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Nanocomposite film of TiO2 nanotube and polyoxometalate towards photocatalytic degradation of nitrobenzene
Materials Research Bulletin 60 (2014) 524–529
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Nanocomposite film of TiO2 nanotube and polyoxometalate towards photocatalytic degradation of nitrobenzene Zhixia Sun, Mingliang Zhao, Fengyan Li *, Tianqi Wang, Lin Xu * Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun 130024, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 June 2014 Received in revised form 13 August 2014 Accepted 8 September 2014 Available online 10 September 2014
The composite film based on polyoxometalates (POMs)-modified TiO2 nanotubes was prepared by electrodeposition method for the photocatalytic degradation of nitrobenzene. The composite film was characterized by field-emission scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, which indicated that the POMs were well introduced into the TiO2 nanotubes. Furthermore, the photocatalytic properties of the TiO2 nanotubes and POMs-modified TiO2 nanotubes were evaluated by the decomposition of nitrobenzene. POMsmodified TiO2 nanotubes showed much higher photocatalytic activity than pure TiO2 nanotubes. These results provide a promising route to effectively photocatalytic degradation of nitrobenzene by POMsmodified TiO2 nanotubes. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Composites Nanostructures Electron microscopy Catalytic properties
1. Introduction Nitrobenzene, a highly toxic aromatic compound, is widely used in industrial processes such as pesticides, dyes and explosives manufacture [1–3]. As an industrial consequence, it appears as contaminant in all kinds of water sources especially in surface water and industrial wastewaters [4]. Also, nitrobenzene is readily absorbed by contact with the skin and by inhalation of vapors, and affects the central nervous system, resulting in headache, vertigo, coma and so on [5]. Therefore, the removal of nitrobenzene from wastewater is a current issue for both fundamental research and practical application. So far, various chemical treatment processes were proposed as the efficient treatment options for nitrobenzene removal, such as photochemical remediation [6], ozonation [7], and photocatalytic degradation [8]. Among these proposed methods, the photocatalytic degradation of aromatic compounds is largely investigated due to the complete mineralization of organic pollutants to harmless inorganic compounds. TiO2 photocatalyst has been widely investigated for the complete degradation of toxic contaminants in water and air [9,10]. Bhatkhande et al. reported photocatalytic degradation of nitrobenzene with TiO2 using UV radiation and investigated the effect of various parameters such as presence of anions, pH, wavelength of light, etc [11]. Furthermore, multidimensional TiO2 showed a higher photochemical reactivity than bulk TiO2 particles
* Corresponding authors. Tel.: +86 431 85098760; fax: +86 431 85099765. E-mail addresses: [email protected] (F. Li), [email protected] (L. Xu). http://dx.doi.org/10.1016/j.materresbull.2014.09.025 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
[12,13]. Recently, using TiO2 nanotubes as photocatalysts have attracted considerable attention [14], because of their unique three-dimensional structures, high surface areas, and high charge transfer rates [15,16]. Tayade and Key evaluated photocatalytic degradation of nitrobenzene with TiO2 nanotubes which were prepared by hydrothermal treatment, demonstrating a higher photocatalytic activity of TiO2 nanotubes than that of TiO2 powder [17]. However, the photocatalytic efficiency of TiO2 nanotubes is still limited by the fast recombination of photogenerated electronhole pairs. Thus, searching for appropriate and effective electron acceptors should be of considerable signification to enhance separation efficiency of electron-hole pairs. Polyoxometalates (POMs), a class of molecular metal-oxo cluster compounds based mainly on Mo, W, and V elements, have shown superior physicochemical properties and various applications in catalysis, materials science and medicine [18,19]. Especially, POMs have an intrinsic characteristic of electron acceptor, which can capture the photogenerated electrons from conduction band (CB) of TiO2. Thus, the recombination of photogenerated electron-hole pairs is suppressed due to the synergistic effect of POMs and TiO2. Ozer and Ferry investigated the use of POMs in TiO2 suspensions to increase the photocatalytic efficiency of 1,2-dichlorobenzene [20]. In addition, it is desirable to develop a heterogeneous photocatalytic system for practical applications. Xie reported a TiO2-POMs nanocomposite photoanode which was synthesized by hydrothermal method. The photoanode exhibited efficient photocatalytic/photoelectrocatalytic degradation for bisphenol A [21]. We also fabricated the POMs/TiO2 nanocomposite films using layer-by-layer
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self-assembly method, which showed great photoelectrochemical performance [22]. Here, we report an easy and quick electrochemical process to modify TiO2 nanotubes with POMs. The electrodeposition method possesses significant technical and economical advantages such as simplicity, low-cost, and the ability to be operated at low temperature [23]. However, the photocatalytic degradation of nitrobenzene using the TiO2-POMs nanocomposite films constructed by electrodeposition has been virtually unexplored so far. In the present work, high density and well-ordered TiO2 nanotube arrays were prepared by electrochemical anodization technique, and Keggin-type POM H3PW12O40 (PW12) was used as chemical modifying reagent. The composite film containing TiO2 nanotubes and POMs (TiO2-POMs) was prepared by an electrodeposition process. We investigated the photocatalytic performance of the TiO2-POMs composite film through the degradation of nitrobenzene. Our results demonstrated that the TiO2-POMs composite film showed higher photocatalytic activity than pure TiO2 nanotubes. This research will provide a promising route to effectively photocatalytic degradation of nitrobenzene and other pollutant in wastewaters. 2. Experimental 2.1. Materials PW12 was prepared according to the reference and identified by UV–vis absorption spectra and cyclic voltammetry [24]. TiO2 nanotubes were synthesized by anodic oxidation according to the reported procedure [25]. 2.2. Preparation of TiO2 nanotubes The TiO2 nanotube arrays were prepared by anodizing high purity Ti foils (99.9% purity) in 1.0 wt% HF electrolyte. First, a rectangle Ti foil (size 30 mm 13 mm, 0.1 mm thick) was ultrasonically cleaned in alcohol and acetone solution for 10 min, respectively. Then the Ti foil was chemically polished in the concentrated HF-HNO3-H2O (molar ratio = 1:3:5) solution for 20 s to form a fresh smooth surface. This Ti foil was used as an anode and Pt foil as a cathode. Two electrodes with a distance of 2.5 cm were submerged into an electrolyte solution containing 1.0 wt% HF and anodized at a constant potential of 20 V with a DC power supply (SK1761SL3A) for 30 min. After the anodizing step, the sample was washed with DI water and dried with N2. It was finally calcined at 723 K for 2 h and cooled to room temperature before characterization. 2.3. Preparation of TiO2-POMs composite film Electrodeposition was carried out using a conventional threeelectrode system with a platinum sheet as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and the TiO2 nanotubes as the working electrode. The electrolyte was a solution of 0.001 M PW12. The pH of the electrolyte was adjusted to 1.0 by HCl. The PW12 film was deposited by applying a constant potential (0.08 V) for 120 min at room temperature. Finally, the obtained TiO2-POMs composite film was calcined at 673 K for 1 h. For comparison, the film of POMs-modified TiO2 nanoparticles (P25) was prepared in a similar way (denoted P25-POMs film). 2.4. Photocatalytic reaction The photocatalytic activity of TiO2-POMs composite film was evaluated by the decomposition of nitrobenzene aqueous solution in a photocatalytic reactor. The photoreaction system consists of a
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quartz reactor with a magnetic stirrer and a 400 W high-pressure mercury lamp with the main emission at 365 nm as an external UV light source. A distance between the lamp and the TiO2-POMs composite film is 5.5 cm. Prior to the photoreaction, the reaction solution was magnetically stirred in the dark for 30 min to reach adsorption/desorption equilibrium. During the photoreaction, the samples were collected at every 30 min for analysis. 2.5. Characterization The UV–vis absorbance of aqueous solutions of nitrobenzene was measured at lmax = 268 nm with a 756CRT UV–vis spectrophotometer. Field-emission scanning electron microscopy (Hitachi S-4800 FEG-SEM) equipped with energy-dispersive X-ray (EDX) analysis was used to investigate the surface morphology and elements composites of samples. The crystallization behavior was examined by X-ray diffraction (XRD) (D/max 2200 VPC diffractometer with a Cu Ka irradiation source). The X-ray photoelectron spectra (XPS) measurements were performed by an Axis Ultra spectrometer (Kratos Analytical Ltd., England) with Al Ka radiation source. The binding energies were normalized to the signal for adventitious carbon at 284.6 eV. The EIS was measured at open circuit voltage with an ac bias signal of 10 mV in the frequency range 102–105 Hz under illumination in 0.1 M Na2SO4 solution. 3. Results and discussion 3.1. Morphology observation In order to obtain detailed surface information about morphology of TiO2 nanotubes and TiO2-POMs composite film, SEM investigation was performed. As shown in Fig. 1A and B, high density, well ordered and uniform nanotube arrays are formed. The average pore diameters of TiO2 nanotubes are about 80–100 nm. Their length and wall thickness are approximately 300 nm and 12 nm, respectively. Fig. 1C is the image of the bottom sides of TiO2 nanotube arrays. It clearly shows that their pore mouth is open on the top layer due to corrosion dissolution by HF, while on the bottom it is closed. The bottom structure of TiO2 nanotubes is identical to the so-called barrier layer (a thin oxidized layer separating the porous layer from the metal substrates). Fig. 1D presents the image of TiO2-POMs composite film. After the electrodeposition, a large number of particulate aggregations anchor to the top of the surface, with some parts are inserted into the tubular structures, which depends basically on the electric field effect. The formation of the aggregations is ascribed to the aggregates of the PW12 polyanions, which is similar to other POMs compsite films [26]. Additionally, the surface of the modified TiO2 nanotube arrays is not destroyed. The above results confirm that POMs aggregates can be successfully introduced into TiO2 nanotube structures by electrodeposition method, forming a TiO2-POMs composite film. 3.2. Energy dispersive X-ray analysis EDX spectrum (Fig. 2) clearly indicates the chemical analysis of the TiO2-POMs composite film. The composite film exhibits the strong K diffraction peaks at 4.51 keV, 4.92 keV and 0.52 keV, corresponding to the elemental of Ti and O, while the diffraction peaks of elemental W appear at 1.82, 2.15, 8.38, and 9.62 keV, and the diffraction peak of elemental P appears at 1.89 keV. Moreover, an approximate ratio of POMs and TiO2 nanotubes is estimated by EDX analysis. The molar ratio of POMs and TiO2 is about 0.006:1. These results further prove that PW12 is well introduced into the TiO2 nanotubes.
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Fig. 1. SEM images of TiO2 nanotubes (A) the top view of the nanotubes, (B) the side view of the nanotubes, (C) the bottom view of the nanotubes, (D) the top view of the TiO2POMs composite film.
3.3. XRD measurement Fig. 3 shows the XRD patterns of TiO2 nanotubes being annealed at 723 K for 2 h and TiO2-POMs composite film for determining the crystal phase. Both the samples exhibit the major characteristic diffraction peaks along (1 0 1) and (2 0 0) planes, indicating the TiO2 nanotubes are in the anatase structure. Furthermore, Fig. 3B displays a strong diffraction peak at 26.1, which is the characteristic peak of PW12 and also means the aggregates of the PW12 polyanions [27]. This indicates that PW12 aggregates are introduced into the TiO2 nanotubes, which is consistent with the results of the SEM investigation.
Additionally, the binding energy for P 2p is observed at 132.4 eV. XPS results again confirm the existence of the POM polyanions in the composite films. This accords well with the results from EDX analysis. 3.5. Photocatalytic activity
In order to further identify the chemical composition and the binding energy of the atoms, we measured the XPS of the TiO2POMs composite film. The presence of P and W elements in the film can be confirmed by the observed signal peaks in Fig. 4. The characteristic peaks located at 36.13 eV and 38.23 eV can be assigned to the W 4f 5/2 and W 4f 7/2, respectively [28].
The photocatalytic performance of TiO2-POMs composite film was investigated using the degradation of aqueous nitrobenzene solution under UV light irradiation. For comparison, the photocatalytic activity of TiO2 nanotubes was also tested under the same conditions. The photocatalytic experiment lasted for 3 h. The initial concentration of nitrobenzene was 50 ppm. The relative decrease of the nitrobenzene concentration, C/C0 against illumination time is shown in Fig. 5A. Here, C0 is the initial concentration of nitrobenzene, and C is the concentration of nitrobenzene after photocatalytic reaction for time. The TiO2-POMs composite film shows better photocatalytic performance than pure TiO2 nanotubes. The nitrobenzene degradation ratio achieves to 86.4% for TiO2-POMs composite film, which is higher than 76.7% for pure TiO2 nanotubes within 3 h treatment. The EDX analysis reveals that
Fig. 2. EDX spectrum of the TiO2-POMs composite film.
Fig. 3. XRD patterns of TiO2 nanotubes (A) and TiO2-POMs composite film (B).
3.4. XPS measurement
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Fig. 4. XPS spectra of the TiO2-POMs composite film W 4f (A) and P 2p (B).
the content of POMs is much smaller than that of TiO2 in the TiO2POMs composite film. These results indicate that POMs can be considered as an efficient electron scavenger to improve the inherent photocatalytic performance of TiO2 nanotubes. Moreover, the TiO2-POMs composite film also displays better photocatalytic activity than the P25-POMs film. The nitrobenzene degradation ratio of only 58.3% is achieved for the P25-POMs film. TiO2 nanotubes possess large specific surface areas, strong adsorption capability, low charge recombination efficiencies and good light harvesting properties when compared with conventional TiO2 powders [29,30]. Thus, POMs-modified TiO2 nanotubes film shows the improved photocatalytic activity. The photocatalytic reaction kinetics follows the Langmuir– Hinshelwood model (Eq. (1)): r¼
dC kr K a C ¼ dt ð1 þ K a CÞ
(1)
where r is the reaction rate, kr is the reaction rate constant, Ka is the adsorption constant, when KaC is very small (KaC 1), Eq. (1) can be simplified to a pseudo-first-order kinetic equation [31] (Eq. (2)): C0 (2) ¼ kr K a t ¼ kt ln C where k is the apparent first-order rate constant. As displayed in Fig. 5B, the apparent first-order rate constants (k) can be easily calculated: 0.009 min1 for TiO2 nanotubes and 0.012 min1 for TiO2-POMs composite film. Obviously, the TiO2-POMs composite film shows a higher rate constant compared to the TiO2 nanotubes. The values of the rate constants are in good agreement with the degradation efficiencies of nitrobenzene using TiO2 nanotubes and TiO2-POMs composite film, respectively. In other words, when POMs were introduced into the TiO2 nanotubes arrays, the fast electron–hole recombination on the surface of TiO2 can be effectively retarded, thus improving the photocatalytic
Fig. 5. (A) Photocatalytic degradation of nitrobenzene by TiO2 nanotubes, the TiO2POMs composite film and P25-POMs film, (B) photocatalytic degradation rate of nitrobenzene for TiO2 nanotubes and the TiO2-POMs composite film.
performance. However, for the pure TiO2 nanotubes, the photocatalytic activity only depends on the inherent properties of TiO2. Hence, POMs are crucial in enhancing the efficiency of photocatalytic degration of nitrobenzene over TiO2 nanotubes. 3.6. Reaction mechanism When TiO2 semiconductor is irradiated by UV photons with energy greater than or equal to the band gap energy, the electrons are promoted from the valence band (VB) of TiO2 into the CB, leaving holes behind. The holes are efficiently scavenged by water molecules or surface hydroxyl groups and then generate hydroxyl radicals (OH). The highly reactive hydroxyl radicals can break C C bond in nitrobenzene and degrade it to some open-ring species and eventually CO2 and H2O [32,33]. In the TiO2-POMs composite photocatalytic system, POMs can act as the efficient electron trappers which can store several electrons per molecule [34]. The redox potential of PW12 and the CB level of TiO2 are +0.22 and 0.5 V versus NHE, respectively [35], thus suggesting that electron transfer from the TiO2 CB to PW12 is a favorable exothermic process. After UV irradiation, the electrons in CB of TiO2 can transfer to the empty d orbits of PW12, forming the reduced PW12. Then the reduced PW12 is reoxidized to PW12 by transferring electrons to oxidants such as oxygen molecules [36– 38]. In this case, PW12 acts as an electron transfer mediator between TiO2 and adsorbed O2. Such effective electron transfer can promote the charge separation and thus retard the fast charge-pair recombination on the surface of TiO2. So the holes have enough time to react with surface water molecule to generate hydroxyl
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possibly attributed to the electron transfer from TiO2 nanotubes to POMs. We expect that the TiO2-POMs composite film will offer promising applications for environmental remediation and renewable energy generation. Acknowledgements
Fig. 6. The mechanism of the photocatalytic reaction.
The authors are thankful for the financial support from the Natural Science Foundation of China (Grant No. 21273031 and 21361027), Jilin Provincial Science and Technology Development Foundation (Grant No. 201201068 and 20140101120JC). This work is also supported by the Fundamental Research Funds for the Central Universities (14QNJJ012). References
Fig. 7. EIS spectra of TiO2 nanotubes and the TiO2-POMs composite film.
radicals (as shown in Fig. 6). For the pure TiO2 nanotubes, the weak photocatalytic activity is due to the fast electron–hole recombination in the TiO2. Therefore, the photocatalytic activity of the TiO2POMs composite film is higher than pure TiO2 nanotubes. To further explore the role of POM in TiO2 photocatalytic system, electrochemical impedance spectra (EIS) measurement was performed by using TiO2 nanotubes and the TiO2-POMs composite film as the working electrodes in solution. EIS Nyquist plots are associated with the charge transfer resistance and the separation efficiency of photogenerated electron–hole pairs [39,40]. A larger circular radius usually corresponds to a larger charge transfer resistance, and thus the lower charge separation efficiency. As shown in Fig. 7, the TiO2-POMs composite film shows the smaller circular radius than the TiO2 nanotubes, indicating the faster interfacial charge transfer and more effective separation of photogenerated electron–hole pairs in the TiO2-POMs composite system. This result further demonstrates that POMs act as “electron scavenger” to transfer the photogenerated electrons, which causes the effective separation of the electron–hole pairs in TiO2, and contributes to the suppression of charge recombination. Therefore, a higher photocatalytic performance can be achieved in TiO2-POMs composite film. 4. Conclusions In this study, the TiO2-POMs composite film has been fabricated for the photocatalytic application by an electrodeposition process. The TiO2 nanotubes were synthesized by anodization method. The experimental results from SEM, EDX and XPS confirmed the presence of POMs on nanotubular structure. The modification of the TiO2 nanotubes with POMs presented a better photocatalytic activity than pure TiO2 nanotubes to degrade nitrobenzene. Such excellent photocatalytic performance is
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Nanocomposite film of TiO2 nanotube and polyoxometalate towards photocatalytic degradation of nitrobenzene Zhixia Sun, Mingliang Zhao, Fengyan Li *, Tianqi Wang, Lin Xu * Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun 130024, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 June 2014 Received in revised form 13 August 2014 Accepted 8 September 2014 Available online 10 September 2014
The composite film based on polyoxometalates (POMs)-modified TiO2 nanotubes was prepared by electrodeposition method for the photocatalytic degradation of nitrobenzene. The composite film was characterized by field-emission scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, which indicated that the POMs were well introduced into the TiO2 nanotubes. Furthermore, the photocatalytic properties of the TiO2 nanotubes and POMs-modified TiO2 nanotubes were evaluated by the decomposition of nitrobenzene. POMsmodified TiO2 nanotubes showed much higher photocatalytic activity than pure TiO2 nanotubes. These results provide a promising route to effectively photocatalytic degradation of nitrobenzene by POMsmodified TiO2 nanotubes. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Composites Nanostructures Electron microscopy Catalytic properties
1. Introduction Nitrobenzene, a highly toxic aromatic compound, is widely used in industrial processes such as pesticides, dyes and explosives manufacture [1–3]. As an industrial consequence, it appears as contaminant in all kinds of water sources especially in surface water and industrial wastewaters [4]. Also, nitrobenzene is readily absorbed by contact with the skin and by inhalation of vapors, and affects the central nervous system, resulting in headache, vertigo, coma and so on [5]. Therefore, the removal of nitrobenzene from wastewater is a current issue for both fundamental research and practical application. So far, various chemical treatment processes were proposed as the efficient treatment options for nitrobenzene removal, such as photochemical remediation [6], ozonation [7], and photocatalytic degradation [8]. Among these proposed methods, the photocatalytic degradation of aromatic compounds is largely investigated due to the complete mineralization of organic pollutants to harmless inorganic compounds. TiO2 photocatalyst has been widely investigated for the complete degradation of toxic contaminants in water and air [9,10]. Bhatkhande et al. reported photocatalytic degradation of nitrobenzene with TiO2 using UV radiation and investigated the effect of various parameters such as presence of anions, pH, wavelength of light, etc [11]. Furthermore, multidimensional TiO2 showed a higher photochemical reactivity than bulk TiO2 particles
* Corresponding authors. Tel.: +86 431 85098760; fax: +86 431 85099765. E-mail addresses: [email protected] (F. Li), [email protected] (L. Xu). http://dx.doi.org/10.1016/j.materresbull.2014.09.025 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
[12,13]. Recently, using TiO2 nanotubes as photocatalysts have attracted considerable attention [14], because of their unique three-dimensional structures, high surface areas, and high charge transfer rates [15,16]. Tayade and Key evaluated photocatalytic degradation of nitrobenzene with TiO2 nanotubes which were prepared by hydrothermal treatment, demonstrating a higher photocatalytic activity of TiO2 nanotubes than that of TiO2 powder [17]. However, the photocatalytic efficiency of TiO2 nanotubes is still limited by the fast recombination of photogenerated electronhole pairs. Thus, searching for appropriate and effective electron acceptors should be of considerable signification to enhance separation efficiency of electron-hole pairs. Polyoxometalates (POMs), a class of molecular metal-oxo cluster compounds based mainly on Mo, W, and V elements, have shown superior physicochemical properties and various applications in catalysis, materials science and medicine [18,19]. Especially, POMs have an intrinsic characteristic of electron acceptor, which can capture the photogenerated electrons from conduction band (CB) of TiO2. Thus, the recombination of photogenerated electron-hole pairs is suppressed due to the synergistic effect of POMs and TiO2. Ozer and Ferry investigated the use of POMs in TiO2 suspensions to increase the photocatalytic efficiency of 1,2-dichlorobenzene [20]. In addition, it is desirable to develop a heterogeneous photocatalytic system for practical applications. Xie reported a TiO2-POMs nanocomposite photoanode which was synthesized by hydrothermal method. The photoanode exhibited efficient photocatalytic/photoelectrocatalytic degradation for bisphenol A [21]. We also fabricated the POMs/TiO2 nanocomposite films using layer-by-layer
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self-assembly method, which showed great photoelectrochemical performance [22]. Here, we report an easy and quick electrochemical process to modify TiO2 nanotubes with POMs. The electrodeposition method possesses significant technical and economical advantages such as simplicity, low-cost, and the ability to be operated at low temperature [23]. However, the photocatalytic degradation of nitrobenzene using the TiO2-POMs nanocomposite films constructed by electrodeposition has been virtually unexplored so far. In the present work, high density and well-ordered TiO2 nanotube arrays were prepared by electrochemical anodization technique, and Keggin-type POM H3PW12O40 (PW12) was used as chemical modifying reagent. The composite film containing TiO2 nanotubes and POMs (TiO2-POMs) was prepared by an electrodeposition process. We investigated the photocatalytic performance of the TiO2-POMs composite film through the degradation of nitrobenzene. Our results demonstrated that the TiO2-POMs composite film showed higher photocatalytic activity than pure TiO2 nanotubes. This research will provide a promising route to effectively photocatalytic degradation of nitrobenzene and other pollutant in wastewaters. 2. Experimental 2.1. Materials PW12 was prepared according to the reference and identified by UV–vis absorption spectra and cyclic voltammetry [24]. TiO2 nanotubes were synthesized by anodic oxidation according to the reported procedure [25]. 2.2. Preparation of TiO2 nanotubes The TiO2 nanotube arrays were prepared by anodizing high purity Ti foils (99.9% purity) in 1.0 wt% HF electrolyte. First, a rectangle Ti foil (size 30 mm 13 mm, 0.1 mm thick) was ultrasonically cleaned in alcohol and acetone solution for 10 min, respectively. Then the Ti foil was chemically polished in the concentrated HF-HNO3-H2O (molar ratio = 1:3:5) solution for 20 s to form a fresh smooth surface. This Ti foil was used as an anode and Pt foil as a cathode. Two electrodes with a distance of 2.5 cm were submerged into an electrolyte solution containing 1.0 wt% HF and anodized at a constant potential of 20 V with a DC power supply (SK1761SL3A) for 30 min. After the anodizing step, the sample was washed with DI water and dried with N2. It was finally calcined at 723 K for 2 h and cooled to room temperature before characterization. 2.3. Preparation of TiO2-POMs composite film Electrodeposition was carried out using a conventional threeelectrode system with a platinum sheet as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and the TiO2 nanotubes as the working electrode. The electrolyte was a solution of 0.001 M PW12. The pH of the electrolyte was adjusted to 1.0 by HCl. The PW12 film was deposited by applying a constant potential (0.08 V) for 120 min at room temperature. Finally, the obtained TiO2-POMs composite film was calcined at 673 K for 1 h. For comparison, the film of POMs-modified TiO2 nanoparticles (P25) was prepared in a similar way (denoted P25-POMs film). 2.4. Photocatalytic reaction The photocatalytic activity of TiO2-POMs composite film was evaluated by the decomposition of nitrobenzene aqueous solution in a photocatalytic reactor. The photoreaction system consists of a
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quartz reactor with a magnetic stirrer and a 400 W high-pressure mercury lamp with the main emission at 365 nm as an external UV light source. A distance between the lamp and the TiO2-POMs composite film is 5.5 cm. Prior to the photoreaction, the reaction solution was magnetically stirred in the dark for 30 min to reach adsorption/desorption equilibrium. During the photoreaction, the samples were collected at every 30 min for analysis. 2.5. Characterization The UV–vis absorbance of aqueous solutions of nitrobenzene was measured at lmax = 268 nm with a 756CRT UV–vis spectrophotometer. Field-emission scanning electron microscopy (Hitachi S-4800 FEG-SEM) equipped with energy-dispersive X-ray (EDX) analysis was used to investigate the surface morphology and elements composites of samples. The crystallization behavior was examined by X-ray diffraction (XRD) (D/max 2200 VPC diffractometer with a Cu Ka irradiation source). The X-ray photoelectron spectra (XPS) measurements were performed by an Axis Ultra spectrometer (Kratos Analytical Ltd., England) with Al Ka radiation source. The binding energies were normalized to the signal for adventitious carbon at 284.6 eV. The EIS was measured at open circuit voltage with an ac bias signal of 10 mV in the frequency range 102–105 Hz under illumination in 0.1 M Na2SO4 solution. 3. Results and discussion 3.1. Morphology observation In order to obtain detailed surface information about morphology of TiO2 nanotubes and TiO2-POMs composite film, SEM investigation was performed. As shown in Fig. 1A and B, high density, well ordered and uniform nanotube arrays are formed. The average pore diameters of TiO2 nanotubes are about 80–100 nm. Their length and wall thickness are approximately 300 nm and 12 nm, respectively. Fig. 1C is the image of the bottom sides of TiO2 nanotube arrays. It clearly shows that their pore mouth is open on the top layer due to corrosion dissolution by HF, while on the bottom it is closed. The bottom structure of TiO2 nanotubes is identical to the so-called barrier layer (a thin oxidized layer separating the porous layer from the metal substrates). Fig. 1D presents the image of TiO2-POMs composite film. After the electrodeposition, a large number of particulate aggregations anchor to the top of the surface, with some parts are inserted into the tubular structures, which depends basically on the electric field effect. The formation of the aggregations is ascribed to the aggregates of the PW12 polyanions, which is similar to other POMs compsite films [26]. Additionally, the surface of the modified TiO2 nanotube arrays is not destroyed. The above results confirm that POMs aggregates can be successfully introduced into TiO2 nanotube structures by electrodeposition method, forming a TiO2-POMs composite film. 3.2. Energy dispersive X-ray analysis EDX spectrum (Fig. 2) clearly indicates the chemical analysis of the TiO2-POMs composite film. The composite film exhibits the strong K diffraction peaks at 4.51 keV, 4.92 keV and 0.52 keV, corresponding to the elemental of Ti and O, while the diffraction peaks of elemental W appear at 1.82, 2.15, 8.38, and 9.62 keV, and the diffraction peak of elemental P appears at 1.89 keV. Moreover, an approximate ratio of POMs and TiO2 nanotubes is estimated by EDX analysis. The molar ratio of POMs and TiO2 is about 0.006:1. These results further prove that PW12 is well introduced into the TiO2 nanotubes.
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Fig. 1. SEM images of TiO2 nanotubes (A) the top view of the nanotubes, (B) the side view of the nanotubes, (C) the bottom view of the nanotubes, (D) the top view of the TiO2POMs composite film.
3.3. XRD measurement Fig. 3 shows the XRD patterns of TiO2 nanotubes being annealed at 723 K for 2 h and TiO2-POMs composite film for determining the crystal phase. Both the samples exhibit the major characteristic diffraction peaks along (1 0 1) and (2 0 0) planes, indicating the TiO2 nanotubes are in the anatase structure. Furthermore, Fig. 3B displays a strong diffraction peak at 26.1, which is the characteristic peak of PW12 and also means the aggregates of the PW12 polyanions [27]. This indicates that PW12 aggregates are introduced into the TiO2 nanotubes, which is consistent with the results of the SEM investigation.
Additionally, the binding energy for P 2p is observed at 132.4 eV. XPS results again confirm the existence of the POM polyanions in the composite films. This accords well with the results from EDX analysis. 3.5. Photocatalytic activity
In order to further identify the chemical composition and the binding energy of the atoms, we measured the XPS of the TiO2POMs composite film. The presence of P and W elements in the film can be confirmed by the observed signal peaks in Fig. 4. The characteristic peaks located at 36.13 eV and 38.23 eV can be assigned to the W 4f 5/2 and W 4f 7/2, respectively [28].
The photocatalytic performance of TiO2-POMs composite film was investigated using the degradation of aqueous nitrobenzene solution under UV light irradiation. For comparison, the photocatalytic activity of TiO2 nanotubes was also tested under the same conditions. The photocatalytic experiment lasted for 3 h. The initial concentration of nitrobenzene was 50 ppm. The relative decrease of the nitrobenzene concentration, C/C0 against illumination time is shown in Fig. 5A. Here, C0 is the initial concentration of nitrobenzene, and C is the concentration of nitrobenzene after photocatalytic reaction for time. The TiO2-POMs composite film shows better photocatalytic performance than pure TiO2 nanotubes. The nitrobenzene degradation ratio achieves to 86.4% for TiO2-POMs composite film, which is higher than 76.7% for pure TiO2 nanotubes within 3 h treatment. The EDX analysis reveals that
Fig. 2. EDX spectrum of the TiO2-POMs composite film.
Fig. 3. XRD patterns of TiO2 nanotubes (A) and TiO2-POMs composite film (B).
3.4. XPS measurement
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Fig. 4. XPS spectra of the TiO2-POMs composite film W 4f (A) and P 2p (B).
the content of POMs is much smaller than that of TiO2 in the TiO2POMs composite film. These results indicate that POMs can be considered as an efficient electron scavenger to improve the inherent photocatalytic performance of TiO2 nanotubes. Moreover, the TiO2-POMs composite film also displays better photocatalytic activity than the P25-POMs film. The nitrobenzene degradation ratio of only 58.3% is achieved for the P25-POMs film. TiO2 nanotubes possess large specific surface areas, strong adsorption capability, low charge recombination efficiencies and good light harvesting properties when compared with conventional TiO2 powders [29,30]. Thus, POMs-modified TiO2 nanotubes film shows the improved photocatalytic activity. The photocatalytic reaction kinetics follows the Langmuir– Hinshelwood model (Eq. (1)): r¼
dC kr K a C ¼ dt ð1 þ K a CÞ
(1)
where r is the reaction rate, kr is the reaction rate constant, Ka is the adsorption constant, when KaC is very small (KaC 1), Eq. (1) can be simplified to a pseudo-first-order kinetic equation [31] (Eq. (2)): C0 (2) ¼ kr K a t ¼ kt ln C where k is the apparent first-order rate constant. As displayed in Fig. 5B, the apparent first-order rate constants (k) can be easily calculated: 0.009 min1 for TiO2 nanotubes and 0.012 min1 for TiO2-POMs composite film. Obviously, the TiO2-POMs composite film shows a higher rate constant compared to the TiO2 nanotubes. The values of the rate constants are in good agreement with the degradation efficiencies of nitrobenzene using TiO2 nanotubes and TiO2-POMs composite film, respectively. In other words, when POMs were introduced into the TiO2 nanotubes arrays, the fast electron–hole recombination on the surface of TiO2 can be effectively retarded, thus improving the photocatalytic
Fig. 5. (A) Photocatalytic degradation of nitrobenzene by TiO2 nanotubes, the TiO2POMs composite film and P25-POMs film, (B) photocatalytic degradation rate of nitrobenzene for TiO2 nanotubes and the TiO2-POMs composite film.
performance. However, for the pure TiO2 nanotubes, the photocatalytic activity only depends on the inherent properties of TiO2. Hence, POMs are crucial in enhancing the efficiency of photocatalytic degration of nitrobenzene over TiO2 nanotubes. 3.6. Reaction mechanism When TiO2 semiconductor is irradiated by UV photons with energy greater than or equal to the band gap energy, the electrons are promoted from the valence band (VB) of TiO2 into the CB, leaving holes behind. The holes are efficiently scavenged by water molecules or surface hydroxyl groups and then generate hydroxyl radicals (OH). The highly reactive hydroxyl radicals can break C C bond in nitrobenzene and degrade it to some open-ring species and eventually CO2 and H2O [32,33]. In the TiO2-POMs composite photocatalytic system, POMs can act as the efficient electron trappers which can store several electrons per molecule [34]. The redox potential of PW12 and the CB level of TiO2 are +0.22 and 0.5 V versus NHE, respectively [35], thus suggesting that electron transfer from the TiO2 CB to PW12 is a favorable exothermic process. After UV irradiation, the electrons in CB of TiO2 can transfer to the empty d orbits of PW12, forming the reduced PW12. Then the reduced PW12 is reoxidized to PW12 by transferring electrons to oxidants such as oxygen molecules [36– 38]. In this case, PW12 acts as an electron transfer mediator between TiO2 and adsorbed O2. Such effective electron transfer can promote the charge separation and thus retard the fast charge-pair recombination on the surface of TiO2. So the holes have enough time to react with surface water molecule to generate hydroxyl
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possibly attributed to the electron transfer from TiO2 nanotubes to POMs. We expect that the TiO2-POMs composite film will offer promising applications for environmental remediation and renewable energy generation. Acknowledgements
Fig. 6. The mechanism of the photocatalytic reaction.
The authors are thankful for the financial support from the Natural Science Foundation of China (Grant No. 21273031 and 21361027), Jilin Provincial Science and Technology Development Foundation (Grant No. 201201068 and 20140101120JC). This work is also supported by the Fundamental Research Funds for the Central Universities (14QNJJ012). References
Fig. 7. EIS spectra of TiO2 nanotubes and the TiO2-POMs composite film.
radicals (as shown in Fig. 6). For the pure TiO2 nanotubes, the weak photocatalytic activity is due to the fast electron–hole recombination in the TiO2. Therefore, the photocatalytic activity of the TiO2POMs composite film is higher than pure TiO2 nanotubes. To further explore the role of POM in TiO2 photocatalytic system, electrochemical impedance spectra (EIS) measurement was performed by using TiO2 nanotubes and the TiO2-POMs composite film as the working electrodes in solution. EIS Nyquist plots are associated with the charge transfer resistance and the separation efficiency of photogenerated electron–hole pairs [39,40]. A larger circular radius usually corresponds to a larger charge transfer resistance, and thus the lower charge separation efficiency. As shown in Fig. 7, the TiO2-POMs composite film shows the smaller circular radius than the TiO2 nanotubes, indicating the faster interfacial charge transfer and more effective separation of photogenerated electron–hole pairs in the TiO2-POMs composite system. This result further demonstrates that POMs act as “electron scavenger” to transfer the photogenerated electrons, which causes the effective separation of the electron–hole pairs in TiO2, and contributes to the suppression of charge recombination. Therefore, a higher photocatalytic performance can be achieved in TiO2-POMs composite film. 4. Conclusions In this study, the TiO2-POMs composite film has been fabricated for the photocatalytic application by an electrodeposition process. The TiO2 nanotubes were synthesized by anodization method. The experimental results from SEM, EDX and XPS confirmed the presence of POMs on nanotubular structure. The modification of the TiO2 nanotubes with POMs presented a better photocatalytic activity than pure TiO2 nanotubes to degrade nitrobenzene. Such excellent photocatalytic performance is
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