60th Annual Report on Research 2015

My research group focuses on understanding the reaction dynamics of graphene functionalization.  Graphene's large surface area and high mechanical strength make it a promising catalytic material.  To fulfill this promise, we need to understand how to control graphene's electronic structure by forming ordered graphene derivatives, and graphene halogenation reactions are excellent model systems to study graphene functionalization.  This research project has two goals: 1) Develop a complete molecular understanding of graphene halogenation reaction dynamics in the gas-phase and in solution and 2) use that knowledge to controllably functionalize graphene, tuning its electronic properties for applications in heterogeneous catalysis.  Graphene chlorination reactions occur on relatively slow timescales and can generate fully functionalized chlorographene and graphene chlorinated on only one side. We have previously performed measurements of one layer graphene photochlorination kinetics in the gas-phase and in water, and observed the formation of chlorographene products.  During the past year, we have expanded on these measurements to further explore the effect of sample annealing and compressive strain on graphene reactivity, we have implemented in situ photodissociation using a 405 nm laser, and we have investigated the role of surface adsorption on graphene functionalization. 

We generate graphene via mechanical exfoliation onto a silicon substrate with a 290 nm SiO₂ layer.  The sample is exposed to Cl₂ gas, and a Xenon lamp or 405 nm laser photodissociates Cl₂ to form reactive Cl radicals. Micro-Raman spectroscopy measures the extent of reaction as a function of irradiation time (Figure 1a).  We also perform these measurements in solution to assess the role of solvent polarity and Cl₂-solvent complexes on photochlorination dynamics (Figure 1b). The Raman G-peak (1580 cm^-1) intensity scales with the number of graphene layers and shifts to higher energy upon electron-transfer between graphene and adsorbed species.  The 2D-peak (2700 cm^-1) shape is sensitive to the number of graphene layers. The D-peak (1350 cm^-1) is absent for pristine graphene, and increases in intensity with greater sp³ hybridization of graphene carbon atoms.  In particular, the I_D/I_G ratio is a sensitive probe of the C-Cl bond density.

We have built on our initial photochlorination measurements to further investigate how annealing our graphene samples affects gas-phase photochlorination kinetics.  Figure 2 shows how the D and G Raman peaks evolve during the photochlorination reaction for a non-annealed sample irradiated with a Xenon lamp.  As the irradiation time increases, the D-peak intensity increases relative to the G-peak, indicating the formation of chlorographene. This I_D/I_G ratio is plotted in the red and blue data points as a function of irradiation time in Figure 3 and shows a reaction that barely proceeds during the first 200 minutes, and then occurs on a timescale of hours, eventually reaching ~3 x 10¹¹ C-Cl bonds/cm². This reaction timescale is unusually slow, so we have more recently investigated how sample preparation influences reaction rate.  While annealing the graphene sample at 290 °C in air prior to chlorine exposure does not appear to affect the reaction rate (Figure 3, green data points), annealing at 400 °C in a reducing atmosphere (5% H₂, 95% Ar) results in substantial product formation at 200 minutes (Figure 3, black data points, emphasized by arrow).  Annealing graphene at 400 °C generates compressive strain in graphene on SiO₂, which we have confirmed by analyzing the G peak and 2D peak frequencies from Raman maps of the annealed samples.  The reaction rate increases because converting the graphene carbon atoms from sp² to sp³ hybridization releases this strain.

We have also added a 405 nm laser to our micro-Raman spectrometer, which we focus directly on the graphene.  As a result, the photon flux at the sample is several orders of magnitude higher, yet the reaction rate does not accelerate by a comparable amount, suggesting that a lack of Cl radicals does not explain our slow reaction and that sample preparation is more influential.

This year we have also confirmed that the graphene photochlorination rate increases by over a factor of two in water.  Since Cl₂ is non-polar and water is highly polar, we expect an excess concentration of Cl₂ on the graphene surface relative to the gas phase, enhancing the concentration of reactive radicals on the surface.  Thus, understanding surface adsorption of Cl₂ on graphene is important for understanding its reactivity.  We have measured the equilibrium constant for surfaced adsorption of nonreactive I₂ on 1L graphene in water.  We use I₂ instead of Cl₂ because we can more easily generate precise concentrations of I₂ in water.  I₂ hole dopes graphene, so we used the G-peak frequency shift to quantitatively determine the hole density as a function of I₂ concentration, and fit the resulting data to a modified Langmuir isotherm to obtain K = 1.7 ± 0.6 M^-1, showing very strong surface adsorption (Figure 4).

During the past year, this grant has supported the efforts of three undergraduate research students.  Katherine Rinaldi (BC ’15) continued her experiments investigating gas-phase photochlorination of graphene and helped train new research students to make samples, measure spectra, and analyze the data.  Katherine began graduate school in physical chemistry in the fall of 2015 at the California Institute of Technology.  Building on previous photochlorination results in water, Rachel Dziatko (BC ’18) measured the surface adsorption of I₂ on graphene in water.  Maria Paley (BC ’17) has expanded our experiments to the semiconducting two-dimensional material MoS₂, creating samples and determining the number of MoS₂ layers. This grant provided travel support for Katherine and Nilam Patel (BC ’16) to present posters on their photochlorination work at the American Chemical Society Spring 2015 National Meeting in Denver.  I anticipate Rachel and Maria presenting their work at the ACS Spring 2016 National Meeting in San Diego. 

The Petroleum Research Fund has been crucial in advancing our graphene photochlorination study.  In the past year, undergraduate research students working in my group have made significant progress studying the role of sample annealing, compressive strain, and surface adsorption on graphene reactivity, and we are currently working complete these measurements and expand these experiments to MoS₂.