4006
J . Phys. Chem. 1984, 88, 4006-4010 systems. While the spectral response of W 0 3 in the visible is insufficient for solar application, it could be improved by sensitizing the particles.
preference over the electron by the R u 0 2 deposit where it oxidizes water according to eq 1. If electrons and holes would react equally fast with RuO,, their recombination would be accelerated, decreasing the yield of the photoreaction. Reaction 5 is energy storing, the value of A,Go (eV) = 0.43 - 0.059pH. In most experiments the starting pH was 4 corresponding to A,Go = 0.2 eV. However, the pH decreases during water oxidation and this can amount to more than 2 units for cases such as the WO,/Ag+ experiment in Figure 1. Thus, the W03/Ag+ photosystem affords significant conversion of light (up to 0.34 eV per absorbed photon) into chemical energy. Such a process could be useful in devices where hydrogen and oxygen are produced in two separate photoreactions. The Ag+/Ag redox system would serve as an electron pool linking the two photo-
Acknowledgment. This work was supported by the Swiss National Science Foundation and The Gas Research Institute, Chicago, IL (subcontract with the Solar Energy Research Institute, Boulder, CO). W.E. thanks the CAPES (MEC-Brazil) for a stipend. We are grateful to Dr. Michael Neumann-Spallart for valuable discussions and to Marco Borgarello for carrying out some of the experiments. We also express our appreciation to B. Senior for help with the SEM measurements. Registry No. AgN03, 7761-88-8; W 0 3 , 1314-35-8; 02,7782-44-7;
H20,7732-18-5.
Fast Electron Storage with Colloidal Semiconductors Functionalized with Polymeric Viologen Takayuki Nakahira*+ and Michael Gratzel Institut de Chimie Physique, Ecole Polytechnique Fgdgrale, Lausanne, Switzerland (Received: September 6, 1983; In Final Form: January 31, 1984)
Stabilizationof colloidal TiO, particles is achieved with poly(viny1pyridine)polymer derivatized with viologen groups. Continuous and laser photolysis technique is applied to study reduction of the viologen (V*+) by conduction-band electrons. Reduced viologen (V'.) is generated within the time duration of the laser pulse at all pH values, indicating very rapid electron transfer. The quantum yield of the reaction approaches unity at sufficiently high converage of the particles with polymer and low laser fluence. Two-electron reduction is observed in neutral or basic medium.
Introduction Recently, there has been a surge of interest'-" in the use of colloidal semiconductor particles as light-harvesting units in artificial photosynthesis and as models for fundamental studies of luminescence phenomena and charge-transfer processes at the semiconductorsolution interface. A particularly intriguing feature of these dispersions is their transparent nature allowing for the application of transient ~pectroscopy'~ to analyze the dynamics of interfacial charge-transfer events. Furthermore, the virtual absence of light scatteirng from colloidal dispersions of compounds such as CdS had led to the discovery of very interesting luminescence phenomena3s9-'*and made their rigorous study and interpretation fea~ib1e.I~ In the present study we pursue our investigations of conduction-band processes involving colloidal TiOz particles. A poly(vinylpyridine) polymer functionalized with viologen groups is introduced as a stabilizing agent for the semiconductor particles. The spatial arrangement achieved with this assembly is such that the viologen moiety is in close proximity to the semiconductor surface. It will be shown that this configuration leads to very rapid interfacial electron transfer and efficient storage of the charge on the redox polymer. Experimental Section Preparation of Colloidal TiO,. Colloidal TiOz (anatase) solution was prepared by adding T i c & (Fluka, priss.) to water at 0 OC, followed by dialysis to ca. pH 3, as described p r e v i o ~ s l y . ~ ~ ' ~ Colloidal TiO, prepared by this method has been well characterized: electron microscopic as well as quasi-elastic light scattering measurements gave an average particle radius of ca. 50 A: The point of zero { potential (ZZP) of the particles has been determined to be pH 4.7. The onset of band gap excitation (3.2 t Visiting scientist from the Department of Applied Chemistry, Faculty of Engineering, CHIBA University, Chiba, Japan.
0022-3654/84/2088-4006$01.50/0
eV) is clearly seen from the absorption spectrum, Le., sharp rise in extinction coefficient below 380 nm. Photoplatinization of TiO, colloid was performed in a similar manner as described earlier: 'J* a deoxygenated 10-mL solution containing 2 g/L TiOz and 0.25 g/L H,PtCl, (pH 1.6) was irradiated in a Pyrex flask with a 450-W Xe lamp. Completion of photoplatinization was indicated after 13 h of irradiation by the fact that H, evolution occurred with a constant rate of ca. 20 pL/h.
(1) D. Duonghong, E. Borgarello, M. Gratzel, J . Am. Chem. SOC.,103, 4685 (1981). (2) K. Kalyanasundaram, E. Borgarello, D. Duonghong, and M. Gratzel, Angew. Chem., 93, 1012 (1981). (3) D. Duonghong, J. J. Ramsden, and M. Gratzel, J . Am. Chem. SOC., 104, 2977 (1982). (4) M. Gratzel and A. J. Frank, J . Phys. Chem., 86, 2964 (1982). ( 5 ) J. Moser and M. Gratzel, Helu. Chim. Acta, 65, 1436 (1982). (6) A. Henglein, Ber. Bunsenges. Phys. Chem., 86, 241 (1982). I (7) J. Kuczynski and J. K. Thomas, Chem. Phys. Lett., 88, 445 (1982). (8) M. Gratzel and J. Moser, Proc. Nutl. Acad. Sci. U.S.A.,80, 3129 (1983). (9) A. Henglein, Ber. Bunsenges. Phys. Chem., 86, 301 (1982). (10) R. Rosetti and L. Brus, J . Phys. Chem., 86, 4470 (1982). (1 1) R. Rosetti, S. M. Beck, and L. E. Brus, J . Am. Chem. SOC.,104,7321 (1982). (12) K. Metcalfe and R. E. Hester, J . Chem. Soc., Chem Commun., 133 (1983). (13) M. A. Fox, B. Lindig, and C. C. Chen, J . Am. Chem. SOC.,104,5828 (1982). (14) M. Gratzel, Acc. Chem. Res., 14, 376 (1981). (15) J. Moser and M. Gratzel, J . Am. Chem. SOC.,105, 6547 (1983). (16) J. J. Ramsden and M. Gratzel, J . Chem. SOC.,Faraday Trans. 1 , 8 0 , 119 (1984). (17) K. Chandrasekaran and J. K. Thomas, Chem. Phys. Lett., 97, 357 (1983). (18) B. Kraeutler and A. J. Bard, J . Am. Chem. SOC.,100,4318 (1978).
0 1984 American Chemical Society
Fast Electron Storage with Colloidal Semiconductors
Preparation of Viologen Polymers. l-Hexadecyl-4,4’-bipyridinium bromide, prepared from 4,4‘-bipyridyl and l-bromohexadecane, was treated with 1,3-dibromopropane in refluxing acetonitrile. l-Hexadecyl-l’-(3-bromopropyl)-4,4’-bipyridinium dibromide, which precipitated, was purified by repeated reprecipitation from methanol into acetonitrile (‘H N M R (CD30D) 6 0.2-1.7 (m, 31 H), 2.1-2.5 (m, 2 H), 3.0-3.3 (t, 2 H), 4.3-4.7 (t, 4 H), 8.2-8.4 (m, 4 H), 8.7-9.0 (m, 4 H)). 1-Methyl-1’-(3bromopropyl)-4,4’-bipyridiniumdibromide was prepared in a similar manner (‘H N M R (DzO) b 2.2-2.6 (m, 2 H), 3.1-3.4 (t, 2 H), 4.3 (s, 3 H), 4.5-4.8 (t, 2 H), 8.2-8.4 (m, 4 H), 8.7-9.0 (m, 4 H)). Treating poly(4-vinylpyridine) ( M , = 68700) with the above-prepared viologen derivatives afforded viologen polymers. a typical procedure is as follows: Poly(4-vinylpyridine) (0.15 g) was added to a D M F solution of l-hexadecyl-l’-(3-bromopropyl)-4,4’-bipryidinium dibromide (0.19 g). The mixture was stirred at 45 OC for 48 h. l-Methyl-l’-(3-bromopropyl)-4,4’bipyridinium dibromide (0.13 g) was then added to one-third of the above reaction mixture, which was further stirred at 45 OC for 48 h. Methyl tosylate (0.26 g) was then added. After being stirred a t room temperature for 24 h, the reaction mixture was poured into ether for precipitation. The dried product was dissolved in water and dialyzed extensively against 0.5 N NaCl solution for anion exchange and then against water. The polymer was freeze-dried. The composition of the polymer was found to be 6 mol % in 1-hexadecyl- l’-propyL4,4’-bipyridinium (Cl6VZ+) and 18 mol % in 1-methyl- l’-propyl-4,4’-bipyridinium (MeV2+) by ‘H FT N M R spectrometry. (In addition to bands due to viologen groups, the backbone poly( 1-methyl-4-vinylpyridinium chloride) gives bands at b 1.0-2.7 (m, 3 H), 4.1 (s, 3 H), 7.1-7.8 (m, 2 H), and 8.2-8.6 (m, 2 H). The composition of the polymers prepared is characterized by a-d, denoting the mole percentage of C16V2+,MeV2+,N-hexadecylpyridinium, and N-methylpyridinium groups, respectively. Polymers prepared and used in the present study are as follows: 6,O-0,94, 6,18-0,76, 6,60-0,34, 0,9-6,85, and 0,O-8,92.
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4007
a, [nml Figure 1. Spectral changes upon UV illumination of a solution containing 0.5 g/L Ti02, 0.09 g / L 6,18-0,76 ([V2+] = 1 X low4M), and 0.91 g/L 2.5 min, (. .) 7.5 0,O-8.92; pH 4.3: (-) 0 s, (-- -) 10 s, (--) 35 s,
.
(-e.-)
min.
Figure 2. Spectral changes upon UV illumination of a solution containing 0.5 g / L Ti02, 0.09. g/L 6,18-0,76 ([V2+] = 1 X M), and 0.91 g/L 30 min. 0.0-8,92; pH 9.7: (-) 0, (-----) 1 (---) 5, (---) 10, and (e..)
Apparatus. A 5-mL sample containing 0.5 g/L TiOz colloid and 1 g/L polymer (viologen polymer viologen-free 0,O-8,92) in a quartz cell was deoxygenated by argon bubbling after its pH was adjusted with HCl or NaOH. Laser flash experiments were performed by using the 347.1-nm flash of a J K 2000 frequency-doubled ruby laser with fast kinetic spectroscopy technique to detect transient species. The incident laser energy per pulse was ca. 50 mJ which at 0.5 g of TiOz/L corresponds to absorption of 2.4 X photons/L of solution. Since the TiOz particle concentration is 3.7 X lo-’ M, the number of photons absorbed in one particle is calculated as ca. 640 at the highest laser intensity employed in this study. This results in a maximal charge carrier density of 1.22 X loz1cm3. Continuous irradiation was carried out with a XBO 450-W Xe lamp (Osram) equipped with a 15-cm water jacket (Pyrex) to remove IR radiation. UV-visible absorption spectra were recoreed on a Cary-219 (Varian) spectrophotometer.
+
Results and Discussion Viologen Polymers as Protective Agents for Ti02 Colloid. It is well-known that polymers of appropriate molecular weights and charge densities act as stabilizers for various inorganic as well as organic colloids in water. The polymers used in the present study were prepared from poly(4-vinylpyridine) of 68 700 in
number-average molecular weight and possess relatively high charge densities along the polymer backbone. They exhibited colloid-stabilizing effect when used in concentrations 2 1 g/L, giving optically clear solutions of TiOz (0.5-1 g/L) at various pHs examined, Le., pH 2-l2,I9 which are stable over at least several weeks. Spectral Changes Caused by Continuous Irradiation. Spectral changes observed with samples containing 6,18-0,76 ( [ P I = 1 x lo4 M) at two different pHs are shown in Figures 1 and 2. At relatively low pH (Figure l ) , bluish purple color, with absorption maxima at 540 and 360 nm, develops upon UV irradiation due to the formation of viologen radical cation monomer and dimer, the latter being the major product: 2o V2+
-
+ eCB-
2v+.
-
V+.
(1)
(V+.),
(2)
After approximately 2.5 min, the system appears to reach a photostationary state involving hole (h’) oxidation of monomeric and dimeric viologen radical cations1*followed by re-reduction by conduction-band electrons (eCB-). An alternative hole reaction competing with the oxidation of viologen radicals is the formation of hydroxyl radicals which are adsorbed to the surface of TiOz particles21,22 ~~
~~
~~~
(19) When unprotected Ti02 colloids coagulate in the pH range of 3.5-9. (20) E. M. Kosower and J. C. Letter, J . Am. Chern. Soc.,86, 5524 (1964).
4008
The Journal of Physical Chemistry, Vol. 88, No. 18. 1984 h+
+ H20
+
(OH*),,j,
+ H+
Nakahira and Gratzel
(3)
The latter are likely to react with the viologen polymer present also at the particle surface leading to its oxidative destruction. In fact, as seen in Figure 1, the yield of viologen radical cations diminishes gradually, suggesting slow decomposition of the viologen moiety during the photostationary cycle. (When irradiated for a prolonged time and then aerated, the solution exhibits a new absorption around 380 nm which was absent prior to irradiation.) This decomposition process will be discussed again in the context of the laser flash experiment. At a higher pH (Figure 2), viologen radical cations are formed more rapidly, the maximum absorption being reached within 1 min. This, however, is followed by a spectral change over a period of approximately 10 min with an isosbestic point at ca. 460 nm. The new absorption is due to the second reduction of viologen radical cation to the doubly reduced form V0.5 (V'.),
-
+ eCB- vo + v+.
(4)
The slower groth of Vo absorption indicates a considerably lower rate for this process: the second reduction proceeds only after virtual completion of the first reduction. After reaching a maximum, the 375-nm absorption of Vo diminishes as irradiation is continued, suggesting again that the viologen moiety decomposes while undergoing a photostationary cycle, Le., hole oxidation of Vo to V+. and subsequent re-reduction by conduction-band electrons to Vo. At an intermediate pH, Le., pH 7.8, the viologen radical cation is also converted to its doubly reduced form. The second reduction at this pH, however, proceeds somewhat more slowly and affords a slightly lower yield of Vo due to decomposition of the viologen moiety during the prolonged irradiation. The above dependence of spectral change on pH is consistent with the shift of conduction-band potential of Ti02 colloid with pH described earlier, Le., EcB = -0.1 1 - 0.059 pH V.','s However, the redox potential for the first reduction step of the viologen is likely to be substantially shifted anodically, compared with Eo = -0.446 V of methylviologen,due to the presence of the positively charged adjacent pyridinium group of the polymer. The shift reduces the overpotential for electron transfer from T i 0 2 to the viologen. By the same reasoning, the second redox potential may also be shifted anodically compared with -0.82 V for methylviologen. (The redox potentials were recently measured by Dr. T. Geiger in our laboratory, using a graphite electrode coated with 6,O-0,94 in 0.1 M KCl. Under the film condition, the values are found to be E l = -0.21 V and E2 = -0.63 V (vs. "E).) The shift in redox potential alone, however, is not sufficient to rationalize the easy formation of Vo a t pH as low as 7.8. (A surfactant derivatized viologen requires at least pH 9 for twoelectron reduction.*) Intimate contact between T i 0 2 and polymer-bound viologen, as well as the increased hydrophobicity of the product leading to its precipitation onto the particle surface, must be responsible for the efficient first as well as second reduction of the latter. Another point to be noted from Figures 1 and 2 is a virtually common maximum concentration of viologen radical cation ([V'.] 2[(V+.),]) that is estimated to be (5 f 0.5) X M by using €540 = 13500 (dimer) and 6750 (monomer) M-' cm-l and e602 = 3360 (dimer) and 13500 (monomer) M-' cm-1.23 The value is about 50% of the initial viologen concentration. At pHs where the second reduction is observed, e.g., pH 9.7, the maximum Vo concentration is again approximately 5,X M (e37s = 40000 M-' Figure 3 shows how the content of viologen in polymer affects the extent of dimer formation. Note that these polymers differ only in the content of "hydrophilic" viologen. The ratios of dimer
+
(21) A. Fujishima and K. Honda, Nature (London),238, 37 (1972). (22) C. D. Jaeger and A. J. Bard,-J. Phys. Chem., 24, 3146 (1979). (23) T. Watanabe and K. Honda, J . Phys. Chem., 86, 2617 (1982). (24) S. Htinig, J. Gross, and W. Schenk, Liebigs Ann. Chem., 20, 324 (1973).
300
400
500
600
700
~[nml
Figure 3. Absorption spectra of samples containing polymers of different viologen contents after ca. 5-min irradiation at pH 5.2. Samples contain 0.5 g/L Ti02 and 1 g/L polymer (viologen polymer + 0,O-8,92) ([V2'] = 1X
M): (-)
6,O-0,94, (-.-) 6,18-0,76,
(-sa-)
6,60-0,34, ( - - - )
before irradiation.
Figure 4. Oscilloscope trace of 602-nm absorption of a sample containing 0.5 g/L TiOz,0.09 g/L 6,18-0,76 ([V"] = 1 X M), and 0.91 g/L 0,O-8,92. Excitation: 347.1 nm. to monomer are estimated to be 27/73 for 6,O-0,94, 60140 for 6,18-0,76, and 67/33 for 6,60-0,34. These ratios do not change significantly with pH. It is rather interesting that even a polymer with only 6% "hydrophobic" viologen still undergoes significant dimer formation, indicating a close proximity of viologen moieties when they are adsorbed ohto Ti02. Laser Flash Experiments. Nonirradiated Samples. As shown in Figure 4, even at a pH as low as 5.3, essentially all electron transfer from T i 0 2 to viologen takes place within the duration of the laser pulse (
