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Thermally Stimulated Light Reflection and Photoluminescence of BaTiO3 Takumi Watanabe, Daiki Hoshi, Masaya Ishida, and Tomonori Ohba Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01615 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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Thermally Stimulated Light Reflection and Photoluminescence of BaTiO3 Takumi Watanabe, Daiki Hoshi, Masaya, Ishida, and Tomonori Ohba* Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan

ABSTRACT

Perovskites have been attracting attention because of their considerable luminescence properties. A conventional perovskite such as BaTiO3 has no intrinsic photoluminescence. Doping with rare metals, nanocrystallization, and addition of organometallic halides induce significant photoluminescence and photovoltages. Here, we report anomalous light reflection and photoluminescence of BaTiO3 on heating. Light absorption shifted from the near ultraviolet region to the visible region on heating. The small emission peaks at around 400–500 nm disappeared and new peaks appeared above 800 nm; the quantum yields of these peaks were less than 1% and more than 7%, respectively.

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1. Introduction Perovskites are good candidates for light-emitting diodes, luminescent solar cells, and photocatalysts.1-6 Conventional perovskites such as BaTiO3 do not have good photoluminescence properties because of their wide band gaps, and broad photoluminescence spectra are observed. However, the carrier mobility can be improved by the introduction of dopants and/or defects.4, 7-8 There is a strong relationship between structural and optical properties. Crystal structures are significantly changed by phase transitions, which are determined by temperature and pressure. Wang and co-workers reported pressure-dependent photoluminescence of organometallic halide perovskites; a red shift of the photoluminescence peak was observed with increasing pressure, and the peak intensity decreased in response to pressure.9 However, the change in the photoluminescence peak was gradual despite the structural transformation. Doping with Bi causes temperature-dependent light emission by changing the energy transfer efficiency.10 This is the result of an increase in the crystal field by thermal expansion. Xia and co-workers reported that an increase in temperature let to a blue shift with broadening bandwidth caused by thermally active phonon-assisted excitation of Eu dopants from a low energy level to a high energy sublevel in the excited state.11 BaTiO3 is a conventional perovskite with ferroelectric properties and a high dielectric constant. BaTiO3 has five different structures, i.e., rhombohedral, orthorhombic, tetragonal, cubic, and hexagonal, in order of increasing temperature. The tetragonal phase at 290–390 K is important because of its ferroelectric and optical properties. Doping is an effective way of increasing the luminescence performance.12-14 Nanocrystalline BaTiO3 synthesized at a low calcination temperature or for a shorter calcination time shows better photoluminescence.15-16 Structural deficiencies and decreased band gaps strengthen the photoluminescence spectra.17-18


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BaTiO3 without dopants and deficiencies can also become luminescent under irradiation with Xrays or gamma-rays, because of creation of vacancies by irradiation.19 Fasasi and co-workers reported that luminescence peaks appeared at around 500 K under gamma-ray irradiation and increased monotonically with increasing irradiation power.20 Highly crystalline BaTiO3 rarely has photoluminescent property at room temperature, but non-crystalline and/or disordered BaTiO3 show photoluminescence at around 550 nm under excitation at 488 nm.21 To the best of our knowledge, temperature-dependent light emission and photoluminescence of crystalline perovskites without dopants and deficiencies have not been previously observed. Here, we report temperature-dependent light reflection and photoluminescence properties of BaTiO3 as well as its crystal structures.

2. Experimental BaTiO3 crystals were synthesized from BaCO3 and TiO2 (purity >98%, Wako Pure Chemical Industries, Ltd., Osaka, Japan). The raw materials were mixed in the stoichiometric proportions needed for BaTiO3, and the mixture was heated at 1073 K for 24 h and then at 1473 K for 48 h after palletization pressing at 60 MPa for 0.5 h. Cl species as an impurity were released by calcination during its synthesis. The BaTiO3 crystal structures were examined by X-ray diffraction (XRD; SmartLab, Rigaku Co., Tokyo, Japan) at 40 kV and 30 mA with Cu Kα radiation at 300–700 K. Raman spectroscopy with a Nd:YAG laser (NRS-3000, JASCO Co., Tokyo, Japan) at 300–700 K was used to evaluate their fluorescence properties. Energy dispersive spectroscopy was measured to evaluate chemical species at 15 kV (JSM-6510A, JEOL Co., Tokyo, Japan). UV-vis reflection spectra of the synthesized BaTiO3 crystals were


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recorded at 300–700 K with a microscopic spectrophotometer (MSV-370, JASCO Co., Tokyo, Japan). Photoluminescence spectra were recorded with a Quantaurus-QY instrument (Hamamatsu Photonics K. K., Shizuoka, Japan).

3. Results and Discussion The XRD patterns of BaTiO3 shown in Figure 1a contain the typical peaks of BaTiO3 at 22.2°, 31.5°, 38.9°, and 45.0–45.4°, corresponding to the (100), (101)/(110), (111), and (002)/(200) faces, respectively. The XRD peaks at 36° and 43° are attributed to the CuO (111) and Cu (111) peaks, respectively, because a Cu sheet was placed under the BaTiO3 as a heat conductor. The lattice parameters were determined by refining the diffraction patterns through the Rietveld method using Rietan-fp, as shown in Figure S1.22 The lattice parameters of the BaTiO3 with space group P4mm were as follows: a = b = 0.399 nm, c = 0.403 nm, and α = β = γ = 90°. The peaks and lattice parameters were perfectly coincident with the referenced data (JCPDS Card #50626) and no other phases were observed, proposing highly crystalline structure.23 The energy dispersive spectroscopy of the synthesized BaTiO3 perfectly coincided with the referenced BaTiO3 crystal (> 99.9%, Kojundo Chemical Laboratory, Saitama, Japan) in Figure S2, proposing that the synthesized BaTIO3 crystal had the same chemical structure to the referenced BaTiO3 crystal and no metal impurities, although the tiny amount of metal impurities beyond lower detection limit was undetectable. Thermodynamic hysteresis of phase transitions was not observed (Figure S3a). The two split peaks at 31.5° and 45.0–45.4°, which are attributed to the tetragonal phase, combined above 400 K because of a phase transition to the cubic phase, as shown in Figure S4. The ab-axes in the unit cell shortened at 400 K, but the c-axis was


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maintained, as shown by the broadened diffraction angles for the (110) and (200) faces, and maintenance of the diffraction angles for the (101) and (002) faces. In addition, the unit cell expanded above 400 K, as shown by the narrowed diffraction angles, as expected.24 The changes with temperature in the Raman spectra (Figure 1b) were the same as those previously reported.25 A broad scattering peak at 260 cm−1 from the A1 mode, sharp scattering peak at 305 cm−1 from the B1 mode, and broad scattering peak at 520 cm−1 from the E and A1 modes were seen below 500 K. The sharp scattering peak disappeared above 500 K and the other broad peaks broadened further. These peak changes are attributed to phase transitions, and their intensities increased in cyclic measurements of Raman scattering (Figure S3b). The XRD patterns and Raman scatterings showed an obvious structural transformation at 400–500 K, which have been reported elsewhere.25-26


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Figure 1. XRD patterns (a) and Raman spectra (b) of BaTiO3 at 300–700 K.

Figure 2 shows optical images of BaTiO3 at 300–700 K. BaTiO3 was white at 300 K, but turned yellow during heating, indicating absorption of blue light on heating. The optical images changed little below 450 K, although the XRD patterns and Raman spectra showed a phase transition from tetragonal to cubic at around 400 K. Cubic-phase BaTiO3 apparently changed color to yellow above 450 K on heating, despite only expansion of its unit cell without a phase transition, suggesting that a change in the band gap was caused by expansion.


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Figure 2. Optical images of BaTiO3 pellets at 300 (a), 350 (b), 400 (c), 450 (d), 500 (e), 600 (f), and 700 K (g).

Figure 3a shows that the reflectance of BaTiO3 was weak below 380 nm, abruptly increased at 400 nm, and achieved a value of 10%, although the reflectance ratio strongly depended on the surface roughness. The reflectance clearly shifted toward longer wavelength with increasing temperature; this suggests absorption of blue light, as expected from the optical images. Figure 3b shows a change in the reflectance between 380 and 500 nm. The reflectance ratio at 400 nm showed the largest decrease as a result of heating. The cubic phase was unchanged above 450 K, while the crystal was expanded continuously. The fermi level was increased by asymmetric structure of BaTiO3 from the crystalline structure and the band gap was decreased by the upward movement of valence bands.21 The expansion and/or thermal fluctuation of crystalline structure might induce the band gap decrease. The reflectance curves at a given temperature for heating and cooling processes changed little, but the reflectance intensities increased slightly in the cooling process (Figure S5). This suggested that BaTiO3 gained luminescence properties by thermal stimulation, because the reflectance was first measured at 300 to 700 K and then at 700 to 300 K. Therefore, if the BaTiO3 crystals gained


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photoluminescence properties, the shift in the reflectance therefore means a shift in the photoluminescence peak, because Maneeshya and co-workers reported that the red peak shift in the UV/vis transmittance spectra is directly related to the red shift of the photoluminescence peak.27 This suggests that heating induces BaTiO3 photoluminescence; photoluminescence of undoped BaTiO3 at ambient temperature has not yet been reported, although noncrystalline BaTiO3 and nanocrystalline BaTiO3 had photoluminescence performance.21, 28

Figure 3. (a) Reflectance spectra of BaTiO3 at 300 (blue), 350 (green), 400 (yellow), 450 (orange), 500 (red), 600 (purple), and 700 K (black) and (b) temperature dependence of reflectance at 380–500 nm.


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The photoluminescence spectra in Figure S6 show a small broad peak at around 500 nm, sharp peaks at 540 and 610 nm, and a large broad peak above 800 nm. Emission peaks in the visible-light region were observed, particularly at 300 K, and disappeared with increasing temperature. Those small peaks were attributed to non-crystalline or disordered BaTiO3, indicating its emission from BaTiO3 interfaces, because its interface is rough even for BaTiO3 crystals.21 The emission above 800 nm abruptly increased at 700 K. The quantum yields were calculated from the ratios of the emission and excitation peaks in the photoluminescence spectra, as shown in Figure 4. Here, the emission peak areas at around 900 nm were not accurately determined because of instrumental limitations. The quantum yields for the emission peaks at around 500, 540, and 610 nm were less than 1%, and decreased clearly with increasing temperature, as mentioned above. The quantum yields for the emission peaks at around 900 nm surprisingly increased at 700 K and achieved values greater than 7% without thermal radiation. This suggests that the temperature-induced shift in the absorption peak of BaTiO3 caused a significant emission at around 900 nm at 700 K, although no photoluminescence has previously been reported without doping and/or disordering.


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Figure 4. Temperature dependence of quantum yields evaluated from emission peaks at around 500 (a), 540 (b), 610 (c), and 900 nm (d) by excitation at wavelengths of 300 (purple), 320 (blue), 340 (green), 360 (yellow), 380 (orange), and 400 nm (red).

4. Conclusion In this study, we investigated the light reflectance and photoluminescence properties of BaTiO3 at 300–700 K. The apparent reflectance above 400 nm shifted to longer wavelength on heating and the phase transition between tetragonal and cubic had little effect on the reflection. The small emission peaks from tetragonal-phase BaTiO3 in the visible region were weakened by a transition to the cubic phase, and anomalous emissions in the long-wavelength visible-light


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region appeared at 700 K. To the best of our knowledge, this is the first report of these unique thermal reflectance and emission properties for highly crystalline BaTiO3 perovskites without dopants. Further studies on the thermally stimulated mechanisms of reflectance and photoluminescence by evaluation of density of state at various temperature are necessary to understand its mechanism and improve the photoluminescence performance.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. The following files are available free of charge. Energy dispersive spectra, XRD, Raman spectra, UV-vis reflection, and photoluminescence spectra (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] (T.O.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT Energy dispersive spectroscopy, UV-vis, and photoluminescence measurements were conducted at the Center for Analytical Instrumentation, Chiba University. This research was supported by the Japan Society for the Promotion of Science KAKENHI (Grant Number 15K12261), the ESPEC Foundation for Global Environment Research and Technology, Sumitomo Foundation, and Shimadzu foundation. REFERENCES (1) Zhang, X.; Wang, W.; Xu, B.; Liu, S.; Dai, H.; Bian, D.; Chen, S.; Wang, K.; Sun, X. W. Thin Film Perovskite Light-Emitting Diode Based on Cspbbr 3 Powders and Interfacial Engineering. Nano Energy 2017, 37, 40-45. (2) Zhou, J.; Hu, Z.; Zhang, L.; Zhu, Y. Perovskite Cspbbr1.2i1.8 Quantum Dot Alloying for Application in White Light-Emitting Diodes with Excellent Color Rendering Index. J. Alloys Compd. 2017, 708, 517-523. (3) Ye, S.; Yu, M.; Yan, W.; Song, J.; Qu, J. Enhanced Photoluminescence of Cspbbr3@Ag Hybrid Perovskite Quantum Dots. J. Mater. Chem. C 2017, 5, 8187-8193. (4) Zhao, H.; Zhou, Y.; Benetti, D.; Ma, D.; Rosei, F. Perovskite Quantum Dots Integrated in Large-Area Luminescent Solar Concentrators. Nano Energy 2017, 37, 214-223. (5) Hou, J.; Cao, S.; Wu, Y.; Liang, F.; Ye, L.; Lin, Z.; Sun, L. Perovskite-Based Nanocubes with Simultaneously Improved Visible-Light Absorption and Charge Separation Enabling Efficient Photocatalytic CO2 Reduction. Nano Energy 2016, 30, 59-68. (6) Do, J. Y.; Im, Y.; Kwak, B. S.; Park, S.-M.; Kang, M. Preparation of Basalt Fiber@Perovskite Pbtio3 Core–Shell Composites and Their Effects on CH4 Production from CO2 Photoreduction. Ceram. Int. 2016, 42, 5942-5951. (7) Yamada, Y.; Kanemitsu, Y. Photoluminescence Spectra of Perovskite Oxide Semiconductors. J. Lumin. 2013, 133, 30-34. (8) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316. (9) Wang, L.; Wang, K.; Zou, B. Pressure-Induced Structural and Optical Properties of Organometal Halide Perovskite-Based Formamidinium Lead Bromide. J. Phys. Chem. Lett. 2016, 7, 2556-2562. (10) Zhou, H.; Wang, Q.; Jin, Y. Temperature Dependence of Energy Transfer in Tunable White Light-Emitting Phosphor Bay2si3o10:Bi3+,Eu3+ for near Uv Leds. J. Mater. Chem. C 2015, 3, 11151-11162. (11) Xia, Z.; Wang, X.; Wang, Y.; Liao, L.; Jing, X. Synthesis, Structure, and Thermally Stable Luminescence of Eu2+-Doped Ba2ln(Bo3)2cl (Ln = Y, Gd and Lu) Host Compounds. Inorg. Chem. 2011, 50, 10134-10142.


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(12) Verma, K. C.; Kotnala, R. K. Lattice Defects Induce Multiferroic Responses in Ce, LaSubstituted Bafe Ti O Nanostructures. J. Am. Ceram. Soc. 2016, 99, 1601-1608. (13) Rabuffetti, F. A.; Culver, S. P.; Lee, J. S.; Brutchey, R. L. Local Structural Investigation of Eu(3 )-Doped BaTiO3 Nanocrystals. Nanoscale 2014, 6, 2909-2914. (14) Hao, J.; Zhang, Y.; Wei, X. Electric-Induced Enhancement and Modulation of Upconversion Photoluminescence in Epitaxial BaTiO3:Yb/Er Thin Films. Angew. Chem. Int. Ed. 2011, 50, 6876-6880. (15) Li, H.; Huang, S.; Zhang, W.; Pan, W. Visible Photoluminescence from Amorphous Barium Titanate Nanofibers. J. Alloys Compd. 2013, 551, 131-135. (16) Zhan, H.; Jiang, X.; Zhu, M.; Li, X.; Luo, Z.; Shu, K. Photoluminescence Activity of Batio 3 Nanocrystals Dependence on the Structural Evolution. J. Cryst. Growth 2016, 433, 8085. (17) Zhang, M.-S.; Yin, Z.; Chen, Q.; Zhang, W. F.; Chen, W. C. Study of Structural and Photoluminescent Properties in Barium Titanate Nanocrystals Synthesized by Hydrothermal Process. Solid State Commun. 2001, 119, 659-663. (18) Moreira, M. L.; Gurgel, M. F. C.; Mambrini, G. P.; Leite, E. R.; Pizani, P. S. Photoluminescence of Barium Titanate and Barium Zirconate in Multilayer Disordered Thin Films at Room Temperature. J. Phys. Chem. A 2008, 112, 8938-8942. (19) Nag Bhargavi, G.; Khare, A. Luminescence Studies of Perovskite Structured Titanates: A Review. Opt. Spectrosc. 2015, 118, 902-917. (20) Fasasi, A. Y.; Balogun, F. A.; Fasasi, M. K.; Ogunleye, P. O.; Mokobia, C. E.; Inyang, E. P. Thermoluminescence Properties of Barium Titanate Prepared by Solid-State Reaction. Sens. Actuator A-Phys. 2007, 135, 598-604. (21) Orhan, E.; Varela, J. A.; Zenatti, A.; Gurgel, M. F. C.; Pontes, F. M. Room-Temperature Photoluminescence of BaTiO3: Joint Experimental and Theoretical Study. Phys. Rev. B 2005, 71. (22) Izumi, F.; Momma, K. Three-Dimensional Visualization in Powder Diffraction. Solid State Phenom. 2007, 130, 15-20. (23) Busca, G.; Buscaglia, V.; Leoni, M.; Nanni, P. Solid-State and Surface Spectroscopic Characterization of BaTiO3 Fine Powders. Chem. Mater. 1994, 6, 955-961. (24) Bolotina, N. B.; Pinkerton, A. A. Temperature Dependence of Thermal Expansion Tensors of Energetic Materials. J. Appl. Crystallogr. 2015, 48, 1364-1380. (25) Hayashi, H.; Nakamura, T.; Ebina, T. In-Situ Raman Spectroscopy of BaTiO3 Particles for Tetragonal–Cubic Transformation. J. Phys. Chem. Solids 2013, 74, 957-962. (26) Smith, M. B.; Page, K.; Siegrist, T.; Redmond, P. L.; Walter, E. C.; Seshadri, R.; Brus, L. E.; Steigerwald, M. L. Crystal Structure and the Paraelectric-to-Ferroelectric Phase Transition of Nanoscale BaTiO3. J. Am. Chem. Soc. 2008, 130, 6955-6963. (27) Maneeshya, L. V.; Thomas, P. V.; Joy, K. Effects of Annealing Temperature on the Photoluminescence of Rf Sputtered Barium Titanate Thin Films. Mater. Sci. Semicond. Process. 2015, 30, 688-693. (28) Moreira, M. L.; Mambrini, G. P.; Volanti, D. P.; Leite, E. R.; Orlandi, M. O.; Pizani, P. S.; Mastelaro, V. R.; Paiva-Santos, C. O.; Longo, E.; Varela, J. A. Hydrothermal Microwave: A New Route to Obtain Photoluminescent Crystalline BaTiO3 Nanoparticles. Chem. Mater. 2008, 20, 5381-5387.


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