Simple Methods for Production of Nanoscale Metal Oxide Films from

Publication Date (Web): March 14, 2013 .... Production of colorful metal oxide films is an easy way to demonstrate these nanoscale structures in educa...
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Simple Methods for Production of Nanoscale Metal Oxide Films from Household Sources Dean J. Campbell,*,† Michelle S. Baliss,† Jordan J. Hinman,† John W. Ziegenhorn,† Mark J. Andrews,‡ and Keith J. Stevenson‡ †

Mund-Lagowski Department of Chemistry and Biochemistry, Bradley University, Peoria, Illinois 61625, United States Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States

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S Supporting Information *

ABSTRACT: Household items coated with various metals or titanium compounds can be heated to produce colorful films with nanoscale thicknesses.

KEYWORDS: General Public, First-Year Undergraduate/General, Upper-Division Undergraduate, Public Understanding/Outreach, Demonstrations, Hands-On Learning/Manipulatives, Nanotechnology, Oxidation/Reduction, Materials Science change. Soap bubbles, films on water, or even a layer of air between two smooth glass surfaces like microscope slides, can exhibit a wide spectrum of colors by interaction of colorless materials with white light. Animal coloration found in some bird feathers and butterfly scales is really structural color, rather than chemical pigmentation, resulting from interference of light reflecting from surfaces in these biological structures.2 Thin metal oxide layers on metals such as titanium, stainless steel, nickel, and chromium can also exhibit structural color. The brightly colored iridescent oxide films of titanium are primarily used as durable decoration (e.g., in jewelry). Thermal and electrochemical methods of production of titanium, niobium, and tantalum oxide films, as well as the correlation between film thickness and color, have been previously published in this Journal3 and elsewhere.4,5 In electrochemical methods, the metal is used as an anode in an aqueous electrolysis cell, where the metal surface is oxidized to produce a thin oxide coating. Controlling the voltage of the cell (e.g., from 5 to 100 V) controls the film thickness and therefore the film color. In thermal methods, the metal is heated to produce thin oxide coatings. Carefully controlling the heating temperature of the metal (potentially more difficult than controlling voltage) controls the film thickness and therefore the film color. Various simple metal items that could be found in a home or a hardware store (rather than ordering metals such as titanium from specialty companies) were surveyed for their ability to

roduction of thin metal oxide films was recently explored as part of an outreach program with a goal of producing nanoscale structures with household items. Nanoscale materials, typically in the size regime of 1−100 nm, are of great research interest because they can possess qualities (e.g., optical properties) that are unlike those of the same materials at either the atomic or bulk scales.1 Thin transparent films can have thicknesses on the order of tens to hundreds of nanometers, making them nanoscale films. In this size regime, light waves that reflect from both the top and bottom surfaces of the films can constructively and destructively interfere to produce a wide variety of colors (Figure 1). How light waves will interfere depends on the light wavelength and such film properties as refractive index and film thickness. As film thicknesses change, the dominant colors that are reflected from the films also

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Figure 1. (Left) Diagram of destructive interference and (right) constructive interference of two light waves. © 2013 American Chemical Society and Division of Chemical Education, Inc.

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produce colored films when heated. This led to an exploration of “titanium” tools at local stores. Many of the “titanium” products available to the nonspecialist are really objects coated with titanium-based ceramics. A fairly common titanium-based coating is the gold-colored titanium nitride. TiN (its composition actually ranges from TiN0.6 to TiN1.2) is hard, yet somewhat slippery, and has been used to coat a variety of cutting surfaces.6,7 Another coating that looks simply like gray titanium metal is actually a mixture of titanium nitride and chromium nitride. So-called “titanium-bonded” products use this coating, patented by the Acme United Corporation.8 The patent contains interesting comments about consumers viewing the gray-coated scissors as having a coating, but still being acceptable for household use. These ceramic films can be deposited from elemental metal or metal compound vapors and elemental nitrogen or ammonia gas by a variety of techniques.8,9 Despite the fact that these “titanium” products contain titanium compounds rather than titanium metal, they can still be used to produce colorful oxide films.

dissolved to a large extent by vinegar or, more quickly, by 1 M hydrochloric acid. Similar color-changing behavior was observed when heating titanium nitride films. A blade from a “titanium-bonded” scissors was placed into the flame of a propane torch and was heated to red heat. Upon cooling, the heated area exhibited yellow, purple, and blue colors (Figure 2). The test scissors

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Figure 2. (Top) Tip of a “titanium-bonded” scissors blade that has been heated with a propane torch; a stainless steel scissors blade (also heated with the torch) is shown for comparison. (bottom) Tip of a “titanium” screwdriving bit that has been heated with a propane torch and an unheated bit for comparison.

PROCEDURE A variety of readily available household objects were heated to successfully produce the colored nanoscale oxide films. Generally, as the metal objects were subjected to more heat, their resulting colors after cooling changed from silvery to yellow to purple to blue to another color (often gray), indicating the formation and thickening of oxide layers that were tens of nanometers thick.3 It is interesting to note that the initial progression of colors with increasing thickness for these films, yellow to purple to blue, can also be observed in soap films, oil slicks, and air spaces between two glass microscope slides. One of the fastest ways to produce oxide films with multiple colors was to heat part of a metal object red hot with a propane torch. Fastest heating was achieved by holding the object with tongs in the hottest part of the flame (the tip of the inner blue cone). As the temperature of the heated portion of the object increased, bands of color would first appear within and then move to the edge of the heated region. Smaller, thinner objects heated more rapidly, therefore their colored oxide films appeared more rapidly, sometimes appearing within tens of seconds. When the heat was removed, the color bands remained on the metal object. Stainless steel objects such as safety razor blades and butter knives, and nickel-plated binder clips, could all be heated in this manner to produce the colored films. Metals such as nickel and titanium were heated under more controlled conditions in the laboratory with a hot plate to produce colors. Again, as the metal objects were subjected to more heat (either by extended exposure to a particular temperature or by exposure to a higher temperature), their resulting colors after cooling changed from silvery to yellow to purple to blue to another color. U.S. coins, such as nickels, dimes, and quarters, could also be heated with a torch or a hot plate to produce colorful oxide layers. When the coins (copper−nickel alloys, minimum 75% copper)10 were placed on a hot plate set to 250 °C, their colors transformed over several minutes through yellow, purple, and blue. The color change was not uniform across the coins: the raised locations of the coins tended to change color first (producing interesting multicolored effects). Although U.S. pennies turned colors, the effect was complicated by the initial coppery color of their cladding10 and also by how easily their zinc cores melted in a propane torch flame. These colorful oxide films can be

could still cut paper after the heat treatment; therefore, the heat treatment did not significantly warp the blades. A “titanium” screwdriving bit (coated with gold-colored titanium nitride) was placed in the flame of the propane torch. The heated area turned a variety of colors, indicating formation of an oxide layer (Figure 2). “Titanium” razor blades also have coatings that change color upon heating. The nearly 2000 °C flame of a propane torch in air is short of the melting point of titanium nitride (2950 °C), but well above the 800 °C at which the substance will oxidize in air.7,11 Chromium nitride decomposes at 1770 °C.12 Electron microscopy and energy-dispersive X-ray (EDX) spectroscopy analyses of the nitride films before and after heating support the hypothesis of nitride-to-oxide conversion (see the Supporting Information). The colored oxide films can be studied by reflectance visible light spectroscopy. Reflectance spectra acquired from shiny, smooth (mirror-like) surfaces had the best signal-to-noise ratios. Constructive and destructive light interference is manifested in the visible light spectrum as a wide oscillation. As the oxide film thicknesses increased, the minima and maxima in the spectral oscillations migrated to longer light wavelengths. The reflectance spectrum is shown (Figure 3) for a titanium nitride-coated cutting blade that has been heated on a hot plate at 500 °C to produce a purple-colored oxide layer (divided by the reflectance spectrum of a glass mirror). At lower heating temperatures, the resulting yellow-colored oxide layer exhibited a reflectance minima at shorter wavelengths, and at higher heating temperatures, the resulting blue-colored oxide layer exhibited minima at longer wavelengths. These reflectance spectra can be compared to those calculated for various theoretical thin films using a program on the Filmetrics.com Web site.13 On this site, information about layered structures such as composition, refractive index, and thickness can be used in the complex-matrix form of Fresnel equations to produce the simulated reflectance spectra. The calculated reflectance spectrum for an 18.5 nm film of titanium(IV) oxide on a titanium nitride surface is also shown in Figure 3. The 18.5 nm thickness was selected for the simulation because the calculated 630

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during coverage of transition metals. The heating to form the oxide film was presented as a combustion reaction, in which the titanium nitride was combined with oxygen gas to produce nitrogen gas and titanium(IV) oxide.14 The stoichiometry of the titanium oxide was predicted in class by pointing out on a periodic table that the highest oxidation number of titanium is +4, due to removal of the 4s and 3d electrons. It was also noted that titanium(IV) oxide is a colorless compound (no d electrons to produce color), providing further evidence that the colors that were observed were due to light interference within the nanoscale films. The demonstration was also used as an opportunity to point out what the “titanium” label in household products can really mean. As part of a materials chemistry laboratory experiment series, students used a visible-light reflectance spectrometer to examine colorful oxide coatings on the torch-heated nickelplated binder clips. Additionally, hundreds of nickel-plated binder clips were heat-colored with a propane torch and distributed at science educational outreach events both at the University of Texas at Austin and the Riverfront Museum in Peoria, IL. Accompanying each clip was a one-page handout explaining how the colors on the clips are due to oxide layers with thicknesses at the nanoscale or near-nanoscale size scale. This handout is described in more detail in the Supporting Information.

Figure 3. Reflectance spectra of the purple portion of heat-treated titanium nitride-coated cutting blade. A simulated reflectance spectrum for an 18.5 nm layer of titanium(IV) oxide on titanium nitride is shown for comparison.

reflectance minimum at ∼530 nm is similar to the minimum in the measured reflectance spectrum, and the calculated intensities were multiplied by three to bring them into the same range as the measured reflectance spectrum. The Supporting Information contains details of a reflectance spectra study of a nickel coin placed on a hot plate at 250 °C.

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ASSOCIATED CONTENT

S Supporting Information *

EDX studies of titanium nitride films, reflectance spectroscopy of the nanoscale oxide films, and heating and distributing heatcolored nickel-plated binder clips. This material is available via the Internet at http://pubs.acs.org.

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HAZARDS Propane torch flames in air can get to nearly 2000 °C,11 clearly presenting a combustion hazard and readily melting some metals (e.g., aluminum). It is recommended that the torch be used in or near a ventilated area such as a fume hood to remove gaseous combustion byproducts. Demonstrators should also remember that many hot metal objects can look like cold metal objects.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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DISCUSSION Neither the elemental metals nor the bulk metal oxides exhibit the varieties of colors observed, especially in the yellow− purple−blue temperature progression. The nanoscale thicknesses of the oxide films on these household items are what give them their multiple colors. Production of colorful metal oxide films is an easy way to demonstrate these nanoscale structures in educational settings. In second-term general chemistry and materials chemistry classrooms, an unheated, titanium nitride-coated cutting blade was shown to the class. A document camera−projection system successfully showed the gold color of the titanium nitride film on the blade. One end of the blade (held with metal tongs) was heated for about 10 s in the hottest portion of a propane torch flame, causing that end to glow red-hot. When the blade was removed from the flame, the end that had been heated was gray, the unheated end was still golden, and the region inbetween exhibited dark yellow, purple, and blue colors from light interference by the nanoscale titanium oxide film. These colors were readily visible with the document camera− projection system (the blade was still held with tongs to prevent any possible residual heat damage). In the second-term general chemistry course, the demonstration was presented

ACKNOWLEDGMENTS This research was supported by nanotechnology outreach efforts by the Center for Nano- and Molecular Science and Technology at The University of Texas-Austin.

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REFERENCES

(1) Campbell, D. J.; Xia, Y. J. Chem. Educ. 2007, 84, 91−96. (2) Campbell, D. J.; Xia, Y. J. Chem. Educ. 2006, 83, 1638−1642. (3) Gaul, E. J. Chem. Educ. 1993, 70, 176. (4) Martin, A. CNS Institute for Physics Teachers. The Clearly Colorful Thin Film Lab (Physics Edition). http://www.cns.cornell. edu/cipt/labs/2010%20labs%20without%20answers/ Clearly%20Colorful%20Lab.wo%20answers.pdf (accessed Feb 2013). (5) Diamanti, M. V.; Del Curto, B.; Pedeferri, M. P. Color Res. Appl. 2008, 33, 221−228. (6) Toth, V. L. E. Transition Metal Carbides and Nitrides; New York: Academic Press: New York, 1971. (7) Pierson, H. O. Handbook of Refractory Carbides and Nitrides: Properties, Characteristics, Processing, and Applications; Noyes Publications: Westwood, NJ, 1996; p 193. (8) Buchtmann, L.; Pia, G. (Acme United Corporation). Coating for Cutting Implements. U.S. Patent 6,988,318, Nov 23, 2003. (9) Xiao, Z. G.; Mantei, T. D. Surf. Coat. Technol. 2007, 177−178, 389.

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(10) United States Mint. Coin Specifications. http://www.usmint. gov/about_the_mint/?action=coin_specifications (accessed Feb 2013). (11) The Engineering Toolbox. http://www.engineeringtoolbox. com/flame-temperatures-gases-d_422.html (accessed Feb 2013). (12) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: Oxford, 1984; p 480. (13) Filmetrics. Reflectance Calculator. http://filmetrics.com/ reflectance-calculator (accessed Feb 2013). (14) Lyutaya, M. D.; Kulik, O. P.; Kachkovskaya, E. T. Powder Metall. Met. Ceram. 1970, 9, 233−235.

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