Synthesis of Cobalt-Based Nanomaterials from Organic Precursors

2.1. Cobalt oxide nanoparticles from oxalate precursors

There are several publications concerning the synthesis of cobalt or cobalt oxide nanoparticles from oxalate precursors. Abdelkader et al. synthesized Co₃O₄ nanoparticles by calcination of cobalt (II) oxalate dihydrate in static air at 500°C for 2 h [14]. Thota et al. employed a sol-gel process to synthesize oxalate precursors and their subsequent thermal treatment to produce Co₃O₄ nanocrystallites. More specifically, cobalt acetate tetrahydrate was dissolved in ethanol at 35–40°C under continuous stirring followed by oxalic acid addition to yield a thick gel. Thermal decomposition of the dried gel in air at 400–600°C for 2 h led to Co₃O₄ [4]. Luisetto et al. have also produced Co₃O₄ by thermal treatment of cobalt oxalate precipitate at 450°C for 2 h in air [2]. de Rivas et al. prepared Co₃O₄ nanoparticles via calcination of oxalate nanorods at 500°C in static air [15]. Cobalt oxalate precursors have been also prepared from cobalt nitrate, cobalt chloride or cobaltous hydroxide carbonate. Shen et al. obtained CoO nanoparticles from the decomposition of corresponding oxalates at 500°C for 4 h in air. Oxalates were prepared via precipitation from a solution of cobalt nitrate, oxalic acid and ethanol [16]. Manteghi et al. synthesized Co₃O₄ nanostructures by thermal decomposition of cobalt oxalate synthesized by precipitation from cobalt nitrate or cobalt chloride and ammonium oxalate in the presence or absence of surfactants [17]. Yuan et al. synthesized Co₃O₄ by annealing cobalt oxalate precursors prepared via hydrothermal and rheological phase reaction methods [18, 19].

2.2. Cobalt oxide nanoparticles from citrate precursors

Manouchehri et al. reported on the synthesis of Co₃O₄ and CoO nanoparticles via solid state ball milling of cobalt acetate and citric acid powders and subsequent calcination of the prepared carboxylate precursors [12]. de Rivas et al. also used ball milling for synthesis of cobalt citrate starting from cobalt carbonate and citric acid [20]. An aqueous sol-gel citrate procedure has been employed by many research groups initiating from cobalt nitrate and citric acid and the obtained precursors were thermally decomposed between 300 and 500°C in static air for 4 h [20–22]. Pudukudy et al. chose a simple solid-state mixing of cobalt nitrate with various modifiers such as citric acid, glucose, sucrose and urea. The components were mixed, pulverized and heated at 80°C for 1 h. The bimodal mesoporous spinel cobalt oxide nanomaterials were prepared by calcination at 300°C for 2 h [23].

2.3. Cobalt and cobalt oxide nanoparticles from malonate precursors

Mohamed et al. synthesized cobalt malonates by precipitation using cobalt chloride and disodium malonate or malonic acid. The precipitates were heated up to 500°C at heating rates between 2 and 20°C min⁻¹ in a dynamic atmosphere of N₂, H₂ or air resulting in cobalt oxides and metallic cobalt [10]. Stefanescu et al. synthesized nanosized cobalt oxides unsupported [3] or dispersed in a SiO₂ matrix [24] by thermal decomposition of compounds resulting from a redox reaction of cobalt nitrate and diols (1,2-ethanediol, 1,3-propanediol). The produced caboxylates were oxalate, glyoxylate and malonate. The redox reaction between the NO₃⁻ ions and -OH groups of each diol took place under controlled heating (90–120°C, 2 h) and the resulting powders were annealed at 350 and 1000°C for 3 h in ambient air.

2.4. Cobalt and cobalt oxide nanoparticles from succinate precursors

Ganguly et al. synthesized cobalt and cobalt oxide nanoparticles from cobalt succinate originated from a reverse micellar method using cobalt nitrate, 1-butanol and CTAB (as the surfactant). The collected product by centrifugation was thermally decomposed under N₂ at 650°C and under air at 500°C for 6 h, yielding pure Co and Co₃O₄, respectively [13].

2.5. Cobalt oxide nanoparticles from tartrate precursors

Bhattacharjee et al. reported the synthesis of Co₃O₄ nanoparticles by thermal decomposition at 400–600°C of cobalt (II)-tartrate complex prepared by a modified sol-gel route employing cobalt nitrate and tartaric acid [25]. Palacios-Hernández et al. prepared CoO and Co₃O₄ nanoparticles by thermal treatment of cobalt tartrate synthesized by precipitation. Cobalt tartrate was obtained by mixing solutions of cobalt nitrate and the disodium salt of L-tartaric acid. The nanoparticles were prepared by thermal decomposition of the metal-organic complex under isothermal conditions at 500°C in the presence of air for 4 h [26].

2.6. Cobalt oxide nanoparticles from other carboxylate precursors

Chen et al. synthesized CoO nanoparticles using cobalt oleate as precursor. Cobalt chloride and sodium oleate were used as initial chemicals and the obtained dried precipitate was dissolved with oleic acid in 1-octadecene. The CoO nanoparticles were produced by heating the mixture up to 320°C and maintaining that temperature for 30 min [27]. Assim et al. presented the synthesis of a series of ethylene glycol-functionalized cobalt (II) carboxylates by an anion exchange reaction of cobalt (II) acetate tetrahydrate with the corresponding acids. Co₃O₄ nanoparticles were prepared by solid state thermal decomposition of Co(CO₂CH₂(OC₂H₄)₂OMe)₂ at 300°C, while different decomposition times of 10, 20 and 30 min were applied [28]. Semenov et al. prepared cobalt composites via thermal treatment under isothermal conditions (335°C for 9 h under Ar) of unsaturated cobalt dicarboxylates (hydrogen allylmalonate, acetylenedicarboxylate, itaconate, maleate, glutaconate, cis-muconate) using cobalt carbonate and an unsaturated dicarboxylic acid aqueous solution [9].

3.1. Spinel mixed metal-oxide nanoparticles from carboxylate precursors

The preparation methods for CoFe₂O₄ nanoparticles require specialized techniques to prevent agglomeration. Diodati et al. synthesized various spinel ferrites (CoFe₂O₄, NiFe₂O₄, ZnFe₂O₄ and MgFe₂O₄) by a wet-synthesis coprecipitation route starting from iron(III) nitrate, cobalt(II) chloride and oxalic acid in order to prepare the oxalate precursors. The precipitate was centrifuged, dried and calcined at 873 or 1173 K for 5 h in air [30]. Many researchers used the redox method to synthesize the carboxylate precursors (oxalate, malonate, succinate, etc.) initiating from iron(III) nitrate, cobalt(II) nitrate and various diols (ethylene glycol, 1,2-propanediol, 1,3-propanediol and 1,4-butanediol). The obtained precursors were calcined at various temperatures in air to cobalt ferrites [11, 31, 32]. In some cases, the synthesized ferrites were dispersed in a SiO₂ matrix using a modified sol-gel and redox reaction method [31, 33–35]. Amiri et al. synthesized CoFe₂O₄/SiO₂ nanocomposites by a procedure described as sol-gel autocombustion method. Iron and cobalt nitrate were dissolved in water and acid solutions (maleic, malonic, trimesic and ascorbic acid) were added separately as well as tetraethoxysilane. The sols were transformed into gel at 40°C for 24 h [36]. Cobalt-zinc ferrite, Co_1−xZn_xFe₂O₄ (x varying from 0 to 1) was obtained by thermal decomposition of the carboxylate precursor, starting from a mixture of Fe(III), Co(II) and Zn(II) nitrates and 1,3-propanediol whereas a redox reaction took place. The precursor was decomposed at 300°C and further annealed in air for 3 h at various temperatures from 350 to 1000°C [37, 38].

Mixed cobalt-manganese oxide is another spinel synthesized by thermal decomposition of carboxylate salts. Papadopoulou et al. obtained Co-MnO catalysts from the pyrolytic decomposition of mixed cobalt-manganese fumarate and gluconate salts. The precursor compounds were prepared by mixing aqueous solution of cobalt and manganese acetate with an aqueous solution of D-gluconic acid or an ethanolic solution of fumaric acid. The corresponding mixed metal/metal-oxide nanoparticles were prepared by thermal decomposition of the organic precursors under inert gas flow in the temperature range of 500–700°C. Cobalt-manganese spinel oxide was also prepared via oxidative treatment of the same mixed carboxylates at 500°C for 2 h [6–8]. Faure et al. prepared Co_xMn_3−xO₄ (0 ≤ x ≤ 3) by controlled decomposition of mixed oxalates, precipitated at room temperature using cobalt nitrate, manganese nitrate and ammonium oxalate. The thermal decomposition of oxalates was carried out near 200°C under O₂/Ar flow and further calcination up to 300°C for 1 h [39].

Wang et al. synthesized CoAl₂O₄ nanoparticles via thermal treatment of Co-Al-glycine precursors, initiating from cobalt and aluminum nitrate and glycine. The precursor was treated under air at 200–1000°C for 2 h [40]. Synthesis of CoCr₂O₄, a member of the chromite spinel family, was reported by Gingasu et al. via thermal decomposition of tartrate and gluconate precursors obtained by precipitation using cobalt (II) nitrate, chromium (III) nitrate and tartaric acid or δ-gluconolactone [41]. Finally, synthesis of metal cobaltite spinels (MgCo₂O₄, NiCo₂O₄) by thermal decomposition of coprecipitated mixed oxalate precursors has been reported [42, 43].

3.2. Perovskite mixed metal nanoparticles from carboxylate precursors

Seyfi et al. synthesized unsupported perovskite samples (i.e. LaCoO₃) by thermal decomposition of citrate complexes using lanthanum(III) nitrate, cobalt(II) nitrate and citric acid as initial compounds. The obtained viscous gel was dried and calcined at 700°C for 5 h under air atmosphere [44].

3.3. Ilmenite mixed metal-oxide nanoparticles from carboxylate precursors

Gabal et al. prepared CoTiO₃ via a coprecipitation method. The mixed oxalate precursor was thermally treated at various temperatures (230–950°C, 30 min and 1000°C, 2 h) and CoTiO₃ nanoparticles were obtained [45].

6.1. Structure and characterization of Co₃O₄ nanoparticles

The majority of publications for the synthesis of cobalt-based nanoparticles from thermal decomposition of organic precursors under air have reported on the formation of the spinel Co₃O₄ phase. The published X-Ray Diffraction (XRD) patterns have reflection peaks that can be perfectly indexed to the cubic phase of Co₃O₄ spinel with the cell parameter α in the range 8.07–8.15 Å and space group Fd3m (Figure 1). In most cases, there were no diffraction peaks related to CoO or metallic cobalt (Co⁰), except for thermal treatment above 900°C, where Co₃O₄ decomposes to CoO. The average crystallite size, estimated by the Debye-Scherrer equation, varied between 5 and 50 nm, depending on the preparation method and the type of cobalt precursor, the thermal treatment conditions, etc. Structural details of Co₃O₄ have been investigated by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). SEM results indicated the agglomeration of Co₃O₄ nanoparticles and various types of morphologies were observed, that is, matchstick type bars [4, 17], nanorods of bamboo stick type [4], truncated hexahedral shape [18], flocculent-like structures [48] and irregularly shaped aggregates (dense clusters) [21, 22, 26, 49]. TEM results were relatively consistent with XRD results concerning the crystallite size (5–50 nm). Palacios-Hernández et al. reported the synthesis of Co₃O₄ with average grain size of 300 nm and a scarce number of nanoparticles with a diameter of 100 nm and smaller, according to TEM. Most nanoparticles had faceted morphology and high crystalline quality as verified by Selected-Area Electron Diffraction (SAED) analysis and High-Resolution Transmission Electron Microscopy (HRTEM) [26]. Pseudo-hexagonal [15, 17, 18, 21], circular and occasionally rectangular [2] or uniform spherical [12] shaped nanoparticles were observed. In general, powders are homogeneously dispersed with dimensions ranging from 50 to 600 nm, in accordance with SEM results, mentioned above. TEM images further validated that the porous architectures—agglomerations—are constructed from the interconnection among the primary crystalline nanoparticles with size of a few nanometers. Furthermore, the regular diffraction spots displayed from SAED pattern, validated the cubic face-centered structure of Co₃O₄ [2, 4, 18, 25]. HRTEM analysis also indicated the single crystalline nature of Co₃O₄ [18, 25].

The textural properties of cobalt oxides are typically determined by N₂ adsorption-desorption at liquid N₂ temperature. The reported Brunauer–Emmett–Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore size distribution of Co₃O₄ nanoparticles differ significantly from sample to sample. Factors like the synthesis method of cobalt precursor, its nature, the treatment temperature and soak time affect their textural properties. The specific surface areas of Co₃O₄ nanoparticles vary from 7 [23] to 120 m² g⁻¹ [21], the pore volume ranges between 0.03 [23] and 0.24 cm³ g⁻¹ [20] while average pore size is between 9 [20] and 68 nm [23]. Wang et al. found that all N₂ sorption isotherms of Co₃O₄ and Co-Ce oxide nanoparticles can be assigned to a type IV isotherm, typical for mesoporous materials, showing a hysteresis loop at P/P₀ = 0.45 [22]. The mesoporous structure of the nanomaterials was also noted by other researchers [15, 17, 20, 23]. Pudukudy et al. synthesized Co₃O₄ spinels possessing a bimodal mesopore distribution in the presence of modifiers [23]. Manteghi et al. also suggested the presence of a secondary porous structure formed by aggregation of primary nanostructures [17]. Another very useful technique for the characterization of cobalt oxide and its precursors is Fourier Transform Infrared spectroscopy (FTIR) spectroscopy. The appearance of characteristic infrared absorption bands of cobalt precursor and final product obtained from thermal decomposition of the precursor validates their formation. The IR spectrum of cobalt oxide exhibits two major bands at ~570 cm⁻¹ (ν₁) and ~670 cm⁻¹ (ν₂) (Figure 2a). The first band (ν₁) is associated with the Co³⁺ vibration in the octahedral hole and the second band (ν₂) is attributed to the Co²⁺ vibration in the tetrahedral hole, which confirms the formation of the Co₃O₄ spinel [12, 17, 23, 24, 48, 49]. Raman spectroscopy has also been utilized for the confirmation of Co₃O₄ formation. Typically, five Raman bands at 198, 484, 522, 622 and 694 cm⁻¹ are visible in the range 100–800 cm⁻¹, which correspond, respectively, to the F¹_2g, E_g, F²_2g, F³_2g and A_1g modes of crystalline Co₃O₄ (Figure 2b). F_2g and E_g modes are associated with the vibration of tetrahedral and octahedral sites, whereas the high-frequency band, A_1g mode, is linked to the occurrence of octahedral sites [14, 15, 20–22, 25].

Τhe surface chemical status and composition of the nanomaterials can be investigated by means of X-ray photoelectron spectroscopy (XPS). According to XPS results, the Co_2p spectrum is decomposed into two distinct peaks and two weak satellites (Figure 3a). The two spin orbit doublets assigned to cobalt oxides located at 780.8 and 795.9 eV can correspond to Co_2p(3/2) and Co_2p(1/2), respectively. The spin orbit doublet of Co_2p(3/2) can be deconvoluted into two peaks at 780.8 and 782.3 eV, which are attributed to Co³⁺_2p(3/2) and Co²⁺_2p(3/2) configurations, respectively. The Co_2p(1/2) spin orbit doublet can also be deconvoluted into two distinct peaks located at binding energies of 795.8 and 797.4 eV and are assigned to Co³⁺_2p(1/2) and Co²⁺_2p(1/2), respectively. The energy difference of 15.1 eV between Co_2p(3/2) and Co_2p(1/2) splitting is characteristic of the Co₃O₄ cubic phase. The presence of the two satellite peaks (Co_sat.) in the vicinity of the two spin orbit doublets, at binding energies of 788.2 and 804.8 eV, further demonstrates the existence of cobalt oxides [22, 55]. The XPS spectrum of O_1s presented in Figure 3b demonstrates a peak at 530.0 eV attributed to lattice oxygen of Co₃O₄ (O_latt., O²⁻) and another one at 531.0 eV associated with adsorbed oxygen onto surface oxygen vacancies and oxygen-containing groups such as hydroxyl species. The additional peak at 532.1 eV can be attributed to physical and chemical adsorption of water on the surface of defects (O_adsorbates) and apparently is not always present in O_1s spectra [22, 55].

Reducibility and acidity-basicity are important properties affecting material behavior in a specific application. Temperature programmed reduction (TPR) of Co₃O₄ nanoparticles shows two main peaks. The first peak is located at 250–360°C and is assigned to reduction of Co³⁺ to Co²⁺ with the concomitant phase change to CoO and the second peak appears at 350–473°C corresponding to reduction of Co²⁺ to Co⁰. Peak temperature deviations in TPR might be due to structural variations of Co₃O₄ nanoparticles (particle size and faceting) and TPR protocols [2, 14, 20–22]. The acidic properties of Co₃O₄ nanoparticles have been studied by NH₃-TPD. Abdelkader et al. performed NH₃-TPD on Co₃O₄ pre-reduced at 400°C for 1 h and reported that the TPD profile contained two peaks, at 104 and 240°C, indicating the presence of two acidic sites [14]. de Rivas et al. carried out NH₃-TPD experiments on Co₃O₄ nanomaterials pretreated in a O₂/He stream at 500°C and observed two ammonia desorption peaks at 190–200°C and 300–305°C as well as a shoulder at 250°C. They concluded that the peak at low temperature could be associated with weak acidic sites and the peak at high temperature with strong acidic sites [20]. In addition to physicochemical characterization of cobalt nanoparticles, some researchers have studied the optical, electrical and magnetic characteristics of the synthesized nanomaterials by means of UV-Vis-NIR Spectrometry [4], SQUID [4], cyclic voltammetry (CV) [17, 18] and electrochemical impedance spectroscopy (EIS) [17].

6.2. Structure and characterization of CoO nanoparticles

As already mentioned above, there is scarce bibliography concerning the preparation of CoO nanoparticles by thermal decomposition of cobalt-organic precursors. XRD results reported by Bartůněk et al. [5] and Manouchehri et al. [12] showed the presence of a face-centered cubic phase with cell parameter α = 4.26 Å and space group F₃3m [12]. In the FTIR spectrum, the appearance of a broad band at 450 cm⁻¹ verified the formation of cubic CoO. According to TEM the as-prepared CoO consisted of two types of structures, small nanoparticles and nanorods [12]. Shen et al. claimed that the CoO nanoparticles exhibited quasi-spherical shape and their measured—from TEM micrographs—average particle size was 18 nm [16]. Chen et al. reported on flower-like CoO nanoparticles with an average particle size of 50 nm [27]. In some cases, synthesis of Co₃O₄ nanoparticles was accompanied by the formation of CoO or metallic cobalt as impurities. Pursuant to XRD results face-centered cubic (space group: F₃3m) [23], monoclinic (space group: C2/m) [23] and tetragonal [9] crystal systems were reported.

6.3. Structure and characterization of metallic Co nanoparticles

Ganguly et al. produced pure metallic Co nanoparticles confirmed by XRD results with average size, as measured by TEM, between 10 and 40 nm [13]. Co composites were also synthesized by Semenov et al. containing, according to XRD data, metallic Co (hexagonal symmetry, unit cell parameters: а = 2.506 Å, с = 4.071 Å) and CoO as admixtures in the main phase of Co₃O₄ [9].

6.4. Structure and characterization of mixed metal-oxide nanoparticles

Dippong et al. synthesized CoFe₂O₄ nanoparticles and concluded that single phase cubic spinel CoFe₂O₄ can be produced at all temperatures initiating from 1,3-propanediol whereas, when initiating from 1,2-ethanediol and 1,2-propanediol elevated temperatures are required. The degree of CoFe₂O₄ crystallization increased with the increase of calcination temperature. The appearance of the intense, symmetric band at 560–585 cm⁻¹ in the FTIR spectra, characteristic of the CoFe₂O₄ phase, confirmed its formation, while, TEM results revealed—in accordance with XRD—the presence of CoFe₂O₄ as nearly spherical nanoparticles with dimensions varying with calcination temperature between 8 and 20 nm [11]. Another publication from the same group focused on the influence of Co/Fe ratio on the oxide phases in CoFe₂O₄ nanoparticles and proved the dependence of nanoparticle size on sample composition (Fe and Co content) and calcination temperature. The diameter of the as-synthesized nanoparticles ranged between 12 and 88 nm and according to TEM micrographs, the spherical nanocrystallites showed an agglomerated conformation [56]. Similar work has been conducted by Stefanescu et al., who concluded that the average particle size of the nanoparticles estimated from XRD varied in the range 10–19 nm [32]. Diodati et al. synthesized CoFe₂O₄ and carried out in-situ temperature-dependent XRD of the oxalate precursors and showed that the XRD patterns of the spinel phase began to be visible at 773–873 K whereas they became sharp and distinct at 1073–1173 K. The crystallite size calculated by means of XRD and TEM was 23 nm (calcination at 873 K) and 185 nm (calcination at 1173 K). Furthermore, TEM micrographs showed that cobalt ferrite nanoparticles retained their identity and size but they had a tendency to form agglomerates. According to XPS and Mössbauer results, the as-synthesized compounds contained surface iron in oxidation state (III) and not in (II). The TPR result showed that the reduction process took place at 700–1173 K and resulted in the reduction of Co(II) and Fe(III) cations to a metallic state [30]. In addition, some researchers have studied the synthesis of CoFe₂O₄ embedded in SiO₂ and according to XRD results the formation of CoFe₂O₄ spinel phase is affected by various factors, that is, annealing temperature, percentage of silica matrix, the precursor type, etc. For instance, Dippong et al. have shown that the nanocrystallite size and crystallinity increased with increase of CoFe₂O₄ and decrease of SiO₂ content [34], with the use of longer chain precursors [31] and the increase of the annealing temperature [36]. The diameter of CoFe₂O₄ nanoparticles, calculated by means of XRD and TEM, ranged between 8 and 32 nm depending on the parameters mentioned above and according to SEM images they were spherical and formed irregular agglomerations [31, 34, 35]. Amiri et al. synthesized CoFe₂O₄ nanoparticles dispersed in a SiO₂ matrix by thermal decomposition of iron-cobalt-carboxylates (maleate, malonate, trimesate and ascorbate) and according to XRD patterns, the thermal decomposition of iron-cobalt trimesate precursor led to the formation of pure cubic spinel CoFe₂O₄ with a crystallite size of 7 nm. The formation of the CoFe₂O₄ phase was also confirmed by FTIR spectra due to the appearance of the characteristic bands at 469 and 694 cm⁻¹. FE-SEM analysis showed irregularly shaped nanoparticles with an average size of 36 nm (Fe-Co-malonate), 38 nm (Fe-Co-ascorbate) and 27 nm spherical nanoparticles with (Fe-Co-trimesate) and 37 nm (Fe-Co-maleate). All nanoparticles formed agglomerations according to SEM images [36]. Another nanomaterial prepared using thermal decomposition of the corresponding carboxylate precursors was Co_1-xZn_xFe₂O₄. Muntean et al. concluded by means of XRD that the as-synthesized nanomaterials consisted of the cubic spinel phase Co_1-xZn_xFe₂O₄ and impurities (CoFe₂O₄, ZnFe₂O₄) except for the thermal decomposition at 1000°C, where Co_0.5Zn_0.5Fe₂O₄ ferrite was obtained as a single, well crystallized phase. The increase in annealing temperature increased the crystallinity and the crystallite size of the nanoparticles. The formation of mixed spinel structure was also confirmed by FTIR spectra due to the presence of characteristic bands at 400 and 600 cm⁻¹ which indicate the anion-cation interaction in octahedral and tetrahedral voids. The average crystallite size varied between 11 and 81 nm depending on the annealing temperature [37]. Similar experiments performed by Stefanescu et al. led to particles with average crystallite size in the range 7–33 nm due to the lower annealing temperature (up to 600°C instead of 1000°C in Ref. [37]) [38].

Mixed cobalt-manganese oxides were investigated by Papadopoulou et al. [6–8]. They employed in-situ XRD to identify the evolving crystalline phases at various stages of pyrolysis of cobalt-manganese gluconate or fumarate precursors and concluded the formation of MnO and metallic Co phases at 550°C due to the thermal decomposition of the fumarate precursor, whereas the decomposition of gluconate precursor above 200°C did not lead to the formation of crystalline phases before 650°C. Same results were obtained by XRD measurements over passivated samples originated from fumarate salts. Thus, the thermal decomposition of both fumarate and gluconate precursors led to the formation of reduced Co-MnO phases in a single step, due to a carbothermal reaction that took place between residual carbon and Co²⁺ ions. Additionally, the specific surface area of all samples obtained from fumarate precursor ranged between 200 and 220 m² g⁻¹ regardless of the pyrolysis temperature, due to the presence of residual carbon in the materials which stabilized their porous structure and prevented extensive sintering of MnO and Co crystallites. Furthermore, the same research group synthesized Co-Mn spinel oxide via oxidative treatment of fumarate precursors and the calcined samples had considerably smaller surface area than pyrolyzed ones in the range of 21–35 m² g⁻¹ [6–8]. Faure et al. synthesized Co_xMn_3-xO₄ (0 ≤ x ≤ 3) by thermal decomposition of oxalate precursor and concluded that the oxides with x < 0.9 are amorphous, while for x > 0.9 all the nanomaterials had a cubic spinel structure. According to TEM and XRD results, the crystallite size ranged between 6 (x = 1.6) and 16 (x = 3) nm. Regarding textural properties, the BET surface area and the pore volume were the highest for x = 2 (S_BET = 270 m² g⁻¹, pore volume = 0.48 cm³ g⁻¹) whereas further increase of cobalt content was associated with a decrease of surface area and pore volume [39].

The structure and physicochemical characteristics of cobalt aluminate (CoAl₂O₄) nanomaterials were investigated by means of XRD, FTIR, TEM and SEM. Wang et al. synthesized Co^IICo^III_xAl_2−xO₄ (x = 0–2) and reported that nanomaterials had spinel cubic structure (Fd3m) and that their crystallite size ranged between 9 and 51 nm depending on Co/Al ratio and pyrolysis temperature. Their formation was further validated by FTIR spectroscopy from the appearance of the characteristic vibrational bands of CoAl₂O₄ (572 and 668 cm⁻¹). According to SEM images the spinel samples were agglomerated in large particles with irregular shape. The BET surface area was 22–69 m² g⁻¹ depending on annealing temperature [40]. Gingasu et al. investigated the physicochemical properties of CoCr₂O₄ nanomaterials synthesized by thermal decomposition of tartrate and gluconate precursors. The reflections of XRD patterns could be indexed to the face-centered cubic cell with the Fd3m space group and the average crystallite size obtained from tartrate and gluconate precursors was 14–18 and 21 nm, respectively. According to SEM and TEM micrographs, the particle size of CoCr₂O₄ obtained from tartrate was in the range 13–35 nm (depending on annealing temperature), while CoCr₂O₄ obtained from gluconate was in the range 16–28 nm showing in each case well-faceted pyramidal or bipyramidal geometry. CoCr₂O₄ from gluconate precursor formed spherical aggregates with an average size 200–400 nm. The formation of cobalt chromite nanomaterials was confirmed by the presence of sharp and intense bands in the range 615–640 and 500–530 cm⁻¹ in FTIR spectra, which are characteristic of Cr-O bonds. According to Raman spectra, the spinel type CoCr₂O₄ has a cubic symmetry where the Co²⁺ and Cr³⁺ ions occupy the tetrahedral and octahedral sites, respectively. Furthermore, the nitrogen adsorption isotherms were of IV type with a H3 hysteresis loop, characteristic for mesoporous materials. The BET surface area and pore parameters for tartrate-derived CoCr₂O₄ were S_BET = 15–51 m² g⁻¹, pore volume = 0.11–0.24 cm³ g⁻¹ and average pore diameter = 21–32 nm, whereas for gluconate-derived CoCr₂O₄ were 17–42 m² g⁻¹, 0.15–0.21 cm³ g⁻¹ and 21–34 nm, respectively [41]. Darbar et al. synthesized MgCo₂O₄ by oxalate decomposition and XRD patterns were indexed to the standard cubic structure with Fd3m space group. The SEM images revealed the agglomeration of MgCo₂O₄ nanoparticles and the BET surface area was measured in the range 4–22 m² g⁻¹ depending on calcination temperature [42]. Kim et al. produced NiCo₂O₄ aggregates by oxalate decomposition and the XRD pattern confirmed the formation of cubic spinel NiCo₂O₄ phase with minor additional impurities (NiO). SEM micrographs showed the presence of various morphologies, such as micro flowers and rod shapes and according to TEM images each microstructure consisted of aggregates of spherical metal-oxide nanoparticles with diameter between 20 and 100 nm [43]. Seyfi et al. reported on the synthesis of LaCoO₃ perovskite catalysts with specific surface area of 5 m² g⁻¹ [44]. On the other hand, Gabal et al. produced purely trigonal ilmenite CoTiO₃ by thermal treatment of cobalt oxalate-TiO₂ precursor. They concluded that at high temperature (1000°C), CoTiO₃ with rhombohedral symmetry (space group R-3 (148)) was formed. According to TEM images, the nanomaterial showed weak agglomeration with heterogeneous morphology in both shape and dimensions and exhibited a larger grain size, 300 nm, compared to that estimated from Scherrer’s equation, 209 nm, due to the agglomeration nature of the nanoparticles as observed from the TEM micrograph. In addition, the specific surface area of the sample was 30 m² g⁻¹ and the adsorption isotherm was classified to type II, characteristic for macroporous adsorbents and had very small hysteresis loop [45].

6.5. Structure and characterization of supported cobalt oxide nanoparticles

The results from XRD analysis of cobalt compounds supported on carbonaceous carriers differ depending on the nature of the compound (Co₃O₄, CoO, Co) but in all cases reflections corresponding to the lattice planes of graphitic materials have been observed. The reflections due to cobalt/cobalt oxide nanoparticles were similar to those mentioned above for unsupported materials or in some cases they were not detected at all. It is obvious that an increase in the carbon content leads to an increase of the graphitic diffraction peaks and simultaneous decrease of cobalt/cobalt oxide peaks. Additionally, with increasing calcination temperature, the diffraction peaks for cobalt/cobalt oxide became sharper in contrast to those for carbon, indicating that the crystallinity of cobalt-based nanoparticles increased and graphite content decreased due to its partial oxidation [47]. According to SEM and TEM micrographs, cobalt-based nanoparticles were spherical, coated with carbon and dispersed in the carbon matrix [54], uniformly distributed on carbon layered structures with particle size of 5–30 nm [53], supported on carbon with random size distribution in the range from 1 to 32 nm [50], embedded in carbon with average particle size of 8–10 nm [51] and encapsulated within carbon nanotubes with an average particle size of 50 nm [52]. N₂ adsorption-desorption isotherms of the as-synthesized nanocomposites exhibited the type IV isotherm with a H3-type hysteresis loop, demonstrating the mesoporous nature of the materials, while the textural properties of the materials varied widely (S_BET = 76–365 m² g⁻¹, pore volume = 0.15–0.96 cm³ g⁻¹, average pore size = 1.5–20 nm) [50, 52–54]. However, Zhou et al. observed a N₂ adsorption isotherm of type V with a H3-type hysteresis loop, while the specific surface area ranged between 160 and 375 m² g⁻¹, the pore volume was 0.58–0.98 cm³ g⁻¹ and the average pore size varied from 4.8 to 6.9 nm depending on the calcination temperature [51]. In many publications, electrical and magnetic properties have been investigated due to the potential use of the as-synthesized materials in Li-ion batteries and as electrocatalysts [47, 52–54].