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Hierarchical Heterostructure of TiO2 Nanosheets on CuO Nanowires for Enhanced Photocatalytic Performance
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Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
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www.elsevier.com/locate/procedia
Procedia Engineering 215 (2017) 180–187
9th International International Conference Conference on on Materials Materials for for Advanced Advanced Technologies Technologies(ICMAT (ICMAT2017) 2017) 9th
Hierarchical Heterostructure of TiO2 Nanosheets on CuO Nanowires for Enhanced Photocatalytic Performance Liangliang Zhua, Connor Kang Nuo Peha, Minmin Gaoa and Ghim Wei Hoa,b,c* a aDepartment
Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, 117583, Singapore bb Engineering Science Programme, National University of Singapore, 9 Engineering Drive 1, 117575, Singapore cc Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602, Singapore
Abstract Photocatalytic water splitting has been considered as one of the major strategies for the production of hydrogen, a renewable and sustainable form of energy. Herein, we report a facile approach to preparing hierarchical heterostructure of TiO22 nanosheets (NSs) supported on CuO nanowires (NWs) for photocatalytic water splitting. Initially, one-dimensional (1D) Cu NWs with diameter of 110 nm were synthesized and facilely controlled via the Maillard reaction. Subsequently, TiO22 NSs with tunable thickness were solvothermally grown on the surface of Cu NWs. 1D core-shell nanostructure with CuO NWs as core and anatase TiO22 NSs as shell was finally obtained by a simple calcination process. CuO NWs in the heterostructure not only played the role of backbones but also served as a great co-catalyst. Photocatalytic performance of the heterostructure was investigated and obvious enhancement was demonstrated in H22 production, compared to the pure TiO22 nanofibers which are achieved by growing TiO22 NSs on TiO22 electrospun fibers. Optimization of thickness of the shell shows that TiO22/CuO composites with ca. 100 nm TiO22 NSs exhibit the highest photocatalytic H22 generation rate that is 9 times higher than that of bare TiO22. The heterostructured nanomaterials have considerable potential to address the environmental and energy issues via generation of clean H22 fuel. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. committee of Symposium 2017 ICMAT. Selection Selection and/or and/or peer-review peer-review under under responsibility responsibility of of the the scientific scientific committee of Symposium 2017 ICMAT. Keywords: Hydrogen; TiO2; CuO; Photocatalysis, Hierarchical
1. Introduction Hydrogen (H22) as a form of clean and renewable energy has drawn dramatically increasing attention in a cleaner environment.[1, 2] Photocatalysis is an efficient and green approach for H 22 production due to its great potential in
* Corresponding author. Tel.: +65-6516-8121; fax: +65-6775-4710.. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. 10.1016/j.proeng.2017.11.007
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resolving environmental and energy shortage issues using the inexhaustible solar energy.[3, 4] The individual semiconductor metal oxide photocatalysts usually suffer from the poor light absorption and the rapid recombination of photogenerated electrons and holes caused by the inappropriate band gap.[5-7] Hierarchical assembly of nanoscale building photocatalyst blocks with a tunable dimension and structure complexity provides an essential strategy to realize the multifunctionality of nanomaterials aiming to enhance the photocatalytic activity by extending the photoresponse range and increase the electron-hole pair separation efficiency.[8] One-dimensional (1D) nanowires (NWs) are ideal material in electronic and optoelectronic applications due to their unique properties, such as semi-directed charge transport, enhanced charge carrier mobility and large surface area. Typically, the electron mobility of 1D semiconductor nanostructures is several orders of magnitude higher than that of corresponding spherical nanoparticles.[5, 8] Moreover, 1D NWs are promising backbone templates in construction of hierarchical heterostructures. Considerable attempts have been made to synthesize a variety of semiconductor-semiconductor hierarchical heterostructures with enhanced photocatalytic activity from 1D backbones. However, cost-effective production methods, in parallel with the high-performance materials, are still the challenges to fulfill the demands of 1D hierarchical photocatalysts. Herein, we present a facile strategy for fabricating 2D-on-1D TiO2 nanosheets (NSs)/CuO nanowires (NWs) hierarchical heterostructures for enhanced photocatalytic performance. First, The Cu NWs used as backbone in hierarchical heterostructure were prepared by the Maillard reaction. Subsequently, directly solvothermal growth of TiO2 NSs on the Cu NWs, not only produced unique porous morphology but also achieved structural stability and enhanced catalytic activity. The as-prepared TiO2/CuO composites were used for photocatalytic H2 generation by sacrificial water splitting, which showed 9 times higher than that of bare TiO2 hierarchical nanostructures. Moreover, the TiO2/CuO composites were shown to be reusable without significant deterioration of its photocatalytic activity. These findings provide the possibility for design and preparation of hierarchical photocatalysts with porous morphology and improved photocatalytic properties for applications in environmental and energy fields. 2. Experimental 2.1. Synthesis of Cu NWs The Cu NWs were synthesized according to our previous work.[9] In a typical run, 0.2 g of anhydrous copper chloride (CuCl2, Sigma Aldrich) was added to 40 ml of deionized (DI) water. 0.72 g of 1-hexadecylamine (HDA, Alfa Aesar) was then added and sonicated with an ultrasonic probe for 3 min until a light blue emulsion was formed. 0.2 g of D(+)-glucose (Sigma Aldrich), 2 g of polyvinylpyrrolidone (PVP, Mwt = 10,000, Sigma Aldrich) and 0.4 g of glycine (VWR) were then added and the emulsion was left to stir for 10 min at 50 °C. Thereafter, the bottle was capped and placed in a pre-heated oven at 102 °C for 4 hours. The NWs were then rinsed with ethanol and centrifuged 4 times until the supernatant turned clear. 2.2. Synthesis of the TiO2 NSs/CuO NWs and TiO2 NSs/TiO2 NFs hierarchical structures. TiO2 NFs were fabricated using the same method as our previous work.[5] TiO2 NSs/CuO NWs and TiO2 NSs/TiO2 NFs hierarchical structures were prepared by a hydrothermal process.[8, 10] The precursor solution was prepared by dissolving 0.1 ml of tetrabutyl titanate (TBT, Sigma Aldrich) into 35 mL mixed solvent of 30 mL isopropyl alcohol (IPA, Tokyo Chemicals Industries) and 5 mL dimethylformamide (DMF, Sigma Aldrich). 3 mg of Cu NWs was then dispersed into the mixed precursor solution. The hydrothermal treatment was subsequently conducted at 180 °C for 2, 3, 5, 10, 15 and 20 hours, respectively. After the reactions, the products were washed with IPA and dried at 55 °C. Finally, the as-prepared composite was calcinated in air at 450 °C for 2 h. TiO2 NSs/TiO2 NFs was fabricated using the same method except using TiO2 NFs as core material.
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2.3. Characterization Crystallographic information about the fabricated products was obtained using X-ray diffraction (XRD, Bruker D5005 X-ray diffractometer with Cu Ka radiation at � = 1.541 Å). Morphology and structural characteristics were studied using field-emission scanning electron microscopy (SEM, JEOL FEG JSM 7001F). The elements were analyzed by energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments). The crystal structure of the composite was investigated by using transmission electron microscopy (TEM, Philips FEG CM300) at 200 kV, and X-ray photoelectron spectroscopy (XPS, VG Thermo Escalab 220I-XL). 2.4. Photocatalytic water splitting 5 mg of photocatalyst, 9 mL DI water and 1 mL methanol were mixed in a 25 ml quartz cylindrical reaction cell and stirred for 30 min to form a homogeneous suspension. The reactor was purged with argon (Ar) gas for 10 min prior to illumination with a 300 W xenon arc lamp of intensity 100 mW cm-2. Gas samples were analyzed using a gas chromatograph (Shimadzu GC-2014AT).
3. Results and discussion The hierarchical heterostructure of TiO2 NSs supported on CuO NWs for enhanced photocatalytic performance has been successfully achieved.The overall procedure is shown in Fig. 1, in which the Cu NWs used as backbone in hierarchical heterostructure were firstly prepared by the Maillard reaction. Fig. 2a shows an SEM image of asprepared Cu NWs with an average diameter of 110 nm. By using Cu NWs as the nuclei, TiO2 NSs can be uniformly grown by a solvothermal synthesis, as shown in Fig. 2b. The TiO2 NSs distributed on the surface of the CuO NWs after calcination in air, forming a 2D-on-1D hierarchical architecture of nanowires core–nanosheets shell, which is further identified by the TEM image in Fig. 2c. The clear lattice fringes in outer nanosheets can be observed in the HRTEM image (Fig. 2d), indicating the crystallinity of the TiO2. The lattice spacing of 0.352 nm is in agreement with the (101) lattice planes of tetragonal anatase TiO2.[5, 11] The crystal structures of Cu NWs and TiO2 NSs/CuO NWs composite were also investigated by XRD and the spectra are shown in Fig. 3a. XRD analyses of the Cu NWs confirms that the products were metallic copper.[9] The diffraction peaks in TiO2 NSs/CuO NWs composite at 25.36, 37.91, 38.70, 48.15, 54.07, 55.18, 62.81, 68.84, 70.35 and 75.27o can be indexed to the (101), (004), (112), (200), (105), (211), (204), (116), (220) and (215) crystal planes of the anatase phase of TiO2, respectively.[12, 13] The crystalline peak which appeared at 2θ = 35.5o corresponds to the CuO crystalline structure.[13] The corresponding EDX spectra in Fig. 3b show apparent Ti peaks at 4.51 and 4.95 keV as well as the Cu peaks at 0.93 and 8.05 keV, which indicates that the growth of TiO2 NSs on Cu NWs.[14] Elemental mapping was also performed to illustrate the core/shell structure. As shown in Fig. 3c, the inner core of CuO is revealed by the Cu element mapping, covered by a uniform sheath consisting of Ti and O elements.
Fig. 1. Schematic illustrations for assembly of TiO2 NSs/CuO NWs hierarchical nanostructures.
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Fig. 2. SEM images of (a) Cu NWs and (b) TiO2 NSs/CuO NWs. (c-d) TEM images of hierarchical TiO2 NSs/CuO NWs.
Fig. 3. (a) XRD and (b) EDX spectra of Cu NWs and hierarchical TiO2 NSs/CuO NWs. TEM mapping of distributed O, Ti and Cu elements in TiO2 NSs/CuO NWs.
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The growth process of TiO2 NSs shell was investigated by changing the hydrothermal treatment time. Fig. 4 depicts the SEM images of hierarchical TiO2 NSs/CuO NWs nanostructures under the different reaction time (2, 3, 5, 10, 15 and 20 h). At the beginning, a thin layer of TiO2 nanoparticles were formed which attributes to the initial nucleation process (Fig. 4a). Over time, the enhancement of the hydrolytic degree of TBT and formation of hydrogen titanate led to the increase of the diameter and the density of TiO2 NSs layers (Fig. 4b-f). By further measuring the diameters of hierarchical TiO2 NSs/CuO NWs, we found that the diameters of the hierarchical TiO2 NSs/CuO NWs increase from ca. 110 to ca. 600 nm, namely, the thickness of TiO2 NSs escalate to ca. 250 nm (Fig. 5a). These samples are labeled as TiO2/CuO-110, TiO2/CuO-150, TiO2/CuO-230, TiO2/CuO-300, TiO2/CuO-490, TiO2/CuO-550, TiO2/CuO-600, respectively.
Fig. 4. SEM images of hierarchical TiO2 NSs/CuO NWs with different diameters at the different hydrothermal reaction times. (a) 2, (b) 3, (c) 5, (d) 10, (e) 15 and (f) 20 h.
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Fig. 5. (a) Average diameter of hierarchical TiO2 NSs/CuO NWs under the different hydrothermal reaction time. (b) H2 amount over the irradiation time for different diameter hierarchical TiO2 NSs/CuO NWs. (c) Stability testing over 3 consecutive cycles.
The photocatalytic activity of these as-prepared hierarchical TiO2 NSs/CuO NWs is determined by H2 production from photocatalytic water splitting.[8, 15] Figure 5b illustrates the time course of H2 production of the hierarchical TiO2 NSs/CuO NWs with different diameters. We found that the H2 evolution rate can be tuned by the thickness of TiO2 NSs layer. Bare CuO NWs (TiO2/CuO-110) shows no activity in photocatalytic H2 evolution due to its intrinsic unsuited bandgap. After TiO2 growth, the H2 evolution rate immensely enhanced and the highest rate of 990.2 ȝPRO g-1 h-1 occurred in TiO2/CuO-300 (blue line), indicating the optimized thickness of TiO2 NSs is ca. 100 nm. Moreover, the reusability of photocatalyst was determined through water splitting by TiO2/CuO-300 for three cycles. As shown in Fig. 5c, no obvious loss of photocatalytic performance was observed, suggesting that the composites are structurally stable. To further determine the role of the CuO core material, a similar hierarchical heterostructure was developed using the electrospun TiO2 NFs as the core material, which referred as to TiO2 NSs/TiO2 NFs. The same TiO2 NSs structure also uniformly grow on the TiO2 NFs, as shown in Fig. 6a and b. The XRD and EDX spectra confirm that TiO2 NSs/TiO2 NFs are pure anatase phase of TiO2 (Fig. 6c and d).[12-14] The H2 production of TiO2 NSs/TiO2 NFs was performed under the same conditions, the rate of which is 108.7 ȝPRO J-1 h-1, much lower than that of TiO2 NSs/CuO NWs (Fig. 6e). This result suggests that Cu NWs are not only used as the backbone of hierarchical structure but also play a pivotal role of co-catalytic function in enhanced photocatalytic performance. An energy level diagram shows the positions of the bands of TiO2 and CuO, as presented in Fig. 6f. Absorption of light greater than the band-gap energy of TiO2 generates electrons and holes in the CB and VB respectively. The sacrificial electron donor consumes the holes in the VB, thus accelerating the electron/holes separation in TiO2. The CB position of CuO below the CB of TiO2 permits the transfer of electrons from the CB of TiO2 to that of CuO, which is served as the reduction sites for capturing the protons to produce H2.[5, 13, 16]
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Fig. 6. (a-b) SEM images of hierarchical TiO2 NSs/TiO2 NFs structure. (c) XRD and (d) EDX spectra of TiO2 NSs/TiO2 NFs. (e) H2 amount over the irradiation time for TiO2 NSs/CuO NWs and TiO2 NSs/TiO2 NFs. (f) Schematic diagram of the photocatalytic H2 generation over the TiO2/CuO heterojunctions.
4. Conclusion In summary, TiO2 NSs/CuO NWs hierarchical nanostructures have been successfully fabricated by a solvothermal method. CuO NWs in the heterostructure not only played the role of backbones but also served as an effective co-catalyst. The as-prepared TiO2 NSs/CuO NWs composites have been explored for hydrogen production from photocatalytic water splitting. Obvious enhancement in the photocatalytic performance of the heterostructure was demonstrated in H2 production, compared to the pure TiO2 hierarchical nanostructures. The optimization of shell thickness shows that TiO2 NSs/CuO NWs composite with ca. 100 nm thickness of TiO2 NSs exhibits the highest photocatalytic H2 generation rate, which is 9 times higher than that of bare TiO2. Such approach towards the synthesis of hierarchical nanostructures may offer an avenue for designing and fabricating various complicated nanostructures and nanocomposites for applications in environment and energy fields. Acknowledgements This work is supported by the Ministry of Education Singapore, R-263-000-B38-112/R-263-000-B63-112 and the National Research Foundation Singapore, Ministry of National Development, R-263-000-C22-277.
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References [1] K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Photocatalyst releasing hydrogen from water, Nature 440 (2006) 295-295. [2] J. Wang, M. Gao, G.W. Ho, Bidentate-complex-derived TiO2/carbon dot photocatalysts: in situ synthesis, versatile heterostructures, and enhanced H2 evolution, J. Mater. Chem. A 2 (2014) 5703-5709. [3] Z. Li, W. Luo, M. Zhang, J. Feng, Z. Zou, Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook, Energy Environ Sci. 6 (2013) 347-370. [4] M.-Q. Yang, Y.-J. Xu, W. Lu, K. Zeng, H. Zhu, Q.-H. Xu, G.W. Ho, Self-surface charge exfoliation and electrostatically coordinated 2D hetero-layered hybrids, Nat. Commun. 8 (2017) 14224. [5] L. Zhu, M. Hong, G.W. Ho, Fabrication of wheat grain textured TiO2/CuO composite nanofibers for enhanced solar H2 generation and degradation performance, Nano Energy 11 (2015) 28-37. [6] Z. Zhang, C. Shao, X. Li, L. Zhang, H. Xue, C. Wang, Y. Liu, Electrospun Nanofibers of ZnO−SnO 2 Heterojunction with High Photocatalytic Activity, J. Phys. Chem. C 114 (2010) 7920-7925. [7] M. Gao, L. Zhu, W.L. Ong, J. Wang, G.W. Ho, Structural design of TiO 2-based photocatalyst for H2 production and degradation applications, Catal. Sci. Tech. 5 (2015) 4703-4726. [8] L. Zhu, M. Hong, G. Wei Ho, Hierarchical Assembly of SnO2/ZnO Nanostructures for Enhanced Photocatalytic Performance, Sci. Rep. 5 (2015) 11609. [9] M. Kevin, G.Y.R. Lim, G.W. Ho, Facile control of copper nanowire dimensions via the Maillard reaction: using food chemistry for fabricating large-scale transparent flexible conductors, Green Chem. 17 (2015) 1120-1126. [10] H.B. Wu, X.W. Lou, H.H. Hng, Titania Nanosheets Hierarchically Assembled on Carbon Nanotubes as High-Rate Anodes for Lithium-Ion Batteries, Chem. Eur. J 18 (2012) 3132-3135. [11] W.-Q. Wu, H.-S. Rao, Y.-F. Xu, Y.-F. Wang, C.-Y. Su, D.-B. Kuang, Hierarchical Oriented Anatase TiO2 Nanostructure arrays on Flexible Substrate for Efficient Dye-sensitized Solar Cells, Sci. Rep. 3 (2013) 1892. [12] L. Wang, Y. Zhao, J. Wang, X. Hong, J. Zhai, L. Jiang, F. Wang, Ultra-fast spreading on superhydrophilic fibrous mesh with nanochannels, Appl. Surf. Sci. 255 (2009) 4944-4949. [13] S.S. Lee, H. Bai, Z. Liu, D.D. Sun, Optimization and an insightful properties—Activity study of electrospun TiO2/CuO composite nanofibers for efficient photocatalytic H2 generation, Appl. Catal. B 140 (2013) 68-81. [14] L. Zhu, C. Fu Tan, M. Gao, G.W. Ho, Design of a Metal Oxide–Organic Framework (MoOF) Foam Microreactor: Solar-Induced Direct Pollutant Degradation and Hydrogen Generation, Adv. Mater. 27 (2015) 7713-7719. [15] W.L. Ong, Y.-F. Lim, J.L. Ting Ong, G.W. Ho, Room temperature sequential ionic deposition (SID) of Ag 2S nanoparticles on TiO2 hierarchical spheres for enhanced catalytic efficiency, J. Mater. Chem. A 3 (2015) 6509-6516. [16] C.K.N. Peh, M. Gao, G.W. Ho, Harvesting broadband absorption of the solar spectrum for enhanced photocatalytic H 2 generation, J. Mater. Chem. A 3 (2015) 19360-19367.
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Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Engineering 215 (2017) 180–187
9th International International Conference Conference on on Materials Materials for for Advanced Advanced Technologies Technologies(ICMAT (ICMAT2017) 2017) 9th
Hierarchical Heterostructure of TiO2 Nanosheets on CuO Nanowires for Enhanced Photocatalytic Performance Liangliang Zhua, Connor Kang Nuo Peha, Minmin Gaoa and Ghim Wei Hoa,b,c* a aDepartment
Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, 117583, Singapore bb Engineering Science Programme, National University of Singapore, 9 Engineering Drive 1, 117575, Singapore cc Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602, Singapore
Abstract Photocatalytic water splitting has been considered as one of the major strategies for the production of hydrogen, a renewable and sustainable form of energy. Herein, we report a facile approach to preparing hierarchical heterostructure of TiO22 nanosheets (NSs) supported on CuO nanowires (NWs) for photocatalytic water splitting. Initially, one-dimensional (1D) Cu NWs with diameter of 110 nm were synthesized and facilely controlled via the Maillard reaction. Subsequently, TiO22 NSs with tunable thickness were solvothermally grown on the surface of Cu NWs. 1D core-shell nanostructure with CuO NWs as core and anatase TiO22 NSs as shell was finally obtained by a simple calcination process. CuO NWs in the heterostructure not only played the role of backbones but also served as a great co-catalyst. Photocatalytic performance of the heterostructure was investigated and obvious enhancement was demonstrated in H22 production, compared to the pure TiO22 nanofibers which are achieved by growing TiO22 NSs on TiO22 electrospun fibers. Optimization of thickness of the shell shows that TiO22/CuO composites with ca. 100 nm TiO22 NSs exhibit the highest photocatalytic H22 generation rate that is 9 times higher than that of bare TiO22. The heterostructured nanomaterials have considerable potential to address the environmental and energy issues via generation of clean H22 fuel. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. committee of Symposium 2017 ICMAT. Selection Selection and/or and/or peer-review peer-review under under responsibility responsibility of of the the scientific scientific committee of Symposium 2017 ICMAT. Keywords: Hydrogen; TiO2; CuO; Photocatalysis, Hierarchical
1. Introduction Hydrogen (H22) as a form of clean and renewable energy has drawn dramatically increasing attention in a cleaner environment.[1, 2] Photocatalysis is an efficient and green approach for H 22 production due to its great potential in
* Corresponding author. Tel.: +65-6516-8121; fax: +65-6775-4710.. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT. 10.1016/j.proeng.2017.11.007
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resolving environmental and energy shortage issues using the inexhaustible solar energy.[3, 4] The individual semiconductor metal oxide photocatalysts usually suffer from the poor light absorption and the rapid recombination of photogenerated electrons and holes caused by the inappropriate band gap.[5-7] Hierarchical assembly of nanoscale building photocatalyst blocks with a tunable dimension and structure complexity provides an essential strategy to realize the multifunctionality of nanomaterials aiming to enhance the photocatalytic activity by extending the photoresponse range and increase the electron-hole pair separation efficiency.[8] One-dimensional (1D) nanowires (NWs) are ideal material in electronic and optoelectronic applications due to their unique properties, such as semi-directed charge transport, enhanced charge carrier mobility and large surface area. Typically, the electron mobility of 1D semiconductor nanostructures is several orders of magnitude higher than that of corresponding spherical nanoparticles.[5, 8] Moreover, 1D NWs are promising backbone templates in construction of hierarchical heterostructures. Considerable attempts have been made to synthesize a variety of semiconductor-semiconductor hierarchical heterostructures with enhanced photocatalytic activity from 1D backbones. However, cost-effective production methods, in parallel with the high-performance materials, are still the challenges to fulfill the demands of 1D hierarchical photocatalysts. Herein, we present a facile strategy for fabricating 2D-on-1D TiO2 nanosheets (NSs)/CuO nanowires (NWs) hierarchical heterostructures for enhanced photocatalytic performance. First, The Cu NWs used as backbone in hierarchical heterostructure were prepared by the Maillard reaction. Subsequently, directly solvothermal growth of TiO2 NSs on the Cu NWs, not only produced unique porous morphology but also achieved structural stability and enhanced catalytic activity. The as-prepared TiO2/CuO composites were used for photocatalytic H2 generation by sacrificial water splitting, which showed 9 times higher than that of bare TiO2 hierarchical nanostructures. Moreover, the TiO2/CuO composites were shown to be reusable without significant deterioration of its photocatalytic activity. These findings provide the possibility for design and preparation of hierarchical photocatalysts with porous morphology and improved photocatalytic properties for applications in environmental and energy fields. 2. Experimental 2.1. Synthesis of Cu NWs The Cu NWs were synthesized according to our previous work.[9] In a typical run, 0.2 g of anhydrous copper chloride (CuCl2, Sigma Aldrich) was added to 40 ml of deionized (DI) water. 0.72 g of 1-hexadecylamine (HDA, Alfa Aesar) was then added and sonicated with an ultrasonic probe for 3 min until a light blue emulsion was formed. 0.2 g of D(+)-glucose (Sigma Aldrich), 2 g of polyvinylpyrrolidone (PVP, Mwt = 10,000, Sigma Aldrich) and 0.4 g of glycine (VWR) were then added and the emulsion was left to stir for 10 min at 50 °C. Thereafter, the bottle was capped and placed in a pre-heated oven at 102 °C for 4 hours. The NWs were then rinsed with ethanol and centrifuged 4 times until the supernatant turned clear. 2.2. Synthesis of the TiO2 NSs/CuO NWs and TiO2 NSs/TiO2 NFs hierarchical structures. TiO2 NFs were fabricated using the same method as our previous work.[5] TiO2 NSs/CuO NWs and TiO2 NSs/TiO2 NFs hierarchical structures were prepared by a hydrothermal process.[8, 10] The precursor solution was prepared by dissolving 0.1 ml of tetrabutyl titanate (TBT, Sigma Aldrich) into 35 mL mixed solvent of 30 mL isopropyl alcohol (IPA, Tokyo Chemicals Industries) and 5 mL dimethylformamide (DMF, Sigma Aldrich). 3 mg of Cu NWs was then dispersed into the mixed precursor solution. The hydrothermal treatment was subsequently conducted at 180 °C for 2, 3, 5, 10, 15 and 20 hours, respectively. After the reactions, the products were washed with IPA and dried at 55 °C. Finally, the as-prepared composite was calcinated in air at 450 °C for 2 h. TiO2 NSs/TiO2 NFs was fabricated using the same method except using TiO2 NFs as core material.
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2.3. Characterization Crystallographic information about the fabricated products was obtained using X-ray diffraction (XRD, Bruker D5005 X-ray diffractometer with Cu Ka radiation at � = 1.541 Å). Morphology and structural characteristics were studied using field-emission scanning electron microscopy (SEM, JEOL FEG JSM 7001F). The elements were analyzed by energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments). The crystal structure of the composite was investigated by using transmission electron microscopy (TEM, Philips FEG CM300) at 200 kV, and X-ray photoelectron spectroscopy (XPS, VG Thermo Escalab 220I-XL). 2.4. Photocatalytic water splitting 5 mg of photocatalyst, 9 mL DI water and 1 mL methanol were mixed in a 25 ml quartz cylindrical reaction cell and stirred for 30 min to form a homogeneous suspension. The reactor was purged with argon (Ar) gas for 10 min prior to illumination with a 300 W xenon arc lamp of intensity 100 mW cm-2. Gas samples were analyzed using a gas chromatograph (Shimadzu GC-2014AT).
3. Results and discussion The hierarchical heterostructure of TiO2 NSs supported on CuO NWs for enhanced photocatalytic performance has been successfully achieved.The overall procedure is shown in Fig. 1, in which the Cu NWs used as backbone in hierarchical heterostructure were firstly prepared by the Maillard reaction. Fig. 2a shows an SEM image of asprepared Cu NWs with an average diameter of 110 nm. By using Cu NWs as the nuclei, TiO2 NSs can be uniformly grown by a solvothermal synthesis, as shown in Fig. 2b. The TiO2 NSs distributed on the surface of the CuO NWs after calcination in air, forming a 2D-on-1D hierarchical architecture of nanowires core–nanosheets shell, which is further identified by the TEM image in Fig. 2c. The clear lattice fringes in outer nanosheets can be observed in the HRTEM image (Fig. 2d), indicating the crystallinity of the TiO2. The lattice spacing of 0.352 nm is in agreement with the (101) lattice planes of tetragonal anatase TiO2.[5, 11] The crystal structures of Cu NWs and TiO2 NSs/CuO NWs composite were also investigated by XRD and the spectra are shown in Fig. 3a. XRD analyses of the Cu NWs confirms that the products were metallic copper.[9] The diffraction peaks in TiO2 NSs/CuO NWs composite at 25.36, 37.91, 38.70, 48.15, 54.07, 55.18, 62.81, 68.84, 70.35 and 75.27o can be indexed to the (101), (004), (112), (200), (105), (211), (204), (116), (220) and (215) crystal planes of the anatase phase of TiO2, respectively.[12, 13] The crystalline peak which appeared at 2θ = 35.5o corresponds to the CuO crystalline structure.[13] The corresponding EDX spectra in Fig. 3b show apparent Ti peaks at 4.51 and 4.95 keV as well as the Cu peaks at 0.93 and 8.05 keV, which indicates that the growth of TiO2 NSs on Cu NWs.[14] Elemental mapping was also performed to illustrate the core/shell structure. As shown in Fig. 3c, the inner core of CuO is revealed by the Cu element mapping, covered by a uniform sheath consisting of Ti and O elements.
Fig. 1. Schematic illustrations for assembly of TiO2 NSs/CuO NWs hierarchical nanostructures.
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Fig. 2. SEM images of (a) Cu NWs and (b) TiO2 NSs/CuO NWs. (c-d) TEM images of hierarchical TiO2 NSs/CuO NWs.
Fig. 3. (a) XRD and (b) EDX spectra of Cu NWs and hierarchical TiO2 NSs/CuO NWs. TEM mapping of distributed O, Ti and Cu elements in TiO2 NSs/CuO NWs.
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The growth process of TiO2 NSs shell was investigated by changing the hydrothermal treatment time. Fig. 4 depicts the SEM images of hierarchical TiO2 NSs/CuO NWs nanostructures under the different reaction time (2, 3, 5, 10, 15 and 20 h). At the beginning, a thin layer of TiO2 nanoparticles were formed which attributes to the initial nucleation process (Fig. 4a). Over time, the enhancement of the hydrolytic degree of TBT and formation of hydrogen titanate led to the increase of the diameter and the density of TiO2 NSs layers (Fig. 4b-f). By further measuring the diameters of hierarchical TiO2 NSs/CuO NWs, we found that the diameters of the hierarchical TiO2 NSs/CuO NWs increase from ca. 110 to ca. 600 nm, namely, the thickness of TiO2 NSs escalate to ca. 250 nm (Fig. 5a). These samples are labeled as TiO2/CuO-110, TiO2/CuO-150, TiO2/CuO-230, TiO2/CuO-300, TiO2/CuO-490, TiO2/CuO-550, TiO2/CuO-600, respectively.
Fig. 4. SEM images of hierarchical TiO2 NSs/CuO NWs with different diameters at the different hydrothermal reaction times. (a) 2, (b) 3, (c) 5, (d) 10, (e) 15 and (f) 20 h.
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Fig. 5. (a) Average diameter of hierarchical TiO2 NSs/CuO NWs under the different hydrothermal reaction time. (b) H2 amount over the irradiation time for different diameter hierarchical TiO2 NSs/CuO NWs. (c) Stability testing over 3 consecutive cycles.
The photocatalytic activity of these as-prepared hierarchical TiO2 NSs/CuO NWs is determined by H2 production from photocatalytic water splitting.[8, 15] Figure 5b illustrates the time course of H2 production of the hierarchical TiO2 NSs/CuO NWs with different diameters. We found that the H2 evolution rate can be tuned by the thickness of TiO2 NSs layer. Bare CuO NWs (TiO2/CuO-110) shows no activity in photocatalytic H2 evolution due to its intrinsic unsuited bandgap. After TiO2 growth, the H2 evolution rate immensely enhanced and the highest rate of 990.2 ȝPRO g-1 h-1 occurred in TiO2/CuO-300 (blue line), indicating the optimized thickness of TiO2 NSs is ca. 100 nm. Moreover, the reusability of photocatalyst was determined through water splitting by TiO2/CuO-300 for three cycles. As shown in Fig. 5c, no obvious loss of photocatalytic performance was observed, suggesting that the composites are structurally stable. To further determine the role of the CuO core material, a similar hierarchical heterostructure was developed using the electrospun TiO2 NFs as the core material, which referred as to TiO2 NSs/TiO2 NFs. The same TiO2 NSs structure also uniformly grow on the TiO2 NFs, as shown in Fig. 6a and b. The XRD and EDX spectra confirm that TiO2 NSs/TiO2 NFs are pure anatase phase of TiO2 (Fig. 6c and d).[12-14] The H2 production of TiO2 NSs/TiO2 NFs was performed under the same conditions, the rate of which is 108.7 ȝPRO J-1 h-1, much lower than that of TiO2 NSs/CuO NWs (Fig. 6e). This result suggests that Cu NWs are not only used as the backbone of hierarchical structure but also play a pivotal role of co-catalytic function in enhanced photocatalytic performance. An energy level diagram shows the positions of the bands of TiO2 and CuO, as presented in Fig. 6f. Absorption of light greater than the band-gap energy of TiO2 generates electrons and holes in the CB and VB respectively. The sacrificial electron donor consumes the holes in the VB, thus accelerating the electron/holes separation in TiO2. The CB position of CuO below the CB of TiO2 permits the transfer of electrons from the CB of TiO2 to that of CuO, which is served as the reduction sites for capturing the protons to produce H2.[5, 13, 16]
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Fig. 6. (a-b) SEM images of hierarchical TiO2 NSs/TiO2 NFs structure. (c) XRD and (d) EDX spectra of TiO2 NSs/TiO2 NFs. (e) H2 amount over the irradiation time for TiO2 NSs/CuO NWs and TiO2 NSs/TiO2 NFs. (f) Schematic diagram of the photocatalytic H2 generation over the TiO2/CuO heterojunctions.
4. Conclusion In summary, TiO2 NSs/CuO NWs hierarchical nanostructures have been successfully fabricated by a solvothermal method. CuO NWs in the heterostructure not only played the role of backbones but also served as an effective co-catalyst. The as-prepared TiO2 NSs/CuO NWs composites have been explored for hydrogen production from photocatalytic water splitting. Obvious enhancement in the photocatalytic performance of the heterostructure was demonstrated in H2 production, compared to the pure TiO2 hierarchical nanostructures. The optimization of shell thickness shows that TiO2 NSs/CuO NWs composite with ca. 100 nm thickness of TiO2 NSs exhibits the highest photocatalytic H2 generation rate, which is 9 times higher than that of bare TiO2. Such approach towards the synthesis of hierarchical nanostructures may offer an avenue for designing and fabricating various complicated nanostructures and nanocomposites for applications in environment and energy fields. Acknowledgements This work is supported by the Ministry of Education Singapore, R-263-000-B38-112/R-263-000-B63-112 and the National Research Foundation Singapore, Ministry of National Development, R-263-000-C22-277.
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