Novel syntheses of wide-band gap semiconducting Cdx Znx-1 Co2O4 of spinel nanostructure: Characterization and optical properties

doi:10.21203/rs.3.rs-4999074/v1

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Novel syntheses of wide-band gap semiconducting Cd_x Zn_x_-1Co₂O₄ of spinel nanostructure: Characterization and optical properties

https://doi.org/10.21203/rs.3.rs-4999074/v1

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Cadmium doped zinc cobaltite spinel structure in the form of Cd_xZn_x-1Co₂O₄ (0 ≤ x ≤ 1) are synthesized by sol-gel method. A good crystallinity of the ZnCo₂O₄ and its dopants samples are indicated by the sharpness and strong peaks of X-ray diffraction patterns. The particle size is independent of the ratio of incorporated Cd into the ZnCo₂O₄ matrix. The FT-IR transmission spectrum of each Cd_x Zn_1-xCo₂O₄ complex shows two strong vibration peaks that are ascribed to the spinel structure's octahedral and tetrahedral. The occurrence of light absorption by Cd_x Zn_x-1 Co₂O₄ is due to the photoexcitation of electrons from O 2p to Co 3d transitions and d-d transition of Co 3d. The obtained values of the optical band gap (E_g) for Cd_x Zn_1-xCo₂O₄ varies from 2.93 eV for ZnCo₂O₄ (x = 0) to 2.15 eV for CdCo₂O₄ (x = 1).

Zinc cobaltite

Spinel structure

Cadmium doped

Sol-gel method

Absorption spectra

Metal oxide complexes as nanoparticles have taken the attentions of many researchers because of their broad and worthy applications in diverse fields. These nanomaterials of semiconducting feature have distinctive properties owing to their great surface to volume relation, effect of quantum confinement and well reaction rate [1, 2]. Mixed oxide nanostructured spinels reveal remarkable of many applications in different fields such as catalysis, optics and electricity [3–5].

Zinc cobaltite (ZnCo₂O₄), as binary metal oxide with normal spinel structure, has been found able to be tuned its chemical, electrical and magnetic properties and its structure characters by doping of suitable amount of different ions into the spinel structure [6]. ZnCo₂O₄ (ZCO) has normal spinel structure with trivalent cobalt ions in octahedral (B) site and divalent zinc ions in tetrahedral (A) sites. Multiple oxidation states have been produced as a result of the presence of dopant cations and two different types host of cations, which further aid in the conductivity of the charges.

The characteristic chemical and physical features of doped spinel zinc cobaltite depend on synthesis method, kind of dopant ion and its concentration, distribution of ions among the octahedral and tetrahedral voids, etc. [7]. Incorporation cations with radii that differ from the host cations alters the system's microstructural properties, which in turn modifies some other properties. Cadmium ion doping into the ZCO matrix causes lattice strain to the spinel structure because Cd²⁺ ions have larger ionic radii (0.95 Å) than those of Co²⁺ ions (0.70 Å), Co³⁺ ions (0.60 Å) and Zn²⁺ ions (0.74 Å). This structural distortion in doped spinel sites is expected to result in a significant change in the optical and electrical properties.

Zinc cobaltite nanostructured material has been broadly used in Li-ion batteries [8], gas sensor [9] and application of electro-catalyst [10] because of their excellent features of electronic structure. ZCO and its based materials belongs to wide-bandgap semiconductors which have bandgaps in the range above 2 eV [11, 12]. Usually, these materials have electronic properties which fall in among the ones of traditional semiconductors (bandgap from 0.6 to1.5 eV) and insulators. Semiconductors of wide-bandgap allow devices to function at a great deal better voltages, temperatures and frequencies than traditional semiconductor substances like gallium arsenide and silicon.

This work aims to synthesize novel cadmium doped zinc cobaltite spinel structure in the form of Cd_x Zn_x-1 Co₂O₄ (x = 0.00, 0.25, 0.50, 0.75, 1.00). The successful synthesis process and characterization of CdZnCo₂O₄ spinel structure is confirmed by spectroscopic measurements. Depending on the Cd dopant ratio in ZnCo₂O₄ matrix, the absorption of light in UV-Vis region is analyzed and the optical band gap is estimated.

2.1 Syntheses

Spinal ZnCo₂O₄ nanoparticles doped with cadmium ions were synthesized by sol-gel method with sodium hydroxide as the chelating agent. The starting materials to synthesize Cd_xZn_1−xCo₂O₄ nanoparticles powder were cadmium chloride hexahyhydrate (CdCl₂.6H₂O), zinc chloride (ZnCl₂), cobalt chloride hexahyhydrate (CoCl₂.6H₂O) and sodium hydroxide (NaOH) in the stoichiometric ratio of (x = 0.00, 0.25, 0.50, 0.75, 1.00) with respect to propellant chemistry. All the reagents were dissolved in the distilled water. Sodium hydroxide solution was added slowly drop by drop to the metallic solutions with continuous stirring. The homogenous solution was prepared at room temperature. The obtained gel was dried overnight in an oven at 60°C. The gel precursor was annealed at 600°C for 2 h to eliminate the organic constituents. Finally, the obtained ZnCo₂O₄ was ground into fine powder.

2.2 Measurements

Elemental, morphological, structural, optical measurement techniques were employed to characterize and analyze the prepared samples. Elemental and morphological examination analysis were performed using a Field Emission Gun Scanning Electron Microscope (FEG-SEM) equipped with Energy Dispersive X-ray (EDX) spectroscopy. Powder X-ray diffraction (XRD) using a Philips PW 1710 diffractometer with Cu Kα radiation determined the crystalline phases present. Fourier Transform Infrared (FTIR) spectroscopy on KBr pellets was conducted using a Nicolet FTIR 6700 spectrometer to identify functional groups within the 400–4000 cm⁻¹ range. Optical properties were assessed through UV-Vis spectroscopy on a JASCO V-770 spectrophotometer within the 200–1000 nm wavelength range.

3.1. Energy dispersive X-ray spectra

The energy dispersive X-ray, EDX, spectra of pure ZCO nanoparticles is shown Fig. 1a. The existence of zinc, cobalt and oxygen in the prepared sample confirms the probability of creating ZnCo₂O₄. Also, the absence of other elements than Zn, Co and O confirms the purity of the material. In the case of Cd doped ZCO, the intensity of the Cd peak reflected depending upon Cd ratio. The peak intensity increases with Cd dopant as 0.25, 0.75, and 1.00 ratios as shown in Fig. 1(b–d).

3.2 Scanning electron microscope (SEM)

SEM images of Cd_xZn_1-xCo₂O₄ are shown in Fig. 2. Sample of ZCO (Fig. 2a) shows agglomerates of nanoparticles of high concentration with irregular shape and sizes. For Cd doped ZCO samples (Fig. 2b, c), the cubic crystal structure of the nanoparticles becomes clearly demonstrated. While for the CdCo₂O₄ sample, the morphology is changed into the non-uniform shaped agglomerated nanoparticle as shown in Fig. 2d.

3.3 X-ray diffraction analysis

X-ray diffraction patterns of undoped ZnCo₂O₄ and Cd doped

samples are shown in Fig. 3. It is found that the observed diffraction data are consistent with the JCPDS file (file no. 23-1390) in the range from 10° to 80° confirming the formation of cubic ZnCo₂O₄ single phase. For the Cd doped samples, the lack of any additional peak indicates the Cd dopant is incorporated in the structure of spinel ZnCo₂O₄. A good crystallinity of the ZnCo₂O₄ and its dopants samples are indicated by the sharpness and strong peaks.

The diffraction peaks at 2θ = 18.96°, 31.24°, 36.83°, 38.48°, 44.80°, 55.68°, 59.38°, and 65.27° shown in Fig. 1 correspond to the reflection planes (111), (220), (311), (222), (400), (422), (511), and (440), respectively, which characterize the spinel structure of ZnCo₂O₄ [14]. Scherrer's equation [14] in the form

$$\:D=\frac{k\:\lambda\:}{\beta\:\:\text{c}\text{o}\text{s}\theta\:}\:,$$

is employed to calculate the average crystal size (D). Here, k is the shape factor constant of value 0.89, λ is the wavelength of used CuK_α radiation (λ = 1.5406 Å), β is the full width at half maximum (FWHM) measured in radians and θ is the angle of Bragg’s diffraction. By using the XRD (311) plane which corresponds to 2θ = 36.83°, D is found to be ~ 19.33 nm for ZnCo₂O₄ (x = 0). This value of D is very close to D ~ 20 nm that obtained for ZnCo₂O₄ synthesized by using the Sol-gel method [15]. As seen from Table 1, the value of D is independent of the ratio of Cd ion that incorporated into the ZnCo₂O₄ matrix.

Table 1

Crystallite size D (nm), strain ε (°), and Dislocation density δ (nm)⁻² for Cd_x Zn_1 − x Co₂O₄ nanoparticles.
x	D nm	ε (°)	δ (nm)⁻²
0.00	19.33	6.45	8.78
0.25	20.56	6.86	7.99
0.50	19.97	6.66	8.23
0.75	23.00	7.68	6.20
1.00	20.21	6.74	8.03

The micro-strain (ε) in the crystal lattice as a structure parameter is calculated on the basis of FWHM from the XRD results using the following equation [16]:

ε = β /4 tan θ (2)

The dislocation density (δ) is given as

$$\:\delta\:=\frac{1}{{D}^{2}}$$

The calculated values of ε and δ are presented in Table 1. The micro-strain and dislocation density behave similar to the crystallite size as Cd ion dopant increases as shown in Table 1.

3.4 FT-IR spectra analysis

Through the use of the vibrational modes present in the synthetic samples, FT-IR spectroscopy has been used to evaluate the bonds. The FT-IR transmission spectra of ZCO and Cd_x Zn_1-x Co₂O₄ are displayed in Fig. 4. The two strong vibration peaks at 579 and 669 cm^-1 are ascribed to the spinel structure's octahedral and tetrahedral complexes, respectively [17]. They correspond to the metal oxygen stretching from tetrahedral and octahedral sites. The absence of an absorption band in the region of higher wavenumbers, that may arise due to organic materials used in the synthesis process, confirms the successful synthesis of the ZnCo₂O₄ spinel. The observed slight shifting of these peaks positions from the pure ZnCo₂O₄ sample is attributed to the change in reduced mass of the system caused by substitution of Cd ions in the site of the host ion.

The observed peak at 1100 cm^− 1 has been attributed to the C–O bond vibration that may participate to the carboxyl group [18]. The broad band at 1626 cm^− 1 is due to the bending vibration modes of the H-O-H bonds of absorbed H₂O molecules being present on the material surface. The broadband in the range of 3432–3470 cm^− 1 that centered at 3446 cm^− 1 is assigned to the stretching variation of O–H bond of water molecules. Similar to the undoped ZnCo₂O₄ sample, the Cd doped samples also display the same peaks.

3.5 Absorption spectra

Important information about the electronic transition mechanism and optical band gap of different materials can be obtained by determining the absorption spectrum in the UV-Vis regions. Figure 5 shows the absorbance (A) as a function of the wavelength (λ) of the incident photons for Cd_x Zn_1-x Co₂O₄ (0.0 ≤ x ≤ 1.0). As shown from Fig. 5, the nanoparticles spinel-type ZCO and its doped show good absorbance of light from 200 nm to 780 nm. The occurrence of light absorption by ZCO has been attributed to the photoexcitation of electrons from O 2p to Co 3d transitions and d-d transition of Co 3d [19]. The absorbance increases as the dopant content of Cd ion increases. In fact, doping a semiconductor (in this case, ZCO) with a metal ion such as Cd^+ 2 tends to modify the semiconductor's valence or conduction band, resulting in intra-band gap levels and band gap narrowing, which in turn increases photon absorption. The enhancement in the absorption of doped samples reveals the light utilization of nanocomposite when ZCO was doped with Cd ions.

As seen from Fig. 5, three distinct absorption characteristics for Cd_x Zn_x-1Co₂O₄ are observed at wavelengths peaking at 268, 532 and 778 nm. The transitions from O 2p of valance band to two states-Co 3d of conduction band are responsible for the two absorption bands of higher energies. Whereas, a d-d transition at Co³⁺ cation located in octahedral site is associated with the transition corresponding the absorption peaking at 778 nm [20]. The photocatalytic activity may be significantly impacted by the last transition [21].

3.6 Optical band gap evaluation

The minimum energy needed, known as band-energy, is necessary for moving electrons from the valance to conduction band. This is the energy difference between a semiconductor's valance and conduction bands. Optical band gap energy (E_g) of the prepared Cd_x Zn_1 − xCo₂O₄ samples are estimated from the Tauc's equation [22] given by,

(αh ν)ⁿ = k (h ν - E_g), (4)

where α is the coefficient of absorption, (h ν) is the incident photon energy, k is a constant dependent material and n is a power index that depends on electronic transition. For allowed and forbidden indirect interband transition, the value of n is 1/2 and 3/2. Whereas n is 2 and 3 for direct allowed and forbidden transition. The values of E_g are calculated by intersecting the baseline of (αhν)² vs. hν plot (shown in Fig. 6) by extrapolating the band line where (αhν)² becomes zero. From the best fit to the experimental data, the obtained values of E_g for Cd_x Zn_1 − xCo₂O₄ varies from 2.93 eV for ZnCo₂O₄ (x = 0) to 2.15 eV for CdCo₂O₄ (x = 1). The optical band gap for ZnCo₂O₄, synthesized by the hydrothermal method, has been reported to be 2.73 eV [23]. In the present study, the decrease of the optical band gap energy with increasing Cd ion dopant can be attributed to structural changes due to introduce of defect states in the lattice structure. The presence of high density levels with energies near the bands can produce band tailing in accordance with reported results for some polycrystalline materials [24].

It has been reported that the band structure of ZCO is described by considering O 2p orbital as the valence band and Co 3d orbital as the conduction band providing a unique internal electronic conversion [25]. Each complex with different Cd ratio of Cd_xZn_x−1Co₂O₄ has two band gaps. Figure 7 shows these two E_g for ZCO as a represented example. These two E_g are estimated in Fig. 7 to be 2.93 and 2.09 eV. The higher E_g corresponds the transition from O 2p of the valance to Co 3d-e_g of the conduction band. The other E_g corresponds the transition from O 2p of the valance to Co 3d-t_2g of the conduction band [20].

ZnCo₂O₄ and Cd-doped ZnCo₂O₄ nanoparticles are synthesized using the sol-gel method. X-ray diffraction patterns confirm the formation of cubic single phase for ZnCo₂O₄ and its dopants samples. The average crystal size of ZnCo₂O₄ nanoparticles is calculated to be around 19 nm. It is independent of the ratio of Cd ion that incorporated into the ZnCo₂O₄ matrix. For each Cd_x Zn_1-xCo₂O₄ complex, there are two absorption bands, resulting in two optical band gaps, due to transitions from O 2p of the valance band to two states-Co 3d of the conduction band. The decrease of the optical band gap energy with increasing Cd ion dopant can be attributed to structural changes due to introduce of defect states in the lattice structure.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.The authors have no conflicts of interest to declare that are relevant to the content of this article.All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.The authors have no financial or proprietary interests in any material discussed in this article.

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Competing interest reported. The authors have no relevant financial or non-financial interests to disclose. The authors have no conflicts of interest to declare that are relevant to the content of this article. All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript. The authors have no financial or proprietary interests in any material discussed in this article.

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You are reading this latest preprint version

Novel syntheses of wide-band gap semiconducting Cd_x Zn_x_-1Co₂O₄ of spinel nanostructure: Characterization and optical properties

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Version 1

Abstract

Figures

1. Introduction

2. Experimental technique

2.1 Syntheses

2.2 Measurements

3. Results and discussion

3.1. Energy dispersive X-ray spectra

3.2 Scanning electron microscope (SEM)

3.3 X-ray diffraction analysis

3.4 FT-IR spectra analysis

3.5 Absorption spectra

3.6 Optical band gap evaluation

4. Conclusion

Declarations

References

Additional Declarations

Status:

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