B. Graphene Fabrication Methods

Ionic and Molecular transport through nanoporous grapheneVivek Saraswat1, Dr. Ulrich F. Keyser2

vs396@cam.ac.uk1. Department of Materials Science & Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK2. Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK

References and Acknowledgement

A. Introduction• Graphene, a 2D honeycomb lattice of carbon atoms, is known to be

impermeable to all gases and ions1,2. Introduction of nanopores, however can enable size selective separation

• Due to its atomic thickness and mechanical strength3 (~1 TPa), nanoporousgraphene (NPG) can achieve 100 times greater permeation rates and sustain 10 times greater hydraulic pressures than currently used RO membranes4. Therefore NPG has a huge potential in gas separation and desalination

• Here we discuss the graphene and nanopore fabrication techniques, gas and ion permeation mechanisms through NPG and present the current project with a representative set of results that have been obtained till date

1. Berry, V. Carbon N. Y. 62, 1–10 (2013)2. Hu, S. et al. Nature (2014).3. Lee, C. et al. Science. 321, 385–388 (2008).4. Cohen-Tanugi, D. et al. Nano Lett. 12, 3602–3608 (2012).5. Bonaccorso, F. et al. Mater. Today 15, 564–589 (2012)

6. Li, W. et al. J. Appl. Phys. 114, 1–17 (2013).7. Sun, C. et al. Langmuir 30, 675–682 (2014).8. Koenig, S. P. et al. Nat. Nanotechnol. 7, (2012).9. Kang, Y. et al. Nanoscale 10666–10672 (2014).10. Liu, Y. et al. J. Appl. Phys. 115, (2014)

11. Cohen-tanugi, D. et al. (2014)12. Walker M.I. et al. Appl. Phys. Lett. 106, (2015)

• Mechanical Exfoliation: Repeated cleaving of graphite layers to obtain single layer graphene. Yields good quality graphene, but not scalable

• Liquid Phase Exfoliation: Exfoliation of graphite in a suitable solvent to obtain graphene flakes. Cheap and scalable but limits the area of graphene

• Chemical Vapour Deposition: Deposition of carbon on metal substrate to obtain graphene. It’s the most promising method to obtain large area graphene inexpensively

Reproduced from 5

C. Creating nanopores in graphene• Oxidative Etching: It involves exposing graphene to reactive oxygen species.

Oxidation commences at existing defects which enlarge into pores. To create sub-nanometer sized pores, graphene can be exposed to UV in presence of oxygen. UV radiation breaks C=C bonds of graphene and creates defects

• Ion-beam etching: Used to create high density nanopores over macroscopic areas of graphene using ion bombardment (eg. Ga+)

• Electron beam etching: Creating nanopores using electron beam of a TEM. Offers precise control over pore size6, but slow and needs to be done manually

D. Gas and Ion permeation mechanisms

Possible molecule trajectories (Reproduced from 7)

Leak rates of different gases before and after nanoporesof certain size were etched in NPG (Reproduced from 8)

Gas Permeation Mechanisms• Total permeation through NPG is a sum of direct flux (molecules cross directly from bulk

phase of one side to other side) and adsorption flux (molecules cross after being adsorbed onto graphene). Certain gases such as CH4 show significant adsorption on NPG, while others such as H2 and He show no surface adsorption

• Studies show that selective permeation is possible if pores of a particular size can be reliably generated (see fig.). Pore functionalization also affects selectivity. By attaching heavy functional groups at pores, it is possible to achieve selective N2 permeation and exclude H2

• Permeation rate is linear at low pressures, but at higher pressures, the adsorbed phase saturates the pores and therefore the permeation rate becomes independent of pressure

• Smaller pores have high energy barriers and are therefore dominated by direct flux. Larger pores have low energy barriers and are therefore dominated by surface adsorption

Ion Permeation Mechanisms• Ionic flux is strongly dependent on pore size and functionalization. Oxygen doped nanopores

are selective to K+. This is due to coulomb coupling between ions and functional groups. • Electric field can also be used to tune ion selectivity to a certain extent. Moreover, it also

depends on the ion itself – heavily hydrated ions (Li+) are less selected

K+ selectivity shown as non zero density of K+ at x=0

(Reproduced from 9)

E. Graphene – A promising RO membrane• Despite decrease in young’s modulus, membrane stress of NPG also decreases with

increasing porosity10. Therefore, NPG can sustain higher pressures at higher porosity• NPG shows near perfect salt rejection for smallest pore sizes. Salt rejection decreases with

increase in pore sizes, but surprisingly also decreases with increase in applied pressure, which is exact opposite of what is observed in diffusive RO membranes.

• Hydroxylated pores show 70% higher permeability than hydrogenated pores but also lower salt rejection due to hydrogen bonding between –OH and ions

• NPG can achieve permeation of 129 L/cm2/day/MPa which is 100 times higher than other RO membranes and can sustain hydraulic pressures of 57 MPa, which is 10 times higher than other RO membranes. NPG therefore has immense potential as RO membrane

Stress distributions in a NPG sheet under increasing biaxial stress (Reproduced from 10)

Performance chart for functionalized nanoporous graphene versus existing technologies (Reproduced from 10)

A typical simulation(Reproduced

from 10)

F. Project Approach & Methodology• Glass nanopore is formed by pulling glass capillary in a

laser capillary puller• This glass nanopore is mounted on a micromanipulator

(which controls its drive) and filled with the ionic solution

• A voltage is applied between electrodes in the nanoporeand the reservoir and the nanopore is lowered onto graphene. Contact is detected when current increases and the micromanipulator drive is stopped

• An inverted microscope is used to obtain in-situ Raman measurements to characterize graphene

Experimental setup for the project(Reproduced from 12)

(i) Graphene is floated on the surface of reservoir by dissolving salt crystal.

(ii) Capillary is pushed into the graphene. Contact is detected by increase in current

(iii) This places a monolayer across the tip(Reproduced from 12)

G. Results and Discussion• The method that will be used has been demonstrated for measuring ionic conductivity across

graphene membranes. This means that it will be possible to carry out experimental investigation of ion selectivity of graphene nanopores and test the published computational results. Results so far have established that graphene seals nanopores of size ~150 nm.

• Following figures are representative selection of experimental results that have been obtained so far

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Measured current

Linear fit of I-V curve

Cu

rre

nt

(nA

)

Voltage (mV)

Bare nanopore resistance=9.73 M

-200 -100 0 100 200

-0.4

-0.2

0.0

0.2

0.4 Measured current

Linear fit of I-V curve

Cu

rre

nt

(nA

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Voltage (mV)

Sealed Resistance=512 M

-60 -40 -20 0 20 40 60-0.3

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-0.1

0.0

0.1

0.2 I-V Curve with bare nanopore

I-V Curve with sealed nanopore

Cu

rre

nt

(nA

)

Voltage (mV)

R=236.6

R=833

1200 1600 2000 2400 2800

1800

2000

2200

2400

2600

Ra

ma

n I

nte

nsity

Raman Shift (cm-1)

D

G

2D

(i)

(ii)

(iii)

(iv)

(i) Raman spectrograph of graphene taken from in-situ Raman spectrometer (ii) I-V curve of Bare nanopore (150 nm nanopore, 1M NaCl) (iii) I- V curve of sealed nanopore (iv) I-V curves of bare and sealed nanopore in pH3 HCl solution

H. Conclusion and Outlook• Nanoporous graphene has huge potential in desalination and gas separation

systems owing to its mechanical strength and extraordinary permeability• Pore geometry and functionalization can be used to tune the selectivity of

NPG to gas molecules and ions• Using ionic current measurements and Raman spectroscopy defect density can

be assessed. Therefore, selectivity of NPG to various ions as a function of defect density can be determined

• Using the proposed setup it will be possible to verify published computational results in the future

The experiments were supported by Biological & Soft Systems group at Cavendish Lab, University of Cambridge. Author is thankful to Michael Walker for providing necessary training and supervision