Work Function Studies of Carbon Nanospikes by XPS and UPS

Carbon nanospikes show excellent energy and selectivity for electrochemical conversion of CO₂ to ethanol and N₂ to NH₃. Reactivity is attributed to an enhanced electric field from the spike topography. We have identified the composition of nanospike surfaces with x-ray photoelectron spectroscopy (XPS) and their absolute work function using ultraviolet photoemission spectroscopy (UPS). Annealing as-grown samples above 275 °C produces a clean surface which has a 4.13 eV work function, a half volt lower than that of flat graphite. Contamination of the spiked surface by exposure to air, oxidation at elevated temperature, or immersion in water or hydrocarbons, increases the work function. Blunting the spikes, by exposure to an oxygen plasma, argon sputtering, or annealing to 800 °C results in a work function close to that of flat graphite. An unusual double onset in the UPS secondary electron intensity is observed on as-grown nanospike samples and is reproduced by absorbing hydrocarbons on clean nanospikes. This double onset appears to be unique to carbon substrates and may originate in inelastic scattering of photoelectrons.