Photoelectrochemical characterisation of porphyrin-coated electrodes

Solar Energy Materials 21 (1991) 317-325 North-Holland

317

Photoelectrochemical characterisation of porphyrin-coated electrodes J. Basu and K.K. Rohatgi-Mukherjee School of Energy Studies and Department of Chemistry, Jadavpur University, Calcutta 700032, India Received 22 December 1989; in revised form 2 July 1990

The electrochemical oxidation of some substituted porphyrins and their metal complexes in organic solvent dichloromethane (CH2CI 2) leads to the formation of electroactive porphyrin films which can produce reproducible photovoltage and photocurrent on illumination. The films are stable in organic solvents but removed by concentrated acids. The films have been characterised by cyclic voltammetry and absorption spectral studies on transparent SnO2 electrodes.

Abbreviations

CV

ZnTPP ZnTVyPP TBAP TBATFB TBAC

Cyclic voltammetry Short circuit current Open circuit voltage Zn(II) a/3 y 8-tetraphenyl porphyrin Zn(II) a/3 7 8-tetra-p-vinylphenyl porphyrin Tetrabutylammoniurn perchlorate (Bu)4NC10 4 Tetrabutylammonium tetrafluoroborate Tetrabutylammonium chloride

1. Introduction

Photoelectrochemical properties of porphyrins have been intensively studied due to their structural analogy to natural plant chlorophyll. Numerous photoinduced reactions involving porphyrins and metalloporphyrins and other dyes have been studied in electrochemical cells for the conversion of solar energy to electrical energy by chemical modification of electrodes [1-5] using their semiconductor properties. The synthesis of tetravinylphenyl porphyrin (TVyPP) was recently taken up by us to bring about electro-initiated polymerisation of these porphyrins on suitable electrodes such that electoactive films with reproducible photovoltage and photocurrent could be produced [6]. 0165-1633/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)

318

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P['¢ ' (haraclertsatlon ~!/ D~Jrph~r~l coated ¢'/,_'~Tr.dc~

2. Experimental procedure The solvents, tetrabutylammonium perchlorate (TBAP), tetrabutylammonium tetrafluoroborate (TBATFB), tetrabutylammonium chloride (TBAC) and other reagents were of G R grade and were used without further purification. TVvPP and its metal complexes were prepared by the method already discussed in a previot> paper [6]. The absorption spectra were recorded at 300 K in a U1 \ -! V I S - N I R Recording Spectrophotometer Shimadzu Model UV-365. The cyclic v o l t a m m e t r \ was carried out by a Wenking 5772 potentiostat. The voltammograms were recorded on a X Y recorder of Houston Model 2000 X Y / f . A saturated calomel electrode was used as reference electrode. The metalloporphyrin films were formed by electrochemical cycling through the metalloporphyrin oxidation wave or by potentiostating the P t / S n O 2 working electrode (dimension 0.5 cm z) at a potential sufficiently positive to oxidise metalloporphines. The number of molecules deposited on the electrode was calculated as reported earlier [6]. The intensity of light falling on the electrode surface at a distance of 5 cm was measured by YSI-Kettering Model 65 Radiometer and was found to be 1.2 × 105 erg c m 2 s ~

3. Results and discussion The electrochemical characteristics of Zn(lI) and Ni(II) complexes of TVyPP in dichloromethane (CH2CI 2) have been investigated via cyclic voltammetry at a platinum electrode [6]. The reductive scanning produced well behaved waves due to reduction of metal and or of the porphyrin ring [7]. But oxidative scanning often produced waves wich grew in size upon successive scans and deposited porphyrin on the electrode surface as evidenced by steady deepening of colour. Fig. 1 shows oxidation-reduction peaks of l m M ZnTVyPP solution in 0.1M TBAP/CH2C12 at a scan rate of 100 mV s 1 on the Pt electrode, the scan range being 400-1400 mV versus SCE. The current grows gradually with each successive scan. A film of ZnTVyPP can also be produced by allowing the current to flow at a fixed potential of 1200 inV. No film formation was observed in Z n T P P solution. The main difference between ZnTVyPP and Z n T P P is that the vinyl group of the former is absent in ZnTPP. Therefore, it is clear that the vinyl group is responsible for the film formation. Films could be deposited on transparent SnO 2 electrode also, using the cyclic voltammetric technique. The absorption spectrum of Z n T V y P P film, fig. 2a, on a SnO2 glass electrode was compared with that of Z n T V y P P in CH2CI:,solution, fig. 2b. The three absorption bands are respectively B (Soret band) and QI and Q0 (/3 and a) peaks of the ~r ~ 7r* transitions of porphyrin dianion [8]. On film formation all the three bands are red shifted by about 5-12 nm. For the Sorer band, a red shift of about 12 nm as well as considerable asymmetric broadening is observed. The absorption m a x i m u m in homogeneous solution appears at 427 nm with a weak shoulder at 395 nm. The red shift and asymmetric broadening of the Soret band is primarily due to aggregation [9] and stacking of porphyrin molecules on the electrode surface. Similar broadening and small red shift is observed for the

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u

319

,

g

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g

1400

I O0

1000

800

600

400

rrV

Fig. 1. Cyclic voltammetric scans at Pt electrodes of 0.5mM ZnTVyPP in 0.1M TBAP/CH2CI 2. scan rate = 100 mV/s.

Q bands also. S o m e contributions m a y be due to partial saturation of vinyl groups for which there is i n d e p e n d e n t evidence from resonance R a m a n spectroscopy [6]. The cyclic v o l t a m m o g r a m s of a Z n T V y P P - f i l m - c o a t e d Pt electrode in C H 2 C 1 2 / 1 M ( B u ) 4 N C 1 0 4 solution (fig. 3) gave a CV curve identical to that of l m M

552 10

08 4~0

0.6

04

380

//

i\

420

460

5gO

500

540

580

620

660

700

Wave length

Fig. 2. Absorption spectrum of (a) ZnTVyPP at C=15•M (- - -) and (b) that of the films on transparent SnO2 electrode ( ).

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Fig. 3. Cyclic voltammetry of ZnTVyPP-film-coated Pt electrode in CH2CI2/1M (Bu)4NC104 solution, scan rate = 100 rnV/s,

ZnTVyPP solution in CH2C12/1M (Bu)4NC104 as solvent (fig. 1) using uncoated Pt as the working electrode. The only difference observed is that the oxidation peak is about 20 mV lower for the former case. The similarity of electrochemical oxidation-reduction waves and comparable potentials of the films, fig. 3 and that of the solution, fig. 1, indicate that the porphyrin structure is retained [10] on the electrode surface. The difference of about 20 mV can be attributed to the fact that the electrochemical environments in the two situations are different. In fig. 1 is also presented the progress of ZnTVyPP film formation when a Pt electrode in a l m M solution of Zn(II) TVyPP in CH2C12/1M TBAP was scanned from 400 to 1400 mV versus SCE. With the growth of the film the voltammograms became progressively less reversible as is evident from increasing AE, the separation between the anodic and cathodic peak potentials. AE is plotted as a function of number of scans in fig. 4a. To determine the amount of electroactive material deposited on the electrode, the electrolysis was interrupted periodically, the electrode removed from the bath, washed and its characteristics determined by cyclic voltammetry using l m M TBAP in CH2C12 as the solvent. The total electroactive material increased steadily up to 15 × 10 t5 molecules/cm 2 after which no further deposition was observed. This amount corresponds to about 250 monolayer equivalents if the porphyrin units are assumed to lie flat and occupy 170 A of surface area [11]. This has been confirmed by the atomic absorption spectroscopic technique as reported earlier [6]. The number of monolayers deposited versus number of scans is plotted in fig. 4b. After about 5 0 - 6 0 scans which took about 20 rain with scan rate 100 m V / s and scan range 400-1400 mV, a prominent break in the slope is observed. The decrease in the slope indicates longer time is required for the

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Z.O 60 No of Scans

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Fig. 4. (a) Plot of AE versusnumber of scans. (b) Plot of number of monolayersversus number of scans. electrons to pass through the film. This may be the reason for irreversibility demonstrated in fig. 4a and could be due to the porphyrin units being poorly oriented for electron migration through the film [11] reducing the diffusion length of the electrons. 3.1. The role of supporting electrolyte

Drastic changes in the voltammograms are observed when tetrabutylammonium tetrafluoroborate (TBATFB) is used as the supporting electrolyte (fig. 5). With TBAP as the supporting electrolyte ZnTVyPP shows only one anodic peak which grows as the film grows (fig. 1). But in TBATFB initially two anodic waves separated by 0.3 V are observed. As the film grows (after 10 scans) only one anodic peak is observed giving a pattern similar to that observed when TBAP is used as the electrolyte. The initial 0.3 V peak separation observed in TBATFB is characteristic of the potential difference between porphyrin mono- and dication formation [11]. Therefore it appears that initially successive one-electron oxidation products are stabilised but after a certain time as the film grows, the properties of the film change and only the monocation [Zn(II) TVyPP] is stabilised. One can therefore conclude that TBATFB can stabilise both the monocation and the dication in the initial stages of film formation; but as the film grows, the resistance of the electrode increases and only the monocation can be stabilised. To further understand the role of supporting electrolyte, cyclic voltammetry was carried out using tetrabutylammonium chloride (TBAC). The C1- being much smaller than BF4- and C104- is expected to give some information regarding the influence of size of the counter ion on the redox properties of ZnTVyPP. The peak potential at which the film is formed by oxidative cycling in a TBAP/CH2C12 solution of Zn(II) TVyPP decreases as compared to the value when the supporting electrolyte is either TBAP or TBAC (fig. 6). The downward shift in peak potential in

322

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1400

1000

600

800

400

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Fig. 5. Cyclic voltammetry scans at Pt electrode of 0.5raM ZnTVyPP in 0.1M T B A T F B / C H 2 C l 2, scan rate = 100 m V / s .

chloride as compared to the value in perchlorate medium can be attributed to ion pairing of the counter ions [12,13]. The order of observed redox potentials in the presence of these three counter ions C104, BF4- and C1- is the same as their respective sizes: C104 > BF4 > C1 . It appears that due to its smaller size, CI can lead to ion pairing. The free ions are likely to be more effective than the ion-pairs [12,13].

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Fig. 6. Oxidative scans of cyclic voltammetry of 0.5mM Zn(II) TVyPP using 0.1M T B A P / C H 2 C I 2 ( ); 0.1M T B A T F B / C H 2 C I 2 ( . . . . . ); 0.1M TBAC/CH2C12 ( - - - ) a s supporting electrolytes.

J. Basu, K.K. Rohatgi-Mukherjee / PEC characterisation of porphyrin-coated electrodes

323

3.2. Photoelectrochemical characterisation of coated electrodes

The photoelectrochemical study of ZnTVyPP film obtained by oxidative scanning for 60 cycles (after which there is very little change in current peak height) was carried out in a solution of pH 2 using KC1 as the electrolyte. A comparison of photovoltage, photocurrent and fill factor of these polymeric films with the values obtained by the dip-dry method are given in table 1. The photoelectrochemical characterisation [14] of these electrodes was carried out by setting up an electrochemical cell with Pt as the counter electrode.

4. Conclusion

The electrochemical characteristics of Zn(II) TVyPP in CH2C12 has been investigated via cyclic voltammetry at platinum- and SnO2-coated glass electrodes. The mechanism of film formation in Zn(II) TVyPP is attributed to the presence of a vinyl group, i.e. - C = C - group, in the p-position of the phenyl substituent of the porphyrin ring. The initial oxidation of the central metal ion electrochemically induces electron transfer from the ring to the - C H = C H 2 group conjugated with the ring, thereby generating - + C H - ' C H 2 radicals. These radicals can propagate the polymerisation chain when once initiated. For a Zn complex, the initiation step is --e

+

PZn(II) ~ P Zn(II). +e

Once the film was formed, it could not be dissolved in any organic solvent although the original solid compound is soluble in almost all organic solvents. This change in solubility does suggest polymer formation. Therefore the film thickness was determined by dissolving the film in hot concentrated acid and measuring the metal content by atomic absorption spectrophotometry. The comparison of the amount of film actually deposited on the electrode surface and the amount of charge passed during cyclic voltammetry shows that the extent of cross-linking is very high. The absorption spectra of Zn(II) TVyPP film deposited electrochemically on SnO 2 electrode, shows marked asymmetric broadening and red shift in the B and Q~ and Q0 bands suggesting stacking interaction between layers of porphyrins deposited on the electrode surface. The film formation of Zn(II) TVyPP on oxidative scanning is therefore by a radical mechanism since the vinyl groups are saturated in the process as confirmed previously by resonance Raman spectroscopy [6]. The film is formed only when the central metal ion is oxidised leading to the oxidation of the porphyrin ring by electron transfer. As the vinyl groups are conjugated with the porphyrin ring via the phenyl group, they are affected only if significant electron transfer occurs from the ring. Thus a stable adherent film is formed by electro-initiation. It is expected that the porphyrin rings are stacked in layers with a slight displacement of the metal center giving a brick-work-like structure [15]. The photochemistry of this reproducible and stable film is very different from that exhibited by the film coated by the

324

J. Basu, K.K. Rohatgl-,l, l u k h e u e v " Pti'(rcharacterisaltotz {/ port hrrtn-( )ateJ. cA', Irod~'s

Table l

Open circuit photopotential 1,i,,, short circuit photocurrent density' Q and other charactcrb, tics ,,i photoelectrochemical cells at T = 300 K (in all the cases, the electrode area was 0.5 t_'m"~ and the liglu intensity falling on the electrode surface was 1.2 × 105 erg cm 2 s ~) Type of photoanode

l~i~ {mV)

J,, ( / * A / c m 2 )

Coating by dip-dry method [14] only

175

0.90

Coating by CV

125 "~ 120 b)

0.428 0.38

PAN c} coated dye

250

370

No. of monolayer equiv.

till factor I). 15

250

0.50 I).39

2.28

-

0.44

2.4

-

0.45

loaded electrode PVP d) coated dye

loaded electrode ~) "} TBAP as supporting electrolyte, b) T B A T F B a) Polyvinyl pyridin, e) Unpublished work.

as supporting electrolyte.

~) Polyacrylonitrile.

dip-dry method directly on the electrode surface or by initial polymer coating of the electrode followed by dye loading (table 1). The interesting observations are that (i) the film obtained by the dip-dry method develops reasonable photovoltage but gives a very small fill factor (0.15), (ii) the dye loaded polyacrylamide or polyvinyl pyridine coated electrode displays much higher photovoltage and photocurrent with fill factor 0,44, and (iii) for an electro-initiated polymer-coated photoelectrode obtained by cyclic voltammetry the fill factor is the highest 0.51, though the photoelectrochemical effect is comparatively low. The low photoelectrochemical effect is apparently due to low absorptivity and high reflectivity of the oriented films and to rapid quenching of the excited electrons through a large number of exciton states and surface states in the metal electrode. The broadening of the absorption band on SnO2 electrode confirms this explanation. In polymer-coated dye-loaded electrodes the dye molecules are randomly distributed and electron transfer can take place by the hopping mechanism and finally the electrons are captured by the electrode via firmly anchored polymer chains. The role of counter ions is very crucial in determining the redox chemistry of these systems as they can control the electrochemistry of films formed by electropolymerisation.

References [1] [2] [3] [4] [5] [6] [7]

J,M. Mountz and H. Ti Tien, Sol. Energy 21 (1987) 291. J. Basu, A.B. Chatterjee and K.K. Rohatgi-Mukherjee, Indian J. Chem. 24A (1985) 550. H. Meier, W. Albrecht, and U. Tschirwitz, Angew. Chem. Int. 11 (1972) 1051. G.A. Chamberlain, Mol. Cryst. Liq. Cryst. 93 (1983) 369. P. Panayotatos, G. Bud, R. Sauers, A. Piechowski and S. Husain, Sol. Cells 21 (1987) 301. J. Basu and K.K. Rohatgi-Mukherjee, Photochem. Photobiol. 48 (1988) 417. R.H. Felton, in: The Porphyrins, Ed. D. Dolphin, Vol. V (Academic Press, N e w York. 1978) p. 513.

J. Basu, K.K. Rohatgi-Mukherjee / PEC characterisation of porphyrin-coated electrodes

325

[8] M. Gouterman, in: The Porphyrins, Ed. D. Dolphin, Vol. III (Academic Press, New York, 1978) p. 165. [9] S. Basu, K.K. Rohatgi-Mukherjee and I. Lopez-Arbeloa, Spectrochim. Acta 42A (1986) 1355. [10] A. Bettectheim, B.A. White, S.A. Raybuck and R.W. Murray, Inorg. Chem. 26 (1987) 1009. [11] K.A. Macor and T.G. Spiro, J. Am. Chem. Soc. 105 (1983) 5601. [12] I. Fajer, D.C. Borg, A. Forman, R.H. Felton, L. Vegh and D. Dolphin, Ann. N.Y. Acad. Sci. 206 (1973) 349. [13] A. Ladwith, S.A. Koes, D.C. Sherrington and P. Bonner, Polymer 22 (1981) 143. [14] J. Basu, A. Bhattacharya, K. Das, A.B. Chatterjee, K.K. Kundu and K.K. Rohatgi-Mukherjee, Indian J. Chem. 22A (1983) 695. [15] H. Kuhn, Pure Appl. Chem. 27 (1971) 421.