Highly ordered nickel and silver nanorods arrays prepared by alumina template assisted electrodeposition were investigated to determine the effect of the array geometry on metal surface hydrophobicity and adhesion forces. The nanorod geometry, clustering and pinning were used to characterize surface hydrophobicity and its modulation. A contribution of metal crystallographic orientation to the surface energy was calculated. To characterize nanorod array surface properties and elucidate the source of the particle adhesion effects has been calculated. The dispersive components of surface tension γ <sub>S</sub><sup style="margin-left:-6px;">D</sup> and surface polarizability k s, as surface features that markedly characterize hydrophobicity and adhesion, were calculated. The highest hydrophobicity was found for Ag nanorods with aspect ratio of 10 then Ni nanorods with aspect ratio 10. The same geometry of nanorods particles resulted in different surface hydrophobicity and it was ascribed to the orientation of Ag and Ni crystals formed on the top of nanorods. Due to high hydrophobicity nanorod array surfaces could be used as an antifouling surface in medicine to select areas on implant surface not to be colonized by cells and tissues.
Surface properties such as morphology are stable features of the surface, compared to chemical modifications which degrade over time [
cos θ r o u g h = r cos θ f l a t (1) [
where r is the roughness factor defined as ratio of the actual surface area to its horizontal projection for Wenzel model. For the alternative Cassie-Baxter model the apparent contact angle is described by (2) [
cos θ r o u g h = f s ( cos θ f l a t + 1 ) − 1 (2) [
where fs is the fraction of the solid surface in contact with the water droplet and is always less than one. A decrease of fs resulted in an increase of θrough, and gradually leads to a hydrophobic or superhydrophobic surface.
One method of increasing hydrophobicity of a surface by increasing the effective area is to coat it with a layer of nanorods. Considering nanorods as having a cylindrical shape as shown in
r = s + n ( 2 π ρ H r ) s = 1 + n ( 2 π ρ H r ) s (3) [
and
f s = n ( π ρ 2 ) s (4) [
where ρ is radius of the cylinder, Hr is cylinder height and s is the two dimensional surface area that the nanorods have grown on and n is number of nanorods in area s [
θ W = cos − 1 [ ( 1 + n ( 2 π ρ H r ) s ) cos θ y ] (5) [
θ C B = cos − 1 [ n ( π ρ 2 ) s ( cos θ y + 1 ) − 1 ] (6) [
The nanorods can be prepared using filler templates. The thin porous anodic alumina oxide (AAO) template derived nanowire/nanopillar arrays are highly-ordered and highly-oriented. They have well-tunable and precisely-controlled structural parameters, including shape, size, interspace, and density, which are all derived from the structural features of the original AAO template [
The polycrystalline nature of Ag and Ni nanorods introduces some variation in their morphology, including length, diameter, and shape [
The aim of the present study was to extend this work and prepare Ag and Ni nanorods by electrodeposition and to determine the influence of composition and of nanorod aspect ratio on their surface hydrophobicity. The crystalline structure of the materials will also be examined and compared with earlier studies.
All operations were performed at atmospheric pressure and 25˚C. All chemicals were p.a. grade. Ultrapure water with resistivity of 18.2 MΩm was prepared with Laboratory purification system (MRC, Ltd. UK). Electron micrographs surfaces were measured using a SEM JEOL JSM 7001F, (JEOL, Japan). Images of water drops were taken with EO 3112-C (Edmund Optics, USA) camera and water contact angles were analyzed by software ImageJ with plugin LB-ADSA. The surface area of nanorod top side was measured by using the measure-particles option in Image J. An Autolab PGSTAT 302N, (Metrohm, Netherlands was used to apply current pulses for metal deposition
Nanorod array silicon wafers were obtained from Brno University of Technology and were coated with a 200 nm tungsten layer and then with a 150 nm high-purity thin aluminum layer (99.99+%) by thermal evaporation (PVD). The thin porous AAO templates were prepared as described earlier [
Electrochemical pulse deposition was used to fabricate the silver nanorods onto nanoporous AAO templates, with either 50 or 500 nm rods. Deposition was performed from 0.2 mol/L AgNO3 (pH 2.5) at 60˚C in two steps each consisting of 20 deposition pulses with a current intensity of 1 mA and a duration of 2 s. The electrolyte was forced to circulate on the AAO template surface for 10 minutes before the pores of the membrane were filled. The current pulses were applied using an Autolab PGSTAT 302 N potentiostat with a standard two-electrode setup. A thin gold layer prepared by evaporation on the AAO membrane was used as a cathode and a platinum electrode as an anode. Finally, the surface was etched in aqueous solution containing 5 ml of 85% H3PO4 and 3 g CrO3 for 6 s at a temperature of 45˚C to remove the AAO.
Nickel nanorods were prepared by electrochemical deposition in modified Watts bath at 55˚C composed of 250 g/L NiSO4∙6H2O, 50 g/L NiCl2∙6H2O. The pH was adjusted with 34 g/L H3BO3 (pH 3.8) until nanorod growth was observed using the same pulse sequence.
Contact angles were measured by dropping water, glycerol, dimethylformamide (DMF) or dimethylsulfoxide (DMSO) onto the surfaces. The droplets were imaged with an EO-3112C colour USB camera (Edmund Optics, USA) and Image J software with plug-in LB-ADSA were used to determine the contact angles.
Silver and nickel coated nanorods were prepared by electrodeposition on AAO templates which were then dissolved. Compared with arrays synthesized by other methods, AAO template-directed nanowire/nanorod arrays are highly-ordered and highly-oriented. They have well-tunable and precisely-controlled structural parameters, including shape, height, size, interspace, and density, which are all guided from the structural features of AAO template. It was possible to accommodate the diameter of the nanorods to 50 (±2) nm and the height to be 50 (±3.5) nm or 500 nm in height, with a corresponding aspect ratios (AR) of 1 or 10, whilst keeping the spacing of the nanorods constant based on the template structure. SEM micrographs of the top surfaces of the array of silver nanorods (A, B) and nickel nanorods (C, D) (
pulled together due to effect of surface tension during drying (most obviously at 500 nm height). The images of the nanorods also showed that the nanorods were aligned parallel to each other. The average spacing between two adjacent rods was approximately 115 nm for Ni 50 and Ni 500 and 146 nm for Ag 50 and Ag 500 with an average nanorods length of 50 and 500 nm and an average diameter of 33 nm for Ni 50 and Ni 500 and 41 nm for Ag 50 and Ag 500. The nanorods density was determined to be 81 rods/µm2 in case of Ag 50 and Ni 50 and 75 rods/µm2 for Ag 500 and Ni 500.
The surface hydrophobicity of the nickel and silver high ordered nanorods surfaces was determined from the surface contact angles for four solutions (water, glycerol, DMSO, and DMF) (
Contact angle | |||||
---|---|---|---|---|---|
water | glycerol | DMF | DMSO | ||
Ag AR 10 | 150 | 112 | 10 | 10 | |
Ag AR 1 | 105 | 98 | 10 | 10 | |
Ni AR 10 | 105 | 110 | 10 | 30 | |
Ni AR 1 | 97 | 77 | 25 | 30 |
Using the solvent properties in
γ S D = ( γ L γ L D ( 1 + cos θ ) − k S γ L P γ L D 2 ) 2 (7)
γ L γ L D ( 1 + cos θ ) = 2 γ S D + k S γ L P γ L D (8) [
The values for Ag nanorods with aspect ratio (AR) 10 were higher than those with an aspect ratio 1 (
C (m−2) | γ (mJ∙m−2) | γ L D (mJ∙m−2) | γ L P (mJ∙m−2) | |
---|---|---|---|---|
Water | 1.3475 × 105 | 72.8 | 21.8 | 51 |
Glycerol | 1.9274 × 105 | 63.4 | 37 | 26.4 |
DMSO | 2.45 × 105 | 44 | 36 | 8 |
DMF | 2.5041 × 104 | 34.4 | 24 | 10.4 |
The surface polarizability (kS) can be derived from the regression equation y = a + xb, where b represents kS and γ S D could be calculated from a2/4 [
The experimental contact angles showed a good correlation with calculated values derived from Equations (1)-(4) (
The results for γ S D , polarizability, adhesion force and contact angle were compared to published values for randomly nanopatterned and highly ordered and oriented Ni and Ag surfaces [
S/mm2 | Θ/˚ | r | φs | fs | Wenzel | Adhesion force/pN | E/TPa | |
---|---|---|---|---|---|---|---|---|
Ag AR 1 | 5.59 | 105 | 1.0001 | 0.0000165 | 0.071 | 104 | 69.6 | 776 |
Ag AR 10 | 1.01 | 150 | 1.001 | 0.0015700 | 0.071 | 149 | 881 | 1005 |
Ni AR 1 | 6.41 | 97 | 1.0001 | 0.0000107 | 0.075 | 96 | 175 | 1.64 |
Ni AR 10 | 5.59 | 105 | 1.001 | 0.0000106 | 0.075 | 92 | 48 | 869 |
Type of prepared surface | WCA/˚ | γ S D /mNm−1 | Polarizability kS | Adhesion force/N | |
---|---|---|---|---|---|
nanostructured nickel | 300 s/−1.1 V | 66 | |||
silver dendrites 10 pulse | pulse time 0.1 s | 63 | 22.4 | 1.1 | |
0.3 s | 74 | 28.9 | 0.76 | ||
0.5 s | 57 | 32.83 | 1.16 | ||
0.7 s | 55 | 37,5 | −0.14 | ||
1 s | 80 | 40.1 | −0.44 | ||
silver dendrites 20 pulse | pulse time 0.1 s | 83 | 28.1 | 0.65 | |
0.3 s | 88 | 31.0 | 0.34 | ||
0.5 s | 105 | 35.5 | 0.14 | ||
0.7 s | 97 | 37.5 | −0.07 | ||
1 s | 111 | 45.4 | −0.41 | ||
silver dendrites 30 pulse | pulse time 0.1 s | 109 | 42.3 | −0.32 | |
0.3 s | 106 | 41.5 | −0.24 | ||
0.5 s | 100 | 33.9 | 0.05 | ||
0.7 s | 112 | 45.6 | −0.41 | ||
1 s | 118 | 47.3 | −0.61 | ||
silver nanorods | AR 10 | 150 | 60.0 | −1.32 | 7.81 × 10−10 |
AR 1 | 105 | 46.9 | −0.29 | 6.96 × 10−11 | |
nickel nanorods | AR 10 | 105 | 39.8 | −0.22 | 4.8 × 10−10 |
AR 1 | 97 | 34.7 | 0.04 | 1.75 × 10−10 |
XRD measurements of the Ag and Ni nanorods confirmed the polycrystalline structure of the surfaces (
The surface energy of the polycrystalline Ni and Ag nanorods was calculated based on crystallographic data with software VESTA (
Wulff projection has been applied and surface energy has been calculated in accordance with Equation (9) [
Crystal structure | a (nm) | Ec (eV) | Miller indices | Surface area (Å2) | γ (eV/Å2) | Weighted surf. energy γ ¯ (eV/Å2) |
---|---|---|---|---|---|---|
Ni | 0.3545 | 4.44 | (111) | 655.764 | 0.955 | 2.207 |
(200) | 213.649 | 0.935 | ||||
(220) | 79.978 | 1.110 | ||||
Ag | 0.4166 | 2.95 | (111) | 439.638 | 0.489 | 1.505 |
(200) | 138.844 | 0.531 | ||||
(220) | 171.809 | 0.394 | ||||
(311) | 50.368 | 0.527 |
γ ¯ = ∑ { h k l } γ h k l A h k l ∑ A h k l (9) [
where:
γ h k l —surface tension for selected plane.
A h k l —area of selected surface plane.
Highly ordered, well-aligned, Ni and Ag nanorod arrays could be prepared by templated electrochemical deposition and retain nanometer-scale features from the template. Wetting data showed that the most hydrophobic material was Ag surface with aspect ratio 10 and a polycrystalline structure with dominant face orientation (111). The surface hydrophobicity increased from Ni AR 1 to Ag AR 10 where the contact angle about 150˚ was measured. Ni nanorod surfaces had lower hydrophobicity and nanorods with a low aspect ratio had a reduced wetting. Thus surface energy decreased with an increase in planar density.
The combination of nanoscale surface geometry and crystallographic plane orientation features could provide important surfaces with modulated hydrophobicity, repellent effects to cells adhesion, and antimicrobial functions as well as self-cleaning processes. In further studies the authors plan to investigate molecular-based surface hydrophobicity and the impact of water molecule orientation.
This research has been financially supported by grant MŠ SR VEGA 1/0074/17, APVV-16-0029.
The authors declare no conflicts of interest regarding the publication of this paper.
Macko, J., Podrojková, N., Hrdý, R., Oriňak, A., Oriňaková, R., Hubálek, J., Vojtuš, J., Kostecká, Z. and Smith, R.M. (2019) Hydrophobicity of Highly Ordered Nanorod Polycrystalline Nickel and Silver Surfaces. Journal of Minerals and Materials Characterization and Engineering, 7, 279-293. https://doi.org/10.4236/jmmce.2019.75020