Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is © The Royal Society of Chemistry 2015
Supporting Information
Superhydrophobic CuO Nanoneedle-covered Copper Surfaces for Anticorrosion Feng Xiao,a Shaojun Yuan,a,* Guanqiu Li,b Bin Liang,a Simo O Pehkonenc TieJun Zhangb,* a
Multi-phases Mass Transfer & Reaction Engineering Lab, College of Chemical Engineering, Sichuan University, Chengdu 610065, China b Department of Mechanical and Materials Engineering, Masdar Institute of Science and Technology, PO Box 54224, Abu Dhabi, United Arab Emirates c Department of Environmental Sciences, University of Eastern Finland, 70211 Kuopio, Finland
*To whom all correspondence should be addressed Tel: +86-28-85990133, Fax: +86-28-85460556 E-mail:
[email protected] (S.J. Yuan)
[email protected] (T.J. Zhang)
S.1. Results and Discussion S3.1. Effect of Electrolyte Concentration. Figure S1 shows the typical SEM images of the CuO nanostructured surfaces obtained by varying the concentration of the KOH solution over 0.5 – 4.0 mol·L-1. The surface morphologies of the CuO nanostructures are found to be strongly dependent on the KOH concentration. At a lower KOH concentration of 0.5 mol·L-1, the anodized copper surface shows pine-like needles with an average length of about 4 – 6 μm and the average diameters of sharp needle tips of about 120 ± 25 nm (Figures S1a and S1b). With increasing the KOH concentration to 2.0 mol·L-1, the average length of nanoneedles increases noticeably to about 7 – 15 μm and the sharp tip of the nanoneedle increases to 170 ± 40 nm in average diameters (Figures S1c and S1d). The amount of CuO nanoneedle array films also appears to be markedly increased and becomes more compact. However, upon increasing the KOH concentration to 3.0 mol·L-1, it is surprising to find that the CuO nanoneedles are disappeared and scroll-like nanorods are formed on the copper surfaces (Figures S1e and S1f). The CuO nanorods are 340 ± 60 nm in an average diameter and about 3 – 5 μm in average length. Further increasing the KOH concentration to 4.0 mol·L-1, the copper surface is covered with dense cuboid nanoparticles (Figures S1g and S1h). The above results are consistent with the fact that the CuO nanoneedle arrays (NNA) are preferably grown on the copper surface under the KOH concentration lower than 2.0 mol·L-1. The orthorhombic Cu(OH)2 nanostructures have been proven unable to form at high NaOH concentration owing to the favorable formation of cuprous oxide (Cu2O) and cupric oxide (CuO).1 Thus, the KOH concentration of 2.0 mol·L-1 is chosen for the subsequent anodization fabrication.
Fig. S1 Representative SEM image at low (×5000) and high (×30000) magnifications of the CuO nanostructured surfaces after anodizing in different KOH concentrations: (a) 0.5 mol·L-1, (b) 2.0 mol·L-1, (c) 3.0 mol·L-1, and (d) 4.0 mol·L-1 at a constant current density of 2.0 mA·cm-2 at 15 ± 1oC for 25 min.
S3.2. Effect of Reaction Temperature. The growth of CuO nanostructures on copper foil was further investigated at various reaction temperature ranging from 5 ± 1oC to 25 ± 1oC, and the representative SEM images are shown in Figure S2. The anodization reaction was carried out in a 2 mol·L-1 KOH solution at a constant current density of 2.0 mA·cm-2 for 25 min. The reaction temperature seems to have little effect on the nanoneedle morphologies, but significantly influence the size and amount of CuO nanoneedle arrays on the copper surface. The average length of CuO nanoneedles increases from about 2 – 5 μm at 5 ± 1oC to 10 – 15 μm at 25 ± 1oC, and the average diameters of the sharp tip of nanoneedles also increase with reaction temperature. On the contrary, the number density of CuO nanoneedle arrays shows an evident decrease with increase in reaction temperature, indicating that the lower temperature faciliates the crystal nucleation to form moare CuO nanoneedles. Taking account of the effect of reaction temperatue on the size and number density of CuO NNA, the reaction temperature used for subsequent anodization process is 15 ± 1oC.
(a)
5.0 kV ×10,000
5 μm
(b)
5.0 kV ×10,000
5 μm
(c)
5.0 kV ×10,000
5 μm
Figure S2 Representative SEM image (×10,000) of the CuO nanostructured copper surface after anodizing in a 2.0 mol·L-1 KOH concentrations at a constant current density of 2.0 mA·cm-2 for 25 min at different temperatures: (a) 5 ± 1 oC, (b) 15 ± 1oC and (c) 25 ± 1oC
S3.3. Effect of Current Density. The electrochemcial anodization of copper foil in a 2 mol·L-1 KOH solution was performed at a current density range of 0.5 – 4.0 mA cm-2 at 15 ± 1oC for 25 min. Figure S3 shows the representative SEM images of the CuO nanostructured surfaces after anodizing at different current densities. At the low current density of 0.5 mA cm-2, black irregular nanoparticles appears to grow on the copper surface (Figure S3a), indicating that the amount of as-formed Cu(OH)2 is not sufficient for the crystallization process.2 It has been reported that electrochemical anodization of copper in an alkaline solution forms a porous Cu2O layer first, and then Cu(OH)2 or CuO films on it. Upon increase the current density to 2.0 mA cm-2, the copper surfaces are completely covered with CuO NNA films (Figure S3b). Further increasing the current density to 4.0 mA cm-2, the number density of CuO NNA is increased due to the increase in nucleation rate, while the length of CuO nanoneedles seems to be decreased (Figure S3c). These results suggest that the increase in current density faciliates the rapid formation of nanoneedle arrays on the copper surfaces. Thus, the current density of 2.0 mA cm-2 is used for the subsequent anodization reaction of copper foils to grow CuO NNA films.
(a)
5.0 kV ×5,000
10 μm
(b)
5.0 kV ×5,000
10 μm
(c)
5.0 kV ×5,000
10 μm
Figure S3 Representative SEM image (×5000) of the CuO nanostructured copper surface after anodizing in a 2.0 mol·L-1 KOH concentrations at 15 ± 1oC for 25 min at a constant current density of (a) 0.5 mA·cm-2, (b) 2.0 mA·cm-2, (c) 4.0 mA·cm-2
(a) O 1s
Oxide
Binding energy (eV)
Hydroxide H2O
528
530
532
534
(b) C 1s
C-H
C-O
282
284
286
O=C-O
288
290
Binding energy (eV) Figure S4 (a) O 1s and (b) C 1s core-level XPS spectra of the CuO NNA-1 surface
Intensity (Arb. Units)
•
• Cu
•
•
•
20
40
60
80
2-Theta (degree) Figure S5 XRD pattern of the pristine Cu substrate
100
120
(a)
(a1)
5.0 kV ×2000
(c)
CA = 51o
20 μm
CA = 113o
(b)
(b1)
5.0 kV ×10,000
5 μm
(d)
Figure S6 Representative SEM images of the pristine Cu substrate surfaces at the low (a, ×2000) and high (b, ×10,000) magnifications. The Insets (a1) and (b1) represent the static water contact angle and optical images of water droplets on the pristine Cu substrates. Images (c) and (d) represent the static water contact angle and optical images of water droplets on the FAS-modified bare Cu surfaces.
Water contact angle (degree)
170
165
160
155
150
0
1
2
3
4
5
NaCl concentration (wt.%) Figure S7. The changes in the static water contact angles of the CuO NNA-1-FAS surfaces as function of the concentration of NaCl solution.
(a)
5.0 kV ×5,000
10 μm
(b)
5.0 kV ×5,000
10 μm
(c)
5.0 kV ×10,000
5 μm
Figure S8 Top-view SEM images of the CuO NNA-1-FAS surfaces after exposure in a 3.5 wt.% NaCl solution for (a) 3 and (b,c) 7 days.
7
Pristine Cu CuO NNA-1 CuO NNA-2 CuO-NNA-1-FAS CuO-NNA-2-FAS
-223 -153 -173 -89 -112
b ca (mV/dec) -85 -114 -135 -94 -114 258 419 177 119 300
bab (mV/dec) 169 184 315 124 283 -250 -246 -226 -147 -134
Ecorrc (mV) -254 -227 -212 -151 -124 14.43 12.93 11.57 1.37 0.99
jcorr. (μA·cm-2) 19.58 10.15 9.11 1.26 0.66 1.804 1.88 1.64 8.48 17.81
Rpd (KΩ) 0.63 1.51 2.33 9.21 21.54 0.34 0.31 0.27 0.032 0.023
0.46 0.24 0.21 0.029 0.016
νe (mm·y-1)
‒ 10.40 19.82 90.51 93.14
ηf (%) ‒ 48.16 53.48 93.56 96.63
bc is the Tafel slope of the cathodic polarization curve, b ba is the Tafel slope of the anodic polarization curve, c Ecorr refers to the potential, at which the current reaches zero under polarization, d Rp refers to the polarization resistance, e ν denotes corrosion rate and is defined as (jcorr×K×Ew)/d×A, where K is a constant that defines the units for the ν, and A, d, and Ew are the coupon area, the density and the equivalent weight of copper, f η denotes inhibition efficiency, and is defined as (jo-jcorr)/jo×100%, where jo and jcorr refer to the corrosion current density of the pristine Cu and surface-modified Cu coupons in a 3.5% NaCl solution, respectively, g pristine Cu refers to a newly-polished copper coupons after pretreatment, h CuO NNA-1 and CuO-NNA-2 represent the CuO nanoneedle array surfaces obtained after anodizing for 25 and 40 min, respectively, in a 2 mol·L-1 KOH solution at a constant current density of 2 mA·cm-2 at 15 oC ± 1o, i CuO NNA-1-FAS and CuO NNA-2-FAS correspond to the fluoro-silanization of the CuO NNA-1 and CuO NNA-2 surfaces with FAS-17, respectively.
a
1
Time (days)
Pristine Cug CuO NNA-1h CuO NNA-2h CuO-NNA-1-FASi CuO-NNA-2-FAS
Samples
Table S1 Analysis of Tafel plots of the pristine Cu, CuO NNA, and FAS-modified coupons after 1 and 7 days of exposure in a 3.5% NaCl solution
Qdl
(a) Rs Rct
W Rs(Qdl[RctW]) Qf
(b) Rs
Cdl Rf Rct Rs(Qf[Rf(RctCdl)]) Qf
(c) Rs
Qdl Rf Rct Rs(Qf[Rf(RctQdl)])
Rs, resistance of the electrolyte solution, Cdl, capacitance of the electrical double layer (EDL), Qdl, constant phase element (CPE) of EDL, Rct, charge transfer resistance of EDL, Qf, constant phase element (CPE) of the surface film, Rf, resistance of the surface film
Figure S9. Equivalent electrical circuits (EEC) used for fitting the EIS spectra of (a) the pristine Cu coupons after 1 day of exposure, (b) the CuO NNA-1 and CuO NNA-2 coupons after 1 and 7 days of exposure, as well as the pristine Cu coupons after 7 days of exposure, and (c) the CuO NNA-1-FAS and CuO NNA-2-FAS coupons after 1 and 7 days of exposure in a 3.5% NaCl solution.
‒ 0.092 0.11 ‒ ‒
Cdl (mF) 1.63 ‒ ‒ 3.36 1.14
Y2·10-4 (Ω-1sn)
Qdl
0.60 ‒ ‒ 0.55 0.71
n1 0.062 ‒ ‒
W (Ω·cm2) ‒ 1.22 1.56 5.27 6.14
Rf (kΩ) Y2·10-4 (Ω-1sn)
Qf n2
‒
0.86 0.59 0.55 0.87 0.82
0.25 0.41 0.44 1.80 2.47
Rct (kΩ)
Pristine Cu 9.21 0.41 0.61 ‒ ‒ ‒ 0.35 7.25 CuO NNA-1 8.17 0.47 0.14 ‒ ‒ ‒ 0.43 7.33 CuO NNA-2 7 8.48 0.43 0.66 ‒ ‒ ‒ 0.85 6.41 CuO-NNA-1-FAS 6.97 1.69 ‒ 0.59 0.69 ‒ 4.42 1.67 CuO-NNA-2-FAS 10.27 1.91 ‒ 0.47 0.76 ‒ 5.49 1.10 a EIS spectrum of the pristine Cu coupons after 1 day of exposure in the 3.5% NaCl solution was fitted with EEC (a) in Figure S6, b EIS data of the pristine Cu coupons after 7 days of exposure and the CuO NNA coupons were fitted with EEC (b) in Figure S6, c EIS data of the FAS-modified CuO NNA coupons after 1 and 7 days of exposure were fitted with EEC (c) in Figure S
7.56 8.68 7.08 8.39 9.68
Rs (Ω) ‒ 0.61 0.70 0.89 0.85
1
Time (days) ‒ 6.50 2.73 1.40 0.93
Pristine Cua CuO NNA-1b CuO NNA-2b CuO-NNA-1-FASc CuO-NNA-2-FASc
Coupons
Parameters
2.14 3.19 7.93 0.64 0.33
2.82 4.69 1.25 3.85 2.23
∑χ2·10-2
Table S2 Fitted parameters of the EIS spectra of the pristine Cu, CuO NNA, and FAS-modified coupons after 1 and 7 days of expsoure in a 3.5% NaCl solution
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