Romain Poupart

Mohamed Guerrouache

Daniel Grande

Benjamin Le Droumaguet
email: ledroumaguet@icmpe.cnrs.fr

Benjamin Carbonnier
email: carbonnier@icmpe.cnrs.fr

Gold Nanoparticles Supported onto Amine-Functionalized In-Capillary Monoliths Meant for Flow-Through Catalysis: A Comparative Study

Keywords: Gold nanoparticles, Catalytic reduction, Monolith, Amino-functionalization, Polymer-supported nano-metal

Monoliths functionalized with ethylene diamine-derived ligands were synthetized within micro-sized channels as highly permeable catalytic supports for the immobilization of in-situ generated gold nanoparticles. The as-obtained hybrids, essentially differing on the grafted amine containing ligands were compared, notably regarding the immobilized nanoparticles morphology and their dispersion at the monolith pore surface. Further, such in-capillary hybrid monoliths were successfully applied as flow-through microreactors for the catalytic reduction of nitroaromatic compounds. The general synthetic route for preparing these composite materials consisted in a three-step procedure involving (i) UV-induced polymerization of N-acryloxysuccinimide and ethylene glycol dimethacrylate in toluene, (ii) surface grafting of a series of ethylene diamine derivatives through nucleophilic substitution of N-hydroxysuccinimide leaving groups, and (iii) successive in situ reduction of tetrachloroauric acid to generate monolith-adsorbed gold nanoparticles. The successful synthesis of such hybrid monoliths was ascertained by in-situ Raman spectroscopy, EDX analysis and SEM observations. Microscopy demonstrated the key role of the grafted amine bearing ligand regarding the morphology, size and surface coverage of the immobilized gold nanoparticles at the monolith pore surface. Monolith-adsorbed gold nanoparticles exhibited good catalytic activities towards the conversion of nitroaromatic compounds into the corresponding amino derivatives in a flow-through process. This study clearly demonstrates the key role of the nature, primary vs. secondary, of the chelating amine in the morphology (shape, size, dispersion) of the supported gold nanoparticles.

Introduction

Miniaturization in materials chemistry had led to the development and the improvement of various strategies and methods to design and tailor performant microreactors.

Among this type of reactors, polymer-based systems have emerged to be very promising for flow-through catalytic processes. [START_REF] Liu | Gold Nanoparticle-Supported Histamine-Grafted Monolithic Capillaries as Efficient Microreactors for Flow through Reduction of Nitro-Containing Compounds[END_REF][START_REF] Poupart | Novel in-capillary polymeric monoliths arising from glycerol carbonate methacrylate for flow-through catalytic and chromatographic applications[END_REF][START_REF] Poupart | Copper nanoparticles supported on permeable monolith with carboxylic acid surface functionality: Stability and catalytic properties under reductive conditions[END_REF][START_REF] Floris | Agglomerated polymer monoliths with bimetallic nano-particles as flow-through micro-reactors[END_REF][START_REF] Floris | Fabrication and characterisation of gold nano-particle modified polymer monoliths for flow-through catalytic reactions and their application in the reduction of hexacyanoferrate[END_REF] The properties of the continuous interconnected porous polymer matrix, also called monolith, which can be easily prepared by polymerization of various commercially available styrenic or (meth)acrylic monomers, can be tuned by grafting of ligands so as to change the interfacial features of the polymeric surface. [START_REF] Poupart | Porous polymers and metallic nanoparticles: A hybrid wedding as a robust method toward efficient supported catalytic systems[END_REF] For instance, the polymer matrix can be functionalized with different chemical groups (e.g. amine, thiol, cyano or carboxylic acid among them) that notably allow for the grafting of several metallic nanoparticles. Svec et al. [START_REF] Svec | Continuous rods of macroporous polymer as highperformance liquid chromatography separation media[END_REF] pioneered the use of such cross-linked polymers as monolithic stationary phases for electrochromatographic and chromatographic applications. [START_REF] Svec | Quest for organic polymer-based monolithic columns affording enhanced efficiency in high performance liquid chromatography separations of small molecules in isocratic mode[END_REF] Since then, the technology has evolved and has been broadened to a plethora of solutes (hydrophobic, hydrophilic, ionic and chiral) that can be now efficiently separated.

On the other hand, metallic nanoparticles are well-known as supported nano-catalysts for various organic reactions: dye reduction, [START_REF] Féral-Martin | Integrative chemistry toward the first spontaneous generation of gold nanoparticles within macrocellular polyHIPE supports (Au@polyHIPE) and their application to eosin reduction[END_REF] nitro compound reduction [START_REF] Harish | Synthesis of Conducting Polymer Supported Pd Nanoparticles in Aqueous Medium and Catalytic Activity Towards 4-Nitrophenol Reduction[END_REF][START_REF] Kuroda | Reduction of 4-nitrophenol to 4-aminophenol over Au nanoparticles deposited on PMMA[END_REF] or C-C cross-coupling such as Heck, [START_REF] Cassol | The Role of Pd Nanoparticles in Ionic Liquid in the Heck Reaction[END_REF][START_REF] Mandal | Pt and Pd Nanoparticles Immobilized on Amine-Functionalized Zeolite: Excellent Catalysts for Hydrogenation and Heck Reactions[END_REF] Sonogashira [START_REF] Venkatesan | Designed Synthesis of Au/Ag/Pd Trimetallic Nanoparticle-Based Catalysts for Sonogashira Coupling Reactions[END_REF] or Suzuki-Miyaura [START_REF] Han | Facile Synthesis of Highly Stable Gold Nanoparticles and Their Unexpected Excellent Catalytic Activity for Suzuki-Miyaura Cross-Coupling Reaction in Water[END_REF] reactions.

Noble metal-based nanoparticles relying on platinum, [START_REF] Mandal | Pt and Pd Nanoparticles Immobilized on Amine-Functionalized Zeolite: Excellent Catalysts for Hydrogenation and Heck Reactions[END_REF] palladium [START_REF] Harish | Synthesis of Conducting Polymer Supported Pd Nanoparticles in Aqueous Medium and Catalytic Activity Towards 4-Nitrophenol Reduction[END_REF][START_REF] Gallon | Palladium Nanoparticles Supported on Polyaniline Nanofibers as a Semi-Heterogeneous Catalyst in Water[END_REF] or silver [START_REF] Jiang | Catalytic Properties of Silver Nanoparticles Supported on Silica Spheres[END_REF] have been the subject of numerous studies. However, gold is so far the most commonly used noble metal for polymer-based heterogeneous catalyzed reactions. [START_REF] Ma | Gold Nanoparticles as Chemical Catalysts[END_REF] One of the major advantages of such monolith -immobilized metallic nanoparticle relies on the facile supported catalyst recovery and the easy reusability of the hybrid systems. Over the last years, a large variety of such nanoparticles@polymer hybrids has been prepared in our laboratory to implement such heterogeneous supported catalytic reactions. Some of us notably demonstrated that catalytic nanoreactors could be obtained upon immobilization of gold nanoparticles at the pore surface of oriented nanoporous polystyrene originating from diblock copolymers possessing a selectively cleavable junction. [START_REF] Droumaguet | Clickable" thiol-functionalized nanoporous polymers: from their synthesis to further adsorption of gold nanoparticles and subsequent use as efficient catalytic supports[END_REF][START_REF] Poupart | Porous Gold Nanoparticle-Decorated Nanoreactors Prepared from Smartly Designed Functional Polystyrene-block-Poly(d,l-Lactide) Diblock Copolymers: Toward Efficient Systems for Catalytic Cascade Reaction Processes[END_REF] It was notably demonstrated that cascade reactions could be envisioned with some of those systems to carry out more complex chemical processes, leading eventually to higher added value molecular transformations. [START_REF] Poupart | Porous Gold Nanoparticle-Decorated Nanoreactors Prepared from Smartly Designed Functional Polystyrene-block-Poly(d,l-Lactide) Diblock Copolymers: Toward Efficient Systems for Catalytic Cascade Reaction Processes[END_REF] Some bulk monoliths have also been prepared from selectively cleavable disulfide containing dimethacrylate crosslinker. [START_REF] Poupart | Gold nanoparticles immobilized on porous monoliths obtained from disulfide-based dimethacrylate: Application to supported catalysis[END_REF] These monoliths could be consecutively modified by chemical reduction of the disulfide bond, affording some available thiol groups at the pore surface of the polymer, further used to immobilize nanogold, thus allowing for a pollutant dye, i.e. Eosin Y, reduction. Different hybrid monolithic systems based on biporous polymeric materials have also been prepared from a functionalizable monomer, i.e. HEMA. [START_REF] Ly | Versatile functionalization platform of biporous poly(2-hydroxyethyl methacrylate)-based materials: Application in heterogeneous supported catalysis[END_REF] Finally, a plethora of examples, in which monoliths are covalently linked to the inner wall of ~100 μm-diameter fused silica capillaries, has been reported in our group. All these in-capillary monoliths relied on a functionalizable monomer that can be modified by a post-polymerization reaction with a chemical graft of interest to allow for further nanometal immobilization. These systems were not only assessed for their flowthrough supported catalysis abilities but also for their separation behaviour. In this context, GCMA- [START_REF] Poupart | Novel in-capillary polymeric monoliths arising from glycerol carbonate methacrylate for flow-through catalytic and chromatographic applications[END_REF] but more importantly NAS-based [START_REF] Liu | Gold Nanoparticle-Supported Histamine-Grafted Monolithic Capillaries as Efficient Microreactors for Flow through Reduction of Nitro-Containing Compounds[END_REF][START_REF] Poupart | Copper nanoparticles supported on permeable monolith with carboxylic acid surface functionality: Stability and catalytic properties under reductive conditions[END_REF][START_REF] Khalil | Gold-decorated polymeric monoliths: In-situ vs ex-situ immobilization strategies and flow through catalytic applications towards nitrophenols reduction[END_REF] polymers have been investigated in different works.

Studies on nanometals immobilized at the pore surface of polymers generally focus on the ability of different chemical function (e.g. thiol, amine, etc) to chelate metallic ions. [START_REF] Cao | Polymer Monoliths with Exchangeable Chemistries: Use of Gold Nanoparticles As Intermediate Ligands for Capillary Columns with Varying Surface Functionalities[END_REF][START_REF] Xu | Porous Polymer Monolithic Column with Surface-Bound Gold Nanoparticles for the Capture and Separation of Cysteine-Containing Peptides[END_REF][START_REF] Guerrouache | Sitespecific immobilisation of gold nanoparticles on a porous monolith surface by using a thiolyne click photopatterning approach[END_REF] Some comparisons were notably achieved regarding the impact of the nature of the chemical function at the pore surface towards metallic nanoparticle immobilization. [START_REF] Westcott | Formation and Adsorption of Clusters of Gold Nanoparticles onto Functionalized Silica Nanoparticle Surfaces[END_REF] Simultaneously, other efforts have been made to fully understand the nucleation and growth processes of the gold nanoparticles in colloidal solution [START_REF] Nguyen | Experimental measurements of gold nanoparticle nucleation and growth by citrate reduction of HAuCl4[END_REF][START_REF] Polte | Nucleation and Growth of Gold Nanoparticles Studied via in situ Small Angle X-ray Scattering at Millisecond Time Resolution[END_REF] or onto a substrate.

Athanassiou research group has reported a lot on the preparation of nanoparticles through the impregnation of chitosan with gold, [START_REF] Spano | In Situ Formation and Size Control of Gold Nanoparticles into Chitosan for Nanocomposite Surfaces with Tailored Wettability[END_REF] silver, [START_REF] Mehrabanian | Laser-induced localized formation of silver nanoparticles on chitosan films: study on particles size and density variation[END_REF] palladium [START_REF] Mehrabanian | Laser-induced in situ synthesis of Pd and Pt nanoparticles on polymer films[END_REF] and platinum [START_REF] Mehrabanian | Laser-induced in situ synthesis of Pd and Pt nanoparticles on polymer films[END_REF] salt precursors and the subsequent reduction into their respective nanoparticles through a lasermediated UV irradiation. Another alternative strategy relies on the reductive properties of the support, such as polystyrene [START_REF] Féral-Martin | Integrative chemistry toward the first spontaneous generation of gold nanoparticles within macrocellular polyHIPE supports (Au@polyHIPE) and their application to eosin reduction[END_REF] or silicone [START_REF] Zhang | In-situ synthesis of poly(dimethylsiloxane)-gold nanoparticles composite films and its application in microfluidic systems[END_REF][START_REF] Sadabadi | Integration of gold nanoparticles in PDMS microfluidics for lab-on-a-chip plasmonic biosensing of growth hormones[END_REF] to generate gold nanoparticles. Finally, thermal annealing after impregnation of the polymer with a metal salt precursor has also been reported to be successfully implemented for the preparation of metallic nanoparticles. [START_REF] Morselli | Ceria/Gold Nanoparticles in Situ Synthesized on Polymeric Membranes with Enhanced Photocatalytic and Radical Scavenging Activity[END_REF] As demonstrated from these studies, the in situ strategy allows for the preparation of a large variety of size-and density-controlled metal nanoparticles within desired areas of polymeric supports. However, no investigation regarding the shape of the metal ligand has been developed so far. In this context, we decided to investigate the effect of amine-derived ligand chemical structure at the pore surface of polymeric monolithic columns on the chelation of gold nanoparticles.

Instrumentation

An HPLC pump (Shimadzu LC-10ATVP) was used to flush monolithic columns with nitroarenes solutions. Spectrolinker XL-1500 UV Crosslinker (Spectronics Corporation) equipped with six lamps (6 × 15W, 365 nm) was used to photoinitiate monomer copolymerization. UV-Vis spectra were recorded on a Cary 60 UV-Vis Spectrophotometer from Agilent Technologies. The chemical structure of monoliths was investigated using a Raman apparatus XPlora One from Horiba Jobin Yvon equipped with a laser emitting at 638 nm. Samples were investigated in different places of the cross sections to control the homogeneity of monoliths. The acquisition time was fixed at 1 min. Scanning Electron Microscopy (SEM) investigation of the materials was performed with a MERLIN microscope from Zeiss equipped with InLens, EBSD, and SE2 detectors using a low accelerating tension (2-3 kV) with a diaphragm aperture of 30 μm. Prior to analyses, the samples were coated with a 4-nm layer of palladium/platinum alloy in a Cressington 208 HR sputter-coater. Energydispersive X-ray spectroscopy (EDX) was performed using a SSD X-Max detector of 50 mm 2 from Oxford Instruments (127 eV for the Kα ray of Mn). (Hybrid) Monolithic capillary columns were cut at different places and small pieces were deposited on a SEM support with double-face adhesive tape. For the EDX analyses, the size of the investigated area was changed and the presented data were obtained for the analysis of 1 µm 2 representing nearly the size of a polymeric globule and represent averaged values obtained measurements performed at 5 different locations from three samples (Table S1). It was notably observed that similar results were observed independently of the probed area. The size of the Au NP was estimated using ImageJ software (Table S2).

Synthesis of in-capillary monoliths

The synthesis of the monolithic columns relied on a two-step process involving (i) the monolith synthesis, ii) functionalization of Poly(NAS-co-EGDMA) with amine containing ligands, and iii) immobilization of gold nanoparticles at the pore surface of amine functionalized Poly(NAS-co-EGDMA) monoliths via the in-situ strategy.

Surface pre-treatment of capillaries

In order to ensure the stability of the monolithic column, the inner wall of the capillaries was submitted to a vinylization step. 3-(Trimethoxysilyl)propyl methacrylate was used as a bifunctional reagent allowing for the covalent attachment of the polymeric material onto the inner wall of the capillaries. The procedure was as follows: fused silica capillaries were treated with 1.0 M NaOH for 1h at room temperature and subsequently heated for 2 h at a temperature of 100 °C. Capillaries were flushed with 0.1 M HCl for 10 min, rinsed with deionized water for 10 min, and then with acetone for 15 min. Thereafter, capillaries were purged with dry nitrogen gas for 2 h at a temperature of 120 °C. A 30 vol.% 3-(trimethoxysilyl)propyl methacrylate solution in toluene was allowed to react overnight with inner silanols at room temperature. Last, the capillaries were rinsed with toluene for 15 min and dried under a stream of nitrogen gas for 1 h. The capillaries thus treated were stored at 4 °C.

In-situ synthesis of porous monolith

The porous monolith was prepared through a polymerization reaction of a mixture consisting of the following combination: NAS as a functional monomer, EGDMA as a crosslinker, toluene as a porogenic solvent, and AIBN as an initiator (1 wt.% with respect to the total amount of monomers). The mixture was sonicated for about 15 min to obtain 15 min to obtain homogeneous solution. The pre-treated capillary was completely filled with the polymerization mixture by immersing the inlet of the capillary into a reservoir and by pushing through the solution under nitrogen pressure (3 bar). After flushing with a large excess of polymerization solution, both ends of the capillary were sealed with rubber septa and the capillary was placed within a Spectrolinker XL-1500 UV and irradiated during 800s. After the polymerization was completed, the septa were removed and the monolith capillary was washed with ACN for 1 h (5 µL.min -1 ) to remove the porogenic solvent.

In-situ functionalization of the porous monolith with amine ligands

The succinimide groups stemming from the NAS moieties were replaced into different amine groups through nucleophilic substitution reaction. The reaction was performed in-situ by flushing the monolith capillary with a solution of EDA, TRIS or AEP (0.3 M ≈ 100 µL in 2 mL of ACN in a reservoir) or TETRA (0.3 M = 200 µL in 2 mL of ACN in a reservoir) pushed under nitrogen pressure (50 bar) during 3 h at room temperature. Thereafter, the monolithic capillary was washed with ACN for 1 h to remove unreacted ligands. The completion of nucleophilic substitution was evaluated through in-situ Raman spectroscopy performed on monolithic capillary samples.

Formation of in situ gold nanoparticles (AuNPs)

The amine-modified monolith was primarily flushed with an HCl solution during 15 min to protonate the amine function into quaternary ammonium. Then, it was flushed with a 1 wt. % HAuCl 4 solution in deionized water until all the capillary was totally coloured in yellow. It was then rinsed with deionized water during 1 h to remove the un-grafted gold ions.

Then, a solution of NaBH 4 (10 mL, 0.1 M) in deionized water was flushed during 2 h. Finally, the capillary was flushed with deionized water for 30 min. A change in the colour appearance of the capillaries was observed as they turned to dark purple suggesting the effective formation of Au 0 .

Reduction of nitroarenes by supported heterogeneous catalysis

A freshly prepared solution containing 200 µL of nitroarenes (i.e. 5 mg of onitrophenol, 4-chloro-7-nitrobenzofurazan or p-nitroaniline in 10 mL H 2 O), 200 µL of NaBH 4

(114 mg in 10 mL H 2 O) in 5 mL of water was injected in the loop of a HPLC pump system, the solution coming out from the in-capillary AuNP-decorated monoliths was collected and then analysed by UV-Vis spectrophotometry. This catalytic flow reactions were performed at a flow rate of 3 µL.min 1 and gold-free monoliths substituted with the amine ligands were used as control monoliths.

Results and Discussion

Functionalization of in-capillary polymer monoliths

Svec research group reported the preparation of poly(glycidyl methacrylate-coethylene glycol dimethacrylate)-based monoliths that were successively chemically modified with thiol functions. [START_REF] Cao | Polymer Monoliths with Exchangeable Chemistries: Use of Gold Nanoparticles As Intermediate Ligands for Capillary Columns with Varying Surface Functionalities[END_REF][START_REF] Xu | Porous Polymer Monolithic Column with Surface-Bound Gold Nanoparticles for the Capture and Separation of Cysteine-Containing Peptides[END_REF] Such sulfhydryl functions were used to tightly immobilize gold nanoparticles. Alternatively, our group took advantage of the amine functions of both cysteamine and ethylene diamine to anchor gold nanoparticles at the pore surface of Nacryloxysuccinimide-based monolithic columns. [START_REF] Guerrouache | Sitespecific immobilisation of gold nanoparticles on a porous monolith surface by using a thiolyne click photopatterning approach[END_REF] On top of that, Liu et al. recently used another imidazole containing ligand, i.e. histamine, to the same purpose. Site-specific immobilization of commercially available preformed gold nanoparticles was achieved upon straightforward dynamic loading of the functionalized monolithic capillary with a gold colloidal solution. [START_REF] Poupart | Gold nanoparticles immobilized on porous monoliths obtained from disulfide-based dimethacrylate: Application to supported catalysis[END_REF] Other strategies to efficiently anchor gold nanoparticles onto porous materials relied on their preparation through an in-situ process involving percolation of a chloroauric acid solution as a AuNP precursor and subsequent reduction of metallic ions with sodium borohydride [START_REF] Poupart | Gold nanoparticles immobilized on porous monoliths obtained from disulfide-based dimethacrylate: Application to supported catalysis[END_REF] or via the Turkevitch method [START_REF] Turkevich | A study of the nucleation and growth processes in the synthesis of colloidal gold[END_REF] involving citrate. [START_REF] Kimling | Turkevich Method for Gold Nanoparticle Synthesis Revisited[END_REF] A two-step process was necessary for preparing amine-bearing porous monoliths within fused silica capillaries (inner diameter of 100 µm), as shown in Scheme 1. After activation of the inner wall of the capillary in the presence of γ-MAPS, a functional reactive acrylic monomer, i.e. Nacryloxysuccinimide (NAS), and EGDMA as a cross-linking agent were copolymerized in the presence of AIBN (1 wt.% with respect to the total amount of co-monomers) and toluene as a radical initiator and a porogenic solvent, respectively. Upon filling the pre-activated capillary with this polymerization mixture, sealing of the extremities with rubber septa and subsequent photo-initiated copolymerization within a UV oven at 365 nm, an in-capillary Poly(NAS-co-EGDMA) monolith was obtained with nucleophile-sensitive N-hydroxysuccinimide activated ester functionality and well-defined interconnected pores in the micrometer-sized range, as reported in previously published results in the field. [START_REF] Khalil | Gold-decorated polymeric monoliths: In-situ vs ex-situ immobilization strategies and flow through catalytic applications towards nitrophenols reduction[END_REF][START_REF] Carbonnier | CEC separation of aromatic compounds and proteins on hexylamine-functionalized N-acryloxysuccinimide monoliths[END_REF][START_REF] Dao | Thiol-yne Click Adamantane Monolithic Stationary Phase for Capillary Electrochromatography[END_REF][START_REF] Guerrouache | In situ functionalization of N-acryloxysuccinimide-based monolith for reversed-phase electrochromatography[END_REF][START_REF] Guerrouache | Functionalization of Macroporous Organic Polymer Monolith Based on Succinimide Ester Reactivity for Chiral Capillary Chromatography: A Cyclodextrin Click Approach[END_REF][START_REF] Guerrouache | Capillary columns for reversed-phase CEC prepared via surface functionalization of polymer monolith with aromatic selectors[END_REF][START_REF] Tijunelyte | Hydrophilic monolith with ethylene glycol-based grafts prepared via surface confined thiolene click photoaddition[END_REF] A post-polymerization modification was then investigated on the as-obtained

Poly(NAS-co-EGDMA) monolith so as to functionalize the N-hydroxysuccinimide functions with nucleophilic amine derivatives from ethylene diamine (Figure S1).

In-situ formation of AuNPs

Immobilization of AuNPs onto porous polymers has been well-described in the literature in the last decades, notably by taking advantage of the chemical functions exhibited at the pore surface, including thiol-, [START_REF] Poupart | Gold nanoparticles immobilized on porous monoliths obtained from disulfide-based dimethacrylate: Application to supported catalysis[END_REF][START_REF] Zhu | Facile and green synthesis of welldispersed Au nanoparticles in PAN nanofibers by tea polyphenols[END_REF] cyano, [START_REF] Zhu | Facile and green synthesis of welldispersed Au nanoparticles in PAN nanofibers by tea polyphenols[END_REF] and amine [START_REF] Liu | Gold Nanoparticle-Supported Histamine-Grafted Monolithic Capillaries as Efficient Microreactors for Flow through Reduction of Nitro-Containing Compounds[END_REF][START_REF] Poupart | Porous Gold Nanoparticle-Decorated Nanoreactors Prepared from Smartly Designed Functional Polystyrene-block-Poly(d,l-Lactide) Diblock Copolymers: Toward Efficient Systems for Catalytic Cascade Reaction Processes[END_REF] functions, to cite but a few. Surprisingly, even if some investigations on the crucial role of the chemical function on the AuNPs adsorption has been reported (thiol vs. amine for instance), [START_REF] Poupart | Gold nanoparticles immobilized on porous monoliths obtained from disulfide-based dimethacrylate: Application to supported catalysis[END_REF] no comparative study has hitherto been investigated on the influence of ligands over AuNPs adsorption. To fill this gap, we thus focused on the effect of different ethylenediamine-based ligands on the shape, morphology, dispersion and coverage density of in-situ generated AuNPs over such amine functionalized Poly(NAS-co-EGDMA)-based monolithic capillaries. The adsorption of AuNPs was achieved through a two-step in-situ strategy. First, chloroauric acid solution (HAuCl 4 ) was percolated through the polymeric monolith. Then, the hydride-mediated reduction of Au 3+ cations was achieved by flowing a NaBH 4 solution through the capillary and allowed for the preparation of hybrid AuNPs@polymeric monoliths. Semi-quantitative EDX analyses of the different resulting amine-functionalized hybrid monolithic capillaries were realized to investigate the surface composition of the porous monoliths after grafting of the amine ligands and in situ synthesis of the gold nanoparticles. Thereby, the prominent features in the EDX patterns are assigned to the characteristic chemical signals of carbon, nitrogen, oxygen and gold (Figure S2). Such profiles are in full agreement with the Raman data and confirm not only the successful grafting of the amine ligands but also the presence of gold at the surface of the functionalized monoliths. To gain further information about the surface composition of the hybrid monoliths, the nitrogen over carbon and gold over nitrogen atomic ratios were calculated for the four types of ligands (Figure 2 and Table S1). The highest N/C value was obtained for the monolith functionalized with 1-(2aminoethyl)piperazine (0.19) that is nearly twice the lowest value obtained for the ethylene diamine functionalized monolith (0.1). Rather similar N/C ratio values (~ 0.14) were obtained for the monoliths functionalized with tris(2-aminoethyl)amine and tetraethylenepentamine.

Interestingly, a very different trend was observed when considering the Au/N atomic ratio.

Indeed, the highest value (15.5) was obtained for the TRIS-functionalized monolith while the AEP-and TETRA-functionalized monoliths exhibited the lowest vales (in the range 6-8).

These results clearly highlight the crucial role of the nature of the amine group, primary amine vs. secondary amine, over the nitrogen/amine content. Indeed, it is noteworthy that the nitrogen-rich AEP-and TETRA-functionalized monoliths, containing only or mainly secondary amines for AEP-and TETRA-functionalized monoliths, respectively, provide a low efficiency for the amine-driven immobilization of gold nanoparticles. In contrast, the TRIS-functionalized monolith bearing two primary amine groups per grafted ligand exhibits the highest efficiency in terms of nitrogen-mediated gold capture. An intermediate behaviour is observed for the ethylene diamine-grafted monolith. Both size, shape, surface density and dispersion of in-situ synthesized AuNPs at the surface of functionalized monoliths was varied depending on the nature, primary vs. secondary, of the amine group (Table S2). The grafting of TRIS on the surface of Poly(NASco-EGDMA) monolith allowed for obtaining low-sized spherical nanoparticles with diameters below 20 nm (9±3 nm), barely distinguishable by SEM, and homogeneously and densely distributed over the monoliths' surface (Figure 3 c). Larger gold nanoparticles with size in the 60 -200 nm range and different shapes were observed on the surface of the EDA-and AEPfunctionalized monolith (Table S2). These nanostructures are sparsely but homogeneously present at the monolith surface. Based on the EDX results, it is assumed that low-sized gold nanoparticle that cannot be observed through SEM analysis are also present at the surface of such monoliths. A very different morphology was observed for the TETRA-functionalized monolith for which a rather inhomogeneous disparity of the nanoparticles diameter along with the presence of many aggregates were obtained.

Supported catalysis associated with reduction of nitroarenes

AuNPs are prone to act as nanocatalysts for the reduction of nitroarene compounds in the presence of sodium borohydride. [START_REF] Kuroda | Reduction of 4-nitrophenol to 4-aminophenol over Au nanoparticles deposited on PMMA[END_REF] To this purpose, different solution of nitroarenes (namely o-nitrophenol, p-nitroaniline and 4-chloro-7-nitrobenzofurazan, see Figure 4 Solutions harvested at the outlet of the four different capillaries were analyzed by offline UV-Visible spectrophotometry (Figure 5). The four hybrid monoliths did not exhibit significant differences in their catalytic efficiency and complete conversion was obtained for o-nitrophenol and p-nitroaniline. This result was rather in contradiction with previous contributions that hypothesized that the smaller the nanoparticles, the higher the yield.

Considering the catalytic reduction of 4-chloro-7-nitrobenzofurazan, a not commonly studied compound, one may conclude that the lowest yield of conversion (32%) was obtained for the TETRA-functionalized monolith exhibiting gold nanoparticle aggregation (Table 1). In contrast, the highest yields of conversion were obtained for the EDA-(73%) and TRIS-functionalized (76%) monoliths for which the gold nanoparticles are chelated by primary amines. Decreasing the flow rate enabled increasing the yield of conversion and complete catalytic reduction of 4-chloro-7-nitrobenzofurazan was obtained for a flow rate of 1µL.min -1 .

Conclusions

The different amine ligands deriving from ethylenediamine exposed at the pore surface of in-capillary monoliths play a key role regarding the subsequent immobilization of gold nanoparticles on such macroporous materials. Even if the gold content immobilized at the interface of monoliths is similar, SEM micrographs clearly demonstrate that the shape and the diameter of adsorbed gold nanoparticles depend on the grafted ligand, while the homogeneity of the dispersion of nanogold and/or the presence of aggregates at the pore surface is also impacted by the nature of the ligand grafted on such porous materials. Finally, such differences in the nanogold shape, diameter and dispersion homogeneity have a significative influence regarding the catalytic reduction of a variety of nitroarenes.

Unexpectedly, best nitroarene reduction yield are obtained with monoliths on which are immobilized AuNPs with the largest size (i.e. EDA and AEP).

This investigation paves the way to the development of tailored amine-functionalized monoliths for the supported heterogeneous catalysis of a wide array of organic reactions.

  Scheme 1. Schematic representation of the synthetic route applied to the design and synthesis of Poly(NAS-co-EGDMA) monoliths grafted with different ethylene diamine derivatives for the immobilization of in-situ generated gold nanoparticles: i) Poly(NAS-co-EGDMA)

  Four different derivatives were assessed, namely ethylene diamine (EDA), tris(2-aminoethyl)amine (TRIS), tetraethylenepentamine (TETRA), and 1-(2-aminoethyl)piperazine (AEP). Flushing of the different amine bearing ligands through the monolithic capillary allowed for a total consumption of the N-hydroxysuccinimide activated ester moieties of the monolith. This was ascertained by in-situ Raman spectroscopy of the in-capillary monoliths before and after postpolymerization modification that pointed out the complete disappearance of characteristic bands of succinimidyl esters present in the pristine poly(NAS-co-EDMA) at 1730 (imide asymmetric stretching), 1780 (imide symmetric stretching) and 1810 cm 1 (activated ester stretching) corresponding to the carbonyl stretching modes, as already published elsewhere.[40-45, and see FiguresS4-6] Additionally, the occurrence of bands assigned to amide bonds (at 1300 cm -1 , 1550 cm -1 and 1650 cm -1 ), as shown in Figure1, confirmed the covalent attachment of the four different ethylene diamine-based ligands at the monolith surface through amide bond coupling.[START_REF] Khalil | Gold-decorated polymeric monoliths: In-situ vs ex-situ immobilization strategies and flow through catalytic applications towards nitrophenols reduction[END_REF] Finally, a band assigned to the -NH stretching mode was observed at 3300 cm 1 in all the amine-functionalized Poly(NAS-co-EGDMA) incapillary monoliths. This clearly demonstrated the efficiency of the grafting onto the functional reactive NHS functionalities by a straightforward nucleophilic substitution.

Figure 1 .

 1 Figure 1. Raman spectra of Poly(NAS-co-EGDMA)-based monoliths a) before or after functionalization with b) EDA, c) AEP, d) TRIS or e) TETRA ligands.

Figure 2 .

 2 Figure 2. a) Nitrogen over carbon and b) gold over nitrogen atomic ratios as determined by EDX for hybrid EDA-, AEP-, TRIS-and TETRA-functionalized monoliths with supported gold nanoparticles.

Figure 3 .

 3 Figure 3. Magnification of SEM pictures of gold nanoparticles immobilized through the in-situ strategy onto a) EDA-, b) AEP-, c) TRIS-and d) TETRA-functionalized Poly(NASco-EGDMA) monoliths. Scale bar = 100 nm.

  ) along with NaBH 4 in water were freshly prepared and injected through the loop of an HPLC pump to a 10 cm long EDA-, AEP-, TRIS-or TETRA-functionalized NAS-based capillary containing in-situ generated AuNPs at a flow rate of 3 µL.min -1 .

Figure 4 .

 4 Figure 4. Chemical structure of the different nitro-containing aromatic compounds reduced by in-capillary flow-through AuNP-catalyzed reaction in the presence of an excess of NaBH 4 .

Figure 5 .

 5 Figure 5. UV-visible absorption spectra recorded in the 200550 nm wavelength range for solutions of a) o-nitrophenol, b) p-nitroaniline and c) 4-chloro-7-nitrobenzofurazan in ethanol supplemented with an excess of NaBH 4 before (black trace) and after percolation through EDA-(red trace), AEP-(pink trace), TRIS-(green trace) or TETRA-(blue trace) functionalized monolithic capillaries with supported gold nanoparticles.

Table 1 .

 1 Yields obtained upon flow-through reduction of the different nitroaromatic compounds in the hybrid microcolumns depending on the grafted ethylene diamine derivative.

	Ligand		Yield (%)
	immobilize d	o-nitro-phenol	p-nitro-aniline	4-chloro-7-nitro-benzofurazan

Acknowledgements

This manuscript is in honor of the 50-year anniversary of the French Polymer Group (Groupe Français d'études et d'applications des Polymères -GFP). Financial support from CNRS and UPEC is gratefully acknowledged. The authors are indebted to UPEC for providing R. P. with a PhD grant.


D Pelati

G Patriarche

O Mauguin

L Largeau

L Travers

F Brisset

F Glas

F Oehler

GaAs (111) epilayers grown by MBE on Ge (111): twin reduction and polarity

Keywords: Molecular Beam Epitaxy, GaAs, Ge, twin, defect, (111), heteroepitaxy

We perform the growth of GaAs (111) epilayers on nominal Ge(111) wafers by molecular beam epitaxy (MBE). The polarity of GaAs is (111)A and homongeneous over the full area, as measured by transmission electron microscopy and high energy electron diffraction. This orientation conflicts with the common growth model for GaAs on Ge(111). Twinned domains are the main defects in our GaAs (111) epilayers. Using cathodoluminescence, we observe that some twin boundaries hold large number of non-radiative recombination centers. During growth, we find that only a narrow domain of As:Ga ratios lead to the growth of smooth and twin-free GaAs (111)A epilayers. At low As:Ga ratio, the surface is rough; while at high As:Ga ratio the epilayers present large densities of twinned domains.

The heteroepitaxy of III-V materials on group IV substrates presents many challenges. First, most of the III-V and group IV materials are cubic crystals of different lattice parameters, which leads to the formation of dense arrays of dislocations at the III-V / IV-IV interface [START_REF] Gerthsen | Mis-434 fit dislocations in gaas heteroepitaxy on (001) Si[END_REF].

Second, the large thermal mismatch between the III-V epilayer and its substrate can lead to cracks after the growth, during the cool down to room temperature [2].

Currently, the direct growth of good quality III-V planar material on silicon is only possible using cunning selective area growth schemes such as InP [3] or InAs [START_REF] Holland | Atomically flat and uni-451 form relaxed III-V epitaxial films on silicon substrate for hetero-452 geneous and hybrid integration[END_REF] on Si (100).

Uniquely, Ge substrates provide a near lattice (-0.9%, a GaAs =5.653Å, a Ge = 5.658Å) and thermal (-2.9%, K GaAs =5.73 10 -6 K -1 , K Ge =5.90 10 -6 K -1 ) match to GaAs, so that GaAs/Ge epilayers are relatively immune to the dislocation-and crack-related problems above. However, even in this favorable configuration, the lower symmetry of the GaAs crystal permits the growth of III-V seeds of different epitaxial relationship with the substrate. If the initial GaAs nucleation is uncontrolled, the final epilayer may consist in individual domains, separated by defective interfaces [START_REF] Lucci | Universal description of III-V/Si epitaxial growth pro-459 cesses[END_REF]. To avoid * Email address: fabrice.oehler@c2n.upsaclay.fr (F. Oehler) this problem, two conditions need to be fulfilled. First, ing to produce double-step surfaces [START_REF] Pukite | Suppression of antiphase domains in the 462 growth of GaAs on Ge(100) by molecular beam epitaxy[END_REF], which may be 33 combined with dedicated nucleation schemes to initiate 34 the growth [START_REF] Sieg | Anti-phase domain-free growth of GaAs on offcut (001) 467 Ge wafers by molecular beam epitaxy with suppressed Ge outd-468 iffusion[END_REF]. In Fig. 6, we investigate the polarity of the GaAs the MBE growth GaAs/Ge(111) using a GaSb buffer to 305 minimize the twin density, in line with other attempts 306 using Sb on GaAs/Si(111) [START_REF] Morohara | Sb irradia-531 tion effect on growth of GaAs thin film on Si (111) substrate[END_REF] and Sb pretreatment in 307 GaSb/Si(111) [START_REF] Toyota | Study 535 of Sb template for heteroepitaxial growth of GaSb thin film on 536 Si(111) substrate[END_REF].

308

The assessment of the epilayer 'quality' with re-309 spect to twin defects is not an easy task. XRD results, From figures 2 and 3, we infer that there is an op-329 timum value of the As:Ga ratio, which is rather unex-330 pected. At low As:Ga ratio (31:1) the surface is rough 331 with some trace of first order twinning (R 1 ), while it is 332 much smoother and slightly less twinned at intermedi-333 ate As:Ga ratio (62:1). The later is coherent with ex-334 isting studies on the homoepitaxy of GaAs(111)A by 335 MBE [START_REF] Esposito | San-539 guinetti, Ehrlich-schwbel effect on the growth dynamics of 540 GaAs(111)A surfaces[END_REF][START_REF] Sato | The growth of high 543 quality GaAs on GaAs (111)A[END_REF][START_REF] Ritzmann | Overcoming ehrlich-schwbel barrier in (111)A GaAs 548 molecular beam epitaxy[END_REF]. Careful observation of the Figure 1 336 in ref. [START_REF] Ritzmann | Overcoming ehrlich-schwbel barrier in (111)A GaAs 548 molecular beam epitaxy[END_REF] shows that small twinned domains form at 337 600 • and low As:Ga ratio, while no twins are observed 338 for higher As:Ga ratios, which is similar to our results.

339

After the initial improvement with increasing V:III 340 ratio (31:1 →62:1), we observe a stark degradation of 341 the surface morphology, Fig. 2, and layer microstruc-342 ture, Fig. 3, at higher As:Ga BEP ratio (94:1). Such 343 optimal As:Ga ratio is not found in homoepitaxial 344 studies [START_REF] Esposito | San-539 guinetti, Ehrlich-schwbel effect on the growth dynamics of 540 GaAs(111)A surfaces[END_REF][START_REF] Sato | The growth of high 543 quality GaAs on GaAs (111)A[END_REF][START_REF] Ritzmann | Overcoming ehrlich-schwbel barrier in (111)A GaAs 548 molecular beam epitaxy[END_REF], which only report a monotonic

24 the

 24 initial covalent bounds between the GaAs crystal 25 and the Ge substrate need to be uniquely determined: 26 either Ge-Ga or Ge-As. Then, the surface of the sub-27 strate needs to present only atomic steps or step bunches 28 which only shift the III-V lattice by one full monolayer 29 or multiple thereof. In the case of GaAs on Ge(100) 30 grown by Molecular Beam Epitaxy (MBE), this often 31 requires the use of offcut Ge wafers and specific anneal-32

35Figure 1 :Figure 2 :

 12 Figure 1: RHEED characterization before and GaAs growth (∼9 nm thick) using 62:1 BEP ratio. (a) Ge(111) 1 × 1 before growth. (b) GaAs 2 × 2 at 550 • C. (c) GaAs 2 × 2 at 635 • C.

89Figure 4 :Figure 4

 44 Fig.1(c). We observe that GaAs (111) RHEED pattern 109

Figure 5 :

 5 Figure 5: (a) SEM-EBSD characterization of a small rough area showing the emergence of two small twinned domains (orange) in the main layer (purple). The orientation of the crystal is indicated by a color relative to its Euler angles with the EBSD detector. The projected cubic unit cell is shown as a pink polygon. Crystal orientations are labeled with matching colors. (b) Large scale SEM-ESBD characterization. Rough and twin area are indicated by red circles. The EBSD color scale is the same as in (a). (c) Room temperature SEM-CL (6 kV) of a small twinned area. The CL color scale is proportional to the integrated GaAs peak intensity. The dotted red line indicates the estimated position of the defective twin boundary.

Figure 6 :

 6 Figure 6: Schematic of (a) GaAs(111)A, and (b) GaAs(111)B atomic structure. (c) Atomically resolved STEM-EDX of the GaAs layer grown at intermediate As:Ga ratio (62:1), viewed along the [110] zone axis. Insert shows the corresponding STEM-HAADF image.

248

  smooth epilayer grown at intermediate BEP ratio (62:1), 249 previously analyzed in Fig. 3 & 4. Fig. 6 (a) and 250 (b) show the crystal structure of GaAs(111)B and 251 GaAs(111)A respectively. For GaAs(111)B, Fig. 6(a), 252 the As atom terminates the As-Ga dumbbell (dotted cir-253 cle) while it is opposite for GaAs(111)A, Fig. 6(b). The 254 direct comparison with the atomically resolved STEM-255 EDX map, Fig. 6(c), demonstrates that the GaAs epi-256 see Fig. 4, do not change the layer polarity.

1 .

 1 Antiphase and twin boundaries

  310

Fig. 3 ,Fig. 5 ,

 35 Fig. 3, only indicate the volume fraction of the twinned 311

  328

350 1 ×Fig. 5 ,Fig. 7 (Figure 7 : 10 400

 157710 Fig. 5, shown that the epilayer is homogeneous on a 369

408 4 .

 4 Conclusion 409We demonstrate the first twin-free and defect-free 410 growth of GaAs(111)A epilayers on nominal Ge(111) 411 wafers by MBE. While the observed GaAs(111)A po-412 larity is unexpected, it is nonetheless homogeneous and 413 robust. Compared to the complex growth procedures 414 used in the case of GaAs/Ge(100), the growth of defect-415 free GaAs on Ge(111) can be carried in a single step at 416 610 • C. No antiphase domains form on the Ge(111) sur-417 face but twinned domains appear if the As:Ga ratio is 418 not adapted. Some twin boundaries hold large number 419 of non-radiative recombination centers and it is critical 420 to limit their density. The later can be minimized by the 421 careful tuning of the As:Ga flux ratio during growth, 422 without the use of any additional chemical element or 423 buffer layer. 424 Acknowledgement 425 The authors thank the Institut Photovoltaique d'Ile 426 de France (IPVF) for financial support under frame-427 work project E.3. The authors also acknowledge ANR 428 Investissement d'Avenir program (TEMPOS Project 429 ANR-10-EQPX-50) for having funded the acquisition 430 of the NANOTEM platform and the TEM-STEM (FEI 431 Titan Themis) used in this work. nal of Applied Physics 114 (3) (2013) 033708 (Jul. 2013).

tions in this temperature range[START_REF] Woolf | The homoepitax-559 ial growth of GaAs(111)A and (111)B by molecular beam epi-560 taxy: an investigation of the temperature-dependent surface re-561 constructions and bulk electrical conductivity transitions[END_REF]. The detailed TEM

on Si and SiGe virtual substrates, Journal of Applied Physics The Figure S1 shows the chemical analysis of the GaAs/Ge interface using STEM-HAADF and EDX. The exact position of the initial GaAs/Ge(111) growth interface is lost due to the strong inter-diffusion of Ge, Ga and As, which extends over ∼2 nm (5-6 ML) in our growth conditions. Yet, the approximative location of the initial GaAs/Ge interface is contained between the two white dashed lines. From Fig. S1(e), we see that the oxygen content is homogeneous over the whole cross section, without any visible variation around the estimated position of the initial interface.

Similarly, the HAADF image show no variation of contrast near or across the approximative position of Ge/GaAs interface.