Pablo Jimenéz-Calvo
email: pablo.jimenez.calvo@fau.de

Yassine Naciri

Anna Sobolewska

Mark Isaacs

Yu Zhang

Amélie Leforestier

Jéril Degrouard

Stéphan Rouzière

Claire Goldmann

Delphine Vantelon

Simon Hettler

Nestor J Zaluzec

Raul Arenal

Pascale Launois

Mohamed Nawfal Ghazzal
email: mohamed-nawfal.ghazzal@universite-saclay.fr

Erwan Paineau
email: erwan-nicolas.paineau@universite-paris-saclay.fr

Ti-Modified Imogolite Nanotubes as Promising Photocatalyst 1D Nanostructures for H 2 Production

Keywords: nanotube, polarization, photocatalysis, hydrogen, multi-scale analysis

Imogolite nanotubes are predicted as a unique one-dimensional material with spatial separation of conduction and valence bands edges but their large band gaps have inhibited their use as photocatalysts. The first step toward using these nanotubes in photocatalysis and exploiting the polarization-promoted charge separation across their walls is to reduce their band gap. Here, we explore the modification of double walled aluminogermanate imogolite nanotubes by incorporation of titanium into the nanotube walls.

The precursor ratio x = [Ti]/([Ge]+ [Ti]) is modulated between 0 to 1. Structural and optical properties are determined at different scales and the photocatalytic performance is evaluated for H 2 production. Although the incorporation of Ti atoms into the structure remains limited, the optimal condition is found around x = 0.4 for which the resulting nanotubes reveal a remarkable hydrogen production of ~1500 µmol g -1 after 5h for a noble metal-free photocatalyst, a 65-fold increase relative to a commercial TiO 2 -P25. This is correlated to a lowering of the recombination rate of photogenerated charge carrier for the most active structures. These results confirm the theoretical predictions regarding the potential of modified imogolite nanotubes as photoactive nanoreactors and pave the way for investigating and exploiting their polarization properties for energy applications.

Introduction

One-dimensional nanostructures such as nanotubes (NTs) have attracted attention in sunlightdriven photocatalytic applications owing to their competitive advantages compared to bulk materials. [1][2][3] 1D nanostructures exhibit intrinsic structural features offering potential solutions to current challenges facing photocatalytic materials, namely charge carrier recombination, low specific surface area, and light-harvesting properties. [4] They have shown fast and long-distance electron transport, a high yield of charge carrier separation, a high reactive surface area, and improved absorbance properties. Owing to these properties, a wide range of 1D-based nanostructure materials have been developed for photocatalytic application. [5][6][7][8][9] These nanostructures provide higher photoactivity but the coupling with metallic nanoparticles or semiconductors with adequate potential energies is required to extend the lifetime of the photogenerated charge carriers. [10][11][12][13] A promising strategy has emerged recently by exploiting the surface charge separation promoted by the electric field caused by an intrinsic dipole moment in the nanostructure. [14] This electric field induces the splitting of excitonic pairs in spatially-separated sites within the photocatalytic material, thus allowing control of the overall drive and kinetics of redox reactions.

In this context, imogolite nanotubes (INTs) with chemical formula (Si,Ge)Al 2 (OH) 3 O 3 R (where R = OH, CH 3 …), appear as alternative materials of well-defined morphology (single or double-walled) with tunable monodisperse diameters and scalable inner/outer interfaces (e.g. hydrophilic or hydrophobic surfaces). [15][START_REF] Paineau | Nanomater. Clay Miner[END_REF][START_REF] Govan | [END_REF] Recently, density functional theory (DFT) studies suggested that INTs could be the ideal photocatalyst, offering at the same time high reactive surface area and natural charge carrier separation thanks to its permanent polarization. [18][19][20] The originality of these nanotubes stems from the atypical spatial separation of the valence (VB) and conduction band (CB) edges across the nanotube walls;

promoting redox reactions at separated locations. [18,19] This unique VB/CB configuration could result in enhanced photocatalytic efficiency. [21][22][23] However, the use of such behavior is inhibited by the high electrostatic band gap of imogolite nanotubes, estimated in the range 3-7 eV, based on tight-binding [24,25] or generalized gradient approximations [18,19] DFT methods, which tend to over or underestimate the band gap, respectively. The wide band gap [24,25] and the band edge energies positions of CB and VB make difficult to perform the proton reduction spontaneously with imogolite nanotubes without any modification of the structure. A promising step is the doping of the nanotube lattice by various metal cations but has been rarely explored. From an experimental point of view, only Fe-doped imogolite nanotubes have been obtained so far, with applications restricted to photoFenton degradation of dyes. [26,27] However, DFT calculations evidenced that doped imogolite-like structures should display a decrease in band gap with the incorporation of different transition metals or rareearth elements. [28,29] In this framework, we explore the potential of aluminogermanate doublewalled INTs (DWINTs) as suitable photocatalysts since the permanent dipole density at the walls results in an electro-negative inner tube while the outer tube displays an electropositive surface. [18] To enhance their optical properties, we dope the nanotube structure with Ti atoms (Figure 1) while preserving the DW shape, allowing to provide promising photoactive imogolite nanotubes for H 2 generation.

Results and Discussion

DWINTs doped with Ti are synthesized by adapting the method proposed by Amara et al. [30] The precursors ratio defined as x = [Ti]/([Ge]+[Ti]) is modulated in the range of x = 0 to 1 (see Experimental section). We first characterized the obtained samples depending on the initial precursor ratio. Morphological investigations by conventional transmission electron microscopy (TEM) reveal the preservation of the nanotube shape up to x = 0.4 (Figure S1), though at the expense of length that is shortened with Ti-doping (Figure S2 and Table S1).

For a higher precursor ratio (x ≥ 0.6), ellipsoid plate-like particles and dense spherical aggregates are dominant (Figure S1) corresponding respectively to AlO(OH) boehmite [31] and TiO 2 anatase nanoparticles [32] , as demonstrated later. High-resolution cryo-TEM observations (Figure 2a) on pristine (x = 0) Ge sample and Ti/Ge samples (x = 0.2 and 0.4) confirm unambiguously the formation of double-walled nanotubes with monodisperse inner (~1.5 nm) and outer (~4.3 nm) diameters (Figure S3 and Table S1). Closer observation also reveals that nanotubes synthesized with Ti may have some structural defects within the walls or occasionally form nanoscrolls (Figure S4). These findings point to the fact that there is a limit to the incorporation of Ti beyond which the tubular structure can no longer be formed. In b and c, the curves have been translated vertically for the sake of clarity.

The structural characteristics of the different Ti/Ge samples are assessed by combining Fourier-transform infrared (FTIR) spectroscopy and wide-angle X-ray scattering (WAXS) experiments. FTIR spectra of the Ti/Ge samples prepared at x = 0.2 and 0.4 are similar to the reference Ge-DWINT sample (x = 0) (Figure 2b). The absorption bands at 915 cm -1 and the doublet at 825 and 800 cm -1 correspond to Ge-O stretching vibrations in the tubular structure of the INT. [33,34] The IR bands at 690 and 550 cm -1 are ascribed to Al-O stretching modes, while the signals at 470 and 420 cm -1 are related to -OH, Ge-O-Al, and O-Ge-O bending modes for aluminogermanate imogolite nanotubes. [34,35] All samples also present a large absorption band in the OH stretching region with two main signals at 3450 and 3100 cm -1 corresponding to external and internal OH groups attached to (OH)Al and (OH)Ge, respectively. [36] Above a 0.4 doping ratio, the INT signature is progressively lost, and new well-resolved absorption peaks occur at 3300, 3080, 1070, 750, 615, and 480 cm -1 attributed to the boehmite structure. [37] The stretching modes of Ti-O-Ti (around 690 cm -1 ) are probably present but overlap with the FTIR signal from boehmite. WAXS experiments also highlight the effect of precursor ratio on the structure of imogolite nanotubes. Scattering intensity profiles reveal broad modulations (Figure 2c and Figure S5). At a low doping ratio (x ≤ 0.4), the scattered intensity below 10 nm -1 mainly displays oscillations characteristic of the form factor of DW nanotubes, [38][39][40] showing that the nanotubes are not aggregated in large bundles.

This DW signature is still present for the sample (x = 0.6). Interestingly, the axial periodicity along the nanotube axis (which results in the asymmetrical peak [20] located around 15 nm -1 ) remains unchanged with increasing the doping ratio up to 0.6. No nanotube signal is observed for x = 0.8 and 0.1. Boehmite and anatase phases are observable for x ≥ 0.4 (Figure S5).

Understanding the local structural modifications of the nanotubes is essential for further investigation of the photoinduced properties of these doped-nanostructures. We performed X- As expected, pure Ge-DWINT (x = 0) does not contain any Ti element. The peak at 1251 eV is assigned to Ge 2p 1/2 whereas the one at 1220.2 eV to Ge 2p 3/2 level of GeO (Figure 3a). [41- 43] The Ti doping INTs with a ratio from 0 to 0.4 induces a noticeable enlargement of the Ge 2p peaks, which can be deconvoluted to two peaks. The new peak at higher energy (1222.4 eV) indicates the formation of Ge with a higher oxidation state as in GeO 2 . [41,44] The Ge 3s peak (Figure S7a) can be deconvoluted into two main peaks centered at 184.1 and 179.1 eV for w ≤ 0.6, which are attributed to Ge-O and Ge-O-Al bonds, respectively. By contrast, the Al 2p signal (Figure 3b) exhibits a single contribution at 74.1 eV, ascribed to Al-O bonding in octahedral sites. [45][46][47] We also notice a slight shift of the Al 2p signal towards higher binding energy with respect to the unmodified Ge-DWINT that could be related to a modification of the distortion in Al sites in boehmite compared to INTs at precursor ratio x > 0.4. The Ti-doping effect is investigated by recording the Ti 2p peak (Figure 3c). For x = 1, the spin-orbit components (2p 3/2 and 2p 1/2 ) of the Ti 2p peak are well deconvoluted into two sharp peaks at 458.8 and 464.1 eV, respectively, characteristic of Ti 4+ in TiO 2 . [48] The measured separation between the Ti 2p 3/2 and Ti 2p 1/2 peaks is 5.7 eV, in agreement with the observed one for stoichiometric TiO 2 . [49] Interestingly, these two components are shifted to lower binding energies (0.3 eV) as the ratio of Ti doping decreases, and the separation between the spin-orbit components is higher as well (5.98 eV), which points to a distortion of Ti sites. The O 1s signal (Figure S7b) exhibits two contributions around 533.0 and 531.9 eV, assigned to Al-O and Ge-O chemical bonds, respectively. [43,47,50] As the doping occurs, the O1s peak becomes larger and shifts toward lower binding energies. This is due to modification of the chemical environment provoked by the formation of O-Ti bond. The progressive evolution of XPS spectra with respect to the doping ratio suggests an incorporation of Ti atoms into the nanotube structure, probably in an octahedral configuration.

To confirm the doping modification, we performed X-ray Absorption Near Edge Spectroscopy (XANES) at the Al K-edge (Figure S8). The first two samples (x = 0 and 0.2) present a large absorption band located at 1571 eV, characteristic of aluminum in an octahedral configuration. [51] With increasing [Ti]/([Ge]+[Ti]) precursor ratio, a lower energy resonance appears at 1567 eV related to the spectral feature of boehmite, which contains Al sites in a distorted octahedral configuration compared to imogolite nanotubes. [52] The amount of boehmite formed depends on the precursor ratio as demonstrated by linear combination fitting of XANES spectra using pure Ge-DWINT (x = 0) and the sample prepared at x = 1 that contains only boehmite as Al species (Figure S8). We also conducted XANES measurements at the Ti K-edge to determine the speciation of Ti atoms and the modification of the local environment for the different Ti/Ge samples (Figure 3d). The curves are quite similar to the one obtained for a reference anatase sample. XANES spectra present several pre-edge features before reaching the edge jump located around 4987 eV. The pre-edge region shows three well-resolved peaks (A 1 -A 3 ), which are attributed to 1s to 3d dipolar transitions of Ti 4+ in octahedral configuration as proposed by Farges et al. [53] The shoulder (B) around 4980 eV is assigned to 1s to 4p transition of the TiO 6 octahedron in agreement with XPS analysis indicating the formation of stoichiometric TiO 2 in its anatase form. These results indicate that titanium incorporated into the walls of the nanotube seems to replace preferentially Al atoms.

The large oscillations that follow the edge jump become better resolved and similar to those of anatase for doping levels above x = 0.6, in agreement with the increasing amount of anatase in the sample.

Finally, scanning transmission electron microscopy (STEM) analyses have been performed in order to further investigate the presence of titanium into the nanotube structure. Figures 4a,b show high-resolution high-angle annular dark field (HAADF) STEM images of nanotubes.

Temporally-resolved hyperspectral X-ray energy dispersive spectroscopy (XEDS) has been performed in different nanotubes. The presence of sparse clusters of titanium at the nanotubes locations has been identified by the presence of minute Ti-K X-ray signals. An example of which can be observed in the XEDS spectrum displayed in Figure 4c. The distribution is rather heterogeneous throughout the nanotubes. In general, the amount of Ti within the nanotubes is extremely low, based upon our maximum observed Ti signal, we estimate that the Ti/Al ratio is < 0.01 when Ti is present. These structural observations prove the local doping of the imogolite structure with Ti atoms We note that titanium in the form of titanium oxide, has also been observed forming small nanoparticles for x = 0.4. After this structural characterization, the evolution of the optical properties is investigated using UV-vis spectroscopy (Figure S9). The samples exhibit an absorption band in the UV range (< 350 nm), typical of the creation of an optical band gap in semiconductors defined as the difference between the valence band edge and conduction band edge. As the Ti ratio increases in the Ti/Ge samples, a redshift is observed, pointing to the doping effect on tuning of the optical band gap (Figure S9). From the Tauc plot analysis (Figure 5a), it can be ascertained that the band gap (E g ) energy drastically decreases from 4.35 eV (for pristine Ge-DWINT) to 3.96 eV (x = 0.2) and then to 3.92 eV (x = 0.4) (Figure 5b). Further increasing of the Ti/Ge ratio steadily decreases the E g down to 3.85 eV. The latter band gap is similar to that obtained for anatase TiO 2 , [49] in agreement with WAXS results. The valence band edge is determined from high-resolution XPS spectra at low binding energy (Figure 5c). Combined with the band gap energy obtained previously, we determine the conduction band edge of the different samples that are reported in the band energy alignment diagram in Figure 5d. The Ti-doping initially induces a downward shift of the VB edge (0.13 and 0.43 eV for 0.2 and 0.4 ratio, respectively), followed by an upward shift when the Ti/Ge ratio further increases. Similar behavior is observed for the CB edge, which shifts down to -0.13 eV (for 0.4 ratio), and up to -1.43 eV. Interestingly, this evolution with an optimum around x = 0.4 can be related to the disappearance of nanotubes in favor to boehmite and anatase phases as shown above. This band gap modification is expected to have a direct effect on the photocatalytic performance of the Ti/Ge samples. In the following, we investigate the photocatalysis properties of Ti-doped imogolite nanotubes.

The H 2 production activity is monitored over time under artificial solar-light irradiation (Figure 6a) to assess the Ti-doping effect on the photocatalytic efficiency of Ti/Ge samples up to x = 0.6 for which the nanotube structure is preserved, knowing that the latter already contains anatase nanoparticles. For comparative purposes, we also measure the H 2 production activity of TiO 2 -P25 nanoparticles (Degussa) used here as a standard noble metal free photocatalyst as well as boehmite nanoparticles. As expected, Ge-DWINTs show negligible photoactivity for H 2 generation, which is due to the low absorbance properties at the energy excitation wavelength of the Xenon lamp as well as the large band gap of the sample. By contrast, Ti-doping samples for x = 0.2 and 0.4 exhibit 1509 and 1472 µmol g -1 of H 2 production, respectively, that is 65 times higher than the commercial TiO 2 -P25 (23 µmol g -1 )

under the same experimental condition. [54] The comparison with tubular TiO 2 photocatalysts is also instructive. Ge and al. provided a comprehensive review on hydrogen production by TiO 2 nanotubes with values ranging from 30 µmol.h -1 .g -1 up to 80 mmol.h -1 .g -1 but with the use of co-catalysts. Therefore, the 1500 µmol.h -1 .g -1 H 2 formation rate of our composite remaines as one of the highest H 2 generation for quasi-bare a, d TiO 2 -based tubular materials without noble metals. [7] The Ti-doping downshifts the CB edges, reaching adequate potential for light absorption and efficient photogenerated charge separation that leads to an enhancement of H 2 generation efficiency. A further increase of the doping ratio impacts both the loss of the nanotube structure (the polarization effect) and the appearance of an inactive boehmite phase (Figure 6a), as evidenced previously, decreasing the photocatalytic activity.

It underlines that the optimal ratio of Ti substitution is around 0.4. The mechanism of H 2 generation is expected to follow a similar pathway than that reported for TiO 2 , [55,56] in the presence of methanol as sacrificial electron donor (SED). Instead of using a metal co-catalyst as a reduction site and electron collector, the natural polarization of Ti-doped nanotubes facilitates the photogenerated charge separation, making the reduction and oxidation sites spatially separated, as illustrated in Figure 6b. Details of the reaction are given in Supporting Information. In brief, the photogenerated charges (e -/h + ) induce dissociative adsorption of SED and hole scavenging, which is followed by hydrogen ion (H + ) release in the intern cavity of the nanotube. The reduction leading to the formation of a H 2 molecule is expected to be localized in the outer surface of the nanotubes, while the oxidation takes a place into the nanotubes (Figure 6b). This is probably a simplified view since the photo-reduction event depends on several parameters such as photo-generated charge-carrier recombination, vibronic coupling between donors (Ti/Ge sample) and acceptors (H 2 O), accessibility of reactants to cite a few. However, at a first approximation, the smaller the energy gap between the donor (CB) and the acceptor (H 2 O-redox potential), the faster the electron-transfer kinetics. [START_REF] Nitzan | Chemical Dynamics in Condensed Phases: Relaxation, Transfer and Reactions in Condensed Molecular Systems[END_REF] Our results show a correlation between the vicinity of the CB edge and the measured H 2 production. The difference between the CB edge and H 2 O reduction redox potential is minimized for x = 0.2, 0.4 while these two samples display the highest photocatalytic activity.

Finally, the beneficial effect of natural polarization in photogenerated charge carrier is evaluated by assessing the transient photocurrent (PC) density and the charge transfer kinetic for each sample (Figure 6c,d). Pristine Ge-DWINTs exhibit the lowest current density, similar to the fluorine doped tin oxide (FTO) electrode, indicating low mobility of photogenerated charge generation as the optical band gap is higher (Figure 6c). Ti-doping of Ge-DWINTs results in an increased PC response. The intensity of the photocurrent during the ON/OFF light illumination cycles increases with Ti/Ge samples, reaching an optimal value for x = 0.4. Further increase in Ti-doping leads to a drastic drop of the photocurrent density, which translates the loss of the tubular structure and thus of the polarization properties. Among all the samples tested, Ti/Ge sample with x = 0.4 catalyst shows the highest PC density, in agreement with optimal photocatalytic H 2 evolution. In addition, we investigate the the rate of OCV decay after turning off the light, the derived average charge carrier lifetime undergoes a significantly slower OCV decay process for Ti-doped samples. The lowest recovering decay was observed for 0.4. The OCV does not recover to the original level even after 700s based on the normalized OCV decay curves (Figure 6d). The transient decay profiles were fitted with a first-order kinetic model to obtain the pseudo-first-order recombination rate constant ( , such as: [START_REF] Kim | [END_REF] where E is the open-circuit voltage decay at any time t while E 0 and E pH represent the dark stationary OCV value and the photo-stationary OCV value, respectively. The values for all photocatalysts are presented in Table S2, showing the lowest decay value for the most active Ti/Ge composite. We observe that the recombination rate constant for the Ti/Ge DWINTs (x = 0.4) photocatalyst (1.42 10 -3 s -1 ) is 5 times smaller than pristine Ge-DWINT (7.01 10 -3 s - 1 ), indicating that holes are more likely to perform water oxidation. In the meantime, the electrons accumulated in the Ti-doping Ge-DWINT (x = 0.4) photocatalyst have a longer lifetime. [START_REF] Kim | [END_REF] The results confirm the effective charge carrier transport and separation when the Ti-doping operates while maintaining the imogolite tubular nanostructure. The presence of Ti atoms, even at a very low amount into the structure, modifies the electronic properties of imogolite nanotubes taking advantage of the natural polarization of the nanotubes, making them a promising photoactive tubular nanostructure.

Conclusion

We investigate the potential of synthetic double-walled imogolite nanotubes for photocatalytic applications. Substitution of Ge precursor by Ti precursors is carried out to transform pristine imogolite from non-photoactive material to a photocatalyst efficient for H 2 generation. Various Ti/Ge samples are prepared by changing the initial precursor ratio. The optimal condition is found around x = 0.4 for which the double-wall shape of the nanotube is preserved. The incorporation of Ti in a very small quantity in the nanotubes modifies its band structure sufficiently to confer them unique photocatalytic properties. The photocatalytic efficiency reveals a high hydrogen production of ~1500 µmol g -1 for noble metal-free photocatalyst, 65-fold higher than the commercial TiO 2 P25. This enhanced H 2 production is attributed to different physico-chemical phenomena, such as preservation of the tubular structure, introduction of Ti atoms within the DWINT structure. This study represents a cornerstone for using modified imogolite nanotubes as novel photoactive nanoplatform and provide the first step toward exploiting the polarization properties of INTs to surpass existent photon conversion efficiencies from conventional materials in energetic and environmental photocatalytic reactions.

Experimental Section

Chemicals: Aluminum perchlorate nonahydrate, (Al(ClO 4 ) 3 .9H2O, Reagent grade, Alfa Aesar), germanium (IV) ethoxide (Ge(OC 2 H 5 ) 4 , ≥99.95% trace metals basis, Sigma Aldrich), titanium (IV) isopropoxide (C 12 H 28 O 4 Ti, 98+%, ACROS Organics™), Urea (CO(NH 2 ) 2 , ACS reagent, >99.0%, Sigma Aldrich), and potassium bromide (KBr, IR grade, Merck) were used as received. The initial aluminum perchlorate concentration is C = 0.2 mol.L -1 . We demonstrated previously that aluminum perchlorate is well-suited to obtain high yield of nanotubes and that perchlorates do not alter their morphology. [34] Within the same solution, an aqueous solution of urea at concentration of 0.2 mol.L -1 is added as a source of hydroxyl ions during the thermal decomposition of CO(NH 2 ) 2 . Solutions are stirred for 1h at room temperature in PFTE beakers. These beakers are placed in autoclaves (Parr Instrument) for hydrothermal treatment under autogenous pressure into an oven at 140 °C for 5 days. After the suspensions are cooled down, all samples are dialyzed against ultrapure water, using 10 kD membranes (Spectra/Por) to remove residuals salts and alcohol in excess until the conductivity is below 5 μS.cm -1 .

Synthesis of

Electron microscopy: Conventional transmission electron microscopy (TEM observations on individual nanotubes were performed on a JEOL1400 microscope operating at 80 kV to avoid any electron beam damage. Dilute aliquot of Ti/Ge samples were prepared at 1 mg.L -1 in ethanol. A drop of this mixture was deposited on a copper grid coated with carbon layer and dried at room temperature. The morphological parameters were measured over 200 individual particles. For cryo-TEM acquisition, a 4 µL of sample solution was deposited onto a holey carbon grid (Quantifoil R2/2) ionized by glow discharge and vitrified into liquid ethane cooled by liquid nitrogen using a Vitrobot Mark IV (Thermo Fisher Scientific) operated at 22 °C and 100% relative humidity. The grids were stored in liquid nitrogen until use. Frozen samples were transferred to a Gatan 626 cryo-holder (Gatan Inc.) and examined at -180°C in a JEOL JEM-2010F at 200 kV. The samples were imaged with a magnification of ×200,000 and were collected with a Gatan Ultrascan 4K CCD camera at 200 nm of nominal defocus under low-electron-dose conditions (9 e -.Å -2 ).

Scanning transmission electron microscopy (STEM) and X-ray energy dispersive spectroscopy (XEDS) measurements were conducted using the Argonne PicoProbe Analytical Electron Microscope (AEM) [59] and the Thermo Fisher Scientific Titan Low-Base probecorrected microscope. The AEM works presented here were conducted at 60 kV using also high-angle annular dark field (HAADF)-STEM imaging. Temporally resolved hyperspectral XEDS was carried out using the ANL XPAD system employing its ultra-sensitivity 4.5 sR detector. During imaging and hyperspectral measurements, the electron beam current on the specimen was < 100 pA.

Fourier-transform infrared spectroscopy (FTIR): FTIR spectra were recorded on dry sample powders using a Nicolet iS50 (Thermo Scientific) spectrometer equipped with a KBr beam splitter and a DTGS/KBr detector. The transmission measurements in the range 4000-400 cm -1 were performed by averaging 200 scans at a resolution of 4 cm -1 . Transparent pressed pellets were prepared by mixing about 1.5 mg of dry powder sample with 150 mg of transparent KBr, to guarantee 1 wt. % of sample inside the final pellet.

Wide-Angle X-ray scattering (WAXS): Prior to measurements, the samples were inserted into borosilicate capillary tubes of 1 mm diameter ( LAS, Sch nwalde bei erlin, Germany) and flame-sealed. WAXS experiments were carried out at the MORPHEUS platform (Laboratoire de Physique des Solides) by using rotating anode generators equipped with multilayer W/Si optics (Osmic). The measurements were performed at two different wavelengths  ( CuKα = 0.1542 nm and  Mo Kα = 0.0711 nm). Two-dimensional (2D) patterns were collected on a MAR345 detector (marXperts GmgH, Germany) with 150 µm pixel size. The sample-todetector distance was set at 300 mm (Cu wavelength) and 150 mm (Mo wavelength). This configuration allows covering a wide range of scattering vector modulus Q = 1-50 nm -1 (Q = 4/ sin() with 2 the scattering angle). The scattered intensity I as a function of Q is obtained from the angular integration of the scattering patterns.

X-ray Absorption Near Edge Structure (XANES): Al and Ti K-edges XANES experiments

were undertaken at LUCIA beamline (Soleil synchrotron, France). [60] The X-ray beam was monochromated using either a pair of KTP (011) crystals (Al K-edge) or Si (111) crystal (Ti K-edge). Spectra were acquired with steps of 0.2 eV in the edge region and of 1 eV behind the edge, at a counting rate of 2 s per step. Data were recorded in fluorescence mode using a mono-element (60 mm²) silicon drift diode detector and they were corrected for the detector dead time. Data processing, normalization and analysis of XANES spectra were performed using the Athena software. [61] X-ray photoelectron spectroscopy (XPS): XPS analysis was performed using a Kratos Axis SUPRA XPS fitted with a monochromated Al Kα X-ray source (1486.7 eV), a spherical sector analyzer and 3 multichannel resistive plates, 128 channel delay line detectors. All data was recorded at 150W and a spot size of 700 x 300 µm. Survey scans were recorded at a pass energy of 160 eV, and high-resolution scans were recorded at a pass energy of 20 eV. Electronic charge neutralization was achieved using a magnetic immersion lens. Filament current = 0.27 A, charge balance = 3.3 V, filament bias = 3.8 V. All data were recorded at a pressure below 10 -8 Torr and at room temperature (294 K). Data was analyzed using CasaXPS (v2.3.19PR1.0). The energy shift due to electrostatic charging was subtracted using the carbon adventitious signal, i.e., the C 1s peak located at 284.8 eV. Peaks were fit with a Shirley background prior to component analysis with the appropriate experimental sensitivity factors of the normalized photo ionization cross-section of the atomic subshells as determined by Scofield. [62] UV-vis spectroscopy: Optical properties of Ti/Ge samples were determined using UV-vis spectroscopy (model Cary 5000 series, Agilent Technologies) equipped with an integrating sphere for diffuse reflection measurements. The maximum reflectance was set to 100% using BaSO 4 as a reference in a wavelength range between 200 to 800 nm.

Hydrogen generation: The production of H 2 from an aqueous methanol solution was carried out following a procedure reported previously. [63] Experiments were performed in a closed quartz reactor and the reaction medium was previously purged with nitrogen gas (30 min) to remove O 2 . The UV-visible light is delivered by a Xenon lamp (Oriel, 300 W) whose spectrum is reported in Figure S10. For these experiments, 20 mg of tested substance was suspended under vigorous stirring in 20 mL of a degassed aqueous solution (H 2 O:MeOH, 3:1 v/v) used as a sacrificial agent. The produced H 2 was quantified by gas chromatography (GC, Agilent Technologies 7820A) using a thermal conductivity detector set at 250°C and nitrogen as carrier gas (flux: 22.5 mL min -1 , column of molecular sieve 5 Å and oven temperature at 50 °C). To assess the morphological stability, Ti/Ge samples with x = 0.2 and 0.4 were recovered after 5 hours of hydrogen generation. After drying, the recycled samples were analyzed by WAXS experiments. The related WAXS patterns (Figure S11) present the same characteristics as original samples with the oscillations representative of the double-walled nanotube form factor as well as the asymmetrical peak related to the axial periodicity along the nanotube axis. Therefore, the structural analysis of the two samples demonstrated that no changes has been observed, attesting of the stability of the Ti/Ge samples.

Photoelectrochemical studies: The electrochemical measurements were conducted in a three electrode rectangular quartz cell that consists of the as-prepared FTO (1 cm 2 ) electrode as the working electrode, Pt wire as a counter electrode, Ag/ AgCl as the reference electrode, and an aqueous Na 2 SO 4 solution (0.5 M, without pH adjustment). The working electrode was prepared as follows. 10 mg of the as-prepared Ti/Ge sample was dispersed in 5 ml of 2propanol, and the suspension was dropcasted on a 1 cm 2 FTO glass plate. Finally, the obtained working electrodes were dried at room temperature overnight. The cell was illuminated by a Xe lamp irradiation using an AM 1.5 solar simulator.

Figure 1 .

 1 Figure 1. (left) Schematic view of a DWINT structure with Al, Ge and O atoms represented in blue, orange and red, respectively. (right) Proposed method to dope imogolite nanotubes with Ti atoms at different precursor ratio x.

Figure 2 .

 2 Figure 2. (a) cryo-TEM images of Ti/Ge samples synthesized at different doping ratios x = 0, 0.2 and 0.4. The insets in panel c are higher magnification images of the tubular structure. Characterization of the different Ti/Ge samples synthesized with varying precursor ratio x. (b) FTIR spectra. The grey curve corresponds to a pure boehmite phase. (c) WAXS patterns. Grey and black arrows, respectively, show an indexation of boehmite and anatase reflections.

  ray photoelectron spectroscopy to investigate the chemical composition and oxidation states of elements at the INTs surface. The survey spectrum of INTs shows the main contributions of Ge 2p, O 1s, and Al 2p (Figure S6).

Figure 3 .

 3 Figure 3. XPS spectra for (a) Ge 2p, (b) Al 2p and (c) Ti 2p. (d) XANES spectra at the Ti Kedge. In all panels, the curves have been translated vertically for the sake of clarity.

Figure 4 .

 4 Figure 4. Scanning transmission electron microscopy (STEM) analyses of imogolite nanotubes. (a-b) High-angle annular dark field (HAADF)-STEM micrographs. (c) XEDS spectrum acquired from the red highlighted area in panel (b) on an individual nanotube. Titanium has been detected in this region of the nanotube as visible from the signal in the Ti-K X-ray emission.

Figure 5 .

 5 Figure 5. Optical and electronic properties of the Ti/Ge samples as a function of the substitution ratio: (a) Tauc plots; (b) evolution of the band gap energy E g ; (c) XPS spectra in the VB region and (d) corresponding energy diagram scheme.

Figure 6 .

 6 Figure 6. Hydrogen evolution reaction under solar-light irradiation for (a) Ge-DWINT and Ti/Ge samples with different ratio. The H 2 production by standard TiO 2 (P25) and boehmite nanoparticles are also reported for comparison. (b) Schematic illustration of the proposed H 2 production mechanism, pointing out the general steps involving electron/hole charge separation/reaction pathways. (c) Transient photocurrent response and (d) normalized opencircuit voltage-decay curves of Ti/Ge samples.

  photovoltage, indicating an efficient charge transport property. Interestingly, when monitoring

  Ti/Ge samples: Germanium (IV) ethoxide and titanium (IV) isopropoxide are mixed at different ratios (x = [Ti]/([Ge]+[Ti]) = 0, 0.2, 0.4, 0.6, 0,8 and 1) under stirring in presence of an aluminum perchlorate solution with an [Al]/([Ge]+[Ti]) ratio equal to 2. [Ti] is the molar concentration of titanium isopropoxide and [Ge] that of germanium (IV) ethoxide.

Acknowledgements

The authors acknowledge the financial support of the French ANR agency (ANR) under grand agreement ANR-18-CE09-0001 (C3PO) and CNRS through the International Emerging Actions program (no. 08216). This work is supported by a public grant from the "Laboratoire d'Excellence Physics Atoms Light Mater" (LabEx PALM) overseen by the French National Research Agency (ANR) as part of the "Investissements d'Avenir" program (reference: ANR-10-LABX-0039-PALM) and by the Region Ile-de-France in the framework of DIM Respore. P.J.-C. received funding from the European Union's Horizon Europe research and innovation program under the Marie Skłodowska-Curie grant agreement No: 101068996. The results used the Imagerie-if core facility supported by l'Agence Nationale de la Recherche (ANR-11-EQPX-0029/Morphoscope, ANR-10-INBS-04/FranceBioImaging; ANR-11-IDEX-0003-02/ Saclay Plant Sciences). Cryo-TEM experiments were supported by the METSA network (Proposal FR3507) and by the French Investissements d'Avenir LabEx PALM (ANR-10LABX-0039PALM). This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 823717 -ESTEEM3 (SIMSINE project). RA acknowledges funding by the Spanish MCIN (PID2019-104739GB-100/AEI/10.13039/501100011033). The work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The X-ray photoelectron (XPS) data collection was performed at the EPSRC National Facility for XPS ("HarwellXPS"), operated by Cardiff University and UCL, under Contract No. PR16195. The authors thank Emmanuel Joussein and Alexandra Courtin for access to LUCIA experiments (SOLEIL, proposal 20180745). Many fruitful discussions with G. Teobaldi are gratefully acknowledged.

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Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.


Erwan Paineau
email: erwan-nicolas.paineau@universite-paris-saclay.fr

Gilberto Teobaldi

Pablo Jiménez-Calvo
email: pablo.jimenez.calvo@fau.de

Imogolite Nanotubes and their Permanently Polarized Bifunctional Surfaces for Photocatalytic Hydrogen Production

Keywords: Imogolite, polarization, band-bending, photocatalysis, hydrogen production

To date, imogolite nanotubes (INTs) have been primarily used for environmental applications such as dye and pollutant degradation. However, imogolite's well-defined porous structure and distinctive electro-optical properties have prompted interest in the system's potential for energy-relevant chemical reactions. The imogolite structure leads to a permanent intrawall polarization arising from the presence of bifunctional surfaces at the inner and outer tube walls. Density functional theory simulations suggest such bifunctionality to encompass also spatially separated band edges. Altogether, these elements make INTs appealing candidates for facilitating chemical conversion reactions. Despite their potential, the exploitation of imogolite's features for photocatalysis is at its infancy, thence relatively unexplored. This perspective overviews the basic physical-chemical and optoelectronical properties of Imogolite nanotubes, emphasizing their role as wide bandgap insulator. Imogolite nanotubes have multifaceted properties that could lead to beneficial outcomes in energy-related applications. This work illustrates two case studies demonstrating a step-forward on photocatalytic hydrogen production achieved through atomic doping or metal co-catalyst. INTs exhibit potential in energy conversion and storage, due to their ability to accommodate functions such as enhancing charge separation and influencing the chemical potentials of interacting species. Yet, tapping into potential for energy-relevant application needs further experimental research, computational, and theoretical analysis.

Introduction

The pressing need to attain carbon neutrality and zero emissions is driving the strong demand for sustainable chemical conversion and use of renewable green energy sources. [START_REF] Teixeira | Renew. Energy Prod. Distrib[END_REF][START_REF] Okonkwo | [END_REF] The goal is to reach 32% consumption by 2030 as established by the EU Energy Directive Council. [START_REF]Promotion of the use of energy from renewable sources[END_REF] Solar energy, holding terawatt potential, stands as a candidate in facilitating the energy transition. [START_REF] Jiménez-Calvo | [END_REF] Notably, significant strides have been made in artificial photosynthesis, driven by photoelectrocatalytic contributions spanning the past two decades, especially in water splitting. [5][6][7][8][9][10][11] As a result, the proliferation of visible-light-responsive high-performance semiconductor materials-either bare or modified-has emerged as an appealing avenue. [12][13][14] Furthermore, the exploration of conventional metal oxides, chalcogenides, carbon-based materials, 1D nano-platforms are typical, functional and active catalysts.

Addressing the issue of photocatalysis involves processes taking place on short time scales but also on restricted distances. [15] For the former, a variety of different time-resolved spectroscopies is now employed for tracking picosecond to microsecond events in photocatalytic materials such as surface photovoltage, transient absorption, or time-resolved spectroscopies (infrared, fluorescence, photoluminescence, microwave conductivity) to cite a few. [15][16][START_REF] Colbeau-Justin | Time-Resolved Microwave Conductivity (TRMC) a Useful Characterization Tool for Charge Carrier Transfer in Photocatalysis[END_REF][18][19][20] Scientists have also worked to understand how fundamental chemical and/or physical principles differ when photocatalytic systems are restricted to nanoscale dimensions. [21][22][23][24] The rise of nanoscience and nanotechnology has offered an ideal playground for exploring these concepts in a variety of different nanoreactors, from nanotubes to more complex three-dimensional porous architectures. [25,26] The strong interest in nanotubes is due to their well-defined, oriented porous structure combined with unique electro-optical properties induced by the rolling up of the atomic lattices. [27] Early on, these iconic nanoscale objects were recognized as ultimate one-dimensional (1D) building blocks for next-generation devices that could outperform current technologies for nanofluidic, molecular sieving or energy storage. [28][29][30][31] In brief, the ion/energy storage capacity is primarily determined by the surface terminal groups, [32] likely hydroxyls in the case of INT. Furthermore, the hollow internal diameter enables ion hosting within the cavity, adding a secondary storage mechanism. As far as photocatalysis is concerned, literature is dominated by studies on TiO 2 nanotubes [33][34][35] although other photoactive metal-oxide semiconductors nanotubes have been proposed. [36] TiO 2 nanotubes stand out as a noteworthy example. [37,38] One prominent TiO 2 nanotubes platform exhibited competitive hydrogen production rates, ranging from 30 µmol h -1 g -1 to 80 mmol h -1 g -1 . [34] However, it's important to note that these rates were achieved in conjunction with the use of co-catalysts. As for bulk materials, coupling nanotubes with metallic nanoparticles is usually necessary to promote the generation and separation of photoexcited carriers in the structure. Although the loading of noble metals as co-catalysts is beyond the scope of this perspective, recent research findings highlight the potential of exploring this avenue as an alternative direction for improving hydrogen production rates. [39,40] Another strategy is to exploit the polarization effect that originates from the non-centrosymmetric arrangement of atoms in the structure. [41,42] The permanent polarization induces positive and negative surfaces spatially separated into the catalyst, which may allow efficient electron-hole separation. Although several photo-active 2D and 3D porous nanostructures with permanent polarization have started to appear in the literature [43,44] the exploitation of 1D polarized nanotubes remains in its infancy. In the context, Imogolite nanotubes (INTs) emerge as a potentially interesting choice, given their prior use in environmental applications, ca. the degradation of dyes and pollutants. [45][46][47] Notably, INTs' cross-sectional separation of the band edges and permanent polarization may be exploited for more elaborate chemical conversion reactions, specifically in energy applications such as hydrogen evolution reaction (HER). [48] In initial section, this perspective emphasizes the primary features and physical-chemical properties of INTs, defining their potential significance as a innovative wide bandgap insulator. In the following section delves two case studies that exemplify groundbreaking outcomes in H 2 production achieved through cutting-edge material modifications: atomic doping and Schottky junction interfacing. Lastly, opportunities, directions, and modifications are proposed to continue unleashing INT's potential.

Main features and physical-chemical properties

A) Bifunctional surface: inner and outer of the tube Imogolite nanotubes are unique 1D nanostructures, first recognized in weathered volcanic soils as true hollow tubes, [49] thirty years before the emergence of the "nanotube era". [50] The chemical composition of INTs is defined as (OH) 3 Al 2 O 3 Si(OH) from outside to inside the tube wall. It consists of an outer dioctahedral Al(OH) 3 wall on which isolated tetrahedral of O 3 Si(OH) are covalently bonded upright to the octahedral vacancy, forming the inner surface of the nanotube wall (Figure 1). The advantage of INTs in contrast to other nanotubes arises from a well-defined minimum of strain energy, [51] which leads to a defined number of tetrahedra along the nanotube circumference. [52] There are no imogolite deposits to speak of, and their extraction from soils requires several purification operations before a sufficient quantity can be obtained. On the other hand, synthetic INTs can be obtained by straightforward methods using low-temperature hydrothermal protocols. [START_REF] Farmer | Dev. {{Sedimentology}}[END_REF][START_REF] Farmer | [END_REF] The benefit is the ability to control or modify the nanotube crystallochemistry by changing the synthesis conditions and obtain new imogolite-like (or geo-inspired) nanostructures. [55,56] One of the most striking examples is the replacement of Si atoms by Ge ones, [57] which allows the direct synthesis of monodisperse nanotubes in diameter with adjustable pore size (1-3 nm), shape (single-walled: SW; or double-walled: DW) and nanotube length through changes in the substitution rate (Si x Ge 1-x ) or the synthesis procedure (Figure 1). [58][59][60][61][62][63] These nanotubes all have the same chirality with a zig-zag configuration. [52,64,65] The peculiar structure of INTs with inner and outer hydroxyl groups also makes them excellent candidates for surface modification. It has been widely documented that the outer wall can be functionalized with various coupling agents such as noble-and/or transition metals [66][67][68] and polymers. [55,69] More interestingly, the inner cavity of hydrophilic INTs can be rendered hydrophobic by a post-synthesis chemical modification. Post-functionalization can be obtained by different silane agents after a perfect dehydration of the hydrophilic INT samples, but the degree of inner surface substitution is lower than 35%. [70] A more convenient way is the direct, template-free, synthesis route by replacing, for instance, the initial alkoxide (prefiguring the tetrahedral layer) by a functionalized one. [71][72][73] Like for hydrophilic INTs, a dependence of the inner cavity diameter with the substitution rate is obtained from the silicon to the germanium analogue end-members with a progressive increase of the nanotube diameter. [74] Interestingly, methylated-INTs roll up into an armchair structure [75] unlike their zig-zag hydroxylated analogs, which may strongly impacted their optical and electronic properties. Overall, synthetic INTs offers a whole family of 1D nanoporous structures with functionalized interfaces, namely single -walled aluminosilicate (SW-AlSi-OH), aluminogermanate (SW-AlGe-OH), methylated aluminosilicate (SW-AlSi-Me) and methylated aluminogermanate (SW-AlGe-Me) nanotubes as well as double-walled aluminogermanate (DW-AlGe-OH) nanotubes (Figure 1). [START_REF] Monet | Nanotubes Géo-Inspirés : Structure Atomique, Transformation En Température et Dynamiques Corrélées Nanotube-Eau Moléculaire[END_REF] Orange tetrahedral correspond to aluminosilicate INTs while green tetrahedral are aluminogermanate INTs. Inner (outer) radius of the nanotubes is provided. Adapted from ref [START_REF] Monet | Nanotubes Géo-Inspirés : Structure Atomique, Transformation En Température et Dynamiques Corrélées Nanotube-Eau Moléculaire[END_REF] .

B) Permanent polarization

Owing to their tubular geometry and compositional radial symmetry, the INTs invariably present a permanent dipole density across the single or double wall interface. This feature, first inferred from electrophoresis measurements by Gustaffson in 2001, [START_REF] Gustafsson | [END_REF] has been directly [78][79][80][81] or indirectly [82] confirmed by Density Functional Theory (DFT) as well as Tight-Binding DFT (TB-DFT) [83] simulations of several members of the imogolite family. Regardless of the zigzag or armchair rolling, the INTs present, at DFT level, accumulation of negative and positive charge on the inner and outer surface, respectively. One notable exception is provided by the results for the SW-AlGe-Me nanotubes, which presents an inverted dipole density with the negative and positive ends pointing towards the outer and inner surface, respectively. [75] As shown in Figure 2, the dipole density across the nanotube walls is accompanied by a marked separation of the Valence Band edge (VBE) and Conduction Band edge (CBE) on opposite sides of the nanotube cavity, which has been suggested to be conducive to the presence of long-range charge-transfer optical excitations. [79,80,84] General, composition agnostic electrostatic models describing the interplay between the permanent dipole density and associated electrostatic potential step, thence polarizing electrostatic field (vide infra) across the nanotube wall(s) have been provided in a prior study. [80] Preliminary simulations indicate such a permanent polarization to be partially resilient to the presence of point-defects, which introduce very local perturbations in the nanotube electrostatics. [78,84] Notably, recent simulations of highly idealized, thence inevitably approximated, structures for the tube ends [81] suggest the INTs can develop, in addition to a radial dipole density, also a longitudinal polarization and ensuing band bending due to physical truncation and ensuing structural relaxation. [85] Figure 2. Schematic representation of the permanent dipole density (μσ) at the wall of imogolite nanotubes, and associated interface electrostatic potential step (ΔV), that can be used to polarize differently reactants inside and outside the NT cavity, offering potential control of their electron-transfer kinetics. The panel shows also the μσinduced real-space separation between the VBE (green) and CBE (red), and intra-wall charge-transfer excitations (wiggly arrow) separating electrons (e -) and holes (h + ) on different sides of the NT-cavity. Picture from Ref.

[G4]. Reproduced with permission. [80] Copyright 2023, Wiley. C) Band-bending of adsorption and desorption species DFT simulation of different hydroxylated and methylated INTs indicates that the dipole density across the wall, and associated electrostatic potential steps (Figure 2), is effective in tuning not only the adsorption geometry and energies of interacting molecules such as H 2 O but also the energy alignment of the molecules' electron acceptor and donor levels. [86] Combined with the demonstrated effects of the nanotubes structure and electrostatic environment in altering the molecular motions at the nanotube-H 2 O interfaces, [87,88] these findings prompt for additional research in the potential of INTs and associated radial polarization for the design and control of electron transfer kinetics at H 2 O interfaces as necessary for improved, up-scalable photocatalytic strategies to hydrogen generation.

The possibility of introducing optical absorption in the visible range of the spectrum by heteroatom doping leading to potentially beneficial photocatalytic pathways not present in the pristine nanotubes has motivated growing computational and experimental research in the subject. [46,[89][90][91][92][93] The nanotube polarization will self-evidently polarize differently different molecular species (with different polarizability). Suggestions have accordingly been put forward also on the potentially beneficial use of INTs as co-photocatalyst i.e. as support for grafted molecular or nanoparticle-based photocatalysts. [79] As the polarizability i.e. response to the electric field from the nanotube dipole density will be invariably different for H 2 O molecules and the grafted photocatalysts, novel and favorable band alignments may/might be engineered and realized, in principle even using systems of known unsuitability for water photo-reduction (e.g. Fe 2 O 3 and WO 3 ) [94] due to their insufficiently high CBE with respect to the H 2 /H 2 O redox potential.

Benefit summary of imogolite nanotubes as photoelectrocatalysts

The distinctive combination of bifunctional surface, permanent polarization, and band-bending modulation brings specific properties in alumino(germano)silicate nanotubes, enabling new chemical potentials for driving the typical energetic reactions, namely H 2 O splitting, CO 2 reduction, and N 2 fixation. The adaptability of nanostructuring single or double walls becomes particularly interesting for improved photon management. Unlike conventional semiconductors, INTs demonstrate the ability to transport photogenerated carriers across their walls. As a result, these multifaceted nanotubes hold the potential to manipulate the chemical potentials of model photocatalytic reactions with minimal material modifications.

Together the polarization, surface, and band-bending of INTs may enable unusual but distinctive effects with promising performance benefits:

-Increase the charge carrier's concentration and continuous flux after photon activation, -Enhancement of carriers' migration to the exposed outer surface since the distance length between the single and double walls are relatively short, -Improvement on time scale of adsorption and desorption species by polarization and surface charge affinity with reactant molecules, -Downward and upward of chemical potentials of model photocatalytic reactions when modified.

Hydrogen production upon photocatalytic and radiolytic activation

Two case studies were selected from two French teams with extensive experience in INT modifications over a decade. These teams have started to explore their catalytic performance using different activation sources. The discussion of this section centers on appreciating the unique aspects of each study, given the substantial differences in experimental conditions, rather than directly comparing the material's modifications nor performance. Particularly, regarding the activation sources (nature: photon or radiation, lamps: monochromatic, full spectrum, and power intensities). Nevertheless, each catalytic case study plays a role in positioning imogolite's in the energy applications (hydrogen evolution reaction) and both are carefully analyzed to reveal its potential as nano-reactors.

Firstly, Jiménez et al. developed Ti-modified aluminogermanate double-wall imogolite nanotubes. [95] The authors reported 1500 μmol -1 g -1 HER (Figure 3a) for the optimal 0.4 Ti/Ge-INT composite with typical sacrificial agent concentration (methanol 33% volume) and full Xe lamp (300 W) irradiation. [95] Interestingly, the present result was obtained without any co-catalyst thus positioning in the metal-free category. The high photoactivity was attributed to low recombination rates and accumulated electrons at the available Ti-sites, which presumably present longer lifetimes.

Rather than relying on metal nanoparticles as co-catalysts and electron collectors, the hybrid active composite's inherent polarization facilitates enhanced photogenerated charge separation. Methanol decomposition functions to block the oxygen evolution reaction (OER) while generating protons simultaneously. These processes occur within the bifunctional surface nanotube, leading to effective spatial separation of useful charge carriers, with proton reduction taking place in the outer wall of the NT.

Secondly, Patra et al. reported a value of 1443 μmol -1 g -1 HER (Figure 3b) in the case of aluminosilicate INT-CH 3 containing 20% Au NPs as co-catalyst with propan-2-ol 20% as sacrificial agent and monochromatic light (254 nm, 15 W) irradiation. [96] Notably, such INT-CH 3 /Au photoactive composite exhibits an efficient metal-semiconductor interface, leading to a substantial gold catalytic surface contribution, while the possibility of polarization and confinement effects from the INT-CH 3 cannot be disregarded. The second source used was gamma radiation from 137 Cs in argon atmosphere (Figure 3c). However, the H 2 production was one order of magnitude lower than when photo-activated, suggesting that the reduction mechanism via radiation does not bring the same benefits as photon to create a feasible metal-semiconductor Schottky barrier. [95] b) Photolytic and c) radiolytic hydrogen production of INTs functionalized with -CH 3 groups and containing Au NPs. [96]

Conclusions and outlook

The chosen case studies on H 2 generation via photocatalytic, photolytic, and radiolytic highlights the experimental proof-of-concept, evidence the clear potential of INTs as a strong candidate to be added to the list of topical semiconductor photoelectrocatalysts. Extensive progress has been achieved in characterizing diverse aspects like physico-chemical, optoelectronic, magnetic, bulk, and surface properties of this nanotube. However, it is crucial to include an examination of charge carrier dynamics in order to complete the comprehensive understanding of its behavior.

Considering the two presented case studies, it is evident that INT holds great potential to be further developed into a widely in-demand metal oxide with remarkable properties, not just for energy conversion but also for energy storage. Although the latter aspect was not covered in this article, interested readers can find comprehensive details elsewhere. [30] On the other hand, the introduction of imogolite double-wall nanotubes provides a novel 1D nanostructure with controlled photon management for activation, opening up potentially exciting avenues for new research and advances in hydrogen/oxygen generation, carbon dioxide reduction, and nitrogen fixation, all of which guarantee further investigations.

On the horizon, there are additional opportunities for incorporating metal dopants, coupling with medium band gap semiconductors, and/or depositing bimetallic catalysts to continue harnessing and exceeding the current efficiencies of solar fuels technologies.

To further and better support future research efforts in the potential of INT for photo-catalytic applications, two lines of simulation development would be very beneficial. First, definition and validation of scalable yet sufficiently accurate routes to modelling realistic electronic ground state interfaces between the nanotube's surfaces, interacting molecules and surrounding media has remained challenging over the last ten to fifteen years. Current advances in DFT-derived machine learning interatomic potentials hold great promise to this end. Second, viable simulation methods for optical electronic excitations in the nanotubes beyond the single-particle picture used so far as well as first principles approaches to excited state (excitonic) dynamics in model Imogolite-water (solvent) nanotube interfaces are currently missing or yet to be applied. While the continuous increase in academically accessible high performance computing resources authorizes outlooks towards the use of many-body approaches to valence-electron optical spectroscopy, progress on the latter front will require intense method development efforts. At least to best of our knowledge, the needed computational machinery is yet to be developed or made available.

As for electrocatalytic applications, it is recommended to measure/report the INTs conductivity and/or resistivity. One can think of implementing significant vacancies or atoms in their lattice that can enhance their conductivity, and therefore electron collection for pure electrocatalysis.

Figure 1 .

 1 Figure 1. (left) Atomic structure of the dioctahedral aluminum layer (blue) and of the location of an SiO 4 tetrahedron (orange). Oxygen atoms are in red and hydrogen ones are white. (right) Representation of the different members of the whole family of imogolite nanotubes with hydroxylated or methylated inner cavity.Orange tetrahedral correspond to aluminosilicate INTs while green tetrahedral are aluminogermanate INTs. Inner (outer) radius of the nanotubes is provided. Adapted from ref[START_REF] Monet | Nanotubes Géo-Inspirés : Structure Atomique, Transformation En Température et Dynamiques Corrélées Nanotube-Eau Moléculaire[END_REF] .

INT-CH 3 /

 3 Au composite has proven another interesting trend in function of size-dependent. Patra et al reported the comparison of 5 and 10 nm average size of Au NPs given a pronounced difference on hydrogen production of 10 and 90 times higher than bare INT-CH 3 /Au. This outcome indicates the presence of metallic Au and highlights the active site on the outer surface of INT-CH 3 . This site effectively converts nearby protons into H 2 .

Figure 3 .

 3 Figure 3. a) Photocatalytic hydrogen production of Ti-modified Imogolite NTs under Xenon lamp illumination,[95] b) Photolytic and c) radiolytic hydrogen production of INTs functionalized with -CH 3 groups and containing Au NPs.[96]

Acknowledgements

E.P. acknowledges financial supports from the French ANR agency (ANR-18-CE09-0001) and CNRS (IEA 08216) as well as M.N. Ghazzal and P. Launois for fruitful discussions related to imogolite nanotubes. G.T. acknowledges support from EPSRC UKRI (EP/I004483/1, EP/K013610/1, EP/P022189/1, EP/P022189/2 and EP/X035891/1). P.J.-C. received funding from the European Union's Horizon Europe research and innovation program under the Marie Skłodowska-Curie Grant Agreement No: 101068996 for his fellowship.

Conflict of interest

The authors declare no conflict of interest.