J. Phys. Chem. 1988, 92,6991-6997
6991
Thermodynamic Stability of n-Ag InSe, in Photoelectrochemical Cells Luisa Peraldo Bicelli Centro di Studio sui Processi Elettrodici del CNR, Dipartimento di Chimica Fisica Applicata del Politecnico. Piazza L. da Vinci, 32, 20133 Milano. Italy (Received: November 23, 1987; In Final Form: April 14, 1988)
n-AgInS%, a ternary chalcogenide recently studied in photoelectrochemicalcells, has been examined as far as its photostability in aqueous electrolytescontaining different redox couples is concerned. The principles of thermodynamics have been followed to evaluate the standard free energy changes and decomposition potentials of all the possible bond cleavage and decomposition photoreactions in the polyiodide and polysulfide electrolyte. In addition, the possible chemical and electrochemical Se/S exchange reactions in the polysulfide electrolyte have been considered. The energy diagram of the semiconductor has been deduced from experimental as well as calculated data. The material stability has been predicted on the basis of the relative position of the hole quasi-Fermi level under open-circuit conditions, the photodecomposition potentials, and the energy level of the redox couple in the electrolyte. The results practically agree with already published experimental data and evidence the substantial instability of n-AgInSe in the considered electrolytes,even after coating its surface with an artificially formed indium oxide layer.
Introduction
TABLE I: Standard Gibbs Free Energy of Formation of Several Ag
For several decades, the Pourbaix phase diagrams' have enabled prediction, on a thermodynamic (TDM) basis, of the equilibrium states of all the possible reactions between a given metal, its ions, and its solid and gaseous compounds in the presence of water; their utility has widely been recognized. Indeed, these diagrams, where the metal itself and its predominant decomposition products are represented on a potential versus pH plot, are used extensively in the field of corrosion as well as in analytical chemistry. In a pioneering work, Park and BarberZ first extended such calculations to semiconducting electrodes (CdS, CdSe, CdTe, Gap, and GaAs) in aqueous solutions of different pH, taking into account the stability conditions predicted by Gerischer3 and by Bard and W r i g h t ~ n . ~More recently, several authors have performed TDM evaluations to determine the stability of semiconductors being part of a regenerative photoelectrochemical (PEC) cell. Thus, both binary5g6and ternary'.* chalcogenides were investigated, and the results compared with experimental data. For performance of TDM calculations, it is in principle necessary to know not only the free energy of formation from the elements of the compound under investigation and of its possible decomposition products but also the parameters for the construction of the semiconductor energy diagram. These are the bandgap, the flatband potential in the examined redox electrolyte, the band bending, and the minority carrier quasi-Fermi level at the surface under the operating conditions (open-circuit, shortcircuit, or maximum output condition). In practice, most of these data are usually lacking, and more so those regarding new materials for which, on the contrary, TDM analysis is particularly significant. Nonetheless, it is still possible to perform the calculations as such W i g values may approximately be determined according to theoretical as well as empirical considerations, so that very useful information may be obtained, in spite of these and previous restrictions. Recently, Dagan and Cahen9 have published preliminary results on the PEC activity of the n-AgInSq/polyiodide and /polysulfide junction, observing a limited output stability of the material. AgInSez has been studied to a remarkably lesser extent than its
and In Compounds,Several Ions, and Water
(1) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd 4.; NACE: Houston, TX,and CEBELCOR, Brussels, 1984. (2) Park, S.;Barber, M. E. J. E/ectroanaI. Chem. 1979,99,67-75. (3) Gerischer, H.J. Electroanal. Chem. Interfacial Electrochem. 1977, 82, 133-143. (4) Bard, A. J.; Wrighton, M. S. J. Electrochem. SOC. 1977, 124, 1706-1 709. ( 5 ) Hodes, G.; Miller, B. J. Electrochem. SOC.1986, 133, 2177-2180. (6) Peraldo Bicelli, L. Electrochim. Acta 1987, 32, 777-783. (7) Cahen, D.; Mirovsky, Y . J. Phys. Chem. 1985,89, 2818-2827. ( 8 ) Peraldo Bicelli, L. Solar Energy Mater. 1987, 15, 77-98. (9) Dagan, G.; Cahen, D. J. Electrochem. Soc. 1986, 133, 1533-1534.
0022-3654/88/2092-6991$01 S O / O
compd
AGOf: kJ mol-'
ref
(-188) (-207.3) (-226.6) -44.35 -40.7 -331.0 -412.5 -11.2 -830.7 -66.2 (-228.0) 77.1 -98.0 -525.0 -313.0 -5 1.6 -51.4 85.8 79.5 73.7 69.1 129.3 108.3 81 (73) (70) (68) 73.6 (66) (65) 12.6 -157.2 -237.1
see text
see text see text 11 11 7 11 11 11 11 7 11 11 11 11 11 11 11 11 11 11 11 7 12
see text 5 5 12 7
see text 13, 14 11 11
"Estimated values are in parentheses. parent compound CuInSez. Nevertheless, it is considered a very interesting material whose behavior is worth being compared to that of CuInSe?. The aim of this work is, therefore, to investigate the TDM stability of AgInSe, electrodes in PEC cells following the method developed by Cahen and Mirovsky,' to discuss the results of such analysis in connection with experimental data, and to compare the same results to those concerning AgInsSes and CuInSez. Thermodynamic Data
As already stated, for these calculations the knowledge of TDM data is required that sometimes are lacking or unreliable. This is more especially true for ternary semiconducting compounds of current interest in photoelectrochemistry. Therefore, as in a 0 1988 American Chemical Society
6992 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988
Peraldo Bicelli
TABLE II: Possible Bond Cleavage Photoreactions for n-AgIaSe2and Related T h e d y n o m i c Quantities
Dhotoreaction ~~
AGO. kJ
Eo(NHE). V
99.6
1.03
67.8
0.23
167.1
0.43
231.8
0.80
296.4
1.54
36 1.O
3.74
~
+ h+ & Ag+(aq) + 0.51n2Se3+ 0.5Se0 2. AgInSe2 + 3h+ 20.5Ag2Se + In3+(aq)+ 1.5Se0 3. AgInSe, + 4h+ 2Ag+(aq) + In3+(aq) + 2Se0 Ag+(aq) + In3+(aq) + 1.5Se0 + 0.5Se24. AgInSe, + 3h+ 5 . AgInSe2 + 2h+ 2Ag+(aq) + In3+(aq)+ Seo + Se26. AgInSe, + h+ & Ag+(aq) + In3+(aq) + 0.5Se0 + 1.5Se27. AgInSe2 + 0.5H20 + h+ 30.5Ag20 + 0.51n2Se3+ 0.5Se0 + H+(aq) 8. AgInSe, + 1.5H20 + 3h+ 0.5Ag2Se + 0.5In2O3+ 1.5Se0 + 3H+(aq) 9. AgInSe2 + 0.5H20 + 4h+ 30.5Ag20 + In3+(aq) + 2Se0 + H+(aq) 10. AgInSe2 + 1.5H20+ 4h+ & Ag+(aq) + 0.5In2O3+ 2Se0 + 3H+(aq) 11. AgInSe, + 2 H 2 0 + 4h+ 20.5Ag20 + 0.5111~0,+ 2Se0 + 4H+(aq) 1. AgInSe2
aq
135.4
1.40
106.1
0.37
203.0
0.53
205.4
0.53
241.2
0.62
TABLE 111: Possible Oxidative Photodecomposition Reactions for n-AgInSe2 in the Polyiodide Electrolyte and Related Thermodynamic Quantities Dhotoreaction AGO. kJ Eo(NHE). V
+ I- + h+ & AgI + 0.51n2Se3+ 0.5Se0 1'. AgInSe2 + 1/,Ij- + 1/,h+ 3 AgI + 0.51n2Se3+ 0.5Se0 2. AgInSe2 + 31- + 3h+ & 0.5Ag2Se + In13 + 1.5Se0 2'. AgInSe2 + 1,- + h+ & 0.5Ag2Se + InI, + 1.5Se0 3. AgInSe2 + 41- + 4h+ 2AgI + InI, + 2Se0 1. AgInSe2
7.9
0.08
-26.6
-0.83
92.6
0.32
-10.8
-0.11
100.2
0.26
+ Y3I3-+ 4/,h+& AgI + InI, + 2Se0
-37.1
-0.29
+ I- + 1.5H20 + 4h+
113.7
0.29
79.2
0.25
227.8
0.59
124.4
0.64
3'. AgInSe2
+ 0.5In2O3+ 2Se0 + 3H+(aq) AgInSe2 + 1/,I< + 1.5H20 + lO/,h+ & AgI + 0.5111203 + 2Se0 + 3H+(aq) AgInSe2 + 31- + 0.5H20 + 4h+ & 0.5Ag20 + In13 + 2Se0 + H+(aq) AgInSe2 + Ij- + 0.5H20 + 2h+ & 0.5Ag20 + InI, + 2Se0 + H+(aq)
4. AgInSe, 4'. 5.
5'.
AgI
previous work on AgIn5Se8 and CuInsSs: we estimated the free energy of formation of AgInS, and of AgInSe,, summing those energies of the two related binaries. This method was previously shown to give values for CuInS, (-249 kJ mol-') and CuInSe, (-202 kJ mol-') in fair agreement, particularly the second one, with experimental data (-218.6 f 15 and -201.2 f 10 kJ mol-', respectively).s We did not consider the experimental and calculated heats of formation of AgInSe, given by Glazov et al.1° since these values are expected to be too high, as found, for example, in the case of CuInSe,. The free energy of the quaternary AgInSSe compound was calculated from that of the ternary compounds according to AGOr(Ag1nSSe) = [AGo,-(AgInS2) + AGof(AgInSe2)]/ 2 where AGOf is the Gibbs free energy of formation in standard conditions, at 298 K. The so-obtained values are collected in Table I, where parentheses are used to emphasize their empirical origin. Other authors, too, estimated TDM values. They refer prevailingly to polysulfaselenide ions whose experimental data are not available, at present. As to S2Se2-and S3Sez-,their AGOf values were here calculated by comparing the trend of several series (S$-,Se:-, SSe:-, and S,SeZ- with n ranging from 1 to 4, and S2Sem2-and S A 2 -with m ranging from 0 to 2) and observing that increasing the total number of atoms the AGOf value decreases and that higher value$ are obtained when S atoms are replaced by Se atoms. The experimental values of the remaining compounds and ions of Table I are mainly taken from the basic compilation of Wagman et al." according to ref 7, where the TDM stability of semicon(IO) Glazov, V. M.; Lebedev, V. V.; Molodyk, A. D.; Pashinkin, A. S. Izu. Akad. Nauk. SSSR, Neorg. Mater. 1979, 15, 1865-1867. (11) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, I.; Bailey, S. M.;Churney, K.L.;Nuttall, R. L.J . Phys. Chem. Ref. Data 1982, 11, Suppl. 2, 38, 52, 56, 61, 133, 160-161.
ducting chalcogenides was first examined systematically.
Results To test the photostability of n-AgInSez electrodes, we examined the possible bond cleavage as well as decomposition photoreactions in several electrolytes. As shown in Tables 11-IV, the anodic half-reaction, only, was considered, and its standard free energy change at room temperature, AGO, was calculated by assuming the hydrogen evolution process (whose standard free energy change is conventionally equal to zero) as the cathodic half-reaction. Therefore, AGO also represents the standard free energy change of the overall process. No intermediate steps were considered since this is a merely TDM approach. Tables 11-IV also show the standard photodecomposition potential referred to the normalized hydrogen electrode, Eo(NHE), calculated, as usual, from AGO. More details are reported in ref 8. In agreement with experimental results on binary and ternary chalcogenides (e.g., ref 15 and 16 and, e.g., ref 17-21, respectively) (12) Greiver, T. N.; Zaitseva, I. G. Zh. Prikl. Khim. (Leningrad) 1967, 40, 1920-1923. (13) Hod%, G. In Energy Resources through Photochemistry and Catalysis; Graetzel, M., Ed.; Academic: New York, 1983; Chapter 13, pp 428-434. (14) Latimer, W. M. Oxidation Potentials, 2nd ed.; hentice-Hall: New York, 1952. (1 5) Gerischer, H. J . Electroanal. Chem. Interfacial Electrochem. 1975, 58, 263-274. Meissner, D.; Benndorf, C.; Memming, R. Appl. Surf.Sci. 1987, 27, 423-436. (16) Hiisser, 0.E.;von Kinel, H.; Uvy, F. J. Electrochem. Soc. 1985,132,
810-814.
(17) Scrosati, B.; Fornarini, L.;Razzini, G.; Peraldo Bicelli, L. J. Elecrrochem. SOC.1985,132, 593-598. (18) Cahen, D.; Dagan, G.; Mirovsky, Y.; Hodes, G.; Giriat, W.; Liibke, M. J . Electrochem. SOC.1985, 132, 1062-1070.
n-AgInSe2 in Photoelectrochemical Cells
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 6993
TABLE IV Possible Oxidative Photodecomposition Reactions for n-AgInSezin the Polysulfide Electrolyte and Related Thermodynamic Quantities photoreaction AGO, kJ Eo(NHE), V
+ HS- + OH- + h+ & 0.5AgzS + 0.51n2Se3+ 0.5SSe” + H 2 0 lb. AgInSe, + y3S32-+ y3h+2O.5Ag2S + 0.51n2Se3+ 0.5S3Se2IC. AgInSe, + 5/S4z- + y4h+Z O.SAg,S + 0.51nzSe3+ 0.5S4Se22a. AgInSe2 + 3HS- + 30H- + 3h+ 2O.5Ag2Se+ 0.51nzS3+ 1.5SSe2- + 3H20 2b. AgInSez + 2S3,- + h+ S 0.5AgzSe + 0.51n2S3+ 1.5S3SeZ2c. AgInSe, + ‘s/ss42+- Y4h+2O.SAg,Se + 0.51n2S3+ 1.5S4SeZ3a. AgInSe, + 4HS- + 40H- + 4h+ 2O.SAg,S + 0.5In2S3 + 2SSe” + 4Hz0 3b. AgInSe, + 8/S,2- + y3H+ 30.5AgzS + 0.5In2S3 + 2S3Se23c. AgInSe, + 5/S4’- + h+ 2O.5Ag2S + 0.51n2S3+ 2S4SeZ4a. AgInSez + S/2HS- + ‘f/,OH- + 4h+ 2O.5Ag2S + 0.5In,O3 + 2SSe2- + 4H20 4b. AgInSe, + 13/6S32+ 30H- + ‘Y3h+-.% 0.5AgzS + 0.5In2O3+ 2S3Se2-+ 1.5H20 4c. AgInSe, + ‘7/*S:- + 30H- + ‘3/h+ & O.5Ag2S + 0.5In2O3+ 2S4Se2-+ 5a. AgInSe, + 7,H.S- + 9/,OH- + 4h+ 20.5AgzO + 0.51n2S3+ 2SSeZ-+ 4Hz0 5b. AgInSe, + 7zS32-+ OH- + 2h+ 20.5AgZ0+ 0.51n,S3 + 2S3Se2-+ 0.5H20 5c. AgInSe, + ‘9/8s4’- + OH- + Y4h+2 0.5Ag20 + 0.5In,S3 + 2s4se2- + 0.5H20 6a. AgInSe, + 0.5HS- + 1.50H- + h+ 20.5Ag20 + 0.51n2Se3+ 0.5SSe2- + H 2 0 6b. AgInSe, + O.5SJz-+ OH- + h+ & 0.5Ag20 + 0.51nzSe3+ 0.5S3SeZ-+ 0.5H20 6c. AgInSe, + O.5Sd2-+ OH- + h+ & 0.5Agz0 + 0.51nzSe3+ 0.5S4Se2-+ 0.5H20 7a. AgInSe, + 1.5HS- + 4.50H- + 3h+ 20.5Ag2Se + 0.5In,O3 + 1.5SSe2- + 3 H 2 0 7b. AgInSe, + 1.5S3,- + 30H- + 3h+ 2O.SAg,Se + 0.5Inzo3 + 1.5S3Se2-+ 1.5H20
-49.8
-0.52
-12.0
-0.37
-7.0
-0.29
-196.4
-0.68
-82.8
-0.86
-68.0
-0.94
-246.6
-0.64
-95.1
-0.74
-75.4
-0.78
-201 .o
-0.52
-151.4
-0.47
-142.6
-0.45
-147.0
-0.38
-29.4
-0.15
-13.3
-0.08
49.8
0.52
53.7
0.56
55.0
0.57
-150.8
-0.52
-139.1
-0.48
7c. AgInSe,
-135.2
-0.47
8a.
-101.4
-0.26
-85.8
-0.22
-80.6
-0.21
la. AgInSe,
aq
aq
8b. 8c.
+ 1.5s4’- + 30H- + 3h+ 30.5AgzSe + 0.5In,O3 + l.5S4Se2- + 1.5H20 AgInSe, + 2HS- + 60H- + 4h+ 20.5Ag20 + 0.5InzO3+ 2SSe2- + 4H20 AgInSe, + 2S3’- + 40H- + 4h+ 20.5Ag20 + 0.51n,03 + 2S3Sez-+ 2 H 2 0 AgInSe, + 2S4’- + 40H- + 4h+ 20.5Ag20 + 0.5In2o3+ 2S4Se2-+ 2Hz0
TABLE V Possible Chemical Se/S Exchange Reactions for n-AgInSez in the Polysulfide Electrolyte and Related Thermodynamic Quantities
chemical reaction
AGO,
+ HS- + OH- 2AgInSSe + Se2- + H20 1b. AgInSe, + 2HS- + 20H- 2 AgInS, + 2Se2- + 2 H 2 0 2a. AgInSe, + S3,- 2AgInSSe + S,SeZ2b. AgInSe, + S32-2AgInS, + SSeZ23a. AgInSe, + S4,- 2AgInSSe + S3SeZ3b. AgInSe, + S4,-2AgInS, + SzSeZ2la. AgInSe,
kJ
AGO / a / kJ/(g-atom)
17.5
2.19
35.0
2.92
-20.0
-2.86
-38.7
-5.53
-18.4
-2.30
-41.7
-5.21
‘ a is the number of reaction atoms.
we assumed throughout the paper that the only species of the compound being oxidized during photodecomposition is Se, which in the case of the polysulfide solution may be solvated by the electrolyte anions, as discussed in the following. As Se atoms in AgInSe, possibly exchange themselves with S atoms of the polysulfide electrolyte, chemical (Table V) and electrochemical (Table VI) exchange reactions were also discussed. In the former case, we also considered the free energy change per (19) Becker, R. S.; Zheng, T.; Elton, J. J . Electrochem. SOC.1985, 132, 1824-1829. (20) Tenne, R.; Mirovsky, Y.; Sawatzky, G.; Giriat, W. J . Electrochem. SOC.1985, 132, 1829-1835. (21) Razzini, G.; Peraldo Bicelli, L.; Arfelli, M.; Scrosati, B. Eleczrochim. Acta 1986, 31, 1293-1298.
reacting g-atom as previously suggested,22to consider somehow the number of atoms participating in the elementary process, since the larger that number, the smaller the reaction probability. A similar correction is performed in practice when dividing the free energy change of an electrochemical process by the net number of transferred charges to obtain the decomposition potential. As usual, TDM values are calculated in standard conditions, which are certainly not the effective experimental conditions, particularly for the reaction products at the very beginning of the measurements.’ Of course, standard values may be adjusted according to the Nernst equation to match experimental conditions, an approach recently followed by Hodes and MillerS in their (22) Wager, J. F.; Jamjoum, 0.;Kazmerski, L. L. Sol. Cells 159-168.
1983, 9,
6994 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988
Peraldo Bicelli
TABLE VI: Possible Oxidative Se/S Exchange Photoreactionsfor n-AgInSe2 in the Polysulfide Electrolyte and Related Thermodynamic Quantities photoreaction AGO, kJ Eo(NHE), V la. AgInSe2
+ 2HS- + 20H- + 2h'
sAgInSSe + SSe2- + 2H20 sAgInS2 + Se?- + 2 H 2 0
+ 2HS- + 2 OH- + 2h' 2a. AgInSe2 + y3S32-+ Y3h' 2 AgInSSe + S3Se22b. AgInSe2 + Y3S3,- + Y3h' & AgInS2 + S2Se22c. AgInSe, + S2- + S32-+ 2h' & AgInSSe + S3Se22d. AgInSe, + S2- + S32-+ 2h+ & AgInS, + S2Se2,3a. AgInSe2 + 5hS42-+ 0 S h + & AgInSSe + S,Se23b. AgInSe, + 5/S42-+ OShf 3 AgInS2 + S3Se2,3c. AgInSe2 + S2- + S,2- + 2h' sAgInSSe + S4Se23d. AgInSe2 + S2- + S42-+ 2h' & AgInS2 + S3Se223e. AgInSe2 + S2- + S3,- + 2h' sAgInSSe + S4Se23f. AgInSe, + SZ2-+ S32-+ 2h' Z=AgInS2 + S3Se2,1b. AgInSe2
aq
significant discussion on the TDM stability of 11-VI chalcogenides in the polysulfide electrolyte. Bond Cleavage Photoreactions. The bond cleavage model may strictly be applied when the top of the semiconductor valence band has a relatively pure anion character and not a mixed anion-cation one as in the case of the Ag-In chalcogenide (up to 17% Ag 4d-orbitals contribution to the otherwise p-like valence bandz3). Here, the photogenerated d-holes may react with the electrolyte from quasiintrametallic d-levels. Keeping the role of the band structure on AgInSq stability well in mind, we tentatively considered the bond cleavage photoreactions owing to the low hybridization degree. Table I1 shows that none of these reactions occurs spontaneously with respect to H+ reduction. In the case of the formation of Ag+(aq) and In3+(aq) ions involving a high number of holes, incomplete Se oxidation was also considered. Such reactions (4-6) are expected to be highly improbable not only on a chemical but also on a TDM basis. By comparing reaction 2 to 1, we observe that in the particular case where the formed ions do not react further it is less difficult to split the bonds between In and Se atoms than between Ag and S e atoms, and this in spite of the In band being located deeply below the uppermost valknce band of AgInSe, and in spite of the high mobility of the monovalent ion expected for silver-containing I-111-VI2 ternary compounds.24 A very high decomposition potential is presented by reaction 7; on the contrary, reaction 8, differing from reaction 7 owing to the formation of In203 instead of In2Se3 (and, of course, of Ag2Se instead of Ag20), has a much smaller value. Such result, which is not unexpected if one compares the AGOf of In203with that of In2Se3(or In2S3;Table I), is a general trend for Ag and Cu ternary chalcogenides* and indicates their tendancy to form indium oxide on their surface. Up to 30 (60) mV more (less) positive decomposition potentials are obtained by assuming that In(OH),'(aq) (In(OH)2+(aq))is formed instead of In203when reactions 8, 10, and 11 take place, as previously observed for other indium-containing chalcogenides."* Polyiodide Electrolyte. Silver and indium iodides as well as oxides were assumed to be formed in the polyiodide electrolyte (Table 111), while the production of Ag+(aq) and In3+(aq) ions was considered as chemically ~ n r e a l i s t i c . ~Reactions ~ in which 13-instead of I- ions are involved occur more likely on a TDM basis, and even negative free energy changes are obtained in some cases (reactions l', 2', and 3'). (23) Shay, J. L.; Tell, B.; Kasper, H. M.;Schiavone, L. M. Phys. Reo. B
1973, 7, 4485-4490.
(24) Tell, B.; Wagner, S.;Kasper, H. M.J. Electrochem. SOC.1977,124, 536-5 37. (25) Cotton, F. A.; Wilkinson, G . Aduanced Inorganic Chemistry; Interscience: New York, 1972.
-123.3
-0.64
-115.3
-0.60
-47.6
-0.74
-70.9
-1.10
-108.8
-0.56
-132.1
-0.68
-37.7
-0.78
-60.0
-1.24
-106.2
-0.55
-128.5
-0.67
-104.5
-0.54
-126.8
-0.66
With both I- and I< as the reacting species, decomposition into indium selenide (reaction 1 and l', respectively) is the most probable process. Table I11 also shows that reactions 2-4 have very close values. Finally, here again, if In(OH),+(aq) or In(OH)2+(aq) are taken as one of the products instead of In203, in both reactions 4 and 4'some 30 mV more or 50 mV less positive values, respectively, are obtained. Polysulfide Electrolyte. According to Licht et a1.,26the currently employed aqueous polysulfide solutions are strongly alkaline S42-,and S3,- ions, while the S2", and prevailingly contain HS-, Ss2-,and S2- ions have lower relative activities, normally below 1% in most conditions. As shown in Table V, several Se/S exchange reactions without charge transfer occur spontaneously and may take place in the dark when the compound is in contact with the electrolyte. The thermodynamically most likely reactions, 2b and 3b, exchange both Se atoms of AgInSe2, the formation of the mixed chalcogenide beiig less probable. The results do not change substantially if reference is made to the free energy variation per number of reacting atoms. Note that reactions l a and l b involving the largest number of reactants are impossible. Examples of electrochemical Se/S exchange photoreactions are collected in Table VI. To limit the discussion to the most probable reactions, we considered only those requesting not more than two different reactants, out of which one at least is present in high concentration in the electrolyte. All these reactions have negative decomposition potentials and, with one exception (reaction l), have more negative values when AgInS, instead of AgInSSe is formed. Moreover, approximately the same results are obtained when two different polysulfide ions are involved. Such processes are expected to be kinetically limited. As to the oxidative photodecomposition reactions possibly taking place in the polysulfide electrolyte (Table IV), we may first consider reactions 1-5, which exchange the Se atoms of the compound with the S atoms of the electrolyte, so that silver and/or indium sulfide are produced. Elemental S e is simultaneously formed, which is further solvated by the sulfur-containing anions of the solution. These reactions mimic mutatis mutandis those occurring in the polyiodide electrolyte and already discussed where silver and/or indium iodides are formed (see Table 111). The results show that reactions 1-5 occur spontaneously and that their decomposition potentials are more (reaction 5 ) or less (reaction 4) affected by the reacting polysulfide ion. Type 4 reactions to Ag2S and In203 occur more likely than type 5 reactions to Ag20 and In2S3. However, this is not always true when In(OH),+(aq) or In(OH)2'(aq) are produced, as 280 or 450 mV more stable electrodes, respectively, are achieved. ( 2 6 ) Licht, S.; Manassen, J. J. Electrochem. SOC.1985,132, 1076-1081. Licht, S.; Hodes, G.; Manassen, J. Inorg. Chem. 1986, 25, 2486-2489.
n-AgInSez in Photoelectrochemical Cells TABLE VII: n-AgInSe2 Parameters (See Text) sulfate polyiodide soh electrolyte 1.24 1.24 E8, eV E,, V vs. NHE -0.39 -0.39 E,, V vs. NHE 0.85 0.85 E,
- EFB,V
E,,, V VS. NHE ~ E F * ( O CV) , VS. NHE EF(+ V VS. NHE EF(OC), V VS. NHE
vB(oc),v
-0.10 -0.29 0.30
-0.10 -0.29 0.30 0.43 -0.08
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 6995 V vs. NHE
A
polysulfide electrolyte 1.24 -1.29 -0.05
-0.2
0.0
-0.10 -1.19 -0.60
0.2
-0.54
0.4
0.21
The second group of photodecomposition reactions (reactions 6-8, Table IV) have silver and/or indium oxide among their products. Here, the decomposition potential is scarcely influenced by the solvating ion, whether HS-, S$-,or Sd2-. Unexpectedly, type 6 reactions have a positive rather than a negative free energy change, evidencing once more that higher decomposition potentials are obtained when A g 2 0 and InzSe3instead of AgzSe and In203 are formed. As to the influence of indium hydroxides, Eo(NHE) increases by about 310 and 500 mV, respectively, when In(OH),+(aq) or In(OH)Z+(aq)is produced in type 7 reactions. The corresponding figures are smaller for type 8 reactions, 230 and 370 mV, respectively. Such large enhancement of the semiconductor stability was previously observed for CuInSz and CuInS? and for CuIn5SJ also. Semiconductor Parameters. Although n-AgInSez has been little studied, it is still possible to approximately evaluate the semiconductor parameters that are necessary to build its energy diagram (Table VII). The bandgap value at 300 K, E*, is that originally determined by Shay et al.,z3 while the flatband potential, EFB, was here evaluated according to Butler and G i n l e ~ ?that ~ is, from the electronegativity of the constituent atoms by assuming the difference between the conduction band edge and the flatband potential, E, - EFe, equal to -0.10 V. The latter value is that theoretically calculated by Cahen and Mirovsky7 for ternary copper chalcogenides having a medium to high donor density. The theoretical flatband potential refers to AgInSez in a hypothetical aqueous solution (e.g., in sodium sulfate) where no ions are adsorbed on the electrode surface. From these data, we then determined the conduction, E,, and the valence, E,, band edges, while the hole quasi-Fermi level at the semiconductor surface under AM 1 illumination and in open-circuit conditions, ~ E F (OC), * was tentatively assumed to differ by -0.55 V from E,, as theoretically estimated for CuInSz and CuInSeZ7and for CdSe.20 Regarding the behavior in the polyiodide electrolyte, Dagan and Cahen9 found the open-circuit photovoltage of n-AgInSe2 to be -0.5 1 V, a value indicative of some Fermi level pinning, as also expected from the trend of the photocurrent-photovoltage characteristics. Indeed, the calculated value, Em - EF(cl) (EF(,,) is the Fermi level of the iodide-iodine redox coupleza),is -0.72 V, assuming that similar to AgInSSes*no I- ion adsorption occurs on the AgInSez surface. Therefore, the height of the potential barrier at the semiconductor surface in open-circuit conditions, V,(OC), evaluated as the difference between the above values, is 0.21 V, and the open-circuit Fermi level, EF(OC), is -0.08 V versus NHE. As to the polysulfide electrolyte, recent results for ternary chalcogenides evidenced a negative shift of the flatband potential with respect to the polyiodide solution,za Le., by -0.9 V for C U I ~and S by~ -0.6 ~ V ~ for ~ CuIn5Sa.s Since experimental data on the silver compounds are not available, we tentatively assumed that such difference is still -0.9 V for AgInSe,. It is worth stressing that as a consequence of such an approach the energy position of the semiconductor flatband and band edges (27) Butler, M. A.; Ginley, D. S . J. Electrochem. Soc. 1978,125,228-232. ( 2 8 ) Dagan, G.; Cahen, D. J. Electrochem. SOC.1987, 134, 592-600. (29) Shen, W.; Siripala, W.; Tomkiewicz, M.; Cahen, D. J. Electrochem. SOC.1986, 133, 107-112.
0.6
2
0 3 9.10 11
Figure 1. Energy scheme of n-AgInSe2in a pH 7 Na2S04solution in flatband conditions under AM 1 illumination. The data are taken from Table VII. The numbered energy levels show the standard photodecom-
position potentials relating to the different bond cleavage photoreactions reported in Table 11. Water (pH 7) redox levels are also indicated. has a certain degree of arbitrariness. Nevertheless, this does not seriously affect the TDM conclusions since it is not the absolute position of but the sign of the difference between the decomposition and the redox Fermi level, i.e., whether the former level lies above or below the latter (and their relative kinetics, of course), that ultimately determines the semiconductor stability. Indeed, this is the approach by Hodes and Miller5 when examining the TDM stability of n-type 11-VI semiconductors. They calculated the difference between the photodecomposition and the electrolyte redox potentials. If such difference turns out to be positive, photodecomposition occurs less likely than competitive electrolyte oxidation, and vice versa. Finally, no information is available at present on the semiconductor open-circuit band bending in the polysulfide electrolyte.
Discussion Figure 1 depicts a schematic energy representation of n-AgInSe, in flatband conditions (since EF(OC) is unknown) and in a pH 7 sodium sulfate solution. It also reports the photodecomposition potential of the bond cleavage photoreactions we previously calculated (Table 11) as well as the Fermi level of the water redox systems, Hz/HzOand HzO/02, in the same electrolyte. Reference is made to the electrochemical scale based on the normalized hydrogen electrode. According to Gerischer3 and to Bard and Wrighton: anodic photodecomposition reactions of an n-type semiconductor are possible from a TDM point of view when their decomposition potential is located between the semiconductor Fermi level and the hole quasi-Fermi level at the surface of the material. As shown in Figure 1, the illuminated compound is not expected to be thermodynamically stable in rest conditions since several bond cleavage reactions have a higher driving force than water oxidation. The most likely process is decomposition to AgzSe, In3+(aq) ions and Se" (reaction 2), as also found for With our choice of the hole quasi-Fermi level, this is just the only possible reaction in open-circuit conditions. Note, however, that such level is expected to shift downward as soon as the density of photogenerated holes at the surface i n c r e a ~ e s .In ~ that case, reaction 8 is the next possible process; as previously seen, its energy level is even higher if In(OH)Z+(aq)instead of In203 is formed. Several photodecomposition reactions may take place in the polyiodide electrolyte, too, as shown in Figure 2, where the relevant energy scheme in flatband and in open-circuit conditions is depicted as well as the Fermi levels of the reactions in Table 111. In addition, such reactions are more likely to occur than the redox process itself, and a highly unstable material is expected, as experimentally found.9 After a passage of several Ccm-z in a PEC cell having n-AgInSez as anode, a surface layer of red Seois formed that, according to microprobe analyses, contains very minor amounts of Ag, In, and iodine. Thus, the decomposition mechanism generally agrees with reactions 1-3 of Table 111, whose
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The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 Vvs. NHE
V vs. NHE
0.6
Peraldo Bicelli
5'
-0.21
I
I
0.8
Figure 4. Energy scheme of n-AgInSezin the polysulfide electrolytein flatband conditions under AM 1 illumination. The data are taken from Figure 2. Energy scheme of n-AgInSez in the polyiodide electrolyte in flatband and in open-circuit conditions under AM 1 illumination. The
data are taken from Table VII. The numbered energy levels show the standard phdtodccompositionpotentials relating to the different oxidative photodecomposition reactions reported in Table 111. The experimental I-/&- redox level is also indicated. V vs NHE
Figure 3. Energy scheme of n-AgInSezin the polysulfide electrolyte in flatband conditions under AM 1 illumination. The data are taken from
Table VII. The numbered energy levels show the standard photodecomposition potentials relating to the different oxidative photodecomposition reactions reported in Table IV. The experimental S2-/S:redox level is also indicated. Fermi levels are in a "dangerous" position (see Figure 2). A similar behavior as to TDM instability and elemental selenium formation was also predicted8 and observed2*J0for n-AgInSSe8. Now, the question is if the presence of an artificially formed indium oxide layer on the electrode surface is able to protect AgInSq from photocorrosion in polyiodide. Indium oxide is, apparently, stable in the neutral electrolyte, while its chemical reaction with I- ions occurs spontaneously in acid solutions and in alkaline solutions the electrochemical oxidation of I- to IO3ions takes place at potentials in the range from 0.4 to 0.7 V versus NHE depending on the reacting oxide.' Preliminary results seem to indicate that air oxidation does not improve n-AgInSe2 performance and stability? as found for n-AgIn,Se8.31 Consistent with TDM data of Table IV, the situation as to stability of n-AgInSe2 in aqueous polysulfide is comparable to that observed in the polyiodide electrolyte. Here too, several reactions are possible (Figure 3), while the redox process may hardly compete with them owing to its unfavorable energy position. However, all the reactions that do not exchange Se with S (reactions 6-8 in Table IV) are less likely on a TDM basis than the redox process itself. ~
Table VII. The numbered energy levels show the standard photodecomposition potentials relating to the different oxidative Se/S exchange photoreactionsreported in Table VI. The experimental S2-/S2- redox level is also indicated. The most probable photodecomposition reactions are those to In2S3plus A g 3 e (type 2 reactions) or plus AgP (type 3 reactions). Because 6 or 8 additional reactant molecules, respectively, are involved per formula unit of the chalcogenide, reactions 2a and 3a are apt to be kinetically limited, notwithstanding the higher concentration of HS-ions with respect to S32-and S2- in the electrolyte. Note that even on a TDM basis, they are less probable than the other type 2 and 3 reactions. However, the decomposition potential of reactions 2a and 3a themselves shifts upward if highly alkaline solutions are employed, and indeed the detrimental effect of the added hydroxide to the polysulfide electrolyte in terms of solution stability (but also of optical adsorption) is well-known in the case of C U I ~ S ~ ~ . ~ ~ The already published results on n-AgInSe2 in polysulfideg substantially agree with this picture. After use in this electrolyte, the electrode surface is severely depleted in Ag and Se, and Auger analyses suggest that at least some of the In is present as part of an oxide. However, from TDM data (Figure 3) In is more likely to form a sulfide. Moreover, even the performance in the optimal (Cs-containing) solution is inferior to that in the polyiodide electrolyte. As a tentative explanation we may recall that, usually, I- ions react much faster with holes than do other redox systems.'* Note that a better performance in polyiodide was also found for n-AgIn5Se8 and n-CuInSe2 and that the former compound was shown to give no PEC response at all in the polysulfide Finally, oxidative Se/S exchange photoreactions are possible in the polysulfide electrolyte in addition to chemical reactions in the dark, as shown in Figure 4 where the Fermi levels of the processes examined in Table VI are indicated together with the energy scheme of n-AgInSe2. The semiconductor surface may be covered by an AgInS2 film whose thickness and porosity determine whether an additional space-charge layer has to be considered. Owing to the lack of experimental data, however, it is difficult to achieve conclusive results. Nevertheless, a discontinuous laycr is expected to be formed since surface processes are prevailingly localized at defect sites associated with weakened or broken bonds which, therefore, are energetically more active (according to Frese et al.33 the decomposition potential of photoanodes shifts negatively a t defect sites). In comparison to CuInSq, A g I n h is expected to show a higher solar-to-electrical conversion efficiency owing to the better tailored value of its bandgap: 1.24 (ref 23) instead of 1.01 (ref 7) eV. Such higher value derives from the lower anion-cation hybridization of the uppermost valence band (17% instead of 34%) and thus from the lower repulsion of the anion p-levels and the noble-metal d-levels.
~~~~
(30)Peraldo Bicelli, L.; Razzini, G.; Arfelli, M.; Scrosati,B. Solar Energy Mater. 1987, 15, 463-414. (31) Razzini, G.; et al., unpublished results.
(32) Gerischer, H. J. Electroanal. Chem. 1983, 150, 553-569. (33) Frese, K. W., Jr.; Madou, M. J.; Morrison, S. R. J . Phys. Chem. 1980,84, 3172-3178.
J. Phys. Chem. 1988, 92,6997-7001 It is the same difference in the bandgap that determines the difference in the electron affinity (EA) that is in the conduction band edge, as the bulk electronegativity ( x ) of the two compounds is practically the same (4.74 and 4.73 eV for the silver and the copper chalcogenide, respectively). In fact, according to Butler and Ginley2’ EA x - E,/2 and, therefore, the values of -0.39 and -0.29 V versus N H E are obtained for AgInSe2 and CuInSe2, respectively. Note that the value of the electron affinity of CuInSez is in good agreement with that calculated by Cahen and Mirovsky’ for the conduction band edge in the polyiodide electrolyte (-0.30 V versus ”E). Since the energy level of the redox couple in the electrolyte is independent of the electrode material, the silver compound turns out to be slightly (-0.10 V) less stable than the copper compound, assuming that no ion adsorption occurs on the semiconductor surface. Moreover, another very important point should be considered. Owing to the low p-d admixture, more involvement of the anion bonding electrons may be expected in the silver than in the copper compound. In other words, lattice Ag-Se bonds are expected to be broken more readily than Cu-Se ones. In addition, both chalcogenides show an appreciable ionic conductivity, particularly the silver c o m p o ~ n d as ~ ~the J ~mobility of the Ag+(aq) ions is higher than that of the Cu+(aq) ions. Thus, (34) Shay, J. L.; Wernick, J. H. Ternary Chalcopyrite Semiconductors, Growth, Electronic Properties and Applications; Pergamon: Oxford, 1975.
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a field-assisted out-diffusion of the cations is to be expected, which leads to a greater surface depletion and lattice instability in AgInSez than in CuInSe2, as primarily suggested by Dagan and Cahem9 Conclusions
In this paper, the energy relations known as thermodynamics are used to show the electrochemical and chemical decomposition reactions that may take place in a PEC cell having n-AgInSe, as the photoanode. The results of such investigation agree with preliminary experimental data. The material is expected to photodecompose and its instability is more pronounced in the polyiodide than in the polysulfide solution since SeO rapidly dissolves in the latter electrolyte, Seobeing the main corrosion product well-known to depress the PEC performance. Moreover, TDM evaluation as well as solid-state considerations evidence that AgInSe? is more prone to hole attack than its copper analogue.
Acknowledgment. I gratefully acknowledge G. Dagan and D. Cahen for their experimental results on AgInSQ (ref 9) on which this work is mainly based. The research was supported by the Progetto Finalizzato Energetica 2, CNR-ENEA Contract No. 86.00908.59 and by Minister0 della Pubblica Istruzione. Registry No. InAgSe’, 12002-76-5;InAgSSe, 1 16128-64-4;Na#04, 7757-82-6; Ag2Se, 1302-09-6; In3+, 7440-74-6; In’s3, 12030-24-9; InAgS2, 12002-75-4;s’-, 18496-25-8;S3’-, 12597-05-6;s,’-, 12597-07-8; HS-,15035-72-0;In203,1312-43-2; In2%,, 12056-07-4;S2Se2-,3168385-9; S3Se2*-,116129-31-8;Ag2S, 21548-73-2; Se, 7782-49-2; H20, 7732-18-5; Ag, 7440-22-4; Se2-, 22541-48-6; I-, 20461-54-5; 13-, 1490004-0; AgI, 7783-96-2; In13, 13510-35-5; Ag20, 20667-12-3; SSe2-, 11089-59-1; S4Se2-, 116129-32-9; S3Se2-,116129-33-0.
Solute-Solute-Solvent Interactlons Studied through the Kirkwood-Buff Theory. 3. Effect of the Pressure on 0,for Some Aqueous and Nonaqueous Mixturest L. Lepori and E. Matteoli* Istituto di Chimica Quantistica ed Energetica Molecolare del C.N.R.,via Risorgimento 35, 56100 Pisa, Italy (Received: October 9, 1987; In Final Form: March 28, 1988)
From thermodynamic data, the Kirkwood-Buff integrals, Gij,have been obtained in the entire mole fracton range for binary mixtures of tetrachloromethane (TCM) with C,HwlOH (n = 1-4), 2-methyl-2-propano1,1,Cdioxane, and tetrahydrofuran. The pressure coefficients of Gij, G’j = aGij/aP,are also reported for the above mixtures as well as for the corresponding aqueous solutions. In addition, the values of the excess local concentration are calculated for all mixtures. In aqueous systems, the tendency to homocoordination is observed for both alcohols and ethers as well as for water and is larger the larger the hydrocarbon-like moiety of the compounds; in TCM-containing mixtures, only alcohol molecules show mutual affinity, which, on the contrary, is larger for the lower alcohols. Gtvcurves indicate that the tendency to homocoordination is practically independent of pressure for TCM mixtures, but for aqueous solutions it decreases sensibly with increasing P at high solute concentrations and increases in very dilute solutions. These results are qualitatively discussed in order to point out the type of interactions that may be responsible for the observed G, and Gtijbehavior.
Introduction
In a previous paper we reported and discussed the first results of our study of solute-solute-solvent interactions based on the Kirkwood-Buff theory.’ By means of thermodynamic quantities such as activity coefficients, partial molal volumes, and compressibilities, for aqueous solutions of a number of organic compounds, we were able to calculate the Kirkwood-Buff integrals, Gjj (ij= 1,2), defined by Gij = s a0 ( g i j - 1)4ar2 d r
(1)
+Preliminary results were presented at the 1st Meeting of the Portuguese Electrochemical Society, Coimbra, Portugal, 1984. To whom correspondence should be addressed.
0022-3654/88/2092-6997$01.50/0
gijbeing the radial distribution function. These parameters, which convey information on the tendency of molecules of species i to become more concentrated or more diluted (with respect to the bulk concentration) in the entire surrounding of a given j molecule, were found useful in characterizing the local environment of various molecules in liquid mixtures. We have now determined GI, values for mixtures of tetrachloromethane (TCM) with each of these compounds: methanol, ethanol, 1-propanol, I-butanol, 2-methyl-2-propano1, tetrahydrofuran (THF), and 1,Cdioxane; in addition, the results of a study of the effect of pressure on G, behavior in mixtures of either water or T C M with each of the above compounds are (1) Matteoli, E.; Lepori, L. J. Chem. Phys. 1984, 80, 2856.
0 1988 American Chemical Society
