J . Phys. Chem. 1987, 91, 3906-3909
3906
TABLE III: A Comparison of Proton-Carbon Distances Obtained from Neutron Diffraction Studies1* with Those Determined from Selective [HAC NOE Measurements and bv Using Eq 3 and 4 C,-H, NOE(C-Ha) r," 8, rXIb8, E,, 8, C(2)-H(3)N 0.15 2.10 2.09 f0.1 C(4)-H(3)N 0.95 2.18 2.11 f0.11 C(2)-H(5')0 0.29 2.95 f0.15 C(4)-H(5')0 0.32 3.08 f0.16 (I
From selective {H,lC NOE measurements.
From eq 3 and 4
4, 5, and 6 by comparison of the experimental results with those previously obtained from neutron diffraction studies.'* A good agreement between our calculated H(3)- -C(2) and H(3)- -C(4) distances and the results obtained with neutron diffraction studies was observed (see Table 111). (05'H)C2,4NOEs have been determined by means of eq 7 and by using the value of (H3)C2,4NOEs in order to account for the (18) Frey, M. N.; Koetzle, T. F.; Lehmann, M. S.; Hamilton, W. C. J . Chem. Phys. 1973, 59, 915.
exchange contribution. The analysis of the selective (H,]C NOE effects induced on the C(2) and C(4) carbonyl carbons by a presaturation of the 05'H resonance requires careful attention, since these effects are correlated to conformational equilibria in solution. On the basis of our experimental results the presence in solution of the dimeric thymidine-thymidine complex stabilized by both 05'H- -C(2)=0 and O5'H- -C(4)=0 hydrogen bonds (Figure 1) can be suggested. In fact the selective (HJC NOEs observed on the C(4) carbon cannot be considered the result of intramolecular interactions because of the long distance between the 05'H proton and the C(4) carbonyl; whereas a 05'H- - O=C(2) intramolecular hydrogen bond could be suggested. On the other hand, due to comparable (H,)C N O E effects observed on C(2) and C(4), it is possible to hypothesize the presence of an equal distribution of intra- and intermolecular hydrogen bonds as pointed out in figure 1. Using this model and eq 4, both the OS'H-carbonyl carbon distances can be determined. The obtained results shown in Table I11 are typical for hydrogen-bond interactions and support the proposed structure of the thymidinethymidine complex. Registry No. Thymidine, 50-89-5.
EPR Study of Hydrated Anatase under UV Irradiation Russell F. Howe* and Michael Gratzel Institut de Chimie Physique, Ecole Polytechnique FedCrale, CH- 1015 Lausanne, Switzerland (Received: November 11, 1986; In Final Form: February 25, 1987)
An in situ EPR study is reported of paramagnetic species produced on UV irradiation of hydrated anatase. Irradiation at 4.2 K in vacuo produces electrons trapped at Ti4+sites within the bulk and holes trapped at lattice oxide ions immediately below the surface. These species decay rapidly in the dark at 4.2 K. In the presence of 01,trapped electrons are removed and the trapped holes are stable to 77 K. Warming to room temperature causes loss of trapped holes and formation of 0,at the surface. Experiments with "0-enriched O2prove that the Oz- is not formed from gas-phase 0,.
Introduction EPR spectroscopy has been widely used to examine paramagnetic species on TiOl surfaces,'g2 particularly with the objective of identifying radicals formed under UV irradiation which may be important in photocatalytic proces~es.~The majority of such EPR studies to date have however employed anhydrous TiO, outgassed at high temperatures, a pretreatment which leaves the surface free of adsorbed H 2 0 and containing isolated hydroxyl groups. In the important area of photoassisted decomposition of water over TiOz-based catalysts," the surface chemistry of the catalysts will be dominated by hydroxyl groups and adsorbed water, yet there is little information available about radical species formed on such hydrated surfaces. The spin-trapping technique has been used to monitor indirectly radicals such as OH and HOZgenerated on photolysis of aqueous TiO, di~persions,~.~ and we have recently reported EPR observation of the Ti3+species responsible for the blue color which develops on irradiation of colloidal Ti02dispersions in the presence of hole scavengers.' The only direct EPR observations of radicals formed on hydrated Ti02surfaces are those of Gonzalez-Elipe et al.,* who have reported poorly resolved spectra of HOz and 0- or 023species generated by irradiation of hydrated anatase in 02,and Anpo et aL9 who observed a spectrum attributed to OH radicals on irradiation of hydrated anatase a t 77 K. An understanding of the reaction pathways available to holes and electrons produced by band-gap irradiation of TiOz and related *Invited Professor on leave of absence from the Department of Chemistry, University of Auckland, Auckland, New Zealand. Address correspondence to this author at the University of Auckland.
materials is essential if improved water dissociation catalysts are to be developed. We have accordingly undertaken a low-temperature in situ EPR study of hydrated anatase surfaces with the objective of identifying the species produced by initial trapping of holes and electrons and their subsequent reaction products. Experimental Section Anatase was kindly provided by Bayer A.G. (crystallite size ca. 10 nm, surface area 145 m2 g-l, sulfate content 0.17%, N a 2 0 content 0.90%). Hydrated samples were prepared by outgassing in vacuo up to 400 OC, calcining in O2at this temperature for 30 min, and then cooling to room temperature in the presence of water vapor (20 mbar). Identical results were obtained with samples which had been exposed to air without pretreatment. Deuteriated anatase was prepared by outgassing a sample in vacuo at 400 "C, exposing to D 2 0 vapor (20 mbar) at this temperature for 1 h, calcining in 0, at 400 OC, and then cooling to room ( 1 ) Iyengar, R. C.; Codell, M . Adu. Colloid Interface Sci. 1972, 3, 365. (2) Howe, R. F. Adu. Colloid Interface Sei. 1982, 18, 1 . (3) Bickley, R. I. In Catalysis; Specialist Periodical Reports; Royal Society of Chemistry: London, 1982; Vol. 5, p 308.
(4) Energy Resources through Photochemistry and Catalysis; Gratzel, M.. Ed.; Academic: New York, 1983. (5) Jaeger, C. D. and Bard, A. J. J . Phys. Chem. 1979, 83, 3146. (6) Harbour, J. R.; Tramp, J.; Hair, M. L. Can. J . Chem. 1985, 63, 204. (7) Howe, R. F.; Gratzel, M. J . Phys. Chem. 1985, 89, 4495. (8) Gonzalez-Elipe, A.; Munuera, G.; Soria, J. J . Chem. SOC.,Faraday Trans. 1 1979, 75, 749. (9) A n p , M.; Shima, T.; Kubokawa, Y . Chem. Lett. 1985, 1799.
0022-3654/87/2091-3906$01.50/0 0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3907
Hydrated Anatase under UV Irradiation
n
A
v Figure 1.
-
A
50 gauss
EPR spectra of (a) hydrated anatase and (b) deuteriated
anatase, irradiated in vacuo at 4.2 K. temperature in the presence of D20vapor (20 mbar). Oxygen enriched to 36.8% in I7O was obtained from Prochem. EPR experiments employed a high-vacuum cell fitted with double grease-free stopcocks, a reactor section and a quartz side arm. In situ EPR spectra were measured at 9 GHz on a Varian E l 15 spectrometer equipped with a Hewlett Packard frequency counter and Bruker N M R probe for field calibration. The sample cell was either mounted in a Lakeshore Cryogenics liquid helium transfer cryostat (4.2 K) or immersed in liquid nitrogen in a quartz insert Dewar (77 K) and irradiated with a 450-W xenon lamp (Pyrex and water filtered) through the irradiation slots of the EPR cavity. A Varian E4 spectrometer was used for ex situ measurements at 77 K.
2003
Figure 2. EPR spectra of hydrated anatase (a) irradiated in 30 mbar of O2 at 77 K; (b) after warming briefly to room temperature; (c) after subsequent evacuation. All spectra recorded at 77 K.
Results Irradiation a f 4.2 K . Irradiation of hydrated anatase samples in vacuo mbar) at 4.2 K produced immediately the EPR spectrum shown in Figure la. This consists of two signals: a high-field axial signal (signal A) and a low-field slightly nonaxial signal (signal B). The relative intensities of the two signals varied somewhat from one experiment to another; in particular the presence of traces of residual oxygen in the cell greatly inhibited the formation of signal A without affecting signal B. The absolute signal intensities did not increase with time after the first several minutes, and both signals decayed rapidly at 4.2 K when the lamp was turned off (half-life ca. 3 min). Both were however restored by further subsequent irradiation. Figure 1 b shows a spectrum obtained on irradiation of a deuteriated anatase sample at 4.2 K. Signal A has in this case a line shape identical with that in Figure l a , but the central component of signal B appeared to be slightly narrowed relative to that from hydrated anatase. Irradiation at 77 K . Irradiation of hydrated anatase samples in vacuo at 77 K produced signals identical with those described above, an order of magnitude less intense. Irradiation at 77 K in the presence of O2 (30 mbar) produced the spectrum shown in Figure 2a, which consists of signal B only (a factor of 5 more intense than that observed on irradiation in vacuo at the same temperature); signal A was not observed at all under these conditions. In the presence of O2signal B was stable in the dark at 7 7 K indefinitely but was immediately lost on warming to room temperature. The spectrum in Figure 2b was recorded at 77 K after brief warming to room temperature. Subsequent evacuation produced the new signal (C) shown in Figure 2c. This new signal was stable at room temperature; it was broadened on addition of
r
50gauss
Figure 3. EPR spectrum of hydrated anatase after irradiation in "0enriched O2 at 77 K and subsequent evacuation at room temperature (recorded at 77 K).
0, (30 mbar) but restored on subsequent evacuation. Further irradiation of the sample in vacuo at 77 K caused a gradual reduction in the intensity of signal C and the reappearance of signal A. A spectrum identical with that in Figure 2c was obtained when the corresponding experiment was conducted with deuteriated anatase. Irradiation of hydrated anatase in the presence of I7Oenriched O2a t 77 K gave signal B with a line shape identical with subsequent evacuation gave the spectrum that obtained with I6O2; shown in Figure 3. This appears to be identical with the signal C obtained with I6O2 (Figure 2c). In particular, there are no new features in Figure 3 which could be due to I7Ohyperfine structure. The possibility that the I70-enriched O2 had become diluted or contaminated was ruled out by using the same sample of I7Oenriched O2to generate the 02-radical anion on a molybdenum
3908 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 TABLE I: EPR Parameters of Paramametic Swcies in TiO, gl
hydrated anatase (this work) A (trapped electrons) B (trapped holes)
g2
g3
1.990 1.990 1.960 2.016 2.012 2.002 2.025 2.009 2.003
c c02-j
TiSCin T10, anatase Sb (ref 11) anatase + Nb (ref 12) colloidal TiO, irradiated (ref 7 ) 0‘ in rutile + G a (ref 18) 0- in rutile AI (ref 18) V centers in hydrated anatase (ref 8) 02- on thermally activated anatase (ref 21) 0,- on UV-irradiated anhydrous anatase (ref 20) 0,- in colloidal TiO, + H,O, (ref 7)
+
+
1.989 1.989 1.992 1.992 1.988 1.988 2.030 2.023 2.026 2.019 2.028 2.016 2.025 2.009 2.021 2.009
1.959 1.962 1.957 2.007 2.003 2.004 2.003 2.001
2.025 2.009 2.003
zeolite catalyst, which showed the expected I7O hyperfine structure.I0
Discussion Identification of Paramagnetic Species. Table I summarizes the g-tensor components of the three signals observed in this work. The high-field signal A formed on irradiation in vacuo is identical with that observed previously7 on irradiation of frozen colloidal TiO2 solutions in the absence of hole scavengers and attributed to interstitial Ti3+cations. A similar signal is found in anatase doped with Sb5+” or Nb5+,I2although Meriaudeau et a1.l3 prefer to assign this signal to substitutional Ti3+ in the anatase lattice. We cannot from our results distinguish between interstitial and substitutional Ti3+, but we note that the extremely narrow line width of the signal (ca. 5 G) and its insensitivity to deuteriation of the anatase indicate that the Ti3+occupies a single type of site located within the interior of the anatase and not at the surface. The low-field signal B also formed on irradiation in vacuo is more difficult to identify. Many authors have suggested that the initial product of hole trapping at hydrated T i 0 2 surfaces will be hydroxyl radicals OH-
+ h+
-+
Howe and Gratzel An alternative possibility which is consistent with the observed EPR parameters of signal B is trapping of positive holes at lattice oxide ions. Trapped hole centers of this type are well-known in the radiation chemistry of oxidesI6 and glasses.” A distinction can be made between a hole trapped at an oxide ion bridging between two cations and a hole trapped at an oxide ion adjacent to a cation vacancy or a surface (V- or V, centers). In both cases the paramagnetic center is best described as an 0- radical anion. The theoretical expressions for the g-tensor components of 0- are, to first orderI6
gz, = ge gxx = ge
where X is the spin-orbit coupling constant for oxygen (0.014 eV),I6 and E, and Ey the energy separations of the orbital containing the unpaired electron @,) from the p x and pu orbitals, respectively. The g-tensor components for signal B can be fitted to these expressions yielding Ex = 2.00 eV and Ey = 2.80 eV, respectively. The closest available models for such a trapped hole are the defect centers reported by ZwingelL8for A1 and Ga-doped rutile, which are described as bridging 0- species: Ti4+0-M3+ The g-tensor components for the doped rutile hole centers are listed in Table I, and the calculated p-orbital splittings are 1.00 and 1.33 eV for Ga3+ and 1.17 and 1.65 eV for A13+. An analogous species with the impurity ion replaced by Ti4+would be expected to show larger splittings of the p-orbitals, consistent with the values calculated from signal B. The slight difference in line width of the central component of signal B between hydrated and deuteriated surfaces suggests that the trapped holes are located close to the surface. Since signal B was not broadened at all by the presence of 02,the trap sites cannot be located in the surface layer itself but may be in the immediate subsurface layer; we suggest the following structure: Ti4+O-Ti4+OH-
OH
and OH radicals have been detected by Bard et aL5 using the spin-trapping technique. The signal attributed to Anpo et aL9 to OH radicals has parameters closely similar to those of signal B. There are however at least two reasons why signal B is unlikely to be due to surface hydroxyl radicals: (a) In other systems, hydroxyl radicals are characterized by large anisotropic ‘H hyperfine couplings. For example, irradiation of hydrated organic crystals produces OH with ‘H hyperfine tensor components A x , = 27 G , A , = 45 G, and A,, = 4 G.I4 In contrast, any ‘H coupling in signal B is 1- than about 5 G. (Anpo et aL9 attribute the two major features of signal B, which are ca. 20 G apart, to ‘H hyperfine splitting, but these features do not shift on deuteriation of the surface.)
+ 2X/Ex
The parameters of signal B are quite different from those of the signals attributed by other authors to trapped holes in TiO,. Gonzalez-Elipe et al.” report formation of V centers in X-irradiated anatase which are described as holes delocalized over several oxide anions; the g-tensor components of these V centers are, however, close to those of trapped holes in impurity-doped rutile (Table I), suggesting that the V centers may be associated with impurity cations in the anatase. Similar species were subsequently reported to be formed in hydrated anatase on UV irradiation.8 Close examination of the complex and poorly resolved spectra in ref 8 and 19 reveals some features which may correspond to signal B. We note that the P-25 anatase used by these authors is a lower purity and lower surface area material than that used here. The ~
Hydrated Anatase under UV Irradiation by adsorption of 0, on thermally activated TiO2,I and by irradiation of anhydrous anatase in the presence of 0 2 , , 0 and the .’ assignment has been confirmed by analysis of 170-enrichedO,O We therefore assign signal C to 0,- on the hydrated anatase surface. The low-field g-tensor component (gzz) of signal C is larger than that of the 02-species formed photochemically on anhydrous anatase (2.025 compared with 2.021), but this difference is consistent with a reduced splitting of the P orbitals by the electrostatic field at the hydrated surface.22 The 0,- on hydrated anatase is certainly present at the surface, since the signal is reversibly broadened by exposure to 30 mbar of 0, (Figure 2). We have not observed the signal attributed by Gonzalez-Elipe et aL8 to the HO, radical on hydrated anatase. Signal C shows no measurable ‘H hyperfine splitting, whereas the H 0 2 radical formed photochemically on a molybdenasilica catalyst was found to have ’H hyperfine tensor components of 18,8, and 14 G.23 The 0; species responsible for signal C on hydrated anatase is thus not protonated under our conditions. An identical species, also unprotonated, was observed by us previously to be formed on irradiation of aqueous colloidal T i 0 2 suspensions in the presence of H2o2.’ The surprising feature of signal C is that it shows no 170 hyperfine splitting when irradiation is carried out in the presence of I70-enriched 02.At the level of I7O enrichment used, the spectrum of an 0,- species formed from gas-phase O2 would , and I 7 Oin~ the ratios contain contributions from l 6 O ~170’60-, 0.400.47:O. 13, and the hyperfine splitting in the singly and doubly labeled species would be expected to be 70-80 G.,, The observed spectrum (Figure 3) showed no trace of either singly or doubly labeled species. This means either that the species giving signal C is not 0,- or that the 0; is not formed from gas-phase 0,. Given the close similarity of signal C to those of known 0, species, we conclude that it is due to an 0,- formed on the hydrated anatase surface from hydroxyl groups and/or adsorbed water, as discussed below. Mechanisms of Formation. We consider here possible pathways for the formation of the three paramagnetic species observed. The primary photochemical event is creation of holes and electrons. At 4.2 K in vacuo a significant fraction of these are trapped and can be observed by EPR; electrons are trapped at Ti4+sites to form Ti3+and holes at subsurface oxide ions to form 0- (we cannot determine the concentrations of trapped holes and electrons since the in situ irradiation does not produce a uniform distribution of spins within the EPR cavity). The trapped charge carriers are not stable in the dark at 4.2 K; we suppose that decay of the Ti3+ and 0- signals on turning off the lamp is due largely to recombination and the observed signals under irradiation represent steady-state concentrations. Adsorbed oxygen functions as a very efficient electron scavenger, since traces of 0, are sufficient to inhibit formation of Ti3+.On anhydrous anatase surfaces a one-electron transfer from Ti3+to adsorbed oxygen forms the 02-radical anion.,’ On the hydrated surface however, 0; is not formed under these circumstances. The final product of O2reduction on irradiation of oxygenated TiO, dispersions in water is H20,24and a similar four-electron reduction may be occurring on the hydrated anatase surface. The correlation observed by Boonstra and MutsaersZ5 between 0, photoadsorption and hydroxyl content of the T i 0 2 surface is consistent with this picture of enhanced reduction of 0, on hydrated surfaces. (21) Naccache, C.; Meriaudeau, P.; Che, M.; Tench, A. J. Trans. Faraday SOC.1971, 67, 506.
(22) Che, M.;Tench, A. J. Adu. Catal. 1983,32, 1. (23) Seyedmonir, S. R.;Howe, R. F. J . Chem. SOC.,Faraday Trans. 1 1984,80, 2269. (24) Gratzel, M., to be published. (25) Boonstra, A. H.; Mutsaers, C. A. J . Phys. Chem. 1976,80, 1694.
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3909 Removal of electrons by adsorbed oxygen stabilizes the trapped holes by preventing recombination. The subsequent disappearance of trapped holes on warming to room temperature is associated with the formation of an 0, species which does not originate from gas-phase 0,. A possible reaction scheme which would account for these observations is as follows h++OH--OH 20H
H 2 0 2+ h+
-
HzOz 0,-
+ 2H+
i.e., reaction of holes with surface hydroxyl groups produces hydroxyl radicals which immediately dimerize to form peroxide; peroxide then traps a further hole to form 02-.Although these suggested reactions must remain for the moment speculative, we note that loss of trapped holes and formation of 0,- occur in a temperature range where hydroxyl radicals would be expected to be extremely mobile on the anatase surface, and spectroscopic evidence for the formation of peroxide during water photolysis has been presented.26 Loss of 0; on further irradiation at 77 K may be due to both oxidation and reduction of adsorbed 0,-
+ h+ 0,- + e02-
-+
0 2
OZ2-
although the simultaneous appearance of a trapped electron signal (Ti3+) suggests that oxidation is the major pathway (and would account for an overall process of oxidation of adsorbed water to oxygen.) Implications for Photocatalysis. The following conclusions can be drawn from the EPR experiments described here concerning fundamental reactions occurring in hydrated anatase under irradiation. (1) The state of hydration of the TiO, surface has a marked influence on the reactivity of holes and electrons produced on band-gap irradiation; anhydrous Ti02 is not a good model for describing aqueous photocatalysis. (2) Trapped electrons are readily scavenged by oxygen, this appears to be a multielectron process rather than the one-electron transfer found on anhydrous surfaces. (3) Trapped holes are much less reactive than trapped electrons, since reaction of holes with surface hydroxyl groups to form OF occurs only on warming above 77 K. (4) The hydroxyl radicals postulated by many authors and detected in spin-trapping experiments are not primary products of hole trapping; their existence on the surface appears to be transient. The in situ EPR experiment is a powerful technique for monitoring the paramagnetic species produced on irradiation of photocatalysts like Ti0,. Extension of these experiments to study, for example, reactions of trapped holes and electrons with other scavengers, or the effects of adding a second catalyst component such as platinum or ruthenium, should lead to a much improved understanding of the elementary steps in aqueous photocatalysis.
Acknowledgment. We acknowledge Dr. J. van der Klink of the Institute of Experimental Physics for permission to use the E l 15 spectrometer. R.F.H. thanks the Ecole Polytechnique Federale for an appointment as “Professeur Invitc”. This work was supported by the Swiss National Science Foundation and the Gas Research Institute, Chicago (subcontract with the Solar Energy Research Institute, Golden, CO). Registry No. Ti4+, 16043-45-1; ” 0 , 13968-48-4; hydrated anatase, 1317-70-0.
(26) Duonghong, D.; Gratzel, M. J . Chem. SOC.,Chem. Commun. 1984, 1597.
