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J. Phys. Chem. C 2008, 112, 8338–8342
Ammonia Responsive Surface Wettability Switched on Indium Hydroxide Films with Micro- and Nanostructures Weiqin Zhu,†,‡ Jin Zhai,*,† Zhongwei Sun,† and Lei Jiang*,† Beijing National Laboratory for Molecular Sciences (BNLMS), Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100864, People’s Republic of China ReceiVed: January 23, 2008; ReVised Manuscript ReceiVed: March 11, 2008
Switchable surface wettability that can be tuned reversibly between superhydrophobicity and superhydrophilicity has been successfully realized on indium hydroxide films with micro- and nanostructures. The films are superhydrophobic in the air with a water contact angle of 150.4 ( 0.9° while they turn superhydrophilic with a water contact angle of 0° after storage in an ammonia atmosphere, which means an ammonia responsive surface wettablity switch. The novel surface wettability is ascribed to the multiple surface structures and surface free energy conversion of the films. This kind of smart surface wettability is promising in developing new types of ammonia detectors simply by virtue of the surface wettability conversion. Introduction With the increasing attention to environment protection, developing simple and effective ways to detect ammonia (NH3 · H2O) has become a noticeable subject. Up to now, gas detectors operate mainly in virtue of resistance or luminescence changes.1,2 Surface wettability changes that are sensitive for the outside atmospheres, however, have rarely been investigated for harmful atmosphere detection.3,4 Indium hydroxide (In(OH)3) is an amphoteric wide band gap semiconductor.5–7 Besides the excellent optical and semiconductive properties, In(OH)3 exhibits subacidity in a basic atmosphere,8 which makes it easy to interact weakly with NH3 · H2O molecules and induce the change of its surface wettability, thus providing a probable way to detect ammonia conveniently. Herein, we reported the ammonia responsive surface wettability that can be switched between superhydrophobicity and superhydrophilicity on In(OH)3 films with micro- and nanostructures. The films show superhydrophobicity in the air while they turn superhydrophilic in an ammonia hydroxide atmosphere. By heating the superhydrophilic films in the air for a period of time, the surface wettability reconverts to superhydrophobicity again. And the reversible conversion shows excellent reproducibility. Since In(OH)3 is an excellent material for industrial application, this kind of switchable surface wettability of In(OH)3 makes it promising to work as new types of ammonia hydroxide detectors, which is much more low-cost and convenient compared with the traditional ammonia sensors. Besides, the study is also of great significance to present a new concept to design gas sensors that can be easily manipulated and effective. Experiment Section The In(OH)3 films were prepared by low temperature hydrothermal synthesis. The experiment was carried out in a well * To whom correspondence should be addressed. Phone: (+86) 1082621396. Fax: (+86) 10-82627566. E-mail:
[email protected] (J.Z.) and
[email protected] (L.J.). † Beijing National Laboratory for Molecular Sciences. ‡ Graduate University of the Chinese Academy of Sciences.
Figure 1. Experiment device for the storage of In(OH)3 films in ammonia hydroxide atmosphere.
closed glass bottle containing an aqueous solution of indium trichloride (InCl3) and urea ((NH2)2CO) with a clean glass substrate standing against the walls of the closed bottle. In order to obtain films with different microstructures, an aqueous solution consisting of (a) 1.875 × 10-2 M InCl3 and (b) 9.375 × 10-3 M InCl3 and 5.625 × 10-2 M (NH2)2CO as well as (c) 1.875 × 10-2 M InCl3 and 0.1125 M (NH2)2CO has been used for the reaction, respectively. After the solution was heated at 95 °C for 24 h, the fully covered glass substrate was moved out of the bottle and thoroughly rinsed with distilled water to remove any possible contamination from residual salts. Then the films were dried at 90 °C. In order to investigate the NH3 · H2O responsive property of In(OH)3, the films were kept in an ammonia hydroxide atmosphere by using the device shown in Figure 1. The morphology of the as-prepared films was observed by the field-emitting scanning electron microscopy (FE-SEM, JSM6700F). The X-ray diffraction (XRD) was performed on Rigaku D/max 2500 using Cu KR radiation. The XPS analysis was performed on a VG Scientific ESCALab220i-XL spectrometer. The high resolution transmitting electron microscopy (HRTEM, Tecnai F30) was employed to investigate the detailed structure of the microstructures. The surface wettability of the as-prepared films was investigated by the water contact angle measurements with OCA20 (Dataphysics) at room temperature. Results and Discussion Morphology and Structure Analysis of the In(OH)3 Films. The SEM images of the as-prepared films were shown in Figure 2a-c. As can be seen, the morphologies of the as-prepared films
10.1021/jp800661h CCC: $40.75 2008 American Chemical Society Published on Web 05/08/2008
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Figure 2. Morphology of the In(OH)3 films prepared by controlling the reaction conditions. (a) Loosely distributed microcubes with the size of 1–3 µm; (b) randomly distributed nanorods with the size of 50–100 nm in diameter and 200–300 nm in length; (c) micro- and nanomultiple structure with microcubes of 1–3 µm in size and nanorods of 100–200 nm in diameter and 1–2 µm in length; (d) XRD patterns of the as-prepared films can be indexed to a body centered cubic (bcc) phase of In(OH)3 (JCPDS 16–0161).
are different from one another at different reaction conditions. Figure 2a is the SEM image of the film prepared by the solution a, from which the loosely distributed microcubes with sharp edges and corners can be observed. The size of the microcubes is evaluated varying in the range of 1–3 µm. Different from the microcubes, the films prepared by using the solution b is mainly composed of nanorods. The nanorods are randomly distributed on the substrate with the size of 200–300 nm in length and 50–100 nm in diameter. For the films prepared via the aqueous solution c, the multiple structures consisting of microcubes and nanorods have been obtained. The SEM image shown in Figure 2c indicates that the films are uniform on large scale area and composed of cube-shaped particles with the size of 1–3 µm and nanorods with 100–200 nm in diameter and 1–2 µm in length. The microcubes and the nanorods stacked with one another to form the micro- and nanostructures. The X-ray diffraction pattern of the as-prepared film was shown in Figure 2d, in which all of the peaks can be indexed to a body centered cubic (bcc) phase of In(OH)3 (JCPDS 16–0161). The sharp diffraction peaks exhibit a well-crystallized film. For the surface analysis of the as-prepared films, X-ray photoelectron spectroscopy (XPS) was used, and the obtained spectra were shown in Figure 3. The peaks in Figure 3a show that the surface of the film mainly consists of In and O. The weak C 1s peak was considered caused by the inevitably carbon contamination in the air. The In 3d5/2 peak with a binding energy of 444.7 eV can be observed in Figure 3b. Further calculation by combining the In 3d5/2 peak together with the In Auger peak indicates that the surface is composed of In(OH)3. The well symmetrical O 1s peak with a binding energy of 531.2 eV shown in Figure 3c suggests that the oxygen species on the surface of the films is single, further confirming that the surface of the film is composed of In(OH)3 only. In order to get more details about the microstructures of the microcubes and nanorods, the high resolution transmitting electron microscopy was employed. The obtained TEM and HRTEM images were shown in Figure 4. The HRTEM images of the microcubes and nanorods shown in Figure 4 exhibits clear lattice fringes with d spacings of 0.393 and 0.298 nm, which is
Figure 3. XPS spectra of the as-prepared films. (a) Spectrum of total elements shows that the film is mainly composed of indium and oxygen; (b) In 3d5/2 peak with the binding energy of 444.7 eV indicates that the surface of the film is composed of In(OH)3; (c) well symmetrical O 1s peak with binding energy of 531.2 eV further confirms that the film surface is composed of In(OH)3 only.
close to the (200) and (220) plane spacings of bcc (bodycentered cubic) In(OH)3. The HRTEM images demonstrate that the microcubes and nanorods might have [100] and [110] preferential growth direction, respectively, and be simply enclosed with crystal faces of {100}, {001}, and {110}. Variation of Surface Wettability with the Microstructure Change. The surface wettability of the as-prepared indium hydroxide (In(OH)3) film was measured by the water contact angle measurement (OCA20, Dataphysics). Figure 5a-c is the pictures of 2 µL water droplets that inhabited on the surfaces of the films with different microstructure. As can be seen, the film composed of In(OH)3 microcubes is easy to be wetted by water droplets. The water contact angle on the film is measured to be 102.2 ( 3.6° (Figure 5a). Distinct enhancement of hydrophobicity can be observed on the surface of the nanorod films. As seen in Figure 5b, the highly hydrophobic In(OH)3 film with water contact angle of 146.2 ( 0.6° has been obtained by virtue of the randomly arrayed In(OH)3 nanorods. For the film consisting of both microcubes and nanorods, the hydrophobicity was intensified remarkably to a superhydrophobic state. The water contact angle of 150.4 ( 0.9° has been measured on the surface (Figure 5c, left). The results of water
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Figure 6. (a) Water droplets are easy to penetrate in the loosely distributed microcube films; (b) the hydrophobicity is remarkably enhanced by the nanostructure because of the increase of air fraction; (c) an air pocket is formed at the interface between the water droplet and the surface of the film, which makes the film show superhydrophobicity.
Figure 4. (a,c) TEM images of In(OH)3 microcube and nanorod; (b,d) HRTEM images of In(OH)3 microcube and nanorod. The lattice fringes with d spacing of 0.393 and 0.298 nm can be indexed to the (200) and (220) plane spacing of bcc In(OH)3, respectively. Insets are the schematic illustration of the crystal planes of {100}, {110}, and {001} on the surface of the microcubes and nanorods.
Figure 5. Surface wettability of the In(OH)3 films. Shape of a 2 µL water droplet on the surface of (a) In(OH)3 microcube films in the air with the water contact angle of 102.2 ( 3.6°; (b) In(OH)3 nanorod film in the air with a water contact angle of 146.2 ( 0.6°; (c) In(OH)3 microcube and nanorod multiple film (left) in the air with a water contact angle of 150.4 ( 0.9° and (right) after being exposed to ammonia hydroxide atmosphere for 20 h with a water contact angle of 0°; (d) smart surface wettability switched between superhydrophobicity and superhydrophilicity shows excellent reproducibility by the alternate storage in the air and ammonia hydroxide atmosphere.
contact angle measurements confirm that the surface wettability of solids can be tuned by the surface microstructure control.
More interestingly, when exposed to an ammonia hydroxide atmosphere for 20 h, the superhydrophobic film turns superhydrophilic with a water contact angle of 0° (Figure 5c, right). By heating the superhydrophilic film in the air at 90 °C for 1 week, the surface wettability returned to be superhydrophobic again. And the cycles can be reproduced (Figure 5d). Mechanism of the Superhydrophobicity of the In(OH)3 Films. As known, surface wettability is mainly dominated by the surface free energy and surface roughness.9,10 According to the HRTEM analysis, the microcubes and the nanorods contained in the films have [100] and [110] preferential growth direction, with the {100}, {001}, and {110} faces exposed to the surface of the films, respectively (insets in Figure 4a,c). Since the distribution of indium ions and hydroxyl groups on these crystal faces may be relatively symmetrical and well bonded, they always have the lowest surface free energy and were thus kept on the surface of the microstructures, which greatly lowered the surface free energy of the whole film.11,12 Besides, because of the strong combination between the hydroxyls, the surface hydroxyls were trapped by the lattice ones, forming a relatively saturated network of hydrogen bonds, which decreases the polarity of the surface hydroxyls greatly and further deduced the surface free energy.13 Therefore, when water droplets contact the surfaces of the films, the low surface free energy prevents them wetting the surface of the microstructures, and the films exhibit excellent hydrophobicity. For a heterogenoeus hydrophobic surface, Cassie and Baxster’s equation has demonstrated how the surface roughness influences the surface wettability of solids.14 According to the equation cos θf ) f1 cos θw - f2 where θf and θw are the water contact angles on the rough and smooth surface respectively, and f1 and f2 are the area fraction of air and solid respectively (f1 + f2 ) 1), the water contact angle (θf) will be greatly enhanced with the increase of air fraction (f2) in the film. That is, with the increase of surface roughness, the air fraction of the heterogeneous surface will be greatly improved, which finally intensifies the hydrophobicity of a homogeneous surface. For the microcube films we prepared, the loose structure makes it easy for water droplets to penetrate in the grooves between microcubes and replace the trapped air, which greatly decreased the air fraction of the films (f2) and thus caused a relatively lower water contact angle (θf) (Figure 6a). In the case of nanorod films, a large amount of nanogrooves were formed because of the random distribution of the nanorods, which could trap a large volume of air. Since the air inhibiting in the nanogrooves is difficult to be discharged, the air would be trapped in the films by water droplets. Thus, the air fraction (f2) in the nanorod films was improved dramatically, and the hydrophobicity was remarkably enhanced according to the equation. However, on a surface with nanoscale roughness, the water droplet was easy
Ammonia Responsive Surface Wettability Switched
Figure 7. Reversible reaction between the ammonia hydroxide molecules (NH3 · H2O) and indium hydroxide (In(OH)3). (a) In the air, the hydrogen atoms on the surface of In(OH)3 are bound by the lattice hydroxyls inside the In(OH)3 crystals; (b) in the ammonia hydroxide atmosphere, the surface hydrogen atoms break the trap of the hydrogen bond network and combine weakly with NH3 · H2O molecules.
to contact the groove bottom when contacting the film, which resulted in the wetting of the nanostructures and the decrease of the trapped air. Therefore, despite the large volume of the air trapped in the grooves of the nanorods, the surface wettability of superhydrophobicity has not been realized on the films only composed of In(OH)3 nanorods (Figure 6b). For the multiple structured films that consist of loosely distributed microcubes and randomly arrayed nanorods, a large volume of air can be trapped in the micro- and nanogrooves in the film. When water droplets contact the surface of the film, the air consisting in the grooves will be trapped in the films at the interface between the water droplet and the microstructures. Moreover, owing to the improvement of surface roughness by microcubes, the water droplets are not easy to contact the groove bottom and an air pocket at the interface between the water droplets and the film surface was thus formed. Therefore, the water droplets will suspend on the surface of the film, and the film will show superhydrophobicity (Figure 6c).15 Mechanism of the Reversible Conversion between Superhydrophobicity and Superhydrophilicity on In(OH)3 Films. Since In(OH)3 is an amphoteric hydroxide, it exhibits acidity with the hydrogen atoms easy to be deprived when placed in basic atmosphere. Therefore, when the as-prepared In(OH)3 films were exposed to an ammonia hydroxide atmosphere, the basic hydroxyl group in the polar NH3 · H2O molecules tends to capture the hydrogen atoms on the surface of In(OH)3 that were formerly bound by the lattice hydroxyls inside the In(OH)3 crystals (Figure 7a) because of the hydrogen bond action. Since the basicity of NH3 · H2O molecules is much stronger than that of In(OH)3, the surface hydrogen atoms would break the trap of the hydrogen bond network to form a relatively strong combination with NH3 · H2O molecules (Figure 7b). Consequently, the NH3 · H2O molecules would anchor on the surface of the crystals and form a layer of ammonia hydroxide on the surface of the micro/nanostructures, which greatly enhanced the polarity of the crystal faces and consequently increased the surface free energy of the films. The hydrophilicity of the films was hence improved. Therefore, when touching the NH3 · H2O anchored films, water droplets will wet the surface of the microstructures because of the increased surface free energy and will penetrate into the voids in the films replacing the trapped air. Thus, the threedimensional capillary effect on rough surfaces was induced, and the wetting state of superhydrophilicity was formed.16 In order to confirm the mechanism, the parallel experiments with dry NH3 and water vapor under the same experimental conditions have been performed at the same time. However, little change has been observed on the surface wettability of the films treated
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Figure 8. Plots of water contact angles varying with the humidity in NH3 atmosphere. The figure indicates that with the increase of NH3 · H2O concentration, the hydrophilicity of the films is intensified, which further confirms that NH3 · H2O molecules play the main role in improving the hydrophilicity of the films.
under such conditions. The films still exhibit superhydrophobicity. This indicates that NH3 · H2O molecules play the main role in improving the hydrophilicity of the films. Furthermore, the humidity dependent experiment in NH3 at atmospheric pressure has been carried out. The plot obtained was shown in Figure 8. From the figure, we can see that the water contact angle decreases with the humidity increasing. That is, with the increase of NH3 · H2O concentration, the hydrophilicity of the films is intensified. The result further confirms that it is the NH3 · H2O molecules that function in improving the hydrophilicity of the films. It has been tested that the pH value on the surface of the In(OH)3 films increased from 5.5 to 6.0 to 7.5–8.0 after being stored in ammonia hydroxide atmosphere, which is ascribe to the NH3 · H2O anchoring. By heating the superhydrophilic film in the air at 90 °C for a period of time, the weak bond between NH3 · H2O molecules and In(OH)3 would be gradually broken and the NH3 · H2O molecules would be decomposed and escape from the surface of In(OH)3. Thus, the surface of the films reconverted to the original state and the wettabilty restored to superhydrophobicity again. Conclusions In summary, we have succeeded in preparing superhydrophobic indium hydroxide films by controlling the surface microstructures via low temperature hydrothermal synthesis. More importantly, the superhydrophobic films can be tuned to superhydrophilic ones just by storing them in an ammonia hydroxide atmosphere. And the superhydrophilic films can reconvert back to the superhydrophobic state after being heated in the air. The reversible conversion shows excellent reproducibility. It has been concluded that the multiple structure of the films and the weak bond formed between NH3 · H2O molecules and In(OH)3 are the main causes for the reversible surface wettability conversion. Such kind of ammonia responsive surface wettability is expected to be useful in developing new types of NH3 · H2O detectors by virtue of the surface wettability change, which has the advantages of being low-cost, easily manipulated, and convenient. Acknowledgment. This work is supported by the National Research Fund for Fundamental Key Projects (2006CB806200, 2006CB932100, and 2007CB936403), the National Natural Science Foundation of China (20571077, 0573120, 20773142, 50533030) and 863 Project (2007AA032348). References and Notes (1) (a) Feng, X.; Irle, S.; Witek, H.; Morokuma, K.; Vidic, R.; Borguet, E. J. Am. Chem. Soc. 2005, 127, 10533. (b) Du, N.; Zhang, H.; Chen, B.; Ma, X.; Liu, Z.; Wu, J.; Yang, D. AdV. Mater. 2007, 19, 1641.
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