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Superhydrophilicity of Highly Textured Carbon Films in Range of pH Values from 0 through 14 Shuyan Gao,* Zhengdao Li, Xiaoxia Jia, and Kai Jiang* College of Chemistry and EnVironmental Science, Henan Normal UniVersity, 46 Jianshe Road, Xinxiang, 453007 Henan, People’s Republic of China ReceiVed: July 25, 2010; ReVised Manuscript ReceiVed: October 5, 2010
In the past few years, the study of superhydrophilic surfaces has attracted considerable attention, due to their great potential in not only fundamental research but also practical applications. However, the bottlenecks in this field are (1) the use of UV irradiation, (2) the chemical modification by high-free-energy materials, and (3) the unavailability of useful superhydrophilic surfaces throughout the range of pH values from 0 through 14. In this article, we describe a method for inducing rough features on carbon surfaces using plasma technique to acquire superhydrophilic character. More interestingly and importantly, the as-prepared films are superhydrophilic for not only pure water but also corrosive liquids, such as acidic and basic solutions. This is the first example of superhydrophilicity over the whole range of pH values without the presence of highfree-energy compounds and in the absence of UV irradiation and might open up new perspectives in preparing novel nanoscale interfacial materials. The method of surface design described here does not require the use of masks or lithography, can be applied to very large surfaces in very short time, and herein offers an inexpensive and rapid method for improving the wettability of materials. 1. Introduction Nature always shows fascinating power by producing peculiar functional materials such as superhydrophobic surfaces by combining the use of low-surface-energy materials with microand nanometer hierarchical rough surfaces.1-4 Well-known examples include lotus leaves and their self-cleaning properties and water striders, which inspire people to attempt to mimic nature by creating superhydrophobic surfaces similar to lotus leaves and water strider legs.5-8 In the past few years, the study of superhydrophilic surfaces has also attracted considerable attention due to their great potential in not only fundamental research, but also practical application in self-cleaning,9-11 antifogging,9 and control of fluid flow based on the special superhydrophilic-superhydrophobic patterns of beetle’s back wings.12-15 Inspired by their promising future, a number of approaches to artificial superhydrophilic surfaces have been developed (e.g., micro- or nanopatterned coatings,16,17 colloidal lithography,18 fractal surfaces,19,20 polymeric nanofibers21) in regard to different theoretical models (mainly the Wenzel22 and Cassie23 models) based on the equilibrium conditions given by Young’s law.3,24 The majority of research on superhydrophilicity has been conducted on TiO2, since its photoinduced superhydrophilicity was discovered in 1997.9 This technique requires an extended period of UV light exposure, typically with an intensity of 1 mW cm-2 and a wavelength of less than 380 nm.6 The superhydrophilicity of TiO2 is lost quickly when placed in a dark environment.9 UV exposure is needed to re-enable the extreme wettability once it is lost in the dark. The high intensity of UV exposure needed makes TiO2 unsuitable for indoor use of antifogging and self-cleaning glass. Additionally, since UV * To whom correspondence should be addressed. E-mail: (S.G)
[email protected]; (K.J.)
[email protected]. Tel: +86-373-3326335. Fax: +81373-3326544.
light is required to produce superhydrophilic surfaces, such technology may not be environmentally friendly and biocompatible.25 On the basis of Wenzel’s equation (eq 1),22 the effect of surface roughness is to amplify the wetting property, that is, roughness makes hydrophilic surfaces more hydrophilic
cos θw ) r cos θγ
(1)
where θw is contact angle (CA) on a rough surface, θγ is the ideal CA on a smooth surface of a sample with the same chemistry, and r is the surface roughness, the ratio of the actual surface area over the projected area. So another approach for forming a stable superhydrophilic surface, without prior UV exposure, is to create a textured surface with the appropriate chemical modification by high-free-energy materials.26-29 Some superhydrophilic surfaces have been produced by these techniques. However, there are several issues originating from the subsequent surface modification, that is, multistep procedures, harsh modification conditions, environmentally unfriendly highfree-energy materials and its contamination, and most importantly, the instability of superhydrophilic effect. In the superhydrophilicity field, there exists another tough issue besides UV irradiation and surface modification, that is, the textured surfaces and TiO2-based superhydrophilic materials cannot survive in different pH environments. Until now, no superhydrophilic surfaces suitable for all pH environments have been prepared in the absence of UV irradiation or without any chemical modifications by high-free-energy materials. As a result, practical applications of such functional materials have not been fully realized and there is a clear need for superhydrophilic surface suitable for all pH environments in the absence of UV irradiation and without chemical modification by highfree-energy materials. Graphite-like carbon has intrinsic properties of thermal and chemical resistance, which can be used in strongly acidic or
10.1021/jp1069384 2010 American Chemical Society Published on Web 10/27/2010
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basic liquids. Therefore, it is expected that the textured carbon film may feature unique wettability performance. Herein, we describe a facile plasma treatment via radio frequency (rf) magnetron sputtering for inducing rough features on carbon surfaces to acquire superhydrophilic character for not only pure water but also corrosive liquids, such as acidic and basic solutions. This is the first example of superhydrophilicity over the whole range of pH values without the help of high-freeenergy compounds and in the absence of UV irradiation and this might open up new perspectives in preparing novel nanoscale interfacial materials.30-32 2. Experimental Section Graphite plates with 99.9% purity were used as original carbon material. After the graphite target was washed in acetone by sonication for 10 min, it was textured by sputtering graphite target using radio frequency (rf) magnetron sputtering for 5 min. Ar gas at 5 sccm (standard-state cubic centimeter per minute) and 5 mTorr was utilized during the sputtering process. The forward rf power was 40 W, and the reflected power was 02 W. The field emission scanning electron microscopy (FESEM) images were obtained on an XL30 ESEM FEG scanning electron microscopy operating at 20 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were taken with a JEOL JEM-2010 transmission electron microscope. The TEM and HRTEM samples were prepared by removing the surface materials from the sputtered target by a scalpel and then transferring onto a copper grid. Raman spectroscopy was carried out using a WITEC CRM200 Raman system. The excitation source is a 532 nm laser with a laser power below 0.1 mW on the sample to avoid laser-induced local heating. The wettability of the as-prepared film was characterized by measuring the water CA with a contact angle meter. A 2 µL water droplet placed on this hierarchically textured film for water CA measurement. CA values were obtained by averaging five measurements on different areas of the sample surface.
Figure 1. FESEM images of the top (A,C) and side (B,D) views of the as-prepared textured carbon films (A,B) and the original graphite (C,D). TEM image (E) and the corresponding HRTEM image (F) of the as-prepared textured carbon film. The scale bar in panels A-D is 200 nm and in panels E and F is 50 and 5 nm, respectively.
3. Results and Discussion Figure 1A,B shows FESEM images from the top and side views of the as-prepared carbon film. We can see a layer of vertical needles with a wedge-shaped morphology, which are almost perpendicular to the surface of the substrate. The average diameters of the top of needles and the interneedle distance are about 12 and 70 nm, respectively. That indicates the film consists of needles with sharp tops and thick bases, which facilitates a high surface roughness accompanied with excellent mechanical strength and adhesiveness. This is considered an ideal morphology for a superhydrophilic film. Compared with the original carbon film (Figure 1C,D), the plasma-treated carbon film is highly textured. The microstructure of the as-grown sample is further analyzed using TEM and HRTEM. TEM image (Figure 1E) clearly reveals a layer of vertical needles with a wedgeshaped morphology, which is well consistent with the FESEM result. The HRTEM image in Figure 1F can give further insight into the details of the structure. The lattice fringes can be clearly distinguished, and the d spacing of 3.45 Å corresponds to the {111} plane of graphite. Micro-Raman spectroscopy was monitored to identify the carbon species of graphite before and after plasma treatment (Figure 2). The typical features in the Raman spectra are the G band at 1575 cm-1 and the D band at 1348 cm-1. The G band is usually assigned to the E2g phonon of C sp2 atoms, while the
Figure 2. Raman spectra of the raw graphite (A) and the as-prepared textured carbon film (B).
D band is a breathing mode of κ-point phonons of A1g symmetry. The intensity ratio (I(D)/I(G)) of D band to G band of the graphite is about 0.55. Plasma treatment increases the I(D)/I(G) to 1.22. Compared with that of the raw graphite, a progressively prominent D band is an indication of increasing disorder, originating from defects associated with vacancies, grain boundaries,33 and amorphous carbon species.34-37 It is instructive to note that after plasma treatment, the 1621 cm-1 band, assigned to the fundamentals corresponding to the high density of phonon states and termed as D′ peak,38 is also clearly observed as an independent peak (Figure 2B). As we know, the graphite crystal composed of regularly stacked infinite planes has space group symmetry D6h. The Raman spectrum of the complete graphite crystal is consistent with the group theoretical prediction assuming D6h symmetry.39 It has been proven that the crystallite size has much importance on the Raman spectra of graphite.40
Superhydrophilicity of Highly Textured Carbon Films
Figure 3. (A-C) Pictures showing the water CAs of the textured carbon films; the pH values are (A) pH 7, (B) pH 1, and (C) pH 14. (D) The water CA of the original carbon film.
In carbon materials with lower graphitization degree, the imperfection of the crystal lattice leads to the breakdown of the wave vector selection rule, and phonons outside the center of the Brillouin zone contribute to Raman scattering.39,41 At the edge plane of graphite, the D6h space group symmetry is not maintained, since the infinitely extended graphite planes are discontinued at the edge plane. The characteristic feature of the observed Raman spectrum, D′ peak, is due to the discontinuity of graphite planes, that is, the micromation of the crystallite size. Thus, the combination of the increase of I(D)/I(G) with the appearance of D′ peak demonstrates the transformation of graphite to nanocrystalline graphite,39,41,42 which is well consistent with the FESEM observation. In graphite, the precise nature of the disorder and defects is of great interest, and their presence can be linked to the extent of π conjugation, concentration of defects, and the modification of the electronic or optical properties, which will be discussed separately. The observation of the superhydrophilic surface was initiated by the CA measurements with a contact angle meter. When a 2 µL water droplet touched this textured film, it spread rapidly along the film, producing a flat surface of water on it. Figure 3A shows the water CA of the textured carbon film (pH 7). The water CA became close to 3.0° within a fairly short time (1000 ms), which is greatly less than that of a fresh graphite plane (80°, Figure 3D)41 and indicates the superhydrophilicity of the film. This result is well consistent with eq 1, which shows that the water CA of the surface decreases with increasing surface roughness when the surface is composed of hydrophilic materials. Herein, the textured structure of carbon film was believed to contribute to the large surface roughness, and therefore, result in the superhydrophilicity. Additionally, the high concentration of defects or dangling bonds is a result of the specific morphology, which originates from the plasma treatment and is also responsible for the superhydrophilicity.42 Figure 3B,C shows the shapes of aqueous solution droplets with pH values of 1 and 14, respectively, on the textured carbon films. Surprisingly, these two droplets also spread rapidly along the film with CAs less than 5° (0 and 2.5, respectively), which demonstrates that the superhydrophilic carbon films can be used throughout the range of pH values from 0 through 14, all pH environments for corrosive liquids. A question arises why and how the graphite substrate can be textured by plasma? Plasma treatment is generally accepted for application in the surface modification of various materials.20,43-46
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Figure 4. FESEM images of the top (A,C) and side (B,D) views of the textured carbon films with rf power of 25 W (A,B) and 50 W (C,D). The scale bar is 200 nm.
Plasma is the fourth state of the matter, generated by a discharge induced in a partial vacuum. It is an excited gas that generally consists of energetic particles (e.g., atoms, ions, molecules, free radicals, electrons and metastable species). The graphite carbon surface with the energetic particles breaks the covalent bonds on the surface of the bombarded carbon and leads to the formation of the surface radicals on the treated carbon. These surface radicals interact with the active plasma species to form various functional groups on the surface of carbon. This process can be ascribed to solid-gas-solid growth mechanism and induces the hierarchically textured surface. To further verify the proposed mechanism, we change the rf power to check the energy-dependence, as shown in Figure 4. When the power was decreased from 40 to 25 W, the energy was so low that just a small quantity of energetic particles was generated so that the texture degree was quite low (Figure 4A,B). When the power was increased from 40 to 50 W, the energy was high enough to produce large amount of energetic particles, which intensively bombarded the surface of the target and quickly formed the hierarchically textured surface (Figure 4C,D). This observation is well consistent with the solid-gas-solid growth mechanism as well as an experimental evidence of the effect of rf power on the texturing process. To optimize the plasma treatment to achieve optimal textured surface, we carried out time-dependent experiments during which samples at different time intervals were checked by FESEM. As shown in Figure 5A,B, at the early stage of sputtering for 1 min the sample surface was just slightly textured. For two-more-minute sputtering (Figure 5C,D), the sample featured progressive texturing effect. With plasma further processing (e.g., 5 min, Figure 1A to B), the texturing becomes complete. Unfortunately, longer plasma (7 and 9 min, Figure 5E to H) results in the damage of the textured surface. On the basis of such observations, the optimal plasma condition is rf power of 40 W and sputtering time of 5 min. To determine the effect of pH values on CAs for the textured carbon films, further studies were developed in detail. Figure 6 shows the relationship between pH values and CAs on the textured carbon films. There is no obvious fluctuation of the CA values. All CA values are in the range from about 0 to 4.1 with little difference coming from experimental error. This result indicates that pH values of the aqueous solution have little or no effect on CAs for as-synthesized carbon films. It confirms that the textured carbon films are superhydrophilic in the pH range from 1 to 14. Graphite-like carbon has intrinsic properties of thermal and chemical resistance, which can be used in
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Figure 7. Water CAs change with time when the textured carbon film was kept in air, showing excellent stability.
the water CA change with time when it was kept in ambient air. The water CA fluctuated just slightly but still retained its superhydrophilicity, even after six months, which substantiates that the as-prepared carbon film possesses a very stable superhydrophilicity. Since the structures of the textures and the material properties of carbon are stable, the superhydrophilicity produced by the combination of carbon micro and nanostructures is stable. 4. Conclusion
Figure 5. FESEM images of the top (A,C,E,F) and side (B,D,F,H) views of the sputtered graphite films with rf power of 40 W and sputtering time of 1 min (A,B), 3 min (C,D), 7 min (E,F), and 9 min (F,H), respectively. The scale bar is 200 nm.
In this article, we describe a method for inducing rough features on carbon surfaces using plasma technique. The resulting surfaces exhibit an excellent hydrophilic character. The method of surface design described here does not require the use of masks or lithography, can be applied to very large surfaces in a very short time, and herein offers an inexpensive and rapid method for the creation of roughness. More interesting and importantly, the as-prepared films are superhydrophilic for not only pure water but also corrosive liquids, such as acidic and basic solutions. This is the first example of superhydrophilicity over a wide range of pH values without the help of high-free-energy compounds and in the absence of UV irradiation, and this might open up new perspectives in preparing novel nanoscale interfacial materials. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 21071047), the Henan Provincial Natural Science Foundation of China (092300410196), and Natural Science Foundation of Educational Department of Henan Province (2008A150014). References and Notes
Figure 6. The relationship between pH values and water CAs on the textured carbon films.
strongly acidic or basic liquids. Nanostructured graphite-like carbon films were reported to be stable in an acidic or basic solution at normal temperature and oxidized only when subjected to strong oxidation treatment, such as reflux at 140 °C in a mixture of concentrated nitric and sulphuric acid (1:3 in volume ratio).41,47 In our study, no obvious changes, including both structure and hydrophilicity, are found when the nanostructured carbon films are dipped into strong alkali and acid for 24 h. The long-term preservation of superhydrophilicity is an important criterion for real applications of superhydrophilic materials. To examine this parameter, the as-prepared textured carbon film was kept in air for half of a year. Figure 7 presents
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