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J. Phys. Chem. 1981, 85,2232-2238
glyoxal), with no reaction. The photoexcited HzCO* may dispose of its excess energy t o the matrix causing local heating and allowing the diffusion of the CO molecule. The relative intensities of the various peaks in the glyoxal photolysis may be used to gain some insight into the relative importance of reactions 1 and 2. It was found in the photolyses of H2C0 in Ar that, for equimolar amounts of H 2 C 0 and CO, the intensity of the C=O stretch of H2C0 is approximately 1.5 times that of the CO frequency as measured by peak areas. Therefore, if reaction 2 were the only process resulting from photolysis, the CO peak should be two-thirds as strong as the stretch of H2C0. However, in the case of the concentrated matrix, these two peaks are of approximately equal intensity throughout the photolysis of the sample. Therefore, CO is produced from an additional source to reaction 2. It has been shown also that the intensities of the C=O stretch frequencies for equal amounts of glyoxal and H2C0 should have roughly the same value. But the loss of glyoxal in the concentrated matrix is not compensated by the increase in H2C0. Thus secondary photolysis of H2C0 is an important reaction H2CO + hv H2 + CO (4) The H2C0 is photolyzed more quickly than expected from previous work: and this may be due to the fact that glyoxal reacts to give a HzCO/CO pair, with CO enhancing the photoreactivity of H2C0. Even if reaction 4 were not important, then reaction 2 would occur to 1.5 times the extent of reaction 1to account for the products. Since reaction 4 is known to occur,z it represents an added source of CO. Therefore, the quantum yield of reaction 2 is a t least 1.5 times as great as the quantum yield of reaction 1. Thus, under the conditions of our experiment, reaction 2 seems to be the more im+
portant pathway for glyoxal photolysis. Quantum Efficiencies of Photolysis. The dimer photolyzes more readily than the major site monomer in the S2 So absorption region (320-260 nm). We are unable to determine the relative quantum efficiencies for product formation, since we were unable to measure relative absorption coefficients of this transition for the dimer vs. the major site monomer. However, it is plausible that the dimer in Ar matrix could have an enhanced absorption coefficient for this highly forbidden transition relative to the monomer in Ar by lowering the symmetry due t o dimerization.
-
Conclusions In the present study it has been found that (1)glyoxal dimers and monomers in different Ar matrix sites are photolyzed more readily than the most common glyoxal monomers; (2) both monomers and dimers give the same photoproducts, H2C0and CO; (3) it is not known whether the higher product yields from photolysis of dimers and perturbed monomers are due to greater quantum efficiency or enhanced absorption of these species; (4) the photochemical threshold for the formation of H2C0 and CO in an Ar matrix appears to lie between 300 and 288 nm on the Sz manifold; and (5) glyoxal A lA, is quenched to 5 3Au in an Ar matrix, with the matrix playing a role similar to that of collision partners in the gas phase. Acknowledgment. This report is based upon research supported by the National Science Foundation under Grant CHE-79-25451 for product studies and the Department of Energy (Office of Basic Energy Sciences) under Contract DE-AT-03-76-ER 70217 for emission studies. The FT IR spectrometer was furnished by the NSF Departmental Research Instrumentation Program.
Electrodic Dye Layers in the Gold-Rhodamine B Photoelectrochemical Cell T. I. Quickenden” and I?.L. Bassett Department of Physical and Inorganic Chemistty, University of Western Australia, Nedlands, W.A., 6009, Australia (Received: January 6, 196 1; In Final Form: March 24, 196 I)
Dye layers have been deposited on a gold electrode from an aqueous solution of rhodamine B (ethanaminium, N-[9-(2-carboxyphenyl)-6-(diethylamino)-3H-xanthen-3-ylidene]-N-ethyl-, chloride), under applied oxidizing potentials in excess of 0.85 V relative to a saturated calomel electrode. Such layers were observed to increase the power conversion efficiency of the gold-rhodamine B photoelectrochemical cell by up to 14 times. The dye layers were shown, by absorption, fluorescence, and fluorescence excitation spectroscopy, to be composed of rhodamine B and its completely deethylated form, rhodamine 110. Photovoltage action spectra were determined for the dye-coated photoelectrodes and were found to resemble the relevant absorption spectra, except for the presence of a photovoltage peak at 400 nm. This peak was not observed in the action spectra of uncoated gold, electrodes immersed in rhodamine B solutions.
Introduction The formation of electrodic dye layers in photoelectrochemical cells containing solutions of fluorescent dyes was first noted briefly by Miller1 in 1962, in his comprehensive study of the iron-thionine cell. However, it is only in the last few years that substantial evidence for the appearance
of dye layers in such systems has become available. In 1978 Yim2 reported that, when cells containing gold electrodes in contact with rhodamine B solutions were irradiated for long periods, a layer of colored material was deposited on the photoelectrode and the photovoltage concurrently increased. Further studies by Quickenden, Yim, and Herring3 on the gold-rhodamine B cell, using a
(1)L. J. Miller, “A Feasibility Study of a Thionine Photogalvanic Power Generation System”, Final Report, Contract No. AF33(616)-7911, Sunstrand Aviation, ASTIA Document No. 282878, 1962.
(2) G. K. Yim, Ph.D. Thesis, University of Western Australia, Nedlands, Australia, 1978.
0022-3654/81/2085-2232$01.25/00 1981 American Chemical Society
Electrodic Dye Layers in the Gold-Rhodamine B Cell
rotating disk photoelectrode, confirmed that a nondiffusive component of the photocurrent gradually became dominant over a period of several days, at the same time as a rhodamine colored layer gradually formed on the electrode. Albery et al.495have recently described procedures for the electrochemical deposition of thionine onto both Pt and SnOz electrodes and have found little evidence to suggest that the coherent layer thus formed is anything but thionine and leucothionine equilibrated therewith. In a subsequent paper, Archer et a1.6 have reported an electrokinetic study of thionine-coated platinum electrodes and have observed that the heterogeneous rate constant for the ferrous/ferric couple at a Pt electrode decreases by more than 4 times when a thionine layer is electrochemically deposited on the electrode. They suggest that the deposited thionine may be anodically polymerized during the coating process, but, as the redox potential of the thionine layer is only slightly displaced from that of thionine in bulk solution, the T orbital structure has evidently suffered little modification. The work of Fujihira et al.7-9bears some relevance to the above studies inasmuch as these workers have chemically bonded monolayers of rhodamine B to T i 0 2 and S n 0 2 semiconductor electrodes. It was found that the linkage of rhodamine B to the electrode via ester groups led to an enhancement of the photocurrent by -2 orders of magnitude. Fox et al.*Ohave recently extended the work of Fujihira using a wider variety of chemical bonding agents and have noted that a rhodamine coating improves the long-term stability of Sn02electrodes. From the point of view of any prospects for solar energy conversion, it is interesting to note that both the electrodic dye layers which form spontaneously3 in the gold-rhodamine B cell and the layers of thionine which can be deposited4+ on Pt or SnOz electrodes result in increased photovoltages and photocurrents. However, the improvements to the power conversion efficiency occasioned by these dye layers have not been determined. One aim of the present study was to determine whether the dye layers observed by Quickenden et al.3 to form spontaneously in the gold-rhodamine B cell could be deposited electrochemically by the method of Albery et a L 4 s 5 or by any other method. The second aim was to study such dye layers to see whether they were composed of rhodamine B molecules or whether the dye molecule had undergone transformation during deposition. The third purpose was to determine the extent to which the power conversion efficiency of the gold-rhodamine B cell is enhanced by the deposition of dye layers on the photoelectrode.
Experimental Section The photoelectrodes and dark electrodes used in the present study were formed by vacuum-depositing gold (99.99% purity) onto 8.5 X 2.0 cm pieces of Perspex. Contact wires were glued to the top of the electrodes by (3) T. I. Quickenden,D. P. Herring, and G. K. Yim, Electrochim. Acta, 25, 1397 (1980). (4) W. J. Albery, A. W. Foulds, K. J. Hall, A. R. Hillman, R. G. Egdell, and A. F. Orchard, Nature (London) 282, 793 (1979). (5) W. J. Albery, A. W. Foulds, K. J. Hall, and A. R. Hillman, J . Electrochem. SOC.127, 654 (1980). (6) M. D. Archer, M. I. C. Ferreira, W. J. Albery, and A. R. Hillman, to be submitted for publication. (7) T. Osa and M. Fujihira, Nature (London), 264, 349 (1976). (8) M. Fujihira, N. Ohishi, and T. Osa, Nature (London),268, 226 (1977). (9) M. Fujihira, T. Osa, D. Hursh, and T. Kuwana, J . Electroanal. Chem., 88, 285 (1978). (10) M. A. Fox, F. J. Nobs, and T. A. Voynick,J. Am. Chem. SOC.,102, 4036 (1980).
The Journal of Physical Chemistty, Vol. 85,No. 15, 198 1 2233 C o n t a c t Wires
a
Vacuum Deposited Gold
T
c]
Perspex
85
7
mm
Flgure 1. Perspex cell used for photoelectrochemical measurements.
using an electrically conducting glue ("silver dag") which was mechanically strengthened with a superficial layer of araldite. The thickness of the deposited gold was kept in the range 19-32 nm and represented a compromise between excessively thin electrodes (transmittance > ca. 65%) which are not electrically continuous and thick electrodes which transmit little light to the solution interface. When necessary, the semitransparent electrodes were glued together to form the photoelectrochemical cell shown in Figure 1. Open circuit voltages and short circuit currents were measured on a calibrated Keithley Model 610c electrometer ( f l % ) and were usually displayed on a chart recorder. Illumination was provided by a xenon lamp (Bausch and Lomb 33-86-20-01)attached to a Bausch and Lomb highintensity monochromator equipped with a UV-visible grating (33-86-79). The monochromator was operated a t a band-pass of 5 nm, and light from it was incident on the cell, in an otherwise light-tight compartment," thermostated at 298.15 f 0.05 K by a flow of deionized water. The beam of light incident on the cell was defined by an aperture of area 3.64 cm2. The incident irradiance was measured with a calibrated photometer (United Detector Technology, Model 181). UV-visible absorption spectra were recorded on a Beckman Acta MIV spectrophotometer, thermostated at 298.15 f 0.1 K. In view of the strongly fluorescent nature of rhodamine solutions, absorption spectra were checked for interference caused by fluorescence by recording spectra with the optical cell at both its maximum distance and its minimum distance from the light detector. These spectra were identical, indicating that they were not being sppreciably distorted by fluorescence from the sample. Fluorescence spectra and fluorescence excitation spectra were recorded on a Perkin-Elmer Model 650-105 fluorescence spectrophotometer, thermostated at 298.15 f 0.1 K. The composition of the electrolyte solution used in the photoelectrochemical cells was as follows: rhodamine B (Laser grade, Eastman Kodak), 1.17 X mol dm-3; (AR), H2S04 (Analar grade), 0.104 mol ~ i m - Fe2(S04)3 ~; 0.078 mol dm-3; FeS04 (A.R.), 1.3 X mol dm-3. The concentration of rhodamine B was close to its maximum solubility in water, and at the pH used the singly charged species should predominate.12 Quickenden and Yim have shown" that, at the concentration of ferric ion used herein, photovoltages and photocurrents are independent of ferric concentration. As high (-0.1 mol dm-3) concentrations of ferrous ions have been found to cause a substantial drop in both photovoltage and photocurrent,'l the concentration thereof was kept at the low value of 1.3 X mol dm-3 provided by the Fez+impurity found in the AR Fe2(S04)3. (11)T. I. Quickenden and G. K. Yim, Sol. Energy, 19, 283 (1977). (12) R. W. Ramette and E. B. Sandell, J. Am. Chem. Soc., 78, 4872 (1956).
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Solutions used for layer deposition did not contain Fez(SO,), or FeS0, but contained the same concentrations of rhodamine B and hydrogen ion.
Results and Discussion Deposition of Dye Layers. Previous work by Quickenden et al.3313has described the spontaneous formation of rhodamine colored dye layers in photoelectrochemical cells containing, inter alia, rhodamine B solution. In the present study the conditions under which such layers could be deliberately deposited were examined. I t was found that in the absence of electrical contact with a second electrode, together with the absence of light, semitransparent gold electrodes showed no visual or absorptiometric evidence of rhodamine deposition despite immersion in rhodamine B solution for periods of time ranging from 48 to 240 h. However, when gold electrodes were irradiated with 528-nm light, of irradiance (1.7 f 0.1) X lo8 photons m-2 s-l, for 50 h without electrical contact, fine crystals of a rhodamine colored material were deposited on the electrode. The surface of the illuminated electrode was neither uniformly or thickly covered, and the deposited material was easily removed with water. It was therefore decided that irradiation without electrical contact was not a successful method of depositing uniform, coherent dye layers. Holmes and Peterson14 have described the formation of rhodamine B films by simple evaporation of a rhodamine B solution, In order to test this method, a rhodamine B solution was placed in a cell with a gold electrode as its base and was evaporated under vacuum. After evaporation, a deep purple coating had formed on the electrode. The dye layer was thick but not uniform and was easily removed when rinsed with water. Hence, evaporation was not a useful means of depositing uniform coherent layers. For electrochemical deposition, two electrodes were fixed into a large Perspex cell containing rhodamine B electrolye solution. The electrodes were potentiostated relative to a saturated calomel electrode (SCE). Before deposition, the electrodes were electrochemically cleaned in situ by the method described by Gileadi et a1.16 at a potential of 1.5 V (SCE). It was found that thick, uniform layers could be deposited after a cleaning procedure which finished with either a positive or negative potential. The former was chosen for subsequent layer preparation. Preliminary experiments showed that, when a potential was applied across two electrodes for longer than a few minutes, a dye layer formed on the positive electrode, but not on the negative electrode. It was also found that uniform layers formed only when the positive electrode was held at a potential between 0.85 and 1.05 V (SCE). Therefore, for all depositions, the positive electrode was held at a constant potential between these two values for 20 min. The electrode potential was monitored, and the current typically fell from 2.9 A m-’ to an approximately steady 0.06 A m-2 in the course of a 20-min deposition at 1.0 V (SCE). After deposition all electrodes were rinsed 10 times for 10 s each in distilled deionized water and left t o air dry. Under the described conditions, uniform dye layers formed on the positive electrode. Layer thicknesses were calculated from the equation A = ecx (13)T.I. Quickenden and G. K. Yim, J. Phys. Chem.,83,2796(1979). (14)W.C.Holmes and A. R. Peterson, J , Phys. Chem. 36,1248(1932). (15)E. Gileadi, E. Kirowa-Eisner, and J. Penciner, “Interfacial Electrochemistry: An Experimental Approach”, Addison-Wesley, Reading, MA, 1975, Chapter 6.
I
I
VOLTAGE
/V
Figure 2. Thicknesses of electrochemically deposlted dye layers formed in a 20-min deposition period as a function of deposition potential (relative to a saturated calomel electrode). The layers were mol dm-3 rhodadeposited from an aqueous solution of 1.17 X mine B in 0.104 mol dm-3 H,SO,.
where A is the absorbance 8f the dye layer at the spectral maximum, c is the concentration of rhodamine B in the solid (6.47 mol drn-,),16 and e is the molar decadic extinction coefficient of solid rhodamine B17 (2.13 X lo3m2 mol-l.18 Layer thicknesses varied from 1to 62 nm, and the color of the layers ranged from deep orange-red to dark purple. The layers were fairly homogeneous in appearance and showed no obvious fine structure at a magnification of 1800. Figure 2 shows the somewhat variable dependence of thickness on applied potential for layers formed in a 20min deposition period. These results show that a t potentials below 0.85 V (SCE) no significant layer deposition occurs and that thickness increases with potential until ca. 1.05 V, after which substantial corrosion appeared to inhibit layer formation. Reproducibility in Figure 2 is poor, and it appears that in some cases layer formation occurs more rapidly than in others. Nevertheless, it is clear that potentials below 0.85 V and above 1.06 V (SCE) are unsuitable for the deposition of rhodamine layers onto gold electrodes, under the conditions listed in Figure 2. Absorption Spectra of Deposited Dye Layers. Figure 3 shows the absorption spectra of variously deposited solid rhodamine B films. Although Absorption spectra for electrochemically deposited rhodamine B are not available in the literature, there is a limited amount of information on rhodamine B films deposited by evaporation from a solution,14by sublimation,18and by chemical reaction with a semiconductor The relevent absorption spectra are shown in Figure 3b. Our own absorption spectrum for a rhodamine B film deposited onto a gold substrate by evaporation of a solution is shown in Figure 3c. This spectrum is similar to the corresponding spectrum obtained by Holmes and Peterson14 although our dimer peak a t 520 nm is rather higher relative to the monomer peak at 568 nm. It is also noted that the monomer-dimer ratio in the sublimed rhodamine B layer of Weigl18 (Figure 3b) is very different from that in the evaporated layers. Bakhshiev and Korovina16have previously noted the wide variability of the optical properties of sublimed organic films. The absorption characteristic4 of our four electrochemically deposited rhodamine B films are shown in Figure 3a. Each absorption spectrum was determined as the difference between the absorbance of the electrochemically (16)N. G. Bakhshiev and V. N. Korovina, Opt. Spektrosk., 22, 35 (1967). (17)This of course assumes that the dye layers are rhodamine B. (18)J. W. Weigl, J. Chem. Phys., 24, 364 (1956).
Electrodic Dye Layers in the Gold-Rhodamine B Cell 0.9
l/ #h--/ u
0.0
400
500
600
700
/
WAVE 1ENGT n nm
Figure 3. Absorption spectra of dye layers: (a) electrochemically deposited from a solution containing 1.17 X low3mol dm-3 rhodamine B and 0.104 mol dm3 H2S04(present work); (b) for layers deposited (1)by chemical reaction,' (2)by sublimation," and (3) by solution evaporation; l4 (c) for a layer formed by evaporation of a solution mol dm-3 rhodamine B and 0.104 mol dm-3 containing 1.17 X H2S04(present work); and (d) for a solution of 2.00X lo-' mol dm-3 rhodamine B in 0.104 mol dm-3 H2S04(present work). Temperature = 298.2 & 2 K for the solid-layer spectra in the present work. Temperature = 298.15 f 0.1 K for (d).
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The Journal of Physical Chemistry, Vol. 85, No. 15, 1981 2235
WAVELENGTH / n m
Flgure 4. Absorption spectra of solutions of the following: (a) electrochemically deposited dye layers in aqueous 0.104 mol dm-3 H2S04, path length = lo3 m; (b) 2.00X 10-'moI dm-3 rhodamine 6 in 0.104 mol dm-3 H2S04, path length = m; (c) rhodamine 110 (from literature2'). Temperature = 298.15 f 0.1 K in (a) and (b).
TABLE I: Spectral Maxima from t h e Measured Absorption, Fluorescence, and Fluorescence Excitation Spectra of Dye Layer Solutionsa fluorescence absorption excitation wavelength, wavelength, nm nm
fluorescence
wavelength, coated electrode and the absorbance of the clean gold nm electrode. These spectra were recorded at 298.1 f 1 K, rhodamine B 5562 2 559 i 2 583i 2 and a check showed that they did not change appreciably layer 1' 554i 2 559i 2 5793 2 over a temperature range of h3 K. The wavelengths of layer 2 550i 2 558i 2 5762 2 the spectral peaks range from 550 to 575 nm and the peaks layer 3 511, 520, 539, vary in width, but these differences were not obviously 5562 2 561i 2 517i 2 correlated with the deposition potential. The absorption layer 4 5062 2 500i 2 5352 2 spectra of the four electrochemically deposited layers are rhodamine 110 5052' 4952 2 528i 2 broadly similar to those of the other solid layers in Figure a Unless superscripted by a reference number, the data 3 and bear the closest similarity to those reported14 and were measured in the present study. observed for layers deposited by evaporation of a rhodamine B solution. B deposited in various ways. It is tempting to surmise that The absorption spectra of the electrochemically deposthe various electrochemically deposited layers may contain ited layers are broader than for a solution of rhodamine more than one substance, and it is shown in the following B (Figure 3d) and have higher dimer-to-monomer peaksection that evidence from the various types of solution height ratios. The absorption maxima for the electrospectra of dissolved layers is consistent with this hypothchemically deposited layers range from 550 to 570 nm, esis. whereas the solution maximum is at 556 nm. Absorption, Fluroescence, and Fluorescence Excitation The above observations allow one to conclude that the Spectra of Dissolved Dye Layers. Ulike the electrochemelectrochemically deposited layers do bear some similarity ically deposited thionine layers of Albery4v5and Archer: to rhodamine B, although a number of disqieting differthe present layers were water soluble, although dissolution ences do exist. On the one hand, it is possible to interpret was often slow and could take up to several weeks in an differences between the spectra of these layers and of unstirred solution. Solutions of the dye layers were preJ ~ ~ ~by~ dissolution in dilute sulfuric acid of the same rhodamine B solutions in terms of the w e l l - k n ~ w n ~ ~ J ~pared spectral broadening and shifts in the wavelength and in concentration (0.104 mol dm-3) as that in the rhodamine the monomer-dimer ratio, which occur when the dye is B solution from which the dye layer was deposited. All concentrated. On the other hand, it is then difficult to solutions used for the above spectra were diluted to the rationalize the spectral differences between the several same maximum absorbance before spectroscopic meaelectrochemically deposited layers, and even between the surements were carried out. literature spectra (Figure 3b) of solid layers of rhodamine Figures 4 and 5 show the absorption spectra and fluorescence spectra, respectively, of solutions of the four electrochemically deposited dye layers. These figures also (19) J. Muto, Keio Eng. Rep., 25, 71 (1972). (20)J. Glowacki, Acta Phys. Pol., 26, 905 (1964). show the corresponding spectra of rhodamine B solution
2236
The Journal of Physical Chemistry, Vol. 85, No. 15, 1981
,
I
Quickenden and Bassett
R
I
I\
.*
ca O .
.
a
3U
.E1 f 20
0
z
1504
t Figure 5. Fluorescence spectra of solutions of electrochemically deposlted dye layers: (a 1-4) dye layer solutions made up to a peak absorbance of 0.223in aqueous 0.104 mol dm3 H,SO,; b) rhodamine B of the same absorbance in aqueous 0.104 mol dm- H2S04; (c) rhodamine 110 of the same absorbance in aqueous 0.104 mol dm-3 H2SO4. In all cases, the wavelength of the exciting light was 365 f 2.5 nm, as indicated by the scattered light peaks at that wavelength. Temperature = 298.15 f 0.1 K.
6
and of a solution of rhodamine 110, the deethylated form of rhodamine B. The spectral maxima from these graphs
rhodamine B
*"w NH2.C I
-
rhodamine 110
are summarized in Table I, which also shows the corresponding maxima taken from fluorescence excitation spectra of the same solutions, obtained at a band-pass of k2.5 nm and a fixed emission wavelength set at the fluorescence maximum. It is clear from the data in Table I and Figures 4 and 5 that the various dye layer solutions differ from one another and that two extreme cases can be observed, as well as two cases which lie in between these two extremes. Solutions of layer 1produce spectra very similar to those from solutions of rhodamine B, and solutions of layer 4 produce spectra similar to those from rhodamine 110 solution, whereas solutions of dye layers 2 and 3 show intermediate characteristics. It thus appears that varying amounts of rhodamine B are deethylated Watanabe the electrochemical deposition.
300
4b0 5b0 WAVELENGTH
6bO
/ nm
Flgure 6. Photovoltage action spectra corrected for the spectral distribution of the exciting light: (0,0)for dye layer coated electrode 1 composed mainly of rhodamine B; p,0)for dye layer 3 containing both rhodamine B and rhodamine 110; (V,V) for an uncoated gold electrode. All voltages were measured relative to an uncoated gold dark electrode, and both the dark and the illuminated electrode were immersed in an aqueous electro e containing 1.17 X mol dm-3 rhodamine B and 0.104 mol dm H2SO4. In each case, the replicate measurements designated by filled and unfilled point symbols are the first and last members of a family of spectral curves whlch slowly drifted between these outside values over periods of 45, 27,and 54 h for the spectra designated by clrcles, squares, and triangles, respectively.
Y
This is not entirely surprising in view of the work of Watanabe et al.,2I who observed that, when powdered CdS is suspended in an aqueous solution of rhodamine B and irradiated with visible light, the dye molecules which are adsorbed on the CdS undergo oxidative deethylation to form rhodamine 110. It is noted that, in the present work, electrochemical deposition of dye layers occurred only under oxidizing potentials. There was no obvious correlation between the deposition potentials (1.05, 1.05, 1.10, and 0.95 V for electrodes 1-4) and the extent of deethylation. Since both rhodamine 110 and rhodamine B appear to exist in the electrochemically deposited dye layers of the present study, it is quite possible that both forms are also present in the variously deposited layers reported in the literature. As rhodamine 110 absorbs and fluoresces a t wavelengths similar to those of the dimer form of rhodamine B, it is possible that the presence of varying amounts of the deethylated material could account for the curiously variable16dimer-to-monomer ratios reported in the literature for rhodamine B layers formed by evaporation of a solution, by sublimation, and by chemical means. Photovoltage Action Spectra. Photovoltage action spectra provide a convenient display of the dependence of photovoltage on the wavelength of the exciting light. Figure 6 shows corrected photovoltage action spectra for (21) T. Watanabe, T. Takizawa, and K. Honda, J.Phys. Chem., 81, 1845 (1977).
Electrodic Dye Layers in the Gold-Rhodamine B Cell
The Journal of Physical Chemistty, Vol. 85,No. 75, 798 1 2237
TABLE 11: Measured Electrical Characteristics and t h e Power Conversion Efficiencies of Gold-Rhodamine B Cellsa -~ -____ power conversion power = 10"Vocisc f , photoelectrode 1o3vOc, v 109isc,A cm-' W efficiency, % 13.1i 1.8 12.6 i 1.8 (3.37 i 0.53) x 10-4 6.68 i 0.07 0.84 t 0.02 ( 2 . 3 8 i 0 . 0 9 ) x 10.' a Using ( a ) an uncoated gold photoelectrode and ( b ) dye-coated electrode 1 ( t h e highest-output coated electrode). In each case, t h e dark electrode was an uncoated gold electrode and both electrodes were immersed in a 1 . 1 7 X 10.' mol d m - 3 mol dm-3 FeSO,. The rhodamine B solution containing 0.104 M aqueous H,SO,, 0 . 0 7 8 mol d m - 3 Fe,(SO,),, and 1 . 3 X illuminated electrode area was 3.64 cm', and the incident flux at 560 n m was (3.53 t 0 . 0 5 ) X lo-' J s - ' (gold photoelectrode) and a t 575 n m was ( 3 . 7 3 i 0 . 0 5 ) X 10.' J s - ' (dye-coated photoelectrode). V,, = open circuit photovoltage; is, = short circuit photocurrent; f = fill factor = 0.5. Errors represent 50% confidence intervals. dye layer 1 gold
1 9 . 2 4 i 0.03 2.53 i 0.02
two gold electrodes which have been coated with dye layers and for an uncoated gold electrode, in cells containing a rhodamine B electrolyte and an uncoated dark electrode in every case. In order to measure these spectra, we first obtained a stable dark voltage. The cell was then illuminated at a band-pass of 5 nm. When the new voltage had become steady, the wavelength of the exciting light was increased. As each new voltage reached a stable value, the wavelength was changed, until a full scan from 350 to 700 nm had been completed. Since the voltage rise times after each wavelength change were often as long as 1h, and since the dark voltage tended to drift uniformly over such periods, it was necesary to check the dark voltage after every two or three wavelength changes. This was done by removing the exciting light unit1 a steady dark voltage was again obtained. Photovoltages were obtained as the difference between the maximum total voltage at each wavelength and the mean of the initial and final dark voltages.22 Wavelength scans were replicated several times for each cell, and the filled and unfilled point symbols in Figure 6 indicate only the first and last scan in each case. The intermediate scans all lay within the envelopes formed by the pairs of spectra, which thus give realistic estimates of the long-term drift errors associated with the measured photovoltages. It is unfortunate that most photoelectrochemical action spectra presented in the literature give no indication of the temporal stability of the measurements. It is clear that the photovoltages obtained with the dye-coated electrodes in Figure 6 are considerably greater than those observed at the clean gold electrode. The maximum photovoltages for layer 3 and layer 1 are respectively 7.3 and 2.6 times the maximum photovoltages obtained with the uncoated gold electrode. The drifts in photovoltage referred to above led to an increase in photovoltage for layer 1, but produced decreases for layer 4 and for the uncoated gold electrode. Absorption spectra of three photoelectrodes after the completion of the action spectra showed that no detectable changes had occurred to the gold photoelectrode but that the thickness of the dye layer on the other two electrodes had decreased somewhat. MinamiB has observed an unexplained increase in photocurrent response over a period of a few days for copper phthalocyanine thin film electrodes. The action spectra in Figure 6 are only representative of the performance of a much wider range of dye-coated electrodes which were fabricated and tested electrically to a greater or lesser extent. Out of 20 cells with dye-coated electrodes which were examined, only 5 gave photovoltages comparable to those in Figure 6. Photovoltages of some of the others were much the same as those obtained for the uncoated gold electrode, whereas others gave no photovoltage at all or gave very unstable photovoltages. The latter two behaviors were probably due to electrical dis(22) T. I. Quickenden and G. K. Yim, J. Phys. Chem., 84,670 (1980). (23) N. Minami, J. Chem. Phys., 72, 6317 (1980).
continuities in the gold substrate. The photovoltage action spectrum of the uncoated gold electrode in Figure 6 is basically similar to the absorption spectrum of rhodamine B solution (Figure 3d). The action spectrum for dye layer 1 is similar to the absorption spectrum of rhodamine B solution, except for the presence of the second photovoltage peak at 400 nm, which does not appear in the absorption spectrum or in the action spectrum of the uncoated gold electrode. Although the major photovoltage peak at 575 nm clearly represents an intrinsic generation process due to the absorption of light by only slightly perturbed rhodamine B molecules, the peak at 400 nm evidently represents an additional process associated with the solid form of the dye or some impurity therein. As would be expected from the presence of comparable quantities of rhodamine 110 and rhodamine B in layer 3, the photovoltage action spectrum contains two maxima in the 500-600-nm region. This result adds further confirmation to the mixed composition attributed to layer 3 from the spectral data in Figures 3-6. It can also be deduced from Figure 6 that the photosensitivity of rhodamine 110-coated gold is not greatly different from that of rhodamine B-coated gold. The ethyl groups on the latter do not evidently play an important role in the photovoltage generation process. As no action spectra have been previously reported for rhodamine B dye layers on metal electrodes (whether deposited electrochemically or otherwise), it is not possible to compare directly the results in Figure 6 with any previous study. Indirect comparison can however be made with the photocurrent action spectra measured by Fujihira et al.7-9 and by Fox et al.1° for layers of rhodamine B deposited by chemical reaction onto semiconducting SnOz electrodes. Unfortunately, the data of the latter workers do not extend below 450 nm and the data of the former extend only to 400 nm, so it is not possible to ascertain clearly whether the 400-nm, peak is present in such work. Fujihira et al.'-9 report a significant broadening in the photocurrent action spectrum compared with the absorption spectrum of a dry, rhodamine B layer on an SnOz substrate. Inspection of Figures 3 and 6 indicates that the opposite effect occurs in the present work with a gold substrate. Power Conversion Efficiencies. The increased open circuit photovoltages shown for the coated electrodes in Figure 6 are associated with considerable enhancement of power output, as shown in Table 11. The fill factor of 0.5 used in this table is taken from the work of Quickenden and Yim.'l It is clear that the best coated-electrode cell studied in the present work shows a 14-fold increase in power output compared with a similar gold-rhodamine B cell containing an uncoated gold photoelectrode. Several reasons can be advanced to explain this improvement. Firstly, as pointed out by Albery and absorption of light close to the (24) W. J. Albery and M. D. Archer, Nature (London),270,399 (1977).
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J. Phys. Chem. 1981, 85, 2238-2243
photoelectrode is a most desirable feature if a high efficiency is to be obtained. A strongly absorbing dye layer on the photoelectrode is one way to meet this requirement. Secondly, Albery and Archer suggest that, if only one electrode is “poisoned” with respect to one redox couple (either the dye couple or the inorganic couple) by adsorbed material, a higher efficiency can be produced. The dye layer on the photoelectrode may well act in this way, and there is evidence4 to this effect in the iron-thionine cell. A third possibility is that the dye layer may constitute a semiconductor electrode with markedly different electrode processes from those which occur at metal electrodes. The latter possibility has particular credibility in the case of thick layers, but further work would be necessary to evaluate the relative merits of the above explanations in the present case. Conclusions It was found that dye layers could be successfully deposited from aqueous rhodamine B solutions onto gold electrodes held a t oxidizing potentials between 0.85 and 1.05 V relative to a saturated calomel electrode. The dye layers thus formed ranged in color from orangered to deep purple and ranged in thickness from ca. 1 to 60 nm. Absorption, fluorescence, and fluorescence excitation spectra indicated that two different substances were commonly present in the dye layers, these being rhodamine
B and its deethylated form, rhodamine 110. Some layers were composed mainly of one of these substances while others contained a mixture of the two, and more control of the deposition conditons woud appear to be necessary if a predictable composition is to be produced. Photovoltage action spectra for dye-coated electrodes were found to resemble the absorption spectra of the several electrodic dye mixtures of rhodamine B and rhodamine 110, although a photovoltage peak at 400 nm is not obviously reflected in the absorption spectrum. The best power conversion efficiency obtained for a gold-rhodamine B photoelectrochemical cell containing a dye-coated electrode was (3.37 f 0.5) X This represented a 14-fold increase on the efficiency of a corresponding cell containing an uncoated gold photoelectrode. Electrochemically deposited dye layers may provide a useful means of improving the efficiencies of other photoelectrochemical cells which are already more efficient than the gold-rhodamine B cell. Acknowledgment. We thank Mr. S. M. Trotman of our Department and Dr. R. J. Marcus, of the Office of Naval Research, United States Embassy, Tokyo, for a number of helpful discussions during the course of this work. The Department of Pharmacology of the University of Western Australia is thanked for making the spectrofluorimeter available.
Crystal Structure and Structure-Related Properties of ZSM-5 D.
H. Olson,” G. T.
Kokotailo, S. L. Lawton,
Mobil Research and Development Corporation, Princeton, New Jersey 08540, and Paulsboro Laboratory, Paulsboro, New Jersey 08066
and W. M. Meler Institute of Crystallography, ETH Zurich, Switzerland (Received: January 15, 198 1; In Final Form: April 20, 1981)
A single crystal X-ray study of an as-synthesized ZSM-5 (Si/Al = 86) material having apparent orthorhombic symmetry Pnma and cell parameters a = 20.07, b = 19.92, and c = 13.42 A has been carried out. The framework topology found agrees with earlier more general descriptions. The three-dimensional channel system consists of straight channels running parallel to [OlO] having 10-rings of ca. 5.4 X 5.6 A free diameter and sinusoidal channels running parallel to [loo] having 10-ring openings of ca. 5.1 X 5.4 A. Diffusion in the [OOl] direction is achieved by movement between these two channels. The structural network can be derived from secondary building units consisting of 12 tetrahedra. The influence of pore structure on hydrocarbon sorption,hydrocarbon product selectivity, and catalyst aging is described.
Introduction Z S M 4 is a representative member of a new class of high-silica zeolites having considerable significance as catalyst materials. Examples of their uses include the conversion of methanol to gasoline, dewaxing of distillates, and the interconversion of aromatic compounds.2 Also, ZSM-5 has been shown to possess unusual hydrophobicity, leading to potential applications in the separation of hydrocarbons from polar compounds, such as water and alcohol~.~ The framework structure and structural features of ZSM-5 have been described previously.4 Herein we report P.O. Box 1025. 0022-3654/81/2085-2238$0 1.2510
the crystal structure analysis of ZSM-5, describe the principal features of the structure, and discuss important structure-dependent properties. Experimental Section A ZSM-5 preparation containing large crystals was synthesized by using established pr0cedures.l SEM mi(1) (a) Argauer, R. J.; Landolt, G. R. U.S. Patent 3702886,1972. (b) Dwyer, F. G.; Jenkins, E. E. U.S. Patent 3941871, 1976. (2) (a) Meisel, S. L.; McCullough, J. P.; Lechthaler, C. H.; Weiez, P. B. Chern. Technol. 1976,6,86. (b) Chang, C. D.; Silvestri, A. J. J.Catal. 1977, 47, 249. ( c ) Chen, N. Y.; Gorring, R. L.; Ireland, H. R.; Stein, T. R. Oil Gas J. 1977, 75, 165. (3) Chen, N. Y. U S . Patent 3732326, 1973. (4) Kokotailo, G. T.; Lawtan, S. L.; Olson, D. H.; Meier, W. M. Nature (London)1978,272, 437.
0 1981 American Chemical Society
