8012
J . Phys. Chem. 1990, 94, 8012-8013
Controlled Photocorrosion of WSe,:
Influence of Molecular Oxygen
D. Mahalu,+ M. Peisach,' W . Jaegermann,g A. Wold,l and R. Tenne*St Department of Materials Research. Weizmann Insriiute of Science, Rehovor 76100, Israel: National Accelerator Center, P.O. Box 72, Faure 7131, Sourh Africa: Hahn- Meitner- Institut, Bereich Strahlenchemie, Glinicker Srrasse 100, D- 1000 Berlin 39. FRC; and Deparimeni of Chemisrry, Brown University, Providence, Rhode Island 0291 2 (Received: June 18, 1990)
(Photo)electrochemical etching (photoetching), which can be considered as a controlled corrosion process, is used to obtain high-quality electrical surfaces on a layered semiconductor (WSe2). The photoetching mechanism was found to depend not only on the reactivity with water but also on the presence or absence of molecular oxygen in the reaction media. A labeling method and nuclear reaction analysis were used to discriminate between the two oxygen sources (water and molecular gas). This new strategy can be useful for studying corrosion mechanisms in general.
Until recently, the high-quality optoelectronic properties of layered semiconductors were associated with the quality of the exposed van der Waals (vdW) surface.',* In a recent work we have shown that mixed surfaces (containing both Ilc and I C crystallographic planes) can be exposed by surface indentation followed by photoelectrochemical etching of WSe23,4(Figure 1). It was noticed that such surfaces (shown to have enhanced photoactivity as well as other unique optical properties4) were obtained only when the electrochemical reaction was carried out in oxygenated solutions (at pH = 1). In this case an anisotropic reaction occurs where the main oxidation products are soluble HSe03- ions and insoluble W03.J Knowing that water molecules participate in the chemical reaction: an intriguing question is, to what extent does molecular oxygen originating from the gas influence the mechanism of the photooxidation process? To address this issue, the two kinds of oxygen sources were discriminated by using a labeling method. Molecular oxygen was found to influence the photocorrosion of WSe,, and a new methodology, for the investigation of corrosion problems in general, was developed. Freshly cleaved n-type WSe, crystals were photoelectrochemically etched' in 1 M HCI solution using a two-compartment cell, under a bias not greater than 500 mV vs SCE. In one series of experiments, the reaction was carried out in 20% H2I80aqueous solution, into which O2or Ar gases were bubbled. In another series, the aqueous solution (H2160) was saturated with a gas The . I8Ocontent, mixture containing ca. 20% l*O, and 80% 1602 in the reaction product deposited on the crystal surface, was determined by nuclear reaction analysis (NRA),' using the 635-keV resonance in the '*O(~,CY)'~N reaction (see Figure 2). The penetration depth of the NRA analysis was estimated to be lower than 300 A. The Se/W ratio, for the same cimpound, was determined by using Rutherford backscattering (RBS) (2.4-MeV a-particle beam) and semiquantitative energy dispersive X-ray spectroscopy (EDS) (1 0-kV electron beam). Supporting evidence for the reaction mechanism was obtained by using Auger and X-ray photoelectron (XPS) spectroscopies, together with I-V characterization. The results from NRA, RBS, and EDS represent a statistical average of numerous experiments and are summarized in Table I. An increased uptake of oxygen from water is obtained, at the electrode surface, in aerated solutions (compare rows 2 and 4 in Table I). The chemical composition of the surface of the photoetched electrode is dependent on the reaction conditions. In deaerated solutions, both the oxygen content and Se/W ratio, on the surface, were not affected by rinsing (compare rows 2 and 3), while a substantial decrease of those values was obtained, due to rinsing, when molecular oxygen was present in the reaction media (compare rows 4 and 5 ) . These results confirm that soluble Weizmann Institute of Science. *National Accelerator Center f Hahn-Meitner-Institut. Brown University. +
TABLE I: Comparison between NRA, RBS, and EDS Results normalized l8ocounts, R from SefW treatment standard RBS EDS 1. freshly cleaved 30 0.85
selenium oxide together with insoluble W 0 3 is formed during photocorrosion of WSe,, in the presence of molecular oxygen. Contrarily, an insoluble mixture of tungsten and selenium compounds is left at the semiconductor surface when the oxidation was carried out in deaerated solutions. Moreover, one can see that l8O, which originated from the dissolved gas, is clearly detected in the oxidation compound (row 6). Samples that were etched in the presence of both H2I80and I8O2exhibited the highest I8O counts after photoelectrochemical etching (row 7 in Table 1 and Figure 2). Note that due to the difference in penetration depth for RBS (ca. 50 rcLm in WSe,) as compared with EDS (ca. 2 r m ) measurements, the latter is more surface sensitive (compare rows 3 and 4 of Table I). Exchange reaction between molecular oxygen and water can be neglected due to the anodic bias and cell configuration. Figure 3 presents the results of XPS analyses. The line shape of the W 4f and 0 Is peaks of the electrode photoetched, under vigorous oxygen bubbling, suggests W 0 3 to be the main insoluble oxidation product.8 In this case, the Se 3d peak corresponds to the background signal from the original WSez surface. A much more complicated situation occurs as a result of photoetching in the absence of 02.In this case the Se 3d peak (see Figure 3.2) suggests that Se in a high oxidation state together with elemental selenium is present on the electrode surface. The 0 1s peak is composed of two contributions: EB ca. 530-531 eV corresponding or OH group and E B ca. 533 eV from oxygen in water to 02molecules.8 Although the W peak is rather similar to the previous case, the low O/W ratio precludes W 0 3 as the main oxidation product. Samples oxidized under argon atmosphere and later exposed to ambient atmosphere, for a prolonged period of time (1) Tributsch, H. Ber. Bunsen-Ges. Phys. Chem. 1977, 81, 361. (2) Lewerenz, H. J.; Heller, A,; DiSalvo, F. J. J. Am. Chem. Sac. 1980, 102. 1877. (3) Tenne, R.; Wold, A. Appl. Phys. L ~ I I 1985, . 47, 707. (4) Jakubowicz, A.; Mahalu, D.; Wolf, M.; Wold, A,; Tenne, R. Phys. Rev. B 1989. 40. 2992. ( 5 ) Kautek, W.; Gerischer, H. Surf. Sci. 1982, 119, 46. (6) (a) Kautek, W.; Gerischer, H. J . Eleciroanal. Chem. 1982, 137,239. (b) Kuhne, H. M.; Tributsch, H . J . Electrochem. SOC.1983, 130, 1449. ( 7 ) Gove, N. B.; Wapstra, A. H . Nucl. Data Tables 1972, 11, 127. (8) Jaegermann, W.; Schmeisser, D. Surf. Sci. 1986, 165, 143. (9) Bahl, 0. P.; Evans, E. L.; Thomas, J. M. Prm. R. Sac. London, A 1967, 306, 53.
0022-3654/90/2094-80 12$02.50/0 0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8013
Letters
o'* counts C 0
1
U
I
I
n I
I
I
1
260
180
I
420
340
Channel number
Figure 1. Photoetched surface on WSez (mixcd surface).
and rinsed, exhibit a spectrum similar with the first case (Figure 3.3). I-V plots were taken during photoelectrochemical etching, for the illuminated WSe2 photoanode, in both the presence and absence of molecular oxygen in solution. The "onset potential" under vigorous oxygen bubbling was found to be ca. 400 mV vs SCE. A typical 100-mV delay of the "onset potential" for the photoelectrochemical reaction was observed in the absence of molecular oxygen, suggesting a different oxidation mechanism in aerated as compared with deaerated solutions. Since photoelectrolysis of water was shown to occur under reverse bias on the vdW surface of WSe2,6a it could be argued that this reaction can provide molecular oxygen during photocorrosion. However, only disulfides of transition metals which can reach high oxidation states (Ru, Pt) are reported to be able to induce oxygen photoevolution, from water, at voltages less than 0.5 V vs SCE.6b This implies that an intermediary species from water decomposition, such as OH', participates in a fast chemical step leading to corrosion, rather than recombine and produce oxygen. Our experiments indicate that molecular oxygen, dissolved in aqueous electrolyte, participates in a 14-hole/electron photocorrosion reaction: WSe2
Y
C
-
+ (2x/4)02 + (9 - x ) H 2 0 + (14 - 2x)h+ W 0 3 + 2HSe03- + (16 - 2x)H+ ( I )
Figure 2. Nuclear reaction analysis for WSe2 photoelectrochemically gas bubbling. etched in 207 H 2 ' * 0 aqueous solution under 20% '*02
w
41 Oeak
41
37
33
6
Se 3d peak .
3
0 1s peak '77 537 529
(1) freshly cleaved
C
4
0
U
400
150
(2)
2010
S
photoetched underargon bubbling (rinsed)
r a t e
(3) like (2) but l a t e r a i r exposed (rinsed)
n
t
S
I
41
.
.
33
37
-
roA
- 6
537
- 5 1. 9
(4) photoetched under oxygen bubbl Ing
binding energy eV
Figure 3. X-ray photoelectron spectroscopy of WSe2 photoelectrochemically etched under various etching conditions.
with AGO= -1409.4
+ (9 - x)237.2
kJ/mol
IOa
(1')
where AGo(WSe2) was estimated (by comparison to WS2 and MoS2) at 95% from AH0(WSe2).Iob Thermodynamic consideration leads to the conclusion that the larger the contribution of molecular oxygen (i.e., larger x in (l')), the more favorable the reaction becomes. In the limit case, the reaction can be carried out in a pure oxygen atmosphere. Nevertheless, kinetic barriers preclude this reaction at room temp e r a t ~ r e .The ~ limited solubility of molecular oxygen in aqueous solutions requires vigorous O2bubbling in order to favor reaction 1. In the absence of dissolved molecular oxygen. the reactions leading to elemental seleniumi1or substoichiometric Se/W oxyare thermodynamically and kinetically favorable (IO) (a) Standard Porentials in Aqueous Solutionr: Bard, A. J.. Parsons, 1985. (b) O'Hare, P. A. G.; Lewis, R. U.: Parkinson, B. A. J. Chem. Thermodyn. 1988, 20. 681. ( 1 I ) Meissner. D.: Renndorf, C.; Memming, R. Appl. S u v . Sci. 1987.27. 423. (12) Schmidt, K. H.; Muller, A. Specrrochim. Acta 1972, 28A. 1892.
R.. Jordan, J.. Eds.; IUPAC:
as was also confirmed by EDS and Auger analysis. In conclusion, this work clearly indicates that molecular oxygen participates in the photocorrosion of WSe2. As a result of the reaction, an enhanced photoresponse surface is exposed. in the absence of molecular oxygen, a mixture of tungsten and selenium oxyhydroxides is produced, which blocks the reactive sites, thus slowing down the corrosion process. The methodology developed in this study is useful for the investigation of photocorrosion reactions on semiconductor surfaces, as well as for corrosion reactions in general.
Acknowledgment. We are indebted to W.Kautek and H. J. Lewerenz for illuminating discussions. Appreciation and thanks are made to G. Hollos and Y. Yurman for their assistance in the NRA and RBS measurements and to the Heavy Oxygen unit of the WIS for providing the l*O. This research was supported by a grant (E1047) from the joint BMFT (FRG)/NCRD (Israel) program. _______
~
~~~~
( I 3) Yoshiike, N.; Kondo, S . J . Electrochem. Soc. 1983, 130, 2283.
