L. Han, Y. Sun, S. Li, C. Cheng, C. E. Halbig, P. Feicht, J. L. Hübner, P. Strasser,* S. Eigler*
In-plane Carbon Lattice-defect Regulating Electrochemical Oxygen Reduction to Hydrogen Peroxide Production over Nitrogen-doped Graphene
Carbon-based materials are considered to be active for electrochemical oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2) production. Nevertheless, less attention is paid to the investigation of the influence of in-plane carbon lattice defect on the catalytic activity and selectivity toward ORR. In the present work, graphene precursors were first prepared from oxo-functionalized graphene (oxo-G) and graphene oxide (GO) with H2O2 hydrothermal treatment, respectively. Statistical Raman spectroscopy (SRS) analysis demonstrated the increased in-plane carbon lattice defect density in the order of oxo-G, oxo-G/H2O2, GO, GO/H2O2. Furthermore, nitrogen-doped graphene materials were prepared through ammonium hydroxide hydrothermal treatment of those graphene precursors. Rotating ring-disk electrode (RRDE) results indicate that the nitrogen-doped graphene derived from oxo-G with lowest in-plane carbon lattice defects exhibited the highest H2O2 selectivity of >82% in 0.1 M KOH. Moreover, high H2O2 production rate of 224.8 mmol gcatalyst-1 h-1 could be achieved at 0.2 VRHE in H-cell with faradaic efficiency of >43.6%. Our work provides new insights for the design and synthesis of carbon-based electrocatalysts for H2O2 production.
C. E. Halbig, R. Lasch, J. Krüll, A. Pirzer, Z. Wang, J. N. Kirchhof, K. I. Bolotin, M. R. Heinrich* S. Eigler*
Selective functionalization of graphene at defect activated sites by arylazocarboxylic tert‐butylesters
The development of versatile functionalization concepts for graphene are currently in the focus of research. With oxo‐functionalization of graphite the full surface of graphene becomes accessible for C‐C bond formation to introduce out‐of‐plane functionality. Here, we present the arylation of graphene by arylazocarboxylic tert‐butylesters, which generates aryl radicals after activation by acids. Surprisingly, the degree of functionalization is related to the concentration of lattice vacancy defects of graphene. Consequently, graphene, which is free from lattice defects is not reactive. The reaction can be applied to graphene dispersed in solvents and leads to bitopic functionalization, as well as monotopic functionalization if graphene is deposited on surfaces. Since the arylazocarboxylic tert‐butylester moiety can be attached to various molecules, the presented method paves the way to functional graphene derivatives with the density of defects determining the degree of functionalization.
K. W. Silverstein, C. E. Halbig, J. Mehta, A. Sharma, S. Eigler and J. M. Mativetsky*
Voltage-Reduced Low-Defect Graphene Oxide: A High Conductivity, Near-Zero Temperature Coefficient of Resistance Material
A highly conductive graphene derivative was produced by using a low-defect form of graphene oxide, oxo-G, in conjunction with voltage-reduction, a simple and environmentally-benign procedure for removing oxygen-containing functional groups. A low temperature coefficient of resistance was achieved, making this material promising for temperature-stable electronics and sensors.
Mit Schwefelsäure von Graphit zu Graphen
Bislang war unklar, wie Graphit Schwefelsäuremoleküle einschließt und zu Graphitsulfat reagiert. Dies haben Chemiker nun mit Ab-initio Moleküldynamiksimulationen geklärt. Soll aus Graphitsulfat wieder Graphen entstehen, entscheiden Korngröße und Schichtfolge, ob und wie gut dies funktioniert.
A. Khannanov, A. Kiiamov, A. Valimukhametova, D. A. Tayurskii, F. Börrnert, U. Kaiser, S. Eigler,* F. G. Vagizov, A. M. Dimiev*
Gamma-iron phase stabilized at room temperature by thermally processed graphene oxide
Stabilizing nanoparticles on surfaces, such as graphene is a growing field of research. Thereby, iron particle stabilization on carbon materials is very attractive and finds applications in charge-storage devices, catalysis, and other applications. In this work, we describe the discovery of iron nanoparticles with the face-centered cubic structure that was believed not to exist at ambient conditions. In bulk, the γ-iron phase is formed only above 917 °C, and transforms back to the thermodynamically favored α-phase upon cooling. Here, with X-Ray diffraction and Mössbauer spectroscopy we unambiguously demonstrate the unexpected room-temperature stability of the γ-phase of iron in the form of the austenitic nanoparticles with low carbon content from 0.60% through 0.93%. The nanoparticles have controllable diameter range from 30 nm through 200 nm. They are stabilized by a layer of Fe/C solid solution on the surface, serving as the buffer controlling carbon content in the core, and by a few-layer graphene as an outermost shell.
C. E. Halbig, O. Martin, F. Hauke, S. Eigler,* A. Hirsch*
Oxo-Functionalized Graphene – A Versatile Precursor for Alkylated Graphene Sheets by Reductive Functionalization
Controlled covalent functionalization of graphene remains a challenging task due to the heterogeneous nature of materials. Functionalization approaches of graphene either lack in quantifying the degree of functionalization or they do not discriminate between covalent and non-covalent functionalization. Here, graphite is oxidized and exfoliated in a three-step procedure and subsequently reduced and functionalized by hexylation. While Raman spectroscopy is powerful to determine the degree of in-plane lattice defects (θLD) and functionalization (θFD), the method fails detecting introduced hexyl groups at a concentration of about 0.03%, next to the pre-existing in-plane lattice defects of 0.7%. However, sensitive thermogravimetric analysis coupled with gas chromatography and mass spectrometry (TGA-GC/MS) can prove the hexylation reaction. The efficiency of functionalization is comparable to reductive functionalization of pristine CVD-graphene and bulk graphite.
V. Cantatore, S. Pandit, V. R. S. S. Mokkapati, S. Schindler, S. Eigler, I. Mijakovic, I. Panas*
Design strategy of a graphene based bio-sensor for glucose
|A novel graphene-based glucose sensor-design is formulated and explored in silico. An ad hoc host molecule is tailored to bind to glucose by multiple hydrogen bonds. A pyridinic core is chosen for this receptor in order to allow for “socket-plug” dative bonding to boron sites of boron doped graphene. The modeling employs DFT (Density Functional Theory) together with an effective aqueous environment to take into account the solvation effect. High selectivity is demonstrated for the suggested host molecule towards glucose as compared to other possible competitors in blood such as fructose, biotin and ascorbic acid. A route to achieve improved sensitivity, exploiting the hydrophilic/hydrophobic properties of the host + glucose system for enhanced selective binding to the hydrophobic boron doped graphene support is discussed.|
S. Seiler, C. E. Halbig, F. Grote, P. Rietsch, F. Börrnert, U. Kaiser, B. Meyer,* and S. Eigler*
Effect of friction on oxidative graphite intercalation and high-quality graphene formation
|Oxidative wet-chemical delamination of graphene from graphite is expected to become a scalable production method. However, the formation process of the intermediate stage-1 graphite sulfate by sulfuric acid intercalation and its subsequent oxidation are poorly understood and lattice defect formation must be avoided. Here, we demonstrate film formation of micrometer-sized graphene flakes with lattice defects down to 0.02% and visualize the carbon lattice by transmission electron microscopy at atomic resolution. Interestingly, we find that only well-ordered, highly crystalline graphite delaminates into oxo-functionalized graphene, whereas other graphite grades do not form a proper stage-1 intercalate and revert back to graphite upon hydrolysis. Ab initio molecular dynamics simulations show that ideal stacking and electronic oxidation of the graphite layers significantly reduce the friction of the moving sulfuric acid molecules, thereby facilitating intercalation. Furthermore, the evaluation of the stability of oxo-species in graphite sulfate supports an oxidation mechanism that obviates intercalation of the oxidant.|
M. Feierabend, G. Berghäuser, M. Selig, S. Brem, T. Shegai, S. Eigler, A. Knorr and E. Malic*
Molecule signatures in photoluminescence spectra of transition metal dichalcogenides
Monolayer transition metal dichalcogenides (TMDs) show an optimal surface-to-volume ratio and are thus promising candidates for novel molecule sensor devices. It was recently predicted that a certain class of molecules exhibiting a large dipole moment can be detected through the activation of optically inaccessible (dark) excitonic states in absorption spectra of tungsten-based TMDs. In this paper, we investigate the molecule signatures in photoluminescence spectra in dependence of a number of different experimentally accessible quantities, such as excitation density, temperature, as well as molecular characteristics including the dipole moment and its orientation, molecule-TMD distance, molecular coverage, and distribution. We show that under certain optimal conditions even room-temperature detection of molecules can be achieved.
Focus Review: Defects in Graphene Oxide as Structural Motive
|Graphene oxide possesses defects of various kinds. The structure of graphene oxide is reviewed with focus on defects in the σ-framework of the hexagonal lattice. We clearly distinguish between on-plane functionalization defects and in-plane lattice defects. Moreover, vacancy defects and hole defects are discriminated. In this context, Raman spectroscopy is introduced as sensitive method for detecting and quantifying defects. Moreover, the visualizing of in-plane lattice defects by transmission electron microscopy at atomic resolution is introduced. Understanding the type and density of defects in graphene oxide bears the potential for advancing the field of research by considering the specific types of defects as structural motives.|