Maïssa K S Barr

Loïc Assaud

Nicolas Brazeau

Margrit Hanbücken

Spyridon Ntais

Lionel Santinacci

Elena A Baranova

Enhancement of Pd Catalytic Activity Toward Ethanol Electrooxidation By Atomic Layer Deposition of SnO 2 onto TiO 2 Nanotubes

Palladium nanoparticles and SnO 2 layers have been grown by two successive steps of atomic layer deposition onto TiO 2 nanotubes (TNTs). The three-dimensional nanostructured catalytic systems have been studied for ethanol electrooxidation in alkaline media. Characterization by scanning and transmission electron microscopies, x-ray photoelectron spectroscopy, and x-ray diffraction indicates the high conformity, the high purity, and the perfect stoichiometry of Pd and SnO 2 deposits onto the TNTs. The electrochemical investigations performed by cyclic voltammetry and chronoamperometry have revealed the beneficial effect of the annealing of the support toward ethanol electrooxidation reaction. The comparison of the catalytic activity with literature shows that such SnO 2 based substrates exhibits highly promising performances.

INTRODUCTION

Direct alcohol fuel cells (DAFCs) have received a great deal of attention for energy generation and storage in portable electronic devices and vehicles. Among the different possible alcohols, ethanol is a promising liquid fuel because it has no toxicity, a high energy density and it can be easily produced from the fermentation of biomass. [START_REF] Rousseau | Direct Ethanol Fuel Cell (DEFC): Electrical Performances and Reaction Products Distribution under Operating Conditions with Different Platinum-Based Anodes[END_REF][START_REF] Antolini | Catalysts for Direct Ethanol Fuel Cells[END_REF][START_REF] Bianchini | Palladium-Based Electrocatalysts for Alcohol Oxidation in Half Cells and in Direct Alcohol Fuel Cells[END_REF] However, the ethanol oxidation kinetics on Pt-based catalysts in acidic media is slow, incomplete and mostly leads to the production of acetic acid and acetaldehyde. The recent development of anion exchange solid electrolytes advanced the development of alkaline DAFCs where the oxidation kinetics is faster and allows using cheaper electrocatalysts than Pt. Pt-free catalysts or Pt alloying with co-catalysts can be used for ethanol oxidation reaction (EOR) in alkaline media. [START_REF] Antolini | Alkaline Direct Alcohol Fuel Cells[END_REF] Within this context, palladium has emerged as an attractive candidate for DAFCs. Pd is more abundant and less expensive, however the EOR is not complete in alkaline media leading to acetate and acetaldehyde formation in addition to small amounts of CO 2 . It is therefore necessary to improve the catalyst to efficiently break the C-C bond at low overpotentials. In order to improve the performances of Pd toward ethanol electrooxidation, bimetallic Pd/M (M = Ru, Au, Ir, Sn, Ni) catalysts have been intensively studied in alkaline media. 5- 8 Another possibility to enhance the performances of Pd based catalyst is the use of a catalyst support with a high surface area and a better stability than conventional carbon black. [START_REF] Bianchini | Palladium-Based Electrocatalysts for Alcohol Oxidation in Half Cells and in Direct Alcohol Fuel Cells[END_REF] The catalyst substrate should promote the formation of well-dispersed and stable Pd particles. The support, in addition to its structural role, can have an electronic effect on the catalyst phase and influence the catalytic activity of metallic particles via metal-support interaction (MSI). [START_REF] Tauster | Strong Metal-Support Interactions -Group-8 Noble-Metals Supported on TiO 2[END_REF][START_REF] Hepel | Nanoporous TiO 2 -Supported Bimetallic Catalysts for Methanol Oxidation in Acidic Media[END_REF][START_REF] Ntais | An X-Ray Photoelectron Spectroscopy Study of Platinum Nanoparticles on Yttria-Stabilized Zirconia Ionic Support: Insight into Metal Support Interaction[END_REF] This in turn will affect the adsorption strength of ethanol and reaction intermediates resulting in improved catalyst activity and in some cases its selectivity. Numerous nanostructured substrates such as carbon microspheres, hollow carbon spheres, nanowires and carbon nanotubes (CNTs) have been investigated to enhance the catalytic efficiency of Pd and to reduce the costs. [START_REF] Xu | Pt and Pd Supported on Carbon Microspheres for Alcohol Electrooxidation in Alkaline Media[END_REF][START_REF] Zheng | Effect of Support on the Activity of Pd Electrocatalyst for Ethanol Oxidation[END_REF] Nevertheless, carbon-based materials are not stable and corrode under reaction conditions. TiO 2 is well known for its high stability and its strong MSI with Pt group metals. [START_REF] Tauster | Strong Metal-Support Interactions -Group-8 Noble-Metals Supported on TiO 2[END_REF] It is thus an interesting support for Pd particles. [START_REF] Huang | Development of a Titanium Dioxide-Supported Platinum Catalyst with Ultrahigh Stability for Polymer Electrolyte Membrane Fuel Cell Applications[END_REF] Among the numerous types of titanium dioxide nanostructures, the nanotubes are interesting because the exhibit a high specific area and a well-defined geometry. TiO 2 nanotubes (TNTs) can be obtained by different methods such hydrothermal reaction, deposition in templates, molecular assembly templating, electrospinning and anodic oxidation. [START_REF] Lee | One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes[END_REF] The later method is straightforward and offers a large control of the tube morphology. In anodic TNTs, the charge transport and collection are facilitated by the nanotubes self-alignment, because they are connected by the Ti substrate. In a previous work, [START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF] it has been shown that anodic TNT arrays can be suitable supports for Pd nanoparticles grown by atomic layer deposition (ALD) and that the annealing of titania improves the electrocatalytic activity of the catalysts. Although a high and stable electroactivity has been measured on such Pd/TNT systems further improvements are proposed in the present work through the addition of a SnO 2 interlayer between TNTs and Pd nanoparticles. SnO 2 has been widely studied for application in sensors, batteries, photovoltaic cell, optoelectronic device and catalytic application. [START_REF] Batzill | The Surface and Materials Science of Tin Oxide[END_REF] There are a number of studies that show a promotional effect of SnO 2 support on alcohol electrooxidation. [START_REF] Wen | The Improved Electrocatalytic Activity of Palladium/Graphene Nanosheets Towards Ethanol Oxidation by Tin Oxide[END_REF][START_REF] Lu | SnO 2 Nanospheres Supported Pd Catalyst with Enhanced Performance for Formic Acid Oxidation[END_REF][START_REF] An | Synthesis and Performance of Palladium-Based Catalysts for Methanol and Ethanol Oxidation in Alkaline Fuel Cells[END_REF][START_REF] An | Electrocatalytic Performance of Pd/SnO 2 -TiO 2 /MWCNT Catalyst for Oxidation of Ethylene Glycol in Alkaline Media[END_REF] A chemical modification of TNT surface by SnO 2 deposition is therefore suitable to enhance the Pd properties toward ethanol electrooxidation. Several techniques can be used to functionalize TNTs with SnO 2 . The SnO 2 has been deposited onto anodic TNTs by solvothermal method, [START_REF] Du | SnO 2 Nanocrystals on Self-Organized TiO 2 Nanotube Array as Three-Dimensional Electrode for Lithium Ion Microbatteries[END_REF] by electrodeposition [START_REF] Wu | Coaxial SnO 2 @TiO 2 Nanotube Hybrids: from Robust Assembly Strategies to Potential Application in Li + Storage[END_REF] or by ultrasonic deposition. [START_REF] Hu | Facile Ultrasonic Deposition of SnO 2 Nanoparticles on TiO 2 Nanotube Films for Enhanced Photoelectrochemical Performances[END_REF] However, ALD remains a method of choice to grown conformal, particles [START_REF] Turkevych | Synergistic Effect between TiO 2 and Ubiquitous Metal Oxides on Photocatalytic Activity of Composite Nanostructures[END_REF] or continuous [START_REF] Jere | Preparation of regularly structured nanotubular TiO 2 thin films on ITO and their modification with thin ALD-grown layers[END_REF][START_REF] Zazpe | Atomic Layer Deposition for Coating of High Aspect Ratio TiO 2 Nanotube Layers[END_REF] oxide layers as well as sulphide [START_REF] Sarkar | In 2 S 3 Atomic Layer Deposition and Its Application as a Sensitizer on TiO 2 Nanotube Arrays for Solar Energy Conversion[END_REF][START_REF] Macak | Antireflection ln 2 O 3 Coatings of Self-Organized TiO 2 Nanotube Layers Prepared by Atomic Layer Deposition[END_REF] films and specially SnO 2 onto such nanostructures. [START_REF] Elam | Atomic Layer Deposition of Indium Tin Oxide Thin Films Using Nonhalogenated Precursors[END_REF][START_REF] Elam | Atomic Layer Deposition of Tin Oxide Films Using Tetrakis(Dimethylamino) Tin[END_REF] To avoid any film contamination, a process with non-halogenated precursor has been used. [START_REF] Elam | Atomic Layer Deposition of Indium Tin Oxide Thin Films Using Nonhalogenated Precursors[END_REF][START_REF] Mullings | Tin Oxide Atomic Layer Deposition from Tetrakis(Dimethylamino)Tin and Water[END_REF] In recent years, many groups have reported the interest of using ALD to grow metallic nanoparticles onto nanostructured substrate. [START_REF] Detavernier | Tailoring Nanoporous Materials by Atomic Layer Deposition[END_REF][START_REF] Elam | ALD for Clean Energy Conversion, Utilization, and Storage[END_REF][START_REF] Weber | Supported Core/Shell Bimetallic Nanoparticles Synthesis by Atomic Layer Deposition[END_REF][START_REF] Lu | Synthesis and Stabilization of Supported Metal Catalysts by Atomic Layer Deposition[END_REF] The amount and the composition of the catalyst are fully controlled, the nanoparticles exhibit the same or better performances than particles grown by other methods. [START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF][START_REF] Goldstein | Enhancing the Nucleation of Palladium Atomic Layer Deposition on Al 2 O 3 Using Trimethylaluminum to Prevent Surface Poisoning by Reaction Products[END_REF][START_REF] Feng | Subnanometer Palladium Particles Synthesized by Atomic Layer Deposition[END_REF][START_REF] Lu | Porous Alumina Protective Coatings on Palladium Nanoparticles by Self-Poisoned Atomic Layer Deposition[END_REF] The aim of the present study is to compare the electrochemical response of Pd catalysts supported by two different nanostructured materials: bare TNTs and SnO 2 coated TNTs. Since the SnO 2 deposition proceeds with a negligible variation of the geometry, the influence of the chemical nature of the substrate is solely expected. As schematically presented on Fig. 1, the SnO 2 layer and Pd nanoparticles have been successively deposited by ALD onto anodic TNTs (Pd/SnO 2 /TNTs).

The morphology, the chemical composition and the crystalline structure of the SnO 2 layer and the Pd catalysts have been investigated by scanning and transmission electron microscopies (SEM and TEM, respectively), x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD). The electroactivity of the Pd/SnO 2 /TNTs systems toward ethanol electrooxidation has been measured by cyclic voltammetry (CV) and chronoamperometry (CA) as a function of the ALD parameters and depending on the crystalline structure of the SnO 2 /TNTs support.

EXPERIMENTAL

Preparation of TiO 2 nanotubes

The TiO 2 nanotubes have been grown by anodization of Ti foils (Advent, 95.6% purity) in an aqueous fluoride containing acidic electrolyte (1 M H 3 PO 4 + 1 M NaOH + 0.5% HF) as described in a previous publication. [START_REF] Assaud | Fabrication of p/n Heterojunctions by Electrochemical Deposition of Cu 2 O onto TiO 2 Nanotubes[END_REF] The electrochemical oxidation was performed under a constant applied potential (U) of 20 V at room temperature using a high-voltage Modulab potentiostat (Solartron Analytical).

Atomic layer deposition of SnO 2

The SnO 2 layer was grown by atomic layer deposition in a Fiji 200 reactor from Ultratech/Cambridge Nanotech. The deposit was achieved at T = 200°C with tetrakis(dimethylamino)tin (TDMASn, 99.99%, from Strem chemicals) and hydrogen peroxide (30% from Sigma-Aldrich). In order to achieve a high growth per cycle (GPC) H 2 O 2 has been chosen as oxygen source (Caution: when performing the ALD process, the H 2 O 2 concentration in the canister can increase due to a distillation phenomenon. The solution can become potentially explosive. A fresh solution should be used for each run). The canister containing the TDMASn was held at T = 80°C. The socalled exposure mode was used to facilitate the deposition onto the whole TNTs. It consisted of isolating the reactor from the pumping system during each precursor pulse. The ALD process was therefore defined as the successive pulse, exposure and purge steps for both precursors. The sequence durations were 1:10:10 s and 1:10:10 s for TDMASn and H 2 O 2 , respectively. The ALD of SnO 2 was performed onto both TNTs and flat Si (100) substrates to facilitate some characterizations. The amount of deposited stannic oxide was controlled by the number of ALD cycles (N SnO2 ). Modification of the crystalline structure of SnO 2 /TNTs was achieved by an annealing at 450°C for 2 h in air.

Atomic layer deposition of Pd

The deposition of the catalyst has been described previously. [START_REF] Assaud | 3D-Nanoarchitectured Pd/Ni Catalysts Prepared by Atomic Layer Deposition for The Electrooxidation of Formic Acid[END_REF] It was carry out in the same reactor with palladium(II)hexafluoroacetylacetonate (Pd(hfac) 2 , 98%, from Strem Chemicals) and formalin (37% formaldehyde in water with 10-15% of methanol from Sigma-Aldrich). The Pd canister was maintained at 90°C, while the temperature of the reaction chamber was set to 200°C.

The pulse, exposure, and purge durations were 1:30:30 s and 3:30:30 s for Pd(hfac) 2 and formalin, respectively. The number of ALD cycles (N Pd ) was varied to adjust the catalyst load.

Characterization Methods

The thickness of the SnO 2 deposit was measured by spectroscopic ellipsometry using a M2000V from J. A. Woollam Inc. The morphology of SnO 2 /TNTs as well as the Pd/SnO 2 /TNTs were characterized by SEM (JSM 6320F, JEOL) and TEM (JEM 3010, JEOL). The crystal structure of the SnO 2 layers and the TiO 2 substrates were analysed by XRD using a Rigaku RU-200 rotating anode x-ray generator (operating power 40 kV/30 mA) equipped with a Xenocs Fox3D Cu 12_INF mirror and a Mar345 image plate detector from Rayonix. Selected-area electron diffraction (SAED) was also performed in the TEM to locally study the crystal structure of the Pd/SnO 2 /TNTs systems.

The chemical composition of the deposits was determined by XPS using a Kratos Axis Ultra DLD with a Hybrid lens mode at 140 W and pass energy of 20 eV using a monochromatic Al K α . The Sn 3d, Pd 3d and O 1s XPS core level spectra were analysed using a fitting routine which decomposes each spectrum into individual mixed Gaussian-Lorentzian peaks using a Shirley background subtraction over the energy range of the fit. The binding energy (BE) was corrected using the C 1s peak at 284.8 eV as an internal standard. The fitting of the Sn 3d was analysed using doublets with spin orbit splitting (SOS) 8.44 and intensity ratio Sn 3d 5/2 :Sn 3d 3/2 = 3/2. The deconvolution of Pd 3d was performed using doublets with SOS 5.3 eV and intensity ratio Pd 3d 5/2 :Pd 3d 3/2 = 3/2, 42 while a peak asymmetry was used in the case of the Pd 3d peak attributed to the metallic state based on the work of Hüfner et al. [START_REF] Hüfner | Core-Line Asymmetries in the X-Ray-Photoemission Spectra of Metals[END_REF]

Electrochemical Characterizations

The electrochemical characterizations of Pd/SnO 2 /TNTs were carried out in the conventional threeelectrode electrochemical cell made of PTFE at room temperature. The BioLogic VSP potentiostat equipped with the EC-Lab software (BioLogic Science Instruments SAS) was used for electrochemical measurements in 1 M KOH with and without 1 M C 2 H 5 OH. The reference electrode used was a Hg/HgO (MMO, E° = 0.098 vs SHE), and the counter electrode was a large surface area Pt mesh. Cyclic voltammetry was carried out at a scan rate of 25 mV s -1 unless otherwise stated. Chronoamperograms were performed by holding the potential at -0.6 V vs MMO for 5 min and then stepping to -0.2 V for 1 h. All the potential are given versus the MMO reference electrode.

RESULTS

SnO 2 deposition onto TNTs

In order, to optimize the SnO 2 deposition different conditions have been investigated, such as the pulse and the purge durations as well as the reactor and the canister temperatures. The influence of those parameters on the growth rate has been studied and adjusted (Fig. S1). In the following, only the N SnO2 was varied to control the thickness of SnO 2 . SEM top views of TNTs with an increasing number of ALD cycles are shown on Figure 2 (N SnO2 = 0, 20, 50, 100 and 150 cycles). The typical morphology of bare TNTs grown in an aqueous electrolyte is observed on Figure 2a. The diameter of the nanotubes is about 70 nm, the length is around 1 μm and the wall thickness is approximately 10 nm (additional SEM images are provided in Figure S2). After ALD of SnO 2 , the wall thickness increases progressively with N SnO2 (Figure 2b-e). For 150 ALD cycles the opening of some TNTs is even fully filled by the deposit. The thickness of the film (t SnO2 ) has been measured by spectroscopic ellipsometry on silicon wafers (Figure S1c). It reveals a linear increase of t SnO2 with the number of ALD cycles. A GPC around 1.9 Å/cycle is then calculated from the slope of the straight evolution. This value is in line with the GPC reported previously by Elam et al. [START_REF] Elam | Atomic Layer Deposition of Tin Oxide Films Using Tetrakis(Dimethylamino) Tin[END_REF] The SEM images show a smooth and homogenous layer of SnO 2 onto the TNTs. They suggest a uniform coating of the TNTs but top views do not provide accurate information about the conformity. Thus, cross sectional observations have been performed by TEM.

The Figure 3 shows TEM cross-sections of TNTs after ALD of SnO 2 (N SnO2 = 100 cycles). A large view presenting the tube mouth and walls is presented in Figure 3a where the TiO 2 appears as light grey while the SnO 2 film corresponds to the darker grey areas (for seek of clarity the SnO 2 layer is highlighted in blue). Although the opening of the voids located between the tubes is much smaller than the inner diameter, the tin dioxide SnO 2 deposition occurs similarly inside and outside the TNTs. The tubes are fully covered by a smooth and continuous layer of SnO 2 . The thickness of the SnO 2 layer is rather constant on the different parts of the tube walls. As seen on Figure 3b, the deposition takes also place conformally at the bottom of the TNTs. The thickness of the SnO 2 layer in the TNTs is approximately 16 nm. It is in agreement with the value (19 nm) obtained on plane Si.

The crystalline structure and the chemical composition have been investigated by XRD and XPS. The XRD patterns of as-grown and annealed SnO 2 /TNT systems are compared on Figure 4.

The diffractogram of the as-grown sample exhibits peaks from the Ti substrate (JCPDS #00-044-1294) and no crystalline TiO 2 is detected since the TNTs are usually amorphous after the anodization process. A small contribution from the rutile phase of SnO 2 (JCPDS #00-41-1445), also called cassiterite is also measured at 26.61°, 33.91° and 51.83° corresponding to the (110), ( 101) and (211) planes, respectively. After annealing at 450°C, two additional peaks ascribed to the plane (100) and (002) of the Ti appears at 35.00 and 38.45°, amorphous TiO 2 becomes crystalline. The peaks ascribed to the anatase structure (JCPDS #00-021-1271) are clearly visible at 25.33°, 48.08°, 53.93°, 55.12° and 62.75° which are attributed to the (101), ( 200), ( 105), ( 211) and (204) planes, respectively. After the thermal treatment, the diffraction peaks from SnO 2 are more intense and the presence of two additional peaks are visible at 37.92° and 39.00°. They correspond to the planes (200) and (111) of the cassiterite, respectively. The SnO 2 peaks are broad.

They suggest a relatively small crystallite average size. As expected the survey spectrum of SnO 2 /TNTs (shown in Figure S3) contains peaks corresponding to Sn, O and C.

The XPS spectrum of Sn 3d is presented in Fig 5a . The peaks centred at 487.1 and 495.5 eV are attributed to Sn 3d 5/2 and Sn 3d 3/2 , respectively. [START_REF] Meng | Preparation of SnO 2 @C-Doping TiO 2 Nanotube Arrays and Its Electrochemical and Photoelectrochemical Properties[END_REF][START_REF] Tian | Encapsulation of SnO 2 Nanoparticles into Hollow TiO 2 Nanowires as High Performance Anode Materials for Lithium Ion Batteries[END_REF][START_REF] Liao | Mesoporous TiO 2 -Sn/C Core-Shell Nanowire Arrays as High-Performance 3D Anodes for Li-Ion Batteries[END_REF] The deconvolution of the Sn 3d peaks reveals the existence of tin atoms in the oxidation state 4+, it can be deduced the presence of stoichiometric SnO 2 . The spectrum of O 1s peak is shown on Figure 5b. It can be deconvoluted with two contributions. The main one, centred at 531 eV, is attributed to O 2-in SnO 2 and the other one positioned at 532.5 eV is ascribed to adsorbed oxygen or to the presence of water. This analysis confirms the high purity of the deposit and the absence of Ti peaks indicates a good coverage of TNT by ALD of SnO 2 . In the following, 75 ALD cycles (i. e. approximately 14 nm) have systematically been used to grow SnO 2 on TNTs. The resulting SnO 2 /TNTs were either used as-grown or after an annealing at T = 450°C.

Palladium deposition

The morphology of the Pd particles deposited onto as-grown SnO 2 /TNTs has been observed by SEM. As shown in the previous part for tin dioxide SnO 2 , ALD is commonly used to grow continuous films. Though the SEM top view (Figure 6a) shows Pd nanoparticles uniformly spread onto the SnO 2 /TNTs and not a continuous metallic film. This phenomenon is expected since there is a high difference of surface energies between the metal and the oxide. [START_REF] Campbell | Ultrathin Metal Films and Particles on Oxide Surfaces: Structural, Electronic and Chemisorptive Properties[END_REF] A similar island-growth mechanism has been observed for Pt deposited by ALD onto SnO 2 nanowires by Lin et al. [START_REF] Lin | Fabrication of Tin Dioxide Nanowires with Ultrahigh Gas Sensitivity by Atomic Layer Deposition of Platinum[END_REF] The deposition of Pd is homogenous along the outer wall of the SnO 2 /TNTs (Fig 6b). It indicates the deposition of the nanoparticles proceeds within the narrow interstitial voids but these SEM observations give no evidence of Pd deposition inside the tubes. A similar morphology has been reported previously for direct ALD of Pd onto TNTs. [START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF] However when the TNTs are covered by the SnO 2 film, the geometry of the palladium catalysts appear more uniform. [START_REF] Neto | Ethanol Electrooxidation on PdIr/C Electrocatalysts In Alkaline Media: Electrochemical and Fuel Cell Studies[END_REF] It is well known that the nature of the substrate influences the ALD of metals. The ALD of Pd on ZnO and Al 2 O 3 has, indeed, revealed that Pd grows faster on ZnO but a better uniformity is observed on Al 2 O 3 . [START_REF] Feng | Palladium Catalysts Synthesized by Atomic Layer Deposition for Methanol Decomposition[END_REF] It is well known that the growth of metallic particles can be affected by the chemical nature and the crystalline structure of the support. [START_REF] Lei | Effects of Chlorine in Titanium Oxide on Palladium Atomic Layer Deposition[END_REF][START_REF] Mackus | Atomic Layer Deposition of Pd and Pt Nanoparticles for Catalysis: on The Mechanisms of Nanoparticle Formation[END_REF] The comparison of the ALD of Pd on bare and SnO 2 -covered TNTs illustrates this effect. While an incubation delay (N Pd = 400 cycles) is observed when depositing Pd on bare TNTs, [START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF] active particles are grown on SnO 2 for a lower number of ALD cycles since an electroactivity is measured for N Pd = 100 cycles (see Figure 9). This can be explained by the action of the formaldehyde that reduces the SnO 2 during the second step of the ALD cycle. The XPS measurements that have been performed on Pd/SnO 2 /TNTs before and after electrochemical characterizations show indeed a modification of the chemical nature of the top surface (Figure 8). The analysis of the Sn 3d 5/2 peak, shown in Figure 8c, indicates the reduction of the SnO 2 to metallic tin since contribution corresponding to the oxidation state 0 replaces the peak attributed to the oxidation state +4 (a detailed analysis of the XPS plots is given below). As the surface of the support is not an oxide but an ultrathin layer of metallic Sn, the nucleation of stable Pd is facilitated. A slight effect of the thermal treatment performed prior to ALD of Pd is also observed. After annealing, one observed the decrease of the particle diameter (e. g.: d Pd varies from 10 to 6 nm for N Pd = 600 cycles, see Figure S4) while the particle density (e. g.: n Pd varies from 6.3 10 9 to 14.2 10 9 particles per cm 2 for N Pd = 600 cycles) is increased. It suggests that the Pd nucleation proceed more easily on crystallized substrate. XPS measurements were performed before and after the electrochemical measurements (EC), on annealed Pd/SnO 2 /TNTs, in order to obtain a better insight into the surface chemical composition of the samples. The analysis of the survey (Figure 8a) shows that, in sample before electrochemical measurements, C, O, Sn and Pd were detected while no Ti was detected due to the attenuation of its signal from the overlayers. The carbon contribution may be attributed to surface contamination or residual carbon from the precursor. [START_REF] Gharachorlou | Palladium Nanoparticle Formation on TiO 2 (110) by Thermal Decomposition of Palladium(II) Hexafluoroacetylacetonate[END_REF] Additionally the O 1s peak is overlapped by the Pd 3p 3/2 making its interpretation rather not straightforward. The deconvoluted Pd 3d and Sn 3d 5/2 before the EC are shown in Figure 8b and8c. The analysis of the Pd 3d 5/2 peak (Figure 8b) reveals the existence of two peaks at 335.5 and 337.5 eV. One, located at the lower binding energy, is attributed to palladium atoms in the metallic state while the one, at higher binding energy, is characteristic of Pd 4+ in PdO 2 . [START_REF] Monyoncho | Role of the Metal-Oxide Support in the Catalytic Activity of Pd Nanoparticles for Ethanol Electrooxidation in Alkaline Media[END_REF] The ratio of relative intensities of the two components Pd 0 :Pd 4+ is 93:7 showing that Pd is mainly in the metallic state. This is slightly different from the analysis of Pd particles grown on TNTs where only metallic Pd has been observed. On the other hand, the analysis of the Sn 3d 5/2 XPS peak shows the existence of three components at 485.1, 486.1 and 487.2 eV attributed to Sn 0 , Sn 2+ and Sn 4+ . [START_REF] Briggs | Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy[END_REF][START_REF] Süzer | Electron Spectroscopic Investigation of Sn Coatings on Glasses[END_REF] The relative intensities of the three components Sn 0 :Sn 2+ :Sn 4+ are 75.0:12.4:12.6. The high amount of Sn 0 was not expected but it can be explained by deposition method of Pd. During the ALD of Pd, the formalin, a reductive precursor, can reduce the tin species at the surface from Sn 4+ to Sn 0 . This phenomenon is different from what has been previously observed on bare TNT surface. [START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF] As predicted by the Ellingham diagram, 56 it indicates that SnO 2 is less stable than TiO 2 toward the reduction by formalin. The metallic tin is essentially localized at the top surface of the film, around 2 nm, which corresponds to the escape depth of the photoelectrons during the XPS experiments. The SAED performed on the Pd/SnO 2 /TNTs have, however, confirmed that the bulk material is composed by SnO 2 (Figure 7d).

After the electrochemical measurements, the analysis of the survey (Figure 8a´) shows that C, Sn and Pd are still detected but the intensities are notably lower. After the EC, a significant amount of potassium is detected on the surface as indicated by the K 2p peak around 293.4 eV. The deconvolution of the Pd 3d peak (Figure 8b´) reveals that beyond the component at lower BEs (Pd 3d 5/2 : 335.1 eV) due to metallic Pd, two additional peaks are present at higher BEs (336 and 337.1 eV). These two peaks are attributed to palladium in the 2+ and 4+ oxidation states related to PdO and PdO 2 and/or hydroxides. [57][START_REF] Gao | Titania-Supported Pd-Cu Bimetallic Catalyst for the Reduction of Nitrite Ions in Drinking Water[END_REF] The relative intensities of the three components Pd:Pd 2+ :Pd 4+ is now 39.8:41.1:19.1. It is therefore evident that after the electrochemical measurements, oxidation of palladium has taken place as well and around 60% of the detected signal is due to oxygenated species. At this point we cannot exclude the formation of surface complexes that contains potassium as well, the K 2p 3/2 is detected around 293.4 eV. In the literature, this binding energy is reported for potassium atoms in ionic compounds and K/Pd. [START_REF] Campbell | Ultrathin Metal Films and Particles on Oxide Surfaces: Structural, Electronic and Chemisorptive Properties[END_REF] It is also well-known that all alkali metals, including potassium, form oxopalladates (K n PdO m ). [START_REF] Lee | Microstructure-Dependent Nucleation in Atomic Layer Deposition of Pt on TiO 2[END_REF] The formation of this complex could explain the decrease of the signal from C, Sn and Pd. Further investigation is necessary to obtain a better insight of the K-based surface species. The analysis of the Sn 3d 5/2 shows the existence of only one component at 487.2 eV, due to SnO 2 . [START_REF] Briggs | Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy[END_REF]

Electrocatalytic activity

Electrochemical response in KOH -Figure 9 shows cyclic voltammograms of Pd/SnO 2 /TNT electrocatalysts and SnO 2 /TNT support without Pd in 1 M KOH. The current density of the bare support is very low indicating, thus, no catalytic activity. The CVs of various Pd loads exhibit the characteristic peaks of Pd oxidation (ca. -0.2 V) and reduction (ca. -0.25 V). The region of hydrogen adsorption, absorption and desorption is visible at more negative potentials. The current density (per geometric area) rises with increasing the number of ALD cycles up to 500 cycles and then decreases because the coalescence of the Pd particles begins. It should be noted that in our previous work carried out on Pd/TNTs, neither oxidation nor reduction peaks of Pd were observed on CVs performed in 1 M KOH for NPd < 500 cycles. [START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF] Conversely, SnO 2 -covered TNTs exhibit an electrochemical response for a Pd loading as low as 100 ALD cycles. This confirms that stable Pd nuclei are formed on such substrate at low N Pd . As mentioned above, this can be ascribed to the reduction of the SnO 2 to metallic tin at the top surface. The enhancement of the current response can also be attributed to the high electronic conductivity of SnO 2 (here 10 -3 -10 -2 W cm while Tighineanu et al. [START_REF] Tighineanu | Conductivity Of TiO 2 Nanotubes: Influence of Annealing Time And Temperature[END_REF] have reported 10 3 -10 4 W cm for TNTs) that allows efficiently interconnect highly dispersed Pd nanoparticles and to MSI interaction between Pd and SnO 2 .

Ethanol electrooxidation on Pd/SnO 2 /TNT: effect of the annealing -In order to compare the intrinsic electrocatalytic activity of Pd/SnO 2 /TNT samples, the current densities were normalized with respect to the electrochemical surface area (ECSA) that is estimated using PdO reduction peak in 0.5 M H 2 SO 4 , as reported previously. [START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF][START_REF] Horkans | The Hydrogen Region of The Cyclic Voltammetry of Pd: The Effect of pH and Anion[END_REF] The ECSA was measured on as-grown and annealed SnO 2 /TNTs with different Pd loads and the values are plotted in Figure S5. It is found that the ECSA increases with N Pd and drops after reaching a maximum. Such evolution is measured for both as-grown and annealed samples but the active area is significantly lower after annealing. This is despite the lower particle size (ca. 2´ lower) and the higher number of particles (ca. 2´ higher), observed for all samples after annealing. [START_REF] Ruiz-Camacho | Platinum Deposited on TiO 2 -C and SnO 2 -C Composites for Methanol Oxidation and Oxygen Reduction[END_REF] This trend is rather unusual, because one would expect the higher number of active surface atoms for the smaller particles in comparison to the larger ones. Furthermore, Sun et al. showed that the crystalline SnO 2, after annealing increased the ECSA if compared to amorphous SnO 2 in as-grown sample, however no morphological difference was found on Pt particles. [START_REF] Chen | Pt-SnO 2 /Nitrogen-Doped CNT Hybrid Catalysts for Proton-Exchange Membrane Fuel Cells (PEMFC): Effects of Crystalline and Amorphous Sno2 By Atomic Layer Deposition[END_REF] In the present case, the evolution of the ECSA seems to be solely related to geometric considerations. Thus the expansion of the ECSA with N Pd can be attributed to the enlargement of the Pd particles (i. e. the rise of their geometric area) while the sudden decrease can be ascribed to their coalescence. The observed decrease of ECSA after annealing could be related to the interaction between SnO 2 and Pd nanoparticles leading to the partial coverage of Pd surface by oxygenated species from SnO 2 . When Pd is supported on SnO 2 the electronic type MSI may raise at the interface of the two solids due to the charge transfer from the support to Pd (from the solid with lower work function (WF) to the solid with higher WF) and/or the higher availability of hydroxyl ions from SnO 2 that adsorb on the catalyst surface leading to lower ECSA, however these oxygenated species could have a promotional effect on ethanol electrooxidation. [START_REF] Monyoncho | Role of the Metal-Oxide Support in the Catalytic Activity of Pd Nanoparticles for Ethanol Electrooxidation in Alkaline Media[END_REF] Figure 10 shows the CVs of as-grown and annealed SnO 2 /TNT with 500 ALD cycles Pd in 1 M KOH + 1 M C 2 H 5 OH. The current density is higher for Pd deposited on the annealed support in the whole range of the anodic potential. The current density increases until it reaches a maximum at 0.251 and 0.275 V for as-grown and the annealed samples, respectively. The decrease of the current density is due to the surface palladium oxide formation, [START_REF] Liang | Mechanism Study of the Ethanol Oxidation Reaction on Palladium in Alkaline Media[END_REF] which is not active for ethanol electrooxidation. On the reverse scan, the Pd activity is recovered at -0.19 and -0.24 V for asgrown and the annealed samples, respectively, due to PdO x reduction to metallic Pd. The current density is much higher for Pd grown on annealed SnO 2 /TNT and the onset potential is shifted negatively, -0.5 and -0.6 V for Pd deposited on as-grown and annealed supports, respectively. The increasing of the catalytic activity can be attributed to several factors: the increased conductivity of both oxides (SnO 2 /TiO 2 ) after annealing and MSI effect. [START_REF] Ruiz-Camacho | Platinum Deposited on TiO 2 -C and SnO 2 -C Composites for Methanol Oxidation and Oxygen Reduction[END_REF] Chen et al. have demonstrated an easier CO oxidation on crystalline Pt/SnO 2 , suggesting a stronger interaction between Pt and crystalline SnO 2 than between Pt and amorphous SnO 2 support. [START_REF] Chen | Pt-SnO 2 /Nitrogen-Doped CNT Hybrid Catalysts for Proton-Exchange Membrane Fuel Cells (PEMFC): Effects of Crystalline and Amorphous Sno2 By Atomic Layer Deposition[END_REF] As shown previously, [START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF] the Pd particles deposited on annealed TNTs support exhibited a higher catalytic activity for ethanol electrooxidation reaction (as seen thereinafter, in Figure 13a, the maximum current ratio is higher than 4).

Ethanol electrooxidation on annealed Pd/SnO 2 /TNT : effect of the Pd charge -Figure 11a shows the linear sweep voltammetry (LSV) of the annealed SnO 2 /TNT with different amount of Pd (N Pd = 300, 400, 500, 600, 800 cycles) in the ethanol containing electrolyte. The current density for ethanol electrooxidation increased with the number of ALD cycles up to 500 cycles and then the decrease occurred. This is illustrated in Figure 11b where the current density at -0.2 V is plotted against the N Pd . This evolution indicates that from 300 to 500 ALD cycles the amount of available active sites for ethanol electrooxidation increases with the number of ALD cycles while at higher number of cycles, the increase of the particles size and the particle coalescence reduce the catalytic activity. Moreover, the MSI effect between Pd and SnO 2 may weaken as the particles become larger thus contributing to the decrease in the rate of electrooxidation. [START_REF] Wieckowski | Particle Size, Support and Promotional Effects[END_REF] A similar behaviour was reported in a previous work, where the catalytic activity of 500 cycles of Pd deposited on TNTs was the highest. Chronoamperograms at -0.2 V for various loading of Pd interfaced with annealed SnO 2 /TNT are presented on the Figure 12. The initial current density and the order of activities are in agreement with LSV results. For the samples with 300 and 800 cycles of Pd, the current density is low at the beginning 0.23 and 0.07 mA cm -2 , respectively. Then it decreases quickly until it reaches a steady-state at 0.16 and 0.05 mA cm -2 , respectively. For the samples with 400, 500, 600 cycles the current density is higher at the beginning 0.35, 0.40 and 0.27 mA cm -2 , respectively and continuously decreases without reaching a steady state after 1 h.

The current percentage loss (without double layer correction) is 32, 50, 55, 45 and 34% for 300, 400, 500, 600 and 800 cycles of Pd, respectively. The current decrease is attributed to Pd oxide and oxopalladate formation as found from XPS analysis on the catalyst after electrochemical measurements (Fig. 8).

Ethanol electrooxidation: effect of the chemical nature and the crystalline structure of the support -Figure 13a compares the effect of the chemical nature of the support and the annealing for 500 cycles Pd/SnO 2 /TNT catalyst. In order to compare the present results with literature, the Pd load (i. e., the mass of Pd, m Pd , per area unit) has been estimated from the ECSA and the d Pd . The LSVs of Pd deposited on as-grown TNTs, annealed TNTs, as-grown SnO 2 /TNTs and annealed SnO 2 /TNTs show that annealing and addition of SnO 2 interlayer enhance the ethanol electrooxidation. The positive effect of the support annealing has already been shown on bare TNTs [START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF] and is further confirmed in this work for SnO 2 -covered TNTs. Since the morphology of the structured support is not significantly modified after the deposition of the thin SnO 2 layer (N SnO2 = 75 cycles), the evolution of the electrochemical response can not be attributed to geometric considerations. The LSVs of the as-grown TNTs and SnO 2 /TNTs catalysts with 500 cycles Pd clearly show that addition of SnO 2 interlayer increases the current density of ethanol electrooxidation. The current density is significantly higher for Pd on SnO 2 /TNTs than for Pd/TNTs and the onset potential is shifted negatively from -0.26 to -0.45 V for as-grown TNTs and SnO 2 /TNTs, respectively. This is in line with previous reports for Pt and Pd catalysts where SnO 2 showed strong contribution towards the improved catalytic activity of the metal. [START_REF] Mao | Carbon-Supported PdSn-SnO 2 Catalyst for Ethanol Electro-Oxidation in Alkaline Media[END_REF][START_REF] Takabatake | Cycle Durability of Metal Oxide Supports for PEFC Electrocatalysts[END_REF] For instance, Ruiz et al. [START_REF] Ruiz-Camacho | Platinum Deposited on TiO 2 -C and SnO 2 -C Composites for Methanol Oxidation and Oxygen Reduction[END_REF] have shown that Pt-SnO 2 -C has higher activity for methanol electrooxidation as compared to Pt-TiO 2 -C.

The high catalytic activity was attributed to the smaller Pt particle size obtained on SnO 2 -C support.

Moreover, Kim et al. have proposed a mechanism in which SnO 2 facilitates the oxidative removal of the intermediates residues on Pt for alcohol electrooxidation. [START_REF] Kim | Influence of Sn Content On PtSn/C Catalysts for Electrooxidation Of C1-C3 Alcohols: Synthesis, Characterization, and Electrocatalytic Activity[END_REF] The addition of Sn species on Pt modifies its electronic environment by influencing the electron affinities of the surface Pt atoms and produces surface oxygenated species which can act as an oxidant source, thereby increasing stability through efficient oxidation of surface poisoning species. An et al. have reported that the catalytic activity of Pd has been enhanced by SnO 2 addition on TiO 2 -MWCT for ethanol and methanol electrooxidation. [START_REF] An | Synthesis and Performance of Palladium-Based Catalysts for Methanol and Ethanol Oxidation in Alkaline Fuel Cells[END_REF] This improvement is due to better particles dispersion and improvement of the resistance level to CO poisoning for methanol electrooxidation due to the increase of the concentration of OH ads species on the Pd surface, which promotes the oxidation of acetaldehyde thus releasing active sites for the reaction. [START_REF] Du | SnO 2 Nanocrystals on Self-Organized TiO 2 Nanotube Array as Three-Dimensional Electrode for Lithium Ion Microbatteries[END_REF] This effect is also visible by comparing annealed SnO 2 /TNTs with annealed TNTs. The current density is further increased for annealed SnO 2 /TNT support. Furthermore, Figure 13b shows a comparison of the current density obtained for 500 cycles of Pd on different supports at -0.2 V. As can be seen, the highest current density is obtained for annealed SnO 2 /TNT with 500 cycles of Pd due to the increased conductivity of both oxides (SnO 2 /TiO 2 ) after annealing, and to the metalsupport interaction between Pd and SnO 2 . The size of Pd clusters is smaller when grown on annealed SnO 2 (for N Pd » 600 cycles, d Pd » 10 nm and d Pd » 6 nm for as-grown and annealed SnO 2 /TiO 2, respectively). The smaller Pd particles may result in the stronger MSI effect. The MSI effect occurs at the interface of the two solids with different work function (WF): 5.2-5.6 and 4.6 eV for Pd and SnO 2 , respectively. [START_REF] Skriver | Ab Initio Work Function of Elemental Metals[END_REF] The difference in WF will result in the charge transfer from the solid with lower WF to the solid with the higher one. The modification of electronic properties of Pd due to the charge transfer from the support and higher availability of oxygenated species or hydroxyl ions (OH -) from SnO 2 to Pd, results in the higher catalytic activity of Pd/SnO 2 /TNT. Figure 14 compares of the activity of the best result of the present study (i. e. Pd/SnO 2 /TNTs, N Pd = 500 cycles after annealing) with similar systems reported in literature. The Pd particles are grown by various methods (electrochemical deposition, electrochemical deposition and milling, chemical deposition or ALD) on TNTs, carbon black, or multiwall CNTs. In some cases, the substrates are covered by SnO 2 . Note that the experimental conditions (i. e. electrolyte composition and scanning rate, see Figure S7) are not exactly identical in all cases but sufficiently closed to be compared. Although the conditions are not advantageous for the present study, the measured activity is very closed to the best reported in literature. [START_REF] Chen | Electrochemical Milling and Faceting: Size Reduction and Catalytic Activation of Palladium Nanoparticles[END_REF] The current density as well as the potential of the forward peak are better than on bare TNTs. [START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF] The comparison also suggests that SnO

  Figure 7d. The pattern is composed by numerous spots and rings. The diffuse rings are attributed to a small amount of SnO 2 , the presence of intense spots on the rings can be attributed to larger crystallite size. The other spots are attributed to the Pd and the anatase phase. The indexation of the different phases is presented in Figure S6. All the main diffraction planes are visible for the three phases and no preferential orientation is noticed.

2

 2 seems to be a more efficient substrate to promote the electroactivity of Pd nanoparticles toward ethanol oxidation.CONCLUSIONIn this work, it has been shown that ALD was successfully used to functionalize TNTs with SnO 2 layer and Pd nanoparticles for ethanol electrooxidation. The depositions of both SnO 2 and Pd, are fully controlled and perfectly conformal onto the TNTs. The ALD of Pd on SnO 2-covered TNTs is facilitated because the top surface is reduced to metallic tin during the deposition of the particles. This synthesis method allows comparing the catalytic activity of Pd nanoparticles toward ethanol electrooxidation on different supports with no significant modification of the three-dimensional geometry. First, the modification of the chemical nature of the support has been investigated by the ALD of SnO 2 as an interlayer in between TNTs and Pd. The comparison between SnO 2 and TiO 2 surface has revealed that the SnO 2 has a promotional effect on Pd catalytic activity for ethanol electrooxidation. The positive impact of the addition of Sn species on Pd catalyst is well known and it is confirmed in this study. This fact can be attributed to several effects (i) the SnO 2 layer can facilitate the OH adsorption, (ii) the SnO 2 extreme surface is reduced to metallic Sn during ALD of Pd which favour the Pd growth and thereby the adsorption and the oxidation of ethanol are accelerated and (iii) MSI between Pd and SnO 2 . Second, the influence of the crystalline structure of the support has been studied. The annealing of the support improves the electrocatalytic properties of Pd. According to this result the promoting effect can be attributed to several aspects: (i) the increased conductivity of the support by the annealing and (ii) MSI effect. The effect of the charge of Pd had also been investigated by varying the number of Pd ALD cycles. The best catalytic activity has been obtained for 500 ALD cycles on annealed SnO 2 /TNTs. The electrochemical characterizations on such Pd/SnO 2 /TNTs systems have evidenced a high current density and low onset potential compared to literature and commercial catalyst. In future studies, it would be interesting to investigate the catalytic properties of Pd deposited on SnO 2 nanotubes without TiO 2 and to investigate the potential effect of the deposition of a thin protective layer to stabilize the Pd activity for long term use. A recent study has shown that ALD of SnO 2 can be used on nanoparticles to enhance Pt catalytic durability for fuel cell application.[START_REF] Marichy | ALD SnO 2 Protective Decoration Enhances the Durability of a Pt Based Electrocatalyst[END_REF]

Figure 1 :

 1 Figure 1: Schematic view of the process. (a) Anodic growth of the TiO 2 nanotubes, (b) ALD of SnO 2 and (c) ALD of Pd.

Figure 2 :

 2 Figure 2: SEM top views of TiO 2 nanotubes covered by a thin SnO 2 layer. The number of ALD cycles (N SnO2 ) is increasing from 0 (a), 20 (b), 50 (c), 100 (d) to 150 cycles (e).

Figure 3 :

 3 Figure 3: TEM cross sections of the opening (a) and the bottom (b) of TNTs after ALD of SnO 2 (a). The scale bar in (b) corresponds to 20 nm.

Figure 4 :Figure 5 :Figure 6 :

 456 Figure 4: X-ray diffractograms of SnO 2 -coated TNTs before (bottom) and after (top) annealing at 450°C

Figure 7 :

 7 Figure 7: TEM images of SnO 2 /TNTs annealed after ALD of Pd (N Pd = 500 cycles): (a) cross section of a single nanotube, (b) top view of several nanotubes, (c) highmagnification of Pd particles on a tube wall and (d) SAED corresponding to the area shown in (b). The various materials are artificially colored in (c) for seek of clarity and the three red lines highlight the Pd crystal rows.

Figure 8 :

 8 Figure 8: Comparison of the chemical composition of the surface before (a,b,c) and after (a´,b´,c´) electrochemical investigations performed on annealed Pd/SnO 2 /TNTs. XPS survey (a,a´), Pd 3d peak (b,b´) and Sn 3d peak (c,c´).

Figure 9 :Figure 10 :Figure 11 :Figure 12 :C 2 H 5 Figure 13 :

 91011122513 Figure 9: CVs of as-grown SnO 2 /TNTs after ALD of Pd for N Pd = 0, 100, 300, 500 and 800 cycles. The electrolyte is 1 M KOH and the scan rate is 25 mV s -1 .

Figure 14 : 75 -

 1475 Figure 14: Comparison of the forward ethanol electrooxidation peak current and potential on Pd catalysts deposited on various types of substrates. The CVs areperformed in alkaline media containing ethanol with a scanning rate ranging from 5 to 50 mV s -1 (see detailed information in FigureS7). Pd/TNT a ,[START_REF] Qin | TiO 2 Nanotube Arrays Supported Pd Nanoparticles forr Ethanol Electrooxidation in Alkaline Media[END_REF] Pd/TNT b ,[START_REF] Chen | Electrochemical Milling and Faceting: Size Reduction and Catalytic Activation of Palladium Nanoparticles[END_REF] Pd/TNT c ,[START_REF] Assaud | Atomic Layer Deposition of Pd Nanoparticles on TiO 2 nanotubes for Ethanol Electrooxidation: Synthesis and Electrochemical Properties[END_REF] Pd/C commercial d ,[START_REF] Rikkinen | Atomic Layer Deposition Preparation of Pd Nanoparticles on a Porous Carbon Support for Alcohol Oxidation[END_REF] Pd/C ALD d ,[START_REF] Rikkinen | Atomic Layer Deposition Preparation of Pd Nanoparticles on a Porous Carbon Support for Alcohol Oxidation[END_REF] Pd/C polyol method e ,[START_REF] Monyoncho | Ethanol Electro-oxidation on Palladium Revisited Using Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS) and Density Functional Theory (DFT): Why Is It Difficult To Break the C-C Bond?[END_REF] Pd/MWCNT f , Pd/SnO 2 /MWCNT f , Pd/TiO 2 /MWCNT f , Pd/TiO 2 +SnO 2 /MWCNT f .[START_REF] An | Synthesis and Performance of Pd/SnO 2 -TiO 2 /MWCNT Catalysts for Direct Formic Acid Fuel Cell Application[END_REF]

ACKNOWLEDGEMENTS

The authors acknowledge V. Heresanu (CINaM) for XRD measurements, D. Chaudanson and S. Nitsche (CINaM) for their valuable help with the electron microscopy. A. Baronnet (CINaM) is also acknowledged for fruitful discussions. R. Sinha Roy and Y Magnin are acknowledged (CINaM) for helping in graphics editing. This collaborative work was supported by the France-Canada Research Fund. M. K. S. Barr and L. Assaud are indebted to the Conseil Régional Provence-Alpes-Côte d'Azur and the CNRS for the PhD grants.

AUTHOR INFORMATION

Corresponding Authors

*To whom correspondence should be addressed: elena.baranova@uottawa.ca; lionel.santinacci@univ-amu.fr