Introduction

The rapid development of industrialization has triggered multiple ecological risks. As a result, it was found that water contamination is at the root of most of the global environmental problems. Due to the extremely large amount of dye effluent released by textile dyeing industries, which produce 700,000 tons of synthetic dyes annually, large volumes of wastewater are discharged into the environment [1]. In general, dyes are hazardous because of their strong resistance to light, temperature, chemicals, and microbial attack [2]. Dye degradation from wastewater can help resolve many environmental issues related to human and aquatic life [3]. Many strategies have been used to remove color pollutants from water, such as chemicals, photochemicals, biological, and physicals to treat the contaminated areas [4, 5]. Among the removal strategies, photodegradation is one of the most effective ways to eliminate pollutants from wastewater [6, 7]. Combining adsorption with photocatalytic degradation is one of the most economical approaches to decontaminating wastewater [8]. In recent years, many materials have been developed in various shapes and structures [9]. Additionally, several semiconductors have been used in the photocatalytic degradation of dyes contaminations, including ZnO, TiO2, CdS, SnO2, CuO, NiO FeO3 etc.[10,11,12]. Among these metal oxides, it was reported that the SnO2 was produced by changing the solvent in the precursor solution using the precipitation method. The comparison investigation found that as the solvent was changed from 23 to 6 nm, the average crystallite size changed, with a tetragonal crystal structure. The photodegradation rate of MB dye revealed that SnO2 nanostructures made using IPA rather than DI and ethanol had the highest photodegradation efficiency [13]. The authors used a simple co-precipitation approach to synthesize Ce-doped ZnO nanostructures with varying doping amounts. Under visible light irradiation, the Zn0.94Ce0.06O sample photocatalyzed Methylene blue (MB) dye was 9.1 times faster than the undoped ZnO. An increase in PL intensity causes an increase in photocatalytic activity. As a result, Ce-doping increases the degradation rate, making ZnO a viable photocatalyst for removing organic contaminants in water [14]. ZnO has many features to become an environmentally friendly metal oxide and properties, such as inexpensive, safe, low toxicity, biodegradable, and easily prepared [15]. Moreover, Zinc oxide (ZnO) has emerged as an effective photocatalyst due to its higher H2O2 generation compared to other metal oxide nanomaterials [16, 17]. ZnO modified by adding dopants metals, such as Ce, Cu, Ag, Mn, Ni, Fe, Sr, and Lanthanides to enhance their photocatalytic activity [18]. The ZnO nanomaterials were synthesized using various strategies, including co-precipitation, combustion, hydrothermal, sol–gel, polyol, and sonochemical [19]. Hydrothermal synthesis is one of the most common methods for obtaining nanomaterials based on solution reactions, and nanomaterials can be produced at a range of temperatures and pressure from low to high [20]. In addition, this method considers environmental safety. It is inexpensive due to its use of water as a solvent and gives us good control over the particle shape and size [21]. There are significant advantages of the hydrothermal synthesis method over others in that the compositions of nanomaterials to be synthesized can be well controlled through liquid phase or multiphase chemical reactions [22]. It was reported that hydrothermal fabrication methods are major routes for synthesizing unique ZnO shapes because it offers both the possibility of rigorous control over crystallite growth and relatively easy upscaling [23]. Four parameters affect crystallization: phase barrier, capping molecules, time, and temperature [24]. The hydrothermal method is also beneficial in a pressure vessel, also known as an autoclave, where varying temperatures and pressure control processing conditions [25]. Researchers have performed doping in the ZnO to obtain a wide range of properties and use for the material. Metal doping is a simple way to increase the efficiency of ZnO nanomaterials in various applications. Firstly, photocatalytic characteristics can be controlled by selecting alkaline earth metal elements as dopants. Indeed, using cations as doping metals with a greater ion radius than Zn+2 radius results in the formation of considerable lattice defects due to ionic radius mismatch between cations and Zn+2 and charge compensation, which may impact the photocatalytic capabilities of ZnO. Additionally, it was known that Sr+2 has a greater radius (2.45Å) than Zn+2 which can affect ZnO's optical characteristics [26]. Moreover, the second group involved the insertion of cations of rare-earth metal within the ZnO matrix that deserve special attention and analyses the light absorption and changes in structural characteristics, defects on the surface, crystallite sizes, and thus the photocatalytic properties; and create intra-4f optical transitions due to the half-filled f-orbital [27, 28]. Optical and photocatalytic properties are recognized to be directly related to one another. Thus, we anticipate seeing these results in the efficiency with which dye is degraded from doped ZnO nanostructures. Specifically, the ZnO doping will produce a larger band gap energy and less charge carrier recombination, improving photocatalytic efficiency [29]. Theoretical calculations based on density functional theory (DFT) have been widely used to establish the relationship between photo electrocatalysts' structure and catalytic activity. These computations used first principles to predict the electronic characteristics of doped ZnO [30].

In this study, we used a hydrothermal method for producing single-doped ZnO and tri-doped ZnO nanomaterials with different metals, such as Sr metal as alkaline earth metals and Sm and La metals as rare earth metals. The obtained composite was investigated to determine the structure, optical properties, electronic properties, morphology, and surface area, and compared these experimental results with the theoretical calculation. Finally, the experimental study of the photodegradation of reactive red 43 and the mechanisms of photodegradation were discussed. Moreover, the DFT calculation showed the difference in crystal structures, electronic properties, and optical properties of the nanomaterials.

Materials and methods

Materials

Zinc acetate dihydrate [Zn (CH3COO)2·2H2O], Lanthanum (III) nitrate hexahydrate [La (NO3)3·6H2O], Samarium (III) nitrate hexahydrate [Sm (NO3)3·6H2O], Strontium nitrate [Sr (NO3)], Sodium hydroxide [NaOH], Acetic acid [CH3COOH] (99.9%), Reactive Red43[C26H17ClN7Na3O11S3], and Distilled water (D.W) used all chemical with no extra purification.

Synthesis technique

Synthesis of pure ZnO nanomaterials (PZ NPs)

Pure ZnO was prepared by dissolving 5gm of Zinc acetate dihydrate in 250 mL of Distilled water under magnetic stirring for 30 min at 25 °C. Then, 2.5 mL acetic acid was added and further stirred for 10min. Then used, NaOH to adjust the pH range (10–11); at this, a white precipitate was formed, followed by 30min of stirring. The mixture was placed in a stainless-steel autoclave lined with Teflon and heated to 160 °C for 3 h. After cooling to room temperature, the yield was removed, filtered, and washed by D.W. many times. Finally, the product obtained was dried at 80 °C.

Synthesis of single doped ZnO as Sm doped ZnO (Sm DZ NPs), la doped ZnO (La DZ NPs), and Sr doped ZnO (Sr DZ NPs) nanomaterials

We generated single doped ZnO by dissolving 5 gm of Zinc acetate dihydrate in 250 ml of Distilled water at room temperature with magnetic stirring for 30 min. The mixture was agitated for 10 min after adding 2.5 ml of acetic acid. As a dopant, add 2.0 mol% Lanthanum (III) nitrate hexahydrate to the preceding mixture and stir until dissolved. Similarly, we completed the experiment as in the case of the synthesis of Pure ZnO, and the yield was labeled as (La DZ NPs). In addition, in the case of preparing Sm doped ZnO, add 2.0 mol% of samarium (III) nitrate hexahydrate labeled as (Sm DZ NPs), while in the case of Sr doped ZnO added 2.0 mol% of strontium nitrate labeled as (Sr DZ NPs).

Synthesis of tri-doped ZnO (la, Sm, Sr Tri- doped ZnO Nano particles (Sm, La, Sr TDZ NPs)

Tri-doped ZnO was prepared by dissolving 5gm of Zinc acetate dihydrate in 250mL of distilled water under magnetic stirring for 30min. at 25 °C. After that, 2.5 mL of acetic acid was added, and the mixture was further stirred for 10 min., then added 2.0 mol% of lanthanum (III) nitrate hexahydrate, 2.0 mol% of samarium (III) nitrate hexahydrate, 2.0 mol% of strontium nitrate (the molar ratio of La: Sm: Sr is 1:1:1) under vigorous stirring until dissolved. Then, NaOH was used to adjust the pH range (10–11) at this white precipitate, followed by stirring for 30min. A Teflon-lined stainless-steel autoclave received the mixture, which was closed for 24h at 200 °C. The yield was washed with D.W. and dried at 80 °C shown in.

Characterization

The synthesized of PZ, Sm DZ NPs, La DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs were affirmed by numerous characterization techniques. The structural properties were investigated by X-ray diffraction (XRD), (PANalytical X'Pert Pro); using Cu-Kα irradiation in a scanning range of 2θ between 10 and 80. The solid phase FT-IR spectrum was investigated by (Perkin Elmer-Spectrum). A Raman spectrum (InVia; Renishaw) was recorded with a laser beam having an excitation wavelength of 514 nm. Additionally, the optical properties were studied by using a spectrophotometer (JASCO V-770), the optical bandgaps were estimated from linear extrapolation of Tauc plots, and photoluminescence (PL) spectroscopy ([RF-5301 Pc) carried with irradiation source at excitation wavelength 345 nm. The binding state of doped metal was distinguished with the X-ray photoelectron spectroscopy (XPS) (HP 5950A, Hewlett-pakard firm). The surface morphology was determined by SEM-(Hitachi, S-4800) attached with Energy-dispersive X-ray (EDX) that using with X-ray photoelectron spectroscopy (XPS-Thermo Electron Corporation Theta Probe) to evaluate the compound elements in the synthesized photocatalysts. The zeta potential is represented by ZS Malvern, A zetasizer attached to an automatic titrator (Malvern MPT-2). By utilizing nitrogen as the adsorbate at 77.4 K in a Fisons SORPTOMATIC 1900 volumetric adsorption–desorption apparatus, the samples' specific surface areas and pore size distributions were calculated.

Theoretical study

The CASTEP module in the materials studio was used to compute this work. The generalized gradient approximation (GGA + U) with the Perdew-Burke-Ernzerh (PBE) function was used to explain the exchange–correlation energy. By employing a cutoff energy of 380 eV and a selection of the Brillouin zone with 5 × 5 × 4 k-points in the Bronkhorst-pack approach. We select the DFT/GGA + U technique. In this study, Ud,zn = 10 eV, Up,O = 6.5 eV, Uf,,o doped metal = 6 eV, which were fine for computation accuracy. Methods of computation of the hexagonal wurtzite structure with the P63mc space group symmetry system is the most stable Zinc oxide at room temperature and pressure. A pair of Zn atoms and a pair of Oxygen atoms make up the primitive cell. PZ, La DZ NPs, and Sr DZ NPs were all modeled in this work using a 2 × 2 × 2 supercell; atoms of La and Sr are exchanged for their neighboring Zn atoms.

Photocatalytic degradation procedure

Photocatalysts activities of PZ, Sm DZ NPs, La DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs investigated degradation of Reactive red 43. Using 20 ppm as the initial concentration in 50 mL of dye, adjust pH at 8, and the photocatalyst was added at a dose of 0.05 g/L. Reactive red 43dye and photocatalyst were magnetically stirred for 1h in the dark to establish absorption/desorption equilibrium. A model UV–visible spectrometer examined the absorption spectrum of an aliquot 3 mL of the deteriorated solution at intervals. The photodegradation of RR43 was studied by measuring the change in absorption intensity using UV–visible spectroscopy every 20 min for a period of 120 min. The experiment was carried out in the same way for both pure and doped ZnO nanomaterials.

Results and discussion

Characterization

X-ray diffraction (XRD)

Figure 1a exhibited diffraction patterns of XRD data for PZ, Sm DZ NPs, La DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs. By analyzing these patterns, crystallographic properties were studied, such as average crystallite size, lattice parameters, and the nature of the sample. We found Card No. 01-079-0207; Zinc Oxide; Hexagonal corresponds well with diffraction peaks obtained for synthesized samples, the peaks of Sm, La, Sr TDZ NPs located at 31.72, 34.36, 36.20, 47.54, 56.53, 62.80, 66.33, 67.86, 72.60, 74.89and 76.91planes (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) can be attributed to the hexagonal wurtzite; this result indicated the doping does not change the crystalline phases of ZnO [31, 32]. No additional peak was detected in all samples, demonstrating the crystallinity and purity of the produced samples [33]. The three most intense peaks (100), (002), and (101) of the total samples are displayed in Fig. 1b as magnified XRD patterns. The single and tri-doped ZnO showed the high three peaks intensity reduced and shifted to the higher 2θ angle when compared with Pure ZnO [34], which indicated that these metal Sm3+, La3+, and Sr2+ ions are incorporated into crystals lattice of the ZnO [35]. The average crystallite size was calculated by using Scherer’s equation:

$$D = \, 0.89\lambda /\beta \, \cos \, \theta$$
(1)

where D is the average crystal size in nm, λ is the Cu-Kα wavelength (0.15406 nm), β is the full width at half maximum, and θ is the direction angle. In Table 1, it was found that an increase in the average crystallite size was due to the addition of dopants Sm3+, La3+, and Sr2+ ions, a similar result reported in [29]. The lattice parameters and volume of single-doped and tri-doped unit cell exhibited values slightly smaller than undoped ZnO, as shown in Table 1. The slight difference in lattice parameters revealed that Sm3+, La3+, and Sr2+ ions are successfully incorporated into the ZnO lattice where it, interstitially, substitutes the Zn2+ ion sites as the ionic radius of Sm3+ (1.08 Å), La3+ (1.06 Å), and Sr2+ (2.45 Å) is larger than that of Zn2+ (0.74 Å). The shift in peaks and changes in lattice characteristics indicate that Sm3+, La3+, and Sr2+ ions are replacing Zn2+ ions [36].

Figure 1
figure 1

(a) The X-ray diffraction patterns (b) Magnified X-ray diffraction patterns of Pure PZ, Sm DZ NPs, La DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs

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Table 1 Crystallite size variation (D), lattice parameter, and unit cell volume of pure, and doped ZnO
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FT-IR spectra

The FT-IR spectroscopy was analyzed to explore the effect of different metals, such as (Sm, La, and Sr) doping, on ZnO, shown in Fig. 2a. The vibrational properties of synthesis samples were measured between 4000 and 400 cm−1. Broad bands are noticed at around 3309.42, 3353.97, 3328.08, and 3422.87 cm−1 for PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs, respectively, and therefore, these bands are corresponding to the stretching vibration of the O–H in water [25]. Furthermore, bands were observed in the range 1502.06, 1395.40, and 700–1100 cm−1corresponding to the starting materials [37]. Additionally, Zn–O stretching absorption bands are represented at 464.63, 455.14, 512.285, 4 65.98, and 514.93 cm−1 for PZ, La DZ NPs, Sm DZ NPs, Sr DZ NP, and Sm, La, Sr TDZ NPs, respectively [38, 39]. Further, the doped nanomaterials exhibited a slight shift in the absorption band, where samarium, lanthanum, and strontium atoms are heavier than the Zn atom. These changes in the band's position can be explained by the changes in the bond length when a Sm, La, or Sr atom is replaced by a Zn atom. Finally, the shifting of these bands indicated that doping was formed.

Figure 2
figure 2

(a) FT-IR spectra (b) Raman spectra of PZ, Sm DZ NPs, La DZ NPs, Sr DZ NPs, Sm, La, Sr TDZ NPs

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Raman spectroscopy

According to group theory, the Raman scattering spectroscopy was used to investigate Nano-scale structures and vibrational properties of doping system. The optical phonon modes in Wurtzite type ZnO at the Brillouin T point are represented by T = A1 + 2B1 + E1 + 2E2, Where A1 and E1 are polar Raman active modes with transverse optical (TO) and longitudinal optical (LO) phonons. At the same time, B1 is the Raman inactive mode, and E 2(low) and E 2(high) are nonpolar. Raman detects crystal stress and defect development [37]. Figure 2b exhibited the Raman spectra for PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, and Sr TDZ NPs, respectively, with the position of phonon vibration frequencies. The strong peaks observed around 441 cm−1 are assigned to the E 2 (high) and showed peaks at 96.50, 98.50, 96.88, 96.25, and 98.84 cm−1 and assigned to the E 2(low). Additionally, peaks at 332.5, 333.75, and 331.25 cm−1 correspond to the multi-phonon scattering process E2(high)-E2(low) mode of vibration, which is a characteristic Raman active for the wurtzite hexagonal phase of ZnO. All the samples show these peaks are due to the Zn and O sub-lattice [34]. Moreover, peaks at 378.75, 383.25, 393.26, 385.5, and 375.76 cm−1 for PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs, respectively, indicating the A1(TO) phonon mode that is usually represented by the defect of the zinc interstitial and oxygen vacancy in the ZnO lattice. Similarly, there are peaks due to E1(LO) mode, which is associated with the impurities and formation of defects at 570.02, 597.57, and 573.25 cm−1 for PZ, La DZ NPs, Sm DZ NPs, and Sr DZ NPs, respectively [40]. In single and Tri-doped, a reduction of intensity and shift in some peaks occurred because of the incorporation of Sm3+, La3+, and Sr2+ into the ZnO lattice. These peaks indicated an increase in defect formation in the synthesized nanomaterials [39].

Ultraviolet–visible spectroscopy (UV–vis)

The UV–vis of PZ, Sm DZ NPs, La DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs were illustrated in Fig. 3a. As represented in Fig. 3a, an absorption band of PZ was observed at 380 nm, but in single-doped samples, as La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs exhibited absorption bands lower than PZ at 375.86, 364.17, 377.25, and 367.67 nm, respectively [41]. All doped ZnO displayed a blue shift toward a lower wavelength than pure ZnO. The band gap of doped ZnO has widened, as evidenced by the blue shift. We also used the Tauc relationship (Eq. 2) to determine the energy band gaps in pure ZnO and doped synthesized ZnO:

$$\alpha h\nu \, = \alpha \left( {h\nu \, - \, E_g } \right)^n$$
(2)

where α is the absorption coefficient, Eg is the optical band gap, and v is the frequency of a photon. We have taken a value of ½ for a direct band gap semiconductor. Tauc's plot of (αhv)2 vs. photon energy (hν) shows the Eg. The calculated Eg values are 2.86, 2.89, 2.93, 2.98, and 3.02 eV for PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and La, Sm, Sr TDZ NPs, respectively shown in Fig. 3b. The Eg of doped ZnO increased compared with pure ZnO as a result of the presence of the oxygen stoichiometry and the quantum confinement effect [29]. Because the value of the band gap rises, the probability that the electron–hole pairs will have greater redox potentials rises along with it, leading to an increase in photocatalytic activity [42].

Figure 3
figure 3

(a) Ultraviolet–visible spectroscopy and (b) Tauc’s plot of Pure and doped ZnO

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Photoluminescence (PL)

Figure 4 depicts the fluctuation in the PL intensity of PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs. and La, Sm, Sr TDZ NPs nanomaterials with incident photon wavelength. In visible region peaks at 448 nm due to the recombination of the free exactions on the ZnO surface [43]. Additionally, the observed peak in the blue region at 469 nm is due to the transition of electrons. Dopant ions Sm3+, La3+, and Sr2+, induce perturbations in the ZnO crystal, and new energy levels are also created by introducing these ions [41]. Moreover, strong peaks related to the high density of oxygen vacancies during the preparation were observed in the green region at 470.56, 470.48, 469.54, 470.52, and 469.53 nm for La DZ NPs, Sm DZ NPs, Sr DZ NPs. and La, Sm, Sr TDZ NPs, respectively. By comparing doped ZnO and pure ZnO, it was found that the PL intensity increased when Sm3+, La3+, and Sr2+ ions were combined into the ZnO crystal structure, which may be due to the defects of oxygen vacancies. It is widely accepted that nanomaterials of ZnO contain defects, according to these results, the synthesized. Nanomaterials possess the capacity to promote photocatalysis.

Figure 4
figure 4

PL spectra of Pure and doped ZnO

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XPS analysis

The valence and chemical composition of an element of a doping metal have been investigated using XPS measurement. All XPS spectra were calibrated for binding energy using the carbon peak at 284.87 eV as a reference. According to Fig. 5a, the undoped ZnO survey spectrum displays peaks assigned to the Zn 2p state, O 1s state, and C 1s state. The XPS spectrum of Zn 2p state for PZ displayed doublet peaks at 1026.20 eV corresponding to 2p3/2 states correlated with the hydroxyl group linkage of zinc within ZnO and 1048.82 eV for 2p1/2 states corresponding to the atomic linkage of zinc with oxygen species. At the same time, the Zn 2p peaks showed a shift in doped samples, such as La DZ NPs at 1022.94 and 1047.37 eV, Sm DZ NPs at 1023.17 and 1046.42 eV, Sr DZ NPs at 1022.16 and 1045.36 eV in Sm, La, Sr TDZ NPs centered at 1022.73 and 1045.68 eV Fig. 5b. The presence of Zn+2 in oxide form is confirmed by an energy difference between spectral lines of 2p1/2 and 2p3/2 that agrees well with the standard value of 22.97 eV [37]. The O 1s region explained the different states and displayed two peaks as adsorption of the hydroxyl ion (OH−) or aqueous species over the surface of ZnO and oxygen vacancies. The first peak displayed a higher binding energy and showed at 536.43, 534.29, 532.22, 532.67, and 532.70 eV for PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, Sm, La, Sr TDZ NPs, respectively. In addition, the lower binding energy related to the oxygen vacancies demonstrated that oxygen vacancies are present at 533.10, 531.72, 531.9, 531.48, and 531.66 eV for PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, Sm, La, Sr TDZ NPs, respectively [29]. Arise new peaks correlate to La metal-doped observed in Figs. 6d and 9d, the binding energy of La 3d for single doped is located at 835.68 and 852.58 eV, while tri-doped appears at peaks 836.76 and 853.52 eV. Moreover, the La 3d showed that the La element existed as a (3 +) valence in the sample, according to the principle method and XPS instrument's manual [44]. Secondly, the Sm metal doped showed a peak in Fig. 7d for a single doped centered at 1082.34 eV. Surprisingly, the third Sr metal doped divided into two peaks for a single doped shown in Fig. 8d centered at 130.45 and 134.59 eV, assignable to 3d 5/2, and 3d 3/2 states, respectively. Similarly, the Sr in tri-doped showed two peaks at 130.85 and 135.22 eV Fig. 9f. These results exhibited that Sm3+, La3+, and Sr2+ ions are successfully incorporated into the ZnO lattice [45].

Figure 5
figure 5

(a) XPS Survey spectra of Pure ZnO, (b) Zn 2p and (c) O 1s

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Figure 6
figure 6

(a) XPS Survey spectra of La DZ NPs, (b) Zn 2p, (c) O 1, and (d) La 3d

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Figure 7
figure 7

(a) XPS Survey spectra of Sm DZ NPs, (b) Zn 2p, (c) O 1s, and (d) Sm 3d

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Figure 8
figure 8

(a) XPS Survey spectra of Sr doped ZnO, (b) Zn 2p, (c) O 1, and (d) Sr 3d

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Figure 9
figure 9

(a) XPS Survey spectra of Sm, La, Sr TDZ NPs, (b) Zn 2p, (c) O 1s, (d) La 3d, (e) Sm 3d, and (f) Sr 3d

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EM, TEM and EDX spectra

The surface morphology of PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs was studied using SEM and TEM analysis. Each scanning electron micrograph has the same magnification and a scale of 20 µ, clearly showing hexagonal ZnO and rod-shaped crystals [46, 47]. The hexagonal shapes don’t change due to uniform disperses of Sm3+, La3+, and Sr2+ ions across the hexagonal lattice (Fig. 10). In addition, the EDX spectra exhibited significant peaks for each Sm, La, and Sr metal, indicating that Sm3+, La3+, and Sr2+ ions are dispersed into the ZnO crystals lattice, according to our experimental measurements (Fig. 11). Additionally, the TEM images of PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, and Sr TDZ NPs showed that the ZnO particles are grown as a rod shape and have average particles size around 10–100 nm as shown in Fig. 12. Finally, the SEM and TEM images confirmed the successfully incorporating of Sm3+, La3+, and Sr2+ ions into the ZnO lattice.

Figure 10
figure 10

SEM images (a) PZ, (b) La DZ NPs, (c) Sm DZ NPs, (d) Sr DZ NPs, and (e) Sm, La, Sr TDZ NPs

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Figure 11
figure 11

EDX spectra (a) PZ, (b) La DZ NPs, (c) Sm DZ NPs, (d) Sr DZ NPs, and (e) Sm, La, Sr TDZ NPs

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Figure 12
figure 12

TEM images (a) PZ, (b) La DZ NPs, (c) Sm DZ NPs, (d) Sr DZ NPs, and (e) Sm, La, Sr TDZ NPs

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Zeta potential and point of zero charge (pHpzc)

It is important to assess the stability of nanomaterials, where the suspension's net electrostatic potential is measured using its zeta potential. The PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, and Sr TDZ NPs had high positive potential values of 17.2, 13.0, 10.1, 13.9, and 10.6 mV, respectively (Fig. 13). In addition, the significant electrostatic repulsion between the values proved to be extremely stable and prevented the values from aggregating. Good nanoparticle stability is often assumed to occur at zeta potential levels between + 30 mV and −30 mV, as reported in the literature [44, 45]. Due to electrostatic attraction, the reactive dye is attracted to the positively charged surface of our doped nanomaterials, resulting in the adsorption of the reactive dye on the nanomaterials' surface.

Figure 13
figure 13

Zeta potential of dispersed for: (a) PZ, (b) La DZ NPs, (c) Sm DZ NPs, (d) Sr DZ NPs, and (e) Sm, La, Sr TDZ NPs in water

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Similarly, the point of zero charge (pHpzc) of PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs were determined by using a salt addition procedure. The adsorbent was added into sodium nitrate solution (50 mL) using both HCl (0.1 M) and NaOH (0.1 M) to maintain a pH range of 2 to 12, and after 24h of shaking at room temperature, and the final pH (pHf) was determined by pH meter. The point of zero charge (pHpzc) of nanomaterials was determined by plotting the graph between initial pHo & ΔpH (pHo – pHf). Additionally, the synthesized nanomaterials PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs showed values of 8.4, 8.2, 8.9, 8.7, and 9.2, respectively (Fig. 14). The surface of nanomaterials has a balance between positive and negative charges at a specific pH, which is denoted as pHpzc. Therefore, if the pH is less than pHpzc, the Nanoparticle surface will be positively charged, and if the pH is more than pHpzc, the surface will have a net negative charge [46]. Finally, it can be concluded that the values Low than pHpzc, the surface of nanomaterials became positive, and therefore, the electrostatic attraction occurred with reactive red 43.

Figure 14
figure 14

Point of zero charge (pHpzc) for pure and doped ZnO

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Surface area

The BET method was used to estimate the pore diameter distribution and specific surface area of PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs (Fig. 15). The BET surface area is also a crucial metric for evaluating metal oxide. Comparing the PZ (SBET = 28.81 m2/g) with Sr DZ NPs (SBET = 37.43 m2/g) showed an increase in BET value. The increased BET surface area of Sr DZ NPs means that reactive dye molecules have a greater chance of adsorption with the Sr DZ NPs [47]. At the same time, La DZ NPs (SBET = 22.40 m2/g), Sm DZ NPs (SBET = 16.79 m2/g), and Sm, La, Sr TDZ NPs (SBET = 6.30 m2/g) exhibited a reduction of the BET surface area value. As a result, surface population density increased proportionally, resulting in a reduction in surface area [48]. Accordingly, the observed specific surface area may be linked to the large pore structure and external area as intra-particle pores, indicating a readily accessible area of both materials. The estimated pore size distribution pattern of the nanomaterials is shown in Fig. 15 (inset). Moreover, the overall pore volume of PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs was evaluated at 0.128, 0.1219, 0.1004, 0.2105, and 0.0277 cm3/g of each average pore size of 17.77, 21.82, 23.57, 22.50 and 17.572 nm, respectively. The addition of La and Sm metal led to a decrease in total pore volume compared to Sr, tri-doped sample [49].

Figure 15
figure 15

Adsorption–Desorption isotherm curve and particle size distribution (inset) of (a) PZ, (b) La DZ NPs, (c) Sm DZ NPs, (d) Sr DZ NPs, and (e) Sm, La, Sr TDZ NPs

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Theoretical study

Crystal structures

The optimized geometry structure of a 2 × 2 × 2 supercell of PZ, La DZ NPs, and Sr DZ NPs, with 32 atoms, is constructed. Choose one metal from the alkaline-earth metals group and another metal from earth metals with different electronic configurations as an example of theoretical study. In ZnO, La atoms are substituted for Zn atoms to create La DZ NPs. In ZnO, Sr atoms are substituted for Zn atoms to create Sr DZ NPs, as illustrated in Fig. 16. The lattice parameters of the supercell models are shown in Table 1 and exhibited an increase in the lattice constants after doping. The theoretical results agree with the experimental results.

Figure 16
figure 16

2 × 2 × 2 supercell of (a) PZ, (b) La DZ NPs, and (c) Sr DZ NPs, where gray balls as the zinc atoms, red (oxygen atoms), blue (Lanthanum atom), and green (Strontium atom)

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Electronic structure

We have estimated the band structure and the partial and total density of states for PZ, La DZ NPs, and Sr DZ NPs to learn more about their electrical structure. We operated the functional generalized gradient approximation (GGA + U) with the Perdew-Burke-Ernzerh (PBE) function, the result of the structure, and plotted gap energy shown in Fig. 17. These data demonstrate the existence of a direct band gap in the system, the value of which varies with the doping type. For PZ, the direct energy gap offered a value is 2.73 eV, proximate to the experimental value of 2.86 eV. In doped ZnO, the Fermi level shifted toward the valence band, which increases the electron carrier concentration of the band and promotes the creation of p-type semiconductors and system conductivity [50]. The calculated 2.77 and 2.85 eV gaps for La DZ NPs and Sr DZ NPs, respectively. These values agree with experimental results. The Burstein-Mott effect is responsible for this widening of the gap [51]. These values fit well with experimental data are 2.89 eV for La DZ NPs and 2.98 for Sr DZ NPs. To explain the formation of these band structures, we computed the total density of states (DOS) and partial density of state (PDOS) for PZ, La DZ, and Sr DZ NPs, as shown in Fig. 18. First, for PZ, the contribution of the O 2s states and Zn 3d states produced the first peak at around 14.89 eV in the valence band. O 2p and Zn 3d states predominate in the higher energy area of the valence band (between 7.41 and 0 eV). States of O 2p make up the top of the valence band. A strong Zn–O bond produced by the high interactions between the Zn 3d state and the 2p state of O, which range from 7.41 to 0 eV. The oxygen 2p, zinc 4p, and zinc 4s also control the bottom of the conduction band. Second, the O 2s, Zn 3d, and La 5p states are the origin of the first valence band peak for La DZ NPs, which are seen at 14.65 eV. O 2p, Zn 3d, and La 4d states dominate the lower valence band, which occupies the energy range from 7.27 to 0 eV, indicating that the orbital bonding relationship was strengthened. The conduction band is dominated by O 2p, Zn 4s, Zn 4p, La 4d, and La5p states. In doped ZnO, the Fermi level moves into the valence band, resulting in a p-type semiconductor. Third, O Zn 3d, and Sr 4p states contribute to the lowest peak at − 14.77 eV in Sr-doped ZnO. The energy range between -7.35 and 0 eV and the energy band toward the top of the valence band are dominated by the O 2p, Zn 3d, and Sr 3d states. Valence band bandwidth increases due to O2p-Sr 3d state interaction. The conduction band widens when electronic states overlap. Doped systems have lower densities of states than PZ, and the Fermi level advances into the valence band, increasing the width of both the conduction and valence bands and the band gap compared to PZ [52, 53].

Figure 17
figure 17

The band structure for: (a) PZ, (b) La DZ NPs, and (c) Sr DZ NPs

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Figure 18
figure 18

Density of state of (a) PZ, (b) La DZ NPs, and (c) Sr DZ NPs

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Optical properties

Density functional theory computed optical characteristics, such as dielectric function. The dielectric function in crystals is the link between electronic transition and electronic structure, reflecting a certain material's band structure and optical properties. The optical properties of semiconductor materials can be described by complex dielectric functions, according to solid state theory. Calculating inertia matrix elements of occupied and non-occupied states and Kramers–Kronig dispersion relations yields dielectric function real and imaginary parts [54, 55]. The electron transition between conduction and valence bands determines the imaginary part. For any material, the imaginary part of the dielectric function is significant. The imaginary part connects the inter-band transition micro-physical process to the solid electronic structure, reflecting the solid energy band structure and other spectrum information. Figure 19 shows the imaginary and real dielectric functions of PZ, La DZ, and Sr DZ NPs. The PZ's imaginary dielectric function exhibits three high peaks (8.82, 12.28, and 14.55 eV) that match other theoretical conclusions [56]. Three peaks may be identified: the first is obtained from the transition between O-2p and Zn-4s, the second from the transition between Zn-3d and O-2p, and the third from the transition between Zn-3d and O-2s. According to the investigation of the density of states and band structure, the electron transition of 4d states for the doped ZnO exhibits a pear peak at about 4.86 eV. Doping causes a slight drop in dielectric constant in the low-energy range.

Figure 19
figure 19

Dielectric Function of PZ, La DZ NPs, Sr DZ NPs Nanomaterials: (a) Real part, and (b) Imaginary part

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Study of photocatalytic activity

RR43 dye degradation was studied in pure and doped ZnO under UV light irradiation to find the best photocatalytic conditions. RR43 dye is extremely stable in water. The degradation of RR43 dye with PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs within constant reaction time from 0 to 120 min, where 0 min in each case is the dark study that is before irradiation with UV light. It has been noticed that Sr DZ NPs exhibited higher photocatalytic degradation at 93.43% Then, PZ at 72.88%, La DZ NPs at 52.54%, Sm DZ NPs at 31.99%, and La, Sm, Sr T DZ NPs 20.55%. On the surface, the Sr2+ ions in the lattice act as electron scavengers, preventing the recombination of electron–hole pairs and extending their lifetime, and can capture photo-induced electrons, slowing electron–hole recombination, resulting in a decrease in recombination rate due to increased defects that develop enhancement of photocatalytic degradation [57]. The degradation is confirmed as the absorbance of dye, and absorbance decreases strongly with Sr DZ NPs, then La DZ NPs, Sm DZ NPs finally Tri-doped. The fastest and most efficient degradation was observed by Sr DZ NPs due to Sr, one of the alkaline-earth metals, which this group considers as the second most reactive family of elements related to electronic configuration.

Effect of photocatalytic dose

An important variable in photodegradation is photocatalytic dosage. We have investigated the effect of photocatalytic dosage on the photodegradation percentage of RR43. The experimental results were performed at 25 °C with UV irradiation using varying concentrations of photocatalytic nanomaterials (0.00, 0.02, 0.05, and 0.07 g/ L) and a constant concentration of RR43 dye (20 ppm/50 mL). As represented in Fig. 20, the photodegradation % increased with increasing adsorbent dosage due to an increase in photocatalytic available active surface area [57].

Figure 20
figure 20

Effect of photocatalytic doses on the photodegradation of RR43 by: (a) PZ, (b) La DZ NPs, (c) Sm DZ NPs, (d) Sr DZ NPs, and (e) Sm, La, Sr TDZ NPs

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Effect of dye concentration

The photodegradation investigation of RR43 was carried out in the presence of 0.05 g/L of photocatalytic nanomaterials under UV light irradiation at various dye solution concentrations (10, 20, 30, and 40 ppm). Figure 21 showed the photodegradation of dye solution concentrations under the same conditions for all photocatalytic nanomaterials. In this study, the Beer-Lambert rule may explain the decrease in photodegradation percentage as the RR43 dye concentration increased, where more dye molecules were absorbed to the surface of the photocatalytic nanomaterials. A reduced photodegradation activity may result from a reduced path length for light entering the solution as dye concentration increases. Furthermore, an increase in the initial concentration of RR43 dye may cause a decrease in the production of OH radicals on the surface, resulting in the RR43 molecules blocking the active sites. [33].

Figure 21
figure 21

Effect of dye concentration on the photodegradation of RR43 by: (a) PZ, (b) La DZ NPs, (c) Sm DZ NPs, (d) Sr DZ NPs, and (e) Sm, La, Sr TDZ NPs

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Effect of solution pH

The effect of solution pH on photocatalytic degradation of RR43 was examined using initial pH values of the aqueous RR43 solution of 3, 5.8, 8, 10, and 12 adjusted by aqueous 0.1M HCl and 0.1M NaOH solutions while keeping all other experimental conditions constant. It was demonstrated that photodegradation depends on dye adsorption on the photocatalytic surface and that dye adsorption is greatly influenced by the pH of the solution. Figure 22 depicts the influence of the initial pH on RR43 degradation. These results were confirmed by measuring the zero-point charge (pHpzc), determined at a pH higher than 8. It was noted that the best degradation percentage of RR43 at pH 8 was recorded at 93.43% for Sr DZ NPs, 72.88% for PZ, 52.54% for La DZ NPs, 31.99% for Sm DZ NPs, and 20.55% for La, Sm, Sr T DZ NPs. The values at pH 8 consider value Low than pHpzc, so the photocatalytic surface will become a positive charge, enhancing electrostatic attraction with reactive red 43 [33]. The values of degradation percentage at pH 10 and 12 were recorded (70.34% and 4.03%) for Sr DZ NPs, (48.73% and 12.71%) for PZ, (25.42% and 6.56%) for La DZ NPs, (11.44% and 9.11%) for Sm DZ NPs and, (11.02% and 9.32%) for La, Sm, Sr T DZ NPs. This decrease in the degradation percentage at pH 10 and 12 may be related to the fact that the surface of the photocatalytic will become a negative charge, which is repulsive with RR43. Finally, at a pH3, the photodegradation percentage decrease showed 82.84% for Sr DZ NPs, 52.11% for PZ, 45.97% for La DZ NPs, 24.15% for Sm DZ NPs, and 13.56% for La, Sm, Sr T DZ NPs, due to Nanoparticle breakdown in acidic pH.

Figure 22
figure 22

Effect of solution pH on the photodegradation of RR43 by: (a) PZ, (b) La DZ NPs, (c) Sm DZ NPs, (d) Sr DZ NPs, and (e) Sm, La, Sr TDZ NPs

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Adsorption kinetic studies

Kinetic studies were carried out using pseudo-first-order models [58] using experimental data (Eq. 3) defines the pseudo-first-order kinetic model:

$$\ln \, \left( {C/C_o } \right) \, = \, --kt$$
(3)

where C represents the dye solution intensity after irradiation, Co represents the preliminary dye solution intensity before irradiation, t represents the time (min), and k (min−1) represents the pseudo-first-order rate constant. The fitting curves were plotted ln(C/Co) versus time [59], which exhibited that the degradation of RR43 dye roughly obeys pseudo-first-order kinetics. We found that the R2 values obtained from the kinetic model offer that the photocatalytic nanomaterials are more fitted with the pseudo-First-order kinetic model due to the higher R2 quantities. The pseudo-first-order rate constant (k) for all photocatalytic nanomaterials with degradation of RR43 recorded a higher value14.84 *10–3(min−1) with Sr DZ NPs, then PZ 7.07 *10–3(min−1), La DZ NPs 3.85 *10–3(min−1), Sm DZ NPs 2.13 *10–3(min−1), and La, Sm, Sr T DZ NPs 1.31*10–3(min−1) shown Fig. 23.

Figure 23
figure 23

(a) Pseudo-first-order kinetic model, and (b) Rate constant of PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, and Sm, La, Sr TDZ NPs on the photodegradation of RR43

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Mechanism of photocatalytic degradation of reactive Red 34

The Sr2+ dopant modifications ZnO, which traps excited electrons in the CB and reduces electron hole-recombination. Through a series of reactions, the electrons trapped by dopants would finally react with ˙O2 to create [˙OH]. Through a series of reactions, the electrons trapped by dopants would finally react with O2 to create [˙OH]. The H2O molecule or [˙OH] adsorbed on the photocatalytic surface would be charged up by the hole in the valence band of doped ZnO, forming [˙OH]. The generated [˙OH] would react with RR43 dye molecules adsorbed on the surface during photocatalytic to destroy the pollutants. Second, rare earth reacts with the electrons in the conduction band when La3+ and Sm3+ are used as dopants. This is because rare earth metals are forceful Lewis’s acids with a partially filled 4f orbital that can trap the electron and further prevent the recombination of charge carriers. Unstable Sm2+ ions lost electrons to oxygen molecules to create active superoxide radicals, which protonated to ˙OOH radicals. The inactive OOH radical is converted into the reactive OH radical. The generation of [˙OH] in doped ZnO can be explained as follows: increasing the optical band gap value keeps photo-generated electrons in the conduction band, enhancing the material's photocatalytic action. In addition, the dopant of La, Sm, and Sr atoms will create intermediate energy levels in the doped lattice, where photo-excited electrons will trap. Additionally, the PL spectrum confirmed that the electron–hole recombination rate was reduced. Scheme 1 depicts the degradation pathway schematically.

Scheme 1
scheme 1

Representation of photocatalytic degradation mechanism of doped ZnO

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Recyclability of the photocatalytic nanomaterials

Recyclability and stability are important parameters for assessing the effectiveness of a photocatalytic process since they allow the photocatalytic to recycle repeatedly for pollutant degradation [60]. Repeating the photocatalytic experiment four times under the same conditions, with the photocatalyst separated from the dye solution via centrifugation after each cycle run, and then using it again after washing and drying, allowed us to evaluate the recyclability of the photocatalytic nanomaterials [33]. After four cycle runs, La, Sm, Sr T DZ, La DZ NPs, and Sm DZ NPs, these compounds showed the photodegradation percentage decreased slightly than Sr DZ NPs and PZ. So, consider La, Sm, Sr T DZ, La DZ NPs, and Sm DZ NPs are more stable due to having rare earth metals. Additionally, shown in Fig. 24, the photodegradation percentage decreases slightly, and the dye degradation is consistent. This reduction could be because photocatalytic is lost when the sample is washed; the photocatalytic nanomaterials showed stability from this result. This current study significantly compares to previously reported studies on degeneration efficiency (Table 2).

Figure 24
figure 24

Photodegradation % of nanomaterials for RR43with four cycle runs

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Table 2 Comparison table of previously reported photocatalysts and their degradation efficiency
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Conclusions

In this work, new nanomaterials such as pure zinc oxide, single doped ZnO nanomaterials (2%), namely as Sm DZ NPs, La DZ NPs, Sr DZ NPs, and tri-doped ZnO as Sm, La, Sr, T DZ NPs were synthesized using hydrothermal method. The structure of the synthesized nanomaterials based on ZnO (single and tri-doped) was confirmed using different spectroscopic techniques, such as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), ultraviolet (UV) light, photoluminescence (PL), scanning electron microscopy (SEM) and energy-dispersive X-ray (XPS), transmission electron microscopy (TEM), and the point of zero charges (pHpzc). The XRD confirmed a hexagonal crystal structure without change in the crystalline phase after doping and exhibited an increase in the average crystallite size. The XRD pattern displayed a shift in peaks and changes in lattice characteristics, indicating that Sm3+, La3+, and Sr2+ ions are replacing Zn2+ ions. The FT-IR spectroscopy showed shifting in the absorption of Zn–O band at 455.14, 512.285, 465.98, and 514.93 cm−1 for La DZ NPs, Sm DZ NPs, Sr DZ NP, and Sm, La, Sr TDZ NPs, respectively, compared to pure ZnO 464.63 cm−1. Moreover, using UV–Vis measurements and the Tauc equation, the energy band gap values were measured experimentally (2.86, 2.89, and 2.98 eV) and calculated theoretically by DFT calculation to afford values of (2.73, 2.77, and 2.85 eV) for PZ, La DZ NPs, Sm DZ NPs, Sr DZ NP, and Sm, La, Sr TDZ NPs, respectively. The theoretical result confirmed the increase in the band gap due to an increase in the atomic radius of the dopant. The PL spectrum displays showed peaks at nearly 470 nm related to the high density of oxygen vacancies. The morphology study (SEM and TEM) showed hexagonal and rod-shaped crystals that do not change with doped nanomaterials. Furthermore, XPS and EDX data exhibited significant peaks for each Sm, La, and Sr metal, indicating that Sm3+, La3+, and Sr2+ ions are dispersed into the ZnO crystals lattice. The Zeta potential demonstrated high positive potential values of 17.2, 13.0, 10.1, 13.9, and 10.6 mV for PZ, La DZ NPs, Sm DZ NPs, Sr DZ NPs, respectively. The BET surface area is also evaluated, and Sr DZ NPs recorded a high value (SBET = 37.43 m2/g). Additionally, the photodegradation of Reactive Red 43 (RR43) dye was studied using different factors, such as photocatalyst dose, dye concentration, and pH. The results revealed that the Sr DZ NPs showed the best photodegradation percentage (93.43%) compared to PZ (72.88%), La DZ NPs (52.54 3%), Sm DZ NPs (31.99%), and La, Sm, Sr T DZ NPs (20.55%). The kinetic studies exhibited that the R2 values are more fitted with the pseudo-first-order kinetic model, and cyclic stability proves the suitability of synthesized nanomaterials under visible light irradiation for the treating of the RR43 dye as polluted wastewaters.