In the Laboratory
Modifying Optical Properties of ZnO Films by Forming Zn1-xCoxO Solid Solutions via Spray Pyrolysis
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Anne K. Bentley, Gabriela C. Weaver, Cianán B. Russell, William L. Fornes, and Kyoung-Shin Choi* Department of Chemistry, Purdue University, West Lafayette, IN 47907; *
[email protected] Susan M. Shih Department of Chemistry, College of DuPage, Glen Ellyn, IL 60137
The concepts of semiconductor electronic band structure (valence band, conduction band, and band-gap energy) are critical to students’ understanding of the chemistry and physics of semiconductors. Experiments that can demonstrate the presence of an energy band gap and its relationship to other structural or compositional factors of a material would greatly contribute to students’ comprehension of semiconductor band structure (1–4). However, incorporation of experiments demonstrating these concepts into the undergraduate curriculum, especially in general chemistry courses, is often limited by the relatively expensive equipment required for solid-state synthesis and characterization (e.g., high temperature furnaces, vacuum systems, and X-ray diffractometers). This article presents a simple and inexpensive experiment that provides students with the opportunity to prepare semiconducting materials and characterize their optical properties in connection with their band-gap energies (Eg) using UV–vis spectroscopy. The experiment was developed as part of the CASPiE project1 in which students carry out experiments related to authentic and current research (5). The experiment presented here is an excerpt of the six-week laboratory module prepared for that application and instead can be carried out in a single laboratory period. The full module (including the experiment described here) has been piloted by general chemistry students enrolled in a CASPiE lab course. The experiment is based on preparing a wide band gap ZnO film and its solid solutions Zn1᎑x CoxO using a simplified spray pyrolysis method. Spray pyrolysis is a solutionbased, inexpensive thin-film synthetic technique that does not require a high-vacuum, high-pressure, or high-temperature environment (6). Solid-state materials are created when the solution-phase precursors are sprayed on hot surfaces and react to form a desired product. Research in spray pyrolysis requires a hot surface capable of precise temperature control and elegant spray systems (e.g., flow control of spray solution as well as a propellant gas) (7, 8). In the experiment presented here the technique has been simplified by using a laboratory hot plate as the heat source and a plastic spray bottle to introduce the solutions. The thin-film morphology created by spray pyrolysis makes it possible to characterize the optical properties of solid materials using UV–vis absorbance spectrometers that are typically available for use in undergraduate labs. The students first prepare a ZnO film, which is a wide band-gap semiconductor (Eg = 3.2 eV) that absorbs only UV light. Incorporation of transition-metal ions such as Co2+ ions into the wurtzite ZnO structure is known to shift the band
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gap of ZnO to the visible region (8). As a result, Zn1᎑x CoxO films will bear colors and their color changes will be directly related to the change of their compositions. These relationships can be easily observed by the naked eye and characterized by UV–vis spectroscopy. Since this experiment demonstrates the band-gap tuning of a wide band-gap material to enhance visible light absorption, it can also be used to introduce solar energy conversion (e.g., photovoltaics, photoelectrochemical cells) using semiconducting materials. The special focus of this general chemistry experiment lies in visualizing the correlation between the composition, band-gap energy, and color of the material, which will provide an integrated understanding of solid-state materials. Experimental Procedure A 0.1 M aqueous solution of Zn(NO3)2 is used as a spray solution. Glass microscope slides serve as the synthesis surface and are used directly from the manufacturer’s box. To heat the glass slides in preparation for synthesis, a hot plate is covered with aluminum foil. A glass slide is placed on the aluminum foil and heated to a target temperature. Crystalline ZnO films can be obtained when the temperature of the hot plate ranges between 300 ⬚C and 400 ⬚C. The corresponding hot plate setting can be selected by directly measuring the surface temperature with a thermocouple. Alternatively the optimum heat setting can be experimentally determined by monitoring the rate of evaporation of the precursor solution upon deposition on the hot glass and confirming the presence of the ZnO band gap in the resulting film (see below). The precursor solution should evaporate instantaneously when it comes in contact with the hot glass substrate. Solid ZnO forms on the slide’s surface as the nitrate decomposes to form NOx gases upon impact. The best quality films are obtained when the solution is sprayed onto the slide from a distance of approximately six inches directly above the slide using a commercially available plastic spray bottle. The total number of sprays determines the thickness of the semiconductor films; twenty or more sprays may be necessary to achieve a uniform thin film of ZnO. Films that demonstrate a sharp absorption edge in UV–vis spectra are uniformly white and opaque. With a band gap of 3.2 eV (λ = 380 nm), ZnO only absorbs light in the UV region. To alter the electronic band structure of ZnO so that visible light can be absorbed, Co2+ ions are used to partly substitute for Zn2+ in the ZnO struc-
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In the Laboratory
Figure 1. UV–vis absorption spectrum of a ZnO thin film synthesized by spraying a 0.1 M Zn(NO3)2 solution on a glass microscope slide twenty times at approximately 300 ºC. The solid line is the absorption spectrum; the dashed lines show how the intersection of the slope of the absorption edge and the slope of the background absorption can be extrapolated to determine the band-gap energy of the material, Eg.
Figure 2. UV–vis absorption spectra of ZnO and Zn1-xCoxO thin films made from solutions in which cobalt nitrate comprised (a) 0%, (b) 2%, and (c) 10% of the total transition metal nitrate concentration (0.1 M).
ture by adding cobalt nitrate to the zinc nitrate solution. The total concentration of both metal nitrate salts is kept at 0.1 M. The absorption spectra of the ZnO and Zn1᎑xCoxO films are obtained using a UV–vis spectrophotometer. Detailed lab instructions and information on supplies can be found in the Supplemental Material.W
states in the band-gap region (9, 10). The color of the Zn1-xCoxO films becomes more intense as the amount of Co increases. For example, a Zn0.98Co0.02O film is light green while Zn0.90Co0.10O is olive green.2 UV–vis measurements of the Zn1᎑xCoxO thin films can be used to draw a connection between the atomic level changes upon incorporation of transition metals and the observable color changes in the films. Figure 2 clearly shows a gradual increase in visible light absorption as the cobalt content in the ZnO structure increases from 0% to 2% to 10%. In addition, their absorption edges appear to be less abrupt than that of the ZnO film. ZnO and CoO possess different structure-types (wurtzite and rock salt, respectively), and there is a thermodynamic limit for Co2+ ions to be stabilized in the tetrahedral coordination geometry found in the wurtzite structure (11). Therefore the solid solution, Zn1-xCoxO, only forms below a particular concentration of cobalt in the precursor solution. When the amount of cobalt exceeds this value, the ZnO lattice becomes saturated with Co substitutions and a separate CoO phase forms. We found the maximum solubility of CoO in ZnO that can be achieved by spray pyrolysis at 300 ⬚C is approximately 10%. When CoO forms as an impurity, it appears as brown–black areas on the slide and can be detected by the naked eye. As a result, the homogeneity of wurtzitebased solid solutions can be adequately assessed by examining the appearance of the films without the use of X-ray diffraction.
Hazards The glass slides should be handled with tweezers at all times to keep the substrates clean and avoid burns. The NOx gases formed by the reaction are a mixture of NO, NO2, N2O3, N2O4, and N2O5, which can irritate the eyes, mucous membranes, and respiratory tract. Therefore the spray pyrolysis synthesis should be carried out in a fume hood. The hot slides may crack on contact with the cool solution. Care should be taken to avoid cuts when handling any cracked or broken slides. Results and Discussion Spraying the zinc nitrate solution on the hot glass surface creates an opaque white film on the slide that can be identified as ZnO via X-ray diffraction (not shown). These films exhibit an abrupt change in the slope of their UV–vis absorption curves at approximately 400 nm, as shown in Figure 1. Students can estimate the band-gap energy of the ZnO thin film from the UV–vis spectrum by extrapolating the slope of the absorption edge to the background absorption level. The wavelength at which the two intersect can be converted to band-gap energy using E = hc兾λ. This method is illustrated in Figure 1. When cobalt nitrate is added to the zinc nitrate solution, the resulting films demonstrate noticeable color changes. This indicates that Co2+ ions are incorporated into the wurtzite ZnO structure forming a solid solution and affecting the electronic band structure of ZnO. The presence of Co2+ ions increases visible light absorption either by decreasing the band-gap energy or by introducing new electronic 1184
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Conclusion This experiment provides students with a simple and effective introduction to the relationship between solid-state electronic structure and optical absorption properties that can be controlled by material composition. Students can observe that the thin semiconductor films change color when the composition of the ZnO-based solid solution changes. Instructors and students can extend the experiment by choosing other transition-metal ions (e.g., Ni2+ and Mn2+) to form solid solutions with ZnO.
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In the Laboratory
Acknowledgments
Literature Cited
The authors thank Kimberly Long for assistance in developing the experiment. Funding was provided by a National Science Foundation Undergraduate Research Center award (CHE-0418902) that supports the Center for Authentic Science Practice in Education (CASPiE). AKB thanks the National Science Foundation for a Discovery Corps postdoctoral fellowship (CHE-0513525).
1. Ellis, A. B.; Geselbracht, M. J.; Johnson, B. J.; Lisensky, G. C.; Robinson, W. R. Teaching General Chemistry: A Materials Science Companion; American Chemical Society: Washington, DC, 1993. 2. Gurnee, E. F. J. Chem. Educ. 1969, 46, 80–85. 3. Munoz-Paez, A. J. Chem. Educ. 1994, 71, 381–388. 4. Fan, Q.; Munro, D.; Ng, L. M. J. Chem. Educ. 1995, 72, 842– 845. 5. Weaver, G. C.; Wink, D.; Varma-Nelson, P.; Lytle, F.; Morris, R.; Fornes, W.; Russell, C.; Boone, W. J. Chem. Educator 2006, 11, 125–129. 6. Mooney, J. B.; Radding, S. B. Ann. Rev. Mater. Sci. 1982, 12, 81–101. 7. Ibanez, J. G.; Solorza, O.; Gomez-del-Campo, E. J. Chem. Educ. 1991, 68, 872–875. 8. Bahadur, L.; Rao, T. N. Sol. Energy Mater. 1992, 27, 327– 360. 9. Jakani, M.; Campet, G.; Claverie, J.; Fichou, D.; Pouliquen, J.; Kossanyi, J. J. Solid State Chem. 1985, 56, 269–277. 10. Fichou, D.; Pouliquen, J.; Kossanyi, J.; Jakani, M.; Campet, G.; Claverie, J. J. Electroanal. Chem. 1985, 188, 167–187. 11. Jayaram, V.; Rajkumar, J.; Sirisha Rani, B. J. Am. Ceram. Soc. 1999, 82, 473–476.
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Supplemental Material
Handouts for the students, notes for the instructor, and additional information about supplies and tips about spray technique are available in this issue of JCE Online. Notes 1. CASPiE is the Center for Authentic Science Practice in Education, an Undergraduate Research Collaborative funded by the Chemistry Division of the National Science Foundation. 2. The compositions of these solid solutions are based on the compositions of the precursor spray solutions. Actual thin-film compositions may vary slightly from the solution compositions when sublimation of zinc occurs above 400 ⬚C.
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