Vertically Aligned Hybrid Core/Shell Semiconductor Nanowires for Photonics Applications

The harvesting of mechanical energy from ambient sources could power electrical devices without the need for batteries. However, although the efficiency and durability of harvesting materials such as piezoelectric nanowires have steadily improved, the voltage and power produced by a single nanowire are insufficient for real devices. The integration of large numbers of nanowire energy harvesters into a single power source is therefore necessary, requiring alignment of the nanowires as well as synchronization of their charging and discharging processes. Here, we demonstrate the vertical and lateral integration of ZnO nanowires into arrays that are capable of producing sufficient power to operate real devices. A lateral integration of 700 rows of ZnO nanowires produces a peak voltage of 1.26 V at a low strain of 0.19%, which is potentially sufficient to recharge an AA battery. In a separate device, a vertical integration of three layers of ZnO nanowire arrays produces a peak power density of 2.7 mW cm(-3). We use the vertically integrated nanogenerator to power a nanowire pH sensor and a nanowire UV sensor, thus demonstrating a self-powered system composed entirely of nanowires.

We describe the use of highly ordered transparent TiO(2) nanotube arrays in dye-sensitized solar cells (DSCs). Highly ordered nanotube arrays of 46-nm pore diameter, 17-nm wall thickness, and 360-nm length were grown perpendicular to a fluorine-doped tin oxide-coated glass substrate by anodic oxidation of a titanium thin film. After crystallization by an oxygen anneal, the nanotube arrays are treated with TiCl(4) to enhance the photogenerated current and then integrated into the DSC structure using a commercially available ruthenium-based dye. Although the negative electrode is only 360-nm-thick, under AM 1.5 illumination the generated photocurrent is 7.87 mA/cm(2), with a photocurrent efficiency of 2.9%. Voltage-decay measurements indicate that the highly ordered TiO(2) nanotube arrays, in comparison to nanoparticulate systems, have superior electron lifetimes and provide excellent pathways for electron percolation. Our results indicate that remarkable photoconversion efficiencies may be obtained, possibly to the ideal limit of approximately 31% for a single photosystem scheme, with an increase of the nanotube-array length to several micrometers.

We demonstrate the design and fabrication of a novel flexible nanoarchitecture by facile coating ultrathin (several nanometers thick) films of MnO2 to highly electrical conductive Zn2SnO4 (ZTO) nanowires grown radially on carbon microfibers (CMFs) to achieve high specific capacitance, high-energy density, high-power density, and long-term life for supercapacitor electrode applications. The crystalline ZTO nanowires grown on CMFs were uniquely served as highly conductive cores to support a highly electrolytic accessible surface area of redox active MnO2 shells and also provide reliable electrical connections to the MnO2 shells. The maximum specific capacitances of 621.6 F/g (based on pristine MnO2) by cyclic voltammetry (CV) at a scan rate of 2 mV/s and 642.4 F/g by chronopotentiometry at a current density of 1 A/g were achieved in 1 M Na2SO4 aqueous solution. The hybrid MnO2/ZTO/CMF hybrid composite also exhibited excellent rate capability with specific energy of 36.8 Wh/kg and specific power of 32 kW/kg at current density of 40 A/g, respectively, and good long-term cycling stability (only 1.2% loss of its initial specific capacitance after 1000 cycles). These results suggest that such MnO2/ZTO/CF hybrid composite architecture is very promising for next generation high-performance supercapacitors.