Effect of metal alloying on morphology and catalytic activity of platinum-based nanostructured thin films in methanol oxidation reaction

Abstract

Direct methanol fuel cells are considered to be promising sources of green power. However, their commercialization is seriously hindered by the high cost of Pt. The use of an alloy is an effective way to solve the problem. According to Duan and Lee et al., nanosheets and nanobranches (similar to dendrimers) are highly favored for superior catalytic performances due to their geometric properties. In this paper a facile and efficient approach for the synthesis of binary, ternary and tetrametallic alloy thin films at a liquid–liquid interface was demonstrated. Transmission electron microscopy exhibited a nanodendritic structure for PtPdNiFeFe₂O₃, a nanosheet structure for PtPdNi or PtPdNiZn, and spherical nanostructures for PdNi, PtNi and PtPdNiSn alloy thin films. The synthesized PtPdNiFeFe₂O₃ nanodendrimers, and PtPdNiZn and PtPdNi alloy nanosheets exhibit a higher electrocatalytic activity toward methanol oxidation than other binary alloy thin films such as a PdNi thin film, and monometallic Pt or Pd thin films which may be attributed to their large surface area. Due to simple implementation, the proposed approach can be considered as a general and powerful strategy to synthesize ternary and tetrametallic alloy electrocatalysts with high surface area.

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1. Introduction

Platinum has been widely studied as an electrocatalyst for the oxidation of small organic molecules, such as methanol and ethanol in direct alcohol fuel cells (DAFCs) which are regarded as promising future power sources.^1–3 However, the high cost and limited supply of Pt constitute major barriers to the wide application of Pt-based electrocatalysts.^4–6 The study of multi-metallic nanoparticles (NPs) has become an important topic in catalysis and Pt and Pd-based NPs are being considered with more attention due to their higher activity in various catalytic reactions related to fuel cells.⁷ Alloying is a strategy that has been used to find non-Pt electrocatalysts that are effective and less expensive for DAFCs. Mixing two or more metals can result in a catalyst that has distinct properties from its monometallic components.⁸ Compared with the single phase of metal nanostructures, mixed metal nanostructures can provide a more effective way for the exploration of catalysts with enhanced performance due to their multiple reaction sites. Therefore, efforts to replace Pt are in progress and only alloys of a few transition metals have shown activity close to or comparable to that of Pt.^9–11 It has been shown that Pd is a promising electrocatalyst because it is cheaper than Pt and is highly electro-active for small organic molecule oxidation.^12–16 Meanwhile, the addition of a third or fourth element, such as a transition metal Ni, Zn or Fe to Pd can obviously improve the overall electrocatalytic activities of Pd because of the trimetallic and tetrametallic promotional effect.¹⁷ The main factors that affect the catalytic activity are electronic and geometric effects, mainly influencing the catalytic performance of noble metal materials. Therefore, besides the components, the size and surface morphology of electrocatalysts are also crucial for their catalytic activities.^18a Nanostructures with different morphologies exhibit different atomic arrangements on the surface resulting in distinct electronic and geometric structures and improving the catalytic activity.^18b Noble metal alloy nanostructures with nanobranch (NB) morphologies such as nanodendrimers are promising candidates for electrocatalytic reactions due to their large surface area and high active sites.^19a Furthermore, nanosheets are an important class of materials that are highly attractive due to their high surface area-to-volume ratio.^19b This characteristic makes nanosheets highly useful for a number of applications including catalysis,²⁰ chemical sensing,²¹ and surface enhanced Raman scattering.²² Based on these factors, constructing noble metal alloy nanostructures is necessary and can improve the catalytic activity. Therefore, PtPdNi and PtPdNiZn alloy nanosheets (ANSs) as new catalysts are expected to attract much interest because of the special conjunct effects of alloys and nanosheets. However, up to now, it still remains a great challenge to find a method to fabricate the well-defined tri and tetrametallic ANSs.

Self-assembly at a liquid–liquid interface is a powerful experimental route to novel nanomaterials and has received intensive attention in obtaining novel structures due to the extraordinary physical and chemical properties of the nanosized structures.^23–40 The oil/water interface strategy is introduced to provide effective platforms for rapid fabrication of large-area self-assembled nanofilms composed of different nanosized structures such as NPs, nanotubes, nanowires and nanosheets at room temperature. Various two immiscible liquids were used to facilitate the interfacial assembly, for a contact angle of 90° of the particles with the water/oil interface which is essential for the interfacial entrapment of nanostructures.^41–43 Previously the effect of different stabilizers was investigated on the size, morphology and catalytic activity of Pt NPs. In the presence of polyvinylpyrrolidone as a stabilizer, nanosheets of platinum were observed.³⁰ Also, the effect of different organoplatinum(II) complexes on the morphology and size of Pt NPs and alloys at the oil/water interface in the absence of any stabilizers was reported.^36,37 Furthermore, we have reported fabrication of Pd NP/reduced graphene oxide (RGO) thin films as effective catalysts for the Suzuki–Miyaura reaction in water,⁴⁴ and demonstrated the use of a liquid–liquid interface as a medium for preparing ultrathin films of magnetic PtFeFe₂O₃ nanohybrid catalysts for methanol electro-oxidation in NaOH medium.^45a We also have recently reported the synthesis of PtPdZn thin film as a catalyst for the Suzuki–Miyaura reaction.^45b Based on the above considerations, we have synthesized spherical PdNi and PtNi binary alloy thin films at a toluene/water interface by the reduction of suitable complexes at the interface. Also, a PtPdNi ternary ANS with a low Pt content via the liquid–liquid interface strategy was synthesized, having the following advantages: (i) the second and third metal elements, such as Pd and Ni, can be easily reduced simultaneously with Pt to form a PtPdNi ternary alloy, and (ii) the PtPdNi ANSs are easily synthesized and most of the catalytically active atoms are on the surface, maximizing the efficiency in their use, which is predominantly favorable for the amount of the precious metals. Furthermore, tetrametallic alloys such as PtPdNiFeFe₂O₃ nanodendrimers, PtPdNiSn NPs and PtPdNiZn nanosheets were synthesized at a toluene–water interface. In this study, we have been able to improve the catalytic performance and minimize the usage of precious metals. The results reported in this paper prove that the fabricated PtPdNi and PtPdNiZn ANSs and PtPdNiFeFe₂O₃ nanodendrimers exhibit remarkable electrocatalytic activity for methanol oxidation. This study should be of high interest and importance for the development of low-cost Pt-based alloy electrocatalysts in the absence of surfactants and templates.

2. Experimental

2.1. Materials and methods

[Ni(acac)₂] and [Zn(acac)₂] (acac: acetylacetonate) were purchased from Merck and Aldrich companies, respectively. NaBH₄ was purchased from Panreac. The complexes [PtCl₂(cod)],⁴⁶ [PdCl₂(cod)]^47a (cod: cis,cis-1,5-cyclooctadiene) and [Fe(acac)₃]^47b were synthesized using the reported procedures. X-ray diffraction (XRD) patterns of the as-prepared electrocatalysts were recorded using a Bruker AXS (D8, Avance) instrument equipped with Cu Kα radiation (λ = 1.54184 Å). Transmission electron microscopy (TEM) images of the electrocatalysts were recorded using a Philips CM-10 TEM microscope operated at 100 kV.

2.2. Preparation of tetrametallic thin films at the toluene–water interface

To prepare the PtPdNiFeFe₂O₃ thin film, a solution containing an equimolar amount (0.25 mM) of each of the complexes [PtCl₂(cod)], [PdCl₂(cod)], [Ni(acac)₂] and [Fe(acac)₃] in toluene (25 mL) was sonicated for 5 min to form a pale orange colored solution. This solution was stood in contact with double-distilled water (25 mL) in a beaker (100 mL). Once the two layers were stabilized, an appropriate volume of aqueous NaBH₄ (10 mL, 0.1 M) was injected into the aqueous layer using a syringe with minimal disturbance to the toluene layer. The onset of reduction was marked by a coloration of the liquid–liquid interface. With the passage of time, the color became more vivid, finally resulting in a film at the liquid–liquid interface. The aqueous and organic layers below and above the film were, however, transparent. A similar procedure was performed to prepare the other thin films, PtPdNiZn or PtPdNiSn, in which [Zn(acac)₂] or [Sn(CH₃)₄], respectively, were used instead of [Fe(acac)₃].

A similar procedure was used to prepare the trimetallic thin film, PtPdNi, using a solution containing an equimolar amount (0.33 mM) of each of the complexes [PtCl₂(cod)], [PdCl₂(cod)] and [Ni(acac)₂] in toluene (25 mL) to form a pale yellow colored solution, or the bimetallic PdNi thin film, using a solution containing an equimolar amount (0.5 mM) of each of the complexes [PdCl₂(cod)] and [Ni(acac)₂] in toluene (25 mL) to form a pale yellow-green colored solution; the PtNi bimetallic and Pt or Pd monometallic thin films were prepared similarly using the appropriate complexes.

2.3. Electrode preparation

To transfer the thin films from the liquid–liquid interface to the surface of a glassy carbon electrode, the toluene phase (top phase) was removed slowly by a syringe and then a solid substrate was inserted into the liquid phase and pulled out. Before use, the glassy carbon electrodes were carefully polished with alumina slurry. Then they were washed with deionized water and cleaned in deionized water and ethanol, respectively, for 20 min by an ultrasonic bath. Inductively coupled plasma (ICP) was performed on Agilent 7500ce quadrupole ICP-AES. The different catalysts, PtPdNiFeFe₂O₃, PtPdNiZn, PtPdNiSn, PtPdNi, PtNi and PdNi, were sampled by treatment with aqua regia. The ICP-MS results indicated that the Pt loading of PtPdNiFeFe₂O₃ was about 9 wt%, that of PtPdNiZn was 10 wt%, PtPdNiSn was 27 wt%, PtPdNi was 51.2 wt%, and PtNi was 67.6 wt%, and the Pd loading of PdNi was about 53.8 wt%. Despite the Pd key role in the catalysis processes such as methanol oxidation, according to our previous experiences in the synthesis of Pd monometallic and Pd/RGO thin films, these films show no considerable methanol oxidation.³⁷

2.4. Electrochemical measurements

The catalysts were characterized using an Autolab Potentiostat/Galvanostat PGSTAT12 (Eco Chemie, Switzerland). All characterizations were conducted at room temperature in a standard three-electrode system using a Ag/AgCl (sat. KCl) reference electrode, a platinum wire counter electrode and glassy carbon coated with the prepared electrocatalysts as a working electrode. However, all potentials in the manuscript were converted to values with reference to a normal hydrogen electrode (NHE). The glassy carbon electrode area diameter was 2 mm. All cyclic voltammetry (CV) was recorded under the same conditions.

3. Results and discussion

Tri and tetrametallic alloy thin films show great promise as electrocatalysts for the methanol oxidation reaction (MOR) in fuel cell anodes. Herein, PdNi and PtNi bimetallic thin films, PtPdNi trimetallic ANSs, and tetrametallic PtPdNiSn NPs, PtPdNiZn ANSs and PtPdNiFeFe₂O₃ nanodendrites were synthesized through an oil–water interface method using a solution containing an equimolar amount of each of the required complexes [PtCl₂(cod)], [PdCl₂(cod)], [Ni(acac)₂], [Sn(CH₃)₄], [Zn(acac)₂] and [Fe(acac)₃] in toluene at room temperature. The aqua solution of NaBH₄ was injected into the interface with minimal disturbance to initiate the reduction and thin film formation was indicated by the interface color change to black; Scheme 1 shows the typical process for PtPdNi trimetallic ANS thin film formation.

Scheme 1 Schematic illustration of the PtPdNi trimetallic ANS thin film formation at the toluene–water interface: (a) an equimolar solution of [PtCl₂(cod)], [PdCl₂(cod)] and [Ni(acac)₂] in toluene, (b) a stabilized mixture of [PtCl₂(cod)], [PdCl₂(cod)] and [Ni(acac)₂] in toluene (top phase) and water (bottom phase) and drop-wise addition of NaBH₄ to the stabilized mixture, (c) thin film of PtPdNi appearing at the toluene–water interface 24 h after adding NaBH₄, followed by removing the toluene (top) phase by a syringe, and (d) transferring the PtPdNi thin film from the liquid–liquid interface to the surface of a glassy carbon electrode.

3.1. PtPdNiFeFe₂O₃ tetrametallic alloy thin film

The crystal structure of the tetrametallic PtPdNiFeFe₂O₃ thin film was investigated by X-ray diffraction (XRD). Fig. 1a shows diffraction peaks recorded for the PtPdNiFeFe₂O₃ thin film. Thus, five diffraction peaks are observed at 2θ = 39.9°, 46°, 67°, 81° and 86° which correspond to (111), (200), (220), (311) and (222), respectively, of the face-centered cubic (fcc) structure for Pt and Pd NPs,³⁷ while the four weak diffraction peaks appearing at 2θ = 43°, 54°, 72° and 83° are assigned, respectively, as the (111), (200), (220) and (311) indices of the Ni(0) NPs.^48,49 Besides, Fig. 1a also exhibits seven weak diffraction peaks at 2θ = 34°, 51°, 53°, 57°, 61°, 65° and 76° corresponding to the planes (110), (113), (024), (116), (018), (214) and (300), respectively, which are usually assigned to α-Fe₂O₃ with a rhombohedral structure.⁵⁰ Also the weak diffraction peaks at 2θ = 44°, 65° and 83°, corresponding to the planes respectively (110), (200) and (211), can belong to Fe(0).⁵¹ These XRD results therefore indicate that the nanohybrid catalyst consists of crystalline Pt, Pd, Ni, Fe and Fe₂O₃. Comparing to the same reflections usually found in Pt thin films, diffraction peaks of the Pt element in the PtPdNiFeFe₂O₃ thin film are significantly shifted, revealing a high level of alloying.^52a

Fig. 1 (a) XRD pattern of the PtPdNiFeFe₂O₃ thin film on glass, and (b) EDAX spectrum of the PtPdNiFeFe₂O₃ thin film.

The chemical composition of the tetrametallic nanoalloy thin film was determined by energy dispersive analysis of X-ray (EDAX), confirming the existence of Pt, Pd, Fe, Ni and O elements for the PtPdNiFeFe₂O₃ thin film (Fig. 1b).

Transmission electron microscopy (TEM) was used to characterize the morphology of the PtPdNiFeFe₂O₃ tetrametallic alloy thin film, confirming the formation of PtPdNiFeFe₂O₃ nanodendrites (Fig. 2a–d), with an average diameter of approximately 21.4 nm for these structures (Fig. 2e).

Fig. 2 (a–d) TEM images of the PtPdNiFeFe₂O₃ nanodendrites, and (e) histogram of particle size distribution.

Previously, a simple approach for the synthesis of highly structured, anisotropic palladium nanostructured dendrites was described using an eco-friendly biomolecule 5-hydroxytryptophan, which acts as both a reducing and stabilizing agent. It was found that the concentration of 5-hydroxytryptophan played a vital role on the morphology of the nanostructured Pd dendrites. This nanomaterial shows enhanced electrocatalytic performance towards the oxidation of formic acid and may be explored in fuel cells.^52b In this study, PtPdNiFeFe₂O₃ nanodendrites were synthesized without using any stabilizer or template at a toluene–water interface.

Mechanism for formation of the PtPdNiFeFe₂O₃ nanodendrites. One of the routes for the synthesis of nanodendrites is seed-mediated diffusion coupled with the aggregation route with a core of one metal attached by the branched arms of another metal.⁵³ The reduction rate is the most important factor for controlling the rate of nucleation and growth. Drop-wise addition of NaBH₄ to the stabilized mixture of [PtCl₂(cod)], [PdCl₂(cod)], [Ni(acac)₂] and [Fe(acac)₃] in toluene (top phase) and water leads to metal precursor reduction. The reduction process occurs according to the values of standard electrode potentials for each metal. Reduction of the platinum(II) precursor is the first reaction that leads to Pt(0) NPs (Scheme 2a). In situ formed seeds of Pt(0) act as nucleation sites for further growth. The second metal precursor is a Pd(II) precursor that leads to Pd(0) by the reduction process occurring and snowman-like³⁷ PtPd bimetallic alloys synthesize on Pt NP seeds at the liquid–liquid interface (Scheme 2b). The aggregation occurs by keeping on with the addition of NaBH₄ with more reduction of platinum and palladium precursors to Pt(0) and Pd(0) (Scheme 2c). In this growth mode, smaller particles with a high surface energy are produced in the nucleation stage through the reduction of metal precursors, and aggregate to each other to minimize the total surface energy. By keeping on with the addition of NaBH₄, dendritic structures are formed (Scheme 2d and e). By the reduction of, respectively, iron and nickel precursors, pre-synthesized branched PtPd seeds were used to generate the alloy PtPdNiFeFe₂O₃ NBs by diffusion of Fe and Ni into the seeds (Scheme 2f) and the tetrametallic nanodendrite structures are well maintained (Scheme 2g).

Scheme 2 Schematic illustration of the mechanism for the formation of PtPdNiFeFe₂O₃ tetrametallic alloy nanodendrites.

3.2. PtPdNiZn tetrametallic alloy thin film

The crystal structure of the tetrametallic PtPdNiZn thin film was investigated by XRD. Fig. 3a shows the diffraction peaks recorded for the PtPdNiZn thin film. Five diffraction peaks appearing at 2θ = 40.1°, 47°, 68°, 82° and 86° corresponding to the planes (111), (200), (220), (311) and (222), respectively, are due to the fcc structure of the Pt and Pd NPs,³⁷ and four weak diffraction peaks appearing at 2θ = 43°, 54°, 72° and 83° are assigned to the (111), (200), (220) and (311) indices, respectively, of the Ni(0) NPs.^48,49 Besides, Fig. 3a exhibits other diffraction peaks for the planes (002), (100), (101), (102), (103), (110) and (004) of the Zn structure.⁵⁴ XRD results confirm that the nanohybrid catalyst from the typical experiment consists of crystalline Pt(0), Pd(0), Ni(0) and Zn(0).⁵⁵ Relative to the same reflections in Pt thin films,³⁰ the diffraction peaks of the Pt element in the PtPdNiZn thin film are shifted, revealing a high level of alloying.^52a

Fig. 3 (a) XRD pattern of the PtPdNiZn thin film on glass, and (b) EDAX spectrum of the PtPdNiZn thin film.

The chemical composition of the tetrametallic nanoalloy thin film was determined by EDAX analysis, confirming the existence of Pt, Pd, Zn and Ni elements in the PtPdNiZn thin film (Fig. 3b).

TEM analysis (Fig. 4a–d) confirms the formation of a nanosheet structure for the PtPdNiZn tetrametallic alloy thin films.

Fig. 4 (a–d) TEM images of the PtPdNiZn nanosheets.

3.3. PtPdNiSn tetrametallic alloy thin film

The crystal structure of the tetrametallic PtPdNiSn thin film was investigated by XRD (Fig. 5a) showing five diffraction peaks corresponding to the planes (111), (200), (220), (311) and (222) for the fcc structure of the Pt and Pd NPs,³⁷ and four weak diffraction peaks are the (111), (200), (220) and (311) indices of the Ni(0) NPs.^48,49 As is expected from the data in the literature for the same reflections for Pt,³⁰ the related diffraction peaks of the PtPdNiSn thin films shifted to higher 2θ values (about 2 degrees), revealing decreased lattice parameters and a high level of alloying.^52a The chemical composition of the tetrametallic nanoalloy thin film was determined by EDAX analysis, confirming the existence of Pt, Pd, Ni and Sn elements in the PtPdNiSn thin film (Fig. 5b). The PtPdNiSn tetrametallic alloy thin film has a spherical shape (Fig. 6a and b show the PtPdNiSn TEM images), with an average diameter of 22.2 nm, and Fig. 6c shows the histogram of the particle size distribution.

Fig. 5 (a) XRD pattern of the PtPdNiSn thin film on glass, and (b) EDAX spectrum of the PtPdNiSn thin film.

Fig. 6 (a and b) TEM images of the PtPdNiSn NPs, and (c) histogram of particle size distribution.

3.4. PtPdNi ternary alloy thin film

The crystal structure of the PtPdNi trimetallic alloy thin film was investigated by XRD (Fig. 7a). The five diffraction peaks appearing at 2θ = 40°, 48°, 68°, 82° and 86° correspond to the (111), (200), (220), (311) and (222) planes of the fcc structure of the Pt and Pd NPs,³⁷ while the four weak diffraction peaks appearing at 2θ = 45°, 54°, 72° and 83° are assigned as the (111), (200), (220) and (311) indices of the Ni(0) NPs.^48,49 The chemical composition of this nanoalloy was determined by EDAX analysis, confirming the existence of Pt, Pd and Ni elements (Fig. 7b).

Fig. 7 (a) XRD pattern of the PtPdNi thin film on glass, and (b) EDAX spectrum of the PtPdNi thin film.

TEM was used to characterize the morphology of the PtPdNi trimetallic alloy thin film; Fig. 8 confirms the formation of PtPdNi ANSs with a thickness of approximately 2 nm, as estimated by measuring the thickness of some puckers in the nanosheets.

Fig. 8 TEM image of the PtPdNi ANSs.

Mechanism for formation of the PtPdNi nanosheets. The mechanism for the formation of the PtPdNi ternary ANSs at the toluene–water interface is described in Scheme 3. Drop-wise addition of aqueous NaBH₄ to the stabilized mixture of [PtCl₂(cod)], [PdCl₂(cod)] and [Ni(acac)₂] complexes in toluene (top phase) and water leads to metal precursor reduction (Scheme 3a). The reduction process occurs according to the values of standard electrode potentials for each metal, and thus the Pt(II) precursor was reduced first leading to Pt(0) NPs (Scheme 3b). This was followed by reduction of the Pd(II) precursor leading to Pd(0) and the snowman-like³⁷ PtPd bimetallic alloys were obtained on the Pt NP cores at the liquid–liquid interface (Scheme 3c). Finally, the third precursor, Ni(II), is converted to Ni(0) and the thin film of PtPdNi appears at the toluene–water interface. After crystal growth, the nanosheets are produced in the liquid–liquid interface (Scheme 3d). The mechanism for the formation of the PtPdNi ANSs may be considered as self-assembly of the three metals at the toluene–water interface (Scheme 3a–d) and the reduced metals are stabilized at the interface between toluene and water due to a decrease in the interfacial energy and without using any template or surfactant.

Scheme 3 Illustration of the synthesis process for the PtPdNi nanosheets at the interface between water and toluene using NaBH₄: (a) drop-wise addition of NaBH₄ as a reducing reagent to the stabilized mixture, (b) reduction of Pt(II) to Pt(0) NPs, (c) reduction of Pd(II) to Pd(0) and formation of a snowman-like shaped PtPd binary alloy thin film, and (d) reduction of Ni(II) to Ni(0) and platinum nanosheets obtained after the crystal growing process.

3.5. PdNi binary alloy thin film

The crystal structure of the PdNi bimetallic thin film was investigated by XRD (Fig. 9). Five diffraction peaks at 2θ = 40°, 46°, 68°, 82° and 86° corresponding to the (111), (200), (220), (311) and (222) planes, respectively, appeared for the fcc structure of the Pd NPs,³⁷ while the four weak diffraction peaks appearing at 2θ = 48°, 54°, 72° and 83° are assigned to the (111), (200), (220) and (311) indices, respectively, of the Ni(0) NPs.^48,49

Fig. 9 XRD pattern of the PdNi bimetallic thin film on glass.

TEM was used to characterize the morphology of the PdNi binary alloy thin film. Fig. 10a–c confirm the formation of PdNi spherical structures, with an average diameter of approximately 24.4 nm (Fig. 10d).

Fig. 10 (a–c) TEM images of the PdNi thin film, and (d) histogram of particle size distribution.

3.6. PtNi binary alloy thin film

The crystal structure of the PtNi bimetallic thin film was investigated by XRD. Fig. 11 shows the diffraction peaks recorded for the PtNi thin film. Five diffraction peaks appear at 2θ = 40°, 46°, 68°, 82° and 86° corresponding to (111), (200), (220), (311) and (222) of the fcc structure for the Pt NPs,³⁷ and the four weak diffraction peaks appearing at 2θ = 48°, 54°, 72° and 83° can be assigned as the (111), (200), (220) and (311) indices of the Ni(0) NPs.^48,49

Fig. 11 XRD pattern of the PtNi bimetallic thin film on glass.

TEM was used to characterize the morphology of the PtNi binary alloy thin film. Fig. 12a–c confirm the formation of PtNi spherical structures. The average diameter of these spherical structures was approximately 25 nm (Fig. 12d).

Fig. 12 (a–c) TEM images of the PtNi thin film, and (d) histogram of particle size distribution.

Heterogeneous catalysis is a surface phenomenon. EDAX, X-ray photoelectron spectroscopy (XPS) and ICP in addition to XRD and TEM were used for investigation of the surface composition. In order to obtain further insight into the chemical state, the interactions and the composition of the outer layers of the thin films, XPS (a more surface sensitive technique) was used for the PtNi thin film that was carried out in an Axis Ultra DLD system by Kratos Analytical using a monochromated Al Kα1 X-ray beam as the excitation source. Fig. S1a in the ESI† presents the XPS spectra in the Pt 4f electron energy range for the PtNi thin film. In conclusion, after comparing the XPS spectra of the PtNi thin film with that of the Pt thin film (Fig. S1a and b†, respectively), we can see a change in the peak position (0.2 eV), indicating interactions of Pt with Ni which is reflected in a small shift of these peaks. Considering this fact, it is important that the surface composition of the thin film may not be equal to that in the bulk. We have thin films containing NPs or nanosheets in this work. Our attempt to synthesize thin films containing bulk particles or sheets for comparison was unsuccessful.

3.7. Electrocatalytic property

One of our main aims in this work is in fact to improve the electrocatalytic performance of the thin films and reduce the usage of precious and expensive metals for commercialization of fuel cells, by developing a suitable route for the synthesis of alloy nanostructures having well defined shapes (Table 1). Thus, cyclic voltammograms of the thin films (PtPdNiFeFe₂O₃ nanodendrites, PtPdNiZn ANSs and PtPdNiSn NPs, the PtPdNi ANSs thin film, and PtNi and PdNi spherical NPs thin films) in 0.5 M H₂SO₄ at a scan rate of 50 mV s⁻¹ (Fig. 13a–18a) were used to investigate the catalytic activity of the thin films by studying the processes of hydrogen adsorption and methanol oxidation on the films. The humps observed on all the diagrams are associated with atomic hydrogen desorption and adsorption (I and IV regions in Fig. 13a–18a). Metal oxide formation and reduction was also observed (II and III regions in Fig. 13a–18a). The electrocatalytic activity of the tetra, tri and bimetallic alloy thin films was characterized by cyclic voltammetry in an electrolyte of 0.5 M methanol and 0.5 M H₂SO₄ at a scan rate of 50 mV s⁻¹. The resulting voltammograms are shown in Fig. 13b–18b. There are three important points in the investigation of the catalytic activity in the methanol oxidation reaction that are described below.

Table 1 Effect of alloying on the morphology of thin films

Entry	Thin film	Morphology	Size (nm)	Ref.
1	Pt	Spherical NPs	2.22	36
2	Pd	Spherical NPs	6.6	37
3	PtPd	Snowman-like shaped nanostructures	37.9	37
4	PtNi	Spherical NPs	25.0	This work
5	PdNi	Spherical NPs	24.4	This work
6	PtPdNi	Nanosheets	—	This work
7	PtPdNiFeFe2O3	Nanodendrites	21.4	This work
8	PtPdNiZn	Nanosheets	—	This work
9	PtPdNiSn	Spherical NPs	22.2	This work

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Current density. The peak current density of the tetrametallic PtPdNiFeFe₂O₃, PtPdNiZn and PtPdNiSn thin films, the trimetallic PtPdNi thin film, the bimetallic PtNi and PdNi thin films and pure Pt NP³⁶ thin film is approximately 1579, 1099, 1027, 863, 687, 240 and 35 mA cm⁻² mg⁻¹, respectively, according to Fig. 13b–18b. It is clear that the catalytic activity of the PtPdNiFeFe₂O₃ thin film is at least 45 times higher than that of the Pt NP film³⁶ (Table 2).

Table 2 Comparison of different electrocatalysts

Electrode	jf/jb ratio	Current density (mA cm^−2 mg^−1)	E (V, NHE)	Mass activity^a (mA mg^−1)	Reference
a Activity per milligram Pt.
Commercial Pt/C	0.57	—	—	100	56a and c
Pt/C	0.605	—	—	—	57
PtRu/C	0.629	—	—	—	57
PtRuCo/C	0.868	—	—	—	57
PtRu/XC-72	1.05	—	0.64	370.11	58a
Pt/C	1.18	—	—	—	58b
PtPd thin film	1.19	—	0.54	558.82	37
PtCeO2	1.20	—	—	—	59
PtAu	1.23	—	—	—	60a
Pt thin film	1.28	35	0.73	47.92	36
PtFe3O4/CeO2	1.32	—	—	—	59
Pd/XC-72	1.73	—	—	—	58
PtRu/C (E-TEK)	1.88	—	0.37	797.18	58a
Pd/Fe2O3	1.98	—	—	—	58
Hexa-Pt/C	2.13	—	—	—	56a
PtRu/CMK-8-II	2.25	—	0.41	383.14	58a
PtRu/CMK-8-I	3.3	—	0.37	505.65	58a
PtNi thin film	1.52	686.8	0.47	1144.46	This work
PdNi thin film	—	240	0.80	—	This work
PtPdNi thin film	2.075	863.3	0.52	515.621	This work
PtPdNiFeFe2O3 thin film	2.85	1579.26	0.59	452.67	This work
PtPdNiZn thin film	3.50	1098.99	0.58	316.89	This work
PtPdNiSn thin film	1.71	1026.82	0.49	447.46	This work

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(j_f/j_b) ratio. It is reported that the ratio of the forward anodic peak current (j_f) to the backward peak current (j_b), j_f/j_b, indicates the tolerance of catalysts toward poisoning species such as adsorbed CO intermediates formed via decomposition of methanol, and a higher ratio indicates more effective removal of the poisoning species on the catalyst surface.³⁶ The j_f/j_b ratios for the thin films PtPdNiFeFe₂O₃ (Fig. 13b), PtPdNiZn (Fig. 14b), PtPdNiSn (Fig. 15b), PtPdNi (Fig. 16b), and PtNi (Fig. 17b), are about 2.85, 3.50, 1.71, 2.075 and 1.52, respectively, and all are larger than those for the ETEK Pt (0.99), another type of commercial Pt/C (0.605), and Pt NP thin films³⁶ (1.28), respectively. Therefore, the PtPdNiZn thin film electrode can lead to more complete methanol oxidation and less accumulation of CO or CO-like species than for other investigated catalysts, commercial Pt and Pt NP thin films due to the higher j_f/j_b ratio, demonstrating that most of the intermediates were oxidized to CO₂ in the forward scan. The PdNi thin film shows no considerable j_f and j_b. The j_f/j_b ratios are compared in Table 2. All of the alloy thin films exhibit a higher j_f/j_b ratio than the Pt monometallic thin film.

Fig. 13 Cyclic voltammograms of the PtPdNiFeFe₂O₃ thin film in (a) 0.5 M H₂SO₄ electrolyte and (b) 0.5 M H₂SO₄ electrolyte containing 0.5 M CH₃OH with a scan rate of 50 mV s⁻¹.

Fig. 14 Cyclic voltammograms of the PtPdNiZn thin film in (a) 0.5 M H₂SO₄ electrolyte and (b) 0.5 M H₂SO₄ electrolyte containing 0.5 M CH₃OH with a scan rate of 50 mV s⁻¹.

Fig. 15 Cyclic voltammograms of the PtPdNiSn thin film in (a) 0.5 M H₂SO₄ electrolyte and (b) 0.5 M H₂SO₄ electrolyte containing 0.5 M CH₃OH with a scan rate of 50 mV s⁻¹.

Fig. 16 Cyclic voltammograms of the PtPdNi thin film in (a) 0.5 M H₂SO₄ electrolyte and (b) 0.5 M H₂SO₄ electrolyte containing 0.5 M CH₃OH with a scan rate of 50 mV s⁻¹.

Fig. 17 Cyclic voltammograms of the PtNi thin film in (a) 0.5 M H₂SO₄ electrolyte and (b) 0.5 M H₂SO₄ electrolyte containing 0.5 M CH₃OH with a scan rate of 50 mV s⁻¹.

Fig. 18 Cyclic voltammograms of the PdNi thin film in (a) 0.5 M H₂SO₄ electrolyte and (b) 0.5 M H₂SO₄ electrolyte containing 0.5 M CH₃OH with a scan rate of 50 mV s⁻¹.

Lower voltage for the onset of current attributed to methanol oxidation. The onset of current attributed to methanol oxidation is at approximately 0.59 V (vs. NHE) for the PtPdNiFeFe₂O₃ thin film, 0.58 V for the PtPdNiZn thin film, 0.49 V for the PtPdNiSn thin film, 0.52 V for the PtPdNi thin film, 0.47 V for the PtNi thin film and 0.80 V for the PdNi thin film, being more negative than that at a pure Pt NP thin film electrode³⁶ (ca. 0.73 V vs. NHE, except for the PdNi thin film). The negative shift indicates that these synthesized alloy thin films have a positive effect on promoting the oxidation of methanol by lowering its overpotential (Table 2).

All of these results strongly demonstrate that the tetra, tri and bimetallic alloy nanostructure thin films exhibit a higher electrocatalytic activity than the Pt thin film and commercial Pt/C^56a catalysts for methanol oxidation due to their higher j_f/j_b ratio and can lead to more complete methanol oxidation. Furthermore, these electrocatalysts can promote the oxidation of methanol by lowering its overpotential. On the other hand, these electrocatalysts exhibit a higher electrocatalytic activity than that of the Pt thin film and commercial Pt/C catalysts for methanol oxidation due to their higher current density (Table 2).

We compare the mass activity of commercial Pt/C and PtRu/C catalysts with our synthesized catalysts in Table 2. The mass activity term is given by the equation:

where A_m is the mass activity of the catalyst, i_0.9 is the current density in mA cm⁻² at 0.9 V, and W is the loading of platinum in mg cm⁻². The value of 0.9 V is chosen to avoid inclusion of any concentration polarization.^56b According to Table 2, PtNi has the highest mass activity.

The following gives descriptions of the reason why alloys are better electrocatalysts for methanol oxidation.

High specific surface areas of the alloys. To improve the catalytic performance, NPs with specific geometric shapes and related high specific surface areas have been interestingly studied in recent years. Noble metal nanocrystals with unique structures such as thin films, nanodendrites and nanosheets with higher active sites exhibit effective catalytic performance (Table 2).

High active sites. The electrocatalytic activities of noble metal nanostructures are ascribed to the existence of a high density of low-coordinated atoms in edges and kinks, which are catalytically active sites on their surfaces.

Synergistic effects. The electronic (synergistic) effects, considering the molecular orbital and band theories, obviously show that the valance and conduction bands of the metals are changed and electron donation from noble metals such as Pt to some other metals was observed.^60b

In this study, in addition to electronic (synergistic) effects that we have in our previous published relevant papers,^37,45b,60c we have increased the catalytic activity of the metal alloys by producing nanodendrites and nanosheets (geometric effect). Therefore, PtPdNiFeFe₂O₃ and PtPdNiSn tetrametallic alloys have a higher electrocatalytic activity due to the electronic (synergistic) effect and their unique morphology (nanodendrite and nanosheet structures with large surface areas).

A typical cyclic voltammogram recorded for the tetrametallic thin film alloy of PtPdNiFeFe₂O₃ is shown in Fig. 19a. The related voltammograms for the PtPdNiZn and PtPdNiSn thin films, the trimetallic alloy PtPdNi thin film, and the bimetallic PtNi and PdNi thin films in 0.5 M H₂SO₄ and 0.5 M CH₃OH at different scan rates ranging from 20 to 100 mV s⁻¹ are shown in the ESI, Fig. S2a–S6a.† An increase in the current density with the scan rate is observed and the peak potentials almost show no change. Fig. 19b and also Fig. S2b–S6b† show that the peak current densities are linearly proportional to the square root of the scan rates, suggesting that the electrocatalytic oxidation of methanol on these alloy thin films is a diffusion-controlled process.^61,62

Fig. 19 (a) Cyclic voltammograms of the PtPdNiFeFe₂O₃ nanodendrite thin film at different scan rates in 0.5 M H₂SO₄ + 0.5 M CH₃OH. (b) Dependence of the peak currents on the square root of the scan rates.

4. Conclusions

Herein, tetrametallic and trimetallic alloy thin films, nanodendrites and ANSs, with a high degree of alloying were synthesized through a liquid–liquid interface method at room temperature showing a remarkable electrocatalytic activity for methanol oxidation. These structures were synthesized without using any stabilizer or template at a toluene–water interface, therefore, the surface of noble metal nanoalloys is not tightly bonded by these agents which will have a negative effect on the catalytic performance. In this study, addition of the non-precious Ni element as a third metal to the PtPd snowman-like shaped bimetallic thin film leads to ternary alloy formation that is a nanosheet. Also, the addition of Fe as a fourth element to the trimetallic thin film leads to a nanodendrite structure exhibiting a high current density and improved methanol oxidation in comparison with the monometallic thin films due to the large surface area. Through the construction of these noble metal alloy thin films, the electrocatalytic performance has been greatly improved and the usage of precious metals has been effectively minimized. The liquid–liquid interface strategy is an easy and inexpensive method for the production of thin films. Our final goal was the synthesis of noble metal electrocatalysts with both improved performance and low cost using non-precious metals. The present method is promising for the synthesis of high performance catalysts for fuel cells and sensors.

Acknowledgements

We would like to thank the Yasouj University Research Council and the Iranian Nanotechnology Initiative Council for their support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04518a

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