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 CdxZn1-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
$$\:D=\frac{k\:\lambda\:}{\beta\:\:\text{c}\text{o}\text{s}\theta\:}\:,$$
1
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 CuKα 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 Cdx Zn1 − 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]:
The dislocation density (δ) is given as
$$\:\delta\:=\frac{1}{{D}^{2}}$$
3
,
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 Cdx Zn1-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 Cdx Zn1-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 Cdx Znx-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 (Eg) of the prepared Cdx Zn1 − xCo2O4 samples are estimated from the Tauc's equation [22] given by,
(αh ν)n = k (h ν - Eg), (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 Eg 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 Eg for Cdx Zn1 − 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 CdxZnx−1Co2O4 has two band gaps. Figure 7 shows these two Eg for ZCO as a represented example. These two Eg are estimated in Fig. 7 to be 2.93 and 2.09 eV. The higher Eg corresponds the transition from O 2p of the valance to Co 3d-eg of the conduction band. The other Eg corresponds the transition from O 2p of the valance to Co 3d-t2g of the conduction band [20].