We describe a fast, efficient, and mild approach to prepare chemically reduced graphene oxide (rGO) at room temperature using divalent europium triflate {Eu(OTf)2}. The characterization of solution-processable reduced graphene oxide has been carried out by various spectroscopic (FT-IR, UV–visible absorption, and Raman), microscopic (TEM and AFM), and powder X-ray diffraction (XRD) techniques. Kinetic study indicates that the bimolecular rate constants for the reduction of graphene oxide are 13.7 ± 0.7 and 5.3 ± 0.1 M–1 s–1 in tetrahydrofuran (THF)–water and acetonitrile (ACN)–water mixtures, respectively. The reduction rate constants are two orders of magnitude higher compared to the values obtained in the case of commonly used reducing agents such as the hydrazine derivative, sodium borohydride, and a glucose–ammonia mixture. The present work introduces a feasible reduction process for preparing reduced graphene oxide at ambient conditions, which is important for bulk production of GO. More importantly, the study explores the possibilities of utilizing the unique chemistry of divalent lanthanide complexes for chemical modifications of graphene oxide.

1. 1

2. 2
Introduction
Chem. Rev. 2014, 114, 7150
Mechanical
exfoliation Chemical oxidation, exfoliation and reduction
Unzippingof
carbonnanotubes
CVD on metal catalysis

3. 3
Introduction
Chemical oxidation, exfoliation and reduction
Chem. Soc. Rev. 2014, 43, 291

4. Develop a fast, efficient and mild chemical method to
reduce graphene oxide
Determination of rate constants of the reduction
Interpretation of mechanism of the reduction
Objectives of present study
4

5. Eu(OTf)2-mediated reduction of rGO
Time progress of reduction
0 min. 2 min. 10 min.
Eu(OTf)2 solution was prepared
in dry-THF or dry-ACN
5
Graphene oxide (GO) Reduced graphene oxide (rGO)

6. GO
rGO
rGO
GO
5 10 15 20 25 30 35 40
Intensity,a.u.
2-Theta (2)
GO
rGO
GP
Characterization of rGO
A band at 270 nm confirms the
formation of rGO
The d-spacing for
rGO is ~3.7 Å
6
UV-visible absorption spectra FT-IR spectra
Powder XRD
GP: Graphite powder
GO: Graphene oxide
rGO: Reduced graphene oxide

7. TEM and AFM images
Characterization of rGO
7Thickness of rGO sheets ~ 1.5–2.6 nm, mostly bilayer rGO were produced
200 nm

8. Possible mechanism
8

9. 0 100 200 300 400 500 600
0.05
0.10
0.15
0.20Absorbance
Time, s
1 2 3 4 5 6
6
8
10
12
14
kobs
x10
2
,s
1
[Eu(OTf)2
]x10
4
, M
0 100 200 300 400 500 600
0.05
0.10
0.15
0.20
Time, s
Absorbance
2 3 4 5
3
4
5
6
7
[Eu(OTf)2
]x10
4
, M
kobs
x10
3
,s
1
Rate constant for Eu(OTf)2-mediated reduction
Plot of absorbance growth trace at 600 nm
Bimolecular rate constant
in THF = 13.7 M−1s−1
Bimolecular rate constant
in ACN = 5.3 M−1s−1
9
𝑨 𝒕 = 𝑨 𝟎 + 𝑪. 𝒆 𝒌 𝒐𝒃𝒔 𝒕
THF
ACN

10. 0 5 10 15 20 25
0.01
0.02
0.03
0.04
0.05
Experimental data points
Fitted line
Absorbance
Time, min
1 2 3 4 5
1.0
1.5
2.0
2.5
kobs
x10
3
,s
1
[N2
H4
.H2
O]x10
4
, M
Experimental data points
Fitted line
0 10 20 30 40 50
0.025
0.030
0.035
0.040
0.045
Experimental data points
Fitted line
Absorbance
Time, min
1 2 3 4 5 6 7 8
0
1
2
3
4
5
Experimental data points
Fitted line
kobs
x10
3
,s
1
[NaBH4
]x10
3
, M
0 5 10 15 20 25 30
0.01
0.02
0.03
0.04
0.05
Experimental data points
Fitted line
Absorbance
Time, min
1 2 3 4 5
0
3
6
9
12
15
Experimental data points
Fitted line
kobs
x10
4
,s
1
[Glucose]x10
3
, M
Rate constant for other reducing agents
Plot of absorbance growth trace at 600 nm
10
N2H4.H2O/NH3 NaBH4 Glucose/NH3

11. Sl.
No.
Reducing
agent
Solvent
Temperature
(oC)
Time
Bimolecular
rate constant,
M−1s−1
Reference
1.
N2H4.H2O +
NH3
H2O 95 1h 1.7
Nat.
Nanotechnol.
2008, 3, 101
2. NaBH4 H2O 125 3h 3.3×10−1 Chem. Mater.
2009, 21, 3514
3.
Glucose +
NH3
H2O 95 1h 1.3×10−1 ACS Nano
2010, 4, 2429
4. Eu(OTf)2
ACN-water RT 5m 5.3 Present work
THF-water RT 5m 13.7 Present work
Rate constant for various reducing agents
11
0
50
100
150
42
108
8
X =
𝒌 𝑬𝒖(𝑶𝑻𝒇) 𝟐
/𝒌 𝑿

12. Why reduction of GO is feasible?
High stability of radical anion through extensive delocalization
shifts the equilibrium towards the intermediate formation
12
∆𝑮 𝒆𝒕 = 𝟐𝟑. 𝟎𝟔 × [𝑬 𝒐𝒙 − 𝑬 𝒓𝒆𝒅] (𝐤𝐜𝐚𝐥/𝐦𝐨𝐥)
Thermodynamics of electron transfer
Free-energy change of ground state electron transfer,
Donor
Acceptor
∆Get, kcal/mol 14.76 −7.84
Eu(OTf)2
(−1.21V)

13. Run [1], μg/mL [2], M [3], M
1 17.39 2×10-4 4.83
2 17.39 4×10-4 4.83
Determination of rate orders
Reaction progress kinetic analysis (RPKA)
1: GO; 2: Eu(OTf)2; 3: H2O
Condition for the experiments
A method to obtain enhanced mechanistic details of any reaction
It can be carried out in synthetically equivalent conditions
Rate orders can be calculated from fewer number of experiments
13Angew. Chem. Int. Ed. 2005, 44, 4302

14. Solvents
Rate orders with respect to various components
GO (1) Eu(OTf)2 (2) H2O (3)
THF −1.03±0.1 0.46±0.05 0.76±0.05
ACN −0.68±0.05 0.44±0.05 0.89±0.05
Determination of rate orders
Rate orders for various components
14
𝑑[𝟒] 𝑑𝑡
[𝟐] 𝑥
= 𝑘 𝑜𝑏𝑠[𝟏] 𝑦
[𝟑] 𝑧
1: GO
2: Eu(OTf)2
3: H2O
4: Product (rGO)
Reaction progress kinetic analysis (RPKA)

15. Understanding the rate orders
Positive rate order for water
𝒌 𝑯
𝒌 𝑫
= 𝟏. 𝟑𝟒in H2O in D2O
Fractional rate order for Eu(OTf)2
Aggregation of Eu(II) in THF
15
According to Eigen mechanism
(protonation occurs from the
Eu(III)-bound H2O molecule),
proton transfer still could be the
rate determining step
250 300 350 400 450 500 550
0.0
0.8
1.6
2.4
3.2
Absorbance
Wavelength, nm
[Eu(II)], M
1.24x10
4
2.49x10
4
4.98x10
4
9.96x10
4
[Eu(II)], M
― 1.24×10−4
― 2.49×10−4
― 4.98×10−4
― 9.96×10−4
0 100 200 300 400 500 600
2.0
2.5
3.0
3.5
[4],g/ml
Time, s
kobs
= 0.0109 s
1
kobs = 0.0109 s−1
0 100 200 300 400 500 600
2.0
2.5
3.0
3.5
kobs
= 0.0082 s
1
[4],g/ml Time, s
kobs = 0.0082 s−1

16. Conclusion
Eu(OTf)2 has been introduced as an efficient reducing agent for
reduction of GO
Fast reduction of GO was confirmed by determining the rate
constants
The rate constant values were compared with that of the other
reducing agents
The mechanistic details have been analyzed by calculating the rate
order for various components
Tufan Ghosh, Sandeepan Maity and Edamana Prasad J. Phys. Chem. B 2014, 118,
5524–5531
16

17. Thank you