Evaluation of Asphaltene Degradation on Highly Ordered TiO2

30 Dec 2010 - It was observed that the water contact angle rose linearly with increasing .... and Verification via Variation in Wettability and FTIR S...
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Evaluation of Asphaltene Degradation on Highly Ordered TiO2 Nanotubular Arrays via Variations in Wettability Xinhu Tang and Dongyang Li* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4 Received October 19, 2010. Revised Manuscript Received December 6, 2010 Photocatalytic degradation of both aquatic and atmospheric organic pollutants on titanium dioxide has been extensively investigated in the past decades, but research on direct photocatalytic degradation of solid-phase organic pollutants is rather limited. In this work, photocatalytic degradation of n-C7 asphaltene, which is composed of solidphase organic substances found in crude oil, on highly ordered TiO2 nanotubular arrays (TNAs) was studied using the wettability as an indicator. It was observed that the water contact angle rose linearly with increasing the concentration of n-C7 asphaltene solution up to 0.02 g mL-1. Further increasing the concentration of n-C7 asphaltene only caused small augment in the contact angle, which eventually became stable around 98°. It is demonstrated that the water contact angle can be used as an indicator to reflect the residual solid-phase organic pollutants within a certain range of pollutant concentration. As observed, n-C7 asphaltene film degraded on TNAs under UV illumination for 60 min, showing complete mineralization of ∼80% of n-C7 asphaltene that was released into air finally. The remaining 20% of asphaltene was partially decomposed into smaller organic molecules, e.g., -C(dO)- and -C(dO)-OH, confirmed by high-resolution X-ray photoelectron spectra analysis. TNAs can be reused to degrade the solid-phase n-C7 asphaltene for a number of cycles without further treatment.

1. Introduction Photocatalytic degradation of organic contaminants on titanium dioxide has been investigated extensively for decades1 and proved to be an effective process for removing both aquatic and atmospheric organic pollutants.2-4 However, research on direct photocatalytic degradation of solid-phase organic pollutants is rather limited, although some relevant studies can be found in the literature, e.g., plastics5-9 and stearic acid.10,11 Theoretically, organic molecules should be in direct contact with the catalyst in order to undergo photocatalytic reactions. Ascribing to the high mobility, gaseous and liquid organic molecules could easily reach the surface of photocatalyst and directly react with lightexcited holes or indirectly degrade via the combination with •OH predominant in aqueous solution.12,13 However, solid-phase organic pollutant is hard to be photocatalytically degraded in a manner similar to that for aqueous or gaseous organic compounds due to the limited mobility and contact between the photocatalyst and the solid-phase pollutant. Some researchers *Corresponding author. Phone: þ1-780-492-6750. Fax: þ1-780-492-2881. E-mail: [email protected].

(1) Gaya, U. I.; Abdullah, A. H. J. Photochem. Photobiol., C 2008, 9, 1–12. (2) Wang, D. F.; Kako, T.; Ye, J. H. J. Am. Chem. Soc. 2008, 130, 2724–2725. (3) Reddy, E. P.; Davydov, L.; Smirniotis, P. Appl. Catal., B 2003, 42, 1–11. (4) Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. Rev. 1993, 22, 417–425. (5) Fa, W. J.; Zan, L.; Gong, C. Q.; Zhong, J. C.; Deng, K. J. Appl. Catal., B 2008, 79, 216–223. (6) Saron, C.; Zulli, F.; Giordano, M.; Felisberti, M. I. Polym. Degrad. Stab. 2006, 91, 3301–3311. (7) Shang, J.; Chai, M.; Zhu, Y. F. Environ. Sci. Technol. 2003, 37, 4494–4499. (8) Zhao, X.; Li, Z. W.; Chen, Y.; Shi, L. Y.; Zhu, Y. F. Appl. Surf. Sci. 2008, 254, 1825–1829. (9) Zhao, X. U.; Li, Z. W.; Chen, Y.; Shi, L. Y.; Zhu, Y. F. J. Mol. Catal. A: Chem. 2007, 268, 101–106. (10) Qiu, S. H.; Starr, T. L. J. Electrochem. Soc. 2007, 154, H472–H475. (11) Jung, H. S.; Lee, J. K.; Nastasi, M.; Kim, J. R.; Lee, S. W.; Kim, J. Y.; Park, J. S.; Hong, K. S.; Shin, H. Appl. Phys. Lett. 2006, 88. (12) Zhao, J.; Yang, X. D. Build. Environ. 2003, 38, 645–654. (13) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1–21.

1218 DOI: 10.1021/la104203f

used polymer-TiO2 composites or mixtures in order to obtain a larger contact area between the photocatalyst and the polymer for facilitating the degradation of the polymers.7-9 This is not a feasible process either, since one has to mix a large amount of TiO2 particles to achieve larger contact area and the degradation occurs only on the surface that is exposed to light. There is also another issue: how to recycle the TiO2 particles from the degraded composites? Furthermore, it is hard to directly determine the degradation state of solid-phase pollutants in the composites. So far, there are only a few indirect methods proposed to evaluate the degradation by measuring the concentrations of volatile organic compounds and CO2 during photocatalytic reactions using a gas chromatograph (GC) equipped with a flame ionization detector7 or FT-IR.10 These methods are, however, inconvenient and/or time-consuming. It is highly desired to develop not only effective solid-phase photocatalytic degradation processes but also alternative methods to assess the degradation of solid-phase pollutants on titanium dioxide film. In recent years, highly ordered TiO2 nanotubular arrays (TNAs) have attracted increasing interests because of their exceptional properties and a wide range of existing and potential applications in elimination of environment pollutants.14-20 The individual one-dimensional nanotube in TNAs harvests sunlight more efficiently than the randomly oriented nanoparticles. Besides, TNAs increase photoexcited charge carrier lifetimes by (14) Hou, Y.; Li, X. Y.; Zou, X. J.; Quan, X.; Chen, G. C. Environ. Sci. Technol. 2009, 43, 858–863. (15) Lin, C. J.; Yu, Y. H.; Liou, Y. H. Appl. Catal., B 2009, 93, 119–125. (16) Ghicov, A.; Schmuki, P. Chem. Commun. 2009, 2791–2808. (17) Macak, J. M.; Zlamal, M.; Krysa, J.; Schmuki, P. Small 2007, 3, 300–304. (18) Seabold, J. A.; Shankar, K.; Wilke, R. H. T.; Paulose, M.; Varghese, O. K.; Grimes, C. A.; Choi, K. S. Chem. Mater. 2008, 20, 5266–5273. (19) Shankar, K.; Basham, J. I.; Allam, N. K.; Varghese, O. K.; Mor, G. K.; Feng, X. J.; Paulose, M.; Seabold, J. A.; Choi, K. S.; Grimes, C. A. J. Phys. Chem. C 2009, 113, 6327–6359. (20) Sanghi, S.; Rani, S.; Agarwal, A.; Bhatnagar, V. Mater. Chem. Phys. 2010, 120, 381–386.

Published on Web 12/30/2010

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one order of magnitude or more. The nanotubular architecture can be optimized for specific light absorption and propagation. Mor et al. demonstrated that hydrogen gas was generated at an overall conversion efficiency of 6.8% in a photoelectrochemical cell based on TNAs with 22 nm pore diameter and 34 nm wall thickness, under 320-400 nm illumination at an intensity of 100 mW/cm2.21 TNAs with a thickness or tube length in the range of tens of micrometers show a photoconversion efficiency of 16.5% under UV illumination (λ = 320-400 nm, 100 mW cm-2).22 Varghese et al.23 demonstrated that hollow TiO2 nanotubes with 135 nm in diameter and a tenth of a millimeter in length could convert carbon dioxide and water vapor into fuels about 20 times faster than that was achieved in previous studies. Therefore, high aspect ratio ordered TNAs appear to be more promising for photocatalytic applications. Superhydrophilicity of TiO2 in particulate or thin film forms has been thoroughly investigated for various industrial applications, such as antifogging mirror, drug delivery, and self-cleaning exterior tiles.24-26 The superhydrophilicity of the TiO2 surface is usually evaluated by the water contact angle, which depends on the surface energy and topography.27 Balaur et al.28 recently reported that a TiO2 array coated with an organic monolayer showed changes in its wetting behavior due to UV-induced decomposition of the coated organic monolayer. The surface of TNAs can be changed from superhydrophilic to superhydrophobic by coating organic molecules, octadecylsilane (C18H37SiH3) or octadecylphosphonic acid [C18H37PO(OH)2]. When illuminated by UV light, the organic monolayer decomposes by chain scission at the functional end of C chains, leading to changes in the wettability of TNAs, which is dependent on the residual organic molecules. Thus, if the organic monolayer is replaced with specific solid-phase organic pollutants, variations in the wetting behavior of TNAs under light illumination could reflect the photocatalytic degradation of the pollutants. In other words, the water contact angle could be used as an indicator to evaluate decomposition of organic pollutants on TNAs. In this work, we investigated photocatalytic degradation of n-C7 asphaltene on TNAs and resulted variations in the wettability of the TNAs. Asphaltenes are commonly described as a solubility class of petroleum that is insoluble in n-alkanes but soluble in aromatics such as benzene or toluene and can cause environmental problems in mitigation of oil spills at sea or ground surface.29,30 n-C7 asphaltene is generally at the high end in molecular weight, polarity, and aromaticity, containing polycyclic sheets cross-linked by alkyl, sulfur, and oxygen bridges. Consequently, it is very stable in nature. So far, little is known about the photocatalytic degradation of asphaltenes.31 (21) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191–195. (22) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18, 11. (23) Varghese, O. K.; Paulose, M.; LaTempa, T. J.; Grimes, C. A. Nano Lett. 2009, 9, 731–737. (24) Miyauchi, M.; Tokudome, H. J. Mater. Chem. 2007, 17, 2095–2100. (25) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431–432. (26) Song, Y. Y.; Schmidt-Stein, F.; Bauer, S.; Schmuki, P. J. Am. Chem. Soc. 2009, 131, 4230–4232. (27) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466–1467. (28) Balaur, E.; Macak, J. M.; Taveira, L.; Schmuki, P. Electrochem. Commun. 2005, 7, 1066–1070. (29) Rudrake, A.; Karan, K.; Horton, J. H. J. Colloid Interface Sci. 2009, 332, 22–31. (30) Mishra, S.; Jyot, J.; Kuhad, R. C.; Lal, B. Appl. Environ. Microbiol. 2001, 67, 1675–1681. (31) Boukir, A.; Guiliano, M.; Asia, L.; El Hallaoui, A.; Mille, G. Analusis 1998, 26, 358–364.

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This study has two objectives, which are (1) to investigate photocatalytic degradation of n-C7 asphaltene on TNAs and (2) particularly to correlate changes in the wettability of n-C7 asphaltene-coated TNAs with the degradation state of the asphaltene under UV illumination. The feasibility to monitor the residual asphaltene using the water contact angle as an indicator was explored. The residues on TNAs after photocatalytic degradation were analyzed with X-ray photoelectron spectroscopy. The photocatalytic stability of TNAs in degrading of solid-phase n-C7 asphaltene was evaluated by testing TNAs’ performance with respect to the number of cycles of reuse.

2. Experimental Details 2.1. Fabrication of TNAs. An anodization process was used to synthesize highly ordered TNAs in organic electrolyte, similar to what we did previously.32,33 Briefly, circular plates of 25 mm in diameter were cut from a titanium foil with a thickness of 2.0 mm (99.2%, Sigma-Aldrich), then ground with silicon carbide papers up to 1200 grit, and finally polished using 0.05 μm alumina powder. A polished titanium sample was mounted into a PTFE sample holder with an exposure area of 1.0 cm2. Prior to anodization, the mounted titanium plate was cleaned with acetone, ethanol, and DI water, successively, in an ultrasonic cleaner. Anodization was performed in a typical two-electrode electrochemical cell with a platinum foil (12 mm 12 mm) used as the counter electrode in an ethylene glycol (99þ%, reagent grade, Fisher Scientific) electrolyte containing 0.25 wt % ammonium fluoride (99.3%, ACS reagent, Fisher Scientific) and 2.0 vol % water for 8 h at room temperature. A direct current power supply (1715A, B&K Precision Corp.) was used to drive the anodization process. All prepared TNAs films were rinsed using DI water and then dried in a nitrogen flow, followed by annealing at 200 °C for 4 h in a programmable tube furnace (F79300, Barnstead|Thermolyne Corp.) with a low heating/cooling rate of 0.50 °C min-1 in air atmosphere. The purpose of using a low heating/cooling rate and low annealing temperature was to minimize the formation of cracks in the TNAs films. 2.2. Characterization. The TNAs were examined under a JEOL JSM6301FXV scanning electron microscope with a field emission electron source operated at 5 kV (FE-SEM). Chemical compositions and bonding states were analyzed by X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS Ultra X-ray photoelectron spectrometer. A monochromatic Al source, operated at 210 W with a pass energy of 20 eV and a step of 0.1 eV, was utilized. All XPS spectra were corrected using the C 1s line at 284.6 eV. Curve fitting and background subtraction were accomplished using the Casa XPS software package (version 2.3.15). The XRD patterns of TNAs were measured through glancing angle X-ray diffraction (Bruker AXS D8 Discover diffractometer with a GADDS area detector). A copper anode target was used, and incidence angle was fixed at 3°. 2.3. Photocatalytic Degradation of n-C7 Asphaltene. 8.0 g of n-C7 asphaltene was dissolved in 10 mL of toluene (laboratory grade, Fisher Scientific) and vigorously stirred for 30 min. The prepared solution was then stored in a sealed glass bottle, which was diluted later to make solutions with different concentrations of n-C7 asphaltene. For asphaltene coating, the annealed TNAs were immersed in an asphaltene solution for 60 s at room temperature. The TNAs coated with asphaltene were then placed in a fume hood for 24 h to have toluene evaporated completely. An UV light source system, consisting of a 150 W xenon arc lamp in ARC lamp housing and a adjustable lamp power supply (LPS220B, Photon Technology International), was used. The output power was set up to 60 W, and the distance from the sample to the light source was fixed at 40 cm. Under the UV illumination, (32) Tang, X. H.; Li, D. Y. J. Phys. Chem. C 2009, 113, 7107–7113. (33) Tang, X. H.; Li, D. Y. J. Phys. Chem. C 2008, 112, 5405–5409.

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Figure 1. Schematic illustration of asphaltene-coated TNAs (A) and hypothetical polycyclic structure of asphaltene molecule34 (B).

Figure 3. XRD patterns of (a) as-fabricated TNAs, (b) TNAs annealed at 200 °C for 4 h, and (c) annealed TNAs after seven cycles’ degradation tests. Figure 2. FE-SEM images of TiO2 nanotubular arrays annealed at 200 °C for 4 h: (A) top view, (B) bottom view, and (C) crosssectional view. degradation of n-C7 asphaltene on TNAs was evaluated by measuring the contact angle of water on the asphaltene-coated TNAs surface at specific time intervals, and XPS was used to obtain supplementary information of asphaltene degradation. The water contact angle was measured in an equilibrium condition using a surface tension instrument (FTA200, First Ten A˚ngstroms). For comparison, the water contact angle of n-C7 asphaltene-coated TNAs stored in darkness for a period of time that was the same as that of UV illumination was also measured. The asphaltene-coated TNAs and a hypothetical molecule of n-C7 asphaltene are schematically illustrated in parts A and B of Figure 1, respectively.

3. Results and Discussion 3.1. Morphology of Fabricated TNAs. The TNAs fabricated by anodization as described in section 2.1 were examined using the FE-SEM. The illustrative top view, bottom view, and cross-sectional views of a typical TNAs film are presented in Figure 2. As shown, this highly ordered and vertically oriented TNAs film is 57 μm in thickness, consisting of individual tubes with 145 nm in mean outer diameter and 27 nm in mean tube’s 1220 DOI: 10.1021/la104203f

wall thickness. The roughness factor (the real surface area per unit nominal surface area) and the length-to-width aspect ratio are 1791 and 393, respectively. It should be indicated that some cracks were observed in the TNAs film, which formed during annealing due to internal stress, although individual tubes keep smooth and were well aligned. These cracks might have negative effects on the photocatalytic activity of TNAs, which however has not been well investigated yet, since up to date no alternative method rather than annealing treatment is available to fabricate fully crystallized TNAs without cracks, which can be used to clarify this issue. To identify the phase transformation of TNAs after annealing at a low temperature of 200 °C, an annealed sample was measured using glancing angle X-ray diffraction. For comparison, both asfabricated TNAs and one annealed TNAs experiencing seven cycles’ photocatalytic tests were also analyzed. Their XRD patterns were presented in Figure 3. As shown, the as-fabricated TNAs are amorphous, consistent with other reported studies.14,17 After heat treatment at 200 °C for 4 h, the TNAs transferred to anatase (ICCD PDF 21-1272). All main diffractions peaks of anatase appeared, as shown in Figure 3, which demonstrated that TNAs had been well crystallized even at the temperature as low as 200 °C. It should be noted that the entire annealing procedure took ∼14 h to complete, including heating and cooling processes at a rather low rate, 0.5 °C/min. Langmuir 2011, 27(3), 1218–1223

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Figure 4. Relationship between the water-contact angle and the concentration of asphaltene.

3.2. Wettability of Asphaltene-Coated TNAs. The wetting behavior of TNAs was evaluated by measuring their equilibrium water contact angle. All wetting-angle measurements were repeated at least four times, over which average values were obtained. It is demonstrated that the contact angle of freshly fabricated TNAs after annealing treatment is 7.1 ( 2.4°, showing a highly hydrophilic characteristic, while the water droplet completely spreads out and flows into the nanotubes after 10 min UV illumination, reflecting the superhydrophilic behavior of the TNAs film, consistent with studies reported in the literature.26,28 This implies that there were some residual organic compounds left on the TNAs, originating from the organic electrolyte used for anodization, which degraded under UV illumination. In order to determine the relationship between the water contact angle and the concentration of n-C7 asphaltene solution, the as-prepared asphaltene solution described in section 2.3 was diluted to make solutions containing 0.4, 0.2, 0.04, 0.02, 0.01, and 0.005 g mL-1 of asphaltene, respectively. A series of TNAs coated with diluted asphaltene solution were then prepared following the procedure described in section 2.3. Water-contact angles of the asphaltene-coated TNAs were measured and results are presented in Figure 4. As shown, the curve of water contact angle vs the concentration of asphaltene solution can be split into two regions: (i) linearly increasing stage (region I); (ii) stable stage (region II). In region I, the water contact angle (θ) increases linearly up to 74.3 ( 2.7° with increasing the concentration of asphaltene solution (C) to 0.02 g mL-1 (the fitted equation θ = 3.95  103C, adjusted R-square=0.98), while in region II (C g 0.04 g mL-1), the water contact angle increases very slowly up to around 98° when the concentration of n-C7 asphaltene is high enough. This result is understandable; Alboudwarej et al.35 have already demonstrated that Athabasca and Cold Lake n-C7 asphaltenes in toluene follows a Langmuir-type adsorption isotherm behavior. Once the adsorption of n-C7 asphaltene saturates on the TNAs, the water contact angle would become constant. Before reaching saturated adsorption, the wetting angle of asphaltene-coated TNAs increased linearly with an increase in the amount of asphaltene, as shown in region I. If the concentration of asphaltene (34) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677–684. (35) Alboudwarej, H.; Pole, D.; Svrcek, W. Y.; Yarranton, H. W. Ind. Eng. Chem. Res. 2005, 44, 5585–5592.

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Figure 5. Representative SEM images of asphaltene-coated TiO2 nanotubular arrays: before (A, 100; B, 2000) and after photocatalytic degradation (C, 2000). Closer views with high magnification are inserted in (B) and (C), respectively.

solution falls into this region, the change in the contact angle under UV illumination could be used as an indicator for assessing the photocatalytic degradation of asphaltene on TNAs. This method for evaluating the photocatalytic degradation of solidphase organic pollutants is fast, convenient, inexpensive, and nondestructive to TNAs as well during measuring the water contact angle. Since asphaltene on the TNAs is degraded, the TNAs could be reused without particular treatment for a number of times, avoiding the inconvenience of, e.g., recycling TiO2 particles from composites or mixtures as discussed earlier. In order to confirm that the changes in the wettability of asphaltene-coated TNAs resulted from photocatalytic degradation of the asphaltene, the surface of TNAs coated with a solution containing 0.04 g mL-1 of n-C7 was examined under a scanning electron microscope. Images of the asphaltene film before photocatalytic degradation taken with two different magnifications (A, 100; B, 2000) and that taken after 60 min UV illumination (C, 2000) are presented in Figure 5. As shown in Figure 5A, an even film of n-C7 asphaltene formed on TNAs after toluene evaporated completely. A closer view illustrated in Figure 5B shows some holes and crevices on the coated asphaltene film, which were caused by cracks formed in TNAs during annealing treatment. After illuminated by UV light for 60 min, the deposited n-C7 asphaltene film degraded and largely removed, showing a morphology (see Figure 5C) similar to that of an asphaltene-free surface as illustrated in Figure 2A. In order to investigate the amount of residual asphaltene, the water contact angles of asphaltene-coated TNAs were measured every 15 min under UV illumination. Results are given in Figure 6. As shown in Figure 6, the water contact angle of TNAs coated with asphaltene decreased from 91.7 ( 2.4° to 17.5 ( 2.8° after UV illumination for 30 min and then decreased continuously, but slowly, down to 0°, and the water droplet entirely spread and entered into the nanotubes after UV illumination for 60 min, which implied that the asphaltene film was fully degraded, since any residual asphaltene would impede the spread of water droplet due to its high hydrophobicity. As shown in Figure 5C (inset), the n-C7 asphaltene film on TNAs disappeared due to photocatalytic DOI: 10.1021/la104203f

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Figure 6. Photocatalytic degradation of n-C7 asphaltene on annealed TNAs under UV illumination. Table 1. Overall Chemical Compositions of Freshly Fabricated (A), n-C7 Asphaltene-Coated (B), and Degraded TNAs under UV Illumination for 60 min (C) content, at. % samples

O

Ti

N

C

S

A B C

62.93 27.55 46.78

19.73 9.89 17.73

0.58 0.86 1.59

16.76 60.19 33.26

1.51 0.63

degradation. To further confirm this, we analyzed changes in surface chemical compositions by examining the X-ray photoelectron spectra of n-C7 asphaltene-coated TNAs before and after 60 min UV illumination. For comparison, X-ray photoelectron spectra of the fresh TNAs were analyzed as well. The overall chemical compositions are listed in Table 1. As shown, the carbon concentration decreased from 60.19 to 33.26 at. % after UV illumination for 60 min. If we assume that the amount of titanium atoms, which have no loss in photocatalytic processes, keeps the same in above three X-ray photoelectron spectra, then ∼80% of asphaltene is estimated to be completely mineralized and turned into CO2 (released into air) by comparing the changes in concentration ratios of carbon to titanium given in Table 1. The highresolution X-ray photoelectron spectra of C 1s, O 1s, and Ti 2p were analyzed to investigate the chemical bonding states of residual organic compounds. Results of the XPS examination are given in Figure 7. As shown in Figure 7A, the relative intensity of peaks of C 1s, Ti 2p, and O 1s changed considerably, resulting from the degradation. The peak of Ti 2p decreased while those of C 1s and O 1s increased, which are directly related to degradation of the n-C7 asphaltene. As shown in Figure 7B, three subpeaks were fitted to the C 1s XP spectrum of fresh TNAs at 284.76 eV, 285.75, and 289.16 eV binding energies with atomic fractions of 61.97, 32.83, and 5.20%, respectively. The first subpeak results from the amorphous pollution carbon (-C-C-) from air.36 The second one at 285.75 eV is in an aliphatic or aromatic C-H environment (=CH-),37 which originates from organic electrolyte. The third peak at 289.16 eV is consistent with binding energy of C in a -COOH environment.38 After coated with n-C7 asphaltene (see Figure 7C), three peaks could be fitted to the C 1s spectrum at

Figure 7. High-resolution XP spectra of C 1s, Ti 2p, and O 1s in fresh TNAs, asphaltene-coated, and degraded TNAs: (A) overlap XP spectra of C 1s, Ti 2p, and O 1s before (solid line) and after (dash line) UV illumination; (B) C 1s in fresh TNAs; (C) C 1s in asphaltene-coated TNAs; and (D) C 1s in degraded asphaltene-coated TNAs. 1222 DOI: 10.1021/la104203f

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Table 2. Chemical States of Carbon and Oxygen in Freshly Fabricated (A), Asphaltene-Coated (B), and Degraded TNAs (C) C1: =CHsamples BE, eV frac, %

C2: -CH2-O or -CdO BE, eV

frac, %

C3: -COOH BE, eV frac, %

Aa 285.75 32.83 289.16 5.20 B 285.63 79.36 287.00 9.52 289.54 11.13 C 285.55 76.02 287.29 13.98 289.36 10.00 a Note: amorphous carbon from air: 284.76 eV, 61.97 at. %.

285.63, 287.00, and 289.54 eV with atomic fractions of 79.36, 9.52, and 11.13%, respectively. The strongest subpeak at 285.63 eV comes from the C in aromatic C-H environment of n-C7 asphaltene. The other two subpeaks result from C in -CdO or C-O and -COOH, respectively.38 The pollutant C-C peak at 284.76 eV disappeared due to its low fraction in amount. After degradation (see Figure 7D), three subpeaks were fitted to the C 1s spectrum at 285.55, 287.29, and 289.36 eV with atomic fractions of 76.02, 13.98, and 10%, respectively. Compared to C 1s XP spectra of freshly coated TNAs, three peaks have close binding energies, indicating they are in similar chemical bonding environments. The above results are summarized in Table 2. However, the atomic fraction of the fitted peak at ∼287 eV increases by 4.5%. It was demonstrated C-H was oxidized into -CdO or C-O on TNAs under UV illumination, consistent with studies reported in the literature.38-40 Therefore, although n-C7 asphaltene itself was degraded completely, some hydrophilic intermediates, e.g., carboxyl, still existed on the TNAs after 60 min UV illumination. The above XPS and SEM analyses well demonstrate the effectiveness of the highly ordered TNAs in degradating n-C7 asphaltene, which was also clearly reflected by changes in the wettability of the TNAs under UV illumination. As a reference, we also examined a polished titanium foil coated with n-C7 asphaltene irradiated by UV. In this case, the wetting angle kept constant and no degradation n-C7 asphaltene was observed, indicating that n-C7 asphaltene was stable under UV irradiation. 3.3. Photocatalytic Stability of TNAs in Degrading Asphaltene. The photocatalytic stability of TNAs in degradation of solid phase n-C7 asphaltene was also evaluated by looking at their performance after repeating usage. The water contact angles were measured at 0, 30, and 60 min of illumination, respectively. In the present work, the photocatalytic degradation processes were repeated up to seven cycles. Results of the wettability measurement are presented in Figure 8. As shown, during the first cycle the TNAs coated with asphaltene of 0.04 g mL-1 showed complete degradation after illuminated under UV for 60 min, although smaller molecules with carboxyl group may remain as discussed earlier. The TNAs were recoated with the same solution of asphaltene and then experienced second degradation cycle for another 60 min UV illumination. The situation remained the same with regard to the degradation of asphaltene on the TNAs. As shown in Figure 8, n-C7 asphaltene appeared to be fully degraded in the initial five cycles, reflected by the fully spreading of water droplet. After that, the water droplet did not spread fully, but the (36) Dedonato, P.; Mustin, C.; Benoit, R.; Erre, R. Appl. Surf. Sci. 1993, 68, 81–93. (37) Sundberg, P.; Larsson, R.; Folkesson, B. J. Electron Spectrosc. Relat. Phenom. 1988, 46, 19–29. (38) Zhu, Y. J.; Olson, N.; Beebe, T. P. Environ. Sci. Technol. 2001, 35, 3113– 3121. (39) Zhu, J.; Yang, J.; Bian, Z. F.; Ren, H.; Liu, Y. M.; Cao, Y.; Li, H. X.; He, H. Y.; Fan, K. N. Appl. Catal., B 2007, 76, 82–91. (40) Zan, L.; Wang, S. L.; Fa, W. J.; Hu, Y. H.; Tian, L. H.; Deng, K. J. Polymer 2006, 47, 8155–8162.

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Figure 8. Photocatalytic stability of TNAs in degrading n-C7 asphaltene (numbers indicate the cycle times).

water contact angles were very small (6.8 ( 1.2°) after seven cycles of repeating usage. It was observed that the water droplet could still fully spread when we extended the time of UV illumination. The good photocatalytic stability of TNAs can also be reflected from the XRD analysis. As shown in Figure 3, the crystalline structure of TNAs (Figure 3c) had no significant change after seven cycles of photocatalytic tests. The above results suggest that TNAs are stable and can be reused in degrading n-C7 asphaltene for a number of cycles without specific treatment.

4. Conclusions Solid-phase photocatalytic degradation of n-C7 asphaltene on highly ordered TiO2 nanotubular arrays under UV illumination was investigated using the water contact angle as an indicator in combination with other analytic instruments. The water contact angle of TNAs rose linearly with increasing the concentration of n-C7 asphaltene solution, which was used to coat the TNAs, up to 0.02 g mL-1. Further increasing the concentration of n-C7 asphaltene, the water contact angle slowly went up to 98°. Under UV illumination, the water contact angle of n-C7 asphaltenecoated TNAs decreased and reached 0° after UV illumination for 60 min, which indicated a complete degradation of the solid-phase pollutant. It was demonstrated that the water contact angle could be used as a quick and feasible indicator to reflect residual solidphase organic pollutants within a certain concentration range. The X-ray photoelectron spectra analysis showed that about 80% or more of n-C7 asphaltene was completely decomposed into CO2 and other smaller hydrophilic organic molecules; e.g., carboxylic acids could remain on TNAs, confirmed by high-resolution spectra of C 1s. The photocatalytic stability tests in the degradation of n-C7 asphaltene showed that the TNAs were stable and could be reused, since all asphaltene appeared to be completely degraded in the initial five cycles, although a small amount of residual asphaltene might be left after that. Acknowledgment. This research was sponsored by the Natural Science and Engineering Research Council of Canada (NSERC). We thank Z. Xu and J. Zhao for their laboratory work in measuring photocatalytic activities of TNAs and thank A. He and S. Xu for their assistances in XPS and SEM measurements. DOI: 10.1021/la104203f

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