Yolk@Shell Nanoarchitectures with Bimetallic ... - ACS Publications

Subscriber access provided by Northern Illinois University

Article

Yolk@shell nanoarchitectures with bimetallic nanocores – Synthesis and electrocatalytic applications Amandine Guiet, Tobias Unmüssig, Caren Goebel, Ulla Vainio, Markus Wollgarten, Matthias Driess, Helmut Schlaad, Jörg Polte, and Anna Fischer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06595 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 29, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Yolk@shell

nanoarchitectures

nanocores

–

Synthesis

with

and

bimetallic

electrocatalytic

applications Amandine Guiet,†,

‡‡,+

Tobias Unmüssig,#,+ Caren Göbel,

†

Ulla Vainio,‡ Markus Wollgarten,§

Matthias Driess,ζ Helmut Schlaad,∂ Jörg Polte, †† and Anna Fischer.* †,#,‡‡ +

authors have equally contributed to the work

†

Department of Chemistry, Straße des 17. Juni 135, Sek TK1, Technische Universität Berlin,

10623 Berlin, Germany ‡‡

Université Bretagne Loire, Université du Maine, Institut des Molécules et Matériaux du Mans

(IMMM), UMR CNRS 6283, avenue O. Messiaen, 72085 Le Mans, France. #

Freiburger Materialforschungszentrum, Albert-Ludwigs-Universität Freiburg, Stefan-Meier-

Strasse 19, 79104 Freiburg, Germany ‡

Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Straβe 1, 21502

Geesthacht, Germany §

Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109

Berlin, Germany ζ

Department of Chemistry – Metalorganics and Inorganic Materials, Straße des 17. Juni 135,

Sek C2, Technische Universität Berlin, 10623 Berlin, Germany ∂

Institute of Chemistry, University of Potsdam, Karl-Liebknecht-Straße 24-25, 14476 Potsdam,

Germany

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces ††

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Institute of Chemistry, Humboldt University Berlin, Brook-Taylor Str. 2, 12489 Berlin, Hahn-

Meitner-Platz 1, 14109 Berlin, Germany ‡‡

Institute of Inorganic and Analytical Chemistry, Albert-Ludwigs-University Freiburg,

Albertstraβe 21, 79104 Freiburg, Germany

KEYWORDS AgAu alloy nanoparticles, tin-rich ITO, yolk@shell materials, nanoreactor, soft-templating, inverse micelles, polystyrene-block-poly(4-vinylpyridine)

ABSTRACT In the present paper, we demonstrate a versatile approach for the one-pot synthesis of metal oxide yolk@shell nanostructures filled with bimetallic nanocores. This novel approach is based on the principles of Hydrophobic Nanoreactor Soft-Templating and is exemplified for the synthesis of various AgAu@tin-rich ITO (AgAu@ITOTR) yolk@shell nanomaterials. Hydrophobic Nanoreactor Soft-Templating thereby takes advantage of polystyrene-block-poly(4vinylpiridine) inverse micelles as two-compartment nanoreactor template, in which the core and the shell of the micelles serve as metal and metal oxide precursor reservoir, respectively. Taking advantage of this sequestration, the composition, size and number of AuAg bimetallic nanoparticles incorporated within the ITOTR yolk@shell can easily be tuned. Taking advantage of the conductivity of the ITOTR shell and the bimetallic composition of the AuAg nanoparticles, the as-synthesized AuAg@ITOTR yolk@shell materials could be used as efficient electrocatalysts for electrochemical glucose oxidation with improved onset potential when compared to their gold counterpart.


Page 2 of 31

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION In the last decade, the design and synthesis of novel and complex functional nanostructures with improved properties has become a major scope in materials research. Among these, yolk@shell materials (a special case of core@shell materials), composed of nanoparticles of a material A encapsulated inside hollow porous spheres of a material B (A@void@B in short A@B), have been shown to be particular interesting for applications in drug delivery,1–3 gas sensing,4,5 battery research,6,7 and catalysis.8,9 Particularly in the field of heterogeneous catalysis (involving energy storage and conversion), metal@support yolk@shell nanostructures are highly promising as their unique architecture helps stabilizing the active nanometallic core (MNP) against particle leaching and sintering; an essential improvement for long term catalytic activity. In this context, a large number of metal nanoparticle@shell nanocomposites (MNP@shell) have been reported - including MNP@Carbon (MNP = Au, Pt),10,11 MNP@Silica (MNP = Au, Ni),12,13 AuNP@TiO214 and AuNP@ZrO2,15,16 – all of them showing improved properties when compared to their non yolk@shell counterparts. For a given reaction and a given support, the activity of MNP@support yolk@shell catalyst can in principle be improved by alloying the monometallic MNP core with a second metal giving rise to synergistic effects. As such for numerous reactions bimetallic MNP have revealed enhanced activity in comparison to their monometallic counterparts.17,18 However so far, only few reports describe the synthesis of catalytic yolk@shell nanostructures with multimetallic nanocores.19–21 This fact certainly relates to the multitude of synthesis steps involved in the synthesis of state-of-the-art MNP@support yolk@shell materials, involving tricky colloidal synthesis, multi-coating processes and aggressive sacrificial template removal; all of these steps seriously limiting the up-scaling and the diversification of MNP@shell yolk@shell materials.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

To overcome these limitations and take full advantage of the unique properties of MNP@Support yolk@shell materials, it seems essential to develop scalable synthetic approaches which allow controlling all functionality determining parameters such as composition, size and loading of the metallic nanocores as well as composition and structure of the supporting shell material. In this context, we recently introduced a supramolecular approach entitled Hydrophobic Nanoreactor Soft-Templating allowing the one-pot synthesis of monometallic AuNP@ITOTR yolk@shell nanostructures.22 ITOTR thereby stands for amorphous tin-rich Indium Tin Oxide, which in contrast to regular ITO combines high conductivity and transparency along with a 40 % decreased indium content.23,24 Hydrophobic Nanoreactor Soft-Templating, in contrast to regular polar micellar templating for sol-gel based mesoporous oxides,25–27 takes advantage of inverse block copolymer micelles, in particular polystyrene-block-poly(4-vinylpiridine) (PS-b-P4VP) inverse micelles, as two-compartment nanoreactor templates. Despite their versatile properties, the use of PS-b-P4VP inverse micelles as hydrophobic nanoreactor template for the synthesis of yolk@shell has only very recently been explored, even though the assembly of hydrophobic metal oxide precursors within the hydrophobic PS domain of such micelles has been previously discussed.28 While the hydrophilic P4VP micellar core of the micelles serves as reservoir for the hydrophilic and acidic metal precursor (i.e. HAuCl4.3H2O), the hydrophobic PS shell serves for the incorporation of the hydrophobic metal oxide precursor (i.e. indium(I)tin(II)tri-tertbutyloxide, ITBO).29 As a result, hybrid metal-metal oxide core-shell precursor micelles are formed (in solution), which in line with their compartmented structure yield multicore AuNP@ITOTR yolk@shell nanostructures after calcination. As such simple co-loading of the hydrophilic P4VP compartment with two different metal precursors should allow the easy synthesis of bimetallic nanoparticles@metal oxide yolk@shell nanostructures (M1M2NP@MOx). In here, we demonstrate the great potential offered by Hydrophobic Nanoreactor SoftTemplating for the synthesis of bimetallic MNP@MOx yolk@shell structures. As exemplified for ACS Paragon Plus Environment

Page 4 of 31

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AgxAuyNP@ITOTR structures, simple variation of the metal precursor loading within the micellar core allows easy tuning of the AgAu nanoparticles composition incorporated within the porous ITOTR yolk@shell structures. As a result, of their bimetallic composition, the AgxAuyNP@ITOTR structures showed an improved onset-potential for glucose electrooxidaton when compared to AuNP@ITOTR reference materials. As such, Hydrophobic Nanoreactor Soft- Templating represents a versatile synthetic tool-box for the design and optimization of yolk@shell nanocatalysts.

Scheme 1: Schematic illustration of the preparation of bimetallic AgAuNP@ITOTR yolk@shell thin films.

RESULTS AND DISCUSSION As depicted in Scheme 1, AgAuNP@ITOTR yolk@shell structures are obtained by simply mixing four components in toluene: PS111-b-P4VP96 as nanoreactor template, AgNO3, AgClO4.H2O and HAuCl4.3H2O as silver and gold precursors respectively, and In(OtBu)3Sn (ITBO)29 as tin-rich ITO (ITOTR)23,24 precursor. In line with the precursors reactivity, an interfacial sol-gel reaction between the metal precursor loaded P4VP core loaded and the ITBO loaded PS shell is expected to take place at the PS/P4VP interface resulting in the formation of ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

metal-metal oxide core-shell hybrid precursor micelles. Spin-coating results in micelle aggregation and thin film formation while subsequent calcination leads to template decomposition, metal reduction and ITOTR formation, yielding, as demonstrated in the following, highly defined AgAuNP@ITOTR yolk@shell particle thin films, composed of size controlled AuAg nanoalloy particles strongly anchored onto the inner surface of closely packed ITOTR hollow spheres. Similarly to our previous work on AuNP@ITOTR materials,22 inverse micelles of PSx-b-P4VPy in toluene (with x = 111 and y = 96 in toluene) were used as nanoreactor soft-templates. Advantageously, as demonstrated for the intramicellar synthesis of various bimetallic colloids including PtFe,30,31 PtAu,31 AuPd,32 PdAg33 and AgAu34, the P4VP micellar core can easily be loaded with multiple hydrophilic metal precursors; necessary condition to achieve multimetallic M1M2@MOx structures via hydrophobic templating. For our aim of synthesizing AuAgNP@ITOTR yolk@shell nanoparticles, silver and gold precursor incorporation within the P4VP core was realized following a two step loading procedure.34 First the micellar core was loaded with the silver precursor (AgNO3 or AgClO4.H2O) followed in a second step by the co-loading of the gold precursor (HAuCl4.3H2O). The silver incorporation thereby likely involves complexation by the pyridine units while the gold incorporation is based on electrostatic interactions between protonated pyridine units and AuCl4- anions.33 Successful loading of the micellar core was verified by small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and local energy dispersive X-ray spectroscopy (EDX) for loadings of 0.35 and 0.7 equivalents per pyridine units of silver (AgNO3 or AgClO4.H2O) and gold precursor (HAuCl4.3H2O) respectively (Figure 1 and Figure S3).


Page 6 of 31

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1: SAXS scattering curves in absolute intensities of unloaded PS111-b-P4VP96 inverse micelles (a, b black), PS111-b-P4VP96 micelles loaded with 0.35 eq. of AgNO3 (b, blue) and 0.35 eq. of AgClO4.H2O (b, red) (Fit in Figure S3) and corresponding TEM micrographs (e, f). SAXS scattering curves in absolute intensities of AgNO3@PS111-b-P4VP96 (0.35 eq. per pyridine unit) inverse micelles after the addition of 0.7 eq. HAuCl4.3H2O per pyridine unit after 4h of stirring (c) and after 2 days of stirring (d) as well as corresponding TEM micrographs (g, h).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

As seen in Figure 1, a), the SAXS curve of the pristine polymer micelles is typical for spherical non-interacting nano-objects and could best be fitted with a core-shell scattering model yielding an overall micellar size of 64 nm, with a 40 nm in diameter sized P4VP core and an approx. 12 nm thick PS shell (Figure S1, Table S1, S2 and S3). As can be seen in Figure 1, b), the addition of silver nitrate (0.35 eq. per pyridine unit) to the micellar solution leads to a large increase of the scattering intensity along with an increase of the scattering length density difference between the micellar core and the surrounding toluene (∆ηcore). This result clearly demonstrates the quantitative and exclusive incorporation of the silver precursor into the P4VP core (Figure S2, Table S3). It is worth noting that a slightly higher increase of the scattering intensity was observed when 0.35 equivalents of silver perchlorate (AgClO4.H2O) were added instead of silver nitrate (AgNO3) (see Figure 1, b)); result which might be related to the higher scattering density contrast of AgClO4.H2O in comparison to AgNO3. In any case, the shape of the scattering profile remains unchanged upon silver precursor incorporation; clear indication that the spherical shape of the micelles is preserved (Figure S2, Table S3). In addition, the slight shift of the scattering minima to lower scattering vectors (q) upon metal incorporation reveals a slight increase of the core size of the micelles upon silver precursor addition (RPS-b-P4VP = 20.0 nm, RAgNO3(0.35)@PS-b-P4VP = 20.3 nm, RAgClO4(0.35)@PS-b-P4VP = 20.7 nm). This shift is slightly more pronounced in case of AgClO4.H2O, which might result from the bulkier character and the presence of water in AgClO4.H2O when compared to AgNO3 (Figure S2, Table S3). The size and shape of the micelles as well as the exclusive incorporation of the silver precursors within the micellar core was additionally verified by TEM (see Figure 1, f)). As shown in Figure 1, e), weakly contrasted spherical structures corresponding to densely packed P4VP domains are


Page 8 of 31

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


observed for unloaded PS111-b-P4VP96 micelles deposited on a TEM grid. Successful silver precursor incorporation within the P4VP domains was beside a large increase in contrast (see Figure 1) revealed by local EDX measurements (see Figure S3). After successful silver incorporation, HAuCl4.3H2O was added to the silver loaded PS111-b-P4VP96 solution. The incorporation of the gold precursor could be followed by eyes and by UV-Vis spectroscopy, as the solution turned progressively from colorless to yellow (Figure 1, c), g) and Figure S4. Gold precursor addition to the silver loaded micellar solution induces a large increase of the scattering intensity reflecting a large increase of the scattering density contrast between the core and the surrounding medium (∆ηcore) (Figure 1, c), Figure S4). We can thus conclude that the gold precursor diffuses homogenously inside the silver loaded P4VP core to form a homogeneous bimetallic precursor distribution, as further confirmed by TEM and EDX (Figure S3). Interestingly, as revealed by TEM (see Figure 1, g) and h)), prolonged stirring for additional 2 days of the bimetallic solution leads to the formation of 20-25 nm sized nanoparticles (Figure 1, h) black circles); probably a result of a reaction occurring between the two metal precursors and the PS-b-P4VP polymer. This spontaneous MNP formation outside the micelles was further corroborated by SAXS measurements. Indeed, the scattering curves for these samples could only be fitted with a bimodal size distribution (see Figure S5, Table S4) taking into account both the micelles and the particles as spherical objects (contribution 1 = 40 nm sized micelles + contribution 2 = 20-25 nm AgAuNP, further details are given in SI). As a result only freshly prepared AgAu@PS111-b-P4VP96 precursor solutions (i.e. directly after the complete incorporation of both precursors and in particular of the gold precursor) were used for the synthesis of AgAuNP@ITOTR yolk@shell materials.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2: a) ASAXS curves of AgNO3(0.35)-HAuCl4(0.7)@PS111-b-P4VP96 recorded at E1 = 11 557 eV (black curve) and E3 = 11 915 eV (red curve) and resulting curve of their subtraction (orange curve). b) Fit of the subtracted ASAXS curve applying a sphere model (i.e. a core-shell model with the shell thickness set to zero) (Details in Figure S6 and Table S5). The spatial distribution of the gold precursor inside the gold and silver loaded P4VP core of the PS111-b-P4VP96 micelles was further investigated using anomalous small angle X-Ray scattering (ASAXS). For this purpose, SAXS curves were recorded at different photon energies, i.e. below and in the close vicinity of the L3 absorption edge of Au (E[L3(79Au)] = 11 919 eV, E1 = 11 557 eV, E2 = 11 873 eV, E3 = 11 915 eV) (see Figure 2, a), b), Figure S6, Table S5). This energy region was chosen as the scattering contribution of the silver precursor and the polymer backbone remains constant (6C K-edge ~ 290 eV,

47

Ag K-edge = 25 514 eV). As such, the

subtraction of the scattering curve measured at E1 = 11 557 eV from the curve measured at E3 = 11 915 eV gives the scattering contribution related solely to the gold loaded domains. As can be


Page 10 of 31

Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


seen in Figure 2, b), the gold contribution can be fitted with a spherical scattering model revealing a gold loaded domain of spherical shape with a size corresponding to the core size determined by SAXS and TEM previously. ASAXS therefore confirms the homogeneous distribution of the gold precursor throughout the silver loaded P4VP micellar core; the prerequisite for the formation of homogeneously alloyed nanoparticles. For the synthesis of AgAuNP@ITOTR yolk@shell thin films, the hydrophobic ITOTR precursor In(OtBu)3Sn (ITBO) was added to the silver and gold co-loaded PS111-b-P4VP96 micellar solutions. In line with our previous observations, the solution turned instantaneously brown upon ITBO addition – indication for an immediate reaction between ITBO and the gold / silver precursors at the PS/P4VP interface.22 Possible reactions might be interfacial oxidoreduction processes (Au3+Cl4- + 2e- Au+Cl2- + 2Cl- (E0 = 0.92 V), Ag+ + e- Ag0 (E0 = 0.80 V) and In+ In3+ + 2e- (E0 = -0.43 V), as revealed previously for HAuCl4.3H2O / ITBO systems by x-ray absorption spectroscopy, 22 as well as partial hydrolysis-condensation reactions of ITBO with intramicellar water molecules from the metal precursors (see

119

Sn-NMR results in our

previous work on AuNP@ITOTR materials). 22 After complete homogenization, the solutions were spin-coated onto selected substrates forming - after solvent evaporation - homogeneous hybrid thin films. As shown in Figure 3, calcination under air at 400 °C for 2 h resulted in concomitant polymer decomposition, metal reduction and metal oxide formation, yielding highly defined yolk@shell structures. These structures are composed, as demonstrated by TEM, EDX and SAED (see Figure 3 and Figure 4) of multiple nanoalloyed AuAg nanoparticles (vide infra) strongly anchored onto the inner surface of tin-rich ITO hollow spheres.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

Figure 3: SEM (left column) and TEM micrographs (middle column) as well as particle size

distribution

(right

column)

of

AgNO3(0.34)Au0.66NP@ITOTR

AgClO4(0.34)Au0.66NP@ITOTR (d, e, f), and AgClO4(0.5)Au0.5NP@ITOTR (g, h,i).


(a,

b,

c),

Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In order to increase the free charge carrier density through the formation of oxygen vacancies, an additional annealing under reducing atmosphere (H2/N2; 10 %:90 % at 300 °C for 90 min) was performed35,36 and four-point conductivity measurements revealed conductivities of 2.8 S.cm-1, 8.1 S.cm-1 and 15 S.cm-1 for AgNO3(0.34)Au0.66NP@ITOTR, AgClO4(0.34)Au0.66NP@ITOTR, and AgClO4(0.5)Au0.5NP@ITOTR respectively (Table S6). In the following, the influence of the Ag:Au precursor ratio incorporated within the micellar core on the composition and size of the formed yolk@shell nanoparticles as well as the influence of the type of silver precursor used was investigated. For an overview, sample names and compositions are listed in Table 1. As revealed by TEM analysis (Figure 3), in all cases AgAuNP@ITOTR yolk@shell structures were formed containing multiple AgAu nanoparticles strongly anchored onto the inner walls of amorphous ITOTR hollow spheres. The ITOTR hollow spheres exhibit a diameter of 40 nm, in line with the size of the P4VP core of the precursor micelles (2Rcore= 40.0 nm) and a shell thickness of roughly 2.5 nm in all cases. For all investigated silver and gold precursor loadings, AuAg nanoparticles with sizes smaller than ca. 10 nm in diameter were formed after calcination at 400 °C (Table 1). This result is surprising considering that the metallic nanoparticles are formed in a very small volume and are separated only by few nanometers and should thus be prone to sinter during thermal treatment. As revealed by TEM analysis (see Figure 3), the metal nanoparticle size as well as the metal nanoparticle number per hollow spheres is influenced by (i) the nature of the metal precursor as well as by (ii) the Ag to Au precursor ratio incorporated within the micellar core. The particles formed for identical Ag and Au precursor loadings and ratios (0.35 eq. of silver precursor and 0.7 eq. of gold precursor) are slightly larger in case AgClO4.H2O is used as silver precursor instead of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

AgNO3. The influence of the Ag to Au ratio on the particle composition was investigated using AgClO4.H2O as silver precursor. As was revealed by TEM (Figure 3), the Ag:Au ratio was set to 0.34:0.66 and 0.5:0.5. Increasing the relative Ag content in the precursor micelle decreases the size of the formed metal nanoparticles from 10.1 ± 2.7 nm for an Ag:Au ratio of 0.34:0.66 down to 6.2 ± 1.7 nm for an Ag:Au ratio of 0.5:0.5. These results indicate that the presence of silver strongly influences the particle formation. High resolution TEM (HR-TEM) measurements (Figure 4) confirmed the crystalline nature of the metallic nanoparticles formed onto the inner surface of the ITOTR hollow spheres. The Fast Fourier Transform (FTT) of the black marked area in Figure 4, b) exhibits two spots, corresponding to a distance of 2.35 Å corresponding to the d(111) lattice distance of metallic Au0 (d111=2.3546 Å) and/or Ag0 (d111=2.3543 Å). (Note: Au and Ag both crystallize in the fcc lattice within the Fm3m space group and have very similar lattice parameters (aAu = 4.0782 Å versus aAg = 4.0862 Å). As such diffraction experiments – either X-ray diffraction or electron diffraction – do not easily allow to discriminate if separated Au and Ag particles or AuAg alloyed particles are formed). In line with our previous work, the FFT of the ITOTR shell (dashed square, Figure 4, c) reveals (independent of the focus) a diffuse ring confirming the rather amorphous nature of the ITOTR shell.22,24 To unambiguously clarify the AuAg alloy formation, HR-TEM and scanning transmission electron microscopy (STEM) coupled EDX analysis have been performed. For this, EDX line scans were recorded across the metal nanoparticles to evaluate the chemical composition of Ag and Au within the particles. AgClO4AuNP@ITOTR yolk@shell materials with an Ag:Au ratio of 0.36:0.64 and 0.5:0.5 were investigated (Figure 5).


Page 14 of 31

Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: a) HR-TEM micrograph of a single AgClO4(0.34)Au0.66NP@ITOTR yolk@shell particle scratched from a thin film calcined at 400 °C for 2 h and H2/N2 (10 %: 90 %) at 300 °C for 90 min. Image b) and c) represent the FFTs (power spectra) of the black and dashed areas respectively.

For both cases, the EDX line scan along a particle shows that the Ag and Au concentration varies in similar way. This result strongly indicates a homogeneous distribution of Au and Ag across the particles. This result could further be corroborated by EDX-SEM-TE mappings (exemplary shown for an AgClO4AuNP@ITOTR sample), revealing a homogeneous distribution of Au and Ag within the particles. Interestingly, for all alloys, the compositions obtained by quantification of the SEM-EDX and STEM-EDX data are in good agreement with the nominal Ag:Au ratio incorporated in the respective micellar solutions (Table 1).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Sample name

Loading

Page 16 of 31

MNP Ø (nm)

Ag:Au

Ag:Au ratio

SEM-EDX

STEM-EDX (for one NP)

STEM-EDX

0.34 : 0.66

8.4±1.7

-

-

-

0.34 : 0.66

10.1±2.7

0.28 : 0.72

0.24 :0.76

0.26 : 0.74

0.5 : 0.5

6.2±1.7

0.46 : 0.54

0.43 0.57

0.42 : 0.58

-

7.6±2.9

-

-

-

Ag:Au

Ag:Au

(average)

0.35 eq. of AgNO3+ AgNO3(0.34)Au0.66NP@ITOTR 0.7 eq. of HAuCl4 0.35 eq. of AgClO4.H2O AgClO4(0.34)Au0.66NP@ITOTR + 0.7 eq. of HAuCl4 0.35 eq. of AgClO4.H2O AgClO4(0.5)Au0.5NP@ITOTR + 0.35 eq. of HAuCl4 AuNP@ITOTR

1.05 eq. of HAuCl4

Table 1: Sample name, loading of the PS111-b-P4VP96 micelles and initial atomic ratio Ag:Au in the micellar solution and MNP compositions of AgAuNP@ITOTR.

Figure 5: EDX line profiles across individual AgAu alloy nanoparticles in a) AgClO4(0.34)Au0.66NP@ITOTR and b) AgClO4(0.5)Au0.5NP@ITOTR. The white arrow indicates the direction of the scan. c) EDX-SEM mapping of AgClO4(0.34)Au0.66NP@ITOTR.


Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To further corroborate the formation of AgAu alloy nanoparticles, UV-Vis measurements were performed.34,37,38 AgNP and AuNP are known for their strongly localized surface plasmon resonance (LSPR) yielding a strong light absorption between 400-450 nm for Ag nanoparticles and 480-600 nm for Au nanoparticles. The exact position of the LSPR depends on the particle size, particle shape, particle surface termination as well as of the medium surrounding the nanoparticles.39,40 In case of a successful alloying, the formed AgAu nanoparticles should exhibit a single LSPR with a maximum located at a wavelength in between the LSPR of pure AgNP and pure AuNP of similar size. In contrast, two separate LSPR absorptions would be obtained for separated Au and Ag41 nanoparticles as well as for Au@Ag core@shell NPs.42 As can be seen in Figure 6, a), all AgAuNP@ITOTR yolk@shell nanocomposites exhibit a single LSPR band which is blue shifted compared to the LSPR band observed for pure AuNP@ITOTR with similar size. This result corroborates the formation of AgAu alloy nanoparticles within the yolk@shell structures. However, a blue shift of nearly constant amplitude is observed for all Ag amounts introduced into the AgAuNP@ITOTR yolk@shell structures. This somehow surprising result can however be explained by the fact that variation of the Ag amount not only influences the composition (as revealed by EDX-line scans) but also the size as well as the distance between the AuAg nanoparticles; all parameters strongly influencing the position of the LSPR absorption. A blue shift would indeed be expected for (i) decreasing NPs sizes and (ii) increasing Ag:Au ratios, while a red shift would be expected for (iii) decreasing NPs spacing.43,44 In any case, the plasmonic properties of the AgAuNP@ITOTR yolk@shell materials and in particular the dependence of the LSPR position of the surrounding medium (air vs. water) could be used to probe the particle accessibility inside the hollow spheres (Figure 6, b)). Indeed,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

according to Mie’s theory,39 any increase of the dielectric constant of the medium surrounding the plasmonic nanoparticles should induce a red shift of the LSPR.39,45 As seen in the UV-Vis spectra in Figure 6, b) (as well as in Figure S7), a notable red shift of the LSPR band is observed for all AgAuNP@ITOTR yolk@shell samples upon transfer from air to water. This result unambiguously demonstrates the penetration of water inside the hollow spheres and therefore the accessibility of the AgAu nanoparticles; necessary condition for potential catalytic applications. Accessibility of the AgAu nanoparticles for bigger molecules than water was further proven by repeating the previous experiment in the presence of ethanol (Figure 6, c)). The AgAuNP@ITOTR sample shows a red shift upon transfer from air to ethanol, thereby proving ethanol penetration within the hollow spheres and hence the porosity of the ITOTR shell.

Figure 6: a) UV-vis spectra of AuAgNP@ITOTR thin films coated on glass substrates. a) AgNO3(0.34)Au0.66NP@ITOTR

(grey),

AgClO4(0.34)Au0.66NP@ITOTR

(pink),

AgClO4(0.5)Au0.5NP@ITOTR (purple) and AuNP@ITOTR (yellow) as pure Au reference. b) UVvis measurements of AgNO3(0.34)Au0.66NP@ITOTR films coated on glass substrates before (red)

and

after

immersion

in

H2O

(blue).

c)

UV-vis

measurements

of

AgNO3(0.34)Au0.66NP@ITOTR films coated on glass substrates before (red) and after immersion in ethanol (blue).


Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Taking advantage of the conductivity of the ITOTR shell and the accessibility of the AgAu nanoparticles, the as synthesized AgAu@ITOTR yolk@shell thin films (in particular AgNO3(0.34)Au(0.66)NP@ITOTR) could be used as efficient electrocatalyst for glucose oxidation. For comparison, the activity of an AuNP@ITOTR reference catalyst with similar particle size (8.4 nm for the alloyed catalyst vs. 7.6 nm for the gold catalyst) was also assessed (Figure S8). As can be seen in Figure 7 (a), the CV (10th cycle) under nitrogen of the AuNP@ITOTR thin film electrode exhibits the characteristic reversible electrochemical behavior of gold, revealing the formation of gold oxide above 1.35 V vs. RHE and the reduction of gold oxide to Au below 1.05 V vs. RHE. In comparison, the CV of the AgNO3(0.34)Au0.66NP@ITOTR electrode reveals similar features, in line with literature reports about gold rich AuAg alloys. Most importantly, no feature could be attributed to surface oxidation of pristine silver nor silver oxide surface reduction, therefore excluding Au and Ag phase segregation within the sample. The origin of the reduction peak at 0.76 V vs. RHE is unclear yet. This peak was however also observed for the AuNP@ITOTR reference electrode and is therefore not associated to the silver component of the sample. Figure 7 (b) shows the CVs in 0.1 M NaOH in the presence of 10 mM glucose for an AgNO3(0.34)Au0.66NP@ITOTR and an AuNP@ITOTR reference thin film electrode (same total metal loading accounting for 1.05 eq. of metal precursor per pyridine unit). As seen by the predominant catalytic waves, both materials catalyze the oxidation of glucose. In case of the AuNP@ITOTR electrode, the onset potential was determined to be 0.49 V vs. RHE, whereas the onset potential of the AgNO3(0.34)Au0.66NP@ITOTR electrode was determined to be 0.43 V vs. RHE. As such a clear improvement of ca. 60 mV in onset potential was achieved for the nanoalloyed sample proving the higher activity of the alloyed catalyst. As both electrodes feature



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

similar particle sizes (AgAu ca. 8.4 nm; Au ca. 7.6 nm), it is more than likely that the improved onset potential results from the bimetallic composition of the nanoparticles rather than from a particle size effect. As a result, the bimetallic catalyst outperforms its monometallic counterpart in the low overpotential region in terms of current density. As such, the alloying with silver improved the catalytic activity of the Au nanoparticles within the AgNO3(0.34)Au0.66NP@ITOTR yolk@shell catalysts. In comparison with literature, the onset potential determined for both the Au@ITOTR and the AgNO3(0.34)Au0.66NP@ITOTR electrode is comparable to nanoparticulate Au and AuAg systems, in which however a much higher metal loading was used.46 Measurements with ITOTR reference thin films, i.e. without any metal nanoparticles, showed no catalytic activity for glucose oxidation (Figure 7 (b)). Therefore the catalytic activity of the Au@ITOTR and AgNO3(0.34)Au0.66NP@ITOTR yolk@shell catalysts can be attributed to the metal nanoparticles, proving thereby again their accessibility via the ITOTR shell.

Figure 7: a) CV (N2, 0.1 M NaOH, 50 mV/s) for AgNO3(0.34)Au0.66NP@ITOTR (black) and AuNP@ITOTR (red) thin films deposited on ITO coated glass substrates. b) CV under 10 mM glucose (N2, 0.1 M NaOH, 50 mV/s) for AgNO3(0.34)Au0.66NP@ITOTR (black), AuNP@ITOTR (red) and ITOTR (blue).


Page 20 of 31

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As such Hydrophobic Nanoreactor Soft-Templating proved to be a powerful approach for the straight forward synthesis of functional yolk@shell electrocatalysts with tunable catalytic activity.

CONCLUSION In this paper, we report the applicability of Hydrophobic Nanoreactor Soft-Templating towards the synthesis of multimetallic M1M2NP@MOx yolk@shell structures as demonstrated for AgAuNP@ITOTR yolk@shell materials. Hydrophobic Nanoreactor Soft-Templating thereby relies on PS-b-P4VP inverse block-copolymer micelles as two-compartment nanoreactor templates. In case of AgAuNP@ITOTR yolk@shell structures, the particle composition as well as the particle size and number within the hollow spheres could be tuned by simply adjusting the loading and ratio of the metal precursors incorporated within the micellar core. As a result of the nanoalloying, the as synthesized AgAuNP@ITOTR materials showed improved catalytic activity for glucose electrooxidation. Hydrophobic Nanoreactor Soft-Templating therefore proves to be an efficient and simple approach for the flexible design of multimetallic M1M2@MOx yolk@shell nanocatalysts.

EXPERIMENTAL SECTION Materials. Tetrachloroauric (III) acid (HAuCl4.3H2O, Roth), silver (I) nitrate (AgNO3, Alfa Aesar) and silver perchlorate (AgClO4.H2O, Alfa Aesar) were used as received and stored in a glove box. Polystyrene-block-poly(4-vinylpyridine), PS111-b-P4VP96 (subscripts denote the average number of repeat units, Mn= 21740 g.mol-1, Mw = 27510 g.mol-1) was synthesized as reported in literature by sequential anionic polymerization of styrene (S) and 4-vinylpyridine (4VP) (initiator: sec-butyllithium/LiCl, solvent: THF, -78 °C).47–49 Note: Mw was used for the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

calculation of the metal loading of the core. The molar ratio S:4VP was determined by 1H-NMR (CDCl3) and the number- and weight-average molar masses were measured by size exclusion chromatography (eluent: THF, PS calibration). The heterobimetallic single source ITOTR precursor indium(I)tin(II)-tert-butyl oxide (denoted as ITBO) was synthesized as previously reported.22–24,29 All experiments and manipulations were carried out in an inert atmosphere glove box from MBraun containing an atmosphere of purified nitrogen (

Yolk@Shell Nanoarchitectures with Bimetallic ... - ACS Publications

Recommend Documents