Langmuir 1993,9, 3291-3297
3291
Surface Modification of n-MoS2 Electrodes with a Viologen Based Redox Polymer: Persistent Attachment of a Polysiloxane via a Thin Sn02 Adhesion Layer Jim Huang and Mark S. Wrighton' Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received February 16, 1993. I n Final Form: September 13,1993@ A procedure for surface modification of n-MoSz electrodes with a polymer formed from hydrolysis of N~'-bis[p-(trimethoxysilyl)benzyl]-4,4'-bipyridinium,BPQ2+,is described. (BPQ2+), is persistently confined onto the surface of n-MoSz crystalsin a sandwich structure, n-MoSz/SnOz/(BPQ2+),.The n-MoSz crystal is first modified by electrochemical deposition of small islands of tin, followed by oxidation to give islands of SnO2. The (BPQ2+),is confined onto the resulting surface via standard procedures for this polysiloxane system: electrochemical reduction of the viologen in aqueous electrolyte. The SnOz at the interface between n-MoSz crystal and (BPQ2+),functions as an adhesive layer to bind the polymer to the MoSz. Without SnOz, the attachment of the coating of (BPQ2+)non MoSz is not durable. Although a coating of SnOz shifts the flat band potential of the electrode -0.1 V negatively (depending on pH and SnO2coverage)the essential behavior of the semicondudorlelectrolyteinterface is not significantlyaltered. (BPQ2+), electrostatically incorporates a high concentration of I- from dilute (0.005 M) aqueous concentration, even when competing with 0.05 M KC1. The photooxidation of the highly concentrated I- at the (BPQ2+),modified n-MoS2 electrode surface suppresses anodic photocorrosionin aqueous solution.
We wish to report the surface modification of n-MoS2 semiconductor electrodes by a polycation formed from hydrolysis of I, NJV'-bis[p-(trimethoxysilyl)benzyl]-4,4'bipyridinium, BPQ2+. Such modification is shown to be useful in protecting n-MoSz electrodes from anodic photocorrosion in aqueous solutions containing a low concentration (0.005 M) of I-.
degree of inherent photostability, because the low energy electronic transitions in these crystals involve only nonbonding d-orbital electrons.2 However, like all n-type semiconductor electrodes investigated to date: group 6 metal dichalcogenide electrodes are thermodynamically unstable toward anodic photocorrosion.2a In aqueous solution, the photoconversion of n-MoS2 electrodes by photogenerated holes (h+)involvesthe reaction of n-MoSz crystals with HzO, eq 1. In a photoelectrochemical cell, MoS,
Crystalline MoS2 belongs to a class of transition-metal dichalcogenides, which are layers of two-dimensional crystals. This class of materials has been investigated as potentially useful electrode materials in rechargeable batteries and solar energy conversion devices.lT2 Group 6 transition-metal dichalcogenides are interesting in terms of potential applications in solar energy conversiondevices, because these crystals have an energy gap that is well matched to the solar spectrum. Photoelectrodes made from these crystals have been used to demonstrate photoelectrochemical cells having high efficienciesfor solar energy c o n v e r ~ i o n .In ~ ~addition, these crystals have a
* Address correspondence to this author. e Abstradpublishedin Advance ACSAbstracts, October 15,1993.
(1)Scrosati, B. Stud. Phys. Theor. Chem. 1988,55 (Semiconductor Electrodes), 457. (2) (a) Tributsch, H.; Bennett, J. C. A. J.Electroanol. Chem.Znterjacial Electrochem. 1977,81,97. (b) Tributach, H. J.Electrochem. SOC.1978, 125,1086. (c)Tributach,H. Sol.EnergyMater. 1979,1,705. (d) Tributach, H. J. Electrochem. Soc. 1981, 128, 1261. (3) (a) Parkinson, B. A.; Furtak, T. E.; Canfield, D.; Kam, K.; Kilne, G.FaradayDiscws. Chem. SOC.1980,70,233. (b) Canfield,D.; Parkinson, B. A. J. Am. Chem. SOC.1981,103,1279. (4) (a) Fan, F.-R. F.; White,H. S.; Wheeler, B. L.; Bard, A. J. J. Am. Chem. SOC.1980,102,5142. (b) White,H. S.; Fan, F.-R. F.; Bard, A. J. J.Electrochem. SOC.1981,128, 1045. (5) (a) Baglio, J. A.; Calabrese, G . S.; Kamieniccki, E.; Kershaw, C. P.; Ricco, A. J.; Wold, A.; Wrighton, M. S.; Zoski, A. J. J.Electrochem. SOC. 1982,129,1461. (b) Simon, R. A.; Ricco, A. J.; Harrison, D. J.; Wrighton, M. S. J.Phys. Chem. 1983,87,4446. (c)Baglio, J. A.; Calabbrese, G. S.; Harrison, D. J.; Kamienicki, E.; Ricco, A. J.; Wrighton, M. S.; k k i , G. P. J. Am. Chem. SOC.1983,105, 2246.
-
+ 18h' + 12Hz0
Moot-
+ 2SO;- + 24H+ (1)
the durability of a semiconductor electrode is determined by the competitive rates of all the thermodynamicallypossible photoreaction processes. The strategy to protect a semiconductor electrode is to promote the desired photoelectrochemical reaction and suppress the anodic photocorrosion. For example, in 15 M LiCl aqueous solution, n-WS2 electrodes can be used for sustained photoelectrochemical oxidation of Cl-.5a The oxidation of a high concentration of C1- enhances the desired photoprocess and the low H2O activity suppresses photocorrosion. In our present study, the electrostatic concentration of anions at (BPQ2+),-modified electrodes7*is explored to protect n-MoSz electrodes against photocorrosion. In a dilute I- electrolyte, a high concentration of I- is present at the surface of (BPQ2+),-modified n-MoSz electrodes, and, at the same time, H2O activity at the modified electrode surface is reduced. The desired photooxidation of I- at (BPQ2+),-modified electrodes suppresses the photocorrosion of n-MoSz. Our resulta with I- are similar to those obtained for stabilizing n-Si based photoelectrochemical cells using Fe(CN)& electrostatically bound in polyviologen.* (6) Bard, A. J.; Wrighton, M. S. J.Electrochem. SOC.1977,124,1706. (7) (a) Hable, C. T.; Crooks, R.; Wrighton, M. S. J.Phys. Chem. 1989, 93,1190. (b) Harrison, D. J.; Wrighton, M. S. J. Phys. Chem. 1984,88, 3932. (c) Lewis, N. S.; Wrighton, M. S. J.Phys. Chem. 1984,88,2009. (d) Dominey, R. N.; Lewis, T. J.; Wrighton, M. S. J. Phys. Chem. 1983, 87, 5354. (e) Dominey, R. N.; Thomas, J. L.; Wrighton, M. S. J. Phys. Chem. 1983,87,5354.
0743-746319312409-3291$04.00/0 0 1993 American Chemical Society
3292 Langmuir, Vol. 9,No. 11, 1993
Scheme I
(BPQ2+),has been confined onto a variety of electrode surfaces in this lab~ratory.~ The BPQ2+monomer is easily polymerized via hydrolysis of the -Si(OMe)s groups. One major difficulty in modifying the n-MoS2 surface with (BPQ2+),is that the van der Waals surface (the surface perpendicular to the c-axis of the crystal) has no oxide functionality to bind the polysiloxane. (BPQ2+),would have to be confined to the electrode surface via van der Waals forces, which we find not to be strong enough to hold the polymer to the electrode surface securely. We have developed a scheme to confine (BPQ2+),onto the n-MoS2 electrode surface using islands of SnO2 as an adhesion promoter, Scheme I. SnO2 is chosen because it is reasonably conductive and (BPQ2+),can be anchored on the Sn027evia the Si(OMe)s functional group. The amount of SnOZ coating is small, so that the essential characteristics of the n-MoS2 semicondudor are not altered at the semiconductor/electrolyteinterface. Experimental Section Electrode Preparation. Naturally occurring single crystals of n-MoS2 were purchased from Climax Molybdenum Co. (Greenwich,CT). Flat crystals of n-MoS2 were peeled apart using fine tweezers and a razor blade to give paper-thin crystals. The areas of peeled crystals were 0.2-0.01 cm2. The peeled crystal was mounted on a copper plate which was connected to a copper wire. The copper wire was put inside a piece of Pyrex tubing. Ohmic contact between a MoS2 crystal and a copper plate was obtained by rubbing one side of the peeled MoS2 crystal with Ga-In eutectic. The crystal was then attached to the copper plate with silver epoxy and cured in an oven at 100 “C for 2 h. The electrodes were completely sealed with white epoxy, leaving only the van der Waals n-MoS2 surface exposed. The electrodes were cleaned by first immersing them in boiling acetone for 1h, then rinsing with distilled H20, immersing in 2 M HCl for 2 min, and finally rinsing again with distilled H20. The electrodes were stored under vacuum. Each electrode was characterized by cyclic voltammetry in CH&N/O.l M [(n-Bu)4N]BF4 electrolyte containing 0.5 mM Nfl,”,W-tetramethyl-p-phenylenediamine (TMPD) in the dark and under illumination. A representative cyclic voltammogram of a well-behaved electrode is shown in Figure 3b. Platinum electrodes were made from platinum disks (0.1 cm2)and were subject to standard pretreatment before Chemicals. TMPD from commercialsources was purified by sublimation. HPLC grade CH3CN was dried by distillation from P205. [(n-Bu)dN)]BF4was dried at 80 “C under vacuum for 24 h prior to use. &Mo(CN)s and BPQ2+C12were available from previous studies.7e All other chemicals were available commercially and were used without further purification. OMNISOLV H20 was used as supplied. Equipment. The reference electrodes were either a Ag/O.Ol M AgNOd0.1 M [(n-Bu)4N]BFJCH&N electrode (for CH3CN solution) or a saturated calomel electrode (for aqueous solution). The counter electrode was a Pt disk (1cm2). Cyclic voltammograms were obtained with either a Pine Instrument Mode RDE4 potentiostat or a PAR Model 173potentiostat driven with a PAR Model 175universal programmer. Data were recorded on either a Houston Instruments Model 2000 X-Y recorder or a Kipp & Zonen BD91 x-y recorder. A digital coulometer was used to (8)Rosenblum, M. D.; Lewis, N. S.J. Phys. Chem. 1984,88, 3103.
Huang and Wrighton monitor the charge passing through the cell. Light sources were either a beam expanded Spectra Physics Model 164 Ar laser a t 514 nm or a beam expanded Coherent Radiation H e N e laser at 632.8 nm. The light intensity of the Ar laser was controlled with a photographicpolarizing filter. The light intensity was measured by either a calibrated Solatex Corp. photometer or a Tektronix J16 digital radiometer equipped with a 56502 probe. Capacitance measurements were made in the dark by employing a PAR model 5204 lock-in analyzer with an internal oscillator which generated a 5-mV peak-peak ac signal at frequencies between 1 and 10 kHz. Calibration was made by using an externally constructed RC circuit. The voltage sweep rate was 1mV/s. Buffered solutions of 0.2 M KCl were used to measure the pH dependence of the flat band potentials: borate (pH 9); phosphate (pH 7); phthalate (pH 5 and 3) and pH 1was achieved with HC1. Auger electron spectra and depth profiles were obtained using a Physical Electronics Model 590A scanning Auger spectrometer. A 5-keV electron beam with a beam current of 0.1-1 pA was used as the excitation source. A Physical Electronics Model 04-303 differential ion gun was used to produce a 2-keV Ar+ ion beam for sputtering. XPS spectra were obtained using a Physical Electronics Model 548ESCA/Augerspectrometer. A 7-V neutralization bias voltage was used throughout the experiments. Au was used as an internal energy reference. Samples were partially coated with Au in a NRC 3117 6-in. bell jar/coater system. A Hitachi Model S-800 scanning electron microscope (SEM) was used to obtain scanning electron microscope pictures. General Electrochemical Procedures. All electrochemical experiments were carried out in a single compartment cell under N2 atmosphere. Solutionswere thoroughly purged with N2 before experiments. Confining of the polymer was carried out by the standard technique of cyclic voltammetry at 50 mV/s?e The stability test of the electrode functionalized with the polymer was done by cycling repeatedly at 50 mV/s the potential of the electrode between the voltages correspondingto the reduced form of the polymer and the oxidized form of the polymer. Coverage of the polymer was determined by integration of the cathodic peak of the cyclic voltammetry waves at a sweep rate of 5 mV/s. The polymer-modified electrodes were dried according to the following procedure: After the electrodes were taken out of the test solution, they were immediately immersed in deoxygenated H20 for 5 min, then rinsed with deoxygenated H20, dried under N2, and stored under vacuum. Deposition of SnO2 on MoS2 Electrodes. Deposition of SnO2 on MoS2 electrodes was carried out by electrodeposition of Sn and then oxidation of the Sn in an oven at 100 OC for 48 h. The plating solution was an aqueous solution of 0.1 M KCl containing 5 mM SnC12. The electrolyte pH was adjusted to 1 by adding HC1and NaOH. During the electrodepositionprocess, the voltage was constantly adjusted between -0.8 and -0.5 V vs SCE in order to keep the current density around 0.2 mA/cm2. The quantity of Sn deposited on the electrodes was monitored by a digital coulometer (typically -5 X 1VC/cm2as an adhesion layer). After oven oxidation, the electrodes were placed in 0.1 M KCl aqueous solution. The electrode potential was held at 0.1 V vs SCE for 5 min to remove any unoxidized Sn.
Results and Discussion a. AttemptedDerivatization of Naked n-MoS2with (BPQ2+)11. Derivatization of the n-MoS2 electrode surface with (BPQ2+),is carried out by cycling the potential of the electrode between 0 and -0.7 V us SCE in an aqueous solution containing the BPQ2+ The growth of the polymer on the electrode is evident from the growth of the current due to the surface confined (BPQ2+),. The modified n-MoS2 electrodes show cyclic voltammograms typical of (BPQ2+),-modified metal electrodes, because the flat band potential of n-MoS2 (0.3 V us SCE) is more positive than the redox potential of (BPQ2+),. Thus, the redox potential of the redox polymer is in the conduction band of MoS2, and MoS2 behaves as a metal toward (BPQ2+),.
Langmuir, Vol. 9, No.11,1993 3293
Surface Modification of n-MoSz Electrodes
I
I
a. S u r v e y
0 I-
z w
(z (z
-000
3 V
-600 -400 -200 BINDING ENERGY, eV
a. Beginning
U
-0.0
-0.6 - 0 . 4 -0.2 0 P O T E N T I A L , V vs. SCE
Figure 1. Cyclic voltammetry for a n-MoSz/(BPQ2+).electrode in 1 M KCl electrolyte at a sweep rate of 50 mV/s: (a) at the beginning of the cyclic voltammetry stability test; (b) after 15 h of the stability test.
The durability of the polymer coatingwas tested in three ways: peal tests, durability during cyclic voltammetry, and comparison of coverage before and after drying. The polymer coating can be easily peeled off by stickinga piece of Scotch adhesive tape to the polymer. Sometimes, isolated small pieces of MoS2 are also peeled off with the polymer. In cyclic voltammetry tests,where the electrode potential is cycled between 0 and -0.75 V us SCE for 15 h, the peak current typically decreases by 30-50'3, Figure 1. A comparison of viologen coverage by cyclic voltammetry before and after drying shows that the coverage typically decreases by more than 50 9% after drying. These results indicate failure of the polymer to adhere persistently to the n-MoS2 crystal surface. However, (BPQ2+)n is known to be durable on other electrode surfaces, including b. SnO2-Coated n-MoSz. The instability of the polymer coating on the naked MoS2 crystal surface leads us to the idea of employing SnO2 as an adhesion promoter between the semiconductor electrode surface and (BPQ2+)n. SnO2 can be coated onto n-MoS2 electrodes as described in the Experimental Section. From SEM characterization of several SnO2 coated electrodes, the SnO2 coating is in the form of islands with a nonuniform size distribution. Typically, islands of Sn of
