Mg and La Co-doped ZnO Nanoparticles Prepared by Sol–gel Method: Synthesis, Characterization and Photocatalytic Activity

In this study, La and Mg doped, and co-doped ZnO nanoparticles were prepared using the sol-gel method. The prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DRS), and N2 physisorption techniques. The XRD results indicated that the prepared nanoparticles can be well adopted by the hexagonal wurtzite structure crystal and there are no second impurity peaks. Studies of the FESEM, EDX and TEM have shown that the samples have uniform spherical-like morphology with a homogenous distribution. The incorporation of La and Mg into the ZnO lattice had no effect on the morphology of the nanoparticles, but a reduction in the size of the grains (≈ 14 nm to ≈ 7 nm) was observed due to the insertion of these ions. The results of N2 physisorption indicated that there was an increase in BET surface area and pore volume for doped and co-doped samples. The results of DRS showed an increase in band gap energy and a blue shift at the absorption edge for doped and co-doped samples. The photocatalytic activity of the prepared catalysts was evaluated in the removal of RhB under UVA irradiation. The results showed that Mg5%-La5%/ZnO had the highest photoactivity (91.18 %) among all samples.

In the heterogeneous photocatalysis, a semiconductor photocatalyst is excited by UV irradiation which forms electron-hole pairs. Hydroxyl radicals and reactive oxygen species (ROS) with high oxidation potential are produced

Synthesis of nanoparticles using sol-gel method
Bimetallic Mg-La/ZnO, monometallic Mg/ZnO and La/ ZnO and pure ZnO were prepared by using sol-gel method. For the co-doped ZnO nanoparticles with 5-6 wt.% Mg and 4-5 wt. % La, at first zinc acetate di-hydrate was slowly dissolved in 100 mL ethanol at 60 °C (with thermal controlling) and ongoing stirring for 30 min until homogeneous solution was obtained. Then, appropriate amounts of Mg(NO 3 ) 2 and La(NO 3 ) 3 dissolved in 20 mL ethanol at 60 °C were used which were continuously stirred for 30 min until homogeneous solution was obtained and was added drop by drop to the ethanolic solution of zinc acetate (where temperature was controlled). The oxalic acid, dissolved in ethanol (40 mL) at 60 °C, was added drop by drop to the ethanolic solution. The mixture was stirred for 2 h. The obtained white colloidal semi-gel dried at 90 °C for 12 h in the oven (E24-Sherwood Co). The dried xerogel was calcinated at 400 °C for 2 h in the electric Furnace (ALF-18-iran, Atbin Co). It was powdered by mortar. Doped Mg/ZnO and La/ZnO nanoparticles with 1-6 wt. % and bare ZnO were also prepared by the similar procedure [4,26,35,62,63].

Photocatalytic experiments
All experiments regarding photocatalytic degradation were carried out in a batch 100 mL volume quartz photoreactor with a 35 × 2.5 cm dimension which was fixed parallel to UVA light source. During the reaction, oxygen gas was entered from the bottom of the photoreactor. UV-A (Hitachi, F15T8/BL, 15 W, λ max = 370 nm) was the irradiation source. During the photocatalytic reaction, evaporation could be neglected because the solution temperature did not increase significantly. For photocatalytic RhB degradation through ZnO nanoparticle (bare, doped and co-doped), 50 mL of aqueous RhB solution with fixed weight of catalyst (20 mg) was sonicated for 15 min in ultrasonic bath (Sonica-2200ETH, Italy, 260W, 40 kHz in dimension of 24×14×10 cm). The aqueous RhB solution (10 mg L -1 ) including the photocatalyst was transferred to the batch quartz photoreactor and bubbled with an oxygen flow to keep the suspension homogeneous (flow rate = 0.5 mL min -1 ) for 15 min in the darkness. The reaction was initiated by turning on the UVA lamp and 5 mL suspension solution was taken at a certain irradiation time (21 min); then, the nanoparticles were separated by centrifuge (Kokusan H-11n). RhB Concentration was analyzed by UV-Vis spectrophotometer (Biowave-S2100, WPA) at λ max = 554 nm. [4,19,26].

Characterization methods
The crystalline phase and average crystallite size of the synthesized ZnO (bare, doped and co-doped) nanoparticles were characterized using XRD technique via analytical-X'pert Pro-X-ray diffractometer. XRD patterns were recorded using Cu kα irradiation (λ = 0.154 nm) operator at 40 kV and 100 mA in the diffraction range (2θ) between 10 to 80º. The average crystallite size, i.e. D (nm), of the obtained nanoparticles was calculated by Scherrer's equation [64]. The morphology and size of particles were determined by FESEM technique (MIRA3-TE-SCAN Co). FESEM was equipped with an energy dispersive X-ray spectroscopy (EDX) system for analyzing the chemical composition of nanoparticles. The morphology and average size of the nanoparticles were analyzed by TEM (EM10C-100kV-Zeiss, Germany). The optical band gap energy of nanoparticles was obtained by UV-Vis diffuse reflectance spectroscopy (DRS, UV-2550, Shimadzu, Japan). The results of the DRS were converted to the Tauc equation. The optical band gap of nanoparticles can be calculated by Tauc equation (Eq. (1)).
where Eg refers to the optical band gap; h denotes Planck's constant; ν refers to the frequency of incident photons; A stands for a constant which is called the band tailing parameter; α refers to the absorption coefficient α π λ = ( ) 4 k ; k is the absorption index or absorbance; λ denotes the wavelength in nm) and n refers to the index which depends on the type of transition and may have values such as 3/2, 1/2, 2 and 3 corresponding to direct forbidden, direct allowed, indirect allowed and indirect forbidden transitions, respectively.
The band gaps of insulators/semiconductors are categorized into allowed direct and indirect band gaps. Hence, the value of n should be 0.5 in equation [1] for allowed direct band gap. The average band gap can be derived by plotting Tauc's graphs between α ν h ( ) 2 versus photon energy hν ( ) and by extrapolating the linear portion of the spectra to the hν axis [65][66][67][68][69][70]. The wavelength of absorption edge (λ) was obtained from equation Eg = 1239 8 . . λ [22,47,63]. N 2 adsorption and desorption experiments were carried out at 77 K using the BET analyzer (Belsorp Mini) so as to measure specific surface area and pore size distribution based on BET (Brunauer-Emmett-Teller) and BJH (Barret-Joyner-Halender) methods.

Results and discussion 3.1 XRD structural studies
The XRD patterns of the nanoparticles ZnO, Mg5%/ZnO, La4%/ZnO, Mg5%-La5%/ZnO and Mg6%-La4%/ZnO are depicted in Fig. 1. All peaks can be well adopted by the hexagonal wurtzite structure crystal according to the standard XRD pattern of ZnO (JCPDS 36-1451) [71]. It appears from the XRD analysis that there are no second impurity peaks corresponding to MgO and La 2 O 3 . The absence of characteristic reflections related to the Mg and La in the XRD pattern of Mg and/or La/ZnO (doped and co-doped) samples may be attributed to the low loading quantity and appropriate incorporation of La and Mg ions in the ZnO lattice [22]. The 2θ value for (101) diffraction peak (ZnO main peak) in the XRD patterns of ZnO, Mg5%/ZnO, La4%/ZnO, Mg5%-La5%/ZnO and Mg6%-La4%/ZnO are 36.343º, 36.265º, 36.343º, 36.369º and 36.447º, respectively. The ionic radius of Mg ions (0.72 Å) is close to the Zn ions (0.74 Å) but the radius of the La ions (1.03 Å) is larger than that of Zn ions. The shift in the 2θ values and the broadening of the diffraction peaks indicate that the doped ions are successfully incorporated into the ZnO lattice. Due to the notable difference between La and Zn ionic radiuses, it is difficult for La to substitute Zn and La ions are incorporated only in the interstitial sites of the ZnO lattice [72]. Nevertheless, Mg might be able to substitute Zn because of the same ionic radius. The average crystallite size of the nanoparticles was measured from X-ray line broadening of the (101) diffraction peak by means of Debye-Scherrer's equation. The results, given in Table 1, obviously indicate the reduction of ZnO crystalline size by doping La and Mg. The reduction in crystallite size is mainly attributed to the presence La-O-ZnO and Mg-O-ZnO on the surface of the doped ZnO, which excludes the growth of crystalline grains [26]. In addition, the doping and co-doping of Mg 2+ and La 3+ ions lead to oxygen vacancies in the ZnO crystalline structure; consequently, its crystallite size decreases. Due to different La and Mg positions in the ZnO lattice, the impact of Mg doping on the crystallite size was more significant and effective than that of La doping. The microstrain (ε) can be calculated using in Eq. (2) [29]. ε β θ = cos 4 (2) As shown in Table 1, there is a slight change in microstrain values. That is, the increased micro-strain in nanoparticles changes the diffraction peak broadening which results in a reduction of particle size and a little shift in the XRD peaks. However, it should be pointed out that the line broadening may be due to the size or microstrain or the interaction between both of them [29]. The cell parameters of prepared samples in Table 1 indicates that the lattice parameter (a) of co-doped nanoparticles are slightly higher than other samples.

FESEM and EDS studies
FESEM is considered to be a useful technique in studying the structural characterization and morphology of nanoparticles. It has information about the growth mechanism, shape and size of the particles [29]. Fig. 2 (a)-(e) illustrates FESEM images and the surface morphology of bare, doped and co-doped Mg, La/ZnO nanoparticles. It shows that the samples have a uniform spherical-like morphology with a homogenous distribution. Indeed, FESEM images obviously demonstrate that the doping and co-doping of Mg 2+ and La 3+ have no impact on the morphology of ZnO nanoparticles; nevertheless, it shows that the size of the grains has decreased, especially in the co-doped sample. In addition to FESEM, EDS analysis was conducted for investigating the chemical composition of nanoparticles. The results of the EDS analysis are depicted in the Fig. 3(a)-(e) and Table 2. EDS spectra of bare ZnO have only two elements, namely Zn and O which indicates that ZnO sample is pure. In addition, doped and co-doped nanoparticles were formed only from Zn, Mg, La and O elements and the purity of the   Table 2. The EDX analysis confirms the presence of Mg and La in ZnO and also indicates good compatibility with the experimental concentration used for the synthesis of nanoparticles.

TEM images
The shape and size of the particles and their distribution for the prepared nanoparticles are depicted in TEM images ( Fig. 4(a)-(e)). TEM images of all samples indicate some agglomeration along with individual particles. According to these images, all prepared photocatalysts have regular distributions and spherical morphology. The size obtained from XRD differs from those of TEM techniques. Whereas the grain size, in TEM, is measured as the distance between visible grain boundaries, the extent of the crystalline region which coherently diffracts X-rays is measured as the grain size in XRD. Consequently, it can be argued that XRD leads to smaller size in comparison with other methods such as TEM and SEM [63]. The particle size distributions based on TEM images for all   Fig. 5. The histograms indicate that the average particle size of ZnO, Mg5%/ZnO, La4%/ZnO, Mg5%-La5%/ZnO and Mg6%-La4%/ZnO is about 28.55, 17.62, 23.71, 16.06 and 13.38 nm, respectively. These values were obtained from 569, 540, 618, 490 and 567 particles, respectively. The results reveal that the particle size of nanoparticles is reduced as a result of doping Mg and La into the ZnO lattice.

BET surface area analysis
Figs. 6(a) and 6(b) depict the N 2 adsorption-desorption isotherms and the pore size distribution for the prepared samples, respectively. According to IUPAC classification, N 2 adsorption-desorption isotherms of the nanoparticles show a type III isotherm which indicates the presence of the mesoporous and macroporous structure [73][74][75][76].
The results regarding BET surface area and total pore volume in Table 3 indicate increased surface area and pore volume for the doped and co-doped samples. In the sol-gel technique, crystal growth was suppressed by the addition of La and Mg dopants. Hence, as acknowledged by XRD results, crystallite size decreases; consequently, surface area increases. Moreover, higher surface area values for the doped samples may be attributed to the removal of nitrate from the ZnO lattice during calcination at 400 °C and the thermal decomposition of La and Mg precursors [10]. Thus, increasing the porosity of the surface enhances the surface area [10].

DRS studies
UV-Vis DRS of nanoparticles are depicted in Fig. 7. The absorption edge of Mg5%/ZnO, La4%/ZnO, and Mg6%-La4%/ZnO are slightly blue shift when compared with bare ZnO nanoparticles. Tauc equation [65,66] was applied for calculating Eg and absorption edge (Fig. 8). The band gap and absorption edge values for nanoparticles are given in Table 4. ZnO band gap value was enhanced by the doping and co-doping of La and Mg elements. The results reveal that doping with Mg lead to 0.072 eV enhancement in the band gap energy. The resulting 0.039 eV difference  in bang gap by La doping may be attributed to the decrease in the particle size. According to XRD and TEM results, there is a noticeable decrease in the particle size of doped and co-doped nanoparticles. As the particle size decreases, the energy levels discrete and, eventually, the band gap energy increases. The increasing in the band gap energy and blue shift in the absorption edge by doping of La and Mg ions may be attributed to the Burstein-Moss effect. This effect is caused by the transition energy in degenerate nanoparticles according to the partial field conduction band [36,77]. Furthermore, shift to lower wavelength or higher energy, blue shift, is due to decreasing the grain size [63]. The blue shift can be attributed to the quantum size effect in nanoparticles [29,77], distortion of host lattice and defect generation [29]. Gopalakrishnan and Muthukumaran [29], found that increasing ZnO band gap with Ni doping may be due to sp-d spin exchange interaction between band electrons and localized spin of the transition metal ions. Osei-Bonsu Oppong et al. [78] argued that, since d-d transition is superior to sp-d transition, La doping in ZnO lattice enhances band gap energy. When the band gap in the nanoparticles enlarges, the separation of the photo-induced electron-hole pairs occurs better [22].

Photocatalytic Activity
The main purpose of the study is to examine the photocatalytic activity of the prepared nanoparticles. The photocatalytic activity of the bare, doped and co-doped samples in the removal of RhB under UVA irradiation has been shown in Table 5. Blank tests results indicated that the removal percent of RhB by UVA alone (photolysis) and adsorption on nanoparticles was negligible.
The results, given in Table 5, obviously reveal that, as Mg doping increases up to 5 wt. %, RhB removal percent increases significantly. The photocatalytic activity of semiconductor nanoparticles is proportional with their band gap energy. That is, higher band gap energy corresponds with stronger electron-hole pairs which, consequently, lead to higher photocatalytic activity [4]. According to DRS results, Mg-doped ZnO has a larger band gap than bare ZnO, which may account for the higher photocatalytic activity of doped samples. Moreover, the increased Mg dopant leads to more crystal deficiency. Crystal deficiency retards recombination of the electron-hole pair and leads to improved photocatalytic activity [4]. Xue et al. [79], reported that most Mg ions were substituted in the Zn sites of ZnO, which resulted in an increase not only in ZnO band gap, but also in the quality of ZnO crystalline.
Prabu and Johnson [80] argued that the formation of intrinsic defects in the ZnS lattice is due to the incorporation of Mg 2+ .
However, Mg content values greater than 5 wt. % decreased the photocatalytic activity of ZnO nanoparticles. Such a reduction is attributed to the aggregation of Mg dopant and the reduction of homogeneous distribution of dopant in the ZnO lattice [81]. As shown in Table 5, it is observed that photocatalytic activity significantly enhanced along with La doping up to 4 wt. %. La 3+ due to existence of a partially filled orbital effectively scavenges photoinduced electrons and produces unstable La 2+ [82].    In air-equilibrated system, electron discharges into the dissolved oxygen to form the O 2 −° and °O H as follows: O H e HO CB 2 2 The produced H O 2 2 in Eqs. (5), (6) reacts with photoinduced electrons which lead to the production of hydroxyl radicals (Eq. (7)). Hydroxyl radicals can effectively participate non-selectively in the degradation of target molecules.
Jia et al. [77], found that La doping produces surface defects and a space charge layer in the surface of La-doped ZnO samples, which hinders recombination of photoinduced electron -hole pairs. However, when the La amount exceeds 4 wt. %, the photocatalytic activity is notably reduced which is attributed to the aggregation of La 2 O 3 and to the negative impact of surface charge layer [77]. Covering ZnO surface from light absorption at higher doping values is another reason for decreasing photocatalytic activity [65]. Tayade et al. [83], maintained that ions with an ionic radius higher than the host element occupy the interstitial positions; however, ions with an ionic radius near the host element occupy the substitutional positions. Also, they found that the occupation of interstitial positions is more effective in the photocatalytic activity than the occupation of the substitutional positions. In the present study, La 3+ ions have an ionic radius which is greater than that of the host element (Zn 2+ ); nevertheless, Mg 2+ ions have an ionic radius which is close to that of Zn 2+ ions. Therefore, La 3+ and Mg 2+ ions are expected to occupy interstitial and substitutional positions in ZnO lattice, respectively. That is why La-doped ZnO has higher photocatalytic activity up to 4 % (La optimum percent) than Mg-doped ZnO. According to the results given in Table 5, co-doped Mg-La/ ZnO has higher photocatalytic activity than mono-doped and bare ZnO. The higher photocatalytic activity of the co-doped Mg-La/ZnO may be due to different sites of La 3+ and Mg 2+ ions in ZnO lattice so that La 3+ in the interstitial positions and Mg 2+ in the substitutional positions. On the other hand, Mg doping increases band gap and ZnO surface area. Consequently, the increased band gap has a critical impact on photocatalytic activity. Firstly, a catalyst with a wide band gap absorbs less light. Due to the  . 7 The DRS-UV-Vis spectra for the prepared nanoparticles. emission wavelength of the lamp (365 nm), the reduced light absorption is not important in this study. According to Table 4, all catalysts can absorb wavelengths less than 390 nm. Secondly, a wider band gap corresponds with better separation and also with stronger photoinduced electron-hole pairs. Thus, it can be argued that a wider band gap improves photocatalytic activity. Increasing the specific surface area with Mg doping is another important factor for higher photocatalytic activity. A higher specific surface area enhances RhB adsorption and active sites, which, consequently, have notable positive impacts on photocatalytic activity. In addition, La with partially filled orbital and a space charge layer in the surface scavenges photoinduced electrons and enhances photocatalytic activity.

Conclusion
In this study, Mg and La doped and co-doped ZnO nanoparticles were successfully prepared by the sol-gel method. The XRD results of nanoparticles did not show the presence of separate metal oxide diffraction peaks for Mg and La phases due to the proper incorporation of doped ions. The FESEM, EDX and TEM results confirmed that all the prepared nanoparticles had a uniform spherical-like morphology with a homogenous distribution; however, the grain size of the nanoparticles decreased due to the doping and co-doping processes. The DRS results indicated a significant increase in Eg for Mg and La doped and co-doped ZnO nanoparticles. N 2 adsorption-desorption results indicated an increase in BET surface area and pore volume for doped and co-doped ZnO nanoparticles. Mg5%-La5%/ZnO had the highest capability for photocatalytic RhB removal in comparison with mono-doped and bare ZnO.