Localized-Plasmon Voltammetry to Detect pH Dependent Gold Oxidation

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Localized-Plasmon Voltammetry to Detect pH Dependent Gold Oxidation Bernhard Steinhauser, Cynthia Vidal, Ruxandra-Aida Barb, Johannes Heitz, Andrei Ionut Mardare, Achim Walter Hassel, Calin Hrelescu, and Thomas A Klar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11355 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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The Journal of Physical Chemistry

Localized-Plasmon Voltammetry to Detect pH Dependent Gold Oxidation Bernhard Steinhauser,† Cynthia Vidal,† Ruxandra-Aida Barb,† Johannes Heitz,† Andrei Ionut Mardare,‡ Achim Walter Hassel,‡ Calin Hrelescu,† and Thomas A. Klar*,† † ‡

Institute of Applied Physics, Johannes Kepler University Linz, 4040 Linz, Austria

Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University Linz, 4040 Linz, Austria

Abstract Localized-plasmon voltammetry (LPV) bears great potential for electrochemical sensing applications beyond conventional cyclic voltammetry. In order to determine the limitations of this method, it is of utmost necessity to investigate the response towards chemical instability of the plasmonic electrode. We therefore investigated electrooxidation of a gold nanowire array with LPV in acidic electrolytes with different pH values. LPV shows excellent agreement with simultaneously recorded cyclic voltammograms up to the onset of oxygen evolution. Beyond that point, LPV still appears to provide meaningful signals. Further, with LPV the pH dependent reduction potentials of electrochemically grown gold oxides were determined and show a linear characteristic over the investigated pH range according to Nernst’s equation.

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INTRODUCTION For conventional cyclic voltammetry (CV), the electric current provides detailed insights in adsorption, desorption and redox processes at a working electrode.1,2 In addition, a large variety of methods, such as quartz crystal microbalance techniques,3,4 atomic force microscopy,5 mass spectroscopy6 or optical methods like ellipsometry7 and spectroscopy of propagating surface plasmons8,9 have been combined with voltammetry to augment the number of sensing applications. Amongst all these methods, localized-plasmon voltammetry (LPV) provides an elegant way of optical readout during voltammetric measurements. It uses the spectral response of localized plasmon resonances (LPR)10 of noble metal nanostructures to an applied electrochemical potential. In general, the plasmon resonance shows a redshift and an increased full-width-at-halfmaximum (FWHM) for more positive electrochemical potentials and a blueshift and decreased FWHM for negative potentials.11–13 It is therefore convenient to use the spectral position and width of the LPR as optical readout channel for voltammetric experiments. The combination of electrochemistry and plasmonics has already been applied to optically monitor the capacitive charging of the plasmonic structure,14–16 adsorption of ionic species,12,13,17–21 deposition of shell materials,14 to investigate electrocatalytic oxidations,22,23 or to monitor redox reactions.13,23–25 LPV bears great advantages for sensing purposes compared to conventional electrochemical techniques, not only because it can be used to give insight to electrochemical processes on a single nanoparticle level,11,13,18 but also because it circumvents the demanding task of detecting charge carrier transport on single nanoparticles electronically.26,27 In the last couple of years, the group of N. Tao developed and applied new plasmonics based counterparts to electrochemical techniques28–31 and invented a method to image electrochemical reactions microscopically with the aid of propagating plasmons.25,32,33 They use the intensity of the microscopic image as sensing channel, while in contrast to that, single particle studies have taken into account the spectral shift and FWHM changes of the plasmon resonances separately.13,18 With some exceptions,13,34 previous LPV studies were not exceeding the onset of hydrogen evolution in the cathodic and oxygen evolution in the anodic potential scans. In order to probe the limitations of LPV, it is necessary to go beyond the onset of oxidation of gold at high electrochemical potentials. The investigation of gold oxides is a surprisingly demanding task which is a direct consequence of the extraordinary chemical stability of gold. While several attempts have 2/19 ACS Paragon Plus Environment

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been made35–41 to clarify oxide growth on polycrystalline gold surfaces as well as on well-defined crystal facets, the exact chemical composition is still debated and no universally accepted interpretation exists for all observed features. According to literature42,43 the basic growth mechanism is that oxygen is adsorbed on the gold surface, followed by a place exchange reaction of an oxygen and gold atom. Thereafter, further reaction occurs to compensate for the increasing electrical field strength in the oxide film due to the increasing electrochemical potential. This leads to the growth of a thin, dense oxide layer consisting of two35 or three36 monolayers of gold-oxide, denoted as α-gold oxide. These reactions set in at approximately 1 to 1.5 V vs. standard hydrogen electrode (SHE) for acidic electrolytes and form a passivating oxide-shell around the gold core. This first type of gold oxide has been studied previously in some LPV reports.13,16,30 It was reported that α-gold oxide formation induces a pronounced redshift along with a significant broadening of the plasmon resonance.13 At higher potentials, a second kind of gold-oxide is formed. This oxide is thicker and is referred to as β-oxide.36 It is an even more complex task to investigate β-oxides, as its oxidation potential is higher than the anodic water decomposition potential. This leads to a huge increase of current in conventional CV due to oxygen gas formation, masking the CV profile of βoxidation. Although literature agrees on the growth of α- and β-oxide, questions about their compositions are still open. Some studies found that α-gold oxide consists of various forms and/or mixtures of oxides and hydroxides with uni- and divalent gold39,44. Other research groups found oxides and hydroxides with trivalent gold, either mixed with divalent gold36,45 or as exclusive oxidation state.35,46–48 The β-gold oxide is thought to contain mostly trivalent gold. The detailed composition is debated as well. Findings range from gold(III) oxide Au2 O3 , gold(III) hydroxide Au(OH) to oxyhydroxide AuOOH, either as sole constituents36,45 or as mixtures.35,44 To our best knowledge, there has only been one report on LPV expanding into these potential ranges in aqueous solution.13 However, they were not focusing on the formation of β-oxide. We present LPV on gold nanowires to optically monitor the pH dependent oxidation and reduction of the gold nanowires. We found that LPV remains in excellent agreement with conventional CV up to the formation of α-gold oxide. Beyond that, the formation of oxygen gas dominates the CV current. LPV however shows signals in this potential regime that can be attributed to the formation of β-gold oxide.



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METHODS PREPARATION OF PLASMONIC GOLD NANOWIRE ELECTRODES The plasmonic gold nanowires were produced as previously reported.49 In brief, a flexible polyethylen-terephtalate (PET) foil (thickness of 50 µm) with ripple-like laser-induced periodic surface structures (LIPSS) was used as substrate. Figure 1a shows an atomic force topography image of a representative LIPSS PET foil. The gold nanowire array was produced by a 70° inclined evaporation of a 15 nm gold layer. A sketch of the evaporation process forming gold nanowires can be seen in Figure 1b. In order to ensure good electric contact to the nanowire array, an additional 50 nm thick gold layer was deposited under vertical evaporation outside a circular area (ca. 5 mm in diameter) while the initial nanowire array inside the circle was preserved by a shadow mask. Although the scanning electron micrograph reveals some discontinuities in the nanowires (Figure 1c), a sufficiently large number of nanowires can be electrically connected due to the additional gold layer. COMBINED OPTO-ELECTROCHEMICAL MEASUREMENTS For the CV measurements, a three electrode setup consisting of the nanowire array as working electrode (WE), a platinum wire as counter electrode (CE) and a commercial silver/silver chloride (Ag/AgCl, 3 M KCl, Sigma-Aldrich) reference electrode (RE), shifted 214 mV vs. standard hydrogen electrode (SHE), was used. The electrodes were connected to a homebuilt potentiostat which was remote-controlled by a PC via a data acquisition board (Meilhaus Electronics MERedLab 1408FS). Ag/AgCl reference electrodes are known for their leakage of trace amounts of chloride ions. One might suspect that this will lead to difficulties because chloride ions show specific electrooxidation mechanisms with gold leading to distinct CV profiles at potentials substantially lower than the onset of hydroxide mediated gold oxidation.50 As we have not detected any signals that indicate reactions with chloride ions in this potential range, we rule out interferences with CV and LPV measurements. In electrochemical measurements, a common source of error is a drifting reference electrode potential. This becomes especially important if the electrodes` electrolyte differs from the electrolyte in the electrochemical cell. We could not observe a potential drift of the reference electrode as shown in the Supporting Information, Fig. S6. Transmission spectra of the gold nanowire array in acetate pH buffer solutions from pH 3.0 – 4/19 ACS Paragon Plus Environment

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Figure 1. Manufacturing steps for the gold nanowire array and principle of the plasmonic and electrochemical measurements. (a) Atomic force micrograph of the surface structured PET foil. (b) Schematic of the inclined gold evaporation to form nanowires. (c) Scanning electron micrograph of the resulting nanowire arrays. (d) Scheme of the spectroscopic and electrochemical measurements. (e) Spectral redshift of the transversal plasmon mode of the gold nanowire array in a pH6 acetate buffer. pH 6.0 were recorded simultaneously to the CV measurements. The collimated output of a halogen lamp (Thorlabs QTM10/M) was focused through the electrolyte onto the nanowire array. Extinction spectra were recorded with a Czerny-Turner spectrometer (Thorlabs CCS175/M). A linear polarizer (Thorlabs GT10) was placed in front of the spectrometer to analyze the transversal LPR of the nanowire array. A schematic of the combined opto-electrochemical experimental setup is illustrated in Figure 1d. Figure 1e shows absorbance spectra of the gold nanowires in a pH 6.0 acetate buffer solution. The exemplary spectra were taken for two different electrode potentials of 0 V and +2 V vs. SHE. Positive charging leads to a redshift and broadening of the LPR. In order to track the changes in the optical response as a function of the applied potential, the extinction spectra were fitted with a Lorentzian function. The resonance energy as well as the spectral width can be retrieved from the fits for each potential and their changes can be investigated during 5/19 ACS Paragon Plus Environment


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electrochemical potential cycles. Spectro-electrochemical measurements were carried out between a minimal potential of -0.2 V and a maximum potential of +2.0 V vs. SHE starting at 0 V with an anodic potential sweep. The potential is changed in 5 mV steps every 0.5 s resulting in a scan rate of 10 mV s−1. (Faster scan rates would smear out the voltammograms and lead to an excessive capacitive charging signal, while slower scan rates render the measurements unacceptably slow. We and others17 found that 10 mV/s is an acceptable compromise.) The recording of the extinction spectra was synchronized with the potential changes, yielding one spectrum per 5mV increment while the electrochemical current was measured concurrently. The electrolytes were sodium acetate/acetic acid buffers with pH values of pH 3.0, 4.0, 5.0 and 6.0 with an acetate concentration of 100 mM. It is known that acetate ions do not contribute to gold oxidation51 which makes the buffer a safe choice to investigate electro-oxidation of gold. The buffers were prepared with ultrapure distilled water (Millipore, 18 MΩ cm), sodium acetate tri-hydrate (CH₃COONa ∙ 3 H₂O, >99%) and glacial acetic acid (CH₃COOH, >99.8%). DATA PROCESSING Electrochemical potential sweeps on polycrystalline metal electrodes induce a recrystallization of the metal film.52 In plasmonic nanostructures this leads to an irreversible blueshift and narrowing of the plasmon resonance with each potential cycle.53 In order to compare individual cycles, one has to correct for this global blueshift. Therefore, we fitted an asymptotic exponential function to the retrieved LPV quantities and subtracted it afterwards. This gives datasets that are corrected for electrochemical annealing. To make the shifts comparable among each other, the shift is set to zero at the beginning of each cycle. A more detailed description as well as original and corrected datasets can be found in the Supporting Information. For each potential, the shift in resonance energy Δ() and spectral width ΔΓ() are given with respect to the resonance energy and the spectral width at the beginning of each cycle. With respect to the particular potential, Δ() and ΔΓ() were averaged over five electrochemical cycles for each buffer solution. Additionally, the negative derivative of the change in resonance energy = −

d d

d

Δ() and the derivative of the change of spectral width Γ = d ΔΓ() were

calculated. It is observed that Δ() and therefore is more stable against experimental noise 6/19 ACS Paragon Plus Environment

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than ΔΓ() and Γ and hence, we concentrate on Δ() and in the following. ΔΓ() and Γ, as well as Δ() and can be found for all investigated buffer solutions in the Supporting Information (Figure S1-S4). RESULTS AND DISCUSSION Although the exact potentials are changing depending on the investigated pH buffer, similar electrochemical mechanisms can be resolved in all electrolyte solutions. As examples, Δ() and are shown in Figure 2a and b alongside the conventional cyclic voltammograms (Figure 2c) for pH 6.0, while Figure 2d,e,f show the respective quantities for the pH 3.0 acetate buffer. In the following, characteristic potentials are given for pH 6.0, and for pH 3.0 in brackets. All potentials are given versus SHE. From 0 V to 1.2 V (1.3 V), the plasmon resonance is being redshifted slightly. At a potential of around 0.3 V (0.4 V), a small kink is observable in Δ(). This has been reported before13,18,54 and is attributed to discharging of the WE (the gold nanowires) until the point of zero charge. From 1.2 V (1.3 V) to ca. 1.4 V (1.6 V), a very prominent decrease in Δ() is observed. In both, the cyclic voltammogram and , these changes are also very eminent. These observations can be attributed to the first step of surface oxidation of gold.13,16 When further increasing the applied potential, oxygen evolution sets in at around 1.5 V (1.7 V). This goes along with a drastic increase of the CV current (Figure 2c). The highest recorded current corresponding to oxygen evolution is about 10 times higher than the maximum of the observed gold oxidation current. Remarkably, Δ() and do not exhibit such pronounced changes in this potential regime (Figure 2a, b). Δ() further decreases with increasing DE until a potential of 2 V. Interestingly, Δ() keeps decreasing non-linearly in the cathodic direction of the potential scan until 1.8 V. From 1.8 V onwards a slight linear decrease can be seen. It has been reported that in potentiostatic and -dynamic scans, rapid growth of β-gold oxide starts between 1.6 V and 2.1 V.36,44,45 So, a plausible explanation for this observed phenomenon during oxygen evolution is the ongoing growth of β-gold oxide which starts in the anodic scan at 1.8 V and continues in the cathodic scan until 1.8 V. From CV, it can be seen that oxygen evolution stops below 1.6 V. However, this is roughly 200-300 mV less than the signal we observed optically. A second indication that the 7/19 ACS Paragon Plus Environment


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Figure 2. LPV of gold oxidation in a pH6 and pH3 acetate buffer solution. (a), (d) Change in plasmon resonance energy ∆E with respect to the applied electrochemical potential. (b), (e) Negative first derivative -dE/dV with respect to the applied electrochemical potential. (c), (f) Cyclic voltammogram recorded alongside the LPV measurements. observed LPV signal is due to gold oxidation is that the ongoing redshift cannot be described consistently by oxygen evolution. Oxygen has in first approximation a refractive index of unity. Therefore gas evolution should lower the refractive index of the ambient and therefore should shift the plasmon resonance towards the blue. As we observe a clear redshift, the Δ() signal cannot be due to the evolution of free oxygen gas. Clearly, LPV reveals interesting electrochemical information at potentials, where CV cannot be used due to water splitting. From 1.6 V, Δ() decreases for pH 6.0 and increases linearly for pH 3.0 until a potential of 0.9 V (1.0 V) is reached. For the CV current, a slight overall decrease is observed in this potential range for both pH buffer solutions. From 0.9 V (1.0 V) downwards, a significant increase of Δ() can be observed. These large changes are observable as pronounced dips in the respective derivative and coincide with the dip in the CV current, and can be pinned to the reduction of previously grown gold-oxide layers. Varying the potential from 0.8 V to -0.2 V and back to 0 V, the onset of 8/19 ACS Paragon Plus Environment

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Figure 3. Left: Comparison of oxidation and reduction signals for conventional CV (black) and LPV (colored) show excellent agreement for four different electrolytes. Right: Reduction signals of the LPVs and fitted Lorentzians associated with gold oxide reduction.

hydrogen evolution can be observed in the CV at approximately 0.3 V (0.7 V). The onset of hydrogen evolution seems to have no influence on Δ() and , similarly as oxygen evolution at high potentials seems to be negligible. LPV and CV show comparable features in a wide potential range. In contrast, the evolution of gaseous species seems to have no effect on the optical signals, while influencing the CV current substantially. For all four studied pH values, the CV and are overlaid in Figure 3 at a potential range where the oxidation peak and the reduction dip are visible concurrently. It can be seen that for both anodic and cathodic scans shares a common baseline, while the CV baselines differ 9/19 ACS Paragon Plus Environment


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significantly for the two directions. With exception of pH 3.0, this behavior can be observed for all other pH values. This means that LPV is insensitive to the macroscopic capacity of the electrochemical cell. The position of the oxidation peak between 1.2 V and 1.5 V in the CV is in very good agreement with the peak occurring in DE. Furthermore, comparison of the reduction dips shows that both and CV show a practically coinciding trace. Not only the peak potentials have excellent correspondence, also the asymmetries in the CVs can be retrieved in . In addition, we observed that the skewness of the reduction dips alters with decreasing pH value. In conventional CV, skewed peaks and dips are an indication for two (or more) not directly resolvable reactions. To identify separable reduction dips, fitting with peak functions was proposed.55 According to literature,36,44,45 gold oxide reduction consists of two individual reduction signals for α- and β-gold oxide. Therefore, we adapted the fitting technique and fitted a sum of two three-parameter (position, width and height) Lorentzians to the skewed reduction dip of DE (Figure 3) to determine the pH dependent reduction potentials separately. The fitted Lorentzians are denoted as A and B respectively. Regardless of the actual gold oxide composition, one can observe two reactions that shift their reduction potentials with pH value. According to Nernst’s equation,2 the reduction potential of an electrochemical half-cell changes with the activity of the involved chemical species and can be calculated by = +

∙ ∙

∙ ln

!Ox !Red

(1)

Here, and denote the activity dependent and the standard reduction potential respectively. Furthermore, " = 8.3145 J K )* mol)* is the universal gas constant, . = 9.6485 ∙ 101 C mol)* is the Faraday constant, 3 the temperature, z the number of charge carriers involved in the reaction and 4Ox and 4Red denote the activities of the respective reactants. It is known that the oxidation and reduction of gold involves hydroxide ions.4,35,36,56 Taking into account that the autodissociation of water connects the pH value directly to the hydroxide concentration, the oxidation and reduction potentials have to shift with pH value according to Nernst’s equation. Nernst’s equation can be rewritten to show linear pH dependence. (2)

= 0 + 7 ∙ pH

where k subsumes the above constants, the conversion factor from natural to decadic logarithm


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Figure 4. Least square fits of the retrieved reduction potentials of the reductions A and B. It is assumed that reduction potentials show (a) two three electron or (b) a one and a two/four electron reaction dependence at the observed pH region. Error bars are dominated by the instrumental error of the potentiostat. =

and a stoichiometric coefficient 9 from the : ; involving reaction such that k = ∙

∙

. At room

∙log (e)

=

temperature (3 = 298.15 K), this simplifies to k = ∙ 59.1 mV∙pH-1 . From the retrieved reduction potentials, two possible pH dependencies can be determined. Figure 4 shows the fitted linear functions for both. One possibility is that both reductions show roughly the same pH dependence meaning that the slopes of the pH dependent reduction potentials are equal within uncertainties (Figure 4a). Assuming the same stoichiometric coefficients, this allows the following interpretation: the same number of electrons is exchanged in reaction 1 and 2 and therefore both reduction potentials shift parallel with varying pH value. Another possibility is that the retrieved reduction potentials are actually intercrossing at a pH value of ca. 4.5, suggesting that different numbers of charge carriers are exchanged. These possibilities can be quantified by fitting Nernst’s equation to the retrieved reduction potentials by the Lorentzians A and B (Figure 4). For future reference, the superscripts (p,1) and (p,2) denote reductions 1 and 2 respectively for the parallel case, while (c,1) and (c,2) are the ones for the intercrossing case. The retrieved fitting (p,1)

values and fitting errors for Nernst’s equation are

(p,2)

= (1020 ? 45) mV,

= (950 ?

30) mV and k(p,1) = (−42 ? 9) mV∙pH-1, k(p,2) = (−41 ? 7) mV∙pH-1 for the parallel case and (c,1)

(c,2)

= (1100 ? 40) mV,

= (870 ? 20) mV and k(c,1) = (−69 ? 8) mV∙pH-1, k(c,2) = (−14 ?

5) mV∙pH-1 for the intercrossing case with the respective correlation coefficients B (p,1) = 0.941, B (p,2) = 0.962 and B (c,1) = 0.999, B (c,2) = 0.983. For the intercrossing case, (c,1) might describe a single electron transfer reaction (C = 1) while 11/19 ACS Paragon Plus Environment


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(c,2) might be a multiple electron transfer reaction (C = 2 or C = 4). However, previous studies never found gold(IV) oxides, so the more probable case is a two electron transfer. The parallel case could describe two three-electron transfer reactions (C = 3) if a stoichiometric coefficient 9 = 2 is assumed. Although the higher correlation coefficients point towards the first interpretation, the latter one seems to be more plausible as gold oxides with an oxidation number of +III have been reported in the majority of studies on gold oxidation.35,36,48 CONCLUSION We have shown that plasmon voltammetry stays in excellent agreement with CV measurements over a wide potential range for all investigated pH values. Together with single particle LPV, the observed pH dependence of the gold reduction potential could be used to enable pH sensing at a nanoscopic level. This nanoscopic localization method is indubitably useful in life sciences to investigate proton concentration gradients e.g. in the respiratory chain. Moreover, we showed that in contrast to conventional CV, LPV seems to be unaffected by evolution of gaseous species. The insensitivity to oxygen evolution allows direct monitoring of the formation of gold oxides beyond the onset of water splitting, such as β-gold oxide, which is not possible to observe with conventional CV. This extends the applicable potential range significantly. Further studies have to show if this insensitivity can be used to investigate additional redox systems with very high oxidation potentials. AUTHOR INFORMATION Corresponding author *E-Mail: [email protected] The authors declare no competing financial interest. SUPPORTING INFORMATION Detailed description of electrochemical annealing correction; changes of the plasmon resonance 12/19 ACS Paragon Plus Environment

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energy, the spectral width, their derivatives and the cyclic voltammograms for the used pH buffers; comparison of reduction dips to show electrode stability; peak current vs. square root of the scan rate for pH 3.0 to 6.0. ACKNOWLEDGEMENT We thank Alfred Nimmervoll for substantial help in constructing the potentiostat for simultaneous CV and LPV. Further, we thank Jan Philipp Kollender for valuable discussions, Bernhard Fragner for technical support as well as Heidi Piglmayer-Brezina for support in the laboratory. We acknowledge funding by the European Research Council (ERC Grant No. 257158 “Active NP”) and the Austrian Klima-und Energiefonds (SolarTrap, Grant No. 843929).

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Localized-Plasmon Voltammetry to Detect pH Dependent Gold Oxidation

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