Unconventional structural and morphological transitions of nanosheets, nanoflakes and nanorods of AuNP@MnO2

Article abstract of DOI:10.1039/c6ta02017h

Two-dimensional (2-D) layered inorganic materials with ultra-high surface area and mechanical strength have shown impressive photo-/electro-catalytic activities. We herein report a facile synthetic strategy to grow monodispersed 2-D MnO₂ nanosheet on an individual gold nanoparticle (AuNP@MnO₂ nanosheet), and demonstrate that the strongly interacted AuNP and MnO₂ nanosheet could greatly improve the electrocatalytic activity of the MnO_x family for electrocatalytic oxygen reduction reactions (ORRs). AuNP@MnO₂ nanosheets were prepared using a hydrothermal reduction of KMnO₄ by citrate ligands capped on AuNPs. Because of the metastability of the layered MnO₂ nanosheets, we observed unconventional structural and morphological transitions of amorphous MnO₂ nanosheets to δ-MnO₂ nanoflakes, and eventually to α-MnO₂ nanorods under hydrothermal conditions. The layered MnO₂ nanosheets underwent a structural expansion to nanoflakes before the curling and re-folding of layered MnO₂ nanosheets occurred. The intermediate states and structural transitions via a layer compression , for the first time, were experimentally recorded at a single-NP scale using electron microscopy. Moreover, we found the electrocatalytic activity of AuNP@MnO₂ nanosheets was enhanced roughly 30-40 times, compared to that of pure MnO₂ nanosheets and AuNPs. The strong interaction of metal-oxide interfaces (MnO₂ nanosheets and AuNPs) was likely responsible for the improved electrocatalytic activity. This interaction of MnO₂ and AuNPs was weakened in the course of hydrothermal treatment where partially positively charged Au⁺ was reduced at elevated temperatures, accompanying with the decrease of ORR activity. This insight into the effect of topological nanostructures and metal-oxide interactions on the electrocatalytic performance of the MnO_x family is believed to illustrate an alternative pathway to develop new efficient electrocatalysts.

Full text of DOI:10.1039/c6ta02017h

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ARTICLE
Unconventional structural and morphological transitions of
nanosheets, nanoflakes and nanorods of AuNP@MnO2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a,b
a
c
a
a,d
Ben Liu, Islam M. Mosa, Wenqiao Song, Haoquan Zheng, Chung-Hao Kuo, James F. Rusling,
a,d
a,d
Steven L. Suib,* and Jie He*
DOI: 10.1039/x0xx00000x
Two-dimensional (2-D) layered inorganic materials with ultra-high surface area and mechanical strength have shown
impressive photo-/electro-catalytic activities. We herein report a facile synthetic strategy to grow monodispersed 2-D
www.rsc.org/
MnO
interacted AuNP and MnO
electrocatalytic oxygen reduction reactions (ORRs). AuNP@MnO
reduction of KMnO by citrate ligands capped on AuNPs. Because of the metastability of the layered MnO
observed unconventional structural and morphological transitions of amorphous MnO nanosheets to δ-MnO
and eventually to α-MnO nanorods under hydrothermal conditions. The layered MnO nanosheets underwent a structural
expansion to nanoflakes before the curling and re-folding of layered MnO nanosheets occurred. The intermediate states
and structural transitions via a “layer compression”, for the first time, were experimentally recorded at a single-NP scale
using electron microscopy. Moreover, we found the electrocatalytic activity of AuNP@MnO nanosheets was enhanced
roughly 30-40 times, compared to that of pure MnO nanosheets and AuNPs. The strong interaction of metal-oxide
2
nanosheet on an individual gold nanoparticle (AuNP@MnO
nanosheet could greatly improve the electrocatalytic activity of the MnO
nanosheets were prepared using a hydrothermal
nanosheets, we
nanoflakes,
2
nanosheet), and demonstrate that the strongly
2
x
family for
2
4
2
2
2
2
2
2
2
2
interfaces (MnO
of MnO
2
nanosheets and AuNPs) was likely responsible for the improved electrocatalytic activity. This interaction
+
2
and AuNPs was weakened in the course of hydrothermal treatment where partially positively charged Au was
reduced at elevated temperatures, accompanying with the decrease of ORR activity. This insight into the effect of
topological nanostructures and metal-oxide interactions on the electrocatalytic performance of the MnO
believed to illustrate an alternative pathway to develop new efficient electrocatalysts.
x
family is
intercalation and chemical exfoliation of δ-MnO
2
in the
1
. Introduction
presence of tetrabutylammonium hydroxide could yield 2-D
1
3
unilamellar nanosheets of MnO
2
. Recently, a bottom-up wet-
The latest development of two-dimensional (2-D) layered
inorganic materials with a thickness of a few nanometers or
less and a lateral size ranging from hundreds of nanometers to
a few centimeters has largely advanced various types of novel
chemical synthesis was developed by utilizing a direct
4-18
1
reduction of KMnO
4
.
Because of their accessible high
nanosheets showed improved
surface area, MnO
2
1
-4
electrochemical performance for Li-ion batteries and
1
catalysts. In particular, 2-D layered materials with ultra-high
surface areas and mechanical strengths have been extensively
9-21
supercapacitors, compared to other MnO
x
species.
5
-11
Alternatively, MnO nanosheets, similar to layered δ-MnO ,
2
2
2-25
utilized as photo-/electro-catalysts directly or as supports.
A
2
are metastable;
and they can transform into other
) by rolling and
boost in catalytic activity was reported for many systems, e.g.
9
5
nitrogen-doped graphene, cobalt oxide/graphene hybrids
polymorphs (e.g. nanowires of α-MnO
2
2
2
1
0
restacking with cations and water. However, a direct
observation of this morphological and structural transition of
and Ni-Fe layered double hydroxides. Of over 30 polymorphs,
Birnessite (or δ-MnO ) is a 2-D layered MnO with edge-shared
2
x
2
MnO , as well as the intermediate states, is very challenging
MnO octahedra filled with metal ions and a single layer of
6
2
water in between layers. Sasaki et al. first reported that the
1
and has been rarely explored.
We herein report a direct observation of the morphological
and structural transition of MnO
nanoparticles (denoted as AuNP@MnO
hereafter). AuNP@MnO nanosheets composed of metallic
AuNP core and MnO nanosheet shell were prepared via a
direct hydrothermal reduction of KMnO . Because of the
metastability of layered MnO nanosheets, unconventional
structural and morphological transitions of amorphous MnO
nanosheets to α-MnO nanorods under hydrothermal
2
nanosheets grown on gold
2
nanosheets
2
2
4
2
2
2
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conditions were directly observed using electron microscopy. towards oxygen reduction reaction (ORR) were recorded in 0.1
Layered MnO nanosheets were “visualized”, for the first time, M KOH solution with a rotating disc working electrode (RDE)
to undergo a morphological expansion to form nanoflakes via a configuration at room temperature. Pyrolytic graphite (PG)
layer compression” before the curling and re-folding of was used as both working and counter electrodes, and a
layered MnO nanosheets occurred. This mechanism is quite standard calomel electrode was used as a reference electrode.
different from the proposed mechanisms for structural An ink of the nanocatalyst was prepared by mixing 4 mg of
2
“
2
2
2, 25
transitions of δ-MnO
such structural changes also allowed us to precisely tune the water/EtOH (1:1). After sonication for 5 min, 85 μL of Nafion
interfacial interaction between AuNPs and MnO that solution (5% by wt. in alcohol and water mixture) was further
dominates the electrocatalytic activity of MnO . The strongly mixed and sonicated for 30 min. The final weight percentages
interacted MnO nanosheets and AuNPs could enhance the of catalysts, carbon black, and Nafion were 43.3%, 10.8%,
electrocatalytic activity of MnO
2
previously.
This careful control over catalyst, 1 mg of carbon black (VulcanXC-72) with 1 ml of
2
2
2
2
roughly 30-40 times.
45.9%, respectively. Then 10 μL of above-prepared solution
was dropped on the PG working electrode and dried before
2
use. The surface area of the PG working electrode is 0.16 cm .
2
The catalysts loading is ~0.24 mg/cm for all samples. The
2
. Experimental
same procedure was used with the commercial Pt/C, but
without the addition of carbon black.
2
.1 Materials. Potassium permanganate (KMnO
chloride hydrate (HAuCl , 99.999%), trisodium citrate, and
sodium borohydride (NaBH , >99%) were purchased from
4
, >99%), gold
4
The number of electron transferred (n) was calculated
according to the Koutecky-Levich (K-L) equation by rotating the
electrode at different rates:
4
Sigma-Aldrich and used without further purification unless
otherwise noted. Deionized water (High-Q, Inc. 103S Stills)
with a resistivity of >10.0 MΩ was used in all experiments. All
chemical reagents were used without further purification
unless otherwise noted.
1
1
1
1
1
=
ꢁ
+
=
+
ꢀ
ꢀꢂ ꢀꢃ ꢄꢅꢆ/ꢇ ꢀꢃ
ꢄ = 0.62ꢈꢉꢊ (ꢌ )ꢇ/ꢍꢎꢏꢆ/ꢐ
ꢋ
ꢋ
ꢀꢃ = ꢈꢉꢑꢊꢋ
2
.2 Synthesis of AuNP@MnO
Preparation of AuNP@MnO
experiment (using AuNP-14 nm an example), 20 mL of KMnO
10 mM) were slowly added to 80 mL of as-made AuNP-14 nm
2
k
where j is the measured current density, j is the kinetic
2
nanosheets. In a typical
L
current, and j is the diffusion limiting current, respectively. ω
4
is the rotation speed of the electrode in rpm, k is the electron
transfer rate constant, B is the reciprocal of the slope of the K-
(
solution (the content of sodium citrate is ~24 mg), and
incubated under room temperature for 1 h. Then, the above
o
solution was heated to 80 C and incubated for another 30 min
L plots, F is Faraday constant (96485 C/mol), C
3
concentration of oxygen in 0.1 M KOH (1.2 × 10 mol/cm ), D
0
is the saturated
-6
0
-5
2
is the diffusion coefficient of O
2
(1.9 × 10 cm /s), and ν is
to grow monodispersed AuNP@MnO
the nanocatalysts were collected by centrifugation twice and
washed by water to remove unreacted KMnO . The as-
obtained AuNP@MnO nanosheets were redispersed into
water to form 0.2 mg/mL of solution for controlled synthesis of
nanoflakes and nanorods. AuNP@MnO nanosheets with
2
nanosheets. After that,
2
kinematic viscosity of the electrolyte (0.01 cm /s).
4
2
.4 Characterizations. SEM images of the nanocatalysts were
2
recorded using a FEI Nova NanoSEM 450 with an accelerating
voltage of 10 kV and a beam current of 10 mA. SEM samples
were prepared by casting the suspension of the materials on a
silicon wafer. TEM and high-resolution TEM (HR-TEM) studies
were carried out using a JEOL 2010 TEM with an accelerating
voltage of 200 kV. Scanning TEM (STEM) mapping and HAADF-
STEM were performed on a Talos F200X Atomic Resolution
Analytical Microscope. TEM and STEM samples were prepared
by casting the suspension of the materials on a carbon coated
copper grid (400 mesh). The wide-angle X-ray scattering (XRD)
2
different diameters of AuNPs were synthesized according to
the above-mentioned procedures.
Hydrothermal treatment of AuNP@MnO
AuNP@MnO nanoflakes, curled nanoflakes and nanorods
were synthesized by hydrothermal treatment of
AuNP@MnO nanosheets. For AuNP@MnO nanoflakes, the
as-prepared solution of AuNP@MnO nanosheets (15 mL) was
2
nanosheets.
2
a
2
2
2
patterns were recorded using
a
Rigaku Ultima IV
sealed in Teflon-lined stainless steel autoclaves (25 mL) and
o
hydrothermally treated under 110 C for 6h. After cooled
diffractometer (Cu Kα radiation, λ=1.5406ꢀÅ) with an operating
voltage of 40ꢀkV and a current of 44ꢀmA. Wide-angle XRD were
down to room temperature, the product was washed with
o
collected over a 2θ range of 8~80 with a continuous scan rate
water for three times. The resulting product was AuNP@MnO
nanoflakes (Fig. 2a). Similarly, curled AuNP@MnO nanoflakes
2
o
-1
of 0.5 min . Ultraviolet-Visible (UV-vis) spectra were collected
with a Cary 60 UV-Vis spectrophotometer. Atomic force
microscopy (AFM) was performed on Bruker MultiMode 8
using tapping mode at room temperature. AFM samples were
prepared on freshly cleaved mica substrate. X-ray
photoelectron (XPS) spectra were recorded on a PHI model
2
were prepared by hydrothermal treatment of AuNP@MnO
0
nanosheets at 135 C for 12 h (Fig. 2c), while AuNP@MnO
2
2
nanorods were obtained by hydrothermal treatment of
0
AuNP@MnO nanosheets at 160 C for 12 h (Fig. 2e).
2
5
90 spectrometer with multi-probes using Al Kα (λ = 1486.6
2.3 ORR evaluation. The electrocatalytic activities of carbon
eV) as the radiation source. The spectra were recorded in the
black, AuNP@MnO nanocatalysts and commercial Pt/C
2
2
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fixed analyzer transmission mode with pass energies of 100 tape mounted on an adhesive Cu tape stuck to a sample stage
and 50 eV for recording survey and high-resolution spectra, placed in the analysis chamber. Binding
respectively. The powder samples were pressed on a carbon
Fig. 1 The nanostructures of AuNP@MnO
AuNP@MnO nanosheets. (f) The size distributions of AuNPs and AuNP@MnO
averaging >200 particles. (g) The change in UV-vis absorption of AuNP-KMnO solution at various reaction times during synthesis.
The UV-vis spectra were recorded at a time interval of 2 mins. The inset of (g) is the UV-vis spectrum of purified AuNP@MnO2
nanosheets in water. (h) The change of peak intensity at 351 nm (square) and the conversion of KMnO (triangle) vs. the reaction
time. The inset in (h) are the optical pictures of reaction mixtures before (left), and after (right) hydrothermal treatment. (i) XPS
spectrum of AuNP@MnO nanosheets and zoomed-in view of Au 4f and Mn 2p peaks.
2
nanosheets. (a) SEM, (b-c) TEM, (d) HR-TEM images, and (e) STEM-EDX mapping of
2
2
nanosheets measured from TEM images by
4
4
2
energies (BEs) were measured for C 1s, O 1s, Au 4f and Mn 2p. Spectrometer Varian Spectra AA 110. The Au@MnO
2
samples
The XPS spectra obtained were analyzed and fitted using (10 mg) were first digested in an acid solution (HCl : HNO
3
=
CasaXPS software (version 2.3.12). Sample charging effects 3:1, vol) for 10 h until the solids were completely dissolved.
were eliminated by correcting the observed spectra with the C The solutions were then diluted with DI water and analyzed by
1s BE value of 284.6 eV. The overlapping Mn 2p and O 1s peaks AAS.
were deconvoluted into several sub-bands by using an optimal
combination of Gaussian (70%) and Lorentzian (30%)
functions. Raman measurements were performed using a
Renishaw 2000 Raman scope attached to a charge-coupled
device camera, with an Ar ion laser (514.4 nm) as the
excitation source. The atomic absorption spectroscopy (AAS)
measurements were performed using an Atomic Absorption
3
. Results and Discussion
AuNP@MnO
reduction of KMnO
AuNPs capped with sodium citrate ligands were directly mixed
2
nanosheets were prepared using a hydrothermal
4
. In a typical synthesis, the pre-synthesized
o
with a KMnO
4
solution and hydrothermally treated at 80 C for
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30 min (see Experimental for synthetic details). As citrate With sodium citrate as the capping ligands of AuNPs, this
ligands on the surface of AuNPs can act as reductants for synthetic method was demonstrated to grow 2-D core-shell
-
MnO
deep purple to dark brown (inset in Fig. 1h and Fig. S1), Interestingly, the diameter of MnO
indicating the formation of MnO nanosheets. Using 14 nm of AuNPs remained in the range of 12-18 nm regardless of
AuNPs as an example, the growth of MnO nanosheets on the AuNP sizes. Again, the importance of limited reducing agents
surface of AuNPs was characterized by SEM and TEM as given on AuNP is to avoid the overgrowth of nanosheets.
in Fig. 1 (see Figs. S2-4 for more images). AuNP@MnO
4
, the color of the mixture solution quickly changed from AuNP@MnO
2
nanosheets with 4-45 nm AuNP cores (Fig. S5-7).
2
nanosheets on the surface
2
2
2
nanosheets have a homocentric core-shell nanostructure in
which a monodispersed AuNP is uniformly covered by a shell
2
of MnO nanosheets (Fig. 1a-c). The STEM energy-dispersive X-
ray (EDX) mapping confirmed the nanostructure and elemental
distributions of AuNP@MnO nanosheets. A dense MnO shell
2
2
grew and fully covered the surface of AuNPs (Fig. 1e). The
weight percentage of MnO2 nanosheets is ~58 wt% from
STEM-EDX mapping (Fig. S4), in good consistency with the AAS
measurements (Table S1). The MnO2 shell is composed of
multilayered nanosheets (roughly 4-7 layers) as seen in the HR-
TEM image (Fig. 1d), corresponded to a thickness of 2~4 nm.
The lamellar d-spacing of nanosheets from the HR-TEM image
is approximately 0.54 nm. The diameter of AuNP@MnO2
nanosheets is 38±8 nm (Fig. 1f) with a fairly uniform coverage
on individual AuNPs. The wrinkles of MnO nanosheets and the
2
centralized location of AuNPs as projected from TEM images
suggest that the MnO nanosheets are loosely packed on the
2
surface of AuNPs. XPS of AuNP@MnO2 nanosheets clearly
shows Mn 2p peaks and Au 4f peaks (Fig. 1i). The intensity of
Au 4f peaks, however, is much weaker compared to that of Mn
2p peaks, indicating that AuNPs are likely embedded in MnO2
nanosheets.
The key to growing MnO nanosheets on an individual AuNP
2
is the utilization of citrate-protected AuNPs as a nucleation
center. The trace amount of sodium citrate on the AuNP Fig.
surface can act as a reducing agent for KMnO4.
No nanosheets under different hydrothermal temperatures. TEM
additional reducing agents were required in our synthesis images of MnO nanomaterial transformed from (a,b) 2-D
system. The reduction of KMnO4 likely occurred MnO nanoflakes with near hexagonal morphology, to (c,d)
2 The morphological evolutions of AuNP@MnO
2
1
8, 26
2
2
heterogeneously only on the surface of AuNPs, as no free curled nanoflakes with partially folding nanotubes/nanorods,
MnO2 nanosheets were observed from TEM images. To and eventually to (e,f) 1-D nanorods. The arrows in (c-d)
monitor the conversion of KMnO and formation process of indicate the curling and folding of MnO
2
nanoflakes.
4
MnO2 nanosheets along AuNPs, time-dependent UV-Vis
spectra of the reaction mixture were recorded (Fig. 1g). The
initial UV-vis absorption peaks at around 310 nm and 560 nm morphological evolution of MnO
When subjected to
a
hydrothermal treatment, the
nanosheets can be recorded
are attributed to the absorption of MnO . The gradual using electron microscopy (Fig. 2). Taking 14-nm AuNP@MnO
2
-
4
2
decrease in intensity at 560 nm can be seen when incubating nanosheets as an example again, upon the hydrothermal
o
o
at 80 C. Meanwhile, a new broad peak at ~351 nm appeared; treatment at 110 C for 6 hr, randomly stacked MnO
its intensity increased rapidly, indicating the formation of nanosheets gradually spread out and became rigid, near-
2
2
7
colloidal MnO
intensity at 351 nm and the conversion of KMnO
from the absorption peak at 560 nm were given in Fig. 1h. The larger in diameter, compared to that of the initial nanosheets;
conversion of KMnO
could only reach up to ~19% regardless while, the thickness of nanoflakes decreased to ~1.4 nm (Fig.
of the reaction time. This low conversion suggests the amount S9), corresponding to 2~3 atomic layers of MnO layered
of KMnO was excessive and the formation of MnO
nanosheets. During this process, AuNPs migrated toward the
2
nanosheets. The kinetic change of the peak hexagonal nanoflakes (Fig. 2b). The diameter of 2-D nanoflakes
calculated increased to ~80 nm (Fig. S8-10), approximately two times
4
4
2
4
2
nanosheets was only dependent on the “insufficient” amount edge of the nanoflakes; and the curling/folding of the
of sodium citrate ligands on AuNPs. Once the surface was nanoflakes could be also seen from the edges of nanoflakes.
o
completely consumed, the growth of MnO
have ceased. Furthermore, the diameter of MnO
2
nanosheets should When increasing the temperature to 135 C for 12 hr, a large
2
nanosheets decrease in diameter of MnO nanoflakes can be observed
2
(
as the shell) is insensitive to the original sizes of AuNP cores. from the TEM images (Fig. 2c-d and Fig. S11). AuNPs were off-
4
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center mostly. The edges of nanoflakes partially curled and ~360 nm for MnO
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2
colloids and the other one at 568 nm for
refolded into 1-D rod-like morphologies (3~5 nm in diameter). the localized surface plasmon absorption of AuNPs. The
The increase in diameter of 1-D rods was likely due to the electronic excitation of MnO layered structures is known to
multiple curling and folding at the edges as shown in Fig. 2d. be determined by the interlayer spacing and the size of MnO
2
2
2
8
The layer spacing of nanoflakes slightly increased to 0.578 nm. colloids. The absorption peak of MnO
2
nanosheets at 360 nm
The lattice fringe of 0.247 nm perpendicular to the layers can firstly blue-shifted to 354 nm (nanoflakes), due to the decrease
be assigned to the (211) plane of α-MnO , indicating that in the layer number of nanosheets; and then red-shifted to
layered Birnessite δ-MnO has partially evaluated into α-MnO 356 nm (curled nanoflakes), because of the re-curling of
With further increasing the hydrothermal temperature to 160 nanosheets. The intensity of the absorption peak became
2
2
2
.
o
29
C, the curled nanoflakes merged together through weaker with the formation of α-MnO
2
nanorods.
interparticle recrystallization. Long MnO nanorods with a Second, the structural transitions of MnO were further
2
2
diameter of 10-20 nm and a length up to several micrometers monitored by XRD (Fig. 3b). Four strong XRD peaks in the
o
were observed; while AuNPs completely moved to the surface range of 38-80 for all samples are ascribed to polycrystalline
of the MnO nanorods (Fig.2e-f). The two d-spacings of 0.705 AuNPs. For AuNP@MnO nanosheets, MnO was absence of
2
2
2
nm and 0.319 nm were seen for MnO nanorods under HR- regular layer stacking and only a weak, a broad XRD peak was
2
o
TEM, corresponding to the (110) and (310) planes of α-MnO2, found at ~12.4 , possibly originating from the lamellar
respectively (Fig. 2f). A slight loss of AuNPs was noted during structure of multilayered nanosheets during drying. Two well-
o
o
the phase transitions of MnO2 as given in Table S1. It is defined XRD signals appeared at 12.0 and 24.4 for
possibly because of the migration of AuNPs to the surface of AuNP@MnO2 nanoflakes, assigned to (001) and (002)
MnO . Some of weakly interacted AuNPs were removed during reflections of Birnessite-type δ-MnO , respectively. For curled
2
2
the sample purification.
AuNP@MnO2 nanoflakes, the peaks of (001) and (002)
reflections did not shift but the intensities of peaks further
increase. Thus, both spread nanoflakes and curled nanoflakes
are mostly assigned to δ-MnO . In the case of AuNP@MnO2
2
nanorods, three diffraction peaks were identified as (110),
(
200), and (310) planes of α-MnO . Overall, the structural
2
evolution of MnO2 nanosheets during the hydrothermal
process was confirmed from amorphous layered structures, to
δ-MnO , and eventually to α-MnO (Fig. 3d).
2
2
Raman spectroscopy was used to further probe the change
in the molecular vibrational behaviour of materials and
support the structural transitions of nanosheets (Fig. 3c). The
very initial MnO nanosheets were amorphous, which only
2
-1
exhibited a single broad Raman signal at 572 cm . This peak
was assigned to the Mn-O lattice vibration of MnO6
2
5, 30
octahedra.
A new Raman peak gradually appeared at
-1
around 645 cm for AuNP@MnO nanoflakes, indicating the
2
formation of Birnessite-type δ-MnO and the stacking of
2
-1
layers. The peak at ~572 cm disappeared and only one sharp
-1
Raman peak appeared at around 652 cm , which confirmed
the structural transition to α-MnO nanorods. This suggested
2
the reorganization of MnO6 octahedra with less lattice
3
distortion after hydrothermal treatment.
0
The structural transitions of MnO polymorphs, as well as
2
the transition mechanism of 2-D layered structures to 1-D
nanorods under hydrothermal treatments, have been reported
Fig. 3 The structural evolutions of AuNP@MnO
vis spectra, (b) XRD patterns and (c) Raman spectra for nanosheets
2
nanosheets. (a) UV-
22, 23, 25
previously.
MnO
nanostructures.
Wang et al. observed the formation of δ-
2
nanosheets as an intermediate to produce 1-D MnO
2
(
(
black), nanoflakes (red), curled nanoflakes (blue) and nanorods
magenta). The diffraction peaks from polycrystalline gold in (b) are
2
2
The rolling/curling of layered lamellar
structures was proposed for the structural transition. Shen et
nanoflakes, and to al. studied the phase transformation of Birnessite MnO to β-
by in situ synchrotron powder XRD. The layer
nanosheets were further “compression” of re-stacked Birnessite MnO along the c
confirmed using UV-vis absorption spectra (Fig. 3a), XRD (Fig. direction was
b) and Raman scattering spectra (Fig. 3c). First, two
absorption peaks of MnO nanosheets could be found, one at
marked with “*”. (d) The change in crystalline structures from
amorphous multilayered nanosheets to δ-MnO
α-MnO nanorods with tunnel structures.
The structural transitions of MnO
2
2
2
3
MnO
2
2
2
2
3
2
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Fig. 4 Electrocatalytic ORR activities of AuNP@MnO
rotation speed of electrodes is 1600 rpm and the loading amount of AuNP@MnO
AuNP@MnO nanosheets (red line) and Pt/C (black line). (c) Current densities of ORR activities at potential of 0.8 V (red) and
.85 V (blue) of Pt/C, Au@MnO , discrete AuNPs and MnO nanosheets. (d) The long-term stability of AuNP@MnO nanosheets
black line) and Pt/C (red line).
2
. (a) LSV curves of commercialized Pt/C (black line) and AuNP@MnO
2
. The
2
2
and Pt/C is 0.24 mg/cm . (b) Tafel plots of
2
0
2
2
2
(
experimentally recorded to be responsible for the formation of lamella (Fig. 2d). The phase transitions of MnO
×2 and/or 1×1 tunnel structures. In a very recent report, influenced by the presence of AuNPs (Fig. S13) or the sizes of
Sinha et al. presented a possible “dissolution-recrystallization” AuNPs (Fig. S5-7). However, when the size of AuNPs was larger
mechanism for the shape transformation of MnO nanosheets (> 20 nm), the layer “compression” of MnO nanosheets
to nanowires. They proposed that the dissolution of became more difficult, as the transition for core-shell
amorphous δ-MnO nanosheets occurred first to detach and AuNP@MnO nanosheets to nanoflakes involved a larger
volume expansion. Overall, aggregation-free MnO nanosheets
on the surface of AuNPs made it possible to visualize the
2
were not
1
2
2
2
5
2
2
diffuse into nanodomains and further recrystallized on α-MnO
nuclei.
2
2
Our results likely fit in to the layer “compression” structural transitions at a single-NP level.
mechanism. The formation of MnO nanoflakes was Since all intermediates of structural evolutions can be
2
conceivably a result of the layer “compression” at the c isolated using different hydrothermal temperatures, it is thus
direction. The most direct observation is that, the increase in possible to evaluate the effect of nanostructures and
the diameter of nanosheets from 38 to 80 nm and the polymorphs of MnO
2
on electrocatalytic activity for the ORR
decrease in layer thickness are evidenced for the remerging of (Fig. 4). To examine ORR activities, cyclic voltammetry (CV) was
o
nanosheets at 110 C. This process was also associated performed on AuNP@MnO
2
nanosheets (14 nm-AuNPs for all
simultaneously with the formation of tunnel structures of electrochemical results) in 0.1 M KOH solution (Fig. S14). In a
MnO
2
nanorods (2×2 tunnel in this case) perpendicular to the
N
2
-saturated electrolyte, the voltammogram without an
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obvious slope for cathodic current density was present; while, S17). The average number of electron transfer for
a well-defined cathodic oxygen reduction peak with a high AuNP@MnO nanosheets is between 3.1 and 2.7 in the
current density appeared in an O -saturated electrolyte, potential range of 0.75-0.35 V vs. RHE, giving an H yield of
indicating that the AuNP@MnO nanosheet is active for ORRs. 45%~65%; this shows a slight decrease when decreasing the
The ORR activity of AuNP@MnO
recorded using linear sweep voltammetric (LSV) scanning with reduction to form H
2
2
2 2
O
2
-
2
nanosheets was further potential. Transition metal oxides generally favor a partial 2e
, as these oxides are highly active in
2
a sweep rate of 5 mV s . As a comparison, similar alkaline media toward the disproportionation reaction of H O ,
2 2
O
-1
2
3
1
measurements were performed on the commercialized Pt/C especially for layered MnO
catalyst (20 wt% of Pt loaded) (also see Fig. S15 for ORR active sites generated by the synergetic coupling of MnO
activity of Vulcan XC-72 carbon black). The ORR current- AuNPs were somehow responsible for the enhanced activity
2
nanosheets. This implies that the
2
and
2
potential curve of AuNP@MnO
2
nanosheets (0.24 mg/cm ) for ORRs.
Several control experiments were carried out to confirm this
shows a lower onset potential and higher current density. The
current density at the reduction potential of 0.9 V (vs. hypothesis. First, the ORR activities of discrete loose-packed
reversible hydrogen electrode (RHE), hereafter) is -1.41 MnO nanosheets and 14-nm AuNPs were examined under the
2
2
mA/cm for AuNP@MnO nanosheets, corresponding to a two- same experimental conditions (Fig. S16 and Table 1). Both of
2
fold higher activity than commercialized Pt/C catalysts (-0.82 them exhibited a low ORR catalytic activity with a half-wave
2
2
31, 32
mA/cm ). The potential at j = -2 mA/cm for AuNP@MnO
potential <0.7 V, similar to the reported results.
nanosheets is 0.88 V, 30 mV higher than that of Pt/C catalysts activity of AuNP@MnO2 nanosheets is roughly 30-40-fold
Fig. 4a). The stability of AuNP@MnO nanosheets was also higher than that of pure MnO nanosheets and 14-nm AuNPs
The
2
(
2
2
studied by current-time (i-t) chronoamperometry at a constant (Fig. 4c). One main reason for the high electrocatalytic activity
potential of 0.6 V (Fig. 4d). A respectable current retention of of AuNP@MnO nanosheets is the decrease of charge transfer
2
>
96% for AuNP@MnO nanosheets was maintained after 24 resistance, compared to pure MnO nanosheets (Fig. S18-19).
2 2
hr, indicating a superior durability of AuNP@MnO nanosheet Second, the ORR activities of AuNP@MnO nanoflakes, curled
2
2
catalysts; while only 50% of activity was found for Pt/C catalyst nanoflakes and nanorods were also evaluated (Fig. 4a) under
after 19 hr. Moreover, the AuNP@MnO2 nanosheets also similar conditions. All ORR polarization curves shifted to lower
showed a better tolerance to methanol crossover effects (Fig. potentials when increasing the hydrothermal temperatures.
2
The potential to reach j = -3 mA/cm is 0.80 V for nanoflakes,
S20).
0.78 V for curled nanoflakes and 0.72 V for nanorods,
Table 1. Summary of the ORR catalytic activities
respectively, indicating a drop of ORR activity over the course
of morphological transitions. The morphological transition
a
b
b
c
Catalysts
E
0
(V)
E
1/2 (V)
0.82
0.81
0.83
0.83
0.65
0.70
0.84
0.69
E
j = -3 mA/cm2 (V)
from nanosheets to nanorods of AuNP@MnO resulted in a
2
AuNP@MnO
AuNP@MnO
AuNP@MnO
AuNP@MnO
MnO nanosheets
2
nanosheets
0.92
0.88
0.90
0.92
0.86
0.86
0.91
0.75
0.84
5
0% loss of activity at the potentials of 0.8 V and 0.85 V (Fig.
c).
2
nanoflakes
0.80
4
2
curled nanoflakes
0.78
2
nanorods
0.72
2
n. a.
AuNP (14 nm)
Pt/C
n. a.
0.83
Vulcan XC-72 carbon
n. a.
a
All catalysts are based on 14 nm AuNP in this investigation.
Results based on LSV curves collected on RDE with a rotation
b
0
rate of 1600 rpm: E is the onset potential; E1/2 is the half-wave
c
2
potential. Potential versus RHE at -3 mA/cm . The rotation
speed of electrodes is 1600 rpm for all samples.
2
Tafel plots of AuNP@MnO nanosheets and Pt/C catalyst
derived from LSV curves are given in Fig. 4b. In the electron-
2
transfer dominating region (j<1 mA/cm ), two well-defined
Fig. 5 Au 4f (a) and Mn 2p (b) XPS spectra of AuNP@MnO
2
.
linear slopes of -71 mV/decade and -136 mV/decade are
present for AuNP@MnO
2
nanosheets. There are likely two
Many previous studies have demonstrated that α-MnO
2
33-36
has
For
kinetic processes occurring on two types of ORR active sites
the best ORR activity among all polymorphs of MnO
pure MnO
reduction of MnO
insertion of edge-shared MnO
terms of the formation of active Mn sites for the O
x
.
3
also see Fig. S18). To gain further insight into the ORR
1
(
2
catalysts, the initial step of electrochemical
via the breaking of oxo bridges and proton
octahedra is important in
kinetics, the kinetic-limiting current j was measured at various
k
2
rotation speeds and the number of electrons transferred (n)
6
for the ORR calculated from Koutecky-Levich (K-L) plots (Fig.
37
2
bonding.
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The stronger Mn-O bonding strength generally makes the To summarize, we described a facile synthesis of AuNP@MnO
initial proton insertion process more difficult. The average Mn- nanosheets and studied their structural and morphological
O bond length of layered MnO is in the range of 0.191~0.194 transitions under hydrothermal treatment. Unconventional
nm, smaller than that of α-MnO nanorods. This suggests morphological transitions of MnO nanosheets to α-MnO
that the oxo bridges in α-MnO nanorods are easier for proton nanorods were observed using electron microscopy at a single-
insertion, in terms of the bonding energy, leading to a higher NP level. The layer “compression” of 2-D layered MnO
ORR activity for α-MnO nanorods, in principle. Therefore, nanosheets was proposed to be responsible for the formation
these results, again, point out that the enhanced ORR activity of MnO nanoflakes as an intermediate. We have
of AuNP@MnO nanosheets is a result of the interaction of demonstrated that the strongly interacted AuNPs and MnO
AuNPs and MnO nanosheets. The synergetic effect from the could significantly enhance their electrocatalytic activity for
interaction of MnO
the ORR activity of AuNP@MnO
We further used XPS spectra to characterize the interaction improvement of the ORR activity for 30-40 fold. The insight
of AuNPs and MnO nanosheets. The high-resolution XPS into the effect of the topological nanostructures and metal-
spectra of Au 4f and Mn 2p regions of AuNP@MnO oxide interactions on the electrocatalytic performance of
nanosheets, nanoflakes and nanorods are given in Fig. 5. The MnO family may illustrate an alternative pathway to develop
Au 4f7/2 peak with a binding energy of ~84.3 eV for new electrocatalysts.
AuNP@MnO nanorods indicates a metallic Au(0) state (Fig.
(a)). The decrease of Au 4f7/2 binding energy ~0.3 eV and the
broadening of Au 4f7/2 peak can be seen for AuNP@MnO
2
2
38
2
2
2
2
2
2
2
2
2
2
+
2
nanosheets and AuNPs is responsible for the ORR. The partially oxidized Au resulted in a strong
nanosheets. interaction at the metal-oxide interface and therefore the
2
2
2
x
2
5
Acknowledgements
2
nanosheets. This shift in the binding energy is interpreted as a
+
result of the presence of partial positive Au species, because
JH thanks the financial support of startup funds from the
University of Connecticut. SLS acknowledges support of the US
Department of Energy, Office of Basic Energy Sciences, Division
of Chemical, Biological and Geological Sciences under grant
DE-FG02-86ER13622. A000. The SEM/TEM studies were
performed using the facilities in the UConn/FEI Center for
Advanced Microscopy and Materials Analysis (CAMMA). This
work was also partially supported by the Green Emulsions
Micelles and Surfactants (GEMS) Center, FEI Company under
of the strong adsorption of AuNPs on the oxygen vacancies of
3
9, 40
MnO
AuNP@MnO
due to the over oxidation of Au atoms on the surface of AuNPs
2
nanosheets via strong electronic interactions.
For
+
2
nanosheets, the formation of Au species is likely
+
4
by KMnO ; while, these Au species can be reduced by Mn
species with a lower oxidation state at elevated temperatures
during hydrothermal treatments. As a comparison, the binding
energy of Mn 2p3/2 peak appears at ~641.8 eV for
an FEI-UConn partnership agreement and
Excellence Award of the University of Connecticut.
a
Research
AuNP@MnO
AuNP@MnO
2
nanosheets, and gradually shifts to ~642.7 eV for
nanorods. A lower binding energy of Mn 2p
2
2
peaks (~0.9 eV) is observed for AuNP@MnO nanosheets. This
suggests that a higher proportion of Mn species with a lower Notes and references
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Table of Contents
A facile approach to growing two-dimensional MnO
gold nanoparticles is reported.
2
nanosheets on
1
0 | J. Name., 2012, 00, 1-3
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