TiO2 heterostructure nanotube arrays for improved photoelectrochemical and photocatalytic activity

Electrochimica Acta 91 (2013) 30–35

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BaTiO3 /TiO2 heterostructure nanotube arrays for improved photoelectrochemical and photocatalytic activity Rui Li, Qiuye Li ∗ , Lanlan Zong, Xiaodong Wang, Jianjun Yang ∗ Key Laboratory for Special Functional Materials, Henan University, Kaifeng 475004, China

a r t i c l e

i n f o

Article history: Received 10 October 2012 Received in revised form 18 December 2012 Accepted 18 December 2012 Available online 27 December 2012 Keywords: TiO2 nanotube array BaTiO3 /TiO2 heterojuction Photoelectrochemistry Photocatalysis

a b s t r a c t BaTiO3 /TiO2 heterostructure nanotube arrays were fabricated by in situ hydrothermal method using TiO2 nanotubes as both template and reactant. The BaTiO3 /TiO2 heterostructures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. Compared with pure TiO2 nanotube arrays, the BaTiO3 /TiO2 heterostructures exhibited enhanced photocurrent under UV light irradiation. The electrochemical impedance spectra (EIS) showed that the impedance arc radius of BaTiO3 /TiO2 heterostructures was much smaller, indicating an improved charge carrier separation ability was achieved. In addition, the BaTiO3 /TiO2 heterostructure nanotube arrays displayed a higher photocatalytic activity for methylene blue degradation. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Highly ordered and well aligned titania (TiO2 ) nanotube arrays fabricated by electrochemical anodization of titanium foil offer a large surface area with an associated increase in geometric and structural order. Owing to the remarkable charge transport property and superior oxidation ability, TiO2 nanotube arrays have attracted much attention in various fields, including, solar cells [1], photoelectrochemistry [2], gas sensing [3], drug delivering [4], lithium-ion battery [5], and photocatalysis [6]. However, the low photo conversion efficiency is still a major barrier restricting the practical application of TiO2 nanotube arrays. Numerous modification methods on TiO2 nanotube arrays have been applied to solve this problem, such as, doping with other elements, deposition with noble-metal nanoparticles, and sensitization by organic dyes. Particularly, coupling of TiO2 nanotube arrays with other semiconductors with a matching energy band structure will facilitate the transfer and separation of photo-generated electron–holes, so as to achieve the purpose of improving the utilization efficiency of the incident light [7–12]. Alkaline earth titanates MTiO3 (M = Ca, Sr, and Ba) with a cubic perovskite structure have been investigated intensively due to their unique dielectric, piezoelectric, and ferroelectric properties, which are of great interest in the technological applications such

∗ Corresponding authors. Tel.: +86 378 3881358; fax: +86 378 3881358. E-mail addresses: [email protected] (Q. Li), [email protected] (J. Yang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.12.073

as capacitors, transducers, actuators, nonvolatile random-access memory devices [11,12]. Ferroelectric materials have internal dipolar fields that separate photogenerated carriers, and this has motivated several investigations of the photochemical properties of ferroelectrics. It has been shown that the distribution of photochemically generated reaction products on ferroelectric surfaces is spatially localized, indicating that electrons and holes are spatially separated in the bulk [13–15]. If the charge separating characteristics of the ferroelectric could be combined with the photocatalytic properties of titania, it might be possible to increase the photolysis efficiency. BaTiO3 is one of the important semiconductors with strong dielectric and ferroelectric properties. And more interestingly, the conduction band and valence band of BaTiO3 is higher than that of TiO2 [16]. In this regard, under UV light irradiation, a proper combination of BaTiO3 and TiO2 can lead to not only transfer of electron from the conduction band of BaTiO3 to that of TiO2 , but also transfer of hole from the valence band of TiO2 to that of BaTiO3 . As such, the improved separation between photogenerated electrons and holes is expected to improve the photocatalytic activity of TiO2 . Therefore, if TiO2 and BaTiO3 are combined together, not only the spatial separation of photo-generated carriers of BaTiO3 can be utilized, but also the effective heterojunction structure could be formed. Thus, the purpose of improving the photo-generated electron–hole pairs will be achieved, and the corresponding photoelectric response and photocatalytic activity would be enhanced. In this study, the BaTiO3 /TiO2 heterojunction nanotube arrays were fabricated by the hydrothermal method using TiO2 nanotubes from the electrochemical anodization as both template and

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Table 1 Preparation conditions of the BaTiO3 /TiO2 heterojunction nanotube arrays. Temperature (◦ C)

Concentration of Ba(NO3 )2 (mol·L-1 )

Reaction time (h)

BT-1 BT-2 BT-3 BT-4

120 150 120 120

0.015 0.015 0.05 0.015

0.5 0.5 0.5 3

initial reactant. The density of BaTiO3 nanostructures can be easily controlled by tuning the precursor concentration or reaction temperature. The BaTiO3 /TiO2 heterostructure nanotube arrays exhibited a higher photocatalytic activity for methylene blue degradation than TiO2 nanotubes, and they also showed a stronger photocurrent and a smaller impedance arc radius. This paper provides a new idea of preparation of heterojucntion photocatalysts, which can be applied to ferroelectric materials with other semiconductors.

BaTiO3

Intensity / a.u.

Sample

TiO2

a b c d e

20

40

60

80

2Theta / degree Fig. 1. XRD patterns of (a) Ti-NT, (b) BT-1, (c) BT-2, (d) BT-3, and (e) BT-4.

2.3. Photoelectrochemical measurements 2. Experimental 2.1. Preparation of samples 2.1.1. Preparation of TiO2 nanotube (Ti-NT) arrays Titanium sheet (purity > 99.6%, 20 mm × 40 mm × 0.25 mm) was washed subsequently by sonication in acetone, isopropanol, and methanol. Then it was etched in a mixture of HF/HNO3 /H2 O (1:4:5 in volume), rinsed with deionized water and dried by N2 blowing. Anodization was carried out at room temperature in a conventional two-electrode cell using a direct current power supply. A Pt meshwork and Ti sheet served as the cathode and anode, respectively. The electrolyte was ethylene glycol containing 0.25 wt% NH4 F and 2 vol% distilled water. The self-organized and well-aligned TiO2 nanotube arrays were fabricated by two-step electrochemical anodization process. The first oxidation step was conducted at 60 V for 1 h, and then the surface TiO2 nanotube arrays were removed by sonication in distilled water and dried under N2 stream. The second oxidation step was that the Ti substrate obtained from the first step was oxidized in the original electrolyte at 60 V for 3 h. After anodization, the samples were sonicated in ethanol and dried under N2 stream. 2.1.2. Fabrication of BaTiO3 /TiO2 heterojunction nanotube arrays In a typical procedure, the as-prepared Ti-NTs were put into an autoclave containing 100 mL of 0.015 M Ba(NO3 )2 aqueous solution. The pH value of the solution was adjusted to be 13 by 1.0 M NaOH solution. The autoclave was heated at 120 ◦ C for 0.5 h, after that, it was taken out and cooled down naturally. The sheets were washed with 0.1 M HCl and deionized water several times and then dried under high purity N2 stream. By this method, the sample of BaTiO3 /TiO2 heterostructures was fabricated, denoted as BT-1. Four BaTiO3 /TiO2 heterojunction nanotube arrays were obtained by adjusting the reaction temperature, time, and the concentration of Ba(NO3 )2 . The detailed preparation conditions of the samples were listed in Table 1. 2.2. Characterization The morphologies of the samples were obtained by scanning electron microscope (SEM, JSM5600LV, Japan) and transmission electron microscopy (TEM, JEM-100CX, Japan). The crystalline structures were determined with an X-ray diffractometer (XRD, Philips X’Pert Pro, Netherland) using Cu K␣ radiation ´˚ ( = 1.54178 A).

The photoelectrochemical properties were characterized in 1 mol L−1 NaOH solution using a three-electrode photoelectrochemical cell with BaTiO3 /TiO2 or TiO2 (20 mm × 18 mm) nanotube arrays as the working electrode, an Ag/AgCl electrode as the reference, and a platinum meshwork as the counter electrode. A 300 W Xenon lamp was used as the incident light source. A scanning potentiostat (IM6ex, Germany) was used to perform a potentiodynamic scan from −1 to 0.5 V vs. SCE at a rate of 10 mV s−1 and measure the generated current. Electrochemical impedance spectroscopy (EIS) experiments were also conducted at the open circuit potential with 5 mV amplitude of perturbation in the frequency range of 10 kHz to 10 mHz under the illumination. 2.4. Evaluation of the photocatalytic activity The photocatalytic activities of the samples were evaluated by analyzing photocatalytic degradation of MB in aqueous solution. The samples were dipped into 30 mL of MB solution with an initial concentration of 10 mg L−1 . The effective area of TiO2 nanotube array films for photocatalytic reaction was 4 cm2 . Prior to light irradiation, the solution was kept in the dark for 2 h to reach an adsorption–desorption equilibrium. During the photocatalytic reaction, the absorbance of MB was measured at a time interval of every 20 min using a UV–vis spectrophotometer (722, Shanghai Jingke Instrument Plant, China) at 664 nm. 3. Results and discussions 3.1. Crystal structure of BaTiO3 /TiO2 heterojunction nanotube arrays The crystal structures of BaTiO3 /TiO2 heterojunciton nanotube arrays as well as pure TiO2 nanotube arrays were revealed by XRD analysis (Fig. 1). The curve a in Fig. 1 revealed that the crystal phase of TiO2 nanotube arrays were anatase with the diffraction peaks at about 2 = 25.5◦ , 37.9◦ , 48.2◦ , 54.1◦ , and 55.0◦ , which could be perfectly indexed to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 1 1) crystal faces of anatase TiO2 (PDF card 21-1272, JCPDS). After hydrothermal reaction with Ba(NO3 )2 aqueous solution at 120 ◦ C for 3 h (shown in curve e), additional diffraction peaks with 2 values of 22.0◦ , 31.4◦ , 38.7◦ , 44.9◦ , 55.9◦ , and 65.5◦ appeared, corresponding to (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 1), and (2 2 0) crystal planes of cubic BaTiO3 , respectively (PDF card 31-0174, JCPDS), indicating that part of anatase TiO2 was successfully converted to cubic BaTiO3 . Additionally, the XRD peaks belonging to TiO2 in

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Ti 2s

400

600

OKLL OKLL

Ba 3d Ba 3d

O 1s

200

Ti 2s

Ti 2p C 1s

Intensity / a.u.

O 2s O 2s 3p Ti 3p Ti Ti 3s Ti 3s

0

decreased gradually, implying that the reaction degree of TiO2 and Ba(NO3 )2 increased gradually.

a: Ti-NT b: BT-1

800

1000

3.2. Morphology of BaTiO3 /TiO2 heterojunction nanotube arrays

b

a

1200

Binding Energy /eV Fig. 2. XPS fully scanned spectra of Ti-NT (a) and BT-1 (b).

the BaTiO3 /TiO2 composites did not shift compared with the pure TiO2 , which could be deduced that the Ba atoms did not substitute Ti and enter into the TiO2 lattices. So, it was obvious that the synthesis route is favor for obtaining multicomponent oxide composite integrating anatase phase TiO2 with the cubic phase BaTiO3 . Moreover, from curve b, we found that the peak intensity of TiO2 decreased evidently, indicating that some part of TiO2 was reacted with Ba(NO3 )2 . However, there was no obvious diffraction peaks belonging to BaTiO3 appeared, which may be due to the small amount of BaTiO3 . This inference was validated by the XPS analysis. The fully scanned spectra of TiO2 and BaTiO3 /TiO2 nanotube arrays were shown in Fig. 2. Compared with the pure TiO2 (curve a), additional two peaks of Ba 3d3/2 and 3d5/2 at 794.259 and 778.926 eV appeared, indicating that BaTiO3 was indeed formed [17]. The mole ratio of BaTiO3 to TiO2 was determined to be 6%. In addition, as the increase of the concentration of Ba(NO3 )2 , or reaction time and temperature(from curve b–e of Fig. 1), the peak intensity of TiO2

The SEM images of TiO2 and BaTiO3 /TiO2 nanotube arrays were presented in Fig. 3. As shown in Fig. 3a, uniform TiO2 nanotubes with a diameter of about 120 nm and length about 20 ␮m were produced by electrochemical anodization. After hydrothermal treatment with Ba(NO3 )2 at 120◦ C for 0.5 h (Fig. 3b), the original nanotube array architecture is retained as denoted in Fig. 3a. However, the diameter of BT-1 decreased slightly in comparison to that of Ti-NT, probably due to volume expansion during the transformation from TiO2 to BaTiO3 or some BaTiO3 nanoparticles filled into the pore. When the reaction temperature increased to 150 ◦ C (Fig. 3c), the compact and vertically aligned nanotube array architecture was destroyed, but most of the nanotube structures were also maintained. When the concentration of Ba(NO3 )2 increased to 0.05 M (Fig. 3d), the nanotube array architecture was nearly collapsed completely, and some agglomerate of porous particles appeared. When the reaction time was prolonged to 3 h (Fig. 3e), TiO2 nanotubes converted to irregular nanoparticles completely. In further investigations of the morphology, some Ti-NT or BT samples were peeled off from the substrate, and their TEM images were shown in Fig. 4. From Fig. 4a, we can clearly see that some TiO2 nanotubes were bundled together, and their average diameter was about 120 nm. These results were in accordance with the compact nanotube array architecture and the displayed diameter in the SEM image. Fig. 4b shows that BT-1 still kept a well nanotube structure, and some nanoparticles formed on or in the TiO2 nanotubes. With the increase of the reaction temperature (Fig. 4c), the consecutive nanotube morphology was destroyed, and the nanoparticles formed a discontinuous TiO2 structure. Fig. 4d showed that the nanotube structure collapsed, and they converted to irregular nanoparticles completely. From the combination of the above results of XRD, SEM, and TEM, we can conclude that BaTiO3 /TiO2 nanotube composites with different morphology, structure and ratio can be obtained by adjusting the detailed

Fig. 3. SEM images of (a) Ti-NT, (b) BT-1, (c) BT-2, (d) BT-3, and (e) BT-4.

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Fig. 4. TEM images of (a) Ti-NT, (b) BT-1, (c) BT-2, and (d) BT-3.

3.3. Photoelectrochemical performance of BaTiO3 /TiO2 heterojunction nanotube arrays It is well known that the interband electron transition is accompanied by relaxation and recombination in TiO2 semiconducting material, and consequently, only part of the photons absorbed by TiO2 can contribute to the generation of a photocurrent [18]. Therefore, it is necessary to take into account the photocurrent–voltage (I–V) characteristics for investigation of the photoelectrochemical properties of different samples. As shown in Fig. 5, the photocurrent density of BT-1 and BT-2 electrodes is higher than that of the pure TiO2 nanotube array electrode. Especially for BT-1, its photocurrent density is about 1.7 times of TiO2 , indicating a lower recombination efficiency of photogenerated electrons and holes was achieved after TiO2 coupling with BaTiO3 . In addition, with the increase of the applied bias potential, the generated photocurrent of BT-1 and BT-2 electrodes also increased. However, the photocurrent density of BT-3 and BT-4 was much lower, especially for BT-4, showing almost no photocurrent. These results indicated that for a composite material, a proper ratio of the two components plays a key role for improving the photo-conversion efficiency. As an effective tool for probing the features of surface-modified electrodes, EIS was further employed to analyze the electron transport properties of BaTiO3 /TiO2 electrode. Fig. 6 shows Nyquist plots of the EIS spectra measured in 1 M NaOH aqueous solution under dark and full xenon lamp irradiation for Ti-NT and

BT-1 electrodes. For both Ti-NT and BT-1, the impedance arc radii in the EIS plane under light were much smaller than that in the dark, implying an improved charge carrier separation under light irradiation [19–22]. In particular, the arc radius for BT-1 electrode was much smaller than that of Ti-NT electrode under both dark and light. This result further demonstrated that BT1 electrode displayed a much higher separation efficiency of photogenerated electron–hole pairs and faster charge-transfer than Ti-NT electrode at the solid–liquid interface. Therefore, coupling TiO2 nanotubes with BaTiO3 with a proper ratio is a promising way to improve the photo-electronic conversion efficiency.

b 0.6

Current (mA/cm2)

experiment parameters, such as, precursor concentration, hydrothermal temperature, reaction time, and so on.

c a

0.3

d e

0.0

-0.3 -1.2

-0.8

-0.4

0.0

0.4

Potential (V) Fig. 5. I–V curves of TiO2 and BaTiO3 /TiO2 heterojunction nanotube arrays (a) Ti-NT, (b) BT-1, (c) BT-2, (d) BT-3, and (e) BT-4.

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R. Li et al. / Electrochimica Acta 91 (2013) 30–35

200

a: Ti-NT b: BT-1

a

150

)

-Z // (KO)

- Z //

100

50

b

a

a: Ti-NT b: BT-1

12

8

dark 4

b

0

0

0

5

10

15

20

25

30

Z / (KO)

0

50

100

150

Z/(

200

250

300

)

Fig. 6. EIS Nyquist plots of Ti-NT (a) and BT-1 (b) under UV light irradiation, and the inset is in dark.

3.4. Photocatalytic activity of BaTiO3 /TiO2 heterojunction nanotube arrays for MB degradation The photocatalytic degradation of MB had been chosen as a model reaction to evaluate the photocatalytic activities of the BaTiO3 /TiO2 heterojunction nanotube arrays, as shown in Fig. 7A. The photolysis rate of MB is very slow (curve f), and only 19% of MB was degraded after 3 h. The degradation yields of MB on all

1.05

A

0.90

f c/c0

0.75

e 0.60 0.45

Light on

a d c b

0.30 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time / h 1.4

B

ka=0.31 1.2

ln(c0/c)

1.0

b c

kb=0.44 kc=0.41

d a

kd=0.33 0.8

ke=0.13

0.6

kf=0.07

0.4

e

0.2

f

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time / h Fig. 7. Concentration change (A) and photocatalytic degradation rate (B) of MB on different samples, (a) Ti-NT, (b) BT-1, (c) BT-2, (d) BT-3, (e) BT-4, and (f) no photocatalyst.

Fig. 8. Proposal of the photocatalytic mechanism of BaTiO3 /TiO2 heterojunctions.

of the samples are higher than the photolysis rate, indicating that they had photocatalytic activity with different level. The order of the photocatalytic activities was BT-1 > BT-2 > BT-3 ≈ Ti-NT > BT-4. Their photocatalytic degradation yields of MB were 72%, 69%, 65%, 63%, and 33%, respectively. This result indicated that the photocatalytic activity of the pure TiO2 nanotube arrays can be improved by coupling with BaTiO3 with a proper ratio. Provided that the bleaching reaction follows a pseudo-first-order reaction [23–25], the degradation rates of MB on different photocatalyststs were calculated, as shown in Fig. 7B. The degradation rates of BT-1 and BT-2 reached 0.44 and 0.41 h−1 , respectively. The enhanced photocatalytic performance of BT-1 and BT-2 nanotube arrays was due to the effective separation of photogenerated electron-hole pairs. This conclusion has been confirmed by the above I–V curve and EIS analysis. While the photocatalytic activity of BT-4 was worse, this may be related to its very little photocurrent of the I–V curves. Moreover, BT-1 and BT-2 still retained partial nanotube morphology, and this one-dimensional structure would facilitate the transfer of the photo-generated carriers [26]. While BT-3 and BT-4 showed the agglomeration of the nanoparticles, the recombination of the photo-generated carriers would be higher. So, the better one-dimensional structure should be another reason for the high photocatalytic activity of BT-1 and BT-2. The photocatalytic mechanism of BaTiO3 /TiO2 heterojunction was proposed in Fig. 8. Under light irradiation, both TiO2 and BaTiO3 could be excited, the generated electrons in BaTiO3 and holes in TiO2 then migrated to the conduction band (CB) of TiO2 and valence band (VB) of BaTiO3 , respectively. This transfer process was thermodynamically favorable because both CB and VB of BaTiO3 are higher than those of TiO2 [16]. As shown in Fig. 8, the CB electrons probably reacted with the dissolved oxygen molecules to yield superoxide radical anion (O2 •− ), which on protonation generated the hydroperoxy (OOH• ), producing hydroxyl radical (OH• ), which was a strong oxidizing radical to decompose the organic dyes[27,28]. On the other hand, the VB holes (h+ ) would react with OH− to produce OH• , both h+ and OH• can oxidize MB molecules in the degradation process. 4. Conclusions In summary, BaTiO3 /TiO2 heterojunction nanotube arrays were fabricated by means of a relatively straightforward in situ hydrothermal reaction, using TiO2 nanotubes as both template and reactant. By adjusting reaction temperature, time, and the precursor concentration, four kinds of different BaTiO3 /TiO2 heterojunction nanotube arrays were obtained. The

R. Li et al. / Electrochimica Acta 91 (2013) 30–35

photoelectrochemical performance of such a vertically aligned heterostructure arrays is strongly dependent on their composition and morphology. BT-1 and BT-2 showed higher photocurrent density than pure TiO2 nanotubes, indicating they have higher photoelectronic conversion efficiency. The electrochemical impedance spectra showed that the impedance arc radius of BT-1 was much smaller than TiO2 nanotube, implying that improved charge carrier separation ability was obtained for BT-1. Moreover, BT-1 showed the best photocatalytic activity for MB degradation, and its yield reached 72% within 3 h. More importantly, this work provides a new idea of preparation of heterojucntion composite photocatalysts, which can be applied to ferroelectric materials with other semiconductors. Acknowledgements The authors gratefully acknowledge the support of the National Natural Science Foundation of China (No. 21103042), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20114103120001), the Scientific Research Foundation (No. 2010YBZR013) and the Postdoctoral Scientific Research Foundation (BH2011054) of Henan University. References [1] Z. Zhou, J. Fan, X. Wang, W. Sun, W. Zhou, Z. Du, S. Wu, Solution fabrication and photoelectrical properties of CuInS2 nanocrystals on TiO2 nanorod array, ACS Applied Materials and Interfaces 3 (2011) 2189. [2] S. Hoang, S. Guo, N.T. Hahn, A.J. Bard, C.B. Mullins, Visible light driven photoelectrochemical water oxidation on nitrogen-modified TiO2 nanowires, Nano Letters 12 (2012) 26. [3] O.K. Varghese, D. Gong, M. Paulose, K.G. Ong, E.C. Dickey, C.A. Crimes, Extreme changes in the electrical resistance of titania nanotubes with hydrogen exposure, Advanced Materials 15 (2003) 624. [4] K.C. Popat, M. Eltgroth, T.J. LaTempa, C.A. Grimes, T.A. Desai, Titania nanotubes: a novel platform for drug-eluting coatings for medical implants, Small 3 (2007) 1878. [5] B. He, B. Dong, H. Li, Preparation and electrochemical properties of Ag-modified TiO2 nanotube anode material for lithium-ion battery, Electrochemistry Communications 9 (2007) 425. [6] O.K. Varghese, M. Paulose, T.J. Latempa, C.A. Grimes, High-rate solar photocatalytic conversion of CO2 and water vapor to hydrocarbon fuels, Nano Letters 9 (2009) 731. [7] C. Wang, C. Shao, X. Zhang, Y. Liu, SnO2 nanostructures-TiO2 nanofibers heterostructures: controlled fabrication and high photocatalytic properties, Inorganic Chemistry 48 (2009) 7261. [8] T. Cao, Y. Li, C. Wang, L. Wei, C. Shao, Y. Liu, Fabrication, structure, and enhanced photocatalytic properties of hierarchical CeO2 nanostructures/TiO2 nanofibers heterostructures, Materials Research Bulletin 45 (2010) 1406. [9] M. Shang, W. Wang, L. Zhang, S. Sun, L. Wang, L. Zhou, 3D Bi2 WO6 /TiO2 hierarchical heterostructure: controllable synthesis and enhanced visible

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