9182 Chem. Commun., 2010, 46, 9182–9184 This journal is c The Royal Society of Chemistry 2010

Honeycomb nanogold networks with highly active sitesw

Blake J. Plowman, Anthony P. O’Mullane,* PR Selvakannan and Suresh K. Bhargava*

Received 7th September 2010, Accepted 11th October 2010

DOI: 10.1039/c0cc03696j

The formation of macroporous honeycomb gold using an

electrochemically generated hydrogen bubble template is

described. The synthesis procedure induces the formation of

highly active surfaces with enhanced electrocatalytic and

surface enhanced Raman scattering properties.

The fabrication of three dimensional porous materials has been

the subject of significant attention due to their wide-ranging

applications in areas such as sensing,1 electrocatalysis,2

superhydrophobic surfaces,3 and their use as surface

enhanced Raman scattering (SERS) substrates.4 Various

approaches to produce such materials have been employed

such as de-alloying,2,5 galvanic replacement6–8 and using a

variety of organic and hard physical templates.9–11 Often these

methods can be time consuming and complicated, and in the

latter case requires the removal of the template after the

synthesis. Recently, it has been reported that an

electrochemical method can be employed to generate in a

rapid and facile manner 3D copper and silver foam

structures. The method exploits the vigorous hydrogen

evolution that occurs at the electrode surface during the

course of the metal electrodeposition process; the formation

of bubbles both at the surface and on the growing deposit acts

as a template that allows for the formation of high surface area

macroporous foams.3,12–16 The advantage of this approach is

that it is rapid, clean and does not result in any contamination

of the metal surface with organic species.

However, to the best of the authors’ knowledge, it has not

been applied to the direct formation of highly porous

honeycomb Au. Indeed, one report claimed that this method

was not applicable to Au.17 Importantly, the applicability of a

Au honeycombmaterial is wide-ranging given the properties of

Au such as biocompatibility and high electrocatalytic and

SERS activity when in the nanostructured form. In this

work, we demonstrate that honeycomb Au structures are

achievable by this hydrogen bubble template method, and

notably that vigorous evolution of hydrogen results in the

formation of highly active metastable states as characterised by

significant Faradaic premonolayer oxide responses which

mediate important electrocatalytic reactions such as glucose

oxidation.

Honeycomb Au films can be achieved from a solution

containing 0.1 to 0.4 M KAuBr4 in aqueous 1.5 M H2SO4. A

150 nm Au substrate prepared by e-beam evaporation onto

quartz was used as the working electrode. In a 3 electrode setup

consisting of a Ag/AgCl reference electrode and a high purity

graphite rod as a counter electrode, Au was electrodeposisted

under constant current density conditions ranging from 1 to

3 A cm�2 for a duration of 5 to 30 s.

Illustrated in Fig. 1 are SEM images of a macroporous Au

structure electrodeposited from a solution containing 0.1 M

KAuBr4 in 1.5 M H2SO4 at a current density of 2 A cm�2 for

30 s. It should be noted that this concentration of H2SO4 was

required to facilitate vigorous hydrogen evolution. It can be

seen from the SEM images that a continuous network of

honeycomb like Au is formed on the substrate (Fig. 1a),

consisting of a layered type structure where the pore size

increases from the bottom to the top of the sample. This is

due to the coalescence of hydrogen bubbles that evolve from

the surface.3 The pore sizes range from 10 to 50 mm in diameter

and the internal wall structure (Fig. 2a) consists of long

dendrites with many branches and nodules where the

backbone tapers to a fine point of around 30 nm. A SEM

image taken at an angle of 451 (Fig. S1, ESIw) indicates clearlythe dendritic nature of the wall structure. The pores are

connected and cross linked, providing a structural integrity

conducive to washing, handling, characterisation and testing.

Fig. 1 SEM images of a honeycomb Au network (a) and a magnified

view of the structure (b). Sample was prepared by depositing at a

current density of 2 A cm�2 for 30 s from a 0.1 M KAuBr4 and 1.5 M

H2SO4 solution.

Fig. 2 (a) SEM image of honeycomb Au deposited at 2 A cm�2 for

30 s from a solution containing 0.1 M KAuBr4 and 1.5 M H2SO4. (b)

XRD of honeycomb Au deposited for 5 s (green), 10 s (blue) and 30 s

(red) at a current density of 2 A cm�2 in 0.1MKAuBr4 in 1.5MH2SO4

normalised to the intensity of the {111} crystal plane.

School of Applied Sciences, RMIT University, GPO Box 2476V,Melbourne, Australia. E-mail: anthony.omullane@rmit.edu.au,suresh.bhargava@rmit.edu.au; Fax: +61 3 99253747;Tel: +61 39925 9940w Electronic supplementary information (ESI) available: SEM imagesof honeycomb Au produced at different times and current density andfrom HAuCl4, additional XRD patterns. See DOI: 10.1039/c0cc03696j

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 9182–9184 9183

The effect of electrodeposition time, current density and Au

salt concentration on the macroporous structure was then

investigated and is illustrated by SEM images in Fig. S2–S4

(ESIw). Upon increasing the electrodeposition time from 5 to

30 s, at a concentration of 0.1 M for KAuBr4 in 1.5 M H2SO4,

and a current density of 2 A cm�2, a gradual increase in the

overall integrity of the macroporous structure can be seen (Fig. S2,

ESIw). The internal wall structure also changes from highly

branched fern like dendrites to more elongated rods with

smaller secondary nodules. The effect of applied current

density, while keeping the time constant at 10 s, is also

similar with a more interpenetrating and better formed

porous honeycomb structure being formed at the higher

current density of 3 A cm�2 (Fig. S3, ESIw). Finally,

increasing the KAuBr4 concentration to 0.4 M results in a

less well defined homogeneous macroporous honeycomb

morphology (Fig. S4, ESIw). Indeed large clumps of Au ca.

250 mm in diameter (Fig. S4b, ESIw) appear across the sample.

Overall, it appears there is a competing process between the

rate of Au electrodeposition and the evolution of hydrogen

bubbles from the evolving electrodeposit which needs to be

tuned for homogeneous formation of micron sized pores.

The XRD patterns for this type of material are shown in

Fig. 2b. It can be seen that the characteristic peaks due to fcc

Au, such as the {111}, {200}, {220}, {311} and {222} facets, are

present. However, when analysed as a function of deposition

time at constant current density an increase can be seen for the

relative ratio in intensity of the higher surface energy facets

{200}, {220} and {311} to the lower surface energy {111} facet.

This effect was also observed as a function of gold salt

concentration (Fig. S5a, ESIw) and current density (Fig. S5b,

ESIw). This enhancement may be due to the increased

elongated growth of the branches of the internal walls, where

such facets more readily occur at sharp edges, corners and

protuberances as reported recently for silver nanomaterials.18

In the synthesis of gold nanomaterials hexadecyltrimethy-

lammonium bromide (CTAB) is a routinely used growth

directing agent.19 Previous studies showed that Br� ions

adsorb strongly to specific crystalline faces of Au,20 and there-

fore during the electrodeposition process the liberated Br� ions

through the reaction, AuBr4�+3e�-Au0+ 4Br�, may also

influence the growth pattern of Au by preferential adsorption

on specific Au planes, resulting in the observed elongated

growth. Similar experiments were carried out using HAuCl4(Fig. S6, ESIw) where the internal wall structure consists of

shorter rods, which strongly indicates the role that Br� ions

play in the synthesis. The overall dendritic nature of the

honeycomb structure is most likely related to the

electrodeposition being carried out under non-equilibrium

conditions which favours the formation of dendrites.21,22

The presence of high surface energy facets such as the {311}

crystal plane in particular and a highly nanostructured

material may be significant for the electrocatalytic and SERS

properties of these materials.18,23 The electrochemical

behaviour of these Au honeycomb substrates was

characterised by cyclic voltammetry (CV) in 1 M H2SO4 and

1 M NaOH (Fig. 3). It can be seen for samples synthesised at

different times that a clear increase in electrochemically active

surface area, as noted by the increase in magnitude of the

monolayer oxide reduction peak at 0.90 V (Fig. 3a), is achieved

that is significantly higher than the unmodified substrate (a 168

fold increase at the 30 s electrodeposition time). When the

double layer region is analysed (�0.20 to 0.70 V) it can be seen

that there are oxidation responses at 0.08 and 0.36 V which are

at significantly lower potentials than the onset of Au2O3 oxide

formation at 1.20 V, which have been shown by Bond et al. to

be Faradaic in nature.24 Under alkaline conditions these

responses are more evident (distinct oxidation responses are

at �0.82, �0.39, �0.17, 0.03 and 0.15 V) as they have been

postulated by Burke to be due to Au*/Au hydrous oxide

transitions, where Au* is the active state of Au (Fig. 3b).

The enhanced and more reversible nature of the responses in

base are due to the greater stability of hydrous oxides in the

presence of a large concentration of OH� ions.24–26 These type

of responses have been observed for Au that has been activated

by prolonged severe cathodic polarisation in the hydrogen

evolution region (5–60 min),24,25,27–29 thermal30 and recently

by sonochemical pretreatment.31 In the former case the active

state is induced by the disruption of the outer layers of Au to

create low co-ordinated sites or clusters in a mechanism akin to

hydrogen embrittlement32 which are easier to oxidise than fully

lattice stabilised bulk Au atoms. Significantly, during the

synthesis method employed here such active sites are induced

in a much shorter time period of 5–30 s. Importantly, the

magnitude of these active state responses increases with

electrodeposition time. This may have significance given that

they have been postulated to mediate electrocatalytic oxidation

reactions.24,29

This hypothesis was then tested for glucose oxidation under

alkaline conditions (Fig. 3c). It can be seen that the current is

markedly higher than that achieved at the unmodified

Au surface and increases with the surface area of the sample.

The onset potential of �0.78 V coincides with that of a

Au*/hydrous oxide transition in the absence of glucose

(Fig. 3b). A significant further increase in the response is

Fig. 3 CVs of unmodified Au (black) and honeycomb Au deposited

for 5 s (green), 10 s (blue) and 30 s (red) in (a) 1 M H2SO4, (b) 1 M

NaOH and (c) 20 mM glucose in 1 M NaOH. (d) SERS spectra of

rhodamine B (Raman spectrum of the powder (brown)).

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9184 Chem. Commun., 2010, 46, 9182–9184 This journal is c The Royal Society of Chemistry 2010

seen at �0.40 V (oxidation to gluconate), which also coincides

with such a transition and supports this electrocatalytic

mechanism. However, when the data are normalised for

surface area it can be seen that there is some reduction in the

specific activity of the material. This may be related to over

oxidation of the active state at the highest surface area material

which competes with the electro-oxidation of glucose. This

highlights the role that not only surface area but also the

tendency of the surface towards facile oxidation plays in

electrocatalytic reactions, as discussed in detail previously.26

This may also have significant impact on the application of Cu

and Ag materials fabricated in this way.21,22

It has been shown previously that anisotropic Au is an

extremely effective substrate material for SERS.23,33–35 The

honeycomb structure presented here shows considerable

anisotropic growth within the wall structure and was

therefore tested for SERS activity. Illustrated in Fig. 3d is

the Raman signal of Rhodamine B powder and the SERS

signal for Rhodamine B on an unmodified and honeycomb Au

substrates prepared by immersing the sample in an aqueous 1

mMRhodamine B solution for 1 h and then washed with water

to remove any non-adsorbed dye. It can be seen that there is no

observable signal at an unmodified Au substrate but a

significant enhancement (enhancement factor of 104

conservatively estimated for the sample prepared at 5 s—see

ESIw) is observed at the Au honeycomb substrate. The sharp

tips at the end of the dendritic branches and secondary nodules

growing out from the main trunk of each dendrite provide

many sites for local electromagnetic field enhancement or the

lightning rod effect36 to occur. Theoretical studies show that

materials with sharp tips may have a localised electric field

value as high as 500 times that of the applied field.35 It is also

known that nanometre sized gaps provide very large

enhancements in the electromagnetic field, generating

significantly improved SERS signals.37 The extensive

crossover of dendritic branches throughout this material may

provide such sites where the dye is immobilised in nanometre

gaps between these branches, thereby contributing to the SERS

signal. This effect was reported recently for honeycomb Au

fabricated by angle resolved nanosphere lithography.38 The

SERS signal intensity from all samples was significant which

indicates that it is the internal wall structure and overlap of

nanometre sized features, rather than the overall macroporous

structure, which is mainly responsible for the enhancement.

The signal intensity was also homogeneous with a relative

standard of deviation of 10%.

In summary, a rapid method for the direct production of

macroporous honeycomb Au using an electrochemically

induced hydrogen bubble template has been described. The

presence of high surface energy crystal facets and active sites as

indicated by extensive hydrous oxide formation controls the

electrocatalytic activity while the formation of an anisotropic

internal wall structure with protruding sharp tips facilitates

good SERS activity. The significant presence of highly active

sites may also play a role in the application of other metals like

Ag and Cu fabricated in this way.

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