J. IV~SLINOWSKI e t al. : Photographic Processes on Bi-Doped AgBr 303

phys. stat. sol. 6, 303 (1964)

Institute of Physical Chemistry, Bulgarian Acaderriy of Sciences

Photographic Processes on Bi-Doped AgBr Monocrystals

BY J. MALINOWSKI, W. PLATIKANOWA, and I. PETKANCHIN

Es wird der EinfluB von Wismut auf die dusbildung des latenten Bildes und den Ent- wicklungsprozeB in Silberbromid untersucht. Es wurden Silberbromid-Einkristalle ver- wendet, auf deren Oberflaehe Wismut durch Verdampfen im Vakuum aufgetragen wurdc. Elementares Wismut katalysiert den EntwicklungsprozeB und ist in dieser Hinsicht sogar aktiver als Silber. Mischkeime von Silber und Wismut sind aktiver als jedes einzelne Metall fiir sich. Auf die Oberflache von Silberbromid-Einkristallen aufgebrachte Wismutionen ver- hindern den EntwicklungsprozeS wie auch die Ausbildung des latenten Bildes.

A study is made of the effect of bismuth on the latent image formation and the develop- ment process in silver bromide. Single cryst,als of silver bromide are used, on the surface of which bismuth is deposited by vacuum evaporation. Elementary bismuth is found to catalyzc the process of development, being even more active than silver in this respect. Mixed nuclci of silver and bismuth are shown to be more active than either of the individual metals. Bis- muth ions applied to the surface of the silver bromide crystals are found to prevent the process of development,, as well as t o hinder the formation of t,he latent image.

1. Introduction The photographic emulsion, as a complicated system, makes rather difficult

the investigation of the elementary stages of the photographic process. The use of single crystals of silver halide, introduced by the Bristol School as a convenient model of the emulsion, made possible a significant progress in our understanding of the intricate nature of the photographic sensitivity [l].

Model investigations so far performed have been directed chiefly to t,he exami- nation of the latent image formation and the nature of chemical sensitization [ 2 ] .

The effect of foreign ions has been investigated in previous works by introducing additions into the crystals [3]. The technique of controlled introduction of a pre- determined quantity of a contamination into the crystal lattice is, however, quite difficult and requires the knowledge of the distribution coefficients of the addition. Apart from that the photographic process is practically a surface process and in many cases only the surface concentration of the impurity may be of any signifi- cance.

A strong desensitization effect of bismuth on silver bromide emulsions has been reported by MALINOWSKI, KARADJOV, and TODOROVA [4]. It has been observed that traces of bismuth ions in the solid phase cause a decrease in the sensitivity of normal photographic emulsions by several orders of ten. This exceptionally strong effect was regarded as convenient for the model investigation of the influence of addions applied on the surface of larger silver bromide crystals.

2. Experimental The silver bromide material for the single cryst'als was synthesized from its

elements following the method given by MALINOWSKI [ 5 ] . Several alterations which proved expedient will be briefly described below.

304 J. MALINOWSKI, It1. PLATIXANOWA, and I. PETKANPHIN

L

J Fig. 1. Apparatus fur the synthesisof silver bromidr I3g. 3. Tai.unri~ apparatus for tlw therim1 t r ~ a t m i m t

(if AgAr

Commercial analytical grade silver was refined by a double electrolysis in a 257" solution of silver nitrate, acidified by nit.ric a d . The electrolytic crystals of silver provide a,n adequate surface and are therefore very convenient for the rapid flow of t,he bromination. The bromine used (analytical grade) was not subjected to furt'her purification, but in fact it was distilled once in the apparatus used for the synthesis.

As shown in Fig. 1, the electrolytic silver is placed into the test tube n of Jena Gerat'eglass 30. The lower end of the test tube has a small bulb blown inwards and punctured on several points. This device proved to be very essential for a smooth procedure of the synthesis. Without it the capillary is sometimes plugged by tiny silver cryst>als, thus bringing the brominat>ion to a st'op. The arm c of the tube is connected with the vacuum system, while to the ot'her arm a bulb with bro- mine b is sealed. A furnace, made by winding tape resistivity wire on glass bubing, is placed around a . It is heated to a temperature of about 450 "C and t,he apparatus is eva.cuated better than lO-5mm Hg. After the whole system has been thoroughly desiccated, the apparatus is sealed off from t>he vacuum system at, the point c .

Photographic Processes on Bi-Doped AgBr Monocrystals 305

The glass-sheathed iron bar is raised with a magnet and then dropped to break the upper part of the bromine bulb. The liquid bromine is heated to 80 "C in a beaker with hot water. An auxiliary glass-tube furnace prevents t2he condensation of the bromine in the lower part of the apparatus. Under these condibions, at a pressure of 1.5 to 2 atm silver is rapidly converted to molten silver bromide, which is filtered successively through the capillary tubes arranged below. After cooling t'he Iast t,est tube is placed in the apparatus shown in Fig. 2, where the silver bro- mide is again melted in vacuum and is pumped at a temperature of 450 "C for 8 to 16 hours [B]. Now the apparatus is sealed off a t point a and the bulb with liquid bromine is broken. After heating for an hour or two in a bromine atmo- sphere, t>he ampoule with the fused silver bromide is sealed off a t point c and placed in the furnace shown in Fig. 3 [7, 81. The aluminium block n is kept a t constant temperature of 450 "C, section b - at 350 "C, while the temperature in section c gradually falls tso about 50 to 60 O C . The glass ampoule with the silver bromide is lowered along this temperature gradient' a t a constant rate of 5 mm/h.

The silver bromide crystal was cut to disks 2 to 3 mm thick by means of a jewe- ler's saw. The disks were abraded with a coarser and a finer emery paper, t'hen polished on a soft cloth moistened in a 10% solut>ion of potassium cyanide. The surface sensitivity of the crystals so treated usually varied widely. A 3 to 4 &in etching at 27 T in i 400/, solution of potassium bromide containing 1 to 2 drops of bromine was proved very useful for diminishing these variations. The crystals were then carefully rinsed with distilled water, dried for a couple of hours a t 25 "C and stored in a bromine atmosphere until use. These crystal disks did not indicate any spontaneous reduction after one mi- nute development in the developer of HEDGES and MITCHELL [9].

Definite amounts of silver and bismuth were ap- plied on the crystal surface by vacuum evaporation. Constant rate of evaporation was secured by using the small crucibles - a graphite one for the silver and an iron one for the bismuth - shown in Fig. 4. These represent effusion vessels in which the surface area of the molten metal is much greater than the cross section of the orifice a. The crucible was heated by a thungsten coil b powered by a ferro- magnetic voltage stabilizer. The temperature was checked by a platinum -rhodium thermocouple c. The crystal onto which the metal vapour were to be condensed rested on a silver plate with narrow slits. These slits could be opened or shut successi- vely by means of a lever manipulated from the out- side in such a manner that to different parts of the crystal surface various quantities of metal were deposited.

Fig. 3. Furnace for growing of singlc crystals

306 J. M.4LINOwSK1, W. PLATIKANOWA, and I. PETKANCHIN

a

0 0 0 0 0 0

0

0 0 0 0 0 0 0 0 0 0 0 0 0

O b

Fig. 4. Crucible fur metal evaporatio~ (magnification i : 1 )

The rate of evaporation of both metals a t various temperatures was determined by analysis of the amount of the metal condensed on glass plates placed instead of the crystals. Silver was determined by extraction titration with dithizon [lo], and bismuth-photocolorimetrically with diethyldithiocarbamate 11 11. The ana- lytical methods used permitted the determination of about 5 pg of metal with an accuracy of 3 to 4 yo, The rate of condensation of the metals on the glass plate a t the temperatures employed was from 1 x to 20 x g min-l with a reproductibility of about 10%.

3. Results In the conventional photographic materials all the impurities are introduced

upon the preparation of the emulsion in the form of ions. The application of metal salts by vacuum evaporation is impracticable on account of the decomposition of some salts a t the temperatures necessary for their evaporation. In the case of bismuth bromide further uncertainty arises because of its hygroscopicity and irreversible hydrolysis. Having this in mind, we preferred to evaporate metalic bismuth, which can readily be converted to bismuth bromide in the presence of bromine vapours [ 121.

3.1 Effect of elementary bisnzuth

It has been shown that the deposition of about 1015 silver atoms per cm2 on a silver bromide crystal creates a developable image [13]. Our preliminary experi- ments proved that bismuth has a similar effect. It was found, however, that the minimum quantity of the metal-silver or bismuth, necessary to produce develop- ability varies for the different crystals. The attempts to establish some treatment for the formation of reproducible crystal surfaces have not been successful so far. In order to compare the catalythic activity of the two metals as regards the pro- cess of development, the following method was applied. Strips with increasing amounts of bismuth were evaporated on each silver bromide crystal. Perpendicu- lar to the bismuth strips on the same crystal were evaporated similar strips of silver. Some examples are shown in Table 1 , where columns 2 and 3 indicate the minimum quantities of silver or bismuth respectively necessary to produce an

Photographic Processes on Bi-Doped AgBr Monocrgstals 307

Crystal KO.

~

103 104 10.5 107 108 111 113

Table 1

6

Development is observed by

Ag atoms/cm2 Bi atoms/cm2

2.0 x 1014 2.0 x 1014 6.0 x 1014 6.0 x 1014 6.0 x 1014 6.0 x 1014 6.0 x 1 0 1 4

4.3 x 1013 4.3 x 1013 4.3 x 1013 4.3 x 1013 14 x 1013 14 x 1013 14 x 1013

Conversion factor

F = [Agl/[Bil

4.7 4.7

14 14 4.3 4.3 4.3

0.7 x 1014

2.3 x 1014

2.4 x 1014 3.8 x 1014 3.8 x 1014

0.7 x 1OI4

2.6 x 1914

image on development. In spite of variations in the absolute values the minimum quantity of bismuth in each case is significantly lower than that of silver - 5 to 6- fold on the average (column 4).

It was observed in these experiments that a developable image is formed on some of the intersections of silver and bismuth stripes, both of which separately do not produce any effect on the process of development (Fig. 5 ) . The four black squares on this figures are formed a t the intersection of silver strips haring 2 x 1014 atoms per om2 and such with 4.3 x 1013 bismuth atoms per cm2. The strips themselves containing silver or bismuth alone are not developable.

It was of interest to evaluate whether the simultaneous presence of the two metals has a greater effect on the development than their separate presence. This comparison, however, isrendereddifficult by the fact that the quantities of silver and bismuth necessary for initiation of the development are quite different. It seemed rational to convert the quantity of bismuth to the quantity of silver. The conver- sion factor P (column 4) should evidently represent the ratio of the minimum amounts of silver and of bismuth which separately create a developable image (columns 2 and 3). This estimation has been done for the intersections where an image is produced only on account of the simultaneous presence of both metals. In column 5 are given the sums of the amounts of silver and of bismuth, the latter converted to silver by means of the factor 3’.

Fig. 5 . Silvcr bromide crystal dcvelopr(1 n f t c r the < ~ w p < > ration of silver and bismuth strips

308 J. MALINOWSKI, W. PLATIKAKOWA, and 1. PETKANCHIN

It was interesting to establish that in all cases the creation of the developable image by the simultaneous action of the two metals requires 2 to 3 times smaller number of atoms than by their individual action. The coniparison of the figures in column 5 with those in column 2 clearly illustrates this synergistic effect.

3.2 Effect of bismuth ion on developnaent

From the experiments described above it is evident that elementary bisniuth has a catalytic action on the development process. The desensitization effect of bismuth observed earlier [4] is t o be ascribed eventually to the bismuth ions. In order to examine the effect of the bismuth ions, the metalic bismuth evaporated on the surface of the silver bromide crystals was brominated. Two limitations where encountered in this procedure. If the brornination was not long enough the larger quantities of bismuth cannot be completely converted to bromide and the remain- ing metal atoms would be responsible for the production of an image on develop- ment. I n the case of a longer bromination (or a t higher temperatures) the danger exists that the smaller quantities of bismuth would disappear from the crystal burface either by evaporation (bismuth bromide is rather volatile) or by diffusion towards the interior of the crystal. Evidently this method seemed impracticable for investigating the effect of widely varying quantities of bismuth bromide applied on the same crystal. Therefore, different crystals were subjected to bromination under various conditions. A silver bromide crystal with different amounts of bismuth ranging from 1013 to IOl5 atoms per cm2 (vertical strips) is shown in Fig. 6. The crystal was brominated for 3 h a t room temperature after which strips with different amounts of silver - 1014 to 1016 atoms per cm2 - were evaporated on the same crystal perpendicularly to the silver strips. As seen from the figure when appropriate ratios of concentrations are reached, the presence of bismuth ions hin- ders the development of the image produced by the available silver. The two top- most horizontal strips, made developable by the presence of silver, are intersected by thc white strips doped with bismuth bromide. At lower bismuth concentration (leftwards) the desensitization becomes weaker and vanishes completely a t higher

Fig. 6. Crystal with bisiiiutli - short term brornitiation; tlic i’apnbilitg fnr dcvclopment prodriced by the siivcr is

destroycd by the bismuth ions

Fig. 7. Smnc as 1’’ig. 6., long temi broininntioii

Photographic Processes on Bi-Doped AgBr Monocrystala 309

concentrations of silver, the amount of which increases downwards. The strips on the extreme right are black again, due to traces of unbrominated bismuth.

The crystal shown in the next Fig. 7, has the same quantities of bismuth eva- porated onto it, but it was brominated 18 h at ambient temperature and 2 h at 100 "C. It is clearly seen that the smaller quantities of bismuth bromide have disappeared. No effect due to these is to be observed on development. The larger quantities, however, are almost completely brominated and prevent the develop- ment even on places containing two orders of magnitude more silver - the two lowest horizontal lines.

These experiments plainly proved that the presence of bismuth ions hinders the process of development. The smaller the amount of silver producing a developable image, the smaller the amount of bismuth ions necessary for its anihilating. The image produced by the minimum quantity of silver is suppressed by the presence of about 3 A ions of bismuth per em2.

3.3 Effect of bismuth ions on latent image formation

The experiments hitherto described proved that the presence of traces of bismuth ions on t8he surface of silver bromide crystals diminishes their capability of spon- t,aneous development, which on its turn is catalyzed by the presence of metal atoms - silver or bismuth. The fundamental photographic process consists in creating developability under the action of light. It was advisable therefore to examine the suppressing effect of the bismubh ions on this primary photochemical process. In order to investigate the process of the formation of the latent image under the action of light bhe crystals were exposed behind slits, with varying opt'i- cal density. In this manner various sect>ions of the same crystal were subjected simultaneously to the action of light with different intensity, a flash lamp being used as a light source.

Since not sensitized silver bromide crystals produce on exposure only but a weak sui.face image, bhe crystals used now were sensibized, sulphur and reduction sensi- tization being employed. The reduction sensit'ization was accomplished by eva- porating silver onto the crystals in an amount less than that giving spontaneous development - 0.6 x 1014 atoms per em2 [13]. The sulphur sensitization was carried out by the method of SAUNDERS, TYLER and WEST [14], based on the fact, t,hat silver bromide dipped into a solution of diacetyltiourea is covered by a mono- layer of this compound. In an alkaline medium the diacetyltiourea is quant'it'a- tively converted to silver sulphide.

The crystal shown in Fig. 8 has received the following treatment. After the eva,poration of bismuth strips the crystal was brominated for an hour. The whole surface was then sensitized by evaporation of silver. The crystal was exposed and developed.

The intensity of the exposure is increasing from bottom to top. It is seen that tmhe image disappears on all intersections of the variously exposed horizontal slit's wit'h the vertical strips containing bismuth ions. It was found that 0.5':. atoms of bismuth per omz are sufficient to prevent the format>ion of a latent image on crystals sensitized by evaporat'ed silver.

The crystal seen in Fig. 9 is a sulphur sensitized crystal. Here too, the horizont'al &ips were exposed to light with different intensity, increasing from bottom to top. The intensity of the light employed here is lower t'han the intensity applied on Fig. 8. Unlike the previous case, the fading out of t,he image caused by Bit+'

310 J. MALIEOWSKI, 11’. PLATIKANOWA, and I. PETKANCHIK

b’ig. 8. Silver sensitimd ( q s t a l ; t he plares containing l.’ig. 9. Crystals srnsitized nitli diacctyl thiourw; t t i r Iiisinuth ions do not Iiiiilrl la tent image on exposure presence of bisniuth ions hinders the latmt imnpr

to l ight forniat ion

is observed only at lower exposures. On the lowest visible strip, the image had vanished on all intersections containing bismuth bromide. On the next strip, the presence of bismuth ions only weakens the image, this weakening being so neglig- ible that it is not clearly discerned on the figure. However, it can be clearly obser- ved in dark field illumination. At higher exposures - the uppermost slit, the effect of the bismuth ions completely disappears.

4. Discussion The experiments described above have shown that it is not only the presence

of elementary silver which makes possible the spontaneous development of silver bromide crystals. Bismuth has been found to be even more active than silver in this respect, five to six times less bismuth atoms being sufficient to produce the same effect.

A synergistic effect, due to the simultaneous presence of bismuth and silver, was observed. It was established that for the formation of a developable image two to three times less atoms per cm2 of silver and bismuth together are necessary than the atoms of each of these metals applied separately. A probable hint for the un- derstanding of this fact is that the atoms of the two evaporatedmetalsformmixeti nuclei with higher activity. The process of development is catalyzed by the metal nuclei, the activity of the mixed catalyst as in many other cases being higher than the activity of the pure metals.

So far no explanation can be offered for the higher activity of bismuth and moreover of the mixed nuclei, nor can it be stated whether this would be of any significance for the photographic process. In this connection, it would be inter- esting to study the behaviour of other metals.

The strong desensitization caused by bismuth-ions observed earlier [4] has been confirmed and some aspects of the mechanism of the effect are discussed below.

In the first place, it, is evident that the presence of bismuth ions hinders the process of development.

Photographic Processes on Bi-Doped AgBr Monocrystals 31 1

For the crystals used and the technique of surface treatment applied here, a developable image could be formed after the evaporation of more than 3 x 1014 atoms of silver per cm2. I n the presence of bismuth ions a developable image is formed only if considerably greater quantities of silver are evaporated on the crystal surface.

The comparison between the number of bismuth atoms causing a developable image - 5 x 1013 atoms per cm2 and the bismuth ions hindering the process of development - 3 x 1013 per om2 points out that these are practically equal. It seems probable that this is the minimum surface concentration of bismuth which affects the process of development. This concentration is with about one order of ten lower than the concentration of silver ions creating developability.

The exposure experiments with sensitized crystals indicate further that bismuth ions not only hinder the process of development but also impede the latent image formation. In the presence of 0.5 x 1013 bismuth ions per em2 - which is another order of ten lower than the concentration preventing the process of development, no image is obtained upon illumination of crystals sensitized with silver. The desensitization effect of bismuth ions is considerably weaker on crystals sensitized with diacetylthiourea. From these experiments it may be concluded that in respect to the process of latent image formation the bismuth ions have an effect opposite to that of silver sulphide which, according to WEST and SAUNDERS [15] acts as a trap of positive holes.

The results outlined above indicate that bismuth ions hinder the two basic steps in the process of formation of the visible photographic image, namely 1. the for- mation of latent image specks and 2. the process of development. These two processes are determined by the transference of electrons to the silver ions

A g + + e + A g , so that bismuth ions are shown to prevent the proceeding of this reduction process. The desensitization effect can be reasonably connected with either of the following assumptions: 1. the reduction of silver ions cannot take place because the injected electrons, either by the exposure or by the developer, are transferred to the bis- muth ions and not to the silver ions; 2. the presence of tervalent bismuth ions diminishes the local Concentration of mobile silver ions decreasing thus the rate of the process of reduction.

With the data available a t present, however, no definite answer can be given for the confirmation of either possibility.

References [l] W. E. GARNER, Chemistry of the Solid State, London 1955 (p. 311). [2] J. M. XTCHELL, Reports on Progress in Physics. vol. 20, 1957 (p. 433). [3] W. WEST and V. J. SAUNDERS, J. phys. Chem. 63, 45 (1959). [4] J. MALINOWSKI, G. KARADJOW, and M. TODOROWA, Bulg. Acad. Sci., Izv. Ins. Phys.

[5] J. MALINOWSKI, J. Phot. Sci. 8, 69 (1960). [6] P. V. CLARK, McD. and J. W. MITCHELL, J. Phot. Sci. 4, 1 (1956). [7] P. BRIDGMAN, Proc. Amer. Acad. Arts Sci. 60, 305 (1925). [ S ] D. C. STOCKBARGER, Rev. sci. Instr. 7. 133 (1936). [9] J. M. HEDGES and J. W. MITCHELL, Phil. Mag. 44, 357 (1953).

Chem. 2, 219 (1962).

LlO] J. MALINOWSKI and G. KARADJOW, Bulg. Acad. Sci., Izv. Inst. Phys. Chem. 3, 89 (1963); J. Phot. Sci. 12, 47 (1964)

21 physica

312

[ l l ] 0. B. BUDEWSKI, private communication. [12] G. BRAUER, Handbuch der praparativen anorganischen Chemie, Stuttgart

[13] J. M. HEDGES and J. W. MITCHELL, Phil. Mag. 44, 323 (1953). [14] V. J. SAUNDERS, R. W. TYLER, and W. WEST, Internationales Kolloquium ubrr

[15] IT. WEST and V. J. SAUNDERS, Phot. Sci. and Eng. 3, 358 (1959).

J. MALIXOWKI et al. : Photographic Processes on Bi-Doped AgBr

1954 (p. 302).

nissenschaftliche Photographie, Ziirieh 1961.

(Receiwd February IS, 1964)