3-Dimensionally
Self-Assembled
Single
Crystalline
Platinum
Nanostructure on Few-layer Graphene as an Efficient Oxygen
Reduction Electrocatalyst
Sreekuttan M Unni a, Vijayamohanan K. Pillai b and Sreekumar Kurungot *a
1. Introduction
The size and shape dependent changes induced in the physico-chemical properties of metal
nanoparticles can be accounted to the electron movement in restricted environment which is
commonly known as quantum confinement. Even diminutive variations in morphologies show
unprecedented alterations in the property characteristics of nanoparticles. Such changes significantly
broaden the scope of the materials for distinctive applications in nanotechnology
1-6
. Fascinated by
such dramatic changes in the properties induced by the variations in the sizes and shapes, development
of appropriate synthetic strategies for metal nanoparticles with desired morphologies has become a
challenging endeavour in nanotechnology3, 4, 7, 8. Platinum (Pt) morphologies in this context are widely
studied because of their excellent tuneable properties that span into various applications 8-11. In energy
portal, Pt catalysts are vital to Polymer Electrolyte Membrane Fuel Cells (PEMFCs), which are
generally powered by hydrogen, methanol, ethanol, formic acid etc
12-14
. Oxygen Reduction Reaction
(ORR) is one of the most challenging quests concerning a PEMFC electrode, for which Pt has become
an inevitable component to minimize the overpotentials 15. However, due to the high cost and
Scheme1. Schematic illustration of the formation of the 3-D self-assembled single crystalline platinum
nanostructure (PtNAGE) on few-layer graphene.
availability issues with Pt, current PEMFC research is mainly focussed on the use of minimal amount
of Pt pertaining to a higher efficiency16. Catalytic efficiency of Pt based nanomaterials can be
enhanced by fine tuning their morphology,
Figure 1. (a-c) HR-TEM images of the PtNAGE supported on graphene 72 h after the commencement
of the reaction taken at different magnifications, inset SAED of PtNAGE supported on graphene
sampled after 72 h (d) size distribution histogram of PtNAGE.
and hence revealing the shape of nanoparticles is significant for assessing the catalytic activity. Many
reports are available on nanowires, nanotubes, nanorods, nanodendrites, mulipods, yolk shells,
nanoflowers, nanocapsules and urchins like structures of Pt and most of these morphologies show
enhanced ORR activity than simple Pt nanoparticles with a size range of 2-4 nm17-35.
Among the different morphologies of Pt, three dimensionally (3-D) assembled nanostructures show
enhanced ORR activity compared to zero dimensional nanoparticles. Pt nanodentrites, nanourchin,
nanowire assembly etc are shown to possess very less defective sites as well as minimum low
coordination sites due to their downy crystalline planes
36, 37
. These factors inevitably reduce the
surface energy and interfacial resistance towards oxygen adsorption. In this regard, the 3-D assembled
nanostructure synthesis is a daunting challenge for the desired applications. Most of the reports
currently dealing with the synthesis of these nanostructures depict the significance of structure
directing moieties. However, for fuel cell catalysis, supported catalysts are preferred. Since most of the
reported literature on Pt nanostructures deals with unsupported particles, development of feasible
strategies for the direct growth of the 3-D assembly of Pt nanostructures without any surfactant on the
support materials is very important.
Slow reduction of Pt ion is very effective for the synthesis of nanowires or dentrites directly on the
support materials. Few reports are available on the direct growth of Pt nanowires, Pt dentrites and Pt
stars on the surface of conducting carbon and carbon nanotubes (CNTs) using HCOOH as a slow
reducing agent 23, 38, 39. Recently, Adzic and Wong et al. reported the synthesis of carbon supported PtPd core-shell nanowires with enhanced ORR activity40. However, these core-shell nanowires were
prepared through multistep processes which include removal of the structure directing groups as a
mandatory requirement in order to activate the surface of the nanowires. Such post synthesis
treatments not only adversely affect the morphologies but it also appeared very difficult to get rid of
the contamination of the noble metal surface by the surfactants.
The work we presented here is important in this context because it involves a unique preparation
strategy of a 3-D assembled platinum nanostructure on graphene nanosheets by using HCOOH as the
reducing agent, devoid of any catalytically harmful structure-directing moieties. It is well known that
graphene has high conductivity and surface area compared to the other allotropes of carbon. 41-43
Graphene itself improves the catalytic activity of Pt by localizing the excess electrons closer to Pt and
thereby making it more favourable for the electron withdrawing reactions. The 3-D self assembled Pt
nanostructure on graphene developed by the approach adopted here shows enhanced ORR activity
which is superior to Pt nanoparticles supported on graphene or on conducting carbon. Moreover, these
PtNAGE are less vulnerable to dissolution, aggregation and ripening during the operating
electrochemical environments. Thus, it may be expected that the combined effects of the nanostructure
and the features of graphene nanosheets such as their high conductivity enhance the total ORR activity.
Figure 2 (a and b) The HR-TEM images of PtNAGE on graphene, (c) represents the SAED pattern of
the image (a).
2. Experimental Section
Materials: Hydrogen hexachloroplatinate hexahydrate (H2PtCl6•6H2O) was purchased from Aldrich
Chemicals. Formic acid (HCOOH) was procured from Rankem Chemicals. Few layer graphene was
purchased from J K Impex, India. All the chemicals were used as received without any further
purification. A copper grid with a carbon support (Icon Analytical) was used for the HRTEM
observations.
Synthesis of 3-D self assembled Pt nanostructure: PtNAGEs were synthesized by the chemical
reduction of Pt precursor with formic acid. All the experiments were conducted in aqueous solution at
room temperature and under ambient atmosphere. In a typical synthesis of PtNAGE (40 wt%) on
graphene sheet, 106 mg hexachloroplatinic acid and 6 mL formic acid were dissolved in 20 mL H 2O
and sonicated with acid treated graphene sheets
Figure 3. HR-TEM image of the PtNAGE in a scale bar of 2 nm.
for 30 minutes. All the aqueous solutions for the reaction were prepared in deionized water. After this
initial dispersion, the solution was stored at room temperature for 72 h. The product was collected by
centrifugation and washed several times with water, and then dried in an oven at 60 oC for further use
in characterization and electrochemical measurements. To study the growth mechanism of the
catalysts, the samples were collected at different time intervals. For the effective comparison, a 40 wt.
% Pt supported graphene catalyst was prepared using a conventional polyol method as reported
elsewhere12 and Pt (40 wt. %) on carbon was purchased from Johnson and Mattey Pvt. Ltd.
Sample Characterization: High-resolution Transmission Electron Microscopic (HRTEM) images
were obtained with a Tecnai-T 30 model at an accelerated voltage of 300 kV. The samples for TEM
were prepared by placing a drop of the catalyst sample in isopropanol onto a carbon coated Cu grid.
The Pt loading was confirmed by Thermo Gravimetric Analysis (TGA). X-ray Diffraction (XRD) was
recorded on an Xpert Highscore Plus instrument using Cu-K radiation at a step of 0.02o (2). Scherrer
and Bragg formula were employed to calculate the mean crystalline size and the lattice parameters of
the catalysts. X-ray Photoelectron Spectroscopic (XPS) measurements were carried out on a VG Micro
Tech ESCA 300o instrument at a pressure of > 1 x 10-9 Torr (pass energy of 50 eV, electron take off
angle of 60o and the overall resolution of ~ 0.1 eV).
Electrochemical studies: The electrochemical properties of the catalysts were measured by cyclic
voltammetry (CV) using Autolab PGSTAT30 (Eco-Chemie) instrument and rotating disk electrode
(RDE) using Pine Research Instrumentation in a conventional three-electrode test cell with reference
hydrogen electrode (RHE) and a platinum foil as the reference and counter electrodes, respectively.
For preparing the working electrode, after polishing the glassy carbon (GC) electrode using 0.3 and
0.05 m alumina slurries, 10 l aliquot of the catalyst slurry made by sonicating 10 mg catalyst in 2 ml
3:2 ethanol-water mixture was drop-coated on the electrode surface. Subsequently, 1.5 l of 0.01 wt.
% Nafion diluted with ethanol was applied on the whole surface of the electrode to yield a uniform
thin film. This electrode was then dried in air and was used as the working electrode for all the
electrochemical studies.
The CV measurements were conducted with a sweep rate of 50 mV s-1 in 0.1 M HClO4 solution. The
electrochemical active surface areas (EASs), were calculated by measuring the charge associated with
the Hads (QH) between 0.0 and 0.4 V and assuming Qref =0.21 mC cm-2. The EAS of Pt was calculated
based on the relation EAS = QH/(Qref x m) where QH is the charge for H adsorption (mC cm-2 ), m is
the Pt loading (mg cm-2 ) on the electrode, and Qref is the charge required for the monolayer adsorption
of hydrogen on a Pt surface (0.21 mC cm-2 ). The adsorption of hydroxyl species was calculated based
on the OHad peak in the CV curves at the potential larger than 0.6 V. Dividing the hydroxyl adsorption
charge by the overall active surface area resulted in the surface coverage of OH ad species. Kinetics of
oxygen reduction reactions of all the catalysts were studied by using rotating disk electrode (RDE)
with platinum loading in the working electrode is 0.102 mg cm-2, in 0.1 M HClO4 using a threeelectrode cell assembly at a scan rate of 10 mV s-1 and electrode rotation speeds of 900, 1200, 1600,
2000 and 2500 rpm. The stability tests were conducted at ambient temperature in 0.1 M HClO4
solution.
Figure 4 X-ray diffraction pattern of the PtNAGE supported on graphene sampled at different reaction
time.
3. Results and Discussion
Morphological characterization was carried out using High Resolution Transmission Electron
Microscopy (HR-TEM). The high magnification images of the formed particles clearly show the 3-D
self assembled nanostructure (figure 1 and 2). A close inspection of the structure of the PtNAGE
reveals that the diameter of the assembly varies from 80 to 100 nm (figure 1d). Interestingly, the HRTEM images reveal single crystalline nature of the formed PtNAGE. Selected Area Electron
Diffraction (SAED) of PtNAGE shows a dotted pattern, which reasserts the single crystalline nature.
This dotted SAED pattern indicates that the nanostructure is not an assembly of nanowires, but
possibly originates from a seed growth mechanism. Sun et. al. have reported a similar mechanism for a
seed assisted growth of single crystalline Pt nanostars involving multiarms originating from the
predeposited seed particles
44
. It has been assumed that the growth mechanism is similar to Pt
nanowire reported on the other carbon allotropes. Nanoparticle assembly grows in the <111> direction
as marked in the HRTEM image shown in figure 3. More interestingly, the crystallographic alignment
of each of the projections is single crystalline with a lattice spacing of 2.2638 Å for the (111) plane.
Typically, HCOOH reduces Pt ion to Pt nanoparticles which get attached to the surface of graphene.
These small nanoparticles act as the nucleation sites for further adsorbing ions and nanoparticles. Since
E{111}<E{100}<E{110} (where ‘E’ stands for the surface energy), most of the Pt particles will be adsorbed
on to the minimum surface energy plane and hence the growth of the petals will be enhanced in the
<111> direction.
The nanoparticles which are reduced initially on the surface of graphene may exhibit mixed faces of
(111) and (100) planes so as to minimize the surface energy. These nanoparticles with truncated
octahedral geometry lead to the growth of the arms in the <111> direction. PtNAGE is thus formed
from the truncated octahedron which has large number of the {111} facets. During the slow reduction
process using HCOOH, at room temperature, preferential anisotropic growth on the (111) plane along
the <111> direction may be justified by the lowest energy principle
38
. However, the slow rate of the
reduction and presence of the graphene support also play significant roles in facilitating the growth
pattern. Further to these observations, X-ray Diffraction (XRD) analysis confirms that the PtNAGE
crystallized in a face centered cubic (fcc) lattice similar to that of bulk Pt (figure 4).
To understand the proposed growth mechanism of the PtNAGE, we collected samples at different time
intervals (10, 35 and 72 h) and performed the HR-TEM analysis. Interestingly, the sample collected 10
h after the commencement of the reaction shows the presence of Pt nanoparticles with truncated
octahedron well dispersed on graphene nanosheets together with free Pt nanoparticles in the reaction
medium (figure S1). This observation suggests that the reduction of Pt ions mainly takes place in the
solution phase and these reduced nanoparticles subsequently get slowly attached to the low energy
surface of the truncated octahedron deposited on the graphene surface. This leads to the one
dimensional growth of Pt along the <111> direction. Anisotropic factor ‘R’ is a useful parameter that
sheds light on the morphology of nanoparticles as X-Ray can scan around 1011 nanoparticles at a time,
which is quite larger than an electron beam could. The ratio D(100)/D(111), defined as the anisotropic
factor R which is the ratio of the growth rates along the <100> and <111> direction
(fwhm111cos(θ200)/fwhm200cos(θ111)), is >1.75 for the lowest energy close packed structure of the
octahedron and 0.58 for the cubic structure
14, 45
. XRD analysis of the nanoparticles after 10 h shows
an R value of ~0.8036, indicating that the formed structure is a truncated octahedron (Figure 4).
Platinised carbon (PtC, 40 wt %) and Pt on graphene (PtGE, 40 wt %) prepared by the polyol method
also have matching R values (~0.8035).
After 35 h, it is observed from the TEM image that the (111) plane of the truncated octahedron starts
growing slowly, resulting into PtNAGE possessing an R value of 0.8274 (figure S2). After 72 h, the R
value of these PtNAGE structures increases to 0.8673, which unambiguously confirms the growth of
the petals along the <111> direction. Even though with the combination of the HR-TEM and XRD
analysis we could postulate the mechanism of the
Figure 5. X-ray Photoelectron spectra of Pt 4f core level of (a) PtNAGE and (b) PtGE.
PtNAGE formation during the slow reduction process, a detailed study is required for a comprehensive
understanding on the coexisting factors in dictating the growth process.
With the aforementioned TEM and XRD data, we can conclude that the Pt nanoparticles with
truncated octahedron planes are forming first by the reduction of Pt ions. These nano particles with the
large number of the {111} faces disperse very well on the surface of the graphene sheets. The
dispersed Pt nanoparticle thereafter acts as seed for the further growth of the nanostructure through the
adsorption of Pt ions. This facilitates the three dimensional growth of the cluster possessing the
PtNAGE morphology. The planar surface morphology of graphene also appeared to play a crucial role
to mobilize the three dimensional self assembling process as we could not observe similar growth
patterns on the other carbon nano morphologies such as CNTs.
X-ray photoelectron spectra (XPS) of Pt 4f core level in PtNAGE and PtGE are given in figure 5.
Different peaks are observed for Pt 4f due to its multiple oxidation states. The Pt 4f(7/2,5/2) peak could
be deconvoluted into three sets of spin-orbit doublets. For PtNAGE, the Pt 4f(7/2,5/2) peaks observed at
71.35, 75.08; 73.79, 78.81, 76.65 and 80.76 eV can be ascribed to the Pt0, Pt2+, Pt4+ moieties
respectively46, 47. Similarly, PtGE also shows three sets of peaks at 71.35, 75.00; 73.92, 78.81, 76.69,
81.17 eV respectively due to the Pt0, Pt2+ and Pt4+ moieties. Additional two sets of doublets of Pt2+ and
Pt4+ are mainly due to the interactions between Pt and oxygen present on graphene and oxygen on Pt
(GE -O-Pt and GE-Pt-O)
48
. XPS spectra of Pt 4f in both PtNAGE and PtGE show similar binding
energy which indicates that interaction between Pt and graphene supports are similar for the system
evolved by the slow reduction as well as by the polyol methods.
Figure 6 (a) Cyclic voltammograms of the three catalysts (40 wt % Pt) in 0.1 M HClO4 at room
temperature with a scan rate of 50 mV s-1; (b) Hydroxyl surface coverage (θOH) for the three catalysts
in 0.1 M HClO4 at a potential higher than 0.6V in room temperature with a scan rate of 50 mVs-1.
Many factors influence the ORR activity of Pt. Few prevailing factors are: (1) Pt-Pt distance (2)
coordination number of Pt and (3) electron density state of the 5d orbital of Pt 49-51. PtNAGE meets all
these criteria and hence shows an enhanced ORR activity. The XRD analysis shows a decreased dspacing of Pt (111) plane compared to the corresponding planes of Pt on carbon (PtC) and graphene
(PtGE) (0.225 nm, 0.226nm and 0.226 nm respectively for PtNAGE, PtC and PtGE). Analysis of the
samples collected at different time intervals clearly depicts the transition in the d-spacing as the one
dimensional growth proceeds. Samples corresponding to the 10 and 35 h after the commencement of
the reaction display the same d-spacing, viz. 0.226 nm, whereas the one sampled after 72 h displays a
slightly lower d-spacing of 0.225 nm. Along with the reduced d-spacing, most of these PtNAGE have
smooth crystalline planes which minimize the low-coordination defect sites which is another criterion
to meet the enhanced ORR and durability characteristics. Randomly localized holes and electrons of
graphene will confine closer to platinum metal nanoparticle after metal decoration
50
. Once this
happens, these localized electrons concurrently modify the electron density of the 5d orbitals and
subsequently facilitate the reduction process of oxygen. More studies to understand the role of the
support materials on modifying the electronic properties of the dispersed metal nanoparticles are
ongoing in our laboratory.
PtNAGE, PtC and PtGE exhibit well defined characteristic peaks for hydrogen desorption and
adsorption, oxide formation and reduction (figure 6a) in cyclic voltammetric (CV) analysis. The
electrochemically active surface areas (EASs) calculated from the charge corresponding to the
hydrogen desorption peaks from the CV analysis are 15.55, 56.80, 20.21 m2 g-1 respectively for
PtNAGE, PtC and PtGE. This difference in EAS is mainly related to particle size. Pt nanoparticle size
is around 4.5 and 8 nm respectively for PtC and PtGE compared to PtNAGE. Due to small size of Pt
particle PtC showing higher EAS compared to rest of thr materials. Hydroxyl (OH) adsorption on
crystalline planes strongly depends on the ORR activity. Strong adsorption of the OH groups always
retards the oxygen reduction process
15, 52
. The OH adsorption at E > 0.6 V (figure 6b); (the area of
adsorption of OH at a potential higher than 0.6V is normalized by EAS 53) of PtNAGE is significantly
low compared to PtGE and PtC. This indicates that the 1-D projections of PtNAGE, which have more
exposed (110) planes due to the growth along the {111} facets of the truncated octahedron seeds do
not bind the OH species too strongly. Consequently this feature provides a favourable adsorption state
required for the oxygen reduction. However, for PtGE and PtC, more exposed planes are (111) and
(100), which have relatively high affinity towards the OH adsorption
54, 55
. The peak potential and the
onset are more positive for PtNAGE on the backward sweep, which indicates fast desorption of the
adsorbed OH species from the Pt surface.
Figure 7 (a) Linear sweep voltammogram of the catalysts in O2 saturated 0.1 M HClO4 solution at
room temperature (1600 rpm and 10 mV s-1 scan) (b) Koutecky-Levich plot of the catalysts at different
rotation speeds at a potential of 0.7 V in O2 saturated 0.1 M HClO4 at room temperature with a scan
rate of 10 mV s-1.
Figure 8 (a) Polarization plots of the catalysts, where kinetic current density (jk) is calculated from the
K-L plots at different potentials in O2 saturated 0.1 M HClO4 at room temperature and a scan rate of 10
mV s-1. (b) Tafel plots corrected to diffusion of the catalysts at the lower current density region derived
from linear sweep voltammogram in O2 saturated 0.1 M HClO4 solution at a scan rate of 10 mV s-1.
The enhanced ORR activity of PtNAGE was studied using a Rotating Disc Electrode (RDE) in O2
saturated 0.1 M HClO4 at a rotation speed of 1600 rpm and a scan rate of 10 mV s-1(figure 7a). Halfwave potential (E1/2) of PtNAGE is 0.81 V, which is significantly higher than the E1/2 of 0.78 V
displayed by PtC and 0.73 by PtGE. Specific activity 56, 57 (i.e. the normalized value with respect to the
EAS) of PtNAGE at a potential 0.85 V is found to be 1.04 A m-2 Pt which is higher than that for
PtGE (0.26 A m-2 Pt) and PtC (0.16 A m-2 Pt). Similarly, mass activity (i.e. the normalized value
with respect to the weight of the catalyst) of PtNAGE at a potential 0.85 V is found to be 15.9 A g-1,
which is superior to PtGE (5.2 A g-1) and PtC (8.8 A g-1). These results unambiguously confirm that,
for PtNAGE, despite its low active surface area, the catalyst quality determined by the specific activity
and mass activity is high by a factor of 6.50 and 1.80 respectively compared to PtC and 4 and 3.05
respectively compared to PtGE (Table 1). As evident from figure 7a, the limiting current density (jL) of
PtNAGE (-5.5 mA cm-2) is also higher than that of PtGE and PtC (-4.2 and -4.5 mA cm-2,
respectively). The more the magnitude of jL, the more enhanced the ORR activity is.
The Koutecky-Levich (K-L) equation was applied to calculate kinetic current density based on the
ORR polarization curves, which can be described as follow:
Table 1. Summary of the electro chemical active area, mass and specific activities for ORR of the
different catalysts prepared in this study
Electrochemical Mass
Catalyst
Specific Limitting
Active Surface activity activity
area (m2g-1)
Current
at 0.85 at 0.85 Density
V
vs. V
RHE
vs (mA cm-
RHE
2
)
(A g-1) (A m-2
Pt)
PtNAGE 15.55
15.9
1.04
-5.51
PtGE
20.21
5.2
0.26
-4.27
PtC
56.80
8.8
0.16
-4.58
1 1 1 1
  
j jl jk jf
where j is the measured current density, jk is the kinetic current density, jl is the diffusion (masstransfer) limited current density and jf is the film diffusion current. Here, jf can be neglected as the
amount of Nafion is significantly low and hence will not affect the limiting current density. In the
laminar flow region, the diffusion current density is a function of the rotational velocity and hence the
above equation may be approximated as follows:
1
1
1


j nFkC O 2 0.62nFAC D 2 3  16  12
O2
O2
Here n is the number of electrons, F is the Farady constant (96485.5 C), A is the area of the electrode
(0.196 cm2) and CO2 is the concentration of the dissolved oxygen in the electrolye solution (1.22x10-6
mol cm-3). This equation is very useful to find out the rate constant of the ORR.
2
jl  0.62nFAC O2 DO32 
1
6

1
2
where n is the number of electrons, F is the Farady constant (96485.5 C) C O2 is the bulk O2
concentration (1.38x10-6 mol cm-3), DO2 is the diffusion coefficient (1.69x10-5 cm2 s-1) of O2 in the
electrolyte, A is the area of the electrode (0.196 cm2), ν is the kinematic viscosity of the electrolyte
(0.01009 cm2 s-1), and ω is the rotation rate of the electrode in radians per second (2πrpm/60). The
value obtained for the kinetic current was independent of diffusion and could be used to evaluate the
intrinsic activity of the catalysts. The plot of the inverse of current density (1/j) as a function of the
inverse of the square root of the rate (ω−1/2), at a particular potential obtained from the hydrodynamic
voltammogram, assists in the evaluation of the useful kinetic parameters such as kinetic current density
(jk), number of electron transfer (n) and rate constant for ORR (k). The slope of the K-L plots (figure
7b) gives crucial insights on the number of electron transfer (n) and in the present case ‘n’ is estimated
to be 3.9, 3.7 and 3.8 for PtNAGE, PtC and PtGE respectively. The nearly parallel nature of the linear
plots corresponding to PtC and PtNAGE shows first order reduction with the possible involvement of
the same reaction mechanism. However, PtGE is found to exhibit a slightly different mode of plot
under the similar conditions. The K-L plots of the catalysts clearly highlight the higher jk value of
PtNAGE compared to the other two catalysts. For example, the jk value measured at 0.7 V for PtNAGE
is found to be 1.3 and 5.1 folds higher than that for PtC and PtGE, respectively. A plot of jk vs. the
applied potential, as shown in figure 8a, clearly highlights the level of superiority maintained by
PtNAGE along the potential window. Most significantly, at higher potentials, the activation loss for
PtNAGE is smaller compared to that of PtC and PtGE. Also, the Tafel plots derived from the
hydrodynamic voltammograms at an electrode rotation speed of 1600 rpm, corrected for diffusion
effect, suggest fine correlation in the mechanism of ORR for both PtNAGE and PtC (figure 8b).
Clearly, the Tafel plot of PtGE exhibits a slight deviation indicating a variation from the mechanism
followed by the other two systems. At the low overpotential region, the jk value of PtNAGE is high
compared to the other two catalysts.
Figure 9. Loss of electrochemical surface area as a function of potential cycling numbers in 0.1 M
HClO4 at room temperature.
This is significant because the current at the low overpotential region determines the efficiency of the
electrocatalyst 58. At 0.95 V, PtNAGE shows 1.86 and 4.01 folds higher jk value compared to PtC and
PtGE respectively. This high jk value at lower overpotential region thus gives conclusive evidence on
the enhanced ORR activity of PtNAGE due to its unique morphological features and modulated
adsorption characteristics. Durability analysis of these catalysts was carried out by potential cycling
between 0.0 to 1.2 V in 0.1 M HClO4 at room temperature with a scan rate of 50 mV s-1 for 250 cycles
(figure 9). This leads to 16% reduction in the EAS of PtNAGE compared to 56 and 59% reduction
observed respectively for PtGE and PtC. The reduction in the EAS of nanoparticles is mainly due to
the ripening where the support also plays a significant role. Potential cycling triggers carbon corrosion
and Pt significantly catalyzes the process. The enhanced durability of the graphene-supported catalysts
compared to PtC reveals that graphene is more resistant to corrosion under the electrochemical
environments.
4. Conclusions
We have presented a facile method for preparing a three dimensionally self assembled platinum
nanostructure on few layer graphene without employing any surfactants and by following simple in
situ reduction of platinic acid by HCOOH. The formed PtNAGEs are single crystalline in nature,
which show enhanced oxygen reduction activity. In a broad perspective, this PtNAGE may also find
other potential applications especially in catalysis and the studies to explore the potential benefits of
the system are underway in our laboratory.
5. Reference
1.
C. Burda, X. Chen, R. Narayanan and M. A. El-Sayed, Chem. Rev., 2005, 105, 10251102.
2.
K.-S. Choi, Dalton Trans., 2008, 5432-5438.
3.
H. Lee, S. E. Habas, S. Kweskin, D. Butcher, G. A. Somorjai and P. Yang, Angew.
Chem. Int. Ed., 2006, 45, 7824-7828.
4.
B. Lim and Y. Xia, Angew. Chem. Int. Ed., 2011, 50, 76-85.
5.
T. J. Kempa, B. Tian, D. R. Kim, J. Hu, X. Zheng and C. M. Lieber, Nano Lett., 2008,
8, 3456-3460.
6.
L. Manna, D. J. Milliron, A. Meisel, E. C. Scher and A. P. Alivisatos, Nat Mater, 2003,
2, 382-385.
7.
T. K. Sau and A. L. Rogach, Adv. Mater., 2010, 22, 1781-1804.
8.
A. Chen and P. Holt-Hindle, Chem. Rev., 2010, 110, 3767-3804.
9.
Z.-Y. Zhou, N. Tian, J.-T. Li, I. Broadwell and S.-G. Sun, Chem. Soc. Rev., 2011, 40,
4167-4185.
10.
M. Subhramannia, K. Ramaiyan and V. K. Pillai, Langmuir, 2008, 24, 3576-3583.
11.
S. Mahima, R. Kannan, I. Komath, M. Aslam and V. K. Pillai, Chem. Mater., 2007, 20, 601603.
12.
B. K. Balan, S. M. Unni and S. Kurungot, J. Phy. Chem. C, 2009, 113, 17572-17578.
13.
Z. Wang and K. Qiu, Electrochem. Commun., 2006, 8, 1075-1081.
14.
S. C. S. Lai and M. T. M. Koper, Phys. Chem. Chem. Phys., 2009, 11, 10446-10456.
15.
J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard and H.
Jónsson, J. Phys. Chem. B, 2004, 108, 17886-17892.
16.
Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. a. Chen and S. Huang, ACS Nano,
2011, 6, 205-211.
17.
A. Takai, H. Ataee-Esfahani, Y. Doi, M. Fuziwara, Y. Yamauchi and K. Kuroda, Chem.
Commun., 2011, 47, 7701-7703.
18.
B. Wu, N. Zheng and G. Fu, Chem. Commun., 2011, 47, 1039-1041.
19.
S. M. Alia, G. Zhang, D. Kisailus, D. Li, S. Gu, K. Jensen and Y. Yan, Adv. Function. Mater.,
2010, 20, 3742-3746.
20.
J. Chen, T. Herricks, M. Geissler and Y. Xia, J. Am. Chem. Soc., 2004, 126, 10854-10855.
21.
T. Herricks, J. Chen and Y. Xia, Nano Lett., 2004, 4, 2367-2371.
22.
X. Huang, H. Zhang, C. Guo, Z. Zhou and N. Zheng, Angew. Chem. Int. Ed., 2009, 48, 48084812.
23.
S. H. Sun, D. Q. Yang, D. Villers, G. X. Zhang, E. Sacher and J. P. Dodelet, Adv. Mater., 2008,
20, 571-574.
24.
N. Tian, Z.-Y. Zhou, S.-G. Sun, L. Cui, B. Ren and Z.-Q. Tian, Chem. Commun., 2006, 40904092.
25.
L. Wang, S. Guo, J. Zhai and S. Dong, J. Phy. Chem. C, 2008, 112, 13372-13377.
26.
W. J. Khudhayer, N. N. Kariuki, X. Wang, D. J. Myers, A. U. Shaikh and T. Karabacak, J.
Electrochem. Soc., 2011, 158, B1029-B1041.
27.
L. Wang, H. Wang, Y. Nemoto and Y. Yamauchi, Chem. Mater., 2010, 22, 2835-2841.
28.
L. Kuai, S. Wang and B. Geng, Chem. Commun., 2011, 47, 6093-6095.
29.
L. Zhang, N. Li, F. Gao, L. Hou and Z. Xu, J. Am. Chem. Soc., 2012.
30.
J. Xu, G. Fu, Y. Tang, Y. Zhou, Y. Chen and T. Lu, J. Mater. Chem., 2012, 22, 13585-13590.
31.
Z. Luo, L. Yuwen, B. Bao, J. Tian, X. Zhu, L. Weng and L. Wang, J. Mater. Chem., 2012, 22,
7791-7796.
32.
J. Kibsgaard, Y. Gorlin, Z. Chen and T. F. Jaramillo, J. Am. Chem. Soc., 2012, 134, 7758-7765.
33.
D. He, K. Cheng, H. Li, T. Peng, F. Xu, S. Mu and M. Pan, Langmuir, 2012, 28, 3979-3986.
34.
S. Ci, J. Zou, G. Zeng, S. Luo and Z. Wen, J. Mater. Chem., 2012, 22, 16732-16737.
35.
H.-J. Jang, S. Hong and S. Park, J. Mater. Chem., 2012, 22, 19792-19797.
36.
C. Koenigsmann, W.-p. Zhou, R. R. Adzic, E. Sutter and S. S. Wong, Nano Lett., 2010, 10,
2806-2811.
37.
Z. Chen, M. Waje, W. Li and Y. Yan, Angew. Chem. Int. Ed., 2007, 46, 4060-4063.
38.
S. Sun, F. Jaouen and J.-P. Dodelet, Adv. Mater., 2008, 20, 3900-3904.
39.
S. Sun, D. Yang, G. Zhang, E. Sacher and J.-P. Dodelet, Chem. Mater., 2007, 19, 6376-6378.
40.
C. Koenigsmann, A. C. Santulli, K. Gong, M. B. Vukmirovic, W.-p. Zhou, E. Sutter, S. S.
Wong and R. R. Adzic, J. Am. Chem. Soc., 2011, 133, 9783-9795.
41.
S. Zhang, Y. Shao, H. Liao, M. H. Engelhard, G. Yin and Y. Lin, ACS Nano, 2011, 5, 17851791.
42.
R. Kou, Y. Shao, D. Wang, M. H. Engelhard, J. H. Kwak, J. Wang, V. V. Viswanathan, C.
Wang, Y. Lin, Y. Wang, I. A. Aksay and J. Liu, Electrochem. Commun., 2009, 11, 954-957.
43.
M. H. Seo, S. M. Choi, H. J. Kim and W. B. Kim, Electrochem. Commun., 2011, 13, 182-185.
44.
S. Sun, G. Zhang, D. Geng, Y. Chen, R. Li, M. Cai and X. Sun, Angew. Chem. Int. Ed., 2011,
50, 422-426.
45.
I. N. Leontyev, S. V. Belenov, V. E. Guterman, P. Haghi-Ashtiani, A. P. Shaganov and B.
Dkhil, J. Phy. Chem. C, 2011, 115, 5429-5434.
46.
D. He, K. Cheng, T. Peng, X. Sun, M. Pan and S. Mu, J. Mater. Chem., 2012, 22, 2129821304.
47.
P. Bera, K. R. Priolkar, A. Gayen, P. R. Sarode, M. S. Hegde, S. Emura, R. Kumashiro, V.
Jayaram and G. N. Subbanna, Chem. Mater., 2003, 15, 2049-2060.
48.
P. Kundu, C. Nethravathi, P. A. Deshpande, M. Rajamathi, G. Madras and N. Ravishankar,
Chem. Mater., 2011, 23, 2772-2780.
49.
M. Shao, A. Peles and K. Shoemaker, Nano Lett., 2011, 11
3714-3719.
50.
W. Qin and X. Li, J. Phy. Chem. C, 2010, 114, 19009-19015.
51.
H.-l. Chou, F.-J. Lai, W.-N. Su, K. C. Pillai, L. S. Sarma and B.-J. Hwang, Langmuir, 2011, 27,
1131-1135.
52.
N. M. Marković, T. J. Schmidt, V. Stamenković and P. N. Ross, Fuel Cells, 2001, 1, 105-116.
53.
Z. Peng and H. Yang, J. Am. Chem. Soc., 2009, 131, 7542-7543.
54.
J. X. Wang, N. M. Markovic and R. R. Adzic, J. Phys. Chem. B, 2004, 108, 4127-4133.
55.
N. M. Markovic, H. A. Gasteiger and P. N. Ross, J. Phy. Chem., 1995, 99, 3411-3415.
56.
W. Li, Q. Xin and Y. Yan, Int. J. Hydrogen Energy, 2010, 35, 2530-2538.
57.
W. Li, W. Zhou, H. Li, Z. Zhou, B. Zhou, G. Sun and Q. Xin, Electrochim. Acta, 2004, 49,
1045-1055.
58.
L. Xiao, L. Zhuang, Y. Liu, J. Lu and H. c. D. Abruña, J. Am. Chem. Soc., 2008, 131, 602-608.