T H E BEHAVIOR O F SILVER IODIDE I N T H E PHOTOVOLTAIC CELL, 11. A IVEW TYPE O F SILVER IODIDE PHOTO-VOLTAIC CELL BY ALLEN GARRISON’
This work on silver iodide is a continuation of an investigation of the cuprous oxide and silver halide photo-voltaic cells2 and the related photochemical and photo-physical properties3, which has been carried on under an appointment of the Research Fellowship Board of the National Research Council. The light-sensitive electrodes used in the firpt experiments with silver iodide were prepared by depositing crystals of the salt in a thin layer over a plate of metallic silver. These electrodes were found to become either positive or negative on illumination depending on certain conditions of the electrode or electrolyte. The experiments recorded in this report were performed on a different type of electrode, one whose properties was in some particulars better adapted to the discovery of the relation between the color or wave-length of the light rays and the extent and nature of the photo-chemical action. Therefore the experiments with this electrode have given results of importance bearing on a phase of the problem which has been only slightly developed.
Experimental Procedure The complete photo-voltaic cell as well as the method of observation was the same as that described in the previous paper on silver iodide4 with the exception of the light-sensitive electrode and its method of preparation. This consisted of a gold plate 2 cm. wide and 3 cm. long over which a thin film of molten silver iodide had been spread and permitted to solidify. The gold acts only as an inert metallic support for the apparently amorphous silver iodide. Electrodes having the same properties were made on plates of both platinum and palladium. Silver can not be used since the molten iodide apparently reacts with the silver and does not solidify in a thin transparent layer as jt does on the other metals. The entire front surface of the electrode was covered uniformly with silver iodide and the entire back surface was covered with paraffin. Thus when the electrode was placed in an electrolyte an electric current could pass between the gold and the solution only through the thin layer of silver iodide. This layer was so thin that it was transparent, had a greenish yellow color Kational Research Fellow in Chemistry. Garrison: J. Phys. Chem. 27, 631 (1923); 28, 333 (1924). 3 Garrison: J. Phys. Chem. 28, 279 (1924). Garrison : J. Phys. Chem. 28, 333 (1924). 1
2
BEHAVIOR O F SILVER IODIDE IS T H E PHOTO-POLTAIC CELL
59
and was a good conductor of electricity. When current was passed into the electrode from the solution iodide ion was liberated into the solution and silver was deposited in dark spots over the surface of the gold. The electrodes were mounted in the photo-voltaic cells and illuminated as has already been described in the previous paper. Light of various colors was obtained by the use of light filters. A large assortment of these was at hand and a good variety of wide and narrow bands in all portions of the visible spectrum could be obtained by using the proper combination of filters. The range of wave-lengths admitted to the cell in each case n7as measured with a direct vision spectroscope.
Experimental Results The light-sensitive electrode consisting of a thin continuous layer of silver iodide on gold in a solution of KI, AgXO, or NHJVO, is obviously an entirely different electrochemical system from that consisting of a silver electrode covered with a layer of the halide in the state cf fine crystals. Therefore it is not surprising that the electrode should have distinctly different properties. In contrast to the silver-silver halide electrode the photo-potential of the gold-silver halide system was always positive provided the electrode did not have metallic silver deposited on it from long continued exposure to light. The changes in potential on illumination were slcw, in case the light intensity was low it required several hours for the potential to reach its maximum value. This suggested that the effect may be due to the heating power of the rays but this was found to be of much smaller magnitude than the photo-potential2 obtained. Thus the temperature coefficient of one of the electrodes in a 0.1 normal solution of iYH,KO, was found to be +.oo09 of a volt per degree while the photo-potential of this system in the light of a 500 candle power lamp at 25 centimeters was +.45 of a volt. The temperature did not rise more than 2OC.
The return of the electrode potential to its original value after the light was removed was also a very slow process requiring several hours. This property of the electrode made possible a very instructive experiment. An electrode in a 0.1normal AgN03 solution was found to have a potential of +0.413 volts relative to the 0.1normal calomel element in the dark. The light-sensitive electrode was then removed from the solution, dried and exposed to light in air behind a water screen for ten minutes. When the light was extinguished it was placed immediately in the solution again and its potential found to be +0.744 volts. After several hours this had fallen to S o . 4 1 3 again. This demonstrates conclusively that the change in potential is not, like the Hallwachs effect, a result of electron emission. A change is produced in the silver iodide by the light and is only slowly reversed in the dark. This photo-chemical change has been shown to involve an increase in solubility. Changes in the constitution of the electrolyte had little effect on the magnitude of the photo-potential. For example, an electrode in a 0.1 normal potassium iodide solution had a dark potential of +o.z587 volts relative to
60
ALLEN GARRISOS
the calomel element and its photo-potential was +0.450 volts while the same electrode in 0.1normal silver nitrate solution had a dark potential of +0.4110 volts and a photo-potential of +0.432 volts. After continued illumination the silver iodide layer became discolored by small spots of silver. After this happened the properties of the electrode changed and it began t o behave like the electrodes prepared on metallic silver. The original properties of the electrode were restored by simply melting the silver iodide and allowing it to solidify again. The Relation between the Light Intensity and the Photo-Potential
It was found that the relation between the maximum values of the photopotential and the intensity of the illuminating beam could not be measured as accurately as was possible in the case of electrodes which responded to the light more rapidly. When light of relatively low intensity was used it required as long as six hours for the electrode potential to rise to its equilibrium value and during this period of time enough permanent decomposition occurred to alter the nature of the electrode. The rough experiments which were possible indicate that the relation is the same form as was obtained for the silver-silver halide electrodes. For low intensities the maximum photopotential was almost linear with the intensity and for higher intensities became nearly constant and independent of the intensity. Since the relation for the maximum values could not be obtained accurately, measurements were made on the rate of rise of the photo-potential. If the values of the photo-potential were plotted against the time the points fell approximately on an exponential curve. The rate of the rise of the photopotential was nearly constant and proportional to the light intensity immediately after the light was turned on but after some time had elapsed the rate became slower and approached zero when the potential was near its maximum value. It was not necessary to solve this exponential curve for its constants for, by limiting the light intensity to moderate values and by confining the observations of rate to the first five minutes of illumination, the rate of rise of the photo-potential could be made to be linear with the light intensity. The slope of this line is therefore a constant which is characteristic of the rate of the photo-chemical reaction proceding in the electrode. Table I contains some of the results which were obtained with a gold-silver iodide electrode in 0.1normal NH4XO3solution. The first column gives the color of the light and the range of wave-lengths included in the illuminating beam for each observation. The second column gives the intensity of the light as recorded by the thermopile. When the light was turned on the potential was recorded a t one minute intervals and an average rate of change in volts per minute was calculated for the first five minutes of illumination. This average rate of rise dE/dt is recorded in the third column. After each illumination the cell was left in the dark until the original potential was restored before another observation was made.
BEHAVIOR O F SILVER I O D I D E Iilr THE PHOTO-TOLTAIC CELL
61
The values in the fourth column are tJherates per unit intensity, that is,
TABLEI The Rate of Rise of Photo-potential and the Light Intensity Wave-lengt h
whit8e ,406- ,790 microns
blue ,402- . 495 microns
I 3.0
. 0 1 20
4.4
.OI
red (narrow) ,645 - I 7 5 0
microns
73
j.0
,0200
6.4 7.0 7.6
,0252
3.7
,0036
5.2
.0048 ,0065
7.2
1j.8 24.5
red ,602- .790 microns
dF, tit
3.4 4.9 7.8
.0278
,0298
.OIjo
,0230
I
