Alberto Azor

Maria Luisa Ruiz-Gonzalez

Francisco Gonell

Christel Laberty- Robert

Marina Parras

Clément Sanchez

David Portehault

José M González- Calbet

Nickel-Doped Sodium Cobaltite 2D Nanomaterials: Synthesis and Electrocatalytic Properties

In this work we report a synthetic pathway to two-dimensional nanostructures of high oxidation state lamellar cobalt oxides with thicknesses of only few atom layers, through the combined use of precipitation in basic water at room temperature and gentle solid state topotactic transformation at 120 °C. The 2D nanomaterials are characterized by X-ray diffraction, nitrogen porosimetry, scanning electron microscopy, transmission electron microscopy and especially scanning transmission electron microscopy coupled to energy dispersive X-ray analysis and electron energy loss spectroscopy to assess the composition of the nanosheets and oxidation state of the transition metal species. We show that the nanosheets preserve high oxidation states Co 3+ and Co 4+ of high interest for electrocatalysis of the Oxygen Evolution Reaction (OER). By combining high Co oxidation state, surface-to-volume ratio and optimized nickel substitution, the 2D nanomaterials produced in a simple way exhibit high OER electrocatalytic activity and stability in alkaline aqueous electrolyte comparable to standard materials obtained in harsh thermal conditions.

INTRODUCTION

Two-dimensional (2D) transition metal oxides exhibit a realm of properties different from their bulk counter parts and are of interest for electronics, catalysis, information storage and energy harnessing. [START_REF] Butler | Progress, Challenges, and Opportunities in Two-Dimensional Materials beyond Graphene[END_REF][START_REF] Kalantar-Zadeh | Two Dimensional and Layered Transition Metal Oxides[END_REF] Many of these 2D nanostructures arise from oxides exhibiting a layered structure that can be scaled down to only few atomic layers, by chemical exfoliation or by surface deposition. [START_REF] Kalantar-Zadeh | Two Dimensional and Layered Transition Metal Oxides[END_REF] Among this latter family of solids, lamellar cobalt oxides A x CoO 2 (with A an alkali ion) possess a robust 2D structure derived from the Delafossite structural type (Figure 1), which can be described as sheets of edge-sharing CoO 6 octahedra interleaved by alkali ions. [START_REF] Viciu | Crystal Structure and Elementary Properties of Na x CoO 2 (x = 0.32, 0.51, 0.6, 0.75, and 0.92) in the Three-Layer NaCoO 2 Family[END_REF] Figure 1. Crystal structures of cobalt oxyhydroxide CoOOH and three-layer Na x CoO 2 cobaltite (P3 structure). Sodium: yellow, cobalt: blue, oxygen: red, hydrogen: white. The coordination of alkali ions in the P3 Na x CoO 2 structure is trigonal prismatic. [START_REF] Viciu | Crystal Structure and Elementary Properties of Na x CoO 2 (x = 0.32, 0.51, 0.6, 0.75, and 0.92) in the Three-Layer NaCoO 2 Family[END_REF] Schematics of the soft chemistry pathway employed in the present work toward nanoplatelets and nanoflakes of cobaltite and Ni-substituted cobaltite, respectively. This structural arrangement allows a high ion mobility and good stability upon ion insertion and deinsertion. These features have boosted the lithiated phases as the most common cathode material in industrially developed lithium-ion batteries, [START_REF] Whittingham | Lithium Batteries and Cathode Materials[END_REF][START_REF] Wang | Characterization of Thin-Film Rechargeable Lithium Batteries with Lithium Cobalt Oxide Cathodes[END_REF][6] which in turn have strongly motivated studies of sodium-intercalated Na x CoO 2 as cathode for Na-ion batteries. [START_REF] Ding | Electrochemical Properties of P2-Phase Na 0.74 CoO 2 Compounds as Cathode Material for Rechargeable Sodium-Ion Batteries[END_REF][START_REF] Baster | Sodium Intercalation in Na x CoO 2-y -Correlation between Crystal Structure, Oxygen Nonstoichiometry and Electrochemical Properties[END_REF] Besides electrochemical storage performances, the degree of occupation of the Na layer controls the charge in the CoO 2 planes, giving rise to different properties as a function of x. Especially, the two-layer form of Na x CoO 2 has been of significant recent interest, as it displays a variety of functional properties, including superconductivity (T C = 4.5 K) in the hydrated phase (x = 0.3), [START_REF] Takada | Superconductivity in Two-Dimensional CoO 2 Layers[END_REF] as well as thermoelectric energy conversion with a large thermopower (100 µV/K at 300 K for γ-Na 0.5 CoO 2 ). [START_REF] Terasaki | Large Thermoelectric Power in NaCo 2 O 4 Single Crystals[END_REF] The three-layer β-Na 0.6 CoO 2 phase has also been scrutinized, especially as conductive additive for nickel-metal hydride batteries [START_REF] Douin | Evolution Mechanism of the Na 0.6 CoO 2 Conductive Additive during Cycling in the Alkaline Electrolyte of Ni-MH Batteries[END_REF] with improved cycling stability.

2D nanostructures of Na x CoO 2 should exhibit enhanced properties related to the increase in the surface-to-volume ratio. For instance, nanostructuration should improve thermoelectric energy conversion by reducing thermal conductivity through phonon scattering at the grain-boundaries while maintaining a high electron mobility and large thermopower. [START_REF] Snyder | Complex Thermoelectric Materials[END_REF][START_REF] Portehault | Facile General Route toward Tunable Magnéli Nanostructures and Their Use as Thermoelectric Metal Oxide / Carbon Nanocomposites[END_REF] Catalytic properties should as well be enhanced. This is especially true for electrocatalysis of the Oxygen Evolution Reaction (OER) occurring at the anode of (phot)electrochemical water splitting devices and metal-air batteries in charge operation. Cobalt-based materials are well-known OER electrocatalysts in alkaline media and large efforts are devoted since few years to understand and improve their activity and stability. [START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF][START_REF] Grimaud | Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution[END_REF][START_REF] Bergmann | Reversible Amorphization and the Catalytically Active State of Crystalline Co 3 O 4 during Oxygen Evolution[END_REF][START_REF] Abidat | Three Dimensionally Ordered Mesoporous Hydroxylated Ni x Co 3-x O 4 Spinels for the Oxygen Evolution Reaction: On the Hydroxyl-Induced Surface Restructuring Effect[END_REF][START_REF] Suen | Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives[END_REF][START_REF] Chen | Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy[END_REF][START_REF] Chen | Activity of Pure and Transition Metal-Modified CoOOH for the Oxygen Evolution Reaction in an Alkaline Medium[END_REF] Besides perovskite [START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF][START_REF] Grimaud | Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution[END_REF] -and spinel [START_REF] Bergmann | Reversible Amorphization and the Catalytically Active State of Crystalline Co 3 O 4 during Oxygen Evolution[END_REF][START_REF] Abidat | Three Dimensionally Ordered Mesoporous Hydroxylated Ni x Co 3-x O 4 Spinels for the Oxygen Evolution Reaction: On the Hydroxyl-Induced Surface Restructuring Effect[END_REF] -derived cobaltites, lamellar cobalt(III) oxyhydroxide CoOOH has been intensively studied. [START_REF] Suen | Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives[END_REF][START_REF] Chen | Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy[END_REF][START_REF] Chen | Activity of Pure and Transition Metal-Modified CoOOH for the Oxygen Evolution Reaction in an Alkaline Medium[END_REF] In most cases, Co 4+ sites are formed during the electrochemical conditions of OER and act as active electrocatalytic species. [START_REF] Chen | Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy[END_REF] Hence, incorporating Co 4+ species in a controlled manner at the synthesis stage would be a step forward in tuning the electrocatalytic properties. The Na x CoO 2 family offers a nice opportunity to reach this level of control due to its ability to accommodate cation substitution, for instance Ni, Mn or Cr. [START_REF] Levi | Lattice Strains in the Layered Mn, Ni and Co Oxides as Cathode Materials in Li and Na Batteries[END_REF][START_REF] Doubaji | Synthesis and Characterization of a New Layered Cathode Material for Sodium Ion Batteries[END_REF] Especially, nickel incorporation can improve the OER electrocatalytic activity of CoOOH due to enhanced adsorption of intermediate species in the OER process. [START_REF] Chen | Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy[END_REF] These considerations have motivated our search for a reliable synthetic pathway towards transition metal-substituted sodium-cobaltites 2D nanostructures, more specifically nanosheets with a limited number of CoO 2 layers.

The stacking of CoO 2 layers in sodium cobaltites is driven by electrostatic interactions between the negative sheets and the alkali Na + cations intercalated.

Therefore, as other similar cases like lepidocrocite-type layered titanates, [START_REF] Besselink | Exfoliation and Restacking of Lepidocrocite-Type Layered Titanates Studied by Small-Angle X-Ray Scattering[END_REF] protonated Ruddlesden-Popper phases [START_REF] Schaak | Prying Apart Ruddlesden-Popper Phases: Exfoliation into Sheets and Nanotubes for Assembly of Perovskite Thin Films[END_REF] or layered manganites, [START_REF] Omomo | Redoxable Nanosheet Crystallites of MnO 2 Derived via Delamination of a Layered Manganese Oxide[END_REF] exfoliation may be possible.

Mechanical and soft chemistry delamination methods applied to weakly Van der Waals bounded two-dimensional compounds [START_REF] Manna | Toward Understanding the Efficient Exfoliation of Layered Materials by Water-Assisted Cosolvent Liquid-Phase Exfoliation[END_REF] are hardly translatable to electrostatically stabilized phases due to stronger cohesion between the layers. The usual delamination strategy for such intercalated compounds is based on consecutive intercalations with progressively larger species in order to decrease electrostatic interactions, until the structure is split apart forming a colloidal suspension. The procedure is the only one reported so far to exfoliate sodium cobaltites [START_REF] Masuda | Exfoliation of Layers in Na x CoO 2[END_REF] produced as bulk solids, [START_REF] Mikami | Crystal Growth of Thermoelectric Material Na x CoO 2-δ by a Flux Method[END_REF][START_REF] Miclau | Low-Temperature Hydrothermal Synthesis of the Three-Layered Sodium Cobaltite P3-Na x CoO 2 (x ∼ 0.60)[END_REF][START_REF] Pétrissans | Solution Synthesis of Nanometric Layered Cobalt Oxides for Electrochemical Applications[END_REF] and very recently manganese-substituted sodium cobaltites, [START_REF] Sakai | Synthesis and Substitution Chemistry of Redox-Active Manganese/Cobalt Oxide Nanosheets[END_REF] but it resulted in a strong reduction of Co species and the formation of CoO nanosheets, thus leading to a loss of the electrocatalytically relevant Co 4+ /Co 3+ couple. A direct synthesis where the individual particles obtained would be as thin as possible, close to single layers, would overcome time-consuming iterative method and, more important, maintain the high mean oxidation state of cobalt.

In this paper, we develop a soft chemistry pathway using water as solvent and solid state topotactic phase transformation, to obtain the thinnest possible layered sodium cobaltite particles. We demonstrate that this simple method allows cobalt substitution by nickel in order to further tune the electrochemical properties. The simple synthesis strategy yields nanoflakes of doped sodium nano-cobaltites as thin as 1.5-3.0 nm (2 to 5 CoO 2 layers), the thinnest reported up to now, as a result from both low temperature synthesis and structural destabilization by the dopant. These nanomaterials exhibit electrocatalytic activity comparable to standard materials, while they are obtained in simpler and milder conditions.

EXPERIMENTAL PROCEDURES

The synthesis strategy is based on a modified procedure from the previous work of Pétrissans et al. [START_REF] Pétrissans | Solution Synthesis of Nanometric Layered Cobalt Oxides for Electrochemical Applications[END_REF] The general approach consists in two steps. First, CoOOH particles are produced in a highly concentrated NaOH aqueous solution. Second, the oxyhydroxide particles surrounded by Na + ions are gently heat-treated in order to complete exchange of protons by sodium cations ions. Based on the Pourbaix diagram of Co, [START_REF] Chivot | New Insight in the Behaviour of Co-H2O System at 25-150 °C, Based on Revised Pourbaix Diagrams[END_REF] Br 2 (E 0 = 1.07 V/ESH), a relatively strong oxidizing agent was chosen.

All reagents were 99.9% grade, commercially available at Aldrich chemicals and used as received. In a typical synthesis, 12 g of sodium hydroxide are dissolved in 50 mL of mili-Q water in a 250 mL three-neck flask under O 2 atmosphere. A 6 M NaOH solution is obtained. The flask temperature is kept at room temperature, and 0.7 mL of Br 2 (14 mmol) is added to the solution, which turns yellow. Immediately afterwards, 5

mL of a (1-x) M Co(NO 3 ) 2 •6H 2 O and x M Ni(NO 3 ) 2 •3H 2 O (x
depending on the desired composition) solution is added dropwise over 10 min directly to the vortex created by vigorous stirring. A black precipitate is formed immediately, which is recovered after centrifugation using a Sigma 30KS centrifuge at 21000 rpm. The precipitate is treated at 120 °C for 24 h under air, producing a black hygroscopic powder. This treatment promotes sodium migration to the interlayer region and subsequent exchange of H + by Na + . Afterwards, a washing step is performed with pure methanol to remove the remaining NaOH (solubility s = 30.8 wt. % at 25 °C33 ), NaBr (s = 15.0 wt. % at 25 °C34 ) and Na 2 CO 3 (s = 0.3 wt. % at 22 °C35 ) side products of the oxidation process. Water is avoided as washing solvent because it would affect the nature of the resulting phase, leading to re-hydrated phases and decreasing the Na content in the final material.

Finally, a selective sedimentation process was carried out on some Nisubstituted samples. In a typical assay, 0.5 g of as prepared sample is dispersed in 30 mL of ethanol (99.99%) and sonicated with a high intensity ultrasonic processor Branson Sonifier Sound Enclosure (time = 30 min, pulse = 0.5 s/0.4 s (on/off), temp. = 60 ºC, ampl. = 40%). Once the sonication is finished, the suspension is left for 30 min.

The supernatant containing the smallest particles is recovered, centrifuged at 21000 rpm and dried at 45 ºC under air overnight. Bulk Na 0.6 CoO 2 was prepared by reacting Na 2 CO 3 and Co 3 O 4 in O 2 at 850 °C for 20 h. Powder X-ray diffraction was carried out on the various synthesized polycrystalline samples using a PANalytical X´PERT diffractometer (Cu λ(K α ) = 1.5405 Å) equipped with a sealed Xenon detector and Bragg-Brentano geometry. The reference patterns were taken from the ICSD file 22285 and reference 3 for CoOOH and Na 0.6 CoO 2 , respectively. The average cation composition was determined by ICP-OES (Perkin Elmer Optima 3300DV). Particle size and morphology were determined on a JEOL JSM 7600F scanning electron microscope, using a graphite holder and "gentlebeam" mode. Transmission electron microscopy (TEM) has been performed on a JEOL JEM 3000F electron microscope operating at 300 kV fitted with and Oxford Inca EDS spectrometer used for compositional analysis. Scanning TEM (STEM) has been performed on a probe-corrected aberration microscope JEOL JEM ARM200cF. The cobalt oxidation state was estimated by energy electron loss spectroscopy (EELS) in a QuantumGIF spectrometer attached to the ARM 200cF microscope.

For electrocatalysis assessment, a rotating disk electrode (RDE) was prepared by coating a glassy carbon (GC) disk electrode (0.07 cm², Radiometer Analytical) previously polished with diamond paste (1 µm, BAS inc.) followed by alumina paste (0.05 µm, BAS inc.) to reach mirror grade. A conductive ink containing the catalyst was then coated over the substrate. This ink was prepared from the catalyst powder, Acetylene Black (AB, Alfa Aesar) (99.9+ %, 75 m²•g -1 ) and Nafion-117 solution (5% in aliphatic alcohol, Sigma-Aldrich). The acetylene black powder was previously treated in nitric acid, by dispersing 500 mg of AB in a 50 mL of HNO 3 20 % at 80 °C overnight under stirring, followed by centrifugation and washing with water and vacuum drying.

The ink was prepared by dispersing 10 mg of catalyst powder in 10 mL of ethanol (99%), along with 10 mg of treated AB. The mixture was sonicated in a water bath for 2 hours. Then, the dispersion was cooled to room temperature and dispersed with a sonic horn for 5 minutes. 435 µL of ion-exchanged Nafion solution (5wt. % in aliphatic alcohols, Aldrich) was then added immediately. The ink was sonicated for another 5 min and remained stable and aggregate free for at least 1 week. After few minutes of horn sonicating in cool conditions, 2 µL of the ink was deposited on the GC substrate.

The film was dried in air for 1 h. The prepared electrode consisted in a black film coated on the GC RDE with an oxide content of 27 µg•cm -² disk .

Electrochemical characterization. The freshly prepared electrode was characterized with a three-electrode setup, with the RDE as the working electrode, a Pt wire as counter electrode and a Ag/AgCl electrode as reference. The setup was connected to a VPS Biologic potentiostat. Prior to electrochemical experiments, the working electrode was hydrated with the 0.1 M KOH electrolyte for 30 min. All experiments were performed in O 2 saturated electrolyte at scan rates of 20 and 100 mV•s -1 with five cycles for linear sweep voltammetry. Each experiment was repeated 3 to 5 times to ensure reproducibility.

RESULTS AND DISCUSSIONS

Cobalt oxyhydroxide (CoOOH) and alkali cobaltites are isostructural. The interlayer gap of the former is occupied by H + ions while the CoO 2 layers are interleaved by Na + in the cobaltites. Hence, our protocol proceeds by the formation of CoOOH nanosheets followed by topotactic transformation through proton exchange with sodium ions.

Briefly, the synthesis method is a soft chemistry procedure relying on an aqueous reaction medium under controlled atmosphere. Gentle thermal treatment (120 °C) in the solid state is performed afterwards to enable structural conversion.

The intermediate products at the different stages of the synthesis have been analysed by powder XRD. The results for a dopant-free assay using Br 2 as oxidant are shown in Figure 2. The raw material, after thermal treatment at 120 °C, shows three phases: the main phase that can be identified as Na 0.6 CoO 2 , and NaBr and Na 2 CO 3 obtained as impurities (Figure 2a). All diffraction peaks of the main phase can be attributed to rhombohedral Na 0.6 CoO 2 where sodium ions are in a trigonal prismatic environment in the interlayer space. After washing with methanol, every NaBr and Na 2 CO 3 diffraction maxima disappeared (Figure 2b). The composition is confirmed by the Na/Co ratio evaluated by ICP-OES, leading to Na (0.6±0.02) CoO 2 . It is worth mentioning that a slight shift of the (003) peak to higher angles can be observed after washing. This effect is associated with possible leaching of Na + ions from the interlayer space. If the washing step is performed before the thermal treatment, CoOOH is obtained (Figure 2c). The width of the (003) reflexion hints at structural disorder and most probably partially substitution of protons by sodium ions. The thermal treatment in the presence of sodium ions is of prime importance to fully exchange protons and enable crystallization of the cobaltite. CoOOH is also made of sheets of edge-sharing For Ni contents higher than 50 at. % phase segregation appears. The average Na content, evaluated by ICP-OES and EDS, is similar to that of the undoped sample. The lamellar morphology of Na 0.6 CoO 2 and Ni-doped samples has been evidenced by SEM and TEM. SEM (Figure 3a) and TEM (Figure 3b no contrast differences can be observed. In order to get information about oxygen and sodium distributions, the ABF image was simultaneously acquired. ABF imaging in STEM has been established as a robust method for observing light atoms like oxygen, lithium or hydrogen. [START_REF] Hojo | Atomic Structure of a CeO 2 Grain Boundary: The Role of Oxygen Vacancies[END_REF][START_REF] Lee | Counting Lithium Ions in the Diffusion Channel of an LiV 2 O 4 Crystal[END_REF] The corresponding ABF image (Figure 5b Atomically resolved EELS chemical maps (see SI.A) were also obtained (Figure 6). Figure 6a shows the HAADF image and the area selected for the EELS study (highlighted in green). The image simultaneously acquired with the spectra and the corresponding sum spectrum are displayed in Figure 6b and 6c, respectively. This [START_REF] Pearson | White Lines and d -Electron Occupancies for the 3 d and 4 d Transition Metals[END_REF] In order to minimize the error due to the spectra background signal, I(L 3 /L 2 ) values were calculated using the amplitude value obtained from the fitting of the second derivative of the M-L 2,3 white lines to a Gaussian curve.

The 1 and2). In addition, the energy of the Co-L 2 , 3 edges for the undoped and Ni-doped samples is located between those of Co(IV) (Ba 2 CoO showing the delafossite structural type have been synthesized. What remains unexplained is why the particle size is so drastically reduced after Ni inclusion in the structure. The connexion between the Ni incorporation and particle size reduction is clear since the compositional analysis shows always Ni content differences in large and small particles. A possible explanation could be the incorporation of structural distortions by the presence of a foreign metal. Assuming high spin configuration for both transition metals, the corresponding ionic radius are 0.53 Å for Co(IV), 0.61 Å for Co(III) and 0.60 Å for Ni(III); all of them in octahedral coordination. [START_REF] Shannon | Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides[END_REF] Incorporation of Ni(III) in the octahedral layers results in larger mismatch between the cations and higher strains that might be exacerbated due to the Jahn Teller effect for Ni(III) in octahedral environment. Because of their high surface-to-volume ratio, especially at the edges, small nanoflakes are expected to stand higher structural strains than bigger particles, and then to accommodate larger amounts of nickel. Such a size decrease upon addition of foreign ions has also been observed in other systems, such as fluorides. [START_REF] Wang | Simultaneous Phase and Size Control of Upconversion Nanocrystals through Lanthanide Doping[END_REF] Our synthetic approach enables incorporating Co 4+ and Ni 3+ species in octahedral environment into layered cobalt oxide materials before using the material for electrocatalysis. Such species are well-known to enhance electrocatalytic activity in the Oxygen Evolution Reaction (OER), especially Co 4+ that are active sites for OER. [START_REF] Bergmann | Reversible Amorphization and the Catalytically Active State of Crystalline Co 3 O 4 during Oxygen Evolution[END_REF][START_REF] Abidat | Three Dimensionally Ordered Mesoporous Hydroxylated Ni x Co 3-x O 4 Spinels for the Oxygen Evolution Reaction: On the Hydroxyl-Induced Surface Restructuring Effect[END_REF][START_REF] Suen | Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives[END_REF][START_REF] Chen | Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy[END_REF][START_REF] Trotochaud | Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution[END_REF] Hence, the layered cobaltite nanostructures appear as designed materials prior to their implementation as OER electrodes in (photo)electrochemical water splitting devices or metal-air batteries, for instance. We have assessed the OER electrocatalytic properties of the as-synthesized nanostructured layered cobaltites with a three-electrode setup in an aqueous 0. (BSCF). The latter compound is known to be highly electrocatalytically active for the OER. [START_REF] Suntivich | A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles[END_REF] First, the material properties were assessed by linear sweep voltammetry (LSV).

After 5 cycles (Figure 8) in the potential range 1.15-1.85 V/RHE at 20 mV s -1 , the i-E curves of the three nanostructured samples reach higher current than bulk Na 0.6 CoO 2 , thus suggesting enhanced activity of the nanoscaled materials compared to the bulk. EELS in the dry samples, it may appear when the materials are put in contact with the aqueous electrolyte, prior to voltammetric measurements. [START_REF] Chen | Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy[END_REF] According to the recorded overpotentials (Table 3), the pure Na 0.6 Ni 0.5 Co 0.5 O 2 nanoflakes are more active than the Na 0.6 Ni 0.5 Co 0.5 O 2 nanoplatelets/nanoflakes mixture, so that activity is increased for larger surface-to-volume ratio and increased Ni content (Table 1). This behaviour is related to previous observations of similar enhancement of the OER activity in Co III oxyhydroxide by doping with Ni, due to the stabilisation of surface hydroxyl groups as intermediates in the OER process. [START_REF] Chen | Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy[END_REF][START_REF] Chen | Activity of Pure and Transition Metal-Modified CoOOH for the Oxygen Evolution Reaction in an Alkaline Medium[END_REF] 3) yield for all the layered cobaltite samples slopes of ca. 30 mV•dec -1 . Such low values are in agreement with previously reported behaviors of Ni-doped cobalt oxides [START_REF] Abidat | Three Dimensionally Ordered Mesoporous Hydroxylated Ni x Co 3-x O 4 Spinels for the Oxygen Evolution Reaction: On the Hydroxyl-Induced Surface Restructuring Effect[END_REF][START_REF] Chen | Activity of Pure and Transition Metal-Modified CoOOH for the Oxygen Evolution Reaction in an Alkaline Medium[END_REF] and suggest similar electrocatalytic mechanisms [START_REF] Suen | Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives[END_REF][START_REF] Shinagawa | Insight on Tafel Slopes from a Microkinetic Analysis of Aqueous Electrocatalysis for Energy Conversion[END_REF] with high electrocatalytic activity of the nanoscaled Ni-doped layered cobaltites. In order to assess the stability of the nanomaterials in operating conditions, chronopotentiometry measurements were performed at a current density of 5 mA cm -2 (Figure 9). Every hour, an LSV cycle was recorded. For the three nanostructured materials, the steady state is reached after ~40 min after each LSV recording, with slight increase in the potential after 5 h. The electrochemical behaviour recorded by LSV every hour is however similar upon time, thus suggesting that the apparent loss of activity, is related to the loss of active matter, presumably by detachment of the nanoparticles from the composite. This is confirmed by the similar Tafel slopes before and after chronopotentiometry (Figure SI.8 and Table S1) at ~30 mV•dec -1 that suggest similar electrocatalytic mechanism, hence showing limited evolution of the electrocatalytic species. According to overpotentials (Table 3), the pure

CONCLUSION

To conclude, we have developed a synthesis method relying on soft chemistry in water to yield 2 to 5 cobalt oxide layers-thick 2D nanostructures of sodium cobaltite, which can be substituted at will with nickel. The relevance of the approach resides in the ability to preserve high oxidation states (Co 3+ /Co 4+ species), even in few layers-thick 2D nanomaterials. These new nanomaterials exhibit significantly higher electrocatalytic activity for the oxygen evolution in alkaline aqueous electrolyte than their bulk counterparts and reach activities and performance stability comparable to reference materials that are obtained through much harsher synthetic conditions. From a more fundamental viewpoint, this study also shows how, in complex oxides, nanostructures can be used as an additional lever to tune compositional ranges, as they accommodate wider substitution ranges than would larger objects.

EELS analysis

The convergence and collection semi-angles used were 18 and 20.3 mrad, respectively.

Chemical maps

To enable simultaneous analysis of O-K (532 eV), Co-L 2,3 (779eV), Ni-L 2,3 (8505 eV), and Na-K (1072eV) edges, an spectrum imaging (30 x 19 spectra) was recorded by using a dispersion of 0.

CoO 6

 6 octahedra and differs from the targeted Na 0.6 CoO 2 by the interlayer distance, which is 4.38 Å and 5.49 Å for CoOOH and Na 0.6 CoO 2 , respectively. Hence, a topotactic transformation occurs readily from the oxyhydroxide to the oxide. Some cobaltite XRD peaks are significantly broader than others, especially the (003) reflection around 16° (2). Such a broadening suggests a 2D morphology with (001) CoO 2 sheets as basal planes, as confirmed below by electron microscopy, and/or disorder, including cation deficiency, substitution in the interlayer space and stacking faults.

Figure 2 .

 2 Figure 2. XRD patterns of Na 0.6 CoO 2 samples: (a) as-prepared sample, (b) sample after washing with methanol and (c) sample washed before thermal treatment. In (a) and (b), (hkl) indexes correspond to Na 0.6 CoO 2 . Squares correspond to NaBr and circles correspond to Na 2 CO 3 . The indexes in (c) correspond to CoOOH.

  ) show that the undoped Na 0.6 CoO 2 compound is obtained as hexagonal nanoplatelets of 40-120 nm basal face and 3-40 nm thickness. In contrast, the addition of nickel provokes a dramatic particle size reduction in all directions, in agreement with XRD. Actually, the TEM image (Figure3c) for a sample with nominal composition Na 0.6 Co 0.5 Ni 0.5 O 2 shows agglomerated nanoflakes with basal faces around 5 nm. The thickness and structural features of such nanoflakes have been assessed by further HRTEM studies (Figure4).

Figure 3 .

 3 Figure 3. Electron micrographs of the Na 0.6 CoO 2 sample: (a) SEM and (b) TEM low magnification image evidencing two different orientations of the nano-platelets in the sample: the basal face and the edge. (c) TEM micrograph of a Na 0.6 Co 0.5 Ni 0.5 O 2 sample showing in the inset a high resolution magnification of an individual nanoflake.

Figure 4 .

 4 Figure 4. HRTEM images of a sample Na 0.6 Ni 0.3 Co 0.7 O 2 : small nanoflakes orientated along two directions. (a) The [001] projection (zone axis). Notice that due to insufficient thickness, nanoflakes boundaries cannot be observed. (b) The [100] projection (zone axis) of the lamellar Na x CoO 2 system.

  ) evidences the introduction of Na in the interlayer space between the Co 1-x Ni x O 2 sheets. It can be better observed on the magnified image Figure 5c on which the crystallographic structure is superimposed. The apparent low quality of the HAADF and ABF images is due to the extremely high instability of the sample under the electron beam even at low voltages (80 kV).

Figure 5 .

 5 Figure 5. Characteristic atomically resolved STEM images of a nanoplatelet in a sample Na 0.6 Ni 0.3 Co 0.7 O 2 with (a) HAADF and (b) ABF detections. (c) Magnified STEM-ABF image with a schematic representation of the atom distribution along this [100] projection.

  spectrum confirms the presence of Na, Ni, Co and O. The corresponding chemical maps are depicted in Figure d-g. In spite of the relatively low quality of these maps due to the sample damage, they suggest that Ni is substituting Co cations.

Figure 6 .Figure 7 .

 67 Figure 6. (a) HAADF detail of a Na 0.6 Ni 0.3 Co 0.7 O 2 nanoplatelets. (b) Simultaneously acquired image. (c) Sum spectrum. (d-g) Chemical maps obtained in the green square in (a) from the Co-L 2,3 , Ni-L 2,3 , O-K and Na-K edges, respectively.

4 )

 4 and Co(III) (LaCoO 3 ) references (Figure SI.5), hence supporting the conclusion from the L 3 /L 2 intensity ratios of mixed valence Co(IV)/Co(III) with no Co(II). The comparison with a reference Co(IV)/Co(III) sample La 0.7 Sr 0.3 CoO 3 (Figure SI.5) is in agreement with this statement. Moreover, the corresponding Ni L 3 /L 2 ratio was also evaluated at 3.1  0.1 for the same region. Comparison with the standards for Ni(II) and Ni(III) (see Table2and SI.A) suggests that all nickel in the sample is Ni(III).

Figure 7 .

 7 Figure 7. Representative EELS spectra for Na 0.6 Ni 0.5 Co 0.5 O 2 , showing the O-K, Co-L 3,2 , Ni-L 3,2 and Na-K edges.

  This was already the case at the first LSV cycle (Figure SI.6). A reversible oxidation event occurs for the Ni-containing samples at 1.3 V/RHE, which increases in intensity when the nanoflakes are purified. Hence, this behavior is characteristic of the nanoflakes that are enriched in Ni. According to the literature, 17,43 it is attributed to the couple Ni(III)/Ni(II). The corresponding oxidation wave was also present in the first scan (Figure SI.6) but at higher potential 1.4 V/RHE, suggesting changes in the local environment of Ni during the first scan. Although Ni(II) was not detected by STEM-

1 (Figure 8 ,

 18 At higher sweep rate (100 mV s -1 , Figure SI.7), similar behaviors are observed. Tafel plot analyses at the fifth LSV cycle for 20 mV s -Figure SI.8 and Table

Figure 8 .

 8 Figure 8. 5 th linear sweep voltammetry scans (20 mV•s -1 ) of bulk BSCF (black), bulk Na 0.6 CoO 2 (red), Na 0.6 CoO 2 nanoplatelets (green), Na 0.6 Ni 0.5 Co 0.5 O 2 mix of nanoplatelets and nanoflakes (blue), Na 0.6 Ni 0.5 Co 0.5 O 2 nanoflakes (orange). The inset shows a zoom in of the onset potential area.

Na 0. 6 Figure 8 )

 68 Ni 0.5 Co 0.5 O 2 nanoflakes remain the most active material. Note also that for the Nisubstituted samples, the Ni(II)/Ni(III) wave observed in the first LSV experiments (disappeared after few hours, thus suggesting structural evolution of the materials. To further assess such evolution in the nanoflakes case, the composite electrode was analyzed by EDX after 6 h of chronopotentiometry measurement. The Co/Ni ratio was maintained, but sodium was lost and replaced by potassium from the electrolyte. Whether potassium becomes part of the catalyst structure or is only sequestered by the ion exchange Nafion polymer is an open question that will be assessed in the near future. All in all, nanoscaling layered cobaltites significantly improved their electrocatalytic activity for the OER. Based on the overpotentials, the resulting material reaches electrocatalytic activity comparable to most active materials, like BSCF. The evolution of the chemical composition suggests restructuration of the nanomaterials, but without loss of electrocatalytic activity.

Figure 9 .

 9 Figure 9. (a) Chronopotentiometry curves at current density 5 mA•cm -2 of Na 0.6 CoO 2 medium size, Na 0.6 Ni 0.5 Co 0.5 O 2 mix of nanoplatelets and nanoflakes, and Na 0.6 Ni 0.5 Co 0.5 O 2 nanoflakes. (b) Linear Sweep Voltammetry curves recorded every hour during the chronopotentiometry measurements for Na 0.6 CoO 2 nanoplatelets, Na 0.6 Ni 0.5 Co 0.5 O 2 mix of nanoplatelets and nanoflakes, and Na 0.6 Ni 0.5 Co 0.5 O 2 nanoflakes: 0 (initial, purple), 1 (black), 2 (red), 3 (green), 4 (blue) and 5 (orange) hours.

Figure SI. 1 .

 1 Figure SI.1. Na x Co 0.5 Ni 0.5 O 2 XRD pattern indexed as Na 0.6 CoO 2 (C2/m). Note that the bad signal-to-noise ratio originates from the fluorescence of cobalt and the emission wavelength of the copper anode of the XRD apparatus.

Figure SI. 2 .

 2 Figure SI.2. HRTEM images of a Na 0.6 CoO 2 sample. (a) Nanoplatelets oriented along [001] and corresponding FFT of the highlighted area. (b) Nanoplatelet oriented along [100] and corresponding FFT of the highlighted area.

Figure SI. 3 .

 3 Figure SI.3. HRTEM images of a Na x Ni 0.3 Co 0.7 O 2 sample: small particles orientated along two directions: (a) the [001] projection (zone axis). (b-d) the [100] projection (zone axis) of the lamellar Na x CoO 2 system.

Figure SI. 4 .

 4 Figure SI.4. (a) HAADF image of a group of nanoflakes representative of a sample Na 0.6 Ni 0.5 Co 0.5 O 2 as well as the FFT of the highlighted area and its structural model. (b) HAADF image of an isolated nanoflake oriented along the [122] direction with its FFT and the structural model along that direction.

5 .Figure SI. 6 .

 56 Figure SI.6. First linear sweep voltammetry scans (20 mV•s -1 ) of bulk BSCF (black), bulk Na 0.6 CoO 2 (red), Na 0.6 CoO 2 nanoplatelets (green), NaNi 0.5 Co 0.5 O 2 mix of nanoplatelets and nanoflakes (blue), NaNi 0.5 Co 0.5 O 2 nanoflakes (orange).

2 Figure SI. 7 .

 27 Figure SI.7. 1 st (top) and 5 th (down) linear sweep voltammetry scans (100 mV•s -1 ) of bulk BSCF (black), bulk Na 0.6 CoO 2 (red), Na 0.6 CoO 2 nanoplatelets (green), NaNi 0.5 Co 0.5 O 2 mix of nanoplatets and nanoflakes (blue), NaNi 0.5 Co 0.5 O 2 nanoflakes (orange).

Figure SI. 8 .

 8 Figure SI.8. Tafel plots at the 5 th linear sweep voltammetry scan (top, 20 mV•s -1 ) and after 5h chronopotentiometry (bottom, 5 mA•cm -2 ) of bulk Na 0.6 CoO 2 (red), Na 0.6 CoO 2 nanoplatelets (green), NaNi 0.5 Co 0.5 O 2 mix of nanoplatelets and nanoflakes (blue), NaNi 0.5 Co 0.5 O 2 nanoflakes (orange).

Table 1 .

 1 Morphological characteristics, composition and oxidation states of the transition metal cations of some samples. The nanoflakes are obtained from the nanoplatelets/nanoflakes (number ratio 10/90) sample by selective sedimentation.

	Nominal	Morphology	Dimensions	Measured Ni/Co	Ni -Co
	composition	(TEM)	(nm)	composition (EDX)	oxidation
					states (from
					EELS,
					Table 2)
	Na 0.6 CoO 2 nanoplatelets 40-120*3-40	Na 0.6 CoO 2	/ -III/IV
	Na 0.6 Ni 0.3 Co 0.7 O 2	nanoflakes nanoplatelets	2-10*1.5-3 40-120*3-40	Na 0.6 Ni 0.30±0.05 Co 0.70±0.05 O 2 Na 0.6 Ni 0.12±0.07 Co 0.88±0.07 O 2	III -III/IV
	Na 0.6 Ni 0.5 Co 0.5 O 2	nanoflakes nanoplatelets	2-10*1.5-3 40-120*3-40	Na 0.6 Ni 0.32±0.03 Co 0.68±0.03 O 2 Na 0.6 Ni 0.05±0.03 Co 0.95±0.03 O 2	III -III/IV
	Na 0.6 Ni 0.5 Co 0.5 O 2	nanoflakes	2-10*1.5-3 Na 0.6 Ni 0.50±0.03 Co 0.50±0.03 O 2	III -III/IV

  resulting Co L 3 /L 2 ratio, measured on 15 different regions to verify reproducibility, was 2.6  0.1 for Na 0.6 Co 0.7 Ni 0.3 O 2 and Na 0.6 Co 0.5 Ni 0.5 O 2 samples, 2.5  0.1 for the undoped Na 0.6 CoO 2 sample. These values have been compared to reference L 3 /L 2 intensity ratios measured for Co(III) and Co(II) standards LaCoO 3 and CoO, respectively (seeTable 2, SI.A and Figures SI.5), and reported for the Co(IV) standard CoSi 2 by Wang et al.[START_REF] Wang | Cobalt Valence and Crystal Structure of La 0.5 Sr 0.5 CoO 2.25[END_REF][START_REF] Wang | Valence State Mapping of Cobalt and Manganese Using Near-Edge Fine Structures[END_REF] The values for the undoped and Ni-doped samples are between

those of Co(III) and Co(IV), thus discarding the presence of Co(II) and showing mixed valence Co(IV)/Co(III) (Tables

Table 2 .

 2 EELS intensities I (L 3 /L 2 ) ratio and corresponding oxidation states for Co, Ni references, and for the as-synthesized Na 0.6 Ni 0.5 Co 0.5 O 2 sample. * value taken from references[START_REF] Wang | Cobalt Valence and Crystal Structure of La 0.5 Sr 0.5 CoO 2.25[END_REF][START_REF] Wang | Valence State Mapping of Cobalt and Manganese Using Near-Edge Fine Structures[END_REF] .

	I (L 3 /L 2 )		Co	Ni
	Co	Ni	oxidation state	oxidation state

  CoO 2 and bulk Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ

	Na 0.6
	1 mol L -1 KOH solution and regular carbon black/Nafion/cobaltite particles
	composite electrodes deposited on glassy carbon as working electrode. Three
	nanostructured samples were analysed (Table 1): first, undoped 40-120 nm-wide and 3-
	40 nm-thick Na 0.6 CoO 2 nanoplatelets; second, the Na 0.6 Ni 0.5 Co 0.5 O 2 sample made of
	10% of the nanoplatelets of the same dimensions and 90% of 2-10 nm-wide, 3-5 CoO 2
	layers-thick nanoflakes; third, these nanoflakes purified by sedimentation. Two
	additional reference samples were produced by solid state chemistry and assessed: bulk

Table 3 .

 3 Overpotentials at current density 5 mA•cm -2 disk and Tafel analysis parameters retrieved from plots Figure SI.8.

	Sample

ACKNOWLEDGEMENT

This work was supported by the Spanish Ministry of Innovation, Science and Technology and Spanish Ministry of Economy and Competitiveness through Research Projects CSD2009-00013, TSI-020100-2011-280, MAT2014-52405-C02-01, and MAT2017-82252-R. We thank the National Center of Electron Microscopy (UCM) for facilities. This work was also partly funded by the CNRS, Sorbonne Université, CNRS, and region Ile de France through Domaine d'Intérêt Majeur C'Nano-DIM Nano-K with the COSMOS project. SUPPORTING INFORMATION. XRD and TEM analyses of a Na 0.6 CoO 2 sample, TEM and STEM-HAADF images of Ni-doped layered cobaltites, details of STEM-HAADF, ABF and EELS characterizations, LSV cycles at different scan rates, Tafel plots.

Supporting Information

Nickel-Doped Sodium Cobaltite 2D Nanomaterials:

Synthesis and Electrocatalytic Properties Alberto Azor 1 , Maria Luisa Ruiz-Gonzalez 1 , Francisco Gonell


Francisco Gonell

Naoual Alem

Peter Dunne

Guillaume Crochet

Patricia Beaunier

Christophe Méthivier

David Montero

Christel Laberty-Robert

Bernard Doudin

David Portehault

oxide nanocrystals and their magnetic properties

Keywords: nanocrystals, manganite, perovskites, molten salts, magnetism

Versatile molten salt synthesis of manganite perovskite

Introduction

If halide perovskites experienced a burst of interest in the last years, [START_REF] Xia | [END_REF][2][3] long-known perovskite oxides are highly regarded materials as they provide a versatile platform to design ionocovalent materials for various properties and applications, from piezoelectricity to catalysis. [4][5][6] Especially, manganite perovskites AMnO3 (A = alkaline earth metals, rare earth metals) are relevant catalysts for the conversion of NOx gases [7] and the oxidation of volatile organic compounds. [8] They are also active electrocatalysts for the reduction of dioxygen at the cathode of fuel cells. [9][10][11] Manganites further exhibit a complex magnetic and magnetotransport behavior that can be tuned by appropriate substitution on the A site of the perovskite or by adjusting the size of the particles. This has translated into diverse studies in magnetism and magnetotransport in general. [12][START_REF] Shao | Proc. Natl. Acad. Sci[END_REF][START_REF] Zhu | [END_REF][15][16] In order both to understand the impact of downscaling on the magnetotransport properties and the catalytic performances of manganite perovskites, surface effects must be exacerbated. Nanoparticles enable maximizing the surface-to-volume ratio and are then ideally suited for these studies. Unfortunately, designing high crystallinity manganites with narrow size distribution is a strong challenge. These iono-covalent solids form only above 240 °C. [17][18][19] contrary to ionic halide perovskites. [START_REF] Xia | [END_REF][2][3] Organic solvents usually react with Mn 4+ species so that they are not suitable to reach mixed valence manganites. On the opposite, some hydrothermal syntheses yield manganites, but only at the microscale. Sol-gel approaches combined to thermal treatments have been used to synthesize manganites. [20][START_REF] Duan | Sensors Actuators[END_REF][22] They produce nanoparticles with a high density of crystal defects that are detrimental to magnetotransport and complicate the interpretation of catalytic properties. Some teams reported the use of molten inorganic salts as high temperature liquid media where manganite nanoparticles. [8,[23][24][25][26][27] The particles were nevertheless poorly faceted and still exhibited crystal defects. Faceted cobaltite particles were obtained the same way but at the submicronic scale. [28] Our team reinvestigated recently the molten salt approach, by setting conditions for narrow size distribution and low density of crystal defects through a burst of nucleation triggered by large heating rates (Scheme 1). [15] As a result, highly crystalline and faceted 20 nm nanocrystals of La2/3Sr1/3MnO3 (LSMO) could be designed. LSMO is however only one of the numerous members of the manganite perovskite family. Overall, the versatility of the molten salt approach is still to be demonstrated for this strategy to become viable in the design of manganite nanocrystals and then to pave the way towards intensive study of catalytic and magnetotransport properties. The aim of the present report is to assess the suitability of the molten salt approach for designing several manganite perovskite nanocrystals. We especially address La2/3Sr1/3MnO3 (LSMO), La2/3Ca1/3MnO3 (LCMO) and La1/3Pr1/3Ca1/3MnO3 (LPCMO) nanocrystals to demonstrate versatility in available substitutions on the A site of perovskite nanocrystals. The magnetization curves of these samples at low temperature hint at a strong effect of the substitution and of nanoscaling on the magnetic properties of these nanomaterials.

Results and Discussion

Manganite perovskite nanocrystals were synthesized by adapting a previously reported procedure that was dedicated to LSMO particles. [15] The approach consists in triggering the reaction between nitrate salts of manganese (II) and La, Pr, Sr or Ca into molten potassium nitrate (Tfus = 334 °C). In order to decrease the particle size and enable relatively narrow size distribution without using organic surface ligands, nucleation events were favored by applying a large heating rate. This was achieved by introducing the powdered mixture initially at room temperature into an oven pre-heated at the set point between 600 and 800 °C. To avoid extensive growth, the reaction medium was also quenched by transferring the reaction crucible from the hot oven to room temperature, enabling solidification of the salt within ca. 5 min. Xray powder diffraction (XRD) shows that LSMO nanoparticles were obtained at 600 °C according to our previous report (Figure 1). [15] In the same conditions, substituting Sr with Ca to obtain LCMO and/or partially substituting La with Pr to obtain LPCMO do not yield pure phases but impurities of birnessite potassium manganese oxide K0.2MnO2•xH2O, CaMnO3 perovskite and La(OH)3 (Figure S1 and Figure S2). In order to achieve complete reaction, the temperature was increased to 750 °C (Figure S1 and Figure S2). However, incomplete reactivity was still observed, as the CaMnO3 phase could still be observed. To avoid this side product, Ca was introduced with a 10 mol. % default. According to XRD (Figure 1), LCMO and LPCMO are obtained as pure phases. Elemental analysis by ICP-OES yields compositions of La0.6Sr0.4MnO3, La0.6Ca0.4MnO3 and La0.4Pr0.3Ca0.3MnO3, close to the expected values. The K/Mn molar ratio was evaluated by Energy Dispersive X-ray analysis (EDX, Figure S3) to 0.05, and 0.04 for LSMO and LCMO, respectively. For LPCMO, the K content was below the detection threshold. According to previous data of atom-scale chemical mapping, which could not detect K substitution into the LSMO nanocrystals, [15] the most probable origin of the K content in LSMO and LCMO is surface contamination from the synthesis medium. Fast Fourier Transform Infrared spectroscopy (FTIR, Figure S4) shows for the three samples a broad band centered at 550 cm -1 , which corresponds to the stretching vibration of Mn-O in MnO6 octahedra. No trace of carbonates could be detected. Scanning electron microscopy (SEM, Figure 2) shows that LSMO, LCMO and LPCMO samples are homogeneously formed of ca. 20, 20 and 40 nm nanoparticles. The LPCMO sample also comprises some sheets apparently made of the assembly of nanoparticles (Figure S5). According to EDX, they correspond to a Ca-poor perovskite phase La0.6Pr0.3Ca0.1MnO3.

Transmission electron microscopy (TEM, Figure 3a-c) highlights cubic nanoparticles of ca. 20, 22 and 38 nm for LSMO, LCMO and LPCMO, respectively. The size distributions are relatively narrow (Figure 3d-f) if one considers the absence of any surface stabilizing ligand, which distinguishes the molten salt approach compared to colloidal syntheses in organic solvents. HRTEM images and the corresponding Fast Fourier transforms are fully consistent with the expected distorted perovskite structures, with space groups R-3c, Pbnm and Pnma for LSMO, LCMO and LPCMO, respectively. HRTEM also allows identifying facets and deciphering the 3D crystal shapes (Figure 3g-i). For all compounds, the cube-like nanocrystals exhibit main facets along the {001}c planes of the perfect cubic perovskite structure (Figure 3j-n). They also show minor facets along the {111}c and {011}c planes of the cubic structure, leading to nanocubes truncated on each edge and corner (Figure 3j-n). Noteworthy, the nanocrystals do not show any evidence of extended crystal defects. We have shown in a previous report that LSMO nanocrystals are single crystals and show only few crystal defects. According to HRTEM, LCMO and LPCMO behave in the same way. No amorphous surface layer was detected. Noteworthy, the LCMO sample also exhibits a minor population of nanocrystals which show overexpressed 110c facets (Figure S6). The composition homogeneity was confirmed by scanning TEM-Energy Dispersive X-ray Spectroscopy (STEM-EDS) mapping (Figures 456). The Mn oxidation states were probed by X-ray photoelectron spectroscopy (XPS), with a probed depth of ca. 10 nm. The Mn 2p3/2 signal can be well described through two quintuplets (Figures 7 andS7) characteristic of Mn 3+ and Mn 4+ in oxide environments, respectively. [29,30] The average Mn oxidation states were then roughly evaluated from the area ratio of these two species, yielding Mn 3.3+ , Mn 3.2+ and Mn 3.2+ in average for LSMO, LCMO and LPCMO, respectively. These values are in agreement with the measured compositions. They are also consistent with previous data recorded for LSMO nanocubes, [15] for which STEM coupled to electron energy loss spectroscopy showed reduction of Mn cations in a ~1 nm surface layer. The O 1s signal (Figure S8) shows the characteristic contributions of lattice oxygen O 2-, but also of hydroxyl HO -and aquo H2O groups adsorbed on the surface of the nanocrystals exposed to air and water during the washing step. The deconvolution is made by using a Mn 3+ (red) and a Mn 4+ (blue) quintuplets.

Strongly correlated systems such as manganites are known to exhibit spatial separation between regions of different magnetic ordering. [12,31] Most commonly, phase separation is observed between an insulating antiferromagnetic charge-ordered (CO) phase and a metallic ferromagnetic (FM) phase. Charge ordering is the result of a periodic arrangement of cations with different oxidation states in the crystal lattice. LSMO has a relatively simple magnetic-electric phase diagram, being a paramagnetic (PM) insulator above its Curie temperature, and a ferromagnetic (FM) metal below, [12] , with no significant CO phase. LCMO on the other hand contains not only a FM metal-to-PM insulator transition, but there is also evidence for the coexistence of an insulating antiferromagnetic charge-ordered (CO) phase close to the Curie temperature for our composition. [12] Finally, LPCMO has a complex phase diagram, [32][33][34][35] with the co-existence of both an antiferromagnetic CO phase and a metallic FM phase over a range of temperatures.

Isothermal magnetization curves at 5 K of the three samples, LSMO, LCMO, and LPCMO (Figure 8a) reveal a mixed magnetic response consisting of both FM and PM signals. Field cooled and zero field cooled measurements, shown here for LSMO (Figure 8b), reveal a commonly observed Curie temperature,Tc = 310 K, using a Curie-Weiss approximation at high temperatures. (Tc = 279 K, 152 K for LCMO and LPCMO respectively). The blocking temperatures exhibit a broad distribution with maxima in the ZFC curves at around 83, 114 and 64 K for LSMO, LCMO and LPCMO respectively. After extrapolation of the high field paramagnetic slopes, the saturation magnetization for all three powders is significantly less than the ideal value of 3.67 µB/f.u. for the bulk phases (Table 1). Less than half the volume is ferromagnetic (VFM). with the rest behaving as a disordered magnetic state. [15] Furthermore, from the high field slopes, one can estimate an effective paramagnetic moment assuming a Curie Law dependence, hence a volume fraction (VPM) of the paramagnetic-like phase. The high field error in using such a Curie model compared to a Langevin or Brillouin function is ~10%. The residue corresponds to a fraction VNM of the materials with no contribution to the particle magnetization. The volume fractions VPM and VNM can be translated into thicknesses tPM and tNM (Table 1) of paramagnetic and non-magnetic shells in two simple models: FM-PM-NM (FPN) and FM-NM-PM (FNP) core-shellshell models. Our previous observations on LSMO already suggested a FM-PM core-shell model. [15] Interestingly, the shell thicknesses are of the order of few Mn-O-Mn bond lengths (corresponding to the ideal perovskite unit cell (001)c lattice spacing: ~0.4 nm). Electronic phase separation (EPS), is a signature phenomenon of LSMO, LCMO and LPCMO, where both ferromagnetic-metal and paramagnetic-insulator phases can coexist. [12] Depending on the composition, EPS typically occurs at the mesoscale (500 -800 nm), [16] but nanoscale EPS is also reported, arising from the interplay between cation inhomogeneities and long-range Coulomb interactions. [36] For a uniform cation distribution, the EPS length scale is of the order of 5-10 ideal perovskite unit cell parameters ((001)c lattice spacing), [36] in agreement with our observations of ca. 3-5 (001)c lattice spacing thicknesses for the paramagnetic layers in LSMO and LCMO for both core-shell-shell models. The PM layer for LPCMO appears thicker with ca. 9 (001)c lattice spacings. These values emphasize the importance of the high crystallinity and low density of crystal defects achieved in these molten salt-derived nanocrystals.

Conclusions

In this report we have successfully designed manganite perovskite oxide nanocrystals of La2/3Sr1/3MnO3, La2/3Ca1/3MnO3 and La1/3Pr1/3Ca1/3MnO3, hence demonstrating the versatility of syntheses in molten salts to reach a range of high crystallinity manganite perovskites and then of magnetic properties. This original synthetic approach enables now to foresee numerous studies addressing the impact of the nanoscale on properties of these materials, by drawing on the abundant data available for bulk manganites or thin films and their properties tunable by cationic substitutions. Indeed, the first magnetic measurements we report highlight magnetic behaviors for the nanocrystals that are strongly different from the bulk phases.

Experimental Section

Nanocrystals synthesis. The LSMO nanoparticles synthesis was carried out as follows, according to a previously described procedure. [15] First, potassium nitrate (65 mmol), strontium nitrate (2.65 mmol), manganese nitrate tetrahydrate (6.5 mmol), lanthanum nitrate hexahydrate (3.86 mmol) were ball milled for 2 min and then dried at 40 °C under vacuum overnight. The powder was then put into an alumina crucible and heated at 600 °C for 2 h. After cooling at room temperature the product was redispersed with deionized water and stirred for 10 min. The suspension was then separated by centrifugation (20 000 rpm, 10 min) 8 times, washed with deionized water until a conductance around 10 μS/cm was reached. The product was dried under vacuum at 40 °C overnight. The LCMO nanoparticles were obtained by a similar procedure except that instead of strontium nitrate, calcium nitrate tetrahydrate was used (1.79 mmol), and the heating dwell step was performed at 750 °C for 1 h. Likewise, the LPCMO particles were obtained by using lanthanum nitrate hexahydrate (1.99 mmol), praseodinium nitrate hexahydrate (1.99 mmol) and calcium nitrate tetrahydrate (1.79 mmol), at 750 °C for 1 h. For all syntheses, the yield was ca. 95 %.

Powder XRD patterns were recorded on a Bruker D8 advance diffractometer operating at the Cu-Kα wavelength. ICSD references used for indexation were 50717, 82820, 96909 for LSMO, LCMO and LPCMO respectively.

SEM was performed on a Hitachi SU-70 microscope equipped with a field emission gun, at 5 kV accelerating voltage and without coating.the samples.

TEM, HRTEM, STEM images and STEM-EDX maps were recorded on a JEM 2100Plus UHR microscope operating at 200 kV, interfaced to a Oxford Instruments AZtex EDS system with an XMax T large area (80 mm 2 ) SDD detector. The powders were dispersed in ethanol, then drop-casted on carbon-coated copper grids. The ICSD references cited in the XRD section were used for indexation of the HRTEM images.

XP spectra were collected on a Omicron Argus X-ray photoelectron spectrometer, using a monochromated AlK (h = 1486.6 eV) radiation source with a 300 W electron beam power. The emission of photoelectrons from the sample was analyzed at a takeoff angle of 45° under ultra-high vacuum conditions (10 -8 Pa). High resolution XP spectra were collected at pass energy of 20 eV. The peak areas were determined after subtraction of a Shirley background. Spectrum processing was carried out using the Casa XPS software package.

Magnetisation loops were measured at 5 K using a commercial Quantum Design Magnetic Properties Measurement System (MPMS) with a vibrating sample magnetometer.

Manganese oxide perovskites are obtained for the first time as highly crystalline, well-faceted nanocrystals. High temperature liquid phase syntheses in molten salts thus offer a nice playground to assess the impact of the nanoscale on the properties of these materials, as exemplified by the magnetic properties of three different manganites.

Scheme 1 .

 1 Scheme 1. Schematic description of the molten salt-mediated synthesis of manganite perovskites.

Figure 1 .

 1 Figure 1. XRD patterns of LSMO, LCMO and LPCMO samples obtained at 600, 750 and 750 °C, respectively.

Figure 2 .

 2 Figure 2. SEM images of (a, b) LSMO, (c, d) LCMO and (e, f) LPCMO samples obtained at 600, 750 and 750 °C, respectively.

Figure 3 .

 3 Figure 3. (a-c) TEM images, (d-f) corresponding diameter distributions, (g-i) HRTEM images and (j-l) corresponding projections of nanocrystals of (a, d, g, j) LSMO, (b, e, h, k) LCMO and (c, f, i, l) LPCMO. (m) 3D model of nanocrystals with indexation of the facets along the ideal cubic perovskite structure. (n) Similar model where the crystal structure is shown, in the case of LSMO.

Figure 4 .

 4 Figure 4. (a) TEM and (b) STEM-HAADF images of LSMO. STEM-EDX (c) O, (d) La, (e) Mn, (f) Sr maps.

Figure 5 .

 5 Figure 5. (a) TEM and (b) STEM-HAADF images of LCMO. STEM-EDX (c) O, (d) La, (e) Mn, (f) Ca maps.

Figure 6 .

 6 Figure 6. (a) TEM and (b) STEM-HAADF images of LPCMO. STEM-EDX (c) Pr, (d) La, (e) Mn, (f) Ca maps.

Figure 7 .

 7 Figure 7. Mn 2p3/2 region of the XPS spectra for LSMO, LCMO and LPCMO.The deconvolution is made by using a Mn 3+ (red) and a Mn 4+ (blue) quintuplets.

Figure 8 . a ) 1 .

 8a1 Figure 8. a) Magnetization curves of LSMO, LCMO, and LPCMO at 5 K, and b) field cooled (FC), zero field cooled (ZFC) magnetization curves of LSMO

Francisco

  Figure S8. O 1s areas of the XPS spectra of LSMO, LCMO and LPCMO samples, with the corresponding deconvolutions into oxo, hydroxo and aquo groups.

Acknowledgements

This work was supported by the French national agency for research (ANR) under the project SALTYSPIN ANR-17-CE09-0005. FG and DP also acknowledge support from region Ile de France through the Domaine d'Intérêt Majeur DIM Nano-K with the project COSMOS. The FEG-SEM facility of Institut des Matériaux de Paris Centre was funded by Sorbonne Université, CNRS and by C'Nano -Ile-de-France. Support of the Région Grand-Est (PD) is also acknowledged.

Supplementary information

Versatile molten salt synthesis of manganite perovskite oxide nanocrystals and their magnetic properties