 2.5wt.%. On average, it can be estimated that there is 1 nanowire every 2 µm at 0.1wt.% ferrocene while there is one nanowire every 0.4 µm at 2.5wt.% (See Table S2). An analysis of structural defects (corking) in the inner channel also shows a decrease in their occurrence for 0.1wt.% as compared to 2.5wt.%, but not in the same proportion as nanowires (Table S2). The iron content of VACNT forests is determined by TGA analysis under air (Fig. 5a) on samples taken in the same central area of the reactor as the ones used for the SEM and TEM analysis above. In single injection, the iron content varies greatly and linearly with the ferrocene concentration, from 0.5wt.% at 0.1wt.% ferrocene up to 9wt.% at 2.5wt.% ferrocene. This value of 0.5 wt% of iron in VACNT sample is a remarkably low value as compared to the literature (see Table 1 and references herein) and is even lower than that of 0.56 recently published in a paper related to a two-step floating CCVD process [START_REF] Kinoshita | Two step floating catalyst chemical vapor deposition including in situ fabrication of catalyst nanoparticles and carbon nanotube forest growth with low impurity level[END_REF]. It confirmed the great increase in the VACNT purity at low ferrocene concentration as observed by TEM above. In double injection, iron content variations are less pronounced as compared to single injection (Fig. 5a) due to the first injection sequence at 2.5wt.%. To verify that the variation in iron content with the ferrocene concentration is not due to a similar variation in iron chemical yield, we estimate the proportion of injected iron found in the final product. In single injection, the incorporated iron represents 40% of the injected iron at 0.1wt.% ferrocene and 50% at 2.5wt.%. Therefore, the incorporation rate of iron remains similar regardless of the ferrocene concentration and cannot be responsible for the high difference in iron content.

The catalytic yield (Fig. 5b) scales as the inverse of the ferrocene concentration for single Regarding the CNT diameter, the distribution does not change with the ferrocene concentration (Fig. 6a), with external and internal average diameters around 35 nm and 9 nm respectively.

Similar results are found in the case of double injection (Fig. S5). In terms of structural quality, the Raman spectra of the samples obtained with 0.1 and 2.5 w.t% ferrocene are compared in Fig. 6b (see Fig. S6 for double injection): G, D and D' bands at 1580 cm -1 , 1350 cm -1 and 1615 cm -1 respectively, as well as the 2D band at 2700 cm -1 are occurring. In multi-walled carbon nanotubes, the D band is associated with defects and disorder and is influenced by the curvature of the walls, while the G band is inherent to in plane tangential stretching of carboncarbon bonds. The ratio between the D and G band intensities (ID/IG) is commonly used as a qualitative measure of the defect density in defective tubes: the higher this ratio, the lower the crystalline level [START_REF] Pimenta | Studying disorder in graphite-based systems by Raman spectroscopy[END_REF][START_REF] Maultzsch | Raman characterization of boron-doped multiwalled carbon nanotubes[END_REF]. This ratio is dependent on the excitation wavelength. The analysis of the Raman spectra recorded at 514 nm on the samples obtained in the current study gives rise to a ID/IG ratio remaining similar whatever the injection mode and the ferrocene concentration.

This ratio is around 0.3 and the lowest value of 0.28 is obtained at 0.25wt.% ferrocene concentration in single injection (Table S1). Such values are low for multi-walled carbon nanotubes produced by CVD and are related to a low defect density as discussed by Meysami et al. [START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study[END_REF]. These values compare well with those reported in Table 1 obtained with similar Raman wavelengths (514 or 532 nm) for samples formed in similar conditions in terms of carbon precursors and synthesis temperatures (750-850°C) [START_REF] Castro | Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition[END_REF][START_REF] Mckee | Dimensional control of multi-walled carbon nanotubes in floating-catalyst CVD synthesis[END_REF][START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study[END_REF]. By contrast, they are much lower than those reported in the range [0.56-0.79] by Kinoshita et al. [START_REF] Kinoshita | Two step floating catalyst chemical vapor deposition including in situ fabrication of catalyst nanoparticles and carbon nanotube forest growth with low impurity level[END_REF] from synthesis at 700°C with acetylene as carbon source. However, it has been shown that the G band in CNT can be affected by defects [START_REF] Maultzsch | Raman characterization of boron-doped multiwalled carbon nanotubes[END_REF], therefore as the 2D band does not represent a disorder-induced mode, the relative intensity of the D and 2D bands (ID/I2D) is also used as a qualitative measure of the defect density. Kinoshita el al. [START_REF] Kinoshita | Two step floating catalyst chemical vapor deposition including in situ fabrication of catalyst nanoparticles and carbon nanotube forest growth with low impurity level[END_REF] found a ID/I2D ratio between 0.8 and 2. In the present work, this ratio is around 0.3 for almost all the ferrocene concentrations, apart for 2.5wt.% of ferrocene which gives rise to a ratio of 0.36. Such low ID/I2D ratio indicates a really good structural quality of the CNT forest that is well preserved and even slightly improved at low ferrocene concentration. This result corroborates the same trend observed by Cho et al. [START_REF] Cho | The influence of reagent stoichiometry on the yield and aspect ratio of acid-oxidised injection CVD-grown multi-walled carbon nanotubes[END_REF] and McKee et al. [START_REF] Mckee | Dimensional control of multi-walled carbon nanotubes in floating-catalyst CVD synthesis[END_REF] with a similar one-step aerosol-assisted process.

VACNT forest density

The mass per unit area of VACNTs collected on the reactor walls in the oven center (Fig. 7a) increases with ferrocene concentration for both single and double injection experiments, meaning that the chemical yield improves in both cases. In single injection, the areal mass is close to 2mg/cm 2 at 0.1wt.% and then increases gradually up to 4 mg/cm 2 . In double injection, the areal mass is already equal to 3 mg/cm 2 at 0.01wt.% ferrocene in the second injection and then rises up to more than 6 mg/cm 2 , with values systematically higher than in single injection.

The gap between the two curves remains approximately constant around 2.5 mg/cm 2 in excess in double injection as compared to single injection. This difference can be attributed to the effect of the first injection sequence with 2.5wt.% in the double injection mode. Reproducibility has been tested for experiments performed with ferrocene concentration of 0.1, 0.25 and 2.5 wt.% in single injection and 0.1, 0.25 and 0.5 wt.% in double injection. The observed deviations do not affect the general trends, particularly for single injection experiments (Fig. S6 in supplementary information).

From measurements of areal masses (Fig. 7a) and forest thicknesses (Fig. 2b and Fig. 3b), the forest density can be determined and increases with the ferrocene concentration, especially in single injection (Fig. 7b). Since nanotube diameters are similar whatever the ferrocene concentration, this density increase must be related to an increase in N, the number density of nanotubes (CNTs/cm 2 ). N can be estimated knowing the mean external diameter of the nanotubes and taking 1.8 g/cm 3 for the intrinsic carbon nanotube density as estimated by helium picnometry. N is found about ~5 10 9 CNT/cm 2 at low ferrocene concentration, but reaches ~1.7 10 10 CNT/cm 2 at 1wt.%. This result suggests that the number density of catalytic nanoparticles increases as well.

Oxidation resistance

The TGA mass-loss profiles are very dependent on the ferrocene concentration (Fig. 8a; see also Fig. S8). In single injection, the profile is strongly shifted towards the high temperatures when the ferrocene concentration decreases. The oxidation temperature (Tox), taken as the temperature of the peak in the TGA graph derivatives (Fig. 8b and Fig. S8), decreases quasilinearly from 685°C down to 550°C when ferrocene increases from 0.1 to 2.5wt.%. Since in single injection the iron content of the sample was previously found to be a linear function of the ferrocene concentration (Fig 5a), Tox appears to be a linear function of the iron content as well (Fig. 8c and Fig. S9). In the case of double injection, the TGA profile exhibits a remarkable shoulder on the high temperature side when the ferrocene concentration in the second injection sequence is lower than 0.5wt.%, so that two Tox (labelled Double1 and Double2) can be identified at low ferrocene concentrations (Fig. 8c andS9). The difference between both values is really significant (nearly 100°C) which points to the existence of two distinct components in the samples as discussed below.

Discussion

The present results show that the one-step aerosol-assisted CCVD process can be operated at very low ferrocene concentration (0.1 wt.%) using an Ar/H2 gas mixture as carrier gas. Using these conditions and a synthesis temperature of 800°C, we demonstrate that this one-step process enables the formation with a high catalytic yield (200) of VACNT forests exhibiting low impurity level (0.5 wt.% of Fe). Such low ferrocene concentration does not involve any reduction of CNT growth rate since it remains higher than 20 µm/min. Compared to the literature (Table 1), it is, to our knowledge, the lowest concentration of ferrocene reported so far for aerosol-assisted CCVD process leading to such growth rates and impressive catalytic efficiency, thus involving the formation of high purity VACNT forests. The growth of a VACNT forest becomes impossible only when the ferrocene concentration falls below 0.1wt.%.

When it increases up to 1wt.%, the growth rate decreases by a factor of 2, while the number density N is multiplied by more than 3 going from 5 10 9 to 1.7 10 10 cm -2 .

Regarding the mechanisms at play, hydrogen plays a significant role in the process of ferrocene decomposition. Indeed, our previous results obtained at 850°C with 2.5wt.% ferrocene [START_REF] Castro | The role of hydrogen in the aerosol-assisted chemical vapor deposition process in producing thin and densely packed vertically aligned carbon nanotubes[END_REF] have shown that including 30% of H2 in the Ar gas flow induces an increase of N from 5 10 9 to 2 10 10 CNT.cm -2 as well as a decrease of the mean CNT external diameter. These results were attributed to the hydrogen induced enhancement of the dissociation rate of ferrocene. As a consequence, there is a fast rise of the iron partial pressure in the reactor and a high level of supersaturation which affects the homogeneous nucleation process of the iron nanoparticles. A much higher number of smaller nuclei forms, which leads to a higher number density of catalytic nanoparticles deposited on the substrate and therefore to a higher density of growing CNTs. Such very efficient effect of hydrogen on the catalytic particles density is responsible for the remarkable decrease in the ferrocene concentration allowing to reach the threshold in density of CNTs needed to form a self-supporting forest structure [START_REF] Bedewy | Collective mechanism for the evolution and self-termination of vertically aligned carbon nanotube growth[END_REF]. Interestingly, in the present work at 0.1wt.% ferrocene in single injection mode, N is equal to 5 10 9 CNT.cm -2 , a value similar to the one obtained without hydrogen at 2.5wt.% ferrocene with the same onestep aerosol assisted CCVD process [START_REF] Landois | In situ time resolved wide angle X-ray diffraction study of nanotube carpet growth: Nature of catalyst particles and progressive nanotube alignment[END_REF]. This number allowing the vertical self-organization of the interacting CNTs is comparable to the value of about 2 10 9 CNTs/cm 2 reported by Bedewy et al. [START_REF] Bedewy | Collective mechanism for the evolution and self-termination of vertically aligned carbon nanotube growth[END_REF]. This threshold is obviously not reached below 0.1wt.%, explaining the absence of VACNT growth at such low concentration. This threshold was also not reached without hydrogen for ferrocene concentration below 1.25wt.%, explaining why no VACNTs were observed at 0.1 and 0.5wt.% by Castro et al. [START_REF] Castro | Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition[END_REF].

In double injection synthesis, the first injection sequence with 2.5wt.% ferrocene determines the density of catalytic nanoparticles. So N is higher from the beginning of the precursor injection and its evolution is less pronounced with the ferrocene concentration increase in the second injection sequence, where a ferrocene concentration as low as 0.05wt.% is sufficient to sustain the growth.

The existence of a clear threshold in the ferrocene concentration allowing the self-organization of VACNTs is responsible for the sharp increase in the growth rate with ferrocene concentration observed for single injection experiments (Fig. 3b). The slow decrease observed after the maximum (Fig. 3b), as also observed by others [START_REF] Ionescu | Hydrogen-free spray pyrolysis chemical vapor deposition method for the carbon nanotube growth: Parametric studies[END_REF][START_REF] Bai | Effects of temperature and catalyst concentration on the growth of aligned carbon nanotubes[END_REF], is generally attributed to an increase in VACNT density. Call et al. [START_REF] Call | The density factor in the synthesis of carbon nanotube forest by injection chemical vapor deposition[END_REF] show that ferrocene concentration dictates forest density but that forest thickness is limited by density presumably due to diffusion-limited precursor supply.

However, in our case, the forest thickness is found proportional to the synthesis duration at least up to 90 min even at 1.25 wt.% ferrocene where the forest density is above 10 10 CNT/cm 2 (Fig. S3). Indeed, the flow rate is too high and the average inter-tube distance too large (between 60 and 150 nm) to encounter feedstock resistance [START_REF] Xiang | Growth deceleration of vertically aligned carbon nanotube arrays: Catalyst deactivation or feedstock diffusion controlled?[END_REF]. Same conclusion was reported by Cho et al. [START_REF] Cho | The influence of reagent stoichiometry on the yield and aspect ratio of acid-oxidised injection CVD-grown multi-walled carbon nanotubes[END_REF] leading them to invoke, as Bai et al. [START_REF] Bai | Effects of temperature and catalyst concentration on the growth of aligned carbon nanotubes[END_REF], a weaker activity of the catalytic nanoparticles due to their size increasing with ferrocene concentration [START_REF] Jourdain | Current understanding of the growth of carbon nanotubes in catalytic chemical vapour deposition[END_REF]. However, in our study, the CNT diameter remains similar whatever the ferrocene concentration, which implies that the catalytic particle size remains also the same [START_REF] Jourdain | Current understanding of the growth of carbon nanotubes in catalytic chemical vapour deposition[END_REF]. In the present range of ferrocene concentration, this size is mainly governed by temperature, H2 content and nature of the substrate, all these parameters being constant here. On the other hand, when the number density of nanotubes increases, the amount of precursors available per nanotube decreases, in principle in inverse proportion to the density. This is not exactly what we are observing here since the mass per unit area that is the product of the two is not constant, but increases. Such result could be rationalized considering that at very low forest density, not all the carbon precursors impinging the substrate area around a given CNT (intertube distance is 150 nm) are able to interact with the catalyst particle, but their proportion increases when the intertube distance decreases. Such effect could explain the observed variation of growth rate.

Regarding the iron content of the VACNT forest, it increases linearly from 0.48 to 8.75 wt.% with the ferrocene concentration in single injection, and the atomic ratio Fe/C in the forest appears to be 10 times the Fe/C ratio in the precursors (Fig. S10). It means that the iron conversion is 10 times higher than the carbon conversion. It should be noted that the contribution of the catalyst nanoparticles themselves to the iron content is very low as is out of principle the case for any VACNT forest (see its estimation in Supplementary Data). The iron incorporated in the material is predominantly found as nanowires encapsulated in the inner channel as a consequence of the continuous refreshment of the catalyst nanoparticles from the iron-based nanoparticles continuously nucleated in the gas phase [START_REF] Landois | In situ time resolved wide angle X-ray diffraction study of nanotube carpet growth: Nature of catalyst particles and progressive nanotube alignment[END_REF] in the one-step aerosolassisted CCVD process. When the ferrocene concentration is decreased, the percentage of the CNT inner channel length occupied by the nanowires (Fig. S4 and Table S2) drastically decreases: it is estimated to be 10 times lower for the optimal concentration of 0.1wt.% ferrocene as compared to 2.5wt.%, which corresponds to an encapsulation frequency which is 3 times lower.. Since the CNT growth rate is two-fold higher at 0.1wt.% than at 2.5 wt.%, it means that the acceleration in the refreshment at high ferrocene concentration does not allow to increase the growth rate but only leads to an increase of the iron content.

Iron can be incorporated in the forest also as nanoparticles found outside the nanotubes. This contribution increases a lot when the ferrocene concentration increases, as observed by SEM and TEM (Fig. 4) and by others [START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study[END_REF][START_REF] Bai | Effects of temperature and catalyst concentration on the growth of aligned carbon nanotubes[END_REF]. Since TEM observations show that the amount of external nanoparticles is negligible in the sample obtained at 0.1wt.% ferrocene, then we can estimate from the TEM analysis of the inner channel (Table S2) that the iron inside the tubes represents about 2/3 of the total iron content for the sample formed with 2.5wt% ferrocene.

Therefore, external nanoparticles contain at most a third of the iron content. Their presence in large quantities is due to the large increase, with the ferrocene concentration, of the number density of iron nanoparticles in the precursor gas flow going towards the substrate through the carpet. To avoid the formation of such large proportion of by-products in the presence of H2, the ferrocene concentration must be kept lower than 1wt.%.

Regarding now the size and crystalline structure of the nanotubes, both TEM and Raman analysis show that their structure and diameter depend only slightly on the concentration of ferrocene in the range studied here. The ID/IG ratio of 0.3 indicates a good structural quality with a low defect density for MWCNTs as observed previously [START_REF] Castro | Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition[END_REF][START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study[END_REF]. That is why it is not possible to interpret the large variation in the oxidation temperature with the ferrocene concentration in terms of variation of the structural properties. Even considering the position of the G-band, which was found to correlate with Tox in the literature [START_REF] Santangelo | Evaluation of crystalline perfection degree of multi-walled carbon nanotubes: Correlations between thermal kinetic analysis and micro-Raman spectroscopy[END_REF], it is not possible to invoke such structure evolution, since here, the G-band position is very stable (Table S1).

Alternatively, the catalytic activity of the iron-based compounds inside the forests during the TGA was also considered in the literature since it is able to change the kinetic parameters of the oxidation [START_REF] Mckee | Dimensional control of multi-walled carbon nanotubes in floating-catalyst CVD synthesis[END_REF][START_REF] Meysami | Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study[END_REF][START_REF] Chiang | Purification and Characterization of Single-Wall Carbon Nanotubes[END_REF][START_REF] Leino | Low-temperature catalytic oxidation of multi-walled carbon nanotubes[END_REF][START_REF] Wang | Tailored cutting of carbon nanotubes and controlled dispersion of metal nanoparticles inside their channels[END_REF]. In the present case, this second interpretation is strongly supported by the strong correlation found between Tox and iron content in single injection synthesis (Fig. 8c). Also, the highest oxidation temperature (680°C) obtained for the sample with the lowest iron content is found very similar to the one of VACNT sample freed from iron after a heat-treatment at 2000°C [START_REF] Pinault | Carbon nanotubes produced by aerosol pyrolysis: growth mechanisms and postannealing effects[END_REF]. Moreover, in double injection, the catalytic activity of the ironbased compounds allows to understand directly the presence of two components in the TGA graphs. When the ferrocene concentration is very different between the two injection sequences, two layers are formed with very different iron contents and different oxidation temperatures.

The highest oxidation temperature is close to those obtained with single injection with a similar low ferrocene concentration. The large variation in oxidation temperature under dry air can be correlated to the iron content and the double peak observed for sample synthesized by double injection underlines the localized catalytic role of iron compounds in the oxidation reaction as mentioned in [START_REF] Wang | Tailored cutting of carbon nanotubes and controlled dispersion of metal nanoparticles inside their channels[END_REF]. It could be used to perform selective oxidation of the carpet top in order to suppress the layer where the CNTs are entangled and to open the tubes. Also, double injection experiments offer the possibility to control independently forest density and growth rate while limiting the iron content. The forest density is determined by the ferrocene concentration in the first injection sequence while the growth rate depends mostly on the ferrocene concentration in the second injection sequence.

Conclusion

Our results show that, in the aerosol-assisted CCVD process, the addition of H2 in the carrier gas results in a significant increase in the number density of catalytic nanoparticles, due to its strong influence on the decomposition kinetics of the ferrocene. As a result, the formation of a self-organized VACNT forest occurs at a much lower ferrocene concentration than without hydrogen and a pristine forest of high purity is formed with high catalytic yield at a performing growth rate. At 800°C, with 30% H2 in Ar, VACNT forest is formed with only 0.1wt.% ferrocene (Fe/C=0.007at%), a value 25 times lower than the usual one of 2.5% optimized without hydrogen. The iron content falls down to 0.5wt.%, so that the catalytic yield is strongly enhanced by a factor of 20, while the growth rate is improved by a factor of 2 and the crystalline structure remains of high quality. A forest density of 5 10 9 CNTs/cm 2 appears to be the threshold for the self-organization of the nanotubes to occur. This number density increases with the ferrocene concentration, due to the increase in the number density of catalytic nanoparticles. The iron content of the forest is directly proportional to the ferrocene concentration. When it decreases, the frequency of encapsulation of iron nanowires in the inner channel of the tubes decreases significantly and the oxidation resistance of the samples is greatly improved, with an increase in oxidation temperature equal to 100°C. Finally, double injection experiments appear to be an attractive approach in order to optimize independently the density and the growth rate of the forest, since the density is fixed by the initial ferrocene

Fig. 1 :

 1 Fig. 1: Experimental set-up used for the one-step aerosol-assisted CCVD process operated at atmospheric pressure. Insert: temperature profile at 800°C measured with a thermocouple moved along the quartz reactor.

Fig. 2 :

 2 Fig. 2: Double injection. (a): SEM images of the VACNT carpets grown on the central substrate (side-view) for increasing ferrocene concentration in the second long injection sequence. (b): mean carpet thickness and growth rate on the substrate (red dot) and on the reactor walls (blue triangle) as a function of ferrocene concentration. Dashed lines are added as eyes guide only. The thickness distribution was measured by SEM on the same samples with 20 measurement points for the CNTs on the walls of the reactor and 3 measurement points for the CNTs on the substrate.

Fig. 3 :

 3 Fig. 3: Single injection. (a): SEM images of the VACNT carpets grown on the central substrate for increasing ferrocene concentration. (b): mean carpet thickness and growth rate on the central substrate (red dot) and on the reactor walls (blue triangle) as a function of ferrocene concentration. Dashed lines are added as eyes guide only. The thickness distribution was measured by SEM on the same samples with 20 measurement points for

Fig. 4 :

 4 Fig. 4: Comparison in single injection between samples obtained with ferrocene concentration of 0.1 and 2.5wt.%. a) SEM image of 0.1%; b) SEM image of 2.5%; c) TEM image of 0.1%; and d) TEM image of 2.5%.

  injection. A very high and maximum catalytic yield about 200 (200g of carbon produced by iron gram) is obtained for the lowest concentration of ferrocene (0.1wt.% ) and then decreases rapidly down to 10 when the ferrocene concentration increases. For double injection, the catalytic yield is varying in a slighter range, between 16 and 35, for the whole ferrocene concentration range.

Fig. 5 :

 5 Fig. 5: Ferrocene concentration effect on a) iron content and b) catalytic yield, for single (full dot) and double (open dot) injection (in this last case, it is the ferrocene concentration of the second long injection sequence). Dashed lines are added as eyes guide only.

Fig. 6 :

 6 Fig. 6: Comparison in single injection between samples obtained with ferrocene concentration of 2.5 (red) and 0.1wt.% (blue), respectively. a) Nanotube external diameter distributions from the TEM analysis of more than 100 CNTs; b) Raman spectra acquired at 514 nm at the center of forest cross-sections.

Fig. 7 -

 7 Fig. 7 -Ferrocene concentration effect on a) mass per unit area of carbon collected on the reactor walls in the center zone, and b) measured mass density, for single (full dot) and double (open dot) injection (in this last case, it is the ferrocene concentration of the second long injection sequence). Dashed lines are added as eyes guide only.

Fig. 8 :

 8 Fig. 8: a) TGA graphs obtained at a temperature rate of 10°C/min and b) their derivatives for single injection samples (blue dashed lines labelled from S_0.1 to S_2.5 according to the ferrocene concentration) compared to the double injection with 0.01wt.% ferrocene in the second injection sequence (orange solid line labelled D_0.01). c) Oxidation temperature as a function of iron content. In double injection the label "Double2" corresponds to the second peak at high temperature when observed in the dTG graph. See also Fig. S8.

Table 1 : Lowest ferrocene concentrations giving rise to VACNT growth reported in the literature for the one-step aerosol-assisted CCVD synthesis. The most relevant experimental parameters are reported, as well as, the growth rate and iron content of the final product when available. Ferrocene concentrations in the precursor solution were converted to wt.%. Note that 1wt.% of ferrocene in toluene corresponds to a Fe/C ratio of 0.07at.%.

 1

		Ferrocene concentration (wt.%)	Carbon source	Carrier gas	T (°C)	Substrate	Growth rate (µm/min)	Iron content (wt.%)	Raman ID/IG  (nm)
	Bai 2005 [33]	0.58 1.14	xylene	Ar/H2 87/13	780	quartz	13.6 20.4	-	--
	Ionescu 2011 [32]	0.1 1	xylene	Ar	750	Sputtered Al Si wafer	0.4 4.7	-	0.49 785 nm 0.68 785 nm
	Cho 2012 [20]	1.5	toluene	Ar	760	quartz	2.5	1.9	0.33* 633 nm
									0.3
	McKee 2009 [19]	0.54	benzene	Ar	750	various	-	0.7 -3	measured at 1.75 wt% Fe
									514 nm
	Meysami 2013 [21]	1	toluene	Ar	800	quartz	16	2.5	0.29 532 nm
	Castro 2010 [15]	1.25 2.5	toluene	Ar	850	quartz	11 20	3.5 3	0.52* 532 nm 0.33* 532 nm
	Castro 2013 [34]	2.5	toluene	Ar/H2 80/20	850 800	quartz	29 23	5.3 6.2	--

* Area ratio

Acknowledgement

The authors would like to thank M. Paternostre (CEA Saclay), P.-E. Coulon (LSI, Ecole Polytechnique Palaiseau) and J. N. Rouzaud (ENS Paris) for access to TEM, HRTEM and Raman analysis facilities, respectively.

concentration in the first short injection sequence while the growth rate can be controlled by the concentration in the second injection sequence.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at


Julie Renoud

Suvasthika Indrajith

Alicja Domaracka

Patrick Rousseau

Patrick Moretto-Capelle

Bernd A Huber

Jean-Philippe Champeaux

Interaction of hydantoin with minority ions of solar wind: O 6+ and He 2+

The laboratory study of prebiotic molecules interacting with solar wind ions is essential to under-stand their role in the emergence of life in the complex context of astrochemistry of circumstellar environments. In this work, we present the first study of the interaction of hydantoin (C 3 N 2 O 2 H 4 , 100 amu) with Solar wind low energy multiply charged ions: O 6+ at 30 keV and He 2+ at 8 keV. The fragmentation mass spectra as well as correlation maps resulting from the collision are presented and discussed in this paper. Prompt and delayed dissosiation from metastable states of the ion-ized molelcule have been observed and corresponding life times mesured. Experimental results are completed by DFT numerical calculations of the energy and structures of the initial state of the molecule (z = 0,1,2,3) that show that the molecule can only support 2 charges before spon-taneously dissociate. Calculations also demonstrate that the hydantoin's ring opens after double ionisation of the molecule wich may enhance its reactivity in the context of biological molecule formation in crimcum stellar environnement. For the major experimentally observed fragmenta-tion pathways (like 44 amu / 56 amu dissociation), Internal Reaction Coordinate calculation was perfomed pointing out the important role of proton transfert in the fragmentation processes.

Introduction

Since the development of space telescopes and the rise of spatial infrared spectroscopy 1 2 3 4 , numerous studies have noted the presence of neutral or cationic complex carbonaceous molecules in interstellar and circumstellar media, recorded in data bases such as 5 6 7 . Some are well identified by their spectral signatures like for example C + or C + 8 9 10 detected in the young In this context, we have focused on the 2,4-imidazolidinedione named hydantoin (C 3 N 2 O 2 H 4 , 100 amu). This prebiotic molecule has been detected in the Murchison and Yamato [START_REF] Shimoyama | Origins of life and evolution of the biosphere[END_REF] meteorites and was synthesized in the laboratory in interstellar ice analogues irradiated by UV photons [START_REF] Marcellus | [END_REF] . Even if hydantoin have not been yet detected directly in nebulae clouds or cometa ionic tails, the simplest amino acid glycine was clearly identified in 2016 60 70 planetary nebula TC1 and reflexion nebulae NGC7023, others are strongly suspected to be present in many nebulae as the family of Polycyclic Aromatic Hydrocarbons 11 12 13 but also organic species such as ethylene oxide 14 . The presence and richness of such molecular complexes reveal the possible existence of a physicochemistry in astrophysical environments related to their interactions with the ionizing radiation of the surrounding stars, either UV photons or stellar winds for circumstellar environments 15 . Organic and pre-biotic molecules have also been detected in several meteorites 16 17 18 . The presence of prebiotic molecules in the circumstellar environment raises the question of their interaction with solar wind particles and the role of theses interactions in the emergence of life. in the comet 67P/Churyomov-Gerasimenko 21 ejecta and calculations showed 22 that hydantoin is a possible precursor of the amino acid glycine after hydrolysis. In the present work, we have looked at the collision induced dissociation of isolated neutral hydantoin by solar wind minority ions O 6+ and He 2+ at 30 keV and 8 keV kinetic energy respectively which correspond to an average solar wind speed of around 600 km.s -1 23 . Obtained mass spectra are compared and discussed. Experimental results are completed by quantum chemistry calculations, for the singly and doubly charged hydantoin cation, in order to investigate fragmentation pathway for the most abundant fragments.

Experimental methods

The experiments were performed at the low-energy ion beam facility ARIBE of GANIL in Caen (France) with a crossed-beam device. The experimental setup is described in details elsewhere 24 , thus, only a brief description is given here. Low-energy ions are produced in an electron cyclotron resonance ion source. They are extracted, mass selected, pulsed (pulse length 1 µs) and transported to the experimental area, where they cross a molecular effusive beam evaporated from an oven device. After collimation, the different beam intensities are 250 pA for 8 keV 3 He 2+ and 200-400 pA for 30 keV 16 O 6+ . After the interaction, cationic products are orthogonally extracted into a modified Wiley-McLaren linear time-of-flight TOF mass spectrometer 25 . After passing the 1m long drift region the mass-over-charge analyzed cations are strongly post-accelerated before impacting a conversion plate. Emitted secondary electrons are guided by a weak magnetic field towards a channel plate detector. In this way, high detection efficiency is obtained independently of the ion mass. The arrival times are digitized with 1 ns resolution in an event-by-event mode (FAST ComTec P7888). An event corresponds to one ion collision. Using a low intensity ion beam allows to obtain conditions where the number of charged particles produced within one ion pulse is negligibly small, thus several fragments detected per ion pulse are due to a single collision by event. Thus, each event is character-C ( 2) O ( 2) N (1) H). Proton transfer from N to O atoms and from CH 2 to CO may gives the lactims and enol tautomer, respectively. In order to support this hypothesis, we have performed DFT calculations of these protomers with the same level of theory as used for the ground state . For each tautomer, we have evaluated the corresponding adiabatic energy ∆E as well as the activation barrier energy AE, referred to the neutral form (Figure 1). Results are presented in Figure 2, energies are given relatively to the ground state.

ized by the number of detected fragments and their TOF values in such a way that the correlation between them can be studied 26 . The molecular beam was produced by evaporation of a commercial powder of hydantoin 98% purity purchased at Sigma-Aldrich in a heated oven. The oven temperature was kept at 393K to ensure sufficient vapor density and avoid thermal decomposition.

3 Quantum chemistry calculations of initials molecular structures The neutral form of hydantoin C 3 N 2 O 2 H 4 molecule in its fundamental state is presented in Figure 1:

C5 N1 O4 C4 C2 N3 O2
Fig. 1 Neutral form of Hyndatoin in the ground state (labels from 27 ).

Geometry was optimized with the GAMESS code using the density functional theory (DFT), with the B3LYP functional and the 6-311G or 6-31G(d,p) basis set.

In our experimental setup, the temperature in the oven used to produce gas phase molecules is 393 K. If we suppose thermal equilibrium, this temperature corresponds to an internal energy given by: Activation energy (AE) barriers corresponding to the transition state of theses tautomers are all more than 2 eV above the neutral form (see Figure 2). Thus, we can reasonably assume that proton transfers cannot be produced by the heating of the molecule during its sublimation but may only occur consecutively to the interaction with O 6+ and He 2+ ions.

Molecular excitation: Reachable charge states, energies and geometries

Vertical and adiabatic ionization energies relative to the ground state of the neutral molecule were calculated with the same functional as for the initial geometries calculation but using two basis sets: 6-311G and 6-31G(d,p). Results are given in Table 1. Eth = (3N -6)kBT

(1) With N = 11 (number of atoms) and kB = 8.617 10 -5 eV.K -1 , this internal energy is then equal to 0.914 eV.

For such thermal energy we may expect that hydantoin molecule can form several tautomers during the heating of the powder in the oven. The ground state presents a keto group (C (4) O (4) C [START_REF] Houck | [END_REF] H 2 ) and two lactam groups (C (2) O ( 2) N (3) For the 6-311G base, calculations are consistent with previous Bacchus-Montabonel 28 results at 9.91 eV and 9.60 eV for the vertical and adiabatic ionisation respectively. The 6-31G(d,p) base gives lower energies than 6-311G basis set except for the hydantoin 3+ . The difference between bases increases in absolute scale with the charge state from δ E=0.27eV to 0.89 eV for vertical transitions. Calculation of the adiabatically relaxed Hyd 3+ molecule does not converge using the DFT method. A stable geometry has been found using MP2 level of theory at 44.14 eV above neutral hydantoin.

The relaxed geometries of hydantoin obtained after Franck-Condon ionization from the ground state of the neutral molecule for the three reachable charge states are presented in Figure 3. The hydantoin ring remains closed for z=1 but not for higher open by breaking the C (4) -C [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] bond. Calculations also show that the C [START_REF] Houck | [END_REF] -N [START_REF] De Graauw | Astronomy and Astrophysics[END_REF] bond can easily rotate forming quasi energetically degenerated rotamers. This strong structural modification leads to a large energy difference between vertical and adiabatic states (4 eV in case of the dication and 6.7 eV in case of the trication (see ular orbitals. Non radiative relaxation like fragmentation and tauto-merisation can then occur: 1). In this case, one may ask if the adiabatic states can be reached without any strong alteration. Dynamical Reaction Coor-100 amu zi + 0 dinates (DRC) calculations were then performed, initialised from the vertical geometry of the Hyd 2+ and Hyd 3+ ionized molecule.

C 3 N 2 O 2 H 4 + X n+ X (n-k)+ + (C 3 N 2 O 2 H k+ ) * (2) Table
With ∑ z = k. The A qi + {A i } + {B j } (3)
i

{ i } cations produced in the interaction
Results are presented in Figure 4. For the dication (a) the ring open and the opened structure oscillates without breaking (calculation have been performed up to 10ps and no breaking occurs). Inversely, the trication (b) breaks into two charged fragments, namely 71 2+ and 29 + . Therefore, theoretically, it should be possible to detect unfragmented Hyd 2+ (m/z=50) but not Hyd 3+ (m/z=33.3).

Experimental results and discussion

4.1 Dissociation mass spectra of hydantoin by stellar wind minority ions If emitted in the gas phase in a circumstellar environment, the hydantoin molecule will interact with solar wind ions. From this interaction it can be multiply ionized due to electron capture and electronically excited if electron capture occurs from inner molec-are mass/charge analysed by time of flight mass spectroscopy (see section 2). The corresponding spectra are presented in Figure 5, for both He 2+ at 8 keV and O 6+ at 30 keV.

The peak structure at m/z=100 corresponds to unfragmented parent hydantoin molecule labelled Hyd 1+ in Figure 5. In both spectra, we observe a very weak signal of unfragmented Hyd 2+ (m/z=50), with branching ratio 0.04% and 0.03% for He 2+ and O 6+ , respectively (see inset in Figure 5, corresponding to a zoom on the m/z=50 peak structure in the O 6+ mass spectra). The ratio is quite low but it shows the existence of the stable hydantoin dictation at microsecond time scale corresponding to the time of flight duration. We do not observe in the mass spectrum Hyd 3+ which would correspond to a peak structure at m/z=33.3. This indicates that the molecule dissociates easily when multiply charged. This is consistent with DRC calculations (see section 3), where we observe the possibility of non dissociation for Hyd 2+ (with a strong structural change) and fragmentation for Hyd 3+ . where SA i (τ) and Stot (τ) are the number of events in the time interval dτ, for the peak structure corresponding to A zi + fragment (between times t1 and t2) and for the total spectrum (between the spectrum time limits). They are given in Table 2 for the principal peak structures as well as relative intensity of each fragment compare to that of the unfragmented Hyd 1+ and possible fragment O (4) or N [START_REF] De Graauw | Astronomy and Astrophysics[END_REF] to O [START_REF] Houck | [END_REF] , indicated by 3b and 3a in Figure 6 2+ collisions. This can be attributed to higher instability due to higher molecular charge state produced in the collision with O 6+ .

In terms of formed cations, both mass spectra for He 2+ and O 6+ are very similar with main fragmentation channels leading to m/z= 28, 29 and 72 cations (see Table 2). The formation of cations at m/z=72 implies the loss of a neutral mass 28 which can correspond to the fragment C [START_REF] Houck | [END_REF] O [START_REF] Houck | [END_REF] or C (4) O (4) (1 and 2 in Figure 5, with no molecular rearrangement) or the fragment CNH 2 including a proton transfer (from C [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] A prompt emission is dominant but for both He 2+ and O 6+ spectra, we observe above peak at m/z=72 a decreasing tail to- Singly charged hydantoin cation (vertical and adiabatic) ward higher time of flight as shown in Figure 7.

dissociation. Only the emission of neutral C (4) O (4) via the opening of the C (4) -N (3) bond of hydantion ring followed by C [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] -C (4) dissociation give rise to a transition state with an activation barrier compatible with the observed meta-stability. For this transition state, we performed an intrinsic reaction coordinate (IRC) calculation with the same level of theory as mentioned before (DFT B3LYP 6-31G(d,p)) to verify the connexion between minima. The corresponding fragmentation path is presented in Figure 8. The transition state is found at 1.86 eV above the adiabatic relaxed geometry of the Hyd 1+ molecule. The dissociated state is only 0.32 eV lower than the transition state. This tail is due to the delayed fragmentation of the hydantoin cation in the extraction zone of the mass spectrometer. The end of the decay tail correspond to the end of the extraction zone of the mass spectrometer (electric field = 24.4 kV.m -1 , length = 11 cm). Knowing the above decay process and the geometric and electronic configuration of the mass spectrometer, we can associate to each time of flight in the tail the corresponding time of residence of the unfragmented hydantoin cation in the mass spectrometer, that is to say its decay time. Zoom in Figure 7 shows the intensity in the tail as a function of the calculated decay time in the case of He 2+ projectile. We observe an exponential decay e -T/τ which leads to a lifetime for the hydantoin cation in the case of He 2+ : Fig. 8 Calculated two body fragmentation path leading to the formation of 72 amu cation via the loss of neutral C (4) O (4) . Energies levels are referenced against the neutral hydantoin molecule.

The observed differences between the Hyd 1+ life time in the case of O 6+ at 30keV and He 2+ at 8 keV can result from differences in internal energy deposited by both projectiles in the molecule. Indeed, considering Transition State Theory [START_REF] Gilbert | Theory of unimolecular and recombination reactions[END_REF] , the lifetime τ (inverse of the unimolecular reaction rate k) can be express as a function of temperature T following:

τ He 2+ = (1.49 ± 0.06) µs (7) 1 1 QT S (T ) ∆E k = = k T e - And in the case of O 6+ : τ h B Qinit (T ) k B T (9)
τ O 6+ = (2.30 ± 0.13) µs

A similar delayed fragmentation was observed in several mulecules after interaction with multiply charged low energy ions 29 30 31 32 .

In order to investigate the origin of this meta-stability, we numerically looked at a possible reaction scheme for the emission where QTS(T ) and Qinit (T ) are the partition functions of the transition state and initial state respectively. ∆E is the activation barrier (energy between the optimized hydantoin singly charged cation and the transition state). For a molecule with N atoms, total partition functions are given by: QTOT = QvibQrot with vibrational and rotational partition functions respectively, defined as:

1 of neutral mass 28 through a transition state. Calculations were performed in order to look for imaginary vibrational frequencies.

We found that the emission of neutral CNH 2 when formed after proton transfert occurs easily but does not go through a transi-

Q vib = ∏ hνi 1 -e -k B T ( 2kB T \ 1/2 ( πIxIyIz ) 1/2 Q rot = (10)
(11) tion state which, considering Transition State Theory, forbids long meta-stability possibilities. Then, the CNH 2 fragment, if emitted, can only contribute to the prompt 72 amu peak signal.

After multiple numerical tentatives of neutral C (2) O [START_REF] Houck | [END_REF] emission via the opening of the C [START_REF] Houck | [END_REF] -N [START_REF] De Graauw | Astronomy and Astrophysics[END_REF] or C [START_REF] Houck | [END_REF] -N (3) bonds, the Hyd 1+ molecular ring always returns to the adiabatic geometry without

h ¯ 2 σ
Vibrational frequencies νi for each mode and moments of inertia in each direction Ix, Iy and Iz are given by GAMESS numerical calculation, for both initial and transition state. Finding the temperature which solves eq. 9 for each lifetime gives the internal energy U = (3N -6)kBT of the molecule: 2.36 eV for O 6+ and

amu

End of extraction zone 2.42eV for He 2+ . Even if this difference between internal energies are small compared to the difference between the lifetimes (due to the exponential parts of eq. 9), this is consistent with a simple Classical Over-the-Barrier model (COB) [START_REF] Bárány | [END_REF] . This model supposes that electron transfer from a molecule to the projectile can occur when the maximum of potential barrier created by the two point charges is lower or equal to the Stark shifted ionisation potential of the molecule. This condition leads to a minimal radius Rn to capture the n th electron given by: 2(q + n -1) 1/2 n 1/2 + n that in case of CO, a fragmentation in 72 + + 28 is energetically lower (1.9 eV and 2.1 eV above Hyd 1+ cation for C (4) O (4) and C (2) O [START_REF] Houck | [END_REF] respectively) than a fragmentation in 72 + 28 + (6.9 eV and 8.1 eV above Hyd 1+ cation for C (4) O (4) and C (2) O [START_REF] Houck | [END_REF] respectively). In case of both CHNH and CH 2 N, the fragmentation leading to 72 + 28 + is energetically lower (5.2 eV and 6.7 eV above Hyd 1+ tautomer cation for CHNH and CH 2 N respectively) and occurs without transition state (exept the proton transfer in case of CHNH). Fragmentation in 72 + + 28 was found to be 12.9 eV and 10.7 eV above Hyd 1+ tautomer cation for CHNH and CH 2 N respectively. These results, displayed in Table 4, indicate that Rn = I n * (12) fragments with m/z = 28 coming from fragmentation of the hy-Where n is the ionisation state of the molecule, q is the initial charge of the projectile (6 for O and 2 for He) and I n * =

In + (qn + 1)/r is the n th Stark shifted ionisation potential of the molecule (In is the n th non-shifted ionisation potential and r is the distance between target and projectile). Calculated Rn for hydantoin molecule and the two projectiles are given in Table 3. dantoin monocation are more likely CHNH + or CH 2 N + than CO + . Another way to form m/z = 28 fragment is a multistep fragmentation, as a CO or CHNH group is often found at the end of higher mass fragments (e.g.: 57 amu: N [START_REF] De Graauw | Astronomy and Astrophysics[END_REF] H-C [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] H2-C (4) O (4) or C [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] H2-N [START_REF] De Graauw | Astronomy and Astrophysics[END_REF] H-C (2) O [START_REF] Houck | [END_REF] , 44 amu: N (3) H-C (4) O (4) or N (3) H-C (2) O [START_REF] Houck | [END_REF] ). As discussed later, the 28 amu fragment may also be formed via direct or multistep fragmentation of multiply charged ions (N [START_REF] De Graauw | Astronomy and Astrophysics[END_REF] H-C [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] H2 (29 amu) and C (4) O (4) (28 amu) are localized at the extremities of the doubly ionized hydantoin (see Figure 3b)).

n Rn (a0) Rn (a0)

+ 28 + C (4) O (4)
1.9 6.9

C (2) O (2)
2.1 8.1 CH NH 12.9 5.2

Thus, the COB model predicts that O 6+ will be able to capture more electrons from the target molecule and as the radius of cap-CH2 N 10.7 6.7

ture is larger than for He 2+ , less energy will be transfered to the molecule.

Cationic fragments m/z=28 and m/z=29

The fragment with m/z = 28 is one of the predominant fragments for both He 2+ and O 6+ , leading to neutral undetected fragments.

A natural way to produce m/q = 28 would be the loss of C (4) O (4) (Fragment 1 in Figure 9a) or C (2) O (2) (Fragment 2 in Figure 9a). Taking into account the possible proton transfer from C [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] to O (4) or N [START_REF] De Graauw | Astronomy and Astrophysics[END_REF] to O [START_REF] Houck | [END_REF] (Figure 2a and 2c repectively), the peak at 28 amu may also correspond to CHNH (Fragment 3b in Figure 9a)

The fragment with m/z=29 is identified as C (2) O (2) H (proton transfer from N [START_REF] De Graauw | Astronomy and Astrophysics[END_REF] to O [START_REF] Houck | [END_REF] or N (3) to O [START_REF] Houck | [END_REF] , see 2a and 2b respectively in Figure 9b), C (4) O (4) H (proton transfer from N (3) to O (4) or C [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] to O (4) see 1a and 1b respectively in Figure 9b) or CH 2 NH with no proton transfer (see 3 in Figure 9b). Calculated dissociation limits relative to the Hyd + cation (stable or totaumer), are presented in Table 5. The only case where a fragmentation 71 + 29 + is energetically advantageous corresponds to the loss of C (2) O (2) H. However, energy differences are significantly lower than for the 28 amu fragment. This indicates that + or CH 2 N (Fragment 3a in Figure 9a) cation, respectively. part of the peak structure at m/z =29 coming from Hyd frag-

3b H H C5 O4 4 N 1 H 3a 1a H H C5 O4 H 1b C4 N 1
mentation is preferentially due to the C (2) O [START_REF] Houck | [END_REF] H ion but may also contain C (4) O (4) H and CH 2 NH + ions. As for the 28 amu fragment, the 29 amu may also be produced by delayed fragmentation or dication fragmentation.

Analysis of the main dissociation path leading to di-

1 N3 C2 N3 C2 H O2 2a
cation and correlated singly charged cations Interaction of neutral hydantoin with Solar wind ions may also

H (a) 2 O 2 2b (b)
lead to dicationic Hyd 2+ ion (see section 3) and dicationic fragments A 2+ . Fragmentation of these dications may contribute to Inclusive mass spectra (i.e fragments from the Hyd k+ molecule whatever the initial charge k is) reveals for both ions some small peaks at m/z=27.5, 28.5 and 35.5(branching ratio 0.04, 0.05, The most intense correlation corresponds to A + /B + = (43 ± 1) + / (28 ± 1) + fragments for both solar wind ions with an important In case of fragmentation with charge separation, coincidence measurements of A + /B + charged fragments have been performed branching ratio difference between O 6+ and He 2+ for 28 + /43 + for each collision event. The Table 6 summarizes the branching ratio (normalized to the whole intensity of the correlated spectrum) of the most intense A + /B + coincidences experimentally observed after double ionisation of hydantoin molecule. The 71 2+ (m/q=35.5 amu) fragment can be attributed to O (4) C (4) N (3) HC (2) O (2) 2+ , via the loss of neutral N (2) HC [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] H2 (29 amu) from the initial Hyd 2+ ion as shown in Figure 12a. DFT (B3Lyp 6-31G(d,p)) geometry optimization of the 71 2+ dication shows that a stable symmetric structure exists (see Figure 12b). No correlation involving 71 2+ /B + were observed but intense correlation 28 + /43 + could be linked to the observed 71 2+ dication. primarily formed 71 2+ into CO + (28 amu) and HNCO + (43 amu).

The corresponding calculated reaction pathway is presented in Figure 13.

intensity of 55 2+ compared to 57 2+ can be related to the higher order process. The loss of one hydrogen should be more probable and conducts to the formation of 56 2+ , unfortunately, it cannot be Figure 10 shows that the branching ratio of 71 2+ is higher in case of O 6+ than He 2+ . The lack of 71 2+ in case of He 2+ may suggest that it dissociate more easily in 28 + + 43 + . In this case, an increasing of peak 28 amu should be observed in the He 2+ spectrum, which is not the case (see Figure 10), but this contribution is certainly overlapped by the fragmentation after single capture.

Another way to form 28 + /43 + correlation would be via initial charge separation: sociate in 28 + + (28 ± 1) + , leading to 28 + /(28 ± 1) + correlations, and participating to 28 amu peak structure. Indeed, a weak signal is observed in the O 6+ correlation map, but not enough data were recorded for He 2+ to observe this correlation island. Surprisingly, this rather intense process implies a proton transfer in the Hyd 2+ molecule before the two body fragmentation is able to form the masses 44 and 56 amu. We then examined a reaction scheme that could lead to this dissociation using the DFT calculation for geometric optimizations and internal reaction coordinate calculations in order to connect the different geometries of each state involved in the dynamics of the dissociation. Results of these calculations are presented Figure 15. A low correlation signal is observed for 43 + /57 + masses, corresponding to this charge separation (see Figure 11 The 57 2+ fragment, arising from Hyd 2+ fragmentation, can correspond to C 2 NOH 3 (without rearrangement, see Figure 14a) and a stable structure for the dication have been found (see Figure 14b).

The fragment 55 2+ corresponds to the loss of two hydrogen As discussed in 3.2, the vertical dication geometry relaxes by opening the C (4) C [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] bond, to form the (a) geometry in Figure 16. A rotamer is then formed via a low transition state (not indicated atoms (from CH 2 group) from the 57 2+ fragment. The smaller in Figure 16), by rotating N [START_REF] De Graauw | Astronomy and Astrophysics[END_REF] H-C [START_REF]Interstellar Molecules -The Cosmic Ice Laboratory -Sciences and Exploration Directorate -NASA's Goddard Space Flight Center[END_REF] H (2) group . This stable rota- tional isomer (b) is 0.2 eV below stable dication. From this Hyd 2+ geometry, a 2 eV transition state (c) leads to a proton transfer from N [START_REF] De Graauw | Astronomy and Astrophysics[END_REF] to O (4) and immediate dissociation in 56 + + 44 + , see Figure 16.

Conclusion

In this work, we report on the fragmentation of the prebiotic molecule hydantoin (C 3 N 2 O 2 H 4 , 100 amu) after collisions with multiply charged minority ions of Solar wind: He 2+ at 8kev and O 6+ at 30keV. Experimental results, obtained at Ganil, are supported by numerical calculations. For both projectiles, major fragments were found at 28, 43 and 72 amu, identified as CNH + , CNOH + and C 2 N 2 OH + , respectively. The role of proton transfer was clearly established in the case of two body dication fragmentation 44 + /56 + . Due to the very low amount of unfragmented Hyd 2+ parent dication and the absence of unfragmented Hyd 3+ , we conclude, in consistence with numerical calculations, that the hydantoin molecule does not resist to charge. This fragility makes the transportation of hydantoin from a circumstellar environment to Earth in gas form in comet outgassing difficult. The fact that the hydantoin geometry is opening when doubly charged implies a high reactivity of the hydantoin dication and makes it interesting for physico-chemistry of circumstellar media, even if no fragments involved in the synthetising of the amino acid glycine have been found in mass spectra. In the following, we would like to study the fragmentation of the hydantoin molecule induced by interacting with majority components of Solar wind: protons and electrons.

a

  Laboratoire Collisions Agrégats et Réactivité, UMR 5589-CNRS Université Paul Sabatier Toulouse III, 118 Route de Narbonne, Bat 3R1B4, 31062 Toulouse Cedex 9, France. b Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, CIMAP, 14000 Caen, France.

3. 1

 1 Initial molecular neutral structures (a) ∆E=1.05 eV AE = 2.77 eV (c) ∆E=1.2 eV AE = 2.75 eV (b) ∆E=1.08 eV AE = 2.79 eV (d) ∆E=1.2 eV AE = 3.61 eV

Fig. 2

 2 Fig. 2 Calculated geometries and energies of the tautomer form of the neutral hydantoin molecule. AE = activation barrier, ∆E = adiabatic energy.

Fig. 3

 3 Fig. 3 Hyndatoin relaxed geometries for the different charge states of the molecule.

Fig. 4

 4 Fig. 4 DRC calculations (B3LYP, 6-31G(d,p)) from the vertical geometry

2

  Mass to charge ratio (m/z) of fragment A z+

2

 2 Analysis of the main dissociation paths leading to mono cations 4.2.1 Fragment m/z=72

  to

Fig. 6

 6 Fig. 6 Formation of m/z = 72 fragment via the loss of neutral CO (1 and 2) or neutral CNH2 (3a and 3b).

(

  Time of flight of fragment A z+ (ns)

Fig. 7

 7 Fig. 7 Intensity distribution on the mass range around m/z =72 for He 2+ projectile. The inset correspond to the delayed tail, fitted by an exponential. In this zoom, the time on abscissa axis delayed fragmentation time of the hydantoin molecule.

Fig. 9

 9 Fig. 9 Location of fragment m/q = 28 (a) and fragment m/q = 29 (b) comming from Hyd 1+ cation.Energy calculations of the dissociation energies, have shown

Fig. 10

 10 Fig. 10 Zoom on the inclusive spectra arising from the fragmentation of charged hydantoin molecule after collision with He 2+ at 8 keV and O 6+ at 30 keV. Peaks corresponding to doubly charged fragments are labelled by stars.

Fig. 11

 11 Fig. 11 Map of the correlation between A + and B + fragments, corresponding to the fragmentation of the hydantoin dication after collision with He 2+ at 8 keV (a) and O 6+ at 30 keV (b).

4. 3 . 1

 31 Dication A 2+ = 71 2+

4. 3 Fig. 12 (

 312 Fig. 12 (a) Localisation of the 71 amu fragment on the hydantoin molecule dication (b) Optimized geometry (numerical calculation) of the 71 2+ fragment.

Fig. 14 (

 14 Fig. 14 (a) Localisation of the 57 amu fragment on the hydantoin molecule dication (b) Optimized geometry (numerical calculation) of the 57 2+ fragment.

Fig. 13

 13 Fig. 13 Calculated fragmentation path from 71 2+ to 28 + + 43 + . Energies levels are in eV and referenced against 71 2+ optimized geometry.

4. 3 . 4

 34 Two body fragmentation: 44 + /56 + correlation One of the most intense A + + correlation with both projectile corresponds to a two body fragmentation of Hyd 2+ (50 amu) forming two mono-cations 44 + amu and 56 + amu (upper right corner of Figure 11 a) and b)).

  )

  .

4. 3 . 3

 33 Dications A 2+ = 55 2+ and A 2+ = 57 2+

Fig. 15

 15 Fig. 15 Calculated fragmentation path corresponding to the two body Hyd 2+ to 44 + + 56 + . Energies levels are in eV and referenced against neutral hydantoin optimized geometry.

Fig. 16

 16 Fig. 16 Two body fragmentation Hyd 2+ (100 2+ ) → 56 + + 44 + , via a proton transfer.

Table 1

 1 Calculated (B3LYP) ionisation energies of hydantoin

	z	Vertical (eV)	Adiabatic (eV)

Table 2

 2 Relative intensities and identification of the cationic products formed by collision of neutral hydantoin with He 2+ at 8 keV and O 6+ at 30 keV 6+ at 30 keV. The inset is a zoom on the m/z=50 peak structure,

	m/z	He 2+		O 6+		Possible Id.
		µi (%)	Rel.	µi (%)	Rel.
	12 13	2,4 0,6	6.2 1.8	3,6 0,4	17.4 1.0	C + C +
	14	2,3	6.0	3,0	13.8	N + /CH +
	15	0,6	1.8	0,6	0.9	N + /NH +
	16	2,5	6.5	3,1	15.4	O +
	24	1,1	2.8	0,9	4.3	C +
	25	0,3	1.0	0,2	1.0	C2H +
	26	2,7	6.9	2,0	9.8	CN + /C2H +
	27	3,1	7.9	2,4	11.4	CNH +
	27.5	0.04	0.1	x	x	C2NOH 2+
	28	8,2	20.5	20,5	93.9	CNH + /CO
	28.5 29	0.05 7,1	0.1 18.1	0.3 7,1	1.4 33.7	C2NOH 2+ CNH + /COH +
	30	0,6	1.7	0,6	3.1	CH2O + /NCH +
	35.5	0.02	0.05	0.14	0.6	C2N2OH 2+ /C NO H 2+
	38	1,3	3.3	1,0	5.2	4 C2N + 2	2 2
	39 40	0,4 0,4	1.0 1.0	0,3 0,4	1.4 2.0	C2NH + C NH +

Fig. 5 Inclusive mass spectra of cations arising from the fragmentation of neutral hydantoin molecule C3N2O2H4 after collision with He 2+ at 8 keV + /C OH corresponding to the unfragmented Hyd 2+ parent ion (intensity is in log

Table 3

 3 Radii of n th electron capture from hydantoin to He 2+ and O 6+ , classical over the barrier calculation

	He 2+	O 6+

Table 4

 4 Dissociation limits for fragmentations Hyd + → 72 (+) + 28 (+) . En-

	1	10.6	16.3	ergies are in eV and referenced against Hyd + stable or corresponding
	2	4.8	8.3	proton transfer tautomer
	3	0 /	5.5
	4 5	0 / 0 /	3.6 2.6	72 + + 28	72
	6	0 /	1.8

Table 5

 5 Dissociation limits for fragmentations Hyd + → 71 (+) + 29(+) . Energies are in eV and referenced against Hyd + stable or corresponding proton transfer tautomer

	Proton transfert	71 + + 29	71 + 29 +
		CH2NH
		3.6	4.6
	N (3) → O (4)	C (4) O (4) H

Table 6

 6 Branching ratio of the major correlation of two mono cations after double ionisation of neutral hydantoin molecule by He 2+ at 8 keV and O 6+ at 30 keV correlation at 20% and 8%, respectively.

Acknowledgements

We would like to thank the EMIE research group (GDR 4479) for participating in the measurements campaign carried out on Cimap's facilities in Caen, France. We also thank the selection committee of Cimap for accepting our proposal and all Cimap staff for their welcome and support in carrying out these experiments.

Notes and references

Conflicts of interest

There are no conflicts to declare.