Research Note pubs.acs.org/IECR
Temperature-Induced Irreversible Change of ZnO Optical Energy Gap Yan Huo and Yun Hang Hu* Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931-1295, United States ABSTRACT: It is generally recognized that the change of ZnO optical properties due to high-temperature treatment is reversible. However, herein, it is reported that high-temperature treatment in air caused an irreversible decrease in ZnO optical energy gap (Eg). The irreversible decrease in Eg was enhanced with increasing treatment temperature. Furthermore, it was found that the irreversible decrease in Eg can be attributed to the enlargement of ZnO crystal particles.
1. INTRODUCTION ZnO, which is a semiconductor with a broad direct band gap and a relatively high exciton binding energy, can be applied for room-temperature ultraviolet (UV) lasers, light-emitting diodes, sensors, solar cells, and photocatalysis.1−6 Furthermore, transition metal-doped ZnO was predicted as a potential candidate in spintronics.7 In addition, piezoelectric zinc oxide (ZnO) nanowire arrays could be employed to convert nanoscale mechanical energy into electric energy.8 It is generally recognized that temperature-induced optical absorption change of crystalline ZnO is reversible. For example, ZnO shows thermochromic behavior, namely, its color changes from white to yellow when heated in air and reverts to white upon cooling again.9 The color change could be attributed to a very small loss of oxygen at high temperatures to form the nonstoichiometric Zn1+xO, where at 800 °C, x = 0.00007.9 Such a small change of ZnO composition can be recovered when it is cooled. However, in this letter, we reported that the optical energy gap of ZnO suffered irreversibly changes during hightemperature treatment in air.
3. RESULTS AND DISCUSSION To evaluate the effect of high-temperature treatment on ZnO energy gap, ZnO powder (purchased from Sigma−Aldrich) was treated at various temperatures (100−1100 °C) in air. Those treated samples were further subjected to UV−vis absorption measurements. As shown in Figure 1, one can see an absorption
2. EXPERIMENTS 2.1. High-Temperature Treatment of ZnO. Zinc oxide (ZnO, purity of 99.99%) was purchased from Sigma−Aldrich. It was treated in air for 4 h at a selected temperature (100−1100 °C), followed by quenching to room temperature. 2.2. UV−Visible Absorption. The ultraviolet−visible (UVvis) absorption spectra of ZnO powders with and without the high-temperature treatment were recorded, using a UV/vis spectrophotometer (Shimadzu, Model UV-2450) in the wavelength range of 200−800 nm at room temperature. Barium sulfate was used as a reference material. 2.3. Powder X-ray diffraction. Powder X-ray diffraction (XRD) was employed to determine the crystal structure change of ZnO that was due to the high-temperature treatment. XRD data were obtained at room temperature and atmospheric pressure, using a Scintag Model XDS2000 powder diffractometer at 45 kV and 35 mA for Cu Kα (λ = 1.5406 Å) radiation with a scan speed of 1° per minute and a range of 20°−80°.
Figure 1. Ultraviolet−visible (UV−vis) absorption spectra of ZnO treated at various temperatures in air.
© 2011 American Chemical Society
edge at 372−380 nm, which is the characteristic of ZnO semiconductor. The absorption is associated with an electronic transition, O2−Zn2+→O−Zn+ ligand-to-metal charge transfer (LMCT).10 However, after treatment at temperatures of >600 °C, the absorption edge shifts to lower energy. The shift was enhanced with increasing treatment temperature. Furthermore, for a direct band gap semiconductor, one can exploit the following relationship between the absorption coefficient (α) near the absorption edge and the excitation energy (hν) to calculate its optical energy gap (Eg) from corresponding UV− vis spectra:11 Received: Revised: Accepted: Published: 1083
September 2, 2011 December 23, 2011 December 23, 2011 December 24, 2011 dx.doi.org/10.1021/ie202001s | Ind. Eng.Chem. Res. 2012, 51, 1083−1085
Industrial & Engineering Chemistry Research
(αhν)2 = A(hν − Eg )
Research Note
ZnO was calculated using the Debye−Scherer equation:28
(1)
D=
where A is a parameter related to effective mass. The energy gaps, which were obtained with eq 1, are listed in Table 1. One
a b
Eg (eV)a
crystal size (nm)b
25 100 200 300 400 500 600 700 800 900 1000 1100
3.239 3.241 3.248 3.243 3.242 3.247 3.238 3.221 3.197 3.144 3.125 3.119
76.637 81.658 82.677 77.826 84.947 83.316 83.473 93.103 89.074 106.541 110.043 115.842
(2)
where λ is the X-ray wavelength, θ the Bragg angle, and β the full width at half-maximum (fwhm) of a diffraction peak. The (100), (002), and (101) diffraction peaks of ZnO were exploited for the calculations. From Table 1, one can see that the size of crystal particles increased when the treatment temperature was increased above 600 °C. Furthermore, the correlation of the ZnO optical energy gap with its crystal size provided a linear relationship (Figure 3). This indicates that the decrease of the optical energy gap can be attributed to the size
Table 1. Optical Energy Gaps and Crystal Sizes of ZnO Treated at Various Temperatures in Air treatment temperature (°C)
0.94λ β cos θ
Calculated from UV−vis absorption spectra (see Figure 1). Calculated from XRD patterns (see Figure 2).
can see that, after the treatment of ZnO at 600 °C or below, its optical energy (Eg) remained almost unchanged. However, after ZnO was treated in air at temperatures of >600 °C, its Eg value decreased. This indicates that the Eg value of ZnO suffered an irreversible decrease when it was treated at a high temperature. Furthermore, the decrease in Eg was enhanced with increasing treatment temperature. This irreversible Eg change of ZnO would be interesting, because it is generally recognized that change in the optical properties of ZnO due to hightemperature treatment is reversible.9 The energy gap (Eg) can be affected by various factors, including crystal defects12−15 and crystal sizes.16−27 It is well-known that high-temperature treatment can cause the enlargement of crystal particles. Therefore, it is reasonable for one to expect that the effect of high-temperature treatment on Eg might be due to the size increase of crystal particles. To confirm this, XRD patterns were obtained for ZnO treated at various temperatures (100−1100 °C). Figure 2 showed that all ZnO samples possess a hexagonal wurtzite structure. Furthermore, the average crystal size (D) of
Figure 3. Optical energy gaps of ZnO versus its crystal size.
increase of crystal particles. It is well-known that when the crystal size is as small as several nanometers, the energy gap increased as the size of the crystal particles decreased.29,30 However, the size dependence of the energy gap has not yet reported for crystal particles larger than 100 nm. The above results show that the size dependence of the energy gap still occurs, even when the crystal particle size is as large as 100 nm. From a molecular orbital point of view, two factors can determine the change of energy gaps with crystal size: the Coulombic interaction shifts the energy gap to lower values as 1/D, while the quantum localization shifts the energy gap to higher energy as 1/D2.20 Thus, the apparent band gap will always increase for sufficiently small crystal size (D), with which the change in energy gap is mainly determined by quantum localization. Indeed, the size dependence of the energy gap was revealed for ZnO nanoparticles smaller than 10 nm in size.29,30 In contrast, when the crystal size (D) is large, the energy gap should decrease, because the Coulombic interaction plays a critical role in energy gap change for the large crystal size (D). This can explain the size dependence of the energy gap for large ZnO particles (>100 nm in size).
4. CONCLUSION In conclusion, this work demonstrated that optical energy gap (Eg) of ZnO suffered an irreversible decrease when it was subjected to a high-temperature treatment in air. Furthermore, the irreversible decrease in Eg was enhanced with increasing treatment temperature. The irreversible change in Eg can be attributed to the enlargement of crystal particles.
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Figure 2. X-ray diffraction (XRD) patterns for ZnO: (a) original, and ZnO heated in air for 4 h at (b) 100, (c) 200, (d) 300, (e) 400, (f) 500, (g) 600, (h) 700, (i) 800, (j) 900, (k) 1000, and (l) 1100 °C.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 1084
dx.doi.org/10.1021/ie202001s | Ind. Eng.Chem. Res. 2012, 51, 1083−1085
Industrial & Engineering Chemistry Research
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Research Note
(22) Anpo, M.; Shima, T.; Kodama, S.; Kubukawa, Y. Photocatalytic hydrogenation of propyne with water on small-particle titania: Size quantization effects and reaction intermediates. J. Phys. Chem. 1987, 91, 4305. (23) Yoneyama, H.; Haga, S.; Yamanaka, S. Photocatalytic activities of microcrystalline titania incorporated in sheet silicates of clay. J. Phys. Chem. 1989, 93, 4833. (24) Hoener, C. F.; Allan, K. A.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. Demonstration of a shellcore structure in layered CdSe−ZnSe small particles by X-ray photoelectron and Auger spectroscopies. J. Phys. Chem. 1992, 96, 3812. (25) Liu, Z.; Davis, R. Investigation of the Structure of Microporous Ti-Si Mixed Oxides by X-ray, UV Reflectance, FT-Raman, and FT-IR Spectroscopies. J. Phys. Chem. 1994, 98, 1253. (26) Chestnoy, N.; Hull, R.; Brus, L. E. Higher excited electronic states in clusters of ZnSe, CdSe, and ZnS: Spin−orbit, vibronic, and relaxation phenomena. J. Chem. Phys. 1986, 85, 2237. (27) Davis, R. J. Synthesis and Characterization of VPI-5 Supported TiO2 Clusters. Chem. Mater. 1992, 4, 1410. (28) Cullity, B. D. Elements of X-ray Diffraction; Addison−Wesley: Reading, MA, 1978. (29) Zhang, L.; Yin, L.; Wang, C.; Lun, N.; Qi, Y.; Xiang, D. Origin of Visible Photoluminescence of ZnO Quantum Dots: Defect-Dependent and Size-Dependent. J. Phys. Chem. C 2010, 114, 9651. (30) Koch, U.; Fojtik, A.; Weller, H.; Henglein, A. Photochemistry of semiconductor colloids. Preparation of extremely small ZnO particles, fluorescence phenomena and size quantization effects. Chem. Phys. Lett. 1985, 122, 507.
ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation (No. NSF-CBET-0931587).
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REFERENCES
(1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Room-Temperature Ultraviolet Nanowire Nanolasers. Science 2001, 292, 1897. (2) Park, W. I.; Yi, G. C.; Kim, J. W.; Park, S. M. Schottky nanocontacts on ZnO nanorod arrays. Appl. Phys. Lett. 2003, 82, 4358. (3) Cao, H.; Xu, J. Y.; Zhang, D. Z.; Chang, S. H.; Ho, S. T.; Seelig, E. W.; Liu, X.; Chang, R. P. H. Spatial Confinement of Laser Light in Active Random Media. Phys. Rev. Lett. 2000, 84, 5584. (4) Bagnall, D. M.; Chen, Y. F.; Zhu, Z.; Yao, T.; Koyama, S.; Shen, M. Y.; Goto, T. Optically pumped lasing of ZnO at room temperature. Appl. Phys. Lett. 1997, 70, 2230. (5) Yu, P.; Tang, Z. K.; Wong, G. K. L.; Kawasaki, M.; Ohtomo, A.; Koinuma, H.; Segawa, Y. Room-temperature gain spectra and lasing in microcrystalline ZnO thin films. J. Cryst. Growth 1998, 184−185, 601. (6) Harbour, J. R.; Hair, M. L. Radical intermediates in the photosynthetic generation of H2O2 with aqueous ZnO dispersions. J. Phys. Chem. 1979, 83, 652. (7) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Zener Model Description of Ferromagnetism in Zinc-Blende Magnetic Semiconductors. Science 2000, 287, 1019. (8) Wang, Z. L.; Song, J. Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays. Science 2006, 312, 242. (9) Wiberg, E.; Holleman, A. F. Inorganic Chemistry; Academic Press: New York, 2001. (10) Bordiga, S.; Lamberti, C.; Ricchiardi, G.; Regli, L.; Bonino, F.; Damin, A.; Lillerud, K. P.; Bjorgen, M.; Zecchina, A. Electronic and vibrational properties of a MOF-5 metal−organic framework: ZnO quantum dot behavior. Chem. Commun. 2004, 2300. (11) Pankove, J. I. Optical Processes in Semiconductors; Dover Publications: New York, 1971. (12) Hauser, A. J.; Zhang, J.; Mier, L.; Ricciardo, R. A.; Woodward, P. M.; Gustafson, T. L.; Brillson, L. J.; Yang, F. Y. Characterization of Electronic Structure and Defect States of Thin Epitaxial BiFeO3 Films by UV−Vis Absorption and Cathodoluminescence Spectroscopies. Appl. Phys. Lett. 2008, 92, 222901. (13) Thevaril, J. J.; O’Leary, S. K. Defect absorption and optical transitions in hydrogenated amorphous silicon. Solid State Commun. 2010, 150, 1851. (14) Sakai, K.; Kakeno, T.; Ikari, T.; Shirakata, S.; Sakemi, T.; Awai, K.; Yamamoto, T. Defect centers and optical absorption edge of degenerated semiconductor ZnO thin films grown by a reactive plasma deposition by means of piezoelectric photothermal spectroscopy. J. Appl. Phys. 2006, 99, 043508. (15) Pereira, A. L. J.; da Silva, J. H. D. Disorder effects produced by the Mn and H incorporations on the optical absorption edge of Ga1−xMnxAs:H nanocrystalline films. J. Non-Cryst. Solids 2008, 354, 5372. (16) Marotti, R. E.; Giorgi, P.; Machado, G.; Dalchiele, E. A. Crystallite size dependence of band gap energy for electrodeposited ZnO grown at different temperatures. Sol. Energy. Mater. Sol. Cells 2006, 90, 2356. (17) Kang, J. Y.; Tsunekawa, S.; Kasuya, A. Size effect on absorption edges of ultrafine SnO2 nanoparticles. Acta Phys. Sinica 2001, 50, 2198. (18) Weber, R. S. Effect of local structure on the UV-visible absorption edges of molybdenum oxide clusters and supported molybdenum oxides. J. Catal. 1995, 151, 470. (19) Brus, L. E. Electron-electron and electron-hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state. J. Chem. Phys. 1984, 80, 4403. (20) Brus, L. E. Electronic wave functions in semiconductor clusters: experiment and theory. J. Phys. Chem. 1986, 90, 2555. (21) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. Pbs in Polymers From Molecules to Bulk Solids. J. Chem. Phys. 1987, 87, 7315. 1085
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