**Figure 10.** Interface of results (corrosion current density—polarization resistance 750/3000 s) obtained according to the

The electrochemical sequence used for corrosion resistance testing and analysis of its effects on cobalt alloy surface condition was: linear polarization at −0.8 to +1 V at 1 mV/s. The corrosion points on the alloy surface of 4, 5, and 6% have irregular shape, having different sizes can

The SEM micrographs obtained for cobalt alloys indicate that the chemical attack (in fresh unpasteurized orange juice) occurs superficially, uniformly over the entire surface of the alloy, and only locally there are a series of superficial, very small corrosion points, which are randomly distributed on surface [14]. The behavior of the investigated alloys is consistent with the results of potentiodynamic measurements indicating low values for instantaneous

The central objective of this study was fulfilled by evaluating improvement of the properties of cobalt alloys, used in medical applications, which can resist longtime in human body, as well as knowledge development of new biocompatible structures [15]. If variants of alloys are not subjected to biological acceptance criteria by the animal body, they cannot be placed in the living organism, no matter how appropriate the properties of biomaterials are. The realization of alloy variants should also consider the possibility of the appearance of basic pathophysiological phenomena, which determine their long-term safety (thrombosis, inflam-

The CoCrMo commercial alloy implanted in subjacent area presents an adipose tissue with canalicular structures and large vessels, separated by membranes of conjunctive tissue with an increased fibroblast population or by islands of conjunctive tissue with similar cell aspect; associated, but and numerous high macrophages, with intracytoplasmic grains (**Figure 12**).

In subcutaneous tissue (**Figure 12a**) after 21 days, can observe a moderate inflammation with a broadcast distribution, rarely nodular, which presents perivascular and perinervous.

alloy, the corrosion points have the

be observed in the 2000× BSE magnification power. At CSi7

large dimensions compared to the experimental alloys analyzed.

**2.11. The clinical study on the biocompatibility of the obtained alloys**

mation, infection and/or induction, and neoplasm challenge) [16].

Stern-Geary equation.

42 Cobalt

corrosion rate (**Table 4**).

In subjacent area of implanted commercial alloy (**Figure 12b**), the skin presents normal aspect, with epidermis, without modifications, and a small diminution of the space occupied by dermis, in which are present numerous adipocytes, striated muscle tissue, but and a hypodermis characterized by the presence of a conjunctive tissue richly vascularized, with numerous active fibroblasts.

In lateral area (**Figure 12c**) of implanted commercial alloy (near pill), the skin has an easy papillomavirus epidermis, dermis is superficial and deep, collagenizated intense and hialinizated. It can be observed in this area, near pill exist a hypodermis and muscular tissue normally striated.

**Figure 12.** CoCrMo commercial alloy implanted: (a) subjacent area; (b) superjacent area; and (c) lateral area.

CSi4 alloy implanted in subjacent area (**Figure 13a**), presents a densification of conjunctive tissue with collagen fibers, parallel between then and numerous active fibroblasts, realizing a fibrous band, which delimits the implanted CSi4 alloy. External to this band, it found a moderate inflammatory infiltrate, numerous macrophages with hemosiderin and numerous congested capillaries.

In superjacent area (**Figure 13b**) of CSi4 implanted alloy, the skin presents a normal aspect.

At the CSi4 alloy, it was not taken in lateral area, because the implanted material was surrounded by a compact film, described at subjacent area.

CSi5 alloy implanted in subjacent area (**Figure 14a**) presents a conjunctive tissue with numerous adiposities and expended venules.

The superjacent area of CSi5 -implanted alloy (**Figure 14b**) presents skin with normal aspect, hypodermis with adiposities and muscular striated tissue.

**Figure 13.** CSi4 implanted alloy: (a) subjacent area and (b) superjacent area.

**Figure 14.** CSi5 implanted alloy: (a) subjacent area; (b) superjacent area; and (c) lateral area.

The skin area is with atrophic epidermis, taking place the diminution of skin axis, and dermis have relatively lax aspect.

The CSi5 alloy implanted in lateral area (near pill) (**Figure 14c**), highlights a skin with atrophic epidermis. In hypodermis, it found collagenized and hialinizated areas with numerous active fibroblasts. In this area, vascular elements appeared (capillary, arterioles, and venules) with reactive endothelium and numerous lymphocytes in lumen.

In comparison with the CSi4 alloy, the CSi5 alloy do not present a conjunctive tissue organized in delimit band of implanted material.

CSi6 implanted in subjacent area of material (**Figure 15a**) presents a hypodermis with adipose tissue separated by bands and/or collagen islands with numerous fibroblasts. These are infiltrating chronic inflammatory, moderately represented big macrophages with grains.

CSi4

44 Cobalt

congested capillaries.

At the CSi4

**Figure 13.** CSi4

**Figure 14.** CSi5

CSi5

 alloy implanted in subjacent area (**Figure 13a**), presents a densification of conjunctive tissue with collagen fibers, parallel between then and numerous active fibroblasts, realizing

alloy, it was not taken in lateral area, because the implanted material was sur-

alloy implanted in subjacent area (**Figure 14a**) presents a conjunctive tissue with numer-

moderate inflammatory infiltrate, numerous macrophages with hemosiderin and numerous

**Figure 12.** CoCrMo commercial alloy implanted: (a) subjacent area; (b) superjacent area; and (c) lateral area.

alloy. External to this band, it found a

implanted alloy, the skin presents a normal aspect.

a fibrous band, which delimits the implanted CSi4

rounded by a compact film, described at subjacent area.

hypodermis with adiposities and muscular striated tissue.

implanted alloy: (a) subjacent area and (b) superjacent area.

implanted alloy: (a) subjacent area; (b) superjacent area; and (c) lateral area.

In superjacent area (**Figure 13b**) of CSi4

ous adiposities and expended venules.

The superjacent area of CSi5

In the case of CSi6 alloy, it is found a densification of conjunctive tissue with collagen fibers, parallel between then and numerous active fibroblasts, realizing a fibrous band, which delimits the implanted material. The fibrous band obtained at CSi6 alloy is thinner than that of CSi alloy.

In the superjacent area of CSi6 -implanted alloy, (**Figure 15b**) can be observed that the skin presents a normal aspect, a hypodermis with aspect similar with that described at subjacent area—densification of conjunctive tissue, under form of fibrous bands, like a wall-organized perimaterial implanted. In the case of this alloy, it is found that the lymphatic vessels are much expended.

In lateral area of CSi7 -implanted alloy (near pill) (**Figure 15c**), the skin presents normal aspect without modifications.

In subjacent area of CSi7 alloy (**Figure 16a**) have infiltrate inflammatory by massive chronic type, arranged diffusive and/or nodular, perivascullar, and perinervous associating elements by acute type and macrophages.

In superjacent area of CSi7 -implanted alloy (**Figure 16b**), the skin presents a normal aspect, but not a normal hypodermis.

The CSi7 alloy implanted in lateral (near pill) presents a densification of conjunctive tissue with collagen fibers, parallel between then, and numerous active fibroblasts (**Figure 16c**).

Due to active fibroblasts was made a fibrous band, which delimits implanted material, but is thinner than CSi4 and CSi6 original variants.

**Figure 15.** CSi6 alloy implanted: (a) subjacent area; (b) superjacent area; and (c) lateral area.

**Figure 16.** CSi7 alloy implanted: (a) subjacent area; (b) superjacent area; and (c) lateral area.

The variants of alloys present aspect of biocompatibility detected by microscopic studies, conducted in same conditions like in the case of CoCrMo commercial alloy. The variants of alloys by CoCrMo system do not present major differences in comparison with commercial alloy. The modifications of tissue consist in appearance of one tissue by grain implanted perimaterial, which can be interpreted like results of conjunctive organizing and repairing of a consecutive injury.

At variants of CSik (k = 4, 5, 6, and 7), alloys exist supplementary, adjacent to implanted alloy, a densification of conjunctive tissue, which formed a delimited structure, type fibrous membrane, which is not significant for general state of tested biological organism, because the inflammation is low.

In the biocompatibility study, it is found that no locally histological modifications are recorded, generated by a possible toxicity of biomaterials, based on cobalt, meaning that these type of alloys can be used successfully in medical applications.

#### **3. Conclusions**

This scientific research presents the following advantages:

The important physical-mechanical characteristics like tensile strength, elasticity modulus, good hardness, and low density ensure the mechanical compatibility of alloy, which is necessary for integration in live tissue and the success for long time of implants.

Both European and National Standards, and also the law requirements from materials domain for medical implanted devices, imposed for biomaterials a high biological compatibility, this being a property influences the behavior of implant in human bodydirectly.

#### **Author details**

Petrica Vizureanu1 \*, Mirabela Georgiana Minciuna1 , Gianina Iovan2 and Simona Stoleriu2

\*Address all correspondence to<EMAIL_ADDRESS>
1 "Gheorghe Asachi" Technical University Iasi, Iaşi, Romania

2 Faculty of Dental Medicine, University of Medicine and Pharmacy "Gr. T. Popa" Iaşi, Iaşi, Romania

#### **References**

The variants of alloys present aspect of biocompatibility detected by microscopic studies, conducted in same conditions like in the case of CoCrMo commercial alloy. The variants of alloys by CoCrMo system do not present major differences in comparison with commercial alloy. The modifications of tissue consist in appearance of one tissue by grain implanted perimaterial, which can be interpreted like results of conjunctive organizing and repairing of a

alloy implanted: (a) subjacent area; (b) superjacent area; and (c) lateral area.

a densification of conjunctive tissue, which formed a delimited structure, type fibrous membrane, which is not significant for general state of tested biological organism, because the

In the biocompatibility study, it is found that no locally histological modifications are recorded, generated by a possible toxicity of biomaterials, based on cobalt, meaning that these

• The increases of chemical homogeneity grade in entire mass for cobalt-base alloy.

• The obtaining of dendrite, biphasic, and uniform structure, which offer an excellent ma-

• The CoCrMo alloy alloyed with Si, obtained by proposed method, present physical, mechanical, and osteo-integration properties superior than the CoCrMo and CoCr alloys, de-

• The ingots obtained by cast, from proposed used alloy, are exactly rigid and nondeformed,

• The remelting installation does not require superheating for melting the alloy, infact it

The important physical-mechanical characteristics like tensile strength, elasticity modulus, good hardness, and low density ensure the mechanical compatibility of alloy, which is neces-

sary for integration in live tissue and the success for long time of implants.

type of alloys can be used successfully in medical applications.

This scientific research presents the following advantages:

(k = 4, 5, 6, and 7), alloys exist supplementary, adjacent to implanted alloy,

consecutive injury.

**Figure 16.** CSi7

46 Cobalt

At variants of CSik

inflammation is low.

**3. Conclusions**

chinability for final products.

veloped by direct alloying method.

reduces the losses of alloying elements.

even at the small used cavities.

**Provisional chapter**

#### **Synthesis of Cobalt-Based Nanomaterials from Organic Precursors Precursors**

**Synthesis of Cobalt-Based Nanomaterials from Organic**

DOI: 10.5772/intechopen.70947

Maria Smyrnioti and Theophilos Ioannides

[12] Minciună MG, Vizureanu P, Achiţei DC, Ghiban B, Sandu AV, Mareci D, Bălan A. Electro-

[13] Stern M, Geary A. Electrochemical polarization. I. A theoretical analysis of the shape of

[14] Vizureanu P. Metode și tehnici de cercetare în domeniu. Studiul materialelor prin

[15] Laboratorul Universitar al Bazei Clinice de Învăţământ Medical Stomatologic. www.

polarization curves. Journal of the Electrochemical Society. 1957;**104**:56-63

microscopie optică. Iasi: Universitatea Tehnică' Gheorghe Asachi ed; 2011

[16] www.cantacuzino.ro/ro/wp-content/uploads/2009/02/catalog-2009.PDF [Internet]

alloys at different simulated physiologi-

chemical behavior of CoCrMo and CoCrMoSi5

umfiasi.ro [Internet]

48 Cobalt

cal medium. Revista de Chimie (Bucharest). 2014;**65**(10):80-87

Maria Smyrnioti and Theophilos Ioannides Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.70947

#### **Abstract**

Development of efficient and low-cost methods for the production of cobalt and cobalt oxide nanoparticles is of great interest. Such nanoparticles are typically prepared via transformation of precursors under controlled conditions. In the case of organic precursors, the production of said nanoparticles takes place through thermal decomposition of the organic moiety. The decomposition pathway of the precursor is greatly dependent on the type (i.e. inert, reducing or oxidizing) of the gaseous atmosphere prevailing during heating, as well as on the heating schedule itself. The characteristics of the organic group have also an important influence on the structure of the final material. The goal of the current work is to present a comprehensive review of the research work focusing on the synthesis of cobalt-based nanomaterials from activation of organic precursors.

**Keywords:** pyrolysis, carboxylates, thermal treatment, cobalt oxides, cobalt, Co3 O4 , nanoparticles, nanomaterials

#### **1. Introduction**

Nanostructured materials exhibit novel properties, which notably differ from those of the corresponding bulk solid, due to the small size effect [1]. Transition metal-oxide nanoparticles have attracted considerable interest on account of their applications in various fields, including catalysis, electronics, coatings, gas sensing, electrochemical devices, solar energy conversion and biomedicine [2, 3]. Several methods have been employed for the synthesis of cobalt nanomaterials, including: (i) microemulsion, (ii) solvothermal method, (iii) thermal decomposition of cobalt precursors, (iv) spray pyrolysis, (v) sol-gel process, etc. [1, 4, 5]. Thermal decomposition is a commonly employed method for preparation of nanoscale cobalt or cobalt

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

oxide, due to process simplicity, short reaction time, and, most importantly, low-cost and efficiency. Such nanoparticles are typically prepared via transformation of precursors under controlled conditions. There are many potential precursors, such as, inorganic and organic salts and metal-organic frameworks (MOFs). The decomposition pathway of the precursor depends on the type of the gaseous atmosphere prevailing during heating, on the heating schedule itself, as well as on the characteristics of the organic group. Co-addition of other metal cations toward a mixed metal-organic precursor provides a straightforward approach to the synthesis of mixed metal oxides. Additionally, supported cobalt-based nanomaterials (i.e. cobalt oxide/graphene composites) can be fabricated. The aim of the current work is to present a comprehensive review of the research focusing on synthesis of cobalt-based nanomaterials from activation of organic precursors.

#### **2. Cobalt and cobalt oxide nanoparticles from carboxylates**

A major group of organic precursors that have been employed for the synthesis of cobalt and cobalt oxide nanoparticles via a thermal decomposition route is cobalt-carboxylate salts. The "carboxylate" term includes several organic species that contain at least one carboxyl group, such as oxalate, citrate, etc. Methods for synthesis of cobalt-carboxylates include (i) sol-gel [2, 4], (ii) intimate mixing of cobalt ions [6–8], (iii) precipitation [9, 10], (iv) hot oxidation-redox reaction [11], (v) ball milling [12] and (vi) microemulsion [13]. Reaction of cobalt acetate with a carboxylic acid, for example, leads directly to the synthesis of cobalt-carboxylate, when the carboxylic acid in question is stronger than acetic acid: Co(Ac)2 + RCOOH → Co(RCOO)2 + 2 AcH (1). This cannot be the case when cobalt nitrate or cobalt chloride is employed, since nitrate and chloride anions are weak conjugate bases of the strong nitric and hydrochloric acids. Nevertheless, many research groups choose to utilize metal-inorganic salts to synthesize cobalt-carboxylate precursors. Thermal treatment of the synthesized carboxylate precursor leads to the removal of the organic species, thus cobalt or cobalt oxide nanoparticles are formed. In the following sections, a review of the synthetic protocols for synthesis of cobalt-based nanomaterials from cobalt-carboxylates is presented.

#### **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 Co3 O4 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 Co3 O4 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 Co3 O4 [4]. Luisetto et al. have also produced Co3 O4 by thermal treatment of cobalt oxalate precipitate at 450°C for 2 h in air [2]. de Rivas et al. prepared Co3 O4 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 Co3 O4 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 Co3 O4 by annealing cobalt oxalate precursors prepared via hydrothermal and rheological phase reaction methods [18, 19].

#### **2.2. Cobalt oxide nanoparticles from citrate precursors**

oxide, due to process simplicity, short reaction time, and, most importantly, low-cost and efficiency. Such nanoparticles are typically prepared via transformation of precursors under controlled conditions. There are many potential precursors, such as, inorganic and organic salts and metal-organic frameworks (MOFs). The decomposition pathway of the precursor depends on the type of the gaseous atmosphere prevailing during heating, on the heating schedule itself, as well as on the characteristics of the organic group. Co-addition of other metal cations toward a mixed metal-organic precursor provides a straightforward approach to the synthesis of mixed metal oxides. Additionally, supported cobalt-based nanomaterials (i.e. cobalt oxide/graphene composites) can be fabricated. The aim of the current work is to present a comprehensive review of the research focusing on synthesis of cobalt-based nano-

A major group of organic precursors that have been employed for the synthesis of cobalt and cobalt oxide nanoparticles via a thermal decomposition route is cobalt-carboxylate salts. The "carboxylate" term includes several organic species that contain at least one carboxyl group, such as oxalate, citrate, etc. Methods for synthesis of cobalt-carboxylates include (i) sol-gel [2, 4], (ii) intimate mixing of cobalt ions [6–8], (iii) precipitation [9, 10], (iv) hot oxidation-redox reaction [11], (v) ball milling [12] and (vi) microemulsion [13]. Reaction of cobalt acetate with a carboxylic acid, for example, leads directly to the synthesis of cobalt-carboxylate, when the carboxylic acid in question is stronger than acetic acid: Co(Ac)2 + RCOOH → Co(RCOO)2 + 2 AcH (1). This cannot be the case when cobalt nitrate or cobalt chloride is employed, since nitrate and chloride anions are weak conjugate bases of the strong nitric and hydrochloric acids. Nevertheless, many research groups choose to utilize metal-inorganic salts to synthesize cobalt-carboxylate precursors. Thermal treatment of the synthesized carboxylate precursor leads to the removal of the organic species, thus cobalt or cobalt oxide nanoparticles are formed. In the following sections, a review of the synthetic protocols for synthesis of cobalt-based nanomaterials from cobalt-carboxylates is

There are several publications concerning the synthesis of cobalt or cobalt oxide nanopar-

nation 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

solved 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

nanocrystallites. More specifically, cobalt acetate tetrahydrate was dis-

O4

O4

by thermal treatment of cobalt oxa-

O4

nanoparticles by calci-

nanoparticles via

materials from activation of organic precursors.

**2.1. Cobalt oxide nanoparticles from oxalate precursors**

ticles from oxalate precursors. Abdelkader et al. synthesized Co3

[4]. Luisetto et al. have also produced Co3

late precipitate at 450°C for 2 h in air [2]. de Rivas et al. prepared Co3

presented.

50 Cobalt

to produce Co3

to Co3 O4 O4

**2. Cobalt and cobalt oxide nanoparticles from carboxylates**

Manouchehri et al. reported on the synthesis of Co3 O4 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 solgel 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 Co3 O4 , respectively [13].

#### **2.5. Cobalt oxide nanoparticles from tartrate precursors**

Bhattacharjee et al. reported the synthesis of Co3 O4 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 Co3 O4 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. Co3 O4 nanoparticles were prepared by solid state thermal decomposition of Co(CO2 CH2 (OC2 H4 )2 OMe)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. Mixed metal-oxide nanoparticles from carboxylate precursors**

Mixed metal oxides contain two or more metallic ions in proportions that may either be defined by stoichiometry or variable. One way to classify mixed metal oxides is according to their crystalline structure (spinels, perovskites, etc.) [29].

#### **3.1. Spinel mixed metal-oxide nanoparticles from carboxylate precursors**

The preparation methods for CoFe2 O4 nanoparticles require specialized techniques to prevent agglomeration. Diodati et al. synthesized various spinel ferrites (CoFe2 O4 , NiFe2 O4 , ZnFe2 O4 and MgFe2 O4 ) 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 CoFe2 O4 /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, Co1−xZnx Fe2 O4 (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].

**2.5. Cobalt oxide nanoparticles from tartrate precursors**

**2.6. Cobalt oxide nanoparticles from other carboxylate precursors**

and an unsaturated dicarboxylic acid aqueous solution [9].

their crystalline structure (spinels, perovskites, etc.) [29].

The preparation methods for CoFe2

O4

nanoparticles by thermal treatment of cobalt tartrate synthesized by precipita-

sition 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

tion. 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

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. Co3

 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

nanoparticles were prepared by solid state thermal decomposition of Co(CO2

**3. Mixed metal-oxide nanoparticles from carboxylate precursors**

**3.1. Spinel mixed metal-oxide nanoparticles from carboxylate precursors**

O4

agglomeration. Diodati et al. synthesized various spinel ferrites (CoFe2

Mixed metal oxides contain two or more metallic ions in proportions that may either be defined by stoichiometry or variable. One way to classify mixed metal oxides is according to

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

) by a wet-synthesis coprecipitation route starting from iron(III) nitrate, cobalt(II)

nanoparticles require specialized techniques to prevent

O4 , NiFe2 O4

, ZnFe2

O4

nanoparticles by thermal decompo-

O4

CH2 (OC2 H4 )2

Bhattacharjee et al. reported the synthesis of Co3

and Co3

52 Cobalt

4 h [26].

OMe)2

and MgFe2

O4

O4

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 Cox Mn3−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 CoAl2 O4 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 CoCr2 O4 , 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 (MgCo2 O4 , NiCo2 O4 ) 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].

### **4. Supported cobalt oxide from carboxylate salts**

Transition metal oxides have been proposed as materials for lithium-ion batteries (LIBs) and it has been reported that the performance of cobalt oxides is further improved if cobalt oxide nanoparticles are supported on carbon (nanofibers, nanoflakes, etc.) [46]. From this point of view, Guo et al. synthesized Co3 O4 /graphite composites by precipitation of cobalt oxalate on the surface of graphite and pyrolysis of the precipitate [47].

#### **5. Cobalt-based nanomaterials from metal-organic frameworks (MOFs)**

A number of recent publications have reported on the synthesis of cobalt-based nanomaterials embedded in carbon, employing metal-organic framework compounds as precursors. Metal-organic frameworks (MOFs) are an emerging class of porous materials constructed from metal-containing nodes and organic linkers [46]. Wang et al. showed that the direct pyrolysis of cobalt nitrate accommodated by impregnation in the pores of ZIF-8 and ZIF-67 yields Co3 O4 nanoparticles [48]. Ashouri et al. utilized the thermal decomposition of cobaltterephthalate MOF precursor as a synthetic route for the fabrication of Co3 O4 nanoparticles [49]. Pei et al. synthesized carbon-supported and Si-doped carbon-supported Co nanomaterials by the thermolysis of Co-MOF-71 at 600°C for 8 h under He flow [50]. Many researchers have utilized MOFs as precursors for the synthesis of cobalt-based nanoparticles embedded in carbon. Zhou et al. prepared nitrogen-doped carbon-embedded Co catalysts by one-step pyrolysis (600–900°C, for 8 h, under N2 ) of ZIF-67 with silica as the hard template [51] while, Khan et al. used direct carbonization of ZIF-12 at 950°C for 6 h under Ar atmosphere [52]. Lu et al. fabricated metallic Co nanoparticles embedded in N-doped porous carbon layers by thermal treatment of macroscale Co-MOFs obtained by using 1,3,5-benzenetricarboxylic acid, triethylamine and nonanoic acid, at various temperatures (800, 900, 1000°C) for 1 h under N2 atmosphere [53]. Qiu et al. obtained cobalt-based nanoparticles coated with a thin carbon shell from Co-MOF-74 via a two-step carbonization where at first the cobalt precursor was heated at 700°C for 2 h under Ar atmosphere to produce cobalt nanoparticles coated with carbon shell and in a second phase the as-synthesized material was calcined either at 500°C for 1 h under CO2 atmosphere to produce carbon-coated Co3 O4 nanoparticles or for 30 min under the same conditions to produce Co/CoO nanomaterials. Pure Co3 O4 nanocrystals were obtained by treating carbon-coated cobalt nanoparticles at 250°C for 2 h under air atmosphere [54].

#### **6. Structure and characterization of cobalt-based nanoparticles**

#### **6.1. Structure and characterization of Co3 O4 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 Co3 O4 phase. The published X-Ray Diffraction (XRD) patterns have reflection peaks that can be perfectly indexed to the cubic phase of Co3 O4 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 Co3 O4 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 Co3 O4 have been investigated by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). SEM results indicated the agglomeration of Co3 O4 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 Co3 O4 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 Co3 O4 [2, 4, 18, 25]. HRTEM analysis also indicated the single crystalline nature of Co3 O4 [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 Co3 O4 nanoparticles

**Figure 1.** Typical XRD pattern of pure Co3 O4 [17].

**4. Supported cobalt oxide from carboxylate salts**

the surface of graphite and pyrolysis of the precipitate [47].

O4

terephthalate MOF precursor as a synthetic route for the fabrication of Co3

view, Guo et al. synthesized Co3

yields Co3

54 Cobalt

under N2

O4

500°C for 1 h under CO2

atmosphere [54].

pyrolysis (600–900°C, for 8 h, under N2

**6.1. Structure and characterization of Co3**

Transition metal oxides have been proposed as materials for lithium-ion batteries (LIBs) and it has been reported that the performance of cobalt oxides is further improved if cobalt oxide nanoparticles are supported on carbon (nanofibers, nanoflakes, etc.) [46]. From this point of

**5. Cobalt-based nanomaterials from metal-organic frameworks (MOFs)**

A number of recent publications have reported on the synthesis of cobalt-based nanomaterials embedded in carbon, employing metal-organic framework compounds as precursors. Metal-organic frameworks (MOFs) are an emerging class of porous materials constructed from metal-containing nodes and organic linkers [46]. Wang et al. showed that the direct pyrolysis of cobalt nitrate accommodated by impregnation in the pores of ZIF-8 and ZIF-67

[49]. Pei et al. synthesized carbon-supported and Si-doped carbon-supported Co nanomaterials by the thermolysis of Co-MOF-71 at 600°C for 8 h under He flow [50]. Many researchers have utilized MOFs as precursors for the synthesis of cobalt-based nanoparticles embedded in carbon. Zhou et al. prepared nitrogen-doped carbon-embedded Co catalysts by one-step

Khan et al. used direct carbonization of ZIF-12 at 950°C for 6 h under Ar atmosphere [52]. Lu et al. fabricated metallic Co nanoparticles embedded in N-doped porous carbon layers by thermal treatment of macroscale Co-MOFs obtained by using 1,3,5-benzenetricarboxylic acid, triethylamine and nonanoic acid, at various temperatures (800, 900, 1000°C) for 1 h

carbon shell from Co-MOF-74 via a two-step carbonization where at first the cobalt precursor was heated at 700°C for 2 h under Ar atmosphere to produce cobalt nanoparticles coated with carbon shell and in a second phase the as-synthesized material was calcined either at

atmosphere to produce carbon-coated Co3

tals were obtained by treating carbon-coated cobalt nanoparticles at 250°C for 2 h under air

30 min under the same conditions to produce Co/CoO nanomaterials. Pure Co3

**6. Structure and characterization of cobalt-based nanoparticles**

**O4**

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

atmosphere [53]. Qiu et al. obtained cobalt-based nanoparticles coated with a thin

 **nanoparticles**

nanoparticles [48]. Ashouri et al. utilized the thermal decomposition of cobalt-

/graphite composites by precipitation of cobalt oxalate on

) of ZIF-67 with silica as the hard template [51] while,

O4

O4

nanoparticles

nanoparticles or for

nanocrys-

O4

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 Co3 O4 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 Co3 O4 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 Co3 O4 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 Co3 O4 spinel [12, 17, 23, 24, 48, 49]. Raman spectroscopy has also been utilized for the confirmation of Co3 O4 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, Eg, F2 2g, F3 2g and A1g modes of crystalline Co3 O4 (**Figure 2b**). F2g and Eg modes are associated with the vibration of tetrahedral and octahedral sites, whereas the high-frequency band, A1g 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 Co2p spectrum is decomposed into two distinct peaks and two weak satellites (**Figure 3a**). The two spin orbit

**Figure 2.** Typical (a) FTIR [17] and (b) Raman [22] spectra of Co3 O4 nanostructures.

doublets assigned to cobalt oxides located at 780.8 and 795.9 eV can correspond to Co2p(3/2) and Co2p(1/2), respectively. The spin orbit doublet of Co2p(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 Co2p(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 Co2p(3/2) and Co2p(1/2) splitting is characteristic of the Co3 O4 cubic phase. The presence of the two satellite peaks (Cosat.) 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 O1s presented in **Figure 3b** demonstrates a peak at 530.0 eV attributed to lattice oxygen of Co3 O4 (Olatt., 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 (Oadsorbates) and apparently is not always present in O1s spectra [22, 55].

Reducibility and acidity-basicity are important properties affecting material behavior in a specific application. Temperature programmed reduction (TPR) of Co3 O4 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 Co3 O4 nanoparticles (particle size and faceting) and TPR protocols [2, 14, 20–22]. The acidic properties of Co3 O4 nanoparticles have been studied by NH3 -TPD. Abdelkader et al. performed NH3 -TPD on Co3 O4 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 Co3 O4 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

**Figure 3.** XPS spectra of (a) Co2p region and (b) O1s region [55].

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 prop-

the pore volume ranges between 0.03 [23] and 0.24 cm3 g−1 [20] while average pore size is

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.

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

23, 24, 48, 49]. Raman spectroscopy has also been utilized for the confirmation of Co3

formation. Typically, five Raman bands at 198, 484, 522, 622 and 694 cm−1 are visible in

of tetrahedral and octahedral sites, whereas the high-frequency band, A1g mode, is linked

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 Co2p spectrum is decomposed into two distinct peaks and two weak satellites (**Figure 3a**). The two spin orbit

O4

nanostructures.

spinels possessing a bimodal mesopore distribution in the presence of

) (**Figure 2a**). The first band (ν1

(**Figure 2b**). F2g and Eg modes are associated with the vibration

nanoparticles vary from 7 [23] to 120 m2 g−1 [21],

sorption isotherms of Co3

) is associated with

spinel [12, 17,

2g and A1g

O4

) is attributed to the Co2+

2g, F3

O4

2g, Eg, F2

O4

O4

between 9 [20] and 68 nm [23]. Wang et al. found that all N2

) and ~670 cm−1 (ν2

the Co3+ vibration in the octahedral hole and the second band (ν2

the range 100–800 cm−1, which correspond, respectively, to the F1

O4

**Figure 2.** Typical (a) FTIR [17] and (b) Raman [22] spectra of Co3

to the occurrence of octahedral sites [14, 15, 20–22, 25].

vibration in the tetrahedral hole, which confirms the formation of the Co3

erties. The specific surface areas of Co3

O4

synthesized Co3

56 Cobalt

bands at ~570 cm−1 (ν1

modes of crystalline Co3

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 F3 3m [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 Co3 O4 nanoparticles was accompanied by the formation of CoO or metallic cobalt as impurities. Pursuant to XRD results face-centered cubic (space group: F3 3m) [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 Co3 O4 [9].

#### **6.4. Structure and characterization of mixed metal-oxide nanoparticles**

Dippong et al. synthesized CoFe2 O4 nanoparticles and concluded that single phase cubic spinel CoFe2 O4 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 CoFe2 O4 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 CoFe2 O4 phase, confirmed its formation, while, TEM results revealed—in accordance with XRD—the presence of CoFe2 O4 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 CoFe2 O4 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 CoFe2 O4 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 CoFe2 O4 embedded in SiO2 and according to XRD results the formation of CoFe2 O4 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 CoFe2 O4 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 CoFe2 O4 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 CoFe2 O4 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 CoFe2 O4 with a crystallite size of 7 nm. The formation of the CoFe2 O4 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 Co1-xZnx Fe2 O4 . Muntean et al. concluded by means of XRD that the as-synthesized nanomaterials consisted of the cubic spinel phase Co1-xZnx Fe2 O4 and impurities (CoFe2 O4 , ZnFe2 O4 ) except for the thermal decomposition at 1000°C, where Co0.5Zn0.5Fe2 O4 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].

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

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

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

CoO or metallic cobalt as impurities. Pursuant to XRD results face-centered cubic (space

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

whereas, when initiating from 1,2-ethanediol and 1,2-propanediol elevated temperatures

tion temperature. The appearance of the intense, symmetric band at 560–585 cm−1 in the

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

on sample composition (Fe and Co content) and calcination temperature. The diameter

O4

3m) [23], monoclinic (space group: C2/m) [23] and tetragonal [9] crystal systems

3m [12]. In the FTIR spectrum,

nanoparticles was accompanied by the formation of

nanoparticles and concluded that single phase cubic

crystallization increased with the increase of calcina-

nanoparticles and proved the dependence of nanoparticle size

phase, confirmed its formation, while, TEM

O4

as nearly spherical

can be produced at all temperatures initiating from 1,3-propanediol

voltammetry (CV) [17, 18] and electrochemical impedance spectroscopy (EIS) [17].

**6.2. Structure and characterization of CoO nanoparticles**

cubic phase with cell parameter α = 4.26 Å and space group F3

**6.3. Structure and characterization of metallic Co nanoparticles**

**6.4. Structure and characterization of mixed metal-oxide nanoparticles**

O4

O4

results revealed—in accordance with XRD—the presence of CoFe2

O4

[27]. In some cases, synthesis of Co3

Dippong et al. synthesized CoFe2

are required. The degree of CoFe2

FTIR spectra, characteristic of the CoFe2

O4

O4

the oxide phases in CoFe2

group: F3

58 Cobalt

Co3 O4 [9].

spinel CoFe2

were reported.

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 Cox Mn3-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 (SBET = 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 (CoAl2 O4 ) nanomaterials were investigated by means of XRD, FTIR, TEM and SEM. Wang et al. synthesized CoIICoIII x Al2−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 CoAl2 O4 (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 CoCr2 O4 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 CoCr2 O4 obtained from tartrate was in the range 13–35 nm (depending on annealing temperature), while CoCr2 O4 obtained from gluconate was in the range 16–28 nm showing in each case well-faceted pyramidal or bipyramidal geometry. CoCr2 O4 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 CoCr2 O4 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 CoCr2 O4 were SBET = 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 CoCr2 O4 were 17–42 m2 g−1, 0.15–0.21 cm3 g−1 and 21–34 nm, respectively [41]. Darbar et al. synthesized MgCo2 O4 by oxalate decomposition and XRD patterns were indexed to the standard cubic structure with Fd3m space group. The SEM images revealed the agglomeration of MgCo2 O4 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 NiCo2 O4 aggregates by oxalate decomposition and the XRD pattern confirmed the formation of cubic spinel NiCo2 O4 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].

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

(0 ≤ x ≤ 3) by thermal decomposition of oxalate precursor and concluded that

(x = 0–2) and reported that nanomaterials had spinel cubic

obtained from tartrate was in the range

obtained from gluconate

O4

O4

) nano-

O4

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 (SBET = 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

materials were investigated by means of XRD, FTIR, TEM and SEM. Wang et al. syn-

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 CoAl2

(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

nate 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

O4

was in the range 16–28 nm showing in each case well-faceted pyramidal or bipyramidal

age 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

nanomaterials synthesized by thermal decomposition of tartrate and gluco-

from gluconate precursor formed spherical aggregates with an aver-

The structure and physicochemical characteristics of cobalt aluminate (CoAl2

Cox

60 Cobalt

Mn3-xO4

and pore volume [39].

x Al2−xO4

TEM micrographs, the particle size of CoCr2

O4

13–35 nm (depending on annealing temperature), while CoCr2

thesized CoIICoIII

O4

geometry. CoCr2

of CoCr2

#### **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 (Co3 O4 , 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 (SBET = 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].

#### **7. Applications of cobalt-based nanoparticles**

The structure and physicochemical characteristics of cobalt-based nanomaterials make them suitable candidates as catalysts for various processes, that is, alcohol reforming, Fischer-Tropsch, oxidation of organic compounds and CO, as oxygen electrocatalysts and as electrode materials in lithium-ion batteries.

There has been a significant effort to decrease the dependence on non-renewable fossil fuels and move toward sustainable energy carriers, such as hydrogen derived from renewable sources. H2 is considered to be the energy carrier of the future and the use of liquid bio-fuels, such as ethanol and methanol, as hydrogen carriers is an attractive option. Thus, H2 production from alcohol steam reforming has been of great interest and has been extensively investigated. Since alcohol reforming is a catalytic process, various catalytic systems have been tested aiming at optimization of their efficiency. Among them, cobalt-based catalysts are promising candidates. Papadopoulou et al. prepared Co-MnO catalysts by pyrolytic decomposition of the corresponding fumarate and gluconate salts and examined them in methanol and ethanol reforming. They concluded that catalysts with the highest cobalt loading were the most active both in methanol and ethanol reforming. The utilization of fumarate-derived Co-MnO catalysts led to complete methanol conversion at 375–400°C (high-temperature methanol reforming compared to copper-based catalysts), while complete ethanol conversion was achieved at 480°C (lowtemperature ethanol reforming compared to noble-metal catalysts). Thus, reforming of both alcohols took place under comparable conditions at temperatures in the range of 400–450°C. The advantages of the proposed method of catalyst preparation were the formation of the reduced active state of the catalysts in a single step and the existence of residual carbon, which hindered sintering and excessive particle growth under synthesis and reaction conditions [6–8]. Abdelkader et al. studied the catalytic activity of Co3 O4 , Fe2 O3 and corresponding mixed Co-Fe in the steam reforming of ethanol and concluded that the mixed Co-Fe sample exhibited higher H2 yield, greater selectivity to CO and CO2 and reduced by-product formation compared to pure Co3 O4 , Fe2 O3 and physical Co-Fe mixture [14].

Rechargeable lithium-ion batteries (LIBs) are the most utilized energy storage devices for portable electronics and one of the most promising electrode materials are cobalt-based nanomaterials in place of carbon. Novel porous Co3 O4 architectures [18, 19], MgCo2 O4 nanomaterials [42], Co3 O4 /graphite composites [47], cobalt-based nanoparticles embedded in nitrogen-doped nanotubes [52] and carbon-coated Co3 O4 nanoparticles [54] have been studied regarding their electrochemical performance. Briefly, Yuan et al. reported on the performance of Co3 O4 polyhedral architectures and showed that they exhibited high discharge capacities for many discharge/charge cycles, due to their porous structure [18]. Yuan et al. synthesized nanosized Co3 O4 and concluded that the advantage of this material over carbon was due to its higher capacity per unit volume (7.5 times in comparison to carbon) and that the particle size affected its electrochemical properties (the optimum average particle size was 37 nm) [19]. Guo et al. studied the electrochemical properties of Co3 O4 /graphite composites and showed that the reversible capacity increased with decrease in graphite content and increase in the calcination temperature, while cycling stability decreased dramatically with decrease in graphitic content [47]. Khan et al. reported on the excellent performance of cobalt oxide nanoparticles embedded in nitrogen-doped carbon nanotubes, which is attributed to the nitrogen doping of carbon nanotubes, the strong interaction between the encapsulated cobalt oxide nanoparticles with the carbon nanotubes, the porosity and the specific surface area of the nanomaterial [52]. In addition, Qiu et al. investigated the electrochemical performance of carbon-coated Co3 O4 nanoparticles in comparison to bare Co3 O4 electrode and concluded that the superior electrode performance of the first was attributed to better dispersion and to the thin carbon shell coating of the nanoparticles on the electrode surface [54].

nanotubes with an average particle size of 50 nm [52]. N2

teries and as electrocatalysts [47, 52–54].

materials in lithium-ion batteries.

renewable sources. H2

option. Thus, H2

Fe2 O3

mixture [14].

**7. Applications of cobalt-based nanoparticles**

that the mixed Co-Fe sample exhibited higher H2

and reduced by-product formation compared to pure Co3

observed a N2

62 Cobalt

therms 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 (SBET = 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.

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 bat-

The structure and physicochemical characteristics of cobalt-based nanomaterials make them suitable candidates as catalysts for various processes, that is, alcohol reforming, Fischer-Tropsch, oxidation of organic compounds and CO, as oxygen electrocatalysts and as electrode

There has been a significant effort to decrease the dependence on non-renewable fossil fuels and move toward sustainable energy carriers, such as hydrogen derived from

of liquid bio-fuels, such as ethanol and methanol, as hydrogen carriers is an attractive

has been extensively investigated. Since alcohol reforming is a catalytic process, various catalytic systems have been tested aiming at optimization of their efficiency. Among them, cobalt-based catalysts are promising candidates. Papadopoulou et al. prepared Co-MnO catalysts by pyrolytic decomposition of the corresponding fumarate and gluconate salts and examined them in methanol and ethanol reforming. They concluded that catalysts with the highest cobalt loading were the most active both in methanol and ethanol reforming. The utilization of fumarate-derived Co-MnO catalysts led to complete methanol conversion at 375–400°C (high-temperature methanol reforming compared to copper-based catalysts), while complete ethanol conversion was achieved at 480°C (lowtemperature ethanol reforming compared to noble-metal catalysts). Thus, reforming of both alcohols took place under comparable conditions at temperatures in the range of 400–450°C. The advantages of the proposed method of catalyst preparation were the formation of the reduced active state of the catalysts in a single step and the existence of residual carbon, which hindered sintering and excessive particle growth under synthesis and reaction conditions [6–8]. Abdelkader et al. studied the catalytic activity of Co3

and corresponding mixed Co-Fe in the steam reforming of ethanol and concluded

is considered to be the energy carrier of the future and the use

production from alcohol steam reforming has been of great interest and

adsorption isotherm of type V with a H3-type hysteresis loop, while the

adsorption-desorption iso-

O4 ,

and physical Co-Fe

yield, greater selectivity to CO and CO2

O4 , Fe2 O3 Lu et al. showed that the utilization of Co-MOFs for the fabrication of metallic Co nanoparticles embedded in nitrogen-doped porous carbon layers led to the production of an efficient electrocatalyst with bifunctional activities toward oxygen reduction and evolution reactions (ORR, OER) in alkaline media due to nitrogen doping and embedded cobalt nanoparticles in the carbon structure [53].

Concerning environmental catalysis, the strict environmental legislation for pollutant emissions and the high-cost of noble-metal catalysts has shifted interest toward the production of effective transition metal-oxide catalysts. Cobalt-based nanomaterials, such as nanocrystalline Co3 O4 , Co-Ce composite oxide, CoFe2 O4 and Co-Mn oxide spinels have been synthesized by thermal decomposition of organic precursors and studied in various pollutant abatement processes. de Rivas et al. have examined nanocrystalline Co3 O4 catalysts in the gas-phase oxidation of chlorinated short chain alkanes and reported on their enhanced catalytic behavior as they exhibited high conversion to deep oxidation products (CO2 , HCl, Cl2 ) at low temperatures with excellent selectivity to CO2 and resistance to deactivation [15, 20]. Similar experiments have been conducted by Wang et al. using Co-Ce catalysts [22]. Liu et al. have demonstrated the excellent activity and stability of Co3 O4 -based catalysts for propane catalytic combustion [21]. Faure et al. synthesized Co-Mn-oxide spinel catalysts and concluded that their high activity for CO oxidation was correlated to both their surface area and cobalt concentration. Among the synthesized nanomaterials, Co2,3Mn0,7O4 exhibited higher activity than cobalt oxide catalysts [39]. Co3 O4 nanoparticles [48] and LaCoO3 nano-perovskite [44] catalysts were also investigated for CO oxidation reaction and exhibited good structural and chemical stability and high activity. Additionally, Ashouri et al. employed MOF-derived Co3 O4 for the catalytic oxidation and epoxidation of olefins and reported that the catalyst showed good catalytic stability and reusability [49]. Diodati et al. synthesized and used CoFe2 O4 in the catalytic oxidation of methane and concluded that the catalytic reaction took place at lower temperature (by about 300 K) in comparison to the uncatalyzed reaction while the cobalt ferrite catalyst exhibited good stability [30]. Gingasu et al. synthesized cobalt chromite catalysts via thermal decomposition of tartrate and gluconate precursors and reported that gluconate-derived CoCr2 O4 was the best catalyst for total oxidation of methane exhibiting high activity and CO2 selectivity [41]. Co nanoparticles embedded in mesoporous nitrogen-doped carbon were investigated by Zhou et al. for the reductive amination of carbonyl compounds with nitro compounds by transfer hydrogenation with formic acid and found them to be active and selective with excellent yields for secondary amines [51]. Another reported application of cobalt-based nanoparticles is in water treatment for the removal of dye pollutants. Advanced oxidation processes and especially Fenton reactions are considered to be among the most feasible pollutant degradation technologies [16, 55].

One of the most significant applications of cobalt nanomaterials in catalysis, is in Fischer-Tropsch synthesis. The Fischer-Tropsch process targets the production of synthetic liquid fuels using coal, natural gas and biomass and hydrogen derived from renewable electricity as feedstocks. Cobalt-based catalysts have been employed to a great extent since they have high selectivity for long chain hydrocarbons and low selectivity for the water-gas shift reaction and they are cost-effective. The reducibility, cobalt dispersion and loading and nanoparticle structure are some of the parameters that affect the catalytic activity and selectivity. Pei et al. synthesized fully reduced carbon-supported cobalt catalysts by pyrolysis of Co-MOF-71 precursor and showed that the as-prepared catalysts had extremely high Co site density, high activity and selectivity with diesel fuels being the main products and high C5+ space-time yields in comparison to conventional cobalt catalysts [50].

#### **8. Conclusions**

Nanostructured cobalt-based materials have attracted considerable interest due to their multiple applications in various fields, like catalysis, electronics, electrochemical devices, etc. Among the synthesized cobalt nanomaterials, Co3 O4 is by far the main nanostructure been reported in the literature, whereas fewer studies have been made regarding CoO, metallic Co, mixed metal-oxide nanoparticles or other types of cobalt-based nanomaterials. A great number of methods have been employed for their synthesis with the thermal decomposition of organic precursors bearing noteworthy advantages as, for instance, simplicity, short processing time and cost-effectiveness. Carboxylate salts (e.g. oxalate, citrate, etc.) as well as metal-organic frameworks (MOFs) are utilized as organic precursors for the production of cobalt oxides, metallic cobalt, mixed cobalt oxides and supported cobalt-based nanomaterials. The nature of organic precursor, the precursor synthesis method, the cobalt content, the combination with other elements, the treatment schedule and temperature and the type of the gaseous atmosphere (oxidizing, inert, reducing) prevailing during heating are some of the factors that affect the physical and chemical properties of the final composite. The relevant nanomaterials have been extensively characterized and analyzed using various techniques (XRD, FTIR, SEM, TEM, XPS, Raman, etc.) and accounts have been reported on their novel properties in comparison to bulk solids. A non-exhaustive list of potential applications of such cobalt-based nanomaterials includes catalysts for Fischer-Tropsch process, alcohol reforming, oxidation of organic compounds and CO, oxygen electrocatalysts and anode materials for Li-ion batteries.

## **Author details**

investigated for CO oxidation reaction and exhibited good structural and chemical stability and high activity. Additionally, Ashouri et al. employed MOF-derived Co3

the catalytic oxidation and epoxidation of olefins and reported that the catalyst showed good catalytic stability and reusability [49]. Diodati et al. synthesized and used CoFe2

in the catalytic oxidation of methane and concluded that the catalytic reaction took place at lower temperature (by about 300 K) in comparison to the uncatalyzed reaction while the cobalt ferrite catalyst exhibited good stability [30]. Gingasu et al. synthesized cobalt chromite catalysts via thermal decomposition of tartrate and gluconate precursors and

in mesoporous nitrogen-doped carbon were investigated by Zhou et al. for the reductive amination of carbonyl compounds with nitro compounds by transfer hydrogenation with formic acid and found them to be active and selective with excellent yields for secondary amines [51]. Another reported application of cobalt-based nanoparticles is in water treatment for the removal of dye pollutants. Advanced oxidation processes and especially Fenton reactions are considered to be among the most feasible pollutant degradation

One of the most significant applications of cobalt nanomaterials in catalysis, is in Fischer-Tropsch synthesis. The Fischer-Tropsch process targets the production of synthetic liquid fuels using coal, natural gas and biomass and hydrogen derived from renewable electricity as feedstocks. Cobalt-based catalysts have been employed to a great extent since they have high selectivity for long chain hydrocarbons and low selectivity for the water-gas shift reaction and they are cost-effective. The reducibility, cobalt dispersion and loading and nanoparticle structure are some of the parameters that affect the catalytic activity and selectivity. Pei et al. synthesized fully reduced carbon-supported cobalt catalysts by pyrolysis of Co-MOF-71 precursor and showed that the as-prepared catalysts had extremely high Co site density, high activity and selectivity with diesel fuels being the main products and high C5+ space-time yields in comparison to conventional cobalt

Nanostructured cobalt-based materials have attracted considerable interest due to their multiple applications in various fields, like catalysis, electronics, electrochemical devices,

ture been reported in the literature, whereas fewer studies have been made regarding CoO, metallic Co, mixed metal-oxide nanoparticles or other types of cobalt-based nanomaterials. A great number of methods have been employed for their synthesis with the thermal decomposition of organic precursors bearing noteworthy advantages as, for instance, simplicity, short processing time and cost-effectiveness. Carboxylate salts (e.g. oxalate, citrate, etc.) as well as metal-organic frameworks (MOFs) are utilized as organic precursors for the production of cobalt oxides, metallic cobalt, mixed cobalt oxides and

O4

is by far the main nanostruc-

etc. Among the synthesized cobalt nanomaterials, Co3

O4

reported that gluconate-derived CoCr2

technologies [16, 55].

64 Cobalt

catalysts [50].

**8. Conclusions**

methane exhibiting high activity and CO2

O4 for

was the best catalyst for total oxidation of

selectivity [41]. Co nanoparticles embedded

O4

Maria Smyrnioti1,2 and Theophilos Ioannides1 \*

\*Address all correspondence to<EMAIL_ADDRESS>
1 Foundation for Research & Technology – Hellas, Institute of Chemical Engineering Sciences (FORTH/ICE-HT), Patras, Greece

2 Department of Chemistry, University of Patras, Greece

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#### **Hydrothermally Produced Cobalt Oxide Nanostructures at Different Temperatures and Effect on Phase Transition Temperature and Threshold Voltage of Nematic Liquid Crystal Host** Hydrothermally Produced Cobalt Oxide Nanostructures at Different Temperatures and Effect on Phase Transition Temperature and Threshold Voltage of Nematic Liquid Crystal Host

DOI: 10.5772/intechopen.70946

Hasan Eskalen, Süleyman Kerli and Şükrü Özgan Hasan Eskalen, Süleyman Kerli and

Additional information is available at the end of the chapter Şükrü Özgan

http://dx.doi.org/10.5772/intechopen.70946 Additional information is available at the end of the chapter

#### Abstract

In this study, cobalt oxide Co3O4 nanostructured material is synthesized by hydrothermal method by using different concentration of cobalt acetate salt at unique hydrothermal reaction time for five different hydrothermal reaction temperatures 105, 120, 140, 160, and 180C. The obtained nanoparticles are annealed at 300C for 5 h. Co3O4 nanostructures are determined by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV-vis spectroscopy. The hydrothermally produced samples were reannealed at 550C, and morphological and structural properties were investigated deeply again. The effect of annealing temperature on morphologies and crystalline structure of cobalt oxide nanoparticle (NP) was also investigated. Nanopyramids and nanorods are two main morphologically obtained structures from the hydrothermal experiment. Nematic liquid crystal mixture E7 is doped with Co3O4 nanorod. Phase transition and threshold voltage of pure and Co3O4 NP-doped E7 LC are examined successfully. It reveals that for Co3O4 NP-doped E7 phase transition temperature and threshold voltage increased very slightly.

Keywords: cobalt oxide, hydrothermal synthesis, band gap, threshold voltage, thermal property

#### 1. Introduction

"There is Plenty of Room at the Bottom" a titled lecture in 1959 was given by Richard Feynman, and this has opened a new era in the field of nanotechnology [1, 2], which can be

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

defined as the engineering of functional systems at the molecular level [3]. Nanoscience and nanotechnology have been growing rapidly due to the fact that nanomaterials are synthesized with new strategies and those synthesized materials are characterized and manipulated with new tools [4]. Nanostructures can be classified into three different categories according to their size and shapes: zero dimensional (0D), one dimensional (1D), and two dimensional (2D) [5, 6]. Nanodots-nanospheres, nanowires-nanorods, and nanosheets-nanoplates are some examples of different shapes and size nanomaterials for this classification. Nanomaterials, unlike to conventional materials, can be physically and chemically manipulated and used in different areas [7, 8] including biology and medicine [9–11], water treatment [12], electronics [13, 14], and optics [15–17].

Magnetic fields more or less impact a certain subclass of NP, which is called as magnetic nanoparticles (MNPs) [18, 19]. MNPs have their own special properties, such as superparamagnetism [20], high mass transference [21], and high field irreversibility [22]. The second most popular MNP, considering to application areas, is cobalt oxide (Co3O4) since these nanoparticles are used in various fields from micro-electronics to drug delivery [7]. Co3O4 is one of the significant transition metal oxides, and the direct optical band gap of bulk Co3O4 is 2.19 eV.

Dispersion of nanomaterials into liquid crystals (LCs) has been a topic of great interest in recent years. There are many distinguished researchers investigating liquid crystals, and some of whose works are focused on doping nanomaterials into liquid crystals [23–29]. Gold, silver, zinc oxide, carbon, and quantum dots nanoparticles are some of the examples of guest nanoparticles. Co3O4 nanomaterials doped nematic liquid crystals (NLC) have only been investigated for spherical morphological Co3O4 nanoparticles to our knowledge [30].

Researchers have focused on synthesizing Co3O4 nanoparticles not only with diverse morphology but also with different methods since any change in production methods, particle size, shape, and structure of Co3O4 nanomaterials could lead producing new nanomaterials for potential applications with unique properties. Up to now, Co3O4 nanoparticles with various morphologies such as nanospheres [31, 32], nanorods [33–36], nanowalls [37], nanoneedles [38], nanobelts [39], nanowires [40], nanoflowers [41], nanotubes [42, 43], nanofibers [44], nanodiscs [45], and nanochains [46] have been synthesized with different approaches, including urea precipitation [47], chemical vapor deposition [48], sol-gel [49], microwave-assisted process [50], wet chemical approach [51], and hydrothermal method [39, 52–54]. Hydrothermal methods are widely used to produce various different Co3O4 nanomaterials. Easily scaling up to industrial demand, requiring low temperature, no need for calcination, being inexpensive, and having a fairly uniform particle size and morphology are some of the advantages of this nanoparticle production method [55]. The term hydrothermal is used when water is used as a solvent, and solvothermal is used when organics are used as solvent [56]. In literature, the effect of hydrothermal reaction times and reaction temperature on morphologies of Co3O4 nanoparticles was examined [57–59].

In this study, new morphologies of Co3O4 have been synthesized by hydrothermal method using Cobalt(II) chloride hexahydrate precursor at different unique temperatures and hydrothermal reaction times. The influence of hydrothermal temperatures on crystal structures and particle morphologies was examined deeply. Five different hydrothermal temperature points were selected between 105 and 180C, and the effect of annealing temperatures was also investigated. The obtained particles were firstly annealed at 300C for 5 h and then reannealed 500C. The prepared Co3O4 NPs were characterized by XRD, SEM, and UV-vis. Moreover, we selected Co3O4 NPs obtained at 120C hydrothermal reaction temperature to investigate how nanorod morphological Co3O4 NPs affect nematic liquid crystal phase transition and threshold voltages.

#### 2. Materials and methods

#### 2.1. Synthesis of Co3O4 NPs

defined as the engineering of functional systems at the molecular level [3]. Nanoscience and nanotechnology have been growing rapidly due to the fact that nanomaterials are synthesized with new strategies and those synthesized materials are characterized and manipulated with new tools [4]. Nanostructures can be classified into three different categories according to their size and shapes: zero dimensional (0D), one dimensional (1D), and two dimensional (2D) [5, 6]. Nanodots-nanospheres, nanowires-nanorods, and nanosheets-nanoplates are some examples of different shapes and size nanomaterials for this classification. Nanomaterials, unlike to conventional materials, can be physically and chemically manipulated and used in different areas [7, 8] including biology and medicine [9–11], water treatment [12], electronics [13, 14],

Magnetic fields more or less impact a certain subclass of NP, which is called as magnetic nanoparticles (MNPs) [18, 19]. MNPs have their own special properties, such as superparamagnetism [20], high mass transference [21], and high field irreversibility [22]. The second most popular MNP, considering to application areas, is cobalt oxide (Co3O4) since these nanoparticles are used in various fields from micro-electronics to drug delivery [7]. Co3O4 is one of the significant transition metal oxides, and the direct optical band gap of bulk Co3O4 is 2.19 eV.

Dispersion of nanomaterials into liquid crystals (LCs) has been a topic of great interest in recent years. There are many distinguished researchers investigating liquid crystals, and some of whose works are focused on doping nanomaterials into liquid crystals [23–29]. Gold, silver, zinc oxide, carbon, and quantum dots nanoparticles are some of the examples of guest nanoparticles. Co3O4 nanomaterials doped nematic liquid crystals (NLC) have only been

Researchers have focused on synthesizing Co3O4 nanoparticles not only with diverse morphology but also with different methods since any change in production methods, particle size, shape, and structure of Co3O4 nanomaterials could lead producing new nanomaterials for potential applications with unique properties. Up to now, Co3O4 nanoparticles with various morphologies such as nanospheres [31, 32], nanorods [33–36], nanowalls [37], nanoneedles [38], nanobelts [39], nanowires [40], nanoflowers [41], nanotubes [42, 43], nanofibers [44], nanodiscs [45], and nanochains [46] have been synthesized with different approaches, including urea precipitation [47], chemical vapor deposition [48], sol-gel [49], microwave-assisted process [50], wet chemical approach [51], and hydrothermal method [39, 52–54]. Hydrothermal methods are widely used to produce various different Co3O4 nanomaterials. Easily scaling up to industrial demand, requiring low temperature, no need for calcination, being inexpensive, and having a fairly uniform particle size and morphology are some of the advantages of this nanoparticle production method [55]. The term hydrothermal is used when water is used as a solvent, and solvothermal is used when organics are used as solvent [56]. In literature, the effect of hydrothermal reaction times and reaction temperature on morphologies of Co3O4

In this study, new morphologies of Co3O4 have been synthesized by hydrothermal method using Cobalt(II) chloride hexahydrate precursor at different unique temperatures and hydrothermal reaction times. The influence of hydrothermal temperatures on crystal structures and particle morphologies was examined deeply. Five different hydrothermal temperature points were selected between 105 and 180C, and the effect of annealing temperatures was also

investigated for spherical morphological Co3O4 nanoparticles to our knowledge [30].

and optics [15–17].

72 Cobalt

nanoparticles was examined [57–59].

All chemicals were used without further purification. A schematic diagram of the synthesis step is given in Figure 1. For the synthesis of the samples, 4.3983 g Cobalt(II) chloride hexahydrate

Figure 1. Schematic diagram for the synthesis procedure of Co3O4.

and 0.12 g Urea (CH4N2O) solution were mixed with 50 ml distilled water. The prepared solution was placed in the autoclave, and it waited in the furnace at 105, 120, 140, 160, and 180C for 6 h. It was precipitated and washed several times with distilled water and dried at 60C for 10 h. The obtained particles were annealed for 5 h at 300C. Moreover, all obtained samples were also annealed at 500C for 1 h to compare the effects of different annealing temperatures on XRD and SEM results of obtained Co3O4 samples.

#### 2.2. Liquid crystal experiment

The LC used was commercially available eutectic mixture E7 (SYNTHON Chemicals, Germany). The nematic to isotropic transition temperature for pure E7 is measured by using polarizing optical microscope integrated with hot stage, and it is found 58.6C. The liquid crystal cells were made by indium-tin oxide-coated optical glass plates with a planar alignment layers. The thicknesses of cells were about 8 μm and an effective area of 1 cm2. The cells were purchased from Instec, Inc. Hydrothermally produced Co3O4 sample at 120C was selected to mixed in host E7 since the morphology of this sample contains nanorods. E7 nematic LC mixture and Co3O4 nanoparticles were dissolved in isopropanol followed by the ultrasonic bath. The mixture was left for 48 h in the furnace at 50C to fully evaporate isopropanol. 0.05% doped sample was filled to LC cell at 60C by capillarity action [25].

#### 2.3. Characterization

The hydrothermally obtained Co3O4 samples' crystalline structures were investigated by a Philips X' Pert Pro X-ray diffractometer (XRD) with Cu-Kα radiation. The morphologies of the samples were examined by scanning electron microscope (Zeiss EVO 10LS). The optical

Figure 2. Synthesis of Co3O4 samples and basic setup of LC light transmittance experiment.

property samples were obtained by a Shimadzu UV-1800 ultraviolet visible spectroscopy (UVvis) in the range of 200–900 nm using distilled water as a reference solvent. The samples were dispersed into distilled water solvent and ultrasonicated before the measurement. The textures of E7 and E7-doped sample were taken using polarizing optical microscope (Eclipse E200, Nikon, Japan) equipped with the digital camera. Temperature was controlled with heating stage (LTS 120, LinkamScientific Instruments, Ltd., England) with a temperature accuracy of 0.1C controlled with PE95 LinkPad. Optical transmittance experiment designed and performed by laser, polarizer, analyzer, and photodiode and experimental scheme is described in detail in the literature [60]. Experimental detail about synthesis of Co3O4 samples and basic setup of liquid crystal optical transmittance experimental is given in Figure 2.

#### 3. Results and discussion

and 0.12 g Urea (CH4N2O) solution were mixed with 50 ml distilled water. The prepared solution was placed in the autoclave, and it waited in the furnace at 105, 120, 140, 160, and 180C for 6 h. It was precipitated and washed several times with distilled water and dried at 60C for 10 h. The obtained particles were annealed for 5 h at 300C. Moreover, all obtained samples were also annealed at 500C for 1 h to compare the effects of different annealing

The LC used was commercially available eutectic mixture E7 (SYNTHON Chemicals, Germany). The nematic to isotropic transition temperature for pure E7 is measured by using polarizing optical microscope integrated with hot stage, and it is found 58.6C. The liquid crystal cells were made by indium-tin oxide-coated optical glass plates with a planar alignment layers. The thicknesses of cells were about 8 μm and an effective area of 1 cm2. The cells were purchased from Instec, Inc. Hydrothermally produced Co3O4 sample at 120C was selected to mixed in host E7 since the morphology of this sample contains nanorods. E7 nematic LC mixture and Co3O4 nanoparticles were dissolved in isopropanol followed by the ultrasonic bath. The mixture was left for 48 h in the furnace at 50C to fully evaporate isopropanol. 0.05% doped sample was filled to LC cell at 60C by capillarity action [25].

The hydrothermally obtained Co3O4 samples' crystalline structures were investigated by a Philips X' Pert Pro X-ray diffractometer (XRD) with Cu-Kα radiation. The morphologies of the samples were examined by scanning electron microscope (Zeiss EVO 10LS). The optical

Figure 2. Synthesis of Co3O4 samples and basic setup of LC light transmittance experiment.

temperatures on XRD and SEM results of obtained Co3O4 samples.

2.2. Liquid crystal experiment

74 Cobalt

2.3. Characterization

X-ray diffraction pattern of Co3O4 samples annealed at 300C for 6 h is given in Figure 3. The diffraction peaks of hydrothermally produced cubic structured sample at 120C are suitable with the values in the standard card (PDF-2, reference code: 01-074-1656) and the Co3O4 particles produced at 180C are partly suitable to this reference code as shown in Figure 3. The others produced Co3O4 particles' apparent crystal structure patents were not observed. It is supposed that this was originated from hydroxide structures and chemical contaminant.

Figure 3. XRD graph of Co3O4 samples annealed at 300C.

Figure 4. XRD graph of Co3O4 samples annealed at 550C.

The obtained Co3O4 particles reannealed at 550C for an hour and crystal structure of reannealed samples were investigated again. The effect of annealing temperature on the crystalline structure of obtained particles is shown in Figure 4. In this figure, XRD peaks of Co3O4 particles become clear, and all samples give at least some of the reference peaks of Co3O4. Hydroxide structures disappeared with reannealing at 550C, and XRD peaks of Co3O4 particles are seen more clearly than samples annealed at 300C.

The morphologies of particles that are produced using hydrothermal method at various temperatures and annealed at 300C for 5 h are seen in Figure 5. In this figure, hydroxide and chemical waste structures are observed. Especially in Figure 5b, nanorod structures are seen. Nanosheet structures as layers are seen in Figure 5d. However, remarkable structures have not been observed for the samples produced at 105, 140, and 180C, which are corresponding to Figure 5a, c, and e.

To observe the effect of annealing temperature, SEM pictures belonging to particles annealed within 550C for 1 h are given in Figure 6. It was observed that the hydroxide compounds and chemical wastes decreased with the effect of the annealing. In Figure 6a and d, octahedral microparticles have been observed clearly, and in Figure 6b, nano/microrods have been shown. Figure 6c shows the octahedral particles in nano level. In Figure 6e, agglomerated nanoparticles are dominantly seen.

Hydrothermally Produced Cobalt Oxide Nanostructures at Different Temperatures and Effect on Phase Transition… http://dx.doi.org/10.5772/intechopen.70946 77

Figure 5. Morphologies of 300C annealed samples produced at (a) 105C, (b) 120C, (c) 140C, (d) 160C, and (e) 180C.

The obtained Co3O4 particles reannealed at 550C for an hour and crystal structure of reannealed samples were investigated again. The effect of annealing temperature on the crystalline structure of obtained particles is shown in Figure 4. In this figure, XRD peaks of Co3O4 particles become clear, and all samples give at least some of the reference peaks of Co3O4. Hydroxide structures disappeared with reannealing at 550C, and XRD peaks of Co3O4

The morphologies of particles that are produced using hydrothermal method at various temperatures and annealed at 300C for 5 h are seen in Figure 5. In this figure, hydroxide and chemical waste structures are observed. Especially in Figure 5b, nanorod structures are seen. Nanosheet structures as layers are seen in Figure 5d. However, remarkable structures have not been observed for the samples produced at 105, 140, and 180C, which are corresponding to

To observe the effect of annealing temperature, SEM pictures belonging to particles annealed within 550C for 1 h are given in Figure 6. It was observed that the hydroxide compounds and chemical wastes decreased with the effect of the annealing. In Figure 6a and d, octahedral microparticles have been observed clearly, and in Figure 6b, nano/microrods have been shown. Figure 6c shows the octahedral particles in nano level. In Figure 6e, agglomerated

particles are seen more clearly than samples annealed at 300C.

Figure 4. XRD graph of Co3O4 samples annealed at 550C.

Figure 5a, c, and e.

76 Cobalt

nanoparticles are dominantly seen.

Figure 6. Morphologies of 550C annealed samples produced at (a) 105C, (b) 120C, (c) 140C, (d) 160C, and (e) 180C.

The optical bad gap (Eg) for the direct transition was obtained using Tauc plot. Optical absorption graph of sampled annealed at 300C is given in Figure 7, left column and optical absorption of sampled reannealed at 550C is given in Figure 7, right column. The direct band gap of 300C annealed Co3O4 NP changing between 3.1 and 3.5; on the other hand, reannealed sampled band gaps were very close to 3.5 eV as interpreted in Figure 7. The measurement results of direct band gaps of reannealed nanoparticles were suitable with the literature [61, 62].

Figure 7. UV-vis spectra and band gaps of hydrothermally obtained particles annealed at 300 and 550C.

Polarizing optical microscope was used to investigate texture of pure and 0.05% nanoparticledoped E7. The different magnification textures of pure E7 are given in Figure 8. The smallest magnification ratio of pure E7 is given in Figure 8a, and the most detailed texture of this LC is shown in Figure 8d.

Phase transitions of 0.05% cobalt oxide nanoparticle-doped E7 LC are illustrated in Figure 9a. The first drop of isotropic liquid appeared at 57.1C, TN, and the last drop nematic disappeared Hydrothermally Produced Cobalt Oxide Nanostructures at Different Temperatures and Effect on Phase Transition… http://dx.doi.org/10.5772/intechopen.70946 79

Figure 8. Optical polarizing image of E7 nematic LC, (a) 40 magnification, (b) 100 magnification, (c) 200 magnification, and (d) 400 magnification.

Figure 9. Texture of pure and NP-doped E7, (a) phase transition, (b) without cross polarizer, (c) with cross polarizer, and (d) E7 at 1 V, (e) 0.05% NP-doped E7 at 1 V.

at 60.6C, TI. The average temperature of nematic-isotropic transition of doped sample is calculated by using equation TNI = 0.5(TN + TI) [63], and found 58.85C. The phase transition difference of pure E7 and NP-doped E7 is found as ΔTNI = 0.2C. Figure 9b and c shows the optical picture of E7 LC without and with cross polarizers, respectively. The texture of pure and doped LC under 1 V applied voltage is given in Figure 9d and e.

Polarizing optical microscope was used to investigate texture of pure and 0.05% nanoparticledoped E7. The different magnification textures of pure E7 are given in Figure 8. The smallest magnification ratio of pure E7 is given in Figure 8a, and the most detailed texture of this LC is

Figure 7. UV-vis spectra and band gaps of hydrothermally obtained particles annealed at 300 and 550C.

Phase transitions of 0.05% cobalt oxide nanoparticle-doped E7 LC are illustrated in Figure 9a. The first drop of isotropic liquid appeared at 57.1C, TN, and the last drop nematic disappeared

shown in Figure 8d.

78 Cobalt

Transmission versus voltage graph of pure and Co3O4 NP-doped LC is illustrated in Figure 10. The wavelength of used laser light in transmission experiment was 650 nm. The prepared LC cells are placed between cross polarizers, and the angle between cross polarizers and LC cell is adjusted to 45, and the output signal was detected by a photodiode detector. Transmission voltage behavior of pure and doped sample is not very different from each other, which

Figure 10. Transmission vs. voltage graph for pure and NP-doped E7.

implies that threshold voltage values are close to each other. The calculated threshold voltage for pure E7 and 0.05% Co3O4 NP-doped E7 are 0.8 V and 0.9 V sequentially.

#### 4. Conclusion

In summary, Co3O4 nanostructures were synthesized using the hydrothermal method with unique hydrothermal reaction time at different temperatures. The crystalline structures, morphologies, and optical absorptions were investigated in detail with XRD, SEM, and UV-vis spectroscopy for two different annealed temperatures. The obtained samples were firstly investigated after being annealed at 300C for 5 h, and the results of SEM and XRD separately indicated that cobalt hydroxide did not decompose fully to form cobalt oxide nanocrystalline. The samples were reannealed at 500C for an hour to investigate deeply. The hydroxide compounds and chemical wastes are removed, and XRD peaks of Co3O4 particles become clear. Obtained morphologies of cobalt oxide nanostructures also changed with calcination. The particles, with a nanorod morphology, produced at 120C hydrothermal reaction temperature and annealed at 500C were selected to doped nematic LC mixture E7. Phase transition and the threshold voltage of pure and Co3O4 NP-doped E7 LC were examined successfully. It reveals that for Co3O4 NP-doped E7 phase transition temperature and threshold voltage increased very slightly.

#### Acknowledgements

The authors thank Kahramanmaras Sutcu Imam University for the continuous motivation to maintain this research. This work was financially supported by Kahramanmaras Sutcu Imam University, (KSU) Scientific Research Projects Coordination Department, under Project No. 2017/4-29D.

### Author details

Hasan Eskalen1 \*, Süleyman Kerli2 and Şükrü Özgan3

\*Address all correspondence to<EMAIL_ADDRESS>
1 Material Science and Engineering, Graduate School of Natural and Applied Science, Kahramanmaras Sutcu Imam University, Kahramanmaras, Turkey

2 Energy System Engineering Department, Kahramanmaras Sutcu Imam University, Kahramanmaras, Turkey

3 Department of Physics, Faculty of Science and Letter, Kahramanmaras Sutcu Imam University, Kahramanmaras, Turkey

#### References

implies that threshold voltage values are close to each other. The calculated threshold voltage

In summary, Co3O4 nanostructures were synthesized using the hydrothermal method with unique hydrothermal reaction time at different temperatures. The crystalline structures, morphologies, and optical absorptions were investigated in detail with XRD, SEM, and UV-vis spectroscopy for two different annealed temperatures. The obtained samples were firstly investigated after being annealed at 300C for 5 h, and the results of SEM and XRD separately indicated that cobalt hydroxide did not decompose fully to form cobalt oxide nanocrystalline. The samples were reannealed at 500C for an hour to investigate deeply. The hydroxide compounds and chemical wastes are removed, and XRD peaks of Co3O4 particles become clear. Obtained morphologies of cobalt oxide nanostructures also changed with calcination. The particles, with a nanorod morphology, produced at 120C hydrothermal reaction temperature and annealed at 500C were selected to doped nematic LC mixture E7. Phase transition

for pure E7 and 0.05% Co3O4 NP-doped E7 are 0.8 V and 0.9 V sequentially.

Figure 10. Transmission vs. voltage graph for pure and NP-doped E7.

4. Conclusion

80 Cobalt

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