Supplementary Information

Catalytically Active Single-Atom Niobium in Graphitic Layers

Xuefeng Zhang1,2†, Junjie Guo3,4†, Pengfei Guan5,6† , Chunjing Liu1, Hao Huang1, Fanghong

Xue1, Xinglong Dong1*, Stephen J. Pennycook3,4, Matthew F. Chisholm3*

1School of Materials Science and Engineering, Dalian University of Technology, Dalian,

Liaoning, 116024, People’s Republic of China

2National Research Council of Canada, 75 Boul. de Mortagne, Boucherville, Québec, J4B 6Y4,

Canada.

3Materials Science & Technology Division, Oak Ridge National Laboratory, P.O. Box 2008 Oak

Ridge, TN 37831, United States of America.

4Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN

37996, United States of America.

5Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD

21218, United States of America.

6Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China.

2

Cu electrode

Power supply

Cooling water

Cathode (carbon)

Methane

Cooling water

Pump

Niobium bulk

Window

- +

Supplementary Figure S1: Schematic diagram of the synthesis equipment used to produce Nb-

in-C complex.

3

Power supply

Cooling water

Cooling water

- +

Metal sheet

a

b

c

Uniformly dispersed NbC clusters

Phase separation of graphite and NbC clusters

Supplementary Figure S2: Schematic diagram for the two kinds of samples collected at

different places. a, Schematic diagram of the synthesis equipment used to produce Nb-in-C and

NbC-in-C complexes. b, TEM images of the Nb-in-C complex collected from the cooling wall of

chamber, and c, NbC-in-C complex collected from the metal sheet without external cooling.

Scale bar, 100 nm.

4

{002}

{311}  {220}

{200}

 

 

 

Intens

ity  (a.u.)

{111}

G raphite

 NbC

20 30 40 50 60 70 80 90 100

NbC -­‐in -­‐C  

 

 

2  T heta  (deg .)

Supplementary Figure S3: X-ray diffraction scan of NbC-in-C complex.

5

a b

Supplementary Figure S4: Microstructure characterization. a SEM images of Nb-in-C

complex. . Scale bar, 200 nm. b, NbC-in-C complex. Scale bar, 100 nm.

6

. .

a b

d e

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50

5

10

15

20

25  S ta tis tic s  G aus s  fit

Frequ

ency

 (%)

C lus ter  S iz e  (nm)

1.6  nm

c

f

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50

5

10

15

20

25

2  nm

 S ta tis tic s  G aus s  fit

C lus ter  S iz e  (nm)

Frequ

ency

 (%)

Carbon

Niobium  carbide

NbC-­‐in-­‐C  

Nb-­‐in-­‐C  

Supplementary Figure S5: Microstructure characterization. a, b and c, STEM images and

cluster size statistics of Nb-in-C complex. Scale bar, 20 nm. d, e and f, NbC-in-C complex. Scale

bar, 50 nm.

7

a b c

d e f

Supplementary Figure S6: Microstructure characterization of the NbC-in-C sample. a,

STEM images of the NbC-in-C collected from the metal sheet without external cooling. a, b and

c present the representative region of niobium carbide clusters; d, e and f correspond to the

region of carbon. It is evident that there is almost no single niobium doping in carbon matrix.

These images unambiguously indicate that the single niobium atoms cannot be frozen at a

moderate cooling condition, and are prone to migrate from the carbon matrix and immerse into

the niobium carbide clusters.

8

150 300 450 600 750 9000

20

40

60

80

100

17.4  wt.%

   

 

T empera ture  (°C)

314 °C

 

Res

idua

l  mas

s  (%

)

300

400

500

600

700

800

900

C 60

G raphite

N -­‐MWC N

T s

F e-­‐MWC N

T s

B -­‐MWC N

T s

MWC N

T s

G raphene

 

 

The

 ons

et  te

mpe

rature  of  o

xida

tion  (o C

)

Nb-­‐in-­‐C

a b

150 300 450 600 750 9000

20

40

60

80

100

20.0  wt.%

   

 

T empera ture  (°C)

310 °C

 

Res

idua

l  mas

s  (%

)

c

Supplementary Figure S7: Thermal oxidation analysis. a and b, TGA curves of as-made Nb-

in-C and NbC-in-C heated in an air atmosphere at a heating rate of 10 oC min−1 from 50 to 900

oC, respectively. c, Comparison of the oxidation temperatures of various carbon materials. 1-3

layer graphene 22, C60 23, MWCNTs 23, B-MWCNTs 24, Fe-filled MWCNTs 24, N-MWCNTs (25),

Bulk graphite 26. The Nb-in-C complex was synthesized by arc-discharge without any impurity;

it thus only consists of niobium and carbon elements in the product. By thermal-oxidation up to

900 oC in a flowing air atmosphere, the final mass reaches a constant value that only corresponds

to stabilized niobium oxide (Nb2O3). Based on these parameters, niobium and carbon in Nb-in-C

complex are calculated to be 13.7 wt% and ~86.3 wt.%, respectively.

9

500 1000 1500 2000 2500

800

900

1000

1100

1200

1300

1400

Nb-­‐in -­‐C  

G

 

 

Intens

ity  (a.u.)

R aman  s hift  (cm -­‐1)

DG  a t  1592  cm -­‐1

D  a t  1330  cm -­‐1

ID/I

G= 0.98

500 1000 1500 2000 2500200

400

600

800

1000

1200

1400G  a t  1573  cm -­‐1

D  a t  1323  cm -­‐1

ID/I

G= 1.17

 

 

Intens

ity  (a.u.)

R aman  s hift  (cm -­‐1)

NbC -­‐in-­‐C  D

G

a b

Supplementary Figure S8: Raman analysis. a and b, Raman spectra of Nb-in-C and NbC-in-C

complexes.

10

-­‐0.8 -­‐0.6 -­‐0.4 -­‐0.2 0.0-­‐5

-­‐4

-­‐3

-­‐2

-­‐1

0

 

J/mA  cm

-­‐2

E /V  vs  A g /AgC l

 400  rpm  800  rpm  1200  rpm  1600  rpm  2000  rpm  2400  rpm

-­‐0.13  V

a b

0.02 0.03 0.04 0.050.2

0.3

0.4

0.5

0.6  -­‐0 .4  V  -­‐0 .5  V  -­‐0 .6  V  -­‐0 .7  V

 

J-­‐1/m

A-­‐1cm

2

ω−1/2/(rpm)−1/2

Supplementary Figure S9: Evaluation of ORR properties of NbC-in-C complex. a Linear

sweep voltammetry curves recorded for Nb-in-C complex in an O2-saturated 0.1M KOH solution

at a scan rate of 5 mVs-1 and different rotation rates. b, Koutecky-Levich plot of J-1 versus w-1/2

at different electrode potentials.

11

200 250 300 350 400 450 500 550 600

364  eV

284  eV

Nb  M3

 

 

E nerg y  los s (eV)

C  K

Nb  M45

206  eV

379  eVNb  M

2

Intensity  (a.u.)

469  eVNb  M

1

Supplementary Figure S10: EELS spectrum of Nb-in-C complex. EELS spectrum obtained

on the region marked by white color square in Fig. 2c. The atomic-scale characterization

indicates that there is no surface oxide on our samples and, thus, the origin of catalysis in our

materials is the niobium-carbon complexes rather than the oxide. To investigate the catalyst

nanoparticles, we performed EELS chemical analysis on the nanoparticle in carbon matrix as

shown in Fig. 1E 39. The spectral analysis reveals the typical fingerprint of Nb. The Nb M4,5

peaks with an edge oneset of ~206 eV stem from transitions of Nb 3d electrons to unoccupied 4f

and 5p states. The nanoparticle with higher ADF contrast is identified to be niobium carbide 40.

No evidence for the presence of other impurities except carbon was found from EELS spectrum;

in particular, there is no evidence of any O edge at 532 eV, indicating the particles are not

oxidized. Carbon (1s) K-absorption edge of the catalyst resembles closely to that of graphite 41.

The sharp peak close to the edge onset at ~284 eV corresponds to the 1s-π* transition whereas

the main peak >290 eV shows the three characteristic 1s-σ * transitions. The blurring of the triple

σ * peaks resembles the trends for single-walled carbon nanotubes and carbon onions in which

the graphitic network is assumed to be highly corrugated 42-44.

12

n h ΔE

1

2

3

4

7

Nb + Graphene@Vn

ΔE= Etotal – ENb – EG@Vn

1.89

1.08

0.35

0

0

1.39

1.27

0

2.83

2.78

Supplementary Figure S11: Various configurations of a Nb atom on a graphene layer.

Schematic of four possible configurations of a single niobium atom incorporated into defects of

various sizes on a single-layer carbon plane. The detailed information in the table shows that the

Nb+G@V3 (graphene with three missing carbon atoms) is the most energetically favourable

configuration.

13

Nb + Graphite@Vn

n 1 3ΔE: 1.38 eV 0

n 4 7ΔE: 2.78 eV 2.76 eV

Supplementary Figure S12: Various configurations of a Nb atom on a tri-layer graphite.

Schematic of two possible configurations of a single niobium atom doped into a three-layer

carbon model system. The calculated change in energy shows that energetics of multi-layer

carbon systems is nearly identical to those calculated for single-layer carbon systems.

14

0.02 0.03 0.04 0.051.2

1.5

1.8

2.1

 

 

 -­‐0 .4  V  -­‐0 .5  V  -­‐0 .6  V  -­‐0 .7  V

J-­‐1/m

A-­‐1cm

2

ω−1/2/(rpm)−1/2

-­‐0.7 -­‐0.6 -­‐0.5 -­‐0.40.0

0.5

1.0

1.5

2.0

2.5

3.0

1.65

1.951.70

 

 

Electron  tran

sfer  num

ber

E /V  vs  A g /AgC l

2.44

0 10000 20000 3000090

92

94

96

98

100

 

 

Relative  Current  (%)

T ime  (s )

a b

c d

-­‐0.8 -­‐0.6 -­‐0.4 -­‐0.2 0.0-­‐0.9

-­‐0.6

-­‐0.3

0.0  400  rpm  800  rpm  1200  rpm  1600  rpm  2000  rpm  2400  rpm

 

 

J/mA  cm

-­‐2

E /V  vs  A g /AgC l

Supplementary Figure S13: Evaluation of ORR properties of NbC-in-C complex. a, Linear

sweep voltammetry curves recorded for NbC-in-C complex in an O2-saturated 0.1M KOH

solution at a scan rate of 5 mVs-1 and different rotation rates. b, Koutecky-Levich plot of J-1

versus w-1/2 at different electrode potentials. c, The dependence of electron transfer number (n)

on potential for NbC-in-C complex. d, Chronoamperometric response curve of NbC-in-C

complex recorded at 0.5 V in an O2-saturated 0.1M KOH at a rotation rate of 1600 rpm.

15

-­‐1 .0 -­‐0 .8 -­‐0 .6 -­‐0 .4 -­‐0 .2 0.0 0.2 0.4

-­‐0 .9

-­‐0 .6

-­‐0 .3

0.0

0.3

 

 

 N2-­‐S a tura ted

 O2-­‐S a tura ted/1M  Methanol

 O2-­‐S a tura ted

J/mA  cm

-­‐2

E /V  vs  A g /A gC l

Supplementary Figure S14: Evaluation of ORR properties of NbC-in-C complex. Cyclic

voltammograms (CV) curves of ORR on Nb-in-C complex in a N2-saturated 0.1M KOH

solution, an O2-saturated 0.1M KOH solution with and without the addition of 1M methanol

(CH3OH) at a scan rate of 100 mVs-1.

16

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1058-1063 (2009).

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