Physical and electrochemical area determination of electrodeposited Ni, Co, and NiCo thin films

Abstract

(ProQuest: ... denotes formulae and/or non-USASCII text omitted; see image)

The surface area of electrodeposited thin films of Ni, Co, and NiCo was evaluated using electrochemical double-layer capacitance, electrochemical area measurements using the [Ru(NH......)......]....../[Ru(NH......)......]...... redox couple, and topographic atomic force microscopy (AFM) imaging. These three methods were compared to each other for each composition separately and for the entire set of samples regardless of composition. Double-layer capacitance measurements were found to be positively correlated to the roughness factors determined by AFM topography. Electrochemical area measurements were found to be less correlated with measured roughness factors as well as applicable only to two of the three compositions studied. The results indicate that in situ double-layer capacitance measurements are a practical, versatile technique for estimating the accessible surface area of a metal sample.

Gira et al. Nano Convergence (2016) 3:6 DOI 10.1186/s40580-016-0063-0

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Web End = Matthew J. Gira1,2, Kevin P. Tkacz1,3 and Jennifer R. Hampton1*http://orcid.org/0000-0003-3953-5435

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Abstract

The surface area of electrodeposited thin lms of Ni, Co, and NiCo was evaluated using electrochemical double-layer capacitance, electrochemical area measurements using the [Ru(NH3)6]3+/[Ru(NH3)6]2+ redox couple, and topographic atomic force microscopy (AFM) imaging. These three methods were compared to each other for each composition separately and for the entire set of samples regardless of composition. Double-layer capacitance measurements were found to be positively correlated to the roughness factors determined by AFM topography. Electrochemical area measurements were found to be less correlated with measured roughness factors as well as applicable only to two of the three compositions studied. The results indicate that in situ double-layer capacitance measurements are a practical, versatile technique for estimating the accessible surface area of a metal sample.

Keywords: Electrodeposition, Ni, Co, NiCo, Alloy , Capacitance, Area, Atomic force microscopy

1 Background

Nanoporous materials are of increasing scientic and technological interest due to a variety of useful properties such as low mass density, high surface area, high strength, and enhanced optical, electrical, thermal, and catalytic behavior. Potential applications of metals with nanoporous morphology include batteries, capacitors, magnetic storage media, lightweight structures, sensors, and water ltration devices [1]. The enhanced surface area and size-dependent reactivity of nanoporous metals also make them a promising area of study for a number of catalytic applications.

An important factor in evaluating the reactivity of a porous metal is the surface area available for reaction. Both increased surface area and changes in intrinsic reactivity can have signicant eects on the overall behavior of a target material. Thus, straightforward and practical area measurement procedures are an essential aspect of catalysis research.

One technique for area measurement is based on the physical absorption of gas molecules to a surface following the theory presented by Brunauer, Emmet, and Teller (BET)[24]. Although this is a well-understood and regularly-used method, BET measurements have limitations, specically the eects that heat treatments may have on the sample being characterized as well as the larger sample sizes needed to achieve the desired sensitivity[5].

Electrochemical techniques for determining surface area have the advantage of being insitu and can be performed just previous to or after any electrochemical reactivity measurements of interest. These techniques fall into two general categories. The rst type uses a surface-limited chemical reaction to quantify the surface area of the electrode. In contrast, the second type measures a physical characteristic that is proportional to the surface area.

Using a surface-limited chemical reaction such as adsorption of hydrogen or carbon monoxide [615], underpotential deposition of a new metallic species [5, 13, 16, 17], or surface oxide formation [9, 1722] to quantify the surface area of the electrode can be quite sensitive. However, a disadvantage is that a particular

*Correspondence: [email protected]

1 Department of Physics, Hope College, Holland, MI 49423, USA Full list of author information is available at the end of the article

2016 Gira et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

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reaction may be specic to the material being assessed. For example, gold oxide formation has been used extensively as a probe of gold electrode surface area, but this method can not be applied directly to an electrode of a dierent composition without considering the extent and potential range of oxide formation on that new material.

Rather than a chemical reaction, a electrochemical characterization using a physical characteristic can be used to quantify the surface area of a working electrode. The current due to a well-characterized redox reaction, such as the reduction of [Fe(CN)6]3 to Fe(CN)6]4, is one such measurement [19, 2325]. Similarly, the electrochemical double-layer capacitance of an electrode, which can be measured either by cyclic voltammetry or by electrochemical impedance spectroscopy, is proportional to its surface area[9, 1922, 2533]. These techniques depend on the conducting nature of the electrode rather than its chemical identity, so to rst approximation they do not depend on the nature of the material being studied. However, the potential range necessary for these measurements must be considered, because the characterization technique itself may aect the structure or composition of the material in question.

Topographic measurements of samples with a scanning tunneling microscope (STM) or atomic force microscope (AFM) can also be used to quantify the surface area of a sample[8, 34, 35]. These methods have the advantage of providing direct quantitative measurements of surface morphology. For AFM in particular, topographic measurements are not sensitive to the nature of the surface being probed. However, scanning probe techniques are local rather than ensemble measurements. Thus, a number of images must be taken for any surface in question to ensure the images are representative of the sample as a whole. For materials with porous morphology, scanning probe microscope measurements are limited, because the local probe can only measure structures which are accessible from the top of the sample. Similarly, if a surface has features smaller than that of the scanning probe tip itself, those features will not be imaged accurately by the technique. However, for materials with simpler morphology, scanning probe measurements provide a nice complement to the other methods described here.

In this work we compare electrochemical methods for determining the surface area of electrodeposited metal thin lms with AFM topographic measurements of the same samples. Electrodeposited nickel, cobalt, and nickelcobalt were chosen for the study because of the interest in these materials as catalysts. The thickness of these lms was varied by controlling the total charge during the deposition process. In this way, the resulting roughness, and therefore surface area, of the material was varied. The resulting lms were characterized using

two electrochemical methods, double-layer capacitance measurements and area determination using a ruthenium-based redox probe. These measurements were compared to the roughness factors extracted from ex situ AFM images of the samples. Correlations between these three measurements were explored, both for the samples with the same composition and for the entire set of samples regardless of composition.

2 Methods

2.1 Electrochemistry

The electrodeposition and electrochemical characterization were performed using an Epsilon electrochemical workstation (Bioanalytical Systems, Inc., West Lafayette, IN, USA) and a custom-built Teon cell with a working electrode area of 0.032 cm2 dened with a Kalrez o-ring[24]. The counter electrode was a coil of platinum wire (Alfa Aesar, Ward Hill, MA, USA) and the reference electrode was an Ag/AgCl (3 M NaCl) electrode (Bio-analytical Systems, Inc., West Lafayette, IN, USA). All of the potentials recorded are with respect to this reference electrode. The electrolyte solutions were created using water that was puried through successive reverse osmosis, deionization, and UV purication stages. All of the chemicals used for these electrolytes were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used as received. Every experiment was carried out at room temperature.

2.2 Deposition

All thin lms were deposited from solutions containing 0.5M H3BO3 and 1M Na2SO4 along with 0.1M NiSO4 for the nickel thin lms, 0.1 M CoSO4 for the cobalt thin lms, or 0.75mM NiSO4 and 0.25mM CoSO4 for the nickelcobalt thin lms. The working electrode substrates were cleaved from a silicon wafer plated with 1000 of gold over a 50 titanium adhesion layer (Platypus Technologies, LLC, Madison, WI, USA). Controlled potential electrolysis was used to step the potential of the working electrode from open circuit to 1000 mV. The deposition was stopped once the desired amount of charge, ranging from 200 to 1000 mC, was achieved in order to vary the thickness of the deposited lms.

2.3 Physical characterization

Physical characterization of the samples consisted of roughness and composition measurements. Atomic force microscope topography was used to measure the roughness of each thin lm. This was completed using a Dimension Icon AFM (Bruker, Santa Barbara, CA, USA) using the ScanAsyst mode and SCANASYST-AIR cantilevers. A minimum of three 10m AFM images (512 pixels512 pixels) were taken of each sample. Nanoscope

Gira et al. Nano Convergence (2016) 3:6

Analysis software (Bruker, Santa Barbara, CA, USA) was used to nd the three-dimensional area of each image. For the NiCo thin lms the elemental composition was measured. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) measurements were completed using a TM3000 Tabletop SEM (Hitachi, Tokyo, Japan) and a Quantax 70 EDS attachment (Bruker, Madison, WI, USA). Images and EDS data were taken at 60 magnication, and Quantax 70 software was used to obtain the Ni and Co compositions from the EDS spectra.

2.4 Electrochemical characterization

Electrochemical characterization consisted of double-layer capacitance and active area measurements. Electrochemical capacitance was measured using cyclic voltammetry (CV) in 0.5M KOH by sweeping from 50 to 350 mV and back to 50 mV. The scan rates were varied between 25 and 400mV/s. The electrochemically active area was also measured using CV. The electrolyte solution was 5mM Ru(NH3)6Cl3 and 1M KCl. The potential was swept from 100 to 600mV and back to 100mV with varying scan rates in the range of 100901mV/s. A minimum of three trials of both experiments were performed for each sample.

3 Results anddiscussion

The goals of this work were to explore the correlations between the AFM-based and electrochemical measurements for samples with dierent roughnesses and therefore dierent areas. The roughness of each of the samples was determined using AFM topographic measurements. Example AFM images are shown in Fig. 1 for samples with a deposited charge of 1000mC. The Ni and Co lms exhibit similar crystallite formation, with the resulting Co features larger and taller than the corresponding Ni ones for the same deposited charge. In contrast, the NiCo lm has a distinct texture with smaller, less compact crystallites.

For each image, the data were attened using a rst order lter to remove sample tilt. Afterwards, the roughness factor, RF, was calculated as RF = AAFM/Aproj, where AAFM is the surface area calculated from the image using the Nanoscope Analysis software and Aproj is the

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(a) Ni

1.0 m

(b) Co

2.5 m

250 nm

(c) NiCo

Fig. 1 Example 10 m 10 m AFM topographic measurements for a Ni, b Co and c NiCo thin lms. Each sample had deposited charge of 1000 mC. The scale bar is 2 m for all the images. The vertical scale is indicated to the right and is dierent for each image. The roughness factors for these images are (a) 1.12, (b) 1.41, and (c) 1.05

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projected (at) area of the measured region, 100m2 in this case. From this calculation, the roughness factor is proportional to the surface area of the sample measured using AFM, but is not specic to the image sized used.

The average RF for the three types of lms are graphed in Fig. 2 as a function of the deposited charge, Q. The approximate average thickness, t, of the lms corresponding to each deposited charge is shown on the upper horizontal axis of the gure. The conversion from deposited charge to thickness was calculated assuming 100% current efficiency from t = Q/(neA), where n = 2 is the number of electrons in the Ni or Co deposition reaction, e is the charge on the electron, A is the dened area of the working electrode, and ~~ is the number density of the deposit. The bulk densities (in g/cm3) and molar masses (in g/mol) of Ni and Co were used to calculate a value of ~~ for each metal. Because the values for Ni and Co are so similar, 9.14 1022 and 9.09 1022cm3 respectively, an average value of ~~ was used to calculate the axis in the gure, corresponding to the assumption of an equal-component alloy. The systematic error for this assumption compared to using the value of ~~ for pure Ni or pure Co is approximately 0.2%.

As seen qualitatively in Fig.2, for the same lm thickness, the Ni samples generally are the smoothest, the Co samples have the roughest topography, and the NiCo alloy samples have intermediate roughness factors. For the Ni and Co samples, the roughness factor generally increases as the thickness of the samples increases, while for the NiCo samples, the roughness uctuates with

deposited charge. For the entire set of samples, regardless of composition, the roughness factors ranged from about 1.05 to 1.4. That is the samples had measured surface areas ranging from 5 to 40% higher than the corresponding projected area.

The compositions of the NiCo thin lms were measured from EDS spectra taken at 60 magnication and are shown in Fig.3 as a function of the average roughness of the samples. The Ni composition of the deposited alloys was generally between 60 and 70 at.%. The fact that the samples have a smaller Ni composition than that of the deposition solution (75 at.%) is attributed to the anomalous codeposition phenomenon which is common for iron group metals[3639]. The decrease in Ni composition with increasing roughness is consistent with the data in Fig.2 where NiCo samples are generally rougher than the Ni samples but smoother than the Co samples.

Electrochemical double-layer capacitance measurements were made on all the electrodeposited samples using CV in KOH electrolyte. Example measurements for a variety of scan rates are shown in the inset of Fig.4, showing the featureless current response expected of a capacitor. For these metals in alkaline electrolytes, a more complex pseudocapactive response corresponding to metal oxide and/or hydroxide redox reactions is often seen[4042]. For the measurements here, however, the potential window used is signicantly negative of that needed for these redox reactions to occur. As a result, the featureless CVs shown in the inset are measured instead. For a given scan rate, v, the average currents during the forward and reverse sweeps were calculated, and half of the dierence between these two values was taken as

t / m

0 2 4 6 8 10 12

Ni

Co

NiCo

1.4

1.3

RF

1.2

1.1

1.0

0 200 400 600 800 1000 1200

Q / mC

Fig. 2 Average roughness factor, RF, of each sample as a functionof the deposited charge, Q. The second horizontal axis indicates the approximate average thickness, t, of the samples. Error bars represent the standard error of the mean for the measurements

75

70

65

Ni/ at. %

60

NiCo Samples

Solution

55

50

1.0 1.1 1.2 1.3

RF

Fig. 3 Ni composition for the NiCo samples as a function of the average roughness factor, RF, of the samples. Composition error bars represent the typical EDS uncertainty. The dashed line indicates the Ni composition in the deposition solution

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the capacitive current, Idl, for that scan rate. This current was linearly dependent on the scan rate, as seen in Fig.4, indicating that the lms acted as simple capacitors in this potential scan range. The measured capacitance, Cdl, was calculated using the time derivative of the denition of capacitance, Idl = Cdlv, as the slope of the linear t[43].

Electrochemical area measurements were made on Ni and NiCo samples with CV using the [Ru(NH3)6]3+/

[Ru(NH3)6]2+ redox couple. Example measurements for a variety of scan rates are shown in the inset of Fig. 5, which show the expected current response for a reversible redox reaction[43]. The ruthenium-based probe was chosen because the potential window for the CV experiment generally does not interfere with the deposited lm. These area measurements could not be made on the Co samples, however, because the CV measurements did not result in reversible redox behavior and the scans in that potential range aected the structure of the lm. The magnitude of the peak cathodic current, Ip, as a function of the scan rate, v, is shown in Fig.5 for an example measurement. The electrochemical area, Aec, of the sample was calculated using the RandlesSevcik equation,

Ip = 0.4463nFAecC(nF/RT)1/2v1/2D1/2, where n = 1 is the number of electrons involved in the redox reaction, F is Faradays constant, C is the bulk concentration of the analyte, R is the molar gas constant, T is the temperature, and D is the diusion constant of the analyte[43]. For [Ru(NH3)6]3+, the measured diusion constant is 7.1 106cm2/s[4446].

The results of these two electrochemical measure

ments, the average Cdl and Aec for each sample, are graphed as a function of the AFM-based measurement

results, average RF, in Fig.6a, b respectively. Because Aec could not be measured for the Co samples, no data for Co are included in Fig.6b.

The results in Fig.6a for all three types of samples show that there is a clear trend towards larger capacitance for rougher samples. There is some uctuation in this correlation between capacitance and roughness, which increases for the rougher samples. Within this level of uctuation, however, the observed trend between capacitance and roughness factor is the same for the group of samples as a whole, regardless of the sample composition or the morphological dierences seen in the AFM topography (Fig.1). This was of particular interest for this study because of the practical importance of determining surface area of materials with a variety of compositions and structures. For these reasons, the results indicate that electrochemical double-layer capacitance is useful as a semi-quantitative measure of the surface area of electrodeposited samples.

In contrast to the capacitance results, the correlation between area measurements and roughness factor, shown in Fig. 6b for the Ni and NiCo samples, is less clear. In particular, although the smoother Ni samples generally have lower capacitance values than the rougher NiCo samples, they have higher measured electrochemical areas.

To explore these observations further, the ratio of average capacitance to average area, Cdl/Aec, was calculated for each of the Ni and NiCo samples. Fig.6c shows this ratio as a function of the average RF of the samples. For the Ni samples, the capacitance-to-area ratio uctuates

100

80

60

100

Current / A

I p / A

40

50

0

-50

20

-1 -750 -500 -250 0 250

00

Potential / mV

0

0 200 400 600 800 1000

v / mV s-1

Fig. 5 Example electrochemical area measurements for a NiCo thin lm. The sample had a deposited charge of 1000 mC. The inset shows CV measurements in 5 mM Ru(NH3)6Cl3 and 1 M KCl at 200, 400, and 800 mV/s. The magnitude of the peak cathodic current, Ip, is t to a square root function vs. scan rate, v, to determine the area, Aec, of the sample using the RandlesSevcik equation

1.25

2

1.00

Current / A

0

-2

0.75

-400 -300 -200 -100 0

Potential / mV

I dl / A

0.50

0.25

0.00 0 100 200 300 400

v / mV s-1

Fig. 4 Example double-layer capacitance measurements for a NiCo thin lm. The sample had a deposited charge of 1000 mC. The inset shows CV measurements in 1 M KOH at 75, 225, and 350 mV/s. The slope of the linear t to the capacitive current, Idl, vs. scan rate, v, is the measured double-layer capacitance, Cdl, for the sample

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between 40 and 75F/cm2 for all roughness factors. This value is larger than, but on the order of 20F/cm2, the specic capacitance value typically used in the literature for a variety of metals and alloys[9, 13, 21, 22, 26, 28, 30 32, 4752]. In contrast, the NiCo lms have even larger capacitance-to-area ratios, between 100 and 500F/cm2, and the ratio tends to increase with increasing roughness factor. The larger ratios for the NiCo lms may be the result of the area measurements being smaller than they

should be. Additional evidence for this interpretation is seen in Fig.6b, where the NiCo area measurements are generally smaller than the Ni area measurements of samples with similar roughness factors.

One explanation for the electrochemical areas of the NiCo samples being underestimated is that in addition to the NiCo lms generally being rougher than the Ni lms, they display a distinct morphology (Fig. 1c). For rougher, more complex morphologies, the assumption of planar diusion which leads to the RandlesSevcik equation may not be accurate. Specically, the thickness of the diusion layer can be as large as 10s of m for the scan ranges and rates used in the area measurements [43]. Thus, for the samples here, with topographic features on the scale of 100s of nm to a few m, some portions of the sample area would not contribute as strongly to the measured current compared to that expected from the simple planar diusion model. On the other hand, double-layer capacitance measurements do not depend on the geometry and extent of the diusion layer. Instead, during capacitive charging and discharging, non-specically adsorbing ions such as K+ and OH can approach an electrode surface as close as the outer Helmholtz plane, generally a distance of 510 [43]. Thus, area measurements may be underestimated in the case of rough, complex topography compared to capacitance measurements of the same sample. This, in turn, would lead to the observed higher capacitance-to-area ratios as well as to the lack of correlation between area and roughness measurements. A similar, but smaller, eect may also explain capacitance-to-area ratios for the smoother Ni samples being slightly higher than is typical in the literature.

4 Conclusions

For the metal thin lms studied here, the results indicate that in situ electrochemical measurements of double-layer capacitance are correlated with the roughness factors extracted from ex situ topographic AFM images. In addition, these measurements can be adapted to a wide variety of metal systems by choosing an appropriate potential range where only capacitive behavior is evident, thus minimizing any permanent eects on the sample. In contrast, the area measurements using a ruthenium-based redox probe are both less correlated with roughness measurements and less broadly applicable.

The uctuations present in the capacitance vs. roughness data do place some limitations on the quantitative nature of the results. Nevertheless, the versatility and simplicity of capacitance measurements make the technique useful as a semi-quantitative measure of the electrochemically accessible surface area of a sample. Ongoing work in our lab aims to explore this method further by looking at additional metals and alloys as well as at the more

(a)

10

C dl / F

Ni Co NiCo

1

0.04

(b)

0.03

A ec / cm

2 0.02

0.01

Ni NiCo

500

(c)

400

-2 300

C dl/A ec / F cm

200

100

Ni NiCo

0 1 1.1 1.2 1.3 1.4RF

Fig. 6 a Average capacitance, Cdl, b average area, Aec, and c ratio of capacitance to area, Cdl/Aec, of each sample as a function of the average roughness factor, RF. Error bars represent the standard error of the mean for the measurements

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complex morphologies with higher roughness factors, such as those produced by electrodeposition through self-assembled colloidal sphere masks. Double-layer capacitance provides a simple, practical, and reliable measure of the accessible surface area of metal and alloy thin lms which can be used to quantify the intrinsic reactivity of these systems towards a variety of catalytic reactions.

Authors contributions

MJG and KPT carried out the experiments and contributed to the data analysis. JRH coordinated the study and helped analyze the data. All authors helped to draft the manuscript. All authors read and approved the nal manuscript.

Author details

1 Department of Physics, Hope College, Holland, MI 49423, USA. 2 Department of Chemistry, Hope College, Holland, MI 49423, USA. 3 Present Address: Department of Chemical Engineering and Materials Science, Universityof California, Irvine, CA 92697, USA.

Acknowledgements

We thank Professors Beth Anderson and TJ Mullen for helpful discussions. This material is based upon work supported by the United States National Science Foundation under Grants No. RUI-DMR-1104725, REU-PHY/DMR-1004811, MRI-CHE-1126462, MRI-CHE-0959282, and ARI-PHY-0963317 as well as by the Jacob E. Nyenhuis Faculty Development Fund of Hope College.

Competing interests

The authors declare that they have no competing interests.

Received: 19 December 2014 Accepted: 14 July 2015

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