Local Coordination and Valence States of Cobalt in Sodium Titanate

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Local Coordination and Valence States of Cobalt in Sodium Titanate Nanoribbons Polona Umek, Carla Bittencourt, Alexandre Gloter, Robert Dominko, Zvonko Jaglicic, Pavel Cevc, and Denis Arcon J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 May 2012 Downloaded from http://pubs.acs.org on May 5, 2012

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The Journal of Physical Chemistry

Local Coordination and Valence States of Cobalt in Sodium Titanate Nanoribbons Polona Umek1,2*, Carla Bittencourt3, Alexandre Gloter4, Robert Dominko5, Zvonko Jagličić6,7, Pavel Cevc1, and Denis Arčon1,8

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Jožef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia

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Center of Excellence NAMASTE, Jamova cesta 39, SI-1000 Ljubljana, Slovenia

Chimie des Interactions Plasma Surface, CIRMAP, University of Mons, 23 Place du Parc, B-7000 Mons, Belgium

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Laboratoire de Physique des Solides, Université Paris Sud, CNRS UMR 8502, F-91405 Orsay, France 5 6

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National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia

Institute of Mathematics, Physics and Mechanics, Jadranska ulica 19, SI-1000 Ljubljana, Slovenia

Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova cesta 2, Si-1000 Ljubljana, Slovenia 8

Faculty of Mathematics and Physics, University of Ljubljana, Jadranska ulica 19, SI-1000 Ljubljana, Slovenia

*E-mail: [email protected], Fax: +386-1-4773-191, Tel: +386-1-4773-500 RECEIVED DATE

ACS Paragon Plus Environment

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ABSTRACT: Co2+-doped sodium titanate nanoribbons were grown under hydrothermal conditions from Co2+-doped TiO2 and NaOH(aq) at 175 °C. The obtained nanoribbons, with diameters of 30−150 nm and length up to several µm, were determined to have a trititanate structure ((Na,H)2Ti3O7). Transmission electron microscopy revealed nanoparticles, not exceeding 15−20 nm in length, as well as hexagonal nanoflakes located on the surface of the nanoribbons. These nanoflakes are most likely originating from β-Co(OH)2 side-product, according to the X-ray diffraction investigation. High-angle annular-dark-filed scanning-transmission electron microscopy combined with electron-energy-loss spectroscopy showed that amount of cobalt in the surface nanoparticles is much higher than in the nanoribbons. X-ray photoelectron spectroscopy (XPS) revealed that atomic concentration of cobalt in the sample is 1.5 wt. %, of which a small amount is in the oxidation state 3+. A detailed electron paramagnetic resonance characterization of this sample proved that Co2+ ions occupy octahedral sites with rhombic distortion in a high-spin S = 3/2 state. Temperature-dependent susceptibility measurement revealed the prevailing paramagnetic behavior, from which a mass ratio 1.3 wt % of Co2+ was obtained, and which is in agreement with the elemental analysis results and the value extracted from the XPS measurements. A weak antiferromagnetic transition at 12 K is associated with β-Co(OH)2 nanoflakes.

KEYWORDS: doping, titanates, nanoribbons, electron microscopy, EELS, magnetic properties, XPS, SQUID, EPR


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1. Introduction For the past few years the interest in alkali titanate nanostructures is rapidly growing due to their potential applications in numerous different fields most notably as catalysis [1, 2], photocatalysis [1, 3 5], magnetism [6, 7], sensors [1, 8] and lithium ion batteries [1, 9−11]. With the aim of improving the electronic and chemical properties of these layered nanostrucutres, different doping approaches were tested and the two most promising are (i) ion-exchange process [12−15] and (ii) in situ doping [5, 15−21]. In the case of an ion exchange synthesized titanate nanostructures are dispersed into a solution of dopant ions where the dopant ions exchange alkali cations intercalated between titanate layers. On contrary, by in situ doping dopant ions are present in the reaction mixture, typically in the form of a doped TiO2. During the reaction they occupy positions between the layers and/or they can even partially substitute Ti4+ in TiO6 octahedra thus directly affecting electronic, optical, or magnetic properties of parent nanostructures [5, 15, 17, 18, 21]. Depending on the local pH values and type of the dopant cation in the ion-exchange process the dopant ions can also form hydroxide clusters on the surface of titanate nanostructures [15], which can act as a potential functional surface centers. Up to now considerable effort has been invested in the synthesis of Co2+-doped sodium titanate [16, 20], Co2+-doped protonated titanate [19−21] nanostructures, and their transformation to Co2+-doped TiO2 nanostructures [11, 16, 20], using both in situ and ion exchange approaches. The motivation behind are improved photo-catalytic or magnetic performances of these nanostructures [1, 3−7]. The later is due to the potential use of these materials in spintronic devices because of the observed room temperature ferromagnetism in Co2+-doped anatase TiO2 films [22, 23]. Nevertheless, controverises in reported data raises certain concerns about the intrinsic origin of ferromagnetism and proposals that cobalt oxide impurities should be taken into account regularly appear in the literature [23−25]. Several reports on Co2+-doped titanate nanotubes and nanoribbons show room temperature ferromagnetic response [16, 19, 20, 21] indicating that magnetism does not depend so much on the details of the


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crystal structure but more on the local Co2+ environment. However, due to the relatively poor cristallinity of Co2+-doped titanate nanotubes it is difficult to verify different models developed for the ferromagnetism considering the highly-ordered anatase TiO2 films [22, 24, 26]. Therefore, there is an urgent need of the Co2+-doped crystalline one-dimensional titanate nanostructures where Co2+-local environment can be uniquely determined and then connected with possible magnetic response. The aim of this work is to shed an additional light on the understanding of the cobalt doping of sodium titanate nanoribbons. More specifically we address the following questions: (i) do the dopant ions (Co2+) accumulate between titanate layers or/and do they exchange Ti4+ atoms in the TiO6 octahedra during reaction, (ii) do the dopant ions form CoTiyOx impurity phases with low titanium content, (iii) do Co2+ ions change oxidation state under applied reaction conditions, (iv) can we distinguish Co2+ local coordination at different sites using electron paramagnetic resonance (EPR) and magnetization measurements, and (v) what would be their magnetic properties? Characterization of the synthesized material with TEM revealed presence of small nanoparticles forming on the surface of the nanoribbons. Further investigations using STEM-HAADF in combination with EELS showed that cobalt is not at all homogenously distributed in the nanoribbons and that the titanium content in the nanoparticles located at the surface of the nanoribbons is lower than in the nanoribbons. Magnetic measurements revealed a predominant presence of paramagnetic Co2+ ions and no indication of ferromagnetism between 2 K and 300 K. The contamination of the nanoribbons with β-Co(OH)2 impurities is seen in XRD and as a weak antiferromagnetic transition at 12 K. Surprisingly, a low amount of cobalt ions is found also in the oxidation state 3+ as revealed by XPS measurements.

2. Experimental part 2.1 Synthesis: Co2+-doped sodium titanate nanoribbons (Co@NaTiNRs) were synthesized in two stages. In the first one, TiO2 doped with 3 wt. % (1.4 at. %) of Co2+ (Co@TiO2) was prepared by the


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sol-gel method using tetraisopropyl titanate [Ti(OCH(CH3)2)4] (Aldrich) as a titanium source, and cobalt(II) nitrate hexahydrate [Co(NO3)2 × 6H2O] (Fluka) as a source of Co2+. Detailed procedure is described elsewhere [15]. In the second stage, Co@NaTiNRs were synthesized from 2 g of Co@TiO2 and 25 mL of 10 M NaOH, using a procedure analogous to that described in ref. 14 and 16. A Teflonlined autoclave was filled to 23 mL and held during 72 hours on 175 °C. The resulting pale bluish to gray product was dispersed into 100 mL of deionized water and filtered. The product caught on the filter was washed with 50 mL of EtOH and dried over night at 100 °C.

2.2. Characterization techniques: The morphology of synthesized product was investigated with field emission scanning (SEM, Carl Zeiss, Supra 35LV equipped) and transmission (TEM, Jeol 2100) electron microscopes. The sample for SEM characterization was prepared by dispersing the product in water and then a drop of a dispersion was deposited on a conductive tape on a sample holder, while for TEM characterization the sample was dispersed in MeOH by using an ultrasonic bath and then 1−2 drops of a dispersion were deposited on a holey carbon grid. The X-ray diffraction (XRD) measurements were performed with a PANalytical X’Pert PRO highresolution diffractometer with CuKα1 radiation (1.5406 Å). The STEM-HAADF (high angle annular dark field scanning transmission electron microscopy) and STEM-EELS (electron energy loss spectroscopy measurements were performed with a dedicated STEM (Vacuum Generators HB 501) equipped with a home-modified Gatan spectrometer. All spectra were recorded in STEM mode with 100 keV incident electrons focused on the specimen. EELS mapping was obtained by rastering a 0.8 nm electron probe at the surface of sample while acquiring EELS spectra with typical time at the order of 10 ms. The Ti L2,3, O-K and Co L2,3 edges were collected simultaneously for further mapping or EELS quantification. The X-ray photoelectron spectroscopy (XPS) measurements were performed in a VERSAPROBE PHI 5000 from Physical Electronics, equipped with a Monochromatic Al Kα X-Ray. The energy


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resolution was 0.6 eV. For the compensation of built up charge on the sample surface during the measurements a dual beam charge neutralization composed of an electron gun (~1eV) and the Argon Ion gun (≤10eV) was used. The samples for XPS measurements were prepared by pressing the sample into a pallet. A conductive double face tape was used to attach the pallet to the sample holder. The X-band electron paramagnetic resonance (EPR) measurements at the Larmor frequency νL ≈ 9.6 GHz were conducted on a commercial Bruker E580 spectrometer equipped with the dielectric ring resonator ER 4118X-MD5. Typically, low microwave powers of 1 mW and modulation fields of 1 G were used in continuous wave (cw) mode at 20 K. Magnetic susceptibility between 2 and 300 K in magnetic field of 1000 Oe and magnetization curves up to 50 kOe at 5 K and 300 K were measured with a Quantum Design MPMS XL-5 SQUID magnetometer. An elemental analysis was performed on the inductively coupled plasma atomic emission spectrometer (ICP-AES). The product (∼50 mg) was prior to analysis dissolved in a mixture of hot conc. HCl acid and H2O2(aq). Elemental composition of the bulk sample revealed that the cobalt content is 1.6 wt. %, sodium 12.8 wt. % and titanium 40.8, wt. %, respectively.

3. Results and Discussion 3.1. SEM and TEM characterization: First, SEM and TEM characterization techniques were employed to determine the morphology and crystallinity of Co@NaTiNR (Figure 1). From these microscopy studies one can conclude: (i) the product grows in the form of nanoribbons as it was expected regarding to the applied reaction conditions [17, 24], (ii) a reaction yield concerning the morphology is very high and (iii) that grown nanoribbons are crystalline. Furthermore, the interplanar distances measured in high resolution TEM images corroborate with trititanate structure ((Na,H)2Ti3O7) of the nanoribbons (Figure 1B). Namely, the interplanar distances parallel to the rod axis are about 0.95


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nm, which agrees with the d value (0.92 nm) of (100) planes of Na2Ti3O7 (JCPD 00-059-066) [27, 28]. The typical widths of the synthesized Co@NaTiNRs are found to be between 30 and 150 nm while the lengths of the nanoribbons vary from 500 nm up to several micrometers. This implies that the low cobalt doping levels have no effect on the average diameter and length as they are comparable to the undoped [27, 28] and Cr3+-doped sodium titanate nanoribbons [17]. Observation at higher magnifications revealed the existence of nanoparticles located at the surface of the nanoribbons (the upper inset in Figure 1B and Figure 1C). These nanoparticles typically do not exceed 15−20 nm in length. We stress that such nanoparticles were not observed in the undoped [27, 28] and in the in situ Cr3+-doped [17] sodium titanate nanoribbons grown under the same reaction conditions, and neither in the in situ Co2+-doped sodium titanate nanotubes [20, 21]. The majority of these nanoparticles have no particular shape (as observed from the upper inset in Figure 1B and Figure 1C) while a minor part appears in a form of hexagonal nanoflakes. These nanoflakes appear to be very unstable under the electron beam indicating that very likely have different chemical composition than the rest of the nanoparticles.

3.2. XRD characterization: XRD diffractograms of the Co2+ doped anatase TiO2 (Co@TiO2) and Co@NaTiNRs are shown in Figure 2. The diffractogram of Co@TiO2 comply with the anatase phase (JCPD 00-002-0406) while all main diffraction peaks in the X-ray diffraction pattern of Co@NaTiNRs can be indexed to Na2Ti3O7 (JCPD 00-059-0666) and thus corroborate with HRTEM results (Figure 1). Broadening of some reflections (i.e. [100] and [300]) arise from nanometer dimensions of the nanoribbons (widths: 30−150 nm, heights of about 20 nm [29]). Weak peaks that cannot be indexed to the trititanate structure may point to the presence of other titanate phases with the formula A2TinO2n-1 (n ≥ 5) where A stands for Na+, Co2+, or H3O+ ions that occupy interlayer spaces. On the other hand, the weak peaks marked with full circles can be easily index to β-Co(OH)2 (JPCD 00-030-0443). The presence of β-Co(OH)2 in Co@NaTiNRs grown at 175 °C under hydrothermal conditions for 72 hours ACS Paragon Plus Environment

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is rather unexpected, since according to TGA and DSC measurements [30] β-Co(OH)2 transforms to CoOOH between 120−190 °C. We thus suggest that sodium titanate nanostructures contribute to the thermal stability of β-Co(OH)2 that is formed as a side product during the reaction. Furthermore, the intensities of β-Co(OH)2 peaks suggest on the preferential orientation in the [001] direction expected for the hexagonal nanoflakes [30].

3.3. XPS characterization: the XPS data recorded in the Co 2p3/2 core level region (Figure 3A) were used to determine the oxidation state of cobalt. The carbon C 1s core level peak at 285 eV of binding energy was taken as the reference. The relative amount of cobalt evaluated using XPS was 1.5 wt. %, which is in accordance with the value obtained by ICP-MS (please see the experimental part). The Co 2p region shows two main peaks separated by near 16 eV as expected for cobalt oxides [21]. Information on the oxidation state can be obtained from a more detailed analysis of the Co 2p3/2 peak. The spectrum in the Co 2p3/2 region shows a main peak centered at 780.5 eV with a low intensity shoulder around 779 eV and a satellite peak with an energy separation from the main peak of 6 eV centered at 786.5 eV (Co shake-up structure). As reported in the literature the main peak is associated with photoelectrons emitted from cobalt atoms in the Co2+ state [21]. The low intensity structure near 779 eV of binding energy suggests a low relative concentration of cobalt atoms in the Co3+ state (~0.3 wt. % considering the area under the Co 2p spectrum). Their presence is not entirely unexpected since Co2+ in alkaline media at elevated temperatures and in the presence of oxygen oxidizes to Co3+ forming CoOOH [29, 30]. The content of Co3+ is extremely low (~0.3 wt. %) as evaluated by XPS. The low content explains the reason we were not able to detect CoOOH in the XRD measurements. A plausible explanation for the Co3+ presence is the partial oxidation of the surface of the β-Co(OH)2 nanoflakes to CoOOH [30]. Titanium binding energies, 458.5 (2p3/2) and 464.3 eV (2p1/2), are compatible to the binding energies for titanium atoms in the 4+ oxidation state. The analysis of the Ti 2p spectra recorded on the ACS Paragon Plus Environment

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undoped sodium titanate nanoribbons (spectrum is not shown here) and Co@NaTiNRs (Figure 3B) revealed that they are identical suggesting that due to the low doping level the electronic states of the titanate matrix is not strongly perturbed.

3.4. HAADF-STEM and EELS characterization: Chemical composition of individual nanoribbons and nanoparticles located on the surface of the nanoribbons was investigated with STEM-HAADF in combination with EELS (Figures 4 and 5). First EELS mapping for O 1s, Ti 2p and Co 2p excitations (Figure 4B) was performed along an individual nanoribbon (see arrow on the Figure 4A) and a nanoparticle located on the surface of the nanoribbon. Chemical profile recorded along the arrow reveals that the highest concentration of cobalt is in the nanoparticle. The interface between the nanoribbon and the nanoparticle is characterized by a sharp drop in the cobalt content and simultaneously a sharp rise in the titanium content. The cobalt content is gradually reduced, nearly to zero, toward to the central part of the nanoribbon. EELS spectra taken over the areas marked in STEM-HAADF images (Figures 5A and B) are presented in Figures 5C and D. The brighter contrast when compared to the nanoribbon surface indicates the presence of few nanoparticles located on the surface of the nanoribbon (Figure 5A). The brighter contrast indicates higher cobalt content in the nanoparticles than in the nanoribbon. This is supported by the EELS spectra taken in the two marked areas (1 and 2, insets in Figure 5A). From the EELS spectra (Figure 5C) it is evident that the nanoparticle contains cobalt while in the area 2, which is a bit further away from the nanoparticle, no cobalt is detected. On the other hand, in the case of the nanoribbon shown in Figure 5B, cobalt is detected in all three investigated areas (Figure 5D). According to the results shown in Figures 4 and 5 one can conclude that the cobalt distribution in the nanoribbons is very inhomogeneous. Similarly inhomogeneous distribution of Co2+ in anatase TiO2 nanorods prepared from Co2+-doped sodium titanate nanorods was observed by Wang et al [11].


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3.5. Characterization with electron paramagnetic resonance: EPR spectroscopy proved to be a very effective probe to investigate the transition metal ions coordination geometry and their valence states in the bulk TiO2 or titanate nanostructures [15, 21, 31, 32]. For a high-spin Co2+(3d7) S=3/2 state in distorted octahedral coordination g-factor tensor becomes very anisotropic as for instance observed for Co2+ doped rutile TiO2 [31]. EPR spectrum of Co@NaTiNRs (Figure 6) is reasonably simulated with anisotropic g-factor tensor giving us g-factor principal values gxx=4.895, gyy=2.794 and gzz=2.025 and anisotropic widths of ∆Hx=378 G, ∆Hy=1427 G, and ∆Hz=673 G. Hyperfine coupling to the nuclear cobalt spin (I=7/2) could not be determined because of the large line widths. Since these g-factor values are comparable to those of Co2+ doped TiO2 our result directly proves incorporation of cobalt species into the nanoribbon matrix as Co2+ ions. We stress that these g-factor values are perfectly consistent with the high-spin S=3/2 state of Co2+ ions in the octahedral environment with rhombic distortion. Since =3.238 is close to 10/3, the covalence reduction may be important in our case [32]. Large linewidths reflect a high degree of site disorder being in agreement with the inhomogeneous Co2+ distribution observed in microscopy data. Our results are qualitatively different from those of Huang et al. [21] where the resonance at g=2.19 has been attributed to Co2+ in the octahedral environment. A closer inspection of the spectrum reveals the existence of another weak resonance at g ≈ 2.003 (inset to Figure 6), which could be attributed to a single electron trapped at the oxygen vacancy. It is believed that oxygen vacancies sitting close to Co2+ sites are essential for the ferromagnetic exchange in cobalt-doped titanate structures. When electron is trapped in such a site, it forms so-called F+-center a key step for the ferromagnetic coupling between Co2+ sites via a donor impurity band exchange model [24]. The relative weakness of g ≈ 2.003 signal suggests a very low concentration of oxygen vacancies in the investigated sample explaining why we could not find any evidences for ferromagnetic response as it will be explained later. There is also no clear signature in the EPR spectra for the reduction of Ti4+ to Ti3+ and simultaneous oxidation of Co2+ to Co3+ (extra small peaks around 4000 Gauss are from the dielectric resonator). Finally, we remark that the absence of β-Co(OH)2 signal in our EPR spectra is


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probably due to its extreme linewidth and its low intensity. In addition, we note that low-temperature EPR spectra measured down to 4 K do not indicate any magnetic ordering of measured Co2+ moments.

3.6. Magnetic properties: Undoped sample of sodium titanate nanoribbons (NaTiNRs) is diamagnetic over entire temperature range, while magnetic susceptibility of Co@NaTiNRs shows predominantly paramagnetic character (Figure 7A). Magnetic susceptibility in the paramagnetic phase (we used data for T > 100 K) can be reasonably simulated with a Curie-Weiss law. The extracted Curie constant C = 6.4·10-4 emu K/g was compared to the theoretical value for Co2+ ions to obtain a mass ratio 1.3 wt % of Co2+ ions in Co@NaTiNRs. We note that this value compares well with those from XPS and ICP-MS experiments. The magnetic response is still largely paramagnetic down to 2 K, which is additionally supported by the isothermal magnetization measurements between -50 kOe and 50 kOe at 5 K and 300 K (Figure 7B). At room temperature the M(H) dependence is linear, while at 5 K is “S”-shaped without reaching the full saturation in the maximal field of 50 kOe. The measured magnetization at 5 K mainly follows the Brillouin function for spin S = 3/2 as expected for the high spin Co2+ states in the octahedral environment and detected in the EPR experiments (Figure 6). No ferromagnetic contribution was detected in isothermal magnetization measurements at 300 K and 5 K. However, microscopy and XRD investigations revealed the presence of hexagonal β-Co(OH)2 nanoflakes. Related to this we note, that bulk β-Co(OH)2 shows an antiferromagnetic transition at 12.6 K [33]. Closer inspection of the magnetic susceptibility data indeed shows a weak anomaly at TN = 12 K due to the antiferromagnetic transition (inset to Figure 7A). Based on the weakness of antiferromagnetic transition and the similarity of TN’s we associate it with the observed β-Co(OH)2 nanoflakes located on the surface of nanoribbons.

Conclusions


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In summary, Co2+-doped sodium titanate nanoribbons were synthesized under hydrothermal conditions from Co2+-doped TiO2 and NaOH(aq), and were investigated with electron microscopy, XRD, XPS, EPR and magnetic susceptibility measurements. From the microscopy and EPR data we conclude that (i) cobalt ions are very inhomogenously distributed in the nanoribbons, and (ii) the cobalt content in the small nanoparticles found on the surface of the nanoribbons is higher than in the nanoribbons itself. EPR and magnetic measurements are consistent with the majority of cobalt ions in the paramagnetic high-spin S=3/2 Co2+ state and are sitting in the distorted octahedral environment. No room temperature ferromagnetism was observed. Furthermore, it was shown that applied reaction conditions enable formation of β-Co(OH)2 on the surface of the titanate nanoribbons and that a minute part of Co2+ oxidizes to Co3+. The presence of β-Co(OH)2 nanoflakes is seen also in magnetization measurements as a weak antiferromagnetic transition at TN = 12 K.

ACKNOWLEDGMENT: The financial support from the Slovenian Research Agency for project J29217 and J2-4034 is gratefully acknowledged. The authors also acknowledge financial support from the European Union under Framework 6 program under a contract for an Integrated Infrastructure Initiative (reference 026019 ESTEEM) and the COST action MP0901 “NanoTP”.


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FIGURE CAPTIONS

Figure 1. A) SEM and B) TEM image of Co2+-doped sodium titanate nanoribbons (Co@NaTiNRs) synthesized at 175 °C. Arrows in the upper inset of the image B are pointing to the nanoparticles located on the surface of the nanoribbon. The average interlayer distance in the lower inset of the image B is 0.95 nm. C) HAADF-STEM image of Co@NaTiNRs taken at larger area. Arrows are pointing to the nanoparticles found at the surface of the nanoribbons.

Figure 2. XRD diffractogram of cobalt doped anatase TiO2 (Co@TiO2) starting material (bottom) and Co2+-doped sodium titanate nanoribbons (Co@NaTiNRs, top). Anatase peaks are marked with A, asterix (∗) other titanate phase with the formula A2TinO2n-1 (n ≥ 5) (A stands for Na+, Co2+ or H3O+), while full circle (•) denote β-Co(OH)2.

Figure 3. A) Co 2p2/3 core level XPS spectrum recorded on Co@NaTiNRs. The inset shows the spectrum recorded on the Co 2p region. The red line is given as guide to the eye. B) Ti 2p core level XPS spectrum recorded on Co@NaTiNRs.

Figure 4. A) HAADF-STEM image of the individual nanoribbon and B) relative chemical composition profile obtained from the EELS analysis of the Ti L, O K and Co L edges along the line shown in panel A.

Figure 5. HAADF-STEM images (A and B) with the corresponding EELS spectra (C and D).

Figure 6. Experimental X-band EPR spectrum measured at 20 K (circles). A solid red line is a spectrum simulation (see text for the details). Insert: expanded region g = 2.000 showing weak resonance


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attributed to oxygen vacancies.

Figure 7. A) Magnetic susceptibility χ and product χT (inset) of in situ Co2+-doped (Co@NaTiNRs) and of undoped sodium titanate nanoribbons (NaTiNRs) measured between 2 K and 300 K in magnetic field of 1000 Oe. B) Isothermal magnetization measurements at 5 K (circles) and 300 K (squares) of in situ Co2+-doped sodium titanate nanoribbons (Co@NaTiNRs). Solid red line is a fit with a Brillouin function for S = 3/2. Inset: 300 K measurement shown in expanded magnetization scale.


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Figure 1

,


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Figure 7


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TABLE OF CONTENTS

Isothermal magnetization measurements at 5 K of in situ Co2+-doped sodium titanate nanoribbons (Co@NaTiNRs). Solid red line is a fit with a Brillouin function for S = 3/2.


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XRD diffractogram of cobalt doped anatase TiO2 (Co@TiO2) starting material (bottom) and Co2+-doped sodium titanate nanoribbons (Co@NaTiNRs, top). Anatase peaks are marked with A, asterix (*) other titanate phase with the formula A2TinO2n-1 (n ≥5) (A stands for Na+, Co2+ or H3O+), while full circle (•) denote β-Co(OH)2. 84x64mm (300 x 300 DPI)



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A) HAADF-STEM image of the individual nanoribbon and B) relative chemical composition profile obtained from the EELS analysis of the Ti L, O K and Co L edges along the line shown in panel A.



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HAADF-STEM images (A and B) with the corresponding EELS spectra (C and D).


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Experimental X-band EPR spectrum measured at 20 K (circles). A solid red line is a spectrum simulation (see text for the details). Insert: expanded region g = 2.000 showing weak resonance attributed to oxygen vacancies. 84x61mm (300 x 300 DPI)



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A) Magnetic susceptibility χ and product χT (inset) of in situ Co2+-doped (Co@NaTiNRs) and of undoped sodium titanate nanoribbons (NaTiNRs) measured between 2 K and 300 K in magnetic field of 1000 Oe. B) Isothermal magnetization measurements at 5 K (circles) and 300 K (squares) of in situ Co2+-doped sodium titanate nanoribbons (Co@NaTiNRs). Solid red line is a fit with a Brillouin function for S = 3/2. Inset: 300 K measurement shown in expanded magnetization scale.


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Local Coordination and Valence States of Cobalt in Sodium Titanate

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