Electrodeposition of metal-trithioanthrone films

281

J. Electroanal. Chem., 249 (1988) 281-290 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

ELECTRODEPOSITION

JEFFERY

OF METAL-TRITHIOANTHRONE

P. SZABO *, PRZEMYSLAW

SZCZECINSKI

**

and MICHAEL

FILMS

COCIVERA

Department of Chemistv, College of Physical Science, Vniversdy of Guelph, Guelph, Ontario NIG 2 WI (Canada) (Received 22nd February 1988; in revised form 30th March 1988)

ABSTRACT The electrochemical reduction of 5H-1,9,9-a-A4-trithiapentaleno-[2,3,4,5-defg]-anthracen-5-one (TTA) involves two one-electron transfer steps occurring at -0.3 and -0.8 V (SCE) in dimethylformamide or bis(Zmethoxyethy1) ether. The number of electrons involved and the diffusion coefficient of mA in the ether was determined by means of chronoamperometry at both a macro- and a microelectrode. Electrodeposition of metal-TTA thin films was accomplished at a variety of electrodes including carbon and indium-tin oxide coated glass. Metal-?TA films could be prepared with Cu, Cd and Ni, and Rutherford backscatteting spectrometry indicates that the metal to TTA ratio is 1. Films studied by scanning electron microscopy appear very smooth with cracks occurring in thicker films. Ni-TTA films deposited on thin-film CdSe that contains cracks appear to cover those cracks and to permit the formation of a Schottky barrier when Au + Pd is deposited on the surface.

INTRODUCTION

Organic surface coatings have been used with n-type semiconductors to improve their stability in liquid junction cells. For example, the stability of the CdSe photoelectrode in ferri/ ferrocyanide solution is enhanced when the electrode surface is coated with a conducting transparent polymer, such as polypyrrole [l]. Also, a thin film of copper(I) thiophenolate on CdS was found to enhance the stability of that electrode greatly in a photoelectrochemical cell employing the ferri/ ferrocyanide couple [2]. Dye sensitization is another application that makes use of organic thin films. Because only high energy light of the solar spectrum is absorbed, wide bandgap semiconductors have small photocurrents but large open circuit voltages are possible. The amount of light absorbed may be increased by depositing several monolayers of an organic dye on the semiconductor surface. An example is sintered * l

Present address: Department of Physics, Simon Fraser University, Bumaby, B.C., Canada. * Present address: Technical University, Warsaw, Poland.

0022-0728/88/$03.50

0 1988 Elsevier Sequoia S.A.

282

ZnO covered with l-2 monolayers of rose bengal dye, which had a reported efficiency of 1.5% in the presence of the 13-/I- redox couple using monochromatic light [3,4]. The use of organic materials as major components in solar cells has received attention because of the potential low cost of both materials and production. Organic semiconductors might be expected to overcome some of the stability problems experienced by inorganic semiconductors in photoelectrochemical cells [5]. Phthalocyanines are organic semiconductors with p-type conductivity [5,6]. Coordinated to various metal ions, they have been studied in both Schottky barrier devices and heterojunction cells with n-CdS. Reported here is a study of a series of novel metal-organic compounds, metal-trithioanthrones, which have been electrodeposited as thin films. Emphasis is placed on characterizing the deposition process and the thin film itself. The electrochemical reduction of the trithioanthrone was studied at macro- and microelectrodes. In the presence of metal ions such as Cu2+, Ni2+ and Cd2+, black thin films are deposited on various electrodes, and composition analysis by Rutherford backscattering spectrometry (RBS) confirmed that the deposits are metal ion-trithioanthrone derivatives. Scanning electron microscopy revealed that the films are very smooth. The nickel-trithioanthrone derivative was tested as a coating for electrodeposited CdSe in both photoelectrochemical and photovoltaic cells. EXPERIMENTAL

Preparation

of trithioanthrone

5H-1,9,9-a-X4-Trithiapentaleno-[2,3,4,5-defg]-anthracen-5-one will be referred to as trithioanthrone (TTA). Trithioanthrone was synthesized as reported previously [7-91 with some modifications. Separate solutions of 5.5 g 1,8-dichloroanthraquinone in 100 ml N,N-dimethylformamide and 14.4 g sodium sulfide nonahydrate in 100 ml water were mixed and heated under reflux for 2 h. Following reflux, the solution was diluted with 500 ml of water and acidified with HCl. The green precipitate that formed was filtered, washed with water, and dried. The crude product (5.6 g) was purified by Soxhlet extraction with CH,Cl, for 2 days. The liquid was allowed to cool at 4O C for 2 days. The resultant precipitate (1.60 g) was filtered and recrystallized from 1,1,2-trichloroethane, m.p. 270-275 o C. The yield was approximately 25%. Analysis of TTA for C, H, 0, and S was performed by M-H-W Laboratories, Phoenix, AZ. (Found: C, 57.5; H, 2.4; 0, 8.2; and S 31.5%. C,,H60S3 requires C, 58.1; H, 2.1; 0, 5.6; and S, 33.68.)

TTA

283

Preparation

of triflate salts

Ni(CF,SO,),, Cu(CF,SO,), and Cd(CF,SO,), were prepared in two-step syntheses. First, Ni(OH),, Cu(OH), and Cd(OH), were prepared by adding excess NaOH to solutions of NiCl, - 6 H,O, CuCl,, and CdCl, respectively. After the metal hydroxides were filtered, washed thoroughly with water, and dried, each was added in excess to separate flasks containing trifluoromethane sulfonic acid (CF,SO,H). After filtration the triflate salt in the filtrate was isolated by rotovap. Each salt was dried at 50” C under vacuum for 24 h before use. The dried Ni(CF,SO,),, CU(CF,SO,)~, and Cd(CF,SO,), were yellow, brown, and white in color, respectively. Solvent preparation

Bis(2-methoxyethyl)ether (diglyme) was either (a) refluxed over lithium aluminum hydride and distilled under vacuum or (b) dried over sodium for 24 h and passed through a column of activated alumina directly into the electrochemical cell. The latter method was found to result in more reproducible electrochemistry and metal-T’TA film formation. Dimethylformamide (DMF) was dried over 3A molecular sieves for 1 week and then passed through a column of activated alumina directly into the electrochemical cell. In all experiments nitrogen (or argon) was bubbled through the solution for 10 min using a stainless steel 16 gauge syringe needle. The needle was then removed from the solution, but the inert gas atmosphere was maintained by nitrogen flow over the solution. Electrode preparation

Platinum wire that was used in cyclic voltammetry (CV) was first cleaned by dipping it in hot (50 o C) nitric acid and rinsing it with water. It was then placed in an oxygen + propane flame for - 30 s, quenched in water, and rinsed with the solvent used for CVs. Films were electrodeposited on the following substrates: copper, platinum, vitreous carbon, graphite, and indium-tin oxide (ITO) coated glass. Copper was etched in a solution that contained 20 g/l KClO, and 68 ml/l cont. HCl. Platinum and vitreous carbon were cleaned in 50 o C nitric acid. Graphite was cleaned by abrasion with emery paper. IT0 glass was cleaned by sonication in a strong detergent (Decon). After cleaning, all substrates were rinsed well with water and then with either diglyme or DMF. An 8 pm diameter carbon fiber microelectrode was fabricated according to a previously described method [lo]. Electrochemical

measurements

The counter electrode was usually a Pt foil (- 1 cm*). The type of working electrode depended on the technique employed: a Pt wire or a vitreous carbon disk was used for cyclic voltammetry; a vitreous carbon disk and a carbon fiber microelectrode were used for chronoamperometry. CVs at Pt and C were very similar. Both exhibited two one-electron transfer steps, but anodic/cathodic peak separations were larger at C. The reference electrode was a saturated calomel

284

electrode separated from the cell by a salt bridge. Ionic contact between the reference electrode and the cell was maintained by means of a leaky Pt joint. Cyclic voltammetry was carried out using a Bioanalytical Systems CVIB Instrument. For chronoamperometric experiments using a 0.07 cm* planar vitreous carbon macroelectrode, a custom-made potentiostat supplied the potential step necessary for the experiment, and data were collected on a Nicolet Digital Oscilloscope. For the time-independent limiting diffusion current measurements using a carbon microelectrode, a PAR 174 polarographic analyzer was used. Metal- TTA and CdSe films For many electrodepositions the solution contained 10 m&i TTA, 10 mM M(CF,SO,), (M = Ni, Cu or Cd), and 0.2 M NaCF,SO, in either diglyme or DMF. However, the reproducibility of the process in regard to film quality was improved when nitrilotriacetate ion was used as a complexing agent for the metal ions. Depositions were carried out at constant potential, from -600 to - 1300 mV vs. SCE. CdSe films were electrodeposited from selenosulfite solution at - 1.10 V vs. SCE for 1 h and annealed in air at 500 o C for 10 min [11,12]. A tbin coating of Ni(TTA) was deposited on the CdSe electrodes by the method described above. Some of the electrodes were tested in photoelectrochemical cells using a 1 M S*- + 1 M S + 1 M OH- solution. For solid state devices having the configuration Ti/CdSe/Au + Pd or Ti/CdSe/Ni(TTA)/Au + Pd, a 20-30 nm gold palladium alloy (60% Au, 40% Pd) film was applied using a Technics Hummer V Sputter Coater. Chemical and physical characterization Rutherford backscattering (RBS) spectra of electrodeposited films were obtained using a 3 MeV Van de Graaff accelerator with a 180” C annular detector. A 1.6 MeV beam of helium nuclei was used to generate spectra. Composition was obtained by fitting simulated spectra to experimental data using the RUMP computer program developed by Doolittle [13]. Scanning electron microscopy (SEM) was performed with an ETEC Autoscan instrument. Dark and light induced current-voltage characteristics were obtained at various sweep rates and recorded by an IBM PC equipped with a Tecmar A/D converter. Digital smoothing of the data was done by a second order 9 point Savitzky-Golay alogorithm [14]. RESULTS AND DISCUSSION

Diffusion coefficient and number of electrons The cyclic voltammogram of TTA at Pt (Fig. 1) shows two reductions at -0.3 and -0.8 V in DMF or diglyme. The peak current of the more positive reduction wave is proportional to the square root of the sweep rate, indicating a diffusion limited process [15]. The separation of the cathodic and anodic waves is 65-75 mV at room temperature and is independent of sweep rate in the range 20-200 mV/s.

285

Ib

21 -500

-2 00

-1000

POTENTIAL/mV(vs.

/

I

1

-400 POTENTIAL/mV

SCE)

-600 (vs. SCEI

Fig. 1. Cyclic volt~o~~ of t~~o~~one (“PTA) at a plater electrode. The solution contained 5.5 m&f ‘ITA and 0.25 M LiCLO, in DMF. The sweep rate was 100 mV/s. Fig. 2. Cyclic voltammogram of TTA on a carbon fiber microelectrode, showing the first wave. The solution contained 10 mM TTA and 1 M NaCF$O, in diglyme. The sweep rate was 100 mV/s.

C~ono~peromet~c rn~ur~rne~ts for TTA indicated that n = 1 for each wave and provided a value for the diffusion coefficient (DrrA) of TTA in diglyme. The method chosen [lo] to determine DrrA uses both the macro- and microelectrode and does not require knowledge of exact concentrations or n. Figure 2 shows an example of a cyclic voltammogram of TTA using a carbon fiber microelectrode for electron transfer at - 0.3 V. The absence of peaks for both oxidation and reduction at this sweep rate of 100 mV,/s is also observed at lower sweep rates. This effect has been observed by others using microelectrodes [16] and is due to a time-independent diffusion current. Approximately 10 s after a potential step, the current i at a planar microelectrode with radius r < 10 pm, reaches a steady-state value i, [lo] and values are given in Table 1. After the potential step for macroscopic planar electrodes, the current decreases with time t according to the Cottrell equation [17]. A plot of i versus t-‘j2 is linear, and values of the slope are given in Table 1 along with the ratio, X= i,/slope.

TABLE 1 Parameters for the determination Species

Ekctrode

TTA TTA Fe(CN):Fe(CN):-

macromicromacromicro-

a Slope of current vs. timelI

of the diffusion coefficient i, /@A)

Slope “/ pA s-“~ 14.4

2.4 37 ptot (Cottreil plot).

of t~thio~throne

113 -

in diglyme

104[X= /s”2 1.7 3.3

(r,/siope)]

286

To determine D, using this equation, ferricyanide ion (FC) has been used as a reference compound for which n = 1 and D,, = 7.63 X lo-6 cm* s-l in water [18]. As a control, D,, was calculated to be 7.97 X 10e6 cm* s-l using the slope of a plot of i vs. t - ‘/* for aqueous ferricyanide at a macroscopic planar working electrode. Each X value is an average of two trials each at two concentrations (10 mM and values were used to calculate a value of 2 m M TTA). The X, and X,, (2.0 f 0.1) x 10T6 cm* s-l for the diffusion coefficient of TTA in diglyme using D ‘I-l-A =

4,

( xTTA/xFC

>”

method, which was developed by Baranski et al. [lo], does not require that the area of each electrode be known or that the same solvent be used for reference and unknown. However, the macro- and microelectrode must be the same for both solvents. The slope and D, were used to determine n = 0.98 for the first wave and n = 1.8 for both waves. Only very low concentrations (- 2 mM) of TTA gave accurate values of n since some TTA remains undissolved at higher concentrations.

This

Film deposition The CV of a solution containing TTA and Ni.*+ has two reduction waves at - 0.4 V and -1.0 V and no oxidation waves. The reduction of Ni*+ alone, on the other hand, has a reduction peak at - 1.2 V as well as a broad oxidation peak at + 0.7 V. Electrodeposition of a black solid occurred at potentials more negative than - 0.6 V vs. SCE when TTA was reduced in the presence of either Ni2+, Cu*+, or Cd’+. If an electrode coated with a thin film of this black deposit is put into an electrolyte solution (1 M NaCF,SO, in diglyme), a flat voltammetric response is exhibited between + 0.5 V and - 1.5 V with the current close to zero, indicating that the deposit is most likely a compound that incorporates the metal ion and TTA. The black films have extremely low solubility in all solvents tested, particularly polar ones such as DMF, acetone and water. RBS of thin films of cadmium-TTA on C exhibited peaks whose surface energies corresponded to Cd, S, 0, C, and Cl. A simulation spectrum of a 250 nm layer containing these elements in the ratio 1: 3: 3 : 14: 0.5, respectively, fit the observed spectrum reasonably well (Fig. 3). Differences between the two spectra at low energies may be attributed to a background signal from the carbon substrate. The presence of Cl is probably due to incorporation of perchlorate ion from the supporting electrolyte. Subtracting the oxygen due to the ClO; ion gives a Cd : S : 0 : C composition of 1: 3 : 1: 14 which is consistent with the Cd : TTA ratio of 1:l. The attainment of high quality films was correlated with a proper voltammetric response for TTA (Fig. 1). Reproducible CVs depended on a dry, deoxygenated solvent and very clean working electrode surface. High temperature (50-100° C) was found to improve the reproducibility somewhat. The ratio of metal to TTA concentrations was an important factor. RBS indicated excess TTA in poor quality films. Using [M*‘] > [TTA] was only partially successful, as this sometimes led to

287 ENERGY

/

MeV

CHANNEL

Fig. 3. Rutherford backscattering spectrum of a cadmium-TTA deposit (jagged line) superimposed on a simulated spectrum (smooth line) of the same material. Tbe simulated spectrum was calculated (see ~~~ent~) assuming (1) a 250 nm thick layer of material with stoic~omet~ Cd : TTA : CIO, = 1: 1: 0.5 and (2) a loo0 run thick layer of carbon substrate.

Fig. 4. Scanning electron micrograph of Ni(‘ITA) films. The rectangle on the left hand side ts shown on (a) Film deposited on carbon for 8 min the right hand side with increased magnification (400 X/1600X). at - 0.5 V (SCE) and 20 OC. (b) film deposited on copper for 3 mm at - 0.7 V (SCE) and 65 o C.

288

excess metal in the film. Excess metal was avoided when nitrilotriacetate ion (NTA) was used as a ligand for the metal ion because the potential for metal deposition was shifted several hundred mV negative. As a result high quality Ni(TTA) films were deposited from a DMF solution in which [NTA] : [Ni”] : [“PTA] = 10 : 10 : 1. The deposition was carried out at - 1.0 V vs. SCE at room temperature. Scanning electron micrographs of Ni(TTA) films showed that thin films (ca. 100 nm) displayed little surface structure at 400 X magnification (Fig. 4). Thicker films (ca. 500 run) had cracks that were apparent at this magnification. Charging of the film surface (indicated by bright spots) suggests a high electrical resistance of portions of the film. Rough measurements of the overall resistance of the film with indium point contacts 0.5 cm apart gave values > 20 MQ. The high resistance of the film may be responsible in part for the cracks that develop during film growth. Electrode coatings The properties

of Ni(TTA) films as electrode coatings were tested in three different ways: (a) the voltammetric response of thin films of Ni(‘ITA) on Pt was compared to bare Pt electrodes using the reversible oxidation of ferrocene in DMF; (b) the voltammetric response of thin film CdSe (in the dark and the light) in 0.5 mM K,Fe(CN), + 0.5 mM K,Fe(CN), + 1 M KC1 solution was compared to the response of CdSe coated with Ni(TTA); and (c) the open circuit photovoltage and short circuit photocurrent of coated CdSe were compared with those of uncoated CdSe in sulfide + polysulfide. For the first two experiments, the CVs showed substantially lower currents for the Ni(TTA) coated electrodes. In addition, coating the CdSe electrodes with Ni(TTA) did not prevent photocorrosion, which turned the semiconductor surface red, when the electrodes were illuminated in ferro/ ferricyanide solution during a potential scan from - 1 V to + 1 V vs. SCE at 100 mV/s. For the third experiment, even very thin coatings (less than 100 nm) had a dramatic negative effect on the open circuit voltage and short circuit current of CdSe in sulfide + polysulfide. Ni(TTA) films deposited on CdSe for 1 to 2 min decrease the short circuit photocurrent by a factor of 5 to 10 and the open circuit photovoltage by a factor of 2. The decrease in the dark and photocurrents for coated electrodes is consistent with the high resistance of the Ni(TTA) film. Since the open circuit voltage is not a function of electrode area, the decreased value for the coated CdSe electrodes suggests that the primary mechanism for electron transfer is through the Ni(TTA) film rather than via uncoated regions on the CdSe. Solid state devices

Thin film CdSe electrodeposited on Ti exhibits cracks, which are visible at high magnification. Solid state devices of the configuration Ti/CdSe/Au + Pd exhibited short circuits, presumably because the Au + Pd alloy made contact with the Ti substrate via the cracks. A very thin film of Ni(TTA) (ca. 100 nm) on a CdSe film prevented short circuits and yet allowed formation of a Schottky barrier. Thus, the current-voltage behavior in the dark was determined for a total of eight 1 mm*

-900 -1500

-1200

-900

Fig. 5. Dark current-voltage

-600

-300

0 VOLTAGElmV

300

600

relationship for a Ti/CdSe/Ni(TTA)/Au+Pd

900

1200

I! IO

solid state cell.

spots having the con~g~ation Ti/ CdSe/Ni(TTA)/ Au + Pd. These were compared with eight spots on the same CdSe film having the structure Ti/CdSe/Au + Pd. All of the latter had linear I-V behavior, which is characteristic of an ohmic contact or a short circuit. Of the eight Ni(‘lTA) coated CdSe cells, however, four exhibited diode I-T/ characteristics (Fig. 5). The lack of short circuits is consistent with Ni(TTA) deposition in the cracks as well as on the surface. In summary, chronoamperometric measurements in diglyme showed that trithioanthrone has two reversible one-electron reductions at - 0.3 V and - 0.8 V vs. SCE, and has a diffusion coefficient of 2.0 X 10T6 cm2 s-*. The use of a microelectrode to determine n and D was found to be both convenient and reproducible. For the electrodeposition of metal-trithioanthrone thin films, it appears that the use of NTA as a ligand for the metal ion may increase the reproducibility of electrodeposition and quality of the films greatly. Rutherford backscatte~g indicates a metal : TTA ratio of 1: 1 can be obtained. CdSe coated with thin Ni(TTA) films performed poorly in sulfide + polysulfide phot~l~tr~hem~cal cells and was susceptible to photocorrosion in ferri/ ferrocyanide cells. On the other hand, Ni(TTA) films could be used to prevent short circuits in CdSe solid state cells without preventing the formation of a Schottky barrier.

290

This work was supported in part by a grant to MC. from the Natural Sciences and Engineering Research Council of Cananda. We are pleased to acknowledge the many useful discussions with Professor A. Baranski and the RBS analysis performed by Dr. K. Shanker. Mr. D. Genner was very helpful in the development of the A/D software. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

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