Functional microporous conducting carbon with a high surface area of about 1230 m 2 g À1 is synthesized by single-step pyrolysis of dead plant leaves (dry waste, ground powder) without any activation and studied for supercapacitor application. We suggest that the activation is provided by the natural constituents in the leaves composed of soft organics and metals. Although the detailed study performed and reported here is on dead Neem leaves (Azadirachta indica), the process is clearly generic and applicable to most forms of dead leaves. Indeed we have examined the case of dead Ashoka leaves as well. The comparison between the Neem and Ashoka leaves brings out the importance of the constitution and composition of the bio-source in the nature of carbon formed and its properties. We also discuss and compare the cases of pyrolysis of green leaves as well as un-ground dead leaves with that of ground dead leaf powder studied in full detail. The concurrent high conductivity and microporosity realized in our carbonaceous materials are key to the high energy supercapacitor application. Indeed, our synthesized functional carbon exhibits a very high specific capacitance of 400 F g À1 and an energy density of 55 W h kg À1 at a current density of 0.5 A g À1 in aqueous 1 M H 2 SO 4 . The areal capacitance value of the carbon derived from dead (Neem) plant leaves (CDDPL) is also significantly high (32 mF cm À2 ). In an organic electrolyte the material shows a specific capacitance of 88 F g À1 at a current density of 2 A g À1 . Broader contextWaste management has always been a big problem in big cities. Most such waste is a rich source of carbon but may contain other elements in different proportions. Usually the waste from natural sources is just burnt producing ash and hazardous gaseous pollution products. If instead it is harnessed to synthesize electronically active carbon, one could use it for value-added products such as materials for supercapacitor electrodes. Supercapacitors have been attracting signicant interest due to their applications in electrical vehicles, digital devices, pulsing techniques etc. In this work we demonstrate the synthesis of high surface area microporous conducting carbon by one-step pyrolysis of dead plant leaves (abundant waste material) without any chemical or physical activation and have examined its properties for supercapacitor application. Although the detailed study performed and reported here is on dead Neem leaves (Azadirachta indica), the process is clearly generic and applicable to most forms of dead leaves. Indeed we have examined the case of dead Ashoka leaves too. With dead Neem leaves we have achieved a high specic capacitance of 400 F g À1 and a energy density of 55 W h kg À1 at 0.5 A g À1 . Moreover, in an organic electrolyte the material shows a specic capacitance of 88 F g À1 at 2 A g À1 .
Zinc oxide (ZnO) nanorods are grown hierarchically on cuprous oxide (Cu 2 O) nanoneedles to form a Cu 2 O/ZnO hetero-nanobrush assembly. This increases the overall aspect ratio, which helps to enhance the field emission properties of the system. Also, the charge separation and transport are facilitated because of the multiple p-n junctions formed at p-Cu 2 O/n-ZnO interfaces and quasi-1-D structures of both the materials, respectively. This helps to significantly enhance the photocatalytic properties. As compared to only Cu 2 O nanoneedles, the Cu 2 O/ZnO hetero-nanobrush shows excellent improvement in both field emission and photocatalytic applications.
Cu(2)O nanoneedles are synthesized on a copper substrate by a simple anodization and reducing ambient annealing protocol. ZnO nanorods are grown on ITO coated glass by a low temperature chemical route. The electronic and photo-response properties of the p-Cu(2)O/n-ZnO flip-chip heterojunction are then studied and analyzed. We show that the I-V characteristic is rectifying and the junction exhibits a good photoresponse (∼120% under 1 V reverse bias) under AM 1.5 (1 Sun) illumination. This nano-heterojunction photo-response is far stronger as compared to that of a pulsed laser deposited thin film p-Cu(2)O/n-ZnO heterojunction, which can be attributed to higher junction area in the former case.
A good high rate supercapacitor performance requires a fine control of morphological (surface area and pore size distribution) and electrical properties of the electrode materials. Polyaniline (PANI) is an interesting material in supercapacitor context because it stores energy Faradaically. However in conventional inorganic (e.g. HCl) acid doping, the conductivity is high but the morphological features are undesirable. On the other hand, in weak organic acid (e.g. phytic acid) doping, interesting and desirable 3D connected morphological features are attained but the conductivity is poorer. Here the synergy of the positive quality factors of these two acid doping approaches is realized by concurrent and optimized strong-inorganic (HCl) and weak-organic (phytic) acid doping, resulting in a molecular composite material that renders impressive and robust supercapacitor performance. Thus, a nearly constant high specific capacitance of 350 F g−1 is realized for the optimised case of binary doping over the entire range of 1 A g−1 to 40 A g−1 with stability of 500 cycles at 40 A g−1. Frequency dependant conductivity measurements show that the optimized co-doped case is more metallic than separately doped materials. This transport property emanates from the unique 3D single molecular character of such system.
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