Abstract
In this study, we grew zinc oxide (ZnO) nanowire arrays on paper substrates using a two-step growth strategy. In the first step, we formed single-crystalline ZnO nanoparticles of uniform size distribution (ca. 4 nm) as seeds for the hydrothermal growth of the ZnO nanowire arrays. After spin-coating of these seeds onto paper, we grew ZnO nanowire arrays conformally on these substrates. The crystal structure of a ZnO nanowire revealed that the nanowires were single-crystalline and had grown along the c axis. Further visualization through annular bright field scanning transmission electron microscopy revealed that the hydrothermally grown ZnO nanowires possessed Zn polarity. From photocatalytic activity measurements of the ZnO nanowire (NW) arrays on paper substrate, we extracted rate constants of 0.415, 0.244, 0.195, and 0.08 s-1 for the degradation of methylene blue at incident angles of 0°, 30°, 60°, and 75°, respectively; that is, the photocatalytic activity of these ZnO nanowire arrays was related to the cosine of the incident angle of the UV light. Accordingly, these materials have promising applications in the design of sterilization systems and light-harvesting devices.
Highlights
The integration of inorganic nanomaterials into soft substrates enables the development of flexible functional devices, including rollup displays, electronic paper, and wearable devices
The zinc oxide (ZnO) NP solutions emitted light under excitation with UV light (Figure 1b), with the intensity of the emitted light increasing with respect to the amount of NaOH that had been added into the solution
The absorption spectra of the ZnO NP solutions (Figure 1c) reveal that the absorption edge of the ZnO NPs appeared near 380 nm, with the absorption increasing with respect to the amounts of added NaOH; a stronger absorption implied a higher concentration of ZnO NPs in the solution
Summary
The integration of inorganic nanomaterials into soft substrates enables the development of flexible functional devices, including rollup displays, electronic paper, and wearable devices. When using paper or paper-like substrates, such systems have a diverse range of possible applications in photodetectors [1], light-emitting diodes [2], piezoelectric nanogenerators [3], paper batteries [4], and electronic paper displays [5]. Paper-like substrates are cheap, light, flexible, and biocompatible; innovative techniques and strategies based on paper-like platforms are just beginning to emerge from laboratorylevel studies. Because of their wide direct band gap (Eg = 3.37 eV) and large exciton energy (60 meV), zinc oxide (ZnO) nanostructures have diverse applications in lightemitting diodes [6], solar cells [7], gas sensors [8], photodetectors [9], and nanogenerators [10,11]. We have observed incident angle-dependence of the photodegradation properties of these ZnO NW arrays on paper substrates
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