Enhanced superhydrophilicity and thermal stability of ITO surface with patterned ceria coatings

Applied Surface Science 329 (2015) 11–16

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Enhanced superhydrophilicity and thermal stability of ITO surface with patterned ceria coatings Mingshan Xue ∗ , Na Peng, Changquan Li, Junfei Ou, Fajun Wang, Wen Li School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China

a r t i c l e

i n f o

Article history: Received 22 September 2014 Received in revised form 3 December 2014 Accepted 20 December 2014 Available online 26 December 2014 Keywords: Ceria coatings Superhydrophilicity Thermal stability

a b s t r a c t Surface wettability of solid materials is significant for both fundamental research and engineering applications. Compared with most existing fabrication methods of superhydrophilic surfaces by UV exposure or chemical modification, in this work, a superhydrophilic ceria coating on ITO substrate is developed by a fast, simple one-step method. It is found that the superhydrophilicity of ceria coatings is strongly dependent on both the patterned microstructures benefiting the capillary effect and the peculiar chemical composition of ceria inducing numerous oxygen vacancies and large surface free energy. Owing to the inherent physical stability of ceria, such a superhydrophilic ceria coating exhibits an excellently thermal stability at both room temperature and higher temperature. These results open up new avenues for the underlying applications of superhydrophilic coatings, such as heat transfer/dissipation. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Surface wetting behaviors of solid materials are one of the most important properties in the field of materials science and surface chemistry [1]. In this aspect, nature has already shown fascinating power by producing peculiar functional materials combining with both chemical composition and geometrical topography. For example, when a liquid droplet contacts with a solid surface, it may remain spherical or spread out on the surface to form a thin liquid film, being anti-wetting or super-wetting. Especially for water, such a solid surface will be superhydrophobic (with contact angle (CA) larger than 150◦ ) or superhydrophilic (with CA less than 5◦ ). In recent years, these materials with excellent wetting behaviors have received more and more attention because of their importance not only in fundamental research but also in industrial fields, such as self-cleaning, anti-reflection, antifogging, oil–water separation, enhanced heat transfer, and so on [2–6]. Inspired by the natural leaves of the lotus, Calathea zebrina or Ruellia devosiana with superhydrophobicity/superhydrophilicity [7], various surface coatings have been widely investigated in order to open up new avenues in preparing novel micro/nanoscale interfacial materials. A number of physical and chemical growth methods have been developed to prepare these artificial materials with excellent wetting behaviors including micro/nanopatterned

∗ Corresponding author. Tel.: +86 791 86453210; fax: +86 791 86453210. E-mail address: [email protected] (M. Xue). http://dx.doi.org/10.1016/j.apsusc.2014.12.145 0169-4332/© 2014 Elsevier B.V. All rights reserved.

coatings, rough/porous surfaces, polymeric nanofibers based on the Wenzel and Cassie models [1,8,9]. It is found that both superhydrophobicity and superhydrophilicity are strongly dependent on the chemical composition and microstructures of the surface coatings with lower or higher surface free energies. Nevertheless, relative to superhydrophobic surfaces, the study on superhydrophilic surfaces is lacking in spite of their potential applications in self-cleaning, antifogging, heat dissipation/transfer. Nowadays there are mainly two methods to obtain superhydrophilic surfaces [4,10]: one is the use of photochemically active materials such as ultraviolet (UV) light exposure to TiO2 making it become superhydrophilic [11], another is the chemical modification by the use of high-surface-energy materials. The former is extremely unstable because the surface coatings readily lose the superhydrophilicity when placed in a dark environment, being unsuitable for their use such as antifogging and self-cleaning glass at night or in a closed space. The latter may refer to multistep processes, environmentally unfriendly materials with high surface energy, or harsh modification conditions [12]. More importantly, the stability of the superhydrophilicity is not obviously enhanced although some effort has been made toward solving this key issue. Consequently, the practical applications of these functional materials have not been fully realized and there is an urgent need for stable superhydrophilic surfaces suitable for different environmental conditions (such as temperature, humidity, radiation) in the absence of UV irradiation and chemical modification [13]. In nature, superhydrophilic plant surfaces can be divided into three kinds [7]: permanent wet surfaces occurring in submerged

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Fig. 1. FESEM image and corresponding EDS spectra of CeO2 coating grown on ITO substrate. Spectra 1 and 2 correspond to the selected positions with a CeO2 island and a micropit, respectively.

plants (such as Anubias barteri), water-absorbing surfaces having porous or multicellular structures (such as Sphagnum squarrosum), and water-spreading surfaces having conical epidermal cells (such as C. zebrina). Especially for the last case, the glands secrete hydrophilic substances in combination with the rough surface morphology to induce a fast water spreading out according to the capillarity phenomena, which is especially worth learning for the design and synthesis of artificial superhydrophilic materials [4,7]. Without UV exposure or surface chemical modification, to obtain a stably superhydrophilic surface is required and expected by choosing some functional materials, especially using some simple methods and processes (such as one-step method). Thus, it will prompt the potential applications of superhydrophilic coatings (e.g., used in the field of heat transfer/dissipation) based on a large contact area between the material surface and the liquid. We noticed that rare earth oxides (REOs) and especially cerium oxide (CeO2 ) coatings have been the subject of extensive studies owing to their unique electronic structures, thermal stabilization and corrosion resistance [14,15]. For instance, the entire lanthanide oxide ceramics series ranging from ceria to lutecia were found to be intrinsically hydrophobic [16]. Mg alloy superhydrophobic surface with CeO2 coatings exhibited an excellent corrosion resistance in NaCl aqueous solution [17]. The soaking of water drops by multi-cracked ceria coatings had an important effect on oxidation resistance of ceria-coated superalloy [18]. According to Wenzel’s theory [19], for a certain material the effect of surface roughness can amplify the wetting behavior, namely, roughness can let hydrophilic surfaces more hydrophilic. Moreover, there is a oneto-one correspondence between the surface polarity and water molecule orientation, namely, the increasing hydrophilicity originates from the increase of surface polarity (increasing surface free energy) [16]. For nanostructured REOs with the unfilled 4f5d orbitals, the high coordinative unsaturation of surface cations and oxygen vacancies (which dominate their electronic and chemical properties) bring a large number of polar sites, enhancing the surface polarity and improving their affinity for water molecules [16,20–22]. Therefore, this tendency may result in the surface superhydrophilicity by means of the formation of hydrogen bonds between the unsaturated cations/oxygen vacancies and interfacial water molecules. In order to validate the hypothesis on the superhydrophilicity of micro/nanostructured REOs, in this work, a fast, facile one-step

electrodeposition method to prepare CeO2 coatings on ITO substrate is presented. The results exhibit that CeO2 coatings are characteristic of excellent superhydrophilicity. As-prepared CeO2 coatings also exhibit an excellently thermal stability at both room temperature and higher temperature. Such a simple method is suitable for the fabrication of superhydrophilic coatings without UV exposure or surface chemical modification, which will extend the potential applications of superhydrophilic coatings. 2. Experimental The substrates of ITO glass (10 /cm, commercially available) sheets with a size of 2 cm × 5 cm were used. These sheets were in-turn cleaned using an ultrasonic cleaner in purified water (18.6 M/cm, for 20 min), alcohol (reagent grade, for 10 min) and acetone (reagent grade, for 10 min), and then dried by blowing nitrogen gas. The electrolytic bath was composed of 0.05 M/L aqueous solution of Ce(NO3 )3 ·6H2 O (Aldrich), and a current density of −1.5 mA cm−2 was applied for the growth of ceria coatings. To control the growth rate, the electrolytic bath was kept in a mediate temperature (50 ◦ C) and the PH of the solution was measured to be about 4 at 50 ◦ C. The bath was not agitated during electrodeposition and the deposition time of 2–120 min was tried. During growth ceria was aggregated on the negative ITO electrode relative to the positive graphite electrode. After growth, the samples were thoroughly rinsed in ethanol and dried in air before any further subsequent operation. The as-deposited samples were annealed in a vacuum drying oven at 400 and 500 ◦ C. The chemical compositions and valence states of the samples were measured using X-ray photoelectron spectroscopy (XPS, Physical Electronics, PHI-5702) with the Al Ka X-ray source (hv = 1486.6 eV) at a base pressure of 10−7 Pa. The surveyed (enlarged) spectra were recorded with a pass energy of 20 (5) eV and an energy step of 0.5 (0.05) eV. The crystal orientation of the samples was characterized by X-ray diffraction (XRD, SMART APEX II) using Cu Ka radiation. The surface microstructures of the samples were observed using field-emission scanning electron microscopy (FESEM, Nova NanoSEM 450, FEI) with attached energy dispersive spectroscopy (EDS). The electron work function (EWF) was measured using Scanning Kelvin Probe (SKP, RHC020, KP Technology Ltd.). A gold Kelvin probe (2 mm diameter) was used and the shield was tied back to the ground. The oscillation frequency of the SKP

M. Xue et al. / Applied Surface Science 329 (2015) 11–16

3. Results and discussion SEM images and corresponding EDS spectra show that the ITO surface is not covered by the uniform ceria film, instead, islands with diameter of several tens of micrometer, as shown in Fig. 1. The bulgy islands at the surface are mainly consisted of Ce and O elements, while the micropits between islands include the contributions from the coatings and the ITO substrate. In some related documents [18,23–26], the similar multi-cracked CeO2 deposits with needle-like morphology have been found by cathodic electrodeposition due to shear stresses between the deposits and the substrate. Fig. 2 shows the chemical composition of the sample surface measured by XPS. Besides the strong signal from Ce and O elements, it also includes the weak signal from the substrate. Combining with the EDS spectrum, it suggests that there may be a thin ceria film with several atom layers thick in the micropit region. In the enlarged Ce 3d XPS spectrum, six peaks, marked as V1 , V2 , V3 , U1 , U2 , and U3 , correspond to three pairs of spin–orbit doublets originating from Ce 4f0 , 4f1 , and 4f2 in the final state, clearly indicating a oxidized cerium oxide, namely, CeO2 [27–29]. In spite of the possible existence of some residual carbonates and nitrites at the topmost surface of ceria [28], combining XPS with EDS spectra, it implies that the ITO surface is mainly covered by CeO2 islands as well as thin CeO2 layer between islands. Fig. 3 shows the SEM images before and after electrodepositing CeO2 coatings. Before electrodeposition, the ITO surface is rather

V3 U1

U3

Intensity (a.u.)

O KLL 3d

Intensity (a. u.)

probe was set to 84 Hz. The SKP gave the value of the WF relative to the Au (EWF = 5.1 eV) probe tip rather than the absolute value. The contact angle (CA) was measured by a contact angle meter (Easydrop, Kruss Instruments GmbH) and the volume of an individual water droplet was fixed at 4 ␮L. The digital photographs and videos of water droplets on sample surfaces were obtained by a digital camera (Fuji, Japan).

13

V1

U2 V2

Ce 3d

930 920 910 900 890 880 BE (eV)

MNN

O 1s

Sn 3d

NVV

In 3d

C 1s

4p

4d 5p

1000

800

600

400

200

0

Binding Energy (eV) Fig. 2. Surveyed XPS spectrum of CeO2 coating on ITO substrate. The inset shows the enlarged Ce 3d core-level spectrum, corresponding to the typical Ce4+ states in CeO2 .

smooth, as shown in Fig. 3(a). After electrodeposition, there are many ceria islands and the surface of these islands looks like a sponge, as shown in Fig. 3(c). Compared with pure ITO surface, the micropits between islands consist of thin CeO2 layer, as shown in Fig. 3(d). For comparison, we measure the static contact angle (SCA) before and after electrodepositing ceria coatings. For pure ITO surface and the surface with ceria coatings, the SCA is 64.34◦ and 0◦ , respectively. To be noticed, for the latter, a water droplet instantaneously spreads out on the surface at the moment of contacting the ceria coatings. In order to analyze the detailed change process of a water droplet on such a material surface, the instantaneous

Fig. 3. FESEM images of (a) pure ITO surface; (b) CeO2 coating; (c) and (d) enlarged CeO2 island and micropit corresponding to (b), respectively. The insets in (a) and (b) show the static contact angle (SCA) after water droplets stabilize at the surface and instantaneous contact angle (ICA) after water droplets drip for comparison.

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Fig. 4. (a) Change of instantaneous contact angle (ICA) at different time after water droplets drip; (b) photographs of spreading of a water droplet with the increase of time; (c) wetting mechanism of ITO surface with CeO2 coating induced by capillary effect; and (d) schematic diagram of spreading of a water droplet on CeO2 coating.

contact angle (ICA) as a function of time after a water droplet contacts the sample surface is monitored by the contact angle meter. For example, after a water droplet drips on pure ITO surface for 0.1 s, the corresponding ICA is 95.15◦ , much larger than the SCA (64.34◦ ) after equilibrium. In contrast, the spreading of a water droplet on ceria coating surface is rather fast. From the dripping of a water droplet to spread out and reach the CA of 0◦ , the time of duration is within 0.5 s. Fig. 4(a) shows the detailed change of the ICA with the increase of time. It is found that the water droplets instantaneously spread out once it contacts the ceria coating (corresponding to Fig. 4(a2) and (a3)). The detailed process of a water droplet spreading out is also recorded using the digital camera by setting the shooting mode with 32 frames per second, as shown in Fig. 4(b). It is clearly observed that the spreading of the water droplet mainly occurs at the moment of contacting the ceria coating. Moreover, in a recent study on the similar multi-cracked ceria coatings [30], the widening of the cracks as well as refinement of ceria grains upon cyclic oxidation of these coatings by putting down 10 ␮L water indirectly testify the superhydrophilicity of ceria coatings because of the enhanced capillary effect. It is noticed that CeO2 has long been considered as one important oxide materials in the field of solid oxide fuel cells, catalysis, and energy conversion, which is strongly dependent on the nature of oxygen storage and release capability of CeO2 [14,31,32]. In other words, the desirable properties of CeO2 are mainly associated with the intrinsic defects (mainly oxygen vacancies) [33]. From the point of view of vacancy formation energy, to form an oxygen vacancy is much easier than to form a cerium vacancy because each cerium cation has eight coordinated oxygen anions while each oxygen anion only has four coordinated cerium cations in the fluorite structure of CeO2 [34]. Oxygen vacancies cause a large number of polar sites at surface, enhancing the surface polarity (increasing the surface free energy) and improving their affinity for water molecules. Thus, the existence of oxygen vacancies at ceria coating surface may play a vital role in enhancing the wettability of ITO surface. It is well known that the concentration of oxygen vacancies in CeO2

is strongly associated with oxygen partial pressure and elevated temperature [14,31–33]. In order to validate the existence of oxygen vacancies in ceria coatings, we monitor the change of O 1s core levels with the increase of annealing temperature by XPS, as shown in Fig. 5(a). The peak shape suggests the existence of at least two species of oxygen. The main peak appearing at lower binding energy (BE) originates from the lattice oxygen in CeO2 [35], while the weak satellite peak at higher BE might be attributed to hydroxyl species, absorbed or defective oxygen species in the literature [36,37]. If the weak satellite peak is originated from hydroxyl species and absorbed oxygen species, they should be obviously weakened even eliminated after annealing at higher temperature because of the easy desorption of hydroxyl and absorbed oxygen species at higher temperature. In fact, as shown in Fig. 5(a), the increase of the intensity of the satellite peak with the increasing temperature indicates that the defective oxygen species are mainly responsible for the satellite peak. Surely, it is inevitable for CeO2 to release significant levels of oxygen and leave oxygen vacancies at elevated temperature, as described by the following defect reaction [38]: Olattice → Ovacancy + 2e− + Oatom

(1) e−

where Olattice , Ovacancy , Oatom and represent oxygen ions, oxygen vacancies, releasing oxygen atoms and electrons in the Ce 4f conduction band, respectively. Using a force-field method [39], it was found that oxygen vacancies could be more stable on (1 1 1), (1 1 0) and (3 1 0) surfaces than these in the bulk crystal. Furthermore, using a first-principle method [40], it was also suggested that the preferred oxygen vacancies located at the surface layer for CeO2 (1 1 0) and at the subsurface layer for CeO2 (1 1 1). The corresponding XRD pattern shown in Fig. 5(b) indicates that CeO2 is mainly grown along [1 1 1], [1 1 0] and [3 1 1] direction, being of advantage to produce oxygen vacancies. In addition, with the decrease in particle size, the large surface area to volume ratio induces that the oxygen vacancy formation energy in ceria islands is lower than that in bulk crystal [14,41]. Just as shown in Fig. 3(c) and (d), the ceria coating consisting of islands plus micropits readily causes the formation of more oxygen vacancies. On one hand,

M. Xue et al. / Applied Surface Science 329 (2015) 11–16

15

Intensity (a. u.)

(a) O 1s

C B Olattice Ovacancy

Substrate Ceria

(331)

(311)

(111)

(220)

Intensity (a. u.)

(b)

537 534 531 528 Binding Energy (eV)

(200)

540

A

D C B

A

20

30

40 50 60 2θ (degree)

70

80

Fig. 5. (a) XPS spectra of O 1s core-level lines in CeO2 coating before annealing (A), after annealing at 400 ◦ C (B) and 500 ◦ C (C). The results testify the increase of oxygen vacancies with increasing annealing temperature. (b) XRD pattern of CeO2 coating on ITO substrate: (A) pure ITO substrate; (B) before annealing CeO2 coating; (C) and (D) after annealing CeO2 coating at 400 ◦ C and 500 ◦ C, respectively.

Fig. 6. (a) Change of the instantaneous contact angle (ICA) at 0.1 s after water droplets drip as a function of annealing temperature. The insets show the corresponding FESEM image and ICA of CeO2 coating after annealing at 500 ◦ C. (b) Change of electron work function (EWF) of CeO2 coating as a function of annealing temperature. The inset gives the two-dimensional image of relative EWF of CeO2 coating after annealing at 500 ◦ C.

these oxygen vacancies bring a number of polar sites and obviously increase the surface free energy, making the increase of surface affinity for water molecules (i.e., tending to form hydrogen bonds with interfacial water molecules). On the other hand, the peculiar microstructures of islands plus micropits of ceria coatings provide the channel for the water spreading because of the capillary effect (the capillary length is 2.7 mm for water) based on Wenzel’s theory [19,42,43]. Therefore, the common roles of the intrinsically chemical composition and geometrical microstructure of CeO2 make such a coating superhydrophilic. The stability of wettable material surfaces is the foundation of their potential applications. In previous studies, the stability of superhydrophilic or superhydrophobic surfaces formed by UV light exposure or chemical modification (such as surface fluorination), especially the thermal stability, is weak. However, the present superhydrophilic ceria coatings exhibit an excellently thermal stability. Fig. 6(a) presents the change of ICA after the water droplet contacts the sample surface for 0.1 s as a function of annealing temperature. Note that the ICA does not increase, in contrast, has a slight decrease with the increase of temperature, i.e., making the superhydrophilic surface more superhydrophilic. It can be

reasonably explained based on the following two points. On one hand, the elevated temperature benefits to aggregate small ceria islands and to form large islands based on island thickening model [44,45], supplying the channel for water spreading based on capillary effect, as shown in Fig. 4(d). The activation barrier of ceria is thermally accessible at higher temperature, and as-induced upstepping makes the density of islands decrease and average size increase with the elevated annealing temperature (the inset in Fig. 6(a)), i.e., the agglomeration and coalescence of ceria islands [42]. On the other hand, the elevated temperature results in more oxygen vacancies based on the above Eq. (1), which increases the surface free energy and enhances the surface affinity for water molecules. Generally speaking, surface free energy plays a vital role in determining the wetting behavior of a solid surface. In the physics of solids, surface free energy quantifies the disruption of intermolecular bonds that occur when a surface is created, and the EWF is defined as the minimum energy required for extracting an electron from the sample surface to a position just outside the sample [46,47]. Thus, there is a strong correlation between the EWF and surface free energy, i.e., a large surface free energy corresponds

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to a small EWF. Generally, the surface free energy is not readily measured exactly by experimental techniques. In contrast, the EWF can be easily measured by SKP or ultraviolet photoelectron spectroscopy (UPS). Fig. 6(b) presents the change of the EWF of ceria coatings as a function of annealing temperature by SKP. During annealing the EWF is slightly decreased, whose change trend is very similar to that of ICA shown in Fig. 6(a). The EWF is essentially sensitive to the chemical composition and surface morphology of material surface [46]. Based on our previous analysis according to the principle of the SKP, the EWF (ФS ) measured by SKP can be written as [48] ˚S = ˚

2 T −e d/(ε0 εr S)

= ˚T −C0 /S

(2)

where ФT , e, d, ε0 , εr , and S are the EWF of the Au tip, the charge of an electron, the distance between the tip and the sample surface, the vacuum and air dielectronic constants, and the area of the microcapacitor consisting of the tip and the sample surface, respectively. C0 = e2 d/(ε0 εr ) is a constant. Thus, ФS of the sample surface is decreased with the decrease of the effective area S. Compared with ceria coatings before annealing, the corresponding effective area S after annealing is smaller because of the aggregation of small ceria islands and the decrease of the density of islands based on island thickening model, resulting in the decrease of the EWF measured according to Eq. (2) [46,47]. Accordingly, the surface free energy of the ceria coatings slightly increases based on the correlation between the EWF and surface free energy, inducing that the superhydrophilicity of ceria coatings keeps stable after annealing. In a word, ceria coating is relatively easy to be superhydrophilic and keep their thermal stability, which is significant for the potential applications of superhydrophilic coatings. 4. Conclusions In summary, the superhydrophilic ceria coating without UV exposure and any chemical modification has been successfully fabricated by one-step electrodeposition and exhibits an excellently thermal stability. The water droplets can rapidly spread out on the ceria coating surface at the moment of contacting, which is strongly associated with both the capillary effect originating from the peculiar topography and the large surface free energy resulting from the oxygen vacancies. The superhydrophilic ceria coatings, exhibiting an excellently thermal stability at high temperature, provide a faster evaporation of water by increasing the contact area of water–air than that at a hydrophilic or superhydrophobic surface, having the potential applications in the field of heat dissipation/transfer.

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The authors acknowledge with pleasure the financial support of this work by the Natural Science Foundation of China (Grant Nos. 21103084 and 51362023) and the Science Foundation for Young Scientists of Jiangxi Province, China (Grant No. 20142BCB23016).

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