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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
One-Pot Synthesis of Pt/Alumina Composites via AC-Bipolar Electrochemistry Hidetaka Asoh,* Sayuri Miura, and Hideki Hashimoto Department of Applied Chemistry, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
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S Supporting Information *
ABSTRACT: A porous alumina film decorated with platinum nanoparticles was formed on an aluminum substrate using one-pot-based on bipolar electrochemistry under an alternating current (AC) electric field. Most previous approaches of the fabrication of metal/porous alumina composites required at least two steps: the formation of a porous alumina film based on an anodic reaction (i.e., anodization) and the subsequent deposition of metal onto porous alumina based on a cathodic reaction (i.e., electrodeposition), and two types of electrolytes for anodization and electrodeposition, respectively. In this study, a Pt/porous alumina composite film was formed on an unconnected aluminum substrate based on a well-balanced redox reaction in a single electrolyte of phosphoric acid under an AC electric field. When the AC electric field was applied between Pt driving electrodes, dissolution of the Pt electrode and deposition of Pt on both sides of the unconnected aluminum electrode continuously proceeded in parallel with the formation of an alumina film. Auger electron spectroscopy was used to analyze the deposition of Pt on the alumina film. This synthetic approach overcomes the drawbacks of conventional methods in terms of the operation efficiency and will lead to the development of various functional composites using one-pot synthesis. KEYWORDS: one-pot synthesis, bipolar electrochemistry, AC electric field, nanocomposite, Pt/alumina resulting in the asymmetrical formation of oxide films (e.g., gradients in the thickness of the porous layer and pore diameter) on a metal substrate used as a bipolar electrode.15,16 However, we focused on bipolar anodization under an AC electric field as uniform surface treatment for aluminum. Consequently, we demonstrated that numerous small aluminum balls can be simultaneously oxidized, resulting in the formation of a porous alumina layer on unconnected aluminum. However, bipolar anodization studies are quite recent, and there are many possible applications in the fields of material science and nanotechnology. In the present study, we continued our preliminary work and tried to fabricate Pt/alumina composite on an unconnected aluminum substrate using one-pot electrochemical synthesis in a single electrolyte. In general, the fabrication of a metal/ alumina/aluminum composite via the wet process requires at least two steps: the formation of a porous alumina film on aluminum based on an anodic reaction and the subsequent deposition of metal onto porous alumina based on a cathodic reaction, as shown in Figure 1a. In addition, these processes are separately conducted in two types of electrolytes; anodization is generally conducted in an acidic electrolyte and metal deposition is performed in a plating bath containing metal ions. Therefore, the specimen needs to be set as anode or cathode depending on the intended use, as shown in Figure 1a.
I
n recent years, there has been renewed interest in bipolar electrochemistry, which deals with reactions occurring on unconnected conductive objects placed between outer electrodes, due to its potential applications in various fields ranging from sensing to the fabrication of functional materials.1−12 The experimental setup for bipolar electrochemistry differs from that of a conventional two- or three-electrode system. The electrochemical reaction proceeds on an unconnected bipolar electrode, placed between the outer two driving electrodes, in an electrolyte using potential gradients and/or pH gradients. Note that this technique does not require a direct electrical connection between the conductive bipolar and external electrodes (i.e., an external power supply). In most of the studies regarding bipolar electrochemistry a direct current (DC) electric field is applied between the driving electrodes, and an inhomogeneous reaction field on the bipolar electrode is used. On the basis of the polarization of both sides of a bipolar electrode under a DC electric field, the electrochemical redox reactions on each part of a bipolar electrode can be controlled. For an overview of bipolar electrochemistry, refer to recent literature.6,11 In our previous study, we investigated the application of an alternating current (AC) electric field for surface coating of aluminum and successfully formed porous alumina films on unconnected aluminum by indirect oxidation under an AC electric field without a direct electrical connection (“ACbipolar anodization”).13,14 A common strategy applied in bipolar anodization is to use the interfacial potential differences between the driving electrodes under a DC electric field, © XXXX American Chemical Society
Received: February 12, 2019 Accepted: March 20, 2019 Published: March 20, 2019 A
DOI: 10.1021/acsanm.9b00268 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Nano Materials
Figure 1. Schematic illustration of the fabrication process of Pt/alumina composites. (a) Conventional two-step process and (b) one-step synthesis via AC-bipolar electrochemistry.
In this study, we examined the feasibility of the following three key reactions during one-pot synthesis in single electrolytes: (i) the formation of the porous alumina film on aluminum used as a bipolar electrode via AC-bipolar anodization; (ii) the simultaneous dissolution of Pt used as driving electrodes; and (iii) the deposition of Pt onto porous alumina based on the reduction of Pt ions, which are eluted from the driving electrode. To our knowledge, the fabrication of metal/porous oxide composites on unconnected metal substrates via one-pot electrochemical synthesis in a single electrolyte has not been reported. From the application standpoint, the proposed method provides a new method to fabricate various functional composites. Figure 1 presents a schematic illustration of the fabrication process of Pt/alumina/aluminum composites. A complete description of the experimental details can be found in the Supporting Information. On the one hand, in conventional anodization processes based on DC or AC electrolysis, a direct electrical connection between the aluminum object and outer power supply is essential for the completion of the electrical circuit, as shown in Figure 1a. On the other hand, the proposed method for bipolar anodization of aluminum and metal deposition via AC electrolysis is conducted in electrochemical cell, as shown in Figure 1b. Figure 2 shows typical current−time curves for AC electrolysis of unconnected aluminum in phosphoric acid with various concentrations at 20 °C, 60 V, and 150 Hz. A potential difference of 60 V and a constant frequency of 150 Hz were the previously defined base conditions for AC-bipolar anodization of aluminum.13,14 Because a thicker oxide film could be formed at frequencies in the middle range of 50−200 Hz compared with other frequencies, the frequency was fixed at 150 Hz in this study. The current densities for 0.1, 0.15, and 0.2 mol dm−3 phosphoric acid were all stable during AC electrolysis for 30 min, that is, ∼760, 1260, and 1440 A m−2,
Figure 2. Change in the AC current density with reaction time. The AC current density was measured for AC electrolysis in phosphoric acid at 20 °C and 60 V at a frequency of 150 Hz for 30 min.
respectively. This transient current suggests that the electrochemical reaction proceeded at a constant rate. The charge, which is the product of the current and time, increased with increasing concentration of the phosphoric acid used as an electrolyte. The concentration dependence of the current coincides with the trend obtained in our previous study.13,14 Figure 3 presents digital photos of the specimens, which were prepared by AC electrolysis in phosphoric acid with different concentrations, as shown in Figure 2. Vivid interference colors were observed in the central part of each specimen. Such unique colors were simultaneously observed on both sides of the aluminum sheet used as a bipolar B
DOI: 10.1021/acsanm.9b00268 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials
Figure 3. Digital photograph of aluminum sheets after AC electrolysis in (a) 0.1, (b) 0.15, and (c) 0.2 mol dm−3 phosphoric acid. The electrolysis conditions are the same as those reported in Figure 2.
electrode. This result indicates that alumina films were formed on both sides of the aluminum. The color tone of the specimens changed from red to purple depending on the position of the aluminum specimen. These varying color tones indicate that the thickness of the formed alumina films was not uniform on the aluminum, as reported previously;14 the current density might have nonuniformly varied outward from the center of the aluminum specimen due to the distortion of the electric field around the aluminum used as a bipolar electrode. In general, anodized aluminum with a relatively thin oxide film is well-known to exhibit a pale interference color.17−21 Color derived from the structure of the film was attributed to the interference of light reflected from the top surface of the films, the interface of the alumina film, and the underlying aluminum. However, the interference color normally observed for alumina film on aluminum is extremely pale and not vivid, as shown in Figure 3. In fact, the interference color observed in the present study was significantly brighter than the expected interference color. Although the same output potential difference of 60 V was applied to the three specimens, the color tone differed distinctly. As Figure 3c shows, among the three specimens, the aluminum treated with high concentrations of phosphoric acid exhibited the most vivid color. To investigate the coloring factors, scanning electron microscopy (SEM) observations and Auger electron spectroscopy (AES) analysis were performed. Figure 4 shows the SEM images of the specimens after AC electrolysis for 30 min. Small pieces from the center of each aluminum specimen were selected for the SEM observations. At a lower current density (i.e., lower concentration of 0.1 mol dm−3 phosphoric acid), particulate precipitates were observed on the top surface of the films (Figure 4a). The high-magnification image in Figure 4b shows porous alumina under the precipitates. The diameter of the pores in anodic alumina is ∼20 nm. The aggregation size of the precipitates, which are sparsely distributed on the porous alumina film, is larger than the pore size of alumina film. The cross-sectional SEM image in Figure 4c shows that the thickness of the porous alumina film that formed on aluminum during AC electrolysis for 30 min is ∼300 nm. The structure of porous alumina, which is similar to that prepared by conventional DC anodization, is characterized by two regions: (i) an outer region consisting of a porous oxide layer and (ii) a thin compact layer adjacent to the aluminum (the so-called barrier layer). When AC electrolysis was conducted under high-current conditions using a higher concentration of 0.2 mol dm−3 phosphoric acid, the thickness of the porous alumina film remained nearly unchanged (Figure 4c). Nevertheless, the
Figure 4. SEM images of the surfaces and cross sections of aluminum specimens after AC electrolysis for 30 min in phosphoric acid with different concentrations. (a−c) 0.1 and (d) 0.2 mol dm−3 phosphoric acid.
amount of the precipitate increased (Figure 4d). The precipitate relatively densely covers the surface of the specimen compared with Figure 4b (see Figure S1). However, both precipitation types are discontinuous films. Figure 5 shows typical AES depth analysis results. The composition on the side close to the substrate indicates that a
Figure 5. AES depth profiles of constituent elements of the composite film formed on aluminum. The specimen was prepared in 0.2 mol dm−3 phosphoric acid at 20 °C and 60 V at a frequency of 150 Hz for 30 min under an AC electric field.
porous film, mainly comprising Al and O, formed on aluminum. The atomic ratio of Al to O of the film is ∼0.67 (i.e., 2/3), indicating the chemical composition of alumina (Al2O3; Al/O = 2/3). The concentration of Pt on the surface layer side is higher than that in the bottom part. On the basis of the depth profile of Pt, the precipitate on the alumina surface shown in Figure 4 is metallic Pt, deposited during AC electrolysis. The AES spectra of the specimen’s Pt MNN evidence the presence of metallic Pt (Figure S2). Because Al, C
DOI: 10.1021/acsanm.9b00268 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials O, and Pt were detected throughout the film, the composite film can be regarded as a Pt-filled alumina layer on aluminum. It is assumed that the Pt was electrochemically deposited from the bottom of the pores through the barrier layer toward the surface layer and finally overflowed, which is consistent with the process of metal deposition in alumina pores based on conventional AC electrodeposition.22 Note that both the formation of porous alumina and the deposition of Pt occurred on the unconnected aluminum substrate and in a one-pot manner (i.e., single electrolyte of phosphoric acid). On the basis of the present strategy, a composite material can be fabricated more swiftly and efficiently than with conventional processes such as two-step fabrication, as shown in Figure 1a. The AES results for the chemical composition of composite film formed on aluminum show that the strong interference color of the specimens in Figure 3 is caused by surface plasmon resonance enhanced by deposited platinum particles. The amount of deposited platinum is considered to be greater at the center than at the edge of the aluminum specimen. Characteristic optical properties of porous alumina decorated with metal nanoparticles have been reported by several research groups.19−21 Such metal/porous alumina composites are of potential use in optoelectronic devices and chemical sensors. Although the optical properties observed in this study are not novel, it should be emphasized that the method of fabricating the composite material represents an unprecedented strategy. The formation mechanism of a composite film on unconnected aluminum is as follows. The main anodic reaction is considered to be the formation of an alumina film (Al → Al3+ + 3e−, 2Al3+ + 3O2− → Al2O3), which is commonly known as the basic phenomenon of anodization of aluminum.23,24 If the conditions are appropriate, it is possible to form a porous alumina film in the same manner as with ordinary DC anodization, even with AC electrolysis. In our previous study, the balance between the anodic current (which contributes to alumina formation) and the cathodic current (which contributes to hydrogen evolution) was speculated to be an important factor affecting the film growth during AC anodization.25 In particular, the suppression of significant hydrogen gas generation is necessary to avoid the exfoliation of the formed film from the aluminum substrate during anodization. However, one of the key cathodic reactions is the reduction of metal ions (i.e., deposition of Pt). Because Pt ions are not present in the solution at the start of electrolysis, the deposition of platinum is caused by the elution of Pt ions from the Pt used as the driving electrode. In other words, the Pt electrode acts as the sacrificial electrode and supplies Pt ions to the electrolyte. Although Pt might be stable and inert in solution, it is well-known that platinum dissolves under harsh operational conditions depending on the potential, temperature, and pH. Therefore, the stability of Pt for a variety of applications, such as catalyst in fuel cells, has been studied for several years.26−30 Importantly, the shortcomings of the nature of Pt can be taken advantage of. The dissolution of Pt can be divided into two processes: (i) electrochemical direct dissolution of Pt (Pt → Pt2+ + 2e−) and (ii) indirect dissolution based on the oxidation of Pt and formation of oxide (Pt + H2O → PtO + 2H+ + 2e−), followed by the chemical dissolution of the oxide (PtO + 2H+ → Pt2+ + H2O).27 When the Pt electrode acts as the anode in this study, the Pt electrode is considered to be
oxidized (i.e., dissolution), and the Pt ions migrate to the cathode side in the electrolyte. As schematically shown in Figure 1b, the produced electrons are consumed for the reduction of Pt ions (Pt2+ + 2e− → Pt). The standard potential of the reaction involving Pt (e.g., Pt2+ + 2e− → Pt, E° = 1.188 V vs standard hydrogen electrode) is a few volts and extremely low compared with the applied potential difference of 60 V. Therefore, Pt can be deposited on the bipolar electrode to complete the redox reaction, despite the unconnected electrode. Because the electric charge periodically switches between positive (anode) and negative (cathode) on both sides of the bipolar electrode under an AC electric field, the ion transport in the cell is complex and remains to be explained. However, on the basis of the AC application, the formation of a Pt/ alumina/aluminum composite could be practically realized through unconnected electrochemical synthesis and in a onepot manner. Although there is room for further investigations, electrochemical synthesis based on AC-bipolar electrochemistry is considered to be a valid method for the fabrication of nanocomposites on different types of unconnected conductive materials, including other metals and semiconductor materials. In summary, we developed a procedure for one-pot electrochemical synthesis of Pt/porous alumina composites via AC-bipolar electrochemistry. When an AC electric field was applied between Pt driving electrodes, dissolution of the Pt electrode and deposition of Pt on both sides of the unconnected aluminum electrode continuously proceeded in parallel with the formation of alumina film. In addition, the formation of a Pt/porous alumina composite film could be achieved on a bipolar electrode based on a well-balanced redox reaction in a single electrolyte of phosphoric acid under an AC electric field. The amount of deposited Pt and/or dissolved Pt increases with increasing concentration of phosphoric acid. The produced Pt/alumina composite film exhibits a strong interference color due to surface plasmon resonance enhanced by deposited platinum particles. The present method is beneficial for the more swift and efficient fabrication of different types of functional composites.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00268.
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A full description of experimental methods and detailed characterization of the precipitate via SEM and AES (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hidetaka Asoh: 0000-0003-0722-9994 Hideki Hashimoto: 0000-0003-4771-1240 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly financed by the Light Metal Education Foundation of Japan. The authors are grateful to M. Ishino for her technical support of this study. D
DOI: 10.1021/acsanm.9b00268 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials
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Metal Films with Nanohole Arrays on Porous Alumina-Aluminum Structures. RSC Adv. 2015, 5, 68143−68150. (22) AlMawlawi, D.; Coombs, N.; Moskovits, M. Magnetic Properties of Fe Deposited into Anodic Aluminum Oxide Pores as a Function of Particle Size. J. Appl. Phys. 1991, 70, 4421−4425. (23) Li, F.; Zhang, L.; Metzger, R. M. On the Growth of Highly Ordered Pores in Anodized Aluminum Oxide. Chem. Mater. 1998, 10, 2470−2480. (24) Lee, W.; Park, S. J. Porous Anodic Aluminum Oxide: Anodization and Templated Synthesis of Functional Nanostructures. Chem. Rev. 2014, 114, 7487−7556. (25) Ishino, M.; Hashimoto, H.; Asoh, H. Effect of Cathodic Current on the Structural Features of Oxide Films formed by AC Anodization of Aluminum. J. Electrochem. Soc. 2017, 164, C939− C944. (26) Bindra, P.; Clouser, S. J.; Yeager, E. Platinum Dissolution in Concentrated Phosphoric Acid. J. Electrochem. Soc. 1979, 126, 1631− 1632. (27) Tang, L.; Han, B.; Persson, K.; Friesen, C.; He, T.; Sieradzki, K.; Ceder, G. Electrochemical Stability of Nanometer-Scale Pt Particles in Acidic Environments. J. Am. Chem. Soc. 2010, 132, 596−600. (28) Yadav, A. P.; Okayasu, T.; Sugawara, Y.; Nishikata, A.; Tsuru, T. Effects of pH on Dissolution and Surface Area Loss of Platinum due to Potential Cycling. J. Electrochem. Soc. 2012, 159, C190−C194. (29) Topalov, A. A.; Cherevko, S.; Zeradjanin, A. R.; Meier, J. C.; Katsounaros, I.; Mayrhofer, K. J. J. Towards a Comprehensive Understanding of Platinum Dissolution in Acidic Media. Chem. Sci. 2014, 5, 631−638. (30) Tian, M.; Cousins, C.; Beauchemin, D.; Furuya, Y.; Ohma, A.; Jerkiewicz, G. Influence of the Working and Counter Electrode Surface Area Ratios on the Dissolution of Platinum un-der Electrochemical Conditions. ACS Catal. 2016, 6, 5108−5116.
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