Introduction

Zinc ferrite (ZF) is one of the famous members of the ferrite’s family, which receives scientific attention due to its interesting magnetic behavior and depends on the synthesis procedure and the particle size [1]. The sol–gel process is suitable for obtaining the ZF nanomaterial, which is responsive to visible light. The ZF has received a lot of interest due to its small band gap of 1.9 eV, good optical absorption, stability, and excellent magnetic response, all of which are important for increased photocatalytic activity [2,3,4,5,6]. Previous studies have shown that combining ZF with optoelectronic materials such as TiO2, ZnO, and NiO creates many nanocomposites with good photocatalytic activity [7,8,9,10]. Notably, ZF is a p-type visible light-driven semiconductor, while anatase TiO2 is a well-known n-type semiconductor with an indirect band gap and a long electron–hole lifetime [11, 12]. The combination of ZF and TiO2 makes p–n heterojunction and can thus extend the absorption to the visible light region [13].

The characterization of tin oxide (SnO2) substance has been the subject of several articles during the past few years [14,15,16,17,18]. Tin oxide is a metal oxide, an n-type semiconductor, and has a wide band gap (3.6 eV) with a carrier concentration of 5.7 × 1020 cm−3, making it a good material for chemical gas sensors. It is an auspicious material because of its optical transparency, high chemical stability, high sensitivity to various toxic gases, and compatibility with microfabrication processes. Its superior optical transparency makes it suitable for an impressive range of applications, e.g., in solar cells, as a catalytic support material, in solid-state chemical sensors, etc. [17, 18].

The structure of monoclinic Ga2O3 has an ultra-wide band gap (Eg ~ 4.8 eV), so it can be used in electrical power, industrial control, automotive electronics, and consumer electronics industries [19]. Furthermore, Ga2O3 is a promising candidate for next-generation high-power semiconductor devices such as Schottky barrier diode [20, 21] and metal–oxide–semiconductor field-effect transistors [22].

Oxide-diluted semiconductors (CdO and NiO) have been extensively investigated due to their interesting magnetic properties for spin valves [23]. Cadmium oxide (CdO) is a promising material for solar cell applications due to its high electrical conductivity and optical transmittance in the visible region of the solar spectrum [24]. CdO is polycrystalline of cubic structure and n-type conductivity with an energy gap of 1.89 eV [25]. Furthermore, CdO nanowires and nanotubes have been synthesized by D. H. Fan [26]. Nickel oxide (NiO) is a transition metal oxide with a cubic lattice structure used in numerous applications such as battery electrodes, photo-electron devices, ion storage materials, gas sensors, magnetic materials, thermoelectric materials, catalysts, fuel cells, dye-sensitized photocathodes, electrochromic films, anticancer properties, cytotoxic activity, and nonenzymatic glucose sensors [27]. NiO is a p-type semiconductor with a band gap width of approximately 3.6 to 4.0 eV [28].

Two-phases, multilayers, and composites are receiving more and more attention because of their versatility. Furthermore, combining different materials is a complicated problem in condensed matter physics and materials science. As a result, many researchers are working on this issue. Michael Lorenz [29], for example, investigated ZnFe2O4–BaTiO3, while Shahrzad Falahatnezhad [30] investigated BiFeO3–ZnFe2O4, Mohammad Bagher [31] investigated ZnFe2O4 nanorods on reduced graphene oxide (rGO), and other ZnFe2O4 nanoparticle-decorated V2O5 was investigated [32]. This work is part of a research project [33,34,35] that studied the relationship between two compounds with different properties and compositions. Here we will examine other nanocomposites with distinct properties and structures together. This study focuses on preparing and characterizing ZF prepared in the presence of traces from the 2nd, 3rd, and 4th oxides such as CdO, NiO, Ga2O3, SnO2, and TiO2. In other words, the ZF is decorated by CdO, NiO, Ga2O3, SnO2, and TiO2, so the physical properties of the ZF may change and can be controlled for different technological applications. In detail, the nanocomposites ZF/RO were synthesized by the sol–gel technique. XRD, FTIR SEM, HR-TEM, VSM, and ESR were used to study the characterization of ZF/RO. Brunauer–Emmett–Teller (BET) was performed to measure the specific surface area. Diffuse reflectance spectroscopy and photoluminescence were performed for the optical properties and photocatalytic performance.

Experimental details

Materials and method

To prepare the nanocomposites ZF/RO in one step, the oxides must be ready and of high quality and then added to the ZF during the preparation. Precursors included zinc nitrate (Zn(NO3)2.6H2O, 98% purity) and ferric nitrate (Fe(NO3)3. 9H2O, 98%). The citric acid (99.5%) was used as a chelating agent. The pH was adjusted using the ammonia solution. The ZF was decorated with CdO, NiO, Ga2O3, SnO2, and TiO2 (STREM CHEMICALS, 99.9%). The purity of all chemicals and reagents was of analytical grade, and they were all used without further purification. Distilled water was used to solubilize all solutions in the preparation and the practical part. Previous works explain the chemical preparation in detail [33,34,35]. The final powder of ZF/RO was grounded to perform XRD, FTIR, SEM, TEM, ESR, VSM, and UV–Vis studies. The weight ratio of ZF to RO was 3:1.

Characterization

Phase analysis for all nanopowder samples was carried out by X-ray diffraction (XRD). An X’pert pro diffractometer took X-ray powder diffraction (XRD) spectra with Cu Kα radiation of 1.5406 Å (Malvern, GH Eindhoven, Netherlands). The obtained X-ray spectra were analyzed using the Rietveld refinement method employing the Maud program [36], where the phase, space group, lattice constant, and crystallite size were determined. The refinement of the structural parameters continued until convergence is reached with the goodness of fit (S) around 1.0.

The changes in the synthesized samples' shape, morphology, and elemental composition were analyzed by field emission scanning electron microscope (FESEM) images and energy-dispersive X-ray analysis (EDX) spectra obtained from ZEISS. The Fourier transform infrared (FTIR) spectra were recorded in the wavenumber range of 400–4000 cm−1, using Perkin Elmer, Spectroscopy 100, USA.

The Brunauer–Emmett–Teller (BET) surface area and the Barrett–Joyner–Halenda (BJH) for pore size distribution measurements were performed using BELSORP-MR 6 (the desorption was adjusted at 195 °C in nitrogen gas).

The magnetic behavior of the produced samples was examined at room temperature using a Lake Shore 7410 vibrating sample magnetometer (VSM) with a maximum magnetic field (H) of 20 kOe. Microscopic magnetic behavior has been evaluated by ESR spectra that are recorded using Bruker EMX ESR spectrometer.

The optical properties of prepared samples were measured by JASCO V-570 UV–Vis spectrophotometer (Japan) with integrating sphere diffuse reflectance accessory between 200 and 1500 nm. Fluorescence and photoluminescence emission spectra were measured using a Shimadzu RF-5301PC spectrofluorometer (Japan).

The photocatalytic activity of the synthesized nanocomposites was performed in a photoreactor glass cell irradiated with a 1000-W halogen lamp with a light filter (λ > 420 nm) as a visible light source. The lamp was fixed at 20 cm above the sample. The photodegradation experiments for the methylene blue dye (MB) were carried out using a photocatalyst/dye content of ~ 0.5 g/l. This solution was kept in the dark for 30 min before starting the photocatalytic tests to reach the adsorption equilibrium. After equilibration, the dye concentration was estimated and donated as Co for initial concentration and Ct for photo-irradiated samples at time regular intervals. The degradation efficiency of colored solutions has been derived from the following equation:

$$\% \, \;Photocatalytic \, \;performance \, = \, \left( {C_{o} - C} \right)/C_{o} \times \, 100$$
(1)

Results and discussion

Structural properties

As seen in Fig. 1, X-ray diffraction (XRD) was used to analyze the structural characteristics and identify the phases of all composites. Figure 1 depicts the Rietveld refined XRD patterns for all composites, which exhibit the profile fitting and separation of overlapping peaks. Where the markers are related to the experimental data, and the solid lines show the calculated data using Rietveld's method, a good agreement between the experiment and the calculated spectra. As can be seen, all peaks are broad, which indicates that all composites are at the nanoscale. According to the Maud program [36] all nanocomposites contain a sizable amount of zinc ferrite and traces of the accompanying oxide. The ZF was detected in all samples with the planes (220), (311), (222), (400), (422), (511), (440), and (533) of the cubic spinel structure (space group Fd-3 m), as well as trace amounts of the associated oxides ZnO, CdO, NiO, Ga2O3, and SnO2, while for the ZF/Ni and ZF/Ti nanocomposites, the structures of FeNi3 and FeTiO2 and the accompanying oxide were observed. In other words, in the ZF/RO nanocomposites, the ZF was more pronounced, along with traces of the accompanying oxide. In some cases, another structure is created from the interaction between the raw materials, i.e., in all cases, heteronanocomposites appear. It is consistent with the literature [7, 11, 32, 37] that another second phase was observed during the preparation of the ZF/oxides composites.

Figure 1
figure 1

Rietveld refined XRD for ZF nanoparticles and ZF/RO nanocomposites

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The Rietveld refinement parameters (phase, space group, concentration, lattice parameters, crystallite size, and lattice strain) are listed in Table 1. Notice that the ratio of oxides matches the amount that was added. The unit cell length of the ZF is slightly different according to the accompanying oxide. This means that there is a mutual force between the RO (the second phase) and ZF, which affects the unit cell of the ZF. This force either expands or contracts the ZF unit cell, as in ZF/Sn (8.4473 Å) or ZF/Ga (8.4283 Å). Also, this force changed the crystallite size of the ZF (25.1–53.6 nm), as shown in Table 1. Therefore, this mutual force may cause a change in the physical properties of the ZF. This force was observed in previous studies [33,34,35], which was the cause of changes in some physical properties of these compounds. The length of the unit cell of ZF varies from earlier studies [7, 11, 32, 37, 38] due to the various preparation methods.

Table 1 Structural parameters
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The morphological analysis

The images of FESEM were analyzed to know the surface morphology of all samples. Figure 2 shows the prepared FESEM images of the nanocomposites ZF/RO powders. The FESEM images show that the dominant morphology of all samples, except for ZF/Cd, is nanoflakes, and the agglomeration phenomenon exists in all samples. The ZF sample is distinguished from the other samples by its larger size. For the FESEM image of ZF/Cd, it is clear that the nanotube structures are dominant on the surface morphology. From the previous literature, it is found that ZF in nanocomposites takes many forms, such as particles [37], fibers [38], rods [31], or tubes [33], depending on the method of preparation and the other component with ZF. The images of FESEM of the ZF/RO nanocomposites revealed that the nanoflakes of ZF have higher dominance than oxides because the oxides have few ratios and were more evenly dispersed over the ZF surface during the nanocomposite preparation. The characteristic EDAX spectra are shown in Fig. 3, and the analysis results are summarized in the inset tables in Fig. 3. The EDAX shows that the prepared samples are highly pure because only the peaks for the elements O, Zn, Fe, and R are present; no other elements are present. The observed composition ratios of metal ions (R: Zn) and iron to oxygen (Fe: O) are consistent with the expected composition ratio.

Figure 2
figure 2

FESEM images for ZF nanoparticle and ZF/RO nanocomposites

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Figure 3
figure 3

EDX spectra for ZF nanoparticle and ZF/RO nanocomposites

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Further nanosize of the particles is also substantial with our TEM studies. TEM is one of the most powerful and versatile techniques for the characterization of nanostructured systems. Figure 4 shows the TEM images, selective area electron diffraction (SAED) patterns, and the histogram of all ZF/RO samples. From the HR-TEM images, as shown in Fig. 4, all samples are at the nanoscale with variations in particle sizes. In detail, ZF sample is characterized by a homogeneous agglomeration of semispherical nanoparticles with an average size of 18 nm. For the ZF/CdO very thin connected nanotubes are homogeneously embedded in semispherical nanoparticles, with a particle size 29 nm. The sample ZF/Ga2O3 is characterized by grouped and dispersed spherical particles within nanotubes having an average size 19.7 nm. The sample ZF/NiO consists of a homogeneous dispersion of semispherical nanoparticles having a size of 27.8 nm. The sample ZF/SnO2 shows different morphology: polygonal particles that are grouped homogeneously. The polygonal particles have an average size of 28.5 nm. The sample ZF/TiO2 consists of polygonal and longitudinal particles that are interconnected and have an average size of 145.5 nm. As shown in Table 1, the particle size from HR-TEM is different from that of the XRD of the ZF/RO system. HR-TEM cannot distinguish between the compounds within the nanocomposite; therefore, the particle size will be the average of all particles. But in the XRD, each compound has distinct peaks that are distinct from the others, so the grain size of any compound will be separate from the others. Consequently, the X-ray grain size is the best and most accurate. The SAED patterns show a series of concentric solid rings with a spotted appearance, indicating the prepared powders are highly crystalline. Furthermore, the sample contains more than one phase, as evidenced by two distinct spot ring patterns that are confirmed by the XRD results.

Figure 4
figure 4

HR-TEM images and SAER for ZF nanoparticle and ZF/RO nanocomposites

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Surface properties

The BET-specific surface area and pore size distribution in the ZF/RO nanocomposites were determined using nitrogen adsorption–desorption isotherms, as shown in Fig. 5. Interestingly, ZF nanocomposite's adsorption–desorption isotherm typically represents type III b (BDDT classification) [39, 40]. The obvious hysteresis loop in the 0.45–1.0 range of relative pressure specifies the adsorption in slit-like structures confirmed by FESEM results. The calculated BET and BJH parameters for each sample are listed in Table 2. Except for ZF/Ti, the surface area of ZF/RO was less than that of ZF samples, according to the BET values, which is consistent with the FESEM images. According to the larger atomic radii of Cd, Ga, Sn, and Ni ions compared to Ti ions, they block the pores and incorporate the molecules between the ZF nanoflakes. As a result, when ZF is decorated with TiO2 traces, the effective surface area increases, and the active sites improve.

Figure 5
figure 5

S. S. A. for ZF nanoparticle and ZF/RO nanocomposites

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Table 2 The BET, BJH parameters
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FTIR analysis

The FTIR spectrum was captured in the wavenumber region of 4000–400 cm−1 to determine our system's chemical bonds and functional groups. As a result of the fundamental lattice vibrations of the crystal lattice, the ferrites have two central bands in the range of 1000–300 cm−1 conformable to the tetrahedral (A-) and the octahedral (B-) sites, represented as ν1 and ν2, respectively [41]. In the range of 600–500 cm−1, the absorption band (ν1) is conformable to cations–oxygen vibrations at the tetrahedral site (A). But in the range of 450–350 cm−1, the absorption band (ν2) is conformable to cations–oxygen vibrations at the octahedral (B) site. The absorption band of tetrahedral (A) site is higher than that of the absorption band at octahedral site (B) because the bond between cations and oxygen inA-site is shorter than the bond between the cations and oxygen in B-site [42].

The FTIR spectrum of ZF/RO nanocomposites is shown in Fig. 6. The only absorption band (ν1) was obtained at the tetrahedral in the range of 530–533 cm−1 since the absorption band (ν2) at the octahedral is not shown in the spectrum. The only absorption band (ν1) at wave numbers of 530–533 cm−1 indicates the presence of the Zn–O bond for ZF nanoparticles. In addition, depending on the accompanying oxide, the curve has a shoulder in the range of 607–645 cm−1. Table 3 shows the chemical bonds, functional groups, and absorption peak wavelengths consistent with the literature [43,44,45,46,47].

Figure 6
figure 6

FTIR spectra for ZF nanoparticle and ZF/RO nanocomposites

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Table 3 FTIR parameters
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As mentioned above, the ZF/RO nanocomposite contains more than one compound with different structures and properties that may affect each other. The vibrational bonds of ZF are affected by the atom mass of the RO; Ti ions have the lighter atomic mass (47.867 a. u.), so the bonds of ZF/Ti have the biggest value of the vibrational bonds (533 cm−1); on the contrary, Sn ions have the heaviest atomic mass (118.71 a. u.), so the bonds of ZF/Sn have the smallest of the vibrational bonds (530 cm−1) as shown in Table 3. This indicates that the atomic mass of RO has caused the strengthening of the metal–oxygen bonds at the tetrahedral sites of ZF. In other words, the change in the atomic mass of the associated oxide with ZF causes an increase or decreases in the A–O bond length at the tetrahedral site of the ZF structure (i.e., a decrease or increase in the ν1 band). So, the confirmed cause of the change in the lattice parameters of ZF/RO (as a result of XRD) is the A–O bond length at the tetrahedral site of the ZF structure.

Magnetic properties

Figure 7 displays the magnetic hysteresis loops for all nanocomposites that were taken into account. As shown in Fig. 7, at the highest applied magnetic field, the full saturation can be observed for all samples, i.e., the M–H curves show a typical S-type shape for all samples. A critical difference in the shape of M–H curves was observed at near zero-field (the inset of Fig. 7). The hysteresis of the ZF and ZF/Ni samples shifted to under zero in the direction of the magnetization axis. The horizontal and vertical shift of the hysteresis loop is due to the interaction of the components of a composite [29]. Table 4 shows the magnetization parameters: saturation magnetization (Ms), coercive force (Hc), remnant magnetization (Mr), the squareness value (Mr/Ms), magnetic moment (μB), and the anisotropy constant (K) values. As shown in Table 4 all samples have remarkable coercivity, remanence magnetization, and saturation magnetization values. However, superparamagnetic materials usually have negligible coercivity values, remanence magnetization, and no full saturation [48]. As a result, all samples exhibit ferrimagnetic rather than superparamagnetic behavior. The variation of coercivity can be correlated to magneto-crystalline anisotropy. The anisotropy constant (K) of the samples can be calculated according to the relation [49]:

$$K = \left[ {H_{c} \times M_{s} } \right]/0.96$$
(2)
Figure 7
figure 7

M-H curves for ZF nanoparticle and ZF/RO nanocomposites

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Table 4 VSM parameters
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Also, the values of Ms and Mr for ZF/Cd, ZF/Ni, and ZF are higher than for the other samples because the particle size of each of them is smaller than that of the other compounds. The literature [50,51,52,53] states that a dropping in the particle size inevitably increases the magnetization value. As a result, the cation distribution undergoes a rearrangement at the nanoscale, making superexchange interaction A–O–B stronger than A–O–A and B–O–B, resulting in an increase in the magnetism in the material. So, the magnetic properties are affected by the size of the particles. Where the relation calculates the magnetic moment (μB),

$$\mu_{B} = \left[ {M \times M_{s} } \right]/5585$$
(3)

where M is the molecular weight of the samples. The variation in the value of μB can be related to the increase and decrease of A–B superexchange interaction, which generally arises due to the disorder in crystal structure such as lattice defects, strain, etc.

Also as shown in Table 4, the squareness ratio (Mr/Ms) for all samples is less than 0.5; this indicates the formation of multidomain grains [33, 35]. The relationship between magnetic properties and particle size is based on the presence of magnetic domains [54]. Magnetic particles would behave as single domain (SD) or multidomain (MD) depending on particle size [55]. SD grains have strong coercivity and remnant magnetization and are magnetically hard [56]. For the multidomain region, domain walls are present and coercivity tends to decrease with an increase in particle size [57]. In a multidomain particle, the magnetization process is regulated by the movement of the domain wall, which loses less energy when the particles are at the nanoscale. Based on the above considerations, we can state that the ZF/RO has a small coercivity, so they tend to be multidomain. So, the particle size controls the magnetic properties of the ZF/RO nanocomposites.

Electron spin resonance (ESR)

Magnetic anisotropy, dipolar interactions, and superexchange interactions all affect the microscopic magnetic performance of magnetic compounds. Electron spin resonance (ESR) is the perfect technique for describing the microscopic magnetic behavior of magnetic compounds.

The X-band ESR spectra of ZF/RO nanocomposites in a continuous microwave frequency and a 600 mT magnetic field are shown in Fig. 8. Two resonance peaks were discovered in the ESR spectra. All ESR spectra display a distinct, strong, sweeping, and asymmetric line. All of the samples exhibit asymmetric behavior because there is more than one phase in the sample, so the random orientation of the magnetic nanoparticles causes an asymmetric ESR spectrum. This resonance peak supports the dilution of secondary phases as observed in XRD patterns. Magnetic nanoparticles with random anisotropy axes also contribute to the samples' broad ESR spectra [58]. Moreover, the combination of Zn2+, Fe3+, and RO ions together is suggested by the large resonance peak. Other systems of Zn-ferrite nanoparticles [59, 60] showed an ESR signal with a broad spectrum and an extensive line.

Figure 8
figure 8

ESR spectra for ZF nanoparticle and ZF/RO nanocomposites

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Table 5 shows the values of the resonance field Hr, peak-to-peak line width ΔHpp, g-factor, the internal field Hin, and spin–spin relaxation time (T1/2) obtained from ESR data. From Table 5, we can see that the resonance field of all samples is different from the resonance field value of isolated free electrons (340 mT and g = 2.0023), which indicates the presence of the interaction between the magnetic nanoparticles. According to Sparks [61] and Anderson [62], one factor influencing the resonance field is the internal field. The internal magnetic field (Hin) originates from three sources: the internal magnetic field produced by demagnetization, the internal magnetic field produced by the interaction of dipole–dipole nanoparticles, and the internal magnetic field of agglomerates. Therefore, the effective magnetic field can be described by the sum of two terms:

$$H_{eff} = H_{ext} + H_{in} = cons\tan t$$
(4)

where Hext is the external applied magnetic field, that is the resonance field read from the ESR spectrometer, i.e., Hext = Hr, and Hin is the internal magnetic field. It is seen from Table 5 that the resonance field is shifted in opposite directions by the change in the internal field. In other words, the resonance field reduces when the internal field increases, and vice versa. This confirms our hypothesis that the internal interactions (the mutual force) between ZF and RO impact the properties of these compounds. Also note that the internal field of the ZF/Cd is the highest of the ZF/RO, which supports the saturation magnetism of the ZF/Cd being the highest of the ZF/RO. Therefore, the effect of the internal interactions (the mutual force) between ZF and RO is responsible for the change in magnetic parameters such as Hr. Also, Table 5 shows the variation of Hr of all the samples associated with several of the grain sizes, which is confirmed by Zakiyah et al. [63]. This result is in line with the earlier prediction that the magnetic properties would change as a result of the particle size change.

Table 5 ESR parameters
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The values of Lande's g-factor and peak-to-peak line width (ΔHpp) (Table 5) can be used to understand how superexchange interactions control the magnetization in magnetic materials. The values of Lande’s g-factor and spin–spin relaxation time T1/2 can be found according to

$$g = hn/\mu_{B} H_{r}$$
(5)
$$1/T = gu\Delta H\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} /2\pi h$$
(6)

where ∆H½ = √3 × ∆Hpp, υ = frequency of electromagnetic radiation, h (ℏ) = Planck’s constant, μB = Bohr magneton, Hr = resonance field, ∆H1/2 = line width at half height of the absorption peak. Table 5 shows that the g-factor values vary depending on the accompanying oxides and are different from the value of the g-factor for a free electron (2.0023), meaning that Zn2+, Fe3+, ions, and the accompanying oxides are not isolated. It is well known that a high line width value and g-factor indicate a rise in dipolar interactions and a fall in superexchange interactions [64]. In other words, when the particle interactions are dominated by dipolar interactions, large line widths, and higher g-values are observed. The variation in g-factor and line width values results from the change in the reaction between compounds ZF and RO. Porosity, morphology, and the presence of secondary phases may all have an impact on the ESR parameters in the current study, as well.

Optical properties

The optical absorption properties of the ZF/RO nanocomposites were investigated via UV–Vis absorption spectra, and the results are presented in Fig. 9. The observed spectra show optical absorption in both UV and visible ranges, which suggests their utility for solar-driven photocatalysis [65]. For all the compounds, the absorption edges of the ZF and its composites are around 700 nm due to charge transfer between O 2p and Fe 3d. Other absorption edges are ranging near IR, indicating that there are different band structures. The basic process of the absorption of light is related to the electronic structures of materials. So, the determination of the band gap is important. Based on spectra and Tauc equation: αhν = (hν − Eg)0.5 [66], the band gap of the ZF, ZF/Cd, ZF/Ga, ZF/Ni, ZF/Sn, and ZF/Ti nanocomposites was estimated by plotting (αhν)2 versus hν (Fig. 9b) as 1.773, 1.789, 1.786, 1.763, 1.822, and 1.835 eV, respectively. The shift in the band gap of the nanocomposite sample compared to the literature may be due to the quantum confinement effect [67]. In other words, the variation of the energy gap is not dependent on the particle size and the type of the added oxide but is also affected by the shape variation. In comparison with other band gaps of ZF (4.5 eV) [68], the considerable changes in the band gap value of the sample are due to the formation of particles in the nanoscale range as well as the shape of the prepared particles [69]. Both nanostructure samples have lower band gap energy and higher wavelength absorption in comparison with the bulk materials. Up to now, band gap values in the range of 2.1–2.4 for conventional synthesized ZF have been reported in previous research [70,71,72,73,74,75]. The low-value band gap up to 1.7 eV in this study may be due to the shape of grains (nanoflakes), but the other work involves other shapes. The band gaps of all samples are the most suitable for photoelectrochemical applications because band gaps are exhibited in the visible region.

Figure 9
figure 9

Optical reflection spectra and optical band gap for ZF nanoparticle and ZF/RO nanocomposites

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According to the results of optical investigations on these nanocomposites, ZF/RO, except ZF/Ni, was characterized by a marked blue shift to ZF due to the Burstein-Moss effect [14, 16]. On the other hand, the red shift in the nanocomposite ZF/Ni may be attributed to the creation of defects as a result of charge compensation, and such defects have their energy levels that are introduced between the valence band and the conduction band, leading to a narrowing of the energy gap. Photoluminescence spectroscopy (PL) is useful for exploring the defect structures in mixed materials because it provides knowledge of the impurities and defects' energy states [75, 76]. Figure 10 illustrates the room temperature PL emission spectra of ZF/RO nanocomposites in the 440–520 nm wavelength range. The PL emission spectra of all samples are shown in Fig. 10, which exhibit a clear and sweeping peak in the light blue region centered at 468 nm and two low heights with weaker peaks in the green light region, one at about 480 nm and the other at 492 nm, respectively. The blue emission around 468 nm can be assigned to the intrinsic defects, particularly interstitial zinc defects [77]. However, the green emission may be due to oxygen vacancies. This is usually assigned to the trap-state emission and singly ionized oxygen vacancy [78]. It is commonly agreed that a lower PL emission spectrum directly indicates a lower recombination rate of photogenerated e−/h+ pairs. Figure 10 shows that the spectral PL intensity of ZF/Sn, ZF/Cd, and ZF/Ti nanocomposites is relatively low compared to the spectrum for the ZF nanocomposite. This suggests a better and efficient separation of the photogenerated electrons and holes, and vice versa for ZF/Ni, and ZF/Ga which shows an increase in the recombination for e−/h+ [79, 80].

Figure 10
figure 10

PL Optical for ZF nanoparticle and ZF/RO nanocomposites

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Photocatalytic activity

To study the ability of the catalysts to the adsorption of the dye molecules it should be determined the active sites on the surface of the catalyst. Figure 11, shows the acidic active sites that can adsorb the target molecules during the chemisorption mechanism. The acidic active sites of the prepared materials were measured by a temperature-programmed desorption technique for the ZF/RO samples. It is clear from the peaks and values in the inset table that the active sites of pristine ZF were decreased from 4.8 to 2.2, 2.6, 2.9, 3.2, and 2.4 mmol/g for ZF/Cd, ZF/Ga, ZF/Ni, ZF/Sn, and ZF/Ti, respectively. Consequently, the presence of metal oxide traces reduces the vacant active site, which lowers the chemisorption of the target molecules, lowers the adsorption of the H2O molecules on the surface of prepared catalysts, and lowers the generation of hydroxyl radicals (•OH) via photo-oxidation [79].

Figure 11
figure 11

TCD for ZF nanoparticle and ZF/RO nanocomposites

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The photocatalytic activities of the synthesized ZF decorated by different oxides of CdO, NiO, Ga2O3, SnO2, and TiO2 were assessed during the photodegradation of harmful molecules, mainly MB. Figure 12a displays the adsorption capacity without illumination for all samples, which is approximately similar and amounted to less than 15%. Upon illumination, for the composite samples, it can be seen that the nanocomposites exhibited revealed photocatalytic activity less than pristine ZF for the MB removal. These decreases occurred due to the deterioration of the physisorption and chemisorption activity. The photocatalytic reaction of MB belongs to the first-order kinetics expressed by: ln(Co/Ct) = kt. Co is the concentration of waste molecule at the adsorption equilibrium, C is the remaining concentration after irradiation time intervals t, and k represents the apparent rate constant, Fig. 12b. The essential factor that could be decreasing the photochemical reaction is the decrease of the active sites on the surface of composites, which decreases the adsorption of the MB molecules on the surface of the composites and then decreases the production of hydroxyl radicals (•OH) through the photo-oxidation process.

Figure 12
figure 12

Photocatalytic degradation for ZF nanoparticle and ZF/RO nanocomposites

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Conclusion

This investigation was carried out to demonstrate how the constituents of the nanocomposites ZF/RO interact with each other. It was revealed through the XRD study that the accompanying oxide had an impact on the ZF unit cell length as well as the size of the nanoparticles. According to the FTIR study, a change in the atomic mass of the accompanying oxide with ZF leads to a change in the A–O bond length at the tetrahedral site of the ZF structure. It was found that each compound's saturation magnetization (Ms), coercive force (Hc), and remnant magnetization (Mr) values varied, which indicates that the decorated oxides had an impact on ZF's magnetic properties. This was supported by the analyses obtained from the ESR, which showed that agglomerates have an interior magnetic field. According to the results of optical investigations on ZF, ZF/Cd, ZF/Ga, ZF/Ni, ZF/Sn, and ZF/Ti nanocomposites, each one has a distinct band gap (1.773, 1.789, 1.786, 1.763, 1.822, and 1.835 eV, respectively), which leads to different optical properties. The variation of energy gap is not dependent on the particle size and the type of the added oxide only; it is also affected by the shape variation. The PL emission spectra of all samples consist of a robust and wide peak centered at around 468 nm in the blue light region and two low heights, less intense peaks located in the green light one at about 480 and the other at 492 nm, respectively.