electrolyte interfaces

Surface Science 163 (1985) 13-18 North-Holland, Amsterdam

13

METALLIC ELECTROREFLECTANCE OBTAINED FROM SINGLE CRYSTALLINE n-RuS2/ELECTROLYTE INTERFACES D. H E R M , H. W E T Z E L and H. T R I B U T S C H Hahn-Meitner-Institut ff~r Kernforschung Berlin, Bereich Strahlencheraie, D-IO00 Berlin 39, Germany

Received 15 April 1985; accepted for publication 25 June 1985

Comparison of electroreflectance(ER) data for the n-RuS2/electrolyte interface with results from photoelectrochemical investigations provides evidence for the appearance of a metallic ER effect originating from a modulation of the surface state charge cartier density due to Fermi level pinning. This finding could be relevant for the interpretation of the catalytic behaviour of these systems.

1. Introduction The attractivity of n-RuS 2 electrodes arises from its stability against photocorrosion and its capability of potential-assisted photoelectrolysis of water. Detailed photoelectrochemical studies on n-RuS 2 electrodes [1] reveal a surface state density (Nss > 1014 cm -2) which is of the order of the electron density of metals. Thus, it m a y be expected that modulation of the surface state charge carrier density could create metal-like ER (MER) due to changes of the dielectric constant of the surface layer. The objective of the present investigation is to prove that under certain conditions such M E R signals can be observed at the n-RuS2/electrolyte interface. Since to date no detailed data of band structure or surface state distribution are available no attempt was made to calculate the shape of such M E R spectra.

2. Experimental n-type RuS 2 single crystals (fee structure) were grown in liquid Bi, as described in ref. [1]. N o attempt was made to orientate the crystals. Thus, (100) and (111) faces, respectively, have been exposed to the electrolyte. In this report we refer only to the investigation of one crystal. We mention that, within the accuracy of our measurements, the differences found for different 0039-6028/85/$03.30 © Elsevier Science Publishers B.V. (North-Hollarid Physics Publishing Division)

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D. Herin et a L / Metallic electroreflectance

orientations are only quantitative. The photoelectrochemical, ER and impedance measurements were performed using standard potentiostatic equipment. All potentials are referred to the saturated calomel electrode (SCE). The experimental arrangements for the ER and impedance measurements are described elsewhere [2]. In order to record the M E R spectra under conditions comparable with those of photocurrent measurements (high light intensity, see also ref. [1]) the electrode was additionally illuminated by an Ar-ion laser (dual beam method) tuned at 476 nm with about 500 m W / c m 2. Due to the intense laser straylight a filter with a cut-off at 2.6 eV had to be used in front of the detector. Therefore the M E R / E R spectra performed with the dual beam method are limited to hv < 2.5 eV. ER was performed with ZaUAc= 0.2 V0_ p sine wave modulation at 20 Hz in the low field region, indication of which is given by the linearity of AR/R versus modulation amplitude and by the fact that variation of the modulation amplitude does not change the lineshape [3-5]. Both conditions were fulfilled. Linearity was obtained with modulation up to 0.6 Vp_p for bias potentials UDC > 0 V and the lineshape remained constant for AUAc~< 1 Vp_ p o

3. Results and discussion

In fig. 1, ER spectra in the range from 1.5 to 2.5 eV for two different electrolyte compositions are displayed. Due to the reasons mentioned above we have to relinquish on a detailed quantitative interpretation but concentrate on the obvious potential-dependent shape of the signals. Apart from the structure it clearly can be deduced that the signal amplitude decreases to zero by polarizing the electrode positive of the potential of the photocurrent onset and that an ER peak develops at - 2 . 4 eV. As will be shown below this peak must be assigned to Franz-Keldysh oscillations whereas the spectra for UDC < 0 V and Uoc < 0.8 V, respectively, are due to MER. In order to demonstrate the potential dependence of the M E R spectra the intensities at 1.7 eV as a function of Uac are displayed in fig. 2 (full lines). The value of A R/R at 1.7 eV as potential-dependent reference was chosen because at this energy no pronounced structure develops for different potentials UDC and a plateau was found between 1.5 and 1.9 eV for the M E R signals of n-RuS 2 electrode immersed into the B r - containing electrolyte. The lack of such a plateau in the case of the " I - electrolyte" needs to be further investigated. For comparison of the M E R data at 1.7 eV with photoelectrochemical measurements the photocurrent/voltage characteristics of n-RuS E obtained with laser illumination of about 500 m W / c m 2 at 476 nm (dashed lines, see also ref. [1]) for two different electrolytes are presented simultaneously in fig. 2. The potential shift of the photocurrent onset agrees with the difference in

D. Herin et al. / Metallic electroreflectance

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the redox potentials of B r - / B r 2 and I - / I ; . Ki~hne et al. [1] deduced Fermilevel pinning for intense illumination in potential ranges negative from the photocurrent onset. In this region the applied potentials, UDc and UAc are almost completely screened within the extension of the surface states. Therefore, due to maximum modulation of the surface state charge density, maximum MER can be observed. For UDc > U~aox Fermi-level pinning is cancelled [1], i.e. the charge density in the surface states remains approximately constant. Thus, UAC drops across the depletion layer and, hence, modulat.es the interband transition of RiaS2. In this potential range MER is quenched and ER

16

D. Herin et a L / Metallic electroreflectance

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signals (Franz-Keldysh oscillations) due to the semiconducting properties of RuS 2 should arise (see below). (The zero-crossing of the MER spectra at 0 and 0.85 V respectively is not yet clarified.) The appearance of ER signals at - 2.45 eV in the range of fixed band edges for UDC = 0.2 and 1.0 V respectively can clearly be seen in fig. 1. The fact that this structure belongs to Franz-Keldysh oscillations can be derived from fig. 3 where the ER signal in the range from 2.0 to 3.1 eV at UDC = 1.1 V is displayed. (Due to the lower signal-to-noise ratio in the case of the dual beam experiments the oscillation at 2.25 eV clearly discernible in this spectrum, is only slightly indicated in the ER spectra shown in fig. 1.) Detailed discussion of the Franz-Keldysh oscillation will be the objective of subsequent investigations. We limit ourselves to mentioning that the onset of these oscillations and the pronounced peak at 2.9 eV indicate direct interband transitions. Since the spectra (Franz-Keldysh oscillations) in the range from 2.0 to 3.1 eV are obtained without additional laser illumination, i.e. photocurrent saturation does occur at lower positive potentials, Fermi-level pinning is already cancelled at potentials UDC < U~aox- The MER spectra obtained in this case (no additional laser illumination) qualitatively exhibit the same spectral feature but are quenched at lower potentials at which Franz-Keldysh oscillations arise.

D. Herm et aL / Metallic electroreflectance

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Fig. 3. Franz-Keldysh oscillations(single beam method, no additional laser illumination was used) between 2.0 and 3.1 eV. No oscillations were found below 2.0 eV. Electrolyte: 1N KBr, IN H2SO4. 4. Conclusions

The appearance of potential-dependent ER signals at the n-RuS2/electrolyte interface for energies below the first direct interband transition in the potential range of Fermi-level pinning provides significant evidence for these signals to be due to modulation of the surface state charge density. Thus, in accordance with photoelectrochemical studies [1] we conclude that the discrepancies observed between the open-circuit photopotential, Uoc, and the difference between flat-band potential and redox potential (Uvb- Ur~aox) is more likely caused by Fermi-level pinning than by hole injection. As known for metals, ER signals due to surface states, which are present in certain crystallographic directions, can probe the electric field within the Helmholtz layer [6]. Thus, the potential-dependent shape of the M E R spectra might analogously provide a means of measuring the potential drop across the Helmholtz layer. It will have to be investigated, whether the favourable properties of n-RuS 2 in potential-assisted light-induced oxygen evolution from water are correlated with a build-up of an electric field in the Helmholtz layer like at oxygen evolving metal electrodes.

Acknowledgements

We like to thank Dr. D.M. Kolb and H.-M. Ki~me for stimulating discussions.

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D. Herin et al. / Metallic electroreflectance

References [1] [2] [3] [4] [5] [6]

H.-M. Ki~hne and H. Tributsch, J. Elcctroanal. Chem., submitted. D. Herm, H. Tributsch and H. Wetzel, submitted. D.E. Aspnes, Surface Sci. 37 (1973) 418. D.E. Aspnes and A.A. Studna, Phys. Rev. B7 (1973) 4605. M. Cardona, Solid State Physics, Suppl. 11 (Academic Press, New York, 1969). W. Boeck and D.M. Kolb, Surface Sci. 118 (1982) 613.