8720 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA J. Phys. Chem. zyxwvuts 1990, zyxwvu 94. 8720-8726 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM Vectorial Electron 1n)ection into Transparent Semiconductor Membranes and Electric Field Effects on the Dynamics of Light-Induced Charge Separation Brian O’Regan,2 Jacques Moser,2 Marc Anderson,’ and Michael Gratzel*?* Institut de Chimie Physique, Ecole Polytechnique F$d?rale de Lausanne. CH- zyxw I015 Lausanne. Switzerland, and Water Cheniistrj*and Material Science lnstit Ute, Unicersity zyxwv of Wisconsin, Madison. Wisconsin 53706 (Rereired: Ma), 7. zyxwvutsrqp 1990) Transparent titanium dioxide membranes (thickness 2.7 pm) were prepared by sintering of 8-nm colloidal anatase particles on a conducting glass support. The dynamics of charge recombination following electron injection from the excited state of RuL, (L zyxwvutsrqpon = zyxwvuts 2,2’-bipyridine-4,4’-dicarboxylic acid) into the conduction band of the semiconductor were examined under potcntiostatic control of the electric field within the space charge layer of the membrane. Biasing the Fermi level of the TiO, positive of the flat-band potential sharply reduced the recombination rate, a 1000-fold decrease being associated with ;I potcntiul chnngc of only 300 mV. Photoelectrochemical experiments performed with the same RuL,-loaded membrane in Kal-containing watcr show the onset of anodic photocurrcnt to occur in the same potential domain. Forward biasing 01‘ the nicmbranc potcntial impairs photosensitized charge injection turning on the photoluminescence of the adsorbed sensitizer. Introduction Tlic dynmics of heterogeneous photochemical electron-transfer reactions are frequently controlled by local electrostatic potential gradients present at the This plays a crucial role in molecular devices for light-induced charge separation and solar energy conversion. In the case of semiconductor-liquid junctions. the dcpletion laycr ficld present within the solid impairs the re- combination of charge carriers formed by light excitation6 With a convcntional scmiconductor electrodes, these kinetic effects of the space chargc arc difficult to monitor directly and so far few timc-resolvcd studies have been reported.’ In the present work. we take advantage of the transparent nature of a newly developed semiconductor membraneE to examine the influence of the depletion layer field on the rate of charge carrier recombination following photoinduced electron injection from surfxc-adsorbed dye molecules. Thin titanium dioxide membranes (I) Water Chemistry and Matcrial Scicncc Institute, University of Wis- consin. Madison. WI 53706. (2) lnstitut de Chimie Physique. Ecole Polytechnique Fidirale de Lau- sannc. IO1 5 I.ausannc. Swit7crland. (3) (a) Gcrischcr. H. Angew. Chem. 1988, 100, 630. (b) Meier. H. J. Phvs. Chem. 1%5,69, 724. (c) Hauffe. K.; Range, J. zyxwvutsr 2. Nuturforsch. B 1968. 238. 736. (d) Watanabe. T.: Fujishima. A,: Honda. K. In Energy Resources Through Photochemistry and Catal.vsis: Academic Press: New York. 1983. (c) Tributsch. H.: Calvin. M. Photochem. Photobiol. 1971. 14. 95. (0 Karnet. P. V.: Fox. M. A. Chem. Phys. Lett. 1983. 102, 379. (8) Memming. R. Prog. Surt Sci. 1984. 17. 7. (h) Krishnan. M.: Zhang. X.; Bard, A. J. J. Am. Chem. Sur. 1984. 106. 7371. (i) Gerischer. H.; Willig. F. Top. Curr. Chem. 1976. 61. 31. (k) Hashimoto. K.: Sakata. T. J. Phys. Chem. 1986. 90. 4474. (4) Spitlcr. M. J. Electroanul. Chem. 1987. 228,69. For related work, cf.: Blosscy. D. F. Phi~ Rei,. 1974. 139. 5183. Willig. F. Chem. Phyc. Left. 1976. 40. 331 (5) (a) Photoinduced Electron 7run.rfer: Fox. M. A,. Chanon, M.. Eds.: Elsevier: Amsterdam, 1988; Part A-4. (b) Gratzel. M. Heterogeneous Phut~,[.hc,,,ri[,trl Elet,rroti Transfer; CRC Prcss: Boca Raton. FL, 1989. (6) (a) Wrighton. M. S. Arc. Chem. Res. 1979. 12. 303. (b) Gerischer. H. Pirre App1. Chem. 1980. 52. 2649. (c) Heller. A. Arc. Chem. Res. 1981. 14. 1.24. (7) (a) Bitterling. K.: Willig. F. J. Electroanal. Chem. 1986. 204. 21 I. (b) Rlnn. M A,: Fit7gerald. E. C.: Spiller. M. T. J. Phys. Chem. 1989, 93. 61 50. (8) We uish to draw aitcntion to studies by Fendler et 31. concerning the formation. chor~icteri7ntion. and photoelectrochemistry of sulfide semicon- ductor particles supported by bilayer lipid membranes, e.g.: Zhao. X. K.: Baral. S.; Rolandi. R.: Fendler. J. H. J. Am. Chem. Soc. 1988. 110. 1012 Bard et al. invcstigatcd thin CdS semiconductor films, c.g.: Finlayson. M. F.: Wheeler. B. L.: Kakuta. N.: Park. K. H.: Bard, A. J.: Fox. M. A,: Webber. S. E.. White. J. M. J. Phj~. Chem. 1985. 89. 5676. Liu. C.: Bard. A. J. J. PhJ.7 Chvnr. 1989. 93. 7749. The TiO? membranes introduced here distin- guiah thcinrclvcs by thcir transparcnt and microporous character. High light-harvcrting cfficicncics arc achicvcd in this fashion at monolayer d l c covcragc allowing for iipplication of timeresolved optical transmission spec- troscupq 0022-3654/90~2094-8720$02.50~0 have been prepared on a conducting glass support allowing for potentiostatic control of the potential gradient within the semi- conductor. RuL3 (L = 2.2’-bipyridine-4,4’-dicarboxylic acid) adheres strongly to the surface of TiO19 and is used as a model chromophore. Time-resolved absorption and transient current nicasurcmcnts are applied for the first time in conjunction with laser photolysis to scrutinize the dynamics of charge carrier formation and recombination events in this system. Experimental Section Prepuration of Transparent TiO, Membranes Supported on Conducting Glass Sheets. Transparent TiO, membranes were produced by deposition of colloidal particles on a conducting glass support. Thc procedure applied was similar to that used for the preparation of unsupported films.I0 TiOz colloid solutions were prepared by hydrolysis of titanium isopropoxide, Ti(OCH(CH,),),, as follows: Under a stream of dry nitrogen, 125 mL of Ti(OCH(CH3)2)4 (Aldrich) was added to a 150-mL dropping funnel containing 20 mL of 2-propanol (Fisher, ACS reagent grade). The mixture was added over IO min to 750 mL of distilled deionized water, stirring vigorously. During the hydrolysis a white precipitate formed. Within 10 min of the alkoxide addition, 5.3 mL of 70% nitric acid (Fisher, ACS rcagcnt) was added to the hydrolysis mixture, still stirring vig- orously. The mixture was then stirred for 8 h at -80 OC. The 2-propanol (and some water) was allowed to evaporate during this time. Approximately 700 mL of stable TiOz colloidal sol resulted from this procedure. The size of the colloidal particles was ca. 8 nm and X-ray diffraction analysis showed them to consist of anatase. Crystallization occurred during the refluxing, the initial TOz precipitate being X-ray amorphous. A portion of the above sol was concentrated under vacuum at room temperature until it was visibly viscous. Depending on the iigc of the sol. the proper viscosity was reached between I50 and 200 g of TiOz per liter. Nonporous Sn02 films ( F doped) on glass ucrc used for electrically conductive supports (provided by Glasstech Solar, Wheat Ridge CO). Membranes were formed on these supports by spin coating at 3000 rpm. Ti02 layers thinner than 0.5 pm did not crack when fired directly in a 400 OC oven. Thicker layers cracked under any firing regime. Membranes up to zyxwvu 1 pm thick were formed by multiple application and firing of (9) ( a ) Desilvestro. J.; Gratzel. M.; Kavan. L.: Moser. J.; Augustynski, J. J. 4m Chem. Soc. 1985, 107. 2988. (b) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J. Ph),.r. Chem. 1986, 90. 1107. (c) Vlachopoulos, N.; Liska. P.; Augustynski. J.: Gratzel, M. J. Am. Chem. Soc. 1988. 110, 1216. (IO) Anderson. 21 A.: Gieselmann. M. J.; Xu, Q. J. Membr. Sci. 1988, 39, 243. 6 1990 American Chemical Society