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Self-Powered Systems Chemie 17.4 % and another based on an array of InP nanopillars with an efficiency of 8.1% have also been demonstrated.[68] Moreover, the performance of solar cells based on NW arrays has been improved strategically by enhancing dye adsorption[69] and reducing surface recombination.[70] Innova- tive structural configurations, such as tandem/core-shell structures and 3D convolution between NWs and light- transmitting media, have also been investigated in efforts to improve the conversion efficiency of solar cells based on 1D nanostructures (Figure 4 b).[39, 71] By taking advantage of the improved photon capturing and photocarrier collection enabled by the engineering of structures at the nanoscale, it is possible to fabricate high- efficiency solar cells, preferably in flexible forms, through scalable processes at very low cost for potential powering applications in future MNSs. However, several technical challenges, such as carrier recombination, mechanical and chemical stability, morphology control, and process scalabil- ity, must be met before 1D-nanostructured solar cells with their promised benefits can be implemented practically. In addition to the advancement of understanding of the funda- mental aspects that affect the solar-cell performance, engi- neering issues, such as scaling, module integration, and device packaging, should be addressed.[62] 3.2. Artificial Photosynthesis for the Production of Solar Fuel Although PV technology enables the direct generation of electricity from sunlight, it is incapable of efficiently produc- ing fuel within which the energy from sunlight can be stored for use at a later time when sunlight is not available.[72] Furthermore, the distribution of sunlight over time is not constant; thus, viable approaches for collecting and storing solar energy in readily accessible media are required.[72b] The conversion of light into stored chemical energy by artificial photosynthesis, a chemical process which replicates natural photosynthesis, has been the focus of much recent attention. Photoelectrolysis, or water splitting, is a form of artificial photosynthesis that converts water into hydrogen and oxygen through the use of sunlight. An artificial photosynthetic system for producing solar fuel typically requires antenna/ reaction center complexes for the generation of electro- chemical potential from sunlight and appropriate catalysts for the oxidization of water and reduction of precursors to hydrogen.[72a] Considerable progress has been made in the design of technologies that mimic natural photosynthesis and the development of efficient water-splitting technology.[73] The use of multijunction configurations has been the pre- dominant approach for the development of efficient photo- electrochemical water-splitting cells, whereby the incorpora- tion of semiconductors with different band gaps offers the advantages of complementary absorption and improved stability.[73c, 74] Efforts have also been focused on the engineer- ing of new materials to optimize processes at both anodes and cathodes and the integration of photoelectrolysis cells with other structures, such as PV cells, which can provide addi- tional voltage to drive the water-splitting reactions and hence increase overall efficiency.[73c, 75] In this way, a monolithic and multijunction integrated PV/electrolysis configuration in a GaInP2/GaAs system was developed with 16% solar-to- hydrogen conversion efficiency.[76] There has been increasing interest in the use of nano- structured materials as the photoanodes in photosynthesis applications as a result of the benefits offered by the large surface area, short lateral diffusion length, and low reflectivity in these nanostructured materials.[77] The use of structured photoelectrodes based on semiconductor NWs with a pre- ferred band gap or an intermediate energy level introduced within the band gap has also been investigated in efforts to improve the water-splitting efficiency through enhanced light absorption in the visible region of interest and enhanced electron-transfer efficiency due to the single-crystalline nature of low-dimensional semiconductor nanostructures.[78] The creation of high-aspect-ratio photoelectrode surfaces by engineering the morphology of nanostructures also enables the use of less expensive catalysts with lower catalytic activities. Furthermore, the ability to modulate the composi- tion of nanomaterials at the atomic scale and to synthesize nanostructures of various morphologies, both in a controlled manner, may improve conversion efficiency in artificial- photosynthesis applications through more effective manage- ment of the light absorption. Issues such as the low photocatalytic efficiency and scaling up of the manufacturing process need to be addressed properly before artificial photosynthetic systems can be deployed in practical applications at low cost. It is equally important to develop artificial photosynthetic systems from earth-abundant materials in an affordable and environmen- tally benign manner for energy conversion and storage.[72b] However, it can be expected that artificial photosynthetic systems will play an important role in providing sustainable and clean energy to MNSs in applications such as environ- mental monitoring and remote sensing. Thus, the ability of artificial photosynthetic systems to convert harvested solar energy into fuel and store it in the system for later applications will enable the operation of self-powered MNSs. 3.3. Harvesting of Thermoelectric Energy The past decades have witnessed an increasing interest in the development of thermoelectric (TE) materials for elec- trical-power generation through direct energy harvesting from natural heat sources and waste heat dissipated from engines.[79] This type of energy harvesting is expected to help reduce the consumption of fossil fuels and hence the emission of CO2. In addition to the macroscale harnessing of heat, attention has also been paid to the scavenging of heat dissipated from humans to support miniaturized sensors in applications such as biomedical monitoring and body-area networks.[80] Electricity generated directly by a temperature gradient, or thermoelectricity, is based on the Seebeck effect, whereby an equilibrium is established between the diffusion of carriers from the hot side to the cold side of the material and the drift of carriers from the cold side to the hot side as a result of the induced electric field. The electrochemical potential that Angew. Chem. Int. Ed. 2012, 51, 2 – 24 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org &&&& These are not the final page numbers! Angewandte Ü ÜPDF Image | Self-Powered Nanosystems
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