Irradiated Zinc Research Articles

EPR Study of Irradiated Zinc Acetate Dihydrate Single Crystals

X-ray irradiation of zinc acetate single crystals at 77°K with subsequent examination at 133°K reveals initially an intense EPR signal due to methyl radicals. Decay of the methyl radicals is accompanied by a corresponding increase in another species shown to be the ·CH2CO2− radical. The proposed mechanism is a simple abstraction of a hydrogen atom by the methyl radical from a neighboring acetate ion. In support of this mechanism, observation of the reaction using Zn(CH2DCO2)2·2H2O indicates an isotope effect. The ·CH2CO2− radicals are produced in two magnetically distinct sites. Spectra taken over a range of temperatures reveal both the slow and rapid exchange of the hydrogen atoms. The barrier to internal rotation is 5.0 ± 1.0 kcal/mole, with less than two entropy units of activation. Parameters characterizing the ·CH2CO2− radical are determined with a 12-parameter least-squares calculation utilizing spectra taken at 55 different crystal orientations. The radical is planar with an HCH bond angle of 116.9°. The coupling constants are compared with other radicals using molecular-orbital calculations.

Photoelectric effects at semiconductor electrodes

Useful electrical work can be derived from photoelectric cells containing zinc oxide electrodes in contact with oxygen-free aqueous solutions containing organic compounds, or from irradiated cadmium sulfide electrodes. Study of these cells has shown that, with electron acceptors at the dark electrode, continuous current results from photolysis of the semiconductor electrode material. Steady photovoltages of 1.8 volt, open circuit, can be maintained over long periods of illumination; steady photocurrents of 1.5 ma can be delivered through 100 ohm external resistance. Zinc oxide electrodes are sensitive to the entire ultraviolet region of the spectrum. The actual operating photovoltages and/or photocurrents obtained deepnd on: (1) the presence of an oxidizable substrate at the irradiated zinc oxide surface, and (2) the nature and concentration of the electron acceptor at the dark electrode, as well as on the light intensity and the external load. Factors which promote the creation of positive holes in zinc oxide, such as doping with copper or silver, favor rapid response to the light and reproducible accumulation of optimum photoelectric effects. Many substances which are efficient electron acceptors at the dark electrode can be re-oxidized by atmospheric oxygen and are thus reversible. Reactions occurring at the irradiated zinc oxide or cadmium sulfide electrodes are not reversible. Formamide and alcohols are suitable substrates for oxidation at the irradiated zinc oxide electrode. At irradiated cadmium sulfide electrodes organic oxidizable materials have no effect; the sulfide itself undergoes photo-oxidation. Ceric ions, in acid solution at the dark, reference, platinum electrode, give the highest photovoltages and photocurrents so far sustained.

Conversion of isopropyl alcohol to acetone on irradiated zinc oxide

Irradiated suspensions of zinc oxide containing isopropyl alcohol form acetone and peroxide provided an adequate concentration of oxygen is present. Initial quantum yields of these products range from 0.25 to 1.50 depending on concentration of alcohol, specific catalyst surface, light intensity, and temperature. The peroxide concentration reaches a limiting value under all conditions, but the acetone accumulates indefinitely. Detailed investigation shows that the products obtained are the result of reactions of radicals in the liquid phase as well as direct surface reactions. The former are particularly important in pure organic liquids. The data obtained are best explained on the assumption that the energy of radiation at 3650 Å, approximately 80 kcal absorbed by zinc oxide, gives rise to an active form of oxygen which initiates both the surface and liquid phase reactions. Consideration of the various possible reactive forms of oxygen which might be produced under these conditions leads to the conclusion that an excited molecular oxygen anion is the most probable active intermediate. This interpretation is consistent with observations on irradiated zinc oxide reported by other investigators and also previous studies carried out in this laboratory. Lack of sufficient oxygen in the reaction mixture, or the instability of the excited molecular oxygen anion in the gas phase, may account for earlier reports of failure to observe vapor phase conversion of isopropyl alcohol to acetone on irradiated zinc oxide. These systems may not have practical value for the formation of peroxide, but the photocatalyzed conversion of isopropyl alcohol to acetone in zinc oxide suspensions is a very efficient process. An attractive feature is the fact that it can be carried out at low temperatures without the intervention of undesirable side reactions.

Szilard-Chalmers reaction in metal phthalocyanines—II

The effects of postirradiation heating on the retention of Zn/sup 65/ were studied for neutron irradiated zinc phthalocyanne and were considered to be anomalous. This behavior was traced to a peculiarity of the extraction methcd for the freed zinc ions from the irradiated phthalocyantne. The method of extracting the unretained metal ions consisted in refluxing the irradiated target with dilute sulluric acid for 2 hr. filtering the solution. and estimating the amount of radioactive metal in solid and solution. This gave reproducible results although it was known that the extraction was incomplete. Funther experiments were conducted using concentrated acid. diluting with ice water, and then heating at 150 C for 4 hr. The results of this extraction are tabulated. (B.O.G.)

The reactions of oxygen at dark and irradiated zinc oxide surfaces

Studies of the kinetics of adsorption of oxygen on zinc oxide over the temperature range from 25 to 390°C have provided evidence for two distinct types of chemisorption, one prevalent below 200 and the other above 300°C. This pattern of activity has been confirmed by measurements of desorption rates over the same temperature range. The mode of preparation and pretreatment of the oxide exert a strong influence on the adsorption behaviour, and these differences are accentuated when the processes of adsorption and desorption are studied in the presence of irradiation in the ultra-violet and visible. Photodesorption of oxygen is confirmed to be the normal behaviour for zinc oxide, but photo-adsorption has been observed under conditions of high excess zinc concentration. The photo-effects are especially marked below 300°C. The adsorption studies have been followed up by experiments on the rate of the intermolecular oxygen reaction 18 O 2 + 16 O 2 ⇌2 18 O 16 O, and on the influence of irradiation on this catalysis. It is evident that irradiation stimulates both adsorption and desorption, but the balance between them depends on the previous history of the specimen. The experiments with heavy oxygen have also included a brief study of oxygen exchange with zinc oxide at 400 to 500°C. The results as a whole are discussed in terms of the model of zinc oxide as an n -type semiconductor with interstitial zinc, and oxygen chemisorbed as O¯ and O 2¯ , respectively, are held to be mainly responsible for the phenomena observed. The relationship with conductivity studies is emphasized and the depletive chemisorption of oxygen, forming a boundary layer, is discussed in some detail. The depletion of electrons is not exhaustive for normal specimens of zinc oxide, and the treatment of this case leads to an expression consistent with the observed kinetics. Several possible mechanisms for photo-adsorption are put forward, and the association with high donor concentrations is discussed. Interstitial zinc diffusing under the influence of the electric field of chemisorbed oxygen is considered to play an important role in specimens heated above 300°C.