Introduction

Despite being one of the most harmful pollutants, dyes are widely used in the fabric, food, plastic, chemical, and tabloid industries. Their discharge into the aquatic environment has a serious impact on living organisms1,2. Color reduces sunlight penetration through water, resulting in decreased photosynthetic activity and decreased biota development. In addition, dyes tend to bind metal ions, resulting in micro toxicity in fish and other living things1,3.

Normally, dyes are barely biodegradable and challenging to be eliminated by conventional approaches. In this context, rhodamine B (RhB) which belongs to the xanthene family, is a highly stable cationic dye due to its rigid heterocyclic structure4. Indeed, the high stability of RhB dye is beneficial for different industrial applications, however, makes its degradation not simple and challenging5,6,7. As a result, providing efficient, environmentally friendly, and cost-effective solutions for the breakdown of such contaminants is critical for the long-term viability of green habitats. This has resulted in a diverse range of techniques being employed to extract dyes from wastewater, including adsorption8,9,10, ultrafiltration11, chemical precipitation12, electrocatalytic breakdown13, and photodegradation3,14,15,16.

Photocatalytic degradation is potentially one of the cheapest, green, and most powerful techniques for the decontamination of water from dye pollutants. In other words, extremely oxidizing conditions can be established without any further required reagents, the only requirement is the supply of aerobic oxygen and a light irradiation source17,18. Electrons (e−), holes (h+), hydroxyl radicals (OH·), and superoxide radicals (O2·−) are all surface-active species that can be generated by photocatalytic degradation. The ability of the photocatalysts to generate surface-active species19,20. An assortment of materials with catalytic activity have been proposed for the photodegradation of pollutants such as graphitic carbon nitride (g-C3N4), TiO2, ZnO, CdS, CaO, MgO, CaTiO3, and MgTiO3, just to name a few3,5,7,15,21,22,23,24,25,26.

Compared to TiO2, the perovskite-type oxides based on titanium with a structural formula of ABO3 are attracting more and more attention in the last decade in photocatalysis due to their intriguing photophysical properties. CaTiO3 and MgTiO3 are utilized in a vast array of applications namely radar telecommunications, capacitors, thermistors, electronics, ceramics, superconductors, nonlinear optics, catalysis, piezoelectric, and dielectric devices27,28,29,30. Besides, they have high photocatalytic degradation activity towards different organic dyes21,25. Ca and Mg are among the most abundant metals on earth and their oxides can be also synthesized from waste materials. This makes them affordable for various important applications. Several methods such as hydrothermal, sol–gel, mechano-chemical, traditional solid-state, and polymeric precursor have been used for the synthesis of CaO, MgO, CaTiO3, or MgTiO37,31,32,33,34. One should emphasize that the catalytic efficiency of these materials is highly reliant on the synthetic procedure and their precursors which of course have a direct impact on the final surface morphology, active sites, and physicochemical properties. Moreover, the performance of these photocatalysts extremely fluctuates from one dye to another7,25,35.

In this study, CaO derived from chicken eggshells waste as well as MgO were synthesized in their pure phases. Furthermore, CaTiO3 and MgTiO3 were obtained through a facile solid-state method by mixing thoroughly the synthesized CaO or MgO with TiO2 before calcination at 900 °C. Spectroscopic and microscopic techniques were used to comprehensively describe the synthesized materials. Moreover, we compared the photocatalytic activity of CaO, MgO, CaTiO3, and MgTiO3 on the degradation of rhodamine B (RhB) which is one of the most challenging dyes due to its high stability. The results might be beneficial for designing photocatalysts with tailored properties and high intrinsic efficiency.

Results and discussion

Characterization

Functional group evaluation

Figure. 1a–d shows the FT-IR spectra of CaO, CaTiO3, MgO, and MgTiO3 respectively. The FT-IR spectrum of CaO (Fig. 1a) displays a sharp peak at 3640 cm−1 that relates to the O–H stretching vibrations in Ca(OH)2. This highly intense peak is formed due to the presence of moisture in the crystal lattice caverns of the CaO system. The absorption peaks at 1477 and 874 cm−1 are attributed to the C–O asymmetrical and symmetrical vibrations. The bands that appeared at 459 cm−1 are due to the existence of Ca–O metal oxide which was transformed from CaCO3 in the eggshells36,37. In the case of CaTiO3, the broad peaks at around 3311 and 1419 cm−1 in Fig. 1b can be accredited to the hydroxyl group vibrations of the CaTiO3. Moreover, the distinctive bands at 540 and 436 cm−1 are typically known for CaTiO3 and they are corresponding to the Ca–Ti–O bending vibrations35,38.

Figure 1
figure 1

FT-IR spectra of (a) CaO, (b) CaTiO3, (c) MgO and (d) MgTiO3.

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The FT-IR spectrum of MgO in Fig. 1c shows a distinctive band at 420 cm−1 which is attributable to the presence of Mg–O metal oxide vibrations. Moreover, the absorption band at 1427 cm−1 is ascribed to the bending vibrations of the OH−39. However, in MgTiO3 bands that occurred at 519 and 457 cm−1 in Fig. 1d can be conferred to the Mg–Ti–O vibrations. Furthermore, the absorption bands at around 702 and 407 cm−1 are attributed to the Ti–O and Mg–O vibrations, accordingly21,40.

Powder XRD examination

The XRD patterns of CaO, CaTiO3, MgO, and MgTiO3 are represented in Fig. 2. The XRD spectrum of CaO (Fig. 2a) exhibits intensive peaks at 2θ = 32.5°, 37.35°, and 53.85° that was matched and indexed to the JCPDS data card (96-101-1328) which fits in the cubic crystal structure. A mixture of CaO and Ca(OH)2 phases are observed; however CaO is the predominant material with lattice parameters of a = b = c = 4.7929 Å and crystallite size of 32–37 nm. For the XRD spectrum of CaTiO3 (Fig. 2b), the high-intensity peaks observed at 2θ = 33.18°, 47.58° and 59.39° match with the JCPDS card: 96-901-3384 proved the phase formation of pure CaTiO3.

Figure 2
figure 2

XRD patterns of (a) CaO, (b) CaTiO3, (c) MgO and (d) MgTiO3.

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The XRD pattern of MgO (Fig. 2c) illustrates the typical peaks of pure MgO that are similar to the JCPDS data card (96-101-1119). Furthermore, MgO has a cubic crystal structure with a crystallite size of about 30–35 nm and lattice parameter values of a = b = c = 4.2039 Å. Figure 2d shows the characteristic peaks of MgTiO3 at 2θ = 33.00°, 35.62°, 40.67°, and 53.77° that match with the JCPDS card: 96-900-3759 and indicate the synthesis of MgTiO3 with the hexagonal crystal arrangement. Table. 1 shows the computed lattice characteristics and crystalline size of the produced materials.

Table 1 Lattice parameters, crystallite size, and crystal structure of the synthesized materials.
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The crystalline size was computed using the Debye-Scherer formula as follows:

$$\mathrm{D }= \frac{0.89\uplambda }{{\upbeta \cos\uptheta }}.$$

In addition, the lattice parameters were determined utilizing the given expression:

$${1}/{\text{d}}^{{2}} = {1}/{\text{sin}}^{{2}} \upbeta ({\text{h}}^{{2}} /{\text{a}}^{{2}} + {\text{k}}^{{2}} {\text{sin}}^{{2}} \upbeta /{\text{b}}^{{2}} + {\text{l}}^{{2}} /{\text{c}}^{{2}} - {\text{2hkcos}}\upbeta /{\text{ab}}),$$
$${\text{d = }}\chi /{\text{2 sin}}\theta ,$$

where D is the average crystallite size, β corresponds to the full width at half maximum, and θ indicates Bragg’s angle.

SEM/EDX analysis

The morphology and elemental composition of CaTiO3 and MgTiO3 were investigated by SEM and EDX. As illustrated in Fig. 3a,b, CaTiO3 micrographs reveal clumps of particles with a rough surface and an uneven shape. In comparison, MgTiO3exhibits a smoother surface with a spherical beads like structure over the surface of the material with non-homogeneous pores distributed throughout the surface (Fig. 3c,d). The SEM images demonstrate the porous nature of the particle which might be due to the elimination of impurities during calcination. The micrographs illustrate that the surface morphology of CaTiO3 and MgTiO3 are distinct, although they were formulated under identical preparatory circumstances. These observations indicate that alkaline earth metals have a significant impact on the final surface morphology of titanates.

Figure 3
figure 3

SEM micrographs of (a,b) CaTiO3 and (c,d) MgTiO3 and their corresponding EDX spectra are shown in (e,f), respectively.

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The EDX spectra of CaTiO3 and MgTiO3were exemplified in Fig. 3e,f, respectively. The peaks observed from the EDX spectrum divulge the existence of the peaks related to Ca, Mg, Ti, and O elements in a stoichiometric ratio. No other elemental peaks were found in the spectrum which indicates the formation of respective titanates without any impurities.

UV-DRS reflectance spectroscopy studies

To identify which fraction of the sun spectrum may be absorbed by semiconductors, it is necessary to calculate the optical bandgap energy (Eg). The diffuse reflectance experiments were used to estimate the Eg of CaO, MgO, CaTiO3, and MgTiO3 from the absorption spectrum (Fig. 4a–d). Furthermore, the Eg was calculated using the transition rate expression for direct bandgap semiconductors. The absorption coefficient for direct bandgap measurements was given by Tauc’s equation:

$$\left( {\alpha h\nu } \right)^{n} = \, A \, \left( {h\nu \, {-} \, E_{g} } \right),$$

where α is the absorption coefficient, hν is the photon energy, and A is a constant. The exponent n in Tauc’s equation depends on the types of transitions41,42. Figure 4a,b validate that, the Egvalue for CaO calculated from Tauc’s plot is 2.80 eV versus SHE, whereas in case of MgO it was found to be around 5 eV. On the other hand, the Egvalues for CaTiO3 and MgTiO3 were observed to be 3.0 eV and 3.12 eV, respectively which are slightly higher than those of their oxides. The Egvalue for CaTiO3 was reported to be in the range of 3.0–3.6 eV versus SHE, while the Egvalue for MgTiO3 was in the range of 3.0–3.2 eV versus SHE21,36,39,43.

Figure 4
figure 4

Tauc’s Plots for bandgap estimation of (a) CaO, (b) CaTiO3, (c) MgO and (d) MgTiO3.

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Photocatalytic activity

Employing ultraviolet illumination, the photocatalytic activity of the newly synthesized CaO, MgO, CaTiO3, MgTiO3 and the mixture of respective titanates was investigated for the degradation of RhB dye in water. As a function of the irradiation period (30–120 min), the dilapidation ability of the as-synthesized materials against RhB dye was compared using the UV–Vis spectra exhibited in Figs. 5a–d and 6. In addition, the absorption of rhodamine B dye has been associated with absorption peaks at 503 and 550 nm, which have been ascribed to the absorption of the de-ethylated and tetra ethylated rhodamine B dye, respectively44. Furthermore, the RhB concentration is proportional to its absorbance; thus, the changes in the absorption peak value at λmax = 550 nm were monitored during the photocatalytic degradation reaction to evaluate the performance of the proposed photocatalysts. The photodegradation efficacy (% E) was calculated using the following equation45:

$$\% {\text{ E }} = \, \left( {{1} - {\text{A}}_{{\text{t}}} /{\text{A}}_{{\text{o}}} } \right) \, \times { 1}00,$$

where At is the absorbance after t min and A0 is the absorbance at time t = 0.

Figure 5
figure 5

UV–Visible spectra for the disintegration of RhB dye by (a) CaO, (b) CaTiO3, (c) MgO, and (d) MgTiO3.

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Figure 6
figure 6

UV–Visible spectra for the disintegration of RhB dye by Mixture (CaTiO3 + MgTiO3).

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As a control experiment, the adsorption of RhB on the synthesized materials was tested in the darkness for 120 min. According to the results, the intense peak at λmax = 550 did not change in indicating that the catalysts do not adsorb RhB. Instead, the intensity of the dye absorption peaks under UV light irradiation intensively decreases in the case of CaO and CaTiO3 even after illumination for 30 min which is correlated to their photocatalytic activity. Furthermore, the photocatalytic activity of CaO and CaTiO3 increases overtime to degrade about 63.13 and 87.64% of RhB solution at 120 min, respectively. In comparison, the synthesized MgO and MgTiO3 do not have high efficacy to deteriorate the dye. The rate of RhB degradation is slow since only the photocatalytic activity of MgO and MgTiO3 are 21.39 and 29.44%, respectively. Whereas the mixture of both the titanates showed the degradation of about 64.62% on UV-light irradiation. The comparative degradation of RhB dye is demonstrated in Table 2 which is given below. From the above spectral data, we plot ln (C/Co) vs time for a detailed comparison of the proposed catalyst under UV radiation in Fig. 7 and the plots follow first-order rate kinetics.

Table 2 Rhodamine B dye decomposition (%).
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Figure 7
figure 7

Kinetic plot of (a) CaTiO3, (b) MgTiO3 and (c) CaTiO3 + MgTiO3.

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Compared to Mg–O, Ca–O has a reduced bond strength46,47 and more active sites are formed during bond cleavage, which is a more crucial aspect in photocatalytic activity16. Furthermore, it is obvious from SEM micrographs (Fig. 3) that the surface of CaTiO3 is rougher than that of MgTiO3indicating that CaTiO3 has a greater surface area. The increased surface area provides further accessible active sites for the photoreduction efficiency of RhB. Moreover, the deterioration efficiency of titanites is higher than that of their corresponding metal oxides, indicating the synergistic effect of Ti in improving the photoactivity of CaO and MgO. One should emphasize that the photocatalytic activity of CaO, MgO, and their titanites strongly depends on several variables such as their synthetic procedure and the investigated dyes. For example, CaO NP derived from eggshell showed high activity towards the photodegradation of methylene blue and toluidine blue compared to a neglectable activity towards RhB7. Interestingly the CaO synthesized in this study through a facile and affordable solid-state method shows a high activity towards the photodegradation of RhB. MgO showed a high photocatalytic degradation activity against reactive yellow (RY) dye43. Furthermore, MgO nanoflakes exhibited excellent performance for the dye degradation of methylene blue and methylene orange48. Moreover, several reports were available on the degradation of RhB employing pure and mixture materials49,50,51. In comparison, it seems that the combustion synthesis of MgO resulted in agglomerated particles with poor photocatalytic activity towards the photodegradation of RhB in this study. However, it might be beneficial to investigate the photocatalytic activity of the synthesized MgO and MgTiO3 towards different dyes. These fundamental insights might help to design photocatalysts with tailored properties and high intrinsic activity.

Chemical oxygen demand (COD)

A COD experiment was conducted to verify that the decolonization of RhB is induced by photocatalytic degradation and not by adsorption. After 2 h treatment, the COD values of the samples treated with CaO, MgO, CaTiO3, MgTiO3, and CaTiO3 + MgTiO3 were determined to be 112, 612, 48, 562, and 96 mg L−1, respectively, as illustrated in Fig. 8. These results showed a significant decrease in the COD value obtained for the sample treated the catalysts compared to the untreated one (742 mg L−1). The observation mentioned above demonstrated using less K2Cr2O7 to oxidize the organic dye pollutants found in the samples. The decrease in COD values proved that the RhB dye was photocatalytically degraded when exposed to UV light. The decline in COD value revealed a trend that was consistent with UV analysis. The catalytic effectiveness of the aforementioned catalysts was demonstrated by the consumption of K2Cr2O7 for the oxidation of treated samples.

Figure 8
figure 8

COD of degraded RhB dye with calcium/magnesium oxides and titanates catalyst under UV radiation.

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Experimental section

Materials and methods

All chemicals including Mg(NO3)2·6H2O(98%), TiO2 (99%), urea (99%), Nitric acid (HNO3) (68%), were of analytical gradient and employed as obtained from SDFCL (SD Fine Chemical Limited) without further refinement.

Synthesis of metal oxides and their corresponding titanates

Preparation of eggshell derived calcium oxide

CaO was extracted from chicken eggshell waste according to the reported procedures in the literature52,53 with some modifications as follows: To eliminate suspended impurities from any organic phases and pathogens, the eggshells were first cleansed with double-distilled water before being seethed at 100 °C for 60 min. The washed eggshells were dehydrated in a hot air oven at 150 °C for 24 h and subsequently were ground manually into a fine powder in a pestle and mortar. After that, the eggshells were calcined in air at 900 °C for 2.5 h to obtain eggshell derived calcium oxide and grounded manually into a fine powder.

Preparation of magnesium oxide

MgO was created utilizing urea as fuel and nitric acid as an oxidizing agent via the solution combustion method. 7.69 g of Mg(NO3)2·6H2O were thoroughly mixed with 3.00 g of urea by stirring in 30 mL of double-distilled water and then 5–6 drops of concentrated nitric acid were added. Afterward, the solution (without drying) was moved into a beaker and heated in a pre-heated furnace at 400 °C for 30 min. Finally, the obtained powder was calcined at 900 °C for 6 h to achieve the pure phase of MgO then, it was cooled down to room temperature and finally ground into a fine powder.

Synthesis of calcium and magnesium titanate

Both CaTiO3 and MgTiO3were synthesized from their corresponding metal oxides through the traditional solid-state approach. TiO2 was used as starting reactant material along with eggshell-derived CaO and MgO, respectively. The synthesized CaO (2.80 g) was mixed and ground thoroughly with TiO2 (3.99 g) before sintering at 900 °C for 6 h. The latter, MgTiO3 was synthesized by grinding the synthesized MgO (2.01 g) with TiO2 (3.99 g). The homogenous powder was then calcined at 900 °C for 6 h to acquire the pure materials of the corresponding titanates; CaTiO3 and MgTiO3.

Characterization

A Shimadzu IR Affinity-1S CE FT-IR spectrophotometer in the range of 4000–500 cm−1 was employed for the functional group analysis of the respective titanates and oxides. The presence of the pure phases of CaO, MgO, CaTiO3, and MgTiO3 was confirmed by the XRD analysis. The XRD patterns were obtained by a D8 advance powder X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with Cu/Kα radiation filter at a wavelength of 1.5406 Å. An EVO 18 Research (Zeiss India) SEM equipped with an energy dispersive X‐ray analysis unit operating at a hastening potential of 20 kV was used for accomplishing the surface morphology along with the elemental composition of the synthesised materials. The photosensitive characteristics of the materials were determined with the aid of UV–Vis diffuse reflectance spectroscopy (Jasco, V-670).

Photocatalytic experiments

To evaluate the degradation capability of the as-prepared materials, 50 mg of the catalysts were supplemented to 100 mL of the 10 ppm rhodamine B dye solution which was prepared from the stock solution. The dye solution and the catalyst were mixed using the magnetic stirrer in a dark condition to bring up the adsorption–desorption between them. Then the mixture was placed inside the Heber immersion type photoreactor in a dark condition and exposed to the UV light irradiation of wavelength 300–400 nm. Then, 10 mL of the solution was stockpiled every 30 min (30, 60, 90, and 120 min) to test the degradation activity as a function of time and the Chemical oxygen demand (COD) was conducted as per the procedure reported earlier16.

Conclusions

The current work dealt with the preparation of calcium and magnesium titanates from their respective oxides using the solid-state approach for the photoreduction of RhB dye under UV–Visible irradiation. The phase formation and purity of the materials were confirmed by the FTIR, XRD, and EDX analyses. SEM micrographs reveal that the surface of MgTiO3 is smoother with agglomerated particles compared to CaTiO3. Diffuse reflectance spectroscopy studies indicate the higher bandgap energy of titanites compared to their alkaline metal oxides. However, the catalytic efficacy of titanates is higher compared to their corresponding alkaline metal oxides, indicating the synergistic effect of Ti. Furthermore, the photocatalytic degradation performance of Ca-based oxides and titanites is much higher than that of Mg-based ones. This might be due to the higher bond strength of Mg–O than Ca–O which in turn makes the bond cleavage of Mg–O much more difficult. Additionally, the surface morphology of CaTiO3 and MgTiO3 should be considered whereas, the agglomeration is higher in the case of MgTiO3 which, indeed decreases the active sites. However, the activity of the MgO might be improved by optimizing the synthetic procedure such as the choice of fuel and reduction in the materialization temperature. Since the photodegradation activity of the materials changes from one dye to another, it is worth trying the photocatalytic activity of the proposed material towards less stable dyes. All of these aspects are valuable for designing effective photocatalysts with tailored properties.