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Two-step alcohothermal synthesis and characterization of enhanced visible-light-active WO3-coated TiO2 heterostructure
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Two-step alcohothermal synthesis and characterization of enhanced visiblelight-active WO3-coated TiO2 heterostructure Kunming Pana,∗∗, Kangning Shana, Shizhong Weia,∗∗∗, Yang Zhaob, Liujie Xua, Jiaming Zhud, Hong-Hui Wub,c,∗ a
Henan Key Laboratory of High-temperature Structural and Functional Materials, National Joint Engineering Research Center for Abrasion Control and Molding of Metal Materials, Henan University of Science and Technology, Luoyang, 471003, China b State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China c Beijing Advanced Innovation Center for Materials Genome Engineering, University of Science and Technology Beijing, Beijing, 100083, China d Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, China
A R T I C LE I N FO
A B S T R A C T
Keywords: WO3-Coated TiO2 Heterostructure Photocatalytic activity
A new kind of WO3-coated TiO2 heterostructure with higher photocatalytic activity was prepared via a novel two-step alcohothermal synthesis process. The effects of the WO3 addition on the TiO2/WO3 products were systematically studied, including analysis of the phases, observation of the morphologies and calculation of the band gaps. Under illumination, the conduction band electrons of TiO2 are excited and accompanied by the generation of positive holes. In this heterostructure, the conduction band electrons of WO3 occupy the nearest valence band of TiO2 and then combine with the internal holes, inhibiting the flow of the electrons transferred from the conduction band of TiO2. Compared with anatase TiO2, the WO3-coated TiO2 heterostructure (weight ratio of WO3:TiO2 = 1:2) with a finer grain size exhibits excellent photocatalytic performance.
1. Introduction As a photocatalyst, TiO2 has been extensively applied to the purification of air and water because of its excellent catalytic activity, nontoxicity, stable properties and low cost [1–6]. However, the main challenge to solar-irradiation-driven applications is the large band gap of 3.2 eV. The utilization rate of solar energy can be as low as 7% in the spectral range < 400 nm, limiting the use of TiO2 for photocatalysis [7]. WO3 has received considerable attention on the potential applications as gas sensors, photocatalyst and electrochromic devices [8–13]. The smaller band gap of 2.7 eV indicates that WO3 can make more efficient use of sunlight, while this smaller band gap also leads to an increase in the recombination rate of excited electron-hole pairs, which degrades the photocatalytic activity of WO3. Therefore, coupling two semiconductors (with different band gaps) should be an efficient way to enhance the photocatalytic activity. In recent years, lots of interesting processes have been reported to support tungsten-doped TiO2 composites for the degradation of organic pollutants. Luo et al. [14] coupled WO3 with TiO2 to obtain a TiO2/WO3
nanocomposite by sol-gel method, and further demonstrated that molecularly imprinted TiO2/WO3 was two times more efficient toward degradation of the target molecule than nonimprinted TiO2/WO3. Additionally, by using the sol-gel method, Qu et al. [15] synthesized TiO2/ WO3 composite nanotubes with higher photocatalytic properties than pure nanotubes (WO3 or TiO2). Leo'n-Ramos et al. [16] prepared Wdoped anatase-phase TiO2 via the sol-gel hydrothermal method, which displayed enhanced photocatalytic activity. However, in recent research, powders were prepared by the sol-gel process, while few studies referred to obtaining products by hydrothermal synthesis. It is well known that hydrothermal synthesis has the characteristics of being efficient, low-cost, easy to operate and environmentally friendly [17–19], but in some cases, hydrothermal processes are not widely used because of the difficulties in selecting raw materials and separating colloidal products [20,26] . For this particular reason, alcohol is chosen as the solvent instead of water to turn the “hydrothermal method” into “alcohothermal synthesis”. In this work, WO3-coated TiO2 heterostructure was prepared via alcohothermal synthesis, the photocatalytic activity and the deformation mechanism of this structure were investigated.
∗
Corresponding author. State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China. Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (K. Pan), [email protected] (S. Wei), [email protected] (H.-H. Wu). ∗∗
https://doi.org/10.1016/j.ceramint.2019.09.192 Received 25 August 2019; Received in revised form 16 September 2019; Accepted 20 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Kunming Pan, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.192
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Fig. 1. Flow chart of experimental procedures.
2. Experimental procedures
3. Results and discussion
Titanium butoxide (abbreviated as TBOT) and ammonium metatungstate (abbreviated as AMT) were used in these experiments to synthesize TiO2 and the TiO2/WO3 heterostructure. For comparison, TiO2 was also prepared by the alcohothermal method using the raw material of TBOT, as shown in Fig. 1. Then, 35 ml of TBOT was added to 80 ml of anhydrous alcohol and stirred vigorously in a glove box with a nitrogen atmosphere for 1 h. At the same time, 5 ml deionized water was dripped slowly into the solution via a pipette. Afterward, the solution was poured into Teflon-lined autoclaves followed by heating at 160 °C for 20 h. The as-obtained products were cleaned and centrifuged repeatedly in both water and alcohol to obtain the precursor. Finally, the powder was obtained after drying at 80 °C for 2 h and calcining at 400 °C for 2 h. The TiO2/WO3 precursor solutions were prepared in the same way as the TiO2 preparation, whereas the required amount of WO3 powders synthesized initially via a typical hydrothermal reaction shown in our previous work [20,27] was added into the solutions. All the samples with designed weight ratios of TiO2 and WO3 were synthesized and processed under the same conditions. The organic pollutant degradation experiment under irradiation was carried out to test the photocatalytic activity of the as-obtained products, while pure TiO2 (Degussa P25) and pure WO3 were used as comparisons. Under an AM1.5 light filter, the simulated sunlight was provided by a 300 W Xe lamp. In this experiment, the contaminant can be simulated by a concentration of 10−5 mol/L rhodamine solution (RhB). After mixing with 100 mL RhB solution, the as-obtained powders were put under the dark condition for 60 min. Then, they were transferred into the illumination system, where a 300 W xenon lamp was served as the illuminative source. The purified solutions were investigated every 15 min to monitor their absorbance. According to formula A = EcL, A is the absorbance value, E is the absorption coefficient, c is the concentration of the solute, and L is the thickness of the liquid layer. In this experiment, since E and L are unchanged, the concentration of RhB solution can be characterized proportionally by A. The phases and morphologies of the powders were observed and analyzed by the methods of XRD, FESEM and HRTEM [28,29]. The element state of the as-prepared powders was explored by XPS analysis. The UV–visible absorption was investigated by a Shimadzu UV-2600 spectrometer, where BaSO4 was employed for a non-absorbing reference. Diffuse reflectance spectra were measured by a Varian Cary 100 with an integration sphere.
Among the three crystalline structures, the anatase structure of TiO2 shows the best photocatalytic performance. The Degussa P25 and powder #1 were determined to be anatase TiO2 (JCPDS No. 65–5714) by the XRD patterns in Fig. 2a. Therefore, the reactions that occurred during alcohothermal synthesis and calcination could be speculated as follows:
Ti(C4 H9 O)4 + 4H2 O= Ti(OH)4 + 4C4 H9 OH
(1)
Ti(OH)4 = TiO2 + 2H2 O
(2)
With tungsten addition, the appearance of monoclinic WO3 peaks indicates the coexistence of WO3 and TiO2 phase (see #2~#4). It is clear that the grain sizes of the samples #1~#4 synthesized by the alcohothermal method are smaller since the FWHM (full width at half maxima) of the peaks were wider than that of P25. These finer grains have a positive effect on the improvement of performance. The XPS spectra were measured as shown in Fig. 2. From Fig. 2b, the compound contained elements of Ti, O and W. In addition, the corresponding high-resolution Ti 2p and W 4f spectra are provided in Fig. 2c and d, respectively. The binding-energy values are measured to be 458.9 eV for Ti 2p3/2 and 464.6 eV for Ti 2p1/2, yielding the characteristics of the Ti(IV) state [30]. In Fig. 2d, the peaks at 37.9 eV and 35.7 eV are confirmed to be the W 4f5/2 and W 4f7/2 of W(VI), respectively. Combined with the results from XRD, the material was proven to be a composite of TiO2 and WO3. Fig. 3 shows some clear 2D crystal lattices in the HRTEM images of the as-prepared powders. In Fig. 3a, the values of the interplanar distance of 0.352 nm and 0.243 nm correspond to the (1 0 1) and (−1 0 3) crystal planes of anatase TiO2, respectively, indicating the existence of pure anatase TiO2. When adding WO3 powder, the phases are proved to be a mixture of TiO2 and WO3 via the HRTEM images in Fig. 3b–d. From the perspective of the high-resolution lattice junction, the two materials were well combined, which provided conditions for the movement of electrons between TiO2 and WO3 and will have a positive contribution to catalytic performance. SEM images of P25 and as-prepared TiO2 samples with different WO3 additions are presented in Fig. 4a–f. Compared with the P25 sample (Fig. 4a), the sample #1 (Fig. 4b) showed a smaller particle size and more uniform morphology, which can be evidence for the advantage of the alcohothermal method. After adding WO3 powder with a mass fraction of WO3:TiO2 to 1:4, large balls with many small particles on the surface appeared, as shown in Fig. 4c. At the higher weight ratio 2
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Fig. 2. (a) XRD patterns for the samples of P25 and TiO2 powders with different WO3 additions. (b) Full spectrum XPS of the sample 3#. High-resolution spectra of (c) Ti 2p region and (d)W 4f region for the sample 3#.
Fig. 3. HRTEM images for the samples of (a) 1#, (b) 2#, (c) 3# and (d) 4#. 3
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Fig. 4. SEM micrographs for the powders of (a) P25, (b) 1#, (c) 2#, (d) 3#, (e) 4#, and (f) pure WO3 [20].
From SEM-EDS analysis on the unique coated structure of sample #3, the particles on the surface (see P1) in Fig. 5 were determined as a mixture of WO3 and TiO2 with a similar peak intensity. However, the intensity of the WO3 peak increased when the detected point was located at P2, the inner particle of the large covered ball, indicating that
of WO3:TiO2 to 1:2, the products changed completely into a covered large ball structure, with more homogenized and refined particles on the surface, as shown in Fig. 4d. However, the products changed into bulky and irregular particles, when the mass ratio of WO3:TiO2 increased to 1:1 and above (Fig. 4e and f).
Fig. 5. SEM-EDS analysis on the sample 3#. 4
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Fig. 6. The formation process of WO3 coated TiO2 structure.
samples scarcely decreased, so interference from self-degradation and physical adsorption on the results were ruled out. From the photodegradation curves, all the RhB solutions degraded completely (C/ C0 < 0.1) in 60 min under simulated sunlight except sample #4 and the pure WO3 samples. Among these catalysts, the #3 sample with the weight ratio of WO3:TiO2 of 1:2 exhibited the best photocatalytic activity. It is necessary to note that the photocatalytic performance of pure TiO2 prepared via the alcohothermal process (#1) was also better than P25, which could be attributed to grain refinement, indicating that this powder preparation of alcohothermal synthesis possesses great advantages. Additionally, the photocatalytic stability was tested via repeating photocatalytic degradation of RhB for continuous 5 cycles, as shown in Fig. 9. The as-obtained sample #3 exhibits excellent durable performance and reusability. UV–visible absorption of sample #3, P25 and as-prepared pure WO3 are shown in Fig. 10. Meanwhile, the corresponding band gaps were calculated by the following Wood and Tauc equation [23]:
the inner particles were WO3. The formation process of the TiO2/WO3 heterostructure was determined as illustrated in Fig. 6. Due to the similar colloid structure with negative charges, WO3 and WO42− can absorb together to form a new colloid group [21,22]. For this group, the measured zeta-potential value of −22.7 mV is in accordance with the above explanation. With positive charges, Ti(OH)4 particles easily cover the surfaces of WO3WO42− colloid groups, as evidenced by the zeta-potential value of 32.1 mV. Subsequently, the Ti(OH)4 on the surface of the WO3 particles dehydrated to be TiO2 after calcination, and the WO3-coated TiO2 structure was formed. To analyze the applicability of these catalyst powders for the decomposition of organic pollutants in wastewater, the photocatalytic properties were studied by the degradation of RhB solution under simulated sunlight, as shown in the photographs and absorption spectra in Fig. 7. Degussa P25 and as-prepared pure WO3 powder were also adopted for comparison (see Fig. 8). The photocatalytic properties of all samples used in this experiment were measured under the same conditions. During the dark portion of the procedure, the absorbance of all
αhν = (hν − Eg )1/2
Fig. 7. Photos and absorption spectra of the RhB solutions in the presence of samples (a) 1#, (b) 2#, (c) 3# and (d) 4#. 5
(3)
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Fig. 11. Schematic of the catalytic mechanism of WO3-coated TiO2 heterostructure.
In the present research, the direct energy gaps of the sample #3, P25 and as-prepared WO3 were determined to be 2.9 eV, 3.2 eV, and 2.7 eV, respectively (see inset Fig. 10), indicating that the band gap value of the TiO2/WO3 composite was between the pure TiO2 and pure WO3. Finally, the concept of a heterojunction was proposed to demonstrate the mechanism of as-obtained TiO2/WO3 powder for catalysis. As shown in Fig. 11, under the illumination condition, an electronic transition accompanied by the generation of positive holes will occur, while some electrons will recombine with the holes. The electrons will definitely utilize the nearest path (the lowest energy cost), so in this heterostructure, the conduction band electrons of WO3 will flow to the nearest valence band of TiO2 and recombine with the internal holes. After the TiO2 valence band is filled with electrons, the transfer of electrons from the conduction band of TiO2 will be suppressed. This heterostructure will reduce the probability of self-recombination of electron holes so that the valence band of WO3 will exert strong oxidizing properties, while the conduction band of TiO2 will exert strong reducibility. On the whole, this heterostructure not only improves the utilization of the sunlight but also hinders the recombination of electron holes. Therefore, the catalytic performance will be improved. The above mechanism can be further confirmed by the representative transient photocurrent response of WO3–TiO2 heterostructure in Fig. 12, where this is a common phenomenon of delay in the photocurrent decay. Generally, this phenomenon is considered to be related to the rate of recombination of excited e−-h+ pairs [24,25]. These delay times are measured to be 25s for WO3–TiO2 heterostructure, 15s for pure TiO2, and 18s for pure WO3. For WO3–TiO2 heterostructure, the prominent increment in the delay time of photocurrent decay indicates that this heterostructure can efficiently separate the excited e−h+ pairs through their heterostructure interfaces.
Fig. 8. The variation in the dye concentrations with time using different catalysts.
Fig. 9. Repeated photocatalytic degradation of RhB for continuous 5 cycles.
4. Conclusions WO3-coated TiO2 catalysts were prepared via alcohothermal synthesis. The WO3 powder prepared initially via one-step hydrothermal synthesis played the role of a template to obtain this coating heterostructure. During the reaction, Ti(OH)4 colloid particles had a high positive charge and covered the surfaces of WO3-WO42− colloid groups by electrostatic interaction. Subsequently, the Ti(OH)4 on the surface of WO3 particles dehydrated to form TiO2 after calcination, and the WO3-coated TiO2 structure was formed. Under the illumination condition, the conduction band electrons of TiO2 became excited, accompanied by the generation of positive holes. In this heterostructure, the conduction band electrons of WO3 occupied the nearest valence band of TiO2 and then combined with the internal holes, inhibiting the flow of electrons that were transferred from the conduction band of TiO2. With finer grain size, the WO3-coated TiO2 heterostructure thus exhibited excellent photocatalytic performance compared with anatase TiO2.
Fig. 10. UV–visible absorption and (inset) the plot of (αhν)2 versus energy of the P25, as-prepared pure WO3 and 3# sample.
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Fig. 12. (a) Representative transient photocurrent response curves and their enlarged parts of (b) P25, (c) TiO2-WO3 heterostructure and (d) pure WO3.
Acknowledgements [10]
The work was supported by Technology Project of Henan Province (No. 172102210042). H. H. Wu acknowledges the financial support from the Natural Science Foundations of China (51901013).
[11]
References [12]
[1] P.H. Mahmood, O. Amiri, S.S. Ahmed, J.R. Hama, Simple microwave synthesis of TiO2/NiS2 nanocomposite and TiO2/NiS2/Cu nanocomposite as an efficient visible driven photocatalyst, Ceram. Int. 45 (2019) 14167–14172 https://doi.org/10. 1016/j.ceramint. 2019.04.118. [2] T. Tao, I.T. Bae, K.B. Woodruff, K. Sauer, J. Cho, Hydrothermally-grown nanostructured anatase TiO2 coatings tailored for photocatalytic and antibacterial properties, Ceram. Int. (2019) In press https://doi.org/10.1016/j.ceramint.2019.08. 017. [3] C. Liu, X. Li, X. Wang, Enhanced photocatalytic activity by tailoring the interface in TiO2-ZrTiO4 heterostructure in TiO2-ZrTiO4-SiO2 ternary system, Ceram. Int. 45 (2019) 17163–17172 https://doi.org/10.1016/j.ceramint.2019.05.271. [4] Y. Liu, J. Lou, M. Ni, C. Song, J. Wu, N.P. Dasgupta, P. Tao, W. Shang, T. Deng, Bioinspired bifunctional membrane for efficient clean water generation, ACS Appl. Mater. Interfaces 8 (2016) 772–779 https://doi.org/10.1021/acsami.5b09996. [5] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications and applications, Chem. Rev. 107 (2007) 2891–2959, https://doi.org/10. 1142/9781786341662_0006. [6] P.V.L. Reddy, B. Kavitha, P.A.K. Reddy, K.H. Kim, Autopsy tissues as biological monitors of human exposure to environmental pollutants. A case study: concentrations of metals and PCDD/Fs in subjects living near a hazardous waste incinerator, Environ. Res. 154 (2017) 296–303 https://doi.org/10.1016/j.envres. 2017.01.014. [7] Y. Nah, I. Paramasivam, P. Schmuki, Doped TiO2 and TiO2 nanotubes: synthesis and applications, ChemPhysChem 11 (2010) 2698–2713 https://doi.org/10.1002/cphc. 201000276. [8] D. Su, J. Wang, Y. Tang, C. Liu, Constructing WO3/TiO2 composite structure towards sufficient use of solar energy, Chem. Commun. 47 (2011) 4231–4233 https:// doi.org/10.1039/C0CC04770H. [9] J. Chu, D.Y. Lu, X.S. Wang, X.Q. Wang, S.X. Xiong, WO3 nanoflower coated with graphene nanosheet: synergetic energy storage composite electrode for
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
7
supercapacitor application, J. Alloy. Comp. 702 (2017) 568–572 https://doi.org/ 10.1016/j.jallcom.2017.01.226. G.H. Go, P.S. Shinde, C.H. Doh, W.J. Lee, PVP-assisted synthesis of nanostructured transparent WO3 thin films for photoelectrochemical water splitting, Mater. Des. 90 (2016) 1005–1009 https://doi.org/10.1016/j.matdes.2015.11.042. R.M. Fernández-Domene, R. Sánchez-Tovar, B. Lucas-Granados, G. RosellóMárquez, J. García-Antón, A simple method to fabricate high-performance nanostructured WO3 photocatalysts with adjusted morphology in the presence of complexing agents, Mater. Des. 116 (2017) 160–170 https://doi.org/10.1016/j.matdes. 2016.12.016. R.F. Mo, G.Q. Jin, X.Y. Guo, Morphology evolution of tungsten trioxide nanorods prepared by an additive-free hydrothermal route, Mater. Lett. 61 (2007) 3787–3790 https://doi.org/10.1016/j.matlet.2006.12.061. R. Huirache-Acuña, F. Paraguay-Delgado, M.A. Albiter, J. Lara-Romero, R. Martinez, Synthesis and characterization of WO3 nanostructures prepared by an aged-hydrothermal method, Mater. Char. 60 (2009) 932–937 https://doi.org/10. 1016/j.matchar.2009.03.006. X. Luo, F. Deng, L. Min, S. Luo, B. Guo, G. Zeng, C. Au, Facile one-step synthesis of inorganic-framework molecularly imprinted TiO2/WO3 nanocomposite and its molecular recognitive photocatalytic degradation of target contaminant, Environ. Sci. Technol. 47 (2013) 7404–7412 https://doi.org/10.1021/es4013596. D. Sun, J. Liu, J. Li, Z. Feng, L. He, B. Zhao, Solvothermal synthesis of spindle-like WO3-TiO2 particles with enhanced photocatalytic activity, Mater. Res. Bull. 53 (2014) 163–168 https://doi.org/10.1016/j.materresbull.2014.02.004. J.A. León-Ramos, D. Kibanova, P. Santiago-Jacinto, Y. Mar-Santiago, M. TrejoValdez, Synthesis, characterization and photocatalytic properties of tungsten-doped hydrothermal TiO2, J. Sol. Gel Sci. Technol. 57 (2011) 43–50 https://doi.org/10. 1007/s10971-010-2322-6. S. Cao, C. Zhao, T. Han, L. Peng, Oxalic acid assisted hydrothermal synthesis and optical absorption property of WO3/TiO2 nanocomposites, J. Mater. Sci. Mater. Electron. 27 (2016) 5635–5639 https://doi.org/10.1007/s10854-016-4471-z. F. Shi, J. Liu, X. Huang, L. Yu, S. Liu, X. Feng, X. Wang, G. Shao, S. Hu, B. Yang, C. Fan, Hydrothermal synthesis of mesoporous WO3-TiO2 powders with enhanced photocatalytic activity, Adv. Powder Technol. 26 (2015) 1435–1441 https://doi. org/10.1016/j.apt.2015.07.019. Z. Tan, K. Sato, S. Ohara, Synthesis of layered nanostructured TiO2 by hydrothermal method, Adv. Powder Technol. 26 (2015) 296–302 https://doi.org/10.1016/j.apt. 2014.10.011. Y. Zhao, K. Pan, S. Wei, B. Zhang, Template-free hydrothermal synthesis of 3D hollow aggregate spherical structure WO3 nano-plates and photocatalytic properties, Mater. Res. Bull. 101 (2018) 280–286 https://doi.org/10.1016/j.materresbull.
Ceramics International xxx (xxxx) xxx–xxx
K. Pan, et al.
[26] J. Leng, Z. Wang, J. Wang, H.H. Wu, G. Yan, X. Li, H. Guo, Y. Liu, Q. Zhang, Z. Guo, Advances in nanostructures fabricated via spray pyrolysis and their applications in energy storage and conversion, Chem. Soc. Rev. 48 (2019) 3015–3072 https://doi. org/10.1039/C8CS00904J. [27] K. Pan, K. Shan, S. Wei, K. Li, J. Zhu, S.H. Siyal, H.H. Wu, Enhanced photocatalytic performance of WO3-x with oxygen vacancies via heterostructuring, Compos. Commun. 16 (2019) 106–110 https://doi.org/10.1016/j.coco.2019.09.003. [28] H. Yang, H.H. Wu, M. Ge, L. Li, Y. Yuan, Q. Yao, J. Chen, L. Xia, J. Zheng, Z. Chen, J. Duan, K. Kisslinger, X.C. Zeng, W.K. Lee, Q. Zhang, J. Lu, Simultaneously dual modifcation of Ni-rich layered oxide cathode for high-energy lithium-ion batteries, Adv. Funct. Mater. 29 (2019) 1808825https://doi.org/10.1002/adfm.201808825. [29] L. Zhao, H.H. Wu, C. Yang, Q. Zhang, G. Zhong, Z. Zheng, H. Chen, J. Wang, K. He, B. Wang, T. Zhu, X.C. Zeng, M. Liu, M.S. Wang, Mechanistic origin of the high performance of yolk@shell Bi2S3@N-doped carbon nanowire electrodes, ACS Nano 12 (2018) 12597–12611 https://pubs.acs.org/doi/10.1021/acsnano.8b07319. [30] G. Liu, H.H. Wu, Q. Meng, T. Zhang, D. Sun, X. Jin, D. Guo, N. Wu, X. Liu, J.K. Kim, Role of the anatase/TiO2(B) heterointerface for ultrastable high-rate lithium and sodium energy storage performance, Nanoscale Horiz (2019) In press https://doi. org/10.1039/C9NH00402E.
2018.01.049. [21] A. Murakado, A. Toyotama, M. Yamamoto, R. Nagano, T. Okuzono, J. Yamanaka, Thermoreversible crystallization of charged colloids due to adsorption/desorption of ionic surfactants, J. Colloid Interface Sci. 465 (2016) 200–270 https://doi.org/ 10.1016/j.jcis.2015. 11.064. [22] K. Pan, Y. Zhao, S. Wei, X. Wu, Synthesis, densification and characterization of ultra-fine W-Al2O3 composite powder, Mater. Char. 142 (2018) 245–251 https:// doi.org/10.1016/j.materresbull.2018.01.049. [23] Y. Keereeta, T. Thongtem, S. Thongtem, Fabrication of ZnWO4 nanofibers by a high direct voltage electrospinning process, J. Alloy. Comp. 509 (2011) 6689–6695 https://doi.org/10.1016/j.jallcom.2011.03.140. [24] Y. Dai, Y. Sun, J. Yao, D. Ling, Y. Wang, H. Long, X. Wang, B. Lin, T.H. Zeng, Y. Sun, Graphene-wrapped TiO2 nanofibers with effective interfacial coupling as ultrafast electron transfer bridges in novel photoanodes, J. Mater. Chem. A 2 (2014) 1060–1067, https://doi.org/10.1039/C3TA13399K. [25] S. Prabhua, L. Cindrellaa, Oh Joong Kwonb, K. Mohanrajub, Photoelectrochemical and photocatalytic activity of TiO2-WO3 heterostructures boosted by mutual interaction, Mater. Sci. Semicond. Process. 88 (2018) 10–19 https://doi.org/10. 1016/j.mssp.2018.07.028.
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Two-step alcohothermal synthesis and characterization of enhanced visiblelight-active WO3-coated TiO2 heterostructure Kunming Pana,∗∗, Kangning Shana, Shizhong Weia,∗∗∗, Yang Zhaob, Liujie Xua, Jiaming Zhud, Hong-Hui Wub,c,∗ a
Henan Key Laboratory of High-temperature Structural and Functional Materials, National Joint Engineering Research Center for Abrasion Control and Molding of Metal Materials, Henan University of Science and Technology, Luoyang, 471003, China b State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China c Beijing Advanced Innovation Center for Materials Genome Engineering, University of Science and Technology Beijing, Beijing, 100083, China d Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, China
A R T I C LE I N FO
A B S T R A C T
Keywords: WO3-Coated TiO2 Heterostructure Photocatalytic activity
A new kind of WO3-coated TiO2 heterostructure with higher photocatalytic activity was prepared via a novel two-step alcohothermal synthesis process. The effects of the WO3 addition on the TiO2/WO3 products were systematically studied, including analysis of the phases, observation of the morphologies and calculation of the band gaps. Under illumination, the conduction band electrons of TiO2 are excited and accompanied by the generation of positive holes. In this heterostructure, the conduction band electrons of WO3 occupy the nearest valence band of TiO2 and then combine with the internal holes, inhibiting the flow of the electrons transferred from the conduction band of TiO2. Compared with anatase TiO2, the WO3-coated TiO2 heterostructure (weight ratio of WO3:TiO2 = 1:2) with a finer grain size exhibits excellent photocatalytic performance.
1. Introduction As a photocatalyst, TiO2 has been extensively applied to the purification of air and water because of its excellent catalytic activity, nontoxicity, stable properties and low cost [1–6]. However, the main challenge to solar-irradiation-driven applications is the large band gap of 3.2 eV. The utilization rate of solar energy can be as low as 7% in the spectral range < 400 nm, limiting the use of TiO2 for photocatalysis [7]. WO3 has received considerable attention on the potential applications as gas sensors, photocatalyst and electrochromic devices [8–13]. The smaller band gap of 2.7 eV indicates that WO3 can make more efficient use of sunlight, while this smaller band gap also leads to an increase in the recombination rate of excited electron-hole pairs, which degrades the photocatalytic activity of WO3. Therefore, coupling two semiconductors (with different band gaps) should be an efficient way to enhance the photocatalytic activity. In recent years, lots of interesting processes have been reported to support tungsten-doped TiO2 composites for the degradation of organic pollutants. Luo et al. [14] coupled WO3 with TiO2 to obtain a TiO2/WO3
nanocomposite by sol-gel method, and further demonstrated that molecularly imprinted TiO2/WO3 was two times more efficient toward degradation of the target molecule than nonimprinted TiO2/WO3. Additionally, by using the sol-gel method, Qu et al. [15] synthesized TiO2/ WO3 composite nanotubes with higher photocatalytic properties than pure nanotubes (WO3 or TiO2). Leo'n-Ramos et al. [16] prepared Wdoped anatase-phase TiO2 via the sol-gel hydrothermal method, which displayed enhanced photocatalytic activity. However, in recent research, powders were prepared by the sol-gel process, while few studies referred to obtaining products by hydrothermal synthesis. It is well known that hydrothermal synthesis has the characteristics of being efficient, low-cost, easy to operate and environmentally friendly [17–19], but in some cases, hydrothermal processes are not widely used because of the difficulties in selecting raw materials and separating colloidal products [20,26] . For this particular reason, alcohol is chosen as the solvent instead of water to turn the “hydrothermal method” into “alcohothermal synthesis”. In this work, WO3-coated TiO2 heterostructure was prepared via alcohothermal synthesis, the photocatalytic activity and the deformation mechanism of this structure were investigated.
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Corresponding author. State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China. Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (K. Pan), [email protected] (S. Wei), [email protected] (H.-H. Wu). ∗∗
https://doi.org/10.1016/j.ceramint.2019.09.192 Received 25 August 2019; Received in revised form 16 September 2019; Accepted 20 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Kunming Pan, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.192
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Fig. 1. Flow chart of experimental procedures.
2. Experimental procedures
3. Results and discussion
Titanium butoxide (abbreviated as TBOT) and ammonium metatungstate (abbreviated as AMT) were used in these experiments to synthesize TiO2 and the TiO2/WO3 heterostructure. For comparison, TiO2 was also prepared by the alcohothermal method using the raw material of TBOT, as shown in Fig. 1. Then, 35 ml of TBOT was added to 80 ml of anhydrous alcohol and stirred vigorously in a glove box with a nitrogen atmosphere for 1 h. At the same time, 5 ml deionized water was dripped slowly into the solution via a pipette. Afterward, the solution was poured into Teflon-lined autoclaves followed by heating at 160 °C for 20 h. The as-obtained products were cleaned and centrifuged repeatedly in both water and alcohol to obtain the precursor. Finally, the powder was obtained after drying at 80 °C for 2 h and calcining at 400 °C for 2 h. The TiO2/WO3 precursor solutions were prepared in the same way as the TiO2 preparation, whereas the required amount of WO3 powders synthesized initially via a typical hydrothermal reaction shown in our previous work [20,27] was added into the solutions. All the samples with designed weight ratios of TiO2 and WO3 were synthesized and processed under the same conditions. The organic pollutant degradation experiment under irradiation was carried out to test the photocatalytic activity of the as-obtained products, while pure TiO2 (Degussa P25) and pure WO3 were used as comparisons. Under an AM1.5 light filter, the simulated sunlight was provided by a 300 W Xe lamp. In this experiment, the contaminant can be simulated by a concentration of 10−5 mol/L rhodamine solution (RhB). After mixing with 100 mL RhB solution, the as-obtained powders were put under the dark condition for 60 min. Then, they were transferred into the illumination system, where a 300 W xenon lamp was served as the illuminative source. The purified solutions were investigated every 15 min to monitor their absorbance. According to formula A = EcL, A is the absorbance value, E is the absorption coefficient, c is the concentration of the solute, and L is the thickness of the liquid layer. In this experiment, since E and L are unchanged, the concentration of RhB solution can be characterized proportionally by A. The phases and morphologies of the powders were observed and analyzed by the methods of XRD, FESEM and HRTEM [28,29]. The element state of the as-prepared powders was explored by XPS analysis. The UV–visible absorption was investigated by a Shimadzu UV-2600 spectrometer, where BaSO4 was employed for a non-absorbing reference. Diffuse reflectance spectra were measured by a Varian Cary 100 with an integration sphere.
Among the three crystalline structures, the anatase structure of TiO2 shows the best photocatalytic performance. The Degussa P25 and powder #1 were determined to be anatase TiO2 (JCPDS No. 65–5714) by the XRD patterns in Fig. 2a. Therefore, the reactions that occurred during alcohothermal synthesis and calcination could be speculated as follows:
Ti(C4 H9 O)4 + 4H2 O= Ti(OH)4 + 4C4 H9 OH
(1)
Ti(OH)4 = TiO2 + 2H2 O
(2)
With tungsten addition, the appearance of monoclinic WO3 peaks indicates the coexistence of WO3 and TiO2 phase (see #2~#4). It is clear that the grain sizes of the samples #1~#4 synthesized by the alcohothermal method are smaller since the FWHM (full width at half maxima) of the peaks were wider than that of P25. These finer grains have a positive effect on the improvement of performance. The XPS spectra were measured as shown in Fig. 2. From Fig. 2b, the compound contained elements of Ti, O and W. In addition, the corresponding high-resolution Ti 2p and W 4f spectra are provided in Fig. 2c and d, respectively. The binding-energy values are measured to be 458.9 eV for Ti 2p3/2 and 464.6 eV for Ti 2p1/2, yielding the characteristics of the Ti(IV) state [30]. In Fig. 2d, the peaks at 37.9 eV and 35.7 eV are confirmed to be the W 4f5/2 and W 4f7/2 of W(VI), respectively. Combined with the results from XRD, the material was proven to be a composite of TiO2 and WO3. Fig. 3 shows some clear 2D crystal lattices in the HRTEM images of the as-prepared powders. In Fig. 3a, the values of the interplanar distance of 0.352 nm and 0.243 nm correspond to the (1 0 1) and (−1 0 3) crystal planes of anatase TiO2, respectively, indicating the existence of pure anatase TiO2. When adding WO3 powder, the phases are proved to be a mixture of TiO2 and WO3 via the HRTEM images in Fig. 3b–d. From the perspective of the high-resolution lattice junction, the two materials were well combined, which provided conditions for the movement of electrons between TiO2 and WO3 and will have a positive contribution to catalytic performance. SEM images of P25 and as-prepared TiO2 samples with different WO3 additions are presented in Fig. 4a–f. Compared with the P25 sample (Fig. 4a), the sample #1 (Fig. 4b) showed a smaller particle size and more uniform morphology, which can be evidence for the advantage of the alcohothermal method. After adding WO3 powder with a mass fraction of WO3:TiO2 to 1:4, large balls with many small particles on the surface appeared, as shown in Fig. 4c. At the higher weight ratio 2
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Fig. 2. (a) XRD patterns for the samples of P25 and TiO2 powders with different WO3 additions. (b) Full spectrum XPS of the sample 3#. High-resolution spectra of (c) Ti 2p region and (d)W 4f region for the sample 3#.
Fig. 3. HRTEM images for the samples of (a) 1#, (b) 2#, (c) 3# and (d) 4#. 3
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Fig. 4. SEM micrographs for the powders of (a) P25, (b) 1#, (c) 2#, (d) 3#, (e) 4#, and (f) pure WO3 [20].
From SEM-EDS analysis on the unique coated structure of sample #3, the particles on the surface (see P1) in Fig. 5 were determined as a mixture of WO3 and TiO2 with a similar peak intensity. However, the intensity of the WO3 peak increased when the detected point was located at P2, the inner particle of the large covered ball, indicating that
of WO3:TiO2 to 1:2, the products changed completely into a covered large ball structure, with more homogenized and refined particles on the surface, as shown in Fig. 4d. However, the products changed into bulky and irregular particles, when the mass ratio of WO3:TiO2 increased to 1:1 and above (Fig. 4e and f).
Fig. 5. SEM-EDS analysis on the sample 3#. 4
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Fig. 6. The formation process of WO3 coated TiO2 structure.
samples scarcely decreased, so interference from self-degradation and physical adsorption on the results were ruled out. From the photodegradation curves, all the RhB solutions degraded completely (C/ C0 < 0.1) in 60 min under simulated sunlight except sample #4 and the pure WO3 samples. Among these catalysts, the #3 sample with the weight ratio of WO3:TiO2 of 1:2 exhibited the best photocatalytic activity. It is necessary to note that the photocatalytic performance of pure TiO2 prepared via the alcohothermal process (#1) was also better than P25, which could be attributed to grain refinement, indicating that this powder preparation of alcohothermal synthesis possesses great advantages. Additionally, the photocatalytic stability was tested via repeating photocatalytic degradation of RhB for continuous 5 cycles, as shown in Fig. 9. The as-obtained sample #3 exhibits excellent durable performance and reusability. UV–visible absorption of sample #3, P25 and as-prepared pure WO3 are shown in Fig. 10. Meanwhile, the corresponding band gaps were calculated by the following Wood and Tauc equation [23]:
the inner particles were WO3. The formation process of the TiO2/WO3 heterostructure was determined as illustrated in Fig. 6. Due to the similar colloid structure with negative charges, WO3 and WO42− can absorb together to form a new colloid group [21,22]. For this group, the measured zeta-potential value of −22.7 mV is in accordance with the above explanation. With positive charges, Ti(OH)4 particles easily cover the surfaces of WO3WO42− colloid groups, as evidenced by the zeta-potential value of 32.1 mV. Subsequently, the Ti(OH)4 on the surface of the WO3 particles dehydrated to be TiO2 after calcination, and the WO3-coated TiO2 structure was formed. To analyze the applicability of these catalyst powders for the decomposition of organic pollutants in wastewater, the photocatalytic properties were studied by the degradation of RhB solution under simulated sunlight, as shown in the photographs and absorption spectra in Fig. 7. Degussa P25 and as-prepared pure WO3 powder were also adopted for comparison (see Fig. 8). The photocatalytic properties of all samples used in this experiment were measured under the same conditions. During the dark portion of the procedure, the absorbance of all
αhν = (hν − Eg )1/2
Fig. 7. Photos and absorption spectra of the RhB solutions in the presence of samples (a) 1#, (b) 2#, (c) 3# and (d) 4#. 5
(3)
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Fig. 11. Schematic of the catalytic mechanism of WO3-coated TiO2 heterostructure.
In the present research, the direct energy gaps of the sample #3, P25 and as-prepared WO3 were determined to be 2.9 eV, 3.2 eV, and 2.7 eV, respectively (see inset Fig. 10), indicating that the band gap value of the TiO2/WO3 composite was between the pure TiO2 and pure WO3. Finally, the concept of a heterojunction was proposed to demonstrate the mechanism of as-obtained TiO2/WO3 powder for catalysis. As shown in Fig. 11, under the illumination condition, an electronic transition accompanied by the generation of positive holes will occur, while some electrons will recombine with the holes. The electrons will definitely utilize the nearest path (the lowest energy cost), so in this heterostructure, the conduction band electrons of WO3 will flow to the nearest valence band of TiO2 and recombine with the internal holes. After the TiO2 valence band is filled with electrons, the transfer of electrons from the conduction band of TiO2 will be suppressed. This heterostructure will reduce the probability of self-recombination of electron holes so that the valence band of WO3 will exert strong oxidizing properties, while the conduction band of TiO2 will exert strong reducibility. On the whole, this heterostructure not only improves the utilization of the sunlight but also hinders the recombination of electron holes. Therefore, the catalytic performance will be improved. The above mechanism can be further confirmed by the representative transient photocurrent response of WO3–TiO2 heterostructure in Fig. 12, where this is a common phenomenon of delay in the photocurrent decay. Generally, this phenomenon is considered to be related to the rate of recombination of excited e−-h+ pairs [24,25]. These delay times are measured to be 25s for WO3–TiO2 heterostructure, 15s for pure TiO2, and 18s for pure WO3. For WO3–TiO2 heterostructure, the prominent increment in the delay time of photocurrent decay indicates that this heterostructure can efficiently separate the excited e−h+ pairs through their heterostructure interfaces.
Fig. 8. The variation in the dye concentrations with time using different catalysts.
Fig. 9. Repeated photocatalytic degradation of RhB for continuous 5 cycles.
4. Conclusions WO3-coated TiO2 catalysts were prepared via alcohothermal synthesis. The WO3 powder prepared initially via one-step hydrothermal synthesis played the role of a template to obtain this coating heterostructure. During the reaction, Ti(OH)4 colloid particles had a high positive charge and covered the surfaces of WO3-WO42− colloid groups by electrostatic interaction. Subsequently, the Ti(OH)4 on the surface of WO3 particles dehydrated to form TiO2 after calcination, and the WO3-coated TiO2 structure was formed. Under the illumination condition, the conduction band electrons of TiO2 became excited, accompanied by the generation of positive holes. In this heterostructure, the conduction band electrons of WO3 occupied the nearest valence band of TiO2 and then combined with the internal holes, inhibiting the flow of electrons that were transferred from the conduction band of TiO2. With finer grain size, the WO3-coated TiO2 heterostructure thus exhibited excellent photocatalytic performance compared with anatase TiO2.
Fig. 10. UV–visible absorption and (inset) the plot of (αhν)2 versus energy of the P25, as-prepared pure WO3 and 3# sample.
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Fig. 12. (a) Representative transient photocurrent response curves and their enlarged parts of (b) P25, (c) TiO2-WO3 heterostructure and (d) pure WO3.
Acknowledgements [10]
The work was supported by Technology Project of Henan Province (No. 172102210042). H. H. Wu acknowledges the financial support from the Natural Science Foundations of China (51901013).
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References [12]
[1] P.H. Mahmood, O. Amiri, S.S. Ahmed, J.R. Hama, Simple microwave synthesis of TiO2/NiS2 nanocomposite and TiO2/NiS2/Cu nanocomposite as an efficient visible driven photocatalyst, Ceram. Int. 45 (2019) 14167–14172 https://doi.org/10. 1016/j.ceramint. 2019.04.118. [2] T. Tao, I.T. Bae, K.B. Woodruff, K. Sauer, J. Cho, Hydrothermally-grown nanostructured anatase TiO2 coatings tailored for photocatalytic and antibacterial properties, Ceram. Int. (2019) In press https://doi.org/10.1016/j.ceramint.2019.08. 017. [3] C. Liu, X. Li, X. Wang, Enhanced photocatalytic activity by tailoring the interface in TiO2-ZrTiO4 heterostructure in TiO2-ZrTiO4-SiO2 ternary system, Ceram. Int. 45 (2019) 17163–17172 https://doi.org/10.1016/j.ceramint.2019.05.271. [4] Y. Liu, J. Lou, M. Ni, C. Song, J. Wu, N.P. Dasgupta, P. Tao, W. Shang, T. Deng, Bioinspired bifunctional membrane for efficient clean water generation, ACS Appl. Mater. Interfaces 8 (2016) 772–779 https://doi.org/10.1021/acsami.5b09996. [5] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications and applications, Chem. Rev. 107 (2007) 2891–2959, https://doi.org/10. 1142/9781786341662_0006. [6] P.V.L. Reddy, B. Kavitha, P.A.K. Reddy, K.H. Kim, Autopsy tissues as biological monitors of human exposure to environmental pollutants. A case study: concentrations of metals and PCDD/Fs in subjects living near a hazardous waste incinerator, Environ. Res. 154 (2017) 296–303 https://doi.org/10.1016/j.envres. 2017.01.014. [7] Y. Nah, I. Paramasivam, P. Schmuki, Doped TiO2 and TiO2 nanotubes: synthesis and applications, ChemPhysChem 11 (2010) 2698–2713 https://doi.org/10.1002/cphc. 201000276. [8] D. Su, J. Wang, Y. Tang, C. Liu, Constructing WO3/TiO2 composite structure towards sufficient use of solar energy, Chem. Commun. 47 (2011) 4231–4233 https:// doi.org/10.1039/C0CC04770H. [9] J. Chu, D.Y. Lu, X.S. Wang, X.Q. Wang, S.X. Xiong, WO3 nanoflower coated with graphene nanosheet: synergetic energy storage composite electrode for
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
7
supercapacitor application, J. Alloy. Comp. 702 (2017) 568–572 https://doi.org/ 10.1016/j.jallcom.2017.01.226. G.H. Go, P.S. Shinde, C.H. Doh, W.J. Lee, PVP-assisted synthesis of nanostructured transparent WO3 thin films for photoelectrochemical water splitting, Mater. Des. 90 (2016) 1005–1009 https://doi.org/10.1016/j.matdes.2015.11.042. R.M. Fernández-Domene, R. Sánchez-Tovar, B. Lucas-Granados, G. RosellóMárquez, J. García-Antón, A simple method to fabricate high-performance nanostructured WO3 photocatalysts with adjusted morphology in the presence of complexing agents, Mater. Des. 116 (2017) 160–170 https://doi.org/10.1016/j.matdes. 2016.12.016. R.F. Mo, G.Q. Jin, X.Y. Guo, Morphology evolution of tungsten trioxide nanorods prepared by an additive-free hydrothermal route, Mater. Lett. 61 (2007) 3787–3790 https://doi.org/10.1016/j.matlet.2006.12.061. R. Huirache-Acuña, F. Paraguay-Delgado, M.A. Albiter, J. Lara-Romero, R. Martinez, Synthesis and characterization of WO3 nanostructures prepared by an aged-hydrothermal method, Mater. Char. 60 (2009) 932–937 https://doi.org/10. 1016/j.matchar.2009.03.006. X. Luo, F. Deng, L. Min, S. Luo, B. Guo, G. Zeng, C. Au, Facile one-step synthesis of inorganic-framework molecularly imprinted TiO2/WO3 nanocomposite and its molecular recognitive photocatalytic degradation of target contaminant, Environ. Sci. Technol. 47 (2013) 7404–7412 https://doi.org/10.1021/es4013596. D. Sun, J. Liu, J. Li, Z. Feng, L. He, B. Zhao, Solvothermal synthesis of spindle-like WO3-TiO2 particles with enhanced photocatalytic activity, Mater. Res. Bull. 53 (2014) 163–168 https://doi.org/10.1016/j.materresbull.2014.02.004. J.A. León-Ramos, D. Kibanova, P. Santiago-Jacinto, Y. Mar-Santiago, M. TrejoValdez, Synthesis, characterization and photocatalytic properties of tungsten-doped hydrothermal TiO2, J. Sol. Gel Sci. Technol. 57 (2011) 43–50 https://doi.org/10. 1007/s10971-010-2322-6. S. Cao, C. Zhao, T. Han, L. Peng, Oxalic acid assisted hydrothermal synthesis and optical absorption property of WO3/TiO2 nanocomposites, J. Mater. Sci. Mater. Electron. 27 (2016) 5635–5639 https://doi.org/10.1007/s10854-016-4471-z. F. Shi, J. Liu, X. Huang, L. Yu, S. Liu, X. Feng, X. Wang, G. Shao, S. Hu, B. Yang, C. Fan, Hydrothermal synthesis of mesoporous WO3-TiO2 powders with enhanced photocatalytic activity, Adv. Powder Technol. 26 (2015) 1435–1441 https://doi. org/10.1016/j.apt.2015.07.019. Z. Tan, K. Sato, S. Ohara, Synthesis of layered nanostructured TiO2 by hydrothermal method, Adv. Powder Technol. 26 (2015) 296–302 https://doi.org/10.1016/j.apt. 2014.10.011. Y. Zhao, K. Pan, S. Wei, B. Zhang, Template-free hydrothermal synthesis of 3D hollow aggregate spherical structure WO3 nano-plates and photocatalytic properties, Mater. Res. Bull. 101 (2018) 280–286 https://doi.org/10.1016/j.materresbull.
Ceramics International xxx (xxxx) xxx–xxx
K. Pan, et al.
[26] J. Leng, Z. Wang, J. Wang, H.H. Wu, G. Yan, X. Li, H. Guo, Y. Liu, Q. Zhang, Z. Guo, Advances in nanostructures fabricated via spray pyrolysis and their applications in energy storage and conversion, Chem. Soc. Rev. 48 (2019) 3015–3072 https://doi. org/10.1039/C8CS00904J. [27] K. Pan, K. Shan, S. Wei, K. Li, J. Zhu, S.H. Siyal, H.H. Wu, Enhanced photocatalytic performance of WO3-x with oxygen vacancies via heterostructuring, Compos. Commun. 16 (2019) 106–110 https://doi.org/10.1016/j.coco.2019.09.003. [28] H. Yang, H.H. Wu, M. Ge, L. Li, Y. Yuan, Q. Yao, J. Chen, L. Xia, J. Zheng, Z. Chen, J. Duan, K. Kisslinger, X.C. Zeng, W.K. Lee, Q. Zhang, J. Lu, Simultaneously dual modifcation of Ni-rich layered oxide cathode for high-energy lithium-ion batteries, Adv. Funct. Mater. 29 (2019) 1808825https://doi.org/10.1002/adfm.201808825. [29] L. Zhao, H.H. Wu, C. Yang, Q. Zhang, G. Zhong, Z. Zheng, H. Chen, J. Wang, K. He, B. Wang, T. Zhu, X.C. Zeng, M. Liu, M.S. Wang, Mechanistic origin of the high performance of yolk@shell Bi2S3@N-doped carbon nanowire electrodes, ACS Nano 12 (2018) 12597–12611 https://pubs.acs.org/doi/10.1021/acsnano.8b07319. [30] G. Liu, H.H. Wu, Q. Meng, T. Zhang, D. Sun, X. Jin, D. Guo, N. Wu, X. Liu, J.K. Kim, Role of the anatase/TiO2(B) heterointerface for ultrastable high-rate lithium and sodium energy storage performance, Nanoscale Horiz (2019) In press https://doi. org/10.1039/C9NH00402E.
2018.01.049. [21] A. Murakado, A. Toyotama, M. Yamamoto, R. Nagano, T. Okuzono, J. Yamanaka, Thermoreversible crystallization of charged colloids due to adsorption/desorption of ionic surfactants, J. Colloid Interface Sci. 465 (2016) 200–270 https://doi.org/ 10.1016/j.jcis.2015. 11.064. [22] K. Pan, Y. Zhao, S. Wei, X. Wu, Synthesis, densification and characterization of ultra-fine W-Al2O3 composite powder, Mater. Char. 142 (2018) 245–251 https:// doi.org/10.1016/j.materresbull.2018.01.049. [23] Y. Keereeta, T. Thongtem, S. Thongtem, Fabrication of ZnWO4 nanofibers by a high direct voltage electrospinning process, J. Alloy. Comp. 509 (2011) 6689–6695 https://doi.org/10.1016/j.jallcom.2011.03.140. [24] Y. Dai, Y. Sun, J. Yao, D. Ling, Y. Wang, H. Long, X. Wang, B. Lin, T.H. Zeng, Y. Sun, Graphene-wrapped TiO2 nanofibers with effective interfacial coupling as ultrafast electron transfer bridges in novel photoanodes, J. Mater. Chem. A 2 (2014) 1060–1067, https://doi.org/10.1039/C3TA13399K. [25] S. Prabhua, L. Cindrellaa, Oh Joong Kwonb, K. Mohanrajub, Photoelectrochemical and photocatalytic activity of TiO2-WO3 heterostructures boosted by mutual interaction, Mater. Sci. Semicond. Process. 88 (2018) 10–19 https://doi.org/10. 1016/j.mssp.2018.07.028.
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