ZINC OXIDE SENSITIZED PHOTOCHEMICAL REDUCTION AND OXIDATION
the limit of detectability of the surface viscometers used and are exceptionally low for polymers adsorbed at the water-air interface. Based on the results of the canal viscometer, their viscosities are probably below surface poise. Even when the monolayers are highly compressed and begin to show visible evidence of film collapse, there is no measurable surface viscosity. A surface viscosity of this low order of magnitude seems quite remarkable for a long-chain polymeric material, particularly one having a molecular weight as high as about 105,000. This must reflect the low intermolecular cohesion that is present in siloxane films, compared with monolayers of proteins and certain linear synthetic organic polymer^.'^ Therefore, it can be concluded that the ability of siloxane polymers to act as defoaming and antifoaming agents is undoubtedly related to their unusually low surface viscosities and their ability to displace the less strongly adsorbed foam-stabilizing materials. Garrett and Zisman3I have reported another remarkable and interesting property of the linear polydimethylsiloxane films, namely, the effect of the films on the so-
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called capillary waves on water. They reported that these films can be very effective in damping the capillary waves, but only a t certain states of compression of the monolayer. These siloxanes gave large “damping coefficient” peaks at areas per molecule corresponding to (1) the initial rapid increase in film pressure with decreasing area, (2) the beginning of the plateau region of the F us. A curve, (3) the inflection point on the plateau, and (4) a t the approach to closest packing of the adsorbed molecules. Between these peaks the monolayers had only a small effect on the amplitude of the capillary waves. It is interesting that surface viscosity shows no such correlation with structure of the monolayer, or that such a remarkable damping of capillary waves can occur at all in the absence of a measurable surface viscosity.
(31) W. D. Garrett and W. A. Zisman, “Damping of Capillary Waves by Monomolecular Layers of Linear Polyorganosiloxanes,” presented at the 150th National Meeting of the American Chemical Society, Colloid and Surface Chemistry Division, Atlantic City, N. J., Sept 16, 1965.
Zinc Oxide Sensitized Photochemical Reduction and Oxidation’i2
by Gerald Oster and Masahide Yamamoto Department of Chemistry, Polytechnic Institute of Brooklyn, Brooklyn, New York
(Received August 8 , 1966)
Ultraviolet radiation below 380 mp causes zinc oxide to reduce many organic compounds. Rate studies were carried out with indophenol. Diphenylpicrylhydrazyl in organic solvents is reduced to the hydrazine if water is present. I n the presence of oxygen the excited zinc oxide causes oxidation. This is ascribed to an autoxidation of a transient species produced in the light-excited solid.
Introduction It has long been appreciated that zinc oxide when illuminated by ultraviolet light Causes chemical oxidations and reductions. For example, Eibner in 19113 found that the reduction of Prussian blue (in the presOf sugars) takes ’lace when zinc Oxide is nated. Baur and Neuweiler in 19274 showed that
ultraviolet-irradiated zinc oxide produces hydrogen Peroxide from oxygen and water. The literature on the photocatalytic properties of zinc oxide is quite extensive (see, for example, ref 5 and 6 ) but is usually (1) Work supported by the U. s. Atomic Energy Commission under Contract AT (30-1)-2206. Presented at the International Conference on Photosensitization in Solids, Chicago, IU., June 23,1984.
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GERALDOSTERAND MASAHIDE YAMAMOTO
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of a descriptive nature. Furthermore, no conclusive experiments regarding the mechanism of the reaction have been presented. The purpose of the present work is to carry out some quantitative rate studies of indophenol reduction with a single sample of zinc oxide whose chemical content and luminescence properties’ are known, where the atmosphere is controlled, and the products of the reaction are characterized. Zinc oxide is an important industrial material (e.g., as a filler in rubbers) and is known to cause photocatalytic destruction.6 It also serves as a photocatalyst for the initiation of vinyl polymerization,8 the detailed mechanism of which is treated el~ewhere.~ Excitation of zinc oxide by X-rays is similar in its effects to ultraviolet excitation,’ and in the present work some comparisons of the chemical effects produced by these two types of radiation are considered.
Experimental Procedures and Results A . Materials and Radiation Sources. Throughout we have used a highly pure zinc oxide (Type SP-500) obtained from the S e w Jersey Zinc Co. This material which is produced by the French process has as its greatest impurity calcium in 10 ppm (iron in 1 ppm). All other materials were obtained from Fisher Scientific Co. or from Eastman Chemical Co. and were, when possible, purified by repeated crystallization. The principal ultraviolet source used mas a GE 100-w AH4 mercury lamp in conjunction with a Wood’s glass filter to isolate the 365-mp lines to give an intensity incident on the sample of 4 X 10-lo einstein/ cm2 sec, as measured on a calibrated thermopile (Eppley Laboratories). For wavelength-dependence studies this lamp and a 100-w intermediate pressure mercury lamp (Hanovia SH) were used (without a filter) with a Bausch and Lomb grating monochrometer. The X-ray source employed was a tungsten target Machlett tube oDerated a t 50 kw and 10 ma to give an intensitv a t the surface of the sample of 8000 rads/hr. B. Reduction of Indophenol. In order to examine the Dhotoreducina DroDertv ” of zinc oxide, sodium 2.6dichiorobenzenone was employed as the indicator. This indophenol absorbs maximally at 600 mp above pH with a extinction coefficient Of l.80 x lo4. At pH 7 its oxidation reduction potential is 0.217 v.10 On reduction it becomes co~or~ess (note: the “lored and the colorless species have extinction lo3and 2.1 coefficients at 365 mp Of 3’7 lo3, respectively). The dye alone is photochemically inactive and the reduced species can be completely reoxidieed by air to the colored species. The zinc oxide powder (average particle diameter
0.3 p ) was added to an aqueous indophenol solution and was flushed with purified nitrogen 20 min prior to and during the irradiation accompanied by magnetic stirring. At various times an aliquot sample was withdrawn and centrifuged in a chemical centrifuge. The absorption a t 600 mp of the clear supernatant was then determined. No appreciable amount of dye was adsorbed to the zinc oxide powder. In Figure 1A is shown the quantum yield of photoreduction of indophenol as a function of wavelength of exciting light. For the calculation of quantum yield, we have assumed that all of the incident light is totally absorbed by the zinc oxide and the small light filtering effect of the dye in the ultraviolet range is also corrected. It is of interest to compare this result with the reflectance spectrum (Figure 1B) of the zinc oxide sample, as determined on a Perkin-Elmer spectrophotometer. The rate of photoreduction of indophenol as function of amount of zinc oxide added is given in Figure 2. The rate was found to be independent of the indophenol A4 concentration within the concentration range and to be proportional to the first power of the intensity (varied by the introduction of fine-mesh wire screens), When the exciting light is removed, the reaction stops immediately (the time scale is on the order of minutes), i.e., there is no posteffect, and it commences immediately when the light is turned on. From a study of the temperature dependence of the rate, it was found that the reaction has an over-all activation energy of 1.2 kcal/mole. Phosphate ions but not phthalate ions stop the photoreduction. It was noticed that an aqueous suspension of zinc oxide which normally is at pH 7.4 becomes acidic (down to pH 6.8) on illumination. C. Reduction of Diphenylpicrylhydrazyl. A zinc oxide suspension in organic solvents (benzene, carbon tetrachloride, and chloroform) will on irradiation with
v
The J O U T of ~Physical Chemistry
(2) Taken in part from the dissertation of M. Yamamoto submitted to the faculty of the Polytechnic Institute of Brooklyn in partial fulfillment of the reauirements for the degree - of Doctor of Philosoohv. _ ” lg6& 359 786 (1911). (3)A. (4) E. Baur and C. Neuweiler, Hetv. Chim. Acta, 10, 901 (1927). (5) C. Ellis, A. A. Wells, and F. F. Heyroth, “The Chemical Action of Ultraviolet Rays,” Reinhold Publishing Corp., New York, N. Y., 1941. (6) H. E. Brown, “Zinc Oxide Rediscovered,” New Jersey Zinc Co., New Pork, N. Y., 1957. (7) G. Oster and M. Yamamoto, J . A p p l . Phys., 37, 823 (1966). (8) M. C. Markham and K. J. Laidler, J . Phys. Chenz., 57, 363 (1953). (9) hI. Yamamoto and G . Oster, J . Polyner sci., A-14, 1683 (1966). (10) W. M. Clark, “Oxidation-Reduction Potentials of Organic Systems,” The Williams & Wilkins Co., Baltimore, Md., 1960, p 130. Eibneri
;k-i ;i;;ri
ZINC OXIDESENSITIZED PHOTOCHEMICAL REDUCTION AND OXIDATION
10.5
z
I&
1
y'
0
x)o
400
500 Mx) WAVE LENGTH I m p )
300
A
400 500 600 W A V E LENGTH ( m ~ l B
Figure 1. (A) Spectral dependence of the quantum yield of zinc oxide sensitized indophenol photoreduction. Indophenol concentration 7.6 X 10-6 M ; zinc oxide concentration 1.5 g/l.; temperature 30". (B) Diffuse reflectance spectrum of zinc oxide. Reflectance standard: MgO.
ZnO ( g l l . )
Figure 2 . Rate of indophenol photoreduction us. amount of zinc oxide added Indophenol concentration 7.6 x 10-6 M ; temperature 30".
ultraviolet light cause 2,2-diphenyl-/3-picrylhydrazyl (DPPH) to become discolored. More specifically, the maximum at 520 mp (molar extinction coefficient of 1.3 X lo4) is decreased and a yellow product is obtained. Water is essential for this reaction. Thus the reaction will not proceed for carefully dried (sodiumtreated) benzene. On addition of small amounts of water, the ral e increases proportionally to the water content. In these studies oxygen was rigorously excluded from the solution. Prior heating a t 300" of the zinc oxide in vacuo had no effect on the rate of reduction of DPPH. The products of the reduction of DPPH in various solvents were examined. Chromatographic separation (on paper using a pentane and carbon disulfide mixture, 1 : l ) revealed that only one product was produced (Rf 0.988 for DPPH and Rr 0.970 for the product, which corresponds to diphenylpicrylhydrazine) . This product absorbs maximally at 320 mp in ethanol or in benzene' The infrared absorption spectrunl Of the product in K13r pellets s h o w a Strong peak at 3.04 p
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regardless of the solvents in which the DPPH was photoreduced, which can be assigned to the N H group in diphenylpicrylhydrazine. On treatment of the product with PbOz one easily obtains DPPH. Although X-rays will cause the decoloration of D P P H in carbon tetrachloride, the reaction proceeds much more rapidly in the presence of zinc oxide. Here again the process is accelerated with trace amounts of water and the product obtained was diphenylpicrylhydrazine. D. Reduction of Metal Ions. Silver ion (in the form of the nitrate) in aqueous solution is rapidly reduced in the presence of zinc oxide under ultraviolet irradiation," and we found that the reaction proceeds at an increased rate even when red light is used, although silver metal deposits gradually at very slow rate in the dark. The product is colloidal metallic silver. With ionizing radiation the product is discernible with dosages as low as 100 rads. Nercuric ion (in the form of mercuric chloride) in aqueous solution is likewise photoreduced with zinc oxide to give grayish colloidal mercury," but in the presence of oxygen one obtains a brownish product which is probably mercuric oxide, and on further irradiation colloidal mercury is obtained. E. Oxidations. Chemically reduced crystal violet (reduced by sodium hydrosulfite) in aqueous solution is readily oxidized to give the colored dye on illumination of zinc oxide. This reaction proceeds only in the presence of oxygen. The oxidation is visible with Xrays of dosages as low as 100 rads. Zinc oxide in aqueous solution also serves as a photocatalyst for the rapid oxidation of p-toluenediamine (to give a brownish product) if oxygen is present. These oxidations can be ascribed to the formation of hydrogen peroxide. This reaction requires the presence of both oxygen and water. Even prior heating at 300" in vacuo of zinc oxide does not sufficiently remove oxygen to stop the reaction.? However, zinc oxide treated in this manner becomes a better photocatalyst for the production of hydrogen peroxide (as shown by iodometric titration) if oxygen is added to the aqueous suspension.
Discussion Many of our results can be interpreted in terms of the photochemical transformation of zinc oxide to zinc (photolysis of zinc oxide). For example, metallic zinc (first treated with HC1 to remove the passive layer) will reduce in the absence of oxygen all of the compounds described in this work. I n the presence of oxygen metallic zinc with moisture will produce hy(11)E. Baur and A. Perret, J. Chim. Phys., 23, 97 (1926); G.A. Korsunovskii, Zh. F i z . Khim., 39, 2136 (1965).
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drogen peroxide. This autoxidation of zinc as well as the decomposition of hydrogen peroxide with metallic zinc has been known for many years.12 In this case zinc cannot be in the metallic state but rather exists as zinc atoms, since prolonged irradiation of zinc oxide does not produce colloidal coloration as is the case with, for example, silver salts. A more favorable scheme is the formation of zinc ion on zinc oxide surface under ultraviolet irradiation. The concentration of zinc ions in solution is increased when an aqueous zinc oxide suspension is illuminated.13 Since our work does not show postirradiation effects, produced zinc ion is very unstable and may not accumulate. The decrease in pH observed with illumination of zinc oxide suspensions is compatible with this result. The inhibitory effect of phosphate ion may be a complexation of photochemically produced zinc ion. I n terms of the electronic theory of photoconductivity of zinc oxide,14 this can be interpreted that ultraviolet excitation produces electrons and holes, the latter having very low mobility (ie., n-type conductivity), and the electrons in the conduction band can transfer to other molecules at the surface of the solid. Hence, the excited zinc oxide solid serves as an electron donor. Ultraviolet light causes a desorption of oxygen from the surface of zinc oxide.13 This would allow direct exposure of the surface to the solvent and hence allow the reaction of zinc ions of the solid with, for example, water or oxygen to take place. It is of interest to note that oxygen quenches the photoconductivity as well as the luminescence of zinc oxide. The quenching effect is considerably greater if water vapor is added to the oxygen.? Apparently, the formation of hydrogen peroxide removes electrons which would normally recombine with holes.
The Journal of Physical Chemistry
GERALD OSTERAND MASAHIDE YAMAMOTO
The rate of the photochemical reduction as a function of exciting wavelength (Figure 1A) follows the absorption spectrum of zinc oxide as seen from the reflection spectrum (Figure 1B). The absorption edge of 380 mp is not infinitely sharp and has a tail into the visible range. This is seen from the silver reduction experiments where extremely small amounts of reduced silver are detectible by eye. In this connection, we have found that red light will quench the low-temperature phosphorescence (as well as thermoluminescence) of zinc oxide? indicating appreciable absorption in the long-wavelength region. The form of the rate curve as a function of zinc oxide concentration (Figure 2) arises from photometric considerations. For low concentrations of the solid much of the incident light is lost by scattering. For the higher concentrations employed, the suspension is so dense the reaction is confined to the front surface and hence the rate is practically independent of the solid content. The fact that the quantum yield is independent of intensity in the range employed shows that the reaction on zinc oxide surface is very rapid and not diffusion limited. The independence of the rate on indophenol concentration indicates merely that the concentration of the indicator is much higher than the concentration of active sites produced on the zinc oxide surface a t any moment. The small activation energy observed may be associated with energy of electron transfer. (12)M. Traube, Ber., 26, 1471 (1893). (13)G. V. Elmore and H. A. Tanner, J. Phys. Chem., 60, 1328 (1956); B. Beranek, E. Barton, K. Smrceck, and I. Sekerka, Collection Czech. Chem. Commun., 25, 369 (1960). (14) See, for example, J. T. Law in “Semiconductors,” N. B. Hanney, Ed., Reinhold Publishing Corp., New York, N. Y.,1959.
