Environ. Sci. Technol. 2009, 43, 3260–3265
Titanium dioxide (TiO2) nanotubes formed by anodization over titanium wires show a significant improvement in photocatalytic activity compared to the nanotubes formed over foils. This is evident when the fractional conversion of a textile dye, methyl orange, increased from 19% over a foil to 40% over wires in the presence of nanotubes of identical dimensions illuminated over the same geometrical area. Higher degradation rates with Pt-TiO2 nanotubes over foils are matched by the Pt-free TiO2 nanotubes over the wires. The higher photocatalytic activity over the anodized wires can be attributed to the efficient capture of reflected and refracted light by the radially outward oriented TiO2 nanotubes formed over the circumference of the titanium wire. The formation of TiO2 nanotubes over wires can be considered as an effective alternate to improve photodegradation rates by avoiding expensive additives.
demonstrated the photocatalytic degradation of a textile dye with TiO2 nanotubes that were prepared by anodization of titanium (Ti) foils (14). The advantages of using TiO2 nanotubes as opposed to a particulate film includes efficient charge separation due to the absence of grain boundaries typical in particulate films (15). Several studies have compared the photoactivity of nanotubes with powder TiO2 and shown that the nanotubes demonstrate improved dye photodegradation than the powders (16, 17). However, the use of Ti in the form of a foil can be expensive. While foils are available in millimeter thicknesses, it is reported that TiO2 nanotubes formed by anodization that are less than a micrometer thick are typically sufficient to perform efficient photocatalysis. Longer nanotubes are not as effective as the shorter nanotubes and in fact reduce photocatalytic activity (18). This means that a significant amount of the foil can remain unused after anodization and result in wastage. One option to consider avoiding this wastage is the formation of TiO2 nanotubes over Ti of other geometries, such as thin wires. There are several advantages to this choice. First, unlike films, a large portion of the Ti can be utilized to form nanotubes if fine wires are used. Second, nanotubes can be formed in all directions over a Ti wire. Therefore, loss of photons attributed to scattering effects in the liquid can potentially be minimized since the nanotubes can absorb reflected and/or refracted light. Finally, there is the possibility that one can achieve greater photocatalytic activity with TiO2 nanotubes formed over Ti wires compared to foils due to improved transport of dye molecules around the radially outward orientated nanotubes. This work presents a comparative analysis of the photocatalytic activity of nanotubes formed over foils and wires by following the degradation of a textile dye, methyl orange (MO). Further, the effects of external bias and the presence of metal nanoparticle deposits over the nanotubes on the photocatalytic dye degradation are also presented.
Introduction
Experimental Methods
Dyes constitute a significant part of the waste produced in textile industries. When the textile dyes are released into water bodies, they can be toxic to the marine plants and species (1). Therefore, textile industry wastes have to be treated prior to discharging them into the environment. A popular and economical method to remove the dyes is to pass the waste stream over a high surface area activated carbon to adsorb the dyes prior to releasing the treated material into the water (2, 3). Alternatively, photocatalytic treatment of the dye using solar radiation in the presence of a suitable photocatalyst can assist in the degradation of the dye to benign byproducts and also help in the minimization of the number of steps required in textile effluent treatment (4, 5). Titanium dioxide (TiO2) is a semiconductor photocatalyst that has been used extensively for waste treatment due to its ability to photodegrade a variety of environmental pollutants (6-10). TiO2 can be used as a powder or as a film for photocatalytic degradation. The immobilization of TiO2 as a film on a suitable substrate can minimize post-treatment (11), provide similar degradation kinetics as a slurry system (12), and also favor improvement of the degradation kinetics in the presence of a suitable external bias (13). Recently, we
Synthesis of TiO2 Nanotubes. Ti foils (99% purity) were obtained from ESPI international and cut into plates of dimensions 1 cm × 3 cm. Ti wires obtained from Aldrich (0.5 and 0.25 mm diameter) were cut into 3 cm strips for anodization. A two-electrode configuration was used for anodization with Ti as the working electrode and platinum (Pt) as the counter electrode. Anodization was performed using a procedure adapted from an earlier work (19). An Agilent power source supplied a constant DC voltage of 20 V between the Ti and the Pt electrode. Ti foils up to 2 cm long were anodized for 60 min. Likewise, 0.5 mm wires of 2 cm length were anodized for 30, 45, 60, and 90 min, and the 0.25 mm wire was anodized for 60 min. After anodization the samples were washed with DI water, annealed at 550 °C in a nitrogen atmosphere for 2 h, and cooled back to room temperature in a box furnace (Thermo Scientific, Lindberg Blue M). Surface Characterization. Scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS) analysis was performed by using a Hitachi S-4700 SEM. Highresolution transmission electron microscope (HRTEM) analysis was performed using a JEOL 2100F. X-ray diffraction (XRD) analysis was performed using a Philips 12045 B/3 X-ray diffractometer with a scan rate of 0.6°/min. Pt Deposition. Pt was deposited onto anodized and annealed TiO2 nanotubes by photoreduction. A 10 mM chloroplatinic acid (obtained from Sigma-Aldrich) in ethanol was used as the stock solution. After the deposition, the
Improved Photocatalytic Degradation of Textile Dye Using Titanium Dioxide Nanotubes Formed Over Titanium Wires ARCHANA KAR,† YORK R. SMITH,‡ AND V A I D Y A N A T H A N ( R A V I ) S U B R A M A N I A N * ,‡ Departments of Electrical and Biomedical Engineering and Chemical and Metallurgical Engineering, University of Nevada, Reno, Nevada 89557
Received November 5, 2008. Revised manuscript received January 18, 2009. Accepted February 11, 2009.
* Corresponding author: phone: (775) 784 4686; fax: (775) 327 5059; e-mail:
[email protected]. † Department of Electrical and Biomedical Engineering. ‡ Department of Chemical and Metallurgical Engineering. 3260
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 9, 2009
10.1021/es8031049 CCC: $40.75
2009 American Chemical Society
Published on Web 03/31/2009
TABLE 1. Dimensions (diameter and length) of the TiO2 Nanotubes Formed over the Anodized and Annealed Ti Foil and 0.5 mm Diameter Wiresa anodization time
diameter (nm)
length (nm)
foil, 60 min wire, 30 min wire, 45 min wire, 60 min wire, 90 min
80 60 70 100 >110
490 300 480 700 >600-800
a Note: Longer anodization of 90 min can cause fragmentation of nanotubes. This is why a range of lengths is provided for the 90 min anodized samples.
catalyst was dried by placing the TiO2 nanotubes over a hotplate at 50 °C till the solvent evaporated. This process was repeated till the desired level of loading was achieved. Photoreduction was subsequently performed by exposing to UV light for ∼15 min. Methyl Orange (MO) Degradation. MO (Fisher scientific) was used as a model compound to monitor the photocatalytic activity of the TiO2 nanotubes. A three-arm cell was used for studying degradation of 20 µM MO in water. Seven milliliters of the dye solution was taken in the 3-arm cell with TiO2 as the working electrode, Pt wire as the counter electrode, and calomel as the reference electrode. A 300 W Xe Ozone-free Newport lamp (model no. 6258) was used to illuminate the photocell with UV-vis light. An aqueous 0.5 M CuSO4 screen was placed between the cell and the source to shield the cell from the far UV and to prevent heating of the solution and maintain the light intensity at ∼100 mW. The degradation experiments were performed in the absence and presence of an external bias with respect to a calomel reference electrode (SCE).
qualitative analysis of the nanotubes using EDS indicates the presence of Ti and oxygen. No residue from the remnants of the electrolyte was detected in the EDS spectrum, confirming the purity of the nanotubes. XRD analysis was performed to identify the structure of the Pt-TiO2 nanotube composites. Compared to the asreceived Ti foil or wire, XRD detects the presence of both anatase (101) and rutile (110) phases of the TiO2 nanotubes after annealing. SEM analysis shows small nanoparticle deposits of 8-10 nm size mostly spread over the rim of the nanotubes. EDS confirms these desposits as Pt nanoparticles. It is noteworthy to mention here that the narrow size distribution and small size Pt particles are expected to be formed using the photodeposition method compared to other methods such as chemical reduction (21). Further, on comparing the nanotube diameter to the Pt particle size, it is not unreasonable to expect that the Pt nanoparticles may deposit within the nanotubes. The HRTEM of the TiO2 nanotubes with the Pt deposits were therefore examined to check for the presence of Pt within the tubes. A view of a representative section is shown in Figure 1. A closer examination of the inside of the tube indicates what appears as Pt spots. Fast Fourier transformation (FFT) analysis of the spot was performed to identify the composition. The insert of Figure 1b shows the FFT of the material at the spot, suggesting that it is crystalline. Further, measurements at the spot indicate a fringe width of 0.226 and 0.198 nm. These values correspond to the (111) and (200) basal planes of Pt, respectively. Thus, HRTEM confirms the presence of crystalline Pt nanoparticles within the TiO2 nanotubes. This also indicates that photodeposition results in the formation of Pt particles as small as 3 nm within the nanotubes. B. Photocatalysis. The photocatalytic activity of the TiO2 nanotubes was examined by monitoring the degradation of MO. The dye degradation after various intervals of time was estimated using eq 1
Results and Discussions A. Surface Characterization. The nanotubes prepared by anodization of Ti foils are tubular with a circular cross section. The diameter and length of the nanotubes are measured as 80 and 490 nm, respectively. The formation of the nanotubes can be attributed to the electrolyte-assisted dissolution and oxidation of the Ti to form ordered TiO2 nanotubes (19). Anodization time, voltage, and anodization medium are generally understood to influence the geometry of the nanotube formation. For example, Xie and co-workers recently demonstrated that an increase in the anodization voltage results in an increase in the diameter and the length of the nanotubes (20). Anodization of the Ti wire also results in the formation of TiO2 nanotubes. The formation of these nanotubes over the wires as a function of anodization time is presented. Supporting Information Figure S1 and Table 1 shows the SEM and dimensions of the nanotubes formed after andoziation for 30, 45, 60, and 90 min over a 0.5 mm diameter Ti wire. The nanotubes formed after anodization for 45 min show almost similar dimensions as the nanotubes formed over the foil. The insert of Supporting Information Figure S1b shows the presence of cavities at various locations. Such cavities can be expected to form as a result of the radially outward growth of the nanotubes during longer periods of anodization. These cavities do not appear on the foils. It can be noted that the nanotube diameter and length increases with the anodization time of the wire. It is interesting to note that anodization of the wire for 90 min shows several small bundles of nanotubes. What begins as small fissures in the samples anodized for 30 min, develops into hollow nanotubes over 45 and 60 min of anodization. At the end of 90 min nanotube bundles appear as small islands radiating outward, decorating the underlying Ti wire. A
% MO conversion )
[
]
Abst)0 - Abst × 100 Abst)0
(1)
where Abst is the MO absorbance at 464 nm obtained after various intervals of time (t). It is worthwhile to mention that MO photodegradation results in the formation of several intermediates (22, 23) that eventually undergo degradation as well to finally form carbon dioxide and water. For comparing the photoactivity of the nanotubes formed over Ti foils and wires, the percent conversion was normalized with respect to the illuminated area of the substrates. Rigorous analysis of the degradation product can be performed using several tools (22-24). Such analyses are beyond the scope of this work. Table 2 shows the percent conversion of the dye in the presence of TiO2 nanotubes formed over the foil and wires anodized for different durations. The photodegradation of MO has been normalized to the total surface area. The foil demonstrates a 19% MO conversion after 60 min of continuous UV-vis illumination. In the case of the wires, one can notice that as the anodization time increases, the conversion normalized to the illuminated surface area increases. The wires anodized for 30 min show a lower conversion than the wires anodized for 45 min. Anodization for 60 min improves the conversion marginally. Such variations in the percent conversions can be attributed to the changing dimensions of the nanotubes formed over the wires (Table 1) due to prolonged anodization. Further, the nanotubes formed over one wire by anodization for 60 min show similar conversion to the foil. The wires anodized for 90 min were not examined for photocatalytic degradation as these nanotube bundles tend to become unstable and delaminate from the titanium backbone when photocatalytic degradation of the dye was attempted. Unlike VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3261
FIGURE 1. (a) HRTEM image of a section of the TiO2 nanotubes containing photodeposited Pt. (b) The details of a representative magnified section of the nanotube interior. The fast Fourier transform (FFT) analysis of a region of interest is shown as the insert of b. The FFT of the region of interest confirms the presence of crystalline Pt deposits.
TABLE 2. Effects of the Anodization Time, Wire Diameter, and Number of Anodized Wires on the Percent MO Photodegradation over 60 min
foil 1 wire 2 wires 3 wires 4 wires a
illuminated surface area (diameter 0.25), cm2
wire diameter ) 0.5 (30 min)
wire diameter ) 0.5 (45 min)
wire diameter ) 0.5 (60 min)
illuminated surface area (diameter 0.25), cm2
wire diameter ) 0.25 (60 min)
2 0.314 0.628 0.942 1.256
11 6.7 5.37 4.1
18.7 31 32.5 40
19a 19 32.9 36.2 43.2
0.157 0.314 0.472 0.628
31.8 38.6 42.3 47.1
All photocatalytic degradation experiments were performed under UV-vis illumination for 60 min.
FIGURE 3. Comparative analysis of the Pt effects on MO degradation over TiO2 formed on foil and 0.5 mm diameter Ti wire. [MO] ) 20 µM. Illumination: UV-vis. Light intensity at photocell surface: 100 mW.
FIGURE 2. (a) Comparison of the changes in the percent photocatalytic degradation of MO in the presence of TiO2 nanotubes formed over a Ti foil and several 0.5 mm diameter wires anodized for 60 min. [MO] ) 20 µM. Illumination: UV-vis. Light intensity at photocell surface: 100 mW. (b) Effects of the change in the diameter of the titanium wire on the percent MO conversion. [MO] ) 20 µM. Illumination: UV-vis. Light intensity at photocell surface: 100 mW. foils where the nanotubes grow parallel to one another and benefit from the support of the adjacent tubes, growth over wires does not offer this support, which causes the nanotubes to become unstable and delaminate after anodization for 90 min. 3262
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 9, 2009
The effects of increasing the illuminated surface area on the dye degradation by adding more wires were examined. Since the 45 min anodized samples show nanotubes with similar dimensions as the nanotubes over the foils (Table 1), the effects of 45 min anodized Ti wires was followed closely. However, since the 45 min anodized wires demonstrate MO conversion similar to the 60 min anodized wire, the effects of multiple anodized wires anodized for all times were also examined. The results of photocatalysis performed using multiple 0.5 mm wires placed such that they juxtapose one another is shown in Figure 2a. The percent conversion of the dye increases as the number of wires increases. It should be noted that the exposed area of the foil to the incident light is the same as the sum of the illuminated surface area of the four wires and the interstitial gap that exists between the wires. Therefore, percent conversion was examined only with up to four wires. An overall conversion of 43% is noted with four wires compared to 19% with one wire at the end of 60 min of illumination. Thus, an increase from 19% to 43% is noted when the foil is replaced by four wires of identical
FIGURE 4. Photocurrent response of the TiO2 nanotubes formed over the (A) 0.5 mm diameter Ti wire and (B) Ti foil in 1 M KOH solution. illuminated area. It is important to mention that repeat experiments using the same wires showed similar results, confirming the reproducibility of this trend and that the catalyst surface does not require regeneration. Similar results were reported in one of our earlier studies with TiO2 nanotubes formed over Ti foils (14). These results confirm that, for the same geometrical surface area of illumination, TiO2 nanotubes formed over wires are more effective for dye degradation compared to foils. Since the wires were noted to improve the efficiency of light absorbance, a pertinent question was can the reduction in the wire diameter influence the dye degradation? Figure 2b shows the comparison of the dye degradation in the presence of TiO2 nanotubes formed over one 0.5 mm and 0.25 mm diameter Ti wires. The 0.25 mm wire shows a higher degradation than the 0.5 mm wire. A significant improvement from 19% to 32% is noted when one wire of 0.25 mm replaces a 0.5 mm wire. When four 0.25 mm diameter wires are used, a 20% higher conversion was noted compared to four 0.5 mm diameter wires. Table 2 shows the comparative summary of the effects of number of wires and the diameter of the wires on the MO degradation at the end of 60 min. Thus, among the different configurations examined so far, the leaner wires of 0.25 mm diameter demonstrate a significant improvement in percent dye degradation compared to either the foil or the 0.5 mm diameter wire. However, one of the issues with the 0.25 mm diameter wire is the challenge to maintain the location of the wires with respect to one another when multiple wires are used due to the difficulty faced in handling such thin wires. Keeping the wires separated is crucial to maximize photodegradation by absorbing light from all directions and will require designing
special fixtures, which was beyond the scope of this work. Therefore, in this work, only 0.5 mm diameter wires were used for further investigation. In an earlier study, we demonstrated that the application of an external bias as low as 0.0 V shows a dramatic improvement in the degradation of MO (14). To examine if a similar improvement in MO degradation can be obtained with TiO2 nanotubes formed over the wires, we performed the photocatalysis at 0.0 V bias. Supporting Information Figure S2 shows the comparison of the degradation over a TiO2 wire and foil under a 0.0 V bias vs SCE. The foil and the 0.5 mm diameter wire show 50% and 84% degradation of MO, respectively. Thus, an improvement of over 30% is noted in the presence of the wire. It is noteworthy to mention that while one wire shows MO degradation similar to the foil under unbiased conditions (Figure 2a, Table 2), biasing at 0.0 V in fact allows the same one wire to demonstrate a higher percent degradation compared to the foil. This can be attributed to the improved charge separation due to the external bias. The effects of Pt addition to the TiO2 nanotubes prepared over the foil and wire was also examined. Pt is known to assist in the photocatalytic degradation of the textile dyes. Pt functions as a sink for the photogenerated electrons and aids in electron-hole separation (25, 26). Besides, it forms a Schottky-type contact with the semiconductor which promotes rapid transport of electrons that in turn facilitates photocatalytic oxidation (27, 28). Supporting Information Figure S3 shows the effects of Pt loading on dye degradation using TiO2 nanotubes formed over the foils. As the Pt loading is raised from 0 to 1 µM, the degradation of the dye increases from ∼20% to 40%. However, a further doubling of the Pt loading from 1 to 2 µM does not proportionally increase the percent dye degradation. This can be attributed to the underutilization of the Pt nanoparticles as a result of them being deposited within the nanotubes. The comparative analysis of the Pt effects over the foil and wire is shown in Figure 3. It can be observed that the Pt deposits over one wire improve the degradation from 19% to 31%. Figure 3 also shows that one wire is not capable of meeting the level of degradation noted when Pt deposits are present over the TiO2 foil (40%). However, it is interesting to note that four anodized wires of 0.5 mm diameter are able to exceed the effects of the Pt-loaded TiO2 (Table 2). Thus, while Pt can prove beneficial to improve MO degradation, changing the geometry can be considered as a smarter alternative to perform degradation without using expensive materials in an economical manner.
SCHEME 1. Improvement in the Photoactivity of the TiO2 Nanotubes Formed over Titanium Wires Can Be Attributed to the Geometry of the Underlying Substrate and the Ability of the Nanotubes To Absorb Reflected (IRL) and Refracted (IRF) Light
VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3263
The higher dye degradation can be attributed to the improvement in the geometry obtained when the wires are located juxtaposed to one another. All the reported higher photocatalytic activity can be attributed to the improved light absorbance by the nanotubes formed over the wires. The fact that the nanotubes are formed on the curvature of the wire as opposed to the flat part of a foil allows the nanotubes to absorb incident, reflected, as well as refracted light from all directions surrounding the wire. Furthermore, the interstitial fissures (Supporting Information Figure S1b) between the nanotubes allow the dye molecules improved access to the photocatalyst surface, which contributes to greater percent conversion. To examine the validity of this hypothesis, we monitored the photocurrent generation using the TiO2 nanotubes formed over the Ti foil and the Ti wire. Higher photocurrent and photovoltage are indicative of better charge separation, improved redox properties at the interface, and/or higher light absorbance. Figure 4 shows the photocurrent obtained after illumination of the foil and wire in a KOH solution. Under otherwise similar conditions, the improvement noted with TiO2 nanotubes formed over one wire compared to the foils highlights the role of geometry on the photoactivity over TiO2 nanotubes. This observation confirms the hypothesis proposed earlier and indicates the benefits of switching to a wire-shaped geometry as opposed to a flat surface for growing TiO2 nanotubes. In summary, efficient absorbance of scattered radiation using the TiO2 nanotubes formed over the wires can be identified as the basis for the higher degradation rates of MO. Scheme 1 shows a pictorial representation of the benefits of the nanotube formation over the wires compared to the foils. The degradation kinetics over the nanotubes formed on wires as well as foils was also examined. A pseudo-first-order kinetic model is reported for photocatalytic degradation of MO over TiO2 nanotubes formed on both wires and foils. A power law model of the form shown in eq 2 was considered for this study (29) n -rMO ) kC MO
(2)
where rMO is the degradation rate, k is the rate constant, CMO is the concentration of the dye, and n is the reaction order. An integral method was applied to resolve the order and rate constant of the data sets (30). A first-order process is observed when a linearized form of the power law model (see Supporting Information Figure S4) was employed. This analysis indicates that the photocatalytic degradation over the nanotubes formed over the foil and wires follow a pseudofirst-order kinetics. Supporting Information Table S1 lists the kinetic parameters for the photodegradation of MO over the foil, wires, and Pt loaded on TiO2 nanotubes. Pseudofirst-order processes for the photocatalytic degradation of azo compounds over TiO2 films formed using nanoparticles and nanotubes has also been observed by other groups (31-34). The maximum degradation rate was observed with four wires anodized on 0.25 mm wires for 1 h, which correlates to over a 3-fold increase in degradation rates in comparison to TiO2 foils. This analysis confirms the applicability of the pseudo-first-order linearized power law model to the experimental data. Alternate approaches such as the LangmuirHinshelwood model can be employed to probe the effect of variation in kinetics due to changes in the surface area caused by geometry. TiO2 nanotubes formed by anodization over titanium wires demonstrate a 2-fold increase in photocatalytic activity compared to TiO2 nanotubes formed over titanium foils. The photocatalytic degradation of a model textile dye, methyl orange, increases from 20% to ∼40% in the presence of nanotubes formed over titanium wires. Further, MO deg3264
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 9, 2009
radation over TiO2 nanotubes on wires matches the degradation in the presence of Pt-loaded TiO2 nanotubes over foils. The improved photoactivity with the nanotubes formed over the foils is attributed to efficient absorbance of reflected and refracted light by the nanotubes as they are formed in a radially outward orientation along a titanium backbone.
Acknowledgments This project was funded in part by the junior faculty research grant and by the office of sponsored projects and research provided as a part of the startup package to the author. The support through these awards is gratefully acknowledged. The assistance provided by Drs. Ming and Ahmedian for the X-ray diffraction and the HRTEM analysis is gratefully appreciated.
Supporting Information Available Four supplementary figures and one supplementary for the discussions presented in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Zollinger, H. Color Chemistry: syntheses, properties, and applications of organic dyes and pigments, 2nd ed.; VCH: New York, 1991. (2) Mondal, S. Methods of dye removal from dye house effluent An overview. Envrion. Eng. Sci. 2008, 25, 383–396. (3) Rai, H. S.; Bhattacharyya, M. S.; Singh, J.; Bansal, T. K.; Vats, P.; Banerjee, U. C. Removal of dyes from the effluent of textile and dyestuff manufacturing industry: A review of emerging techniques with reference to biological treatment. Crit. Rev. Env. Sci. Technol. 2005, 35, 219–238. (4) Vautier, M.; Guillard, C.; Herrmann, J. M. Photocatalytic degradation of dyes in water: Case study of indigo and of indigo carmine. J. Catal. 2001, 201, 46–59. (5) Lachheb, H.; Puzenat, E.; Houas, A.; Ksibi, M.; Elaloui, E.; Guillard, C.; Herrmann, J. M. Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania. Appl. Catal. B: Environ. 2002, 39, 75–90. (6) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69–96. (7) Arslan, I.; Balcioglu, I. A.; Bahnemann, D. W. Heterogeneous photocatalytic treatment of simulated dyehouse effluents using novel TiO2-photocatalysts. Appl. Catal. B: Environ. 2000, 26, 193–206. (8) Pelizzetti, E.; Minero, C. Mechanism of the Photooxidative Degradation of Organic Pollutants over TiO2 Particles. Electrochim. Acta 1993, 38, 47–55. (9) Bhatkhande, D. S.; Pangarkar, V. G.; Beenackers, A. A. C. M. Photocatalytic degradation for environmental applications - a review. J. Chem. Technol. Biot. 2002, 77, 102–116. (10) Schmelling, D. C.; Gray, K. A.; Kamat, P. V. The influence of solution matrix on the photocatalytic degradation of TNT in TiO2 slurries. Water. Res. 1997, 31, 1439–1447. (11) Guillard, C.; Disdier, J.; Monnet, C.; Dussaud, J.; Malato, S.; Blanco, J.; Maldonado, M. I.; Herrmann, J. M. Solar efficiency of a new deposited titania photocatalyst: chlorophenol, pesticide and dye removal applications. Appl. Catal. B: Environ. 2003, 46, 319–332. (12) Dijkstra, M. F. J.; Michorius, A.; Buwalda, H.; Panneman, H. J.; Winkelman, J. G. M.; Beenackers, A. A. C. M. Comparison of the efficiency of immobilized and suspended systems in photocatalytic degradation. Catal. Today 2001, 66, 487–494. (13) Vinodgopal, K.; Bedja, I.; Kamat, P. V. Nanostructured semiconductor films for photocatalysis. Photoelectrochemical behavior of SnO2/TiO2 composite systems and its role in photocatalytic degradation of a textile azo dye. Chem. Mater. 1996, 8, 2180–2187. (14) Sohn, Y.; Smith, Y.; Misra, M.; Subramanian, V. Electrochemically assisted photocatalytic degradation of methyl orange using anodized titanium dioxide nanotubes. Appl. Catal. B: Environ. 2008, 84, 372–378. (15) Liu, Z.; Zhang, X.; Nishimoto, S.; Jin, M.; Tryk, D.; Murakami, T.; Fujishima, A. Highly Ordered TiO2 Nanotube Arrays with Controllable Length for Photoelectrocatalytic Degradation of Phenol. J. Phys. Chem. C 2008, 112, 253–259.
(16) Macak, J. M.; Zlamal, M.; Krysa, J.; Schmuki, P. Self-organized TiO2 nanotube layers as highly efficient photocatalysts. Small 2007, 3, 300–304. (17) Xie, Y. B. Photoelectrochemical application of nanotubular titania photoanode. Electrochim. Acta 2006, 51, 3399–3406. (18) Liu, Z.; Zhang, X.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami, T.; Fujishima, A. Highly ordered TiO2 nanotube arrays with controllable length for photoelectrocatalytic degradation of phenol. J. Phys. Chem. C 2008, 112, 253–259. (19) Raja, K. S.; Gandhi, T.; Misra, M. Effect of water content of ethylene glycol as electrolyte for synthesis of ordered titania nanotubes. Electrochem. Commun. 2007, 9, 1069–1076. (20) Xie, Z. B.; Adams, S.; Blackwood, D. J.; Wang, J. The effects of anodization parameters on titania nanotube arrays and dye sensitized solar cells. Nanotechnology 2008, 19. (21) Kozlova, E. A.; Vorontsov, A. V. Influence of mesoporous and platinum-modified titanium dioxide preparation methods on photocatalytic activity in liquid and gas phase. Appl. Catal. B: Environ. 2007, 77, 35–45. (22) Baiocchi, C.; Brussino, M. C.; Pramauro, E.; Prevot, A. B.; Palmisano, L.; Marci, G. Characterization of methyl orange and its photocatalytic degradation products by HPLC/UVVIS diode array and atmospheric pressure ionization quadrupole ion trap mass spectrometry. Int. J. Mass Spectrom. 2002, 214, 247–256. (23) Dai, K.; Chen, H.; Peng, T. Y.; Ke, D. N.; Yi, H. B. Photocatalytic degradation of methyl orange in aqueous suspension of mesoporous titania nanoparticles. Chemosphere 2007, 69, 1361– 1367. (24) Zhiyong, Y.; Keppner, H.; Laub, D.; Mielczarski, E.; Mielczarski, J.; Renken, A.; Kiwi, J. Photocatalytic discoloration of Methyl Orange on innovative parylene-TiO2 flexible thin films under simulated sunlight. Appl. Catal. B: Environ. 2008, 79, 63–71.
(25) Lee, J. S.; Choi, W. Y. Photocatalytic reactivity of surface platinized TiO2: Substrate specificity and the effect of Pt oxidation state. J. Phys. Chem. B 2005, 109, 7399–7406. (26) Bahnemann, D. W.; Hilgendorff, M.; Memming, R. Charge carrier dynamics at TiO2 particles: Reactivity of free and trapped holes. J. Phys. Chem. B 1997, 101, 4265–4275. (27) Lam, S. W.; Chiang, K.; Lim, T. M.; Amal, R.; Low, G. K. C. The effect of platinum and silver deposits in the photocatalytic oxidation of resorcinol. Appl. Catal. B: Environ. 2007, 72, 363– 372. (28) Hufschmidt, D.; Bahnemann, D.; Testa, J. J.; Emilio, C. A.; Litter, M. I. Enhancement of the photocatalytic activity of various TiO2 materials by platinisation. J. Photochem. Photobiol. A 2002, 148, 223–231. (29) Subramanian, V.; Kamat, P. V.; Wolf, E. E. Mass-transfer and kinetic studies during the photocatalytic degradation of an azo dye on optically transparent electrode thin film. Ind. Eng. Chem. Res. 2003, 42, 2131–2138. (30) Fogler, H. S.; Gurmen, M. N. Elements of chemical reaction engineering; Prentice Hall: New York, 2006; Vol. 2. (31) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865– 3868. (32) Daneshvar, N.; Rabbani, M.; Modirshahla, N.; Behnajady, M. A. Kinetic modeling of photocatalytic degradation of Acid Red 27 in UV/TiO2 process. J. Photochem. Photobiol. A 2004, 168, 39– 45. (33) Guettai, N.; Amar, H. A. Photocatalytic oxidation of methyl orange in presence of titanium dioxide in aqueous suspension. Part II: kinetics study. Desalination 2005, 185, 439–448. (34) Zhuang, H. F.; Lin, C. J.; Lai, Y. K.; Sun, L.; Li, J. Some critical structure factors of titanium oxide manotube array in its photocatalytic activity. Environ. Sci. Technol. 2007, 41, 4735–4740.
ES8031049
VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3265
