4276
J . Phys. Chem. 1990, 94, 4276-4280
Wide-Range Tuning of the Titanium Dioxide Flat-Band Potential by Adsorption of Fluoride and Hydrofluoric Acid Chong Mou Wang and Thomas E. Mallouk* Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712 (Received: September 8, 1989; In Final Form: January 5, 1990)
The adsorption of fluoride, (HF),F ions, and hydrofluoric acid at the surface of n-type Ti02-,F, ( x = 0.001) electrodes was studied by electrochemical methods. Adsorption of F from acetonitrile/tetraethylammonium fluoride (TEAF) solution follows a Langmuir isotherm above a fractional coverage of about 0.5, and the equilibrium constant for adsorption is 8300 M-l. Specific adsorption of fluoride ions, at millimolar concentration in acetonitrile, results in a large negative shift in the flat-band potential, as manifested by capacitance measurements and cyclic voltammetry of R ~ ( b p y ) , ~ +Fluoride . ions are strongly complexed by HF in acetonitrile to form (HF),F ions ( n = 1, 2 ) , which are also specifically adsorbed at the Ti02,F,(001) surface. The flat-band potential can be shifted by ca. 2 V, from -1.8 V vs Cu/CuF2 in pure acetonitrile/OS M TEAF to +0.2 V in acetonitrile/0.9 M TEAF/36 M HF. Addition of relatively small amounts (ca. 2 M) HF to acetonitrile/OS M TEAF causes the valence band edge to shift to a sufficiently positive potential that elemental fluorine can be evolved photoelectrochemically.
Introduction To date, hundreds of studies of semiconductor photoelectrochemistry have been conducted, and its principles are well establi~hed.l-~A common goal unifies most of this research, namely, the creation of stable and highly efficient photocatalysts and photoelectrochemical devices. In order to achieve this goal, it is necessary to understand the chemistry of the semiconductor/liquid interface. The control of interfacial chemistry is essential because practical devices require elimination of photocorrosion reactions,62' optimization of junction barrier height^,^ and in some cases elimination of surface defect state-related properties such as Fermi level pinning.8 A variety of physical technique^^-^' and much theoretical e f f ~ r t ~have ~ - ~ been ' applied to this problem, and it
(1) Gerischer, H. Pure Appl. Chem. 1980, 52, 2649. (2) Nozik, A. J. Annu. Rev. Phys. Chem. 1978, 23, 1 1 17. (3) Wrighton, M. S . Acc. Chem. Res. 1979, 12, 303. (4) (a) Heller, A. Acc. Chem. Res. 1981, 14, 154. (b) Heller, A,; Miller, B.: Thiel, F. A. Appl. Phys. Lett. 1981, 38, 282. (5) Bard, A. J . J . Phys. Chem. 1982, 86, 172. (6) Bard, A. J.; Wrighton, M. S. J . Elecfrochem. SOC.1977, 124, 1706. (7) Gerischer. H. J . Electroanal. Chem. 1977, 82, 133.
(8) Bard, A. J.; Bocarsly, A. B.; Fan, F.-R. F.; Walton, E. G.; Wrighton, M. S. J . Am. Chem. SOC.1980, 102, 3671. (9) Tomkiewicz, M. J . Elecfrochem. SOC.1980, 127, 1518. (IO) Vithanage, R.; Finklea, H. 0. J . Electrochem. SOC.1984, 131, 799. ( I 1) Finklea, H. 0.:Abruiia, H.; Murray, R. W. Ado.. Chem. Ser. 1980, 184, 253. (12) Hayes, T . R.; Evans, J. F. J . Phys. Chem. 1981, 85, 3550. (13) Hayes, T . R.; Evans, J. f. Surf. Sci. 1985, 159, 466. (14) Pujadas, M: Salvador, P. J . Electrochem. SOC.1989, 136, 716. ( I 5 ) Mishra, K. K.; Ossec-Asare, K. J . Electrochem. Soc. 1988, 135, 2502. (16) Gorochov, 0.:Stoicoviciu, L. J . Electrochem. SOC.1988, 135, 1159. (17) Semkow, K. W.; Pujare, N. U.; Sammells, A. F. J . Electrochem. SOC. 1988, 135. 1142. (18) 1-emasson, P.: Van Huong, C . N. J . Electrochem. SOC.1988, 135, 2080. (19) Kamat, P. V.; Fox, M. A. J . Phys. Chem. 1983, 87, 59. (20) Nogami, G.; Nishiyama, Y . J . Electrochem. SOC.1988, 135, 3038. (21) Nagasubramanian, G.;Wheeler, B. L.; Bard, A. J . J . Elecfrochem. SOC.1983, 130, 1680. (22) Munnix, S.: Schmeits, M. Phys. Rev. B 1983, 28, 7342. (23) Powell, R. A.; Spicer, W. E. Phys. Reo. E 1976, 13, 2601. (24) Kowalski, J. M.; Johnson, K. H.; Tuller. H. C. J . Elecfrochem. SOC. 1980, 127, 1969. ( 2 5 ) Nogami, C . J . Electrochem. Soc. 1986, 133, 525.
0022-3654/90/2094-4276$02.50/0
- -
CB
CB
Em
a"3
E"*
NA+
+
-vB
NA+
IAV
-vB
has been shown that chemical modificati~n~,".~~,~~-~~ of the semiconductor/solution interface is an important and effective strategy. For example, modification of small band gap semiconductors such as GaAs, CdS, and InP with such reagents as S2-,Se2-, 1-/13-, V3+i2+,Ru3+,etc., has been found to facilitate interfacial charge transfer (in competition with photocorrosion reactions), thereby increasing the efficiency of solar energy c o n ~ e r s i o n . ~ ~ ~ ~ - ~ ~ ~
~~
(26) Mehandru, S. P.; Anderson, A. B. J . Electrochem. SOC.1989, 136, 158. (27) Santangelo, P. G.; Miskelly, G. M.; Lewis, N. S. J . Phys. Chem. 1988, 92, 6359. (28) (a) Osa, J.; Fujihira, M. Nature (London) 1976, 264, 349. (b) Fujihira, M.; Ohishi, N.; Osa, T. Nature (London) 1977, 268, 226. (29) (a) Tributsch, H. J . Electrochem. SOC.1978, 125, 1086. (b) Gobrecht, J.; Tributsch, H.; Gerischer, H. J . Electrochem. SOC.1978, 125, 2085. (30) Gerischer, H.: Willig, F. Top. Curr. Chem. 1976, 61, 31. (31) Finklea, H. 0.; Murray, R. W. J . Phys. Chem. 1976, 83, 353. (32) Hodes, G.; Manassen, J.; Cahen, D. Nature (London)1976, 261,403. (33) Rolison, D. R.; Murray, R. W. J . Electrochem. SOC.1984, 131, 337. (34) (a) Ellis, A. B.; Bolts, J. M.; Kaiser, S. W.; Wrighton, M. S. J . Am. Chem. SOC.1977, 99, 2848. (b) Calabrese, G. S.; Wrighton, M. S. J . Am. Chem. Soc. 1981, 103,6273. (c) Baglio, J. A,; Calabrese, G. S.; Kamieniecki, E.: Wold, A,; Wrighton, M. S.; Zoski, G. D. J . Electrochem. SOC.1982, 129, 1461. ( 3 5 ) Turner, J. A,: Parkinson, B. A. J . Electroanal. Chem. 1983, 150, 61 I.
0 1990 American Chemical Society
Tuning of the T i 0 2 Flat-Band Potential Specific adsorption plays a key role in these effects, in that charge transfer. adsorbed species can either catalyze39or An additional important consequence of specific adsorption is the shifting of the semiconductor flat-band potentials. This effect can be used to advantage in two ways, as illustrated for an n-type semiconductor in Scheme I. First, the barrier height (and therefore the attainable photovoltage) can be increased for a redox couple that has its formal potential in the midgap region. Second, the flat-band potential can be shifted in order to electrolyze a redox couple that is otherwise too positive (or too negative, in the case of a p-type semiconductor) in the electrochemical series to be reactive at the illuminated semiconductor surface. For wide band gap semiconductors such as Ti02, a limited number of reagents are available for surface chemical modification. In a previous paper, we reported that elemental fluorine could be evolved photoelectrochemically with high efficiency at n-type Ti02-, and TiO,-,F, electrodes, in anhydrous hydrofluoric acid/sodium fluoride.43 In acetonitrile/fluoride solutions, no fluorine evolution occurs with the same electrodes because of an unusually large negative shift of the T i 0 2 flat-band potential.44 The very negative flat-band potential (-2.10 V vs N H E ) in acetonitrile and positive value (+0.6 V vs the H 2 / H F couple) in anhydrous hydrofluoric acid imply strong specific adsorption of fluoride and hydrogen fluoride to the Ti02surface. The possibility of flat-band-potential tuning of Ti02using these reagents appeared particularly interesting to us in light of recent findings that easily oxidized organic molecules are photochemically fluorinated in acetonitrile/Ti02/AgF suspension^.^^ It was found that compounds which have a first oxidation potential negative of the Ti02 valence band edge in this medium are photocatalytically fluorinated, while less easily oxidized compounds are unreactive. Shifting the flat-band potential to more positive values, as shown in the lower half of Scheme I, might allow this chemistry to be extended to less easily oxidized molecules. We have therefore undertaken a study of the adsorption of fluoride and hydrofluoric acid at the (001) surface of Ti02-,F, electrodes, the results of which are reported in this paper. We find that fluoride ions adsorb very strongly to this surface from acetonitrile solution, causing a large negative flat-band-potentia! shift even at submillimolar concentration. Fluoride ions desorb in acetonitrile as HF is added, forming first adsorbed HF2- and then other ( H F ) , F ( n > 1) species as the ratio of H F to fluoride is increased. The concomitant positive shift in flat-band potential is reflected in Mott-Schottky plots and in the photoelectrochemistry of solution-phase redox couples, including the fluoride/fluorine couple.
Experimental Section The preparation, electrochemistry, and photochemistry of single-crystal Ti02-,F, ( x = 0.001) electrodes followed procedures described p r e v i o ~ s l y . Crystals ~ ~ , ~ ~ were cut to expose the (001) face, and results reported herein refer to adsorption and electrochemistry at that surface. An additional Teflon FEP reservoir was connected to the previously described electrochemical cell for adjusting solution volume and the concentration of electrolyte. (36) (a) Fan, F.-R. F.; Bard, A. J. J . Am. Chem. SOC.1980, 102, 3677. (b) Fan, F.-R. F.; Bard, A. J. J . Electrochem. SOC.1981, 128, 945. (37) (a) Schneemeyer, L. F.; Wrighton, M. S. J . Am. Chem. SOC.1979, 101,6496. (b) Schneemeyer, L. F.; Wrighton, M. S. J . Am. Chem. Soc. 1980, 102, 6964. (38) Natan, M. J.; Thackeray, J. W.; Wrighton, M. S. J . Phys. Chem. 1986, 90, 4089. (39) (a) Tufts, B. J.; Abrahams, 1. L.; Santangelo, P. G; Ryba, G. N.; Casagrande, L. G.; Lewis, N. S . Nature (London) 1987, 326,861. (b) Lunt, S. R.; Casagrande, L. G.; Tufts, B. J.; Lewis, N. S . J. Phys. Chem. 1988, 92, 5766. (40) Tench, D. M.; Yeager, E. J . Electrochem. SOC.1974, 121, 318. (41) Frese, K. W., Jr.; Canfield, D. G. J . Electrochem. SOC.1974, 121, 318. (42) Micka, K.; Gerischer, H. J . Electroanal. Chem. 1972, 38, 397. (43) Wang, C. M.;Mir, Q.-C.; Maleknia, S . ; Mallouk, T. E. J. Am. Chem. SOC.1988, 110, 3710. (44) Wang, C. M.; Mallouk, T. E. J . Phys. Chem. 1990, 94, 423. (45) Wang. C. M.; Mallouk, T. E. J . Am. Chem. SOC.1990, 112, 2016.
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4211 4
0
~
,
I W = 6282 s e b ’
0
20 N-
0 A A
4
3
2
1
0
-1
0.0 M 0.077 mM 0.15mM 0.29 mM 0.44 mM 0.71 mM 1.8 mM 3.96 mM 37.7 mM
1 -2
-3
V VS. CuiCuF2 Figure 1. Mott-Schottky plots for TiO,_,F, ( x 0.001) electrodes in acetonitrile/O.l M TEAP containing 0-37.7 m M TEAF.
The electrolyte, tetraethylammonium fluoride (TEAF), was prepared from the hydrate (obtained from Aldrich) by recrystallization/vacuum filtration five times with anhydrous diethyl ether/acetonitrile. The salt was dried over P 2 0 5under vacuum ( Torr) and then checked electrochemically with a Pt-disk electrode in dry acetonitrile. Flat-band potentials were determined from Mott-Schottky plots, as previously described.M In some experiments a saturated calomel electrode (SCE), and in other cases a fluorinated copper wire electrode, was used as a reference; the potential of the latter was checked against the ferrocene/ferrocenium couple by adding ferrocene to the cell immediately after each experiment. The potential of the copper electrode was consistently -0.35 V vs SCE in acetonitrile/TEAF and +0.35 V vs the onset of hydrogen evolution at a clean platinum electrode in anhydrous hydrofluoric acid/fluoride solutions. For measurements of flat-band potential vs hydrogen fluoride concentration, H F was added to the cell in two different forms. At lower concentrations ([HF] < 0.5 M), TEAF.3HF was used instead of anhydrous H F (Matheson). TEAF.3HF was made by vacuum transfer of excess anhydrous H F to TEAF in Teflon (translucent FEP) tube sealed at one end. After mixing, the excess H F was pumped away through a sodalime trap, and the salt was dried in vacuo over P2O5 for 2 days. The ratio of H F to TEAF was found to be 3.1 1 f 0.12 by titration with standard 0.1 N NaOH. 1.0 M TEAF.3HF solutions were prepared in acetonitrile and stored in an argon atomosphere drybox before use. For higher concentrations, liquid H F was added directly from a Teflon FEP tube sealed at one end and connected to the cell via stainless steel Swagelok fittings. Tris(2,2’-bipyridine)ruthenium( 11) hexafluorophosphate, Ru(b~y),~+.2PF~-, was precipitated from an aqueous solution of the chloride salt (Aldrich) with NH4PF6and recrystallized in absolute ethanol and then dried over P2OSin vacuo. Tetraethylammonium perchlorate (TEAP) was synthesized from perchloric acid and tetraethylammonium hydroxide. The TEAP was extracted with CH2C12and the combined extracts were washed twice with 0.5 M aqueous KHC03. The salt was recrystallized twice from CH2Cl2and dried in vacuo for 2 days over P2OS.
Results and Discussion Adsorption of Fluoride. Figure 1 shows typical Mott-Schottky plots obtained with Ti02-,F, electrodes in acetonitrile/O. 1 M TEAP solutions to which varying amounts of TEAF have been added. These plots were recorded at a frequency of 1 kHz. While there is some frequency dispersion in the slope of the I / c Z vs potential c u r ~ e sthe , ~ intercept, which gives the flat-band potential, is relatively invariant with frequency. From these plots the flat-band potential is calculated according to the Mott-Schottky equation where the space-charge capacitance, C,,, is expected to be the smallest series capacitance in the circuit (and hence approximately equal to the measured capacitance C ) at potentials sufficiently positive of EFB. E is the electrode potential, EFBthe flat-band
4278
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
Wang and Mallouk
2w' 1
0 0
5
10
15
[ F - l - l , mM-1 Figure 2. Reciprocal of the fractional coverage of fluoride (PI) vs reciprocal concentration in 0.1 M TEAP. Solid line gives Kadc= 8300 M-I. potential, ND the donor density, and c the dielectric constant of TiO,, taken to be The Mott-Schottky plots show a significant negative shift of EFB with increasing fluoride concentration, even at submillimolar concentration, implying that fluoride is much more strongly adsorbed than perchlorate. The most negative value of the flat-band potential, -1.8 V vs Cu/CuF2, is essentially reached at a concentration of I .80 mM, and increasing the fluoride ion concentration to 37.7 mM has no further effect. If we assume that the shift of EFB is due to the change in potential drop across the Helmholtz layer, we can express the relationship between the flat-band-potential shift and the quantity of charge specifically a d ~ o r b e d ~byl . (2). ~ ~ Here E F B O is the flat-band potential in the
10 I
.
.
.
V vs SCE 0 , , ,
! ,
1
,
-1 0
l
,
.
,
,
- 20 l ,
,
Figure 3. Cyclic voltammetry of 1 mM Ru(bpy)32+,scan rate 20 mV/s, in the dark at (a) a platinum disk electrode in 0.1 M TEAP; (b) a TiO2,F, electrode in 0.1 M TEAP (c) platinum in 0.1 M TEAP, 5 m M TEAF; and (d) TiO,-,F, in 0.1 M TEAP, 5 mM TEAF.
(6) coverage vs reciprocal concentration in Figure 2, with the assumption that in the millimolar range the activity of fluoride equals its concentration, we obtain a straight line from which an equilibrium constant Kads = 8300 M-l is calculated for adsorption of fluoride. At the lowest concentration of floride, 0 is anomalously low, well off the line formed by the other data points. Attempts to fit all the data (including this point) to other isotherms, e.g., a Frumkin isotherm$1 were not successful, and we hypothesize that the anomaly is caused by the competitive, but weaker, adsorption of perchlorate. At very low coverage of fluoride, each fluoride ion that adsorbs must displace a perchlorate ion. Displacement
of perchlorate is energetically unfavorable, and the equilibrium constant for adsorption of fluoride will be correspondingly low. Since perchlorate is much larger than fluoride, all the perchlorate ions will be displaced when the fluoride coverage 0, is only partial. At this point the equilibrium constant for fluoride adsorption effectively increases to the observed value of 8300 M-I. From Figure 2 it appears that this transition occurs at a fluoride coverage of approximately 0.5. The flat-band-potential shift of TiO, caused by adsorption of fluoride is also manifested in the cyclic voltammetry of solution-phase redox couples. The band-edge potentials of semiconductor photoelectrodes can be estimated with some accuracy from the photoelectrochemistry of a series of couples with potentials between the conduction and valence bands or from the electrochemistry of a single species with multiple oxidation/reduction waves near the flat-band potentia1.35b-37,48-50In this case Ru( b ~ y ) , ~is+ an appropriate example of the latter, since it can undergo three reversible ligand-centered reductions in the potential range of interest and additionally one reversible metal-centered oxidation at more positive potential. Figure 3 shows the electrochemistry of Ru(bpy)3Z+at TiO,,F, and clean platinum electrodes. In perchlorate solution, reversible waves are observed for three ligand-centered reduction/oxidations at both Pt and TiO,F,. This behavior implies that the flat-band potential is sufficiently positive of even the first reduction wave (-1.29 V vs SCE) that reversible reduction and oxidation can occur in the dark. When fluoride is added to the solution in sufficient quantity to give 0, = 1, the electrochemistry at Pt is unchanged (indicating little interaction between F and Ru(bpy),,+ in solution), but all three reductions occur at more negative potentials and are less reversible at Ti02-xF,. This behavior may be rationalized according to Scheme 11. When ,!?'redoxof the couple lies negative of EFB(as is the case for Ru(bpy)32+in perchlorate solution), the density of states of both the oxidized and reduced forms is sufficiently large, at the potential of the conduction band, that reversible electron transfer can occur in both directions. However, when the flat-band potential is shifted negative by adsorption of fluoride, well past the third reduction potential of R ~ ( b p y ) , ~electron +, transfer between the semiconductor and the redox couple becomes sluggish because of poor energetic overlap
(46) Morrison, S. R. Electrochemistry at Semiconducror and Oxidized Metal Electrodes; Plenum: New York, 1980; Chapters 2 and 5. (47) Thackeray, J. W.; Natan, M. J.; Ng, P.; Wrighton, M. S. J . Am. Chem. SOC.1986, 108, 3570.
(48) Frank, S. N.; Bard, A. J. J . Am. Chem. SOC.1975, 77, 7427. (49) Kohl, P. A,; Bard, A. J. J . Am. Chem. SOC.1977, 99, 7531. (50) Lin, M. S.;Hung, N.; Wrighton, M. S. J . Electroanal. Chem. 1982, 135, 121
(2) E~~ - EFB' = Q a d s / c H absence of adsorbate, Qad,is the quantity of adsorbed charge, and CH represents the Helmholtz layer capacitance, which is assumed to be constant. The surface coverage, 0, can then be calculated from (3),
0 = (EFB- E F B ' ) / ( E F B-~EFB')
(3)
where EFBS is the flat-band potential at saturation coverage, Le., where 0 = 0 is in turn related to the activity, a, of the adsorbing species, and its electrochemical potential, p, by (4), 0 = a exp(-,ii/RT) ,ii= L./ zFE,
+
(4) (5)
where ,ii is related to the chemical potential, p, of the adsorbate, its charge, z , the potential drop across the Helmholz layer, EH, and Faraday's constant, F, by (5).47 In practice the electrostatic contribution to the electrochemical potential, zFEH, can often be i g n ~ r e d , and ~ ' the chemical potential can be taken to be independent of c o ~ e r a g e . ~In~ .this ~ ~ case the adsorption follows a simple Langmuir isotherm, (6). By plotting the reciprocal 0-l = Kad;'a-I
+1
Tuning of the Ti02 Flat-Band Potential
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4279
SCHEME 11: Density of States Diagram for the Oxidized and Positive of EFB Reduced Forms of a Couple with Eoledox CB
U
3
9
3 ui >
> m LL
w
0 0
0.2
0.4
0.6
0.8
1.0
[HFI/([H F l + [ F - l ) density of states+
of available states.51 Electron transfer to the oxidized form of the couple is slow since its manifold of states lies primarily positive of the conduction band edge, where the semiconductor has essentially zero denstiy of states, and a significant overpotential for reduction is observed. The states corresponding to the reduced form are even more positive, by an amount corresponding to the reorganization energy. The three reductions of R ~ ( b p y ) , ~at+ TiO,-,F, are therefore less reversible with adsorbed fluoride than they are with adsorbed perchlorate ion. While the first reduction wave is irreversible under these conditions, consistent with Scheme 11, the second and third reductions appear quasi-reversible, even though they occur at potentials positive of EFBin this medium (indicated by the arrow in Figure 3). It is interesting to note that the oxidation of Ru(bpy):+, which is reversible at Pt, cannot be observed at Ti02,F, electrodes. In the case of 0.1 M TEAP containing no fluoride, this wave is the midgap region, and with added fluoride it lies positive of the valence band potential. Charge transfer to couples in the midgap region is often observed with oxide semiconductors such as n-type Ti0248,49 and is thought to be mediated by deep-level surface states. The lack of charge transfer in this case is consistent with our previous conclusion, drawn from impedance measurements,” that this surface of Ti02-,F, contains an unusually low density of midgap states. Adsorption o f ( H F ) , F Species. The important point that is learned from Figures 1-3 is that adsorption of fluoride at the Ti02-,F, surface is strong and results in a large negative shift of the flat-band potential: the change in flat-band potential caused by adding ca. 2 mM fluoride to 0.1 M TEAP in acetonitrile is approximately 1.4 V. Fluoride is known to be a very strong base in acetonitrile, capable of deprotonating even such weak acids as nitr~methane.~ It ~is therefore not surprising that it should show strong specific adsorption at the surface of TiO,-,F,, which has good Lewis acid sites associated with Ti4+. I n dilute solution in acetonitrile the Bransted acidity of H F will be very low, since fluoride acts as a strong Bransted base, and since the autoprotolysis constant of acetonitrile is exceedingly on the order of 10-26-10-32.H F can nevertheless act as a good Lewis acid, forming such species as (HF),F, which can be detected in solution and (for n 5 5) isolated in the solid state.54
Figure 4. Flat-band potential of TiO,-xFx vs [HF] in acetonitrile/TEAF. The analytical concentration of T E A F is 0.5 M for the first point ( [ H F ] = 0) and is approximately constant a t 0.89-0.93 M for other points.
The stepwise equilibrium contants for forming these species are relatively small in water (on the order of 0.25 for n = 1, 255)but are significantly larger in nonaqueous solutions.56 To our knowledge no data on these formation constants exist for acetonitrile solutions. It is thought that, in anhydrous HF, efficient solvation of fluoride to make ( H F ) , F is responsible for the “superacid” properties of that s ~ l v e n t , ~and ’ an extremely large Hammett acidity constant, H,,is estimatedS8(on the order of -14) for anhydrous HF/1 M F. Strong complexation of fluoride by H F and increasing acidity of the solvent as H F is added to acetonitrile have an important effect on the flat-band potential of Ti02-,F,, as shown in Figure 4. In this figure the flat-band potential is plotted against the ratio of H F added to total fluoride (the abcissa thus resembling that of an acid-base titration), in order to show the stepwise formation of ( H F ) , F species. This experiment was done at roughly constant fluoride ion concentration (0.9 M), Le., under conditions relevant to the photochemical fluorination reactions reported in ref 45. The flat-band potential rises sharply with increasing concentration of HF, and reaches a plateau at -1.0 V for HF/total fluoride ratios of 0.15-0.50 (or from n = 0.18 to 1.OO, expressed as (HF),F). The flat-band potential rises again with addition of more HF, possibly showing a second plateau near 0.65 HF/total fluoride (Le., n = 2), and the rises monotonically to a value of +0.18 V in 36 M H F ( n = 41). While a rigorously quantitative treatment of this complex system would be difficult, especially since we cannot make the assumption that activity equals concentration in these concentrated solutions, a qualitative explanation of the data shown in Figure 4 is possible. As H F is added to a solution containing excess fluoride, we expect efficient complexation of fluoride ion to form bifluoride, HF2-, with little free H F present.56 The flat-band potential and therefore the surface charge density are essentially constant as the solution concentration is varied between 21% HFy/79% F and 100%HF;. It is reasonable to conclude that over this range of solution composition a single species (Le., HFY) is adsorbed at the Ti02-,F, surface with approximately unit coverage. The fact that this plateau in EFBis reached when only about of the fluoride in solution has been converted to bifluoride implies that the latter binds even more strongly than fluoride to the TiO,,F, surface. The value of E , in the plateau region, -1 .O V vs Cu/CuF,, is consistent with the size of the bifluoride ion, (54) Mootz, D.; Boenick, D. Z . Anorg. Allg. Chem. 1987, 544, 159. (55) McTigue, P.; O’Donnell, T. A,; Verity, B. Aust. J . Chem. 1985, 38,
(5 1) Gerischer, H. In Physical Chemistry: An Advanced Treatise; Eyring, H., Henderson, D., Jost, W., Eds.; Academic: New York, 1970; Vol. 9A. (52) Rozhov, 1. N.; Knunyants, 1. L. Dokl. Akad. Nauk SSSR 1971,199, 614. (53) Rondinini, S . ; Longhi, P.; Mussini, P. R; Mussini, T. Pure Appl. Chem. 1987, 59, 1693.
1797. (56) Luxenberg, P.; Kim, J. I. Z . Phys. Chem. (Wiesbaden) 1980, 121, 173. ( 5 7 ) O’Donnell, T. A. J. Fluorine Chem. 1984, 25, 75. (58) Devynck, J.; Ben Hadid, A,; Fabre. P. L.; Tremillon, B. Anal. Chim. Acta 1978, 100, 343.
4280
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 I'
'
t
"
I
"
"
I
'
"
"
" '
-
(a) [HF]/[HF]+[F-]= 0, EFB -1.8 __----
-
[
t
(b) [HF]/[HF]+[F-]= .22, Em
- -1.0
r
L
t
/
,,
accounts for, respectively, about 40 and 70% of the solution volume, and we expect significant protonation of the Ti02-*F, surface by uncomplexed HF. The protonation of surface oxide will contribute a further positive shift to EFB. Ultimately, in pure HF/NaF solution, EFBis shifted to about +0.25 V vs Cu/CuF2." Photoelectrochemical Fluorine Evolution in Acetonitrile ( H F ' ) E Solutions. We previously observed that fluorine can be generated photoelectrochemically at Ti02-*F, in H F / N a F solutions, but not in acetonitrile/TEAF, since the valence band edge potential in pure acetonitrile/TEAF is shifted negative of the F / F , formal p ~ t e n t i a l . ~Addition ~ . ~ ~ of small amounts of H F to acetonitrile/TEAF has a large effect on the flat-band potential and therefore on the valence band edge potential; adding H F should allow one to tune the band positions so that photoelectrochemical oxidation of F is possible. This is illustrated in Figure 5, which shows i-V traces recorded with Ti02-xF, under 365-nm illumination as H F is added to acetonitrile/OS M TEAF. The formal potential of the F-/F2 couple is +2.47 V vs Cu/CuF, in HF/I.O M NaF43and probably close to this value in acetonitriIe/OS M TEA+(HF),F- solutions. Since the band gap of Ti02 is 3.0 eV, we would ideally expect a threshold for photoelectrochemical fluorine evolution at EFB= -0.5 V. Experimentally, we see negligible photocurrent at E F B = -1.8 and -1.0 V but find that fluorine is evolved when E F B L -0.4 V (Le., for formal H F concentrations L 1.2 M or, expressed as (HF),F, for n L 2.4). Referring to Scheme 11, the bands are shifted by addition of H F in such a way that the F / F 2 couple lies between the valence and conduction band edges, and F is thus photoelectrochemically oxidizable.
/
i t!
Wang and Mallouk
Conclusions
,
- - - - V vs. CuICuFz 1 .o
2.0 I
1
t
1
I
,
3
'
1
0.0 ,
*
2
1
I
a
,I
Figure 5. Photocurrent/potential curves for TiO,-xFx electrodes in acetonitrile/TEAF with varying amounts of added H F . The analytical concentration of T E A F is kept constant at 0.5 M. Solid lines: dark current; dashed lines: current under 365-nm mercury-xenon lamp illumination. Scan rate 50 mV/s.
which is intermediate between that of fluoride and perchlorate. The flat-band potential, which reflects the saturation coverage via (2) and (3), is intermediate between the potential obtained with pure fluoride (-1.8 V) and pure perchlorate (-0.4 V). The apparent existence of a second plateau near n = 2 and E F B = -0.7 V suggests the formation of H2F3-ions in solution which again are strongly adsorbed. Past this point the data are too sparse to reveal additional structure in the curve, so we cannot say if other ( H F ) , F ions are dominant species at higher H F concentrations. At the two highest H F concentrations shown in Figure 4, H F
We have shown that fluoride and ( H F ) , F species ( n = 1 , 2 ) are strongly adsorbed at the (001) surface of titanium dioxide. By controlling the composition of the solution, Le., the value of n in (HF),F-, the flat-band potential can be tuned over a range of ca. 2 V. Addition of relatively small amounts of H F to acetonitrile/TEAF has the effect of shifting the valence band edge potential to very positive values, ultimately more positive than the fluorine evolution potential in this medium. These results may have significant implications for the photocatalytic fluorination of organic molecules, since only those that are relatively easily oxidized are reactive in pure acetonitrile/AgF/Ti02 suspension^.^^ The application of this approach to the photocatalytic system is presently being explored. Acknowledgment. This work was supported by a grant from the donors of Petroleum Research Fund, administered by the American Chemical Society, and by grants from the National Science Foundation (PYI Award 8657729) and the Robert A. Welch Foundation. T.E.M. also acknowledges support from an Alfred P. Sloan Foundation Research Fellowship.
