Article pubs.acs.org/ac
Observing Electrochemical Dealloying by Single-Nanoparticle Collision Rui Hao and Bo Zhang* Department of Chemistry, University of Washington, Seattle, Washington 98195, United States S Supporting Information *
ABSTRACT: We report direct observation of electrochemical and thermal dealloying processes of individual metal alloy nanoparticles (NPs). Electrochemical dealloying of single Ag−Hg alloy NPs was achieved in a basic solution (e.g., pH 13) by oxidizing Hg under controlled potentials. Ag can also be oxidized during single-particle collision. However, it requires elevated potentials. The strong basic environment promoted the formation of metal oxides during collision leading to a unique core−shell type nanostructure which was further confirmed by transmission electron microscopy (TEM). In thermal dealloying, Hg was evaporated due to the use of a high-energy electron beam and the process was imaged in situ inside a TEM. Both electrochemical and thermal dealloying processes resulted in the transformation of an amorphous NP to a more stable Ag−Hg alloy nanocrystal. This work demonstrates that NP collision can be a useful tool to study dealloying processes of various nanomaterials at a single-particle level.
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addressed. Most of the previous dealloying methods have used NP ensembles25−27 which yield averaged results and dealloying of single NPs has rarely been observed. Among methods to characterize electrochemical properties of single metal NPs, the use of an ultramicroelectrode (UME) to record single NP collision events has quickly gained enormous attention.10 This method monitors the electrochemical response of a UME in a solution which usually contains both redox molecules and NPs. Adsorption of a single NP on the electrode can be captured and identified as a quick change (enhancement or inhibition)10 in the electrochemical signal. Several detection mechanisms have been explored in the literature which include the popular electrocatalytic amplification,28−36 direct oxidation/reduction of the NP itself,37,38 detection by current blockage39,40 or electrode area amplification,41 and open-circuit potential.42−44 Our group has used fastscan cyclic voltammetry (FSCV)45 to obtain chemical resolution and electron-transfer kinetics46 in single-particle collision. On the basis of the tremendous success of single NP collision, we anticipate that it would be a facile method to observe electrochemical dealloying processes at single NPs. Amalgam metallurgy, consisting of facile amalgamation and dealloying processes, has been used for thousands of years.47 In this work, we chose Ag−Hg NPs as a unique platform to investigate the process of electrochemical dealloying. The standard potentials of the reduction reactions, Hg 2+ + 2e− = Hg(l) and Ag+ + e− = Ag(s), are 850 and 799.6 mV vs NHE,
e report the observation of electrochemical and thermal dealloying of individual Ag−Hg alloy nanoparticles (NPs) through NP collision on a gold microelectrode. Ag−Hg alloy NPs were prepared by a galvanic replacement reaction between metallic Ag NPs and Hg(NO3)2. Electrochemical oxidation of Ag−Hg alloy NPs in a basic solution at relatively low potentials resulted in selective removal of Hg from the alloy NPs. Oxidation of Ag, on the other hand, required slightly increased potentials at this condition. Single-NP dealloying was achieved through NP collision on a gold microelectrode and the results were confirmed by NP imaging in transmission electron microscopy (TEM). TEM results also indicated the formation of amorphous oxide due to oxidation of Hg and Ag. An interesting thermal dealloying process was also revealed when Ag−Hg alloy NPs were exposed to a highly focused electron beam (e-beam) in TEM. Both electrochemical and thermal dealloying processes resulted in the transformation of an amorphous alloy NP to a stable Ag−Hg nanocrystal. Because of their unique physical and chemical properties, metal NPs have been extensively investigated in numerous areas.1−4 In the area of (electro)catalysis,5−11 for example, several approaches have been used to engineer NP composition,12 structure,13 and surfaces14,15 in order to improve their catalytic activity and stability.16 Among various methods, preparation of alloy NPs with controlled crystal structure and facets17 as well as engineering the morphology18−20 and composition21,22 by selective element removal are particularly attractive. In these studies, metal dealloying is a key process which must be avoided during the use of the alloy NPs18 and carefully controlled in the selective removal of certain elements.15,23,24 As such, detailed investigation and understanding of metal dealloying at the nanoscale should be © XXXX American Chemical Society
Received: May 27, 2016 Accepted: August 1, 2016
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Clamp connected to a 1322a Digidata digitizer running at 100 kHz data acquisition rate. Electron Microscopy. TEM and HRTEM images were obtained on an FEI Tecnai G2 F20 electron microscope.
respectively. The 50 mV potential difference drives galvanic replacement reaction between Ag NPs and Hg2+. The small potential difference, however, also makes it challenging to strip Ag from the Ag amalgam. Moreover, since the oxidation potential of Hg is slightly higher, it cannot be selectively removed prior to Ag at standard conditions. However, in an alkaline solution, the reduction potentials for the reactions HgO + H2O + 2e− = Hg(l) + 2OH− and Ag2O + H2O + e− = 2Ag(s) + 2OH− are 97.7 and 342 mV vs NHE, respectively. Therefore, at high pH, the larger potential difference, ∼250 mV, would allow one to electrochemically dealloy an otherwise less active metal (Hg) from the alloy. Herein, we demonstrate the observation of electrochemical dealloying of silver amalgam at a single-NP level. Electrochemical single-NP dealloying was carried out by amperometry on a gold microelectrode. Single-particle dealloying events were detected as individual current spikes in an amperometry trace. Under basic pH, Hg can be oxidized prior to Ag by controlling electrode potential and it generates HgO forming a core−shell type structure. The formation of metal oxide could hinder or even completely prevent further oxidation of NPs. A thermal dealloying process of single silver amalgam NPs under E-beam irradiation was also observed during TEM observation of NPs. TEM imaging of the Ag−Hg alloy NPs and their electrochemical products further supports our hypothesis about electrochemical dealloying. Moreover, our results indicated that, by changing the chemical environment such as the solution pH, concentrations of specific ions or ligands, one can possibly extract an otherwise less active metal from alloy NPs. This study has demonstrated an important method for better understanding, at a single-NP level, the dealloying processes of metal alloy NPs. We anticipate that it will be useful for studying many other systems including AgAu, PtCu, or PtFe.
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RESULTS AND DISCUSSION Formation of Ag−Hg Alloy Nanoparticles. The standard potentials for the reactions: Hg2+ + 2e− = Hg(l) and Ag+ + e− = Ag(s) in a neutral or acidic solution are 850 mV and 799.6 mV vs the normal hydrogen electrode (NHE), respectively. The small potential difference, ∼50 mV, would allow galvanic replacement reaction to occur between Ag NPs and Hg2+ ions at standard states to form Ag−Hg alloy NPs. This process of direct galvanic replacement has been previously used by Whitby and co-workers to form Ag−Hg alloy NPs, and their results showed that it may take 3 min for an 11 nm Ag particle to reach the equilibrium.49 For the preparation of Ag−Hg NPs, we used citrate-capped, 80 nm Ag NPs (nanoComposix) mixed with mercury(II) nitrite (Hg(NO2)2) solution. The concentration of Ag NPs was 4 × 108 particles/mL corresponding to a total concentration of Ag0 of 10 μM in the reaction solution. Two concentrations of mercury(II), 50 or 100 μM, were used in the preparation. This galvanic replacement reaction should proceed to equilibrium when sufficient Hg2+ ions are consumed and Ag+ concentration is increased in the bulk solution. In this work, however, the reaction was only allowed to proceed for 1 min before the solution pH was quickly adjusted to 13 from ∼7 by adding NaOH solution to quench the reaction. The galvanic replacement reaction was terminated by rapid formation and decomposition of Hg(OH)2 to an insoluble product HgO in a basic solution, according to the following decomposition reaction:
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Hg(OH)2 = HgO + H 2O
EXPERIMENTAL SECTION Chemicals and Materials. Sodium hydroxide (NaOH, 99.9%, Fisher Scientific), mercury nitrate (Hg(NO3)2, 99%, Fluka), perchloric acid (HClO4, 70%, Sigma-Aldrich), nitric acid (HNO3, 70%, Macron), and 80 nm silver NPs (nanoComposix, Inc.) were all used as received from the manufacturers. A diluted mercury nitrate solution (1 mM) was made by adding a small amount of perchlorate acid (10 μM) to avoid hydrolysis. All aqueous solutions were made using deionized water (>18 MΩ cm) obtained from a Barnstead Nanopure water purification system. Preparation of the Gold Microelectrode. A 25 μm gold microelectrode was prepared according to a previously published procedure.48 A small piece of gold wire (diameter = 25 μm, Alfa Aesar) was placed in a 10 cm-long piece of glass capillary tubing (o.d. = 1.5 mm, i.d. = 1 mm, Sutter) and sealed on a hydrogen flame. A gold microelectrode was obtained after polishing the electrode to a mirror finish. Electrochemical Measurements. Steady-state cyclic voltammograms (CVs) were recorded using a computer controlled Chem-Clamp voltammeter/amperometer (Dagan) and an in-house virtual instrumentation program written in LabView 8.5 (National Instruments). A desktop Dell PC equipped with a PCI-6251 (National Instruments) data acquisition card was used for date acquisition. Homemade Ag/Ag2O electrodes, which were fabricated by dipping a silver wire (0.1 mm diameter, Alfa Aesar) in nitric acid, were used as reference electrodes for all electrochemical measurement. Amperometric traces were collected using a Dagan Chem-
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When the pH of the solution was raised to 13, the remaining mercury ions in the bulk would quickly precipitated in the form of insoluble HgO (Ksp 3.6 × 10−26).50 This would result in a quick decrease in concentration of Hg2+ and termination of the galvanic replacement reaction. The formation of Ag−Hg alloy NPs could also be facilitated by the slow diffusion of Hg in solid Ag when the outer surface of the Ag NP is completely covered by Hg. A rough estimation reveals that it may take 20 min for Hg atoms to diffuse a distance of 40 nm (the radius of the Ag NPs) in solid silver, assuming that diffusion coefficient of Hg in Ag is 10−10 cm2 s−1.49,51 Hence, considering the large size of Ag NPs and the short reaction time, only part of the silver NP may be oxidized and replaced by Hg. Therefore, Ag−Hg alloy NPs instead of mercury drops were obtained. Although, there is small chance that individual mercury drops may be generated during this process, a rapid phase separation of high density liquid mercury and water would be expected. Thus, no specific approaches were taken to separate nanodrops from the alloy NPs. We found that the morphology and composition of the product Ag−Hg alloy NPs depended on the Hg2+ ion concentration in the initial solution as illustrated in Figure 1a. With lower mercury concentration (50 μM), the product Ag− Hg alloy NPs show well-crystallized polyhedron structures while higher mercury concentration (100 μM) results in amorphous spherical shape NPs. Figure 1d shows the product (AH5 NPs, denoting a mole ratio of 5 between Hg and Ag) of the reaction between 10 μM Ag and 50 μM Hg2+ for 1 min. B
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lattice spacing for (100) planes in the HRTEM image (Figure 1c). In order to further verify the presence of Ag and Hg in the spherical amorphous NPs, we investigated their dealloying processes by in situ TEM. The dealloying is believed to be driven by an electron-beam induced thermal effect at nanoscale in HRTEM. This technique has been widely used to anneal nanostructures based on the thermal effect and is powerful for in situ investigation of temperature dependent behaviors of nanomaterials including decomposition52 or crystallization.53 In this work, the thermal effect of focused e-beam was used to heat and evaporate Hg from the Ag−Hg alloy NPs. The boiling point of mercury (∼630 K) is very low under the standard atmospheric pressure and it is anticipated to be significantly lower in a high vacuum environment such as inside a TEM specimen chamber (1 × 10−7 Torr, Tecnai F20 TEM). Radhakrishnan and co-workers reported rapid evaporation of Hg when nanodrops of Hg were placed under TEM observation with low magnification (×10 K) at 23 °C.54 Therefore, it could be possible that Ag−Hg amalgams are thermally dealloyed during e-beam illumination. In this study, we found no obvious morphology changes to the AH5 or AH10 NPs at relatively low magnifications (e.g., ×100 K) indicating neither of these NPs were simple Hg nanodrops and the beam energy was insufficient to remove Hg. The alloy NPs were further examined under highly focused ebeam irradiation. The AH5 nanoparticles remained unchanged for several minutes at ultrahigh magnification (×900 K) with a 200 kV accelerating voltage. In this case, the heat generated by the e-beam may still be insufficient to selectively remove Hg from well-crystallized Ag−Hg alloy. On the other hand, we found that both the size and shape of the spherical amorphous AH10 NPs transformed when imaged at the lower magnification (×700 K) at 200 kV. Figure 1f−j shows a series of images taken from an AH10 particle taken at a 1-s time interval from which one can clearly observe the quick transformation process of the AH10 NP. The NP gradually shrunk and a small polyhedron NP was seen at the end of the dealloying process. The smaller size and its clear crystallinity of the final product particle may indicate dealloying of Hg from Ag−Hg alloy NP. The whole dealloying process was fast and only lasted several seconds and no further morphology changes were observed at even higher magnifications. The final NPs had a somewhat irregular shape and appear to be Ag−Hg alloy. HRTEM results show lattice fringe spacing of 2.04 Å (Figure 1j) and at 2.24 Å (Figure S2f), which correspond to the lattice planes of (422) and (420) of cubic phase Ag2 Hg3 (JCPDS 11-0067), respectively. The transformation shown here should be considered as in situ precipitation of Ag2Hg3 alloy from a solid solution of Ag in Hg. Thermal dealloying of single silver amalgam NPs was successfully observed and these NPs were furtherly investigated by electrochemical dealloying in the following section. Electrochemical Dealloying through Single-NP Collision. In order to confirm the overturned and increased potential difference in alkaline solution, CV responses were obtained on a 25 μm gold disk microelectrode with and without modification by Ag NPs and Ag−Hg alloy NPs (AH10) in an Ar-purged NaOH solution at pH 13 using an Ag/Ag2O reference electrode. Silver NPs or AH10 NPs were drop-casted on the surface of the gold electrode by adding 10 μL of the solution of NPs on the polished surface of the electrode. Figure 2 shows a direct comparison of the CVs recorded on all three
Figure 1. (a) Schematic illustration of the preparation of amalgam NPs by galvanic replacement reaction. (b) TEM image and (c) HRTEM image of commercial 80 nm silver NPs; (d) TEM image and (e) HRTEM image of AH5 silver−mercury alloy NP; (f−j) TEM images of the dealloying process of the AH10 silver−mercury alloy NPs with 1 s interval and (k) HRTEM image of the thermally stable Ag−Hg alloy NP in part j.
The polyhedron shape of the NPs suggests that the Ag−Hg alloy NPs were still crystalline with a lower Hg content. The HRTEM images further reveal that the NPs are hexagonal phase AgHg (JCPDS 34-0624) alloy NPs with 2.35 Å lattice spacing of (300) planes (Figure 1e). Figure S1 shows an image at relatively low magnification of several AH10 NPs (product of 1 min reaction between 10 μM Ag and 100 μM Hg2+) which exhibited uniform spherical shape with diameters around 40− 60 nm. TEM results at higher magnification (Figure 1f and Figure S2a) suggest the NPs are amorphous. This is in agreement with literature result of noncrystalline alloy from higher loadings of mercury.49 As a control, TEM images (Figure 1b) confirmed the diameters of the commercial silver NPs are ∼80 nm. The NPs are well-crystallized hexagonal silver (JCPDS 41-1402) which could be confirmed by the 2.5 Å C
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Figure 2. CV responses in Ar-purged 0.1 M NaOH solution of a bare 25 μm gold electrode (black), Ag NPs-modified gold electrode (red), and AH10 Ag−Hg alloy NPs-modified gold electrode (blue). The scan rate was 100 mV/s.
electrodes (bare gold, black curve; gold modified with Ag NPs, red curve; and gold modified with AH10, blue curve). As shown in the red CV, the bare gold electrode had a small anodic wave appeared at an onset potential of ∼350 mV. In alkaline solutions, the Au3+ ions will be precipitated in form of hydrated auric oxide, Au2O3·3H2O or Au(OH)3, and the oxidation potential of gold in alkaline solution can be calculated by the equation: E (V) = 1.48 − 0.059 × pH,55 which should be 0.39 V vs Ag/Ag2O. Therefore, the small additional wave observed at 350 mV should be due to the oxidation of gold and formation of thin gold oxide layer in an alkaline solution. Compared to the bare gold, the Ag-NP-modified electrode showed an additional anodic wave at around 500 mV vs Ag/ Ag2O. This additional anodic current is believed to be due to the oxidation of 80 nm silver NPs on gold. A positive 500 mV potential difference was observed between oxidation of Ag NPs and the Ag/Ag2O reference electrode. This large position shift may be due to a combination of the formation of a gold oxide layer, a layer of Ag2O at the gold/silver contact, and the inhibition effect of the surface ligands on the NPs. Interestingly, the AH10-NP-modified gold electrode exhibited two consecutive peaks at around 250 mV and 450 mV, respectively. The first wave at 250 mV should be due to selectively oxidation of Hg from the alloy NPs and the second wave should be due to the oxidation of the remaining Ag and Hg. The CV results of the AH10 NPs suggested that Hg can be selectively oxidized from the Ag−Hg alloy prior to silver oxidation resulting in an apparent electrochemical dealloying. Thus, further amperometric detection of the dealloying process of during NP collision can be anticipated. On the basis of these results, we set off to study electrochemical dealloying of single Ag−Hg alloy NPs using the method of single particle collision on a 25 μm diameter gold microelectrode. Figure 3a is a cartoon of detection of single Ag−Hg alloy NPs on a gold electrode when a NP collides with the electrode surface. The Hg is selectively oxidized when the potential is sufficiently high to oxidize Hg but lower than the critical potential for Ag oxidation. HgO is formed at high pH forming a nonconductive layer which may inhibit further oxidation. Here, NP-collision was carried out at 300 mV vs Ag/Ag2O in a solution of 0.1 M NaOH purged with argon. Figure 3b displays three i−t traces showing collision events recorded from 80 nm Ag, AH5, and AH10 NPs. Under this potential, no collision events were detected from pure Ag NPs, while both of the
Figure 3. (a) Cartoon showing the electrochemical dealloying process at a gold microelectrode via NP collision. (b) Amperometric traces of Ag, AH5, and AH10 NPs (from bottom to top) collected at 300 mV vs Ag/Ag2O on a 25 μm gold electrode in an argon-purged 0.1 M NaOH solution. (c) Histogram of the number of detected collision events vs total peak charge for AH10 (blue) and AH5 NPs (purple). The inserts are typical current spikes recorded in the i−t traces for AH10 (blue) and AH5 (purple).
amalgam NPs exhibited numerous detection events. NP collision events in Figure 3b were identified as sharp current spikes, indicating quick oxidation of Hg and dealloying of single Ag−Hg alloy NPs. Two typical current spikes were pulled out from the AH5 and AH10 NPs traces and were shown as inserts in Figure 3c. The detection pulses are characterized by a quick increase in current followed by a somewhat slower current decay with pulse duration of about 8−10 ms at full width half max (fwhm). The shape of the current response resembles some of the typical amperometry pulses collected from oxidizing or reducing a limited pool of redox species, such as those recorded during detection of single exocytosis events in neurochemistry56 and detection of single redox-filled vesicles.57 These pulses are believed to be due to oxidation and consumption of Hg in the alloy NPs. Figure 3c shows the D
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trace in Figure 4a is accompanied by a representative current spike given as an insert in Figure 4b which exhibits similar asymmetric pulse shape and duration, ∼10 ms (fwhm). Each detection event is again characterized by a quick current increase reaching a maximum followed by a slow decay to the baseline. The detection events collected at 500 mV are significantly higher than that at lower potentials indicating faster oxidation kinetics and more materials were oxidized at this potential. Figure 4b gives a comparison of detection events in terms of total charge integrated from the current pulse at three potentials. We found that the average charges for the events collected at 300 and 400 mV traces are 34 ± 40 fC and 33 ± 20 fC, which are quite similar. However, the average charge detected at 500 mV is 84 ± 45 fC significantly higher than the two lower potentials, which again indicates more materials were oxidized at this higher potential. It is worthy to notice the detected event frequency at 400 mV was more than that at 300 mV according to the traces in Figure 5a and the
histograms of integrated peak charges in the traces of AH5 and AH10 NPs. The average charges of the peaks from recordings of AH5 and AH10 NPs (from 10 traces each) are 37 ± 19 fC and 34 ± 41 fC. The charges of the peaks in the traces are similar but are far less than the charges estimated assuming an entire 80 nm Ag NP was oxidized (∼2.5 pC, see the Supporting Information). Considering that the solution was basic and both of silver oxide and mercury oxide are highly insoluble (Ksp = 4 × 10−11 and 3.6 × 10−26, respectively),50,58 formation of an insulating metal oxide layer, which could slow down or even completely stop the electrochemical reaction, should be responsible for the asymmetrical spikes and low average charges. The facts that similar average charges were seen for AH5 and AH10 and less than 2% of the available charges were detected indicate that the majority of the alloy NP was protected from further oxidation during collision. This is likely due to the formation of the highly insoluble HgO. Figure 4a shows a comparison of detection events of AH10 NPs collected on a same gold microelectrode at three different potentials: 300, 400, and 500 mV vs Ag/Ag2O. Each detection
Figure 5. (a) TEM image and (b) HRTEM image of an AH10 NP after undergoing electrochemical dealloying at 300 mV, (c) TEM image, and (d) HRTEM image of an AH10 NP after electrochemical dealloying at 500 mV. Electrochemical dealloying was performed on a gold electrode for 1 min in an argon-purged solution of 0.1 M NaOH.
histogram in Figure 5b. Considering the NPs are negatively charged, a higher collision frequency would be reasonable with increased detection potential due to a possible migration effect. Figure S3 shows detection of pure silver NPs (80 nm) on a gold microelectrode at this condition which confirmed that detection of silver NPs by collision-induced oxidation can only be observed at 500 mV. Hence, the extra charges detected from AH10 NPs at 500 mV should be due to oxidation of silver. One should note that even for pure Ag NPs, the average charge in the detection events is still less than 50% the predicted total charge based on the size of the NPs. This also indicates that the oxidation of Ag (and Hg) is more complicated and is likely hindered by the formation of Ag2O (and HgO). The amperometric detection results of the collision events at different potentials are in good agreement with the previous CV results in Figure 2. The collision events observed at 300 and 400 mV could be attributed to single-NP dealloying process of Ag−Hg alloy NPs.
Figure 4. (a) Amperometric traces of AH10 alloy NPs collected on a 25 μm gold electrode at three potentials showing detection of single collisional events in an argon-purged solution of 0.1 M NaOH with Ag/Ag2O as a reference electrode. The inserts are typical current spikes recorded in the i−t traces. (b) Histogram showing the number of detected collisional events vs total charge integrated in each current spike for AH10 NPs at three potentials. E
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was observed during TEM observation of NPs. TEM results of the Ag−Hg alloy NPs and their products in electrochemical oxidation support our hypothesis about electrochemical dealloying. Moreover, our results indicate that, by changing the chemical environment such as the solution pH, concentrations of specific ions or ligands, one can possibly dealloy a less active metal from an alloy NP. This study has demonstrated a useful method for better understanding dealloying processes of alloy NPs. This method may find extensive use in future studies toward understanding many other metal alloys including AgAu, PtCu, or PtFe at a single-NP level.
We have performed additional NP analysis by TEM in order to further confirm the electrochemical dealloying processes. Pure silver and AH10 NPs were both collected from a 3 mm diameter bulk gold electrode after the electrode was held at a constant potential (300 mV and 500 mV) for 1 min in the respective NP solutions. The silver NPs collected at 300 mV showed no obvious change in morphology or composition as evidenced in Figure S4a,b. However, after electrochemical oxidation at 500 mV for 1 min, silver NPs with void structure and low contrast shell could be readily seen confirming the oxidation of silver and formation of a layer of silver oxide (Figure S4c,d). Figure 5a displays a TEM image of an AH10 NP after 1 min oxidation at 300 mV. The NP is somewhat more irregular compared to the spherical shape AH10 NPs prior to oxidation observed in Figure 1f. A low contrast shell type structure surrounding the irregular shaped core was developed during the electrochemical oxidation and can be clearly seen in Figure 5a. The HRTEM image in Figure 5b indicates that the irregular core is hexagonal phase AgHg with 2.35 Å lattice spacing of (300) planes (JCPDS 34-0624) and the low contrast shell could be mercury oxide. Interestingly, Figure 5c shows a TEM image of an AH10 NP after 1 min oxidation at 500 mV which exhibits similar core−shell structure. The remaining core exhibited a phase separation. Right half of the NP core, which was marked as a pink area and surrounded by low contrast oxide layer can be identified as hexagonal silver (JCPDS 41-1402) by the 2.04 and 2.42 Å lattice spacing corresponding to (103) and (101) planes while the other half (blue area) should be hexagonal AgHg (JCPDS 34-0624) with 2.98 Å lattice space for (112) planes (Figure 5c,d). The HRTEM results suggested that the half of the NPs was completely dealloyed and that there was still a small fraction of silver left unoxidized which could be due to the hindering effects of the insulating oxide. It is important to note that there are obvious differences between thermal and electrochemical dealloying processes since the way of removing mercury atoms are drastically different. In thermal dealloying, mercury leaves the alloy NP by thermal evaporation in the form of Hg0 and the driving force is thermal energy deposited from the focused electron beam. In electrochemical dealloying, however, Hg atoms are first oxidized upon NP contacting with the electrode and then redeposited around the NP in the form of HgO which could prevent further oxidization of the remaining NP. The identification of Ag0 is strong evidence for this hypothesis. Nevertheless, in the case of AH10 NPs, both of the dealloying processes involved morphology transformation from a larger spherical amorphous structure to smaller irregular crystalline ones.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b02072. Additional TEM images of Ag and Ag−Hg alloy NPs showing the thermal and electrochemical dealloying processes, more amperometry traces showing detection of silver NPs at pH 13, and calculation of the total charge obtainable from oxidizing an 80 nm Ag particle (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: 206 543 1767. Fax: 206 685 8665. E-mail: zhang@ chem.washington.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the AFOSR MURI (Grant FA9550-14-1-0003). Part of this work was conducted at the University of Washington NanoTech User Facility, a member of the National Science Foundation, National Nanotechnology Infrastructure Network (NNIN).
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CONCLUSIONS In summary, the electrochemical dealloying process was studied through single NP collision at a gold microelectrode. As a model system for this study, silver−mercury alloy NPs were easily prepared via a galvanic replacement reaction between 80 nm silver NPs and Hg2+. Under basic pH conditions, Hg can be selected oxidized from the alloy NPs at lower applied potentials and both Hg and Ag can be oxidized when the potential is further increased. Oxidation of Hg and Ag generates metal oxidize forming a shell type structure which could hinder or completely stop further oxidation of NPs. Thermal dealloying process of single silver amalgam NPs under E-beam irradiation F
DOI: 10.1021/acs.analchem.6b02072 Anal. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.analchem.6b02072 Anal. Chem. XXXX, XXX, XXX−XXX
