High resolution
STEM
and
EELS
investigation of
N
‐doped carbon allotropes decorated with noble metal atom catalysts
Authors: Stambula Samantha , Bugnet Matthieu , Cheng Niancai , Lushington Andrew , Sun Xueliang , Botton Gianluigi 
Published: December 2016

Graphene's unique properties make this material an ideal catalyst support for use in the proton exchange membrane fuel cell (PEMFC). Graphene offers increased electrical conductivity and a larger surface area for catalyst deposition compared to other catalyst support material, such as carbon black.[1,2]. Utilizing graphene as the electrode support also results in increased chemical stability due to the sp
2
bonding; however, this precludes the availability of dangling bonds for chemisorption, thus leading to a poor Pt distribution and the formation of large nanoparticles (NPs). Functionalization can be used to introduce nucleation sites into the graphene lattice, where N‐dopants have been shown to increase the Pt‐C binding energy.[3] The atomic layer deposition (ALD) technique creates ultra‐small NPs (<1 nm) which, when combined with the enhanced catalyst binding energy from the N‐doped graphene support can produce stable Pt clusters/atoms, resulting in increased Pt utilization while subsequently reducing the cost.[4]

To fully understand and design a more efficient PEMFC the material must be characterized at the atomic level. This can be accomplished by the use of aberration‐corrected transmission electron microscopy (TEM). High resolution TEM (HRTEM) can be utilized to observe the structure of the graphene lattice, while high‐angle annular dark‐field (HAADF) scanning transmission electron microscopy (STEM) can be used to examine the Pt clusters' size and distribution. Furthermore, electron energy loss spectroscopy (EELS) can be utilized to examine local chemical composition and bonding of the probed atoms from nanometer scaled areas in order to reveal the coordination of the N‐dopant species in the graphene lattice.[4] Here we used a FEI Titan 80‐300 Cubed TEM equipped with aberration correctors of the probe and imaging forming lens, and a monochromator for optimal imaging at a low accelerating voltage.

Low‐energy condition HRTEM (figure 1) and STEM imaging were used to investigate thermally‐exfoliated graphene (figure 2). From these observations, we deduced that the graphene lattice maintained its short range order; however, the long range order was lost due to the presence of steps, folds, defects, and incomplete exfoliation.[4] The mass‐thickness dependence in HAADF imaging confirmed the presence of numerous steps and ledges in the N‐doped graphene nanosheets. More importantly the Z‐contrast in the HAADF images revealed that Pt was present as stable single atoms and clusters on the N‐doped graphene and that Pt NPs were not detected.[4] Lastly, using EELS and the N‐K edge fine structures, we probed the distribution of N‐dopants within the nanosheets (figure 3). Significant variations in the local concentration of N‐dopants were observed among the graphene sheets, in which an inhomogeneous distribution was discovered. Such effects have not been observed in previous broad beam techniques.[4] To futher investigate the effect of N‐doping, mutliwalled N‐doped CNTs (N‐CNTs) were produced and ALD was utilized to deposit Pd. Initial investigations showed the stabilization of single Pd atoms with N‐doping; however, small NPs were also formed (figure 4). It was demonstrated with EELS that many of the N‐CNTs were filled with N
2
gas. Using controlled evacuation of the multiwalled CNTs induced by the electron beam, we were able to reveal the intrinsic N‐type doping within the CNT walls.