Samuel Ahoulou

Neus Vilà

Sébastien Pillet

Dominik Schaniel

Alain Walcarius
email: alain.walcarius@univ-lorraine.fr

Coordination polymers as template for mesoporous silica films: a novel composite material Fe(Htrz) 3 @SiO 2 with remarkable electrochemical properties

Keywords: spin crossover complex, mesoporous silica film, nanocomposite, Fe(Htrz)3, electrocatalysis

Introduction

One of the key scientific questions for chemists of the 21 st century is "How far can we push chemical selfassembly?". [START_REF] Service | How Far Can We Push Chemical Self-Assembly[END_REF] Inspired from Nature, molecular self-assembly is ubiquitous in designing complex biological systems and it has emerged in the past decades as an elegant bottom-up method for fabricating nanostructured materials. [START_REF] Whitesides | Molecular Self-Assembly and Nanochemistry: a Chemical Strategy for the Synthesis of Nanostructures[END_REF][START_REF] Lin | Self-Directed Self-Assembly of Nanoparticle/Copolymer Mixtures[END_REF] Self-assembly usually refers to the spontaneous organization of molecular building blocks or nano-objects owing to noncovalent interactions, [START_REF] Whitesides | Self-Assembly at All Scales[END_REF] giving rise to structurally welldefined ensembles. [START_REF] Whitesides | Molecular Self-Assembly and Nanochemistry: a Chemical Strategy for the Synthesis of Nanostructures[END_REF] Since the first report on ordered mesoporous silicates generated by a surfactant directed cooperative self-assembly approach, [START_REF] Kresge | Ordered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Template Mechanism[END_REF] the preparation of nanostructured inorganic solids with organic templates has become a platform for new generations of functional hybrid materials. [START_REF] Wan | On the Controllable Soft-Templating Approach to Mesoporous Silicates[END_REF][START_REF] Hoffmann | Silica-Based Mesoporous Organic-Inorganic Hybrid Materials[END_REF][START_REF] Farrell | Self-Assembled Templates for the Generation of Arrays of 1-Dimensional Nanostructures: From Molecules to Devices[END_REF] Beside the widely used ionic or nonionic surfactants and amphiphilic block copolymers extensively used as templates to fabricate well-defined nanomaterials with controlled structural dimensions and shape, [START_REF] Wan | On the Controllable Soft-Templating Approach to Mesoporous Silicates[END_REF][START_REF] Di Renzo | Handbook of Porous Solids[END_REF][START_REF] Soler-Illia | Block Copolymer-Templated Mesoporous Oxides[END_REF][START_REF] Han | Anionic Surfactant Templated Mesoporous Silicas (AMSs)[END_REF][START_REF] Wu | Synthesis of Mesoporous Silica Nanoparticles[END_REF] other types of structure-directing agents have appeared in view of extending the characteristics of mesoporous silica-based materials, thus widening the scope of their application. For example, one can cite surfactant/swelling-agent micellar templates for getting ultra-large pores, [START_REF] Kruk | Access to Ultralarge-Pore Ordered Mesoporous Materials through Selection of Surfactant/Swelling-Agent Micellar Templates[END_REF] the combination of soft and hard templates or the use of solid lipid nanoparticles for the elaboration of hierarchical porous structures, [START_REF] Stein | Perspective on the Influence of Interactions Between Hard and Soft Templates and Precursors on Morphology of Hierarchically Structured Porous Materials[END_REF][START_REF] Kim | Solid Lipid Nanoparticle -Functional Template of Meso-Macrostructured Silica Materials[END_REF] the use of ionic liquids to form mesoporous ionogels, [START_REF] Viau | Ionic Liquid Mediated Sol-Gel Synthesis in the Presence of Water or Formic Acid: Which Synthesis for Which Material?[END_REF] or the resort to functionalized surfactants which may lead to functional hybrid materials in one step. [START_REF] Li | Functional-Template Directed Self-Assembly (FTDSA) of Mesostructured Organic-Inorganic Hybrid Materials[END_REF] From that last point of view, even if organically functionalized mesoporous silica can be obtained from either post-synthesis grafting or co-condensation using appropriate organosilane reagents or as periodic mesoporous organosilica, [START_REF] Hoffmann | Silica-Based Mesoporous Organic-Inorganic Hybrid Materials[END_REF][START_REF] Yao | Periodic Mesoporous Organosilicas[END_REF] a straightforward way to functional mesoporous materials would be via the use of non-surfactant molecular templates that would fill the mesopores while acting as porogen during the formation of the silica network. [START_REF] Nukatsuka | Spontaneous Formation of Mesoporous Silica Films Using Non-Surfactant Template, and Optimization with Doehlert Designs, for Adsorbent for Polycyclic Aromatic Hydrocarbons[END_REF][START_REF] Wang | Columnar Liquid Crystals Self-Assembled by Minimalistic Peptides for Chiral Sensing and Synthesis of Ordered Mesoporous Silica[END_REF] The spontaneous formation of ordered mesoporous silica using non-surfactant templates remains however highly challenging, especially if wishing to prepare a silica-based materials containing the molecule alone (i.e., without another porogen), as sol-gel processing in the presence of non-templating organic additives usually leads to their molecular dispersion in the silica matrix, resulting in disordered mesoporous materials. [START_REF] Wang | Unified Formation Mechanism of Disordered Mesoporous Silica, Structured by Means of Nontemplating Organic Additives[END_REF] An exception is the particular case of cyclodextrins, which are known to self-assemble and form aggregates in aqueous solutions, [START_REF] Ryzhakov | Self-Assembly of Cyclodextrins and Their Complexes in Aqueous Solutions[END_REF] enabling an ordered aggregation with silica (mostly through wormlike mesopore systems [START_REF] Polarz | From Cyclodextrin Assemblies to Porous Materials by Silica Templating[END_REF] ) to form functional nanostructures. [START_REF] Bleta | Cyclodextrin-Based Supramolecular Assemblies: a Versatile Toolbox for the Preparation of Functional Porous Materials[END_REF] Though not belonging to the surfactant family, it should be emphasized that such cyclic oligosaccharides can be considered as amphiphilic molecules as cyclodextrins are characterized by a hydrophobic interior and a hydrophilic exterior, [START_REF] Saenger | Structures of the Common Cyclodextrins and Their Larger Analogues -Beyond the Doughnut[END_REF] making even more scarce the mesoporous silica-based materials formed from non-surfactant/amphiphilic templates.

Here we would like to evaluate the possible incorporation of a spin crossover complex, Fe(Htrz)3 (Htrz = 1,2,4,-1H-triazole), into a mesoporous silica matrix during its formation. Iron-based spin crossover complexes have received considerable attention due to their numerous potential applications in optical, electronic, catalysis and/or sensing. [START_REF]Spin Crossover in Transition Metal Compounds I-III[END_REF][START_REF] Halcrow | Spin-Crossover Materials: Properties and Applications[END_REF] However, the design of spin-crossover based devices requires an appropriate processing of the materials at the nanoscale in the form of supported nanostructures, nanoparticles dispersed in soft media, nanocomposites, molecular thin films. [START_REF] Kumar | Emerging Trends in Spin Crossover (SCO) Based Functional Materials and Devices[END_REF][START_REF] Molnar | Spin Crossover Nanomaterials: From Fundamental Concepts to Devices[END_REF] Within the whole spincrossover family of molecular compounds, the Fe(II)-triazole coordination polymers of the general formula [Fe(Rtrz)3]X2 (Rtrz is a functionalized 4-R-1,2,4,-1H-triazole derivative, and X is a counter ion) certainly present the most appealing optical, magnetic, and electronic characteristics for technological applications (i.e. close to room temperature bistability, large hysteresis, ease of chemical preparation and chemical tunability). [START_REF] Roubeau | Triazole-Based One-Dimensional Spin-Crossover Coordination Polymers[END_REF] Triazole derivatives are known to coordinate with iron(II) in solution to form coordination polymers, where the iron(II) centers can adopt two different spin states, that is, the low spin (LS) and high-spin (HS) states that are interconvertible upon external stimuli (temperature, pressure, light-irradiation). As recently proved by X-ray crystallography [START_REF] Grosjean | The 1D Polymeric Structure of the [Fe(NH2trz)3](NO3)2.nH2O (with n=2) Spin Crossover Compound Proven by Single Crystal Investigations[END_REF] , Fe(II)-triazole systems self-assemble as infinite chains (or finite polynuclear Fe(II) oligomers, e.g. dimers or trimers) [START_REF] Bräunlich | Polynuclear Iron(II)-Aminotriazole Spincrossover Complexes (Polymers) In Solution[END_REF] of Fe(II) cations triply bridged by triazole ligands (scheme 1); this structural connectivity also takes place in solution and metallogels to some extent. [START_REF] Bräunlich | Polynuclear Iron(II)-Aminotriazole Spincrossover Complexes (Polymers) In Solution[END_REF][START_REF] Echeverria | Structure of a Spin-Crossover Fe(II)-1,2,4-Triazole Polymer Complex Gel in Toluene. Small Angel Neutron Scattering and Viscoelastic Studies[END_REF] The ability to isolate and maintain such linear complexes in appropriate media or nanocomposite would provide interesting functional spin crossover molecular wires. Several strategies based on self-organized molecular assembly have been proposed, such as functionalizing the triazole ligand with long alkyl chain or large lipophilic substituents, or using lipophilic counter-ions to generate metallogels or liquid crystals. [START_REF] Roubeau | Thermoreversible Gels as Magneto-Optical Switches[END_REF][START_REF] Gaspar | Spin Crossover in Soft Matter[END_REF][START_REF] Matsukizono | Lipid-Packaged Linear Iron(II) Triazole Complexes in Solution: Controlled Spin Conversion via Solvophobic Self-Assembly[END_REF][START_REF] Kuroiwa | Self-Assembly and Functionalization of lipophilic Metal-Triazole Complexes in Various Media[END_REF] These exhibit magnetic and optical properties (and even thermal hysteretic behavior and photofunctionality) [START_REF] Kume | Photoresponsive molecular wires of Fe(II) Triazole Complexes in Organic Media and Light-Induced Morphological Transformations[END_REF] originating from the parent spin-crossover precursors. Alternatively, nanoparticles (or at least coordination clusters) of Fe(II)-triazole complexes have been incorporated into mesoporous silica-based matrices to obtain monolith or thin film nanocomposites. [START_REF] Voisin | Preserving the Spin Transition Properties of Iron-Triazole Coordination Polymers within Silica-Based Nanocomposites[END_REF][40][START_REF] Faulmann | A Facile Route for the Preparation of Nanoparticles of the Spin-Crossover Complex [Fe(Htrz)2(trz)](BF4) in Xerogel Transparent Composite Film[END_REF][START_REF] Durand | Room Temperature Bistability with Wide Thermal Hysteresis in a Spin Crossover Silica Nanocomposite[END_REF] However, while the porosity in terms of pore size distribution and pore surface functionalization can be tuned and controlled, these nanocomposites do not present any ordering or adjustable spatial organization of the confined spin crossover entities. Mesoporous materials can be manufactured in several morphologies, but the thin film one offers many advantages in view of insertion into devices for target applications. [START_REF] Innocenzi | Mesoporous Thin Films: Properties and Applications[END_REF] Another important feature is the control of mesopores orientation over a macroscopic scale in order to get well-defined positions of guest molecules and highly controlled molecular events in the channels. [START_REF] Cho | Macroscopically Oriented Porous Materials with Periodic Ordered Structures: From Zeolites and Metal-Organic Frameworks to Liquid-Crystal-Templated Mesoporous Materials[END_REF] Ordered mesoporous silica thin films are usually prepared by evaporation-induced self-assembly (enabling to tune the final mesostructure by an appropriate control of the experimental conditions used [START_REF] Grosso | Fundamentals of Mesostructuring Through Evaporation-Induced Self-Assembly[END_REF] ), but getting mesochannels orthogonally oriented onto the underlying support has required the development of more specific methods. [START_REF] Walcarius | Electrochemically Assisted Self-Assembly of Mesoporous Silica Thin Films[END_REF][START_REF] Teng | Highly Ordered Mesoporous Silica Films with Perpendicular Mesochannels by a Simple Stöber-Solution Growth Approach[END_REF][START_REF] Kao | A General Method for Growing Large Area Mesoporous Silica Thin Films on Flat Substrates with Perpendicular Nanochannels[END_REF] Among them, the electrochemically assisted self-assembly (EASA) approach has proven to be a simple, versatile and fast way to generate vertically aligned mesoporous silica membranes onto electrode surfaces. [START_REF] Walcarius | Electrochemically Assisted Self-Assembly of Mesoporous Silica Thin Films[END_REF][START_REF] Goux | Oriented Mesoporous Silica Films Obtained by Electro-Assisted Self-Assembly[END_REF] Such oriented hexagonally packed mesopores offer an ideal configuration for applications (e.g., in the sensors field, [START_REF] Walcarius | Mesoporous Materials-Based Electrochemical Sensors[END_REF][START_REF] Yan | Vertically Ordered Silica Mesochannel Films: Electrochemistry and Analytical Applications[END_REF] where their vertical alignment ensures fast diffusion processes and high sensitivity [START_REF] Zhou | Two Orders-of-Magnitude Enhancement in the Electrochemiluminescence of Ru(bpy)₃²⁺ by Ver cally Ordered Silica Mesochannels[END_REF][START_REF] Nasir | Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat[END_REF] ), as otherwise reported for other types of oriented films (based on mesoporous metal [START_REF] Li | Electrochemical Deposition: An Advanced Approach for Templated Synthesis of Nanoporous Metal Architectures[END_REF][START_REF] Li | Surfactant-Directed Synthesis of Masoporous Pd Films with Perpendicular Mesochannels as Efficient Electrocatalysts[END_REF][START_REF] Wu | New Trend on Mesoporous Films: Precise Controls of One-Dimensional (1D) Mesochannels Toward Innovative Applications[END_REF] or metal oxides [START_REF] Wu | New Trend on Mesoporous Films: Precise Controls of One-Dimensional (1D) Mesochannels Toward Innovative Applications[END_REF][START_REF] Weng | A High-Speed Passive-Matrix Electrochromic Display Using a Mesoporous TiO2 Electrode with Vertical Porosity[END_REF][START_REF] Oveisi | A Mesoporous γ-Alumina Film with Vertical Mesoporosity: The Unusual Conversion from a Im3m Mesostructure to Vertically Oriented γ-Alumina Nanowires[END_REF] ). They exhibit unprecedented molecular sieving properties for an amorphous film, [START_REF] Vilà | Molecular Sieving with Vertically-Aligned Mesoporous Silica Films and Electronic Wiring Through Isolating Nanochannels[END_REF] along with charge selectivity features, [START_REF] Lin | Gated Molecular Transport in Highly Ordered Heterogeneous Nanochannel Array Electrode[END_REF][START_REF] Karman | Amplified Charge Transfer for Anionic Redox Probes Through Oriented Mesoporous Silica Thin Films[END_REF] making them very promising for use as permselective or antifouling membranes, for instance. [START_REF] Serrano | Mesoporous Silica Thin Films for Molecular Sieving and Electrode Surface Protection Against Biofouling[END_REF][START_REF] Yang | Molecular Filtration by Ultrathin and Highly Porous Silica Nanochannel Membranes: Permeability and Selectivity[END_REF][START_REF] Zhou | In Vivo Monitoring of Oxygen in Rat Brain by Carbon Fiber Microelectrode Modified with Anti-fouling Nanoporous Membrane[END_REF] They can be obtained in organically-functionalized forms by using organosilane reagents and/or exploiting multi-step click chemistry approaches, [START_REF] Etienne | Oriented Mesoporous Organosilica Films on Electrode: a New Class of Nanomaterials for Sensing[END_REF][START_REF] Vilà | Electrochemically Assisted Generation of Highly Ordered Azide-Functionalized Mesoporous Silica for Oriented Hybrid Films[END_REF][START_REF] Vilà | Clickable Bifunctional and Vertically-Aligned Mesoporous Silica Films[END_REF] but to date no attempts to incorporate organo-functional moieties as "simple molecular additives" to the synthesis medium for possible incorporation into such oriented mesoporous films have been made. Otherwise, sol-gel electrodeposition has been applied to the entrapment of large compounds such as redox proteins or enzymes within silica films, [START_REF] Nadzhafova | Direct Electrochemistry of Hemoglobin and Glucose Oxidase in Electrodeposited Sol-Gel Silica Thin Films on Glassy Carbon[END_REF][START_REF] Wang | Electrochemically Assisted Deposition of Sol-Gel Bio-Composite with Co-Immobilized Dehydrogenase and Diaphorase[END_REF] but the resulting biocomposite coatings were not ordered at all as these biomolecules are not expected to be involved in self-assembly and mesoporous silica templating.

The aim of the present work is thus to investigate the use of the spin crossover Fe(Htrz)3 complex (scheme 1) as additive and/or template for the preparation of mesoporous silica thin films by EASA, the ultimate objective being to evidence a real template role of such molecular species in designing ordered composite films containing only the Fe(Htrz)3 complex in an stable/active form. By varying the amount of Fe(Htrz)3 in the starting sol solution (in addition to TEOS and CTAB), we will show that low concentrations (≤ 3 mM) ensure to keep a vertically aligned mesostructure with Fe(Htrz)3 complexes co-incorporated with the surfactant species, whereas larger contents (5 mM) lead to wormlike mesoporous films filled with Fe(Htrz)3 species that are durably immobilized as a real template in the silica matrix. The film composition and structure are confirmed by XPS, FTIR and TEM. Their electrochemical properties are characterized by cyclic voltammetry, showing remarkable operational stability at high potential scan rates for the wormlike Fe(Htrz)3-templated films, which can be exploited in electrocatalysis, for instance.

Experimental Section

Chemicals and reagents

Tetraethoxysilane (TEOS, 98%, Alfa Aesar), ethanol (95-96%, Merck), HCl (37% Riedel de Haen), NaNO3 (99%, Fluka), cetyltrimethylammonium bromide (CTAB, 99%, Acros), have been used as received. The Fe(Htrz)3 complex was prepared from Fe(BF4)2 and 1,2,4,1H-triazole as previously described. [START_REF] Durand | Room Temperature Bistability with Wide Thermal Hysteresis in a Spin Crossover Silica Nanocomposite[END_REF] Indium-tin oxide (ITO) plates (surface resistivity 8-12 Ω) were purchased from Delta Technologies. Analytical grade ferrocene dimethanol (FcMeOH, Alfa Aesar) was used as redox probes for electrochemical monitoring of mass transport through the films. H2O2 (30%, WWR Chemicals) was used for electrocatalytic studies.

Preparation of the films

The mesoporous silica thin films have been electrochemically generated on ITO electrodes according to the previously reported EASA procedure, [START_REF] Walcarius | Electrochemically Assisted Self-Assembly of Mesoporous Silica Thin Films[END_REF][START_REF] Goux | Oriented Mesoporous Silica Films Obtained by Electro-Assisted Self-Assembly[END_REF] which was adapted for incorporating the Fe(Htrz)3 complex.

The starting sol was a hydroalcoholic solution (10 mL H2O + 10 mL ethanol) containing 100 mM of the silica precursor (TEOS), 32 mM of CTAB as template, 0.1 M NaNO3 as electrolyte, and variable concentrations of the Fe(Htrz)3 complex (from 0 to 5 mM). The pH was adjusted to 3 by adding 0.1 M HCl. After a first step of hydrolysis for 2.5h, a cathodic potential of -1.3 V was applied for 20 s to the ITO working electrode (optimized values [START_REF] Goux | Oriented Mesoporous Silica Films Obtained by Electro-Assisted Self-Assembly[END_REF] ), leading to the growth of the mesoporous silica or the Fe(Htrz)3silica nanocomposite layers. The electrode surface was then thoroughly rinsed with water and aged overnight at 130°C. Extraction of the CTAB was performed by immersing the film electrode in an ethanol solution containing 0.1 M HCl for 15 min.

Apparatus

All the electrochemical experiments were performed using a μAutoLab III potentiostat (Eco Chemie) monitored by the GPES software. A three electrode configuration was employed for all measurements, including the electrogeneration, evaluation of the permeability properties, and characterization of the Fe(Htrz)-functionalized silica-based mesoporous films. The voltammetric curves were recorded using a platinum rod as counter electrode, a Ag/AgCl reference electrode (Metrohm) and ITO plates as working electrodes where the hybrid films were deposited. The electrochemically-assisted deposition was carried out using a home-made electrochemical cell with a configuration allowing placing the working electrode at the bottom (see Scheme S1 in Supporting Information). A stainless steel counter electrode and AgClcoated silver wire pseudo-reference electrode completed the electrochemical setup.

Mesostructure, morphology, and thickness of the films have been analyzed by transmission electron microscopy (TEM) using a ACCEL ARM 200F microscope at an acceleration voltage of 200 kV. X-ray Photoelectron Spectroscopy (XPS) characterizations were performed using a KRATOS Axis Ultra X-ray photoelectron spectrometer (Kratos Analytical, Manchester, UK) equipped with a monochromated AlKα X-ray source (hν = 1486.6 eV) operated at 150 W. Fourier transform infrared spectroscopy (FTIR) measurements were performed using a Nicolet 8700 apparatus equipped with a diffuse reflectance accessory (Smart Collector).

Results and discussion

Preliminary observations

Figure 1 shows typical cyclic voltammograms recorded for mesoporous silica films prepared from sol solutions containing increasing contents of the Fe(Htrz)3 complex (in addition to TEOS and CTAB). The assynthesized films revealed distinct behavior depending on Fe(Htrz)3 concentration in the synthesis medium (Fig. 1A): the films obtained from [Fe(Htrz)3] up to 2 mM are characterized by rather ill-defined signals with almost no reversibility whereas the curves observed for more concentrated media ([Fe(Htrz)3] ranging from 3 to 5 mM) gave rise to well-defined peaks on both forward and reverse scans.

After treatment in HCl/ethanol (which is expected to remove the CTAB template from the film [START_REF] Goux | Oriented Mesoporous Silica Films Obtained by Electro-Assisted Self-Assembly[END_REF] ), the situation was significantly different, with still well-defined responses for the more concentrated films but no noticeable signals for the electrodes covered with nanocomposites prepared from [Fe(Htrz)3] lower than 2 mM (Fig. 1B). It seems therefore that the Fe(Htrz)3 complexes incorporated into the materials from the most diluted sol solutions are washed out of the films along with the surfactant template while those entrapped from the most concentrated sol solutions survive the solvent extraction step. In this latter case, the intensity of the voltammetric signals (which can be related to the amount of Fe(Htrz)3 in the film) is proportional to the Fe(Htrz)3 concentration used to prepare the films. These preliminary results (Fig. 1) demonstrate the successful incorporation of Fe(Htrz)3 complexes into the mesoporous silica films, most probably due to favorable electrostatic interactions between the negatively charged silica precursors and the positive complex, but they also suggest significant behavioral differences depending on the amount of entrapped Fe(Htrz)3 species.

. Structural characterization and permeability properties of the films

From the EASA procedure used to prepare the films, [START_REF] Walcarius | Electrochemically Assisted Self-Assembly of Mesoporous Silica Thin Films[END_REF][START_REF] Goux | Oriented Mesoporous Silica Films Obtained by Electro-Assisted Self-Assembly[END_REF] one already knows that their expected structure is made of well-organized and hexagonally-packed mesopore channels vertically aligned onto the electrode surface, which is confirmed here from TEM micrographs and the electron diffraction pattern (see top and cross-section views Fig. S1 in Supporting Information). This oriented mesostructure is maintained up to 3 mM Fe(Htrz)3 in the synthesis medium (Fig. 2A-C), as also supported by GISAX characterization (see Fig. S2 in Supporting Information), suggesting that the incorporation of few amounts of this complex does not affect the final mesostructured (yet with some quality loss for the sample prepared from 3 mM Fe(Htrz)3, see Fig. 2C). However, larger Fe(Htrz)3 contents led to clearly less ordered but still regular wormlike mesostructures (see, e.g., Fig. 2D for a film prepared from 5 mM Fe(Htrz)3), yet uniformly deposited over the whole electrode surface and keeping constant thickness over the whole membrane area; the wormlike structure can be also evidenced by XRD (see Fig. S2 in Supporting Information), giving rise to a lattice parameter of 4.7 nm (consistent with TEM). One can even notice some stigmata of the vertical orientation in these highly loaded films; see top of cross-section view in Fig. 2D). The film thickness is independent on the Fe(Htrz)3 concentration in the sol solution (i.e., 1105 nm), indicating that the rate of film growth is not affected by the presence of the complex. Permeability measurements can bring additional information on the film properties, notably by comparing data obtained before and after treatment in HCl/ethanol (Fig. 3). Electrodes covered with template-based mesoporous silica films in general, [START_REF] Etienne | Molecular Transport into Mesostructured Silica Thin Films: Electrochemical Monitoring and Comparison Between p6m, P63/mmc, Pm3n Structures[END_REF][START_REF] Etienne | Electrochemical Approaches for the Fabrication and/or the Characterization of Pure and Hybrid Templated Mesoporous Oxide Thin Films: A Review[END_REF] and the oriented ones in particular, [START_REF] Karman | Amplified Charge Transfer for Anionic Redox Probes Through Oriented Mesoporous Silica Thin Films[END_REF][START_REF] Etienne | Electrochemical Approaches for the Fabrication and/or the Characterization of Pure and Hybrid Templated Mesoporous Oxide Thin Films: A Review[END_REF] have typical electrochemical signatures when using selected redox probes in solution. This is illustrated in part A of the figure for the ferrocenedimethanol probe (Fc(MeOH)2), showing before template removal a reversible signal around +0.5 V corresponding to the Fc(MeOH)2/Fc(MeOH)2 + system solubilized in the surfactant phase and, after CTAB extraction the voltammetric response typical for Fc(MeOH)2 on ITO at about +0.3 V because the probe is now able to cross the film to reach the electrode surface (peak currents are even slightly larger for the film electrode than on bare ITO because Fc(MeOH)2 + cations are likely to be accumulated onto the negatively charged silica walls). The potential shift observed for the film before template extraction is explained by the stabilization of the neutral Fc(MeOH)2 probe in the surfactant phase (making it more difficult to be oxidized into Fc(MeOH)2 + that is repelled by the cationic surfactant, in agreement with previous observations made for the electrochemistry of ferrocene derivatives in micellar media [START_REF] Ryabov | Mechanism of a "Jumping Off" Ferricenium in Glucose Oxidase-D-Glucose-Ferrocene Micellar Electrochemical Systems[END_REF] ). We have thus repeated these experiments using the film electrodes containing various Fe(Htrz)3 contents, respectively before (Fig. 3B) and after treatment in HCl/ethanol (Fig. 3C), which is in principle likely to remove the template from the films. For as-synthesized films, the voltammetric response to Fc(MeOH)2 probe was dramatically decreased or even totally suppressed (Fig. 3B). Only small and ill-defined signals were observed with films prepared from Fe(Htrz)3 concentrations lower than 2 mM. This indicates that the presence of Fe(Htrz)3 complex in the mesoporous film restricts or even totally prevents the redox probe from reaching the underlying electrode surface, suggesting that few or no Fc(MeOH)2 species are likely to be solubilized in the template phase containing increasing contents of Fe(Htrz)3 complex. The absence of response also indicates that the nanocomposite films are well recovering the whole electrode surface area and that they are crack-free (otherwise, the probe would have been able to enter the defects and give a response when reaching the electrode). After treatment in HCl/ethanol, usually applied to extract the surfactant template and make the mesopore channels open to free diffusion of the probe (as for undoped mesoporous silica film, see red curve on Fig. 3A), the Fe(Htrz)3-doped films behaved differently as a function of the Fe(Htrz)3 content (Fig. 3C). For the films prepared at Fe(Htrz)3 concentrations up to 3 mM, surfactant template removal seems to be effective as the Fc(MeOH)2 probe gave rise to well-defined voltammetric signals at about +0.3 V. Yet, their intensity is decreasing when passing from low to high Fe(Htrz)3 contents, suggesting increasingly limited diffusion rates for the probe through the nanochannels. Such restricted mass transport, which was already observed with mesoporous silica films grafted with increasing amounts of organo-functional groups, [START_REF] Guillemin | Electrogeneration of Highly Methylated Mesoporous Silica Thin Films With Vertically-Aligned Mesochannels and Electrochemical Monitoring of Mass Transport Issues[END_REF] is thought to be due to residual Fe(Htrz)3 complexes that were not removed from the film during CTAB removal by solvent extraction (consistent with results of Fig. 1B) thus occupying some space in the mesopore channels and impeding somewhat the diffusion of Fc(MeOH)2.

Another argument supporting the restricted electrochemical response of Fe(Htrz)3-doped films is the larger anodic-to-cathodic peak potential separation (by ca. 40 mV) relative to the unmodified mesoporous silica film. Even more surprising is the complete blocking of films prepared from Fe(Htrz)3 concentrations larger than 3 mM (see, e.g., blue curve in Fig. 3C, corresponding to 4 mM Fe(Htrz)3), suggesting that the classical HCl-ethanol treatment for surfactant extraction is ineffective for the removal of the complex from such highly concentrated films. This suggests a good stability of the incorporated Fe(Htrz)3 species in the hybrid material, which is thought to be due to a combination of effects (triazole ligand-driven formation of a coordination polymer, physical entrapment of polymer chains in the mesopore channels, favorable electrostatic interactions between the positive complexes and negative silica walls). The only way to deeply unclog the mesopores and reveal the film permeability to the redox probe was to calcine the nanocomposite material (see Fig. S3 in Supporting Information), yet with some iron remaining in the matrix (1 %, as revealed by EDX analysis). Note that treatment in a more concentrated acid solution (i.e., 1 M HCl) also resulted in some film permeability but without removing all complex species from the material (see Fig. S4 in Supporting Information). This unexpected behavior might be of interest in view of confining durably such spin crossover complex into a mesoporous silica matrix. In order to understand better these phenomena, spectroscopic analyses have been performed, aiming at the evaluation of mesopore composition respectively before and after template extraction.

Mesopores composition and effect of solvent extraction on template removal

XPS was first used to investigate the surface composition and the chemical environment of the elements likely to be part of the Fe(Htrz)3-doped mesoporous silica films. XPS survey spectra (not shown) indicate the presence of C1s, O1s, N1s, Si2p and Fe2p signals, the intensity of which evolving with the amount of Fe(Htrz)3 complex and varying upon treatment in HCl/ethanol (see Table S1 in Supporting Information).

The most important conclusions can be drawn from the analysis of narrow scan regions corresponding to N1s and Fe2p signals (Fig. 4). From the analysis of Fe2p core level spectra (Fig. 4A), one can see a gradual decrease in signal intensity for both 2p3/2 and 2p1/2 peaks respectively located at the binding energies 711.1 and 724.5 eV (corresponding to a 2p3/2-Fe 2p1/2 splitting of 13.4 eV, along with their characteristic satellites at a distance of 5 eV, which are consistent values for Fe-1,2,4-triazole compounds [START_REF] Mazalov | Study of Electronic Structure of Spin-Transition Complexes by XPS and Mössbauer Spectroscopy[END_REF] ). After HCl/ethanol treatment, almost no change was observed for the sample prepared from 5 mM Fe(Htrz)3

(decrease in the Fe atomic content within the margin of error), suggesting no significant loss of the complex upon solvent extraction, whereas the films prepared from Fe(Htrz)3 concentrations lower than 2-3 mM suffered from dramatic leaching and even complete removal of Fe(Htrz)3 species from the mesoporous silica material. These data are in agreement with preliminary electrochemical observations (Fig. 1B). This is also supported by C1s signals showing notably a line at 288.8 eV (corresponding to N-C=N centers in the triazole ring [START_REF] Fettkenhauer | Synthesis of Efficient Photocatalysts for Water Oxidation and Dye Degradation Reactions Using CoCl2 Eutectics[END_REF][START_REF] Danyıldıza | Voltammetric Sensor Based on Glassy Carbon Electrode Modified With 1H-1,2,4-Triazole-3-thiol Coating for Rapid Determination of Trace Lead Ions in Acetate Buffer Solution[END_REF] ) which remained intact after solvent extraction for the 5 mM sample, whereas the lines characteristic of the CTAB template (C-C at 284.5 eV and C-N at 286.0 eV) dramatically decreased upon solvent extraction of the films prepared from lower Fe(Htrz)3 concentrations (see Fig. S5 in Supporting Information), but such data have to be taken with great care because of the existence of carbon-containing contamination and residual ethoxy groups resulting in C1s peaks at similar binding energies. [START_REF] Finšgar | EQCM and XPS Analysis of 1,2,4-Triazole and 3-Amino-1,2,4-Triazole as Copper Corrosion Inhibitors in Chloride Solution[END_REF][START_REF] He | Design and Fabrication of Highly Ordered Ion Imprinted SBA-15 and MCM-41 Mesoporous Organosilicas for Efficient Removal of Ni 2+ From Different Properties of Wastewaters[END_REF] In addition, considering previous XPS studies involving iron, the 2p3/2 peak for high-spin Fe(III) and Fe(II) compounds is broadened compared to metallic Fe 0 or low-spin Fe(II) [START_REF] Furlani | Spectroscopic Studies of FeCl3-Doped Polymers of Polyphenylacetylene[END_REF] due to the inclusion of electrostatic interactions, spin-orbit coupling between the 2p core hole and unpaired 3d electrons of the photoionized Fe cation and crystal field interactions. [START_REF] Gupta | Calculation of Multiplet Structure of Core p-Vacancy Levels[END_REF][START_REF] Gupta | Calculation of Multiplet Structure of Core p-Vacancy Levels[END_REF] The 2p3/2 peaks from low-spin Fe(II) compounds display no multiplet interactions because all six 3d electrons are spin paired. [START_REF] Nesbitt | X-ray Photoelectron Spectroscopic study of a Pristine Pyrite Surface Reacted With Water Vapour and Air[END_REF] The broadened peaks observed on the Fe2p spectra in addition to the binding energies observed at 711.1 and 724.5 eV seem to be in good agreement with the presence of Fe(III) in the mesoporous films (despite the iron-triazole complex introduced in the starting sol solution used to prepare the films is a low spin Fe(II) complex). This suggests that an oxidation of Fe(II) into Fe(III) centers takes place during the generation of the films, and this will be demonstrated and discussed later on (see beginning of Section 3.4). From Fe2p signals, on can also see some correlation between the iron content in the mesoporous films with the Fe(Htrz)3 concentration in the starting sol (see Table S1 in Supporting Information). The analysis of N1s core level spectra (Fig. 4B) further confirms the above observations. At low Fe(Htrz)3 contents, the only visible contribution to the N1s core level is due to the presence of the surfactant template, appearing in the form of a single peak located at 402.4 eV (assigned to the ammonium groups originating from CTAB). [START_REF] Jia | Effects of CTAC Micelles on the Molecular Structures and Separation Performance of Thin-Film Composite (TFC) Membranes in Forward Osmosis Processes[END_REF] This signal disappeared after HCl/ethanol treatment, confirming surfactant removal (as for unmodified mesoporous silica films [START_REF] Nasir | Electrografting of 3-Aminopropyltriethoxysilane on Glassy Carbon Electrode for the Improved Adhesion of Vertically Oriented Mesoporous Silica Thin Films[END_REF] ). Increasing the Fe(Htrz)3 content at 2-3 mM and above led to considerable decrease of this signal with concomitant growth of an additional peak at 400.1 eV which can be ascribed to the triazole moieties. [START_REF] Mazalov | Study of Electronic Structure of Spin-Transition Complexes by XPS and Mössbauer Spectroscopy[END_REF][START_REF] Danyıldıza | Voltammetric Sensor Based on Glassy Carbon Electrode Modified With 1H-1,2,4-Triazole-3-thiol Coating for Rapid Determination of Trace Lead Ions in Acetate Buffer Solution[END_REF] For the highest Fe(Htrz)3 content (5 mM), its intensity remained almost unchanged, confirming the strong binding of the complex to the mesoporous silica film, which acted thus as a real template during film growth owing to favorable electrostatic interactions between the cationic complex and the negatively charged silica precursors (electrostatic attraction has been also claimed for Fe(Htrz)3 in Nafion, [START_REF] Nakamoto | Demonstration of the Thermally Induced High Spin-Low Spin Transition for a Transparent Spin Crossover Complex Film [Fe(II)(H-trz)3]-Nafion (trz = triazole)[END_REF] for instance). The residual N1s contribution at 402.4 eV (with same intensity before and after HCl/ethanol treatment) cannot be unambiguously explained as it can be due to the amine functions of the triazole ring interacting with the silanol groups (via H-bonding or protonation, as reported for cyclam-functionalized silica materials [START_REF] Etienne | Multiarm Cyclam-Grafted Mesoporous Silica: A Strategy to Improve the Chemical Stability of Silica Materials Functionalized With Amine Ligands[END_REF] ) or to some remaining CTA + species not washed out by solvent extraction and likely to act therefore as co-template.

FTIR analyses were also performed to confirm XPS observations, notably to check if information gathered from surface composition is also true for the bulk film. The results (see Fig. S6 in Supporting Information) indicate the presence of Fe(Htrz)3 (from typical vibrations of =C-H, -N-H, -C=N and -N=N groups [START_REF] Trivedi | Characterization of Physical, Spectral and Thermal Properties of Biofield Treated 1,2,4-Triazole[END_REF][START_REF] Mokadem | Triazole Containing Magnetic Core-Silica Shell Nanoparticles for Pb 2+ , Cu 2+ and Zn 2+ Removal[END_REF] ) with no or negligible amount of CTAB in the sample prepared from 5 mM of the complex, which remained in the film after HCl/ethanol treatment, whereas only the surfactant template was noticeable in the film generated from 0.5 mM Fe(Htrz)3 and it was completely removed after solvent extraction. This supports conclusions made from XPS data. FTIR results also show the absence of signal in the region 3300-3500 cm -1 corresponding to OH groups (from silanol or water) when the film contained CTAB as the template (the signal only occurred after HCl/ethanol treatment), while it was clearly visible for the sample obtained when Fe(Htrz)3 acted as the template due to its more hydrophilic character. From all the above results, one can thus clearly distinguish two cases: the nanocomposite films exhibiting a wormlike mesostructure filled with Fe(Htrz)3 species that are durably immobilized as a real template in the silica matrix (i.e., as generated from a sol solution containing a high concentration of the iron triazole coordination polymer), and the vertically aligned hexagonal mesostructure with Fe(Htrz)3 complexes coincorporated with the surfactant species (as obtained at Fe(Htrz)3 contents ≤3 mM) which suffer from important leaching of Fe(Htrz)3 species from the film upon solvent extraction. Note that durable immobilization of the complex does not mean an improved chemical stability of the hybrid material in comparison to pure mesoporous silica films, as the silica walls are still constituted of amorphous silica gel which can suffer from some degradation upon prolonged use in aqueous media, especially at high pH values. In the following section, we will focus only on the film electrode prepared from 5 mM Fe(Htrz)3 in order to point out the possible interest of such strongly immobilized complex as an electroactive template for electrocatalytic purposes.

Electrochemical characterization and electrocatalytic properties of immobilized Fe(Htrz)3

It appears from preliminary electrochemical experiments (Fig. 1B) and Fe2p XPS data (Fig. 4A) that the immobilized Fe(Htrz)3 complex in the mesoporous silica material is in the form of Fe(III) although the Fe(II) coordination polymer was used in the synthesis medium. The explanation of such unexpected observation has to be found in the film generation procedure by EASA involving a potential-driven pH increase at the electrode/solution interface (i.e., to induce the condensation of silica precursors at pH above 8) [START_REF] Shacham | Electrodeposition of Methylated Sol-Gel Films on Conducting Surfaces[END_REF][START_REF] Sibottier | Factors Affecting the Preparation and Properties of Electrodeposited Silica Thin Films Functionalized With Amine or Thiol Groups[END_REF] and taking into account the pH-dependent chemistry of Fe(Htrz)3 and its possible oxidation. [START_REF] Faulmann | A Facile Route for the Preparation of Nanoparticles of the Spin-Crossover Complex [Fe(Htrz)2(trz)](BF4) in Xerogel Transparent Composite Film[END_REF][START_REF] Kamebuchi | Development of pH-Sensitive Spin-crossover Iron(II) Complex Films, [FeII(diAMsar)]-Nafion: Manipulation of the Spin State by Proton Concentration[END_REF] This is confirmed from cyclic voltammetry experiments performed with the Fe(II) form of the complex in aqueous solution at distinct pH (see Fig. S7 in Supporting Information) showing that it is intact at pH 3.5 (colorless and oxidizable species) whereas the solution turned yellowish at pH 9 due to the chemical oxidation of Fe(II) into Fe(III), resulting in a well-defined cathodic response. One can therefore conclude that the Fe(Htrz)3 complex acting as template during the growth of the mesoporous silica films is in its oxidized Fe(III) form, as a consequence of the electrochemical reduction of water leading to the production of OH -species at the vicinity of the electrode surface in the EASA process. [START_REF] Walcarius | Electrochemically Assisted Self-Assembly of Mesoporous Silica Thin Films[END_REF][START_REF] Goux | Oriented Mesoporous Silica Films Obtained by Electro-Assisted Self-Assembly[END_REF] Figure 5 depicts cyclic voltammograms of the film electrode prepared from 5 mM Fe(Htrz)3 as recorded at various potential scan rates (Fig. 5A) and under multiple successive cycling conditions at the same scan rate (Fig. 5B). This latter shows remarkable operational stability (no noticeable change in peak currents), which is rather unique for redox species weakly bonded/entrapped in a porous silica matrix confined to an electrode surface for which significant amounts of the electroactive probes are usually lost in solution upon electrochemical excitation, resulting in signals of lower intensity. [START_REF] Wang | Durable Cofactor Immobilization in Sol-Gel Bio-Composite Thin Film for Reagentless Biosensors Using Dehydrogenases[END_REF][START_REF] Fattakhova Rohlfing | Functionalized Mesoporous Silica Films as a Matrix for Anchoring Electrochemically Active Guests[END_REF] This is advantageous compared to organic-inorganic hybrid films elaborated from (sometimes sophisticated) covalent binding strategies to ensure durable immobilization of guest species in mesoporous silica films, [START_REF] Vilà | Clickable Bifunctional and Vertically-Aligned Mesoporous Silica Films[END_REF][START_REF] Fattakhova Rohlfing | Functionalized Mesoporous Silica Films as a Matrix for Anchoring Electrochemically Active Guests[END_REF][START_REF] Font | Periodic Mesoporous Silica Having Covalently Attached Tris(Bipyridine)Ruthenium Complex: Synthesis, Photovoltaic and Electrochemiluminescent Properties[END_REF][START_REF] Rafiee | Hydroquinone Functionalized Oriented MCM-41 Mesochannels at the Electrode Surface[END_REF] as the present method operates on the basis of a simple and single-step growth of the nanocomposite material.

The operational stability extended up to high potential scan rates (>5 V s -1 ), yet becoming slightly less reversible, while potential scan rates lower than 100 mV s -1 resulted in the progressive decrease in cathodic peak currents with concomitant vanishing of the anodic counterparts on scan reversal (see Fig. S8 in Supporting Information) suggesting possibly lower affinity of the Fe(II) form of the complex for the mesoporous silica matrix. The voltammetric signals are located at -0.35 V vs. Ag/AgCl, with cathodic-toanodic peak potential difference equal to 0.21 V for potential scan rates up to 400 mV s -1 and increasing somewhat above (see Fig. S8 in Supporting Information), and both cathodic and anodic peak currents are directly proportional to the potential scan rate (see inset in Fig. 5A) indicating a thin layer behavior (i.e., all electroactive species in the film are transformed during the scan, without diffusional restrictions). The fact that the thin layer behavior is maintained at high scan rates (far beyond 1 V s -1 ) and the good stability of current responses upon multisweep are indicators of the exceptional performance of such films for which the Fe(Htrz)3 coordination polymer not only plays the role of template but also contributes to stabilize the complex species in an active form in the hybrid nanocomposite. It was not the case of previously reported electroactive guests anchored into mesoporous silica films (which were usually characterized by diffusion-controlled charge transfer), [START_REF] Fattakhova Rohlfing | Functionalized Mesoporous Silica Films as a Matrix for Anchoring Electrochemically Active Guests[END_REF][START_REF] Song | Electrode Modification With Spin-Coated Films of Mesoporous Molecular Sieve Silicas[END_REF] for which dramatic decrease in current responses were observed (e.g., 70% drop in peak currents after 2 h in blank electrolyte for a mesoporous silica film doped with electrostatically-bound [Ru(bpy)3] 2+ species). [START_REF] Song | Electrode Modification With Spin-Coated Films of Mesoporous Molecular Sieve Silicas[END_REF]

Figure 5. (A)

Cyclic voltammograms recorded at various potential scan rates (in the 0.2 to 1.2 V s -1 range) in 0.1 M NaNO3 aqueous solution using a mesoporous silica film modified ITO electrode prepared from 5 mM Fe(Htrz)3 complex (after film treatment in 0.1 M HCl in ethanol); the variation of peak currents with potential scan rate is shown in the inset. (B) 20 consecutive cyclic voltammograms recorded at 1 V s -1 using the same electrode in the same medium as in (A).

Quantitative evaluation of the amount of Fe(Htrz)3 complex in the film can be achieved from the integration of peak currents in Fig. 5, leading to a charge of ca. 27 µC, corresponding to 2.810 -10 mol Fe(Htrz)3, which corresponds to a surface concentration of  = 1.410 -9 mol cm -2 (geometric surface area of the electrode = 0.196 cm 2 ). Taking into account the film diameter (5 mm), thickness (110 nm) and density (0.8 g cm -3 for MCM-41 [START_REF] Edler | The Structure and Dynamics of Hydrogen Adsorption in Mesoporous MCM-41[END_REF] ), this led to an estimated value of 1.610 -4 mol g -1 Fe(Htrz)3 in the film, if assuming that all of them are electroactive (i.e., a reasonable hypothesis for such species confined in the mesopore channels for which the electron transfer mechanism should be the electron hopping between adjacent Fe(Htrz)3 moieties). Considering the pores density for such films prepared by EASA (i.e., 75 000 pores µm -2 , [START_REF] Walcarius | Electrochemically Assisted Self-Assembly of Mesoporous Silica Thin Films[END_REF] which is also consistent with the top TEM view on Fig. 2D for the wormlike mesostructure) and the surface concentration determined above from voltammetry ( = 1.410 -9 mol cm -2 , corresponding to 8.410 14 molecule cm -2 ), one can estimate roughly an amount of 110-120 complex units per mesopore, which is compatible with a coordination polymer made of Fe(Htrz)3 complexes having a length of the same order of magnitude as the film thickness (assuming a complex size of about 1 nm in diameter, as estimated from XRD data). This suggests the filling of all mesopore channels over the whole film thickness and supports further the idea of a template role played by Fe(Htrz)3 in the mesoporous silica film formation. The direct electron transfer is not possible due to the insulating character of the silica walls, but the high density of Fe(Htrz)3 complexes very close to each other in the film (i.e., 5 times more than the maximum amount of ferrocene groups that can be covalently-bonded to a similar mesoporous silica film, [START_REF] Vilà | Electrochemically Assisted Generation of Highly Ordered Azide-Functionalized Mesoporous Silica for Oriented Hybrid Films[END_REF] for instance) enables effective electron hopping-based charge transfer.

Just to illustrate the potential interest of nanocomposite mesoporous films made of a supramolecular electroactive template such as the Fe(Htrz)3-based coordination polymer, we provide here a preliminary investigation of their electrocatalytic behavior towards the reduction of hydrogen peroxide. In the presence of H2O2, the reversible signal characteristic of the immobilized [Fe(Htrz)3] 3+/2+ system changed significantly, showing an increase in the intensity of the cathodic peak while the anodic signal almost disappeared (see Fig. S9 in Supporting Information). This is due to the electrocatalytic reduction of H2O2 by the electrogenerated [Fe(Htrz)3] 2+ species which are thus no more available to be reoxidized on scan reversal. This can be exploited for the amperometric detection of H2O2, by applying a constant potential (i.e., -0.4 V) to the electrode placed in a stirred electrolyte solution to which increasing concentrations of H2O2 are added, and measuring the resulting current responses as depicted in Figure 6. As shown, no detectable response was observed at bare ITO because the direct reduction of H2O2 requires more cathodic potential values, whereas well-defined current jumps occurred upon injections of H2O2 aliquots of increasing concentrations when using a Fe(Htrz)3-doped mesoporous silica film electrode (Fig. 6A).

Comparing the curves obtained from film electrodes prepared from either 3 mM or 5 mM Fe(Htrz)3 in the synthesis medium (Fig. 6B) reveals that both exhibited an electrocatalytic response to H2O2, but the sensitivity obtained with the 5 mM film was larger than for the 3 mM film, confirming the advantages offered by the highly loaded material. The responses were fast, reaching steady-state in few seconds (i.e., >90% of the signal obtained after 3 and 5 s, respectively for films prepared from 5 and 3 mM Fe(Htrz)3). Focusing on the most sensitive film (generated using 5 mM Fe(Htrz)3), a linear response was observed in the H2O2 concentration range from 1 to 160 µM (see inset in Fig. 6B).

Conclusions

Using a coordination polymer as template, we have designed a general one-step elaboration procedure of mesoporous silica composite thin films based on electrochemically assisted self-assembly. Depending on the concentration of the templating spin-crossover Fe(Htrz)3 coordination polymer in the starting sol, two distinct situations have been reached. For low Fe(Htrz)3 concentration (≤ 3 mM), the vertically aligned mesostructure is formed and Fe(Htrz)3 complexes are co-incorporated with the surfactant species, but most of them are leached out from the film upon surfactant extraction. On the contrary, for high Fe(Htrz)3 concentration (typically 5 mM), a wormlike mesoporous film is obtained in which Fe(Htrz)3 species are strongly immobilized within the silica matrix, acting as a novel type of structure-directing agent. For the optimized situation, a density of Fe(Htrz)3 complexes as high as 1.6x10 -4 mol g -1 has been estimated, roughly corresponding to an amount of 110 complex units per mesopore channel. This enables efficient electron-hopping charge transfer through the entire film thickness. As a consequence, the resulting Fe(Htrz)3@SiO2 composite offers remarkable electrochemical properties, characterized by high operational stability, maintained at high scan rates with a good stability of current response upon multisweep. The potential interest of the composite has been illustrated through its electrocatalytic performance towards the detection of hydrogen peroxide.

These results pave the way to the design of new functional and multifunctional silica composite films, using coordination polymers with appealing physical properties (magnetic, optical, electronic) as template. The synthesis method appears to be very simple and versatile, and optimizing the elaboration conditions could lead to improving the control of the mesostructuration level and pore orientation to get functional molecular entities durably confined in an attractive geometry for applications. One can expect the use of other coordination polymers as template to get in a single step other kinds of organicinorganic hybrids exhibiting new or improved properties (not only the electrochemical ones) and we are also confident that spin cross-over characteristics of such immobilized iron-triazole complexes might be exploited in near future.

Scheme 1 .

 1 Scheme 1. Chemical structure of the Fe(II)-triazole coordination polymer (counter-anions are omitted).

Figure 1 .

 1 Figure 1. Cyclic voltammograms recorded at 0.5 V s -1 in 0.1 M NaNO3 aqueous solution using mesoporous

Figure 2 .

 2 Figure 2. TEM micrographs (top views on the left and cross-sections on the right) of mesoporous silica

Figure 3 .

 3 Figure 3. (A) Cyclic voltammograms recorded at 100 mV s -1 in 1 mM Fc(MeOH)2 (+0.1 M NaNO3) aqueous

Figure 4 .

 4 Figure 4. High-resolution XPS spectra for (A) iron (2p) and (B) nitrogen (1s) obtained using mesoporous

- 1 .

 1 0 -0.8 -0.6 -0.4 -0.2

Figure 6 .

 6 Figure 6. Amperometric responses recorded at bare ITO or using ITO electrodes modified with

Acknowledgments

Financial supports from the CNRS, the French PIA project "Lorraine Université d'Excellence" (reference ANR-15-IDEX-04-LUE) and the CPER (program SusChemProc) are acknowledged. We also thank Jaafar Ghanbaja for TEM imaging, Aurélien Renard for recording XPS spectra, and Lionel Richaudeau for GISAXS measurements. SA acknowledges a PhD fellow from the Université de Lorraine.

Supporting Information description

Scheme of the electrochemical cell used for film preparation, TEM of unmodified mesoporous silica film, GISAXS of the films, permeability data after film calcination and treatment in 1 M HCl, quantitative XPS data (film composition as a function of Fe(Htrz)3 concentration), additional characterization data (C1s XPS spectra, FTIR spectra, respectively before and after treatment with ethanol/HCl), pH effect on CV of Fe(Htrz)3 complex in solution, additional CV curves for Fe(Htrz)3-modified films at various scan rates, CV characterization of the electrocatalytic detection of hydrogen peroxide at the modified electrode.


Taisiia Sikolenko

Christelle Despas

Neus Vilà

Alain Walcarius
email: alain.walcarius@univ-lorraine.fr

Thickness control in electrogenerated mesoporous silica films by wet etching and electrochemical monitoring of the process

Keywords:

come

Thickness control in electrogenerated mesoporous silica films by wet etching and electrochemical monitoring of the process

Introduction

Nanostructured electrode surfaces have gained importance in recent years for applications in various fields (e.g., energy conversion and storage [1,2], electronics and optoelectronics [3,4], sensors and biosensors [5,6]). Electrodeposition methods have played a special role in producing a wide range of nanostructured materials [7][8][9], which sometimes exhibit improved characteristics in comparison to materials produced by conventional techniques [10]. In particular, silica-based nanomaterials have attracted considerable interest from the electrochemistry community because these materials can be prepared in a variety of functionalized forms with tailored and organized structures at the nanoscale [11][12][13]. In this context, ordered mesoporous silica thin films have been generated on electrodes using chemical or electrochemical synthetic methods [14,15] and their performance was dramatically dependent on all features likely to affect mass transfer processes [14][15][16][17][18], the most critical point being the necessity to produce coatings with pores accessible from the film surface [19].

The best way to achieve this goal is by precise control of pore orientation, and an ideal configuration is given by films with uniaxially oriented one-dimensional mesochannels, with promising applications notably for electroanalytical purposes [13,20]. The most versatile methods available to date to produce small-pore (∅ 2-3 nm) vertically aligned mesoporous silica films on electrodes are: (i) Stöber-solution growth [21] and (ii) electrochemically assisted self-assembly (EASA) [22]. Oriented films with organo-functional groups covalently bonded to the silica walls can be also generated by EASA [23]. Although the thickness of electrogenerated mesoporous silica deposits can be tuned to some extent in the range of typically 100 nm by adjusting the synthesis parameters (i.e., deposition time and silica precursor concentration), scaling-up has been restricted by the formation of silica aggregates when attempting to get thick films (due to bulk precipitation in the diffusion layer), while scaling-down to ultra-thin films was not possible due to the absence of self-assembly at very short deposition times, t dep , or very low precursor concentration (no film growth at t dep < 10 s and silane concentration < 75 mM [24]). For instance, one can generate non-ordered ultrathin silica films by electrodeposition from very dilute precursor solutions [25], but nanostructured deposits cannot be produced as the EASA method requires a surfactant content above the critical micellar concentration and a silane/surfactant molar ratio larger than 0.78 [24]. To circumvent the first limitation, we have recently proposed a multilayer electrogeneration approach which makes it possible to obtain films with thicknesses of up to 400 nm [26].

Here, we now propose a post-treatment strategy to decrease the thickness of mesoporous silica films prepared by EASA. Although some approaches for thickness control of nanostructured silica membranes have been previously reported, they are not applicable to either vertically-aligned or small-pore mesoporous silica thin films [27][28][29][30][31].

The objective of the present work is thus to evaluate how tuning the thickness of oriented mesoporous silica films generated by EASA can be realized by wet etching using dilute ammonium fluoride (NH 4 F) solutions. Stronger etchants (such as HF or NaOH) must be avoided because they lead to complete dissolution of the silica or non-uniform attack leading to rough layers [32,33], whereas NH 4 F appears to be soft enough for the purpose of slow film etching (as recently shown for hydrophobic and large-pore 3-dimensional mesoporous materials [34]). The results of film etching have been monitored by profilometry, electrochemical methods (cyclic voltammetry) and electron microscopy analyses, using ferrocene-functionalized mesoporous materials. This can be more informative than operating with non-functionalized silica films, as the voltammetric response of the ferrocene groups can be used to monitor the etching process.

Experimental section

Preparation of ferrocene-functionalized and oriented mesoporous silica films on ITO electrode

Azide-functionalized silica films were first generated on indium-tin oxide (ITO) plates (surface resistivity 8-12 Ω, Delta Technologies) by EASA [23]. The sol was made of an ethanol:water solution (1:1) containing 80 mM TEOS (tetraethoxysilane, 98%, Alfa Aesar), 20 mM AzPTES (azidopropyltriethoxysilane, prepared as in [23]), 32 mM CTAB (cetyltrimethylammonium bromide, 99%, Acros Organics) and 0.1 M NaNO 3 , which was set to pH 3 using 1 M HCl (Prolabo), and aged for 2 h at room temperature. Film deposition was performed potentiostatically at a negative potential of -1.3 V for 20 s, using a three-electrode system (ITO working electrode, silver rod as pseudo-reference electrode and stainless steel plate as counter electrode) and a μAutolab III potentiostat operated by GPES software. The resulting films were rinsed with distilled water and kept overnight at 130 °C. After template extraction (in 0.1 M HCl ethanol solution for 15 min), the covalent bonding of the ferrocene species was achieved by a 'click reaction' between azide-containing films and ethynylferrocene. This was carried out in DMF/H 2 O media (12 ml dimethylformamide (DMF, 99.94%) + 8 ml water) containing 10 mg ethynylferrocene (97%, Aldrich), 3 mg copper acetate dihydrate (UCB) and 7.5 mg ascorbic acid (99.7%, Merck). The reaction was conducted at room temperature for 20 h in the absence of light. The films were cleaned to remove traces of the catalyst using an ethanol solution of 0.05 M diethyldithiocarbamate trihydrate (DDTC, Sigma Aldrich) for 30 min, and then rinsed with ethanol.

Etching procedure and film characterization

Ferrocene-functionalized mesoporous silica films were etched with a 200 μl drop of 0.05 M ammonium fluoride (NH 4 F, 98%, Prolabo) under cover for the chosen time (in the 0-10 h range, under quiescent conditions). After etching, the samples were washed thoroughly with deionized water and air-dried. Their thickness was estimated from profilometry measurements, performed before and after etching, using the Dektak XT apparatus with Vision 64 software (Brüker). Their electrochemical response was characterized by cyclic voltammentry (CV) in 0.1 M NaNO 3 using an EM Stat2 potentiostat (PalmSens). Some samples were imaged by electron microscopy (surface analysis by SEM using the JEOL JCM-6000 microscope and mesostructure observation by TEM with Philips CM20 microscope at an acceleration voltage of 200 kV).

Results and discussion

Fig. 1A illustrates schematically the film preparation process, which involves the electrochemically assisted self-assembly (EASA) of an ordered, azide-functionalized and vertically aligned mesoporous silica membrane and its subsequent click coupling with ethynylferrocene [23]. EASA is a known electrodeposition method based on the application of a cathodic potential to an electrode immersed in a hydrolyzed sol solution containing silica precursors (TEOS and AzPTES in this case), which condense due to the electrochemically induced local pH increase on the cathode, with a cationic surfactant (CTAB) acting as a structure-directing agent, ensuring the growth of hexagonally-packed mesochannels orthogonal to the underlying electrode surface [22][23][24]. The supporting electrolyte, NaNO 3 , also contributes to the cathodic generation of OH -ions [35][36][37]. Fig. 1B shows that the azide-functionalized film was uniformly deposited over the whole electrode surface area and was crack-free (no voltammetric signal recorded for the Ru (NH 3 ) 6 3+ redox probe prior to surfactant removal, while ferrocene dimethanol (Fc(MeOH) 2 ) is likely to be detected only due to the possible solubilization of such neutral species in the surfactant phase; see blue curves in Fig. 1B). After template removal, the film was highly porous (see the large voltammetric signals observed for both Ru(NH 3 ) 6 3+ and Fc(MeOH) 2 probes in Fig. 1B, red curves), confirming the good accessibility of azide groups for further functionalization with ferrocene moieties. The effectiveness of click coupling is demonstrated by the well-defined voltammetric signal obtained for the ferrocene grafted film (thanks to electron hopping between adjacent redox centers in the mesopore channels) whereas no noticeable response was observed with the azide functionalized membrane (Fig. 1C). When operating at a low potential scan rate (< 10 mV s -1 ), all ferrocene groups are electrochemically accessible (the voltammetric peak currents are characteristics of thin-layer behavior [38]), thus providing a good method for quantitative monitoring of film thickness variations upon etching. This is illustrated in Fig. 2A, which shows a progressive decrease in peak currents on increasing the duration of film treatment in NH 4 F prior to voltammetric analysis in the electrolyte solution. This suggests a continuous film etching, at a rather slow rate, as the progressive dissolution of the silica network would result in leaching more and more ferrocene into the solution and thus there would be less ferrocene in the film available for voltammetric detection (Fig. 2B). In order to confirm effective film etching (and not just ferrocene loss from the material), additional film thickness measurements (Fig. 3) have been performed by profilometry on scratched samples to evaluate thickness variations upon reaction with NH 4 F. In spite of rather noisy responses (Fig. 3A), originating mainly from the presence of silica particles/aggregates on top of the films (as is common for EASA-based deposits [22,24]), a clear trend can be drawn on the basis of multiple profilometry measurements made on various films etched for different periods of time (Fig. 3B). As shown, prolonged treatments in NH 4 F resulted in a progressive decrease in film thickness, confirming the suitability of NH 4 F for controlled wet etching of the silica material [34], in contrast to stronger reagents such as HF (which leads to complete dissolution of the silica film within seconds) or NaOH (which is known to rapidly destroy the structure of mesoporous silica [39]). The evolution of both electrochemical (Fig. 2B) and profilometry (Fig. 3B) data being similar, one can conclude that cyclic voltammetry is a satisfactory technique for monitoring the effectiveness of wet etching applied to a mesoporous silica film bearing redox-active moieties such as ferrocene. The quite large error bars do not mean that the thickness is hard to control; these are due to the fact that the etching experiments were carried out using numerous different starting films which already exhibited quite a large distribution in thickness and ferrocene content.

Effective etching was also demonstrated using electron microscopy (Fig. 4). Imaging by SEM reveals not only that the mesostructured thin film is dissolved by NH 4 F but also that the overlaying aggregates/particles tend to disappear upon prolonged etching (Fig. 4A), consistent with the roughness decrease observed by profilometry (Fig. 3A). TEM views confirm that the regular mesostructure and pore orientation are maintained upon moderate wet etching and that the process is uniform over the whole film surface area (uniform film thickness), but also that prolonged etching leads to some damage in the mesostructure (becoming wormlike instead of hexagonal) (Fig. 4B). Note that in spite of the loss in quality of the mesostructure, the ferrocene groups in the material remained electrochemically accessible, yet led to less welldefined CV responses (Fig. 2A). The progressive shape changes in the CV curves and shift in peak currents towards more anodic values are in agreement with restricted transport phenomena reported previously for such oriented mesoporous silica films bearing redox moieties [38].

Conclusion

In this work, we have demonstrated the possible wet etching of electrogenerated, vertically-aligned and ferrocene-functionalized, mesoporous silica thin films by treatment in NH 4 F. The process has been characterized by profilometry measurements and electrochemical monitoring, both of which confirm the progressive decrease in film thickness upon increasing the contact time with etchant solution. The integrity of the regular mesostructure and pore orientation were maintained, except for very long etching times when some loss of order was observed while keeping the redox probes attached to the silica framework electrochemically accessible.

Fig. 1 .

 1 Fig. 1. (A) Scheme showing the preparation of ferrocene-functionalized films by EASA and click coupling; (B) CV curves recorded in 0.5 mM Ru(NH 3 ) 6 Cl 3 (left) or 0.5 mM Fc(MeOH) 2 (right) using (a) a bare ITO or (b,c) an ITO covered with azide-functionalized mesoporous silica film (b) before or (c) after surfactant removal (supporting electrolyte: 0.1 M NaNO 3 ; potential scan rate: 50 mV s -1 ); (C) CV curves recorded in 0.1 M NaNO 3 using ITO electrodes covered with mesoporous silica film bearing azide (N 3 ) or ferrocene (Fc) groups.

Fig. 2 .

 2 Fig. 2. (A) CV curves recorded in 0.1 M NaNO 3 (at 5 mV s -1 ) using an ITO electrode covered with mesoporous silica film bearing ferrocene groups, which had been treated in 0.05 M NH 4 F for 0, 1, 2, 4, 7 and 10 h. (B) Variation of the anodic signal with etching time (average of results obtained with six different films) and schematic illustration of the film thickness decrease upon etching (red pentagon = ferrocene). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3 .

 3 Fig. 3. (A) Typical profilometry measurements (red, before etching, and green, after etching); (B) Variation of film thickness upon etching over increasing periods of time (eight different films, six distinct profilometry measurements per film). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4 .

 4 Fig. 4. (A) SEM and (B) TEM imaging (1) before and (2,3) after moderate and long etching times, corresponding to film thicknesses of about 50 and 20 nm, respectively.

Acknowledgements

The authors gratefully thank Liang Liu (LCPME, Nancy) for advice in profilometry measurements and Jaafar Ghanbaja (IJL, Nancy) for TEM analysis. T.S. acknowledges a PhD grant from the University of Lorraine.