Preparation, Characterization and Photocatalytic Activity of La-Doped Zinc Oxide Nanoparticles

Lanthanum (La)-doped zinc oxide nanoparticles were synthesized with different La concentrations by employing a gel combustion method using poly(vinyl alcohol) (PVA). The as-synthesized photocatalysts were characterized using various techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), photoluminescence (PL) spectroscopy, and UV–visible absorption spectroscopy. The average size of ZnO nanoparticles decreased from 34.3 to 10.3 nm with increasing concentrations of La, and the band gap, as evaluated by linear fitting, decreased from 3.10 to 2.78 eV. Additionally, it was found that the photocatalytic activity of doped samples, as investigated by using methyl orange dye under visible lights, improved in response to the increase in La concentration. The decomposition of methyl orange reached 85.86% after 150 min in visible light using La0.1Zn0.9O as the photocatalyst.


Introduction
Among n-type semiconductors, zinc oxide is known for its advantageous characteristics, including a wide direct band gap (3.27 eV), large exciton binding energy (about 60 meV), high optical gain at room temperature, great saturation velocity, piezoelectricity, and pyroelectricity. Zinc oxide is also non-toxic, biocompatible, and simple and inexpensive to synthesize [1]. These properties of zinc oxide make it an important II-VI semiconductor. Due to a myriad of versatile applications in technical manufacturing, cosmetics, and the pharmaceutical industry, zinc oxide nanomaterials, such as nanoparticles or zinc oxide-based semiconductors, have been extensively studied [2,3].
Zinc oxide nanomaterials have been synthesized by different techniques such as the co-precipitation method [4,5], sol-gel method [6][7][8], combustion method [9], hydrothermal method [10], and thermal decomposition [1], depending on the desired structure. ZnO, when modified, could be utilized as a base material for diluted magnetic semiconductors [11], gas sensors [12,13], and photocatalysts [14][15][16][17]. In photocatalytic applications, ZnO nanoparticles were used to remove numerous azo dyes, such as methylene blue [18], methyl orange [17], rhodamine B [19], and methyl red [20].  , could be assigned to the hexagonal wurtzite structure of ZnO [28,33,34]. In addition, the XRD spectra indicated the absence of lanthanum oxide or Zn-La alloys formation [35], and also indicated a trend of continuously broadening XRD peaks of La-doped ZnO in response to increasing La load. Therefore, it is suggested that the dopant was dispersed uniformly in the ZnO matrix, causing no alteration to the crystal structure [25,36]. Table 1 compares pure ZnO and La-doped ZnO with different dopant concentrations, in terms of crystallite sizes and lattice parameter values. It is interesting to note that the crystallite sizes and lattice parameter values of La-doped ZnO, as resulting from the Scherrer equation, were lower than those of pure ZnO, which is inversely proportional to the La doping. This was thereby consistent with previous studies [23,25,28]. The reduction of particle size and lattice parameters of La-doped ZnO could be attributable to the diffusion and growth impairment of ZnO crystal grains, which is caused by the the ionic radius 0.74 and 1.26 Å, respectively, for Zn 2+ and La 3+ [23,28]. These size characteristics make La-doped ZnO materials beneficial regarding photocatalytic activity.  [23,28]. These size characteristics make La-doped ZnO materials beneficial regarding photocatalytic activity.

Energy Dispersive X-ray Analysis (EDX) Study
EDX results showing the chemical purities and elemental composition of the La0.05Zn0.95O materials are displayed in Figure 2. The EDX spectra of the nanoparticles indicated the existence of Zn, O, and La elements, as confirmed by their corresponding peaks and the absence of other characteristic peaks. The produced sample was also devoid of other element impurities, due to the EDX data showing that there was 38.65% and 4.8% of zinc and lanthanum, respectively.   Figure 2. The EDX spectra of the nanoparticles indicated the existence of Zn, O, and La elements, as confirmed by their corresponding peaks and the absence of other characteristic peaks. The produced sample was also devoid of other element impurities, due to the EDX data showing that there was 38.65% and 4.8% of zinc and lanthanum, respectively.   Figure 3 shows Transmission Electron Microscopy (TEM) images of the pure and La-doped ZnO samples. The drastic difference between the TEM images of samples synthesized with varying La concentrations suggests that intensifying the La doping on ZnO reduces the size of ZnO nanoparticles and induces no change in morphology. This reduction in crystalline size was also supported by the XRD results.

Figure 2.
Energy dispersive X-ray (EDX) spectrum of La0.05Zn0.95O nanoparticles. Figure 3 shows Transmission Electron Microscopy (TEM) images of the pure and La-doped ZnO samples. The drastic difference between the TEM images of samples synthesized with varying La concentrations suggests that intensifying the La doping on ZnO reduces the size of ZnO nanoparticles and induces no change in morphology. This reduction in crystalline size was also supported by the XRD results.

UV-Visible Absorption Study
The optical properties of pure ZnO and ZnO doped with different La concentrations were explored using UV-visible absorption spectroscopy. Figure 4 shows that the absorbance in the ultraviolet region (200-400 nm) was high for both pure ZnO and La-doped ZnO. In the visible region

UV-Visible Absorption Study
The optical properties of pure ZnO and ZnO doped with different La concentrations were explored using UV-visible absorption spectroscopy. Figure 4 shows that the absorbance in the ultraviolet region (200-400 nm) was high for both pure ZnO and La-doped ZnO. In the visible region (400-800 nm), the absorbance of La-doped ZnO samples was improved. From the spectra, the band gap of La x Zn 1−x O samples were determined from the Kubelka-Munk equation via conversion to absorbance [3]. For the synthesized catalyst, the band gap energy E g (eV) was calculated using the following equation.
where h, c, and λ max are the Planck constant (6.62.10 −34 J.s.photon −1 ), speed of light (3 × 10 8 m.s −1 ), and wavelength at the absorption edge (nm), respectively [37]. Table 2 shows the band gap results of both pure ZnO and La-doped ZnO. Evidently, the band gap of La-doped ZnO decreased gradually from 3.1 to 2.78 eV while La concentration increased from 0 mol % to 10 mol %. This is in line with the results of La-doped TiO 2 in a previous study [38]. Regardless of the higher radius of La 3+ in comparison with that of Zn 2+ , La 3+ could easily enter ZnO lattices due to the low decomposition temperature of nitrate (126 • C), which facilitates the movement of the formed La 2 O 3 into ZnO lattices during the annealing process. La 3+ ions that enter the lattice could either take over the Zn 2+ position or be situated in the interstitial voids [39].
where h, c, and λmax are the Planck constant wavelength at the absorption edge (nm), respectively [37]. Table 2 shows the band gap results of both pure ZnO and La-doped ZnO. Evidently, the band gap of La-doped ZnO decreased gradually from 3.1 to 2.78 eV while La concentration increased from 0 mol % to 10 mol %. This is in line with the results of La-doped TiO2 in a previous study [38]. Regardless of the higher radius of La 3+ in comparison with that of Zn 2+ , La 3+ could easily enter ZnO lattices due to the low decomposition temperature of nitrate (126 °C), which facilitates the movement of the formed La2O3 into ZnO lattices during the annealing process. La 3+ ions that enter the lattice could either take over the Zn 2+ position or be situated in the interstitial voids [39].

Photoluminescence Spectrum Analysis
The measurements of photoluminescence (PL) spectra of La-doped ZnO nanoparticles were carried out under excitation of 325 nm at room temperature. Figure 5 presents the PL spectrum of 1% La-doped ZnO sample. The PL spectrum showed an emission maximum at 369 nm with very strong intensity, which is attributed to the emission of pure ZnO. When La 3+ ions were doped into ZnO crystal lattices, interstitial zinc ions and oxygen vacancies were formed along with the incorporation of La 3+ [40]. The emission peaks at 439, 451, and 469 nm are attributed to the transition between the vacancies of oxygen and interstitial oxygen [28,40]. The peaks at 482 and 494 nm may have originated from the ionized oxygen vacancies in the valance. The results show that the doping of La into ZnO nanoparticles was successful.

Photoluminescence Spectrum Analysis
The measurements of photoluminescence (PL) spectra of La-doped ZnO nanoparticles were carried out under excitation of 325 nm at room temperature. Figure 5 presents the PL spectrum of 1% La-doped ZnO sample. The PL spectrum showed an emission maximum at 369 nm with very strong intensity, which is attributed to the emission of pure ZnO. When La 3+ ions were doped into ZnO crystal lattices, interstitial zinc ions and oxygen vacancies were formed along with the incorporation of La 3+ [40]. The emission peaks at 439, 451, and 469 nm are attributed to the transition between the vacancies of oxygen and interstitial oxygen [28,40]. The peaks at 482 and 494 nm may have originated from the ionized oxygen vacancies in the valance. The results show that the doping of La into ZnO nanoparticles was successful.

Photocatalytic Activity of La-Doped ZnO Nanoparticles
Comparison of the photocatalytic efficiency of methyl orange (MO) between pure ZnO and Ladoped ZnO nanoparticles was performed in the presence of visible light at different irradiating

Photocatalytic Activity of La-Doped ZnO Nanoparticles
Comparison of the photocatalytic efficiency of methyl orange (MO) between pure ZnO and La-doped ZnO nanoparticles was performed in the presence of visible light at different irradiating intervals. The results shown in Figure 6 suggest an improved photocatalytic efficiency of MO in response to increasing irradiation time and La concentration. Higher photocatalytic activities of the La-doped ZnO photocatalysts, compared to that of pure ZnO, were observed, confirming the enhancing effect of La doping on the photocatalytic activity of ZnO photocatalysts. This effect was also observed by L. Elsellami et al. in the case of the 4-nitrophenol degradation in the presence of the La-doped TiO 2 and visible light [38].  Figure 7 plots the variations of ln(Co/Ct) with respect to irradiation time (t) for all the samples. Visually, the relationship could be observed to assume a linear function, which suggests that a pseudo-first-order reaction is suitable for describing the photodegradation of MO [37,41]. The estimation parameters, including k values and the coefficient of determination (R 2 ), are shown in Table 3. The increase in the La concentration, from 1.0 mol % to 10 mol %, resulted in a gradual increase in rate constant, from 0.0039 to 0.013 min −1 . Peak photocatalytic activity was observed at 10 mol % La-doped ZnO, with a rate constant four times larger than that of pure ZnO. Thus, the cause for the improved photocatalytic performance could be two-fold. First, it could be attributable to the combinational effect of La and ZnO. Second, the decreased size and band gap energy of nanoparticles could suppress the recombination rate of the electron-hole pairs.  The photocatalytic efficiency of MO reached 85.86% after 150 min under irradiation of visible light using La 0.1 Zn 0.9 O as a photocatalyst. Table 1 indicated that the average crystallite size and lattice parameter values of La 0.1 Zn 0.9 O were smaller than that of the other samples. In addition, Figure 2 indicated that the band gap energy of La 0.1 Zn 0.9 O at 2.78 eV was smaller than that of the other samples. This suggests that both particle size and the specific surface areas of ZnO nanoparticles could be efficiently enhanced by La doping. The doping could increase the number of electrons/holes escaping to the surface of the La 0.1 Zn 0.9 O nanoparticles and, consequently, improve the photodegradation efficiency of La 0.1 Zn 0.9 O.
The rate of reactions could be described as follows: where C t and k is the concentration of MO and rate constant of the reaction, respectively. The subscript t denotes time interval. By integration, the equation could be transformed as follows: Figure 7 plots the variations of ln(C o /C t ) with respect to irradiation time (t) for all the samples. Visually, the relationship could be observed to assume a linear function, which suggests that a pseudo-first-order reaction is suitable for describing the photodegradation of MO [37,41]. The estimation  Table 3. The increase in the La concentration, from 1.0 mol % to 10 mol %, resulted in a gradual increase in rate constant, from 0.0039 to 0.013 min −1 . Peak photocatalytic activity was observed at 10 mol % La-doped ZnO, with a rate constant four times larger than that of pure ZnO. Thus, the cause for the improved photocatalytic performance could be two-fold. First, it could be attributable to the combinational effect of La and ZnO. Second, the decreased size and band gap energy of nanoparticles could suppress the recombination rate of the electron-hole pairs.

Preparation of La-Doped ZnO Nanoparticles
A gel combustion method was employed to produce the La-doped ZnO nanoparticles. A typical experimental procedure is described as follows: After PVA was dissolved in deionized water, a prespecified amount of Zn(NO3)2.4H2O, La(NO3)3.6H2O, according to mol ratio correlative, was added into the above solution with vigorous stirring to form a mixed solution. Afterwards, the mixed solution underwent stirring for 4 hours to form the gel, which was subsequently subjected to thermal treatment at 80 °C for 10 hours in an oven. The produced white precipitate was calcined at 500 °C for 3 hours with a heating rate of 5 °C /min [35].

Characterizations
A D8 Advance diffractometer (Brucker, Madison, WI, USA) was used to produce XRD spectra, characterizing the phase of the obtained powder with Cu Kα radiation (λ = 1.5406 Å) in a 2θ angle ranging from 20° to 80° with a step of 0.03°. The crystallite size, D of ZnO, was determined using the Scherrer formula, in which λ is the X-ray wavelength (0.1504 nm), k is Scherrer's constant (k = 0.89), β is the full width at half maximum observed in radians, and θ is the angle of diffraction of the (101) peak with the highest intensity.

Preparation of La-Doped ZnO Nanoparticles
A gel combustion method was employed to produce the La-doped ZnO nanoparticles. A typical experimental procedure is described as follows: After PVA was dissolved in deionized water, a pre-specified amount of Zn(NO 3 ) 2 .4H 2 O, La(NO 3 ) 3 .6H 2 O, according to mol ratio correlative, was added into the above solution with vigorous stirring to form a mixed solution. Afterwards, the mixed solution underwent stirring for 4 hours to form the gel, which was subsequently subjected to thermal treatment at 80 • C for 10 hours in an oven. The produced white precipitate was calcined at 500 • C for 3 hours with a heating rate of 5 • C /min [35].

Characterizations
A D8 Advance diffractometer (Brucker, Madison, WI, USA) was used to produce XRD spectra, characterizing the phase of the obtained powder with Cu Kα radiation (λ = 1.5406 Å) in a 2θ angle ranging from 20 • to 80 • with a step of 0.03 • . The crystallite size, D of ZnO, was determined using the Scherrer formula, in which λ is the X-ray wavelength (0.1504 nm), k is Scherrer's constant (k = 0.89), β is the full width at half maximum observed in radians, and θ is the angle of diffraction of the (101) peak with the highest intensity.

Photocatalytic Degradation of Methyl Orange
The conditions in which the photocatalytic experiments were conducted included continuous circulation mode, ambient temperature, and at the natural pH of methyl orange (MO) solution (pH = 4). Prior to the photocatalytic reaction, the suspension underwent stirring for 45 min in the dark condition to achieve adsorption/desorption equilibrium. Following that, 50 mg of La x Zn 1−x O was dispersed in 100 mL of MO aqueous solution (10 mg·L −1 ), and the suspension was exposed to visible irradiation in 150 min using a 20 W Compact Lamp (Philips) with 130 cd. The concentration of MO was determined at 464 nm using an ultraviolet-visible spectrophotometer (UV-1700 Shimadzu, Kyoto, Japan). To determine the photocatalytic efficiency of MO (H), the following equation was adopted.
where C o is the equilibrium concentration of MO (mg·L −1 ) measured following dark adsorption and C t is the concentration of MO after irradiation for time t.

Conclusions
Pure ZnO and La x Zn 1−x O (x = 0.01, 0.05, 0.10) were synthesized via the gel combustion method. The amount of lanthanum significantly affected the morphology, optical properties, and photocatalytic activity of ZnO nanoparticles. Transmission electron micrographs showed that the synthesized samples had spherical morphology and that particle size was inversely proportional to La molar percentage.
The doping of lanthanum into the ZnO lattice induced a decline in the band gap of ZnO, from 3.1 to 2.78 eV. These results suggest that the La-doped ZnO nanoparticles could be promising for optic applications extending into the visible wavelength range. The La 0.1 Zn 0.9 O nanoparticles had the highest photocatalytic degradation of MO (85.86%) after 150 min of visible light irradiation. The kinetics of MO photodegradation were consistent with being a first-order reaction.