Resultsanddiscussion解读

3.

Results and discussion

Cyclic voltammogram (CV) of Pt electrodeposition on GC

electrode in 19.3 mM H2PtCl6/DESs solution at 80

?

C is shown in

Fig.

1

.

From

this

figure,

in

the

negative- going

potential

scan,it

can be clearly observed that two current peaks of reduction

occurred

at

near

?

0.93

V

and

?

1.29

V

(vs.

Pt),

in

comparison

with

the voltammogram (inset of

Fig. 1

) recorded on the same GC

substrate in DESs. X-ray photoelectron spectra (XPS) results

demonstrated

that

these

two

reduction

peaks

are

corresponding

to

the electrochemical reduction of Pt(IV) to Pt(II) and Pt(II) to

Pt(0), respectively (

Fig. S1 and Table S1 in supplementary

information

). This is consistent with the electrodeposition

behaviors of Pt in hydrophilic 1-n-butyl-3-methylimidazolium

tetrafluoroborate (BMIMBF4) and hydrophobic

1-n-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6)

room- temperature ionic liquids

[36]

.

Fig. 2

a displays the typical SEM image of Pt nanoflowers

electrodeposited

on

GC

by

using

CV

method

in

19.3

mM

H2PtCl6/DESs

solution at 80

?

C. It can be seen that the Pt nanoflowers with

sharp

petals

were

homogeneously

formed,

and

their

size

was

about

200 nm. The crystal structure of Pt nanoflowers was further

investigated

by

high

resolution

transmission

electron

microscopy

(HRTEM).

Fig. 2

b shows the TEM image of a single Pt nanoflower,

and the inset is the corresponding selected area electron

diffraction (SAED) pattern, which indicates that the petals of

as- prepared

nanoflowers

possess

the

single-crystalline

structure.

The

HRTEM

image

of

a

petal

marked

in

Fig.

2

b

is

displayed

in

Fig.

2

c. The continuous fringe pattern further verifies the single

crystalline property of the petal. The lattice spacing of 0.23

nm agrees with the distancebetween two

{

1 1 1

}

planes of Pt. As

compared to other Pt nanoflowers and nanothorn assemblies

reported previously

[16

–

19 ,37]

, the unique characteristic of

as-prepared Pt nanoflowers is the formation of high density of

atomic

steps

at

the

edge

of

the

petals

(

Fig.

2

c),

which

are

crucial

for the enhanced activity of Pt nanoflowers toward ethanol

electrooxidation.

The

energy

dispersive

X-ray

spectroscopy

(EDX)

analysis of Pt nanoflowers confirms the presence of only Pt, C

and

O

elements

(

Fig.

2

d),

indicating

no

DESs

residue

on

the

surface

of Pt nanoflowers.

The effect of deposition conditions, namely,the precursor

concentration,CV number of cycle, scan rate and temperature on

the

size

and

morphology

of

Pt

nanostructures

electrodeposited

in

DESs

was

examined.

Fig.

3

shows

the

SEM

images

of

Pt

nanostructures

prepared

by

using

the

different

concentrations

of

H2PtCl6.

We

can

see that, at the H2PtCl6 concentration of 1.93 mM, the

quasispherical

Pt

NPs

were

formed

(

Fig.

3

a),

and

their

size

ranged

from 45 to 95 nm. When the H2PtCl6 concentration was 5 mM, the

flowerlike Pt NPs without sharp petals and several cubic Pt NPs

appeared

(

Fig.

3

b).

Further

increasing

the

H2PtCl6

concentration

to 10 mM,the sharp petals started to appear at the edge of the

Pt

nanoflowers

(

Fig.

3

c).

Finally,

the

perfect

Pt

nanoflowers

with

sharp petals were homogeneously formed at the H2PtCl6

concentration

of

19.3

mM

(

Fig.

3

d).

The

concentration

dependence

of the above Pt nanostructures maybe results from the high

viscosity of DESs

[25,26]

, which decreased the mass

transportation of reactive species in DESs, leading to the

difficult formation of Pt nanoflowers with sharp petals at the

lower H2PtCl6 concentration.

Among all deposition conditions, the CV number of cycle

exerts a leading influence on the process of particle growth.

Different Pt nanostructures generated by various CV number of

cycle were obtained, as shown in

Fig. 4

. Some irregular

quasi- spherical nanoparticles with low surface coverage were

produced in the lower CV number of cycle (

Fig. 4

a and b), which

acted as the nuclei for subsequently producing Pt nanoflowers

[17,19,38]

. Since the nucleation process is relatively slow and

irreversible, newly deposited Pt favors growing on the small Pt

cores

instead

of

generating

more

new

nuclei

[39]

.

Increasing

the

CV number of cycle would result in complex monodisperse

nanoflowers with more sharp petals and larger size (

Fig. 4

c).

Further

increasing

the

CV

number

of

cycle

to

80

cycles,the

perfect

Pt nanoflowers with sharp petals were formed on the GC substrate

(

Fig.

3

d).

When

the

CV

number

of

cycle

was

increased

to

100

cycles,

the aggregation phenomenon of Pt nanoflowers was observed (

Fig.

4

d). In addition, the scan rate plays important roles in the

electrodeposition of Pt owing to its possible influence on the

anisotropic growth of the lower scan rate of 1