Ionic Conduction of Silver Halide

Transcription

1 J. Soc. Photogr. Sci. Technol. Japan, Vol. 47, No. 1, 1984 Ionic Conduction of Silver Halide Emulsion Microcrystals Peng BI-XIAN, Peng YAN-BIN, Li ZHEN-XING, Wu XUE-HAN, Wang RONG-QIN, Fan SU-YUE, Chen LI-CHUAN and Jia YAN-MING Institute of Photographic Chemistry, Academia Sinica, Beijing, China (Received 20th July, 1983, Accepted for Publication 1st February, 1984) Abstract The photographic emulsion which consists of conductive silver halide microcrystals embeded in isolating gelatin can be regarded as a heterogeneous medium showing the interface polarization called the Maxwell-Wagner effect. The effects of various measuring conditions, such as gelatin type, degassing degree, moisture content upon the ionic conductivity of emulsion microcrystals have been studied with the aid of dielectric loss method. The change of dielectric loss of photographic emulsions versus frequency was determined as a function of raw material quality, iodide content, the addition of various chemical sensitizers and the addition of different stabilizers. From these measurements, ionic conductivity and in some cases, the space charge characteristics of a series of silver halide emulsion microcrystals are presented and discussed. 1. Introduction According to the Gurney and Mott theory, motions of interstitial silver ions play an important role in latent image formation. Dielectric loss measurement developed by Van Biesen1) offers a convenient method to examine the ionic conductivity of silver halide microcrystals in photographic emulsion. By measuring the conductance and capacitance as a function of applied frequency, several authors have been able to examine the influence of a series of factors on the real and imaginary parts of the dielectric constant for gelatin-silver halide system. From their results, they have determined variati0ns in the conductivity for cubic AgCl and AgBr microcrystals between 0.1 and 1.0ƒÊm in size.1,2) More recently Takada and Tani (1974), Hoyen (1976), Van Hulle and Meanhout Van der Vorst (1977, 1979) and Travot (1980) have investigated influence of adsorbed dyes, dopants and crystal habit upon the ionic conductivity and the space charge region.3-8) In present paper we studied the influence of pag, iodide content, sulfur, reduction, gold and combined sulfur and gold sensitization, and the addion of the stabilizers on the ionic conductivity of the microcrystals of the model and/or commercial emulsions. 2. Theory of the Method The method of the measurement is based on the fact that an emulsion sheet in an evacuated state can be regarded as a heterogeneous medium, consisting of the conductive silver halide grains and isolating gelatin binder. Such a heterogeneous medium would be expected to have a dielectric dispersion, called Maxwell-Wagner effect.9) As shown theoretically3) and experimentally,4,5) a maximum can be fonnd in the curve of dielectric loss ƒã" versus frequency ƒö. This maximum occurs for ƒömax E7=1 and the value ƒömax is proportional to the conductivity ƒð2 of the grain (1) where s i is the dielectric constant of the gelatin, the dielectric constant of silver

2 Vol. 47, No. 1, 1984 Ionic Conduction of Silver Halide Emulsion Microcrystals halide, q the volume concentration of silver halide in the sample, N a constant relative to ƒã'1, ƒã'2 and n, the shape factor for silver halide grains, ƒð2 the ionic conductivity of silver halide grains, and ƒö=2ƒîf, the angular frequency. Travot et al.2) showed that the ionic conductivity can be expressed by: (2) and then calculated using the Cole and Cole diagram (ƒã'max is the value of ƒã' at resonance). If we take the potential (ƒó) to be zero in the bulk, the surface potential (eƒós) is given by Tan and Hoyeni10) as follows: (3) where ƒ Gi and ƒ GƒÒ is the formation energy of an interstitial and a vacancy respectively, k the Boltzman constant, T the absolute temperature. Since the ionic conductivity on silver halide is mainly due to silver interstitials it becomes (4) with Ui migration energy of interstitial ions. Knowing that (5) where ƒ GF is the formation energy of a Frenkel pair, it follows from equations (3), (4) and (5) For AgBr, using the value of Ui and ƒ GF determined by Muller (1965)11) (6) (7) So from the temperature dependence of the ionic conductivity, or directly from the dependence of f max versus 1/T, it is possible to deduce, in a simple way, the surface potential. 3. Experimental 3.1 Sample preparation The silver halide emulsions are prepared by the double jet method with a controlled silver ion concentration or by single jet method (in the case of commercial emulsions) and if necessary, the emulsions are ripened in the presence of sulfur, reduction or gold sensitizers. Following precipitation or afterripening, the emulsions are coated on a sheet of base, chilled at Ž and then air-dried for about hours. The thickness of a dry sample is about 200ƒÊm. Peeling off the base the circular samples mm in diameter are trimmed. The volume concentration is calculated from the weight of the sample and the density of gelatin and silver halide. 3.2 Dielectric measurement The dielectric measurements are performed in the frequency range between 1 khz and 3 MHz, using Ando Electric type TR-10C. Before measuring, the air-dried emulsion layer is located in a chamber, and then, the chamber is evacuated to 10-3 torr. The dielectric loss is calculated from the conductance and capacitance measured and the thickness of the emulsion layer. The electron microscope JEM-100B is used to observe the grains by the replica method. 4. Results and Discussion 4.1 Gelatin type As is shown in equation (1) the ionic conductivity of silver halide is related to dielectric constant of gelatin The value used in ƒð2 calculation, however is rarely reported in literature except.5) In order to obtain the accurate i we determined the dielectric constant ƒã'1 of various types of gelatin, i.e. inert, deionized and modified (AP, 5% ash) under various degassing conditions and different frequencies. The obtained results are shown in Fig. 1. Fig. 1 Dielectric constant versus frequency for low calcium gelatin ( ), deionized gelatin ( ) and PA gelatin ( œ)

3 Pena BI-XIAN, Pena YAN-BIN, Li ZHEN-XING, T. Soc. Photogr. Sci. Technol. Japan Wu XUE-HAN, Wang RONG-QIN, Fan SU-YUE, Chen LI-CHUAN and Jia YAN-MING (1) (2) (3) Fig. 2 Dielectric loss versus frequency for low calcium gelatin (1) and deionized gelatin (2), and PA modified gelatin (3) 1. air-dried, torr, torr, torr, torr It can be seen from Fig. 1 that, as expected, the dielectric constant of gelatin depends on frequency and its nature. The dielectric constant of gelatin ƒã'1 varies also with a condition of degassing of the sample. Among the three kinds of gelatins used, the phthalated gelatin has the smallest ƒã'1 This phenomenon may be caused by the following factor, that is, the excessive ash content may weaken the salt bond formed between the amphoteric ions decreasing ƒã'1. In general, the dielectric loss measurement of the emulsion is made under 10-3 torr, it would be preferable and reasonable to take =3.5 in the ƒð2 calculation. ƒã'1 It is very interested from Fig. 2 that the value of dielectric loss of gelatin is strongly dependent upon the frequency. The peak f max shifts toward the range of lower frequency by 1-2log units with the increase of evacuation degree, from 10-2 to 10-5 torr. In our opinion this experimental fact may be one of the important reasons why the sensitivity of a film may increase to some extent after the vacuum treatment. On this increasing effect we will discuss elsewhere is a very hydrophilic protein and in general, the air-dried gelatin silver emulsion layer contains 8-12% water. The dielectric constant of water is 84, being more than 20 times as large as that of gelatin. But up to now none has done work in this direction. We carefully investigated the influence of water content upon the dielectric loss of the emulsion. The moisture content determination method is described in paper.12) Obtained results are illustrated in Fig. 3 (the emulsion used is of polydispersed grains, halide composition Bro. 96I0.04, pbr 4, grain size 6.00 p). From Fig. 3 the following conclusion can be drawn: when the moisture content is 4%, the dielectric loss of water plays an dominant role, depressing the dielectric loss peak of gelatin-silver halide emulsion. When the moisture content decreases to 1.3%, fmax further shifts toward the range of higher frequency, ƒã'max in the Cole-Cole digram continues to decrease, only when the mosture content drops to %, the fmax remains constant but ƒã'max still tends to decrease (Fig. 4). Thus, strict control of the moisture content in detail. 4.2 Moisture content of the sample by evacuation (in general 10-3 torr, 5 `6 hours) is the key for obtaining It is known that the gelatin macromolecule accurate value of ionic conductivity.

4 Vol. 47, No. 1, 1984 Ionic Conduction of Silver Halide Emulsion Microcrystals in affecting the electric property of the semiconductor materials, including silver halide. As the starting point of this systematic research we dealed with the effect of purity of the two main starting materials i.e. gelatin and silver nitrate upon the ionic conductivity of the emulsion microcrystals. We used two kinds of gelatin and silver nitrate. Their main organic and/or inorganic impurity content was determined by high pressure liquid chromatography, atomic absorption spectroscopy and other routine techniques. The analytical results are summarized in Table 1. Table 1. The purity of gelation and AgNO3 (Ppm) Fig. 3 Dielectric loss versus moisture content of emulsion layer. 1. air-dried 2. 4% moisture content % moisture content % moisture content % moisture content Fig. 4 Cole-cole diagram for emulsions with different moisture content % moisture content % moisture content % moisture content % moisture content % moisture content 4.3 Effect of the starting materials It is known that the doping effect of foreign impurities plays an important role It is of great interest that the purity of these two starting materials greatly affects the fmax and the ionic conductivity. The ionic conductivity of silver halide prepared with gelatin and silver nitrate of higher purity is more than 6 times greater as compared with the emulsion prepared with lower purity. This problem needs further investigation, because it is of great significance not only in the theoretical aspect but also in the practical usage. The starting materials used in this paper were purified as much as possible to exclude any impurities. 4.4 pag. effect Some author13,14) have studied the effect of pag on the ionic conductivity of monodispersed cubic and octahedral silver halide emulsion. It was found that the ionic conductivity increased with increasing pag. Except the octahedral emulsion, we studied the effect of pag on the ionic conductivity of the commercial emulsion (Table 2). It is found that in the case of octahedral emulsion,

5 Peng BI-XIAN, Peng YAN-BIN, Li ZHEN-XING, J. Soc. Photogr. Sci. Technol. Japan Wu XUE-HAN, Wang RONG-QIN, Fan SU-YUE, Chen LI-CHUAN and Jia YAN-MING Table 2. Effect of pbr on ionic conductivity when pbr drops from 8 to 3, the ionic conductivity increased 12 times (7.04 ~ but in the case of the commercial emulsion only 2.8 times (3.3 ~ ~10-6). This experimental fact was examined and confirmed by several authors, but various researchers interpreted it from different point of view. In our opinion, the effect of pag is to change the surroundings of the grains by modifying the surface which, probably, at high pag, injects silver ions into interstitial position and at low pag, traps interstitial silver ions from the inside of the grain in a way to get the right electrical neutrality. In order to get the satisfactory explanation it is necessary to determine the surface and subsurface ion constitution by some surface analysis tools, such as SIMS etc. 4.5 Iodide content effect J. V. Burt15) and S. Takada16) investigated the effect of iodide content on the ionic conductivity of AgBr (I) microcrystals. The iodide content used in their microcrystals (cubic habit, grain size 0.70 and 1.10ƒÊm) is very small, 1 M %. It was found that the ionic conductivity increases nonlinearly with iodide content. Since the iodide content plays an fundamental role in crystal defect, consequenly in latent image formation, we made a systematic survey. The iodide content added, covers an extremely wide range, from 1 M % to 15 M %. The conductivity results are illustrated in Fig. 5. It can be seen from Fig. 5 that: (1) With the increase of the iodide content the peak f max shifts toward the higher frequency step by step. When the iodide content varies from 1 M % to 5 M %, the fmax shifts comparatively slowly. When Fig. 5 Dielectric loss versus frequency for emulsions with different iodide content the iodide content is more than 5 M %, the fmax increases almost linearly, in this case, the ionic conductivity of AgBr (I) microcrystals increase propotionally to the iodide content. The ionic conductivity of the emulsion grains containing 15 M % is 9 times as great as that of the grains having 1 M% iodide. (2) In all the ca ses the absorption peak

6 Vol. 47, No. 1, 1984 Ionic Conduction of Silver Halide Emulsion Microcrystals is asymmetric as against the vertical line going through the maximum of the losses. This asymmetry is attributed to an asymmetric distribution of the relaxation time, which themselves are associated with a distribution of the conductivity around an average value. The more the iodide content is, the more asymmetric the absorption peak is. The asymmetry can be explained by a continuously decreasing conductivity of AgBr (I) grains from the surface of the grain down to the center of it. Our measurements point to a continuous distribution of the conductivity, the conductivity decreasing slowly towards the center of the grain. If there was an abrupt transition we should have measured two absorption peaks. (3) Fig. 6 shows the dielectric losses as a function of log fmax at four different temperatures. As decreasing temperature the absorption peak shifts toward lower frequency, whereas the maximum value of the Fig. 6 Dielectric loss peaks at different temperature losses is slightly dependent on temperature. Temperature dependence of f max allows us to determine the activation energy of conduction, ƒ E. The ƒ E in the case of 0 M and in the case of 4 M % is 0.42 ev and 0.39 ev respectively. Thus the addition of iodide lessens the conduction activation energy and makes the interstitial silver ions formation easier. (4) As Travot2) and Takada3) indicated the conductivity varies linearly with the surface area/volume ratio and is inversely proportional to the edge length of the grains. We examined the effect of the iodide content on the average grain size of the emulsion micracrystals (Table 3). The grain size increases as the iodide content increases from 1 to 4 M % ( ii), but decreases as the iodide content increases from 4 to 15 M % ( p). It con be seen from the ionic conductivity and grain size results that in general the ninefold change of the ionic conductivity as the iodide content increases from 1 to 9M % is mainly attributed to the change of the interstitial silver ion numbers. The grain size is only of secondary effect. 4.6 Chemical sensitizers effects The influence of sulfur, reduction, gold and sulfur-gold combined sensitization on the ionic conductivity are also studied. Table 4 shows the chemical sensitization condition (the emulsion used is of monodispersed octahedral grain, grain size 0.4 p, pbr 4.0), Table 5 gives the results. It was found that sulfur sensitization and reduction sensitization only decrease the value of f max by 0.1 log unit. Table 3. Infiuence of iodide content on average grain size of emulsions

7 Peng BI-XIAN, Peng YAN-BIN, Li ZHEN-XING, J. Soc. Photogr. Sci. Technol. Japan Wu XUE-HAN, Wang RONG-QIN, Fan SU-YUE, Chen LI-CHUAN and Jia YAN-MING Table 5. Influence of chemical sensitization on log fmax The sensitizers containing gold salt and thiocyanide induce a drop of value, fmax by 0.3 log unit. The combined sulfur and gold sensitization induce an additive drop of fmax by 0.3 log unit. All the results mentioned above are interpreted as that anions of the agents added in the emulsion are adsorbed by the emulsion grains on positive kink sites and the adsorption causes the drops of fmax. It is well known interstitial silver ions are formed through the process discribed by the equation: (8) where Site (Ag+) and Site (Br-) are positive and negative kink sites, respectively, and Ago represents an interstitial silver ion. The anions which interact selectively with site (Ag+), or site (Br-) influence the equilibrium of equation (8), hence the ionic conductivity. S2O3=, SO3=, CNS- react with silver ions and form stable complexes. So they must be adsorbed to the silver ions on the surface, especially on the site (Ag+) of silver halide grains and the adsorption causes a drop of ionic conductivity. In our experiment the amount of CNS- added is more than that of sulfur and reduction sensitizers, so it deduces a larger drop of ionic conductivity. To search the reason why gold sensitization causes a larger drop of fmax, NH4CNS is alone added to the octahedral emulsion and the emulusion is ripened at 50 C for 60 minutes, after ripening the pag value of the emulsion is adjusted to 7 immediately. The amounts of NH4CNS added and corresponding fmax is shown in Table 6. From this Table it can be seen that NH4CNS causes the drop of fmax instead of HAuC14. Takada's13) results on sulfur sensitization of octahedral AgBr emulsion are different from ours in two points: 1. larger drop of fmax at the very beginning of the sensitization process. 2. increasing of the value of fmax during the sensitization process. The reason for these difference may be due to : 1. in our experiment less S2O3= is added. 2. our emulsion is sensitized at rather lower temperature. 4.7 Stabilizer effect The influence of the addition of 5-methyl- 7-hydroxy-2, 3, 4-triazoindolizine (stabilier 1) and 1-pheyl-5-mercaptotetrazole (stabilizer 2) on the ionic conduction of the octahedral Table 7. Dependence of log fmax on the anddition of the stabilizer in the emulsion

8 Vol. 47, No. 1, 1984 Ionic Concuction of Silver Halide Emulsion Microcrystals silver bromide emulsion grain (edge length 0.4 pm) is studied. The addition of 0.8 g/m AgBr of stabilizer 1 or 0.9 g/m AgBr of stabilizer 2 in the emulsion respectively increases the ionic conduction activation energy and surface potential to 0.50 ev and ev, which are derived from the slope of the straight line in Fig. 7. Fig. 7 Temperature dependence of log fmax octahedral emulsion oct. emul. +triaz. oct. emul. œ +tetroaz. 4.7 DNA effect For many years gelatin was used as manufactured and contained the correct amount of all the active components. But little by little it was recognized that it would be preferable to have the gelatin act only as a binding agent and to add to it, at the right time, controlled amount of the necessary sensitizer and restrainers. Today DNA is widely used as the grain growth restrainer and the antifoggant during the 1st ripening in the practice. We studied the influence of DNA amount added in the 1st and 2nd ripening on ionic conductivity of silver halide emulsion. It is shown that in the addition case of 1st ripening log fmax shifts to the higher frequency with the increase of DNA amount added (Table 8), the activation energy of conduction lessens from 0.40 to 0.37 ev when the added amount of DNA increases from 0 to 0.4 g/m AgBr. This relation is mainly due to the strong adsorption of DNA on the silver halide microcrystals, which is clearly demonstrated by the adsorption investigation of DNA on silver bromide sol with ultra-violet spectrophotometric method (Table 9) and the great crystal growth-restraining power of DNA during the 1st ripening. The more the amount of DNA added, the less the grain size formed, hence the greater the ionic Table 8. Influence of DNA on log fmax Table 9. Adsorption of DNA on AgBr sol D1: Optical density of DNA solution (268 nm). D2: Optical density of DNA solution after AgBr sol's absorption and it's centrifugal sedimentation

9 Peng BI-xIAN, Peng YAN-BIN, Li ZHEN-XING,T. Soc. PhotoEr. Sci. Technol. Japan Wu XUE-HAN, Wang RONG-QIN, Fan SU-YUE, Chen LI-CHUAN and Jia YAN-MING Fig. 8 ƒã" versus log f by addition of DNA to emulsion after 1st ripening and washing, amount added (g/m AgBr) 1. 0, , , , conductivity obtained. On the contrary when DNA is added to an emulsion only after the 1st ripening and washing and incubated at 50 Ž for one hour, the log fmax keeps constant with the increase of DNA amount from 0 to 0.8 g/m AgBr (Fig. 8). This implies that it is very difficult for such a macromolecule as DNA to reach the subsurface of silver halide grains to exert it's effect on the ionic conductivity. 5. Conclusions The ionic conduction of the monodispersed octahedral emulsion and the emulsion containig iodide is studied by the measurements of dielectric loss in the frequency range of 1 khz-5 MHz. (1) The dielectric constant of gelatin e1, depends on it's nature, frequency and degassing degree. It is preferable to take el =3.5 in ch calculation. (2) Strict control of the moisture content of the emulsion layer by evacuation (in general, 10-3 torr, 5-6 hours, 1% moisture content) is the key for obtaining accurate value of ionic conductivity. (3) The purity of silver bromide and gelatin affects the ionic conductivity of the emulsion microcrystals greatly. This may be associated with the doping effect of the divalent cations impurities present in these starting materials. (4) The ionic conductivity of octahedral and commercial emulsions increases with increasing pag. When pbr drops from 8 to 3 the ionic conductivity increases 12 times in the case of octahedral emulsion, but it increases only 2.8 times in the case of commercial emulsion. (5) With the increase of iodide content the fmax shifts towards the range of higher frequencies. When the iodide content varies from 1 M % to 5 M %, the fmax shifts comparatively slowly, when iodide content is more than 5 M % the fmax, consequently the ionic conductivity increases almost linearly. (6) In the case of AgBr (I) emulsion, the absorption peak is asymmetrical as against the vertical line going throug the maximum of the losses, the more the iodide content is, the more asymmetric the absorption peak is. This asymmetry is attributed to the asymmetric distribution of the relaxation time, which themselves are associated with a distribution of the conductivity around the average value. (7) The influence of the chemical sensitizers on the fmax, i.e. on the ionic conductivity is studied. Sulfur sensitization, reduction sensitization, gold sensitization and combined sulfur and gold sensitization induce a drop of fmax by 0.1, 0.1, 0.3 and 0.3 log unit respectively. (8) The additions of stabilizer 1 and 2 shift fmax toward to lower frequency, increase the ionic conduction activation energy and surface potential to 0.5 ev and ev respectively. (9) The addition of DNA in 1st ripening shifts log fmax toward the range of higher frequency (from 4.50 to 4.80) and lessens activation energy of ionic from 0 to 0.4 g/m

10 Vol. 47, No. 1, 1984 Ionic Conduction of Silver Halide Emulsion Microcrystals AgBr. This is due to the DNA's crystalgrowth-restraining power. The addition of DNA during the 2nd ripening has no effect on the ionic conductivity. References 1) Van Biesen, J. App. Phys., 41, 1910 (1970). 2) G. Travot et al., J. Phys. E: Sci. Instrum., 13, 1231 (1980). 3) S. Takada, Jpn. J. Appl. Phys., 12, 1970 (1973). 4) S. Takada and T. Tani, J. Appl. Phys., 45, 4767 (1974). 5) Harry A. Hoyen, J. Appl. Phys., 47 (9), 3784 (1976). 6) M. E. Van Hulle and W. P. Meanhout Van der Vorst, Phys. Stat. Sol. (a), 39, 253 (1977). 7) Ibid., 52, 277 (1979). 8) S. Takada, J. Soc. Photogr. Sci. Tech. Jpn., 44, 81 (1981). 9) R. W. Sillars, JIEE, 80, 378 (1937). 10) Y. T. Tan and H. A. Hoyen, Surf. Soc., 36, 242 (1973). 11) P. Muller, Phys. Stat. Sol., 12, 775 (1965). 12) Peng Bi-xian, Peng Yan-bin, Wu Xue-han and Li Zhen-xing, Photographic Science and Photochemistry, No. 2, 1 (1983). 13) S. Takada, 2nd International Symposium on Model Investigation of the Photographic Process and New Photoregistering Systems, 1980, p ) F. P. Chen, K. C. Chang, L. Corben, M. Falxa and B. Levy, Phot. Sci. Eng., 26, 15 (1982). 15) J. V. Burt, Phot. Sci. Eng., 21, 245 (1977). 16) S. Takada, J. Soc. Photogr. Sci. Tech. Jpn., 42, 112 (1979).