Preparation and Characterization of Copper-Doped Cobalt Oxide

J. Phys. Chem. B 2006, 110, 24021-24029

24021

Preparation and Characterization of Copper-Doped Cobalt Oxide Electrodes A. La Rosa-Toro,† R. Berenguer,† C. Quijada,‡ F. Montilla,§ E. Morallo´ n,*,† and J. L. Va´ zquez† Departamento de Quı´mica Fı´sica e Instituto UniVersitario de Materiales, UniVersidad de Alicante, Apartado 99, E-03080 Alicante, Spain, Departamento de Ingenierıá Textil y Papelera, UniVersidad Polite´ cnica de Valencia, Plaza de Ferra´ ndiz y Carbonell, E-03801 Alcoy (Alicante), Spain, and Instituto de Biologıá Molecular y Celular, UniVersidad Miguel Herna´ ndez, AVenida de la UniVersidad s/n, E-03202 Elche (Alicante), Spain ReceiVed: July 7, 2006; In Final Form: September 13, 2006

Cobalt oxide (Co3O4) and copper-doped cobalt oxide (CuxCo3-xO4) films have been prepared onto titanium support by the thermal decomposition method. The electrodes have been characterized by different techniques such as cyclic voltammetry, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). The effect on the electrochemical and crystallographic properties and surface morphology of the amount of copper in the oxide layer has been analyzed. The XPS spectra correspond to a characteristic monophasic Cu-Co spinel oxides when x is below 1. However, when the copper content exceeds that for the stoichiometric CuCo2O4 spinel, a new CuO phase segregates at the surface. The analysis of the surface cation distribution indicates that Cu(II) has preference for octahedral sites.

1. Introduction Conductive transition metal oxides (TMOs) form an important and diverse family of materials which have attracted large attention in many fields of technological interest owing to their outstanding electronic, optical, magnetic, and catalytic properties.1-3 Tetragonal rutile-like TMOs containing noble metal cations have been successfully applied as anode coatings (commonly known as dimensionally stable anodes, DSAs) in a number of important electrolytic processes, such as oxygen and chlorine evolution or the electrochemical oxidation of biorefractory organic pollutants in wastewaters.4-11 However, spinel-type cobalt oxidebased coatings have emerged as a cheaper alternative to DSA electrodes in alkaline media, because they possess long-term stability under anodic conditions, good electrical conductivity and high electrocatalytic activity toward oxygen/chlorine evolution reactions,2,4,12-16 and the oxidation of organic compounds17-21 and cyanide.22 In addition, these materials have found potential use in a host of solid-state applications, such as gas sensors, magnetic materials, electrochromic devices, coatings in electrodes for lithium-ion rechargeable batteries and fuel cells, solar absorbers, and heterogeneous catalysts.23-31 Substitution of cobalt ions by foreign divalent metal ions can result in spinel structures with enhanced catalytic activity and stability. For this reason, the morphology, crystallinity, composition, and electrochemical properties of binary spinel oxides of the type MxCo3-xO4 (with M d Ni, Cu, Mn, etc.)15,19,24,31-35 and of ternary spinel oxides involving Ni-Cu-Co14,36,37 or CuZn-Co38 have been the subject of extensive research in an attempt to establish clearly defined composition/structure/ properties correlations. Among them, copper-substituted cobaltite spinels combine advantageously high stability and activity with low cost and availability.14,15,32,34,37 Despite the fact that * Corresponding author: E-mail: [email protected]. Tel.: 34-965909590. Fax: 34-965903537. † Universidad de Alicante. ‡ Universidad Polite ´ cnica de Valencia. § Universidad Miguel Herna ´ ndez.

binary Cu-Co spinel oxides prepared as thin films on conducting substrates have been thoroughly investigated in the literature, most of the data available on their structure, morphology, and electrochemical properties refer to CuCo2O4. Only a few reports deal with the characterization of thin films of mixed Cu-Co oxides with true Cu/Co ratios other than ∼0.5.15,37 In particular, the work by Angelov et al.39,40 and Li et al.41 remains among those that most systematically studied the variation of the crystalline structure and surface composition of CuxCo3-xO4 powders upon a wide range of x values. It is widely accepted that Cu-Co mixed oxides tend to form a single phase with a partially inverted spinel structure,31,34,37,39,41 and the segregation of new cobalt and/or copper oxide phases strongly depends on the Cu/Co ratio in the precursor salt as well as the calcination temperature. However, the distribution of the mixed cationic valences and the surface morphology, which determine the catalytic activity and other physicochemical properties, are rather variable because they are profoundly conditioned by the preparation method (either chemical coprecipitation,41,42 thermal decomposition,14,31-33,36-40 spray pyrolysis,33,34 or sol-gel routes15), the chemical nature of the precursor and substrate, and the annealing atmosphere. Therefore, the characterization of thin films prepared with a particular method and under concrete experimental conditions is necessary to understand their electrochemical performance. This work is aimed at the study of the electrochemical properties and stability under anodic conditions of copper-doped cobalt oxide spinels of nominal composition CuxCo3-xO4 with x ranging systematically between 0 and 1.5. The binary oxides were prepared in the form of thin films by the thermal decomposition of mixed nitrate salt precursors impregnated onto Ti. The effect of the copper content on the surface redox activity and service life of Cu-Co mixed spinels will be reported and correlated to their microstructure, composition, and crystalline state as determined by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray diffraction (XRD). Also, a study of the surface composition and valence

10.1021/jp0642903 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/04/2006

24022 J. Phys. Chem. B, Vol. 110, No. 47, 2006

La Rosa-Toro et al.

state of copper as well as its lattice site preferences will be undertaken by means of X-ray photoelectron spectroscopy (XPS). 2. Experimental Section 2.1. Preparation of Cobalt Oxide Electrodes. Pure Co3O4 in powder form was supplied by Aldrich. Copper-cobalt spinel oxide electrodes of nominal composition CuxCo3-xO4 (with 0.0 < x e 1.5) were prepared as thin films by thermal decompostion of proper salt precursors onto a Ti support. The salt precursors were made up of Co(NO3)2‚6H2O (A.C.S Aldrich) and Cu(NO3)2‚3H2O (MERCK p.a.) dissolved in absolute ethanol (J.T. Baker). The nitrate salts were mixed in stoichiometric amounts according to the desired nominal composition. The total metallic cation concentration was kept constant at 0.5 M. Prior to the decomposition process, Ti plates (1 × 1 × 0.05 cm, Goodfellow 99.6% purity) were degreased in acetone, etched in a boiling 10% oxalic acid solution for 1 h, and finally rinsed with distilled water. The precursor solution was spread over the Ti surface with the aid of a brush. The solvent was dried at 70 °C and the electrode was subsequently calcined at 350 °C for 10 min for the thermal decomposition of the salt and metal oxide formation to be accomplished. The Ti sheets were coated with succesive layers of the oxides by repeating this procedure. A final annealing step was carried out for 1 h at the same temperature. Photoelectron spectroscopy analysis showed negligible amounts of N on the catalyst surface, which suggests full precursor decomposition during the calcination step. The most suitable number of pyrolysis steps were found to correspond to an oxide loading ranging between 3.00 and 3.50 mg cm-2 as determined by weight difference. The surface texture of the electrodes was studied by scanning electron microscopy (Hitachi S-3000N) that was coupled to a Rontec X-ray detector for energy dispersive X-ray microanalysis. X-ray diffraction measurements were performed using a Seifer JSO-DEBYEFLEX 2002 diffractometer with a Ni-filtered Cu KR radiation (λ ) 1.541 Å). The profile intensities were measured step by step (0.05° in 2θ) for a whole time of 3000 s. Cell parameters were calculated by a computer program using the peak position relevant to the KR1 monochromatic radiation, obtained by fitting the experimental range with a pseudo-Voigt function per peak plus a background line. Line broadening analysis was performed to determine the volume-weighed average crystallite size. The optimized pseudo-Voigt function obtained for each diffraction peak is Fourier transformed and deconvoluted from the instrumental broadening by Stokes’ method.8 Electrochemical measurements were carried out in a conventional three electrode cell, the counter electrode was a platinum electrode, and the potentials were referred to a reversible hydrogen electrode (RHE) immersed in the same solution. The aqueous 0.1M NaOH solutions were prepared from Merck p.a. and ultrapure water (Purelab Ultra from ElgaVivendi, 18.2 MΩ cm). Cyclic voltammograms were obtained at a constant sweep rate of 20 mV s-1. The current densities were calculated using the geometric area of the electrodes (2 cm2). The XPS spectra have been obtained in a VG-Microtech Multilab electron spectrometer by using the unmonochromatized Mg KR (1253.6 eV) radiation from a twin anode source operated at 300 W. Photoelectrons were collected into a hemispherical analyzer working in the constant energy mode at pass energy of 50 eV. The base pressure in the analysis chamber was maintained at 5 × 10-10 mbar. The binding energy (BE) scale

Figure 1. SEM image of a Ti/Co3O4 electrode prepared with 25 pyrolisis processes.

TABLE 1: Service Life in the Accelerated Test Performed at 100 mA cm-2 in 1M NaOH Solution for Several Ti/ CuxCo3-xO4 Electrodes nominal copper content x

number of deposition steps

service life (efficiency, Ah mg-1)

0 0 0 0 0.2 0.5 0.8 1 1.5

10 17 25 30 25 25 25 25 25

114 112 184 66 160 150 122 114 119

was referenced against the main C1s line of adventitious impurities set at 284.6 eV. Peak energies were given to an accuracy of (0.2 eV. All XPS curves were fitted with mixed Gaussian (70%)-Lorentzian (30%) line shape functions after nonlinear Shirley background subtraction. Peak areas were normalized by using appropriate atomic sensitivity factors. The analyzed samples were conducting well enough to prevent surface charging effects. 3. Results and Discussions 3.1. Electrode Stability. One of the main problems in the use of metal oxide electrodes in the treatment of wastewater is the short service life under anodic polarization conditions in aqueous media. Accelerated service life tests were performed by anodic polarization of the different electrodes in a 1M NaOH solution at 100 mA cm-2. The anode potential was monitored as a function of time and a rise of at least 5 V over the initial potential value was regarded as a sign of electrode deactivation. Table 1 shows the service life for different electrodes, expressed in terms of efficiency, that is, as the ratio of the total charge passed to the oxide loading (Ah mg-1). Undoped cobalt spinel electrodes showing the best performance are obtained after 25 deposition steps. The same number of deposition processes was employed in the fabrication of Cu-Co binary oxides. It can be observed that the introduction of copper in the oxide layer maintains the service life within the same order of magnitude, yet the lifetime is shortened as the amount of copper is increased. In all cases, the amount of titanium detected by EDX is lower than 1% indicating that the titanium support is practically covered when 25 deposition steps are applied. According to the typical sampling depth in EDX analysis, an estimated thickness of at least 1 µm could be assumed for the oxide coating. 3.2. SEM and EDX Characterization. Figure 1 shows the SEM micrograph of a Ti/Co3O4 electrode prepared after 25

Copper-Doped Cobalt Oxide Electrodes

J. Phys. Chem. B, Vol. 110, No. 47, 2006 24023

Figure 2. SEM images of Ti/CuxCo3-xO4 electrodes prepared with 25 pyrolisis processes: (a) x ) 0.2, (b) x ) 0.5, (c) x ) 0.8, (d) x ) 1.0.

Figure 3. SEM images of (a) oversaturated Ti/CuxCo3-xO4 and (b) Ti/CuO electrodes prepared with 25 pyrolisis processes.

deposition steps. The substrate is uniformly covered by the oxide and the coating surface appears rather smooth and compact, especially when compared to those obtained by others onto Ti or Ni substrates via thermal decomposition or sol-gel methods.15,37 Figure 2 shows the morphology of Ti/CuxCo3-xO4 electrodes with 0.0 < x e 1.0. If the appearance of macrodefects are disregarded, the surface retains the smooth and compact texture at low Cu doping levels (Figure 2a). Upon increasing the amount of incorporated Cu, the surface becomes progressively more porous and rough (Figure 2b-d). At x ) 1.5, the surface undergoes a noticeable change and reaches a granular and highly porous morphology (Figure 3a), which resembles that observed for a Ti/CuO electrode (Figure 3b) prepared under the same conditions as the cobalt oxide electrodes. Further data from XRD and XPS (see below) suggest that Cu(II) ions enter the cobalt spinel lattice to form a solid solution until the stoichiometric (i.e. Cu-saturated) CuCo2O4 is obtained. At oversaturation (x > 1.0), a surface segregation of a new CuOlike phase seems to occur, which would explain the drastic morphological change observed in Figure 3a. The bulk composition of the CuxCo3-xO4 coatings was determined by EDX microanalysis. Table 2 summarizes the nominal ratio of copper to cobalt, together with the experimental ratio as determined by EDX. A fairly good correlation exists between the nominal and EDX Cu/Co values at x ) 0.5, 0.8,

TABLE 2: Nominal and Experimental Atomic Ratios for Ti/CuxCo3-xO4 Electrodes Obtained by EDX (Bulk) and XPS (Surface) Techniques atomic ratio x 0.0 0.2 0.5 0.8 1.0 1.5

nominal Cu/Co 0.07 0.20 0.36 0.50 1.00

(Cu/Co)EDX 0.04 0.19 0.37 0.51 0.56

(Cu/Co)XPS

(O/M)EDX

(O/M)XPS

0.11 0.35 0.78 0.78 6.12

1.47 2.80 1.67 2.00 1.25 1.36

1.64 1.42 1.38 1.51 1.38 1.37

and 1.0. In the case of the copper oversaturated cobalt spinel coating (x ) 1.5), the Cu/Co ratio determined by EDX is only slightly higher than that of the stoichiometric copper cobaltite. Because the sampling depth of this EDX analysis is at least 1 µm, it could be concluded that both spinels possess a similar bulk composition. This conclusion apparently contradicts the assumption that a segregated CuO-like phase may occur in oversaturated copper spinels, unless the segregated phase was much thinner than the total sampling thickness probed by EDX. XPS data will assist in supporting this proposal. The formation of a distinct Cu-rich phase in spinel oxides has been often reported to occur when either the annealing temperature is higher


La Rosa-Toro et al.

Figure 4. Stabilized cyclic voltammograms of the Ti/CuxCo3-xO4 electrodes in 0.1M NaOH solution: (a) x ) 0, (b) x ) 0.2, (c) x ) 0.5, (d) x ) 0.8, (e) x ) 1, (f) x ) 1.5. V ) 20 mV s-1.

than 300-350 °C32,41 or the Cu/Co ratio in the oxide is well above 0.5.39,41,42 The O/M values (O the oxygen and M the total amount of metal) determined by EDX also are listed in Table 2. It is clear that the amount of oxygen is higher than the theoretical one for all studied spinels, including pure Co3O4. It was pointed out earlier40,41 that the oxygen excess is inherent to cobalt spinels and could be related to structural defects involved in the conduction mechanism. However, rather divergent O/M values were obtained for the different copper cobalt spinels. We attribute this scattering to severe contamination of some samples after prolonged atmospheric exposure and during handling. As will be discussed further below, surface O/M values derived from XPS appear much less dispersed, because the amount of O-containing contaminants could be estimated from carbon photolelectron peaks.

3.3. Cyclic Voltammetry. Figure 4 shows the cyclic voltammograms of Ti/CuxCo3O4 electrodes in a 0.1M NaOH solution. In the case of the Ti/Co3O4 electrode, two characteristic redox peaks are observed in the voltammogram at a half-wave potential of 1.20 V (A1/C1) and 1.48 V (A2/C2), respectively. The sharp current rise occurring at potentials above the A2 peak corresponds to the oxygen evolution reaction (OER). As previously reported in the literature,28,32,33,43,44 the two redox features below the OER can be assigned to Co(II) T Co (III) and Co (III) T Co(IV) surface transitions according to the following reversible half-reactions:

A1/C1: Co3O4 + OH- + H2O T 3CoOOH + e-

(1)

A2/C2: CoOOH + OH- T CoO2 + H2O + e-

(2)


J. Phys. Chem. B, Vol. 110, No. 47, 2006 24025 TABLE 3: Lattice Parameters and Unit Cell Volume for the Different Ti/CuxCo3-xO4 Electrodes Calculated from Diffraction Patterns lattice parameter

Figure 5. X-ray diffraction patterns for thermally prepared Ti/CuxCo3-xO4 electrodes, Ti/CuO electrodes, and pure Ti.

yet the nature of the A1/C1 redox couple could also be related to33,44

A1/C1: Co(OH)2 + OH- T CoOOH + H2O +e-

(3)

As copper is incorporated into the cobalt spinel lattice (0.0 < x e 1.0), the first redox couple (A1/C1) progressively vanishes, and both the second transition and the OER shift to less positive potentials. This latter fact could be associated with an electrocatalytic effect in the OER by the copper in the electrode composition. When x exceeds 1.0, i.e., for a copper oversaturated spinel, a drastic change in the voltammogram profile is observed, which is characterized by the complete loss of both Co3O4 distinctive redox couples and a significant increase in the capacitive charge. Moreover, the OER appears at even less positive potentials and the voltammogram becomes more irreversible. These voltammetric changes parallel the modifications observed at the morphological level (Figure 3a), and hence they could be attributed to the growth of a CuO-like surface phase. Also, the marked enhancement of the surface roughness should lead to an increase in the surface active area and, consequently, in the measured voltammetric charge. 3.4. X-ray Diffraction. Figure 5 shows the X-ray diffraction patterns of Ti/CuxCo3-xO4 electrodes with different levels of Cu doping (0.0 < x e 1.5). For comparison purposes, the figure also includes the diffractograms of the Ti substrate and a Ti/ CuO electrode fabricated by following the same experimental procedure as for the cobalt spinel electrodes. The XRD for sample Ti/CuO shows reflections at 2θ ) 35.45, 38.73, 38.92, and 58.31 that could be indexed to a monoclinic CuO phase according to the data corresponding to a pure CuO phase (JCPDS-ICDD 05-0661). The XRD for sample Ti/Co3O4 shows reflections at 2θ ) 31.29, 36.88, 44.84, 59.40, and 65.29° that could be indexed to a cubic spinel lattice belonging to the Fd3m space group. Both the position and the relative intensities of the above diffraction lines are in agreement with data corre-

electrodes

a(Å)

V(Å3)

crystallite size (Å)

Co3O4 (JCPDS) Ti/Co3O4 Ti/Cu0.5Co2.5O4 Ti/Cu0.8Co2.2O4 Ti/CuCo2O4 Ti/Cu0.5CuCo1.5O4

8.084 8.093 8.112 8.128 8.130 8.134

528.29 530.06 533.80 536.97 537.36 538.16

99 81 58 38 45

sponding to a pure Co3O4 phase (JCPDS-ICDD 9-418). Extra lines should be attributed to reflections at (002) and (103) planes of the underlying Ti substrate. Upon increasing the Cu content, the XRD patterns remain essentially unchanged, and interestingly no peaks corresponding to CuO can be distinguished. No evidence for the formation of a CuO crystalline phase was found at any Cu content, not even when the stoichiometric amount was superseded. Even though the SEM and XRD results are apparently conflicting, these results can be conciliated if it is assumed that CuO segregates in oversaturated spinels to form either an amorphous surface phase or a crystalline phase with particles of very small size, making it hardly detectable by XRD. The unit-cell parameter for the cobalt oxides has been calculated according to the Bragg’s formula for face-centered cubic crystals and by using the main indexed planes of the cobalt spinel, namely, (220), (311), (400), (511), and (440). The resulting mean values of the lattice parameter as well as the unit cell volume are shown in Table 3 for the different electrodes prepared in this work. The cell parameter of the thermally prepared Co3O4 electrode is slightly higher than that reported for standard powders (JCPDS-ICDD 9-418). This deviation may reflect the influence of the Ti support and the particular method of preparation on the cystallographic properties of the oxide. In fact, Fradette et al.33 reported that the lattice parameter of Cu0.9Co2.1O4 electrodes prepared on Ni was higher than on Ti, and Gautier et al.34 showed a significant variation of the cell size of CuCo2O4 formed by spray pyrolysis on glass depending on the annealing atmosphere. In our case, a continuous increment in the cell parameter is observed as the amount of copper in the oxide layer is increased. This result can be explained by taking into account the ionic radius of copper and cobalt in different coordination sites (0.73 Å and 0.62 Å for Cu(II) in octahedral and tetrahedral coordination, respectively, against 0.65 Å and 0.57 Å for Co(II) at low-spin octahedral sites and tetrahedral sites, respectively). Then, the increase in the unit cell parameter indicates a substitution of the cobalt by copper, although it cannot be ascertained whether Cu(II) occupies octahedral sites, tetrahedral sites, or both of them in the pristine cobalt oxide. Finally, a significant line broadening is observed in the diffraction pattern of copper cobaltites as the x value is increased. The average crystallite size, calculated according to the Scherrer formula, was found to decrease with the amount of copper in the spinel up to x ) 1.0 (Table 3) and then to increase slightly for the oversaturated spinel oxide. The overall decrease in the crystallite size also would explain the enhancement of the surface active area in copper-substituted Co spinels. 3.5. XPS Measurements. Because electrochemical chargetransfer is essentially an interfacial phenomenon, an account of the surface chemistry of CuxCo3-xO4 would assist in the understanding of the role of the dopant. X-ray photoelectron spectroscopy is a surface-sensitive probe that can provide valuable information on the surface composition and allow


La Rosa-Toro et al. TABLE 4: Summary of Core-Level Co 2p and Cu 2p Photoelectronic Data in Ti/CuxCo3-xO4 Electrodes Co 2p Co 2p3/2

∆Coa

Cu 2p3/2

∆Cua

Isat/Imain

0.2 0.5 0.8 1.0 1.5 Ti/Co3O4 Co3O4 Ti/CuO CuO

779.9 779.8 779.8 779.8 779.9 779.9 780.3

9.2 9.4 9.7 9.6

934.4 934.4 934.3 934.3 933.8

7.8 7.5 7.6 7.6 9.1

0.62 0.65 0.62 0.62 0.50

933.8 933.6 47 933.8 42

9.1

0.47

a

Figure 6. Co 2p XPS spectra of a set of Ti/CuxCo3-xO4 electrodes.

Figure 7. Cu 2p XPS spectra of a set of Ti/CuxCo3-xO4 electrodes.

distinction of the different local atomic environments (i.e valence state and coordination) of an element. 3.5.1. QualitatiVe Analysis. Wide scan spectra (not shown) of a set of Ti/CuxCo3-xO4 electrodes (0 < x e 1.5) display the characteristic core-level photoemission lines of Co, Cu, O and C (adventitious contamination). No evidence for Ti was found, which is in accordance with EDX data that the coating is homogeneous and thick enough to cover completely the underlying substrate and prevent it from direct electrolyte contact. The detailed XPS spectra of the Co 2p and Cu 2p transitions for the series of thermally prepared Ti/CuxCo3-xO4 electrodes are stacked in Figures 6 and 7, respectively. For comparison, the Cu 2p spectrum of a Ti/CuO specimen prepared by following the same experimental protocol also is included in Figure 7. The binding energies of the main Co 2p3/2, Cu 2p3/2, and the energy shift of their respective shake-up satellites, ∆,

Cu 2p

x

∆ ) BEmain

peak

9.5 9.3

- BEsat.

are summarized in Table 4. Available data for pure powder oxides are also tabulated for comparison. The Co 2p spectrum of a Ti/Co3O4 electrode (Figure 6, top) shows two broad and asymmetric main peaks separated by a spin-orbit splitting of 15.1 eV. The Co2p3/2 component is centered at 779.9 eV (fwhm ) 3.1 eV) and is accompanied by a shake-up satellite of low intensity shifted by 9.5 eV on average to higher BEs. The satellite is characteristic of paramagnetic Co(II) high-spin complexes. The general line shape of the Co 2p spectrum and the position of the above spectral features are all in close agreement with data reported earlier for pure Co3O4 crystals with spinel structure in which the 2p3/2 peak lies between 779.6 and 780.2 eV.37,40-42,45 In our case, the main Co 2p3/2 component in Ti-supported Co and Cu-Co mixed oxides appears shifted to lower BEs compared to the spectrum of pure Co3O4 taken in the same spectrometer (Table 4). This shift may stem from either an interaction with the support or from particle size effects. In particular, unexpected core-level shifts can be observed for oxide particles in the nanoscale domain owing to final state contributions in the photoemission process, related to altered Madelung potential at the atom in a cluster of limited size in comparison with the field in an infinite periodic lattice.3 Despite XRD data that reveal our thermally prepared oxides are composed of nanocrystallites, it still needs to be confirmed that they are true nanostructured materials, i.e., the typical size of an agglomerate of crystallites is within the nanometric scale. Therefore, it is difficult to ascertain at the present time the cause for the observed Co core-level shift in Ti-supported Cocontaining oxides. It should be stressed that the absence of a strong satellite at 5.0-6.0 eV above the main 2p3/2 component allows the growth of CoO or Co(OH)2 inactive phases to be ruled out.37,45,46 Accordingly, the surface redox transition at 1.20 V in Figure 4 should be exclusively attributed to the reaction in eq 1. As found previously by others,36,37,40 no significant change in both the spectral profile and the BE of the main peaks and satellites is observed as the amount of copper increases. This result suggests that Cu(II) enters the oxide lattice to form a solid solution, thus preserving the spinel structure at the whole range of nominal compositions. This result also is shown by the absence of difraction lines other than those for cobalt spinel in the XRD patterns (Figure 5). The case of spinel with x ) 1.5 (Figure 6, bottom spectrum) requires detailed discussion, which will be given further below. The Cu 2p spectrum of copper cobaltites where 0 < x e 1 (Figure 7) shows the typical p3/2-p1/2 spin-orbit doublet and an intense shake-up satellite at the high binding energy side, characteristic of paramagnetic Cu(II) compounds. The line shape of the core-level main peaks appears broad and asymmetric,


J. Phys. Chem. B, Vol. 110, No. 47, 2006 24027 a subject of some controversy and has been assigned to a wide variety of species such as surface OH- originating from either hydroxylation or as a part of surface oxyhydroxydes, chemisorbed oxygen, oxygen ions in low coordination, and oxygencontaining surface contamination.49,50 In our case, most of the lateral structure appears to be contributed by O-containing carbon functionalities present in adventitious contamination. The amount of oxygen contamination can be estimated from the C1s peak (Figure 8b) as follows:51

[O]cont ) [C-OH] + 2[COOH]

Figure 8. Representative O1s and C1s XPS spectra of a Ti/CuxCo3-xO4 electrode (x ) 0.8).

with a clearly defined shoulder at the lower BE side. These features have been widely recognized as characteristic of monophasic Cu-Co mixed oxides.31,36-38,40,41 The Cu 2p shape of copper cobaltites with x e 1 contrasts with that of CuO supported on Ti, which displays narrow and fairly symmetric main peaks.47 The shake-up satellite in CuO also presents a different spectral profile with two overlapped but readily discernible peaks. On the contrary, the satellite appears as a broad single feature in copper cobaltites. In addition, the Cu 2p3/2 peak in CuxCo3-xO4 (934.4 eV) is shifted by ca. 0.5 eV above the main core level in CuO (Table 4). Moreover, the satellite-main peak energy gap, ∆Cu, is lowered by ca. 1.5 eV with respect to CuO, and their intensity ratio is noticeably increased (Table 4). According to Sawatzky theory,48 the shorter separation between the Cu 2p3/2 line and its satellite peak and the higher value of the Isat/Imain ratio points to a decrease in the covalent character of the Cu-O bond in copper cobaltites as compared to CuO. When x ) 1.5, i.e., the copper content is higher than that for the stoichiometric CuCo2O4 spinel (Table 2), the structure and energy position of the Cu 2p photoemission signal departs from that recorded up to x e 1 and mimics that of cupric oxide. This change strongly suggests that a new CuO phase segregates at the surface of copper cobaltites with x exceeding 1.0. This is in line with the drastic change observed in the surface morphology (Figure 3a) which adopts a grainy and highly porous texture resembling that of CuO (Figure 3b). A representative spectrum of the O1s line in copper cobaltites is depicted in Figure 8a. The spectrum shows an intense main peak with a shoulder at the high BE side and can be fitted with two components at 529.5 ( 0.2 eV and 531.3 ( 0.2 eV. The first contribution has been unequivocally assigned to lattice oxygen ions in Co-containing spinels45,49,50 and other transition metal oxides.50 By contrast, the origin of the lateral structure is

(4)

where the surface concentration of carbon functional groups is derived from the intensity area, divided by the carbon sensitivity factor, of the respective fitted peaks in the C1s spectrum. Upon subtracting [O]cont, the amount of high BE oxygen that truly belongs to the analyzed material diminishes to no more than 5-10% of the total oxygen. 3.5.2 QuantitatiVe Analysis. The Cu/Co and the O/M (where M stands for Co + Cu) ratios in the bulk (obtained from EDX measurements) and at the surface (determined by XPS) are compared in Table 2 for pure and copper-doped cobalt spinel oxides. The surface oxygen content was derived from integrated O1s photolelectron peaks after correction from the carbonrelated oxygen impurities, as specified above. The surface Cu/Co atomic ratio is in excess over the bulk ratio at 0 < x e 1. The surface copper enrichment increases as x is raised and seems to stabilize at x ∼ 0.8-1.0. Foreign metal enrichment at the surface has been described earlier for Cuand Ni-substituted cobalt oxides prepared by thermal decomposition.24,33,36,40,41 For Cu-Co spinel oxides with x ) 1.5, the Cu/Co ratio found by XPS rises sharply above the nominal expected value, while the bulk value remains slightly higher than that corresponding to the stoichiometric copper cobaltite, CuCo2O4. This finding further supports the development of a surface CuO phase in Cu-Co mixed spinels when the nominal composition exceeds x ) 1.0, leaving behind a mixed oxide of composition close to CuCo2O4. In other words, it seems to be confirmed that the Cu-Co spinel structures reach saturation at a Cu/Co ratio of 0.5 under our experimental conditions. When the saturation ratio is superseded, the segregation of a surface CuO phase is favored. Under this assumption, a simple estimate worked out on the basis of the experimental bulk composition for CuxCo3-xO4 (where x ) 1.5) suggests that the oxide thin film should be composed of about 10% CuO by mole (or about 4% CuO by weight). Because the copper-rich crystalline phase occurs at the very surface and represents a very low percentage of the material analyzed, it is not surprising that its characteristic difracttion lines were too weak to be detected by XRD. The coexistence of two different crystalline phases would explain both the remarkable variations of the Cu 2p photoelectron spectrum and the unchanged Co 2p spectrum. Furthermore, the segregation of a CuO surface phase may be at the origin of the observed change in the voltammetric profile and in the surface morphology at x ) 1.5. In regard to the O/M atomic ratio, both bulk and surface data are higher than the theoretical stoichiometric value (1.33). A possible source for the surface oxygen excess might be some overestimation in the measured O1s peak area owing to overlapping with the Co L3M23M45 Auger transition when Mg KR radiation is used. However, it has been pointed out by many researchers that oxygen excess is inherent to the family of cobalt spinels.40,41 Nonstoichiometric oxygen is thought to be closely related to the electrocatalytic properties of structure defect sites


La Rosa-Toro et al.

Figure 9. Representative fitting of a Cu 2p XPS spectrum of a Ti/ CuxCo3-xO4 electrode (x ) 0.8).

TABLE 5: Binding Energies and Surface Relative Amount of the Various Cu Cationic Species in Ti/CuxCo3-xO4 Electrodes, 0.0 < x e 1.0 Cu+ x

BE/eV

0.2 0.5 0.8 1.0

932.1 931.9 931.9 931.8

Cu2+ (B)

Cu2+ (A)

relative relative relative Cu+2(A)/ % BE/eV % BE/eV % Cu+2(B) 14.6 15.3 18.4 16.9

934.4 934.2 934.1 934.1

81.0 74.7 71.1 69.8

936.0 936.0 936.0 936.0

4.4 10.0 10.5 13.3

0.23 0.34 0.41 0.42

in which a part of the M+2 and M+3 are converted into M+3 and M+4 ions and hence increasing the electrical conductivity.4,37 3.5.3 Surface Cation Distribution. Cobaltites of the formula Co3O4 adopt a normal spinel crystal structure, which can be envisioned as a fcc packing of oxygen anions with Co(III) filling half the octahedral interstitial sites, and Co(II) filling one-eighth of the tetrahedral sites. The substitution of Co ions by foreign divalent transition metals is known to promote an inhomogeneous distribution of cations and produce a partially inverted spinel structure with foreign and cobalt ions occupying both octahedral and tetrahedral sites. In the case of Cu-doped cobaltites, the oxidation state distribution can be formally written as Cux-λ+2Co1-x+2Coλ+3Cuλ+2Co2-λ+3O4,38 0 < x e 1, where λ stands for the degree of inversion of the spinel structure. In this formula, cations preceding brackets are filling tetrahedral sites (A sites) and those within brackets occupy octahedral sites (B sites). It is well-established that the true stoichiometry as well as the cation distribution effectively condition some properties of technological interest of the spinel oxides, such as the catalytic activity. Therefore, the determination of the distribution of cations over tetrahedral and octahedral sites is an important issue in the surface chemistry of these materials. In principle, the sensitivity of XPS to atomic local environments should make it a powerful tool to elucidate valence states and lattice site preferences at the surface. The analysis of the various lattice sites and valence states of Cu cations was done by employing curve fitting procedures to the Cu 2p3/2 line. An example of peak fitting of a representative Cu 2p spectrum is given in Figure 9. In all CuxCo3-xO4 spinels with x e 1, the main Cu 2p3/2 peak was deconvoluted by using three components at 932.0, 934.2, and 936.0 ( 0.2 eV (Table 5), which have been found to correspond to Cu(I), Cu(II) in octahedral sites and Cu(II) in tetrahedral sites respectively.14,33,36,41 The presence of cuprous ions in binary Cu-Co spinel oxides prepared by thermal decomposition was frequently reported by earlier authors.14,36,42 In particular, XPS studies of coppercontaining manganite52 and ferrite53 spinels have attributed Cu

signals at about 932.0 eV to Cu(I) species in tetrahedral environments. However, Cu+ ions can arise from the wellknown X-ray induced reduction of Cu(II) to Cu(I), which was reported to increase in the presence of an overlayer of adventitious carbon.54 In the calculations below, we will assume that Cu(I) species originate from reduction of Cu(II) at tetrahedral sites, and hence they should be computed as such to obtain the relative occupancy of tetrahedral to octahedral sites. In all cases, the peak at 934.2 eV is the most prominent one (Table 5), which means that Cu(II) distributes preferentially over sites with an octahedral coordination. The experimental values of the Cu2+(A)/Cu2+(B) atomic ratio (Table 5), where Cu2+(A) signifies all tetrahedrally coordinated cationic Cu, show that Cu(II) ions have preference for octahedral sites, yet the relative amount of tetrahedrally coordinated Cu(II) rises slightly as x increases. It also is confirmed that mixed Cu-Co spinels with x ) 0.8-1.0 possess similar surface composition, and moreover a rather similar distribution of the mixed oxidation states. These results are reasonable, because the well-documented Jahn-Teller distortion of octahedral Cu(II) removes the degeneracy of the eg orbital, lowering the energy of the dz2 orbital below that of the dx2-y2 orbital. Then two valence electrons can pair in the stabilized dz2 orbital and the resulting geometry is more energetically favored. Also, the energy site preference (defined as the difference between crystal field stabilization energies for octahedral and tetrahedral coordinations) for Cu(II) is close to that for Co(III) which tends to fill octahedral sites.41 This preferred occupancy may explain that the electrocatalytic activity of Ti/CuxCo3-xO4 electrodes toward OER is enhanced when x is increased within the range 0 < x e 1. It has been found for numerous substituted spinels that divalent ions at surface octahedral sites exceed substantially in activity divalent ions at surface tetrahedral sites.31,55 4. Conclusions Cobalt spinel oxides doped with different amounts of copper were prepared by thermal decomposition of the nitrate precursors. These electrodes were characterized by SEM, cyclic voltammetry, XRD, and XPS techniques. Accelerated service life tests of the Ti/CuxCo3-xO4 electrodes were performed by anodic polarization in 1M NaOH solution. Undoped cobalt spinel electrodes show the best performance after 25 deposition steps. The introduction of copper in the oxide layer maintains the service life within the same order of magnitude. The surface morphology becomes gradually more porous and rough as the copper content increases up to x ) 1.0. At x ) 1.5, SEM micrographs show a granular and highly porous surface that is very much like that of a Ti/CuO electrode prepared under the same conditions. The voltammetric behavior shows two redox peaks in the case of the Ti/Co3O4 electrode at a half-wave potential at 1.2V and 1.48V corresponding to the Co(II) T Co(III) and Co(III) T Co(IV) transitions, respectively. As copper is incorporated into the cobalt oxide layer, the first redox couple decreases, and both the second process and the oxygen evolution reaction shift to less positive potentials indicating an electrocatalytic effect by the copper in the electrode composition. XRD diffraction analysis shows that the Ti/CuxCo3-xO4 electrodes display a similar diffraction pattern corresponding to a monophasic Cu-Co mixed spinel oxide, which indicates that the Cu(II) ions are filling interstitial sites within the bulk structure of the cobalt oxide. As a consequence, the unit cell is expanded and the average crystallite size is enlarged.

Copper-Doped Cobalt Oxide Electrodes The analysis of the surface composition by XPS confirms that Cu(II) is effectively incorporated into the spinel lattice to form a solid solution up to x ) 1.0, as deduced from characteristic Cu 2p and Co 2p core-level spectra. When x ) 1.5, the structure and energy position of the Cu 2p photoemision signal changes from that recorded with copper doped cobaltites with x e 1 and is similar to that of CuO. Nevertheless, the Co 2p region remains unchanged. These results strongly suggest that a new Cu O phase segregates at the very surface of CuCo spinel oxide at this copper content. This new phase may be the origin of the change in the voltammetric profile and in the surface morphology. The analysis of surface cation distribution indicates that the Cu(II) ions have preference for octahedral sites, even though the relative amount of Cu(II) at tetrahedral positions rises as the amount of copper increases in the oxide layer. Acknowledgment. Finantial support by the Generalitat Valenciana (GV05/136) and Ministerio de Educacioń y Ciencia (MAT2004-1479) projects are gratefully acknowledged. References and Notes (1) Cox, P. A. Transition Metal Oxides: An Introduction to Their Electronic Structure and Properties; Clarendon Press: Oxford, 1992. (2) Tejuca, L. G.; Fierro, J. L. F.; Tascon, J. M. AdVances in Catalysis; Academic: New York, 1989; Vol. 36. (3) Fernańdez-Garcıá, M.; Martıńez-Arias, A.; Hanson, J. C.; Rodriguez, J. A.; Chem. ReV. 2004, 104, 4063-4104. (4) Trasattti, S., Ed. Studies in Physical and Theoretical Chemistry. Electrodes of ConductiVe Metallic Oxides, Part A-B; Elsevier Science Publishers: Amsterdam, The Netherlands, 1980/1981. (5) Trasatti, S. Electrochim. Acta. 2000, 45, 2377-2385. (6) Vicent, F.; Moralloń, E.; Quijada, C.; Va´zquez, J. L.; Aldaz, A.; Cases, F. J. J. Appl. Electrochem. 1998, 28, 607-612. (7) Montilla, F.; Moralloń, E.; De Battisti, A.; Va´zquez, J. L. J. Phys. Chem. B 2004, 108, 5036. (8) Montilla, F.; Morallon, E.; De Battisti, A.; Benedetti, A.; Yamashita, H.; Vazquez, J. L. J. Phys. Chem. B 2004, 108, 5044-5050. (9) Montilla, F.; Moralloń, E.; Va´zquez, J. L. J. Electrochem. Soc. 2005, 152 (10), B421-B427. (10) Rodgers, J. D.; Jedral, W.; Bunce, N. J. EnViron. Sci. Technol. 1999, 33, 1453-1457. (11) Szpyrkowicz, L.; Juzzolino, C.; Kaul, S. N. Water Res. 2001, 35, 2129-2136. (12) Burke, L. D.; McCarthy, M. M. J. Electrochem. Soc. 1988, 135, 1175-1179. (13) Castro, E. B.; Gervasi, C. A. Int. J. Hydrogen Energy 2000, 25, 1163-1170. (14) Tavares, A. C.; Cartaxo, M. A. M.; Pereira, M. I. D.; Costa, F. M. J. Electroanal. Chem. 1999, 464, 187-197. (15) Singh, R. N.; Pandey, J. P.; Singh, N. K.; Lal, B.; Chartier, P.; Koenig, J. F.; Electrochim. Acta 2000, 45, 1911-1919. (16) Boggio, R.; Carugati, A.; Lodi, G.; Trasatti, S. J. Appl. Electrochem. 1985, 15, 335-349. (17) Cox, P.; Pletcher, D. J. Appl. Electrochem. 1990, 20, 549-553. (18) Cox, P.; Pletcher, D. J. Appl. Electrochem. 1991, 21, 11-13. (19) Rios, E.; Nguyen-Cong, N.; Marco, J. F.; Gancedo, J. R.; Chartier, P.; Gautier, J. L. Electrochim. Acta 2000, 45, 4431-4440. (20) Casella, I. G.; Guascito, M. R. Electrochim. Acta 1999, 45, 11131120. (21) Casella, I. G. J. Electroanal. Chem. 2002, 520, 119-125. (22) Stavart, A.; Lierde, A. V. J. Appl. Electrochem. 2001, 31, 469474.

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Preparation and Characterization of Copper-Doped Cobalt Oxide

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