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Langmuir 1997, 13, 5082-5088
Primary Events in the Photocatalytic Deposition of Silver on Nanoparticulate TiO2 M. R. V. Sahyun Department of Chemistry, University of Wisconsin, Eau Claire, Wisconsin 54702
N. Serpone* Department of Chemistry and Biochemistry, Concordia University, 1455 de Maisonneuve Boulevard West, Montre´ al, Que´ bec, Canada H3G 1M8 Received January 7, 1997. In Final Form: June 6, 1997X The photocatalytic deposition of silver from ethanol solution on TiO2 nanoparticles prepared with a chemisorbed surface alkoxide layer has been examined in real time by picosecond-resolved transient absorption spectroscopy. This photocatalyst formation of surface-trapped photoelectron states, hypothesized to be Ti(III), can be followed on the time scale of the experiment (e10 ns). Loss of these electrons to recombination is inconsequential, presumably owing to sacrificial hole trapping by the surface alkoxide states. Silver deposition occurs on the same time scale, and the pseudo-first-order rate constant for growth of the silver(0) transient absorption is the same as for the disappearance of the Ti(III) states under these conditions. We infer that one-electron, inner sphere reduction of Ag(I) by Ti(III) is rate determining in the formation of the colloidal silver deposit. These particles must accordingly grow by a sequence of alternating electronic and ionic events analogous to those hypothesized to be involved in latent image formation in silver halide photography. The quantum yield for silver deposition under our conditions was estimated as ca. 0.8, much higher than that reported by other authors (refs 1, 2, and 6).
1. Introduction
Abstract published in Advance ACS Abstracts, August 1, 1997.
Photocatalytic deposition of silver on anatase powder dispersed in water yields metallic silver spheroids of ca. 20 Å diameter comprising ca. 250 atoms and exhibiting the characteristic plasmon resonance absorption spectrum of colloidal silver.6 These authors define two limiting cases for the mechanism of colloidal metal particle formation: (a) aggregation of concurrently formed silver atoms; (b) a sequence of alternating electronic and ionic events which build up the silver(0) particle in a fashion similar to the latent image cluster in silver halide photography.9 There remains considerable ambiguity with respect to the role of the photocatalyst itself. Herrmann and coworkers6 estimated apparent quantum yields of formation of silver(0) of up to 0.16. (However, serious difficulties are encountered with the measurement of absolute quantum yields in particulate dispersions.10) From such estimates and the linear dependence of the rate of product formation on light intensity, it was inferred that binary recombination is not a major loss process with the Degussa P25 photocatalyst (about 60% of the photogenerated electrons in aqueous dispersions of this TiO2 recombine in ∼50 ps 11,12). By contrast, laser flash experiments13,14 on nanocrystalline TiO2 preparations have shown that photoelectrons are trapped immediately on generation, but that free photoholes may in some cases have a longer lifetime, leading to elimination of the electrons by free hole-trapped electron recombination.13-15 We identify the
(1) Fleischauer, P. D.; Kan, H. K. A.; Shepherd, J. R. J. Am. Chem. Soc. 1972, 94, 283. (2) (a) Hada, H.; Yonezawa, Y.; Ishino, M.; Tanemura, H. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2677. (b) Hada, H.; Yonezawa, Y.; Saikawa, M. Bull. Chem. Soc. Jpn. 1982, 55, 2010. (3) Ohtani, B.; Okugawa, Y.; Nishimoto, S.-I.; Kagiya, T. J. Phys. Chem. 1987, 91, 3550. (4) Ohtani, B.; Zhang, S.; Handa, J.; Kajiwara, H.; Nishimoto, S.; Kagiya, T. J. Photochem. Photobiol. A: Chem. 1992, 64, 223. (5) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (6) Herrmann, J.-M.; Disdier, J.; Pichat, P. J. Catal. 1988, 113, 72. (7) Tahiri, H.; Serpone, N. J. Adv. Oxid. Technol. 1996, 1, 179. (8) (a) Jonker, H.; Kippel, C. J.; Hutman, H. J.; Janssen, J. G. F.; van Beek, L. K. H. Photogr. Sci. Eng. 1969, 13, 1. (b) Berman, E. Photogr. Sci. Eng. 1969, 13, 50. (c) McLeod, G. L. Photogr. Sci. Eng. 1969, 13, 93. (d) Tabei, H.; Nara, S., Japan Patent 73,114,370 (1973).
(9) (a) Mitchell, J. W. J. Photogr. Sci. 1983, 31, 148. (b) Gurney, R. W.; Mott, N. F. Proc. R. Soc. (London) 1938, A164, 151. (10) (a) Serpone, N.; Terzian, R.; Lawless, D.; Kennepohl, P.; Sauve´, G. J. Photochem. Photobiol. A: Chem. 1993, 73, 11. (b) Agugliaro, V.; Schiavello, M.; Palmisano, L. Coord. Chem. Rev. 1993, 125, 173. (11) Colombo, D. P., Jr.; Bowman, R. M. J. Phys.Chem. 1996, 100, 18445. (12) van de Ven, J.; Serpone, N., unpublished results, 1995. (13) Nozik, A. J.; Memming, R. J. Phys. Chem. 1996, 100, 13061 and references cited therein. (14) Serpone, N.; Lawless, D.; Khairutdinov, R. F; Pelizzetti, E. J. Phys. Chem. 1995, 99, 16655. (15) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69.
The photocatalytic deposition of metallic silver on TiO2 from silver salt solution has been of considerable interest for mechanistic reasons1-4 for the applicability of the technology to silver recovery, e.g., from waste photographic effluents.5-7 The chemistry has also been proposed as the basis for several photoimaging processes.8 Studies conducted on large particles, e.g., single crystals of TiO2 or TiO2 photoconductive electrodes,1,2 has led to an electrochemical model for the process of silver deposition. In developing this model, Fleischauer and coworkers1 assumed electron transfer from the conduction band under flat band conditions to adsorbed Ag(I) ions, without intervention of deep-trapped electron states. Like Herrmann et al.,6 Ohtani and co-workers3,4 inferred involvement of Ag-O-Ti surface states formed by chemisorption of Ag(I) at specific active sites on the TiO2 surface. They specifically analyzed a commercially available, nanoparticulate TiO2 photocatalyst comprising both anatase and rutile phases (Degussa P25) and estimated an areal density of active sites corresponding to ca. 20 Å2 per site. Insofar as these are basic sites, they may be associated primarily with the rutile phase of the photocatalyst; rutile has more basic sites than anatase on an areal density basis.3 X
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Photocatalytic Deposition of Silver on TiO2
trapped electrons with Ti(III) states,14-16 which accordingly are intermediate in the cathodic half-reactions of photocatalysis. Both these results contradict the inferences from the studies of silver photodeposition. The purpose of the present study was to resolve this dichotomy by observing formation of a silver(0) deposit on nanoparticulate TiO2 under picosecond-resolved laser flash photolysis conditions. For this work we selected as photocatalyst ca. 4 nm anatase particles prepared by hydrolysis of Ti(O-iso-C3H7)4 according to the methods of Henglein and co-workers.17,18 This material can be dispersed to form a transparent solution (which somewhat simplifies the problem of measuring quantum yields). Studies by microwave photoconductivity (time-resolved dielectric loss) indicated that conductivity, hence a population of mobile photoelectrons, persists for up to microseconds following flash exposure of a solution of these particles.19 On the other hand, recombination tends to occur in TiO2 nanoparticles prepared by other methods, e.g., TiCl4 hydrolysis, on the subnanosecond time scale.14 2. Experimental Details Preparation of Nanoparticulate TiO2. Preparation and handling of the photocatalyst were carried out under subdued incandescent lighting to minimize photochemical changes in the material. In a modification of the previously published procedures,17,19 tetraisopropoxy orthotitanate (Aldrich, reagent grade) was dissolved in 1-propanol (to facilitate subsequent solvent removal by azeotropic distillation) and added dropwise with stirring to 1 M HNO3. This mineral acid was chosen to promote hydrolysis and to avoid halide contamination of the photocatalyst and thereby minimize the possibility that formation and photolysis of AgCl on the catalyst surface might be intermediate in the observable photochemistry. A sufficient volume of propanol was used to ensure a 70/30 ratio of propanol/water at the conclusion of the addition. The reaction mixture was stirred until it became clear (usually 2 h), and the solvent was removed by vacuum distillation to leave a free-flowing white powder. The dry powder was not fired or calcined further prior to analysis or use. A detailed analytical characterization of the product is described in the Results and Discussion section. In all cases, a mass balance in excess of 100% was found to result in our preparations, even if the powder was exhaustively dried for 16 h at 45 °C and 0.7 Torr in a vacuum oven. This treatment did not change the absorption spectrum of the product on redispersion. In this case the product yield was 122% of theory for TiO2. The solid product was analyzed by electron spectroscopy for chemical analysis (ESCA). We found Ti, C, O, Cl, and F. The source of the halide contamination was traced to the starting titanate ester, which is presumably manufactured from TiCl4. Calculated for TiO1.75X0.5‚C3H7OH: Ti, 13.8; O, 37.9; X, 6.9; C, 41.4. Found: Ti, 13.0; O, 40.0; X, 6.3; C, 41.0. The 1:1 stoichiometry of titania:propanol corresponds approximately to two alkoxy groups for each surface Ti(IV) ion, insofar as in 40 Å anatase spheres (see below) approximately half of all Ti(IV) ions are on the surface. Laser Flash Photolysis Experiments. For the laser flash photolysis experiments the powder was redispersed in absolute ethanol (EM Omnisolv spectroscopic grade) under intense ultrasonication. Solutions were initially made up with 0.15 g of TiO2/L and then diluted 1:10 with ethanol to yield solutions 2 × 10-4 M in TiO2 for the spectroscopic experiments. These solutions were clear to the naked eye but did exhibit a Tyndall effect when viewed under bright, collimated illumination. The silver ion was introduced as silver acetate made up as 0.001 M in freshly opened, Karl Fischer grade pyridine. Increments thereof (100 µL from a microliter syringe) were added to the TiO2 solution (10 mL) to (16) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988, 92, 5196. (17) Bahnemann, D. W.; Henglein, A.; Lilie, J.; Spanhel, L. J. Phys. Chem. 1984, 88, 709. (18) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 241. (19) (a) Warman, J. M.; de Haas, M. P.; Pichat, P.; Serpone, N. J. Phys. Chem. 1991, 95, 8858. (b) Martin, S. T.; Herrmann, H.; Hoffmann, M. R. J. Chem. Soc., Faraday Trans. 1994, 90, 3323.
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Figure 1. Determination of the indirect optical band gap, hν0, for nanoparticulate TiO2 by extrapolating the plot of the square root of the linear absorption coefficient, measured on a solution of the particles vs photon energy, hν. generate the desired silver ion concentrations in the range 8 × 10-6 - 1.6 × 10-4 M. Owing to introduction of pyridine as a cosolvent, the pH of the solution varied from 6.3 to 8.0 over this concentration range, in all cases on the anionic side of the point of zero charge (pH ) 5.0) reported for similarly prepared colloids.16 Absorption spectra indicated no new bands as the result of the addition of the AgOAc-pyridine solution to the nanosol. The experimental setup and procedure for picosecond-resolved laser flash photolysis have been described in detail in previous publications from our laboratories.20 Photolyses were carried out at 355 nm using the frequency tripled output of the passively mode-locked Nd:YAG pump laser amplified to 2.2 ( 0.3) mJ/ pulse and focused to a 3 mm spot on the 2 mm quartz sample cuvette containing the nanosol. Laser pulse width was ca. 30 ps (full width at half-maximum (fwhm)). Laser pulse energies incident on the sample were modulated, as required, using neutral density filters calibrated at 355 nm. At this wavelength the TiO2 absorbed ca. 10% of the incident intra-band-edge radiation. A minimum of eight flash exposures were made on each sample at each delay time, and the individual spectra recorded at a given delay time were computer averaged. Approximately 3% of the total sample volume was irradiated in a single-flash exposure; thorough mixing of the sample between exposures was found to be essential to reproducible results. Analytical Methods. Crystalline phases were analyzed by a wide-angle X-ray-scattering technique using Cu KR radiation on a Phillips vertical diffractometer with proportional detector registry of the scanned radiation. For the proton NMR analysis a sample of the powder (30 mg) was dispersed ultrasonically in 0.6 mL of CD3OD. The dispersion was then broken by centrifugation, and the clear supernatant was collected for analysis by 500 MHz NMR. For quantitative analysis the sample was spiked with an amount of 1-propanol equivalent to the amount of propanol added with the titania sample on the basis of the empirical formula determined for the product (see below).
3. Results and Discussion Analytical Characterization of TiO2 Photocatalyst. The square root of the linear absorption coefficient, R, for the redispersed TiO2 powder in ethanol as determined from the UV absorption spectrum of the solution scaled linearly with the photon energy, hν, as expected14 for an indirect band solid. The plot shown in Figure 1 extrapolated to an optical band edge hν0 ) 3.34 eV (375 nm) that is strongly suggestive of anatase TiO2. From the Brus equation21 parameterized for anatase TiO2,16 this (20) (a) Serpone, N.; Sharma, D. K.; Moser, J.; Gra¨tzel, M. Chem. Phys. Lett. 1987, 136, 47. (b) Serpone, N.; Jamieson, M. A.; Sharma, D. K.; Danesh, R.; Bolletta, F.; Hoffman, M. Z. Chem. Phys. Lett. 1984, 104, 87. (21) Brus, L. E. J. Phys. Chem. 1986, 90, 2555.
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Figure 2. Proton NMR spectrum recorded at 500 MHz of the supernatant from nanoparticulate TiO2 exchanged with CD3OD. See text.
band gap corresponds to a mean particle diameter of ca. 40 Å. The band-edge shift is consistent with the reported band-edge shifts observed by other authors16,22 in TiO2 nanoparticles prepared by hydrolysis of orthotitanate esters, but not with observations on nanoparticles prepared from TiCl4.14 X-ray diffractometry of our samples revealed only reflections characteristic of crystalline anatase TiO2. A Scherrer line width analysis23 inferred a mean monocrystalline phase dimension of ca. 40 Å in agreement with our interpretation of the absorption spectroscopy. Other authors19 have reported obtaining TiO2 particle sizes in the range of 20-40 Å by this preparative technique. Essentially complete coverage of the titania particles by chemisorbed propanol (or propoxide) may account for the relative ease of redispersion of the nanoparticles. To establish the origin of these propoxy groups, the surface alkoxy groups were exchanged with CD3OD, and the solution was analyzed by proton NMR. The 500 MHz proton spectrum is shown in Figure 2. The principal features of the spectrum are triplets at 1.04 and 3.63 ppm and a sextet at 1.66 ppm which match the proton resonances of n-propanol in chemical shifts, splittings, and relative intensities. Another triplet at 1.18 ppm is coupled to the methylene resonance at 2.5 ppm; we assign these peaks to propanal. The propanal most likely results by photocatalytic oxidation of surface 1-propoxy groups during NMR sample preparation and analysis. No resonances identifiable with 2-propanol were observed. The major signal at ca. 1.4 ppm is due to the solvent. When the sample was spiked with authentic 1-propanol in an amount equivalent to that added with the titania according to the empirical formula inferred above, the magnitude of the propanol resonances increased ca. 50(22) Bahnemann, D. W. Isr. J. Chem. 1993, 33, 115. (23) Klug, H. P.; Alexander, L. E. X-Ray Diffraction Procedures, 2nd ed.; Wiley: New York, 1974; p 618.
fold. Thus under these conditions the magnitude of the resonances assigned to the 1-propanol derived from the TiO2 correspond to ca. 2% of the total propanol present in the sample. The intensity of these resonances furthermore did not change whether the titania sample was allowed to remain in contact with the CD3OD for a few minutes or for 24 h. We infer that all the 2-propoxy groups in the starting material exchanged with the reaction solvent under conditions of colloid synthesis, but that in the product powder no more than 2% of the surface 1-propoxy groups are exchangeable, e.g., with CD3OD. Chemisorption of alcohols to titania surfaces is wellknown.24-26 Alcohols adsorb exclusively by dissociation, i.e., titanium alkoxide formation, on rutile surfaces,25 but they adsorb both dissociatively and nondissociatively on anatase.24 FTIR analysis of alcohols chemisorbed on anatase excludes hydrogen bonding as the nondissociative mechanism.25 Formation of Ti-alkoxides has been shown to be intermediate in the thermal oxidation of alcohols on TiO2, which yields primarily epoxide products.26 We are inclined to associate the exchangeable 1-propanol with that bound nondissociatively and the unexchangeable 1-propanol with that bound dissociatively. Since surface complexation strongly enhances interfacial electron transfer rates on colloidal semiconductors,27 we expect that the chemisorbed alkoxy groups should strongly influence the observable photophysics of the nanoparticles by providing sacrificial hole-trapping centers. Polyvinyl alcohol (PVA) has been shown to be a sacrificial hole trap when PVA is used to stabilize TiO2 nanosols in a photocatalytic application.17 (24) Jackson, P.; Parfitt, G. D. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1443. (25) Ramis, G.; Busca, G.; Lorenzelli, V. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1591. (26) Kantoh, T.; Okazaki, S. Bull. Chem. Soc. Jpn. 1981, 54, 3259. (27) Moser, J.; Punchihewa, S.; Infelta, P. P.; Gra¨tzel, M. Langmuir 1991, 7, 3012.
Photocatalytic Deposition of Silver on TiO2
Figure 3. Light-absorbing transients assigned to Ti(III) surface states on TiO2 observed following laser flash photolysis (355 nm, 30 ps fwhm) of the nanosol at delay times 500 ps and 1, 2, 5, 7, and 10 ns.
Figure 4. Linear dependence of the intensity of the lightabsorbing transients at 620 nm assigned to Ti(III) surface states, as recorded 10 ns after laser flash photolysis of the TiO2 nanosol, on laser pulse energy, I.
Photophysics of the Photocatalyst. When the 2 × 10-4 M photocatalyst solution was subjected to laser flash photolysis at 355 nm (pulse energy, I ) 2.2 mJ/pulse) strong light-absorbing transients were observed which persisted longer than 10 ns. Representative transients are shown in Figure 3.28 The spectral distribution with λmax ) 620 nm is typical of those transients assigned to trapped electrons in TiO213,29 and typically interpreted as Ti(III) surface states.14-16 The magnitude of the transient absorption, ∆A at λmax, scaled linearly with intensity, I, when the pulse energy was varied. This result is illustrated in Figure 4. It suggests that neither binary recombination, prior to carrier trapping, nor biphotonic excitation mechanisms play a significant role in the photophysics of the TiO2 powder. In both cases dependence of ∆A on I n would be expected, with n < 1 in the former case and n > 1 in the latter. (28) The transient absorption spectral behavior observed for the TiO2 colloids examined here is in contrast to the features observed for similar colloids prepared from the arrested hydrolysis of TiCl4 (ref 14). In these latter systems, the time interval over which formation of Ti(III) states (trapped electrons, e-tr) is observed is limited by binary recombination of electrons and holes, which is complete by ca. 200 ps in nanoparticle TiO2. In our present system, where the photoholes are sacrificially scavenged by the surface alkoxy states, it becomes possible to observe the continued formation of Ti(III), or equivalent, states by electron trapping for up to 5 ns and beyond. (29) (a) Bahnemann, D. W.; Henglein, A.; Spahnel, L. Faraday Discuss. Chem. Soc. 1984, 78, 78. (b) Hilgendorff, M.; Bahnemann, D. W.; Memming, R. J. Chem. Soc., Faraday Trans., in press.
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Figure 5. Pseudo-first-order kinetics for appearance of the light-absorbing transients assigned to Ti(III) following laser flash photolysis.
The intensity of the transients furthermore suggests a high quantum yield for the formation of Ti(III). Quantitative conversion of absorbed photons (1 mJ/pulse corresponds to absorption of 1.6 × 10-8 einstein cm-3 under our conditions) into trapped electrons requires a molar extinction coefficient approaching 1 × 105 M-1 cm-1 at 620 nm to account for the observed signals. An estimate of this extinction coefficient approximately 1 order of magnitude lower has previously been reported.30 The present estimate is consistent with the (in general) highly allowed character of electronic transitions associated with localized surface states of nanoparticles.31 The persistence of the trapped electron signals to 10 ns and beyond can be accounted for by efficient removal of the photoholes at the particle interface by reaction with the chemisorbed alkoxy groups and alcohol solvent. The Ti(III) states thus remain intact until they are consumed by dissolved reagents, e.g., O227,32 which does not occur on the time scale of our experiments. Formation of the Ti(III) states followed pseudo-first-order kinetics as shown in Figure 5 with rate constant k1 ) 0.21 ns-1. The present material is thus quite different to other reported TiO2 preparations wherein electron trapping occurs more-orless instantaneously with carrier photogeneration13,14 and the Ti(III) states are short-lived. Photocatalytic Reduction of Silver. In the presence of silver acetate-pyridine a new feature is observed in the transient absorption spectrum of the laser flash excited TiO2 nanosol. This is an absorption which occurs in the blue and near-UV spectral region where colloidal metallic silver typically absorbs.33 Typical results are shown in Figure 6. The spectral distribution of the absorption assigned to silver was deduced by subtraction from the transient absorption spectra, as recorded, of the appropriately scaled absorption spectra for the Ti(III) states, as obtained in the study of the photophysics of the nanosol, above, e.g., Figure 3. The exact position of the spectral absorption maximum, λmax, as well as the intensity, ∆∆A, of the silver band varied with both delay time after the flash at which the spectrum was recorded and with the concentration of silver ions in the photolysis sample. These (30) Moser, J. Ph. D. Dissertation, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland, 1986; see also ref 14. (31) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226 and references cited therein. (32) Brown, G. T.; Darwent, J. R. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1631. (33) (a) Chernov, S. F. Opt. Spectrosc. (U.S.S.R.) 1985, 59, 141. (b) Chernov, S. F.; Zakharov, V. N. J. Mod. Opt. 1989, 36, 1541. (c) Subramanian, S.; Nedeljkovic, J. M.; Patel, R. C. J. Colloid Interface Sci. 1992, 150, 81.
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Figure 6. Transient absorption spectra recorded following laser flash photolysis of the TiO2 nanosol in the presence of silver acetate-pyridine (cf. Figure 3) at delay times 1, 2, 5, 7, and 10 ns (bottom to top). See text for meaning of dashed line deconvolution of the spectrum at 10 ns delay. Table 1. Characteristics of Absorption Spectra of Colloidal Silver Product from the Photocatalytic Reduction of Silver Ions on TiO2 [Ag(I)] 8 × 10-6 M: λmax (nm) ∆∆A 2 × 10-5 M: λmax (nm) ∆∆A 5 × 10-5 M: λmax (nm) ∆∆A 8 × 10-5 M: λmax (nm) ∆∆A 1.2 × 10-4 M: λmax (nm) ∆∆A 1.6 × 10-4 M: λmax (nm) ∆∆A
t ) 2 ns
t ) 5 ns
t ) 7 ns
t ) 10 ns
450 0.20
435 0.34
430 0.38
0.05
455 0.15
440 0.26
430 0.42
0.08
460 0.14
445 0.27
425 0.47
465 0.11
460 0.18
440 0.24
415 0.45
460 0.14
445 0.26
430 0.34
