From oxo-Functionalized Graphene to Porous Graphene and Graphene Quantum Dots

Abstract

This cumulative dissertation is divided into two main topics: 1) preparation of porous graphene and revealing the mechanism of pore formation; 2) investigation of solvent effects on the optical properties of graphene quantum dots (GQDs). The research was conducted in collaboration with the groups of 1) Dr. Ute Resch-Genger from the Federal Institute for Materials Research and Testing; 2) Prof. Dr. Andrey Turchanin from the Institute of Physical Chemistry and Abbe Center of Photonics at Friedrich Schiller University Jena; 3) Prof. Dr. Ute Kaiser from the Central Facility of Electron Microscopy at Ulm University. The complete results and experimental details are included in the attached publications in section 5 and section 6. 1) Porous graphene via chemical etching by hydroxyl radicals Porous graphene is a new class of graphene materials that has received favorable attention from researchers due to its unique properties, such as large specific surface area. However, current synthetic methods like laser lithography and catalyst activation have some problems, such as tedious operation or poor controllability. Therefore, a controlled wet chemical method for preparing porous graphene in large quantities is urgently needed. To achieve scalable preparation with easier manipulation, specific chemical etching would be one of the available choices. Here, a simple and controllable method is developed for producing solution-processable porous graphene derivatives. We identifie oxo-functionalized graphene (oxo-G) with its regulated, low density of in-plane vacancy defects as a suitable precursor. Hydroxyl radicals are generated by the photolysis of hydrogen peroxide under ultraviolet (UV)-irradiation, which etch pores on the basal plane of oxo-G, initiating the pore formation preferentially at defect sites. Correspondingly, oxo-G flakes with lateral dimensions of µm-size and pores with tunable sizes between 5 nm and 500 nm are accessible. Based on results from atomic force microscopy (AFM), transmission electron microscopy (TEM), ultraviolet/visible (UV/Vis) spectroscopy, Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (NMR) spectroscopy, and statistical Raman spectroscopy (SRS), a possible mechanism for pore formation is proposed. The electrophilic addition and oxidation reaction between hydroxyl radicals and oxo-G close to defect sites is the basis for etching. The porous graphene materials can serve as membranes and provide ideas for the tunability of two-dimensional materials. 2) Solvent effects on the optical properties of GQDs Due to their special optical features, GQDs have gained a growing amount of interest and have found widespread use, for instance, in the biomedical, chemical, and environmental fields as sensors and markers. However, the structure-related features of GQDs resulting in solvent-dependent optical properties are currently unclear. Using oxo-functionalized graphene and p-phenylenediamine as precursors, we synthesized long-wavelength emitting GQDs with a size of approximately 3.6 nm by a solvothermal approach. The structure and surface characteristics of as-synthesized GQDs were analyzed by TEM, AFM, FTIR, Raman, and XPS. Subsequently, the effect of solvent polarity and proticity on the optical characteristics of GQDs containing -OH, -NH2, -COOH, and pyridine surface groups was studied. A plausible luminescence mechanism is presented based on the absorption and fluorescence (FL) results of GQDs. The observed alterations in FL with respect to the position of the maximum, quantum yield, and decay kinetics in protic and aprotic solvents, are attributed to polarity effects, intramolecular charge transfer processes, and hydrogen bonding. Additionally, the optical sensing potential of GQDs for trace amounts of water was evaluated. Our systematic spectroscopic analysis will facilitate the rational design of GQDs and provide more information on the fluorescence process of carbon-based fluorescent nanomaterials.