Document

Electron Transfer Kinetics on
Graphene/Graphite and MoS2
Matěj Velický*
Robert A.W. Dryfe
School of Chemistry,
University of Manchester,
Oxford Rd, Manchester, M13 9PL, UK
*matej.velicky@manchester.ac.uk
2
University of Manchester
Relevant properties of graphene/2D materials
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Carbon electrochemistry – light and inexpensive devices
(electrode material, fuel cells, Li-ion batteries, micro-sensors)
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Graphene – an ideal electrode material
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high electron mobility, electron & thermal conductivity
specific surface area
flexibility
almost transparent
Relevant applications
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Ultrathin electrodes
Sensors (chemical/biological/mechanical)
Supercapacitors (surface charge)
Fuel cells (electrocatalysis)
Solar cells (transparency)
3
University of Manchester
Graphene electrochemistry
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BUT a fundamental property remains - electrode kinetics !!
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Electron transfer rate between graphene surface and a molecule, k0.
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Carbon exhibits slower kinetics than most metals
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How does graphene compare to other carbon materials?
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What are the active sites on graphene/graphite (basal plane vs. edges)?
ox
e-
C.E. Banks, R.G. Compton, Analyst 131 (2006) 15
higher k0:
faster electrode kinetics
red
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University of Manchester
Electron transfer kinetics on graphene
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Controversial topic – even for bulk graphite (HOPG)
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Contradicting results coming from different groups.
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Studies on liquid-phase exfoliated and CVD graphene. [1-5]
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Very little on high-quality, mechanically exfoliated graphene.[6-7]
[1] W. Li, C. Tan, M.A. Lowe, H.D. Abruña, D.C. Ralph, ACS Nano 5 (2011) 2264.
[2] A.T. Valota, P.S. Toth, Y. J. Kim, B.H. Hong, I.A. Kinloch, K.S. Novoselov, E.W. Hill and R.A.W. Dryfe, Electrochim. Acta 110 (2013), 9-15.
[3] A.G. Guell, N. Ebejer, M.E. Snowden, J.V. MacPherson, P.R. Unwin, JACS 134 (2012) 7258.
[4] C. Tan, J.Rodríguez-López, J.J.Parks, N.L.Ritzert, D.C. Ralph, H.D. Abruña, ACS Nano 6 (2012), 3070.
[5] M.S.Goh, M.Pumera, Chemistry – An Asian Journal 5 (2010), 2355.
[6] R. Sharma, J.H.Baik, C.J.Perera, M.S.Strano, Nano Letters 10 (2010), 398.
[7] A. T. Valota, I. A. Kinloch, K. S. Novoselov, C. Casiraghi, A. Eckmann, E. W. Hill and R. A. W. Dryfe, ACS Nano 5 (2011), 8809-8815.
ox
e-
red
k0
5
University of Manchester
Preparation of 2D material electrodes
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‘Scotch tape’ method, high optical contrast
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Large flakes required (> 100 µm)
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Well-defined basal plane surface
Geim/Novoselov 2004
Brightfield/darkfield optical microscopy
6
University of Manchester
Experimental setup
•  Flakes of varied thickness are
mechanically exfoliated onto
Si/SiO2 substrate. !
!
•  electrical contact is made
using silver epoxy and copper
wire. !
!
50 µm •  micro-droplet is injected on
the surface of a flake!
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University of Manchester
Solid/liquid electrochemistry
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BASAL plane of graphene – working electrode
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Voltammogram (I-V curve): diffusion and electron transfer
V
top-gate configuration
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Aqueous electrolyte solution of a redox mediator
K4Fe(CN)6, Ru(NH3)6Cl3, (NH4)2IrCl6
1/ 2
2
⎛ α n F Dν ⎞ − αRTF n ΔEp
⎟⎟ e
k = 2.18 ⎜⎜
⎝ R T ⎠
0
ψ = k0
RT −0.5
ν
πnFD
‘electrode kinetics’
(electron transfer rate)
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University of Manchester
Flake characterisation: Raman spectroscopy and AFM
5 µm
•  Flake thickness!
•  Defect-free !
basal plane surface!
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University of Manchester
Dependence of kinetics on flake thickness:
obscured by the local surface conditions
bulk graphite
graphene
bulk graphite
graphene
•  electron transfer kinetics are found to be
largely independent of the thickness of
graphene/graphite flakes. !
!
Velický, M.; Dryfe, R. A. W, et al.; ACS Nano, 8 (10), 2014, 10089-10100.
•  Instead, large site-to-site variation of the
kinetics is observed, even on the surface of
the same flake. !
10
University of Manchester
Surface ageing effects
Large increase in the
kinetics is observed on
freshly cleaved graphite
due to surface ageing.!
A.N.Patel, P.R. Unwin, et al. JACS, 134 (2012), 20117.
Freshly cleaved surface: ~70-fold increase in kinetics!!
11
University of Manchester
XPS analysis of the graphite surface
freshly cleaved graphite
aged graphite
•  Fluctuation of electroactivity due to
significant reactivity/functionalization
of graphene upon exposure to air, as
confirmed by XPS and EDX analyses.!
•  Large variation in concentration and
hybridisation of carbon atoms is found
on freshly cleaved graphite!
Velický, M.; Dryfe, R. A. W, et al.; ACS Nano, 8 (10), 2014, 10089-10100.
•  ‘Averaged’ concentration is observed
on aged graphite!
12
University of Manchester
Kinetics measurements on bulk MoS2 and graphite
Comparison between:
• 
• 
• 
pristine basal plane
defective basal plane
freshly cleaved surface
13
University of Manchester
Comparison of kinetics on MoS2 and graphite
•  Semiconducting MoS2 exhibits slower
kinetics than graphite. !
•  Differences between pristine and
defective basal plane of graphite and
MoS2 studied.!
•  Freshly cleaved surface generally
faster kinetics than aged one. !
14
University of Manchester
Surface activity related to oxygen and contaminants concentration
• 
XPS, EDX and Raman
15
University of Manchester
Conclusions
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Basal planes of both graphene and MoS2 are electrochemically active !
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No clear correlation between graphene thickness and electrode kinetics!
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Surfaces of both materials age rapidly upon exposure to air
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Many other questions remained unanswered: active sites on 2D materials, time-scale
of surface degradation, photoelectrochemistry of MoS2 and other TMDCs.
Acknowledgments Peter Toth
Anna Valota
Adam Cooper
Hollie Patten
Stephen Worrall
Artem Mishchenko
Mark Bissett
Dan Bradley (Liverpool)
Sheng Hu
Greg Auton
Andrew Rodgers
Thanasis Georgiou
Liam Britnell
Fred Withers
Huafeng Yang
Antonios Oikonomou
Rob Dryfe
Ian Kinloch
Kostya Novoselov
Ernie Hill