The cadmium sulfide powders were prepared from aqueous solutions of cadmium sulphate with sulphate acidity of 0N to 8N. Their mean particle sizes are about 0.1 to 4. 3 microns. According to X-ray diffraction patterns, all CdS powders were found to have hexagonal structure. The specimen with the smallest mean particle size has a light-yellow-collored appearance and one with the largest size, an orange-red-collored appearance. Their reflection spectra have been measured.
Using an interdigital electrode, the photoconductivity of packed powders was measured under various conditions. The current-voltage curves show that: 1) the current varies linearly with the applied voltage in an ohmic relationship at low voltages, below about 10 V (100 V/cm across the electrode gap), and 2) the current varies as a higher power of the voltage than unity at higher voltage than 10 V. The photocurrent varies as a power of about 0.6 with exciting light intensity. The spectral response curve extends from 400 mμ to 700 mμ, and has a peak at about 550 mμ for all the specimens. The sensitivity, however, depends considerably on the particle size. The powder of large particles has the higher sensitivity.
It is proposed on the basis of the data that the particle size-dependence of sensitivity is caused by the difference of electron-hole recombination coefficient. The powders of larger particles (which have small specific surface areas) have small recombination coefficients, and so they show high sensitivity. While, ones of smaller particles (which have large surface areas) have large recombination coefficients, and so they show low sensitivities. It is concluded consistently that the most part of recombination centers is distributed at the surface of particles.

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It has previously been reported that only when a polar (particularly an anionic) group was introduced into a merocyanine dye, adsorption and spectral sensitization occurred in zinc oxide electrophotography.
The possibility of chemical interactions between an acid merocyanine dye having a carboxymethyl group and zinc oxide grains has also been described in the previous paper of this series.
It is the purpose of this paper to report the probability of chemisorption of organic carboxylic acids including the acid merocyanine dye on zinc oxide surfaces by describing further experimental results obtained mainly from the infrared spectroscopic methods.
Zinc salt of the acid merocyanine dye was prepared to compare the infrared absorption spectrum with that of the dye which was adsorbed on zinc oxide, and it was concluded that the dye was converted to the zinc salt as a result of adsorption. Other organic carboxylic acids, e.g. stearic acid, gl. acetic acid, polymethacrylic acid, behaved likewise.
In addition to the zinc oxide-disk method of measuring infrared spectra of adsorbed molecules, the Attenuated Total Reflection method was applied in order to eliminate the possibility of mechanochemical changes of the adsorbate-adsorbent system at the disk making.
It is emphasized that chemical interactions between zinc oxide and acid dyes have to be taken into consideration when the spectral sensitization in zinc oxide electrophotography is discussed.

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The adsorption and desorption of oxygen on ZnO powder under illumination has been studied at various temperature.
The type of chemisorbed oxygen on ZnO is controlled to obtain a low temperature type or a high temperature type, by the pre-treatment. The potential change induced by the illumination is measured with the static electrode surface potentiometer. The activation energy of the oxygen chemisorbed on ZnO is obtained about 0.1 eV and 0.4 eV for the low temperature type.
In the sample of the high temperature type, the photo-adsorption is obserbed at the temperature over about 90°C.
At the same intensity of illumination, the potential change for the sample of the high tamperatuee type is about twice as large as the low temperature type.
The number of chemisorbed ion for the low temperature type is obtaimed about 1.3×10¹⁰/cm² at room temterature.
It made clear that the potential change induced by the illumination was the Barrier photo-effect.

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