Chunyu Li

Erwan Paineau

Francois Brisset

Sylvain Franger

Christophe Colbeau-Justin

Mohamed Nawfal Ghazzal
email: mohamed-nawfal.ghazzal@u-psud.fr

François Brisset

Photonic titanium dioxide film obtained from hard template with chiral nematic structure for environmental application

Keywords: cellulose nanocrystals, light harvesting, evaporation induced self-assembly, iridescent film, chiral nematic structure, hard template, phenol degradation

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

Photocatalysis is attracting more and more attention owing to its environmentally friendly approach toward the conversion of light energy into chemical energy and under mild reaction environments. Photocatalysis is considered as a promising solution to energy shortage or environmental issues and has been successfully applied in a wide range of applications, ranging from hydrogen production [START_REF] Chen | Plasmonic Photocatalyst for H 2 Evolution in Photocatalytic Water Splitting[END_REF][START_REF] Linic | TiN Modulator 45 Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy[END_REF][START_REF] Wang | Positioning the Water Oxidation Reaction Sites in Plasmonic Photocatalysts[END_REF] to pollutants elimination [START_REF] Ghazzal | Photocatalytic degradation of Rhodamine 6G on mesoporous titania films: Combined effect of texture and dye aggregation forms[END_REF][START_REF] Ghazzal | A simple procedure to quantitatively assess the photoactivity of titanium dioxide films[END_REF][START_REF] Guillard | Photocatalytic degradation and mineralization of a malodorous compound (dimethyldisulfide) using a continuous flow reactor[END_REF] or for CO 2 reduction [START_REF] Li | A critical review of CO2photoconversion: Catalysts and reactors[END_REF][START_REF] Li | Recent Advances in Heterogeneous Photocatalytic CO 2 Conversion to Solar Fuels[END_REF]. Among all the semiconductors (SC) used for photocatalysis, TiO 2 remains the most widely used SC, recognized for, low cost, high chemical stability throughout a wide pH range and robustness under UV illumination. However, because of its wide band-gap (3.2 eV for anatase), it can only absorb in the UV light (< 400 nm), which accounts for 3-5% of the total solar energy reaching the earth's surface, but also involves a fast recombination of electron-hole pairs [START_REF] Tamaki | Dynamics of efficient electron-hole separation in TiO 2 nanoparticles revealed by femtosecond transient absorption spectroscopy under the weak-excitation condition[END_REF][START_REF] Ghazzal | Effect of SiN x diffusion barrier thickness on the structural properties and photocatalytic activity of TiO 2 films obtained by sol-gel dip coating and reactive magnetron sputtering[END_REF]. Thus, the practical application of pure TiO 2 as photocatalyst (PC) materials is seriously impeded. To tackle this issues, various strategies have been adopted in the last decade, for improving the photocatalytic efficiency of TiO 2 in particular by coupling with other seminconductors [START_REF] Bessekhouad | UV-vis versus visible degradation of Acid Orange II in a coupled CdS/TiO 2 semiconductors suspension[END_REF][START_REF] Ghazzal | Study of mesoporous CdSquantum-dot-sensitized TiO 2 films by using X-ray photoelectron spectroscopy and AFM[END_REF] and metallic co-catalyst nanoparticles [START_REF] Linic | TiN Modulator 45 Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy[END_REF][START_REF] Liu | Energy-level matching of Fe(III) ions grafted at surface and doped in bulk for efficient visible-light photocatalysts[END_REF][START_REF] Liu | Enhanced photoactivity with nanocluster-grafted titanium dioxide photocatalysts[END_REF][START_REF] Kowalska | Visible light-induced photocatalytic reaction of goldmodified titanium(iv) oxide particles: action spectrum analysis[END_REF][START_REF] Verbruggen | Plasmonic gold-silver alloy on TiO2photocatalysts with tunable visible light activity[END_REF]. Furthermore, considering the speed of light (~1-3 × 10 8 m s -1 ) passing through nanosized TiO 2 photocatalyst, it leaves less than hundred femtoseconds to TiO 2 to be activated.

One strategy that appears very efficient is to design TiO 2 in a photonic structure, which requires neither the modification of TiO 2 nor the coupling with other semiconductors. The pioneering work of Yablonovich proposed the basics of the extraordinary properties of photonic crystals (PCs) [START_REF] Yablonovitch | Inhibited Spontaneous Emission in Solid State Physics and Electronics[END_REF]. The optical enhancement properties of photonic crystals focused on solar-energy applications have been extensively explored these years [START_REF] Mihi | Origin of light-harvesting enhancement in colloidal-photoniccrystal-based dye-sensitized solar cells[END_REF][START_REF] Guldin | Dye-sensitized solar cell based on a three-dimensional photonic crystal[END_REF][START_REF] Mihi | Transfer of preformed three-dimensional photonic crystals onto dye-sensitized solar cells[END_REF][START_REF] Nishimura | Standing wave enhancement of red absorbance and photocurrent in dye-sensitized titanium dioxide photoelectrodes coupled to photonic crystals[END_REF][START_REF] Halaoui | Increasing the Conversion Efficiency of Dyesensitized TiO2 Photoelectrochemical Cells by Coupling to Photonic Crystals[END_REF][START_REF] Xie | Photonic crystals for sensitized solar cells: fabrication, properties, and applications[END_REF]. Depending on geometry of the structure, photonic crystals can be divided into one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) structures. In particular, 3D artificial photonic crystals are considered as one of the most potential materials due to their permittivity modulation along all three directions. Furthermore, their complete photonic band-gap could reflect electromagnetic waves in all directions [START_REF] Ghazzal | Porosity control and surface sensitivity of titania/silica mesoporous multilayer coatings: applications to optical Bragg resonance tuning and molecular sensing[END_REF][START_REF] Ghazzal | Tuning the selectivity and sensitivity of mesoporous dielectric multilayers by modifiying the hydrophobic-hydrophilic balance of the silica layer[END_REF]. It is well-recognized that photonic nanostructures improve the Bragg scattering in the material and have the ability to reduce the group velocity of photons [START_REF] Chen | Supporting information for Synergy of Slow Photon and Chemically Amplified Photochemistry in Platinum Nanocluster Loaded Inverse Titania Opals Jennifer I[END_REF]. This interesting concept applied to photocatalytic system was suggested some years ago by combining, in the same structure, the optical properties of photonic crystals with the unique photocatalytic behaviour of TiO 2 [START_REF] Wang | Surface plasmon resonance of gold nanocrystals coupled with slow-photon-effect of biomorphic TiO2photonic crystals for enhanced photocatalysis under visible-light[END_REF][START_REF] Liu | Slow Photons for Photocatalysis and Photovoltaics[END_REF][START_REF] Wu | High photocatalytic activity enhancement of titania inverse opal films by slow photon effect induced strong light absorption[END_REF][START_REF] Deparis | Light harvesting in photonic crystals revisited: why do slow photons at the blue edge enhance absorption?[END_REF][START_REF] Zhang | Coupling surface plasmon resonance of gold nanoparticles with slow-photon-effect of TiO2 photonic crystals for synergistically enhanced photoelectrochemical water splitting[END_REF][START_REF] Chen | Tracing the effect of slow photons in photoisomerization of azobenzene[END_REF][START_REF] Chen | Amplified photochemistry with slow photons[END_REF][START_REF] Zhao | Blue-edge slow photons promoting visible-light hydrogen production on gradient ternary 3DOM TiO2-Au-CdS photonic crystals[END_REF]. This appealing idea could be used to design photocatalytic systems of SC materials with enhanced light interaction time, resulting in higher photogenerated charge carriers through "slow-photon effect". In this case, the energy of the slow photons must overlap with the absorbance spectrum of the semiconductor [START_REF] Wu | High photocatalytic activity enhancement of titania inverse opal films by slow photon effect induced strong light absorption[END_REF][START_REF] Chen | Amplified photochemistry with slow photons[END_REF].

Interestingly, slowing down the photon motion also increase the optical path length. Hence, TiO 2 would have more time to absorb photons leading to higher photogenerated electron-hole pairs density (enhancement of the absorbance factor).

Up to now, opal, inverse opal and butterfly wings have been extensively used as an organic template to fabricate photonic materials [START_REF] Wang | Surface plasmon resonance of gold nanocrystals coupled with slow-photon-effect of biomorphic TiO2photonic crystals for enhanced photocatalysis under visible-light[END_REF][START_REF] Sordello | Improved Photochemistry of TiO 2 Inverse Opals and some Examples[END_REF][START_REF] Chen | Synergy of slow photon and chemically amplified photochemistry in platinum nanocluster-loaded inverse titania opals[END_REF]. Many researches reported the synthesize of photonic TiO 2 with opal or inverse opal fashion. Synthesis of 3D inverse opal TiO 2 photonic crystals with large surface area using polystyrene opal templates in sandwich method or using convective self-assembly of polystyrene microspheres by repeat atomic layer deposition of titanium dioxide were reported [START_REF] Gu | Fabrication of High-Quality Opal Films with Controllable Thickness[END_REF][START_REF] Kubrin | Bottom-up Fabrication of Multilayer Stacks of 3D Photonic Crystals from Titanium Dioxide[END_REF]. TiO 2 -based photocatalysts fashioned as inverse opals with tuneable Bragg peak reflection showed an enhancement of the photocatalytic activity for dyes degradation [START_REF] Wu | High photocatalytic activity enhancement of titania inverse opal films by slow photon effect induced strong light absorption[END_REF][START_REF] Chen | Synergy of slow photon and chemically amplified photochemistry in platinum nanocluster-loaded inverse titania opals[END_REF]. The photoactivity improvement was attributed to the intensified dye sensitization because of slow photon effect on the red and blue edges of the photonic band-gaps [START_REF] Deparis | Light harvesting in photonic crystals revisited: why do slow photons at the blue edge enhance absorption?[END_REF]. The photocatalytic activity of PC materials formed from phototonic crystals showed a dependence of the illumination wavelength and the irradiation angle. Thus, the irradiation wavelength matching the blue or the red edges of the Bragg reflection showed highest photoactivity for dye degradation. Meanwhile, synthesis of biomorphic TiO 2 photonic crystals were explored by using butterfly wings as a template and the subsequent deposition of gold nanoparticles as co-photocatalyst, that show a significant increase for the photodecomposition of methyl orange dye under visible light illumination [START_REF] Wang | Surface plasmon resonance of gold nanocrystals coupled with slow-photon-effect of biomorphic TiO2photonic crystals for enhanced photocatalysis under visible-light[END_REF]. Coupling slow photon effect with surface plasmon resonance enhances the photocatalytic activity.

In this study, titanium dioxide as a photocatalyts was elaborated using mesoporous silica film with chiral nematic structure as a hard template. The Bragg reflection of the hard template was tuned using variable ratio of SiO 2 /CNCs in the visible light. The obtained TiO 2 showed a birefringence suggesting a transfer of the 3D structure from the hard template to the photocatalyst. The study of the dynamic of photogenerated charges and the photocatalytic properties showed an enhancement of the light harvesting and the photoefficiency.

Experimental section

Materials

All chemicals were used as received without further purification. Titanium isopropoxide (TTIP, 97%), Tetraethyl orthosilicate (TEOS, 98%), and cellulose nanocrystals were extracted from pure ramie fibers as previously reported [START_REF] Habibi | Bionanocomposites based on poly(ε-caprolactone)-grafted cellulose nanocrystals by ring-opening polymerization[END_REF] and ethanol was purchased from VWR Chemicals.

Deionized water (Milli-Q water under 18.2 MΩ) was used in the experiment.

Synthesis of SiO 2 /CNCs composite films with chiral nematic structure

The composite films were prepared following the procedure reported elsewhere [START_REF] Shopsowitz | Biopolymer templated glass with a twist: Controlling the chirality, porosity, and photonic properties of silica with cellulose nanocrystals[END_REF][START_REF] Shopsowitz | Free-Standing Mesoporous Silica Films With Tunable Chiral Nematic Structures[END_REF]. Briefly, the composite films were obtained by combining the CNCs (3.3 wt%) and TEOS via evaporation induced self-assembly method (EISA). TEOS were mixed with 1 ml ethanol under stirring at room temperature for 10 min and then added dropwise to 5 mL of aqueous CNCs solution, readily sonicated for 10 min. The TEOS/CNCs weight ratios were ranged from 0.5 to 3 wt% as detailed in Table 1. The solution was aged under stirring for 1 hour at room temperature before free-casting the solution in petri dishes.

Two-step hard template method

The protocol used in this work was inspired by the method detailed in [START_REF] Shopsowitz | Hard templating of nanocrystalline titanium dioxide with chiral nematic ordering[END_REF]. Mesoporous silica films with chiral nematic structure were elaborated by the calcination of silica/CNCs nanocomposite films. The nanocomposite films were dried at 150 °C for 2 h and then at 500 °C during 4 h with a rate of 2 °C.min -1 . An aqueous TTIP-HCl solution was loaded into the mesoporous silica films using the incipient wetness method. After each loading step, the films were dried and annealed at 180°C. After five impregnation/drying cycles, the silica/titania films were calcined at 500°C for 4h to obtain a crystalline product. The silica templates were then removed by dissolving them in 2M NaOH (aq) for 1 day, resulting in mesoporous TiO 2 film with photonic structure.

Characterization

The morphologies and nanostructures of synthesized samples were characterized by scanning electron microscope (SEM) ZEISS Supra 55VP FEG-SEM at 1 kV scanning electron microscope operating at 2 kV and working distance between 3 and 5.7 mm.

The purity of CNC was controlled by Wide-Angle X-Ray Scattering (WAXS) experiments.

Measurement was carried out on a copper rotating anode generator (λ Cu Kα = 1.5418 Å, Rigaku

Corp., Japan) monochromatized with a multilayer W/Si optics (Osmic) a monochromatic beam of 1 x 1 mm² at the sample position [START_REF] Paineau | Effect of ionic strength on the bundling of metal oxide imogolite nanotubes[END_REF]. Two-dimensional WAXS pattern was collected on a MAR345 detector (marXperts GmbH, Germany) with 150 μm pixel size, placed at a sampleto-detector distance of 200 mm. The typical accessible range of scattering vector modulus Q was 0.1 -3 Å -1 (Q = 4π/λ sin(θ), where λ is the incident wavelength and 2θ is the scattering angle). Scattered intensity I as a function of the scattering vector modulus Q is obtained by angular integration over the 2D scattering patterns using homemade software. Experimental resolution can be approximated by a Gaussian with Full-Width at Half Maximum (FWHM)

~0.013 Å -1 .
Optical properties of CNC-based films were studied using UV-Vis-NIR spectroscopy (model Cary 5000 series from Agilent Technologies) equipped with an integrating sphere for diffuse and total reflection measurements. The diffuse-reflectance spectra were recorded by mounting the film perpendicular to the beam light. The maximum reflectance was set to 100% using BaSO 4 as a reference in a wavelength ranged between 300 to 800 nm.

Polarized optical microscopy (POM) observations were obtained at room temperature using a ZEISS Axio Observer Z1 equipped with 10× (0.25 NA), 20× (0.5 NA) and 40× (0.6 Korr)

objectives. The images were recorded using ZEN software 3.2 lite.

The dynamics of charge-carrier and lifetimes in TiO 2 were determined by microwave absorption experiments using the Time Resolved Microwave Conductivity (TRMC) method after UV illumination (λ max =360 nm) [START_REF] Colbeau-Justin | Structural influence on charge-carrier lifetimes in TiO2 powders studied by microwave absorption[END_REF]. After the illumination by means a laser pulse, free photogenerated electrons and holes leads to a perturbation of the initial microwave absorbance.

The temporal evolution of conductivity signal reflected by the sample is related to the lifetime of the photogenerated carriers, which can be evaluated considering the following equation:

(

where ∆n i (t) is the number of excess charge-carriers i at time t and μ i is their mobility. The sensitivity factor A.

The photocatalytic activity of synthesized TiO 2 was evaluated by following the photodegradation of phenol as a probe molecule in water (50 ppm) under UV-vis illumination using a Xenon lamp (Oriel 300 W). The photocatalytic degradation of phenol was carried out in a quartz reactor (1 × 1 × 4.5 cm 3 ) using 3.5 mg of photocatalyst suspended in 3.5 mL of an aqueous solution. During the photocatalytic experiments, the solution was continuously stirred and bubbled a fixed flow of oxygen. Before starting the photocatalytic test, the slurry solution was aged for 20 minutes under magnetic stirring in the dark to reach the adsorption and desorption equilibrium. The phenol concentration was followed by HPLC (Agilent 1260 infinity quaternary liquid chromatograph) equipped with a UV detector set at 260 nm for phenol analysis.

Results and discussions

Cellulose nanocrystal

Cellulose nanocrystal extracted from tunicate under sulfuric acid hydrolysis were used to synthesize the photonic films with evaporation induced self-assembly method. During acid hydrolysis, the sulfate ester group (CNC-OSO 4 H) replaces the hydroxyl group (CNC-OH) of cellulose [START_REF] Beck | General procedure for determining cellulose nanocrystal sulfate half-ester content by conductometric titration[END_REF]. The morphology of the CNCs after extraction has been determined by SEM observations (Figure 1a). The image shows agglomerated rod-like nanocrystals, which the respectively, characteristics of cellulose I β without any other by-products [START_REF] Ureña-Benavides | Wide-Angle X-ray Diffraction of Cellulose Nanocrystal-Alginate Nanocomposite Fibers[END_REF].

Preparation of Mesoporous Silica as a hard template

Hybrid solution containing silica precursor and CNCs was prepared in water. The negatively charged sulfate group creates an electrostatic repulsion form isotropic and anisotropic phase [].

A well-ordered hybrid mesostructured SiO 2 /CNCs is obtained after free casting the solution by using the EISA method. It is well-known that CNC form spontaneously birefringent chiral nematic droplets (or tactoids) when dispersed in suspensions [START_REF] Revol | Helicoidal self ordering of cellulose microfibrils in aqueous suspension[END_REF]. In the present case, upon evaporation of the solvent, the tactoids coalesce leading to the self-organization of CNCs together with a polycondensation of the TEOS precursor [START_REF] Shopsowitz | Biopolymer templated glass with a twist: Controlling the chirality, porosity, and photonic properties of silica with cellulose nanocrystals[END_REF][START_REF] Wang | Structure and transformation of tactoids in cellulose nanocrystal suspensions[END_REF]. The self-assembly process was performed at pH = 4.8, which was found to be a critical parameter for the stabilization of the chiral nematic structure. Indeed, the pH of the solution in composite films is preferred to be near the isoelectric point of silica material . SiO 2 /CNCs composite films are fisrt characterizad by polarized optical microscopy (Figures 2a-d). Whatever the SiO 2 /CNCs weight ratio, all composite films display strong birefringence. However, the birefringence is not homogeneous but rather presents a patchwork of regions of different blue-purple color with macrodomaines differing by their size, which is typical of spherulitic behavior of fibrillar crystals in which the molecules are packed with their axes perpendicular to the fibrillar axis [START_REF] Saha | Photonic Properties and Applications of Cellulose Nanocrystal Films with Planar Anchoring[END_REF]. Furthermore, the fingerprint of chiral nematic structutre are clearly visible ( EISA method enables to elaborate iridescent film with good homogeneity as showed in Figure 3a-d. Based on the Bragg law, when the half of helical pitch much exactly equal to the wavelength of incident light, the light cannot propagate through the material but it is rather reflected. Thus, the films display iridescent color due to the light reflecting in the visible region.

Figure 4a presents the UV-visible spectra recorded for the hybrid films prepared at different wieght ratio. The maximum wavelength of the Bragg peak reflection is red-shifted with increasing silica/CNCs weight ratio. The two components of the composite silica/CNCs films, SiO 2 and crystalline cellulose, have similar refractive index (n = 1.46 and 1.54, respectively).

One can therefore expect only small variation of the average refarctive index, when CNCs and SiO 2 are associated in the resulting composite films. In the present case, the observed red-shift must be predominantly caused by the increase in helica pitch P between CNCs with increasing the silica content. Indeed, SiO 2 coveres the CNCs nanoparticles and thus increases the helical pitch [START_REF] Shopsowitz | Free-Standing Mesoporous Silica Films With Tunable Chiral Nematic Structures[END_REF]. After calcination at 500°C, the cellulose nanocrystals are mineralized and the resulting silica films with a chiral nematic structure are obtained. Indeed, POM observations depicted in Figure 4a clearly highligth the irridescence caused by the the retention of the chiral nematic structure in the inorganic SiO 2 films. Furthermore, higher magnification of Figure 4b evidences the fingerprint texture caracteristic of the chiral nematic ordering.

Nanocrystalline TiO 2 obtained from hard templating of SiO 2 with chiral nematic structure Hard template method has been used to elaborate mesoporous TiO 2 with chiral nematic structure. The cellulose biotemplate was removed by both calcination of silica/CNCs films at 500 °C under air. After filling the porous SiO 2 structure with titanium precursor, the silica was selectively etched under highly basic solution leaving only inorganic mesoporous TiO 2 films.

EDX analysis of the HT-TiO 2 is presented in Figure 5.a showing that the etching process removed efficiently the silica hard template. Indeed, only TiO 2 was observed and no silica was present in the material with a trace of Na and Cl left due to the acidic medium used for the TTIP stabilization and the after the basic treatment used for the silica etching. The morphology of the cross section of the films has been analyzed by SEM (Figure 5.b). The film presents a mesoporous structure with a size about of 5-15 nm in correlation with the expected diameter of the initial CNCs biotemplate. However, these observations the hierarchical mesostructured of the film shows no evidence of the helicoidal chiral nematic structure. This would be attributed to the orientation of the films during the SEM analysis.

POM micrographs were used to study the synthesized TiO 2 after etching SiO 2 under basic solution. Polarized light is a contrast-enhancing technique that improves the quality of the image obtained with birefringent materials that can be useful for both quantitative and qualitative studies for a wide range of anisotropic specimens. POM observation of HT-TiO 2 using POM, the films show typical birefringence of TiO 2 with chiral nematic structure as reported elsewhere [START_REF] Shopsowitz | Hard templating of nanocrystalline titanium dioxide with chiral nematic ordering[END_REF]. The strongly birefringent of the films suggested the long-range anisotropy of TiO 2 with the retention of the hierarchical structure. The color evolving from silver (Figure 6.a and 6.b), gold (Figure 6.c) to clear blue (Figure 6.d). The color change could be due to selective reflection, since the variable initial Bragg peak position of the composite SiO 2 /CNCs used as a hard template were used (showed in Figure 3).

Photophysical and photocatalytic properties of synthesized TiO 2 nanomaterials

Time Resolved Microwave Conductivity (TRMC)

Electronic property of the synthesized TiO 2 was studied by TRMC technique. TRMC is used to study the excess of photogenerated charge-carriers and their dynamics [START_REF] Colbeau-Justin | Structural influence on charge-carrier lifetimes in TiO2 powders studied by microwave absorption[END_REF][START_REF] Martin | Time-resolved Microwave Conductivity[END_REF]. The chargecarrier dynamics (electronic properties) of the samples were studied at the excitation wavelength at 360 nm. The laser energies reaching the samples was 1.3 mJ.cm -2 . The TRMC signal is mainly due to mobile electrons reaching the surface after excitation. After the illumination of TiO 2 , electron and hole are produced, which modify the conductivity of the semiconductor. Knowing that holes are heavier than electrons with limited mobility, they remain localized in the bulk. Thus, the TRMC signal is mainly due to electrons [START_REF] Brauer | Dynamics of Interfacial Charge Transfer States and Carriers Separation in Dye-Sensitized Solar Cells: A Time-Resolved Terahertz Spectroscopy Study[END_REF]. synthesized by one-pot method. These results highlight the beneficial effect of the chiral nematic structure on the charge-carriers density produced after each excitation. Higher TRMC signal intensity indicates that the density of electrons produced at the surface is enhanced. The 3D structure act as an optical cavity, which increases the light scattering in the photonic film.

These results indicate that the modified TiO 2 harvest more light by taking advantage of slow photon effect or scattering effect. Furthermore, compared with mesoporous TiO 2 films, the decay of ImTiO 2 show faster decay, which can be attributed to high trapping sites ratio in the mesostructure.

Photocatalytic degradation of Phenol

The photoactivities of the photonic TiO 2 through two-step hard template were evaluated by following the degradation of phenol under UV-visible illumination. Before illumination, no adsorption was observed after stirring the solution in dark. The films elaborated using the hard template method "HT-TiO 2 " showed better photocatalytic efficiency compared to mesoTiO 2 film, except the one obtained with ratio 0.5. Optimal photocatalytic efficiency was observed for HT-TiO 2 obtained using the weight ratio 2 and up to 70 % of the phenol was decomposed after 2 h. The photoefficiency is 1.5 fold-higher for that sample (HT-TiO 2 -2), a sample that also shows higher production of charge carriers density. The enhancement of the photocatalytic activity could be correlated to the enhancement of the improvement of the photogenerated charge carriers evidenced by TRMC. The improvement of the light scattering combined to the slow-photon effect increase the time that illumination photons are in contact with TiO 2 . This enables TiO 2 to increase its light harvesting leading to higher production of photogenerated charge carriers. In solution, the production of higher concentration of OH° and O 2 °-responsible to the degradation of phenol enhance the kinetic of photocatalytic reaction. Thus, the improvement of the photocatalytic efficiency is due to the improvement of the light harvesting due multiple scattering or "slow-photon effect".

Conclusion

In the present work, different methods were successfully used to elaborate TiO 2 film showing a birefringence indicating the retention of helicoidal structure using hard template method. The resultant photocatalysts were elaborated using hard silica film, already obtained using variable ratio and showing tuneable stopbands. The photonic TiO 2 films were synthesized with a primary aim to improving light harvesting process in photocatalysis. The replicas of chiral nematic structure were confirmed by POM and SEM. We also investigate electronic properties of our samples by TRMC. It was found that by taking advantage of the photonic film, HT-TiO 2 films exhibited higher light-harvesting efficiency than the mesoTiO 2 . This enhancement can be attributed to the increase of light scattering or slow photon effect, which is was found in agreement with the photocatalytic efficiency improvement.

  diameter of each nanorode is estimated to be ranged in 200-300 nm in length. The crystallinity of CNC sample was assessed by WAXS measurements. WAXS diagram presents a sharp peak at Q = 1.604 Å -1 (d-spacing d = 3.92 Å) and two overlapping peaks at Q = 1.051 Å -1 (d = 5.98 Å) and Q = 1.175 Å -1 (d = 5.35 Å), corresponding to the and planes,

Figure 2 .

 2 b) confirming the formation of the 3D structure during the EISA. To clearly determine the organization of CNCs in the resulting film, cross section of a hybrid SiO 2 /CNCs film (SiO 2 /CNCs-2) was observed by SEM (Figure2.e). Successive bands are observed, which correspond to the twist of periodic CNCs structure, perpendicular to the surface of the films, similar to that observed in pure chiral nematic CNC film[START_REF] Beck | Controlled production of patterns in iridescent solid films of cellulose nanocrystals[END_REF]. This observation confirms the presence of a chiral nematic ordering in the SiO 2 /CNCs composite films.

  The signal display the apparition of charge-carriers during the pulse and their lifetime (decay) after the pulse by phenomena; such as recombination or trapping. TRMC signals of HT-TiO 2 samples obtained from variable hard template showing Bragg peaks reflection with variable wavelength maximum is displayed Figure 7.a. The TRMC signal shows an increase of the intensity right after the illumination indicating a large photogenerated charge carriers. After 20 s, the signal decay suggested a recombination and surface trapping of the photogenerated charges. As a comparison, and in order to highlight the effect of the transferred helicoidal chiral structure into the TiO 2 film on the photogenerated charges, mesoporous TiO 2 film with no chiral nematic structure was prepared by one-pot method (mesoTiO 2 ). The intensity of the TRMC signals of the HT-TiO 2 under UV illumination 4 fold-higher compared to the mesoporous TiO 2 films

Figure 1 .

 1 Figure 1. (a) SEM micrograph and (b) WAXS diagram obtained on a CNC film realized by the EISA method.

Figure 2 .

 2 Figure 2. POM image of composite photonic films of SiO2/CNCs in weight ratio 0.5(a), 1 (b) 2 (c) and 3 (d) showing the birefringence and the fingerprint of the chiral nematic structure. (e) SEM image of the cross section of composite SiO2/CNCs films with a weight ratio of 2 showing the chiral nematic structure.

Figure 3 :

 3 Figure 3: Photographs of SiO2/CNCs composite films at weight ratio (a) 0.5 (b) 1 (c) 2 (d) 3 and (c) UV-vis reflectance spectra showing variable Bragg peak position red-shifted as the ratio of SiO2/CNCs increases.

Figure 4 :

 4 Figure 4: (a) POM observations of structure in the inorganic SiO2 films showing birefringence(b) higher magnification of region in (a) evidences the fingerprint texture caracteristic of the chiral nematic ordering.

Figure 5 :Figure 6 :

 56 Figure 5: (a) EDX analysis of HT-TiO2-2 film after the silica etching under basic solution. (b) SEM images of of the cross-section of HT-TiO2 film.

Figure 7 :

 7 Figure 7: (a) TRMC signals of mesoporous TiO2 and HT-TiO2 after excitation at λ=360 nm. (b) Photocatalytic degradation of phenol (50 ppm) under UV-visible light for TiO2 prepared via hard-templating and one-pot method.

Table 1 .

 1 Compositions of silica/CNCs films with different weight ratio

	Sample

Weight ratio SiO 2 /CNCs TEOS (ml) Ethanol (mL) Calculated SiO 2 (g) 3.3 wt% 5mL CNCs (g) SiO 2 /CNC-0.5

		0.5	0.3	1	0.08	0.17
	SiO 2 /CNC-1	1	0.58	1	0.17	0.17
	SiO 2 /CNC-2	2	1.16	1	0.33	0.17
	SiO 2 /CNC-3	3	1.74	1	0.45	0.17

Acknowledgements

MNG is grateful to Marie-Claire Schanne-Klein from "Laboratoire d'optique et biosciences", université Paris-Saclay, for the full access to the polarized optical microscope and to Youssef Habibi from "Department of Materials Research and Technology (MRT)", Luxembourg, for cellulose nanocrystals samples.