Tuning Physical and Optical Properties of ZnO Nanowire Arrays Grown on Cotton Fibers

This article reports the first systematic study on the quantitative relationship between the process parameters of solution concentration ratio, structure, and physical and optical properties of ZnO nanowires grown on cotton surfaces. To develop a fundamental understanding concerning the process-structure-activity relations, we grew a series of well-defined, radially oriented, highly dense, and uniform single-crystalline ZnO nanorods and nanoneedles on cotton surfaces by a simple and inexpensive two-step optimized hydrothermal process at a relatively low temperature. This process involves seed treatment of a cotton substrate with ZnO nanocrystals that will serve as the nucleation sites for subsequent anisotropic growth of single crystalline ZnO nanowires. All of the ZnO nanowires exhibit wurtzite crystal structure oriented along the c-axis. For investigating structure-controlled properties, seed-to-growth solutions concentrations ratio ([S]/[G]) of the synthesis process was varied over six different values. Superhydrophobicity was achieved for all morphologies after 1-dodecanethiol modification, which was highly durable after prolonged UV irradiation. Durability of the ZnO materials under laundry condition was also verified. Variation of the [S]/[G] ratio resulted in a morphological transform from nanorods to needle-like structures in conjunction with a drastic change in the physical and optical properties of the ZnO modified cotton surfaces. Higher [S]/[G] ratios yielded formation of ZnO nanoneedles with high degree of crystallinity and higher aspect ratio compared to nanorods. Increasing [S]/[G] ratio resulted in the amount of ZnO grown on the cotton surface to drop significantly, which also caused a decrease in the surface hydrophobicity and UV absorption. In addition, room temperature photoluminescence measurements revealed that the band gap of ZnO widened and the structural defects were reduced as the morphology changed from nanorods to nanoneedles. A similar trend was observed in the UV-vis absorption of nanorods and nanoneedles, the onset of the latter exhibiting a blue-shift that correlates with the widening of band gap with nanoneedle formation.