J . Phys. Chem. 1985,89, 4495-4499
4495
EPR Observation of Trapped Electrons in Colloidal TiO, Russell F. Howe* and Michael Gratzel Institut de chimie Physique, Ecole Polytechnique FZdZrale, CH- 1015 Lausanne, Switzerland (Received: May 15, 1985)
An EPR study is reported of paramagnetic species formed on irradiation of colloidal Ti02 solutions. Electrons produced by band-gap irradiation are trapped as surface Ti3+species in the presence of suitable hole scavengers. In the absence of hole scavengers, trapping occurs only at interior sites and the resulting interstitial Ti3+species are unstable at room temperature. In acidic solution, all of the trapped electrons are accounted for as Ti3+,whereas in alkaline solution only 10% of the electrons are detected as EPR-visible Ti3+.
Introduction There is growing interest in the use of colloidal Ti02 particles as mediators in the light-induced cleavage of water, when loaded with appropriate catalysts, and as models for studying chargetransfer processes a t the liquid-semiconductor interface.’ The primary photochemical event occurring in such systems on irradiation is production of holes and electrons, but much remains to be understood about the subsequent reactions that these may undergo, such as recombination, trapping, transfer across the semiconductor-liquid interface, and reaction with adsorbed molecules. Several groups have reported the appearance of an intense blue color when colloidal T i 0 2 is irradiated in the presence of molecules which can function as hole scavengers.24 The absorption spectrum associated with the blue color shows a maximum rather than a continuously increasing absorption with wavelength, indicating that it is due at least in part to trapped electrons rather than free electrons in the conduction band. Kolle et a1.6 have measured extinction coefficients for the electron absorption and have shown that both the extinction coefficient and the wavelength of the absorbtion maximum vary with the pH of the solution, suggesting that electron trap sites are located at the surface of the colloidal particles. This conclusion is supported by the observation that the appearance of the spectrum can differ between different colloid preparation^.^^^,^ The trap sites have not, however, been definitely identified to date. In this paper we describe an EPR investigation of UV-irradiated T i 0 2 colloids. Although EPR spectroscopy has been extensively used to study one-electron redox processes in dry solid anatase and rutile,’ aqueous colloidal systems have not to our knowledge been previously examined by this technique.
Experimental Section Colloidal solutions of T i 0 2were prepared by hydrolysis of TiC14, as previously described.* Acidic solutions (pH 2.2) were prepared containing 13 or 5.5 g of T i 0 2dm-3; the average particle diameters were determined by quasi-elastic light scattering to lie between 10 and 15 nm. Alkaline solutions (pH 10.6) were obtained by rapidly adding N a O H solution to the 5.5 g dm-3 acidic solution in the presence of 1 g of poly(viny1 alcohol) (PVA) dm-3 to stabilize the colloid; the average particle diameter was measured to be 200 nm in the alkaline solution. An alkaline solution containing no PVA was prepared at a lower concentration (1.5 g dm-3). Deuterated colloids were prepared by substituting D 2 0 in the hydrolysis procedure; an acidic solution was prepared containing 7.1 g dm-3 (pD 2.5) and an alkaline solution containing 2 g dm-3 (pD 10.6). Colloid solutions were degassed prior to irradiation with argon. EPR spectra were measured on a Varian E l 15 spectrometer at 9 GHz. Microwave frequencies were measured with a Hewlett Packard frequency counter, and the magnetic field was calibrated with a Bruker N M R probe. For room temperature spectra a *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.
quartz aqueous solution cell was used. Spectra at 77 and 4.2 K were obtained from frozen solutions in 5-mm quartz tubes. For concentration measurements the EPR tube was fitted as a sidearm on a 1-cm quartz UV-visible cell. Irradiation and measurement at 77 K were carried out in quartz insert dewar containing liquid nitrogen; a Lakeshore Cryogenics liquid helium transfer cryostat was used for irradiation and measurement at 4.2 K. Spin concentrations were determined by numerical double integration of the first-derivative spectra and comparison with a frozen aqueous solution of Cu2+in an identical sample tube; the absolute accuracy is estimated to be f20%. Simulated EPR spectra were obtained by using the program POW9 on an IBM 4341 computer. A 450-W xenon lamp was used for irradiation, with Pyrex glass and water filters to remove the far-UV and infrared components. UV-visible spectra of colloid solutions were measured on a Perkin-Elmer Hitachi 340 spectrophotometer. The concentrations of electrons were calculated from the extinction coefficients previously determined;6 for acidic solutions e = 1100 mol-] dm3 cm-I at 700 nm, and for alkaline solutions e = 800 mol-’ dm3 cm-’ at 800 nm. The absolute accuracy of the concentration measurements is estimated to be *lo%.
Results Photolysis of the degassed acidic solutions of colloidal T i 0 2 in the presence of PVA (1-5 g dm-3) produced the characteristic blue color after a few minutes. No EPR signals could be detected from the blue solutions at room temperature, but at 77 K the spectra shown in Figure 1 were obtained (by freezing the solutions after photolysis). These consist of two overlapping signals: a broad slightly asymmetric signal centered on g = 1.92, and a much less intense and narrower feature at g = 1.988 which appears as a shoulder. The g = 1.988 feature was more pronounced in lower concentration solutions: the parallel and perpendicular components of the g = 1.92 signal were also more clearly resolved at lower concentration (Figure 1b). Identical line shapes were obtained when spectra were recorded at 4.2 K. Figure I C shows a spectrum obtained from a deuterated acidic colloid solution. The frozen (1) Recent reviews on this subject include: Gratzel, M. Angew. Chem., In?. Ed. Engl. 1985. Kalyanasundaram, K.; Gratzel, M. “Chemistry and Physics of Solid Surfaces V”, Howe, R., Vanselow, R., Eds.; Springer Verlag: Berlin, New York; 1984. (2) (a) Duonghong, D.; Borgarello, E.; Gratzel, M. J. A m . Chem. SOC. 1981,103,4685. (b) Duonghong, D.; Ramsden, J.; Gratzel, M. J. Am. Chem. SOC.1982, 104, 2911. (3) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 241. (4) Bahnemann, D.; Henglein, A,; Lilie, J.; Spanhel, L. J. Phys. Chem. 1984, 88, 709. ( 5 ) Dimitrijevic, N. M.; Savic, D.; Micic, 0.;Nozik, A. J. J . Phys. Chem. 1984, 88, 4278. (6) Kolle, U.; Moser, J.; Gratzel, M. Inorg. Chem., in press. (7) See, for example, Iyengar, R. D.; Codell, M. Adu. Colloid Inrerface Sci. 1972, 3, 365, and references therein. ( 8 ) Moser, J.; Gratzel, M. J. Am. Chem. SOC.1983, 105, 6547. (9) Nilges, M. Ph.D. Thesis, University of Illinois of Urbana-Champaign, 1979.
0022-365418512089-4495$0 1.5010 0 1985 American Chemical Society
4496 The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 10
,.+3
Ti +3 . A
’
Howe and Gratzel
1
I
h
“
I/ 0
6
3
9
12
[ELECTRONS] / i O - 4 M O L OMb3
Figure 3. Concentrations of Ti3+measured by EPR vs. concentrations of electrons measured from UV-visible spectra: 0 , pH 2.2, 13 g d ~ n - ~ ; *, pH 2.2, 5.5 g dm-’; 0,pH 10.6, 5 g dm-?.
1 -
100 gauss
Figure 1. EPR spectra of colloidal TiOz solutions at pH 2.2 containing PVA after irradiation at room temperature (recorded at 77 K): (a), 13 g of Ti0, dm-); (b), 5.3 g of TiO, dm-’; (c), deuterated colloid in D20, 7 g of Ti0, dm-’.
I -
100 gauss
I 1.988
I 1.957
Figure 4. EPR spectra of colloidal TiO, solutions irradiated at 77 K: (a) pH 2.2, 5.5 g dm-’, containing PVA; (b) same solution without PVA.
gl/ Figure 2. EPR spectra of colloidal TiO, solutions at pH 10.6 containing PVA after irradiation at room temperature (recorded at 77 K): (a) 5 g of Ti02 dm-’; (b), deuterated colloid in D20,2 g of TiO, dm-’. 91
colloid solutions could be warmed to room temperature and cooled again at 77 K without loss of the blue color or the EPR signals. With solutions which had not been thoroughly outgassed, however, standing at room temperature caused a fading of the color and loss of the EPR signals measured on cooling again to 77 K. Figure 2 shows spectra recorded at 77 K in similar experiments with alkaline colloid solutions. In alkaline solution the broad signal (g = 1.93) is more anisotropic and significantly less intense than in acid solution, and the g = 1.988 feature is now clearly resolved
as the perpendicular component of an axially symmetric signal with the corresponding parallel component of g = 1.957. In alkaline solution the g = 1.93 signal shows a small but significant reduction in line width for the deuterated colloid (Figure 2b). The line shapes in alkaline solution also were unchanged on measuring at 4.2 K. The concentrations of the paramagnetic species responsible for the broad g = 1.93 signals were measured as described in the Experimental Section, and the concentrations of electrons determined by measuring UV-visible spectra of the same solutions prior to freezing. A plot of the spin concentrations vs. total electron concentrations is shown in Figure 3. In acid solution, the paramagnetic species accounts, within experimental error, for all of the electrons responsible for the blue color, whereas in alkaline solution only about 10% of the electrons are detected by EPR. The narrow axial signal was neglected in these calculations since its integrated intensity corresponds to no more than 5% of the broad signal in alkaline solution, and correspondingly less in acidic solution. Irradiation of a frozen acidic solution containing PVA at 7 7 K gave the spectrum shown in Figure 4a. In this case the narrow
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 4491
Trapped Electrons in Colloidal T i 0 2 TABLE I: EPR Parameters of Ti3+Signals system
g,
(Ti3+)interatitial in colloidal TiO2
1.988
(Ti3+)syrfam in colloidal T i 0 2 at p H 2.2 (PVA, I-, or acetate as scavenger) (Ti3+),urfaoc in colloidal T i 0 , at pH 2.2 (methanol as scavenger) (Ti3+)surfaw in colloidal TiO, at p H 10.6 (PVA as scavenger) hvdrated anatase irradiated at 4.2 KI2 ~ 3 in+ solid anatase" Ti3+ in solid anatase" Ti3+ in Pt-TiO, Ti3+ in silicate glassesI4 Ti(H20)63+in frozen s o l ~ t i o n ' ~
1.925 1.930 1.945 1.989 1.990 1.992
comments
gll
1.957 stable at room temperature only in the presence of a hole scavenger 1.885 1.885 1.880 1.959 unstable at room temperature 1.959 doped with S b or N b 1.962 doped with N b g ,, = 1.92 g,, = 1.927
1.892
1.988
note inversion of g tensor
2.025
,.+3
-
I
100 gauss
V I
2002
Figure 6. EPR spectra of colloidal TiO, solution at pH 2.2 containing mol of H 2 0 2 dm-) after irradiation at 77 K (recorded at 77 K).
1
II
Figure 5. EPR spectra of colloidal TiO, solutions at pH 2.2 irradiated at room temperature (recorded at 77 K): (a) containing 0.1 mol of Idm-'; (b) containing methanol (20 VOI %).
axial signal is most prominent, although the broad signal can still be detected. Subsequent warming to room temperature and cooling again to 77 K left the spectrum unchanged. The corresponding experiment with acidic solutions containing no PVA gave only the narrow signal (Figure 4b). On warming to room temperature this signal was lost but could be restored by further irradiation at 77 (K. Identical spectra were obtained on irradiation of acidic solutions without PVA at 4.2 K, and on irradiation of alkaline solutions at 4.2 or 77 K. The importance of PVA as a hole scavenger in forming the broad signal at g = 1.93 was investigated by irradiating acidic colloid solutions containing other potential hole scavengers at 77 K or room temperature. For example, Figure 5a shows a spectrum recorded at 77 K after irradiation at room temperature of a colloid solution containing 0.1 mol dm-3 of iodide; this consists of the same two signals found with PVA (cf. Figure lb). The irradiated solution was blue in color, and the intensity of the EPR signals comparable in magnitude with that of the corresponding PVA solutions (5.5 g of T i 0 2 dm-3). Irradiation in the presence of iodide at 77 K gave only the narrow axial signal; no new signals from species such as 1,- were detected. A blue color was also produced on irradiation of acidic colloid solutions containing methanol (20 ~ 0 1 % ) Figure ; 5b shows the spectrum of such a solution recorded at 77 K. The intensity of the EPR signals is comparable to that of PVA solutions; however, the g = 1.92 signal is in this case significantly narrower than that in Figure 1. Irradiation of methanol-containing solutions at 77 K produced spectra in which the narrow axial signal was predominant (with identical g-tensor
components to that observed with or without PVA) although the g = 1.92 signal was also detected. Irradiation of acidic colloid solutions in the presence of acetate mol dm-3) at room temperature produced a barely detectable blue color and gave EPR signals identical with those obtained from PVA solutions but an order of magnitude less intense. Irradiation at 77 K in the presence of acetate produced, in addition to the narrow axial signal, a weak four-line signal centered on g = 2.002 due to methyl radicals (intensity ratio 1:3:3:1, splitting of 23 G). Figure 6 shows the only EPR signal detected when acidic colloid solutions containing 10" mol of H202dm-3 were irradiated a t 77 or 4.2 K. This signal has the line-shape characteristic of an orthorhombic g tensor (gl = 2.025, g2 = 2.009, g3 = 2.002). It was lost on warming to room temperature and was not observed when solutions irradiated at room temperature were subsequently cooled at 77 K. In the presence of peroxide no EPR signals were observed at g < 2, and no blue color developed on irradiation (the colloid solutions are a faint yellow color in the presence of peroxide).
Discussion Four different EPR signals at g C 2 have been observed when various colloidal T i 0 2 solutions were irradiated. The g values of all four signals indicate that they are due to Ti3+species (Table I). The g-tensor components of the narrow axial signal are closely similar to those reportedl0JI for signals obtained from anatase doped with niobium or tantalum and attributed to either substitutional'O or interstitial" Ti3+. An identical signal has been observed by us to be formed on UV irradiation of hydrated anatase at 4.2 or 77 K.12 The extremely narrow line width of this signal and its insensitivity to changes in pH suggest that it is due to Ti3+ located in the interior of colloid particles. The colloid particles consist of a mixture of anatase and amorphous Ti02; thus some interstitial Ti4+cations are likely to be present which could trap electrons producing interstitial Ti3+. The number of interstitial Ti3+ species produced is small; the maximum concentrations measured (ca. 10" mol dm-3) are of the same order of magnitude as the concentrations of colloid particles (e.g. 3 X lod Mol dm-3 for a solution containing 5 g of Ti02dm-3 at pH 2.2). Thus, on (10) Meriaudeau, P.; Che, M.; Jorgensen, C . K. Chem. Phys. Leu. 1970, 5 , 131. (11) Kiwi, J.; Suss, J. T.; Szopiro, S. Chem. Phys. Lett. 1984, 106, 135. (12) Gratzel, M.; Howe, R. F., to be submitted for publication.
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The Journal of Physical Chemistry, Vol. 89, No. 21, 1985
n
v
(1.885
Figure 7. Computer simulated Ti3+signals: (a) acid colloid, 13 g dm-) (Figure la), Gaussian line widths W, = 35 G, WIl = 75 G; (b) alkaline colloid (Figure 2a), Gaussian line widths W, = 50 G, W,, = 75 G; (c) acid colloid 5 g dm-3 containing methanol (Figure 5b), Gaussian line widths W, = 25 G, Wll = 35 G.
average, only one interstitial Ti3+species is produced per colloid particle in acidic solution. The g-tensor components of the broad EPR signals at g = 1.93 and g = 1.92 could not be determined by inspection because of the large line widths and overlapping of the parallel and perpendicular components. Computer simulations of the signals were therefore undertaken. Figure 7 shows the best fit simulations of the observed spectra in Figures la, 2a, and 5b, and the corresponding g-tensor components are listed in Table I. (The interstitial Ti3+signal was not included in the simulations.) The shapes of the simulated spectra, in particular the relative intensity of the gll feature, were found to depend strongly on the line widths used in the simulation (Guassian line shape functions were employed). The differences between the observed signal shapes for acidic colloid solutions containing different concentrations of Ti02 (Figure 1a,b, or acidic colloid solutions containing PVA or I- as hole scavenger (Figure lb, Figure Sa), are due entirely to differences in line widths. The values of the g-tensor components for the colloid Ti3+ signals are significantly lower than those normally found for Ti3+ in solid Ti02, with the exception of a signal reported for platinized Ti02at g = 1.92 with a line width of 100 G, which was attributed to Ti3+ formed in the vicinity of platinum cluster^.'^ Similar parameters have been reported for Ti3+ in silicate glasses (Table I). The g-tensor components can be fitted to the first-order expres~ions'~ for a d1 ion in a tetragonally distorted octahedral crystal field: gll
= ge = 8A/A
where X is the spin-orbit coupling constant for Ti3+(154 cm-'), (13) Huizinga, T.; Prins, R. J . Phys. Chem. 1981, 85, 2156. (14) Iwamoto, N.; Hidaka, H.; Makino, Y . J . NonCrysr. Solids 1983, 58,
131.
Howe and Gratzel 6 the energy separation of the 2E, doublet from the 2Bzrground state (due to tetragonal distortion), and A the 2B?gto 2B,, energy separation. For acidic colloid solutions containing PVA, I-, or acetate as a hole scavenger, A = 10500 cm-' and 6 = 4000 cm-I. The corresponding values for acidic colloid containing methanol are 10500 and 4260 cm-I, respectively, and for the alkaline colloid containing PVA, 10000 and 5400 cm-I, respectively. The major difference between the three signals is in the value of g,, i.e., the amount of tetragonal distortion of the octahedral symmetry. The signal obtained at pH 10.6 shows significantly larger tetragonal distortion than those from acidic colloids. The parameters of the Ti3+signals obtained from both acidic and alkaline colloids are completely different from those of Ti(H20),3+in frozen aqueous s o l ~ t i o n ,where '~ trigonal distortion of the octahedral symmetry produces an inverted g tensor (gi
