Optical properties of Cr doped ZnAl₂O₄ nanoparticles with Spinel structure synthesized by hydrothermal method

All chemicals used in this work were of analytical grade and were used without any further purification. Zn_1−xCr_xAl₂O₄ nanopowders were prepared via hydrothermal method, as shown in figure 1. In a typical procedure, according to the chemical formula Zn_1−xCr_xAl₂O₄ (x = 0, 0.01, 0.03, 0.05 and 0.07) and the molar ratio of metal cations Zn:Cr:Al = (1−x):x:2, weighed stoichiometric amounts of Zn(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O and CrCl₃·6H₂O used as raw materials, dissolved in distilled water to obtain mixed metal salt solution, and evenly dispersed by magnetic stirring at room temperature. Meanwhile, dropped NaOH solution into the above solution to adjust the pH value of the mixed solution to be 12, subsequently a certain amount of CTAB slowly added to the above mixed solution, all the above processes are carried out under constant magnetic stirring. Then the resultant mixture was transferred to and sealed a 100 ml Teflon-lined stainless-steel autoclave and kept at 200 °C for 24 h. After the reaction was completed, the autoclave was naturally cooled down to room temperature. Thereafter, the resulting precipitate was separated centrifugally and washed several times with distilled water and absolute alcohol to remove excess surfactants and impurities, respectively, the precipitate was dried in a vacuum drying oven at 60 °C for 12 h to obtain the precursors. Finally, the Zn_1−xCr_xAl₂O₄ sample can be obtained by heat treatment the precursors at sintering temperature of 700 °C for 6 h.

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The crystal phases of the samples were identified by x-ray powder diffraction (Rigaku corporation, Japan, D/Max-2400) using Cu Kα radiation (λ = 0.15406 nm) operating at 40 kV and 30 mA in the diffraction angle range of 20–80°with scanning rate 0.005 ° s⁻¹ and step size 0.02°. The morphology and microstructure were characterized by scanning electron microscope (SEM, JEOL JSM-6701F) and high-resolution transmission electron microscopy (HRTEM, JEM-2010). The chemical composition analysis of the products was measured by x-ray energy dispersive spectroscopy (XEDS). The optical absorption and bandgap energy of the samples were characterized by ultraviolet-visible (UV–vis) spectrophotometer (PERSEE TU-1901) using BaSO₄ as reference. Fourier transform infrared spectroscopy (FT-IR) analysis were performed by Nexus 670 FT-IR spectrophotometer in the range of 4000–500 cm⁻¹ using KBr powder. The composition and chemical states of elements were studied by x-ray photoelectron spectroscopy (XPS) measurement was carried out on a PHI-5702 multi-functional x-ray photoelectron spectrometer. The photoluminescence (PL) spectra were recorded by PerkinElmer spectrophotometer (LS-55) with an excitation wavelength of 320 nm.

Figure 2 show the XRD patterns of Zn_1−xCr_xAl₂O₄ (x = 0, 0.01, 0.03, 0.05 and 0.07) samples. It is clearly observed from figure 2 that the diffraction peaks of pure ZnAl₂O₄ at 2θ = 31.5°, 36.8°, 44.9°, 47.3°, 56.2°, 58.1°, 65.3°, 73.6° and 77.3° are well indexed to (220), (311), (400), (331), (422), (511), (440), (620) and (533) crystal planes of single-phase ZnAl₂O₄ cubic spinel structure with lattice parameters a = b = c = 0.8048 nm, which is in accordance with that of the standard spectrum (JCPDS Card No. 05-0669). In addition, there is no extra diffraction peaks of other impurities such as Cr clusters or Cr oxides are detected within the detection limit of XRD, ZnAl₂O₄ is the only phase detected in all the heated samples. which indicating that all Cr ions are successfully incorporated into Zn²⁺ ions sites without changing the parent ZnAl₂O₄ structure, and the purity of the product is relatively high.

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The crystallite size of the samples is calculated using the most intense diffraction peak (311) by means of Scherrer formula: $D=\tfrac{K\lambda }{B\,\cos \,\theta },$ where D is the crystallite size, K = 0.9 is the Scherrer constant, λ = 1.54056 Å is the x-ray wavelength, B is the corrected full width at maximum and θ is the diffraction angle. The average crystallite size of the samples was summarized in table 1. It is clear that that the crystallite size gradually decreases with the increase of Cr doping concentration, indicating that Cr doping have a regulating effect on the crystallite size.

The interplanar crystal spacing and the lattice constants of the sample can be obtained according to the crystalographic calculation formula and the Bragg formula, the diffraction angle, lattice parameters, interplanar crystal spacing and crystallite size of (311) peak for Zn_1−xCr_xAl₂O₄ samples as shown in table 1. It is obviously that the diffraction angle of the (311) peak for doped samples is slightly shifted towards lower angle with the increase of Cr doping content, the lattice parameters and interplanar crystal spacing of doped samples decrease with Cr doped content increase. The lattice constant is distorted, and exhibit lattice expansion. which may be attributed to the incorporation of Cr ions are doped into ZnAl₂O₄ host lattice, and ionic radius of Cr³⁺ (0.63 Å) smaller than Zn²⁺ (0.74 Å), which eventually led to the lattice distortion. These results show that the lattice parameters and particle size of the sample can be controlled by doping, and then the microstructure and physical properties of the sample can be controlled and adjusted.

The typical SEM images of pure ZnAl₂O₄ and Zn_0.95Cr_0.05Al₂O₄ sample as shown in figure 3. It is obviously that the morphology of pure ZnAl₂O₄ is mainly irregular spherical or ellipsoid particles with average particle size below 50 nm. Due to the existence of van der Waals force, coulomb force or chemical bond cooperation between nanoparticles, there is a certain degree of agglomeration phenomenon. compared with pure samples, the morphologies of pure ZnAl₂O₄ and doped ZnAl₂O₄ samples are basically the same, but Cr doped sample show a tendency to possess smaller particle size and more uniform size distribution, and the doped ions do not have significant effect on the morphology of the products.

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The microstructure and morphology of the as-prepared samples were performed by HRTEM, figure 4(a) shows a representative HRTEM images of Zn_0.95Cr_0.05Al₂O₄, it can be seen that the morphology of Zn_0.95Cr_0.05Al₂O₄ sample is mainly irregular ellipsoid particles with rough surface and irregular edges. Figure 4(b) represents the local magnification of Zn_0.95Cr_0.05Al₂O₄ sample, it clearly find the orderly and flawless crystal plane stripes, and the interplanar spacing about 0.284 nm, which corresponds to the (311) crystal planes of cubic spinel structure ZnAl₂O₄, indicating that Zn_0.95Cr_0.05Al₂O₄ are well-crystallized and grew selectively along the (311) crystal planes, which is consistent with the XRD results.

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The selected area electron diffraction (SAED) pattern of Zn_0.95Cr_0.05Al₂O₄ as shown in figure 4(c), the SAED pattern of the samples consists of a series of regular concentric rings with different clear and obvious radius due to the sample possess polycrystalline structure. The polycrystalline diffraction rings from inside to outside correspond to the crystal plane of ZnAl₂O₄ (220), (311), (400), (331), (422), (511) and(440) , respectively. The results of SAED and HRTEM further indicated that the obtained samples with spinel structure, which further indicating that all Cr³⁺ entered ZnAl₂O₄ lattice and replaced Zn²⁺, which is consistent with the XRD analysis results.

Figure 4(d) is the particle size distribution diagram of Zn_0.95Cr_0.05Al₂O₄ nanoparticles, it can be seen from the figure that the particle size distribution is relatively uniform, mainly distributed in the range of 10–50 nm, and the average particle size is about 25 nm.

Figure 5 show the XEDS spectra of pure ZnAl₂O₄ and Zn_0.95Cr_0.05Al₂O₄ sample. From the XEDS spectra, it is observed that pure ZnAl₂O₄ samples mainly exhibit characteristic peaks corresponding to Zn, Al and O elements (figure 5(a)), and the atomic percentage (At %) of Zn/Al/O is found to be 14.27 /25.51/ 60.22%, respectively, as shown in inset of figure 5(a). Figure 5(b) confirms the existence of Cr elements in addition to Zn, Al and O element for Zn_0.95Cr_0.05Al₂O₄ sample, while no Cr element was found in the spectrum of pure ZnAl₂O₄. The quantitative atoms and weight percentages of the associated elements for Zn_0.95Cr_0.05Al₂O₄ sample from XEDS results as shown in inset of figure 5(b), reveals Zn/Cr/Al/O atomic percentage is 9.24/3.83/25.74/61.23%, respectively. The Atomic% and Weight of the constituent elements are found to be almost close to the nominal stoichiometry within the range of experimental error. Which also confirmed the presence of Zn, Al, O and Cr elements only and shows the purity of the synthesized materials. XEDS results further verified the conclusion of XRD, indicating that Cr-doped spinel ZnAl²O⁴ nanocrystalline were successfully prepared by hydrothermal method, and Cr³⁺ successfully used as doping agent replaced Zn²⁺ and entered ZnAl₂O₄ matrix.

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In order to further determine the composition and element chemical states of Zn_1−xCr_xAl₂O₄ samples, the sample was analyzed by x-ray photoelectron spectroscopy (XPS). Figure 6(a) shows the full XPS spectra, revealing the existence of Zn, Al, Cr, O, and C elements in Zn_0.95Cr_0.05Al₂O₄ sample. Figures 6(b)–(f) shows illustrates the high resolution XPS spectra of Zn 2p, Al 2p, O 1s, Cr 2p and C 1s, respectively. As seen from figure 6(b), the Zn 2p XPS spectrum shows two sharp peaks at 1022 and 1045 eV correspond to Zn 2p_3/2 and Zn 2p_1/2, respectively, which indicates that Zn exists in the combination state of +2 valence. The Al 2p peak of the sample appearing around 74.5 eV in figure 6(c). The C 1s XPS spectrum (figure 6(f)) presents the peak at 284.8 eV is attributed to the hydrothermal reaction process without washing clean organic carbon. The O 1s peak about 532 eV shown in figure 6(d) can be fitted into three peaks at about 531.1, 532.5 and 533.7 eV respectively. The main peaks centered at 531.1eV are attributed to the contribution of oxygen in the crystal lattice, which is the energy spectrum of Zn-O and Al-O bonds in ZnAl₂O₄ lattice. The peak at 532.5 eV is related to the oxygen defect in ZnAl₂O₄ crystal. which indicates that there are some oxygen vacancies in the sample. While the peak at 533.7 eV is caused by free oxygen on the surface of the sample or oxygen molecules in the pollutant surface of the hydrated oxide (such as adsorbed pollutants, chemical adsorbents and –CO₃, H₂O and O₂, etc) [35]. Figure 6(e) is Cr 2p_3/2 XPS spectra, there are two main peaks appeared around 577.1 and 588.5ev, which correspond to the binding energy between Cr 2p_3/2 and satellite peaks, respectively. The Cr 2p_3/2 XPS spectra can be fitted into three peaks at 576.9, 577.3 and 588.5 eV respectively. Therefore, the peak at 576.9 eV corresponds to the Cr ion of the octahedron, and the peak at 577.3 eV corresponds to the Cr ion of the tetrahedron. The Cr2p XPS results clearly show that the Cr ions mainly occupied the tetrahedral site, while only a small number of Cr ions occupy the octahedral position in Zn_0.95Cr_0.05Al₂O₄ sample.

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The UV–vis absorption spectra of Zn_1−xCr_xAl₂O₄ samples recorded in range 250–850 nm as depicted in figure 7(a). pure ZnAl₂O₄ exhibit the absorption peak in the wavelength range of 300–400 nm. Compared to pure ZnAl₂O₄, doped Zn_1−xCr_xAl₂O₄ samples exhibit a broad absorption peak near 500 nm, which is the characteristic peak of Cr³⁺ in the tetrahedral sites [36], this peak corresponds to the transition from ⁴A₂(F) to ²A₁(G) of 3d electron of Cr ion in the field of tetragonal crystal, and this peak causes the sample to appear green-blue [37]. With the increase of Cr concentration, the intensity of the absorption peak gradually increased. The UV–vis absorption spectra are consistent with the fitting results of Cr 2p in XPS, at low concentration, the proportion of Cr ion concentration at the tetrahedral position is relatively low. With the increase of Cr ion concentration, more Cr ions replace Zn ions at the tetrahedral position. Therefore, this will lead to further increase of characteristic peak strength of Cr ion at the tetrahedral position.

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The band gap Eg of Zn_1−xCr_xAl₂O₄ (x = 0, 0.01, 0.03, 0.05 and 0.07) samples can be calculated according to Tauc formula [38]: ${(\alpha hv)}^{{\rm{n}}}=A(hv-{E}_{g})$ The relationship curve between (αhν)² and hν as shown in figure 7(b) The band gap Eg of Zn_1−xCr_xAl₂O₄ (x = 0, 0.01, 0.03, 0.05 and 0.07) samples are 3.13 eV, 2.99 eV, 2.89 eV and 2.97 eV, respectively. The band gap of pure ZnAl₂O₄ (3.23 ev) is smaller than that of corresponding block ZnAl₂O₄ (3.8 eV) due to the quantum limiting effect. In addition, with the increase of Cr ion concentration, the band gap of the doped sample decrease and exhibit red shift. The phenomenon of redshift is mainly due to the sp-d interaction between the electrons in the band and the local electrons in the d-shell of the substituted cation, which makes the conduction band potential decrease and the valence band potential increase, thus leading to the decrease of the band gap. Furthermore, the radius of Cr ion (0.063 nm) is less than that of Zn ion (0.074 nm), the band gap of Zn_1−xCr_xAl₂O₄ samples from 0 to 7% gradually decreases with the increase of Cr ion concentration.

FT-IR spectra of Zn_1−xCr_xAl₂O₄ (x = 0, 0.01, 0.03, 0.05 and 0.07) samples recorded in the range of 400–4000 cm⁻¹ as shown in figure 8, It can be seen that two obvious absorption peaks appear near 3448 and1635 cm⁻¹ for all samples, FT-IR spectra of pure and doped ZnAl₂O₄ samples are basically the same. The wide absorption band near 3448.3 cm⁻¹ corresponds to the stretching vibration of O-H group, and the band at 1635 cm⁻¹ can be assigned to the bending vibration of H-O-H group, both of which are related to water molecules on the surface of the sample [26]. In addition, there are three sharp absorption peaks appear near 490 cm⁻¹, 553 and 689 cm⁻¹, which belong to the characteristic peaks of spinel structure ZnAl₂O₄. Among them, the peak at 553 cm⁻¹ are corresponds to the Zn–O bond asymmetric stretching vibration mode of the tetrahedron positions, while the peak at 490 and 689 cm⁻¹ are attributed to the Al-O bond asymmetric stretching vibration and the bending vibration mode of the octahedron positions, respectively [25]. In addition, with the increase of Cr concentration, the intensity of FT-IR spectra for Zn_1−xCr_xAl₂O₄ samples decreases slowly in the range of 400–700 cm⁻¹ region, and the peaks moving toward long wavelength direction and occurs red shift. This is mainly due to the cation distribution in ZnAl₂O₄ nanocrystals changes significantly when Cr ions entered ZnAl₂O₄ lattice and replaced Zn ions and the particle size reaches the nanoscale scale, some Al³⁺ enters the A position of tetrahedron, and Cr³⁺ enters the B position of octahedron, which leads to the generation of ion logarithm of super exchange between A and B [10], and introduce new interstitial structural defects in spinel structure, the new introduced structural defects will lead to redshift of the spectra, which is consistent with the UV–vis analysis results.

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Figure 9 shows PL spectra of Zn_1−xCr_xAl₂O₄ (x = 0, 0.01, 0.03, 0.05 and 0.07) samples with excitation wavelength of 300 nm at room temperature. It was observed that Zn_1−xCr_xAl₂O₄samples exhibited three emission spectra at 406, 465, and 576 nm, respectively. Pure ZnAl₂O₄ exhibit a broad blue emission band centered at 406 nm (3.05 eV), which can be attributed to excitonic NBE emission, and the emission energy is close to the band gap of ZnAl₂O₄ (3.08 eV), indicating the sample possesses better crystallization. the blue emission peak centered at 465 nm for the samples is associated with oxygen vacancy. Compared with pure ZnAl₂O₄, the blue peak of doped sample moves in the direction of long wave and occurs red shift, which may be due to the substitution of Zn²⁺ ions by Cr³⁺ ions in ZnAl₂O₄ lattice will introduce new impurity levels mainly between O vacancy level and valence band, leading to slight lattice distortion, resulting in energy change during energy level transition. In addition, the photoluminescence peak is attributed to the recombination of electron-hole related to intrinsic defects, the PL intensity of the doped sample decrease sharply and occur quenching phenomenon with the increase of Cr ion concentration. This is mainly due to the decrease of zinc gap and the increase of Cr³⁺ concentration after the replacement of Zn²⁺ by Cr³⁺, the octahedral and tetrahedral gap formed by O atom accumulation are occupied by metal ions respectively, forming a compact structure, which is likely to form Zn, Al, O and other interstitial structural defects in spinel structure, photoelectron preferentially occupies the trap center caused by Cr³⁺, leading to luminescence annihilation.

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Figure 10 shows the Commission International DeI'Eclairage (CIE) chromaticity diagram of Zn_1−xCr_xAl₂O₄ samples, which are calculated via fluorescence spectra under the excitation wave of 320 nm. The CIE coordinates values of Zn_1−xCr_xAl₂O₄ (x = 0, 0.01, 0.03, 0.05 and 0.07) nanoparticle are shown inset of figure 7, which are located in green-blue region. It can be seen from the chromaticity diagram, the chromaticity distribution of the sample can be controlled by controlling the doping concentration of the sample, and the peak decreases regularly with the increase of the doping concentration, demonstrating Zn_1−xCr_xAl₂O₄ nanoparticle exhibit excellent light-emitting ability.

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