2805
J . Phys. Chem. 1986, 90, 2805-2807
Detection of Surface States Associated with Adsorbed Hydrogen Peroxide on TiO, by Impedance and Electrolyte Electroreflectance Measurements I. J. Ferrer, H.Muraki; and P.Salvador* Instituto de Catrilisis y Petroleoquimica, C.S.I.C.,Serrano, 119, 28006-Madrid, Spain (Received: February 14, 1986)
The interaction of Ti02with hydrogen peroxide at acidic and basic pH was studied by impedanceand electrolyteelectroreflectance (EER) measurements at the semiconductor-aqueous electrolyte interface. The shift toward negative values of the flat-band potential, as well the changes observed in the EER spectrum in basic medium, was attributed to the generation of extrinsic surface states associated with chemisorbed H0; species. Because of the weaker interaction of neutral H 2 0 2molecules with the Ti02 surface, no perturbation of the potential distribution at the Ti02-electrolyte interface could be detected at acidic PH.
Introduction Indirect experimental evidence for the generation of hydrogen peroxide from photogenerated surface OH' radicals as an intermediate step of water photoelectrolysis at the T i 0 2 and SrTi03 electrodes has been given recently.'V2 It is known, on the other hand, that hydrogen peroxide interacts strongly with the T i 0 2 surface producing appreciable changes in the reflection spectra of the semiconductor; even small amounts of chemisorbed H202have a marked effect on the reflectivity of T i 0 2 sample^.^ We have proposed that chemisorbed H202 molecules give rise to surface states which are involved in inelastic processes of charge exchange either with the T i 0 2 valence band (photooxidation reaction leading to O2evolution: (H202)s+ 2h+ve 0,' 2H+) or with the conduction band (electroreduction 20H-).495 Further verification of reaction: (H202),+ 2e-ce this claim will necessarily include the use of new physical techniques. Siripala and Tomkiewicz have recently demonstrated that electrolyte electroreflectance (EER) is a powerful tool for the "in situ" detection of intrinsic surface states at the Ti02-electrolyte interface.6 By combining the information obtained from EER and impedance measurements, we have been able to show, for the first time, that the distribution of potential at the Ti02-electrolyte interface at basic pH is appreciably perturbed by the presence of hydrogen peroxide. This is attributed to the presence of extrinsic surface states associated with chemisorbed H02- species.
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+
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Experimental Section n-Ti02 thin-film electrodes were prepared by thermal oxidation in air, at 550 OC, for 0.5 h, of a 99.9% purity Ti disk (0.1 cm thick, 1-cm diameter) previously polished to a mirror finish. The film's cm.' A thickness was estimated to be of the order of three-electrode Pyrex cell with a Pt counter electrode and a reference saturated calomel electrode (SCE) was used. Two quartz windows allowed the light passage with an incident angle of 4 5 O on the Ti02surface. The electrolytes were prepared with ultrapure Millipore-Q water and analytical grade chemicals. In all the experiments the electrolytes were deoxygenated. The ionic strength was kept constant in electrolytes of different composition. The light source was a 150-W xenon lamp followed by a monochromator. Measurements of the depletion layer capacitance of the semiconductor (C,) in contact with the electrolyte were obtained by Tomkiewicz's method.* The experimental setup for electrolyte electroreflectance spectroscopy (EER) measurements was similar to that described by Cardona et aL9 The dc bias was applied to the semiconductor with a Wenking POS 73 potentiostat. The ac voltage was superimposed on it by means of a wave form generator (Leader LAG-125). The signal was detected with an EGG-9503 lock-in amplifier and registered with an X-Y recorder (YEW-30331 3). 'On leave from the Department of Chemical Engineering, Tokyo Institute of Technology.
0022-3654/86/2090-2805$01 S O / O
Results and Discussion Figure 1 compares the plots of C2vs. Vin Na2S04electrolytes at acidic and basic pH, with and without addition of hydrogen peroxide. The measurements were obtained at 10 kHz, the maximum frequency allowed by our experimental setup at which it can be reasonably assumed that C = C,.g The plots are linear, which indicates the absence of Fermi level pinning produced by fast surface states. The flat-band potential (V,) is obtained from the intercept with the potential axis. In the absence of hydrogen peroxide, V, was measured as -0.22 and -0.70 V (SCE) at pH 3.4 and 11.3, respectively, which are reasonable values for TiO2.I0 This corresponds to an observed shift of about 60 mV per pH unit, in good agreement with the expected Nernstian behavior." It is also remarkable in Figure 1 that V, is not changed by the addition of H202at acidic pH, while it shifted almost 200 mV toward negative potentials at basic pH. This shift of the flat-band potential is attributed to an increase of the net negative charge on the semiconductor surface (inner Helmholtz layer) due to hydrogen peroxide specific adsorption, according to the reaction (Ti-H20),
-
+ (H02-)aq
(Ti-H02-),
+ H20
(1)
Le., water molecules coordinated to surface Ti ions are replaced by H02- ions, the stable form of hydrogen peroxide in basic medium. At acidic pH, no change of the net surface charge should be expected if H202displaces adsorbed H20molecules. According to the expression for the Helmholtz layer capacitor
A Q = CHAV
(2)
where the Helmholtz layer capacitance is CH N 40 pF/cm2,6 a V , shift AV = 0.2 V corresponds to a net accumulation of surface C. This approximately represents negative charge AQ = a surface concentration of H02-ions of 1013-10'4 cmW2,i.e., between 5% and 10% of a monolayer, in good agreement with the values found by Boonstra and Mutsaers for TiO, aqueous suspension~.~ EER measurements confirm these results. In fact, in Figure 2 the EER spectra of a thin T i 0 2 film in 0.2 M N a 2 S 0 4at pH 11, with and without addition of H202,are compared. In the band gap region both spectra are identical and have a shape that agrees (1) P. Salvador and C. Gutitrrez, J . Phys. Chem., 88, 3696 (1984). (2) P. Salvador and C. Gutitrrez, Surf. Sci. 124, 398 (1983). (3) A. H. Boonstra and C. A. H. A. Mutsaers, J . Phys. Chem., 79,1940 (1975). (4) P. Salvador and F. Decker, J . Phys. Chem., 88, 6116 (1984). (5) P. Salvador, J . Phys. Chem., 89, 3863 (1985). (6) W. Siripala and M. Tomkiewicz, Phys. Rev. Lett., 50, 443 (1983). (7) S. E. Lindquist, A. Lindgren, and 2. Yan-Ning, J. Electrochem. SOC., 132, 623 (1985). (8) M. Tomkiewicz, J . Electrochem. SOC.,126, 1505 (1979). (9) M. Cardona, K. L. Shaklee, and F. H. Polluk, Phys. Rev., 154, 696 (1967). (10) M. A. Butler and D. S.Ginley, J . Muter. Sci., 15, 1 (1980). (11) M. A. Butler, and D. S. Ginley, J . Electrochem. SOC.,125, 228 (1978).
0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 13, 1986
Letters
o 0 2 3 M Na2S04 (pH 3L) o 023M Na2S0,,01M H 2 0 2 ( p H 3 6) A 0 23M Na2S04 (pH 11 3)
0.01 J -1.0
I
I
-a5
I
I
1
0.0
0.5 ,V,
V (SCE) Figure 1. Mott-Schottky plots for a n-TiOz polycrystalline thin film in contact with aqueous electrolytes, at acidic and basic pH, with and
WE)
Figure 3. Dependence of the EER signal, at 4.0-eV photon energy, on the applied bias for n-Ti02 in contact with different electrolytes: (a) 0.23 M Na2S04(pH 3.4) without and with addition of 0.1 M H202,(b) 0.23 M Na2S04(pH 11,3),and (c) 0.2 M Na2S04+ 0.1 M H202(ph 11.3). V,, = 200 mV rms, u = 65 Hz.
without addition of H202( u = 10 kHz). pinning due to surface states fast enough to equilibrate with the semiconductor bulk at the used modulation frequency. According to Aspnes,’* for a fully depleted space charge layer, in the “lowfield” regime where the amplitude of the EER signal is proportional to the modulating voltage, the electroreflectance signal is given by
where ND is the donor concentration, VaCis the fundamental harmonic component of the ac applied voltage across the depletion layer, t, is the static dielectric constant, and L(ho) is the spectral line shape function. Assuming a density of fast enough surface states, N,, Tomkiewicz and Siripalai4showed that eq 3 becomes -101
Figure 2. EER spectra for the n-Ti0,-electrolyte junction at basic pH in (a) 0.23 M NaZSO4(Vdc = 0 V (SCE)) and (b) 0.2 M NaZSO4+ 0.1 M HZO2( Vdc = 0 V (SCE)). The modulation was a 200-mV rms sinusoidal wave form with P = 65 Hz.
well with that of a TiOzsingle crystal,12 showing a strong, broad peak around 4 eV. Additional structure is observed at higher energies, starting at -3.5 eV. The main feature of these spectra in the sub band gap region is the considerable increase of AR/R under addition of HzOz to the electrolyte, keeping constant its ionic strength. Figure 3 shows the dependence of the 4-eV peak amplitude on the dc applied bias, for the same electrolytes as in Figure 2. Two features are remarkable in this figure. On the one hand, the dc voltage at which AR/R changes sign, which should correpsond to the flat-band potential,13 agrees very well, for all electrolytes, with the corresponding Vb values obtained from the MottSchottky plots. On the other hand, it can be seen that, at basic pH, the signal amplitude decreases dramatically toward the reverse bias when H,02 is added to the electrolyte. This decrease of AR/R has been observed before with single crystal and polycrystalline chalcogenide ele~trodes.’~-’’ It was attributed to Fermi level (12) K. Vos and H. J. Krusemeyer, J . Phys. C, 10, 3893 (1977). (13) Y . Hamakawa, P. Handler, and F. A. Germano, Phys. Rev., 167,709 ( 1 968). (14) M. Tomkiewicz and W. Siripala, J . Electrochem. Soc., 131, 736 (1984).
where dVis the modulating potential (Vat in (3)). In the absence of surface states (N, = 0) eq 4 becomes identical with (3), and an EER signal independent of V, is predicted, provided that the line shape does not change significantly with the applied bias (V,). However, as dNs/dVincreases, a part of the ac potential will fall across the Helmholtz layer giving rise to a modulation of the semiconductor band edge’s position, which results in a decrease of the modulation amplitude on the depletion layer and also of AR/R. The electroreflectance signal will become zero when (q/CH)(dNi,/dV) = 1, as then no modulation voltage appears across the space charge layer. These concepts can be applied to the experimental data of Figure 3. The linear relation between AR/R and VaCin the range of 50-500 mW rms, for 30 1 Y 1 100 Hz, under depletion conditions (vdc - Vb > 0.5 V), together with the insensitivity of the spectral line shape to Vd,, indicates that the “low-field” approximation is valid for all the experiments of Figure 3. The rapid quenching of ARIR with increasing band bending (vdc > -0.6 V (SCE)) observed in curve c, for basic pH in the presence of (15) R. P. Silverstein, F. H.Pollak, J. K. Lyden, and M. Tomkiewicz, Phys. Rev. B: Condens. Matter, 24, 7397 (1981). (16) R. P.Silverstein, J. K. Lyden, M. Tomkiewicz, and F. H. Pollak, J . Vac. Sci. Technol., 19, 406 (1981). (17) P. Lemasson, C. Hinnen, N. R. de Tacconi, and N. Van Huong, J . Electrochem. Sor., 132, 2405 (1985). (18) D. E. Aspnes, Phys. Rev. Left., 28, 913 (1972); Surf. Sei., 37, 418
(1973).
2807
J . Phys. Chem. 1986, 90, 2807-2808 H 2 0 2 , can, therefore, be interpreted as an effect of band gap surface states associated with chemisorbed HOT ions that communicate with the conduction band. This seems to be in agreement with our interpretation' of the luminescence experiments by Nakato et al.I9 that the band at 1.47 eV in the T i 0 2 luminescence spectrum should be a consequence of a radiative recombination process between conduction band electrons and chemisorbed hydrogen peroxide molecules ((HOC), + H 2 0 2e-cB 30H-).' It is surprising that those surface states generated by adsorption of H 2 0 2neutral molecules, at acidic pH, have not any appreciable influence on the EER signal in the range of 0.2 V C V,, C 0.7 V, as ARIR appears to be independent of the applied bias (curve a of Figure 3). This might indicate that the relaxation of adsorbed H202species is slow with respect to v and that the surface states do not follow the perturbation. It seems reasonable that the interaction of neutral H202molecules with surface Ti ions is weak compared with the strength of the Ti;'+-H02- bond. In fact, we have shown that the effective cross section of adsorbed H202 molecules for capture of conduction band electron is a t least 1 order of magnitude smaller than their geometric cross section, which indicates a relatively poor electronic coupling moleculeadsorbate.' In any case, surface states associated with the adsorbed hydrogen peroxide are too slow to follow the ac perturbation imposed to the system during impedance measurements (v = 10 kHz), which explains the linearity of the Cvs. Vplots (apparent absence of Fermi level pinning), even at basic pH.
+
(19) (1983).
Finally, we propose that the increase of the sub band gap EER signal observed in Figure 2 when H202is added to the electrolyte is probably the result of optical transitions between filled band gap surface states associated with the adsorbed H02- species and the conduction band: HO2- + 2hv
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0 2
+ H+ + 2e-cB
Unfortunately, because of the limitation of our illumination system, we could not extend the EER spectrum to wavelengths longer than 700 nm where the maximum amplitude of the EER signal should appear according to our interpretation' of Nakato et al.'s luminescence data.lg New EER experiments are in progress in our laboratory in order to determine the energy distribution of surface states associated with the adsorbed hydrogen peroxide molecules. In conclusion, by combining the information obtained from impedance measurements and electroreflectance spectroscopy at the TiO,-aqueous electrolyte interface, we have been able to show that the interaction of dissolved hydrogen peroxide with the semiconductor surface is strong enough to remove H 2 0molecules solvating surface Ti ions, giving rise to extrinsic surface states located in the midgap region. These observations are in agreement with recent results concerning the photo- and electroluminescence behavior of T i 0 2 in contact with different aqueous electrolyte^.^^'^
Acknowledgment. This work was partially finacned by the US-Spain Joint Committee for Scientific and Technological Cooperation, under Contract No. CCA 831038. We thank Dr. C. GutiCrrez for the experimental facilities given for obtaining EER spectra.
Y.Nakato, A. Tsmura, and H. Tsubomura, J . Phys. Chem., 87,2402
A Proton-Transfer Laser A. U. Acuiia,* A. Costela, and J. M. Mufioz Instituto de Quimica Fhica "Rocasolano", C.S.I.C. Serrano 119, 28006 Madrid, Spain (Received: February 27, 1986)
Stimulated radiation was generated by an active medium where the population inversion results from excited-state intramolecular proton-transfer reaction. The operation of a salicylamide pulsed laser, built on this principle, is described. The laser shows a 5% energy conversion efficiency when pumped with 5-20 mJ of 308-nm radiation.
Introduction The presence in salicylic derivatives of a weak fluorescence band with an abnormal Stokes shift (ca. 10000 cm-I) was correctly interpreted by Weller' as arising from a fast proton reaction, in the electronically excited state. Much later it was found (see ref 2 and references therein) that the emitting photoproduct (a zwitterionic or a quinonic species) can be produced with high yields in a large variety of salicylic derivatives, either in gas phase or in solution. In addition, we provided evidence that the electronic excitation induced the shift of a proton along an intramolecular hydrogen bond which already existed in the ground state. In some of the molecules that give rise to excited-state proton transfer, the new excited species Z has no counterpart in the ground state and exhibits a high fluorescence quantum yield with a short lifetime. Therefore, these molecules should have a high cross section for stimulated emission. With this in mind, we optically pumped salicylamide in a relatively simple laser resonator and succeeded in detecting stimulated emission originating from the
SCHEME I
(1) (a) Weller, A. Natunvissenschuften 1955.42, 187. (b) Weller, A. Z . Elektrochem. 1956,60, 1144. (c) Weller, A. Prog. React. Kine?. 1961, 1 , 189.
excited proton-transfer r e a ~ t i o n . ~(See ~ , ~Scheme I for the proposed photochemical mechanism.)
0022-365418612090-2807$01.50/0
0 1986 American Chemical Society
