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Najdovski, Ilija & O’Mullane, Anthony(2014)The effect of electrode material on the electrochemical formation of porouscopper surfaces using hydrogen bubble templating.Journal of Electroanalytical Chemistry, 722-723, pp. 95-101.

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https://doi.org/10.1016/j.jelechem.2014.03.034

1

The Effect of Electrode Material on the Electrochemical Formation of Porous Copper

Surfaces using Hydrogen Bubble Templating

Ilija Najdovski1,2 and Anthony P. O’Mullane1,3†*

1 School of Applied Sciences, RMIT University, GPO Box 2476V, Vic 3001, Melbourne

Australia

2 Centre for Advanced Materials and Industrial Chemistry, School of Applied Sciences, RMIT

University, GPO Box 2476V, Vic 3001, Melbourne Australia

3 Present address: School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia

† ISE member

Email: anthony.omullane@qut.edu.au

Keywords: electrodeposition; copper; catalysis; electrocatalysis; porous metal

Abstract

The electrodeposition of copper onto copper, gold, palladium and glassy carbon (GC) electrodes

via a hydrogen bubble templating method is reported. It is found that the composition of the

underlying electrode material significantly influences the morphology of the copper

electrodeposit. Highly ordered porous structures are achieved with Cu and Au electrodes,

however on Pd this order is disrupted and a rough randomly oriented surface is formed whereas

on GC a bubble templating effect is not observed. Chronopotentiograms recorded during the

2

electrodeposition process allows bubble formation and detachment from the surface to be

monitored where distinctly different potential versus time profiles are observed at the different

electrodes. The porous Cu surfaces are characterised with scanning electron microscopy, X-ray

diffraction and cyclic voltammetric measurements recorded under alkaline conditions. The latter

demonstrates that there are active sites present on electrodeposited copper whose coverage and

reactivity depend on the underlying electrode material. The most active Cu surface is achieved at

a Pd substrate for both the hydrogen evolution reaction and the catalytic reduction of ferricyanide

ions with thiosulphate ions. This demonstrates that the highly ordered porous structure on the

micron scale which typifies the morphology that can be achieved with the hydrogen bubbling

template method is not required in producing the most effective material.

3

1. Introduction

The fabrication of materials via electrochemical methods for various applications; such as

electrocatalysis [1-4], hydrogen storage [5, 6], photocatalysis [7-9] and sensing [10-14] is an

increasingly attractive field of research due to the relatively simple, clean, cheap and efficient

processing methods involved. There are many factors which influence the electrodeposition

process such as the choice of potentiostatic or galvanostatic conditions, background electrolyte,

precursor concentration, presence/absence of additives and temperature [15]. It is this plethora of

experimental conditions which makes electrodeposition such an attractive method for controlling

the properties of an electrodeposited material whether it be metallic, inorganic or organic in

nature. However, an important but often overlooked parameter is the choice of substrate upon

which the material is deposited and can often influence the morphology of the final structure and

therefore its functionality and applicability [16-18]. For example, even on the same electrode

material, it has been shown that the deposition of Ni using a constant current of 25 mA/cm2

formed fine grain, high strength structures on an annealed Cu substrate as opposed to large,

weakly adhered crystals on cold-rolled Cu [19]. This variation was attributed to the different

exposed crystalline surfaces of the annealed and cold-rolled Cu substrates ([100] and [110]

respectively) which facilitate the formation of different nucleation sites for the growing nickel

structures.

A relatively new approach for the electrodeposition of high surface area metal deposits is

using hydrogen bubble templating. This offers a quick and easy route to the formation of porous

metallic surfaces where hydrogen evolution is initiated at the working electrode during the course

of metal deposition. The first reports were on the deposition of copper foam [20, 21] and has

since been extended to various other metals and bimetallic combinations including Ag [22], Pt

[23], Au [24], Pd [25], Pb [26], Cu/Sn [27], Cu/Pd [10], Cu/Au [28], Au/Pt [29], Au/Pd [30]. In

the majority of these examples little emphasis has been placed on the role of the working

4

electrode material in influencing the morphology of the deposit. However, in one study on the

deposition of porous copper onto copper it was concluded that the roughness of the electrode

influenced the final porosity of the structure where a nodular surface promoted the formation of

smaller pores with thinner wall structures compared to that of a smooth surface [31]. It is

envisaged that the use of different electrode materials would also have significant implications on

the morphology of the final material electrodeposited under these conditions.

Therefore, in this work, we investigate the effect that various different substrates have

on the electrodeposition of copper under high overpotential conditions and evaluate them for their

suitability as electrode materials for the hydrogen bubble templating approach. Substrate

electrode materials were chosen to provide a large range for their effectiveness in

electrocatalysing the hydrogen evolution reaction, namely glassy carbon (GC), Au, Cu and Pd.

The influence of copper morphology on electrocatalytic and catalytic activity is then investigated

for the hydrogen evolution reaction and reduction of ferricyanide to ferrocyanide using

thiosulphate ions respectively.

2. Experimental

Solutions containing CuSO4, H2SO4, sodium thiosulphate (Na2S2O3) and potassium ferricyanide

(K3[Fe(CN)6]) (Ajax Finechem) were used as received and made up with deionized water

(resistivity of 18.2 MΩ cm) purified by use of a Milli-Q reagent deioniser (Millipore).

Electrochemical experiments were conducted at (20±2) ˚C with a CH Instruments (CHI 760C)

electrochemical analyser. Copper foil (0.158 cm2 area; 99.999% purity purchased from

Goodfellow) when used as the working electrode, was first immersed in diluted HNO3 (10% v/v)

to remove any surface oxides and then washed with acetone (Chem-Supply) and methanol

(Merck) followed by drying in a stream of nitrogen gas prior to use. For gold and palladium

electrodes they were prepared by e-beam evaporation (Balzers™ electron beam evaporator) of a

5

10 nm Ti adhesion layer on a silicon (100) substrate (Montco Silicon Technologies) followed by

150 nm of Au or Pd. Prior to use the films were cleaned by successive immersions in acetone,

methanol and Milli-Q water for five minutes each, and then dried under a stream of nitrogen. The

reference electrode in all cases was Ag/AgCl (aqueous 3M KCl). For the electrodeposition of Cu,

an inert graphite rod (3 mm diameter, Johnson Matthey Ultra “F” purity grade) was used as the

counter electrode to avoid any possible contaminants from electrodissolution [32]. All

electrochemical measurements commenced after degassing the electrolyte solutions with nitrogen

for at least 10 min prior to any measurement. SEM measurements were performed on an FEI

Nova SEM instrument. Prior to imaging, samples were thoroughly rinsed with Milli-Q water and

dried under a flow of nitrogen. To measure film thickness the samples were tilted at a 40º angle

and then imaged. XRD measurements were carried out on a Bruker AXS X-ray diffraction

system operating at a voltage of 40 kV and current of 40 mA with CuKa radiation. The reaction

of 1 mM K3[Fe(CN)6] with 0.1 M Na2S2O3 was carried out in a beaker (100 ml vessel) containing

a total volume of 24 ml of solution under stirring conditions using a stirrer bar at the bottom of

the beaker rotated at 800 rpm with the sample fixed in the solution. The geometric area of the

sample was 0.158 cm2 in all cases. The progress of the reaction was monitored by taking aliquots

from the reaction mixture and performing UV-visible spectroscopy using a Varian (Cary 50) in a

cuvette of 1 cm path length.

3. Results and discussion

The electrochemical deposition of copper was conducted galvanostatically using GC, Cu, Au and

Pd electrodes by applying a constant cathodic current of 3 A cm-2 in an electrolyte containing 0.4

M CuSO4 in 1.5 M H2SO4. The use of this current density value and electrolyte composition has

been shown previously to produce a well adhered and highly porous copper foam on a copper

substrate [21, 33, 34]. Under these conditions it appears that there is a balance between the

6

evolution of hydrogen from the surface and the rate of metal electrodeposition. It is expected that

the metal deposition rate is not likely to be the dominant factor in influencing the morphology of

the deposit as potentials well below that of the Cu2+/0 process (0.34 V vs SHE) are attained as

discussed below but rather the nature of hydrogen evolution from the surface given that the

exchange current density for this reaction at these materials have been measured to be 1.3 x 10-4,

1.6 x 10-6, 6.3 x 10-7 A cm-2 for Pd, Cu, Au respectively [35] and 1.6 x 10-6 A cm-2 for GC [36].

These values can only be treated as an indicator of potential activity as the surface area during the

course of the reaction will continually be changing. Also given that this is an inner sphere

electron transfer process the presence of defect or active sites on the surface may play a

significant role in enhancing the electrocatalytic activity for a particular metal for the HER which

would influence how hydrogen is evolved from the surface and therefore bubble size and

residence time.

This is reflected in the linear sweep voltammograms recorded at Cu, Au and Pd electrodes

in 1.5 M H2SO4 where the order of activity is Pd > Au > Cu in terms of both earlier onset

potentials and current density (Figure 1). For the case of Pd, a noisy response is observed which

is attributed to vigorous gas evolution perturbing the current response. Therefore at the materials

used in this study the evaporated Au film is a more effective electrocatalyst than Cu foil for the

HER. The active site behaviour of gold in acidic electrolytes has been well documented by Burke

et al [32, 37, 38] who have shown that gold is a very effective electrocatalyst for a variety of

reactions. This is also consistent with previous work from this laboratory which investigated the

active site behaviour of evaporated thin films of gold [2, 39]. Given the difference in activity of

these materials for the HER it is therefore expected that the morphology of the resultant copper

layers will be influenced.

7

Figure 1: Linear sweep voltammograms recorded at 10 mV s-1 in 1.5 M H2SO4 at a Cu (1), Au (2) and Pd (3) electrode.

Changing the substrate material had a profound influence on the morphology of the

electrodeposit (Figure 2). The most well defined and layered structures were seen on the Cu

substrate (Figure 2A); which had a fairly even pore size distribution of 63 µm (Table 1). The

diameter of only the outermost layer of pores was measured via inspection of SEM images. The

pores grow in size from the bottom to the top of the deposit due to the coalescence of hydrogen

bubbles which occurs with time and has been discussed in detail elsewhere [33]. The internal wall

structure is heavily populated with nano-tipped dendrites (Figure 2E) which are primarily due to

diffusion limited aggregated growth. Two factors influence this type of growth; the amount of

escaping hydrogen from the surface of the substrate, and the concentration of metal salt at the

substrate surface. It is known that the surface tension of the electrolyte affects the morphology of

porous copper created via this approach by changing the break-off diameter of the hydrogen

bubble [33]. However surface tension is not expected to play a role in this study as in all cases the

concentration of H2SO4 and CuSO4 as well as the temperature of the electrolyte is the same which

amounts to an equivalent surface tension for all experiments. As hydrogen evolution from the

surface occurs, a turbulent layer is created above the substrate where vigorous mixing of the

solution occurs. This mixing forms a layer where metal deposition is impeded by the escaping

8

hydrogen causing a disturbance in the formation of new nucleation sites. As less nucleation sites

are formed, any growth from the spaced out nucleation sites would grow as branches and

eventually form dendrites as further branches evolve. For the case of gold the formation of a

porous copper surface with significantly smaller pores compared to the Cu case was observed

with an average size of 18 µm (Figure 2B). The morphology of the internal walls is also

perturbed (Figure 2F) and it can be seen that globular like structures are formed in comparison to

the dendrites observed in the case of the copper substrate. For the Pd case patchy growth was

observed with the formation of sporadic pores with an average diameter of 28 µm (Figure 2C).

However it should be noted that porosity is evident within the electrodeposited material. The

internal wall structure contains a mixture of plates and globular like structures (Figure 2G)

presumably due to the highly turbulent conditions that would not favour dendritic growth. Finally

in the case of the GC substrate, the adhesion of Cu to the surface is poor, and the production of

hydrogen gas from the surface dislodged large amounts of the Cu deposit which was visible in the

bottom of the electrochemical cell after the experiment. The material which is deposited on the

surface is not particularly porous, although there is some evidence of this in the centre of Figure

2D. It mainly consists of quite large sheets of copper which are greater than 500 µm in length.

Within the region that demonstrates some porosity, small globular like structures are formed with

some elongated growth also observed (Figure 2H).

Substrate Average pore size (µm) 15 s deposition

Average pore size (µm) 30 s deposition

Cu 63 ± 6 73 ± 7

Au 18 ± 2 62 ± 5

Pd 28 ± 2 -

GC - -

Table 1: Pore sizes for copper electrodeposited onto different substrates for different times

9

Figure 2: SEM images of copper electrodeposited from 0.4 M CuSO4 in 1.5M H2SO4 at – 3 A

cm-2 for 15 s on Cu (A, E), Au (B, F), Pd (C, G) and GC (D, H) substrates (scale bar is 500 µm in

A-D and 5 µm in E-H).

10

In an effort to monitor the evolution of the structure during the course of the reaction

chronopotentiograms were recorded (Figure 3). It can be seen that are significant differences in

the E v t profiles for each of the substrates. It is immediately apparent that the data is consistent

with the linear sweep voltammograms recorded in Figure 1. To maintain a constant current

density of – 3 A cm-2 the potential attained by the system is most negative for GC and least

negative for Pd as expected and follows the trend GC < Cu < Au < Pd. The amount of hydrogen

evolved from each system is presumably comparable as the current from the metal deposition

process is likely to be constant at the potentials involved in the deposition process which are

significantly lower than the standard reduction potential of Cu2+/0. Even though the amount of

hydrogen evolved from the surface is comparable the profile of the E v t curves offers significant

insights into the electrodeposition process. The individual spikes seen in the curves can be

attributed to the growth of a hydrogen bubble and its dislodgement from the electrode surface.

For the case of Cu quite distinct spikes are seen whose width increases with time reflecting the

increase in pore size as copper is deposited further out from the underlying substrate. As

mentioned previously this is due to the coalescence of hydrogen bubbles with time. For the case

of gold the intensity of the spikes in potential are not as pronounced which most likely indicates

that hydrogen bubbles are not coalescing to as great as an extent as the case for copper which

therefore results in a smaller pore size (Figure 2B). For Pd this trend continues with a relatively

flat E v t profile suggesting bubble coalescence is not occurring to an extent that is sufficient to

set up a template over which copper can be electrodeposited and this results in the sporadic

deposition seen in Figure 2C. Finally in the case of GC very large changes in the E v t profile can

be seen which indicates that very large bubbles are created on the surface during the reaction

which inhibits metal deposition on the surface and results in the very patchy growth seen in

Figure 2D.

11

Figure 3: Potential versus time recorded at Pd, Au, Cu and GC electrodes during the

galvanostatic deposition of copper at -3 A cm-2 for 15 s in a solution of 1.5 M H2SO4 and 0.4 M

CuSO4.

Upon increasing the deposition time to 30s, the existence of several porous copper layers

became more pronounced on the copper substrate (Figure S1) and the average pore size increased

from 63 to 73 µm which is expected due to the longer time that hydrogen bubbles have to

coalesce and form a larger template (Table 1). For the Au substrate a highly porous copper

surface was formed (Figure S1B) with an average pore size of 62 µm which was remarkably

similar to the surface formed on copper after 15 s (Figure 2A). This can be attributed to the fact

that after 15 s copper is being electrodeposited onto copper and the underlying gold surface is no

longer playing a significant role in determining the rate of hydrogen evolution. However, on the

Pd electrode a rough copper deposit is formed (Figure S1C) with no clear evidence of well-

defined pore formation which implies that the initially deposited Cu does not offer a suitable seed

layer to induce pore formation as seen in the case of Au. For the GC surface the increased

deposition time does not affect the morphology and large sheet like structures are again seen

(Figure S1D) with some evidence of a porous material formed in patches on the surface. The poor

adhesion of copper to GC under these conditions means that Cu is continuously being removed

12

from the surface during the course of the experiment as seen by the deposit that can be collected

from the bottom of the electrochemical cell post reaction.

X-ray diffraction experiments were undertaken and shown in Figure 4 where in all cases

peaks associated with metallic copper (Cu(111) and Cu(200)) are observed. For the Au and Pd

electrodes, peaks associated with Au(111) and Pd(111) are evident due to the penetrating nature

of the X-rays. However on Cu and GC electrodes there is also evidence of surface oxide

formation not seen in the case of Au and Pd electrodes. For Cu there is a peak due to Cu2O and

on GC a peak for CuO is quite prominent. Given that highly dendritic copper with sharp tips and

edges is formed for the case of the copper substrate (Figure 2E) suggests that it may be more

prone to oxidation, in particular under the localised alkaline conditions induced by the

consumption of protons, compared to the larger globule like structures formed on Au and Pd. For

GC the intensity of the CuO peak suggests a significant amount of oxide formation occurs which

is reflected in the diminished ratio of the Cu(111) : Cu(200) peak intensity as seen in the case of

Au and Pd electrodes. This result is quite intriguing and unexpected and may be due to the

oxidation of freshly cleaved copper that is continuously being detached from the surface.

However this is still only speculative in nature.

Figure 4: XRD pattern of Cu electrodeposited from 0.4 M CuSO4 in 1.5M H2SO4 at – 3 A cm-2

for 15 s on various substrates.

13

Electrochemical measurements allow the active nature of nanostructured materials to be

investigated and it was recently demonstrated that porous copper electrodeposited onto copper

foil under these conditions exhibited significant Faradaic responses in the double layer region

under alkaline conditions [40]. Illustrated in Figure 5 are cyclic voltammograms recorded at

copper foil and electrodeposited copper on Cu, Au, Pd and GC electrodes. A reduced potential

limit was employed to prevent the possible electrodissolution of copper at higher potential values

that could be redeposited in the reverse sweep. On the forward sweep peak 2 is attributed to the

oxidation of Cu to Cu2O and the increase in current until the end of the forward sweep is due to

the oxidation of both Cu and Cu2O to a mixture of CuO and Cu(OH)2. On the reverse sweep peak

3 is attributed to the reduction of CuO and Cu(OH)2 and peak 4a to the reduction of Cu2O. Once

the poorly conducting Cu2O has been removed any remaining CuO material is reduced and gives

rise to peak 4b. This behaviour is consistent with previous reports on the electrochemical

behaviour of copper in alkaline solution [41, 42]. However there is a distinct difference observed

on all the porous copper surfaces compared to the copper foil and that is the presence of peaks in

the double layer region labelled 1a-c. This is consistent with the work of Burke et al. who

investigated copper electrodes that had been electrochemically activated to produce a highly

disrupted and defect rich surface [41]. This results in the facile oxidation of copper at specific

sites at potentials more negative than that for Cu2O formation which were later confirmed to be

Faradaic in nature by Bond et al [43]. It is also consistent with previous work from this laboratory

investigating the electrodeposition of porous copper on copper [40]. Therefore the

electrodeposition of copper under these experimental conditions of high overpotential and

hydrogen evolution promotes the formation active copper at all electrodes. However there is a

difference in the active site responses observed on both Pd and Au compared to Cu. It can be seen

that the onset potential for the active site responses (peaks 1a-c) and the formation of Cu2O is at

more negative potentials for the Au and Pd cases compared to Cu. This is likely to be a

combination of two processes; the surface is more active in the former cases and oxidation is

14

more facile and because Cu2O has already formed on Cu by a chemical process as evidenced by

the XRD data (Figure 4) some of the active sites have already been oxidised. Therefore there are

fewer active sites available for electrochemical oxidation. This observation may have significant

implications for the catalytic activity of these materials as Cu2O is generally a poorer catalyst

compared to metallic copper for many reactions [44]. It should be noted that due to the

dislodgement of copper from the GC substrate and the irreproducible nature of the deposit on this

surface, it was not investigated for its electrochemical or catalytic behaviour.

Figure 5: Cyclic voltammograms recorded in 1 M NaOH at a sweep rate of 50 mVs-1 for copper

electrodeposited from 0.4 M CuSO4 in 1.5M H2SO4 at – 3 A cm-2 for 15 s on Cu, Au and Pd

electrodes.

The electrocatalytic behaviour of these porous copper materials electrodeposited for 15 s

(Figure 2) was investigated with the HER. The most electrocatalytically active surface was for

copper deposited onto the Pd substrate (Figure 6). This demonstrates that a highly ordered and

open porous structure, as seen for copper deposited on copper (Figure 2A), does not guarantee the

most effective material. It should be noted that the data is normalised to the geometric area of the

substrate. An attempt to estimate the electrochemically active surface area via the charge

associated with thallium under potential deposition was severely hampered by the early onset of

15

the HER at these samples [45]. However looking at the charge associated with Cu2O formation

for porous copper (Figure 5) over the potential range of -0.5 to -0.3 V gives comparable values

for all the porous samples indicating that the surface area is very similar despite the significant

differences in morphology. In Figure 5 the onset potential for processes 1a-c and Cu/Cu2O occur

at more negative potentials for copper deposited on Au and Pd compared to Cu indicating a more

active surface as discussed previously [46, 47]. Therefore the presence of such metastable surface

atoms with high activity would significantly influence the HER as seen for the case of copper

deposited on Au and Pd.

Figure 6: Linear sweep voltammograms recorded at 10 mV s-1 in 1.5 M H2SO4 at porous copper

electrodeposited at -3 A cm-2 for 15 s from 0.4 M H2SO4 and 1.5 M H2SO4 onto Cu (1), Au (2)

and Pd (3) electrodes.

The underlying electrode may play some part in the activity seen for the HER, however

from side on SEM imaging it can be seen that these films that were electrodeposited for 15 s are

relatively thick (Figure 7). The thickness for Cu on Cu varies from 40 – 60 µm, Cu on Au from

50 – 60 µm and Cu on Pd from 30 – 50 µm. Notably, the underlying electrode is still exposed in

the case of Cu electrodeposited on Cu and due to the extensive dendrite formation is quite porous

(Figure 7A) however this becomes less so for Cu on Au where the film is denser (Figure 7B) and

16

for Cu on Pd (Figure 7C) the underlying Pd electrode is completely covered by quite a dense

layer in particular at the copper/palladium interface. Therefore it is expected that for Cu

electrodeposited on Pd that the HER is dominated by Cu and a contribution form the underlying

Pd electrode can be ruled out. This is also assumed for the following study of these materials for

their heterogeneous catalytic activity.

The heterogeneous catalytic activity of porous copper was investigated by monitoring the

reduction of ferricyanide ions with thiosulphate ions. The reaction can be written as

2[Fe(CN)6]3− + 2S2O32− → 2[Fe(CN)6]4− + S4O6

2− (1)

The reduction of ferricyanide is easily observed by UV-vis spectroscopy by monitoring an

absorbance peak at 420 nm [48, 49] which diminishes with time during the course of the reaction

(Figure 8a). This reaction is generally agreed to be under surface control where the reaction rate

is governed by the electron transfer between ferricyanide and thiosulphate ions through the

surface of the catalyst [48, 50, 51]. It should be noted that although this reaction is

thermodynamically favourable (when the standard reduction potential of the redox couples are

compared: S4O62-/2S2O3

2- E0 = 0.080 V and Fe(CN)63-/Fe(CN)6

4- E0 = 0.361 V vs SHE) it is

kinetically limited and the reaction only proceeds very slowly in the absence of a catalyst where

full conversion to ferrocyanide is still not achieved after several hours which was experimentally

verified. A typical reaction rate in the absence of a catalyst is reported to be 1.5 x 10-4 min-1 [52].

The reaction is assumed to be first order due to the large excess of thiosulphate used during the

reaction.

17

Figure 7: SEM images of copper electrodeposited from 0.4 M CuSO4 in 1.5M H2SO4 at – 3 A

cm-2 for 15 s on (A) Cu, (B) Au and (C) Pd.

Figure 8: Time dependent UV-vis spectra monitoring the catalytic reduction of 1 x 10-3 M

K3Fe(CN)6 with 0.1 M Na2S2O3 by (a) porous Cu electrodeposited on a Pd substrate and (b) plot

of ln(At/A0) versus Time for the same reaction for porous copper electrodeposited on Cu (■), Au

(●) and Pd (▲).

It can be seen that the highest reaction rate (Figure 8b), determined by taking the slope of

the linear part of the plot of ln(A0)/At) versus Time (after 6 mins) where A0 is the absorbance

measured at the beginning of the reaction and At the absorbance at time t, is achieved at Cu

which has been electrodeposited on to Pd (1.5 x 10-3 s-1). The sluggish reaction for the first 6

minutes in all cases is often referred to as an induction time. In previous work on heterogeneous

18

catalytic reaction of nanoparticles this induction time has been related to surface restructuring

effects, however the nature of this type of surface restructuring is unknown and still speculative

in nature. It has been suggested that it may be related to a shift of single atoms or to a concerted

rearrangement of surface atoms [53, 54]. The next highest reaction rate was achieved at Cu

electrodeposited on Cu (1.1 x 10-3 s-1) and finally Cu electrodeposited on Au (4.7 x 10-4 s-1). As

was the case for the HER the formation of a regular porous structure on the micron scale did not

guarantee the best performance. The presence of a rough and porous structure on the nanoscale

(Figure 2G) provides a better surface for both electrocatalytic and heterogeneous catalytic

reactions. This increased activity may again be related to the type of active sites present on the

surface as determined by the cyclic voltammetric measurements which showed that Cu

electrodeposited onto Pd is more reactive than the other samples facilitating the adsorption of

reactants on to the surface prior to electron transfer. In addition to the Pd electrode being totally

covered by Cu (Figure 7C) it should be noted that the reaction rate determined at an unmodified

Pd substrate was 5 x 10-5 s-1 and therefore would not be a factor in determining the overall

reaction rate. Also noteworthy is that complete conversion of ferricyanide to ferrocyanide is

achieved after 12 minutes for the best sample in a large solution volume of 24 ml where most

reports in the literature for this reaction are confined to a UV-visible cuvette. The fabrication of

such an active copper surface free from any significant oxide formation may have many more

applications in the area of heterogeneous catalysis given that it is easily recoverable from the

reactant solution.

4. Conclusion

The composition of the underlying electrode upon which copper is electrodeposited via the

dynamic hydrogen bubble templating approach significantly influences both the morphology and

activity of the final material. At Pd a poorly ordered surface is achieved on the micron scale but

19

porosity is still achieved on the nanoscale. At surfaces such as copper and gold the formation of

highly regular pores across the entire substrate can be achieved. However, at GC a templating

effect is not observed and poor adhesion to the surface results in the loss of material. It appears

that the nature of the initial hydrogen evolution from the surface in terms of bubble coalescence

and residence time affects the morphology throughout the deposition process. This work also

demonstrated that the formation of a highly regular porous structure on the micron scale,

typically observed for the hydrogen bubble templating method, is not necessary for the

fabrication of a functional copper surface as demonstrated by the HER and the reduction of

ferricyanide with thiosulphate ions where the highest activity was achieved at the rough and more

randomly oriented surface. The presence of active sites as probed by cyclic voltammetric

measurements suggest that it is the nature of the active sites on the surface which dictate the

material’s applicability as a catalyst and electrocatalyst.

Acknowledgements

AOM gratefully acknowledges funding through a Future Fellowship from the Australian

Research Council (FT110100760). The authors acknowledge the facilities, and the scientific and

technical assistance, of the Australian Microscopy and Microanalysis Research Facility at the

RMIT Microscopy & Microanalysis Facility.

References

[1] S. Murugesan, A. Akkineni, B.P. Chou, M.S. Glaz, D.A. Vanden Bout, K.J. Stevenson, ACS Nano, 7

(2013) 8199-8205.

[2] B.J. Plowman, A.P. O'Mullane, S.K. Bhargava, Faraday Discuss., 152 (2011) 43-62.

[3] J. Seo, D. Cha, K. Takanabe, J. Kubota, K. Domen, Phys. Chem. Chem. Phys., (2014).

20

[4] S.T. Bliznakov, M.B. Vukmirovic, L. Yang, E.A. Sutter, R.R. Adzic, J. Electrochem. Soc., 159 (2012)

F501-F506.

[5] K.W. Cho, H.S. Kwon, Catalysis Today, 120 (2007) 298-304.

[6] S. Shi, J.-Y. Hwang, Int. J. Hydrogen Energy, 32 (2007) 224-228.

[7] J. Wang, G. Ji, Y. Liu, M.A. Gondal, X. Chang, Catal. Commun., 46 (2014) 17-21.

[8] S. Wei, J. Shi, H. Ren, J. Li, Z. Shao, J. Mol. Catal. A, 378 (2013) 109-114.

[9] G. Wu, W. Zhai, F. Sun, W. Chen, Z. Pan, W. Li, Mater. Res. Bull., 47 (2012) 4026-4030.

[10] I. Najdovski, P.R. Selvakannan, A.P. O'Mullane, S.K. Bhargava, Chem. Eur. J., 17 (2011) 10058-

10063.

[11] P. Bartlett, J. Gardner, R. Whitaker, Sens. Act. A, 23 (1990) 911-914.

[12] B.J. Plowman, S.K. Bhargava, A.P. O'Mullane, Analyst, 136 (2011) 5107-5119.

[13] C.-J. Yang, F.-H. Lu, Langmuir, 29 (2013) 16025-16033.

[14] D.D. Yao, J.Z. Ou, K. Latham, S. Zhuiykov, A.P. O’Mullane, K. Kalantar-zadeh, Cryst. Growth Des.,

12 (2012) 1865-1870.

[15] Y.D. Gamburg, G. Zangari, Theory and Practice of Metal Electrodeposition, Springer New York,

2011.

[16] Y. Liu, Y. Liu, R. Mu, H. Yang, C. Shao, J. Zhang, Y. Lu, D. Shen, X. Fan, Semiconductor science

and technology, 20 (2005) 44.

[17] J. Lin-Cai, D. Pletcher, J. Electroanal. Chem. Interfac. Electrochem., 149 (1983) 237-247.

[18] X. Huang, Y. Li, H. Zhou, X. Zhong, X. Duan, Y. Huang, Chem. Eur. J., 18 (2012) 9505-9510.

[19] F. Ebrahimi, Z. Ahmed, Mater. Characterisation, 49 (2002) 373-379.

[20] N.D. Nikolic, K.I. Popov, L.J. Pavlovic, M.G. Pavlovic, Surf. Coat. Technol., 201 (2006) 560-566.

[21] H.-C. Shin, M. Liu, Chem. Mater., 16 (2004) 5460-5464.

[22] S. Cherevko, X. Xing, C.-H. Chung, Electrochem. Commun., 12 (2010) 467-470.

[23] A. Ott, L.A. Jones, S.K. Bhargava, Electrochem. Commun., 13 (2011) 1248-1251.

[24] B.J. Plowman, A.P. O'Mullane, P.R. Selvakannan, S.K. Bhargava, Chem. Commun., 46 (2010) 9182-

9184.

[25] G.-M. Yang, X. Chen, J. Li, Z. Guo, J.-H. Liu, X.-J. Huang, Electrochim. Acta, 56 (2011) 6771-6778.

21

[26] S. Cherevko, X. Xing, C.-H. Chung, Appl. Surf. Sc., 257 (2011) 8054-8061.

[27] H.C. Shin, M. Liu, Adv. Funct. Mater., 15 (2005) 582-586.

[28] I. Najdovski, P.R. Selvakannan, S.K. Bhargava, A.P. O'Mullane, Nanoscale, 4 (2012) 6298-6306.

[29] J. Liu, L. Cao, W. Huang, Z. Li, ACS Appl. Mater. Interf., 3 (2011) 3552-3558.

[30] J. Liu, L. Cao, Y. Xia, W. Huang, Z. Li, Int. J. Electrochem. Sci., 8 (2013) 9435-9441.

[31] R. Kim, D. Han, D. Nam, J. Kim, H. Kwon, J. Electrochem. Soc., 157 (2010) D269-D273.

[32] L.D. Burke, A.P. O'Mullane, V.E. Lodge, M.B. Mooney, J. Solid State Electrochem., 5 (2001) 319-

327.

[33] N.D. Nikolić, K.I. Popov, Hydrogen Co-deposition Effects on the Structure of Electrodeposited

Copper Electrodeposition, in: S.S. Djokic (Ed.) Modern Aspects of Electrochemistry, vol. 48, Springer

New York, 2010, pp. 1-70.

[34] J.-H. Kim, R.-H. Kim, H.-S. Kwon, Electrochem. Commun., 10 (2008) 1148-1151.

[35] W. Sheng, M. Myint, J.G. Chen, Y. Yan, En. Environ. Sci., 6 (2013) 1509-1512.

[36] U. Sim, T.-Y. Yang, J. Moon, J. An, J. Hwang, J.-H. Seo, J. Lee, K.Y. Kim, J. Lee, S. Han, B.H.

Hong, K.T. Nam, En. Environ. Sci., 6 (2013) 3658-3664.

[37] L.D. Burke, Gold Bull., 37 (2004) 125-135.

[38] L.D. Burke, L.M. Hurley, V.E. Lodge, M.B. Mooney, J. Solid State Electrochem., 5 (2001) 250-260.

[39] B.J. Plowman, M.R. Field, S.K. Bhargava, A.P. O’Mullane, ChemElectroChem, 1 (2013) 76-82.

[40] M. Mahajan, S.K. Bhargava, A.P. O’Mullane, Electrochim. Acta, 10 (2013)186-195.

[41] L.D. Burke, M.J.G. Ahern, T.G. Ryan, J. Electrochem. Soc., 137 (1990) 553-561.

[42] D. Reyter, M. Odziemkowski, D. Belanger, L. Roue, J. Electrochem. Soc., 154 (2007) K36-K44.

[43] M.J.A. Shiddiky, A.P. O’Mullane, J. Zhang, L.D. Burke, A.M. Bond, Langmuir, 27 (2011) 10302-

10311.

[44] R. Prucek, L. Kvitek, A. Panacek, L. Vancurova, J. Soukupova, D. Jancik, R. Zboril, J. Mater. Chem.,

19 (2009) 8463-8469.

[45] E. Norkus, A. Vaškelis, I. Stalnioniene, J. Solid State Electrochem., 4 (2000) 337-341.

[46] L.D. Burke, A.M. O'Connell, R. Sharna, C.A. Buckley, J. Appl. Electrochem., 36 (2006) 919-929.

[47] L.D. Burke, T.G. Ryan, J. Electrochem. Soc., 137 (1990) 1358-1364.

22

[48] D. Li, C. Sun, Y. Huang, J. Li, S. Chen, Science in China Series B: Chemistry, 48 (2005) 424-430.

[49] J.H. Tuttle, J.H. Schwartz, G.M. Whited, Appl. Environ. Microbiol., 46 (1983) 438-445.

[50] P.L. Freund, M. Spiro, J. Chem. Soc., Faraday Trans. 1, 82 (1986) 2277-2282.

[51] M. Mahajan, S.K. Bhargava, A.P. O'Mullane, RSC Advances, 3 (2013) 4440-4446.

[52] Y. Li, J. Petroski, M.A. El-Sayed, J. Phys. Chem. B, 104 (2000) 10956-10959.

[53] S. Wunder, F. Polzer, Y. Lu, Y. Mei, M. Ballauff, J. Phys. Chem. C, 114 (2010) 8814-8820.

[54] X. Zhou, W. Xu, G. Liu, D. Panda, P. Chen, J. Am. Chem. Soc., 132 (2009) 138-146.