J . Phys. Chem. 1988,92, 464-461
464
of silver with the use of more recent experimental data on the optical constants of silver, put the maximum SERS enhancement figures at lower values than the IO6 figure initially calculated. The 103-104 enhancements for single spheres of silver are more in line with observations that have shown similar enhancements for both silver and gold. With the uncertainty in the purity of silver on and in colloidal silver particles due to the presence of oxide and other silver ion complexes, these ideal estimates almost certainly need to be revised downward. The electrodynamic theory for two interacting silver spheres is not complete. At the small particle limit, calculations have been made that give larger enhancements for multiplets than for single spheres as in the calculations for ellipsoidal single particles.49 For the two-sphere case, however, the assumption of point dipoles for the spheres necessarily gives peak resonance SERS intensities located at the single-sphere absorption maxima instead of the coupled sphere frequency. Additional diple-dipole coupling terms that become important only when pairwise interaction occurs between neigh-
-
(49) Kerker, M.;Siiman, 0.;Wang, D.4. J . Phys. Chem. 1984,88, 3168. (SO) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise group 3-12, and the p-block elements comprise groups 13-18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., I11 3 and 13.)
-
boring particles in aggregates include
in which p / ' ) (pi(2))in the equivalent dipole at the center of particle 1 (2) and himol(2)(himo1(')) is the induced dipole of a molecule on the surface of particle 2 (1). As a crude first approximation, we tentatively suggest that the net SERS enhancement for molecules on the surface of particles in aggregates may be expressed as a product of enhancements from adjacent single particles
normalized to a per adsorbed molecule basis, and summed over all adjacent pairs. Some compensation factor, a,also needs to be made for the number of neighboring particles about any particular reference particle. In any case, the product of calculated enhancements for single particles of silver with an upper limit of 106-108 is more in agreement with observed enhancements of lo6 or greater for nonchromophoric adsorbates on silver.
Acknowledgment. This work was supported by Army Research Office Grant DAAG29-85-K-0102. Registry No. Ag, 7440-22-4; GO:-, 13907-45-4; WO?-, 14311-52-5; M O , O ~12274-10-1; ~~, HzW120406,12207-61-3; PW,,O,'-, 12534-77-9; PO,)-,14265-44-2; P2OTe,14000-31-8; HPO?-, 14066-19-4; H4SiW,,0 4 0 , 12027-38-2; PMol2O4o3-,12379-13-4.
Discovery of Reversible Photochromism in Titanium Dioxlde Using Photoacoustic Spectroscopy. Implications for the Investigation of Light-Induced Chargeseparation and Surface Redox Processes in Titanium Dioxide James G. Highfield* and Michael Gratzel Institut de Chimie Physique, Ecole Polytechnique Fgdgrale de Lausanne, Lausanne, Switzerland, CH- 101 5 (Received: April 29, 1987; In Final Form: July I , 1987)
Utilizing a conventional photoacoustic spectrometer in a pump-probe arrangement, band-gap excitation of powdered titanium dioxide in a moist, oxygen-free atmosphere results in a strong photochromic response, characteristic of small polaron formation. The reactivity and remarkable lifetime of such excited species are demonstrated in their subsequent dark reduction of methylviologen. Results from two independent methods indicate carrier densities in the range 10'9-1020 ~ m - Electron-hole ~. recombination follows first-order kinetics, suggesting that detrapping of surface-trapped holes is rate controlling. The presence of water vapor delays the recombination process, extending decay constants from seconds to minutes.
Introduction In recent years, the search for efficient catalysts for light-driven endergonic chemical reactions has been actively pursued in this laboratory and elsewhere,' with the ultimate objective of developing an economically viable process of solar energy conversion and storage. While investigations of semiconductor photocatalysts in colloidal, Le., transparent, form by techniques such as laser flash photolysis and transmission spectrophotometry have yielded valuable information on the efficiency and dynamics of interfacial charge transfer and the optical absorption characteristics of transient excited species,2 there is a need for complementary techniques for in situ studies of such systems in more readily available form, and under typical screening conditions, e.g., as opaque powders in contact with gaseous reactants and product^;^ or suspended in liquid reactant^^,^ under continuous illumination. * Present address: Laboratoire de Chimie Technique, Ecole Polytechnique FCdCrale de Lausanne, Lausanne, Switzerland CH-1015. 0022-3654/88/2092-0464$01.50/0
In the past 10 years, photoacoustic spectroscopy (PAS) has found increasing application in the study of light-absorption and deexcitation processes in condensed-phase media, and its advantages over alternative optical methods, such as reflectance spectroscopy, in the UV-visible-near-infrared region are well documented? In its most popular (dispersive) configuration, utilizing an intensity-modulated broad-band exciting source, with synchronous gas-coupled microphonic detection, PAS is probably the most versatile "parent" of a growing family of techniques based (1) Energy Resources Through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic: New York, 1983. (2) Kalyanasundaram, K.; Gratzel, M.; Pelizzetti, E. Coord. Chem. Rev. 1986, 69, 57. (3) Yamaguti, K.; Sato, S . J . Chem. Soc., Faraday Trans. 1 1985, 81, 1237. (4) Kiwi, J.; Gratzel, M. J . Phys. Chem. 1984, 88, 1302. ( 5 ) Pichat, P.; Herrmann, J.-M.; Disdier, J.; Courbon, H.; Mozzanega, M.-N. Nouu. J . Chem. 1981, 5 , 627. (6). Rosencwaig, A. Photoacoustics and Photoacoustic Spectroscopy; Chemical Analysis, Vol. 57; Wiley-Interscience: New York, 1980.
0 1988 American Chemical Society
Reversible Photochromism in Titanium Dioxide
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 465
on calorimetric detection of light absorption via subsequent nonradiative relaxation of excited states7 The prospects for photothermal deflection spectroscopy (PDS) for in situ investigations of the photoelectrode/electrolyteinterface also seem good.' In this article we report interesting observations of long-lived, reversible photochromism in band gap (UV) irradiated titanium dioxide powders of commercial and laboratory origin. In view of evidence we will present for subsequent (dark) interfacial electron transfer, and the close correspondence of the photoexcited spectrum to a conventional PA spectrum of the same compound subjected to controlled (quantitative) chemical reduction, we believe that the photochromism has its origin in a high density of charge carriers in the bulk of the semiconductor and is, therefore, a phenomenon whose fundamental importance would seem to have been overlooked until now.
Experimental Section The instrumental arrangement consisted of a conventional, dispersive, single-beam PAS spectrometer together with a second (Xe arc) light source for continuous illumination of the sample. As only the probe (PAS) beam is modulated, the additional lamp acts essentially as an optical pump to create excited states whose absorption spectra may be measured by the probe beam in the conventional manner. The individual components of the system are all commercially available, but the photoacoustic cell (EDT Research) was modified to permit purging and, thus, control of the cell atmosphere. Data acquisition and spectral normalization were automatically performed by using a suitable programmed microcomputer (Hewlett Packard HP85) interfaced to the lock-in analyzer (EG and G, Model 5206) and plotter (Hewlett Packard, HP7475A). Full details will be presented in a later paper.9 The PA spectra were obtained at a modulation frequency of 20 Hz, a spectra half-band width of 18 nm, and a lock-in analyzer time constant of 1 s. Wavelength scanning rates were 1 or 2 nm s-l for spectra recorded up to 1000 or 2000 nm, respectively. With a data acquisition rate of 1 point every 5 s, this resulted in PA spectra containing 150 data points. Except where explicitly stated, blocking filters were used routinely to filter out the higher order (shorter) wavelengths passed by the monochromator. The standard experimental procedure was to purge the sample (-50 mg) in situ with the coupling gas of interest for at least 15 min prior to study. The inlet valve was closed and excess gas pressure was released via a pinhole exit valve. The cell was then sealed and left to reach equilibrium for 10 min. The "dark", Le., conventional, photoacoustic signal was observed to reach a constant value after 2-3 min. Exposure to H 2 0 vapor was achieved by pipetting an excess (-50 pL) of liquid HzO into the base of the internal body of the cell prior to insertion of the sample tray and subsequent purging. Thus, samples were always exposed to a constant (assumed saturated) vapor pressure of H 2 0 before spectrum acquisition and/or photoexcitation. The pump beam was passed through a 10 cm path length HzO filter to avoid signal instabilities which probably arose from the dc heating effect due to absorption of IR radiation by HzOvapor in the cell. The growth of the TiOZphotoresponse was followed (typically at 950 nm) to equilibrium over a period ranging from 5 to 30 min, depending on the sample and conditions, before spectrum acquisition. After blocking the UV (pump) beam, decay curves at constant wavelength were obtained by feeding the signal from the lock-in analyzer directly to a potentiometric chart recorder. The electron-trapping experiment was conducted in a purgeable, airtight, Pyrex reactor as described el~ewhere.~ A 5 mL volume of a neutral aqueous solution M) of methylviologen, MV2+, was introduced into the base of the reactor and 12 mg of rutile (Bayer; 70 m2 g-l) was loaded, remote from the solution, into a side arm of the same vessel. The reactor was purged by bubbling H e through the solution for 1 h. The rutile sample was then (7) 4th International Topical Meeting on Photoacoustic, Thermal and Related Sciences; Technical Digest, Ville D'Esterel, QuCbec, August 1985. (8) Wagner, R. E.; Wong, V. K. T.; Mandelis, A. Analyst 1986, I l l , 299. (9) Highfield, J. G.; Gratzel, M.; Pichat, P. J . Phys. Chem., to be submitted.
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Figure 1. An example of photochromism in TiO, (Degussa, P-25)induced by band-gap (UV)excitation.
subjected to irradiation for 1 h under continuous purging. During this time, the sample become blue-gray in appearance. The irradiation was then blocked and the colorless viologen solution was immediately allowed to contact and stand over the powder. A blue coloration instantaneously developed, initially at the interface, which gradually dispersed into the solution. The absorption spectrum of the blue solution was monitored in situ periodically by decanting the liquid back into the main body of the reactor and placing this liquid in the path of the probe beam above the PA cell, which contained carbon black. In this arrangement, the PA cell acts as a radiometric detector and provides a simple and convenient means to measure absorption spectra of nonscattering samples, as reported elsewhere.1° The characteristic absorption spectrum of the methylviologen radical cation, MV'', was observed and the absorbance was measured at Y~~~ = 602 nm. To generate a reference spectrum of reduced TiO,, 1.5 g of Bayer rutile was loaded into a conventional vacuum frame, pretreated at 700 O C in O2to clean the surface of oxidizable (organic) contaminants, cooled to 25 O C , and evacuated. Reduction was effected in H2 (50 Torr) up to 700 O C , monitoring the consumption of H2 (100 pmol), and formation of H 2 0 (90 pmol), trapped at 77 K. The sample was then evacuated at 700 OC and cooled to 25 O C and the uptake of O2 was measured (20 pmol) prior to discharge and transfer to the PA cell. Based on the assumption that O2 is chemisorbed on reduced TiOzto form the 0, specie^,'^ the above treatment was estimated to liberate E lozo electrons in the bulk of the sample, occupying -0.4 cm3. Thus, a value for the carrier density of 2.5 X 1020cm-3 was obtained.
Results and Discussion A typical example of photochromism in Ti02, in this case Degussa P-25, is shown in Figure 1. Spectrum a shows the band edge of the material, which extends to -420 nm, and a weak tail which almost disappears at longer wavelength upon insertion of a UV cutoff filter at 530 nm. Spectrum b shows the effect of leaving out the UV filter: two weak, broad features appearing at -780 and 850 nm. Thus, the UV component of the vestigial amount of stray light passed by the monochromator is apparently sufficient to populate electron-trapping states within the band gap. Spectrum c shows the dramatic effect of high-power irradiation with the pump beam, developing a quasi-continuum spectrum. This absorption was clearly responsible for the blue-gray color of the sample, which could easily be seen if the light beams were blocked. The two weak features at 780 and 850 nm were also seen, superimposed on the stronger continuum background. The origin of the latter absorption was clearly different as high-power irradiation in the absence of H 2 0 vapor generated only a weak continuum along with the same weak features at 780 and 850 nm. (10) Adams, M. J.; Highfield, J. G.; Kirkbright, G. F. Anal. Chem. 1977, 49, 1850.
Highfield and Gratzel
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
466 l.m
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When a UV filter (A < 450 nm) was interposed, the continuum decayed slowly over several minutes, thus demonstrating its origin in band-gap excitation. The effect was clearly reversible under these conditions as the growth/decay cycle could be repeated indefinitely. Corresponding photochromic effects have been induced in a variety of other TiOz samples of commercial or laboratory origin, though the steady-state spectral intensities vary significantly. Indeed in one notable case, that of an amorphous material of high-surface area (320 m2 g-l), obtained by controlled hydrolysis of aqueous Tiel4 with NaOH, there was no detectable photoresponse. Furthermore, this TiO, also showed no activity in water photolysis upon plantinization." Bayer rutile was selected for further investigation on account of its strong photoresponse. Figure 2 shows a comparison of typical (UV) photoexcited spectra (b, c) with conventional (dark) spectra of unreduced (a) and reduced (d, e) samples. All the spectra are remarkably similar (except (a)), showing a shallow maximum in the continuum at 11 10 nm. In view of its general resemblance to spectra reported elsewhere,', the absorption is tentatively attributed to small polaron formation, Le., a Ti3+ center, stabilized by an associated local anion distortion. This point will be taken up in greater detail in a longer paper, to f0ll0w.~ On the assumption of a homogeneous distribution of such charge carriers, their comparable spectral intensities lead to a provisional estimate of carrier density, accumulated by band-gap excitation, of the order of lozo ~ m - ~This . value is remarkably high, approaching that observed in colloidal TiOz, irradiated in the presence of hole scavengers. I 3 The availability of the electrons, thus created, for interfacial charge transfer was clearly demonstrated in their dark reduction of methylviologen, MV2+, to the blue radical cation, MV".' Methylviologen is frequently used as a scavenger for Ti02 conduction band electrons and the kinetics for this process have been extensively studied.I4 In water, the MV2+/+couple has a standard potential of -0.41 V (NHE) and the driving force for electron transfer from the TiOz conduction band is ca. 0.1 eV. A steady-state absorbance A = 0.18 was measured at 602 nm for the latter in a solution path length I = 0.6 cm. Taking t N 11 000 M-1 cm-l ,14 the concentration of MV" = 2.8 X M , giving a total of 1.4 X lo-' mol. As 12 mg TOz occupies 3 X cm3 ) , gives a provisional value (assuming a density of -4 g ~ m - ~this of "available carrier density" in photoexcited Bayer rutile of -2.8 X lOI9 ~ m - ~ In. view of competition with electron-hole recombination, and possible chemical losses, e.g., reaction of MV'+ with traces of dissolved 02,the two independent estimates are in good ( 1 I ) Koslowski, R.; Gratzel, M. unpublished results. (12) Bogomolov, V. N; Mirlin, D. N. Phys. Status Solidi 1968, 27, 443. (13) KBlle, U.; Moser, J.; Gratzel, M. Znorg. Chem. 1985, 24, 2253. (14) Grltzel, M. In Energy Resources Through Photochemistry and Catalysis: Academic: New York, 1983; p 83.
2
'
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Figure 3. Kinetics of charge recombination in Ti02: decay of PAS signal at 950 nm, after cutoff of UV excitation, in the absence or presence of
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agreement. Assuming that the holes are surface-trapped, this experiment also yields an estimate for the surface-state density of lOI3 cm-2, a value similar to that reported by WilsonI5 in photoelectrochemical studies of TiO, anodes in aqueous media. The explanation for the enhancement of the photochromic effect by water vapor can be seen most clearly in Figure 3, which shows decay curves for various samples. (TiO, (sph) denotes a sample in monodispersed (- 1 pm) spherical form, supplied by Montedison.) The presence of physically adsorbed, or weakly hydrogen-bonded, water reduces the electron-hole recombination rate by at least an order of magnitude, an observation which seems difficult to explain simply on the basis of "band-bending" considerations. Due to their small size (-450 A) and the fact that they are undoped the Ti0, particles are unlikely to develop a depletion layer in contact with ambient gas or water. In view of the observed first-order decay kinetics, and the high efficiency of interfacial electron transfer, it would appear that detrapping of the trapped-hole state is rate controlling in the recombination process, as was also concluded by Wi1s0n.I~ The sensitivity of the trapped holes to the ambient atmosphere clearly indicates that they are located at, or close to, the surface, possibly as p-peroxo species formed by the pairing up of two adjacent 0- ions.* The presence of 0,markedly suppressed the photochromism, as exemplified by Degussa P-25in Figure 4, but the effect was completely reversible even after 3 h of continuous irradiation. As evidence exists for both photoadsorption'6 and photodesorption"
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( 15) Wilson, R. H. J . Electrochem. SOC.1980, 127, 228. (16) ChC, M.; Tench, A. J. Advances in Catalysis; Academic: New York, 1983; vol. 32, p 47 and references cited therein.
J. Phys. Chem. 1988, 92, 467-470 of O2 over Ti02 it would seem reasonable to speculate that O2 may act as a recombination center at the surface under these conditions. In conclusion, we believe that this extraordinary photochromic effect in Ti02,reported here in brief, is of fundamental importance in photocatalysis and warrants thorough investigation. Possibilities that immediately spring to mind are studies of sensitization, Le., charge injection into T i 0 2 by visible-light-absorbing surface compounds or dopant ions; the effect of surface-deposited metals; ~~
(17) Pichat, P. In Heterogeneous and Homogeneous Photocatalysis; Pelizzetti, E., Serpone, N., Eds.; Reidel: Dordrecht, in press.
467
and more fundamental characterization of the trapping states and the role of the chemical state, and physical texture, of the TiOz surface in controlling electron-hole recombination. Acknowledgment. We thank Dr. C. Dimitropoulos and Professor w. Benoit in the Dgpartement de Physique for the use and adaption of the single-beam PAS spectrometer, and Dr P. P. Infelta for his generous assistance in the development of the software programming for data acquisition and manipulation. This work was supported by a grant from the Gas Research Institute, Chicago (subcontract with S.E.R.I., Golden, CO) and by the Swiss Office of Energy. Registry No. Ti02, 13463-67-7; H20, 7732-18-5; methylviologen, 1910-42-5.
Room Temperature Photoassisted Formation of Hydrogen-Molybdenum Bronzes with an Alcohol as a Hydrogen Source Pierre Pichat, * Marie-Noelle Mozzanega, Ecole Centrale de Lyon, Equipe CNRS Photocatalyse, BP 163, 69131 Ecully Cedex, France
and Can Hoang-Van Universitd Claude Bernard Lyon I, U A 231 du CNRS de Catalyse AppliquPe et Cindtique H?tProg?ne, 69622 Villeurbanne Cedex, France (Received: May 18, 1987)
The possibility of using the photosensitive properties of a highly divided MOO, (30 m2 8-I) to prepare hydrogen-molybdenum bronzes has been explored. Ultraviolet illumination of MOO, suspensions in an alcohol (methanol, 2-propanol) as a source of hydrogen atoms produces Ho,~&~OO~. Addition of Ti02 allows the formation of H0,93M003, which indicates interparticle hydrogen atom transfer. The presence of Pt metal, either directly deposited on MOO, or by adding Pt/TiO,, suppresses any hydrogen-molybdenum bronze formation. The origin of the detrimental role of Pt, which contrasts with the necessary presence of a group VI11 (groups 8-10) metal for the preparation of bronzes by hydrogen spillover, is discussed.
Introduction Molybdenum trioxide is capable of incorporating hydrogen, thus forming four phases of bronze H,MoO, in the range 0 < x 6 2l which, inter alia, are catalysts for h y d r ~ g e n a t i o n ~and - ~ isomerization5 reactions. These bronzes are usually prepared from single crystals or from powders of low surface area at temperatures below 373 K by using atomic hydrogen which can be generated by various ways6 cathodic reduction, reaction of HCl on Zn, hydrogen plasma, mercury photosensitization,' and dissociative chemisorption on an appropriate metal supported on MOO, (at least during the activation step'). At 473-488 K, the action of gaseous methanol on MOO, of high surface area also results in the formation of bronze^.^^'^ On the other hand, molybdenum trioxide is a photosensitive n-type semiconductor with a band gap of ca. 3.15 eV. Our intention was to determine whether this photosensitivity can be used to produce a bronze. Indeed, hydrogen is abstracted from gaseous or liquid alcohols by means of photocatalysts formed from group VI11 (groups 8-1 0)29metals deposited on semiconductor oxides, (1) Birtill, J. J.; Dickens, P. G. Muter. Res. Bull. 1978, 13, 31 1. (2) Sermon, P. A.; Bond, G. C. J . Chem. Soc., Faraday Trans. 1 1980, 76, 889. (3) Marcq, J. P.; Wispenninckx, X.;Poncelet, G.; Fripiat, J. J. J . Catal. 1982, 7 3 , 309. (4) Benali, R.; Hoang-Van, C.; Vergnon, P. Bull. SOC.Chim. Fr. 1985, 417. (5) Hoang-Van, C.; Benali, R.; Vergnon, P., unpublished work. (6) Fripiat, J. J. Surface Properties and Catalysis by Non-Metals; Bonnelle, J. P., et al., Eds.; Reidel, Dordrecht, 1983; p 477. (7) Fleisch, T. H.; Mains, G. J. J. Chem. Phys. 1982, 7 6 , 780. (8) Erre, R.; Van Damme, H.; Fripiat, J. J. Surf. Sci. 1983, 127, 48. (9) Vergnon, P.; Tatibouet, J. M. Bull. SOC.Chim. Fr. 1980, 1-45. (10) Guidot, J.; Germain, J. E. React. Kinet. Catal. Lett. 1980, 15, 389.
0022-3654/88/2092-0467$01.50/0
such as titanium dioxide."*12 Over UV-illuminated TiO, without deposited metal, an evolution of H2 is also observed, but it is not a catalytic phenomenon. Accordingly, the purpose of this study was to find out whether MOO, suspended in a liquid alcohol, and activated by photons of suitable energy, can yield hydrogen bronzes at room temperature.
Experimental Section Materials. MOO, was prepared by decomposing Mo02C12 vapors in an oxygen stream using a flame r e a ~ t 0 r . l ~After calcination at 673 K in O2 for 15 h, the powder obtained had a surface area of ca. 30 m2 g-' and was nonporous, which enabled a complete exposure to illumination. Its composition and structure were verified by its Raman spectrum and X-ray diffractogram which revealed the orthorhombic phase. Ti02 was nonporous Degussa P.25 (50 m2 g-l; mainly anatase). Procedures. Bronze formation was carried out in a cylindrical static slurry photoreactor (volume ca. 100 cm3) with an optical Pyrex window at its base, transmitting wavelengths >300 nm. Illumination was provided by a Philips HPK 125 W mercury lamp through a 2.5-cm water-circulating cuvette. The radiant power at the entrance of the photoreactor was in the range 45-50 mW as measured with a UDT 21A radiometer calibrated against a microcalorimeter. Unless otherwise indicated, 120 mg of MOO, was dispersed in 10 cm3 of methanol. The suspension was deaerated by using a rotatory pump before illuminating and was (1 1) Pichat, P. ACS Symp. Ser. 1985, 278, 21 and references cited therein. (12) Pichat, P.; Herrmann, J.-M.; Disdier, J.; Courbon, H.; Mozzanega, M.-N. Nouu. J . Chim. 1981, 5, 627.
(13) Vergnon, P.; Bianchi, D.; Benali, R.; Coudurier, G. J . Chim. Phys. 1980, 7 7 , 1043.
0 1988 American Chemical Society
