Exploration of properties (crystallographic, morphological, optical) of nano cobalt aluminate synthesized by facile sol–gel method: Effects of sintering temperature

Monmon Podder; Md. Rassel Moni; Md. Lutfor Rahman; Bristy Biswas; Nahid Sharmin; Mahmuda Hakim; Moksodur Rahman; Md. Sahadat Hossain; Md. Farid Ahmed

doi:10.1016/j.arabjc.2024.105601

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03 2024

:17;

105601

doi:

10.1016/j.arabjc.2024.105601

Exploration of properties (crystallographic, morphological, optical) of nano cobalt aluminate synthesized by facile sol–gel method: Effects of sintering temperature

Monmon Podder^{^a,^c}, Md. Rassel Moni^{^c}, Md. Lutfor Rahman^{^a}, Bristy Biswas^{^a}, Nahid Sharmin^{^a}, Mahmuda Hakim^{^b}, Moksodur Rahman^{^a}, Md. Sahadat Hossain^{^a}, Md. Farid Ahmed^{^a,⁎}

a Institute of Glass and Ceramic Research and Testing (IGCRT), Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhaka-1205, Bangladesh

b Biomedical and Toxicological Research Institute, Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhaka-1205, Bangladesh

c Department of Chemistry, Comilla University, Cumilla-3506, Bangladesh

⁎Corresponding author. ahmedfarid@bcsir.gov.bd (Md. Farid Ahmed)

Received: 2023-8-23, Accepted: 2024-1-1,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The present work reports on the facile sol–gel synthesis of CoAl₂O₄ nanoparticles using cobalt nitrate and aluminum nitrate as the metal salts and citric acid and glycerol as chelating agents. The consequence of sintering temperature on the structure, particle size, morphology, composition, zeta potential, color, and reflectance spectra at neutral pH condition were explored with various instrumental techniques. The CoAl₂O₄ nanoparticles were characterized with Powder X-ray diffraction (PXRD), Fourier-transform infrared spectroscopy (FT-IR), Field emission scanning electron microscopy (FESEM), Energy dispersive spectroscopy (EDS), Nanoparticle size analyzer, Ultraviolet–Visible Diffuse Reflectance Spectroscopy (UV–Vis-DRS) and CIE-Lab colorimetric method. From the data of PXRD, the crystallite size and peak profile analysis were investigated by four different models. It was found that the average crystallite size varied in the range of 28.01–320.97 nm and single-phase cubic CoAl₂O₄ was formed without any impurity. The average particle size of CoAl₂O₄ nanoparticles was discerned from the FESEM images using ImageJ software and from a nanoparticle size analyzer. The average particle sizes were increased with the increasing sintering temperature. The values of zeta potential found in all of the samples represented high dispersion stability. From the colorimetry of CoAl₂O₄ nanoparticles, the CIE-La*b* output illustrated high purity and brightness and the spectra.

Keywords

Sol–gel

CoAl2O4

XRD

FESEM

PXRD

CIE-Lab*

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1 Introduction

Nano-sized cobalt aluminate (CoAl₂O₄) also familiar as Thenard’s blue is an environmentally friendly inorganic compound having mainly a normal spinel structure in which Co²⁺ ions are arranged in tetrahedral positions and Al³⁺ ions are in octahedral sites (Peymannia et al., 2014; Sirotyuk et al., 1998; Zayat and Levy, 2000; Zhou et al., 2018 ). Cobalt aluminate exhibits outstanding characteristics such as high thermal stability, high mechanical resistance, dispersion, color strength, impermeability and non-migration, which leads to potential diverse application (Dhak and Pramanik, 2006; X. Zhao, L. Zhang, P. Xiong, Wenjing Ma, Na Qian, Wencong Lu, 2015). In various applications such as ceramic pigments, electrochemical sensors, thin films, catalysts, and color filters for automotive lamps (A. Walsh, Y. Yan, M.M. Al-Jassim, S.-H. Wei, J, 2008; Carta et al., 2005; D. Rangappa, T. Naka, A. Kondo, M. Ishii, T. Kobayashi, T. Adschiri, 2007; Deraz and Fouda, 2013; Desouza et al., 2009; Ji et al., 2001; Maurizio et al., 2010; P.M.T. Cavalcante, M. Dondi, G. Guarini, M. Raimondo, G. Baldil, Dyes Pigments, 2009; Rangappa et al., 2007; W.S. Cho, M. Kakihana, 1999; X.L. Duan, D.R. Yuan, Z.H. Sun, C.N. Luan, D.Y. Pan, D. Xu, M.K. Lv, 2005 ).These diverse uses are enabled by the material's distinct optical characteristics. It has also been successfully used in coloration of ceramic samples, decorative and protective coatings and in paint (Pan Z, Wang Y, Huang H, Ling Z, Dai Y, Ke S., 2015; Peymannia et al., 2014; Tong Y, Zhang H, Wang S, Chen Z, Bian B, 2016; Zhang A, Mu B, Luo Z, Wang A, 2017 ).

Recently, many a variety of synthesis technique such as polymeric sol–gel, co-precipitation route, micro emulsion, hydrothermal process,combustion etc. used for the preparation of nanocrystalline cobalt aluminate (D. Rangappa, T. Naka, A. Kondo, M. Ishii, T. Kobayashi, T. Adschiri, 2007; F.L. Yu, J.F. Yang, J.Y. Ma, J. Du, Y.Q. Zhou, 2009; J. Chandradass, M. Balasubramanian, K.H. Kim, 2010; L. Gama, M.A. Ribeiro, B.S. Barros, R.H.A. Kiminami, I.T. Weber, A.C.F.M. Costa, 2009 ). Most of these methods except sol–gel require highly sophisticated apparatus and are expensive, high thermal process, low productivity with inhomogeneity. Compared to other methods, it was ascertained that the sol–gel processes high reproducibility route and it improves the purity of CoAl₂O₄ with high degree of homogeneity. It also produces the nanoparticles with small particle size distribution at lower temperature synthesis process (F. Davar, M.S. Niasari, 2011; F. Meyer, R. Hempelmann, S. Mathur, M. Veith, J. Mater, 1999).

Parvin Imanipour et all studied the possibility of vanadium substitution on Co lattice sites in CoFe₂O₄ synthesized by sol–gel autocombustion method (Imanipour et al., 2020). A process was optimized to study the structural and magnetic properties of MnFe₂O₄ doped by Zn and Dy using Taguchi method (Shayestefar et al., 2022). Zinc and Vanadium Co-Substituted CoFe₂O₄ nanoparticles were synthesized by using the Sol-Gel auto-combustion method (Imanipour et al., 2022).

Several authors have utilized the sol–gel route to fabricate CoAl₂O₄ nanoparticles. For instance, Stranger and Orel employed the sol–gel technique, utilizing ethyl acetoacetate as the chelants, to prepare CoAl₂O₄ nanoparticles (U.L. Stangar, B. Orel, M.J. Krajnc, 2003). Cui et al. described a study where they reported the fabrication of a series of spinel nanoparticles using the sol–gel process (Cui H, Zayat M, Levy D., n.d.).

From the previous studies, it is observed that the synthesis temperature is strongly affected the color of CoAl₂O₄ (I.S. Ahmed, S.A. Shama, M.M. Moustafa, H.A. Dessouki, A.A. Ali, 2009; M. Zayat, D. Levy, 2000; U.L. Stangar, B. Orel, M.J. Krajnc, 2003 ). In detail, at relatively low temperature synthesis process, the green color of CoAl₂O₄ is obtained but at relatively high temperature, the color changes to blue. Some authors shown that the blue color is originates from the Co²⁺ in tetrahedral co-ordination (Karmaoui M, Silva NJ, Amaral VS, Ibarra A, Millan A, Palacio F, 2013) and violet and green color originate from the octahedral Co²⁺ and Co³⁺ coordination (Herrero M, Benito P, Labajos FM, Rives V., 2007; Tielens F, Calatayud M, Franco R, Recio JM, Pérez-Ramírez J, Minot C, 2009). Some suggested the color is affected due to the crystalline Co₃O₄ existing (Štangar UL, Orel B., 2003). There are different types of materials which are used in the modification of internal and external properties of materials for various purposes. The fruitful applications of any material are mainly depended on the properties of the material. During the synthesis process, reaction parameters and supporting elements are varies to get desired properties. Magnetic properties were changed with the variation of temperature (Choudhary et al., 2021b).

In the current study, sol–gel method is used to prepare nano crystalline CoAl₂O₄. It was found that blue color spinel CoAl₂O₄ were prepared without using any dopant. The impact of varying sintering temperature on the structural, morphological, dispersion stability, and optical characteristics of CoAl₂O₄ was assessed using various characterization techniques. These included powder X-ray diffraction (PXRD) for structural analysis, Fourier transform infrared spectroscopy (FT-IR) for chemical bonding analysis, scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS) for morphological and elemental composition analysis, a nanoparticle size analyzer for particle size distribution analysis, UV–Vis absorption spectroscopy for studying absorption properties, and CIE-La*b* colorimetric analysis for evaluating color characteristics. These techniques collectively provided insights into the changes occurring in the material as a result of different sintering temperatures.

2 Experimental

2.1

2.1 Materials

Nano cobalt aluminates were synthesized from precursors cobalt nitrate-hexahydrate [Co(NO₃)₂·6H₂O], aluminum nitrate-nanohydrate [Al(NO₃)₃·9H₂O], glycerol (GC,C₃H₈O₃), citric acid (C₆H₈O₇) and aq·NH₃ (25 %), which were purchased from Merck, Mumbai with 98 % purity.

2.2

2.2 Synthesis

In a traditional sol–gel method, 0.01 mol of Co(NO₃)₂·6H₂O and 0.02 mol of Al(NO₃)₃·9H₂O were dispersed in 300 mL of deionized water. The solution was heated to 75 °C while vigorously stirring for 30 min. A mixture containing a chelating agent, citric acid, at a concentration of 0.03 mol/L, and fuel glycerol, was mixed to the above solution. The resulting solution was then stirred for 2 hrs. Thereafter, the pH of the solution was adjusted to ∼ 7 by adding aq·NH₃ drop by drop. The solution was vaporized at 85 °C until it formed a thick gel-like substance. The product was then dried in an electrical oven at 120 °C for 5 hrs. At the end, the resulting black powder was collected and subjected to calcination at temperatures ranging from 1000, 1100, 1200, 1300 and 1400 °C for 4 hrs. The samples that followed were labeled as S1, S2, S3, S4, and S5, in that order. Graphical representation is visualized in Fig. 1.

2.3

2.3 Characterization

The PXRD patterns of the materials were investigate by powder X-ray Diffractometer (Smart Lab SE, Rigaku, Japan) with Cu-Kα radiation. The XRD peaks observed from 10 °C to 80 °C with a scan speed 20°/ min. FE-SEM and EDX were investigated by Field Emission Scanning Electron Microscopy (Model 7610f, Jeol, Japan). The infrared spectra were analyzed using FT-IR spectrometer (Model-IRAffinity-1S, MIRACLE 10, Shimadzu, Japan) at room temperature and analysis was conducted using KBr pellets, covering a frequency range of 400–4000 cm⁻¹. The absorbance spectra were measured by UV–Vis spectra and the colorimetric parameters L*, a* and b* were determined by UV–visible Spectrophotometer (Brand:Perkin Model: Elmer Lambda 1050⁺). The Nanoparticle Size Analyzer (Model:nanoParticaSZ-100-S2, Brand:HORIBA scientific Ltd, Japan) was used to measure the particle size and zeta potential.( ).

The pictorial representation of the process of synthesizing CoAl2O4 nanoparticles through the sol–gel method.

3 Results and discussion

3.1

3.1 XRD analysis

The PXRD patterns of the prepared samples at various calcination temperatures are given in Fig. 2. The PXRD patterns showed different characteristic peak at 2θ values of 31.16°, 36.72°, 44.65°, 55.45°, 59.12° and 64.99° which could be assigned to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) & (4 4 0) of single phase cubic CoAl₂O₄ respectively. Additionally, it was noticed that the relative intensity of the CoAl₂O₄ peaks progressively rises as the annealing temperature increases. This trend correlates with an augmentation in crystallinity. The results are good agreement to spinel cubic CoAl₂O₄ (JCPPS Card no.01–082-2243). No impurity was found in the XRD peaks which proved that the sintering temperature (1000–1400 °C) was adequate for the synthesis of highly pure CoAl₂O₄. The Rietveld refinement analysis has been performed to determine the structural properties of the synthesized cobalt aluminate nanoparticles (Afshari et al., 2019; Darvishi et al., 2023; Mohebbi et al., 2023 ).

PXRD patterns and Rietveld refinement of CoAl2O4 nanoparticles sinteredd at (1000–1400)°C.

The x-ray peak profile analysis and average crystallite sizes were calculated using various methods, including the Classical Scherrer model (C-S), Williamson-Hall (W-H)model, Halder-Wagner (H-W) model, and Size-Strain Plot (SSP) procedure.

3.1.1

3.1.1 The Debye-Scherrer equation

Using the X-ray diffractogram patterns, the crystallite size is measured from Scherrer’s model Eq. (1) (Choudhary et al., 2021a; Sen et al., 2020b).

(1)

D_{C - S =} \frac{k λ}{β_{D} c o s θ}

Where, D_C-S refers to the crystalline size (nm), k represents a shape-dependent constant with a value of 0.94, and X-ray wavelength is denoted by λ represents x-ray wavelength (λ = 0.154060 nm), β_D corresponds to the peak width at half maximum intensity (FWHM), and θ signifies the diffraction angle based on Bragg's law.

The values of crystallite size of the CoAl₂O₄ nanoparticles using Scherrer method are shown in Table 1.

Table 1 Record of the obtained CoAl₂O₄ nanoparticles crystallite size by Scherrer, W-H, H-W and SSP for various calcination temperatures.

Sample ID	D_C-S(nm)	D_W-H(nm)	D_H-W(nm)	D_SSP(nm)
S1	44.9	28.01	45.02	45.02
S2	80.0	56.59	77.03	77.03
S3	102.1	108.32	99.04	99.04
S4	122.8	175.51	113.65	113.65
S5	134.9	320.97	180.61	180.61

3.1.2

3.1.2 W-H model

The W-H analysis refers to simplified methods that distinguish between nano crystallite size and strain-dependent peak broadening by examining the width of peaks as a function of 2θ (Mote et al., 2012).The peak widening in nanoparticles resulting from lattice strain can be determined using the Stokes-Wilson equation (Sen et al., 2020b; Stokes and Wilson, 1944). In this model, lattice strain was also been estimated in addition to the crystallite size. As peak broadening is dependent on the lattice strain and domain size, this model is considered more accurate and the expression of microstructural property (Hossain and Ahmed, 2023).

(2)

β_{strain} = 4 t a n θ

The total broadening is observed through the sum of β_strain and β_D.

(3)

B_{hkl} = β_{Strain} + β_{D}

Where, β_hkl is the instrumental corrected broadening, β_S is due to strain induced broadening and β_D is the peak widening based crystallite size. By substituting the values of βstrain and β_D from equations (2) and (3) into equation (4), we can calculate.

(4)

β_{hkl} = 4 t a n θ + k λ / D c o s θ

By rearranging the Equation (4),

(5)

β_{hkl} c o s θ = 4 t a n θ + k λ / D_{W - H}

Equation (5) represents the Williamson-Hall equation as a linear equation, y = mx + c. By graphing between β_hklcosθ and 4sinθ, as shown in Fig. 3, and obtained slope and intercept gives Halder-Wagner crystallite size and strain (Yadav et al., 2016). The values of crystallite size are tabulated in Table 1.

Williamson-Hall plot of cobalt aluminate nanoparticles for different sintering temperature.

The linear lines obtained from the W-H plot do not exhibit a strong correlation, which indicated by the correlation factor of R² is 0.38071, 0.19497, 0.00976, 0.00587, 0.21115 for 1000 °C, 1100 °C, 1200 °C, 1300 °C, 1400 °C, respectively. For the first two samples (S1 and S2) negative strain was found and for the rest of the sample positive strain was found. Negative and positive values of the strain carried good evidence for the compressive and tensile strain of the crystallites (Hossain and Ahmed, 2023).

3.1.3

3.1.3 H-W model

The H-W model provides another procedure that incorporates the integral width β* of the corresponding lattice point and d* as the interplanar spacing for the corresponding cell. This model allows for the determination of both the crystallite size (D_H-W) and lattice strain (ε_H-W) using β_hkl and the interplanard-spacing (distance among neighboring planes) through the application of H-W analysis (Sen et al., 2020b).

(6)

{(\frac{β^{*}}{d^{*}})}^{2} = \frac{k β^{*}}{D_{H - W} {(d^{*})}^{2}} + {(2 €_{H - W})}^{2}

(7)

β * = \frac{β c o s θ}{λ}

(8)

d^{*} = \frac{2 s i n θ}{λ}

By substituting the values of β* and d* from Eqs. (7) and (8) into equation (6), we can calculate

(9)

{(\frac{β c o s θ}{s i n θ})}^{2} = \frac{k λ}{D_{H - W}} \frac{β c o s θ}{\sin^{2} θ} + 16 {€_{H - W}}^{2}

Modifying the Equation Eq. (9),

(10)

{(\frac{β}{t a n θ})}^{2} = \frac{k λ}{D_{H - W}} \frac{β c o s θ}{\sin^{2} θ} + 16 {€_{H - W}}^{2}

Equation (10) can be understood as a straight-line equation, y = mx + c, where the slope represents kλ / D_(H-W) and the intercept corresponds to 16 ${(€_{H - W})}^{2}$ (Sen et al., 2020a). In the H-W method, a graph has been constructed with βcosθ/sin²θ vs (β/tanθ)², as depicted in Fig. 4. The linear fitting allows for the calculation of crystallite size and lattice strain from the slope and intercept, respectively (Leila et al., 2018).These values have been determined and are tabulated in Table 1.

Halder-Wagner plot of cobalt aluminate nanoparticles for different sintering temperature.

The linear fitting using the H-W method exhibit a good fit, as indicated by the correlation factor values of R². For the respective temperatures of 1000 °C, 1100 °C, 1200 °C, 1300 °C, and 1400 °C, the correlation coefficient values are 0.9828, 0.9876, 0.96274, 0.79793, and 0.16467. These high correlation coefficients suggest a strong linear relationship between the variables in the H-W plot for temperatures up to 1300 °C, while the correlation becomes weaker at 1400 °C.

3.1.4

3.1.4 SSP model

The SSP model is utilized to determine the crystallite size and strain of nanoparticles. This technique assumes the crystal structure to be isotropic (R. Sivakami, S. Dhanuskodi, R. Karvembu, 2016; Sen et al., 2020b). In SSP analysis, the crystallite size is pointed utilizing the Lorentzian function, while the lattice strain is described by the Gaussian function (Ahemen et al., 2016; Kafashan, 2018).

(11)

{(d β c o s θ)}^{2} = \frac{K}{D_{ssp}} (d^{2} β c o s θ) + {(\frac{€_{ssp}}{2})}^{2}

Where, in Eq. (11), K is a constant. By plotting (dβcosθ)² vs (d²βcosθ) as displayed in Fig. 5, the values of D_SSP (crystallite size get from the slope) and €_SSP (lattice strain derived from the y-intercept) have been calculated and are presented in Table 1 (Mohammed et al., 2020).

SSP plot of cobalt aluminate at (1000–1400)°C for different calcination temperature.

The SSP method demonstrates a strong fit, as evidenced by the correlation factor (R²) of 0.9828, 0.9876, 0.96275, 0.79793, and 0.16468 for S1, S2, S3, S4, and S5, respectively. These high correlation coefficients indicate a good agreement between the SSP analysis and the data, suggesting that the SSP method is a reliable approach for determining crystallite size and strain in these samples.

The obtained results for the structural parameters show good agreement between the SSP and H-W plot methods. The data is accurately fit in both of these methods, with all high-intensity points aligned with the linear adaptation (Irfan et al., 2018; R. Sivakami, S. Dhanuskodi, R. Karvembu, 2016). This alignment is more pronounced compared to the W-H method. Therefore, it is evident that the SSP and H-W methods are superior in terms of accuracy. The crystallite size for CoAl₂O₄ nanoparticles derived from the Scherrer model, W-H, H-W, and SSP techniques are relatively similar to each other. This consistency further strengthens the reliability of the SSP and H-W methods for determining crystallite size at various calcination temperatures. It can be observed that sample S1 has the smallest crystallite size among the analyzed samples.

3.2

3.2 FTIR analysis

Fig. 6 displays the FTIR absorption bands for samples S1 (1000 °C), S3 (1200 °C), and S5 (1400 °C). Upon analysis, it is observed that three prominent absorption bands are identified at roughly 650 cm⁻¹, 550 cm⁻¹, and 430 cm⁻¹, respectively. These bands indicate the presence of CoAl₂O₄ spinel in the materials (Zayat and Levy, 2000).The sharp two bands, located around 600 cm⁻¹, are attributed to the intrinsic stretching vibrations of tetrahedral metal complexes. On the other hand, the lowest band around 400 cm⁻¹ is affiliated with octahedral metal complexes (Wang et al., 2004). Moreover, broad bands in the vicinity of 3462 cm⁻¹ represent the –OH stretching vibrational modes.The band at 1630 cm⁻¹ because of vibrational mode of absorbed water molecules (Ge et al., 2013). Additionally, small peaks observed around 2922 cm⁻¹ and 2850 cm^-1complementary to the stretching vibrations of C–H bonds (Sharma and Ghose, 2014).

FTIR absorption band of CoAl2O4 nanoparticles for 1000 °C, 1300 °C and 1400 °C.

The existence of CO₂ is indicated by the absorption band obtained at 2337 cm⁻¹. On the other hand, small absorption band obtained at 1384 cm⁻¹ is corresponds to left over nitrogen groups arise from the combustion reaction (Wei and Chen, 2006). Notably, in all the samples, observed vibrational modes confirm the existence of spinel CoAl₂O₄ nanoparticles.

3.3

3.3 Morphological analysis

The surface morphology of the samples was analyzed by using Field Emission Scanning Electron Microscopy (FE-SEM). Fig. 7 displays the FE-SEM images and histograms depicting the distribution of particle sizes for different calcination temperatures. The FE-SEM images revealed that the surfaces of the nanoparticles obtained were extensively agglomerated and exhibited various shapes. The degree of agglomeration in CoAl₂O₄ increased as the calcination was raised from 1000 °C to 1400 °C, primarily due to the augmented surface energy, resulting in larger particle sizes. Increasing the calcination temperature proved advantageous in promoting complete reactions and enhancing the stability and uniformity of the particles. The particle size distribution was found to be irregular and non-uniform. Through the utilization of FESEM micrographs and ImageJ Software (Mazzoli and Favoni, 2012), the average nanoparticle sizes of all materials were determined to range from 23.25 nm to 237.14 nm, as presented in Table 2. The particle sizes of the synthesized materials were increased with the increment of temperature. Similar types of results were also noticed in the case of crystallite size estimated from XRD data. Thus, it can be predicted that the augmented temperature influenced the formation of large crystallites as well as extended agglomeration. In addition to the formation of large particles, the surface roughness was also increased which is clearly visualized form the images.

FE-SEM image of CoAl2O4 nanoparticles (for different calcination temperature).

Table 2 List of average particle size distribution obtained from FE-SEM for CoAl₂O₄ nanoparticles.

Sample ID	Particle Size(nm)
S1	23.25
S2	41.12
S3	43.83
S4	97.82
S5	237.14

3.4

3.4 Compositional analysis of CoAl₂O₄ nanoparticles

EDS was utilized to verify the elemental and compositional analysis of the CoAl₂O₄ nanoparticles. The atomic weight percentages at 15 KeV, as depicted in Fig. 8 and Table 3, indicate that the experimental atomic ratio of cobalt (Co) to aluminum (Al) is approximately 1:2 for all the samples. The EDS pattern of CoAl₂O₄ revealed that the Co/Al ratio of samples S1, S2, S3, S4, and S5 is relatively close to 0.5, except for the S1 sample. Upon conducting the composition analysis, it was confirmed that the presence of Co, Al, and O is free from any impurities.

EDS results of CoAl2O4 nanoparticles at different sintering temperature.

Table 3 Atomic Weight percentage of cobalt aluminate nanoparticles obtained from EDS spectra.

Sample ID	Atomic Weight (%)		Co/Al
Sample ID	Co	Al	Co/Al
S1	35.89	28.65	1.25
S2	17.85	28.60	0 0.62
S3	23.65	31.03	0 0.76
S4	17.47	29.24	0 0.60
S5	17.39	28.83	0 0.60

3.5

3.5 Zeta potential analysis of CoAl₂O₄

The zeta potential results for all samples are displayed in Table 4, and these findings are visually represented in Fig. 9. When the values fall within the range of higher + 30 mV and lower −30 mV, it indicates that the nanoparticles are stable in the dispersed solvent (Madhavi et al., 2013). The study reveals that all the samples exhibit a high level of stability in water. As stated in Ohshima's model, smaller particles tend to have higher zeta potential values, while larger particles have lower values. This is attributed to the more intense density of oppositely charged ions surrounding smaller particles compared to larger ones (Holmberg et al., 2013). The sample S1 demonstrates the highest value (-59.4 mV) due to its smaller particle size of 23.25 nm, as observed through FE-SEM, which is significantly smaller than the other obtained nanoparticles.

Table 4 List of zeta potentialvalues of CoAl₂O₄ nanoparticles.

Sample ID	Zeta potential (mV)
S1	−59.4
S2	−55.3
S3	−46.9
S4	−43.1
S5	−53.6

Zeta potentials of CoAl2O4 nanoparticles at different sintering temperature.

3.6

3.6 Nanoparticle size analysis of CoAl₂O₄

Table 5 showcases the particle size distribution of CoAl₂O₄ nanoparticles, as obtained through the dynamic light scattering model utilizing a nano-particle size analyzer. It is worth noting that the average particle size measured by the nano-particle size analyzer is higher compared to the measurements obtained through FESEM. This disparity can be accredited to the fact that the nanoparticles were dispersed in a 0.005 % water solution during the analysis, causing the nano-particle size analyzer to provide the hydrodynamic diameter, which tends to be larger than the dehydrated diameter observed in FESEM (Bootz et al., 2004; de la Calle et al., 2019). Despite this difference, the trend of increasing particle size aligns with the observations made in FESEM. Specifically, with the increase of calcination temperature the particle size gradually increases within the range of 3130 to 4204.3 nm.

Table 5 Particle size of CoAl₂O₄ nanoparticles at different sintering temperature.

Sample ID	Average Particle Size (nm)
S1	3130.0
S2	3217.8
S3	3373.5
S4	3800.0
S5	4204.3

3.7

3.7 Color property analysis of CoAl₂O₄ at different sintering temperature

The color characteristics of the particles were evaluated using colorimetric data represented by CIE-La*b* values, which are listed in Table 6. The intensity of the blue color in the particles was primarily determined by the b* parameter, where a more negative value indicated a stronger blue hue. With higher in calcination temperature, the b* value steadily rose from −13.74 to −27.36, accompanied by an increase in chroma from 29.28 to 36.40. This demonstrates that the sintering temperature had a remarkable impact on the color characteristics of the nanoparticles. Notably, the S5 sample, calcinated at 1400 °C, exhibited the most intense blue color (b* = −27.36) and the highest saturation (36.40), indicating exceptional color purity and brightness. .

Table 6 CIE-L*a*b* data of CoAl₂O₄ nanoparticles at various sintering temperature.

Sample ID	L	a*	b*	Chroma
S1	61.03	−2.33	−13.74	29.28
S2	61.49	−0.74	−16.84	29.81
S3	61.37	0.23	−17.64	29.67
S4	60.03	2.47	−21.81	28.16
S5	66.83	5.45	−27.36	36.40

Table 7 Comparative study of the optical properties for the similar materials.

Cobalt Aluminate	Band Gap Energy	Reference
Zn- and Cu-doped cobalt aluminate	3.1–4.29 eV	(Pathak et al., 2021)
Semiconductor CoAl₂O₄	1.83 eV	(Boudiaf et al., 2020)
Cobalt Aluminate (C12 &C13)	2.20 – 2.25 eV	(Gingasu et al., 2018)
Cobalt Aluminate	1.89–1.95 eV	This study

To further assess the color characteristics, the reflectivity of the CoAl₂O₄ nanoparticles was analyzed in the visible range, as depicted in Fig. 10. The intensity of the reflected peak in the blue region (400–492 nm) significantly increased with higher sintering temperatures, aligning with the colorimetric data and confirming the color changes observed (Song et al., 2017).

Reflection spectra and Chromaticity of CoAl2O4 nanoparticles in the visible wavelength range.

3.8

3.8 Band gap energy

The optical Band-gap energy (Eg) of the obtained nanoparticles was calculated through Kubelka-Munk (K-M) function, which is expressed as follows Eq. (12):

(12)

F (R) = \frac{{(1 - R)}^{2}}{2 R}

Where, the Kubelka-Munk (K-M) functionEq. (13), denoted as F(R). In this function, R represents the percentage reflectance, while F(R) is directly linked to the material's absorption co-efficient. The Band gap analysis of all the obtained samples was performed on their solid forms. The energy Band gap is a critical parameter in semiconductors, as it provides valuable information regarding their potential applications in optoelectronic devices.

(13)

F (R) \infty α = \frac{{(h ϑ - E_{g})}^{n}}{h ϑ}

Where, E_g represents the band gap, hϑ signify the energy of photon, n represents the optical transition. In the case of a direct allowed transition, the value of n is assigned as 1/2. The Eg is determined by graphing (αhϑ)² vs hϑ, as illustrated in Fig. 11 (Debnath and Das, 2020). Optical band gap of nano ferrites normally varied form 3.7 to 5.0 eV (Lal et al., 2020). The calculated values for the Eg using the Kubelka-Munk (K-M) function fall within the range of 1.89 to 1.95 eV. A comparative study is registered in Table 7.

The optical Band gap energy of CoAl2O4 nanoparticles.

4 Conclusion

The CoAl₂O₄ nanoparticles were synthesized successfully using metal salts and citric acid and glycerol as chelating agents. Phase purity was confirmed from the X-ray diffraction patterns as well as Rietveld refinement. The average crystallite size from four different methods was in the range from 28.01 to 320.97 nm. FESEM images and nanoparticle size analysis demonstrated that the particle size of CoAl₂O₄ nanoparticles increased with higher calcination temperatures. The zeta potential measurements indicated high dispersion stability for all the samples, suggesting good colloidal stability. Moreover, the colorimetric data using CIE-La*b* values revealed the high purity and brightness of the CoAl₂O₄ nanoparticles. The findings contribute to the understanding of the relationship between synthesis parameters, sintering temperature, and the resulting properties of CoAl₂O₄ nanoparticles, offering potential uses in variety of fields such as photocatalysis, energy storage, and sensors. Further research could focus on exploring the specific applications of these nanoparticles and optimizing their synthesis for desired properties.

CRediT authorship contribution statement

Monmon Podder: Investigation, Formal analysis, Software, Data curation, Writing – review & editing, Writing – original draft. Md. Rassel Moni: Validation, Resources, Writing – review & editing. Md. Lutfor Rahman: Validation, Resources, Writing – review & editing. Bristy Biswas: Validation, Resources, Writing – review & editing. Nahid Sharmin: Project administration, Supervision, Writing – review & editing, Funding acquisition. Mahmuda Hakim: Validation, Resources, Writing – review & editing. Moksodur Rahman: Writing – review & editing. Md.Sahadat Hossain: Validation, Resources, Writing – review & editing. Md. Farid Ahmed: Conceptualization, Methodology, Resources, Supervision, Project administration, Visualization, Writing – review & editing.

Acknowledgements

This work was supported by the Ceramic Raw Materials & Ceramic Materials Testing Division, Institute of Glass and Ceramic Research &Testing (IGCRT), Strengthening of Institute of Glass and Ceramic Research & Testing (SIGCRT) of BCSIR project, IGCRT and Biomedical and Toxicological Research Institute (BTRI), Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhaka, Bangladesh.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Show Sections

[1] Afshari M., Rouhani Isfahani A., Hasani S., Davar F., Jahanbani Ardakani K., . Effect of apple cider vinegar agent on the microstructure, phase evolution, and magnetic properties of CoFe _{2 O} 4 magnetic nanoparticles. Int. J. Appl. Ceram. Technol.. 2019;16:1612-1621.
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[2] Ahemen I., De D.K., Dejene F.B., Viana B., . White light tunable emissions from ZnS: Eu 3+ nanophosphors over 330–465 nm excitation range for white LED applications. Mater. Res. Express. 2016;3:045016
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[3] Ahmed I.S., Shama S.A., Moustafa M.M., Dessouki H.A., Ali A.A., . Synthesis and spectral characterization of CoxMg1-xAl2O4 as new nano-coloring agent of ceramic pigment. Spectrochim. Acta A. 2009;74(3):665-672.
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[4] Bootz A., Vogel V., Schubert D., Kreuter J., . Comparison of scanning electron microscopy, dynamic light scattering and analytical ultracentrifugation for the sizing of poly(butyl cyanoacrylate) nanoparticles. Eur. J. Pharm. Biopharm.. 2004;57:369-375.
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[5] Boudiaf S., Nasrallah N., Mellal M., Belabed C., Belhamdi B., Meziani D., Mehdi B., Trari M., . Synthesis and characterization of semiconductor CoAl2O4 for optical and dielectric studies: application to photodegradation of organic pollutants under visible light. Optik. 2020;219:165038
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[6] Carta G., Casarin M., El Habra N., Natali M., Rossetto G., Sada C., Tondello E., Zanella P., . MOCVD deposition of CoAl2O4 films. Electrochim. Acta. 2005;50:4592-4599.
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[7] Cavalcante P.M.T., Dondi M., Guarini G., Raimondo M., Baldil G., Pigments D., . Colour Performance of Ceramic Nano-pigments. Dyes Pigments. 2009;80:226-232.
[Google Scholar]

[8] J. Chandradass, M. Balasubramanian, K.H. Kim, 2010. https://www.sciencedirect.com/science/article/pii/S0925838810016968. J Alloys Compd 506, 395–399.

[9] Cho W.S., Kakihana M., . Crystallization of ceramic pigment CoAl2O4 nanocrystals from Co-Al metal organic precursor. J. Alloys Compd.. 1999;287:87.
[Google Scholar]

[10] Choudhary B.L., Hasan P.M.Z., Darwesh R., Kumar S., Dalela S., Dolia S.N., Alvi P.A., . Low temperature field dependent magnetic study of the Zn0. 5Co0. 5Fe2O4 nanoparticles. J. Magn. Magn. Mater.. 2021;536:168102
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[11] Choudhary B.L., Kumari N., Kumari J., Kumar A., Dolia S.N., . Relaxation mechanism in Ni0.5Zn0.5Fe2O4 nanocrystalline ferrite at a lower temperature. Mater. Lett.. 2021;304:130731
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[12] Cui H, Zayat M, Levy D., n.d. Sol-Gel Synthesis of Nanoscaled Spinels Using Propylene Oxide as a Gelation Agent. J. Sol-Gel Sci. Technol. 35, 175–181.

[13] Darvishi M., Hasani S., Mashreghi A., Rezvan M.T., Ziarati A., . Application of the full factorial design to improving the properties of CoFe2O4 nanoparticles by simultaneously adding apple cider vinegar and agarose. Mater. Sci. Eng. B. 2023;297:116754
[Google Scholar]

[14] Davar F., Niasari M.S., . Synthesis and characterization of spinel-type zinc aluminate nanoparticles by a modified sol–gel method using new precursor. J. Alloys Compd.. 2011;509:2487-2492.
[CrossRef] [Google Scholar]

[15] de la Calle I., Soto-Gómez D., Pérez-Rodríguez P., López-Periago J.E., . Particle size characterization of Sepia Ink Eumelanin biopolymers by SEM, DLS, and AF4-MALLS: A comparative study. Food Anal. Methods. 2019;12:1140-1151.
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[16] Debnath S., Das R., . Study of the optical properties of Zn doped Mn spinel ferrite nanocrystals shows multiple emission peaks in the visible range –a promising soft ferrite nanomaterial for deep blue LED. J. Mol. Struct.. 2020;1199:127044
[CrossRef] [Google Scholar]

[17] Deraz N.M., Fouda M.M.G., . Synthesis, structural, morphological properties of cobalt- aluminum nano-composite. Int. J. Electrochem. Sci.. 2013;8:12.
[Google Scholar]

[18] Desouza L., Zamian J., Darochafilho G., Soledade L., Dossantos I., Souza A., Scheller T., Angelica R., Dacosta C., . Blue pigments based on CoxZn1−xAl2O4 spinels synthesized by the polymeric precursor method. Dyes Pigments. 2009;81:187-192.
[CrossRef] [Google Scholar]

[19] Dhak D., Pramanik P., . Particle size comparison of soft-chemically prepared transition metal (Co, Ni, Cu, Zn) aluminate spinels. J. Am. Ceram. Soc.. 2006;89:1014-1021.
[CrossRef] [Google Scholar]

[20] Duan X.L., Yuan D.R., Sun Z.H., Luan C.N., Pan D.Y., Xu D., Lv M.K., . Preparation of Co2+-doped ZnAl2O4 nanoparticles by citrate sol–gel method. J. Alloys Compd.. 2005;386:311.
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[21] L. Gama, M.A. Ribeiro, B.S. Barros, R.H.A. Kiminami, I.T. Weber, A.C.F.M. Costa, 2009. Synthesis and characterization of the NiAl2O4, CoAl2O4 and ZnAl2O4 spinels by the polymeric precursors method. J Alloys Compd 483, 453–455. http://dx.doi.org/10.1016%2Fj.jallcom.2008.08.111.

[22] Ge D.-L., Fan Y.-J., Qi C.-L., Sun Z.-X., . Facile synthesis of highly thermostable mesoporous ZnAl _{2 O} 4 with adjustable pore size. J. Mater. Chem. A. 2013;1:1651-1658.
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[23] Gingasu D., Mindru I., Patron L., Marinescu G., Ianculescu A., Surdu V.-A., Somacescu S., Preda S., Oprea O., . Synthesis of cobalt aluminate nanoparticles by combustion methods using cinnamon bark extract. Rev. Roum. Chim.. 2018;63:459-466.
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Exploration of properties (crystallographic, morphological, optical) of nano cobalt aluminate synthesized by facile sol–gel method: Effects of sintering temperature

Abstract

Keywords

Sol–gel

CoAl2O4

XRD

FESEM

PXRD

CIE-L*a*b*

1 Introduction

2 Experimental

2.1 Materials

2.2 Synthesis

2.3 Characterization

3 Results and discussion

3.1 XRD analysis

3.1.1 The Debye-Scherrer equation

3.1.2 W-H model

3.1.3 H-W model

3.1.4 SSP model

3.2 FTIR analysis

3.3 Morphological analysis

3.4 Compositional analysis of CoAl2O4 nanoparticles

3.5 Zeta potential analysis of CoAl2O4

3.6 Nanoparticle size analysis of CoAl2O4

3.7 Color property analysis of CoAl2O4 at different sintering temperature

3.8 Band gap energy

4 Conclusion

CRediT authorship contribution statement

Acknowledgements

Declaration of competing interest

References

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CIE-Lab*

3.4 Compositional analysis of CoAl₂O₄ nanoparticles

3.5 Zeta potential analysis of CoAl₂O₄

3.6 Nanoparticle size analysis of CoAl₂O₄

3.7 Color property analysis of CoAl₂O₄ at different sintering temperature