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J O U R N A L
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PHYSICAL CHEMISTRY Registered i n
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@ Copyright, 1966, by the American Chemical Society
VOLUME 69, NUMBER 3 MARCH 15, 1965
Spectral Sensitization of Chemical Effects in Solids’
by Jean Bourdon Centre de Recherche8 Kodak-Pathe, Vincennes, Seine, France
(Received October 6 , 1964)
The processes of spectral sensitization of the chemical effects in silver bromide and zinc oxide are described and discussed along the following lines. In what respect is the intrinsic photochemical effect modified when sensitized by a dye? I n what respect does the photochemical effect modify or perturb the spectral sensitization process? In the case of silver bromide, photoholes generated intrinsically are mobile into the crystal. This behavior can be altered considerably when they are generated by light absorbed by the dye. On the other hand, surface bromide ions seem to play an important role in the spectral sensitization process. In the case of zinc oxide, the present available data render it difficult to reach a definite conclusion on the reciprocal influence of oxygen photodesorption and the spectral sensitization process.
Many solids exhibit a chemical modification when illuminated by ultraviolet or visible light. Among these photosensitive solids are silver halides, mercury salts, many salts of heavy metallic ions2* (organic salts in particular), zinc oxide, and many organic solids2b such as photochromic compounds or photosensitive high polymers (fabrics and textiles being a practical example of the latter). The chemical mcldifications which are observed can be very different, varying from the photolysis of silver halidesa to the photodesorption of oxygen from zinc oxide, the photo cross linking of certain high po1ymers14 the photochromism in some organic solids, and the photoionization of organic molecules in a solid matrix. (See in the second part of this paper the reasons for including ZnO in photochemically sensitive solids.) Some of the photosensitive systems can be spectrally sensitized by dyes and this is particularly the case for silver halides, mercurous ~ x a l a t ezinc , ~ oxide, and some organic materials.6
The purpose of this review is not to give a comprehensive description of the various photosensitive systems described above, which is beyond the scope of this paper, but to present the main aspects of the mechanism of the only two systems which have been the subject of thorough studies: silver halides and zinc oxide. We
(I‘) Presented to the International Conference on Photosensitization in Solids, Chicago, Ill., June 22-24, 1964. (2) (a) P. Glafkidhs, “Chimie Photographique,” P. .Mantel, Paris, 1957, p. 350; (b) H.S. Gilmour. “Physics and Chemistry of the Organic Solid State,” Vol. I , Interscience Publishers, Inc., New York, N. Y., 1963,p. 330. (3) C. E. K. Mees, “The Theory of the Photographic Process,” Macmillan and Co., Ltd., London, 1954 (see also the new edition by T. H. James, in preparation). (4) L. M. Minsk, J. G. Smith, W. P. Van Deusen, and J . F. Wright, J . Appl. Polymer Sci., 2 , 302 (1959). (5) P. A. Van der Meulen and R. H. Brill, Phot. Sei. Eng., 2 , 121 (1958). (6) E. M. Robertson, W. P. Van Deusen, and L. M. Minsk, J. Appl. Polymer Sei., 2 , 308 (1959).
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intend in particular to discuss the problem along the following lines. The process of spectral sensitization of inorganic photosensitive solids such as silver halides or zinc oxide goes through an electronic step, as in the case of any dye-sensitized photoelectric effect in a semiconductor. This electronic step is followed by the chemical step itself, which is, for silver bromide, the formation of a latent image (Ago) and evolution of bromine, and for zinc oxide, the desorption of ~ x y g e n . ~ No differences should therefore be found between the mechanism of spectral sensitization of these two systems (chemical transformation or electronic effect) and as a matter of fact, they have been, for the most part, the object of the same studies and the same discussions.*-”J In spite of this consideration, it should be of interest to try to answer the following questions. (1) In what respect is the chemical effect which is produced by irradiation in the intrinsic absorption band of the solid modified when the irradiation is given in the absorption band of the sensitizer? (2) In what respect does the chemical effect modify or perturb the spectral sensitization process? The influence of photolytic silver on the spectral sensitization process is not considered. (3) I n relation to the second question, what is the part taken by the photochemical transformation in the regeneration of electron donor levels, which appear to be essential to this process? (4) Is there energy transfer or electron transfer? Though here somewhat outside the subject, a discussion on spectral sensitization mechanism would not be complete without a few comments on this highly controversial subject. In spite of the many points of similarity between spectral sensitization of silver halides and of zinc oxide, for reason of convenience, these two topics will be discussed separately. The following report will deal mostly with cases of spectral sensitization by dyes in a molecular state of adsorption, excluding therefore, unless specifically mentioned, cases of spectral sensitization by dye films.
A. Silver Bromide I . Intrinsic process, The chemical effect initiated by light in silver bromide is a photolysis resulting in the formation Of metallic Silver and eVOlUtiOn Of bromine. For very weak illuminations, the metallic silver will be described as a latent image: for stronger illuminations, very large amounts of silver and bromine are released; this is called print-out‘ In both cases’ the photochemical reactions are written The Journal of Physical Chemistry
JEAN BOURDON
Bre-
+ hv +e- + Bro (positive hole) + Ag+ -+ Ago +latent image
BrO +Br + Br2 (evolution of photolytic bromine) (surface) The mechanism of formation of the latent image has been the object of many studies and very good review papers have been given on the subject.“ We will accept, for the present discussion, the mechanism as described by Gurney and Mott, one of the best pieces of evidence, among others, being the experiments of Haynes and Shockley,l 2 later repeated and very thoroughly developed by WebbI3 and Hamilton, et aL1* These experiments consist of the displacement, in the silver halide sample, of electrons and holes by a pulsed applied field synchronized with the excitation flash. They showed without ambiguity that the absorbed photon creates a mobile hole and a mobile electron, the latter resulting finally in the formation of the latent image. I I . Spectral Sensitization of Silver Bromide.3 , l 5 West and Carroll recently have written a very good review on the subject16 and hence only a few points will be developed here to recall the main aspects of the problem. A. Conditions of Spectral Sensitization (Figure 1 ) . A dye molecule will be a good sensitizer for the photolysis and the photoconduction of silver bromide if several requirements, arising from the following considerations, are met. The energy of the photon absorbed by the dye has to be transferred (as energy or as an electron) to the silver bromide crystal, the efficiency of the transfer being decreased by any competing factors which will interfere with it and increased if the dye possesses the corresponding required qualities. ~~~
~
~
(7) D. B. Medved, J . Chem. Phys., 28, 870 (1958). (8) R. C. Nelson, J . O p t . SOC.Am., 46, 13, 1016 (1956); 51, 1182, 1186 (1961). (9) J. Meier , “Die Photochemie der organischen Farbstoffe,” Springer-Verlag, Berlin, 1963: (a) p. 273; (b) p. 288. (10) (a) A. Terenin, E. Putseiko, and I. Akimov, J . chim. phys.. 716 (1957); (b) A. Terenin and I. Akimov, Z . physik. Chem. ILeipsig), 217, 307 (1961). (11) W. F. Berg, “Photographic Science Symposium, Zurich, 1961,” Focal Press, London, 1963,p. 27; T. H. James, “The Theory of the Photographic Process,” new edition in preparation. (12) J. R. Haynes and W. Shockley, Phya. Rev., 82, 935 (1951). (13) J. H. Webb, J . Appl. Phys., 26, 1309 (1955). (14) J. F. Hamilton, F. A. Hamm, and L. E. Brady, ibid., 27, 874 (1956);J. F. Hamilton and L. E. Brady, ibid.. 30, 1902 (1959). (15) B. H. Carroll, Phot. Sci. Eng., 5, 65 (1961). (16) W. West and B. H. Carroll, “The Theory of the Photographic Process,” new edition by T. H. James, in preparation.
SPECTRAL SENSITIZATION OF CHEMICAL EFFECTS IN SOLIDS
707
FLUORESCENCE
f
THERMAL
DEACTIVATION
VACUUM LEVEL hS
oev
jl
INTERMOLECULAR I NTERACTION5
-SUPERSENSITIZATION
ENERGY TRANSFER
I
L
Br Ag
ELECTRON TRANSFER
c
Figure 1. F a t e of the absorbed energy in the spectral sensitization.
Figure 2. Energy level diagram of the d y e s i l v e r bromide system.
( a ) Coplanarity. Any substituent perturbing the planarity of the dye molecule inhibits the existing fluorescence, if any, by favoring thermal energy degradation processes and therefore the sensitizing efficiency. To be a good sensitizer, a dye molecule has to present a coplanar structure resulting eventually in fluorescent properties and better sensitizing ability. (b) Adsorption. Poor adsorption prevents good transfer. A strong adsorption of the dye is shown by measurements of heat of adsorption and by the observation of the ultraviolet and visible absorption spectrum of the adsorbed dye. The latter usually shows a strong modification, compared to the unadsorbed dye, indicating a dye-crystal interaction.” Another competing process which decreases the efficiency of transfer from the dye to the substrate can come from the strong interaction between dye molecules when present in a state of aggregation (J-aggregate). Adding a supersensitizerl* allows the energy “stored” in the dye layer to be transferred to the crystal. Recent experiments at low teiiiperaturelY demonstrate very well the poor interaction between J-aggregate and crystal. (c) Respectiw Energy Leiiels 0.J the Dye and the Crystal.20 This requirement is to be considered only in the case of a mechanism of vlectron transfer and is discussed later.
B. Mechanism of Spectral Sensitization o j Silver Bromide (Figure 2 ) . ( a ) Role and Behavior of t h Positive Hole. Since the dye sensitizer is adsorbed at the surface of the silver bromide, the absorption of photons by the dye results in the formation of a latent image in a fashion similar to the one resulting from absorption of photons in the intrinsic absorption ba.nd of the silver bromide (one can observe, in particular, internal and surface latent images equally well in both cases). This observation shows that after absorption of a photon by the dye, an electron has been excited into the conduction band of the halide and ultimately combined with a silver ion to give a silver atom. However, the energy of the photon absorbed by the dye (1 to 2.5 e.v.) is inferior to that corresponding to the absorption edge of the silver bromide (2.5 e.v.). The donor level from which the electron is excited has therefore to be located in the forbidden gap, 1 to 2.5 e.v. below the bottom of the conduction band; conse(17) S.Boyer, J . chim. phys., 381 (1960); W. West and L. A. Geddes,
J. Phys. Chem., 68, 837 (1964). (18) R. Brllnner, A. Oberth, and G. Scheibe, 2. Wiss. Phot., 50, 283
(1955). (19) H. Frieser, A. Graf, and D. Eschrich, Z . Elektrochen., 65, 870 (1961). (20) W. West, ref. 11, p. 71.
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quently, a positive hole cannot be created by the primary absorption act of the photon in the valence band of the silver bromide, unless by a two-quantum mechanism. In fact, after the sensitization act, apositive hole should be left a t the intermediate level in the forbidden gap, being trapped either a t the dye site as a dye positive radical ion (electron transfer), or a t the surface of the AgBr crystal as an empty donor level (energy transfer). The fact that the positive hole, whatever its nature may be, is directly ‘(formed” on a trap at an energy level located in the forbidden gap probably explains the impossibility of exciting the fluorescence of dye-sensitized silver bromide by illumination in the dye absorption band, as was noted by West.m A similar observation was made by Akimovzl with dye-sensitized AgX, TlX, PbO, and ZnO. This intrinsic trapping of the positive hole should result in its inability to move through the crystal. This has been demonstrated recently by Saunders, Tyler, and W e ~ byt experiments ~ ~ ~ ~on ~dyed sheet crystals of silver bromide. The authors showed that photoelectrons, formed by illumination either in the absorption band of silver bromide or in the absorption band of the dye, could be moved deep irito the crystal by a pulsed electric field. Positive holes showed a very different behavior; with the proper reversed electric field and irradiation with ultraviolet or blue light, mobile holes were observed inside the crystal, but with visible light, no sign of hole mobility could be observed and as was pointed out by the authors, the process of spectral sensitization in these crystals is not accompanied by the formation of mobile holes. These experiments seem almost conclusive in spite of some halogen acceptance properties of the dye when illuminating the AgBr crystal with the 254-mp mercury line (a nonpenetrating radiation). They are also in agreement with the observation of e.p.r. signalsz4 arising in dyed silver bromide powders illuminated a t liquid nitrogen temperature, if one accepts the hypothesis that trapped holes would be responsible for such e.p.r. signals. However, these observations seem to be in contradiction with those made by Terenin and co-workerslO in their studies on the Dember effect of dye-sensitized silver bromide powders. These authors claim, indeed, that only a positive charge-carrier internal photoelectric effect can be observed with such samples. In fact, it appears that these two experiments are not in contradiction but they represent two different aspects of the same phenomenon. In West’s experiments, z z the samples used are highly purified monocrystals of silver bromide melted into thin sheets. The J O U T ~ of ZPhysical ~~ Chemistry
JEANBOURDON
Such crystals have, without any doubt, very few physical or chemical defects able to trap electrons, allowing the authors to observe their displacement. In Terenin’s experiments, the samples are microcrystalline powders of silver bromide characterized by a high specific surface and probably by many physical defects, all factors being favorable to electron trapping. In confirmation of this view, Hamiltonz5 has shown in particular that photoelectron lifetimes are shorter in small crystals than in large crystals, this effect being due to their trapping by superficial silver ions. On the other hand, such high specific area samples would be very sensitive to the presence of adsorbed bromine, which creates surface acceptor levels, as suggested by Terenin and co-workers.26-z8 All of these factors are therefore very unfavorable to the appearance of an electronic photocurrent but very favorable to that of a hole photocurrent. Two hypotheses can be brought forward to explain the observation of a dye-sensitized hole migration. (1) Surface Migration. Photoholes resulting from the light absorption by the dye, intrinsically trapped at the dye site as suggested by West,zzcould have some mobility a t the surface of the silver bromide itself in the quasi-conduction band which exists as a consequence of the high specific surface of these microcrystals and of the presence of bromine. (2) Internal Migration. Photoholes initiated in the valence band by excitation of electrons from that band to surface acceptor levels (bromine), as suggested by Terenin, would be able to migrate inside the crystal. This latter hypothesis suggests the possibility of a two-quantum mechanism in spectral sensitization of silver bromide and introduces the question of regeneration of electron donor levels in this process. ( b ) Regeneration of Electron Donor Levels. The problem has been raised by the observation of Leszinski,Z9 who found that by irradiation of dye-sensitized AgBr, many silver atoms can be formed for each dye (21) I. A. Akimov, Zh. Nauchn. i Prikl. Fotogr. i Kinemutogr., 4, 64 (1959). (22) V. Saundera, R. W. Tyler, and W. West, Turin Photography Conference, 1963. (23) W. West, J. Phys. Chem., 66, 2398 (1962). (24) W. C. Needler, R. L. Griffith, and W. West, Nature, 191, 902 (1961). (25) J. F. Hamilton and L. E. Brady, J. Phys. Chem., 66, 2384 (1962). (26) I. A. Akimov, Dokl. Akad. Nauk SSSR, 121, 311 (1958). (27) A. Terenin and I. A. Akimov, “Scientific Photography,” Photographic Science Symposium, Liege, 1959, Pergamon Press, London, 1962, p. 532. (28) A. Akimov, Fiz. Tverd. Tela, 4, 1138 (1962). (29) W. Leszinski, Z . W i s s . Phot., 24, 261 (1926).
SPECTRAL SENSITIZATION OF CHEMICAL EFFECTSIN SOLIDS
709
molecule adsorbed on the silver bromide surface, The Such ions would provide a large supply of electrons and particularly high value of 160 silver atoms formed per in fact a carbocyanine dye molecule adsorbed flat dye molecule was found later by Eggert30 with some that is to say, is in possible interaction covers 155 infrared sensitizers. with 14 bromide ions and when adsorbed on the edge From the latter facts arises the evidence that, in (J-aggregate), it covers 74 A.z36and is therefore in some cases a t least, electron donor levels, belonging interaction with seven bromide ions. either to the dye molecule or to the crystal surface, These figures are, of course, an upper limit, the dyeshould be regenerated and that the photochemical crystal interaction taking place only between specific transformation taking place in the silver bromide parts of the dye and energy-rich bromide ions. should have an influence on such a process. Several hypotheses have been made to explain the Some data permit the following statement of the occurrence of energy-rich bromide ions, the most often problem. The area of an average silver bromide grain cited being the location of bromide ions at dislocations in a highly sensitive emulsion is 1 p2. At the optimum and the effect of adsorption of the dye on the energy dye coverage (30 to 80% of its surface), it will adsorb levels of the bromide ions. in area, about 5 X lo5 to lo6 dye molecules each 74 Accepting the hypothesis of energy-rich superficial (3-3’-diethylthiacarbocyaninebromide in the J-aggrebromide ions as electron donors, it has to be recalled gate). that the regeneration process has to take place only in On the other hand, several authors have measured cases of heavy or long exposures, conditions which rethe number of quanta necessary to render a silver brosult in a photolysis of the crystal. The surface of such mide grain developable: the most sensitive grains of a a crystal should therefore present a continuous modifihighly sensitive emulsion : 4 quanta/grain31; half of the cation by elimination of the bromine by “bromine acgrains of a similar emulsion: 10 to 25 q ~ a n t a / g r a i n ~ ~ ceptors” ; and of silver ions to metallic silver. Such a process contributes in a very efficient way to the reaverage grain of a moderately sensitive emulsion: up to 500 q ~ a n t a / g r a i n . ~ ~ newal of the surface and therefore to the regeneration These numbers are very low compared to the number of the electron donor sites. of adsorbed dye molecules just mentioned, and show In such a discussion, one should not neglect bromine without any doubt that the regeneration of electron acceptors which have to be present to protect the dye donors is not necessary for the formation of the latent and which certainly can be efficient electron donors. image in practical photography, but there still reThese various considerations lessen the problem of mains the question of whether or not, in the individual the electron donor regeneration and at the same time sensitizing act, the mechanism is such that the dye is render the “two-quantum mechanism” mentioned destroyed. above less probable, if not less useful. Anyhow, this regeneration process seems to be necesAs has been said, the possibility of this mechanism sary, in the practical sense, in two cases: (a)thephotolysis conies from Terenin’s worklob which emphasizes the role of silver bromide with the formation of visible metallic of adsorbed bromine as an electron acceptor level or as silver (print-out), if an efficient halogen acceptor has an electron donor level (as adsorbed bromide ions). been added to prevent a rapid dye destruction (no In such a mechanism, a first quantum absorbed by serious consideration of possible spectral sensitization the dye would raise an electron from the valence band by photolytic silver has been given in these experito the bromine acceptor level, a second quantum raising ments) ; (b) studies of spectrally sensitized photoit to the conduction band. However, it is difficult to electric effects in silver bromide. imagine that such a process would operate in practical Mechanism of Electron Donor Regeneration. As West cases a t low exposure levels and it is more probable has written,2a “The donor levels are a t present little that, according to the population of the donor-acceptor more than a concept which secures conservation of (30) J. Eggert, W. Meidinger. and H. Arens, Hela. Chim. Acta, 31, energy in spectral sensitization.” However, the work of 1163 (1948). Carroll and c o - w o r k e r ~showing ,~~ that the efficiency of (31) A . Marriage, J . Phot. Sci., 9, 93 (1961). spectral sensitization is not influenced by chemical (32) G. Farnell and J. Chanter, ibid., 9, 81 (1961) sensitization, suggests that the donor levels are not (33) C. E. K. Mees, “The Theory of the Photographic Process,” Macmillan and Co., Ltd., London, 1954, p. 122. modified by chemical impurities and that they may be(34) B. H . Carroll, E . A. MacWilliams, and R. B. Henrickson. long, in an intrinsic way, to the silver bromide surface ref. 1 1 , p. 68. itself, It has been proposed35 that the donor sites (35) N. F. Mott and It. W. Gurney, “Electronic Processes in Ionic could be superficial bromide ions, having an energy conCrystals,” 2nd Ed., Oxford University Press, London, 1948, p. 242. tent somewhat higher than those of the regular lattice. (36) E. Klein and F. Moll, Phot. Sci. Eng., 3 , 232 (1959). Volume 69. .\‘umber
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JEAX BOCRDOX
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levels, one would observe either electron photoconduction or hole photoconduction (cf. above discussion of West's and Terenin's experiments). One should remember also that examples of spectral sensitization3' with a quantum yield close to unity excludes such a process, at least for those examples. Finally, the fact that the reciprocity failure in the region of dye absorption is the same as that in the region of halide absorption for the dyed emulsion imposed an almost insuperable difficulty against a two-quantum process. (c) Electron or Energy Transfer (Figure 3 ) . This subject has been the topic of many discussions and still remains the most controversial point of the spectral sensitization process. For a good coverage of the problem, one should refer to ;\/Ieie~-,~ Terenin,lob Pl;elson,s or to the recent and comprehensive review given by West and CarrolL'6 Some comments will be made here on some recent developments. The problem is essentially a question of the origin of electron donor sites.
r
I
1 TRANSFER
I
Considerations from studies made on systems sensitized by adsorbed dye molecules in the molecular state would be in favor of an energy transfer mechanism,lob one of the strong arguments being that the sign of the charge carrier of the sensitized semiconductor does not depend upon the sign of the charge carrier of the dyelob; another argument for energy transfer arises from the value of the ionization potential of the dye measured in the vapor phase, a value which makes an electron transfer impossible. However, a new observation gives support to the possibility of electron transfer even in the case of a dye molecule in a molecular state of adsorption. Several a ~ t h o r shave ~ * ~shown ~ ~ recently that organic molecules (carbocyanines in particular) exhibit, in rigid media, a delayed fluorescence when excited in the long wave length absorption band, this fluorescence being interpreted as a photoionization of the molecule. Such a photoionization results in a positive radical ion and in a trapped electron, which recombine later with the emission of fluorescence. Kern3*points out that such a process could occur at the surface of the silver bromide, corresponding to the transfer of electrons from the dye to the crystal. In conclusion, it seems likely, therefore, that in the present state of our knowledge, all possibilities exist for an electron transfer mechanism both in cases of dye film, and in cases of isolated molecules, though no argument can definitely exclude the energy transfer mechanism.
B. Mechanism of Spectral Sensitization of Zinc Oxide
Figure 3. Schematic representation: energy or electron transfer?
In the context of the preceding discussion, if the electron is provided by the dye, one has an electron transfer; if the electron is provided by a donor level a t the crystal surface, one has an energy transfer (Figure 3). Studies made on systems of silver halide (or other inorganic photoconductors) sensitized with dye films seem to give support to an electron transfer mechanism.*, In particular, all data on photoconductivity, photovoltaic effect, ionization potential of dye films, and the relative energy levels of the dye film and the silver bromide are consistent with the possibility of such a mechanism. The Journal of Physical Chemistry
Zinc oxide possesses a light sensitivity which appears as an internal photoelectric effect, and as photoconductive properties, these effects are accompanied by a photodesorption of the oxygen adsorbed a t the surface. The oxygen photodesorption from the zinc oxide surface can be considered as a chemical phenomenon since it results from the transformation of chemisorbed oxygen, 02-,to physisorbed oxygen, 02, followed by the oxygen desorption by thermal activation. When zinc oxide is sensitized by a dye, photoelectric effects and photoconductivity are observed by illumination in the absorption band of the dye as well as in the (37) W. West, J. chim. phys., 672 (1958); W. West and B. H. Carroll, J . Chem. Phys., 15, 529 (1947). (38) J. Kern, F. Dorr, and G. Scheibe, 2. Elektrochem., 66, 462 (1962). (39) (a) A. H. Kalantar and A. C. Albrecht, J . Phys. Chem., 66, 2279 (1962); E. C. Lim and G. W. Swenson, J . Chem. Phys., 36, 118 (1962); E. C. Lim and W.-Y. Wen, ibid., 39, 847 (1963); (b) W. West, personal communication.
SPECTRAL SENHITIZATIOK OF CHEMICAL EFFECTS IN SOLIDS
intrinsic absorption band of the zinc oxide. The spectral sensitization process and the oxygen photodesorption process both being surface phenomena, a discussion on the relationship between the two problems is therefore relevant to the present review, the purpose being to see under what conditions the two processes interact or if oxygen photodesorption is only a secondary factor. As a preliminary to the discussion of the subject itself, the main characteristics of zinc oxide will be recalled briefly along with the views generally accepted on the mechanism of its intrinsic photoelectric properties. Zinc oxide is an n-type seniiconductor.a Its semiand photoconductive properties are considered to originate from excess zinc, ionized a t room temperature as interstitial zinc ions or as oxygen ion vacancies.41 This situation can be described by the equations Zn
Zn+
+ e-
and
O2 (phys.)
+ e-
-+
02-(chem.)
It appears from the second equation that the semiconductive properties of zinc oxide are strongly dependent upon the presence of adsorbed oxygen a t its surface ; the electrons coming from excess zinc thermally ionized are captured by physisorbed oxygen which, as a consequence, becomes chemisorbed with creation of a potential barrier layer. The light excitation in the intrinsic absorption band (A < 385 nip) of zinc oxide produces an electron-hole pair; a mobile t:lectron appears in the conduction band, and the corresponding hole, apparently of very low is readily trapped by chemisorbed oxygen 02-(chem.)
+ e- + hole +O2 (phys.) + eL-_--d
electron-hole pair the latter reaction being the cause of the oxygen photodesorption. Hauffea concludes that in cases of very strong illuminations, even oxygen ions belonging to the crystal lattice could be desorbed, resulting in a real photolysis of zinc oxide. This point deserves verification. On the other hand, i \ l e d ~ e dremarks ~~ that the weak photoconduction of zinc oxide excited by irradiation between 400 and 600 mp could be due to a direct photoions adsorbed at the surface ionization of 0 - and 02and occupying energy levels in the forbidden gap of ZnO. Spectral Sensitization of Zinc Oxide. Recent work has shown that several aspects of spectral sensitization are
711
similar for zinc oxide and silver bromide and in particular, in the following instances. Dye Adsorption and Desensitization. Markevich and P ~ t z e i k ohave ~ ~ shown indeed that the adsorption isotherms from alcoholic solutions of erythrosin are represented by the Langrnuir equation and those of trypaflavine by the Freundlich equation. The maximum photoelectric effect sensitized by the dye is reached when 30 to 40y0of the zinc oxide surface is covered by the dye. A higher coverage value results in a desensitizing action of the dye. Supersensitization. 45 When mixtures of two sensitizing dyes were used (1,l’-diethyl-2,2’-quinocyanine iodide and 3-ethyl-2-(p-dimethylaminostyryl)benzothiazoliuni iodide, for example) , an increased sensitization of the photoconductivity was observed compared to the process sensitized by one of the two dyes only. Fluorescence Quenching of the Dye Sensitizer by A d sorption. As a general rule, effective sensitization of photolysis or of an internal photoelectric effect results in fluorescence quenching of the sensitizer, whereas no such quenching is observed when the sensitizer is adsorbed on photoinsensitive substrates such as Si02 and BaO. A k i m ~ vhas ~ ~reported such experiments on the following dyes : fluorescein derivatives, rhodamine B, methylene blue, chlorophyll, and three carbocyanines. This observation confirms the effective transfer of light energy (as energy or as a charge) to the photosensitive substrate. The heavy atom effect does not seem to be the reason for fluorescence quenching of the dyes by a singlettriplet conversion process. These experiments therefore favor the participation of the excited singlet state of the sensitizer in the sensitization process. Similar conclusions had been reached previously by West with silver bromide.47 Ability of a Dye Molecule to Perform the Sensitization Act M a n y Times. AMatejechas shown that up to 250 photoelectrons can be formed for each dye molecule.48 Photoinduced E.p.r. Signals in Dye-Sensitized Z i n c Oxide.49 According to Baranov, et al.,49the observed ~
~~
~~
(40) K. Hauffe, J . Phot. Sci., 10, 321 (1962). (41) E. Mollwo and F. Stockmann, Ann. Phys., (6) 3, 240 (1948). (42) D. B. Medved, J . Chem. Phys., 28, 870 (1958). (43) D. B. Medved, J . Phys. Chem. Solids, 20, 255 (1961). (44) N. N . Markevich and E. K. Putzeiko, Russ. J . Phys. Chem., 36, 1297 (1962). (45) S. Namba and Y. Hishiki, Rept. Inst. Phys. Chem. Res. T o k y o , 39, 27 (1963). (46) I. A. Akimov, Zh. Nauchn. i Prikl. Fotogr. i Kinematogr.. 4, 64 (1959). (47) W. West, “Scientific Photography.” Pergainon Press, London, 1962, p. 557. (48) R. Matejec, ref. 11, p. 289; 2. Elektrochem., 65, 783 (1961)
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March 196h
JEAN BOURDON
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signals are indicative of a possibility of photosensitization of the oxygen desorption process.
Discussion Several mechanisms of spectral sensitization are actually proposed; all of theni agree on the liberation of electrons in the conduction band, following the sensitization act, but they differ on the origin of the electron and on the role played by oxygen in the process. Classical Theory. This consists of the sensitized photoexcitation of an electron from a donor level in the forbidden gap to the conduction band of zinc oxide. The donor level is provided either by the ZnO surface (energy transfer) or by the dye (electron transfer). This theory is supported by various authors14*,50each proposing a different mechanism for the regeneration of donor levels. Terenin, Putzeiko, and Akimov’O gave much attention to the role of adsorbed oxygen as an electron acceptor arid an electron donor level. They studied, in particular, the Deniber effect by the condenser method on dyed ZnO powders. They report that the presence of oxygen a t the surface of zinc oxide was necessary to observe a sensitized internal photoelectric effect under intermittent illumination. The proposed mechanism consists of a sensitized photoionization of the chemisorbed oxygen (as 0- or
02-) hu
sensitizer +sensitizer* (excited) sensitizer*
+
02-
+0
2
+ e- + sensitizer
The reverse dark reaction causes a regeneration of electron donor levels and therefore allows the observation of the sensitized photoelectric effect. Theory of Grossweiner and DudlcowshiS1 The merit of the present theory is that it is a completely new approach to the problem. It is based on photoconductivity and photovoltaic measurements of zinc oxide films sensitized by eosin films, both having a thickness of about 100 8. The studies were made after the samples were carefully freed from oxygen. Various photoconductivity measurements, including light intensity, temperature, and wave length dependence have been performed and it was observed in particular that the dye-sensitized photoresponse was proportional to the zinc oxide dark current. The following mechanism, consistent with these data, was proposed by the authors. Eosin is a positive charge carrier photoconductor. Excitation by red or green light produces holes in the dye layer which are injected in the zinc oxide layer and The Joiunal of Physical Chemistry
trapped a t the interstitial Zn+ ions, thus forming Zn+2 ions. Assuming, by analogy with the F-center in potassium chlorideS2that the capture cross section of the Znf2 ions is smaller than that of Zn+,S3the reduction in the concentration of Zn+ should result in an increase of the lifetime of the electrons present in the zinc oxide. This niechanisni can be applied to ZnO if conduction electrons are provided only from excess zinc, and are therefore limited in number. Of course, such a model is valid only for a p-type sensitizer and cannot be extended, a priori, to n-type sensitizers.
Conclusion Striking differences exist between the Terenin and the Grossweiner spectral sensitization theories and these differences deserve some discussion. As Grossweiner has pointed out, a usable conductivity of zinc oxide can be obtained only by thorough elimination of oxygen to release conduction electrons. This oxygen removal is achieved by heating a t 350’ under vacuum or by ultraviolet irradiation. On the other hand, Terenin, et al.,lO working with dyed zinc oxide powders, noted that adsorbed oxygen is indispensable to the observation of a photoelectric response by the condenser method. The discrepancy probably arises from the different methods and the different samples used and, as a matter of fact, one has to bear in mind that the condenser method allows one to observe only the instantaneous change of photovoltage produced by each flash of light but does not give any measure of the total photovoltage created in the sample by a succession of flashes when the recombination time is slow compared to the dark period as is the case here. With due consideration of these facts, several hypotheses can be proposed for the possible role of oxygen. (1) The oxygen acts as an electron donor as chemisorbed 02-and as an electron acceptor as physisorbed 0 2 , the observation of the photoelectric effect depending upon the electronic population of the donors as suggested by Terenin. I n such a case, one can suppose that Terenin’s mechanism would be operative in the presence of oxygen and Grossweiner’s or another one, in its absence. The possibility then arises as two different (49) E. V. Baranov, 1‘. E. Kholmogorov, and A. N. Terenin, Dokl. Akad. Nauk S S S R , 146, 125 (1962). (50) J. W. Weigl, ref. 11, p. 345. (51) S.J. Dudkowski and L. I. Grossweiner, J . O p t . Soe. Am., 54, 486 (1964). (52) H. Fedders, M. Hunger, and F. Luty, J . Phys. Chem. Solids 2 2 , 299 (1961). (53) R. H. Bube, “Photoconductivity of Solids.” John Wiley and Sons, Inc., New York. N. Y . , 1960, p. 195.
SPECTRAL SENSITIZATION OF CHEMICAL EFFECTS IN SOLIDS
processes of ZnO spectral sensitization according to the state of the surface. In relation to this point, it is interesting to note that the presence of electron-acceptor n i o l e ~ u l e other s ~ ~ than oxygen produces a strong enhancement of the sensitized photocurrent. Are these effective because they compete with oxygen for adsorption sites and thus decrease the electron trap efficiency, assuming that adsorbed oxygen had the highest electron trapping ability, or do they provide a more efficient donor-acceptor systeni for spectral sensitization? (2) The oxygen acts only as an electron trap, as decreasing the over-all photoresponse, physisorbed 02, but permitting observation of the Dember effect in intermittent light by trapping photoelectrons during the dark period. In that case, oxygen would not be operative in the sensitized process itself. As a conse-
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quence, other sensitization processes would be required for the excitation of electrons. Either a process in which electron donor levels would be, for example, the oxygen ions belonging to the ZnO lattice surface, or Grossweiner’s process, if such a process, related more to a solid-state effect between two semiconductors, could be applied to the case of sensitization by dye molecules.
Acknowledgments. The author wishes to thank Professor L. I. Grossweiner and the Organization Coninlittee of the Photosensitization Conference (Chicago, Ill., June 1964) for their invitation and financial support, and Dr. W. West and Dr. M. Van Horn of The Eastman Kodak Co. for their helpful suggestions and criticism. (54) E. Inoue, T. Yamagichi, and 66, 428 (1963).
I. Maki, Kogyo Kagaku Zasahi,
Volume 69, Number 8
March 1966
