944
Langmuir 1998, 14, 944-950
In Situ Raman Studies on Cathodically Deposited Nickel Hydroxide Films and Electroless Ni-P Electrodes in 1 M KOH Solution Yi Ling Lo and Bing Joe Hwang* Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, 106, Taiwan, Republic of China Received January 9, 1996. In Final Form: December 4, 1997 Three types of working electrodes, as-deposited nickel hydroxide, deactivated nickel hydroxide, and electroless Ni-P electrode, are studied in this work. The surface state of the electrodes varying with potential are investigated using cyclic voltammetry techniques combined with in situ Raman spectroscopy in 1 M KOH solution. As-deposited nickel hydroxide is electrooxidized to the oxides of higher state with Raman characteristic bands, 475 and 55 cm-1, and the corresponding reaction is assigned to be a R-Ni(OH)2/γ-NiOOH redox reaction in the potential range between 350 and 500 mV. When the film is deactivated by voltammetric cycling, the phases, β-Ni(OH)2, and some untransformed R-Ni(OH)2 are present. β-Ni(OH)2 will be electrooxidized to β-NiOOH, which has Raman characteristic bands of 480 and 560 cm-1 in the anodic sweep between 350 and 450 mV. The redox pair on the electroless Ni-P electrode is recognized as β-Ni(OH)2/β-NiOOH because of the identical Raman bands with β-NiOOH, namely, 480 and 560 cm-1, after being electrooxidized.
Introduction Nickel electrodes has been applied to electrochromic devices,1,2 alkaline batteries,3 and electrocatalyst.4,5 Most of the applications are on the basis of the redox pair, Ni(OH)2/NiOOH. Previous literature reveal that the properties of nickel electrodes depend on the structure of nickel hydroxides which relates closely to the preparation conditions.6 Two main structures, crystallized β-Ni(OH)2 as well as amorphous R-Ni(OH)2, are identified by applying the techniques such as X-ray diffraction (XRD),7 extended X-ray absorption fine structure (EXAFS) analysis,8 and X-ray adsorption near-edge structures (XANES) analysis,9,10 etc. These studies focus on identifying the structure of the redox pair and determining the structure parameters. However, the structural changes under applying potentials are not well-understood. Raman spectroscopy is a powerful technique for distinguishing the structural difference between the redox pair.11-15 Densilvestro and co-workers14,15 have characterized the redox states of a * To whom all correspondence should be addressed. (1) Lampert, C. M.; Omstead, T. R.; Yu, P. C. Solar Power Mater. 1986, 14, 161. (2) Fantini, M.; Gorenstein, A. Solar Power Mat. 1987, 16, 487. (3) Karnath, P. V.; Dixit, M.; Indira, L.; Shukla, A. K.; Kumar, V. G.; Munichandraiah, N. J. Electrochem. Soc. 1994, 141, 2956. (4) Amjad, M.; Pletcher, D.; Smith, C. J. Electrochem. Soc. 1977, 124, 203. (5) Lu, P. W. T.; Srinivasan, S. J. Electrochem. Soc. 1978, 125, 1416. (6) Hopper, M. A.; Ord, J. L. J. Electrochem. Soc. 1973, 120, 183. (7) Oliva, P.; Leonardi, J.; Laurent, J. F. J. Power Sources 1982, 8, 229. (8) Pandya, K. I.; O’Grady, W. E.; Corrigan, D. A.; McBreen, J.; Hoffman, R. W. J. Phys. Chem. 1990, 94, 21. (9) McBreen, J.; O’Grady, W. E.; Tourillon, G.; Dartyge, E.; Fontaine, A.; Pandya, K. I. J. Phys. Chem. 1989, 93, 6308. (10) Pandya, K. I.; Hoffman, R. W.; McBreen, J.; O’Grady, W. E. J. Electrochem. Soc. 1990, 137, 383. (11) Cornilsen, B. C.; Karjala, P. J.; Loyselle, P. L. J. Power Sources 1988, 22, 351. (12) Melendres, C. A.; Xu, S. J. Electrochem. Soc. 1984, 131, 2239. (13) Johnston, C.; Graves, P. R. Appl. Spectrosc. 1990, 44, 105. (14) Densilvestro, J.; Corrigan, D. A.; Weaver, M. J. J. Phys. Chem. 1986, 90, 6408. (15) Densilvestro, J.; Corrigan, D. A.; Weaver, M. J. J. Electrochem. Soc. 1988, 135, 885.
nickel hydroxide film by in situ surface enhanced Raman spectra (SERS). In their investigations, emphasis is placed on the film thickness effect to clearly deduce conditions for obtaining observable Raman signals. Two peaks are found after oxidation of nickel hydroxide, 480 and 560 cm-1 peaks. The other literature11,12 have investigated the oxidation states of a nickel oxide electrode by mapping the Raman spectra of nickel oxide compounds with well-known structures. Metal nickel electrodes are good electrocatalysts for oxygen evolution,5,16 organic oxidation,4,17 and organic synthesis,18 etc. The electrocatalytic capability of nickel electrode comes from the redox reaction under applying anodic potential in alkaline solution. Nickel is easily deposited on a conductive or nonconductive substrate by electroless deposition method.19,20 Electroless deposits also show high electrocatalytic capability for application to OER19 and organic oxidation.20 However, the redox pair type formed on the electroless Ni-P electrode during electrooxidation process have not been explored yet. The performance of a nickel electrode relates closely to their structure; consequently, it is important to explore the structure of the electroless Ni-P electrode during oxidation. In the present investigation, characterization of anodic oxidation reaction of R-Ni(OH)2 and β-Ni(OH)2 on gold in 1 M KOH is achieved by in situ SERS. The Raman spectra of the electroless Ni-P electrode in 1 M KOH are taken without surface enhancement. The oxidation state of the electrodes would be identified by mapping the Raman spectra with the spectra of nickel hydroxide films. Experimental Section Preparation of Nickel Hydroxides. Nickel hydroxide was cathodically deposited on the electrochemically roughened gold substrate. The electrochemical roughening procedure to generate (16) Miao, H. J.; Piron, D. L. J. Appl. Electrochem. 1991, 21, 55. (17) Fleischmann, M.; Korinek, K.; Pletcher, D. J. Electroanal. Chem. 1971, 3, 39. (18) Manandhar, K.; Pletcher, D. J. Appl. Electrochem. 1979, 9, 707. (19) Lo, Y. L.; Hwang, B. J. J. Appl. Electrochem. 1996, 26, 1. (20) Lo, Y. L.; Hwang, B. J. J. Electrochem. Soc. 1995, 142, 445.
S0743-7463(96)00025-X CCC: $15.00 © 1998 American Chemical Society Published on Web 01/24/1998
Nickel Hydroxide Films and Ni-P Electrodes SERS activity on gold was the same as those described in the previous literature.21,22 After the activation procedures the gold substrate was then rinsed with redistilled water. Nickel hydroxide films were deposited on the pretreated substrate by galvanostatic deposition at cathodic current 0.08 mA/cm2 in 0.01 M Ni(NO3)2 for 20 s at room temperature. The deposited Ni(OH)2 film was rinsed and subsequently transferred to 1 M KOH solution for taking in situ Raman spectra. Preparation of Electroless Ni-P/SnO2/Ti Electrodes. Electroless Ni-P films were deposited on a titanium substrate with tin oxide coating as interlayer for improving adhesion of Ni-P to the substrate. A titanium sheet was first etched in 6 N HCl solution after being cleaned with detergent. Tin oxide films were subsequently coated on the etched titanium substrate by thermal decomposition at 450 °C after being dipped in the SnCl2 solution. The detailed procedures have been described in our previous studies.19,20 Electroless Ni-P deposits were plated from the deposition bath: 0.1 M NiSO4, 0.2 M NaH2PO2, 0.2 M sodium citrate, and 0.5 M (NH4)2SO4 at pH 9.0 and 85 °C, under a stirring rate of 400 rpm. All of the chemicals used were of analytical grade (Janssen Chemical Co.). Redistilled water was used. The cyclic voltammetry measurements were carried out using an electrochemical system (BAS 100B). Raman spectra were taken via a Renishaw Raman Microscope system 2000 at room temperature. The light source is a He-Ne laser with a 6328 Å excitation line. The signal was detected by a charge couple device (CCD). The resolution of the system was about 1 cm-1.
Results and Discussion Raman Spectra of As-Deposited Nickel Hydroxide in Alkaline. As-deposited nickel hydroxide film prepared by cathodic galvanostatic deposition from Ni(NO3)2 solution is identified as R-Ni(OH)2.6,23 The structure of R-Ni(OH)2 containing many water molecules is amorphous and, the formula of R-Ni(OH)2 is generally written as 3Ni(OH)2‚ 2H2O.24 Nickel hydroxide was deposited on an activated gold substrate to obtain significant Raman spectra in this work. R-Ni(OH)2 film was obtained from 0.01 M Ni(NO3)2 solution by cathodic galvanostatic deposition on a gold substrate at 0.08 mA/cm2 for 20 s with a geometrical area of 0.283 cm2. According to the investigation of Densilvestro and co-workers,14,15 the film thickness would affect SERS activity and the optimal film thickness is corresponding to a charge density around 1 mC/cm2. The charge density passed for the deposition of R-Ni(OH)2 is about 1.6 mC/ cm2 in this work. In this condition, the SERS activity of the deposited film is suggested to be an optimum. Figure 1 shows the cyclic voltammogram obtained for the R-Ni(OH)2 film in 1 M KOH, scan rate 1 mV/s. The corresponding in situ Raman spectra are demonstrated in Figure 2. In the potential range of anodic sweep between 0 and 300 mV (vs Ag/AgCl), the voltammogram indicates no significant current until the potential larger than 350 mV. Raman spectra shows that two main peaks, 455 and 490 cm-1, are observed in the potential range of 0-300 mV, and the peaks shift toward higher Raman frequency (a blue shift) with increasing potential. Another new small peak around 555 cm-1 is detected at 300 mV. This peak becomes more remarkable at 350 mV. However, the other (21) Tadayyoni, M. A.; Gao, P.; Weaver, M. J. J. Electroanal. Chem. 1986, 198, 125. (22) Desilvestro, J.; Weaver, M. J. J. Electroanal. Chem. 1986, 209, 377. (23) Chen, R.-R.; Mo, Y.; Scherson, D. A. Langmuir 1994, 10, 3933. (24) McBreen, J. In Modern Aspects of Electrochemistry; Conway, B., White, R., Bockris, J. O’M., Eds.; Plenum Press: New York, 1981; Vol. 21, p 34. (25) Jackovltz, J. F. In Proceedings of the Symposium on the Nickel Electrode; Gunther, R. G., Gross, S., Eds.; Electrochemistry Society: Pennington, NJ, 1982; p 48. (26) Nahn, F.; Floner, D.; Boden, B.; Lamy, C. Electrochim. Acta 1987, 32, 1631.
Langmuir, Vol. 14, No. 4, 1998 945
Figure 1. Cyclic voltammograms for R-Ni(OH)2 film in 1 M KOH, scanning rate 1 mV/s at room temperature.
peak, 455 cm-1, fades away at 350 mV. The intensity ratio of peaks 455 to 495 cm-1 is listed in Table 1. Peak 455 cm-1 and peak 490 cm-1 grow and decline at the potential range between 0 and 200 mV, respectively. With sweeping potential to 350 mV, the peak ratio of 450/490 cm-1 decreases with increasing potential. When the potential arrives at 400 mV, the peaks of 455 and 490 cm-1 disappear; on the other hand, two remarkable bands, 475 and 555 cm-1, are observed. At the potential 300 mV, small anodic current arises in the CV response; meanwhile, the peak of 555 cm-1 appears in the corresponding Raman spectra. The anodic peak occurs at ca. 400 mV, the structure changes obviously as shown in Raman spectra. Two remarkable Raman peaks are observed at 475 and 555 cm-1, respectively, at the potential range between 400 and 550 mV. These two peaks show blue shifts as anodic potentials increase, i.e., from 473 and 555 cm-1 (at 400 mV) to 477 and 559 cm-1 (at 550 mV). When the potential is swept toward the cathodic direction, the peaks shift backward from their original positions, i.e., from 477 and 559 cm-1 (at 550 mV) to 474 and 555 cm-1 (at 300 mV), respectively. However, the characteristic Raman peaks which correspond to the anodic (400 mV) and cathodic peaks (300 mV) in CV are at the same frequency, 473 and 555 cm-1. As the potential sweeps toward the cathode continuously, the two sharp peaks get broad and then split into three less remarkable peaks, 453, 496, and 562 cm-1, at the potential of 250 mV. With the potential toward the cathode (Figure 2b), the peak at 560 cm-1 disappears at ca. 150 mV and the subsequent Raman spectrum reveals that the surface structure is similar to its initial state; i.e., both the final state (0 mV) and the initial state (OCP) have the peak ratios of 450/490 cm-1 larger than unity, as indicated in Table 1. However, the peak ratio of 455/490 cm-1 decreases and the 490 cm-1 peak moves toward a positive Raman shift, i.e., from ca. 491 to 495 cm-1 after the surface undergoes an oxidationreduction process. R-Ni(OH)2 is an amorphous, hydrated, and nonstoichiometric intermediate compound.7 The compound commonly prepared by electrochemical precipitation from nitrate baths is regarded as highly hydrated, and the dehydrated films correspond chemically to NiO.1 Three bands are observed in IR spectra for the R-form of nickel hydroxide; they are δOH, γOH, and νNiO, respectively, at
946 Langmuir, Vol. 14, No. 4, 1998
Lo and Hwang Table 1. Peak Intensity Ratio of 450/490 cm-1 and 475/555 cm-1 Varying with the Applied Potential (vs Ag/AgCl) for As-Deposited r-Ni(OH)2a potential (mV)
peak position (cm-1)
450/490 cm-1
OCP 50 (f) 100 (f) 150 (f) 200 (f) 250 (f) 300 (f) 350 (f) 400 (f) 450 (f) 500 (f) 550 (f) 500 (r) 450 (r) 400 (r) 350 (r) 300 (r) 250 (r) 200 (r) 150 (r) 100 (r) 50 (r) 0
455,491 453,490 453,490 450,491 450,491 453,496 453,496,555 453,496,555 473,555 475,560 476,557 477,559 476,558 476,559 475,559 475,555 474,555 453,496,562 453,496,562 453,495 450,494 458,495 448,495
1.145 1.652 0.775 1.104 0.993 0.894 0.800 0.711
475/555 cm-1
1.094 1.139 1.073 1.067 1.052 1.101 1.074 1.145 1.092 0.643 0.455 0.253 0.556 0.833 1.059
a f, forward scanning direction in cyclic voltammetric measurement; r, reverse scanning direction in cyclic voltammetric measurement; OCP, under condition of open circuit potential.
characteristic Raman bands for as-precipitated nickel hydroxide (R-Ni(OH)2) are 455 cm-1 for Ni-OH symmetric stretching vibration and 490 cm-1 for Ni-O vibration. The intensity ratio and position of these two bands would vary with the applied potential, as presented in Table 1. Since R-Ni(OH)2 is a hydrated compound, it would be dehydrated easily by applying an anodic potential. The dehydration-hydration reaction would be expressed as follows: At low applied potential regime (about OCP ∼200 OH Ni OH
Figure 2. In situ Raman spectra for R-Ni(OH)2 film in 1 M KOH at different potentials corresponding to cyclic voltammograms in Figure 1: (a, top) anodic scan; (b, bottom) cathodic scan from 550 to 0 mV.
wavelengths of 400-700 cm-1.7 In this work, two Raman bands of ca. 455 and 490 cm-1 are observed under open circuit potential (OCP) in 1 M KOH. The 455 cm-1 band is assigned to a symmetric Ni-OH stretching mode.14 This assignment is also verified by isotopic technique.27 Melendres and Xu12 show the characteristic Raman bands for NiO compound are 497 and 540 cm-1. According to Melendres and Xu’s study, the 490 cm-1 band is most likely due to Ni-O vibration. Consequently, the two (27) Visscher, W.; Barendrecht, E. Electrochim. Acta 1980, 25, 651.
k(E)
Ni
O + H2O
(1)
mV), the surface bonding state is unstable due to the dehydration-hydration reaction. This phenomenon results in the growth and decline of the two Raman bands, 455 cm-1 for Ni-OH vibration and 490 cm-1 for Ni-O vibration, at the potential regime between OCP and 200 mV. However, the bonding state gets stable with increasing anodic potential, having a tendency for dehydration. Initially, the intensity of Raman band at 450 cm-1 is larger than that of 490 cm-1; however, the peak of 450 cm-1 decays but the 490 cm-1 peak grows with increasing anodic potential in the range between 200 and 350 mV. Both peaks show a blue shift as the potential increases (from 200 to 350 mV). A CV result indicates no significant current at the potential region between 0 and 300 mV; nevertheless, remarkable variations appear in Raman spectra. These experimental results suggest that the surface structure of the nickel hydroxide electrode proceeds with reconstruction via a dehydration-hydration reaction to obtain a more stable surface state at the potential region less than 350 mV. The Raman bands move toward positive Raman shift as anodic potentials are increased. This phenomenon means that the degree of the process for structure reconstruction increases with increasing anodic potentials. As a result, arrangement of the surface structure gets orderly. Another new Raman band appears at a Raman shift of 555 cm-1 at an anodic potential of 300
Nickel Hydroxide Films and Ni-P Electrodes
mV. It is supposed that the surface structure begins changing due to another reaction different from the dehydration-hydration reaction (1) at the potential of ca. 300 mV. The intensity of 550 cm-1 increases with anodic potential. When the applied potential arrives at 400 mV, i.e., around the anodic peak potential, two remarkable Raman bands are observed, 475 and 555 cm-1. These two bands show a blue shift as the anodic potential is increased, e.g., from 473 (at 400 mV) to 477 cm-1 (at 550 mV) and from 555 (at 400 mV) to 559 cm-1 (at 550 mV), respectively. Thereafter, the bands shift backward from their original positions when the applied potentials are swept toward the cathode. Oxidation reactions proceed on the electrode surface at the anodic potential region between 400 and 550 mV. According to the study of Melendres and Xu,12 475 and 555 cm-1 are the characteristic Raman peaks for Ni2O3‚2H2O. Moreover, Cornilsen et al.11 have reported that 476 and 559 cm-1 are the characteristic Raman peaks for the species called “γ-charged active mass”. Figure 2a indicates that the surface of electrode begins oxidizing when the anodic potential arrives at ca. 300 mV. The oxidation current is very small, as shown in the CV response of Figure 1; however, variations in Raman spectra are observable at 300 mV. With increasing anodic potential, the oxidation of electrode surface keeps proceeding; simultaneously, the oxidation product of R-Ni(OH)2, which is called the “γ-active species”, is charged. Consequently, the characteristic Raman bands 473 and 555 cm-1 at 400 mV arise from the oxidation product, the γ-active species, and Raman characteristic bands 477 and 559 cm-1 at 550 mV from the charged oxidation product, the “charged γ-active species”. According to the investigation of Jackovitz,25 eg Ni-O bending vibration appears at 480 cm-1 and a1g Ni-O stretching vibration at 560 cm-1. Consequently, it infers that the Raman scattering comes from the Ni-O bonding vibration of γ-NiOOH at the potential range between 400 and 550 mV. γ-NiOOH converting from the oxidation of R-Ni(OH)2 is a loose structural and hydrous compound with stacking layer by layer.7,24 The Raman scattering bands thus show blue shifts due to the structural characteristics of γ-NiOOH during electrooxidation. When potentials are swept to 300 mV in the cathodic scan, the surface state is still in an oxidation state, i.e., having γ-species with 474 and 555 cm-1 being the characteristic Raman frequency. However, most of the electrode surface turns back to nickel hydroxide, and there are revealed three peaks, 453, 496, and 562 cm-1, in Raman spectra as the potential sweeps backward to 200 mV, as shown in Figure 2b. At this potential, species on the electrode surface include a majority of nickel hydroxide (reduced from γ-species) and a minority of unreduced γ-species, having the bending vibration of a1g Ni-O stretching at 562 cm-1. In the forward scanning from 300 to 550 mV, the surface is at the oxidation state, having both Ni-O stretching and Ni-O bending vibrations for γ-species, the oxidation product. But in the backward scanning from 500 to 250 mV, the surface is at the reduction state, having only Ni-O stretching vibration for γ-species, the unreduced species. Moreover, repeating the cyclic voltammetric scanning following the first scanning of the as-deposited film, the Raman band of 562 cm-1 which decays at the potential of 150 mV in the first cycle is still observable at the reduction potential of 50 mV in the second cycling. On the other hand, for nickel hydroxide species, the degree of surface hydration increases gradually with sweeping potential toward the cathode; consequently, the
Langmuir, Vol. 14, No. 4, 1998 947
Figure 3. Cyclic voltammograms for deactivated nickel hydroxide film in 1 M KOH, scanning rate 1 mV/s at room temperature.
peak intensity of the 455 cm-1 band for Ni-OH vibration increases with increasing cathodic potential. Nevertheless, the band arising from Ni-O stretching vibration shifts from 491 to 495 cm-1 after undergoing the oxidation-reduction process. It implies that the surface state of the electrode would not return to its original state under the experimental conditions. Ni-O stretching is the main vibration on the surface structure of the electrode at E ) 0 mV (vs Ag/AgCl), as indicated from the intensity ratio of 450/495 cm-1 decaying after proceeding with oxidationreduction process. It follows that the surface state of the electrode will hardly be restored to its original state as the cycling number increases. Not only is this fact is related to the appearance of more electrochemical stable species with cycling but also time (cycling number) is an important factor in the transformation. In order to realize the structural variations during the transformation, we next studied the in situ Raman spectra of deactivated nickel hydroxide produced from as-deposited film treated by cycling in alkali. Raman Spectra of Deactivated Nickel Hydroxide in Alkaline. The deactivated nickel hydroxide film is produced from as-deposited Ni(OH)2 via cyclic voltammetric procedure at 100 mV/s in 1 M KOH for 100 cycles. Figure 3 shows the cyclic voltammogram for the deactivated Ni(OH)2 film in 1 M KOH, scan rate 1 mV/s. The corresponding in situ Raman spectra are shown in Figure 4a,b. The deactivated film has the characteristic Raman bands at 475 and 557 cm-1. For the as-deposited film, the characteristic bands are at 450 and 490 cm-1 under OCP condition. This is the difference between as-deposited and the deactivated Ni(OH)2 films. The surface structure of Ni(OH)2 film changes after oxidation-reduction cycling for 100 cycles at a scanning rate of 100 mV/s. When the anodic potential is applied, the structure of the deactivated film changes, as indicated in Table 2. The structure is unstable at the anodic potentials from 50 to 350 mV. At this potential range, ca. 500 cm-1 is the main Raman band which has been identified as Ni-O vibration, as discussed in the previous section. However, the characteristic band for Ni-OH at 455 cm-1 is not observed. It is clear that the deactivated nickel hydroxide is a “dehydrated” compound with characteristic vibration of Ni-O bonding, not Ni-OH vibration. As the potential arrives at 300 mV, the surface of the electrode begins to be oxidized, as
948 Langmuir, Vol. 14, No. 4, 1998
Lo and Hwang Table 2. Raman Spectra Varying with the Applied Potential (vs Ag/AgCl) for the Deactivated Ni(OH)2 Filma potential (mV)
peak position (cm-1)
OCP 50 (f) 100 (f) 150 (f) 200 (f) 250 (f) 300 (f) 350 (f) 400 (f) 450 (f) 500 (f) 550 (f) 500 (r) 450 (r) 400 (r) 350 (r) 300 (r) 250 (r) 200 (r) 150 (r) 100 (r) 50 (r) 0
475, 557 506, 559 506 458, 506, 543 503 500, 547 497, 550 503, 559 477, 560 479, 561 479, 560 482, 558 479, 560 479, 560 479, 560 478, 559 475, 555 471, 554 495, 555 497, 555 497, 553 497, 553 497, 553
a
Figure 4. In situ Raman spectra for deactivated nickel hydroxide film in 1 M KOH at different potentials corresponding to cyclic voltammograms in Figure 3: (a, top) anodic scan; (b, bottom) cathodic scan from 550 to 0 mV.
indicated from the characteristic band at 555 cm-1 in the Raman spectra of Figure 4a. The anodic current in the cyclic voltammogram is very small; however, the variation in Raman spectra is obvious at the potential scanning range from 0 to 350 mV. Two remarkable characteristic Raman bands, 477 and 560 cm-1, are observed at 400 mV, which is around the peak potential of anodic oxidation, as indicated in the cyclic voltammogram (Figure 3). With increasing potential up to 550 mV, the peak 477 cm-1 shifts to 482 cm-1; thereafter, the peak shifts from 482 to 471 cm-1 as the potential sweeps from 550 backward to
480/560 cm-1
1.042 1.006 0.969 1.079 1.000 0.982 0.998 1.059 0.953 0.667
See abbreviation definitions in footnote a of Table 1.
250 mV. Although the cathodic current is very small at 200 mV as seen in the cyclic voltammogram, structural variation occurs, as revealed in Raman spectra. Two Raman bands, 495 cm-1 for Ni-O vibration and 555 cm-1 for O-Ni-O stretching vibration, are observed at the cathodic potential of 200 mV. It is noteworthy that the peak at 555 cm-1 appears at the anodic potential of 300 mV and still can be seen at the cathodic potential of 0 mV. These phenomena on the as-deposited film in the first cycle study and the deactivated films are different. There exist two phases, R-Ni(OH)2 and β-Ni(OH)2: a more stable one usually called β-Ni(OH)2 and a more unstable structure of hydrated R-Ni(OH)2. R-Ni(OH)2 will convert to β-Ni(OH)2 by fast charging according to a Bode diagram.24 From the CV-Raman study, the main bonding types for β-Ni(OH)2 are Ni-O and O-Ni-O stretching vibration. Moreover, β-Ni(OH)2 is also concluded to be less hydrous than R-Ni(OH)2 in this study, as indicated from the weakened band of Ni-OH vibration in Raman spectra. As has been discussed above, 475 and 555 cm-1 are the characteristic Raman bands for γ-NiOOH, the oxidation product of R-Ni(OH)2. And 480 and 560 cm-1 are the bands for β-NiOOH, the oxidation product of β-Ni(OH)2. Two cathodic bands which locate at 400 and 300 mV are observed in the cyclic voltammogram of Figure 3. As the potential continues sweeping to 300 mV (peak A in the CV response), the characteristic bands are 475 and 555 cm-1. It is consequently concluded that peak A in CV results from the reduction of γ-NiOOH and peak B from the reduction of β-NiOOH. As was pointed out in the Introduction, the NiOOH species can exist in the β-NiOOH structure and in the other form called γ-NiOOH. The β-NiOOH is a relatively well-defined material, while the γ-form represents a whole series of compounds exhibiting a large intersheet distance, of general formula AxHy(H2O)zNiO2 (x, y e 1). “A” represents alkali ions (mainly K+ and Na+), and water molecules are intercalated between the NiO2 slabs.7,24 β-NiOOH probably has some absorbed and adsorbed water. However, TGA data are very limited.24 Fast charging R-Ni(OH)2 will be transformed from R-Ni(OH)2 into β-Ni(OH)2. And γ-NiOOH is also produced on an overcharge of β-Ni(OH)2, particularly when the charging
Nickel Hydroxide Films and Ni-P Electrodes
Langmuir, Vol. 14, No. 4, 1998 949
Figure 5. Cyclic voltammograms for electroless Ni-P electrode in 1 M KOH, scanning rate 1 mV/s at room temperature.
is carried out at high rates in alkali.24 Raman spectrum analysis indicates that the characteristic bands for γ-NiOOH are at 475 and 555 cm-1, and those for β-NiOOH are at 480 and 560 cm-1. The energy for exciting the Ni-O vibration in β-NiOOH is higher than that in γ-NiOOH. It stands to reason that the structure of β-NiOOH is more defined than the structure of γ-NiOOH. The structure of γ-NiOOH is disorderly due to water molecules and alkali ions being contained between the intersheet which distorts the construction of γ-NiOOH. Consequently, the Raman exciting energy for γ-NiOOH is lower. γ-NiOOH and β-NiOOH are Raman distinguishable. Raman Spectra of Electroless Ni-P/SnO2/Ti Electrode in Alkaline. The film prepared by the electroless deposition technique contains non-metal atoms such as P or B which depends on the type of a reducing agent utilized in deposition bath. It is well-known there are two forms of nickel redox pairs, r-Ni(OH)2/γ-NiOOH and β-Ni(OH)2/β-NiOOH. For a nickel wire electrode, r-Ni(OH)2 will be formed during the first oxidation stage and then it will be converted to β-Ni(OH)2 when the subsequent oxidation-reduction process proceeds.5 To decide what difference in the oxide form of an electroless Ni-P electrode from that of a pure nickel wire electrode, in situ Raman spectra of the electroless Ni-P film in alkaline were taken. These results were then compared with the spectra obtained from the as-deposited as well as the deactivated films. An electroless Ni-P/SnO2/Ti electrode was treated with cyclic voltammetry in 1 M KOH to form a Ni(OH)2 film. Figure 5 shows the cyclic voltammogram for the electroless Ni-P deposits in 1 M KOH at a scan rate of 1 mV/s. The corresponding in situ Raman spectra are shown in Figure 6a,b. The quantity of Ni(OH)2 formed on the electroless Ni-P deposits is too thin to exite Raman scattering; consequently, no obvious Raman bands are observed during the potential range between 0 and 400 mV. There appear no Raman bands until peak current arises at about 450 mV in the CV response. Only two Raman characteristic bands, ca. 490 and 560 cm-1, are observed through the whole potential scanning range. The Raman shift of peak 560 cm-1 is nearly invarient with potential and fades as the cathodic potential is reduced to 350 mV. However, a band position that shifts to high Raman frequency is observed with an increase in anodic
Figure 6. In situ Raman spectra for electroless Ni-P electrode in 1 M KOH at different potentials corresponding to cyclic voltammograms in Figure 5: (a, top) anodic scan from 0 to 550 mV; (b, bottom) cathodic scan from 550 to 0 mV.
potential, i.e., from 480 (at 450 mV) to 494 cm-1 (at 550 mV). Thereafter, this band shifts back to 490 cm-1 at the cathodic potential of 400 mV, as indicated from Table 3. Both of the peaks decay at the cathodic potential of 350 mV. As compared with Figure 4a,b, the film shows the characteristic Raman peaks at frequencies of 490 and 560 cm-1 at the peak potential 450 mV at which Ni(OH)2 is electrooxidized to NiOOH. These results are consistent with those obtained with Raman spectra of β-Ni(OH)2 electrooxidized to β-NiOOH. It means that the major type of Ni(OH)2 film that forms on the surface of the electroless
950 Langmuir, Vol. 14, No. 4, 1998
Lo and Hwang
Table 3. Characteristic Raman Bands for the Electroless Ni-P/SnO2/Ti in 1 M KOH at Various Potentials (vs Ag/AgCl)a potential (mV) OCP 100 (f) 200 (f) 300 (f) 400 (f) 450 (f) 500 (f) 550 (f) a
peak position (cm-1)
potential (mV)
peak position (cm-1) 494, 560 494, 560 490, 560
480, 560 486, 558 491, 562
500 (r) 450 (r) 400 (r) 350 (r) 300 (r) 200 (r) 100 (r) 0
See abbreviation definitions in footnote a of Table 1.
Ni-P/SnO2/Ti electrode is the β-form nickel hydroxide. However, the Raman bands locate more positive frequency for the Ni-P/SnO2/Ti electrode. It means that the structure of nickel oxide on the Ni-P electrode is more defined than that of the β-NiOOH film transformed from deactivated Ni(OH)2. Similar results have already been obtained from spectroscopic studies on the nickel wire surface whose surface oxide type is also the β-form.26,27 The surface state is important for applications of an electrode. For cathodically deposited R-Ni(OH)2 film, a larger number of electrons are exchanged per nickel atom during the redox reaction of R-Ni(OH)2/γ-NiOOH because the oxidation state of nickel in γ-NiOOH is close to 3.5. On the basis of this reason, R-Ni(OH)2 is suitable for application to the nickel-positive electrode in the battery industry.3,7 On the other hand, the electrocatalytic activity of β-NiOOH for OER is better than γ-NiOOH.19 Previous study shows that the electroless Ni-P electrodes possess comparable activity for OER with Raney nickel and IrO2.19,28 The observations of Raman spectra in this work, taken together with previous studies on electrocatalytic activity of electroless Ni-P electrodes,19,28 lead to the tentative conclusion that the better electrocatalytic activity of electroless Ni-P is ascribed to β-NiOOH on the electrode. As has been confirmed in the previous work, the preanodization of a nickel electrode yields the β-NiOOH film, which is the “right type of oxide” providing active sites for the oxygen evolution reaction.5 Since β-NiOOH is a good electrocatalyst for organic synthesis18 and oxygen evolution,5 the electroless Ni-P electrode is more suitable for an anode in organic synthesis and oxygen evolution than for a positive electrode in the battery industry. Summary This study investigates the surface reaction of three types of working electrodes, as-deposited nickel hydroxide, deactivated nickel hydroxide, and electroless Ni-P electrode, in KOH via in situ Raman spectrum. The following important results are obtained. 1. R-Ni(OH)2 is cathodically deposited from 0.01 M Ni(NO3)2 at constant current density 0.08 mA/cm2 on surface enhanced gold substrate. The current is very small (28) Lo, Y. L.; Hwang B. J. Electrochem. Soc. 1996, 143, 2158.
between 0 and 250 mV, as shown in a cyclic voltammogram; however, two characteristic Raman bands are observed at 450 and 490 cm-1. These two peaks grow and decline with each other in the potential range between 0 and 250 mV. The band of 450 cm-1 comes from the vibration of Ni-OH and 490 cm-1 for Ni-O vibration. It concludes that the surface proceeds from the dehydrationhydration process as the potential sweeps from 0 to 250 mV. When the potential is larger than 250 mV, the current increases at 555 cm-1. It implies that the surface begins being oxidized at 300 mV, although the current is small. A remarkable CV peak arises at 400 and 550 mV; it comes from the oxidation of Ni(OH)2 to NiOOH. At this potential range, the characteristic Raman bands are 475 and 555 cm-1, which come from Ni-O bending vibration and stretching vibration, respectively. The Raman bands shift at OCP condition, showing blue shifts after undergoing an oxidation-reduction process. 2. The deactivated Ni(OH)2 film is produced by treating R-Ni(OH)2 film with fast cycling. The cyclic voltammogram of the deactivated film shows one anodic peak at 400 mV and two reduction peaks at ca. 400 (peak B) and 300 mV (peak A). The current is very small, and the Raman spectra show the surface state is unstable at the potential between 0 and 350 mV. The main Raman band is 500 cm-1 for Ni-O vibration at this potential region, and no Ni-OH vibration is observed. The deactivated film consists of the dehydrated form of R-Ni(OH)2. Bands at 477 and 560 cm-1 are the characteristic Raman bands corresponding to the anodic oxidation peak at 400 mV. On the other hand, the Raman bands are 475 and 555 cm-1 with respect to the reduction peak A at 300 mV, and 480 and 560 cm-1 are to the peak B at 400 mV. It indicates that peak A is from the reduction of γ-NiOOH to R-Ni(OH)2 and peak B is from the reduction of β-NiOOH to β-Ni(OH)2. The structure of β-NiOOH is more defined than that of γ-NiOOH; consequently, the energy for exciting Ni-O vibration is larger than that for γ-NiOOH. This results in a blue Raman shift in β-NiOOH/β-Ni(OH)2 transformation. 3. No Raman band relevant Ni(OH)2 is found on the Ni-P/SnO2/Ti electrode between 0 and 400 mV because the film is too thin to produce an obvious Raman vibration band. Two Raman bands appear at 490 and 560 cm-1 at 450 mV. These two bands locate closely to the characteristic Raman bands of β-NiOOH. It indicates that the major type of nickel oxide on the electroless Ni-P/SnO2/ Ti electrode is the β-form when the electrode is electrooxidized in KOH. β-NiOOH on the Ni-P electrode is better constructed than the β-NiOOH transformed from R-Ni(OH)2. Acknowledgment. The authors wish to thank the National Science Council (Grant NSC 84-2214-E-011-018) of the ROC for the financial support and National Taiwan University of Science and Technology of the Republic of China. LA9600255
