Photocorrosion of strontium titanate photoanodes

Solar Energy Materials 5 (1981) 437444 North-Holland Publishing Company

PHOTOCORROSION

437

OF STRONTIUM TITANATE PHOTOANODES

C. W. de K R E U K * , J. L. B. de G R O O T Central Laboratory TNO, Deftt, The Netherlands

and A. M A C K O R lnstituteJor Organic Chemistry TNO, Utrecht, The Netherlands

Received 30 July 1981 The photocorrosion of a strontium titanate photoanode was studied under acidic, neutral and alkaline conditions. It was found that in 0.5 M H2SO4, SrTiO3 corrodes very rapidly upon band-gap irradiation with an external bias. Assumingthat the photocorrosion is a two-hole process releasing strontium ions, we calculated that the corrosion corresponds to 10~,, of the photocurrent. In the dark or under open-circuit conditions no substantial corrosion is found. In a neutral medium the corrosion is much less, although still considerable. Chips of titanium oxide appear on the surface due to the loss of strontium. Under alkaline condit~ns no substantial corrosion was found.

1. Introduction Strontium titanate is c o m m o n l y considered to be a suitable material for the study of photoanodes in photoelectrochemical cells (PEC) for the cleavage of water by means of solar light [1]. A SrTiO3 anode needs no bias voltage and it is supposed to possess a high resistance to corrosion. Tench and Raleigh [2], however, observed loss of Sr from SrTiO3 surfaces under dark conditions in acidic aqueous solutions. In our experiments [3] we found a rapid photocorrosion in an acidic medium and even in a neutral medium a considerable photocorrosion. Independently Schwerzel et al. [4] observed a severe photocorrosion of SrTiO3 under the combined action of acetic and sulfuric acids, while little (if any) photocorrosion was observed in aqueous sulfuric acid, which is not in agreement with our own observations. We therefore conducted a further study into the corrosion behaviour of n-type strontium titanate in aqueous electrolytes. For this purpose we chose single crystals which were either nominally pure or homogeneously doped with lanthanum chromite. It was shown that this dopant considerably improves the response of SrTiO3 photoanodes to visible light [5-7]. The two materials, partially reduced in a H2 atmosphere, were studied, together with a single crystal of SrTiO3 made conductive by doping with niobium ions. *Present address: Institute for Physical Chemistry TNO, P.O. Box 108, 3700 AC Zeist, The Netherlands. 0165-1633/81/0000-0000/$02.75 (~) 1981 North-Holland

438

C. W. de Kreuk et al. / Photocorrosion qt SrTi03 photoanodes

2. Experimental The experiments were performed under nitrogen atmosphere in a PTFE electrochemical cell provided with a quartz window. The photoanode was fixed in the wall opposite to the window. In order to prevent leakage, even in prolonged experiments, this electrode is mounted between two viton O-rings (for details see fig. 1). After two months no leakage was observed, as was indicated by a low dark current (smaller than 10 nA/cm 2 at 1.0 V vs. Ag/AgCI). Ohmic contact was obtained by a vapour deposited Au layer of 0.1 gm thickness. In the experiments we used a three electrodes system controlled by a home-made potentiostat (developed particularly for low current measurements). A platinum ring mounted around the quartz window served as a counter electrode. Potentials are referred to a Ag/AgC1 electrode in 1 M NaCI (double junction). The experiments were performed with single crystals of SrTiO3 (Commercial Crystal Labs; formerly National Lead) nominally pure (a) or doped with Nb (b, 1000 ppm) or LaCrO3 (c, 500 ppm as Cr, found 406 ppm by AAS). The homogeneous distribution of the lanthanum and chromium ions was confirmed by Secondary-Ion-Mass-Spectrometry (SIMS). Samples (a) and (b) were reduced in a H2/N2 (3:1) atmosphere at 1000°C for at least 5 h to make them sufficiently conducting. All samples were cut perpendicular to the growth-axis into disks of 1.2 mm thickness and they were mechanically polished with A1203 powders ( < 1/~m). The corrosion behaviour was studied in an acidic (0.5 M H2SO4), neutral (0.5 M Na2SO4) or alkaline (1.0 M NaOH) medium, respectively. The light-source was a high-pressure xenon lamp (Osram, XBO 450 W). For the analysis of the crystal surface we used a scanning electron microscope (SEM, Cambridge Stereoscan 180), equipped with an accessory for Energy Dispersive Analysis by X-rays (EDAX), and X-ray diffraction (Philips X-ray Generator PW 1140/00/60 with Goniometer PW 1050/25).

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cell- wall (back side)

----

SrT~O3 electrode

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Au layer Ol pm

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Fig. 1. Part of the electrochemical cell.

C. W. de Kreuk et al, / Photocorrosion of SrTlOs photoanode,s

439

During the experiments in acidic medium the titanium and chromium concentrations in the electrolyte solution were measured by AAS (Perkin-Elmer 430 with graphite oven P.E.-HGA 500).

3. Results

In fig. 2 some SEM pictures of SrTiOa are shown after prolonged band-gap irradiation at a potential of 1 V (Ag/AgCl). The photocurrents were 1-10 mA/cm 2. The results in 0.5 M H2SO 4 (fig. 2a-c) indicate that serious photocorrosion occurs under these conditions. Typical etched structures appear locally (fig. 2a, b) and SrSO4 crystals are formed as shown by the presence of Sr and S signals in EDAX (fig. 2c). Moreover the EDAX measurements indicate that the Sr/Ti ratio has decreased locally. In 0.5 M Na2SO4 the corrosion is much less severe. In this case chipping of the crystal surface is observed, however (fig. 2d). These chips (thickness roughly 0.2-0.4 /~m) were isolated from the crystal and analysed by X-ray diffraction and EDAX. They are void of strontium and they were found to consist largely of amorphous titanium oxide (plus some anatase). In 1 M N a O H no degradation was found over extended periods of time. One could observe, however, that the reflecting properties of the surface change upon irradiation which points to some change of surface composition. This is a subject of further study. More information on the corrosion behaviour of SrTiOa in acidic medium was obtained from determination of the titanium concentration in the electrolyte solution by AAS. These experiments were performed in the dark or with band-gap irradiation, at open circuit or with a potential of 1.0 V. The photocurrent densities were 1-10 mA/cm 2. During irradiation with the external bias and shortly afterwards, the titanium concentration in the electrolyte increases. The final titanium concentration was less than l0 mg per liter. In the dark and at open circuit the increase of Ti is negligible. The average depth of corrosion was calculated from the titanium concentration in solution and is plotted against the amount of transported charge (fig. 3). The corrosion rate is found to be proportional to the amount of transported charge, irrespective of the time in which this charge is transported. This is concluded from measurements, in which the photocurrent was regulated by varying the light-intensity (between l03 and l04 W/m 2) and by applying various cut-off filters (transmitting light above 280-360 nm). No difference in corrosion rate was found between "reduced" SrTiO3 and the Nb-doped material. Doping of SrTiOa by LaCrOa seems to promote the corrosion a little. Analysis of the electrolyte by AAS reveals that within experimental errors, Cr and Ti dissolve in proportion to their presence in the crystal. 4. Discussion

Photocorrosion of n-type semiconductors used as a photoanode is a general phenomenon whereby photogenerated holes in the valence band reach the surface

C. W. de Kreuk et al. / Photocorrosion of Sr'l~Oa photoanodes

441

Fig. 2. SEM pictures of SrTiO~ after prolonged irradiation. (al (cl in 0.5 M HzSO 4. (d) in 0.5 M Na2SO4.

442

C. W. de Kreuk et al. / Photocorrosion q[ Sr770.~ phomanodes

depth of corrosion,/Jm

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t

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20 40 50 fransported charge coulomb/cm2 Fig. 3, Depth of corrosion as a function of the amount of transported charge through the electrode.

to react with the photoanode itself, in competition with components of the electrolyte e.g. water to give oxygen. It has been investigated, for instance, for both small band-gap materials such as CdS [8, 9] and for Ti02 [10, 11] having a wide band-gap. In this competition one may expect a pH-influence, due to the availability of hydroxyl ions at the surface. For SrTiO3 Scaife [12] has calculated that it is thermodynamically stable against photoanodic decomposition in the pH-range 5.9-14. We found, however, that even in a neutral medium photocorrosion of SrTiO3 occurs. This is still compatible with Scaife's predictions if one takes into account the decrease of pH at the crystal surface due to H ÷ ions formed by the oxidation of water: H 2 0 + 2h+__,2H + +~O2. 1

(1)

We have calculated (Fick's Law) the surface pH in our experimental conditions to be 3-4, assuming a diffusion-layer thickness of 10- 3 cm. From our experimental results a simplified model for the photocorrosion can be derived. The first step is absorption of a photon followed by the formation of a hole electron pair. SrTiOs h~ ,SrTiOs (h + + e ).

(2)

The electron is transported to the cathode to produce hydrogen, the hole may either produce oxygen from water or it may be used for the decomposition of the photoanode. As a first step in the photocorrosion process we assume a reaction of the anode with holes arriving at the surface to give Sr 2 + ions:

C. W. de Kreuk et al. / Photocorrosion of SrT~03 photoanodes

S r T i O 3 + 2 h + -~ S r 2 + + T i O 2 + 1 0 2 . T h e Sr 2 +

443

(3)

ions will react with sulfate ions:

Sr 2 + + SO ]- ~SrSO4+.

(4)

Reactions (3) and (4) explain the formation of SrSO4 crystals and TiO2 chips. The large effect of acid on the corrosion rate, however, together with the observed dissolution of titanium would indicate a further reaction of the TiO2 to form titanyl ions which are soluble in acid medium. For this reaction either a photochemical or a chemical reaction is possible: Y i O 2 + 2 h + ~ T i O 2 + + ~1O 2 ,

(5)

TiO2 + 2H + ~ T i O 2 + + 1 0 2.

(6)

From the fact that in a neutral medium, where titanyl ions do not dissolve, TiO2 is found at the anode surface, combined with the high corrosion rate in 0.5 M H2SO4, we conclude that reaction (6) is most likely to occur in acidic medium. Based on this scheme (reaction (3), (4) and (6)), in which the corrosion is initiated by the loss of strontium, two holes are used per disappearing SrTiO3 unit. We have calculated from the data in fig. 3 that roughly 10~o of the available holes is used for the photodecomposition. In the four-hole process (reaction 5) this number would have been 200,0. So far it has been generally assumed that SrTiO3 photoanodes are stable under various conditions, following the observation by Wrighton et al. [13], which was made in alkaline solutions. The present investigation, supported by Schwerzel's results [4], clearly shows that this assumption is invalid for acidic and neutral electrolytes and that substantial photocorrosion occurs under these conditions.

Acknowledgements The authors are grateful for the kind hospitality of the Department of Solid State Chemistry of the State University of Utrecht, where part of the materials were obtained and machined. The photocorrosion measurements were carried out in the Physics Department of MT-TNO at Delft. Assistance of Ing. P. J. Nederveen and discussion with Drs. Ch. A. Kruissink are thankfully acknowledged. This work was financially supported by VEG-Gasinstituut N.V.

References [1] See, e.g., review articles by M. S. Wrighton, Chem. Eng. News 57 (1979129 and by H. P. Maruska and A. K. Ghosh, Solar Energy 20 (19781 443. [2] D. M. Tenth and D. O. Raleigh, NBS Special Publication 455, Proc. Workshop held at NBS, Gaithersburg, MD [1975) p. 229.

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('. W. de Kreuk et al. : Photocorrosion O/ Sr7703 photoanode~x

[3] A preliminary report of this work was presented: A. Mackor, G. Blasse, J. Schoonman, P. H. M de Korte, C. W. de Kreuk and J. van Turnhout, Book ~f Abstracts, 3rd Intern. Conf. on Photochem Conversion and Storage of Solar Energy, Boulder, CO 11980) p. 317. [4] R. E. Schwerzel, E. W. Brooman, H. J. Byker, E J. Drauglis. D. D. Levy, L. E. Vaaler and V. E. Wood, Book of Abstracts. 3rd Intern. Conf. on Photochem. Conversion and Storage of Solar Energy. Boulder, CO (1980) p. 351. [5] G. Blasse, P. H. M. de Korte and A. Mackor. J. Inorg. Nucl. Chem. 43 (1981) 1499. [6] A. Mackor and G. Blasse, Chem. Phys, Lelt. 77 !1981~ 6 [7] P. H. M. de Korte, R. U. E. 't Lam and G. Blasse. J. lnorg. Nucl. Chem., in press. [8] R. Williams, J. Vac. Sci. Technol. 13 (1976) 12. [9] H. Gerischer, J. Electroanal. Chem. 58 11975) 263. [10] L. A. Harris and R. H. Wilson, J. Electrochem Soc. 123 (1976) 1010. [11] M. E. Gerstner, ibid. 126 11979) 944. [12] D. E. Scaife. Solar Energy 25 (1980)41 [13] M. S. Wrighton, A. B. Ellis, P. T. Wolczanski, D. E. Morse, H. B. Abrahamson and D. S. Ginley, J. Am. Chem. Soc. 98 11976)2774.