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Letter
Visible-Light-Driven Photocatalytic Hydrogen Generation on Nanosized TiO-II Stabilized by High-Pressure Torsion 2
Hadi Razavi-Khosroshahi, Kaveh Edalati, Masashige Hirayama, Hoda Emami, Makoto Arita, Miho Yamauchi, Hidehisa Hagiwara, Shintaro Ida, Tatsumi Ishihara, Etsuo Akiba, Zenji Horita, and Masayoshi Fuji ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01482 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on July 1, 2016
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ACS Catalysis
Visible-Light-Driven Photocatalytic Hydrogen Generation on Nanosized TiO2-II Stabilized by High-Pressure Torsion Hadi Razavi-Khosroshahi1,*, Kaveh Edalati2,3, Masashige Hirayama4, Hoda Emami2, Makoto Arita3, Miho Yamauchi2,4, Hidehisa Hagiwara2,5, Shintaro Ida2,5, Tatsumi Ishihara2,5, Etsuo Akiba2,6, Zenji Horita2,3 and Masayoshi Fuji1 1
Advanced Ceramics Research Center, Nagoya Institute of Technology, Gifu, Japan
2
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka, Japan
3
Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka, Japan
4
Department of Chemistry, Faculty of Sciences, Kyushu University, Fukuoka, Japan
5
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka, Japan
6
Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, Fukuoka, Japan
Supporting Information Placeholder ABSTRACT: Photocatalytic activity of pure TiO2 is limited to ultraviolet (UV) light due to wide bandgap of anatase and rutile phases. The bandgap of high-pressure phases of TiO2 can theoretically coincide with the visible light, but these phases are unstable at ambient pressure. In this work, the high-pressure TiO2-II (columbite) phase with large fractions of oxygen vacancies was stabilized by inducing plastic strain to anatase under 6 GPa. The material could absorb visible light as a consequence of bandgap narrowing by ~0.7 eV. Formation of nanosized TiO2-II enhanced the hydrogen generation efficiency under visible light, and the efficiency improved after removing the oxygen vacancies by annealing.
leads to increase in the light absorption, these methods do not necessarily result in photocatalytic activity under visible 9-10 light due to the defect-induced recombination losses. Narrowing the bandgap of pure TiO2 using dopant-free ap11-13 proach has been in the spotlight in recent years. Theoretical studies suggested that high-pressure phases of TiO2 have 14-16 low bandgaps which can coincide with the visible light. For example, TiO2-II (columbite) phase with the orthorhombic structure may have a bandgap as narrow as 2.59 17 eV. An anatase/TiO2-II mixture can also theoretically exhib18 it a bandgap lower than anatase. However, TiO2-II forms at pressures higher than 2 GPa and is unstable at ambient pres19 sure (see phase diagram of TiO2 in Figure 1a). 20,21
Keywords: Titanium Oxide (Titania); TiO2-II (Columbite); High-Pressure Torsion (HPT); Severe Plastic Deformation (SPD); Hydrogen Generation; Visible Light Active Photocatalyst Titanium dioxide (TiO2) is a semiconductor with excellent 1 2 photocatalytic and photovoltaic properties. These optical properties are strongly influenced by crystal structure and lattice defects. TiO2 has three main phases under ambient pressure: rutile and anatase with the tetragonal structures (bandgap: 3.0-3.2 eV); and brookite with the orthorhombic 3 structure (bandgap: 3.3 eV). Among the TiO2 polymorphs, anatase is the best photocatalyst because of its large electron effective mass, which leads to a low mobility of charge carri3 ers. Despite its good photocatalytic features, the application of pure anatase has been limited to the UV range of sunlight due to its wide indirect bandgap. To enhance solar energy absorption, an ideal photocatalyst should have a bandgap of 4 5 6 ~2 eV. Ion implantation , metal loading, and doping with 7,8 cations or anions are several approaches to reduce the bandgap. Although bandgap narrowing by these methods
High-pressure torsion (HPT) method, as schematically shown in Figure 1b, is an effective technique for stabilizing high-pressure phases at ambient pressure. In this method, high pressure and large plastic strain are simultaneously applied to a sample between two rotating anvils. The large plastic strain and resultant formation of lattice defects (vacancies, dislocations and grain boundaries) change the thermody20-24 namic stability of phases at ambient condition. Earlier reports showed that high-pressure phases could be stabilized at ambient pressure in several materials: orthorhombic phase 20 22 (black phosphorous) in P, diamond-like phases in C, ω 23 24 phase in Zr and Ti, cubic phase in BaTiO3, and so on. Inspired by earlier studies on the effectiveness of HPT method in stabilizing high-pressure phases at ambient pres20-24 sure, the HPT method is employed in this study for stabilizing TiO2-II. The effect of TiO2-II formation on the bandgap and photocatalytic activity of TiO2 is experimentally studied for the first time. It is found that the stabilization of TiO2-II phase is effective to reduce the bandgap and enhance the hydrogen generation under the visible light. The HPT process was conducted at room temperature on pure (99.8%) anatase powder with an average particle size of
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The results of thermal annealing, as summarized in Figure 2, indicate several important points. First, the fraction of TiO2-II, as shown in Figure 2a, decreases with increasing the
~150 nm. Plastic strain was introduced under 6 GPa by rotating the lower HPT anvil with respect to the upper anvil for 4 turns. The HPT-processed materials had a disc shape with 10 mm diameter and 0.8 mm thickness. To reduce the fraction of lattice defects, the HPT-processed discs were further annealed at 300-1100 °C for 1 h with the heating/cooling rates of 10 °C/min. Phase transformations were investigated by X-ray diffraction (XRD) using the CuKα radiation and by Raman spectroscopy with a 532 nm laser. Defect nature was evaluated by examining the color of samples, differential scanning calorimetry (DSC), photoluminescence (PL) spectroscopy with excitation wavelength of 350 nm, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) with 10 GHz microwave at liquid nitrogen, electron energy loss spectroscopy (EELS), and Fourier transform infrared (FTIR) spectroscopy. Microstructures were examined by transmission electron microscopy. UV-Vis diffuse reflectance spectroscopy was used to estimate the bandgap. Photocatalytic hydrogen evolution was examined in a solution of 500 g 3 3 3 TiO2 sample, 25 cm CH3OH, 225 cm H2O and 1.325 cm H2PtCl6.6H2O (19.5 mM) under UV light using a Xenon lamp and under visible light using the Xenon light equipped with a cold mirror and a 420 nm cutoff filter. The active surface area of the samples was measured using pulsed nuclear magnetic resonance (NMR) method (see the supporting information).
Figure 1. (a) Phase diagram of TiO2; (b) Schematic illustration of HPT; and (c) Raman spectra of TiO2 before/after HPT. Raman spectra, as shown in Figure 1c, reveal that TiO2-II forms after HPT processing under 6 GPa and remains stable at ambient pressure. Figure S1 confirms that grains after HPT processing are nanosized. The color of sample after HPT processing changed from white to dark green, as shown in 25 Figure 1c, indicating that oxygen vacancies are formed. The formation of oxygen vacancies is consistent with earlier reports on the formation of vacancies in HPT-processed ox20,24 26 ides and metals. It was suggested that vacancies are formed during HPT due to the effect of strain on accumulation of lattice defects, while they can remain metastable because the vacancy migration energy increases under high 27 hydrostatic pressure. Although oxygen vacancies on the surface have a positive effect on bandgap narrowing of 13,28 TiO2, oxygen vacancies in the bulk usually act as recombination centers for electrons and holes and reduce the pho29 tocatalytic efficiency. In order to eliminate the oxygen vacancies and study the effect of TiO2-II on photocatalytic activity, the HPT-processed samples were annealed in the air.
Figure 2. (a) Phase fractions against annealing temperature; (b) Heat flow in DSC against temperature; (c) PL spectra; and (d) EPR spectra for different TiO2 samples. temperature, but it remains as high as 35 wt.% even after annealing at 500 °C. Second, after annealing, the color of HPT-processed disc changes to yellow at 300 °C and to white at 500 °C. The change of color to white indicates that the 25 oxygen vacancies were annihilated. Third, an exothermic peak corresponding to oxygen vacancy annihilation with a
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ACS Catalysis annealing at 500 °C (with TiO2-II but without oxygen vacancies). In all samples, below a certain wavelength, known as absorption edge, an intense absorption occurs. It is obvious that the absorption edge of initial powders is ~400 nm, while it shifts to the visible light region and reaches ~470 nm after HPT processing. An appreciable tail absorbance at wavelengths higher than 500 nm is also observed in the HPTprocessed sample. The tail absorbance is due to the existence of oxygen vacancies which form localized states in the 35-38 After annealing, the tail absorbance disappears bandgap. because of annihilation of oxygen vacancies. The blue shift of absorption edge after annealing should be due to partial 36 phase transformation from TiO2-II to anatase.
total energy of 170 J/mol appears at ~300 °C in DSC curve of Figure 2b. Since the formation energy for one oxygen vacancy 30 in TiO2 is in the range of 4.05-4.89 eV, the fraction of oxygen vacancies in the HPT-processed TiO2 should be at least 0.01 atom.%. Fourth, in PL spectra of Figure 2c, a peak at 430 nm with a distinct shoulder at 470 nm appears. The shoulder 31 is indicative of shallow traps on oxygen vacancies. PL intensity increases after HPT processing and decreases after annealing, proving that vacancies form after HPT processing 32 and annihilate during annealing. Since PL intensity corresponds to radiative recombination of conduction band electrons and valence band holes, lower PL intensity can mean 32 more carriers are available for photocatalytic reactions. Fifth, as shown in Figure 2d, EPR signals with g values of 2.004 and 1.989 corresponding to oxygen vacancies and par3+ 33,34 amagnetic Ti appear after HPT processing. , while these signals diminish after annealing. No appreciable signal with g 3+ = 1.930 (corresponding to Ti on surface) or g = 1.93 (corresponding to Ti interstitials) are detected in the EPR spectra, 3+ indicating that Ti is mainly formed in the bulk. Formation of oxygen vacancies as point defects after HPT processing, and their annihilation after annealing is also evident from the XRD and Raman peak shifts as in Figures S2(b) and S3(b), from the XPS peak broadenings as in Figure S4, from the Ti-L signal splitting in the EELS results as in Figure S5, and from the light absorbance in the FTIR spectra as in Figure S6. Evolution of crystallite size (representing grain boundaries as planar defects) and lattice strain (representing dislocations as linear defects) are summarized in Table S1.
Bandgap of samples are estimated by Kubelka-Munk theo39 ry and summarized in Figure 3b. The estimated bandgaps are 3.1, 2.4, and 2.7 eV for samples before HPT processing, after HPT processing and after annealing, respectively. The estimated bandgap for the initial powders is consistent with 3 the reported bandgap of anatase. The bandgap after HPT 15,17 processing is close to the calculated bandgap of TiO2-II. The decrease in the bandgap after annealing is due to a decrease in the fractions of TiO2-II and oxygen vacancies. A comparison between the bandgaps of initial powders and that of the annealed sample suggests that the formation of TiO2-II is effective for narrowing the bandgap.
Figure 4. Photocatalytic hydrogen generation under (a) UV light and (b) visible light for different TiO2 samples. Photocatalytic hydrogen generation by water splitting was examined as a measure of photocatalytic activity of TiO2. Since HPT processing causes consolidation of ceramic pow20,40 ders, the samples were crushed and their active surface area was measured by pulsed NMR. The surface areas were 2 4.0, 2.5, and 1.9 m /g for samples before HPT processing, after HPT processing, and after annealing, respectively (see Table S2). It should be noted that the active sites for water
Figure 3. (a) UV-Vis diffused reflectance spectra, and (b) bandgap estimation for different TiO2 samples (α: absorption coefficient: h: Planck’s constant, ν: light frequency). Figure 3a shows the UV-Vis diffused reflectance spectra of samples before HPT processing (without TiO2-II), after HPT processing (with TiO2-II and oxygen vacancies) and after
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splitting are located in regions where H2PtCl6.6H2O is decomposed and 1 wt.% Pt deposited on the surface (see Figures S7). Photocatalytic hydrogen generation as a function of time under UV and visible lights are shown in Figures 4a and 4b, respectively. As can be seen, hydrogen generation rate under UV light is slower for the HPT-processed sample comparing with that for the anatase powder. However, the hydrogen generation rate is faster for the HPT-processed sample when illuminated by visible light. Hydrogen generation rate both under UV and visible lights significantly improves when the HPT-processed sample is annealed at 500 °C. It should be noted that TiO2-II was stable after photocatalytic tests, as shown in Figure S8.
5. 6. 7. 8. 9. 10. 11. 12. 13.
The improvement of hydrogen generation under visible light in this study is a consequence of bandgap narrowing due to the formation of TiO2-II. Although the annealed sample exhibits less visible light absorption as compared with the HPT-processed sample, the best photocatalytic efficiency was achieved for the annealed sample. XRD, Raman, XPS, and EPR results confirmed the presence of oxygen vacancies both in the bulk and on the surface of sample after HPT. Although oxygen vacancies on the surface of TiO2 can act as shallow traps for charge carriers and prolong the life time of electron39,41,42 hole pairs, which leads to better photoactivity, oxygen vacancies in the bulk usually act as recombination centers and decrease the lifetime of charge carries and deteriorate 29,43 the photoactivity. After annealing, oxygen vacancies eliminate both from surface and bulk, but the sample still absorbs a range of visible light because of the presence of TiO2-II, which has a narrower bandgap than anatase.
14. 15. 16. 17. 18. 19. 20. 21. 22.
In summary, the high-pressure TiO2-II phase was successfully stabilized at ambient pressure by HPT processing. The bandgap was narrowed down and the resultant material was able to split the water and generate hydrogen using a photocatalytic reaction under visible light.
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AUTHOR INFORMATION
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Corresponding Author
*
[email protected] 27.
Notes
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The authors declare no competing financial interests.
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ASSOCIATED CONTENT
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Supporting Information
This supporting information is available free of charge via the Internet at http://pubs.acs.org. Experimental details, Figures S1-S8, Table S1, Table S2
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ACKNOWLEDGMENT
33.
This work was supported in part by the ALCA, Japan, in part by CREST, JST, Japan, and in part by the MEXT, Japan (No. 26220909 and No. 15K14183). The HPT was conducted in the IRC-GSAM center at Kyushu University.
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Figure 1. (a) Phase diagram of TiO2; (b) Schematic illustration of HPT; and (c) Raman spectra of TiO2 before/after HPT. 393x321mm (96 x 96 DPI)
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Figure 2. (a) Phase fractions against annealing temperature; (b) Heat flow in DSC against temperature; (c) PL spectra; and (d) EPR spectra for different TiO2 samples. 342x900mm (96 x 96 DPI)
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Figure 3. (a) UV-Vis diffused reflectance spectra, and (b) bandgap estimation for different TiO2 samples (α: absorption coefficient: h: Planck’s constant, ν: light frequency). 352x497mm (96 x 96 DPI)
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Figure 4. Photocatalytic hydrogen generation under (a) UV light and (b) visible light for different TiO2 samples. 351x511mm (96 x 96 DPI)
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