Langmuir 1997, 13, 4621-4627
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Characterization of Bulk and Surface Composition of CoxNi1-xOy Mixed Oxides for Electrocatalysis Yu. E. Roginskaya,† O. V. Morozova,† E. N. Lubnin,† Yu. E. Ulitina,† G. V. Lopukhova,† and S. Trasatti*,‡ Karpov Institute of Physical Chemistry, Vorontsovo pole 10, 103064 Moscow, Russia, and Department of Physical Chemistry and Electrochemistry, University of Milan, Via Venezian 21, 20133 Milan, Italy Received September 19, 1996. In Final Form: April 14, 1997X
Films of general composition CoxNi1-xOy with x ) 0, 11.1, 22.2, 33.3, 44.4, 55.6, 66.7, 77.8, 88.9, and 100 mol %, prepared by thermal decomposition of 2-propanol solutions of Ni and Co nitrates on Ni supports, have been characterized by X-ray diffraction (XRD), infrared spectroscopy (IRS), and X-ray photoelectron spectroscopy (XPS). These techniques have provided the surface chemical composition, the crystalline structure, and the oxidation state of Co and Ni in the bulk as well as on the surface. The results have revealed, in addition to previous work where NiO and NixCo2-xO4 solid solutions were found to be the predominant crystalline phases, that Ni and Co hydroxides interact in the surface layer of nanosized crystallites of the oxide phase. The interaction in the hydroxylated surface has a redox character and results in two oxidation states (+2 and +3) for both Ni and Co ions. This work has shown that Co hydroxide fragments are localized in the periphery of Ni hydroxide domains. The redox interaction between the hydroxides prevents Ni ions from being oxidized to higher valency states in anodic electrocatalytic processes, which results in improved anodic corrosion stability of Ni + Co mixed oxides.
1. Introduction Co3O4 and NiCo2O4 have long been known to be active electrocatalysts for oxygen evolution as well as reduction in alkaline electrolytes.1 On the other hand, data on the application of these oxides to organic and inorganic electrosyntheses are scanty. The effect of the composition and of the conditions of preparation on the electrochemical properties of Ni + Co oxides in the whole composition range has previously been discussed in a few works.2,3 In one of the papers,2 where the cryochemical synthesis of oxide powders was performed, the system included NiO + CoO solid solutions, solid solutions based on the spinel phase, together with unreacted NiO as well as Co3O4. If electrodes are prepared from Ni and Co hydroxides,3 the same phase composition is found except in the region with 60-70 mol % Co, where the monophasic spinel NiCo2O4 exists. The surface composition and the structure of NiCo2O4 layers prepared at various temperatures (300-500 °C) and various calcination times (1-10 h) have been determined by Haenen et al.4 by XPS, XRD, and DTA. X-ray analysis and DTA4,5 have shown that NiCo2O4 decomposes at T > 400 °C with formation of NiO, which results in the enrichment of the NiCo2O4 surface with NiO. Conversely, Co3O4 decomposes with formation of CoO only at about * Author for correspondence: fax +39.2.26603-224; e-mail
[email protected]. † Karpov Institute of Physical Chemistry. ‡ University of Milan. X Abstract published in Advance ACS Abstracts, July 15, 1997. (1) Tarasevich, M. A.; Efremov, B. N. In Electrodes of Conductive Metallic Oxides; Trasatti, S., Ed.; Elsevier: Amsterdam, 1980; Part A, p 221. (2) King, W. J.; Tseung, A. C. C. Electrochim. Acta 1974, 19, 439. (3) Trunov, A. M.; Peskov, V. A.; Uminskii, M. V.; Rakityanskaya, O. F.; Bakulina, T. S.; Kotseruba, A. I. Sov. Electrochem. 1975, 11, 509. (4) Haenen, J.; Visscher, W.; Barendrecht, E. J. Electroanal. Chem. 1986, 208 (Parts I-III), 273-341. (5) Mari, C. M.; Gilardoni, G.; Carugati, A.; Trasatti, S. In Extended Abstracts, 32nd ISE Meeting; Dubrovnik/Cavtat, 1981; p 96.
S0743-7463(96)00912-2 CCC: $14.00
900 °C.6 The oxidation state of Ni has been ascertained to be +2 on the basis of the Ni 2p core line binding energy (BE). Cobalt ions are predominantly in the state of lowspin Co3+; in addition, some paramagnetic high-spin Co2+ have been found. The electrode coatings in refs 4 and 5 were prepared by thermal decomposition of aqueous or 2-propanol solutions of the Co and Ni nitrates. These precursors normally provide the most reproducible electrochemical properties. However, some variations in the experimental procedure (e.g., (a) application of the solution layer by layer onto the cold support with intermediate drying and heating,7 (b) spray pyrolysis at higher temperatures,8 (c) alternation of the type of solvent and heat treatment just at the final temperature,9 (d) preconcentration of the solutions of the Co and Ni precursors10) resulted in different compositions of the electrode surface. This is due to hydrolytic processes in solution or during the early stages of the heat treatment, which lead to different local compositions as well as different microstructure of the Ni and Co mixed oxides. Upon hydrolysis, Ni and Co hexaaquo cations become hydroxylated, polymerize, and form the nuclei of hydrated oxides before completion of nitrogen oxide evolution. Even the thermolysis of the dry crystalline hydrate Ni(NO3)2‚ 6H2O forms intermediate aquo- and hydroxynitrate complexes (-[Ni(H2O)4](NO3)2 at 145 °C, -Ni2(OH)2(NO3)2‚2H2O at 230 °C, and -Ni2(OH)2(NO3)2‚H2O at 245350 °C).11 Therefore, among the products of decomposition of Co and Ni nitrates one can anticipate the presence of partly dehydrated oxides such as MO‚H2O, M(OH)x, or (6) Koumoto, K.; Yanagida, H. Jpn. J. Appl. Phys. 1981, 20, 445. (7) Carugati, A.; Lodi, G.; Trasatti S. J. Electroanal. Chem. 1983, 143, 419. (8) Singh, R. N.; Koenig, J. F.; Poillerat, G.; Chartier, P. J. Electrochem. Soc. 1990, 137, 1408. (9) Haenen, J.; Visscher, W.; Barendrecht, E. J. Appl. Electrochem. 1985, 15, 29. (10) De Chialvo, M. R. G.; Chialvo, A. C. Electrochim. Acta 1993, 38, 2247. (11) Maneva, M.; Petrov, N.; Pankova, M. J. Therm. Anal. 1990, 36, 577.
© 1997 American Chemical Society
4622 Langmuir, Vol. 13, No. 17, 1997
MOOH, besides MO, in an amount depending on the hydrolytic history (M ) metal). For the above reasons, it is essential to correlate the electrochemical properties of mixed-oxide electrodes with the physicochemical properties determined on precisely the same samples prepared with the same procedure in the same laboratory. Although similar studies of mixed Ni and Co oxide characterization do exist in the literature, they cannot be extrapolated beyond purely qualitative considerations. The structure and the surface composition of Ni oxides and Ni hydrated oxides, the latter as a component of the nickel hydrated electrode in batteries, have been intensively investigated in recent years.12-14 A number of IRS, Raman, XPS, and EXAFS data have revealed the oxidation state of Ni in these compounds. Ni oxyhydroxides (β and γ) and their composites proved to be the best materials for the Ni hydrated electrode, the addition of Co hydroxide making the battery action more reversible. The change of the structure and oxidation states at charge-discharge and the role of Co(OH)2 in the stabilization of the oxidation state of Ni in Ni oxyhydroxide have been investigated. These data could be used to elucidate the possible mechanism of Ni and Co oxide interaction in the surface layer of oxides of the NiO-Co3O4 system. In this work, the actual chemical composition, the structure, and the Co and Ni states of oxidation in the bulk as well as on the surface have been investigated in a wider range of Ni/Co ratios than in refs 4 and 10 with the aim to disclose possible correlations between physicochemical properties of the active layers prepared by thermodecomposition of Co and Ni nitrates dissolved in 2-propanol solution and their electrochemical performances in electrosynthesis. 2. Experimental Section 2.1. Oxide Samples. Ten samples of general composition xCo + (100 - x)Ni oxide with x ) 0, 11.1, 22.2, 33.3, 44.4, 55.6, 66.7, 77.8, 88.9, and 100 mol % were prepared by thermal decomposition of Ni(NO3)‚6H2O + Co(NO3)‚6H2O 2-propanol solutions (c ≈ 0.2 mol dm-3). The solution was mechanically spread onto Ni (or quartz) supports, layer by layer, dried at 85 °C, and preliminarily calcined at 250 °C. The procedure was repeated until a nominal thickness of ca. 4 µm was achieved. The final calcination was performed at 400 °C for 2 h. The samples for IR spectroscopy were prepared on quartz at 250 °C as above and then calcined at 300, 400, and 500 °C (and 800 °C for 100% Ni). The active layer was finally scratched from the support and analyzed after mixing with CsI or Nujol. 2.2. Instrumentation. Structure, chemical composition, and oxidation state of the coatings were studied using the following instrumentation: (a) DRON-3M X-ray diffractometer with Cu KR radiation and Ni filter for X-ray diffraction analysis (XRD). (b) Perkin-Elmer-580 IR spectrometer with computer DATA STATION 3600 (IRS). Samples were prepared in the form of pellets with CsI or Nujol mulls between CsI plates. (c) VG-ECSALAB-5 spectrometer for photoelectron spectroscopy (XPS) measurements, carried out by irradiating the sample with a Mg KR X-ray source (hν ) 1253.1 eV). Spectrometer linearity and absolute energy position were established using high-purity metals as reference standards after argon ion sputtering with no annealing. The spectrometer energy scale was calibrated to give Au4f7/2 (for sputtered Au foil), Ag3d5/2, and (12) McBreen, Y.; O’Grady, W. E.; Tourillon, G.; Dartyge, D.; Fontaine, A.; Pandja, K. I. J. Phys. Chem. 1989, 93, 6308. (13) Pandya, K. I.; O’Grady, W. E.; Corrigan, D. A.; McBreen, J.; Hoffman, R. W. J. Phys. Chem. 1990, 94, 21. (14) Ezhov, B. B.; Malandin, O. G. J. Electrochem. Soc. 1991, 138, 885.
Roginskaya et al. Cu2p3/2 peak positions at 83.8, 368.0, and 932.4 ( 0.1 eV binding energy (BE), respectively, as external standards.15,16 Most of the spectra were taken at 50 eV pass energy, giving a Ag3d5/2 peak with a full width at half-maximum (FWHM) of 1.1 eV. Spectra taken at 50 eV pass energy were usually acquired using a 10 or 20 eV BE window containing the elemental peak of interest. All BEs were calibrated against the C 1s line at 284.6 ( 0.1 eV as an internal reference. XPS spectra were smoothed using a linear background. Deconvolution of the spectra was accomplished using a Gauss function curve-fitting routine. The surface mole fractions of Ni, Co, and O were calculated by dividing the area of the relative XPS peak, corrected for the experimental sensitivity, by the sum of comparable factors for all the components of the sample detected at a constant pass energy. The experimental sensitivity factors were measured from Ni, Co, NiO, and Co3O4. The resulting value is a true measure of the relative composition. The spectrometer analytical chamber was isolated from the preparation chamber by a Viton-sealed valve. The vacuum system maintained a residual pressure of 5 × 10-10 Torr in the former. Temperature treatments of samples were performed within the preparation chamber. The temperature of the sample was raised in steps up to 400 °C. The sample probe was heated by an electric resistance and the temperature controlled by means of a Chromel-Alumel thermocouple.
3. Results 3.1. X-ray Diffraction. Pure Oxide Coatings. The sample with 100% Ni oxide contains two crystalline phases: NiO, with the lattice parameter of the cubic unit cell a ) 0.4180 nm, close to the value in the literature,17 and Ni oxyhydroxide, γ-NiOOH. The latter is strongly amorphous as revealed by a diffuse line with d ) 0.6330.718 nm. γ-NiOOH (ASTM Box N6-75) shows the highest peak at d ) 0.6897 nm. The presence of hydrated oxide forms (NiOOH) on the surface of NiO crystallites has been shown18 by XPS for samples prepared at low temperature. The only phase in the sample with 100% Co is a cubic spinel Co3O4 with a ) 0.8084 nm, in agreement with other data19,20 for powders. This spinel phase differs from the Co3O4 phase obtained by Belova et al.21 some years ago, who used an aqueous solution of Co(NO3)2 for the film preparation and found an additional X-ray reflection due to a rhombic distortion of the spinel structure. The distortion was attributed to the presence of crystalline defects at the grain boundaries rather than in bulk Co3O4. Differences in the morphology of Co3O4 layers have been reported22 as aqueous and 2-propanol solutions of Co(NO3)2 are decomposed: the crystallite size in the former case was half that of the latter. Since the volume of defective surface layers is smaller for larger crystallites, one can conclude that the spinel phase in the films of the present work is presumably less defective than in the films studied previously21 which were prepared from aqueous solutions. Mixed-Oxide Coatings. The results of the X-ray analysis are summarized in Table 1. The unit cell parameters of the crystalline phases are given in Figure 1. The parameter a of the NiO cubic phase does not change in the composition range explored (x ) 0-55.6%), pointing to (15) Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Riggs, D., Seach, M. P., Eds.; Wiley: New York, 1983. (16) Bird, R.; Swift, P J. Electron Spectrosc. Relat. Phenom. 1980, 21, 227. (17) ASTM Diffraction Data Card File, Box N 4-835, Washington, 1967. (18) Roberts, M. W.; Smart, R. St. C. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2957. (19) ASTM Diffraction Data Card File, Box N 9-418, Washington, 1970. (20) Ivanova, A. C.; Dzis’ko, V. A.; Moroz, E. M.; Noskova, S. P. Kinet. Katal. 1985, 26, 1193. (21) Belova, I. D.; Roginskaya, Yu. E.; Venevtsev, Yu. N. Zh. Neorg. Khim. 1983, 28, 3009. (22) Trasatti, S. Electrochim. Acta 1991, 36, 225.
Characterization of CoxNi1-xOy Mixed Oxides
Figure 1. Dependence of unit cell parameters on composition: (1) NiO, (2) NiδCo3-δO4, and (3) Ni-Co spinel20 in NiO + Co3O4 mixed-oxide layers. Table 1. Bulk Phase Composition of Ni1-xCoxOy Mixed Oxides from IRS and XRD x/mol %
phase composition
0 11.1 22.2 33.3 44.4 55.6 66.7 77.8 88.9 100.0
NiO + γ-NiOOH + β-Ni(OH)2 NiO + γ-NiOOH + β-Ni(OH)2 NiO + NiCo2O4 NiO + NiCo2O4 NiO + NiCo2O4 NiO + NiCo2O4 NiO + NiCo2O4 NiδCo3-δO4 NiδCo3-δO4 Co3O4
the absence of NiO-based solid solutions. Otherwise, substitution of Co2+ or Co3+ for Ni2+ would have resulted in a noticeable change of the unit cell parameter, since the ionic radii of the Co ions are 0.065 and 0.055 nm, respectively, the Ni2+ radius being 0.069 nm.23 Co oxide involved in coatings with 11.1% Co was not observed in crystalline form. It is probably embedded into amorphous NiOOH. At Co content 22.2%, NiOOH disappears, but a spinel phase appears. For samples with x ) 22.2-44.4%, the unit cell parameter of the spinel phase does not change, being 0.8152 ( 0.0008 nm. With a further increase in Co content, a decreases linearly up to x ) 88.9%. The value of a in the monophase sample where Ni/Co ) 1/2 is close to the parameter of the spinel NiCo2O4 for which a ) 0.8128 nm.24 The change of a for oxides prepared by different procedures3,25 shows that the spinel is a solid solution and its composition and structure are influenced by the preparation conditions. It is known that the ion distribution between tetrahedral and octahedral sites determines the structural parameters of a spinel, in turn depending on the oxidation state of the ions, and on the preparation conditions.26 Nonlinearity in the plot of a versus x in the composition range 44.4-100% Co and an abrupt increase of a in the vicinity of pure Co3O4 are probably caused by the fact that the structure of a spinel solid solution does not match that of the oxide in our layer. Coexistence of NiO and spinel in the middle of the range, coupled with XPS data (discussed below) which indicate surface enrichment in (23) Wells, A. F. Structural Inorganic Chemistry; Mir: Moscow, 1987; Vol. 1, p 374. (24) ASTM Diffraction Data Card File, Box N 2-1074, Washington, 1960. (25) Knop, O.; Reid, K. I. G.; Sutarno; Nakagawa, Y. Can. J. Chem. 1968, 46, 3463. (26) Tretjakov, Yu. D. Thermodynamics of Ferrites; Khimiya: Leningrad, 1967; p 304.
Langmuir, Vol. 13, No. 17, 1997 4623
Figure 2. Average crystallite size of (1) NiO and (2) Co(Ni,Co)2O4 solid solution in NiO + Co3O4 mixed-oxide layers.
Ni oxides of the monophase sample with 66.7% Co, suggests that some Ni oxide in samples with 66.7 and 77.8% Co does not fit into the spinel lattice, but exists as a separate phase (poorly crystalline or amorphous). This view is confirmed by IRS. The observed increase of a with increasing Ni oxide content in comparison with Co3O4 may be explained by the incorporation of the large Ni2+ ion, preferring the octahedral sites, into the Co3O4 spinel lattice (formally Co[NiδCo2-δ]O4). The replacement of Co3+ ions (r ) 0.055 nm) by Ni3+ ions (r ≈ 0.056 nm) would not result in the observed effect. The Ni2+ substitution for Co3+ ions results in a redistribution of the other ions and in a change of their oxidation state. Many models of ion distribution in NiCo2O4 have been proposed, but none has been unambiguously proved. For instance,
Co3+hs[Ni2+hsCo3+ls]O4
ref 25
Co2+hs[Ni3+lsCo3+ls]O4
ref 27
Co2+0.9Co3+0.1[Ni2+0.9Ni3+0.1Co3+]O2-3.2O-0.8 ref 2 where subscripts hs and ls stand for high spin and low spin, respectively. Since NiCo2O4 possesses a metallictype conduction and ferromagnetism, the possibility of an indirect electron exchange Co3+-O-Co4+ or Ni3+-O-Ni4+ should not be ruled out. A distribution suggesting formation of the Co4+ ions may be the following:
Co2+[Ni2+δCo3+2-2δCo4+δ]O4 where δ is the actual Ni content in the spinel structure. Figure 2 shows the FWHM of the (220) X-ray peak of NiO and the (044) peak of Co[NiδCo2-δ]O4 in various layers. For 30-60% Co a remarkable FWHM increase is probably caused by a decrease in the crystallite size up to some nanometers. Such a mutual inhibition of crystallite growth occurs in the two-phase region. 3.2. IR Spectroscopy. NiO Spectra. The IR transmittance spectra of films with 100 mol % Ni oxide, calcined at 300, 400, 500, and 800 °C, are shown in Figure 3 in the region 250-800 cm-1. The spectra of the film at T ) 300 °C, over the whole explored region, 250-4000 cm-1, indicates (i) a wide absorption band with a maximum at (27) Blasse, G. Philips Res. Rep. 1963, 18, 383.
4624 Langmuir, Vol. 13, No. 17, 1997
Figure 3. IR spectra of NiO treated at (1) 300, (2) 400, (3) 500, and (4) 800 °C.
cm-1,
OH-
3580 characteristic of vibrations linked by H bonding; (ii) a deformation band of OH- at 1630 cm-1; and (iii) a very diffuse intense peak at 420 cm-1 with a shoulder at 560-580 cm-1 in the region of M-O (metal-oxygen) vibrations. At higher calcination temperatures up to 500 °C, the main M-O band splits into two sub-bands (420 and 470 cm-1). At 800 °C, when X-rays reveal only crystalline NiO, the intensity of the band at 465 cm-1 increases abruptly, the band at 430 and 580 cm-1 being visible as a shoulder. The bands corresponding to OH- group vibrations disappear. The IR spectrum of β-Ni(OH)2 has been shown28,29 to include intense and sharp bands, characteristic of the vibration of hydrogen bonds in OH- and H2O groups (3650 and 1670 cm-1), and in the region of the M-O bands (430440 cm-1 and 520-550 cm-1). For γ-NiOOH,28,29 which includes Ni(+3) states and does not contain H-bonded OH- groups, the intensity of OH- group vibrations is so small that they cannot be detected, whereas M-O bands are in the region 560-570 cm-1 (most intense) and 620630 cm-1 (weak). The spectra of the oxide films calcined in the temperature range 300-500 °C can be considered as a superposition of poorly crystalline β-Ni(OH)2 and γ-NiOOH, and of progressively ordered NiO, which essentially agrees with the conclusions drawn from XRD analysis. As mentioned above, hydrated oxides are the result of the formation of Ni oxyhydroxide nuclei already during the first stages of thermolysis. The microheterogeneous phaseshydrated Ni oxidesobtained at 300 °C involves the products of the known series of topotactical transformations Ni(OH)2-NiOOH-Ni. The nucleus of crystallites consists of the more ordered oxide, while the less ordered hydrated oxides are localized on the surface. Co3O4 Spectra. IR (absorption) spectra of films with 100 mol % of cobalt oxide calcined at 300, 400, and 500 °C are identical with the spectra of spinel Co3O4.30 Two high-frequency, intense bands at 665 and 570 cm-1 are interpreted as M-O vibrations in octahedral sites of the spinel structure, while weaker narrow bands at 390 and (28) Kober, F. P. J. Electrochem. Soc. 1965, 112, 1066. (29) Kober, F. P. J. Electrochem. Soc. 1967, 114, 215. (30) Busca, G.; Guidetti, R.; Lorenzelli, V. J. Chem. Soc., Faraday Trans. 1990, 86, 989.
Roginskaya et al.
Figure 4. IR spectra of NiCo2O4 treated at (1) 300, (2) 400, and (3) 500 °C.
220 cm-1 are interpreted as related to tetrahedral sites.31 There is no indication of OH- group absorption. The position and the intensity of the absorption bands do not change as the calcination temperature increases, which shows the formation of a well-organized Co3O4 structure already at 300 °C. NiCo2O4 Spectra. IR (absorption) spectra of films with 33.3 mol % of nickel oxide and 66.7 mol % of cobalt oxide calcined at 300, 400, and 500 °C are shown in Figure 4. Absorption spectra have been obtained by accumulation. The presence of high back-scattering is a notable feature of these spectra. This is a result of intraband absorption by free charge carriers, and it points to the metallic nature of the main component of this phase, i.e., NiCo2O4. Two intense bands at 560 and 650 cm-1 are similar to those of Co3O4 (570 and 665 cm-1, respectively). The lowfrequency vibration decrease correlates with the metaloxygen distance increment as indicated by the increase of the spinel unit cell parameter. The latter can be explained by assuming that the larger Ni ions replace Co ions in octahedral sites. The optical density of bands at ν ) 560 and 650 cm-1 is practically constant at all calcination temperatures, which points to the formation of the spinel structure already at 300 °C. In addition, a third band at ν ) 465 cm-1 shows up at all temperatures. This band is probably due to traces of NiO not detectable by XRD. The intensity of the band increases with temperature from 300 to 500 °C. In the sample calcined at 300 °C some NiO not included in the spinel structure is probably present: the stronger bands at 400 and 500 °C might be related to increasing crystallinity; on the other hand some additional NiO can be produced by decomposition of the spinel NiCo2O4. 3.3. X-ray Photoelectron Spectroscopy. Table 2 summarizes the XPS data of surface composition. The surface layer appears to be enriched with oxygen for all samples, which is typical of partly hydrated oxides and is thought to be related to oxygen adsorption from H2O dissociation. The BE of such an oxygen species is higher than the BE of O 1s in OH groups and is equal to 532 eV. This kind of oxygen amounts to ca. 5% of the common oxygen and does not affect the relation between oxygen (31) Preudhomme, J.; Tarte, P. Spectrochim. Acta 1971, 27A, 1817.
Characterization of CoxNi1-xOy Mixed Oxides
Langmuir, Vol. 13, No. 17, 1997 4625
Table 2. XPS Surface Composition of Ni + Co Mixed-Oxide Layers nominal composition, Co/mol % 0 22.2 22.2a 66.7 66.7a 100 a
Table 3. XPS Parameters of Ni1-xCoxOy Mixed Oxides
mol % Co 22.7 32.7 6.1 6.7 30.4
Ni
O
C
30.8 7.2
55.2 57.1 56.3 56.7 55.9 61.7
14.0 13.0 11.0 11.0 10.7 7.9
26.2 26.7
After further heating at 400 °C.
Co content, x/mol %
Isat/Ipa
SOH/Sox, %b
b/ac
Co 2p3/2 Co 2p1/2
(Co 2p3/2)/ (Co 2p1/2)
0 22.2 22.2d 66.7 66.7d 100
0.05 0.12 0.14 0.24 0.05
42 44 41 37 29 42
1.55 1.91 1.67 2.42 1.55
15.3 15.3 15.6 15.8 15.3
2.91 2.41 2.41 2.13 2.49
a Intensity ratio of satellite peak at 786 eV to Co 2p 3/2 peak at 780 eV. b Ratio of OH groups to lattice O2-. c Asymmetry of the Co 2p3/2 line. d After further heating at 400 °C.
Table 4. Binding Energy of Main Lines of XPS Spectra of Ni1-xCoxOy Mixed Oxides Co content, x/mol % 22.2 22.2a 66.7 66.7a 100 a
Figure 5. Co 2p spectra of mixed-oxide layers. Co content (mol %): (a) 100; (b) 22.2; (c) 22.2, after further heating at 400 °C; (d) 66.7; (e) 66.7, after further heating at 400 °C.
in OH groups and lattice oxygen. Remarkably, the sample with 22.2% Co indicates surface enrichment in Co and, after further heating, Co is the only metal ion on the surface. Conversely, surface enrichment in Ni oxide is shown by the sample with 66.7% Co (the nominal atomic composition of NiCo2O4 is Ni 14.3%, Co 28.6%, O 57.1%). The latter result agrees with other data of XPS spectra. It is interesting to note that the amount of impurity carbon contained on the surface decreases monotonically from 14 to 8% as the Co content varies from 0 to 100% and does not correlate with the real surface composition. This suggests that the source of carbon impurities is probably the absorption of carbon contamination (hydrocarbons) in the spectrometer. The quantity of absorbed carbon depends on the porosity of the oxide layers which can monotonically vary from NiO to Co3O4. Carbonaceous residues coming from 2-propanol during the thermal decomposition step can be a substantial contribution. The carbon contamination is present on the surface of only the as-prepared samples since it is effectively removed by argon ion bombarding. The relatively low width (FWHM 1.6 eV) and the symmetry form of the carbon core level 1s indicate that all carbon atoms on the surface are chemically equivalent. There are no low-BE shoulders due to carbidic structures, or peaks at 3-5 eV higher BE that might indicate the presence of Cd, -C-O-, or CO3 groups. The BE of C 1s, equal to 284.6 eV, is characteristic of hydrocarbon contamination. The lack of correlation between the amount of carbon and the composition of the sample surface suggests that the source of carbon is to be sought in the conditions of sample preparation. Co 2p Spectra. (a) Pure Co3O4. The spectrum of spinorbit doublet Co 2p3/2 and Co 2p1/2 for the sample with 100% Co oxide is shown in Figure 5. The spectrum parameters are summarized in Tables 3 and 4 together with those of the samples with 22.2 and 66.7% Co oxide. The BE of the main peak Co 2p is 780.0 eV, its asymmetry b/a ) 1.55, its FWHM ) 2.8 eV, and the doublet separation (∼15.3 eV) for the sample with 100% Co oxide
Ni 2p3/2 BE/eV 855.6 855.8, 854.2 856.1, 854.7
Co 2p3/2 (FWHM) BE/eV
O 1s BE/eV
779.9 (2.8) 780.2 (3.2) 780.2 (3.2) 780.6 (3.9) 780.0 (2.8)
531.5, 529.3 532.2, 529.6 531.3, 529.5 531.5, 529.9 531.4, 529.6
After further heating at 400 °C.
agrees with the features of the spectra of Co3O4.32 The Co 2p peak asymmetry is the result of the overlapping of the intense line for Co3+ in octahedral spinel sites, with the twice weaker line for Co2+ in tetrahedral sites; the BE of the latter is only 1 eV higher than that of the former. The value of the doublet separation, ∼15.3 eV, and the practical absence of the satellite line at 786.0 eV are characteristic features of the low-spin state of Co3+ in octahedral sites and enable this state to be distinguished from Co3+ in a high-spin state, or from Co2+ in the same oxygen environment. The intense, well-resolved peak in the O 1s spectrum at 531.4 eV, besides the lattice oxygen peak at 529.6 eV, points to the presence of hydroxyl groups on the Co3O4 surface.33 Since the process of thermodecomposition, dehydration, and crystallization of the precursor in Co3O4 comes to completion at lower T (already at 300 °C, according to IRS and XRD) than in NiO, it seems reasonable to deduce that the hydroxyl group is a ligand, substituting O in the Co3O4 surface, but it is not involved in a separate phase (e.g., CoOOH). However, an unambiguous conclusion is difficult to draw, since Co 2p for Co3+OOH has the same BE34 as Co3+ ion in the spinel structure. (b) Mixed Oxides. The Co 2p spectrum for the 22.2% sample before further heating is identical with the spectrum of the sample with 100% Co oxide (Figure 5), i.e., its surface contains Co3O4. However, the changes in the Co 2p spectrum after heating and the spectrum parameters for the 66.7% sample (before and after heating) suggest that Co2+ states appear in octahedral sites, i.e., either CoO or Co(OH)2. The changes, compared to Co3O4, are the following: (i) a remarkable increase in the intensity of the satellite line at 786.0 eV, (ii) an enhancement of the Co 2p peak asymmetry due to the 1-2 eV higher BE for the Co 2p peaks in CoO, (iii) an increase of the peak width, and (iv) a doublet separation arising from the presence of the unresolved multiplet Co 2p2/3 in CoO. Co2+ ions have the t2g5eg2 configuration and S ) 3/2, in which d-d and O(2p) f Co(3d) charge transfer excitations may arise that enhance the number of final states, thus resulting in the multiplet structure of the Co 2p doublet (32) Chuang, T. J.; Brundle, C. R.; Rice, D. W. Surf. Sci. 1976, 59, 413. (33) Valeri, S.; Battaglin, G.; Lodi, G.; Trasatti, S. Colloids Surf. 1986, 19, 387. (34) McIntyre, N. S.; Cook, M. G. Anal. Chem. 1975, 47, 2208.
4626 Langmuir, Vol. 13, No. 17, 1997
Roginskaya et al.
Figure 6. Ni 2p spectra of mixed-oxide layers. Ni content (mol %): (a) 100; (b) 77.8.
Figure 7. Ni 2p spectra of mixed-oxide layers. Ni content (mol %): (a) 77.8; (b) 77.8, after further heating at 400 °C.
and satellite lines. The weak resolution of the multiplet is the reason for line broadening and doublet separation enhancement (up to 16.0 eV). It is to be stressed that reduced species of Co oxide are found only in the mixed oxides while they are not observed in Co3O4 (even if calcined). This implies that the interaction between Co oxide and Ni oxide microregions is the probable reason for CoO or Co(OH)2 appearance in samples with 22.2 and 66.7% Co oxide. Ni 2p Spectra. The Ni 2p2/3 spectrum for the electrode with 100% of Ni oxide is shown in Figure 6. It consists of a main line at 854.0 eV and two lines at higher BE (855.6 and 861.5 eV, respectively). The O 1s spectrum shows the presence of two resolved, intense lines: the main line is related to lattice oxygen, and the other one at higher BE (531.5 eV) is related to OH. This kind of spectrum for NiO agrees with literature works;34-40 however, there exist more than a single view about the origin of high-BE components in O 1s and Ni 2p spectra. They are alternatively interpreted (i) as a consequence of a NiO disordered surface with [NiO6]3- defects,35-37 (ii) as a result of the presence of an oxyhydroxide (∼NiOOH) in the overlayer,38 or (iii) as due to the Ni surface layer where some oxygen atoms have been replaced by hydroxyl groups.36 At any rate, the Ni 2p and O 1s spectra obtained by thermodecomposition of β-NiOOH,39 where the oxyhydroxide is present together with NiO, do not differ from the spectra of the NiO surface exposed to H2O vapor and whose composition, on the basis of isotope exchange measurements, has been estimated to be NiO1H0.2 (i.e., the hydroxide phase is absent).36 The Ni 2p3/2 and O 1s spectra for β-NiOOH and for β-Ni(OH)2 are identical; both consist of only one line: Ni 2p3/2 at 855.6 and 856.0 eV, respectively, and O 1s at 531.5 eV. For Ni2O3 the BE of the Ni 2p3/2 line is 855.6-855.9.41 Taking into consideration the data of XRD and IRS for samples with 100% Ni oxide which have revealed some hydrated forms of NiO besides dry NiO (mixtures or solid solutions of γ-NiOOH and Ni(OH)2), it is reasonable to expect these amorphous compounds to be present on the surface of NiO crystallites. The spectrum of the sample with 22.2% Co oxide confirms this point.
In the sample with 22.2% Co oxide (Figure 6), as already noted above, the Ni 2p peak intensity is strongly reduced, and the intensity ratio between the main line and the line at 855.6 eV is changed, the latter being the more intense one, while the line at 854.0 eV shows up as a shoulder. The spectrum as a whole is analogous to the case of the superposition of the Ni 2p2/3 lines for NiOOH and NiO as the hydroxide phase dominates,39 or for a strongly hydroxylated surface of NiO.36 After heating, the Ni signal disappears probably as a result of complete coverage by cobalt oxide. As NiO is covered by Co oxide, the last free area is the topmost part of NiO crystallites, which is enriched with Ni oxyhydroxide or Ni hydroxide. In the sample with 66.7% Co oxide, already before heating but more remarkably after heating, both an increase in the Ni 2p3/2 BE (Figure 7 and Table 4) and a reduction of the hydroxide O 1s intensity (Table 3) in comparison with the sample without Co are observed. This result should be interpreted as an increase in the NiOOH content in the hydroxylated layer of NiO.
(35) Hufner, S.; Wertheim, G. K. Phys. Rev. B 1973, 8, 4857. (36) Norton, P. R.; Tapping, R. L.; Goodale, J. W. Surf. Sci. 1977, 65, 13. (37) Tomellini, M. J. Chem. Soc., Faraday Trans. 1 1988, 84, 3501. (38) Roberts, M. W.; Smart, R. S. C. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2957. (39) Moroney, L. M.; Smart, L. M.; et al.; J. Chem. Soc., Faraday Trans. 1 1983, 79, 1769. (40) Kim, K. S.; Winograd, N. Surf. Sci 1974, 43, 625. (41) Lee, W. W.; Sutula, R. A.; Ferrando, W. A.; Anderson, C. R.; Lee, R. N. In The Nickel Electrode; Gunther, R. G., Cross, S., Eds.; ECS Proc. Vols. 82-4; Electrochemical Society: Pennington, NY, 1982; p 243.
4. Discussion The differences between bulk and surface composition for the samples with 22.2 and 66.7% Co can be explained in terms of the different nature of the dominating crystalline phases in each layer. The main crystalline phase of the sample with 22.2% Co has been found to be NiO with a hydrated overlayer phase (Ni hydroxide and Ni oxyhydroxide) retained even at 500 °C. Data14 are available pointing to the interaction between β-Ni(OH)2 or its partly oxidized form, Ni(OH)2‚NiOOH, with Co3+ oxyhydroxide in Ni-Cd batteries. The interaction proceeds by intergrowth of CoOOH fragments with the layer structure of nickel oxyhydroxide, the former being preferably localized at the periphery of layer crystallites. Peripheral CoOOH hinders intercalation processes limiting the oxidation of the Ni component up to high oxidation states. The inhibition of Ni ion oxidation to high valence (3.5-4) results in good reversibility during chargedischarge processes. In the case of electrocatalysis in anodic processes, prevention of Ni ion transition to high states of oxidation may impart high corrosion stability to Ni-Co oxides. In the layers with 66.7% Co, abundant NiO with Ni(OH)2 and NiOOH is localized on top of NiCo2O4 crystallites (Table 2). The interaction of Ni hydroxide regions with part of Co3+ in the spinel results in the reduction of Co3+ to CoO or Co(OH)2. Since the composition of the surface does not match the bulk composition for the layer with 66.7% Co, i.e., Co:Ni ) 2:1, the problem of ion distribution in tetrahedral and
Characterization of CoxNi1-xOy Mixed Oxides
octahedral spinel sites and of their oxidation states could not be solved. This work has revealed that the existence of two oxidation states of Ni ions (+2 and +3) in the surface of NiO-Co3O4 layers is inconsistent with the XPS data of Haenen et al.4 but agrees with the findings of King and Tseung.2 Their presence may explain the observation of redox processes (for instance, in cyclic voltammetry) in the system.
Langmuir, Vol. 13, No. 17, 1997 4627
The composition of the surface layers has been established to be as follows:
5. Conclusions As in our previous work,42 microheterogeneity (coexistence of various crystalline and amorphous phases of nanosized particles) has been found in NiO-Co3O4 oxide layers. Two states of oxidation have been found for both nickel and cobalt ions. Some of these states are the result of redox interaction of Co and Ni hydrated microregions. (42) Roginskaya, Yu. E.; Morozova, O. V. Electrochim. Acta 1995, 40, 817.
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