PHOTOCONDUCTION IN MICROCRYSTALLINE ZINC OXIDE
767
Hypersensitization of Photoconduction in Microcrystalline Zinc Oxide’
by Eiichi Inoue, Hiroshi Kokado, and Takashi Yamaguchi Graphic Engineering Laboratory, Tokyo Institute of Technology, Tokyo, Japan
(Received November 4 , 1964)
A discussion is given of two types of sensitization for photoconduction in microcrystalline zinc oxide. Sensitization by organic acid or acid anhydride is supposed to take place similarly to that by oxygen: the electron transfer between a positively charged interstitial zinc ion and a negatively charged organic acid radical ion is suggested. Optical sensitization, the other type of sensitization, also is studied with various dyestuffs and photodesorption of oxygen is observed during that process. Between the efficiencies for photodesorption and for sensitized photoconduction, a parallel relation is found. When acid anhydride or other electron-affinitive substances and dyestuff coexist on zinc oxide surface, an anomalous sensitizing effect is observed. This “hypersensitization” is considered to be caused by an interaction between the dye and the electron-affinitive molecule. Weigl’s mechanism for optical sensitization is favored on the basis of the experimental results.
Introduction Microcrystalline zine oxide exhibits remarkable photoconduction as long as its surface is covered with chemisorbed oxygen, but it turns out to be a quite poor photoconductor as the oxygen is removed from the surface.2 For this reason, chemisorbed oxygen may, in a sense, be regarded as a chemical sensitizer for zinc oxide photoconduction. Several other electron-affinitive niolecules (EAJI) such as nitrogen oxide, nitrogen peroxide, and quinones have been proved to be effective for this kind of s e n s i t i ~ a t i o n . ~The , ~ mechanism of sensitization by these substances has been well understood since the work of Ruppel and co-workers2: EAJI adsorbed on a zinc oxide surface produces an acceptor level for conduction electrons. A depletion layer formed there lowers the dark conductivity. Destruction of the depletion layer takes place during the illuniination of zinc oxide with light shorter than 4000 A., increasing t8heelectron density. I t will be of interest to see the sensitizing effect when two or more different kinds of EAM coexist on the surface of zinc oxide. It has been reported that further sensitization is possible if an adequate second electronaffinitive adsorbate is given to the surface which already has cheniisorbed oxygen.6 A quite different type of sensitization is the optical sensitization by dyestuffs or the dye sensitization. The mechanism for this phenomenon has been widely
discussed for zinc oxide by several investigators,6-8 though it is not yet fully understood. A distinct point is, however, that the dye sensitization can be observed only with light absorbed by the dye molecules. An interacting sensitization of EAM and dyestuff has been found to improve further the photoconduction performance of zinc ~ x i d e . ~We ~ . shall ~ call such a cooperative sensitization “hypersensitization of photoconduction.” In the first section of the present paper, work done concerning the sensitization by benzoic acid, phthalic anhydride, and other electron-affinity substances will be described. In the second section, the optical sensitization by various dyestuffs will be discussed in relation to photodesorption of oxygen from dyed zinc (1) Presented to the International Conference on Photosensitization in Solids, Chicago, Ill., June 22-24, 1964. (2) W. Ruppel, H. J. Gerritsen, and A. Rose, Helv. Phys. Acta, 30, 495 (1957).
(3) H . Kokado, E. Inoue, T. Yamaguchi, and K. Takahashi, Bttll. Chem. SOC.Japan, 34, 705 (1961). (4) A. Terenin, E. Putzeiko, and I. Akimov, J. chim. phys., 54, 716 (1957). (5) (a) E. Inoue, I. Maki, and T. Yamaguchi. Kogyo Kagaku Zasshi, 6 6 , 428 (1963); (b) E. Inoue and T. Yamaguchi, Bull. Chem. SOC. Japan, 36, 1573 (1963).
(6) J . W. Weigl, Internationales Kolloquime iiber Wissenschaftliche Photographie, Zurich, September 1961, (7) R. hlatejec, Z . Elektrochem., 6 5 , 783 (1961). (8) S. J. Drudkowski and L. I. Grossweiner, J . Opt. SOC.A m . , 54, 486 (1964).
Volume 69,Kumber 3 March 1965
EIICHIINOUE, HIROSHIKOKADO, A N D TAKASHI YAMAGUCHI
768
10-6
IO -L--
ATMOSPHERIC AIR I REDUCED PRESSURI AIR
Io-
~
i
I
(-Io-
10d,
E,
*-
g
lo-
L
6 7-U
I 0-
_1 t
IO
3.
t f
v
I
IO
- 1 1
LIGHT
LIGHT
IO I
I
Time, min.
Figure 1. Typical response curves for photoconduction:
_ - --, basic system (ZnO with chemisorbed oxygen); , basic system 0.01 mole % benzoic acid; ___ , basic system 0.01 mole % phthalic anhydride;
+ +
L
I
I
I
I
I
0 1 2 3 4 5 Phthalic anhydride concentration X 10-2, mole %.
I
6
Figure 3. Dark and photocurrents in ZnO us. concentration , dark current; - - - -, of pht,halic anhydride added: photocurrent; 0 , in atmospheric air; 0, in reduced-pressure l excitation: 365 mp, 1 X w./cm.2; air ( ~ 1 0 - mm.); ZnO used: no. 2 .
excitation: 365 mp, 1 X lo-‘ w./cm.z.
oxide, and in the last section the hypersensitization will be discussed.
I. Effect of Organic Acid and Acid Anhydride on Photoconduction of Zinc Oxide Having Chemisorbed Oxygen
10-1
----e-
o,&
IO
I
o*c
-1.1
IO 0
I
l
l
I
,
I
2
3
4
5
6
Benzoic acid concentration X 10-2, mole To.
Figure 2. Dark and photocurrents in ZnO us. concentration of , dark current; - - - -, benzoic acid added: photocurrent; 0 , in atmospheric air; 0, in reducedpressure air (-10-1 mm.); excitation: 365 mp, 1 x 10-4 w./cm.2; ZnO used: no. 2 .
The Journal of Physical Chemistry
I t has been reported that phthalic anhydride, iodine, and p-chloroanil have a sensitizing effect for the photoconduction of zinc oxide in the intrinsic region of light absorption, as coadsorbed with o ~ y g e n . ~The ~ , ~thermal activation energy of EAM adsorbed zinc oxide was, in most cases, found to be a little lower compared to that of the basic systeiii (zinc oxide with cheinisorbed oxygen), indicating that the adsorption had changed the surface barrier height of zinc A further experiment was carried out to investigate the mechanism of sensitization by phthalic anhydride which has been found to be iiiost efficient in the previous work. Attention was paid especially to comparison of phthalic anhydride with benzoic acid. The iiiicrocrystalline zinc oxide used was Merck’s reagent, -0.1-0.3 p in particle size. The adsorption of phthalic anhydride or benzoic acid was carried out in the dark froin an alcoholic solution. During this procedure, no attempt was made deliberately to remove cheinisorbed oxygen already present on the surface of zinc oxide; therefore most of the oxygen was considered
PHOTOCONDUCTION IN MICROCRYSTALLINE ZINC OXIDE
"
- 1
IO
0
0 0
I I 1 I I 2 3 4 Benzoic acid concentration X 10-8, mole %.
769
-
I 5
Figure 4. Dark and photocurrents in ZnO us. concentration of benzoic acid added (in atmospheric air): 0, dark current; 0, photocurrent; excitation: 365 mp, 1 X w./cm.l; ZnO used: no. 1.
2
0
4
6
8
Concentration X 10-3, mole %.
Figure 6. Dark and photocurrents in ZnO us. concentration of phthalic acid or benzoyl peroxide added (in atmospheric , phthalic acid; - - - -, benzoyl peroxide; air): 0 , dark current; 0, photocurrent; excitation: 365 mp, 1 x 10-4 w./cm.-Z; ZnO used: no. 2.
-a IO
c
g
0 I 2 3 4 Phthalic anhydride concentration X 10-8, mole yo.
5
Figure 5 . Dark rind photocurrents in ZnO us. concentration of phthalic anhydride added (in atmospheric air): 0 , dark current; 0, photocurrent; excitation: 365 mp, 1 X w./cm.?; ZnO used: no. 3.
to reiliain chemisorbed. The samples prepared were coated in the dark on glass plates to a thickness of -30-50 p. Two aluminum strips used as electrodes were placed 0.5 cni. apart on each of the specimens to accomplish a surface-type arrangement. In order to eliiiiinate any past irradiation history of the samples, every speciiiien was kept in the dark for 3 weeks or iiiore
4
ok"'
II
2I
3 1
4L
0 Initial amount added X 10-1, mole/g. of ZnO.
Figure 7. Consumption of benzoic acid by ZnO. The broken line shows the expected relation for 2: 1 consumption.
before its electric current was measured with a vibrating reed electrometer. A high pressure iiiercury laiiip coupled with an ultraviolet filter was used as the light source for photocoriduction measurements. Figure 1 illustrates typical photoresponse curves of the two kinds of zinc oxide, together with the basic system. Zinc oxide samples employed for these iiieasureiiients (no. 2) seem to have an inversion layer with surface ptype characteristics as noticed by Ciiiiino and coVolume 69, Yumber 9
March 1965
EIICHIINOUE, HIROSHI KOKADO, AND TAKASHI YAMAGUCHI
770
0
2
I
3
4
5
Time, min.
Initial amount added, mole/g. of
ZnO.
Figure 8. Adsorption of phthalic anhydride by ZnO: 0 , in the dark; 0, under illumination with a mercury lamp. The broken line shows the expected relation for 1:1 consumption.
Figure 10. Photodesorption of oxygen from dyed ZnO with rose bengal: A, 0.002 mole %; B, 0.001 mole %; C, 0.0005 mole yo; D, 0.01 mole %; E, 0.005 mole % of rose bengal added; excitation: -510-700 mp; 1.5 X w./cm.z.
IONIZATION VACUUM GAUGE
E
3
@
SHUTTER
THERMOSTAT
-
COLOR- FILTER
CUPRIC SULFATE SOLUTION FILTER
-e-
LIGHT SOURCE (XENON ARC)
Figure 9. Experimental setup for photodesorption measurement.
worker^.^ The dramatic decrease of photocurrent with time in atmospheric air and the sharp decrease of current immediately after rapid evacuation of the specimen container, followed by the slow increase, indicate the presence of an inversion layer. Figures 2 and The Journal of Physical Chemistry
'
'
"""'
IO3
IO2
Rose bengal concentration, mole %.
---
SAMPLE
0,L.
Figure 11. Correspondence of oxygen photodesorption and photosensitivity of dyed ZnO with rose bengal; excitation: -510-700 mp, 1.5 X 10-3 w./cm.z.
3 show the dependence of dark and photocurrents upon the amount added of the two adsorbates. When only a small amount is added (less than 0.005 mole %), organic acid and acid anhydride affect the conductivity of zinc oxide essentially in the same manner, while a little difference appears as the amount is increased. The difference may be attributed to a difference in the adsorption processes of the two substances which will be described later. A more detailed relation between the current and the amount of addition in the region up to 0.005 mole yo was studied in atmospheric air (Figures 4 and 5). It should be noted that the curves in Figure 4 which were consistent with those measured in atmospheric air in Figure 2 were obtained with zinc oxide no. 1 that had a less pronounced inversion layer than no. 2, while the curves in Figure 5 which reproduced those in reduced-pressure air in Figure 3 were obtained with zinc oxide no. 3 that had a normal de(9) A. Cimino, E. Molinari, and F. Cramarossa, J. Catalysis, 2 , 315
(1963).
PHOTOCONDUCTION IN MICROCRYSTALLINE ZINC OXIDE
,"X
771
droxide solution, whereas untreated zinc oxide did not. The difference in adsorption behavior between acid and acid anhydride is possibly responsible for the difference in the sensitizing behavior at the higher concentrations In Figures 2 and 3. Possible mechanism of the sensitization of zinc oxide by organic acid and acid anhydride are listed below. For benzoic acid
'"'F
Light intenaity, w./cm.*.
Figure 12. Light intensity dependence of the oxygen photodesorption rate; ZnO used: dyed with 0.002 mole 7' rose bengal; wave length of light: -510-700 mp.
For phthalic anhydride
pletion layer. This is understandable by assuming that the inversion layer of zinc oxide no. 2 was destroyed by illuminating and outgassing. Curves similar to Figure 4 were obtained for phthalic acid-adsorbed and benzoyl peroxide-adsorbed zinc oxides (Figure 6). An attempt was made to find any difference in adsorption behavior between benzoic acid and phthalic anhydride. A weighed amount of zinc oxide was thrown into toluene or alcoholic solutions of different concentration of benzoic acid or phthalic anhydride and the change in solute concentration after stirring for 1 hr. was plotted against the initial concentration. The concentration was determined spectroscopically at the wave length of 295 nib in the case of benzoic acid and of 290 nib in the case of phthalic anhydride. A stirring t h e of 1 hr. was experinientally confirmed to be enough to establish the adsorption equilibrium for both adsorbates. From the results shown in Figures 7 and 8, it is obvious that most of the added benzoic acid was consumed by zinc oxide up to the molar ratio of 2 : l to zinc oxide. On the other hand, only a few per cent of phthalic anhydride was adsorbed. The intensification of adsorption by light from the mercury lamp is considered to be an indication of chemisorption. No effect of light was observed on the adsorption of benzoic acid. These results strongly suggest that benzoic acid reacts with zinc oxide in toluene. 2C6H6COOH
+ ZnO + (C6H&OO)zZn + HzO
(1)
When the amount of benzoic acid is small, however, interstitial zinc: atoms will be attacked first by the acid.
+
C6H6COOH Zn(interstitia1) ---f C6H,COOZn+(interstitial)
+
+ 0.5Hz
(2)
Evidence for the first reaction wm given by the fact that benzoic acid-treated zinc oxide (in toluene) callipletely dissolved in diluted aqueous ammonium hy-
The second possibility assumes catalytic oxidation of acid anhydride and after that sensitization takes place just as well as in the first one. This mechanism is plausible to interpret the similarity of acid and acid anhydride in sensitizing behavior a t the lower concentrations. The mechanism, however, can be decided only from additional experimental information. In a recent publication, Alarkevich and Putseiko'O concluded that chemisorbed oxygen served as the adsorption center for acidic dyes. This also provides a quite interesting suggestion for the present case. There is nothing to disprove the reasoning that the adsorption of phthalic anhydride takes place similarly to that of acidic dyes.
11. Desorption of Oxygen during Optical Sensitization Photodesorption of oxygen from zinc oxide has been reported by several workers. 11-13 The same phenomenon was observed for the first time in the course of the optical sensitization with various dyestuffs. The experimental apparatus used is illustrated in Figure 9. (10) N . N. iMarkevich and E. K. Putseiko, Kinetika i Kataliz, 4, 307 (1963). (11) D. B. Medved, J . Chem. Phys., 28, 870 (1958). (12) Y. Fujita and T. Kwan, Bull. Chem. Soc. J a p a n , 31, 379 (1958). (13) A. Terenin and y, Solonitzin, Discussions Faraday Sot,, 28, 28
(1959).
Volume 69, Number 3
March 1985
EIICHIIXOUE, HIROSHI KOKADO, A N D TAKASHI YAMAGUCHI
772
IC
x
5
a' B
I
2 x
x S
2
I 3.0
3.1
3.2
3.3
3.4
3.5
5
x
1/T X 10-*/"K.
Figure 13. Temperature dependence of the oxygen photodesorption rate; ZnO used: dyed with 0.002 mole yo rose bengal; excitation: 510-700 mp, 1.5 X 10-3 w./cm.z.
0
Time, min
Figure 15. Photodesorption of oxygen from ZnO dyed with n-type dyestuffs; RH, rhodamine B; 31, methylene blue; A, acridine orange; F, fuchsine; C, crystal violet; concentration: 0.002 mole %; excitation. -510-700 mp, 1.5 X w./cm.2.
I
3
2
Time, min.
Figure 14. Photodesorption of oxygen from ZnO dyed with p-type dyestuffs; RB, rose bengal; P, phloxine; ER, erythrosine B; U, uranine; EO, eosine; concentration: 0.002 mole yo; excitation: -510-700 mp, w./cm.z. 1.5 X
The aniount of oxygen photodesorbed was measured via the pressure increase in the sample container and plotted against the illumination time. Figure 10 shows the results obtained with various concentrations of rose bengal. The satisfactory correspondence between the rate of photodesorption and the sensitized photocurrent in Figure 11 strongly indicates that the two processes are common in origin. Additional evidence for that is given by the saiiie light intensity dependence of the two processes (Figure 12). A value of 0.04 e.v. was obtained for the thermal activation energy of photodesorption in the temperature range in Figure 13. The photodesorhed gas was examined with a inass spectrometer and also with a gas chromatograph. Within the experimental accuracy of 1%, no decomposition fragments from the dye molecules were found. The aniount of photodesorbed oxygen was compared for various kinds of n-type and p-type dyestuffs. As is observed in Figures 14 and 15, the aniount increased in the order: The Journal of Physical Chemistry
Figure 16. Mechanisms for the optical sensitization: ( 1 ) Weigl's mechanism: A e +. A - (in the dark); D hr +. D + e (into the conduction band); D+ A- +D A. ( 2 ) Terenin's mechanism: D h v P-. D*; D* A - (or T - ) +. D A (or T) e; A (or T) e + A - (or T-) (in the dark). (3) When the dye is absent: A e +. A - (in the dark); hv + @ e; A @I -+ A. D, dye molecule; A, electron-affinitive molecule (oxygen, phthalic anhydride, or others); T, electron trap other than A ; e,hole.
+ +
+
+ +
+
+
+
+
+ +
+
+
eosin, uranine, erythrosine B, phloxine, rose bengal for p-type dyestuffs, and crystal violet, fuchsine, acridine orange, methylene blue, rhodamine B for n-type dyestuffs, in accord with the order of increasing sensitizing efficiency for photoconduction. At the present stage, it is still too difficult to determine the niechanisin of optical sensitization. In light of the present experiment, however, two of several proposed mechanism seen1 to survive : one proposed by WeigP (Figure 16 (1)) which involves the electroil transfer from dye molecules to zinc oxide and the other proposed by Tereriin and co-workers4 (Figure 16 ( 2 ) )
773
PHOTOCOKDUC'lYO?U' IN 3IICROCRYSTALLINE Z I N C OXIDE
2oc 0
v uv
h
I
2
3
4
5
Time, min.
Figure 17. Phoiodesorption of oxygen from ZnO dyed with rose bengal: V, -510-700 mp excited; UV, -360-370 mp excited: rose bengal concentration: 0.002 mole %; light intensity: '7.5 X w./cm.2.
which involves the energy transfer. Another mechanism suggested by Drudkowski and Grossweiner,8 who applied the concept of a compensated acceptor, is of interest but only for cheniisorbed oxygen-free zinc oxide.
111. Interacting Sensitization of EAM and Dyestuff-Hypersensitization It has been established that the desorption of oxygen from the zinc oxide surface occurs during the dye sensitization of photoconduction. On the other hand, the photodesorption of oxygen froin zinc oxide under ultraviolet light illuniination is well known to increase the electron density (Figure 16 (3)). The rate of oxygen release by ultraviolet light was compared with that by visible light of the same intensity (Figure 17). The fact that oxygen can be desorbed by either ultraviolet or visible light provides a possibility of interacting sensitization when dyed zinc oxide is exposed to light which includes both regions of wave length. If the rate-determining step for photoconduction is the bimolecular recombination of oxygen a t o m discharged a t the surface of zinc oxide, the coirradiation would greatly increase its opportunity because of higher atomic concentration and, consequently, the photocurrent would be higher than the suiii of those under separate irradiation. Figure 18 shows that this is actually the case. This superposing eff ect14should be distinguished from the hypersensitization effect described below. The term "hypersensitization" is given to the phenonienon that coaddition of EAM and dyestuff yields an enoriiious increase of photocurrent in zinc oxide in the visible region. This was observed with most of the electron-affinitive substances exaniined. The spectral response of hypersensitization is reproduced in Figure 19. Here, supposedly, the electron-
3
2
I
4
Time, min.
Figure 18. Superposing effect of photoconduction: 0, ZnO with chemisorbed oxygen 0.002 mole 70
+
+
rose bengal; XI ZnO with chemisorbed oxygen 0.002 mole yo rose bengal 0.03 mole yc p-chloranil; V, 577 mp excited; UV, 365 mp excited.
+
4
2
-
X 3 I .9
.1 . '"
fx 2
..-
I
.-?
a
2 ' 305 0
400
450
500
550
600
Wave length, mp
Figure 19. Hypersensitization of zinc oxide photoconduction+ yo rose bengal); 0, basic system phthalic anhydride (0.02 mole 70); A, basic system iodine (0.03 mole X, basic system p-chloranil (0.03 mole 70); light intensity: 4 X w./cm.2, 0, basic system (dyed ZnO with 0.002 mole
+ +
x);
Volume 69,Number 9
+
March 1965
SUSUMW NAMBA AND YASUSHI HISHIKI
774
affinitive molecule cooperates with the dye molecule in the optical sensitization process; in other words, the optical sensitization is intensified by the electron-affinitive molecule. This appears to favor Weigl's mechanism for optical sensitization, in which a direct interaction between a positively charged dye ion and a negatively charged electron-affinitive molecule ion is postulated. This niechanisni would allow one to interpret the photosensitivity in ternis of the nature and the surface density of both the dye and the electron-affinitive molecule. It is surprising that the wave length of effective light was so widely exheiided that the photore-
sponse curves cover a new region where only negligible sensitivity had been observed before the coaddition of EAM. The reason for that is not yet clear. The negligible photosensitivity might be emphasized by the interaction between the two adsorbates, or a new absorption due to the interaction might arise there. Acknowledgment. The authors wish to thank N r . Isaniu Maki for his earnest assistance with the experimental details. (14) Terenin and his co-workers,4 who also observed this superposing effect, ascribed it to an accumulation of trapped carriers caused by the simultaneous exposure to ultraviolet light.
Color Sensitization of Zinc Oxide with Cyanine Dyes'
by Susumu Namba and Yasushi Hishiki The Institute of Physical and Chemical Research, Bunkyo-ku, Tokyo, J a p a n
(Received Octoher 6 , 1964)
The photoconductivity of zinc oxide can be supersensitized by dyeing the surface with concentrated solutions of cyanine dyes or combinations of two cyanine dyes which have been used in the field of photographic sensitization. The time response of the photoconductivity of zinc oxide can be modified over the range of two decades by the effect of the adsorbed dyes.
Introduction The photoconductivity of zinc oxide can be sensitized for wave lengths longer than the fundamental absorption wave length of the host crystal by the effect of adsorbed dyes on the ~ u r f a c e . ~It, ~also can be supersensitized by dyeing the surface with a concentrated solution of a cyanine dye or conibinations of two cyanine dyes.*s5 Although niany kinds of cyanine dyes were used to sensitize the photoconductivity of zinc oxide, the following special features of interest are pointed out. (1) Sonie dyes show the supersensitizing character in the presence of J-band aggregation on the surface of zinc oxide which is similar to the case of silver halides. (2) Sonie dyes show the supersensitizing character under the existence of other special dyes (which is similar to The Journal of Phyaicnl Chemistry
the case of silver halide). (3) The time response of photoconductivity of zinc oxide is fast when ZnO is sensitized by some particular group of dyes, and is slow when ZnO is sensitized by other groups of dyes.
Experimental ( 1 ) Zinc Oxide Powder Cell. As shown in Figure
l(a), ZnO powder cells were prepared by applying ZnO paste on a glass substrate on which aluminum electrodes ~
~~~
(1) Presented to the International Conference on Photosensitization
in Solids, Chicago, 111.. June 22-24, 1964. (2) J. A. Amick, R C A Rev., 20, 770 (1959). (3) S.J. Dudkowski and L. I. Grossweiner, .I. Opt. SOC.Am., 54, 486 (1964).
(4) Y . Hishiki, et al., Rept. Inst. P h y s . Chem. Res. (Tokyo), 36, 386 (1960). (6) 9. Namba and Y . Hishiki, ibid., 39, 27 (1963).
