Gold nanosheets synthesized with red marine alga: Actinotrichia fragilis as efficient electrocatalysts toward formic acid oxidation

Article abstract of DOI:10.1039/c6ra14691k

In this study, a simple and green synthesis of gold nanosheets was explained when red marine alga extract (Actinotrichia fragilis) collected from the Persian Gulf was used as both a reducing and shape controlling agent. Various nanosheet shapes such as a

Full text of DOI:10.1039/c6ra14691k

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Gold nanosheets synthesized with red marine alga Actinotrichia fragilis as
efficient electrocatalysts toward formic acid oxidation
a*
b
b
a
SafiehMomeni , Afsaneh Safavi , RahelehAhmadi , IrajNabipour
a
Persian Gulf Marine Biotechnology Research Center, The Persian Gulf Biomedical Sciences Research Institute,
Bushehr University of Medical Sciences, Bushehr 75147 Iran
b
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
*
Corresponding author. Fax: +98 77 33328724.
Eꢀmail address: safieh.momeni@gmail.com (S. Momeni).
Abstract
In this study, a simple and green synthesis of gold nanosheetsis was explained when red marine
alga extract (Actinotrichiafragilis) collected from Persian Gulf was used as both reducing and
shape controlling agent. Various nanosheet shapes such as triangle, truncated triangular, hexagon
and polygon were synthesized via this seaweed extract. The size, morphology and crystal nature
of the asꢀprepared nanostructures were characterized by field emission scanning optical
microscopy (FESEM), atomic force microscopy (AFM), Xꢀray diffraction (XRD) and FTꢀIR
spectroscopy. AFM analysis showed that the Au nanosheets have a thickness ranging from 10ꢀ15
nm. The asꢀprepared Au nanosheets were used as an electrocatalyst for the fabrication of an
electrochemical sensor for oxidation of formic acid. The Au nanosheets modified carbon ionic
liquid electrode (AuNS/CILE) was designed and its catalytic activity was investigated toward
formic acid oxidation. The results imply that the AuNS/CILE shows excellent electrocatalytic
activity and good stability for formic acid oxidation in direct pathway without CO poisoning.
These results, direct pathway formic acid oxidation reaction without
CO poisonous
intermediates together with a simple and green route for synthesis of Au nanosheets, show that
the AuNS/CILE is one of the most promising system for application in direct full cells.
Keywords:Gold nanosheets; Carbon ionic liquid electrode; Formic acid; Sensor; Red alga.
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Introduction
Nanomaterials with no spherical shapes such as cube, wire/rod, plate and belt have been shown
physical and chemical properties that are distinctly different from conventional spherical
1
ꢀ3
nanoparticles. On the other hand, the properties of nanomaterials are strongly dependent on
4
their size, shape and crystallinity. As a result, shape controlled synthesis of nanostructures has
been paid increasing attention and there is fundamental interest in developing new methods for
generating anisotropic nanostructures. Gold nanostructures with controllable morphologies have
5
6
7,8
many identified applications in catalysts, optics, surfaceꢀenhanced Raman substrate (SERS),
9
10
tipꢀenhanced Raman substrate and medical applications. Among the various morphologies of
gold nanoparticles, twoꢀdimensional Au nanosheets are a particularly interesting class of
1
1ꢀ15
nanostructures with a desired degree of anisotropy.
The main interest in these gold
nanosheets relies on their potential applications in gas sensors, inducing hyperthermia in tumors,
1
6ꢀ18
infrared absorbing optical coatings, and basic building block for nanodevices.
Therefore,
fabrication of gold nanosheets with effective but simple preparative methods has been paid
considerable attention.
Generally, chemical techniques have been employed to prepare gold nanosheets, particularly
1
9
20
21
seedꢀmediated growth approach, templateꢀassisted synthesis, microwave heating, and oneꢀ
2
2
pot procedure. Currently, there is increasing interest in the fabrication of nanomaterials using
2
3
biomolecules and bioorganisms. However, there are only few reports on the biosynthesis of
gold nanostructures with high anisotropy. In biological synthesis, various microorganism and
2
4
plant extracts were known to produce nanomaterials either intracellularly or extracellularly.
Due to the complexity of biological reactions, the actual mechanisms of their performance and
the presence of any effective structureꢀdirecting agent in the synthesis of nanoparticles are often
not clear. Marine biological species such as yeast, fungi, sponge and alga have been found to be
2
5
capable of synthesizing nanostructures. Unicellular green alga (Chlorella vulgaris) and
2
6
multicellular brown alga (Sargassum sp.) were used for synthesis of gold nanosheets by Lee et
2
7
al. They showed that a protein with a molecular weight of approximately 28 kDa in crude
extract of green alga participates in the nucleation and growth of gold nanostructures into the
2
5
specific shape and size. Sastry et al. reported a method for synthesis of gold nanoplates by
2
8
using an extract of Lemograss leaves as a reducing and shapeꢀdriving agent.
2

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Fuel cells are potential devices in the conversion between chemical energy and electronic energy
using hydrogen or small organic molecules as fuels. Electroꢀoxidation of small organic
2
9
30
31ꢀ34
molecules such as formic acid , ethanol and methanol
has been greatly investigated for
the development of direct formic acid fuel cells (DFAFCs) and direct methanol fuel cells
3
5ꢀ37
(DMFCs).
Direct liquid fuel cells such as DMFCs and DFAFCs are considered as potential
power generator for portable electronic devices such as laptop computers. Compared with
methanol, formic acid has attracted much more attention due to its high theoretical open circuit
3
8
voltage, less toxicity, nonꢀflammability and low crossover flux through polymer membrane.
These unique characteristics of formic acid allow DFAFC to have a combination of higher
voltage and higher power densities. Moreover, alkaline fuel cells (AFCs) with the use of alkaline
anion exchange membranes and oxygen reduction reaction (ORR) on the cathode have attracted
3
9ꢀ41
much attention.
Electroꢀoxidation of formic acid (FA) to CO follows a dualꢀpath mechanism. The first involves
2
the dehydrogenation path (direct pathway) with formate anion as the reactive intermediate;
while,the other pathway proceeds by the dehydration of FA (indirect pathway) with the
formation of the CO poisoning intermediate and subsequent oxidation to CO at higher
2
4
2
potentials. Platinum (Pt) and palladium (Pd) have been considered as the most common
35, 36
catalysts for formic acid oxidation.
However, pure Pt can be easily poisoned by CO
intermediats (indirect pathway is predominant on Pt) and its catalytic activity decreased
significantly and inhibits the direct FA oxidation. In contrast, Pd possesses higher catalytic
activity than Pt for FA oxidation, but is much less stable in acidic solutions. As shown in a
previous investigation, the formation of CO intermediates needs continuous Pt surface, while for
4
3
the direct oxidation pathway, the presence of discrete Pt atoms is sufficient. Unfortunately, Pt
and Pd are expensive metals and their available resources in the earth are limited. Thus, many
reports are focused on the synthesis of economical and active catalysts to further decrease the
cost of fuel cells.
A common strategy to improve the catalytic activity in dehydrogenation pathway of FA
oxidation is to make the PtꢀM (M = Au, Pd, Ru, Bi, etc) alloy or bimetallic coreꢀshell
44ꢀ48
nanoparticles, which could enhance COꢀtolerance capability during the FA oxidation.
These
facts show that a Ptꢀbased catalyst has reinforced the direct dehydrogenation pathway and has
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weaken the indirect dehydration pathway simultaneously. Among various Ptꢀbased catalysts,
bimetallic PtꢀAu nanocatalysts with different structures such as PtꢀAu alloy, Pt modified Au and
4
9ꢀ51
Au@Pt have attracted more interest.
Introducing Au dopant could reduce the number of
adsorption sites for CO through the segregation of Pt sites and thus improves the catalytic
4
3
activity toward FA oxidation. In addition, the presence of Au could enhance the stability
against dissolution by raising the Pt oxidation potential. To further decrease the consumption of
noble metals and improve their catalytic activity, the key method is the binding of nanocatalyst
5
2
53
onto the surface of conductive substrates such as carbon black, carbon nanotube and graphene
5
4
55
nanosheets. Zhang et al. prepared graphene nanosheets supported PtꢀAu nanoparticles and
found that the PtꢀAu/graphene has superior catalytic activity for FA oxidation. To the best of our
knowledge, there have been only few reports on using Au nanostructures for the electrocatalytic
5
6
56
oxidation of FA till now. Ramarajet al. showed that the morphology of Au nanostructures is
an important factor in the electrocatalytic oxidation of FA. In their report, Au nanoparticles
electrodeposited on electrochemically reduced graphene oxide modified glassy carbon
electrode(GCE) showed electrocatalytic activity for formic acid oxidation but single Au
nanostructureꢀmodified GCE failed to catalyze the formic acid oxidation.
According to the previous reports, oxidation of formic acid has been intensively investigated in
acidic media and only a few studies have focused on the oxidation of formic acid in alkaline
5
7,58
ꢀ
media.
Osawa et al. investigated the electrooxidation of HCOOH/HCOO over a wide range
5
9
of solution pH. Their results showed that the FA oxidation current exhibits a volcanoꢀshape pH
dependence with a peak at a pH close to the pK of formic acid (3.75). On the other hand, the
a
best oxidation performance of formic acid can be achieved at pH ≈ pK . Further examination in
a
6
0
single cell experiments was performed by Lee et al. over a wide pH range. They showed that
the maximum cell performance was found at a pH slightly higher than the pK of FA. The peak
a
current of FA oxidation is then suppressed in the alkaline media due to OH adsorption on the
surface and surface oxidation.
In this report, we describe a rapid, green and oneꢀstep wet chemical technique to produce largeꢀ
scale gold nanosheets using crude extract of red marine alga (Actinotrichia fragilis) as a green
reducing and shape controlling agent. Various nanosheet shapes such as triangular, truncated
triangular, hexagon and polygon were synthesized via this seaweed extract. Detailed
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investigations on the influences of temperature and reagent concentrations have been found to
arrive the optimal growth conditions for the gold nanosheets. The asꢀprepared Au nanosheets
were used as an electrocatalyst for the fabrication of electrochemical sensor for oxidation of
formic acid. Herein, we designed Au nanosheets modified carbon ionic liquid electrode
(AuNS/CILE) catalyst and investigated its electrocatalytic activity toward formic acid oxidation
in alkaline media. Au nanosheets showed an excellent electrocatalytic activity for FA oxidation
on the surface of AuNS/CILE. One of the most distinctive features of electrooxidation of FA on
AuNS/CILE is elimination of the indirect FA oxidation pathway and the significant tolerance
against CO poisoning.
Experimental
Materials
Formic acid, sodium hydroxide (NaOH), chloroauric acid (HAuCl ),ethanol, 1ꢀiodooctane,
4
pyridine and diethyl etherwere used as received from Merck. Graphite powder (mesh size<100
lm),ammonium hexafluorophosphate was purchased from Fluka. All compounds were used as
received without further treatment. Double distilled water was used throughout the experiment.
Instrumentation
The morphology and size of the synthesized gold nanosheets were examined by field emission
scanning optical microscope (Hitachi Sꢀ4160 FESEM) at an accelerating voltage of 20 kV.
Structural characterization was performed by Xꢀray diffraction using an Xꢀray diffractometer
(D8, Avance, Bruker, axs diffractometer). The diffraction angle covered 20ꢀ90 degree with a step
size of 0.05 degree. The thickness and surface topography of nanosheets were characterized by
atomic force microscopy (JPK, Nano Wizard II AFM) using a silicon probe at room temperature.
The UVꢀvisꢀNIR spectroscopy was measured on a Specord analytic jena spectrophotometer. The
electrochemical measurements were carried out in a conventional three electrode cell. A carbon
ionic liquid electrode (CILE) and Au nanosheets modified CILE (1.8 mm in diameter) were used
as the working electrodes. The counter electrode and the reference electrode were platinum wire
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and Ag/AgCl electrode, respectively. The electrochemical properties were recorded with an
Autolab system (EcoꢀChemie, Utrecht, The Netherlands) equipped with GPES software (Ecoꢀ
Chemie, Utrecht, The Netherlands). All of the electrochemical measurements were carried out at
room temperature. The electrochemical measurements were performed in a 0.1 M NaOH
solution.
Electrode preparation
Carbon ionic liquid electrode was prepared by handꢀmixing of graphite powder and nꢀoctyl
6
1
pyridinum hexafluoro phosphate (OPyPF ) with a ratio of 50/50 (w/w) as described previously.
6
Au nanosheets modified carbon ionic liquid electrode was prepared using of graphite powder,
ionic liquid, and Au nanosheets (50%:40%:10%, wt %), respectively. The paste was packed into
the cavity of a Teflon tube (1.8 mm diameter) and the electrical contact was established via a
stainless steel handle.
Synthesis of Au nanosheets
Actinotrichia fragilisred alga was collected along the Bushehr coast of the Persian Gulf
(southwestern Iran) in May 2015. The red alga was transported to the laboratory, cleaned
carefully in fresh water and then by distilled water to remove sand and salts. After cleaning, the
algae were dried in shade at room temperature for 6 days. Prior to the experiments, the dried
seaweed was ground to a fine powder. About 2.0 g of the red alga powder was added to 200 mL
of deionized water and the mixture was boiled for 15 min. The seaweed extract was passed
through Whatman No.1 filter paper and the filtrates were centrifuged at 12000 rpm for 10 min.
The prepared extract was used for synthesis of Au nanosheets.
In a typical synthesis experiment of Au nanosheets at room temperature, crude extract (9.5 mL)
was added to different volumes of an aqueous solution of chloro auric acid (HAuCl , 50ꢀ500 µL,
4
0.05 M) and the volume was adjusted to 10 mL by the addition of an appropriate volume of
distilled water. The reaction was carried out under stirring at 500 rpm at room temperature for 10
h. The color of the mixture of algal extract and aqueous HAuCl solution changed from yellow to
4
shimmery yellow state, indicative of changes in the metal oxidation state. The resulting products
were isolated by centrifugation (12000 rpm for 15 min). The process of centrifugation and
redispersion were repeated several times with water and ethanol to remove residual reactants in
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the crude extract. The product was then dried. The prepared Au nanosheets were subjected for
the detailed characterization to determine their properties.
Results and Discussion
The current study is focused on the synthesis of gold nanosheets using red marine alga extract
and their application as an electrocatalyst for the fabrication of an electrochemical sensor for
oxidation of formic acid. The algal crude extract was a lightꢀyellow liquid. After mixing of
HAuCl (300µL, 0.05 M) with 9.5 mL of seaweed extract at room temperature, the color
4
gradually changed during reaction time (10 h) with the appearance of glittering Au nanosheets.
The UVꢀvis absorption spectra and digital photographs of a mixture of gold ions and crude
extract were obtained at multiple time intervals from the beginning of the reductive reaction as
shown in Fig.1A. The spectroscopic change indicates a change in the gold oxidation state (Au
(
III) was reduced to Au(0)). It is well known that optical properties of Au nanoparticles are
6
2
strongly dependent on their size and shape. In general, spherical gold nanoparticles exhibit only
one surface plasmon resonance peak around 550 nm, whereas anisotropic Au nanoparticles have
6
2
more than two adsorption peaks because of mutually different dipole resonance. As has been
illustrated in Fig.1A, the band at 315 nm attributed to HAuCl ion solution gradually decreases
4
and a broad band over 500 nm begins to appear that implies the formation of nanostructures with
anisotropic shape and very wide size distribution. The morphology of the prepared Au
nanosheets was characterized by FESEM to confirm the sheet nature of the products. Fig.1B
shows the FESEM images of gold nanosheets prepared using red marine alga extract. FESEM
images present the sheet structure with different shapes including hexagon, triangles, truncated
triangular and polygonas well as small spheres. The size of gold nanosheets was in the
micrometer scale (0.5ꢀ3µm along their longest edge) and their thickness was in the nanometer
scale with flat surface. It is worth noting that the synthesized Au nanosheets were transparent
which could be attributed to their thin nature (10ꢀ20 nm in thickness). Generally, red marine alga
extract contains both reducing and stabilizing agents, so that the in situ synthesis of gold
nanoparticles is simply feasible.
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The generation of Au nanosheets was found to be dependent on the appropriate concentration of
the precursor (HAuCl ). The effect of the amount of HAuCl on the size and morphology of the
4
4
synthesized Au nanosheets was investigated at different amounts of HAuCl (50–500 µL, 0.05
4
M) in the presence of 9.5 mL of red marine alga extract. The surface plasmon resonance (SPR)
spectra change gradually with varying the amount of HAuCl . As shown in Fig.2, the variation in
4
the SPR of Au nanoparticles synthesized by the reduction of different concentrations of HAuCl4
solution exhibits that Au nanostructures with different shapes have been synthesized. At low
concentrations of HAuCl (100µL of 0.05 M), a purple colloidal stable gold nanostructure was
4
obtained with an intense SPR band at 540 nm that is mainly due to the presence of Au
nanospheres. At HAuCl amounts above 100 µL, the absorbance spectrum displayed a broad
4
band in the visible and nearꢀinfrared region and gold nanosheets were obtained that resulted in
golden glittering precipitate (Fig.2, inset photograph).With increasing the amounts of HAuCl4,
the SPR band at longer wavelengths extended to the infrared region that indicates the decrease in
the number of nanospheres and formation of large gold nanosheets. Fig.3 shows the FESEM
images of gold nanostructures prepared with different amounts of HAuCl . Fig.3A shows that the
4
resulting gold product at a low concentration of HAuCl (100 µL, 0.05 M) is mainly consisted of
4
nanospheres with size of about 30ꢀ50 nm. Fig.’s 3B–D show the FESEM images of the obtained
products at HAuCl amounts above100 µL. FESEM images show that the products are
4
dominated by microꢀsized Au nanosheets with regular shapes (hexagon, triangles, truncated
triangular) and low amounts of nanospheres.
To investigate the crystalline structure of Au nanosheets, XRD analysis was performed. XRD
analysis was used for the product obtained from the addition of 300 µL HAuCl (0.05 M) to 9.5
4
mL of red marine alga extract. As shown in Fig.4A , the XRD peaks appeared at (111), (200),
(220) and (311)plane (2θ 35–100) that are assigned to the face centered cubic (fcc) structure of
gold nanoparticles (JCPDS card no. 04ꢀ0783). In addition, the relative diffraction intensities of
either (200)/(111) and (220)/(111) are much lower than the conventional values (0.1 versus 0.53
and 0.15 versus 0.33, respectively). These results imply that the preferential growth direction of
Au nanosheets is along the gold (111) facets.
FTIR analysis was performed to recognize the nature of biomolecules that bound specifically on
ꢀ1
the surface of Au nanosheets (Fig. 4B).The strong peak at 3343 cm is attributed to the free OꢀH
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ꢀ1
group. The band at 2924 cm is due to the alkane CꢀH stretching or is assigned to the secondary
ꢀ1
ꢀ1
amine. The peak appearing at 1435 cm is assigned to the –CꢀOꢀ group and a band at 1642 cm
might result from conjugated C=O group of the red alga polysaccharides. The absorption band at
ꢀ1
ꢀ1
1
027 cm can be assigned to the S=O stretching and the band at 875 cm is corresponded to –C–
6
3
O–SO of the sulfated polysaccharides. These results suggest that polysaccharide containing
3
compounds in the crude extract are adsorbed on the surface of Au nanosheets.
Surface topography and thickness of the Au nanosheets were investigated by AFM. Fig. 4C
shows the AFM image of the synthesized Au nanosheets with the addition of 300 µL HAuCl to
4
9.5 mL of seaweed extract. AFM showed atomically flat surface and thickness on the scale of
nanometers which give them optical transparency. In AFM analysis, a number of Au nanosheets
produced by this method were examined and showed a thickness ranging from 10ꢀ20 nm with an
edge length of 0.5ꢀ4 µm.
Temperature plays an important role in the generation of Au nanosheets. In fact, the kinetic of
gold ion reduction and the rate of the nucleation and growth of the nanoparticles are strongly
o
dependent on the temperature. The selected temperatures were 0, 25 and 50 C. The effect of
temperature was investigated at two concentrations of HAuCl (100 and 300 µL of 0.05 M)
4
solution. At room temperature, a solution of 1.5 mM HAuCl (300 µL, 0.05 M) produced gold
4
nanosheets with wellꢀdefined hexagonal and triangular shapes as shown in Fig.5A. By increasing
o
o
the temperature from 25 C to 50 C, the reduction reaction rate increased sharply and the color of
the solution changed from yellow to purple. Fig.5B shows the FESEM image of gold
o
o
nanostructures obtained at 50 C, which show at 50 C, the products were predominantly
spherical nanoparticles and the yield of Au nanosheets decreased dramatically. In a second
o
experiment, the temperature of the reaction was decreased to 0 C for a gold ion solution with a
concentration of 0.5 mM (100 µl, 0.05 M HAuCl ). At this concentration, mainly spherical gold
4
o
nanoparticles were produced at room temperature. By decreasing the temperature to 0 C, the
rate of HAuCl reduction reaction considerably decreased, which resulted in the formation of
4
wellꢀdefined hexagonal, truncated triangular and triangular Au nanosheets. Fig.5D shows the
o
SEM image of the product obtained at 0 C, which contains triangular and hexagonal nanosheets.
Red alga extract was used as a cheap, nontoxic, and green reagent for seedꢀless synthesis of Au
nanosheets. To further evaluate the influence of the amount of crude extract used on the
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generation of Au nanosheets, five samples were prepared by adding different amounts of crude
extract (1.0 mL, 2.5mL, 5.0 mL, 7.5mL and 9.5mL of extract (2.0 g red alga in 200 mL water))
to the HAuCl solution while other experimental conditions were kept at their optimum values.
4
As shown in Fig. 6, the reduction of gold ions and formation of gold nanoparticles occurred at
every employed concentration of seaweed extract and the main products were Au nanosheets
with smooth surfaces for HAuCl amount of 300 µL. However, by increasing the amount of
4
crude extract, the yield of production of Au nanosheets was increased. Higher yields of gold
nanosheets were obtained for crude extract amounts of 7.5ꢀ9.5 mL.
In this study, synthesis of gold nanosheets was performed using red marine alga extract. As
shown in previous reports, algal extract contains various compounds such as polysaccharides
6
4,65
(carrageenan), polyphenols, alkaloids, amides and amines.
It is worth noting that the crude
extract of red alga provided both the reducing agent(s) and the shape controlling agent for
synthesis of anisotropic gold nanoparticles. The formation of Au nanosheets was initiated by the
generation of seeds and then the appropriate capping agents are adsorbed on the selected facets
of gold seeds and greatly decrease the growth on these facets relative to the others. As a result,
anisotropic Au nanosheets with different shapes were formed. However, the exact growth
mechanism of Au nanosheets has not been thoroughly explored due to the diversity of
compounds available in a crude extract of red marine alga. According to the literature, Chlorella
vulgaris green alga and Sargassum sp. brown alga were used for synthesis of Au nanosheets. In
this study, it is shown that Actinotrichia fragilis red marine alga could also be used for synthesis
of Au nanosheets in large scale.
The catalytic activities of the asꢀprepared Au nanosheets were investigated for FA oxidation with
electrochemical measurements. Au nanosheets synthesized by the proposed method were used
for construction of an Au nanocomposite electrode (AuNS/CILE).The catalytic activity of
AuNS/CILE toward FA oxidation was investigated in acidic and alkaline media. But the best
oxidation performance of FA was achieved in 0.1 M NaOH solution containing FA (0.1 M).
After addition of FA to the NaOH solution, pH of solution was measured that reached to 5.5.
These results were in agreement with previous reports that FA oxidation exhibits a pH
dependence and shows the best performance at a pH close to the pK . The cyclic
a
ꢀ1
voltammograms, at 50 mVs in a potential window of ꢀ0.5 to 1.0 V versus Ag/AgCl are
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presented in Fig.7A. For comparison the result for FA oxidation with CILE under the same
conditions has also been shown in Fig.7A, curve a. However, at the bare CILE electrode, no peak
was observed for the FA oxidation (Fig.7A curve a). FA oxidation on the surface of AuNS/CILE
(Fig.7A, curve b) is appeared with onset potential at 0.3 V and a sharp peak centered at around
0.64 V in the forward scan (peak I) and a second peak that is often related to the CO poisoning
was not shown in forward scan. In the reverse scan, a large anodic peak was observed almost at
about the same peak potential (0.67 V) of the anodic peak in the forward scan (peak II). The
anodic peak in the reverse scan can also be associated with the direct FA oxidation and assigned
to the same reaction. Cyclic voltammogram of AuNS/CILE electrode in the 0.1 M NaCl solution
(pH 5.5) in the absence of FA is shown in Fig.7A, curve c. It can be seen that AuNS/CILE shows
two peaks at around +1.0 and +0.6 V in 0.1 M NaCl solution (pH 5.5) in the absence of FA. The
peaks that appear in the base line of AuNS/CILE are attributed to the oxidation and reduction of
Au nanosheets in the structure of electrode. The catalytic activity of AuNS/CILE toward FA
oxidation was investigated in 0.5 M phosphate buffer solution (PBS, pH 5.5), (Fig. 7B). As
shown in Fig. 7, the modification of CILE with AuNS could enhance the catalytic activity of the
electrode for FA oxidation. Surprisingly, when AuNS was used in the construction of the
electrode, the oxidation of FA was performed exclusively with the direct pathway
(dehydrogenation mechanism) and the indirect peak (dehydration mechanism, CO oxidation
peak) was not appeared. In fact, Au nanosheets with sharp edges and corners can increase the
rate of oxidation of FA via a direct CO pathway.
2
+
ꢀ
+
ꢀ
Direct pathway: HCOOH
Indirect pathway: HCOOH
HCOOads + H + e
CO + 2H + 2e (dehydrogenation)
2
+
ꢀ
COads + H O
CO + 2H + 2e (dehydration)
2
2
4
5
As shown in previous reports, the morphology of Au nanostructures is an important factor in
the electrocatalytic oxidation of FA. Ramaraj et al. showed that Au nanoparticles
electrodeposited on electrochemically reduced graphene oxide modified GCE showed
electrocatalytic activity for formic acid oxidation; but Au nanostructures modified GCE failed to
catalyze the formic acid oxidation. In our study, Au nanosheets with (111) high index facets
were evaluated as a catalyst for oxidation of FA. However, one of the most distinctive features of
this study for electrooxidation of FA on AuNS/CILE is the elimination of the indirect FA
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oxidation pathway. Table 1 shows a comparison between AuNS/CILE and previously reported
electrodes for oxidation of formic acid.
Fig.7C shows ten repetitive cyclic voltammograms recorded at AuNS/CILE in a formic acid
solution. It can be seen that the peak current density increased during successive scans. The
increase in the oxidation current might be due to the preconcentration of formic acid into the
6
6
ionic liquid component of the electrode prior to any electron transfer process.
The stability and the activity of the electrodes were investigated by chronoamperometry.
Chronoamperometric measurements of oxidation of FA at the bare CILE and AuNS/CILE were
examined by maintaining the electrode at a potential of 0.6 V for 6000 s continuous electrolysis.
Fig.8A shows a typical chronoamperometric response of 0.1 M FA and 0.1 M NaOH at the
surface of CILE and AuNS/CILE. The current density of FA oxidation at AuNS/CILE is higher
than CILE and the current increased slowly within 0ꢀ3000 s and remained efficiently unchanged
after 6000s of continuous electrolysis. These results indicate the high catalytic activity and
stability of AuNS/CILE and the significant tolerance against CO poisoning.
Electrochemical impedance spectroscopy has been used to investigate the electrocatalytic
activity of AuNS toward FA oxidation. Fig. 8B shows the Nyquist plot of FA (0.1 M FA in 0.1
M NaOH) oxidation on CILE and AuNS/CILE at open circuit potential (OCP) and 0.3 V. The
diameter of the impedance arc dramatically decreased in AnNS/CILE which indicates that the Rct
on the AuNS/CILE is much smaller than CILE. In Fig. 8, no impedance behavior was appeared
in the negative part of Nyquist plot, indicating a good tolerance towards CO poisoning
intermediates. With increasing the electrode potential from OCP to 0.3 V, the diameter of the arc
decreased that shows faster electron transfer kinetic at the onset potential.
The electrocatalytic activity of metal nanostructures is strongly dependent on their size, shape
and exposed surfaces. The anisotropic effects of gold nanostructures were studied in a previous
6
7
report and emphasized the importance of the anisotropic effects of crystallographic plane. Chen
et al. showed that the reactive sites of gold nanoplates are mainly lowꢀcoordination metal sites
6
8
and follow the trend of corner regions> edge regions> flat surface facet regions. In this study,
due to the presence of sharp edges and corners and high conductivity of Au nanosheets,
AuNS/CILE shows excellent electrocatalytic activity toward formic acid oxidation with
elimination of the indirect pathway.
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Conclusion
In summary, micrometerꢀsized Au nanosheets were efficiently synthesized by a oneꢀstep room
temperature reduction of HAuCl aqueous solutions with red marine alga extract. Au nanosheets
4
with wellꢀdefined hexagonal, truncated triangular and triangular shapes were synthesized in high
yields with the lateral size in the micrometer scale (0.5ꢀ4µm along their longest edge) and
thickness in the nanometer scale with flat surface. This preparation method is very simple, green
and economical and only the crude extract of red marine alga is required. The asꢀprepared Au
nanosheets were used as an electrocatalyst for fabrication of electrochemical sensor for oxidation
of formic acid. AuNS/CILE exhibited substantially efficient catalytic activity, good stability and
strong poisoning tolerance to CO intermediate during the electrooxidation of formic acid in
alkaline media. Thus, the direct pathway FA oxidation reaction without the formation of
poisonous intermediates together with a simple and ecofriendly synthesis of Au nanosheets by
red marine alga extract seems to be effective for application in direct full cells.
Acknowledgments
The authors wish to express their gratitude to Shiraz University Research Council, for the
support of this work. This project was partly supported by Iran National Science Foundation
(Research Chair Award N. 95/INSF/44913).
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Legends to the Figures
Fig.1 (A)UVꢀvis absorption spectra of synthesis of Au nanosheets prepared after different
reaction times with the red marine alga extract at 25 ˚C: (b) 2 h, (c) 4 h, (d) 6 h, (e) 7 h, (f) 8h
and (g) 10 h.The inset shows photographs (a) before and (b–g) after the addition of 300 µL
HAuCl (0.05 M) to a 9.5 mL solution of crude extract at different reaction times and (B)
4
FESEM images of synthesized Au nanostructures.
Fig.2 (A)Photograph images and (B) UVꢀVis absorption spectra ofsynthesized gold
nanostructures with the addition of (a) 100, (b) 200, (c) 300,(d) 400, (e) 500 µL HAuCl (0.05
4
M) to a 9.5 mL of red marine alga extract solution.
Fig.3 FESEM images of synthesized Au nanostructures with addition of (A) 100, (B) 200, (C)
3
00, (D) 400 µL HAuCl (0.05 M) to a 9.5 mL of red marine alga extract solution.
4
Fig.4 (A) XRD pattern, (B) XPS spectra and (C) AFM image of synthesized gold nanosheetswith
the addition of 300 µL HAuCl to a9.5 mL of red marine alga extract solution. The panelshows
4
the height profiles along the line in the AFM image.
Fig.5 FESEM images of Au nanosheets synthesized at different reaction temperatures, (A) and
(
C) 25, (B) 50 and (D) 0 ˚C. Synthesis conditions: (A–B) 300 and (C–D) 100 µL HAuCl4
0.05M) was added to a 9.5 mL of red marine alga extract solution.
(
Fig.6 FESEM images of synthesized gold nanostructures with additionof 300 µL HAuCl (0.05
4
M) to a 10 mL solution containing (A) 1.0 (B) 2.5 , (C) 5.0 and (D) 7.5 mL of red marine alga
extract.
Fig.7 (A)Cyclic voltammograms at a (a) bare CILE and (b) AuNS/CILE in 0.1 M NaOH in the
presence of 0.1 M formic acid (pH 5.5) and (c) AuNS/CILE in 0.1 M NaCl (pH 5.5) in the
ꢀ1
absence of 0.1 M formic acid(scan rate: 50 mVs ), (B) Cyclic voltammograms of AuNS/CILE in
ꢀ
0.5 M PBS (pH 5.5) in the absence(a) and presence (b) of 0.1 M formic acid (scan rate: 50 mVs
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1
)
and (C) Cyclic voltammograms of 0.1 M of formic acid solution in 0.1 M NaOH for ten
ꢀ1
consecutive scans at AuNS/CILE (scan rate 50 mVs ).
Fig.8 (A)Chronoamperometry of 0.1 M NaOH in the presence of0.1 M formic acid solution at a
working potential of 0.6 V (vs Ag/AgCl) for (a) CILE and (b) AuNS/CILE and (B) Nyquist plots
of the formic acid oxidation on the (a, b) AuNS/CILE and (c) CILE electrode in 0.1M NaOH and
0.1M formic acid at (a) 0.6 V and (b, c) open circuit potential.
Table 1. Comparison of the electrocatalytic behavior of AuNS/CILE for formic acid oxidation
with some of the previously reported Pd, Pt and Au modified electrodes
ꢀ
2
E in forward scan (V)
p
J (mA cm )
Electrode
Electrolyte
Ep1
Ep2
0.69 V
0.6 V
0.69 V
ꢀ
J /J
Ref
[69]
[70]
[56]
[50]
[41]
[71]
p1 p2
Pt–Au/RGO/CF
PtAuCu alloy NPs
Au NPs/RGO/GCE
Pt Au alloyed NPs
0.5 M H SO4
0.33 V
0.32 V
0.46 V
0.5 V
2.57
~ 1
~ 1
ꢀ
2
0.5M H SO4
2
0.5 M KOH
0.5M H SO4
5
0
50
2
NiO /Au/Pt/GC
0.5 M Na SO4
0.28 V
0.2 V
ꢀ
∞
x
2
AuPt@Pd/C
0.5 M Na SO4
0.65 V
~ 2
2
PtꢀonꢀAu/SnO2ꢀ
CNTs/GC
1
M HClO4
0.3 V
0.3 V
0.3 V
ꢀ
0.7 V
~ 2
~ 2
~ 2
[72]
[73]
[54]
Porous Au/Pt alloy NPs
Pd–Au NPs/graphene
nanoplatelets
0.5 M H SO4
2
0
.5 M H SO4
0.65 V
2
This
AuNS/CILE
0
.1 M NaCl
0.64 V
ꢀ
∞
work
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A
B
Fig. 1
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2.5
2.0
1.5
1.0
0.5
0.0
a
b
c
d
e
1
2
00µL
00µL
300µL
4
5
00µL
00µL
b
c
e
a
d
400
500
600
700
800
900
1000
Wavelength (nm)
Fig. 2
2
0

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Fig. 3
2
1

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2
1
1
00
50
00
(111)
A
B
4
4
42
4
0
(
200)
(
222)
(
220)
38
5
0
(
310)
3
6
3
0
40
50
60
ꢀTheta (degree)
70
80
90
4000
3000
2000
1000
2
ꢀ1
Wavenumber (cm
)
C
Fig. 4
2
2

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Fig. 5
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3

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Fig. 6
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4

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1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
b
A
c
a
ꢀ
0.2
ꢀ
0.4
ꢀ0.2
0.0
0.2
0.4
0.6
0.8
1.0
E (V) vs Ag/AgCl
1
.2
.0
.8
.6
.4
.2
.0
b
1
0
0
0
0
0
B
a
ꢀ
0.2
0.4
ꢀ
ꢀ0.4
ꢀ0.2
0.0
0.2
0.4
0.6
0.8
1.0
E (V) vs Ag/AgCl
2
2
1
1
0
0
.5
.0
.5
.0
.5
.0
C
1
0
1
ꢀ0.4
ꢀ0.2
0.0
0.2
0.4
0.6
0.8
1.0
E (V) vs Ag/AgCl
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Fig. 7
800
600
400
200
0
A
B
80
60
40
20
0
c
6
5
4
3
2
1
0
b
a
a
b
0
2
4
6
8
10 12 14
Z' (KOhm)
a
0
1000
2000
3000
4000
5000
6000
0
50
100
150
200
250
300
350
Time (s)
Z' (KOhm)
Fig. 8
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Electrocatalytic oxidation of formic acid using gold nanosheets biosynthesized
with red marine alga Actinotrichia fragilis
a*
b
b
a
Safieh Momeni , Afsaneh Safavi , Raheleh Ahmadi , Iraj Nabipour
a
Persian Gulf Marine Biotechnology Research Center, The Persian Gulf Biomedical Sciences Research Institute,
Bushehr University of Medical Sciences, Bushehr 75147 Iran.
b
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Formic acid oxidation
Au Nanosheets
Red marine alga
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
E (V) vs Ag/AgCl