Article pubs.acs.org/JPCC
Tuning the Wettability of Indium Oxide Nanowires from Superhydrophobic to Nearly Superhydrophilic: Effect of OxygenRelated Defects Kavita Yadav,† Bodh Raj Mehta,† Kolluru V. Lakshmi,‡ Saswata Bhattacharya,† and Jitendra P. Singh*,† †
Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Department of Chemistry and Chemical Biology and The Baruch ’60 Center for Biochemical Solar Energy Research, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, United States
‡
S Supporting Information *
ABSTRACT: Herein, the preparation of indium oxide nanowires (IO NWs) having superhydrophobic to nearly superhydrophilic water wetting capability without using any chemical coating is demonstrated. The oxygen-related defects strongly determine the giant variation in water wetting on IO NWs. It is found that the oxygen-rich IO NW surfaces are more favorable for wetting with water. The IO NWs were synthesized under three different ambient growth conditions, namely, oxidizing (IO_W), inert (IO_Ar), and reducing (IO_H2), by using a chemical vapor deposition system. The deposition parameters were calibrated to obtain nanowire morphology. The observed static water contact angles were 8° ± 5°, 134° ± 4°, and 168° ± 2° for the IO_W, IO_Ar, and IO_H2 samples, respectively. The effect of oxygen-related defects on the wettability of IO NW surfaces has been examined by photoluminescence, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy measurements. The results presented herein show that the oxygen-deficient IO NWs show superhydrophobic behavior, whereas stoichiometric IO NWs show nearly superhydrophilic nature. In addition, the ultraviolet light induced wetting transition from hydrophobic to hydrophilic is also studied.
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dissociative adsorption over TiO2 and ZnO surfaces.20 In contrast, there are few studies available to show that oxygen plasma treatment or oxygen annealing converts the surface of ZnO nanowires from hydrophobic to superhydrophilic.26,28,29 In these studies, contradictory results have been shown, and the idea behind water wetting is still unclear. These contradictory reports in the literature on the wettability of metal oxide surfaces suggest that the nature of defects that are responsible for the water wetting properties of metal oxide surfaces and the underlying mechanism remain unclear. Therefore, it is necessary to conduct a detailed study of the water dissociation process to gain insight into this phenomenon. Once the water wetting mechanism becomes clear, then the idea can be utilized to prepare controlled superhydrophobic and superhydrophilic metal oxide nanostructures for future nanodevice applications. Being a wide band gap semiconductor, indium oxide (IO) is widely used in optoelectronic devices and nanoscale chemical sensors and biosensors.30−33 There is no need for waterproof
INTRODUCTION Superhydrophobic and superhydrophilic surfaces with water contact angles greater than 150° and less than 5°, respectively, are often found in nature on plant leaves and insect wings. The plant and insect cuticles provide amazing structural and chemical modifications of surface wetting properties that result in superhydrophobic to superhydrophilic behavior.1−3 It is extremely difficult to wet the superhydrophobic surfaces. In contrast, the superhydrophilic surfaces display widespread wetting behavior. These interesting phenomena have stimulated extensive research to design artificial bioinspired superhydrophobic and superhydrophilic surfaces due to their countless applications.4−15 In addition, it is also important to understand the fundamental mechanism behind water wetting on different types of surfaces. There exists a vast body of literature on the wetting properties of metal oxide surfaces16−27 in which only a few reports have tried to explain the water wetting mechanism. For example, Schaub et al.16 have previously reported that oxygen vacancies are active sites for water dissociation on rutile TiO2(110) through proton transfer to a proximal oxygen atom. It has been found that water molecules that are coordinated to oxygen vacancy sites lead to © XXXX American Chemical Society
Received: April 7, 2015 Revised: June 17, 2015
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Figure 1. (a−c) SEM images of IO NWs prepared under different ambient growth conditions. (d−f) Corresponding contact angles of static water droplets of 5 μL volume.
Billerica, MA). The cw EPR spectroscopy measurements were conducted with an ER 4116-DM dual-mode resonator (Bruker BioSpin) and an E900 continuous-flow helium cryostat (Oxford Instruments, Oxfordshire, U.K.). The operating microwave frequency was 9.64 GHz. The EPR spectra were acquired with a modulation frequency of 100 kHz and a modulation amplitude of 0.5−1.0 G. The Fourier transform infrared (FTIR) spectroscopy measurements were done using Thermo Scientific Nicolet iS 50 system. The wettability (static and rolling) of a water droplet on different IO NW samples was characterized using an optical contact angle setup. The UV light illumination was carried out by using a 9 W Phillips UV B lamp with wavelength in the range between 280 and 315 nm. The distance between the sample and UV lamp during illumination was 6 cm, and the measurements were done at room temperature.
coating if the superhydrophobic IO NWs will be utilized for electronic devices. On the other hand, superhydrophilic IO NWs can enhance the sensing response of biosensors because most of the organic dies and molecules can be easily dissolved with water. Therefore, the preparation of superhydrophobic and superhydrophilic IO NWs without any chemical coating is required to enhance the performance of smart nanodevices based on IO NWs. Also, water wetting studies on IO nanowires are rare.30,31 In the present study, we have synthesized IO NWs in three different environments to control the oxygen vacancy defects. The experimental results show that IO NWs with oxygen-rich surfaces display nearly superhydrophilic wetting behavior, whereas the oxygen-deficient IO nanowires are superhydrophobic in nature. The proposed water wetting mechanism is that the necessary and sufficient condition for water adsorption over the surface of the nanowires is the presence of oxygen atoms in close proximity to the acidic cationic sites which can easily attract hydrogen atoms from water molecules and thus increase the rate of water dissociation.
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RESULTS AND DISCUSSION Morphology and Water Wetting Study. The SEM images of IO samples prepared in oxidizing (IO_W), inert (IO_Ar), and reducing (IO_H2) growth conditions are shown in Figure1a−c, respectively. The SEM images reveal that all three samples are comprised of a nanowire-like morphology. The diameter and length of the nanowires in the three samples range from 40 to 110 nm and from 6 to 15 μm, respectively. The X-ray diffraction (XRD) spectra (not shown here) of these samples illustrate that the intense peaks arise from cubic IO with a lattice constant a = 10.11 Å (JCPDS 71-2195). There are no traces of indium metal present in the samples. We have performed contact angle measurements on the samples at room temperature under ambient atmospheric conditions. A water droplet of 5 μL was used for each study, and the contact angle measurements were performed at five different positions for each sample. Parts d−f of Figure 1 show the contact angles of IO NWs prepared in different ambient conditions. The values of the contact angles of the water droplets on IO_W, IO_Ar, and IO_H2 surfaces are 8° ± 5°, 134° ± 4°, and 168° ± 2°, respectively. The IO_H2 NW sample not only displays superhydrophobic behavior but also demonstrates a rolling off angle of 3° (video S1, Supporting Information). These results indicate that the nanowires in IO_W samples which were prepared in the presence of water vapor (oxidizing agent) display nearly superhydrophilic behavior, whereas the IO nanowires that were prepared in the presence of an inert
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EXPERIMENTAL SECTION The IO NWs were synthesized under different ambient growth conditions using a horizontal tube furnace via the chemical vapor deposition (CVD) method. The tube furnace was purged with argon gas for 20 min and then heated to a temperature of 900 °C. In2O3 powder (99.5% pure) mixed with carbon (1:1) was used as a precursor with gold-coated (∼5 nm) silicon as the substrate. The three different ambient growth conditions that were used during the growth of different IO nanowire samples were (a) Ar gas mixed with water vapor (IO_W), (b) only Ar gas (IO_Ar), and (c) Ar gas mixed with hydrogen gas (50 sccm) (IO_H2). The flow rate of Ar gas was constant at 200 sccm in all of the cases, and the reaction time was 1 h. The morphological and structural characterizations of assynthesized samples were performed by scanning electron microscopy (SEM; ZIESS EVO 50) and high-resolution transmission electron microscopy (HRTEM; Tecnai G20Stwin 200 kV). The photoluminescence (PL) measurements were performed on IO NWs using a Horiba Jobin Yvon Lab RAM (HR 800 Evolution) system where a He−Cd laser of 325 nm wavelength and 30 mW was used as the excitation source. The X-band electron paramagnetic resonance (EPR) spectra were recorded on a custom-built continuous wave (cw)/pulsed X-band Bruker Elexsys 580 spectrometer (Bruker BioSpin, B
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Figure 2. HRTEM images of IO NWs prepared under different ambient growth conditions.
argon atmosphere (IO_Ar) and in a reducing hydrogen atmosphere (IO_H2) display hydrophobic and superhydrophobic behavior, respectively. The surface chemical composition and geometric architecture play important roles in deciding the water wetting of nanostructures.34 Recently, we have observed that the nanorod alignment affects the wetting properties of IO nanorods. The randomly distributed IO nanorods show a contact angle value of 133.7°, whereas the vertically aligned IO nanorods have a superhydrophobic surface with a contact angle of 159.3°.35 It is important to note that in the present case all three types of samples contain almost similar nanowire morphologies with a random distribution. Therefore, it can be easily assured that the morphology of the nanostructures does not play a significant role in determining the giant variation in their wetting behavior. It has been previously reported that IO NWs prepared in an inert Ar atmosphere display hydrophobic behavior,30,35,36 which is in agreement with the current results observed for the IO_Ar sample. The main focus of the present study is to analyze the effect of defects in IO_W (nearly superhydrophilic) and IO_H2 (superhydrophobic) samples that determine the respective wetting behavior. Surface Crystallography of IO NWs. We analyzed the crystallography of the different IO NWs by HRTEM measurements. The HRTEM images reveal that the lattice spacings, d, for the IO_W, IO_Ar, and IO_H2 samples are 0.506, 0.418, and 0.715 nm, respectively, which correspond to the (200), (211), and (110) planes of cubic IO as shown in Figure 2. The HRTEM images confirm that the IO NWs in the three samples are crystalline in nature but have different growth directions. The NWs on IO_W samples have a ⟨100⟩ growth direction, whereas NWs on IO_H2 samples have a growth direction of ⟨110⟩. Surface Defects of IO NWs. We performed PL and EPR spectroscopy analysis to evaluate the oxygen vacancy defects that are present in the IO NW samples. The PL spectra for the three different IO NW samples are shown in Figure 3. The most intense peak that is observed in the PL spectra at 520 nm (green emission) is attributed to oxygen vacancy defects.37,38 The PL spectra illustrate that the IO_H2 nanowires are rich in oxygen vacancy defects. However, oxygen vacancy defects are almost absent in the IO_W NW sample. Another peak at 590 nm in the PL spectra corresponds to the indium interstitial defects.39,40 We have performed EPR spectroscopy measurements to further analyze the oxygen vacancy defects in these samples. The peak at 3445.5 G in the EPR spectra (Figure 4) at a g value of 2.006 corresponds to paramagnetic oxygen vacancy defects in indium oxide.41,42 Interestingly, IO_W NWs do not display any measurable EPR signal, whereas an intense peak was observed in the EPR spectrum of IO_H2 NWs. Thus, the EPR spectroscopy results confirm that IO_H2 NWs are highly oxygen-deficient, whereas there is a lack of oxygen vacancy
Figure 3. Photoluminescence spectra of IO NWs prepared under (a) reducing, (b) inert, and (c) oxidizing ambient growth conditions.
Figure 4. EPR spectra of IO NW samples grown under different ambient growth conditions.
defects in IO_W NWs. This is in agreement with the PL results. The presence of water vapor during growth of IO NWs in IO_W samples maintains an oxygen-rich atmosphere, which results in the growth of IO NWs with rare oxygen vacancy defects. In contrast, the presence of hydrogen gas during growth of IO NWs reduces the IO reactant vapor species during growth, which results in the growth of oxygen-deficient IO NWs (IO_H2). Upon comparison of the oxygen vacancy defects with the water wetting behavior on the surfaces of IO NWs, the surface of oxygen-rich IO NWs (IO_W) exhibits nearly superhydrophilic behavior, whereas the highly oxygendeficient IO NW (IO_H2) surface is superhydrophobic in nature. FTIR spectrometry measurements were performed to further investigate the role of reactive ionic species present on IO NW surfaces on the wetting behavior. Figure 5a shows the FTIR spectra of IO NWs prepared under different ambient growth conditions. The spectra were recorded in transmittance mode in which the IO NWs were isolated from the Si substrate and dispersed in a KBr pellet. The IR peak that is positioned C
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Figure 5. (a, left) FTIR spectra of IO NWs prepared in different ambient growth conditions and (b, right ) FTIR spectra of the IO_H2 nanowire sample before and after annealing in oxidizing growth conditions for 1 h at 500 °C.
Figure 6. (a) Photoinduced water wettability transition from hydrophobic to hydrophilic on the IO_Ar NW sample, (b) water contact angle recovery on the dark-stored IO_Ar sample, (c) cyclic transition of water wettability with UV light illumination and dark storage, and (d) EPR spectroscopy of the IO_Ar NW sample (i) before and (ii) after UV light illumination.
between 3300 and 3600 cm−1 is assigned to the hydroxyl group for both dissociated and molecular water that is adsorbed on the surface, and the peak at 1626 cm−1 is due to the H−O−H bending mode of molecular water.43 These observations reveal that the molecular and dissociated adsorption of water is higher on the IO_W NW sample surfaces, whereas the decreased intensity of both the bands in IO_Ar and IO_H2 NW samples suggests decreased adsorption of molecular and dissociated water on these samples. The higher contact angle is consistent with the lower adsorption and dissociation of water on these samples. To directly correlate the effect of the surface oxygen to the wetting properties of IO NWs, the IO_H2 NW sample was annealed in the presence of water vapor for 1 h at 500 °C. It was found that, after annealing, the contact angle decreased from 168° to 25° with an increase in the intensity of the IR peak that is positioned between 3300 and 3600 cm−1 as shown in Figure 5b. The decrease in the contact angle with an increase
in the intensity of the IR signal between 3300 and 3600 cm−1 due to increased water adsorption indicates that water wetting is higher on the oxidized surfaces. We have also investigated the role of oxygen vacancy defects on the water wetting behavior of IO NWs by studying the ultraviolet (UV) light induced wettability transition measurements. Figure 6 shows the UV light induced wettability transition measurements on IO_Ar NWs. The UV light illumination was carried out by using a 9 W Phillips UV B lamp with wavelength in the range between 280 and 315 nm at room temperature. The photogeneration of a hydrophilic surface depends on the duration of UV light illumination as shown in Figure 6a. On UV light illumination for about 50 min, the contact angle decreases from 134° ± 4° to 13° ± 2°. When these hydrophilic samples were stored in the dark for 45 days under ambient atmospheric conditions, the surfaces gradually turned back to hydrophobic (Figure 6b). The IO_Ar NWs also D
DOI: 10.1021/acs.jpcc.5b03346 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C display a reversible cyclic wetting transition from hydrophobic to hydrophilic and vice versa with UV light exposure and dark storage as shown in Figure 6c. However, sample degradation was observed after the third cycle of UV light exposure and dark storage. EPR spectroscopy was utilized to analyze the change in the oxygen vacancy defects before and after illumination with UV light. The EPR spectra in Figure 6d indicate that, after the UV illumination of the IO_Ar NW sample, the EPR signal corresponding to the oxygen paramagnetic defects was reduced by about 50%. The UV illumination generates electron−hole pairs in IO. These electrons and holes either recombine or react with surface species. The photogenerated electrons increase the adsorption of oxygen on the nanowire surface.44 The adsorbed oxygen molecule could dissociate at an oxygen vacancy site and passivate it, but it may also leave an oxygen adatom on the nanowire surface.45 In this way, the adsorbed oxygen can increase the oxygen adatoms on nanowire surfaces. The adsorbed oxygen molecules on the nanowire surfaces trap more photogenerated electrons and improve the separation of electrons and holes. The freely available photogenerated holes trap hydroxyl groups. The increased amount of hydroxyl groups enhances hydrophilicity on the surface of the nanowires.46,47 The increased oxygen adsorption and water wetting with UV light illumination provide direct evidence of increased water wetting in the presence of an oxygen-rich surface of IO NWs. This indicates that when the NW surface is oxygen rich or there is a lack of oxygen vacancy defects, the surface becomes nearly superhydrophilic. We performed first-principles-based DFT calculations of both the 100 and 110 surfaces of In2O3. All of the surfaces in our study were modeled in terms of supercells of suitable dimensions in the x- and y-directions, and a layer of vacuum of 20 Å was applied on top of the surface. The electronic structure and total energy were calculated using the fhi-aims code, which is an all-electron code using a numerical atom-centered basis set.48 The exchange and correlation functional was taken using generalized gradient approximations (GGAs) as in Perdew− Burke−Ernzerhof (PBE) implementation.49 It is well-known that GGA functionals are not very good for metal oxide systems or surfaces.50,51 Therefore, the energetics were calculated with the more advanced HSE06 functional.52 The k-mesh was generated by the Monkhorst−Pack method, and all results were tested for convergence with respect to the mesh size (6 × 6 × 1). In all calculations, self-consistency was achieved with a 0.1 meV convergence of the total energy. van der Waals corrections were taken into account as per the Tkatchenko−Scheffler scheme.53 To understand the charge transfer, Hirshfeld charges were compared as partitioned among the different atoms. We noticed from analyzing the Hirshfeld charges that O atoms are electron-rich, while In atoms are electron-deficient. Therefore, when a water molecule comes closer to the In2O3 surface, there are three possible sites (that we have considered) where it can be adsorbed, viz., the (a) In site, (b) O vacancy site, and (c) O site. We found that when the H2O molecule is adsorbed at the In site, the lone pair of H2O is partially transferred to the nearby In atom and electropositive H atoms experience a feeble electrostatic force from the nearby O atoms as shown in Figure 7a. This force is definitely not sufficient for breaking the O−H bond of the H2O molecule. Therefore, a stand-alone H2O molecule remains stable on the In site of the surface. On the other hand, in the presence of an O vacancy, the In atoms share the additional electron at the vacancy and
Figure 7. Ball and stick model of H2O adsorption of different sites on the 100 (a−c) and 110 (d−f) planes of the In2O3 surface. Gray, red, and white balls represent In, O, and H atoms, respectively. (a, d) H2O adsorption on the In site, (b, e) H2O adsorption on the O vacancy site, and (c, f) H2O adsorption on the O site.
become less electropositive. In this case the situation is almost similar to the case we have described in Figure 7a, but the bond strength of the H2O molecule on top of the In site is a little less (bond elongated by 0.01 Å than that in Figure 7a). This is because the In atom after sharing the electron at the O vacancy as shown in Figure 7b has become less positive, so its interaction with the lone pair of H2O is less than in the case where the H2O molecule is adsorbed at the In site (Figure 7 a). Finally, we have noticed a completely different situation of H2O adsorption at the O site, where the water molecule becomes totally dissociated into H and OH as the bond is broken due to strong electrostatic attraction from electron-rich O atoms at the In2O3 surface with one of the electropositive H atoms of H2O (Figure 7 c). Out of the three cases described above, we have found that the H2O dissociation at the O site (Figure 7c) is energetically the most favorable situation. It is favored by 1.39 eV (HSE06+vdW) (0.89 eV (PBE+vdW)) over that at the In site (Figure 7a). This clearly indicates that the chance of water dissociation occurring is significantly more if the surface oxygen concentration is high and vice versa. Therefore, we conclude that if the IO surface is O-rich (for both 100 and 110 surfaces), it should be superhydrophilic, while if the surface is O-deficient, the behavior should be superhydrophobic. This analysis is consistent with our experimental findings where the contact angle was found to be small (or large) in O-rich (or Odeficient) surfaces, thereby causing the surface to be nearly superhydrophilic (or superhydrophobic). IO has a bixbyite-type crystal structure. The lattice is bodycentered cubic with a space group of Ia3. The unit cell has a lattice constant a of 10.11 Å, and each unit cell contains 80 atoms. Each In atom displays 6-fold coordination to its neighboring oxygen atoms. Forty-eight oxygen atoms in the cell display tetrahedral coordination to four neighboring In atoms. There are two nonequivalent position of In atoms: In1 and In2. The In1−O bond length is constant (0.2174 nm), and the bond length of In2 to O varies between 0.2127 and 0.2230 nm.54 Water molecules are physically adsorbed on the acidic sites (cations) on the metal oxide surface. Higher water dissociation occurs with an increase of the concentration of hydroxyl groups, which leads to higher water wetting on the metal oxide surfaces.47 The dissociation of water molecules in the form of hydroxyl groups can be explained in terms of the availability of E
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proximal acidic (metal cations) and basic (O2−) sites.18 If the surface is stoichiometric, the probability of dissociation of water molecules will be higher as the hydrogen atoms of the water molecules will be attracted by the oxygen atoms at the IO surface. This leads to the dissociation of the water molecules and results in two hydroxyl species which will be chemisorbed on the IO surfaces. The next layer of water can be easily adsorbed on the hydroxyl species. In this way a stoichiometric surface with a lack of oxygen vacancy defects displays a nearly superhydrophilic surface. On the other hand, if oxygen vacancies are present on the IO surface in proximity of the In ions, this decreases the possibility of water dissociation. This is most likely because of the lack of oxygen atoms on the surface that could accept hydrogen from the dissociated water molecules. In turn, the decrease in water dissociation increases the water contact angle and superhydrophobicity. These results indicate that water dissociation is higher on a stoichiometric surface in comparison with a highly oxygen-deficient surface.
CONCLUSIONS In conclusion, superhydrophobic and nearly superhydrophilic IO NWs were synthesized without using any surface chemical coating. IO NWs with different oxygen concentrations were synthesized using a CVD system. The contact angle values obtained on the oxygen-rich, oxygen-deficient, and highly oxygen-deficient IO NWs are 8° ± 5°, 134° ± 4°, and 168° ± 2°, respectively. The current findings show that the IO NW wetting properties can be varied by simply controlling the oxygen defect concentration. This study indicates that oxygenrich IO NWs are energetically more favorable for water wetting, whereas the highly oxygen-deficient IO NWs exhibit superhydrophobic behavior with a rolling off angle of ∼3°. This novel approach to synthesize superhydrophobic and nearly superhydrophilic IO NWs may be useful to produce other metal oxide nanowires with controlled water wetting properties which may have potential applications in future nanodevices. ASSOCIATED CONTENT
S Supporting Information *
Video S1 showing the rolling off water droplet on the IO_H2 NW sample. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b03346.
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REFERENCES
(1) Bixler, G. D.; Bhushan, B. Fluid Drag Reduction with Shark-Skin Riblet Inspired Microstructured Surfaces. Adv. Funct. Mater. 2013, 23, 4507−4528. (2) Liu, X.; Liang, Y.; Zhou, F.; Liu, W. Extreme Wettability and Tunable Adhesion: Biomimicking beyond Nature? Soft Matter 2012, 8, 2070−2086. (3) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Bioinspired Surfaces with Special Wettability. Acc. Chem. Res. 2005, 38, 644−652. (4) Yu, S.; Guo, Z.; Liu, W. Biomimetic Transparent and Superhydrophobic Coatings: From Nature and beyond Nature. Chem. Commun. 2015, 51, 1775−1794. (5) Zhu, H.; Guo, Z.; Liu, W. Adhesion Behaviors on Superhydrophobic Surfaces. Chem. Commun. 2014, 50, 3900−3913. (6) Ueda, E.; Levkin, P. A. Emerging Applications of Superhydrophilic-Superhydrophobic Micropatterns. Adv. Mater. 2013, 25, 1234−1247. (7) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. Superhydrophobic Surfaces: From Structural Control to Functional Application. J. Mater. Chem. 2008, 18, 621−633. (8) Cao, L. Superhydrophobic Surface: Design, Fabrication, and Application. Doctoral Dissertation, University of Pittsburgh, 2010. (9) Nosonovsky, M.; Bhushan, B. Superhydrophobic Surfaces and Emerging Applications: Non-Adhesion, Energy, Green Engineering. Curr. Opin. Colloid Interface Sci. 2009, 14, 270−280. (10) Liu, K.; Yao, X.; Jiang, L. Recent Developments in Bio-Inspired Special Wettability. Chem. Soc. Rev. 2010, 39, 3240−3255. (11) Lai, Y.; Tang, Y.; Gong, J.; Gong, D.; Chi, L.; Lin, C.; Chen, Z. Transparent Superhydrophobic/Superhydrophilic TiO2-Based Coatings for Self-Cleaning and Anti-Fogging. J. Mater. Chem. 2012, 22, 7420−7426. (12) Seo, J.; Lee, S.; Han, H.; Jung, H. B.; Hong, J.; Song, G.; Cho, S. M.; Park, C.; Lee, W.; Lee, T. Gas-Driven Ultrafast Reversible Switching of Super-Hydrophobic Adhesion on Palladium-Coated Silicon Nanowires. Adv. Mater. 2013, 25, 4139−4144. (13) Li, C.; Guo, R.; Jiang, X.; Hu, S.; Li, L.; Cao, X.; Yang, H.; Song, Y.; Ma, Y.; Jiang, L. Reversible Switching of Water-Droplet Mobility on a Superhydrophobic Surface Based on a Phase Transition of a SideChain Liquid-Crystal Polymer. Adv. Mater. 2009, 21, 4254−4258. (14) Li, C.; Zhang, Y.; Ju, J.; Cheng, F.; Liu, M.; Jiang, L.; Yu, Y. In Situ Fully Light-Driven Switching of Superhydrophobic Adhesion. Adv. Funct. Mater. 2012, 22, 760−763. (15) Leung, K. C. F.; Xuan, S.; Lo, C. M. Reversible Switching between Hydrophilic and Hydrophobic Superparamagnetic Iron Oxide Microspheres via One-Step Supramolecular Dynamic Dendronization: Exploration of Dynamic Wettability. ACS Appl. Mater. Interfaces 2009, 1, 2005−2012. (16) Schaub, R.; Thostrup, P.; Lopez, N.; Lægsgaard, E.; Stensgaard, I.; Nørskov, J.; Besenbacher, F. Oxygen Vacancies as Active Sites for Water Dissociation on Rutile TiO2 (110). Phys. Rev. Lett. 2001, 87, 266104. (17) Meyer, B.; Rabaa, H.; Marx, D. Water Adsorption on ZnO (1010): From Single Molecules to Partially Dissociated Monolayers. Phys. Chem. Chem. Phys. 2006, 8, 1513−1520. (18) Henderson, M. A. Structural Sensitivity in the Dissociation of Water on TiO2 Single-Crystal Surfaces. Langmuir 1996, 12, 5093− 5098. (19) Fryar, J.; McGlynn, E.; Henry, M. O.; Cafolla, a A.; Hanson, C. J. The Dominant Role of Adsorbed Fluid Layers on the Polar Surfaces of ZnO in Ambient Atmospheric Conditions. Nanotechnology 2004, 15, 1797−1801. (20) Sun, R.; Nakajima, A.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Photoinduced Surface Wettability Conversion of ZnO and TiO2 Thin Films. J. Phys. Chem. B 2001, 105, 1984−1990. (21) Hu, H.; Ji, H.-F.; Sun, Y. The Effect of Oxygen Vacancies on Water Wettability of a ZnO Surface. Phys. Chem. Chem. Phys. 2013, 15, 16557−16565.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K.Y. kindly acknowledges the Council of Scientific and Industrial Research (CSIR), India, for a senior research fellowship. We thank the Department of Information Technology (DIT) sponsored Nanoscale Research Facility (NRF), Indian Institute of Technology Delhi (IIT Delhi), India (B.R.M. and J.P.S.), and the Office of Basic Energy Sciences, United States Department of Energy (Grant DE-FG0207ER15903) (K.V.L.), for support. F
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(44) Chou, C.; Wu, Y.; Lin, C. RSC Advances Oxygen Sensor Utilizing Ultraviolet Irradiation Assisted ZnO Nanorods under Low Operation. RSC Adv. 2014, 4, 52903−52910. (45) Epling, W. S.; Peden, C. H. F.; Henderson, M. A.; Diebold, U. Evidence for Oxygen Adatoms on TiO2(110) Resulting from O2 Dissociation at Vacancy Sites. Surf. Sci. 1998, 412−413, 333−343. (46) Pakdel, A.; Bando, Y.; Golberg, D. Plasma-Assisted Interface Engineering of Boron Nitride Nanostructure Films. ACS Nano 2014, 8, 10631−10639. (47) Simonsen, M. E.; Li, Z.; Søgaard, E. G. Influence of the OH Groups on the Photocatalytic Activity and Photoinduced Hydrophilicity of Microwave Assisted Sol-Gel TiO2 Film. Appl. Surf. Sci. 2009, 255, 8054−8062. (48) Blum, V.; Gehrke, R.; Hanke, F.; Havu, P.; Havu, V.; Ren, X.; Reuter, K.; Scheffler, M. Ab Initio Molecular Simulations with Numeric Atom-Centered Orbitals. Comput. Phys. Commun. 2009, 180, 2175−2196. (49) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (50) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207−8215. (51) Richter, N. a.; Sicolo, S.; Levchenko, S. V.; Sauer, J.; Scheffler, M. Concentration of Vacancies at Metal-Oxide Surfaces: Case Study of MgO (100). Phys. Rev. Lett. 2013, 111, 1−5. (52) Bhattacharya, S.; Levchenko, S. V.; Ghiringhelli, L. M.; Scheffler, M. Stability and Metastability of Clusters in a Reactive Atmosphere: Theoretical Evidence for Unexpected Stoichiometries of MgMOx. Phys. Rev. Lett. 2013, 111, 1−5. (53) Tkatchenko, A.; Scheffler, M. Accurate Molecular van der Waals Interactions from Ground-State Electron Density and Free-Atom Reference Data. Phys. Rev. Lett. 2009, 102, 6−9. (54) Morales, E. H.; He, Y.; Vinnichenko, M.; Delley, B.; Diebold, U. Surface Structure of Sn-Doped In2O3 (111) Thin Films by STM. New J. Phys. 2008, 10, 125030.
(22) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Studies of Surface Wettability Conversion on TiO2 Single-Crystal Surfaces. J. Phys. Chem. B 1999, 103, 2188−2194. (23) Lira, E.; Hansen, J. Ø.; Huo, P.; Bechstein, R.; Galliker, P.; Lægsgaard, E.; Hammer, B.; Wendt, S.; Besenbacher, F. Dissociative and Molecular Oxygen Chemisorption Channels on Reduced Rutile TiO2 (110): An STM and TPD Study. Surf. Sci. 2010, 604, 1945− 1960. (24) Gercher, V. A.; Cox, D. F. Water Adsorption on Stoichiometric and Defective SnO2 (110) Surfaces. Surf. Sci. 1995, 322, 177−184. (25) Batzill, M. Surface Science Studies of Gas Sensing Materials: SnO2. Sensors 2006, 6, 1345−1366. (26) Xie, S.; Zhao, Y.; Jiang, Y. Laser-Induced Hydrophobicity on Single Crystal Zinc Oxide Surface. Appl. Surf. Sci. 2012, 263, 405−409. (27) Kushwaha, A.; Kalita, H.; Aslam, M. Effect of Oxygen Annealing on the Surface Defects and Photoconductivity of Vertically Aligned ZnO Nanowire Array. World Acad. Sci. Eng. Technol. 2013, 7, 258− 263. (28) Meng, X. Q.; Zhao, D. X.; Zhang, J. Y.; Shen, D. Z.; Lu, Y. M.; Dong, L.; Xiao, Z. Y.; Liu, Y. C.; Fan, X. W. Wettability Conversion on ZnO Nanowire Arrays Surface Modified by Oxygen Plasma Treatment and Annealing. Chem. Phys. Lett. 2005, 413, 450−453. (29) Zhang, J.; Liu, Y.; Wei, Z.; Zhang, J. Mechanism for Wettability Alteration of ZnO Nanorod Arrays via Thermal Annealing in Vacuum and Air. Appl. Surf. Sci. 2013, 265, 363−368. (30) Shen, G.; Liang, B.; Wang, X.; Chen, P. C.; Zhou, C. Indium Oxide Nanospirals Made of Kinked Nanowires. ACS Nano 2011, 5, 2155−2161. (31) Zhong, M.; Zheng, M.; Zeng, A.; Ma, L. Direct Integration of Vertical In2O3 Nanowire Arrays, Nanosheet Chains, and Photoinduced Reversible Switching of Wettability. Appl. Phys. Lett. 2008, 92, 093118. (32) Zamarreño, C. R.; Sanchez, P.; Hernaez, M.; del Villar, I.; Fernandez-Valdivielso, C.; Matias, I. R.; Member, S.; Arregui, F. J. Sensing Properties of Indium Oxide Coated Optical Fiber Devices Based on Lossy Mode Resonances. IEEE Sens. J. 2012, 12, 151−155. (33) Qurashi, A. In2O3 Nanostructures and their Chemical and Biosensor Applications. Arabian J. Sci. Eng. 2010, 35, 125−145. (34) Pan, J.; Song, X.; Zhang, J.; Shen, H.; Xiong, Q. Switchable Wettability in SnO2 Nanowires and SnO2@SnO2 Heterostructures. J. Phys. Chem. C 2011, 115, 22225−22231. (35) Yadav, K.; Mehta, B. R.; Singh, J. P. Superhydrophobicity and Enhanced UV Stability in Vertically Standing Indium Oxide Nanorods. Appl. Surf. Sci. 2015, 346, 361−365. (36) Shen, G.; Liang, B.; Wang, X.; Huand, H.; Chen, D.; Wang, Z. L. Ultrathin In2O3 Nanowires with Diameters below 4 nm: Synthesis, Reversible Wettability Switching Transistor Applications. ACS Nano 2011, 5, 6148−6155. (37) Zhou, H.; Cai, W.; Zhang, L. Photoluminescence of Indium− Oxide Nanoparticles Dispersed within Pores of Mesoporous Silica. Appl. Phys. Lett. 1999, 75, 495. (38) Ko, T.; Chu, C.; Chen, J.; Lu, T.; Kuo, H.; Wang, S. Tunable Light Emissions from Thermally Evaporated In2O3 Nanostructures Grown at Different Growth Temperatures. J. Cryst. Growth 2008, 310, 2264−2267. (39) Kumar, M.; Singh, V. N.; Singh, F.; Lakshmi, K. V.; Mehta, B. R.; Singh, J. P. On the Origin of Photoluminescence in Indium Oxide Octahedron Structures. Appl. Phys. Lett. 2008, 92, 7−10. (40) Yadav, K.; Mehta, B. R.; Singh, J. P. Presence of Metal-Oxide Interface Enhanced Photoluminescence from In−In2O3 Core−Shell Nanorods. RSC Adv. 2015, 5, 1581−1586. (41) Peng, X. S.; Meng, G. W.; Zhang, J.; Wang, X. F.; Wang, Y. W.; Wang, C. Z.; Zhang, L. D. Synthesis and Photoluminescence of SingleCrystalline In2O3 Nanowires. J. Mater. Chem. 2002, 12, 1602−1605. (42) Zheng, M. J.; Zhang, L. D.; Li, G. H.; Zhang, X. Y.; Wang, X. F. Ordered Indium-Oxide Nanowire Arrays and Their Photoluminescence Properties. Appl. Phys. Lett. 2001, 79, 839. (43) Henderson, M. A. An HREELS and TPD Study of Water on TiO2(110): The Extent of Molecular versus Dissociative Adsorption. Surf. Sci. 1996, 355, 151−166. G
DOI: 10.1021/acs.jpcc.5b03346 J. Phys. Chem. C XXXX, XXX, XXX−XXX
