Functionalization of Self-Organized TiO2 Nanotubes with Pd

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Functionalization of Self-Organized TiO2 Nanotubes with Pd Nanoparticles for Photocatalytic Decomposition of Dyes under Solar Light Illumination Susanta K. Mohapatra, Narasimharao Kondamudi, Subarna Banerjee, and Mano Misra* Chemical and Materials Engineering, UniVersity of NeVada, Reno, NeVada 89557 ReceiVed April 21, 2008. ReVised Manuscript ReceiVed July 10, 2008 Self-organized, vertically oriented TiO2 nanotube arrays prepared by the sonoelectrochemical anodization method are functionalized with palladium (Pd) nanoparticles of ∼10 nm size. A simple incipient wetness method is adopted to distribute the Pd nanoparticles uniformly throughout the TiO2 nanotubular surface. This functionalized material is found to be an excellent heterogeneous photocatalyst that can decompose nonbiodegradable azo dyes (e.g., methyl red and methyl orange) rapidly (150-270 min) and efficiently (100%) under ambient conditions using simulated solar light in the absence of any external oxidative radicals such as hydrogen peroxide.

Introduction Nanoparticles are of considerable interest because of their unique electronic, optical, sensing, and catalytic properties. All of these attributes are due to the quantum size effect of nanopartcles.1-3 In particular, the high surface area to volume ratio of the metal nanoparticles makes them highly attractive materials for catalysis. Palladium (Pd) nanoparticles are of great interest because of their high reactivity and selectivity in hydrogenation and Heck reactions.4-6 A key challenge in the application of these nanoparticles in catalysis is agglomeration (which causes deactivation of the catalyst) and separation. Attempts have been made to overcome the above stated problems by synthesizing the nanoparticles in porous materials such as alumina, silica, polymers, and zeolites.7-9 However, the catalyst particles inside the pores of the support have limited accessibility to the reactants, and the separation problem is still a major issue for industrial applications. In recent years, self-organized, vertically oriented titania (TiO2) nanotubes have been of great interest because of their high surface area, good charge-transport properties, and ease of use. These nanotubes are widely used in the photoelectrolysis of water, hydrogen storage, sensor materials, and biomedical applications.10-18 These 1D TiO2 nanotubes, with both internal and * Corresponding author. E-mail: [email protected]. (1) Lewis, L. N. Chem. ReV. 1993, 93, 2693. (2) Alivisatos, A. P. Science 1996, 271, 993. (3) Kim, S.; Park, J.; Jang, Y.; Chung, Y.; Hwang, S.; Hyeon, T. Nano Lett. 2003, 3, 1289. (4) Selvam, P.; Mohapatra, S. K.; Sonavane, S. U.; Jayaram, R. Appl. Catal., B 2004, 49, 251. (5) Selvam, P.; Sonavane, S. U.; Mohapatra, S. K.; Jayaram, R. Tetrahedron Lett. 2004, 45, 3071. (6) Nakao, R.; Rhee, H.; Uozumi, Y. Org. Lett. 2005, 7, 163. (7) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364. (8) Konya, Z.; Puntes, V. F.; Kiricsi, I.; Zhu, J.; Ager, J. W., III.; Ko, M. K.; Frei, H.; Alivisatos, P.; Somorjai, G. A. Chem. Mater. 2003, 15, 1242. (9) Huang, J.; Jiang, T.; Gao, H.; Han, B.; Liu, Z.; Wu, W.; Chang, Y.; Zhao, G. Angew. Chem., Int. Ed. 2004, 43, 1397. (10) Mohapatra, S. K.; Misra, M.; Mahajan, V. K.; Raja, K. S. J. Phys. Chem. C 2007, 111, 8677. (11) Mohapatra, S. K.; Misra, M.; Mahajan, V. K.; Raja, K. S. J. Catal. 2007, 246, 362. (12) Pillai, P.; Raja, K. S.; Misra, M. J. Power Sources 2006, 161, 524. (13) Park, J. H.; Kim, S.; Bard, A. J. Nano Lett. 2006, 6, 24. (14) Park, J.; Bauer, S.; von der Mark, K.; Schmuki, P. Nano Lett. 2007, 7, 1686. (15) Albu, S. P.; Ghicov, A.; Macak, J. M.; Hahn, R.; Schmuki, P. Nano Lett. 2007, 7, 1286.

Figure 1. Schematic showing the synthesis of Pd nanoparticles on TiO2 nanotube arrays by the incipient wetness method.

external surface areas being available for reaction, are promising materials for surface functionalization.19-21 The integrated electrode (with TiO2 nanotubes standing on the Ti surface) gives better stability and geometrical exposure to react better with the substrate compared to other TiO2 architectures. We report here the synthesis of highly disperse Pd nanoparticles on vertically oriented TiO2 nanotube arrays (Pd/TiO2). A schematic of the process is shown in Figure 1. The catalytic activity of Pd/TiO2 is tested for the decoloration of two organic dyes, viz., methyl red (MR) and methyl orange (MO). The vertically oriented TiO2 nanotubes serve to absorb solar light efficiently, and the welldispersed nanoparticles on the surface of the nanotubes help in the rapid decoloration of the azo dyes. This catalyst offers a cost-effective, easy method by utilizing solar energy and the ease of handling of the catalyst. This process is also cheaper because the catalyst is recyclable. The selection is made to use (16) Paulose, M.; Prakasam, H. E.; Varghese, O. K.; Peng, L.; Popat, K. C.; Mor, G. K.; Desai, T. A.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 14992. (17) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69. (18) Chanmee, W.; Watcharenwong, A.; Chenthamarakshan, C.; Kajitvichyanukul, P.; de Tacconi, N. R.; Rajeswar, K. J. Am. Chem. Soc. 2008, 130, 965. (19) Mohapatra, S. K.; Misra, M. J. Phys. Chem. C 2007, 111, 11506. (20) Mohapatra, S. K.; Mahajan, V. K.; Misra, M. Nanotechnology 2007, 18, 445705. (21) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Varghese, O. K.; Grimes, C. A. Langmuir 2007, 23, 12445.

10.1021/la801253f CCC: $40.75  2008 American Chemical Society Published on Web 08/27/2008

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this catalyst for the above reaction because azo dyes are environmentally harmful and there are not many efficient catalysts available to combat the above problem. Even though some heterogeneous photocatalysts have been reported for the same use, they are lacking in one or all of the problems such as employing monochromatic UV light, lesser decomposition efficiencies, longer decomposition times, large amounts of catalysts, and/or using external oxidative radical-generating sources such as hydrogen peroxide.22-24

Materials and Methods Synthesis of TiO2 Nanotubes. Nanotubular TiO2 arrays are formed on the Ti surface by modifying our earlier reported sonoelectrochemical anodization method.11 Ti foils (ESPI, 0.2 mm thick, 99.9% purity) are anodized in 300 mL of electrolytic solution (0.14 M sodium fluoride and 0.5 M phosphoric acid) using ultrasonic waves (100 W, 42 kHz, Branson 2510R-MT) at 20 V for 1 h at a steady-state current density of 1.5 mA/cm2. The above process is carried out using a two-electrode system (6.75 cm2 Ti foil as the anode and Pt gauge (Aldrich, 100 mesh) as the cathode; distance 4.5 cm). After anodization, the samples are repeatedly washed with distilled water to remove the occluded ions, dried in an air oven, and processed for the characterization and functionalization of the Pd nanoparticles. Functionalization of TiO2 Nanotubes with Pd Nanoparticles. The functionalization of the Pd nanoparticles on the above synthesized TiO2-nanotube/Ti disk is carried out by the incipient wetness method using ethanolic palladium chloride (PdCl2, Aldrich, 99.999% purity) solution. For this purpose, TiO2 nanotubular arrays are activated (to remove adsorbed moisture and atmospheric gases) in an air oven at 130 °C for 12 h. This sample is then soaked in a dilute solution of PdCl2 (0.5 wt % in ethanol) for 30 min under ultrasonication. The preactivation and ultrasonication processes help the salt solution to diffuse through the nanochannels of TiO2. This is then dried under a vacuum overnight to remove the ethanol. The above vaccuumdried sample is then heated in a chemical vapor deposition (CVD, FirstNano) furnace at 500 °C for 2 h under a reducing (10% H2 in argon) atmosphere. The flow of H2 and argon is maintained at 20 and 200 sccm, respectively. This heat treatment process converts the TiO2 nanotubes to anatase and reduces the Pd salt to metallic Pd. Henceforth, the Pd-functionalized TiO2 nanotubes will be noted as Pd/TiO2. For comparison, a pure (without Pd functionalization) TiO2 nanotubular sample is also made on Ti foil. In this case, the asanodized TiO2 nanotubes (prepared under similar conditions as described for Pd/TiO2) are annealed in an oxygen atmosphere (500 °C for 3 h) for crystallization. The geometrical area of both samples is kept constant. Characterization. A field-emission scanning electron microscope (FESEM, Hitachi S-4700) is used to analyze the morphology and distribution of the nanotubes and nanoparticles. The images are taken at an accelerating voltage of 20 kV. High-resolution transmission electron microscopic studies (HRTEM, JEOL 2100F) are carried out at 200 kV. A scanning transmission electron microscopy (STEM) equipped with ESVision software is used for mapping and determining the crystal distribution of the samples. A small amount of sample is placed on a carbon-coated copper grid and is used for HRTEM and STEM analyses. Energy-dispersive X-ray (EDX) analysis is carried out using an Oxford detector. A selected-area electron diffraction (SAED) pattern is obtained and fast Fourier transformations (FFT) are carried out to find the crystal phases. Diffuse reflectance ultraviolet and visible (DRUV-vis) spectra of TiO2 samples are measured from the optical absorption spectra using a UV-vis spectrophotometer (UV-2401 PC, Shimadzu). Fine BaSO4 powder is used as a standard for the baseline, and the spectra are (22) Ni, Y.; Tao, A.; Hu, G.; Cao, X.; Wei, X.; Yang, Z. Nanotechnology 2006, 17, 5013. (23) Zhonghai, Z.; Yuan, Y.; Shi, G.; Fang, Y.; Liang, L.; Ding, H.; Jin, L. EnViron. Sci. Technol. 2007, 41, 6259. (24) Formo, E.; Lee, E.; Campbell, D.; Xia, Y. Nano Lett. 2008, 8, 668.

Figure 2. Experimental setup for the photodegradation of dyes using the Pd/TiO2 catalyst under simulated solar light illumination.

recorded in the range of 200-800 nm. Glancing-angle X-ray diffraction (GXRD) is carried out using a Philips 12045 B/3 diffractometer. The target used in the diffractometer is copper (λ ) 1.54 Å), and the scan rate is 1.2 deg/min. STEM, FFT, SAED, and XRD results are provided in the Supporting Information. Photocatalytic Degradation of Azo Dyes. The photocatalytic activity of the Pd/TiO2 catalyst is tested for the degradation of azo dyes. A schematic of the experimental setup used is shown in Figure 2. A 300 W solar simulator (69911, Newport-Oriel Instruments) is used as a light source. An optical AM 1.5 filter (Newport) is used to illuminate to 1 sun intensity (100 mW/cm2, measured by thermopile) on the catalyst. The degradation of the azo dyes (MR and MO) is carried out in a quartz cell (10 cm × 9 cm × 3 cm) containing 50 mL of a 0.024 mM (0.2 mg in 50 mL) solution of MR. The cell is kept 15 cm from the light source. Catalytic testing is done by suspending the catalyst plate (Pd/TiO2 on a Ti plate of geometrical area of 6.75 cm2) through an alligator clip in the dye solution under ambient conditions. The solution is stirred continuously during the whole process. The decomposition of azo linkage is monitored spectroscopically with a UV-vis absorption spectrophotometer (Shimadzu UV-2401PC). The measurements are carried out in fast mode (350 to 700 nm in 61.5 s) using 0.5 mL of dye solution in every 15 to 30 min interval. After each measurement, the solution is poured back into the cell and allowed to degrade further. The absorption values of the dyes are recorded with water as a reference. The catalyst plate (Pd/TiO2 nanotubes on the Ti foil) is removed from the solution, washed in distilled water, and dried at 70 °C overnight for recycling purposes.

Results and Discussion Formation Mechanism and Characterization of TiO2 Nanotubes. TiO2 nanotubes are prepared on the Ti metal by the sonoelectrochemical anodization method. The formation of these nanotubes goes through three major steps as described below: Step I (Formation of the PassiVe Layer). In an aqueous acidic medium (here H3PO4), Ti oxidizes to form a thin layer of TiO2 on the Ti metal by eq 1.

Ti + 2H2O f TiO2(anodic) + 2H2(cathodic)

(1)

Step II (Breakage of the PassiVe Layer). Although TiO2 is stable thermodynamically in a pH range of 2-12, the presence of a complexing ligand (e.g., fluoride ion, F-, here from NaF) leads to substantial dissolution by eq 2.

TiO2 + 6F- + 4H+ f [TiF6]2- + 2H2O

(2)

This complex formation leads to breakage in the passive oxide layer, with pit formation leading to the formation of self-standing

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Figure 3. SEM image of the self-organized, vertically oriented TiO2 nanotube arrays. The inset shows the cross-sectional view of the nanotube arrays.

nanotube arrays on the metal surface. The self-organized nanotubes formed on the Ti surface as the result of a combined effort as shown in eqs 1 (field-assisted) and 2 (chemical dissolution). Step II depends on the chemical composition and pH of the solution and also on the agitation process used for synthesis. Ultrasonic waves help F- break the TiO2 layer (eq 2) as well as facilitate the effusion and diffusion of ions in the electrolyte for faster kinetics. Step III (RepassiVation). Nanotube formation continues in this process until an equilibrium is established between the electrochemical etch rate (eq 1) and the chemical dissolution rate (eq 2), which is called repassivation (i.e., nanotube formation stops). Detailed information about the formation mechanism of TiO2 nanotubes by the anodization method has been described previously.11,25-27 Figure 3 shows the SEM images (surface and cross-sectional views) of oxygen-annealed TiO2 nanotubular arrays. The average diameter of these nanotubes is found to be ∼80 nm, and the tube length is in the range of ∼600 nm. The wall thickness of the TiO2 nanotubes is found to be in the range of 15-20 nm. It is also observed (Figure 3, inset) that TiO2 nanotubes are compact (nanotubes are well attached to each other) and one-dimensionally oriented (straight). The nanotubes are found to be uniform throughout the titanium surface. The as-anodized TiO2 nanotubes are generally amorphous in nature; however, they crystallize after annealing. X-ray diffraction (not shown here) and TEM studies (not shown here) show that the oxygen-annealed TiO2 nanotubes are polycrystalline in nature (anatase and rutile).19 The phase transformation of anatase to rutile is favored in the oxygen atmosphere because of the elimination of oxygen vacancies through the diffusion of oxygen ions into the TiO2 lattice. A detailed discussion of the effect of annealing conditions on the crystallization of the nanotubes and more characteristics of TiO2 nanotubes has been described previously.11,19,28,29 The as-anodized TiO2 nanotubes are used to functionalize with Pd nanoparticles and are compared with pure TiO2 nanotubes (oxygen-annealed). (25) Cai, Q.; Paulose, M.; varghese, O. K.; Grimes, C. A. J. Mater. Res. 2005, 20, 230. (26) Raja, K. S.; Misra, M.; Paramguru, K. Electrochim. Acta 2005, 51, 154. (27) Maca´k, J. M.; Tsuchiya, H.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 2100. (28) Heald, E. F.; Weiss, C. W. Am. Mineral. 1972, 57, 10. (29) Funk, S.; Hokkanen, B.; Burghaus, U.; Ghicov, A.; Schmuki, P. Nano Lett. 2007, 7, 1091.

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Figure 4. SEM image of Pd-nanoparticle-functionalized TiO2 nanotube arrays. The inset shows the EDX spectrum of the Pd/TiO2 surface.

Figure 5. TEM image of a single TiO2 nanotube functionalized with Pd nanoparticles with an average size of 10 nm.

Characterization of Pd/TiO2 Nanotubes. Figure 4 shows the SEM image of the surface of Pd-nanoparticle-functionalized TiO2 nanotubes. The dispersed PdCl2 solution on TiO2 nanotube arrays (as-anodized) after reduction in a H2 atmosphere forms uniformly distributed metallic Pd nanoparticles on TiO2 nanotube arrays, as shown in the SEM image (Figure 4). The Figure shows a narrow particle size distribution of the Pd nanoparticles (∼10 nm) on the TiO2 nanotubular surface. The structural integrity and morphology of the TiO2 nanotubular arrays remain unaltered after the synthesis of the Pd nanoparticles. EDX (Figure 4, inset) analysis shows ∼1.25 wt % Pd on the TiO2 nanotube arrays. The uniformity of the Pd dispersion is obtained via EDX analysis of the Pd/TiO2 surface at four different places. It is worth pointing out that the high porosity of the TiO2 nanotube arrays is advantageous for making tiny uniform nanoparticles. Furthermore, TEM measurements are carried out to get a more precise size distribution of Pd nanoparticles throughout the TiO2 nanotube surface (both inside and at the top of the nanotubes). Figure 5 shows the TEM image of a single TiO2 nanotube with uniform Pd nanoparticles inside the nanotube. STEM (dark-field) image (Figure S1) and mapping (Figure S2) also supported the homogeneous distribution of Pd nanoparticles throughout the nanotube arrays. Direct evidence of the single-crystalline nature of Pd and TiO2 nanotubes is obtained from the HRTEM image and SAED pattern. Figure 6 shows a single Pd nanoparticle

Self-Organized TiO2 Nanotubes

Figure 6. HRTEM image of a Pd nanoparticle embedded inside a TiO2 nanotube.

embedded inside a TiO2 nanotube. It shows two different lattice planes with spacings of 0.35 and 0.22 nm. These two planes correspond to the (101) plane of anatase TiO2 and the (111) plane of metallic Pd.10,30,31 This is further confirmed by FFT and the SAED pattern (Figure S3). The latter is obtained from an individual Pd nanoparticle by directing the electron beam perpendicular to its spherical surface. Similarly, the SAED pattern of a TiO2 nanotube is taken from an individual TiO2 nanotube. These above results confirm that Pd/TiO2 consists of singlecrystalline TiO2 nanotubes as well as metallic Pd nanoparticles. The present results indicate that a uniform distribution of metal nanoparticles can be achieved on TiO2 nanotube arrays using this simple process. The formation of Pd nanoparticles and the crystallization of TiO2 nanotubes are further confirmed by XRD measurements. X-ray structure analysis (Figure S4) shows typical peaks corresponding to anatase TiO2 and metallic Pd. The main diffraction peaks for the anatase TiO2 (101) and Pd (111) nanoparticles are observed at 2θ values of 24 and 40.07°.32 The DRUV-vis studies (Figure S5) of pure TiO2 nanotubes shows absorption only in the UV region (200-400 nm).19 However, Pd/TiO2 nanotubes shows light absorption in both the UV (200-400 nm) and visible regions (a shoulder at ∼450 nm) with a band edge up to 800 nm. The absorption of Pd nanoparticles depends on the size and shape of the particles. Pd particles of ∼6 nm are reported to absorb in the UV region only;33 however, larger particles and clusters of Pd particles absorb in the visible region.30 Xia and co-workers observed an enhancement in the visible light absorption of Pd nanostructures by changing the morphologies (nanocages and nanoboxes) of the particles.34 Shah and co-workers35 reported the visible light absorption of TiO2 nanoparticles by Pd2+ doping. Therefore, the visible light absorption of Pd/TiO2 nanotubes might be due to the formation of Pd clusters or the partial doping of Pd ions inside the anatase matrix; however, further investigation is necessary to find the (30) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y.; Yin, Y.; Li, Z.-Y. Nano Lett. 2005, 5, 1237. (31) Xiong, Y.; McLellan, J. M.; Chen, J.; Yin, Y.; Li, Z.-Y.; Xia, Y. J. Am. Chem. Soc. 2005, 127, 17118. (32) Wang, M.; Guo, D.-J.; Li, H.-L. J. Solid State Chem. 2005, 178, 1996. (33) Ho, P.-F.; Chi, K.-M. Nanotechnology 2004, 15, 1059. (34) Xiong, Y.; Wiley, B.; Chen, J.; Li, Z.-Y.; Yin, Y.; Xia, Y. Angew. Chem. 2005, 117, 8127. (35) Shah, S. I.; Li, W.; Huang, C.-P.; Jung, O.; Ni, C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6482.

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Figure 7. UV-vis spectra of methyl red (MR) degradation in the presence of Pd/TiO2 nanotubes under simulated solar light illumination. MR (0.2 mg) dissolved in 50 mL of water is used as the initial solution (2.33 × 10-5 M). The complete degradation of MR occurred in ∼150 min. This is one of the fastest degradations of MR using a heterogeneous catalyst and solar light. Degradation is carried out under ambient conditions without the combination of oxidant and potential.

Figure 8. Fraction of MR left after irradiation under different experimental conditions: (a) Pd/TiO2 nanotubes, (b) TiO2 nanotubes, (c) light source without catalyst, and (d) Pd/TiO2 nanotubes without light source. The photocatalytic activity of Pd/TiO2 nanotubes is found to be almost twice that of pure TiO2 nanotube arrays. There is no significant degradation of MR observed without light and catalyst. The degradation follows first-order kinetics; however, because of the formation of amines (after ∼80% degradation), a sudden decay in dye concentration is observed.

exact reason. The UV absorption in Pd/TiO2 is due to the isolated Pd nanoparticles as well as the TiO2 nanotubes whereas the shoulder in the visible region is due to the clustering of the Pd nanoparticles. Photodegradation of Dyes Using the Pd/TiO2 Nanotubular Catalyst. To investigate the catalytic activity of the Pd/TiO2 catalyst, the decoloration of MR and MO are tested. The decoloration is monitored by UV-vis measurements in a certain time interval. The presence of the n f π* band indicates the presence of diazo linkage. The decay of this band at 524 nm (ε ) 5111 mol-1 cm-1) for MR (Figure 7) and at 467 nm (ε ) 26 050 mol-1 cm-1) for MO (Figure S6) is monitored over time. It is interesting that 100% decomposition of both the dyes is observed over Pd/TiO2 under solar light illumination. Complete conversion is attained in 150 min for MR and 270 min for MO (Figures 7 and 8). For the studied dyes, the action of light alone or of the catalyst in the absence of light is not significant (Figure 8). Interestingly, under similar experimental conditions TiO2 nanotubes also decomposed both dyes, however with a slower rate compared to that of Pd/TiO2. The initial 80% degradation follows first-order kinetics (C/C0 vs time plot, Figure 8); however,

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because of the formation of amines, a sudden change in dye concentration is observed for both TiO2 and Pd/TiO2 catalysts. This is confirmed by a change in the pH of the solution. The initial pH of the MR solution changes from 4.3 to 8.0 after 80% completion of the reaction. This is due to the formation of aromatic amines (which are basic in nature and biodegradable) from azo compounds. The photocatalytic degradation of azo compounds involves either oxidation or reduction. TiO2-involved decompositions are shown to follow a reductive mechanism.36 Tada et al.37 reported the reduction involving TiO2 to be a 2e- reduction, which converts the azo functional group to the hydrazo functional group. The presence of a metal that has reductive properties such as Pd will help to reduce the hydrazo functional group further by completely cleaving the azo linkage.38 Kovalenko et al.39 reported that Pd in the Pd/TiO2 system acts as a better reducing agent. Electrondensity transfer of TiO2 to Pd makes the metal a better reducing agent.39 In addition, 2e- photocatalytical reduction of TiO2 further enhances the process of cleaving the azo linkage. Effect of Pd Loading. The effect of Pd concentration on the photoactivity of the Pd/TiO2 catalyst is tested by varying the Pd amount from 0.71 wt % (0.25 wt % PdCl2 in ethanol is used) to 2.38 wt % (1 wt % PdCl2 in ethanol is used) in the catalyst. The SEM images (surface view) of these catalysts are shown in Figure S8. Pd/TiO2 with 0.71 wt % Pd shows spherical Pd particles with lower concentration compared to the catalyst discussed earlier with 1.25 wt % Pd (Figure 4). However, the catalyst with 2.38 wt % Pd shows nanorod-type crystals with length ∼150 nm and diameter 20-30 nm. These nanorods of Pd might have formed by the conjugated nanoparticles of Pd at the higher concentrations. More in-depth investigation will be carried out in the future to evaluate the morphology of Pd nanoparticles with the concentration of the precursor. Nevertheless, it is observed that the photoactivity of the Pd/TiO2 catalyst decreases with high Pd loading (2.38 wt %). The catalyst at high Pd loading takes more than 5.5 h for a complete MR degradation (under identical conditions) compared to the catalyst with 1.25 wt % Pd loading. The higher surface coverage of Pd particles decreases the accessibility of the active sites inside the TiO2 nanotubes, which reduces the photoactivity. The catalyst with 0.71 wt % Pd loading shows the complete photodegradation of MR in 170 min, which is comparable to the catalyst with 1.25 wt % Pd loading. Therefore, to obtain good photoactivity from the Pd/TiO2 catalyst, the Pd loading can be kept around 1.25 wt %. Kinetics and Recycling Tests. The photodegradation of dyes at low concentration follow first-order kinetics (eq 3)

( )

ln

C0 ) kt C

Table 1. Effect of Light Sources on the Photocatalytic Degradation of MR (2.33 × 10-5 M) after a 1 h Experiment Using TiO2 and Pd/TiO2 Nanotubes as Catalysts light source

TiO2 nanotube (%)

Pd/TiO2 nanotube (%)

36.5

55.6

8.7

32.0

UV (330 ( 70 nm, 13.9 mW/cm2)a visible (520 ( 46 nm, 5.27 mW/cm2)a a

Purchased from Edmund optics.

effect of highly disperse Pd nanoparticles and the geometry of the TiO2 nanotubes (which absorb light efficiently and each nanotube acts as a nanoreactor). Furthermore, the photodegradation of MR using UV and visible light sources is carried out to understand the effect of these components on the photoactivity of Pd/TiO2 nanotubes. For these purposes, various filters are used in between the AM 1.5 filter and the catalyst. The details of the filters are mentioned in Table 1. All experiments are carried out for 1 h. The activity of Pd/TiO2 nanotubes is found to be superior compared to that of TiO2 nanotubes under both UV and visible light illumination. The photoactivity of Pd/TiO2 in the UV region is a combined effect of both TiO2 nanotubes and the isolated Pd nanoparticles (in which UV photons are also absorbed). However, in the visible light region the conjugated Pd nanoparticles and/or Pd-doped sites play a significant role in the decomposition of MR, whereas TiO2 alone did not show significant activity in this region. There is no significant reduction in photoactivity observed when the Pd/TiO2 catalyst is used four times in a row for the degradation of MR. SEM and DRUV-vis studies of the used Pd/TiO2 catalyst showed features similar to those of the original catalyst. This showed the stability and true heterogeneous nature of this catalyst under these experimental conditions. It is reported that the diameter of the TiO2 nanotubular arrays can be tuned from a few nanometers to more than a hundred nanometers and the length can also be varied from a few hundred nanometers to micrometers.10,16,27 It is also reported that by the anodization method other metal oxide and mixed metal oxide nanotubes can be synthesized.40,41 These varieties of nanotubes with variable absorption and charge-transport properties can also be used for the further enhancement of the photocatalytic activity of the TiO2 nanotubes. In addition to Pd, other sensitizing elements also can be employed for the degradation dyes and toxic organic compounds.42-46

Conclusions (3)

where C0 is the initial concentration of the reactant, C is the concentration of the reactant at time t, t is the irradiation time, and k is the reaction rate constant (min-1). In the current investigation, the plot of ln(C0/C) versus time gives a straight line, which indicates that the decomposition of the dyes follows first-order kinetics (Figure S8). The rate constant (slope of the straight line obtained in Figure S8) of MR degradation using Pd/TiO2 (0.012 min-1) is found to be 2.18 times higher compared to that of pure TiO2 nanotubes (0.0055 min-1). The higher activity of the Pd/TiO2 catalyst is due to a synergistic (36) Bonancea, C. E.; do Nascimento, G. M.; de Souza, M. L.; Temperini, M. L.; Corio, P. Appl. Catal., B 2008, 77, 339. (37) Tada, H.; Kubo, M.; Inubushi, Y.; Ito, S. Chem. Commun. 2000, 977. (38) Mandal, S.; Roy, D.; Chaudhari, R. V.; Sastry, M. Chem. Mater. 2004, 16, 3714. (39) Kovalenko, N. A.; Petkevich, T. S. J. Appl. Spectrosc. 1999, 66, 88.

Self-organized and vertically oriented TiO2 nanotube arrays are successfully functionalized with Pd nanoparticles using the incipient wetness method. A uniform distribution of singlecrystalline Pd nanoparticles of average particle size 10 nm on TiO2 nanotubes (∼80 nm diameter and ∼600 nm length) is observed. The Pd/TiO2 catalyst is found to possess excellent photoactivity for the decoloration of azo dyes (where the (40) Mohapatra, S. K.; Raja, K. S.; Misra, M.; Mahajan, V. K.; Ahmadian, M. Electrochi. Acta 2007, 53, 590. (41) Mohapatra, S. K.; Banerjee, S.; Misra, M. Nanotechnology 2008, 19, 315601. (42) Sun, B.; Vorontsov, A. V.; Smirniotis, P. G. Langmuir 2003, 19, 3151. (43) Tada, H.; Konishi, Y.; Kokubu, A.; Ito, S. Langmuir 2004, 20, 3486. (44) Nath, S.; Ghosh, S. K.; Panigahi, S.; Thundat, T.; Pal, T. Langmuir 2004, 20, 7880. (45) Kemell, M.; Pore, V.; Ritala, M.; Leskela, M.; Linden, M. J. Am. Chem. Soc. 2005, 127, 14178. (46) Awazu, K.; Fujimaki, M.; Rockstuhl, C.; Tominaga, J.; Murakami, H.; Ohki, Y.; Yoshida, N.; Watanabe, T. J. Am. Chem. Soc. 2008, 130, 1676.

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degradation rate is more than 2 times faster than for the pure TiO2 nanotubes), namely, MR and MO, using simulated solar light in the absence of any sacrificing agent (e.g., hydrogen peroxide). A 100% decoloration of MR and MO is observed in 150 and 270 min, respectively. A Pd loading of around 1.25 wt % is observed to be the best concentration for the maximum degradation of dyes. The catalyst is found to be active in both

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the UV and visible light regions of the solar spectrum. The integrated Pd/TiO2 catalyst can be reused many times without much change in activity and structural integrity. Supporting Information Available: Figures S1-S8. This material is available free of charge via the Internet at http://pubs.acs.org. LA801253F