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Electrowetting Properties of ZnO and TiO2 Nanostructured Thin Films Evie L. Papadopoulou,*,† Alexios Pagkozidis,†,§ Marios Barberoglou,†,‡ Costas Fotakis,†,‡ and Emmanuel Stratakis†,§ Institute of Electronic Structure and Lasers-Foundation for Research and Technology–Hellas, P.O. Box 1385, GR-71110 Heraklion, Crete, Greece, Physics Department, UniVersity of Crete, Heraklion GR-71409, Greece, and Materials Science and Technology Department, UniVersity of Crete, Heraklion GR-71003, Greece ReceiVed: March 23, 2010; ReVised Manuscript ReceiVed: April 26, 2010
Electrowetting experiments have been performed on ZnO and TiO2 nanostructured thin films grown on crystalline Si and subsequently coated by a thin dielectric layer. The contact angle is decreased upon the application of an electric field between the film and a water droplet placed on the dielectric surface until saturation is observed. The electrowetting phenomenon is reversible, with the droplet returning to its initial hydrophobic state, when the applied potential is decreased. For the actuation of the droplet, low voltage, on the order of a few volts, is required. Introduction Control of surface wettability by application of external stimuli has drawn a lot of attention in the last years due to a wide range of potential applications. The dynamical manipulation of the behavior of liquids on surfaces, including the contact angle, droplet mobility, and effective area of the solid-liquid interface, leads to functional surfaces, the wettability of which can be reversibly switched between hydrophobicity and hydrophilicity. One way to change the wettability of a surface is to apply an electric field between the surface and a liquid droplet placed on it. This is called electrowetting. When an external electric field is applied between a hydrophobic solid surface and a droplet, the surface energy is lowered due to the net electric charge appearing at the interface, inducing hydrophilicity.1 Potential application of such “smart” surfaces range from intelligent microfluidics and laboratory-on-chip devices to controllable drug delivery, adjustable lenses, self-cleaning surfaces, etc.2-6 Commonly, in an electrowetting experiment, a layered structure is used, consisting of a conductive film covered by an insulating layer of macroscopic thickness.7 This type of electrowetting configuration is termed EWOD (electrowetting-ondielectric). The insulating layer is usually composed of an inorganic dielectric material such as SiO2 or Si3N4 or organic polymers, whereas the hydrophobicity is induced by coating the dielectric by a hydrophobic layer such as fluoropolymers or resists.8 The main requirements for efficient electrowetting are the low hysteresis of the hydrophobic layer in conjunction with high initial contact angle, as well as the high quality of the dielectric layer. There are many reports in the literature on the EWOD of various superhydrophobic structures, mainly on Si9-12 or carbon13-15 nanostructures. From these reports it becomes obvious that reversibility in air of the electrowetting behavior * Corresponding author. Telephone: +30-2810391132. Fax: +302810301305. E-mail:
[email protected]. † Institute of Electronic Structure and Lasers-Foundation for Research and Technology. ‡ Physics Department, University of Crete. § Materials Science and Technology Department, University of Crete.
is not only a challenge but a necessity for the technological application of electro-switchable surfaces. Despite the amount of work that has been performed in metal oxides, mainly ZnO and TiO2, regarding the switching of their wetting state using UV light,16-19 not great attention has been paid in electrowetting experiments using ZnO20 or TiO2 as conductive electrodes. Wettability switching using UV light is a procedure that requires a long time. Tens of minutes are normally needed in order to induce hydrophilicity in a metal oxide hydrophobic surface by UV irradiation, whereas the recovery of the hydrophobicity takes up to several days, when the surface is stored in the dark. On the other hand, switching the wettability of a surface using electric field requires only a few seconds, which is consistent with their application in devices where the switching must take place within seconds. The first report, to our knowledge, of EWOD on metal oxides appeared recently by Campbell et al.,19 who reported electrowetting experiments using ZnO nanorods. Also in this report, instant reversibility was only attained when voltages less than 35 V were applied, for which the contact angle change was small. EWOD on transparent electrodes is an add-on on the properties of transparent metal oxides and widen the applications of transparent oxides on electroswitchable surfaces. In the present work we report on the electrowetting behavior of ZnO and TiO2 nanostructured thin films grown by pulsed laser deposition (PLD). ZnO and TiO2 thin films were both subjected to electrowetting experiments and their performances were compared. In both cases a reduction of contact angle upon the application of an electric field was observed for relatively low voltages. The effect is reversible in the case of TiO2 films, with the droplet returning to its initial wetting state, upon decreasing the applied bias. Experimental Details ZnO and TiO2 nanostructured thin films have been grown by conventional pulsed laser deposition (PLD) in flowing oxygen environment. A KrF excimer laser (Lambda Physik, λ ) 248 nm, τ ) 34 ns pulse duration, 600 mJ/pulse maximum) was used for the ablation delivering pulses at a repetition rate of 10 Hz. The base pressure prior to deposition was less than
10.1021/jp1026114 2010 American Chemical Society Published on Web 05/14/2010
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TABLE 1 name
PO2 (mbar)
Tsub (°C)
F (J/cm2)
no. of pulses
ZO1 TO1 TO2
0.005 0.05 0.5
650 650 650
1.5 3 3
2500 2000 2000
thickness (nm) 100 145 120
10-4 Pa. The ablated material was deposited on crystalline Si substrates that were placed parallel to the target at a distance of 4 cm and heated up to 650 °C using a resistive heater. The samples were cooled to room temperature in the same oxidized environment as during deposition. The PLD parameters are shown in Table 1. We have shown in the past that these growth conditions result in nanostructured thin films, with uniform surface morphology and good stoichiometry.21,22 The electrowetting measurements were performed in a typical electrowetting setup, as depicted in Figure 1. The sample is placed on a metallic lower contact which conducts the electrical energy upon application of electric field. A nonconductive, insulating layer is sandwiched between the lower electrode and the upper electrode, which is immersed into the conductive liquid drop. For the upper electrode, we used a thin Au wire, immersed into the liquid droplet (4 µL) gently placed on the sample surface. The liquid used was deionized water, with a small amount of salt 100 mM of KCl added to it, in order to improve electrical conductivity. The principle of electrowetting is that when an electrical potential is applied between the upper and the lower electrode, charge accumulation takes place in the solid-liquid interface resulting in a change in the surface tension, and consequently rendering the hydrophobic surface more hydrophilic. When the applied potential exceeds certain value, water electrolysis is observed due to the occurrence of high field induced dielectric breakdown. Here, we present results before water electrolysis takes place. In the present experiments the insulating material was a 200 nm layer of SiNx, deposited by thermal evaporation. Since the SiNx surface is hydrophilic, hydrophobicity in the samples was induced by deposition of a few DMDCS ((CH3)2SiCl2, silane group) monolayers on the SiNx dielectric. For silanization, the samples were placed in a flask containing 0.5 mL of dimethyldichlorosilane (DMDCS) reagent. Hydrophobic DMDCS monolayers were subsequently deposited on the sample’s surface through adsorption reactions. The vapor-phase reactions were carried out overnight at room temperature. The hydrophobized wafers were rinsed with toluene, ethanol, 1:1 ethanol/water, deionized water, ethanol, and deionized water and were finally dried in a clean oven at 120 °C for 30 min.23
Figure 1. Electrowetting setup.
The system of the DMDCS layer and the SiNx layer can be visualized as two capacitors in series, and the total capacitance is determined by that of the smaller capacitor. Since the thickness of the low dielectric constant DMDCS layer is ∼100 times smaller than the thickness of the SiNx layer, the capacitance of the former might be neglected. As a result, the EWOD effect occurs primarily due to the SiNx layer. The morphology of the surfaces was examined by field emission scanning electron microscopy (FE-SEM JEOL7000). The uniformity of the surfaces was confirmed by looking at different areas of the films in the FE-SEM. The exact thickness of the dielectric layer was determined by cross-sectional FESEM images. AFM measurements (not shown here) were also performed in order to determine the mean roughness that was found to be 4-5 nm in TiO2 films and 7 nm in the ZnO film. All measurements took place in ambient conditions at room temperature. Results and Discussion Figure 2 depicts the FE-SEM images of the surfaces of the different TiO2 (TO1 and TO2) and ZnO (ZO1) nanostructures. Figure 2a,b depicts TiO2 nanostructured thin films, grown on flat Si. The surface of the films consists of nanosized grains, the size of which depends on the partial oxygen pressure during growth.20 As seen from the FE-SEM images, in Figure 2a,b, as the partial oxygen pressure is increased, the size of the nanoparticles comprising the film is decreased. Hence, the main diameter of the grain for the film grown at 0.05 mbar is about 20 nm, while for the films grown at 0.5 mbar is approximately 15 nm. The surface morphology of a ZnO thin film deposited on a flat Si substrate is presented in Figure 2c. It consists of nanosized grains of average diameter of approximately 100 nm, with a regular, hexagonal shape. Initially, the setup depicted in Figure 1 was used to perform classical electrowetting experiments, on samples similar to the aforementioned, but without the dielectric layer. It was seen, that no electrowetting behavior took place, and electrolysis of the liquid droplet occurred at low voltages. Consequently, all films were covered by a dielectric SiNx layer and subsequently by a silane layer that induces hydrophobicity. The contact angle was measured as a function of the applied voltage. The contact angles before the application of any voltage was about 100-105° for the TiO2 samples and 95° for the ZnO sample. Upon the application of the external electric field, the apparent contact angle started decreasing in all samples, until at threshold applied voltage it reaches a minimum value. Measurements were repeated at different locations on the surface revealing similar behavior, indicating uniformity of the coating. The aforementioned EWOD behavior is depicted in Figure 3, for samples TO2 and ZO1. The initial contact angle is shown for 0 V, followed by its progressive reduction as the applied bias is increased. The phenomenon is reversible in the case of TiO2. Figures 4a and 5a show the progressive decrease of the apparent contact angle with increasing applied bias for the TiO2 nanostructured thin films. In the case of sample TO1, there is a threshold voltage which must be overcome in order for the droplet to be actuated. Hence, the droplet remains unaffected by the externally applied voltage, and it is actuated only after the applied voltage exceeds a threshold value, V0, of approximately 13.5 V. The reduction of the droplet’s CA is about 30%. In contrast, in sample TO2 the decrease of the initial contact angle starts upon the application of the electric field and it is approximately 15%.
Electrowetting Properties of ZnO and TiO2
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Figure 2. FE-SEM images of the surfaces of the samples (a) TO1, (b) TO2, and (c) the cross section of TO1 where the different layers can be distiguished and (d) ZO1.
Figure 3. 4 µL of water on (a) TiO2 (sample TO2) and (b) ZnO (sample ZO1) for different applies biases. The progression of the contact angle as a function of the applied bias is shown for increasing and decreasing bias cases.
Figure 6 shows the electrowetting performance of ZnO nanostructured film. Also in this case, the initial contact angle of 96° reduces gradually to 80° (∆θ ≈ 16°) when the externally applied voltage increases from 0 to 60 V, leading to a decrease of 16% in the value of the initial contact angle. It is very important to notice here that the actuation voltage is very low, for both materials, a property which is desirable for the EWOD applications.1 An applied voltage of the order of a few volts is enough to actuate the droplet. Especially for
sample TO2, a voltage of 15 V is enough to induce a change of 15% in the wettability of the surface. Electrowetting is theoretically described by the LippmannYoung equation2,24,25 which describes the dependence of the contact angle, θ, on the applied potential, V
cos θ ) cos θ0 +
1 1 ε0ε 2 V 2 γLG d
(1)
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Figure 4. Electrowetting behavior of sample TO1. The change of the contact angle as a function of the applied voltage, at (a) increasing bias and (b) decreasing bias. The threshold voltage for electrowetting was approximately 13.5 V. Inset: the corresponding cosine of the contact angle versus the applied voltage. The threshold voltage has been subtracted. The solid lines are the corresponding fits to the Lippmann equation.
where θ0 is the initial contact angle (before the application of the electric field), γLG is the surface tension on the liquid-air interface, ε0 is the vacuum permittivity (8.854 × 10-12 F/m), ε is the dielectric constant of the material and d is the thickness of the dielectric layer. In the case of a nonzero threshold actuation voltage, V0, the V in eq 1 becomes (V - V0). The insets in Figures 4a, 5a, and 6 show the cosine of the contact angle plotted against the square of the (V - V0)2. It is seen that in all cases, with the exception of the case of TO1, the dependence is linear, following the behavior predicted by the Lippmann-Young’s equation. Furthermore, we have calculated the slope of the linear relation using eq 1, for γLG ) 72 dyn/cm (for water), ε ) 6.5 for amorphous SiNx,26 d ) 200 nm, and it was found to be approximately 3.9 × 10-3 [V-2]. The corresponding slopes obtained from the fit to experimental data were 1.1 × 10-3 and 3.2 × 10-3 [V-2] for samples TO1 and TO2, respectively, close to the theoretical value. However, this is not the case for ZO1 where the slope of the fit was 1 order of magnitude lower, 1.5 × 10-4 [V-2]. In the frame of the classical electrowetting theory, i.e., electrowetting on a metal, the offset voltage V0 is explained as the voltage needed to overcome the spontaneous charging at zero bias and it comes from the creation of an electric double layer in the metal-liquid interface.8 However, in the present
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Figure 5. Electrowetting behavior of sample TO2. The change of the contact angle as a function of the applied voltage, at (a) increasing bias and (b) decreasing bias. Inset: the corresponding cosine of the contact angle versus the applied voltage. The solid lines are the corresponding fits to the Lippmann equation.
Figure 6. Electrowetting behavior of ZnO sample, for increasing bias. Inset: the corresponding cosine of the contact angle versus the applied voltage. The solid lines are the corresponding fits to the Lippmann equation.
case, a dielectric layer lies between the two electrodes and charge trapping occurs at the insulator-liquid and/or the insulator-metal oxide interfaces.27,31 Given that the insulating layer in both TiO2 samples is identical since it was deposited
Electrowetting Properties of ZnO and TiO2 simultaneously, the only difference between the samples is in the TiO2 layers that were deposited using different deposition parameters. In fact, the partial oxygen content during deposition is known to affect the density of oxygen vacancies in the film.28 The difference in the two samples is the TiO2 layer which was deposited using different deposition parameters. Therefore, it is suggested that charge becomes trapped in or on the insulating layer.31 Furthermore, it has been shown that in nanocrystalline TiO2 films there exist a large amount of surface traps, created during fabrication.29,30 In our case, the lower partial pressure in which TO1 was grown is expected to have induced a larger amount of oxygen vacancies, the films to be less stoichiometric and therefore there are more surface traps. This may result in a thicker electric double layer to be created in the TiO2/SiNx interface, which causes electric charge trapping, which in turn, results in the occurrence of V0. This is depicted in Figure 4a, where there is a deviation between the experimental data and the theoretical fit. According to eq 1, total spread of the liquid drop on the surface should be possible at high voltages. Nonetheless, in the case of TiO2 film, saturation of the contact angle takes place at approximately 25 V for TO1 and at about 13 V for TO2. For voltages above this value the contact angle becomes independent of the applied voltage and total wetting of the surface is inhibited. This saturation effect has been observed in the literature and many explanations have been proposed, however, the phenomenon has not been explained unambiguously yet.31-34 It is noteworthy that no saturation of the contact angle takes place in the ZnO sample. On the contrary, electrolysis of the liquid drop starts before the saturation of the contact angle sets on. Theory predicts reversibility upon removal of the applied voltage. This means that once the applied voltage is removed, the liquid drop should attain its original shape and initial apparent contact angle. In the present case, reverse electrowetting was observed in the TiO2 films. Decreasing the applied voltage, results in the recovery of the initial hydrophobic state, as shown in Figures 4b and 5b. However, the reversibility was only partial and the contact angle attained after reducing the voltage back to zero was lower than the initial contact angle, before any voltage application. The insets of Figures 4b and 5b depict the cosine of the contact angles versus the square of the applied voltage, for the reverse electrowetting. The theoretical fit (red line) shows that the data are in good agreement to the predicted behavior described by eq 1. In the case of ZnO no reversibility was observed. The droplet reaches a minimum value of 78° when the applied voltage is increased to 60 V, however, decreasing the applied voltage to zero results in further decrease of the contact angle. This irreversibility might be due to the increased surface roughness of the ZnO film or to other dissipative forces inhibiting the motion of the droplet.10 Microscopic chemical inhomogenity35 of the insulating layer might yet be another reason for the observed irreversibility. Conclusions Reversible electrowetting at very low voltages is demonstrated when nanostructured ZnO and TiO2 thin films were used as conductive electrodes. Reversibility of the electrowetting effect as well as the actuation of the liquid drop at so low voltages are technologically very important, since it allows the use of these transparent electro-switchable surfaces in potential applications, including microfluidics. Furthermore, the PLD
J. Phys. Chem. C, Vol. 114, No. 22, 2010 10253 technique is widely used in the metal oxide growth and can be applied in a wide range of substrates for the growth of various nanostructures. Finally, considering that metal oxide coatings are also photoswitchable, this report may be useful for the design of doubly functional coatings with capabilities of both lightand electric field-induced wettability switching. Acknowledgment. This work was supported by the Integrated Initiative of European Laser Research Infrastructures LASERLAB-II (Grant Agreement No. 228334).Ms. Aleka Manousaki is acknowledged for the FE-SEM pictures. Dr. George Konstantinidis is acknowledged for the thermal evaporation of the dielectric layer. References and Notes (1) Moon, H.; Cho, S.-K.; Garrell, R. L.; Kim, C. J. J. Appl. Phys. 2002, 92, 4080. (2) Lippmann, G. Ann. Chim. Phys. 1875, 5, 494. (3) Hayes, R. A.; Feenstra, B. J. Nature 2003, 425, 383. (4) Srinivasan, V.; Pamula, V. K.; Fair, R. B. Lab Chip 2004, 4, 310. (5) Acharya, B. R.; Krupenkin, T.; Ramachandran, S.; Wang, Z.; Huang, C. C.; Rogers, J. A. Appl. Phys. Lett. 2003, 83, 4912. (6) Berthier, J. and Silberzan, P. Microfluidics for Biotechnology; Artech House: Norwood, 2006. (7) Berge, B. C. R. Acad. Sci. III 1993, 317, 157. (8) Mugele, F.; Baret, J.-C. J. Phys.: Condens. Matter 2005, 17, R705. (9) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20, 3824. (10) Bahadur, V.; Garimella, S. V. Langmuir 2008, 24, 8338. (11) Verplanck, N.; Galopin, E.; Camart, J.-C.; Thomy, V. NanoLett. 2007, 7, 813. (12) Lapierre, F.; Thomy, V.; Coffinier, Y.; Blossey, R.; Boukherroub, R. Lanmuir 2009, 25, 6551. (13) Zhu, L.; Xu, J.; Xiu, Y.; Sun, Y.; Hess, D. W.; Wong, C.-P. J. Phys. Chem. B 2006, 110, 15945. (14) Kakade, B.; Mehta, R.; Durge, A.; Pillai, V. Nano Lett. 2008, 8, 2693. (15) Dhindsa, M. S.; Smith, N. R.; Heikenfeld, J.; Fowlkes, J. D.; Doktycz, M. J.; Melechko, A. V.; Simpson, M. L. Langmuir 2006, 22, 9030. (16) De Sun, R.; Nakajima, A.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem. B 2001, 105, 1984. (17) Papadopoulou, E. L.; Barberoglou, M.; Zorba, V.; Manousaki, A.; Pagkozidis, A.; Stratakis, E.; Fotakis, C. J. Phys. Chem. C 2009, 113, 2891. (18) Gras, S. L.; Mahmud, T.; Rosengarten, G.; Mitchell, A.; Kalantarzadeh, K. ChemPhysChem 2007, 8, 2036. (19) Vernardou, D.; Stratakis, E.; Kenanakis, G.; Yates, H. M.; Couris, S.; Pemple, M. E.; Koudoumas, E.; Katsarakis, N. J. Photochem. Photobiolo. A 2009, 202, 81. (20) Campbell, J. L.; Breedon, M.; Latham, K.; Kalantar-zadeh, K. Langmuir 2008, 24, 5091. (21) Walczak, M.; Papadopoulou, E. L.; Sanz, M.; Manousaki, A.; Marco, J. F.; Castillejo, M. Appl. Sur. Sci. 2009, 255, 5267. (22) Papadopoulou, E. L.; Zorba, V.; Pagkozidis, A.; Barberoglou, M.; Stratakis, E.; Fotakis, C. Thin Solid Films 2009, 518, 1267. (23) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (24) Welters, W. J. J.; Fokkink, L. G. J. Langmuir 1998, 14, 1535. (25) Ivosevic, N.; Zutic, V. Langmuir 1998, 14, 231. (26) Sato, T.; Mitsui, M.; Yamanaka, J.; Nakagawa, K.; Aoki, Y.; Sato, S.; Miyata, C. Thin Solid Films 2006, 508, 61. (27) Herbertson, D. L.; Evans, C. R.; Shirtcliffe, N. J.; McHale, G.; Newton, M. I. Sensors and Actuators A 2006, 130-131, 189. (28) Zhou, X.-S.; Lin, Y.-H.; Li, B.; Li, L.-J.; Zhou, J.-P.; Nan, C.-W. J. Phys. D: Appl. Phys. 2006, 39, 558. (29) Zhao, H.; Zhang, Q.; Weng, Y.-X. J. Phys. Chem. C 2007, 111, 3762. (30) Boschloo, G.; Fitzmaurice, D. J. Phys. Chem. B 1999, 103, 2228. (31) Verheijen, H. J. J.; Prins, M. W. J. Langmuir 1999, 15, 6616. (32) Vallet, M.; Vallade, M.; Berge, B. Eur. Phys. J. B 1999, 11, 583. (33) Shapiro, B.; Moon, H.; Garrell, R. L.; Kim, C. J. J. Appl. Phys. 2003, 93, 5794. (34) Verplanck, N.; Coffinier, Y.; Thomy, V.; Boukherroub, R. Nanoscale Res. Lett. 2007, 2, 577. (35) Tokkerli, Ph.D. Thesis; Droplet Microfluidics on a Planar Surface; Technical Research Centre of Finland, 2003.
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