Sabyasachi Patra
email: sabyasachi.patra@cea.fr

Delphine Schaming

Pierre Picot

Marie-Claire Pignié

Jean-Blaise Brubach

Lorette Sicard

Sophie Le Caër
email: sophie.le-caer@cea.fr

Antoine Thill
email: antoine.thill@cea.fr

Inorganic Nanotubes with Permanent Wall Polarization as Dual Photo-Reactors for Wastewater Treatment with Simultaneous Fuel Production

Keywords: charge separation, photo-catalysis, confinement, imogolite, wastewater management, energy conversion

Photocatalytic production of fuels, even in small quantities, from the mineralization of hazardous pollutants, is a promising and renewable way of recycling wastewater. In the present work, the potential of methyl functionalized inorganic aluminosilicate nanotubes (methyl imogolite/Imo-CH3) as photocatalytic nanoreactors for this application is demonstrated. Using the phototoxic polycyclic aromatic hydrocarbon dibenzo(a,h)anthracene (DBAN) as a model pollutant, we show that DBAN molecules can be efficiently trapped inside Imo-CH3 nanotubes from an aqueous medium to undergo subsequent oxidative photo-degradation under UV light. The kinetics of this photo-degradation were shown to depend strongly on both the initial DBAN concentration in the nanotubes and the presence/absence of dissolved dioxygen. The photo-degradation process followed a complex mechanistic pathway, consisting of combined photo-oxidation and photocycloaddition reactions, where detection of carbon dioxide (CO2) as a photo-oxidation product confirmed the mineralization of encapsulated DBAN. CO and CH4 molecules were also formed, however these could arise from the further photo-reduction of CO2 on the external surface of the nanotubes. Moreover, dihydrogen (H2) was produced upon UV illumination under anaerobic conditions due to water reduction reactions on the external surfaces of Imo-CH3 nanotubes. The 2 possible mechanistic pathways of these processes are proposed, and the dual capability of Imo-CH3 nanotubes for simultaneous pollutant degradation and H2 production is then demonstrateda rare feat for a single photocatalyst material.

Introduction

Since the pioneering discovery of photocatalytic water splitting on TiO2 electrodes by Fujishima and Honda in 1972, [START_REF] Fujishima | Electrochemical Photolysis of Water at a Semiconductor Electrode[END_REF] the utilization of solar energy for various applications has been intensively studied with two central domains of interest: energy and environment. [START_REF] Chen | Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications[END_REF][START_REF] Fujishima | Titanium dioxide photocatalysis[END_REF][START_REF] Chatterjee | Visible light induced photocatalytic degradation of organic pollutants[END_REF] During the past few decades, tremendous efforts have been made to identify low cost and better performing photocatalytic platforms that are commercially practical, and a large number of semiconducting materials such as TiO2, [START_REF] Chen | Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals[END_REF] CdS, [START_REF] Zong | Enhancement of Photocatalytic H2 Evolution on CdS by Loading MoS2 as Cocatalyst under Visible Light Irradiation[END_REF] CuS, [START_REF] Xu | Nanoporous CuS with excellent photocatalytic property[END_REF] BiVO4, 8 WO3, [START_REF] Chen | Hierarchical WO3 Hollow Shells: Dendrite, Sphere, Dumbbell, and Their Photocatalytic Properties[END_REF] ZnO, [START_REF] Vaiano | Enhanced photocatalytic removal of phenol from aqueous solutions using ZnO modified with Ag[END_REF] GaN, [START_REF] Jung | Photocatalysis Using GaN Nanowires[END_REF] ZnO:GaN solid solution, [START_REF] Maeda | Photocatalyst releasing hydrogen from water[END_REF] perovskites, [START_REF] Zhang | Inorganic perovskite photocatalysts for solar energy utilization[END_REF] and g-C3N4 [START_REF] Wang | A metal-free polymeric photocatalyst for hydrogen production from water under visible light[END_REF] have been explored. One approach to achieve efficient solar conversion is to modify the base photocatalyst through the addition of dopants [START_REF] Khlyustova | Doped TiO2: the effect of doping elements on photocatalytic activity[END_REF] and/or co-photocatalysts, [START_REF] Schubert | Immobilization of Co, Mn, Ni and Fe oxide cocatalysts on TiO2 for photocatalytic water splitting reactions[END_REF] whether throughout the material or on its surface. However, the energy conversion efficiency of such photocatalysts to date has still not reached the expected threshold for commercial use, [START_REF] Chen | Particulate photocatalysts for overall water splitting[END_REF] and thus, the quest for new photocatalytic materials is ongoing.

Semiconducting materials with hollow nanostructures are particularly appealing in photocatalysis due to several advantages, such as enhanced light harvesting, better charge separation due to a reduced charge transfer distance across the thin wall of the material and higher rates of surface reactions due to increased surface areas. [START_REF] Xiao | Hollow Nanostructures for Photocatalysis: Advantages and Challenges[END_REF] Additionally, the occurrence of polarization in a photocatalyst material can promote charge separation by facilitating the migration of electrons and holes in opposite directions. [START_REF] Chen | The Role of Polarization in Photocatalysis[END_REF] Recent theoretical predictions by Teobaldi and coworkers of the existence of a permanent polarization across the walls of imogolite nanotubes make it an increasingly attractive material for photocatalysis, i.e. one which processes the advantageous attributes of both hollowness and polarization. [START_REF] Poli | The potential of imogolite nanotubes as (co-) photocatalysts: a linear-scaling density functional theory study[END_REF][START_REF] Elliott | Chemically Selective Alternatives to Photoferroelectrics for Polarization-Enhanced Photocatalysis: The Untapped Potential of Hybrid Inorganic Nanotubes[END_REF] Very recently, the predicted effects of wall polarization in imogolite were experimentally proven by our group using radiolysis of confined water. [START_REF] Pignie | Confined Water Radiolysis in Aluminosilicate Nanotubes: The Importance of Charge Separation Effects[END_REF] Imogolite is a natural aluminosilicate nanotube with the general formula Al2O3Si(OH)4. [START_REF] Yoshinaga | Imogolite in some ando soils[END_REF] These nanotubes are single-walled with a highly monodisperse external diameter of 2.6 nm, [START_REF] Paineau | A liquid-crystalline hexagonal columnar phase in highly-dilute suspensions of imogolite nanotubes[END_REF][START_REF] Amara | Single-step formation of micron long (OH)3Al2O3Ge(OH) imogolite-like nanotubes[END_REF] and are completely hydrophilic on the external and internal surfaces. They have a band gap around 5.2 eV, [START_REF] Zhao | Energetic Minimum Structures of Imogolite Nanotubes: A First-Principles Prediction[END_REF][START_REF] Poli | The Role of Cation-Vacancies for the Electronic and Optical Properties of Aluminosilicate Imogolite Nanotubes: A Non-local, Linear-Response TDDFT[END_REF] due to which, electrons and holes are produced after the absorption of high-energy electromagnetic radiation. Their one-dimensional morphology may also favor the efficient diffusion of photo-generated electrons and holes. [START_REF] Jung | Photocatalysis Using GaN Nanowires[END_REF][START_REF] Jang | Solvothermal Synthesis of CdS Nanowires for Photocatalytic Hydrogen and Electricity Production[END_REF][START_REF] Kenanakis | Light-induced photocatalytic degradation of stearic acid by c-axis oriented ZnO nanowires[END_REF][START_REF] Chen | Enhanced visible light photocatalytic performance of ZnO nanowires integrated with CdS and Ag2S[END_REF][START_REF] Hong | High Piezophotocatalytic Efficiency of CuS/ZnO Nanowires Using Both Solar and Mechanical Energy for Degrading Organic Dye[END_REF][START_REF] Liu | High-Yield Synthesis of Ultralong and Ultrathin Zn2GeO4 Nanoribbons toward Improved Photocatalytic Reduction of CO2 into Renewable Hydrocarbon Fuel[END_REF][START_REF] Soto-Vázqueza | Synthesis and photocatalytic activity of TiO2 nanowires in the degradation of p-aminobenzoic acid: A comparative study with a commercial catalyst[END_REF] The structure being hollow with a thin wall, electrons are driven towards the external surface and the holes inside of the nanotubes. [START_REF] Poli | The potential of imogolite nanotubes as (co-) photocatalysts: a linear-scaling density functional theory study[END_REF][START_REF] Elliott | Chemically Selective Alternatives to Photoferroelectrics for Polarization-Enhanced Photocatalysis: The Untapped Potential of Hybrid Inorganic Nanotubes[END_REF][START_REF] Pignie | Confined Water Radiolysis in Aluminosilicate Nanotubes: The Importance of Charge Separation Effects[END_REF] This property can be particularly interesting to the use of imogolite as a photocatalytic or cophotocatalytic nanoreactor for (reduction-oxidation) Redox reactions favoring the oxidation (resp. reduction) reaction inside (resp. outside) the nanocavity.

Recent studies have shown that the diameter, length and surface properties and reactivity of imogolite nanotubes can be tuned. [START_REF] Yucelen | Shaping single-walled metal oxide nanotubes from precursors of controlled curvature[END_REF][START_REF] Yamamoto | Preparation and properties of poly(methyl methacrylate)/imogolite hybrid via surface modification using phosphoric acid ester[END_REF][START_REF] Thomas | Biosurfactant-mediated one-step synthesis of hydrophobic functional imogolite nanotubes[END_REF][START_REF] Bac | Surface-modified aluminogermanate nanotube by OPA: synthesis and characterization[END_REF][START_REF] Ma | Preparation and properties of PVC/PMMA-g imogolite nanohybrid via surface-initiated radical polymerization[END_REF][START_REF] Bonelli | Surface properties of alumino-silicate single-walled nanotubes of the imogolite type[END_REF][START_REF] Shafia | Al/Fe isomorphic substitution versus Fe2O3 clusters formation in Fe-doped aluminosilicate nanotubes (imogolite)[END_REF][START_REF] Shafia | Isomorphic substitution of aluminium by iron into single-walled alumino-silicate nanotubes: A physico-chemical insight into the structural and adsorption properties of Fe-doped imogolite[END_REF][START_REF] Shafia | Reactivity of bare and Fe-doped alumino-silicate nanotubes (imogolite) with H2O2 and the azo-dye Acid Orange 7[END_REF][START_REF] Bahadori | Photo-activated degradation of tartrazine by H2O2 as catalyzed by both bare and Fe-doped methyl-imogolite nanotubes[END_REF] Among the possible modifications, functionalization of the internal walls with alkyl groups is particularly appealing, where the nanotube cavity becomes hydrophobic and the external surface retains its hydrophilicity. [START_REF] Bottero | Synthesis and characterization of hybrid organic/inorganic nanotubes of the imogolite type and their behaviour towards methane adsorption[END_REF][START_REF] Boyer | Aqueous dispersion state of amphiphilic hybrid aluminosilicate nanotubes[END_REF][START_REF] Picot | Behaviour of hybrid inside/out Janus nanotubes at an oil/water interface. A route to self-assembled nanofluidics?[END_REF][START_REF] Picot | Exploring Hybrid Imogolite Nanotube Formation via Si/Al Stoichiometry Control[END_REF] Indeed, such methylfunctionalized imogolite nanotubes (Imo-CH3) can capture small organic molecules very efficiently, [START_REF] Amara | Tunable-Diameter, Metal Oxide Nanotubes for Trapping of Organic Molecules[END_REF][START_REF] Picot | Dual internal functionalization of imogolite nanotubes as evidenced by optical properties of Nile red[END_REF] making them a promising material to pre-concentrate organic pollutants from contaminated water. [START_REF] Nasi | Hybrid organic-inorganic nanotubes effectively adsorb some organic pollutants in aqueous phase[END_REF] Importantly, this chemical modification of the internal surface does not prevent the formation of an intra-wall electric field. [START_REF] Monet | Structural resolution of inorganic nanotubes with complex stoichiometry[END_REF] Therefore, the Janus character of Imo-CH3 nanotubes with an hydrophobic internal core and an hydrophilic external surface with strong wall polarization provides a unique opportunity to perform oxidative photo-degradation of encapsulated organic pollutants inside the nanotubes with simultaneous reduction reactions on the external surface.

In this work, we investigated the predicted curvature-induced charge separation in hybrid methyl-functionalized imogolite nanotubes using a photocatalytic Redox reaction relevant to the treatment of water contaminated with organic pollutants. As a model pollutant, we chose the highly phototoxic polycyclic aromatic hydrocarbon (PAH), dibenzo(a,h)anthracene (DBAN). [START_REF] Alves De Lima Ribeiro | QSAR model of the phototoxicity of polycyclic aromatic hydrocarbons[END_REF] PAHs are ubiquitous in environment and are of significant concern due to their carcinogenicity and toxicity, [START_REF] Kriek | Polycyclic aromatic hydrocarbon-DNA adducts in humans: relevance as biomarkers for exposure and cancer risk[END_REF] affecting primarily the lungs, [START_REF] Bosetti | Occupational exposures to polycyclic aromatic hydrocarbons, and respiratory and urinary tract cancers: a quantitative review to 2005[END_REF] bladder [START_REF] Burstyn | Bladder cancer incidence and exposure to polycyclic aromatic hydrocarbons among asphalt pavers[END_REF] and skin. [START_REF] Siddens | Polycyclic aromatic hydrocarbons as skin carcinogens: comparison of benzo[a]pyrene, dibenzo[def,p]chrysene and three environmental mixtures in the FVB/N mouse[END_REF] These molecules are mainly formed by incomplete combustion of organic materials and are produced by various industries. The hazards of PAH increase upon exposure to visible or UV light, a phenomenon known as photo-toxicity. 56- 58 Enzymatic metabolization of small PAH molecules is the most conventional way to convert them into easily eliminated water-soluble compounds, [START_REF] Baird | Carcinogenic polycyclic aromatic hydrocarbon-DNA adducts and mechanism of action[END_REF] but larger PAHs such as DBAN, having at least four aromatic rings, are extremely resistant to enzymatic degradation. [START_REF] Alves De Lima Ribeiro | QSAR model of the phototoxicity of polycyclic aromatic hydrocarbons[END_REF][START_REF] Manahan | Environmental Chemistry[END_REF][START_REF] Lehner | Benzylic carbonium ions as ultimate carcinogens of polynuclear aromatic hydrocarbons[END_REF][START_REF] Baird | Environmental Chemistry[END_REF] Taking advantage of the Janus character of the nanotubes and of the predicted curvatureinduced charge separation across the nanotube wall, the aims of the present work were to (i) encapsulate relatively large DBAN molecules inside the Imo-CH3 cavity, (ii) study their possible degradation by photocatalytic oxidation and (iii) use the latter as a driving force to produce fuels such as dihydrogen (H2). The encapsulation and loading concentration of DBAN in Imo-CH3 nanocavity was studied by UV-Vis absorption spectroscopy and small angle X-ray scattering (SAXS). Using combined UV-Vis and fluorescence spectroscopies, the photochemical reactions induced in DBAN encapsulated in aqueous solutions of Imo-CH3 were analyzed as a function of various experimental parameters (e.g. the nature of the gaseous atmosphere and the influence of the loading concentration) and compared to those obtained for DBAN in bulk hexane. The gaseous degradation products were measured along with the production of some valuable fuels, and a model for the photo-degradation and fuel production mechanism was proposed.

Experimental

Materials

Aluminum-tri-sec-butoxide (97%), trimethoxymethylsilane (TMMS; 98%), hydrochloric acid (HCl; 37%), potassium bromide (FT-IR grade; ≥ 99%) and dibenzo(a,h)anthracene (DBAN; 97%) were purchased from Sigma-Aldrich and used without further purification. Ultra high purity Ar (99.9999%) and O2 (99.9999%) gas cylinders were purchased from Air Products. MilliQ water (18.2 MΩcm at 25°C) was used throughout the present work.

Synthesis of Imo-CH3 nanotubes

Imogolite nanotubes with a hydrophobic internal cavity (Imo-CH3) were synthesized using a procedure inspired by Bottero et al. [START_REF] Bottero | Synthesis and characterization of hybrid organic/inorganic nanotubes of the imogolite type and their behaviour towards methane adsorption[END_REF] Briefly, aluminum-tri-sec-butoxide was added to a solution of hydrochloric acid (75 mM) in the presence of trimethoxymethylsilane (TMMS). The molar ratios of Al:HCl:Si were fixed at 2:1:1.2. The mixture was initially turbid and became clear after vigorous stirring at ambient temperature for an hour. This solution was subsequently kept in an oven at 90 o C for 5 days. The resultant solution was slightly turbid and consisted of imogolite nanotubes as evidenced by their birefringence under cross-polarized light. The imogolite solution was further purified by dialysis (molecular weight cutoffs of 6-8 kDa) against milliQ water until the conductivity of the permeate solution was reduced to 4 μS cm -1 or less. Due to the hydrophilic external surface, Imo-CH3 nanotubes form a stable colloidal dispersion in water.

Encapsulation of DBAN in the Imo-CH3 cavity

DBAN (C22H14, molecular weight of 278.3 gmol -1 ) is a five-ring polycyclic aromatic hydrocarbon with extremely low solubility in water. [START_REF] Abou-Naccoul | Aqueous solubility (in the range between 298[END_REF][START_REF] Ruelle | Enhancement of the solubilities of polycyclic aromatic hydrocarbons by weak hydrogen bonds with water[END_REF] DBAN was mixed with Imo-CH3 in aqueous solution (Imo-CH3, concentration of 7.5 mgmL -1 ) and kept under continuous stirring. The progressive encapsulation of DBAN in Imo-CH3 nanotubes was followed regularly with a UV-Vis spectrophotometer (UB-2450, Shimadzu) and a HORIBA fluoromax-4 spectrofluorometer. A solution of pristine imogolite nanotubes of the same concentration was also kept under stirring and used as the reference for the UV-Vis spectroscopy measurements. In order to avoid the saturation of absorption and fluorescence intensity, the samples were diluted with Milli-Q water. A quantitative measure of the encapsulation was obtained from the increase in internal electron density of the nanotubes obtained from small angle X-ray scattering (SAXS) performed on a laboratory X-ray beamline (Xeuss 2.0). The distance between the sample and the detector was 45.6 cm. For the SAXS measurements, the nanotube dispersions were introduced in sealed glass capillaries with a diameter of 1.5 mm. The counting time was 2 h for each sample. Radial averaging of scattered X-rays was carried out to obtain plots of the scattering intensity as a function of the scattering wave vector (q), where q is equal to 4πsinθ/λ (λ being the incident wavelength and 2θ the scattering angle). The raw SAXS profiles were corrected using the scattering contributions of water, the empty capillary and the electronic background, respectively.

Photochemical studies

UV illumination of aqueous solutions of DBAN encapsulated in Imo-CH3

UV illumination of the various solutions was performed in a standard quartz cuvette with a stopper (optical path equal to 1 cm) and by using a benchtop illumination chamber equipped with eight 15 W low pressure mercury lamps (OSRAM G15T8/OF, ozone free) emitting at 253.7 nm. The incident photon intensity at 253.7 nm was measured by a potassium ferrioxalate actinometer solution [START_REF] Boltona | Determination of the quantum yields of the potassium ferrioxalate and potassium iodide-iodate actinometers and a method for the calibration of radiometer detectors[END_REF] (0.006 M) and found to be 6.9 x 10 -5 EinsteinL -1 s -1 (equivalent to 4.15 × 10 19 photons.L - 1 s -1 ). The uncertainty of actinometry measurement was found to be equal to 7.5 %. The average temperature of the solutions was ~30 °C and did not vary significantly during the illumination. The cuvette was taken out of the illumination box at regular time intervals. The corresponding UV-Vis absorption spectra and fluorescence spectra were recorded. UV illuminations were performed for DBAN loaded in Imo-CH3 solutions at three different loading concentrations (9 × 10 -6 , 3.1 × 10 -5 and 8.1 × 10 -4 molg -1 where the DBAN amount [mol] is expressed as a function of the imogolite quantity [in g]; as deduced from SAXS) and under different gaseous environments (O2, air and Ar).

In the case of the experiments performed under O2 or Ar, the solutions were first bubbled for 30 minutes with respective gases and the cuvette was immediately closed.

UV illumination of DBAN dissolved in hexane

In order to understand the specific role played by the polarized wall of Imo-CH3 nanotubes on the photochemical reactions taking place in DBAN under alkyl environment (methyl groups on the internal wall of Imo-CH3), control experiments were performed using a DBAN solution (1.1 × 10 - 5 molL -1 ) in a nonpolar alkane (hexane). The experimental conditions were the same as those described in the previous section.

Detection and measurement of gaseous degradation products

H2, CH4, CO and CO2 gases were quantified by gas chromatography (Agilent µ-GC-R3000, SRA instrument) using argon as a carrier gas. Before illumination, the samples, placed in a Pyrex glass ampule comprising a standard quartz cuvette bottom with a 1 cm optical path length, were degassed for 30 min by argon or dioxygen bubbling, depending on the required gaseous environment. They were then outgassed at approximately 5 × 10 -3 mbar and subsequently filled with 1.5 bar of argon or dioxygen, depending on the desired gaseous environment. This operation was repeated five times for each sample.

Results and Discussion

Imogolite nanotubes are made of a curved octahedral gibbsite-like Al(OH)3 layer on the external surface that is covalently linked to isolated silicon tetrahedron units (SiO3(OH)) on the inner surface via mutually shared oxygen atoms (Figure 1a). [START_REF] Monet | Structural resolution of inorganic nanotubes with complex stoichiometry[END_REF] In the case of methyl imogolite (Imo-CH3), the hydroxyl group in the silicon tetrahedral unit on the internal surface is replaced by a methyl group (SiO3(CH3)), making the internal surface hydrophobic (Figure 1b). The morphology of Imo-CH3 nanotubes is well characterized by various techniques such as FT-IR, SAXS and TEM, as extensively discussed in previous publications from our group. [START_REF] Picot | Exploring Hybrid Imogolite Nanotube Formation via Si/Al Stoichiometry Control[END_REF][START_REF] Amara | Tunable-Diameter, Metal Oxide Nanotubes for Trapping of Organic Molecules[END_REF][START_REF] Picot | Dual internal functionalization of imogolite nanotubes as evidenced by optical properties of Nile red[END_REF][START_REF] Le Caër | Dynamics in hydrated inorganic nanotubes studied by neutron scattering: towards nanoreactors in water[END_REF] CH3 (e-g). The DBAN loading concentrations are: 9 x 10 -6 mol.g -1 (e), 3.1 x 10 -5 mol.g -1 (f) and 8.1 x 10 -4 mol.g -1 (g).

Encapsulation of DBAN in Imo-CH3 nanotubes

DBAN is a pale yellowish crystalline solid (see the chemical structure of DBAN in Figure 1c). Pristine Imo-CH3 solution is nearly transparent and colorless (Figure 1d). The slight turbidity is attributed to the residual presence of aluminum hydroxide as an impurity that could not be removed by dialysis. [START_REF] Picot | Exploring Hybrid Imogolite Nanotube Formation via Si/Al Stoichiometry Control[END_REF] After equilibration of Imo-CH3 solutions with DBAN, the color of the solutions turn to orange as the DBAN concentration increases (Figure 1e-g). In order to confirm and quantify the centrifugation. At high scattering vector (q), the SAXS profile presents the characteristic oscillations of single walled Imo-CH3 nanotubes, evidencing the good quality of the nanotubes used in the present study. The position of the intensity minima at different q values and the characteristic deformation of the first oscillation confirm the alkyl functionalization of the Imo-CH3 nanotubes. [START_REF] Amara | Tunable-Diameter, Metal Oxide Nanotubes for Trapping of Organic Molecules[END_REF] Further, the shift of the first minimum to higher scattering vector (highlighted by the vertical line in Figure 2) confirms the encapsulation of DBAN inside the nanotube cavities, as also evidenced by the increase in the internal electron density of the nanotubes (see also Table S1; Electronic Supplementary Information). [START_REF] Amara | Tunable-Diameter, Metal Oxide Nanotubes for Trapping of Organic Molecules[END_REF] Table S1 summarizes the parameters used for the S2). Noteworthily, the infrared intensity of the different vibrational bands of DBAN seems to be significantly reduced upon encapsulation in Imo-CH3 nanotubes. This may be attributed to the dielectric screening by the polar Imo-CH3 wall, as already observed for molecules encapsulated in carbon nanotubes. [START_REF] Kazachkin | Dramatic Reduction of IR Vibrational Cross Sections of Molecules Encapsulated in Carbon Nanotubes[END_REF] The detection/quantification of DBAN encapsulated in Imo-CH3 nanotubes was also tested thanks to elution with acetone, followed by gas chromatography coupled to mass spectrometry analysis (GC-MS). However, the high sublimation/boiling point (the boiling point of DBAN being equal to 524 o C) and the low vapor pressure of DBAN ruled out the feasibility of elution and of the GC-MSbased quantification of DBAN in the present concentration range. Furthermore, the complete elution of DBAN from Imo-CH3 cavity using acetone was found to be difficult. Therefore, a quantitative estimation of encapsulated DBAN by elution is challenging.

Since DBAN is an aromatic hydrocarbon, it exhibits intense characteristic UV-Vis absorption and fluorescence emission spectra in organic solvents. [START_REF] Tropp | A sensor array for the discrimination of polycyclic aromatic hydrocarbons using conjugated polymers and the inner filter effect[END_REF] Figure 3a shows the UV-Vis absorption spectrum of DBAN in hexane, a nonpolar aliphatic hydrocarbon. Multiple maxima in a single absorption band are attributed to electronic excitations in different vibrational sublevels. The UV-Vis absorption spectrum of DBAN in hexane can be roughly divided into two parts, consisting of two sets of maxima, labelled A-C (EES-I) and D-L (EES-II), and associated with different electronic excitation states, [START_REF] Jones | The Ultraviolet Absorption Spectra of Anthracene Derivatives[END_REF] with the most intense absorption peak at 297 nm (Peak F, Figure 3a).

The relative absorbance of different vibrational levels changes with the increase of the DBAN concentration in hexane (Figure S4, Electronic Supplementary Information), suggesting co-facial stacking [START_REF] Carter-Fenk | Reinterpreting p-stacking[END_REF] between DBAN molecules at the highest concentrations. It is worth noting that the UV-Vis absorption spectrum of DBAN in hexane is very close to the spectrum measured in dimethylformamide (DMF), a polar organic solvent. [START_REF] Tropp | A sensor array for the discrimination of polycyclic aromatic hydrocarbons using conjugated polymers and the inner filter effect[END_REF] In hexane, the molar extinction coefficient at λmax (ε297nm) is found to be 1.52 × 10 5 Lmol -1 cm -1 (Figure 3a), which is similar to the value measured in DMF (1.5 x 10 5 Lmol -1 cm -1 ). [START_REF] Tropp | A sensor array for the discrimination of polycyclic aromatic hydrocarbons using conjugated polymers and the inner filter effect[END_REF] The increase in electron density in DBAN-loaded tubes, as compared to pristine Imo-CH3 ones (Figure 2), can be used to estimate the amount of trapped organic molecules. However, the internal electronic density of the pristine nanotubes is not equal to zero, which suggests that they are initially occupied by some residues of the synthesis such as alcohols. Therefore, DBAN encapsulation may lead to the exclusion of these small molecules from the nanocavity. This suggests that our calculation underestimates the actual DBAN loading concentration in the nanotubes. However, it gives a reasonable order of magnitude of this value. Using it and the corresponding UV-Vis absorption spectrum, the molar extinction coefficient (ε) of DBAN encapsulated in Imo-CH3 suspensions could be calculated (see Electronic Supplementary

Information for details of calculation). It was found to be 1.29 × 10 4 Lmol -1 cm -1 at peak F, which is roughly ten times lower than in hexane. Knowing the molar extinction coefficient, the loading concentration (in molg -1 ) of DBAN in Imo-CH3 suspensions could then be systematically obtained. The UV-Vis spectra of DBAN encapsulated in aqueous Imo-CH3 solutions are displayed in Figures 3(b-d) for various loading concentrations. The relative loading concentration of DBAN in these three samples was 1, 3 and 90, respectively. Roughly, these spectra share some common features with that of DBAN in hexane (Figure 3a). However, the absorption peaks shown in Figures 3(b-d) are less structured than those measured in hexane. This phenomenon is even more obvious as the DBAN concentration increases. Some slight modifications in the relative intensities of peaks E and F, and in the position of the different maxima are reported in Figure S5 and in Table S3, respectively, in Electronic Supplementary Information.

Interesting information was also obtained from fluorescence spectroscopy measurements. Similar to the UV-Visible spectra, the fluorescence emission spectra of DBAN encapsulated in Imo-CH3 solutions were broader than the spectrum measured in hexane (Figure 4) and with a change in relative intensities (Figure S6, Electronic Supplementary Information). Moreover, a significant bathochromic shift of the peak maxima (Table S4, Electronic Supplementary Information) was also observed upon encapsulation. Fioressi et al. studied the absorption and fluorescence spectra of benzopyrene (BP), a similar polycyclic aromatic hydrocarbon to DBAN, both in hexane and adsorbed on a silica surface. [START_REF] Fioressi | Effects of cluster formation on spectra of benzo[a]pyrene and benzo[e]pyrene[END_REF] The authors also observed a spectral broadening with a bathochromic shift in the case of BP adsorbed on a surface. This was attributed to BP-surface interaction and to a reversible BP-BP clustering in the absence of any solvent. The same may be true in the case of DBAN. Thus, the detected broadening of bands when DBAN is encapsulated inside the nanotubes can be attributed to the DBAN-Imo-CH3 internal surface interaction. The observed bathochromic shift as a function of increasing DBAN loading concentration can be attributed to the formation of stacked DBAN aggregates within the nanotube cavity via DBAN-DBAN co-facial weak adsorption. This clustering effect is also corroborated by the evolution of the color of the DBANloaded Imo-CH3 solution, as it changes from colorless to orangish red upon loading (Figure 1e-g).

This suggests a decrease in the HOMO-LUMO gap for the DBAN clusters, which is in agreement with the theoretical calculations of Fioressi et al. in the case of BP. [START_REF] Fioressi | Effects of cluster formation on spectra of benzo[a]pyrene and benzo[e]pyrene[END_REF] The UV-Vis absorbance of DBAN saturated water is negligible due to the extremely low aqueous solubility of DBAN (Figure S11a; Electronic Supplementary Information). The fluorescence emission spectrum of DBAN in water, as shown by Lehto et al., [START_REF] Lehto | Photolysis of polycyclic aromatic hydrocarbons (PAHs) in dilute aqueous solutions detected by fluorescence[END_REF] was also different from that measured in organic solvents such as hexane (Figure 4a) or DMF. [START_REF] Tropp | A sensor array for the discrimination of polycyclic aromatic hydrocarbons using conjugated polymers and the inner filter effect[END_REF] These differences are attributed to the presence of DBAN in the form of aggregates/clusters in aqueous medium due to its poor solubility. It is noteworthy that the solubility of DBAN is increased significantly in aqueous medium in the presence of Imo-CH3 nanotubes. Indeed, the nanotubes with the Janus surface character play a similar role to a micelle or to different organic supramolecular hosts such as cyclodextrin, cucurbit[n]uril, etc. and solubilize the otherwise insoluble DBAN molecules in the hydrophobic nanocavities. Although the nanotubes are dispersed in aqueous media, the DBAN molecules are in a polar organic environment [START_REF] Picot | Dual internal functionalization of imogolite nanotubes as evidenced by optical properties of Nile red[END_REF] due to the wall curvature with the methyl groups on the internal wall of Imo-CH3, and a negligible volume of water vapor may enter in the hydrophobic nanochannels. [START_REF] Liao | Tuning the properties of confined water in standard and hybrid nanotubes: An infrared spectroscopic study[END_REF] This explains why the UV-Vis absorption and the fluorescence emission spectra of DBAN encapsulated in Imo-CH3 are similar to those measured in hexane.

However, as already mentioned before, the spectral features of encapsulated DBAN tend to disappear with the increase of DBAN loading in the nanotubes, due to same type of clustering effects. Noteworthily, the exchange of encapsulated DBAN from the nanotubes to external water is expected to be negligible due to the extremely poor solubility of DBAN in water. Moreover, once encapsulated, the DBAN molecules may also undergo adsorption on the hydrophobic wall.

Therefore, encapsulation and adsorption may be difficult to discriminate in the present system.

Photochemical studies

UV illumination of DBAN in hexane

The internal wall of Imo-CH3 nanotubes provides an alkyl environment that is polarized due to the wall curvature. [START_REF] Zhao | Energetic Minimum Structures of Imogolite Nanotubes: A First-Principles Prediction[END_REF][START_REF] Poli | The potential of imogolite nanotubes as (co-) photocatalysts: a linear-scaling density functional theory study[END_REF][START_REF] Picot | Behaviour of hybrid inside/out Janus nanotubes at an oil/water interface. A route to self-assembled nanofluidics?[END_REF][START_REF] Picot | Dual internal functionalization of imogolite nanotubes as evidenced by optical properties of Nile red[END_REF] Hence, to isolate the surface effect of the polarized wall of Imo-CH3 on the photochemical reactions of DBAN, it is essential to understand these reactions in a bulk, nonpolar alkane environment. It is worth noting that there is very limited literature on the photolysis of DBAN in organic and aqueous media. Low et al. studied the photodegradation of various PAHs, including DBAN, in different organic solvents with varying polarity. [START_REF] Low | Solvent induced photodegradation as a source of error in the analysis of polycyclic aromatic hydrocarbons[END_REF] They demonstrated that, under UV illumination with a 254 nm lamp, the half-life of DBAN in nonpolar hexane is 77 times longer than in a polar solvent such as methanol. This suggests that DBAN is resistant to photodegradation in non-polar or less polar environment. Deaerated solutions of PAHs in different organic solvents are known to form stable photo-dimers when exposed to UV light. [START_REF] Becker | Unimolecular Photochemistry of Anthracenes[END_REF][START_REF] Bouas-Laurent | Photodimerization of anthracenes in fluid solution: structural aspects[END_REF][START_REF] Bouas-Laurent | Photodimerization of anthracenes in fluid solutions: (part 2) mechanistic aspects of the photocycloaddition and of the photochemical and thermal cleavage[END_REF][START_REF] Singh | Infrared spectra of photodimers of anthracene, benzo[a]anthracene, and tetracene[END_REF][START_REF] Han | Light-and heat-triggered reversible luminescent materials based on polysiloxanes with anthracene groups[END_REF][START_REF] Librando | Photodegradation of Anthracene and Benzo[a]anthracene in Polar and Apolar Media: New Pathways of Photodegradation[END_REF][START_REF] Tanabe | Remarkable acceleration of template-directed photodimerisation of 9-phenylethynylanthracene derivatives assisted by complementary salt bridge formation[END_REF][START_REF] Saito | Formation of polycyclic aromatic hydrocarbon grains using anthracene and their stability under UV irradiation[END_REF] Therefore, it is important to study the UV photolysis of DBAN in hexane under different gaseous environments.

Figure 6a shows the evolution of the UV-Vis absorption spectra of an air-saturated solution of DBAN in hexane upon illumination. The rapid decrease of the UV-Vis absorption intensity suggests a photo-cycloaddition reaction, as already shown for several anthracene derivatives. [START_REF] Han | Light-and heat-triggered reversible luminescent materials based on polysiloxanes with anthracene groups[END_REF] It corresponds here to the loss of the conjugated ᴨ system of DBAN due to the dimerization at the 7 and 14 positions of the DBAN structure (see Figure 1c). The decrease in absorbance is initially a slow process (Figure 6a), followed by a quick drop after roughly 40 minutes of illumination. It is worth noting that the "induction time" required for the absorbance value to drop suddenly is very dependent on the atmosphere: it is minimum when the sample is pre-bubbled with Ar, then increases when the sample is under air and further increases when it is pre-bubbled with dioxygen (Figure S7 in Electronic Supplementary Information). Therefore, dissolved dioxygen clearly plays an important role in this process. During the initial slow absorbance decrease phase, dissolved dioxygen in the organic solvent likely leads to the formation of unstable endoperoxides or hydroperoxide intermediates by a reversible reaction between DBAN and singlet dioxygen, thereby protecting the DBAN molecules against photo-cycloaddition. [START_REF] Tanabe | Remarkable acceleration of template-directed photodimerisation of 9-phenylethynylanthracene derivatives assisted by complementary salt bridge formation[END_REF][START_REF] Miller | Photolysis of polycyclic aromatic hydrocarbons in water[END_REF] Consequently, the stage at which the rapid cycloaddition takes place refers to the moment at which dissolved dioxygen in the solution under illumination is fully depleted. It can be observed that, the relative intensity between the two most intense absorption peaks at 285 and 295 nm decreases during the "induction time" (see Figure 6a and Figure S8 in Electronic Supplementary Information). As discussed previously, stacking of DBAN molecules in hexane and in the Imo-CH3 cavity led to an increase in the relative absorbance between 285 and 295 nm (see Figure S4 in Electronic Supplementary Information and Figure 3).

Hence, the decrease in relative absorbance (Abs285nm/Abs295nm) during the "induction time" in Figure 6a and S8 (Electronic Supplementary Information) suggests de-stacking of DBAN molecules while depleting the dissolved dioxygen via the formation of endoperoxide / hydroperoxide derivatives, under UV illumination. and fluorescence emission spectra of encapsulated DBAN evolved very differently (Figure 7) to that measured in bulk hexane (Figure 6). Indeed, steady changes in intensity were detected in all cases, except for very short illumination times for the highest concentration (Figure 7c). In this latter case (Figure 7c), the rapid decrease in absorption intensity after a short illumination time can be attributed to photo-cycloaddition of close DBAN molecules within the nanotube. This is similar to the processes observed for DBAN in hexane (Figure 6a). The absence of "induction time" in the case of encapsulated DBAN (Figure 7c) is possibly due to very high local concentration of DBAN inside the nanotubes and also to a lesser availability of dioxygen molecules inside the hydrophobic cavity. The fact that in all cases, the signals decay to zero at the highest illumination times suggests the photo-oxidation of DBAN molecules under UV irradiation, leading to the formation of molecules that do not absorb in this spectral range or to gaseous products. The presence of isosbestic points (Figure 7(a-c)) suggests the formation of intermediate compounds during the photo-oxidation process of DBAN, absorbing around 240-245 nm and 400 nm. Figure S9 (Electronic Supplementary Information) provides the temporal evolution of absorbance at different wavelengths. This evidences that the concentration of the intermediates goes through a maximum, before decreasing. These intermediates are finally converted to non-absorbing molecules or to gaseous products. Depending on the extent of nanotube loading concentration and on the gaseous atmosphere, the photochemical processes are complex and the photo-cycloaddition and photoredox phenomena may take place as collective non-separable events. As opposed to the photoredox reaction involving the charge pairs produced in Imo-CH3 nanotubes, the DBAN photocycloaddition reaction is promoted by UV light itself, without any charge transfer with the host nanotubes. It is worth noting that the concentration of intermediates or of the final non-gaseous molecules produced inside Imo-CH3 nanotubes was below the detection limit of conventional characterization techniques (FT-IR, 1 H NMR, etc.) and could not be characterized in the present work. However, a quantitative characterization of the gaseous products was possible by micro gas chromatography and is presented in Section 3.3. However, the decay shape depended on the loading concentration and could not be fitted with simple first or second order rate laws, suggesting a complex mechanism by superposition of the two reactions of photo-oxidation and photo-cycloaddition. Indeed, for the lowest loading concentration, the absorbance slowly decreased to zero. On the contrary, the sample with the highest loading concentration exhibited a rapid initial absorbance loss, followed by a much slower decay. Obviously, the reaction mechanisms in question are very complex. However, we can obtain an estimate by simplifying the system and considering a first-order oxidation and a second-order cycloaddition as the two dominant phenomena that may be contributing to the loss of the UV-Vis absorbance of encapsulated DBAN under UV illumination. The decay rate of the DBAN concentration can then be expressed as:

Figure 8. Kinetics of photo-oxidation and photo-cycloaddition (evolution of the absorbance of peak F, marked with an asterisk (*) in

(Eq. 1)

where 𝑘 1 and 𝑘 2 represent the rate constants for the photo-oxidation and the photo-cycloaddition reactions, respectively. The solution of Equation 1 can be expressed as follows:

(Eq. 2)

with C a constant.

The rate constants 𝑘 1 and 𝑘 2 at varying loading concentrations and gaseous atmospheres, as summarized in Table S5 (Electronic Supplementary Information), have been obtained from the exponential fitting of [𝐷𝐵𝐴𝑁] -1 vs. time plots as shown in Figure S10 (Electronic Supplementary Information). The good quality of the fit validates the proposed kinetic model.

1 [𝐷𝐵𝐴𝑁] = - 2𝑘 2 𝑘 1 + 2𝑘 2 𝑘 1 𝑒 𝑘 1 (𝑡-𝐶)
It is interesting to observe that the values of 𝑘 1 and 𝑘 2 depend on the experimental conditions. They vary with the DBAN loading and globally decrease when the DBAN loading increases, corresponding to a restriction in diffusion motions upon loading in the nanotubes. While 𝑘 1 is weakly dependent on the loading, 𝑘 2 strongly decreases upon loading. This can be attributed to the bimolecular character of the cycloaddition reaction. increases upon loading (and the same for 𝑘 1 [𝐷𝐵𝐴𝑁]). The accelerated kinetics are thus due to the large change in concentration, which largely compensates for the decrease in the rate constants.

As the presence/absence of dissolved dioxygen plays a role in the processes under consideration, photochemical studies of encapsulated DBAN were also carried out in solutions prebubbled with either dioxygen or argon prior to UV illumination (Figure 8d and 8e, respectively).

In the case of the system pre-bubbled with O2, the extent of the decrease in relative absorbance during the initial cycloaddition phase (initial rapid decrease) was reduced. This suggests that gaseous O2 molecules enter the nanotube cavity and protect the internal DBAN molecules from photo-cycloaddition by forming endoperoxide/hydroperoxide intermediates in a similar mechanism to that described in Section 3.2.1. This makes photo-oxidation the dominant mechanism in this case. The photo-oxidation in the presence of dioxygen proceeds via the peroxide intermediates with a rate constant slightly lower than in air atmosphere. Conversely, for the system pre-bubbled with Ar, the external solution and the internal nanotube cavity do not contain O2. This causes the photo-cycloaddition to take place dominantly in the nanotube cavity within a short initial illumination.

Evidently, the kinetics of the photo-cycloaddition depend also on the diffusion coefficient of DBAN inside Imo-CH3 nanocavities. For example, Lopatkin et al. derived a kinetic model for the photo-dimerization of anthracene in a polymer matrix by considering the diffusion coefficient of anthracene molecules taking part in the dimerization reaction. [START_REF] Lopatkin | Kinetics of the photodimerisation of anthracene in an inhomogeneous polymer matrix[END_REF] In the present work, the diffusion of DBAN has not been considered separately for the sake of simplicity. However, the effect of diffusion is implicit from the dependence of k2 on the loading concentrations. The weak dependence of 𝑘 1 on the loading concentration suggests that the presence of DBAN in monomeric or aggregated form (DBAN-DBAN co-facial stacking) has a weak influence on the kinetics of photo-degradation. This is in agreement with the conclusion of Kahan et al., [START_REF] Kahan | Photolysis of Polycyclic Aromatic Hydrocarbons on Water and Ice Surfaces[END_REF] who showed that the clustering of PAHs has no effect on the photo-degradation kinetics.

Table S5 (Electronic Supplementary Information) also summarizes the quantum yield of photo-oxidation of DBAN in Imo-CH3 cavity under varying loading concentrations and gaseous atmospheres. The quantum yield at a given wavelength is defined as the number of DBAN molecules undergoing hole (h + ) mediated photo-oxidation per photon absorbed by Imo-CH3. The photo-oxidation quantum yield (ɸ 1 ) under the UV light source (peak wavelength at 253.7 nm) was calculated using Equation 3. [START_REF] Lehto | Photolysis of polycyclic aromatic hydrocarbons (PAHs) in dilute aqueous solutions detected by fluorescence[END_REF][START_REF] Chu | Photodechlorination of Polychlorobenzene Congeners in Surfactant Micelle Solutions[END_REF][START_REF] Tsui | Quantum yield study of the photodegradation of hydrophobic dyes in the presence of acetone sensitizer[END_REF]

ɸ 1 = 𝑘 1 2.303×𝐼 𝜆,0 ×ԑ 𝐼𝑚𝑜,𝜆 ×𝑙 (Eq. 3)
where 𝑘 1 represents the first order photo-oxidation rate constant (time -1 ); 𝐼 𝜆,0 denotes the photon intensity at 253.7 nm (6.9 × 10 -5 EinsteinL -1 s -1 ); ԑ 𝐼𝑚𝑜,𝜆 represents the molar absorptivity of the Imo-CH3 nanotube unit cell at 253.7 nm (1071.3 Lmol -1 cm -1 ), and 𝑙 is the cell path length (1 cm).

The quantum yield (ɸ 1 ) values obtained for DBAN photo-oxidation inside Imo-CH3 nanotubes (Table S5; Electronic Supplementary Information) are similar or slightly higher (for the lowest loading concentrations) than the quantum yield of direct DBAN photolysis in water (approximately 2.5 × 10 -3 ). [START_REF] Lehto | Photolysis of polycyclic aromatic hydrocarbons (PAHs) in dilute aqueous solutions detected by fluorescence[END_REF] However, as the light source was nearly monochromatic and the band gap of Imo-CH3 (~ 5.2 eV) [START_REF] Zhao | Energetic Minimum Structures of Imogolite Nanotubes: A First-Principles Prediction[END_REF] is slightly higher than the peak energy of the light (4.9 eV for a 253.7 nm wavelength), only a small fraction of the absorbed light was used to produce electrons and holes in Imo-CH3. However, it was revealed to be efficient enough to induce charge separation in Imo-CH3 nanotubes. Also, the band gap of Imo-CH3 may be reduced in presence of encapsulated DBAN due to the possible charge transfer from DBAN to Imo-CH3 nanotube, [START_REF] Majidi | Band gap modulation of graphene and graphyne via tetracyanoethylene adsorption[END_REF][START_REF] Akdim | Understanding effects of molecular adsorption at a single-wall boron nitride nanotube interface from density functional theory calculations[END_REF][START_REF] Makuła | How To Correctly Determine the Band Gap Energy of Modified Semiconductor Photocatalysts Based on UV-Vis Spectra[END_REF]

Analysis of photo-degradation products

Even if the loss of UV-Vis absorbance and fluorescence emission upon UV illumination of DBAN encapsulated in Imo-CH3 suggests the loss of conjugated double bonds in DBAN, it does not give any insight into its fate and into the possible production of gases. Therefore, various gases (H2, CO, CO2 and CH4) were identified and, if present, quantified by micro gas chromatography (μ-GC).

The sample with an initial loading concentration of 9 × 10 -6 molg -1 was illuminated for different times and the produced gases were measured. The experiments were performed under O2 or Ar atmosphere. After illumination under O2 atmosphere, CO2 and CO were detected by μ-GC (Figure 9). The formation of CO2 suggests the mineralization of the encapsulated DBAN under UV illumination. It is worth noting that the illumination of the pristine Imo-CH3 solution also produced CO2 and CO, but with a lower yield as compared to the DBAN-loaded Imo-CH3 sample. In the case of pristine Imo-CH3 suspension, the production of gases during illumination could be due to the photo-degradation of small alcohols inherently present as impurities in the cavity of the nanotubes. As the CO2 yield in the pristine Imo-CH3 sample did not increase significantly from 3 to 5 hours of illumination, the photo-degradation of the organic impurities can be assumed to be nearly completed in 3 hours. In the pristine sample, the saturation of the CO2 yield and the significantly low overall yield (59.9 μmolg -1 ) of carbon containing gases (CO2 + CO) after 5 hours of UV illumination as compared to the carbon concentration in Imo-CH3 (5100 μmolg -1 ) rule out the possibility of photo-degradation of the methyl groups inside Imo-CH3. The formation of a small concentration of CO, under O2 atmosphere, suggests a further photo-reduction of CO2 (formed inside nanotube cavity) on the external surface of the nanotubes. In addition to CO, trace amounts of H2 (0.48 μmolg -1 ) and CH4 (1.35 μmolg -1 ) were also detected after only 5 hours of illumination, which indicate that photocatalytic water reduction and CO2 conversion occurred on the external wall under the O2 atmosphere. When DBAN was encapsulated in the nanotubes, the overall yield of carbon containing gases (CO2 + CO + CH4) produced after 5 hours of illumination (78.45 μmolg -1 ; corrected from the gas production due to pristine Imo-CH3) suggests that 40% of the initial DBAN concentration was transformed to small gaseous products after 5 hours. Clearly, the actual percentage of DBAN mineralization after 5 hours of illumination may be higher, as other possible gaseous and non-gaseous products, such as HCOOH, HCHO, CH3OH, C2H4, C2H5OH….

might have also been formed during the CO2 conversion. However, they could not be detected by μ-GC and gas chromatography coupled to mass spectrometry (GC-MS). The yields of H2 and CH4 from water and CO2 reduction, respectively, are significantly low under O2 atmosphere due to the electron scavenging by O2. These photo-reduction reactions are, in general, performed in deaerated solutions. [START_REF] Singh | A review on H2 production through photocatalytic reactions using TiO2/TiO2-assisted catalysts[END_REF][START_REF] Li | Toward High-Value Hydrocarbon Generation by Photocatalytic Reduction of CO2 in Water Vapor[END_REF] On the contrary, photocatalytic degradation of organic compounds does not take place in the absence of dioxygen. [START_REF] Long | Historical development and prospects of photocatalysts for pollutant removal in water[END_REF] Our spectroscopic studies a clue that the present photo-reactor can photo-oxidize the encapsulated DBAN molecules even without dioxygen (see Figure 8e and Figure S10e, Electronic Supplementary Information). In order to verify whether the present photo-reactor can degrade the encapsulated DBAN molecules even without dioxygen, specific gases (CO, CO2, CH4 and H2) were identified and, when present, quantified after illumination of DBAN-loaded Imo-CH3 suspensions under argon atmosphere (Figure 10). Interestingly, CO2 was produced, which suggests the oxidative mineralization of encapsulated DBAN under UV illumination, even without dioxygen. This is attributed to the exceptionally strong oxidizing potential of Imo-CH3 due to its wall curvature. [START_REF] Poli | The potential of imogolite nanotubes as (co-) photocatalysts: a linear-scaling density functional theory study[END_REF] Also, the CO2 conversion products such as CO (2 e -transfer) and CH4 (8 e -transfer) were detected in considerably larger amounts when the samples were under Ar atmosphere than under O2 atmosphere, suggesting further photo-reduction of CO2 on the external surface of the nanotube by the conduction band electrons (𝑒 𝐶𝐵 -). [START_REF] Poli | The Role of Cation-Vacancies for the Electronic and Optical Properties of Aluminosilicate Imogolite Nanotubes: A Non-local, Linear-Response TDDFT[END_REF][START_REF] Poli | The potential of imogolite nanotubes as (co-) photocatalysts: a linear-scaling density functional theory study[END_REF] Remarkably, it is then possible to couple these two otherwise incompatible reactions, i.e, pollutant degradation and CO2 conversion, to produce valuables from wastewater. In addition to the production of carbon-containing gases, dihydrogen (H2) was also produced (Figure 10). This demonstrates that in the absence of dissolved O2, water molecules can also act as electron acceptors and undergo two electron transfer reduction reactions by receiving conduction band electrons (𝑒 𝐶𝐵 -) on the external surface of Imo-CH3 nanotubes. The photocatalytic degradation of encapsulated DBAN in absence of dioxygen and the simultaneous production of H2 and CH4 demonstrate the unique role of Imo-CH3 nanotubes to act as dual-photocatalytic nanoreactors for the simultaneous degradation of pollutants and production of fuels. These results suggest that by using the Janus-type Imo-CH3 photocatalytic reactor, organic pollutants could be successfully used as sacrificial agents for the production of dihydrogen and methane, avoiding the need of using electron donors such as organic acids and alcohols. [START_REF] Christoforidis | Photocatalytic Hydrogen Production: A Rift into the Future Energy Supply[END_REF] Similarly, the trace organic impurities in the pristine Imo-CH3 cavity led to production of CO2 and other conversion products such as CO and CH4 under UV illumination with an Ar gaseous atmosphere (Figure 10). Note, a heat treatment to fully remove these organic impurities from Imo-CH3 could not be applied, as this may destroy the methyl functional groups. The overall yield of carbon-containing gases (CO2 + CO + CH4) produced under argon atmosphere (33.7 μmolg -1 ; corrected from the gas production due to pristine Imo-CH3) suggests that 17% of the initial DBAN concentration was transformed to those small gaseous products after only 5 hours of illumination. The ability of Imo-CH3 nanotubes for CO2 and H2O reduction reactions can be corroborated qualitatively by comparison of the energy band edges of Imo-CH3 with the redox potentials of various couples. So far, the band diagram of imogolite has not been reported experimentally. In contrast and as already mentioned before, the band structure of the material has been theoretically calculated [START_REF] Zhao | Energetic Minimum Structures of Imogolite Nanotubes: A First-Principles Prediction[END_REF] and the band gap energy value has been reported as being equal to 5.2 eV. The effect of defects on the band structure of the material has also been studied by DFT calculations. [START_REF] Poli | The Role of Cation-Vacancies for the Electronic and Optical Properties of Aluminosilicate Imogolite Nanotubes: A Non-local, Linear-Response TDDFT[END_REF] Even if the exact energy band edges of Imo-CH3 is not known, it can be predicted using the well-known Matsumoto's correlations. [START_REF] Matsumoto | Energy Positions of Oxide Semiconductors and Photocatalysis with Iron Complex Oxides[END_REF] Indeed, Matsumoto established linear correlations between band gap energy value and band edges for a variety of semiconductor oxides. With a band gap energy value (Eg) of Imo-CH3 equal to 5.2 eV, the conduction (Ec) and valence (Ev) band edges of Imo-CH3 nanotubes are calculated to be equal to -1.37 V and 3.83 V vs. NHE, respectively. Obviously, the proton-assisted multi-electron CO2 reduction steps [START_REF] Li | Recent Advances in Heterogeneous Photocatalytic CO2 Conversion to Solar Fuels[END_REF][START_REF] Siahrostami | One-or Two-Electron Water Oxidation, Hydroxyl Radical, or H2O2 Evolution[END_REF] such as CO2/HCOOH (-0.61 V vs. NHE at pH 7), CO2/CO (-0.53 V vs. NHE at pH 7), CO2/HCHO (-0.48 V vs. NHE at pH 7), CO2/CH3OH (-0.38 V vs. NHE at pH 7), CO2/CH4 (-0.24 V vs. NHE at pH 7), are thermodynamically allowed on the Imo-CH3 external surface due to the less negative redox potentials of these couples as compared to the conduction band edge of Imo-CH3 (Ec). Also, the water reduction/oxidation reactions [START_REF] Singh | Systematic review on applicability of magnetic iron oxideseintegrated photocatalysts for degradation of organic pollutants in water[END_REF] such as H + /H2 (-0.41 V vs. NHE at pH 7), H2O/O2 (+0.83 V vs. NHE at pH 7), H2O/ . OH (+2.27 V vs. NHE at pH 7) are also thermodynamically allowed. Furthermore, and in order to confirm the photo-reduction of CO2 on the Imo-CH3 surface, a degassed aqueous solution of DBAN-loaded Imo-CH3 was pre-bubbled with ultra-pure CO2 and subjected to UV illumination for 2 hours. The gaseous atmosphere was argon. Interestingly, the CO and CH4 yields were found to increase by 20% as compared to the experiment with no CO2, while the H2 yield decreased by a factor of 3. This confirms that CO2 undergoes photo-reduction on the Imo-CH3 surface, alongside with H2 production by H2O photo-reduction. The added CO2 molecules scavenge the photoelectrons due to reduction on the external surface of Imo-CH3, thereby reducing the H2 yield.

Obviously, some other gaseous/non-gaseous CO2 reduction products, that could not be detected by µ-GC, may have also been formed.

Moreover, the Imo-CH3 photo-reactor has been found to be highly stable under high-energy UV illumination, as evidenced by UV-Vis spectroscopy and FT-IR characterization of the nanotubes before and after illumination (Figures S12-S13; Electronic Supplementary Information).

Possible mechanisms of DBAN photo-degradation inside the Imo-CH3 photo-reactor

The photo-oxidation mechanisms of DBAN inside the Imo-CH3 cavity are expected to be different from the mechanisms acting in bulk water. Due to the difficulty in fully characterizing the intermediates and any non-gaseous products produced in small quantities inside Imo-CH3 nanotubes, any possible mechanistic pathways are qualitatively discussed (Table 1). Upon absorption of photons with an energy higher than the band gap value of Imo-CH3, electron-hole pairs are produced across the wall of Imo-CH3 nanotubes (R1). These charges are predicted to be efficiently separated by the curvature-induced wall polarization. [START_REF] Poli | The potential of imogolite nanotubes as (co-) photocatalysts: a linear-scaling density functional theory study[END_REF][START_REF] Elliott | Chemically Selective Alternatives to Photoferroelectrics for Polarization-Enhanced Photocatalysis: The Untapped Potential of Hybrid Inorganic Nanotubes[END_REF][START_REF] Pignie | Confined Water Radiolysis in Aluminosilicate Nanotubes: The Importance of Charge Separation Effects[END_REF] Nevertheless, the internal DBAN molecules are also excited (DBAN * ) by UV radiation (R2). The excited DBAN molecules may be de-excited by several photophysical and energy transfer pathways (R3) or may undergo photo-cycloaddition to form dimers via the formation of excimers (R4) and subsequent deexcitation of the dimers (R5). The excited DBAN molecules may also undergo chemical reactions by hole/electron transfer reactions (R6). [START_REF] Miller | Photolysis of polycyclic aromatic hydrocarbons in water[END_REF] As a few gaseous dioxygen molecules could be present in the Imo-CH3 cavity under equilibrium, in addition to the direct electron transfer from DBAN/DBAN * to Imo-CH3, another important pathway could be the formation of highly reactive singlet dioxygen ( 1 O2) by energy transfer between DBAN * and the ground state of O2 (R9), followed by the formation of endoperoxide/hydroperoxide derivatives (R10). [START_REF] Miller | Photolysis of polycyclic aromatic hydrocarbons in water[END_REF] These peroxide intermediates may also undergo further oxidation by ℎ 𝑉𝐵 + or participate in reactions with . OH radicals (R20) and be finally mineralized. In the case of solutions pre-bubbled with O2, the nanotube cavities are expected to be occupied with a sufficient number of O2 molecules, thereby making the endoperoxide/hydroperoxide intermediate pathway an important one. On the contrary, for solutions pre-bubbled with argon, direct electron transfer from DBAN* (R6) and (DBAN)2* (R11) to Imo-CH3 is likely to be the main pathway. However, even in Ar-bubbled solutions, the production of small concentrations of endoperoxide/hydroperoxide intermediates cannot be ruled out, as a minor concentration of O2 may be formed within the nanotube cavity by possible oxidation of water molecules, which are residually present inside the nanotube cavity as water vapor (R7). However, the concentration of formed O2, if any, is below the detection limit of the μ-GC. In the case of the air-saturated system, both of these mechanistic pathways can exist. Moreover, the possibility of direct water photolysis using a short wavelength UV radiation could also generate reactive species (for example . OH, O2 .-etc.) and hydrated electrons in the DBAN-Imo-CH3 aqueous solution. [106][107][108][109] However, the yields of the species formed by direct water photolysis are expected to be much lower than those of the species produced by Imo-CH3, and this route is therefore neglected here. The gaseous products such as CO, CH4 and H2 are formed by further photo-reduction of CO2 and water on the external surface of the nanotubes (R21-23). Scheme 1 shows the major possible photodegradation pathways of DBAN encapsulated in Imo-CH3 nanotubes.

Scheme 1. Illustration of possible major photo-degradation pathways of DBAN encapsulated in

Imo-CH3 nanotubes in aqueous medium.

Conclusions

The present work utilizes and demonstrates the predicted charge separation effect in inorganic aluminosilicate nanotubes (imogolite). It establishes the promising potential of Janus hybrid methyl imogolite nanotubes (Imo-CH3) as dual photocatalytic nanoreactors for simultaneous and efficient pollutant mineralization and production of fuels such as H2, CH4).

Electronic Supplementary Information

TEM images, SAXS data, FT-IR characterization, additional UV-Vis absorption and fluorescence emission data, fitting of kinetic curves, stability of nanoreactor.

  Similar characterizations have been performed for the Imo-CH3 nanotubes synthesized in the present work. The conventional and cryo-transmission electron microscopy (TEM and Cryo-TEM) images of the Imo-CH3 nanotubes, synthesized in this work, are given in Figure S1 (Electronic Supplementary Information). The conventional TEM image shows fibrous spatial arrangement of the Imo-CH3 nanotube bundles and the Cryo-TEM image shows the highly monodisperse single-walled Imo-CH3 nanotubes.

Figure 1 .

 1 Figure 1. 3D model of natural imogolite (a) and methyl imogolite with hydrophobic core (b) [blue: aluminum octahedron; yellow: silicon tetrahedron; red: oxygen atom; grey: methyl group], chemical structure of DBAN (c), photographs of Imo-CH3 aqueous solution (7.5 g.L -1 ) (d), and aqueous solution of varying concentrations of DBAN encapsulated in Imo-CH3 (e-g). The DBAN

  within the hydrophobic nanocavity of the Imo-CH3 nanotubes, SAXS characterization of the solutions was carried out.

Figure 2 .

 2 Figure 2. SAXS profiles of pristine Imo-CH3 (a) and of DBAN encapsulated in Imo-CH3 (b) in aqueous solution (Imo-CH3 concentration is 7.5 mgmL -1 ). Dashed curves show the simulated SAXS profiles. The vertical line demonstrates the shift of the first intensity minimum to higher scattering vector upon loading, which confirms the encapsulation of DBAN inside the nanotubes. Comparison between curve (a) and (b) enables estimation of the amount of DBAN present inside the nanotubes.

Figure 2

 2 Figure 2 displays the SAXS profiles of pristine Imo-CH3 in aqueous medium (a) and of an aqueous solution of Imo-CH3 nanotubes stirred with an excess of DBAN for 15 days (b). Prior to SAXS investigation, the undissolved DBAN powder was removed from the solution by

  simulated SAXS profiles of pristine and DBAN-loaded Imo-CH3 nanotubes. The encapsulation of DBAN in Imo-CH3 nanotubes was also evidenced by infrared spectroscopy as discussed in Electronic Supplementary Information (FiguresS2, S3and Table

Figure 3 .

 3 Figure 3. UV-Vis absorption spectrum of DBAN (1.1 × 10 -5 molL -1 ) in hexane (a), and of DBAN encapsulated in Imo-CH3 suspensions (b-d). The DBAN loading concentrations were: 9 × 10 -6 molg -1 (b), 3.1 × 10 -5 molg -1 (c) and 8.1 × 10 -4 molg -1 (d). The absorbance of the DBAN-loaded Imo-CH3 solutions (b-d) was adjusted close to unity after dilution with ultra-pure water. The dilution factors were 1, 3 and 50 for b, c and d, respectively. EES-I and EES-II correspond to two

Figure 5

 5 Figure 5 qualitatively gives a picture of the relative molecular crowding (DBAN) inside the Imo-CH3 nanotube cavity in three different loading concentrations.

Figure 4 . 14 Figure 5 .

 4145 Figure 4. Fluorescence emission spectrum of DBAN (1.1 × 10 -5 molL -1 ) in hexane (a), and of DBAN encapsulated in Imo-CH3 in aqueous medium with a loading concentration of 9 × 10 -6 molg - 1 (b), 3.1 × 10 -5 molg -1 (c) and 8.1 × 10 -4 molg -1 (d). All solutions were excited at 295 nm. They were diluted with ultra-pure water to avoid the saturation of the fluorescence intensity. The dilution factors were 1, 3 and 50 for b, c and d, respectively. Multiple maxima labelled as II -IV are ascribed to transitions involving different vibrational levels.

Figure 6 . 3 . 2 . 2 . 73 Figure 7

 6322737 Figure 6. Evolution of UV-Vis absorption (a) and fluorescence emission (b) spectra of DBAN in hexane (initial concentration: 1.1 × 10 -5 molL -1 ) during illumination with UV light (253.7 nm).For fluorescence measurements, the sample was excited at 295 nm. The experiments were performed with air-saturated solutions in a stoppered quartz cell.

Figure 7 .

 7 Figure 7. Evolution of UV-Vis absorption spectra of DBAN encapsulated in Imo-CH3 nanotubes in aqueous medium with an initial loading concentration of 9 × 10 -6 molg -1 (a), 3.1 × 10 -5 molg -1 (b) and 8.1 × 10 -4 molg -1 (c) under illumination with UV light (253.7 nm). Evolution of the corresponding fluorescence emission spectra of DBAN encapsulated in Imo-CH3 nanotubes in aqueous medium with an initial loading concentration of 9 ×10 -6 molg -1 (d). The experiments were performed in a stoppered quartz cell with air-saturated solutions. The asterisk (*) indicates peak F, located between 295 and 300 nm.

Figure 8

 8 Figure8shows the time variation of the value of the DBAN absorbance at peak F at time t with respect to its value at time 0 (see Figure3afor the nomenclature of DBAN absorption peaks), encapsulated in Imo-CH3 nanotubes in aqueous solution under UV illumination. No matter the loading concentration or atmosphere, the absorbance decreased to zero upon UV illumination.

Figure 7

 7 photochemical processes.

  photo-conversion of DBAN inside the Imo-CH3 reactor.

Figure 9 .

 9 Figure 9. Amount of gases (CO and CO2) produced during UV illumination of DBAN-loaded Imo-CH3 aqueous suspension under O2 atmosphere. The amount of gases produced by illumination of pristine Imo-CH3 aqueous solution under identical conditions are also shown for comparison. The error bars are given on the figure. They include the uncertainty associated with μ-GC measurements (3.5 to 7.6 %) and with the chemical actinometry (7.5%).

Figure 10 .

 10 Figure 10. Evolution of the concentration of various gases (CO2, CO, CH4 and H2) produced during UV illumination of DBAN-loaded Imo-CH3 and pristine Imo-CH3 solutions under argon atmosphere. The error bars are given on the figure. They include the uncertainty associated with μ-GC measurements (3.5 to 7.6 %) and with the chemical actinometry (7.5 %).

  Using charge distribution calculations from density functional theory (DFT), Poli et al. have shown that the outer and inner surfaces of imogolite nanotubes are partially positively and negatively charged, respectively, which introduces a strong polarization across the nanotube wall. This suggests that the most probable reaction pathway for the DBAN or the excited DBAN molecules (DBAN * ) is an electron transfer to the Imo-CH3nanotube with the subsequent formation of a radical cation (DBAN .+ ) (R6). Owing to the high LUMO energy level of DBAN (-1.49 eV), 100 another de-excitation route could be the electron transfer from the LUMO level of DBAN to the conduction band of Imo-CH3, which is analogous to dye-sensitized photocatalysts.101 Although this pathway is only energetically allowed if the conduction band edge of Imo-CH3 is lower in energy than the LUMO of DBAN. Unfortunately, the band structure of Imo-CH3 has been only predicted theoretically by DFT calculations until now,[START_REF] Poli | The potential of imogolite nanotubes as (co-) photocatalysts: a linear-scaling density functional theory study[END_REF][START_REF] Zhao | Energetic Minimum Structures of Imogolite Nanotubes: A First-Principles Prediction[END_REF] and an experimental validation of the band structure is needed to accurately compare the LUMO level of DBAN with the energy position of conduction band edge of Imo-CH3. Also, as the bottom of the conduction band arises from the electronic states of the hydroxyl moieties on the outer wall of Imo-CH3,[START_REF] Zhao | Energetic Minimum Structures of Imogolite Nanotubes: A First-Principles Prediction[END_REF] this de-excitation pathway of encapsulated DBAN is expected to be less probable than electron transfer to the top of the valence band (corresponding to electronic states located on the inner wall of Imo-CH3).[START_REF] Zhao | Energetic Minimum Structures of Imogolite Nanotubes: A First-Principles Prediction[END_REF] Besides, there may be a few water molecules (in vapor state) present inside the nanocavity which can undergo hole (h + )-mediated oxidation to form O2 (E 0 = +0.83 V vs. NHE 99 at pH 7) (R7) or . OH radicals (E 0 = +2.27 V vs. NHE[START_REF] Singh | Systematic review on applicability of magnetic iron oxideseintegrated photocatalysts for degradation of organic pollutants in water[END_REF] at pH 7) (R8) inside the nanocavity. The formation of DBAN radical cations (DBAN .+ ) inside Imo-CH3 may also attract water molecules into the hydrophobic nanocavity by possible interaction with the lone pair electrons on the oxygen atom of water molecules, and thereby, increase the yield of internal water oxidation. The holes in valence band (ℎ 𝑉𝐵 + ) and the . OH radicals are indeed powerful oxidants. These highly reactive . OH radicals can ensure complete mineralization (R20) of the encapsulated DBAN molecules (or of the intermediates formed by the hole (ℎ 𝑉𝐵 + )-mediated oxidation of DBAN) by C-C bond scission via stepwise . OH radical adsorption and H abstraction pathway. 102,103 Figure S14 (Electronic Supplementary Information) presents the possible mechanistic pathways for the mineralization of DBAN. Concurrent with the oxidation reactions inside, O2 or H2O molecules on the external surface of the nanotubes can trap the photo-generated electrons in the conduction band (𝑒 𝐶𝐵 -) (R12-19). The dioxygen reduction reaction on the external surface may proceed in multiple pathways involving electron transfer from the external surface of the nanotube and reactions with co-adsorbed water molecules. This may also lead to the formation of strong oxidants such as . OOHand . OH radicals (R17, R19).[104][105][106] These radicals are likely to enter into the nanotube cavity and participate in the mineralization of the encapsulated DBAN (R20).

  phototoxic model pollutant, dibenzo(a,h)anthracene (DBAN), that was subsequently decomposed inside the Imo-CH3 nanocavity by photocatalytic oxidation under UV light. The encapsulated DBAN molecules underwent two parallel processes, photo-oxidation and photo-cycloaddition. Due to these parallel photochemical processes, the kinetics of photo-degradation of DBAN inside Imo-CH3 is complex and depends on the initial DBAN concentration, but also on the concentration of dissolved dioxygen. UV degradation of DBAN or of the photo-dimers/oligomers inside Imo-CH3 nanotubes produces gases, such as CO2, CO and CH4 along with H2. The formation of CH4 and H2 evidences the role of the external surfaces of the nanotubes as photo-reduction centers.By comparing the predicted energy band edge positions of Imo-CH3 nanotubes with popular existing photocatalytic systems, such as TiO2, CdS, CdSe, ZnS, SnO2, ZrO2, 110 Imo-CH3 is clearly a large band gap material. Its band edges are close in energy position to those of ZrO2, owing to a very similar band gap energy value.111,112 However, Imo-CH3 possesses numerous advantages due to its (i) environmentally benign chemical composition, (ii) low cost of synthesis, (iii) hollow Janus nanostructure with an hydrophobic internal cavity favoring the encapsulation of organic species and an hydrophilic external surface favoring the dispersion of the nanotubes in water, (iv) favored curvature-induced charge separation across the polarized wall, (v) tunable surface property etc.Further studies on the band gap reduction of doped Imo-CH3, better charge separation by using metal nanoparticle reactive centers on the external wall or suitable photoactive antenna molecules could enable such nano-photo-reactors to work efficiently under the solar spectrum. The present discovery of hybrid Imo-CH3 as a dual photocatalytic nano-reactor could strengthen imogolitebased photocatalytic research and accelerate many technological breakthroughs for solar photocatalysis by combining two popular but otherwise disconnected photochemical reactions (i.e.

Table S5 (

 S5 Electronic Supplementary Information) also shows that, as expected, 𝑘 1 depends on the presence/absence of dioxygen whereas 𝑘 2 does not. It is worth noting that even if 𝑘 2 decreases upon loading, 𝑘 2 [𝐷𝐵𝐴𝑁][START_REF] Chen | Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications[END_REF]

Table 1 .

 1 Possible photo-redox reactions involving the photo-degradation of DBAN together with fuel production in an Imo-CH3 photo-reactor under UV illumination. 𝐶𝑂 2 + 2𝐻 + + 2𝑒 -→ 𝐶𝑂 + 𝐻 2 𝑂 R21 𝐶𝑂 2 + 8𝐻 + + 8𝑒 -→ 𝐶𝐻 4 + 2𝐻 2 𝑂 R22 4𝑒 𝐶𝐵 -+ 4𝐻 2 𝑂 → 2𝐻 2 + 4𝑂𝐻 - R23

		Production of charge pairs
		𝐼𝑚𝑜𝐶𝐻 3 + ℎ𝜈 → 𝑒 𝐶𝐵 -+ ℎ 𝑉𝐵 +			R1
	Photo-oxidation inside the cavity			Photo-reduction on the external surface
	𝐷𝐵𝐴𝑁 + ℎ𝜈 → 𝐷𝐵𝐴𝑁 *	R2	𝑂 2 + 𝑒 𝐶𝐵 -→ 𝑂 2 .-	R12
	𝐷𝐵𝐴𝑁 * → 𝐷𝐵𝐴𝑁		R3	𝑂 2 .-+ 𝐻 2 𝑂 + 𝑒 𝐶𝐵 -→ 𝑂𝑂𝐻 -.	+ 𝑂𝐻 -.	R13
	𝐷𝐵𝐴𝑁 + 𝐷𝐵𝐴𝑁 * → (𝐷𝐵𝐴𝑁 * ) 2 𝐸𝑥𝑐𝑖𝑚𝑒𝑟	R4	𝑂𝑂𝐻 -.	+ 𝑒 𝐶𝐵 -→ 𝑂 𝑠𝑢𝑟𝑓 -.	+ 𝑂𝐻 -.	R14
	→ (𝐷𝐵𝐴𝑁) 2 *
	(𝐷𝐵𝐴𝑁) 2 * → (𝐷𝐵𝐴𝑁) 2	R5	.	𝑂 𝑠𝑢𝑟𝑓 -	+ 𝐻 2 𝑂 + 𝑒 𝐶𝐵 -→ 2 𝑂𝐻 -	R15
	𝐷𝐵𝐴𝑁 (𝑜𝑟 𝐷𝐵𝐴𝑁 * ) + ℎ 𝑉𝐵 + → 𝐷𝐵𝐴𝑁 .+ →→	R6	𝑂𝑂𝐻 -.	+ 𝐻 2 𝑂 → 𝐻 2 𝑂 2 + 𝑂𝐻 -.	R16
	→ 𝑝𝑜𝑙𝑦𝑚𝑒𝑟
	2𝐻 2 𝑂 + 4ℎ 𝑉𝐵 + → 𝑂 2 + 4𝐻 +	R7	𝑂 2 .-+ 𝐻 + → 𝑂𝑂𝐻 .	R17
	𝐻 2 𝑂 + ℎ 𝑉𝐵 + → 𝑂𝐻 .	+ 𝐻 +	R8	.	𝑂𝑂𝐻	+ 𝑂𝑂𝐻 .	→ 𝐻 2 𝑂 2 + 𝑂 2	R18
	𝐷𝐵𝐴𝑁 * + 𝑂 2 → 𝐷𝐵𝐴𝑁 + 𝑂 2 1	R9	𝐻 2 𝑂 2 + 𝑒 𝐶𝐵 -→ 𝑂𝐻 .	+ 𝑂𝐻 -	R19
	𝐷𝐵𝐴𝑁 + 𝑂 2 → → 𝑝𝑒𝑟𝑜𝑥𝑖𝑑𝑒 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 1	R10
	(𝐷𝐵𝐴𝑁) 2 * + ℎ 𝑉𝐵 + → 𝑟𝑎𝑑𝑖𝑐𝑎𝑙 𝑐𝑎𝑡𝑖𝑜𝑛 → →	R11
	→ 𝑝𝑜𝑙𝑦𝑚𝑒𝑟
		Mineralization inside the cavity
	𝐷𝐵𝐴𝑁/(𝐷𝐵𝐴𝑁) 2 /𝑝𝑜𝑙𝑦𝑚𝑒𝑟𝑠/𝑖𝑛𝑡𝑒𝑟𝑚𝑒𝑑𝑖𝑎𝑡𝑒𝑠 + 𝑂𝐻 .	→ → → 𝐶𝑂 2 + 𝐻 2 𝑂	R20
		Fuel conversion on the external surface

Acknowledgements

An Ile de France DIM RESPORE 2019-20 and CEA Bottom Up 2020-21 grant funded this study by the project GRANITE and PHOTOTUBE, respectively. S.P. gratefully acknowledges the DIM RESPORE and the CEA for the post-doctoral fellowship. Dr. Frederic Gobeaux is sincerely acknowledged for the transmission electron microscopy images. Authors thank Dr. Mark Levenstein for reading the manuscript and making insightful comments. The project proposal GRANITE, submitted jointly by S.P. and A.T., was awarded "Marie Sklodowska-Curie Actions Seal of Excellence" from the Marie Skłodowska-Curie actions call H2020-MSCA-IF-2018.

Conflict of Interest

The authors declare no competing financial interest.