New Insight into the Vibrational Behavior of Nickel ... - ACS Publications

J. Phys. Chem. C 2008, 112, 2193-2201

2193

New Insight into the Vibrational Behavior of Nickel Hydroxide and Oxyhydroxide Using Inelastic Neutron Scattering, Far/Mid-Infrared and Raman Spectroscopies J. L. Bantignies,† S. Deabate,‡ A. Righi,§,⊥ S. Rols,† P. Hermet,| J. L. Sauvajol,† and F. Henn*,§ Laboratoire des Colloı¨des, Verres et Nanomate´ riaux, UMR CNRS 5587, UniVersite´ Montpellier II, Place Euge` ne Bataillon, 34090 Montpellier Cedex 5, France, Institut Europe´ en des Membranes, UMR 5635, UniVersite´ Montpellier II, Place Euge` ne Bataillon, 34090 Montpellier Cedex 5, France, Institut Charles Gerhardt, UMR CNRS 5253, UniVersite´ Montpellier II, Place Euge` ne Bataillon, 34090 Montpellier Cedex 5, France, and Laboratoire de Physique The´ orique des Mate´ riaux, UniVersite´ de Lie` ge, B-5, 4000 Sart-Tilman, Belgium ReceiVed: July 24, 2007; In Final Form: NoVember 15, 2007

The electrochemical properties of β-type nickel (oxy)hydroxides Ni(OH)2 and NiO(OH) are known to depend significantly on their structural features. In this work, these features are investigated by means of inelastic neutron scattering and infrared and Raman spectroscopies. Our experiments probe both inter- and intramolecular (40-4000 cm-1) vibrational ranges and particular attention is paid to the low-frequency (40-300 cm-1) domain, where new features are observed. Namely, the presence of an IR active mode at around 130 cm-1, characteristic of the oxidized β-NiO(OH) phase, is observed for the first time. The effects of temperature (10-300 K) and deuteration are also measured in order to provide deeper insight into the nature of the vibrational states. A detailed comparison of all of the collected data allows us to propose a new assignment of the vibrational features observed in both β-Ni(OH)2 and β-NiO(OH) and to quantify the proton dynamic relevance on the vibrational modes. We show that the interlayer interactions are mostly electrostatic and, hence, do not involve hydrogen bonding. We also point out that, in the oxidized β-phase, protons cannot be localized so that they can be considered as fully labile. To sum up, this investigation emphasizes that proton delocalization and crystalline disorder, which arise concomitantly upon oxidation, are the key factors that explain the evolution of the vibrational features observed between the oxidized and reduced β-type forms.

1. Introduction For many years, the highly reversible Ni(OH)2/NiO(OH) redox system has been used extensively as the active mass of the positive electrode of various secondary alkaline batteries, namely, Ni-Cd, Ni-MH, and Ni-H2.1 The redox mechanism of the nickel oxide electrode (NOE) is based on the reversible insertion-deinsertion of protons and electrons:

Ni(OH)2 T NiO(OH) + H+ + e-

(1)

The rate of the solid-state reaction (eq 1) is usually recognized to be mass-transfer-limited,2-5 which means that the electrochemical properties of this system are largely influenced by the mobility of protons that are assumed to hop between oxygen atoms located at the surface of the interlayer space. It is therefore straightforward to assume that the electrochemical performances of the Ni(OH)2/NiO(OH) redox couple rely on both the proton and electron transports, which are believed to depend signifi* To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +33 4 67 14 48 55. † Laboratoire des Colloı¨des, Verres et Nanomate ´ riaux, UMR CNRS 5587, Universite´ Montpellier II. ‡ Institut Europe ´ en des Membranes, UMR 5635, Universite´ Montpellier II. § Institut Charles Gerhardt, UMR CNRS 5253, Universite ´ Montpellier II. | Universite ´ de Lie`ge. ⊥ Present address: Departamento de Fı´sica, Instituto de Cie ˆ ncias Exatas, Universidade Federal de Minas Gerais, CP 702, 30123-970, Belo Horizonte MG, Brazil.

cantly on the crystallographic characteristic. The highly disordered character of the oxidized state, NiO(OH), revealed by X-ray diffraction,6 makes it difficult to access its exact crystallographic structure. Vibrational spectroscopies, that is, infrared and Raman, have thus been employed in order to get further information about the local structure. In that way, an attempt to correlate the vibrational features to the electrochemical features was even proposed.7-9 However, many questions remain posed and more work about the local structure and its connection to the electrochemical activity is still required. The different nickel (oxy)hydroxide crystallographic phases that can participate to the NOE redox process (eq 1) have been described well by Bode et al.’s scheme.10 All phases exhibit the same layered structure formed by slabs of edge-sharing NiO6 octahedra packed along the c-axis direction, with weak interlayer forces. The present paper is only focused on the β-Ni(OH)2 and β-NiO(OH) phases, denoted as β(II) and β(III), respectively, which are currently used in rechargeable batteries sold on the market. The β(II) form, isomorphous with brucite Mg(OH)2, crystallizes in the hexagonal system (P3hm1,D33d space group), with ABAB oxygen packing11 (Figure 1). In the ideal structure, protons are located in tetrahedral “HO4” sites in the interslab space, just below or above the oxygen atoms. Because of its disorder feature, the structure of the β(III) form remains partly unknown. When β(II) is oxidized into β(III), X-ray diffraction patterns exhibit the disappearance of all the non-00l lines (Figure 2). The selective line broadening induced by the oxidation process is consistent with the presence of a

10.1021/jp075819e CCC: $40.75 © 2008 American Chemical Society Published on Web 01/24/2008

2194 J. Phys. Chem. C, Vol. 112, No. 6, 2008

Figure 1. Structure of β-Ni(OH)2: brucite-like layers consist of an hexagonal planar arrangement of Ni(II) cations with octahedral coordination of oxygen.

Bantignies et al. sensitivity to Ni-O and O-H group vibrations in which the main characteristic depends on the oxidation state.5,7-9,18-33 Infrared spectroscopy in the mid-frequency range, that is, 3004000 cm-1, is documented less because the bands are relatively broad and the frequency shifts are difficult to observe.34-36 In the far-infrared (FIR) domain, that is, below 300 cm-1, no investigation is found in the literature. In this frequency range, important information can be gained about the lattice vibrations associated with torsional motion of the hydroxide octahedral37 and about the presence of hydrogen bonds. So, one expects the investigation of the low-frequency vibrational modes to be particularly important for understanding the structural features of the β(II) phase and their evolution toward the β(III) phase. The detailed information about the vibrational dynamics of nickel (oxy)hydroxides is still an open question. In this work, we aim to provide new insight into the vibrational features and their relationship to structural and conductivity properties. To reach that goal, we investigated the vibrational dynamics of hydrogenated, well-crystallized β(II), and chemically oxidized β(III) phases by Raman, middle, and far-infrared spectroscopy and inelastic neutron scattering (INS). To highlight the spectral weight of the protons in the vibrational dynamics, we also investigated deuterated phases using INS. After a brief survey of the state of the art, we detail the results obtained by INS, giving the generalized density of states (GDOS) of both β(II) and β(III) and their corresponding deuterated phases. Then, the comparison of the GDOS with infrared and Raman vibrational modes leads to a new attribution of the active modes. The results suggest electrostatic interlayer interactions in both phases and proton delocalization into the interlayer space for the β(III) phase. 2. State of the Art

Figure 2. X-ray powder diffraction patterns of the H-β(II) and H-β(III) crystalline phases studied in this work.

high degree of structural disorder along the ab-crystallographic plane direction, whereas crystallographic features are more or less preserved along the c-axis direction (perpendicular to the layer surface). Also, the interlayer distance (the c parameter of the hexagonal lattice) increases slightly from ∼4.6 to ∼4.85 Å,11 suggesting the decreasing of the interlayer adhesion forces. Anyhow, the lack of well-defined Bragg reflections makes it unrealistic to propose a unique and accurate structural model for β(III).12,13 It has also been emphasized that β-type reduced and oxidized phases have very different conductivity properties. The reduced β(II) exhibits a very poor conductivity, that is, between 10-13 and 10-15 Ω-1‚m-1 at 293 K, which is assumed to be due to proton transport via structural defects.14 Oppositely, β(III) is known to be a conductive, that is, ∼10-3 Ω-1‚m-1, electronic semiconductor.15 In addition, vibrational spectroscopy studies suggest that protons are strongly attached to the β(ΙΙ) structure via covalent O-H bonds while they become free upon oxidation.5,16 These observations are in accordance with the results reported from coupled structural characterization and electrochemical cycling investigations,6,17 which show that a sharp potential drop occurs upon discharge of NOE when the amount of insulating β(II) phase overpasses a critical value. All of these facts clearly evidence that there is a close relationship between structural, vibrational, protonic/electronic transport and, hence, electrochemical properties. Raman spectroscopy has been used widely to probe the vibrational modes of β(ΙΙ) and β(ΙΙΙ) phases because of its

The interlayer vibrational modes of β(II)-Ni(OH)2 can be initially interpreted on the basis of the brucite structure (D33d space group). The factor group analysis predicts 15 phonons at the center of the Brillouin zone (3 acoustics and 12 optics). Four Raman (2A1g + 2Eg) and four infrared (2A2u + 2Eu) active bands are expected (Figure 3).38 Various assignment schemes have been proposed for the infrared and Raman spectra of β(II). Table 1 summarizes the experimentally observed FTIR and Raman bands of Mg(OH)2 and β-Ni(OH)2, together with their attribution to the different symmetries, according to various authors. Symmetry-related selection rules do not apply in INS and, hence, all modes should be observed. 2.1. Inelastic Neutron Scattering. The INS investigation carried out by Baddour-Hajean et al.16 revealed that all bands, appearing below 500 cm-1 in the β(ΙΙ) phase, vanish in β(ΙΙΙ). In the latter, a broad intense band centered around 830 cm-1 is, however, observed. The authors16 claimed that this band corresponds to a quasi-isotropic oscillation of protons. They concluded that protons, though not specifically bound to any oxygen atom, are embedded in potential wells located in the interslab space; that is, H+ are placed in specific lattice sites with high local symmetry. However, this investigation was carried out on the frequency domain above 200 cm-1 and no data were reported concerning the lower frequency range that is known, as for far-infrared, to probe intermolecular as well as soft network vibrational modes. 2.2. Raman Spectroscopy. Five or even six Raman-active modes were reported for the experimental spectra of the β(ΙΙ) phase (Table 1),7-9,18,19,22-24,28,29,31,32 whereas, at the gamma point, the factor group theory predicts only four Raman-active modes, that is, 2A1g + 2Eg (Figure 3).38

Behavior of Nickel Hydroxide and Oxyhydroxide

J. Phys. Chem. C, Vol. 112, No. 6, 2008 2195

Figure 3. Intramolecular vibrational modes in brucite-type M(OH)2 compounds (R for rotational lattice modes, T for translational lattice modes), after ref 38.

TABLE 1: Observed FTIR and Raman Bands (cm-1) of Mg(OH)2 and β(II)-Ni(OH)2, in the Mid-Infrared Frequency Domain β(II)-Ni(OH)2

Mg(OH)2 IR 36, 39 3698 562 462 368

36

35

this study

A2u 3635 A2u 3639 A2u 3636 A2u A2u(T) 549 Eu(R) 530 Eu(R) 530 A2u(T) or Eu(T) A2u(T) or Eu(t) 451 A2u(R) 452 A2u(T) 440 A2u(T) or Eu(T) Eu(T) and Eu(R) 348 Eu(T) 350 Eu(T) 332 Eu(R) Raman 40

3654 803

A1g Eg(R)

445 282

A1g(T) Eg(R)

8, 9, 18, 19, 24 3680 3600 3570 510 445 310

A1g Eg(R) A1g(T) Eg(T)

32 3600 3580 880 517 450 315

A1g Eg(R) Eg(T) A1g(T)

this study 3601 3581 508 447 313

A1g Eg(T) or A1g(T) Eg(T) or A1g(T) Eg(R)

In the region of the hydroxyl stretching vibrations (νOH), two modes were detected and the most intense around 3570 cm-1 was assigned to the A1g mode.24 The attribution of the unexpected second one, centered at 3600 cm-1, is not clear but it was subsequently assigned to the νOH vibration of hydroxyl groups located near structural defect sites8,23 or to isolated water molecules adsorbed at the surface of the Ni(OH)2 nanocrystallites.9 Another additional νOH line was also reported in badly crystallized β(II), around 3680 cm-1, and ascribed to free OH groups located at the nanocrystallite surface.8,28 In the region of the NiO lattices modes, three bands were reported.5,7-9,18,19,22-24,29,31,32 The two first bands at lower frequencies were assigned to translational lattice modes:18 the first one, centered around 310 cm-1, to the Eg(T) mode of the Ni-OH lattice vibration and the second, located around 445 cm-1, to the A1g(T) mode due to νNi-OH. Nevertheless, this interpretation remains controversial because others authors32 suggested A1g(T) attribution for the 315 cm-1 mode and Eg(T) attribution for the 450 cm-1 one (see Table 1). The third feature, around 510 cm-1, which is known to be very sensitive to the β(II) crystallization degree, was assigned to the presence of structural defects8 and/or to a Eg(R) mode.9,19 All of the well-resolved Raman peaks observed in β(II) were reported to vanish for β(III), whereas two new lines, centered around 470 and 550 cm-1, could be seen.7,23,29 These features were assigned to specific lattice vibrations of NiO2 sheets

between which delocalized protons are not associated with oxygen atoms.29 The concomitant disappearance of the INS peaks in that region16 further questions the origin of the 310315 cm-1 band observed in β(II), which was initially ascribed to Eg(T) or A1g(T) symmetry (Table 1). In principle, the spectral activity of both Eg(T) and A1g(T) lattice translational modes should not be or very slightly dependent on the departure of protons (see Figure 3). 2.3. Infrared Spectroscopy. Infrared spectroscopy in the mid-frequency range, that is, 300-4000 cm-1, is far less documented than Raman and even no paper dealing with vibrational dynamics in the far-infrared domain, that is, below 300 cm-1, can been found in the literature. The group factor theory predicts 4 IR-active modes,35 which were observed in Mg(OH)2 at 368 (mixed Eu(T) andEu(R)), 462 (A2u(T) or Eu(T)), 562 (A2u(T)), and 3698 cm-1 (A2u)36,39 and in β(II)-Ni(OH)2 at 348-350 (Eu(T)), 451-452 (A2u(R) or A2u(T)), 530-549 (Eu(R)), and 3635 cm-1 (A2u)35,36 (Table 1). As far as we know, no attribution for the IR bands of the oxidized β(III) phase has been reported. 3. Experimental Section 3.1. Material Preparation. The hydrogenated β(II) phase, noted H-β(II), was purchased directly from Aldrich. It is a welldefined and crystallized material made of micrometer-sized spherical particles that consist of a porous aggregate of nanocrystallites. More information about its structural features will be found elsewhere.6,31 This compound was chemically oxidized using the usual procedure,17 that is, by adding 150 mL of 13% NaClO solution to 1.5 g of the hydroxide powder and 150 mL of H2O at 65 °C for 4 h. After washing and drying, the resulting compound was identified as pure β(III)-NiO(OH) phase (noted H-β(III)) by X-ray powder diffraction (Figure 2). The deuterated phases, noted D-β(II) and D-β(III), were synthesized as followed.17 First, 100 mL of 1 M nickel sulfate D2O solution was added to 125 mL of 2 M NaOD/D2O solution at 70 °C for 2 h. The obtained green precipitate (D-β(II)) was washed with D2O and dried at 55 °C for 15 h. Then, to obtain the corresponding oxidized phase (D-β(III)), 1.5 g of D-β(II) was added to 150 mL of a LiOCl/D2O solution under refluxing at 65 °C for 4 h. The precipitate was washed with NaOD solution and dried for 12 h. 3.2. Inelastic Neutron Scattering. INS measurements were performed on the MiBeMol “time-of-flight” spectrometer built on the cold neutron guide at the Orpheé reactor at the Laboratoire León Brillouin (CEA Saclay, France).41 This


Bantignies et al.

Figure 4. GDOS, obtained by INS at 293 K (MiBeMol spectrometer), of the D- (a), H- (b) β(II), and β(III) phases, in the [0-600 cm-1] frequency range.

spectrometer allows detection of scattered neutron in the [0-1600 cm-1] energy and [10°-160°] angle ranges and has an angular and energy variable resolution. For the experiments reported in this work, an incident neutron wavelength of 5 Å was used in order to obtain the maximum of incoming flux. The maximum resolution of the spectrometer (fwhm ∼ 90 µeV) is achieved at elastic scattering so that precise information is obtained in the low-frequency [0-150 cm-1] range. Above this range, the low-resolution only allows us to follow the frequency dependence of the non-resolved components. The experiments were performed at 293 K, and specific aluminum containers were used. After proper normalization, the spectra were corrected from an empty Al standard. The quantity measured from such an experiment is called the neutron cross section and contains all of the information (structure and dynamics) of the sample under study. In particular, the generalized density of states (GDOS) can be derived directly from the cross section. This quantity is known to be very appropriate when discussing the vibrations of a powder sample.42 In the following, data obtained on the hydrogenated and deuterated β(II) and β(III) samples will be presented in the form of GDOS. The intensityderived GDOS was normalized to the mass of sample in the beam so that the relative intensity of the different spectra are directly comparable. Note that, because of the peculiar neutronnucleus interaction, neutron spectroscopy is highly sensitive to hydrogen atoms. The neutron cross section is 80 barns for hydrogen, which is much higher than that for any other atomic species by a factor of about 10. Thus, measurements are very sensitive to H/D exchange and vibrational modes involving hydrogen displacement can be highlighted by comparing the responses obtained for hydrogenated to deuterated samples. To gain more accurate information in the mid-frequency [200-1200 cm-1] range, we performed additional INS experiments on the hydrogenated β(II) and β(III) phases by using the indirect geometry time-of-flight spectrometer “TOSCA”, at the ISIS pulsed spallation neutron source of the Ruterford Appleton Laboratory. The scattered energy was kept fixed at 3.10 meV. The incident energy, covering the energy range from 3 to 500 meV, yields the density of states, in the 30-4000 cm-1 range, with a significantly better resolution than that obtained with the MiBeMol spectrometer. The experiments on TOSCA were performed at 10 K.

3.3. Raman Spectroscopy. Room-temperature Raman spectra were obtained using the Ar/Kr laser excitation line at 514.5 nm of a Jobin-Yvon T64000 spectrometer, equipped with a liquidnitrogen-cooled charge-coupled device (CCD) detector. To avoid the heating and degradation of the sample, we limited the incident power to 500 µW. The scattered light was collected in a backscattering configuration by a microscope using a 100x objective (laser spot ∼1 µm). The instrumental resolution was 2 cm-1. 3.4. Infrared Spectroscopy. Mid-FTIR transmittance measurements, between 600 and 4000 cm-1, were carried out on a Bruker IFS 113V spectrometer equipped with a N2-cooled MCT detector, a Globar source, and a KBr beam splitter. The spectral resolution was 2 cm-1, and 64 scans were co-added for each spectrum. Samples were pellets made of a ground mixture of dried KBr and 1% weight of β(II) or β(III) powder, pressed up to 5 tons. Far-FTIR transmittance measurements, between 40 and 600 cm-1, were carried out using a He-cooled Bolometer detector, a Mercury source, and a Composite Mylar 6 micrometers coated with germanium thick beam splitter. The spectral resolution was 2 cm-1, and 256 scans were co-added for each spectrum. Samples for far-infrared were pellets made of a ground mixture of dried polyethylene and 10% weight of β(II) or β(III) powder, pressed up to 5 tons. All infrared measurements were performed as a function of temperature, using a liquid helium cryostat. The 10-300 K range was investigated by steps of 30 K, with a temperature accuracy of 1 K. 4. Results and Discussion 4.1. Inelastic Neutron Scattering. The GDOS of the H- and D-forms of the β(II) and β(III) phases, from INS data obtained with the MiBeMol spectrometer, are reported in Figure 4. Let us first focus on the comparison between the GDOS of D-β(II) and D-β(III) in the [0-600 cm-1] frequency range (Figure 4a). These phases exhibit comparable GDOS, where the main unresolved contributions are a band around 130 cm-1 and a double peak structure centered at 430 cm-1, with a shoulder around 310 cm-1. Compared to D-β(III), the GDOS of H-β(III) exhibits a general upshift of the main features (as expected from the isotopic effect) and slight differences in their relative intensities. The similarities between the GDOS of D-β(II), D-β(III), and H-β(III) indicate that all of their INS spectral



Figure 5. Comparison of GDOS obtained by INS at 293 K (MiBeMol spectrometer) (a) and at 10 K (TOSCA spectrometer) (b), in the [200-1200 cm-1] frequency range.

contributions in the [0-600 cm-1] range mainly involve vibrations of the NiO2 lattice, which is very close (at the local scale) for the reduced and oxidized materials.11 In contrast, important differences can be observed between the GDOS of the H-β(II) and H-β(III) phases (Figure 4b). If, as underlined previously, several contributions seem to be present in the GDOS of both phases, then significant changes of their intensity are observed in this case. The intensity of the low-frequency band around 130 cm-1 clearly increases in H-β(II). The same behavior is found for the contribution at 430 cm-1 and for the shoulder around 315 cm-1. Figure 5 compares GDOS of the [200-1200 cm-1] range, obtained with MiBeMol and TOSCA spectrometers (Figure 5a and b, respectively). The high-resolution experiments performed at ISIS (Figure 5b) allow us a better assignment of all of the vibrational features of the hydrogenated phases appearing above 150 cm-1. The GDOS of H-β(II) displays well-defined components located around 310 and 460 cm-1 (Figure 5b). The intensity of both of these features decreases strongly in H-β(III). According to our previous attribution of these modes to NiO lattice vibrations, this intensity decreasing can be interpreted as the consequence of the drastic reduction of the scattering cross-section in the oxidized H-β(III) phase. A detailed analysis of the [600-1200 cm-1] frequency range of H-β(II) shows another well-defined component, at 670 cm-1 (Figure 5b). This band, previously assigned to the degenerate antisymmetric O-H bending mode (Eu(R)),16 disappears in the H-β(III) phase. Because the Eu(R) vibrational mode directly involves vibrations of protons (Figure 3), its vanishing in H-β(III) can be related to the breaking of the iono-covalent O-H bonds upon oxidation and consequent delocalization of protons. Finally, another broad structure around 870 cm-1 is observed in the H-β(II) phase (Figure 5a and b), which was previously assigned to the contribution at the GDOS of the symmetric O-H bending mode (Eg(R)).16 A stronger component can also be observed in the same frequency range for H-β(III) (Figure 5b), which has been assigned, in that case, to a quasi-isotropic oscillation of protons not bound to oxygen atoms but located in lattice sites with high local symmetry.16 It must be emphasized that our experiments carried out on deuterated phases show a broad component in the same frequency range for both D-β(II) and D-β(III) phases (Figure 5a), meaning that NiO lattice modes also contribute to the GDOS in this spectral domain. Thus, on

the basis of all of the INS information obtained in the [6001200 cm-1] range, the excess of intensity observed with the H-β(III) phase around 870 cm-1 (Figure 5) can be interpreted as arising from excitations involving proton vibrations overlapping with NiO lattice modes. Starting from these relevant observations, it is now possible to analyze in a clearer manner the different groups of vibrations observed in the infrared and Raman spectra. 4.2. IR and Raman [200-800 cm-1] Frequency Range. In that domain, the Raman signal of H-β(II) shows, as expected from selection rules,38 two main lines centered at 313 and 447 cm-1 and a third weak one around 508 cm-1 (Figure 6a). The corresponding infrared lattices modes are observed at 332, 440, and 530 cm-1 (Figure 6b). In the H-β(III) phase, the 313 cm-1 Raman band vanishes. This result enlightens the main role played by protons in this vibrational mode and therefore proves that this bands cannot be ascribed to translational lattice mode Eg(T)8,9,18,19,24 or A1g(T),32 as proposed previously (see also Table 1). Consequently, we assign this mode to Ni-OH rotational lattice vibrations (Eg(R)) because this is the only one expected, in this frequency range, to be sensitive to the presence of H+; that is, it involves proton motion regarding the oxygen position (see Figure 3). This assignment is also in agreement with GDOS measured on TOSCA spectrometer that exhibits, for the H-β(III) phase, a strong intensity decrease of the 310 cm-1 feature (Figure 5b). The same behavior is observed for the corresponding infrared band, around 332 cm-1 (Figure 6b), which can thus be assigned to the Eu(R) vibrational mode (Figure 3). Raman bands observed at 447 and 508 cm-1 in H-β(II) remain after oxidation, though shifted toward higher frequencies, that is, 471 and 549 cm-1, respectively (Figure 6a). This behavior should correspond to translational lattice modes, weakly sensitive to protons (see Figure 3). However, this study cannot conclude between Eg(T) or A1g(T) attribution for these modes (Table 1). A computational simulation is definitively needed to gain deeper insight into the origin of these vibrational features. IR data show the same behavior (Figure 6b). The H-β(II) vibrational bands at 440 and 530 cm-1 are observed at 450 and 550 cm-1 in H-β(III), around the same position found in the corresponding Raman spectrum. These IR modes can therefore be ascribed to A2u(T) or Eu(T) symmetries (Figure 3). It should be mentioned that the spectral attributions proposed above are


Bantignies et al.

Figure 6. Raman (a) and FTIR (b) spectra of H-β(II) and H-β(III) lattice vibrational modes observed in the mid-infrared frequency domain.

Figure 7. Temperature dependence, measured using FTIR in the [10290 K] range, of the most intense lattice vibrational modes appearing in the mid-IR domain (νj > 500 cm-1) for the H-β(II) (a) and H-β(III) (b) phases.

in agreement with INS spectra, where the presence of a 460 cm-1 contribution can be observed for both the H-β(II) and, with decreased intensity and slight upshift, H-β(III) phase (Figure 5b). As pointed out previously, the strongly decreasing intensity of this mode in the oxidized material corresponds to the removing of one proton out of two (eq 1) upon oxidation and to the consequent, drastic reduction of the scattering crosssection in the H-β(III). To gain more insight into the lattice vibrational modes appearing in this frequency range, we investigated the temperature dependence of the β(II) and β(III) infrared bands located at 530 and 565 cm-1, respectively (Figure 7). The β(III) contribution has a pseudo-harmonic behavior, that is, a frequency decrease with the increase of temperature, while the 530 cm-1 band observed for β(II) exhibits a very slight narrowing and frequency upshift upon heating (see also Figure 8). These behaviors, which appear opposite at first sight, can be explained by on one hand the nature of the corresponding vibrations and on the other hand the disordering into the proton sublattice when β(II) is oxidized in β(III). The 565 cm-1 vibration of β(III) corresponds to a pure NiO6 octahedral lattice mode. At low as well as at high temperature, the NiO6 lattice is surrounded by protons that are delocalized and highly mobile. The increase of temperature has no influence on this disorder so that this mode

Figure 8. Frequency shift as a function of temperature (10-290 K) of the most intense lattice modes appearing in the mid-IR domain (νj > 500 cm-1) for the H-β(II) (solid squares) and H-β(III) (open squares) phases.

undergoes a standard pseudo-harmonic evolution. On the contrary, the 530 cm-1 vibration of β(II), which is assigned to a Ni-OH translational lattice mode, is more strongly dependent on the way protons are attached to the oxygen atoms or, in other words, on the way protons are engaged into the interlayer cohesion. In pure β(II), protons are well attached to oxygens so that O-H stretching vibration modes are clearly seen (see Figure 10). Upon increasing temperature, the O-H bond becomes weaker with an expected peak broadening (see Figures 11). It thus results a slight increase of the oxygen negative charge and then a weak but significant re-enforcement of the Ni-O bond. 4.3. IR [40-200 cm-1] Frequency Range. Intermolecular vibrations and H-bonding are basically probed in this frequency range, namely, between 110 and 160 cm-1.43 In the [40-200 cm-1] domain, no Raman features have been detected in either β(II) or β(III) phases and no infrared modes have been observed for the reduced β(II) phase (Figure 9a). So, we can rule out any charge transfer between OH groups belonging to opposite layers, via the formation of H bonds, in β(II). This assumption will be confirmed by the results obtained in the νOH vibration domain (see Section 4.4.). On the contrary, β(III) exhibits a significant feature around 126 cm-1 (Figure 9a). Noteworthy,



Figure 9. Far-FTIR spectra of H-β(II) and H-β(III), carried out at room temperature and 10 K (a), and frequency shift as a function of temperature (10-290 K) of the vibrational mode observed for H-β(III) in this spectral range (b).

Figure 10. Raman (a) and FTIR (b) spectra of H-β(II) and Η-β(III) in the νOH vibrational modes frequency domain.

INS measurements show the presence of a band close to the same frequency, that is, 130 cm-1 (Figure 4), whose behavior is independent of the material oxidation state and on the H/D exchange. These results suggest that this low-frequency IR mode, which is not predicted from the group theory, occurs in the β(III) phase because of the loss of crystallinity and, hence, of the selection rules. When studying the temperature dependence of the 126 cm-1 infrared band, we observe an expected pseudo-harmonic behavior with an upshift of 5 cm-1 at 10 K with respect to the room temperature (Figure 9b). It seems rather clear that this IR mode does not directly involve proton motions. BaddourHadjean et al.16 attributed this INS contribution to an acoustic mode in zone edge. Accordingly, we can assume that, in the far-infrared domain, the 126 cm-1 band arises from an acoustic mode that is infrared-inactive in β(II) for symmetry reasons and becomes active in β(III) owing to the loss of crystallinity associated to the β(II) f β(III) oxidation process. 4.4. IR and Raman νOH Vibrations (3500-3700 cm-1 Domain). Two allowed normal modes, both corresponding to hydroxyl stretching vibrations, are expected in this frequency

range: the Raman-active A1g and the infrared-active A2u38 (Figure 3). In this work, the former (in-phase νOH) is observed at 3581 cm-1 (Figure 10a) and the latter (out-of-phase νOH) at 3636 cm-1 (Figure 10b). Worth noting, both Raman and FTIR spectra of the β(III) phase do not exhibit νOH vibration. This observation is consistent with the breaking of the O-H ionocovalent bonds upon oxidation of the reduced phase and with the resulting delocalization of protons inside the β(III) interlayer lattice space.5,29 The Raman spectrum of β(II) (Figure 10a) exhibits an additional, weaker mode at 3601 cm-1, which is normally Raman-forbidden. The interlayer interactions could explain this anomaly if, as claimed by recent speculations,35,44 hydrogen bonds played an important role in the layers’ cohesion. However, previous theoretical and experimental studies about brucite-type solids have examined the question on the nature of the interlayer interaction and concluded that this is dominated by coulomb and dispersion forces.45-47 To gain better insight into this question, a variable-temperature FTIR study was performed on the H-β(II) sample. The effect of temperature on the OH stretching vibration is displayed in Figure 11. The upshift of


Bantignies et al.

Figure 11. Temperature dependence (10-290 K) of the FTIR spectrum of H-β(II) (νOH frequency domain) (a) and the corresponding frequency shift of the νOH vibrational mode (b).

the β(II) ν(OH) mode, from 3636 to 3643 cm-1, is observed upon cooling. This behavior is related to a typical strengthening of the O-H bonds when the temperature decreases. If O-H bonds were engaged in H-bonding, then a specific frequency downshift would be observed.48 This result definitively attests that interlayer hydrogen bonding is very weak, or even nonexistent, in β-type nickel hydroxides. As a consequence, the unexpected Raman band at 3601 cm-1 (Figure 10a) has to be attributed to νOH vibrations of structural defect sites in the extended layer structure of Ni(OH)2, for example, free OH groups located at the particle’s surface. Let us now recapitulate the main results of this study. Among the different infrared and Raman-active vibrational modes expected from the factor group analysis (Figure 3),38 four involve opposite displacement of proton and oxygen (the Eg(R), A1g, Eu(R), and A2u modes) while the remaining ones are related to the displacement of the whole OH group (Eg(T), A1g(T), Eu(T), and A2u(T)). The evolution of the IR and Raman spectral features, observed in β phases as a function of the Ni oxidation state, can be interpreted as the consequence of the vanishing of all bands related to those vibrational modes involving opposite O and H motions. Likewise, INS spectra show that modes involving vibrational displacements of the whole OH group remain in β(III) but with strongly weakened scattering cross sections. The absence, in both infrared and Raman spectra of the oxidized phase β(III), of any vibrational contribution to the νOH modes frequency domain is, then, particularly meaningful. By combining all of the experimental results reported above, it can finally be proposed that protons are free inside the β(III) phase lattice, that is, not bonded to a given oxygen atom of the NiO2 structure. None of the vibrational features observed in this work for β(III) allow us to locate protons in a given structural site, different from the previous proposition of Baddour-Hajean et al.17 Thus, protons intercalated in the β(III) lattice have to be considered fully labile. The presence of completely delocalized protons has to be taken into account for a better understanding of the structural defects featuring the disordered stacking of the slabs in the β(III) phase. The disappearance of iono-covalent O-H groups, whose orientation is normal to the layer surface (Figure 1), could be responsible for the weakening of the interlayer friction forces along the ab-crystallographic plane direction and, hence, for

the misalignment of the laminar structure along the basal plane. Then, the disappearance of all the “two-dimensional” hk0 diffraction lines in the X-ray diffraction patterns of β(III) phases12,13 (see also Figure 2) would be related to the random orientation of the layers along the c-crystallographic axis, according to the presence of long-range structural defects like turbostratic disorder and stacking defaults, observed previously for badly crystallized Ni(II) hydroxides and some other layered minerals.11,49,50 Concerning vibrational dynamics, the increasing of structural disorder, concomitant to the departure of protons and related to the β(II) f β(III) solid-state oxidation process, would yield to the broken of the spectral selection rules, thus allowing the appearance of new vibrational features in the farinfrared domain and making the infrared and Raman spectra closer to the GDOS. 5. Conclusions The experimental vibrational study reported here allows us to identify the influence of the crystallographic disorder and of the proton dynamic in the infrared, Raman, and INS spectra of β(II) and β(III) phases. Neutron scattering experiments provide insight into the origin of the Raman and infrared modes. The β(II) f β(III) oxidation leads to a vanishing of the intramolecular features directly associated to the hydroxyls groups and consequently significantly modifies the activity of vibrational modes in the far-infrared domain. We show, for the first time, that a low-frequency band, evidenced by far-infrared spectroscopy, is the signature of the β(II) f β(III) transition. Comparison of INS data obtained on H/D-β(II) and -β(III) samples clearly emphasizes that this feature is not associated to protons but is likely due to the occurrence of crystalline disorder in the ab-crystallographic plane and to the resulting loss of symmetry due to stacking defaults. The appearance of this low frequency infrared mode is concomitant to the disappearance of the hydroxyl stretching modes whose the temperature dependence undoubtedly outlines a quasi-harmonic behavior incompatible with their contribution to interlayer H-bonding. So, it can be concluded that the interlayer interactions in β(II) are not due to H-bonding between OH groups facing each other in the interlayer space but mainly to electrostatic interaction. Finally, it must be pointed out that none

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New Insight into the Vibrational Behavior of Nickel ... - ACS Publications

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