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Dip-coating for dodecylphosphonic acid derivatization on aluminum surfaces: an easy approach to superhydrophobicity.

Abstract An easy approach of rapid dip-coating to superhydrophobic aluminum (Al) surfaces is reported. Dodecylphosphonic acid (DDPA) layers were formed on periodic micro-column-patterned Al surfaces by a rapid dip-coating in a 2 mM DDPA solution in trichloroethylene. It was demonstrated that DDPA layers were derived on Al substrates in a couple of seconds and superhydrophobic Al surfaces were achieved with water static contact and sliding angles of 153[degrees] and 5[degrees], respectively. Atomic force microscope and X-ray photoelectron spectroscopy (XPS) measurements showed that multilayer and island structural DDPA layers were formed on Al substrates. XPS analyses demonstrated that DDPA headgroups were connected with Al by P-O-Al bonds which contributed to the excellent durability of the superhydrophobic Al surfaces.

Keywordss Superhydrophobicity, Dodecylphosphonic acid (DDPA), Dip-coating, Self-assemble


Extensive attention had been paid to superhydrophobic Al surfaces due to their potential applications in architecture, passivation layers of electronic elements, and so forth. (1-6) Since the blank Al surfaces are intrinsically hydrophilic, special surface pattern and chemical modification are necessary to make them superhydrophobic. In the surface roughing step, several electrical methods such as anion exchange method, anodization combined with low-temperature plasma treatment were adopted to create micro/nano-scale hierarchical structures, (7,8) and chemical etch techniques were carried out in various etchants such as NaOH solution and dislocation etchant to prepare biomimetic surfaces. (9,10) Methods mentioned above have the advantages of low cost and commercialization. However, when accurate structure size is needed, especially in the research phase, these processes are quite difficult to control. Lithography is an optimal choice for preparing certain surface patterns which will benefit the fundamental study of the impact of surface structure on the wettability. In the surface modification step, widely low-surface-energy materials such as fluoroalkvl silane, (11,12) teflon, (13) perfluorooctyltriethoxysilane, (14) and other materials were used to achieve superhydrophobicity combined with functions such as anti-icing and anticondensation. (15) However, the adhesion between Al and these materials is limited and needs to be improved by special processes. Hence, extensive attention has been drawn to studies on the bonding mechanism between Al and the low-surface-energy coatings. (16)

We obtained superhydrophobic Al surfaces via dodecylphosphonic acid [DDPA, [CH.sub.3][([CH.sub.2]).sub.12]P(O) [(OH).sub.2]] monolayer derivatization and found that DDPA was connected to Al mainly by bidentate P-O-Al bonds. (17) The strong bonds between DDPA and Al lead to excellent durability and provide huge potential applications of superhydrophobic Al surfaces. However, the rinsing process during self-assembly monolayer derivatization would increase the cost. (18) Therefore, canceling the following rinsing process under the precondition of superhydrophobicity has advantages of lower cost and better convenience. In the present work, we obtained superhydrophobic Al surfaces via two steps of (1) creating a micro-column pattern (2) dip-coating with DDPA solution without any rinsing process. Atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) measurements showed multilayer and island structural DDPA layers derivatization. Water static contact and sliding angles of superhydrophobic Al surface were 153[degrees] and 5[degrees], respectively.


Materials and sample preparation

Semiconductor grade single-crystal n-type Si (100) wafers were cut into 15 x 15 [mm.sup.2] coupons. After ultrasonic cleaning in acetone for 15 min followed by sequentially rinsing with deionized water (18.2 M[OMEGA]; MilliQ), these coupons were dried in a dry [N.sub.2] stream and treated by a 5 min UV/ozone to remove residual organics.

SU-8 photoresist was spin-coated onto the cleaned Si surfaces and patterned by micro-column array. The period of the column array is 25 pm, and the diameter and height of each column are 16 and 19 [micro]m, respectively. Detailed processes are described in references (19) and (20). Approximate 100 nm Al layers were deposited onto SU-8 surfaces by RF magnetron-sputtering to obtain micro-column-patterned Al surfaces. The Al surfaces were cleaned sequentially with methanol and deionized streams and treated by a 5 min UV/ozone to remove residual organics and to oxidize the surface to a depth of about 3 nm simultaneously. The treated Al surfaces were used immediately to DDPA derivatization to avoid contamination which will weaken or even block the interaction between the DDPA headgroups and the Al surfaces.

Crystalline DDPA powder (from Alfa Aesar, 93% purity, Ward Hill, MA) was heated to 100[degrees]C to eliminate moisture prior to use. In the derivatization process, the micro-column-patterned Al surfaces were immersed into a 2 mM DDPA solution for a fast dipcoating of 3 s and dried in a [N.sub.2] stream without any rinsing process. The process differs from the authors' previous work (17) in which the micro-column-patterned Al surfaces were immersed into the DDPA solution for 5 min and followed by successive alcohol and deionized water rinsing and a final [N.sub.2] stream drying. The main difference between the present and the previous work was the lack of rinsing process which would result in different layer structures demonstrated below.

Surface characterization

Scanning electron microscopy (SEM) and AFM were conducted to characterize the morphology of Al, SU-8, and DDPA surfaces, respectively. In the case of AFM characterization, since it is difficult to measure surfaces with micrometer scale roughness, we used "flat" Al and DDPA surfaces as control samples, i.e., flat (pattern-free) SU-8 layers covered by Al and DDPA layers. The images were obtained by dynamic force mode AFM at scan speed of 5 [micro]m/s. The data points in each image were 256 x 256 obtained in an area of 2 x 2 [micro][m.sup.2]. The AFM was carried out in an ambient with relative humidity of 50%.

XPS measurements

XPS (Kratos AXIS ULTRA) was carried out in an ultrahigh vacuum chamber (base pressure below [10.sup.-9] mbar) equipped with monochromatic Al [K.sub.[alpha]] radiation driven by 15.0 kV and 150 W. Samples were grounded to prevent charging and charge compensation was also applied. The survey scans were conducted in an energy range of 0-1100 eV with pass energy of 80 eV, while the core level single spectra were collected with pass energy of 20 eV and acquisition time of 120 s. The binding energy was calibrated by the contaminated [C.sub.1,s] v photoelectron peak (achieved from blank cleaned Si surface) at 284.7 eV. The deconvolution of high resolution XPS peaks has been done by mixed Gaussian-Lorentzian fitting after Shirley background subtraction. (21) Curve fitting and analyses were performed by using the standard CASA XPS and XPSPEAK processing softwares.

Surface wetting properties characterization

Water contact angle (WCA) and water sliding angle (WSA) measurements were done with a Rame'-Hart's Model 100-00 Digidrop contact angle measurement (CAM) system. All CAMs were carried out with deionized [H.sub.2]O (18.2 M[OMEGA], MilliQ) at ambient of 75-90% relative humidity and 23-25[degrees]C. The volume and diameter of the water droplet used were about 5 [micro]L and 1.1 mm, respectively. On each sample, at least 3 spots were tested, and the averaged WCAs were adopted. All contact angles have an absolute error or uncertainty of [+ or -] 3[degrees]. The CAMs were done immediately and a few months after the coating and drying for the fresh and the aged surfaces, respectively. The mechanical durability of the prepared superhydrophobic Al surfaces was tested by cycled pressure water flushing. In each cycle, the samples were continuously flushed for 5 to 10 min with 0.1 mPa waterspout and dried at 80[degrees]C for 1 h followed by WCA and WSA measurements.

Results and discussion

Surface morphology

In the micrometer scale, SU-8, Al/SU-8, and DDPA/ Al/SU-8 surfaces showed no obvious difference in SEM images. For simplicity, Fig. 1 only shows the images of (a) DDPA/Al/SU-8 surface and (b) Al/SU-8 profile. The photo-etched SU-8 surface exhibited a micro-column array with a period of 25 [micro]m. The diameter and height of each column were 16 and 19 pm, respectively. The profile image clearly showed the interface of Al and SU-8 layers, and the thickness of Al layer could be measured to be about 110 nm. Since the thicknesses of Al and DDPA layers were relatively small, coating of Al and DDPA layers did not change the morphology of SU-8 surfaces. Hence, the final samples could be regarded as micro-column-patterned Al surfaces with DDPA modification.


Figures 2a and 2b are AFM images of the surfaces of Al and DDPA/Al control samples, respectively, and c is the corresponding sections. The roughness of blank Al surface was about 3 nm, while that of DDPA/Al was about 4 nm and had about 14 nm height island structural microaggregates. Since the thickness of DDPA monolayer is about 2 nm, the morphology of DDPA/Al surface could be described as multilayer and island structures, which were further demonstrated by XPS measurements.

Surface chemical composition

Figure 3a shows the XPS spectra of Al/SU-8 and DDPA/Al/SU-8 surfaces. The spectrum of Al/SU-8 agrees well with that of blank Al reported. (22) In the spectrum of DDPA/Al/SU-8, the peaks of phosphorus (P) are clearly seen in the binding energy range of 175-200 eV and the DDPA derivatization on Al surface is thus identified. (22)

The [C.sub.1s] peak in Fig. 3b in the binding energy range of 284-289 eV was fitted with two components: a main state at 286.8 eV with a full width half maximum (FWHM) of 1.15 eV and a very low intensity tail at higher binding energy of 287.66 eV with FWHM of 0.71 eV. The two peaks are attributed to the presence of C-C and C=0 bonds, respectively. The locations and shapes of [C.sub.1s] peaks agree with the report of Irina Gouzman et al., (23) in which [C.sub.1s] line-shape of a multilayer octadecylphosphonic acid (ODPA. [C.sub.1s] in both of ODPA and DDPA has the same chemical environment) film is asymmetric and includes a main peak at 285.6 eV and a very low intensity tail al 286.8 eV. Hence, the peak in Fig. 3b indicates that the DDPA films were structured by an island multilayer rather than a monolayer. The position shift of the [C.sub.1s] line is due to the variation in film thickness. (23)

More than one solution of curve fitting existed when analyses of the [O.sub.1s] core level line in Fig. 3c were carried out. By comparison with references (5) and (6), a reasonable scheme was adopted in which [O.sub.1s] core level line was separated to three smaller peaks: the peak at the binding energy of 532.0 eV has an FWHM of 1.97 eV which represents the main contribution of P-O bonds; the peak at 532.65 eV, with the maximum intensity and an FWHM of 1.65 eV, consists of different bonding states including OH, Al oxide and others; and the peak at 533.9 eV with an FWHM of 2.00 eV was attributed to P=0 bonds. We demonstrated in our former work that the bonding configuration of phosphorus in the phosphonate headgroups was mainly bidentate in the case of DDPA monolayer derivatization on Al surfaces, i.e., contribution of P-O in [O.sub.1s] peak was relatively stronger than that of P=0. (17) However, in the present work, P=0 bonds presented in the peak at 533.9 eV (non-bonded to the surface) were obviously increased. The consequence of more P=0 states than that of P-O suggested the existence of multilayer and island structure of DDPA layer.

Figure 3d shows an Al metal peak at binding energy of 72.86 eV, as well as an [Al.sub.2][O.sub.3] peak at 73.69 e V. There is also a peak at 75.94 eV, which was attributed to the presence of native Al[O.sub.x]. The results agree well with the reported peak separation of 2.8 eV. (24,25) Aluminum oxide was essential to the condensation reaction between Al and phosphonate to form P-O-Al bonds.


Figure 3e shows the PZs peak. The observed P2y binding energy is 192.94 eV. The peak shape agrees well with reference (23) in which multilayer and bulk structures were reported, and the low binding energy peak at 180.99 eV was attributed to the Al plasmon loss.

The positions, FWHM, and atomic percent concentration (APC) are listed in Table 1. P was detected in the spectra which verified the presence of phosphonate molecules on Al surface. The APC ratio of [C.sub.1s]/[P.sub.2s] is 12.43, which agrees well with the report of Hoque et al. (24) Thus, the XPS data support the conclusion of DDPA multilayer and island structure chemically derived on Al surface which differed from the monolayer structure reported in reference (17). A schematic structure of DDPA multilayered surface with island microaggregates is shown in Fig. 4.

Surface wetting properties

The insert of Fig. 4 shows a WCA of 153 [+ or -] 3[degrees] of the DDPA derived and column-patterned Al surface. The WSA was approximately 5[degrees] (not shown). Meanwhile, for a DDPA derived flat Al surface, we detected a WCA of 113 [+ or -] 3[degrees]. Therefore, the superhydrophobicity of the Al surface is attributed to both the column pattern and the low-surface-energy material derivatization.

We found that the flat blank Al surface was hydrophilic and had a WCA < 10[degrees] immediately after the UV/ozone cleaning. The column-patterned Al surface also had a WCA < 10[degrees] after UV/ozone cleaning. It is because the surface energy was very high after the UV/ozone cleaning, so water wetted the whole surface and no air pockets left in the gaps among columns. Therefore, both the flat and rough blank Al surfaces have very high surface energy after UV/ozone cleaning. However, since the WCA on the flat blank Al film was very small (<10[degrees]), the roughness effect which follows the Wenzel equation was undetectable.

In order to survey the impact of aging on surface wetting properties, samples were simply placed in laboratory in ambient environment for 3 months without any special treatment. We found that aged flat Al surfaces had a larger WCA of 83[degrees], which was due to adsorption of hydrocarbons. The Wenzel equation predicts a WCA < 83[degrees] on the aged column-patterned Al surface. On the other hand, the Cassie-Baxter equation predicts a WCA > 90[degrees] in a metastable state. (26) Actually, the WCA we obtained of this aged columnpatterned Al surface was 140[degrees]. Thus, the water droplet was in the Cassie state. When we took the WSA into account, we found the water droplet stuck onto the surface even if the sample was turned around. This phenomenon was described as high hysteresis. (27,28) According to Balu et al., the hysteresis is a combination state of a Cassie state on the micro-scale and a Wenzel state on the nano-scale. (28) Despite the microcolumn pattern, there were nano-scale Al aggregates formed on the surfaces during the sputter process which contributed to the presence of the Wenzel state. (17)




In the case of DDPA derivatization, because of the low surface energy and hydrophobicity of DDPA surface, the effect of Wenzel state disappeared. As a consequence, only the micro-scale Cassie state dominated the surface performance. Hence, the water droplet (with WCA of 153 [+ or -] 3[degrees] and WSA of 5[degrees]) beading up on the freshly cleaned DDPA derived, column-patterned Al surface was in the Cassie-Baxter state, meaning that there were air pockets trapped among the columns.

Figure 5 shows the WCA of the superhydrophobic Al surface slightly increased from 156[degrees] to 157[degrees] in the first 5 min. From 5 to 45 min, the curves maintained smooth except a slight decrease from 156[degrees] to 155[degrees] at 30 min. After 45 min, the WCA decreased to 155[degrees]. Meanwhile, the WSA had a decrease from 5[degrees] to 3[degrees] in the beginning 15 min and maintained at 5[degrees] from then on. The curve fluctuation was corresponding to the changes of the DDPA layer during the flushing process. In the first 5 min, DDPA molecules were continuously removed except those that made up the bottom monolayer and the structure of multilayer and island transformed to monolayer simultaneously. As a consequence, the surfaces showed a higher WCA and a lower WSA. In the second cycle, from 5 to 10 min, DDPA molecules located at the edge of the columns which bonded relatively weaker to the Al surface were removed. Up to 15 min, all unstable factors were erased and the curves remained unchanged. The slight decrease of WCA at 30 min was tentatively attributed to measuring error since the WSA did not change at that time and the WCA remained unchanged at 20 and 35 min, respectively. Although the DDPA monolayer was likely to be slowly destroyed after 45 min, the durability of the superhydrophobic Al surfaces was great due to the tight chemical bonds between DDPA headgroups and Al.

The lithography used in present work for column pattern can be easily controlled and reproduced, which will benefit fabrication of novel surfaces, for example, anisotropic surfaces. Hence, there is significance for fundamental studies. The approach to superhydrophobicity by using organophosphonic acid as surface-energy control agent will be accelerated due to the easy process and strong adhesion to Al substrate.


In summary, periodic SU-8 photoresist micro-columns were used to fabricate Al patterned surfaces, and multilayer and island structural DDPA were conducted to control the surface energy of Al surfaces. The applicability of surface roughing and low-surface-energy reagent modification to superhydrophobicity had been demonstrated. Superhydrophobic Al surfaces had been achieved with WCA and WSA of 153[degrees] and 5[degrees], respectively.

Dip-coating used in this work has the benefit of low cost and convenience which has potential applications in self-cleaning field. Furthermore, the strong adhesion which comes from the chemical bonds of P-O-Al provide the system of Al and organophosphonic acid an opportunity in general use of superhydrophobic materials in more broad areas.

DOI: 10.1007/s11998-015-9729-5

Y. M. Hu, Y. Zhu (El), W. Zhou ([mail]), J. H. Yi, T. Shen

Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China


W. Zhou


Y. M. Hu

School of Engineering, Dali University, Dali 671003, China

H. Wang

Faculty of Mechanical Engineering, Kunming Metallurgy College, Kunming 650031, China

S. S. Xin, W. J. He

Kunming Institute of Physics, Kunming 650031, China

Acknowledgments The authors thank Dr. Dequan Yang for helpful discussion in XPS analyses. This work was partially supported by the National Natural Science Foundation of China (NSFC) under 21104028 and the 2014 Competitive Grant Program of Oversea Returnees Research Projects. The authors also acknowledge the support by the Natural Science Foundation of Yunnan Province of China under 2011FZ015 and 2011FZ051, Scientific Research Foundation of Dali University, and University-Enterprise Cooperation Pre-research Project of Kunming University of Science and Technology under Jinchuan 201211, and the young talents support program of Kunming University of Science and Technology.


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Table 1: Binding energy, FWHM of XPS peaks, and atomic percent
concentration (APC, at.%) of elements C, O, Al, and P. Elemental
ratio of [C.sub.1S]/[P.sub.2S] is also presented

Elements                      [C.sub.1s]               [O.sub.1s]

                        C-C      COOR       P-O       OH        P=O

Binding energy (eV)   286.53    287.66    532.00    532.65    533.90
FWHM (eV)               1.15      0.71      1.97      1.65      2.00
APC (at. %,            48.46                         28.14
  [+ or -]2%)

Elements                           [Al.sub.2p]

                      [Al.sub.2P]   Al[O.sub.x]   [Al.sub.2]

Binding energy (eV)        72.86         73.69        75.94
FWHM (eV)                   0.71          1.43         1.55
APC (at. %,                19.50
  [+ or -] 2%)

Elements                 [P.sub.2s]       [C.sub.1s]/

Binding energy (eV)   180.99    192.94
FWHM (eV)               7.06      2.75
APC (at. %,             3.90                   12.43
  [+ or -] 2%)
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Author:Hu, Y.M.; Zhu, Y.; Zhou, W.; Wang, H.; Yi, J.H.; Xin, S.S.; He, W.J.; Shen, T.
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Jan 1, 2016
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