H2O2 Catalyst of Nanoporous Gold from AlâAu

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On the High Activity Methanol / HO Catalyst of Nanoporous Gold from Al-Au Ribbon Precursors with Various Circumferential Speeds Hui Xu, Kechang Shen, Shuai Liu, Xiaoguang Wang, Lai-Chang Zhang, Jingyu Qin, and Weimin Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06314 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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The Journal of Physical Chemistry

On the High Activity Methanol / H2O2 Catalyst of Nanoporous Gold from AlAl-Au Ribbon Precursors with Various Circumferential Speeds Hui Xu,a Kechang Shen,a Shuai Liu,a Laichang Zhang,b Xiaoguang Wang,c Jingyu Qin,a and Weimin Wang*a a Key

Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry

of Education, Shandong University, Jinan 250061, China b School

of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, Perth,

WA6027, Australia c

Laboratory of Advanced Materials & Energy Electrochemistry, Taiyuan University of

Technology, Taiyuan 030024, China

*Corresponding author. E-mail address: [email protected] (W. M. Wang)

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ABSTRACT We have prepared nano-porous gold (np-Au) with a three dimensional (3D) bicontinuous interpenetrating ligament-channel structure by dealloying the melt spun Al2Au ribbon precursors with three different circumferential speeds (Sc). With increasing Sc, the lattice constant (a0) of precursors decreases. After the dealloying procedure, the np-Au samples have an increasing a0 and a decreasing pore size with increasing Sc. There exists the heredity of the preferred orientation factors (F) between precursors and np-Au samples. The cyclic voltammetry (CV) curves of methanol electro-oxidation reaction (MOR) on np-Au samples are related to their F and show a higher activity with a higher Sc. In addition, np-Au with a lower pore size exhibit a higher sensitivity for the concentration of H2O2 in phosphate buffered solutions (PBS), which reaches 73.4 µA mM-1cm-2 with Sc = 18.3 m/s. These results suggest that we can change the pore size of the dealloyed np-Au by adjusting the Sc of the precursors, and then enhance the catalytic activity of np-Au.


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1. INTRODUCTION As we know, porous solids, such as zeolites and other molecular sieves, contain intracrystallite/framework cavities and channels that produce microporous (pore diameter, D＜2 nm), mesoporous (2 nm＜D＜50 nm), and macroporous (D＞50 nm) structures, and have demonstrated excellent potential as materials for use in many separation and catalytic processes.1-7 The pore sizes of these porous materials may play a critical role in controlling separation and catalytic selectivity due to their shape and size selectivity.2, 4, 8 Among these various nanoporous materials, nanoporous gold (np-Au) fabricated by electrochemical dealloying possesses a selforganized, self- supporting three- dimensional (3D) nanoarchitecture, and has been gaining increasing attention as a multifunctional material useful in a wide range of applications.9-15 The catalytic activity of np-Au mainly depends on nanopore size.10 Furthermore, the property of nanoporous materials greatly depends on the size of ligament/channel. Hence, the size control of nanopore is particularly important for their better application in areas such as catalysis, sensing, fuel cells, lithium batteries and supercapacitors. There are several methods to control the nanopore size of nanoporous materials.16 In choosing dealloying as the preparation method, the size of nanoporous materials can be modified by the alteration of the component of the precursor alloy,17-19 dealloying solutions,1, 20, 21 dealloying time11 and the dealloying temperatures.8 Generally, the as obtained np-Au performs a typical characteristic length scale of ligaments/channels of np-Au with several tens of nanometers, especially under free corrosion conditions. We have fabricated np-Au through doping a certain amount Pt/Pd in precursor alloy when performing dealloying in acid and alkaline solutions respectively.2 Qian and Chen8 had fabricated np-Au at a lower dealloying temperature (-20 ℃) and even within a shorter dealloying 3 ACS Paragon Plus Environment


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time (1 h or 10 min). However, there exists little research on the size control of np-Au through regulating the circumferential speeds (Sc) of the rapid solidification technology. It is known that there are several methods of preparing porous gold such as template methods and colloid crystal,22, 23

porous alumina24 and organic skeleton25 can be used as the templates of the np-Au. Compared

with these template methods, the dealloying with the rapid solidified technology has several advantages. Rapid solidification (RS) was first introduced by Duwez et al.,26 and melt spinning is the most commonly used technique to achieve rapid solidification and to produce long and continuous ribbons. First, this method is simple, convenient and an easy-operation. Second, the high cooling rates of rapid solidification processing from the melts would be of the order of 104 to 108 K s-1,27 which leads to special refined microstructures different from that under conventional conditions. Third, rapid solidified technology leads to a non-equilibrium solid with a high solubility of solute elements.28, 29 In addition, cooling rate gradient is developed in the melted metal in the direction of vertical to the roll surface due to the rapid solidification, which results in the grain stress concentration.30 Hence, incorporation of rapid solidification technique to the preparation of the nanoporous gold is expected to retain the merits of the rapidly quenched alloys. Generally, the experiment characterization of the morphology of np-Au is critical for quantifying the surface defect concentrations, lattice stress, and its correspondence to local surface curvature.10 In the present paper, we choose Al-Au binary alloy (Al0.66Au0.34, at%) as the precursor alloy. The Al0.66Au0.34 ribbons with different Sc were prepared by rapid solidification technology. We try to find the relation between the microstructures of precursors and dealloyed Au samples. Meanwhile, by operating the electrochemical experiments, we try to find the electrocatalytic activity of dealloyed Au in methanol electro-oxidation reaction (MOR) and oxydol reduction reaction. 4 ACS Paragon Plus Environment

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2. METHODS AND MATERIALS 2.1. Precursors Preparation and Dealloying Procedure. The prealloyed precursor (Al0.66Au0.34, at%) was designed according to the nominal composition of a single-phase Al2Au intermetallic compound. Figure S1 shows the Al-Au binary phase diagram. Al0.66Au0.34 ingots were prepared by arc-melting the pure bulk metals (Au: 99.99%, Al: 99.99%) in argon atmosphere and using recycled water cooling. Then, the ingots (about 15-20 g) were melted by high-frequency induction heating in a quartz glass tube and melt-spun onto a copper roller sized 0.35 m at the circumferential speed (Sc) of 11.0, 14,7 and 18.3 m/s. The schematic diagram of single roller melt spinning apparatus can be seen in Figure S2.The ribbons with thickness of 20-40 µm, width of 2-4 mm and length of several centimeters were obtained. Here, the cooling rate of ingots is lower compared with the rapid solidified ribbons. Meanwhile, the cooling rate of ribbons increases with increasing Sc. The ribbons are thinner at a higher Sc than at a lower Sc. In order to observe the microstructural feature of samples, the mosaic block of ingot and ribbons were prepared by using cold mounting and embedding resins. The ingot and ribbons were polished in the section and length direction. The rapidly solidified Al0.66Au0.34 alloy ribbons were dealloyed in a 5 wt % HCl aqueous solution first at 90 ± 5 ℃ until no obvious bubbles emerged. This procedure was carried out once again in order to further leach out the residual Al in the samples. The typical dealloying time was less than 1h. Finally, the nanoporous Au samples were obtained.

2.2. Microstructural Characterizations. The phases of the precursor alloys (Al0.66Au0.34) and as-dealloyed samples were identified using X-ray diffraction (XRD) with Cu kα radiation in 5 ACS Paragon Plus Environment


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the 2θ range from 10 to 90 degree. The lattice constants of Al2Au were obtained from the Rietveld analysis method with deducting the kα2 lines. The lattice constants of np-Au obtained from ribbon precursors were calculated by the extended Bragg equation:31

a0 =

λ 2sin θ

(1)

h2 + k 2 + l 2 ,

where λ is the radiation wavelength (λ=0.1542 nm); θ is the diffraction angle; and h, k, and l are the crystal plane indices. In present samples, the preferred orientation factors F of (111) plane of Al2Au are investigated by the Lotgering method:32

F=

P − P0 ∑ I( h00) and P = ∑ I0( h00) , ,P = 0 1 − P0 ∑ I (hkl ) ∑ I0(hkl )

(2)

where I(h00) and I(hkl) are the integral intensities of diffraction peaks for composites samples and randomly oriented samples, respectively; and P and P0 are the ratios of integral intensities of (h00) planes to those of all (hkl) planes for composites samples and randomly oriented samples, respectively. The microstructure and chemical composition of the precursor alloy and np-Au were determined with a scanning electron microscope (SEM). Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) patterns were used to characterize the microstructure of the specimens. We randomly selected and measured four hundred pores of np-Au prepared by dealloying Al0.66Au0.34 alloy ribbons with the different Sc by “Test Graph” software. The corresponding pore size Gaussian distribution of the np-Au was analyzed. The vertical axis represents the frequency of pores with different size while the horizontal axis represents the pore size. Some TEM specimens were observed using high-resolution TEM (HRTEM). The fast Fourier transform (FFT) patterns were obtained from the HRTEM images using Gatan software. 6 ACS Paragon Plus Environment

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2.3. Electrochemical Measurements. The electrochemical measurements were operated in a standard three-electrode cell using an electrochemical work station (CHI660E, Chenhua instrument Ltd., Shanghai). All the measurements were only conducted on one sides of the as-dealloyed ribbons and their another sides were covered by silicone rubber. The silicon rubber also has the fixation function to the ribbons because such as-dealloyed specimens are much fragile compared with the rapid solidification ribbons. The dealloyed Au samples were used as the working electrodes. The counter electrode was a bright Pt plate, and one saturated calomel electrode (SCE), one Hg/HgO electrode (MMO) and one Hg/Hg2SO4 electrode (MSE) were used as the reference electrode, depending on the experimental requirements. All experiments were performed at room temperature (298 K). In the cyclic voltammetry measurements, the electrolytes were 0.5 M KOH + 0.5 M methanol and 0.5 M H2SO4 + 0.5 M methanol, respectively. The scanning rate was 50 mV/s. The potentiodynamic polarization measurements were carried out in the same electrolyte solutions with the scan rate 1 mV/s. In order to study the application of np-Au in the detection of oxydol, cyclic voltammetry was also performed in 0.1 M phosphate buffer solution (PBS, pH=7), which is served as supporting electrolyte to avoid excessive decomposition of the oxydol to water and oxygen, with different concentrations oxydol. The scanning rate was 50 mV/s. All current densities were normalized by the real surface of the np-Au electrodes.

3. RESULTS AND DISCUSSION 3.1. Microstructure of Al0.66Au0.34 Precursors. Figure 1 shows the XRD patterns of the Al0.66Au0.34 ingot and melt spun ribbons with Sc= 11.0, 14.7 and 18.3 m/s. Both ingot and ribbons consist of intermetallic phase identified as Al2Au-type (PDF No.17-0877). Three strongest 7 ACS Paragon Plus Environment


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diffraction peaks at 25, 42 and 51 degree correspond to the (111), (220) and (311) crystal faces, respectively. Table 1 lists the position of diffraction peak (111) and the corresponding interplanar spacing (2θ(111) and d(111)), lattice parameter a0 and preferred orientation factors F(111) and F(220) of Al2Au phase for original ingot and ribbons. In order to obtain the error of lattice parameter a0, the XRD analysis of ribbon precursors were repeated, as shown in Figure S3. Here, the cooling rate of solidification for four samples is in the order: ingot < ribbon with Sc = 11.0 m/s < ribbon with Sc = 14.7 m/s < ribbon with Sc = 18.3 m/s. The 2θ(111) and F(111) of samples increase and their d(111), a0 and F(220) tend to decrease with increasing cooling rates. According to Refs 33 and 34, the compressive stress increases with the decreases of lattice constant. Meanwhile, the compressive stress concentrates on the inner of ribbon samples.35 Hence, the compressive stress of present ribbons increases with increasing Sc. Also, the preferred orientation factors F(111) of ribbons increases with increasing Sc, but their F(220) decreases at the same time. The SEM images of air side and section fracture surfaces of ribbon precursors are shown in Figure 2. The grain boundaries are clearly shown in Figure 2a, b, and c. Except the majority Al2Au phase, there are several second phases distributing near the grain boundaries. The fraction of second phase becomes higher increase with increasing Sc, which is consistent with the Rietveld analysis of XRD patterns of ribbons. The mean grain sizes of ribbons with Sc = 11.0, 14.7, and 18.3 m/s are approximately 12.8, 8.1, and 4.7 µm, respectively, showing an descending tendency with increasing Sc. In addition, SEM images of section fracture surface of ribbons display that the columnar crystals rather than dendrites align along the normal direction of the ribbon plane. Metallographical sample appearances and the corresponding SEM images of the Al0.66Au0.34 ingot and the ribbons with different Sc are shown in Figures S4 and S5. 8 ACS Paragon Plus Environment

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The TEM and HRTEM images as well as SAED patterns of Al0.66Au0.34 ribbons are shown in Figures 3 and S6. The sample edge with Sc = 11.0 m/s seems brighter and thinner than that with Sc = 18.3 m/s, indicating that the former sample has a higher ductility than the latter sample. Besides, the corresponding SAED patterns (insets of Figure 3a and c) consisting of polycrystalline diffraction rings are indexed as fcc Al2Au, which is consistent with the XRD pattern in Figure 1. The HRTEM images of ribbon (Figure 3b and d, S6e and f) exhibit the polycrystalline structure. The corresponding FFT patterns (insets of Figure 3b and d) are indexed as Al2Au phase and consistent with XRD patterns as well. The (200) plane spacing of one selected nanocrystal with Sc = 11.0 m/s is 0.301 nm (Figure 3b) and is greater than the value with Sc = 18.3 m/s (0.299 nm, Figure 3d), implying that the degree of lattice contraction of the ribbon with Sc = 18.3 m/s is larger than that of Sc = 11.0 m/s, agreeing with the XRD results (Table 1).

3.2. Microstructure of Nano-Porous Gold (Np-Au). Figure 4 shows the XRD patterns of as-dealloyed samples from the ribbon with various Sc, which display a single fcc Au phase and are consistent with the standard pattern of fcc Au (PDF No. 04-0784). Meanwhile, five diffraction peaks are identified as (111), (200), (220), (311) and (222) faces in turn. However, the intensity of (111) peak of ribbons with Sc = 18.3 m/s is higher than that of others, showing that Au phase exhibits a stronger prior growth of (111) faces with a higher Sc. In addition, the position of diffraction peak (111) and the corresponding interplanar spacing (2θ(111) and d(111)), lattice parameter a0 and preferred orientation factors F(111) and F(200) of dealloyed Au samples with different Sc and pure Au bulk are listed in Table 2. The (111) peak of dealloyed Au samples slightly shifts to the lower diffraction angles with increasing Sc and their 2θ(111) are smaller than the standard Au; hence, their interplanar spacing d(111) and a0 are larger compared to the corresponding ones of standard Au. 9 ACS Paragon Plus Environment


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These parameters reflect that Au atoms in dealloyed Au are deviated from the equilibrium position of pure Au, which may be induced by both the original inner compressive stress in ribbons and the stress triggered in the dealloying process. According to the results of Schofield and co-workers,33 np-Au has two regions: tension regions with the larger lattice parameters (a0 > 0.408 nm and compression region with the smaller lattice parameters (a0 < 0.408 nm) after dealloying; the bonds tend to stretch with positive curvature and to contract with negative curvature, giving rise to tensile and compressive stress, respectively. Hence, the tensile stress is dominant in present dealloyed Au samples since their a0 are larger than 0.408 nm (Table 2). On the other hand, as we mentioned before, in the ribbon with Sc = 18.3 m/s, the inner compressive stress is higher than other Sc; however, the samples present tensile stress after dealloying, suggesting that Au atoms in the precursor with a higher Sc need overcome a higher initial compressive pressure in the rearrangement process. In addition, the F(111) of dealloyed Au increases with increasing Sc similar to F(111) of ribbon precursor (Tables 1 and 2). Here, the (111) peak of dealloyed Au is one of these strongest peaks and locates at 2θ < 40 degree (Figure 4), which is consistent with the (111) peak in ribbon precursors (Figure 1). Meanwhile, the F(200) of dealloyed Au samples decreases with increasing Sc and is similar to F(220) of ribbon precursors. Here the (200) peak is another three-strongest peak of dealloyed Au and locates at about 45 degree (Figure 4), which is corresponding to the (220) peak in ribbon precursors (Figure 1). Hence, the change behavior of F of the dealloyed Au is related to that of ribbons, which reflects the heredity between precursors and dealloyed samples. Figures 5 and S7 show the SEM images and corresponding statistical histogram of the size distribution of the dealloyed porous gold made from ribbon precursors with various Sc. The insets in 10 ACS Paragon Plus Environment

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Figure 5a, b and c show the corresponding zoomed out images. The nanoporous gold (np-Au) exhibits a bicontinuous interpenetrating ligament – channel structure with the ligament spacing, i.e. pore size between 5 to 75 nm. As shown in Figure 5d, e and f, the mean pore sizes are 45 ± 12, 35 ± 17 and 27 ± 25 nm with Sc =11.0, 14.7 and 18.3 m/s, respectively, i.e. the pore size of np-Au decreases with increasing Sc. Apparently, the smaller the mean pore size, the larger the absolute value of the positive or negative curvatures. In addition, the range of pore sizes i.e. the gap between the largest pore sizes and the smallest pore sizes of np-Au increases with the increasing Sc, which means that the np-Au has an increasing roughness parameter as the Sc increases according to Ref 36. Moreover, the second phase in Figure 2a, b, and c increase with increasing Sc. According to the Refs 37, 38, the existence of second phase leads to a smaller ligament /pore sizes of np-Au in the dealloying process compared with dealloying the single-phase Al2Au, which demonstrates that the Sc – dependent ligament / pore size can be partially ascribed to the contribution of second phase. The TEM and HRTEM images of np-Au samples with Sc =11.0 and 18.3 m/s are presented in Figures 6 and S8. Three-dimensional bicontinuous interpenetrating ligament (dark skeleton) – channel (bright region) structure exists in np-Au samples; moreover, the size of channels (bright region) with Sc = 18.3 m/s is smaller compared with Sc = 11.0 m/s (Figure 6a and c), which is in agreement with the SEM images in Figure 5. Meanwhile, the corresponding SAED patterns (insets in Figure 6a and c) verify the crystalline spots as fcc Au with an approximate single-crystalline nature. The HRTEM images consist of the regularly arranged planes, showing the single-crystalline nature (Figure 6b and d, S8e and f). The corresponding FFT patterns verify the ligaments of samples as fcc Au (insets of Figure 6b and d). The (200) plane spacing of one selected crystal in np-Au with Sc = 11.0 m/s is about 0.200 nm (Figure 6b), which is smaller than the value (0.202 nm) 11 ACS Paragon Plus Environment


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with Sc = 18.3 m/s (Figure 6d) and consistent with the XRD data in Table 2. In the dealloying process, the compressive stress is triggered in the dealloyed layer, which hinders the diffusion and rearrangement of inner Au atoms of the samples.39, 40 The greater the initial compressive pressure in ribbons, correspondingly, the harder the reorganization of Au atoms and the smaller the ligament spacing. Indeed, the reorganization of atoms at lower temperature during dealloying AuCu film is more difficult than that at high temperature, resulting in a nanoporous Au with finer ligament and pore at low dealloying temperature.41 Hence, it is understood that the pore size (i.e. ligament spacing) of np-Au decreases with increasing Sc (Figure 5b, d and f).

3.3. Electrochemical Properties of Np-Au. The consequences of the electro-oxidation catalytic for methanol and the detection to oxydol of np-Au electrodes are now discussed. The geometry area of each np-Au electrode in electrochemical tests was 0.2 cm2. Based on the charge needed to form surface gold oxide monolayer and the equation: 42 Areal = Q/Q0 as well as Q0 = 386 µC cm-2, the real surface areas were estimated to be about 1.21, 1.43, and 1.64 cm2 for np-Au with Sc = 11.0, 14.7, and 18.3 m/s, respectively. The detailed information about the experiments for real surface measurements can be found in the supporting information (Figures S9 and S10). All current densities were calculated by the real surface areas obtained from Figure S9 in this paper.

3.3.1 Methanol Electro-oxidation Reactions (MOR). Figure 7 shows the cyclic voltammetry (CV) curves of the np-Au dealloyed from the precursor with different Sc in alkaline and acidic solutions. The cyclic voltammograms (CVs) of np-Au electrodes with the different Sc in solutions without methanol can be found in supporting information (Figure S11) in order to compare the effects of the addition with methanol. In 0.5 M KOH + 0.5 M methanol solution, two 12 ACS Paragon Plus Environment

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different potential regions occur in forward scan direction: one from -0.4 to 0.5 V，and the other starts at about 0.6 V. It has been indicated previously that methanol oxidation in Au electrode proceeds in two potential regions with different mechanisms.43-45 At lower potential from -0.4 to 0.5 V, taking np-Au dealloyed by ribbons with the Sc = 18.3 m/s as an example, the major peak Emp at about 0.3 V involves the reaction that methanol is mainly oxidized to formates via an overall four-electron-transfer reaction:43, 44 CH3OH + 5 OH- = HCOO- + 4 H2O + 4 e-

(3)

Here, the methanol oxidation on the oxide-free np-Au surface begins at about -0.4 V and proceeds in a slow reduction rate until about 0 V due to the chemisorption of a small amount − of OH ads . As the potential sweeps positively, more OH- anions adsorb onto the np-Au surface and

react with gold surface atoms, resulting in the formation of a small subpeak at Esub before the major peak at Emp. This subpeak indicates the formation of called “pre-oxidation precursors”, i.e. Au-

OH (1ads− λ )− , where “ads” and λ refer to the chemisorbed species on np-Au and the charge-transfer coefficient ranging from 0 to 1, respectively.46 According to the results of Borkowska,44 the formation of small prewave (Esub) in CV curves is associated with the Au (111) face. There is no subpeak in the CV profile of np-Au electrode with Sc = 11.0 m/s, which may be related to the lower F(111), resulting the lower ability of the chemisorbed OH- anions and a lower activity for methanol electro-oxidation compared with other counterparts. In CV curves, the shoulder peaks Esh can be seen after the major peaks, and the shoulder peak of np-Au dealloyed by the ribbon with Sc = 11.0 m/s is most obviously compared to others. According to the results of Hamelin,47 in the low potential region (region A), the main peak is usually associated with the face index (111) of Au, and the shoulder peak Esh in right side is attributed to the 13 ACS Paragon Plus Environment


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face index (100). As shown in Table 2, the preferred orientation factor F(200) of (200) face of np-Au samples decreases with the increase of Sc, but their F(111) increases. As expected, the relative height of the shoulder peak of np-Au samples decreases with increasing Sc (Figure 7a). At lower potential range up to 0.5V, methanol oxidation is inhibited in the beginning of AuO formation from the Au-

OH (1ads− λ )− .48 Hence, the current density decreases after the major peaks with the consumption of AuOH (1ads− λ )− species to form relatively dense and ordered AuO (Figure 7a). In short words, both methanol electro-oxidation and Au surface oxidation occurs in the first potential region. In the second forward scanning region (> 0.6 V), a sharp increase in the anodic current for methanol oxidation is observed, which is associated with the reaction of the anodic oxidation of methanol to form carbonates by gold oxides44, 45, 49: CH3OH + 8 OH- = CO32- + 6 H2O + 6 e-

(4)

Here, instead of the four-electron-transfer reaction, a six-electron-charge-transfer process generates in oxidized np-Au surface. This reaction mechanism of np-Au allows the reaction intermediates produced at low potentials to be further oxidized.50 Besides, the total rising rate of the oxidation current of np-Au increases with the increase of Sc (Figure 7a), indicating a more active state of np-Au electrode with a smaller pore size. This result can be explained by Ref 36: the np-Au with a lower size has a higher roughness parameter and then a higher catalytic activity. The np-Au with Sc = 11.0 m/s has a higher F(200) than other two np-Au samples (Table 2). According to the results of Hamelin,47 in the high potential region (region B), the first peak is related to the index (100) face with the second peak related to the index (111) face. Hence, the CV curve of the np-Au with Sc = 11.0 m/s exhibits a peak at 0.7 V (i.e. the lower potential range in the second forward scanning region), while this peak is merged into the second peak in case of Sc = 14 ACS Paragon Plus Environment

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14.7 m/s and Sc = 18.3 m/s (Figure 7a). The negatively-going potential scan shows that the electrochemical reduction of the gold surface oxides leads to a reduction peak centered on 0 V. With the removal of dense AuO, Au- OH (1ads− λ )− species are regenerated on the np-Au surface, stimulating the re-initiation of the methanol oxidation according to reaction (3).44, 49 Thereby, after the potential reaches ∼ 0 V, the absolute values of corresponding oxidation current rises immediately, which is consistent with Ref 49. The redox peak shift negatively and its absolute height increases with the increase in Sc, which is consistent with the potential shift in positive scan direction (Figure 7a) and confirms the higher methanol electro-oxidation activity of np-Au with a higher Sc. In Figure 7b, the onset potentials for both Emp and further methanol oxidation shift negatively with increasing of Sc. It is known that the negatively shifts of Emp is positively related to the Au atoms oxidation rates.50 Hence, the np-Au from the precursor with a higher Sc was oxidized more readily than that with a lower Sc. At the reverse sweep, one found that decreasing the pore size of np-Au electrodes would cause a slight negative shift of the reduction peak potentials, indicating that np-Au with a higher Sc has a more active surface state and a higher catalytic ability for methanol electro-oxidation. In 0.5 M H2SO4 + 0.5 M methanol solution, the oxidation peaks marked as Emp, Esh and E’ in positively scan region from 1.1 – 1.4 V (inset Figure 7b) are ascribed to the oxidation of surface Au atoms and methanol, according to Ref 48 The relative height of Esh of samples changes with Sc, similar to the situation in alkaline solution (Figure 7a), which is related to F(200) (Table 2 and Ref 47). It has been indicated previously that methanol oxidation in Au electrode proceeds in two potential regions with different mechanisms.43-45 At a low potential, methanol is mainly oxidized to 15 ACS Paragon Plus Environment


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formats via an overall four-electron-transfer reaction (eq. 3). And at a high potential, methanol oxidation is associated with the reaction of the anodic oxidation of methanol to form carbonates by gold oxides. Instead of the four-electron-transfer reaction in the surface of nanoporous gold, a six-electron-transfer process (eq. 4) generates in oxidized np-Au surface. According to the Ref 48, it is clear that the methanol electro-oxidation begins at around 1.1 V. On the other hand, once AuOH formation begins at Emp = 1.15 V, the methanol oxidation (eq 3) current reaches a maximum value, which coincides with the beginning of the Au- OH (1ads− λ )− formation on surface of np-Au. At potentials beyond Esh = 1.25 V, the methanol oxidation is being inhibited according to the AuO formation, which leads to another methanol oxidation (eq. 4) current occurs in at E ’= 1.35 V. This is similar to the redox peak in alkaline solution. Apparently, both the surface oxidation and subsequent reduction take place at much higher positive potentials in acidic solutions with respect to alkaline solutions. Meanwhile, the absolute current density of oxidation and reduction is considerably lower than in alkaline solutions, which implies that np-Au exhibits a much higher electrocatalytic activity in alkaline media than that in acidic media and is consistent with earlier work.50 As such, the absorption of oxygen species (such as OH- anion) on the Au surface has a potential effect on the methanol electro-oxidation reaction (MOR) in both alkaline and acid solutions. In addition, according to the results of Hamelin,47 the CV curves of Au (111), (110), and (100) faces have characteristically different current profiles for the formation of a monolayer of oxide and its reduction at the higher potential range. The anodic current of Au (111) was larger than Au (110) and (100), which means that Au (111) is more active than other faces at higher potentials related to the methanol electro-oxidation reaction. In present work, the F(111) (Table 2) and catalytic efficiency 16 ACS Paragon Plus Environment

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(Figure 7) of np-Au increase with increasing Sc, which is qualitatively consistent with Hamelin´s work. In order to better reveal the advantages of np-Au with large surface special area and different sizes pores for MOR, the schematic of the methanol electro-oxidation reaction in alkaline solution of np-Au with Sc = 18.3 m/s is shown in Figure 8. Figure 8 shows the process model of methanol electrocatalytic oxidation on the surface of np-Au. The unique porous structure provides a high special surface area and the interconnected nanoporous channels can effectively facilitate charge/ion transfer and mass transport for the electrocatalysis. Simultaneously, the 3D continuous large pores could supply more active sites for the adsorption of ion and methanol and accelerate the methanol transport. With the addition of OH- in the solution, methanol could be oxidized to CO 32 − and H2O through losing 6 electrons. In order to further compare the influence of spinning speed activity of the np-Au electrode on MOR, Tafel polarization analysis was shown in Figure S12. The results of Tafel analysis are consistent with the MOR.

3.3.2 Oxydol Reduction Reaction. The CV curves of np-Au electrodes in 0.1 M phosphate buffer solution (PBS) with pH = 7.0 toward the detection of oxydol are shown in Figure 9. Different from the little current response of np-Au in single PBS solution (Figure 9a), after adding 5 mM H2O2, the current responses increase siginificantly (Figure 9b). Such significant current increase confirms the active catalytic performance of np-Au toward H2O2 reduction and the potential for nano-porous gold to work as sensor materials for H2O2 detection. According to the literature,51, 52 the mechanism for H2O2 electro-reduction can be indicated as following: H2O2 + e− → OHad + OH−

(5)

OHad + e− → OH−

(6) 17 ACS Paragon Plus Environment


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2 OH− + 2 H+ → 2 H2O

(7)

According to Refs 50 and 53, apparently, the current density of np-Au electrodes increases with increasing Sc under the same H2O2 concentration in the reduction potential region due to the higher surface areas of np-Au with a higher Sc. Apparently, the height of reduction current cathodic peak increases with the increase of H2O2 concentrations from 0 to 5 mM (Figure 9c, d and e). It can be seen that the current density is proportional to the concentration of H2O2. The liner regression equation for the concentration range of np-Au electrodes with increasing Sc were obtained as expressions (8) − (10), respectively:

I (mA) = -46.3 × 10-3 c -24.3 × 10-3, with Sc = 11.0 m/s and R2 = 0.96

(8)


(9)


(10)

where c is the concentration of H2O2. The absolute slope values, i.e. sensitivities of np-Au with Sc = 11.0, 14.7 and 18.3 m/s are 46.3, 55.6 and 73.4 µA mM-1cm-2, respectively, which indicates that np-Au electrode dealloyed from a higher Sc exhibits a higher sensitivity to H2O2 concentration. In addition, as shown in Figure 9c, d and e, the potential of reduction current peak of np-Au shifts negatively as Sc increases, which is consistent with the consequence of MOR (Figures 7) and Ref 50. Hence, np-Au with a smaller pore size exhibits a better catalytic activity for methanol and H2O2 due to the higher active surface states and higher defect densities, which is consistent with the high curvature and stress inner the np-Au with high Sc.

CONCLUSIONS Nano-porous gold with a three dimensional (3D) bicontinuous interpenetrating ligament-channel 18 ACS Paragon Plus Environment

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structure has been prepared by dealloying the melt spun Al2Au ribbon precursors with different circumferential speeds (Sc). The Al2Au precursors have a decreasing lattice constant (a0), a decreasing preferred (220) orientation factor F(220), and an increasing F(111) with increasing Sc, indicating an increase of compressive stress in the ribbons. After the dealloying procedure, the nanoporous gold (np-Au) samples have an increasing F(111) and a decreasing F(200) with decreasing Sc, which shows the heredity between precursors and np-Au samples. Meanwhile, the np-Au samples have an increasing a0 and a decreasing pore size with increasing Sc. Methanol molecules can be oxidized on np-Au surface through two different mechanisms, both are dependent on the F of (100) and (111) faces and pore size of np-Au samples. And Au (111) face plays an important role in improving the catalytic performance on methanol eletro-oxidation reaction. Moreover, np-Au exhibits a good linear responding as well as high sensitivity for the concentration of H2O2 in phosphate buffered solutions (PBS). These results indicate that np-Au with a smaller pore size has a higher catalytic activity no matter for methanol electro-oxidation or detection of H2O2. In other words, we can change the pore size of the dealloyed np-Au by adjusting the Sc of the precursors; and then we can enhance the catalytic activity of np-Au.

ASSOCIATED CONTENT Supporting Information: Schematic illustration of the Al-Au binary phase diagram and single roller melt spinning apparatus. Repeated XRD patterns of ribbon precursors with different Sc. Metallographical sample appearance and the corresponding SEM images of the Al0.66Au0.34 ingot and the rapidly solidified ribbons with different Sc. TEM and HRTEM image of the rapidly solidified Al0.66Au0.34 ribbons with Sc = 18.3 m/s. SEM images of nanoporous gold (np-Au) prepared 19 ACS Paragon Plus Environment


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by dealloying the rapidly solidified Al0.66Au0.34 ribbons with Sc = 18.3 m/s. TEM and HRTEM images of nanoporous gold (np-Au) prepared by dealloying the rapidly solidified Al0.66Au0.34 ribbons with Sc = 18.3 m/s. CV curves of np-Au electrodes measured in 0.5 M H2SO4 solution. Scan rate: 50 mV/s. Pb UPD CV curves of np-Au electrodes measured in 0.01 M Pb(ClO4)2, 0.1 M NaClO4, and 0.01M HClO4 solution. Scan rate: 5 mV/s. Cyclic voltammograms (CVs) of np-Au electrodes with the different Sc in 0.5 M KOH solution and 0.5 M H2SO4 solution. Scan rate: 50mV/s. Tafel curves of np-Au with the different ligament spacing size in the 0.5M KOH + 0.5M Methanol solution and the 0.5M H2SO4 + 0.5M Methanol solution. Scan rate: 1mV/s.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work is supported by National Natural Science Foundation of China (Nos. 51471099 and 51571132), National Key Research Program of China (Grant No. 2016YFB0300501)，and National Basic Research Program of China (973 Program) (No. 2012CB825702 )


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Figures and Tables

Figure 1. XRD patterns of the original Al0.66Au0.34 ingot and rapidly solidified ribbon precursors with three different circumferential speeds (Sc).



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Figure 2. SEM images of ribbons with Sc of (a and d) 11.0, (b and e) 14.7 and (c and f) 18.3 m/s. a, b, and c, air side surface; d, e, and f, section fracture surface. Scale bar, 4 µm.


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Figure 3. TEM and HRTEM image of ribbon precursors with (a and b)Sc =11.0 m/s and (c and d) 18.3 m/s. Scale bar of (a) and (c), 100 nm. Scale bar of (b) and (d), 5 nm. The insets show SAED and FFT patterns of corresponding TEM and HRTEM images, respectively. Scale bar of the insets in (a) and (c), 5 1/ nm.



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Figure 4. XRD patterns of the dealloyed Au from precursors with different Sc as well as pure Au.


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Figure 5. SEM images and statistical histogram of the size distribution of nanoporous gold (np-Au) prepared by dealloying ribbons precursors with (a and d)Sc = 11.0 m/s, (b and e)14.7 m/s and (c and f)18.3 m/s. Scale bar of (a), (b) and (c), 200 nm. The insets in (a), (c) and (e) show the corresponding zoom out images. Scale bar of insets in (a), (b) and (c), 1 µm.



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Figure 6. TEM and HRTEM image of nanoporous gold (np-Au) prepared by dealloying ribbon precursors with (a and b)Sc = 11.0 m/s and (c and d)18.3 m/s. Scale bar of (a) and (c), 100 nm. Scale bar of (b) and (d), 5 nm. The insets show SAED and FFT patterns of corresponding TEM and HRTEM images, respectively. Scale bar of the insets in (a) and (c), 5 1/ nm.


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Figure 7. Cyclic voltammograms (CVs) of np-Au electrodes with the different Sc in (a) 0.5 M KOH + 0.5 M methanol solution and (b) 0.5 M H2SO4 + 0.5 M methanol solution, respectively. Scan rate: 50 mV/s. The inset in (b) shows the enlarged CV parts.

Figure 8. Schematic illustration of the methanol electro-oxidation reaction in alkaline solution of np-Au with Sc = 18.3 m/s.



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Figure 9. Cyclic voltammograms (CVs) of np-Au electrodes with various Sc in the PBS solution with (a) 0 and (b) 5 mM H2O2. CVs curves of np-Au electrode with Sc of (c) 11.0 m/s, (d) 14.7 m/s and (e)18.3 m/s in neutral PBS solution of various H2O2 concentrations. (f) The current response vs. H2O2 concentrations of np-Au samples. Scan rate: 50 mV/s.


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Table 1. The Position of Diffraction Peak (111) and The Corresponding Interplanar Spacing (2θ(111) and d(111)), Lattice Parameter a0 and Preferred Orientation Factors F(111) and F(220) of Al2Au Phase for Original Ingot and Ribbons with Sc =11.0, 14.7 and 18.3 m/s, Respectively Samples d(111)(nm) a0(nm) F(111) F(220) 2θ(111)(deg.) Ingot Ribbon, 11.0 m/s Ribbon, 14.7 m/s Ribbon, 18.3 m/s

25.506 25.550 25.574 25.600

0.34894 0.34834 0.34802 0.34768

0.60123 0.60090 ± 0.0001 0.60089 ± 0.0001 0.60088 ± 0.0001

-0.142 0.007 0.022 0.228

0.036 -0.012 -0.036 -0.042

Table 2. The Position of Diffraction Peak (111) and The Corresponding Interplanar Spacing (2θ(111) and d(111)), Lattice Parameter a0 and Preferred Orientation Factors F(111) and F(100) of np-Au Samples Prepared by dealloying Al0.66Au0.34 Ribbon Precursors with Different Sc and Pure Au Bulk Samples d(111)(nm) a0(nm) F(111) F(200) 2θ(111)(deg.) Pure Au (PDF No. 04-0784) Dealloyed Au, Sc =11.0 m/s Dealloyed Au, Sc =14.7 m/s Dealloyed Au, Sc =18.3 m/s

38.184 38.020 38.018 38.017

0.23550 0.23648 0.23649 0.23650

Table of Contents (TOC) Image


0.40790 0.40959 0.40960 0.40962

0.0278 0.0490 0.1992

0.0416 0.0259 0.0179


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49x34mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. SEM images of ribbons with Sc of (a and d) 11.0, (b and e) 14.7 and (c and f) 18.3 m/s. a, b, and c, air side surface; d, e, and f, section fracture surface. Scale bar, 4 µm. 69x71mm (300 x 300 DPI)



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Figure 3. TEM and HRTEM image of ribbon precursors with (a and b)Sc =11.0 m/s and (c and d) 18.3 m/s. Scale bar of (a) and (c), 100 nm. Scale bar of (b) and (d), 5 nm. The insets show SAED and FFT patterns of corresponding TEM and HRTEM images, respectively. Scale bar of the insets in (a) and (c), 5 1/ nm. 74x74mm (300 x 300 DPI)


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Figure 4. XRD patterns of the dealloyed Au from precursors with different Sc as well as pure Au. 50x36mm (300 x 300 DPI)



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Figure 5. SEM images and statistical histogram of the size distribution of nanoporous gold (np-Au) prepared by dealloying ribbons precursors with (a and d)Sc = 11.0 m/s, (b and e)14.7 m/s and (c and f)18.3 m/s. Scale bar of (a), (b) and (c), 200 nm. The insets in (a), (c) and (e) show the corresponding zoom out images. Scale bar of insets in (a), (b) and (c), 1 µm. 85x100mm (300 x 300 DPI)


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Figure 6. TEM and HRTEM image of nanoporous gold (np-Au) prepared by dealloying ribbon precursors with (a and b)Sc = 11.0 m/s and (c and d)18.3 m/s. Scale bar of (a) and (c), 100 nm. Scale bar of (b) and (d), 5 nm. The insets show SAED and FFT patterns of corresponding TEM and HRTEM images, respectively. Scale bar of the insets in (a) and (c), 5 1/ nm. 75x75mm (300 x 300 DPI)



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Figure 7. Cyclic voltammograms (CVs) of np-Au electrodes with the different Sc in (a) 0.5 M KOH + 0.5 M methanol solution and (b) 0.5 M H2SO4 + 0.5 M methanol solution, respectively. Scan rate: 50 mV/s. The inset in (b) shows the enlarged CV parts. 42x16mm (300 x 300 DPI)


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Figure 8. Schematic illustration of the methanol electro-oxidation reaction in alkaline solution of np-Au with Sc = 18.3 m/s. 49x32mm (300 x 300 DPI)



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Figure 9. Cyclic voltammograms (CVs) of np-Au electrodes with various Sc in the PBS solution with (a) 0 and (b) 5 mM H2O2. CVs curves of np-Au electrode with Sc of (c) 11.0 m/s, (d) 14.7 m/s and (e)18.3 m/s in neutral PBS solution of various H2O2 concentrations. (f) The current response vs. H2O2 concentrations of np-Au samples. Scan rate: 50 mV/s. 87x107mm (300 x 300 DPI)


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H2O2 Catalyst of Nanoporous Gold from AlâAu

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