Free-Standing Metal Films Prepared via Electroless Plating at Liquid

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Free-Standing Metal Films Prepared via Electroless Plating at Liquid-Liquid Interfaces Toshihiko Tsuneyoshi, Yu Yohaze, Takaichi Watanabe, and Tsutomu Ono Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02822 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Free-Standing Metal Films Prepared via Electroless Plating at Liquid-Liquid Interfaces Toshihiko Tsuneyoshi, Yu Yohaze, Takaichi Watanabe, Tsutomu Ono* Department of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1, Tsushima-Naka, Kita-Ku, Okayama 700-8530, Japan *E-mail: [email protected]

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GRAPHICAL ABSTRACT


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ABSTRACT

We report a simple preparation of free-standing metal films via electroless plating (ELP)

at the liquid-liquid (L-L) interface between an aqueous electroless plating solution and an

organic solvent. The use of ELP does not require any external energy in the form of heating

and stirring. We find that the affinity of the organic solvent for the palladium nanoparticles

(PdNPs) as catalysts and the vertical position of the organic and aqueous phases in the

biphasic system are important considerations for synthesizing a robust copper film.

Specifically, 1,2-dichloroethane which has an appropriate affinity for PdNPs and higher

density than water was found to be a good candidate for use as the organic phase in this

system. However, a poor-quality copper film was obtained in the system with 1-hexanol

as the organic phase. We also controlled the micro-scale surface structure of the copper

films by using different concentrations of the injected PdNP dispersion. A high density of


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PdNPs caused smaller regions of metal growth, which contributed to the formation of

smoother metal films. Moreover, under the optimal synthesis condition we confirmed the

electrical conductivity of the obtained copper film to be 1.16107 m. We believe that

this metal film preparation represents a promising way to produce a range of metal film

structures through the use of flexible L-L interfaces as templates.


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INTRODUCTION

Electroless plating (ELP) is a wet chemical process, which involves deposition of a metal

film onto a substrate surface through in-situ chemical reduction of metal ions [1, 2]. ELP

is widely used to prepare various surface-modified materials such as composite fibers [3-

5] and flexible electronics [6, 7], which have hybrid-functionalities owing to the

combination of substrate materials (e.g., polymers and ceramics) and deposited metals.

In addition, ELP allows colloidal materials to be coated with metal films, which is difficult

to achieve by electroplating methods that require a connection to an external electronic

source [2]. Indeed, many metal-composite colloidal materials, including core-shell

particles and microcapsules synthesized by ELP, have been studied for decades owing to

their high performance as catalysts and conductive materials [8-19]. We have recently

reported on the synthesis of metal-coated microcapsules with tunable magnetic

properties, which contained a liquid core and metallic shell [18]. The microcapsules


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showed a magnetic response induced by the ELP. This response enabled the phase change

materials to act as a heat-pump system in an external magnetic field.

In ELP processes, in-situ metal precipitation is initiated by activation of reductants

by a dehydrogenation reaction, which is catalyzed by noble metal like palladium at the

substrates surface [1, 2]. Catalytic nanoparticles are necessary for ELP and the conditions

of the catalytic NPs adsorption onto the substrate are important. The deposition of

catalytic NPs on substrate surfaces should be sufficiently dense and durable to form a

continuous metal film on the surface. The NPs strongly adsorb to the solid surface by

chemical or physical bonding to catalytically promote the formation of a dense and

durable layer by ELP. However, unstable substrates, such as L-L interfaces that cannot

support catalytic have not been considered as candidates for substrates of metal film

formation by ELP.


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Particle adsorption at interfaces has been studied for the last century, because

particles adsorbed at interfaces represent a good platform for novel materials synthesis.

The droplets stabilized by adsorbed particles, known as Pickering emulsions, were discovered at the beginning of the 20th century by Pickering and Ramsden [20, 21].

Approximately 100 years later, Dinsmore et al. synthesized microcapsules consisting of

adsorbed particles denoted as colloidosomes, which derived from Pickering emulsions

[22-26]. Colloidosomes are functional microcapsules with size-selective permeability

owing to the spaces between particles packed at the interface. In these materials,

micrometer-sized particles are used owing to their high adsorption stability at interface

[27-29]. NPs adsorption at interfaces has also been studied recently, because of the high

potential of NPs in electronic, optical, and magnetic applications [30-40]. Mohwald et al.

reported that surface-functionalized gold nanoparticles strongly adsorb at L-L interfaces

and the resulting films have a metallic luster [36, 37]. Reincke et al. also presented


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spontaneous assembly and structural control of NPs at water-oil interface by changing

the electrical charge of the particles, which could be applied to the fabrication of 2D and

3D materials for optoelectrical or magnetic applications [38]. In addition, metal

nanoparticles films prepared via Langmuir–Blodgett films are good substrates for Raman-

imaging [39, 40].

The technique of in-situ metal deposition at L-L interface has been studied for long

time since Faraday reported reduction of metal at the water-carbon disulfide interface

[41-45]. Recently, Nishi et al. reported synthesis of Au and Pt nanofiber at ionic liquid-

water interface [42-44]. Although these studies have presented smart way to prepare

various shape of metal at L-L interface, few reports are available on the fabrication of

continuous and robust metal film. Several groups have reported on the synthesis of

continuous metal films at soft fluid interfaces by interfacial crystallization of metals, cross-

linking of NPs and liquid phase deposition (LPD) [46-51]. Rao et al. reported the in-situ


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crystallization of metals at organic solvent-water interfaces, in which reactants in both

phases contacted and reacted at the interface [46-48]. The obtained metal films were

ultrathin and lacked free-standing character; however, high metal-like conductivities were

achieved by the formation of pure metal films. Russell et al. successfully synthesized free-

standing films containing gold NPs, which were prepared via cross-linking of

nanoparticles assembled at air-liquid interfaces [49]. However, these metal films, showed

weak metallic properties owing to the high content of polymer in the films. The LPD

approach reported by Mizuhata et al. addressed the trade-off between durability and

purity in metal films prepared at L-L interfaces but their approach was limited to forming

metal oxide films, such as TiO2 and SnO2 [51]. Therefore, the formation of free-standing

films of pure-metals at an L-L interface has yet to be achieved, without several reports

using the electrode connected to fluid interface that needs complex equipment [52].


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Here we describe the formation of free-standing metal films by ELP at catalytic

nanoparticles adsorbed to a L-L interface. The L-L interface in a biphasic system consisted

of electroless plating solution and an appropriate organic solvent, which enabled the

synthesis of continuous and robust metal films. They were formulated by the continuous

reduction of metal precursors provided from plating solution. This is quite different from

the previous challenges for metal film formation using interfacial reaction requiring

reactants feeding from both phases [41-45]. We report on the appearance, surface

structure, and characteristics of metal-films fabricated under different synthesis

conditions, organic solvents, and catalytic NP concentrations. This report provides insight

into the mechanism of ELP at L-L interfaces and highlights the potential applications of

these novel metal films.


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EXPERIMENTAL SECTION

Materials. Sodium tetrachloropalladate (II) (Na2PdCl4), 1,2-dichloroethane (DCE), 1-

hexanol, copper (II) sulfate, formaldehyde aqueous solution (37wt%), potassium sodium

(+)- tartrate tetrahydrate and sodium hydroxide solution (5 N) were obtained from Wako

Pure Chemicals. A triblock copolymer, poly(ethyleneglycol)-block-poly(propyleneglycol)-

block-poly(ethyleneglycol) with an average molecular weight of 5800 g/mol (Pluronic P-

123) was obtained from Sigma-Aldrich. All reagents were used as received. Elix water,

obtained from a Milli-Q system (Millipore), was used in all experiments. Sample bottles

used for film synthesis were hydrophobized with trimethoxy (octadecyl) silane (Sigma-

Aldrich) to prevent undesirable adsorption of nanoparticles and precipitated metals to the

glass wall. Sample bottles were dried at 110°C for 1 day after immersion in 1 wt%

trimethoxy (octadecyl) silane in toluene solution for 4 h. These sample bottles were used

after washing with ethanol and hexane (Wako Pure Chemicals).


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Synthesis of PdNPs. Pluronic stabilized palladium nanoparticles (PdNPs) were

synthesized according to a procedure reported by Piao et al. [53]. The PdNPs were

obtained in a 1-day reaction starting with rapid injection of 1 mL of a sodium

tetrachloropalladate aqueous solution (1 M) into a vigorously stirred 100 mL Pluronic P-

123 aqueous solution (2 wt%) at room temperature. We changed the particle

concentration of this dispersion by controlling the molar ratio of Na2PdCl4 and Pluronic.

The concentrations of the injected Na2PdCl4 solutions (1 mL) were 3.4510−2, 6.9010−2,

0.345, 1.73, and 3.45 M, which gave a range of molar ratios against Pluronic (0.345 mol,

[Na2PdCl4]: [Pluronic]) in the reaction system, namely, 1:1000, 1:50, 1:100, 1:20, and 1:10,

respectively. The obtained PdNP dispersions were used as prepared without any

purification.

Observation of morphologies and adsorption behaviors of PdNPs. The size and

morphology of the prepared PdNPs were observed with a transmission electron


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microscope (TEM) (JEM-2100F; JEOL). We observed the assembly behavior of PdNPs at

DCE-water and 1-hexanol-water interfaces according to the method presented by

Kutuzov et al. [54] to reproduce and confirm the behavior of PdNPs at the organic solvent–

plating solution interfaces under actual experimental conditions without a subsequent

plating reaction (Supporting Information Scheme S1). A TEM grid supporting organic

solvent was immersed gently into the dilute PdNP aqueous dispersion for 1 min. The TEM

grid was carefully pulled out and then dried overnight. The TEM grids used for all the

samples in these measurements were copper grids with a collodion membrane (200 mesh,

Nisshin EM).

Preparation of metal films at L-L interface. The reaction field, liquid-liquid (L-L)

interface was composed of organic solvents (DCE, 1-hexanol; oil phase) and electroless

plating solutions (aqueous phase) in a sample bottle. The composition of the copper

electroless plating solution was as follows: 7.5 g/L of copper (II) sulfate, 22 mL/ L of


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formaldehyde aqueous solution, 85 g/L of potassium sodium (+)- tartrate tetrahydrate,

(the pH of the plating solution was adjusted to be 12.5 by addition of sodium hydroxide

solution). An organic solvent (DCE or 1-hexanol) contacted the plating solution (2 mL

each) in a hydrophobized sample bottle to form a biphasic system. The PdNP aqueous

dispersion (20 L) was then injected gently onto the L-L interface with the use of a

micropipette. After injection, the plating reaction immediately started at the L-L interface

because of the high catalytic activity of PdNPs for the copper electroless plating reaction

(Supporting Information Scheme S2). The obtained metal films were collected with a pair

of tweezers after 60 min of reaction. We studied the effects of organic solvents and

concentration of the PdNP dispersion on the metal film formation and its characteristics.

Characterization of the obtained PdNP dispersions. The particle concentration of

obtained PdNP dispersion was calculated based on diameters measured from TEM images

and the weight of deposited nanoparticles after ultracentrifugation. The obtained PdNPs


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were centrifuged (20°C, 60 min, 30000 rpm; 72,260 G) three times in an ultracentrifugation

machine (HIMAC CP100MX with angle rotor P100AT2; Hitachi Koki). The deposited

nanoparticles were dried under reduced pressure and weighed with a balance. The particle

concentration was calculated by the following equations:

𝑊𝑑𝑝

𝑁𝑝 = 𝑊𝑝 × 𝑣,

6𝑊𝑑𝑝

= 𝜋𝑑

𝑎𝑣

3

,

𝜌𝑣

where 𝑁𝑝 is the particle concentration (particles/mL), 𝑊𝑑𝑝 is the total weight of

deposited nanoparticles after centrifugation (g), 𝑊𝑝 is the weight of one palladium

nanoparticle estimated from the average diameter (𝑑𝑎𝑣, nm) of nanoparticles measured from TEM images and the density of palladium (12.0 g/cm3 at r.t.), and 𝑣 is the volume of

the dispersion that originally contained these nanoparticles (mL).


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Characterization of the prepared metal films. The obtained metal films were

characterized in terms of their morphology, material composition, surface roughness,

crystalline character and electrical conductivity as follows.

The morphology of the obtained metal films was examined with a field-emission

scanning electron microscope (FE-SEM) (S4700; Hitachi) operating at an applied voltage

of 1.0 kV.

The elemental surface composition of the prepared metal films was determined by

X-ray photoelectron spectroscopy (XPS). XPS measurements were performed with an S-

Probe ESCA apparatus (Fusion Instruments) equipped with a monochromated 𝐴𝑙𝐾𝛼 X-ray

source, after Ar-ion sputtering of samples dried for 10 s.

The surface roughness of the metal films was studied with an atomic force microscope

(AFM, SPA-400; Seiko Instruments Inc.). The AFM measurements were conducted in contact


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mode with a micro-cantilever (SI-DF3; Seiko Instruments Inc.) over a 5 m  5 m area on the

metal films. The height change of a line scan of the sample surface was also examined in contact

mode and the average roughness was calculated according to the equation below:

1

𝑛

𝑅𝑎 = 𝑛∑1|𝑍(𝑖) ― 𝑍𝑐|,

where 𝑅𝑎 is the average roughness (nm), 𝑛 is the number of measured points, 𝑍(𝑖)

is the height, and 𝑍𝑐 is average height on the measured line (nm).

The crystallinity of the obtained metal films was examined by X-ray diffraction

(XRD) with the use of a RINT-2500 diffractometer (Rigaku). Measurements were

conducted at an applied voltage of 40 kV and a current of 200 mA under CuK radiation.

A reflection-free Si sample holder was used for all XRD studies. The size of the copper

crystals was estimated from the XRD peaks corresponding to Cu (111) crystal with the

Scherrer equation [55]:


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𝐾𝜆

𝐷 = 𝐵cos 𝜃,

where 𝐷 is the single crystal diameter, 𝜆 is the wavelength of CuK radiation (1.54056 Å),

𝐵 is the full width at half maximum (FWHM) of an XRD peak, and 𝐾 is the Scherrer constant

(0.9).

The electric conductivities of the obtained metal films were studied by a four-terminal

method with an electrical resistance meter (RM3545; HIOKI). The measurements were conducted

for the dried metal films at room temperature.


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RESULTS AND DISCUSSION

An aqueous dispersion of PdNPs was obtained (Supporting Information Figure S1, S2 and

Table S1) and used as the initiator for metal film preparation. The addition of the PdNP

dispersion to the organic-water interface of the biphasic system is shown in Figure 1(a)

and (d). After injection of the PdNP dispersion, the nanoparticles stabilized by amphiphilic

block copolymer adsorbed at the L-L interface to reduce the surface energy of the system

(Supporting Information Table S2) [27]. As shown in Figure 1(b) and (e), there was a

difference in the assembly behavior of PdNPs on the interfaces between the DCE-water

and 1-hexanol-water. At the DCE-water interface, PdNPs slightly aggregated whereas for

at the 1-hexanol-water interface PdNPs were well-dispersed. We attribute this difference

to the affinities of poly(ethylene oxide) (PEO) chains, which created hydrophilic corona

that surrounded the PdNPs [56]. The PdNPs surrounded by the PEO chains achieved a

uniform dispersion at the interface between the water and 1-hexanol, which dissolved and


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dispersed the PEO chains more easily than did DCE. We explain these difference

solubilities in terms of Hansen solubility parameters of the components (Supporting

Information, Table S3) [57]. Hence, the Pluronic stabilized PdNPs formed aggregates at

the DCE-water interface but were finely dispersed in 1-hexanol. This manner of PdNP

assembly at the L-L interface might influence the characteristics of the resulting metal

films, as will be discussed later. After the plating reaction proceeded for 60 min,

continuous films with a metallic luster were formed at L-L interfaces in both biphasic

systems with DCE and 1-hexanol as the organic phase [Figure 1(c) and (f)]. The metal film

obtained at the DCE-water interface showed both mechanical robustness and some

brittleness owing to its metallic nature. This metal film could be picked up with a pair of

tweezers, indicating successful preparation of a free-standing metal film at the L-L

interface. In contrast, metal films obtained at the 1-hexanol-water interface lacked

mechanical toughness owing to the formation of a thinner metal film at the interface. We


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attribute this difference to the condition of PdNPs at the interface. The highly disperse

nature of PdNPs at the 1-hexanol phase was disadvantageous for metal film formation

because of the low particle density caused by steric repulsion between each nanoparticle.

Conversely, the PdNPs in the plating solution-DCE interface were densely aggregated at

the interface such that a thicker metal film formed with many points capable of initiating

the catalytic reaction.


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Figure 1. Preparation of metal films at planar L-L interfaces in biphasic systems. PdNPs

were injected into biphasic systems with (a) DCE-water and (d) 1-hexanol-water interfaces.

The aqueous phase contained Cu electroless plating solution. TEM images of the assembly

behavior of the PdNPs at (b) DCE-water and (e) 1-hexanol-water interfaces. The assembly

behavior of PdNPs at DCE-water and 1-hexanol-water interfaces was observed to

reproduce and confirm the behavior of PdNPs at the organic solvent–plating solution


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interfaces under actual experimental conditions without a subsequent plating reaction.

Photographs of continuous metal films formed at (c) DCE-water and (f) 1-hexanol-water

interfaces after a 60-min electroless plating reaction.


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The morphologies of the obtained metal films in each system were observed by

FE-SEM (Figure 2). We found distinct differences in the surface structure of metal films

prepared with DCE and 1-hexanol as the organic phase [Figure 2(a) and (d)]. The metal

films formed at the DCE-water interface had a rough surface with some holes and

adsorbed particles, which we assumed were caused by the spherical-shaped growth of

the deposited continuous metal film. Conversely, we observed dendritic structures on the

surface of the metal films obtained at the 1-hexanol-water interface. These differences in

the surface structure directly affected the appearance of metal films; the metal films had

a relatively smooth surface when prepared with DCE and exhibited a metallic luster;

however, films prepared with 1-hexanol had a rough surface and showed no luster. The

films also had different thicknesses. The metal film formed at the DCE-water interface

grew thicker than that formed at the 1-hexanol-water interface (Film thickness: 40827

nm vs. 374 nm) [Figure 2(b) and (e)]. From these results, we proposed two mechanisms


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of metal film formation [Figure 2(c) and (f)]. In the system with DCE-water, the metal film

formation started with in-situ metal precipitation, followed by growth of metallic copper

from the desorbed catalytic PdNPs [Figure 2c (i)]. In addition, reduced and copper metal

in the bulk solution then deposited on the L-L interface. This deposition strongly

contributed to the formation of a metal film through a mechanism analogous to the phase

separation of melamine-formaldehyde resin used for microencapsulation [58] [Figure 2(c)

(ii) and (iii)]. The deposited metal grew continuously to form a thick metal membrane by

in-situ metal growth [Figure 2(c) (iv)]. Conversely, for the film formed in the system with

1-hexanol, precipitated metal settled at the bottom of the sample bottle, which did not

deposit onto the L-L interface [Figure 2(f) (i’)-(iii)]. The in-situ metal growth without any

other deposition resulted in poor quality films with dendritic metal precipitation. We

found that in-situ metal precipitation at the L-L interface with 1-hexanol proceeded by

dendritic growth (Supporting Information, Figure S3). In general, dendritic growth of


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crystals occurs under diffusion-limited reaction conditions, when a highly-active agent is

present in a relatively dilute solution [60]. The large number of PdNPs might have caused

an absence of copper ions around the L-L interface and induced dendritic growth under

the diffusion-limited conditions.


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Figure 2. SEM images of surface structure (a) and (d) and cross-section (b) and (e), and

schematic illustration of metal film formation (c) and (f) in systems with DCE and 1-hexanol

as the organic phase, respectively. Insert photographs in (a) and (d) show the appearance

of metal films with scale bars showing 1 cm.


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Metal films synthesized at the oil-water interface had different surface structures

on both sides. Photographs of the metal films from each side prepared at the DCE-water

interface are shown in Figure 3(a). Although both sides had a macroscopically similar

appearance, XPS spectra from each side of the metal film clearly showed the presence of

palladium only on the side adjacent to the organic phase [Figure 3(b)]. This result supports

the mechanism we proposed above; the metal films formed by adsorption of PdNPs at

the DCE-water interface and the following metal growth was directing into the aqueous

phase, which contained the metal and reductant source. FE-SEM images of the surface

structures of both sides of the film also indicated this mechanism of film formation [Figure

3(c) and (d)]. The film surface adjacent to the aqueous phase had a relatively rough surface

due to deposition of precipitated particles from bulk solution. Conversely, the organic

phase of the metal film had a smooth and compressed surface, which indicates that metal

growth toward the organic phase was prevented.


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Figure 3. A metal film formed at L-L interface with Cu plating solution and DCE; (a)

appearance of the obtained metal film and both sides of the collected film. (b) XPS spectra

of both sides of the obtained film from SEM images of these surfaces [(c) plating solution

side, (d) DCE side]. In the XPS spectrum of the surface contacted to the DCE, peaks at 335

and 340 eV were detected corresponding to Pd 3d.


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The surface roughness of the metal films was affected by the particle concentration

of the injected PdNP dispersions. Table 1 summarizes the synthesis conditions of the

PdNP dispersion and the characteristics of the PdNP dispersion in terms of particle size

and particle concentration. There were no notable differences in the size of the PdNPs

among these conditions (Supporting Information, Figure S4). However, particle

concentrations of each PdNP dispersion (𝑁𝑝) increased as the concentration of metal

precursor in Na2PdCl4 solution was increased from 6.51015 to 2.11017 particles/mL. In

this nanoparticle synthesis, Pluronic acted not only as a stabilizer of the obtained PdNPs

but also as a reducing agent of the palladium precursors [56]. Therefore, the palladium

precursors were immediately reduced for nucleation in Pluronic-rich conditions and few

precursor ions were used for the following particles growth. Pluronic-rich conditions also

ensured the stability of the PdNPs and contributed to the formation of uniform

nanoparticles. Notably, when the molar ratio of [Na2PdCl4]: [Pluronic] was 1:10, we could


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not measure the particle size because the obtained particles were severely aggregated

owing to the lack of Pluronic to reduce and stabilize all the injected palladium.

We prepared metal films at the interface of the plating solution and DCE using

these PdNP dispersions. The surface structure of the obtained metal films for each

dispersion was studied by SEM and AFM. The surface structures formed from the use of

different particle densities at the interface suggested that a higher particle density led to

smaller metal precipitates [Figure 4(a)-(c)]. For the metal precipitates formed at a high

PdNP density the metal precursors were mainly consumed for nucleation, and formed

many small metal precipitate domains in the films. At the lowest particle density (i.e.,

6.51015 particles/mL), a metal film was not formed because an insufficient amount of

PdNPs was supplied to L-L interface. AFM images of the film surfaces also showed that

the surface roughness decreased as the PdNP concentration in the dispersions increased

[Figure 4(d)-(f)]. The change in height of the white line in each AFM image was measured


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and the results are plotted in Figure 4(g). We confirmed that the surface roughness

decreased as the PdNP concentration at the interface increased. The average roughness

(𝑅𝑎) calculated from these height changes also showed slight decrease; the 𝑅𝑎 values were

26, 22, and 20 nm when the concentrations of PdNPs were 1.61016, 5.31016, and 2.11017

particles/mL, respectively [Figure 4(h)].


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Table 1. Diameter, stability, and calculated particle concentration 𝑁𝑝 of PdNP aqueous

dispersion obtained under the same synthesis conditions, with [Na2PdCl4]: [Pluronic]

molar ratios of 1: 1000, 1: 500, 1: 100, 1: 20, and 1: 10.

Entr y

1

2

3

4

5

Na2PdCl4 (A) (mol)

Pluronic (B) (mol)

3.45×104

6.90×104

3.45×103

1.73×102

3.45×102

0.345

Molar ratio A: B

Diamete r (nm)

Particle Concentration 𝑁𝑝 (particles/mL)

1: 1000

4.4

6.5×1015

1: 500

3.7

1.6×1016

1: 100

4.1

5.3×1016

1: 20

4.2

2.1×1017

1: 10

N/D

N/D


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Figure 4. SEM and AFM images of the surface of metal films exposed to the organic phase,

which were obtained with PdNP dispersions having different particle concentrations 𝑁𝑝:

(a, d) 1.61016, (b, e) 5.31016, and (c, f) 2.11017 particles/mL. (g) Line profiles of the height

of the surface scanned along the white lines in the AFM images [purple, blue, and black

lines corresponds to (d), (e) and (f)]. Scale bars show 1 m. Average roughness, 𝑅𝑎 of the

metal films calculated from the line profiles (g) is shown in (h).


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We conducted an XRD study of the metal films prepared with different PdNP

dispersion particle concentrations. Figure 5 shows peaks at 2  43.3 corresponding to

the Cu (111) crystal in metal films, which had similar peak shapes. We calculated the

crystallite diameter in the metal films from the FWHM of the Cu (111) XRD peaks using

the Scherrer equation (Table 2, Supporting Information, Figure S4). The diameters of Cu

(111) crystals decreased from 35.9 to 30.0 nm as the PdNP concentration was increased

from 1.61016 to 5.31016 particles/mL. The size change of the crystals corresponded with

the roughness change of the metal films described above. A higher density of PdNPs lead

to smaller metal crystallites. In addition, we measured the electrical conductivities of these

metal films by the four-terminal method with an electrical resistance meter (Table 2). The

electrical conductivities of metal films prepared with 𝑁𝑝: 5.31016 and 2.11017

particles/mL (Entries 3 and 4) were 1.16107 and 6.62107 m respectively, whereas the

electrical conductivity for the metal film prepared with 𝑁𝑝: 1.61016 (Entry 2) was too brittle


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Page 38 of 54

to be measured under the pressure of the measurement probes. We attribute the

brittleness of the metal film to the low content of Pluronic polymers, which stabilized the

PdNPs. The Pluronic content of the metal films might provide mechanical durability to the

film owing to the flexible nature of the polymers. These values for the electric

conductivities are comparable to those of composite copper membranes (electrical

conductivity of pure copper: 1.68108 m [60]), which indicates successful preparation

of electrically conductive metal films by our process. In this preparation, the electrical

conductivities increased as the concentration of catalytic PdNPs was increased. We

attribute this trend to the small size of the copper crystallites and the amount of

composite materials included in the metal films. The metal film consisted of a large

number of small Cu (111) crystals, which were produced from numerous PdNPs, and had

many crystal interfaces. The many interfaces prevented transfer of electrons and


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contributed to the high electrical conductivities. Moreover, the PdNPs and Pluronic

polymers included in the metal films might decrease the electric conductivity.

Figure 5. XRD patterns for obtained metal films, which were obtained with different

particle concentrations for the PdNP dispersion 𝑁𝑝: (a) 1.61016, (b) 5.31016, and (c)


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Page 40 of 54

2.11017 particles/mL. Peaks corresponding to the Cu (111) crystals were detected at 2 

43.3 with a Cu-K radiation source.


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Table 2. Calculated crystal diameters of Cu (111) crystals and electric conductivities of the

metal films, which were obtained with different PdNP concentrations 𝑁𝑝: (a) 1.61016, (b)

5.31016, and (c) 2.11017 particles/mL. The crystal diameters were calculated with the

Scherrer equation based on the XRD peaks detected at 2  43.3 with a Cu-K radiation

source. Entr y

Particle Concentration 𝑁𝑝 (particles/mL)

Crystal Diameter (nm)

Electric Conductivity (m)

2

1.6×1016

35.9

N/D

3

5.3×1016

35.6

1.16107

4

2.1×1017

30.0

6.62107


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Page 42 of 54

CONCLUSIONS

We successfully prepared free-standing metal film via electroless plating at L-L interfaces

composed of an organic solvent and aqueous plating solution. The affinity of the organic

solvent for the catalytic PdNPs and the vertical position of the organic phase and aqueous

phase in the biphasic system were important factors affecting the synthesis of robust

metal films. Slightly aggregated PdNPs formed thin metal films with a large number of

total adsorbed nanoparticles in the DCE-water system. Conversely, well-dispersed PdNPs

were poorly adsorbed at the L-L interface, owing to the high affinity of the solvent (1-

hexanol) for the PdNPs. This system led to the formation of thin metal films without free-

standing ability. For film formation in the biphasic system, it is important to use higher

density organic solvent like DCE than aqueous plating solution, which promotes

deposition from the bulk solution induced by gravity. Therefore, organic solvents with an

appropriate affinity for the catalytic nanoparticles and a density higher than water are


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good candidates for the organic phase in this system. We could also control the surface

morphology of the metal films by changing the concentration of the injected catalytic

nanoparticle dispersion. A high density of PdNPs induced growth of small metal

crystallites, which contributed to the formation of smooth metal films. Moreover, we

confirmed the electrical conductivities of metal films obtained under the optimal synthesis

condition to be 1.16107 m. Conductivity also varied with the PdNP density.

This simple preparation of metal films is an important expansion of the usage of soft L-L

interfaces as templates. Various L-L interfaces might be used as templates for such metal films.

For example, metal microcapsules can be prepared through encapsulation of oil droplets with a

metallic shell by this technique. This metal film preparation might also enable mass production of

commercial metal films with low-energy costs and no need for external thermal or mechanical

energy.


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Page 44 of 54

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the @@@.


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

Tel: +81-86-251-8072, Fax: +81-86-251-8072.

Author Contributions

T.T., Y.Y., T.W. and T.O. designed the experiments. Y.Y. performed the experiments and

analyzed the data. T.T., Y.Y., T.W., and T.O. interpreted the results and wrote the

manuscript. T.T. and T.O. designed the research.

Notes

The authors declare no competing financial interest.


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Page 46 of 54

ACKNOWLEDGMENT

We thank S. Matsumoto for assistance conducting XPS measurements and useful discussions, M.

Yoneda for assistance conducting TEM measurements, and M. Nakanishi for assistance with XRD

measurements and other useful advice. This work was supported by the JSPS KAKENHI (Grant

number: 16K1445908, 26289291).


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Free-Standing Metal Films Prepared via Electroless Plating at Liquid

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