A One-Pot Approach to Mesoporous Metal Oxide Ultrathin Film

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A One-Pot Approach to Mesoporous Metal Oxide Ultrathin Film Electrodes Bearing One Metal Nanoparticle per Pore with Enhanced Electrocatalytic Properties Amandine Guiet,† Tobias Reier,† Nina Heidary,† Diana Felkel,† Benjamin Johnson,‡ Ulla Vainio,§ Helmut Schlaad,∥ Yilmaz Aksu,†,⊥ Matthias Driess,† Peter Strasser,† Arne Thomas,† Jörg Polte,# and Anna Fischer*,† †

Department of Chemistry, Technical University Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany Inorganic Chemistry Department, Fritz Haber Institute, Faradayweg 4-6, 14195 Berlin, Germany § Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Strasse 1, 21502 Geesthacht, Germany ∥ Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424 Potsdam, Germany ⊥ Department of Material Science and Engineering, Akdeniz University, Faculty of Engineering , Dumlupinar Bulvari, 07058 Antalya, Turkey # Humboldt University of Berlin, Brook-Taylor-Strasse 2, 12489, Berlin, Germany ‡

S Supporting Information *

ABSTRACT: The controlled incorporation of single metal nanoparticles within the pores of mesostructured conducting metal oxide ultrathin films is demonstrated, taking advantage of the controlled metal precursor loading capacities of PS-b-P4VP inverse micellar templates. The presented one-pot approach denoted as Evaporation-Induced Hydrophobic Nanoreactor Templating (EIHNT) unusually involves the nanostructuration of the metal oxide via the hydrophobic shell of the micellar template, while the concomitant nanostructuration of the metal is achieved via its confinement in the hydrophilic micellar core. This approach is applied to tin-rich ITO and gold, to yield unique mesoporous tin-rich ITO ultrathin film electrodes remarkably loaded with one size-controlled gold nanoparticle per pore. Interestingly, the resulting tin-rich ITO-supported gold nanoparticles exhibit improved catalytic activity and durability in electrocatalytic CO oxidation compared to similarly sized gold nanoparticles supported on conventional ITO coatings. KEYWORDS: inverse micelles, mesoporous thin films, electrocatalysis, gold nanoparticle, tin-rich ITO

1. INTRODUCTION

via the thermal decomposition at low temperature of indium(I) tin(II) tri-tert-butoxide12 (ITBO), advantageously combines a high conductivity and a 40% decreased indium content.10,11 The availability of conducting metal oxides in general, and in particular of this novel ITOTR, as mesoporous thin film electrodes with tunable and uniform pore sizes is of particular interest for the immobilization and stabilization of metallic NPs and the further development of electrocatalytic systems. Existing preparative approaches to mesoporous metal oxidesupported NPs are either based on metal precursor impregnation into preformed mesoporous supports or on one-pot approaches, in which metal precursors or preformed NPs are introduced into the mixed template/metal oxide precursor sol.13−16 However, these strategies offer only limited simultaneous control over the collective properties of the

The design of high surface area conducting metal oxide thin film electrodes containing monodisperse, well-distributed, and stable metal nanoparticles (NPs) is a challenging issue in electrocatalysis.1 In fuel cells conductive carbon materials are commonly used as catalyst supports for NPs. System stability is limited, however, because of oxidative carbon corrosion and NP agglomeration/ripening favored by weak metal−carbon interactions.2,3 Conductive metal oxides are promising alternative supports, as they offer higher stability in oxidizing environments along with tunable charge carrier properties as required for electrocatalysis. In addition, oxide supports can positively influence the catalytic activity and stability of supported metallic NPs via strong metal−support interactions or synergistic effects.4−6 In this context, tin oxide and indium tin oxide (ITO) have been employed as improved conductive catalyst supports.7−9 Recently, amorphous but conductive tinrich ITO (ITOTR) was reported as a promising alternative to regular ITO.10,11 This novel ITOTR material, which is formed © 2013 American Chemical Society

Received: April 8, 2013 Revised: September 17, 2013 Published: October 22, 2013 4645

dx.doi.org/10.1021/cm401135z | Chem. Mater. 2013, 25, 4645−4652

Chemistry of Materials

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Scheme 1. Evaporation-Induced Hydrophobic Nanoreactor Templating (EIHNT) Using Preformed PS-b-P4VP Inverse Micelles as Dual Templates for Concomitant Metal Oxide and Metal Templating

inverse micellar templates can be used for the facile and tailored synthesis of mesostructured metal oxide ultrathin films bearing one metallic NP per pore (Scheme 1, d → e → f). Moreover, the latter hydrophobic templating approach offers the possibility to structure, in a highly defined manner, metal oxides with complex compositions, in particular those which are not accessible through regular sol−gel processes. This novel approach, denoted as evaporation-induced hydrophobic nanoreactor templating (EIHNT), is exemplified in the following by the synthesis of mesostructured tin-rich ITO (ITOTR) ultrathin films loaded with one gold nanoparticle per pore. The asobtained ultrathin film electrodes, with an unusual tin-rich composition and unique structural features, exhibit improved activity and durability for the electrocatalytic oxidation of CO.

supported NPs, such as (i) NP size distribution, (ii) NP spacing, (iii) NP dispersion/location/accessibility, and (iv) NP loading, crucial parameters defining the activity and stability of the resulting NP catalyst.17 Mesoporous metal oxide thin films are typically synthesized through evaporation-induced self-assembly (EISA).18 In EISA, sol−gel-based metal oxide formation is combined with the assembly of hydrophilic metal oxide precursor sols within the hydrophilic domain, typically poly(ethylene oxide), of selfassembled amphiphilic block copolymer mesophases.19−26 Consequently, one straightforward approach to exclusively incorporate metal NPs within the pores of mesoporous metal oxide thin films involves the incorporation of hydrophobic molecular metal precursors into the hydrophobic domain of the micellar pore template.27,28 However, micelle formation occurs often only during the solvent evaporation step, affording balanced kinetics of micelle formation/organization, metal precursor incorporation, and metal oxide condensation. To overcome those difficulties, the use of preformed block copolymer micelles in solution with controlled metal precursor loading capacities would be an effective alternative approach. Polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) forms highly stable inverse micelles (PS shell, P4VP core) in apolar solvents at very low concentrations (cmc ≈ 10−7 M).29 In those micelles, the hydrophilic P4VP micellar core provides a defined compartment for the controlled incorporation of a broad range of hydrophilic metal precursors (e.g., HAuCl4, AgNO3, H2PtCl6, Na2PdCl4). Accordingly, PS-b-P4VP inverse micelles are widely employed as exotemplate/nanoreactor in metallic NP synthesis30−32 and for the preparation of metal NP arrays via micelle nanolithography.33−37 Despite their versatile properties, the utilization of PS-b-P4VP inverse micelles as hydrophobic endotemplates for the synthesis of mesostructured metal oxide thin films has yet to be explored, even though the assembly of hydrophobic metal oxide precursors within the hydrophobic PS domain of PS-b-PMMA mesophases has been previously discussed.38 In this work, we revisit the idea of evaporation-induced hydrophobic assembly between hydrophobic metal oxide precursors (i.e., nonhydrolyzed metal alkoxides) and preformed PS-b-P4VP inverse micelles in solution and apply it for the synthesis of mesostructured metal oxide ultrathin films (Scheme 1, a → b → c). We further demonstrate that, when loaded with a hydrophilic molecular metal precursor, these

2. EXPERIMENTAL SECTION 2.1. Materials. Tetrachloroauric(III) acid (HAuCl4·3H2O, Roth) was used as received and stored in a glovebox. Polystyrene-blockpoly(4-vinylpyridine) (PS111-b-P4VP96) was synthesized as reported in literature29 by sequential anionic polymerization of styrene and 4vinylpyridine (initiator: sec-butyllithium/LiCl, solvent: THF, −78 °C). The molar ratio S:4VP = 1.15:1 was determined by 1H NMR (CDCl3). The number- and weight-average molecular weights (Mn = 21700 g/mol and Mw = 27500 g/mol, respectively), as well as the polydispersity index (Mw/Mn = 1.27) were determined by size exclusion chromatography (THF, PS calibration). Indium(I) tin(II) tri-tert-butyloxide (denoted as ITBO), the single source heterobimetallic tin-rich ITO (denoted as ITOTR) precursor, was synthesized as previously reported.10,11 Briefly, to a suspension of InBr (2.73g, 14 mmol) in anhydrous toluene (inert conditions) was added a concentrated toluene solution of Na(OtBu)3Sn (3.61g, 10 mmol) under vigorous stirring. The reaction mixture was refluxed at 110 °C for 48 h. After cooling to room temperature, insoluble byproducts were removed by filtration. The solvent of the filtrated solution was removed under vacuum, and the obtained white product was recrystallized in pentane and stored at −24 °C. The obtained precursor was characterized by 1H NMR (C6D6, 200.13 MHz, δ = 1.36 ppm [1s, 27 H, OC(CH3)3]) and 119Sn{1H} NMR (C6D6,149.21 MHz, δ = −78 ppm). 2.2. Synthesis of Mesoporous ITOTR and Au NP-Loaded Mesoporous ITOTR Thin Films. For the synthesis of mesoporous ITOTR thin films (mp-ITOTR), PS111-b-P4VP96 was dissolved in a dry and oxygen-free glovebox in anhydrous toluene and stirred overnight. To this inverse micellar template solution was added a concentrated ITBO solution so that m(PS-b-P4VP)/m(ITBO) = 0.02. The resulting solution was stirred for 20 min and subsequently spin-coated at 3000 to 6000 rpm on selected substrates (Si-wafers, glass, or ITO-coated 4646



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performed at a scan rate of 300 mV s−1 with CO flowing above the electrolyte. 2.5. Methods. NMR spectroscopy was performed on a Bruker ARX 200 (1H 200 MHz) and ARX 400 (1H 400 MHz, 119Sn 149.21 MHz) NMR spectrometer. UV−vis spectra were acquired on a PerkinLambda 20 spectrometer under inert atmosphere. X-ray Diffraction (XRD) was performed with a Bruker-AXS D8 Advanced diffractometer with DAVINCI design using Cu Kα radiation (λ = 1.5418 Å) equipped with a LynxEye-detector. Thermogravimetric analysis (TGA) was performed on a Prototyp Thermobalance Linseis L 81/ II in corundum type crucibles under synthetic air (80% N2, 20% O2, gas flow 5 L h−1). Dynamic light scattering (DLS) measurements were performed with an ALV/LSE-5004 correlator and an ALV CGS-3 goniometer with a laser light source (He−Ne laser, 632.8 nm). Measurements were performed at 25.0 °C at scattering angles of 50°, 70°, 90°, 110°, and 130°. Data analysis was performed by inverse Laplace transformation of the electric autocorrelation functions g1 (CONTIN). g1 is obtained from the experimental intensity autocorrelation functions using the Siegert relation. Small-angle Xray scattering (SAXS) was conducted at the B1 beamline of the DORIS III synchrotron source at DESY (Hamburg, Germany). Measurements were made, with a sample-to-detector distance of 3.6 m , at a photon energy of 11.5 keV (λ ≈ 0.1 nm), using a single photon counting pixel detector Pilatus 1 M (Dectris). The two-dimensional images were azimuthally integrated to one-dimensional curves, corrected for sample transmission and background scattering. The data were normalized into an absolute intensity scale using a glassy carbon reference. The quartz capillaries for SAXS measurements were filled with sample solution and sealed in a glovebox and stored on dry ice prior to measurements. The polymer concentration was kept constant. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) measurements were recorded on a TECNAI G220 S-TWIN electron microscope operated at 200 kV, equipped with an EDAX EDX system (Si(Li) SUTW detector, at an energy resolution of 136 eV (for MnK(α)). For sample preparation a drop of diluted solution was deposited onto a carbon-coated copper grid and the excess removed with a filter paper, leaving behind a thin micellar film. Scanning electron microscopy (SEM) was performed using a JEOL 7401F equipped with an inlens secondary electron detector. Image J Version 1.39u (http://rsbweb.nih.gov/ij) was employed to determine the pore diameters, the Au0 NP size distribution, and the FFTs of the SEM micrographs. X-ray photoelectron spectroscopy (XPS) measurements were performed using Mg Kα radiation in a vacuum chamber (base pressure ∼5 × 10−9 mbar) equipped with a Phoibos detector from Specs. All measurements were calibrated with clean, sputtered gold foil (BE(Au0 4f5/2) = 84 eV). Quantitative XPS ratios were calculated, taking into account ionization cross section, number of scans, electron mean free path, and the analyzer transmission function.

glass slides 8−10 Ω/sq (Aldrich)). After solvent evaporation, the spincoated films were dried for 2 h in the glovebox and finally calcined under air (400 °C, 2 h, heating rate: 5 °C min−1) and reducing atmosphere (300 °C, 90 min; 10% H2, 90% N2). For the preparation of mp-Au0NP-ITOTR films, the same procedure was applied. In this case, however, HAuCl4-loaded PS111-b-P4VP96 inverse micelles (HAuCl4@PS111-b-P4VP96) loaded with 0.2 equiv of HAuCl4 per pyridine unit were used as template. For the preparation of HAuCl4@ PS111-b-P4VP96 inverse micelles, HAuCl4 was added to a PS111-bP4VP96 micellar solution, prepared as previously described, and stirred for at least 24 h. The HAuCl4 incorporation into the micellar cores resulted in a yellow coloration of the initially colorless solution. 2.3. Preparation of Au NP Arrays on ITO. Au NP arrays on ITO (Au0NP-ITO) were prepared by depositing a monolayer of HAuCl4@ PS111-b-P4VP96 inverse micelles (0.2 equiv of HAuCl4 per pyridine unit) onto ITO-coated glass slides (Aldrich). The polymer concentration of the dip-coating solution was 8 mg/mL, and the withdrawing speed was 100 mm/min. After drying, the blockcopolymer micelle-modified ITO slide was treated with oxygen plasma (120 W, 0.67 mbar O2 pressure, 2 h). 2.4. Electrochemical Measurements. Gold Characterization by Cyclic Voltammetry. All the electrodes were rinsed with ethanol and Milli Q water and dried under nitrogen flow. Electrochemical measurements were performed with a μAutolab III potentiostat controlled by the GPES software in a three-electrode cell with the prepared thin film electrodes as working electrodes, a Pt wire as counter electrode, and an Ag/AgCl (3.0 M KCl) as reference electrode. To determine the electrochemical gold surface area, the mpAu0NP-ITOTR electrodes were electrochemically cycled under nitrogen atmosphere (six consecutive cycles) in 0.1 M NaOH at a scan rate of 50 mV s−1. The sixth cyclovoltamogramm (CV) was used to estimate the electrochemical gold surface area of the electrodes by calculating the charge needed to reduce the formed surface gold oxide and the reported value of 400 μC/cm2 associated to this process. Electrochemical Measurements of 6-(Ferrocenyl)hexanethiol (FcSH) Immobilized on mp-Au0NP-ITOTR. The respective electrodes were immersed for 72 h in freshly prepared ethanol solutions of 1 mM of 6(ferrocenyl)hexanethiol (Fc-SH) (Sigma-Aldrich). To ensure the removal of any unbound Fc-SH from the electrode surface, the electrodes were immersed in fresh ethanol for 2 h, washed with Milli Q water, and dried under nitrogen flow prior to use. The redox activity of the immobilized Fc-SH was measured in a 0.1 M HClO4 aqueous solution by cyclovoltammetry (CV) using a scan rate of 10 mV s−1. Electrocatalytic CO Oxidation. Electrochemical measurements were performed in a three-compartment electrochemical glass cell with a Luggin capillary and glass frit separating the working and counter electrode compartments. A reversible hydrogen electrode (RHE) (Gaskatel), in the same electrolyte, served as reference and a platinum mesh as counter electrode. The 0.1 M NaOH electrolyte was prepared from sodium hydroxide (Sigma-Aldrich, 99.99%) and deionized water (18 MOhm cm at room temperature). The potential was controlled by a SP-200 potentiostat from BioLogic. The reference gold bulk electrode (cylinder, 5 mm diameter, 99.99%, Pine) was ground (SiC 400, Buehler) and polished successively using diamond pastes (9 μm, 3 μm, 1 μm, Buehler) and alumina (0.05 μm, Buehler) to a mirrorlike surface finish with a semiautomatic polishing machine (Buehler). For the electrochemical measurement, the gold electrode was sealed in a interchangeable Teflon rotating disk electrode holder (Pine) and cleaned for 10 min in a 3:1 mixture of H2SO4 (98%, Roth) and H2O2 (35%, Roth) prior to the experiment. The mp-Au0NPITOTR ultrathin films coated on ITO-coated glass slides were contacted at their upper part using a copper tape with conductive glue (Plano). The electrodes were immersed at 1.2 V into the electrolyte, which was previously degassed for 15 min by nitrogen bubbling. Subsequently, the potential was shifted to 0.75 V, and 30 potential cycles were performed from 0.75 to 1.65 V at a rate of 100 mV s−1. The gold surface area was determined as previously described, using the 30th cycle in this case. Then the electrolyte was CO saturated for 15 min by CO bubbling at an electrode potential of 1.2 V. Afterward, 60 potential cycles between 0.75 and 1.65 V were

3. RESULTS AND DISCUSSION Amorphous tin-rich ITO (ITOTR) coatings with 40% decreased indium content and high conductivities in the range of 200 S/ cm were recently reported as promising alternatives to ITO coatings.10,11 Transparent amorphous ITOTR is formed between 200 °C and 400 °C via the thermal decomposition of indium(I) tin(II) tri-tert-butoxide (ITBO), a hydrophobic bimetallic alkoxide (see XRD and TGA measurements in S.I.1 and S.I.2, Supporting Information).12 Mesoporous ITOTR (mp-ITOTR) as well as mesoporous ITOTR ultrathin films bearing one gold nanoparticle per pore (mp-Au0NP-ITOTR) were prepared with evaporation-induced hydrophobic nanoreactor templating (EIHNT) by simply spincoating precursor solutions composed of toluene, PS-b-P4VP inverse micelles, ITBO, and, if desired, HAuCl4, onto selected substrates (Scheme 1a,d). After solvent evaporation and film drying, ITBO/template hybrid thin films were formed (Scheme 1b,e). Calcination of the films at 400 °C resulted in ITOTR 4647



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Figure 1. SAXS scattering curves (absolute intensities) of (a) PS-b-P4VP inverse micelles in toluene (black curve), (b) PS-b-P4VP inverse micelles after incorporation of HAuCl4 inside the P4VP core (orange curve, HAuCl4@PS-b-P4VP), (c) PS-b-P4VP inverse micelles after ITBO addition (green curve, ITBO/PS-b-P4VP), and (d) HAuCl4@PS-b-P4VP inverse micelles after ITBO addition (khaki curve, ITBO/HAuCl4@PS-b-P4VP) and (e, f) corresponding UV−vis spectra. All fits of the SAXS data can be found in Supporting Information.

that highly defined PS-b-P4VP inverse micelles with an approximately 40 nm P4VP core (in diameter) and a 12 to 22 nm thick swollen PS shell are formed in solution. These PS-b-P4VP inverse micelles with two distinct compartments are used to simultaneously achieve the nanostructuration of (i) the metallic component via the hydrophilic P4VP domain and (ii) the metal oxide component via the hydrophobic PS domain. Gold precursor loading of the micellar core (Scheme 1d) was performed by stirring the micellar solution for 24 h in the presence of HAuCl4 (0.2 equiv of HAuCl4 per pyridine unit). The toluene-insoluble HAuCl4 is incorporated through the formation of an intramicellar [AuCl4]−/[C5H6N]+ complex, resulting in a yellow coloration of the solution, as verified by UV−visible spectroscopy (Figure 1e). The exclusive and homogeneous HAuCl4 incorporation within the P4VP core of the micelles was evidenced by SAXS (Figure 1b). Although the scattering curves before and after HAuCl4 addition (Figure 1b, black curve and orange curve, respectively) exhibit the same scattering profile, the scattering intensity increases with the HAuCl4 addition. This correlates with an increased electron density contrast between the P4VP core and the surrounding (PS shell and solvent), a result of the exclusive HAuCl4 incorporation within the micellar core (see S.I.4 Table 3, Supporting Information, Δηcore (HAuCl4@PS‑b‑P4VP) ∼ 1.9 × Δηcore (PS‑b‑P4VP)). The shape and size of the micelles is mainly unaffected by the metal precursor incorporation (Rh(HAuCl4@PS‑b‑P4VP) ≈ 42 nm, SI. 3, Rcore(HAuCl4@PS‑b‑P4VP) ≈ 20.5 nm, ΔRshell(HAuCl4@PS‑b‑P4VP) = 13.9 nm, Figure 1b and S.I. 4 Table 3, S.I.11, Supporting Information). For the synthesis of mesoporous ITOTR ultrathin films, ITBO was added to PS-b-P4VP and HAuCl4-loaded PS-b-P4VP (HAuCl4@PS-b-P4VP) template solutions. The template to ITBO mass ratio was adjusted to 20 wt % [m(PS-b-P4VP)/ m(ITBO) = 0.2]. The DLS and SAXS data analysis revealed no

formation and template removal (Scheme 1c). In the case of HAuCl4-loaded micelles, calcination of the films resulted in gold salt/HAuCl4 reduction and gold/Au-NP formation (Scheme 1f). To increase the charge carrier concentration in the ITOTR matrix, the samples were additionally annealed at 300 °C under H2/N2, as reported previously.10,11 In the present study, PS111-b-P4VP96 (number/weight average molecular weight: Mn = 21700 g/mol, Mw = 27500 g/mol) was used as template. Dissolving the polymer in anhydrous toluene results in the formation of inverse micelles with a P4VP core and a swollen PS corona, because toluene is a selective solvent for polystyrene (Scheme 1a). Dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) measurements were conducted to determine size and structure of the micelles. From DLS measurements the hydrodynamic radius of the formed micelles (Rh) was determined to be approximately 42 nm (2Rh(DLS) ≈ 84 nm, S.I. 3 and S.I. 11, Supporting Information). SAXS measurements, sensitive to electron density contrast variations on the nanometre scale, were additionally conducted to determine the size and in particular the structure of the micelles in solution. Modeldependent data analysis using a spherical core−shell model revealed the formation of inverse spherical micelles, composed of an approximately 20 nm in radius P4VP core (Rcore = 20 nm, Δηcore = 9.08 × 109 cm−2, S.I. 4 Table 3, Supporting Information) and an appoximately 12 nm thick swollen PS shell (ΔRshell = 12 nm, Δηshell = 9.92 × 108 cm−2, S.I. 4 Table 3, Supporting Information). The overall size of the micelles determined by SAXS is smaller than the hydrodynamic radius determined by DLS (Rcore + ΔRshell ≈ 32 nm vs Rh ≈ 42 nm). This discrepancy most likely results from an underestimation of the PS shell in the model-dependent SAXS evaluation, because the chosen core−shell model does not take into account the increasing swelling of the PS shell. In any case, we can conclude 4648



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Figure 2. TEM micrographs of (a) PS-b-P4VP and (b) HAuCl4-loaded PS-b-P4VP inverse micelles deposited from the solution onto a carboncoated copper grid. TEM micrographs of (c) PS-b-P4VP and (d) HAuCl4-loaded PS-b-P4VP inverse micelles assembled onto a carbon-coated copper grid in the presence of ITBO in the precursor solution. SEM micrographs of (e) mp-ITOTR and (f) mp-Au0NP-ITOTR after calcination (400 °C; left inset: SEM of a single Au0 NP in a ITOTR pore; right inset: FFT).

42 nm, S.I. 3, Supporting Information, Rcore(ITBO/PS‑b‑P4VP) ≈ 20.5 nm, Δηcore = 9.89 × 109 cm−2, ΔRshell(ITBO/PS‑b‑P4VP) = 13.9

significant change in shape and size for unloaded PS-b-P4VP micelles after ITBO addition (Figure 1c, RhITBO/PS‑b‑P4VP) ≈ 4649



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Figure 3. (a) Cyclic voltammograms (CV) of mp-ITOTR (green), mp-Au0NP-ITOTR (red), and polycrystalline gold electrodes (inset) in 0.1 M NaOH. In the plots, the current densities are normalized to the geometric (geo) surface of the electrodes. (b) XPS spectra of mp-Au0NP-ITOTR after calcination, revealing the formation of Au0 NPs. (c) Au0 NP size distribution of mp-Au0NP-ITOTR determined from SEM measurements. (d) CVs of 6-(ferrocenyl)hexanethiol (Fc-SH) immobilized on a polycrystalline gold electrode (inset) and on mp-Au0NP-ITOTR (red curve). No Fc-SH signals are observed on mp-ITOTR (green curve) after exposure to Fc-SH. For all electrodes, blank measurements prior to Fc-SH immobilizations were performed (dashed curves). (e) CVs in 0.1 M NaOH after saturation with CO (60th scan, 300 mV/s) measured for mp-Au0Np-ITOTR (red line) and Au0 NP arrays on ITO (Au0NP-ITO, blue line). Polycrystalline gold does not show significant activity for CO oxidation (black line) as well as mp-ITOTR (inset, green line). In the present case, for a comparison of the activity, the currents are normalized to the Au surface area determined by CV (Au). (f) Current densities (at a potential of 1.015 V) plotted versus scan number (increasing time) measured in 0.1 M NaOH after CO saturation for mp-Au0NP-ITOTR (red curve) and Au0NP-ITO (blue curve).

nm, Δηshell = 9.34 × 108 cm−2, S.I. 4 Table 3, Supporting Information). In contrast to HAuCl4 addition, the addition of ITBO to unloaded PS-b-P4VP micelles does not lead to a substantial increase in scattering intensity at small q (Figure 1c, green curve). Thus, in contrast to HAuCl4, ITBO does not substantially penetrate the P4VP core of the micelles (see S.I. 4 Table 3, Supporting Information, Δηcore ITBO/PS‑b‑P4VP ≈ 1.1 Δηcore PS‑b‑P4VP) and is therefore distributed in the surrounding PS corona as well as in the solvent toluene, a necessary condition to achieve the ITOTR nanostructuration via the hydrophobic PS shell. When ITBO is added to HAuCl4-loaded micelles (i.e., ITBO/HAuCl4@PS-b-P4VP), the solution turns immediately dark-orange (S.I. 5, Supporting Information). The color change does not relate to the formation of Au0 NPs, as no characteristic localized surface plasmon absorption between 500 and 550 nm is observed (Figure 1f). The overall size of the HAuCl4-loaded micelles is barely affected by ITBO addition (2RITBO/HAuCl4@PS‑b‑P4VP(DLS) = 84 nm, S.I. 3, Supporting Information), and the ITBO/HAuCl4@ PS-b-P4VP colloidal solution remains stable (see DLS and SAXS analysis in S.I. 3 and S.I. 4 Table 3, Supporting Information). Only a minor size increase of the P4VP core (10% compared to unloaded micelles) is observed (Figure 1d, brown curve, Rcore(ITBO/HAuCl4@PS‑b‑P4VP) = 21.9 nm vs Rcore(HAuCl4@PS‑b‑P4VP) = 20.5 nm, S.I. 4 Table 3, Supporting Information). This might result from a small ITBO incorporation within the PS/HAuCl4@P4VP interface, an assertion supported by the slight increase of scattering intensity at larger scattering vectors (q) (brown curve versus orange curve, Figure 1d). Considering the hydrate/acidic nature of the gold precursor (i.e., HAuCl4·3H2O), ITBO could indeed undergo a hydrolysis condensation reaction at the PS/

HAuCl4@P4VP interface. However, as no major increase in the scattering intensity is observed at low q (Δηcore (HAuCl4@PS‑b‑P4VP) ≈ Δηcore(ITBO/HAuCl4@PS‑b‑P4VP), S.I. 4 Table 3, Supporting Information), we conclude that the major fraction of ITBO is homogeneously distributed within the solvent and the hydrophobic PS corona (as in the case of unloaded micelles) while the molecular gold precursor remains homogeneously distributed within the micellar core. Consequently, we could achieve a precise and controlled precursor distribution within the two nanocompartments of these inverse micelles in solution, the necessary condition for hydrophobic nanoreactor templating. Spin coating the respective colloidal micellar precursor solutions onto silicon or glass substrates leads to homogeneously patterned hybrid ultrathin films (Scheme 1b,e). At low polymer concentration, inhomogeneous templating is observed (S.I. 6, Supporting Information). As a result of the high stability of the micelles in solution, single isolated pores (reflecting the size of the P4VP core) are imprinted into the ITOTR matrix. Increasing the polymer concentration to 20 wt % leads to regularly templated thin films with a homogeneous substrate coverage. The exclusive location after solvent evaporation of the hydrophobic ITBO precursor in the hydrophobic PS domain of the PS-b-P4VP micellar template was verified by transmission electron microscopy (see Figure 2). For the TEM study, PS-b-P4VP (Figure 2a), HAuCl4@PS-bP4VP (Figure 2b), ITBO/PS-b-P4VP (Figure 2c), and ITBO/ HAuCl4@PS-b-P4VP (Figure 2d) precursor solutions were coated onto carbon-coated copper grids. In the absence of ITBO only the P4VP or the HAuCl4-loaded P4VP core of the micelles can clearly be observed (Figure 2a,b; core diameter ≈ 41 nm, core-to-core distance ≈ 50 nm, S.I. 7, Supporting Information). The surrounding contracted PS domains with a 4650



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arrays with similar NP size (≈8 nm) and NP spacing (≈50 nm) were prepared via micelle nanolithography on commercial ITO substrates (Au0NP-ITO, S.I. 13, Supporting Information).42,43 For this purpose, a monolayer of HAuCl4@PS-b-P4VP micelles was deposited onto an ITO-coated glass slide and subjected to an oxygen plasma treatment.43 This procedure is reported to favor the formation of Au0 NPs with a substantial amount of gold oxide stabilized at their surface, especially true for small Au NPs in combination with metal oxide supports.43 In contrast, as evidenced by XPS, the Au NPs in mp-Au0NP-ITOTR produced by EIHNT do not expose gold oxide on their surface after the synthesis (Figure 3b). This difference between the two types of samples could be evidenced by CV. While in the first cathodic scan a strong gold oxide reduction peak is present for Au0NPITO, this peak is absent in the case of mp-Au0NP-ITOTR electrodes (S.I. 14, Supporting Information, controlled immersion of the electrodes at 1.2 V vs RHE, a potential at which no gold oxidation or gold oxide reduction takes place). Compared to the Au0NP-ITO electrodes, the performance of electrocatalytic CO oxidation by the mp-Au0NP-ITOTR electrodes is highly increased (Figure 3e). The measured specific CO oxidation current density for mp-Au0NP-ITOTR electrodes is initially 5 times and after 60 cycles 1 order of magnitude higher than for Au0NP-ITO. It reflects not only a higher activity but also an improved stability of the ITOTR-supported Au NPs (1st scan: 42.5 mA/cm2Au vs 7.8 mA/cm2Au and 60th scan: 10.5 mA/cm2Au vs 0.8 mA/cm2Au for mp-Au0Np-ITOTR and Au0NpITO respectively, Figure 3f). Indeed, the residual specific current density after 60 cycles accounts for 25% of the initial value for mp-Au0NP-ITOTR and only for 10% for Au0NP-ITO. As in both cases the Au0 NPs have similar sizes (7 nm for mpAu0NP-ITOTR vs 8 nm for Au0NP-ITO), the observed increased activity does not seem to be size related but rather depends on (i) the preparation method, influencing the anchoring of the Au0 NPs on the support and/or the amount of formed gold oxide and/or (ii) the nature and structure of the conductive substrate. Compared to commercial ITOs with low tin oxide content (5−15 mol.%), the amount of tin oxide in ITOTR is ≥50 mol % (XPS, S.I. 12, Supporting Information). As tin oxide has been shown, at least in the gas phase, to promote the CO oxidation over supported Au0 NPs,44 this could explain the observed increased activity.

thickness of 10 nm are barely visible. By contrast, in the presence of ITBO, close packed micellar structures with ITBOrich PS domains are formed (S.I. 8, Supporting Information), as evidenced at higher magnification (Figure 2c,d, insets) and by local EDX measurements (S.I. 9, Supporting Information). In all cases, the structure and size of the micelles determined by TEM are in very good agreement with the size and structure determined by SAXS and DLS. As a result of the exclusive incorporation of the ITBO precursor within the hydrophobic PS domains of the micelles, calcination of the hybrid ITBO/PSb-P4VP and ITBO/HAuCl4@PS-b-P4VP ultrathin films at 400 °C yielded mesoporous ultrathin films (Scheme 1c,f). The formation of ITOTR and Au0 after calcination was verified by XPS (Figure 3b). No evidence was found for the formation of gold oxide. The mp-ITOTR and mp-Au0NP-ITOTR ultrathin films exhibit similar porous structure with large spherical pores imprinted by the P4VP domains of the template micelles (pore diameter ≈ 33 nm; pore wall ≈ 18 nm; average pore-to-pore distance ≈ 50 nm; Figure 2e,f, S.I. 10). The FFT of the SEM images (insets Figure 2e,f) revealed that the porous structure is in between a 2D hexagonal and cubic packing of spherical pores. At higher magnification, the silicon substrate can be identified through the P4VP imprinted pores, indicating the formation of a porous ITOTR monolayer (Figure 2e,f), further confirmed by crosssection SEM measurements (S.I. 6c, Supporting Information). Remarkably, as a result of the exclusive location of the HAuCl4 precursor within the P4VP micellar core, the mp-Au0NP-ITOTR ultrathin films are homogeneously loaded with one Au0 NP per pore with a mean diameter of ∼7 nm (Figure 2f, Figure 3c). The functionality of the as-prepared mp-Au0NP-ITOTR electrodes was evaluated for electrochemical applications such as the (i) selective confined immobilization of thiol terminated redox active probe molecules and (ii) electrocatalytic CO oxidation. The accessibility and anchorage of the Au0 NPs within the pores was investigated with cyclic voltammetry (Figure 3a). The presence of the oxidation and reduction peaks of gold and gold oxide (Eox = 1.53 V and Ered = 1.06 V vs RHE in 0.1 M NaOH) indicates that the Au0 NPs are electrochemically accessible and well contacted with the conductive ITOTR matrix. In the following, if not stated differently, the current densities are normalized to the gold surface area, evaluated from the amount of charge consumed during the reduction of the surface gold oxide formed upon cycling and the charge associated with this process (400 μC/cm2).39,40 Taking advantage of the highly selective thiol−gold interaction, high amounts of 6-(ferrocenyl)hexanethiol (Fc-SH) could be selectively immobilized on the Au0 NPs and electrochemically addressed (Figure 3d, calculated surface coverage ΓFc‑SH = 3.73 × 10−10 mol/cm2Au).41 In addition, the as-prepared mp-Au0NP-ITOTR nanocomposite electrodes show high activity and improved durability for electrocatalytic CO oxidation in CO saturated alkaline media (0.1 M NaOH; Figure 3e,f). While the mp-ITOTR electrodes are inactive for CO oxidation (Figure 3e, green curve), an intense CO oxidation current with a peak current maximum around 1 V (vs RHE) is measured for the mp-Au0NP-ITOTR electrode (Figure 3e, red curve, scanning speed = 300 mV/s, 60th cycle).42,43 After a rapid initial decay (within the 10 first cycles), the specific CO oxidation current density levels down from initially ≈42 mA/cm2Au to a constant but still high value of ≈10.5 mA/cm2Au (Figure 3f). For comparison, 2D Au0 NP

4. CONCLUSION Evaporation-induced hydrophobic nanoreactor templating (EIHNT) has been demonstrated to be a powerful synthetic strategy for the controlled incorporation of single metal NPs within mesoporous conducting metal oxide ultrathin films, taking advantage of the controlled metal loading capacity of PSb-P4VP inverse micelles. This one-pot approach allowed the facile synthesis of unique mesoporous tin-rich ITO ultrathin film electrodes typically loaded with one gold nanoparticle per pore, a notable synthetic challenge. Electrodes prepared with this novel synthetic approach showed very high activity and stability for CO electro-oxidation, when compared to those reported in the literature. Finally, as PS-b-P4VP inverse micelles allow the incorporation of a broad range of NP precursors, this approach opens access to a wide range of tailored multifunctional mesoporous metal−metal oxide nanocomposite thin films for which the function is determined by the crosscombination of the matrix and the filling material. 4651



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ASSOCIATED CONTENT

S Supporting Information *

XRD of tin-rich ITO powders calcined at different temperatures, TGA of the different thin film components, DLS, SAXS model and fits, UV−vis data of the polymer micelles in solution, particle size distributions, SEM and CVs of AuNP arrays on ITO, EDX spectra, and detailed XPS analysis (pdf). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 (0)30-314-22302. E-mail: anna.fischer@tu-berlin. de. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Cluster of Excellence “Unifying Concepts in Catalysis” EXC-314 (supported by the Deutsche Forschungsgemeinschaft and administered by the Technical University Berlin) is gratefully acknowledged. In addition J.P. acknowledges generous funding by the Deutsche Forschungsgemeinschaft within the project PO 1744/1-1. The ZELMI of the Technical University Berlin is gratefully acknowledged for access to the TEM facility and Sören Selve for helpful discussions. Dr. Ralph Krähnert and his group members are gratefully acknowledged for training and access to the SEM facility.

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ABBREVIATIONS ITBO, indium(I) tin(II) tri-tert-butoxide; ITO, indium tin oxide; ITOTR, tin-rich ITO; mp-ITOTR, mesoporous tin-rich ITO ultrathin film; mp-Au0NP-ITOTR, mesoporous tin-rich ITO ultrathin films loaded with one gold nanoparticle per pore; PS-b-P4VP, polystyrene-block-poly(4-vinylpyridine); NP, nanoparticle; EIHNT, evaporation-induced hydrophobic nanoreactor templating; EISA, evaporation-induced self-assembly; CV, cyclic voltammograms; Fc-SH, 6-(ferrocenyl)hexanethiol

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REFERENCES

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A One-Pot Approach to Mesoporous Metal Oxide Ultrathin Film

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