1748
J. Phys. Chem. 1991, 95, 1748-1755
Light- Induced Dioxygen Reduction at Thin Fllm Electrodes of Various Porphyrins D. Schlettwein, M. Kaneko, A. Yamada, RIKEN Institute, Hirosawa, Wako-Shi, Saitama 351, Japan
D. Wohrle,**tand N. I. Jaeger* Institut fur Organische und Makromolekulare Chemie and Institut fur Angewandte und Physikalische Chemie, Universitat Bremen, Leobener Str. NW 2, 2800 Bremen 33, FRG (Received: July 27, 1989; In Final Form: September 14, 1990)
Thin films of porphyrins and porphyrin analogue compounds such as phthalocyanines, 1, tetraphenylporphyrins, 2, and naphthalocyanines, 3, coated on IT0 in contact with an aqueous electrolyte were investigated in the photoreduction of dioxygen under illumination with visible light. The films (thickness 10-500 nm) were obtained by casting and vapor deposition procedures. Especially zinc phthalocyanine dispersed in poly(viny1idene fluoride) exhibited a significant cathodic photocurrent with a high selectivity toward dioxygen and a good stability. Besides the photoaction spectrum and the current pulse at the onset of illumination, the influences of the film thickness, concentration of dioxygen, and light intensity were investigated. The mechanisms of the charge carrier generation in the bulk material and of the surface processes leading to the photoreduction of dioxygen to H202 are discussed.
Introduction Phthalocyanines (Pc, 1) function as active components in processes driven by visible light: photosensitization in solution,'.2 photoreductions or photooxidations in photoelectrochemical cells,3-" electrophotographic appli~ations,'~J~ and photovoltaic ~ e I l s . ' ~ -Thin ~ ~ films of phthalocyanines in contact with an electrolyte solution can exhibit reversible chargdischarge processes in the absence of redox couples.'*22 Photoelectrochemical cells in the presence of various redox couples in aqueous solution have been intensively investigated with thin films of metal-free or metal-containing phthalocyanines obtained by vapor deposition and drop or spin coating techniques. The interface between the phthalocyanine as molecular semiconductor and the electrolyte forms a junction which is active in photoconversion. Sensitized photocurrents both cathodic and anodic are observed depending upon the applied potential. The spectral dependence of the cathodic photocurrent follows the absorption spectrum if the light is incident from the electrolyte side. In photoelectrochemical cells containing vapor-deposited phthalocyanine films, a linear relationship exists between the open-circuit photopotential and the electrochemical potential of the redox c o ~ p I e . ~Both * ~ *positive ~ and negative photopotentials are observed. Casted films consisting of metal-free phthalocyanine dispersed in a polymer binder exhibit the highest short-circuit photocurrents with redox couples whose redox levels are located within the band gap of the semiconductor.8 It is important to note that unsubstituted phthalocyanines behave as p-type materials. The acceptor dioxygen is the most important dopant incorporated into phthalocyanine The presence of the so-formed defect sites is a fundamental prerequisite for a good performance of phthalocyanines in photovoltaic/photoelectrochemical cells and for their electrical conductivity as well as photoconductivity. The interaction of Pc's with O2is composed of multiple steps. Two or more radicals are formed in the P C . ~ * ~ In the dark, phthalocyanines with the central metals Fe(I1) and Co(l1) are very active electrocatalysts for the dioxygen reduction in fuel cell reaction^.^^-*^ Few results about photoinduced reductions of O2by thin films (thickness 10-300 nm) of different niobium,30platin~m,~' or phthalocyanines on polished goldlo have been reported. Quite different magnitudes of the photoeffect in the dioxygen electroreduction are described. A detailed and comparative study, however, is missing. The im*Author to whom correspondence should be addressed.
'lnstitut fiir Organische und Makromolekulare Chemie.
* lnstitut fiir Angewandte und Physikalische Chemie.
portance of the "dopant" dioxygen for the performance of phthalocyanines in the solid state and the easy availability of this weak oxidant justify detailed investigations of the O2reduction in photoelectrochemical cells.32 In this paper the photoinduced (1) Darwent, J. R.; Douglas, P.; Harriman, A.; Porter, G.; Richoux, M . C. Coord. Chem. Rev. 1982, 44, 83. (2)Wohrle, D.; Gitzel, J.; Krawczyk, G.; Tsuchida, E.; Ohno, H.;Okura, I.; Nishisaka, T. J. Makromol. Sci.-Chem. A 1988,25, 1227. (3) Klofta, T.J.; Danziger, J.; Lee, P.; Pankow, J.; Nebesny, Y. W.; Armstrong, N. R. J. Phys. Chem. 1987,91,5646. (4)Klofta, T.J.; Sims, T. D.; Pankow, J. W.; Danziger, J.; Nebesny, K. W.; Armstrong, N. R. J . Phys. Chem. 1987, 91,5651. ( 5 ) Klofta, T. J.; Rieke, P. C.; Linkous, C. A.; Buttner, W. J.; Nanthakumar, A.; Mewborn, T. D.; Armstrong, N. R. J . Electrochem. Soc. 1985, 132,2134. (6)Rieke, P. C.;Armstrong, N. R. J . Am. Chem. Soc. 1984, 106, 47. (7) Leempoel, P.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1983,87,2948. (8)Loutfy, R. 0.; Mclntyre, L. F. Can. J . Chem. 1983, 61, 72. (9)Giraudeau, A.; Fan, F.-R. F.; Bard, A. J. J . Am. Chem. Soc. 1980, 102, 5137. (IO) Tachikawa, H.; Faulkner, L. R. J. Am. Chem. Soc. 1978,100,4379. (11) Mezza, T. M.; Linkous, C. L.; Shepard, V. R.; Armstrong, N. R.; Nohr, R.; Kenney, M. J . Electroaof. Chem. 1981,124, 311. (12) Loutfy, R. 0.; Hsiao, C. K.; Hor, A. M.;DiPaola-Baranyl, G. J. Imaging Sci. 1985,29, 148. (13) Takano, S.;Enokida, T.; Kakata, A.; Mori, Y. Chem. Lett. 1984, 2037. (14)Simon, J. J.; Andre, J. J. Molecular Semiconductors; Springer-Verlag: Berlin, 1985. (15)Chamberlain, G. A. Sofar Cells 1983, 8, 47. (16)Tang, C.W. Appl. Phys. Lett. 1986.48, 183. (17)Loutfy, R. 0.; Sharp, J. U.;Hsiao, C. K.; Ho, R. J. Appl. Phys. 1981, 52. 5218. (18) Minami, N.; Sasaki, K.; Tsuda, K. J . Appl. Phys. 1983,54, 6764. (19) Wohrle, D.; Schumann, B.; Schmidt, V.; Jaeger, N. I. Makromol. Chem., Macromol. Symp. 1987,8, 195. (20) Wohrle, D.; Schmidt, V.; Schumann, B.; Yamada, A.; Shigehara, L. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 975. (21) Kahl,J. L.;Faulkner, L.R.;Dwarakanath, K.; Tachikawa, H. J . Am. Chem. SOC.1986,108, 5434. (22) Wohrle, D.; Kaune, H.; Schumann, B.; Jaeger, N. Makromol. Chem. 1986, 187,2947. (23) Yaeger, E. Electrochim. Acta 1984,29, 152. (24) Wohrle, D.Adv. Polym. Sci. 1983,50, 48. (25) Jahnke, H.; Schonborn, M.; Zimmermann, G. Top. Curr. Chem. 1976,61,133. (26) van der Putten, A.; Elzing, A.; Visscher, W.; Barendrecht, E. J . Electroanal. Chem. 1986. 205, 233. (27)Hirai, T.;Yamaki, J.; Yamaji, Y. J. Appl. Electrochem. 1985, 15, 441. (28) Hirai, T.; Yamaki, J. J. Electrochem. Soc. 1985, 132, 2125. (29)Alferov, G. A.; Sevastyanov, V. I. Elektokhimiya 1975, 11, 827. (30) Sevastyanvov, V. I.; Alferov, G. A.; Asanov, A. N.; Komissarov, G. G. Biofzika 1975,20, 1004. (31)Alferov, G. A.; Sevastyanov, V. I. Zh. Fiz. Khim. 1976, 50, 214. (32)Kaneko, M.;Wohrle, D.; Schlettwein, D.; Schmidt, V . Makromol. Chem. 1988,189, 2419.
0022-3654/91/2095-1748$02.50/00 1991 American Chemical Society
Light- Induced Dioxygen Reduction reduction of dioxygen at porphyrins and analogous compounds is described in more detail. Experiments with a variety of electrode materials such as tetrabenzo[b,g,l,q]-5,10,15,20-tetraazaporphyrins (phthalocyanines; Pc, l), 5,10,15,20-tetraphenylporphyrins(2),
tetranaph~ho[2~~-b2’,3’-g:2’’,3‘‘-12‘’’,3’”-q]-5,10,15,20-tetraazaporphyrins (naphthalocyanines; 3) were performed to study the influence of the central metal and the organic ligand. In order to prepare thin films of these compounds dispersed in various polymers the films of 1-3 were mainly prepared by a drop coating technique. The combination with polymers offers the advantage
R
)I’
N’
-1 (M = Zn, AI@), Ga(F), Mn, Fe, Co, Cu, H;,
2 (M = Zn, Fe, Co, H),
3 (M = ZnTV(0); R = -H, -C(CHJJ of preparing more flexible films of higher mechanical stability. In addition, polar polymers can improve the activity of organic photovoltaic and photoelectrochemical ~ e l l s . ~ Exemplarily, ~’~~~* one active electrode is optimized for the O2 photoreduction in composition and thickness. Details on the photoreduction like the photoaction spectrum and the influences of light intensity and O2 concentration are given. The reaction is discussed in view of the p-type character of the bulk and of the adsorption properties of the electrode surface. Experimental Section Materials. Compounds 1 and 2 were purchased commercially and used without further purification. Synthesis of compounds 3 was performed as described elsewhere.33 Tris(2,2’-bipyridine)ruthenium(II) dichloride hexahydrate was prepared from ruthenium trichloride and 2,2’-bi~yridine.~~Nafion (5 wt % solution in lower aliphatic alcohols and 10% water; Aldrich), poly(acrylonitri1e) (PAN; Bayer), polystyrene (PS; Aldrich), poly( 1-vinylcarbazole) (PVCz; Janssen), poly(viny1 chloride) (PVC; Janssen), poly(viny1idene fluoride) (PVDF; Aldrich), poly(2-vinylpyridine) (PVP; Aldrich) were used as obtained commercially. Water for electrolyte preparation was doubly distilled; KN03, K2HP04,KH2P04. Methylviologen dichloride, K3Fe(CN),, p-benzoquinone, Oz, Ar, C 0 2 , N,N-dimethylacetamide (DMA), and pyridine were purchased in analytical grade. (33) Kmhev, E.; Puchnova, E.; Lukyanets, E. A. Zh. Org. Khim 1971,41, 396. (34) Kaneko, M.; Yamada, A. Phorochem. Photobiol. 1981, 33,193.
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 1749 Electrode Preparation. I T 0 of high electrical conductivity (20 ohm/square) was obtained from Matsuzaki Shinku Co. Ltd. cut into plates of 1 X 1.5 cmz and cleaned with distilled water and methanol. Solutions of 0.5 X IO4 to 1.7 X lo4 mol/L 1, 2, or 3 and 0.02-0.10 g/L of one of the polymers mentioned above in DMA or pyridine served as coating solutions. For electrode preparation, 10-70 pL of the solution were dropped on an I T 0 plate. The solvent was removed in vacuo (mbar) at 70 OC using a glass tube oven. Assuming the film density to be the mean value of the individual densities, layer thicknesses were derived from calculated coverage data obtained from the coated amount. The validity of this method was confirmed by measuring the layer thicknesses and roughnesses of various films with a Sloan Dektak I1 A. Vapor-deposited films were prepared by using a Veeco VE-7700 vacuum system at Torr. The film thickness was determined in situ using a Kronos QM-3 11 thickness monitor. After the appropriate coating procedure, a square of 1 cm2 electroactive area was contacted by a glass-covered copper wire using conducting adhesive Rex bond T-700 (Muromachi Kagaku Kogyo Co. Ltd.). This connection and the nonactive electrode area were sealed with Ciba-Geigy Araldite Rapid epoxy resin. Electron Spectroscopy. Electronic spectra were recorded on a Perkin Elmer 554 spectrophotometer. Due to different reflections and scattered light, a background represented by a baseline has been subtracted assuming the absorbance at 860 and 500 nm as zero. Integral absorbance data were determined by integrating the absorption spectra graphically between 500 and 860 nm using a Maho planimeter. Emission and excitation spectra were recorded on a Hitachi fluorospectrophotometer MPF-4 with the film sample placed diagonally in a quartz cell. Electronic spectra of films were recorded on I T 0 or quartz substrates, respectively. Photoelectrochemistry. The electrochemical experiments were performed in a gas-tight 10-mL glass cell using the appropriate working electrode, a saturated calomel reference electrode (SCE), and a platinum counter electrode. A Hokuto Denko HA-301 potentiostat connected with a Hokuto Denko HB-104 function generator and a Rikadenki RW-21T X-Y recorder was used. The light source was an Ushio 500-W xenon arc lamp. Toshiba neutral-density filters were used to vary the light intensity without changing the spectral distribution. The light intensity was measured with a Kipp & Zonen CAl-754373 instrument. Illumination was performed from the electrolyte (front side mode)?7498 If not mentioned otherwise the electrolyte was 0.5M aqueous KN03. In some measurements phosphate buffers were used. The oxygen concentration was varied by mixing the appropriate flows of oxygen and argon using precision flow meters. The corresponding oxygen concentration in the aqueous electrolyte was calculated from solubility data assuming the validity of Henry’s law. The gas mixture was bubbled through the cell for 20 min prior to each measurement. During the measurements the electrolyte was kept unstirred. Current densities were calculated from the geometrical surface area of the electrode. All potentials are given vs NHE. The redox potentials at pH 5 (EpH5 ) were calculated. Results Casting produced homogeneously colored films for pure compounds as well as for mixtures with polymers. The film roughness increased with the thickness (-40 nm at 200-nm films, -80 nm at 500-nm films). N o pinholes or cracks could be detected by use of an optical microscope at a magnification of 1200. The films were transparent to visible light. The optical density of all the cast films was below 1 in the studied wavelength region. The films showed significant differences in the electronic absorption spectra compared to those obtained in solution and are more similar to those of vapor-deposited films. This is exemplarily shown for 1 (M = Zn) in Figure 1. Whereas the dissolved 1 exhibits the Q-band transition at 668 nm, it is known that layers with different polycrystalline structures lead to different electronic spectra due to a different Davydov splitting. Both the a-and 0-modifications
1750 The Journal of Physical Chemistry, Vol. 95, No. 4, 1991
Schlettwein et al.
Absarban
Ill
I 111
-2o0mV
NHE
-200 mV
NHE
//
Yo0 mV
1
EDH 5
-6M) mV
n
1 \ 500
7w
900 Wavelength / nm
Figure 1. Visible absorption spectra in arbitrary units of 1 (M = Zn): (i) 1 dissolved in DMA; (ii) dropcast thin film (thickness 250 nm) of 1 (50 wt W )in mixture with PVDF; (iii) dropcast thin film (thickness 130 nm) of 1; (iv) dropcast thin film (thickness 250 nm)of 1 (50 wt %) in mixture with PS; (v) vapor-deposited film (thickness 50 nm) of 1.
characterized by a slip stack orientation of adjacent phthalocyanine rings give two distinguishable bands in the visible region showing specific intensities for each polymorphic s t r u c t ~ r e . ~ ~The -~~ vapor-deposited films showed the absorption bands of the amodification at 620 and 690 nm with the 620-nm band being of higher intensity.37 A similar absorption spectrum was observed for a casted film of 1 (M = Zn) in PS. In the 8-polymorphic structure the bands are shifted toward longer wavelengths with the band at longer wavelength now being of higher intensity.",36,37 This tendency was observed for the other cast films especially with PVDF as embedding polymer. Therefore, it can be concluded that different polymorphic structures were obtained depending on the procedure applied and on the kind of polymer used. Independent of the applied technique, all the prepared films can be regarded as doped by dioxygen. Undoped phthalocyanine films can only be obtained under UHV conditions.I4 Cyclic voltammetry was performed in an oxygen-saturated aqueous electrolyte (pH 5 ) between +580 and -120 mV vs N H E at a scan rate of 20 mV s-I in the dark and under illumination. Figure 2a depicts the cyclic voltammograms of a pure ZnPc (1; M = Zn) film obtained by vapor deposition, Figure 2b that of a ZnPc film in PVDF obtained by casting. For both films the most efficient layer thickness which depends on the coating (35) Meshkova, G. N.; Vartanyan, A. T.; Sidorov, A. N. Opr. Spectrosc.
(USSR) 1977, 43.42.
(36) Hollebone, B . R.; Stillman, M. J. J. Chem. Soc., Faraday Trans. 2
i__w.x- ,. 74., 21137 - .- . .
(37) Lucia, E. A.; Verderame, F. D. J. Chem. Phys. 1968, 48, 2674. K.J. Chem. Soc., Faraday Trans. I 1977,
(38) Meshitsuka, S.;Tamaru. 73, 236, 760.
W
Figure 2. (a) Cyclic voltammogram at 20 mV s-' at a vapor-deposited 50-nm thin film of 1 (M = Zn) in dioxygen-saturated electrolyte: ---, in the dark; -, illumination with 400 mW/cm2. (b) Cyclic voltammograms at 20 mV s-I at a dropcast 250-nm thin film of 1 (M = Zn)in PVDF (weight ratio 1:l) in the presence of (i) dioxygen (saturated solution, 1.3 X lo-) M); (ii) methylviologen dichloride M); (iii) carbon dioxide (saturated solution, 3 X M); ---, in the dark; -, illumination with 400 mW/cm2. (c) Cyclic voltammograms at 20 mV s-' at a dropcast 250-nm thin film of 1 (M = Zn) in PVDF (weight ratio 1:l) in the presence of (i) p-benzoquinone (lo4 M); (ii) p-benzoquinone M); (iii) potassium ferricyanide (lo4 M); (iv) potassium ferricyanide (lo-) M); ---, in the dark; -, illumination with 400 mW/cm2.
procedure (see below) was chosen. When the electrode potential was scanned negatively in the dark no significant cathodic current corresponding to the reduction of dioxygen was observed.I0 The same situation was evident when the electrode was scanned negatively under inert gas (argon) in the dark and also under illumination. A significant cathodic current could be obtained only under illumination and in the presence of dioxygen. The cyclic voltammogram shows that the photoreduction occurs irreversibly. In a paper about O2photoreduction at a film of ZnPc embedded in PVCz it was reported3* that the highest currents under illumination were obtained at quite cathodic potentials (--1 V vs NHE). The present measurements were carried out in a potential region of negligible dark currents which can also be considered as an optimization toward a photosensor. At -120 mV vs N H E a cathodic photocurrent density as high as 0.1 mA/cm2 was observed for the cast films. This value is in the order of
Light-Induced Dioxygen Reduction
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 1751
TABLE I: Dioxygen Reduction by Various Porphyrin Electrodes# porphyrin compound 1 ( M = 2H) 1 (M = AI(CI))
1 (M 1 (M 1 (M 1 (M 1 (M 2 (M 2 (M 2 (M 2 (M 3 (M
= Mn) = Fe) = Co) = Zn) = Ga(F)) = 2H) = Fe)
= Co) = Zn)
= V ( 0 ) ; R = -H) 3 ( M = Zn; R = -H) 3 ( M = Zn; R = -C,H9)
drop-coated filmsb current density/pA cm-, ratio illumin ( i i ) photocurr (ii- id) (ii/id) dark ( i d ) 75 I2 250
30 240 18 4
80
33 250 40 240
130
13 13
9 40 20
IO
12
9
13
44 15 40
12
23
IO
5 21 0
1.1 2.8 1.o
IO
1.3 1 .o
0 102
vapor-deposited filmse current density/pA cm-* ratio dark ( i d ) illumin (ii) photocurr (ii - id) ( i i / i d )
7.2 2.3
5 27 7 2
3.1 1.5
35 5
1.2 4.9 I .5
27 11
3.1 1.9
8
23 26 120 32 11 46
13 75 22
3 10 13 3 5 6
31 21 8
7
15
2.9 2.0 1.6 1.5 3.7 4.6 2.4 9.0 1.6 1.7 2.4
13 45
IO 8
36 18 24
3
IO
4
17
IO
Measurements in aqueous 0.5 M KNO, at pH 5 . Current densities during cyclic voltammetry at an electrode potential of -120 mV vs NHE. Film thickness I30 nm. e Film thickness 50 nm. magnitude that can be estimated for the current limited by the diffusion of dioxygen in a resting electrolyte. Consequently the currents could be increased by stirring the electrolyte. Photocorrosion of the ZnPc can be excluded as a significant source of the observed currents. In long-time experiments lasting several hours the total charge (1.5 X mol e-) was high compared to the coated amount of ZnPc present in the film (5.2 X lo-* mol). No change in the electronic spectra could be detected following the photoelectrochemical experiment. Excitation of the phthalocyanine induces the photocurrent at the ZnPc/PVDF electrode. The photoaction spectrum normalized to photon numbers (Figure 3) in principle resembles the absorption spectrum of 1. A more detailed interpretation reveals a deviation from the absorption spectrum insofar as illumination with light of the main absorption band’s wavelength does not lead to the highest photocurrent. The latter is produced by light closed to the smaller absorption band at 630 nm. A third maximum of the photocurrent appears near 760 nm (- 1.6 eV) where the absorption of the film is very small. Other acceptors like [Fe(CN),13-, 1 ,4-benzoquinone (BQ), methylviologen dichloride (MV2+),and carbon dioxide were used in photoelectrochemical experiments at a ZnPc/PVDF electrode in order to compare their activity with that of O2 (Figure 2, b and c). An outstanding photocurrent was observed for the O2 reduction (EpHs(02/H202)= 390 mV vs NHE). The other acceptors covering a large part of the electrochemical potential scale (E,” ~([Fe(cN),]~-/[Fe(cN),1‘ = +460 mV; Ep”5(BQ/ HQ) = 420 mV vs NHE; E p H S (MV2+/MV‘+) = -440 mV; EpHS(C02/C204H2)= -770 mV vs NHE) show smaller differences between the cathodic current in the dark and under illumination, even though their redox potentials are also located in the ZnPc bandgap region. MV2+ and C02exhibit small currents in the dark which are only slightly altered by illumination. [Fe(cN),],- and BQ already show high currents in the dark but only in the case of BQ a significant photoeffect is observed which was also measured for H2Pc on Sn02.’ The large differences in the photoeffects of the [Fe(CN),13-, benzoquinone, and dioxygen reduction, all having quite similar redox potentials, are surprising and give rise to the idea of the importance of surface kinetics and a specific interaction of 1 and O2 For [Fe(CN),]> both the shape of the cyclic voltammogram and the separation of the peak potentials are in good agreement with reported results for ZnPc on a gold substrate.’O An increase of the concentration of [Fe(CN)6]* or BQ from IO-, to mol L-I results in a behavior expected for reactions limited by diffusion in the electrolyte or by charge transfer at the electrode as the currents increase approximately proportional to the acceptor concentration (Figure 2c). This would not be expected for a process being limited by charge transport in the bulk material. In addition, the photocurrent in the dioxygen reduction was found to be strongly dependent on the concentration in the
1
Absorbance
Photocurrent
P I
I
(-
I
I P I
I
\ \ \
Figure 3. Spectral distribution of the steady-state cathodic photocurrent at -60 mV vs NHE due to dioxygen reduction (-O-) at a thin film of ZnPc/PVDF (weight ratio 1:1, thickness 250 nm). The solid line represents the visible absorption spectrum (in arbitrary units).
electrolyte (Figure 4), while the dark current varies only insignificantly. Under inert gas (argon) no photocurrent could be detected. Hydrogen peroxide could be detected as the product of the dioxygen photoreduction. Its formation during the cathodic sweep was estimated from a rise of the anodic current, also in the dark, on the back sweep. In addition, H202could be chemically detected by discoloration of an aqueous KMnO, solution and by a specific reaction with titanyl sulfate.# In the second step, H202 must be electrochemically reduced to water because addition of H202under inert gas resulted in high dark current densities in the cathodic sweep. Due to the outstanding results in the dioxygen photoreduction, the influences of electrode materials and experimental conditions on this reaction were investigated in detail. Changing the electrolyte concentrations (0.1-1 M KNO,) or the kind of cation (Li+, K+) or anion (NO), CI-) did not show any influence on the cathodic photocurrent. Variation of the pH value between 1 .O and 13.0 gave a maximum photocurrent at pH 5. Therefore, measurements described further on were conducted in 0.5 M K N 0 3 at pH 5 under a dioxygen concentration of 1.3 X IO-’ mol L-l corresponding to a pure 0,atmosphere (760 Torr) at room temperature. Table I compares various compounds 1-3 (without polymer) obtained by casting from solution and vapor deposition in the O2 reduction. The cyclic voltammograms obtained were of similar shape but differed strongly in the measured currents especially under illumination. Because detailed information on the band positions (flat band potentials) at the porphyrin surface in contact with an electrolyte are not available for the studied compounds, the current densities at -120 mV vs N H E are listed to compare
1752 The Journal of Physical Chemistry, Vol. 9.5, No. 4, 1991 current density I y A ern-2
Schlettwein et al. TABLE 11: Dioxygea Reduction by Various Electrodes of 1 (M = Zn) in Mixture with Polymers‘
polymer P,VPY PS Nafion PAN PVC PVCZ PVDF
current density/rA cm-’ photocurrent dark ( i d ) illumination (ii) (ii - id) 5
5 9 4 4 5 6
ratio (iilid)
7 12
2
1.4
7
2.4
30
21
14 25
IO
3.3 3.5 6.3 7.0 16.7
21
35
30
100
94
Weight ratio I:]; thickness 250 nm. Measurements in aqueous 0.5 M KNO, at pH 5. Current densities during cyclic voltammetry at a potential of -120 mV vs NHE. current
density /PA cm-2
Figure 4. Dependence of steady-state cathodic currents of ZnPc/PVDF films (weight ratio 1:1, thickness 250 nm; electrode potential -60 mV vs
NHE) on the dioxygen concentration: illumination with 400 mW/cmz.
dark current; 0 , current under
the electrode materials. The influence of the thickness of vapor-deposited films was studied for films of ZnPc. Films up to 50 nm gave stable voltammograms, and the photocurrent density increased with the film thickness. Thicker films gave higher photocurrents for the first scan but during subsequent scans the current density decreased to the value obtained for the 50-nm film. A thickness of 50 nm was therefore chosen to compare the efficiency of various porphyrin films prepared by vapor deposition. In the case of drop-cast films the influence of the layer thickness (the coated amount of dyestuff) was investigated for films of a ZnPc/PVDF mixture (see below). In order to compare dropcast films of various porphyrins, a film thickness of 130 nm was casted. This corresponds to the optimal amount of ZnPc found in the case of ZnPc/PVDF. When comparing various complexes 1 it can be seen that the zinc and the aluminum complex exhibited the highest photocurrents and the highest ratios of the current under illumination to that in the dark. The metal-free phthalocyanine shows higher dark currents which are slightly enhanced under illumination. MnPc, FePc, and CoPc are well-known as electrocatalysts for the dioxygen reduction in fuel cell reactions. Their high dark current cannot be improved under illumination. Ga(F)Pc gives a moderate ratio of the current under illumination to that in the dark but the currents are quite small. As in the case of the zinc complex of 1, also those of 2 and 3 yielded a high photocurrent and a high ratio of the current under illumination to that in the dark (Table I). This indicates that the influence of the central metal seems to be similar for the studied porphyrin compounds and that the central metal plays an important role in the charge carrier generation and/or in the surface kinetics. Compared to the other ring systems the phthalocyanines exhibited the highest photocurrents. Casting of 1 in mixtures with various polymers produced smooth films of reasonable mechanical stability. The dark currents were lower in comparison to films without polymers (Tables I and 11). This is explained by the reduced electronic conductivity of polymer-cast films. A comparison of the photoelectrochemical activities shows the general trend that polyvinyl compounds containing polar substituents are advantageous. Above all, films of 1 in PVDF showed high photocurrents and the highest values of the ratio of current under illumination to that in the dark. Therefore, these films were investigated in more detail in cyclic voltammetry and steady-state experiments at an applied potential of -60 mV vs NHE under variation of the experimental conditions. The dark current rose linearly with the concentration of ZnPc (Figure 5) which shows that better intermolecular contact of ZnPc particles leads to a higher electrical conductivity?2 The current density under illumination of films made from various mixtures
02
05
20
10
welght rotlo
ZnPc
PVDF
Figure 5. Dependence of steady-statecathodic currents for the dioxygen reduction (electrode potential -60 mV vs NHE) on the concentration of ZnPc in PVDF (film thickness 250 nm): m, dark current; 0,current under illumination with 400 mW/cm2.
of ZnPc and PVDF rose with increasing concentration of the complex. The slope is higher at smaller ZnPc concentrations, indicating that an increase of the ratio ZnPc:PVDF above 0.5 or 1.0 leads to a lower efficiency of ZnPc in generating the photocurrent. Variation of the film thickness at a constant weight ratio of ZnPc and PVDF (1:l) leads to a sharp limiting value of the photocurrent for thicknesses higher than 250 nm (Figure 6). In contrast, the amount of the absorbed light in the Q-band region of phthalocyanines in the film was continuously increasing with the film thickness (Figure 6). It can therefore be concluded that 250 nm of the film efficiently take part in the whole process of the photoreduction of dioxygen. For the photoreduction of pbenzoquinone a t electrodes of metal-free 1 vapor deposited on Sn02, active layers of 200-500 nm were found to exhibit the highest photocurrents.’ From Figure 6 it can be seen that the dark current is independent on the layer thickness. The photocurrent varied with the light intensity (Figure 7) following a power law with an exponent of 0.65. Values between 0.5 and 1.0 for other redox couples at phthalocyanine thin film electrodes obtained by vapor deposition were explained by monomolecular and bimolecular recombination processes of charge carriers.14 For a 250 nm thick film obtained by casting ZnPc and PVDF (weight ratio 1:l) the conversion efficiency at 622.5 nm was determined by using an interference filter of half-height widths of 13.5 nm and a maximum transmission of 26.5%. The incident light intensity at the electrode was 1.05 mW cm-2 corresponding mol cm-2 photons. From electronic spectra an to 5.5 X uptake of 4.2 X lo4 mol cm-2 s-I photons was calculated. From the photocurrent of 6.6 pA cm-2 at -60 mV vs NHE, it was calculated that -2% of the absorbed photons lead to a transfer of an electron from the surface of the electrode. A very fast response of the cathodic photocurrent in less than 1 s was observed under dioxygen when switching the light on and off (Figure 8). A steady state was reached after less than 30 s.
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 1753
Light-Induced Dioxygen Reduction integral absorbow
current density / P A cm-2
I
I
on
off
la1
I50
00 Light
50
d
...
m I
250
500
3 750
film thicknsss / nm
Figure 6. Cathodic currents for the dioxygen reduction at -120 mV vs NHE measured during cyclic voltammetry under illumination with 400 mW/cm* and amount of absorbed light versus the thickness of ZnPc/ PVDF (weight ratio 1:l) film electrode: W, dark current; 0,photocurrent; A, amount of absorbed light. photocurrent density / UA cm-2
1
'i
io
100 lo00 lght intensity / mW cm-2
Figure 7. Dependence of steady-state cathodic photocurrent obtained at ZnPc/PVDF film (weight ratio 1:1, thickness 250 nm) for the dioxygen reduction at -60 mV vs NHE (in 0.5 M KNO, at pH 5) on the incident light intensity.
Discussion The function of the back electrode I T 0 is to provide a nearly ohmic contact to the p-conducting porphyrins.*J4 The active part for the cathodic photoreduction of dioxygen is the porphyrin film in contact with the electrolyte. The strong influence of the central metal on the efficiency in the dioxygen photoreduction (Table I) is explained by the importance of the central metal for the photoelectrical properties and the coordinating capability of a porphyrin to 0,.For porphyrins containing Zn(II), Al(III), V(IV), the influence of incorporated dioxygen is even more spectacular on the photoelectrical properties than on the dark proper tie^.^^^' Illumination in the presence of O2results in a marked increase in photoconductivity due to enhanced charge carrier generation. In contrast, the photoconductivity of Mn(I1)-, Fe(I1)-, Co(I1)containing porphyrins is depressed in the presence of 0,due to the formation of ionized charge-transfer complexes which reduce the mobility of charge carriers by acting as localized trapping state^.'^,^^ These porphyrins are known to catalyze the 0,reduction in the dark. In these cases the rate-determining step was (39) Day, P.; Screeg, G.; Williams, R. J. P. Nature 1963, 197, 589.
on
off
Figure 8. Current changes induced by switching the illumination (400 mW cm-2) on and off. Measurements at ZnPc/PVDF film (weight ratio 1:1, thickness 250 nm) at -60 mV vs NHE.
found to be the interaction between the metal of the complex and dioxygen. The high dark currents are not significantly altered by illumination. The axial coordinating capabilities of the other central metals is smaller and decreases further in the order of ZnPc > CuPc > N~Pc~.~@' The metal-free phthalocyanine does not interact with 02.41 Ga(F)Pc behaves as a p-type semiconductor with a high acceptor c~ncentration.~ But the cofacial stacking of adjacent rings with a strong Ga-F-Ga bond may inhibit axial metal coordination of 02. Therefore, porphyrins with Zn as central metal turned out to be the most suitable for the 0,photoreduction. The phthalocyanine ring system provided the highest photocurrents. One of the interesting features of the present system is the question whether dioxygen is reduced at the interfacial contact between the bulk phase and the electrolyte or also in the bulk of the film since the thickness of the active layer was found to be in the order of 250 nm (Figure 6). For single crystals of phthalocyanines the diffusion constant of 0,is as small as 7.2 X 10-9 cmz s-I at 5 15 K.42 Assuming an Arrhenius type temperature dependence of the diffusion coefficient and a typical activation energy of 80-120 kJ mol-', the mean diffusion distance of dioxygen at room temperature after 1 h can roughly be estimated to be about 50-2 nm. For fluorinated polyalkylenes the permeability constants of the dioxygen diffusion are higher but still very low compared to less polar pol~mers.4~The polymers are known not to be swollen with water. Therefore, the presupposition for a rapid diffusion of 0,in the films is not fulfilled. Some additional experiments were conducted to confirm this situation: The triplet state of the tris(2,2'-bipyridine)ruthenium(II) complex ( [ R ~ ( b p y ) ~ ] is ~ +known ) to be quenched by 0,diffused into polymer membranes such as methyl methacrylate copolymer, Nafion, silk etc. The Stern-Volmer plots of the quenching show proportionality to the O2c o n ~ e n t r a t i o n . In ~ ~this ~ ~ study, ~ thin films (thickness 250 and 1000 nm) of PVDF containing 10 or 1 wt 5% Ru(bpy),Cl, were used to measure the emission intensity at 610 nm during excitation of the ruthenium complex at 450 nm. The films were in contact with 0.5 M KN03. Under inert gas (argon) and dioxygen the emission signal is of the same intensity and not dependent on the concentration of 0,in the electrolyte. It can be concluded that for the photoreduction process no 0,is continuously diffusing into the bulk film and that only the interface (40) Day, P.; Williams, R. J. P. J . Chem. Phys. 1962, 37, 567. (41) Contour, J. P.; Lefant, P.; Vijh, A. K. J . Catal. 1973, 29, 8. (42) Yasunaga, H.; Kojima, K.; Yohda, H.; Takeya, K. J . Phys. Soc. Jpn. 1974, 37, 1024. (43) Egli, S.; Ruf,A.; Buck, A. Swiss Chem. 1984. 6, 3. (44) Kaneko, M.; Hayakawa, S. J . Macromol. Sci. Chem. A 1988, 25, 1255. (45) Kaneko, M.; Nakamura, H. Makromol. Chem. 1987, 188, 2011. (46) Bornmann, J. A. Chem. Phys. 1954, 27, 604. (47) Meier, H. Organic Semiconductors; Verlag Chemie: Weinheim, FRG, 1974. (48) Jander, G.; Blasius, E. Lehrbuch der analytischen und priiparativen anorganischen Chemie; Hirzel Verlag: Stuttgart, 1988. (49) Jaeger, C. D.; Fan, F. F.; Bard, A. J. J . Am. Chem. SOC.1980, 102, 2592.
Schlettwein et al. 3-15-
EpHs IC4'c2W)
-10-
91
I
I
Zn Pc elec ttulyte Figure 9. Energy diagram of the photoreduction of dioxygen on ITO/ IT0
ZnPc (l)/dioxygen (aqueous electrolyte pH 5 ) .
between the bulk and the electrolyte is responsible for the O2 reduction. This can also be seen from the fact that the photocurrent is influenced by both light intensity and oxygen concentration in the electrolyte. In the charge carrier generation process in the bulk of the films, however, dioxygen plays an active role as a dopant of the p-type porphyrins. The electrical conductivity is altered, the activation energy of conduction is lowered, and the Fermi level is moved closer to the valence band in the presence of dioxygen.I4J0 For phthalocyanines in the intrinsic case the width of the band gap, in a slip-stack orientation of adjacent molecules, is around 2.0 eV. Illumination results in a S -SI transition, Le., the formation of MPc* located about 0.2 eV below the conduction band edge. The ionized intermediate MPc'+, 02'-is formed by electron transfer. An energy of -0.4 eV is needed for the generation of free charge carriers.I4 At zinc phthalocyanine electrodes the existence of two sets of surface states located about 0.6 and 1.0 eV above the valence band edge is derived from photoelectrochemical experiments7** and potential-dependent impedance measurement^.^' Specifically adsorbed dioxygen is assumed to provide the surface states. This hypothesis is supported by surface photovoltage measurements leading to the determination of two different bonding states of dioxygen at phthalocyanine surfaces.14 The photoaction spectrum (Figure 3) shows the importance of states in the bandgap region either in the bulk or at the surface. Light of longer wavelengths (760 nm) than the maximum absorption contributes significantly to the generation of the photocurrent in the O2reduction. The corresponding energy of 1.6 eV would not be sufficient to excite an electron from the valence to the conduction band and therefore can be correlated with a transition inside the bandgap. The photogeneration of charge carriers is schematically represented in an energy level diagram (Figure 9) before contact is established. The band positions are based on reported values estimated from impedance measurem e n t ~ and ~ ' the observation of photocurrents dependent on the
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(50) Heilmeier, G . H.; Harrison, S . E. Phys. Reu. 1963, 132(5), 2010. (51) Fan, F'.-R.; Faulckner, L. R . J . Am. Chem. Sor. 1979, 101, 4179. (52) Verkhovsbya, K. A.; Danz, R.; Fridkin, V. M. Sou. Phys. Solid State 1987, 29(7), 1268.
(53) Memming, R. Electroanal. Chem. 1979, I I , I . (54) Milazzo, G. Tables of Standard Electrode Potentials; Wiley: New York, 1978.
electrolyte redox p ~ t e n t i a l . ~Using . ~ 0.5 M K N 0 3 as an inert electrolyte we found 850 mV vs N H E (-5.8 eV) to be the OXidation potential of ZnPc which is in good agreement with the literature.55 Surface ionization energies of about 5 eV are reported for metal phthalocyanine^.^^^^^ Although the three values are quite different, the chosen value (5.4eV) seems to be reasonable also in view of the high dark currents due to hole injection by [Fe(CN)),I3- and BQ. Reliable information about the amount of bending and the thickness of the space charge region is missing because the reported small thickness of the space charge region which is derived from impedance measurements83I7 does not correspond to the material's low electrical conductivity and the mobility of charge carrier^.'^*^^ The latter would lead to space charge regions of higher thickness than the film thickness. In the present situation it appears impossible to decide whether the thickness of the photoactive region may be equated to the thickness of the space charge region or depends on the mean diffusion lengths of excitons. The kind of polymer that is used as polymer binder has a strong influence on the photocurrent in O2 reduction (Table 11). The current under illumination differs strongly between various polymer binders, although the dark currents are quite similar. Besides the influence toward a crystallization of ZnPc in a more favorable modification (Figure l), polar polymers can have an active effect on photocurrent efficiencies in phthalocyanine photoelectrochemical cells as was shown for the performance of photovoltaic type cell^.^^-^^ The cathodic photocurrent of ZnPc in PVDF is much higher in comparison to other polymers. PVDF has a dipole moment of 2.1 D. The polar groups produce a sizable electrical field (as high as lo6 V cm-I) in the immediate neighborhood of the repeating monomer units.Is It is reasonable to assume that this field in the microenvironmental site enhances the electron/hole separation and contributes to the increase of the photocurrent. This is supported by the fact that films of pure PVDF themselves show a small photovoltaic effect.52 Besides charge carrier generation and separation, also recombination would take place. The latter process may be predominant for path lengths >250 nm. A suppression of recombination processes is another possibility to explain the influence of PVDF. Figure 8 shows the short response time of the electrode toward illumination. The sharp rise of the photocurrent and the overshoot are explained by a fast reduction of O2adsorbed at the surface starting from an equilibrium coverage and relaxing to a steadystate value. Proof for this hypothesis is drawn from a comparison with a model based on the presupposition that the photoelectrochemical current is proportional to the charge carrier density in the bulk material as calculated for CoPc on S n 0 2 and experimentally verified in the oxidation of oxalate. In that case the current rises significantly slower when the light is switched on and no overshoot appears.38 The experiments show a high selectivity of the photoeffect at the ZnPc/PVDF electrode toward dioxygen as acceptor in solution. In principle, all acceptors having a redox potential within the ZnPc bandgap and a sufficient overlap of states with the conduction band should give a photocurrent (Figure 9). Hz02should therefore produce no photoeffect as confirmed by our study. The high dark currents in the case of [Fe(CN))J3- and BQ which are only slightly altered by illumination indicate a sufficient overlap with the ZnPc valence band. Although it has a similar redox potential as [Fe(CN))6I3- and BQ, the behavior of O2 indicates that there is no overlap. If it is taken into account that the reduction of O2 to H 2 0 2 involves two electrons, the redox potential has to be regarded as an average value of two one-step potentials.53 There This would is a potential of -0.563 V vs N H E listed for 02/02-.54 locate the first step of O2 reduction in the region of MV2+ and C 0 2 which would well explain the low dark current since the maximum acceptor density would be found at higher energies than the ZnPc valence band edge. If 02-trap levels and surface states play a decisive role, it is reasonable, that MV" and C 0 2only show ~~~
~
~
( 5 5 ) Green, J. M.; Faulckner, L. R. J . Am. Chem. Sor. 1983, 105, 2950. (56) Pope, M. J . Chem. Phys. 1962, 36, 2810.
J. Phys. Chem. 1991, 95, 1755-1759
a small photoeffect because an overlap is only given with the conduction band. The much higher current under illumination of O2compared to MV2+ and C02can again be explained by the nature of the surface states. Dioxygep is assumed to cause the surface states, and reduced oxygen species are found on the surface.I4 No charge transfer seems to be necessary from the ZnPc surface to solvated species in solution, but reduced oxygen species from the film surface could diffuse into the electrolyte and are replaced by dioxygen molecules from solution. The surface states are populated by electrons only under illumination so that the described mechanism only works under illumination of the surface even though O2 is adsorbed also in the dark.
1755
To conclude, it has been shown that zinc phthalocyanine, especially when embedded in poly(viny1idene fluoride), leads to an efficient photoreduction of dioxygen. A high ratio of the current under illumination to that in the dark is obtained in a short response time. A high sensitivity toward dioxygen and visible light is observed. The electrodes exhibit excellent mechanical and electrochemical stability.
Acknowledgment. We gratefully acknowledge financial support for D.S. by the Deutscher Akademischer Austauschdienst (DAAD) for a research visit to Japan. We are thankful to R. Memming (Hannover, FRG) for intensive and fruitful discussions.
Temperature Rises Produced by a Molecular Beam Striking a Platinum Surface. 2 T. Taot and E. F. Greene* Department of Chemistry, Brown University, Providence, Rhode Island 0291 2 (Received: August 10, 1990)
Measurements of the temperature rises produced when molecular beams strike Pt surfaces are interpreted to give information on the probability of the transfer of energy as a result of collisions. Previous work introducing the method is supplemented by showing the effect of roughening the surface by the deposition of Pt black, of varying the temperature of the nozzle from which the beam emerges, and of adding internal energy to the molecules of the beam by irradiating them with an infrared laser. Experiments with binary gas mixtures provide a method for determining relative accommodation coefficients for the two components without any need for external calibration.
Introduction In an earlier paper,' here denoted I, we reported measurements of temperature rises AT produced by molecular beams issuing from a nozzle a t room temperature and then striking a Pt surface initially also a t room temperature. Interpretation of the results gave values for a coefficient y (closely related to cy, the classical energy accommodation coefficient) that measures the fraction of the scattered molecules whose energy is determined by the temperature of the surface. Because this method of learning about energy transfer between gases and solids differs in several respects from other experiments previously reported by others (see the review by Goodman and Wachman2 and other references in I), we describe here further experiments that characterize it more fully. In particular we show the effect on the AT when ( I ) the nozzle source of the beam is heated above room temperature, (2) the polycrystalline Pt surface struck by the beam is roughened by electrochemical deposition of Pt black, and (3) some of the molecules in a beam of SF6are excited by the absorption of infrared radiation from a laser. Apparatus and Procedure In most respects the'apparatus and procedure are the same as those described in I. Briefly stated, a manipulator in a highvacuum system permits translation of a small Pt surface in three Cartesian directions relative to the exit hole of the nozzle from which the molecular beam emerges. After the beam is turned on, Tj, the temperature of the Pt (measured by thermocouple TCl), rises and in about 3 min becomes steady. This rise, AT = T j Tb,where Tb is the temperature of the background (measured on thermocouple TC3), increases as n, the beam flux reaching the Pt surface, is increased, e.g., by increasing P,, the pressure in the nozzle, or by decreasing x, the distance from the nozzle along the axis of the beam. Asymptotic values AT, = lim (n m) AT obtained by short linear extrapolation of A T 1 versus either x 2 or P i ' gives the value of the rise, AT, or ATp, respectively, when losses due to radiation and heat conduction from the Pt are negligible. The main differences from I are that there is a heater
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'Present address: Teknor Apex, Attleboro, MA 02703.
0022-3654191 12095- 1755$02.50/0
for the nozzle so that its temperature T,, need no longer be kept at Tb,the ambient one, a second Pt surface coated with Pt black is mounted 3.5 mm nearer the nozzle and offset from the polycrystalline one so that AT can be recorded by using either surface during a given run, and there is provision for using other nozzles. One of these permits vibrational excitation of the molecules of SF6by their absorption of radiation from a CO, laser, while the other is simply a round hole in a thin Ni foil. The Pyrex nozzle from I is heated by closely wound turns of resistance wire, and T, is measured on thermocouple TC2 (chromel-alumel) that is inserted into the nozzle from the rear so the molecules that are to form the beam flow over the junction just before they emerge into the vacuum chamber. The tip of the nozzle passes through a hole in a radiation shield that reduces the heating of TCI by radiation from heated nozzles. The Pt black surface is prepared by first making TCI as in I by spot welding together two (2.0 X 2.2 mm2) polycrystalline Pt foils 25-pm thick to enclose the junction between one alumel and two chrome1 wires that support TCl in its frame. Next, additional Pt is deposited electrochemically from a solution of 0.10 g of PtClz (Alfa Products) in 2 mL of concentrated HCI (12 M) that is diluted to 10 mL with deionized water. The normal rapid dissolution of the fine alumel wire (25 pm) in the acid can be slowed sufficiently by applying the potential before placing T C l in the solution and keeping the time of electrolysis short. In 3 min a current density of 1.3 A cm-, at 3.6 V gives good blackening over about 90% of the surface. (Better coverage would be desirable but is difficult to achieve.) The blackened Pt is then immediately rinsed in deionized water and dried in air. The coating typically increases the mass of T C l from 0.060 to 0.080 g. To produce an increase in the vibrational energy of the molecules of an SF6beam, we use a C 0 2laser (wavelength 10.6 pm) (1) Greene, E. F.; Tao, T.; Thantu, N. J . Phys. Chem. 1989,93, 6778. Note: In 1 on p 6782,column 2, y i should be X i in the expression for Emnd and a square bracket, [, is missing before c in the equation defining the rate coefficient c'; also there are four errors in eq 15 that are corrected in eq 2 of this paper. (2)Goodman, F. 0.; Wachman, H. Y.Dynamics of Gas-Surface Scattering; Academic: New York, 1976;Chapter 10.
0 1991 American Chemical Society
