Many reactions involve metals, especially noble metals or metal oxides as catalysts. Although metal-based catalysts have been playing a major role in various industrial processes, they still suffer from multiple competitive disadvantages, including their high cost, susceptibility to gas poisoning, and detrimental effects on the environment. Owing to their wide availability, environmental acceptability, corrosion resistance, and unique surface properties, certain carbon nanomaterials have recently been demonstrated to be promising metal-free alternatives for low-cost catalytic processes. This perspective highlights recent progresses in the development of carbon-based metal-free catalysts. Scheme 1. ODH of (1) Ethylbenzene and (2) Alkanes (a) and ORR in Alkaline (b) and Acidic (c) Media

Abstract

As an atomically thin sheet of carbon atoms packed in a twodimensional (2D) honeycomb lattice with excellent electronic, thermal, and mechanical properties, graphene has shown great potential for a wide range of applications. Examples include the use of graphene and its derivatives as transparent conductive electrodes or active materials in solar cells, counter electrodes in dye-sensitized solar cells, electrocatalysts for oxygen reduction in fuel cells, high-performance electrodes in supercapacitors, batteries, actuators, and sensors. [1, 2] Of particular interest, Guo et al. [2f] reported a significant advancement in the development of layered graphene/quantum dots for highly efficient solar cells. Stoller et al.[1j] produced graphene-based supercapacitors free from any conducting filler with a specific capacitance of 135 F g À1 in aqueous electrolytes. We also demonstrated that N-doped graphene could act as a metal-free electrode with a much better electrocatalytic activity, long-term operational stability, and tolerance to crossover effect than platinum for oxygen reduction in alkaline fuel cells.[2b] By using graphene as a superior dimensionally compatible and electrically conductive component, Guo et al. [2g,h] further constructed a smart graphene-based multifunctional biointerface for cell growth as well as in situ selective and quantitative molecular detection. There is now a pressing need to integrate graphene sheets into multidimensional and multifunctional systems with spatially well-defined configurations, and hence integrated systems with a controllable structure and predictable performance. This requires controlled functionalization of graphene sheets at the molecular level, which is still a big challenge.The recent availability of solution-processable graphene by exfoliation of graphite into graphene oxides (GOs), followed by solution reduction, [3] has allowed the functionalization of graphene sheets through various solution reactions.[4] As far as we are aware, however, there is still no report on the asymmetric functionalization of graphene sheets by attaching different chemical moieties to their two opposite surfaces. The asymmetric functionalization, if realized, should significantly advance the self-assembling of graphene sheets into many new multidimensional and multifunctional systems with molecular-level control. Herein, we report for the first time a simple but effective asymmetric modification method for functionalizing the two opposite surfaces of individual graphene sheets with different nanoparticles (NPs) in either a patterned or nonpatterned fashion. The resultant asymmetrically modified graphene sheets with ZnO and Au NPs on their two opposite surfaces were demonstrated to show a strong photodependent diode rectifying behavior.We have previously developed a polymer masking technique for asymmetric functionalization of carbon-nanotube sidewalls by sequentially masking vertically aligned carbon nanotubes twice, with only half of the nanotube length being modified each time. [5,6]...

Nobel metal composite aerogel fibers made from flexible and porous biopolymers offer a wide range of applications, such as in catalysis and sensing, by functionalizing the nanostructure. However, producing these composite aerogels in a defined shape is challenging for many protein-based biopolymers, especially ones that are not fibrous proteins. Here, we present the synthesis of silk fibroin composite aerogel fibers up to 2 cm in length and a diameter of ~300 μm decorated with noble metal nanoparticles. Lyophilized silk fibroin dissolved in hexafluoro-2-propanol (HFIP) was cast in silicon tubes and physically crosslinked with ethanol to produce porous silk gels. Composite silk aerogel fibers with noble metals were created by equilibrating the gels in noble metal salt solutions reduced with sodium borohydride, followed by supercritical drying. These porous aerogel fibers provide a platform for incorporating noble metals into silk fibroin materials, while also providing a new method to produce porous silk fibers. Noble metal silk aerogel fibers can be used for biological sensing and energy storage applications.