Work Function Lowering of Graphite by Sequential Surface Modifications: Nitrogen and Hydrogen Plasma Treatment

Graphite-related materials play an important role in various kinds of devices and catalysts. Controlling the properties of such materials is of great significance to widen the potential applications and improve the performance of such applications as field emission devices and catalyst for fuel cells. In particular, the work function strongly affects the performance, and thus development of methods to tune the work function widely is urgently required. Here, we achieved wide-range control of the work function of graphite by nitrogen and hydrogen plasma treatments. The time of hydrogen plasma treatment and the amount of nitrogen atoms doped beforehand could control the work function of graphite from 2.9 to 5.0 eV. The formation of a surface dipole layer and the nitrogen-derived electron donation contributed to such lowering of the work function, which is advantageous for applications in various fields.

Ar plasma for 10 min (red), HOPG treated by N plasma for 10 min (green), HOPG treated by H plasma for 10 min (blue), HOPG treated by N plasma for 30 min followed by H plasma treatment for 10 min (light blue). All spectra have been normalized with respect to the intensity of G peak (1581 cm -1 ).
To investigate difference of defect introduction by plasma species, Raman spectra of the plasma treated HOPG were compared. An increase of the D peak (1353 cm -1 ) is observed after all kinds of plasma treatment, which means the plasma treatment introduces defects on HOPG surface. These defects are derived from vacancies, hetero doping, and sp 3 bonding carbon atoms. The ID/IG of hydrogenated HOPG (blue) was increased to around 0.7 just by irradiating H plasma for 10 min, which is much larger than S4 that of N and Ar plasma (Figure S 1). Previous reports said defect of sp 3 bond shows larger ID/IG than that of vacancy and N doping 1,2 . Therefore, the large increase in ID/IG by H plasma should indicates the formation of sp 3 structure of hydrogenation. On the other hand, the N + H plasma treated HOPG showed smaller D peak than H plasma HOPG. That is because the many surface carbon atoms were replaced with nitrogen atoms due to its highly doping amount, and the amount of sp 3 carbon decreased.   This thermal treatment removes pyrrolic N that contributes p electron into the π system 5 . Schwartz has reported that heating pyridine in a hydrogen-containing atmosphere forms piperidine, in which C -N bonds are broken and rings are opened, and then finally decompose into the molecules of NH3 7 . Exposing to an active hydrogen plasma promotes chemical reactions like such heating under a hydrogen atmosphere. In our N doped HOPG samples, therefore, it can be thought that H plasma eventually desorbs the doped nitrogen as NH3, and decomposes pyridinic N in the similar process in the study of piperidine.
In addition, we should mention that core level depth (binding energy) might be shifted with Fermi level shift, because the binding energy is calculated from Fermi level.
However, in our analysis of N 1s peak fitting, the apparent change of the N 1s binding energy was not observed. It is because the work function is changed not only by Fermi level shift but also by vacuum level shift 8 . While the Fermi level shift changes the binding energy, the vacuum level shift by the surface dipole layer does not change that. Therefore, we consider that a large portion of the work function change was caused by the decrease of vacuum level.

Figure S 5 Raman spectra of hydrogenated HOPG samples, before (black) and after (red)
thermal dehydrogenation at 600°C. These spectra were normalized by G peak intensity.
We investigated the surface state of HOPG after thermal dehydrogenation. Before the annealing, hydrogenated HOPG showed ID/IG ~ 0.79 (black). After dehydrogenation (red), defects of about ID/IG ~ 0.14 are still left. Elias et al. reported that hydrogen atoms are eliminated by heating above 350 °C 9 . In addition, the reduced work function also returned to its original state. These facts indicate the residual D peak does not originate from hydrogenated-sp 3 bonds, but originated from vacancies. It is reported that carbon atoms are removed together with hydrogen desorption in the form of CH4 when graphene is etched by hydrogen plasma 10,11 . Likewise, carbon desorption seems to be occurred S10 even in the dehydrogenation process, and finally leave vacancies. It can be explained a reason why the D peak remains in graphene even after desorption of hydrogen atoms. the work function is more effectively reduced when the doping amount is larger.

S13
In addition, we would like to discuss comparison of other nitrogen-contained carbon materials. In our study, the graphite surface disordered by long time plasma irradiation should be similar to amorphous carbon nitride (a-CN). Photoelectron spectroscopy cannot measure a-CN due to its insulation, while its work function could be estimated as 5.2 eV using Kelvin Probe Force Microscopy (KPFM) 13 . We had obtained consistent results in heavily doped graphite, which showed the work function of 5.2 eV when doping amount was 32%. In addition, there is a report about hydrogenated a-CN surface. Although its work function was not directly measured by KPFM, Saitoh et al. reported that the work function of field emission characteristics was greatly reduced by hydrogenation 14 .