Electrochemical Synthesis of Mesoporous AuâCu Alloy Films with

Research Article www.acsami.org

Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Electrochemical Synthesis of Mesoporous Au−Cu Alloy Films with Vertically Oriented Mesochannels Using Block Copolymer Micelles Asep Sugih Nugraha,†,‡,§ Victor Malgras,*,‡ Muhammad Iqbal,‡,§ Bo Jiang,‡ Cuiling Li,‡ Yoshio Bando,‡,∥ Abdulmohsen Alshehri,⊥ Jeonghun Kim,†,# Yusuke Yamauchi,*,†,#,∇ and Toru Asahi*,§ †

College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China International Center for Materials Nanoarchitectonics (WPI-MANA) & International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan ∥ Australian Institute for Innovative Materials (AIIM), University of Wollongong, Squires Way, North Wollongong, New South Wales 2500, Australia ⊥ Department of Chemistry, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia # School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia ∇ Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea

Downloaded via NEW MEXICO STATE UNIV on July 3, 2018 at 11:33:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: We synthesized Au−Cu bimetallic alloy films with a controlled mesoporous architecture through electrochemical deposition using an electrolyte solution containing spherical polymeric micelles. The composition of the alloy films can be easily controlled by tuning the ratio between the Au and Cu species present in the electrolyte solution. At low Cu content, cage-type mesopores are formed, reflecting the parent micellar template. Surprisingly, upon increasing the Cu content, the cage-type mesopores fuse to form vertically aligned one-dimensional mesochannels. The vertical alignment of these mesopores is favorable for enhanced mass and ion transfer within the channels due to low diffusion resistance. The atomic distribution of Au and Cu is uniform over the entire film and free of any phase segregation. The as-synthesized mesoporous Au−Cu films exhibit excellent performance as a nonenzymatic glucose sensor with high sensitivity and selectivity, and the current response is linear over a wide range of concentrations. This work identifies the properties responsible for the promising performance of such mesoporous alloy films for the clinical diagnosis of diabetes. This micelle-assisted electrodeposition approach has a high degree of flexibility and can be simply extended from monometallic compounds to a multimetallic system, enabling the fabrication of various mesoporous alloy films suitable for different applications. KEYWORDS: mesoporous materials, mesoporous metals, gold, copper, alloys, micelles, block copolymers, electrocatalysts

■

INTRODUCTION

catalytically active sites and give rise to superior performances.10 The direct electro-oxidation of glucose is an attractive research field for many kinds of applications including the development of direct glucose fuel cells and glucose sensors for medical use such as early diagnostic for diabetes patients.11 Several metals such as platinum (Pt), gold (Au), and their alloy electrodes have been reported as highly active electrocatalysts toward the electro-oxidation of glucose.12−15 Although the oxidation of glucose by Pt can occur at moderate potentials,

Mesoporous films consisting of vertically aligned mesochannels have received considerable attention because of their good molecular accessibility for rapid mass transport. These characteristics are ideal for electroanalytical chemistry and separation technology. Several strategies have been employed to obtain these structures, such as high magnetic field,1,2 electroassisted self-assembly,3,4 air flow,5 confined effect,6,7 and optimal evaporation-induced self-assembly.8,9 However, the material composition has been mostly limited to carbons, silica, organosilica, titania, and alumina. Combining the known stability of metallic framework with a design consisting of vertical mesochannels would enable a large number of © XXXX American Chemical Society

Received: April 5, 2018 Accepted: June 6, 2018

A

DOI: 10.1021/acsami.8b05517 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

resistance. The electrocatalytic performance of the mesoporous Au−Cu films toward the electro-oxidation of glucose is evaluated in alkaline solution with optimized applied potential where the current response of glucose appeared to be higher than other possible interferents.

the Pt electrode tends to be poisoned by the adsorbed products during the catalytic process.11 Therefore, a substitute such as Au is interesting due to its resistance to corrosion or oxidation, nontoxicity, and high sensitivity at a relatively low negative oxidation potential.13 Moreover, gold nanostructures show higher resistance to poisoning owing to the large surfaceto-volume ratio.16 The cost-effectiveness of using precious metal such as gold in the fabrication of electrode, however, becomes an obstacle when it comes to industrial production. Considerable efforts have been made to develop Au-based catalysts with designed shapes and compositions to obtain a high electrocatalytic activity while using a minimal amount of gold.17−19 A promising strategy consists of introducing cheap metals in the Au catalyst and forming a multicomponent Aubased alloy catalyst, while improving the overall electrocatalytic activity and sensing performance. It has been frequently reported that copper (Cu) shows a similar catalytic tendency to Au and could partially substitute the precious metal in such applications.20 Cu electrodes show high sensitivity toward glucose oxidation reaction, but the catalysis takes place at a more positive potential compared to Au.21,22 Therefore, combining Au and Cu in one bimetallic system is expected to reduce the fabrication cost compared to monometallic Aubased devices while improving the selectivity and sensitivity caused by the synergistic electronic response and the catalytic benefits from both components.23,24 Several approaches to prepare mesoporous alloy films have been reported so far. A soft-templating method using lyotropic liquid crystals made of highly concentrated surfactants is often used to form mesoporous structures.25 The high viscosity of the micellar system, however, limits this approach.26 Porous metal particles can be obtained from an aerosol consisting of nonaqueous solutions of metal oleates, followed by thermal decomposition and sintering process to remove the organic residues.27 Similar nanoporous structures have been fabricated through a dealloying approach where the most chemically active element is selectively removed.28 We developed recently a facile and reliable synthetic procedure to assemble mesoporous Au films with a high electrocatalytic activity toward glucose oxidation.17,18 Combining electrochemical deposition, polymeric micelle assembly, and facile solvent extraction circumvents the limitations caused by the high viscosity found in typical liquid crystal systems, thus enabling accurate control over the size of the pores by tuning the size of the micelles. In addition, the numerous unsaturated sites present on the pore surface, known as kink/step sites, are directly responsible for improving the electrocatalytic performance. This strategy has a high degree of flexibility and can be simply extended from a monometallic compound to a multimetallic system, enabling the fabrication of various mesoporous alloy films suitable for different applications. In this study, we synthesize Au−Cu bimetallic alloy films with a controlled mesoporous architecture through an electrochemical deposition approach using an electrolyte solution containing spherical polymeric micelles. The compositions of the mesoporous Au−Cu alloy films can be easily controlled by tuning the ratio between the Au and Cu species present in the electrolyte solution. At low Cu content, cage-type mesopores are formed, reflecting the parent micellar template. At higher Cu content, the cage-type mesopores fuse to form vertically aligned one-dimensional (1D) mesochannels. The vertical alignment of mesopores is favorable for enhanced mass and ion transfer within the channels due to low diffusion

■

EXPERIMENTAL SECTION

Synthesis of Mesoporous Au−Cu Alloy Films. The electrolyte solution contained polystyrene-block-polyethylene oxide (PS-b-PEO) micelles as a pore-directing agent, as well as HAuCl4 and CuSO4 as the gold and copper precursors, respectively. The electrolyte solutions used during electrodeposition were prepared by dissolving 10 mg of PS-b-PEO diblock copolymer (the molecular weight of the PS and PEO blocks are 18 000 and 7500, respectively) in 2 mL of tetrahydrofuran (THF) at 50 °C. After adding 1 mL of ethanol, aqueous HAuCl4 (40 mM) and CuSO4 (40 mM) solutions were slowly incorporated under constant stirring. Various [AuCl4]−/Cu2+ compositions were employed: 100:0, 90:10, 75:25, 50:50, 25:75, and 10:90 (Table 1). A 0.5 mL H2SO4 and 3 mL water were added to

Table 1. Relationship between the Ratio of Precursors and the Final Film Compositions sample name

Au/Cu precursor molar ratio

film composition Au/Cu molar ratio

Au100 Au94Cu6 Au87Cu13 Au78Cu22 Au49Cu51 Au41Cu59

100:0 90:10 75:25 50:50 25:75 10:90

100:0 94:6 87:13 78:22 49:51 41:59

increase the ionic conductivity in the electrolyte solution.20 The mixture was gently stirred for 30 min at room temperature, until complete dissolution of the metal precursors, indicating a successful integration of the Au and Cu species within the PEO region of the micelle. The synthesis of mesoporous Au−Cu alloy films was carried out through an electrochemical deposition at a constant applied potential of −0.5 V (vs Ag/AgCl) for 1000 s without stirring by using an electrochemical workstation (CHI 842B electrochemical analyzer, CHI Instruments). The electrodeposition was performed in a standard three-electrode cell system with a gold-coated silicon wafer substrate, typical area of 0.45 cm2 (0.3 cm × 1.5 cm), as the working electrode, Ag/AgCl (saturated KCl) as the reference electrode, and a platinum wire as the counter electrode. After electrodeposition, the asprepared films were immersed in THF at 40 °C and then thoroughly rinsed with deionized water. Characterization. Scanning electron microscopy (SEM) observations of the as-deposited mesoporous Au−Cu alloy films were carried out using a Hitachi HR-SEM SU8000 microscope at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) images were taken using a JEOL JEM-2100F microscope at an accelerating voltage of 200 kV. The wide-angle X-ray diffraction (XRD) patterns of the as-deposited Au−Cu alloy films were obtained with a Rigaku SmartLab XRD with Cu Kα radiation. The X-ray photoelectron spectroscopy (XPS) was carried out by a PHI Quantera SXM (ULVAC-PHI) with Al Kα radiation. The C 1s peak at 285.0 eV was used to calibrate all the XPS spectra. Cyclic voltammetry (CV) and amperometry (i−t) measurements were performed by using CHI 842B electrochemical analyzer (CH Instruments).

■

RESULTS AND DISCUSSION Synthesis of Mesoporous Au−Cu Alloy Films. Mesoporous Au−Cu films are electrochemically deposited on conductive Au−Si substrates (i.e., working electrode) by applying a fixed potential of −0.5 V (vs Ag/AgCl) in the

B


Research Article


through hydrogen bonding.20,29 When an electrostatic potential is applied between the reference electrode and the working electrode, the metal-rich micelles can be directed toward the cathode, and the metal ions are reduced to their metallic species. The subsequent template extraction is carried out by immersing the as-synthesized Au−Cu films in THF to dissolve the organic template from the mesoporous films. Figure 2 shows the top-surface SEM images of the asprepared Au−Cu films with different compositional ratios. We clearly confirm the presence of mesopores in the Au100, Au 94 Cu 6 , Au87 Cu 13 , Au 78 Cu22 , Au 49 Cu 51 , and Au 41 Cu59 samples. The mesopores are uniformly distributed over the entire films, free of any cracks or voids. The average pore size is around 28 nm (Figure S2), which corresponds roughly to the size of the micelles (Figure S1).29 Unlike the other compositions, mesoporous Cu100 could not be assembled with a well-defined mesoporous structure under our experimental conditions. Indeed, at a deposition potential of −0.5 V, the low reduction rate drives the micellar templates to break down, resulting in the early formation of large Cu crystals (Figures S3 and S4).30,31 In the present study, the deposition time is maintained at 1000 s for all the films. Therefore, the thickness depends only on the starting compositions. The thickness of the mesoporous Au100, Au94Cu6, Au87Cu13, Au78Cu22, Au49Cu51, and Au41Cu59 films is 420, 240, 170, 160, 130, and 120 nm, respectively, with a growth rate of 0.42, 0.24, 0.17, 0.16, 0.13, and 0.12 nm s−1. This result highlights the dependence of the deposition rate on the original Cu content in the electrolyte (Figure S5), also supported by the amperometric data in Figure S3. The final mesoporous structure strongly depends on the soft template. The surfactant assembly consists of spherical and cylindrical micelles.10,32 To understand how the mesopores are formed inside the films, cross-sectional SEM observations on all the mesoporous films were carried out. As shown in Figure 3, each film exhibits a certain degree of mesoporosity with uniformly sized mesopores. For Au100, Au94Cu6, and Au87Cu13 (Figure 3a−c), the cage-like mesoporous structure appears to replicate the sacrificial PS-b-PEO spherical micelle assembly

electrolyte containing the metal precursors (Au and Cu sources) and the sacrificial soft-template consisting of polystyrene-b-polyethylene oxide (PS-b-PEO) polymeric micelles. The impact of the electrolyte stoichiometry is studied by varying the Au/Cu molar ratio used during the preparation of the mesoporous Au−Cu alloy films. To determine the exact composition of the as-synthesized mesoporous films, the atomic ratio between Au and Cu was measured through the inductively coupled plasma analysis. As shown in Table 1 and Figure 1, the final Au content in the films is proportional to the

Figure 1. Relationship between the precursor and film compositions.

Au species content dissolved in the starting electrolyte solutions. Because the standard redox potential of [AuCl4]−/ Au (+1.00 V vs standard calomel electrode (SCE)) is higher than that of Cu2+/Cu (+0.34 V vs SCE), the Au precursor is preferentially reduced under these conditions. In our approach, the mesoporous structure can be easily obtained through the co-reduction of the two metal precursors, which selectively interact with the ethylene oxide moieties in the polymeric micelles (Figure S1). The formation of aqua− metal complexes, consisting of metal species and water, interact with the ethylene oxide shell domains of the micelles

Figure 2. SEM images of the films of the mesoporous (a) Au100, (b) Au94Cu6, (c) Au87Cu13, (d) Au78Cu22, (e) Au49Cu51, and (f) Au41Cu59 films. The starting Au/Cu precursor ratios are shown in parentheses. C


Research Article


Figure 3. Cross-sectional SEM images of the mesoporous (a) Au100, (b) Au94Cu6, (c) Au87Cu13, (d) Au78Cu22, (e) Au49Cu51, and (f) Au41Cu59 films.

Figure 4. (a) Wide-angle XRD patterns of the mesoporous Au and Au94Cu6 films. (b) Wide-angle XRD patterns of the mesoporous Au−Cu films with different compositions resolved between 36 and 44°.

(Figure S1). The Au78Cu22 and Au49Cu51 films (Figure 3d,e) possess well-ordered 1D cylindrical channels, which are likely originating from the spherical micelles merging to form cylindrical micelles, as clearly observed in Figure S6. A previous study by Zhao et al. supports these observations by reporting that the spherical micelles of a PS-b-PEO diblock copolymers-based soft template can fuse to form cylindrical core−shell micelles resulting in 1D cylindrical mesostructured by adjusting the compositional ratio between PS-b-PEO diblock copolymers and the precursor.32 In addition, the

hydrophobicity of the used Au-coated Si substrate might also facilitate the formation of 1D cylindrical mesochannels by eliminating preferential interactions between the PEO moieties and the substrate. This factor can accelerate the continuous growth of mesochannels along the vertical direction during the reduction process (Figure S7).33,34 Our results imply that the spherical micelles approach and further attach on the working electrodes, where they merge along the electric field orientation to form 1D cylindrical channels. The fusion of spherical micelles is likely triggered by the shearing forces D


Research Article


Figure 5. XPS spectra of (a) Au 4f and (b) Cu 2p from the mesoporous Au−Cu alloy films with different composition ratios.

Figure 6. Cross-sectional TEM images of the mesoporous Au49Cu51 film. (a) Bright-field TEM image, (b) high-resolution TEM image, (c) HAADF-STEM image, and corresponding elemental mapping images: (d) Au, (e) Cu, and (f) merged.

952.3 and 932.4 eV (Figure 5b). This suggests the complete reduction of the metal precursors. We note that the Au 4f peaks are slightly shifted toward higher binding energies in the mesoporous alloyed films. Similar results have been observed previously for Au−Cu alloy nanoparticles.36 This shift indicates that the Au and Cu electronic structure has been changed in the alloy system, supporting that both metals have been atomically mixed without phase segregation. The binding strength of intermediate species is strongly influenced by the electronic structure of the catalyst. Alloying Au with Cu tends to lower the d-band (relative to the Fermi level), maintaining the maximum catalytic activity during the reaction.37 High-angle annular dark-field scanning TEM (HAADFSTEM) and energy-dispersive X-ray spectroscopy (EDS) elemental mapping are conducted to confirm the elemental distribution and the arrangement of mesopores within the cross-section of the mesoporous Au49Cu51 film. Figure 6a highlights the tubular mesochannels oriented perpendicularly to the substrate. This structure gives significant advantages when it comes to electrocatalytic applications because of the high accessibility from the surface through the whole film thickness, thus leading to an increased ion and mass transfer (the detailed discussion is given in the following section). From the high-resolution TEM (Figure 6b), highly crystallized walls can be observed in the mesoporous Au49Cu51 film, with a (111) interplanar spacing of 0.23 nm (fcc Au−Cu alloy), thus supporting our previous XRD analysis. 38 The spatial distribution of both Au and Cu is uniform through the entire

induced under the optimal deposition rate upon increasing the Cu content (Figure S8). Vertically oriented mesochannels have a great advantage for enhanced mass and ion transfer within the channels due to low diffusion resistance. However, after further increasing the Cu content, the slow deposition rate results in a substantial collapse of the mesoporous structure, as mentioned above (Figure 3f). The wide-angle XRD patterns were measured to investigate the crystalline structure of the mesoporous Au−Cu films. In Figure 4a, the representative XRD patterns of mesoporous Au100 and Au94Cu6 films highlight the presence of the (111), (200), (220), and (311) diffraction peaks of the Au facecentered cubic (fcc) crystal structure. The formation of a single-phase Au−Cu alloy in the mesoporous films can be confirmed from the gradual shift in the peak lying between that of pure Au fcc and pure Cu fcc (Figure 4b). This originates from the lattice constant shrinking from 4.065 to 3.597 Å, following the Vegard’s law. Moreover, the (111) diffraction peak is significantly broader in the bimetallic systems, suggesting a smaller crystallite size than in the monometallic films.35 To better understand the electronic states describing the chemical bonds in each film, XPS measurements are performed. As shown in Figure 5a, the 4f5/2 and 4f7/2 peaks of the Au100 film, located at binding energies of 83.8 and 87.5 eV respectively, can be attributed to the presence of metallic Au0. On the other hand, the presence of metallic Cu0 in all the alloys can be confirmed due to the 2p1/2 and 2p3/2 located at E


Research Article


Figure 7. (a) Time-dependent amperometry following successive the addition of glucose with the mesoporous Au49Cu51 alloy, mesoporous Au, and nonporous Au49Cu51 alloy films. The concentration ranges from 0 to 19 mM at 0.3 V (vs Ag/AgCl). (b) Corresponding calibration curves. The current response of the dynamic detection of glucose was normalized by the geometrical electrode area.

oxidation of glucose on mesoporous Au−Cu alloy films is complex: the onset potential of glucose electro-oxidation to gluconolactone is found at a more positive potential (e.g., ca. 0.0 V for Figure S10b, ca. 0.2 V for Figure S10c−f), which then undergoes further oxidation. The peak located between −0.2 and 0.0 V should be assigned to the oxidation of Cu.23 In contrast with the mesoporous Au film, the glucose oxidation taking place on the mesoporous Au−Cu films reaches a more positive potential with a higher current response than mesoporous Au film: 0.3 V (2.76 mA cm−2), 0.49 V (5.16 mA cm−2), 0.52 V (6.81 mA cm−2), 0.53 V (8.15 mA cm−2), and 0.54 V (5.21 mA cm−2) for the mesoporous Au94Cu6, Au87Cu13, Au78Cu22, Au49Cu51, and Au41Cu59 films, respectively. Although the mesoporous Au−Cu films show similar electrochemical behavior, the highest current density is obtained from the mesoporous Au49Cu51 film, indicating the enhanced catalytic activity toward the electro-oxidation of glucose.10,23,41 Figure S11 also shows the increment of anodic current response of glucose electro-oxidation on the mesoporous Au49Cu51 film with different glucose concentrations. It becomes clear that the current peak of glucose oxidation is shifted positively compared to mesoporous Au film (Figure S10a). The presence of electroactive interferents, such as uric acid (UA), ascorbic acid (AA), and other sugars, for instance, maltose, can alter the nonenzymatic glucose detection. These species can also be oxidized at a moderately high anodic potential, thus exhibiting a current response overlapping with that of glucose and leading to overestimated detection level values.11 It is then of primary importance to determine a suitable detection potential favoring the current response of glucose oxidation over that of the interferents. Figure S12 shows the current response after addition of 0.1 mM glucose, 10 μM maltose, 5 μM UA, or 5 μM AA on the vertically oriented mesoporous Au49Cu51 electrode in 0.1 M NaOH at detection potentials ranging from 0.2 to 0.7 V. From these results, 0.3 V is selected as the best potential for glucose oxidation to achieve the highest selectivity. The sensitivity of mesoporous Au−Cu alloy films toward the electro-oxidation of glucose is measured by amperometry at 0.3 V for various glucose concentrations ranging from 0 to 0.65 mM (Figure S13). The mesoporous Au−Cu electrodes responded promptly (∼3 s) to the addition of glucose, showcasing the rapidity of the process. From the corresponding calibration curves, the highest sensitivity, 642.5 μA cm−2 mM−1, is found to be attributed to the mesoporous Au49Cu51

film, without any occurrence of phase segregation, as confirmed by EDS elemental mapping (Figure 6c−f). Electrochemical Analysis. The nonenzymatic detection of glucose by amperometry keeps raising interest due a simple and rapid data processing. In this technique, electrodes with high electroconductivity and large surface area, active for the electro-oxidation of glucose, are critical to achieve highly performant glucose sensing. In this context, mesoporous Au− Cu alloy films with well-defined mesopores and uniform Au− Cu alloyed phase are expected to exhibit high sensitivity and selectivity for glucose electro-oxidation over a wide range of glucose concentrations. In this study, several techniques were tested to evaluate the performance of mesoporous Au−Cu alloy films with various compositions toward glucose electrooxidation. At first, the electrochemically active Au surface area (ECSA) for each film is calculated by integrating the charge density associated with the reduction of gold oxide species formed during the negative sweep of the cyclic voltammetry (CV) in an acid solution, assumed to be 400 μC cm−2 (Figure S9).39 The volume-normalized ECSAs are 95.2, 226.5, 233.4, 236.9, 237.5, and 81.5 m2 cm−3 for the mesoporous Au, Au94Cu6, Au87Cu13, Au78Cu22, Au49Cu51, and Au41Cu59 films, respectively. The electrocatalytic activity of mesoporous Au− Cu films toward the electro-oxidation of glucose is determined by using CV in an alkaline solution, where the as-synthesized mesoporous Au−Cu films with different compositions are employed as the working electrode. As can be observed from Figure S10, the CV curves show the electrochemical behavior of the mesoporous Au, Au94Cu6, Au87Cu13, Au78Cu22, Au49Cu51, and Au41Cu59 films in 0.1 M NaOH containing 5 mM glucose at a scan rate of 50 mV s−1 at room temperature. In the case of pure Au (Figure S10a), the formation of gold hydroxide from the chemisorption of hydroxide anions on the gold surface is believed to be the catalytic component. As the voltage is swept forward, the oxidation reaction of glucose to gluconolactone takes place from −0.3 V and reaches a maximum current of 3.65 mA cm−2 at 0.25 V, as the OH− anions are known to be chemisorbed in that potential range. The current density then decreases due to the formation of Au oxide, which occupies the active sites and prevents glucose oxidation to take place. A high current density is also observed in the backward sweep, starting from 0.2 V, as Au oxide is gradually reduced from that potential.11,40 As seen in Figure S10b−f, the voltammogram characteristic of the mesoporous Au−Cu alloy films with different compositions change significantly from the mesoporous Au film. The elecroF


Research Article

ACS Applied Materials & Interfaces Table 2. Summary on Glucose Sensors Based on the Au- and Cu-Based Nanomaterials materials a

AuCu/CNTs nanoporous Au−Cu Pd−Au clusters Ni−Cu/TiO2 nanotube arrays mesoporous Pt−Au alloy Cu/Au NPs Cu nanowires/CNTsa mesoporous Au mesoporous Cu mesoporous Au−Cu

applied potentials

sensitivity (μA mM−1 cm−2)b

0.34 V vs SCE 0.2 V vs SCE −0.1 V vs SCE 0.6 V vs Ag/AgCl 0.3 V vs Ag/AgCl −0.03 V vs Ag/AgCl 0.55 V vs Ag/AgCl 0.2 V vs Ag/AgCl 0.55 V vs Ag/AgCl 0.3 V vs Ag/AgCl

22 50 75.3 1590 352 40.8 1995 291.6 606 643.6

limit of detection (μM) 4 50 5 6 0.05 0.26 4.13 0.2 1.5

linear range (mM)

refs

0.08−9.26 0−12 0.1−30 0.01−3.2 0−11 0−10 0−3 0.01−10 0−10 0.01−19

43 21 44 37 17 45 46 15 47 this work

a

CNTs = carbon nanotubes. bSensitivity = the values normalized by geometrical electrode areas (cm2) except for AuCu/CNTs (ref 43).

film. The sensitivities for other compositions are 95.5, 168.9, and 496.3 μA cm−2 mM−1 for Au, Au87Cu13, and Au78Cu22, respectively. To further emphasize on the importance of our mesoporous Au−Cu catalyst, the current response toward glucose detection of the mesoporous Au49Cu51 alloy film is benchmarked against the mesoporous Au film and a nonporous Au49Cu51 alloy film (template free). Figure 7 shows that the mesoporous Au49Cu51 alloy film exhibits a linear dependence between the current response and the glucose concentration (from 0.01 to 19 mM), with a correlation coefficient of R2 = 0.991, as well as a substantially higher sensitivity (643.6 μA cm−2 mM−1) than that of the mesoporous Au and nonporous Au49Cu51 alloy films (98.5 μA cm−2 mM−1 (R2 = 0.996) and 340.1 μA cm−2 mM−1 (R2 = 0.992), respectively). The limit of detection of the mesoporous Au49Cu51 alloy films was estimated to be about 1.5 μM, with a signal-to-noise ratio of 3. The high performance of glucose sensor from the mesoporous Au−Cu film is compared with other published glucose sensors based on the Au- and Cunanomaterials (Table 2). Applying stronger negative potentials ensures the elimination of interferents during the measurement of glucose concentration using nonenzymatic glucose sensor. It is clearly understood that our mesoporous Au−Cu film shows excellent performance at a relatively strong negative potential, with high selectivity, high sensitivity, and good linearity over a broad range of glucose concentrations with a low detection limit.42

tubular mesochannels can also be obtained by using different alloy compositions such as Au−Ni (Figure S14). Therefore, we believe that this approach can be widely adapted to other alloy compositions and serves as a proof of concept for the extended production of vertically aligned mesoporous metallic films through electrodeposition in the presence of micelles.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05517. TEM images of the polymeric micelles, pore size distribution of the mesoporous films, amperometric plots for the deposition of mesoporous films with different compositions, SEM images of the deposited Cu films, relationship between the film growth rate and precursor composition, cross-sectional SEM images of the mesostructured/mesoporous Au−Cu films, schematic illustration of formation of vertically oriented mesochannels, CV and amperometric measurements of the mesoporous films, investigation of ECSA (Au) and volume-normalized ECSA (Au), and cross-sectional SEM images of the mesoporous Au−Ni films (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

■

*E-mails: [email protected] (V.M.). *E-mail: [email protected] (Y.Y.). *E-mail: [email protected] (T.A.).

CONCLUSIONS In summary, we reported the fabrication of mesoporous Au− Cu alloy films with vertically aligned mesochannels and with various compositions through electrodeposition process. The method requires the application of a suitable constant potential, utilizes block copolymer micelles as sacrificial template to form spherical (or cylindrical) pores, and does not require thermal treatments to remove the organic template. Instead, the as-synthesized films are immersed in THF to yield well-defined mesoporous structures. The interaction between the aqua−metal complex and the ethylene oxide group of the polymeric micelles enables the preparation of Au−Cu alloy system under typical electrodeposition conditions. The assynthesized mesoporous Au−Cu films have vertically aligned mesoporous and uniform atomic distribution of Au and Cu over the entire film. As a result, the mesoporous Au−Cu films provide the large surface area with highly accessible pores for the guest species, which leads to excellent performance for nonenzymatic glucose sensor compared to other Au- and Cubased nonenzymatic glucose sensors. Such vertically oriented

ORCID

Cuiling Li: 0000-0003-2283-579X Yusuke Yamauchi: 0000-0001-7854-927X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Australian Research Council (ARC) Future Fellow (FT150100479), JSPS KAKENHI (17H05393 and 17K19044), and the research fund by the Suzuken Memorial Foundation. The authors would like to thank New Innovative Technology (NIT) for helpful suggestions and discussions.

■

REFERENCES

(1) Tolbert, S. H.; Firouzi, A.; Stucky, G. D.; Chmelka, B. F. Magnetic Field Alignment of Ordered Silicate-Surfactant Composites and Mesoporous Silica. Science 1997, 278, 264−268.

G


Research Article


by Electrochemical Soft Templating. Angew. Chem., Int. Ed. 2016, 55, 12746−12750. (21) Li, X.; Zhub, Q.; Tonga, S.; Wanga, W.; Songa, W. SelfAssembled Microstructure of Carbon Nanotubes for Enzymeless Glucose Sensor. Sens. Actuators, B 2009, 136, 444−450. (22) Li, Y.; Fu, J.; Chen, R.; Huang, M.; Gao, B.; Huo, K.; Wang, L.; Chu, P. K. Core−Shell Tic/C Nanofiber Arrays Decorated with Copper Nanoparticles for High Performance Non-Enzymatic Glucose Sensing. Sens. Actuators, B 2014, 192, 474−479. (23) Li, X.; Qiu, H.-J.; Wang, J. Q.; Wang, Y. Corrosion of Ternary Mn-Cu-Au to Nanoporous Au-Cu with Widely Tuned Au/Cu Ratio for Electrocatalyst. Corros. Sci. 2016, 106, 55−60. (24) Kim, D.; Resasco, J.; Yu, Y.; Asiri, A. M.; Yang, P. Synergistic Geometric and Electronic Effects for Electrochemical Reduction of Carbon Dioxide Using Gold-Copper Bimetallic Nanoparticles. Nat. Commun. 2014, 5, No. 4948. (25) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Mesoporous Platinum Films from Lyotropic Liquid Crystalline Phases. Science 1997, 278, 838−840. (26) Malgras, V.; Ataee-Esfahani, H.; Wang, H.; Jiang, B.; Li, C.; Wu, K. C.-W.; Kim, J. H.; Yamauchi, Y. Nanoarchitectures for Mesoporous Metals. Adv. Mater. 2016, 28, 993−1010. (27) Xiao, Q.; Sohn, H.; Chen, Z.; Toso, D.; Mechlenburg, M.; Zhou, Z. H.; Poirier, E.; Dailly, A.; Wang, H.; Wu, Z.; Cai, M.; Yunfen. Nanoarchitectures for Mesoporous Metals, Mesoporous Metal and Metal Alloy Particles Synthesized by Aerosol-Assisted Confined Growth of Nanocrystals. Angew. Chem., Int. Ed. 2012, 51, 10546−10550. (28) Ding, Y.; Chen, M.; Erlebacher, J. Metallic Mesoporous Nanocomposites for Electrocatalysis. J. Am. Chem. Soc. 2004, 126, 6876−6877. (29) Li, C.; Dag, Ö ; Dao, T. D.; Nagao, T.; Sakamoto, Y.; Kimura, T.; Terasaki, O.; Yamauchi, Y. Electrochemical Synthesis of Mesoporous Gold Films Toward Mesospace-Stimulated Optical Properties. Nat. Commun. 2015, 6, No. 6608. (30) Attard, G. S.; Göltner, C. G.; Corker, J. M.; Henke, S.; Templer, R. H. Liquid-Crystal Templates for Nanostructured Metals. Angew. Chem., Int. Ed. 1997, 36, 1315−1317. (31) Iqbal, M.; Li, C.; Wood, K.; Jiang, B.; Takei, T.; Dag, Ö ; Baba, D.; Nugraha, A. S.; Asahi, T.; Whitten, A. E.; Hossain, M. S. A.; Malgras, V.; Yamauchi, Y. Continuous Mesoporous Pd Films by Electrochemical Deposition in Nonionic Micellar Solution. Chem. Mater. 2017, 29, 6405−6413. (32) Luo, W.; Zhao, T.; Li, Y.; Wei, J.; Xu, P.; Li, X.; Wang, Y.; Zhang, W.; Elzatahry, A. A.; ALghamdi, A.; Deng, Y.; Wang, L.; Jiang, W.; Liu, Y.; Kong, B.; Zhao, D. A Micelle Fusion-Aggregation Assembly Approach to Mesoporous Carbon Materials with Rich Active Sites for Ultrasensitive Ammonia Sensing. J. Am. Chem. Soc. 2016, 138, 12586−12595. (33) Koganti, V. R.; Rankin, S. E. Synthesis of Surfactant-Templated Silica Films with Orthogonally Aligned Hexagonal Mesophase. J. Phys. Chem. B. 2005, 109, 3279−3283. (34) Koganti, V. R.; Dunphy, D.; Gowrishanker, V.; McGehee, M. D.; Li, X.; Wang, J.; Rankin, S. E. Generalized Coating Route to Silica and Titania Films with Orthogonally Tilted Cylindrical Nanopore Arrays. Nano Lett. 2006, 6, 2567−2570. (35) Blosi, M.; Ortelli, S.; Costa, A. L.; Dondi, M.; Lolli, A.; Andreoli, S.; Benito, P.; Albonetti, S. Bimetallic Nanoparticles as Efficient Catalysts: Facile and Green Microwave Synthesis. Materials 2016, 9, 550. (36) Yin, J.; Shan, S.; Yang, L.; Mott, D.; Malis, O.; Petkov, V.; Cai, F.; Ng, M. S.; Luo, J.; Chen, B. H.; Engelhard, M.; Zhong, C. J. Gold− Copper Nanoparticles: Nanostructural Evolution and Bifunctional Catalytic Sites. Chem. Mater. 2012, 24, 4662−4674. (37) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Nørskov, J. K. Surface Electronic Structure and Reactivity of Transition and Noble Metals. J. Mol. Catal. A: Chem. 1997, 115, 421−429. (38) He, R.; Wang, Y.; Wang, X.; Wang, Z.; Liu, G.; Zhou, W.; Wen, L.; Li, Q.; Wang, X.; Chen, X.; Zeng, J.; Hou, J. G. Facile Synthesis of

(2) Yamauchi, Y.; Sawada, M.; Noma, T.; Ito, H.; Furumi, S.; Sakka, Y.; Kuroda, K. Orientation Of Mesochannels in Continuous Mesoporous Silica Films by a High Magnetic Field. J. Mater. Chem. 2005, 15, 1137−1140. (3) Walcarius, A.; Sibottier, E.; Etienne, M.; Ghanbaja, J. Electrochemically Assisted Self-Assembly of Mesoporous Silica Thin Films. Nat. Mater. 2007, 6, 602−608. (4) Goux, A.; Etienne, M.; Aubert, E.; Lecomte, C.; Ghanbaja, J.; Walcarius, A. Oriented Mesoporous Silica Films Obtained by ElectroAssisted Self-Assembly (EASA). Chem. Mater. 2009, 21, 731−741. (5) Shan, F.; Llu, X.; Zhang, Q.; Wu, J.; Wang, Y.; Bian, F.; Lu, Q.; Fei, Z.; Dyson, P. J. A Facile Approach for Controlling the Orientation of One-Dimensional Mesochannels in Mesoporous Titania Films. J. Am. Chem. Soc. 2012, 134, 20238−20241. (6) Yamaguchi, A.; Uejo, F.; Yoda, T.; Uchida, T.; Tanamura, Y.; Yamashita, T.; Teramae, N. Self-Assembly of a Silica-Surfactant Nanocomposite in a Porous Alumina Membrane. Nat. Mater. 2004, 3, 337−341. (7) Platschek, B.; Keilbach, A.; Bein, T. Mesoporous Structures Confined in Anodic Alumina Membranes. Adv. Mater. 2011, 23, 2395−2412. (8) Koh, C. W.; Lee, U. H.; Song, J. K.; Lee, H. R.; Kim, M. H.; Suh, M.; Kwon, Y. U. Mesoporous Titania Thin Film with Highly Ordered and Fully Accessible Vertical Pores and Crystalline Walls. Chem. Asian J. 2008, 3, 862−867. (9) Ma, C.; Han, L.; Jiang, Z.; Huang, Z.; Feng, J.; Yao, Y.; Che, S. Growth of Mesoporous Silica Film with Vertical Channels on Substrate Using Gemini Surfactants. Chem. Mater. 2011, 23, 3583− 3586. (10) Li, C.; Jiang, B.; Miyamoto, N.; Kim, J. H.; Malgras, V.; Yamauchi, Y. Surfactant-Directed Synthesis of Mesoporous Pd Films with Perpendicular Mesochannels as Efficient Electrocatalysts. J. Am. Chem. Soc. 2015, 137, 11558−11561. (11) Toghill, K. E.; Compton, R. G. Electrochemical Non-enzymatic Glucose Sensors: A Perspective and an Evaluation. Int. J. Electrochem. Sci. 2010, 5, 1246−1301. (12) Song, Y.-Y.; Zhang, D.; Gao, W.; Xia, X. Nonenzymatic Glucose Detection by Using A Three-Dimensionally Ordered, Macroporous Platinum Template. Chem. - Eur. J. 2005, 11, 2177−2182. (13) Comotti, M.; Pina, C. D.; Falletta, E.; Rossi, M. Aerobic Oxidation of Glucose with Gold Catalyst: Hydrogen Peroxide as Intermediate and Reagent. Adv. Synth. Catal. 2006, 348, 313−316. (14) Liu, Z.; Huang, L.; Ma, H.; Ding, Y. Electrocatalytic Oxidation of D-Glucose at Nanoporous Au and Au-Ag Alloy Electrodes in Alkaline Aqueous Solutions. Electrochim. Acta 2009, 54, 7286−7293. (15) Kurniawan, F.; Tsakova, V.; Mirsky, V. M. Gold Nanoparticles in Nonenzymatic Electrochemical Detection of Sugars. Electroanalysis 2006, 18, 1937−1942. (16) Shu, H.; Cao, L.; Chang, G.; He, H.; Zhang, Y.; He, Y. Direct Electrodeposition of Gold Nanostructures onto Glassy Carbon Electrodes for Non-enzymatic Detection of Glucose. Electrochim. Acta 2014, 132, 524−532. (17) Nugraha, A. S.; Li, C.; Bo, J.; Iqbal, M.; Alshehri, S. M.; Ahamad, T.; Malgras, V.; Yamauchi, Y.; Asahi, T. Block-CopolymerAssisted Electrochemical Synthesis of Mesoporous Gold Electrodes: Towards a Non-Enzymatic Glucose Sensor. ChemElectroChem 2017, 4, 2571−2576. (18) Li, C.; Jiang, B.; Chen, H.; Imura, M.; Sang, L.; Malgras, V.; Bando, Y.; Ahamad, T.; Alshehri, S. M.; Tominaka, S.; Yamauchi, Y. Superior Electrocatalytic Activity of Mesoporous Au Film Templated from Diblock Copolymer Micelles. Nano Res. 2016, 9, 1752−1762. (19) Li, C.; Wang, H.; Yamauchi, Y. Electrochemical Deposition of Mesoporous Pt-Au Alloy Films in Aqueous Surfactant Solutions: Towards A Highly Sensitive Amperometric Glucose Sensor. Chem. Eur. J. 2013, 19, 2242−2246. (20) Li, C.; Jiang, B.; Wang, Z.; Li, Y.; Hossain, M. S. A.; Kim, J.; Takei, T.; Henzie, J.; Dag, Ö ; Bando, Y.; Yamauchi, Y. First Synthesis of Continuous Mesoporous Copper Films with Uniformly Sized Pores H


Research Article

ACS Applied Materials & Interfaces Pentacle Gold−Copper Alloy Nanocrystals and Their Plasmonic and Catalytic Properties. Nat. Commun. 2014, 5, No. 4327. (39) Trasatti, S.; Petrii, O. A. Real surface Area Measurements in Electrochemistry. Pure Appl. Chem. 1991, 63, 711−734. (40) Zhong, G.; Zhang, W.; Sun, Y.; Wei, Y.; Lei, Y.; Peng, H.; Liu, A.; Chen, Y.; Lin, X. A Nonenzymatic Amperometric Glucose Sensor Based on Three Dimensional Nanostructure Gold Electrode. Sens. Actuators, B 2015, 212, 72−77. (41) Qiu, H. J.; Shen, X.; Wang, J. Q.; Hirata, A.; Fujita, T.; Wang, Y.; Chen, M. W. Aligned Nanoporous Pt-Cu Bimetallic Microwires with High Catalytic Activity Toward Methanol Electrooxidation. ACS Catal. 2015, 5, 3779−3785. (42) Wang, J.; Thomas, D. F.; Chen, A. Nonenzymatic Electrochemical Glucose Sensor Based on Nanoporous PtPb Networks. Anal. Chem. 2008, 80, 997−1004. (43) Liu, D.; Luo, Q.; Zhou, F. Nonenzymatic Glucose Sensor Based on Gold−Copper Alloy Nanoparticles on Defect Sites of Carbon Nanotubes by Spontaneous Reduction. Synth. Met. 2010, 160, 1745− 1748. (44) Shen, C.; Su, J.; Li, X.; Luo, J.; Yang, M. Electrochemical Sensing Platform Based on Pd−Au Bimetallic Cluster for NonEnzymatic Detection of Glucose. Sens. Actuators, B 2015, 209, 695− 700. (45) Shi, H.; Zhang, Z.; Wang, Y.; Zhu, Q.; Song, W. Bimetallic Nano-Structured Glucose Sensing Electrode Composed of Copper Atoms Deposited On Gold Nanoparticles. Microchim. Acta 2011, 173, 85−94. (46) Huang, J.; Dong, Z.; Li, Y.; Li, J.; Wang, J.; Yang, H.; Li, S.; Guo, S.; Jin, J.; Li, R. High Performance Non-Enzymatic Glucose Biosensor Based on Copper Nanowires−Carbon Nanotubes Hybrid for Intracellular Glucose Study. Sens. Actuators, B 2013, 182, 618− 624. (47) Li, C.; Jiang, B.; Wang, Z.; Li, Y.; Hossain, M. S. A.; Kim, J. H.; Takei, T.; Henzie, J.; Dag, Ö ; Bando, Y.; Yamauchi, Y. First Synthesis of Continuous Mesoporous Copper Films with Uniformly Sized Pores by Electrochemical Soft Templating. Angew. Chem., Int. Ed. 2016, 55, 12746−12750.

I


Electrochemical Synthesis of Mesoporous AuâCu Alloy Films with

Recommend Documents

Electrochemical Synthesis of Mesoporous AuâCu Alloy Films with