3538
A. KALDOR AND G. A. SOMORJAI
Photodecomposition of Lead Chloride
by A. Kaldor and G. A. Somorjai Department of Chemistry, University of California, Berkeley, California 94720 (Received May 19, 1966)
Lead chloride, PbClZ, undergoes photodecomposition in the temperature range 25-225" under vacuum or in a nonreactive ambient. The threshold photon energy which is necessary to induce the reaction shifts sharply to longer wavelength as a function of temperature and it is smaller than the energy at the absorption edge of lead chloride. The photoreaction is a strong function of the light intensity and virtually independent of temperature in the studied range. It is proposed that charge transfer controls the photoreaction rate which occurs via chloride ion vacancies. Several types of chloride ion vacancies can exist in the lead chloride lattice which are electron traps in the energy range 4.2-4.4 ev above the valence band. At temperatures above 225" thermal decomposition of lead chloride occurs readily under nonequilibrium conditions. Oxidation at elevated temperatures results in the formation of oxychloride compounds a t the lead chloride surface which renders it insensitive to further photodecomposition.
Introduction The photodecomposition of solids exhibits most of the important features of solid-state reactions. The complex over-all reaction can be divided into three major steps: (1) the creation and trapping of free carriers at defect centers or at lattice sites, ( 2 ) surface and/or bulk diffusion of atoms or vacancies away from the lattice site where the charge trapping occurred, and (3) the removal of some of the reaction products from the solid surface. Since the electromagnetic radiation which induces photodecomposition is absorbed strongly by the solid in most cases, the photochemical reaction is often limited to a narrow layer at the surface of the solid. Such complexity makes it difficult to investigate the mechanism of photodecomposition in the solid state. This may be the reason for the scarcity of such investigations in inorganic solids. Most of the photodecomposition studies have been carried out on one type of compound, silver halides. The mechanism of photoreaction for these compounds has been discussed in several more recent papers.'v2 We have investigated the photodecomposition of compounds other than silver halides. I n this paper, we shall discuss the photodecomposition of lead chloride, PbC12. When the white solid is irradiated by light of suitable Wavelength a t temperatures, T 2 25", under vacuum or in a nonreactive ambient, lead precipitates The Journal of Physical Chemistry
and the surface turns to a dark gray color. The net photoreaction which occurs at the solid surface may be written as PbClz(s)
-
\hv
hu
Pb(s)
L Pb(s)
+ Clz(vapor)
+ 2Cl(vapor)
(1)
The studies which were carried out to understand the mechanism of this reaction can be divided into the following groups: (a) identification of the reaction products and studies of the structure of lead chloride; (b) optical studies of the absorption characteristics of lead chloride and their variation with temperature; optical studies of the wavelength, intensity, time, and temperature dependence of the photodecomposition rate; (c) studies of the effects of ambients [vacuum, Nz, Hz, PbClz(vapor), 0 2 1 on the photoreaction.
Experimental Section Properties of PbCk and Experimental Procedure. Lead chloride, PbClZ, melts at 501", it is o r t h o r h ~ m b i c , ~ ~ ~ (1) C. E. K. Mees and T. H. James, "The Theory of the Photographic Process," The Macmillan Co., New York, N. Y., 1966. (2) J. W. Mitchell, J . Phot. Sci., 5, 49 (1957). (3) H. Braekken, 2.Krist., 83, 222 (1932). (4) R.L. Sass, E. G. Brackett, and T. E. Brackett, J . Phys. Chem., 67, 2863 (1963).
PHOTODECO~fPOSITIONO F
LEADCHLORIDE
3539
surface plane. Spectroscopic analysis of the high and the anion-cation distance varies from 2.80 to 3.70 purity PbClz powder showed the presence of only Cu A. The chloride ions occupy nine different positions and Fe impurities, in concentration less than 10 ppm. about the lead ionse4 The compound shows ionic A high-pressure mercury lamp (PEK llO), was used character, has a wide band gap5 (Egap 5.28 ev a t as a light source for the photodecomposition and the 2,j0),and exhibits ionic c o n d u ~ t i v i t y . ~ , ~ optical transmission measurements. The absorption Conductivity studies on single crystals doped with curves were obtained on a Cary 14 recording spectroAgC1, KCl, BiC13, and LaCL indicate that chloride ion photometer. The temperature dependence of the abvacancies are the dominant mobile charge carriers in sorption edge was monitored using a grating monothe range 25-400" although the presence of interstitial chromator and a photoconductor detector. For the lead ions and vacancy aggregates which may participate optical reflection studies, a calibrated brightness spotin the charge transport cannot be ruled out completely. photometer was employed. I n studies of the waveThe activation energy of formation of chloride ion of the photodecomposition where length dependence is ,reported to be Ev,,(formation) = 0.52 vacancy, V C ~ large light intensities ( I = 2 X lo3 to 2 X 105 pw/cm2) ev18while the activation energy for the diffusion of V C l were necessary, cutoff filters mere used. is Ev,,(diffusion) = 0.30 ev. The mobility of chloride The reaction products were analyzed in a hydrocarion vacancies, p, in the undoped crystal8 is given by bon-free ultrahigh-vacuum system using a quadrupole p = [(a X 102)/T] exp(-00.35/kT) cmz v-I sec-'. Thus, using these values, the diffusion constant of V C ~ mass spectrometer. The sensitivity of the instrument mass unit in is torr and has a resolution of and its equilibrium concentration can readily be calcuthe mass range 5-500 mass units. lated in the studied temperature range. The activaNost of the studies of the photodecomposition and of tion energies of lead ion vacancy formation and migrathe absorption characteristics of PbC12were carried out tion are appreciably greater8fgthan the corresponding under vacuum or in a nonreactive atmosphere (nitrovalues for the anion vacancy. gen). Identical results were obtained in measurePbClz undergoes thermal decomposition at tempem ments which were performed in air at temperatures of tures T 2 200" with the precipitation of lead as a second 25-225". Above that, temperature, however, the phase. The rate of thermal decomposition is slow a t T oxidation of PbClz takes place at an appreciable rate as = 200" (-1 hr) but increases rapidly with increasing to affect the photoreaction. The effect of an oxidizing temperature. Decomposition occurs only if the haloambient on the photodecomposition will be discussed gen reaction products are removed from the surface, later. Le., under vacuum or in other ambients under nonequiil/lass Xpectrometuk Study of the Photo- and Thermal librium conditions. When the solid is in equilibrium Decoinposition of PbC12. The opaque evaporated with PbCl? vapor, thermal decomposition does not films of lead chloride were illuminated at 25" in u l t m occur. 300 mp and high vacuum by light of wavelength X Vacuum-evaporated thin PbClz layers (20 & 5 p ) intensities1 = 2 X lo3 to 2 X lo5pw/cm2. The mass mere used in most of our studies to provide a large surface area for the photoreaction to occur. The films spectrometer trace shows that the major gaseous products of the photodecomposition are C1 and Ch. HC1 were vaporized under high vacuum ( 225") the apparent absence of the photodecomposition can similarly be observed. This effect is due to the oxidation of lead chloride which results in the formation of oxychlorides, zPbO 'yPbC12 which have similar light absorption characteristics to that of lead chl0ride.1~ Sine oxychloride compounds of lead have been reported" which form under different oxidizing conditions. The oxychlorides, unlike lead chloride, do not readily decompose in light. Therefore, their formation on the surface of lead chloride renders the thin film insensitive to further photoreaction.
Discussion Lead chloride decomposes upon illumination under vacuum or in a nonreactive ambient in the temperature range 25-225". It is apparent that among the three major reaction steps leading to the photodecomposition, charge trapping by lead and chloride ions is the rate-limiting step. This can be seen from the strong light intensity dependence of the photoreaction. The separation of lead and chlorine atoms by diffusion during the trapping lifetime of the charge carriers is sufficiently rapid at room temperature that it has little effect on the photodecomposition rate. The lack of a strong temperature dependence of the photoreaction in the studied temperature range supports this conclusion. Diffusion most likely occurs via chloride ion vacancies which are known to have the lowest activation energy of formation and diffusion among the ions and ion defects in the lead chloride lattice. The diffusion rates of the crystal constituents via (16) R. H. Bube, "Photoconductivity of Solids," John Wiley and Sons, Inc., New York, N. Y., 1960. (17) J. W. Mellor, "Inorganic and Theoretical Chemistry," Vol. 7 , Longmans, Green and Co., London, 1927, p 736.
PHOTODECOMPOSITION OF LEADCHLORIDE
3543
not been studied as yet. One mechanism may be the chloride ion vacancy mechanism are so great that expressed as thermal decomposition of lead chloride occurs rapidly at temperatures ( T > 225") where the chloride ion cl-(lattice) V c l S vel~1.f (4 vacancy concentration becomes an appreciable fraction of the atomic density (-101' vacancies/cm3). zPb2+(lattice) y V ~ l + At - 195", however, due to the exponential tempera[Pbz2+Vci,-]+XPbJ. yVc1 (b) ture dependence of the diffusion rate, the diffusion of chloride and lead atoms can be the rate-limiting where C1-(lattice) and Pb*+(lattice) are the anion step in the photodecomposition as it was indicated by and the cation in their lattice positions. The photonthe low-temperature experiments. induced charge trapping at chloride ion vacancies is The final major step in the photodecomposition followed by charge transfer between the chloride ion involves the removal of chlorine from the crystal vacancy and the cation. The first step results in the lattice. This process which can control the rate of removal of chlorine from the lattice while the second photodecomposition of other groups of compoundslS step leads to the precipitation of lead. The minimum does not seem to affect the lead chloride photoreacnumber of cations and charged vacancies (z and y) tion. There is no indication that the rate of the photowhich, when aggregate, result in the precipitation of decomposition is affected in any way by using nonlead, has not been determined. ;\lore chloride ions reactive ambient (nitrogen, argon) instead of a vacuum. can subsequently diffuse from the bulk to the illumiIt seems likely that the hole mobility in lead chloride nated surface via the chloride ion vacancy mechanism is so low that the chlorine atom which undergoes charge to participate in the photoreaction. transfer may vaporize instead of releasing the hole to The photodecomposition reaction seems to have the neighboring chloride ions. a different mechanism under conditions of lower light Since charge trapping seems to control the photointensity (I < 7 X lo4 pw/cm2) from that at high light decomposition of lead chloride, verifying the mechlevels. At low intensities, photodecomposition occurs anism of the charge transfer is of primary importance. with low quantum efficiency (D = IO.''). At high We find that the absorption edge of lead chloride is a light intensities, many lead atoms precipitate as a weak function of temperature (Figure 2, a) while result of the absorption of a single photon at the lead the threshold photon energy which is necessary to chloride surface (D = P - 7 . One may suggest that a t cause photodecomposition changes sharply with temhigh light levels the surface density of chloride ion perature (Figure 2, b). These measurements indicate vacancies becomes so great that their average distance that photon energies which are smaller than the energy becomes smaller than 3, the mean displacement of a t the absorption edge can induce photodecomposition. the chlorine atoms. Therefore, the crystal lattice The excited electrons must be captured by trapping collapses and the observed superlinear growth of the centers which are in the forbidden gap 4.2-4.4 ev above lead aggregates is observed. the valence band. It is likely that these centers are There is a strong temperature dependence of the khloride ion vacancies. The irreversible broadening threshold energy which is necessary to induce photoof the absorption peak a t E = 4.6 ev upon heat treatdecomposition. The minimum energy shifts from the ment of the lead chloride films under vacuum, nitroultraviolet to the visible range of the electromagnetic gen, or lead vapor* is due to the formation of chloride spectra within AT = 200". This can be explained by ion vacancies. The broad energy range in which these the presence of the different types of chloride ion vacantrapping centers can exist in the forbidden gap is due cies in a broad energy range in the forbidden gap which to the many different types of atomic positions the can all serve as electron traps. This unique characterchloride ions can occupy in the lead chloride l a t t i ~ e . ~ istic of lead chloride could certainly be important if The color centers thus formed, Vcl-, can induce imthe substance is to be considered for purposes of phomediate charge transfer and the subsequent pretography. cipitation of the neighboring lead ions. The experiments were carried out using undoped It is difficult to describe the mechanism of the lead chloride thin films. It is expected that impurities photodecomposition of PbC12 by a simple reaction which increase the efficiency of trapping and the scheme since there are competing processes in each step trapping lifetime of the free carriers will further acwhich lead to the formation of the final products. celerate the photodecomposition. The incorporation of Several possible mechanisms can be suggested since the nature of lattice defects in PbC12, charge trapping processes, and the precipitation kinetics of lead have (18) G. A. Somorjai, Surface Sci., 2, 298 (1964).
+
+
+
+
Volume 70,Number 1 1
November 1966
3544
ALGIRDG. LEIGAAND JAMES N. SARMOUSAKIS
some impurities, such as monovalent cations (for example, T1+) can increase the chloride ion vacancy
concentration, and thus may accelerate the rate of thermal decomposition.
The Effect of Certain Salts on the Aqueous Solubilities of 0-,
m-, and p-Dinitrobenzene
by Algird G . Leigal and James N. Sarmousakis Department of Chemistry, New York University, New York, New York
(Received May 19, 1966)
The aqueous solubilities of 0-, m-, and p-dinitrobenzene at 25“ were found to be 0.1247, 0.5328, and 0.0618 g/l., respectively. The activity coefficients, f i , of the nonelectrolytes in aqueous solutions of sodium chloride, sodium p-toluenesulfonate, and tetraethylammonium chloride were obtained as a function of salt concentration, C,. Using the equation logfi = k,C,, the limiting slopes, k,, were obtained for each dinitrobenzene with each of the three salts. It was found that the net dipole moment of the dinitrobenzenes was not a significant factor in determining the salting out parameter, IC,, but rather that distinct differences were obtained between the behavior of the orlho isomer and that of the other two which behaved similarly. At higher salt concentrations, the solubilities of 0- and mdinitrobenzene in sodium p-toluenesulfonate were found, first, to increase and then to decrease greatly beyond about 1 M ; however, the solubility of the para isomer increased monotonously. I n tetraethylammonium chloride solutions, the soiubilities of m- and pdinitrobenzene went through a maximum while that of the ortho isomer increased monotonously as the salt concentration increased. It was concluded that the sharp breaks in the solubility curves with the large anion salt resulted from the formation of a complex between the nonelectrolyte and the salt.
I. Introduction The aqueous solubility of a nonelectrolyte has been found generally to be dependent on the concentration and type of salt present in solution. Some salts cause the solubility to decrease, “salting out” the nonelectrolyte, and other types of salts can enhance the nonelectrolyte solubility, causing “salting in.” The activity coefficient of a nonelectrolyte in a salt solution may be taken as a measure of the salt effect on the solubility of the nonelectrolyte. The activity coefficient is defined by ai = fiC,, where ai and f i are the activity and the activity coefficient of the nonelectrolyte in a solution of salt concentration C,. For a saturated aqueous The JournaE of Physical Chemistry
solution of a sparingly soluble nonelectrolyte, the activity coefficient may be taken as unity. I n this case, the activity equals the nonelectrolyte solubility, Sio. The activity coefficient of the nonelectrolyte in a salt solution is then given by f i = Sio//siwhere Si is the solubility in the salt solution. In sufficiently dilute nonelectrolyte solutions, the expression for the activity coefficient2 becomes
(1) Xerox Corp., P.0.Box 1540,Rochester, N. Y. 14603. (2) F. A. Long and W. F. McDevit, Chem. Rev., 51, 119 (1952).
