5544
Langmuir 2006, 22, 5544-5547
Modification of Mesoporous TiO2 Electrodes with Cross-Linkable B12 Derivatives Simona Asaftei and Lorenz Walder* Institute of Chemistry, UniVersity of Osnabru¨ck, Barbarastrasse 7, 49069 Osnabru¨ck, Germany ReceiVed February 27, 2006. In Final Form: April 26, 2006 The modification of mesoporous TiO2 film electrodes with vitamin B12 derivatives (e.g., 1, 2, or 3) yields electrodes with interesting sensing and electrocatalytic properties. So far, only coordinative bonding between the B12 derivatives and the metal oxide surface was used, and B12 was lost under conditions of extended electrocatalysis [1. Schulthess, P.; Ammann, D.; Simon, W.; Caderas, C.; Stepanek, R.; Krautler, B. HelV. Chim. Acta 1984, 67 (4), 1026-1032. 2. Mayor, M.; Scheffold, R.; Walder, L. HelV. Chim. Acta 1997, 80 (4), 1183-1189. 3. Stepanek, R. Ph.D.; ETH: Zu¨rich, 1987].1-3 We report here on a procedure that yields highly improved stabilities of the electrocatalysts toward reductive expulsion from the mesopores. It is based on cross-linking the B12 derivatives (4 or 5) equipped with multiple reaction sites in the TiO2 mesopores. The cross-linkers are multiple functionalized, one of them assisting the electron transfer from TiO2 to the Co centers via redox shuttling. The modified electrodes show high electrocatalytic reactivity toward organic halides and highly improved stability.
Vitamin B12 and related macrocyclic Co complexes, which mimic the vitamins reactivity, are used as electrocatalysts in organic synthesis,4-8 in wastewater treatment,9,10 and in sensing applications.11-15 Originally, B12 was used as a homogeneous electrocatalyst, but later, much effort has been directed toward the immobilization of vitamin B12 or one of its derivatives on electrode surfaces. A homogeneous electrocatalyst requires usually concentrations of ∼1 mol % as compared to the substrate, thus, stabilities expressed by turn over numbers of ca. 100 are needed. On the other hand, if the electrocatalyst is electrodesurface confined, turnover numbers in the range of 10 000100 000 are needed; that is, much higher catalyst stabilities are required. The “stability” of the surface-confined catalyst involves chemical stability in the course of catalysis as well as stability of its electrode surface confinement. To make electrocatalysis more durable, the density of electrocatalyst on the electrode surface can be increased by two principle techniques. Polymeric multilayers of electrocatalysts on flat electrodes or mono and multilayers on three-dimensional electrodes have been reported. * To whom correspondence should be addressed. E-mail: lowalder@ uos.de. Phone: +49 (0) 541 969 24 95. Fax: +49 (0) 541 969 33 0. (1) Schulthess, P.; Ammann, D.; Simon, W.; Caderas, C.; Stepanek, R.; Krautler, B. HelV. Chim. Acta 1984, 67 (4), 1026-1032. (2) Mayor, M.; Scheffold, R.; Walder, L. HelV. Chim. Acta 1997, 80 (4), 1183-1189. (3) Stepanek, R. Ph.D.; ETH: Zu¨rich, 1987. (4) Scheffold, R. In Modern Synthetic Methods; Spring-Verlag: New York, 1983; Vol. 3, pp 355-439. (5) Lund, H.; Baizer, M. Organic Electrochemistry, 3rd ed.; Marcel Dekker Inc.: New York, 1991; pp 809-875. (6) Njue, C. K.; Rusling, J. F. Electrochem. Commun. 2002, 4 (4), 340-343. (7) Murakami, Y.; Hisaeda, Y. Bull. Chem. Soc. Jpn. 1985, 58 (9), 26522658. (8) Scheffold, R.; Dike, M.; Dike, S.; Herold, T.; Walder, L. J. Am. Chem. Soc. 1980, 102 (10), 3642-3644. (9) Ahuja, D. K.; Gavalas, V. G.; Bachas, L. G.; Bhattacharyya, D. Ind. Eng. Chem. Res. 2004, 43 (0888-5885), 1049-1055. (10) Chang, B.-V.; Chiang, C.-W.; Yuan, S.-Y. Chemosphere 1998, 36 (3), 537-545. (11) Steiger, B.; Ruhe, A.; Walder, L. Anal. Chem. 1990, 62 (7), 759-766. (12) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1994, 66 (3), 391-398. (13) Florido, A.; Daunert, S.; Bachas, L. G. Biosens. Chem. Sens. 1992, 487, 175-185. (14) Ruhe, A.; Walder, L.; Scheffold, R. Makromolekulare Chem.-Macromol. Symp. 1987, 8, 225-233. (15) Fuchs, J., (FZKA 5816), 1-110 pp. CODEN: WBFKF5 ISSN:. Report written in German. CAN 126: 148093 AN 1996: 675827 CAPLUS, Wissenschaftliche Berichte-Forschungszentrum Karlsruhe 1996, (0947-8620).
In the case of B12-modified electrodes, a mixed long chain cobester derivative combined with an organoalkyl silane monolayer with a very low apparent surface concentration (Γ) was prepared on an ITO (indium tin oxide) electrode.16 A trialkoxysilyl B12 derivative was covalently linked to an oxidized Pt surface showing Γ ) 1.6 × 10-10 mol/cm2.17 Modifications of electrodes with a polymeric epoxy resin formed from a B12 diamine derivative and a diepoxy component showing Γ values of 1 × 10-11 to 1 × 10-9 mol/cm2 have been reported.11,18 Further, polypyrrole derivatives of B12 on flat gold or glassy carbon electrodes have been reported.19 Hydrophobic heptapropyl cobester was trapped in a silica sol-gel layer on an ITO electrode (6 × 10-11 mol/cm2).20 Recently, we have shown that the inner surface of mesoporous TiO2 electrodes can be modified with an EDTA derivative of cobester c-acid that coordinates via three carboxylic groups to TiO2.21 The B12 molecules on these three-dimensional electrodes exhibit surface concentrations in the range of 0.1 to 3 × 10-8 mol/cm2, depending on the TiO2 film thickness, and they exhibit reasonable stability thus allowing preparative scale electrocatalysis, e.g., the photoassisted radical-type intramolecular cyclization.21 Rusling et al. have extended these studies using 8-amino-cob(III)yrinic acid c-lactam (vitamin B12 hexacarboxylate) on TiO2; they reported Γ ) 1.1 × 10-8 mol/cm2 and demonstrated electrocatalytic applications, i.e., another radicaltype intramolecular cyclization and reductive eliminations.22 However, both research groups observed a loss of the electrocatalyst from the semiconductor, especially during extensive reductive electrocatalysis because polarizing the TiO2 with a negative potential reduces its affinity toward the carboxylic acid anchors of the electrocatalyst drastically. In this work, we present a new method to stabilize the vitamin B12 derivatives on the pore walls of TiO2 by cascade-type cross(16) Ariga, K.; Tanaka, K.; Katagiri, K.; Kikuchi, J.; Shimakoshi, H.; Ohshima, E.; Hisaeda, Y. Phys. Chem. Chem. Phys. 2001, 3 (16), 3442-3446. (17) Shimakoshi, H.; Tokunaga, M.; Kuroiwa, K.; Kimizuka, K.; Hisaeda, Y. Chem. Commun. 2004, 50-51. (18) Ruhe, A.; Walder, L.; Scheffold, R. HelV. Chim. Acta 1985, 68 (5), 13011311. (19) Otten, T.; Darbre, T.; Cosnier, S.; Abrantes, L.; Correia, J.; Keese, R. HelV. Chim. Acta 1998, 81 (6), 1117-1126. (20) Shimakoshi, H.; Nakazato, A.; Tokunaga, M.; Katagiri, K.; Ariga, K. Dalton Trans. 2003, 2308-2312. (21) Mayor, M.; Hagfeldt, A.; Gra¨tzel, M.; Walder, L. Chimia 1996, 50, 4749. (22) Mbindyo, J. K. N.; Rusling, J. F. Langmuir 1998, 14 (24), 7027-7033.
10.1021/la060549x CCC: $33.50 © 2006 American Chemical Society Published on Web 05/27/2006
Letters
Langmuir, Vol. 22, No. 13, 2006 5545 Scheme 1. B12 Derivatives
linking. We have developed a similar technique recently for the modification of TiO2 electrodes with electrochromic materials and for the modification of antimony-doped tin oxide (ATO) with ferrocene derivatives.23,24 In case of the B12 modification, the cross-linking procedure inhibits the electrocatalyst from diffusing out of the mesopores, i.e., no loss of electrocatalyst from its support and no accompanying reddish coloration of the solvent/electrolyte is observed even upon extended times of electrocatalysis. The B12 or cob(II)ester derivatives (1-5), presented in Scheme 1, are all candidates for the modification of TiO2. The monoacid cob(II)ester-c-acid (1),1 the triacid cob(III)ester-CO-ethanolamine-EDTA (2),2 and the heptaacid cob(III)yrin-heptaacide (3)3 have earlier been used as purely coordinative modifiers for TiO2. We now add the known heptol (4) and the hexaiodide (5) as two candidates with cross-linking potential.25 The mesoporous TiO2 film electrodes on FTO glass were made according to known procedures; for details, see the Supporting Information.26-28 After immersion of a TiO2 electrode in a millimolar solution of the vitamin B12 derivatives with one, three, or seven carboxylic acid functions, i.e., compounds 1-3, for 1 h, the electrodes E-1 to E-3 were obtained with an orange-redcolored TiO2 layer. Obviously, the three compounds are strongly adsorbed onto the inner surface of the TiO2 mesopores. The surface concentration calculated from absorbance spectroscopy at 525 nm directly after modification was Γ ) 1.1 × 10-7 mol cm-2 for E-2 (surface concentrations are based on the macroscopic projected area of the electrode, see Table 1). The Γ values for E-3 and E-1 are smaller (for details on the determination of surface concentrations, see the Supporting Information). The mono acid is a weak coordinator, and the coordination of the hepta acid requires much more space, thus diminishing the apparent surface concentration. The electroactivity of the electrodes was checked in propylene carbonate/0.5 M TBAP (23) Moller, M. T.; Asaftei, S.; Corr, D.; Ryan, M.; Walder, L. AdV. Mater. 2004, 16 (17), 1558. (24) Asaftei, S.; Walder, L. Electrochim. Acta 2004, 49 (26), 4679-4685. (25) Steiger, B.; Walder, L. HelV. Chim. Acta 1992, 75 (1), 90-108. (26) Cummins, D.; Boschloo, G.; Ryan, M.; Corr, D.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 2000, 104 (48), 11449-11459. (27) Hillebrandt, H.; Wiegand, G.; Tanaka, M.; Sackmann, E. Langmuir 1999, 15 (24), 8451-8459. (28) Campus, F.; Bonhote, P.; Gratzel, M.; Heinen, S.; Walder, L. Sol. Energy Mater. Sol. Cells 1999, 56 (3-4), 281-297.
between 0 and -0.9 V vs Ag/AgCl using cyclic voltammetry. Only the Co(II)/Co(I) wave can be observed on TiO2, because the electrode behaves as an isolator at potentials more positive than -0.1 V. The best response is observed again for the tri acid on TiO2 (E-2), and much smaller responses are obtained for electrodes E-1 and E-3 with mono and hepta acid, respectively (Figure 1a). The fact that E-2 shows a much higher surface concentration than E-3 is explained by the coordination mode and the difference in space requirements of the two B12 derivatives, i.e., 3 sitting flat on the TiO2 electrode and 2 being anchored side-on with much more conformational freedom allowing tighter packing. Surface concentrations calculated from the integration of the faradayic charge under the CV wave are much lower than Γ calculated from the absorbance. This “inconsistency” is related to the fact that the absorption spectra were taken in air, directly after adsorption of the compounds and CN- coordination, whereas under electrochemical conditions after conditioning (CV cycling), a fraction of the coordinated compounds is expelled from the pores, especially when the electrode is polarized at a negative potential. Additionally, another fraction of the B12 units observed spectroscopically on TiO2 seems not to be electrochemically active, i.e., missing electrochemical contact. This desorption process increases especially under conditions of electrocatalysis. It is not observed when using the new B12 electrodes with crosslinked subunits (see below). The preparation of electrodes with cross-linked B12 subunits is shown in Scheme 2 (for details of synthesis see the Supporting Information). It is based on a solid phase supported synthesis using the inner walls of the TiO2 mesopores as the solid support. The electrode is sequentially exposed to multifunctional compounds with electrophilic (blue in Scheme 2) and nucleophilic (green) reaction centers. The first treatment of the TiO2 involves a compound with a TiO2-anchoring group (red) and with at least one further electrophilic or nucleophilic side chain, e.g., trimesic acid chloride (6) with the -COCl group exhibiting both anchoring and electrophilic properties. The next step involves the reaction of the resulting surface with the nucleophilic hepta-alcohol B12 derivative 4. Formation of ester linkages occurs smoothly (without addition of a base). Due to the spatial arrangement of the hydroxyl terminated side chains in 4, outward-looking groups survive as free hydroxyl groups, ready to react in the next step with trimesic acid chloride to yield E-6-(4-6)1 with outward acylated and partially cross-linked B12 units. The sequential reaction with compounds 4 and 6 was repeated to yield E-6-(4-6)2 and E-6-(4-6)3. The surface concentrations obtained for E-6-(4-6)1 are in the same range as the one observed for E-3 because of the comparable binding mode for the single layer. The B12 layer grows smoothly with the cascade number n as followed by CV and by vis-absorption spectroscopy (Figure 1b and Table 1). Again, only ca. 10-20% of the surface concentration observed spectroscopically is electrochemically accessible. However, and this is in contrast to the pure coordinative immobilization, we did not observe the decoordination of the B12 derivative from the electrode and concomitant reddish coloration of the solvent upon extended potential cycling. The preparation of the E-7-(5-8)n electrodes follows the same idea. The TiO2 electrode is first equipped with the anchoring bipyridine 7 with one open nucleophilic reaction site (Scheme 2). E-7 is then treated with the hexaiodide derivative 5, yielding E-7-5. The optimum reaction time for this step was studied (Figure 2). The amount of surface confined B12 units was found to increase with reaction time up to 6 h, to go through a maximum, and to decrease again for longer reaction times.
5546 Langmuir, Vol. 22, No. 13, 2006
Letters
Table 1. Surface Concentration of Co Centers and Surface Charge on TiO2 E-1 0.30 0.008 a
E-2 1.1 0.11
E-3 0.83 0.09
E-7-(5-8)1 0.39 1.07
E-7-(5-8)2 Co-units Γ x 10 0.88
E-7-(5-8)3 -7
E-6-(4-6)1
E-6-(4-6)2
E-6-(4-6)3
0.68
1.15
1.3
0.09
0.16
0.21
2 a
[mol/cm ] 1.2
Co + viologen units Γ x 1.22
10-7
2 b
[mol/cm ] 1.33
From vis-absorption in dry state at λ ) 525 nm, ) 2512. From CV after conditioning in 0,5M TBAP/PC + 0.1% HAc, at V ) 20 or 5 mV/s. b
Figure 1. Cyclic voltammograms of B12-modified TiO2 electrodes, (a) coordinative modification E-1 (solid line), E-2 (dotted line) and E-3 (broken line); (b and c) cross-linked B12 derivatives E-6-(4-6)n and E-7-(5-8)n, respectively, (solid line, n ) 1; dotted line, n ) 2; broken line, n ) 3). E-1, E-2, E-3, and E-6-(4-6)n were measured in PC/0.5 M TBAP + 0.1% HAc, at V ) 20 mVs-1; and E-7-(5-8)n in MeCN/0.5 M LP + 0.1% HAc, at V ) 5 mVs-1. Scheme 2. Preparation of E-6-(4-6)n and E-7-(8-5)n by Cascade Type Surface Modification of TiO2 Electrodes
This is explained by the superposition of two effects, i.e., (i) kinetics of the C,N-bond formation between the terminal iodo alkyl side chains and the surface-confined bipyridine nitrogen and (ii) the desorption of anchored molecules and their loss into the bulk of the MeCN solvent. The optimized reaction time was used for the preparation of E-7-(5-8)n. As shown in Table 1, the surface concentration of B12 centers increases smoothly with the number of cascade steps n. Assuming complete reaction, each B12 derivative is linked to six bipyridinium units. Their reduction from the dicationic to the monocationic state occurs in the same potential range as the Co(II)/ Co(I) reduction wave. Thus, charge under the Co(II)/Co(I) wave may include up to six viologen units per B12 center, a value that is smaller, if one corrects for cross-linking and mono-alkylated bipyridinium
moieties present in the layer. Assuming a similar amount of Co centers in E-7-(5-8)3 and in E-6-(4-6)3, we find from the spectroscopic results in Table 1
[vio2+ + Co]/[Co] ) 1.33/0.21 ) 6.33 leading to [vio2+] ) 5.33 [Co] thus in qualitative agreement with the theory. A possible advantage of the bipyridine spacer is that it is electroactive in the same potential range as the Co(II)/Co(I) wave. It means that the viologen subunits can electrochemically wire the B12 subunits to the semiconductor surface and facilitate electron transfer. The modified electrodes were tested by cyclic voltammetry and exhaustive electrolysis with respect to the electrocatalytic
Letters
Langmuir, Vol. 22, No. 13, 2006 5547 Table 2. Exhaustive Electrolysis of Vicinal Dibromides on 0.5 cm2 B12 Modified TiO2 Electrodesa electrode E-6-(4-6)1 E-7-(5-8)1 E-7-(5-8)2 E-7-(5-8)3 E-7-(5-8)3
nsubstrate Qcalc/Qfound ncat substrate [µmol] [C] [µmol] TONb TOFc DBEt DBCH DBCH DBCH DBCH
232 146 146 146 73
44.8/2.7 28.2/25.2 28.2/26.4 28.2/24.6 14.1/12
0.05 0.05 0.065 0.08 0.08
260 2600 2104 1586 750
787 156 191 106 374
a DBCH ) dibromocyclohexane, DBEt ) 1,2 dibromoethane in propylene carbonate/0.5 M TBAP at U ) -1.1 to -1.2 V vs Ag/AgCl. b TON ) turn over number (Q/2F)/n(cat). c TOF ) turn over frequency ) TON/t (h-1).
Figure 2. Growth of the B12-layer as a function of reaction time of E-7 in a solution of 4 mg/mL 5 in MeCN as monitored by visabsorption spectra; black, TiO2 electrode (E); red, E-7 and E-7-5; pink, 1 h; yellow, 3 h; green, 6 h; dark blue, 9 h; light blue, 12 h. All electrodes were exposed to KCN before measurement. Inset: Γ vs reaction time calculated from ∆ absorbance at 613 nm using ) 12 880.
Figure 3. Electrocatalytic reduction of 1,2-dibromoethane in PC/TBAP (0.1 M); (a) CVs of [DBEt] ) 23 mM at a bare TiO2electrode (E) (broken line), at the viologen modified TiO2 electrode (E-7) (dotted line), at the B12-modified electrode (E-7-(5-8)3 (solid line); V ) 20 mV/s; (b) preparative electrolysis on E-6-(4-6)1 of 232 µmol DBEt in propylene carbonate/TBAP (0.1 M) yielding 300 TON/Co center; breakdown at 6% of total conversion (see text); stirred electrolyte, E ) -1.2 V vs Ag/AgCl.
reduction of vicinal dihalides (dibromoethane (DBEt) and transdibromocyclohexane (DBCH)) according to
RsC(H,Br)sC(H,Br)sR + 2 e- f RsC(H)dC(H)sR + 2 BrThe CVs of the electrodes E (bare TiO2), E-7 (viologen layer on TiO2) and E-7-(5-8)3 (viologen-cross-linked B12) in the presence of DBEt is shown in Figure 3a. In contrast to the unmodified or viologen-modified electrodes, E-7-(5-8)3 shows effective electrocatalysis. In Figure 3b, the time dependence of the electrocatalytic current for the reduction of 232 µmol DBEt with E-6-(4-6)1 is shown. After 20 min (Q ) 2.7 C), the electrocatalysis breaks down. A possible reason could be the formation of polymeric compounds from DBEt, ethylene, and/or intermediates of the catalysis. The small but steady decrease of the current in the middle part of the experiment (Figure 3b) is typical for an electrocatalytic reaction in which the substrate
is used up at a constant rate. In this region, the electrochemical regeneration of the active catalyst is the main rate determining step, whereas in the final sigmoidal part of the plot, the diffusion of the substrate determines the rate. Generally, similar results are obtained for the DBEt reduction using multilayer electrodes of type E-6-(4-6)1-3 or E-7-(5-8)1-3. The electrocatalytic reduction of DBCH at E-7-(5-8)n electrodes is not hampered by a breakdown of catalysis. The charge found for complete conversion is in agreement with the amount of substrate, and high turn-over numbers are achieved (Table 2). Rusling et al. found for coordinated B12/TiO2 a turnover frequency (TOF) of 100 h-1.22 For our cross-linked electrodes E-7-(5-8)n, TOF values in the same range or slightly better were observed. In all cases with the DBCH substrate, it was possible to reactivate electrocatalysis after total consumption of the substrate by addition of a small amount of DBCH. The color of the electrode becomes green after complete consumption of the substrates, and it turns again red if another portion of alkyl halide is added. The intensity of this color change is similar to that observed at the start of the experiment indicating again more or less full presence of the Co(II)/Co(I) redox couple after prolonged catalysis. As earlier shown with ferrocene or viologens derivatives, crosslinking of the electrocatalyst on the pore walls of a mesoporous TiO2 electrode can help to hold the modifier in place. Higher stability is achieved mainly because catalyst bleeding is substantially eliminated. Even if the catalyst loses its coordinative interaction with TiO2, it stays mechanically entrapped in the mesopores, because the cascade reaction yields oligomeric catalyst units which have a larger diameter than the tunnels interconnecting the mesopores. The proof of the function of the bipyridinium units acting as a molecular wire for electron transfer between the semiconductor and the B12 subunits is under investigation. Acknowledgment. S.A. and L.W. thank Dr. D. Corr NTERA (Dublin) for providing the colloidal TiO2 paste. Supporting Information Available: 1. Electrode surface modification (synthetic details); 2. analytical electrochemistry and spectroelectrochemistry; 3. controlled potential electrolysis; 4. determination of surface concentrations. This material is available free of charge via the Internet at http://pubs.acs.org. LA060549X
