Nanocrystalline cobalt hydroxide oxide: Synthesis and characterization with SQUID, XPS, and NEXAFS

Journal of Alloys and Compounds 824 (2020) 153925

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Nanocrystalline cobalt hydroxide oxide: Synthesis and characterization with SQUID, XPS, and NEXAFS Alexander Kudielka *, Martin Schmid , Benedikt P. Klein , Clemens Pietzonka , J. Michael Gottfried , Bernd Harbrecht €t Marburg, Fachbereich Chemie, Hans-Meerwein-Straße 4, 35032, Marburg, Germany Philipps Universita

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 November 2019 Received in revised form 16 January 2020 Accepted 18 January 2020 Available online 20 January 2020

Nanocrystalline cobalt hydroxide oxide (nc-CoOOH) particles were synthesized via a precipitationoxidation mechanism with different oxidation agents. Since the choice of the oxidative compound is of great practical importance, we compared the properties of the nc-CoOOH particles obtained from the oxidation with O2 and Br2. The particles from both synthesis routes were fully characterized with X-ray powder diffraction (XRPD), infrared spectroscopy (IR), superconducting quantum interface device (SQUID) measurements, X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) measurements. It was found that the choice of the oxidation agent greatly affects the properties of the resulting nanoparticles; the most significant differences concern particle size and magnetic moment, due to residual Co2þ. Although both, Br2 and O2, lead to the formation of nc-CoOOH, particles obtained from the synthesis involving O2 show a three times larger particle size than their counterparts obtained from the synthesis using Br2 (24 nm vs. 8 nm). Furthermore, using Br2 during the synthesis leads to an 8-fold increase of the amount of residual Co2þ impurities, which, in fact, induce a magnetic moment. A possible enrichment of those Co2þ impurities on the surface of the nanoparticles is discussed based on XPS and NEXAFS data. © 2020 Elsevier B.V. All rights reserved.

Keywords: Nanoparticle Cobalt hydroxide oxide XRPD XPS NEXAFS SQUID

1. Introduction The development of inorganic materials with special morphologies in the nanometer regime is of significant interest in material science and industry. It is well known that the synthesis conditions have a great impact on the properties of the nanomaterial. The intrinsic properties of nanomaterials depend sensitively upon many factors, such as the precise chemical composition, the crystal structure and microstructural features, the degree of crystalline order, the defect chemistry, the size and size distribution of the particles, the shape of the particles, and the porosity of the material [1]. CoOOH, a mineral named heterogenite [2], is often investigated as a multifunctional material with several potential applications. Examples include uses as catalyst [3], CO sensor [1,4e6], precursor for electrode materials in energy storage [7e9], precursor for other cobaltoxides [6], high performance supercapacitor [10], and potential candidate for electrocatalytic water oxidation [11e13].

* Corresponding author. E-mail address: [email protected] (A. Kudielka). https://doi.org/10.1016/j.jallcom.2020.153925 0925-8388/© 2020 Elsevier B.V. All rights reserved.

CoOOH is isotype to NaHF2 and CrOOH. The 3R-modification (space group R3m) has a rhombohedral symmetry with lattice parameters a ¼ 2.851 Å and c ¼ 13.15 Å [14] (Fig. 1). The edge linked CoO6 octahedrons build layers perpendicular to the c-axis with a AABBCC anion stacking sequence of hexagonal close-packed layers of O-atoms [14]. The H-atoms occupy positions between two Oatoms of different layers, leading to a connection of the layers by strong H-bonds (OeH$$$O ¼ 2.50 Å [14]). In an ABC (ccp) cation stacking pattern of the layers, as in CoOOH, anions of adjacent layers directly oppose each other. Several ways to synthesize CoOOH are described in the literature. The most commonly used procedure involves the oxidation of dissolved Co(OH)2 with H2O2 or NaOCl in NaOH or KOH solution [1,8,11,15,16]. Other reported possibilities are the oxidation of freshly prepared Co(OH)2 with air in NaOH [14], the oxidation under acidic conditions with (NH4)2S2O8 [3], the electrochemical oxidation of Co(OH)2 [8] or Co [17], via chemical bath deposition (CBD) on nickel foil [18] or at higher temperature via oxidation of [Co(NH3)6]2þ [10] and by hydrothermal treatment of Co(OH)2 in KOH under O2 pressure [8]. However, it is not unambiguously clear how far these different synthesis routes lead to CoOOH materials

2

A. Kudielka et al. / Journal of Alloys and Compounds 824 (2020) 153925

2.2. Experimental details 2.2.1. X-ray powder diffraction experiments X-ray powder diffraction (XRPD) patterns of the CoOOH samples were measured at room temperature in a range from 15 to 100 2q with a step size of 0.026 using an X’PertPro PW 3064/60 diffractometer (PANalytical) operating in Bragg-Brentano geometry with Co-Ka radiation (6930 eV). Diffracted X-rays were detected using a PIXel detector. The powder was placed on a silicon single crystal, which was rotated during the measurement. Rietveld refinements were carried out employing the X’Pert HighScore Plus software. 2.2.2. SQUID measurements Magnetic measurements were conducted with a SQUID magnetometer MPMS (Quantum Design). Application of reciprocal susceptibility were taken from zero field cooled measurement at 5000 Oe in a temperature range of 5e350 K. For temperaturedependent measurements, the sample was filled into a gelatin capsule and fixed with a straw as sample holder.

Fig. 1. Structure of 3R CoOOH with displayed stacking sequence (cobalt: green, oxygen: blue, hydrogen: red). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

with comparable properties. In this study, we apply a simple precipitation oxidation method, either with Br2 or O2 as oxidation agent, and fully characterize the resulting CoOOH nanomaterials. We find that the particular choice of the oxidation agent significantly affects the chemical composition and particle size of the resulting nc-CoOOH. Furthermore, we find that, depending on the choice of the oxidation agent, a magnetic moment is present in the - ideally - diamagnetic CoOOH materials. We examined this behavior in detail as this is, to the best of our knowledge, the first study that explicitly addresses magnetic phenomena in nc-CoOOH. 2. Synthesis and experimental details 2.1. Synthesis As reported in a previous paper [19] the synthesis route comprises two steps: First, the precipitation of Co(OH)2 in alkaline medium and second, the subsequent oxidation at room temperature. The first applied synthesis route [14] consists of the precipitation of Co(OH)2 in alkaline medium (Equation (1)), followed by an oxidation with air at room temperature (Equation (2)). The product yielded by these reactions is in the following referred to as CoOOH . Co2þ þ 2OH / Co(OH)2

(1)

2 Co(OH)2 þ ½ O2 / CoOOH þ H2O

(2)

The second method [20] also starts with the precipitation of Co(OH)2 (Equation (1)) but the subsequent oxidation at room temperature is carried out with bromine (Equation (3)). The product obtained with this synthesis route is labeled CoOOH . 2 Co(OH)2 þ 2 OH þ Br2 / 2 CoOOH þ 2H2O þ 2 Br

(3)

2.2.3. Density measurements Density measurements were performed with a gas pycnometer AccuPyc II 1340 from micromeritics. The powder was filled into a 0.1 cm3 insert and purged with helium for 20 cycles. A measurement consisted of 10 cycles with a sensitivity of 0.025 kPa/min. 2.2.4. Elementary analysis The CHN analysis was carried out with a vario MICRO cube (Elementar) through combustion of the samples. The O content was measured with a rapid OXI CUBE from Elementar. The Co content was determined by a 4200 MP-AES instrument from Agilent. The samples were tested for a possible bromine content via m-RFA with a Torando M4 (Bruker). 2.2.5. X-ray photoelectron spectroscopy The X-ray photoelectron spectra (XPS) were acquired with monochromatized Al Ka (1486 eV) radiation using a Phoibos 150 electron energy analyzer from SPECS. The samples were introduced into an ultrahigh-vacuum chamber with a base pressure in the low 1010 mbar range. The sample powders were either directly pressed onto an Al foil or, alternatively, a water based slurry of the sample material was prepared and dried onto an Ag carrier foil. For the fitting of the spectra, an asymmetric Pseudo-Voigt function was used [21,22]. 2.2.6. Near-edge X-ray absorption fine structure measurements Near-edge X-ray absorption fine structure (NEXAFS) measurements were performed on the HE-SGM beamline at BESSY-II synchrotron facility in Berlin. The sample powder was glued to the sample holder using carbon tape. The X-ray absorption was measured indirectly via an electron yield detector [23], which counts (i) Auger electrons that are created by the decay of the excited state after photon absorption and (ii) the secondary electrons [24] that are caused by Auger electrons and photoelectrons. If a retarding field is applied between detector and sample, electrons with a low kinetic energy may be excluded from contributing to the signal, this case is referred to as partial yield detection. Without retarding field, all electrons contribute to the signal (total yield). Both detection methods were used for this work. 3. Results and discussion An analysis of the CoOOH and CoOOH

A. Kudielka et al. / Journal of Alloys and Compounds 824 (2020) 153925

samples with X-ray powder diffraction shows that both compounds show no residual impurities, such as Co(OH)2 or Co3O4, and crystalize in the space group R3m (3R-modification). Fig. 2 displays the measured diffraction patterns together with the calculated reflex pattern associated with the 3R-modification [14]. An analysis of the full width at half maximum of the (003) and (110) reflexes allows to calculate the particle size according to Scherrer’s equation [25]. The average particle size obtained for CoOOH was 24 nm, while CoOOH shows a significantly smaller particle size of 8 nm. The particle size determined via Scherrer’s equation separately for the (003) and (110) reflexes allows also to quantify the ratio of the facets showing (003) and (110) orientation, this ratio is referred to as aspect ratio. An aspect ratio of 1 is associated with nanoparticles, where the surface equally comprises (110) and (003) facets. In fact, an aspect ratio of 1 is found for CoOOH . In contrast, CoOOH shows an aspect ratio of 2.5, a clear indication that the CoOOH nanocrystals are preferentially exhibiting (110) surface orientations (Table 1). The extraction of the lattice parameters (Fig. 1) from the XRPD data reveals that the lattice parameter ‘a’ is in both samples nearly identical (CoOOH : 2.8576 (4) Å and CoOOH : 2.8527 (3) Å). However, the lattice parameter ‘c’ deviates slightly with a value of 13.137 (3) Å in CoOOH and 13.331 (5) Å in CoOOH (Table 1). Measurements of the density and chemical composition of the nanoparticles reveal that there are deviations from the nominal stoichiometry in both cases. In the literature [14], a density of 4.95 g/cm3 was found for CoOOH. In our experiments, CoOOH has a density of 4.43 (3) g/cm3, while CoOOH has a density of 4.81 (7) g/cm3 (Table 1). This result may arise from excess mass (due to intercalation or remaining wash water or CO2 from air). A detailed analysis of the chemical composition of both samples (Table 2) shows that in particular CoOOH has excess O and H atoms, which could be associated with the presence of Co2þ in nc-CoOOH. A comparison of the IR spectra of CoOOH and CoOOH is shown in Fig. 2 (right). Both spectra exhibit a sharp band at ~564 cm1, indicating CoeO bending mode [14]. A broad band between 1000 and 2000 cm1 was found, indicating a H bending mode [14]. Importantly, there were no bands for Co(OH)2 or Co3O4 (cobalt spinel). Co(OH)2 exhibits bands at ~489 cm1 (Co2þ-O in octahedral ligand field) and at ~3630 cm1 (free OH-

3

Table 1 Particle sizes, aspect ratios, lattice parameters, and densities of CoOOH and CoOOH .

/nm aspect ratio a/Å c/Å density/g cm3

CoOOH

CoOOH

24 2.5 2.8527 (3) 13.137 (3) 4.81 (7)

8 1 2.8576 (4) 13.331 (5) 4.43 (3)

Table 2 Comparison of the composition from CoOOH and CoOOH with stoichiometric CoOOH.

Co/wt% O/wt% H/wt% C/wt% N/wt% Br/wt% Composition

Theory

CoOOH

CoOOH

64.1 34.8 1.1 0 0 0 CoOOH

61.3 37.5 1.2 0 0 0 CoO1.09(OH)1.16

59.1 39.3 1.4 0 0 0.2 CoO1.08(OH)1.37

group) [26].The cobalt spinel Co3O4 has bands at 581 cm1 and 666 cm1, none of which are present [27]. While the analysis of the chemical composition and the density measurements gave indications for a deviation of both samples from the ideal stoichiometry, the chemical origin of these deviations is not clear with the methods applied so far. However, it appears likely that these differences are associated with the presence of Co2þ within the crystal structure. In particular, the higher H- and O-content for CoOOH indicates a larger Co2þ content in this sample, which should directly change the magnetic properties of the nanoparticles. In the following, the presence of Co2þ was proven and its amount quantified with various independent experimental methods: (i) Measurement of the magnetic susceptibility (CoOOH and CoOOH ), and (ii) X-ray Photoelectron Spectroscopy (XPS) as well as Near Edge X-ray Absorption Fine Structure (NEXAFS) for CoOOH . An analysis of the susceptibility of both samples reveals significant differences between CoOOH and CoOOH (Fig. 3). The susceptibility measured for CoOOH is

Fig. 2. Left: XRPD patterns of as-prepared CoOOH (black) and CoOOH (red) between 15 and 100 2q measured with Co Ka, compared with calculated pattern of the 3R-modification of CoOOH (blue). Right: IR spectra of CoOOH (black) and CoOOH (red) between 4000 and 375 cm1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

4

A. Kudielka et al. / Journal of Alloys and Compounds 824 (2020) 153925

Fig. 3. Temperature dependency of the reciprocal susceptibility, fitted with the modified Curie Weiss equation (5), for CoOOH (black) and CoOOH (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

significantly higher than the corresponding value for CoOOH . However, also CoOOH shows a non-negligible magnetic behavior. To quantitatively understand the observed phenomena in both samples, a modified Curie-Weiss law with Curie-Weiss susceptibility cCW and temperature-independent paramagnetism c0 was used to calculate the magnetic moments of the as prepared samples (Equation (4)).

c ¼ cCW þ c0

(4)

The temperature dependence of the reciprocal susceptibility of the samples was fitted according to Equation (5). There, n is the number of the moment carriers per formula unit CoOOH, qCW denotes the Curie-Weiss temperature, and neff represent the number of the Bohr magnetons.




cCW ¼

neff

.

2:828

2

negligible magnetic interactions. Nonlinear behavior in the high temperature range from 50 K can be explained via temperature independent paramagnetism c0. This behavior is not obvious for CoOOH . With increasing magnetic moment, the influence of c0 decreases and the curve approximate the ideal linear Curie behavior. As shown in Table 3, a similar c0 is found regardless of the magnetic moment. The chemical composition of the CoOOH sample was further examined by X-ray photoelectron spectroscopy (XPS),a highly surface sensitive method [28]. A detailed analysis of the Co 2p3/2 and 2p1/2 core levels (Fig. 4) allows to draw conclusions about the oxidation states of the cobalt species within the sample and, in

n (5)

T  QCW

Least-squares fits of the measured susceptibilities with Equation (5) (Fig. 3) show that both samples clearly exhibit a paramagnetic behavior and reveal effective magnetic moments neff of 0.409 (3) mB and 1.105 (4) mB per formula unit for CoOOH and CoOOH , respectively. This magnetic behavior cannot be explained by the presence of Co3þ ions, since Co3þ ions are diamagnetic in an octahedral ligand field. However, a magnetic moment would be associated with the presence of incompletely oxidized d7 high spin Co2þ. Taking into account that Co2þ ions carry an effective magnetic moment of 3.87 mB in an octahedral ligand field, one can use the measured susceptibilities to estimate the Co2þ content. This procedure leads to a Co2þ content of 8.1% in CoOOH and 1.1% in CoOOH . In Table 3, the different magnetic parameters are summarized. The analysis also revealed slightly negative Curie-Weiss temperatures, an indication for

Table 3 Comparison of magnetic data of CoOOH and CoOOH .

neff/mB n (Co2þ)/% c0/104 cm3 mol1 qCW/K

CoOOH

CoOOH

0.409 (3) 1.1 1.96 6.4 (5)

1.105 (4) 8.1 1.48 9.8 (6)

Fig. 4. Co 2p XP spectra with deconvolution into the Co2þ and Co3þ spectral features for [Co(NH3)6]Cl3 (top), CoOOH (middle) and Co(OH)2 (bottom).

A. Kudielka et al. / Journal of Alloys and Compounds 824 (2020) 153925

particular, in the near surface region. Co2þ and Co3þ ions exhibit, due to their different configuration in the d-subshell, significantly different spectral fingerprints in their 2p photoelectron spectra. While Co3þ shows relatively narrow 2p3/2 and 2p1/2 lines with minor satellites, Co2þ generally exhibits broad 2p3/2 and 2p1/2 lines accompanied by intense satellite structures [29]. The complex spectra of Co2þ arise from unpaired electrons in the d-subshell which participate in final-state coupling effects with the 2p core hole after photoionization [30e33]. Since XPS is a quantitative method, it is in principle possible to deconvolute the total signal into a Co3þ and Co2þ component. In order to obtain the signal patterns of pure Co2þ and Co3þ compounds, spectra of Co(OH)2 (reference for Co2þ) and [Co(NH3)6]Cl3 (reference for Co3þ) were recorded. A least-squares fit reveals that the spectral pattern of CoOOH comprises 88% Co3þ and 12% Co2þ components (Fig. 4). This analysis is consistent with the finding that the magnetic moment in CoOOH is caused by Co2þ ions present in the material. It is important to note that one cannot directly associate the 12% contribution of the Co2þ signal to the total signal with a stoichiometric amount of 12% Co2þ in the bulk sample. It is easily possible that Co2þ ions are enriched on the surface, while the bulk material would contain exclusively the Co3þ species. In such a situation, careful depth profiling would be necessary to obtain depth-concentration profiles for the Co2þ species. However, such an analysis would be highly speculative as the information necessary to treat the curvature of the nanoparticles is not readily accessible [34,35]. Clearly, a complementary method which allows to distinguish between homogeneous and layered nanocrystals is required. X-ray absorption (NEXAFS) measurements at the Co L-edge (Fig. 5) were performed (i) to clarify if the particles show a similar composition in the bulk and in the near surface region and (ii) to elucidate details of the electronic structure of the cobalt species in the CoOOH sample. The first point was addressed by examination of spectra collected in total electron yield (bulk sensitive) and partial electron yield (surface sensitive) modes. The second issue was examined by a comparison of the NEXAFS spectrum of CoOOH with the spectra of Co(OH)2 (Co2þ) and [Co(NH3)6]Cl3 (Co3þ). To examine the surface and bulk composition of the sample separately, total and partial yield spectra of CoOOH were recorded. To maintain surface sensitive conditions, a retardation field of 150 eV was applied at the electron yield detector. This allowed only secondary electrons with a kinetic energy higher than 150 eV and lower than 770 eV (kinetic energy of Co LIIIVV Auger excitation without any energy loss) to contribute to the signal. Since the escape depth varies between ~0.5 nm (150 eV) and ~1.5 nm (770 eV) in this energy range [36], the surface of the nanoparticles (with an average diameter of 8 nm) should have a higher contribution to the signal. It should be noted that the NEXAFS detector does not distinguish between elastically and inelastically scattered electrons (as long as their energy is high enough to overcome the retarding field); therefore, the actual escape depths may be somewhat different. In contrast to partial yield, total yield detection also includes extremely slow secondary electrons with escape depths by far exceeding the particle diameter. The fact that total and partial yield spectra look identical indicates that the distribution of Co2þ and Co3þ is homogeneous within the nanoparticles. The absence of surface enrichment leads to the conclusion that the contribution of the Co2þ XPS signal to the total Co 2p XPS signal can in fact be directly associated with a concentration of 12% of the Co2þ species. Considering that the Co2þ signal pattern (Fig. 4) does not show an easily distinguishable peak, but only to a broad signal at the same energy as Co3þ and a slight increase in satellite

5

Fig. 5. Co L-edge X-ray absorption spectra of CoOOH (black) compared with the spectra of Co(OH)2 (orange) and [Co(NH3)6]Cl3 (blue). CoOOH was measured in total yield (solid line) and partial yield (dashed line). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

intensity (at ~788 eV), the uncertainty of the fit result is rather high. In addition, the background subtraction is complicated by an underlying Auger peak. Thus, the concentration of 12% obtained by XPS is in reasonable agreement with the concentration of 8.1% derived from magnetic data and confirms the previous results regarding the chemical nature of the nanoparticles. It is also possible to extract limited information regarding the amount of Co2þ in the CoOOH sample from the NEXAFS data. A comparison with the Co L-edges of similar Co2þ (Co(OH)2) and Co3þ ([Co(NH3)6]Cl3) compounds reveals that the CoOOH sample does not show the signature of a pure Co3þ compound. The characteristic shoulder of Co3þ spectra (see [Co(NH3)6]Cl3 and Ref. [37]) at approximately 2.5 eV below the main line (~779 eV) is reduced in the CoOOH sample. This points towards the presence of a further Co2þ component and shows that the CoOOH sample is not a pure Co3þ system. However, there is no further direct indication for Co2þ in the NEXAFS spectrum, since the Co2þ signature (see Co(OH)2) is rather broad and has a strong overlap with the features observed in the Co3þ samples. 4. Summary The presented simple precipitation-oxidation reaction in basic solution provides non-stoichiometric nc-CoOOH with various

6

A. Kudielka et al. / Journal of Alloys and Compounds 824 (2020) 153925

particle sizes and a varying amount of residual Co2þ. Both samples contain an excess of intercalated and/or remaining wash water or CO2 from air. This reduces the density and leads to the compositions CoO1.09(OH)1.16 (sample CoOOH ) and CoO1.08(OH)1.37 (sample CoOOH ), as determined by density measurements and elementary analysis. XRPD and IR spectra provide no indication for residual impurities such as Co(OH)2 or Co3O4. If cobalt(II)hydroxide is oxidized with Br2, particles with an average particle size of 8 nm are obtained, in contrast to a particle size of 24 nm yielded by oxidation with air. SQUID measurements prove that both samples contain a magnetic moment that is incompatible with d6 Co3þ in an octahedral ligand field. It is shown with XPS and NEXAFS that the magnetic moment in CoOOH arises from incompletely oxidized Co2þ in the structure. With this information, it is possible to calculate the Co2þ content starting from the SQUID results, which yield 8.1% for CoOOH and 1.1% for CoOOH . The results of the total and partial yield NEXAFS measurement give no indication for surface enrichment of Co2þ. In conclusion, the choice of the synthesis conditions of the CoOOH nanoparticles enables a targeted tailoring of their properties, including particle size, morphology, composition, and magnetic moment. In particular, the magnetic moment of the nanoparticles provides hope for a potential application as a nanomagnet. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Alexander Kudielka: Conceptualization, Investigation, Writing - original draft, Writing - review & editing, Visualization, Formal analysis. Martin Schmid: Investigation, Writing - original draft, Writing - review & editing, Visualization, Formal analysis. Benedikt P. Klein: Investigation, Writing - original draft, Writing - review & editing, Visualization, Formal analysis. Clemens Pietzonka: Investigation. J. Michael Gottfried: Writing - review & editing, Resources, Supervision. Bernd Harbrecht: Conceptualization, Writing - review & editing, Resources, Supervision. Acknowledgements We thank BESSY-II/Helmholtz-Zentrum Berlin (HZB) for the allocation of synchrotron radiation beamtime. References [1] J. Yang, H. Liu, W.N. Martens, et al., Synthesis and characterization of cobalt hydroxide, cobalt oxyhydroxide, and cobalt oxide nanodiscs, J. Phys. Chem. C 114 (2010) 111e119. [2] M. Deliens, H. Goethals, Polytypism of heterogenite, Miner. Mag. 39 (1973) 152e157. [3] C.-H. Chen, S.F. Abbas, A. Morey, et al., Controlled synthesis of self-assembled metal oxide hollow spheres via tuning redox potentials: versatile nanostructured cobalt oxides, Adv. Mater. 20 (2008) 1205e1209. [4] B. Geng, F. Zhan, H. Jiang, et al., Facile production of self-assembly hierarchical dumbbell-like CoOOH nanostructures and their room-temperature CO-GasSensing properties, Cryst. Growth Des. 8 (2008) 3497e3500. [5] S. Zhuiykov, V. Dowling, The nanostructured Au-doped cobalt oxyhydroxide based carbon monoxide sensor for ire detection at its earlier stages, Meas. Sci. Technol. 19 (2008), 024001. [6] G. Salek, P. Alphonse, P. Dufour, et al., Low-Temperature carbon monoxide and propane total oxidation by nanocrystalline cobaltoxides, Appl. Catal., B 147 (2014) 1e7.

[7] W.K. Hu, X.P. Gao, M.M. Geng, et al., Synthesis of CoOOH nanorods and application as coating materials of nickel hydroxide for high temperature NiMH cells, J. Phys. Chem. B 109 (2005) 5392e5394. [8] V. Pralong, A. Delahaye-Vidal, B. Beaudoin, et al., Oxidation mechanism of cobalt hydroxide to cobalt oxyhydroxide, J. Mater. Chem. 9 (1999) 955e960. [9] V. Pralong, A. Delahaye-Vidal, B. Beaudoin, et al., Electrochemical behavior of cobalt hydroxide used as additive in the nickel hydroxide electrode, J. Electrochem. Soc. 147 (2000) 1306e1313. [10] D.S. Dhawale, S. Kim, D.-H. Park, et al., Hierarchically ordered porous CoOOH thin-film electrodes for high-performance supercapacitors, ChemElectroChem 2 (2015) 497e502. [11] J. Huang, J. Chen, T. Yao, et al., CoOOH nanosheets with high mass activity for water oxidation, Angew. Chem. Int. Ed. 54 (2015) 8722e8727. [12] M. Bajdich, M. García-Mota, A. Vojvodic, et al., Theoretical investigation of the activity of cobalt oxides for the electrochemical oxidation of water, J. Am. Chem. Soc. 135 (2013) 13521e13530. [13] M. García-Mota, M. Bajdich, V. Viswanathan, et al., Importance of correlation in determining electrocatalytic oxygen evolution activity on cobalt oxides, J. Phys. Chem. C 116 (2012) 21077e21082. [14] R.G. Delaplane, J.A. Ibers, J.R. Ferraro, et al., Diffraction and spectroscopic studies of the cobaltic acid system HCoO2-DCoO2, J. Chem. Phys. 50 (1969) 1920e1927. [15] F. Barde, M.R. Palacin, B. Beaudoin, et al., New approaches for synthesizing g III-CoOOH by soft chemistry, Chem. Mater. 16 (2004) 299e306. [16] R.L. Penn, A.T. Stone, D.R. Veblen, Defects and disorder: probing the surface chemistry of heterogenite (CoOOH) by dissolution using hydroquinone and iminodiacetic acid, J. Phys. Chem. B 105 (2001) 4690e4697. [17] D. Petzold, Thermoanalytische und Kalorimetrische Untersuchung von CoOOH und CoOOD, J. Therm. Anal. 30 (1985) 391e398. [18] E. Hosono, S. Fujihara, I. Honma, et al., Synthesis of the CoOOH fine nanoflake film with the high rate capacitance property, J. Power Sources 158 (2006) 779e783. [19] A. Kudielka, S. Bette, R.E. Dinnebier, et al., Variability of composition and structural disorder of nanocrystalline CoOOH materials, J. Mater. Chem. C 5 (2017) 2899e2909. [20] G.F. Hüttig, R. Kassler, Zur Kenntnis des Systems Kobalt(3)oxyd-Wasser, Z. Anorg. Allg. Chem. 184 (1929) 279e288. [21] M. Schmid, H.P. Steinrück, J.M. Gottfried, A new asymmetric Pseudo-Voigt function for more efficient fitting of XPS lines, Surf. Interface Anal. 46 (2014) 505e511. [22] M. Schmid, H.P. Steinrück, J.M. Gottfried, A new asymmetric Pseudo-Voigt function for more efficient fitting of XPS lines (vol 46, pg 505, 2014), Surf. Interface Anal. 47 (2015) 1080. [23] G. Bracco, B. Holst, Surface Science Techniques, Springer Verlag, Berlin Heidelberg, 2013. €lkel, B. Golzhauser, A. Muller, et al., Influence of secondary electrons in [24] M. Vo proximal probe lithography, J. Vac. Sci. Technol. B Microelectron. Nanometer Struct. 15 (1997) 2877. €sse und der inneren Struktur von Kolloid[25] P. Scherrer, Bestimmung der Gro € ntgenstrahlen, Go €ttinger Nachr. (1918) 98e100. teilchen mittels Ro [26] Y. Zhu, H. Li, Y. Koltypin, et al., Preparation of nanosized cobalt hydroxides and oxyhydroxides assisted by sonication, J. Mater. Chem. 12 (2002) 729e733. [27] Y. Chen, Y. Zhang, S. Fu, Synthesis and characterization of Co3O4 hollow spheres, Mater. Lett. 61 (2007) 701e705. [28] C.J. Powell, A. Jablonski, Progress in quantitative surface analysis by x-ray photoelectron spectroscopy: current status and perspectives, J. Electron. Spectrosc. Relat. Phenom. 178e179 (2010) 331e346. [29] Y.G. Borod’ko, S.I. Vetchinkin, S.L. Zimont, et al., Nature of satellites in X-ray photoelectron spectra XPS of paramagnetic cobalt(II) compounds, Chem. Phys. Lett. 42 (1976) 264e267. [30] T. Ivanova, A. Naumkin, A. Sidorov, et al., X-ray photoelectron spectra and electron structure of polynuclear cobalt complexes, J. Electron. Spectrosc. Relat. Phenom. 156e158 (2007) 200e203. [31] M. Schmid, J. Zirzlmeier, H.P. Steinrück, et al., Interfacial interactions of iron(II) tetrapyrrole complexes, J. Phys. Chem. C 115 (2011) 17028e17035. [32] D.C. Frost, C.A. McDowell, I.S. Woolsey, X-ray photoelectron spectra of cobalt compounds, Mol. Phys. 27 (1974) 1473e1489. [33] V. Nefedov, The multiplet structure of the Ka 1,2-lines of transition elements, Bull. Acad. Sci. USSR, Phys. Ser. (1964) 724e730. [34] M. Mohai, I. Bertoti, Calculation of overlayer thickness on curved surfaces based on XPS intensities, Surf. Interface Anal. 36 (2004) 805e808. [35] S.V. Merzlikin, N.N. Tolkachev, T. Strunkus, et al., Resolving the depth coordinate in photoelectron spectroscopy - comparison of excitation energy variation vs. angular-resolved XPS for the analysis of a self-assembled monolayer model system, Surf. Sci. 602 (2008) 755e767. [36] M.P. Seah, W.A. Dench, Quantitative electron spectroscopy of surfaces: a standard data base for electron inelastic mean free paths in solids, Surf. Interface Anal. 1 (1979) 2e11. [37] F.M.F. De Groot, M. Abbate, J. Van Elp, et al., Oxygen 1s and cobalt 2p x-ray absorption of cobaltoxides, J. Phys. Condens. Matter 5 (1993) 2277e2288.