C2JM31485A 11950..11956 - c2jm31485a.pdf

Highly ordered tailored three-dimensional hierarchical nano/microporous

gold–carbon architectures

Sirilak Sattayasamitsathit,

Aoife M. O’Mahony,

Xiaoyin Xiao,

Susan M. Brozik,

Cody M. Washburn,

David R. Wheeler,

Wei Gao,

Shelley Minteer,

Jennifer Cha,

D. Bruce Burckel,

Ronen Polsky

and Joseph Wang

Received 10th March 2012, Accepted 13th April 2012

DOI: 10.1039/c2jm31485a

The preparation and characterization of three-dimensional hierarchical architectures, consisting of

monolithic nanoporous gold or silver films formed on highly ordered 3D microporous carbon supports,

are described. The formation of these nano/microporous structures involves the electrodeposition or

sputtering of metal alloys onto the lithographically patterned multi-layered microporous carbon,

followed by preferential chemical dealloying of the less noble component. The resulting hierarchical

structure displays a highly developed 3D interconnected network of micropores with a nanoporous

metal coating. Tailoring the nanoporosity of the metal films and the diameter of the large micropores

has been accomplished by systematically changing the alloy compositions

via

control of the deposition

potential, plating solution and coarsening time. SEM imaging illustrates the formation of unique

biomimetic nanocoral- or nanocauliflower-like self-supporting structures, depending on the specific

preparation conditions. The new 3D hierarchical nano/microporous architectures allow for enhanced

mass transport and catalytic activity compared to common nanoporous films prepared on planar

substrates. The functionality of this new carbon–gold hierarchical structure is illustrated for the greatly

enhanced performance of enzymatic biofuel cells where a substantially higher power output is observed

compared to the bare microporous carbon substrate.

1. Introduction

Micro- and nanoporous structures have attracted considerable

recent interest due to their remarkable chemical/physical prop-

erties.

1–4

In particular, three-dimensional (3D) structures of

micro- and nanoscale materials have received enormous atten-

tion owing to a wide range of potential applications, including

energy storage,

plasmonics,

sensing

7,8

and catalysis.

9,10

Here we describe the first example of a 3D hierarchical porous

carbon–gold architecture based on a judicious combination of

micro- and nanostructures: new interferometric lithographically

fabricated 3D microporous carbon

11,12

and nanoporous Au or

Ag films.

3,4,13

Within this new hierarchical architecture, we can

tailor both the microporosity of the carbon substrate (indirectly

by varying the thickness of the alloy film) and the nanoporosity

of the porous gold film (by modifying the deposition and deal-

loying conditions). Such fine tuning of both the micro/

submicrometre-sized building blocks and the nanometre-sized

assemblies of the resulting hierarchical architectures leads to

great versatility towards diverse applications.

The lithographically fabricated 3D porous materials (used in

the present work and developed recently at Sandia National

Laboratory) display an attractive highly ordered open-porous

carbon microstructure to support the nanoporous metal

films.

11,12

The open-cell morphology and structural regularity of

these multilayered microporous carbons offer many structural

and hydrodynamic advantages such as a facile support for

catalytic nanoparticles and diffusion profiles that lead to

enhanced internal mass transport within the structures.

Coating this 3D highly periodic microporous carbon substrate

with nanoporous metal layers, through electrodeposition of

alloys and dealloying these metal films, provides a high-area

catalytic metal surface with nanoscale pore structures and high

surface-to-volume ratios.

Such nanoporous metal layers are commonly prepared on 2D

substrates

via

electrodeposition of a single-phase two-component

alloy M1

followed by electrochemical or chemical

etching of the less noble component.

3,4,13,14

Ding and Erlebacher

combined nano- and micro-porous Au films using a two-

dimensional substrate. The present work, in contrast, aims at

producing a 3D hierarchical structure of micropores and

Department of Nanoengineering, University of California, San DiegoLa

Jolla, CA, 92093 USA. E-mail: josephwang@ucsd.edu

Sandia National Laboratories, Department of Biosensors and

Nanomaterials, PO Box 5800, Albuquerque, NM 87185, USA. E-mail:

rpolsky@sandia.gov

Department of Chemistry and Materials Science and Engineering,

University of Utah, Salt Lake City, UT 84112, USA

11950 |

J.Mater.Chem.

, 2012,

, 11950–11956

This journal is

ª

The Royal Society of Chemistry 2012

Dynamic Article Links

Journal of

Materials Chemistry

Cite this:

J.Mater.Chem.

, 2012,

, 11950

www.rsc.org/materials

PAPER

Published on 18 April 2012. Downloaded by California Institute of Technology on 24/01/2018 22:37:14.

View Article Online

/ Journal Homepage

/ Table of Contents for this issue

nanopores with well-defined repeated micro-periodicity by

preparing a nanoporous gold film over the lithographically

fabricated multi-layered microporous carbon substrate.

The preparation of the new nano/micro-hierarchical metal–

carbon architectures relies on the electrodeposition or sputtering

of a binary alloy, Au–Ag or Ag–Al, onto the microporous

carbon support followed by preferential etching of the Ag or Al

sacrificial metal, respectively (Fig. 1). Control of the micropo-

rosity is achieved through different rates of the gold-reordering

during the etching and coarsening step for different levels of the

sacrificial metal. Consequently, alloy segments with a lower gold

content lead to thinner nanoporous layers and thus to larger

micropores of the overall scaffold. The combination of electro-

deposition of the alloy onto the 3D microporous carbon with the

preferential dealloying processes thus results in an attractive

route for preparing tailor-made highly ordered hierarchical

architectures, resembling nanocoral- or nanocauliflower-like

structures. Such new structures combine the attractive structural

regularity of lithographically fabricated 3D porous carbon

materials with the high surface-to-volume ratios of nanoscale

pore metal structures, and allow for high mass transport access

and catalytic activity for a diverse range of applications, ranging

from fuel cells to chemical sensors. These attractive features and

advantages are described below using a biofuel cell constructed

with the new hierarchical structure compared to the one based on

the microporous carbon surface.

2. Experimental section

2.1 3D porous substrate preparation

A porous carbon substrate with 3D nanostructure was obtained

from Sandia National Lab (Sandia National Laboratories,

Albuquerque, NM). 3D porous carbon electrodes were rinsed

with isopropanol (Fisher Scientific, Fair Lawn, NJ) and ultra-

pure water followed by drying under N

stream. These porous

carbon substrates were coated with Ti (20 nm) and Au layer

(30 nm) using an E-beam evaporator (Temescal BJD 1800 E-

beam Evaporator) as an adhesive layer for the nanoporous Au

coating and were used as a working electrode for all experiments.

2.2 Nanoporous Au synthesis

The nanoporous Au film was synthesized employing an electro-

deposition and sputtering technique. The mixtures of gold and

silver plating solution (Orotemp 24 RTU RACK and 1025

RTU@4.5 Troy/Gallon) (85/15 ratio) were used for electrodepo-

sition processes. The three deposition potentials were

demonstrated (

0.5 V,

0.75 V and

1.0 V) and two deposition

times were investigated (120 and 300 s). The potential was

controlled by a

m

-Autolab type II (Eco Chemie, Utrecht, The

Netherlands) and a conventional three-electrode electrochemical

cell. A platinum wire was used as a counter-electrode and Ag/AgCl

(in 1 M KCl) was used as a reference electrode. The ability to tailor

nanoporous structure using different plating ratios (5/5, 6/4, 7/3

and 8/2) and constant deposition potential (

0.9 V, 300 s) was also

employed. All plating solution in this work was diluted with

ultrapure water in the ratio of 1 : 1 before use. The silver compo-

nent was etched (dealloyed) using HNO

(70%) for 15 min and the

porous structure was thoroughly rinsed with ultrapure water. To

generate nanoporous Au using the sputtering technique, the

Denton Discovery 18 was used. The compositions of alloys were

co-sputtered onto the PC substrate using different Au and Ag

target gun power settings. DC power for Au was 100 and 150 W,

RF power for Ag was 400 and 350 W, respectively. The sputter was

performed at room temperature under vacuum of 5

Torr

and flow Ar to 3.6 mT. Au shutter was opened 30 s before Ag

shutter. The rotation speed was 65. The sputter time was 3 min.

2.3 Nanoporous Ag synthesis

The ability to generate nanoporous Ag was also presented here

using the sputtering technique. The fabrication processes were

carried out at room temperature under a vacuum of 5

and flow Ar to 3 mT. The Ag shutter was opened 100 s before the

Al shutter. The sputter time was 20 min. The DC power for Al

was 120 W and RF power for Ag was 50 W. The film was etched

in H

(1 M, 2 h).

2.4 Characterization

Scanning electron microscopy (SEM) images were taken using

a Phillips XL30 ESEM instrument using an acceleration potential

of 20 kV. Metal compositions were qualified by Oxford EDX

attached to the SEM instrument and operated by Inca software.

The electrochemicalactivesurface areaofthe nanoporousgoldwas

determined from cyclic voltammogram by the scanning nano-

porous gold electrode in a N

-saturated H

(0.1 M) (Fisher

Scientific, Fair Lawn, NJ) at a scan rate of 50 mV s

to measure the

chargeintegrationunderoxidereductionpeakandcomparedtothe

electrochemical active surface area of the alloy one.

2.5 Biofuel cell construction

The nanoporous gold-modified 3D porous carbon employed as

a bioanode was prepared by depositing the Au–Ag alloy using

Fig. 1

Creation of bicontinuous nanoporous surfaces along with a large open-pore macrostructure on highly ordered 3D porous carbon using bimetal

alloy deposition followed by chemical etching of less noble metal.

This journal is

ª

The Royal Society of Chemistry 2012

J.Mater.Chem.

, 2012,

, 11950–11956 | 11951

Published on 18 April 2012. Downloaded by California Institute of Technology on 24/01/2018 22:37:14.

View Article Online

a deposition potential of

0.75 V for 3 min and an 85/15 (Au/Ag)

ratio. The film was then dealloyed in 70% HNO

for a time of 15

min. The Meldola’s blue-glucose dehydrogenase (MDB-GDH)-

modified anode was fabricated by immersing the nanoporous

Au-3D PC into a MDB solution for 30 min, followed by casting

4.5

m

L of GDH (2 U

m

) mixed with 1% BSA (the mixed ratio

2 : 1) on the electrode, and dropping 6

m

L of 40 mM glutaral-

dehyde. The biofuel cell cathode was prepared by electro-

depositing Pt nanoparticles onto a Pt disk electrode from

a plating solution containing 4 mM H

PtCl

in 0.5 M H

using a deposition potential of

0.3 V for 180 s. Phosphate

buffer (0.1 M, pH 7.0) containing 20 mM NAD

was used as an

electrolyte and glucose (30 mM) was used as a fuel.

3. Results and discussion

3.1 Preparation of nanoporous metal decorated 3D

microporous carbon substrate

Initial attempts to prepare hierarchical structures involved the

creation of a nanoporous silver film on the multilayered micro-

porous carbon through sputtering of a binary Ag–Al alloy

followed by the dealloying the Al using 1 M H

for 2 h. The

formation of the pores during the dealloying process involved

etching of the less noble metal causing surface diffusion of the

nobler element along the alloy–electrolyte interface.

13–17

Fig. 2 displays scanning electron microscopy (SEM) images of

the preparation of the nanoporous silver onto a lithographically

patterned highly ordered porous carbon. Fig. 2A shows the bare

porous carbon substrate before depositing the Ag–Al alloy. The

pore diameter is noted to be 890 nm. The corresponding image

after sputtering the alloy is shown in Fig. 2B. This SEM image

indicates that the coverage of the Ag–Al sputtered film over the

carbon surface is relatively homogeneous, resulting in a pore

diameter of 700 nm. EDX analysis indicates Ag and Al atomic

contents of 35% and 65%, respectively. Cross-section images

from previous studies on silver sputtered films onto the same

substrates showed that the sputtering process is not simply line-

of-sight and produces coatings on undersides as well, most likely

due to the open nature of the structures.

Fig. 2C shows the

image of the nanoporous silver film after etching the aluminium,

and displays a maze-like morphology of nanopores and liga-

ments, characteristic of the dealloying process. The Al dissolu-

tion leaves behind a nanoporous Ag with interconnected pore

characteristics, closely resembling natural sea coral. The result-

ing nanoporous Ag film has an average pore size of about 32 nm

and ligament widths of 70 nm. The overall structure has

a micropore diameter of 790 nm. Such an increase in the

micropore diameter, from the alloy film (700 nm) to the deal-

loyed film (790 nm), reflects the variation of microstructure

porosity through the Ag-reordering process. Overall, these

examinations reveal that an ultra-rough Ag nanocoral structure

can be prepared from an aluminium-rich Ag–Al alloy film and

could be extended to different Ag–Al alloy ratio films to tune the

nanoporosity, thickness and ligament size.

Subsequent studies focused on the formation of nano- and

micro-pores by coated nanoporous gold onto the 3D macro-

porous carbon through vapor deposition as well as co-electro-

deposition of a binary Au–Ag alloy. The formation of the Au–Ag

alloy films over the 3D substrate penetrated into 3 layers of the

microporous carbon, as was observed previously using this

technique,

unlike electrodeposition of Pt nanoparticles from

previous studies.

After dealloying in 70% HNO

for 15 min

silver undergoes depletion not only on the surface but at

appreciable depths within the film, leading to the reordering/

coarsening of the Au adatoms into island growth and nucle-

ation.

4,13–17

The sponge-like morphology of the resulting nano-

porous gold consists of interconnecting ligaments with diameters

on the order of tens of nanometres (exact dimensions discussed in

next sections). A nanoporous Au structure of uniform pore size

and distribution, closely resembling the highly rough

morphology of cauliflower, is thus observed. The film thickness

for each deposition method was measured before and after Ag

dissolution. This thickness does not change significantly after the

dealloying process. Hence, a negligible variation in the micro-

porosity between the alloy and dealloy films is also observed,

with a small (3%) increase in diameter for the sputtered sample

and a 2% increase for the electrochemically deposited sample.

3.2 Tailoring nano/microporosity

Due to self-diffusion and rearrangement of Au atoms at the

electrolyte–metal interface,

13,14,16,20,21

the Au ligaments were

tailored through control of the dealloying time. Fig. 3 displays

SEM images of the nanoporous Au on the 3D porous carbon

obtained from electrodeposition of Au/Ag (85/15) plating solu-

tion and deposited at

0.75 V for 180 s followed by dealloying in

70% HNO

. Increasing the dealloying times from (A) 15 min to

(B) 15 h and (C) 30 h allows a significant increase in coarsening of

the Au adatoms, resulting in enlargement of the Au ligament

from 10 nm to 40 nm.

Fig. 2

Creation of nanoporous surfaces on a large open-micropore structure using sputtering of an Ag–Al alloy film followed by chemically dealloying

of the Al component to transform dense film to bicontinuous nanoporous film.

Conditions

: Sputtering time 20 min for Ag–Al alloy film and dealloying

using 1 M H

for 2 h. (A) Porous carbon, (B) Ag–Al alloy film and (C) porous Ag on 3D porous carbon.

11952 |

J.Mater.Chem.

, 2012,

, 11950–11956

This journal is

ª

The Royal Society of Chemistry 2012

Published on 18 April 2012. Downloaded by California Institute of Technology on 24/01/2018 22:37:14.

View Article Online

Fig. 4 displays close-up images of the morphology of the

nanoporous gold on the 3D macroporous carbon substrate,

prepared from both sputtering and electrodeposition techniques.

Sputtering of the Au–Ag alloy film (10 min) followed by deal-

loying leads to a nanoporous Au film, outlined in Fig. 4A.

Such sputtering conditions offer unimodal pore sizes in the

range of 10–20 nm and ligament networks with widths in the range

of 15–30 nm. Fig. 4B displays SEM images of nanoporous Au

prepared by a 5 min electrodeposition at

0.75 V from the mixture

of Au and Ag plating solution at the ratio of 85/15, and dealloying

the Ag using 70% HNO

for 15 min. Such an image shows

a sponge-like nanoporous structure with unimodal pore size in the

range of 7–20 nm and ligament network with ligament widths in

the range of 10–20 nm, resembling a nanocoral-like structure.

In this work, however, the electrochemically deposited alloy

displays a rougher morphology with higher porosity than that of

the sputtered substrate which should be more favorable for many

applications. The higher density of pores and interconnectivity

can be attributed to a more homogeneous layer of alloy formed

during electrodeposition. In the following sections we will discuss

the influence of different Au/Ag ratios, electrochemical deposi-

tion potentials, film shrinkage after the dealloying step and

characterization of the morphology of each substrate for

a variety of experimental conditions.

The nanoporosity of bicontinuous metal films over the 3D

porous carbon substrate can be tailored through the variation of

the deposition potential that leads to the formation of alloy films

with different metal compositions. The ability to control metal

contents in the alloy film was investigated and the results are

illustrated in Fig. 5. The experiments were carried out over

a potential range of

0.5 to

1.25 V using a 3 min deposition

time and an Au–Ag mixture (ratio of 85/15). The different

reduction rates of silver and gold at different deposition poten-

tials lead to varying Au and Ag compositions

13,14,16,20,21

and hence

a porous film with varying pore sizes and film thicknesses. The

alloy film and the pore morphology at plating potentials of

0.5,

0.75 and

1.0 V were characterized with SEM (data not shown

here) and EDX analysis (shown in Fig. 5D). The resulting EDX

analysis indicated that the alloy film contained 7.5%, 28% and

40% atomic gold for deposition potentials of

0.5,

0.75 and

1.0 V, respectively.

After the dealloying, wide open pores with thin network

connections are observed for a deposition potential of

0.5 V,

shown in Fig. 5A. The decrease in the film thickness, from

300 nm for the alloy film to 100 nm for the dealloyed porous film,

is noteworthy, and is attributed to the high Ag content of up to

91 atomic%, and the concomitant coarsening of the Au adatoms

into a thin porous film. This has the effect to increase the

micropore diameter from 650 nm (alloy) to 850 nm (dealloy).

Pore sizes in the range of 13–40 nm and ligament sizes in the

range of 4–12 nm are observed. A high relative range in pore size

as well as a large maximum diameter and small ligament size are

observed for this deposition potential.

The corresponding dealloyed nanoporous structure at

0.75 V

is displayed in Fig. 5B. The morphology shows a much more

highly ordered porosity than that observed in Fig. 5A, and is

more characteristic of the sponge-like nanoporous gold. A

minimal change in the film thickness (and thus the micropore

diameter) is observed before and after the dealloying step,

reflecting a higher percentage of Au in the mixture. A coral-like

homogeneous structure was achieved with pore sizes in the range

of 7–20 nm and ligaments sizes in the range of 10–20 nm. This

reflects the larger extent of gold co-deposition at more negative

potentials associated with the different standard reduction

potentials of the silver and gold plating solutions. Finally, the

image in Fig. 5C shows the morphology of the dealloy film at

a deposition potential of

1.0 V which corresponds to 40 atomic

% Au (EDX analysis, Fig. 5D).

The pore formation is less dense with smaller sizes than those

previously observed in Fig. 5A and B, yielding pore sizes in the

range of 4–8 nm and ligaments sizes in the range of 12–32 nm.

However, solid crystalline features still remain on the film surface

and thus little nanoporosity is observed. The ligament size varies

substantially due to the presence of solid crystals and thus liga-

ment widths calculated are not reflective of an actual structure

with high porosity. As with Fig. 5B, a minimal change in the film

Fig. 3

SEM images of nanoporous Au on 3D porous carbon at different dealloying times, (A) 15 min, (B) 15 h and (C) 30 h, using Au

–Ag

plating

solution and deposited at

0.75 V for 180 s.

Fig. 4

Creation of nanoporous Au surfaces using (A) sputtering and (B)

electrodeposition of bimetallic alloy (Au–Ag) film on highly ordered 3D

porous carbon followed by chemically dealloying less noble metal (Ag).

This journal is

ª

The Royal Society of Chemistry 2012

J.Mater.Chem.

, 2012,

, 11950–11956 | 11953

Published on 18 April 2012. Downloaded by California Institute of Technology on 24/01/2018 22:37:14.

View Article Online

thickness and thus of the micropore diameter is observed due to

the higher percentage of Au present in the alloy compared to that

in Fig. 5A. These images show that control of ligament size can

be achieved by increasing the percentages of gold deposited onto

the carbon surface which is achieved by varying the metal

precursor concentration. The atomic% Au is always lower than

Ag due to the more negative reduction potential of the Au

compared to that of Ag. The increase in the Au content (

i.e.

lower Ag content) at higher deposition potentials is clearly

reflected through a significant decrease in porosity attributed to

a lower need for reordering of Au during the dealloying step.

Tailoring nano/microporosity with alloy ratios of the plating

solution which allows tuneable morphology was also investi-

gated. Here, nanoporous Au films of varying thicknesses were

obtained from Au–Ag solution mixtures with different ratios

(using a potential of

0.9 V for 5 min) 6/4, 7/3, and 8/2. However,

the pore sizes and distributions of nanoporous Au are noticeably

unchanged at approximately 7–20 nm for each sample and

ligament sizes were 10–20 nm. The nanoporous structure itself

retains its nanocoral-like features, thus retaining the remarkably

high surface-to-volume ratios and self-supporting characteristics

(SEM images not shown here). The EDX analysis of alloy films

displays the atomic percentages of Au

,Au

, and

. Interconnected nanoporous Au can be fabricated

from Ag-rich film having Ag content about 60–90% (10–40

atomic% Au). Outside of these ranges (>40% and <10% Au) the

sponge-like nature of the nanoporous film was not observed.

3.3 Electrochemical characterization of nanoporous gold

electrodes for varying deposition potentials

Cyclic voltammetry was used to characterize the electrochemi-

cally active surface area (ECSA) of nanoporous gold structures

prepared at different electrochemical deposition potentials, as

well as for a nanoporous gold substrate formed by co-sputtering.

In these experiments, the surface area of the nanoporous gold

electrodes was examined in 0.1 M H

solution. The CV was

scanned over a potential range of 0.0 to 1.5 V (

vs.

Ag/AgCl) at

a scan rate of 50 mV s

. The resulting cyclic voltammograms,

shown in Fig. 6, are characteristic of the oxidation of gold in this

acidic solution, whereby an oxidation signal is observed for the

anodic scan due to the formation of gold oxide and a reduction

signal for gold oxide is observed at a potential of

0.75 V on the

reverse scan. This characteristic signal was observed for the

following nanoporous Au electrode surfaces prepared from: (a)

sputtering using gun power 100 W, and electrodeposited at

potentials (b)

0.5 V, (c)

0.75 V and (d)

1.0 V. It is clear that

the magnitude of the Au oxide reduction signal increases with

increasingly negative deposition potentials. The increase in this

signal is attributed to the increase in the overall electrochemically

active surface area of the electrode. The ECSA was calculated by

integrating the charge under the reduction signal using a GPES

software and assuming a specific charge of 386

m

Ccm

for Au

oxide reduction.

7,22

The inset of Fig. 6 displays ECSA of nano-

porous Au under different prepared conditions. The increasing

value of the Au oxide reduction signal in Fig. 6, upon changing

the deposition potential from

0.5 V to

0.75 V and then to

1.0 V, is expected. The film thickness for the latter two elec-

trode surfaces was greater than that of the former, in accordance

with the presence of substantially more surface area. The increase

in ECSA from a deposition potential of

0.75 V to

1.0 V is also

noteworthy since the sample deposited at

0.75 V shows clearly

more porous features to that deposited at

1.0 V. However, the

sub-surface porosity is not clearly visible from the SEM image

along with EDX (Fig. 5) and CV shows that the Au loading at

1.0 V is higher than at

0.75 V. The ECSA value for the

sputtered substrate is 2 cm

and it is in between that of the

electrochemically deposited samples at

0.5 and

0.75 V. The

porosity of the sputtered substrate shows a much more crystal-

line morphology in many areas of the sample, with an overall less

exposed active area of the gold.

The ESCA values increase from 0.3, 4.5 and 13.0 cm

a deposition potential of

0.5,

0.75 and

1.0 V. This outlines

Fig. 5

SEM images of a nanoporous Au film on 3D porous carbon using Au

–Ag

plating solution deposited for 180 s at different deposition

potentials: (A)

0.5 V, (B)

0.75 V, (C)

1.0 V, dealloying 15 min in 70% HNO

, and (D) EDX analysis showing metal compositions in the alloy film at

different deposition potentials.

Fig. 6

CV of nanoporous gold on a 3D porous carbon electrode in 0.1

with a scan rate of 50 mV s

; (a) Au-modified 3D-porous

carbon using the sputtering technique compared with nanoporous Au

prepared using the electrodeposition technique at deposition potentials of

(b)

0.5 V, (c)

0.75 V and (d)

1.0 V; inset displays ECSA of nano-

porous Au at different prepared conditions.

11954 |

J.Mater.Chem.

, 2012,

, 11950–11956

This journal is

ª

The Royal Society of Chemistry 2012

Published on 18 April 2012. Downloaded by California Institute of Technology on 24/01/2018 22:37:14.

View Article Online

the major advantage of the incorporation of a highly developed

three-dimensionally interconnected porous network of micro-

and nano-pore films, compared to the two-dimensional flat

polycrystalline electrode. The largest enhancement of the nano-

porous gold is observed for a deposition potential of

1.0 V,

which shows a 108-fold enhancement over the planar Au disk

electrode (ESCA 0.12 cm

). The large utilization of Au in the

other samples is attributed to the synergy effect of nanoporous

gold on the microporous carbon displaying the extremely high

surface-to-volume ratios that can be achieved using the deal-

loying method of a binary metal mixture.

3.4 Biofuel cell application

The functionality and advantages of the new 3D hierarchical

nano/microporous architectures were illustrated for a greatly

enhanced performance of an enzymatic biofuel cell compared to

the bare 3D carbon macroporous carbon. For this purpose we

constructed for the first time a biofuel cell system employing

hierarchical nanoporous Au-modified microporous carbon

structures by coating it with a Meldola’s blue-glucose dehydro-

genase (MDB-GDH) layer. First, MDB was coated onto a 3D

hierarchical nano/microporous substrate, followed by immobi-

lization of GDH using glutaraldehyde as a cross-linker. The

resulting 3D bioelectrocatalytic electrode provides both nano-

pores to support the biocatalytic sites and micropores to support

effective mass transport of the glucose fuel. As a result, and as

illustrated in Fig. 7, the new 3D bioelectrocatalytic nano/

microporous anode, coupled to the Pt nanoparticle-modified Pt

disk cathode, provided a high power density output of 45

m

, with the open circuit voltage of 0.46 V, which is 10 times

larger than the power observed at the MDB-modified 3D

microporous carbon surface (4

m

Wcm

). This new 3D hierar-

chical nano/microporous architecture can be extended for a wide

range of applications in sensor technology to energy

applications.

4. Conclusions

In conclusion, we have demonstrated the formation of hierar-

chical nanoporous Au and Ag structures on a microporous

lithographically defined 3D multilayered carbon scaffold from

the deposition of a binary alloy (

e.g.

, Au–Ag and Ag–Al)

followed by etching of the Ag and Al, respectively. The

dissolution and reordering processes result in the formation of

nanopores in combination with microporous substrates with

unique surface morphologies, leading to highly nodular cauli-

flower- and coral-like nanostructures. We have illustrated the

ability to tailor the thickness and porosity of monolithic

nanoporous gold along with the control of the microporous

architecture through controlled electroplating conditions

i.e.

deposition potentials and plating solutions. Such a fine control

of the physical characteristics of the resulting nanoporous gold

film reflects the different extents of gold-reordering during the

coarsening process. The deposition of such variable nano-

porous films onto the microporous carbon substrate combines

the high surface-to-volume ratio and favorable catalytic char-

acteristics of nanoporous gold with the open-cell morphology

of carbon foams to offer many structural and hydrodynamic

advantages compared to common nanoporous metal structures

on planar surfaces. Finally, we demonstrated the enhanced

functionality of the new 3D hierarchical nano/microporous

architectures toward biofuel cell systems, where the new 3D

bioelectrocatalytic anode provided a substantially higher power

density output (45

m

Wcm

) compared to the corresponding

3D microporous carbon substrate. These bioinspired hierar-

chical coral- and cauliflower-like nanostructures hold also

considerable promise for Surface-Enhanced Raman Spectros-

copy (SERS)

chemical and biological sensing, as well as to

future electronic and energy-storage devices.

Acknowledgements

This work was supported by the Laboratory Directed Research

and Development program at Sandia National Laboratories and

the National Science Foundation (Award Number CHE-

1057562). A.O’ M. was partially supported by DOE BES

DE-SC0004937.

Notes and references

1 J. E. G. J. Wijnhoven, S. J. M. Zevenhuizen, M. A. Hendriks,

D. Vanmaekelbergh, J. J. Kelly and W. L. Vos,

Adv. Mater.

, 2000,

, 888.

2 J. Zhang, Y. Li, X. Zhang and B. Yang,

Adv. Mater.

, 2010,

, 4249.

3 J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov and K. Sleradzki,

Nature

, 2001,

410

, 450.

4 Y. Ding and J. Erlebacher,

J. Am. Chem. Soc.

, 2003,

125

, 7772.

5 Y. Yu, L. Gu, X. Lang, C. Zhu, T. Fujita, M. W. Chen and J. Maier,

Adv. Mater.

, 2011,

, 2443.

6 S. Ahl, P. J. Cameron, J. Liu, W. Knoll, J. Erlebacher and F. Yu,

Plasmonics

, 2008,

, 13.

) S. Ben-Ali, D. A. Cook, S. A. G. Evans, A. Thienpont,

P. N. Bartlett and A. Kuhn,

Electrochem. Commun.

, 2003,

, 747;

(

) S. Campuzano and J. Wang,

Electroanalysis

, 2011,

, 1289.

8 R. Szamocki, S. Reculusa, S. Ravaine, P. N. Bartlett, A. Kuhn and

R. Hempelmann,

Angew. Chem., Int. Ed.

, 2006,

, 1317.

9 A. Wittstock, V. Zielasek, J. Biener, C. M. Friend and M. B

€

aumer,

Science

, 2010,

327

, 319.

10 V. Zielasek, B. J

€

urgens, C. Schulz, J. Biener, M. M. Biener,

A. V. Hamza and M. B

€

aumer,

Angew. Chem., Int. Ed.

, 2006,

8241.

11 D. B. Burckel, C. M. Washburn, A. K. Raub, S. R. J. Brueck,

D. R. Wheeler, S. M. Brozik and R. Polsky,

Small

, 2009,

, 2792.

12 X. Xiao, M. E. Roberts, D. R. Wheeler, C. M. Washburn,

T. L. Edwards, S. M. Brozik, G. A. Monta

~

no, B. C. Bunker,

D. B. Burckel and R. Polsky,

ACS Appl. Mater. Interfaces

, 2010,

3179.

Fig. 7

Power density curves of (a) (GDH-MDB)-modified 3D micro-

porous carbon electrode and (b) a (GDH-MDB)-modified 3D nano/

micro-porous electrode in 0.1 M PBS (pH 7.0) containing 20 mM NAD

and 30 mM glucose.

This journal is

ª

The Royal Society of Chemistry 2012

J.Mater.Chem.

, 2012,

, 11950–11956 | 11955

Published on 18 April 2012. Downloaded by California Institute of Technology on 24/01/2018 22:37:14.

View Article Online

13 C. Ji and P. C. Searson,

Appl. Phys. Lett.

, 2002,

, 4437.

14 C. Ji and P. C. Searson,

J. Phys. Chem. B

, 2003,

107

4494.

15 A. J. Forty,

Nature

, 1979,

282

, 597.

16 R. Laocharoensuk, S. Sattayasamitsathit, J. Burdick,

P. Kanatharana, P. Thavarungkul and J. Wang,

ACS Nano

, 2007,

, 403.

17 Z. Zhang, Y. Wang, Z. Qi, W. Zhang, J. Qin and J. Frenzel,

J. Phys.

Chem. C

, 2009,

113

, 12629.

18 X. Xiao, J. Nogan, T. Beechem, G. A. Monta

~

no, C. M. Washburn,

J. Wang, S. M. Brozik, D. R. Wheeler, D. B. Burckel and

R. Polsky,

Chem. Commun.

, 2011,

, 9858.

19 S. Sattayasamitsathit, A. M. O’Mahony, X. Xiao, S. M. Brozik,

C. M. Washburn, D. R. Wheeler, J. Cha, D. B. Burckel, R. Polsky

and J. Wang,

Electrochem. Commun.

, 2011,

, 856.

20 A. Bulbarello, S. Sattayasamitsathit, A. G. Crevillen, J. Burdick,

S. Mannino, P. Kanatharana, P. Thavarungkul, A. Escarpa and

J. Wang,

Small

, 2008,

, 597.

21 (

) C. Ji, G. Oskam, Y. Ding, J. D. Erlebacher, A. J. Wagner and

P. C. Searson,

J. Electrochem. Soc.

, 2003,

150

, C523; (

) X. Hu,

R. Wang, Y. Ding, X. Zhang and W. Jin,

Talanta

, 2010,

, 1737.

22 S. Trasatti and O. A. Petrii,

J. Electroanal. Chem.

, 1992,

327

, 353.

23 P. L. Stiles, J. A. Dieringer, N. C. Shah and R. P. Van Duyne,

Annu.

Rev. Anal. Chem.

, 2008,

, 601.

11956 |

J.Mater.Chem.

, 2012,

, 11950–11956

This journal is

ª

The Royal Society of Chemistry 2012

Published on 18 April 2012. Downloaded by California Institute of Technology on 24/01/2018 22:37:14.

View Article Online