Publications

 

Link: Google citations

Details of full list of publications:

2018    2017    2016    2015    2014    2013-2004    Patents

Selected Publications:

15)

S. Seiler, C. 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

14)
F. Grote, C. Gruber, F. Börrnert, U. Kaiser, S. Eigler*

Thermal disproportionation of oxo-functionalized graphene

 

Graphene production by wet-chemistry is an ongoing scientific challenge. Controlled oxidation of graphite introduces oxo- functional groups that enable processing and graphene formation after reductive defunctionalization. Although thermal processing yields conductive carbon, a ruptured and undefined carbon lattice is produced as a consequence of CO2 formation. This thermal process is not understood, but it is believed that graphene is not accessible. Here, we thermally process oxo-functionalized graphene (oxo-G) with a low (4-6%) and high degree of functionalization (50-60%) and find on the basis Raman spectroscopy and transmission electron microscopy performed at atomic resolution (HRTEM) that thermal processing leaves as major process an intact carbon framework behind with a density of lattice defects as low as 0.8%. We attribute this finding to reorganization effects of oxo-groups. This finding holds out the prospect of thermal graphene formation from oxo-G derivatives.

13)
A. Dimiev, S. Eigler
Edited Book: Graphene Oxide -Fundamentals and Applications
Due to its unique properties, graphene oxide has become one of the most studied materials of the last decade and a great variety of applications have been reported in areas such as sensors, catalysis and biomedical applications.
This comprehensive volume systematically describes the fundamental aspects and applications of graphene oxide. The book is designed as an introduction to the topic, so each chapter begins with a discussion on fundamental concepts, then proceeds to review and summarize recent advances in the field. Divided into two parts, the first part covers fundamental aspects of graphene oxide and includes chapters on formation and chemical structure, characterization methods, reduction methods, rheology and optical properties of graphene oxide solutions. Part Two covers numerous graphene oxide applications including field effect transistors, transparent conductive films, sensors, energy harvesting and storage, membranes, composite materials, catalysis and biomedical applications. In each case the differences and advantages of graphene oxide over its non-oxidised counterpart are discussed. The book concludes with a chapter on the challenges of industrial-scale graphene oxide production.
Graphene Oxide: Fundamentals and Applications is a valuable reference for academic researchers, and industry scientists interested in graphene oxide, graphene and other carbon materials.
12)
B. Butz,* C. Dolle, C. E. Halbig, E. Spiecker, S. Eigler*

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

Pure and simple:

Highly pure oxo-functionalized graphene is efficiently synthesized by the use of PTFE equipment.The structural integrity of the underlying carbon framework is confirmed by atomic-resolution transmission electron microscopy and the gently electron-beam induced release of surface functional groups is systematically studied as an alternative approach to chemical reduction.

 

11)

A. Naumov,* F. Grote, M. Overgaard, A. Roth, C. E. Halbig, K. Nørgaard, D. M. Guldi, S. Eigler*

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.

J. Am. Chem. Soc. 2016, 138, 11445.

10)
S. Eigler*
Controlled Chemistry of Oxo-Functionalized Graphene
Graphene is the best-studied 2D material available. However, its production is still challenging and the quality depends on the preparation procedure. Now, more than a decade after the outstanding experiments conducted on graphene, the most successful wet-chemical approach to graphene and functionalized graphene is based on the oxidation of graphite. Graphene oxide has been known for more than a century; however, the structure bears variable large amounts of lattice defects that render the development of a controlled chemistry impossible. The controlled oxo-functionalization of graphene avoids the formation of defects within the σ-framework of carbon atoms, making the synthesis of specific molecular architectures possible. The scope of this review is to introduce the field of oxo-functionalizing graphene. In particular, the differences between GO and oxo-functionalized graphene are described in detail. Moreover analytical methods that allow determining lattice defects and functional groups are introduced followed by summarizing the current state of controlled oxo-functionalization of graphene.
9)
H. Pieper, S. Chercheja, S. Eigler, C. E. Halbig, M. Fillipović, A. Mokhir*
Endoperoxides revealed as origin of toxicity of graphene oxide
Potential biomedicinal applications of graphene oxide (GO), for example, as a carrier of biomolecules or a reagent for photothermal therapy and biosensing, are limited by its cytotoxicity and mutagenicity. It is believed that these properties are at least partially caused by GO-induced oxidative stress in cells. However, it is not known which chemical fragments of GO are responsible for this unfavorable effect. We generated four GOs containing variable redox-active groups on the surface, including Mn2+, C-centered radicals, and endoperoxides (EPs). A comparison of the abilities of these materials to generate reactive oxygen species in human cervical cancer cells revealed that EPs play a crucial role in GO-induced oxidative stress. These data could be applied to the rational design of biocompatible nontoxic GOs for biomedical applications.

8)

S. Eigler*
Graphite Sulphate – A Precusrsor to Graphene
Graphite sulphate is used as a precursor to graphene for the first time. The positively charged graphene layers react with water to yield a processable graphene derivative. The unprecedented low density of defects is determined to be 0.06% on average and may open the way for electronic applications.
7)
S. Eigler,* A. Hirsch
Chemistry with Graphene and Graphene Oxide – Challenges for Synthetic Chemists
The chemical production of graphene as well as its controlled wet chemical modification is a challenge for synthetic chemists. Furthermore, the characterization of reaction products requires sophisticated analytical methods. In this Review we first describe the structure of graphene and graphene oxide and then outline the most important synthetic methods that are used for the production of these carbon-based nanomaterials. We summarize the state-of-the-art for their chemical functionalization by noncovalent and covalent approaches. We put special emphasis on the differentiation of the terms graphite, graphene, graphite oxide, and graphene oxide. An improved fundamental knowledge of the structure and the chemical properties of graphene and graphene oxide is an important prerequisite for the development of practical applications.

Angew. Chem. Int. Ed. 2014537720-7738;

Angew. Chem. 20141267852-7872.

6)
S. Eigler,* S. Grimm, A. Hirsch 
Investigation of the Thermal Stability of the Carbon Framework of Graphene Oxide
In this study, we use our recently prepared graphene oxide (GO) with an almost intact σ-framework of carbon atoms (ai-GO) to probe the thermal stability of the carbon framework for the first time. Ai-GO exhibits few defects because CO2 formation is prevented during synthesis. Ai-GO was thermally treated before chemical reduction and the resulting defect density in graphene was subsequently determined by statistical Raman microscopy. Surprisingly, the carbon framework of ai-GO is stable in thin films up to 100 °C. Furthermore, we find evidence for an increase in the quality of ai-GO upon annealing at 50 °C before reduction. The carbon framework of GO prepared according to the popular Hummers’ method (GO-c) appears to be less stable and decomposition starts at 50 °C, which is qualitatively indicated by CO2-trapping experiments in μm-thin films. Information about the stability of GO is important for storing, processing, and using GO in many applications.

Chem. – A Eur. J. 201420984-989.

5)
S. Eigler,* Y. Hu, Y. Ishii, A. Hirsch
Controlled Functionalization of Graphene Oxide with Sodium Azide
We present the first example of azide functionalization on the surface of graphene oxide (GO), which preserves thermally unstable groups in GO through the mild reaction with sodium azide in solids. Experimental evidence, by 15N solid-state NMR and other spectroscopic methods, indicates the substitution of organosulfate with azide anions as the reaction mechanism

Nanoscale 2013512136-12139.

4)

S. Eigler,* S. Grimm, F. Hof, A. Hirsch 

Graphene oxide: a stable carbon framework for functionalization

The effect of NaOH and HCl on the stability of the carbon framework in graphene oxide (GO) after substitution or etherification reaction in GO is demonstrated at 10 °C and 40 °C, respectively. Our results allow the preparation of functionalized GO based architectures with an intact σ-framework of carbon atoms.

J. Mater. Chem. A 2013111559-11562.

3)
S. Eigler,* S. Grimm, M. Enzelberger, A. Hirsch 
Graphene Oxide: Efficiency of Reducing Agents
The efficiency of reducing agents for the reduction of graphene oxide (GO) could be probed by scanning Raman spectroscopy. A film of graphene flakes derived from GO was probed to be graphene like. We also focus on the surface quality of reduced GO (rGO).
2)
S. Eigler,* M. Enzelberger, S. Grimm, P. Hofmann, W. Kroener, C. Dotzer, M. Röckert, J. Xiao, C. Papp, O. Lytken, H. -. Steinrück, P. Müller, A. Hirsch 
Wet Chemical Synthesis of Graphene
A suitable technology for the preparation of graphene based on versatile wet chemistry is presented for the first time. The protocol allows the wet chemical synthesis of graphene from a new form of graphene oxide that consists of an intact hexagonal σ-framework of C-atoms. Thus, it can be easily reduced to graphene that is no longer dominated by defects.

Adv. Mater. 2013253583-3587.

1)
S. Eigler,* C. Dotzer, F. Hof, W. Bauer, A. Hirsch
Sulfur Species in Graphene Oxide
The structure of graphene oxide (GO) is of crucial importance for its chemical functionalization. However, the sulfur content present in GO prepared by Hummers’ method has only been addressed by a few authors so far. It has been reported that hydrolysis of sulfur species takes place and that stable sulfonic groups are present in graphite oxide. In this manuscript, in contrast to earlier reports, sulfate species are identified that are covalently bound to GO and still present after extensive aqueous work-up. Additionally, we exclude the possibility that sulfonic groups are present in GO as major species after aqueous work up. Our results are based on bulk characterization of graphene oxide by thermogravimetry and subsequent analysis of the decomposition products using mass spectroscopy and infrared spectroscopy. Up to now, the combustion temperature between 200 and 300 °C remained almost unaddressed. In a temperature dependant experiment we reveal two main decomposition steps that differ in temperature and that are closely related to the sulfur species in GO. While the decomposition, between 200 and 300 °C, is related to the degradation of organosulfate, the other one, between 700 and 800 °C, is assigned to the pyrolysis of inorganic sulfate. Furthermore, organosulfate is to some extent responsible for the reactivity of GO. Therefore, the structural model of GO was extended by adding organosulfate in addition to epoxy and hydroxyl groups, which are predominantly covalently bound above and below the carbon skeleton. Furthermore, the identification of organosulfate groups beneath epoxy groups makes new molecular architectures feasible and can be used to explain the properties of GO in various applications.