Cathodic electrodeposition of se on ti electrodes

ELSEVIER Journal of Electroanalytical Chemistry 415 (1996) 123- 132

Cathodic electrodeposition of Se on Ti electrodes

S. Cattarin, F. Furlanetto, M.M. Musiani * IPELP CNR, 4 Corso Stati Uniti., I-35127 Padova, Ituly

Received 5 February 1996; revised 19 April 1996

Abstract

The electrodeposition of Se on Ti has been studied as a function of SeO, concentration, presence of a surfactant (sodium n-dodecylsulphate), electrode illumination and temperature. This process is characterised by (i) a reduction peak followed by a “passive” and a “transpassive” domain in cyclic voltammetry, (ii) a slow current decay followed by an abrupt drop in chronoamperometry, and (iii) a sudden transition from a less to a more negative arrest potential in chronopotentiometry. This behaviour is specific to Ti cathodes and is not observed for other electrode materials. In the less negative potential domain, Se film growth stops at a limiting thickness which increases for deposits of better electronic conductivity. Se deposits may be grown further in the “transpassive” potential domain. The reaction mechanism in the different potential domains is presumably different.

Keywords: Se; Ti; Cathode; Electrodeposition; SeOz

1. Introduction

The electrodeposition of selenium has been reviewed by Zhdanov [I]. The interest in Se films is associated with their semiconducting properties and light sensitivity. Sev- eral years ago it was shown that cathodic reduction of selenious acid is the best way to obtain Se films either in the so-called metallic form [2] or in the amorphous insulating form 131, although anodic deposition from selenide baths has also proved possible [4]. The decisive factor in determining the structure and electrical properties of the films is temperature: amorphous, insulating Se (resistivity 10’ R cm) is obtained at room temperature, while hexagonal, semiconducting Se (resistivity 1 R cm> is deposited at 55°C and above (with optimum nucleation rate at 90°C) P,51.

Se electrodeposition is also interesting because it is a side reaction in the cathodic synthesis of metal selenides [6], which has been investigated extensively as an inexpen- sive method of obtaining thin films of polycrystalline materials suitable for solar energy conversion, especially CdSe [6-91. Indeed, when MSe (where M*+ is a divalent

’ Corresponding author.

cation) is deposited according to the overall reaction H,SeO, + M2++ 6e-+ 4Hf-+ MSe + 3H,O by a mechanism involving Se*- as an intermediate, selenide and selenious species react chemically to yield elemental selenium 161. The control of the deposit stoichiometry is thus a delicate matter, which has been approached in several ways, for example, in the case of CdSe, (i) the use of very low Se(IV)/Cd(II) ratios in the electrolytes, aimed at favouring the reaction of selenides with Cd(U) [6], (ii> the use of soluble Se(O) compounds as the Se source (e.g. selenosulphate, unable to undergo chemical reaction with selenides [9]), (iii) sequential monolayer electrodeposition [IO], etc. Very low Se(N) concentrations limit the deposition rate so that methods like brush plating have been devised in order to speed up the CdSe growth [l I].

Se electrodeposition has been used as a step in the synthesis of photoactive In,Se, thin films by sequential deposition of elemental Se and In layers followed by thermal annealing [12]. This method, used in our labora- tory for the preparation of III-V compounds (see for example Refs. [ 13- 1511, requires a non-reactive substrate which is often Ti. By studying Se electrodeposition onto Ti electrodes from electrolytic baths similar to those described by Graham et al. [3], we have observed peculiar electrochemical behaviour. Its study is the object of the present paper.

0022.0728/96/$15.00 Copyright 0 1996 Elsevier Science S.A. All rights reserved. PII SOO22-0728(96)04707-9

124 S. Catrurin et aL/Journul o)~Elecfroanalytical Chemistry 415 (1996) 123-132

2. Experimental

SeO, and sodium n-dodecylsulphate (SDS) are com- mercially available and were used as received. Ti electrodes of different shape and dimensions were used for different experiments: 0.2 and 0.8 cm* area disc electrodes with a Teflon sheath were used for most electrochemical experiments run at room temperature; Ti sheet electrodes (2cm’) were used for both high temperature experiments and for preparing deposits to be analysed by SEM-EDX, XPS and X-ray diffraction. A large Ti cylinder (10 cm* area) was used to measure the coulombic efficiency of Se deposition, from the weight increase and deposition charge. In the following, when current and charge are not referred to unit area, the electrode area is specified in the figure captions. Ti electrodes were abraded with graded emery paper and rinsed before use.

Two-compartment cells were used, with the working and Pt counter electrodes in the same compartment; a saturated calomel electrode (SCE) placed in the second compartment and connected through a Luggin capillary was used as the reference electrode. The solutions, prepared by deionised and purified water (Millipore Milli-RO system), were thermostatted (25°C if not specified) and not deaerated.

Most electrochemical experiments were carried out with a PAR 273A potentiostat-galvanostat. The a.c. impedance experiments were performed with a 1250 frequency response analyser and a 1286 electrochemical interface, both from Solartron. Impedance plots were obtained under galvanostatic control, covering a frequency range of 1 Hz to 50 kHz with five points per decade. Scanning electron micrographs were obtained with a Philips XL 40 scanning electron microscope equipped with an EDAX PV99 energy-dispersive X-ray spectrometer. X-ray diffractograms were obtained with a Philips PW3710 instrument equipped with a holder for thin films, a flat plate collimator and a flat monochromator (graphite). Cu Ka radiation was em- ployed (40 kV and 30 mA, narrow divergence slit l/6”). All spectra were obtained with a grazing incident beam at a fixed angle of 2”. XPS spectra were recorded in a Perkin-Elmer PHI 56OOci spectrometer using non-mono- chromated Al Ka radiation (1486.6 eV). Photoelectro- chemical experiments were performed with a standard set-up using a xenon lamp as light source. In experiments under white light a power of 300 to 400mW cm-* was measured at the sample position. Photospectra were obtained at - 0.6 V in the solution used for electrodeposition.

3. Results

3.1. Cyclic voltammetry

The electrodeposition of Se onto Ti disc electrodes from SeO, solutions of three different concentrations (3.0, 1.0

0.0 -

2 , -0.3 -

-0.6-

-0.9. ' -0.75 -0.50 -0.25 0.0

E/V

Fig. 1. Cyclic voltammetries obtained on Ti (0.2cm’) in SeO, solutions of different concentrations: (a) 3 M, (b) 1 M and (c) 0.3 M. NaHSO, was

added, when necessary, to make the total concentration 3M. pH 0.8 (by NaOH when necessary). Sweep rate lOmVs- ’

and 0.3 M) was studied by cyclic voltammetry. When the SeO, concentration was below 3 M, NaHSO, was added to yield an overall 3 M concentration. The pH value was 0.8 for all solutions (NaOH was added when necessary) so that Se(IV) was present in solution as H,SeO, [16]. Additional variables in this study were the concentration of a sut-fac- tant (SDS, already used by Graham et al. [3]), illumination of the electrode and temperature of the electrolytic solutions.

Fig. 1 shows the voltammograms obtained for the different SeO, concentrations at 25°C in the dark, with no SDS added. In all cases. a reduction peak followed by “passivation” is observed. Both the onset and the passivation potentials are shifted towards negative values for lower SeO, concentrations. Passivation is less sharp in dilute solutions. By extending the sweep to more negative potentials, a “transpassive” current is observed. Taking into account the effect of the different concentrations of Se&, the potentials at which the current peak and transpassive current are observed agree with those ascribed in Ref. [6] to four-electron and six-electron reduction of Se(IV) on non-reactive cathodes (Pt, Au and carbon). No oxidation peak is observed in the reverse sweep.

Different electrode materials were compared with Ti. Vitreous carbon, Ni and Au showed a cathodic current at E < - 0.5 V, but no passivation. A peak was observed on Ag, but at a much less negative potential (onset at 0.05 V(SCE)). An electrode process in this potential domain has been observed previously on Ag in dilute SeO, solutions and attributed to the formation of Ag,Se [6].

Fig. 2 shows the voltammograms obtained in 3.OM SeO, solution, in the presence of 0.01% or 0.1% SDS (the curve obtained without surfactant is also shown for comparison). The presence of the surfactant induces remark- able changes: the peak current increases significantly, the maximum being reached at more negative potentials; the reduction charge flowing before passivation increases; and the transpassive current becomes much smaller. In less concentrated solutions comparable effects were observed.

S. Cone-in et ul./Journal ofElectrounalytical Chemistry 415 (19961 123-132

E/V

-1.0 -0.5 0

E/V

Fig. 2. Cyclic voltammetries obtained on Ti (0.2cm’) in 3M SeO, Fig. 4. Effect of temperature ((a) 30, (b) 70 and (c) 90°C) on the cyclic

solutions containing (a) no, (b) 0.01% and (c) 0.1% SDS. Sweep rate voltammetry obtained on Ti (2cm’) in 1 M SeO, + 2M NaHSO,. Sweep

10nlVs~‘. rate IOmVsK’.

Fig. 2 also shows that a small cathodic current was observed in the reverse sweep around - 0.3 V. Such a current was larger for more negative switching potentials.

Fig. 3 accounts for the effect of electrode illumination either in the absence (a) or in the presence (b) of SDS. For the solution devoid of surfactant, illumination increases the peak current and, more markedly, the current beyond the peak (“passive” current). The photocurrent observed in this potential range on an Se electrode in acid solution free of Se(IV) has been ascribed [ 171 to Se photocorrosion to give H,Se; in the presence of excess Se(IV), Se will be obtained either by reduction of Se(IV) or by compropor- tion of the latter with selenide. The illumination effect is much weaker for SDS-containing solutions. It is worth noting that Graham et al. prepared Se layers in the presence of SDS, although their aim was to obtain highly

I I I 1

The charges associated with the voltammetric peak under different experimental conditions are shown in Table 1. At room temperature, the charge increases by factors of approximately 6 and 15 when SDS is added to the 3M solutions in 0.01% and 0.1% amounts respectively. The SeO, concentration is of lesser importance: the passivation charge is somewhat higher at lower concentrations, but remains of the same order of magnitude. Table 1 does not report data on Se deposition from SDS-free solutions under illumination because, in this case, passivation is not complete (Fig. 3(a)) and the peak charge is not well-defined. In the presence of SDS, illumination increases the passivation charge moderately (by a factor of 1.2 to 1.6). The passivation charge increases strongly with tempera-

Table 1 -0.75 -0.50 -0.25 0.0 Passivation charges measured in cyclic voltammetry (IOmVs-‘)

E/V

I I I I

0.0 -

2 , 4.0 -

-8.0 - b-

I -0.75 -0.50 -0.25 0.0

E /V

Fig. 3. Effect of electrode illumination on the cyclic voltammetry obtained on Ti (0.2cm’) in 3M SeO, solutions containing (a) no or (b)

0.1% SDS. Sweep rate IOmVs-‘.

photoactive materials [3]. The behaviour shown in Fig. 3 is also representative of that observed with less concentrated solutions.

Fig. 4 compares the cyclic voltammograms obtained on a 2cm2 Ti sheet electrode at 30, 70 and 90°C in 1 M SeO, solution. The higher temperatures, known to favour the deposition of metallic Se [4], induce much larger current peaks but do not significantly alter the general shape of the curves.

SeO, /M SDS/% Temperatore/“C Light Passivation charge/ mCcm-’

3.0 0.0 25 off 60 1 .o 0.0 2.5 off 60 0.3 0.0 25 off 95 3.0 0.1 25 off 780

1.0 0.1 25 off 950 0.3 0.1 25 off 1100 3.0 0.01 25 off 350 3.0 0.1 25 on 950 1.0 0.1 25 on 1450 3.0 0.0 1 25 on 500 1.0 0.0 60 off 1700 1.0 0.0 90 off 3300 1.0 0.1 60 off 4500

126 S. Catturin et ai./Journal of Electromalytical Chemistry 415 (19961 123-132

ture; a simultaneous effect of high temperature and SDS is observed. By assuming that a four-electron reduction of Se(IV) is occurring without side-reactions (see below), the charges in Table 1 correspond to the formation of Se layers 0.025 and 0.4 km thick at 25°C for no and 0.1% SDS respectively. At 60°C the film thickness reaches 0.65 and 1.6 pm without and with 0.1% SDS respectively, but SDS is detrimental to the morphology.

Cyclic voltammograms were obtained in 3 M SeO, solution as a function of the sweep rate. A single passivation peak was detected in the usual range; the corresponding charge increased from 45 to 75 mC cm-’ as the sweep rate decreased from 200 to 1 mV s- ’ . The angular speed of the rotating disc electrode had little effect: the cathodic peak was somewhat larger at 100 than at 0 rev min- ’ and then remained unaltered at angular speeds up to 3600 rev min- ’ .

3.2. Chronoamperometry

Chronoamperometric curves were obtained by stepping the electrode potential from O.OV to a value in the range -0.3 to - l.OV. Some typical results for the 3 M SeO, + 0.1% SDS solution at room temperature are shown in Fig. 5. Although the time scale of the observed phenomena depends on potential, all curves exhibit similar shapes: the current decreases smoothly at first and then drops suddenly to almost zero. The initial current increases and the time required for passivation decreases as the potential is made more negative. The charge required for passivation is much larger in the presence than in the absence of SDS. In the latter case it is almost independent of potential, and of the order of 60 mC cme2, while in the former it is close to 1 Ccm-* for potentials up to -0.6V and then decreases when the potential becomes more negative.

The effect of electrode illumination is to delay passivation. This is very limited in the presence of SDS, but more marked in its absence, in both cases being greater at less negative potentials. Chronoamperometric curves obtained at temperatures in the range 60 to 90°C and -0.7V show

r I o- .

2 1 -10 -

t/s Fig. 5. Chronoamperometric curves obtained on Ti (0.2cm’) in 3M SeO, +O.l% SDS. Initial potential O.OV. Final potentials are indicated

on the figure.

I I I I -

O-17 -

2 -25-

\

-50 -

-75 ; I , ,I

100 203 3M t/s

0

-10

-15 0 la, 200 3cn

t /s Fig. 6. Top: chronoamperometric curve obtained on Ti (O.Zcm*) in I M SeO? +2M NaHSO, at 60°C. Initial potential O.OV, final potential

- 0.7 V. Bottom: expanded views of chronoamperometric curves obtained with final potential (a) - 0.65, (b) - 0.7 and (c) - 0.75 V.

a marked passivation at short times, followed by a small peak, suggesting a nucleation phenomenon (Fig. 6). The passivation charge (measured at the first sudden drop) is much larger at 60°C than at room temperature, reaching 3Ccm -* in the voltammetric peak region and decreasing at more negative potentials.

3.3. Chronopotentiometry

The time evolution of the electrode potential upon application of a constant current was followed for various combinations of experimental parameters. A typical chronopotentiometric response, obtained for the 3 M SeO, + 0.1% SDS solution in the dark at room temperature, is shown in Fig. 7. It shows the features common to the responses observed under other conditions: upon applica-

Fig. 7. Chronopotentiometric curve obtained on Ti (0.2cm’) in 3M SeO, + 0. I % SDS in the dark. Imposed current density - 7.5 mAcmm2.

S. Cattarin et al./Journal of Electroanalytical Chemistry 415 (1996) 123-132 127

tion of the reduction current the electrode potential fixes at a value E, in the range of the peak observed in cyclic voltammetries, which slowly drifts towards more negative values, until a sudden potential jump (to more negative values) occurs after a well-defined transition time 7. Fi- nally, the potential decays somewhat to less negative values and stabilises at a potential E2 (though, under some conditions, with fluctuations). In the absence of SDS, E2 is less well-defined, as a steady drift towards more negative values takes place.

The transition time 7 decreases as the current increases, as shown in Fig. 8. The log T-log j plots are quite close to straight lines, the slope of which lies between - 1.15 and - 1.45. The transition time increases strongly with SDS concentration, being, in the presence of 0.1% SDS, 10 times longer than in its absence. Fig. 8 shows that for the 3 M SeO, -t 0.1% SDS solution, illumination of the electrode approximately doubles the transition time at all imposed currents. For the SDS-free solution the effect of light is stronger: at each current density the transition time becomes more than three times longer. The transition time is almost independent of SeO, concentration, but depends markedly on temperature: the same transition time is obtained at 60 and 25°C for currents in a ratio 20: 1. SDS also affects T at high temperatures.

The charge transferred up to the transition time for an imposed current density of 1 mAcm-’ is 85, 350 and 2100mC cm- ’ for a 3M SeO, solution containing no, 0.01% and 0.1% SDS respectively at 25°C. These charges are of the same order of magnitude as those measured in cyclic voltammetry (Table 1). The current efficiency of galvanostatic Se deposition from the 3 M SeO, + 0.1% SDS solution was measured (from the weight increase of large-area Ti electrodes and the charge), and found to increase from 80 to 100% in the range 0.5 to 4 mA cme2, then remaining at 100% for currents up to the explored limit of 20mAcm-*. The observed dependence of the transition time on current density implies that the charge

Fig. 8. Dependence of transition time on the imposed current density on dark (0, 0) and illuminated (X , +) electrodes in 3M SeO, solutions

containing no (0, +) or 0.1% (0, X) SDS.

E, / v

Fig. 9. j-E, plots for Se deposition from 3 M SeO, (+I and 3M SeO, + 0.1% SDS (X) solutions under illumination.

transferred to the electrode at the moment of passivation is not constant, i.e. the limiting thickness of the film grown at the transition time decreases as its growth rate increases.

Chronopotentiometric experiments also allow log j-E plots to be built up for both the first and the second arrest potential. With this aim, potential data were corrected for the ohmic drop, the uncompensated resistance being measured as the real high frequency limit of the impedance. E, (measured at f = 7/2) is generally in the potential domain where the passivation peak is found in cyclic voltammetry. Fig. 9 shows that in a log j-E, plot the data points pertaining to 3 M SeO, and 3 M SeO, + 0.1% SDS solutions fall on the same straight line, the slope of which is about 180mV per decade. The large value of this slope may be due to a resistive film on the electrode surface [18]. The log j-E, plots for 3, 1 and 0.3 M solutions are found at progressively more negative potentials, and have comparable slopes. The effect of electrode illumination on E, is rather small for both SDS-containing and SDS-free solutions.

E, values are more scattered than those of E,, and sometimes cannot be identified unambiguously due to significant drift. For the same current density, E, is markedly less negative for SDS-free than for SDS-containing solutions. The effect of light is rather small for the 3 M SeO, + 0.1% SDS solution (10 to 40mV for the same current density), but much more marked in the absence of SDS, as shown in Fig. 10. Here, E2 is markedly less negative under illumination than in the dark. In addition, it may be seen that when the illumination is chopped, E, changes reversibly. This experiment shows that a part of the applied overpotential is necessary to overcome the Se film resistance, which is decreased by illumination.

Chronopotentiometric experiments were also carried out using Ti disc electrodes rotating at angular speeds between 100 and 2500 rev min- ‘. Neither the shape of the curves nor the transition time were modified significantly.

Electrode materials other than Ti gave rise to chronopotentiometric transients of different shapes: a gradual de-

128 S. Catturin et al./Journal of Electrounulyticol Chemistry 415 (1996) 123-132

crease in potential was observed for Ni and Au, while Ag exhibited behaviour rather similar to that of Ti, as previously reported by de Becdelievre et al. [19]. According to these workers, the deposits consisted of superimposed layers of Ag,Se and amorphous Se. It was suggested that, on the less negative arrest potential, Ag,Se was formed by reaction of elemental Ag and Se, controlled by diffusion of Ag+ ions through a layer of growing thickness. The transition to a more negative potential was ascribed to the inability of Ag+ to diffuse at the rate imposed by the current, leading to deposition of non-conducting Se. Sur- prisingly, these workers [19] reported that no relation was found between the thickness of the Ag,Se layer and the transition time, although from the curves in their paper the charge transferred at the transition time appears to be constant, implying constant thickness (since the deposition efficiency was 100%). A diffusion controlled process would imply an inverse proportionality between charge and current.

3.4. A.c. impedance

Fig. 7 shows that, under appropriate conditions, the electrode potential remains fairly stable at well-defined arrest potentials during the galvanostatic deposition of Se, except around the transition time. Therefore, a.c. impedance measurements may be performed either before or after the transition, although the frequency range cannot be ex- tended to a very low limit. In the present paper, the discussion of preliminary impedance data is limited to some high frequency features which are reproducibly observed by sweeping the frequency in either direction. A full interpretation of the plots is not attempted.

Fig. 11 shows two impedance plots obtained on a 0.8 cm* Ti disc in a 3 M SeO, + 0.1% SDS solution under an imposed current density of 5 mAcme2: plot (a) was obtained before and plot (b) after the transition. In other experiments the impedance was measured at a single frequency (30 kHz) at 30 s intervals during a time longer than

I dark

I I r._l 100 200

t/s

Fig. 10. Comparison of chronopotentiometric transients obtained on Ti

(0.2cm’) in 3M SeO, solution in the dark and under illumination,

emphasising also the effect of chopped illumination for times longer than

the transition time. lmposed current density - 1 mAcm-*.

c: lo- 250 Hz

2 h . I

2- z 5- 0 0 O O o. .- D 30 kHz o”

Ooo 4 Hz

E - T \RO

Oooi

0 , ’ 1. I I. I. 0 5 10 15 20 25 30

Real Part/R

-lo\. , , . , , . , , . I., . 0 lo xl 30 40 50 al 70 Bo

Real ParUR

Fig. 11. Nyquist plots of the a.c. impedance measured at a Ti electrode

(0.8cm2) in a 3M SeO, t0.195 SDS solution; j= -5mAcm-*. (a) before and (b) after the transition.

the transition time. As Fig. 1 l(a) shows, before the transition the impedance measured at 3OkHz is purely real. It increases linearly with time for f < 7, starting from a value (measured before the onset of electrodeposition) obviously corresponding to the electrolyte resistance. If this increase is ascribed to the formation of a resistive film, a resistivity close to 4000 Q cm can be obtained from the impedance increase (0.16 fi cm2) and the estimated film thickness (4 x 1O-5 cm). This resistivity value is intermediate between those reported in the literature for amorphous and hexagonal Se. After the transition, the impedance measured at 3OkHz does not correspond to the high frequency limit. However, Fig. 1 l(b) shows that the diagram extrapo- lates to a value close to 6 a, not far from that measured before the transition.

Analysis of the plots in Fig. 11 shows that the capacitance associated with the high frequency loops changes dramatically with the transition: it is about 40 and 0.3 FFcm-* before and after the transition respectively. While the first value is typical of a double layer capacitance, the second is more reasonably ascribed to a dielectric film on the electrode surface. If so, a sudden change in the properties of the Se deposit at the transition time is suggested by the impedance results.

3.5. Characterisation of the deposits

Since galvanostatic electrodeposition of Se clearly shows two different stages, at E, and E, respectively, the

S. Catturin et al./Journd of Elrctroumlyticul Chemistry 415 (1996) 123-132 129

possibility may be considered that different products are formed before and after the transition time. Therefore, a characterisation of the deposits was performed. EDX and XPS analyses of samples obtained from 3 M SeO, + 0.1% SDS, with electrolysis times either half or twice the transition time, showed that they consisted of pure Se. In agreement, the current yield was constant for different deposition times, both shorter and longer than the transition time, always approaching the theoretical value for the four-electron reduction of Se(N). The photospectrum of a deposit taken after the transition time is shown in Fig. 12. The plot of (Q.E. hv)‘.’ vs. hv, shown in the inset and appropriate for an indirect transition, provides an energy gap value of 2.1 eV, in agreement with the literature [20,21]. The same photospectrum was also obtained when different electrode materials were used instead of Ti.

The morphology of deposits was studied by SEM as a function of electrolysis duration (either shorter or longer than the transition time), illumination, presence of SDS and temperature (Fig. 13). At room temperature, samples obtained in the dark (a> were fairly smooth. Under illumination the deposit roughness tended to increase, especially after the transition time (b), when some solid material also appeared in the solution. For comparable electrolysis times,

5

1.6 1.8 2.0 ’ 2.2 2.4 2.6

energy I eV

400 500 600 700 800

lambda / nm

Fig. 12. Photospectrum of the deposit obtained from 3M SeO, with j= - 1 mAcm-’ after the transition. The spectrum was recorded at

- 0.6V in the same solution. Inset: indirect transition plot.

Fig. 13. SEM images of Se deposits obtained at room temperature from

3 M SeO, solutions. Experimental conditions: (a) 25°C. 0. I % SDS, dark, t < 7; (b) 25°C. no SDS, light, I > 7; (c) 60°C. no SDS, dark, t < 7.

similar morphology was obtained with or without SDS (though the th’ k rc ness was much higher in the former case>. Although to the naked eye the samples obtained at high temperature were grey and metallic rather than red [2,3,5], the morphology of an Se deposit obtained at 60°C without

130 S. Catturin et aL/Journal of Elrctrounulyticul Chemistry 415 f 1996) 123-132

SDS (c) is very close to that of a sample grown with SDS at room temperature.

X-ray diffraction showed that all Se deposits obtained at room temperature were amorphous, whatever the other experimental conditions (SDS, light). Samples prepared at 60 and 90°C in the absence of SDS showed the typical pattern of hexagonal Se [22].

4. Discussion

Cyclic voltammetry (Figs. l-41, chronoamperometry (Fig. 5) and chronopotentiometry (Fig. 7) show that Se deposition onto Ti electrodes shears many phenomenologi- cal similarities with the anodic passivation of metals. At room temperature and in the dark, a potential sweep towards negative values or potentiostatic deposition in the potential range - 0.2 to - 0.9 V leads to almost complete suppression of current flow. Galvanostatic deposition occurs initially in the range - 0.15 to - 0.5 V, and then at potentials more negative than - 1 V, corresponding to a sort of transpassive region.

In both potential ranges (that of the peak and the transpassive region) Se is deposited. However, it is proba- ble that two different mechanisms of Se(N) deposition operate:

- four-electron reduction to elemental Se at less negative E;

- six-electron reduction to selenide, followed by reaction of the latter with Se(N) to yield elemental Se, at more negative E [6].

The overall stoichiometry is the same for both paths, but the deposition potential has some relevance to the deposit morphology. Owing to passivation, the Se deposit thickness reaches a limiting value unless the transpassive region is entered.

The charges associated with passivation for the same set of experimental conditions are of the same order of magnitude whatever the controlled electrochemical variable (current or potential). Instead, the passivation charge, and therefore the attainable Se thickness, depend strongly on other variables: increasing in the presence of SDS, under

Table 2 Best fit values of j” and k = T( j - j”) according to Eq. (1)

SeO,/M SDS/%

3.0 0.0 I.0 0.0

0.3 0.0

3.0 0. I 1.0 0.1 3.0 0.01

1.0 0.0 3.0 0.0

3.0 0.1

Temperature/“C Light

25 off 25 off

25 off

25 off 25 off 25 off

60 off 25 on

25 on

-j /mAcm-2

Fig. 14. Experimental 7 - j data for galvanostatic Se deposition from 3 M SeO, + 0.1% SDS under illumination compared with Eq. (I) (solid line).

electrode illumination and at high temperature. While the passivation charge is greatly affected, none of these variables qualitatively alters the main features of the process.

A similarity between Se deposition and passivation phenomena is also found in the dependence of the transition time on the imposed current in chronopotentiometric experiments. In most passivation processes the transition time (generally called the induction or arrest time) is linked to the current by a relation of the form [23]

7=k(j-jy (1) where n is generally 1 or 2. For n = 1 a constant charge, k = T( j - j’), is needed for the formation of a passivating film; j” is a current not contributing to passivation, usually interpreted as a dissolution component. Fig. 14 shows that Eq. (1) may be fitted to experimental data by assuming n = 1 and taking j” and k as adjustable parameters. Table 2 reports the adjusted values of j” and k = dj - jO>, showing that SDS, electrode illumination and high temperature increase both j” and T( j - j”).

j’/mAcm-’ T(j- j”)/mCcm-’

0.029 81 0.02 I 56

0.020 59

0.27 980 0.52 IOQO 0.17 300

1.6 700 0.10 190

0.63 1900

S. Catturin et al./Journal of Electroanalyticol Chemistry 415 (1996) 123-132 131

Despite the reported similarity between Se deposition and anodic passivation, there are marked differences in the physical processes involved:

(i) typical passive layer thicknesses are in the nanome- tre range, whereas Se layers may be more than three orders of magnitude thicker. Thus, the former case is close to a 2D blocking, while a 3D layer is clearly involved in the latter;

(ii) the growth of a passivating oxide requires transport of at least one ionic species, while electron transport across the film may be sufficient for Se deposition.

All experiments show that all factors improving Se conductivity delay (though do not prevent) passivation. Therefore, the low conductivity of Se is likely to be responsible for the passivation process. However, even under conditions leading to the lowest conductivity, the behaviour of Se is not that of a perfect electronic insulator permeable to ions for which the electrochemical reaction occurs at the substrate/deposit interface. In this case the current varies as f-‘I* . m potentiostatic electrolyses, while the transition time in galvanostatic electrolyses is proportional to iC2 [24].

The resistance of the Se deposit, which may explain the high apparent Tafel slope of 180mV per current decade (Fig. 9) [18], is expected to cause a potential drop across the layer but not to prevent electron flow. Indeed, electrochemical processes occurring on thick (5 to 600p,m) Se films are characterised by linear current-potential relations [25]. However, a continuous increase of the Se layer resistance during deposition, proportionally to its thickness, cannot explain the observed discontinuous behaviour. For instance, a linearly growing potential (without any sharp transition) would be observed in a galvanostatic deposition controlled by a resistance proportional to the deposit thickness.

A tentative explanation of the observed discontinuity may be the following: Se deposits consist of domains of different conductivity (the structure and conductivity of electrodeposited Se is known to depend in a subtle way on experimental variables [2,3,5] and electrode material [26,27]). Current flow across the films is ensured by interconnected conductive domains. As the film grows, it has a tendency to lose order with deposition of increas- ingly insulating material. Passivation occurs when no conductive path connecting the TilSe and Selsolution inter- faces is left. In this framework, the effect of experimental variables may be understood: the higher the temperature, the larger the relative amount of conducting domains; electrode illumination imparts photoconductivity to insulating domains. It is possible that SDS, besides influencing deposit morphology (and structure [27]), which becomes similar to that of hexagonal Se (Fig. 13), also affects conductivity. Although XRD cannot detect any effect of SDS on crystallinity, the lower photoactivity exhibited by Se layers grown in the presence of SDS might indicate higher dark conductivity. For a given set of experimental

conditions, higher currents, likely to cause faster loss of order, lead to passivation by transfer of smaller charges. The SeO, concentration and mass transport regime have little or no relevance since the overall behaviour is controlled by the electronic properties of the deposit, not the electrochemical reaction.

The a.c. impedance data suggest that the deposit properties change discontinuously at the transition. If the high frequency loop of Fig. 1 l(b) is interpreted as due to a parallel combination of resistance and capacitance of the deposited film, the overall film resistivity should be close to lo6 0 cm, if its thickness is assumed to be that reached at the transition time. A calculation of the dielectric constant based on the same assumptions would yield 130, a value about 20 times larger than the literature value for Se. One may speculate that in order to obtain the correct dielectric constant one should assume that the low capacitance is determined by a layer much thinner than the deposit (a low capacity is not measured before the transition). The resistivity of this layer should be correspond- ingly higher than that calculated above. In this view, the deposit would consist of two layers of different resistivi- ties: 4 X lo3 and > lo6 fi cm respectively. The formation of an insulating layer on top of a fairly conducting Se deposit also agrees with the appearance of a marked photoactivity only after the transition time in the galvanostatic experiments. The high sensitivity of E, to chopped light, while E, is less sensitive, would mean that insulating Se is more photoactive.

Once this layer has formed, Se deposition may continue only if the potential becomes negative enough to enter the transpassive region. Here local reduction of the passive film may take place (besides six-electron reduction of Se(IV)), as possibly indicated by the cathodic currents observed in the reverse sweep of cyclic voltammetries performed with very negative cathodic limits (also visible in Figs. 2 and 3) and by the decay of the potential immediately following the overshoot at the transition time (Fig. 7).

The specificity of Ti in determining the described behaviour deserves some discussion. Ti has been used little as a cathode material for Se deposition [ 121, and no special effect on kinetics was described. A major role of other electrode materials has been previously observed: Graham et al. have reported that the Se appearance and structure depend strongly on substrate [3]; de Becdelievre et al. [25] have shown that crystalline Se, usually obtained at high temperature, may be grown at 25°C on a crystalline Se cathode; Modolo et al. 1271 have shown that the presence of an oxide layer on a Pt electrode induces the growth of crystalline (monoclinic) Se, while an amorphous deposit is formed in its absence. The same authors have also reported that SDS favours Se electrocrystallisation. Therefore, it is conceivable that Ti (or Ti dioxide) is able to affect the structure and therefore the properties of Se deposits, even if a major effect, detectable by XRD, is not evident.

132 S. Cutturin et al./ Journal of Electroanulytical Chemistry 415 (1996) 123-132

5. Conclusions

The electrodeposition of Se onto Ti seems to occur through two different reaction paths in two different potential regions, as on other substrates 16,271, but in addition exhibits some features not observed with different cathode materials. Deposition at the less negative potentials (presumably occurring via four-electron reduction) leads to a limiting thickness which varies between 25 nm and some micrometres, depending on experimental variables. Since this limiting thickness increases with all factors that en- hance Se conductivity, the formation of a resistive layer may be considered the most likely reason for the observed cathodic “passivation”. However, the continuous increase of the layer resistance proportionally to its thickness cannot account for the sudden passivation. We therefore pro- pose a discontinuous variation in the layer conductivity associated with the formation of a thin, highly resistive layer. Experiments providing an in-situ monitoring of the film properties (a.c. impedance and photoelectrochemical experiments) agree with the formation of such a layer at the transition time.

The results of this investigation are not sufficient to clarify why Ti cathodes induce peculiar electrochemical behaviour not observed on other cathode materials, but confirm earlier reports of a strong sensitivity of the properties of electrodeposited Se layers to substrate nature.

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Cathodic electrodeposition of se on ti electrodes