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 *[email protected] 2 University of Manchester Relevant properties of graphene/2D materials   Carbon electrochemistry – light and inexpensive devices (electrode material, fuel cells, Li-ion batteries, micro-sensors)   Graphene – an ideal electrode material           high electron mobility, electron & thermal conductivity specific surface area flexibility almost transparent Relevant applications           Ultrathin electrodes Sensors (chemical/biological/mechanical) Supercapacitors (surface charge) Fuel cells (electrocatalysis) Solar cells (transparency) 3 University of Manchester Graphene electrochemistry   BUT a fundamental property remains - electrode kinetics !!   Electron transfer rate between graphene surface and a molecule, k0.   Carbon exhibits slower kinetics than most metals   How does graphene compare to other carbon materials?   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 4 University of Manchester Electron transfer kinetics on graphene   Controversial topic – even for bulk graphite (HOPG)   Contradicting results coming from different groups.   Studies on liquid-phase exfoliated and CVD graphene. [1-5]   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   ‘Scotch tape’ method, high optical contrast   Large flakes required (> 100 µm)   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! 7 University of Manchester Solid/liquid electrochemistry   BASAL plane of graphene – working electrode   Voltammogram (I-V curve): diffusion and electron transfer V top-gate configuration   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) 8 University of Manchester Flake characterisation: Raman spectroscopy and AFM 5 µm •  Flake thickness! •  Defect-free ! basal plane surface! 9 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   Basal planes of both graphene and MoS2 are electrochemically active !   No clear correlation between graphene thickness and electrode kinetics!   Surfaces of both materials age rapidly upon exposure to air   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