The phase identification of each sample was accomplished by carrying out XRD and the crystallite size was calculated using Debye- Scherrer equation
$$D=\frac{0.9\lambda }{\beta cos\theta }$$
λ is the wavelength of X-ray used, β is the full width at half maxima in radian of the most prominent peak, θ is half the peak center of the most prominent peak.
The XRD pattern of NiFe2O4, MoO3 and NiFe2O4 /MoO3 composite is given in Fig. 1(a). The spectra of NiFe2O4 indicate the presence of planes (111), (220), (311), (222), (400), (422), (511), (440), (620) and (533) corresponding to 2θ 18.570, 30.360, 35.740, 37.354, 43.439, 53.919, 57.460, 63.083, 71.5418, 74.820 as per JCPDS Card No. 86-2267 (Heiba et.al (2015); Kambale et.al 2009; Anwar et.al 2020). The crystallite size was found to be 18.3 nm. Comparison with JCPDS card No. 05-0508 reveals that the synthesized α-MoO3 possesses orthorhombic structure (Krishnamoorthy et.al 2014; Nagabhushana et.al 2014). The peaks at 18.217, 23.501, 25.847, 27. 455, 33.844, 35.583, 39.147, 46.493, 49.405, 58.968, 64.922, 65.357, 67.704 existence of planes (020), (110), (040), (021), (101), (041), (150), (210), (002), (112), (042), (171), (190) is confirmed in the XRD pattern. The peaks corresponding to both the constituent phases are visible in the XRD spectrum of the composite. The peaks labeled as M indicate those of MoO3 and N corresponding to NiFe2O4.
The FTIR spectra of samples are shown in Fig. 1(b). NiFe2O4 and NiFe2O4 /MoO3 samples show bands at 420 & 587 cm− 1 corresponding to Ni-O bond at tetrahedral site and Fe-O at octahedral respectively arising from the spinel structure of nickel ferrite (Hema et.al 2020; Jacob et.al 2011). Whereas for MoO3 there is only a single peak at 540 cm− 1 attributing to the Mo-O bond. Additional peaks near 1000 cm− 1 for MoO3 and NiFe2O4 /MoO3 composite indicate the presence of Mo-O bonds (Maheswari et.al 2017). All other dips are those arising from the alcohol segments from remnants of ethylene glycol or ethanol i.e., at 3431.47 cm− 1 due to OH stretching; 1382 cm− 1 assignable to O-H bending; 2334 cm− 1 attributed to CO2 stretching; 1624 cm− 1 as a result of C = C stretching and 1462 cm− 1 because of C-H bending (Thankachan et.al 2013). Details of groups present in samples corresponding to FTIR peaks are listed in Table 1.
Table.1. FTIR peak details.
FTIR Absorption Wavenumber
cm− 1
|
NiFe 2O4
|
MoO3
|
NiFe2O4 /MoO3
|
Group
|
420
|
✓
|
✗
|
✓
|
Ni-O tetrahedral
|
540.25
|
✗
|
✓
|
✗
|
Mo-O
|
587.31
|
✓
|
✗
|
✓
|
Fe-O octahedral
|
865.97
|
✗
|
✓
|
✗
|
C-H bending
|
1000.70
|
✗
|
✓
|
✓
|
Mo-O
|
1463.59
|
✓
|
✗
|
✗
|
C–H bending
|
1624.87
|
✓
|
✓
|
✓
|
C = C stretching
|
2334.60
|
✓
|
✗
|
✓
|
O = C = O stretching
|
3431.47
|
✓
|
✓
|
✓
|
O-H stretching
|
The TEM image of ferrite as shown in Fig. 2.(a) indicates the particles are more or less spherical in shape with uniform size. The concentric ring-like SAED pattern in Fig. 2.(b) suggests that the NiFe2O4 formed is polycrystalline in nature. The SEM image of α - MoO3 is shown in Fig. 2.(c) confirms the nanorod morphology with its diameter ranging from 82.46 nm to 170.88 nm. The FESEM image of NiFe2O4 /MoO3 composite is illustrated Fig. 3.(a). The agglomerated particles and rods are seen in the image of nanocomposite. Also, the EDAX result authenticates that the composite contains all the elements nearly in the expected ratio. The EDAX spectra is given as Fig. 3.(b) and the data are displayed in Table.2.
Table.2. Percentage of elements in NiFe 2O4 /MoO3 obtained by EDAX
Element
|
Line Type
|
Wt%
|
Atomic %
|
Expected Percentage
|
O
|
K series
|
31.55
|
68.13
|
68.64
|
Fe
|
K series
|
19.1
|
11.82
|
12.18
|
Ni
|
K series
|
9.97
|
5.87
|
6.09
|
Mo
|
L series
|
39.38
|
14.18
|
13.09
|
Total:
|
|
100
|
100
|
|
The XPS data of NiFe2O4 reveals the presence of Ni 2p1/2, Ni 2p3/2, Fe 2p1/2, Fe 2p3/2 and O 1s orbitals at 872.45 eV, 854.8 eV, 723.85 eV, 710.55 eV and 530.4 eV respectively (shown in Fig. 4.a-c). (Sankaranarayanan et.al 2020) Whereas two peaks corresponding to 3d3/2 and 3d5/2 at binding energy 234.05 eV and 230.95eV along with O 1s peak at 528.9 eV were observed for MoO3 (shown in Fig. 4.d,e) ((Thomas et.al 2021). The nanocomposite exhibits peaks at 530.1 eV, 235.65 eV, 232.5 eV, 723.85 eV, 710.75 eV, 872.05 eV and 854.8 eV of O 1s, Mo 3d3/2, Mo 3d5/2, Fe 2p1/2, Fe2p3/2, Ni 2p1/2 and Ni 2p3/2 respectively (shown in Fig. 4.f-i). The peak of Mo 3d shifts to higher binding energy in composite, which can be attributed to a decrease in electron density around the nucleus (Krishna et.al 2022).
The DRS UV-vis spectra of samples are shown in Fig. 5. (a). The spectra show reflectance peaks in the visible region. The Tauc plot for indirect band gap materials assumes that the energy-dependent absorption coefficient α can be expressed by the following equation
$${A\left(\alpha h\vartheta \right)}^{2}=h\vartheta -{E}_{g}$$
where Eg is the band gap (Makula et.al 2018). The absorption coefficient α is calculated from the DRS data using the formula
$$\alpha =\frac{K}{S}=\frac{{(1-R)}^{2}}{2R}$$
where K and S are absorbance and scattering coefficients, R is the reflectance percentage.
The band gap is calculated from [αhν]2 versus hν plot. The Tauc plots are shown in Fig. 5. (b), (c) and (d) and calculated band gaps are 3.15 eV, 2.25eV and 3.0 eV for NiFe2O4, MoO3 and NiFe2O4/MoO3 samples respectively.
The photocatalytic study is shown in Fig. 6.a. It reveals that all the three samples exhibit good activity in UV light. The maximum absorbance wavelength for congo red dye is at 492 nm and the degradation is inferred from the decrease in the absorbance value.
In order to determine the reactive species involved in the reaction, scavenger studies were carried out. Isopropyl alcohol (IP) was used as a scavenger for. OH free radical, EDTA for superoxide (. O2− ) radical, benzoquinone (BQ) for holes and DMSO for electrons. As indicated by Fig. 6. b and c the degradation was most retarded by IP confirming. OH free radical as the reactive species involved. The photocatalytic degradation of congo red dyes based on the release of. OH free radicals from the electrons excited to the conduction band from valence band via photoexcitation (Kirankumar et.al 2017). The reaction mechanism proposed for photocatalytic degradation is as follows (Ali et.al 2020):
$$Catalyst + h\vartheta \to {e}_{CB}^{-}+{h}_{CB}^{+}$$
$${H}_{2}O+{h}_{VB}^{+}\to {OH}^{∎}+{H}^{+}$$
$${O}_{2}+{e}_{CB}^{-}\to {O}^{-2}$$
$${O}^{-2}+{H}^{+}\to H{O}_{2}^{∎}$$
$$2H{O}_{2}^{∎}\to {H}_{2}{O}_{2}+{O}_{2}$$
$${H}_{2}{O}_{2}\to 2\text{O}{H}^{-}$$
$${OH}^{-}+Dye\to C{O}_{2}+{H}_{2}\text{O}$$
The percentage of dye degraded after time t is calculated using the equation:
$$Degradation \% =\frac{{C}_{O}-{C}_{t}}{{C}_{O}} \times 100$$
where C0, is the initial concentration, Ct is the concentration at time t.
The dye solution without any catalyst showed 4.6% degradation, NiFe2O4 containing sample exhibit 49.4% of degradation, MoO3 degraded 52% and nanocomposite degraded by 74.5% of dye in 30 minutes. While MoO3 was added to the dye solution the color of the solution instantly changed into blue. This is supported by the degradation study by the shift of the peak at 492 nm to 550 nm. When MoO3 is added to H2O, it gets converted to molybdic acid and congo red is a dye indicator which turns bluish violet at acidic pH (Rehman et.al 2012). In the case of composite no visible change in the color of dye solution was observed which shows molybdic acid is not produced when composite is added. NiFe2O4 being a wide band gap material exhibits a lower degradation rate. Among the three catalysts, MoO3 owes the least band gap and the reaction takes place in acidic pH with efficiency lower than the composite. Whereas NiFe2O4 /MoO3 even though possess a band gap intermediate to their constituents showcased greater degradation percentage. Hence, all the samples definitely serve as a photocatalyst in Congo red degradation under UV light. The increasing order of the catalytic efficiency is NiFe2O4, MoO3 and NiFe2O4 /MoO3 composite.
The antibacterial activity shown by the samples is illustrated in Fig. 7. Among the three preparations tested nickel ferrite showed an inhibition zone of 13mm against E.coli and 12mm against S.aureus, nanocomposite of NiFe2O4/MoO3 showed a good inhibition zone of 15mm against E.coli and 13mm against S.aureus and molybdenum trioxide exhibited maximum inhibition zone of 16mm against both E.coli and S.aureus. Numerous studies have reported that NiFe2O4 nanoparticles have the least amount of antibacterial activity when compared to other spinel ferrite structures. The results are included in the Table. 3. The differences in the makeup and structure of bacterial cell walls are reflected in the diversity of bacterial activity (Bhosale et.al 2018). Molybdenum oxide (MoO3 ) nanoparticles have previously been shown to have an antibacterial impact on a few different microorganisms. It has been suggested that the MoO3 antibacterial activity results from the production of hydroxonium ions (H3O+) from molybdic acid (H2MoO4) which create an acidic environment (Krishnamoorthy et.al 2013; Krishnamoorthy et.al 2014; Zollfrank et.al 2012). The H3O+ ions damages the DNA thereby inhibiting the protein production and thereby causing cell destruction (Farooq et.al 2023).
Table.3. Inhibition zone diameter on E.coli and S.aureus on NiFe2O4, MoO3 and NiFe2O4 /MoO3
Sl. No.
|
Name of the Compound
|
Inhibition zone diameter (mm)
|
E.coli
|
S.aureus
|
1.
|
NiFe2O4
|
13
|
12
|
2.
|
NiFe2O4/MoO3 nanocomposite
|
15
|
13
|
3.
|
MoO3
|
16
|
16
|
4.
|
Control (DMSO)
|
10
|
11
|