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Redox-Switchable Superhydrophobic Silver Composite Arun Kumar Sinha,† Mrinmoyee Basu,† Mukul Pradhan,† Sougata Sarkar,† Yuichi Negishi,‡ and Tarasankar Pal*,† ‡ †
Tokyo University of Science, Tokyo, Japan Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India
bS Supporting Information ABSTRACT: Unique packaging of Ag2O on the surface of polycrystalline AgCl allows fabrication of a new useful, superhydrophobic composite material. This pure inorganic material with surface porosity of submicrometer aperture size fabricates air pockets, which make the composite material superhydrophobic. The new material behaves like lotus leaves, butterfly wings, or water strider’s leg in relation to superhydrophobicity. Visible light induces photoreduction of solid Ag2O surface layer and generates Ag(0), making the composite surface superhydrophilic. Reoxidation of Ag(0) on the composite surface gives back the hydrophobicity that represents the redox-switchable wetting property of the material.
’ INTRODUCTION Superhydrophobic materials currently find numerous applications governed by both their chemical composition and geometry in the solid states.1,2 In 1997, Fujishima and co-workers3 first reported a thin TiO2 film showing a water contact angle of 72° before ultraviolet irradiation. After irradiation, water droplets spread out on the film, resulting in 0° contact angle. Superhydrophobic materials are of two types: natural and synthetic. Superhydrophobic systems in nature include lotus leaves, butterfly wings, and water strider’s legs, while artificial surfaces with superhydrophobicity typically feature micro/nanostructured morphologies bearing low surface free energy.4 Gao and Jiang5 reported the superhydrophobic property of water strider’s legs (Gerris remigis), where the water contact angle was 167.6°. The presence of special hierarchical structure with large numbers of oriented tiny hairs with fine nanoapertures on the legs of the water strider become more important in inducing water resistance.5 Barthlott and Neinhuis6 were first to explain the superhydrophobicity of lotus leaves. They reported that this unique property is caused by the surface papillae. The micrometer-sized surface papillae are composed of micro- and nanometer-scale hierarchical structures that are fine-branched nanostructures on top of micropapillae. Water droplets stand almost spherical on them and roll off easily, which is usually referred to as the “lotus effect”.6 Butterflies easily shake off water droplets from their hydrophobic wings. Such property also originates from the special microstructures on their wings. These surface microstructures on the wing can show self-cleaning properties.7 Wettability of metal oxide surfaces is influenced by both the surface geometry and the chemical composition. Until now, r 2011 American Chemical Society
UV-light-driven inorganic oxide surfaces such as TiO2,8 ZnO,9 WO3,10 V2O5,11 and SnO212 show the superhydrophobic/superhydrophilic transition. Upon UV light exposure, the hydrophobic oxide surface becomes hydrophilic, and in the dark, the hydrophilic surface gradually changes back to hydrophobic, and this transition is reversible. Factors contributing to this phenomenon include surface roughness, chemical modification, and photochemical reduction of the oxide surface. Besides UV light illumination, X-rays have been observed to be an effective stimulus that can tune successfully the wettability of a variety of inorganic materials (ZnO, p-Si, Al2O3, SrTiO3, TiN, ZnS, CuO, and Cr2O3). The wettability can be repetitively switched between on and off by X-ray exposure.13 When a copper film is modified with the n-alkanoic acids, CH3(CH2)nCOOH (n = 1, 2, 3, ..., 16), the surface wettability between superhydrophobicity and superhydrophilicity is achieved.14 Another example of a chemically modified hydrophobic material is “magic sand”15,16 a commercially available material made from regular sand by liquid silicone spray-coating to make the sand hydrophobic. A tiny amount of magic sand floats on water like a film but larger amounts sink in water, sticking together and resisting movement. In this article, we present for the first time the wettability property of Ag2OAgCl composite material that has been prepared via a simple inorganic reaction. The well-structured nanoporous Ag2OAgCl is obtained from freshly prepared solid AgCl powder from a strong NaOH medium. Without any Received: April 26, 2011 Revised: August 1, 2011 Published: August 02, 2011 11629
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Scheme 1. Packing of Ag2OAgCl Composite Material from Single Precursor AgCl with NaOH
chemical modification, the purely inorganic Ag2OAgCl composite material shows a high water contact angle (WCA = 150°). Upon visible light exposure, the black material (Ag2OAgCl) is changed to brown superhydrophilic AgAgCl. When AgAgCl is reoxidized to Ag2OAgCl, the superhydrophobicity property returned. So the transition between superhydrophobicity/superhydrophilicity is observed to be redox-reaction-switchable.
’ EXPERIMENTAL SECTION Materials. All the reagents were of AR grade. Triple-distilled water was used throughout the experiment. AgNO3 and HCl were obtained from E Merck. AgCl was prepared from those precursors following simple inorganic procedure. Solid NaOH was purchased from BDH Company, Mumbai, India. All the reagents were used without further purification. Analytical Instruments. Powder X-ray diffraction (XRD) was done in a PW1710 diffractometer, a Philips, Holland, instrument. The XRD data were analyzed by use of Joint Committee on Powder Diffraction Standards (JCPDS) software. The chemical state of the element on the surface was analyzed by a VG Scientific Escalab MK II spectrometer equipped with a Mg Kr excitation source (1253.6 eV) and a five-channeltron detection system. Fourier transform infrared (FTIR) studies were performed with a Thermo-Nicolet continuum FTIR microscope. Field emission scanning electron microscopy (FESEM) analysis was performed with a supra 40, Carl Zeiss Pvt. Ltd. instrument and an energy-dispersive spectrometry (EDS) machine (Oxford, Link, ISIS 300) attached to the instrument was used to obtain the composition. Transmission electron microscopy (TEM) analysis was performed with an H-9000 NAR instrument, Hitachi, using an accelerating voltage of 300 kV. Water droplets (50500 μL) were dispensed carefully onto the Ag2OAgCl composite surfaces by use of a micropipet. The average contact angle value was evaluated by measuring at five different positions of the same sample by digital still camera (Sony Cyber-shot 8.1 megapixels) in sidewise fashion. During this experiment there was no disturbance such as mechanical jerking or air flow.
Figure 1. XRD analysis of (a) Ag2OAgCl composite material and (b) AgAgCl composite material. for some time. The grinding is done until the sample floats on water as a fine powderlike film. We have adopted a solid-state photochemical approach for the preparation of AgAgCl from the Ag2OAgCl composite material. The black Ag2OAgCl material is placed on a Petri dish as a thin layer and is exposed to visible light of a 200 W bulb, with the material kept at a distance of 10 cm from the light source. Ag2 OAgCl ðblackÞ þ visiblelight f AgAgCl ðbrownÞ After 10 h, the black material is changed into a brown mass. The brown powder upon analysis indicates the formation of AgAgCl. The brown mass AgAgCl reverts back to black Ag2OAgCl when the brown AgAgCl (1.0 g) is ground in an ethanolwater mixture with an agate mortar and pestle.
Preparation of Ag2OAgCl and AgAgCl Composite Material. To an aqueous solution of NaOH (5 mL and 35 M) in a
’ RESULTS AND DISCUSSION
50 mL beaker was added dry AgCl (0.1750 g) powder in portions with stirring. This changes the color of AgCl from white to black. The whole mass of AgCl is not converted to Ag2O, but the surface of AgCl becomes covered with Ag2O layers, leaving a robust coreshell morphology (Scheme 1).
Characterization of Materials. In recent developments in materials chemistry, it is being emphasized to control the surface property of the materials, with the reproducibility and structural complexity taken into consideration. We have discovered a methodology for the synthesis of porous Ag2O shell on AgCl, which shows superhydrophobicity without any surface modification by a long-chain alkyl group. Upon visible light exposure, the black material (Ag2OAgCl) is changed to brown AgAgCl and becomes superhydrophilic. Both the synthesized materials are characterized by various physical methods. X-ray Diffraction Analysis. X-ray diffraction (XRD) analysis is carried out with the black Ag2OAgCl composite and shown in Figure 1a. All patterns matched very well with the JPCDS
AgCl þ NaOH ð35 MÞ f Ag2 OAgCl The shell of the Ag2O layer inhibits percolation of NaOH and hence resists further reaction of AgCl core even if the material is kept under NaOH solution for a week and occasionally stirred. The resulting black powder is washed several times repetitively with distilled water and ethanol and dried under vacuum at room temperature. In our reaction, the product yield is 82% (determined gravimetrically). The black mass is finely ground (in ethanol/water = 1:3) with an agate mortar and pestle
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Figure 2. XPS spectra of Ag2OAgCl indicate (a) the presence of Ag1+ (before tungsten-light irradiation) and (b) the presence of Ag0 after tungstenlight irradiation of Ag2OAgCl, indicating AgAgCl formation.
Figure 3. (ac) FESEM images at different magnifications and (d) EDS spectrum of black composite material.
standard data of AgCl (JCPDS file 31-1238). The peaks at (2θ) 27.82°, 32.24°, 46.25°, 54.81°, and 57.56° were assigned to the (111), (200), (220), (311) and (222) planes of the cubic phase17 of AgCl crystal and are labeled in black. The peaks at (2θ) 33.02°, 38.24°, and 65.70° were assigned to the (111), (200), and (311) planes of cubic structure18 of Ag2O crystal and are labeled in red (JCPDS 012-0793). The observed XRD pattern reveals that the intercalation of the silver chloride particle in the Ag2O matrix represents a core. The generation of Ag(0) on the surface of the material by visible light has been proved by powder X-ray diffraction, which is shown in Figure 1b. The black composite material Ag2OAgCl, upon visible light exposure, changes to brown AgAgCl, which has diffraction peaks at 38.1°, 44.3°, and 64.2°. These four diffraction peaks matched well with the (111), (200) and (220) crystalline planes of metallic silver, which are labeled in red.17 The positions of the four peaks could be well-indexed to the cubic phase of Ag (metallic Ag, JCPDS file 65-2871). No other phases such as Ag2O or AgO were observed in AgAgCl
composites, indicating that the sample only contained the phase of metallic Ag and AgCl. X-ray Photoelectron Spectroscopic Analysis. To investigate the chemical state of silver in black Ag2OAgCl composite, we carried out X-ray photoelectron spectroscopy (XPS) measurements (Figure 2a). The XPS peaks for Ag in AgCl in the case of the Ag2OAgCl composite were centered at 369.51 and 375.55 eV, which described Ag 3d5/2 and Ag 3d3/2, respectively. On the other hand, the peaks for Ag in Ag2O (367.91 and 373.88 eV, which described Ag 3d5/2 and Ag 3d3/2, respectively) in the Ag2OAgCl composites are also negatively shifted from the pure Ag2O. This might be due to the interaction between Ag2O and AgCl. After photoreduction, we have also investigated the oxidation state of silver for the brown AgAgCl composite, and the XPS spectrum is shown in Figure 2b. The XPS peaks for metallic Ag(0) in the composite AgAgCl were centered at 368.01 eV (Ag 3d5/2) and 374.02 eV (Ag 3d3/2), indicating the presence of Ag(0). On the other hand, the XPS peaks for Ag of AgCl in the case of AgAgCl composite were centered at 369.98 eV 11631
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Figure 4. (a) Surface image of Ag2OAgCl and (b) silver nanoparticle formation on the composite material upon visible light irradiation, that is, AgAgCl.
Figure 5. (a, b) TEM images of Ag2O(shell)AgCl(core) and (ce) lattice fringes of (c) shell, (d) core, and (e) Ag0.
(Ag 3d5/2) and 375.95 eV (Ag 3d3/2). The peaks show negative shifts with respect to pure metallic silver and pure AgCl due to the interaction between Ag(0) and AgCl.17,19 The above XRD analysis confirms that Ag2O and AgCl coexisted in the Ag2OAgCl composite and Ag(0) and AgCl coexisted in the AgAgCl samples.
Field Emission Scanning Eelectron Microscopic Analysis. The morphology of the as-prepared black Ag2OAgCl composite can be seen in FESEM images at different magnifications (Figure 3). The sample has an interesting stonelike morphology, which results in the aggregation of Ag2O nanoparticles. The closeup image shows that the morphology of the stonelike material is an assembly of particles that arises from the packed spherical particles of Ag2O on a AgCl core. The composite material has a length of several micrometers. Figure 3b,c shows highly magnified images that identify individual particles consisting of hierarchical but densely packed well-oriented nanoparticles. The composite is additionally characterized by energydispersive X-ray spectroscopy (EDS), which confirms that Ag, Cl, and O (other Au and C come from gold coating and carbon basement) are present as constituents (Figure 3d). Upon visible light exposure, the black material Ag2OAgCl changes to brown AgAgCl. The photon from visible light reduces the upper layer oxide (Figure 4a) and forms Ag(0) on the surface of AgCl. The surface morphology of AgAgCl is shown in Figure 4b. Typical spherical Ag nanoparticles appear on the AgCl surface as individual entities. Standard and High-Resolution Transmission Electron Microscopic Analysis. The composite material Ag2OAgCl is shown in Figure 5a,b as examined by TEM analysis. The crystalline shell and core part of the composite are confirmed from the lattice gap as shown in Figure 5panels c and d, respectively. Furthermore, the HRTEM images reveal that the interplanar distance of 2.00 Å correlates with the interplanar distance of the (220) plane of Ag2O, while the interplanar distance of 2.45 Å correlates with the interplanar distance of the (200) plane of AgCl. After visible light irradiation, the surface Ag2O is converted to Ag(0) on the composite surface. The lattice fringe with an interplanar spacing is observed to be 0.234 ( 0.01 nm, which is consistent with the interplanar distance of the (111) plane of Ag(0) (Figure 5e). Wettability Properties of Ag2OAgCl Composite Material. The cavity of the stonelike structure is found to be within the nanometer range. Interestingly, the material shows superhydrophobicity and experimentally the powder exhibits a nanoporous structure with submicrometer apertures, similar to natural 11632
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superhydrophobic materials, and retains its hydrophobicity for extended periods.1,2,6 The surface wettability of the Ag2O AgCl composite has been studied by measuring the water contact angle (CA) as shown in Figure 6. The water droplet on top of the black powder shows CA ∼ 150°, which indicates the superhydrophobic character of the composite material (Figure 6a). A large volume of water (0.5 mL) can easily ride on the black sample (0.75 g) for long periods of time, showing a strong water-repellent property (Figure 6c and video file 1 in Supporting Information). The water droplet can easily roll on the hydrophobic powder and in turn collects a thin layer of black powder that rides on the droplet (Figure 7). We have also performed an FTIR study of the sample before and after the hydrophobicity test and noticed that there is no dissociative water adsorption. The FTIR spectrum is shown in Figure 8. The material remains stable and dry for years together (tested for 2 years at regular intervals) in adverse weather conditions. The sample loses its hydrophobicity in
∼10 h and becomes hydrophilic upon direct exposure to visible light out of a 200 W electric bulb (video file 2 in Supporting Information). Upon visible light exposure, the material Ag2O AgCl changes to AgAgCl and the nanoporous architecture of Ag2O (in Ag2OAgCl) was photochemically modified to AgAgCl structure bearing spherical silver(0) on the surface. The surface morphology of AgAgCl is shown in Figure 4b. Due to the presence of individual Ag(0), the cross section of the nanocapillary is increased, so the air cushion is unable to resist the water droplet and the sample becomes hydrophilic. The change from superhydrophobicity to superhydrophilicity in terms of contact angle is represented in Figure 9. Interestingly, when the photoirradiated hydrophilic powder is dispersed in an ethanolwater mixture and ground repetitively in a mortar with a pestle for ∼3 h, it regains its original hydrophobic property (Scheme 2). This novel reversible hydrophobicity/hydrophilicity is controlled by a redox reaction, in which the redox cycle (Ag+ f Ag0)/(Ag0 f Ag+) is driven repetitively by light and dissolved oxygen, respectively (Figure 10). We have taken a comparative corrosion account also with small pieces of clean iron plates. The plates in open air corrode easily, but the plates with black powder covering together with a water droplet as the rider resist corrosion. Because of the nanoporous network, the nanostructured Ag2OAgCl should trap air in the apertures. The hierarchical roughness and trapped air are responsible for the hydrophobic behavior of the composite material, and thus the water droplet rides on the surface of the powder as mentioned earlier. Due to the presence of trapped air in the composite material, there is an internal reflection at the airwater interface as shown in Figure 6c. A large mass
Figure 6. Measurement of (a) water contact angle, (b) upper view, and (c) capacity of water hydrophobicity (0.5 mL).
Figure 8. FTIR characterization of composite material before (green spectrum) and after (red spectrum) hydrophobicity testing.
Figure 7. Digital pictures represent (a) composite material, (b) water on the composite, and (c) rolling water droplet on the composite. 11633
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Figure 9. Variation of contact angle with visible light irradiation time.
Scheme 2. Redox-Switchable Superhydrophobicity and Superhydrophilicity of Silver Composite
Figure 11. Digital picture represents composite material (0.20 g) easily floating on water; the material is highly hydrophobic.
with nanoporous surface is sufficient to create a hydrophobic surface. According to Wenzel’s law,2b the very high water CA value of 150° must be related to the minute surface roughness of the composite bearing air pockets. The air pockets completely block the penetration of water droplets through the Ag2O-derived grooves. Again, consideration of the superhydrophobicity of the heterogeneous surface composed of an airsolid matrix speaks for the Cassie and Baxter formulation. Here it is idealized that a large fraction of air trapped in the grooves forms a filmlike cushion at the filmwater interface that prevents the penetration of the water droplet into the grooves. A uniform film of air acts like a wrapper on a superhydrophobic solid surface, which is analogous to the air film on raindrops. In either case, a film of air remains physisorbed and makes the frictional force of similar nature.
’ CONCLUSIONS In conclusion, Ag2OAgCl with special nanoscale hierarchical surface structures has been prepared. The inorganic material shows a remarkable surface wettability that guarantees superhydrophobicity, a challenge in surface chemistry. Cooperation of the nanoporous surface, orientation of the crystal planes, and the air pockets in the nanostructure are responsible for the novel hydrophobic behavior. This arrangement of the coreshell composite as a coating would protect nonaquatic instruments if they are showered by water drops. Our discovery may be helpful to design miniature nonaquatic devices, simple oilwater separation systems, and nonwetting materials. ’ ASSOCIATED CONTENT
bS Figure 10. Reversible change of contact angle with reduction and oxidation reaction.
(0.20 g) of the material easily floats on water, as shown in Figure 11 and video file 3 in Supporting Information. The reason behind floating is the presence of air pockets in the sample. Commercial Ag2O or AgCl does not show hydrophobicity at all, but intelligent packaging of the two components
Supporting Information. Three video files, showing superhydrophobic behavior of Ag2OAgCl and superhydrophilic behavior of AgAgCl and floating behavior of Ag2OAgCl. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail
[email protected]. 11634
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’ ACKNOWLEDGMENT We are thankful to the Council of Scientific and Industrial Research (CSIR), Department of Science and Technology (DST), the University Grant Commission (UGC), and the Indian Institute of Technology, Kharagpur, for financial assistance. ’ REFERENCES (1) (a) Han, J. T.; Lee, D. H.; Ryu, C. Y.; Cho, K. J. Am. Chem. Soc. 2004, 126, 4796. (b) Hikita, M.; Tanaka, K.; Nakamura, T.; Kajiyama, T.; Takahara, A. Langmuir 2005, 21, 7299. (c) Han, J. T.; Xu, X.; Cho, K. Langmuir 2005, 21, 6662. (2) (a) Xin, B.; Hao, J. Chem. Soc. Rev. 2010, 39, 769. (b) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (c) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (3) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T Nature 1997, 388, 431. (4) Feng, X.; Jiang, L. Adv. Mater. 2006, 18, 3063. (5) Gao, X.; Jiang, L. Nature 2004, 432, 36. (6) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (7) Wagner, T.; Neinhuis, C.; Barthlott, W. Acta Zool. 1996, 77, 213. (8) Feng, X. J.; Zhai, J.; Jiang, L. Angew. Chem., Int. Ed. 2005, 44, 5115. (9) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62. (10) Wang, S.; Feng, X.; Yao, J.; Jiang, L. Angew. Chem., Int. Ed. 2006, 45, 1264. (11) Lim, H. S.; Kwak, D.; Lee, D. Y.; Lee, S. G.; Cho, K. J. Am. Chem. Soc. 2007, 129, 4128. (12) Zhu, W.; Feng, X.; Feng, L.; Jiang, L. Chem. Commun. 2006, 2753. (13) Kwon, Y.; Weon, B. M.; Won, K. H.; Je, J. H.; Hwu, Y.; Margaritono, G. Langmuir 2009, 25, 1927. (14) Wang, S. T.; Feng, L.; Liu, H.; Sun, T. L.; Zhang, X.; Jiang, L.; Zhu, D. B. ChemPhysChem 2005, 6, 1475. (15) Sarquis, J. L.; Sarquis, M.; Williams, J. P. Teaching Chemistry with Toys: Activities for Grades K9; Terrific Science Press: Middletown, OH, 1995, 183. (16) Goldsmith, R. H. J. Chem. Educ. 2000, 77, 41. (17) Xu, H.; Li, H.; Xia, J.; Yin, S.; Luo, Z.; Liu, L.; Xu, L. ACS Appl. Mater. Interfaces 2011, 3, 22. (18) Wang, X.; Wu, H. F.; Kuang, O.; Huang, R. B.; Xie, Z. X.; Zheng, L. S. Langmuir 2010, 26, 2774. (19) Bai1, J.; Li1, Y.; Yang1, S.; Du1, J.; Wang1, S.; Zhang, C.; Qingbiao Yang, O.; Chen, X. Nanotechnology 2007, 18, No. 305601.
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