Cristina Botas

Raman spectroscopy for the study of reduction mechanisms and optimization of conductivity in graphene oxide thin films

Highly reduced few-layer graphene oxide films with conductivities of up to 500 S cm−1 are obtained. The thin films with an optimized compromise between sheet resistance (3.1 kΩ sq−1) and transparency (around 80% to 90%) are suitable for touch screens and transparent electrodes in OLEDs. We discuss the effects of low temperature annealing and chemical reduction on the properties of the films and present an optimized reduction process that allows the original 2D/G Raman intensity ratio of few-layer graphene to be recovered. The Raman spectrum of graphene oxide is found to be only related to oxygen-free graphene-like regions with Raman bands at 1130 and 3155 cm−1 that probably involve C–H vibrations of rings and edges, while a band at 1700 cm−1 is assigned to irregular rings such as Stone Wales defects. All the bands involve resonant Raman processes and disappear in highly reduced samples. Clear correlations of the D band width with the sp2 content in thin films and resistivity have been observed, indicating that this is a good Raman parameter for evaluating the quality of the samples. The structural defects produced by the release of embedded water and some of the oxygen functional groups during annealing are detrimental for intra-grain conductivity but greatly enhance inter-grain connectivity.

Sn– and SnO2–graphene flexible foams suitable as binder-free anodes for lithium ion batteries

With the objective of developing new advanced composite materials that can be used as anodes for lithium ion batteries (LIBs), herein we describe the synthesis of novel three dimensional (3D) macroporous foams formed by reduced graphene oxide (rGO) and submicron tin-based particles. The aerogels were obtained by freeze/freeze-drying a suspension of graphene oxide (GO) in the presence of a tin precursor and its subsequent thermal reduction under an argon atmosphere. The materials exhibited a 3D-macroporous structure formed by the walls of rGO decorated with Sn or SnO2 particles depending on the temperature of calcination. Self-standing compressed foams were directly assembled into coin cells without using any metallic support to be evaluated as binder-free anodes for LIBs. The homogeneous dispersion and intimate contact between the Sn-based particles and graphene walls were confirmed by scanning electron microscopy (SEM). The performance of SnO2–rGO composite materials as anodes for LIBs showed higher specific capacity compared with rGO and metallic Sn-containing samples, reaching a reversible capacity of 1010 mA h g−1 per mass of the electrode at 0.05 A g−1 and good capacity retention (470 mA h g−1) even at 2 A g−1 (∼2 C), among the highest reported for similar systems. The SEM images of selected electrodes after 50 charge–discharge cycles showed that even though SnO2 submicron particles were pulverized into small nanoparticles they remain intact upon cycling.

Graphene-based technologies for energy applications, challenges and perspectives

Here we report on technology developments implemented into the Graphene Flagship European project for the integration of graphene and graphene-related materials (GRMs) into energy application devices. Many of the technologies investigated so far aim at producing composite materials associating graphene or GRMs with either metal or semiconducting nanocrystals or other carbon nanostructures (e.g., CNT, graphite). These composites can be used favourably as hydrogen storage materials or solar cell absorbers. They can also provide better performing electrodes for fuel cells, batteries, or supercapacitors. For photovoltaic (PV) electrodes, where thin layers and interface engineering are required, surface technologies are preferred. We are using conventional vacuum processes to integrate graphene as well as radically new approaches based on laser irradiation strategies. For each application, the potential of implemented technologies is then presented on the basis of selected experimental and modelling results. It is shown in particular how some of these technologies can maximize the benefit taken from GRM integration. The technical challenges still to be addressed are highlighted and perspectives derived from the running works emphasized.

Tailored graphene materials by chemical reduction of graphene oxides of different atomic structure

Graphene materials with different characteristics in terms of sheet size and defects (structural and/or functional groups) were obtained by the reduction with hydrazine of two graphene oxides with similar oxygen content, but with functional groups of different type and location. The oxides were prepared from two synthetic graphites with distinct crystalline structure. Our research has obtained experimental evidence of a greater reactivity of the oxygen functional groups located in the interior of the aromatic domains on the basal planes (mainly epoxy) and a lower reactivity of oxygen functional groups located at the edges (mainly carboxyl and OH). Furthermore, these edge-located groups were found to be responsible for hydrogen bonding lateral interactions between sheets (these occur through the residual OH groups), which cause a substantial increase in the size of the reduced graphene oxide with respect to that of the parent graphene oxide. These results offer a way to tailor the characteristics of graphene materials for diverse applications.

Reconstruction of the carbon sp2 network in graphene oxide by low-temperature reaction with CO

Low-temperature (−176 °C) CO adsorption on graphene oxide and partially reduced graphene oxide sheets was investigated in a combined IR spectroscopic and DFT study. The reactivity of the carbon vacancies in the network was observed to be extremely high, causing the CO molecules to dissociate in a barrier-less process that leads to the reconstruction of the sp2graphene network. After the adsorption of CO on the graphene oxide materials, the intensity of the FTIR bands is lower in the case of partially reduced graphene oxide sheets, indicating that there are fewer active sites available for CO interaction.

Silicon-Reduced Graphene Oxide Self-Standing Composites Suitable as Binder-Free Anodes for Lithium-Ion Batteries

Silicon-reduced graphene oxide (Si-rGO) composites processed as self-standing aerogels (0.2 g cm-3) and films (1.5 g cm-3) have been prepared by the thermal reduction of composites formed between silicon nanoparticles and a suspension of graphene oxide (GO) in ethanol. The characterization of the samples by different techniques (X-ray diffraction, Raman, thermogravimetric analysis, and scanning electron microscopy) show that in both cases the composites are formed by rGO sheets homogeneously decorated with 50 nm silicon nanoparticles with silicon contents of ∼40% wt. The performances of these self-standing materials were tested as binder-free anodes in lithium-ion batteries (LIBs) in a half cell configuration under two different galvanostatic charge-discharge cutoff voltages (75 and 50 mV). The results show that the formation of a solid electrolyte interphase (SEI) is favored in composites processed as aerogels due to its large exposed surface, which prevents the activation of silicon when they are cycled within the 2 to 0.075 V voltage windows. It is also found that the composites processed in the form of self-standing films exhibit good stability over the first 100 cycles, high reversible specific capacity per mass of electrode (∼750 mAh g-1), areal capacities that reach 0.7 mAh cm-2, and high Coulombic efficiencies (80% for the first charge-discharge cycle and over 99% in the subsequent cycles).

Highly packed graphene–CNT films as electrodes for aqueous supercapacitors with high volumetric performance

The increasing complexity of portable electronics demands the development of energy storage devices with higher volumetric energy and power densities. In this work we report a simple strategy for the preparation of partially reduced graphene oxide/carbon nanotube composites (prGO–CNT) as highly packed self-standing binder-free films suitable as electrodes for supercapacitors. These carbon-based films are easily obtained by the hydrothermal treatment of an aqueous suspension of graphene oxide and CNTs at 210 °C and then compacted under pressure. The prGO–CNT films, which had an apparent density as high as 1.5 g cm−3, were investigated as binder-free electrodes for aqueous supercapacitors using 6 M KOH solution as the electrolyte. The results show that the presence of merely 2 wt% of CNTs produces a significant enhancement of the capacitance retention at high current densities compared to the CNT-free samples, and this improvement is especially relevant in systems formed using electrodes with high mass loadings. Volumetric capacitance values of 250 F cm−3 at 1 A g−1 with outstanding capacitance retention (200 F cm−3 at 10 A g−1) were achieved using the prGO–CNT electrodes with an areal mass loading above 12 mg cm−2.

Pathways Towards High Performance Na-O2 Batteries: Tailoring Graphene Aerogel Cathode Porosity & Nanostructure

Fundamental understanding of the physical phenomena and electrochemical reactions occurring in metal–air batteries is critical for developing rational approaches towards high-performing Na–O2 battery cathodes. In this context, air cathode porosity plays a key role in battery performance, influencing oxygen supply and hence oxygen reduction and evolution reaction kinetics (ORR/OER). Graphene-based aerogels offer great versatility as air-cathodes due to their low density, high electronic conductivity and adjustable porosity. Reduced graphene aerogels with different porosities are examined where high meso-macroporosity and a narrow macropore size arrangement exhibit the best electrode performance among all studied materials (6.61 mA h cm−2). This is ascribed to the particular macroporous 3D structure of graphene-based electrodes, which favours the diffusion of oxygen to the defect sites in graphene sheets. An outstanding cycle life is achieved by using the pore-tuned cathode, leading to 39 cycles (486 h) at 0.5 mA h cm−2 with very low overpotential (250 mV) and efficiency over 95%. The cyclability is further increased to 745 h (128 cycles) by decreasing the capacity cut-off. This study shows that tuning of material porosity opens a new avenue of research for achieving Na–O2 batteries with high performance by maximizing the effective area of the electrodes for the ORR/OER.