Construction Of Biomimetic Structures On Copper Surface And Manipulation Of The Wettability

Wettability of a solid surface is one of the most important properties of materials, which depends on both geometrical microstructures and chemical composition. In recent years, inspired by"lotus effect", researchers have been paying increased attention to superhydrophobic surfaces with a water contact angle (CA) higher than 150o and a low contact angle hysteresis (CAH).Up to now, many significant techniques have been developed to produce superhydrophobic surfaces by mimicking the lotus leaf. Some examples of this approach include replication of plant leaves, sol-gel process, self-assembly technique, electrodeposition, electrospin, hydrothermal synthesis, thermal evaporation, phase separation process, and so forth. Most of these techniques, however, are costly and time-consuming, and often involve complicated multistage processes, and it still remains a great challenge to develop a facile and time-saving method for the fabrication of superhydrophobic surfaces.In this dissertation, we report on three simple and convenient techniques for fabricating superhydrophobic surfaces on copper substrates.1. Cu(OH)2 nanotube quasi-arrays, nanorod quasi-arrays, Cu(OH)2 nanorod quasi-arrays/CuO microflowers hierarchical structures, and CuO peony-like structures have been fabricated on copper substrates via alkali assistant surface oxidation technique. The morphologies, microstructures, and crystal structure were analyzed by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). Meanwhile, the formation mechanism of the above-mentioned structures was discussed based on experimental and theoretical analysis. It was found that nanoparticles of Cu(OH)2 were first generated on the copper substrate in a solution of NaOH and (NH4)2S2O8; and then Cu(OH)2 nanorod arrays were formed based on copper substrate. At last, Cu(OH)2 nanorods transformed to Cu(OH)2 nanotubes and CuO nanoslices. Synchronously, flower-like CuO microspheres were deposited from the bulk solution of NaOH and (NH4)2S2O8 during the growth of nanorods. So that, the Cu(OH)2 nanorod quasi-arrays/CuO microflower hierarchical structures were formed. The reaction mechanism of Cu in the mixed solution of NaOH and (NH4)2S2O8 was suggested as follows:The formation of Cu(OH)2 micro- and nanostructures on copper surfaces involves inorganic polymerization (poly-condensation) reactions under alkaline and oxidative conditions. At first, a corrugated sheet is formed by edge sharing of the distorted Cu-O octahedron which is formed from Cu2+ and OH-. Then the sheets stack together through hydrogen bonds forming Cu(OH)2 particles and rods. However, under highly basic condition, the interlayer H-bond linkage at the sheet edges can be weakened, causing stresses in the layers. Therefore, the sheets are rolled so as to relieve the stresses, resulting in the formation of tubular structures. In fact, the corrugated layer structure of Cu(OH)2 is unstable against oxolation because oxygen atoms are either pentacoordinated or tricoordinated. At a reaction time of more than 3 h, the Cu(OH)2 tubes are transformed to CuO slices, owing to the cleavage of the interplanar hydrogen bonds.The wettability of the rough surface on copper substrates was changed from surperhydrophilicity to superhydrophobicity by chemical modification with stearic acid (STA) or 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PDES). The static contact angles (CAs) for water on both of the modified surfaces were larger than 150o, which was closely related to the chemical modification and rough structure of the surfaces. Compared with the STA-modified surface, the PDES-modified surface had a lower CA hysteresis (CAH) and adhesion for water droplets, possibly due to the lower surface free energy of PDES than STA.2. Three dimensional (3D) honeycomb-like porous structures made up of nanoslices of hydroxy cupric phosphate heptahydrate (Cu8(PO3OH)2(PO4)4·7H2O) single crystals were constructed by a facile assistant surface oxidation technology. The pore size of the 3D structure ranges from hundreds of nanometers to two micrometers, and the thickness of the two dimensional (2D) nanoslices is about 50~100 nm. The wettability of the porous surfaces was transformed from superhydrophilic to superhydrophobic by chemical modification with octadecanethiol (ODT) or PDES. It was found that the 3D porous structures of the surfaces helped to amplify the wettability. The static CAs for water on both of the modified surfaces were larger than 160o. Compared with the surface modified with ODT, the PDES-modified surface has lower CAH for water droplets.3. Cu2(OH)3NO3 microslice arrays were self-organized on copper foils via a facile solution-immersion method. The growth process has been carefully investigated. It was found that the growth of the Cu2(OH)3NO3 microslice arrays experienced a intermediate stage of Cu2O microcrystals. The wettability of the as-prepared surfaces was transformed from superhydrophilic to superhydrophobic by chemical modification with PDES. After being modified with PDES, the static CA for water was higher than 160o and the sliding angle was lower than 5o.The above procedure is fairly facile to operate and no special technique or equipment is required. Moreover, the procedure is time-saving and the reagents are cheap, while as-fabricated superhydrophobic surfaces are very stable even in a long term preservation. Hopefully, this process could be used to prepare a series of novel multifunctional materials with potential industrial applications such as corrosion prevention, drag reduction, self-cleaning and so forth.