Pt Cu nanocrystals with hexa-pod morphology and their ...We utilized a facile solvothermal approach to synthesize the hexa-pod PtxCuy NCs. [Pt(NH3)4](NO3)2 and Cu(NO3)2 were used as

Nano Res

1

PtxCuy nanocrystals with hexa-pod morphology and

their electrocatalytic performances towards oxygen

reduction reaction

Yujing Li1,2 (), Fanxin Quan2, Enbo Zhu3, Lin Chen2, Yu Huang3,4, and Changfeng Chen1,2

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0834-7

http://www.thenanoresearch.com on June 9, 2015

© Tsinghua University Press 2015

Just Accepted

This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been

accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance,

which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP)

provides “Just Accepted” as an optional and free service which allows authors to make their results available

to the research community as soon as possible after acceptance. After a manuscript has been technically

edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP

article. Please note that technical editing may introduce minor changes to the manuscript text and/or

graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event

shall TUP be held responsible for errors or consequences arising from the use of any information contained

in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®),

which is identical for all formats of publication.

Nano Research DOI 10.1007/s12274-015-0834-7

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

1 Nano Res.

TABLE OF CONTENTS (TOC)

PtxCuy nanocrystals with hexa-pod morphology and their electrocatalytic performances towards oxygen reduction reaction

Yujing Li1,2*, Fanxin Quan2, Enbo Zhu3, Lin Chen2, Yu Huang3,4, Changfeng Chen1,2

1. State Key Laboratory of Heavy Oil, China

University of Petroleum, Beijing, Changping,

102249, China.

2. Department of Materials Science and

Engineering, College of Science, China University

of Petroleum, Beijing, Changping, 102249, China.

3. Department of Materials Science and

Engineering, University of California-Los Angeles,

Los Angeles, California, 90095, United States.

4. California NanoSystems Institute, University of

California-Los Angeles, Los Angeles, California,

90095, United States.

Hexa-pod PtxCuy synthesized with wet chemistry display

superior electro-catalytic activity towards oxygen reduction

reaction due to the existence of high-index facets.



reduction reaction

Yujing Li1,2 (), Fanxin Quan2, Enbo Zhu3, Lin Chen2, Yu Huang3,4, Changfeng Chen1,2

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

Oxygen reduction

reaction; Bimetallic alloy;

Morphology control; Fuel

cell Electro-catalysis;

High-Index.

ABSTRACT

Synthesis of bimetallic PtxCuy nanocrystals (NCs) with well-shaped hexa-pod

morphology through wet chemistry approach is reported. As-synthesized

convex NCs around 20nm in size expose by low-index (111) facets on seeds and

various high-index facets on pods. The growth mechanism involves preferred

growth along crystallographic direction on cuboctahedral seeds. The

synthetic protocol can be applied to synthesis of PtxCuy NCs with various Cu/Pt

ratios. The electro-catalytic activities of hexa-pod PtxCuy NCs supported on

carbon black towards oxygen reduction reaction (ORR) are studied. Hexa-pod

PtCu2/C catalysts exhibit the highest specific activity (3.7 mA/cm2Pt) and mass

activity (2.4 A/mgPt), the highest compared with data reported from literatures

on PtxCuy. Comparison with PtxCuy in other morphologies indicates that the

enhanced activity originates from morphology factor. Existence of high-index

facets, as well as abundant edges and steps on pods, could reasonably explain

the enhanced catalytic activity. The hexa-pod PtxCuy/C catalysts also show high

stability in morphology and activities after accelerated durability tests. As-

synthesized hexa-pod PtxCuy NCs are highly potential cathode electro-catalysts

for proton exchange membrane fuel cells.

Nano Research DOI (automatically inserted by the publisher)

Research Article

Address correspondence to Yujing Li, Email: [email protected]

Carbon supported Pt nanoparticles are currently the

most widely accepted cathode catalysts for proton

exchange membrane fuel cells (PEMFCs), but Pt-

based alloy nanocrystals (NCs), mainly Pt alloyed

with transition metal M (denoted as PtM, M=Ni, Co,

Fe, Mn, etc.), have gained enormous attention in the

past decade due to their improved catalytic activities

compared with pure Pt in oxygen reduction reaction

(ORR).[1-10] The drastically improved electro-

catalytic performance of PtM alloys can mainly be

attributed to surface electronic effect, and geometric

effect induced by the surface strain or atomic

arrangement.[11-13] It has also been reported by

literatures that, for PtM alloys, the formation of Pt-

skin structure is crucial in achieving enhanced ORR

activities.[14, 15] It is generally accepted that the

formation of Pt-skin structure leads to the modified d-

band electronic structure and shortened Pt-Pt distance

on surface, both of which could benefit the adsorption

of oxygen molecules and breakage of O-O bond.[2, 16-

20]

Among various PtM alloys for PEMFC cathode

application, PtxCuy is a special alloy family that has

not received much attention.[21-24] Strasser et al

reported that electro-catalytic activities of PtxCuy

towards ORR can also be drastically enhanced

through voltammetric de-alloying approach, and

hypothesized that lattice-contracted alloy core

stabilizes the Pt-rich skin with surface strain and

hence improves the surface catalytic activity.[25, 26]

The same group also found that when increasing

Cu/Pt ratio for PtxCuy alloy, the catalysts show

elevated ORR activities in acidic media but declined

activity in alkaline electrolyte.[27] Cu atoms on the

surface remain stable in alkaline media, but tend to

deplete in acid, leading to the formation of ORR-

preferred Pt-rich skin in only acidic media, which

explains their observed distinctive behaviors in

different electrolytes. Stephens and Chorkendorff et al

engineered a Cu/Pt(111) near-surface alloy with Cu

atoms in subsurface up to 1 monolayer, and

demonstrated 8-fold enhancement of ORR activity

over Pt(111) with appropriate amount of deposited

Cu.[28] They proposed that the presence of subsurface

Cu can weaken the binding of surface Pt atoms to OH*

intermediates and expedite the ORR. From literatures,

PtxCuy alloys have clearly demonstrated enhanced

ORR activities via surface de-alloying.[29-31]

Nonetheless, to achieve ORR activities from PtxCuy

catalysts as high as those reported from PtxNiy-based

alloys remains difficult.

Both theoretical and experimental studies have

pointed out that, for Pt-based alloy catalysts, their

catalytic activities strongly depend on exposed

facets.[14, 32] For instance, Stamenkovic and

Markovic et al found that for single crystalline Pt3Ni

film, Pt3Ni(111) facet shows one-order-of-magnitude

enhancement in ORR activity compared with Pt(111)

surface, which is attributed to modified surface d-

band electronic structure induced by atomic

arrangement on (111) facet and Pt-rich surface

compositional profiles as mentioned above.[14] For

nano-sized catalysts, morphology of catalyst particle

determines exposed facets, and therefore affects their

activities towards ORR.[33-35] In addition, it is also

known that atoms at corners and edges of catalyst

particle, determined by morphology as well, situated

in special strain and bonding states and thus could

show distinctively high catalytic activities.[32, 36]

Therefore, controlling the morphology of alloy NCs is

crucial in optimizing catalytic performance and

investigating catalytic mechanism.[37-39] Large

amount of studies have been placed on morphology-

controllable syntheses of PtNi, PtCo, and PtFe alloy

families, while relative literatures are limited for PtCu

alloy. Earlier literatures on PtCu nanocatalysts

generally show irregular morphologies.[21, 23] Wang

and Li et al reported a unique synthetic protocol of

synthesizing PtxCuy with various Cu/Pt ratios in

different shapes, including mainly spheres, truncated

octahedron, flowers etc, demonstrating the possibility

to control the morphology of PtCu NCs.[40] Fang et al

reported the synthesis of octahedral and cubic PtCu

NCs with the typical oleylamine/oleic acid protocol

with the presence of tungsten hexacarbonyl.[41, 42]

Saleem and Wang et al employed a two-step synthetic

approach, and obtained ultrathin PtCu nanosheets,

based on which PtCu nanocones could further be

obtained by controllable rolling of nanosheets.[43]

However, none of the above work correlates

morphology of PtCu NCs with their catalytic activities

towards ORR, leading to the lack of data regarding

morphology-activity relation.

Herein, we demonstrate a facile and robust approach

for harvesting uniform monodisperse PtxCuy NCs

with hexa-pod morphology in various Cu/Pt ratios.


3 Nano Res.

As-synthesized hexa-pod PtxCuy NCs are enclosed

mainly by high-index crystallographic facets,

hypothetically in a seeding scheme. The electro-

chemical activities towards ORR are determined for

the PtxCuy NCs with Cu/Pt ratio of 2/3, 1/1, and 2/1

supported on commercial carbon black. The highest

specific activity, 3.7 mA/cm2Pt, and mass activity, 2.4

A/mg Pt, are achieved with PtCu2/C, both of which are

higher than the best ever reported activities obtained

from PtCu alloy nano-catalysts.

We utilized a facile solvothermal approach to

synthesize the hexa-pod PtxCuy NCs. [Pt(NH3)4](NO3)2

and Cu(NO3)2 were used as metal precursors, and 1,2-

propanediol (PDO) was used as solvent and reducing

agent for the synthesis. After numerous trials, we

found that a combination of polyvinylpyrrolidone

(PVP) and sodium bromide (NaBr) with appropriate

ratio as capping agent can be used to obtain NCs with

hexa-pod and truncated octahedral (TO)

morphologies. Previously reported shape-controllable

syntheses of PtxCuy NCs were mostly conducted in

non-polar organic solvents such as oleylamine, hence

the NCs commonly show hydrophobic surface and

need further treatment by either ligand exchange or

acid treatment for the purpose of obtaining best

electrochemical activity.[33, 39] In this work, the

reaction is conducted in hydrophilic solvent, and

PVP/NaBr is used as capping agent combination,

endowing the as-synthesized PtxCuy NCs with

hydrophilic surface. Besides, capping species PVP and

NaBr can be easily washed off to minimize their

possible hindering effects on catalytic activities.

Therefore, the synthetic protocol is purely green and

favored by the electrochemical application as well.

The morphology of the as-synthesized PtCu2 NCs is

characterized by transmission electron microscope

(TEM). Fig. 1(a) represents the typical view of the

sample at low magnification, indicating that NCs are

distributed within a narrow size window around 20

nm and all NCs show pod-like projections. At higher

magnification, NCs show well-defined morphology.

The NC at lower right clearly shows the hexa-pod

morphology, and other NCs in the same image also

show 2D projections of hexa-pod along different zone

axes. By examining 50 NCs, it is found that over 92%

of the NCs show projections of hexa-pod, indicating

fairly high yield of hexa-pod NCs with this synthetic

protocol. A schematic of hexa-pod NC with smoothed

facets is shown in the inset of Fig. 1(b). Elemental

mapping images of the NCs under scanning

transmission electron microscope (STEM) are shown

in Fig. 1(c), indicative of uniform distribution of Pt and

Cu in NCs. Atomic resolution TEM image of a NC

lying along zone axis is shown in Fig. 1(d-f). A

close view of the pod highlighted by red rectangle can

be found in Fig. 1(e). According to fast Fourier

transformation (FFT) in Fig. 1(f), surface on the pod

can mostly be assigned to low index facets such as (111)

(yellow), (100) (orange), and (220) (light green) with

abundant atomic steps and kinks. The NCs grow into

well-defined morphology but most facets are

smoothed out.

Figure 1. Typical view of as-synthesized PtCu2 NCs at low

magnification (a), high magnification (b) under TEM, as well as

elemental mapping (c) under STEM . High resolution TEM image

of one PtCu2 NC aligned in zone axis is shown in (d) with

close view of highlighted red square region in (e) and its fast

Fourier transformation (FFT) in (f). Color code in (e): Yellow for

(111) facet, Orange for (100) facets, and Light green for (220)

facets.

To further confirm the of hexa-pod morphology, TEM

sample was tilted in α direction to capture the

projections of NCs along different zone axes, as shown

in Fig. 2. Images were recorded at every 10-degree tilt

from 10˚ to -30˚ in the direction illustrated in Fig. 2(f),

by denoting horizontal position as 0˚. A polygon

model with similar morphology to a real NC, e.g. with

simulated lengths of the six pods, was constructed as

shown in Fig. 2(f). The selected model NC can be

found on the left in Figs. 2(a-e). The 3D model was

tilted in the same way as the real NC on the sample


4 Nano Res.

stage, and the 3D schematics were displayed as insets

at corresponding angles. It can be implied through

comparison that the 3D models show similar

projections to the real NC viewed from different zone

axes, confirming the hexa-pod morphology of the

PtCu2 NCs.

Figure 2. TEM images taken when sample holder is tilted at -30o

(a), -20o (b), -10o (c), 0o (d) and 10o (e) along α direction. (f)

Schematic of hexa-pod 3D model tilted in the highlighted α

direction.

Understanding how the NCs grow into hexa-pod

morphology is of great importance to explore the

growth mechanism. To better understand the growth

mechanism, a time evolution experiment was carried

out to capture the morphology of NCs growing at

different stages, e.g. 30min, 2h and 6h, as shown in Fig.

3. When taken at 30min, most NCs show TO and

octahedral morphologies with the sizes around 9 nm.

A small amount of NCs start to show pod-like

structure with one or two pods as in the inset of Fig.

3(a).

Figure 3. TEM images of PtCu2 NCs captured from reactions

stopped at 30min (a), 2h (b), and 6h (c).

It is commonly known that TO NCs can easily

transform into octahedron via preferable growth

along crystallographic direction, which makes

the transition between TO and octahedron difficult to

be discernible. It is a reasonable hypothesis that the

NCs start from TO seeds, and evolve into octahedron

through the faster growth in direction. From

30min to 2h, all NCs develop into multi-pod

morphology. For a TO NC, it has six (100) facets which

tend to shrink and disappear by the continuing fast

growth of direction. Eventually, the six (100)

facets can extrude and grow into pods, leading to the

convex morphology shown in Fig. 3(b). At 6h, the NCs

show obvious pod structure with longer pods. A close

look into Fig. 3(c) indicates that NCs show uniform

hexa-pod morphology, with pods varying in length on

each NC.

Figure 4. Schematic of growth mechanism illustrating the

morphology evolution of the hexa-pod PtxCuy NCs.

Through the time evolution experiment, we propose

that due to faster growing rate in

crystallographic directions, TO seeds quickly

transform into octahedral NCs that sit in a meta-stable

state due to the continuing fast growth in

directions, and eventually evolve into hexa-pod

morphology. The growth trajectory can be illustrated

more clearly as the schematic in Fig. 4. It can be easily

deduced from the hypothesis that the surface of the

hexa-pod NCs strongly depend on their growth stage,

i.e. pod length. Therefore, they can hardly be assigned

to a fixed crystallographic facet. Assuming that the

hexa-pod NCs were perfect polygons with sharp

facets, the NC should be enclosed by 32 facets,

including 8 facets on the seed with and other 24 facets

on the pods. It could be calculated that interfacial

angle is 70.6˚ between two (111) facets, and 54.7˚

between (111) and (100) facet, both of which are higher

than the maximum interfacial angle that could

possibly be between the facets on the pod (highlighted

with green dashed lines in Fig. 4) and (111) facets

(highlighted with yellow dashed lines). It means that

the surface on the pod can be neither (111) nor (100),

indicating that the pod should be enclosed by facets

with higher crystallographic indexes. Through tilt of

TEM sample holder, two high resolution TEM images

of selected PtCu2 NCs viewed from zone axis

shown in Fig. S1 (Supporting Information). Surface

highlighted by red dashed line can be designated to

high index crystallographic facets such as (420), (460),

(710), and (750).[44] It is noteworthy that the convex

NCs in this work commonly show smoothed pods,

adding that pods differ in length, meaning that pods


5 Nano Res.

of NCs are enclosed by high-index facets with various

indexes.

Since the growth mechanism has been proposed, it is

of our interest to see if the composition of the alloy

NCs can be tuned. In this work, PtxCuy hexa-pod NCs

with three Cu/Pt ratios, determined by energy

dispersive X-Ray spectroscopy (EDX) equipped in

TEM and inductively coupled plasma optical emission

spectrometer (ICP-OES), are shown in Fig. 5. For the

alloy products with Cu/Pt ratio 2/1, 1/1 and 2/3, the

starting precursors, Cu(NO3)2 and [Pt(NH3)4](NO3)2,

were added with the Cu/Pt ratio at 3/1, 1/1, and 1/3.

The ratio difference between products and precursors

comes from the reduction potential difference of Cu2+

and Pt(NH3)42+ in PDO. In earlier work reported by

Wang and Li et al, synthesis was carried out at

temperatures generally above 240˚C when all Cu2+

ions can be fully reduced.[40] In this work, all

syntheses were conducted at 180˚C, leading to

incomplete reduction of precursors. Well-shaped

hexa-pod PtCu NCs can only be formed for PtCu2

alloy (Fig. 5(c)), while mixed multi-pod morphologies

can be formed for Pt3Cu2 and PtCu alloys as in Fig. 5(a-

b). However, it can be clearly seen for mult-pod Pt3Cu2

and PtCu alloy NCs that, they show longer pods. EDX

spectra of Pt3Cu2 and PtCu2 NCs can be found in Fig.

S2-S3.

Figure 5. TEM images of Pt3Cu2 (a, d), PtCu (b, e), and PtCu2 (c,

f).

It is an obvious trend that when increasing Cu/Pt ratio,

NCs tend to show more regular and uniform

morphology. It is possible due to the fact that

overgrowth could occur when more Pt sources exist,

while slower reaction kinetics could benefit

morphology evolution at the scenario with more Cu

sources.[45] To avoid misunderstanding, all PtxCuy

alloy NCs are denoted as hexa-pod NCs because most

NCs still show six pods for Pt3Cu2 and PtCu alloys.

The growth mechanism of PtCu NCs is also explored

by time evolution experiment as shown in Fig. S4. The

hexa-pod PtCu NCs also start from octahedral seeds

and evolve gradually into hexa-pod alloy NCs,

indicating that PtCu and PtCu2 share similar growth

mechanism towards hexa-pod NCs. It can be

concluded that this protocol can be applied to hexa-

pod PtxCuy alloy NCs with various Cu/Pt ratios. X-ray

diffraction (XRD) patterns of PtCu and PtCu2 are

shown in Fig. S5. For PtCu2, the shift of (111)

diffraction peak verifies that the inclusion of more Cu

atoms leads to the shrink of unit cell.

As comparisons, PtxCuy NCs with two other

morphologies were synthesized to explore the effect

of morphology on ORR activities. By switching the

solvent to 1, 4-butanediol (BDO), uniform and

monodisperse TO and near-spherical (NS) irregular

PtCu NCs, shown in Fig. S6, can be obtained simply

by manipulating the Cu/Pt feeding ratio of precursors.

It has been mentioned that, at 180˚C, PDO and PVP

are not stong enough to reduce all Pt and Cu

precursors. The reducibility is even weaker for BDO at

this temperature, so the overgrowth can hardly occur

when using BDO, leading to the formation of

truncated octahedral NCs and NS NCs. The Cu/Pt

ratio is 1/1 for both alloy samples, determined by EDX

and ICP.

To maximize the active surface area and enhance the

stability of hexa-pod NCs, all PtxCuy NCs were loaded

onto high-surface-area carbon black (Vulcan XC-72,

CB), as shown in Fig. S7. The catalyst loadings are

determined by total masses, with the weight

percentage at 20%. According to Fig. S7, loadings of

PtxCuy NCs on CB are highly uniform. No aggregation

can be found in TEM view.

To evaluate ORR activities of the CB-supported NCs,

the catalysts were loaded onto glassy carbon electrode

(GCE). Masses of loaded noble metals (referred to Pt

only) were the same (12 μg/cm2) for all catalysts.

Catalyst films on GCE were prepared by covering the

pre-formed catalyst film with thin Nafion film.

Electrochemical surface areas (ECSAs) were

determined by integrating hydrogen

adsorption/desorption peak in cyclic voltammetric

(CV) curves (between ~0.05 and 0.40 V vs. reversible

hydrogen electrode denoted as RHE) shown in Fig.

4(a), by assuming the monolayer hydrogen adsorption.


6 Nano Res.

It is worth noting that the CVs in Fig. 6(a) were

recorded after the curves were stabilized. The initial

de-alloying cycles, e. g. 1st, 2nd, and 10th cycles, are

shown in Fig. S8. Although the characteristic

desorption peak of under-potentially deposited (UPD)

hydrogen is not well-shaped as on pure Pt surface,

they can still be recognizable even during the 1st cycle,

indicating the presence of substantial Pt atoms on NC

surface for all catalysts. Surface Cu/Pt ratio

determined by X-ray photoelectron spectroscopy (XPS)

is 5/11 for PtCu2 sample, which deviates from bulk

ratio but agrees well with the indication from

hydrogen UPD in CV. Since the reaction temperature

is not high enough to reduce all metallic precursors,

and, Cu atoms are more reductive than Pt at such

temperature, it results in the displacement reaction

between Cu atoms at surface and Pt ions in solution,

leading to the Pt-rich structure. The low anodic peaks

between 0.5 and 0.75V on the 1st and 2nd cycle are

attributed to selective Cu dissolution from NC surface.

The cathodic peaks associated with the reduction of Pt

oxide mixed with re-deposition peak of Cu atoms,

split on 2nd cycle into distinguishable peaks. The peak

around 0.75V related to reduction of Pt oxide shifts to

the positive potential, while the shoulder peak at 0.6V

gradually disappear on 10th cycle, implying

undetectable Cu dissolution from NC surface at this

stage.[25] CV curves in Fig. 6(a) are recorded once the

electrochemical activation curves stabilize, which

resemble that of pure Pt and indicate the formation of

Pt-rich surface.

Figure 6. CV curves (a) and polarization curves (b) of Pt3Cu2,

PtCu, and PtCu2. Inset of (b): Tafel plot of the PtCu/C electro-

catalysts compared with commercial JM Pt/C.

ORR activities were determined by rotating disk

electrode (RDE) approach. The electrode was

polarized between 0.1 and 1V in oxygen-saturated

0.1M HClO4 electrolyte via linear scanning

voltammetry (LSV) at the rotating rate of 1600 rpm.

Commercial Johnson Matthey (JM) Pt/C (20wt% Pt)

catalyst was selected as comparison. It can be seen in

Fig. 6(b) that onset potentials of PtCu/C catalysts are

at least 50 mV more positive than JM Pt/C catalyst,

among which Pt3Cu2/C and PtCu2/C show further 15

mV more positive onset potential than that of PtCu/C.

Mass-transport-corrected specific activities between

0.88 and 0.96 V can be found through Tafel plot in the

inset of Fig. 6(b). The log j-V curve of JM Pt/C catalyst

shows two linear components, implying that oxygen

molecules probably experience a transition between

two distinct mechanism regimes when the potential is

elevated from 0.88 to 0.96 V. For PtxCuy/C catalysts,

their Tafel plots show high linearity and indicate

single-reaction-mechanism and high oxygen

reduction rates at the potential range.[46] It can be

clearly seen that the specific activities of hexa-pod

PtCu/C catalysts are higher than that of JM Pt/C by at

least one order of magnitude. The calculated number

of transferred electrons is 3.86 per O2 molecule, as

shown in Fig. S9. For PtxCuy/C catalysts with TO and

NS PtCu NCs, their ORR activities were also

determined which can be found in Fig. S10. With

almost the same composition, hexa-pod PtCu/C

shows over 3-time enhancement in specific activity

than those of TO and NS PtCu/C catalysts, and 4-time

enhancement in mass activity. Eliminating the

possible effect induced by composition, the

enhancement in activity mainly originates from the

morphology of NCs. It has been reported in abundant

literatures that atoms located at corners and edges of

catalysts are more active catalytic sites. In this work,

the hexa-pod NCs obviously show higher ratio of

atoms at corners and edges according to Fig. 1(d),

which possibly make the surface highly active.[32] In

addition, as is mentioned above, hexa-pod PtCu NCs

expose partially by high-index facets, which could

also endow the surface with higher activity towards

ORR. Specific and mass activity values of various

catalysts at 0.9V are listed in Tab. 1. The highest

activities were achieved by PtCu2/C catalyst, with

specific activity up to 3.7 mA/cm2Pt and mass activity

2.4 A/mgPt, both of which are the highest-ever

reported for PtxCuy alloys, and also higher than

commercial Pt and other Pt-based alloy catalysts.[10,

35] Table 1. ECSAs, specific and mass activities of hexa-pod Pt3Cu2,

PtCu, PtCu2.and TO PtCu.

Sample ECSA

(m2/g)

Specific

Activity

Mass

Activity


7 Nano Res.

(mA/cm2Pt) (A/mgPt)

Pt3Cu2 55.3 3.4 1.6

PtCu 51.7 3.0 1.2

PtCu2 63.0 3.7 2.4

TO PtCu 52.3 0.8 0.3

Durability is also a crucial factor when evaluating the

practical usability of catalysts. Accelerated durability

tests (ADTs) were carried out for all materials. The

catalysts were polarized with CV curves between 0.6

and 1.1 V at the scanning rate of 50 mV/s, in 0.1 M

HClO4 with continuous oxygen bubbling to keep

electrolyte saturated. ORR activities were determined

every 4000 cycles until it reaches 20000 cycles. The

50th and 20000th cycles are shown in CV and LSV in

Fig. S11-S13 for Pt3Cu2/C, PtCu/C, and PtCu2/C,

respectively. Activities of PtCu2/C catalyst shows most

drop in activity, 32% in specific activity and 50% in

mass activity. With the decrease of Cu/Pt ratio, the

activity declines less during ADT, confirming that

PtxCuy/C with high Cu content tend to have Cu atoms

dissolved easily, thus activity tends to drop faster. For

Pt3Cu2/C, it loses 11.8% in specific activity to 3.0

mA/cm2Pt, and 25% in mass activity to 1.2 A/mgPt. The

amount of degradation, as well as the final activities

after ADT, still meets the DOE targets on durability for

fuel cell cathode catalyst. As a fair comparison, ADT

was also measured for TO PtCu/C catalyst. It is

interesting that PtCu/C with TO NC actually shows

enhanced specific activity during ADT. We can be

inferred from LSV in Fig. S14 that kinetic current at 0.9

V remains almost unchanged. As a result, the

declining ECSA leads to higher specific activity, while

the mass activity remains almost the same. The

microstructures of catalysts after ADT were

characterized with TEM, as shown in Fig. S15. The TO

NCs seem to be smoothed after ADT and show near-

spherical shape without distinguishable increase in

size. For hexa-pod NCs, they tend to show smoothed

pods after ADT without change in size, indicating the

high stability of hexa-pod NCs at ADT operation

condition.

Conclusion

To summarize, we synthesized PtxCuy NCs with hexa-

pod morphology. The synthetic protocol can be

employed to synthesize PtxCuy NCs with various

Cu/Pt ratios. The six pods on NCs grow from (100)

facets of TO seeds, along crystallographic

direction. The hexa-pod NCs expose by (111) and

other facets with high indexes. Due to the high ratio of

atoms at corners and edges, as well as the existence of

high index facets, hexa-pod NCs supported on carbon

black show drastically enhanced catalytic activities

towards ORR, with the highest specific activity 3.7

mA/cm2Pt and mass activity 2.4 A/mgPt obtained at 0.9

V by hexa-pod PtCu2/C catalysts. ADTs show high

stability for alloys with lower Cu/Pt ratio. The highest

durability was obtained from hexa-pod Pt3Cu2/C, with

11.8% lost in specific activity and 25% lost in mass

activity after ADT. The high catalytic performance of

hexa-pod PtxCuy NCs endows them with potential

application as cathode catalysts in PEMFC.

For more instructions, please see our web page at

http://www.thenanoresearch.com/.

Acknowledgements

We acknowledge the Microstructure Laboratory for

Energy Materials (MLEM) at CUP for the technical

support with TEM. We also acknowledge the funding

support from Natural Science Foundation of China

(No. 21303265), Ph.D. Programs Foundation of

Ministry of Education of China (No. 20130007120012)

and Young Talent Award of CUP (No. YJRC-2013-46).

Electronic Supplementary Material: Supplementary

material, such as Characterization, additional graphs

and electrochemistry data, is available in the online

version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

(automatically inserted by the publisher). References

[1] Gasteiger, H. A.; Markovic, N. M. Just a Dream-or Future Reality? Science 2009, 324, 48-49.

[2] Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. Enhancement of the Electroreduction of Oxygen on Pt Alloys with Fe, Ni, and Co. J. Electrochem. Soc. 1999, 146, 3750-3756.

[3] Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl. Catal., B 2005, 56, 9-35.

http://dx.doi.org/10.1007/s12274-***-****-*


8 Nano Res.

[4] Huang, X. Q.; Zhao, Z. P.; Chen, Y.; Chiu, C. Y.; Ruan, L. Y.; Liu, Y.; Li, M. F.; Duan, X. F.; Huang, Y. High Density Catalytic Hot Spots in Ultrafine Wavy Nanowires. Nano Lett. 2014, 14, 3887-3894.

[5] Chen, C.; Kang, Y. J.; Huo, Z. Y.; Zhu, Z. W.; Huang, W. Y.; Xin, H. L. L.; Snyder, J. D.; Li, D. G.; Herron, J. A.; Mavrikakis, M.; Chi, M. F.; More, K. L.; Li, Y. D.; Markovic, N. M.; Somorjai, G. A.; Yang, P. D.; Stamenkovic, V. R. Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces. Science 2014, 343, 1339-1343.

[6] Chen, Y.; Yang, F.; Dai, Y.; Wang, W.; Chen, S. Ni@Pt core-shell nanoparticles: Synthesis, structural and electrochemical properties. J. Phys. Chem. C 2008, 112, 1645-1649.

[7] Wakabayashi, N.; Takeichi, M.; Uchida, H.; Watanabe, M. Temperature dependence of oxygen reduction activity at Pt-Fe, Pt-Co, and Pt-Ni alloy electrodes. J. Phys. Chem. B 2005, 109, 5836-5841.

[8] Huang, Q.; Yang, H.; Tang, Y.; Lu, T.; Akins, D. L. Carbon-supported Pt-Co alloy nanoparticles for oxygen reduction reaction. Electrochem. Commun. 2006, 8, 1220-1224.

[9] Kang, Y.; Murray, C. B. Synthesis and Electrocatalytic Properties of Cubic Mn-Pt Nanocrystals (Nanocubes). J. Am. Chem. Soc. 2010, 132, 7568-7569.

[10] Li, J.; Wang, G.; Wang, J.; Miao, S.; Wei, M.; Yang, F.; Yu, L.; Bao, X. Architecture of PtFe/C catalyst with high activity and durability for oxygen reduction reaction. Nano Res. 2014, 7, 1519-1527.

[11] Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Norskov, J. K. Alloys of platinum and early transition metals as oxygen reduction electrocatalysts. Nat. Chem. 2009, 1, 552-556.

[12] Norskov, J. K.; Abild-Pedersen, F.; Studt, F.; Bligaard, T. Density functional theory in surface chemistry and catalysis. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 937-943.

[13] Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Norskov, J. K. Changing the activity of electrocatalysts for oxygen reduction by tuning the surface electronic structure. Angew. Chem. Int. Ed. 2006, 45, 2897-2901.

[14] Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Improved oxygen reduction activity on Pt3Ni(111) via increased surface site availability. Science 2007, 315, 493-497.

[15] Sha, Y.; Yu, T. H.; Merinov, B. V.; Shirvanian, P.; Goddard, W. A., III Mechanism for Oxygen Reduction Reaction on Pt3Ni Alloy Fuel Cell Cathode. J. Phys. Chem. C 2012, 116, 21334-21342.

[16] Zhang, Y.; Duan, Z.; Xiao, C.; Wang, G. Density functional theory calculation of platinum surface segregation energy in Pt3Ni (111) surface doped with a third transition metal. Surf. Sci. 2011, 605, 1577-1582.

[17] Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.; Yu, C.; Liu, Z.; Kaya, S.; Nordlund, D.; Ogasawara, H.; Toney, M. F.; Nilsson, A. Lattice-strain control of the activity in dealloyed core-shell fuel cell catalysts. Nat. Chem. 2010, 2, 454-460.

[18] Min, M. K.; Cho, J. H.; Cho, K. W.; Kim, H. Particle size and alloying effects of Pt-based alloy catalysts for fuel cell applications. Electrochim. Acta 2000, 45, 4211-4217.

[19] Mukerjee, S.; Srinivasan, S. Enhanced electrocatalysis of oxygen reduction on platinum alloys in proton exchange membrane fuel cells. J. Electroanal. Chem. 1993, 357, 201-224.

[20] Zheng, F.; Wong, W.-T.; Yung, K.-F. Facile design of Au@Pt core-shell nanostructures: Formation of Pt submonolayers with tunable coverage and their applications in electrocatalysis. Nano Res. 2014, 7, 410-417.

[21] Xiong, L.; Kannan, A. M.; Manthiram, A. Pt-M (M = Fe, Co, Ni and Cu) electrocatalysts synthesized by an aqueous route for proton exchange membrane fuel cells. Electrochem. Commun. 2002, 4, 898-903.

[22] Xiong, L.; Manthiram, A. Effect of atomic ordering on the catalytic activity of carbon supported PtM (M = Fe, Co, Ni, and Cu) alloys for oxygen reduction in PEMFCs. J. Electrochem. Soc. 2005, 152, A697-A703.

[23] He, T.; Kreidler, E.; Xiong, L. F.; Ding, E. R. Combinatorial screening and nano-synthesis of platinum binary alloys for oxygen electroreduction. J. Power Sources 2007, 165, 87-91.

[24] Tseng, C.-J.; Lo, S.-T.; Lo, S.-C.; Chu, P. P. Characterization of Pt-Cu binary catalysts for oxygen reduction for fuel cell applications. Mater. Chem. Phys. 2006, 100, 385-390.

[25] Liu, Z. C.; Koh, S.; Yu, C. F.; Strasser, P. Synthesis, dealloying, and ORR electrocatalysis of PDDA-stabilized Cu-rich Pt alloy nanoparticles. J. Electrochem. Soc. 2007, 154, B1192-B1199.

[26] Srivastava, R.; Mani, P.; Hahn, N.; Strasser, P. Efficient oxygen reduction fuel cell electrocatalysis on voltammetrically dealloyed Pt-Cu-Co nanoparticles. Angew. Chem. Int. Ed. 2007, 46, 8988-8991.


9 Nano Res.

[27] Oezaslan, M.; Hasche, F.; Strasser, P. PtCu3, PtCu and Pt3Cu Alloy Nanoparticle Electrocatalysts for Oxygen Reduction Reaction in Alkaline and Acidic Media. J. Electrochem. Soc. 2012, 159, B444-B454.

[28]Stephens, I. E. L.; Bondarenko, A. S.; Perez-Alonso, F. J.; Calle-Vallejo, F.; Bech, L.; Johansson, T. P.; Jepsen, A. K.; Frydendal, R.; Knudsen, B. P.; Rossmeisl, J.; Chorkendorff, I. Tuning the Activity of Pt(111) for Oxygen Electroreduction by Subsurface Alloying. J. Am. Chem. Soc. 2011, 133, 5485-5491.

[29] Dutta, I.; Carpenter, M. K.; Balogh, M. P.; Ziegelbauer, J. M.; Moylan, T. E.; Atwan, M. H.; Irish, N. P. Electrochemical and Structural Study of a Chemically Dealloyed PtCu Oxygen Reduction Catalyst. J. Phys. Chem. C 2010, 114, 16309-16320.

[30] Mani, P.; Srivastava, R.; Strasser, P. Dealloyed binary PtM3 (M = Cu, Co, Ni) and ternary PtNi3M (M = Cu, Co, Fe, Cr) electrocatalysts for the oxygen reduction reaction: Performance in polymer electrolyte membrane fuel cells. J. Power Sources 2011, 196, 666-673.

[31] Ge, X. B.; Chen, L. Y.; Kang, J. L.; Fujita, T.; Hirata, A.; Zhang, W.; Jiang, J. H.; Chen, M. W. A Core-Shell Nanoporous Pt-Cu Catalyst with Tunable Composition and High Catalytic Activity. Adv. Funct. Mater. 2013, 23, 4156-4162.

[32] Wu, J.; Qi, L.; You, H.; Gross, A.; Li, J.; Yang, H. Icosahedral Platinum Alloy Nanocrystals with Enhanced Electrocatalytic Activities. J. Am. Chem. Soc. 2012, 134, 11880-11883.

[33] Wu, J.; Zhang, J.; Peng, Z.; Yang, S.; Wagner, F. T.; Yang, H. Truncated Octahedral Pt3Ni Oxygen Reduction Reaction Electrocatalysts. J. Am. Chem. Soc. 2010, 132, 4984-4985.

[34] Li, H.; Wang, J.; Liu, M.; Wang, H.; Su, P.; Wu, J.; Li, J. A nanoporous oxide interlayer makes a better Pt catalyst on a metallic substrate: Nanoflowers on a nanotube bed. Nano Res. 2014, 7, 1007-1017.

[35] Si, W.; Li, J.; Li, H.; Li, S.; Yin, J.; Xu, H.; Guo, X.; Zhang, T.; Song, Y. Light-controlled synthesis of uniform platinum nanodendrites with markedly enhanced electrocatalytic activity. Nano Res. 2013, 6, 720-725.

[36] Narayanan, R.; El-Sayed, M. A. Shape-dependent catalytic activity of platinum nanoparticles in colloidal solution. Nano Lett. 2004, 4, 1343-1348.

[37] Choi, S. I.; Shuifen, X.; Shao, M.; Odell, J. H.; Ning, L.; Peng, H. C.; Protsailo, L.; Guerrero, S.; Park, J.; Xia, X.; Wang, J.; Kim, M. J.; Xia, Y. Synthesis and Characterization of 9 nm Pt−Ni Octahedra with a Record High Activity of 3.3 A/mgPt for the Oxygen Reduction Reaction. Nano Lett. 2013, 13, 3420-3425.

[38] Cui, C.; Gan, L.; Li, H.-H.; Yu, S.-H.; Heggen, M.; Strasser, P. Octahedral PtNi Nanoparticle Catalysts: Exceptional Oxygen Reduction Activity by Tuning the Alloy Particle Surface Composition. Nano Lett. 2012, 12, 5885-5889.

[39] Zhu, H.; Zhang, S.; Guo, S.; Su, D.; Sun, S. Synthetic Control of FePtM Nanorods (M = Cu, Ni) To Enhance the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2013, 135, 7130-7133.

[40] Wang, D.; Peng, Q.; Li, Y. Nanocrystalline Intermetallics and Alloys. Nano Res. 2010, 3, 574-580.

[41] Bromberg, L.; Fayette, M.; Martens, B.; Luo, Z. P.; Wang, Y.; Xu, D.; Zhang, J.; Fang, J.; Dimitrov, N. Catalytic Performance Comparison of Shape-Dependent Nanocrystals and Oriented Ultrathin Films of Pt4Cu Alloy in the Formic Acid Oxidation Process. Electrocatalysis 2013, 4, 24-36.

[42] Zhang, J.; Yang, H.; Martens, B.; Luo, Z.; Xu, D.; Wang, Y.; Zou, S.; Fang, J. Pt-Cu nanoctahedra: synthesis and comparative study with nanocubes on their electrochemical catalytic performance. Chem. Sci. 2012, 3, 3302-3306.

[43] Saleem, F.; Zhang, Z.; Xu, B.; Xu, X.; He, P.; Wang, X. Ultrathin Pt-Cu Nanosheets and Nanocones. J. Am. Chem. Soc. 2013, 135, 18304-18307.

[44] Tian, N.; Zhou, Z.-Y.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity. Science 2007, 316, 732-735.

[45] Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem. Int. Ed. 2009, 48, 60-103.

[46] Marayama, J.; Abe, I. Application of conventional activated carbon loaded with dispersed Pt to PEFC catalyst layer. Electrochim. Acta 2003, 48, 1443-1450.


Nano Res.

Electronic Supplementary Material



reduction reaction

Yujing Li1,2 (), Fanxin Quan2, Enbo Zhu3, Lin Chen2, Yu Huang3,4, Changfeng Chen1,2

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Synthesis of hexa-pod PtCu nanocrystals (NCs): For the synthesis of hexa-pod PtCu NCs, Pt[Pt(NH3)4](NO3)2

and Cu(NO3)2 were used as metallic precursors. Appropriate amount of precursors were dissolved in 14 mL 1, 2-

propanediol (PDO) which also serves as reducing agent. Polyvinylpyrrolidone (PVP) and sodium bromide (NaBr)

were also dissolved in PDO as stabilizing and capping agent. The starting solution was stirred for 10 minutes

and transferred into a Teflon-lined autoclave. The solution was heated from room temperature up to 190 oC at

the rate of 6 oC/min and kept at this temperature for 6 hours, and then cooled down in the air. The products were

separated from the reaction solution by centrifuge. The NCs were dispersed in 2 mL of ethanol, precipitated by

15 mL of acetone, sonicated for 10 min, and then centrifuged at 8000 RPM for 10 min. The washing procedure

was repeated three times. The NCs were stored in ethanol at the concentration of 1mg/mL. The ratios of added

precursors Pt[Pt(NH3)4](NO3)2/Cu(NO3)2 are 3/1, 1/1, and 1/3 for the synthesis of Pt3Cu2, PtCu, and PtCu2,

respectively.

For the synthesis of truncated octahedral and near-spherical PtCu NCs, all conditions were the same as above

except that 1, 4-butanediol (BDO) was used as solvent and reducing agent. The ratios of added precursors

Pt[Pt(NH3)4](NO3)2/Cu(NO3)2 are 1/3 and 1/1 for the synthesis of truncated octahedral and near-spherical PtCu

NCs, respectively.

Preparation of carbon-supported catalysts: Carbon black (Vulcan XC-72, CB) is used as the catalyst support.

For a typical catalyst preparation, CB was dissolved in ethanol at a concentration of 1 mg/mL, and sonicated for

30 min. Appropriate amount of NCs dissolved in ethanol was sonicated and mixed with the CB solution, with

the NC/CB mass ratio fixed at 1/4, further sonicated for another 1 hour, and then stirred overnight. The catalysts

were then collected by centrifuge at the rate of 8000 rpm, washed with ethanol, and then dried under a N2 stream.

As-prepared catalysts were stored in ethanol or isopropanol (IPA).

Characterization: Transmission electron microscopic (TEM) images with α angle tilt was taken on JEOL JEM

2100 at an accelerating voltage of 200 kV. High resolution transmission electron microscopy (HRTEM) and

energy dispersive X-Ray spectroscopy (EDX) were taken on FEI TECNAI F-20 field emission microscope at an


Nano Res.

accelerating voltage of 200 kV. The fast Fourier Transformations (FFTs) were obtained by high resolution image

under TEM. The simulated FFT was also obtained by constructing the same crystal structure with WinXMorph,

along the same zone axis with the real image. Both FFTs were aligned in the same angle (overlapped) to determine

the index of the exposed facets on the pod. All TEM samples were made by casting corresponding NC solution

on carbon film supported on molybdenum grid.

Electrochemical measurement: All electrochemical measurements were carried out in a home-made three-

electrode cell. Catalyst inks were prepared by dispersing NC/CB in IPA/H2O (volume ratio: 3/1) with the

concentration of 2 mg/mL. Appropriate amounts of inks, were pipetted onto a rotating disk electrode (Pine

Instrumentation) made with glassy carbon (GCE) with diameter of 5 mm. The total loading masses of Pt were

controlled at the level of 12 μg/cm2. In this work, the electrode was prepared by a two-step approach. The carbon-

supported electro-catalysts form the first layer, followed by a Nafion® film formed by drying 10 μL of 100-time-

diluted Nafion® solution on top. The catalysts were electrochemically de-alloyed by applying cyclic voltammetric

(CV) scans between 0 and 1.1 V (vs. reversible hydrogen electrode, RHE) in 0.1 M HClO4 electrolyte, at the scan

rate of 50 mV/s. Electrochemical surface area (ECSA) were determined from CV curves recorded. The charges

induced by the adsorption/desorption of hydrogen species were calculated by integrating the area approximately

between 0.05 and 0.4 V (depending on the shape) under the CV curve. ORR was studied by recording linear scan

voltammetic (LSV) curves between 0 and 1.1 V in oxygen-saturated HClO4 electrolyte at the rotating rate of 1600

rpm. The electrolyte was bubbled with oxygen prior to, and during the measurement. The rotation rate

dependent polarization curves were recorded at the rotating rates of 400, 900, 1600, and 2500 rpm. The durability

tests were conducted by applying CV scans up to 4000 cycles each time at the scan rate of 50 mV/s, and recording

CV curves as mentioned above, and then studying ORR afterwards. After each set of measurement, another

round of ADT was applied, and a total of 20000 CV scans was applied to each catalyst for the evaluation of

durability.


Nano Res.

Figure S1. High resolution TEM images of (a) and (b) PtCu2 NCs projected from zone axis. Inset

of (a) is the 3D schematic of a hexa-pod particle with the same projection as the NC. Inset of (b) is

lattice in selected region showing that the zone axis of the NC is .

Figure S2. EDX spectrum PtCu2 by focusing electron beam on one NC.


Nano Res.

Figure S3. EDX spectrum PtCu2 by focusing electron beam on one NC.

Figure S4. TEM images of PtCu NCs captured from reactions stopped at 30min (a), 2h (b), and 6h (c).


Nano Res.

Figure S5. XRD patterns of PtCu2 (red) and PtCu (black), compared with pure Pt and Cu as shown in

black and blue lines.

Figure S6. TEM images of truncated octahedral (TO) PtCu NCs (a, b) and near-spherical (NS) PtCu

NCs (c, d).


Nano Res.

Figure S7. TEM images of CB supported Pt3Cu2 (a, b), PtCu (c, d), and PtCu2 (e, f) NCs.

Figure S8. Comparison of the 1st, 2nd and 10th CV curve of Pt3Cu2 (a), PtCu (b) and PtCu2 (c).

Figure S9. Polarization curves of PtCu2/C catalyst at the rotating rates of 400, 900, 1600 and 2500

rpm (a), and Levich-Koutecký plot of PtCu2/C compared with JM Pt/C at 0.5 V (b).


Nano Res.

Figure S10. Polarization curves (a) and Tafel plots (b) of hexa-pod PtxCuy/C catalysts, truncated

octahedral and near-spherical PtCu NCs.

Figure S11. CV curves (a) and polarization curves (b) of 50th and 20000th cycles during ADT for hexa-

pod Pt3Cu2/C catalysts.


Nano Res.


pod PtCu/C catalysts.


pod PtCu2/C catalysts.


Nano Res.

Figure S14. CV curves (a) and polarization curves (b) of 50th and 20000th cycles during ADT for TO

PtCu/C catalysts.

Figure S15. TEM images of TO PtCu/C (a, b) and hexa-pod Pt3Cu2/C (c, d) catalysts.

Address correspondence to Yujing Li, Email: [email protected]


Nano Res.

0834_Manuscript_2015-6-8

Documents

Pt Cu nanocrystals with hexa-pod morphology and their ...We utilized a facile solvothermal approach to synthesize the hexa-pod PtxCuy NCs. [Pt(NH3)4](NO3)2 and Cu(NO3)2 were used as