- Email: [email protected]
Surface and photoelectrochemical studies of semiconducting MoS2
SURFACE
AND PHOTOELECTROCHEMICAL SEMICONDUCTING MoS,
STUDIES
OF
S. M. AHMED Mineral Sciences Laboratories, Canada Centre for Minerals and Energy Technology, Department Mines and Resources, 555 Booth St., Ottawa, Ontario, Canada (Received
infinnl
form
13 October
of Energy,
1981)
A&tract-Redox reactions with CU(NH~)~~/‘~, Fe(CN)~-/4-, Fe’+“+ and IT/I2 were studied on 11-C surfaces of MoS, using rdes. From electrophoretic measurements, the zpc was found to occur at pH 2. The role of potential-determining ions and the effects of specific adsorption of ions on the electrode behaviour have been discussed. The change of potential with pH was - 30 mV/pH unit for MoS, electrode in contact with a solution saturated with molybdenum and S2- ions. . From the information available, an energy level diagram has been constructed. This diagram is in agreement with the observed behaviour that electron exchange for the first three redox couples occurs with the conduction band, except that the electron transfer from the reduced species, Fe(CN)z- and Fe’+, 1x1the reverse direction (oxidation) is partly and totally restricted, respectively. The charge transfer behaviour in the ease of I; /I, couple, aDDcarS to be much more complex, most probably involving adsorption of 1. on the surface.- _ __
ring, n-type MoS, were finely crushed using an agate mortar and pestle. The powder was leached with 2 M HCl and washed with triple distilled water until the washings were free of Cl-. A Zeta Meter (Zeta Meter, New York) was used to measure the electrophoretic mobility in the presence of 0.01 M KNO, as an indifferent electrolyte. The original Zeta Meter electrodes were replaced by palladium electrodes which were electrolytically charged with hydrogen to half of their capacity. The preparation and use of these electrodes have been described elsewhereC5, 6-J.Use of such partly charged palladium electrodes prevents gas evolution during electrophoretic measurements, and enables use of high current densities, provided the electrode polarity is reversed for equal durations of time during measurements. The equilibrium potentials of MoS, electrodes with respect to see were measured, in the absence of any added redox couple, as a function of pH ih 0.1 M K2S04 solutions buffered with borate, and recorded as a function of time on a strip chart recorder. Two Beckman Research pH/volt Meters were used, one for monitoring the pH and another to record the potential. For reasons to be clarified later, the experimental solutions were saturated For at least 24 h in nitrogen atmosphere with a precipitate of molybdenum sulphide (MoS,) prepared by passing H2S into a solution of ammonium molybdate (0.01 M) in HBr containing S “/, formic acid at 0°C. To prevent changes in the rest potential of the electrode due to air exposure or pretreatment, all pH-potential readings were taken in the same solution without removing the electrode, after only pH adjustment. The solubility of MoS, was also determined as a function of pH by measuring molybdenum dissolved in 0.01 M KC1 solutions that were saturated with MoS, powder for 24 h in nitrogen atmosphere. Atomic absorption was used for molybdenum analysis. A 150 W xenon lamp was used for illumination of the electrode under potentiostatic conditions.
INTRODUCTION In earlier work by Tributsch[i, 21 MO& has been reported to exhibit superior stability to photocorrosion in photoelectrochemical (PEC) cells and good efficiency in solar energy conversion. It was subsequently discovered[3,4] from the measurements of the exchange current densities for various redox reactions that the inertness of MO& was limited only to the basal plane (surfacesI-C axis) of the layered, hexagonal MoSz structure, whereas the surfaces parallel to the C-axis (11-C) were highly reactive. Also, in studies of redox reactions it was shown that both forward and reverse reactions occurred readily in the case of the Cu(NHI)z+/t+ couple. In the case of Fe(CN)z-‘4and Fe”+“+, while the electron transfer occurred readily in the forward direction (reduction), it was partly and totally blocked in the reverse direction, respectively. The present work reports further studies of the redox reactions on the 11-C surfaces of MO&, determinations of the zero point of charge (zpc), and the effect of pH on the reversible double layer at the MoS,-electrolyte interface. Specific effects of certain ions on the photocurrents of MoS, have also been investigated on both 11-C andl-C surfaces. From the information obtained, an energy level diagram has been constructed and the nature of charge transfer established.
EXPERIMENTAL Experimental details of electrode preparation and most electrochemical measurements have been described earlier[3]. Fat measurements of the electrophoretic mobility, samples of the naturally occur-
0
Minister of Supply and Services, Canada,
1981. 707
708
S. M.
RESULTS The current-voltage
behaviour
Results of rotating disk electrode (rde) studies For the Cu(NH3)<+/l*, Fe(CN)z-‘4P, Feat’*+ and I; /I* couples on the 11-Csurfaces of MoS, are shown in Figs l-3 and 4 respectively. Although the anodic side of the voltammogram for the Fe’+ oxidation is not shown in Fig. 3, there was no evidence for any
AHMED
detectable Fe2+ oxidation. As seen in Figs l-3 the diffusion limited currents (iL) are reached easily in both forward and reverse directions in the case of the copper ammonia couple (Fig. 1) and in the forward direction for the other two coupks (Fig. 2,3). A plot of iL usthe square root of the rotation speed yielded linear plots and the diffusion constants calculated from these plots using Levich’s equation, were found to be comparable with the known values, within the experimental error.
Fig. 1. Current-voltagecurves at differentrotation speedsfor the Cu(NH& +I+ ‘ couple 011MoS, surface 11-C; pH 11.O, electrolyte 0.5 M NH,CI plus excess ammonia;sweep rate 102 mV mitt- t ; concn.,cupric 1.O K lo-’
M cuprous 1.05 x lo-‘M.
Fig. 2. Current-voltage curves at different rotation speeds for Fe(CN):-‘*each, on MoSz surfams (1C; pH 7; electrolyte0.5 hl KCl; sweep
couple; concn 5 x 10m3M
rate 102mV mm-‘.
Surface and photoelectrochemical
MOSS,
OT
Fc2+/3+
studies of semiconducting MoS,
709
II-C (001
U)
im _-
0 0.2
04
0
-0.2
-a4
-0.6
-08
VOLTS (SCE)
Fig. 3. Current-voltage curvesat differentrotation speeds for Fe l+j3+ reduction (0.01 M), on MoS, surface (I-C; pH 2.1; electrolyte
0.5 M KCI; sweep rate 135 mV min-‘.
Results of rde studies for the anodic oxidation of iodide to I2 (E” = + 0.293 V see) in dark, on 11-C and 1-C surfaces of MoS, are shown in Fig. 4in the anodic range only. Electron transfer occurs readily in both directions although diffusion limited currents were not reached readily. On illumination a large shift in potential for the onset of photocurrents (i,,h) ( - 600 mV) in I- solutions followed by large photo-
242
7.26 N 'E o 9,68 E
I
110 I=
f
-- IPI 0
------‘Ka
0.075 (*
S04+0.0Q5M I
I
I
KI
/
,’
0
16.94 3 0.t
-
/’ / /
I-
c
Photo Currents Oork Currents
I
KC,
O,SM
2
KBr
0.1 M
3 Kl O.IM + 0.5 MK C,
0.6
I
I
1
0.4
0.2
0
VOLTS
t-5
(S.C.E.l
Fig. 5. Voltammogram showing dark and photocurrents in Cl- Br- and I- solutions on MoS,I-C surfaces (naturally occurring n-type, 11-Csurfacesalso exposed). 150 W Xe lamp was used, sweep rate 100 mV min-‘.
3-a
I
I
0.7
0.6
I 0.4
0.2
VOimTS (SE) Fig. 4. Current-voltagecur at differentrotation speedsof MO& in KI(0.005 M) plus K,SO, (0.5 M) solutions at PH- 7. Sweep rate 100 mV min- ‘. currents compared to Br- and Cl- (Fig. 5) are most noteworthy and have been observed by earlier workers also[2]. In the absence of any oxidizable ions such as I-, the holes generated from illumination have been shown to react as follows[2]. MoS, + BH,O + lXh+ + MO(W) + 250:-
+ 16H+ (1)
and partly as, MoS,+2H,O+4h+
shown plotted vs pH in Fig. 6. MoSz acquires a zero surface charge at pH 2 in an indifferent eIcctrolyte solution which also gets saturated with the dissolved lattice ions from the surface during the experiments. Hence the zero point of charge (or zpc) for MoS, occurs at pH 2. In solutions of pH > 2 the surface acquires a strong negative charge resulting in a negative zeta potential (Fig. 6) which reverses its sign to positive in the presence of 1O-3 MCu’*, Pb2’ or Cd’+ due to their specific adsorption on the surface. However, these cations and Fe3+ in lo-’ M concentrations have no effect on the surface charge in the pH range above the zpr, while below the zpc the zeta potential becomes positive. The behaviour is similar to that of galena having a pH,, = 2.4, pS&; = 19[7]. In ammonical solutions, the copper ammonia complex is so strongly
-+MoS,+0,+4H+.
The zero point of charge oJ.MoS,
(2)
and the pH-potential
WhiOflShip
potentials as calculated from the electrophoretie mobility of MoS, using standard equations are Zeta
MoS*
---
14-52
1
‘5 E ._
/
ill
adsorbed
on MoS2
that the surface
charge is seen to become zero, in Fig. 6A. In Fig. 7, the electrode potentials of MoS, (see) in an indifferent electrolyte solution, saturated with molybdenum and S2- ions are shown plotted us pH. This plot gives a slope of -30 mV/pH unit. Without presaturation of the electrolyte with molybdenum and S’-, as described previously, the equilibrium was hard
S. M.
710 Cuz+
P
A
1 _
AHMED
Pbz+8
Fe’*
Cl
CP’ lo-~u
-0
x c$+ l
lo-%4
* CttfNH&+
IO%
Na2S.
0 Pb**or l
Na2S
Cd2’
Pb*+ lO-3
Fe3+
10-M
10-3M M
Fig. 6. Diagram showing the effect of different ions on the clectrophoretic mobility of MoS, in 10m3M K, SO, solutions.
Fig. 7. Variation of MO& electrode potential (see) with PH. to obtain and the pH-potential plots showed inconsistent behaviour most probably due to an unstoichiometric dissolution of the surface. The amount of MO”+ ions dissolved in 0.01 M KC1 solution from MO&, as a function of final pH, under equilibrium conditions, is shown in Table 1. Table 1. Molybdenum ions dissolved in 0.01 M KCI solutions from MoS, as a function of final
pH, at equilibrium
MO dissolved,
PH
(pm01 1-l)
1.1 3.1 8.0 9.8 13.1
125 2 109 375 360
DISCUSSION The reversible double layer (MO%)
on a semiconductor
sulphide
When a metal sulpbide (or oxide) is brought into contact with an electrolyte solution, the initial reaction
is surface hydration followed by a rapid (_ 1 min) acid-base dissociation of the surface groups giving rise to surface charge ( +) and an ionic double layer at the interface. This is followed by a slow dissolution of the lattice ions from the surface and a pH-dependent ion hydrolysis in solution, which continues until equilibrium is established between the two phases. These two stages of equilibria have been well established for oxides[8]. As some of these ions could exchange electrons with the bulk semiconductor (see later), the Fermi levels in the two phases would be equalized. The primary potential-determining (p-d) ions are the constituents of the lattice itself, although their concentration in solution depends on pH as a result of ion hydrolysis and degree of dissociation. As a number of both cationic and anioic p-d ions are involved in the pH-potential equilibria of sulphides and oxides, mixed potentials are normally set up in most cases. The overall rates of electron exchange at these equilibrium processes are given by the exchange currents (i” =T=T) which can be determined experimentally[3]. In most cases the exchange currents and hence the electron transfer rates are low and the equilibria are also slow to be established. A dynamic equilibrium can be assumed in some cases where the change of electrode potential and pH with time is small and linear. At a characteristic pH or pS’-, known as the zero point of charge (zpc), the concentration of the cationic and anionic p-d ions remain equal and the surface groups undissociated, when the surface charge and hence the field strength (d4/dx) will be zero. The solubility of the material is also known to be minimum at the pHfpc, as seen in Table 1. The zpc of the reversible, Ionic double layer is the counterpart of the flat band potential associated with the space charge region. In principle, the surfacedissolution/adsorption processes on MO& can give rise to hydrated molybdenum ions in several oxidation states, the surface itself being negatively or positively charged depending on the surface hydrolysis and the degree of acid/base dissociation of the surface groups. 2(MoS&+ 2(MoS&
(MO&),+
Mo(IV) + 2S2-
(3)
+ (MoS,);~-
+ Ma(VI) + 2Sz-
(4)
Surface and photoelectrochemical studies of semiconducting MoS,
2(MoS&
= (MO&);
+ Mo(III) + 2s’ -, etc., (5)
S*- -tH+ =+HSMO”+ + 6H20 = Mo(H,O)“,+ .
(6) (7)
In our preliminary investigations of MoS, electrodes by cyclic voltammetry with MO(W) added to the test solutions (as bromide; ammonium molybdate dissolved in HBr), well defined redox peaks in the range of - 350 to - 750 mV see [Mo(VI)=MO(IV)] were obtained at PI-I - 1.7. These peaks disappear when the same solution of Mo(V1) is reduced by Zn to MO(W) before the experiment, when new peaks appear in the - 900 mV see range. The E”,z,s.sbeing = - 0.72 V see, Eredol values of the p-d ions lie in the conduction band range of MoS, (Fig. 8). This behaviour of electron exchange between the semiconductor and the p-d ions is still under investigation at present. There is evidence that some of the p-d ions can alter the Helmholtz layer potential in the adsorbed state by virtue of their excess surface charge without any electron exchange with the bulk. For example, addition of Ti!+ ion to a cell with a semiconductor TiO, electrode in acid medium, changes the equitibrium electrode potential strongly to more negative values, changes the flat band potential and also reduces the open circuit photovoltages measured against a see. However, Ti3+ (E”, Ti3+‘*+ = 0.06 V nhe) under the same conditions was Found to have no effect whatsover on the i-V behaviour of TiO, as found from rde studies. The change of Galvani potential in this case by adding Ti3+ was due to excess Ti-Ogroups introduced on the surface. Depending on the chemistry of metal hydroxy complexes, the effects of cationic p-d ions on the Fermi levels, Galvani potentials and possibly on band bending could be practically eliminated at high pH as these are precipitated out and only anions such as S2- for sulphides and 02- or OH- for oxides will be potential determining. Hence, the equilibrium potential (E,) for MoS, in the absence of any added redox couple, may be written as. RT
E,=En--Inasl. 2F in the pH range > pH,,
where the effect of cations can
711
be neglected. Hence the variation of the electrode potential @,with usI- and pH ( - 30 mV/pH unit, Fig. 7) is given as: RT d@, = --dlno,,., (9) 2F d4& = O.O295dlna,+
= -O.O295d(pH).
HS-
= I..I+ +Sf-; a.& =
K,
K, = aH”as~a HS-
. uHSaH*
--.
K
27:s
-
E
(11) (12)
*H+
Hence, A@,
= A&
= - 0.0295 (pH, - pH,,);
(13)
assuming that the entire change in the electrode potential 4w occurs in the Helmholtz region, A#M = A~!I”; pH, being the pH at equilibrium. As seen earlier part of this potential drop could be manifested as change in the Fermi level. A detailed study of the role of the primary p-d ions, in this respect is needed. The double layer effect on the photoelectrochemical behauiour The double layer on sulphides is similar to that on oxides as reviewed earlier[9] except that sulphides being less ionic than oxides, the surface charge densities in the former case are one order of magnitude lowerC9, lo] than in the latter ( - 60-70 pC cm- * for 3ODmV for oxides). Counter ions which can NH form insoluble sulphides can get easily adsorbed on the negatively charged surfaces of MO& as described in the previous section. The zpc can be shifted by severai hundred mV by specific adsorption of suitable inorganic and organic reagents, in the inner Helmholtz layer. The observed shift in the potential For the onset of photocurrents on MO& in the presence of I-, as shown in Fig. 5, is most probably due to the specific adsorption of I- which would shift the flat band potential.
Fig. 8. Energy level diagram of MoS, showing schematically positions of the Ehcx levels relative to the conduction band (E,) and valence band (E,) edges at different pH values. For details see text. El\
(10)
This is because the concentration of HS- remains practically constant up to pH 11 and as1- is inversely proportional to aHl, as shown below;
712
S. M. AHMED
__
Charge transfer reactions
on n-type
MO&
From the measured values of the zpc and the flat band potentials ( j’bp) obtained by the capacitative phase shift method[2,7], the energy level diagrams at DH 2.0.7.0 and I 1.5 have been derived. as shown in Fig. ‘s, afte; allowing - 29.5 mV/pH for ;he change in t& Helmholtz layer potential (Ad” = 0 at pH 2, the zpc) according to equation (1 l), U,,,(sce,
where U,,,(sce} located and &is
= (4,-t-
A&,)
- 4.74,
(14)
is the level at which the jbp(sce) is
the work function of the semiconductor. The E” (see) values of the redox couples and the conduction band edge, E,at the flat band condition, at pH 2.0, 7.0 and 11.0 at which the redox couples were investigated, are also shown in Table 2. The band gadis taken as 1.1 eV based on recent investigations by Kautek et a/.[41 and an average separation of 0.2 eV between the Fermi level and E, for the naturally occurring n-type MoSz has been used, the fbp being 0.3 V (see) for the 11-Csurface[7] and 0.5 V (see) for the I-C MoS,[2, 71. The data shown in Fig. 8 and Table 2 are for ) 1-Csurfaces, for which the redox reactions are presented in Figs i--3; the position of E,is 0.2 eV lower for theI-C surfaces in the energy level diagram. The reorganizational energy (A), used in Fig. 8 for Fe(CN)z“- is taken from published work[12] while for other couples it is schematic. The E” value for the copper couple depends on NH3 concentration, or pH[lS]. Hence E’at pH lOand 11Saregiven inTable 2 and the latter value is used in Fig. 9. The anodic decomposition of MoS, occurs at - a7 V (WC). Table 2 Redox couple Fe2+l,+ Fe(CN).?-‘*Cu(NH,):+“+ Cut NHa),5+/1+ s - /‘SD 1-/t,
PH
E” (SW) (V)
E, (We) (V)
!p”v,
2.0 7.0 10.0 11.5 14.0 7.0
0.53 0.12 - 0.0 - 0.224 - 0.76 0.293
0.1 - 0.05 -0.14 -0.18
> 0.74 0.74 -
- 0.05
The E, edge (Table 2, Fig. 8) is located slightly below well above (
forward direction (cathodic) for all the three couples, while electron transfer in the reverse direction (anodic) from the reduced species to the conduction band should be moderate for Fe(CN)z and poor or nil for Fe*+. This is in agreement with the experimental work as seen from Figs l-3. Specific adsorption of Cu(NH& + on MoS, (Fig. 6) could also contribute to the reduction of the energy barrier for electron transfer
and metal-like behaviour of the copper ammonia couple. The slow electron injection from Fe(CN)zinto the conduction band, may also be promoted through other mechanisms such as thermal activation of the energy levels in solution, charge pick-up by
surface states and the possible variation of the band edge position at different voltages or variations in concentration of the p-d ions as discussed earlier. The Fe2+ (uncomplexed) level appears to be too far below ( c 0.45 eV) the E,edge for Fe2+ oxidation to occur in preference to surface oxidation and/or anodic dissolution of MoS, (Edsc = 0.7 V see). Electron injection into the valence band is also not possible as E, edge lies well below the Ed=. The charge transfer from I; /I2 appears to involve a complex mechanism and is strongly influenced by specific adsorption of I- on the surface. In the preparation of synthetic MO&, iodine may be used for vapour transport to promote growth of single or larger crystals. The foregoing studies have shown that the inclusion of residual iodine, as molybdenum iodide, would result in an increase in dark currents, particularly when 11-C surfaces are produced in large proportions. This would consequently decrease the overall energy conversion efficiency of the electrode. This difficulty was actually experienced in our attempt to sinter pressed MoS, pellets in the presence of iodine. However, by taking advantage of the higher reactivity of 11-C surfaces compared to that ofl-C surfaces, if the former can be rendered inactive by selective chemisorption of suitable reagents, then it might be possible to reduce the dark currents and increase the efficiency.
Attempts in this direction are being made. Alternatively, bromine can be used in the vapour transport and sintering processes. However, if one wants to take advantage of the high photocurrents in the presence of I-, the dark currents due to 11-C surfaces will have to be minimized by suitable means. Acknowledgements-Part oftheexperimental work described herein was carried out at the Fritz-Haber-Institut, MaxPlanck-Gesellschaft, Berlin. The author is thankful to Professor Gerischer for providing the research facilities and for critical comments and to Dr. A. H. Webster, Head, Physical Chemistrv Section, CANMET, Ottawa for helpful dis&ssions.
REFERENCES 1. H. Tributsch and J. C. Bennett, J. elecrroanal. Chem 81, 97 (1977). 2. H. Tributsch, J. electrochem. Sot. 125, 1086 (1978). 3. S. M. Ahmed and H. Gerischer, Electrochim. Acta 24,705 (1979). 4. W. Kautek, H. Gerischer and H. Tributsch, Ber. BlmsengesPhys. Chem. 83, loo0 (1979). 5. R. Neihof, J. Cvlloid Inferface Sci. 30, 128 (1969). 6. S. M. Ahmed, Mines Branch Technical Bulletin TB 140, Department of Energy, Mines and Resources, Ottawa, Canada 1971. 7. S. M. Ahmcd, unpublished work. 8. S. M. Ahmed and D. Maksimov, Can. J. Chem. 46,384l (L968).
9. S. M. Ahmed, in Anodic Behaviour of Metals and Semiconductors Series, Vol. 1, Ch. 4, Oxides and Oxide Films. (Edited by J. W. Diggle) Marcel Dekker, New -York (i972). _ 10. S. M. Ahmed, J. phys. Chem. Irhaca 73, 3546 (1965). 11. A. J. Nozik, Proc. 2nd World Hydrogen Energy Conference, Zurich, 21-24 August, p. 1217 (1978). 12. R. A. L. Vanden Berghe, F. Cardon and W. P. Games, Surf. Sci. 39, 368 (1973). 13. J. J. Lingane, Chem. Rev. 29, 1 (1941). Publication N.B.S. Special 455 14. H. Gerischer, Eleclrocotnlysis on Non-metallic Sur/aces, Proc. NBS Workshop, Gaithersburg, 9-12 December 1975 (1976).
AND PHOTOELECTROCHEMICAL SEMICONDUCTING MoS,
STUDIES
OF
S. M. AHMED Mineral Sciences Laboratories, Canada Centre for Minerals and Energy Technology, Department Mines and Resources, 555 Booth St., Ottawa, Ontario, Canada (Received
infinnl
form
13 October
of Energy,
1981)
A&tract-Redox reactions with CU(NH~)~~/‘~, Fe(CN)~-/4-, Fe’+“+ and IT/I2 were studied on 11-C surfaces of MoS, using rdes. From electrophoretic measurements, the zpc was found to occur at pH 2. The role of potential-determining ions and the effects of specific adsorption of ions on the electrode behaviour have been discussed. The change of potential with pH was - 30 mV/pH unit for MoS, electrode in contact with a solution saturated with molybdenum and S2- ions. . From the information available, an energy level diagram has been constructed. This diagram is in agreement with the observed behaviour that electron exchange for the first three redox couples occurs with the conduction band, except that the electron transfer from the reduced species, Fe(CN)z- and Fe’+, 1x1the reverse direction (oxidation) is partly and totally restricted, respectively. The charge transfer behaviour in the ease of I; /I, couple, aDDcarS to be much more complex, most probably involving adsorption of 1. on the surface.- _ __
ring, n-type MoS, were finely crushed using an agate mortar and pestle. The powder was leached with 2 M HCl and washed with triple distilled water until the washings were free of Cl-. A Zeta Meter (Zeta Meter, New York) was used to measure the electrophoretic mobility in the presence of 0.01 M KNO, as an indifferent electrolyte. The original Zeta Meter electrodes were replaced by palladium electrodes which were electrolytically charged with hydrogen to half of their capacity. The preparation and use of these electrodes have been described elsewhereC5, 6-J.Use of such partly charged palladium electrodes prevents gas evolution during electrophoretic measurements, and enables use of high current densities, provided the electrode polarity is reversed for equal durations of time during measurements. The equilibrium potentials of MoS, electrodes with respect to see were measured, in the absence of any added redox couple, as a function of pH ih 0.1 M K2S04 solutions buffered with borate, and recorded as a function of time on a strip chart recorder. Two Beckman Research pH/volt Meters were used, one for monitoring the pH and another to record the potential. For reasons to be clarified later, the experimental solutions were saturated For at least 24 h in nitrogen atmosphere with a precipitate of molybdenum sulphide (MoS,) prepared by passing H2S into a solution of ammonium molybdate (0.01 M) in HBr containing S “/, formic acid at 0°C. To prevent changes in the rest potential of the electrode due to air exposure or pretreatment, all pH-potential readings were taken in the same solution without removing the electrode, after only pH adjustment. The solubility of MoS, was also determined as a function of pH by measuring molybdenum dissolved in 0.01 M KC1 solutions that were saturated with MoS, powder for 24 h in nitrogen atmosphere. Atomic absorption was used for molybdenum analysis. A 150 W xenon lamp was used for illumination of the electrode under potentiostatic conditions.
INTRODUCTION In earlier work by Tributsch[i, 21 MO& has been reported to exhibit superior stability to photocorrosion in photoelectrochemical (PEC) cells and good efficiency in solar energy conversion. It was subsequently discovered[3,4] from the measurements of the exchange current densities for various redox reactions that the inertness of MO& was limited only to the basal plane (surfacesI-C axis) of the layered, hexagonal MoSz structure, whereas the surfaces parallel to the C-axis (11-C) were highly reactive. Also, in studies of redox reactions it was shown that both forward and reverse reactions occurred readily in the case of the Cu(NHI)z+/t+ couple. In the case of Fe(CN)z-‘4and Fe”+“+, while the electron transfer occurred readily in the forward direction (reduction), it was partly and totally blocked in the reverse direction, respectively. The present work reports further studies of the redox reactions on the 11-C surfaces of MO&, determinations of the zero point of charge (zpc), and the effect of pH on the reversible double layer at the MoS,-electrolyte interface. Specific effects of certain ions on the photocurrents of MoS, have also been investigated on both 11-C andl-C surfaces. From the information obtained, an energy level diagram has been constructed and the nature of charge transfer established.
EXPERIMENTAL Experimental details of electrode preparation and most electrochemical measurements have been described earlier[3]. Fat measurements of the electrophoretic mobility, samples of the naturally occur-
0
Minister of Supply and Services, Canada,
1981. 707
708
S. M.
RESULTS The current-voltage
behaviour
Results of rotating disk electrode (rde) studies For the Cu(NH3)<+/l*, Fe(CN)z-‘4P, Feat’*+ and I; /I* couples on the 11-Csurfaces of MoS, are shown in Figs l-3 and 4 respectively. Although the anodic side of the voltammogram for the Fe’+ oxidation is not shown in Fig. 3, there was no evidence for any
AHMED
detectable Fe2+ oxidation. As seen in Figs l-3 the diffusion limited currents (iL) are reached easily in both forward and reverse directions in the case of the copper ammonia couple (Fig. 1) and in the forward direction for the other two coupks (Fig. 2,3). A plot of iL usthe square root of the rotation speed yielded linear plots and the diffusion constants calculated from these plots using Levich’s equation, were found to be comparable with the known values, within the experimental error.
Fig. 1. Current-voltagecurves at differentrotation speedsfor the Cu(NH& +I+ ‘ couple 011MoS, surface 11-C; pH 11.O, electrolyte 0.5 M NH,CI plus excess ammonia;sweep rate 102 mV mitt- t ; concn.,cupric 1.O K lo-’
M cuprous 1.05 x lo-‘M.
Fig. 2. Current-voltage curves at different rotation speeds for Fe(CN):-‘*each, on MoSz surfams (1C; pH 7; electrolyte0.5 hl KCl; sweep
couple; concn 5 x 10m3M
rate 102mV mm-‘.
Surface and photoelectrochemical
MOSS,
OT
Fc2+/3+
studies of semiconducting MoS,
709
II-C (001
U)
im _-
0 0.2
04
0
-0.2
-a4
-0.6
-08
VOLTS (SCE)
Fig. 3. Current-voltage curvesat differentrotation speeds for Fe l+j3+ reduction (0.01 M), on MoS, surface (I-C; pH 2.1; electrolyte
0.5 M KCI; sweep rate 135 mV min-‘.
Results of rde studies for the anodic oxidation of iodide to I2 (E” = + 0.293 V see) in dark, on 11-C and 1-C surfaces of MoS, are shown in Fig. 4in the anodic range only. Electron transfer occurs readily in both directions although diffusion limited currents were not reached readily. On illumination a large shift in potential for the onset of photocurrents (i,,h) ( - 600 mV) in I- solutions followed by large photo-
242
7.26 N 'E o 9,68 E
I
110 I=
f
-- IPI 0
------‘Ka
0.075 (*
S04+0.0Q5M I
I
I
KI
/
,’
0
16.94 3 0.t
-
/’ / /
I-
c
Photo Currents Oork Currents
I
KC,
O,SM
2
KBr
0.1 M
3 Kl O.IM + 0.5 MK C,
0.6
I
I
1
0.4
0.2
0
VOLTS
t-5
(S.C.E.l
Fig. 5. Voltammogram showing dark and photocurrents in Cl- Br- and I- solutions on MoS,I-C surfaces (naturally occurring n-type, 11-Csurfacesalso exposed). 150 W Xe lamp was used, sweep rate 100 mV min-‘.
3-a
I
I
0.7
0.6
I 0.4
0.2
VOimTS (SE) Fig. 4. Current-voltagecur at differentrotation speedsof MO& in KI(0.005 M) plus K,SO, (0.5 M) solutions at PH- 7. Sweep rate 100 mV min- ‘. currents compared to Br- and Cl- (Fig. 5) are most noteworthy and have been observed by earlier workers also[2]. In the absence of any oxidizable ions such as I-, the holes generated from illumination have been shown to react as follows[2]. MoS, + BH,O + lXh+ + MO(W) + 250:-
+ 16H+ (1)
and partly as, MoS,+2H,O+4h+
shown plotted vs pH in Fig. 6. MoSz acquires a zero surface charge at pH 2 in an indifferent eIcctrolyte solution which also gets saturated with the dissolved lattice ions from the surface during the experiments. Hence the zero point of charge (or zpc) for MoS, occurs at pH 2. In solutions of pH > 2 the surface acquires a strong negative charge resulting in a negative zeta potential (Fig. 6) which reverses its sign to positive in the presence of 1O-3 MCu’*, Pb2’ or Cd’+ due to their specific adsorption on the surface. However, these cations and Fe3+ in lo-’ M concentrations have no effect on the surface charge in the pH range above the zpr, while below the zpc the zeta potential becomes positive. The behaviour is similar to that of galena having a pH,, = 2.4, pS&; = 19[7]. In ammonical solutions, the copper ammonia complex is so strongly
-+MoS,+0,+4H+.
The zero point of charge oJ.MoS,
(2)
and the pH-potential
WhiOflShip
potentials as calculated from the electrophoretie mobility of MoS, using standard equations are Zeta
MoS*
---
14-52
1
‘5 E ._
/
ill
adsorbed
on MoS2
that the surface
charge is seen to become zero, in Fig. 6A. In Fig. 7, the electrode potentials of MoS, (see) in an indifferent electrolyte solution, saturated with molybdenum and S2- ions are shown plotted us pH. This plot gives a slope of -30 mV/pH unit. Without presaturation of the electrolyte with molybdenum and S’-, as described previously, the equilibrium was hard
S. M.
710 Cuz+
P
A
1 _
AHMED
Pbz+8
Fe’*
Cl
CP’ lo-~u
-0
x c$+ l
lo-%4
* CttfNH&+
IO%
Na2S.
0 Pb**or l
Na2S
Cd2’
Pb*+ lO-3
Fe3+
10-M
10-3M M
Fig. 6. Diagram showing the effect of different ions on the clectrophoretic mobility of MoS, in 10m3M K, SO, solutions.
Fig. 7. Variation of MO& electrode potential (see) with PH. to obtain and the pH-potential plots showed inconsistent behaviour most probably due to an unstoichiometric dissolution of the surface. The amount of MO”+ ions dissolved in 0.01 M KC1 solution from MO&, as a function of final pH, under equilibrium conditions, is shown in Table 1. Table 1. Molybdenum ions dissolved in 0.01 M KCI solutions from MoS, as a function of final
pH, at equilibrium
MO dissolved,
PH
(pm01 1-l)
1.1 3.1 8.0 9.8 13.1
125 2 109 375 360
DISCUSSION The reversible double layer (MO%)
on a semiconductor
sulphide
When a metal sulpbide (or oxide) is brought into contact with an electrolyte solution, the initial reaction
is surface hydration followed by a rapid (_ 1 min) acid-base dissociation of the surface groups giving rise to surface charge ( +) and an ionic double layer at the interface. This is followed by a slow dissolution of the lattice ions from the surface and a pH-dependent ion hydrolysis in solution, which continues until equilibrium is established between the two phases. These two stages of equilibria have been well established for oxides[8]. As some of these ions could exchange electrons with the bulk semiconductor (see later), the Fermi levels in the two phases would be equalized. The primary potential-determining (p-d) ions are the constituents of the lattice itself, although their concentration in solution depends on pH as a result of ion hydrolysis and degree of dissociation. As a number of both cationic and anioic p-d ions are involved in the pH-potential equilibria of sulphides and oxides, mixed potentials are normally set up in most cases. The overall rates of electron exchange at these equilibrium processes are given by the exchange currents (i” =T=T) which can be determined experimentally[3]. In most cases the exchange currents and hence the electron transfer rates are low and the equilibria are also slow to be established. A dynamic equilibrium can be assumed in some cases where the change of electrode potential and pH with time is small and linear. At a characteristic pH or pS’-, known as the zero point of charge (zpc), the concentration of the cationic and anionic p-d ions remain equal and the surface groups undissociated, when the surface charge and hence the field strength (d4/dx) will be zero. The solubility of the material is also known to be minimum at the pHfpc, as seen in Table 1. The zpc of the reversible, Ionic double layer is the counterpart of the flat band potential associated with the space charge region. In principle, the surfacedissolution/adsorption processes on MO& can give rise to hydrated molybdenum ions in several oxidation states, the surface itself being negatively or positively charged depending on the surface hydrolysis and the degree of acid/base dissociation of the surface groups. 2(MoS&+ 2(MoS&
(MO&),+
Mo(IV) + 2S2-
(3)
+ (MoS,);~-
+ Ma(VI) + 2Sz-
(4)
Surface and photoelectrochemical studies of semiconducting MoS,
2(MoS&
= (MO&);
+ Mo(III) + 2s’ -, etc., (5)
S*- -tH+ =+HSMO”+ + 6H20 = Mo(H,O)“,+ .
(6) (7)
In our preliminary investigations of MoS, electrodes by cyclic voltammetry with MO(W) added to the test solutions (as bromide; ammonium molybdate dissolved in HBr), well defined redox peaks in the range of - 350 to - 750 mV see [Mo(VI)=MO(IV)] were obtained at PI-I - 1.7. These peaks disappear when the same solution of Mo(V1) is reduced by Zn to MO(W) before the experiment, when new peaks appear in the - 900 mV see range. The E”,z,s.sbeing = - 0.72 V see, Eredol values of the p-d ions lie in the conduction band range of MoS, (Fig. 8). This behaviour of electron exchange between the semiconductor and the p-d ions is still under investigation at present. There is evidence that some of the p-d ions can alter the Helmholtz layer potential in the adsorbed state by virtue of their excess surface charge without any electron exchange with the bulk. For example, addition of Ti!+ ion to a cell with a semiconductor TiO, electrode in acid medium, changes the equitibrium electrode potential strongly to more negative values, changes the flat band potential and also reduces the open circuit photovoltages measured against a see. However, Ti3+ (E”, Ti3+‘*+ = 0.06 V nhe) under the same conditions was Found to have no effect whatsover on the i-V behaviour of TiO, as found from rde studies. The change of Galvani potential in this case by adding Ti3+ was due to excess Ti-Ogroups introduced on the surface. Depending on the chemistry of metal hydroxy complexes, the effects of cationic p-d ions on the Fermi levels, Galvani potentials and possibly on band bending could be practically eliminated at high pH as these are precipitated out and only anions such as S2- for sulphides and 02- or OH- for oxides will be potential determining. Hence, the equilibrium potential (E,) for MoS, in the absence of any added redox couple, may be written as. RT
E,=En--Inasl. 2F in the pH range > pH,,
where the effect of cations can
711
be neglected. Hence the variation of the electrode potential @,with usI- and pH ( - 30 mV/pH unit, Fig. 7) is given as: RT d@, = --dlno,,., (9) 2F d4& = O.O295dlna,+
= -O.O295d(pH).
HS-
= I..I+ +Sf-; a.& =
K,
K, = aH”as~a HS-
. uHSaH*
--.
K
27:s
-
E
(11) (12)
*H+
Hence, A@,
= A&
= - 0.0295 (pH, - pH,,);
(13)
assuming that the entire change in the electrode potential 4w occurs in the Helmholtz region, A#M = A~!I”; pH, being the pH at equilibrium. As seen earlier part of this potential drop could be manifested as change in the Fermi level. A detailed study of the role of the primary p-d ions, in this respect is needed. The double layer effect on the photoelectrochemical behauiour The double layer on sulphides is similar to that on oxides as reviewed earlier[9] except that sulphides being less ionic than oxides, the surface charge densities in the former case are one order of magnitude lowerC9, lo] than in the latter ( - 60-70 pC cm- * for 3ODmV for oxides). Counter ions which can NH form insoluble sulphides can get easily adsorbed on the negatively charged surfaces of MO& as described in the previous section. The zpc can be shifted by severai hundred mV by specific adsorption of suitable inorganic and organic reagents, in the inner Helmholtz layer. The observed shift in the potential For the onset of photocurrents on MO& in the presence of I-, as shown in Fig. 5, is most probably due to the specific adsorption of I- which would shift the flat band potential.
Fig. 8. Energy level diagram of MoS, showing schematically positions of the Ehcx levels relative to the conduction band (E,) and valence band (E,) edges at different pH values. For details see text. El\
(10)
This is because the concentration of HS- remains practically constant up to pH 11 and as1- is inversely proportional to aHl, as shown below;
712
S. M. AHMED
__
Charge transfer reactions
on n-type
MO&
From the measured values of the zpc and the flat band potentials ( j’bp) obtained by the capacitative phase shift method[2,7], the energy level diagrams at DH 2.0.7.0 and I 1.5 have been derived. as shown in Fig. ‘s, afte; allowing - 29.5 mV/pH for ;he change in t& Helmholtz layer potential (Ad” = 0 at pH 2, the zpc) according to equation (1 l), U,,,(sce,
where U,,,(sce} located and &is
= (4,-t-
A&,)
- 4.74,
(14)
is the level at which the jbp(sce) is
the work function of the semiconductor. The E” (see) values of the redox couples and the conduction band edge, E,at the flat band condition, at pH 2.0, 7.0 and 11.0 at which the redox couples were investigated, are also shown in Table 2. The band gadis taken as 1.1 eV based on recent investigations by Kautek et a/.[41 and an average separation of 0.2 eV between the Fermi level and E, for the naturally occurring n-type MoSz has been used, the fbp being 0.3 V (see) for the 11-Csurface[7] and 0.5 V (see) for the I-C MoS,[2, 71. The data shown in Fig. 8 and Table 2 are for ) 1-Csurfaces, for which the redox reactions are presented in Figs i--3; the position of E,is 0.2 eV lower for theI-C surfaces in the energy level diagram. The reorganizational energy (A), used in Fig. 8 for Fe(CN)z“- is taken from published work[12] while for other couples it is schematic. The E” value for the copper couple depends on NH3 concentration, or pH[lS]. Hence E’at pH lOand 11Saregiven inTable 2 and the latter value is used in Fig. 9. The anodic decomposition of MoS, occurs at - a7 V (WC). Table 2 Redox couple Fe2+l,+ Fe(CN).?-‘*Cu(NH,):+“+ Cut NHa),5+/1+ s - /‘SD 1-/t,
PH
E” (SW) (V)
E, (We) (V)
!p”v,
2.0 7.0 10.0 11.5 14.0 7.0
0.53 0.12 - 0.0 - 0.224 - 0.76 0.293
0.1 - 0.05 -0.14 -0.18
> 0.74 0.74 -
- 0.05
The E, edge (Table 2, Fig. 8) is located slightly below well above (
forward direction (cathodic) for all the three couples, while electron transfer in the reverse direction (anodic) from the reduced species to the conduction band should be moderate for Fe(CN)z and poor or nil for Fe*+. This is in agreement with the experimental work as seen from Figs l-3. Specific adsorption of Cu(NH& + on MoS, (Fig. 6) could also contribute to the reduction of the energy barrier for electron transfer
and metal-like behaviour of the copper ammonia couple. The slow electron injection from Fe(CN)zinto the conduction band, may also be promoted through other mechanisms such as thermal activation of the energy levels in solution, charge pick-up by
surface states and the possible variation of the band edge position at different voltages or variations in concentration of the p-d ions as discussed earlier. The Fe2+ (uncomplexed) level appears to be too far below ( c 0.45 eV) the E,edge for Fe2+ oxidation to occur in preference to surface oxidation and/or anodic dissolution of MoS, (Edsc = 0.7 V see). Electron injection into the valence band is also not possible as E, edge lies well below the Ed=. The charge transfer from I; /I2 appears to involve a complex mechanism and is strongly influenced by specific adsorption of I- on the surface. In the preparation of synthetic MO&, iodine may be used for vapour transport to promote growth of single or larger crystals. The foregoing studies have shown that the inclusion of residual iodine, as molybdenum iodide, would result in an increase in dark currents, particularly when 11-C surfaces are produced in large proportions. This would consequently decrease the overall energy conversion efficiency of the electrode. This difficulty was actually experienced in our attempt to sinter pressed MoS, pellets in the presence of iodine. However, by taking advantage of the higher reactivity of 11-C surfaces compared to that ofl-C surfaces, if the former can be rendered inactive by selective chemisorption of suitable reagents, then it might be possible to reduce the dark currents and increase the efficiency.
Attempts in this direction are being made. Alternatively, bromine can be used in the vapour transport and sintering processes. However, if one wants to take advantage of the high photocurrents in the presence of I-, the dark currents due to 11-C surfaces will have to be minimized by suitable means. Acknowledgements-Part oftheexperimental work described herein was carried out at the Fritz-Haber-Institut, MaxPlanck-Gesellschaft, Berlin. The author is thankful to Professor Gerischer for providing the research facilities and for critical comments and to Dr. A. H. Webster, Head, Physical Chemistrv Section, CANMET, Ottawa for helpful dis&ssions.
REFERENCES 1. H. Tributsch and J. C. Bennett, J. elecrroanal. Chem 81, 97 (1977). 2. H. Tributsch, J. electrochem. Sot. 125, 1086 (1978). 3. S. M. Ahmed and H. Gerischer, Electrochim. Acta 24,705 (1979). 4. W. Kautek, H. Gerischer and H. Tributsch, Ber. BlmsengesPhys. Chem. 83, loo0 (1979). 5. R. Neihof, J. Cvlloid Inferface Sci. 30, 128 (1969). 6. S. M. Ahmed, Mines Branch Technical Bulletin TB 140, Department of Energy, Mines and Resources, Ottawa, Canada 1971. 7. S. M. Ahmcd, unpublished work. 8. S. M. Ahmed and D. Maksimov, Can. J. Chem. 46,384l (L968).
9. S. M. Ahmed, in Anodic Behaviour of Metals and Semiconductors Series, Vol. 1, Ch. 4, Oxides and Oxide Films. (Edited by J. W. Diggle) Marcel Dekker, New -York (i972). _ 10. S. M. Ahmed, J. phys. Chem. Irhaca 73, 3546 (1965). 11. A. J. Nozik, Proc. 2nd World Hydrogen Energy Conference, Zurich, 21-24 August, p. 1217 (1978). 12. R. A. L. Vanden Berghe, F. Cardon and W. P. Games, Surf. Sci. 39, 368 (1973). 13. J. J. Lingane, Chem. Rev. 29, 1 (1941). Publication N.B.S. Special 455 14. H. Gerischer, Eleclrocotnlysis on Non-metallic Sur/aces, Proc. NBS Workshop, Gaithersburg, 9-12 December 1975 (1976).