Langmuir 1995,11, 1375-1380
1375
Silver Photoelectrodeposition at p-MoSea Raul J. Castro and Carlos R. Cabrera" Department of Chemistry a n d Materials Research Center, P.0. Box 23346, Rio Piedras Campus, University of Puerto Rico, S a n Juan, Puerto Rico 00931-3346 Received January 25, 1994. I n Final Form: January 3, 1995@ Silver has been photoelectrodeposited at p-MoSez surfaces. These surfaces were analyzed by electron spectroscopyfor chemical analysis (ESCA),auger electronspectroscopy(AES),scanning electronmicroscopy (SEM), and photovoltammetric methods. AES depth profiles and ESCA spectra show that MoSez reacts with silver by a displacement reaction that produces AgzSe and Moos. The dissolution of the Moo3 is evident from the AES depth profiles. Photovoltammetric studies of p-MoSez with different amounts of electrodepositedsilver were done in 5 mM&No3 and 0.25 M I(no3aqueous solutions. A continuous shifi in the onset photopotential for the reduction of Agf, with a decrease in the photovoltage, was observed concomitant with an increase in the photoelectrodeposited silver. This photoelectrochemical change is mainly due to the surface reaction of silver atoms with MoSez,which creates the semimetallic AgzSe phase, thus increasing the conductivity of the substrate.
Introduction The interactions of semiconductor surfaces with transition metals have been the subject of intense studies, mainly under ultrahigh vacuum (UHV).' In most of these studies, metal atoms are deposited onto semiconductor surfaces by vacuum deposition method^.^-^ Recently, interest has been focused on the photoassisted electrodeposition of metals at semiconductor surfaces with the idea of creating Schottky or p-n junctions and metal clusters on semiconductors by photoelectrochemical methods. The electrochemical deposition of transition metals on semiconductors have been studied by Memming2 and other^.^ Metal atoms deposited on semiconductor surfaces may react in different ways due to the chemical and physical properties of the semiconductor and the metal atoms being deposited. In some instances, a metal overlayer may be produced (no interaction) or diffusion and reaction with the substrate, to form a new phase, may occur. For example, Cu diffuses and reacts with the surface of SnSz to form a new phase of stoichiometry C u s n S ~ .On ~ the other hand, no reaction occurs when Cu is deposited on
WS2.5 Under UHV, no interfacial reaction has been observed between basal planes of transition metal dichalcogenides of group VI and noble metals. In these cases, an atomically abrupt interface is f ~ r m e d . The ~ , ~electrolytic deposition of noble metals onto semiconductor surfaces of group VI is not commonly studied since the same behavior as in UHV metal vapor deposition is expected. However, Uosaki and co-workers found a different electrochemical behavior for CdSe electrodes by both methods: electrolytic and metal vapor deposition.' They found that CdSe electrodes modified by vacuum evaporation of Ag behave as semiAbstract published inAdvanceACSAbstracts,March 15,1995. (1)Jaegermann, W.; Tributsch, H. Prog. Surf. Sci. 1988,29,1. (2)Reineke, R.;Memming, R. Surf. Sci. 1987,192,66. (3) (a) Allongue, P. In Physics and Applications of Semiconductor Electrodes Covered with Metal Clusters;Conway, B. E.,Bockris, J. O'M., White, R. E., Eds.; Modern Aspects ofElectrochemistry; Plenum Press: New York, 1992;Vol. 23,Chapter 4. (b)Schlesinger,R.; Rogaschewski, S.; Janietz, P. J. Phys. Status Solidi A, 1990,120,687. (4)(a) Ohuchi, F. S.; Jaegermann, W.; Parkinson, B.A. Surf. Sci. 1988,194,L69; (b) Ohuchi, F. S.; Jaegermann, W.; Parkinson, B.A. Ber. Bunsenges. Phys. Chem. 1989,93,29. (5)Jaegermann, W.; Ohuchi, F. S.; Parkinson, B. A.Surf. Sci. 1988, 201, 211. (6)Bortz, M. L.; Ohuchi, F. S.; Parkinson, B.A.Surf. Sci. 1989,223, 285. (7)Uosaki, K ;Yoneda, R.; Kita, H. J . Electroanal. Chem. 1990,283, 167. @
0743-746319512411-1375$09.0010
conductors. On the other hand, the CdSe electrodes treated in electrolytic solution of AgN03 have ohmic characteristics. Due to the layered structure ofMoSez,strong anisotropic properties are observed. The physical and chemical behaviors are characterized by its two dimensionality. The surface reactivity and photoelectrochemical behavior of these materials are dependent on the surface quality, as well. For example, it has been reported that the solar energy conversion efficiency and photointercalation processes are dependent on the surface morphology.8 In previous studies we have found that copper interacts preferentially with rough surfaces, Le., exposed edges, of p - M o S e ~ .We ~ now report on the surface reaction of photoelectrodeposited silver on p-MoSez. This surface reaction has been analyzed by techniques such as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES).In addition, photovoltammetric methods were used to study the interaction between silver atoms and the semiconductor surface of p-MoSez. Our results indicate that photoelectrodeposited silver atoms react with p-MoSez, leading to the formation of AgzSe and MoO3.
Experimental Section Singlecrystals of p-MoSe2,grownby a chemicalvapor transport technique, were used. Details on the synthesis and characterThe ization of these crystals have been reported previo~sly.~ electrodes were prepared by making an ohmic contact with graphite paint (SPI supplies) between a copper wire and the semiconductorcrystal. We used graphite paint instead of silver paint to avoid the presence of silver in our samples since it might interfere with the ESCA and AES analyses. Although it is known that silver paint makes a good ohmic contact between p-MoSez and a copper wire,"J we found that graphite conductive paint makes an adequate ohmic contact, as well. Torr-Seal epoxy (Varian)was used to cover the contact area and the edges of the electrode while a surface area in the range of 0.01-0.12 cm2 was left uncovered. The copper wire was placed inside a 6 mm diameter Pyrex tube and sealed with Torr-Seal epoxy for mechanical stability. All experiments were done with freshly made electrodes. The MoSez crystals were not cleaved or etched before being used. The electrochemical apparatus consisted of a PrincetonApplied Research (PAR)173 galvanostatlpotentiostat with a digital (8)Kautek, W.;Gerischer, H. Electrochim. Acta 1982,27,1035. (9) Castro, R.J.;Cabrera, C. R. J . Electrochem. SOC.1992,139,3385. (10) Etman, M. J . Electrochem. SOC.1988,135,1115.
0 1995 American Chemical Society
1376 Langmuir, Vol. 11,No. 4,1995
Castro and Cabrera
coulometer Model 179, a PAR 175 programmer, and an X-Y recorder (SoltecModel VP-6423s). For the photoelectrochemical experiments a 250 W quartzhalogen lamp (Oriel Model 66181) with a water filter (Oriel Model 6123)was used as a light source. A three-compartment electrochemical cell with a flat optical window was used in all experiments. A large area gauze was used as the counter electrode. All potentials are referenced with respect to the saturated calomel electrode (SCE). All cyclic voltammetry (CV)experiments were done at a scan rate of 20 mVIs. Electrolyte solutions were prepared by using reagent grade chemicals. The water used in our experiments was previously distilled and pumped through a Nanopure system (Barnstead), giving18 MQ cm water. All electrochemicalmeasurements were done at room temperature. Theelectrolyte solutions were purged with nitrogen for at least 15 min prior to electrochemical experiments. AES measurements were done with a Physical Electronic Auger microprobe(PHI Model 600) at a current of 81 ,uA and a voltage of 10 kV. An Ar+ ion gun, at a current of 25 mA, was used in AES depth profile analyses. The sputtering yield of the Ar+ ion gun was measured by analyzing a silicon wafer with an oxide layer of 1000 A. The calculated sputtering rate was 96 kmin. We used this value although the sputtering yield for MoSe2 should be different. However, we always use relative values instead of absolute values in order to compare our results. ESCA measurementswere donewith a Perkin-Elmer (PHIModel 560)instrument which had a Mg K a X-ray radiation source. For high-energy resolution studies a pass energy of 50 eV was used. The spectrometer was calibrated to the Au 4f7n (EB= 83.8 eV) energy level. Scanning electron micrographs (SEM)were done with a JEOL JSMJ300FV at 3 keV electron gun voltage. The SEMs in this article (Figure 1) have a magnification of 3000x and the bar on the pictures has a dimension of 10 pm.
I ’?
Results and Discussion Photovoltammetry. The p-MoSe2 electrodes used for silver photoelectrodeposition had different degrees of imperfection. The presence of surface imperfections was observed in the cyclic voltammograms of p-MoSep in 0.25 M KN03 solution, with and without illumination (see Figure 2A). Cyclic voltammograms scanned from +1.0 to -0.2 V vs SCE showed no cathodic peak current with or without illumination; however, a n anodic current was obtained a t potentials more positive than +0.7 V vs SCE. This is due to the oxidation of the semiconductor surfaces, which depends on the presence of surface imperfections or defects. Cyclic voltammetric measurements were done with a p-Moses electrode in a solution containing 5 mM AgNO3 and 0.25 M KN03. A cathodic wave, which corresponds to the reduction of Ag+, was observed under illumination (see Figure 2B). The shape of this cathodic wave shows either steady state (i.e., as in radial diffusion)or diffusion limited behavior (i.e., as in linear diffusion), depending on the electrode being used. The cathodic peak potential (Ep,&for electrodes showing diffusion limited behavior, was a t +0.30 V vs SCE. The Eonset for the photoelectrochemical reduction ofAg+varied between +0.49 and +0.60 V vs SCE. This range of Eonset values as well as the differencein cathodicwave shapes obtained with different electrodes is due to the density of imperfections of the electrode surfaces. The value of +0.60 V vs SCE, for the EoWt, compares favorablywith the&, of p-MoSe2 reported previously for a single ~ r y s t a l .The ~ small difference between the flat band potential and the onset potential may be due to a negative potential shift of the Em during i1l~mination.l~-l3 The maximum photovoltage, which we (11)Etman, M. J. Phys. Chem. 1986,90,1844. (12)Meisser, D.;Lavermann, I.; Memming,R.; Kastening, B. J. Phys. Chem. 1988,92,3484. (13) Sinn,C.; Meisser, D.; Memming, R. J. Electrochem. Soc. 1990, 137, 168.
Figure 1. Scanning electron micrographs of (A)a pure sample of p-MoSe2where imperfections are seen and (B)p-MoSepafter silver electrodeposition under illumination (Q= 0.84 mC, t = 90 s).
define as the difference between the E,, at a Pt electrode and the Eonset of the semiconductor under illumination, was 340 mV. Without illumination, the cyclic voltammogram in Figure 2B shows a cathodic peak for the reduction ofAg+at a more negative potential. The current of this peak was smaller than the one observed under illumination. The Eonwt for the reduction of Ag+, without illumination, was at +0.22 V vs SCE. A difference of 380 mV in Eonwt between the voltammograms done with and without illumination is obtained. After consecutive cyclic voltammograms under illumination between +1.0 and -0.2 V vs SCE, the Eonset for the reduction of Ag+ shifted toward a more negative potential. The Eonset values began at +0.55 V and ended a t +0.47 V vs SCE after 15consecutive cycles. Following these consecutive cycles, a cyclic voltammetric measurement was done without illumination and in the same solution. A dramatic change was observed in the voltammetric behavior of p-MoSe2 (see Figure 2C). Without illumination, a well-defined cathodic peak was observed for the reduction of Ag+. I t resembled the peak observed under illumination but shifted toward a more negative potential. Also, the Eonwt under illumination shifted toward a less positive potential compared to the first cyclic voltammogram done under illumination. This shift inEOwtcaused a decrease in photovoltage. The initial photovoltage was 316 mV and after consecutive cyclic voltammograms it decreased to 252 mV. The cathodic peak current for the reduction ofAg+without illumination
Langmuir, Vol. 11, No. 4, 1995 1377
Silver Photoelectrodeposition a t p-MoSez I
A
I
1
1
I
600.0
400.0
200.0
0
b
I 10
uA
B a
1000.0
800.0
Binding Energy (eV)
Figure 3. ESCA survey spectrum (Mg Ka)for a pure sample of p-MoSe2.
b
C
a
I 10
I +1.Z
1
I
I
+0.7
c0.2
-0.3
E
(
UA
V vs SCE)
Figure 2. (A) Cyclic voltammogramsof the electrolyte solution (0.25 M mo3)at a p-MoSez electrode (b)with and (a)without illumination. (B)Same conditions as in part A but in &No3 (5 mM)/KN03 (0.25M) solution. (C) Same as part B, but at p-MoSe2 after silver photoelectrodeposition. The scan rate was 20 mV/s. also increased, but it was still less than the one observed under illumination. This cathodic peak, without illumination, was stable and reproducible after continuous cyclic voltammograms in AgN03 solutions. These results show that silver, once at the surface, interacts irreversibly with p-MoSez. This is reflected by the fact that an anodic stripping of Ag was not observed with or without illumination. In addition, after several voltammetric cycles under illumination in a 5 mM AgNO3 and 0.25 M KN03solution the p-MoSez electrode behaved as a semimetallic electrode. Therefore, the electronic properties of p-MoSez changed concomitant with a n increase in the amount of silver irreversibly photoelectrodeposited on the surface of the electrode. Moreover, this is seen in the decrease of the photovoltage and in the increase of the cathodic peak current when the cyclic voltammogram was done without illumination. This electrochemical change may be explained by a surface reaction of Ag with MoSez. This reaction may yield the formation ofAgzSe, which is a semimetallic material. Since this reaction may increase the density of imperfections,
in addition to the formation of AgzSe, the voltammetric behavior of p-MoSez should resemble that of a metal. However, although there are changes in the electronic properties, photovoltage and photocurrent are still seen under illumination. This rules out the formation of a metallic overlayer because the surface is still having semiconducting properties. SEM of silver photoelectrodeposited surfaces (bulk deposition), shown in Figure lB, shows that the electrodeposition occurred mainly at the basal planes of MoSez. ESCA Analyses. Surface analyses with ESCA and AES were done to MoSe2 surfaces with and without photoelectrodeposited silver. These analyses gave us a n understanding of the surface reaction occurring a t MoSez after silver photoelectrodeposition. The ESCA spectrum of p-MoSez in which the electrode potential was continuously cycled between +1.0 and -0.2 V vs SCE in a 0.25 M KN03 solution is shown in Figure 3. The spectrum shows, beside the binding energy peaks of Mo and Se, carbon and oxygen contamination. The latter elements are introduced by the epoxy resin and by sample handling, i.e., exposure to air. Jaegermann showed that these contaminants are easily removed from the surface by etching with Ar' ion sputtering and that they do not change the electronic properties of the semicond~ctor.'~ The binding energy values obtained for Mo 3d512 and 3d312 energy levels were 228.5 and 231.6 eV, respectively. The energy difference between these two ESCA peaks was 3.1 eV. On the other hand, the binding energy for the Se 3d energy level was 54.1 eV. These Mo and Se binding energy values match, very closely, those obtained for a pure sample of MoSez that has not been exposed to electrochemical treatment.14-16 ESCA analyses were done to a p-MoSe2 surface with photoelectrodeposited silver at a n applied potential of -0.200 V vs SCE for 160 s in a 5 mM AgN03 and 0.25 M KNOBsolution. As mentioned before, the ESCA spectrum of p-MoSez, without photoelectrodeposited silver, had two binding energy peaks for Mo. This corresponds to an oxidation state of +4, as expected (see Figure 4A).15 On the other hand, after silver photoelectrodeposition, the ESCA spectrum showed three peaks for Mo, as shown in Figure 4B. The binding energy peak a t 228.2 eV is due to Mo 3d512 in a n oxidation state of +4. The other two (14) Jaegemann, W.; Schmeisser, D. Surf. Sci. 1986, 165, 143. (15)Grim, S.0.; Matienzo, L. J. Inog. Chem. 1975, 14, 1014. (16) Moulder, J. F.; Stickle, W. F.;Sobol, P. E.; Bomben, K. D. In Handbook ofX-Ray Photoelectron Spectroscopy;Chastain,J., Ed.;Perkin
Elmer Corporation,Physical Electronics Division: Eden F'raire, MN, 1992.
Castro and Cabrera
1378 Langmuir, Vol. 11, No. 4, 1995 I
I
n
A
I
I
Mo 3d
li
I
I
I
I
I
240.0
236.0
232.0
228.0
224.0
Binding Energy 1
I
I
372.0
376.0
I
I
I
368.0
364.0
360.0
Binding Energy
(e\') I
I
3
2
I
Mo 3d
B
(e\!)
Figure 5. ESCA spectrum of the Ag (3d512, 3 d d doublet of silver at the surface of p-MoSez. Table 1. ESCA Binding Energy Values (eV) for the Main Peaks of p-MoSez and p-MoSez(&) samples Mo 3dm Mo 3 d 3 ~ Ag 3dm Ag 3 d 3 ~ Se 3d
I
A. p-MoSez(Ag) 1 2 3 B. p-MoSez 1 2 3
232.0 231.9 232.2
235.3 235.1 235.3
228.5 228.4 228.6
231.6 231.6 231.6
232.0 228.5 228.3
235.2 231.6 231.4
367.6 367.6 367.5
373.6 373.6 373.5
54.1 54.0 54.1 54.2 54.2 54.2
C. average
p-MoSez (Ag) 3
1
I
I
I
236.0
232.0
228.0
224.0
'
p-MoSez D. literaturea a
Binding Energy (eV) Figure 4. ESCA spectra of the Mo region (Mo 3d~z, Mo 3d3d (A) before and (B) after silver photoelectrodeposition. peaks appeared a t 232.0 and 235.2 eV. The peak at 232.0 eV may correspond to Mo4+. However, the peak a t 235.2 eV is due to a shift toward a higher binding energy of the Mo 3d levels. This binding energy value for Mo 3d is consistent with a n oxidation state of +6. Therefore, the peak that appeared a t 232.0 evincludes 3d electrons from Mo4+and Mo6+. Consequently, the peak intensity ratio between the binding energy peak a t 232.0 and 228.2 eV increases after silver photoelectrodeposition. This change in peak ratio suggests a decrease in the amount of Mo4+ present on the surface. The formation of Mo oxide in aqueous solution, for semiconductors of the type MoX2 (X = S, Se, or Te), has been previously reported by Jaegermann.14 Therefore, these ESCAresults show the presence of MoSe2 and Moos a t silver photoelectrodeposited pMoSez. The presence of a potassium salt of M004~-was considered as a possible Mo oxide compound on the surface; however, potassium was not present in the ESCA spectrum. This may be due to the high solubility of K2Mo04 in water. The existence of two oxidation states for Mo in the Agl MoSe2 samples may be explained by the deposition mode of Ag. The SEM of the Ag/MoSe2 surface shown in Figure 1B indicates that the deposition ofAgforms small islands randomly spread over the surface. This means that a full coverage of Ag was not obtained. Since the size of the X-ray beam of the ESCA instrument is roughly 1mm, the Mo 3d binding energy peak seen in Figure 4B will include photoelectrons from reacted and unreacted Mo. The ESCA spectra for Se 3d before and after silver
367.6
373.6
368.3b
374.3b
54.1 54.2 54.6
Values reported in the literature. b Values for Ag".
electrodeposition are quite similar. The binding energies were 54.1 and 54.2 eV for the samples with and without silver, respectively. This very small change in the binding energy reflects that there was no major change in the oxidation state of Se. This agrees with our proposition that we are forming a semimetallic AgZSe phase. Silver binding energies were measured for the Ag photoelectrodeposited p-MoSez surfaces, a s well. The ESCA spectrum for the Ag 3d level is shown in Figure 5. The binding energies of Ag 3d512 and 3d312 are a t 367.5 and 373.6 eV, respectively. These values are shifted to lower binding energies when compared to the binding energies of silver metal that are a t 368.3 and 374.3 eV (see Table 1). It is possible that this shift in the silver signal is due to band bending. This band bending can arise from the difference between the metal and semiconductor work functions. However, the band-bending model does not explain our XPS results for Mo and Se. The peak position ofAg 3d5/2and 3d312 energy levels suggests that silver may exist as Ag2Se or Ag20, a t the surface of p-MoSe2, since the binding energy values match with those reported in the literature for these silver compounds (see Table 1).16J7 However, as we will see below, AES depth profile analyses of the silver photoelectrodeposited MoSe2 show the presence of AgpSe instead of Ag20. A E S Analyses. AES and depth profile analyses with Ar+ sputtering were carried out on the electrochemically treated and silver photoelectrodeposited p-MoSez electrodes. The AES spectrum, shown in Figure 6A, is for a pure p-MoSez sample. The spectrum shows the presence (17) Leung, L. K.;Komplin, N. J.;Ellis, A. B.; Tabatabaie, N. J . Phys. Chem. 1991, 95, 5918.
Langmuir, Vol. 11, No.4,1995 1379
Silver Photoelectrodeposition a t p-MoSez
I
IA
n
j
12.5
0
v
x 10.0 ._ 4.-
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400
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250C Ciepth d i s t c v c e (4'1
1200
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Kinetic Energy, eV
9.0
1
IB
75 60
1 1
L5
3.0
0
2
Kinetic Energy. eV Figure 6. A E S spectra of p-MoSez (A) before and (B)after silver photoelectrodeposition (Q= 0.42mC, t = 10 s) at -0.25 V vs SCE.
of Mo and Se and of C and 0. Figure 6B shows the AES spectrum of a silver photoelectrodepositedf l m of p-MoSez. After silver photoelectrodeposition, the AES signal at 351 eV (kinetic energy) confirms the presence of Ag a t the surface of p-MoSez. A typical AES depth profile for a pure sample, which was electrochemically treated in KNO3, is shown in Figure 7A. The AES depth profile reveals a monotonic increase of Mo and a decrease of Se concentration until both Auger signals stabilize. As shown in Figure 7B, after silver photoelectrodeposition, the Se and Mo distribution change due to the presence of silver. After a depth profile of 2500 the Auger signals for Mo, Se, and Ag were still present. It is important to notice that the decrease in the silver signal is not steplike as expected for a n inert interface. On the other hand, the C and 0 Auger signals disappear from the spectrum after sputtering about 500 A of the surface with h+. Therefore, this rules out any possibility of silver oxide formation in the bulk of the semiconductor. However, at the surface of the semiconductor we might have a mixture of AgzO and AgzSe. It is known that the continued exposure of Ag, a t high temperatures, to the atmosphere may create a n oxide layer. This has been confirmed by STM analyses.18 Figure 8A,B shows AES depth profiles of the Se and Ag signals with respect to the Mo signal, a t different silver photoelectrodeposition times. The electrodeposition time
A,
(18)Obretenov, W.; Hopfener, M.; Lorenz, W. J.; Budevski, E,; Staikov, G.; Siegenthaler, H.; Surf. Sei. 1992,271, 191.
-
li;
1.5 Y
Mo
'A
001
C
I
C I -,=
6Gil '20C 180C 2439 Depth d i s t a r c e ( A )
e
3CC0
Figure 7. AES depth profiles for p-MoSez (A) before and (B) aRer silver photoelectrodeposition (Q = 0.90 mC, t = 90 s) at -0.25 V vs SCE.
ranged from 0 to 160 s a t an applied potential of -0.200 V vs SCE under illumination. The solution used in this experiment was 5 mM AgNO3 and 0.25 M KN03. Evidently, from the depth profiles of Figure 8A, the Se to Mo Auger peak ratio increases concomitant with a n increase in the amount of silver photoelectrodeposited a t p-MoSez. On the other hand, as the amount of photoelectrodeposited silver increases, the ratio of Ag to Mo AES peaks increases (see Figure 8B). From these data we have concluded that we are having a Se enriched surface layer and the AgMoSez reaction is occurring at the bulk of the semiconductor. This means that silver is being intercalated concomitant with the surface reaction. From the AES depth profiles, the silver signal is seen over 2000 A in depth. This Ag and Se enrichment is due to the formation of a AgZSe layer a t the surface as suggested by our ESCA data that was discussed previously. In addition, we believe that Mo dissolves from the surface as M004~-or Moos since the Se to Mo AES peak-to-peak ratio increased concomitant with the amount of silver photoelectrodeposited (see Figure 8A). The amount of Mo decreased while that of Se was kept constant since the AgzSe compound is insoluble in water.
Conclusions Surface analyses and photoelectrochemistry of the silver photoelectrodeposited p-MoSez electrodes show that silver reacts irreversibly with the semiconductor surface. The photoelectrochemical behavior of p-MoSez shows that p-MoSezchanges from a semiconductingto a semimetallic surface as the amount of photoelectrodeposited silver increases. ESCA analyses of the silver-modified surface
Castro and Cabrera
1380 Langmuir, Vol. 11, No. 4, 1995
AES analyses, Ag&e is the only silver-containing species present at p-MoSe2 after the first 100 A. The possibility of silver oxide formation was ruled out from the AES depth profiles. These data can be explained by the following displacement reaction mechanism: Ag' MoSe, a
n
!
Ago
\
MoSe,
\ i
MoSe;-
+ hv
+ eZ=
+ hf t Ag'
Z= Ago
MoSe,
+ h+ + e-
(at the interface)
+ 2e- t MoSe,'-
(at t h e interface)
+ 4Ag' + 3H,O 2Ag,Se
+ MOO, + 6H' + 4e-
The process begins with the reduction of Ag+ on MoSe2 surfaces. This step explains the cathodic current observed in the voltammograms. If a photon is absorbed at the chalcogenide interface, a hole-electron pair is produced. Holes can be captured by Agoproducing Ag+ a t defects or atomic imperfections. Silver ion diffuses to the bulk, and in the presence of water, Ag2Se is formed and Mo4+ is oxidized to MoOa. The net result is the formation of AgzSe and Moo3. The overall reaction would be MoSe,
+ 4Agf + 3H,O 2Ag,Se
+ MOO, + 4H' + H,
This proposed mechanism is consistent with our experimental data. We are currently studying this system by STM since the reaction of Ag with MoSe2should involve the creation of atomic defects or atomic regions where Ag2Se and Moo3 are formed. In addition, we are interested in the effect of atomic defects in the photoelectrodeposition mechanisms a t p-MoSez.
J C ! C
333
63C
933 1200 '500 '8OC 2 00
L??F!" c s t g n r e 5 ) Figure 8. AES depth profile of p-MoSez showing relative intensities at different silver photoelectrodeposition time. (A) Se/Mo relative intensity vs depth (A) and (B) Ag/Mo relative intensity vs depth (A).
of p-MoSe2 showed that silver had a n oxidation state of +1and that Mo had oxidation states of +4 and +6. From
Acknowledgment. We gratefully acknowledge the generous support from NSF-EPSCOR, grant number EHR-9108775, and NSF-Rimi, grant number HRD9353197. We are grateful to Mr. 0. Resto (University of Puerto Rico, Rio Piedras Campus) for the XPS measurements. We would also like to acknowledge Mr. Bruce Rothman and Ms. Xue-Qin Wang for the work done at the ion-scattering central facility of the MRL a t the University of Pennsylvania which is supported by the National Science Foundation under grant number DMR91-20668. R. J. Castro was a Kodak fellow and actually a fellow of the Graduate Assistance in Areas of National Need (Department of Education), grant number P200A20135. LA940084N
