Reaction mechanism and kinetics for CO₂ reduction on nickel single atom catalysts from quantum mechanics

Experiments have shown that graphene-supported Ni-single atom catalysts (Ni-SACs) provide a promising strategy for the electrochemical reduction of CO₂ to CO, but the nature of the Ni sites (Ni-N₂C₂, Ni-N₃C₁, Ni-N₄) in Ni-SACs has not been determined experimentally. Here, we apply the recently developed grand canonical potential kinetics (GCP-K) formulation of quantum mechanics to predict the kinetics as a function of applied potential (U) to determine faradic efficiency, turn over frequency, and Tafel slope for CO and H₂ production for all three sites. We predict an onset potential (at 10 mA cm⁻²) U_(onset) = −0.84 V (vs. RHE) for Ni-N₂C₂ site and U_(onset) = −0.92 V for Ni-N₃C₁ site in agreement with experiments, and U_(onset) = −1.03 V for Ni-N₄. We predict that the highest current is for Ni-N₄, leading to 700 mA cm⁻² at U = −1.12 V. To help determine the actual sites in the experiments, we predict the XPS binding energy shift and CO vibrational frequency for each site

High On/Off Ratio Graphene Nanoconstriction Field Effect Transistor

We report a method to pattern monolayer graphene nanoconstriction field effect transistors (NCFETs) with critical dimensions below 10 nm. NCFET fabrication is enabled by the use of feedback controlled electromigration (FCE) to form a constriction in a gold etch mask that is first patterned using conventional lithographic techniques. The use of FCE allows the etch mask to be patterned on size scales below the limit of conventional nanolithography. We observe the opening of a confinement-induced energy gap as the NCFET width is reduced, as evidenced by a sharp increase in the NCFET on/off ratio. The on/off ratios we obtain with this procedure can be larger than 1000 at room temperature for the narrowest devices; this is the first report of such large room temperature on/off ratios for patterned graphene FETs.Comment: 18 pages, 6 figures, to appear in Smal

Photoluminescence and Band Gap Modulation in Graphene Oxide

We report broadband visible photoluminescence from solid graphene oxide, and modifications of the emission spectrum by progressive chemical reduction. The data suggest a gapping of the two-dimensional electronic system by removal of π-electrons. We discuss possible gapping mechanisms, and propose that a Kekule pattern of bond distortions may account for the observed behavior

Large-area epitaxial growth of curvature-stabilized ABC trilayer graphene.

The properties of van der Waals (vdW) materials often vary dramatically with the atomic stacking order between layers, but this order can be difficult to control. Trilayer graphene (TLG) stacks in either a semimetallic ABA or a semiconducting ABC configuration with a gate-tunable band gap, but the latter has only been produced by exfoliation. Here we present a chemical vapor deposition approach to TLG growth that yields greatly enhanced fraction and size of ABC domains. The key insight is that substrate curvature can stabilize ABC domains. Controllable ABC yields ~59% were achieved by tailoring substrate curvature levels. ABC fractions remained high after transfer to device substrates, as confirmed by transport measurements revealing the expected tunable ABC band gap. Substrate topography engineering provides a path to large-scale synthesis of epitaxial ABC-TLG and other vdW materials

Regulating Top-Surface Multilayer/Single-Crystal Graphene Growth by “Gettering” Carbon Diffusion at Backside of the Copper Foil

A unique strategy is reported to constrain the nucleation centers for multilayer graphene (MLG) and, later, single-crystal graphene domains by gettering carbon source on backside of the flat Cu foil, during chemical vapor deposition. Hitherto, for a flat Cu foil, the top-surface-based growth mechanism is emphasized, while overlooking the graphene on the backside. However, the systematic experimental findings indicate a strong correlation between the backside graphene and the nucleation centers on the top-surface, governed by the carbon diffusion through the bulk Cu. This understanding steers to devise a strategy to mitigate the carbon diffusion to the top-surface by using a carbon “getter” substrate, such as nickel, on the backside of the Cu foil. Depth profiling of the nickel substrate, along with the density functional theory calculations, verifies the gettering role of the nickel support. The implementation of the backside carbon gettering approach on single-crystal graphene growth results in lowering the nucleation density by two orders of magnitude. This enables the single-crystal domains to grow by 6 mm laterally on the untreated Cu foil. Finally, the growth of large-area polycrystalline single layer graphene, free of unwanted MLG domains, with significantly improved field-effect mobility of ≈6800 cm^2 V^(−1) s^(−1) is demonstrated

Optoelectronic properties of single-walled carbon nanotubes and their aggregates

Precise determination of (n,m) abundance and their optical spectroscopy characteristics of single wall carbon nanotubes (SWNTs) is of pivotal importance for their future involvement in high-end electronic and optoelectronic devices as well as in biosensory applications. ^ We first developed a methodology to correlate photoluminescence (PL) (n,m)-intensities with (n,m)-SWNT abundance using two dimensional fluorescence spectroscopy of non-covalently functionalized SWNTs sample. Using the extremely sharp diameter-distributed Co-MCM-41 SWNT sample, in conjunction with spectral reconstruction of its near infrared ES11 absorption spectrum, we have been able to independently assess the accuracy of two current theories. Moreover, the reconstruction of the ES11 absorption spectrum provides additional insights of (n,m)-absorption linewidths and the existence of zigzag (n,0) nanotubes. ^ We also established a methodology to reconstruct the radial breathing modes (RBM) of SWNTs. Using the difference between experimental and E ii(n,m) values of isolated SWNTs. A quantitative assessment was obtained that can be attributed to bundling and possibly excitonic effects. Furthermore, the relative electron-phonon interaction matrix elements (Mph) for 28 (n,m) semiconducting SWNTs species were extracted from the resonance Raman cross sections of individually dispersed HiPco SWNTs. The observed Mph pattern was fitted according to nanotube family (i.e. (2n +m)=constant) and modality ( i.e. mod(n-m,3)=1 or 2) using an empirical equation based on trigonal warping effects. Moreover, the corresponding absolute values of the electron-phonon interaction matrix element were also obtained in both individually dispersed and aggregated states, by utilizing the correlation between RBM overtone and RBM fundamental intensity ratio as a function of laser energy. These values were further used to obtain the Huang-Rhys factor and absolute Mph values for these nanotube species. ^ Recently, we observed a new class of intermediate frequency modes (IFMs) associated with the ES22 and ES11 optical transitions of bundled HiPco SWNTs. Step-like dispersive behavior was observed for these IFMs, along with associated clusters of RBM overtones at higher frequencies. The observed IFM maxima were found to obey a resonance behavior based on a combination of the ES22 and E S11 transition energies, scaled by the inverse diameter of the respective nanotube. Moreover, we found the RBM overtone and IFM Raman intensities are very sensitive to the aggregation state. These observations significantly extended the capability of resonant Raman spectroscopy.

Optoelectronic properties of single-walled carbon nanotubes and their aggregates

Precise determination of (n,m) abundance and their optical spectroscopy characteristics of single wall carbon nanotubes (SWNTs) is of pivotal importance for their future involvement in high-end electronic and optoelectronic devices as well as in biosensory applications. ^ We first developed a methodology to correlate photoluminescence (PL) (n,m)-intensities with (n,m)-SWNT abundance using two dimensional fluorescence spectroscopy of non-covalently functionalized SWNTs sample. Using the extremely sharp diameter-distributed Co-MCM-41 SWNT sample, in conjunction with spectral reconstruction of its near infrared ES11 absorption spectrum, we have been able to independently assess the accuracy of two current theories. Moreover, the reconstruction of the ES11 absorption spectrum provides additional insights of (n,m)-absorption linewidths and the existence of zigzag (n,0) nanotubes. ^ We also established a methodology to reconstruct the radial breathing modes (RBM) of SWNTs. Using the difference between experimental and E ii(n,m) values of isolated SWNTs. A quantitative assessment was obtained that can be attributed to bundling and possibly excitonic effects. Furthermore, the relative electron-phonon interaction matrix elements (Mph) for 28 (n,m) semiconducting SWNTs species were extracted from the resonance Raman cross sections of individually dispersed HiPco SWNTs. The observed Mph pattern was fitted according to nanotube family (i.e. (2n +m)=constant) and modality ( i.e. mod(n-m,3)=1 or 2) using an empirical equation based on trigonal warping effects. Moreover, the corresponding absolute values of the electron-phonon interaction matrix element were also obtained in both individually dispersed and aggregated states, by utilizing the correlation between RBM overtone and RBM fundamental intensity ratio as a function of laser energy. These values were further used to obtain the Huang-Rhys factor and absolute Mph values for these nanotube species. ^ Recently, we observed a new class of intermediate frequency modes (IFMs) associated with the ES22 and ES11 optical transitions of bundled HiPco SWNTs. Step-like dispersive behavior was observed for these IFMs, along with associated clusters of RBM overtones at higher frequencies. The observed IFM maxima were found to obey a resonance behavior based on a combination of the ES22 and E S11 transition energies, scaled by the inverse diameter of the respective nanotube. Moreover, we found the RBM overtone and IFM Raman intensities are very sensitive to the aggregation state. These observations significantly extended the capability of resonant Raman spectroscopy.

Turning off Hydrogen To Realize Seeded Growth of Subcentimeter Single-Crystal Graphene Grains on Copper

Subcentimeter single-crystalline graphene grains, with diameter up to 5.9 mm, have been successfully synthesized by tuning the nucleation density during atmospheric pressure chemical vapor deposition. Morphology studies show the existence of a single large nanoparticle (>similar to 20 nm in diameter) at the geometric center of those graphene grains. Similar size particles were produced by slightly oxidizing the copper surface to obtain oxide nanoparticles in Ar-only environments, followed by reduction into large copper nanoparticles under H-2/Ar environment, and are thus explained to be the main constituent nuclei for graphene growth. On this basis, we were able to control the nanoparticle density by adjusting the degree of oxidation and hydrogen annealing duration, thereby controlling nucleation density and consequently controlling graphene grain sizes. In addition, we found that hydrogen plays dual roles on copper morphology during the whole growth process, that is, removing surface irregularities and, at the same time, etching the copper surface to produce small nanoparticles that have only limited effect on nucleation for graphene growth. Our reported approach provides a highly efficient method for production of graphene film with long-range electronic connectivity and structure coherence

Turning off Hydrogen To Realize Seeded Growth of Subcentimeter Single-Crystal Graphene Grains on Copper

Subcentimeter single-crystalline graphene grains, with diameter up to 5.9 mm, have been successfully synthesized by tuning the nucleation density during atmospheric pressure chemical vapor deposition. Morphology studies show the existence of a single large nanoparticle (>∼20 nm in diameter) at the geometric center of those graphene grains. Similar size particles were produced by slightly oxidizing the copper surface to obtain oxide nanoparticles in Ar-only environments, followed by reduction into large copper nanoparticles under H₂/Ar environment, and are thus explained to be the main constituent nuclei for graphene growth. On this basis, we were able to control the nanoparticle density by adjusting the degree of oxidation and hydrogen annealing duration, thereby controlling nucleation density and consequently controlling graphene grain sizes. In addition, we found that hydrogen plays dual roles on copper morphology during the whole growth process, that is, removing surface irregularities and, at the same time, etching the copper surface to produce small nanoparticles that have only limited effect on nucleation for graphene growth. Our reported approach provides a highly efficient method for production of graphene film with long-range electronic connectivity and structure coherence