Zinc nitrate hexahydrate (Zn(NO₃)₂⋅6H₂O), praseodymium nitrate hexahydrate (PrN₃O₉⋅H₂O), trisodium citrate dihydrate (C₆H₅Na₃O₇⋅2H₂O), and sodium hydroxide (NaOH) were used for the chemical synthesis. All these reactants were analytic reagents and no further purification was performed in the work. Deionized water was employed in all the synthesis processes.

A series of Zn_1−xPr_xO (with x = 0, 0.02, 0.04, 0.06) photocatalysts was synthesized by co-precipitating with nitrates. In a typical run, zinc nitrate hexahydrate and praseodymium nitrate hexahydrate were dissolved in 300-ml deionized water in a given value of substance ratio and agitated magnetically to obtain well-proportioned and stable solutions. After 15 min and 60 min, respectively, trisodium citrate dihydrate and sodium hydroxide were added to the solution. After 2 h, the magnetic mixer was turned off and the white precipitate was produced. Then, the precipitates were filtered separately and washed repeatedly with absolute ethyl alcohol to wipe off the unreacted chemicals. The product was then dried at 80 °C for 14 h. In this way, nanoparticle samples of ZnO, Zn_0.98Pr_0.02O, Zn_0.96Pr_0.04O, and Zn_0.94Pr_0.06O were produced. Finally, these samples were heated at a temperature of 240 °C for 2 h.

The structural characterization of the synthetic photocatalysts was carried out with an x-ray diffractometer (D8 Advance, BRUKER) by using Cu Kα radiation (λ = 0.1542 nm). The 2θ range is 20°–80° and the scanning step is 0.02°. The morphologies of the samples were observed by SEM (SIRION, FEI) along with EDS. The chemical compositions of the samples as well as the valences of elements were investigated by XPS (Thermo Fisher ESCALAB 250 Xi) and all spectra were calibrated with the C 1s core level spectrum at a binding energy of 284.6 eV. The infrared spectra were recorded by Fourier transform infrared (FTIR) spectroscopy (FTIR5700, Thermo USA) in a wavenumber range of 500 cm⁻¹–4000 cm⁻¹ to investigate the chemical bonds of the photocatalysts. The PL spectra were surveyed on a Raman spectrophotometer (RM1000, Renishaw) by using an Ar ion laser with an excitation wavelength of 325 nm. The UV-vis DRS spectra in a wavelength of 300 nm–600 nm were recorded through a UV-vis spectrophotometer (Lambda 35, PerkinElmer) with an integrating sphere for determining the bandgap of photocatalysts.

The photocatalytic properties of the samples were investigated by the photodegradation for RhB dyes. In a typical run, 5 mg of photocatalysts were suspended in 10 ml of RhB solution with a concentration of 10 mg/L. Then, the suspension was gently stirred for 30 min in the dark to make the photocatalyst well dispersed and to reach adsorption–desorption balance. Next, the suspensions were irradiated by a mercury-tungsten combined light lamp (300 W, involving the UV and visible light region to simulate the solar conditions). A 2-ml suspension was sampled at 15-min intervals for centrifuging at a speed of 5000 rpm for 5 min. Then, the supernatant liquid of the solution was poured into a quartz glass, and the UV-vis absorption spectra ranging from 650 nm to 450 nm in wavelength were recorded by an S2500 spectrophotometer (UV-2550, SHMADZU) to monitor the process of photodegradation. The concentration of RhB was determined by the absorbance value of the solution located at 554 nm of the UV-vis spectrum.

Figure 1(a) shows the XRD spectra in a range 20°–60° of the undoped ZnO and Pr-doped ZnO samples. The sharp and narrow peaks mean that the samples obtained are polycrystalline.^[26] It can be seen that all the diffraction peaks of the crystallographic planes (100), (002), (101), (102), and (110) in the diffraction patterns are consistent with the data of the JCPDS No. 36-1451,^[28] which indicates that samples have the hexagonal wurtzite structure and Pr doping does not cause evident change of the ZnO crystal structure. There is no additional peak in the diffraction spectrum, which means that no secondary impurity phases are found within the sensitivity of the technique. Also no characteristic peak of praseodymium oxide is found in the Pr-doped ZnO sample, which is believed to result from the low doping concentration and the location of dopants at Zn sites.

Fig. 1.

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	Fig. 1. (color online) (a) XRD spectra in a range 20°–60° of pure ZnO and Pr-doped ZnO nanoparticle samples. The bottom shows the values from standard JCPDS No. 36-1451 card of pure ZnO, (b) XRD spectra in a range 30°–33° of pure ZnO and Pr-doped ZnO nanoparticle samples.

It is worth noting that in a range of 30°–33° of the XRD spectrum (Fig. 2(b)), the peak position of the (100) plane has a small shift towards lower angles after doping. This slight shift of the diffraction peak results from the expansion of the ZnO lattice, which is caused by the difference in radius between praseodymium ion (1.01 Å) and zinc ion (0.74 Å). The variation of the ZnO lattice parameter is because of the substitution of praseodymium ions for zinc sites.^[2,3]

Fig. 2.

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	Fig. 2. SEM images of (a) undoped ZnO and (b) 2-mol% Pr-doped ZnO shown on a scale of 2 μm, (c) pure ZnO, and (d) 2-mol% Pr-doped ZnO shown on a scale of 10 μm, and (e) EDS spectrum of the 2.0-mol% Pr-doped ZnO sample.

The average crystallite sizes of the as-obtained nanoparticles were estimated by the peaks at a diffraction angle of 36° by using Debye–Scherrer’s equation: 1 where K is the Scherrer constant (K = 0.89), λ is the x-ray wavelength (λ = 1.54 Å), β is the diffraction angle of 36° (0.628 rad), and D is the corresponding full width at half maximum (FWHM) value of the diffraction peak. The crystallite sizes of undoped ZnO, 2.0-mol% Pr-doped ZnO, 4.0-mol% Pr-doped ZnO, and 6.0-mol% Pr-doped ZnO are 23.35 nm, 18.65 nm, 21.55 nm, and 18.57 nm, respectively. It is obvious that the crystallite size decreases after doping of Pr, which is because Pr³⁺ would hinder the diffusion and growth of ZnO crystallites, consistent with the fact that Pr³⁺ has a larger ionic radius than Zn²⁺ (see Ref. [2]).

Figures 2(a) and 2(b) show the SEM images of undoped ZnO and 2.0-mol% Pr-doped ZnO on the scale of 2 μm, while figures 2(c) and 2(d) show those two on a scale of 10 μm. The dopant of Pr does not significantly change the morphology of ZnO, and the 2-mol% Pr-doped ZnO sample is selected as the representative of the doped samples. It can be seen that the samples consist of numerous macro aggregates. The agglomerated particles are mostly spherical and inhomogeneous in size, which is because of the aggregation of ZnO nanoparticles and the growth of irregular nanocrystals in the preparation process.^[2] Figure 2(e) shows the EDS result, revealing that the samples are composed predominantly of Zn and O but with a trace amount of the Pr element.

The elemental composition and chemical bonds in Pr-doped ZnO samples are measured and the incorporation of Pr into ZnO lattice was confirmed by the XPS analysis. The XPS scan in a binding energy range of 0 eV–1300 eV of the 2.0-mol% Pr-doped ZnO shows that the photocatalyst consists mainly of Zn, O, and Pr element^[27] (Fig. 3(a)). From the Zn 2p core level spectrum of the 2-mol% Pr-doped ZnO (Fig. 3(b)), the peak of Zn 2p_3/2, and the peak of Zn 2p_1/2 located, respectively, at 1020.8 eV and 1043.9 eV are in line with the standard binding energy of Zn and can be attributed to Zn–O bonds. The O 1s spectrum (Fig. 3(c)) is deconvolved with Gaussian Lorentzian mixed functions, giving two peaks. One is located at 530.93 eV in accordance with the Zn–O–Zn bond, and the other is positioned at 529.53 eV and attributed to the Pr–O–Pr bond consistent with the binding energy of Pr₂O₃, which again confirms the successful doping of Pr into the ZnO lattice. The Pr 3d_3/2 and Pr 3d_5/2 peaks located, respectively, at 953.7 eV and 932.8 eV (Fig. 3(d)) agree with the standard binding energy of Pr.

Fig. 3.

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	Fig. 3. (color online) (a) XPS survey of 2.0-mol% Pr-doped ZnO, core level spectra of (b) Zn 2p, (c) O 1s, and (d) Pr 3d.

To identify the presence of functional groups, FTIR spectra of pure and Pr-doped ZnO photocatalysts are measured, and the results are shown in Fig. 4. The peak at 510 cm⁻¹ results from ZnO vibration,^[26] which is a characteristic absorption peak of ZnO. The broad peaks at 3418 cm⁻¹–3500 cm⁻¹ and 1384 cm⁻¹–1600 cm⁻¹ correspond to the stretching and bending vibrations of OH in the adsorbed water molecules, respectively, which proves the presence of Pr³⁺ indirectly because of its water absorption.^[2] No evident impurity is found in the samples, as there are no further distinguished peaks associated with other functional groups.

Fig. 4.

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	Fig. 4. (color online) FTIR spectra of pure and Pr-doped ZnO nanoparticle samples.

To investigate the optical properties, room temperature photoluminescence is measured in the wavelength range of 350 nn–800 nm as shown in Fig. 5. The spectra each have a strong ultraviolet (UV) peak at 400 nm, along with a broad weak peak centered at 550 nm. The UV peak apparently originates from a recombination of free excitons.^[29] The main feature is an evident decrease of the UV peak intensity with Pr doping, which indicates a decrease in the recombination rate of the photogenerated carriers, and this may result from the doped Pr³⁺ ions that act as traps to capture the photogenerated electrons or holes. The weak visible emissions are most likely associated with the singly ionized oxygen vacancies in the products, which result in the recombination of photogenerated holes with the electrons of the oxygen vacancies. It has been shown that more defects could be produced in the nanoparticle samples due to the incorporation of the dopants into the ZnO lattice,^[30] for example, the substitution of praseodymium ions for zinc ones leads to the generation of oxygen defects for keeping electrical neutrality.^[31] Hence, the doping of Pr into the ZnO lattice significantly weakens the band edge emission and increases the visible emission.^[26] Finally, according to the wavelength of the ultraviolet emission peak, the bandgaps of the pure and 2.0-mol% Pr-doped ZnO are estimated at 3.12 eV while those of the 4.0-mol% and 6.0-mol% Pr-doped ZnO are calculated to be 3.11 eV. The bandgap is not significantly changed with the doping of Pr, because of the small doping level; but with an even larger amount of Pr dopant, such as 6.0 mol%, the bandgap reduction is still observable.

Fig. 5.

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	Fig. 5. (color online) PL spectra of the doped and undoped ZnO samples.

To investigate the optical absorption properties, the UV-vis DRS spectra in a range 300 nm–600 nm are measured as shown in Fig. 6(a). The optical bandgaps of the prepared photocatalysts are estimated using the Tauc equation (for direct bandgap semiconductor): 2 where A is the absorbance of UV-vis diffuse reflectance, E_g is the bandgap, and K is a constant. The value of E_g is not affected by the value of K. Through extrapolating the linear region of the plot (Ahν)² ∼ hν, the optical bandgap of the samples can be estimated in a range of 3.11 eV–3.12 eV (Figs. 6(b)–6(e)). The bandgap for Pr-doped ZnO slightly decreases after the incorporation of moderate amounts of Pr dopant compared with for undoped ZnO. The slight decrease of bandgap may result from the impurity levels due to the substitutional Pr³⁺ in ZnO. It is easier for Pr-doped ZnO with narrower bandgaps than for undoped ZnO to generate more electron-hole pairs and hydroxyl radicals.^[21]

Fig. 6.

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	Fig. 6. (color online) (a) Absorption spectra of pure and Pr-doped ZnO samples, the Tauc plots given bandgaps of (b) pure, (c) 2-mol%, (d) 4-mol%, and (e) 6-mol% Pr-doped ZnO.

To study the photocatalytic properties of the synthesized samples with different Pr dopant concentrations, the photocatalytic degradation process of rhodamine B in aqueous solution under a simulated sunlight illumination is measured as shown in Figs. 7(a)–7(d). Figures 8(a)–8(b) show the photodegradation curves (C/C₀ versus time, where C and C₀ are the instantaneous and initial concentration of RhB, respectively) and degradation rates of rhodamine B by using various photocatalysts irradiated with simulated sunlight for 45 min. The degradation rate η of the RhB solution is calculated by using the following equation: 3 where C_t is the final concentration of RhB. It can be seen that the concentration of the RhB dye declines dramatically during the photodegradation process compared with that of the pure RhB solution. It is obvious that adding the dopant of Pr leads to enhanced photocatalytic activity, and the 6.0-mol% Pr-doped ZnO photocatalyst exhibits a high degradation efficiency, up to 93.75% degradation after 45-min irradiation. It is interesting that 2.0-mol% Pr-doped ZnO photocatalyst (69.12% degradation) does not exhibit better photocatalytic performance compared with undoped ZnO (79.87% degradation). The photocatalytic performance of photocatalyst can be influenced to an extent as big as several factors. The improved photocatalytic activities of 4.0-mol% and 6.0-mol% Pr-doped ZnO may result from the decrease of recombination rate of the photogenerated electron-hole pairs under the influence of the doped ions, which may capture the photogenerated electrons or holes in the photocatalytic process. Another reason is the narrowing of bandgap after doping. The photodegradation of rhodamine B is in accordance with the first-order kinetics at low concentration, which is based on the Langmuir–Hinshelwood model expressed as 4 where k is the reaction rate.^[32] The kinetic rate of undoped ZnO is 3.220% min⁻¹ and for 6.0-mol% Pr-doped ZnO, 5.490% min⁻¹ [Fig. 9(b)], which exhibits that the photocatalytic property obtained from the best dopant quantity is enhanced by 1.7 times.

Fig. 7.

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	Fig. 7. (color online) UV-vis absorption spectra of the photodegradation of rhodamine B by (a) undoped, (b) 2-mol%, (c) 4-mol%, and (d) 6-mol% Pr-doped ZnO photocatalysts.

Fig. 8.

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	Fig. 8. (color online) (a) Photodegradation and (b) degradation rates of rhodamine B by using various photocatalysts irradiated with simulated sunlight for 45 min.