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Schematic diagrams representing potential of a, b ZnO nanoparticles with grain-boundary junctions in the presence of dry air and NO2 gas, respectively. c, d ZnO nanoparticles with necked junctions in the presence of dry air and NO2 gas, respectively. Thickness of the arrows is related to the amount of electron current carriers. Reprinted with permission from [59]. © 2009 Elsevier B.V
