Yige Yan

Valérie Keller
email: nkeller@unistra.fr

Nicolas Keller

On the role of BmimPF6 and P/F-containing additives in the solgel synthesis of TiO2 photocatalysts with enhanced activity in the gas phase degradation of methyl ethyl ketone

Keywords: Photocatalysis, TiO2, sol-gel synthesis, ionic liquid, synthesis mechanism

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

On the role of BmimPF6 and P/F-containing additives in the sol-gel synthesis of TiO2 photocatalysts with enhanced activity in the gas phase degradation of methyl ethyl ketone

Yige Yan, Valérie Keller, Nicolas Keller

To cite this version:

Yige Yan, Valérie Keller, Nicolas Keller. On the role of BmimPF6 and P/F-containing additives in the sol-gel synthesis of TiO2 photocatalysts with enhanced activity in the gas phase degradation of methyl ethyl ketone. Applied Catalysis B: Environmental, 2018, 234, pp.56-69. 10.1016/j.apcatb.2018.04.027. hal-02354994

Introduction

Heterogeneous photocatalysis on semi-conductor is a research field which gained importance over the last decades due to increasing challenges in environmental concerns and energy demands [START_REF] Hoffmann | Environmental applications of semiconductor photocatalysis[END_REF][START_REF] Anpo | The design and development of highly reactive titanium oxide photocatalysts operating under visible light irradiation[END_REF][START_REF] Enesca | Enhanced UV-Vis photocatalytic performance of the CuInS2/TiO2/SnO2 hetero-structure for air decontamination[END_REF]. In the frame of the search for sustainable and green approaches, the photocatalytic oxidation has been shown to be promising for environmental reactions such as water detoxification or gaseous pollutant removal [START_REF] Lichtin | Photocatalytic oxidative degradation of vapors of some organic compounds over TiO2[END_REF][START_REF] Turchi | Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack[END_REF][START_REF] Zhao | Photocatalytic oxidation for indoor air purification: a literature review[END_REF][START_REF] Pelaez | A review on the visible light active titanium dioxide photocatalysts for environmental applications[END_REF]. Among usually studied semiconductors, the wide band-gap anatase TiO2 still remains considered as the most efficient photocatalyst under UV-A illumination.

A variety of strategies have been implemented for enhancing the photocatalytic activity of TiO2, and factors such as the surface area [START_REF] Tellez | Comparison of Hombikat UV100 and P25 TiO2 performance in gas-phase photocatalytic oxidation reactions[END_REF], the presence of bulk or surface defects [START_REF] Lu | Ti 3+ defect sites on TiO2 (110): production and chemical detection of active sites[END_REF],

the type and density of surface chemical states [START_REF] Keller | On the modification of photocatalysts for improving visible light and UV degradation of gas-phase toluene over TiO2[END_REF][START_REF] Verbruggen | Factors driving the activity of commercial titanium dioxide powders towards gas phase photocatalytic oxidation of acetaldehyde[END_REF] and furthermore the TiO2 morphology [START_REF] Tachikawa | Evidence for crystal-face-dependent TiO2 photocatalysis from single-molecule imaging and kinetic analysis[END_REF], have been reported to affect the photocatalytic oxidation performances.

The simple, cost effective and versatile sol-gel synthesis method has already been largely applied for preparing with high synthesis yield, photocatalysts with controlled and tunable bulk and surface physico-chemical properties, including a high homogeneity, fine-scale and controllable morphology [START_REF] Hench | The sol-gel glass transformation of silica[END_REF][START_REF] Wang | Review of the progress in preparing nano TiO2: an important environmental engineering material[END_REF]. Applying controlled modifications of the synthesis parameters or adding additive compounds has been shown to allow especially a fine tuning of the catalyst morphology [START_REF] Loryuenyong | Sol-gel derived mesoporous titania nanoparticles: Effects of calcination temperature and alcoholic solvent on the photocatalytic behavior[END_REF][START_REF] Malinger | Crystallization of anatase nanoparticles from amorphous precipitate by a continuous hydrothermal process[END_REF][START_REF] Grandcolas | Porogen template assisted TiO2 rutile coupled nanomaterials for improved visible and solar light photocatalytic applications[END_REF][START_REF] Bosc | A simple route for low-temperature synthesis of mesoporous and nanocrystalline anatase thin films[END_REF][START_REF] Bosc | Mesostructure of anatase thin films prepared by mesophase templating[END_REF].

Room temperature ionic liquids (IL) are an exceptional type of chemical compounds which are composed nearly by only ions at room temperature. They take advantage of a negligible vapor pressure, a high chemical stability, non flammability properties and a recyclability for being considered and used as green solvents with tunable properties in reaction media for various synthesis and catalytic applications [START_REF] Wang | Synthesis of iron oxide nanoparticles using a freshly-made or recycled imidazolium-based ionic liquid[END_REF][START_REF] Xie | The multifunctional roles of the ionic liquid [Bmim][BF4] in the creation of cadmium metal-organic frameworks[END_REF]. Most of the attractiveness of IL relates to the possibility to tune properties by choosing their adequate cation/anion association [START_REF] Vioux | Use of ionic liquids in sol-gel; ionogels and applications[END_REF].

IL received a growing interest in the field of inorganic chemistry, e.g. metal electrodeposition [START_REF] Endres | Ionic liquids: solvents for the electrodeposition of metals and semiconductors[END_REF], ionothermal synthesis [START_REF] Zhu | Ionothermal synthesis of hierarchical ZnO nanostructures from ionic-liquid precursors[END_REF] and sol-gel chemistry. By acting as aging control agent, catalyst, co-solvent, or via an underrated nanostructure shaping ability resulting from their large size organic cation and from a non-coordination anion combination configuration, they can be used as structure-directing additives for synthesizing inorganic nanostructures [START_REF] Xie | The multifunctional roles of the ionic liquid [Bmim][BF4] in the creation of cadmium metal-organic frameworks[END_REF][START_REF] Lee | Synthesis of iron oxide nanoparticles with control over shape using imidazolium-based ionic liquids[END_REF]. They have recently attracted a significant interest as a new kind of template agent in various types of synthesis of TiO2. Yoo et al. [START_REF] Yoo | Preparation and characterization of mesoporous TiO2 particles by modified sol-gel method using ionic liquids[END_REF] have shown that the use of 1-Butyl-3methylimidazolium hexafluorophosphate (BmimPF6) can result in synthesizing smaller TiO2 nanoparticles with a better crystallinity and a better thermal resistance. Although thermally stable mesoporous anatase TiO2 nanoparticles with a high surface area and small size crystallites have been obtained via the participation of IL like BmimPF6, the large amount of expensive IL required in the process (IL/Ti atomic ratio larger than 3) is a major economical drawback when compared to the use of conventional templating agents or surfactants. Choi et al. [START_REF] Choi | Preparation of thermally stable mesostructured nanosized TiO2 particles by modified sol-gel method using ionic liquid[END_REF] were able to use 1/100 of the IL quantity used by Yoo et al., but both the BET surface and the photocatalytic activity were dramatically reduced.

To our knowledge, a detailed mechanism including the structuration role of the BmimPF6 IL involved in sol-gel or solvothermal reactions was not described till now. Among scarce reports, Zhao et al. [START_REF] Zhao | Shape-and size-controlled synthesis of uniform anatase TiO2 nanocuboids enclosed by active {100} and {001} facets[END_REF] have investigated the role of BmimBF4 IL for obtaining shapecontrolled TiO2 crystals in a hydrothermal synthesis, and they have shown that both the fluorine and the Bmim + ions released from the ionic liquid played a role of capping agent and favor the {001} and {100} facet growth, respectively. But unfortunately the TiO2 crystals were too large (crystal size diameter around 100-500 nm) for maintaining a high surface area, so that they might not be efficiently used in environmental depollution processes. Even if the capping mechanism of BmimBF4 IL was well demonstrated, the same mechanism cannot be directly extrapolated to BmimPF6, as PF6 -differs from BF4 -in many chemical properties [START_REF] Freire | Hydrolysis of tetrafluoroborate and hexafluorophosphate counter ions in imidazolium-based ionic liquids[END_REF].

So both the role played by BmimPF6 and the detailed mechanism in the TiO2 sol-gel synthesis still remain to be proposed.

In this work, shape-and size-engineered TiO2 with highly crystallized nanoparticles and enhanced photocatalytic performances was firstly synthesized by a sol-gel route using a small amount of BmimPF6. Further, the role played by BmimPF6 -and especially that played by the phosphorus (P) and the fluorine (F) constituents -on the control of the TiO2 properties was studied, by replacing BmimPF6 by controlled combinations of the individual components of the ionic liquid. Gaseous butanone (methyl ethyl ketone, MEK), a typical malodorous pollutant of dwelling home indoor air whose presence results from its use as solvent in the production of many consumer products such as protective coatings and adhesives, glues, varnishes, plastics, textiles, paints or magnetic tapes, was taken as gas phase model VOC for assessing the photocatalytic activity of the TiO2. Details on the mechanism of this well-documented photocatalytic degradation reaction can be found elsewhere [START_REF] Raillard | TiO2 coating types influencing the role of water vapor on the photocatalytic oxidation of methyl ethyl ketone in the gas phase[END_REF][START_REF] Vincent | Remediation of olfactory pollution by photocatalytic degradation process: Study of methyl ethyl ketone (MEK)[END_REF][START_REF] Vincent | Abatement of volatile organic compounds using an annular photocatalytic reactor: Study of gaseous acetone[END_REF]. Commercial Aeroxide® TiO2 P25 from Evonik was used as reference TiO2 powder. Correlation between the sol-gel synthesis mechanism involving IL and/or IL replacement additives, the physicochemical properties of the synthesized photocatalyst and the photocatalytic activity under UV-A light was proposed. In a typical ionic liquid assisted sol-gel synthesis, 5 g of titanium (IV) isopropoxide (TTiP, Ti(O i Pr)4, 97%, Aldrich) was mixed in 10 g of propan-2-ol (AnalaR Normapur, >99.5%, VWR Chemicals), before 0.09 g of BmimPF6 ionic liquid (>99%, Carl Roth) was added corresponding to a BmimPF6/Ti molar ratio of 0.03. After stirring for 30 min, a BmimPF6/alcohol two-phase mixture was formed since BmimPF6 is not miscible with either TTiP or propan-2-ol. Then 6.2 g of 2.2 mol/L acetic acid solution as acid catalyst was further added dropwise. A white milky precipitate was formed immediately upon hydrolysis process and the solution was kept under stirring for 1 h, before being further sealed by parafilm and aged at room temperature for 18 h or 6 days without any stirring. After filtration of the powder from the rest of the solvent, the solid was dried for 2 h at 100 o C, and the light yellow powder obtained was washed under stirring in 100 mL acetonitrile overnight, dried overnight at 100°C and further vacuum filtrated with 2 x 50 mL distilled water for 10 min, before being dried again for 2 h at 100 o C. The so-obtained white powder was finally calcined in air at 550 o C for 2 h with a heating rate of 5 o C/min in order to remove fluorine adsorbed on the sample surface [START_REF] Jung | TiO2 nanotube fabrication with highly exposed (001) facets for enhanced conversion efficiency of solar cells[END_REF]. Table 1 summarizes the labeling of TiO2 samples.

Experimental

For comparison, reference sol-gel TiO2 was synthesized by following the same protocol in the absence of any BmimPF6 IL.

Sol-gel synthesis of TiO2 with ionic liquid replacement additives.

The role of Bmim + , phosphate anion, and fluoride anion -three constitutive components of the BmimPF6 ionic liquid -was studied by replacing BmimPF6 by two among three selected chemical compounds, namely 1-Butyl-3-methylimidazolium chloride (BmimCl (BC), ≥95%, Aldrich), sodium fluoride (NaF, ACS reagent, ≥99%, Sigma-Aldrich) and phosphoric acid (H3PO4 (PA), analytical reagent, ≥85%, R.P.Normapur). They provided to the synthesis media the Bmim + cation, the fluoride anion (F -) and the phosphate anions, respectively. The molar ratio of each additive to titanium was fixed at 0.03, similarly to the IL/Ti ratio.

Two aging durations were applied, i.e. 18 h and 6 days, and the calcination temperature was fixed at 550 o C, while the other parameters as well as the synthesis protocol were identical to those used in the BmimPF6-assisted TiO2 sol-gel synthesis. Depending on the two additives selected in each combination and on the aging duration, the TiO2 samples were labeled as BCPA 18h, BCPA 6d, BCNaF 18h, BCNaF 6d, PANaF 18h and PANaF 6d.

Characterization methods

The P/Ti bulk atomic ratio of the TiO2 samples was obtained by Inductively Coupled Plasma -Atomic Emission Spectroscopy (ICP-AES) performed at IPHC (Strasbourg, France).

The X-ray diffraction (XRD) patterns of the samples were recorded on a D8 Advance Brucker powder diffractometer in a q/q mode and using the Ka1 radiation of Cu at 1.5406 Å. The surface area measurements were carried out on an ASAP2010 Micromeritics Tristar 3000 analyser using N2 as adsorbent at 77K. X-ray photoelectron spectroscopy (XPS) characterization was performed on a Thermo VG Scientific equipped with X-Ray source of Al Ka radiation (hv = 1486.6 eV). Transmission Electron Microscopy (TEM) analysis was performed using a Philips CM200 in standard mode observation, equipped with thermo-ionic LaB6 filament, operating at a 200 kV acceleration voltage. Details on the characterization devices can be found as SI 1.

Gas-phase photocatalytic tests

The photocatalytic performance and behavior of TiO2 samples are assessed by their ability to degrade and mineralize MEK under UV-A irradiation in a single-pass continuous gas flow reactor, with a flat configuration derived from that detailed in the ISO 22197 standard series [START_REF]) --Test method for air-purification performance of semiconducting photocatalytic materials --Part 2: Removal of acetaldehyde, Fine ceramics (advanced ceramics, advanced technical ceramics) --Test method for air-purification performance of semiconducting photocatalytic materials[END_REF] (Figure SI 2).

50 mg of TiO2 powder, corresponding to a surface density of 1 mg.cm -2 , was evenly coated by adding drop-wisely onto a glass plate (100 mm length, 50 mm width and 3 mm thickness for fitting into the reactor) and simultaneously evaporating a TiO2 ethanoic suspension to dryness. Prior to that, the suspension was stirred overnight and sonicated for 30 min. Finally, the TiO2-coated glass plate was dried overnight at 100°C. This TiO2 surface density was previously determined as being adequate for evaluating the activity of photocatalysts in such operating conditions [START_REF] Masson | β-SiC alveolar foams as a structured photocatalytic support for the gas phase photocatalytic degradation of methylethylketone[END_REF].

Two synthetic air flows were bubbled at ambient temperature and atmospheric pressure through two temperature-controlled saturators containing MEK (butan-2-one, Sigma-Aldrich, >99%) and distilled water, respectively, and mixed with an additional synthetic air flow to set an inlet MEK concentration at 100 ppmv, relative humidity (RH) at 50% and a total air flow rate at 227 ml/min, corresponding to a velocity of 1.33 cm/s -1 and a residence time of 7.5 s in the reaction zone of the reactor. A test temperature of 25°C was maintained throughout the test. The photocatalytic performances were obtained by on-line quantifying both inlet and outlet flows using a R3000A gas micro-chromatography (SRA instruments), equipped with thermal conductivity micro-detectors, allowing quantification of MEK, water, CO2 and acetaldehyde.

The photocatalysts were firstly exposed for 2 h to the inlet polluted air flow with no illumination until dark adsorption equilibrium was reached, before the UV-A illumination was switched on. The UV-A illumination was provided by a 8W UV-A blacklight lamp (Sylvania Blacklight Blue F8W/BLB T5), with a spectral peak centered on 365 nm, so that the photocatalytic material was exposed to an irradiance of 3.0 mW.cm -2 , as recorded using a wideband spectroradiometer (RPS900-W ILT). The catalytic activity was evaluated under steady state conditions, typically achieved after 3 h of irradiation. No change in activity was observed for all samples for 24 h after reaching steady state conditions. The absence of any deactivation during the MEK oxidation allowed the TiO2 photocatalysts to be re-used if necessary.

The photocatalytic activity was expressed in terms of MEK conversion (CMEK), of selectivities to acetaldehyde (SAc) and CO2 (S "# $ ), as main reaction intermediate byproduct and final oxidation product, respectively, as well as of CO2 mineralization yield (Y "# $ ), calculated according to Eqs. (1-4):

C )*+ (%) = ([)*+ /0 ]2[)*+ 345 ]) [)*+ /0 ] * 100 (1)
S 9: (%) = [9: 345 ] ([)*+ /0 ]2[)*+ 345 ]) * ; * 100 (2)
S "# $ (%) = ["# $ 345 ] ([)*+ /0 ]2[)*+ 345 ]) * = * 100 (3)
Y "# $ (%) = ["# $ 345 ] [)*+ /0 ] * = * 100 (4)
All the photocatalytic tests have been performed twice, and the photocatalytic data corresponded to the averaged data, with a relative accuracy of ± 5%. The dried sample after 6 days of aging synthesized without BmimPF6 already showed the presence of partially crystallized anatase TiO2 phase with a small mean crystallite size of 6 nm. However, the presence of BmimPF6 favored the crystallization of TiO2 during the aging step, while the mean crystallite size was slightly increased, from 6 nm for the reference solgel TiO2 sample to 7-8 nm when TiO2 was synthesized in the presence of BmimPF6.

Characterization of photocatalysts

After calcination at 550 o C, the crystallinity of the sol-gel TiO2 sample synthesized in the absence of BmimPF6 strongly increased, together with a large increase of the mean crystallite size to 19 nm, close to that of 22 nm shown by the anatase phase in TiO2 P25. By contrast, only a moderate increase in the mean crystallite size of TiO2 synthesized in the presence of BmimPF6, from 7-8 nm to 10-11 nm, was observed after 6 days of aging, whatever the aging duration. The presence of BmimPF6 provided to the materials thermal resistance and the crystal growth during the calcination was hindered.

The TiO2 materials displayed a bulk P/Ti molar ratio of 0.01, lower than the P/Ti ratio of the precursor mixture. After calcination of TiO2 samples at 550°C, no residual fluorine was detected by high pressure ion chromatography, in agreement with Jung et al [START_REF] Jung | TiO2 nanotube fabrication with highly exposed (001) facets for enhanced conversion efficiency of solar cells[END_REF]. Adsorbed fluorine has to be removed to avoid passivation of the photocatalyst surface before use [START_REF] Jung | TiO2 nanotube fabrication with highly exposed (001) facets for enhanced conversion efficiency of solar cells[END_REF][START_REF] Liao | Activating the single-crystal TiO2 nanoparticle film with exposed {001} facets[END_REF].

Whether ionic liquid was involved or not in the synthesis, all anatase TiO2 samples displayed after calcination a band gap energy of 3.2 ± 0.1 eV derived from the Diffuse Reflectance UV-vis spectra, similar to that of the TiO2 P25 reference (not shown).

Surface area and porosimetry properties.

Whether BmimPF6 was used or not, dried TiO2 displayed a high and non-microporous surface area within the 251-294 m 2 /g range, resulting from a small mean crystallite size. The specific surface area of the reference sol-gel TiO2 was drastically decreased after calcination down to 50 m 2 /g, close to that of the TiO2 P25 reference, as a result from similar mean crystallite sizes. By contrast, when BmimPF6 was used, TiO2 showed a different behavior, and calcined TiO2 samples were able to maintain a high non-microporous specific surface area of 97-110 m 2 /g, despite the use of a high calcination temperature.

The influence of the presence of BmimPF6, of the aging duration and of the calcination on the N2 sorption isotherms and the pore size distribution of TiO2 are shown in Figure 2. When BmimPF6 was used, the presence of mesopores within the materials is suggested by the presence of mainly type-IV isotherms. Also, H2-type of hysteresis was observed, corres-ponding to interconnected mesopores, with non-uniform size or shape. This differs from the isotherm obtained on the non-porous TiO2 P25. When synthesized in the absence of BmimPF6, dried TiO2 displayed an isotherm with a H3-type hysteresis, characteristic of aggregates or agglomerates of particles forming slit shaped pores with nonuniform size and/or shape, whereas the calcination at 550 o C resulted in the appearance of a H1-type contribution, corresponding to the formation of more uniform and larger mesopores with a monomodal distribution centered on 15 nm. By contrast, when BmimPF6 was used, the H2-type hysteresis was mainly turned into a H1-type hysteresis after calcination, with a monomodal pore size distribution centered on about 10 nm. Here, the presence of BmimPF6 ionic liquid during the synthesis and aging seems to be a key factor for controlling the pore size and the surface area. In addition, the use of BmimPF6 also led to a unique shape evolution with the aging time.

The nanoparticle morphology turned from a round-like shape to a more square-like and faceted shape when increasing the aging duration from 18 h to 6 days (Erreur ! Source du renvoi introuvable.d), while maintaining constant the mean crystallite size. This shape change could result from the better crystallinity achieved after a longer aging step, or from the capping ability of F -present in the synthesis medium. This capping ability associated with TiO2 crystallization has been reported using hydrofluoric acid [START_REF] Yang | Anatase TiO2 single crystals with a large percentage of reactive facets[END_REF][START_REF] Yang | Hydrothermal stability of {001} faceted anatase TiO2[END_REF] or F-containing ionic liquid such as BmimBF4 [START_REF] Zhao | Shape-and size-controlled synthesis of uniform anatase TiO2 nanocuboids enclosed by active {100} and {001} facets[END_REF] as additive, although no sol-gel synthesis was concerned. Since the hexafluorophosphate anion of BmimPF6 is the only F source in the synthesis, the involvement of BmimPF6 in the aging duration-dependent crystal growth control, especially via the F -, will be further investigated.

Surface analysis.

Erreur ! Source du renvoi introuvable. shows the XPS spectra of Ti2p, O1s, P2p and F1s orbital regions recorded on dried and calcined TiO2 samples. Whether BmimPF6 was used or not, and whether the TiO2 sample was dried or calcined, the Ti2p region pattern shows the typical Ti 2p 3/2 -Ti 2p 1/2 doublet located at 458.3 eV and 464.0 eV with a spin-orbit coupling constant of 5.7 eV, and assigned to Ti 4+ (Ti-O) in an octahedral in typical TiO2 crystalline structure. No higher energy contribution that could be attributed to tetrahedral Ti-O-P bonds [START_REF] Alfaya | Highly dispersed phosphate supported in a binary silica-titania matrix: Preparation and characterization[END_REF][START_REF] Yu | Synthesis and characterization of phosphated mesoporous tianium dioxide with high photocatalytic activity[END_REF] was observed. Similarly, the O1s spectra showed usual contributions from TiO2 lattice O 2-in O-Ti bond and O-H surface groups, at 529.2 eV and 531.2 eV, respectively. The OH content was strongly reduced after calcination, in agreement with the decreasing surface area.

For dried TiO2, the P2p spectra did not reveal any P contributions. By contrast, the P2p spectra of IL 6d 550 TiO2 showed unambiguously the presence of P, with a P 2p 3/2 -P 2p 1/2 doublet at 132.9 and 133.8 eV (with a spin-orbit coupling constant of 0.87 eV), characteristic of P in a pentavalent oxidation state (P 5+ ), and usually attributed to phosphate groups [START_REF] Splinter | XPS characterization of the corrosion films formed on nanocrystalline Ni-P alloys in sulphuric acid[END_REF][START_REF] Bortun | Synthesis and characterization of two novel fibrous titanium phosphates Ti2O(PO4)2.2H2O[END_REF][START_REF] Sudarsan | PO4 3tetrahedra in SbPO4 and SbOPO4: a 31P NMR and XPS study[END_REF].

The P/Ti surface atomic ratio was calculated at 0.06. Before calcination, the phosphates were probably finely dispersed within the TiO2 bulk, so that its surface content remained very low.

The calcination at 550°C could cause partial migration of phosphates species from the bulk to the surface, with consequently an increase in the P/Ti surface atomic ratio to a level overcoming that of the bulk, and observable by XPS. This was in agreement with the migration of phosphorous to the TiO2 surface observed by Elghniji et al. for temperatures higher than 550°C [START_REF] Elghniji | Chemical modification of TiO2 by H2PO4 -/HPO4 2anions using the sol-gel route with controlled precipitation and hydrolysis: enhancing thermal stability[END_REF]. However, the presence of P as Ti-O-P bond contribution at higher binding energy was not observed on the Ti2p spectra, like for the uncalcined samples, possibly due to the low content of P in TiO2 samples.

No trace of residual nitrogen from Bmim + was observed in the dried or calcined TiO2 samples, confirming the efficiency of the acetonitrile/water washing procedure for the cationic part of BmimPF6. By contrast, residual F has been detected for the dried sample, with

F1s orbital peak at 683.3 eV. This may consist in F adsorbed on the material surface rather than in F incorporated in the TiO2 host as substituent of O 2-, for which the F-Ti-O contribution at 688.3 eV was never observed whatever the TiO2 samples [START_REF] Liu | Fluorinated semiconductor photocatalysts: tunable synthesis and unique properties[END_REF]. This residual content diminished strongly after calcination, with a F/Ti surface atomic ratio decreasing from 0.18 to below the detection limit. This confirmed the full desorption of residual F from the TiO2 surface during the calcination step at 550°C [START_REF] Jung | TiO2 nanotube fabrication with highly exposed (001) facets for enhanced conversion efficiency of solar cells[END_REF].

TiO2 synthesized with IL replacement additives

Structure properties.

The XRD patterns of the dried and calcined TiO2 samples synthesized with replacement additives are reported in Erreur ! Source du renvoi introuvable. and their main bulk physico-chemical properties are reported in Table 2. Firstly, whatever the replacement additive combination and the aging duration, only anatase was observed as crystallized phase in dried and calcined materials. Only samples prepared with P-containing additives, i.e.

BCPA and PANaF samples, have shown an inhibition effect of the crystallite size growth during the calcination. Those samples maintained after calcination a relatively small mean crystallite size, around 10-11 nm vs 8 nm for the dried samples, associated to a high surface area within the 90-130 m 2 /g range. By contrast, the P-free BCNaF TiO2 sample had a strong mean crystallite size increase after calcination, whatever the aging duration, from 5 and 8 nm before calcination to 24 and 19 nm after calcination. This was correlated to a medium surface area of 31 and 46 m 2 /g, respectively. Also, the use of P-containing additives resulted in a better crystallinity of dried TiO2 samples after the aging step than TiO2 samples aged in the absence of any phosphorous. Interestingly, these characteristics are similar to those brought by BmimPF6.

Whatever the IL replacement additives used in the synthesis, all anatase TiO2 samples displayed after calcination a band gap energy of 3.2 ± 0.1 eV, similar to that obtained when using ILs in the synthesis (not shown).

Surface analysis.

XPS analysis has been performed on both P-containing BCPA-, and PANaF-derived TiO2 samples (Figure 6 increased after calcination from 0.05 to 0.1 and from 0.06 to 0.09 for the BCPA-and PANaFderived TiO2 samples, respectively. The increase in the P/Ti surface ratio after calcination till a value overcoming that of the bulk measured at 0.01 for PA-derived samples, could result from a phosphate migration from the bulk to the surface, like it was proposed for TiO2 synthesized in the presence of BmimPF6.

Morphological characterization.

The role of P in limiting the crystal size during calcination was also visualized in TEM by comparing TiO2 materials prepared with and without P-containing additives, i.e. PANaF and BCNaF (Erreur ! Source du renvoi introuvable.), for which mean crystallite sizes around 12-15 nm and 22-25 nm, respectively, were observed, in agreement with the behavior observed by XRD. shown in Table 3. The performances were compared to those achieved with TiO2 P25 and with the reference sol-gel TiO2, i.e. TiO2 synthesized in the absence of BmimPF6.

The reference TiO2 P25 showed a MEK conversion of 37% with selectivities to CO2 and acetaldehyde of 36% and 34% respectively, resulting in a CO2 mineralization yield of 13%. It had a slightly higher activity than the reference sol-gel TiO2 that achieved a MEK conversion of 32%, with a CO2 mineralization yield of 10%. Considering that both materials have similar main characteristics in terms of specific surface area, crystallite size and crystallinity, the presence of anatase-rutile phase mixture in the TiO2 P25 reference could be put forward for explaining the slight superiority of the P25 photocatalyst [START_REF] Hurum | Explaining the enhanced photocatalytic activity of Degussa P25 mixed-phase TiO2 using EPR[END_REF], although we know that the literature can be controversial when explaining the activity of the commercial P25.

By contrast, TiO2 synthesized in the presence of BmimPF6 showed a higher activity in terms of MEK conversion and CO2 mineralization yield compared to both TiO2 P25 and reference sol-gel TiO2, with conversions of 48-50%, and CO2 mineralization yields of 17% and 25%. 6 days of aging led to a higher CO2 selectivity of 52% compared to that achieved on both the same sample aged for 18 h, similar to that achieved with the reference sol-gel TiO2.

Also, dried TiO2 synthesized in the presence of BmimPF6 showed increased crystallinity, close to that of calcined samples, yet, no activity has been noticed. Passivation of the catalyst by fluorine in such small amount is unlikely a factor here. The absence of activity probably resulted from the presence of amorphous TiO2 phases remaining within the catalysts (Figure SI 4).

Whatever the TiO2 samples, the carbon balance was not closed by MEK, CO2 and acetaldehyde as the main reaction intermediate (Table SI 5). As reported in the literature, the carbon balance was probably closed by other reaction intermediates, that could not be detected due to their small quantities or to the sensitivity of the analytical system [START_REF] Raillard | TiO2 coating types influencing the role of water vapor on the photocatalytic oxidation of methyl ethyl ketone in the gas phase[END_REF][START_REF] Vincent | Remediation of olfactory pollution by photocatalytic degradation process: Study of methyl ethyl ketone (MEK)[END_REF][START_REF] Vincent | Abatement of volatile organic compounds using an annular photocatalytic reactor: Study of gaseous acetone[END_REF][START_REF] Liu | Fluorinated semiconductor photocatalysts: tunable synthesis and unique properties[END_REF].

TiO2 synthesized with IL replacement additives

The results of the gas-phase photocatalytic degradation of MEK for calcined TiO2 materials synthesized in the presence of two additives among BC, PA and NaF, are shown in Table 3.

The P-containing "BCPA 6d 550" and "PANaF 6d 550" photocatalysts exhibited a high MEK conversion of 52% and 53%, respectively, whereas the P-free "BCNaF 6d 550" counterpart showed a lowest MEK conversion of 31%. In addition to a high MEK conversion, the PANaF 6d 550 TiO2 led to a high CO2 selectivity of 54% (with a low acetaldehyde selectivity of 9%, resulting in a high mineralization yield of 28%), when compared to the lower CO2 selectivity and to the higher acetaldehyde selectivity achieved on the high MEK conversion BCPA counterpart (42% and 28%, respectively). Similarly to TiO2 synthesized in the presence of BmimPF6, the increase in the aging duration from 18 h to 6 days resulted in an increase in the CO2 selectivity from 41% to 54% for the PANaF sample.

The morphological analysis of PANaF 6d dried sample provided by TEM images in Figure8 showed an anisotropic spindle-like morphology for the crystals. This morphology could derive from the capping ability of F -anions present in the reaction medium during aging, since neither Na + nor PA is known for inducing a shape modification effect for TiO2 crystals in solution. Discussion on the capping ability of F -during the synthesis of TiO2 samples will be made in relationship with the activity of the photocatalysts.

Here, the results obtained did not allow us to evidence a possible direct role of Bmim + on the photocatalytic activity of TiO2 after calcination.

Correlation between IL-, P-, F-derived TiO2 properties and photocatalytic activity

Whether BmimPF6-or P-containing additives were involved, the increase in the surface area (and inversely the decrease in the mean crystallite size) of TiO2 photocatalysts was strongly beneficial to the improvement of the MEK conversion. Also, the photocatalytic activity of BmimPF6-and P-derived TiO2 samples displayed a similar aging durationdependency. At a similar MEK conversion level, an increase in the CO2 selectivity and in the CO2 mineralization yield was observed for both TiO2 sample series when the aging duration was increased. Since the samples displayed approximatively a similar surface area and a similar mean crystallite size, this dependency might be rather related to the morphology change of TiO2 crystals during aging.

Despite a decrease in the isoelectric point from 6 to 3.5 has been observed with the incorporation of phosphate in TiO2 samples (TiO2 vs. IL-TiO2 and PA-TiO2), no significant change in the adsorption behavior was observed during the adsorption period, except a slight increase in the amount of MEK and H2O adsorbed at the dark adsorption equilibrium, that was directly related to the increase in the TiO2 surface area.

The performances shown by the "PANaF 6d 550" photocatalyst were not only the highest observed among the TiO2 photocatalysts synthesized with the use of BmimPF6 replacement additives, but also they were slightly superior to those achieved over the most active TiO2 photocatalyst synthesized with BmimPF6. So, the high photocatalytic activity of TiO2 samples might result from the presence of P and F elements in BmimPF6 and only the combined influence of those two elements might result in a comparable or even higher photocatalytic activity than that obtained on TiO2 synthesized in the presence of BmimPF6. This enhancement of the photocatalytic activity could be related to the beneficial TiO2 physicochemical properties evolution reported early, through a room-temperature crystallization mechanism during the aging in presence of P and F elements.

Discussion

P and F are hypothesized to be the two elements in BmimPF6 and in additives playing a key-role in the sol-gel synthesis of TiO2 nanomaterials. We have noticed three major impacts of their presence in the synthesis media that influenced the physico-chemical properties of TiO2 and affected the photocatalytic behavior. The activity of TiO2 photocatalysts:

showed an aging-duration dependency.

was influenced by a F-induced modification of the crystal shape, especially the CO2 selectivity.

showed a dependency with a P-induced size modification, especially the MEK conversion.

Room temperature aging effect

Surface area and XRD analyses indicated together an aging duration-dependent crystal size evolution in the TiO2 sol-gel synthesis at room-temperature. We assumed that this resulted from a room temperature self-crystallization process of TiO2 in solution during the aging step. Conventionally, most of the crystallization processes of TiO2 materials from amorphous to anatase were achieved and studied in high-pressure and high-temperature hydrothermal conditions, although studies have reported possible low-temperature sol-gel approaches for synthesizing anatase crystals by aging the sol [START_REF] Seo | Effect of aging agents on the formation of TiO2 nanocrystalline powder[END_REF][START_REF] Qi | Room-temperature synthesis of single-phase anatase TiO2 by aging and its self-cleaning properties[END_REF]. In one recent review about crystal growth of inorganic functional materials, Zhang et al. stated that the classical growth of crystals occurs through atom-by-atom or monomer-by-monomer addition to an inorganic or organic template, or through the dissolution of unstable phases (small particles or metastable polymorphs) and re-precipitation of the more stable phase [START_REF] Zhang | Recent advances in oriented attachment growth and synthesis of functional materials: concept, evidence, mechanism, and future[END_REF]. This implies in the case of TiO2, that amorphous TiO2 formed upon hydrolysis of the Ti precursor would go redissolution and re-crystallization to achieve amorphous to anatase phase transition, which has been evidenced in the work done by Ayral and coll. [START_REF] Bosc | A simple route for low-temperature synthesis of mesoporous and nanocrystalline anatase thin films[END_REF][START_REF] Bosc | Mesostructure of anatase thin films prepared by mesophase templating[END_REF].

Based on hydrolysis-polycondensation mechanisms in sol-gel process [START_REF] Su | Sol-gel preparation and photocatalysis of titanium dioxide[END_REF], we hypothesized a mechanism for our particular sol-gel synthesis underlying the aging durationdependent TiO2 crystal growth as follows: as water with a relatively high H2O/Ti molar ratio was added into the TTIP/isopropanol solution, TTIP was hydrolyzed immediately before the formed Ti(OR)n-x(OH)x started quickly to condense for forming the initial Ti-O-Ti network, observed as a white precipitate (polymerization form of Ti(OR)n-x(OH)x). So normally, well dispersed small amorphous TiO2 nanoparticles (precipitated as excess solute) should be stabilized thanks to the coordination ability and the specific adsorption of acetic acid on TiO2 [START_REF] Livage | Sol-gel synthesis of heterogeneous catalysts from aqueous solutions[END_REF][START_REF] Polleux | Ligand functionality as a versatile tool to control the assembly behavior of preformed titania nanocrystals[END_REF]. These very small size nanoparticles could serve as heterogeneous nucleation sites for the further crystallization. However, we hypothesized that the single use of acetic acid does not prevent those nucleation seeds from redissolution in acidic environment, and they are likely involved in a re-dissolution/re-crystallization equilibrium. This hypothesis is supported by the relatively low crystallinity of additive-free anatase TiO2 crystals aged for 6 days, resulting probably from the non-stability of nucleation seeds which prevents the formation of better crystallized crystals in a limited time. A lack of solute should not be a cause here, since the reaction medium is saturated by continuously-formed TiO2 solute during the course of the aging period. Compared to studies reported above [START_REF] Bosc | A simple route for low-temperature synthesis of mesoporous and nanocrystalline anatase thin films[END_REF][START_REF] Bosc | Mesostructure of anatase thin films prepared by mesophase templating[END_REF][START_REF] Seo | Effect of aging agents on the formation of TiO2 nanocrystalline powder[END_REF][START_REF] Qi | Room-temperature synthesis of single-phase anatase TiO2 by aging and its self-cleaning properties[END_REF][START_REF] Zhang | Recent advances in oriented attachment growth and synthesis of functional materials: concept, evidence, mechanism, and future[END_REF], in which lower crystallinity In most of the cases for TiO2 synthesized via a traditional sol-gel route, the calcinationinduced phase transformation comes along with a brutal crystal size increase and a sharply diminished porosity. Here, as a longer aging duration usually leads to more crystallized materials already before calcination, it might be expected that the longer-aged samples would exhibit a lower crystal size increase during the calcination step, since the already crystallized materials do not grow the way that the amorphous materials do. This contributed to the superior resistance against calcination in terms of crystal size growth shown by the longeraged samples. This size advantage can have a very important influence on the photocatalytic activity of the sample, as small crystals provide a higher specific surface area, while a better crystallinity provides less crystal defects which are often considered as acting as charge recombination centers.

It was worth noting that the use of BmimPF6 or of P-containing additive boosted the crystallization rate during aging, resulting in a better TiO2 crystallinity compared to the TiO2 sample synthesized without any additive before calcination. The way that phosphorous might have taken actions on size modifications of TiO2 nanocrystals during the aging step will be detailed below.

Effect of F on TiO2 properties and photocatalytic behavior

The TEM images of the TiO2 photocatalysts synthesized with BmimPF6 after calcination showed a unique shape evolution with the increasing aging duration. Also, TEM analysis on dried PANaF-assisted TiO2 samples aged for 6 days (Figure 8) showed a spindle-like shape of the nanocrystals. Although the crystals did not show the mostly reported capped bi-pyramid crystal shape obtained in many papers [START_REF] Yang | Anatase TiO2 single crystals with a large percentage of reactive facets[END_REF][START_REF] Yang | Hydrothermal stability of {001} faceted anatase TiO2[END_REF][START_REF] Liu | Fluorinated semiconductor photocatalysts: tunable synthesis and unique properties[END_REF][START_REF] Chen | Shape evolution of highly crystalline anatase TiO2 nanobipyramids[END_REF][START_REF] Tian | Raman spectroscopy: A new approach to measure the percentage of anatase TiO2 exposed (001) facets[END_REF][START_REF] Sofianou | Tuning the photocatalytic selectivity of TiO2 anatase nanoplates by altering the exposed crystal facets content[END_REF][START_REF] Wang | Role of point defects on the reactivity of reconstructed anatase titanium dioxide (001) surface[END_REF][START_REF] Luan | Long-lived photogenerated charge carriers of 001-facet-exposed TiO2 with enhanced thermal stability as an efficient photocatalyst[END_REF], the capping ability of F -on exposing {001} facet is not limited only to form flat "bi-pyramid" TiO2 crystals. It was reported that the etching chemistry of F -on TiO2 depends on many parameters such as the concentration of the capping agent or both duration and temperature of the aging [START_REF] Zhao | Shape-and size-controlled synthesis of uniform anatase TiO2 nanocuboids enclosed by active {100} and {001} facets[END_REF][START_REF] Chen | Shape evolution of highly crystalline anatase TiO2 nanobipyramids[END_REF][START_REF] Zhang | Anatase TiO2 crystal facet growth: mechanistic role of hydrofluoric acid and photoelectrocatalytic activity[END_REF].

E.g. Zhang et al. [START_REF] Zhang | Anatase TiO2 crystal facet growth: mechanistic role of hydrofluoric acid and photoelectrocatalytic activity[END_REF] have obtained rod-like anatase crystals instead of bipyramid sheet-like crystals in HF solution at 180 o C at relative higher pH values. Also, Yu et al. [START_REF] Yu | Ionic liquid assisted chemical strategy to TiO2 hollow nanocube assemblies with surface-fluorination and nitridation and high energy crystal facet exposure for enhanced photocatalysis[END_REF] have shown that a large quantity of BmimBF4 resulted in the formation of spindle-like TiO2 crystals, while a lower quantity led to the typical capped bi-pyramid shape. Therefore, at room-temperature and atmospheric pressure, it might be expected that the action mode of F -on the control of the TiO2 crystal growth and shape are much different than in hydrothermal conditions. To our knowledge, crystal shape control on TiO2 anatase crystal by using F-derived capping agent in a mild condition has never been reported, so that great difficulty might be expected when comparing our TiO2 crystal morphology to that demonstrated in many papers.

The PF6 -anions of the BmimPF6 ionic liquid is the only source of F available. It has been reported that the hydrolysis of BmimPF6 into phosphates anions and HF can take place in acidic solution [START_REF] Freire | Hydrolysis of tetrafluoroborate and hexafluorophosphate counter ions in imidazolium-based ionic liquids[END_REF][START_REF] Swatloski | Ionic liquids are not always green: hydrolysis of 1-butyl-3-methylimidazolium hexafluorophosphate[END_REF]. In our case, traces of F have been observed by XPS in TiO2 before calcination when BmimPF6 is used, as a clue that the PF6 -ion could be hydrolyzed into phosphate and fluoride ions in our acidic reaction medium (pH around 3-4).

The exposed {001} facets of anatase TiO2 crystals resulting from the capping effect of F - are hypothesized to increase the photocatalytic mineralization of MEK thanks to a favored adsorption of certain VOC molecules [START_REF] Lv | Fluorine ions-mediated morphology control of anatase TiO2 with enhanced photocatalytic activity[END_REF]. Although we did not find studies on preferential adsorption of MEK on different anatase crystal facets, studies reported on the favored dissociative adsorption of methanol [START_REF] Fang | On the unusual properties of anatase TiO2 exposed by highly reactive facets[END_REF] and formaldehyde [START_REF] Zhou | Effects of adsorbed F, OH, and Cl ions on formaldehyde adsorption performance and mechanism of anatase TiO2 nanosheets with exposed {001} facets[END_REF][START_REF] Liu | First-principles study of formaldehyde adsorption on TiO2 rutile (110) and anatase (001) surfaces[END_REF] on the {001} facet by using density functional theory (DFT) calculations. These molecules were not detected as intermediate products in the photocatalytic degradation of MEK in the present study, but there are chemical similarities between formaldehyde and acetaldehyde, the main detected intermediate by-product, so that we hypothesized that a higher percentage of exposed {001} facet would result in a higher acetaldehyde adsorption rate, and therefore in a higher probability for performing a full mineralization of MEK into CO2. This may explain the increase in the CO2 selectivity associated to the increase in the aging duration for the BmimPF6-or F-assisted TiO2 photocatalysts.

In addition, we cannot rule out a beneficial effect of exposing {001} facets on the conversion, as the recombination of photogenerated electrons and holes was reported to be lowered by spatial separation of redox sites, thanks to a selective migration of electrons and holes to specific exposed crystal faces (anisotropic electron flow) [START_REF] Murakami | Shape-controlled anatase titanium (IV) oxide particles prepared by hydrothermal treatment of peroxo titanic acid in the presence of polyvinyl alcohol[END_REF]. Tachikawa et al. [START_REF] Tachikawa | Evidence for crystal-face-dependent TiO2 photocatalysis from single-molecule imaging and kinetic analysis[END_REF] have used a single-molecule, single-particle fluorescence approach to propose that the oxidation reaction mainly occurs on the {001} facets, while the reduction reaction would mainly occur on the {101} facets. In fact, this anisotropic photogenerated electron flow has been predicted by density functional theory (DFT) calculations with the conduction band potential of anatase (101) being slightly lower than that of anatase (001) [START_REF] Li | Mechanism and activity of photocatalytic oxygen evolution on titania anatase in aqueous surroundings[END_REF]. However, a too large ratio between the {001} and {101} exposed planes could decrease the photocatalytic activity due to an unbalanced oxidation-reduction site number [START_REF] Xiang | Pivotal role of fluorine in enhanced photocatalytic activity of anatase TiO2 nanosheets with dominant (001) facets for the photocatalytic degradation of acetone in air[END_REF]. In our case, it could not be ruled out that anatase TiO2 crystals with enlarged but not over-exposed {001} facets could also enhance the overall oxidation of MEK, by reducing the charge recombination.

Effect of P on TiO2 properties and photocatalytic behavior

To investigate the role of P in P-modified TiO2 samples, a high P/Ti ratio up to 0.4 has been applied for the synthesis of TiO2 with PANaF additive. According to XRD analysis, by contrast to its counterpart sample when a P/Ti molar ratio of 0.03 was used, the crystallization spectra. Firstly, the Ti 4+ -O contribution was slightly shifted to higher binding energy at ca.

459.1 and 464.8 eV when compared to that of P-low or P-free samples (about 0.6 eV).

Secondly, in addition to the characteristic Ti 4+ -O doublet, the Ti2p spectra displayed the appearance of an additional higher energy doublet at about 460.5 and 466.2 eV. Yu et al.

attributed this second additional peak to Ti 4+ in a tetrahedral environment, indicating possible Ti-O-P bonds in titanium phosphate phase [START_REF] Yu | Synthesis and characterization of phosphated mesoporous tianium dioxide with high photocatalytic activity[END_REF]. The binding energy shift value between the Ti-O-P and the Ti 4+ -O peak in our results excided by that of the work of Yu (1.5 eV higher vs 0.9 eV in Yu et al.). Along with a higher peak intensity of the second doublet, such differences could result from the use of an even higher P/Ti ratio when compared to that used Along with other groups, they have reported that the relatively high binding energy value of the polyphosphate-metal center bond resulted from the strong transfer of electron density between the P atom in the complex phosphate form, and the 3d transition metal central atom.

So, we preferred to hypothesize that two types of P chemical environment might exist in the TiO2 sample synthesized with a high P/Ti ratio. The first one could consist of dispersed phosphates in TiO2 environment, which has resulted in a higher binding energy shift of the Ti 4+ -O doublet. The second contribution consists of a large-size titanium phosphate phase zone embedded in TiO2 matrix, which results from the agglomeration of polymerized phosphate chains with Ti centers.

Even when a low P/Ti ratio of 0.03 was used with a resulting bulk P/Ti ratio of 0.01, for which phosphate species were finely dispersed after calcination at 550 o C and thus could not be observed via XRD, the electronic chemical state of Ti centers is still expected to be influenced by the presence of the phosphates within the TiO2 crystallites. Considering that, we would like to propose a complete mechanism of P action in P-modified TiO2 samples.

Mechanism of P action with BmimPF6 as P source.

Since the beginning of the aging process, the partially released phosphate and the F -ions from BmimPF6 were present in the reaction medium along with the (Ti(OR)n-x(OH)x)

hydrolyzed titanium precursor species. The F -ions usually do not have the chelating ability to bind to more than one Ti atom. So, because of the strong interaction and high affinity between Ti centers and phosphates, the titanium phosphate precipitation was formed immediately upon contact of just-released phosphates with the titanium precursor prior to the formation of a Ti-O-Ti network, no matter the amount of P. Ti-phosphate bond is very stable, and was shown to resist to high temperature treatment [START_REF] Bhaumik | Mesoporous titanium phosphate molecular sieves with ionexchange capacity[END_REF]. We think that it is less probable for such a stable material to undergo re-dissolution in an acidic medium like it was described early. So, the aforementioned re-dissolution/re-crystallization equilibrium for the condensed Ti-O-P-O-Ti network is shifted towards re-crystallization through the presence of phosphorous. In another way to say, if the condensed Ti-O-P-O-Ti polymers serve as nucleation centers for further crystal growth, the suppression of the re-dissolution direction favors the stability of the future nucleation sites.

We propose that in the early stage of the aging step, a large number of Ti centers and phosphates reacted to form a large number of Ti-O-P-O-Ti clusters with the help of acetic acid, and those future nucleation centers were preserved in number, because of their superior stability. The room temperature crystallization of anatase crystals further carried on during aging. Note that the mean crystal size of P-involved TiO2 samples (e.g. PANaF-derived TiO2)

did not significantly grow during the aging step. This suggests that the initial crystallization of Ti-O-P-O-Ti already occurred after 18 h (even if partially). Provided that the raw Ti material quantity was similar at the beginning of the crystallization whatever the synthesis condition, a larger number of P-involved stable nucleation centers result in a smaller final crystallized crystal size compared to the synthesis without any P-containing additives.

Mechanism of P action with PA as P source.

The action mode and the influence of P on the TiO2 crystal growth is believed to be similar when using PA instead of BmimPF6. The main difference is that the whole amount of phosphate ions could be available earlier in the synthesis media at the beginning of the crystallization process in the case of PA, since hydrolysis of BmimPF6 is needed prior to the release of phosphates when BmimPF6 is used. This is thought to be in favor of a faster formation and an earlier stabilization of the Ti-O-P-O-Ti nucleation centers, that might consequently lead to a better final crystallization of TiO2 nanocrystals. Erreur ! Source du renvoi introuvable. (bottom) summarizes the proposed room temperature crystallization mechanism of TiO2 in the presence of BmimPF6 or P/F-containing additives during the aging step. In the following sections, the effects of this P-induced advantage will be segmented in a detailed way.

P-induced thermal resistance for TiO2 photocatalysts.

We have proposed that phosphorous provided thermal resistance to the materials during the calcination even when the P content is low. Unusual crystal shape-and size-change during calcination were noticed for the "PANaF 6d" sample. Spindle/rod-like nanocrystals were visualized by TEM images before calcination, with a mean length near 10 nm and a mean width around 3-4 nm, while after calcination, the crystals changed of morphology to a larger spherical shape with a mean diameter around 12 nm. We can note that the crystal growth upon calcination mainly occurred in terms of width, rather than of length. Since the crystallinity of the crystals before calcination was already rather good, the crystal width increases unlikely through the conversion of amorphous phase into crystallized TiO2. We propose thus that the crystal growth during calcination may be caused by the sintering of already-crystallized crystals along the length in direction. As example, this means that a final spherical TiO2 crystal with a diameter of 12 nm could schematically result from the sintering of three rod-like-shape crystals stacked together along their length direction during calcination.

This P-induced limited crystal size growth mechanism is responsible for the smaller crystal size and the larger specific surface area of such TiO2 photocatalysts compared to the Pfree TiO2 materials after calcination. We hypothesized that this behavior is responsible for the enhancement of the MEK conversion rate due to the exposure of more adsorption sites. The limitation of the crystal size growth during the BmimPF6 ionic liquid template-based sol-gel synthesis of TiO2 was already reported, but no explanation was proposed till now [START_REF] Yoo | Preparation and characterization of mesoporous TiO2 particles by modified sol-gel method using ionic liquids[END_REF][START_REF] Choi | Preparation of thermally stable mesostructured nanosized TiO2 particles by modified sol-gel method using ionic liquid[END_REF].

According to us, this is the most important feature brought by P in terms of boosting the photocatalytic activity of TiO2. However, we cannot rule out a direct positive effect of negatively-charged phosphate anions on the photocatalytic activity, since they have been already reported being capable to attract photogenerated holes, thus preventing electron-hole recombination [START_REF] Zhao | Surface modification of TiO2 by phosphate: effect on photocatalytic activity and mechanism implication[END_REF].

Conclusion

Highly crystallized anatase TiO2 nanoparticles were prepared with crystallite size and shape control via a BmimPF6 ionic liquid or PA/NaF assisted sol-gel synthesis. We showed that BmimPF6 ionic liquid could be efficiently substituted by cheap PA and NaF additives in an adequate ratio in the sol-gel synthesis for synthesizing TiO2 photocatalysts without altering the main physico-chemical properties and the photocatalytic activity. We proposed a correlation between the main bulk and surface physico-chemical properties and the photocatalytic activity of the TiO2 materials, and highlighted a similar combined role of phosphate and fluoride ions, whether they are brought by the BmimPF6 ionic liquid or by the PA and NaF additives. When synthesized with an extended aging time, TiO2 synthesized in the presence of phosphate and fluoride ions displayed a higher MEK conversion and a twice higher mineralization yield in the gas phase degradation of MEK under UVA light, in comparison to the TiO2 Aeroxide P25 reference and the sol-gel TiO2 counterpart synthesized in the absence BmimPF6.

In both cases, the formation of a titanium hydroxide network in the early stage of the solgel synthesis was proposed to result in a size control of TiO2 crystallites and thus in a higher specific surface area, in favor of a higher MEK conversion rate, while the fluoride ions were proposed to cause an anisotropic TiO2 crystal growth during the aging step of the sol gel synthesis, in favor of a higher selectivity to CO2 through a favored adsorption of intermediate products of MEK degradation on the exposed TiO2 {001} facets.

P 2p

Binding energy (eV) IL 6d 550 IL 6d

F 1s

Binding energy (eV)

3. 1 . 3

 13 Morphological characterization. TEM images of Erreur ! Source du renvoi introuvable. show the influence of the presence of BmimPF6 on the TiO2 nanocrystal morphology after 6 days of aging and calcination at 550 o C, and TiO2 P25. The mean crystal size of the reference sol-gel TiO2 was around 20 nm, similar to that of TiO2 P25, but with a moderately homogeneous distribution. This crystal size distribution heterogeneity could result from a fast crystal growth during the calcination step. By contrast, the TiO2 sample prepared with BmimPF6 (Erreur ! Source du renvoi introuvable.c,d) displayed a smaller average crystal size with a more homogeneous distribution (Erreur ! Source du renvoi introuvable.b), in agreement with the XRD analysis.

4. Photocatalytic activity 4 . 1

 41 TiO2 synthesized via BmimPF6 IL assistanceFirstly, no MEK oxidation occurred neither with the sole UV-A irradiation in the absence of TiO2, nor in the presence of TiO2 in the dark. The MEK conversion, the selectivities to acetaldehyde and CO2, as well as the CO2 mineralization yield achieved on calcined TiO2 synthesized in the presence of BmimPF6 and with an aging duration of 18 h and 6 days are

  TiO2 materials were synthesized, we might propose that no better crystallized anatase TiO2 nanocrystals might be obtained thanks to only acetic acid and an appropriate aging duration at room temperature. A schematic illustration of the room temperature crystallization mechanism of TiO2 in the absence of IL and additives during the aging step is presented in Erreur ! Source du renvoi introuvable. (top): A. Initial condensation to form polymerized Ti-O-Ti chains; B. Those chains could go re-dissolution to form smaller size segments of chains in acidic condition; C. Seeds of nucleation could be formed by ordered arrangement of those small segments; D. If those seeds are stabilized, crystallization can occur from the seeds by re-arranging the rest of small chains in an ordered manner to minimize energy.

  Figure top) allowed the crystallization of new titanium phosphate phases in addition to

in

  Yu et al. (0.4 vs. 0.1). The strong increase in the O1s peak area with change of the envelope profile was attributed to extra oxygen brought by phosphates as the calculated O/Ti surface atomic ratio increased up to 4.5-4.8, in agreement with studies in which PA was used during sol-gel syntheses of TiO2[START_REF] Yu | Synthesis and characterization of phosphated mesoporous tianium dioxide with high photocatalytic activity[END_REF][START_REF] Zhao | Surface modification of TiO2 by phosphate: effect on photocatalytic activity and mechanism implication[END_REF][START_REF] Korosi | Structural properties and photocatalytic behaviour of phosphate-modified nanocrystalline titania films[END_REF]. The P2p spectra showed also an additional doublet contribution at about 135.0 and 135.9 eV beside the contribution corresponding to P 5+ of usual phosphate group at 133.5 and 134.4 eV. This additional P-related doublet peak was rarely described in the literature of P-derived TiO2 till now. Zhao et al.[START_REF] Zhao | Surface modification of TiO2 by phosphate: effect on photocatalytic activity and mechanism implication[END_REF] have noticed a new P2p3/2 contribution at 134.5 eV for PO4 3--treated TiO2, assigned to PO4 3-species bonded by bidentate form (bi-phosphate). Works of Gasik et al.[START_REF] Gasik | The investigation of phosphorus chemical bond in the manganese-ore materials[END_REF], and Lyakishev et al.[START_REF] Lyakishev | The nature of chemical bonding of phosphorus in monomineral fractions and concentrates of manganese ores[END_REF] have assigned new P2p contributions at 135.8 eV to P within island, ring-shaped and chain-like orthophosphates or diorthophosphate motives (complex polyphosphate like [Р3O9] 3-, [Р2O7] 4- and [Р3О10] 5-) connected to central transition metal atoms, in their cases with manganese.

Figure 1 :Figure 2 Figure 2 : 3 Figure 3 :

 12233 Figure 1 : XRD patterns of TiO2 IL0 6d, IL 18 h and IL 6d samples, (A) after the drying

4 Figure 4 :

 44 Figure 4

Figure 6 :

 6 Figure 6 : P2p XPS patterns of BCPA 6d, BCPA 6d 550, PANaF 6d and PANaF 6d 550.

Figure 7 :Figure 9 :

 79 Figure 7 : TEM images of (a) BCNaF 6d 550 and (b) PANaF 6d 550.

Figure 10

 10 Figure 10

Figure 10 :

 10 Figure 10 : (Top) XRD pattern of PANaF-TiO2 synthesized with a PA/Ti ratio of 0.4 and a calcination temperature of 800 o C (the NaF/Ti ratio was fixed at 0.03). Black diamonds are anatase phase, magenta stars are Ti4P6O23 phase, and hollow triangles are TiP2O7 phase. (Bottom) Ti2p, O1s and P2p XPS patterns of PANaF-TiO2 synthesized with a P/Ti ratio equal to 0.4 : (a) dried sample; (b) sample after calcination at 550 o C.

2.1 Catalyst preparation 2.1.1 Ionic liquid assisted sol-gel synthesis of TiO2.

3.1 TiO2 synthesized via BmimPF6 IL assistance 3.1.1 Structural properties.

	The XRD patterns of the IL-derived TiO2 samples are shown in Erreur ! Source du
	renvoi introuvable.. Their main bulk physico-chemical properties such as the mean
	crystallite sizes derived from XRD measurements and the specific surface areas are reported
	in Table 1. XRD patterns show only anatase phase as crystalline phase, with or without
	addition of BmimPF6, and whether the samples were calcined at 550 o C or only dried at 100 o C.

  and FigureSI 3). The TiO2 sample prepared with BCNaF are not reported here. The spectra of Ti2p, and O1s orbitals for dried and calcined TiO2 did not show any strong

differences compared to those recorded on TiO2 synthesized with BmimPF6 (Figure

SI 3)

. By contrast, in BCPA-and PANaF-derived TiO2 samples, the presence of P has been confirmed in dried and calcined TiO2 (Erreur ! Source du renvoi introuvable.), with the characteristic doublet of P2p3/2-P2p1/2 orbitals corresponding to P-O bond in non-polymerized phosphates, at 132.9/133.8 eV (BCPA dried), at 133.2/134.1 eV (BCPA calcined), at 132.9/133.8 eV (PANaF dried) and at 133.3/134.2 eV (PANaF calcined). The P/Ti surface atomic ratio

Table 1 :

 1 Influence of the presence of BmimPF6 during the sol-gel TiO2 synthesis on the BET specific surface area and the mean crystallite size of TiO2 photocatalysts (BmimPF6/Ti molar ratio of 0.03, aging duration of 18 h and 6 days). Corresponding to the TiO2 anatase phase. c The mean TiO2 crystallite size, i.e. the average size of the coherent diffracting domains, was derived from XRD measurement through the Scherrer equation applied to the {101} peak at 25.7 o with the usual assumption of spherical crystallites.

	Sample name	BmimPF6/Ti molar ratio	bulk	P/Ti molar ratio surface	Aging duration	BET surface area (m 2 /g) Dried samples Calcined samples	Mean crystallite size (nm) c Dried samples Calcined samples a
	P25	-	-	-	-	-	55 a	-	22 b
	IL0 6d	0	-	-	6 days	286	50	6	19
	IL 18h	0.03	0.01	n.m. d	18 h	294	110	7	10
	IL 6d	0.03	0.01	0.06	6 days	251	97	8	11

a Aeroxide P25 TiO2 was used as it. b d not measureable

Table 2 :

 2 Specific surface areas and mean crystallite sizes of TiO2 samples synthesized by combination of replacement additives for BmimPF6, in the assisted sol-gel route. The two replacement additives in one set of combination for each synthesis shared same molar ratio to Ti.

	Sample name	Aging duration	BET surface area (m 2 /g)	Mean crystallite size (nm) (Mean crystallite size before calcination)
	BCNaF 18h 550	18 h	31	24 (5)
	BCNaF 6d 550	6 days	46	19 (8)
	BCPA 18h 550	18 h	91	11 (-a )
	BCPA 6d 550	6 days	109	10 (8)
	PANaF 18h 550	18 h	108	10 (8)
	PANaF 6d 550	6 days	131	11 (8)

a not measureable

Table 3 :

 3 Results of the gas-phase photocatalytic degradation of MEK for calcined TiO2 synthesized with BmimPF6 and replacement additives. The corresponding specific surface areas are also reported.

	Sample name	Aging duration	BET surface area (m 2 /g)	MEK conversion (%)	CO2 selectivity (%)	Acetaldehyde selectivity (%)	CO2 mineralization yield (%)
	P25	-	55	37	34	36	13
	IL0 6d 550	6 days	50	32	30	25	10
	IL 18h 550	18 h	110	50	34	31	17
	IL 6d 550	6 days	97	48	52	30	25
	BCPA 6d 550	6 days	109	52	41	26	22
	BCNaF 6d 550	6 days	46	31	42	28	13
	PANaF 18h 550	18 h	108	56	41	23	23
	PANaF 6d 550	6 days	131	53	54	9	28

Acknowledgements

This work is part of the CLEANCOAT project funded by the CARNOT MICA (Materials Institute Carnot Alsace). A. Rach and M. Wolf (ICPEES) are thanked for their contributions on the photocatalytic test setup. P. Bernhardt and V. Papaefthymiou (ICPEES) are acknowledged for XPS measurements. L. Vidal (IS2M) is thanked for performing TEM analysis.


Stephane Monfray
email: stephane.monfray@st.com

Sébastien Cremer

Nathalie Vulliet

Emmanuel Dubois

Florian Domengie

Sebastien Jan

Francoise Baille

Frederic Boeuf

N Vuillet

Optimization of deep rib high speed phase modulators on 300mm industrial Si-photonics platform

Keywords:

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INTRODUCTION

Silicon photonics technology provides unreached data densities and integration level while taking benefits from low cost and industrial manufacturing infrastructure of CMOS electronics [START_REF] Boeuf | Silicon Photonics R&D and Manufacturing on 300-mm Wafer Platform[END_REF]. In this paper, O-band Silicon PN junction modulators have been designed and integrated. In order to improve light confinement in the core for maximizing the modal overlap with PN junction, we optimized a Deep Rib architecture. The purpose of this paper is to demonstrate the optimization of the modulator to obtain the best optical performances and speed with optimized implants and structure thanks to a DOE (Design Of Experiments) analysis, and to propose a new combination of well implants (named Z-implants) with the optimized Deep Rib geometry to reach OMA (Optical Modulation Amplitude) value as high as -2dBm with high cut-off frequency. To extract the intrinsic cut-off frequency (fc) of the modulator PN junction, a so-called OPEN deembedding procedure has been applied to eliminate RC parasitics associated to the probing pads and interconnect layers from the topmost Metal4 to the inner Metal2 layers. This involves two distinct Sparameters measurements of the active device and of the passive OPEN structure, respectively. By subtracting the admittance of the OPEN from that of the modulator PN junction, the intrinsic RC product is subsequently easily determined to yield fc. All RF measurements were performed with a reverse PN junction bias of 1.8V. The figure 3 shows that by reducing the distance to the waveguide of the Arm implants combined with a high dose (x20 vs industrial reference [START_REF] Boeuf | Silicon Photonics R&D and Manufacturing on 300-mm Wafer Platform[END_REF], R can be largely reduced leading to high fc. However, this optimization has to take into account the impact on optical performances (in particular losses) induced by the doping modification. To explore this, we decided to monitor the OMA (Optical Modulation Amplitude) value for all structures of the DOE. OMA is defined as the difference between the optical power levels (Phigh and Plow) of the signal. The value of OMA is calculated based on the extracted HSPM parameters: α=total device Losses (dB/mm), PS=device Phase Shift (°/mm) and L=device length (mm). We highlighted that by increasing by a factor 20 (N=1e19cm -3 ) the implant dose in the access arms down a distance to the waveguide d=250nm, the cutoff frequency could be highly improved (fig. 3) without penalty on optical performances (fig. 4). For shorter distance, higher losses lead to OMA decrease.

II. DOE DEVICES INTEGRATION ON 300MM

III. OPTIMIZED DEVICES INTEGRATION WITH VERTICAL PN IMPLANTS

In order to determine the best structure, we first performed TCAD simulations of all DOE structures to evaluate the access resistance, while other parameters (capacitances, losses, phaseshift) where measured on the fabricated devices. Thanks to the DOE analysis, we determined that the best optical performances was obtained for Xj=60nm and a width of 400nm. Experimentally, the closest measured device had a width of 370nm and was highlighted as the best point of our OMA vs fc (=1/2PiRC) figure of merit (fig. 5). The same DOE was then measured with the RF structures (fig. 6) to extract the exact fc with a reverse bias of 1,8V, and confirmed the best performances for our identified experimental geometry (W=370nm, Xj=60nm, d_Arm implant distance=250nm) with the highest Arm implant dose (x20 vs the previous platform). Then we optimized the value of the Waveguide (Wells) implants by increasing sequentially the dose of Pwell and Nwell vs the reference. The figure 7 show that we improved OMA by increasing both values by a 1,5 factor. Finally, compared to our previous industrial platform (Rib modulators in [START_REF] Boeuf | Silicon Photonics R&D and Manufacturing on 300-mm Wafer Platform[END_REF]) we can see on figure 8 that the optimized Deep Rib structure improves the OMA by 1,6dB with an improved cut off frequency (1/2PiRC). The figure 9 provides OMA vs L characteristic for the best device. IV.

NEW DEEP RIB DEVICES INTEGRATION WITH Z IMPLANTS

We propose in this section the combination of our best Deep-Rib geometry with Z-implants (combination of depths of vertical N-well implants ) to improve the coverage between the highly confined mode in the Deep-Rib structure and the depletion zone of the PN junction (fig. 10). The same DOE was analyzed with these Z-implants and the best geometry was the same as previously identified, but with Xj=140nm to ensure the best overlap with the junction. Fig. 11 & 12 shows the optical performances of the structure and compare it with the previous points. One can see that this approach improves OMA by 2,15dB compared to our previous platform, by assuming a slight decrease of the cut-off frequency. The figure 13 provides OMA vs L characteristic for the best device. V. CONCLUSION This paper shows optimized deep Rib modulators for 400G and above applications. DOE integration allowed us to define the optimal structure and implants to guaranty high speed (high cutoff frequency) coupled with high optical performances, to propose highly performant silicon PN junction modulator with OMA improved by 1,6dB and up to 2,15dB compared to the industrial state-of-the-art. The best Deep Rib geometry with Zimplants has OMA (Optical Modulation Amplitude) value as high as -2dBm (L=3mm) with 54GHz@0V cut-off frequency.

  PHOTONICS PLATFORM A Design Of Experiments (DOE) for Deep Rib modulators was tested on our 300mm Si-Photonics platform, including Ge Photodiodes, Si & SiN waveguides and transitions (see Fig.1) for CWDM circuits. The Design of Experiments (DOE) for Deep Rib modulators was first established with variations on the junction position Xj (from 20nm to 160nm), on the width of the modulator (W from 320 nm to 600nm) and on the distance d of access arms implants to the guide (from 100nm to 400nm) with different levels of doping (Process of reference Arm Doping x1 and highly doped Arm Doping x20), see figures 2a and 2b.

Fig. 2a :

 2a Fig.2a: Parameters used in the DOE definition. We studied the width (W) impact, the junction (Xj) position and the Arm implants Dose and position (d = d_Arm).

Fig. 2b :

 2b Fig.2b: Fabricated HSPM devices with Deep Rib structure. Total guide thickness=300nm and Arm thickness=50nm are constant parameters for the DOE.

Fig 3

 3 Fig 3 shows the measured cutoff frequency in function of d distance, for two levels of Arm Doping (x1 and x20) vs previous industrial reference [1])

  𝑂𝑀𝐴 (𝑑𝐵𝑚) = 10. 𝐿𝑜𝑔 10 (𝑃ℎ𝑖𝑔ℎ -𝑃𝑙𝑜𝑤)

Fig 4 :

 4 Fig 4: impact of the Arm implant distance from the Si guide on the optical performances (OMA), for two levels of Arm doping (reference and x20).

Fig 5 :

 5 Fig 5: DOE results for different geometrical and implants parameters. Best structure for compromise OMA vs cut-off frequency is identified. L=2,4mm.

Fig 6 :

 6 Fig 6: Confirmation of best geometry confirmed with cut-off frequencies measured at 1,8V reverse bias on dedicated RF structures. L=2,4mm.

Fig. 7 :

 7 Fig.7: (left) Impact of Si guide Pwell implant increase for a given Nwell value and (right) Impact of Si guide Nwell implant increase for the best Nwell implant. OMA is optimum for the highest values. L=2,4mm.

Fig 8 :

 8 Fig 8: Impact of well implants increase for the best HSPM geometry. OMA is improved from -3,25dBm to -2,85dBm on the Deep Rib structure. L=2,4mm. Compared to [1], OMA is improved by 1,6dB.

Fig 9 :

 9 Fig 9: OMA vs L for the Optimized geometry and optimized implants (Arm x20) & (Nwell x1,5) & (Pwell x1,5) vs ref implants [1].

Fig 10 :

 10 Fig 10: Proposal of new Z-implant structure on Deep-Rib best architecture. Z-implants are optimized to ensure a better electrostatic coverage of the optical mode (left) vs the classical vertical implants (right).

  Fig 11: Optical performances of the new HSPM Deep Rib with Zimplants measured on the best design (W=370nm, d_Arm=250nm, Xj=140nm)

Fig 13 :

 13 Fig 13: OMA vs L for the optimized geometry, optimized implants (Arm x20) & Z-implants on Deep Rib structure.

Fig 14 :

 14 Fig 14: OMA vs L for the Optimized Deep Rib geometry and optimized implants (Arm x20) & Zimplants compared to the previous industrial platform [1].

Fig 15 :

 15 Fig 15: VPi*LPi for the Optimized Deep Rib geometry and optimized implants (Arm x20) & Zimplants compared to the previous industrial platform [1].

Fig.1: 300mm Si Photonic Platform including Si & SiN waveguides, adiabatic transition, Thick M4 Cu level and Deep Rib HSPM modulators. Integrated Ge PD is not shown on the picture.

This work was partially supported by H2020 project N° 688516 COSMICC and project IRT Nanoelec.


Danylo Babich

Keiki Fukumoto

Benoit Corraze

Julien Tranchant

Maciej Lorenc

Hervé Cailleau

S-Y Koshihara

Laurent Cario

Etienne Janod
email: etienne.janod@cnrs-imn.fr

Unusually long carrier lifetime in a Mott insulator revealed by time-resolved Photoemission Electron Microscopy

Mott insulators display puzzling insulator to metal transitions under electric field. We study here the Mott insulator GaV4S8 using time-resolved Photoemission Electron Microcopy and reveal unusually long carrier lifetimes, supporting the mechanism of electronic avalanche.

The study in non-equilibrium conditions of an emblematic class of quantum materials, the Mott insulators, is currently developing into an active and fruitful branch of solid state physics. This research field opens up many perspectives by revealing unexpected physical properties based on new control parameters. Mott insulators excited either by ultrafast infrared or THz laser pulses [START_REF] Yamakawa | Mott transition by an impulsive dielectric breakdown[END_REF][START_REF] Giorgianni | Overcoming the thermal regime for the electric-field driven Mott transition in vanadium sesquioxide[END_REF], or by electric fields [START_REF] Cario | Electric-Field-Induced Resistive Switching in a Family of Mott Insulators: Towards a New Class of RRAM Memories[END_REF]display indeed non-trivial insulator to metal transitions (IMT's) which origin goes beyond the well-established bandwidth-and filling-controlled mechanisms [START_REF] Imada | Metal-insulator transitions[END_REF]. Interestingly such transitions could be used to build up a new electronics based on Mott insulators, the so-called Mottronics, including breakthrough devices such as Mott memories [START_REF] Cario | Electric-Field-Induced Resistive Switching in a Family of Mott Insulators: Towards a New Class of RRAM Memories[END_REF] and artificial neurons for artificial intelligence [START_REF] Adda | Mott insulators: A large class of materials for Leaky Integrate and Fire (LIF) artificial neuron[END_REF]. A common feature of out-of-equilibrium IMT's in Mott insulators is that they are based on the massive excitation of electrons from the Lower (valence) to the Upper (conduction) Hubbard Band. For example, recent studies proposed that the multiplication of carrier in a Mott insulator placed under electric field occurs thanks to impact ionization and electronic avalanche [START_REF] Guiot | Avalanche breakdown in GaTa4Se8-xTex narrow-gap Mott insulators[END_REF][START_REF] Diener | How a dc Electric Field Drives Mott Insulators Out of Equilibrium[END_REF]. However the efficiency of such mechanisms might vanish if carrier lifetime is too small, i.e. if electron-hole recombination due for example to defects is too fast. In classical semiconductors, the carrier lifetime varies from the millisecond in ultrapure silicon to the nanosecond in heavily doped silicon [START_REF] Tyagi | Minority carrier recombination in heavily-doped silicon[END_REF][START_REF] Del Alamo | Modelling of minority-carrier transport in heavily doped silicon emitters[END_REF]. As usual purity levels of Mott insulators are far from the one of ultrapure classical semiconductors, one might naively expect very small carrier lifetime in the nanosecond range. Experimentally, the carrier lifetime in Mott insulators has been scarcely explored, partly due to the lack of appropriate technique to measure it. This situation has recently evolved, with the advent of time-resolved photoemission electron microscopy (tr-PEEM), a technique well-suited to determine carrier lifetime [START_REF] Fukumoto | Femtosecond time-resolved photoemission electron microscopy for spatiotemporal imaging of photogenerated carrier dynamics in semiconductors[END_REF]. We report here a tr-PEEM study of the canonical Mott insulator GaV4S8. GaV4S8 is a Mott insulator exhibiting a lacunar spinel structure (see Figure 1a) built from a NaCl-like arrangement of GaS4 tetrahedra and V4S4 cubane-like units. In each V4S4 cluster, one out of the seven V 3d electrons is unpaired and lies on the threefold degenerate molecular t2 level. In the solid, the three narrow bands deriving from these t2 molecular levels are split by the on-site electron-electron repulsion, leading to an insulating state with a Mott-Hubbard gap of 0.3  0.1 eV [START_REF] Guiot | Avalanche breakdown in GaTa4Se8-xTex narrow-gap Mott insulators[END_REF]. The application of electric field pulses in GaV4S8 leads to an IMT, opening the possibility to use this compound in memory applications [START_REF] Cario | Electric-Field-Induced Resistive Switching in a Family of Mott Insulators: Towards a New Class of RRAM Memories[END_REF]. We performed tr-PEEM experiments at room temperature on the KEK experimental setup [START_REF] Fukumoto | Femtosecond time-resolved photoemission electron microscopy for spatiotemporal imaging of photogenerated carrier dynamics in semiconductors[END_REF] on a freshly cleaved GaV4S8 single crystal synthesized as described in Ref. [START_REF] Janod | Negative Colossal Magnetoresistance Driven by Carrier Type in the Ferromagnetic Mott Insulator GaV4S8[END_REF]. Figure 1b provides a schematic description of the electron pathway in a pump-probe tr-PEEM experiment. A first pump laser, either 1.2 eV / 180 fs / repetition rate 500 Hz or 1.1 eV / 10 ns / 200 Hz, promotes electrons in the conduction band. A second laser, the probe, with a tunable delay with respect to the pump expels the excited electrons from the conduction band toward the vacuum level, where electrons are accelerated and detected. A typical tr-PEEM experiment presents the ratio of photoelectrons number detected with and without pump laser versus the time delay between the pump and the probe. Figure 1c shows the results of a first experiment carried out without any pump aiming at establishing the work function  of GaV4S8 and performed by varying the probe energy between 4 and 4.8 eV. This curve displays a clear threshold effect consistent with a work function close to  = 4.6  0.1 eV. For the tr-PEEM experiments, we chose a probe energy of 4.47 eV slightly below the  value, in order to detect only the excited electrons in the Upper Hubbard Band without any pollution by those in Lower Hubbard Band. A first tr-PEEM experiment performed with the 1.2 eV / 180 fs pump laser reveals a striking feature: there is no detectable relaxation of the signal shown in Figure 1d in the 0-100 ps time windows after the pump laser. To confirm this result, a second series of experiment was carried out using the 1.1 eV / 10 ns pump laser. Figure 1e-f shows that the relaxation indeed occurs over a much longer timescale, with a fair description of the relaxation as the sum of three exponential decay terms with1 = 43 ns, 2 = 270 ns and 3 = 4 µs. By comparison with classical semiconductors [START_REF] Tyagi | Minority carrier recombination in heavily-doped silicon[END_REF][START_REF] Del Alamo | Modelling of minority-carrier transport in heavily doped silicon emitters[END_REF], the long carrier lifetime found here might look surprising in a Mott insulator possessing likely a high level of native defects. However, a comparison with other strongly correlated insulators might be more instructive to clarify its origin. Among the scarce studies of carrier lifetime in such compounds, extremely different values are observed, going from the sub-picosecond range in the Mott insulator 1T-TaS2 [START_REF] Perfetti | Time Evolution of the Electronic Structure of 1T-TaS2 through the Insulator-Metal Transition[END_REF] to 1-20 µs in the strongly correlated insulator VO2 [START_REF] Miller | Unusually long free carrier lifetime and metal-insulator band offset in vanadium dioxide[END_REF]. As proposed in Ref. [START_REF] Miller | Unusually long free carrier lifetime and metal-insulator band offset in vanadium dioxide[END_REF], a possible clue is the difference of nature and symmetry of the valence and conduction bands states. In a "oneband" Mott insulator like 1T-TaS2, the Lower and Upper Hubbard Bands derive from the same band and therefore display the same symmetries. In this case, the electron-hole recombination is permitted and hence very fast. Conversely, in a "three-band" Mott insulator like GaV4S8, the top of the LHB and the bottom of the UHB are most probably made of different and orthogonal V3d bands (see Suppl. Mater. of Ref. [START_REF] Camjayi | First-Order Insulator-to-Metal Mott Transition in the Paramagnetic 3D System GaTa4Se8[END_REF] and Figure 1a). This would result in a mostly forbidden interband transition and hence in a slow electron-hole recombination. Even if further theoretical and experimental work is required to confirm this conjecture, the possibility to tune the carrier lifetime in Mott insulators by more than six orders of magnitude is of prime interest for artificial intelligence applications. The carrier lifetime is indeed controlling the operating frequency of artificial neurons based on Mott insulators [START_REF] Adda | Mott insulators: A large class of materials for Leaky Integrate and Fire (LIF) artificial neuron[END_REF]. In summary, we have detected unusually long carrier lifetime in the multiband Mott insulator GaV4S8 using the tr-PEEM technique. This result is important since it supports the proposed mechanism of electronic avalanche [START_REF] Guiot | Avalanche breakdown in GaTa4Se8-xTex narrow-gap Mott insulators[END_REF][START_REF] Diener | How a dc Electric Field Drives Mott Insulators Out of Equilibrium[END_REF] and will guide the future development of new hardware components in artificial intelligence.

© 2 0Figure 1 :

 21 Figure 1 : (a) crystallographic structure of GaV4S8 and molecular orbital diagram associated to the V4 tetrahedral cluster. (b) schematic description of the electron pathway in a tr-PEEM experiment. (c) PEEM energy scan of the probe laser without any pump on a GaV4S8 single crystal. (d-f) tr-PEEM signal vs various ranges of time delay between the pump and the probe in GaV4S8 at room temperature.

This work was carried out within the French-Japanese International Associated Laboratory IM-LED (CNRS).