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Effects of electrochemical reduction of polycrystalline TiO2 photoelectrodes in acidic solutions
Solar Energy Materials 15 (1987) 367-382 North-Holland, Amsterdam
367
EFFECTS OF ELECTROCHEMICAL REDUCTION OF POLYCRYSTALLINE TiO 2 PHOTOELECTRODES IN ACIDIC S O L U T I O N S Sten-Eric L I N D Q U I S T a n d A n d e r s L I N D G R E N Institute of Physical Chemistry, University of Uppsala, Box 532, S-751 21 Uppsala, Sweden
Christofer L E Y G R A F Swedish Corrosion Institute, Box 5607, S-114 86 Stockholm, Sweden
Received 30 January 1987; in revised form 19 March 1987 PolycrystaUine TiO2 film electrodes prepared by thermal oxidation were electrochemically reduced at cathodic potentials in acidic solutions. Changes in the spectral distribution of the quantum efficiency and the current-voltage characteristics at different stages of reduction were observed and related to ESCA (electron spectroscopy for chemical analysis) spectra of the surface and SIMS (secondary ion mass spectroscopy) depth profile analysis. Whereas a modest electrochemical reduction had a large effect on the anodic photoresponse in the short wavelengths range (250-330 nm) and almost none on the response at wavelengths close to the bandgap, more heavy reduction also affected the spectral response close to the bandgap and shifted the edge of the action spectrum towards shorter wavelengths. ESCA showed that the relative amount of hydrogen (present as hydroxy and/or oxyhydroxy groups) at the surface increased upon electrochemical reduction. SIMS profile measurements revealed that hydrogen was accumulated in a narrow (approximately 100-150 nm wide) zone just underneath the TiO2-surface facing the electrolyte. The photoinduced cathodic currents that could be registered at the "as prepared" electrodes vanished upon electrochemical reduction. The observations were interpreted in terms of an increased dopant density due to implanted hydrogen in a low doped top layer (the recombination layer) formed during the preparation of the virgin electrodes. The observations are in qualitative agreement with the predictions that can be made from accepted theoretical expressions for a liquid junction as described by Popkirov and Schindler [Solar Energy Mater. 13 (1986) 161].
1. Introduction H e a t i n g in v a c u u m or reduction with h y d r o g e n at elevated t e m p e r a t u r e are s t a n d a r d procedures to increase the c o n d u c t i v i t y of b o t h single crystal a n d polycrystalline s e m i c o n d u c t i n g oxide electrodes. Electrochemical r e d u c t i o n has also b e e n used for the same purpose. Studies of the latter t r e a t m e n t o n the p h o t o e l e c t r o c h e m ical properties of T i O 2 have b e e n p e r f o r m e d b y a n u m b e r of researchers (see refs. [1-5] a n d refs. therein). G i n l e y a n d K n o t e k [1] studied the change in the spectral d i s t r i b u t i o n of the q u a n t u m efficiency at different stages of r e d u c t i o n of single crystal T i O 2 electrodes i n the range 3 3 0 - 4 5 0 nm. T h e y also investigated the effect of r e d u c t i o n in the 0 1 6 5 - 1 6 3 3 / 8 7 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
368
S.-E. Lindquist et al. /' E[~bcts ~)[reduction of TIO: photoeh,¢tron~s
presence of D 2. Thus they could monitor the change in relative amount of H and D in the crystals by observing the absorption bands of the stretching frequencies of OH and OD in the IR spectra of TiO 2. Depth profiling by electron-stimulated desorption (ESD) measurements showed that in the electrochemically doped surface there was a high density of hydrogen in a shallow surface layer. Weber, Schumacher and Dignam [2] studied the effect of hydrogen loading by cathodic electrochemical treatment of reactivity sputtered TiO 2. They registered up to a tenfold increase in the overall photoresponse for electrodes illuminated with unfiltered light from a xenon lamp. They ascribed the improved photocurrent to an increase in the (minority) carrier diffusion length. They made current-voltage and impedance measurements, but no analysis of the spectral distribution of the quantum efficiency. One of the most extensive studies of single crystal TiO 2 as electrode in PEC cells was made by Wilson and co-workers [6,7]. In a brief but elucidating review Harris and Schumacher [3] sort out the different chemical states introduced into the TiO 2 lattice by reduction in H 2. They also suggest how these states affect the properties of a TiO 2 electrode. In the present work we will report on electrochemical reduction of polycrystalline TiO 2 electrodes. In particular the effect of the reduction on the anodic photocurrent and the photoinduced cathodic currents has been investigated. Anodic action spectra at different stages of reduction will be presented. ESCA analysis of the TiO 2 surface and SIMS depth profile measurements have been performed. The origin of the observed change in the photoresponse of the electrodes with reduction will be discussed with reference to these measurements.
2. Experimental 2.1. Materials
The titanium plate used in the preparations of the electrodes was the same as described before [8]. All solutions were made from reagent grade chemicals and double or triple distilled water, 2.2. Electrode preparation
1 mm thick circular titanium plates (diameter: 20 rmn) were polished to mirror brightness [8]. The plates were washed by ultrasonic agitation in an alkaline detergent (RBS 25, Labkemi AB, Stockholm) water solution. They were thoroughly rinsed with distilled water. The TiO 2 layers were formed by thermal oxidation in air at atmospheric pressure and at temperatures ( T ) in the range 5 0 0 - 7 0 0 ° C [8]. The electrodes were mounted in an O-ring sealed plexiglass holder exposing approximately 1.8 cm 2 to the electrolyte. The electrochemical reduction was performed at constant electrode po*~,nfials in 0.05 molar H2SO 4 solutions. The electrodes were stored under air in a closed vessel. They were carefully washed with water before they were mounted in the cell for current-voltage or
S.-E. Lindquist et al. / Effects of reduction of TiO: photoelectrons
369
Table 1 Preparation parameters and some properties of the T i O 2 electrodes used in the present work. All films were prepared by thermal oxidation in air of atmospheric pressure. The titanium plates were polished with 0.25 ~tm diamond paste in the last step before the oxidation procedure. The oxidation time was in all cases 4.00 h. Treatment after oxidation
Oxidation conditions Electrode
T
Film thickness
No.
(°C)
(~m)
1 2 3 4 5 6
685 695 685 685 710 710
1.1 -
7
508
0.10
-
Reduced, see table 2 Reduced 4 min at - 2.00 V vs SCE None Reduced 10 min at - 2.0 V vs SCE None Polished with diamond paste (0.25 I~m) to a TiOE-film thickness of 0.62 ~ m None
action spectra registrations. The preparation parameters of all electrodes are summarized in table 1.
2.3. Apparatus A conventional three electrode system in a cell with a Suprasil quartz window was used in the photoelectrochemical measurements. The counter electrode was separated from the electrolyte by a glass frit. The equipment for registration of the current-voltage characteristics and spectral distribution of the quantum efficiency has been described elsewhere [8]. The surface analysis was made with a Leybold Heraeus E S C A / A u g e r spectrometer L H 2000, with an Al-anode emitting soft X-rays with an energy of 1486.6 eV, and with photoelectrons being analyzed by means of a hemispherical analyzer. Under operation the base pressure was below I x 10-8 mm Hg. SIMS analysis was carried out on a Cameca, IMS 3S3F apparatus. The mass spectra and depth profile measurements were registered with negative secondary ions. 10 kV Cs + ions were used in the primary ionisation beam. The pressure in the system was routinely 3 x 10 -9 mm Hg. The analyzed areas were typically 60 ~m in diameter. 3. Results
3.1. Action spectra Fig. 1 shows a set of action spectra * of a polycrystalline TiO2 electrode (No. 1) (see table 1) oxidized in air at 6 8 5 ° C for 4.00 h. The oxidation conditions were • The action spectra were registered using a Schott U G 5 b a n d p a s s filter (in the range 250-380 nm) and a G G 375 cutoff filter (in the range 380-600 nm) in the b e a m from the monochromator.
S.-E. Lmdquis! eta/. / Effects of reduction o/TiO 2 photoelectr~ms
370
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300
350
400
450
Fig. 1. The quantum efficiency, q~, vs wavelength, ~, for TiO2 electrode No. 1, at different stages of reduction in 0.05 molar HzSO4 at cathodic potentials. Experimental parameters and symbols are given in table 2. The spectra were registered in a 0.100 molar phosphate buffer (pH = 5.8) at an electrode potential of 0.500 V vs SCE. The insert shows curves 1-4 in a semilogarithmic plot to elucidate the changes in the sub-bandgap response.
chosen to given an approximately 1 ~m thick TiO 2 film. Curve 1 shows the spectrum of the as prepared electrode, curves 2 - 5 show the recorded spectra after various successive electrochemical reductions of the same electrode in 0.05 molar H 2 S O 4. Experimental parameters with reference to fig. 1 are summarized in table 2. The decrease in q u a n t u m efficiency in the short wavelength range (250-300 nm, curve 1) is typical for electrodes prepared at higher temperatures [8]. The electrochemical reduction recovers the q u a n t u m efficiency in this range (curves 2 and 3) and the action spectrum successively reaches a shape congruent with the absorption spectrum of TiO 2. At longer reduction periods and at more cathodic potentials a decrease in q u a n t u m efficiency is registered in the long wavelength range 3 3 0 - 4 0 0 nm. Also note that after the first reduction sequence there is a marked increase in the sub-bandgap response (see inserted logarithmic plot in fig. 1). In the subsequent sequences the s u b - b a n d g a p response remains fairly constant.
S.-E. Lindquist et al. / Effects of reduction of TiO2 photoelectrons
371
Table 2 Summary of the experimental parameters used in the electrochemical reduction of electrode No. 1. The reduction was made at constant electrode potential in 0.05 molar H2SO4. The table gives the electrode potential, U, and the total transferred charge, Q, during various sequences of reduction. It also gives the symbols with reference to the action spectra given in fig. 1. Sequence of reduction
U (V vs SCE)
Q (A s cm-2)
Symbol in fig. 1
1 2 3 4 5
- 0.75 - 0.75 - 2,00 -2,50
0 0.016 0.035 2.8 57
zx [] x © •
3.2. Current-voltage characteristics and action spectra
The current-voltage ( i - U ) curves shown in fig. 2 were recorded before and after the reduction of electrode No. 2 prepared by oxidation in air at 695 o C for 4.00 h. It was heavily loaded with hydrogen by passing a charge of 18 A s cm -2. This charge was passed at approximately constant current density during 4 min at a potential of - 2 . 0 0 V vs SCE. Both curves in fig. 2 were recorded in a 0.1 molar, N2-purged N a O H solution. The electrode was illuminated by intermittent light from a 450 W xenon lamp, furnished with U G 5 Schott band pass filter transmitting light in the range 250-400 nm. Curve (a) was recorded at the as prepared electrode. Beginning at 0.00 V vs SCE anodic photocurrents were registered down to a potential of about - 0 . 8 V vs SCE. Below this potential photoinduced cathodic currents were registered. A cathodic wave, showing a peak at about - 1 . 1 V vs SCE, was observed. On the reversed scan the cathodic peak almost disappeared. This behaviour is typical of polycrystalline TiO 2 electrodes prepared by thermal oxidation at relatively high temperatures ( > 600 ° C) and long oxidation periods [8-11]. As was shown earlier in an RRDE-experiment [10] the cathodic wave at - 1.1 V vs SCE can be explained by the reduction of the products (mainly 02 from water oxidation) produced in light at potentials above about - 0 . 8 V. Curve (b) shows the i - U curve of electrode No. 2 after electrochemical reduction. Although the light also in this case was chopped throughout the whole potential range there were no photoinduced cathodic currents registered below - 0 . 8 V vs SCE. However, a huge cathodic wave at about - 1 . 0 V vs SCE (which disappeared on the reversed scan) shows that the 02 present in the solution (photogenerated while the electrode was swept through the more positive potentials, 0 ~ - 0 . 8 V vs SCE) was reduced effectively by electrons from the conduction band at potentials below - 0 . 8 V vs SCE. In fig. 3A the action spectra (in arbitrary units) registered before and after the reduction of electrode No. 2 can be compared. These spectra were recorded at 0.3 V vs SCE (well positive of the flatband potential) in the potential range where the spectral distribution of quantum efficiency has been shown to be independent of electrode potential and the photocurrent proportional to the light flux [12]. The
S.-E. Lindquist el al. / Effects of reduction of TiO: photoe/ectron.s
372
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electrode No. 2 in a 0.100 m o l a r The light b e a m from a the 450W SCE), but was left on d u r i n g the 10 m V / s .
large increase in quantum efficiency of the reduced electrode observed in the short wavelength range (250-330 nm) and the decrease at longer wavelengths (close to the bandgap) is evident from the figure. In order to obtain an estimated value of the relative change in the effective depletion layer width, w, upon reduction, data points in fig. 3A were replotted (fig. 3B) according to eq. (1): (
) 1/2 = (
)l/2( h. -
(1)
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(2)
a x = A(hl,
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- Eg)'/2/hu.
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Fig. 3. (A) Action spectra of electrode No. 2 (a) before and (b) after reduction. (B) Data in (A) replotted according to eq. (1).
In these relations fix is the quantum efficiency, Z p is the minority carrier diffusion length, h is the Planck's constant, v is the frequency of the light, n is an integer equal to 4 for an indirect transition [15], Eg is the bandgap, A is a constant, and w is expressed by: w = (2eeo/eNo)l/2(U
-
Ufb) 1/2,
(4)
In eq. (4) [14] e is the relative dielectric constant, e0 is the permittivity of free space, N D is the dopant density, U is the electrode potential and Ufb the flatband potential. The relative change in depletion layer width expressed as (Was preparedWreduced)Wasprepared was determined to be 0.43 for the present reduction. (Typical values of w0, defined as w at U - Ufb = 1 V, for as prepared electrodes of the same type as No. 2 are approximately 0.3 ~m [8]). It can be estimated from fig. 3B that the Eg-values of electrode No. 2 is somewhat smaller than the expected 4.8 × 10 -19 J or 3.05 eV [16] as determined from the temperature dependence of conductivity on single crystals of TiO2. Some influence from sub-bandgap transitions in the optical measurements may well explain the discrepancy.
374
S.-E. Lindquist et al. / Ef]ects o/reduction of TiOe photoelectron.~
3. 3. Effect of electrochemical reduction on the film thickness The electrode thickness was determined by measuring maxima and minima in the interference pattern in the visible and infrared region [8]. Repeated measurements on similar electrodes indicate a small (50-100 nm) decrease of the optical film thicknesses after the electrochemical reduction. Work is in progress to clarify if this effect is a consequence of a real decrease in film thickness due to etching or caused by changes in the optical properties of the film.
3.4. ESCA analysis ESCA analysis was performed on a number of electrodes prepared in the temperature range 500-700°C. Both as prepared and reduced electrodes were analyzed. The photoelectron peaks around 458.5 eV (Ti(2p3/2)) and 529.9 eV (O(ls)) have been studied in some detail. (The values of the given binding energies are all calculated assuming the C(ls) peak to be at 284.6 eV [17]). Table 3 summarizes some typical results. The Ti(2pl/2 ) peak was observed at 5.7 eV higher binding energy than the Ti(2p3/2) peak. All these peaks are typical of pure TiO 2 and they were present in the ESCA spectra of all the investigated electrodes. The ESCA spectra in fig. 4 show the O(ls) peaks at 529.9 + 0.1 eV (labelled B) of electrodes No. 3 and No. 4. The latter electrode was electrochemically reduced before the analysis. This electrode exhibits an additional peak at 532.2 + 0.2 eV labelled A. This peak was present in the spectra of all the reduced electrodes. It was ascribed to some hydroxy or oxyhydroxy configuration. The reduction procedure also affects the ratio of the intensities of the Ti(2p3/2 ) and O(ls) peaks significantly (see table 3). The interpretation of this will be discussed further on.
3.5. S I M S analysis 3.5.1. Mass spectra Fig. 5 shows the negative ion mass spectrum of electrode No. 5 registered as described above. (Note that the ion intensities are given on a logarithmic scale.) The main features of the spectrum are the same as found by Pefia and et al. [18]. Some prominent peaks are listed and assigned in table 4. Ions containing impurities are shown to the right in the list. A detailed discussion of the spectrum is beyond the
Table 3 Data from the ESCA-analysis Electrode No.
Binding energy/eV Ti(2p3/2)
O(ls)
Amplitude ratio Ti(2p3/2)/)O(ls)
3 4 7
458.4 458.5 458.5
529.9 529.9, 532.2 529.8
0.78 0.83 0.77
S.-E. Lindquist et al. / Effects of reduction of TiO2 photoelectrons
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Fig. 4. O(ls) photoelectron spectrum of the as prepared electrode No. 3 and the electrochemically reduced electrode No. 4. Both electrodes show a peak at 529.9 eV (labelled B), but only the reduced electrode (No. 4) show the satellite peak at 532.2 eV (labelled A). scope of the present work. The following main features should however be noted. The stable titanium isotopes (natural abundance 46Ti 7.93%, 47Ti 7.28%, 48Ti 73.94%, 49Ti 5.51% and 5°Ti 5.34%) are represented in sequences around the most abundant peaks at mass number (m/e) 48, 64, 80, 96 and 112. Hydrogen (m/e = 1) is present also in combinations with oxygen at m/e = 17, 18, and 19 as O H - , O H ~ and O H 3. The presence of m / e = 39 ( A r - ) indicates that some influence of background from air must be taken into account. The good resolution of the instrument allowed for good separation of T i - , S O and O~- at role = 48 (see insert of fig. 5) and S - and O~- at role = 32. The latter was of importance for the depth profile measurements of Ti and S (vide infra). N o significant difference between the reduced and as prepared electrodes could be deduced directly from the separate mass spectra.
3.5.2. Depth profile measurements 3.5.2.1 The depth profiles correlation to surface roughness. Fig. 6 shows SIMS depth profile measurements of some typical electrodes, Nos. 4-6. Electrode No. 5 was analyzed as prepared, No. 6 was polished after the oxidation procedure (with 0.25 ~tm diamond paste in the last step) to approximately half its initial thickness, and No. 4 was electrochemically reduced before the measurements (see table 1). The ion intensities, I, of a few representative mass numbers in the TiO 2 spectrum were followed as a function of the sputtering time t. The sputtering time depends on the morphology of the TiO 2 film but can be regarded as more or less proportional to the depth of the crater formed by the sputtering beam. As seen from the profiles in fig. 6 there is no sharp boundary line between the titanium and the oxide layer. The end point of the last fall-off region in the curves of role = 17, 32 and 35 has here been
376
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40
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90
100
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120
130
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--m/e Fig. 5. Mass spectrum of the polycrystalline TiO 2 film of electrode No. 5. For preparation parameters see table 1. Table 4 List of negative ions of most abundant isotopes in the TiO 2 films m/e
Ions from TiO 2
1 12 13 16 17 18 19 32 35 48 64 80 96 112
Impurity ions H C CH
O (Main peak)
0 2
Ti, 03 TiO TiO 2 Ti 2, TiO3 Ti20, TiO 4
OH OH 2 OH3 S C1 SO (S2) (TiSO)
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Fig. 6. SIMS profiles of the negative ions of OH (role =17), S (m/e = 32), C1 (role = 35) and Ti (m/e = 48). Electrodes No. 5 was analysed as prepared, No. 4 and No. 6 were reduced and polished respectivelybefore the SIMS analysis. For further preparation parameters, see table 1. taken as an approximate measure of the sputtering time that corresponds to the film thickness. Knowing the film thickness from optical measurements (see ref. [8] and references therein) the sputtering rate can thus be estimated. The t i t a n i u m / t i t a n i u m oxide interface was quite rough as was shown by diffuse- and total-reflectance measurements on as prepared and polished TiO 2 films in the UV and visible region [19]. In the interfacial region Magneli-phase titanium oxides (Ti,O2,_1) most probably are present. These will also affect the shape of profile in this region. Profile measurements were made at m / e = 48 and 64, corresponding to the negative ions of Ti and TiO. The ion intensity at m / e = 1 was to low to achieve reliable measurements, so role = 17 was followed to trace O H - and thus the depth profile of hydrogen. Sulphur and chlorine, present as impurities, were traced at m / e 32 and m / e 35. The mass spectrometer was (as mentioned above) during the profile measurements operated at high resolution, so that O~- and S - at m / e = 32, and S O - , 0 3 and T i - at m / e = 48 etc. were well separated. F r o m the curves in fig. 6 it can be seen, that impurities like sulphur, m / e = 32, and chorine, m / e = 35, tend to be transported backwards into the material during the oxidation procedure. A comparison of the concentration profiles of these impurities in the as prepared electrode No. 5 and the polished electrode No. 6 also shows marked differences in the region close to the surface. While the ion intensities of the impurities have a very low concentration at the surface in the case of the polished electrode, they are high in the case of the as prepared (No. 5) and reduced (No. 4) electrodes, and fall off to distinct minima after a sputtering time of about 0.4-0.6 ks. It is known from surface profile measurements [8] that the surface of films prepared under similar conditions as those described here, have a distance between tops and valleys of about 0.1-0.2 /~m. With a sputtering rate of = 0.3
378
S.-E. Lmdquist et a L / kJ'j~'cts oj reduction ~/ 770: photoelectron~
n m / s (estimated as described above) the position of the minima of the concentration profiles of the impurities S ( m / e = 32) and CI (role = 35), correspond well with that distance. It is proposed that the high ion intensities of S ( m / e = 32) and C1 (role -- 35) found close to time zero are due to sulphur and chlorine adsorbed at the surface. Although the details of the mechanism behind the found pattern of the depth profiles of the impurities in the surface region are somewhat obscure, it is evident from a number of runs that the deep minimum appearing in the profiles of these impurities is related to the surface roughness of the samples. The roughness of the TiO 2 films at both its interfaces ( a i r / T i O 2 and TiO2/Ti) as indicated by the SIMS profile data given above is in agreement with what can be deduced from optical reflectance measurements made in the range 200-600 nm on as prepared and polished TiO 2 films [19]. 3.5.2.2. The hydrogen depth profile. By comparing the O H ion ( m / e = 17) profiles of electrodes No. 4 and No. 5 we can see that hydrogen introduced by the electrochemical reduction is accumulated in a region just below the outmost surface region of the TiO2 film (see also ref. [19]). The distinct maximum at --- 0.9 ks was found in the O H - ion profile of all electrochemically reduced electrodes, but it was very small or nonexistent at the as prepared electrodes. The m a x i m u m was not present in the profiles of O at m / e = 16. Observing the position of the maximum of the O H -~ ion profile of electrode No. 4 one may speculate that the hydrogen preferentially enters the electrode surface via the valleys between the crystals. If so, it may be an indication of a nonuniform distribution of the current at the surface. 4. Discussion
4.1. The effects of reduction on the UV action spectrum It is evident from the present and earlier work that hydrogen can be implanted into TiO 2 films by electrochemical methods. The successive increase of the photoresponse in the short wavelength range (250-330 nm) is accompanied by an increase in surface concentration of hydroxy a n d / o r oxyhydroxy groups (as shown by the ESCA analysis). The ESCA studies show that there is also a significant increase in the peak ratio of Ti(2p3/2)/O(ls) upon reduction. This is consistent with the former result and can be interpreted as an effect of hydrogenation of oxygen atoms at the outmost surface region. On heavy reduction an increased concentration of hydrogen just below the TiO 2 surface region is observed, as shown by the SIMS profile measurements. At this stage of reduction a substantial shift in the edge of the action spectrum towards shorter wavelengths is registered, and the electrode becomes more conductive. Harris and Wilson [3] proposed that the extra conductivity they observed at the cathodically treated TiO 2 single crystals they investigated could be attributed to the diffusion of H atoms into the crystal. Hydrogen donates electrons to the conduction band leaving interstitial protons in the TiO 2 lattice. They also observed that some hydrogen disappeared in a few hours time if the crystal was just left standing in air or in the electrolyte.
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379
The ESCA and SIMS analysis of the present electrodes were for practical reasons performed several days after the electrodes had been reduced. A decrease in hydrogen content as observed by Harris and Wilson could thus not be verified. The SIMS measurements made with more than one year interval gave within experimental error the same result. The hydrogen we observed can therefore be regarded as relatively firmly bound in the TiO 2 film. Unfortunately the chemical state of the hydrogen present deeper in the electrode cannot be derived from the SIMS measurements. Notice that the constellations of atoms found in the ions of the spectrum not necessarily are present as such in the analyzed sample. They may have been formed during the sputtering procedure. Referring to the long time stability of the reduced electrodes it is reasonable to assume that hydroxyl groups bound to the crystal lattice are present also deeper in the TiO 2 film and not only at the surface. The firmly bound hydrogen in OH-groups will contribute to a more permanent increase of the dopant density, similar to that achieved after treatments at elevated temperatures in hydrogen atmosphere. The accumulation of hydrogen in a narrow region close to the surface upon reduction is consistent with the observations of Ginley and Knotek [1]. They implanted deuterium by what they call electrochemical doping, and used electronstimulated desorption measurements to monitor the depth profile. They found (at single crystals) a surface layer 60-200 nm thick with very high concentration of deuterium. It was earlier suggested [8] that the low quantum efficiencies registered in the short wavelength range of TiO 2 electrodes prepared at higher temperatures, would be due to a low doped top-layer (recombination layer) formed in the oxidation procedure. It was proposed that electron-hole recombination in this layer or at the surface was the cause of the reduced efficiency in the short wavelength range. The observations made in the present work are consistent with such a model. Remembering that the absorption coefficient [20-22] in the short wavelength range (250-330 nm) is very high *, the recovery of the photoresponse in this range at moderate reduction (curve 2 and 3 in fig. 1) can be understood as follows: The increased dopant density in the top-layer upon reduction results in higher conductivity and an increased local field strength. This gives a more efficient charge separation in the top-layer. Furthermore the diffusion of electrons and holes to the surface and the concomitant recombination at the surface is suppressed. Thus a decrease in the recombination rate close the electrode/electrolyte interface is achieved and subsequently a recovery of the photoresponse in the short wavelength range is registered. It is also interesting to note in this connection that the recovery of the photoresponse in the short wavelength range at moderate reduction takes place without any notable change in the position of the edge (towards longer wavelengths) of the action spectra. This indicates that there is no significant change in the depletion layer width, w, at this early stage of reduction. This means that w (and thus * The high absorption coefficientbelow 330 nm results in a very small penetration depth, 1/ct x < 100 am.
380
S.-E. Lindquist et a L / EfJects of reduction ~[ TiO~ photoeh'ctrons
U - U~, see eq. (4)) depends primarily on the dopant density deeper in the TiO, layer and not so much on the properties at the surface. The conditions for reduction of electrode No. 1 was not optimized. Nevertheless one can easily imagine (see fig. 1) that by a proper adjustment of the parameters in the cathodic treatment it should be possible to get electrodes with a high quantum efficiency in the whole investigated spectral range. Popkirov and Schindler [23] have in a recent paper discussed the spectral dependence of the quantum efficiency of thin film semiconductor electrodes. As a model for their calculations they used TiO 2 thin film electrodes. They found from theoretical considerations that at very low dopant densities there is a pronounced decrease in quantum efficiency in the action spectrum in the short wavelength range. Although they assume a homogeneously doped semiconducting film with a completely flat surface, the fall-off in the short wavelength range they predict from the model is qualitatively consistent with what we observe at the as prepared electrodes. The decrease in quantum efficiency in the short wavelength range is caused by majority carriers which are photogenerated close to the surface and reach the interface by diffusion against the electric field. In our case some contribution to the backcurrent due to the surface roughness cannot be excluded. Popkirov and Schindler conclude "that low doped material should be used to get a cell with high efficiency in the long wavelength range. To overcome the opposing effect of diffusing majority carriers and still maintain a high long wavelength response a very thin surface layer shoald be additionaly doped in order to increase the local field near the interface". Actually the present results seem to confirm this conclusion, and show that it is true even for a polycrystalline electrode with a rough surfaces. On heavier reduction the hydrogen penetrates deeper into the electrode. A considerable shift of the edge of the spectrum towards shorter wavelengths is observed (fig. 1, curve 4 and 5, and figs. 2 and 3). The response in the short wavelength range, however, remains high. This must be interpreted as a decrease in the depletion layer width, w. Our interpretation is here consistent with that of Harris and Wilson [3] but differs from that of Ginley and Knotek [1]. They suggested that the decrease in quantum efficiency in the long wavelengths range (330-400 nm) of their "cathodically aged" single crystals, indicates that the hydrogenated surface region acts as if it has many recombination centers. The results from the electrodes investigated in the present case rather imply that recombination in the surface region decrease. Unfortunately the action spectra given by Ginley and Knotek do not show the response below 330 nm (the short wavelength range), which contains information about on the conditions of the photogenerated carriers in the first 100-150 nm of the TiO 2 at the surface. It is therefore not possible to determine whether the decrease in photoresponse in the long wavelength range (330-400 nm) they observe on the cathodically aged electrodes is the result of a narrowing depletion layer width due to an increased dopant density close to the surface or (and), as they propose, increased recombination at the surface.
S.-E. Lindquist et al. / Effects of reduction of TiO e photoelectrons
381
4.2. The sub-bandgap response
Increased sub-bandgap response was observed by Goodenough and co-workers [24] after mechanical polishing of sintered SrTiO 3 electrodes. They showed that the electrodes after the polishing procedure could be reverted to their original characteristics after etching in concentrated HNO 3 at 120 o C. The weight loss measured after 5 rain etching corresponded to dissolution of about 0.5 ~m of the thickness of the material from the surface. The removal of such a layer was sufficient to decrease the response in the visible region to zero. In a previous paper from our laboratory [8] it was shown that the sub-bandgap response was increased at polycrystalline electrodes upon mechanical polishing. As in the present case for the reduced electrodes (fig. 1) the effect was most pronounced after the first treatment. Apparently the electrochemical reduction as performed in the present case does also introduce similar changes in the surface region. The almost constant quantum efficiency in the sub-bandgap region after the first treatment indicated that we are dealing with an effect which is due to changes in the outermost parts of the interfacial region. 5. Summary Electrochemical reduction in acidic solutions gives means to modify the photoresponse of thermally grown TiO 2 electrodes. The observed changes upon reduction with respect to the spectral distribution and changes in photocathodic response can qualitatively be understood as follows: (1) The reduction at cathodic potentials generates an increased amount of hydroxy a n d / o r oxyhydroxy groups in the surface region. (2) SIMS depth profile measurements show that on prolonged reduction hydrogen is accumulated in an 100-150 nm thick surface layer. (3) The effect of hydrogen is to increase the dopant density in a low doped surface layer (the recombination layer) formed during the oxidation procedure. Thus the charge separation close to the electrode/electrolyte is improved and a dramatic increase in the anodic photoresponse in the short wavelength range (250-330 nm) is observed. (4) With prolonged reduction hydrogen penetrates deeper into the electrode. This results in a decrease in the depletion layer width and the edge of the action spectrum is shifted towards shorter wavelengths. (5) The photoinduced cathodic current observed at the as prepared electrodes is related to the low dopant density in the recombination layer, as it vanishes when the surface of the electrode is doped with hydrogen.
Acknowledgement Hans Odelius at the Institute of Physics, Chalmers University of Technology, G~Steborg, Sweden, is greatly acknowledged for his assistance with the SIMS measurements.
S.-E. Lindqui.~t et al. / k,i/~ects o/ reduction ~J/ fiO: photoelectrons
382
This work was supported (NFR).
by the Swedish National
Science Research Council
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[101 [11] [12]
[13] [14] [15] [16] [17]
[18] [19] [20] [21] [22] [23] [24]
D.S. Ginley and M.L. Knotek, J. Electrochem. Soc. 126 (1979) 2163. M.F. Weber, L.C. Schumacher and M.J. Dignam, J. Electrochem. Soc. 129 (1982) 2022. L.A. Harris and R, Schumacher, J. Electrochem. Soc. 127 (1980) 1186. Y. Matsumoto, T. Shimizu, A. Toyoda and E. Sato, J. Phys. Chem. 86 (1982) 3581. H.-R. Sprfinken, R,. Schumacher and R.N. Schindler, Faraday Disc. 70 (1980) 55. L.A. Harris, M.E. Gersmer and R.H. Wilson, J. Electrochem. Soc. 126 (1979) 850. R.H. Wilson, L.A. Harris and M.E. Gerstner, J. Electrochem. Soc. 126 (1979) 844. S.-E. Lindquist, A. Lindgren and Zhu Yan-Ning, J. Electrochem. Soc. 132 (1985) 623. S.-E. Lindquist and Hilmar Vidarsson, Light Induced Cathodic Currents at Polycrystalline TiO 2 Electrodes, in the Book of Abstracts, The Electrochemical Society Spring Meeting, Toronto, Canada (May 12-17, 1985). S.-E. Lindquist and Hilmar Vidarsson, J. Mol. Catal. 38 (1987) 131. Y. Matsumoto, J. Kurimoto, T. Shimizu and E. Sato, J. Electrochem. Soc. 128 (1981) 1040. H. Vidarsson and S.-E. Lindquist, An RRDE Investigation of Light Induced Cathodic Currents at Polycrystalline TiO 2 Electrodes in the Presence of Dioxygen. Part lI. Shifts in Onset Potential, to be published. W.W. G~irtner, Phys. Rev. 116 (1959) 84. M.A. Butler, J. Appl. Phys. 48 (1977) 1914. E.J. Johnson, in: Semiconductors and Semimetals, Vol. 3, eds. R.K. Willardson and A.C. Beers (Academic Press, New York, 1967) chap. 6. D.C. Cronemeyer, Phys. Rev. 87 (1952) 876. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, eds., Handbook of X-Ray Photoelectron Spectroscopy, Perkin Elmer Corporation, Eden Prairie, Minnesota, USA (1979). J.L. Pe~a, M.H. Farlas and F. S~nchez-Sinencio, J. Electrochem. Soc. 129 (1982) 94. G. Manneberg, S.-E. Lindquist and M. Kull, J. Lightwave Technol. 5 (1987) 236. D.M. Eagels Jr., Phys. Chem. Solids 25 (1964) 1243. S.-E. Lindquist, B. Finnstr~m and L. Tegnrr, J. Electrochem. Soc. 130 (1983) 351. M. Cardona and G. Harbeke, Phys. Rev. 137 (1965) A1467. G. Popkirov and R.N. Schindler, Solar Energy Mater. 13 (1986) 161. B.T. Chang. G. Campet, J. Claverie, P. Hagenmuller and J.B. Goodenough, J. Solid State Chem. 49 (1983) 247.
367
EFFECTS OF ELECTROCHEMICAL REDUCTION OF POLYCRYSTALLINE TiO 2 PHOTOELECTRODES IN ACIDIC S O L U T I O N S Sten-Eric L I N D Q U I S T a n d A n d e r s L I N D G R E N Institute of Physical Chemistry, University of Uppsala, Box 532, S-751 21 Uppsala, Sweden
Christofer L E Y G R A F Swedish Corrosion Institute, Box 5607, S-114 86 Stockholm, Sweden
Received 30 January 1987; in revised form 19 March 1987 PolycrystaUine TiO2 film electrodes prepared by thermal oxidation were electrochemically reduced at cathodic potentials in acidic solutions. Changes in the spectral distribution of the quantum efficiency and the current-voltage characteristics at different stages of reduction were observed and related to ESCA (electron spectroscopy for chemical analysis) spectra of the surface and SIMS (secondary ion mass spectroscopy) depth profile analysis. Whereas a modest electrochemical reduction had a large effect on the anodic photoresponse in the short wavelengths range (250-330 nm) and almost none on the response at wavelengths close to the bandgap, more heavy reduction also affected the spectral response close to the bandgap and shifted the edge of the action spectrum towards shorter wavelengths. ESCA showed that the relative amount of hydrogen (present as hydroxy and/or oxyhydroxy groups) at the surface increased upon electrochemical reduction. SIMS profile measurements revealed that hydrogen was accumulated in a narrow (approximately 100-150 nm wide) zone just underneath the TiO2-surface facing the electrolyte. The photoinduced cathodic currents that could be registered at the "as prepared" electrodes vanished upon electrochemical reduction. The observations were interpreted in terms of an increased dopant density due to implanted hydrogen in a low doped top layer (the recombination layer) formed during the preparation of the virgin electrodes. The observations are in qualitative agreement with the predictions that can be made from accepted theoretical expressions for a liquid junction as described by Popkirov and Schindler [Solar Energy Mater. 13 (1986) 161].
1. Introduction H e a t i n g in v a c u u m or reduction with h y d r o g e n at elevated t e m p e r a t u r e are s t a n d a r d procedures to increase the c o n d u c t i v i t y of b o t h single crystal a n d polycrystalline s e m i c o n d u c t i n g oxide electrodes. Electrochemical r e d u c t i o n has also b e e n used for the same purpose. Studies of the latter t r e a t m e n t o n the p h o t o e l e c t r o c h e m ical properties of T i O 2 have b e e n p e r f o r m e d b y a n u m b e r of researchers (see refs. [1-5] a n d refs. therein). G i n l e y a n d K n o t e k [1] studied the change in the spectral d i s t r i b u t i o n of the q u a n t u m efficiency at different stages of r e d u c t i o n of single crystal T i O 2 electrodes i n the range 3 3 0 - 4 5 0 nm. T h e y also investigated the effect of r e d u c t i o n in the 0 1 6 5 - 1 6 3 3 / 8 7 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
368
S.-E. Lindquist et al. /' E[~bcts ~)[reduction of TIO: photoeh,¢tron~s
presence of D 2. Thus they could monitor the change in relative amount of H and D in the crystals by observing the absorption bands of the stretching frequencies of OH and OD in the IR spectra of TiO 2. Depth profiling by electron-stimulated desorption (ESD) measurements showed that in the electrochemically doped surface there was a high density of hydrogen in a shallow surface layer. Weber, Schumacher and Dignam [2] studied the effect of hydrogen loading by cathodic electrochemical treatment of reactivity sputtered TiO 2. They registered up to a tenfold increase in the overall photoresponse for electrodes illuminated with unfiltered light from a xenon lamp. They ascribed the improved photocurrent to an increase in the (minority) carrier diffusion length. They made current-voltage and impedance measurements, but no analysis of the spectral distribution of the quantum efficiency. One of the most extensive studies of single crystal TiO 2 as electrode in PEC cells was made by Wilson and co-workers [6,7]. In a brief but elucidating review Harris and Schumacher [3] sort out the different chemical states introduced into the TiO 2 lattice by reduction in H 2. They also suggest how these states affect the properties of a TiO 2 electrode. In the present work we will report on electrochemical reduction of polycrystalline TiO 2 electrodes. In particular the effect of the reduction on the anodic photocurrent and the photoinduced cathodic currents has been investigated. Anodic action spectra at different stages of reduction will be presented. ESCA analysis of the TiO 2 surface and SIMS depth profile measurements have been performed. The origin of the observed change in the photoresponse of the electrodes with reduction will be discussed with reference to these measurements.
2. Experimental 2.1. Materials
The titanium plate used in the preparations of the electrodes was the same as described before [8]. All solutions were made from reagent grade chemicals and double or triple distilled water, 2.2. Electrode preparation
1 mm thick circular titanium plates (diameter: 20 rmn) were polished to mirror brightness [8]. The plates were washed by ultrasonic agitation in an alkaline detergent (RBS 25, Labkemi AB, Stockholm) water solution. They were thoroughly rinsed with distilled water. The TiO 2 layers were formed by thermal oxidation in air at atmospheric pressure and at temperatures ( T ) in the range 5 0 0 - 7 0 0 ° C [8]. The electrodes were mounted in an O-ring sealed plexiglass holder exposing approximately 1.8 cm 2 to the electrolyte. The electrochemical reduction was performed at constant electrode po*~,nfials in 0.05 molar H2SO 4 solutions. The electrodes were stored under air in a closed vessel. They were carefully washed with water before they were mounted in the cell for current-voltage or
S.-E. Lindquist et al. / Effects of reduction of TiO: photoelectrons
369
Table 1 Preparation parameters and some properties of the T i O 2 electrodes used in the present work. All films were prepared by thermal oxidation in air of atmospheric pressure. The titanium plates were polished with 0.25 ~tm diamond paste in the last step before the oxidation procedure. The oxidation time was in all cases 4.00 h. Treatment after oxidation
Oxidation conditions Electrode
T
Film thickness
No.
(°C)
(~m)
1 2 3 4 5 6
685 695 685 685 710 710
1.1 -
7
508
0.10
-
Reduced, see table 2 Reduced 4 min at - 2.00 V vs SCE None Reduced 10 min at - 2.0 V vs SCE None Polished with diamond paste (0.25 I~m) to a TiOE-film thickness of 0.62 ~ m None
action spectra registrations. The preparation parameters of all electrodes are summarized in table 1.
2.3. Apparatus A conventional three electrode system in a cell with a Suprasil quartz window was used in the photoelectrochemical measurements. The counter electrode was separated from the electrolyte by a glass frit. The equipment for registration of the current-voltage characteristics and spectral distribution of the quantum efficiency has been described elsewhere [8]. The surface analysis was made with a Leybold Heraeus E S C A / A u g e r spectrometer L H 2000, with an Al-anode emitting soft X-rays with an energy of 1486.6 eV, and with photoelectrons being analyzed by means of a hemispherical analyzer. Under operation the base pressure was below I x 10-8 mm Hg. SIMS analysis was carried out on a Cameca, IMS 3S3F apparatus. The mass spectra and depth profile measurements were registered with negative secondary ions. 10 kV Cs + ions were used in the primary ionisation beam. The pressure in the system was routinely 3 x 10 -9 mm Hg. The analyzed areas were typically 60 ~m in diameter. 3. Results
3.1. Action spectra Fig. 1 shows a set of action spectra * of a polycrystalline TiO2 electrode (No. 1) (see table 1) oxidized in air at 6 8 5 ° C for 4.00 h. The oxidation conditions were • The action spectra were registered using a Schott U G 5 b a n d p a s s filter (in the range 250-380 nm) and a G G 375 cutoff filter (in the range 380-600 nm) in the b e a m from the monochromator.
S.-E. Lmdquis! eta/. / Effects of reduction o/TiO 2 photoelectr~ms
370
~X(O} _' '-' .6
!g~x I
.5
[-s
I .4 i }(f
. . . . .
300
&O0
500
.3
.2
.0
l 250
nm
300
350
400
450
Fig. 1. The quantum efficiency, q~, vs wavelength, ~, for TiO2 electrode No. 1, at different stages of reduction in 0.05 molar HzSO4 at cathodic potentials. Experimental parameters and symbols are given in table 2. The spectra were registered in a 0.100 molar phosphate buffer (pH = 5.8) at an electrode potential of 0.500 V vs SCE. The insert shows curves 1-4 in a semilogarithmic plot to elucidate the changes in the sub-bandgap response.
chosen to given an approximately 1 ~m thick TiO 2 film. Curve 1 shows the spectrum of the as prepared electrode, curves 2 - 5 show the recorded spectra after various successive electrochemical reductions of the same electrode in 0.05 molar H 2 S O 4. Experimental parameters with reference to fig. 1 are summarized in table 2. The decrease in q u a n t u m efficiency in the short wavelength range (250-300 nm, curve 1) is typical for electrodes prepared at higher temperatures [8]. The electrochemical reduction recovers the q u a n t u m efficiency in this range (curves 2 and 3) and the action spectrum successively reaches a shape congruent with the absorption spectrum of TiO 2. At longer reduction periods and at more cathodic potentials a decrease in q u a n t u m efficiency is registered in the long wavelength range 3 3 0 - 4 0 0 nm. Also note that after the first reduction sequence there is a marked increase in the sub-bandgap response (see inserted logarithmic plot in fig. 1). In the subsequent sequences the s u b - b a n d g a p response remains fairly constant.
S.-E. Lindquist et al. / Effects of reduction of TiO2 photoelectrons
371
Table 2 Summary of the experimental parameters used in the electrochemical reduction of electrode No. 1. The reduction was made at constant electrode potential in 0.05 molar H2SO4. The table gives the electrode potential, U, and the total transferred charge, Q, during various sequences of reduction. It also gives the symbols with reference to the action spectra given in fig. 1. Sequence of reduction
U (V vs SCE)
Q (A s cm-2)
Symbol in fig. 1
1 2 3 4 5
- 0.75 - 0.75 - 2,00 -2,50
0 0.016 0.035 2.8 57
zx [] x © •
3.2. Current-voltage characteristics and action spectra
The current-voltage ( i - U ) curves shown in fig. 2 were recorded before and after the reduction of electrode No. 2 prepared by oxidation in air at 695 o C for 4.00 h. It was heavily loaded with hydrogen by passing a charge of 18 A s cm -2. This charge was passed at approximately constant current density during 4 min at a potential of - 2 . 0 0 V vs SCE. Both curves in fig. 2 were recorded in a 0.1 molar, N2-purged N a O H solution. The electrode was illuminated by intermittent light from a 450 W xenon lamp, furnished with U G 5 Schott band pass filter transmitting light in the range 250-400 nm. Curve (a) was recorded at the as prepared electrode. Beginning at 0.00 V vs SCE anodic photocurrents were registered down to a potential of about - 0 . 8 V vs SCE. Below this potential photoinduced cathodic currents were registered. A cathodic wave, showing a peak at about - 1 . 1 V vs SCE, was observed. On the reversed scan the cathodic peak almost disappeared. This behaviour is typical of polycrystalline TiO 2 electrodes prepared by thermal oxidation at relatively high temperatures ( > 600 ° C) and long oxidation periods [8-11]. As was shown earlier in an RRDE-experiment [10] the cathodic wave at - 1.1 V vs SCE can be explained by the reduction of the products (mainly 02 from water oxidation) produced in light at potentials above about - 0 . 8 V. Curve (b) shows the i - U curve of electrode No. 2 after electrochemical reduction. Although the light also in this case was chopped throughout the whole potential range there were no photoinduced cathodic currents registered below - 0 . 8 V vs SCE. However, a huge cathodic wave at about - 1 . 0 V vs SCE (which disappeared on the reversed scan) shows that the 02 present in the solution (photogenerated while the electrode was swept through the more positive potentials, 0 ~ - 0 . 8 V vs SCE) was reduced effectively by electrons from the conduction band at potentials below - 0 . 8 V vs SCE. In fig. 3A the action spectra (in arbitrary units) registered before and after the reduction of electrode No. 2 can be compared. These spectra were recorded at 0.3 V vs SCE (well positive of the flatband potential) in the potential range where the spectral distribution of quantum efficiency has been shown to be independent of electrode potential and the photocurrent proportional to the light flux [12]. The
S.-E. Lindquist el al. / Effects of reduction of TiO: photoe/ectron.s
372
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6
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electrode No. 2 in a 0.100 m o l a r The light b e a m from a the 450W SCE), but was left on d u r i n g the 10 m V / s .
large increase in quantum efficiency of the reduced electrode observed in the short wavelength range (250-330 nm) and the decrease at longer wavelengths (close to the bandgap) is evident from the figure. In order to obtain an estimated value of the relative change in the effective depletion layer width, w, upon reduction, data points in fig. 3A were replotted (fig. 3B) according to eq. (1): (
) 1/2 = (
)l/2( h. -
(1)
Eq. (1) was derived from the G~irtner-Butler relation [13,14], given by eq. (2), and the wavelength (~) dependence of the absorption coefficient (ax) [13] given by eq. (3) below, assuming o~xLp << 1 and ax w << 1. Eq. (1) is thus only valid in a range close to the bandgap: q~x= (1 - (1/(1 + axLp)) e x p [ - a x w ] ;
(2)
a x = A(hl,
(3)
- Eg)'/2/hu.
S.-E. Lindquist et al. / Effects of reduction of TiO2 photoelectrons i
i
I
373
I
o b
L
o
1
I
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o
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o
o
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I
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5.5
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In these relations fix is the quantum efficiency, Z p is the minority carrier diffusion length, h is the Planck's constant, v is the frequency of the light, n is an integer equal to 4 for an indirect transition [15], Eg is the bandgap, A is a constant, and w is expressed by: w = (2eeo/eNo)l/2(U
-
Ufb) 1/2,
(4)
In eq. (4) [14] e is the relative dielectric constant, e0 is the permittivity of free space, N D is the dopant density, U is the electrode potential and Ufb the flatband potential. The relative change in depletion layer width expressed as (Was preparedWreduced)Wasprepared was determined to be 0.43 for the present reduction. (Typical values of w0, defined as w at U - Ufb = 1 V, for as prepared electrodes of the same type as No. 2 are approximately 0.3 ~m [8]). It can be estimated from fig. 3B that the Eg-values of electrode No. 2 is somewhat smaller than the expected 4.8 × 10 -19 J or 3.05 eV [16] as determined from the temperature dependence of conductivity on single crystals of TiO2. Some influence from sub-bandgap transitions in the optical measurements may well explain the discrepancy.
374
S.-E. Lindquist et al. / Ef]ects o/reduction of TiOe photoelectron.~
3. 3. Effect of electrochemical reduction on the film thickness The electrode thickness was determined by measuring maxima and minima in the interference pattern in the visible and infrared region [8]. Repeated measurements on similar electrodes indicate a small (50-100 nm) decrease of the optical film thicknesses after the electrochemical reduction. Work is in progress to clarify if this effect is a consequence of a real decrease in film thickness due to etching or caused by changes in the optical properties of the film.
3.4. ESCA analysis ESCA analysis was performed on a number of electrodes prepared in the temperature range 500-700°C. Both as prepared and reduced electrodes were analyzed. The photoelectron peaks around 458.5 eV (Ti(2p3/2)) and 529.9 eV (O(ls)) have been studied in some detail. (The values of the given binding energies are all calculated assuming the C(ls) peak to be at 284.6 eV [17]). Table 3 summarizes some typical results. The Ti(2pl/2 ) peak was observed at 5.7 eV higher binding energy than the Ti(2p3/2) peak. All these peaks are typical of pure TiO 2 and they were present in the ESCA spectra of all the investigated electrodes. The ESCA spectra in fig. 4 show the O(ls) peaks at 529.9 + 0.1 eV (labelled B) of electrodes No. 3 and No. 4. The latter electrode was electrochemically reduced before the analysis. This electrode exhibits an additional peak at 532.2 + 0.2 eV labelled A. This peak was present in the spectra of all the reduced electrodes. It was ascribed to some hydroxy or oxyhydroxy configuration. The reduction procedure also affects the ratio of the intensities of the Ti(2p3/2 ) and O(ls) peaks significantly (see table 3). The interpretation of this will be discussed further on.
3.5. S I M S analysis 3.5.1. Mass spectra Fig. 5 shows the negative ion mass spectrum of electrode No. 5 registered as described above. (Note that the ion intensities are given on a logarithmic scale.) The main features of the spectrum are the same as found by Pefia and et al. [18]. Some prominent peaks are listed and assigned in table 4. Ions containing impurities are shown to the right in the list. A detailed discussion of the spectrum is beyond the
Table 3 Data from the ESCA-analysis Electrode No.
Binding energy/eV Ti(2p3/2)
O(ls)
Amplitude ratio Ti(2p3/2)/)O(ls)
3 4 7
458.4 458.5 458.5
529.9 529.9, 532.2 529.8
0.78 0.83 0.77
S.-E. Lindquist et al. / Effects of reduction of TiO2 photoelectrons
.
.
.
.
i
375
•
L~ I
5;0.0 ....
535.0
1
i
i
,
5 '0.0
i
525.0
BINDING ENERGY/eV
Fig. 4. O(ls) photoelectron spectrum of the as prepared electrode No. 3 and the electrochemically reduced electrode No. 4. Both electrodes show a peak at 529.9 eV (labelled B), but only the reduced electrode (No. 4) show the satellite peak at 532.2 eV (labelled A). scope of the present work. The following main features should however be noted. The stable titanium isotopes (natural abundance 46Ti 7.93%, 47Ti 7.28%, 48Ti 73.94%, 49Ti 5.51% and 5°Ti 5.34%) are represented in sequences around the most abundant peaks at mass number (m/e) 48, 64, 80, 96 and 112. Hydrogen (m/e = 1) is present also in combinations with oxygen at m/e = 17, 18, and 19 as O H - , O H ~ and O H 3. The presence of m / e = 39 ( A r - ) indicates that some influence of background from air must be taken into account. The good resolution of the instrument allowed for good separation of T i - , S O and O~- at role = 48 (see insert of fig. 5) and S - and O~- at role = 32. The latter was of importance for the depth profile measurements of Ti and S (vide infra). N o significant difference between the reduced and as prepared electrodes could be deduced directly from the separate mass spectra.
3.5.2. Depth profile measurements 3.5.2.1 The depth profiles correlation to surface roughness. Fig. 6 shows SIMS depth profile measurements of some typical electrodes, Nos. 4-6. Electrode No. 5 was analyzed as prepared, No. 6 was polished after the oxidation procedure (with 0.25 ~tm diamond paste in the last step) to approximately half its initial thickness, and No. 4 was electrochemically reduced before the measurements (see table 1). The ion intensities, I, of a few representative mass numbers in the TiO 2 spectrum were followed as a function of the sputtering time t. The sputtering time depends on the morphology of the TiO 2 film but can be regarded as more or less proportional to the depth of the crater formed by the sputtering beam. As seen from the profiles in fig. 6 there is no sharp boundary line between the titanium and the oxide layer. The end point of the last fall-off region in the curves of role = 17, 32 and 35 has here been
376
S.-E. Lindquist et al. / Effects o/reduction o[ 7)'02 photoe/ectr(m~s 1E6
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90
100
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120
130
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140
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--m/e Fig. 5. Mass spectrum of the polycrystalline TiO 2 film of electrode No. 5. For preparation parameters see table 1. Table 4 List of negative ions of most abundant isotopes in the TiO 2 films m/e
Ions from TiO 2
1 12 13 16 17 18 19 32 35 48 64 80 96 112
Impurity ions H C CH
O (Main peak)
0 2
Ti, 03 TiO TiO 2 Ti 2, TiO3 Ti20, TiO 4
OH OH 2 OH3 S C1 SO (S2) (TiSO)
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i I
0 t ks
~ 2
L 3
IE2 0
I
2
r
Fig. 6. SIMS profiles of the negative ions of OH (role =17), S (m/e = 32), C1 (role = 35) and Ti (m/e = 48). Electrodes No. 5 was analysed as prepared, No. 4 and No. 6 were reduced and polished respectivelybefore the SIMS analysis. For further preparation parameters, see table 1. taken as an approximate measure of the sputtering time that corresponds to the film thickness. Knowing the film thickness from optical measurements (see ref. [8] and references therein) the sputtering rate can thus be estimated. The t i t a n i u m / t i t a n i u m oxide interface was quite rough as was shown by diffuse- and total-reflectance measurements on as prepared and polished TiO 2 films in the UV and visible region [19]. In the interfacial region Magneli-phase titanium oxides (Ti,O2,_1) most probably are present. These will also affect the shape of profile in this region. Profile measurements were made at m / e = 48 and 64, corresponding to the negative ions of Ti and TiO. The ion intensity at m / e = 1 was to low to achieve reliable measurements, so role = 17 was followed to trace O H - and thus the depth profile of hydrogen. Sulphur and chlorine, present as impurities, were traced at m / e 32 and m / e 35. The mass spectrometer was (as mentioned above) during the profile measurements operated at high resolution, so that O~- and S - at m / e = 32, and S O - , 0 3 and T i - at m / e = 48 etc. were well separated. F r o m the curves in fig. 6 it can be seen, that impurities like sulphur, m / e = 32, and chorine, m / e = 35, tend to be transported backwards into the material during the oxidation procedure. A comparison of the concentration profiles of these impurities in the as prepared electrode No. 5 and the polished electrode No. 6 also shows marked differences in the region close to the surface. While the ion intensities of the impurities have a very low concentration at the surface in the case of the polished electrode, they are high in the case of the as prepared (No. 5) and reduced (No. 4) electrodes, and fall off to distinct minima after a sputtering time of about 0.4-0.6 ks. It is known from surface profile measurements [8] that the surface of films prepared under similar conditions as those described here, have a distance between tops and valleys of about 0.1-0.2 /~m. With a sputtering rate of = 0.3
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S.-E. Lmdquist et a L / kJ'j~'cts oj reduction ~/ 770: photoelectron~
n m / s (estimated as described above) the position of the minima of the concentration profiles of the impurities S ( m / e = 32) and CI (role = 35), correspond well with that distance. It is proposed that the high ion intensities of S ( m / e = 32) and C1 (role -- 35) found close to time zero are due to sulphur and chlorine adsorbed at the surface. Although the details of the mechanism behind the found pattern of the depth profiles of the impurities in the surface region are somewhat obscure, it is evident from a number of runs that the deep minimum appearing in the profiles of these impurities is related to the surface roughness of the samples. The roughness of the TiO 2 films at both its interfaces ( a i r / T i O 2 and TiO2/Ti) as indicated by the SIMS profile data given above is in agreement with what can be deduced from optical reflectance measurements made in the range 200-600 nm on as prepared and polished TiO 2 films [19]. 3.5.2.2. The hydrogen depth profile. By comparing the O H ion ( m / e = 17) profiles of electrodes No. 4 and No. 5 we can see that hydrogen introduced by the electrochemical reduction is accumulated in a region just below the outmost surface region of the TiO2 film (see also ref. [19]). The distinct maximum at --- 0.9 ks was found in the O H - ion profile of all electrochemically reduced electrodes, but it was very small or nonexistent at the as prepared electrodes. The m a x i m u m was not present in the profiles of O at m / e = 16. Observing the position of the maximum of the O H -~ ion profile of electrode No. 4 one may speculate that the hydrogen preferentially enters the electrode surface via the valleys between the crystals. If so, it may be an indication of a nonuniform distribution of the current at the surface. 4. Discussion
4.1. The effects of reduction on the UV action spectrum It is evident from the present and earlier work that hydrogen can be implanted into TiO 2 films by electrochemical methods. The successive increase of the photoresponse in the short wavelength range (250-330 nm) is accompanied by an increase in surface concentration of hydroxy a n d / o r oxyhydroxy groups (as shown by the ESCA analysis). The ESCA studies show that there is also a significant increase in the peak ratio of Ti(2p3/2)/O(ls) upon reduction. This is consistent with the former result and can be interpreted as an effect of hydrogenation of oxygen atoms at the outmost surface region. On heavy reduction an increased concentration of hydrogen just below the TiO 2 surface region is observed, as shown by the SIMS profile measurements. At this stage of reduction a substantial shift in the edge of the action spectrum towards shorter wavelengths is registered, and the electrode becomes more conductive. Harris and Wilson [3] proposed that the extra conductivity they observed at the cathodically treated TiO 2 single crystals they investigated could be attributed to the diffusion of H atoms into the crystal. Hydrogen donates electrons to the conduction band leaving interstitial protons in the TiO 2 lattice. They also observed that some hydrogen disappeared in a few hours time if the crystal was just left standing in air or in the electrolyte.
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The ESCA and SIMS analysis of the present electrodes were for practical reasons performed several days after the electrodes had been reduced. A decrease in hydrogen content as observed by Harris and Wilson could thus not be verified. The SIMS measurements made with more than one year interval gave within experimental error the same result. The hydrogen we observed can therefore be regarded as relatively firmly bound in the TiO 2 film. Unfortunately the chemical state of the hydrogen present deeper in the electrode cannot be derived from the SIMS measurements. Notice that the constellations of atoms found in the ions of the spectrum not necessarily are present as such in the analyzed sample. They may have been formed during the sputtering procedure. Referring to the long time stability of the reduced electrodes it is reasonable to assume that hydroxyl groups bound to the crystal lattice are present also deeper in the TiO 2 film and not only at the surface. The firmly bound hydrogen in OH-groups will contribute to a more permanent increase of the dopant density, similar to that achieved after treatments at elevated temperatures in hydrogen atmosphere. The accumulation of hydrogen in a narrow region close to the surface upon reduction is consistent with the observations of Ginley and Knotek [1]. They implanted deuterium by what they call electrochemical doping, and used electronstimulated desorption measurements to monitor the depth profile. They found (at single crystals) a surface layer 60-200 nm thick with very high concentration of deuterium. It was earlier suggested [8] that the low quantum efficiencies registered in the short wavelength range of TiO 2 electrodes prepared at higher temperatures, would be due to a low doped top-layer (recombination layer) formed in the oxidation procedure. It was proposed that electron-hole recombination in this layer or at the surface was the cause of the reduced efficiency in the short wavelength range. The observations made in the present work are consistent with such a model. Remembering that the absorption coefficient [20-22] in the short wavelength range (250-330 nm) is very high *, the recovery of the photoresponse in this range at moderate reduction (curve 2 and 3 in fig. 1) can be understood as follows: The increased dopant density in the top-layer upon reduction results in higher conductivity and an increased local field strength. This gives a more efficient charge separation in the top-layer. Furthermore the diffusion of electrons and holes to the surface and the concomitant recombination at the surface is suppressed. Thus a decrease in the recombination rate close the electrode/electrolyte interface is achieved and subsequently a recovery of the photoresponse in the short wavelength range is registered. It is also interesting to note in this connection that the recovery of the photoresponse in the short wavelength range at moderate reduction takes place without any notable change in the position of the edge (towards longer wavelengths) of the action spectra. This indicates that there is no significant change in the depletion layer width, w, at this early stage of reduction. This means that w (and thus * The high absorption coefficientbelow 330 nm results in a very small penetration depth, 1/ct x < 100 am.
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S.-E. Lindquist et a L / EfJects of reduction ~[ TiO~ photoeh'ctrons
U - U~, see eq. (4)) depends primarily on the dopant density deeper in the TiO, layer and not so much on the properties at the surface. The conditions for reduction of electrode No. 1 was not optimized. Nevertheless one can easily imagine (see fig. 1) that by a proper adjustment of the parameters in the cathodic treatment it should be possible to get electrodes with a high quantum efficiency in the whole investigated spectral range. Popkirov and Schindler [23] have in a recent paper discussed the spectral dependence of the quantum efficiency of thin film semiconductor electrodes. As a model for their calculations they used TiO 2 thin film electrodes. They found from theoretical considerations that at very low dopant densities there is a pronounced decrease in quantum efficiency in the action spectrum in the short wavelength range. Although they assume a homogeneously doped semiconducting film with a completely flat surface, the fall-off in the short wavelength range they predict from the model is qualitatively consistent with what we observe at the as prepared electrodes. The decrease in quantum efficiency in the short wavelength range is caused by majority carriers which are photogenerated close to the surface and reach the interface by diffusion against the electric field. In our case some contribution to the backcurrent due to the surface roughness cannot be excluded. Popkirov and Schindler conclude "that low doped material should be used to get a cell with high efficiency in the long wavelength range. To overcome the opposing effect of diffusing majority carriers and still maintain a high long wavelength response a very thin surface layer shoald be additionaly doped in order to increase the local field near the interface". Actually the present results seem to confirm this conclusion, and show that it is true even for a polycrystalline electrode with a rough surfaces. On heavier reduction the hydrogen penetrates deeper into the electrode. A considerable shift of the edge of the spectrum towards shorter wavelengths is observed (fig. 1, curve 4 and 5, and figs. 2 and 3). The response in the short wavelength range, however, remains high. This must be interpreted as a decrease in the depletion layer width, w. Our interpretation is here consistent with that of Harris and Wilson [3] but differs from that of Ginley and Knotek [1]. They suggested that the decrease in quantum efficiency in the long wavelengths range (330-400 nm) of their "cathodically aged" single crystals, indicates that the hydrogenated surface region acts as if it has many recombination centers. The results from the electrodes investigated in the present case rather imply that recombination in the surface region decrease. Unfortunately the action spectra given by Ginley and Knotek do not show the response below 330 nm (the short wavelength range), which contains information about on the conditions of the photogenerated carriers in the first 100-150 nm of the TiO 2 at the surface. It is therefore not possible to determine whether the decrease in photoresponse in the long wavelength range (330-400 nm) they observe on the cathodically aged electrodes is the result of a narrowing depletion layer width due to an increased dopant density close to the surface or (and), as they propose, increased recombination at the surface.
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4.2. The sub-bandgap response
Increased sub-bandgap response was observed by Goodenough and co-workers [24] after mechanical polishing of sintered SrTiO 3 electrodes. They showed that the electrodes after the polishing procedure could be reverted to their original characteristics after etching in concentrated HNO 3 at 120 o C. The weight loss measured after 5 rain etching corresponded to dissolution of about 0.5 ~m of the thickness of the material from the surface. The removal of such a layer was sufficient to decrease the response in the visible region to zero. In a previous paper from our laboratory [8] it was shown that the sub-bandgap response was increased at polycrystalline electrodes upon mechanical polishing. As in the present case for the reduced electrodes (fig. 1) the effect was most pronounced after the first treatment. Apparently the electrochemical reduction as performed in the present case does also introduce similar changes in the surface region. The almost constant quantum efficiency in the sub-bandgap region after the first treatment indicated that we are dealing with an effect which is due to changes in the outermost parts of the interfacial region. 5. Summary Electrochemical reduction in acidic solutions gives means to modify the photoresponse of thermally grown TiO 2 electrodes. The observed changes upon reduction with respect to the spectral distribution and changes in photocathodic response can qualitatively be understood as follows: (1) The reduction at cathodic potentials generates an increased amount of hydroxy a n d / o r oxyhydroxy groups in the surface region. (2) SIMS depth profile measurements show that on prolonged reduction hydrogen is accumulated in an 100-150 nm thick surface layer. (3) The effect of hydrogen is to increase the dopant density in a low doped surface layer (the recombination layer) formed during the oxidation procedure. Thus the charge separation close to the electrode/electrolyte is improved and a dramatic increase in the anodic photoresponse in the short wavelength range (250-330 nm) is observed. (4) With prolonged reduction hydrogen penetrates deeper into the electrode. This results in a decrease in the depletion layer width and the edge of the action spectrum is shifted towards shorter wavelengths. (5) The photoinduced cathodic current observed at the as prepared electrodes is related to the low dopant density in the recombination layer, as it vanishes when the surface of the electrode is doped with hydrogen.
Acknowledgement Hans Odelius at the Institute of Physics, Chalmers University of Technology, G~Steborg, Sweden, is greatly acknowledged for his assistance with the SIMS measurements.
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This work was supported (NFR).
by the Swedish National
Science Research Council
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