J. Phys. Chem. C 2009, 113, 603–609
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Nanoporous Gold Thin Film: Fabrication, Structure Evolution, and Electrocatalytic Activity Hua Dong*,† and Xiaodong Cao†,‡ Joint Department of Biomedical Engineering, NC State UniVersity and UNC Chapel Hill, Raleigh, North Carolina 27695, and Department of Wood and Paper Science, NC State UniVersity, Raleigh, North Carolina 27695 ReceiVed: September 30, 2008; ReVised Manuscript ReceiVed: NoVember 9, 2008
This paper demonstrates a new and simple approach to fabricate nanoporous structure on gold thin films (∼1.5 µm) via electrochemical alloying and dealloying reactions in ZnCl2/DMSO electrolyte. The relevant parameters affecting the formation of nanoporous structure in the alloying/dealloying process in organic electrolyte and the post-treatment in sulfuric acid solution are investigated in detail by simultaneous electrochemical and field emission scanning electron microscopy (FESEM) characterization. Herein, a new strategy for direct estimation of the end-point in the dealloying process is developed, assuring the success of preparing nanoporous gold (NPG) films with the highest purity and porosity. Our method is especially useful for fabrication of nanoporous structure on ultrathin Au films without any further deposition of Au. The asprepared NPG thin films are then used as anodic catalysts for borohydride oxidation in the direct borohydride fuel cell (DBFC), which show much higher catalytic activities in contrast to planar gold electrode, indicative of the promising applications in actual DBFC systems. We believe that our work offers a new route to solve the current problems encountered in the development of DBFCs. 1. Introduction Due to its high surface-to-volume ratio, excellent stability, and biocompatibility, nanoporous gold (NPG) has attracted considerable attention in the recent years,1-5 and there is an increasing interest in the discovery of new methods to fabricate NPG materials as well as new technologies to process them into useful forms. The common approaches reported in the literature are template-based synthesis by use of polycarbonate membranes,6 colloidal crystals,7 self-assembled surfactants,8 anodic porous alumina/silica,9 echinoid skeletal structures,10 and even dynamic hydrogen bubbles11 as templates. These fabrication techniques offer a high degree of control over the pore size and microstructure periodicity but are generally difficult and time-consuming to implement. Alternatively, etching of binary alloys (also known as dealloying), especially the Au-Ag alloy, has emerged recently as an efficient route to fabricate NPG, where the less noble component like Ag is selectively dissolved from the Au-Ag alloy frame, leading to an open bicontinuous nanostructure comprised almost entirely of gold.12-14 Compared with template-based fabrication methods, dealloying of Aucontaining binary alloy is much easier to operate and thus more suitable for manufacturing production.15,16 The current paper describes a simple and effective way to fabricate nanoporous structure on ultrathin gold films (∼1.5 µm) without any template and Au deposition from solution; i.e., the preparation was performed via electrochemical alloying/dealloying of Au-Zn alloy in dimethyl sulfoxide (DMSO)/ZnCl2 electrolyte. Compared with other solvents reported in the literature,17,18 DMSO is inexpensive, nontoxic, and thus frequently used in organic electrochemistry. The high boiling point of DMSO (189 °C) also enables the rapid formation of the * To whom correspondence should be addressed. E-mail: hdong2@ ncsu.edu. Tel.: 1-919-2710922. Fax: 1-919-5133814. † Joint Department of Biomedical Engineering, NC State University and UNC Chapel Hill. ‡ Department of Wood and Paper Science, NC State University.
Au-Zn alloy at elevated temperatures without any special protection from humidity and oxygen. Furthermore, our method does not involve any toxic cyanide that is often employed to fabricate Au-Ag alloy in solution.19 In this study, both electrochemical and SEM characterization are used to investigate vividly and systematically the alloying/dealloying process including the evolution of nanoporous structure on the planar gold film surface and the influences of temperature, potentials, and scan rate. To the authors’ knowledge, this has never been reported before and is quite useful for the understanding of the mechanism hidden in the process. Besides, we develop herein a new strategy to estimate the completion of dealloying reaction, assuring the success of NPG fabrication with the highest purity and porosity. The as-prepared NPG thin films are then used as anodic catalysts for BH4- oxidation in the direct borohydride fuel cell (DBFC). Our primary results show much higher catalytic activities of NPG films in contrast to planar gold electrode, indicative of the promising application in actual DBFC systems. 2. Experimental Section 2.1. Reagents and Instrumentation. Zinc chloride, dimethyl sulfoxide (DMSO), sulfuric acid, sodium borohydride, sodium hydroxide, zinc plate, and platinum plate were purchased from Sigma. Pure gold thin film was obtained by sputtering titanium (10 nm) and gold (1.6 µm) on a cleaned silicon wafer. All aqueous solutions were prepared using deionized water (Millipore). The electrochemical experiments were performed on an Autolab potentiostat/galvanostat controlled with the GPES software (Eco Chemie B.V. Utrecht, The Netherlands). Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectrometer (EDX) measurements were conducted using JEOL-6700F (Japan) and Oxford INCA. X-ray diffraction (XRD) was carried out on D/max-2400 Rigaku (Japan) with Cu Ka radiation (k ) 1.54178 Å) operating at 50.0 kV and 200.0 mA.
10.1021/jp8086607 CCC: $40.75 2009 American Chemical Society Published on Web 12/19/2008
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Figure 1. Cyclic voltammograms of gold thin films in ZnCl2/DMSO electrolyte at different temperatures: 1st cycle (s), 5th cycle (- -), 10th cycle (- · - · -) 15th cycle (s · s), 20th cycle (- · · -). Scan rate: 50 mV/s.
2.2. Fabrication of NPG Thin Film Electrode. All processes for the fabrication of NPG film were carried out in the ZnCl2/DMSO electrolyte in air. The electrolyte was prepared by dissolving anhydrous ZnCl2 in DMSO at room temperature with the concentration of ZnCl2 set at 1.5mol/L. The organic electrolyte was not deaerated and dewatered in the whole fabrication process. Multicyclic electrochemical alloying/dealloying on sputtered gold electrode was realized in a threeelectrode cell, which consisted of a Pt plate as the counter electrode and Zn plate as the reference electrode, respectively. All of the three electrodes were placed in the same cell without a salt-bridge connection. After electrochemical treatment, the gold electrode was taken out and cleaned quickly by DMSO, ethanol, and deionized water in sequence. 2.3. Electrochemical Measurements in the Direct Borohydride Fuel Cell (DBFC). Cyclic voltammograms (CV) of BH4- on NPG film were measured with a three-electrode system, i.e., NPG film as working electrode, Hg/HgO as reference electrode, and Pt sheet as counter electrode. Discharge behavior of BH4- was examined in prototype NaBH4-air cells, using MnO2 as a cathode catalyst. Since MnO2 showed sufficient electrocatalytic activity for oxygen reduction and indiscernible catalytic activity for the electrooxidation and hydrolysis of the BH4- ion, no ion exchange membrane like Nafion was employed in our experiment. 3. Results and Discussion 3.1. Electrochemical Alloying/Dealloying of Gold Thin Film in the ZnCl2/DMSO Electrolyte. In our experiment, an ultrathin gold film sputtered on a silicon wafer (thickness: ∼ 1.5 µm) was used to fabricate nanoporous structure to increase the utility ratio of gold atoms in electrocatalytic reactions. Moreover, the plane structure of the electrode also favors the design of fuel cells. Figure 1 shows the cyclic voltammetry (CV) curves of gold thin films in ZnCl2/DMSO electrolyte at various temperatures. When the potential moved from +1.4 to -0.4 V
(vs Zn2+/Zn), these voltammograms showed an obvious reduction wave, corresponding to the deposition of Zn on the gold surface. The transfer coefficient (R) for Zn2+ reduction was calculated as 0.12 from the plot of cathodic peak potentials versus the natural logarithm of the scan rate (Figure S1, Supporting Information). During the reverse scan, there was only a single anodic peak (∼0.4 V) at 50 °C resulting from the stripping of Zn. When temperature increased to 70 °C, the anodic wave became broader with CV cycles, and the peak potential moved toward the positive side (∼0.52 V), announcing the commencement of Au-Zn alloying. With the succession of CV scans at 90 °C, another anodic peak appeared at 0.65 V after 15 cycles, which could be attributed to the oxidation of Au-Zn alloy. Further increase in temperature, for example 110 °C, resulted in the appearance of the second anodic peak in a shorter time (10th cycle). This implies the formation of Au-Zn alloys is more effective at higher temperature. In summary, the above observation reveals that the Au-Zn alloy can be formed on the gold surface at temperatures higher than 70 °C, and the alloying reaction accelerates with the growth of temperatures. Our results are similar to those reported in the literature20 except that the critical temperature for the formation of the Au-Zn alloy is ∼30 °C lower, suggesting the advantages of our method over those developed before. The surface morphology of gold thin films after electrochemical treatments at different temperatures was characterized by FESEM (Figure 2). Figure 2a shows that the sputtered gold grains were closely packed side by side, and the in-plane grain size was ∼200 nm. After 20 cycles of CV scans in 50 °C, the grain boundaries in Figure 2b became rough while the main body of the grains remained unchanged. As the temperature increased to 70 °C, the rough zone expanded from the boundaries to the center of the grains due to the formation of Au-Zn alloy (Figure 2c). This proves that the alloying reaction on the sputtered gold surface first happened on the grain boundaries; in other words, the gold atoms located at the grain
Nanoporous Gold Thin Film
Figure 2. SEM characterization of the gold surface (plan-view) after cyclic voltammetry for 20 cycles at different temperatures (scan rate: 50 mV/s): (a) untreated gold surface; (b) 50 °C; (c) 70 °C; (d) 90 °C; (e) 110 °C; (f) 130 °C. All images were recorded with the same magnification of 50 000. The scale bar shown in (a) is 100 nm and applicable to all images.
boundaries were more active to form alloy with the deposited Zn than those at the grain center. The tendency of roughening continued at elevated temperatures until a porous structure appeared at the grain boundaries at 90 °C and eventually spread to the whole surface at 110 and 130 °C, as shown in Figure 2d, 2e, and 2f, respectively. It is obvious that the porous feature was caused by the repetitive cycles of the alloying and dealloying process, i.e., the repetitive injection and withdrawal of Zn in the Au crystal lattice. This was further confirmed by the color change of the gold thin film, which was initially golden (110 °C) gradually due to the formation of nanoporous structure.18 Another interesting phenomenon is that no grain boundaries could be seen on the gold surface after treatment above 110 °C, indicative of the leveling effects of CV cycling on the film structure. The similarity between the surface morphologies of gold films obtained at 110 and 130 °C exhibits the saturation of the alloying/dealloying reaction. Therefore, if not stated elsewhere, the highest temperature to fabricate NPG film in the following experiments was set at 110 °C. In addition to temperature, other parameters like scan rate, scan cycles, and potential range also influence dramatically the formed structure of the gold thin films (Figures S2-S4, Supporting Information). Figure S2 displays the top views of the gold surface prepared at various scan rates from 10mV/s to 100mV/s with the increment of 10mV/s under 110 °C. Comparison of these images shows that the decrease in scan rate would increase the porosity of the as-prepared Au films and the depth of these pores. Since the decrease in scan rate prolonged the reaction time between Au and deposited Zn, the amount of Au-Zn alloy formed at lower scan rate was much
J. Phys. Chem. C, Vol. 113, No. 2, 2009 605 more than that generated at higher scan rate, leading to higher porosity and deeper reacted Au layer. Similarly, increasing scan cycles (Figure S3, Supporting Information) as well as widening negative potential window (Figure S4, Supporting Information) could also reinforce the formation of Au-Zn alloy via longer reaction time and thus achieve the same effects like decreasing scan rate. 3.2. Structure Evolution in Sulfuric Acid Solution (PostTreatment). Although most of the Zn atoms (∼90%) could be stripped out of Au-Zn alloy by anodic oxidation, the residual Zn was, according to our experiment, very difficult to remove even after electrochemical dealloying at a high positive potential for a long time in the organic electrolyte. The possible reason can be ascribed to the slow surface diffusion coefficient of Au atoms in organic solvent, which restricts their local rearrangement after dissolving the outmost Zn from the alloy and thus prevents the exposure of the inner Zn. In contrast, the surface diffusion of gold in acidic solutions such as concentrated nitric acid and perchloric acid was reported to be quite fast.21 As a result, the general methods used in the literature for depletion of less noble metal from Au-based alloys are either chemical etching in nitric acid12 or electrochemical etching in perchloric acid with the potentiostatic mode.22 However, both of them have a deficiency; i.e., they cannot judge the end-point of the dealloying reaction directly, leading to either inadequate or excessive dealloying. Compared with inadequate dealloying, excessive dealloying is much worse due to the structure coarsening effect which significantly reduces the porosity of NPG film. Herein we develop a new electrochemical post-treatment method that can determine the end-point of dealloying in a straightforward way, i.e., CV scan of Au-Zn alloy in 0.5 mol/L of H2SO4 solution. It is well-known that the purity of Au can be characterized by the CV curves in H2SO4 solution. Thus, if a suitable scan range is chosen which covers the characteristic peaks of Au and the critical potential of dealloying,21 the completion of the dealloying process can be easily estimated by comparing the CV curve with that of pure gold. One may argue that elemental analysis would be a better option to judge the end-point of dealloying. However, the elemental analysis only measures the elements in the ultrathin outmost layer of gold films and cannot give the information of the inner layer. Actually, we indeed met the situation that the elemental analysis results showed no residual Zn while the current peak caused by Zn was still observed clearly in CV measurement. Figure 3 shows the CV curves of the sample prepared by electrochemical alloying/dealloying at 110 °C. In the first cycle, there were a couple of redox peaks with the anodic peak appearing at 1.38 V (vs SCE) and cathodic peak appearing at 0.84 V. Apparently, the anodic peak can be assigned to the stripping of Zn from the Au-Zn alloy as well as the oxidation of remaining Au, while the cathodic peak corresponds to the reduction of gold oxide formed at anodic scan. With the successive scans, the anodic peak at 1.38 V gradually decreased and finally disappeared after 50 cycles, implying the complete stripping of Zn from the gold film or, namely, the end of the dealloying process. The new anodic peaks at 1.05 V and 1.24 V are caused by the oxidation of NPG film, and the potential shift of the Au reduction peak toward the positive side further proves the increased purity of NPG film during the post-treatment process. The completion of dealloying does not ensure that the nanoporous structure of gold film would not change thereafter. Actually, its surface area and porosity continued to decrease with the further CV scans, as demonstrated by the declined
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Figure 3. Cycle voltammograms of gold thin film in 0.5 mol/L of H2SO4 solution. The gold film was first treated by an electrochemical alloying/dealloying process in ZnCl2/DMSO electrolyte at 110 °C for 20 cycles. Scan rate: 10 mV/s.
Figure 5. Plan-view and cross-sectional SEM images of gold films after treatment in H2SO4 solution with optimum time. The pretreatment of these gold films are the same as that in Figure 2: (a) and (c) 50 °C; (b) and (d) 70 °C; (e) and (g) 90 °C; (f) and (h) 110 °C. Scan rate: 50 mV/s.
Figure 4. SEM images of gold thin film after different CV cycles in 0.5mol/L of H2SO4 solution: (a) 20 cycles; (b) 50 cycles; (c) 75 cycles; (d) 100 cycles; (e) 150 cycles; (f) 200 cycles. The NPG films were first treated by electrochemical alloying/dealloying at 10 mV/s for 20 cycles under 110 °C.
reduction peak, indicative of structure coarsening of NPG materials in H2SO4. However, this coarsening process can be quenched immediately by removing the NPG film from sulfuric acid and washing with deionized water, making it possible for ex situ SEM characterization of structure evolution. Figure 4 shows SEM photos of the gold films upon various CV cycles of 20, 50, 75, 100, 150, and 200, respectively. The pretreatment of these gold films was conducted in ZnCl2/DMSO electrolyte at a rate of 10 mV/s for 20 cycles under 110 °C. After CV scans for 20 cycles in H2SO4 solution, the resultant gold film in Figure 4a was still similar to the precursor. Prolonged immersion in acid for 50 cycles generated a pore structure with
the size of 10-15 nm (Figure 4b). Although the dealloying process was completed at this stage based on our CV data, the ligaments and pores continued to become larger (Figure 4c) and yielded an interconnected three-dimentional (3D) network after 100 cycles (Figure 4d). The mechanism for the structure evolution in this process could be explained by Erlebacher’s work, where the Monte Carlo model was used to stimulate Au-Ag dealloying in HNO3.23 In brief, the nanoporosity in metals is due to an intrinsic dynamical pattern formation process. That is, the gold atoms dissolved from the alloy/electrolyte interface are chemically driven to aggregate into clusters and islands by a phase separation process, which continuously opens up regions of virgin alloy and allows the dissolution front to penetrate through the bulk of the alloy. However, it should be noted that continual exposure of the 3D network to acidic solution would make the structure collapse into a 2D porous structure (Figure 4e) and finally coalesce into a plane frame with low porosity (Figure 4f). Similar results were also observed when the scan rates of alloying/dealloying in ZnCl2/DMSO electrolyte increased to 50 and 100 mV/s (Figures S5-S8, Supporting Information). The gold thin films prepared at various temperatures in ZnCl2/ DMSO electrolyte (under the same conditions as those shown in Figure 2) were also post-treated in acidic solution with the results illustrated in Figure 5. In contrast to the image in Figure 2b, the morphology of gold film prepared at 50 °C did not show much variation but became coarser after post-treatment. This may be possibly caused by the local alloying of Zn with a thin layer of surface gold atoms (∼1-5 nm). As the temperature increased to 70 °C, Au-Zn alloying occurred at the grain boundaries, resulting in the formation of ligaments with the size of ∼40 nm in the same place (Figure 5b). The area covered by the tubular ligaments continued to grow, which was in ac-
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TABLE 1: Real Surface Area of NPG Films control 50 °C 70 °C 90 °C 110 °C 110 °Ca normalized charge for 0.456 0.467 1.027 2.685 3.231 gold oxide reduction (mC/cm2) roughness factor 1.027 1.052 2.313 6.047 7.277
7.307 16.457
110 °C stands for the gold film fabricated by electrochemical alloying/dealloying for 20 cycles at a scan rate of 10 mV/s. The others are fabricated at 50 mV/s under the indicated temperatures. All the NPG films are post-treated in H2SO4 solution until Zn was completely stripped out of the films. a
Figure 8. Cyclic voltammograms of BH4- (0.01 mol/L) on NPG films fabricated at 110 °C (a) and flat gold thin film (b). Scan rate: 100 mV/s.
Figure 6. XRD patterns of (a) Au-Zn alloy, (b) porous gold after dealloying, (c) sputtered gold film, and (d) zinc.
cordance with the larger amount of Au-Zn alloy formed at higher temperatures (Figure 5e and 5f). On the basis of these findings, we may conclude that the tubular ligaments can only evolve from the region once covered by Au-Zn alloy with a certain thickness. As stated above, the ligaments are formed via surface diffusion of gold atoms released from alloys when the dealloying process proceeds. Since the surface diffusion coefficient of gold atoms in acidic solution is ∼10-14 cm2/s, they are unlikely to diffuse over a long distance on an experimental time scale.12 Consequently, there are no such ligaments on the surface where enough movable gold atoms cannot be produced due to the lack of alloy. In addition, it should be pointed out that the adhesion between the gold thin film and the substrate was very strong during the post-treatment process although the volume of the film changed owing to the formation of nanoporous structure.24 In our experiment, we never found that the gold thin films were peeled off the substrate, ensuring their applications as an anode in DBFC.
3.3. Surface Area and XRD Characterization of NPG Films. The porosity of NPG films is usually evaluated by the roughness factor, i.e., the ratio between the real surface area and the geometric area. In the case of gold, the real surface area is calculated from the charge for reduction of gold oxide in H2SO4 solution and a conversion factor of 444 µC/cm2.25 Table 1 lists the roughness factor of NPG films fabricated under various temperatures. (See Supporting Information for the relevant CV curves in H2SO4, Figure S9) As can be seen, the roughness factor, or namely, the real surface area, increased with the fabrication temperatures. At a scan rate of 50 mV/s, the real surface area of NPG film fabricated at 110 °C was ∼7.3 times higher than that of the gold film at negative control. When temperature was constant, the real surface area increased with the decrease of scan rate; i.e., the highest porosity of NPG film was realized at 10 mV/s under 110 °C. To examine the crystallographic feature of the as-prepared NPG films, we conducted X-ray diffraction (XRD) analyses of the sputtered gold film, Au-Zn alloy, NPG film after complete dealloying, and pure Zn (Figure 6). The XRD pattern shows that the preferred orientation on sputtered gold film was (111) and (222). After Zn deposition on gold surface at 110 °C, a series of weak diffraction peaks appeared in the range of 39-80° (see Supporting Information for enlarged profile, Figure S10), among which the peaks at 40-42° resulted from Au-Zn alloy and the peaks at 44.27°, 64.40°, and 77.43° could be ascribed to the other crystal planes of gold: (200), (220), and (311). Since the crystal structures of Zn and Au are different (Zn: hexagonal close-packed; Au: cubic close-packed), the multicycles of the
Figure 7. Plan-view of NPG films post-treated by different conditions: (a) CV scan in 0.5 mol/L of H2SO4 for 100 cycles; (b) CV scan in 0.5 mol/L of H2SO4 for 400 cycles; (c) CV scan in 0.5 mol/L of H2SO4 for 100 cycles and then CV scan in 1 mol/L of NaOH for 300 cycles.
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Figure 9. Apparent current density (mA/cm2) of NPG thin films fabricated at different temperatures in 0.05 mol/L NaBH4 + 1 mol/L NaOH solution after discharging for 1 h. (Inset) The discharge curves of NPG film in the first 1 h.
alloying/dealloying process between Zn and Au would inevitably cause a new crystal plane of gold exposed to the surface. When Zn was stripped out via post-treatment in H2SO4 solution, the NPG film retained the XRD patterns of gold shown in the Au-Zn alloy, which is in agreement with the results observed in the Au-Ag alloy; i.e., the initial crystal face orientation of alloy is preserved during dealloying and coarsening.26 3.4. Electrocatalytic Anode in DBFC. One of the most promising applications of nanoporous materials is their usage as electrocatalysts in fuel cells, which allow not only fast mass transfer of ion/gas through the electrolyte/electrode interface but also rapid electrochemical reaction rate due to the large active surface area. Compared to the nanoparticle-modified electrode prepared by the adsorbing methods, the nanoporous metal electrode provides better electron transfer and thereby significantly increases the performance of fuel cells. However, the serious coarsening of NPG film in acidic solution limits its application as an electrocatalyst in the proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC).27,28 In contrast, we observe that the coarsening effect would be eliminated dramatically as the pH value of the solution rises. Figure 7 compares the structure of NPG films upon longterm CV cycling in acidic and alkaline solution. Evident coarsening took place after a CV scan in 0.5 mol/L of H2SO4 for 400 cycles, whereas no significant change was found in 1 mol/L of NaOH under the same conditions. This observation intrigues our interest to use NPG film as an electrocatalytic anode in the alkaline fuel cell. Among various alkaline fuel cells developed so far, the direct borohydride fuel cell (DBFC), comprised of BH4- oxidation at the anode and O2 reduction at the cathode, is now recognized as a preferable system for portable devices thanks to its high equilibrium cell voltage (1.64 V), high theoretical energy density (9.25 Wh/g NaBH4), and extremely fast anodic kinetics.29-32 Great progress has been achieved in the past few years for the development of DBFC.33-35 At present, the biggest obstacle for its commercialization is the simultaneous hydrogen generation during the electrooxidation of BH4- ions on most of the usual electrodes like Pt, Pd, Ru, and Ni, making the actual capacity much less than the expected eight-electron oxidation of the BH4-
ion. Moreover, the evolution of hydrogen gas also brings safety issues in the design and management of the fuel cell system. This problem can be addressed by using Au as an electrocatalytic anode since the reaction of the BH4- ion on the Au surface is proven to be complete 8e oxidation (no hydrogen generation as shown in eq 1).36-39 Unfortunately, the voltage of DBFC based on a Au anode was not high enough compared with that using Pt or Ni as electrocatalysts for the reason that the electrooxidation of the BH4- ion on the Au surface occurs at a more positive potential and thereby decreases the voltage output of the fuel cell. Hence, how to decrease the polarization of BH4electrooxidation on the gold surface is of critical importance for the development of DBFC. The utilization of NPG materials may possibly provide a way out.
BH4- + 8OH- f BO2- + 6H2O + 8e
(1)
Figure 8 depicts the CV curves of 0.01 mol/L of NaBH4 in 1 mol/L of NaOH solution. One can find that the electrochemical behavior of BH4- on NPG films was fairly complex, as characterized by a number of oxidation peaks. In the positive direction, the first oxidation peak on both NPG and flat gold films occurred at -0.30 V (vs Hg/HgO), followed by a second anodic peak at ∼0.51 V. On the reverse scan, interestingly, an additional oxidation peak was observed with a peak potential of ∼0.3 V. According to the literature,36,40 the first anodic peak on the positive scan could be attributed to the direct 8e oxidation of BH4-, and the second peak at ∼0.51 V was due to the oxidation of reaction intermediates on the partially oxidized Au surface. The sharp oxidation peak on the negative scan was caused by adsorbed BH3OH- that could appear as an intermediate during the oxidation of BH4- on Au. Despite the similar shape of the CV curves between NPG and flat gold films, what we care about here is the initial oxidation potential of BH4(peak onset) on the positive scan, which exhibits the electrocatalytic activity of the two anodes. It can be seen obviously from Figure 8 that the initial oxidation potential of BH4- on NPG film (-0.74 V) was ∼0.17 V more negative than that on the flat gold film (-0.57 V), and this value still stayed the same
Nanoporous Gold Thin Film even after the peak current was normalized, indicative of the promising application in actual DBFC systems. To further confirm our observation, the discharge curves of 0.05 mol/L of NaBH4 on NPG films at -0.5 V vs Hg/HgO were measured at room temperature in a prototype NaBH4-air cell, using MnO2 as the cathode catalyst.38 These data can be analyzed in three aspects: apparent current density, real current density, and mass specific activity. As shown in Figure 9, the apparent current density of BH4- discharge on flat gold film fell from 1.40 mA/cm2 in the beginning to almost zero (0.038 mA/cm2) after 1 h. In contrast, the apparent current density on NPG films fabricated at 50, 70, 90, and 110 °C after discharging for 1 h were 2.02, 4.88, 5.92, and 9.12 mA/cm2, respectively, which were 53, 128, 156, and 240 times higher than that of the flat gold film. Even if considering the real surface area of NPG films (Table 1), the real current density of NPG films at these temperatures were still much higher. For example, the real current density was 0.98 mA/cm2 at 90 °C and 1.25 mA/cm2 at 110 °C compared with 0.037 mA/cm2 of flat Au. In our experiment, the load of Au was 2.9 mg/cm2 (the density of Au is 19.32 g/cm2). Thus, the mass specific activity of flat gold and NPG films was calculated as 0.01 mA/mg (flat gold film), 0.69 mA/mg (50 °C), 1.68 mA/mg (70 °C), 2.04 mA/mg (90 °C), and 3.14 mA/mg (110 °C). We believe that the extra electrocatalytic activities of NPG can be mainly attributed to the nanoscale size of the electrocatalysts.41,42 Besides, the synergistic effect between Au and a trace amount of Zn in gold films and/or the particular orientation of gold atoms on the film surface after the alloying/dealloying process may also contribute. Further experiments are still going on to evaluate the possibility of fabricating nanoporous structure on a thinner gold film like 500 nm and the long-term performance of NPG film as an anode in DBFC. Results will be reported in a separate paper. 4. Conclusions In this work, we demonstrate a simple approach to fabricate NPG thin films via electrochemical alloying/dealloying reactions in ZnCl2/DMSO electrolyte. In comparison to traditional template-based methods, our technique is especially suitable for fabrication of nanoporous structure on ultrathin gold films without any further deposition of Au. The relevant factors affecting the formation of nanoporous structure in the alloying/ dealloying process in organic electrolyte and the subsequent treatment in sulfuric acid solution are investigated in detail. We find that the NPG films prepared by our method show a nanoporous structure similar to those obtained from the Au-Ag alloy, indicating the same mechanism (“intrinsic dynamical pattern formation”) in these two dealloying processes. In addition, preliminary experimental results of the as-prepared NPG films as an electrocatalytic anode for borohydride electrooxidation are shown here, which exhibit much higher electrocatalytic activities toward this reaction and thereby can be used as an anode in DBFC. Acknowledgment. The authors would like to thank Prof. Changming Li (Nanyang Technological University, Singapore) for assistance with field emission scanning electron microscopy. Supporting Information Available: The influence of experimental conditions like scan rate, scan cycles, and potential range on nanoporous structure of gold is described. This material is available free of charge via the Internet at http://pubs.acs.org.
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