Top‐down methods are of key importance for large‐scale graphene and graphene oxide preparation. Electrochemical exfoliation of graphite has lately gained much interest because of the simplicity of ...execution, the short process time, and the good quality of graphene that can be obtained. Here, we test three different electrolytes, that is, H2SO4, Na2SO4, and LiClO4, with a common exfoliation procedure to evaluate the difference in structural and chemical properties that result for the graphene. The properties are analyzed by means of scanning transmission electron microscopy (STEM), Raman spectroscopy, and X‐ray photoelectron spectroscopy. We then tested the graphene materials for electrochemical applications, measuring the heterogeneous electron transfer (HET) rates with a Fe(CN)63−/4− redox probe, and their capacitive behavior in alkaline solutions. We correlate the electrochemical features with the presence of structural defects and oxygen functionalities on the graphene materials. In particular, the use of LiClO4 during the electrochemical exfoliation of graphite allowed the formation of highly oxidized graphene with a C/O ratio close to 4.0 and represents a possible avenue for the mass production of graphene oxide as valid alternative to the current laborious and dangerous chemical procedures, which also have limited scalability.
Which electrolyte? Three different electrolytes, H2SO4, Na2SO4, and LiClO4, are tested with a common exfoliation procedure to evaluate the difference in structural and chemical properties that result for graphene and graphene oxide. Use of LiClO4 during the electrochemical exfoliation of graphite allowed the formation of highly oxidized graphene with a C/O ratio close to 4.0 and represents a possible avenue for the mass production of graphene oxide as valid alternative to the current laborious and dangerous chemical procedures with limited scalability.
3D printing fabrication methods have received lately an enormous attention by the scientific community. Laboratories and research groups working on analytical chemistry applications, among others, ...have advantageously adopted 3D printing to fabricate a wide range of tools, from common laboratory hardware to fluidic systems, sample treatment platforms, sensing structures, and complete fully functional analytical devices. This technology is becoming more affordable over time and therefore preferred over the commonly used fabrication processes like hot embossing, soft lithography, injection molding and micromilling. However, to better exploit 3D printing fabrication methods, it is important to fully understand their benefits and limitations which are also directly associated to the properties of the materials used for printing. Costs, printing resolution, chemical and biological compatibility of the materials, design complexity, robustness of the printed object, and integration with commercially available systems represent important aspects to be weighted in relation to the intended task. In this review, a useful introductory summary of the most commonly used 3D printing systems and mechanisms is provided before the description of the most recent trends of the use of 3D printing for analytical and bioanalytical chemistry. Concluding remarks will be also given together with a brief discussion of possible future directions.
Graphical abstract
The oxygen reduction reaction (ORR) is of high industrial importance. There is a large body of literature showing that metal‐based catalytic nanoparticles (e.g. Co, Mn, Fe or hybrid Mn/Co‐based ...nanoparticles) supported on graphene act as efficient catalysts for the ORR. A significant research effort is also directed to the so‐called “metal‐free” oxygen reduction reaction on heteroatom‐doped graphene surfaces. While such studies of the ORR on nonmetallic heteroatom‐doped graphene are advertised as “metal‐free” there is typically no sufficient effort to characterize the doped materials to verify that they are indeed free of any trace metal. Here we argue that the claimed “metal‐free” electrocatalysis of the oxygen reduction reaction on heteroatom‐doped graphene is caused by metallic impurities present within the graphene materials.
Carbon materials: Heteroatom‐doped graphene surfaces are used as electrocatalysts for the oxygen reduction reaction. The claimed “metal‐free” electrocatalysis of the oxygen reduction reaction is caused by metallic impurities (see picture) present within the graphene materials.
MoS2 and other transition metal dichalcogenides (TMDs) have recently gained a renewed interest due to the interesting electronic, catalytic, and mechanical properties which they possess when ...down‐sized to single or few layer sheets. Exfoliation of the bulk multilayer structure can be achieved by a preliminary chemical Li intercalation followed by the exfoliation due to the reaction of Li with water. Organolithium compounds are generally adopted for the Li intercalation with n‐butyllithium (n‐Bu‐Li) being the most common. Here, the use of three different organolithium compounds are investigated and compared, i.e., methyllithium (Me‐Li), n‐butyllithium (n‐Bu‐Li) and tert‐butyllithium (t‐Bu‐Li), used for the exfoliation of bulk MoS2. Scanning transmission electron microscopy (STEM), Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) are adopted for a comprehensive characterization of all materials under investigation. In addition, catalytic properties towards the hydrogen evolution reaction (HER) and capacitive properties are also tested. Different organolithium compounds exhibit different extent of Li intercalation resulting in different degrees of exfoliation. The inherent electrochemical behavior of MoS2 consisting of significant anodic and cathodic peaks as well as its capacitive behavior and catalytic properties towards hydrogen evolution reaction are strongly connected to the exfoliation compound used. This research significantly contributes to the development of large‐scale synthesis of electrocatalytic MoS2‐based materials.
Different Li intercalation compounds used to exfoliate MoS2
generate materials with significantly different electrochemical and catalytic properties. The inherent electrochemistry of MoS2 with significant anodic and cathodic peaks, its capacitive behavior, and its catalytic properties towards the hydrogen evolution reaction (HER) are strongly connected to the exfoliation compound used.
3D‐printing represents an emerging technology that can revolutionize the way object and functional devices are fabricated. Here the use of metal 3D printing is demonstrated to fabricate bespoke ...electrochemical stainless steel electrodes that can be used as platform for different electrochemical applications ranging from electrochemical capacitors, oxygen evolution catalyst, and pH sensor by means of an effective and controlled deposition of IrO2 films. The electrodes have been characterized by scanning electrode microscopy and energy dispersive X‐ray spectroscopy before the electrochemical testing. Excellent pseudocapacitive as well as catalytic properties have been achieved with these 3D printed steel‐IrO2 electrodes in alkaline solutions. These electrodes also demonstrate Nernstian behavior as pH sensor. This work represents a breakthrough in on‐site prototyping and fabrication of highly tailored electrochemical devices with complex 3D shapes which facilitate specific functions and properties.
On‐site design and fabrication of metal electrodes for different electrochemical applications is possible, thanks to metal 3D printing. Helical‐shaped steel electrodes—fabricated through a selective laser melting technology—are electrochemically modified with IrO2 films and used as capacitors, pH sensors, and catalysts for oxygen evolution reaction, demonstrating excellent performance.
Graphene materials are generally prepared from the exfoliation of graphite oxide (GO) to graphene oxide, followed by subsequent chemical or thermal reduction. These methods, although efficient in ...removing most of the oxygen functionalities from the GO material, lack control over the extent of the reduction process. We demonstrate here an electrochemical reduction procedure that not only allows for precise control of the reduction process to obtain a graphene material with a well‐defined C/O ratio in the range of 3 to 10, but also one that is able to tune the electrocatalytic properties of the reduced material. A method that is able to precisely control the amount and density of the oxygen functionalities on the graphene material as well as its electrochemical behaviour is very important for several applications such as electronics, bio‐composites and electrochemical devices.
Film studies: Electrochemical reduction allows for precise tuning of the surface composition of graphene oxide films by altering the content of the oxygen functionalities (see figure). The electrochemical activity of graphene oxide modified electrodes can therefore be adjusted to specific levels according to the desired application.
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