J. Phys. Chem. 1986, 90, 1107-1 115
1107
Colloidal Semiconductors in Systems for the Sacrificial Photolysis of Water: Sensitization of TiO, by Adsorption of Ruthenium Complexes D. Neil Furlong,* Darrell Wells, and Wolfgang H. F. Sasse CSIRO Division of Applied Organic Chemistry, Melbourne. Australia (Received: July 19, 1985)
Adsorbed tris(2,2'-bipyridine-4,4'-dicarboxylic acid)ruthenium(II) molecules (Ru"(acid),) sensitize the production of H2 that occurs when Pt/TiO, dispersions are illuminated with visible light in the presence of EDTA as sacrificial electron donor. Ru"(acid), adsorbs on Ti02 at pH values below the isoelectric point of the Ti02 (6.1) in response to electrostatic attraction between its dissociated species and positively charged T i 0 2 particles. At pH 3, sensitization increases with the density of Ru"(acid), in the first adsorbed layer but decreases as subsequent multilayers are deposited. Multilayers can be deposited at pH 3 because of the very low solubility of Ru"(acid),. Sensitization decreases when the pH is increased above 3 because less Ru"(acid), adsorbs at higher pH. Sensitization by Ru"(acid), at pH 10, with TEA as donor, is much less effective because no adsorption occurs on negative TiOz particles. Better sensitization at pH 10 is attained when Ru"(acid), is first adsorbed at pH 3 and the particles are then dried and redispersed at pH 10. Ru(bpy),'+ did not give effective sensitization at pH 3 or 10 despite some adsorption at pH 10. EDTA adsorbs strongly onto Ti02 and suppresses the adsorption of Ru"(acid),. EDTA displaces Ru"(acid), anions into solution at pH 4;at pH 3 protonated Ru"(acid), molecules remain on particles but are displaced from the first adsorbed layer. Displacement by EDTA leads to decreased sensitization by Ru"(acid),-rapidly if EDTA is preadsorbed onto TiOz, slowly if RuIT(acid), is first adsorbed. In the latter case sensitization also decreases with time because the production of H2 causes the pH to increase which leads to desorption of Ru"(acid),. Desorption of Ru"(acid), from the first adsorbed layer is not reversed by readjustment of the pH to 3 because the EDTA present will preferentially adsorb. Therefore, in contrast to band-gap systems, sensitized systems are not regenerated by readjustment of pH and addition of electron donor. Clearly, sensitization requires adsorption of sensitizer in the first adsorbed layer in addition to favorable redox potentials for electron injection from sensitizer into the conduction band of the TiOz particles. Multilayers of sensitizer do not assist electron injection but instead reduce the amount of light available to the first adsorbed layer.
Introduction In recent years the spectral sensitization1 of semiconductors has been actively studied, particularly in solar energy conversion systems involving the photolysis of water to produce hydrogen (H2). Sensitization enables the use of semiconductors that are not intrinsic absorbers of visible light and do not suffer destructive photodecomposition.2 A variety of semiconductor electrodes have been studied, for example, using Zn0,3-5 SIIO,,~.~ SrTi03,8-10and TiOz,1'-'5 with sensitization by surface dye layers or metal dopants. The need to have inexpensive and stable semiconductor catalysts with high specific catalytic activity has led to the use of colloidal dispersions of Ti02. The photocatalytic properties of T i 0 2 have long been known in the paint industry.I6 The photogenerated charge centers in TiOz are thermodynamically suited to promote both the reduction of aqueous hydrogen ions and the oxidation of aqueous hydroxyl i0ns.l' In recent years Gratzel and co-workers18-26and other^^'-^^ have ( I ) Carroll, B. H. Photogr. Sci. Eng. 1977, 21, 151. (2) Gerischer, H. J . Electroanal. Chem. 1977, 82, 133. (3) Broich, B.; Heiland, G. Surf. Sci. 1980, 92, 247. (4) Matsumura, M.; Mitsuda, K.; Tsubomura, H . J . Phys. Chem. 1983, 87, 5248. ( 5 ) Shimidzu, T.; Iyoda, T.; Koide, Y . ;Kanda, N. N o w . J . Chim. 1983, 7 , 21. (6) Ghosh, P. K.; Spiro, T. G. J . A m . Chem. SOC.1980, 102, 5543. (7) Mernrning, R. Surf. Sci. 1980, 101, 551. (8) Mackor, A,; Blasse, G. Chem. Phys. Lett. 1981, 77, 6. (9) Breddels, P. A.; Blasse, G. Chem. Phys. Lett. 1981, 79, 209. (IO) Chang, B.-T.; Campet, G.; Claverie, J.; Hagenmuller, P. Bull. Chem. Sor. Jpn. 1984, 57, 2574. (11) Clark, W. D. K.; Sutin, N . J . A m . Chem. SOC.1977, 99, 4676. (12) Spitler, M. T.;Calvin, M. J . Chem. Phys. 1977, 66, 4294. (13) Hamnett, A.; Dare-Edwards, M . P.; Wright, R. D.; Seddon, K. R.; Gocdenough, J. B. J . Phys. Chem. 1979, 83, 3280. (14) Dare-Edwards, M. P.; Goodenough, J. B.; Harnnett, A,; Seddon, K. R.; Wright, R. D. Faraday Discuss. Chem. SOC.1980, 70, 285. (15) Giraudeau, A,; Fau, F.-R. F.; Bard, A. J. J . Am. Chem. SOC.1980, 102, 5137. (16) Renz, C. Helo. Chim. Acta 1921, 4 , 961. (17) Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 50. (18) Borgarello, E.; Kiwi, J.; Pelizetti, E.; Visca, M.; Gratzel, M. Nature 1981, 289, 158.
reported on the sensitized production of H2 from aqueous dispersions of TiOz. In early work adsorbed ruthenium complexes were said to sensitize T i 0 , in both nonsacrificial'8-20 and sacrif i ~ i a (Le., l ~ ~in the presence of an added donor) systems. Subsequently metal dopants?l surface hydroxyquinoline complexes,22 surface photoderived ruthenium complexes,23and interparticle electron transfer25 were shown to lead to sensitization. Most r e ~ e n t l ythe ~ ~electrostatic .~~ adsorption of has reemerged as a means of promoting "highly efficient"26 and "advanced"29 sensitization. Light scattering prevents the accurate calculation of quantum efficiencies of H2production from colloidal dispersions. However, the rate ( R )of production of H2 from various systems can be validly compared if the intensity of the incident light is comparable, which appears to be so for all the reports cited With the exception of one early result18 R values for to 5 X nonsacrificial systems have been in the range 5 X mol rnin-' cm-, for the past several years. R values for sacrificial systems, when optimized with regard to the concentration of the donor, have been around mol min-' cm-,. Clearly the nonsacrificial photolysis of water is still relatively inefficient and the development of better catalysts for the oxidation (19) Duonghong, D.; Borgarello, E.; Gratzel, M . J . Am. Chem. Soc. 1981, 103, 4685. (20) Borgarello, E.; Kiwi, J.; Pelizetti, E.; Visca, M.; Gratzel, M. J . A m . Chem. SOC.1981. 103. 6324. (21) Borgarello, E.;Kiwi, J.; Gratzel, M.; Pelizetti, E.; Visca, M. J . Am. Chem. SOC.1982, 104, 2996. (22) Houlding, V. H.; Gratzel, M. J . Am. Chem. SOC.1983, 105. 5695. (23) Duonghong, D.; Serpone, N.; Gratzel, M. Helu. Chim Acta 1984,67, 1012. (24) Moser, J.; Gratzel, M. J Am. Chem. SOC.1984, 106, 6557. (25) Serpone, N ; Borgarello, E.; Gratzel, M. J . Chem. SOC.,Chem. Commun. 1984, 342. (26) Moser, J.; Humphry-Baker, R.; Desilvestro, J.; Liska, P ; Gratzel, M.; Augustynski, J. J . Am. Chem. Soc., in press. (27) Hashimoto, K.; Kawai, T.; Sakata, T. Nouu. J . Chim. 1983, 7 , 249. (28) Borgarello, E.; Pelizetti, E.; Ballardini, R.; Scandola, F. Nouu. J . Chim. 1984;8, 567. (29) Shimidzu. T.;Ivoda, T.; Koide, Y . J . Am. Chem. SOC.1985, 107, 35. (30) Furlong, D. N : A u s t . J . Chem. 1982, 35, 911 (31) Furlong, D. N.; Sasse, W H . F. Colloids Surf. 1983, 7 , 29. (32) Furlong, D. N.; Sasse, W. H. F. Colloids Surf. 1983, 7 , 115
0022-3654/86/2090-1107$01.50/00 1986 American Chemical Society
1108 The Journal of Physical Chemistry, Vol. 90, No. 6, 1986
of water remains as a major step toward efficient solar photolysis. Sacrificial systems may however find application in areas where a need exists for an oxidative process other than that of water. Recent reports indicate that irradiated dispersions of TiO, may find application in waste water treatment33s34and oxidative synt h e s i ~ ; ~of~ particular ?,~ interest in synthesis is the possibility of selectivity toward reaction product^.^ The present study reports on the sensitization of TiO, by ruthenium complexes. Sensitization is achieved by electrostatic adsorption of these complexes and assessed by monitoring the production of H2 in the presence of an electron donor (EDTA or TEA). Our work focuses on the tendencies of all species present to accumulate at the TiO,/aqueous solution interface and the effects that competitive adsorption can have on the performance of the system.
Experimental Section Materials. ( a ) Titanium Dioxide. Degussa P25 (anatase, specific surface area 5 1 m2 g-l, average particle size ca. 30 nm) was used, as received, for all adsorption and electrophoresis experiments. Photolysis experiments were performed using P25 and TiO, H1, the latter being prepared by acid hydrolysis of titanium isopropoxide and cleaned by ultrafiltration at pH 3.5 before use.38 H I consists of anatase particles of average 9.0 nm size and was chosen for photolyses because of its relatively low scattering of light. We have discussed p r e v i ~ u s l ythe ~ ~similarity ~ ~ ~ in the surface characteristics between P25 and H I . ( b )Platinum. Platinum sols (designated BI4O) were prepared by the reduction of H,PtCl, at 90 "C by using aqueous sodium citrate. Excess citrate was removed by ion exchange.40 The sols consist of particles of 2.0 nm average size. (c) Ruthenium Complexes. Tris(2,2'-bipyridine)ruthenium(II) dichloride (henceforth designated Ru(bpy),*+) was prepared as described by Braddock and M e ~ e r . ~ ' Tris(diethy1 2,2'-bipyridine-4,4'-dicarboxylate)ruthenium(II) bisperchlorate (Ru"(ester),) was prepared as described by S a ~ s e . ~This , ester was shown by HPLC to be converted into tris(2,2'-bipyridine-4,4'dicarboxylic acid)ruthenium( 11) (Ru"(acid),) after stirring at pH 1 1 for 2 h. All ruthenium complexes were analytically pure. ( d ) Other. Analytical reagent (AR) grade disodium ethylenediaminetetraacetic acid (EDTA) and triethanolamine (TEA) were used as electron donors. Solutions of electrolytes (NaNO,) and acid and base (HCI, KOH) were also prepared from AR grade chemicals. Triply distilled water (maximum conductivity 0.9 pS cm-I) and research grade gases were used. Methods. ( a ) Photolysis. A 5-cm3 volume of dispersion was illuminated at 25 "C with a 300-W UV boosted xenon lamp (Cermax LX3OOuv). The light beam (incident power 0.1 W) was always filtered by 11 cm of water (to remove infrared radiation) and a Pyrex filter (absorbance >4 at wavelengths below 300 nm). For sensitization experiments a 420-nm filter (absorbance >4 at wavelengths below 420 nm) was in place in order to eliminate band-gap excitation of TiO, particles (wavelengths below 370 The rate of formation of H, was determined by gas c h r ~ m a t o g r a p h ywith ~ ~ a precision of IO-" mol min-' cm-,. For most experiments the TiO, sol, Pt sol, sensitizer, and donor solutions were mixed together under specified pH conditions. For example, the designation TiO,/Ru"(acid),/pH 7 3/Pt/EDTA shows the sequence of mixing of components, the pH of mixing
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(33) Peyton, G. R.; De Berry, D. W. U S . Patent 874938, 1982. (34) Barbeni, M.; Pramauro, E.; Pelizetti, E.; Borgarello, E.; Gratzel, M.: Serpone, N. N o m . J . Chim. 1984, 8, 547. (35) Fujihara, M.; Satoh, Y . ;Osa, T. Nature 1981, 293, 206. (36) Ait-Ichou, I.; Formenti, M.; Pommier, B.; Teichner, S. J . J . Caral. 1985, 91, 293. (37) Harada, H.; Sakata. T.; Ueda. T. J . Am. Chem. SOC.1985,107, 1773. (38) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J . Phys. Chem. 1985, 89, 626. (39) Furlong, D. N.: Wells, D.; Sasse, W. H. F. J . Phys. Chem. 1985, 89, 1922. (40) Furlong, D. N.; Launikonis, A,; Sasse, W. H. F.; Sanders, J . V . J . Chem. SOC., Faraday Trans. 1 1984, 80, 571. (41) Braddock, J. N.;Meyer, T. J. J . A m . Chem. SOC.1973, 95. 3158. (42) Sasse. W. H. F., unpublished results.
Furlong et al. and any changes made to the pH of the mixture. Unless otherwise specified the mixing time was 30 min. For a few experiments TiO,/Pt aggregates and TiO,/sensitizer mixtures were dried in air prior to being redispersed by sonicating in water and mixed with other components. The concentration of TiO, for all photolysis experiments was 0.43 g dm-, (ca. 3 X lo'' particles dm-3) mol dm-, (also ca. 3 X lo" particles and of Pt was 1.2 X dm-3). ( b ) Adsorption/Desorption. Uptakes of Ru"(acid), by TiO, (P25) were determined by difference. Dispersions were equilibrated at 25 " C for 30-60 min and the TiO, then removed by centrifugation (6000 rpm, 30 min). Blank experiments confirmed that TiO,(P25) was quantitatively removed by such centrifugation. The concentration of Ru"(acid), in aqueous solution was determined from the absorbance at 467 nm (Cary 217 UV/vis spectrophotometer) (c) Microelectrophoresis. Electrophoretic mobility was determined by using a Rank Bros. Mark I1 apparatus in conjunction with a flat ~ e 1 1 . ~The ~ , average ~~ deviation in measured mobility varied from 0.05 to 0.1 bm/s per V/cm, the larger deviations occurring when mobilities were small (
