Energetics and dynamics of the interface of RuS2 and implications for photoelectrolysis of water

J. Electroanal Chem., 201 (1986) 2633282 Elsevier Sequoia S.A., Lausanne - Printed

263 m The Netherlands

ENERGETICS AND DYNAMICS OF THE INTERFACE OF RuS, IMPLICATIONS FOR PHOTOELECTROLYSIS OF WATER

H.-M. KUHNE

and H. TRIBUTSCH

Hahn-Mertner-lnstrtut (Received

AND

ftir Kernforschung Berlrn. BereIch Struhlenchemre,

10th May 1985; m revised form 4th November

D-1000 Berlin 39 (F.R.G

)

1985)

ABSTRACT RuSz (Eo = 1.3 eV), grown from liquid Bi, has proved to be a stable and fairly efficient photoanode for a potential assisted photoelectrolysis of water using vtstble and near infrared light. Its valence band has a quite pure d-character. Holes generated on d-states in the valence band lead to the formation of Ru-based surface states which induce interfacial coordination bonding. The average positive charge accumulated m these complexes determines the energetic position of the energy bands wtth the consequence that the band edges are shifted with application of an electrode potential until their position hecomes stabilized by electron injection from the electrolyte. This conclusion is derived from capacity measurements performed in the region of hght Intensity hmited and diffusion controlled anodic photocurrents (rotating disk experiments). The energetic hmitation of RuS, photoelectrodes has mainly to be seen in the position of the surface states which are formed too high above the edge of the valence band For the present this compound appears to be a very attracttve model system for research on low photon energy photooxidatton of water.

INTRODUCTION

RuS, photoanodes have been shown to be of interest for the photoelectrolysis of water and halogenic acids [l-4]. Good stability was found during the photoevolution of oxygen from water at the thermodynamically unstable compound [3-51. The purity of the d-character of Ru-states, which form the upper valence band edge of RuS,, seems to be of decisive importance in the kinetic inhibition of possible corrosion reactions [5,6]. A major advantage for the use of RuS, in a potential assisted photoelectrolysis device would be its catalytic activity for oxygen and chlorine evolution. At the semiconductor/electrolyte interface, however, considerable potential losses occur. which are accompanied by an unpinning of the band edges. Instead of the formation of an inversion layer [7] we had to assign this shift of band edges to a high concentration of Ru-based surface states, the position of which is situated in the forbidden energy gap of RuS, [3]. The central importance of these surface states for charge transfer catalysis and energetic limitations of RuS, photoanodes is reflected in recently performed laser pulse studies [8], and it will be discussed from a more photoelectrochemical point of view in this paper. 0022-0728/86/$03.50

0 1986 Elsevier Sequoia

S.A.

EXPERIMENTAL

Single crystalline RuS2 was grown in liquid Bi. In a typical experiment, 1 g RuS, powder (for preparation see refs. 3,9) and 40 g Bi (freshly reduced in H,/Ar at 700°C for 2 h) were sealed in a quartz tube (lOph Torr). The melt was kept at 1080°C for 10 h (perpendicular position of the ampoule), then slowly cooled down (2”C/h). At a temperature of 800°C the furnace was switched off. Bi was removed in 30% HNO,; the crystals of RuS, were rinsed several times with HNO, and water. The reagents (Bi: m6N5, Ru: M3N. S: T5N5) were supplied by Ventron. For the exact crystal growth procedure and analysis see ref. 9 (main impurity is Bi, content up to 1% of the lattice atoms). Without addition of any doping the material is n-type. Ohmic back contacts were made using a Ga/In alloy. The crystals were mounted onto a Vespel cylinder with a brass core by means of Ag epoxy and encapsulated with epoxy resin (Scotchcast 3M XR 5241). Typical surface areas exposed to the solution were of the order of 1 mm2. A standard photoelectrochemical setup was used for current-voltage and capacity measurements (potentiostat: Bank POS 73, lock in: PAR 5204, chopper: PAR 9479, light source: Oriel W-Hal 25OW, XY-recorder: Rohde&Schwarz ZSK2). Rotating disk experiments were performed using Pine Instrument equipment. Photocurrent spectra were taken with a computerized setup (HP85, step motor drive: HMI ST70, monochromator: Kratos GM 252). Quantum efficiencies were determined using a laser spot (633 nm), focused onto the crystal (different light intensities, intensity measurement by a UDT 80 X Opto-Meter). Chemicals were of analytical grade and water triply deionized. A mercury/mercurous sulfate electrode (MSE) served as the reference electrode. All potentials are given with respect to SHE by adding +0.65 V to the measured potentials. RESULTS

Dynamics

of current-voltage

behavrour

The photoelectrochemical behaviour of Bi-grown n-type RuS, crystals is very similar to earlier results obtained on material formed in liquid Te [l-4]. In many cases no photoeffects are observed prior to the first oxygen evolution at the crystals. The electrodes in contact with sulfuric acid behave like a metal with a high catalytic activity for oxygen evolution. During the oxidation reaction, dark currents decrease and a simultaneous onset of anodic photocurrents is observed. The electrode soon stabilizes and assumes the qualities of an n-type semiconductor, which is able to photooxide water with visible and infrared light (Fig. 1). The spectral sensitivity of Bi-grown RuS, extends to photon energies of about 1.3 eV (Fig. 2). Deviations in the energy threshold compared to crystals containing traces of iron [2.3] are small but remarkable. The absolute sensitivity for long wavelength photons

265

-03

0

1

electrode

c

J

potential (SHE)/V

Fig. 1 Current-voltage curves of n-RuS, under mterrmttent illumination (W-halogen 250 W). Positive sweep rate 10 mV SC’ (a, b), 20 mV SC’ (c). (a) First polarisatlon (0.1 M K2S04): (b) after (a) and subsequent oxygen evolution at U = + 1.95 V/SHE for 5 mm; (c) typical charactenstlcs of a stabllised Bi-grown photoelectrode (0.5 M H,SO,) Arrow indicates the detachment of large 0, bubbles. insert corresponds to higher photocurrent density (focused light)

varies,

probably

depending

on the solid

state

quality.

There

is, however,

no evidence

for an energy gap in the visible part of the solar spectrum (cf. ref. 1). Measurements of the monochromatic quantum efficiency at 633 nm of the samples investigated hardly yield values above 0.15 (15%). even after correction for reflection losses. It is interesting to compare the penetration depth of photons (Y-’ with the space charge layer extension W. In a separate experiment, the absorption coefficient (Yat 633 nm has been determined ellipsometrically and exceeds 3.6 X lo5 cm-‘, corresponding to (Y- ’ < 28 nm. For a surface barrier height of 650 mV (i E,/e) and a typical donor concentration of 1.4 X 10” cme3 (see Discussion) we

photon

energy

/eV

Fig. 2. Photocurrent spectrum of BI-grown RuS, V (SHE). The energy gap EG IS Indicated

single crystals.

0.5 M H2S04,

electrode

potential

+ 1.8

obtain W = 23 nm [lo]. Hence, light is almost completely absorbed in the depletion layer. Current-voltage characteristics of Bi grown n-RuS, show a pronounced photocurrent saturation, as can be seen in Fig. 3. Because light absorption in all electrolytes used is negligible, photocurrent saturation is reached at about the same current densities. Note the considerable shift of the photocurrent onset of more than 1 V and the obvious kinetic inhibition of the

12

8

- 0.2

0.2

0.6

1.0

1.4

electrode potential

m the presence of different 3 Photocurrent-voltage characteristics of n-RuS, Photocurrent saturation 1s regularly observed. Illummatlon W-Hal ca. 170 mW cm-‘. were obtained by dissolvmg the potassmm salts in 0.5 M H,SO,.

k.HE electron donors. “HI” and “HBr”

267

125-

25

-L

I! eLectrode

potential

I VSHE

Fig. 4. Current-voltage behawour of RuSz in 1 M KI +OS M H,SO,. (W-Hal ca. 1.8 W cme2, positive sweep rate: 10 mV s-’ ). Electron injection mto the electrolyte leads to dark reduction of photogenerated 1;

mterrmttently dluminated from the conduction band

O2 evolution. With respect to iodine and bromine formation there is an overpotential of about 0.4 V (determined from the difference between half-wave and redox potentials). The importance of electron injection from the conduction band, when the electrode is biased more negatively than the thermodynamic redox potential, can be seen in the current-voltage characteristics under intermittent illumination (Fig. 4). Photogenerated I, is readily reduced in the dark leading to the cathodic.wave between photocurrent onset and Ezd (I -/I 1 ). An analogous behaviour is observed of for solutions of [Fe(CN),14-. Fe 2+, Br- or Ce3+. The reversal of photooxidation Cl- or H,O is much less evident. In order to explain why oxidation and reduction processes can occur simultaneously for different redox species over a wide range of electrode potentials, it has to be concluded that the energy bands are shifting. Because the photopotential of n-RuS2 is rather limited, there is quite a large potential region in sulfuric acid, where no photooxidation processes other than corrosion are thermodynamically possible. Accordingly, no net photocharge is observed over a full period in fast cyclic voltammograms (Fig. 5).

268

06-

I

0

02

04

06

0.8

electrode potential I

I.0

1;

VSHE

Fig. 5. Current-voltage characteristics of dluminated smgle crystal RuS, m 0.5 M H,SO, (02-saturated atmosphere) at varying scan rates (directlon mdlcated by arrows). Illummatlon (633 nm. ca. 50 mW cm-‘) mcreases both anodic and cathodic currents whtch compensate themselves over a full cycle m the voltammograms.

The hysteresis of the observed currents can be assigned to reversible surface controlled processes at the interface. Obviously there are no signs for irreversible reactions like corrosion. The peak structure of Fig. 5 is in fairly good agreement with data obtained for single crystal RuO, [ll], indicating that the same adsorption and reaction sites are present on both compounds. Earlier XPS measurements performed in our laboratory likewise gave evidence for a slight Ru-oxide coverage (a few monolayers) on RuS, photoanodes [5]. The potential region where, in the presence of electron donors anodic photocurrents are observed, depends mainly on the respective thermodynamic redox potential (cf. Figs. 3 and 4). Interestingly, anodic photocurrents can also be detected if electron acceptors only are added to the solution. During the negative sweep a cathodic dark current onset is found some 200 mV positive from Erzd. which corresponds to the reduction of the acceptor present. Intermittent illumination reveals that the reduced species can be reoxidized by photogenerated holes (for Fe’+/Fej+, cf. insert in Fig. 6). It is important to note that anodic photocurrents disappear in the potential region where the dark current becomes limited due to diffusion control in solution. The cathodic currents, reduced under illumination, were detected by the lock-in technique as anodic photocurrents

269

-1.0

-05

0

05

10

15

20

-

electrode potential/ VSHE 0.5 Fig. 6. Lock-m measurement of anodic photocurrent on n-RuS, (rotatmg electrode. 50 Hz) ( -) M H,SO,: (- - -) 1 M Fe3+ (0.5 mol Fe,(SO,), in 0.5 M H,SO,). Negative sweep rate’ 10 mV s-‘, chopper frequency 3.2 Hz. Insert: negative CV sweep m &lute Fe3+ +0.5 A4 H,SO, (internuttent dluminatlon). Photocurrents correspond to 0, evolution (Cl > 1 V) or to reoxldatlon of in-situ reduced H+ or Fe’+ (left photocurrent scale apphes to solid lme. the right one to the hroken lme).

for Fe3+-containing and pure H,SO, (Fig. 6). The photocurrents can formally be understood as reoxidation of in-situ reduced H+ (solid line) and Fe3+ (dashed curve). Both increased electrolyte concentration as well as electrode rotation enlarge the potential region of the occurrence of anodic photocurrents. We were interested in how far into the negative direction the anodic photocurrents extend. The photocurrent onset during hydrogen evolution is found to be at about - 1.1 V/SHE (rotating n-RuS, electrode). For non-rotating electrodes, the results become strongly dependent upon sweep direction (cf. Fig. 5 in ref. 4), an effect apparently due to a change of electrolyte composition in the region near the surface. With rotating electrodes this effect is much less pronounced. Capacity measurements

under limited photocurrent

condrtions

To determine the exact position of energy bands at the semiconductor-electrolyte interface, one usually performs capacity measurements [IO]. The flatband potential in the dark, as determined by the Mott-Schottky approximation [12], however, loses its significance if band edge positions shift during the photoreaction. Kelly and Memming [13] have demonstrated that capacity measurements performed on illuminated semiconductor electrodes in the potential range of photocurrent saturation can be evaluated in the same way as experiments in the dark. Because the minute periodic potential modulation does not influence the total Faradaic current density, the equivalent circuit representing the junction remains

270

-t I

I

I

I

I

.

‘;”

.I

U

HCI .;‘,.’ ., ,* .I /’

15 -

I 0

0.5

1.0

1.5

2.0

electrode potential

_

2.5 / V (SHE)

Fig. 7. Mott-Schottky plots of illummated n-R& m the presence of different electron donors, obtamed from capacity measurements m the photocurrent saturation region at 3 kHz Solutions and illummatlon as m Fg. 3. A conslderable shift of band edges under dlummatlon 1s obwous from extrapolated apparent flatband potentials (mdlcated by arrows).

unaffected under illumination. This is because the shunt resistor representing the steady-state charge transfer is parallel to the space charge capacitor, but assumes infinity for di/dU = 0 (photocurrent saturation). Capacity measurements of illuminated RuS, photoelectrodes were taken in contact with different electron donors. The Mott-Schottky plots obtained are displayed in Fig. 7. Over a wide potential range (AU > f E,/e) a linear Mott-Schottky behaviour is observed. The extrapolated apparent flatband potentials under illumination lJ& are indicated by arrows. U& clearly depends on the redox species which is being photooxidized during the measurement. The slopes of the straight lines are different for different RuS, crystals and reflect the concentration of donors ionized in the depletion layer. Increasing the light intensity by two orders of magnitude (linearly correlated with photocurrent density) does not affect the slopes significantly but a slight positive shift of U,*, ( - 200 mV for oxygen evolution) can be inferred from the measurements. The determination of the flatband potential in the dark U,, usually does not yield unambiguous results. In ref. 3 only a slight variation of U,, due to anions added to the solution (e.g. PO:-, I- ) was found. indicating no specific adsorption in the potential region of hydrogen evolution. With p-type material of higher defect density Mott-Schottky plots were obtained which were clearly correlated with dark current onset and the redox potential of different redox couples (poor barrier quality) [14]. It seems as if a “real” flatband potential (cf.

271

Discussion, section D) was measurable only in the dark with samples of good solid state properties, whereas under current flow (photoinduced or due to defects) the conduction band position becomes stabilized at potentials somewhat negative (n-RuSz. depending on the conditions of operation) of the redox potential of the solution. When impedance measurements on n-RuS, electrodes under illumination are extended to potentials where no photocurrent saturation is achieved, strong frequency dispersion of the electrode response is observed. At lower frequencies (300 Hz) electrode capacities of > 30 PF cmP2 are measured under illumination (values not corrected for surface roughness). The smaller dark capacities are probably due to a high resistance of the depletion layer; the displacement current is carried out only by majority carriers via the conduction band. It is interesting to note that certain peak structures, observed in capacity measurements at a reverse bias before photocurrent onset, are similar to the fast cyclic voltammograms of RuS, (cf. Fig. 5) which show reversible surface reactions at the photoelectrode. Frequency dispersion and light intensity dependence become small when photocurrent saturation is reached. We conclude that in this case the potential drop in surface states and hence the corresponding compensation in the Helmholtz layer remains constant and the Mott-Schottky approximation can be applied. From the

1

I

0.5 electrode

1

I

10

15

2

potential /V(SHE)

Fig. 8. Photocurrents limlted by mass transport m the electrolyte. KI (ca. 2 mM) m 0.5 M H,SO,. Oxidation products: (a) I,. (b) IO,. (c) 10;/Oz. Increasmg the hght mtenslty affects photocurrents only m region (c) (0, evolution). Illummatlon as in Fig. 3.

212

results in Fig. 7 we are able to derive the exact band edge positions of RuS, during the photoreaction with different redox species. A different kind of photocurrent limitation occurs in dilute electrolytes (Fig. 8). The pronounced transient behaviour in region (a) is due to limited mass transport of I- in solution. The sharp photocurrent rise in region (b) corresponds most probably to the possibility of photogenerated holes reacting with iodide ions directly to 10; (Ee,d = + 1.085 V [15], 6-electron transfer, cf. Discussion). Diffusion control is overcome only in region (c) after onset of oxygen evolution. Here, the photocurrent increases linearly with light intensity, in contrast to (a) and (b) where mass transport is limiting.

/=- 5

l0

' 0

I, 05

electrode

10

potential

15

I

'0 20

VSHE

plots of 9. Capacity measurements (bold hne). photocurrent density and Mott-Schottky RuS, m 5 mM KI +OS M H,SO,. Modulation frequency 3 kHz, positwe sweep rate 4 mV s-’ (a), (b): rotating disk experiments (50 Hz). (a) + (b): hght mtenslty reduced, (b) + (c): rotation switched off. Curve (c) 1s slightly distorted because of 0, covering of the electrode

273

We chose a rotating disk experiment to perform capacity measurements under conditions of time- and potential-independent photocurrents. For the same arguments used to construct the equivalent circuit for impedance measurements under photon flux limited photocurrent saturation (see above), we are able to determine the band edge positions in the case of diffusion controlled photocurrents. Figure 9a represents the behaviour of a rotating RuS, photoanode. The light intensity was adjusted such that photocurrents are diffusion controlled only in the region of the II/I, reaction (corresponding to region (a) of Fig. 8). The capacitance hyperbola of the Mott-Schottky region extends in a negative direction only to the I/IO, reaction, which is not diffusion controlled. Reducing the light intensity to about half of the original value shifts the capacitance curves towards the I-/I, region (Fig. 9b). Then we simply switched off the electrode rotation. The Mott-Schottky plots are displaced to the O? reaction, the only one which is not limited by electrolyte diffusion in this case (Fig. SC). The extrapolated values for U& ( indicated by arrows in Fig. 9) are positively displaced by about 300 mV with respect to Fig. 7. This is regularly observed with electrodes of high donor density as used in this case (2.8 x 1019 cmP3). DISCUSSION

(A) Electrochemical

stabilization

of photoelectrodes

An oxidative alteration of the electrode surface is obvious from Fig. 1. Oxygen evolution during the alteration process is clearly visible. The Tafel plots of the dark current onset, however, yield an apparent exchange current density of > 10m4 A crnd2 [16]. This is more than 6 orders of magnitude larger than values for films of TiO,-based RuOz [17], one of the best electrocatalysts for oxygen evolution. Tafel slopes for the anodic reaction (U < 1.45 V/SHE) range between 40 mV for RuOz films and 120 mV for single crystals [18]. Higher slopes (- 200 mV) have been attributed to corrosion processes (U > 1.5 V/SHE) [17]. Such values are usually found for RuS, electrodes, which undergo the first anodic polarisation. Furthermore. in most cases dark currents are observed even at potentials < 1.2 V/SHE. This is clear evidence that the RuS, surface itself is involved in the overall electrochemical reaction during the stabilisation process of the photoanode. The time necessary to decrease the dark currents and for build-up of the characteristic photocurrent voltage behaviour of Fig. lc varies from sample to sample. Figure la shows an intermediate case between immediately stabilised electrodes and the more typical case of some minutes of necessary oxygen evolution (oxidation of iodide does not alter the electrode properties). All crystals investigated so far had in common that dark currents decreased simultaneously with the appearance of photoeffects. This indicates that the quality of the surface barrier improves during the oxidation process. Dark currents will preferably flow through channels of high defect density (“resonance tunneling” [19]). These defects might simultaneously act as centres of efficient minority carrier recombination [28]. Corrosive processes will

274

therefore preferentially remove electrode material of poor electronic quality, thus decreasing dark currents with simultaneous increase of photocurrents. If a stabilised electrode (Fig. lc) is mechanically polished (3 pm diamond paste), the behaviour becomes similar to most freshly mounted samples (no photoeffects). It is known [lo] that by these means electronic defects are introduced into the depletion layer of semiconductors. During renewed oxygen evolution photoeffects reappear. The quenching of the photoeffect of RuS, by a metallic film has been reproduced by depositing a 10 nm thick Pd layer onto a photoactive RuS, surface. The photocurrent was thereby completely eliminated and the original electrochemical Schottky barrier turned into an ohmic contact. The characteristic of Fig. lc, once achieved, is not subject to variations with time. RuS, photoanodes prove to be essentially stable against oxygen evolution. A detailed analysis of sensitivity towards photocorrosion is given in refs. 3.4. (B) Solid state properties The spectral sensitivity of the Bi-grown n-RuS, single crystals used in our experiments extends into the infrared part of the solar spectrum. From Fig. 2 we derive an energy gap of 1.3 eV, a value which is identical to results on p-RuS,, obtained by As doping of the compound [21]. The transition at the absorption edge turns out to be indirect, the plot ($Jzv)“’ vs. hv yields a straight line ($: photocurrent efficiency, hv: photon energy). To determine the charge carrier concentration N,, in our crystals, we evaluated the slope of the Mott-Schottky extrapolations in Fig. 7. This is possible because photoinduced electron-hole pair generation does not influence the concentration of excess carriers under steady-state conditions; only deep electron traps which can be ionized by light could contribute to the result. With Ns, = 2(dU,,/dC$)/ec<,A

(I)

we obtain a donor concentration of 1.4 X 10” cmp3 [lo]. The slope dC,-,‘/dU,, was taken to be lOI V-’ Fp2 cm4 (cf. Fig. 7). e = electron charge, co = permittivity of free space and A = electrode surface. For the dielectric constant E of RuS2 which is not yet known, a typical value of 10 was assumed [22]. For the calculation of the Fermi energy E, in RuS, we adopt the classical approximation for nondegenerate semiconductors [23]. The effective density of states at the bottom of the conduction band No = (2/h3)(2m:kT)3’2

(2)

All constants have their usual meaning. Replacing the effective mass rn: with that of the free electron, Nc becomes for T = 300 K: N, = 2.5 X 1019 cm- 3. The position of E, below the conduction band edge (E,,) EcB - E, = kT In( Nc/N,) yields 0.075 eV.

(3)

275

Quantum efficiencies I#I on Bi-grown RuS, are moderate. A value of < 0.15 at 633 nm is consistently observed, apparently not dependent on light intensity if measurements are performed in the realm of photocurrent saturation. Furthermore, the result is identical for crystals of different donor density and does not change even with p-type material [21]. By estimation of both the space charge layer width W and light penetration depth of the simple Gartner model [20], which lX-I. we can rule out the applicability demands unity collection efficiency for W > a-‘. $I= 0.15 = constant could only be understood for either a minority carrier diffusion length L = 5 nm and W-C a-l or With N,, = 1.4 X 10’” cmP3 we derive W= 28 nm [lo] W~4.5 nm and L-CC’. which does not fit the conditions derived above. Other models. which account for competition between charge transfer and recombination processes at the surface [24] or in the space charge layer [25.26], are based on the assumption of a Hall-Shockley-Read mechanism [27,28] which should be susceptible to variations of the Fermi level. This is not observed in our experiments. For the present, we are not able to understand the recombination pathway in RuSz photoanodes. The decay of photogenerated electron-hole pairs into Mott-Wannier excitons before charge separation could be a possible explanation [29]. In the first low temperature fluorescence experiments with Bi-grown RuSz samples, clearly resolved exciton transitions were indeed observed [30]. To discuss the charge transfer processes at the semiconductor-electrolyte interface we will not consider further the recombination mechanism in the potential range of photocurrent saturation. (C) Surface states as centres for charge transfer and recornhination The domain of anodic photocurrents extends from about - 1 V to at least + 2 V with respect to the hydrogen scale (Fig. 6). When suitable electron donors are present, photocurrents are stable (no transient behaviour) and good diode characteristics are observed (Fig. 3). Dark currents usually remain low even under strong anodic bias (no breakdown). indicating low defect densities in the material. The photovoltage, however, is limited by a pronounced forward current (Fig. 4) an effect apparently typical for reversible redox couples and independent of whether the electron acceptor carries a positive or negative charge. Consequently, the chemical nature and charge [31] of redox species are not systematically correlated with a shift of band edges of RuSz electrodes, which obviously has to be considered to explain our results. Thermal activation of majority carriers to overcome barrier heights up to 300 mV can considerably reduce the oc-photovoltage [7]. Likewise, tunneling processes [10,32] might contribute to this limitation of RuSz electrodes. especially for higher doping densities. We emphasize, however, that the superposition of forward and reverse currents (Fig. 4) does not depend on the absolute position of the level of the (reversible) redox species in solution. which leads us to the conclusion that the

276

relative energetic position of redox couple and band edges is always about the same when the photocurrent sets. This is in contrast to the classical theory of fixed band edges [23]. In order to distinguish whether this unpinning of bands is due to the formation of an inversion layer [7,33] or caused by surface states [34] (Fermi level pinning [35]) we refer to the results of Fig. 6. Anodic photoeffects are observed as long as the simultaneous cathodic dark forward current does not reach diffusion limited values. Due to this diffusion polarization there is some uncertainty about the effective when redox potential at the surface E&. Anyhow, we will encounter contradictions, we analyze the observed currents on the basis of the classical model of isoenergetic exchange currents between bands and redox levels at the interface [23.36]. In this model, a finite net cathodic forward current ic, reflects the competition of isoenergetic electron capture (which prevails) and injection by the redox couple. Illumination, which affects very little the quasi-Fermi energy for majority carriers under moderate band bending, has no significant influence on i(-,. Anodic photocurrents correspond to a net zvB carried by minority carriers in the valence band. Diffusion polarization which can be reduced in a rotating disk experiment should shift E&, towards the positive direction. The threshold of anodic photocurrent onset, however, is shifted in the opposite (negative) direction, which cannot be rationalized on the basis of the classical theory [23]. An even more drastic example is the equivalent case of 0, evolution at p-RuS, [21]. The cathodic light reaction is, similarly to Fig. 6, superposed to the anodic water decomposition (dark reaction) up to about + 3 V (SHE). Thermodynamics and the experience of an irreversible 0, reduction conflict with the assumption of direct exchange currents between bands and a free solution species. We consider these results as evidence for the existence of surface states acting both as centres for surface recombination (mutual annihilation of i,, and iv,), and mediators of charge transfer of holes to the electrolyte. In this model, photogenerated minority carriers are detected by the lock-in technique as long as they reduce the forward current by surface recombination. They are easily compensated by majority carriers and no photocurrents are observed, when there is diffusion control in the electrolyte. (0) Chemical nature and energetic positlon of surface states By performing capacity measurements of the illuminated interface [13], we were able to determine the band edge positions of RuS, during the photoreaction with different electron donors. The apparent flatband potentials U&, indicated by arrows in Fig. 7, are correlated to the respective photocurrent onset (Fig. 3). The physical relevance of U&, however, is limited to the domain of photocurrent saturation, as there are no signs of an accumulation layer formation. when U,?, is crossed over during a negative sweep. Consequently, before reaching photocurrent saturation, we are varying the energetics for charge transfer at the RuS,-electrolyte interface (metal-like behaviour) rather than exclusively the entropy term (statistics

277

of charge carriers), as is to be expected for an ideal Schottky barrier [23]. For the same reason, we cannot approximate the shape of i-cl characteristics in the region of photocurrent rise using the classical model including surface recombination [24]. The shift of U& (Fig. 7) can be attributed to charge trapping at the surface with a resulting voltage change AU = U& - U,, in the Helmholtz double layer [13.37] (U,,= “real” flatband potential, measured in the dark). AU. the surface charge Q,. through eqn. (4) [lo]: and Helmholtz capacity C,, are interrelated

Q, = C,&J

(4a)

or Nsse = Cuu (U& - U,, 1

(4b)

with N,, = surface state density. There is some ambiguity about the value of U,, [1,3]. For an estimation of Ns,. which is positively charged during the photoreaction with water, we assume an averaged value of U,, = - 1 V (SHE), which coincides with the anodic photocurrent onset in Fig. 6. With U& = + 1 V (Fig. 7) for oxygen evolution from sulfuric acid and a typical value of lop5 F cm-’ for C,, [lo] we obtain from eqn. (4b): N,, = 1.25 X lOI cmp2. This surface state density is equivalent to the assumption that 40% of Ru-atoms exposed to the electrolyte on a fresh (lOO)-surface (a, = 0.561 nm [38]) are positively charged. It is evident that such a high surface covering must be detectable in cyclic voltammograms (Fig. 5). Two aspects are to be considered: (1) increasing hysteresis for increasing sweep rate and (2) peak positions, which do not depend upon sweep velocity or sweep direction, indicating surface controlled reactions. Single crystal RuO, electrodes show almost identical features [ll]. Accordingly. we can assign the shoulder close to the cathodic H, evolution to a H-adsorption/desorption and the double peak structure at 650.. .900 mV to a charge-induced reorganization of the topmost oxide layer. Current densities in Fig. 5 are not perfectly linear with the sweep rate u (H, and 0, are present in the region near the surface). Therefore, the reversible displacement current must be somewhat lower than measured current densities. We assume a reduced value for the average current density i = 0.2 mA cm-2 for LJ= 2.5 V s-l. The electrode capacitance due to surface controlled reactions becomes C, = ;/LI = 8 x 10e5 F cmp2. a factor of 8 higher than the value adopted for C,, (cf. eqn. 4b). However. the current densities in Fig. 5 have not been accounted for surface roughness, which might partially explain the observed deviations from an expected lower value (cf. Results). Cyclic voltammetric results as well as the high catalytic activity for 0, evolution suggest that the chemistry of surface states corresponds to that of Ru-oxide complexes (or a thin Ru oxide layer, e.g. RuO,). This is confirmed by XPS data [5.6]. With charge transfer mediated by such an interfacial layer, the electrode stability must be governed by the anodic dissolution rate of the Ru oxide. RuO, is known to be a rather stable electrocatalyst for oxygen evolution from water [39]. Moreover, from a more theoretical point of view, the good stability of RuS,-photoanodes can be related to the relatively pure d-character of the upper valence band edge [5,6]. From this concept there is a much higher contribution of Ru wave

278

functions to a localized surface state with an energetic position in the forbidden band gap than contributions of sulfur p-orbitals, an important pre-supposition for chemical stability of these surface complexes, which have to store positive holes without undergoing corrosion [40]. From Fig. 5 it is evident that the chemical composition of the interface may change upon electrode polarization. Besides the metal-OH bond strength [41], such electrochemical alterations of surface layers [42] seem to be of considerable importance for the catalytic activity for oxygen evolution. Varying chemical composition should be reflected in a different energetic position of surface states in the forbidden band gap [23]. Observed differences in the equilibrium oc-photovoltages lJ,,, which are also very susceptible to forward current changes [7,10.32], however, were too insignificant to infer a shift in surface state energies for different potential regions. A comparison of n-RuS, (U, = 0.25-0.4 V) with p-RuS, (U, = 0.45-0.55 V [21]) suggests a donor level position slightly (- 0.15 V) above the middle of the band gap (comparable kinetic limitations of U,, assumed). The classification as donor states (in spite of their energetic vicinity to the conduction band) was made because a flatband potential of -1 V (SHE) with uncharged (occupied) donor states corresponds quite well to expectations. based on comprehensive experimental data [lo]. Acceptor states would carry a negative charge for n-RuS, with a considerable influence on U,,. A rough estimation of the surface state position Es, is also possible by assuming the same overvoltage for oxygen evolution for RuS, and RuOz. U* = 0.96 V for i=12mAcmP’ (Fig. 7). For comparable experimental conditions: GRuO,) = 1.55 V [17]. Hence. the surface states are situated (1.55 - 0.96 + 0.08) eV = 0.67 eV below the conduction band edge, about 0.2 eV within the estimation based on q,1,, measurements. The correction of 0.08 eV corresponds to the distance between E, and the conduction band edge (cf. Section B). (E) Band edge postion

controlled by charge transfer

It has been mentioned that any bias on RuS, before photocurrent onset primarily changes the energetics at the interface rather than the entropy term. This can be shown directly by a rotating disk experiment (Fig. 9). Three different anodic reactions dominate the photocurrent-voltage characteristics in the electrolyte used for the experiment. Two of the electrode processes are easily identifiable, namely iodine formation (colour) and oxygen (gas) evolution at elevated potentials (region c in Fig. 8). The rate of the third reaction is clearly related to the iodide concentration in the solution. Moreover, no gas evolution is visible at an electrode polarisation corresponding to region b in Fig. 8. Therefore, iodine in a higher oxidation state has to be considered the oxidation product. The mass transport limited photocurrents are in good agreement with the assumption that iodate (IO;, EP,,, = 1.085 V) is formed. For the 6-electron transfer to IO; a six times higher mass transport limited photocurrent has to be expected than for formation of iodine. As the latter will form immediately in the acid solution when IO; encounters I- ions (outside the diffusion

layer), iodate itself has to be considered an intermediate reaction product which is not analytically detectable in ex-situ experiments. In Fig. 9, it is interesting to see how the band edges shift, as long as the respective interfacial oxidative reaction is governed by mass transport in solution, but immediately become fixed when this limitation is overcome (U& indicated in Fig. 9). With the assumption of minority carrier charge transfer mediated by surface states (Section C), band shifting becomes rather evident. Holes, which are trapped at the surface and cannot be transferred to the solution for any reason, must be annihilated by an equivalent forward current (surface recombination). During the positive sweep, however, the energy of electrons in the conduction band is steadily decreasing and cannot compensate the hole flux to the surface. Holes become stabilized in surface states and cause a shifting of band edges until dynamic to a kinetically stabilized surface equilibrium is re-established. U& corresponds potential which cannot be anodically passed, because charge transfer from the electron donor in solution immediately neutralizes excess surface charge. Further increase of anodic bias will then be compensated only in the space charge double layer (Mott-Schottky region). To construct a qualitative energy band model for charge transfer processes at n-RuS, in the different potential regions, we make four basic presuppositions: (1) There is a high density of surface states (5%). in which positive charges can be accumulated to cause a shift of band edges; (2) Minority carrier charge transfer can be described to occur exclusively between SS and redox species in solution; (3) At moderate anodic current densities in the dark the potential drops to a large extent in the depletion layer (fast exchange currents between SS and redox couple): (4) We extend the Fermi level concept [22] qualitatively to the near-surface region. We conclude that pEg and “EF merge in SS if there is a total surface recombination. Four different situations (dark and illuminated semiconductor) are depicted in Fig. lOa-d. To compare the relative band edge positions better, the conduction band level in the dark is indicated by a dot in either case. Net charge transfer reactions are accounted for by arrows. (a) Open circuit photovoltage measurement. Band edge positions are determined by exchange currents between solution and both semiconductor bands and SS. ,,ET and .ET are bent in the near-surface region due to recombination processes. Photovoltage is basically limited by the energetic position of SS (Fermi level pinning [351): (b) Strong negative polarization (cf. measurement reaction is limited by majority carrier transfer through voltage drop in the depletion layer). Holes are trapped electrons (reduced dark current = “anodic photocurrent”); (c) Bias between cathodic and anodic decomposition steady-state currents are expected. The surface barrier

of Fig. 6). Cathodic dark the surface barrier (ohmic in SS and recombine with potentials of water. No will change under illumina-

dark

illuminated

-3

3

l-l?,* _________---

Ftg. 10. Energy band dtagrams of n-RuS, illustratmg the dtfferent charge transfer processes in the dark and under illuminatton. Shown are redox levels m solutton. Ru-based surface states. quast-Fermi levels for electrons (- - -) and holes (. .). Net charge transfer reactions are indicated by arrows, the conductron band edge m the dark by a black dot, bent Fermi levels account for potenttal drops and recombmation. (a) Open circutt measurement of photovoltage; (b) strong negattve polartzatton (cf. Ftg. 6): (c) external bras between cathodic and anodic decomposition potential of water: (d) potential regton of anodtc photocurrent rise.

tion until hole flux is compensated by electrons. Anodic transients (when light is switched on) are reflective for the interface capacitance. Cathodic transients (light off) can be explained correspondingly. (d) Strong positive polarization. For the high depletion layer resistance, low dark currents are expected. Photocurrent rise is parallel to increased band bending and decreased surface recombination. The band edge position under illumination is determined by charge transfer kinetics (overvoltage). Like ,,Ec, ,,E: also depends on illumination for high band bending (cf. intrinsic case [23]). The region of high energy holes is limited by the light penetration (no hole diffusion against the potential barrier).

281 CONCLUSION

Due to its good stability towards corrosion and high catalytic activity for oxygen and chlorine evolution, n-RuS, is an interesting photoanode material for combined photoanode/photocathode or potential assisted solar electrolysis devices. The solar energy conversion efficiency for n-RuS, is considerably limited by low photovoltages. The corresponding small surface barrier height is due to Ru-based surface complexes, the energetic positon of which is slightly above the middle of the RuS, energy gap. Minority carriers are trapped in these surface states. A shift of band edges and efficient surface recombination are the dominating interfacial processes until energetic requirements for charge transfer to the solution are matched (metallic-type behaviour). In the domain of photocurrent saturation, band edges are fixed (typical semiconductor behaviour). This interpretation has recently been supported by high sensitivity electroreflectance measurements on n-RuS, electrodes [43.44]. The advantage of charge trapping in surface states without causing anodic corrosion is that the flat band potential does not have to be adjusted to the desired redox reaction. The photovoltage, which is obtainable to drive photoelectrolytic processes, remains constant for all redox couples of comparable charge transfer kinetics [45]. For catalysis of more electron transfer reactions like water oxidation, interfacial coordination of intermediates is indispensable [40]. This coordination, induced by photogenerated holes on RuS,, is followed by an energy dissipating reorganization of surface complexes (hole trapping). Therefore, research might have to be directed to materials. which allow a higher delocalization of surface charge involved in interfacial coordination bonding (semiconducting cluster compounds) [461. ACKNOWLEDGEMENTS

The authors are grateful to Dr. H. Goslowsky for performing the ellipsometrical determination of the absorption coefficient of RuS,. For a helpful discussion of the manuscript we thank Dr. H.J. Lewerenz and Dr. W. Jaegermann. This work was supported by a grant of the B.M.F.T. (No.O3E-8375A). REFERENCES 1 2 3 4 5 6 7 8 9

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