2174
JOHN H. KENNEDY AND EMILBOODMAN
thus attained from a single low-temperature configuration and the disordering aspect of the transition process will be characterized by an entropy change of R In 2. Since there is no energy difference between forms, either might, of course, exist exclusively in a separate crystal. The shape of the excess heat capacity through the transition has features common to both Schottky (rounded maximum) and X (the slopes above and below the transition) anomalies. X anomalies arise where the disordering phenomenon is cooperative, Schottky anomalies where it is not (for a more complete discussion see Fowler).13 The shape of this transition in [2.2]paracyclophane suggests, therefore, that it is characterized by a weakly cooperative disordering phe-
nomenon. The proposed molecular twisting would affect only nearest neighbors and so the transition shape is not inconsistent with the proposed mechanism. In the absence of definitive physical evidence this mechanism cannot be regarded as other than tentative. Nmr and ir spectroscopy throughout the transition region might well indicate the truth of the matter, and X-ray structural information as a function of temperature would be of great interest. Acknowledgments. The authors are grateful for the support of the United States Atomic Energy Commission throughout this research. They also thank Mrs. Suong-Sik Kim for her help with the calculations. (13) R . H.Fowler, “Statistical Mechanics,” Cambridge University Press, Cambridge, 1936,p 807.
Photoelectric Effects in Solid Electrolyte Materials
by John H. Kennedy Chemistrv Department, Uniaersity of California, Santa Barbara, California 93106
and Emil Boodman Bissett-Berman Corporation, Santa Monica, California (Receiued September 22, 1969)
The photoelectric responses of silver sulfide bromide, Ag3SBr, and silver sulfide iodide, Ag3SI,have been observed as a function of wavelength. A fast initial photocurrent is followed by a lower steady-state current. From the amount of silver deposited coulometrically during light exposure, the photocurrent was shown to be predominantly ionic in nature. The results are ascribed to a photoreaction with sulfide ions, producing silver ion photocurrent.
Introduction Photoelectric effects with solid silver halides have been studied by several investigators both in polycrystalline materia11f2 and single crystal materiaL3 As part of our studies on electrochemical properties of solid electrolyte materials, we have examined the effect of light impinging upon an electrolyte surface coated with a partially transparent gold electrode. The solid electrolytes studied were primarily Ag,SBr and Ag3SI, but results are compared with those from similar experiments carried out with AgBr, AgI, Ag2S, and RbAgJS. Silver sulfide halides could have interesting photoreaction behavior as well as electrochemical behavior, since they are well defined compounds, but may demonstrate properties related to the parent compounds AgBr, AgI, and AgzS. Electrical conductivity has been shown to be almost entirely ionic, unlike Ag2S, yet orders of magnitude higher than the correIt will be shown that photosponding silver The Journal of Physical Chemistry
electric properties are also significantly different from the parent compounds. Photoreactions of AgBr and AgzShave been described by slightly different mechanisms, but in essence the net result is the same. Tan and Trautweiler3 view the initial reaction in AgBr to be the production of an electron-hole pair with the holes being trapped near the point of creation by reaction with AgBr to form BrO and silver ion interstitials. The more mobile photoelectrons diffuse into the bulk material and eventually become trapped by reaction with a silver ion interstitial (1) 6. E.Sheppard, W. Vanselow, and V. C. Hall, J . Phys. Chem., 33, 311, 1403 (1929). (2) W. Cooper, J . Phys. Chem., 66, 857 (1962). (3) Y. T.Tan and F. Trautweiler, J. Appl. Phys., 40, 66 (1969). (4) V. Reuter and K. Hardel, Naturwissenschaften, 48, 161 (1961). (6) T. Takahashi and 0. Yamamoto, EZectrochim. Acta, 11, 779 (1966). (6) J. H . Kennedy and F. Chen, J . Electrochem. floc., 116, 207 (1969).
PHOTOELECTRIC EFFECTS IN SOLIDELECTROLYTE MATERIALS to form Ago. The net reaction is the formation of bromine atoms near the surface, silver atoms in the bulk, and a net flow of silver ion interstitials from the surface into the bulk material. Photoresponse begins a t wavelengths below 500 nm. For silver sulfide, Scholz postulated that photons eject electrons directly from sulfide ion donors to form So near the surface.’ The photoelectrons migrate into the bulk and combine with an acceptor such as interstitial silver ions to form silver atoms. The surface would build up an excess of silver ions, and these would migrate in the form of silver ion interstitials into the bulk. As can be seen, the net reaction would be analogous to the AgBr reaction. However, Ichimescu and Suciu report a photoconductivity maximum at 1050 nm8 which is much lower in energy compared to the AgBr reaction.
Experimental Section Materials. Ag3SBr and Ag3SI were prepared by the method described previously6 using analytical reagent grade silver salts. Silver and gold electrodes, with the exception of the gold minigrid screen, were prepared from 99.99% pure powders. The minigrid screen was a 45% transparent electro-formed grid made by Buckbee-Mears and consisted of 1000 lines/ in. Photoelectric Cells. Pellet-type cells were prepared by compressing powders with a Perkin-Elmer KBr pellet die. The silver or gold electrodes were formed from micron-sized powders pressed at 1000 psi. While the silver pellet was in the press, the powdered electrolyte was placed into the die cavity and tamped in place. The gold minigrid was placed on the tamped electrolyte powder and the entire assembly, silver pellet, electrolyte, and gold minigrid, was brought to a full applied pressure of 24,000 psi. The completed pellet measured 13.5 mm diameter by approximately 0.80 mm thickness. The photoresponse electrode was the gold minigrid, while the solid silver or solid gold electrodes were counterelectrodes. The pellets had a resistance of 5-10 ohms as measured with a 1 kHz signal. Electrical contact to the pellet was made by using gold-plated beryllium-copper lead springs. Photoconductivity Measurements. A potential was applied between the gold grid and counterelectrode with the grid being positive. After allowing the current to reach a steady-state value, the grid electrode was exposed to light from either a Honeywell Prox-olite xenon flash tube or a Bausch and Lomb Nicholas microscope illuminator, Model 31-33-56, for continuous illumination. For wavelength studies, a Bausch and Lomb monochromator was incorporated between source and photocell. The source for wavelength studies was a Sylvania Sungun Lamp (FAW) with a temperature of 6200°K and 27,000 candle power a t the beam center. Data given are uncorrected for spectral output. Since
2175
spectral output for the particular lamp used was not measured, data given are uncorrected for spectral output. The lamp type has a spectral peak energy ca. 500 nm and drops off rapidly above 1000 nm. Correction of the data would enhance the near-infrared peaks with some shift toward higher wavelength. Photocurrent response was recorded on an Electro Instruments Model 520 X-Y Plotter. By using open-circuit conditions in place of an applied voltage, photovoltaic response was also measured with the same instrumentation. Coulometry Measurements. Pellets of Ag3SBr sandwiched between a gold grid electrode and a gold counterelectrode were fabricated, and the gold counterelectrode was then split to form a three-electrode system. The third electrode acted primarily as an auxiliary, but as it became silver plated, this electrode could also be considered a silver reference electrode. The other solid gold electrode, designated electrode two, was used to collect silver during the experiments, which was then determined coulometrically. The grid, electrode one, was the photoresponsive electrode. First, electrodes one and two were brought to a positive potential us. electrode three, an operation called “anodic clearing.” Following the “clearing” operation, electrode one was held at a positive potential us. electrode two, while electrode three was disconnected. Blank runs were carried out in the dark, but normally this step was with the grid electrode exposed to light. After a period of time of 2-10 min, electrode two was “read-out” coulometrically a t a constant current us. electrode three. When silver was completely stripped from electrode two, the potential rose sharply, and the current was automatically stopped when electrode two was +0.500 V us. electrode three. This operation was the same as that used for solid electrolyte coulometers.6~9 Coulombs of charge were determined from the constant current and time for this read-out and was compared with the coulombs of charge during the light exposure period calculated from the area under the current-time curve.
Results and Discussion Photoresponse was observed with Ag3SBr pellets when the transparent gold electrode was exposed to flashes from a xenon tube 2 in. away. With 250 mV applied between the electrodes (transparent gold electrode positive), peak currents of 22 p A were recorded. The current decreased as the strobe distance increased, but the peak current appeared saturated for distances less than 2 in. When continuous illumination was used, two effects were noted. An inital peak current was reached in (7) A . Scholz, Ann. Phys. (Leipzig), 19, 175 (1956). (8) A. 1chimesci.i and P. Suoiu, Rev.Roum. Phgs., 12, 917 (1967). (9) J. H. Kennedy, F. Chen, and A. Clifton, J . Electrochem. Soc., 115, 918 (1968). Volume 74,Number 10 M a y 14, 1970
PHOTOELECTRIC EFFECTS IN SOLIDELECTROLYTE MATERIALS
2177
Table I : Integration of Photocurrent Results of Ag3SBr r INITIAL PEAK CURREl
Duration,
' STATE C
sec
PA
mC
450 360 500 400 300 330 300
50 90 60 60 45 80 48
15.6 23.7 18.8 18.0 10.0 21.4 11.0
Silver plated, mC
17.8 23.2 17.2 20.2 11.4 21.6 13.3*
Diff,
%"
+14 -2 -9 12 +11 $1 $21
+
1ENT
I
I 800
9Q)
IMO
1100
WLVELENGTH, nm
Figure 2. Photoconductivity of Ag3SI. Applied voltage 75 mV, circuit resistance 100 ohms, illuminated by Sylvania FAW 6200'K color temperature lamp.
CHOTO-CURRENT
50
Charge,
a Positive difference assigned for more silver found than Silver st,ripped charge calculated by integrating photocurrent. 2.5 hr after exposure, showing storage capability. From ref 6, Figure 10 it is seen that a positive error is expected after storage, amounting to about 1.5 mC. If this correction is included, the silver-plated value becomes 11.8 and the difference is +7%.
7
0
Photocurrent-Peak current,
-
100
IS0
200
230
300
APPLIEDVOLTAGE, mi
Figure 3. Photocurrent dependence on applied voltage: A, Ag3SBr peak current at 1100 nm; B, AgaSBr steady state current at 650 nm; C, AgsSI peak current at 1100 nm; D, AgSSI steady state current a t 650 nm.
may be useful for measuring light signals with storage of the information in the form of plated silver which can be read out by anodic stripping a t a later time. Based on these results, the following mechanism is postulated. Photons produce photoelectrons by reaction with sulfide ions near the gold minigrid electrode and making it become more negative a t open circuit. When held positive, the minigrid electrode collects
the electrons which then flow through the electrical circuit to the counterelectrode. In previous studies, the light-exposed surface was not kept positive, which allowed the photoelectrons to diffuse into the bulk material. The applied voltage dependence, shown in Figure 3, suggests that photoionization occurs more readily as the local Fermi level at the minigrid is raised (less positive voltage), but that the existence of at least some positive potential aids the electrode collection process. Sulfide ions at the electrode surface would be especially active and account for the initial peak currents shown in Figure 1 and 2. The lower steady-state currents, on the other hand, are supported by migration of electrons produced away from the electrode surface toward the positive electrode. At the counterelectrode, silver ions migrating to the cathode are reduced to silver metal. Since a buildup of excess silver ions would occur a t the anode as sulfide ions are oxidized to sulfur, there would be a net flow of silver ions from anode to cathode which is the same as that postulated in the previous photovoltaic studies with AgBr or Ag2S. Since the photoelectrons are collected by the positive gold electrode, the photocurrent is ionic in nature. The process depends on fast migration of silver ions and helps to account for the high response of Ag3SBr and Ag3SI compared to AgBr, AgI, or AgzS. For compounds of low conductivity, the rate-controlling step could actually be the migration of silver ions since the migration of photoelectrons toward the anode mentioned above would also depend on a cooperative migration of charge carriers in the electrolyte to preserve charge balance. However, the high conductivity of Ag8SBr and AgsSI should be ample to support the low photocurrents observed. The wavelength dependence suggests that the photoreaction is primarily due to reaction with sulfide. The peak response a t 1100-1200 nm is close to the 1050-nm peak reported for Ag2S. It must be remembered that Volume 74, Number 10 May 14, 1970
2178
LOUISFISHMAN AND RAYMOND D . MOUNTAIE
Ag3SX does not contain AgzS as such, but does contain sulfide ions surrounded by silver ions as does AgzS. A shift in the maximum would not be surprising since the sulfide ion environment is somewhat different. The additional small peak found for Ag,SBr between 600 and 700 nm may be a contribution from a photoreaction involving bromide or possibly a second ionization of sulfur ions. The maximum response for AgBr films was recently reported to be 365 nm,ll the same as AgI.” However, some response has been observed close to 500 nm for AgBr3 and may be shifted to higher wavelength by the change in environment. Photoreaction with iodide appears to be absent al-
though there was some response in the visible region which could include a contribution from such a reaction. I n conclusion, the photoreaction appears to be primarily with sulfide, but the ionic current which flows is supported by the facile migration of silver ions present in Ag,SX compounds.
Acknowledgment. The authors thank Mr. Currie of Bissett-Berman Corp. for carrying out many of the experiments described. (11) B. U. Barshchevskii, Piz. Tverd. Tela, 10, 3689 (1968).
Activity Coefficients of Solutions from the Intensity Ratio of Rayleigh to Brillouin Scattering1
by Louis Fishman and Raymond D. Mountain Institute for Basic Standards, National Bureau of Standards, Washington, D. C. 2OfB4
(Received December 29, 2989)
The ratio of the intensity of Rayleigh to Brillouin scattered light for a binary mixture with internal degrees of freedom is determined by calculating the frequency spectrum of fluctuations in concentration, temperature, and pressure. Thermodynamicfluctuation theory and linearized hydrodynamic equations, modified to include the internal degrees of freedom through a frequency-dependent volume viscosity, are employed in the calculation. The intensity ratio is of interest as it may be used to determine activity coefficients. A method of doing so when the system contains internal degrees of freedom is described.
Introduction Miller has pointed out how light scattering experiments, in which the Rayleigh and Brillouin components of the scattered light are resolved, may be used to determine the activity coefficients of binary solutions.za The ratio J of the intensity of the Rayleigh to the Brillouin scattered light was found from thermodynamic fluctuation theoryzbto be
-J - -
( b ~ / ’ d c ) ~, p~ / ’ ( b / ’ ~ ~ ,T ) p
ksT
polPs [
+
@ ~ / ~ p ) T , c
+ ( b ~ / d T ) ~ p , c T o / C(1),
(Ta~~T/~oCp)(b€/dT)p,el’
Here l c ~ Tis Boltzmann’s constant times the absolute temperature, E is the dielectric constant (measured at optical frequencies), p is the chemical potential, and c is the concentration as defined by Landau and Lif~ h i t z ,C~p is the specific heat a t constant pressure, p is the density, p is the pressure, Ps is the adiabatic compressibility, and aT is the isothermal expansion coefficient of the mixture. The subscript o indicates an equilibrium quantity. The appropriateness of this The Journal of Physical Chemistry
result was confirmed by a dynamical calculation made by Deutch and one of the author^.^ In a subsequent paper, Miller and Lee reported measurements of the intensity ratio for several dilute solutions.6 It is not possible to apply eq 1 directly to most of these measurements as several of the solvents chosen exhibit internal degrees of freedom (thermal relaxation or structural relaxation). In recognition of this situation, Miller and Lee heuristically developed a modified (1) Contribution of the Kational Bureau of Standards, not subject to copyright. (2) (a) G. A. Miller, J . Phys. Chem., 71, 2305 (1967); (b) L. D. Landau and E. M . Lifshite, “Statistical Physics,” Addison-Wesley Publishing Co., Inc., Reading, Mass., 1958, Chapter 12. (3) L. D. Landau and E. M. Lifshitz, “Fluid Mechanics,” AddisonWesley Publishing Co., Inc., Reading, Mass., 1959, Chapter 6. For 1 g of solution p = pl/ml - pdmz where p i and pz are the chemical potentials and ml and m2 are the masses of the two species. If ni is the number of particles of substance one in 1 g then c = nim~. (4) R. D. Mountain and J. M. Deutch, J. Chem. Phys., 5 0 , 1103 (1969). (5) G. A. Miller and C. S. Lee, J. Phys. Chent., 72, 4644 (1968).
