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Electrocatalysis of CO2 reduction at surface modified metallic and semiconducting electrodes
101
J. Electroanal. Chem., 209 (1986) 101-107 Elsevier Sequoia S.A.. Lausanne - Printed
in The Netherlands
ELECTROCATALYSIS OF CO, REDUCTION AT SURFACE METALLIC AND SEMICONDUCTING ELECTRODES
CARLOS Department (Received
R. CABRERA of Chemrst~, 26th December
and HkCTOR
MODIFIED
D. ABRUfiA
Baker Laboratory,
ComeN Unrversrty. Ithaca, NY 14853 (U.S.A.)
1985; in revised form 1st April 1986)
ABSTRACT The use of surface modified metallic and semiconducting electrodes for the electrocatalytic and photoelectrocatalytic reduction of CO, is demonstrated using electropolymerized films of [Re(CO),(vbpy)Cl] (v-bpy 1s 4-vinyl.4’-methyl-2,2’-bipyridine). Large turnovers (ca. 600) were obtained for films on metallic electrodes (e.g. Pt). On sermconductor electrodes (p-Si and polycrystalline thin films of p-WSe,) somewhat lower (450) turnovers were obtained. In both cases CO was the predominant reduction product with essentially unit current efficiency.
INTRODUCTION The reduction of CO, to CO, formate and other products, at energies close to those dictated by thermodynamics, represents a formidable task due to the fact that systems capable of multi-electronic transfers and with appropriate binding properties must be designed. In addition, the mechanism must involve low energy pathways so that high energy intermediates are precluded. A number of systems based on transition metal complexes have been shown to achieve these transformations [l-9]. Recently, Lehn and co-workers [lo-121 demonstrated that systems based on [Re(CO),(bpy)Cl] and related materials were effective in the photochemically and electrochemically driven reduction of CO, to CO and, in some cases. formate. Meyer and co-workers [13-171 have further shown that a number of other derivatives are also active in this respect and that there can be two different pathways involved in the reduction process. We recently presented an extensive electrochemical and mechanistic study of [Re(CO),(dmbpy)Cl] (dmbpy is 4,4’-dimethyl-2,2’-bipyridine) and their relation to the electrocatalytic reduction of co, [X3]. Given that the coordination environment involves a bipyridine ligand and that we have developed a systematic way for electrodeposition of transition metal
0022-0728/86/$03.50
0 1986 Elsevier Sequoia
S.A.
102
complexes based on vinylbipyridine (v-bpy) (vinylbipyridine is 4-vinyl,4’-methyl2,2’-bipyridine) derivatives [19,20], we decided to investigate the potential of using v-bpy derivatives of the above mentioned complexes to achieve the electrocatalysis of CO, reduction at surface modified metallic and semiconducting electrodes. EXPERIMENTAL
[Re(CO),(L)Cl] (L = 4-vinyl,4’-methyl-2,2’-bipyridine or 4,4’-dimethyl-2,2’-bipyridine) were prepared according to the procedure of Wrighton and co-workers [21] by substituting phenanthroline for the desired ligand. Acetonitrile (Burdick and Jackson Distilled in Glass) was dried over 0.4 nm molecular sieves. Tetra-n-butyl ammonium perchlorate (TBAP) (G.F. Smith) was recrystallized three times from ethyl acetate and dried in vacua at 70°C for 72 h. All other reagents were of at least reagent grade quality and were used without further purification. Boron doped p-Si single crystal electrodes were provided with ohmic contacts by depositing a thin layer of gold on the back side. They were contacted with a piece of copper wire with conductive silver paint (Acme). The wire was inserted into 6 mm glass tubing and the electrode was masked with Torr-Seal (Varian) except for the surface to be studied (typically 0.05 cm’). Prior to use, the electrodes were etched in 48% HF for 60 s and afterwards they were rinsed with water, methanol and acetone. Polycrystalline thin films of p-WSe, were prepared and mounted as described previously [22,23]. Platinum electrodes sealed in glass or glassy carbon (Union Carbide) electrodes mounted in teflon shrouds were employed. Electrodes were cleaned by polishing with 1 pm diamond paste (Buehler) followed by rinsing with water and acetone. Transparent tin oxide on glass electrodes (PPG) were ultrasonically cleaned with hexanes, methanol, acetone and water. Electrical contact was made with conducting silver paint (Acme). Electrochemical experiments were performed with P.A.R. models 173 Potentiostat, 179 Digital Coulometer and 175 Universal Programmer or with an IBM EC 225 voltammetric analyzer. Data were recorded on either Soltec or Hewlett-Packard X-Y recorders or on a Nicolet digital oscilloscope. Conventional three compartment electrochemical cells, separated by medium porosity frits and provided with Pyrex windows were employed. Experiments were performed in acetonitrile containing 0.1 M TBAP as supporting electrolyte. All potentials are referenced to the sodium saturated calomel electrode (SSCE) without regards for the liquid junction. Experiments were conducted at room temperature; 24 -t 2°C. Semiconductor electrodes were illuminated with a beam expanded 5 mW He/Ne laser (Spectra Physics). Intensities of illumination were determined using an EG&G Electrooptics Model 450 Radiometer. Gas chromatographic experiments were performed on a Hewlett-Packard model 700 gas chromatograph with thermal conductivity detector on 183 cm column packed with spherocarb (Analabs). Helium was used as a carrier gas.
103 RESULTS
Similar to vinyl-bipyridine derivatives of other transition metal complexes [20], [Re(CO),(v-bpy)Cl] undergoes electroreductively initiated polymerization to give rise to stable electroactive polymer films. However, due to a dimerization reaction that follows the reduction of the complex [17,18], care must be exercised in the electrodeposition. Specifically, we find that the electrode potential needs to be scanned at about 0.5 V/s and that the lower limit should be no further than - 1.70 V. Under these conditions, the electropolymerization is very smooth and the deposit on the electrode surface has a bright yellow color characteristic of the complex. The presence of dimerization is characterized by greenish-yellow deposits due to the deep green color of the dimer (see Fig. 2C). Figure 1A shows a series of cyclic voltammograms for a platinum electrode in contact with a 1 mM solution of [Re(CO),(v-bpy)Cl] in acetonitrile +O.l M TBAP. The increase in current on each consecutive sweep is indicative of the electrodeposition process. Figure 1B shows a cyclic voltammogram of the modified electrode in supporting electrolyte only. The polymer film on the electrode surface shows a well behaved electrochemical response and the coverage of the complex on the electrode surface can be estimated from the charge under the voltammetric wave. Electrodeposition could also be effected on a transparant tin oxide electrode. The spectrum of such a deposit (shown in Fig. 2A) compares very well with that of the complex (Fig, 2B). The absorbances at 600 and 450 nm are due to dimer formation (see Fig. 2C for a
E/V vs SSCE E/V vs SSCE Fig. 1 (A) Consecutwe cyclic voltammograms at 0.5 V/s for a platinum electrode (0.04 cm’) in contact wth a 1 mM solution of [Re(CO),(v-bpy)Cl] in acetonitrile+O.l M TBAP. (B) Cychc voltammogram at 0.5 V/s m acetonitrile+O.l M TBAP for a platmum electrode modified with a polymeric film of
[Re(CO),(v-bpy)Cll.
WAVELENGTH/
WIZVELENGTH
,“rn
nm
W*“ELENFTtl
,
“IT
Fig. 2. (A) Spectrum of a polymeric film of [Re(C0)3(v-bpy)C1] deposited on a transparent tin oxide electrode. (B) Spectrum of [Re(C0)3(v-bpy)C1] in acetomtrile solution. (C) Spectrum of dimer in acetonitrile.
spectrum of the dimer in acetonitrile). (Large iR drops in this case precluded the use of fast potential sweeps.) Analogous to a recent report by Meyer and co-workers 1161, we find that metallic electrodes (Pt, glassy carbon) modified with films of [Re(CO),(v-bpy)Cl] are active in the catalytic reduction of CO, in acetonitrile + TBAP (0.1 M). From cot&metric experiments during CO2 reduction and knowledge of the surface coverage, we estimate a turnover of about 600 after which the catalytic activity is lost. Gas chromatographic analysis of the reduction products indicated essentially a quantitative yield of CO with a current efficiency of over 95%.
105
+____L I
-10
-05
E/V vs SSCE Fig. 3. Cyclic voltammograms for a single crystal p-S electrode in contact with a 1 mM solution of [Re(CO),(dmbpy)Cl] in acetonitrile + 0.1 M TBAP. (A) After purging with mtrogen and in the absence of illumination; (B) same as (A) except under dlummation with a beam expanded He/Ne laser: (C) same as (B) except after purging with CO,.
From our ongoing interest in photoelectrochemical processes, we were also interested in ascertaining whether these polymer films could be used on semiconductor electrodes for the photoelectrocatalytic reduction of CO,. However, before attempting the use of polymeric films, we first characterized the photoelectrochemical behavior of semiconductor electrodes in contact with acetonitrile solutions of [Re(CO),(dmbpy)Cl] (dmbpy is 4,4’-dimethyl-2,2’-bipyridine; a non-polymerizable analog). Figure 3 shows a series of cyclic voltammograms for a p-Si single crystal electrode in contact with a 1 mM solution of the complex. Trace A represents the response after deaerating the solution with N, and in the absence of illumination and as expected, there is essentially no current flow due to the fact that there are no electrons (minority carriers) at the interface [24]. Curve B shows the response when the electrode is illuminated with a beam expanded He/Ne laser (incident power is about 22 mW/cm2). In this case, a well developed cathodic wave with an onset potential of about -0.80 V can be observed. Since at a metallic electrode the first reduction wave for the complex is at - 1.43 V, this represents an underpotential of about 0.6 V. Curve C shows the response when the electrode is illuminated and the solution is saturated with CO,. In this case, the onset potential moves further in the positive direction, and more notably, the current is greatly enhanced, indicative of the catalytic effect. Given the radiant intensity incident on the electrode suface, this represents a conversion efficiency of about 3.6%. We have performed long term irradiations and we find that the reduction product is CO with essentially unit current efficiency. There is, however, some decomposition of the complex in solution to yield what we identify as the acetonitrile complex [Re(CO),(dmbpy) CH,CN]+) based on spectroscopic analysis of the solution before and after electrolysis. (The acetonitrile complex has a characteristic absorption at 345 nm [14].) Having established a strong photoelectrocatalytic effect towards CO, reduction with the complex in solution, we attempted the same process but on surface modified semiconductor electrodes. In this case we employed both single crystal
106
I
1
-05
-1.0 E/V
-1 5
vs SSCE
Fig 4. Cychc voltammograms in acetomtrile+O.l M TBAP for a thin film p-WSe2 electrode modified with a polymeric thm film of [Re(CO),(v-bpy)Cl]. (A) After purging with nitrogen and m the absence of rllumination; (B) same as (A) except under illumination wth a beam expanded He/Ne laser; (C) same as (B) except after purging wth CO,.
p-Si and polycrystalline thin films of p-WSe,. The deposition was effected from acetonitrile solutions of the complex (1 mM) by scanning the potential of the electrode between 0.0 and - 1.2 V vs. SSCE while the electrode was under illumination. This gave rise to deposits that were very similar to those obtained on metallic electrodes. Figure 4A shows a cyclic voltammogram (in the absence of illumination) for a surface modified polycrystalline thin film p-WSe, electrode in contact with an acetonitrile solution (0.1 M TBAP) that had been deaerated with N2 and as expected, essentially no current flows for the reasons previously mentioned. Upon illumination (Fig. 4B) a well developed cathodic wave can be observed with an onset potential of about -0.8 V. In the presence of CO, (Fig. 4C) the onset potential moved to about -0.65 V and a greatly enhanced photocathodic current peak was observed, consistent with a strong catalytic effect. Catalytic currents could be sustained with turnovers of the order of 450. The reason for the lower number of turnovers (relative to surface modified metallic electrodes) is not clear at this point but we believe that it is due in part to the difficulty in estimating the coverage of the deposit accurately. Similar effects were observed for surface modified p-Si electrodes. As for the surface modified metallic electrodes, gas chromatographic analysis established that CO was the predominant product obtained with essentially unit current efficiency. Although the catalytic activity of these materials is not very long lived, it points to the fact that a very strong catalytic effect can be achieved on these modified interfaces by suitably controlling the reactivity of transition metal complexes and by making use of the ability to immobilize them on electrode surfaces. This is particularly attractive for polycrystalline thin films of materials such as p-WSez since they can be prepared as large area thin films [22,23] and as such can potentially represent a viable means of achieving large scale conversion of solar energy.
107 ACKNOWLEDGEMENTS
We gratefully acknowledge support of this work’by the Materials Science Center at Cornell University as well as conversations with B.P. Sullivan and Professor T.J. Meyer (University of North Carolina at Chapel Hill). We thank Anne I. Breikss for the synthesis of the complexes. REFERENCES 1 B. Ftsher and R. Eisenberg, J. Am. Chem Sot., 102 (1980) 7361. 2 M. Tezuka, T. Yajma. A. Tsuchiya, Y. Matsumoto, Y. Uchida and M. Hidat, J. Am. Chem. Sot., 104 (1982) 6834. 3 C M. Lieber and N.S. Lewis, J. Am. Chem. Sot.. 106 (1984) 5033. 4 S. Slater and J.H. Wagenknecht. J. Am. Chem. Sot., 106 (1984) 5367. 5 R. Eisenberg and D.E. Hendrickson, Adv. Catal. 28 (1979) 79. 6 J.A. Ibers. Chem. Sot. Rev., 11 (1982) 57 7 J.M. Lehn and R. Ztessel, Proc. Natl. Acad. SCI. US A., 79 (1982) 701. 8 M.P. Beley, J.-P. Collin, R. Ruppert and J.-P. Savage, J. Chem. Sot. Chem. Commun., (1984) 1315. 9 M.G. Bradley, T. Tysak. D.J. Graves and N.A. Vlachopoulos. J. Chem. Sot. Chem. Commun., (1983) 349 10 J. Hawecker. J M. Lehn and R. Ztessel, J. Chem. Sot. Chem. Commun . (1983) 536. 11 J. Hawecker. J.M. Lehn and R. Ziessel, J Chem. Sot Chem. Commun.. (1984) 328. 12 J. Hawecker, J.M. Lehn and R. Ztessel, J Chem. Sot. Chem. Commun., (1985) 56. 13 T.D. Westmoreland, H. LeBozec. R.W. Murray and T.J. Meyer, J. Am. Chem. Sot., 105 (1983) 5952. 14 J.V. Caspar and T.J. Meyer, J. Phys. Chem.. 87 (1983) 952. 15 B.P. Sulhvan and T.J. Meyer. J. Chem. Sot. Chem. Commun.. (1984) 1244. 16 T.R. O’Toole, L.D. Margerum. T.D Westmoreland. W.J. Vinmg. R.W Murray and T.J. Meyer, J Chem. Sot. Chem. Commun.. (1985) 1416. 17 B.P Sulhvan. C.M. Bolinger, D. Conrad, W.J. Vmmg and T.J Meyer, J. Chem Sot. Chem. Commun., (1985) 1414. 18 A.I. Breikss and H.D. Abruira, J. Electroanal. Chem., 201 (1986) 347. 19 H.D. Abrufia. A.I. Breikss and D.B. Collum. Inorg. Chem . 24 (1985) 987 20 (a) H.D. Abruiia. P. Denisevich, M. Umaira, T.J. Meyer and R.W. Murray, J. Am. Chem. Sot.. 103 (1981) 1; (b) P. Demsevtch. H.D. Abruiia, C.R. Leidner. T.J. Meyer and R.W. Murray, Inorg. Chem . 21 (1982) 2153. 21 MS. Wrtghton and D.L. Morse, J. Am Chem. Sot., 96 (1974) 998. 22 H.D. Abruiia and A.J. Bard. J. Electrochem. Sot.. 129 (1982) 673. 23 C.R. Cabrera and H.D Abruira, J. Phys. Chem., 89 (1985) 1279. 24 S.R Mornson, Electrochemistry at Semiconductors and Oxidtzed Metal Electrodes, Plenum, New York. 1980.
J. Electroanal. Chem., 209 (1986) 101-107 Elsevier Sequoia S.A.. Lausanne - Printed
in The Netherlands
ELECTROCATALYSIS OF CO, REDUCTION AT SURFACE METALLIC AND SEMICONDUCTING ELECTRODES
CARLOS Department (Received
R. CABRERA of Chemrst~, 26th December
and HkCTOR
MODIFIED
D. ABRUfiA
Baker Laboratory,
ComeN Unrversrty. Ithaca, NY 14853 (U.S.A.)
1985; in revised form 1st April 1986)
ABSTRACT The use of surface modified metallic and semiconducting electrodes for the electrocatalytic and photoelectrocatalytic reduction of CO, is demonstrated using electropolymerized films of [Re(CO),(vbpy)Cl] (v-bpy 1s 4-vinyl.4’-methyl-2,2’-bipyridine). Large turnovers (ca. 600) were obtained for films on metallic electrodes (e.g. Pt). On sermconductor electrodes (p-Si and polycrystalline thin films of p-WSe,) somewhat lower (450) turnovers were obtained. In both cases CO was the predominant reduction product with essentially unit current efficiency.
INTRODUCTION The reduction of CO, to CO, formate and other products, at energies close to those dictated by thermodynamics, represents a formidable task due to the fact that systems capable of multi-electronic transfers and with appropriate binding properties must be designed. In addition, the mechanism must involve low energy pathways so that high energy intermediates are precluded. A number of systems based on transition metal complexes have been shown to achieve these transformations [l-9]. Recently, Lehn and co-workers [lo-121 demonstrated that systems based on [Re(CO),(bpy)Cl] and related materials were effective in the photochemically and electrochemically driven reduction of CO, to CO and, in some cases. formate. Meyer and co-workers [13-171 have further shown that a number of other derivatives are also active in this respect and that there can be two different pathways involved in the reduction process. We recently presented an extensive electrochemical and mechanistic study of [Re(CO),(dmbpy)Cl] (dmbpy is 4,4’-dimethyl-2,2’-bipyridine) and their relation to the electrocatalytic reduction of co, [X3]. Given that the coordination environment involves a bipyridine ligand and that we have developed a systematic way for electrodeposition of transition metal
0022-0728/86/$03.50
0 1986 Elsevier Sequoia
S.A.
102
complexes based on vinylbipyridine (v-bpy) (vinylbipyridine is 4-vinyl,4’-methyl2,2’-bipyridine) derivatives [19,20], we decided to investigate the potential of using v-bpy derivatives of the above mentioned complexes to achieve the electrocatalysis of CO, reduction at surface modified metallic and semiconducting electrodes. EXPERIMENTAL
[Re(CO),(L)Cl] (L = 4-vinyl,4’-methyl-2,2’-bipyridine or 4,4’-dimethyl-2,2’-bipyridine) were prepared according to the procedure of Wrighton and co-workers [21] by substituting phenanthroline for the desired ligand. Acetonitrile (Burdick and Jackson Distilled in Glass) was dried over 0.4 nm molecular sieves. Tetra-n-butyl ammonium perchlorate (TBAP) (G.F. Smith) was recrystallized three times from ethyl acetate and dried in vacua at 70°C for 72 h. All other reagents were of at least reagent grade quality and were used without further purification. Boron doped p-Si single crystal electrodes were provided with ohmic contacts by depositing a thin layer of gold on the back side. They were contacted with a piece of copper wire with conductive silver paint (Acme). The wire was inserted into 6 mm glass tubing and the electrode was masked with Torr-Seal (Varian) except for the surface to be studied (typically 0.05 cm’). Prior to use, the electrodes were etched in 48% HF for 60 s and afterwards they were rinsed with water, methanol and acetone. Polycrystalline thin films of p-WSe, were prepared and mounted as described previously [22,23]. Platinum electrodes sealed in glass or glassy carbon (Union Carbide) electrodes mounted in teflon shrouds were employed. Electrodes were cleaned by polishing with 1 pm diamond paste (Buehler) followed by rinsing with water and acetone. Transparent tin oxide on glass electrodes (PPG) were ultrasonically cleaned with hexanes, methanol, acetone and water. Electrical contact was made with conducting silver paint (Acme). Electrochemical experiments were performed with P.A.R. models 173 Potentiostat, 179 Digital Coulometer and 175 Universal Programmer or with an IBM EC 225 voltammetric analyzer. Data were recorded on either Soltec or Hewlett-Packard X-Y recorders or on a Nicolet digital oscilloscope. Conventional three compartment electrochemical cells, separated by medium porosity frits and provided with Pyrex windows were employed. Experiments were performed in acetonitrile containing 0.1 M TBAP as supporting electrolyte. All potentials are referenced to the sodium saturated calomel electrode (SSCE) without regards for the liquid junction. Experiments were conducted at room temperature; 24 -t 2°C. Semiconductor electrodes were illuminated with a beam expanded 5 mW He/Ne laser (Spectra Physics). Intensities of illumination were determined using an EG&G Electrooptics Model 450 Radiometer. Gas chromatographic experiments were performed on a Hewlett-Packard model 700 gas chromatograph with thermal conductivity detector on 183 cm column packed with spherocarb (Analabs). Helium was used as a carrier gas.
103 RESULTS
Similar to vinyl-bipyridine derivatives of other transition metal complexes [20], [Re(CO),(v-bpy)Cl] undergoes electroreductively initiated polymerization to give rise to stable electroactive polymer films. However, due to a dimerization reaction that follows the reduction of the complex [17,18], care must be exercised in the electrodeposition. Specifically, we find that the electrode potential needs to be scanned at about 0.5 V/s and that the lower limit should be no further than - 1.70 V. Under these conditions, the electropolymerization is very smooth and the deposit on the electrode surface has a bright yellow color characteristic of the complex. The presence of dimerization is characterized by greenish-yellow deposits due to the deep green color of the dimer (see Fig. 2C). Figure 1A shows a series of cyclic voltammograms for a platinum electrode in contact with a 1 mM solution of [Re(CO),(v-bpy)Cl] in acetonitrile +O.l M TBAP. The increase in current on each consecutive sweep is indicative of the electrodeposition process. Figure 1B shows a cyclic voltammogram of the modified electrode in supporting electrolyte only. The polymer film on the electrode surface shows a well behaved electrochemical response and the coverage of the complex on the electrode surface can be estimated from the charge under the voltammetric wave. Electrodeposition could also be effected on a transparant tin oxide electrode. The spectrum of such a deposit (shown in Fig. 2A) compares very well with that of the complex (Fig, 2B). The absorbances at 600 and 450 nm are due to dimer formation (see Fig. 2C for a
E/V vs SSCE E/V vs SSCE Fig. 1 (A) Consecutwe cyclic voltammograms at 0.5 V/s for a platinum electrode (0.04 cm’) in contact wth a 1 mM solution of [Re(CO),(v-bpy)Cl] in acetonitrile+O.l M TBAP. (B) Cychc voltammogram at 0.5 V/s m acetonitrile+O.l M TBAP for a platmum electrode modified with a polymeric film of
[Re(CO),(v-bpy)Cll.
WAVELENGTH/
WIZVELENGTH
,“rn
nm
W*“ELENFTtl
,
“IT
Fig. 2. (A) Spectrum of a polymeric film of [Re(C0)3(v-bpy)C1] deposited on a transparent tin oxide electrode. (B) Spectrum of [Re(C0)3(v-bpy)C1] in acetomtrile solution. (C) Spectrum of dimer in acetonitrile.
spectrum of the dimer in acetonitrile). (Large iR drops in this case precluded the use of fast potential sweeps.) Analogous to a recent report by Meyer and co-workers 1161, we find that metallic electrodes (Pt, glassy carbon) modified with films of [Re(CO),(v-bpy)Cl] are active in the catalytic reduction of CO, in acetonitrile + TBAP (0.1 M). From cot&metric experiments during CO2 reduction and knowledge of the surface coverage, we estimate a turnover of about 600 after which the catalytic activity is lost. Gas chromatographic analysis of the reduction products indicated essentially a quantitative yield of CO with a current efficiency of over 95%.
105
+____L I
-10
-05
E/V vs SSCE Fig. 3. Cyclic voltammograms for a single crystal p-S electrode in contact with a 1 mM solution of [Re(CO),(dmbpy)Cl] in acetonitrile + 0.1 M TBAP. (A) After purging with mtrogen and in the absence of illumination; (B) same as (A) except under dlummation with a beam expanded He/Ne laser: (C) same as (B) except after purging with CO,.
From our ongoing interest in photoelectrochemical processes, we were also interested in ascertaining whether these polymer films could be used on semiconductor electrodes for the photoelectrocatalytic reduction of CO,. However, before attempting the use of polymeric films, we first characterized the photoelectrochemical behavior of semiconductor electrodes in contact with acetonitrile solutions of [Re(CO),(dmbpy)Cl] (dmbpy is 4,4’-dimethyl-2,2’-bipyridine; a non-polymerizable analog). Figure 3 shows a series of cyclic voltammograms for a p-Si single crystal electrode in contact with a 1 mM solution of the complex. Trace A represents the response after deaerating the solution with N, and in the absence of illumination and as expected, there is essentially no current flow due to the fact that there are no electrons (minority carriers) at the interface [24]. Curve B shows the response when the electrode is illuminated with a beam expanded He/Ne laser (incident power is about 22 mW/cm2). In this case, a well developed cathodic wave with an onset potential of about -0.80 V can be observed. Since at a metallic electrode the first reduction wave for the complex is at - 1.43 V, this represents an underpotential of about 0.6 V. Curve C shows the response when the electrode is illuminated and the solution is saturated with CO,. In this case, the onset potential moves further in the positive direction, and more notably, the current is greatly enhanced, indicative of the catalytic effect. Given the radiant intensity incident on the electrode suface, this represents a conversion efficiency of about 3.6%. We have performed long term irradiations and we find that the reduction product is CO with essentially unit current efficiency. There is, however, some decomposition of the complex in solution to yield what we identify as the acetonitrile complex [Re(CO),(dmbpy) CH,CN]+) based on spectroscopic analysis of the solution before and after electrolysis. (The acetonitrile complex has a characteristic absorption at 345 nm [14].) Having established a strong photoelectrocatalytic effect towards CO, reduction with the complex in solution, we attempted the same process but on surface modified semiconductor electrodes. In this case we employed both single crystal
106
I
1
-05
-1.0 E/V
-1 5
vs SSCE
Fig 4. Cychc voltammograms in acetomtrile+O.l M TBAP for a thin film p-WSe2 electrode modified with a polymeric thm film of [Re(CO),(v-bpy)Cl]. (A) After purging with nitrogen and m the absence of rllumination; (B) same as (A) except under illumination wth a beam expanded He/Ne laser; (C) same as (B) except after purging wth CO,.
p-Si and polycrystalline thin films of p-WSe,. The deposition was effected from acetonitrile solutions of the complex (1 mM) by scanning the potential of the electrode between 0.0 and - 1.2 V vs. SSCE while the electrode was under illumination. This gave rise to deposits that were very similar to those obtained on metallic electrodes. Figure 4A shows a cyclic voltammogram (in the absence of illumination) for a surface modified polycrystalline thin film p-WSe, electrode in contact with an acetonitrile solution (0.1 M TBAP) that had been deaerated with N2 and as expected, essentially no current flows for the reasons previously mentioned. Upon illumination (Fig. 4B) a well developed cathodic wave can be observed with an onset potential of about -0.8 V. In the presence of CO, (Fig. 4C) the onset potential moved to about -0.65 V and a greatly enhanced photocathodic current peak was observed, consistent with a strong catalytic effect. Catalytic currents could be sustained with turnovers of the order of 450. The reason for the lower number of turnovers (relative to surface modified metallic electrodes) is not clear at this point but we believe that it is due in part to the difficulty in estimating the coverage of the deposit accurately. Similar effects were observed for surface modified p-Si electrodes. As for the surface modified metallic electrodes, gas chromatographic analysis established that CO was the predominant product obtained with essentially unit current efficiency. Although the catalytic activity of these materials is not very long lived, it points to the fact that a very strong catalytic effect can be achieved on these modified interfaces by suitably controlling the reactivity of transition metal complexes and by making use of the ability to immobilize them on electrode surfaces. This is particularly attractive for polycrystalline thin films of materials such as p-WSez since they can be prepared as large area thin films [22,23] and as such can potentially represent a viable means of achieving large scale conversion of solar energy.
107 ACKNOWLEDGEMENTS
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