Photoelectrochemical in situ studies of passive films on cadmium

Materials Chemistry and Physics, 14 (1986) 455-470

PHOTO~LECTROCHEMICAL

IN SITU STUDIES

455

OF PASSIVE

FILMS

ON CADMIUFl

M. METIKOS-HUKOVIC

Institut of Electrochemistry, Faculty of Technology, University of Zagreb, (Yugoslavia) Received November 4, 19‘85; accepted December 17, 1985.

ABSTRACT The formation, growth, reduction and solid state properties of passive films on Cd in a polysulfide solution were studied in situ under dark and modulated light conditions using impedance, cyclic voltammetry, optical methods and SEM analysis. It was found that a thin barrier photoactive film of cadmium sulfide is formed under controlled potential scans. At more positive potentials, a thicker layer containing both sulfide and hydroxide or oxide is initially deposited, enhancing photoactivity and n-type semiconductor behavior. The semiconductor properties of these passive films are discussed on the basis of capacitance and photoelectrochemical measurements. Anodic CdS films are not suitable for photovoltaic purposes because of their limited thickness and small crystal size. The results of this study suggest that thicker films containing hydroxide or oxide species can be equilibriated to a more effective composition for photoanodes by appropriate post-treatment.

INTRODUCTION Cadmium chalcogenides such as CdS, CdSe and mixtures thereof are important anode materials for photoelectrochemical cells. Recently, the thin film growth of these compounds has become of interest in the manufacture of low-cost photocells. Such films have been produced by vacuum deposition, spray pyrolysis, chemical deposition, painting, electrophoretic deposition and electrocrystallization [l-91. In the photoelectrochemical conversion of solar energy, electrolyte-semiconductor

junctions appear particularly attractive. Poly-

crystalline semiconductors are more economical than monocrystals 0X4-0584/86/$3.50

0 Elsevier Sequoia/Printed inThe Netherlands

456

for utilizing the easily formed solid-liquid photovoltaic junctions in these cells. Much has already been reported on cadmium chalcogenides

[lo-151

as semiconductor electrodes. The band gaps of these substances are suitable for converting solar energy into electrocity. A S2-/Szredox system is a suitable electrolyte for a cadmium chalcoyenide electrode [16] RESULTS AND DISCUSSION Figure

1 illustrates the general features of the passivation of

cadmium in sulfide solutions under a controlled potential sweep. Three characteristic potential regions can be distinguished during the anodic scan. Current in the initial potential region shows a sharp maximum which corresponds to the growth of a cadmium sulfide layer of monomolecular dimensions - the monolayer region.

1200pA

1

!U

-7.6

-1.2

-0.8

-0.4

0

0.4

0.8 22 E/V vs.S.c.f.

Fig. 1 Current vs. potential plot during a cyclic sweep at 100 mV/s for Cd electrode in 0.1 M Na,S solution. ‘ In the plateau region, the current is nearly constant, indicating a thickening of the film with a constant electric field. The charge required in the plateau region corresponds to a film thickness of about 5-8 nm.

The anodic decomposition of CdS to Cd(OH)2 or Cd0 takes place in the transpassive region. The thickness of this film can reach 100 nm or more. The sharp peak on the anodic sweep of the cyclic voltammogram shown in Fig. 1 is characteristic of the anodic film nucleation processes and its spread over the surface [17]

In earlier reports,

we have noted a similar sharp peak for bismuth sulfide in this electrolyte [18,,191.The charge passed (Q = 227 K/cm -2) is appropriate for a monomolecular layer. A unit cell of CdS is probably deposited by two-dimensional nucleation and growth mechanism. The formation of monolayer anodic phases has been well established for amalgam electrodes [20]. The essential features of the monolayer region of CdS or Cd(OH)2 on cadmium amalgams are identical to those of polycrystalline electrodes. The total peak charge, Fig. 1, was unaffected by the manner of electrode preparation and was also independent of the solution concentration. There was also no significant variation caused by the scan rate -1 applied in the region from 10 to 100 mVs . Considering that the electrodeposition of cadmium sulfide from the sulfide electrolyte may be described by the following reaction: Cd + S2(aq)'

CdScsj + 2e-

(1)

the corresponding Nernst relation is: E

rev

EEO

- 0.0295 log aS2-

(2).

Variations of the anodic nucleation potential, EAN, with the sulfide ion concentration and pH parallel the reversible potential of the CdS/Cd couple (eqn. 2). The anodic nucleation overpotential

r7AN = EAN - Erev

(3)

was about +90 mV in 0.1 mol dmm3 and about +50 mV in 1 mol dms3 sulfide solutions. The anodic nucleation potential also depends on the surface preparation. The current/potential profile in Fig. 1 is typical of freshly polished specimens. The current maxima increase with the scanning rates, but the corresponding potentials (Fig. 2). The monolayer appears to be

remain practically unchanged,

essentially complete after the first large peak. The second CdS layer then grows before the current enters the plateau region.

458

80 f/fiA 40 -

o-

-40

-

-30

-

-720

-

-160

-

-200 -

L

I

I

I

-1.8 -1.6 -14

I

I

I

I

I

I

I

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0 F/V vs.s.cx

Fig. 2. Cyclic voltammograms for Cd electrode in 0.1 M Na2S solution. Sweep rate 10, 30, 50, 70, 90 and 100 mV/s.

The integration of current during the anodic sweep (Fig. 1) indicates that the field strength in

the film is high in the

plateau region (ii: = lo7 Vcm-1 f. This system is considered to be in a steady kinetic state with a constant current and electrical field, both of which are uniform throughout the layer. The growth of an anodic film under such conditions of high field strength has been described by a number of theories. The best correspondence of empirical data for thin anodic CdS films grown on cadmium under a constant potential is obtained by the direct logarithmic law [7, 21-231. The current rises sharply in thetranspassiveregion.On

the basis

of thermodynamic data as well as our experience that at a given formation occurs at the same potential in the absence pH CdKW2 of sulfide, we believe that the sharp rise in anodic current signifies the decomposition of the CdS to Cd(OHJ2 according to CdS + 2 OH--

Cd0 + H20 + S f 2e-

Additional information on the passive layer can also be obtained by removing the films by cathodic stripping. In Fig. 3

(4)

at dE/dt = 3 mVs

-1

, two reduction peaks appear. The first at

-1.25 V corresponds to an oxygen-rich reduction Layer of cadmium hydroxide, while the second at cca -1.65 V is caused by the reduction of CdS to metal, i.e. to the formation of a porous metallic Cd layer.

I -2.0

I

,

I

I

I

-1.6

-12

-0.8

-0.4

0

I

4

0.4 0.8 E/V 9. S.C.E.

Fig. 3. Current vs. potential plot during a cyclic sweep at 3 mV/s for Cd covered by anodic film in 0.1 M Na2S solution.

Detailed investigations have demonstrated that the maximum reduction potential and the amount of demonstrated current are independent of the rate of electrolyte stirring. However, the position of the reduction peak depends on the anodic limit and moves toward more negative potentials with increases in the sulfide ion concentration and the sweep rate. By means of cyclic voltammetry and other techniques, we were able to monitor the progress of the reduction process in situ with great sensitivity. The results obtained demonstrate that the reduction to metal occurs in the solid state according to the previously described mechanism [24-271. Taking into consideration the semiconducting properties of the solid phase as well as the thermodynamic and kinetic criteria, it was concluded that the reduction of anodically formed oxide films on antimony and bismuth occurs by electron injection

460

ovex the low metal/oxide barrier in the film to the film/electrolyte interface. Here electrons are trapped in the anionic vacancies, which become metal nucleation centers. This reduction model was recently also found to be valid for anodic sulfide films on Ag f281, Pb [291 and Sb [301, although in our opinion the last case involves the reduction of oxide to metal rather than to sulfide. Processes involving the formation and reduction of an adherent film showed characteristic increases in current with increases in the potential sweep rate (Fig. 2). The processes of nucleation, thickening and reduction of the cadmium sulfide films were independent of the rate of stirring, showing that a process of the direct nucleation of a film onto the surface, and not of dissolution/precipitation, was present. Figure 4 shows the results of linear sweep experiment, in which the chopped light method was used to separate the photosensitive of the current. The three characteristic regions component 'ph described above (see Fig. 1) are clearly distinguished on this current-potential curve as well as in Fig. 4.

I

-1.6

-7.4

-12

t

I

I

I

I

-1.0 -0.8 -0.6 -0.4 -0.2

I

0‘

I

0.2

I

0.4

I

0.6

I

0.8

I

1.0

Fig. 4. Photocurrent during a linear sweep at 10 mV/s for Cd electrode in 0.1 M Na2S solution. Electrode illuminated by 100 W tungsten lamp via slowly rotating chopper (0.5 Hz).

461 which

The photocurrent, increases

transpassive interphase

current

photocurrent

I

and continues

The resulting

current under

for the layer

to increase

that

flows

across

illumination

growth

region, in the the

is composed

and electronic

ph:

+1

a

ph

The dark equation

current

(1). The

by the electric

Ia is ionic

light

toward

reduce

in character

generated

and corresponds

electron-hole

Electrons

field.

the metal/semiconductor band migrate they

at the end of the monolayer

region

of an electrode

region. boundary

of the ionic

I=1

appears

in the plateau

pairs

in the conduction

interface,

while

the sulfide

ions

to elementary

are separated

band move

the holes

the semiconductor/electrolyte

to

to

in the valence boundary

sulfur,

where

either

directly

or indirectly:

S2- + 2h+

CdS,hV

CdS + 2h+

7

In order

reactions

0

;

SO + xs2-

-

Gil

(5)

Cd 2+ + So.I

So + xs2-

-

s2x+1

(6)

to use metal/semiconductor

it is very

cells,

important

the free sulfur

the S2-'ions holes

S

from Na2S

and generating

formation

which

CdS crystalline corrosion

species

carry free

remains

sulfur

electrodes.

in which

are produced.

in photoelectric of the above

In reaction

discharging

the current,

two

(51,

the surface

at the site of semiconductor

unchanged

of the cadmium

electrodes

to determine

by the process,

Reaction

surface

with

as found

(6) may be considered simultaneous

for the to be

sulfur

production. During

the anodic

electrolyte, film occurs without

phase,

caused

The sharp

is connected

is initially

If the anodic

independent thickening

ions.

potentials

which

species.

of cadmium

pH the decomposition

at the same potential

sulfide

critical

polarization

at a given

of the oxide

fact similar

increase with

composed

scan

layer field

Cd(OHj2

in current

the formation

of sulfide

is continued,

of the potential

by the electric

at which

in a polysulfide (corrosion)

at these

as well

as hydroxide again

(Figs. 1 and 4), indicating involving

some transport

to the galvanostatic

method,

This

becomes

a

processes

experiment

in which

but

of a new oxide

the current

in the layer.

of the

is formed

is in

a constant

462 current is applied and the resulting rate of potential increase is determined

[31].

Peter [71, on the other hand, maintains that in the transpassive region, only the structure of the film is changed in the buffered sulfide solution (0.1 mol dms3 Na2S + 0.1 mol dm -3 NaHC03). According to Peter, the growth of anodic CdS films on polycrystalline Cd takes place in two stages. Initially, a barrier film grows to a thickness of about 5 nm. When the electrode potential exceeds the critical value, the second stage of film growth begins and a porous polycrystalline CdS layer forms over the original barrier layer. This type of film can be thickened to 500 nm or more by a diffusion-controlled process. Photocurrent and photopotential curves obtained during the passivation of Cd in Na2S solutions are presented in Fig.5.

-150

0.5 c

$0 0.4

-100 0.3

0.2 -50 0.1

0 E/V vs.S.C.E.

Fig. 5. Photocurrent and photopotential vs. potential for Cd electrode in 0.1 M Na S solution. Electrode illuminated 2 by 100 W tungsten lamp. The results of the modulated light response are consistent with the results of cyclic voltammetry, Figs. 1 and 4. An abrupt increase and E amplitude occurs at the potentials where CdS in the I ph and Cd(O$ 2 transitions are expected. Further information on the potential regions associated with changes in photoactivity and the

463

200 I//LA 100 0 -100 -200 -300 -400 -500 I

I

,

-1.6 -1.4 -7.2

I

I

-1.0 -0.8

I

-0.6

I

I

-0.4 -0.2

I

0

I

0.2

I

0.4

1

I

0.6 0.8 E/V vs S.C.E.

Fig. 6. Linear sweep voltammogram for Cd covered by anodic film in 0.1

M

Na2S solution. Sweep rate 10 mV/s. Electrode

illuminated by 100 W tungsten lamp via slowly rotating chopper (0.5 Hz).

range over which light-sensitive materials are stable can be obtained by removing the anodic films by cathodic stripping, Fig. 6. On reversing the sweep, the photocomponent of the current continuously diminishes. A very slight photoeffect follows the reduction peak of the hydroxide films and sulfide films, until the potential of -1.6 V is reached, where the reduction of CdS to Cd metal is completed. At this potential, of course, the photocomponent of the current drops to zero since the metal electrodes exhibit no photoeffects. During potentiodynamic film formation, the capacitance and resistivity were measured

(Fig. 7). After the monolayer region, the capacitance drops abruptly, and then more gradually as the potential nears the plateau region. In the transpassive region, it becomes almost independent of the potential. The drop in capacity is connected with the nucleation of the CdS film, indicating that a compact film at the electrode surface is formed, after the

PA

R,/n 30 -

50

25-

70

20 -

50

IS -

0 -1.6

-1.2

-0.8

-0.4

0

0.4

u”“;f. -

s. c.f

Fig. 7. Linear sweep voltammogram at 100 mV/s, capacitance and resistance at 5 kHz for Cd electrode in 0.1 M Na2S solution.

charge of 227 pCcm

-2

has been passed, as required for monolayer

formation. The semiconducting properties of the solid phase were studied under conditions at which electronic conductivity prevailed. The results of photoelectrochemical impedance measurements were analyzed using a simple Schottky barrier model of the semiconductor-electrolyte interface. CdS is an n-type semiconductor with a band gap of 2.4 eV corresponding to the light wave length of 517 nm. In the potential range, where the electrode is covered with a CdS layer, the capacity follows the Mott-Schottky regime (see Fig. 8), and the data can be fitted to the Mott-Schottky equation: ND

= 0.5 x 1o18

cmq3,

and to the f .at band potential

Lfb =

-1.4 V vs. SCE.

.

-0.6

Fig. 8. Flat band potential determination. Photocurrent, photopotential and Mott-Schottky plot for anodic Cds films on 0.1 mol dm

-3

Na2S solution.

The flat band potential, which follows from the photocurrent curves (Fig. 9)‘ shows the specific interaction of the sulfide ion with the CdS surface. This effect contributes significantly to the appearance of a well-defined photocurrent wave (Fig. 9) [32, 331. The photocorrosion problems of the illuminated CdS/sulfide-polysulfide interface have been analyzed from the thermodynamic and kinetic standpoints [2, 341. The stabilization of the Cd-chalcogenide electrodes by polysulfide is well known, and our calculations show that anodically-formed CdS should be stable in 0.1 and 1.0 mol/dm3 Na2S solutions (Fig. 10).

The quantitative description of the photoelectrochemical behavior of the anodic CdS/sulfide-polysulfide redox couple interface can best be given by considering the energy-level diagram

(Fig. 10). This figure clearly shows that the EF,

466

and pEdecomp are far above the valence band edge and both processes are therefore thermodynamically possible at an illuminated electrode. The formation of a polysulfide is a much faster reaction for kinetic reasons and can therefore prevent t:he decomposition of CdS.

The flat band potential used for the energy level diagram (Fig. 10) construction was estimated using three measurements: capacity, photocurrent and photopotential. The results, summarized in Fig. 7, are in accordance and yield a flat band potential value of -1.4 2 0.05 V (S.C.E.).

-1.6 -1.2 -0.8 -0.4

0

0.4 (18

1.1

E/V"S.f.E.

vs. patential characteristic for Cd Fig. 9. Photocurrent covered by anodic film in 0.1 M Na2S solution. Electrode illuminated by 100 W tungsten lamp.

467 E/eV

Conduction band

E/V

-3 ,E~,,,mp,(CdS+2e---+Cd+S$I

-1

E F,redox(ZSiq +2h+ ?sf: Si,,) 0 __--pEdccomp.(CdS+2h+ --+Cd2,;+Sj Valenceband

“‘+,,,,,(2H,O +f,h+*

CH&,+O,I

Fig. 10. Energy correlation between band edges and the Fermi energies of electrode reactions in 0.1 mol dmD3 Na2S for anodic CdS films.

CONCLUSION The purpose of this investigation was to monitor the essential characteristics of Cd passivation in sulfide electrolytes under controlled scan conditions by means of in situ methods. This enabled us to: i) identify reactions which form photosensitive materials at certain potentials and ii) define the potential regions over which the formed photosensitive materials are stable. In some experiments, chopped light was used to separate out the photosensitive component of the current. Based on photoelectrochemical and capacitance measurements, as well as on data from the literature, the mechanism and the kinetics of the formation and reduction of anodic layers on Cd in a sulfide electrolyte are discussed in some detail. The growth kinetics follow various laws, depending on whether the film growth is produced by an applied potential. The first anodic process involves the adsorption of sulfide onto the surface and the formation of thin photoactive sulfide films. By continuing the anodic scan, a second layer composed of both sulfide and hydroxide

468

or oxide is formed. These results as well as those reported in the literature suggest that growing anodic CdS films with efficiencies comparable to monocrystals under these conditions is very difficult. However, thicker films containing hydroxide or oxide species can be equilibrated to a more effective composition by appropriate post-treatment. The semiconductor properties of the solid phase were studied under conditions where electronic conductivity prevailed. Capacity, photocurrent and photopotential measurements show that the behavior of passive cadmium electrodes in sulfide solutions is determined by the solid state properties of the anodic nonstoichiometric films. The characteristics of n-type semiconductors were established. In the potential region, where the electrode is covered by a CdS layer, the capacity data accord with the Schottky-Mott equation. Reduction of anodically formed films occurs in the solid phase, according to the solid phase mechanism. The cathodic decomposition occurring at the CdS/electrolyte interface results in the deposition of the porous Cd metallic layer. This problem iS discussed in respect to thermodynamic and kinetic data.

ACKNOWLEDGEMENT The author wishes to express gratitude Mr. M.Ceraj-Ceric and J. BoZiEevic, dipl.eng. for their help in present experiments.

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