Nidhal Fessi

Mohamed Faouzi Nsib

Luis Cardenas

Chantal Guillard

Frederic Dappozze

Ammar Houas

Francesco Parrino
email: francesco.parrino@unitn.ithighlights.

Leonardo Palmisano

Gilles Ledoux

David Amans

Frédéric Dappozze

Yves Chevalier
email: yves.chevalier@univ-lyon1.fr

Surface and Electronic Features of Fluorinated TiO 2 and Their Influence on the Photocatalytic Degradation of 1-Methylnaphthalene

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In the last decades, heterogeneous photocatalysis has been applied in different fields ranging from environmental remediation, 1-4 synthesis of high value added compounds, [5][6][7] self-cleaning surfaces, 8-11 dye-sensitized solar cells, 12, 13 and photodynamic therapy 14,15 inter alia. TiO2 is widely used as photocatalyst in most of these applications due to its high photoactivity, low cost, chemical inertness, photostability and "green" features. 16 UV light excitation of TiO2 induces formation of photo-generated electrons (ecb -) and holes (hVb + ) in the conduction and valence band, respectively.

The photogenerated charges, in turn, induce degradation of organic molecules (e.g. pollutants) in the surrounding liquid medium by direct interfacial electron transfer (IFET) or by indirect oxidation upon formation and release of highly oxidizing species 17 (OH • , O2 •-, H2O2, NO3 • ) produced by interaction with oxygen and water.

However, despite the huge amount of papers published so far on environmental remediation by TiO2 photocatalysis, large-scale applications for water treatment are hardly viable due to the large volume of effluents to be treated, and to the low reaction rates exhibited in the presence of highly concentrated pollutants. Coupling photocatalysis with other advanced oxidation processes can be an economically sustainable alternative due to the efficiency enhancement resulting from the synergy between the concomitant processes. [18][19][20] On another hand, photocatalysis alone can be competitive with traditional remediation processes for niche applications, such as the degradation of nearly insoluble pollutants in aqueous media downstream to physical separation plants. In fact, oil separation is a time demanding step in conventional plants and does not generally provide the required level of decontamination in reasonable treatment times. In this regard, it has been recently demonstrated that the degradation of insoluble pollutants in aqueous emulsions stabilized by bare TiO2 is ca. 50 times faster than the degradation observed in biphasic systems. 21 This remarkable result is the starting point of the present investigation on surface fluorinated TiO2 (TiO2-F). The outstanding photocatalytic efficiency of TiO2-F, reported in low concentrated pollutant solutions, along with its robust and stable features and facile preparation methods, make it a promising candidate for the degradation of poorly soluble pollutants in aqueous media. To this end, we deeply investigated the electronic and surface properties of TiO2-F, responsible for its photocatalytic efficiency, and tested its activity for the degradation of 1-methylnaphtalene, a recalcitrant polycyclic aromatic hydrocarbon.

Reaction of fluoride anions onto the TiO2 surface results in substitution of hydroxyl groups with fluoride groups, yielding a TiO2-F material bearing Ti-F species at its surface. [START_REF] Minero | Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Anions. 1. Hydroxyl-Mediated and Direct Electron-Transfer Reactions of Phenol on a Titanium Dioxide-Fluoride System[END_REF] This strongly influences adsorption processes at the water-semiconductor interface, and remarkably affects the photocatalytic degradation of organic species depending on their affinity to the modified surface. As a result, the rate of photocatalytic degradations has been reported either to increase [START_REF] Minero | Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Ions. 2. Competitive Reactions of Phenol and Alcohols on a Titanium Dioxide-Fluoride System[END_REF][START_REF] Oh | Isotope Studies of Photocatalysis TiO2-Mediated Degradation of Dimethyl Phenylphosphonate[END_REF][START_REF] Park | Effects of TiO2 Surface Fluorination on Photocatalytic Reactions and Photoelectrochemical Behaviors[END_REF][START_REF] Park | Enhanced Remote Photocatalytic Oxidation on Surface-Fluorinated TiO2[END_REF][START_REF] Wang | Change of Adsorption Modes of Dyes on Fluorinated TiO2 and Its Effect on Photocatalytic Degradation of Dyes under Visible Irradiation[END_REF] or to decrease [START_REF] Vohra | Effects of Surface Fluorination of TiO2 on the Photocatalytic Degradation of Tetramethylammonium[END_REF] for surface fluorinated TiO2 with respect to the bare material. Among the various parameters affecting the photocatalytic activity, surface and electronic features are some of the most relevant.

As far as surface features are concerned, it is worth to mention that most of the molecules photocatalytically degraded in the presence of TiO2-F so far reported, were soluble in water while less attention has been paid to the photodegradation of non-polar molecules.

One example is the photodegradation of stearic acid in the presence of TiO2-F catalyst. [START_REF] Park | Enhanced Remote Photocatalytic Oxidation on Surface-Fluorinated TiO2[END_REF] However in this case, the photocatalyst and the substrate were separated by a thin air layer, in order to demonstrate the migration of the radical oxygen species which can behave as air-borne oxidants. For this reason, a part of this work is devoted to investigating the surface properties of TiO2-F material and their influence on the degradation of non-polar pollutants.

With regards to electronic properties of TiO2-F, it has been often reported that surface fluorination results in a selectively addressed interfacial electron transfer. For instance, Mrowetz and Selli 29 observed a higher hydroxyl radical production in the presence of TiO2-F compared with bare TiO2 and justified this result by considering the inhibited surface trapping of the photogenerated holes and the consequently more efficient hole induced direct water oxidation. In fact, surface fluorine groups enhance the Lewis acidity of the Ti 4+ groups, which effectively adsorb molecular water. [START_REF] Minella | Effect of Fluorination on the Surface Properties of Titania P25 Powder: An FTIR Study[END_REF] Similarly, Minero et al. [START_REF] Minero | Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Ions. 2. Competitive Reactions of Phenol and Alcohols on a Titanium Dioxide-Fluoride System[END_REF] reported faster photocatalytic degradation of phenol in the presence of surface modified TiO2-F compared to pristine TiO2 due to the enhanced formation of OH • radicals. Maurino et al. [START_REF] Maurino | Sustained Production of H2O2 on Irradiated TiO2 -Fluoride Systems[END_REF] reported the sustained production of hydrogen peroxide (up to 1.3 mM) in the presence of TiO2-F due to the competition of fluoride with superoxide/peroxide species at the surface of TiO2 which consequently inhibited the degradation of the photogenerated H2O2 which, therefore, accumulated in the system. However, the reported examples focus on electronic effects of the surface modification without any energetic characterization.

As mentioned above, the enhanced photocatalytic activity has been mainly attributed to changes in the interfacial electron transfer path rather than to intrinsic energetic differences of the photogenerated charges. This conclusion has been justified by various experimental evidences. It is generally reported that the UV-vis spectrum of TiO2-F does not show significant novel electronic transitions with respect to pristine TiO2. [START_REF] Mino | Beyond Shape Engineering of TiO2 Nanoparticles: Post-Synthesis Treatment Dependence of Surface Hydration, Hydroxylation, Lewis Acidity and Photocatalytic Activity of TiO2 Anatase Nanoparticles with Dominant {001} or {101} Facets[END_REF] Moreover, the substitution of terminal -OH groups with isoelectronic -F ones does not produce variation of the charge balance, and the system is silent to highly sensitive techniques such as electron paramagnetic resonance. 33 X-ray photoelectron spectroscopy (XPS) enables the detection of surface fluorine atoms, but it is often not sensitive enough to retrieve information on possible electronic interactions between fluorine and TiO2. On the other hand, UV photoelectron spectroscopy (UPS) is a more surface sensitive tool 34 that provides detailed information on the electronic density distribution of the valence band of semiconductive materials, which is not accessible by XPS owing to its low photoionization cross-section. This technique has been here applied for the first time to surface-fluorinated TiO2 and highlighted the existence of intra band gap energy states close to the valence band of TiO2.

The present investigation aims to correlate electronic and surface properties of the surface fluorinated TiO2 with its activity towards the degradation of poorly soluble substrates dispersed in aqueous suspensions. 1-Methylnaphtalene (1-MN) has been used as a model pollutant to this aim. It is a widely used model for studying hazards of polycyclic aromatic hydrocarbons in environment. [START_REF] Achten | Overview of Polycyclic Aromatic Compounds (PAC)[END_REF][START_REF] Neff | Response of Marine Animals to Petroleum and Specific Petroleum Hydrocarbons[END_REF]

Experimental

Materials Synthesis and Reagents

TiO2 nanoparticles were synthesized by using a simple sol-gel method. [START_REF] Behnajady | Investigation of the Effect of Sol-Gel Synthesis Variables on Structural and Photocatalytic Properties of TiO2 Nanoparticles[END_REF] In a typical synthesis, 19 mL of titanium tetraisopropoxide (Ti[OCH(CH3)2]4 97%, Alfa Aesar) were dissolved in 4 mL of methanol (MeOH 99.9%, Aldrich). The obtained solution was sonicated in an ultrasonic bath (Elma, T460/H, 35 kHz and 170 W). The hydrolysis process was then performed by adding dropwise 74 mL of deionized water under reflux and magnetic stirring. After 3 h refluxing time, the obtained white gel was filtered and washed several times using ethanol and deionized water, dried at 100 °C for 18 h, and finally calcined at 400 °C for 4 h to obtain the pristine TiO2 material, labelled below simply as TiO2.

The TiO2-F photocatalyst was prepared by wet impregnation of acidic fluoride solution. [START_REF] Herrmann | Uber die Chemie der Oberflache des Titandioxids. IV Austausch von Mydroxidionen gegen Fluoridionen[END_REF] 1 g of TiO2 was dispersed in 50 mL of a solution containing 4% sodium fluoride (NaF 99%, Aldrich) and a suitable amount of nitric acid (HNO3 99.5%, Fisher Scientific) to reach pH 3.2. [START_REF] Minero | Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Anions. 1. Hydroxyl-Mediated and Direct Electron-Transfer Reactions of Phenol on a Titanium Dioxide-Fluoride System[END_REF] The powder was centrifuged and washed several times with diluted HNO3 solutions (pH 3.2) and dried at 80 °C for 2 h.

Commercial Evonik P25 ® TiO2 was used as reference photocatalyst. This sample is comprised of anatase (80%) and rutile (20%), presents a specific surface area of ca. 50 m 2 •g -1 , and mean crystallite size of 30 nm. 1-Methylnaphtalene (95%) was purchased from Sigma-Aldrich and used as received without further purification. Surface charge changes upon pH variation and the isoelectric point of the samples were determined by means of electrophoresis measurements by using a Zetasizer NanoZS (Malvern, UK). The zeta potential, , was calculated from the electrophoretic mobility uE using the Henry relationship under the Smoluchowski approximation for particles radius larger than the Debye length:

Characterization Methods

The morphology and particle size of

𝒖 𝑬 = 𝜺 𝟎 𝜺 𝜻 𝜼 (1)
where η is the dynamic viscosity of water (η = 0.887 mPa•s at 25°C), ε0 the vacuum permittivity, and ε the dielectric constant.

Diffuse reflectance UV-vis absorption spectra of the samples were recorded in the range 200-800 nm using a Shimadzu UV-2401PC spectrophotometer equipped with an integrating sphere. BaSO4 standard was used as the reference.

X-ray photoelectron spectroscopy (XPS) was performed by using an Al Kα (1486.6 eV) source fitted to an Axis Ultra DLD spectrometer. High-resolution core levels analysis revealed chemical states attributable to C 1s, Ti 2p, F 1s and O 1s. The spectrum decomposition was carried out using a combination of Voigt functions with an overall full-width at half maximum (FWHM) of 1.6 eV.

Ultraviolet photoelectron spectroscopy (UPS) was performed by using the methodology developed by Cardenas et al. [START_REF] Maheu | UPS and UV Spectroscopies Combined to Position the Energy Levels of TiO2 Anatase and Rutile Nanopowders[END_REF] UPS spectra were obtained by using a gas discharge UV lamp installed to an Axis Ultra DLD detector. UV lamp can deliver photons of 21.2 eV (He I) and 40.8 eV (He II). UPS measurements were collected using the He I resonance line as photon source, based on its high photoionization cross-section suitable for valence band analysis. Colloidal suspensions of TiO2 and TiO2-F powders dispersed in pentane (4 g•L -1 ) were deposited by drop-casting method on an Ag foil. The Ag surface was previously prepared using Ar + sputtering under ultra-high vacuum, allowing the Fermi level step and "4d" states to be easily observed by UPS. Ag UP spectrum was used for internal calibration to determine the absolute energy level positions of TiO2 and TiO2-F. Photoelectrochemical measurements were performed in a 150 mL Pyrex reactor. 60 mg of the photocatalyst were added to 100 mL of a 0.1 M NaNO3 solution and sonicated for 2 min. A platinum electrode and an Ag/AgCl were used as working and reference electrodes, respectively. The suspension was degassed for 30 min by bubbling N2 and 25 mg methyl viologen (1,1'-dimethyl-4,4'-bipyridinium dichloride) were added. The pH value was adjusted using HNO3 and NaOH solutions. During the potentiometric titration the suspension was irradiated by an UV lamp (Philips HPK 125 W), and the values of pH and potential (V) were measured by using a pH meter Thermo Orion 720A and a multimeter Metex3800, respectively. Photoluminescence spectra were recorded by exciting the samples with a 10 mW CW laser model UF-F-355 (355 nm) from CNI laser, focused on a spot of 2 mm diameter. The sample was placed on a Linkam stage THMS600 in which the temperature could be varied between 77 K and 873 K. The luminescence spectra were collected thanks to an optical fiber, and fed into a Jobin-Yvon TRIAX320 monochromator coupled to a Newton EMCCD from Andor. The resolution of the system was 2 nm for all of the performed measurements. Time-resolved photoluminescence was measured on the same apparatus.

The excitation was performed by using an OPO laser from EKSPLA (NT230-50-SH) with a repetition rate of 50 Hz. The emission light was separated by a TRIAX320 monochromator and fed to a Peltier cooled Hamamatsu R943-02 photomultiplier (PMT).

The PMT signal was then sent to a digital oscilloscope from Lecroy (model waverunner LT372). The signal was averaged over 4000 laser shots. Measurements were performed on two different time ranges in order to cover a large range of lifetimes.

The amount of fluorine in samples was determined by means of inductively coupled plasma optical emission spectrometry (ICP-OES, Axial ICP-OES Vista Agilent). Details on XRD, SEM and BET measurements are reported in Supporting Information.

Photocatalytic Degradation of 1-MN

Photocatalytic experiments were carried out in a double-jacketed cylindrical Pyrex batch photoreactor (diameter: 10 cm) containing 160 mL of a suspension of photocatalyst (1 g•L -1 ) in an aqueous solution of 1-MN (10 mg•L -1 ). The reaction temperature was adjusted at 298 K. The suspension was stirred at 500 rpm and illuminated by a PLL lamp UVA-cantered at 365 nm (8 mW•cm -2 ) placed horizontally below the photoreactor.

Samples were collected at pre-specified time intervals and filtered through a Millipore PTFE 0.45 μm filter before liquid (HPLC) and gas (GC) chromatography analyses.

The evolution of the 1-MN concentration in the liquid phase was followed by injecting 50 μL into an HPLC system model 1290 Infinity (Agilent Technologies) equipped with a G4220B binary pump, a 1290 Infinity autosampler, a G4212B diode array detector (DAD), a G1321B fluorescence detector and a Polycyclic Aromatic Hydrocarbon (PAH) column (Agilent: 4.6×100 mm, 1.8 μm). Ultrapure water (solvent A) and acetonitrile (solvent B) were employed as the mobile phase with a flow rate of 1 mL•min -1 as follows: started with 60% A and 40% B, changed to 10% A and 90% B over 10 min, and finally, in 19 min take back to 60% A and 40% B for 1 min. Data were collected by using the MassHunter software (Agilent). The concentration of 1-MN and degradation by-products in the reactor head space was detected by coupling the photoreactor online with an electronic-nose Heracles II (Alpha MOS France) equipped with a CombiPal autosampler. An electronic nose consists of a fast Gas Chromatography equipped with two columns working in parallel mode: a non-polar column (MXT5: 5% diphenyl, 95% methylpolysiloxane, 10 m length and 180 μm diameter) and a slightly polar column (MXT1701: 14% cyanopropylphenyl, 86% methylpolysiloxane, 10 m length and 180 μm diameter) connected to a dual Flame Ionization Detector. 5 mL of gas flow was collected in the head space cell (remote sampling site of the photoreactor) and injected in a cold Tenax ® trap maintained at 15 °C. The gas desorbed from the trap at 240 °C was then flowed through the column by a H2 carrier gas. The columns were maintained at 35 °C for 60 s, then brought to 260 °C at 3 °C•s -1 heating rate and kept at that temperature for 30 s before cooling down. Data were processed using AlphaSoft v14 software.

Results and Discussion

Structural and Morphological Characterization

X-ray diffraction patterns of both the TiO2 and TiO2-F photocatalysts (Figure S1 in Supporting Information) show the presence of anatase structure (JCPDS card: 21-1272) and the mean crystallite size of the nanoparticles, calculated by using the Debye-Scherrer equation, was about 11 nm. It is evident that fluorination does not produce structural changes in the bulk of the sample, the structural modification being merely near to the surface.

The morphology of TiO2 and TiO2-F samples was observed by TEM (Figure 1) and SEM (Figure S2 in Supporting Information) analyses. Surface fluorination did not affect the morphology. In fact, both the samples were made of aggregates of small faceted elementary nanoparticles with quite narrow size distribution, being the mean crystallite size of ca. 10 nm, according to the value calculated from the XRD patterns.

Nitrogen adsorption and desorption isotherms (Figure S3 in Supporting Information) of both TiO2 and TiO2-F samples showed the classical type IV adsorption isotherm, suggesting the presence of a mesoporous structure. At high relative pressure from 0.45 to 0.80, the isotherms exhibited hysteresis loops of H2 type indicating the existence of ink-bottle type pores with narrow necks and wide bodies. The specific surface area calculated by using the Brunauer-Emmet-Teller (BET) equation was 103 m 2 •g -1 for both the samples. The pore size distribution, calculated by using the Barrett-Joyner-Halenda (BJH) equation was narrow and the average pore diameter was about 5 nm for both samples. Since the calculated pore size has the same order of magnitude of the elementary particles size, the retrieved porosity can be considered as deriving from interstices between the elementary nanoparticles. These results give evidence that surface fluorination does not induce modification of morphology and bulk structure.

Surface Properties

Surface fluorination of TiO2 can be considered as a simple ligand exchange between F - ions and surface hydroxyl groups (Equation 2). [START_REF] Herrmann | Uber die Chemie der Oberflache des Titandioxids. IV Austausch von Mydroxidionen gegen Fluoridionen[END_REF] TGA experiments of TiO2 and TiO2-F samples up to 900 °C are summarized in Figure 3.

Ti-OH

+ F -→ Ti-F + OH - (2)
Heating up to 450 °C caused almost identical mass losses (ca. 4.5%) for both samples. In this range two mass loss stages are evident. The first, in the range 40-200 °C is related to the removal of physically adsorbed water. [START_REF] Mueller | Surface Density of SiO2 and TiO2 by Thermogravimetric Analysis[END_REF][START_REF] Kim | Control of Hydroxyl Group Content in Silica Particle Synthesized by the Sol-Precipitation Process[END_REF] The slight deviation between TiO2 and TiO2-F samples in this range corresponds to a difference of surface hydration, being the TiO2-F sample less hydrophilic than the pristine one. This is in agreement with the relevant literature. [START_REF] Mino | Beyond Shape Engineering of TiO2 Nanoparticles: Post-Synthesis Treatment Dependence of Surface Hydration, Hydroxylation, Lewis Acidity and Photocatalytic Activity of TiO2 Anatase Nanoparticles with Dominant {001} or {101} Facets[END_REF] The second step between 200 and 450 °C is attributed to the mass loss of water formed from condensation of surface hydroxyl groups.

Electronic Properties

The electronic properties of pristine and surface fluorinated TiO2 were investigated by the joint use of spectroscopies and photoelectrochemical techniques.

Diffuse reflectance spectroscopy in the UV-vis range provides information on the band gap of the material, i.e. on the excitation energy of electrons from the valence to the conduction band. Figure 5 shows that fluorine surface modification of TiO2 led to a slight shift of absorbance towards the visible region compared to pristine TiO2.

The absorbance has been calculated as a function of the reflectance (R'∞) according to the Kubelka-Munk equation (Equation 3), where α and s are the absorption and the scattering coefficients, respectively

𝐹(𝑅′ ∞ ) = (1-𝑅 ′ ∞ ) 2 2𝑅 ′ ∞ = 𝛼 𝑠 (3)
The inset of Figure 5 Opposite evidences (no optical changes) have been reported by other authors, [START_REF] Czoska | The Nature of Defects in Fluorine-Doped TiO2[END_REF] so that this aspect needs to be further investigated. In our case, the mild preparation procedure makes fluorine insertion into the bulk of TiO2 less probable; however, formation of Ti-F-Ti surface species cannot be in principle excluded. However, the binding energy of the Ti-F bond is shifted by few hundreds meV with respect to literature values, 43,44 indicating a contamination by atmospheric oxygencontaining species (water, CO2, organics) interacting with the fluorinated surface of both TiO2-F and TiF4. This could induce a variation of the chemical environment at the surface and influence the binding energy of the Ti-F bond, possibly leading to misinterpretation due to the spectral resolution and differential charge effects on the surface. In order to check against this hypothesis, both the TiF4 and TiO2-F samples have been treated with Ar + sputtering under ultra-high vacuum at 1 keV for 10 and 30 min. Ar + etching removes from the surface air-borne molecules, leaving the sole F bound to Ti atoms. As a consequence of Ar + sputtering, the signals of the Ti-F bond of both TiF4 and TiO2-F shifted towards higher binding energies (of ca. 0.4 eV for TiO2-F and 0.6 eV for TiF4) in agreement with literature data. Ti-F bonds were kept in both cases even after prolonged Ar + etching (30 min, green line Figures 6b andc) thus providing clear demonstration that stable covalent Ti-F bonds have been formed at the surface of TiO2-F. Notably, the F 1s peak at 684.3 eV, observed in the non-sputtered sample, has been previously assigned to combination of non-resolved peaks of bound fluorine atoms and adsorbed fluoride anions. [START_REF] Yeh | Atomic Subshell Photoionization Cross Sections and Asymmetry Parameters: 1  Z  103[END_REF] However, according to our results, the presence of residual adsorbed fluoride anions was discarded because of the similar behavior observed with pure TiF4 that does not contain fluoride anions.

Finally, even if fluorine insertion into the bulk of anatase can be excluded because of the absence of the characteristic F 1s peak at 688.3 eV, 42 a weak F 1s peak at ca. 690 eV might correspond to very small amounts of bridging fluorine (Ti-F-Ti) at the surface of TiO2. Such signal has been observed in case of bulk fluorination, e.g. for bulk F-doped TiO2 samples prepared by sol-gel synthesis by using NH4F as a precursor. [START_REF] Yu | Effects of F -Doping on the Photocatalytic Activity and Microstructures of Nanocrystalline TiO2 Powders[END_REF] The trace amount of Ti-F-Ti groups which could be detected by XPS analysis, justifies that only slight changes were observed by the poorly sensitive UV-vis diffuse reflectance spectroscopy technique, and the contradictory results often reported in literature on the optical properties of surface fluorinated TiO2.

In order to gain further evidences on the possible electronic interactions between surface fluorine species and the semiconductor structure of TiO2, UPS analysis was performed by using the particular expertise developed by Cardenas et al. [START_REF] Maheu | UPS and UV Spectroscopies Combined to Position the Energy Levels of TiO2 Anatase and Rutile Nanopowders[END_REF] This technique affords spectra of electronic states related to the valence band against the vacuum level with spectral resolution (0.15 eV) more than five times higher than XPS (0.6 eV). Indeed, the photoionization cross-section of titanium is ca. 10,000 times higher for UV excitation (21.2 eV) than for X-rays (1486.6 eV). [START_REF] Yeh | Atomic Subshell Photoionization Cross Sections and Asymmetry Parameters: 1  Z  103[END_REF] Moreover, as a lower photoelectron energy range is probed, only electrons coming from the extreme surface of the material can be excited and the technique results very surface sensitive (the analyzed depth is ca. 1 nm). For these reasons, UPS is a powerful technique for the purpose of the present investigation.

Both TiO2 and TiO2-F samples produce a broad signal between 13 and 2 eV (Figure 7a).

TiO2 spectrum exhibits three bands. The broad band at ca. 6 eV and the one at 8.5 eV correspond to hybridization between Ti and nonbonding and bonding O 2p orbitals. [START_REF] Asahi | Electronic and Optical Properties of Anatase TiO2[END_REF] The small band at around 11 eV can be attributed to surface OH groups. [START_REF] Sanjinés | Electronic Structure of Anatase TiO2 Oxide[END_REF] The band assignment for TiO2-F sample is much more complex. The observed differences between TiO2 and TiO2-F can be attributed to the contribution of F 2p orbitals. [START_REF] Valentin | Trends in Non-Metal Doping of Anatase TiO2: B, C[END_REF] Notably, signal related to trivalent Ti 3+ 3d species was expected at around -1 eV from EF as it has been observed with reduced TiO2 and bulk-doped TiO2; 4953 it has not been observed in both present samples, confirming the XPS results. Notably, the band at 11 eV is negligible in the TiO2-F sample further confirming the above mentioned substitution of surface OH groups with covalently bound fluorine atoms. By magnifying the spectrum in the region of the edge of the valence band (Figure 7b), it is evident that photoelectrons can be excited at lower energy in the case of TiO2-F with respect to pristine TiO2. This important upward shift of the valence band by ca.

EFB (pH=7) = 𝐸 0 MV 2+ /MV +• + 0.059 (pH0 -7) ( 4
)
where 𝐸 0 MV 2+ /MV +• is the standard potential of the redox couple MV 2+ /MV +• equal to -0.445 V vs (NHE). [START_REF] Wardman | Reduction Potentials of One-Electron Couples Involving Free Radicals in Aqueous Solution[END_REF] The values obtained were -0.53 ± 0.4 V vs (NHE) for TiO2-F and -0.56 ± 0.4 V vs (NHE) for pristine TiO2. Being the reproducibility of ± 0.04 V for the determination of flat band potential, no remarkable differences in the absolute potential of the photogenerated electrons can be observed between the two samples, so that it can be concluded that the intermediate energy states disclosed by UPS analysis are located close to the valence band. respectively. Authors also report that the extent of these changes decreases by decreasing the fluorination degree of the surface, but are fully visible already for coverages of ca.

30%. All of these reports, even if controversial with regard to the nature of the energy states, confirm that surface fluorination can induce remarkable changes of the order of ca.

1 eV, supporting the experimental results hereby reported.

The classical theory of semiconductor junctions by Tamm and Shockley provides support

showing how slight modifications of the surface cause large effects on bulk electronic properties. The influence of electronic states at the surface of semiconductors is first the formation of well-defined 'intrinsic' surface electronic states within the band gap. 60, [START_REF] Lüth | Surfaces and Interfaces of Solid Materials[END_REF] The second consequence is bending the VB and CB. As the Fermi levels of bulk and surface electrons are the same, electrons filling surface states from the subsurface region leave there a depletion region that causes upward band bending of an n-doped semiconductor.

Band bending coming from the electrostatic effect of the space charge is the same for VB and CB, so it does not change the band gap energy. Such surface phenomena are operating at all semiconductorvacuum interfaces. Band bending can extend over a large part of the band gap of bulk materials. [START_REF] Lüth | Surfaces and Interfaces of Solid Materials[END_REF] It is weak for small semiconductor particles because of the confinement of the electrical double layer in the space charge region. [START_REF] Zhang | Band Bending in Semiconductors: Chemical and Physical Consequences at Surfaces and Interfaces[END_REF] So, surface states set bulk properties by 'Fermi level pinning'. Chemical modification of the surface strongly affects bulk electronic properties and photocatalytic activity in that way.

The intermediate energy levels result in different photoluminescence emission behavior of

TiO2-F with respect to the pristine TiO2 material. Emission spectra of TiO2 and TiO2-F have been recorded at different temperatures ranging from -190 °C to 20 °C. Figure 10 shows them at the limit temperatures considered. The spectra recorded at intermediate temperatures are reported in Figure S5 in Supporting Information. The differences between the two samples obviously appeared at low temperatures, while the behavior is virtually the same at room temperature. For both the samples emission was weak between 380 and 400 nm while it increased at longer wavelength.

Therefore, emission from free excitons is less probable, as expected for an indirect bandgap semiconductor such as TiO2, while most emitted photons have longer wavelengths. Such emission has been ascribed to the presence of empty levels of Ti 3+ within the band gap, slightly below the CB. 63, [START_REF] Dozzi | Fluorine-Doped TiO2 Materials: Photocatalytic Activity vs Time-Resolved Photoluminescence[END_REF] The higher emission observed at low temperature for the TiO2-F sample with respect to bare TiO2 confirms the existence of intermediate energy levels disclosed by UPS spectroscopy. This effect is visible at low temperature because vibration quenching occurring at low temperature decreases nonradiative de-excitation, thus revealing the stronger emission of TiO2-F at longer wavelengths. Light emission by radiative decay of the excited states was measured by time resolved photoluminescence spectroscopy, which gave information on the life-time of the excited species. This information is of high relevance since efficient photocatalytic activity generally requires long living photoexcited states. Measurements were performed at 500 nm and -190 °C (Figure 11). The emission decay of the excited states is faster for TiO2-F than TiO2. The mean relaxation time, 𝜏̅ estimated as the time when half the energy has been emitted through Equation 5 was 0.24 ± 0.02 µs for TiO2 and 0.13 ± 0.02 µs for TiO2-F.

∫ 𝐼(𝑡) 𝑑𝑡 𝜏 ̅ 0 = 1 2 ∫ 𝐼(𝑡) 𝑑𝑡 ∞ 0 (5)
Photoluminescence decays were not purely exponential. This can be ascribed to a distribution of several types of excited states emitting light at the same wavelength, or to a distribution of non-radiative de-excitation paths (recombination channels) from the same state. In order to discriminate these two cases, the non-exponential decay was modelled using a classical Kohlrausch stretched exponential [START_REF] Kohlrausch | Theorie des elektrischen Rückstandes in der Leidener Flasche[END_REF] (Figure S6 in Supporting Information) according to Equation 6𝐼(𝑡) = 𝐼(0) exp (-(

𝑡-𝑡 0 𝜏 ) 𝛽 ) (6)
where  is a time constant,  describes the width of the distribution (0    1), and t0 is the start time hidden by the excitation pulse. The  exponent characterizes the width of the distribution and have nearly the same value for TiO2 and TiO2-F, i.e.  = 0.080  0.002. A distribution of radiative excited states would be characterized by quite a large value of  (0.5 <  < 1). 66- [START_REF] Brown | Origin of Stretched-Exponential Photoluminescence Relaxation in Size-Separated Silicon Nanocrystals[END_REF] The very low value of  reveals a broad distribution of non-radiative deexcitation paths coming from an increased density of non-radiative centers.

Non-radiative de-excitation is attenuated at low temperature (77 K), so that radiative transitions were detected with higher sensitivity. Since the shorter lifetime observed for TiO2-F was ascribed to a faster non-radiative decay coming from a larger density of recombination paths, the luminescence quantum yield should be lower for TiO2-F.

Therefore, the origin of higher luminescence observed for TiO2-F is a larger density of defects emitting light. As a whole, fluorination causes both increased densities of defects emitting light and non-radiative centers. The presence of intermediate energy states within the band gap of TiO2-F causes faster de-excitation with prevalent non-radiative character compared to bare TiO2.

Photocatalytic Activity

Figure 12 shows the kinetics of 1-MN photodegradation in the presence of pristine TiO2, TiO2-F and commercial P25 TiO2 (Evonik) under UV light. P25 TiO2 was used as a reference in order to compare the photocatalytic activity of TiO2 and TiO2-F samples with a commercial benchmark. The suspension containing 1-MN was maintained in the dark under stirring during 180 min before irradiation. 1-MN adsorption reached equilibrium after ca. 30 min for all samples. Notably, the initial concentration of 1-MN was below its solubility limit in water. Tests in biphasic conditions are ongoing and will be subsequently reported. Under UV light irradiation degradation of 1-MN started. Figure 12 reports the 1-MN concentration decrease in the liquid phase and in the head-space gas, demonstrating the effective degradation of the substrate. The degradation of the 1-MN was faster in the presence of TiO2-F than both pristine TiO2 and P25.

The present results show that fluorination of TiO2 results in enhanced 1-MN degradation with respect to both pristine TiO2 sample and to the benchmark TiO2 P25.

The activity of a photocatalyst is the result of the complex interplay between various factors. In the present case both pristine and surface fluorinated TiO2 possess similar morphological and structural properties, so that the higher photocatalytic efficiency of TiO2-F can be associated with the surface and opto-electronic features.

As far as the surface features are concerned, it has been shown from TGA analysis that the surface of the fluorinated sample is slightly less hydrophilic that the pristine TiO2. This would favor the adsorption of 1-MN, as it results from the slightly higher amount of 1-MN adsorbed under dark conditions (Figure 12a).

𝛤 =

(𝐶 tot -𝐶 res )𝑉 𝑀 𝐴 sp (7) where M/V = 1 g•L -1 is the dispersed amount of catalyst and Asp = 103 m 2 •g -1 is its specific area. The equilibrium constant for 1-MN adsorption was estimated assuming a Langmuir-type adsorption isotherm (Equation 8).

𝐾 =

𝛤 (𝛤 max -𝛤) 𝐶 res (8) The surface concentration at full monolayer coverage was 2.8 µmol•m -2 , as calculated from the molecular area of 1-MN adsorbed flat to a surface (60 Å 2 ). 70 By using concentrations expressed in mol•L -1 , the equilibrium constant obtained through Equation 8 is K = 3200 (LogK = 3.5), which corresponds to a standard free energy of adsorption much larger than thermal energy: adsG 0 = -20 kJ•mol -1 (8 RT).

Finally, surface fluorination might enhance photocatalytic activity because of the better dispersibility of the photocatalyst in aqueous medium, [START_REF] Kim | Reactivity of Surface Fluorinated TiO2 and TiAl Particles[END_REF] leading to a better contact with 1-MN molecules in the medium. The observed shift of the point of zero charge (PZC) upon fluorination from 6.2 to 4.0 makes TiO2-F particles negatively charged at neutral pH (between 6 and 7), whereas TiO2 particles are quasi-neutral (PZC = 6.2). Electrostatic repulsions between TiO2-F particles provide a better dispersion and efficiently stabilize the dispersion during the photocatalytic process.

As far as the electronic properties are concerned, we hereby demonstrated the existence of localized intra band gap energy levels within ca. 1.3 eV above the valence band for the TiO2-F sample. The resulting slightly different optical features of the fluorinated sample with respect to the pristine TiO2 can be beneficial to the photocatalytic activity. Thus, excitation can occur at lower energy of the incident radiation and these centers can behave as active sites promoting photocatalytic activity. The higher defect density observed for the TiO2-F caused higher emission and faster charge recombination compared to bare TiO2. De-excitation occurs through both radiative and non-radiative pathways. So it can be inferred that the higher density of electronic defects may be beneficial to the photocatalytic activity, regardless the slightly shorter life time of the electron-hole pairs. The presence of intermediate energy states can also explain the reported enhanced generation of singlet oxygen in the presence of TiO2-F. 72, [START_REF] Daimon | Formation and Behavior of Singlet Molecular Oxygen in TiO2 Photocatalysis Studied by Detection of Near-Infrared Phosphorescence[END_REF] Energy emission from these states can induce excitation of oxygen from the triplet to the singlet state that would hardly occur in bare TiO2 due to the mismatch between the band gap energy and the oxygen excitation energy. BET measurements. Nitrogen adsorption and desorption isotherms of the synthesized photocatalysts were measured using a Micrometrics TRISTAR 3000. The specific surface area was assessed using the Brunauer-Emmett-Teller (BET) equation in the relative pressure range p/p 0 between 0.05 and 0.3. Porosity was calculated from the desorption branch above 0.4 relative pressure by the Barrett-Joyner-Halenda (BJH) method. Adsorption and desorption isotherms showed the classical type IV adsorption isotherm suggesting the presence of a mesoporous structure (Figure S3). At high relative pressure from 0.45 to 0.80, the isotherms exhibited a hysteresis loop of type H2, indicating the existence of ink-bottle type pores with narrow necks and wide bodies. The specific surface area calculated using the BET equation was 103 m 2 •g -1 .

Conclusions

Identical results were obtained with TiO 2 -F (not shown).

S4

The inset in Figure S3 shows the pore size distribution calculated from the desorption branch of the isotherm using the BJH equation. The pore size distribution of porous volume dV/dD was narrow and the average pore diameter was about 5 nm. Whatever the temperature, exciton photoluminescence was not observed. Photoluminescence at low wavelengths was stronger for TiO 2 -F than TiO 2 , especially at low temperatures where radiative deexcitation is more visible. where τ is a time constant, β describes the width of the distribution (0 ≤ β ≤ 1), and t 0 is the start time hidden by the excitation pulse. β = 1 for a single excited state yielding a purely exponential decay. The stretched exponential decay has been often used as a model accounting for the distribution of emission decays 15-18 and also many other relaxation phenomena in disordered media. 19,20 Experimental data was taken for times longer than 10 ns for which the excitation pulse did not perturb emission signal. Adjustment of the three parameters t 0 , τ and β (Table S1) yielded a good fit (with R 2 close to 1) over the whole experimental time scale (Fig. S6). For small values of β, the time constant τ does not have a clear physical meaning. The mean lifetime 𝜏̅ of the excited state is estimated as the time when half the energy has been emitted

∫ 𝐼(𝑡) 𝑑𝑡 The low values of β originated from a broad distribution of deexcitation pathways leading to a broad distribution of exponential decays. Such pathways were predominantly non-radiative because the luminescence intensity was weak. Indeed the mean luminescence decay rate 1 𝜏̅ ⁄ is defined by contributions to emission of all emitting sites {i} following their own individual lifetime 𝜏 i . For each site {i}, 𝜏 i is a combination of radiative and non-radiative lifetimes:

1 𝜏 i = 1 𝜏 i,rad + 1 𝜏 i,non-rad (S4)
Accordingly, the quantum yield equal to the ratio τ/τ rad was low. As a whole, the short nonradiative life-times contribute much to the luminescence decay, even though non-radiative deexcitation does not emit light.

  TiO2 samples were determined by means of a JEOL JEM 1220 transmission electron microscope (TEM) operating at 120 kV. The samples were dispersed in water (0.1 wt%), deposited onto a thin Formvar film supported on a Cu grid, and dried in open air before TEM observation. Thermogravimetric analyses (TGA) were performed with a Setaram SETSYS Evolution 12 thermoanalyzer coupled via a heated (ca. 150°C) fused silica capillary with Pfeiffer Omnistar quadrupole mass spectrometer (MS). TGA-MS studies were performed by heating the samples (40 mg in Pt-based open crucibles) from room temperature to 1100 °C at a constant rate of 10 °C•min -1 under nitrogen gas (50 cm 3 •min -1 STP).

Fig 1 .

 1 Fig 1. TEM micrographs of synthesized pristine TiO2 (A) and TiO2-F (B).

Equation 2

 2 FigureS3in Supporting Information), results have been expressed in the form of fluorine surface density. In agreement with Minero et al.[START_REF] Minero | Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Anions. 1. Hydroxyl-Mediated and Direct Electron-Transfer Reactions of Phenol on a Titanium Dioxide-Fluoride System[END_REF] the maximum fluorine surface density of 3 fluorinated surface sites per nm 2 was achieved at pH = 3.2 (Figure2). This value corresponds to the substitution of ca. 50-60% of the total Ti-OH surface groups in the pristine TiO2 (hydroxylation degree of 4.8-6.1 Ti-OH groups per nm 2 ).

Fig 2 .

 2 Fig 2. Amount of fluorinated sites obtained at different pH values of the starting NaF solution used for the modification. The experimental error has been estimated as 0.3 nm 2 in all experiments though no systematic repetitions have been performed.

39 , 40

 3940 Dehydration is almost complete at 500 °C. However, by considering the mass spectrometry analysis of the gases released in this temperature range, it is evident a slight mass drift up to 600 °C, showing further dehydration at higher temperatures. A third mass loss of about 3 wt% occurred only for the TiO2-F sample in the range 450-600 °C. It was attributed to dehydrofluorination of the surface, as confirmed by the mass spectroscopy analysis of the HF-containing released gases between 450 and 600 °C. It was not possible to quantify the absolute amount of H2O and HF released due to the unknown ionization efficiency of each compound. Moreover, being H2O and HF simultaneously released, these data could not be used to confirm the fluorination extent retrieved by ICP-EOS analysis.

Fig 3 .

 3 Fig 3. TGA thermographs of pristine TiO2 (black line) and TiO2-F (red line). The green and blue lines show the amount of released H2O and HF, respectively, from TiO2-F obtained by means of MS analysis. Notably, MS data are reported in arbitrary units, and do not refer to the vertical axis.

Fig 4 .

 4 Fig 4. Zeta potential of TiO2 and TiO2-F particles in aqueous suspensions as a function of pH.

  shows the plot of [F(R'∞) hν] 1/2 versus the incident photon energy (hν). The extrapolation of the linear part of the plot to the x-axis, defines the band gap energy of the samples considered as indirect band gap semiconductors. The values of the band gap retrieved through this procedure were 3.23 and 3.16 eV for TiO2 and TiO2-F, respectively. This difference is rather small but discloses slight changes in the electronic structure of the surface fluorinated sample with respect to the pristine one. However, a global shift keeping the band gap quasiconstant can take place; this does not prevent a redistribution of spectral weight concomitant with the formation of new electronic levels around of the pseudo Fermi level. It is worth to mention that no difference in terms of carbon or nitrogen impurities deriving from the preparation procedure could be detected by comparing FTIR (data not shown) and XPS analysis of pristine and fluorinated samples, so that the small changes in the UV-vis absorption should be attributed to the presence of fluorine. Though the absorption edge was only slightly affected by surface fluorination, Figure5shows a different optical behavior of the two powders. Similarly, Park and Choi 25,26 gave evidence of a higher light absorption (mainly in the UV region) when the surface of TiO2 was fluorinated. This result has been related to the higher amount of adsorbed water in the case of TiO2 with respect to TiO2-F which enhances the amount of reflected photons and consequently reduces the absorbance with respect to the fluorinated sample. This hypothesis was also inferred in our case by taking into account the lower hydrophilicity of the TiO2-F sample experimentally observed in the previous section by means of thermo-gravimetric analysis. On the other hand, insertion of fluoride into the crystal lattice of TiO2 is sometimes reported to increase the light absorption and to cause a slight red shift in the band gap compared with pristine TiO2.[START_REF] Yu | Effects of F -Doping on the Photocatalytic Activity and Microstructures of Nanocrystalline TiO2 Powders[END_REF]

Fig 5 .

 5 Fig 5. UV-vis diffuse reflectance spectra of synthesized pristine TiO2 (black) and TiO2-F (red).Plots of ( h) 1/2 versus h (inset).

Figure 6

 6 Figure6shows the Ti 2p and F 1s core level signals related to pristine TiO2, TiO2-F, and a commercial TiF4 sample purchased from Sigma-Aldrich. In some cases, samples have been sputtered under Ar + etching treatment.The different core levels of Ti in TiO2 and TiO2-F are shown in Figure6a. The spin orbit states of Ti 2p3/2 at 458.2 eV and Ti 2p1/2 at 464.0 eV can be attributed to Ti 4+ species and they are present both in the TiO2 and TiO2-F sample. However, no trace of Ti 3+ species coming from fluorine insertion is visible at 457.0 eV in the Ti 2p level of the spectrum of TiO2-F, confirming that, if present, Ti-F-Ti groups on the TiO2 surface are below the detection limit of XPS. Similarly, no Ti(II) species, which would produce a signal at 455.2 eV, could be seen.The analysis of the F 1s state for TiO2-F surface (Figure6b, blue line) revealed a single contribution at 684.3 eV which, as expected, is absent in the spectrum of pristine TiO2 (data not shown). The spectrum of TiF4 taken as reference (Figure6c, blue line) showed the same state at 684.3 eV, confirming the presence of Ti-F bonds on the surface of TiO2-F sample.

Fig 6 .

 6 Fig 6. (a) Ti 2p core levels in TiO2 (black line) and TiO2-F (red line) samples. F 1s core levels in TiO2-F (b) and TiF4 (c) samples before (blue line), after Ar + sputtering for 10 min (black line), and for 30 min (green line).

1 . 3

 13 eV indicates the presence of intermediate intra band gap energy levels in the fluorinated sample. Such states, appearing as the presence of a tail ranging between 1.3 and 2.6 eV in Figure 7, are localized close to the valence band. In order to estimate the position of these intermediate energy states with respect to the conduction band, the absolute position of the conduction band edge was determined.

Fig 7 .

 7 Fig 7. UPS spectra of TiO2 and TiO2-F. Binding energy is expressed with respect to the vacuum energy level. (a): Wide UPS scan between 13 and -2 eV. (b): Local spectral zone between 4.5 and -0.6 eV.

Fig 8 .

 8 Fig 8. Effect of pH on the photovoltage developed by irradiation of TiO2 (left) and TiO2-F (right). Each experiment was repeated 3 times and reproducibility was better than ± 20 mV.

Fig 9 .

 9 Fig 9. Schematic diagram representing the electronic structure of TiO2 and TiO2-F. The light blue part shows the position of the intermediate energy levels inside the band gap.

Fig 10 .

 10 Fig 10. Photoluminescence spectra of pristine TiO2 (black lines) and TiO2-F (red lines) at +20 °C and -190 °C. The 355 nm excitation wavelength has been cut off with a long-pass filter at 380 nm. The inset shows the photoluminescence spectra of TiO2 (red line) and TiO2-F (black line) recorded at +20 °C in linear scale.

Fig 11 .

 11 Fig 11. Time-resolved photoluminescence of pristine TiO2 (black line) and TiO2-F (red line) at 500 nm and 77 K.

  Even though the surface features dramatically change under irradiation, the proximity of 1-MN molecules to the surface of TiO2 could be beneficial for the photocatalytic degradation both if it occurs directly through photogenerated holes or indirectly through OH • radical mediated oxidation. It must be mentioned, that surface fluorinated TiO2 produces higher amount of OH • radicals with respect to pristine TiO2 because the holes in the valence band are not able to oxidize surface fluorine groups because the oxidation potential of the F • /F -couple is too high (3.6 V). Therefore, hole-induced oxidation of water producing OH • radicals is favored.

Fig 12 .

 12 Fig 12. Degradation of 1-MN over pristine TiO2, TiO2-F and P25 under UV irradiation. Concentration of 1-MN in liquid phase (a) and in the head-space gas phase (b) was analyzed by HPLC and by electronic nose GC analysis, respectively.

  photocatalyst for the photocatalytic degradation of polycyclic aromatic hydrocarbon 1methylnaphtalene. Evidence has been given that the morphology and the structural features of TiO2 do not change upon fluorination. On the other hand, significant modifications have been induced as far as the surface and opto-electronic features are concerned. In particular, it has been shown that fluorination reduces the hydrophilic character of TiO2 and shifts the point of zero charge of TiO2 towards lower pH values.Moreover, it has been highlighted for the first time that surface fluorination induces intra band gap energy states within 1.3 eV above the valence band. Combination of UPS and photoelectrochemical measurements specified the position of such energy levels close to the valence band. These features have been related with the photocatalytic activity of the fluorinated TiO2 sample for the degradation of 1-methylnaphtalene, which is ca. twice higher with respect to both the pristine TiO2 sample and to the commercial Evonik P25.

Figure S2 .

 S2 Figure S2. SEM surface morphology of pristine TiO 2 .

Figure S3 . 12 S5Figure

 S312 Figure S3. Nitrogen adsorption/desorption isotherms of pristine TiO 2 . Inset shows the pore size distribution.

Figure S5 .𝑡

 S5 Figure S5. Photoluminescence spectra of TiO 2 and TiO 2 -F at different temperatures between 20°C and -190°C.

  small value of β (broad distribution) can lead to a very small value of the parameter 𝜏̅ . Most distributions of excited states yield 0.5 ≤ β ≤ 1. 15-18,21 The error on 𝜏̅ was estimated as twice the width of the laser light pulse.

Acknowledgment

NF acknowledges a fellowship for travelling from the Tunisian Ministry of Higher Education and Scientific Research as a support to this international collaborative project.

Supporting Information

X-ray diffraction, SEM, BET, XPS and photoluminescence measurements that are not included in the main text.

Conflicts of interest

There is no conflict of interest to declare. Table S1. Parameters of the best fit of the stretched exponential (Eq. S2) to the time-resolved photoluminescence at 500 nm for delays longer than 10 ns.

Supporting Information

Surface and Electronic Features of Fluorinated


V K Guda

D Chablat
email: damien.chablat@cnrs.fr

C Chevallereau
email: christine.chevallereau@ls2n.fr

Safety in a Human Robot Interactive: Application to Haptic Perception

. For application considered in this paper, the cobot carries several specimens of texture on its end-effector, to allow contact between a finger of the user and the robot.

Safety is an important aspect in Human Robot Interactions (HRI) [5], even with the use of cobots, because contacts are expected. The purpose of this paper is to introduce a new methodology to define the basic placement of the robot in relation to the human body and for the planning and control of movements during HRIs to ensure safety.

Introduction

Safety is important in interactions between human and robots (HRI). Robots can perform powerful and fast movements that can be dangerous to the humans around them. Involuntary contact between these robots and humans is a threat. This is particularly important in a virtual reality context where humans equipped with a headset will not be able to anticipate the robot's movements. Today more than ever, men work closely with robots. In the case of intermittent contact interface ICI, contact is inevitable between humans and robots. Cobots are best suited to such a scenario, but in terms of human safety, accident prevention can always be improved [START_REF] Cherubini | Collaborative manufacturing with physical human-robot interaction[END_REF]. These robots are designed to work at limited speeds during potential contacts. Moreover, it must be ensured that the desired contacts with the robot during interaction will not result into an necessary restart of the robot after a safety stop [START_REF] Long | An industrial security system for human-robot coexistence[END_REF].

The model of haptic perception is the human sense of touch. The deep sensibility is an integral part of this human sense. It includes the senses of force, position and movement of the different parts of the human body. In our case the objective is development of a new generation of haptic devices called intermittent contact interfaces (ICI) [START_REF] Cruz | Contributions to the design of a 6 dof contactless sensor intended for intermittent contact haptic interfaces[END_REF]. ICI are based on the principle of controlling a robot to follow the user's movements remotely, and only come into contact when a haptic interaction must be created, for example to simulate a contact between his hand and the virtual environment. The combination of visual virtual reality and haptic device can provide users with the sensation of touching virtual objects of varying shapes, using pseudo-haptic effects [START_REF] Ban | Modifying an identified curved surface shape using pseudo-haptic effect[END_REF][START_REF] Lécuyer | Simulating haptic feedback using vision: A survey of research and applications of pseudo-haptic feedback[END_REF].

Context of the study

The context of the study is the evaluation of the perceived quality of a virtual car interior during the first design phases. In a given scenario, the user sits in the real world for a visual virtual reality experience inside the car. The user wears a helmet and cannot see the robot, which explains the safety problem (Figure 1). While the user is trying to interact with the virtual object in the environment, the robot must come and position a sample of the material associated with the local surface, in order to provide a tactical sense of touching the object [START_REF] Mercado | Entropia: Towards infinite surface haptic displays in virtual reality using encountered-type rotating props[END_REF][START_REF] Salazar | Altering the stiffness, friction, and shape perception of tangible objects in virtual reality using wearable haptics[END_REF]. A motion capture system based on HTC vive trackers is used to know the position of the body and especially the hand used for interaction as well as the position of the chair and the robot [START_REF] Tomić | Human to humanoid motion conversion for dual-arm manipulation tasks[END_REF] (Figure 2). Currently, the prop can carry six different materials (Figure 3). The robot is fixed on a 75 cm high table and the user sits on a seat 60 cm above the floor. So in this scenario, ideally we want the robot workspace to cover all the virtual environment (in this case the passenger compartment of the car) the user could interact. Figure 4 depicts the interior of a Dacia Duster and three areas to touch. In this process the robot should avoid unwanted collisions with the user. In this context we have two types of contacts: (i) task contacts and (ii) unwanted contacts.

Task contacts are contacts that are related with the actual task of the robot and therefore they are expected and mainly initiated by the robot. They are located on the frontiers of the virtual environment and occurs between the endeffector and the human's hand.

Unwanted contacts are collisions that happen when the robot end-effector or robot body comes in contact with the user while it is not expected in the task description.

To manage these collisions, we come up on a strategy for safety of user in ICI scenario. We consider multiple control factors like planning, workspace /working posture and placement.

Planning: There are two aspects of planning of robot motion in this scenario: (i) reaching the desired next ICI location safely, (ii) being able to track the user hand at the frontier of the virtual environment without having collision with the user, to prepare a future motion of the ICI to its desired location as soon as possible. Having a better planning control helps in avoiding collisions.

Workspace / Working posture: Constraining the robot's working posture for the entire workspace reduces the risk of collisions. In practice this avoid reconfiguration of the robot to change of working posture (such as elbow up or elbow down configuration) and such large amplitude motion can lead to collision.

Placement: The placement of the robot with respect to the user and the environment has to be defined in order that the robot is able to carry the prop to the limit of the virtual environment that the user wants to explore while avoiding collision with the human. It is better if all the part of the robot is keep outside of the interior of the virtual car. This requirement is especialy difficult to reach for the region defined by S 3 (see Figure 4). Find a convenient location for the robot is an not easy task and it is the main subject of the paper. [START_REF] Kim | Versatile encountered-type haptic display for vr environment using a 7-dof manipulator[END_REF] Base placement of the robot for reach the desired poses

Requirement

The base placement of the robot, is defined in order to be able to place the prop at the point that the human want to explore. Since the robot arm studied has dimension which are close to the dimension of human arm, the ideal robot location would be in the location where the user is seated. However to avoid collisions, we cannot place the robot there. In fact, we would like the robot to be located outside the passenger compartment of the virtual car. It is expected that the user body (specially the chest and neck) should be outside the workspace of the robot, such that there will be no possibility of contact between body parts of the user and the robot's arm. The robot placement should be such that it can reach all the desired contact regions in the required orientation.

Methodology

The Robot The cobot used in this paper is UR5 from Universal Robots [START_REF] Ostergaard | Lightweight robot for everybody [industrial activities[END_REF]. It is a 6 DOF robot able to interact with humans in a shared space or to work Fig. 4. Example of virtual environment and a set of regions to be reached safely in close proximity. We constrain the posture of the robot to stay in a single aspect, i.e. a particular configuration [START_REF] Wenger | Cuspidal and noncuspidal robot manipulators[END_REF]. In this case, it is elbow up. This helps us in avoiding crossing singularities, during motions. The selection of the specific posture is done by studying the various reachability ranges of different postures and selecting the posture with maximum reachability range without collision between the arm and its support. Now, for the posture of the robot defined as elbow up, the workspace of the robot is defined as the position location of the effector with respect to the base of the robot, defining a range of orientations for the end-effector.

The workspace is calculated using the ROS package Reuleaux, via a discrete set of spheres of 5 cm diameter. For each center of the sphere and desired orientation of the end-effector, the inverse geometric model is used (with imposed posture) and if a solution exist, the sphere is conserved. At this point, we have all the information about each point in the workspace that can be reached with the preset posture of the robot. If several orientation of the end effector is desired, the intersection of the workspace for each orientation is defined. The workspace of the robot for two orientations and the intersections of both orientations are illustrated on Figure 6.

These orientations under test are obtained as a factor of the surfaces of the prop that will be used for this tests. The prop that we consider has 6 surfaces. Figure 5 shows the major frames of interest in this test scenario. the TCP frame is the frame at the end of robot without any attachment. We will need to consider two transforms from the TCP to the surface of the prop in contact. For the S 2 Region as shown in Figure 7. We will considering that all the six surfaces of the prop can be touched. In this region we can have maximum surface interaction with robot prop to compare the feeling with several texture. When building the workspace, the table linked to the robot is taken into account as an obstacle, but since the pose of the robot in the environment is not known, the other obstacles are not considered. The Task For interaction between the virtual environment and robot tool, lets consider three major regions in the passenger compartment of the car. These regions are defined as S 1 , S 2 and S 3 located at the extremes of the car for the driver: the door, the dash board and the seat next to the user/driver. In the three regions S 1 , S 2 and S 3 , we define four points P 1 , ..., P 4 that we want the robot to reach.

For S 1 region as shown in Figure 8, which is the door close to the user, we want for the robot being able to reach this surface with the prop surface 6. This correspond to the orientation 1. The points must belong to the workspace W 1 shown in Figure 6 For the S 2 region, shown in Figure 7, we can have a maximum surface interaction with robot prop, we imposed that the surface 6 or surface 1 to 5 of the prop can be presented in order to test several material. A rotation of the last axis of the robot allows to change the surface of the prop from surface 1 to 5. To achieve this, the points has to be reachable by the two orientations. The points must belong to the workspace W 3 shown in Figure 6.

Finally, the S 3 region is defined in the middle of the seat close to driver, as shown in Figure 9. For this task, the points must belong to workspace W 2 shown in Figure 6. 5 Obstacles avoidance

In the previous study, the obstacle of the environment (the chair and the human) was not taken into account since the location of the robot was not known. Thus the next step is to check for the selected pose location (in Figure 10) that the points can be reached when the real environment are taken into account.

In this environment, we introduce a model of the chair and the human in rest position. For the user's body, the dimensions considered, for user cover 75 percentile of men and women of age group 25-55 in France. The volume dimensions of the cuboid are 185 cm in height and 60 cm in width and 30 cm in thickness (Figure 14).

First the location of the robot corresponding to the actual height of the table is considered. For each pose location, the following task is considered: go from home position to each points successively.

From several candidate poses of the robot, the task cannot be achieved. For others, the task can be achieved. Two main problems were encountered:

An illustration of the result is shown for the case illustrated in Figure 14. The configuration of the robot to reach a point on the passenger chair is shown in Figure 15, the robot is close to the legs of the human. The configuration of the robot to reach a point on the dashboard is shown in figure 16, the robot configuration is convenient, it is far from the human. To improve, the robot base placement, we can change the height of the table used. Several tests are done to determine the effect of changing the height of the table, on the position of robot base and its effect on the inclusion of the robot in the passenger compartment of the car. For performing the tests, the task points shown in Figure 17 are considered.

When the robot tries to reach the chair task points, we can see from Figure 15 that the robot goes to the passenger compartment of the car. Conversely, the same problem does not occur in Figure 19, as the height to the table is reduced by 15 cm. Similar result is shown in Figure 18 with reduced base table dimensions. It can be concluded that it is better to use a table of 60 cm height that 75 cm height for the given scenario. In this article, a collaborative robot is used as a haptic interface with intermittent contact. This interface is used when designing car passenger compartment to choose the textures of the materials used. The work presented allows to choose a robot placement so that the end-effector reaches the areas of interest in the environment while ensuring the absence of collision between the robot and the user. An algorithm has been proposed define convenient placement of the robot base in order to reach several areas of interest. First results are obtained using a given height of the table that support the robot base. The results show an accessibility of the task but also show a robot insertion in the passenger compartment. Changing this height setting prevents this intrusion and therefore improves user safety.

Fig. 1 .

 1 Fig. 1. Conceptual scheme of the experimental platform

Fig. 2 .

 2 Fig. 2. The complete system setup for human robot interaction

Fig. 3 .

 3 Fig. 3. (a) The UR5 robot with a prop attached to the end-effector and (b) The prop used to carry six sample textures. The lateral surface of the prop are denoted 1 to 5 and can be changed by a rotation of the last axis of the robot. The frontal surface of the prop is denoted 6. A displacement of the robot is required to present a lateral surface and then the frontal surface at the same cartesian location

Fig. 5 .Fig. 6 .

 56 Fig. 5. The prop used in this experiment

Fig. 7 .Fig. 8 .Fig. 10 . 11 .Fig. 12 .Fig. 13 .

 7810111213 Fig. 7. S2 -region

Fig. 14 .

 14 Fig. 14. Base location in blue and tasks points with the desired orientation in pink, 75 cm table height

Fig. 15 .

 15 Fig. 15. Interaction at chair, 75 cm height Fig. 16. Interaction at dash, 75 cm height

Fig. 17 .

 17 Fig. 17. Base location in blue and tasks points with the desired orientation in pink, 60 cm height

Fig. 18 .

 18 Fig. 18. Interaction at dash, 60 cm height Fig. 19. Interaction at chair, 60 cm height

Acknowledgement

This work was funded under the LobbyBot project, ANR-17-CE33 [21]. The authors of the article thank the members of the project for their help in carrying out this work, Lionel Dominjon, Sandrine Wullens, Allexandre Bouchet, Javier Posselt, Maud Marchal, Anatol Lecuyer and Victor Rodrigo Mercado Garcia.

The Robot Placement Now to find the best base placement of the robot, we discretize the possible position of the robot. The set of possible location are expressed as a set of sphere of 8 cm of diameter that represent the paving of the space.

If the robot is fixed on the floor, the paving can include only sphere at the level of the floor. In more general case, as the one studied here, a 3D paving can be considered. The vertical coordinate of the best pose of the robot will defined the good height for the table supporting the robot.

The task is defined as previously by the set of points that we want to reach with a given orientation. The workspace generated for each orientation of region is also used (This is called as reachability map [START_REF] Zacharias | Capturing robot workspace structure: representing robot capabilities[END_REF]). A point can be reach if it belongs to the worskpace associated to the desired orientation (for example W 1 ).

The algorithm developed is based on the Reuleaux package [START_REF] Makhal | Reuleaux: Robot Base Placement by Reachability Analysis[END_REF] available via ROS [START_REF] Quigley | Ros: an open-source robot operating system[END_REF] and using libraries [START_REF]Kdl kinematics and dynamics library[END_REF]20].

The methodology can include any number of point tasks. For each point associated with one orientation i.e. one workspace W i , all the sphere inside of the workspace is covered and placed at the point studied, and the corresponding pose of the robot is registered. The location of the based is associated to the sphere paving the possible placement of the robot. The number of point reachable from this base location is increased.

After considering all the point of interest (12 point in our study), the possible placement of the robot to achieve the task is defined as the sphere where the number of reachable is maximum.

From this set of base locations, we remove the base locations that coincide with the location of the user. Depending on the number of desired points that can be reached, the points are categorized. This gives the possible robot base location more suited for this scenario. Test were performed to find the robot base placement for given set of point the robot has to reach with a defined orientation. In the tested scenario, the dimension of the table under consideration is 75 cm high and 53 cm long and wide. The base of the robot is located in the middle of the table to ensure its stability. A total of 12 task points have been defined: four task points in S 3 region (Chair), four task points in S 1 region (Door), and four task points in S 2 region (Dash board) with two sets of different orientation.

The possible robot base location are illustrated by blue sphere in Figure 10. The base location are outside the passenger compartment of car. Multiple robot base locations were found that could reach all the desired positions.