PRL 100, 256102 (2008)
week ending
27 JUNE 2008
PHYSICAL REVIEW LETTERS
Influence of Nanotips on the Hydrophilicity of Metallic Nanorod Surfaces
D.-X. Ye* and T.-M. Lu
Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180-3590, USA
T. Karabacak
Department of Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas 72204, USA
(Received 9 November 2007; published 24 June 2008)
The hydrophilicity of vertically aligned metal nanorods with sharp nanotips were investigated
experimentally. Ruthenium and platinum nanorod arrays were deposited on flat silicon substrates using
oblique angle sputter deposition. We show that the effects of nanotips on nanorods should be considered in
the ‘‘hemiwicking’’ model for hydrophilic metallic samples. With the influence of nanotips, we successfully explained the experimental contact angles of water sessile drops on metallic nanorod surfaces. Our
experiments confirm that the shape of the nanorods is an important parameter in determining the
hydrophilicity of the nanostructured surfaces.
DOI: 10.1103/PhysRevLett.100.256102
PACS numbers: 68.08.Bc
The study of the liquid-solid interface thermodynamics,
including the energetics, chemistry, and structure of interfaces, with a focus on wetting phenomena, still attracts
considerable attention [1,2]. In particular, the contact angle
of a connected drop of liquid resting on a planar solid
surface has been related to the interfacial energies of the
surface [1]. The prediction of contact angles can be obtained from the consideration of a thermodynamic equilibrium between the three phases on the surface: the solid
phase of the substrate (S), the liquid phase of the droplet
(L), and the vapor phase of the ambient (V). We denote the
interfacial energy of the solid-vapor interface as SV , the
interfacial energy of the solid-liquid interface as SL , and
the interfacial energy of the liquid-vapor interface as LV .
Under thermodynamic equilibrium, minimization of the
total interfacial energy of this three-phase system yields
the classical Young law, cos0 SV SL =LV , for
the contact angle 0 [1– 4].
The equilibrium contact angle 0 determined by
Young’s law is experimentally observable only in an ideal
thermodynamic system under particular conditions such as
the homogeneity and flatness of the bounding surfaces [5].
In practice, however, the bounding surfaces may be rough
and may contain more than one element. Therefore, the
equilibrium contact angle measured on a real surface often
deviates from the prediction of Young’s law. There are two
models, Wenzel’s model [6] and the Cassie-Baxter model
[7], that have been developed to describe the contact angle
on a real surface.
Recently, superhydrophobic surfaces with a measured
contact angle > 150 have been realized by several researchers using different methods including material synthesis and surface patterning [8]. However, limited effort
has been made to achieve the superhydrophilic state of
solid surfaces with nanoscale or microscale roughness [9–
12]. Martines et al. fabricated superhydrophilic SiO2 pillars with different tip curvature using the combination of
e-beam patterning and plasma dry etching processes [9].
0031-9007=08=100(25)=256102(4)
Fan et al. reported the work of fabricating vertical aligned
Si nanorods using oblique angle electron evaporation technique with substrate rotation [10]. There was no documented study of the water wettability of metallic films with
nanoscale or microscale roughness, although there are
many important applications of such a film in hydrogen
production and storage, surface catalyst, and heat transfer,
etc.
In this Letter, we studied the water wettability of metallic nanorod surfaces created by oblique angle sputter deposition, which is similar to the technique used by Fan et al.
[10]. In our experiments, Ru and Pt cathodes (about
12.7 cm in diameter) and Si substrates were housed in a
high vacuum dc magnetron sputtering system. The system
was maintained at an Ar pressure of 2.0 mTorr during the
deposition. The sputtering power used was 200 W. As such,
the deposition rates of Ru and Pt nanorods were measured
to be 4:0 0:2 and 9:2 0:1 nm= min , respectively. The
Si substrates were mounted on a stepper motor with a
distance as large as 15 cm measured from the center of
the cathode. The rotation speed was set to 30 revolutions
per minute for all the experiments. The deposition flux
arrived at the Si substrates at an angle of 85 with respect
to the substrate surface normal. With this oblique incident
flux and the substrate rotation, the deposited materials on
Si substrates formed a porous columnar structure in the
nanometer scale, or ‘‘nanorods,’’ due to the shadowing effect and limited surface diffusion. The individual
metallic nanorods fabricated using our method have been
shown to possess a single-crystal structure [13]. A clear
pyramidal-shaped nanotip was also formed on the top end
of the nanorod as depicted by scanning electron microscopy (SEM) [13]. For example, in Fig. 1, we presented the
high resolution SEM micrographs of selected Ru and Pt
nanorods with a nanotip.
A wide range of nanorod heights were covered for water
wettability studies, from a few nanometers to about 500 nm
for Ru nanorod arrays and about 800 nm for Pt nanorod
256102-1
© 2008 The American Physical Society
PRL 100, 256102 (2008)
week ending
27 JUNE 2008
PHYSICAL REVIEW LETTERS
FIG. 1. Scanning electron microscopy (SEM) images of short
(a) ruthenium (Ru) nanorods and (b) platinum (Pt) nanorods. The
insets are the cross-sectional images of the Ru and Pt nanorods.
The tall nanorods in (c) and (d) have a sharp apexlike nanotip.
The scale bars of the SEM micrograph are 100 nm.
arrays, as listed in Table I. The nanorods grow with the
deposition time and the pyramidal-shaped tips develop and
grow as well. We observed that, however, the shape of the
tips are similar to each other as they grow; that is, the facets
of the tips self-aligned to a specific crystal plane due to the
evolution of the single-crystal texture [13]. The water
contact angle measurements were carried out following
the commonly used sessile drop method. In our experiments, a drop of 5 l deionized water was deposited on the
surface of our nanostructured samples. The measured contact angles on the two types of metallic nanorods (Ru and
Pt) demonstrated that the contact angles strongly depend
on the morphology of the nanostructured surfaces. For
short deposition time, the surface resembles a rough sur-
face [Figs. 1(a) and 1(b)], but for long enough deposition
time, the surface should be treated as a heterogeneous
surface. Thus, the behavior of the water contact angles
follows different models.
In the case of rough surfaces, as shown in Fig. 2(a), the
experimentally measured contact angles can be related to
the equilibrium contact angles 0 by Wenzel’s law as
cos r cos0 [6], where the surface roughness factor r
is defined as the ratio of the total surface area of a rough
surface and the corresponding projected area 0 on a
horizontal plane, i.e., r =0 . The surface roughness
factor r can dramatically change the contact angles on a
solid substrate [14,15].
In the case of heterogeneous surfaces and porous surfaces, the approach of Cassie and Baxter has good agreement with experimental results [7,15]. The Cassie-Baxter
theory assumes that a heterogeneous surface is composed
of patches with different surface energies. The observed
contact angle is a weighted average of the equilibrium
contact angles of all the patches with their surface area
concentrations as the weights. In a heterogeneous system
with two components i and j, the fraction of the surface is
assumed to be i and j with i j 1. The equilibrium contact angles of ideal flat surfaces Si and Sj are 0i
and 0j , respectively. The Cassie-Baxter theory gives the
contact angle of the two-component system as [7,15]
arccosi cos0i j cos0j :
(1)
In general, a porous surface can be treated as a composite
surface either with solid and air for a hydrophobic surface
or with solid and liquid for a hydrophilic surface. For a
rough hydrophilic surface, 0i 0 and i for the
flattop solid part, and 0j 0 for the water part; thus, the
contact angle given by Eq. (1) is
arccos cos0 1:
(2)
TABLE I. The average height H, diameter a, and separation d of Ru and Pt nanorods measured from the SEM images.
Sputter time t (min)
Height H (nm)
10
20
25
30
60
90
120
45:2 2:1
86:6 3:7
105:7 4:5
121:6 6:2
252:5 3:7
370:2 9:6
480:6 9:6
5
8
15
35
60
75
90
48:3 1:6
81:2 3:2
145:0 4:6
327:2 5:7
566:8 9:3
701:6 7:8
820:0 6:9
Diameter a (nm)
Separation d (nm)
16:5 4:3
36:1 5:3
31:1 4:4
46:1 5:9
56:7 13:2
96:0 10:5
114:1 20:7
22:4 3:5
40:4 7:1
36:3 6:8
50:3 5:8
65:5 12:1
129:2 19:3
288:5 40:8
25:2 4:1
32:7 4:8
55:1 5:7
125:8 12:3
145:8 13:6
162:3 15:0
205:1 27:7
30:0 2:8
41:2 3:7
67:5 4:5
166:7 13:5
203:5 13:3
264:9 14:3
350:6 37:2
Ru nanorods
Pt nanorods
256102-2
PRL 100, 256102 (2008)
PHYSICAL REVIEW LETTERS
FIG. 2. Schematics of (a) Wenzel’s law of water contact angle
on a rough surface and (b) ’’hemiwicking’’ model of water
contact angle on a hydrophilic surface with nanotips. The tips
are supposed to be ‘‘dry’’ outside of the sessile drop of water.
The contact angles of nanorod samples can be described using
Wenzel’s law when the nanorods are short. The contact angles
then follow the hemiwicking model as the nanorods grow to
longer ones.
Equation (2) can also be derived from the minimization of
the surface energy of the composite interface [16]. The
contact angles can be changed by the arrangement of the
surface fraction of the solid, as suggested by the CassieBaxter model. It is necessary to point out that Eq. (2) is
valid only for a smooth composite interface; modified
equations can be obtained for a geometrically clear defined
interface [9,15,17].
In our present work, the metallic nanorod arrays with
sharp tips were randomly arranged on the Si substrates
with a characteristic average separation d of adjacent nanorods [18]. Therefore, unlike the conventional flattop composite ‘‘hemiwicking’’ model [described by Eq. (2)], our
nanorod’s water wicking behavior is schematically shown
in Fig. 2(b). Here we assume that the surface of the tips is
smooth and the pyramidal surface remains dry. The total
pyramidal surface can be estimated from the SEM images
of the samples. We measured the average of diameter a and
separation d of the nanorods. The tilted angle of the
facets of the nanotips was measured with respect to a
horizontal plane from the cross-sectional SEM images.
The angle is about 44.5 for Ru nanorods and about
35.3 for Pt nanorods. We approximated the individual
nanorod as a pillar with a pyramidal-shaped apex and
obtained a2 =d2 and r 1= cos. Thus, Eq. (2)
should be modified to give
2 a cos0
1 1 :
(3)
arccos
d2 cos
In Fig. 3, we show the side view shape of water drops
deposited on Ru nanorod arrays with different heights of
the nanorods. The contact angle of a water drop on a
smooth Ru film was determined to be 0 65:9 1:5
in our experiment. From the CCD images, the contact
angle decreased dramatically from 0 to about 30 with
short Ru nanorods; the contact angle decreased slowly
thereafter. The same behavior of the change of contact
angles on Pt nanorod arrays has been observed in our
experiment. The contact angles were predicted a priori
using Wenzel’s law and Eq. (3) with the parameters of
the nanorods listed in Table I.
week ending
27 JUNE 2008
FIG. 3 (color online). Images of water sessile drops sit on Ru
nanorod samples with different rod heights. A drop of 5 l
deionized water was deposited on each samples for the measurement of contact angles. From (a) to (d), the height of Ru
nanorods increases while the measured contact angle of the
drops decreases. From (a) to (d), the nanorod height is about
45.2, 121.6, 252.5, and 370.2 nm, respectively.
The measured and predicted contact angles of the Ru
and Pt nanorod samples using Wenzel’s law and the hybrid
model are shown in Figs. 4(a) and 4(b), respectively. In the
first region in Fig. 4(a), the Ru nanorod arrays started to
grow from 0 to less than 100 nm in height and demonstrated a Wenzel-type wetting behavior; i.e., the measured
contact angles can be predicted using Wenzel’s law. From
the cross-sectional SEM images, the tips of the Ru nanorods did not develop clearly yet at this early growth stage.
The roughness factor r was thus estimated as r 1 aH=d2 , where H is the average height of the nanorods,
by assuming the shape of nanorods to be a cylinder with a
flat top. Similar behavior of the Pt nanorods was observed
and presented in the first region in Fig. 4(b). In this part, the
height of nanorods has an effect on the surface roughness
factor, as well as the diameter and the separation of the
nanorods. The growth of the diameter and separation of
nanorods are coherently related through the shadowing
effect. Therefore, the ratio a=d is roughly constant.
When the nanorods reach a critical height, Wenzel’s equation is no longer valid. Wenzel’s equation predicts a complete wetting state of the samples with relatively short
nanorods, about 100 nm for Ru samples and about
200 nm for Pt samples. However, our measurements
show a great difference from this scenario. In our case,
the decrease of the contact angles is gradual after Wenzel’s
region in the curves in Figs. 4(a) and 4(b). That is, the
wetting mechanism changes at a certain height.
The second part of the curves in Fig. 4(a) shows a nonflat
shape that is attributed to the influence of the development
of the nanotips on top of each nanorod. We could calculate
and predict this part of the contact angles using the abovementioned hybrid model of hemiwicking as given by
Eq. (3). The parameters for the calculation were listed in
Table I and no adjustable parameter was used. The calcu-
256102-3
PRL 100, 256102 (2008)
PHYSICAL REVIEW LETTERS
week ending
27 JUNE 2008
This work is financially supported by National Science
Foundation under Grant No. NIRT-0506738. We thank
Meral Reyhan, Milan Begliarbekov, and Li Li for their
assistance in the contact angle measurements. We also
thank Dr. Gwo-Ching Wang of Rensselaer Polytechnic
Institute for valuable suggestions.
FIG. 4. Measured contact angles on (a) Ru nanorods and
(b) Pt nanorods with varying nanorod height H. The heights of
the nanorods were measured from the SEM cross-sectional
images of corresponding samples. The contact angles cannot
be predicted by a single model. Specifically, the experimental
data of contact angles were predicted using Wenzel’s law for
short nanorods and hemiwicking model for long nanorods.
lated results show an excellent agreement with the measured contact angles. In Eq. (3), there is no parameter
height H and it has no effect on the change of contact
angles. Instead, the change of the contact angles depends
on the average diameter a and separation d of the nanorods, as well as the shape of the growing nanotips. We also
presented the predicted curve in Fig. 4(b) for the Pt nanorod samples. Finally, a small contact angle was measured
for large heights in our experiment, indicating a possible
superhydrophilic state of the Ru and Pt nanorod arrays.
However, this state is retarded by the development of the
nanotip of metallic nanorods. A significantly tall nanorod
has to be grown in order to achieve the complete wetting of
metallic nanorod surfaces.
In conclusion, we have demonstrated that Ru and Pt
nanostructured surfaces can be wetted by water if the surface is engineered to consist of tall nanorod arrays. The
measured contact angles on these surfaces depend greatly
on the morphology of the nanorods. Even the pyramidshaped tips of the nanorods play an important role in the
wettability of the metallic nanorod arrays. However, one
should be aware that not all metallic surfaces can be
tailored to be hydrophilic by growing nanorod arrays [19].
*[email protected]
[1] P. G. de Gennes, Rev. Mod. Phys. 57, 827 (1985).
[2] A. W. Adamson and A. P. Gast, Physical Chemistry of
Surfaces (Wiley-Interscience, New York, 1997); Contact
Angle, Wettability and Adhesion, edited by K. L. Mittal
(VSP, Utrecht, The Netherlands, 1993).
[3] T. Young, Philos. Trans. R. Soc. London 95, 65 (1805).
[4] G. Alberti and A. DeSimone, Proc. R. Soc. A 461, 79
(2005).
[5] R. Finn, Equilibrium Capillary Surfaces (Springer-Verlag,
New York, 1986).
[6] R. N. Wenzel, Ind. Eng. Chem. 28, 988 (1936); J. Phys.
Colloid Chem. 53, 1466 (1949).
[7] A. B. D. Cassie and S. Baxter, Trans. Faraday Soc. 40, 546
(1944).
[8] Z. Wang, L. Ci, P. M. Ajayan, and N. Koratkar, Appl.
Phys. Lett. 90, 143117 (2007); D. Quéré, Rep. Prog. Phys.
68, 2495 (2005), and the references therein.
[9] E. Martines, K. Seunarine, H. Morgan, N. Gadegaard,
C. D. W. Wilkinson, and M. O. Riehle, Nano Lett. 5,
2097 (2005).
[10] J.-G. Fan, X.-J. Tang, and Y.-P. Zhao, Nanotechnology 15,
501 (2004).
[11] G. McHale, N. J. Shirtcliffe, S. Aqil, C. C. Perry, and M. I.
Newton, Phys. Rev. Lett. 93, 036102 (2004).
[12] J. Bico, C. Tordeux, and D. Quéré, Europhys. Lett. 55, 214
(2001); J. Bico, U. Thiele, and D. Quéré, Colloids Surf. A
206, 41 (2002).
[13] T. Karabacak, A. Mallikarjunan, J. P. Singh, D.-X. Ye,
G.-C. Wang, and T.-M. Lu, Appl. Phys. Lett. 83, 3096
(2003); F. Tang, T. Karabacak, P. Morrow, C. Gaire, G.-C.
Wang, and T.-M. Lu, Phys. Rev. B 72, 165402 (2005); P.
Morrow, F. Tang, T. Karabacak, P.-I. Wang, D.-X. Ye,
G.-C. Wang, and T.-M. Lu, J. Vac. Sci. Technol. A 24, 235
(2006).
[14] T. Onda, S. Shibuichi, N. Satoh, and K. Tsujii, Langmuir
12, 2125 (1996).
[15] D. Quéré, Physica (Amsterdam) 313A, 32 (2002).
[16] J. Bico, C. Marzolin, and D. Quéré, Europhys. Lett. 47,
220 (1999).
[17] M. Nosonovsky and B. Bhushan, Microsystem
Technologies 11, 535 (2005).
[18] F. Tang, T. Karabacak, L. Li, M. Pelliccione, G.-C. Wang,
and T.-M. Lu, J. Vac. Sci. Technol. A 25, 160 (2007); C.
Yang, U. Tartaglino, and B. N. J. Persson, Phys. Rev. Lett.
97, 116103 (2006).
[19] M. E. Abdelsalam, P. N. Bartlett, T. Kelf, and J. Baumberg,
Langmuir 21, 1753 (2005); N. J. Shirtcliffe, G. McHale,
M. I. Newton, and C. C. Perry, Langmuir 21, 937 (2005).
256102-4