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.
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.
The formation, stability, and decomposition of CO2 intercalated
graphene oxide was analyzed by FTIR, TGA-MS, TGA-IR, AFM, and SEM
for the first time. We found that the formation starts at 50 °C
and develops up to 120 °C. The formation process can be best
observed by FTIR spectroscopy, and the product is stable at ambient
conditions. At higher temperatures, the decomposition of CO2 intercalated graphene oxide occurs due to the release of water,
CO2, and CO that can be monitored by TGA-MS and TGA-IR
analysis. AFM and SEM images can visualize the formation of blisters
in GO films that become instable at 210 °C. We further prepared
graphene oxide with a low water-content and found that the formation
of CO2 was significantly suppressed and CO became the major
species responsible for the weight loss. In addition we prepared 18OH2 treated graphene oxide to elucidate the formation
process of CO2 and found C16O18O
by TGA-MS analysis that proves the crucial role of water during CO2 formation. From these experiments we propose that hydrate
species are key-intermediates for the formation of CO2.
Hence, it seems likely that rearrangement reactions that can proceed
via hydrate intermediates, known from organic chemistry, are probably
responsible for the formation of carboxylic acids at the edges of
graphene oxide sheets after sonication of graphite oxide. Further,
our investigations prove that graphene oxide is less stable than shown
by TGA measurements. This has a high impact on the electronic properties
of reduced graphene oxide, especially for all those using it for electronic
applications.
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 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,
a 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 insights for the design and synthesis
of carbon-based electrocatalysts for H2O2 production.
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