國立臺灣師範大學微奈米光機電系統實驗室 -1- Z. Y. Chen, NTNU ME Evaluation Criteria for Reduced Graphene Oxide Ref: Dachao Luo, Guoxin Zhang, Junfeng Liu, and Xiaoming

國立臺灣師範大學微奈米光機電系統實驗室

-1-

Z. Y. Chen, NTNU ME

Evaluation Criteria for Reduced Graphene OxideEvaluation Criteria for Reduced Graphene OxideRef: Ref: Dachao Luo, Guoxin Zhang, Junfeng Liu, and Xiaoming Sun, Dachao Luo, Guoxin Zhang, Junfeng Liu, and Xiaoming Sun, J. Phys. Chem. C, 2011.J. Phys. Chem. C, 2011.

Advisor: Chii-Rong Yang Advisor: Chii-Rong Yang Student: Zi-Ying Chen (Student: Zi-Ying Chen ( 陳姿穎陳姿穎 ))

Department of Mechatronic EngineeringDepartment of Mechatronic EngineeringNational Taiwan Normal UniversityNational Taiwan Normal University

Date: 10/20/2014Date: 10/20/2014


-2-

Z. Y. Chen, NTNU ME

OutlineOutline

Abstract

Introduction

Experimental

Result and discussion

Conclusions


-3-

Z. Y. Chen, NTNU ME

AbstractAbstract


-4-

Z. Y. Chen, NTNU ME

AbstractAbstract

The oxidation in this chemical method induces a variety of defects and oxygen-containing functional groups such as hydroxyl and epoxide, resulting in degradation of the electronic properties of graphene. Meanwhile, the reduction process can lead to the occurrence of aggregate, ion doping, and so on.

The samples were systematic compared by four aspects: dispersibility, reduction degree, defect repair degree, and electrical conductivity. On the basis of the comparison, a simple evaluation criterion was proposed for qualitatively judging the quality of RGO.


-5-

Z. Y. Chen, NTNU ME

AbstractAbstract

Here we present a simple classification as followed:

Reducing agents

hydroquinone, dimethylhydrazine, hydrazine, sodium borohydride, sulfur-containing compounds, aluminum powder, vitamin C, hexamethylenetetramine, ethylenediamine (EDA), polyelectrolyte, reducing sugar, protein, sodium citrate, carbon monoxide, Fe, and norepinephrine.

Performing under various conditions

acid/alkali, thermal treatment, and others like microwave, photocatalytic, sonochemical, laser, plasmas, bacterial respiration, lysozyme, tea solution.

Electrochemical -

Two-step reduction -


-6-

Z. Y. Chen, NTNU ME

IntroductionIntroduction


-7-

Z. Y. Chen, NTNU ME

IntroductionIntroduction

In this study, RGOs were prepared by six typical reduction

methods: N2H4·H2O, NaOH, NaBH4, solvothermal, high-

temperature, and two-step.

On the basis of the comparison, a simple evaluation criterion was

proposed for qualitatively judging the quality of RGO.


-8-

Z. Y. Chen, NTNU ME

ExperimentExperiment


-9-

Z. Y. Chen, NTNU ME

GO Synthesis and Purification (1/4)GO Synthesis and Purification (1/4)

Nature graphite (5 g) was grounded with NaCl (100 g) for 30 min.

NaCl was washed away using water with vacuum filtration.

The remaining graphite was heated at 60 °C in oven for 24 h to

remove any water.


-10-

Z. Y. Chen, NTNU ME


at RTstirred for 24 h

The dried solid +

H2SO4 (115 mL) Chiller

+ 15 g of KMnO4

kept below 20 °C for 1.5 h.

+ NaNO3 (0.5 g)

for 20 min


-11-

Z. Y. Chen, NTNU ME


Increase to 35 ~ 40 °C and keep for 1 h. Increase to 70 °C and keep for 30 min.Add water (15 mL), increase to 100 °C and keep for

20 min.Add water (15 mL) and keep for 30 min.Add water (200 mL). Add ice water (500 mL), this step can dilute and cool

down the system to 50 °C. ∼After 15 min, 50 mL of 30% H2O2 was added to the

flask under vigorous stirring. This suspension was stirred at room temperature for

1 h.

Put into the oil bath


-12-

Z. Y. Chen, NTNU ME


The suspension was centrifuged at low speed for 3 times (4500 rpm,

15 min) and washed with 5% HCl solution for 2 rounds and then

centrifuged at high speed for 3 times (12000 rpm, 60 min) with

distilled water (first step we add 0.1 g of NH4Cl).

Finally, the GO was kept in a vacuum desiccator with self-

indicating silica gel for 1 week.

self-indicating silica gel


-13-

Z. Y. Chen, NTNU ME

Reduction (1/3)Reduction (1/3)

Hydrazine hydrate

GO dispersion (25.0 mL, 0.05 wt %)+ water (25 mL)+ hydrazine solution (11.0 μL, 80 wt %)+ ammonia solution (175.0 μL, 28 wt %)

After being vigorously shaken or stirred for a few minutes, the vial was put in a water bath (95 °C) for 1 h.

NaOH reduction

GO dispersion (0.5 mg/mL, 75 mL)+ NaOH solution (1 mL, 8 M)

The vessel was kept at 70 °C and mild sonicated for a few minutes.

NaBH4 reduction

graphite oxide (75 mg)+ water (75 g)+ sodium borohydride (600 mg in 15 g of water) + 5 wt % sodium carbonate solution. (pH being adjusted to 9–10)

The mixture was then kept at 80 °C for 1 h under constant stirring.


-14-

Z. Y. Chen, NTNU ME


SolvothermalGO aqueous solution (35 mL of 0.5 mg/mL) was transferred to a Teflon-lined autoclave and heated at 180 °C for 6 h.

High-temperature

Put the GO powder in porcelain boat and then place it into the edge of long quartz tube.

The sample was flushed with argon for 10 min. The porcelain boat was quickly inserted into the

middle of long quartz tube furnace preheated to 900 °C and held in the furnace for 30 s.


-15-

Z. Y. Chen, NTNU ME


Two-step

100 mg of dry GO sample was dispersed in deionized water to give a 1.0 mg/mL colloidal solution.

The pH value of this solution was adjusted to 9–10 by 5 wt % sodium carbonate solution. (NaBH4 reduction)

Sodium borohydride (800 mg) was directly added into 100 mL of GO dispersion under magnetic stirring, and the mixture was kept at 80 °C for 1 h.

Reduced product was separated by filtration and washed with large amount of water several times to remove most residual ions.

This partially reduced GO was kept in a vacuum desiccator with phosphorus pentoxide for 2 days and redispersed in concentrated sulfuric acid and heated to 120 °C with stirring for 12 h.

After cooling down, the dispersion was diluted with deionized water. Further annealed at 900 °C under gas flow of Ar for 15 min. (High-

temperature)


-16-

Z. Y. Chen, NTNU ME

Result and discussionsResult and discussions


-17-

Z. Y. Chen, NTNU ME

CriterionCriterion

The samples were systematically compared by four aspects:

a. Dispersibility

b. Reduction degree

c. Defect repair degree

d. Electrical conductivity


-18-

Z. Y. Chen, NTNU ME

AFM imagesAFM images

Tapping-mode AFM images (2 × 2 μm2, scale bar: 400 nm).

GO N2H4

NaOH

Solvothermal

NaBH4

High-temperature

Two-step NaBH4

two-step


-19-

Z. Y. Chen, NTNU ME

DispersibilityDispersibility

1 week after


-20-

Z. Y. Chen, NTNU ME

UV–vis absorption spectroscopyUV–vis absorption spectroscopyUV–vis absorption spectroscopy gives us an average reflection of

reduction degree of RGO by different reduction methods. To further reveal the details, XPS was employed to analyze the GO and RGO.

UV–vis absorption spectraN

2H4

GO

NaO

H

Sol

voth

erm

al

NaB

H4

Hig

h-t

emp

erat

ure

Tw

o-st

ep

→


-21-

Z. Y. Chen, NTNU ME

XPS spectraXPS spectraGO N2H4

NaOH

Solvothermal

NaBH4

High-temperature

Two-step

XPS spectra and Values of C/O atomic ratios obtained by the XPS analysis for GO (sample 1) and different methods reduced RGO (from sample 2 to 7: N2H4, NaOH, NaBH4, solvothermal, high-temperature, two-step).


-22-

Z. Y. Chen, NTNU ME

GO N2H4 NaOH NaBH4 SolvothermalHigh

temperatureTwo-step

C–O 0.31 0.39 0.28 0.45 0.17 0.36 0.30

C═O 0.23 0.08 0.13 0.16 0.26 0.01 0.05

N2H

4

GO

NaO

H

Sol

voth

erm

al

NaB

H4

Hig

h-t

emp

erat

ure

Tw

o-st

ep

On the basis of the above results, we can rationally design the reaction process for transforming the functional groups of GO to improve the reduction degree, for example, by adjusting the environment (e.g., pH) to convert some functional groups into carboxyl or hydroxyl groups and then using a reducing agent to further restore the structure.

↑ ↑ ↑


-23-

Z. Y. Chen, NTNU ME

Raman spectraRaman spectra

Here we borrow the D/G ratio of single-layer graphene prepared by mechanical exfoliation of graphite (about 1.0, HOPG is about zero) as a reference to solve this problem.

N2H

4

GO

NaO

H

Sol

voth

erm

al

NaB

H4

Hig

h-t

emp

erat

ure

Tw

o-st

ep


-24-

Z. Y. Chen, NTNU ME

Electrical Conductivity Electrical Conductivity


temperatureTwo-step

electrical conductivity

[S/m]insulator 156.2 3.6 0.006 4.8 232.1 267.8


-25-

Z. Y. Chen, NTNU ME

ConclusionConclusion


-26-

Z. Y. Chen, NTNU ME

ConclusionConclusion (1/2)(1/2)


temperatureTwo-step

Dispersibility Good Bad Good Good OK Bad Bad

Reduction degreeUV–vis

absorption

- OK Bad Bad Bad Good Good

Reduction degreeXPS

- Bad Bad Bad Bad Good OK

Defect repair degreeRaman

- OK Good OK OK Bad Good

Electrical conductivity

insulator 156.2 3.6 0.006 4.8 232.1 267.8


-27-

Z. Y. Chen, NTNU ME

ConclusionConclusion (2/2)(2/2)Under this evaluation criterion, the two-step method appears the

best one for better RGO synthesis in the six reduction methods

compared. RGO reduced by the two-step method has better reduction degree,

defect repair degree and electrical conductivity, but the relatively

weak dispersibility and tedious preparation process still need to

improve. In practice, the reduction degree and the defect repair degree are

the two key factors for effective reduction of GO.


-28-

Z. Y. Chen, NTNU ME

Thank you for your attention!Thank you for your attention!

敬請批評與指教敬請批評與指教

Documents

國立臺灣師範大學微奈米光機電系統實驗室 -1- Z. Y. Chen, NTNU ME Evaluation Criteria for Reduced Graphene Oxide Ref: Dachao Luo, Guoxin Zhang, Junfeng Liu, and Xiaoming