Article pubs.acs.org/JPCC
Kinetics and Physicochemical Process of Photoinduced Hydrophobic ↔ Superhydrophilic Switching of Pristine and N‑doped TiO2 Nanotube Arrays Rajini P. Antony, Tom Mathews,* S. Dash, and A. K. Tyagi Thin Films and Coating Section, Surface and Nanoscience Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India 603102 S Supporting Information *
ABSTRACT: Visible-light-active near superhydrophobic ↔ superhydrophilic switching vertically aligned anatase TiO2 and TiO1.84N0.14 nanotube array thin films were synthesized by anodizing Ti foils in ethylene glycol + NH4F + water electrolyte-containing urea as nitrogen source. Compared with pristine TiO2, Ndoped TiO2 nanotube arrays underwent hydrophobic to superhydrophilic transition faster under sunlight. The UV-light-induced hydrophobic-to-superhydrophilic conversion rates in the hydrophobic and hydrophilic regimes for pristine and N-doped samples are 0.2178 min−1, 0.9534 min−1 and 0.5185 min−1, 1.376 min−1 respectively. The corresponding sunlight induced conversion rates for the pristine and N-doped samples are 0.015 min−1, 0.061 min−1 and 0.013 min−1, 0.152 min−1 respectively. The reverse hydrophobic conversion, when kept in dark, was found to follow a single curve with rates 8.94 × 10−5 and 4.06 × 10−5 min−1 for the pristine and doped samples respectively. The major physicochemical process behind the hydrophobic ↔ superhydrophilic transition is found to be decomposition of surface carbonaceous species and their adsorption, respectively, by surface X-ray photoelectron spectroscopy before and after photoirradiation.
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INTRODUCTION Surface wetting is an important property of materials and plays important role in nature and technology. The surfaces on which water droplets remain as almost perfect spheres (contact angles ≥150°) and roll off such surfaces without leaving any residue are termed superhydrophobic, and the surfaces on which water droplets spread completely (contact angles ≤5−10°) are termed superhydrophilic.1 Since the discovery of photoinduced hydrophobic-to-hydrophilic conversion of titanium dioxide layers/thin film surfaces by Wang and coworkers in 1997,2 TiO2 thin films have attracted much attention as photofunctional materials for self-cleaning, antifogging, antibacterial, and stain-proofing agents.3,4 It has also been observed from contactangle measurements of water droplets on TiO2 thin-film surfaces that the reversible conversion (hydrophobic ↔ hydrophilic) can be achieved by keeping the irradiated samples in the dark.5−8 The surfaces that are able to switch between superhydrophobic and superhydrophilic are of importance9 because of their potential application to enhance rapid water motion,10 improve microfluidic devices,11 and create smart membranes.12 Recently vertically aligned TiO2 nanotube arrays (TNTAs) have attracted much scientific interest because of their aspect ratio, wall thickness, and alignment-dependent properties that can be exploited in technological applications like dyesensitized solar cells,13 sensors,14,15 field-emission devices,16,17 and photoassisted hydrogen generation by water splitting.18 As of now, the best method for synthesizing vertically aligned © 2013 American Chemical Society
TNTAs is anodization of Ti foil/coating. The main advantage of this technique is easy tuning of tube aspect ratio, wall thickness, inner diameter, and tube-to-tube spacing by changing applied voltage and duration of anodization for a given electrolyte composition.19 Late studies on photoinduced reversible hydrophobic ↔ hydrophilic behavior of TNTAs as well as altering of WCA by coating organic monolayer coupled to UV irradiation are reported.20−23 Pure TiO2 films can only be activated under UV light because of their large band gap (3.2 eV), which limits their widespread application because UV light accounts for only 5% of solar spectrum. A great deal of research has focused on doping TiO2 with both transition metal cations and anions to lower the threshold energy for photoexcitation,. The identified promising dopants are N, S, C, and B anions, which introduce p states above the valence band maximum.24−27 Among the anions, N is found to be more promising.25 Substitution of O2− ions by N3− ions is more effective due to the comparable ion size and electronegativity.28 Because of the comparable ion size, the lattice distortion will be less and hence large numbers of recombination centers are not generated. To date, there are only a few reports on synthesis, characterization, and reversible hydrophobic ↔ hydrophilic switching of N-doped TiO2 films.29−32 To the best of our knowledge, no reports regarding Received: January 22, 2013 Revised: March 15, 2013 Published: March 15, 2013 6851
dx.doi.org/10.1021/jp400718t | J. Phys. Chem. C 2013, 117, 6851−6860
The Journal of Physical Chemistry C
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the fabrication of photoinduced hydrophobic ↔ hydrophilic switchable vertically aligned N-doped TNTAs have been published. In the present study, we report the photoinduced reversible hydrophobic ↔ hydrophilic switching behavior of vertically aligned pristine and N-doped TNTA, correlation between morphology and WCA, kinetics of near-superhydrophobic ↔ superhydrophilic transitions, and the physicochemical process behind the wetting ↔ dewetting behavior.
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Article
RESULTS AND DISCUSSION Structure, Morphology, and Composition. On the basis of the results of our previous study on synthesis and characterization of pristine and N-doped TNTAs,17,19,33 the synthesis of N-doped TNTAs was carried out by anodizing Ti foils in ethylene glycol + 2.5 wt % H2O + 0.5 wt % NH4F + 0.2 wt % urea electrolyte. The as-prepared samples upon XRD analyses were found to be amorphous. The samples on annealing at 400 °C in air for 2 h transformed to the anatase phase (Figure 1a of the Supporting Information). The formation of the anatase phase was further confirmed by Raman spectroscopy (Figure 1b of the Supporting Information). Raman spectra of the N-doped TNTAs synthesized at different voltages and annealed at 400 °C match those reported in the literature.34,35 Compared with pristine TiO2, the highly intense Eg mode (145 cm−1) of N-doped TNTAs showed a red shift. This can be ascribed to the change in the force constant due to nitrogen doping. The high-resolution N-1s X-ray photoelectron spectra of Ndoped TiO2, taken at a sputter depth of 3 min to obtain reliable intensity of substitutional and interstitial nitrogen, shown in Figure 1 confirm substitutional N-doping. The spectra were
EXPERIMENTAL SECTION
Titanium metal foils (10 mm × 10 mm × 0.5 mm) (Alfa Esar) thoroughly washed with distilled water, sonicated in ethanol as well as acetone, and dried in nitrogen stream were anodized in the electrolyte, ethylene glycol containing 0.5 wt % ammonium fluoride and 2.5 wt % of water, as well as in ethylene glycol +0.5 wt % ammonium fluoride +2.5 wt % of water electrolyte containing 0.2 wt % of urea using platinum as the cathode. The electrodes were kept 1.5 cm apart. Potentiostatic anodization was conducted at different voltages, viz. 20, 30, 40, 50, and 60 V at room temperature by ramping up to the specific end potentials with a ramping speed of 1 V/s and holding it for 2 h. The time-dependent anodization current was recorded with a computer-controlled multimeter (HP 34401A, USA). The pH of the electrolyte was measured by using a pH meter. After each anodization, the samples were sonicated in ethanol and then dried in nitrogen stream. The dried samples were annealed in air at 400 °C for 2 h. The surface morphology, crystal structure, and composition of the samples were analyzed using field-emission scanning electron microscope (FEG Quanta, Philips, Netherlands) having EDAX facility, X-ray diffractometer (Bruker D8 Discover, Germany), and X-ray photoelectron spectrometer (M/s SPECS, Germany), respectively. From the partial lift off of the nanotube layer, the cross-sectional images were taken and the thickness variations were calculated. The X-ray photoelectron spectroscopic analysis of the annealed TNTAs was performed at 1486.74 eV using Al Kα as the X-ray source. The spectrometer was calibrated using a standard silver sample. Data were processed using Specslab2 software. The binding energy (BE) of the C 1s transition from contaminated C at 284.7 eV was used as the reference to account for any charging of the sample, and the peak positions were compared with standard values for the identification of different elements and their oxidation states. Raman spectra were recorded using a Renishaw Micro Raman spectrometer, equipped with a confocal microscope with an “argon ion laser” operating at 514.5 nm at a power level of 200 mW. The optical characterization of the nanotube powders was determined using the UV−vis diffuse reflectance spectrometer (Shimadzu UV 2401). The reflectance spectra were taken over the range 800 to 200 nm at a scan rate of 100 nm/s. BaSO4 was used as the reference. The WCA experiments were carried out using a contact-angle meter equipped with CCD camera (Holmarc, HO-IAD-CAM-01, India). The sessile drop method using distilled water was adopted. The volume of the water droplets dispensed was approximately 2 to 3 μL. The image of the droplet, which is in contact with the sample surface, is captured using the CCD camera. From the images, the contact angles were measured using Image J software.
Figure 1. High-resolution N-1s spectrum of the N-doped TiO2, taken at a sputter depth of 3 min.
analyzed using the product of Gaussian and Lorenzian functions with the mixing parameter “m” being 30% Lorenzian. The high-resolution N-1s spectra consist essentially of three peaks, viz. one high intensity asymmetric peak around 396.8 eV and two low intensity peaks at 400.2 and 403.1 eV, respectively. The asymmetric high-intensity peak has been deconvoluted to high-and low-intensity peaks at 396.8 and 397.6 eV, respectively. The different N-1s BE values represent different electron density around nitrogen, suggesting various N environments in the lattice. On the basis of literature reports, the two deconvoluted peaks in the range 396−398 eV (396.8 and 397.6 eV) and peaks at 400.2 and 403.1 eV can be assigned to N3− substituting for O2− (NO), interstitial nitrogen (Ni), and surface-adsorbed NOx species.36−38 From the high-resolution Ti-2p, O-1s (Figure 2 of the Supporting Information), and N1s spectra, all obtained at 3 min of sputter depth, the atom fraction of Ti, O, and N was calculated using the equations XN = (I(N‐1s)/S N)/[(I(Ti‐2p)/STi) + (I(O‐1s)/SO) + (I(N‐1s)/S N)] 6852
(1)
dx.doi.org/10.1021/jp400718t | J. Phys. Chem. C 2013, 117, 6851−6860
The Journal of Physical Chemistry C
Article
Figure 2. (a−e) Surface topography of the N-doped samples synthesized at 20, 30, 40, 50, and 60 V and annealed at 400 °C for 2 h. Inset shows images of water droplets reflecting variation in water contact angle with increase in tube diameter. (f) Typical cross-sectional FESEM image of the annealed sample grown at 60 V.
Table 1. Tube Parameters Evaluated from the FESEM Images Using Image Analysis Softwarea applied voltage (V) 20 30 40 50 a
diameter (D) (nm) 47.7 71.45 97.41 105.8
± ± ± ±
5.45 7.25 12.69 11.03
wall thickness (w) (nm) 4.23 4.9 7.9 8.46
± ± ± ±
0.80 0.82 1.76 1.63
height (h) (nm) 2.39 2.69 6.8 8.3
± ± ± ±
0.26 00.54 0.27 1.25
intertubular distance (x) (nm)
surface density ρ (/μm2)
± ± ± ±
193 93 62 50
13.67 22.26 29.8 38.5
1.93 3.35 2.86 3.54
Surface density ‘ρ’ is the number of nanotubes per square micrometer of the anodized Ti surface.
X Ti = (I(Ti‐2p)/STi)/[(I(Ti‐2p)/STi) + (I(O‐1s)/SO) + (I(N‐1s)/S N)]
(2)
XO = (I(O‐1s)/SO)/[(I(Ti‐2p)/STi) + (I(O‐1s)/SO) + (I(N‐1s)/S N)]
(3)
as TiO1.813N0.14, where IN‑1s is the peak area of the N-1s peak corresponding to O−Ti−N (NO), ITi‑2p is the sum of the areas of the Ti2p deconvoluted peaks, and IO‑1s is the area of the O-1s corresponding to bonded oxygen (Figure 2 of the Supporting Information). The surface topography of the N-doped samples synthesized at 20, 30, 40, 50, and 60 V and annealed at 400 °C for 2 h as well as a typical cross-sectional FESEM image of the annealed sample grown at 60 V are shown in Figure 2. The formation of vertically aligned nanotube arrays is clear from the topographical and cross-sectional FESEM images. The tube parameters evaluated from the FESEM images using image analysis software are given in Table 1. Wetting Studies under UV and Sun Light. The water contact angles, before irradiation, obtained for vertically aligned anatase pristine and N-doped TNTAs are ∼140 and ∼125°, respectively, and are high compared with those reported in the literature for pristine TNTAs.20−23 It is clear from Table 1 that tube diameter varies with synthesis voltage. Figure 3 depicts the change in contact angle with tube diameter for the N-doped samples. In the case of nanotube arrays, of tube diameters in the range 45 to 100 nm, the observed contact angles are ∼125°, whereas for nanotube arrays of ∼106 and ∼130 nm tube diameters, the observed contact angles are ∼92 and ∼53, respectively (Figures 2 and 3). The relatively high contact-angle values despite the very large porosity can be due to the cushion effect provided by air trapped in the vertically aligned nanotubes, which prevent
Figure 3. Variation in contact angle with tube diameter for N-doped TiO2.
the water droplet from penetrating into the tube. The low contact-angle values observed in the case of nanotube arrays synthesized at 50 and 60 V (∼106 and ∼130 nm tube diameters) can be due to the formation of precipitates on the top surface (Figure 3 of the Supporting Information). In addition, as the tube diameter increases, a decrease in the air cushion effect is expected, which reduces the wetting contact angle (WCA). All samples consisting of nanotubes of tube diameter up to ∼97 nm showed hydrophobicity. According to the Cassie and Baxter relation39,40 the WCA can be calculated using the equation cos θ* = fs cos θ − fv
(4)
where θ* is the real contact angle, fs is the fraction of the surface area made up of solid, which corresponds to the TiO2 surface area forming the nanotubes with respect to the total surface area of the sample including intertube voids and circular openings, f v is surface fraction occupied by air and is equal to (1 − fs), and θ is the contact angle on a smooth polycrystalline 6853
dx.doi.org/10.1021/jp400718t | J. Phys. Chem. C 2013, 117, 6851−6860
The Journal of Physical Chemistry C
Article
Table 2. Measured Contact Angles and Those Obtained from the Cassie and Baxter Relation Using the fs and *fs Valuesa tube diameter (D) (nm)
CA measured (deg)
fs
*fs
CA calculated using fs
CA calculated using *fs
47.7 71.5 97.4 105.8 130.0
131 121 125.3 92 53
0.1634 0.1266 0.1479 0.1299 0.1648
0.1280 0.1066 0.1584 0.1582 0.1433
144.150 148.846 146.303 147.82 144.315
148.033 151.029 144.636 145.791 146.954
Contact angles were calculated using the equation cos θ* = fs cos θ − f v, where θ* is the real (measured) contact angle, fs is the fraction of the surface area made up of solid and is calculated from the equation, fs = 2π w(w + D)/31/2 (D + 2w +x)2 whereas *fs is calculated using the equation *fs = πρ [((D/2) + w)2 − (D/2)2]. a
Figure 4. Variation of WCA with time: (a) pristine and doped TNTA under UV light, (b) pristine and doped TNTA in dark, (c) pristine and Ndoped TNTA under sunlight, and (d) pristine and N-doped TNTA in dark after sunlight irradiation.
corresponding fs and *fs values calculated using eqs 5 and 6, and measured contact angles along with those computed using eq 4 are given in Table 2. The contact angles measured and calculated using the value of 82° for θ in eq 4 for nanotube arrays of tube diameters 48, 71, and 97 are in reasonable agreement. The deviation from the calculated value can be attributed to the presence of HO-Ti species on the surface of TNTA films, as evidenced from XPS. The substantial deviation in contact-angle values for arrays consisting of large-diameter nanotubes (106 and 130 nm) can be due to the presence of precipitate on the surface (Figure 3 of the Supporting Information) and reduction in air cushion effect because of large pore size. The photoinduced hydrophobic-to-superhydrophilic transition was studied by first irradiating the nanotube arrays, synthesized at 40 V and annealed in air at 400 °C for 2 h, with UV light. The rationale behind the selection of samples synthesized at 40 V is that of all of the synthesized samples the samples synthesized at 40 V have the maximum surface uniformity and homogeneity in terms of tube diameter, intertube spacing, and tube wall thickness. Because in the present study the nanotube arrays were synthesized in an organic medium, contamination of surfaces with organics is
anatase TiO2.41 The values of contact angle of water on freshly prepared, clean, and smooth polycrystalline anatase-TiO2 surface (ideal Young contact angle θ) obtained by various groups are contradicting 72, 60, 52, 50, 15,
