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Superhydrophobic surfaces with photocatalytic activity under UV and visible light irradiation
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Superhydrophobic surfaces with photocatalytic activity under UV and visible light irradiation Ju Ha Lee 1 , Eun Ji Park 1 , Dae Han Kim, Myung-Geun Jeong, Young Dok Kim ∗ Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e
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Article history: Received 20 March 2015 Received in revised form 8 April 2015 Accepted 11 May 2015 Available online xxx Keywords: Superhydrophobicity Photocatalysis Visible light PDMS TiO2
a b s t r a c t Hydrophobic PDMS-coated silica nanoparticles were mixed with photocatalytically active N-doped TiO2 and distributed on the glass substrate. This facile method created films with simultaneous superhydrophobic and photocatalytic activity under UV and visible light. The effective decomposition of methylene blue and 2-methylisoborneol photocatalyzed by these films was confirmed. This superhydrophobicity is expected to provide resistance against dust particle accumulation, improving the durability of photocatalytic activity in wastewater treatments. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobicity is the physical property referring to the repelling of water with a water contact angle exceeding 150◦ . This property results from dual surface roughness in micro- and nano-meter scales superimposed on a hydrophobic surface functionality, similar to the microscopic structure of lotus leaf surfaces [1–4]. Many different strategies for fabricating artificial superhydrophobic surfaces have been reported in the literature over the last decade. These include distribution of hydrophobic nanoparticles on solid surfaces [5–7] and micro- and nano-structuring of flat surfaces combined with surface termination with hydrophobic molecules [8–11]. Superhydrophobic surfaces have drawn particular attention due to their unique self-cleaning behaviors, as dust particles on superhydrophobic surfaces can be swept away by water droplets on the surface [7,12–14]. Moreover, superhydrophobic surfaces have been demonstrated as anti-fouling [15–17] and corrosion-resistant in aqueous media [18,19]. In this study, we make attempt to incorporate superhydrophobic material into photocatalytic water treatment system, which can enhance durability of photocatalysts by preventing deposition of particles and biofilms on the surface. Photocatalytic processes can eliminate organic pollutants from wastewater. In photocatalysis,
∗ Corresponding author. Tel.: +82 31 299 4564; fax: +82 31 290 7075. E-mail address: [email protected] (Y.D. Kim). 1 These authors contributed equally to this work.
semiconductive oxides absorb light to form electron–hole pairs that interact with O2 and H2 O. Strongly oxidizing agents such as O2 − and OH radicals are formed and can participate in the decomposition of organic pollutants. TiO2 is one of the most widely used photocatalysts due to its high photocatalytic activity and chemical stability. However, the wide band gap of TiO2 (3.2 eV) allows absorption of only UV light, which constitutes a small portion of the solar spectrum. Many studies have suggested strategies for fabricating visible light-responsive TiO2 -based photocatalysts, such as doping TiO2 with non-metallic elements such as N, C, F, and S [20–22] and preparing TiO2 /noble-metal hetero-structures [23–26]. It is seemingly difficult for a surface to have simultaneous photocatalytic and superhydrophobic activity because the reactive oxygen species responsible for the oxidation of organic pollutants can be formed at water/photocatalyst interface, i.e., chemical interaction between water and photocatalyst surface is needed for showing high photocatalytic activity, whereas superhydrophobicity is caused by limited interaction between water droplets and a surface. However, photocatalysis relies on molecular interaction between water and a solid surface, whereas superhydrophobicity macroscopic interaction between water and a solid surface. A surface consisting of only 50–70% hydrophobic domains with dual surface roughness can show superhydrophobicity, and such surfaces do not fully inhibit molecular-level interaction with water [27–29]. This study shows the facile preparation of films consisting of inert and hydrophobic materials on which photocatalysts are added to result in both superhydrophobic and photocatalytic activity
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under UV and visible light. These films maintain their superhydrophobicity and self-cleaning properties in sunlight, while both visible and UV irradiation of the solar spectrum can be exploited for photocatalysis. 2. Experimental 2.1. Sample preparation To coat PDMS on SiO2 nanoparticles (purity 99.8%, mean particle size = 12 nm, Sigma–Aldrich), fluidic PDMS (Sylgard 184, Dow Corning) and SiO2 nanoparticles with a weight ratio of 1:1 were placed in a stainless steel chamber. PDMS was placed on the bottom of the chamber, and SiO2 was placed on top of a stainless steel mesh above PDMS that physically separated it from the SiO2 . Chamber temperature was maintained at 300 ◦ C for 12 h. Fluidic PDMS was evaporated and then deposited on SiO2 nanoparticles to result in the formation of hydrophobic thin films on SiO2 nanoparticles. TiO2 powder (Degussa P-25) in a quartz holder was placed in a quartz reactor and treated with constant ammonia gas flow at 200 sccm and 600 ◦ C for 5 h, resulting in N-doping of TiO2 (N-TiO2 ). Photocatalytically active films with superhydrophobicity were prepared through the following process. First, PDMS adhesives (PDMS:curing agent = 10:1) were homogeneously dispersed on a glass substrate (2 × 2 cm2 ) and partially cured by heating at 60 ◦ C. After 40 min, the mixtures of PDMS-coated SiO2 and N-TiO2 with various ratios were distributed on top of PDMS adhesives and pressurized to improve adhesion between the particles and PDMS adhesives. PDMS adhesives were completely cured, and particles that had not adhered to the glass were removed using compressed air. 2.2. Characterization Contact angles on films consisting of PDMS-coated SiO2 and N-TiO2 were measured using a Theta Optical Tensiometer (KSV Instruments, Ltd) equipped with a digital camera connected to a computer. Young-Laplace curves were employed as a fitting method. Three microliters of distilled water was dropped onto the surface for each measurement, and the average value of water contact angles at three different positions on a sample was evaluated. The stability of water contact angles for films consisting of N-TiO2 and PDMS-coated SiO2 mixtures with different ratios (N-TiO2 : 0, 10, 30, 40, 50, 60, 80, and 100%) was tested. Each sample was placed in a petri dish with 20 ml of distilled water and irradiated with visible (blue light emitting diode (LED): ∼455 nm) or UV (365 nm) light for up to 48 h. The samples were dried for 1 min under dark conditions before contact angle measurement was performed every 2 h for a stability test. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface composition and chemical state of pelletized N-TiO2 . XPS analysis was performed in an ultra-high vacuum (UHV) chamber with base pressure maintained lower than 3.0 × 10−10 Torr. All XPS spectra were obtained at room temperature using a Mg K␣source (1253.6 eV) and a concentric hemispherical analyzer (CHA, PHOIBOS-Has 3500, SPECS). In addition, the UV–vis absorption spectra of N-TiO2 were measured using diffused reflection spectroscopy (DRS, UV-3600, SHIMADZU). A N-TiO2 pellet was used for DRS analysis, and BaSO4 was used as a reference. The obtained reflectance value (R) was converted to absorbance based on the Kubelka–Munk equation (Eq. (1)). F(R) =
(1 − R)2 2R
(1)
Fig. 1. Schematic description of the set-up for photocatalysis experiments.
Surface structural information of the film with N-TiO2 and PDMS-coated SiO2 ratio of 5:5 was obtained by scanning electron microscope (SEM, JSM-7100F, JEOL) equipped with energy dispersive X-ray spectroscopy (EDS) and atomic force microscope (AFM, NTEGRA spectra, NT-MDT)).
2.3. Photocatalysis experiments Adsorption and photocatalytic degradation of methylene blue (MB) over films consisting of N-TiO2 and PDMS-coated SiO2 mixtures with different ratios (N-TiO2 : 0, 10, 30, 50, 80, and 100%) were investigated by monitoring the absorbance of MB in the UV–vis range as a function of time. A UV–vis spectrometer (OPTIZNE 3220UV) was used to record absorbance values at the maximum absorbance wavelength of MB to determine MB concentrations. Fig. 1 shows the experimental set-up used for adsorption and photocatalysis experiments. A petri dish was divided into three subsections by water-penetrable metal meshes. Two sample pieces (2 × 2 cm2 area for each) were placed in the middle part, and two magnetic bars were located at the edges to homogeneously mix the solution inside the petri dish. The petri dish was first kept in dark conditions for 4 h to saturate MB adsorption on the sample surface. Subsequently, the petri dish was exposed to blue LED or UV lamp irradiation (photocatalysis step) for 8 h with moderate stirring. Absorption spectra of the solution were measured every 1 h during the photocatalysis step. Photocatalytic degradation of 2-methylisoborneol (MIB) in aqueous solution was also tested using the set-up mentioned above for MB experiments (Fig. 1). Photocatalysis experiments of 2-MIB were carried out with a film consisting of a 5:5 mixture of N-TiO2 and PDMS-coated SiO2 . The initial concentration of 2-MIB aqueous solution was 1 ppm. Solid phase microextraction (SPME) and gas chromatography (GC) were used to determine the concentration of 2-MIB at different times in adsorption and photocatalysis experiments. In the SPME process for extracting 2MIB, 5 ml of each 2-MIB aqueous solution in a sealed glass vial were heated at 70 ◦ C with moderate magnetic stirring. Divinylbenzene (DVB)/carboxen (CAR)/PDMS fiber (SUPELCO) was exposed to vapor in the vial for 30 min. The 2-MIB extracted by the fiber was directly injected into the GC and heated for desorption, qualitative analysis, and quantitative analysis of the desorbed 2-MIB. To obtain each data point in the 2-MIB adsorption and photocatalysis experiment, a 2-MIB aqueous solution with a concentration of 1 ppm was freshly prepared. This solution was maintained under dark and/or illumination conditions for a certain time in the presence of PDMS coated-SiO2 /N-TiO2 samples. The GC (HP6890, Agilent Technologies) was equipped with a 5% phenyl methylsiloxane capillary column (DB-5, 30 m × 0.25 mm, Agilent Technologies), methanizer, and flame ionization detector (FID).
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Fig. 2. (a) Optical images of bare and PDMS-coated SiO2 in vials with water, (b) water contact angle on the surface of PDMS-coated SiO2 .
3. Results and discussion 3.1. Characterization of bare and modified nanoparticles and photocatalyst films To verify the hydrophobic properties of SiO2 nanoparticles covered with PDMS-coating, PDMS-coated SiO2 was dispersed in distilled water, and its behavior was compared with that of bare SiO2 (Fig. 2(a)). Because SiO2 is hydrophilic and denser than water, bare SiO2 particles can be mixed with water and then placed at the bottom of a water-containing vial. In contrast, PDMS-coated SiO2 floated on the water surface, indicating that the SiO2 surface became completely hydrophobic with PDMS coating. The mean water contact angle of PDMS-coated SiO2 distributed on a flat glass slide was determined to be 157◦ (±2◦ ), indicating that the surface was extremely repellent to water (Fig. 2(b)). Note that the surface prepared in the same way with bare SiO2 showed a water contact angle of 110◦ (±2◦ ), and this is much lower than the respective value of PDMS-coated SiO2 (Supporting information 1). Most likely, distribution of 10–50 nm
Intensity (a.u.)
N 1s C 1s
900
600
300
0
405
Binding Energy(eV)
465
460
455
450
Binding Energy(eV)
445
Kubelka-Munk (a.u.)
Intensity (a.u.) 470
400
395
390
Binding Energy(eV) TiO2 N-TiO2
(c)
N-TiO2
(b) Intensity (a.u.)
Ti 2p
1200
nanoparticles on the surface resulted in dual surface roughness due to the sizes of individual particles overlapping with the submicrometer scale roughness caused by agglomerated particles. Such surface dual roughness in combination with the hydrophobic properties of PDMS-coating resulted in superhydrophobic behavior. The surfaces of TiO2 particles before and after NH3 treatment were analyzed by XPS to characterize the compositions and electronic states of surface elements. Before NH3 treatment, no N core level peaks were observed (Fig. 3(a)). However, a N 1s peak centered at 396.3 eV was seen after NH3 treatment, which corresponds to Ti N bond formation (Fig. 3(a and b)) [20,30–32]. The Ti 2p XPS spectrum of TiO2 in Fig. 3(c) shows a Ti 2p3/2 core level peak centered at 458.8 eV, which corresponds to Ti4+ in the TiO2 lattice [33]. Upon NH3 treatment, shoulders were noted at lower binding energies (identified with a circle in Fig. 3(c)). This finding indicates that Ti4+ is reduced to Ti3+ , Ti2+ , and Ti+ due to incorporation of N into the TiO2 lattice during NH3 treatment. This result is in agreement with previous suggestions that substitutional N is stabilized by the presence of oxygen vacancies in TiO2 [34,35]. UV–vis absorption spectra of TiO2 and N-TiO2 in diffuse reflectance mode are shown in Fig. 3(d). No TiO2 absorption of visible light from 400 to 800 nm was observed before N-doping. Conversely, N-TiO2 absorbed a broad wavelength range of visible light. This difference is attributed to two distinctive effects. The substitutional doping of N 2p states contributes to a localized mid state above the valence band of TiO2 formed by linear combination with O 2p states, which enhances the absorbance at 400–500 nm [36,37]. Oxygen vacancies verified by XPS can form multiple unoccupied mid band gap states below the conduction band, resulting in absorption of N-TiO2 over the 500 nm region [37–39]. PDMS-coated SiO2 and N-TiO2 were mechanically mixed and distributed on the surfaces of PDMS adhesives during PDMS curing. The surface structure and EDS mapping images of film surfaces consisting of N-TiO2 and PDMS-coated SiO2 with a ratio of 5:5 are
TiO2 N-TiO2
O 1s
(a)
3
(d)
TiO2 N-TiO2 400
600
800
Wavelength (nm)
Fig. 3. (a) survey XPS spectra of bare and N-doped TiO2 , (b) N 1s spectrum of N-doped TiO2 , (c) Ti 2p spectra of bare and N-doped TiO2 , (d) optical absorption spectra of bare and N-doped TiO2 obtained in reflectance mode. Y-axis was converted based on the Kubelka–Munk function.
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Fig. 4. AFM (a), SEM image (b), and EDS mapping images of a surface of PDMS-coated SiO2 and N-TiO2 . (c) Si, (d) Ti, (e) C, (f) O.
shown in Fig. 4(a)–(f). In the AFM image (Fig. 4(a), a submicrometerscale surface roughness (a root mean square roughness of 496 nm) due to the agglomerated structures of individual nanoparticles dominates, even though the individual nanoparticles are visible in the SEM image of Fig. 4(b). EDS mapping images in Fig. 4(c)–(f) show that N-TiO2 and PDMS-coated SiO2 are not heterogeneously agglomerated but are homogeneously mixed and uniformly distributed on the surface. 3.2. Photo-stability of superhydrophobicity and photocatalytic activity The photo-stability of films with different mixing ratios of PDMS coated-SiO2 and N-TiO2 nanoparticles (N-TiO2 : 0, 10, 30, 40, 50, 60, 80, and 100%) was evaluated under UV and visible light irradiation (Fig. 5). Before UV or visible light irradiation, all of the as-prepared films showed superhydrophobicity with water contact angles exceeding 150◦ . It is worth mentioning that use of bare TiO2 instead of N-TiO2 in the film also resulted superhydrophobic property (Supporting information 1) [28]. Both dual surface roughness and hydrophobic functionality are required to
achieve superhydrophobicity. Dual surface roughness originated in the superhydrophobic surfaces because of the intrinsic sizes of nanoparticles and micrometer sizes of aggregated nanoparticles (Fig. 4(a) and (b)). In addition, the hydrophobic functionality of the surface resulted from a combination of PDMS adhesives, PDMS-coated SiO2 nanoparticles, and organic residue molecules from the atmosphere adsorbed on N-TiO2 [40,41]. The organic residue molecules are most likely alkane species from the atmosphere terminated by hydrophobic CH3 groups, which has been also reported in the literature, yet cannot be spectroscopically (e.g., using XPS or FT-IR) identified in the present work due to the signal overlap with the CH3 group of PDMS. Because locations covered by SiO2 and TiO2 and uncovered areas were all terminated by hydrophobic entities, the film surfaces showed superhydrophobic properties independent of the ratio of PDMS-coated SiO2 and N-TiO2 . Upon UV light exposure for 48 h, films consisting of more than 60% PDMS-coated SiO2 maintained superhydrophobicity, whereas samples with less PDMS-coated SiO2 showed significant decreases in water contact angles (Fig. 5(a)). It should be noted that PDMS layers are highly stable under UV light, whereas hydrophobic
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Fig. 5. Change in water contact angle of various surfaces consisting of different ratios of PDMS-coated SiO2 and N-TiO2 as a function of light exposure time. (a) UV, (b) visible light.
Fig. 6. Removal of MB as a function of light irradiation time by various photocatalysts with different ratios of PDMS-coated SiO2 and N-TiO2 . (a) UV, (b) visible light.
organic residue molecules on N-TiO2 are reactive and easily decomposed [42]. Only films with a dominant PDMS-layer with respect to the reactive organic residue molecules on TiO2 can demonstrate highly stable superhydrophobicity under UV light. It is notable that when the film dominantly consisted of N-TiO2 (80 and 100%), one can observe superhydrophilicy with a water contact angle close to 0◦ after long UV-irradiation, since then N-TiO2 surface can becomes hydrophilic due to the surface OH-termination. In contrast to the UV experiment results, all samples maintained superhydrophobicity under visible light irradiation (Fig. 5(b)). The organic residue molecules on TiO2 are apparently only destroyed under UV irradiation and survive under visible light. Borrás et al. observed a similar phenomenon that the hydrophobic-to-hydrophilic conversion of TiO2 surfaces was easier under UV light irradiation than with visible light exposure [42]. It was suggested that the oxidizing power of N-TiO2 holes induced by UV light is stronger than that induced by visible light, allowing more facile decomposition of organic molecules by UV light. This suggestion is also valid to justify our observations. The detailed behaviors of the photocatalytic degradation of superhydrophobicity under UV light are noteworthy. Because superhydrophobicity is maintained for certain periods and a sudden decrease in water contact angle was found for some samples, induction periods seem to exist for photocatalytic degradation of the hydrophobic layers. At the initial stage of the experiment, water chemisorption was almost completely inhibited by the hydrophobic surface layers that heavily reduce photocatalytic degradation of hydrophobic molecules on TiO2 . As reaction time increased, the number of free TiO2 sites increased, causing more interaction with
water and higher photocatalytic activity. The process demonstrated here is regarded as autocatalytic. Photocatalytic degradation of MB under UV light irradiation on films with different ratios of N-TiO2 (N-TiO2 : 0, 10, 30, 50, 80, and 100%) (Fig. 6((a) and (b)) was also evaluated. For the experiments with different light sources, catalytic performance increased as the ratio of N-TiO2 on the film increased. It is worth noting that use of bare TiO2 instead of N-TiO2 resulted in a much less photocatalytic activity under blue LED (Supporting information 2). Therefore, one can conclude that the N-doping of TiO2 is essential for enhancing visible-light responses of photocatalytic activity, even though the bare TiO2 also shows some photocatalytic activity under visible light, most likely due to the visible light absorption caused by impurities. As shown in Fig. 7, MB decomposition is a first-order reaction of MB concentration. Under these experimental conditions, reaction rates are governed by MB concentration rather than properties of light sources (wavelength, number of photons). The rate constant for the MB decomposition almost linearly changed as a function of the amount of TiO2 (Fig. 7(c)). When the portion of N-TiO2 becomes lower, photocatalytic activity decreases, whereas increase in the relative amount of TiO2 makes superhydrophobicity unstable under UV. The ∼5:5-mixture of PDMS-coated SiO2 and N-TiO2 can be regarded as the film with an almost maximum photocatalytic activity with stable superhydrophobcity. When such surfaces are kept in contact with aqueous media under sun light, photocatalytic purification of water with a high durability of phtocatalytic activity can be realized: high resistance of the surface towards particle accumulation on the
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Fig. 8. Decrease in 2-MIB concentration as a function of time under UV and visible light irradiation. The weight ratio of PDMS-coated SiO2 and N-TiO2 in the photocatalyst was 5:5.
Fig. 7. Values in the y-axis of Fig. 6(a) and (b) were converted into logarithm and displayed in (a) and (b), respectively, as a function of time. In (c) the rate constants of each reaction in (a) and (b) are summarized.
surface and biofilms can be achieved by the superhydrophobicity. We actually showed that our superhydrophobic and photocatalytically active surface can be utilized for the removal of 2-MIB which is one of the origins of undesirable earthy flavor and musty odor in drinking water. As shown in Fig. 8(a) and (b), the concentration of 2-MIB rapidly decreased as a function of visible or UV light irradiation time. Similar to MB, the 2-MIB decomposition rate is almost independent of the light source. This is probably due to 2-MIB-concentration-dependent kinetics under the experimental conditions. Further evaluation of our data in Fig. 8 shows that the decomposition of 2-MIB can be described as a first order reaction of the concentration of 2-MIB, analogous to the situation of MB (Fig. 9).
Fig. 9. Values in the y-axis of Fig. 8 were converted into logarithm and displayed as a function of time.
4. Conclusion We fabricated films with simultaneous superhydrophobic properties and photocatalytic activity using PDMS-coated SiO2 and N-TiO2 mixed in a proper ratio, which is ∼5:5. Under both UV and visible light irradiation, superhydrophobicity was sustained. Photocatalytic activity causing MB decomposition in aqueous solution was confirmed under UV and visible light conditions. In addition, performance of these superhydrophobic and photocatalytically
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active surfaces towards photocatalytic decomposition of 2-MIB, a cause of the undesired odor of the drinking water, was demonstrated. We suggest that our materials can be promising building blocks of water treatment systems. Acknowledgements This work was supported by the Agency for Defense Development through the Chemical and Biological Defense Research Center (CBDRC) and Civil-Military Technology Cooperation Program. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.05. 020 References [1] [2] [3] [4] [5] [6] [7] [8]
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Superhydrophobic surfaces with photocatalytic activity under UV and visible light irradiation Ju Ha Lee 1 , Eun Ji Park 1 , Dae Han Kim, Myung-Geun Jeong, Young Dok Kim ∗ Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e
i n f o
Article history: Received 20 March 2015 Received in revised form 8 April 2015 Accepted 11 May 2015 Available online xxx Keywords: Superhydrophobicity Photocatalysis Visible light PDMS TiO2
a b s t r a c t Hydrophobic PDMS-coated silica nanoparticles were mixed with photocatalytically active N-doped TiO2 and distributed on the glass substrate. This facile method created films with simultaneous superhydrophobic and photocatalytic activity under UV and visible light. The effective decomposition of methylene blue and 2-methylisoborneol photocatalyzed by these films was confirmed. This superhydrophobicity is expected to provide resistance against dust particle accumulation, improving the durability of photocatalytic activity in wastewater treatments. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Superhydrophobicity is the physical property referring to the repelling of water with a water contact angle exceeding 150◦ . This property results from dual surface roughness in micro- and nano-meter scales superimposed on a hydrophobic surface functionality, similar to the microscopic structure of lotus leaf surfaces [1–4]. Many different strategies for fabricating artificial superhydrophobic surfaces have been reported in the literature over the last decade. These include distribution of hydrophobic nanoparticles on solid surfaces [5–7] and micro- and nano-structuring of flat surfaces combined with surface termination with hydrophobic molecules [8–11]. Superhydrophobic surfaces have drawn particular attention due to their unique self-cleaning behaviors, as dust particles on superhydrophobic surfaces can be swept away by water droplets on the surface [7,12–14]. Moreover, superhydrophobic surfaces have been demonstrated as anti-fouling [15–17] and corrosion-resistant in aqueous media [18,19]. In this study, we make attempt to incorporate superhydrophobic material into photocatalytic water treatment system, which can enhance durability of photocatalysts by preventing deposition of particles and biofilms on the surface. Photocatalytic processes can eliminate organic pollutants from wastewater. In photocatalysis,
∗ Corresponding author. Tel.: +82 31 299 4564; fax: +82 31 290 7075. E-mail address: [email protected] (Y.D. Kim). 1 These authors contributed equally to this work.
semiconductive oxides absorb light to form electron–hole pairs that interact with O2 and H2 O. Strongly oxidizing agents such as O2 − and OH radicals are formed and can participate in the decomposition of organic pollutants. TiO2 is one of the most widely used photocatalysts due to its high photocatalytic activity and chemical stability. However, the wide band gap of TiO2 (3.2 eV) allows absorption of only UV light, which constitutes a small portion of the solar spectrum. Many studies have suggested strategies for fabricating visible light-responsive TiO2 -based photocatalysts, such as doping TiO2 with non-metallic elements such as N, C, F, and S [20–22] and preparing TiO2 /noble-metal hetero-structures [23–26]. It is seemingly difficult for a surface to have simultaneous photocatalytic and superhydrophobic activity because the reactive oxygen species responsible for the oxidation of organic pollutants can be formed at water/photocatalyst interface, i.e., chemical interaction between water and photocatalyst surface is needed for showing high photocatalytic activity, whereas superhydrophobicity is caused by limited interaction between water droplets and a surface. However, photocatalysis relies on molecular interaction between water and a solid surface, whereas superhydrophobicity macroscopic interaction between water and a solid surface. A surface consisting of only 50–70% hydrophobic domains with dual surface roughness can show superhydrophobicity, and such surfaces do not fully inhibit molecular-level interaction with water [27–29]. This study shows the facile preparation of films consisting of inert and hydrophobic materials on which photocatalysts are added to result in both superhydrophobic and photocatalytic activity
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under UV and visible light. These films maintain their superhydrophobicity and self-cleaning properties in sunlight, while both visible and UV irradiation of the solar spectrum can be exploited for photocatalysis. 2. Experimental 2.1. Sample preparation To coat PDMS on SiO2 nanoparticles (purity 99.8%, mean particle size = 12 nm, Sigma–Aldrich), fluidic PDMS (Sylgard 184, Dow Corning) and SiO2 nanoparticles with a weight ratio of 1:1 were placed in a stainless steel chamber. PDMS was placed on the bottom of the chamber, and SiO2 was placed on top of a stainless steel mesh above PDMS that physically separated it from the SiO2 . Chamber temperature was maintained at 300 ◦ C for 12 h. Fluidic PDMS was evaporated and then deposited on SiO2 nanoparticles to result in the formation of hydrophobic thin films on SiO2 nanoparticles. TiO2 powder (Degussa P-25) in a quartz holder was placed in a quartz reactor and treated with constant ammonia gas flow at 200 sccm and 600 ◦ C for 5 h, resulting in N-doping of TiO2 (N-TiO2 ). Photocatalytically active films with superhydrophobicity were prepared through the following process. First, PDMS adhesives (PDMS:curing agent = 10:1) were homogeneously dispersed on a glass substrate (2 × 2 cm2 ) and partially cured by heating at 60 ◦ C. After 40 min, the mixtures of PDMS-coated SiO2 and N-TiO2 with various ratios were distributed on top of PDMS adhesives and pressurized to improve adhesion between the particles and PDMS adhesives. PDMS adhesives were completely cured, and particles that had not adhered to the glass were removed using compressed air. 2.2. Characterization Contact angles on films consisting of PDMS-coated SiO2 and N-TiO2 were measured using a Theta Optical Tensiometer (KSV Instruments, Ltd) equipped with a digital camera connected to a computer. Young-Laplace curves were employed as a fitting method. Three microliters of distilled water was dropped onto the surface for each measurement, and the average value of water contact angles at three different positions on a sample was evaluated. The stability of water contact angles for films consisting of N-TiO2 and PDMS-coated SiO2 mixtures with different ratios (N-TiO2 : 0, 10, 30, 40, 50, 60, 80, and 100%) was tested. Each sample was placed in a petri dish with 20 ml of distilled water and irradiated with visible (blue light emitting diode (LED): ∼455 nm) or UV (365 nm) light for up to 48 h. The samples were dried for 1 min under dark conditions before contact angle measurement was performed every 2 h for a stability test. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface composition and chemical state of pelletized N-TiO2 . XPS analysis was performed in an ultra-high vacuum (UHV) chamber with base pressure maintained lower than 3.0 × 10−10 Torr. All XPS spectra were obtained at room temperature using a Mg K␣source (1253.6 eV) and a concentric hemispherical analyzer (CHA, PHOIBOS-Has 3500, SPECS). In addition, the UV–vis absorption spectra of N-TiO2 were measured using diffused reflection spectroscopy (DRS, UV-3600, SHIMADZU). A N-TiO2 pellet was used for DRS analysis, and BaSO4 was used as a reference. The obtained reflectance value (R) was converted to absorbance based on the Kubelka–Munk equation (Eq. (1)). F(R) =
(1 − R)2 2R
(1)
Fig. 1. Schematic description of the set-up for photocatalysis experiments.
Surface structural information of the film with N-TiO2 and PDMS-coated SiO2 ratio of 5:5 was obtained by scanning electron microscope (SEM, JSM-7100F, JEOL) equipped with energy dispersive X-ray spectroscopy (EDS) and atomic force microscope (AFM, NTEGRA spectra, NT-MDT)).
2.3. Photocatalysis experiments Adsorption and photocatalytic degradation of methylene blue (MB) over films consisting of N-TiO2 and PDMS-coated SiO2 mixtures with different ratios (N-TiO2 : 0, 10, 30, 50, 80, and 100%) were investigated by monitoring the absorbance of MB in the UV–vis range as a function of time. A UV–vis spectrometer (OPTIZNE 3220UV) was used to record absorbance values at the maximum absorbance wavelength of MB to determine MB concentrations. Fig. 1 shows the experimental set-up used for adsorption and photocatalysis experiments. A petri dish was divided into three subsections by water-penetrable metal meshes. Two sample pieces (2 × 2 cm2 area for each) were placed in the middle part, and two magnetic bars were located at the edges to homogeneously mix the solution inside the petri dish. The petri dish was first kept in dark conditions for 4 h to saturate MB adsorption on the sample surface. Subsequently, the petri dish was exposed to blue LED or UV lamp irradiation (photocatalysis step) for 8 h with moderate stirring. Absorption spectra of the solution were measured every 1 h during the photocatalysis step. Photocatalytic degradation of 2-methylisoborneol (MIB) in aqueous solution was also tested using the set-up mentioned above for MB experiments (Fig. 1). Photocatalysis experiments of 2-MIB were carried out with a film consisting of a 5:5 mixture of N-TiO2 and PDMS-coated SiO2 . The initial concentration of 2-MIB aqueous solution was 1 ppm. Solid phase microextraction (SPME) and gas chromatography (GC) were used to determine the concentration of 2-MIB at different times in adsorption and photocatalysis experiments. In the SPME process for extracting 2MIB, 5 ml of each 2-MIB aqueous solution in a sealed glass vial were heated at 70 ◦ C with moderate magnetic stirring. Divinylbenzene (DVB)/carboxen (CAR)/PDMS fiber (SUPELCO) was exposed to vapor in the vial for 30 min. The 2-MIB extracted by the fiber was directly injected into the GC and heated for desorption, qualitative analysis, and quantitative analysis of the desorbed 2-MIB. To obtain each data point in the 2-MIB adsorption and photocatalysis experiment, a 2-MIB aqueous solution with a concentration of 1 ppm was freshly prepared. This solution was maintained under dark and/or illumination conditions for a certain time in the presence of PDMS coated-SiO2 /N-TiO2 samples. The GC (HP6890, Agilent Technologies) was equipped with a 5% phenyl methylsiloxane capillary column (DB-5, 30 m × 0.25 mm, Agilent Technologies), methanizer, and flame ionization detector (FID).
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Fig. 2. (a) Optical images of bare and PDMS-coated SiO2 in vials with water, (b) water contact angle on the surface of PDMS-coated SiO2 .
3. Results and discussion 3.1. Characterization of bare and modified nanoparticles and photocatalyst films To verify the hydrophobic properties of SiO2 nanoparticles covered with PDMS-coating, PDMS-coated SiO2 was dispersed in distilled water, and its behavior was compared with that of bare SiO2 (Fig. 2(a)). Because SiO2 is hydrophilic and denser than water, bare SiO2 particles can be mixed with water and then placed at the bottom of a water-containing vial. In contrast, PDMS-coated SiO2 floated on the water surface, indicating that the SiO2 surface became completely hydrophobic with PDMS coating. The mean water contact angle of PDMS-coated SiO2 distributed on a flat glass slide was determined to be 157◦ (±2◦ ), indicating that the surface was extremely repellent to water (Fig. 2(b)). Note that the surface prepared in the same way with bare SiO2 showed a water contact angle of 110◦ (±2◦ ), and this is much lower than the respective value of PDMS-coated SiO2 (Supporting information 1). Most likely, distribution of 10–50 nm
Intensity (a.u.)
N 1s C 1s
900
600
300
0
405
Binding Energy(eV)
465
460
455
450
Binding Energy(eV)
445
Kubelka-Munk (a.u.)
Intensity (a.u.) 470
400
395
390
Binding Energy(eV) TiO2 N-TiO2
(c)
N-TiO2
(b) Intensity (a.u.)
Ti 2p
1200
nanoparticles on the surface resulted in dual surface roughness due to the sizes of individual particles overlapping with the submicrometer scale roughness caused by agglomerated particles. Such surface dual roughness in combination with the hydrophobic properties of PDMS-coating resulted in superhydrophobic behavior. The surfaces of TiO2 particles before and after NH3 treatment were analyzed by XPS to characterize the compositions and electronic states of surface elements. Before NH3 treatment, no N core level peaks were observed (Fig. 3(a)). However, a N 1s peak centered at 396.3 eV was seen after NH3 treatment, which corresponds to Ti N bond formation (Fig. 3(a and b)) [20,30–32]. The Ti 2p XPS spectrum of TiO2 in Fig. 3(c) shows a Ti 2p3/2 core level peak centered at 458.8 eV, which corresponds to Ti4+ in the TiO2 lattice [33]. Upon NH3 treatment, shoulders were noted at lower binding energies (identified with a circle in Fig. 3(c)). This finding indicates that Ti4+ is reduced to Ti3+ , Ti2+ , and Ti+ due to incorporation of N into the TiO2 lattice during NH3 treatment. This result is in agreement with previous suggestions that substitutional N is stabilized by the presence of oxygen vacancies in TiO2 [34,35]. UV–vis absorption spectra of TiO2 and N-TiO2 in diffuse reflectance mode are shown in Fig. 3(d). No TiO2 absorption of visible light from 400 to 800 nm was observed before N-doping. Conversely, N-TiO2 absorbed a broad wavelength range of visible light. This difference is attributed to two distinctive effects. The substitutional doping of N 2p states contributes to a localized mid state above the valence band of TiO2 formed by linear combination with O 2p states, which enhances the absorbance at 400–500 nm [36,37]. Oxygen vacancies verified by XPS can form multiple unoccupied mid band gap states below the conduction band, resulting in absorption of N-TiO2 over the 500 nm region [37–39]. PDMS-coated SiO2 and N-TiO2 were mechanically mixed and distributed on the surfaces of PDMS adhesives during PDMS curing. The surface structure and EDS mapping images of film surfaces consisting of N-TiO2 and PDMS-coated SiO2 with a ratio of 5:5 are
TiO2 N-TiO2
O 1s
(a)
3
(d)
TiO2 N-TiO2 400
600
800
Wavelength (nm)
Fig. 3. (a) survey XPS spectra of bare and N-doped TiO2 , (b) N 1s spectrum of N-doped TiO2 , (c) Ti 2p spectra of bare and N-doped TiO2 , (d) optical absorption spectra of bare and N-doped TiO2 obtained in reflectance mode. Y-axis was converted based on the Kubelka–Munk function.
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Fig. 4. AFM (a), SEM image (b), and EDS mapping images of a surface of PDMS-coated SiO2 and N-TiO2 . (c) Si, (d) Ti, (e) C, (f) O.
shown in Fig. 4(a)–(f). In the AFM image (Fig. 4(a), a submicrometerscale surface roughness (a root mean square roughness of 496 nm) due to the agglomerated structures of individual nanoparticles dominates, even though the individual nanoparticles are visible in the SEM image of Fig. 4(b). EDS mapping images in Fig. 4(c)–(f) show that N-TiO2 and PDMS-coated SiO2 are not heterogeneously agglomerated but are homogeneously mixed and uniformly distributed on the surface. 3.2. Photo-stability of superhydrophobicity and photocatalytic activity The photo-stability of films with different mixing ratios of PDMS coated-SiO2 and N-TiO2 nanoparticles (N-TiO2 : 0, 10, 30, 40, 50, 60, 80, and 100%) was evaluated under UV and visible light irradiation (Fig. 5). Before UV or visible light irradiation, all of the as-prepared films showed superhydrophobicity with water contact angles exceeding 150◦ . It is worth mentioning that use of bare TiO2 instead of N-TiO2 in the film also resulted superhydrophobic property (Supporting information 1) [28]. Both dual surface roughness and hydrophobic functionality are required to
achieve superhydrophobicity. Dual surface roughness originated in the superhydrophobic surfaces because of the intrinsic sizes of nanoparticles and micrometer sizes of aggregated nanoparticles (Fig. 4(a) and (b)). In addition, the hydrophobic functionality of the surface resulted from a combination of PDMS adhesives, PDMS-coated SiO2 nanoparticles, and organic residue molecules from the atmosphere adsorbed on N-TiO2 [40,41]. The organic residue molecules are most likely alkane species from the atmosphere terminated by hydrophobic CH3 groups, which has been also reported in the literature, yet cannot be spectroscopically (e.g., using XPS or FT-IR) identified in the present work due to the signal overlap with the CH3 group of PDMS. Because locations covered by SiO2 and TiO2 and uncovered areas were all terminated by hydrophobic entities, the film surfaces showed superhydrophobic properties independent of the ratio of PDMS-coated SiO2 and N-TiO2 . Upon UV light exposure for 48 h, films consisting of more than 60% PDMS-coated SiO2 maintained superhydrophobicity, whereas samples with less PDMS-coated SiO2 showed significant decreases in water contact angles (Fig. 5(a)). It should be noted that PDMS layers are highly stable under UV light, whereas hydrophobic
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Fig. 5. Change in water contact angle of various surfaces consisting of different ratios of PDMS-coated SiO2 and N-TiO2 as a function of light exposure time. (a) UV, (b) visible light.
Fig. 6. Removal of MB as a function of light irradiation time by various photocatalysts with different ratios of PDMS-coated SiO2 and N-TiO2 . (a) UV, (b) visible light.
organic residue molecules on N-TiO2 are reactive and easily decomposed [42]. Only films with a dominant PDMS-layer with respect to the reactive organic residue molecules on TiO2 can demonstrate highly stable superhydrophobicity under UV light. It is notable that when the film dominantly consisted of N-TiO2 (80 and 100%), one can observe superhydrophilicy with a water contact angle close to 0◦ after long UV-irradiation, since then N-TiO2 surface can becomes hydrophilic due to the surface OH-termination. In contrast to the UV experiment results, all samples maintained superhydrophobicity under visible light irradiation (Fig. 5(b)). The organic residue molecules on TiO2 are apparently only destroyed under UV irradiation and survive under visible light. Borrás et al. observed a similar phenomenon that the hydrophobic-to-hydrophilic conversion of TiO2 surfaces was easier under UV light irradiation than with visible light exposure [42]. It was suggested that the oxidizing power of N-TiO2 holes induced by UV light is stronger than that induced by visible light, allowing more facile decomposition of organic molecules by UV light. This suggestion is also valid to justify our observations. The detailed behaviors of the photocatalytic degradation of superhydrophobicity under UV light are noteworthy. Because superhydrophobicity is maintained for certain periods and a sudden decrease in water contact angle was found for some samples, induction periods seem to exist for photocatalytic degradation of the hydrophobic layers. At the initial stage of the experiment, water chemisorption was almost completely inhibited by the hydrophobic surface layers that heavily reduce photocatalytic degradation of hydrophobic molecules on TiO2 . As reaction time increased, the number of free TiO2 sites increased, causing more interaction with
water and higher photocatalytic activity. The process demonstrated here is regarded as autocatalytic. Photocatalytic degradation of MB under UV light irradiation on films with different ratios of N-TiO2 (N-TiO2 : 0, 10, 30, 50, 80, and 100%) (Fig. 6((a) and (b)) was also evaluated. For the experiments with different light sources, catalytic performance increased as the ratio of N-TiO2 on the film increased. It is worth noting that use of bare TiO2 instead of N-TiO2 resulted in a much less photocatalytic activity under blue LED (Supporting information 2). Therefore, one can conclude that the N-doping of TiO2 is essential for enhancing visible-light responses of photocatalytic activity, even though the bare TiO2 also shows some photocatalytic activity under visible light, most likely due to the visible light absorption caused by impurities. As shown in Fig. 7, MB decomposition is a first-order reaction of MB concentration. Under these experimental conditions, reaction rates are governed by MB concentration rather than properties of light sources (wavelength, number of photons). The rate constant for the MB decomposition almost linearly changed as a function of the amount of TiO2 (Fig. 7(c)). When the portion of N-TiO2 becomes lower, photocatalytic activity decreases, whereas increase in the relative amount of TiO2 makes superhydrophobicity unstable under UV. The ∼5:5-mixture of PDMS-coated SiO2 and N-TiO2 can be regarded as the film with an almost maximum photocatalytic activity with stable superhydrophobcity. When such surfaces are kept in contact with aqueous media under sun light, photocatalytic purification of water with a high durability of phtocatalytic activity can be realized: high resistance of the surface towards particle accumulation on the
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Fig. 8. Decrease in 2-MIB concentration as a function of time under UV and visible light irradiation. The weight ratio of PDMS-coated SiO2 and N-TiO2 in the photocatalyst was 5:5.
Fig. 7. Values in the y-axis of Fig. 6(a) and (b) were converted into logarithm and displayed in (a) and (b), respectively, as a function of time. In (c) the rate constants of each reaction in (a) and (b) are summarized.
surface and biofilms can be achieved by the superhydrophobicity. We actually showed that our superhydrophobic and photocatalytically active surface can be utilized for the removal of 2-MIB which is one of the origins of undesirable earthy flavor and musty odor in drinking water. As shown in Fig. 8(a) and (b), the concentration of 2-MIB rapidly decreased as a function of visible or UV light irradiation time. Similar to MB, the 2-MIB decomposition rate is almost independent of the light source. This is probably due to 2-MIB-concentration-dependent kinetics under the experimental conditions. Further evaluation of our data in Fig. 8 shows that the decomposition of 2-MIB can be described as a first order reaction of the concentration of 2-MIB, analogous to the situation of MB (Fig. 9).
Fig. 9. Values in the y-axis of Fig. 8 were converted into logarithm and displayed as a function of time.
4. Conclusion We fabricated films with simultaneous superhydrophobic properties and photocatalytic activity using PDMS-coated SiO2 and N-TiO2 mixed in a proper ratio, which is ∼5:5. Under both UV and visible light irradiation, superhydrophobicity was sustained. Photocatalytic activity causing MB decomposition in aqueous solution was confirmed under UV and visible light conditions. In addition, performance of these superhydrophobic and photocatalytically
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Please cite this article in press as: J.H. Lee, et al., Superhydrophobic surfaces with photocatalytic activity under UV and visible light irradiation, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.05.020