Thomas Cottineau

Mireille Richard-Plouet

Annabelle Rouet

Eric Puzenat

Hari Sutrisno

Yves Piffard

Pierre-Emmanuel Petit

Luc Brohan
email: luc.brohan@cnrs-imn.fr

Photosensitive Titanium Oxo-polymers: Synthesis and Structural Characterization

TiOCl 2 acid solution hydrolyzes N,N-dimethylformamide (DMF) at room temperature, leading to the formation of dimethylammonium chloride (DMACl) and methanoic acid. During hydrolysis, a slow consumption of hydrochloric acid enables a gentle control over the inorganic polymerization. This onepot process allows the preparation of novel hybrid organic-inorganic sols and gels based on titanium oxide. The hydrolytic inorganic polymer was structurally characterized by XAS, X-ray diffraction, and TEM measurements, and a chemical composition was proposed for sol and gel, based on FT-IR, Raman spectroscopies, and 1 H and 13 C NMR experiments for the organic part. Results on the kinetics and growth mechanisms for nanostructured sols and gels based on titanium oxide are presented to explain the respective roles of TiOCl 2 and DMF in this double hydrolysis reaction.

Introduction

Over the past decade, because of the potential applications in the field of environmental protection, the photochemistry of TiO 2 has been a fast growing area both in terms of research and commercial activity. 1 This level of research activity can be appreciated through the exponential increase in relevant research literature produced. [2][3][4] In addition to the white pigment properties of rutile and anatase (e.g., paints and cosmetic products), titanium dioxide is used in heterogeneous catalysis and photocatalysis (water purification, air cleaning), in photoelectrochemical solar cells, for the production of hydrogen 5 and electricity, 6 as an active layer in the design of electrochromic devices, [START_REF] Granqvist | Handbook of Inorganic Electrochromic Materials[END_REF] as a gas sensor, as a corrosionprotective coating, in ceramics, and in electric devices such as varistors, to name a few. But in such applications, the performance of titanium oxide could be optimized with specific nanostructural control over the morphology of the material. Under UV irradiation, an electron-hole pair is generated in TiO 2 . Technological devices are then based on chemical reactions that take place at the TiO 2 /adsorbate interface and induced electron transfers resulting from this pair. Because of the large band gap of TiO 2 (3.2 eV), only 10% of the solar energy is absorbed. A major objective for future work is the development of a semiconductor photocatalyst film, which would be able to utilize visible as well as UV light. Nanoscience has the potential to provide entirely new classes of materials with capabilities that transcend this limitation and generate the performance breakthroughs required for a viable economy based on sustainable energy. [START_REF] Dresselhaus | In Basic Research Needs for the Hydrogen Economy[END_REF][START_REF] Hutzler | Annual Energy Outlook 2004 with Projection to[END_REF] Most chemical routes to TiO 2 nanomaterials are based on transformations in solution such as sol-gel processing, hydroor solvothermal syntheses, metal organic decomposition (MOD), or in the vapor phase, chemical vapor deposition (CVD). Currently used precursors of titanium dioxides are molecules that already contain metal-oxygen bonds, [START_REF] Ivanda | [END_REF][11][12] namely, titanium alkoxides Ti(OR) 4 (R ) saturated or unsaturated organic group, alkyl, or aryl), titanium oxide acetylacetonates, or titanium salts [13][14][15][16] TiX 4 (X ) Cl, Br, NO 3 ), and the oxosalt TiOSO 4 . The most commonly used organic precursors are titanium alkoxides (Ti(OR) 4 ), which are dissolved in an alcohol and hydrolyzed by the addition of water under acidic, neutral, or basic conditions. Condensation reactions involving the hydroxyl ligands produce polymers composed of Ti-O-Ti bonds. Titanium alkoxides are very sensitive to moisture and the gelation times of titanium alkoxides are o, the order of seconds or minutes. The hydrolysis rate of Ti(OEt) 4 (k h ) 1 × 10 -3 mol -1 s -1 ) is extremely fast [START_REF] Livage | In Sol-Gel Technologies for Glass Producers and Users[END_REF] and thus the nucleation and growth are never separated into two steps. In spite of the advantages of sol-gel techniques for the preparation of materials, [START_REF] Livage | [END_REF][19][20][21] namely, low-temperature processing, homogeneity, versatility, and flexible rheology, a detailed understanding of the hydrolysis-condensation reactions leading to 3D framework growth has not yet been achieved. Chemical precursors and the nature of the solvents play a key role for the sol-gel formation. Starting from the TiOCl 2 precursor in an acidic aqueous solution, we have shown that the control of polycondensation at the various stages of the synthesis allows to obtain sols, gel, or solids for which the Ti-O networks dimensionality is well-defined as 0D, 1D, 2D, or 3D. [22][23][24][25] Because of the enhanced surface area to volume ratio, the nanostructured sols and gels in nonaqueous solvents produce singular photoelectrochemical properties that are drastically different from their bulk counterparts. These materials can absorb photons with a lower energy than the bandgap energy of titanium dioxide, and thus a significant increase in the limiting efficiency of conventional solar cells is expected. We report herein the preparation in a one-pot process and characterization of novel and highly photosensitive sols and gel based on titanium oxide.

Experimental Section

Sol-Gel Formation. The sol-gel process, being a low-temperature method, allows the preparation of hybrid organic-inorganic compounds. Our direct low-temperature processes for a highly photosensitive sol-gel based on titanium oxide involve a controlled hydrolysis of titanium species using anhydrous N,N-dimethylformamide ((CH 3 ) 2 NCHO, 99.8%, Aldrich, noted DMF) as solvent. Titanium oxychloride solution stabilized in an acidic medium (TiOCl 2 • 1.4HCl • 7H 2 O, with concentration [Ti 4+ ] ) 4.85 mol L -1 , Millennium) was chosen as precursor instead of pure TiCl 4 because of the strong moisture sensitivity of the latter. In a typical experiment, hybrid organic-inorganic sol is usually prepared by adding droplets of DMF to an ice-cooled TiOCl 2 solution to reach a titanium concentration range between 0.05 and 1.42 mol L -1 (noted <<TiDMF>> sol). The reaction mixture, which slowly becomes viscous, is stored in a closed container at room temperature. After aging for 13 days, the <<TiDMF>> sol turns into syruplike liquid and is noted <<TiDMF>> aged sol. Further heating at 70 °C for several hours converts the transparent sol into transparent and waxy solid, hereafter denoted <<TiDMF>> gel. The most common diagnostic test of gelation is to turn a test tube containing the sample upside-down and then to note whether the sample flows under its own weight. It is assumed that a sample having a yield stress (gel) will not flow, whereas a viscous but elastic sample (sol and aged sol) will show appreciable flow. [START_REF] Macosko | Rheology: Principles, Measurements and Applications[END_REF] In a typical preparation, 12 mL of DMF were introduced using a syringe into a closed test tube containing 5 mL of 4.85 mol L -1 TiOCl 2 solution; the Ti concentration is thus 1.42 mol L -1 . The sol was allowed to hydrolyze for 24 h under stirring at 25 °C, and it was then placed in an oven at 70 °C for 24 h to obtain the transparent gel.

Depending on the concentration, these gels can precipitate after a few days or some weeks. For a Ti concentration larger than 1.42 mol L -1 , an opaque white product is formed that contains a mixture of a transparent polymeric phase and an amorphous white precipitate or a white precipitate of anatase in the case of very high concentration.

FTIR and FT-Raman Absorption Spectroscopy. <<TiDMF>> sol and gel were further studied by infrared and Raman spectroscopies. IR spectroscopy was performed using a FTIR Nicolet 20SXC spectrometer in the range 400-4000 cm -1 . A drop of sol was deposited between two KBr pellets, the gel was ground with dried KBr and then pressed in order to obtain a pellet (1.3 cm in diameter) suitable for transmission mode. In ATR mode, a slight pressure applied on sample holder allows us to distinguish the solid and liquid components of the gel.

Raman spectra were taken on a Bruker RFS 100 FT Raman spectrometer with a 1064 nm laser. All the samples were set in quartz Suprasil cells. The spectra were collected at room temperature with a step scan of 2 cm -1 .

NMR. All 1 H and 13 C NMR spectra were acquired at room temperature on an ARX spectrometer operating at 400 MHz. The recycle times used are 2 s for NMR 1 H and 50 s for NMR 13 C. The last one has been fixed to obtain the good proportions between each carbon in DMF. First, 1 H NMR spectra were recorded separately for dimethylammonium chloride (DMACl) in aqueous solution, methanoic acid, HCOOH, and DMF. Then, HCl was added to each sample in order to obtain acidic conditions close to those of <<TiDMF>> sol ([Ti 4+ ] ) 1.42 mol L -1 ). A 1 H- 13 C correlation spectrum has been realized to attribute the peaks obtained in 13 C NMR. Fits of peaks were carried out with dmfit 2002 program, 27 using Lorentzian curves. All solutions were maintained in darkness at room temperature, in tubes 5 mm in diameter. Locking was performed with a D 2 O set in an inner tube for the quantitative analysis, in order to prevent any perturbation of the system by this extra solvent.

XAS. XAS spectra were collected at the titanium K-edge in fluorescence mode at the Hasylab facility (Hamburger Synchrotronstrahlungslabor at DESY) on an A1 beam line, at FAME-B30B ohio1/ycm-ycm/ycm-ycm/ycm99907/ycm3303d07z xppws 23:ver.3 11/28/07 14:01 Msc: cm-2007-02531q TEID: clp00 BATID: cm1a26 or BM29, (ESRF, Grenoble) for EXAFS and XANES spectra of <<TiDMF>> sol and gel, with a seven-cell Si(Li) detector and 30-or 13-cell Ge detector, respectively. We have checked that the Ti concentration is low enough in our samples to avoid any autoabsorption corrections. The <<TiDMF>> sol and gel, in their oxidized state (i.e., not UV irradiated), were contained in a small Teflon cell covered with a 5 µm sheet of kapton, assuring the airtightness. For EXAFS and XANES experiments performed at Hasylab, a two-crystal Si (111) monochromator was used. Harmonic rejection was carried out by detuning the second crystal to 60% of maximum transmission. At FAME, a double-crystal Si (220) monochromator was used together with two Rh-coated Si mirrors for harmonic rejection. At BM29, the monochromator was a double crystal Si (111) and harmonic rejection was performed using two Si mirrors. Commercial TiO 2 (rutile and anatase) was mixed with the appropriate amount of boron nitride BN to obtain, in transmission detection, an absorption jump close to 1 across the edge. XANES spectra were normalized by assigning a value of 1 to the absorbance at half of first EXAFS oscillation. XANES peaks were extracted by fitting the background with a cubic polynomial function. The resulting peaks were fitted with a pseudovoigt function, the Gaussian part was fixed to a suitable value and not refined (30%). The width of the peaks were refined but constrained together. For EXAFS spectra, the kχ(k) function was extracted from the absorption coefficient using a cubic spline function. 28 The Fourier Transforms were performed on k 3 χ(k) with a Kaiser-Bessel window (τ ) 3) in the k-range 4.5-12 Å -1 for <<TiDMF>> gel and 5-12 Å -1 for <<TiDMF>> sol. Using FEFF7 Code, 29 we calculated EXAFS oscillations for different clusters corresponding to usual Ti-O and Ti-Ti distances, namely, in the 1.90-2.20 Å range for the first shell and between 3.15 and 3.80 Å for the second shell, corresponding to connection between octahedra sharing edges or corners. All the fits were undertaken with Michalowicz's EXAFS signal treatment and refinement programs. 30 X-ray Diffraction (XRD). X-ray diffraction patterns of sols and gel, enclosed in a Lindeman capillary, were collected at room temperature in a Debye-Scherrer geometry using an INEL diffractometer equipped with a 120°curved CPS 120 position-sensitive detector and a quartz monochromator (CuKR1 radiation: λ ) 1.540598 Å).

TEM. The structure and morphology of sol and gel were investigated by electron microscopy. <<TiDMF>> sol and <<TiDMF>> gel, crushed beforehand in DMF, were deposited on a copper grid covered with a carbonaceous membrane allowing the observation in transmission electron microscopy. A highresolution electron microscopy (HREM) study was carried out with a Hitachi H9000NAR electron microscope operating at 300 kV with a Scherzer resolution of 1.8 Å. Image calculations were performed using the MACTEMPAS program. 31 To reduce irradiation damage produced by the electron beam on the sample, the size of the condenser aperture was reduced. In spite of these precautions, the observation of the samples has proved to be difficult because of the high vacuum present in the column. Therefore, a gradual transformation of <<TiDMF>> gel was observed in the course of the experiment. Further details will be exposed in Results and Discussion.

Results and Discussion

Interestingly, these sols and gel exhibit original optical and electronic properties that will be described and discussed in a forthcoming paper. The resulting samples show high transparency and exhibit a strong red coloration upon irradiation with UV or daylight that changes to orange-yellow and finally turns dark blue after prolonged illumination, as shown in Figure 1. When UV excitation was removed, the photogenerated species exhibit a long lifetime that can reach several years in a closed test tube and at room temperature. The singular photoelectrochemical properties of nanostructured sols and gels offer new possibilities in areas such as photochemical conversion and storage of solar energy, in both third-generation photovoltaic cells and photobatteries (ultracapacitors).

The structure and chemical composition of the transparent <<TiDMF>> sols and gel have been investigated using several techniques according to the organic or inorganic constituents.

Characterization of the Organic Component. FT-IR and Raman Spectroscopies. Strong mineral acids are effective reagents for the hydrolysis of amides at about 100 °C. The products are usually a mixture of the amine and carboxylic acid according to the following equation 32 30 h, [Ti 4+ ] ) 1.42 mol L -1 ) are reported in Figure 2 A sol aged for 5 months exhibits the same IR spectrum as a gel. The main bands and their assignment [34][35][36][37][38] are gathered in a table in the Supporting Information. New bands are observed when TiOCl 2 is dissolved into DMF (Figure 2b), namely, at 1364, 832, and 692 cm -1 for the most intense, together with 1307, 1120, 962, 909, and 879 cm -1 . These bands may be related to the formation of hydrated [(CH 3 ) 2 N-CHO, nH 2 O] and protonated [(CH 3 ) 2 N + )CHOH]Cl -intermediates involved in the hydrolysis reaction of DMF. 39 During the sol-gel transition, the intensity of these peaks strongly decreases for the most intense, whereas the less intense disappear. Other peaks related to dimethylammonium chloride (1022 cm -1 ) and methanoic acid (MA) are evidenced when a sol is aged for 4 days (Figure 2c). Their intensities increase when maturation time increases and upon heating. We note also the presence of three other bands at 1555, 1367, and 889 cm -1 . These bands are not detected in the liquid part of the gel but their intensities are clearly enhanced in the solid part. They can be attributed to methanoate species (M) adsorbed on the inorganic framework enclosed into the gel. IR evidence of vibration bands attributable to methanoic acid and amino-chloride salt suggests that part of the dimethylformamide was modified during the hydrolytic process according to reaction 1. In the freshly prepared sol, the intensity of the carboxylic acid vibrations is rather weak but clearly enhances upon heating. Stretching C-O vibra-tions of methanoate ions are evidenced at ν as ) 1620 cm -1 and ν s ) 1364 cm -1 for a 4 day aged sol, whereas they are shifted to ν as ) 1555 cm -1 and ν s ) 1373 cm -1 in the case of gel. Usually, carboxylate anions can coordinate metal cations in different ways: monodentate, bidentate (chelate), or bridging two cations. The value of ∆ ) ν asν s enables us to discriminate between these different complexes. 33 In the case of a 4 day aged sol, ∆ ) 256 cm -1 is in the range of monodentate complexation, whereas in the gel, ∆ ) 182 cm -1 corresponds to a bridging or ionic carboxylate. The latter is observed only in the solid part of the gel, indicating that Ti atoms are complexed only in the polymerized inorganic component of the gel.

R 2 NCHO + HCl + H 2 O f R 2 NH 2 Cl + HCOOH (1) (R ) H,
Figure 3 shows the Raman spectra of the aged sol <<TiDMF>> 1.42 mol L -1 and the (DMF + HCl) solution, along with those of DMF, DMACl, and methanoic acid. For comparison, a Raman spectrum of TiOCl 2 in acidic solution was also recorded. As expected, the Raman spectra of DMF/HCl solution (Figure 3e) and aged sol <<TiDMF>> (Figure 3d) are quite similar. They appear to consist of DMF component bands ((Figure 3c), whereas the additional peaks can be mainly assigned to methanoic acid (Figure 3a) and DMACl (Figure 3b). In comparison to pure DMF, the characteristic frequencies of DMF/HCl and <<TiDMF>> are slightly shifted as already underlined from the IR study. This trend, observed for DMF in many aqueous systems, 40,41 is due to water molecules coordinated to DMF molecules. The intensity of DMACl vibrations at 891 cm -1 (ν s CN) and 1022 cm -1 (F ip (CH 3 )) becomes progressively higher with increasing titanium concentration and hydrolysis spread (aged sol and gel). On The formation of methanoic acid and dimethylammonium chloride has also been checked and quantified using proton NMR spectroscopy. 1 H and 13 C NMR Spectroscopies. Liquid 1 H and 13 C NMR spectra of <<TiDMF>> sol clearly show that the DMF decomposes into dimethylammonium chloride and methanoic acid. The attribution of peaks has been made by comparison with the parent phases. The 1 H and 13 C NMR chemical shifts of DMACl, HCOOH, and DMF without addition of HCl, are summarized in Table 1. + . The addition of HCl to DMACl (Figure 4a) and HCOOH (Figure 4b) slightly affects the signals with respect to those observed without acid. In the DMACl + HCOOH + HCl spectrum (Figure 4c), peaks characteristic of the protons of DMACl and HCOOH appear as well as those corresponding to the protons of H 3 O + , NH 2 + , and H 2 O. A comparison of this spectrum with that of DMF + HCl (Figure 4d) confirms that the hydrolysis reaction of DMF in acidic medium led to the formation of methanoic acid and dimethylammonium chloride. Indeed, six peaks can be distinguished in spectrum d. Their position and attribution are summarized in Table 1, together with the results for <<TiDMF>> sol. In the 1 H NMR spectrum of <<TiDMF>> sol (Figure 4e)), peaks characteristic of DMF (3.10, 3.30, and 8.42 ppm), DMACl (2.89 ppm), and HCOOH (8.51 ppm) are clearly evident. The relative surface area between the methyl groups of DMF and those of DMACl accounts for the progress of the hydrolysis reaction of DMF (at this stage, a small amount of DMF is hydrolyzed). However, some significant differences were evidenced. The peak of the -NH 2 + protons appears only in the <<TiDMF>> sol spectrum at 8.96 ppm (and not in the DMACl spectrum or DMACl + HCl spectrum). This could be explained by the difference between solution concentrations and/or to the presence of inorganic species. For the DMACl spectrum, the DMACl is diluted in water (and HCl) and thus less concentrated than in the DMF + TiOCl 2 solution. In this case, it is considered that the -NH 2 + protons have the same chemical shifts as H 2 O (and H 3 O + ) protons. Furthermore, a shift of the peak related to protons H 3 O + is observed (due to dilution with D 2 O for signal locking) together with a broadening of the peaks of the -CHO protons of DMF.

For the 13 C NMR spectrum of <<TiDMF>> sol aged for 2 days (Figure S1 in the Supporting Information), the peaks located at 160.1, 32.5, and 27.1 ppm are assigned to DMF, C of the aldehyde group and C of methyl, respectively, whereas those located at 29.7 and 158.7 ppm are characteristic for DMACl and HCOOH, respectively.

QuantitatiVe Analysis of ,TiDMF. Sol-Gel Formation. It is possible to follow the kinetic evolution of the DMF hydrolysis by estimating the amount of the various products from 1 H or 13 C NMR and Raman spectroscopies.

The normalized amount of DMACl, calculated either from the 1 H or 13 C spectra, is determined by the ratio between the -CH 3 peak areas (at 2.89 and 29.7 ppm for 1 H and 13 C spectra, respectively) of DMACl, and the area of the -CH 3 peaks related to the DMF + DMACl components. Similarly, the quantity of methanoic acid can also be evaluated by the ratio between the area of the peak at 8.51 ppm and the area of the peaks at 8.51 and 8.42 ppm in the 1 H spectrum and area of the peak at 158.7 ppm divided by the area of the peaks at 158.7 and 160.1 ppm in the 13 C spectrum. From these data, the evolution of the concentration in DMF can be plotted (Figure 5(left)). After 56 days, the sol becomes very viscous and no signal is detectable with the liquid NMR conditions. The amount of consumed DMF corresponds to 36% of the initial content, at room temperature. The global reaction can thus be written

TiOCl 2 •1.4HCl•7H 2 O + x(CH 3 ) 2 NCHO f "TiO 2 , zH 2 O polymer" + y(CH 3 ) 2 NH 2 Cl + yHCOOH + (x -y)(CH 3 ) 2 NCHO + (3.4 -y)HCl + (6 -y-z)H 2 O (2)
With x ) 6.4 and y ) 2.3 at the end of this experiment for [Ti 4+ ] ) 1.42 mol L -1 . Furthermore, the time evolution of the acidity in the solution was followed by plotting the chemical shift of the peak of the acidic protons (H 3 O + , NH 2 + ) group in 1 H NMR spectra. The quantity of HCl consumed during the acidobasic reaction induces a decrease in acidity of the medium. This is reflected in 1 H NMR spectra by a decrease in the chemical shift of H 3 O + ions (g, Figure 5(right). Interestingly, the increased pH value has a direct consequence on polycondensation of inorganic species: the stability of TiOCl 2 is altered and an inorganic condensation can occur. As evidence on XAS experiments, the TiOCl 2 solution is hydrolyzed into polymeric form of titanium oxides, which is schematically summarized by the reactions

TiOCl 2 + 2H 2 O f TiO(OH) 2 + 2HCl (3) TiO(OH) 2 , (z -1)H 2 O f "TiO 2 , zH 2 O polymer" (4)
Raman spectroscopy was also undertaken to quantify the advancement of hydrolysis because the ν s (CN) vibration of dimethylammonium chloride (891 cm -1 ) and ν s (C′ 2 N) of dimethylformamide (866 cm -1 ) are well-separated and their relative surface areas are proportional to the (CH 3 ) 2 NH 2 Cl/ DMF molar ratio. This assertion has been confirmed by recording reference spectra of (CH 3 ) 2 NH 2 Cl mixed with DMF in a different ratio. To determine this area, we fitted the peaks by a pseudo Voigt function (50% Gaussian) after removing the background. The results for sols that were aged for various times and for gels are also gathered in Figure 5(left), together with the NMR results. The evolution of the DMF concentration could be followed on a longer duration.

In addition, we could get additional information on the gel that was not available with the liquid NMR study. The hydrolysis extent reaches more than 50% in agreement with the stoichiometric proportion, which is close to the maximum (53% with x ) 6.4, y ) 3.4, z e 2.6), as shown in eq 2. At equilibrium, all hydrochloric acid of TiOCl 2 solution is released, leading to the hydrolysis of DMF and the formation of dimethylammonium chloride and methanoic acid. The decrease in DMF concentration can be accounted for with a linear variation (0 order kinetics) during the first 8 days and then with an exponential decay (first order kinetics). The kinetics of gelation can be significantly accelerated by an annealing treatment at 70 °C. Figure 5(inset) shows the ex situ evolution of the (CH 3 ) 2 NH 2 Cl/DMF molar ratio inferred from Raman experiments. The induction time for gelation is reduced by about 2 orders of magnitude because the sol-gel transformation operates after 36 h, whereas at room temperature, a gel may be obtained within 7 months. It is worth noting that during this discontinuous thermal treatment, required for the ex situ measurements, the gelation time is longer compared with that of the continuous reaction (24 h). Characterization of the Inorganic Component. From the above IR, Raman, and NMR studies of <<TiDMF>> sols and gels, it is found that the hydrolysis of DMF solvent into DMACl and methanoic acid is catalyzed by the TiOCl 2 acid solution. Without stirring and annealing, the hydrolysis process is limited by the diffusion rate and proceeds slowly over several days, in such a way that the various steps of the inorganic polymerization can be easily controlled. The induced gelation phenomena have been investigated by XAS, X-ray diffraction, and TEM analysis.

XAS Experiments. The local arrangements of <<TiDMF>> polymeric sols and gel around Ti atoms were characterized by analysis of the XAS signal recorded at the titanium K-edge. The structure of the inorganic polymer mainly depends on the initial titanium concentration and on the hydrolytic conditions, especially the aging and thermal treatment. Prior to the EXAFS signal being analyzed, information concerning the local symmetry around Ti atoms can be inferred from the XANES spectrum and particularly from the pre-edge structure analysis (figure 6).

At the titanium K-edge, freshly prepared sol, matured sol, and gel present three pre-edge peaks in agreement with Ti atoms located in an octahedral environment. 42 In the case of freshly prepared sols, these three peaks are decomposed into three contributions: for a sol with [Ti 4+ ] ) 0.1 mol L -1 , A1 is located at 4968.7 eV, A3 at 4970.8 eV, and finally, B at 4973.2 eV; for a sol with [Ti 4+ ] ) 1.42 mol L -1 , the positions are A1 at 4968.9 eV, A3 at 4971.0 eV, and B at 4973.6 eV. These three peaks are close to the positions of the pre-edge structures in rutile. A matured sol and a gel exhibit the same features, and four contributions are needed to account for the pre-edge structures: A1 located at 4968.6 eV, A2 at 4970.4 eV, A3 at 4971.5 eV (sol), and 4971.7 eV (gel), and finally, B at 4973.5 eV (sol), 4973.8 eV (gel). These positions are very close to those of anatase (4968.6, 4970.4, 4971.6, and 4973.8 eV, indicating that the local symmetry around Ti atoms is similar (D 2d group). However, in anatase, A2 appears as a shoulder of A3 (I A2 /I A3 ) 0.6).

(42) Farges, F.; Gordon, E.; Brown, Jr.; Rehr, J. J. Phys. ReV. B 1997, 56, 1809. In our samples, its intensity is higher than the intensity of A3, I A2 /I A3 ) 1.3 and 1.25 for sol and gel, respectively. The attribution of A2 still remains unclear. In rutile form, A2 is fairly undetectable, and only A3 is observed. The pre-edge peaks are attributed to a quadrupolar transition of a coreelectron toward the t 2g orbitals for the A1 peak, mainly to dipolar transitions but also to quadrupolar contributions toward e g orbitals for A3, and purely to dipolar transition for B. 43 Five-fold coordinated Ti atoms were reported to give a pre-edge peak at 4970.5 eV, 42 which is close to A2 position. In nanometric anatase, the intensity of peak A2 with respect to peak A3 is reported to increase when the size of anatase nanoparticles 44 and the crystalline domain size inside the nanoparticles 45 decrease because of an increasing proportion of Ti atoms sitting in 5-fold coordination. Therefore, we infer that because of the nanometric dimensions of the aged sol and gel, A2 exhibits a higher intensity than reported in bulk anatase, in agreement with previous results. However, EXAFS fitting was conducted with Ti atoms lying in an octahedral oxygenated coordination because the intensity and position of the pre-edge peaks are close to the intensity observed for octahedral environment. 42 Attempts to fit the experimental data with one shell around Ti, as first neighbor, failed and then two contributions were considered. As depicted in Figure 7 As previously mentioned, these distances can correspond to Ti-O and Ti-OH or Ti-OH 2 bonds, respectively. Fitting the second neighbors of Ti atoms allows us to differentiate two pairs of Ti-Ti distances located at 3.17 Å (3.15 Å) and 3.39 Å (3.37 Å). The former agrees with octahedra sharing edges, whereas the longest is observed when octahedra are connected by corners. These results suggest that the TiO 6 arrangement in aged sol and gel with [Ti 4+ ] ) 1.42 mol L -1 is different from that found in freshly prepared sols with the same concentration. X-ray Diffraction. X-ray measurements, displayed in Figure 8, show that the polymeric sols and gel obtained in various concentration ranges remain poorly crystallized. For low concentrations of the sol samples ([Ti 4+ ] ) 0.1 mol L -1 ), only two Bragg reflections are observed: a weak peak in the low-angle region, located at d ) 14.23 Å and a broad peak centered at d ≈ 3.7 Å. At higher concentration [Ti 4+ ] ) 1.42 mol L -1 , two main Bragg reflections also appeared for most of the aged sols and gels studied: an intense peak located at d ) 19 Å and an often rather broad peak located at d ≈ 3.6-3.7 Å. The aged sols and gels exhibit a quite similar XRD patterns. The presence of a low-angle reflection suggests that these compounds could adopt a lamellar structure.

Structural Models for << TiDMF >> Sols and Gel. Because XAS and X-ray diffraction do not give sufficient information to elucidate the packing in the fibers at the symmetric level, the structure and morphology of sols and gel in various concentrations were investigated by transmission electron microscopy. The latter clearly demonstrates that these <<TiDMF>> compounds self-assemble into highly ordered polymers at the nanometer scale, and the highresolution images reveal the structures of the titania sols and gel. (a) << TiDMF >> Sol (0.1 mol L -1 ). Figure 9 shows some electron micrographs of the weakly concentrated sol. The sol exhibits a ribbonlike morphology with a very regular shape and high aspect ratio (Figure 9a): length 180 nm, width 9 nm,and thickness 3 nm (deduced from the calculated image). The experimental image was processed in order to improve the observed contrasts and the filtered image is a cf, refined under constraint; f, refined parameters; F, fixed parameters; N, number of atoms at a distance R from the central Ti atom for each shell; σ, Debye-Waller factor; ∆E, shift in origin of energy. b F ) 1.30 × 10 -2 ; χv 2 ) 2.3 × 10 -4 (see text). c F ) 1.34 × 10 -2 ; χv 2 ) 1.9 × 10 -4 (see text). d F ) 1.81 × 10 -2 ; χv 2 ) 3.9 × 10 -4 (see text). e F ) 2.65 × 10 -2 ; χv 2 ) 7.4 × 10 -4 (see text). depicted in Fgure9b with the Fourier tansform (Figure 9c). The two-dimensional (2D) structure of the polymer consists of nanosized thin layers with a periodicity of 3.5 Å, deduced from the distance between the parallel dark striations on the filtered image. Among the numerous titanates with layered perovkite-type structures, only one can be retained with respect to the XAS and X-ray diffraction results. The two Ti-Ti distances at almost 3.5 and 3.75 Å with both neighboring Ti atoms suggest that the structure is built from single layers as in NaLaTiO 4 . 46 Unfortunately, the relative stacking of layers in the perpendicular direction does not agree with the experimental contrast observed in HRTEM. The tetragonal symmetry disagrees with different Ti-Ti distances, and the P4/nmm space group implies a shift of (½,½) in the (110) plane from one layer to the other, leading to more homogeneous contrasts along the [100] and [010] directions. Consequently, the structural model chosen refers to KFeF 4 layered perovkite (Figure 9e), which crystallizes in the orthorhombic system with the Pm2 1 n space group and matches with results found by XAS and XRD. Most likely, the structure consists of a layered arrangement of distorted TiO 6 octahedra sharing corners in which each layer is stabilized by hydrogen bonds between water molecules and Ti-OH 2 bonds along a direction parallel to the long axis of the layers, i.e., the [010] direction. This conclusion is further supported by the fact that structures with very regular shape and high aspect ratio arise from a strong anisotropic growth process. Using a perovskite-like structure in projection along [001] (Figure 9e), the observed contrasts on the filtered image fit quite well with that calculated for a defocus value of 85 nm and a lamellar thickness t ) 3 nm (inset of Figure 9b). The lack of contrast along the [010] direction, results from a 1.8 Å periodicity of TiO atomic columns, which is close to the resolution limit. The calculated electron diffraction pattern (Figure 9d) in projection along [001] is in reasonable agreement with the Fourier transform image (Figure 9c). For the weakly concentrated sol, the inorganic part crystallizes in the orthorhombic system with the Pm2 1 n space group and approximate cell parameters a ≈ 7.1 Å, b ≈ 7.5 Å, and c ≈ 14 Å. The proposed structural model does not preclude the presence of the Ti-OH bonds, H 3 O + , or other species inserted into the interlayer space. Interestingly, the rigid lamellar structure of Figure 9c can be rolled and transformed into an annular disk as in Figure 9f. The main structural features of both morphologies are retained with respect to the distances on electron diffraction patterns and the width of the layer (9 nm). The lamellar to disk transformation suggests that TiO 6 octahedra undergo additional distortion with small tilts compared to the regular layered perovskite (Figure S2 in the Supporting Information). Assuming that the cohesion necessary for the stability of lamellar structures is provided by hydrogen bonds, the decreased amount of intercalated water molecules reduces the hydrophilic volume fraction and the transition from layer to disk structures occurs to minimize the interfacial energy between Ti-O network and water molecules. Thus the digital print morphology can be explained as arising from breaking of some hydrogen bonds characteristics of rigid segment along [010].(b) Freshly Prepared << TiDMF >> Sol. At higher concentration ([Ti 4+ ] ) 1.42 mol L -1 ), the structure of freshly prepared sol further self-assembles into arrangements with shortrange order, which has not been evidenced during the HRTEM experiments. From XAS results, it is clear that the main packing motif is probably similar to the arrangement encountered in a diluted sol. That no structure could be isolated by TEM suggests that the extension of the 2D layers is limited in this case, probably because of the highly acidic medium during the first rapid step of hydrolysis. (c) <<TiDMF >>, Aged Sol and Gel. For both samples, we observed long thin fibers, which form an entangled network. The experimental image (Figure 9) reveals that the structure of <<TiDMF>> gel seems to present a unidimensional character (1D): the polymer fibers are wound concentrically in the manner of a cotton ball. The presence of an important disorder in the direction perpendicular to the stacking of fibers appears on the electron diffraction pattern as two diffuse lenticular spots. The average periodicity, inferred from the spacing between fibers, is close to 3.5 Å.The effect of beam-exposure on the polymeric fibers leads, after a few minutes, to the coalescence of the TiO network and the generation of small crystallites reaching ∼10 nm in diameter. Their orientation disorder gives rise to diffraction rings in the Fourier transform spectrum. From an estimate of their intensity and a relative measurement of their position, one can identify the (101) and (004) planes of anatase (Figure S3 in the Supporting Information). This suggests that the crystallites mainly extend in the (010) plane. A similar effect to that of beam exposure is obtained upon heating a sol at a temperature higher than 70 °C.In a first approximation and taking into account the previous XAS results, the structural model proposed is based on the FeUS 3 type structure, which crystallizes in the orthorhombic symmetry with the Cmcm space group. The inorganic part of aged sol and gel is built from a single chain in which distorted TiO 6 octahedra share edges (two short Ti-Ti distances at 3.15-3.17 evidence that the major part of the methanoate group, formed during the decomposition of DMF, is coordinated to Ti atoms either in a bidentate fashion or via ionic interaction. Because in a freshly prepared sol the methanoate species are mainly monodentate bonded to Ti cations, we infer that during aging, condensation of the inorganic framework leads to the formation of bridging unit between two 6-fold coordinated Ti atoms. These layers are stacked along the c axis and held together by multiple hydrogen bonds between the apical µ 2 -oxygens of each octahedron and CH groups along the [001] direction, via chloride ions inserted into the layer. Because the inorganic arrangement was kept during the sol to gel transformation, the gel structure is stabilized by hydrogen bonds between DMACl groups and/or DMF. Hydrogen bonds can occur between a proton donor group -CH 3 , Ti-OH, and an electron-donating Cl atom or CdO group. This conclusion is further supported by the fact that in crystalline dimethylammonium chloride, each methyl group is hydrogen bonded to two chlorine atoms 47 (Figure 11).

Conclusion

The route for polymer synthesis, starting from titanium oxo-salt precursor in DMF solvent, is a powerful method for the design of new nanomaterials. At this stage of investigation, the sol-gel formation involves a double hydrolysis process that operates on both organic (DMF) and inorganic (TiOCl 2 ) parts. Hydrolysis of DMF in TiOCl 2 acidic medium leads to the formation of DMACl and HCOOH, and at the same time, the consumption of hydrochloric acid enables a gentle control over the inorganic polymerization. As a first step in our kinetic study of the highly concentrated sol, the DMF hydrolysis is mainly concerned and monodentate methanoate species are adsorbed onto the Ti-O framework. Because of the low pH values, the sol consists of two phases, i.e. a continuous phase (DMF, DMACl, HCOOH, H 2 O, HCl) in which the other phase, containing Ti-O/OOCH nanoribbons with octahedra sharing corners only as in KFeF 4 arrangement, is dispersed. The increasing amount of DMACl and methanoic acid then induces a rapid decreasing in the acidity. In the second step, the Ti-O framework slowly condenses to form aged sol and gel. The 2D structure contains octahedra sharing both edges and corners, on which bidentate methanoate species are adsorbed. After a thermal treatment at 70 °C, the fibers form a three-dimensional network in the DMF/DMACl mixture, thereby turning the aged sol into a gel. It is worth noting that the growth of the layered structures is intimately correlated to the mono-or bidentate nature of adsorbed methanoate species and to the titanium concentration. The present study clearly shows that this one-pot process allows a careful control over the hydrolysis and condensation reactions because the gelation times of <<TiDMF>> sols are of the order of days or months, depending on the thermal treatment. Therefore, the nucleation and growth mechanisms are well-separated. The size and structure of nanoribbons in sol-gel can be manipulated and controlled by adjusting the suspension pH and using suitable dispersants. These nanostructured sols and gel exhibit original optical and electronic properties that will be studied in the future.

Supporting Information Available: Figures S1-S3 and an additional table (PDF)

. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 1 .

 1 Figure 1. Coloration time dependence (red to dark blue) of <<TiDMF>> sols upon daylight irradiation at 300 K.

Figure 2 .

 2 Figure 2. tk;4FTIR spectra of various titanium compounds and organic species: (a) DMF+H2O, (b) freshly prepared sol, (c) 4 day aged sol, (d) <<TiDMF>> gel, (e) liquid (red) and solid (black) part of gel (MA, methanoic acid; M, methanoate).

Figure 3 .

 3 Figure 3. Raman spectra of various titanium compounds and organic species: (a) methanoic acid; (b) DMACl, 2, 1022 cm -1 <118>(CH3)ip; ), 880 cm -1 νs(CN); (c) DMF; (d) 4.26 mol L -1 HCl in DMF solution; (e) 1.42 mol L -1 <<TiDMF>> gel; (f) TiOCl2 acid solution.

Figure 4 ,

 4 shows the spectra obtained for (a) DMACl + HCl, (b) HCOOH + HCl, (c) HCOOH + DMACl + HCl, (d) DMF + HCl, and (e) <<TiDMF>> sol after aging for 2 days. The peak of the -(CH 3 ) 2 protons of DMACl is split into a triplet because of the multiplicity rule of the signals: such protons are coupled with the two protons of -NH 2

Figure 4 . 1 H

 41 Figure 4. 1 H NMR spectra of various mixtures: (a) dimethylammonium chloride (DMACl) and HCl; (b) methanoic acid and HCl; (c) DMACl, methanoic acid, and HCl; (d) dimethylformamide (DMF) and HCl; (e) <<TiDMF>> sol aged for 2 days, containing TiOCl 2 • 1.4 HCl, 7H2O, and DMF with concentration [Ti 4+ ] ) 1.42 mol L -1 .

Figure 5 .F

 5 Figure 5. Kinetics of DMF hydrolysis at room temperature (left scale) determined by 1 H and 13 C NMR ( 1 H from 0, HCOOH peak at 8.28 ppm; O, CH 3)2NH2Cl peak at 2.69 ppm; and 13 C from 4, HCOOH peak at 158.7 ppm; 3, (CH 3)2NH2Cl peak at 29.7 ppm) and by Raman spectroscopy (4, ν s(CN)DMACl/νs(C′2N)DMF surface area ratio), reduction in acidity (g, right scale), for TiOCl 2/DMF sol with concentration [Ti 4+ ] ) 1.42 mol L -1 . Inset: at 70 °C for the same <<TiDMF>> sol and deduced from Raman experiments.

  , (a) a diluted sol and (b) a more concentrated, freshly prepared one exhibit similar extracted signals. For the same concentration [Ti 4+ ] ) 1.42 mol L -1 , (c) aged sol and (d) gel also exhibit quite similar signals. The extracted signals of EXAFS spectra clearly differ from a and b to c and(d samples. The results of the best fit are summarized in Table 2. The inverse Fourier transforms of the experimental and calculated spectra can be compared in Figure 7(left); the comparison of the imaginary part and the module of the Fourier transform of the experimental and calculated spectra can be made from Figure 7(right).(a) Freshly Prepared Sols. The 6-fold coordinated titanium atoms of concentrated sol ([Ti 4+ ] ) 1.42 mol L -1 ) are built from one short (1.79 Å) and five rather long Ti-O distances, i.e., 2.15 Å. In a more diluted sol (0.1 mol L -1 ), the best simulation is obtained from one Ti-O distance at 1.84 Å and five Ti-O distances at 2.19 Å for the first neighbors. The shortest bond is typical of a Ti-O bond observed in oxides (1.86Å < d < 2.03Å), whereas the longest distance can be attributed to Ti-OH or Ti-OH 2 . Two Ti-Ti (43) (a) Joly, Y.; Cabaret, D.; Renevier, H.; Natoli, C. R. Phys. ReV. Lett. 1999, 82, 2398. (b) Danger, J.; Le Fèvre, P.; Magnan, H.; Chandesris, D.; Bourgeois, S.; Jupille, J.; Eickhoff, T.; Drube, W. Phys. ReV. Lett. 2002, 88, 243001-1-4. (44) Luca, V.; Djajanti, S.; Howe, R. F. J. Phys. Chem. B 1998, 102, 10650. (45) Hanley, T. L.; Luca, V.; Pickering, I.; Howe, R. F. J. Phys. Chem. B 2002, 106, 1153.

Figure 6 .

 6 Figure 6. Titanium K-edge XANES spectra: (a) TiO2 rutile, TiOCl2, <<TiDMF>> freshly prepared sol ([Ti 4+ ] ) 1.42 mol L -1 ) and diluted sol ([Ti 4+ ] ) 0.1 mol. L -1 ), (b) TiO2 anatase, <<TiDMF>> aged sol-gel with concentration [Ti 4+ ] ) 1.42 mol L -1 .

Figure 7 .

 7 Figure 7. EXAFS spectra of <<TiDMF>> samples. Inverse Fourier transforms of experimental, filtered (in the 1.2-4 Å range), and calculated (left panels), imaginary part (right upper panel), and module of the Fourier transform (right lower panel) filtered and calculated signals: (a) diluted sol ([Ti 4+ ] ) 0.1 mol L -1 ), (b) freshly prepared sol,(c) aged sol, and (d) gel with concentration [Ti 4+ ] ) 1.42 mol L -1 . The FT are not corrected from phase shifts.

Figure 8 .

 8 Figure 8. XRD patterns of <<TiDMF>> diluted sol with concentration ([Ti 4+ ] ) 0.1 mol L -1 ) and aged sol and gel with concentration [Ti 4+ ] ) 1.42 mol L -1 .

Figure 9 .

 9 Figure 9. TEM studies of <<TiDMF>> diluted sol: (a) low-magnification image; (b) filtered high-resolution image along [001] showing the polymeric structure; the inset calculated image is for 3 nm thick foil at 85 nm defocus; (c) Fourier transform of the image; (d) calculated ED pattern with (001) zone axis; (e) structural model along [001]; (f) experimental image showing a digital print morphology.

  Å). Such chains, running along [010], are linked together by sharing corners (two Ti-Ti distances at 3.37-3.39 Å) in the [100] direction, resulting in polymeric layers parallel to the (001) planes (a,b plane) (Figure 10). FT-IR analysis shows (46) (a) Toda, K.; Kameo, Y.; Kurita, S.; Sato, M. J. Alloys Compd. 1996, 234, 19-25. (b) Byeon, S.-H.; Park, K.; Itoh, M. J. Solid State Chem. 1996, 121, 430.

Figure 10 .

 10 Figure 10. HRTEM experimental image of <<TiDMF>> aged sol and gel.

  Acknowledgment.A.R. is indebted to the Région Pays de la Loire for a grant. Dr. Jean-Luc Rehspringer, IPCMS-GMI-Strasbourg; Dr. Fabrice Odobel and Gilbert Nourisson, LSO-Nantes; and Dr. Anne-Marie Marie are thanked for their help in TG/MS, 1 H NMR, and TEM experiments, respectively. Dr. Françoise Villain (LURE, France) is thanked for contributions to XAS experiments and fitting. We acknowledge Desy (Deutsches Elektronen-Synchrotron) and Dr. Edmund Welter (Hasylab, Hamburg) for assistance during experiments (Projects I-04-041 EC and I-05-087 EC). This work was supported by the European Community-Research Infrastructure Action under the FP6 "Structuring the European Research Area" Program (through the "Integrated Infrastructure Initiative "Integrating Activity on Synchrotron and Free Electron Laser Science) and by the French Ministère de la Recherche through the ACI Energie and ACI Nanosciences. We also acknowledge the European Synchrotron Radiation Facility for provision of beamtime (Projects CRG 30-02-733 and CH-2220) and thank Dr. Hervé Palancher, Dr. Olivier Proux (beamline FAME), and Dr. Carmello Prestipino (beamline BM29) for assistance in XAS data collection.

CM702531Q(

  47) Lindgren, J.; Olovsson, I. Acta Crystallogr. 1968, 24, 549.

Figure 11 .

 11 Figure 11. Structural model for aged sol and gel ([Ti 4+ ] ) 1.42 mol L -1 ) that is based on the FeUS 3 type structure.

  alkyl, aryl, etc.) S. Liu et al. reported also that the hydrolysis of DMF into dimethylamine is catalyzed by polyoxometalate, 12-molybophosphate acid.33 In our experiment using TiOCl 2 in DMF, because of the presence of HCl to stabilize the oxychloride solution, formation of dimethy-

lammonium chloride is expected. Its presence together with methanoic acid was detected by infrared, Raman, and NMR spectroscopies.

For comparison, infrared spectra for dimethyformamide slightly hydrated, <<TiDMF>> sol (freshly prepared and 4 days aging) and <<TiDMF>> gel (heated at 70 °C for (28) Winterer, M. J. Phys. IV 1997, 7, C2-243. (29) Rehr, J. J.; Mustre de Leon, J.; Zabinsky, S. I.; Albers, R. C. J. Am. Chem. Soc. 1991, 113 (14), 5135. (30) Michalowicz, A. EXAFS pour le Mac, Logiciels pour la Chimie; Société Française de Chimie: Paris, 1991; p 102. (31) O'Keefe, M. A.; KilaasR. MacTempas, V1.70 and Crystalkit V1.77, HRTEM Image Analysis; Lawrence Berkley National Laboratory: Berkeley, CA, 1987. (32) Challis B. C.; Challis, J. A. The Chemistry of Amides; Zabicky, J., Ed.; Interscience Publishers: London, 1970. (33) Liu, S.; Wang, C.; Zhai, H.; Li, D. J. Mol. Struct. 2003, 654, 215.

Table 1 . Assigments of 1 H Chemical Shifts for DMACl, HCOOH, and DMF in Aqueous Solution, Or in the Presence of HCl and Assigments of 1 H and 13 C NMR Chemical Shifts for <<TiDMF>> Sol Aged for 2 Days

 1

	1 H (ppm)	1 H (ppm)	1 H (ppm)	13 C (ppm)
	in water	with HCl	<<TiDMF>> sol	<<TiDMF>> sol	DMACl	HCOOH	DMF
	2.74	2.78	2.89	29.7	-(CH 3)2
	2.85 and 3.02	2.96 and 3.14	3.10 and 3.30	27.1 and 32.5			-(CH3)2 doublet
	4.82	6.58	8.96		-NH 2 + , H2O
	5.0		7.27			-CO 2H, H2O
	8.08	8.17	8.42	160.1			-HCO
	8.25	8.34	8.51	158.7		-HCO

Table 2 . Refinement Results of EXAFS spectrum Performed at the Titanium K-Edge for Various <<TiDMF>> Transparent Sols and Gel a

 2

	-1b

shell element N (cf) σ (× 10 2 ) (Å) (f) R (Å) (f) ∆E (eV) (F)

Diluted Sol with Concentration [Ti 4+ ] ) 0.1 mol L

ohio1/ycm-ycm/ycm-ycm/ycm99907/ycm3303d07z xppws 23:ver.3 11/28/07 14:01 Msc: cm-2007-02531q TEID: clp00 BATID: cm1a26


Orbitrap mass spectrometry of synthetic exoplanetary particles

Introduction

Organic aerosols are present in every substantial planetary atmosphere in the solar system, but so far the studies of aerosols in exoplanet atmospheres are still largely in their infancy for two reasons: [START_REF] Moses | Disequilibrium carbon, oxygen, and nitrogen chemistry in the atmospheres of HD189733b and HD209458b[END_REF] efforts to characterize the atmospheres of exoplanets have focused on the best current targets for exoplanets, which are mainly hot Jupiter-like planets where hydrocarbon destruction rates greatly inhibit the formation of complex molecules [START_REF] Moses | Disequilibrium carbon, oxygen, and nitrogen chemistry in the atmospheres of HD189733b and HD209458b[END_REF] and (2) aerosol formation is a complicated process that is still difficult to replicate in the models.

Aerosols affect the chemistry, dynamics and flux of radiation in planetary atmospheres. They can therefore provide organic material to the surface and/or modify the atmospheric temperature and consequently have an impact on the habitability of the planet. Decades of analogous studies of planetary aerosols (notably those of Titan, satellite of Saturn or the primitive Earth) have shown that they can contain complex molecules that serve as bricks for the construction of organized systems [START_REF] Hörst | Formation of Amino Acids and Nucleotide Bases in a Titan Atmosphere Simulation Experiment[END_REF]. However, little is known about the size distribution, optical properties, and particle composition that can form in the super-Earth and mini-Neptune atmospheres. Thus, laboratory studies on atmospheric particles of exoplanets are essential for the interpretation of future spectroscopic observations and the characterization of the atmospheres of these worlds.

Exoplanet particle production by the PHAZER setup

Laboratory analogues were synthesized at Johns Hopkins University by exposing to an AC glow discharge nine different gas mixtures with various compositions varying from 100x to 10000x solar metallicity calculated at thermodynamic equilibrium at 300, 400 and 600 K, that flow continuously through a stainless-steel reaction chamber. The produced particles are retrieved from c mb ' inside wall and wrapped in foil [START_REF] Hörst | Haze production rates in super-Earth and mini-Neptune atmosphere experiments[END_REF]. More details about sample production and compositions can be found in [START_REF] He | Photochemical Haze Formation in the Atmospheres of Super-Earths and Mini-Neptunes[END_REF].

Unveiling the molecular diversity with Orbitrap mass spectrometry

A Thermo Scientific LTQ Orbitrap XL (resolving p w W m/∆m∼100,000 at m/z=400u) available in Univ. Grenoble Alpes coupled with an electrospray ionisation source (ESI) was used to acquire very high-resolution mass spectra of exoplanets particle soluble organic matter. About 1 mg of particles is dissolved in 1 mL of methanol to generate a primary solution that is vortexed during several minutes and then centrifugated at 10,000 rpm for 10 minutes. 500 µl of the soluble fraction is mixed with 500 µl of methanol to obtain the final solution that is injected in the Orbitrap. Mass spectra were acquired in both positive and negative mode with mass range from m/z=50-300 u and m/z=150-450 u. Only three samples out of the nine analysed show prominent features in their mass spectra and about 3000 stoichiometric formulas are obtained in each spectrum using the Attributor software developed at IPAG [START_REF] Orthous-Daunay | Ultraviolet-photon fingerprints on chondritic large organic molecules[END_REF]. The starting gas mixture composition for the three samples are shown in Table 1.

SAMPLE NAME

GAS MIXTURE COMPOSITION

Results and Conclusions

From the stoichiometries, some different approaches can be performed: (1) extract the elemental composition of the attributed molecules and link the differences to the gas mix variation, (2) display some proxy representations to understand the evolution of the incorporation of nitrogen and oxygen with the gas mix variation and even (3) compare the stoichiometric formulas to known molecules and select the proper chromatographic methods to confirm or not their presence. Here we will focus on finding the proper proxy to see the differences between samples and link them to the gas mix composition.

We propose in Figure 1 a representation based on ESI negative mode analysis on the higher mass range (m/z ranging between 150 and 450 u) that compares the incorporation of nitrogen and hydrogen compared to the oxygen one. For all the three gas mix compositions, N and H contents are equivalent between samples, roughly at 9 and 10% respectively. T " 000x -600 K" mp ws a higher amount of incorporated oxygen, corresponding to the lowest O and highest C content, at 63 and 19% respectively. O " 000x -300 K" ws a higher N+H content, corresponding to the lowest C and highest O content, at 8 and 72% respectively. The difference in heteroatom composition seems to correlate to the carbon content and anticorrelate to the oxygen content of the gas mixture. As the gas mixture composition is complex, correlate the variation to one compound only seems difficult to achieve.

Other proxies can be used to check for other sample features, such as the unsaturation with the Double Bound Equivalent (DBE) or Aromaticity Index (Xc) representations [START_REF] Yassine | Structural characterization of orga-nic aerosol using Fourier transform ion cyclotron resonance mass spectrometry: Aromaticity equivalent approach: Characterization of organic aerosol using FTICRMS[END_REF].

Figure 1 :

 1 Figure 1: Comparison of incorporation of nitrogen and hydrogen compared to oxygen in ESI negative analysis on the second mass range.

Table 1 :

 1 Composition of the starting gas mixture composition for the three samples.

	1000X -	66%H 2 O, 6.6%CH 4 ,
	300 K	6.4%N 2 ,4.9%CO 2 , 16%He
	1000X -	56%H 2 O, 11%CH 4 , 10%CO 2 ,
	400 K	6.4%N 2 , 1.9%H 2 , 14.7%He
	1000X -600 K	42%H 2 , 20%CO 2 , 16%H 2 O, 5.1%N 2 , 1.9%CO, 1.7%CH 4 , 13.3%He

Acknowledgements


Juan Uriel Esparza

Amit Raj Dhawan

Rafael Salas-Montiel

Willy Daney De Marcillac

Jean-Marc Frigerio

Bruno Gallas

Agnès Maître

Differential reflectivity spectroscopy on single patch nanoantennas

We present an experimental technique adapted to characterize individual metallic nanostructure in terms of differential reflectivity spectroscopy. We analyze gold patch nanoantennas holding different morphological properties. Our experimental methodology shows steady and reliable results consistent with classical analytical approximations and simulation methods. This technique allows to identify absorption properties of metallic nanostructures commonly associated to surface plasmon resonances. By contrasting the light absorbed solely by the metallic antenna with respect to a surrounding reference medium, we found that some antennas show absorption of almost 50% of the incident light across the range of the visible spectrum. Plasmonic patch nanoantennas are promising systems in which the confinement of the electromagnetic field inside the dielectric gap strongly modify the local density of states.

Differential reflectivity spectroscopy (DRS) is a nondestructive, surface analytical technique used to identify the optical absorption signature of samples with a reflecting substrate, within a given spectral range from the near-UV to the IR region. The comparison of the sample under investigation with a reference, under same conditions of illumination, provides sensitive spectral information, so that, even the smallest inhomogeneity change between samples may lead to a different reflectogram signature. As a consequence, DRS has been used for several purposes such as the study of intrinsic and doping materials (n-or p-type) [START_REF] Hummel | Differential reflectance spectroscopy in analysis of surfaces[END_REF] , the formation of metallic clusters during thin vapor deposition [START_REF] Lazzari | In situ study of a thin metal film by optical means[END_REF][START_REF] Martin | Silver particle sizes and shapes as determined during a deposit by in situ surface differential reflectance[END_REF][START_REF] Simonot | In situ optical spectroscopy during deposition of Ag:Si3N4 nanocomposite films by magnetron sputtering[END_REF] , the anisotropy in metallic surface structure [START_REF] Mochán | Optical reflectance anisotropy of Ag and Au (110) single crystals[END_REF][START_REF] Aspnes | Anisotropies in the aboveband-gap optical spectra of cubic semiconductors[END_REF] and analysis of multi-polar effects in alkali metals such as potassium nanoparticles [START_REF] Beitia | Multipolar plasma resonances in supported alkali-metal nanoparticles[END_REF][START_REF] Román-Velázquez | Substrate effects on the optical properties of spheroidal nanoparticles[END_REF] . In addition, DRS technique can also probe the optical properties of individual plasmonic nanostructures [START_REF] Kaniber | Surface plasmon resonance spectroscopy of single bowtie nano-antennas using a differential reflectivity method[END_REF] . DRS technique offers an straightforward implementation and an alternative with respect to others optical surface techniques 10 such as transmission spectroscopy [START_REF] Proust | Optical monitoring of the magnetoelectric coupling in individual plasmonic scatterers[END_REF] , dark-field microscopies [START_REF] Li | Plasmonic particleon-film nanocavities: a versatile platform for plasmon-enhanced spectroscopy and photochemistry[END_REF] , spatial modulation spectroscopy [START_REF] Devkota | Spatial modulation spectroscopy imaging of nano-objects of different sizes and shapes[END_REF][START_REF] Devadas | Detection of single gold nanoparticles using spatial modulation spectroscopy implemented with a galvo-scanning mirror system[END_REF] .

In the present work, we propose a DRS technique for the analysis of differential reflectivity on single metallic nanoantennas exhibiting plasmonic resonances which are highly dependent on the morphological properties of the particle and the surrounding medium [START_REF] Noguez | Surface plasmons on metal nanoparticles: The influence of shape and physical environment[END_REF][START_REF] Kelly | The optical properties of metal nanoparticles: The influence of size, shape, and dielectric environment[END_REF][START_REF] Sosa | Optical properties of metal nanoparticles with arbitrary shapes[END_REF] . Thanks to this spectroscopic technique, we measure the plasmonic signature of individual patch nanoantennas with different morphological features across the visible spectral range.

We fabricated 4x4 matrix of gold (Au) nanopatch supported by a 40 nm poly(methyl methacrylate) (PMMA), with index of refraction n = 1.45, deposited on a Au substrate using an in-situ laser etching lithography technique inspired by [START_REF] Belacel | Controlling spontaneous emission with plasmonic optical patch antennas[END_REF] and described in reference [START_REF] Dhawan | Extreme multiexciton emission from deterministically assembled single-emitter subwavelength plasmonic patch antennas[END_REF] . The a) Electronic mail: agnes.maitre@insp.upmc.fr spectroscopic ellipsometry data for the gold permittivity are plotted in supporting information (Fig. ??). The Au patches are disks with a 20 nm thickness and a typical diameter of 1 µm. Fig. 1a shows a scanning electron microscope (SEM) image of 16 patch antennas. Although they were fabricated with the same lithographic parameters, they exhibit slightly different morphological properties (diameter, defects, etc.), probably due to inhomogeneities in the resist layers used for the photolithography and to lift-off processes. In Fig. 1b, we show a scanning electron microscope image corresponding to antenna 9. We note that the nanodisk is not perfectly circular. Indeed, the patch has a slight asymmetry and the border edge is discontinuous. Therefore, we define an effective diameter d eff as the diameter of the patch disk not taking into account asymmetries and defects of the nanostructures (yellow circumference on Fig. 1b). We found that the effective diameter of antenna 9 corresponds to a diameter d eff = 830 nm. The experimental setup consists of a confocal reflection microscope with a halogen lamp as a source of light (Fig. 2). The wide-spectrum lamp (ranging from 350 to 800 nm) is partially reflected by a 50:50 beam splitter inside the microscope. The reflected light is then focused by an objective (0.80 NA and 100x), which illuminates the sample. The field of view of the optical system has a diameter of 0.265 mm. The light reflected by the sample is then collected by the same objective and focalized in the aperture of a circular pinhole of 100 µm diameter. The pinhole, placed at the conjugated image plane of the sample, is employed as a spatial filter to reject most of the light reflected by the sample and to collect exclusively the light reflected by a 1 µm disk centered on the nanodisk. The lateral precision for positioning the pinhole is estimated to be 5 µm that corresponds in the conjugated plane, where the sample lies, to 50 nm. The reflected light is then collimated and coupled to an optical fiber connected to a Jobin Yvon HR 460 spectrometer equipped with a 150 g/mm grating. The linear dispersion of our instrument is 14.1 [nm/mm], corresponding to 0.36 [nm/px] taking into account the physical size of the spectrometer CCD. For differential reflectivity spectroscopy technique, the lamp is first switched off and replaced by a 405 nm laser diode that illuminates the sample on a 500 nm radius spot size. The sample is raster scanned across the xyplane and the fluorescence is collected within the spectral range ranging from 584 nm to 676 nm in order to obtain a spectroscopic cartography of the fluorescence.

Fluorescence imaging helps to distinguish the Au nanodisk from another materials surrounding the antenna. Figure 3a shows a fluorescence scanning image of a single Au patch antenna of 945 nm effective diameter. In the image we traced a circle of 1 µm diameter indicating the area collected by the spatial filter. From the image, we observe a bright spot generated at the bottom right edge defect of the patch antenna that efficiently scatters light. Furthermore, we note low-level fluorescence dots around the patch antenna from lift-off resist that remains from the fabrication process [START_REF] Molard | Red-nir luminescent hybrid poly(methyl methacrylate) containing covalently linked octahedral rhenium metallic clusters[END_REF] . Figure 3b shows the corresponding SEM image of the same nanoantenna. From both images we can correlate the bright fluorescence spot of Fig. 3a with the edge defect observed in Fig. 3b. Finally, we can identify in both the fluorescence and SEM images the position of the patch antenna.

Once the antenna is identified by fluorescence imaging, we switch the illumination source and replace the laser by the white lamp. We register the reflected spectra at two positions on the sample. In the first one, the microscope pinhole is optically conjugated to the patch antenna. In the second one, pinhole is conjugated with a zone some micrometers far away from the antenna, acting as reference. We define the differential reflectivity spectroscopy (DRS) as the relative light reflected by antenna with respect to the reference:

DRS = ∆R ant R ref = R ant -R ref R ref , (1)
where R ant is the spectrum reflected by the patch antenna and R ref that of a reference consisting of a gold surface and the dielectric layer without Au disk (Fig. 4a). Because the only difference between antenna and reference is the presence of the Au disk, DRS quantifies the decrease of reflectivity, and therefore the relative absorption due to the presence of Au nanodisk with respect to the Au substrate and dielectric layer. We performed multiple measurements for different patch nanoantennas exhibiting different shapes and diameter but equal dielectric layer thickness value of 40 nm. In Fig. 4b we show the differential reflectivity spectra of seven patches. We observe that all curves are characterized by a predominant dip around 500 nm. At this wavelength, the DRS varies between 30% and 50%, reaching almost an efficiency of 50% for antennas 4, 9, and 10. The error bars on the graphs in Fig. 5 represent the standard deviation of five consecutive measurements on the same antenna in order to get rid of lamp intensity fluctuations. Some antennas exhibit similar effective diameter but substantial difference in DRS. This feature is the case of Ant 2 and 10, with 30% and 47% of extrema values of DRS, respectively. Beyond the main absorption peak around 500 nm, the spectra present inflections and different slopes. To further investigate these antennas, we compare SEM and optical dark field images (Fig. 6). We see scattered light from the circular edge of the disks. In the case of the antenna 2, we distinguish a homogeneous blue ring around the nanostructure, whereas for antenna 10 we clearly observe a bright white spot on the left-side. This scattering spot is directly related with the asymmetry defect observed in the SEM image. These results imply the influence of features such as asymmetric defects on the edges that induce hot spot increasing absorption displayed on reflectivity analysis.

To model the DRS of the antenna, we first consider the patch antenna as infinite in transverse direction by replacing the thin Au disk by an infinite Au layer of same thickness. We calculate numerically the reflection of a non-polarized plane wave normally incident on this "infinite patch" and on the reference one (i.e. without the top Au layer). One-dimensional finite-difference timedomain method, implemented by Meep 21 , was used to numerically calculate the reflectivity spectra. The dielectric layer index was 1.45, corresponding to PMMA.

Figure 7a shows the results of the numerical calculations of the reflectance in different configurations: bare Au substrate, "infinite patch" in the case of 40 nm and 60 nm dielectric layer thickness. The differential reflectivity spectroscopy for both systems is calculated according to Eq. ( 1) and plotted in Fig. 7b. Both graphs reproduce well the experimental results (Fig. 4b). We observe a dip around 500 nm with values in differential reflectivity of DRS = -0. tric thickness, respectively. By comparing both curves, we can observe that the increase of the thickness in the dielectric layer modifies the amplitude of the DRS dip but not the wavelength position. At this point, we will turn our discussion towards two main points: the origin of the dip at λ = 500 nm and the DRS variation with thickness dielectric layer. Figure 7a shows that the absorption of gold changes quickly within the interval 500 nm < λ < 550 nm, but is quite flat for λ < 500 nm, and for λ > 550 nm. This is in accordance with the calculated DRS shown in Fig. 7b: the appearance of the dip around λ = 500 nm corresponds mainly to this abrupt change in the absorption properties of the gold material (see suppl. info.) whereas the DRS remains smooth for both regions exhibiting slight variation of the absorption. Indeed, the differential reflectivity spectroscopy estimates the relative difference between reference reflectivity and the curves "infinite patch" reflectivity in Fig. 7a. The differential reflectivity curves of Fig. 7b shows that the DRS of the antenna with 60 nm of dielectric layer is larger than that for 40 nm, as the resonance and spatial distribution of energy are not equivalent in both configurations. To evaluate the absorption dissipated by the "infinite patch" at λ = 500 nm, we calculated the power absorbed per unit volume across the antennas (Fig. 7c), defined as [START_REF] Jackson | Classical electrodynamics[END_REF] :

P abs (z, ω) = -0.5ω 0 |E(z, ω)| 2 Imag[ (z, ω)] , (2)
where ω 0 is the frequency, E is the electric field, and Imag( ) is the imaginary part of the Au permittivity. The total absorption in the volume is higher for the "infinite patch" with 60 nm gap than that with 40 nm.

This model is convenient to describe the main features in the reflectivity curve of those "infinite" patch nanoantenna. In this approach however, as it is based on the multiple interference between Au layers, any variation in size or shape of the nanostructures is not taken into account. To overcome this limitation for more realistic nanostructures simulations, we used 3D FDTD simulations. We used the full vectorial 3D FDTD method that supports the simulation of Maxwell's equations in cylindrical coordinates for the patch antenna that has continuous rotational symmetry around the z axis. The method is implemented by the freely-available software Meep [START_REF] Oskooi | Meep: A flexible freesoftware package for electromagnetic simulations by the FDTD method[END_REF] . By analogy with the Bloch's theorem, the field in a continuous rotational symmetry structure can be expressed as the field multiplied by an angular dependence in the form exp(imφ) for some integer m = 0, 1, 2,. . . , which is related to the angular momentum. Thus, the field can be expressed as:

E(r, φ, z) = E(r, z) exp(imφ) . (3)
The field can be therefore expanded into its radial and azimuthal modes and identified with the indices (r, m), respectively. By placing a closed surface of flux monitors surrounding the nanoantenna, we calculate all incoming powers first without and then with the antenna.

To obtain patch antenna absorption, we subtract the two results and normalize to the incident flux.

For the simulation, we use a linearly-polarized plane wave normally incident along the z direction. We first calculated the reflectance for the three different conditions presented beforehand: Au, 40-nm-thick dielectric layer on Au, and the reflectivity of a single nanoantenna of 1 µm in diameter (Fig. 8a). We can observe that the reflectivity response of the single antenna (40 nm thin dielectric layer) is quite similar to that of the reference calculated in the previous section (Fig. 7a). We observe that numerical results for the DRS dip around 500 nm agree roughly for finite size antenna ("infinite patch"). On the contrary, the DRS exhibit additional three dips around 600 nm, 709 nm, and 909 nm for the finite size patch, which are not present for the infinite patch. Because of the micrometer size of the structure, we attribute these dips to high-order resonance modes of the single patch. In the same Figure 8a we confirm that the maximum of absorption of the single nanoantenna corresponds to the peaks of the DRS for these 3 additional dips, whereas it is not the case for wavelengths around 500 nm, for which the absorption does not present a maximum but varies quickly with the wavelength. It confirms the fact that the dip in DRS at that wavelength is not due to the resonance of the finite size of the patch but is a signature of the abrupt change of gold absorption in the vicinity of λ = 500 nm. While the peaks at 709 nm and 909 nm are outside our detection bandwidth, the peak at λ = 600 nm is weakly detected (Fig. 4). To understand this feature and to investigate the influence of the radius size and edge defects on the DRS and absorption properties of the patch antennas, we conducted FDTD simulations. The spectra red-shift for higher radius size, with 14.8 nm shift for every 10 nm increase on the radius. Gold edge defects also generate a red-shift but with higher values when the defect is placed at the upper-half height of the patch antenna relative to that placed at the middle. In contrast, dielectric defects do not produce any significant change (see suppl. info.). These simulations demonstrate that the consequences of various defects in real antennas is a broadening and flattening of absorption peaks, experimentally smoothing the weak peak around 600 nm.

To identify the modes of the field that we excite, we record the distribution of the mode fields at the resonant frequencies (Fig. 8). The mode fields are mainly oriented along the vertical z-axis with main component E z . We confirm that high-order modes are excited as r = 2, 3, and 4 (Fig. 8b). However, the azimuthal index m is not clearly identified with the distribution of the modes. We suppose that the azimuthal index is m = 1 as the excitation a plane wave oriented along the r axis. In other words, dipoles are excited preferentially. In Figure 9, we plotted the angular dependencies of the mode field for r = 2, 3, 4 with m = 1. In summary, we described the experimental setup used to perform differential reflectivity spectroscopy on single nanoantennas. The DRS compares the absorption loss of the metallic nanoantenna with respect to a reference (i.e. a dielectric layer on a gold substrate) and quantifies the effect of the patch nanodisk. We found that for nanoantennas fabricated with similar parameters, the shape, effective diameter, and wavelength of maximum absorption can be slightly different between each structure with qualitative similar DRS but quantitative difference for the DRS maximum value. Indeed, they may present some non symmetric morphological defects inducing hot spots which have an influence on the absorption properties of the antenna and therefore on the DRS. Based on numerical simulations, we found that the amplitude of the differential reflectivity is strongly dependent of the dielectric gap between the metallic layers. Additionally, FDTD simulations of single patch antennas reveal the excitation of additional resonance modes, observed only for finite-size antennas. This surface method provides an efficient tool to characterize the optical response of a nanostructure and offers this flexibility for other structured materials. No significant changes were obtained for PMMA edge defects on the DRS.

FIG. 1 .

 1 FIG. 1. Gold patch antennas. Scanning electron microscope image of (a) an array of 4x4 gold patch antennas laying on gold substrate and (b) zoom around nanoantenna 9. The effective diameter is d eff = 830 nm (limited by a yellow circle). Scale bar is 5 µm and 200 nm in (a) and (b), respectively.

FIG. 2 .

 2 FIG. 2. Experimental setup used to perform specular reflectivity measurements. The confocal reflection microscope collects the light reflected from the individual antennas. Inset: spectrum of the halogen source.

FIG. 3 .

 3 FIG. 3. Identification of the patch antenna with scanning fluorescence imaging. (a) Scanning laser fluorescence image of a patch nanoantenna. The bright spot is generated at an edge defect that efficiently scattered light. (b) Corresponding scanning electron microscope image of the single patch nanoantenna in (a). The opaque dots around the patch antenna are lift-off resist residuals from the fabrication process.

FIG. 4 .

 4 FIG. 4. Differential reflectivity spectra of single patch antennas. (a) Scheme of illumination for the measurements of the reflectivity of the patch antenna and reference areas. (b) Differential reflectivity spectra of the different patch antennas within the array, measured individually. The thickness of PMMA is h = 40 nm and the effective diameter of the antennas is reported in Fig.5.

  FIG. 5. Extrema DRS values. The error bars represent the standard deviation of five consecutive measurements.

10 FIG. 6 .

 106 FIG. 6. Influence of asymmetrical path antennas on reflectivity measurements. (a) Optical dark-field and (b) scanning electron microscope images of antenna 2 with effective diameter 946 nm. (c) Optical dark-field and (b) SEM images of antenna 10 with effective diameter 932 nm. Patch antennas with equivalent effective diameter exhibit different DRS.

FIG. 7 .

 7 FIG.7. "Infinite patch" antenna analysis. (a) Calculated reflectivity of a gold substrate, 40-nm-thick (solid green line) and 60-nm-thick (solid blue line) dielectric layer on gold substrate. (b) Calculated differential reflectivity spectra of "infinite patch" antennas with 40 nm and 60 nm dielectric layer obtained from (a). (c) Spatial distribution of the power dissipated by the gold "infinite patch" antennas and substrate.

FIG. 8 .

 8 FIG. 8. Electromagnetic simulations of a gold patch antenna (radius = 500 nm). (a) Calculated differential reflectivity spectroscopy and absorption of the patch antenna. Distribution around the patch antenna of the z-component of the electric field and the φ-component of the magnetic field at wavelength (b) λ = 909.93 nm, (c) λ = 709.02 nm, and (d) λ = 615.64 nm. Scale bar is 500 nm in (b)-(d).

1 FIG. 9 .

 19 FIG. 9. Calculated real part of the z-component of the electric field and φ-component of the magnetic field at (a) 909.93 nm (r = 2), (b) 709. 02 nm (r = 3), and (c) 615.64 nm (r = 4). The azimuthal index is m = 1.

FIG. 3 .

 3 FIG.3. Influence of edge defects on the DRS and absorption of patch antennas. An antenna with radius 500 nm and with squared edge defect at the middle height.

FIG. 4 .

 4 FIG.4. Influence of edge defects on the DRS and absorption of patch antennas. The edge defect is placed at the upper half height of the patch antenna. The simulations were conducted with patch antenna with radius 500 nm.

FIG. 5 .

 5 FIG. 5. Influence of PMMA edge defects on the DRS and absorption of patch antennas. The edge defect is placed at the upper half height of the patch antenna. The simulations were conducted with patch antenna with radius 500 nm.

ACKNOWLEDGMENTS

Computations were carried out with the resources of the French Regional Grand-Est HPC Center ROMEO. J. U. E. would like to thank Loïc Becerra for technical assistance in SEM measurements.

DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.

SUPPORTING INFORMATION for

Differential reflectivity spectroscopy on single patch nanoantennas Juan Uriel Esparza, 1 Amit Raj Dhawan, 1 Rafael Salas-Montiel, [START_REF] Lazzari | In situ study of a thin metal film by optical means[END_REF] Willy Daney de Marcillac, [START_REF] Hummel | Differential reflectance spectroscopy in analysis of surfaces[END_REF] Jean-Marc Frigerio, [START_REF] Hummel | Differential reflectance spectroscopy in analysis of surfaces[END_REF] Bruno Gallas, [START_REF] Hummel | Differential reflectance spectroscopy in analysis of surfaces[END_REF] and Agnès Maître This document provides supporting information concerning the influence of radius size and edge defects on the DRS and absorption spectra. In addition, we provide the gold permittivity used for calculations.

a) Electronic mail: agnes.maitre@insp.upmc.fr

GOLD PERMITTIVITY

The permittivity of gold was measured with the use of spectroscopic ellipsometry in a wavelength range ranging from 400 nm to 1000 nm. The measured data were fitted to the Lorentz susceptibility model, with five oscillators. The fitted values were used for FDTD calculations. Below is plotted the permittivity of the gold used in the experiments and simulations.

INFLUENCE OF RADIUS SIZE AND EDGE DEFECT OF THE PATCH ANTENNA ON THE DRS AND ABSORPTION SPECTRA

To investigate the influence of the radius size and edge defects on the DRS and absorption spectra of the patch antenna, we conducted FDTD simulations. The spectrum is red-shifted for higher radius size and edge defects. We also performed numerical calculations of the influence of PMMA edge defects on the DRS and absorption spectra (Fig. 5).


Sarah M Hörst
email: sarah.horst@jhu.edu

Chao He

Nikole K Lewis

Julianne I Moses

Eliza Miller-Ricci Kempton

Patricia A Mcguiggan

Mark S Marley

Caroline V Morley

Jeff A Valenti

Véronique Vuitton

Xinting Yu

Haze formation in the atmospheres of super-Earths and mini-Neptunes

.

Introduction

Super-Earths and mini-Neptunes are the most common type of planet in our galaxy, but their atmospheres remain mostly unexplored due to a lack of solar system analogue. With the launch of the Transiting Exoplanet Survey Satellite (TESS) and the impeding launch of the James Webb Space Telescope (JWST) we are beginning in a new era in planetary atmospheric science as we will be provided with the opportunity to study previous unexplored phase space. In preparation for these observations we have undertaken a series of laboratory simulation experiments using the Planetary HAZE Research (PHAZER) chamber (Figure 1) to study atmospheres spanning a wide range of temperatures and atmospheric compositions as shown in Figure 2. The experiments we will present span a range of temperatures (300-600 K) and initial gas mixtures (100 to 10,000x solar metallicity)

Experimental Setup

Our Planetary Haze Research (PHAZER) Chamber at Johns Hopkins University can be operated at a broad range of temperatures and pressures using a variety of reactant gases and two different energy sources. A schematic of the experimental setup is shown in Figure 1. The pressure in the reaction cell is measured and maintained at 1 mbar. The temperature of the gases and chamber are controlled by a custom heating system and measured with multiple thermocouples at the entry, exit, and inside in the chamber. While in the chamber, the reactant gases are exposed to one of two energy sources: a cold plasma generated by an AC glow discharge within the chamber or FUV photons produced by a hydrogen lamp attached to a port on the chamber. The gas phase products flow out of the chamber where they are measured by the residual gas analyzer (RGA). The solid phase products collect in the chamber; the collection technique depends on the requirements for subsequent analysis.

Results and Conclusions

We find that all experiments produce haze particles and that there are a wide range of production rates spanning at least three orders of magnitude, which indicates that we should expect some atmospheres to be hazy, while others should be relatively free from haze. We find that for both the cold plasma and UV energy sources the 300 K, 1000x metallicity experiments have the highest production rates [START_REF] He | Photochemical Haze Formation in the Atmospheres of super-Earths and mini-Neptunes[END_REF][START_REF] Hörst | Haze Production Rates in super-Earth and mini-Neptune Atmosphere Experiments[END_REF].

We see that visual inspection of the particles indicates that they will vary in optical properties as they are different colors in the optical [START_REF] He | Laboratory Simulations of Haze Formation in Cool Exoplanet Atmospheres: Particle Color and Size Distribution[END_REF].

We find that particles range from ~10 nm to 180 nm depending on the experimental conditions and that the width of the size distribution also depends on the initial conditions of the experiments [START_REF] He | Photochemical Haze Formation in the Atmospheres of super-Earths and mini-Neptunes[END_REF][START_REF] He | Laboratory Simulations of Haze Formation in Cool Exoplanet Atmospheres: Particle Color and Size Distribution[END_REF].

We find that in some cases our experiments simultaneously produce molecular oxygen (O 2 ) and organics. We also find that organics are produced in all experiments, even those that do not contain methane in the initial gas mixture. [START_REF] He | Gas Phase Chemistry of Cool Exoplanet Atmospheres: Insight from laboratory simulations[END_REF]

Figure 1 :

 1 Figure 1: Schematic of the PHAZER setup configured to use the cold plasma energy source.

Figure 2 :

 2 Figure2: The experiments we will present span a range of temperatures (300-600 K) and initial gas mixtures (100 to 10,000x solar metallicity)

Acknowledgements

This work was supported by the NASA Exoplanets Research Program Grant NNX16AB45G. C.H. was supported by the Morton K. and Jane Blaustein Foundation.


Véronique Vuitton
email: veronique.vuitton@univ-grenoble-alpes.fr

Laurène Flandinet

Sarah Moran

Chao He

François-Régis Orthous-Daunay

Cédric Wolters

Sarah Hörst
email: sarah.horst@jhu.edu

Orbitrap mass spectrometry of synthetic (exo-)planetary organic haze

Introduction

Aerosols are present in every substantial planetary atmosphere of the solar system and in that of exoplanets. They affect the chemistry, dynamics and flux of radiation in these atmospheres. They can transport organic material to the surface, act as a UV shield, modify the temperature and consequently have an impact on the habitability of the planet [START_REF] Marley | Clouds and hazes in exoplanet atmospheres in Comparative Climatology of Terrestrial Planets[END_REF].

The structure, composition and formation processes of organic aerosols are still largely unknown. A most illustrative example is that of Titan. Although the Cassini-Huygens mission provided a wealth of data on Titan's atmosphere and surface, in situ mass spectrometry as well as remote spectroscopy data provided only sparse information about the aerosols' molecular composition. Photochemical models cannot describe species with mass-to-charge ratios (m/z) above about 100 u because of the increasing complexity of the chemical network and the lack of appropriate kinetic data [START_REF] Vuitton | Chemistry of Titan's atmosphere in Titan: Interior, Surface, Atmosphere and Space Environment[END_REF].

The best way to study the molecular composition of planetary hazes is to synthesise analogs in the laboratory, called tholins, and to analyse them with stateof-the-art analytical instruments. Indeed, aerosol analogues made by exposing N 2 and CH 4 to various energy sources have been extensively characterised, revealing a bulk composition C/N∼1.5-3 and amine, imine, nitrile functional groups as well as heteroaromatic compounds [START_REF] Cable | Titan tholins: Simulating Titan organic chemistry in the Cassini-Huygens era[END_REF].

CO is an important component in many N 2 /CH 4 atmospheres: Titan, Pluto, Triton and probably super-Earths. Therefore, CO is the main atmospheric carrier of oxygen, an essential element of life as we know it. Previous laboratory experiments have shown that CO has an impact on the size and production rate of the tholins and that oxygen can be efficiently incorporated to form prebiotic molecules [START_REF] Hörst | Formation of amino acids and nucleotide bases in a Titan atmosphere simulation experiment[END_REF][START_REF] Fleury | Influence of CO on Titan atmospheric reactivity[END_REF][START_REF] Hörst | The effect of carbon monoxide on planetary haze formation[END_REF][START_REF] He | Carbon monoxide affecting planetary atmospheric chemistry[END_REF]. However, the impact of oxygen at the molecular level remains to be investigated.

2 Haze production -The PHAZER setup Laboratory analogues were synthesised at Johns Hopkins University by exposing to an AC glow discharge six different gas mixtures of N 2 , 5% CH 4 and variable amounts of CO (from 0 to 5%). The gases are cooled down to about 100 K and flow continuously (10 sccm) through a stainless steel reaction chamber where the pressure is held at 2 Torr. After 72h of discharge flow, the red/brown film deposited on the wall of the reaction chamber is collected under a dry N 2 atmosphere. The tholins are kept in a glove box and wrapped in foil to avoid exposure to air and light, respectively [START_REF] He | Carbon monoxide affecting planetary atmospheric chemistry[END_REF].

Molecular composition -The Orbitrap™ mass spectrometer

Very high-resolution mass spectrometry data were acquired at Univ. Grenoble Alpes with a Thermo LTQ Orbitrap XL (resolving power FWHM m/∆m ∼ 100,000 at m/z = 400 u) coupled with an electrospray ionisation (ESI) source. About 1 mg of tholins is diluted in 1 mL of methanol to generate a primary solution that is shaken and centrifuged at a spin rate of 10,000 rotations / min for 10 min. 500 µL of the soluble fraction is mixed with 500 µL of methanol to generate a secondary solution that is injected in the Orbitrap. Mass spectra of the samples are obtained in both positive and negative mode, in the 50-300 and 150-500 m/z range [START_REF] Somogyi | The role of ultrahigh resolution Fourier Transform Mass Spectrometry (FT-MS) in astrobiology-related research: Analysis of meteorites and tholins[END_REF]. The stoichiometry of each individual molecule making up the samples (about 3000 per sample) is obtained using the Attributor software developed at IPAG [START_REF] Orthous-Daunay | Ultraviolet-photon fingerprints on chondritic large organic molecules[END_REF] EPSC Abstracts Vol. 13, EPSC-DPS2019-PREVIEW, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019

These composition measurements provide some understanding of the chemical mechanisms by which CO affects particle formation and growth. Once a molecular formula has been assigned to each peak, the percentage in mass of element X (where X = H, C, N, O) in each sample is calculated by summing the number of element X in each peak weighted or not by its intensity [START_REF] Bonnet | Structure and composition of HCN polymer through high resolution mass spectrometry[END_REF]. As seen in Figure 1, as the initial abundance of CO increases, the O content increases while the N content decreases and the C and H content remains the same, in agreement with results from the elemental analysis of the bulk [START_REF] He | Carbon monoxide affecting planetary atmospheric chemistry[END_REF]. The Double Bond Equivalent (DBE) and Aromaticity Index (Xc) are proxies to determine the number of unsaturation (double/triple bonds, cycles) in a molecule based on the number of each element (H, C, N, O) [START_REF] Yassine | Structural characterization of organic aerosol using Fourier transform ion cyclotron resonance mass spectrometry: Aromaticity equivalent approach: Characterization of organic aerosol using FTI-CRMS[END_REF]. They show that the unsaturation does not depend on the number of O atoms in the molecule and that a large fraction of condensed aromatics (PAHlike) and aromatics are present in all samples.

In the context of the quest for biosignatures, development of very high-resolution mass spectrometers for spaceflight capable of in situ sampling of atmospheres and surfaces is mandatory [START_REF] Briois | Orbitrap mass analyser for in situ characterisation of planetary environments: Performance evaluation of a laboratory prototype[END_REF].

Figure 1 :

 1 Figure 1: Mass percentages of H, C, N, O in the six samples with increasing abundance of CO in the initial gases.

Acknowledgements

This work is supported by the French National Research Agency in the framework of the Investissements d'Avenir program (ANR-15-IDEX-02), through the funding of the "Origin of Life" project of the Univ. Grenoble-Alpes and the French Space Agency (CNES) under their Exobiology and Solar System programs. Cédric Wolters acknowledges a PhD fellowship from CNES/ANR (ANR-16-CE29-0015 2016-2021). Chao He was supported by the Morton K. and Jane Blaustein Foundation.