OPTICAL SENSITIZING BY DYES
411
There are a number of frequently overlooked pH changes which occur in aqueous sucrose-pectin-acid systems, especially in the region where gels can form. Data to be submitted in a later paper will show several of the individual factors affecting these pH changes and point out the importance of considering these factors when postulating a mechanism for gel formation. For instance, i t will be shown that when sucrose is added to an acidified pectin sol the pH may decrease as much as 0.10 unit or in some owes may rise 0.20 pH higher than the original value, depending upon the pH region, the sucrose content, and upon whether or not gelation occurs. In the region of proper balance for gel formation, there is, in addition, a time factor which must be considered when making pH measurements. REFERENCES (1) KRUYT,H. R . : Colloids, a Teztbook, 2nd edition, p. 200. John Wiley & Sons, Inc., New York (1930). GENE:J. Phys. Chem. 34, 410-17 (1930). (2) SPENCER, (3) TARR,L. W.: Univ. of Delaware Agri. Expt. Sta. Bull. No. 134 (February, 1923).
PHOTOVOLTAIC CELLS WITH SILVERSILVER BROMIDE ELECTRODES. I11
OPTICALSENSITIZING BY DYES^ S. E. SHEPPARD, W. VANSELOW, AND G . P. HAPP Koduk Research Laboratories, Eastman Koduk Company, Rochester, New York Received July 94, 1039
In the previous papers of this series (6, 7) there have been described the photovoltaic phenomena observed with a cell of two silver-silver bromide electrodes, one exposed to light, the other kept dark; the electrodes were connected electrolytically, usually by a dilute potassium bromide solution, and externally either through a vacuum tube voltmeter or a three-stage amplification oscillograph. In the present investigation these instruments have been replaced by an Einthoven string-galvanometer. A detailed description of the apparatus and operation is being presented elsewhere.2 With this there have been studied the photovoltaic effects obtained when the silver bromide of the illuminated electrode is dyed with a sensitizing l
Communication No. 742 from the Kodak Research Laboratories.
* In preparation.
412
8. E. SHEPPARD, W. VANSELOW AND G. P. HAPP
dye; the light incident might then have a spectral composition limited to that absorbed by the silver bromide alone, or to that absorbed by the adsorbed dye alone. Sensitizing dyes could be chosen whose light absorption (when adsorbed to silver halide) is well separated from that of silver bromide itself. The nature of the adsorbed dye layer a t silver halide surfaces has been dealt with recently by Leermakers, Carroll, and Staud (3) and by Sheppard, Lambert, and Walker (5). Here it may be noted that the adsorption may be practically complete (irreversible) or incomplete (reversible), depending upon the number and position of solubilizing (polar) groups in the molecule. In the present investigation it was desirable to have a completely adsorbed dye and a t such an adsorption density that optical shielding effects would not be present. The dye ~ h o s e n 2,2'diethyl-8,~ methylthiacarbocyanine bromide, of the following structure:
S N
H
I
I
/b
C=C--C==C--C CHs \+N/
CzHs
BF
\
\ CB"
is satisfactory in these respects. The source of light was a mercury arc (Cooper Hewitt, quartz). It was operated a t 150 volts potential drop across the terminals, and a t 21.2 cm. distance from the front surface of the silver-silver bromide electrode. The spectral distribution of the light from this source is largely discontinuous (4), with a relatively faint continuous background. I n these experiments, limited spectral regions were isolated by the use of light filters of known transmissions. The filters used, their approximate spectral range, and the energy transmitted from the mercury arc to the photocell surface are given in the following tables. Table 1 refers to what are termed, inclusively, the first series of relevant experiments, and table 2 to the second series. There was no difference in principle between these, but several details were different, which will be noted as required. In addition, with each of the combinations in table 1 there was also present an infrared absorption filter, consisting of a cell of 1 cm. internal thickness filled with 5 per cent cupric chloride solution. The investigations of Leermakers, Carroll, and Staud (3) have shown very definitely that the absorption spectrum of the dyed silver halide corresponds accurately to its spectral sensitivity. In figure 1 is shown the spectral reflectance curve for silver bromide dyed with dye 1%.
* Dye IV b; see reference 5.
413
OPTICAL SENSITIZING BY DYES
TABLE 1
Ezperiments i n series IC
1
BILTPR
BPECPBAL RANQE TRAN81118810N
1
ENERQY TRANSMITTED clod
“Neutral” D1. . . . . . . . . . . . . . . . . . Wratten No. 61., . . . . . . . . . . . . . . j. Wratten No. 61 plus D l . . . . . . . . .i Corning 986.. . . . . . . . . . . . . . . . . . . . Corning 988 plus D l . . . . . . . . . . . . .
Total Maximum 530 mp Same Ultraviolet up to 400 mp Same
per second per mm.9
55.45 52.25 12.80 50.25 1.71
~
Cooper Hewitt lamp 10,474was used. TABLE 2
Experiments i n series II* FILTER
a g a p a reand p u mm.2
Cf.figure 1
Wratten No. 16 cemented in optical B glass.. . Wratten No. 16 plus Neutral D1 as required. . Purple Corex “A” 986 (-1) (Corning Glass CO.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
* Cooper Hewitt
lamp 50,184 was used.
h
EY L v
EY
i m
xa
400
5w
boa
700
V I Y E L C W e T Y IN UlLLlUlCrmNS
FIG.1. Transmittances of Wratten filters No. 61 and No. 16 superposed on the reflectance curve. Curve 1, transmission of Wratten filter No. 16; curve 2, transmission of Wratten filter No. 61; curve 3, reflectance of silver bromide (in gelatin) sensitized with dye IVb, 1.00 mg. of dye per 100 cc. of emulsion; curve 4, transmission of Corning filter, Red Purple Corex “A” No. 986, thickness 5.02 mm.
414
8. E. SHEPPARD, W. VANSELOW AND Q. P. HAPP
The reflectance curve corresponds, inversely, to the absorption (and sensitizing). It will be seen that there are two well-marked absorption bands with maxima at 520 mp and 590 mp, respectively. In figure 1 are shown the transmittances of filters Wratten No. 61 (first series) and Wratten No. 16 superposed on the reflectance curve. Evidently in the first series the band at 520 mp was fairly well isolated, there being little TABLE 3
Preparation of electrodes OPERATION
EBRIE0
SERIDB I1
1
Mechanical and heat treatment . . . . . . . . . Polished with 400-mesh carborundum paper Annealing Heated 2 hr. a t 125"C., cooled slowly. Polished on wet MgO (Shamva)
Insulating, back face, etc.. . . . . . . . . . . . . . . . Paraffin (Parowax) Bromination, . . . . . . . . Electrolytic: paired electrodes as anode; P t cathode electrolyte 0.5 per cent KBr solution, 0.12 per cent HBr. Initial current 20 milliamperes over 2 x 100 mm.' anode surface. p.d. = 0.8 volt. Time = 5.0 sec Thickness of AgBr layer. . . . . . . . . . . . . . 0.00025 mm. Aging. . . . . . . . . . . . . . . In water saturated with AgBr (1) 7 days for undyed (2) few minutes for dyed Dyeing time 4 days Three changes of water satuWashing rated with AgBr in 3 days after dyeing
As in I
Heated gradually over 8hr. up to 525"C., cooled to room temperature. Polished aa in I with carborundum paper. Annealed aa in I. Polished aa in I
As in I As in I
0.00021 mm. 1day in 0.001N KBr solution saturated with AgBr Varied up t o 4 days Three changes of 0.001 N KBr solution saturated with AgBr in 24 hr. following dyeing
or no radiation shorter than 480 mp, or longer than 580 mp, while in the second series the band at 590 mp was secured free from all radiation of less than 500 mp, and with only a small amount at 520 mp. Another experimental difference between series I and series I1 was in the preparation of the electrodes (see table 3). This subject has been rather fully discussed in Papers I and I1 (6,7)and only some minor modifi-
416
6. E. SHEPPARD. W. VANBELOW AND G. P. HAPP
negative response) as the concentration of soluble bromide of the electrolyte is decreased. The next table (table 6) gives results obtained when the electrodes, both undyed and dyed, were illuminated with ultraviolet plus visible radiation, and also with ultraviolet alone. The ultraviolet radiation of experiment KO. 3 was isolated by a Corning glass filter (Corex “A” 986 (-1)) of 5.02 mm. thickness, the transmission of which is shown in figure 1. The TABLE 4 Series IA dAxinum NEQATIVE RESPONSE NO.
ELECTROLYTE‘
Undyed
RADlATIONf
Dyed
rolls
mlr
a0118
N KBr 0.001 N KBr 0.001 N KBr 0.001 N KaSO, 0.001 N KBr 0.01 N S a S O i 0.001 N KBr 0.10 N S a x 0 2
0 . 0028
0.M)Ol 0 ,om2 O.OOO2
0.0075 0.m70
0.0010
0.0135
0,0005
0.0080
0.1
530 530 530 530 530
* The electrolyte was in each case saturated with respect to silver bromide. t Radiation transmitted by Wrattrn filter S o . 61 (cf. figure 1): TABLE 5 Series IB M A X I M U M NEQATIVE RESPONSE NO.
Undyed
1
ELECTROLYTE
RADIATION
Dyed
rolta
volta
0.002
O.OO60
0.002
0.0075
0. 0025
0.021
mu
O.OOO1 N KBr 0.01 N KaN02 O.OOO1 N K B r 0.10 N XaNO2 O.oooO1 N KBr 0.10 N NaSOl
530 530 530
large “sensitization” which the dye has conferred with ultraviolet light is not optical sensitizing, since this radiation is strongly absorbed by silver bromide itself. It is to be attributed to the efficient halogen-accepting capacity of the dye. This has been demonstrated elsewhere in an investigation of the photolysis of dyed silver bromide by Sheppard, Lambert, and Walker (5). In that investigation it was found that tkiese sensitizing dyes very readily form labile bromine addition compounds, which give up
MAXIMUM NEGATIVE RESPONSE
1
-
NO.
-
oolra
Cd&
1 2
0.001 0.OOO
0.0091 0.0043
0.1 N KBr 0.1 N KBr
3
O.OOO8
0.0087
0.1 N KBr
BLXCTROD= DYlD
0
RADIATION
IL~BOLTTE
Dyed
Undyed
.VERAGE MAXIMUM NEGATIVE RESPONBE
-
-
No NaNOI
NaNOI
roltd
colt8
0.01 N
\VERAOB nxn TO REACH MAXIMUM NEGATIVE POTENTIAL
NO 1.01 N YaNOl XaNOi
__
E d 8
Seonds
VERAQE T I M E TO BTART POSITIVE ACTION
No NaNOI
NsNOI
ascorda
seconds
0.01 N
tive action in 6 sec.
0.01 N
No NaNOI
NaNOI
8EOnda
8SeOnda
Varied
Varied
0.31
0.0005
0
AVERAGE TIME TO RECROW ZERO
No positive action in 6 sec.
No posi-
0.49
1.0003
Ultraviolet plus visible Ultraviolet plus visible (at low intensity) Ultraviolet alone
minufu
1 1 5
1.0068
30
0.08
0.0156
30
Approximately 2
0.06
Approximately 2.25
0.12
0.12
D ,0077
1.68 Approximately 2.38
0.12 0.06
0.0137
1.70 0.05
0.05 0.12
3.0066
5
0.07
0.07 0.0131
Approximately 2
0.08
how8
3
X o t quite I
0.07
0.07
0.0100
a t 6 sec.
0.0110
4
0.0130
4
-
0.06
0.0175
3 24 24
0.07
0.0203
0.08
0.0199 __ __
-_
Varied
0.10
0.10
Varied
0.06
Not quite 0 a t 6 sec.
0.07
Approximately 6
0.08
0.07
0.07
-
__
417
Approximately 6
418
s. E. WEPPARD, t.VANSELOW AND G. P. HAPP
bromine to more permanent halogen acceptors. In the study of the photolysis of silver bromide by ultraviolet or blue light i t was also found that the sensitizing dyes acted as halogen acceptors. The behavior of the silverailver bromide electrode in light is, therefore, entirely consistent with the conclusions reached in the study of the photolysis. These conclusions were fortified by the results of series I1 of experiments on the dyed electrodes, in which, as previously noted, some differences were made in the technique of preparing them. Furthermore, the
PERIOD OF DYEIN6 (MVS)
FIQ.3
hQ. 4
FIG.3. Relation of negative photopotential to time of dyeing. Curve 1, photocell electrolyte, 0.001 N potassium bromide saturated with silver bromide; curve 2, same with addition of 0.01 N sodium nitrite. FIG.4. Relation of negative photopotential to time of dyeing. Curve 1, photocell electrolyte, 0.001 N potassium bromide saturated with silver bromide; curve 2, same with addition of 0.01 N sodium nitrite.
electrodes were illuminated in the longer wave length band-maximum absorption a t 560 mp-by means of Wratten filter No. 16, with little or no radiation shorter than 520 mp (cf. figure 1). Well-marked optical sensitizing of the negative or electronic photovoltaic effect was shown here also. The principal results are collected in table 7. Two principal observations are resumed in the curves of figure 3 and figure 4. I n figure 3 the maximum negative potential difference has been plotted against time in minutes of dyeing, i.e., the initial period. It is evident that there is an initid very rapid adsorption of the dye, followed
OPTICAL SENSITIZING BY DYES
419
by a much slower process which only increases the sensitivity very gradually. In figure 4, where the times of treatment in the dye bath have been extended to days, the same phenomenon is evident, with the difference that when a halogen acceptor, sodium nitrite, was included in the electrolyte, the sensitizing appeared to have approached saturation or equilibrium within 12 to 24 hr. From this it seems probable that the two phases of the dyeing process are (i) very rapid adsorption, as already observed in dyeing silver bromide sols, and (ii) a slower penetration of dye into fissures of the microcrystalline layer. This slower process continues to sensitize in the absence of other halogen acceptor, by furnishing more halogen acceptor for bromine produced by the primary optical sensitizing effect. DISCUSSION
.The previous investigation (6, 7) of the silver-silver bromide electrode in light absorbed by silver bromide had led to the following theory of the photovoltaic effect: The absorption of a quantum releases an electron from a bromide ion of the crystal, which becomes photoconducting. Such electrons can travel through the crystal, and, reaching the metallic silver base, tend to charge this negatively. The excess electron pressure is instantly revealed as a current through the circuit as the negative photopotential difference. The continuance of a current in this sense requires a compensatory process: this is considered to be ( a ) neutralization of Ag+ ions in the dark electrode la$er, ( b ) equivalent emigration of Brions from the “dark” electrode into solution, and ( c ) transport of equivalent Br- ions into the “light” electrode silver halide layer, thereby annulling any space charge due to Ag+ ions left unbalanced by the displacement of photoelectrons. The entry of the photoelectrons into the subjacent silver metal consists, in the language of the quantum-mechanical “zone” model of semiconductors (10, l ) , first, in raising an electron into the previously empty conductance band of the crystal, in which it is mobile, and then the “spilling over” of these electrons into the lower levels of the metallic silver. The electrolytic section of the circuit described above accords with the observation that the magnitude of the negative potential difference is diminished as the concentration of bromide ion in solution is raised. Evidently higher bromide-ion concentration would lower the bromide-ion “solution tension” of the dark electrode, and thus reduce the current. The genesis of the reverse positive current or potential difference in terms of the attack of bromine upon silver has already been explained (6, 7). The reaction Ag
+ 3 Brr = Ag+Br
420
6 . E. SHEPPARD, W. VANSELOW AND G. P. HAPP
may be formulated (see figure 5) according to the Wagner model for such a corrosion reaction ( 8 ) , viz., aa an “inverted electrolysis”; as though the system were an electric element with an E.M.F. corresponding to the free energy of the reaction, and in which the circuit is completed through an electronic conduction component of the corrosion layer, in this case silver bromide. Evidence in favor of this conception in the attack of halogens on silver has been presented by Wagner (9). Since it involves a reverse movement (reverse to the sense of the primary negative photovoltaic effect) of electrons outward from the (silver) metal through the silver bromide layer toward the bromine, in compensation of a migration of (silver) cations through the layer toward the bromine, it is evident that the E.M.F. generated should be, as actually found, opposed to the negative photovoltaic effect. SP
*q*
Bc-
Ri
= internal resistance of corrosion layer
*i
R. = external resistance through metal
FIQ.5 DYE-SENSITIZING OF PHOTOVOLTAIC EFFECT
The emission of electrons by the dyed silver bromide might be ascribed either directly to the dye, or as indirectly from bromide ions of the crystal activated by light-absorbing dye molecules. This has been discussed in recent papers on the photolysis (5), and reasons have been given in favor of some sort of “exciton” transmission from the dye molecule, with subsequent, rather than direct, electron liberation. The present investigation, while confirming the conclusions reached in the photolytic study, does not furnish definite evidence for or against the “direct emission” hypothesis for optical sensitizing. The secondary sensitizing effect of dyes aa halogen acceptors is well confirmed by the photovoltaic studies. SUMMARY
1. An investigation has been made of the effect of an optically sensitizing (cyanine) dye on the photovoltaic effect with silver-silver bromide electrodeg.
OPTICAL SENSITIZING n Y DYES
42 1
2. I t was established that definite optical sensitizing of the primary negative, or photoelectronic, effect was effected. Light absorbed by the adsorbed dye,-and which is not absorbed by silver bromide itself,produced relatively large (electronic) photocurrents. 3. At the same time, it was shown that the adsorbed dye acts as a halogen acceptor, diminishing the (reverse) positive effect, and this function occurs whether the active radiation is that absorbed by silver bromide or by the dye. 4. The results are in excellent agreement with those obtained in the study of the photolysis of dye-sensitized silver halide. 5. The results give further confirmation of the theory previously proposed by the authors for the photovoltaic effect with silver-silver halide electrodes. It is again shown that this effect can be analyzed into an initial photoelectronic effect, produced by photoelectrons released from the silver halide passing into the silver metal, and a relatively delayed chemical effect, viz., the reaction of halogen with the silver. This latter process produces an E.M.F. opposing the primary effect. 6. The results, a t present, do not permit a conclusion as to whether the electrons are initially released from the dye by absorption of light and recovered from bromide ions of the crystal (with release of bromine), or whether the dye merely transmits sufficient energy to the crystal to separate electrons and halogen. REFERENCES (1) CONDON, E. U.:Phys. Rev. 64, 1089 (1938). (2) KOLTHOFF, I. M., A N D O’BRIEN,A. S.: J. Am. Chem. SOC.61, 3409,3414 (1939). (3) LEERMAKERS, J. A., CARROLL, €3. H., AND STAUD,C. J.: J. Chem. Phys. 6, 878
(1937). (4) MCALIBTER, E. D.: Smithsonian Inst. Pub. 3187, Vol. 87,No. 17,p. 9 (January
16, 1933). (5) SHEPPARD, S. E . , LAMBERT, R. H., AND WALKER, R.D.: J. Chem. Phys. 7,265,
426 (1939). (6) SIfEPPARD, 8. E., VANSELOW, w., AND HALL,v . c . : J. Phys. Chem. 33, 1403 (1929). (7) VANSELOW, W., AND SHEPPARD, S. E . : J. Phys. Chem. 39,331 (1929). (8) WAQNER, C.: Z.physik. Chem. B21,25 (1933). (9) WAQNER,C.: Z.physik. Chem. Bsz, 447 (1936). (10) WILSON,A. H.: Proo. Roy. SOC. (London) AlSS, 458 (1931).
