Langmuir 1989,5, 17-22
17
Articles Angle-Resolved Infrared Spectroelectrochemistry. 1. An in Situ Study of Thin-Film Nickel Oxide Electrodes Gholamabbas Nazri,* Dennis A. Corrigan, and Shyam P. Maheswari Electrochemistry Department, General Motors Research Laboratories, Warren, Michigan 48090 Received July 7, 1987. I n Final Form: July 20, 1988 Angle-resolvedinfrared spectroelectrochemistry, a new technique based on attenuated total reflectance (ATR), allows in situ depth profiling at the electrode/electrolyte interface. Varying the angle of incidence of the infrared beam at the sensing surface of the ATR element allows various depths from the electrode surface to be sampled. This technique was used to study the structure of hydrated nickel oxide films. In the reduced state, both (Y and fl nickel hydroxides were found, indicating the presence of the (Y phase at the metal/film interface and the /3 phase at the film/solution interface. A sandwich-type structural model for the hydrated nickel oxide is proposed it has an open structure at the top surface and a more compact structure at the electrode/film interface. The spectra of the film after ita transformation into the oxidized state showed the presence of nickel oxyhydroxide. Both the reduced and oxidized forms were found to contain water in the film structure.
Introduction An understanding of electrochemical systems requires knowledge of the nature of the electrode/electrolyte interface. To this end, many high-vacuum surface techniques such as AES,ESCA, SIMS, ISS, EELS, and LEED have been applied to surface films a t electrode surfaces. However, there are difficulties with these methods due to the changes of the electrode environment and loss of electrode potential control during transference of the electrode to vacuum.ld Even without these problems, it has also been demonstrated that the vacuum and electron bombardment in these techniques can modify the structure of surface film~.'8~Nonetheless, an advantage to the use of vacuum surface techniques is that they allow depth profiling of the surface layers on electrodes by the use of ion sputtering. Ion sputtering, however, can change the composition of the surface layer due to the preferential removal of ions and atom^.^.^*"' In addition, other complications due to phenomena such as segregation, diffusion, and plasma chemistry can take place during sputtering.11J2 To avoid the above-mentioned difficulties with vacuum surface techniques, several in situ techniques have recently been developed to study the electrode/solution interface.13-18 These techniques have proven useful in ana(1) McIntyre, N. S. In Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy;Briggs, D.,Seah, M. P., Eds.; Wiley: New York, 1983. (2) Yeager, E. B.Surf. Sci. 1980, 101, 1. Faraday Trans. 1 (3) Hall, H.Y.; Sherwood, P. M. A. J. Chem. SOC., 1984,80, 135. (4) Lynch, D. W. J.Electroanal. Chem. 1983,150, 229. (5) OGrady, W. E.; Woo, M. Y. C.; Hugans, P. L.; Yeager, E. J. Vac. Sci. Technol. 1977, 14, 365. (6) Wieckowski, A.; Rosasco, S. D.;Schardt, B. C.; Stickney, J. L.; Hubbard, A. T.Inorg. Chem. 1984,23, 565. (7) Nazri, G.;Cahan, B. D.; Kuroda, K.; Yeager, E.; Mitchell, T. Electrochemical Society Meeting, Montreal, Canada, May 5, 1982. (8) Nazri, G.; Muller, R. H. J. Electrochem. SOC. 1985, 132, 2050. (9) Holloway, P. M.; Bhattacharya, R. S. SIA Surf. Interface Anal. 1981, 3, 118. (10)Strop, S.; Holm, R. J. Electron Spectrosc. Relat. Phenorn. 1979. 183, 16. (11) Strop, S. Spectrochim. Acta 1985, 408, 745. (12) Holm, R.; Strop, S. Appl. Phys. 1977, 12, 101.
0743-7463/89/2405-0017$01.50/0
lyzing the nature of the surface layer or adsorbed molecules on electrode s u r f a ~ e s . ~ItJ ~would also be useful to be able to probe the spatial distribution of materials as a function of depth in electrode surface layers. In this work, we introduce angle-resolved infrared spectroelectrochemistry, which allows in situ depth profiling of the electrode surface layers. Hydrous metal oxides are thought to form multilayer surface films, which could be studied by this technique. In particular, hydrous nickel oxide films are of interest because they have recently shown promise for electrochromic window app1i~ations.l~Electrochromic materials change color in response to change in oxidation state, which can be achieved by control of the electrode potential. Hydrous nickel oxide can be switched between a "colored" oxidized state, which absorbs light throughout the visible spectra region yielding a neutral bronze tint, and a "bleached" reduced state, which is nearly transparent in the visible region. The electrochemistry of hydrous nickel oxide has been studied extensively because of its importance to the battery industry. It is now generally accepted that several phases can be involved in the redox reactions? Q-NI(OH)~
1
+
OH-
y - N 1 0 0 H 4- H 2 0 i- e-
aging in water
P-NI(OH)~
+
OH-
I
aging at anodic potenlisl
P-NIOOH
+
H20
+
(1 )
e~-
(13) Fleischmann, M.; Hill, I. R. In Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., &.; Plenum: New York, 1982; p 275. (14) Kolb, D. M. In Surface Polaritons; Agronovich, V. M., Mills, D. L., Eds.; North Holland New York, 1982; p 299. (15) McIntyre, J. D. E. In Aduances in Electrochemistry and Electrochemical Engineering;Muller, R. H., Ed.;Wiley: New York, 1973; Vol. 9. (16) Boordman, A. D. Electromagnetic Surface Modes; Wiley: New York, 1982. (17) Pons, B. S.;Davidson, T.;Bewick, A. J.Electroanal. Chem. 1982, 140, 211. (18) Habib, M. A.; Bockris, 3. OM. Langmuir 1986, 2, 388. (19) Carpenter, M. K.;Conell, R. S.; Corrigan, D. A. Sol. Energy Mater. 1987, 16,333.
0 1989 American Chemical Society
18 Langrnuir, Vol. 5, No. 1, 1989 Incoming IR Beam (Low Angle)
Nazri et al.
Outgoing IR Beam (Low Angle) Outgoing IR Beam (High Angle)
IR Beam
(High Angle)
Electrolvte Film Substrate
h
i
F i g u r e 1. Schematic diagram showing the principle of angleresolved IR spectroscopy; the penetration depth is greater with lower angle of incidence 4 than with higher angle of incidence oh.
11'1
These phase transformations are important to aging processes both for battery applications and for electrochromic applications. In this work, the angle-resolved IR spectroelectrochemistry was used to follow these transformations and monitor them spatially.
Experimental Section
Theory The infrared beam is directed at an ATR element, which probes the electrode surface as shown in Figure 1. Briefly, various depths into a film at the electrode surface can be sampled by changing the angle of incidence, which changes the penetration depth of the evanescent wave. When electromagnetic radiation impinges on the boundary of two phases (with different refractive indices), it will be totally reflected when the angle of incidence is greater than the critical angle. When the total reflection occurs, an evanescent wave is set up in the reflecting phase in which intensity decays exponentially in the direction normal to the interface. According to Hansen21i22and Harri~k~~~~~
E = E, exp(-z/d,)
(2)
where Eois the electric field intensity a t the interface, z is the distance from the interface, and d, is a parameter called the penetration depth. The penetration depth is the distance normal to the interface where the electric field intensity drops to l / e of the value Eoat the interface. The value of d, has been calculated by using Gauss and Fresnel's equations and can be written as22-25 d, = (A/2r)(sin2 8 - n21
IR Beam Figure 2. In situ spectroelectrochemical cell showing working electrode (WE) probed by ATR element, counter electrode (CE), and reference electrode (RE).
1
(3)
where 6 is the angle of incidence of the infrared beam and n21is the ratio of the refractive index of the reflecting phase to that of the incident phase. This equation, which shows the dependence of the penetration depth on the angle of incidence, is the key to this technique. Thus, by varying the angle of incidence, it should be possible to vary the sampling depth in phases next to the ATR element. (20)Oliva, P.; Leonardi, J.; Laurent, J. F.; Delmas, C.; Braconnier,J. J.; Figlarz, M.; Fievet, F.; deGuibert, A. J. Power Sources 1982,8, 229. (21)Hansen, W. N. In Advances in Electrochemistry and Electrochemical Engineering; Muller, R. H., Ed.; Wiley: New York, 1973;Vol. 9. (22)Haneen, W. N. Spectrochim.Acta 1965,21, 815. (23)Harrick, N. J. Internal Reflection Spectroscopy; Harrick Scientific Corp.: Ossining, NY, 1979. (24)Harrick, N. J. J. Opt. SOC. Am. 1965, 55, 851. (25)Lampert, C. M.; Omstead, T. R.; Yu, P. C. Proceedings of SPIE-International Society for Optical Engineering; Lampert, C. M., Ed.; SPIE-International Society for Optical Engineering: Bellingham, WA, 1985;Vol. 562,p 15.
Analytical grade chemicals were used for solution preparation. Instrumentation for electrochemical experiments included a Princeton Applied Research Model 273 potentiostat, a HI-TECH wave generator, and an Esterline Angus X-Y-T recorder. Nickel oxide thin films were deposited on a nickel disk working electrode with a diameter of 1.5 cm by an anodic depositidn procedure described in detail e l s e ~ h e r e .Briefly, ~~ films were formed by potential sweep cycling in a stirred solution prepared by mixing 20 mL of NiS04 and 20 mL of NH40H. Here, to produce films about 100 nm thick, 10 cycles between 0 and 1.5 V vs SCE at 5 mV/s were used. After deposition, the nickel oxide thin-film working electrode was rinsed and transferred to the spectroelectrochemical cell with 1 M KOH electrolyte. This cell, shown in Figure 2, allows positioning of the working electrode adjacent to the ATR element. A platinum wire was used as the counter electrode. The a-Pd-H electrode as the reference was positioned to minimize ohmic drops. The ATR element was ZnSe (from Harrick Scientific). An IBM Model 98 Fourier transform infrared spectrometer was used for the spectroscopic measurements. Spectra were obtained in a nitrogen-purged environment. Each spectrum was comprised of 512 scans with a resolution of 4 cm-l. Collection time for spectra a t each angle of incidence was about 20 min. To obtain spectra of the nickel oxide thin films, the working electrode was pushed flush to the ZnSe ATR element. The distance between the electrode and the cell window was about 1 pm. This distance was maintained by placing three to five carbon fibers (1-wm diameter) between the cell window and the electrode surface. First, spectra were obtained in the reduced state. The electrode potential was held at 0.1 V vs W E , and spectra were obtained a t increasing angles of incidence (45', 60°, 70°, Eo, 88'). Then, the film was oxidized by sweeping the potential to 0.5 V vs SCE at 5 mV/s. Spectra of the oxidized phase were obtained at decreasing angles of incidence @8', 85', 70°, 60°, 45'). A spectrum of a nickel working electrode (in the spectroelectrochemical cell) with no deposited film was used as a reference for the in situ measurements. For standards, bulk /3 nickel hydroxide WEE obtained as reagent grade nickel hydroxide from Alfa Products. It was identified as /3 nickel hydroxide by X-ray diffraction (XRD). Other samples were prepared as described below. T o prepare CY nickel hydroxide, 8 mL of 15 M NH40H (about a fivefold excess) was added dropwise to stirred solution of 25 mL of 1M NiSO1. After the solution was kept standing for 5 days, the green nickel hydroxide precipitate was filtered and rinsed with 500 mL of distilled water. XRD confirmed the presence of CY nickel hydroxide. T o prepare /3 nickel oxyhydroxide, 1.5 mL of bromine (a threefold excess) in 75 mL of 1 M KOH was added to a stirred suspension of 1.85 g of /3 nickel hydroxide in 25 mL of distilled
Angle-Resolved Infrared Spectroelectrochemistry
Langmuir, Vol. 5, No. I, 1989 19
f
Coloration
0.6 0.4 0.2 -
3 -
700
0-
0.2 -
0.4 -
Bleaching
-0.2
0
0.2
0.4
0.6
0.8
E (V) vs SCE
Figure 3. Cyclic voltammogram of anodically deposited nickel oxide in 1 M KOH electrolyte at a sweep rate of 50 mV/s.
Wave Number (cm.1)
Figure 5. In situ infrared spectra of nickel oxide thin film in the reduced state (0.1 V vs SCE in 1 M KOH) as a function of angle of incidence in the 0-H stretch region.
E
'E I E
Results and Discussion
e
t-
'80
660 540 Wave Number (cm-4)
420
Figure 4. In situ infrared spectra of nickel oxide thin film in the reduced state (0.1 V vs SCE in 1 M KOH) as a function of
angle of incidence in the Ni-O stretch region. Solid and dashed arrows indicate bands for a and j3 nickel hydroxide, respectively. water. This reaction mixture was stirred for 6 h and then allowed to set for 1 week. The black precipitate was then filtered and rinsed 5 times with 40 mL of 1 M KOH. (Rinsing with water decomposed the sample rapidly.) The sample was then vacuum dried in a desiccator. XRD confirmed the presence of j3 nickel oxyhydroxide. To prepare y nickel oxyhydroxide,7 g of K2S208(a threefold excess) was added to a stirred suspension of 1.85 g of @ nickel hydroxide and stirred for 6 h. After standing for 1week, the black precipitate was filtered and rinsed 5 times with 40 mL of 1 M KOH. XRD confirmed the presence of y nickel oxyhydroxide. However, XRD lines for j3 nickel hydroxide were also present. Bulk powder samples of the nickel hydroxides and oxyhydroxides were diluted with about 20 parts KBr and pressed into pellets. Infrared spectra of these bulk samples were obtained by using the transmission mode.
The oxidation (coloration) and the reduction (bleaching) of the film were performed in the 1 M KOH electrolyte. Redox processes associated with coloration and bleaching are indicated by peaks in the cyclic voltammogram of Figure 3. Coloration occurred during the oxidation peak at 0.42 V, and bleaching occurred during the reduction peak at 0.25 V. These results taken in the spectroelectrochemical cell closely resemble previous results.25 The infrared spectra of the reduced state of the film at various angles of incidence are shown in Figures 4 and 5. These results are interpreted by comparison to spectra of bulk a and p nickel hydroxide powders given in Figures 6 and 7. In the Ni-0 stretch region (Figure 4), the infrared peaks are around 680 cm-' and are thus assigned to the a nickel hydroxide, and bands around 513 and 457 cm-l are assigned to p nickel hydroxide. The p nickel hydroxide bands at 513 and 457 cm-' only appeared at the highest angles of incidence, 85O and 88O. According to theory, the shallowest penetration depths result from these high angles of incidence. Therefore, the p nickel hydroxide observed at these high angles can be located closest to the ATR element, i.e., at the outer film surface. Note also that at all angles of incidence the a nickel hydroxide bands at 680 cm-l predominate this spectral region. This suggests that most of the reduced film is present in the a nickel hydroxide form. In the 0-H stretch region (Figure 5), spectra of the film in the reduced state show a sharp 0-H band at around 3630 cm-' and a broad band for bonded water around 3400 cm-'. By comparison to standard spectra in Figure 7, we see that the sharp band at 3630 cm-' is characteristic of 0 nickel hydroxide and the broad 3400-cm-l band is
20 Langmuir, Vol. 5, No. 1, 1989
720
800
Nazri et al.
560
640
480
400
Wave Number (cm-')
Figure 6. Infrared spectra of a and @ nickel hydroxide powders in the Ni-0 stretch region (transmission mode).
"7 v, I
30
-
660
,
I
,
540
I
420
Wave Number (cm-1) I"
pNi(0H)z
Figure 8. In situ infrared spectra of nickel oxide thin film in the oxidized state (0.5 V vs SCE in 1 M KOH)as a function of angle of incidence in the Ni-0 stretch region.
Figure 7. Infrared spectra of a and ,9 nickel hydroxide powders in the 0-H stretch region (transmission mode).
highest angles of incidence. This is also evidence for /3 nickel hydroxide at the outer surface of the film, although interference from solvent absorption may occur in this spectral region. The broad 3400-cm-' band observed in a nickel hydroxide is indicative of hydrogen-bonded water in the film structure. The small sharp bands above 3630 cm-' are similar to those observed for water in the gas phase.26 Thus, this may be evidence for free water (without hydrogen bonding) trapped in the film or on the film surface. The most important result for the reduced films is the angular dependence of the spectra. This work provides evidence for a sandwich-type structure with a layer of a nickel hydroxide in the inner part of the film closest to the nickel substrate and /3 nickel hydroxide in the outer part of the film next to the electrolyte. It is known that a nickel hydroxide is formed by electrochemical deposition procedures.m Furthermore, it is known that, by aging these films in aqueous electrolytes, the a nickel hydroxide is transformed into /3 nickel hydroxide. Since the electrolyte participates in this transformation, it is reasonable to expect that the /3 phase forms first in the outer parts of the film next to the electrolyte. Thus, this work may provide evidence for the first time as to how this important transformation occurs spatially. The infrared spectra of the oxidized state of the film are shown in Figures 8 and 9. By comparison to reference spectra for y and /3 nickel oxyhydroxides in Figures 10 and
characteristic of a nickel hydroxide. Relative to the broad 3400-cm-' band, the 3630-cm-' band was strongest at the
(26) Fox, J. J.; Martin, A. E.Proc. R. SOC.(London) 1940, A174,234.
I
/-
a-Ni(OH)a
I
i
I
I
I
3400
I :
I I
I
I
I
I
I
I 1
'
4000
1-
I
\\
I
3600
f
I
I
I
f
3200 Wave Number (cm-')
2800
Langmuir, Vol.5, No. 1, 1989 21
Angle-Resolved Infrared Spectroelectrochemistry
T-NiOOH
800
720
640 560 480 Wave Number (cm-')
400
Figure 10. Infrared spectra of the /3 and y nickel oxyhydroxide powders in the Ni-0 stretch region (transmission mode).
4000
3600 3200 Wave Number (cm-1)
Figure 9. In situ infrared spectra of nickel oxide thin film in the oxidized state (0.5 V vs SCE in 1 M KOH) as a function of angle of incidence in the 0-H stretch region. 11, the infrared peak around 580 cm-' is seen to be characteristic of both y and 0 oxyhydroxide. At high angles of incidence, bands grow in a t around 480 and 580 cm-l. The 480-cm-' band may be an indication of 0nickel oxyhydroxide forming at the film outer surface (see Figure 10). Alternatively, it may be due to 0nickel hydroxide, which also has bands in this region. In the 0-H stretch region shown in Figure 9, bands are shown for bound water around 3400 cm-'. This is expected, particularly for y nickel oxyhydroxide. (However, solvent absorption may interfere with this result.) It is interesting that again bands appear which could indicate free (not hydrogen-bonded) water molecules. These are seen most clearly at high angles of incidence (see bands around 3800 cm-l in Figure 9). On the basis of the angle-resolved IR spectra, we can propose a structural model for the electrochromic nickel oxide film as it is undergoing electrochemical aging. Our model is illustrated by the schematic diagram of Figure 12. The spectra of the reduced phase show a nickel hydroxide as the predominant phase, which we show as a compact layer next to the substrate surface. We see the transformation into 0 nickel hydroxide occurring in an open surface layer adjacent to the electrolyte. This irregular layer would contain trapped water and electrolyte sites as well as 0nickel hydroxide crystallites (not shown in Figure 12). This is supported by the results showing 0nickel hydroxide as well as free water located near the surface of the film.
0
r
E e L
t
4000
3800
3600
3400
3200
3000
I
I
2800
2600
Wave Number (cm-')
Figure 11. Infrared spectra of the B and y nickel oxyhydroxide powders in the 0-H stretch region (transmission mode).
The spectra of the oxidized state are also consistent with the proposed structure. While the infrared spectra of the
Langmuir 1989, 5 , 22-26
22
observe these multiple layes directly by conventional vacuum surface techniques. Burke and co-workers have found electrochemical evidence for the formation of multiple-layer oxide films on the surface of iridium, rhodium, and gold as well as nickel.2g30 However, likewise, direct physical evidence for these layers is lacking. The angle-resolved IR spectroelectrochemistry provides an opportunity to obtain physical measurements of multiple-layer films in these difficult yet important electrochemical systems. No attempt has been made in this work to calculate the relative thickness of each layer. The quantitative measurements of the optical density of each layer require knowledge of optical constants of the materials in the film.
Metal Substrate
Compact
Open Surface Layer
Layer
NiOOH
HZO
Anion
Figure 12. Schematic diagram showing sandwich structure of hydrated nickel oxide film. two oxidized phases are very similar, there is some evidence for the presence of nickel oxyhydroxide a t the outer surface. This is expected from eq 1 since the oxidation of p nickel hydroxide in the outer layer of the film should yield p nickel oxyhydroxide. Accordingly, the bulk of the film in the compact inner layer would be y nickel oxyhydroxide formed by oxidation of a nickel hydroxide in part of the film. Of more general interest is the application of the angle-resolved IR spectroscopy to other hydrated metal oxide films. Nazri, Yeager, and Cahan have found electrochemical evidence for the formation of multiple layers on hydrous iron oxide films.27 However, it was not possible to (27) Nazri, G.;Yeager, E.; Cahan, B. D. Gou. Rep. AD-A116422,1982.
Conclusions Angle-resolved infrared spectroelectrochemistry was developed and successfully used for in situ depth profiling at the electrode/electrolyte interface. The results of this work showed that the electrochromic nickel oxide in the oxidized (colored) state has an oxyhydroxide structure and in the reduced (bleached) state contains both and a phases of nickel hydroxides. A sandwich structure is evident, particularly in the reduced state with a nickel hydroxide in the inner part of the film near the nickel substrate and p nickel hydroxide in the outer part of the film near the electrolyte interface. It is expected that this technique can also be used to probe multiple layers in other metal oxide films. Acknowledgment. We thank Drs. D. M. MacArthur and M. K. Carpenter for helpful discussions, R. S. Conell for preparation of the bulk standard samples, and Dr. J. L. Johnson for XRD characterization of the bulk standards. Registry No. NiOOH, 12026-04-9; Ni(OH)2, 12054-48-7; ",OH, 1336-21-6;NiS04, 7786-81-4; K2S208,7727-21-1; Br2, 7726-95-6; KOH, 1310-58-3;nickel oxide, 11099-02-8;hydrated nickel oxide, 12627-60-0. (28) Burke, L. D.; Twomey, T. A. M. J.Electroanal. Chem. 1982,134, 353. (29) Burke, L. D.; OSullivan, E.J. M.J. Electroanal. Chem. 1981,117, 144. (30) Burke, L. D.; Hopkins, G. P. J. Appl. Electrochem. 1984,14,679.
Photoelectrochemistry in Particulate Systems. 11. Reduction of Phenosafranin Dye in Colloidal Ti02 and CdS Suspensions K. R. Gopidas and Prashant V. Kamat* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received June 8, 1988 The photoelectrochemicalreduction of an azine dye has been carried out at Ti02and CdS semiconductor colloids under band-gap excitation. A laser flash photolysis technique has been employed to characterize the transients formed after the laser pulse excitation and to elucidate the mechanism of the interfacial charge-transfer process in these colloidal semiconductor systems. The formation of the radical anion of phenosafranin confirmed the interfacial charge transfer to be a one-electron reduction process. The quantum yield for the reduction of phenosafranin was found to be 0.05 in colloidal TiOz and 0.02 in colloidal CdS suspensions. Steady-state photolysis of TiOz colloids containing phenosafranin, which led to the formation of the leuco dye, is also described. Introduction In recent years a considerable interest has been shown in employing semiconductor particles as photocatalysts to
carry out the chemical transformations of organic and inorganic compounds in aqueous and nonaqueous media (see, for example, ref 1). Under band-gap excitation, the
0743-746318912405-0022$01.50/0 0 1989 American Chemical Society
