Bifunctional gold-manganese oxide nanocomposites: Benign electrocatalysts toward water oxidation and oxygen reduction

Article abstract of DOI:10.1039/c4ra05240d

Gold-manganese oxide nanocomposites were synthesised by seed-mediated epitaxial growth at the water/n-heptane interface under mild reflux conditions. These nanocomposites exhibit efficient electrocatalytic activity toward the water oxidation reaction (WOR) and the simultaneous oxygen reduction reaction (ORR) at a low overpotential (η ≈ 370 mV) and under neutral pH conditions. This journal is

Full text of DOI:10.1039/c4ra05240d

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Bifunctional gold–manganese oxide
nanocomposites: benign electrocatalysts toward
Cite this: RSC Adv., 2014, 4, 41976
water oxidation and oxygen reduction†
Received 2nd June 2014
Accepted 21st August 2014
*
Hasimur Rahaman, Koushik Barman, Sk Jasimuddin and Sujit Kumar Ghosh
DOI: 10.1039/c4ra05240d
www.rsc.org/advances
Gold–manganese oxide nanocomposites were synthesised by seed- capable of oxidizing water to drive this energetically highly
mediated epitaxial growth at the water/n-heptane interface under unfavorable reaction (DHꢁ ¼ 572 kJ molꢀ1).10 Several protocols
mild reflux conditions. These nanocomposites exhibit efficient elec- have been adopted in the literature to design efficient, inex-
trocatalytic activity toward the water oxidation reaction (WOR) and the pensive and robust electrocatalysts for the water oxidation
simultaneous oxygen reduction reaction (ORR) at a low overpotential reaction based on metallic platinum,11 various oxides and
(h z 370 mV) and under neutral pH conditions.
complexes of iridium,12 ruthenium,13 nickel,14 cobalt,15 iron16
and copper complexes,17 and to evaluate the oxygen evolution
activity in acidic and/or alkaline conditions Amongst the
materials considered so far for water oxidation catalysts, rst
row transition metal oxides can are abundant and have
reasonable stability compared with their noble metal
analogues.18
1. Introduction
Industrial scale production of clean energy could replace nite
fossil fuels with abundant, renewable, environmentally benign
energy sources and would lead to the survival of the planet in a
sustainable manner that poses a signicant challenge to
humanity in the 21st century.1,2 There is a growing need for
electrocatalytic water oxidation to produce dioxygen for the
conversion of electrical energy to stored chemical energy in the
form of fuels.3,4 Plants utilize complex catalytic systems for the
breakdown of water into its elements. In nature and in many
protocols designed for articial photosynthesis, water oxida-
tion, 2H2O / 4H+ + O2 + 4eꢀ, is a key step and hence our better
understanding of these systems could pioneer new designs of
effective catalysts.5,6 The activity of electrocatalysts for water
oxidation is of fundamental importance for the development of
promising energy conversion technologies, including inte-
grated solar water-splitting devices, water electrolyzers, and
lithium–air batteries.7 Therefore, to meet future challenges,
water oxidation has become an important area of research,
which is inspired by energy challenges and bioinspired by the
emerging understanding of photosystem II.8,9 The salient
features of the technical impediments to such a reaction is the
requirement of efficient and inexpensive electrocatalysts
In particular, manganese oxides are materials of consider-
able importance due to their interesting structural, magnetic
and transport properties that arise from their excellent struc-
tural exibility combined with novel chemical and physical
properties.19 Among the series of manganese oxides available in
various oxidation states of manganese (II, III, IV), Mn3O4 (haus-
mannite) has been found to be an effective and inexpensive
catalyst in a number of oxidation and reduction reactions.20 On
the other hand, gold nanoparticles have also attracted
increasing attention due to their unique properties, such as
high biocompatibility, tunable electronic and optical behavior,
good conductivity and high catalytic activity, which make them
fundamental building blocks for the development of innovative
functional materials.21 The assembly of different nanomaterials
with specic optical, magnetic, or electronic properties to
multicomponent composites can change and even enhance the
properties of the individual constituents.22 Bifunctional
composite nanostructures containing gold have found
tremendous importance in the eld of nanocatalysis due to the
rich surface chemistry of gold.23 The fabrication of nano-
composites containing two or more different functionalities has
also started attracting attention due to their enhanced catalytic
properties.24
Department of Chemistry, Assam University, Silchar 788011, India. E-mail: sujit.
kumar.ghosh@aus.ac.in; Fax: +91-3842-270802; Tel: +91-3842-270848
Several reports have been published in the literature using
manganese oxides and complexes as electrocatalysts for water
oxidation. Dismukes et al.25 described the thermodynamic and
† Electronic supplementary information (ESI) available: Experimental details,
cyclic voltammogram, digital camera images and reaction schemes. See DOI:
10.1039/c4ra05240d
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mechanistic aspects that Nature appears to use for catalyzing in In this communication, we have explored the controllable
vitro water oxidation by photosynthesis using bioinspired and integration into gold–manganese oxide nanocomposites by
photoactive Mn4O4-cubane clusters. Drawing inspiration from seed-mediated epitaxial growth at the water/n-heptane inter-
the cubane-like CaMn(4)O(x), the biological catalyst found in face, and also investigated the electrocatalytic activity of the
the oxygen evolving centre in the photosystem II, Gorlin and combinatorial catalysts toward water oxidation and oxygen
Jaramillo26 investigated the electrocatalytic activity of nano- reduction at low overpotential (370 mV) and, most importantly,
structured manganese oxide surfaces that exhibited similar under neutral pH conditions.
oxygen electrode activity to the best known precious metal
nanoparticle catalysts, viz., platinum, ruthenium and iridium.
2. Experimental section
a
recent communication, Jaramillo and colleagues27
In
2.1. Reagents and instruments
demonstrated the addition of Au to MnOx to produce an order
of magnitude higher turnover frequency than that of the best
pure MnOx catalysts and a local, rather than bulk, interaction
between Au and MnOx, which led to the observed enhancement
in the activity of the reaction. Zaharieva and co-authors28
showed that the binuclear manganese molecular complex
[(OH2)(terpy)Mn(m-O)2Mn(terpy)(OH2)]3+, which is the most
prominent structural and functional model of the water-
oxidizing manganese complexes operating in plants and cya-
nobacteria, could be supported on montmorillonite clay and,
using Ce(IV) as a chemical oxidant, the complex could act as one
of the best manganese-based molecular catalyst toward water
oxidation. Wiechen et al.29 reported the syntheses of layered
manganese oxides, where the interlayer cations, viz., K-, Ca-, Sr-
and Mg-containing birnessites, were varied and also observed
that the oxygen-evolving complex required the presence of
calcium in their structures to achieve maximum catalytic
activity. Spiccia and co-workers30 showed that for the nano-
particulates of manganese oxides, formed in Naon polymer,
the catalytic activity toward the water oxidation was dependent
on the dispersivity of the nanoparticles. They also reported
synthetic methodologies for the preparation of highly active
mixed valent MnOx catalysts by the partial oxidation of crys-
talline MnIIO nanoparticles and analysed the catalytic activity in
water splitting devices.31 In a review, the group elucidated the
perspectives of a cluster that contains four manganese and one
calcium ions bridged by ve oxygen atoms in a distorted chair-
like arrangement in the current structural and mechanistic
understanding of the oxygen evolving complex in photosystem
II.32 Being inspired by the structural diversity of manganese
oxides that occur naturally as minerals in at least 30 different
crystal structures, Dismukes and colleagues33 chose to system-
atically compare eight synthetic oxide structures containing
Mn(III) and Mn(IV) with cubic phases, and concluded that elec-
tronically degenerate Mn(III) imparts lattice distortions, due to
the Jahn–Teller effect, that are hypothesized to contribute to the
structural exibility, which is important for catalytic turnover in
water oxidation at the surface. Suib and group34 compared the
catalytic activity of mixed valent porous amorphous manganese
oxides, cryptomelane-type tunnel manganese oxides, and
layered birnessite as water oxidation catalysts, and observed
that amorphous manganese oxides exhibit signicantly higher
turnovers compared to tunnel and layered structures. In spite of
all these investigations with molecular and solid-state electro-
catalysts capable of mediating water oxidation, many funda-
All the reagents used were of analytical reagent grade. Gold(III)
chloride trihydrate (HAuCl4$3H2O, $99.9%), trisodium citrate
($99%), manganese acetate tetrahydrate (Mn(ac)2$4H2O), 4-
aminothiophenol (4-ATP), and phosphoric acid were obtained
from Sigma Aldrich and were used as received. Sodium
perchlorate, sodium hydroxide, methanol, n-heptane, and
ammonia were purchased from Sisco Research Laboratories,
India, and were used without further purication. An aliquot of
0.1 M phosphate buffer saline (PBS) was prepared by mixing an
equimolar solution of phosphoric acid and sodium perchlorate,
followed by the dropwise addition of sodium hydroxide.
Double-distilled water was used throughout the course of the
investigations. The temperature was 298 ꢂ 1 K for all
experiments.
Absorption spectra were recorded in a Shimadzu UV-1601
digital spectrophotometer (Shimadzu, Japan), taking the
sample in a 1 cm well-stoppered quartz cuvette. Transmission
electron microscopy (TEM) was carried out on a JEOL JEM-2100
microscope with a magnication of 200 kV. Samples were
prepared by placing a drop of solution onto a carbon coated
copper grid and drying overnight under vacuum. Dark eld
scanning tunneling electron micrograph (DF-STEM) and
selected area electron diffraction (SAED) patterns were obtained
using the same instrument. Energy dispersive X-ray (EDX)
analysis was performed on an INCA Energy TEM 200 using an X-
ray detector. Fourier transform infrared (FTIR) spectra were
recorded in the form of pressed KBr pallets in the range (400–
4000 cmꢀ1) on a Shimadzu-FTIR Prestige-21 spectrophotom-
eter. X-ray diffraction (XRD) pattern was obtained using a D8
ADVANCE BROKERaxs X-ray diffractometer with CuKa radia-
ꢁ
ꢀ
tion (l ¼ 1.5418 A); data were collected at a scan rate of 0.5
minꢀ1 in the range of 10–80ꢁ. Electrochemical measurements
were performed by a CHI-660C electrochemical workstation. A
Ag/AgCl electrode (in 3.0 M KCl) and a Pt wire were used as the
reference and auxiliary electrodes, respectively. Catalytic reac-
tions were performed by the immobilized nanoparticles or
nanocomposites over a 4-aminothiophenol monolayer modied
gold working electrode, with 0.1 M phosphate buffer saline
(pH ꢃ 7.5) used as the electrolyte and at a scan rate of
100 mV sꢀ1
.
2.2. Synthesis of the nanomaterials
2.2.1 Synthesis of gold nanoparticles (NPs). Gold nano-
mental questions and practical challenges remain, and clear particles were synthesized by the Frens citrate reduction
improvements are needed in cost, durability, and overpotential. procedure,35 where the standard procedure for the preparation
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of 10 nm gold nanoparticles is as follows. An aliquot of 50 mL
aqueous solution of HAuCl4$3H2O (0.25 mM) is heated to
boiling and 0.5 mL of trisodium citrate (1%) is added. In about
25 s, the boiling solution turns faintly blue (nucleation). Aer
approximately 70 s, the blue color suddenly changes into a
brilliant red, indicating the formation of gold particles. The
boiling was continued for half an hour and then cooled to room
temperature.
2.2.2 Synthesis of gold–manganese oxide nanocomposites
(NCs). Gold–manganese oxide nanoparticles were synthesised
at the environmentally benign water/n-heptane interface under
mild reux conditions. In 25 mL binary solvent mixture of water
and n-heptane (3 : 1 v/v), 2.5 mM Mn(ac)2$4H2O was added and
Fig. 1 (a–c) Transmission electron micrographs of Au, Mn3O4 and
Au–Mn3O4 nanoparticles, respectively; (d) dark field scanning
tunneling electron micrographs, (e) energy dispersive X-ray and (f)
selected area electron diffraction pattern of Au.
ꢁ
brought to reux (ca. 65–70 C) under stirring. Aer about 30
min, 150 mL of ammonia was added, and immediately aer, 1.0
mL of preformed gold nanoparticles (0.25 mM) was added
dropwise for 10 min to the solution under reux. The reuxing
was continued overall for 1.5 h. Aer the addition of the gold
colloid, the color of the sol slowly began to change, and nally, a
brownish red coloration was seen at the end of the reaction.
Then, the heating was stopped and the mixture was stirred for
12 h at room temperature. The particles obtained were retrieved
from the solvent mixture by centrifugation at 10 000 rpm for 15
min and subsequently, redispersed into water. The dispersion
was found to be stable for a month when stored in a vacuum
desiccator. Manganese oxide nanoparticles were synthesised
following the same procedure, but without the addition of any
gold nanoparticles.
from the (111) plane of the fcc-structured gold and the strong
ring pattern corresponds to the (101), (103) and (211) planes of
the tetragonal hausmannite structure. In addition, a combined
multireection results, due to the Au–Mn3O4 composite
formation, conrming the crystallinity of the resultant
materials.19
Nowadays, Au, Mn3O4 and Au–Mn3O4 nanoparticles are
employed to investigate the electrocatalytic activity toward the
water oxidation reaction. A detailed procedure of the modi-
cation of the electrodes is described in ESI 3.†
Fig. 2 displays the cyclic voltammogram of 4-ATP/gold (red),
Au NPs/4-ATP/gold (green), Mn3O4 NPs/4-ATP/gold (blue) and
Au–Mn3O4 NCs/4-ATP/gold (black) electrodes in PBS at
pH ꢃ 7.5. A specic amount (20 mg) of the catalysts was dis-
solved in 10 mL of water in each case. These dispersions were
then employed for the loading of catalysts by dipping the elec-
trodes and allowing them to equilibrate overnight under
vacuum. It was observed that the Au–Mn3O4 nanocomposites
(black) caused a shi of the oxidation potential in the less
3. Results and discussion
In the present experiment, the water/n-heptane binary solvent
mixture plays an important role in the evolution of gold–
manganese oxide nanocomposites by epitaxial growth without
any external stabilizing agents.36 The controllable integration of
gold and manganese oxide into single nanostructures was
characterised by absorption, Fourier transform infrared (FTIR)
spectroscopy and by X-ray diffraction (XRD) techniques, which
revealed the epitaxial growth of manganese oxide on the surface
of gold nanoparticles, as described in ESI 2.†
The morphology, composition and crystallinity of the parti-
cles are depicted in Fig. 1. Transmission electron micrographs
(panels a, b and c) of gold, manganese oxide and gold–manga-
nese oxide show that the particles are 10 ꢂ 2 nm, 15 ꢂ 3 nm and
20 ꢂ 5 nm, respectively. In the image of the nanocomposites,
the Au particles appear black, while Mn3O4 are light colored,
because Au has a higher electron density and allows fewer
electrons to transmit.19 Dark eld scanning tunneling electron
micrographs (DF-STEM) (panel d) highlight the epitaxial growth
of manganese oxide on gold nanoparticles. The energy disper-
sive X-ray spectrum (panel e) of Au–Mn3O4 particles reveals that
the particles are composed of Mn, C, O, Cu and Au elements.
Among these elements, the signals of Mn, O and Au result from
the Mn3O4 and Au particles, which form the product and the
signals of C, while the O and Cu elements come from the
precursor and the supporting TEM grid. From the SAED pattern
Fig. 2 Cyclic voltammograms of water oxidation in the presence of 4-
ATP/gold (red), Au NPs/4-ATP/gold (green), Mn3O4 NPs/4-ATP/gold
(blue) and Au–Mn3O4 NCs/4-ATP/gold (black) electrodes in PBS at
of the composites, it is clear that a bright reection appears pH ꢃ 7.5.
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positive potential and a large increase in the current height
compared to that of the only Au (green) or Mn3O4 nanoparticles
(blue). In spite of the ideal electrocatalytic process at its ther-
modynamic potential (for example, at 0.62 V vs. Ag/AgCl,
pH ꢃ 7.0 and 1 atm O2), the actual electrode reaction occurs
at a more positive potential (i.e. overpotential) whose magni-
tude reects the electrode kinetics of the solution. It is seen that
Au NPs/4-ATP/gold, Mn3O4 NPs/4-ATP/gold, and Au–Mn3O4
NCs/4-ATP/gold electrodes display an anodic response at +1.13
V (h z 0.51 V, pH ꢃ 7.5), +1.12 V (h z 0.50 V, pH ꢃ 7.5), and
+0.998 V (h z 0.37 V, pH ꢃ 7.5) vs. Ag/AgCl, respectively, in the
potential window of 0.3–1.3 V, signifying water oxidation in 0.1
M PBS (pH ꢃ 7.5) and a corresponding cathodic peak at ꢃ+ 0.5 V
for O2 + 4H+ + 4e / 2H2O at pH ꢃ 7.5, which is indicative of the
reversibility of the process,3 whereas no such signal appears for
4-ATP/gold electrode. It was noted that the particles synthesized
in the present experiment are stable for a couple of weeks
without any signicant agglomeration or precipitation of the
particles. Aer the deposition of the particles at the electrodes,
the catalytic activity of the particles was measured and the
electrodes were rinsed in distilled water aer the experiments. It
was noted that the electrodes retained their catalytic activity
even aer 48 h of the loading of the catalysts. Therefore, it could
be conceived that the particles do not suffer from dissolution or
corrosion during the measurement of their electrocatalytic
activity. However, assuming that the particles are nearly
spherical and that the densities of Mn3O4 and Au–Mn3O4 are
4.86 and 12.09 g cmꢀ3 (taking an average of the density of Au
and Mn3O4) respectively, the number of Mn3O4 and Au–Mn3O4
particles is ca. 2.3 ꢄ 1012 and 3.8 ꢄ 1011, respectively, and the
corresponding surface areas are ca. 1625 ꢄ 1012 and 477 ꢄ 1012
nm2, respectively.37 Therefore, it is evident that Au–Mn3O4
composites are better electrocatalysts than Mn3O4 particles.
The turnover frequency (TOF) of the catalysts could be
calculated using the equations:38
kcat ¼ Icat/Q
(3)
The catalytic current densities for Au, Mn3O4, and Au–Mn3O4
are 3.8, 5.8 and 17.4 mA, respectively, and the charges obtained
by integrating the cathodic peak for Au, Mn3O4, and Au–Mn3O4
is 2.4, 2.6, and 3.8 mC, respectively. Therefore, the turnover
frequencies corresponding to Au, Mn3O4, and Au–Mn3O4 are ca.
1.6, 2.2, and 4.6 sꢀ1, respectively. The catalytic activity for water
oxidation by Au–Mn3O4 nanocomposites is considered as not
only combining the properties of both noble metal and metal
oxides, but also the unique collective and synergistic effects
of its component parts, compared to single component
materials.27,39
Variation of the gold particle size in the nanocomposites
shows that the catalytic activity increases with a decrease in the
particle size of the nanocomposites (ESI 4†); this study elicits
the reproducible electrocatalytic activity of the nanocomposites.
Therefore, it could be conceived that Au–Mn3O4 composites are
more efficient catalysts for the water oxidation reaction than the
individual Au or Mn3O4 nanoparticles. The enhanced catalytic
activity of the Au–Mn3O4 catalysts could be attributed to the
benecial presence of higher amounts of oxidizable gold
species and surface oxygen vacancies, resulting from the strong
interaction between Au and the reactive surface of Mn3O4
nanoparticles.40 An increase in the 5d vacancy of Au increases
the interaction of O2 and Au, thereby, enhancing the catalytic
activity of Au in the composites.41 However, interestingly, it was
noted while the electrode was cycled to the cathodic potential
aer several scans through the catalytic anodic wave, an irre-
versible peak was observed at ca. ꢀ0.001 V to ꢀ0.2 V for
different modied electrode systems, as presented in Fig. 3.
ꢀ
This peak could be attributed to the O2/O2 couple42 and indi-
cates that O2 evolved from water oxidation on the electrode
surface. A digital camera photograph showing the evolution of
oxygen gas during water oxidation and the corresponding cyclic
voltammogram of the Au–Mn3O4 NCs/4-ATP/gold electrodes in
normal and N2-saturated PBS at pH ꢃ 7.5 is shown in ESI 5.†
Icat ¼ nFAkcatGcat
Gcat ¼ Q/nFA
(1)
(2)
where, Icat is the catalytic current density, n is the moles of
electron transfer, F is the Faraday constant, A is the geometric
surface area of the underlying anodic surface, kcat is the rate
constant of the electron transfer, G is the surface coverage in
moles per square centimeter, and Q is the charge obtained by
Ð
integrating the cathodic peak (¼ ifdt, where if is the Faradaic
current). However, the concept of turnover frequency is based
on the number of active sites of the catalysts. In the above
equations, the active sites have been replaced by surface area,
which is an apparent value; therefore, the calculated TOFs
provide only apparent values rather than the real TOFs of the
catalysts. In addition, for the catalyst of Au–Mn3O4, the active
sites for Au are different from those for Mn3O4 and, as a result,
the calculated TOF is a hybrid or average of that of the
constituent particles. From eqn (1) and (2), we can estimate the Fig. 3 Cyclic voltammograms for oxygen reduction in the presence of
4-ATP/gold (blue), Au NPs/4-ATP/gold (green), Mn3O4 NPs/4-ATP/
electroactive site turnover rate during O2 evolution as:
gold (red) and Au–Mn3O4 NCs/ATP/gold (black) electrodes in PBS at
pH ꢃ 7.5.
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Under ambient conditions, the current height due to oxygen
Cyclic voltammetry data of water oxidation and oxygen
reduction is higher than in the N2-sparged solution, which reduction in the presence of the nanomaterials are summarized
authenticates that the origin of this peak originated from O2 in Table 1. The overpotential of different manganese-based
reduction. In a likewise manner to water oxidation, the higher systems as a function of experimental conditions has been
cathodic peak current and lower reduction potential with the discussed elsewhere.27 A comparative account of the pH
Au–Mn3O4 NCs (black) electrode rather than with the Mn3O4 NP conditions of the experiments and the overpotential of the Au–
(red) and Au NPs (green)- modied gold electrodes authenticate Mn3O4 composite and some other electrocatalysts is presented
that Au–Mn3O4 composites are better ORR catalysts than the in ESI 6.†
individual Au or Mn3O4 particles.37 The Au–Mn3O4 particles
Fig. 4 illustrates the cyclic voltammograms at varying pHs of
upon exposure to oxygen form radical species on the surface of the PBS with the Au–Mn3O4 NCs/4-ATP/Au electrode. A slight
the catalysts. The ability to form such radical species in the change of current height was observed for the O2/O2ꢀ couple at
presence of oxygen leads to enhanced performance of the Au– ꢀ0.18 V, but the anodic peak potential, as well as the peak
Mn3O4 composites in the oxygen reduction reaction.43 It is also current, varying with the change in pH of the solution (Fig. 4A).
likely that oxygen can dissociate on the Au surface and spill over A plot of potential vs. pH and the current vs. the pH (Fig. 4B)
from Au to the oxygen vacancies in the oxide, which synergis- show that the potential is optimum at pH ꢃ 7.5, and that the
tically promotes the adsorption and dissociation of O2.44 electrocatalytic activity for water oxidation increases with the
Therefore, the different surface structural features clearly increasing pH (5.5–9.5) of the solution, respectively.
determine the strength of the metal–support interaction and,
In conclusion, the synthesis of stabiliser-free gold–manga-
thus, the catalytic activity. The overpotentials for Au, Mn3O4, nese oxide nanocomposites by seed-mediated epitaxial growth,
and Au–Mn3O4 are 514, 505, and 370 mV, respectively. When Au employing an environmentally benign water/n-heptane inter-
interacts with transition metal oxides to form a reduced oxide face paves a facile strategy through surface attachment for
and an oxidized metal at the interface of the two materials,45,46 it combinatorial catalyst design. We have successfully overcome
dissolves at an oxidizing potential relevant to the water oxida- the key challenge in recent research of electrocatalytic water
tion reaction.47 Therefore, the presence of Au during the water oxidation, surprisingly, at a nearly neutral pH (pH ꢃ 7.5) and a
oxidation reaction could lead to an enhancement in the elec- low overpotential of 370 mV, which is beyond the typical range
trocatalytic activity of Mn3O4 particles.
of many homogeneous water oxidation catalysts (600–900 mV).
The oxidation of water to dioxygen is one of the key reactions As manganese oxides are available in various oxidation states
that need to be fully understood for water splitting devices. A and exhibit extensive biomimetic chemistry with oxygen, this
reasonable mechanism for the water oxidation and oxygen result adds a new feather and illuminates ample opportunities
reduction reactions on the nanocomposite surface could be in water oxidation electrocatalysis using a wide variety of inex-
enunciated as follows. From an electrochemical perspective, pensive and earth-abundant materials. This facile and
this reaction can be divided into two half reactions, namely,
water oxidation and proton reduction.48 The formation of H2O2
from such species suggests that hydrolysis of an O–O bonded
species proceeds more rapidly than the additional oxidation
steps needed to form O2. In the presence of the catalysts, an
O–O bonded intermediate undergoes rapid electron transfer in
the material or to the electrode to enable the selective evolution
of O2. In the present experiment, upon the addition of gold to
the metal oxide system, the gold-metal oxide perimeter inter-
face acts as a site for activating the reactants. Therefore, it could
be conceived that the very strong metal–support interactions
aer Au deposition49 create a unique interface, which results in
enhanced activity for the water oxidation and oxygen reduction
reactions.
Fig. 4 (A) Cyclic voltammograms of water oxidation in 0.1 M PBS at
pH ꢃ 5.5 (blue), 6.5 (brown), 7.5 (green), 8.5 (red), and 9.5 (black) with
Au–Mn3O4 modified electrodes; and (B) Profile showing the variation
of current and potential as a function of pH.
Table 1 Cyclic voltammetry data of water oxidation and oxygen reduction in the presence of nanomaterials
Water oxidation
1st oxygen reduction
2nd oxygen reduction
Catalysts
potential (Eox, H O), V
Ipaa (mA)
potential (Ered, O ), V
Ipc (1)a (mA)
potential (Ered, O ), V
Ipc (2)a (mA)
2
2
2
Au
Mn3O4
Au-Mn3O4
1.134
1.125
0.998
ꢀ4.5
ꢀ6.8
ꢀ17.1
0.562
0.515
0.546
3.3
6.3
15.6
ꢀ0.189
ꢀ0.139
ꢀ0.001
2.9
3.8
11.6
a
Where, Ipa and Ipc denote the irreversible peak currents at the anode and cathode, respectively.
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Acknowledgements
We gratefully acknowledge nancial support from DST, New
Delhi (Project no.:SR/FT/CS-68/2010).
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