Formation of Au Nanoparticles at the Counter Electrode During the

Subscriber access provided by EAST TENNESSEE STATE UNIV

Article

Formation of Au-Nanoparticles at the Counter-Electrode During the Oscillatory Oxidation of Methionine on a Gold Electrode Jeffrey G. Bell, and Jichang Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04666 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 21, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Formation of Au-Nanoparticles at the Counter-Electrode During the Oscillatory Oxidation of Methionine on a Gold Electrode Jeffrey G. Bell and Jichang Wang* Department of Chemistry and Biochemistry, University of Windsor, Windsor, ON, N9B 3P4 Abstract Oscillatory behavior was uncovered during the electrochemical oxidation of methionine in a potassium chloride solution on a gold electrode. Linear sweep voltammograms illustrate that it is Au rather than Au oxide which facilitates the oxidation of methionine. The passivation of Au electrode surface due to its oxidation causes the development of a negative differential resistance in the system, which consequently leads to the emergence of oscillations in current. Under certain conditions more complicated reaction behavior in the form of bursting oscillations could be seen. Gold dissolution was found to occur during the oscillatory process, which led to coating of the platinum counter electrode by gold nanoparticles, as confirmed with elemental analysis studies and scanning electron microscopy measurements. The as-prepared gold nanoparticle modified electrode is able to differentiate dihydroxybenzene isomers in their electrochemical analysis, a great improvement over flat Au electrode.

* Email: [email protected] ; Fax: 1-519-9737089

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

1. Introduction Sulfur-containing compounds have very important functions in a variety of areas ranging from industrial applications to biochemical processes.1-3 Methionine is one of two sulfur containing proteinogenic α-amino acids (cysteine being the other) and is crucial to many biological functions such as its important role as an initiator in protein synthesis.4 In biological systems its oxidation plays an important role in protein stability and can lead to biological damage depending on the nature of the oxidant involved.5 Apart from biology, methionine has been found to form self-assembled monolayers (SAMs) on gold surfaces,6 suggesting that a study of its oxidation on a gold electrode could provide useful information to material chemists researching applications of methionine – gold SAMs. In the area of nonlinear electrochemical dynamics sulfur compounds have also attracted a great deal of attention due to that the access of multiple oxidation states can potentially lead to rich dynamical behavior. Both simple and complex oscillations have been reported in sulfur based electrochemical systems, such as thiosulfate, thiourea, hydrogen sulfide, and recently hydroxymethanesulfinate.7-10 Strong interaction between sulfur compounds and Au may lead to the corrosion of Au electrodes. In corrosion driven nonlinear electrochemical systems, upon increasing the applied potential, the system undergoes three distinct regions, which become visible on polarization curves. Firstly, the current increases as the metal is oxidized in the active region; the abrupt decrease in current at the Flade potential caused by the onset of the passive region; finally, at sufficiently high applied potentials the passive layer is destroyed due to the formation of oxygen bubbles caused by the oxidation of hydroxide ions.11 The passivation of the electrode causes the presence of negative differential resistance (NDR), which is a

2


Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


prerequisite for seeing electrochemical oscillations.12,13 Electrochemical impedance spectroscopy has been utilized to determine the presence of NDR and Nyquist diagrams have been useful in locating regions of NDR in various systems such as methanol, formaldehyde, as well as many sulfur compounds in various electrolytes.14-21 Depending on the location of their NDR, electrochemical oscillators are classified differently, whether it is hidden on a positive slope (HN-NDR) or whether it is on a negative branch (N-NDR). This study reports on the discovery of electrochemical oscillations during the oxidation of methionine on a gold electrode. Dissolution of the gold electrode was found to play a significant role in the nonlinear behavior and, surprisingly, led to the coating of the counter electrode with gold nanoparticles. It thus provides a new example of application of nonlinear dynamics to materials science, which has been a topic of great interest for chemists who wish to use the inherent instabilities in these systems.22-27 One existing example of using nonlinear behavior in materials science is the polymerization of acetylnitrile in a solution containing acidic bromate, malonic acid, and cerium (BelousovZhabotinsky Reaction).28,29 The polymerization was found to only occur in the oscillatory regime and that bromine dioxide was responsible for the periodic termination and polymerization process. Electrochemically, Switzer et al. found that multilayers of Cu/Cu2O could form during electrodeposition experiments, which were accompanied by spontaneous oscillations of the electrode potential.30 Interestingly, they found that the composition of the formed layer was dependent on the oscillating electrode potential.

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

As a test case the as-prepared gold nanoparticle modified electrode was used for the oxidation of hydroquinone and pyrocatechol in solution, showing excellent catalytic behavior and yielding a good peak-to-peak separation of the two isomers. 2. Experimental Electrochemical experiments were performed on a Voltalab PGZ100 system (Radiometer Analytical, USA) and CHI 760E (CHI Instruments, Texas, US). Polycrystalline gold and platinum electrodes with diameters of 2.0 mm (CHI Instruments) were applied as working electrodes. The counter electrode was a platinum wire (Shanghai Ruosull Technology Co., LTD) and a saturated calomel electrode (SCE) was applied as the reference electrode. Before each experiment, the working electrode was polished with consecutively finer grades of alumina powder, rinsed with double distilled water, cleaned by an ultrasonic cleaner (Branson 1510, USA) for 10 minutes, and again rinsed with double distilled water. The three electrodes were placed in the traditional triangle configuration and all electrochemical experiments were performed at room temperature (22 ± 2 oC). Reagents l-methionine (C5H11NO2S, 98+%), potassium chloride (KCl, 99+%), hydroquinone (C6H6O2, 99%) and pyrocatechol (C6H6O2, 99%) were purchased from Sigma Aldrich. All reactions contained 0.05 M l-methionine in 0.1 M KCl, unless otherwise stated. Reaction volumes were held constant at 60.0 mL, and all experiments were performed under ambient conditions (no purging with inert gases). Electrochemical impedance spectroscopy experiments were measured in the frequency range of 100 kHz to 10 mHz (unless otherwise stated) with an amplitude of 5 mV and 12 points per frequency decade were collected. Scanning electron microscopy (SEM) and energy-

4


Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dispersive X-ray spectroscopy (EDS) were performed with a Quanta 200 FEG microscope (FEI, Inc.). Mass Spectroscopy measurements were taken on a Micromass LCT - electrospray ionization time-of-flight mass spectrometer. The parameters used in differential pulse voltammetry (DPV) experiments were 10 mV increments, 50 mV pulse amplitude, 200 ms pulse width and 500 ms pulse period. 3. Results and Discussion 3.1 Nonlinear behavior Figure 1 shows linear sweep voltammetry (LSV) of a Au electrode in a 0.1 M KCl solution without (a) and with (b) the presence of methionine. Voltammogram (a) indicates that the oxidation of Au surface begins when the applied potential reaches about 0.95 V (vs SCE) and such an activity increases continuously until the applied potential reaches 1.4 V, where a sudden drop in the current density is seen due to the passivation of the surface. As can be seen in (b), the presence of methionine drastically changes the shape of the LSV, where the oxidation activity becomes visible at the applied potential as low as 0.65 V. This result suggests that it is Au atom that facilitates the electro-oxidation of methionine. Indeed, as the applied potential reaches a level where Au oxidation starts to occur, the oxidation activity of methionine begins to level off and then decreases slightly as the applied potential moves higher, resulting in the development of a negative differential resistance in the studied system. The flat methionine “oxidation peak” overlaps with the region where gold is being actively oxidized, which suggests that the oxidation of gold is playing a role in the oxidation kinetics of methionine, leading to the development of a NDR. Notably, when the passivation of Au surface occurs at the potential of 1.25 V, the oxidation current in (b) does not drop as much as methionine-free

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

system seen in (a). This is likely because of the strong interactions between Au ions and sulfur compounds, which regenerates active Au surface by dissolving Au oxides. In Figure 2 LSV of Au electrode in methionine and KCl solution was performed at various scan rates. At 100 mV/s there is an oxidation peak centered at approximately 1.1 V. The oxidation peak becomes broader as the scan rate is decreased to 25 mV/s (ii). At a scan rate of 1.0 mV/s (iii) oscillations in current become observable on the oxidation peak and disappear during the abrupt decrease in the anodic current. Scanning at such a slow scan rate as 1.0 mV/s mimics potentiostatic conditions, and utilizes the applied potential as a control parameter taking the system through various dynamical regimes. The above result indicates that oscillatory behavior can take place in this system even in the absence of external resistor. To further confirm that the system indeed possesses NDR, electrochemical impedance measurements were conducted at various potentials along the positive and negative branches seen in Figure 2(iii). Along the positive slope, from 0.6 V to 0.9 V there is no evidence of any hidden NDR (HN-NDR) properties, where a semi-circle and a line were found at high and low frequencies respectively. This indicates that both electron transfer processes as well as mass transportation (diffusion) of methionine from the bulk solution to the electrode surface are important. In the regions where oscillations were observed, approximately 1.0 V - 1.2 V the EIS spectra were expectedly unstable, however, once EIS measurements were taken at potentials on the sharp negative branch the EIS became stable. Figure 3 presents an EIS spectrum conducted at 1.25 V, which shows the impedance crossing the imaginary axis and intersecting with the negative

6


Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


axis before intersecting again at -2000 Ω. These results clearly indicate that the system possesses NDR and this new electrochemical oscillator falls into the N-NDR class. In N-NDR type oscillators the double layer potential ØDL acts as an essential variable driving the oscillatory behavior. As proposed above, during the electrochemical oxidation of methionine in potassium chloride solution, Au oxides may strongly interact with sulfur compounds, dissolving into the bulk solution. This is confirmed by visible pitting on the Au surface in our study. Gold dissolution has been found earlier to occur during the electrochemical oxidation of sulfur containing species, but has also been observed in solutions containing halogens ions.31-33 To gain insights into the possible oxidation products high-resolution mass spectroscopy analysis was conducted. These results indicated that the electrochemical oxidation of methionine (C5H11NO2S) results in the formation of the compounds methionine sulfoxide, (m/e 165, C5H11NO3S), as well as methionine sulfone (m/e 181, C5H11NO4S). The formation of methionine sulfoxide and methionine sulfone corresponds to consecutive two-electron oxidation processes, as shown in Scheme 1.

Scheme 1 - Oxidation Pathway for the Electrochemical Oxidation of Methionine.

Figure 4 shows i-t curves demonstrating the effect that changing the applied potential has on the methionine system and interestingly no external resistance is required to maintain the oscillatory behavior. In Figure 4a, at an applied constant potential of 0.8

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

V, there are very small amplitude damped oscillations before the system becomes stable for the duration of the experiment. Once the applied potential is increased to 1.0 V simple oscillations can be seen to emerge. Increasing the potential to 1.1 V and 1.2 V (Figure 4c and 4d) simple oscillations occur and once the applied potential is 1.3 V (Figure 4e) oscillations no longer exist. The frequency of the oscillations does not change between 1.0 V and 1.2 V and is approximately 90 mHz; however, the amplitude of the oscillations can be seen to increase as the potential is increased from 1.0 V to 1.2 V. It should be noted that theoretically systems which fall into the classification of an N-NDR type electrochemical oscillator require the incorporation of external resistance, of a magnitude matching the resistance where the first intersection with the negative axis in an EIS spectrum occurs, in order to sustain oscillatory behavior. The occurrence of oscillations in this system without external resistance is likely due to the fact that the concentration of supporting electrolyte is sufficiently low, such that the solution resistance overcomes the necessity for the inclusion of external resistance. To test this hypothesis, a solution of methionine containing 1.0 M KCl (ten times original supporting electrolyte concentration) was studied with EIS, which similarly to 0.1 M KCl solutions showed the presence of NDR. The solution resistance of this solution was found to be 22 Ω, and the impedance was found to cross the negative axis at approximately -330 Ω. An LSV of the methionine oxidation in 1.0 M KCl is shown in Figure 5a with and without the presence of external resistance, dashed line and solid line respectively. As can be seen, no oscillations in current emerge at this concentration of supporting electrolyte when no external resistance is incorporated. However, once an appropriate resistor was utilized to compensate for the shift in solution resistance, the oscillatory regime emerged.

8


Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5b shows an i-t curve of the methionine oxidation in 1.0 M KCl with and without the presence of an external resistor of magnitude 330 Ω. As can be seen, oscillations in current are absent without the external resistor in series (dotted line), however, once the resistor is in series, stable oscillations emerge, giving more evidence that this new methionine electrochemical oscillator falls into the N-NDR class. Figure 6 shows the effect that increasing the applied potential to 1.28 V has on the electrochemical oxidation of methionine in the higher KCl concentration solution (1.0 M). With the incorporation of the 330 Ω resistor complex behavior in the form of bursting oscillations were uncovered. Bursting type oscillations, commonly found during the firing of neurons occur through the presence of multiple time scale processes occurring simultaneously, and their discovery in this system illustrates that the electrochemical oxidation of methionine can support very rich dynamical behavior 3.2 Formation of Au nanoparticles at the counter electrode As stated above, gold electrode dissolution occurs during the electrochemical oxidation of methionine, which causes significant pitting on the electrode surface. The dissolution is likely driven by the formation of Au ion-sulfur compound complexes, which are soluble in solution. When operated under potentiostatic conditions at which the system exhibited nonlinear behavior, the Pt counter electrode changed color to yellowish. Scanning electron microscopy (SEM) of the Pt counter wire after potentiostatic oxidation of methionine for 6 h showed that nanoparticles with nearly uniform size and shape were formed (Figure 7a). In Figure 7b energy-dispersive X-ray spectroscopy (EDS) measurements confirmed that the as-deposited nanoparticles consisted of Au. This observation indicates that the Au-sulfur complexes have been driven by the electrical

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

field to migrate toward the counter electrode and got reduced there. So far, our preliminary exploration has not demonstrated any connection between the mode of oscillations and the morphology of Au nanoparticles. As a case study, this Au-nanoparticle modified electrode, fabricated through the nonlinear dynamical reaction, was applied towards the detection of hydroquinone (HQ) and pyrocatechol (PC) isomers. Hydroquinone and pyrocatechol are two toxicological compounds that often coexist in environmental samples, making the ability to detect both isomers in solution an important research. Figure 8 shows the differential pulse voltammogram (DPV) in a solution consisting of 2.0 mM HQ and 2.0 mM PC in 0.1 M KCl. As can be seen, neither the bare Au (dashed line) nor the bare Pt (dotted line) electrodes can detect both HQ and PC simultaneously, showing the presence of an unresolved single peak. However, when the Au-nanoparticle coated electrode was used (solid line), the two isomers become individually detectable (HQ at approximately 0.05 V and PC at 0.11 V). The ability to differentiate the two isomers is due to their different degrees of interaction with the rough electrode surface generated by Au-nanoparticles. Also, in comparison to the bare electrodes, the modified electrode shows excellent catalytic properties as the oxidation of HQ and PC occurs at lower potentials. 4. Conclusions This study illustrates that electrochemical oxidation of methionine on a gold electrode is mediated by Au atoms rather than Au oxides. The occurrence of Au oxidation at high potentials causes the development of a negative differential resistance in the oxidation of methionine. The presence of NDR consequently allows spontaneous oscillations in current to emerge in the studied system. Oscillations in current have so far

10


Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


been found to occur with or without the presence of external resistance, depending on the overall solution resistance. Notably, more complicated oscillatory behavior such as bursting phenomenon was seen as the external resistance was used. Results presented in Figures 4 and 6 also illustrate that as the concentration of supporting electrolyte KCl is increased from 0.1 to 1.0 M, there is a drastic change in the observed nonlinear behaviour. Such a change could be caused by the poisoning of the electrode surface34 or enhanced dissolution of Au electrode.31-33,35 To shed light on the cause, K2SO4 was also used as the supporting electrolyte, in which no spontaneous oscillations were seen under otherwise identical reaction conditions. It suggests that chloride-enhanced Au dissolution may have played an important role in the development of oscillations.31,32 EIS characterization suggests that this new electrochemical oscillator fits the classification of an N-NDR type. The formation of complexes between Au ions and methionine and methionine oxidation products leads to the pitting of Au electrode. Interestingly, the Au complexes migrate toward the counter electrode and get reduced there, forming nearly uniform size Au nanoparticles. Since the concentration of Au ions in the electrolyte solution is affected and related to the oscillatory activity at the working electrode, it represents a unique way of fabricating Au nanoparticles or Au-modified electrochemical sensors. Our preliminary investigation demonstrates that the as-prepared Au nanoparticles coated electrode is capable of differentiating hydroquinone isomers, outperforming smooth surfaced Au or Pt electrodes. Since the mode of oscillations is associated with the production rate of Au atoms, which determines the nucleation and growth of Au particles, nanoparticles with different morphology may potentially be obtained at the counter electrode. The result that

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

the oxidation process is facilitated by Au rather than Au oxides provides useful guidance in developing and understanding electro-oxidation of other sulfur compounds. In addition, it also presents an effective way of designing new electrochemical oscillators, in particular those involving surface corrosion.

Acknowledgements The researchers would like to thank support by Natural Sciences and Engineering Research Council of Canada (NSERC).

12


Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References 1.

Gutscher, M.; Pauleau, A. L.; Marty, L.; Brach, T.; Wabnitz, G. H.; Samstag, Y.; Meyer, A. J.; Dick, T. P. Real-Time Imaging of the Intracellular Glutathione Redox Potential. Nat. Methods 2008, 5, 553-559.

2.

Hall, A. H.; Dart, R.; Bogdan, G. Sodium Thiosulfate or Hydroxocobalamin for the Empiric Treatment of Cyanide Poisoning? Ann. Emerg. Med. 2007, 49, 806813.

3.

Aylmore, M. G.; Muir, D. M. Thiosulfate Leaching of Gold – A Review. Electrochim. Acta 2001, 14, 135-174.

4.

Kozak, M. Comparison of Initiation of Protein Synthesis in Procaryotes, Eucaryotes, and Organelles. Microbiol. Rev. 1993, 47, 1-455.

5.

Drazic, A.; Winter, J. The Physiological Role of Reversible Methionine Oxidation. Biochimica et Biophysica Acta. 2014, 1844, 1367-1382.

6.

Humblot, V.; Tielens, F.; Lugue, N. B.; Hampartsoumian, H.; Methivier, C.; Pradier, C. Characterization of Two-Dimensional Chiral Self-Assemblies L- and D- Methionine on Au(111). Langmuir 2014, 30, 729-734.

7.

Du, Z.; Gao, Q.; Feng, J.; Lu, Y.; Wang, J. Dynamic Instabilities and Mechanism of the Electrochemical Oxidation of Thiosulfate. J. Phys. Chem. B 2006, 110, 26098-26104.

8.

Zhao, Y.; Wang, S.; Varela, H.; Gao, Q.; Hu, X.; Yang, J.; Epstein, I. R. Spatiotemporal Pattern Formation in the Oscillatory Electro-Oxidation of Sulfide. J. Phys. Chem. C 2011, 115, 12965-12971.

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

9.

Page 14 of 28

Feng, J.; Gao, Q.; Lv, X.; Epstein, I. R. Dynamic Complexity in the Electrochemical Oxidation of Thiourea. J. Phys. Chem. A 2008, 112, 6578-6585.

10.

Bell, J. G.; Wang, J. Nonlinear Instabilities During the Electrochemical Oxidation of Hydroxymethanesulfinate. Electrochim. Acta 2016, 222, 678-684.

11.

Orlik, M. Self-Organization in Electrochemical Systems I: General Principles of Self-Organization. Temporal Instabilities; Springer: Berlin, 2012.

12.

Strasser, P.; Eiswirth, M.; Koper, M. T. M. Mechanistic Classification of Electrochemical Oscillators – An Operational Experimental Strategy. J. Electroanal. Chem. 1999, 478, 50-66.

13.

Bonnefont, A.; Morschl, R.; Bauer, P.; Krischer, K. Electrochemical Impedance Spectroscopy of Patterned Steady States on Electrode Surfaces. Electrochim. Acta 2009, 55, 410-415.

14.

Lee, J.; Eickes, C.; Eiswirth, M.; Ertl, G. Electrochemical Oscillations in the Methanol Oxidation on Pt. Electrochim. Acta. 2002, 47, 2297-2301.

15.

Han, L.; Ju, H.; Xu, Y. Ethanol Electro-Oxidation: Cyclic Voltammetry, Electrochemical Impedance Spectroscopy and Galvanostatic Oscillation. Int. J. Hydrogen Energy. 2012, 37, 15156-15163.

16.

Mukouyama, Y.; Nakanishi, S.; Chiba, T.; Murakoshi, K.; Nakato, Y. Mechanisms of Two Electrochemical Oscillations of Different Types, Observed for H2O2 Reduction on a Pt Electrode in the Presence of a Small Amount of Halide Ions. J. Phys. Chem. B 2001, 105, 7246-7253.

14


Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17.

Vedharathinam, V.; Botte, G. G. Experimental Investigation of Potential Oscillations During the Electrocatalytic Oxidation of Urea on Ni Catalyst in Alkaline Medium. J. Phys. Chem. C 2014, 118, 21806-21812.

18.

Seland, F.; Tunold, R.; Harrington, D. A. Impedance Study of Formic Acid Oxidation on Platinum Electrodes. Electrochim. Acta 2008, 53, 6851-6864.

19.

Wolf, W.; Lübke, M.; Koper, M. T. M.; Krischer, K.; Eiswirth, M.; Ertl, G. Experimental and Theoretical Description of Potentiostatic Current Oscillations During H2 Oxidation. J. Electroanal. Chem. 1995, 399, 185-196.

20.

Potkonjak, N. I.; Nikolić, Z.; Anić, S. R.; Minić, D. M. Electrochemical Oscillations During Copper Electrodissolution/Passivation in Trifluoroacetic Acid Induced by Current Interrupt Method. Corros. Sci. 2014, 83, 355-358.

21.

Hromadová, M.; Pospíšil, L.; Sokolová R.; Fanelli, N. New Electrochemical Oscillator Based on the Cation-Catalyzed Reduction of Nitroaromatic Radical Anions. Electrochim. Acta. 2009, 54, 4991-4996.

22.

Jiménez-Prieto, R.; Silva, M.; Pérez-Bendito, D. Analyte Pulse Perturbation Technique: A Tool for Analytical Determinations in Far-From-Equilibrium Dynamic Systems. Anal. Chem. 1995, 67, 203-212.

23.

Zhang, Y.; Hu, G.; Song, J. Identification of Two Aliphatic Position Isomers Between α- and β- Ketoglutaric Acid by Using a Briggs-Rauscher Oscillating System. Anal. Chem. 2015, 87, 10040-10046.

24.

Hanke-Rauschenbach, R.; Mangold, M.; Sundmacher, K. Nonlinear Dynamics of Fuel Cells: A Review. Rev. Chem. Eng. 2011, 27, 23-52.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

25.

Page 16 of 28

Zhang, J.; Datta, R. Sustained Potential Oscillations in Proton Exchange Membrane Fuel Cells with PtRu as Anode Catalyst. J. Electrochem. Soc. 2002, 149, A1423-A1431.

26.

Zhang, J.; Datta, R. Higher Power Output in a PEMFC Operating Under Autonomous Oscillatory Conditions in the Presence of CO. Electrochem. SolidState Lett. 2004, 7, A37-A40.

27.

Orlik, M. Self-Organization in Nonlinear Dynamical Systems and its Relation to the Materials Science. J. Solid State Electrochem. 2009, 13, 245-261.

28.

Pojman, J. A.; Leard, D. C.; West, W. W. Periodic Polymerization of Acrylonitrile in the Cerium-Catalyzed Belousov-Zhabotinskii Reaction. J. Am. Chem. Soc. 1992, 114, 8298-8299.

29.

Washington, R. P.; West, W. W.; Misra, G. P.; Pojman, J. A. Polymerization Coupled to Oscillating Reactions: (1) A Mechanistic Investigation of Acrylonitrile Polymerization in the Belousov-Zhabotinsky Reaction in a Batch Reactor. J. Am. Chem. Soc. 1999, 121, 7373-7380.

30.

Switzer, J. A.; Hung, C. J.; Huang, L. Y.; Miller, F. S.; Zhou, Y.; Raub, E. R.; Shumsky, M. G. Bohannan, E. W. Potential Oscillations During the Electrochemical

Self-Assembly

of

Copper/Cuprous

Oxide

Layered

Nanostructures. J. Mater. Res. 1998, 13, 909-916. 31.

Zheng, J.; Huang, W.; Chen, S.; Niu, Z.; Li, Z. New Oscillatory Phenomena During Gold Electrodissolution in Sulfuric Acid Containing Br- or in Concentrated HCl. Electrochem. Commun. 2006, 8, 600-604.

16


Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


32.

Zhang, S.; Nicol, M. J. An Electrochemical Study of the Dissolution of Gold in Thiosulfate Solutions Part I: Alkaline Solutions. J. Appl. Electrochem. 2003, 13, 767-775.

33.

Chai, L.; Okido, M.; Wei, W. Effect of Na2SO3 on Electrochemical Aspects of Gold Dissolution in Alkaline Thiourea Solution. Hydrometallurgy 1999, 53, 255266.

34.

Yang, J. P.; Song, Y.; Varela, H.; Epstein, I. R. Epstein; Bi, W.; Yu, H.; Zhao, Y.; Gao, Q. The Effect of Chloride on Spatiotemporal Dynamics in the Electrooxidation of Sulfide on Platinum, Electrochim. Acta. 2013, 98, 116-122.

35.

Jia, Y.; Zhao, Y., He, Y.; Pan, C.; Ji, C.; Bi, W.; Gao, Q. Chemisorbed SulfateDriven Oscillatory Electro-Oxidation of Thiourea on Gold, J. Phys. Chem. C 2015, 119, 24837–24843

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

Figure Captions Figure 1. Linear sweep voltammograms at a scan rate of 1.0 mV/s in a solution of 0.1 M KCl with a methionine concentration of (a) 0.0 M and (b) 0.05 M. Figure 2. Linear sweep voltammograms at (i) 100 mV/s (ii) 25 mV/s and (iii) 1 mV/s in a solution consisting of 0.05 M methionine and 0.1 M KCl. Figure 3. Electrochemical impedance spectrum at an applied potential of 1.25 V in a solution consisting of 0.05 M methionine and 0.1 M KCl. Figure 4. Potentiostatic experiments performed with applied potentials of (a) 0.80 V, (b) 1.0 V, (c) 1.1 V, (d) 1.2 V, and (e) 1.3 V. Other reaction conditions were: [methionine] = 0.050 M and [KCl] = 0.1 M with no external resistance. Figure 5. (a) Linear sweep voltammogram in a solution of 0.05 M methionine and 1.0 M KCl without external resistance (dashed line) and with an external resistor of magnitude 330 Ω. (solid line), (b) Potentiostatic experiments conducted at 1.25 V without external resistance (dashed line) and with an external resistor of magnitude 330 Ω (solid line). Figure 6. Potentiostatic experiment in a solution of 0.05 M methionine and 1.0 M KCl conducted at an applied potential of 1.28 V with an external resistor of magnitude 330 Ω. Figure 7. (a) SEM image showing a Pt wire electrode coated with Au nanoparticles (inset shows SEM of bare Pt wire) and (b) EDS spectrum of the Pt wire electrode coated with Au nanoparticles. 18


Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Differential pulse voltammogram measurements of a 2.0 mM hydroquinone and 2.0 mM pyrocatechol solution on a bare Pt wire electrode (dotted line), a bare Au electrode (dashed line) and a Pt wire electrode coated with Au nanoparticles (solid line). The solution also contains 0.1 M KCl. Figure 1

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

Figure 2

20


Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

Figure 4

22


Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

Figure 6

24


Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

Figure 8

26


Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

TOC Graphic

28


Formation of Au Nanoparticles at the Counter Electrode During the

Recommend Documents