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2007, 111, 11506-11510 Published on Web 07/19/2007
Enhanced Photoelectrochemical Generation of Hydrogen from Water by 2,6-Dihydroxyantraquinone-Functionalized Titanium Dioxide Nanotubes Susanta K. Mohapatra and Mano Misra* Chemical and Metallurgical Engineering, MS 388, UniVersity of NeVada, Reno, NeVada 89557 ReceiVed: April 12, 2007; In Final Form: June 7, 2007
Titanium dioxide, TiO2, nanotubes prepared by sonoelectrochemical anodization method are functionalized with 2,6-dihydroxyantraquinone (anthrafavic acid). The functionalization takes place by chemical condensation of the Ti-OH hydroxyl groups present on the TiO2 nanotubular surface with the phenolic hydroxyl groups of anthrafavic acid forming an inorganic-organic hybrid material. The condensation results in an intramolecular ligand-to-metal charge-transfer transition that leads to an enhancement in absorption, and an absorption band is observed in the visible region. This hybridization is responsible for an increase in the photoelectrochemical generation of hydrogen from water up to 30%.
Introduction
Materials and Methods
Titania (TiO2) in various forms is widely used for photoelectrolysis of water to generate hydrogen and oxygen.1-3 Recently, there is a growing interest in the application of titania nanotubes prepared by anodization method for the photoelectrochemical generation of hydrogen by photoelectrolysis of water.4-9 These self-organized one-dimensional TiO2 nanotubes prepared by anodization create a better opportunity to harvest sunlight more efficiently than the randomly oriented nanoparticles or the nanotubes prepared by the sol-gel process.5,10 This property of titania nanotubes prepared by the anodization method makes them a versatile catalyst for photoelectrolysis of water. TiO2, in a native state, has a band gap of 3.0-3.2 eV that can absorb light in the ultraviolet range (i.e., wavelength below 400 nm), which corresponds to only ca. 4-5% of the energy of sunlight. Thus, several strategies and methods such as doping of external ions and dye sensitization, etc. have been found to increase the absorption of TiO2 nanotubular materials in the visible light region and thus the overall photoactivity.4-11 TiO2 is known to react with phenolic compounds such as catechol, 2-naphthol, salicylic acid, and 1,1′-binaphthlene-22′-diol.12-15 This enhances the absorption of TiO2 to visible region (i.e., red shift) and thus visible light-driven photoelectrolysis. This works by chemical condensation of the Ti-OH hydroxyl groups, present on the TiO2 surface with the phenolic hydroxyl(s) to give an inorganic-organic hybrid material.11 The extension of the absorption band to the visible region is due to the intramolecular ligand-to-metal charge-transfer transitions.12,14 In this communication, we report for the first time an enhancement in the visible light absorption of the titania nanotubes by surface modification with 2,6-dihydroxyantraquinone (Scheme 1). These functionalized titania nanotubes are found to be more active to generate hydrogen by photoelectrolysis of water than the intrinsic TiO2 nanotubes.
Synthesis of TiO2 Nanotubes. Anodization of titanium, Ti, is carried out by our earlier reported procedures.8,16 Nanotubular TiO2 arrays are formed by anodization of the Ti foils (ESPI) in 300 mL electrolytic solution using ultrasonic waves (100 W, 42 KHZ, Branson 2510R-MT). The solution is prepared by mixing distilled water, 0.14 M sodium fluoride (NaF, Fischer) and 0.5 M phosphoric acid (H3PO4). The pH of the solution is found to be ∼2.1. A two-electrode configuration is used for anodization. A flag-shaped platinum (Pt) electrode (thickness, 1 mm, area, 3.75 cm2) serves as the cathode. The distance between the two electrodes is kept at 4.5 cm in all the experiments. The anodization is carried out by applying a potential of 20 V using a rectifier (Agilent, E3640A). During anodization, ultrasonic waves are irradiated onto the solution continuously. A sonoelectrochemical anodization method is preferred rather than a conventional magnetic stirring method as the previous method gives better quality nanotubes with clean surfaces, which are essential for functionalization.8 A detail discussion on the advantages of the sonoelectrochemical anodization method is available in the literature.8,9 The anodization current is monitored continuously using a digital multimeter (METEX, MXD 4660A). The anodization is carried out for 30 min. The anodized samples after anodization are properly washed with distilled water to remove the occluded ions, dried in an air oven, and processed for characterization. The TiO2 nanotubes are annealed using two different atmospheres, viz., oxygen and nitrogen at 500 °C for 2 h. This converts the amorphous TiO2 nanotubes to crystalline phases. Synthesis of 2,6-Dihydroxyantraquinone (DHA) Functionalized TiO2 Nanotubes. Three different sets of titanium discs containing TiO2 nanotubes, viz., as-anodized, N2-annealed, and O2-annealed are selected for fabrication with DHA. The selection of these materials is based on their wide variety of materials properties. The as-anodized TiO2 nanotubes are amorphous in nature, N2-annealed TiO2 nanotubes are generally anatase, and O2-annealed TiO2 nanotubes are mostly rutile. In
* To whom correspondence should be address. E-mail: misra@ unr.edu.
10.1021/jp0728712 CCC: $37.00
© 2007 American Chemical Society
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J. Phys. Chem. C, Vol. 111, No. 31, 2007 11507
SCHEME 1: Surface Sensitization of TiO2 Nanotubes by 2,6-Dihydroxyantraquinonea
a
This enhances the absorption of the TiO2 nanotubes in the visible region and photoelectrolysis of water.
a typical experiment, four sets of TiO2 nanotubular discs are suspended in a methanolic solution of DHA (4 mM solution, 25 mL). The resulting solution in a three-necked round bottomed flask of 50 mL volume is refluxed for 8 h under an argon atmosphere. After the reaction is over, the solution is cooled to room temperature, and then the titanium discs containing TiO2 nanotubes (as-anodized, N2-annealed, and O2-annealed) are taken out. They are washed in methanol and are dried overnight under vacuum to remove the occluded DHA from the samples. These hybrid nanotubes are then processed for characterization and photoactivity test. Characterization. A field emission-scanning electron microscope (FESEM, Hitachi, S-4700) is used to analyze the morphology of the nanotubes before and after functionalization. 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 baseline, and the spectra are recorded from 200 to 850 nm range. Glancing angle X-ray diffraction (GXRD) is done to evaluate the crystalline TiO2 phases using a Philips-12045 B/3 diffractometer. The target used in the diffractometer is copper (λ ) 1.54 Å). The scan rate used for GXRD analysis is 1.2 deg/min. Fourier transform infrared (FTIR) measurement is carried out by a BIORAD FTS 6000 spectrophotometer using KBr pellet technique. Photoelectrolysis of Water. Experiments on H2 generation by photoelectrolysis of water are carried out in a glass cell with photoanode (nanotubular TiO2 specimen) and cathode (platinum foil) compartments. The compartments are connected by a fine porous glass frit. The reference electrode (Ag/AgCl) is placed closer to the anode using a salt bridge (saturated KCl)-Luggin probe capillary. The cell is provided with a 60 mm diameter quartz window for light incidence. The electrolyte used is 1 M KOH. A computer-controlled potentiostat (SI 1286, England) is employed to control the potential and to record the photocurrent generated. A 300 W solar simulator (69911, NewportOriel Instruments, USA) is used as a light source. An AM 1.5 filter is used to get the one sun intensity, which is illuminated in all the experiments. The samples are anodically polarized at a scan rate of 5 mV/s under illumination, and the photocurrent is recorded.
Figure 1. FESEM image of N2-TiO2-2,6-dihydroxyantraquinone nanotube arrays. (a) Front view and (b) cross-sectional view of the functionalized nanotubes.
Results and Discussion Figure 1 shows the SEM images of DHA-functionalized N2annealed TiO2 nanotubular arrays (N2-TiO2-DHA), which shows similar features to the parent N2-TiO2 nanotubular arrays. The average diameter of these nanotubes is found to be ∼80 nm, and the tube length in the range of 600-650 nm. The wall thickness of the titania nanotubes is found to be in the range of 15-20 nm. It is also observed from the Figure 1 that TiO2 nanotubes are compact (nanotubes are well attached to
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Figure 2. GXRD pattern of (a) N2-annealed TiO2 nanotubes and (b) O2-annealed TiO2 nanotubes. The annealing was carried out at 500 °C for 2 h.
Figure 4. DRUV-vis absorption spectra of (a) O2-annealed TiO2 nanotube arrays and (b) 2,6-dihydroxyantraquinone-functionalized TiO2 nanotube arrays annealed under O2. A large red shift of ∼50 nm is observed after functionalization.
Figure 3. DRUV-vis absorption spectra of (a) N2-annealed TiO2 nanotube arrays and (b) 2,6-dihydroxyantraquinone-functionalized TiO2 nanotube arrays annealed under N2. The absorption in the visible region is enhanced up to 140 nm after functionalization. A new shoulder around 500 nm and band edge extension up to 850 nm is also observed due to the hybrid material.
each other) and one dimensionally oriented (straight). No surface deposition of the DHA compound is observed. GXRD studies of the as-anodized TiO2 nanotubular arrays are found to be amorphous in nature. The TiO2 nanotubular arrays annealed under N2 and O2 atmosphere mostly form anatase and rutile (Figure 2), respectively. DRUV-vis spectra of the N2-TiO2-DHA nanotubes is shown in Figure 3. It can be seen from Figure 3 that there is a significant increase in absorption in the visible region (∼140 nm red shift) that occurs after the DHA adsorbed on the TiO2 surface. The absorption tail is found to be extended to 850 nm compared to the parent TiO2 at 550 nm. This works by chemical condensation of the hydroxyl groups present on the TiO2 nanotubular surface, TiOH with the phenolic hydroxyl(s), to produce an inorganicorganic hybrid material.15 Water is the only byproduct from this process (Scheme 1). This enhances the absorption band to the
visible region by the intramolecular ligand-to-metal chargetransfer transitions. Similar results are also observed for TiO2 nanoparticle catalysts after functionalization with catechol or binaphthol compounds.12,13,15 However, the visible light absorption obtained using DHA (absorption onset ∼850 nm) is significantly higher compared to the other reported compounds (e.g., Ti-salycilate ) 450-500 nm, Ti-catechol ) 600 nm, and Ti-binaphthol ) 550-600 nm).12,13,15 This is due to the better conjugation of the anthraquinone ring compared to the benzene and naphthalene rings. The above observation is further supported by the FTIR measurements (Figure 4). Figure 4 shows the FTIR spectrum of the pure N2-TiO2 nanotubes, DHA and N2-TiO2-DHA. The spectrum (a) in Figure 4 shows a typical TiO2 pattern with Ti-O-Ti stretching frequency around 600 cm-1. Spectrum (c) shows the characteristic peaks corresponding to 2,6-dihydroxyanthraquinone. Spectrum (b), which corresponds to N2-TiO2DHA, shows the peaks corresponding to TiO2, DHA, and a new peak around 1054 cm-1. This peak, which is absent in both the parent TiO2 and the organic compound (DHA) spectra is indicative of the formation of Ti-O-C types of bonding. The above observation is in line with our earlier discussed UV-vis studies. In addition to the above observations, a split in the peaks corresponding to DHA is also observed. This might be due the asymmetric nature of the DHA after it bonded to the TiO2 surface. To confirm that the new peak is formed due to the functionalization of the DHA with TiO2 and not just occluded particles inside the TiO2 nanotubes, a sample is also prepared by room temperature mixing of the DHA solution and TiO2 nanotubes. Interestingly, this sample did not show any such peak around 1054 cm-1. The above observations confirm the presence of the DHA in the TiO2 nanotubes in the bonded form and not just occluded particles. The TiO2 sample annealed under O2 atmosphere (mostly rutile) also showed a similar enhancement in absorption in the visible region after DHA absorption (Figure 5). A red shift of ∼50 nm is observed after the TiO2 nanotubes are modified by
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J. Phys. Chem. C, Vol. 111, No. 31, 2007 11509
Figure 6. Variation of photocurrent density vs applied potential (potentiodynamic plot) of (a) DHA-N2-TiO2 under no illumination, (b) DHA functionalized on as-anodized TiO2 nanotubes under the illumination of solar spectrum (AM 1.5 filter), (c) N2-TiO2 under illumination of solar spectrum, and (d) DHA functionalized N2-TiO2 nanotubes under the illumination of solar spectrum. The photoanode prepared by the functionalization of titania nanotubes with 2,6dihydroxyantraquinone (DHA) generates ∼30% more photocurrent than the photoanode prepared by the untreated titania nanotubes.
Figure 5. (A) FTIR spectra of (a) N2-TiO2, (b) 2,6-dihydroxyantraquinone, and (c) N2-TiO2-2,6-dihydroxyantraquinone nanotubes. (B) is the expanded form (1000-1300 cm-1) of (A).
DHA. The absorption onset is found to be around 600 nm for O2-TiO2-DHA compared to the parent O2-TiO2 where the onset is observed around 400 nm. It can be seen from both of the figures that irrespective of TiO2 forms (anatase or rutile), the absorption of titania nanotubes in the visible region of the solar spectrum increases significantly after fabrication of the surface with DHA. However, the pattern and extent of shifting are found to be different. In the case of N2-TiO2-DHA, the absorption is increased by a new shoulder around 500 nm, which is extended until 850 nm. On the other hand, in the case of O2-TiO2-DHA, the red shift is observed by a shift in the charge-transfer transition (O2- f Ti4+). Further studies are necessary to find the differences of interaction of the anatase and rutile phases of TiO2 with DHA. A preliminary photoelectrochemical activity test for photoelectrolysis of water to hydrogen and oxygen using this hybrid TiO2 nanotubular photocatalysts is carried out. The N2-TiO2DHA nanotubes are tested for this purpose. Figure 6 summarizes the results of photoelectrochemical hydrogen generated in terms of the photocurrent obtained from the photoelectrochemical cell using N2-TiO2-DHA nanotubes as photoanode and the Ptfoil using the illumination of one sun intensity as cathode. As can be seen from the Figure 6, the photocurrent density obtained from the N2-TiO2-DHA photoanode is ∼1.85 mA/cm2 (at 0.2 VAg/AgCl), which is almost 30% higher than the unmodified N2TiO2. The higher photoactivity of the N2-TiO2-DHA photoanode than the N2-TiO2 photoanode is because the number of photoelectrons generated by the former photoanode is higher than the latter. In the case of the N2-TiO2 photoanode, the photoelectrons are generated by the UV portion of the solar spectrum by band gap (BG) mechanism (electron jumps from the valence band to conduction band by absorbing the energy from the UV photons). On the other hand, in the case of N2TiO2-DHA, in addition to the above photoelectrons generated
Figure 7. A schematic of a cross section of a nanotube showing the generation of electron by the absorption of sunlight by the hybrid TiO22,6-dihydroxyantraquinone nanotube arrays. (I) Electron generated by charge-transfer mechanism, and (II) electron generated by band gap mechanism.
by the BG mechanism, photoelectrons are also generated by the charge-transfer mechanism (electron transition from the highest occupied molecular orbital, (HOMO) to lowest unoccupied molecular orbital (LUMO)) from the surface complexation with DHA (Figure 7).17 These HOMO f LUMO electron transitions can be possible using the lower energy portion of the solar spectrum (visible light). This is supported by our preliminary Mott-Schottky measurements where the charge carrier density in N2-TiO2-DHA photoanode is found to be higher than the N2-TiO2 photoanode. Because of this, the photoactivity of N2-TiO2-DHA nanotubes is higher than the photoanode prepared from N2-TiO2 nanotubes. The dark-current density (without illumination) is also shown in Figure 7, which is found to be
