We use micro-Raman spectroscopy to measure the surface adsorption and electron-transfer dynamics between graphene and I2 molecules in solution. Solvated I2 molecules can adsorb onto graphene via weak van der Waals forces. We measure the degree of charge-transfer between these adsorbed molecules and graphene as a function of I2 concentration and solvent identity. Concentration dependent measurements in cyclohexane, benzene, and water generate solution-phase isotherms with two distinct increases in surface concentration. For the initial, rapid increase, fits to the data produce K = 1200 M-1 in cyclohexane, K = 26,000 M-1 in benzene, and K = 180,000 M-1 in water for surface adsorption. The large equilibrium constant in water can be understood by the unfavorable interactions between nonpolar I2 and polar water driving I2 molecules to the graphene surface. Having a larger equilibrium constant in benzene than cyclohexane suggests that I2-benzene complexes enhance surface adsorption and electron transfer, a surprising result. As the single-layer I2 surface coverage stabilizes after this first increase, the G-peak splits for bilayer graphene, showing unequal doping on the top and bottom graphene layers. Thus, the initial rapid and subsequent slower increase in the I2 surface coverage is due to adsorption on the top and bottom surfaces, respectively, showing that the substrate-graphene interface impedes molecular diffusion, in contrast to gas-phase I2 doping experiments.

Asymmetric solvent-mediated electron-transfer chemical doping of graphene

253rd National Meeting (2017)

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