Platinum-based catalysts have been considered the most effective electrocatalysts for the hydrogen evolution reaction in water splitting. However, platinum utilization in these electrocatalysts is extremely low, as the active sites are only located on the surface of the catalyst particles. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their efficiency by utilizing nearly all platinum atoms. Here we report on a practical synthesis method to produce isolated single platinum atoms and clusters using the atomic layer deposition technique. The single platinum atom catalysts are investigated for the hydrogen evolution reaction, where they exhibit significantly enhanced catalytic activity (up to 37 times) and high stability in comparison with the state-of-the-art commercial platinum/carbon catalysts. The X-ray absorption fine structure and density functional theory analyses indicate that the partially unoccupied density of states of the platinum atoms' 5d orbitals on the nitrogen-doped graphene are responsible for the excellent performance. S ecuring renewable and reliable sources of clean energy is one of the world's foremost challenges. Addressing this challenge is not only critical for the global economy but will also aid in the mitigation of environmental and health hazards caused by fossil fuels 1 . Hydrogen is the cleanest fuel available and is believed to be one of the most promising energy sources of the twenty-first century 2,3 . However, the majority of the hydrogen produced today is derived from steam-reformed methane, which is sourced from fossil reserves and produces a substantial amount of CO 2 (ref. 4). The production of hydrogen from water electrolysis is a promising alternative to the current CO 2 -emitting fossil fuel-based energy systems 5,6 .Platinum (Pt)-based catalysts are generally considered to be the most effective electrocatalysts for the hydrogen evolution reaction (HER) 5,7 . Unfortunately, Pt is expensive and scarce, limiting the commercial potential for such catalysts. The development of active, stable and inexpensive electrocatalysts for water splitting is a key step in the realization of a hydrogen economy, which is based on the use of molecular hydrogen for energy storage.Significant effort has been devoted to the search of non-preciousmetal-based HER catalysts, including sulfide-based materials 8-11 , and C 3 N 4 (refs 12-14). Although these candidate materials show promising activities for the HER, the activities of these catalysts in their present form are insufficient for industrial applications 15 .To overcome the challenges associated with the Pt HER catalysts and to drive the cost of H 2 production from water electrolysis down, it is very important to markedly decrease the Pt loading and increase the Pt utilization efficiency. Currently, supported Pt nanoparticles (NPs) are typically used to promote Pt activity towards the HER. Unfortunately, the geometry of the NPs limit the majority of the Pt atoms to the particle core, deeming them ineffect...

A platform for producing stabilized Pt atoms and clusters through the combination of an N-doped graphene support and atomic layer deposition (ALD) for the Pt catalysts was investigated using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was determined, using imaging and spectroscopy techniques, that a wide range of N-dopant types entered the graphene lattice through covalent bonds without largely damaging its structure. Additionally and most notably, Pt atoms and atomic clusters formed in the absence of nanoparticles. This work provides a new strategy for experimentally producing stable atomic and subnanometer cluster catalysts, which can greatly assist the proton exchange membrane fuel cell (PEMFC) development by producing the ultimate surface area to volume ratio catalyst.

In this study, we demonstrate that bipolar electrochemistry is a viable strategy for “wireless” electrochemical intercalation of graphite flakes and further large-scale production of high-quality graphene suspensions. Expansion of the graphite layers leads to a dramatic 20-fold increase in the yield of high-shear exfoliation. Large graphite flakes, which do not produce graphene upon high shear if left untreated, are exfoliated in a yield of 16.0 ± 0.2%. Successful graphene production was confirmed by Raman spectroscopy and scanning transmission electron microscopy, showing that the graphene flakes are 0.4–1.5 μm in size with the majority of flakes consisting of 4–6 graphene layers. Moreover, a low intensity of the D peak relative to the G peak as expressed by the I D / I G ratio in Raman spectroscopy along with high-resolution transmission electron microscopy images reveals that the graphene sheets are essentially undamaged by the electrochemical intercalation. Some impurities reside on the graphene after the electrochemical treatment, presumably because of oxidative polymerization of the solvent, as suggested by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. In general, the bipolar electrochemical exfoliation method provides a pathway for intercalation on a wider range of graphite substrates and enhances the efficiency of the exfoliation. This method could potentially be combined with simultaneous electrochemical functionalization to provide graphene specifically designed for a given composite on a larger scale.

Electron microscopy has always played an important role in the development and the understanding of new materials. In the last ten years there have been significant advancements in instrumentation, enabling improved studies of materials at the nanoscale. In the area of catalysts and energy storage materials, detailed microscopy of material structure, composition and bonding at the nanometer length scale are needed to optimize material properties and performance. Here we summarize recent examples of work related to the study of nano-alloy catalysts used in proton exchange membrane fuel cells and commercial Li ion battery materials, illustrating the crucial role of imaging and spectroscopy for the characterization of these materials.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Electron microscopy has always played an important role in the development of new materials and for understanding properties of complex functional materials. The recent developments in instrumentation have significantly improved the insight that such techniques can provide, particularly for nanoscale materials and for fundamental studies related to bonding and electronic structure. In the area of functional materials, namely energy storage and conversion materials, plasmonic structures, and quantum materials, detailed microscopy is needed to optimize material properties and to understand their electronic properties. Here we highlight recent examples of work related to the study of functional materials, illustrating the crucial role of imaging and spectroscopy for the characterization and understanding of these materials. Using an aberration‐corrected TEM equipped with electron energy loss spectroscopy (EELS), we have studied the mechanism of cluster formation following atomic layer deposition on graphene nanosheets. We have also shown, with electron energy loss near‐edge structures (ELNES), that it is possible to detect the presence of N dopant atoms at different atomic sites [1]. With high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) and EELS, we have studied the evolution of alloy catalysts following in‐situ and ex‐situ annealing procedures. Starting with a disordered PtFe nanoparticle, we captured the ordering transformation, showing evidence of the formation of ordered Pt and Fe rich planes, and evidence of both Pt and Fe‐rich shells over an ordered core (Figure 1) [2]. We also showed that the Pt surface segregation induces local strain and atomic displacements [2] (Figure 2) that can be further correlated to the enhanced activity of the material [3,4]. Using in‐situ heating, it has also been possible to study the alloying phenomena of AuPt nanoparticles showing evidence of full miscibility starting at 200ºC (Figure 3), well below the thermodynamically expected temperature. At high‐temperature, we have also detected the formation of unexpected ordered structures (Figure 4). Furthermore, we found that the annealing leads to mostly phase separation and monolayer surface segregation [5]. In a related catalyst system, we have been able to study the evolution of catalysts and hybrid supports, visualizing the presence of single atom dissolution of catalysts [6]. Similar approaches have been used to study the structure of LiNi x Mn y Co 1‐x‐y O 2 (known as “NMC”) and (Li rich) NMC compounds. In this work, using a combination of HAADF‐STEM and EELS, together with multiple‐linear least squares fitting, we have demonstrated the mechanisms of charge compensation, following electrochemical cycling and the presence of monolayer‐like surface changes in the valence of transition metal ions. STEM imaging and ELNES demonstrate the presence of local heterogeneities in the Li and transition metal distribution and in the local carriers distribution. The same techniques are used to probe the localization of charges in a variety of high‐temperature superconductors [7,8]. Finally, examples of plasmonic imaging of hybridization phenomena in metallic nanostructures, together with rigorous simulations of the optical response, will be shown [9]. These examples highlight the power and versatility of analytical techniques in the TEM to solve important materials science and fundamental physics problems.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Electron energy loss spectroscopy (EELS) has dramatically evolved since the development of electron monochromators, faster detectors within improved spectrometers, all this within more stable aberrationcorrected transmission electron microscopes. These developments allow the acquisition of spectra with atomic resolution and spectroscopic quality that is sufficient to probe the changes in chemical bonding at near-atomic level so that "real materials" questions can be studied. Through energy loss near-edge structure analysis, EELS provide useful chemical state information so that changes in materials, for examples at interface, under extreme high-temperature conditions, or at defects can be probed. This contribution aims to review some of the applications of this technique by demonstrating examples of this work related to the study of interfaces in complex oxides, the quantification of spectra obtained at high-spatial resolution, the detection of spectroscopic changes in nanomaterials subjected to intense light irradiation and the plasmonic response of nanostructures.Experiments were carried out on two aberration-corrected FEI Titan microscopes (a double-corrected Titan 80-300 Cubed, and an "image-corrected" Titan 80-300), both equipped with monochromators and EELS spectrometers (Quantum 966 and Tridiem 865 respectively), achieving down to 70 meV energy resolution. The capability of mapping at the atomic scale is not simply revealing local changes in composition but it allows us to identify the termination of the surface of substrates, and the chemical species that are in direct contact with the substrate (Figure 1). For the particular case of La 2/3 Ca 1/3 MnO 3 (LCMO) grown on YBa 2 Cu 3 O 7-d (YBCO) [1], our work has shown that the last atomic plane in YBCO is Ba, while the first atomic plane in LCMO is Mn (Figure 1). Similarly, it is also possible to use EELS mapping to provide unambiguous information on the site preference of transition metals in oxides whereby, in a Ca 2 FeMnO 5 , Mn is found on octahedral sites and Fe on tetrahedral sides (Figure 2). In disordered systems, this approach has allowed us to directly map the distribution of implanted Pr atoms in SrTiO 3 and has demonstrated the expected statistical distribution (Figure 3) of atoms that cannot be resolved based on purely high-angle annular dark-field (HAADF)-contrast imaging [2]. For surfaces, we have been able to determine the valence state of surface atoms in reconstructed SrTiO 3 [3] A good demonstration of the very high energy resolution application in energy loss near-edge structures is shown in our studies of the C K edge in single-wall and multiwall carbon nanotubes [4]. With the improved energy resolution, we have demonstrated that, simultaneous intense light irradiation (in-situ) and acquisition of energy-loss spectra, there is a significant change in the excitonic peak portion of the σ* fine structure [4]. This high-sensitivity demonstrates that the local heating and the charge carriers generated by infra-red photons significantly m...