Christian E. Halbig

Graphene Oxide: A One- versus Two-Component Material

The structure of graphene oxide (GO) is a matter of discussion. While established GO models are based on functional groups attached to the carbon framework, another frequently used model claims that GO consists of two components, a slightly oxidized graphene core and highly oxidized molecular species, oxidative debris (OD), adsorbed on it. Those adsorbents are claimed to be the origin for optical properties of GO. Here, we examine this model by preparing GO with a low degree of functionalization, combining it with OD and studying the optical properties of both components and their combination in an artificial two-component system. The analyses of absorption and emission spectra as well as lifetime measurements reveal that properties of the combined system are distinctly different from those of GO. That confirms structural models of GO as a separate oxygenated hexagonal carbon framework with optical properties governed by its internal structure rather than the presence of OD. Understanding the structure of GO allows further reliable interpretation of its optical and electronic properties and enables controlled processing of GO.

Extending the environmental lifetime of unpackaged perovskite solar cells through interfacial design

Solution-processed oxo-functionalized graphene (oxo-G1) is employed to substitute hydrophilic PEDOT:PSS as an anode interfacial layer for perovskite solar cells. The resulting devices exhibit a reasonably high power conversion efficiency (PCE) of 15.2% in the planar inverted architecture with oxo-G1 as a hole transporting material (HTM), and most importantly, deploy the full open-circuit voltage (Voc) of up to 1.1 V. Moreover, oxo-G1 effectively slows down the ingress of water vapor into the device stack resulting in significantly enhanced environmental stability of unpackaged cells under illumination with 80% of the initial PCE being reached after 500 h. Without encapsulation, ∼60% of the initial PCE is retained after ∼1000 h of light soaking under 0.5 sun and ambient conditions maintaining the temperature beneath 30 °C. Moreover, the unsealed perovskite device retains 92% of its initial PCE after about 1900 h under ambient conditions and in the dark. Our results underpin that controlling water diffusion into perovskite cells through advanced interface engineering is a crucial step towards prolonged environmental stability.

Highly Intact and Pure Oxo-Functionalized Graphene: Synthesis and Electron-Beam-Induced Reduction

Controlling the chemistry of graphene is necessary to enable applications in materials and life sciences. Research beyond graphene oxide is targeted to avoid the highly defective character of the carbon framework. Herein, we show how to optimize the synthesis of oxo-functionalized graphene (oxo-G) to prepare high-quality monolayer flakes that even allow for direct transmission electron microscopy investigation at atomic resolution (HRTEM). The role of undesired residuals is addressed and sources are eliminated. HRTEM provides clear evidence for the exceptional integrity of the carbon framework of such oxo-G sheets. The patchy distribution of oxo-functionality on the nm-scale, observed on our highly clean oxo-G sheets, corroborates theoretical predictions. Moreover, defined electron-beam irradiation facilitates gentle de-functionalization of oxo-G sheets, a new route towards clean graphene, which is a breakthrough for localized graphene chemistry.

Towards the Synthesis of Graphene Azide from Graphene Oxide

In the last decades, organic azides haven proven to be very useful precursors in organic chemistry, for example in 1,3-dipolar cycloaddition reactions (click-chemistry). Likewise, azides can be introduced into graphene oxide with an almost intact carbon framework, namely oxo-functionalized graphene (oxo-G1), which is a highly oxidized graphene derivative and a powerful precursor for graphene that is suitable for electronic devices. The synthesis of a graphene derivative with exclusively azide groups (graphene azide) is however still a challenge. In comparison also hydrogenated graphene, called graphene or halogenated graphene remain challenging to synthesize. A route to graphene azide would be the desoxygenation of azide functionalized oxo-G1. Here we show how treatment of azide functionalized oxo-G1 with HCl enlarges the π-system and removes strongly adsorbed water and some oxo-functional groups. This development reflects one step towards graphene azide.

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.Scalable graphene production from graphite via an intercalation-oxidation-reduction process is still hampered by low reproducibility and many lattice defects. Here, the authors show that reducing molecular friction by using highly crystalline graphite and mild oxidizing conditions is the key to high quality graphene.

Oxo-Functionalized Graphene as a Cell Membrane Carrier of Nucleic Acid Probes Controlled by Aging

We applied a fluorescein-containing oligonucleotide (ON) to probe surface properties of oxidized graphene (oxo-G) and observed that graphene-like patches are formed upon aging of oxo-G, indicated by enhanced probe binding and by FTIR spectroscopic analysis. By using a recently developed fluorogenic endoperoxide (EP) probe, we confirmed that during the aging process the amount of EPs on the oxo-G surface is reduced. Furthermore, aging was found to strongly affect cell membrane carrier properties of this material. In particular, freshly prepared oxo-G does not act as a carrier, whereas oxo-G aged for 28 days at 4 °C is an excellent carrier. Based on these data we prepared an optimized oxo-G, which has a low-defect density, binds ONs, is not toxic, and acts as cell membrane carrier. We successfully applied this material to design fluorogenic probes of representative intracellular nucleic acids 28S rRNA and β-actin-mRNA. The results will help to standardize oxidized graphene derivatives for biomedical and bioanalytical applications.

Oxo-Functionalized Graphene: A Versatile Precursor for Alkylated Graphene Sheets by Reductive Functionalization

Controlled covalent functionalization of graphene remains a challenging task owing to the heterogeneous nature of materials. Functionalization approaches for 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. Although Raman spectroscopy is powerful to determine the degree of in-plane lattice defects (θLD ) and functionalization (θFD ), the method fails at 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 chemical vapor deposition (CVD)-graphene and bulk graphite.

Hidden Defects and Unexpected Properties of Graphene — How Advanced TEM Contributes to Materials Development

1. Institute of Microand Nanostructure Research & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany. 2. Interdisciplinary Center for Molecular Materials (ICMM) & Computer Chemistry Center (CCC), Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany. 3. Chair for Applied Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany. 4. Lehrstuhl für Theoretische Festkörperphysik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany. 5. Department of Chemistry and Pharmacy and Central Institute of Materials and Processes (ZMP), Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany. 6. Institut für Chemie und Biochemie, Organische Chemie, Freie Universität Berlin, Erlangen, Germany. 7. Molecular Foundry/National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA, United States. 8. Visiting Scholar, Materials Science & Engineering, Stanford University, Stanford, CA, United States.