Structure, microstructure, optical and photocatalytic properties of Mn-doped ZnO nanoparticles

In this work, Zn1−xMnxO samples were synthesized by the hydrothermal method by varying the dopant ratio from x = 0.01 to 0.05 with 0.01 increment. Structural, optical and photocatalytic properties of Mn-doped ZnO were analyzed based on their concentration dependence. Structural behavior of the nanoparticles was studied by X-ray diffraction technique and it was found that Zn1−xMnxO samples were hexagonal Wurtzite structure with no secondary phase. The effect of Mn element in Zn1-xMnxO composition was clarified by calculating lattice parameters, cell volumes, stress, microstrain, the locality of the atoms dislocation density, displacement of atoms and bond length. SEM images revealed random agglomeration. The band gap and Urbach energies of Zn1−xMnxO structures were determined and discussed. Different models were used to calculate refractive index and the refractive index was found in the range of 2.05–2.72. Mn-doped ZnO nanoparticles exhibited lower degradation rate constants (k = 0.0004–0.0009 min−1) than that of pure ZnO (k = 0.0014 min−1). Doping Mn increased the Urbach energy value.


Introduction
The increasing demands on energy and technological applications question new materials with different doping combinations which are critical on their physical and chemical characteristics. Zinc oxide (ZnO) a widely used material with different dopant elements still keeps the importance of varying miscellaneous properties depending on synthesizing conditions, dopant rate and elements [1][2][3][4][5]. With the property of wide band-gap (3.37 eV), semiconductor ZnO occupies a special place at detector technologies with the blue and UV spectral ranges and solid-state light sources [6] and the references therein. Among the laser material technology, ZnO is one of the conventional materials for creating laser and UV light-emitting diodes with the property of 60 meV binding energy which is higher than the existing binding energy of GaN (25 meV) [6]. High radiation properties, thermal and chemical resistance make ZnO a preferable material in the transparent contacts of solar cells. Besides these features, ZnO and its doped forms are used in acoustic, magnetic, electric, optic, cosmetic, and nuclear applications [7][8][9][10][11].
Moreover, due to fast reset time, high response, photoconductive gain, and larger effective area, ZnO-based photodetectors have an important role in commercial applications when compared to the conventional or GaN [12][13][14] or SiC [15] photodetector technologies. In addition, to tailor the intrinsic physical properties such as electronic properties and luminescence, relevant doping elements and their rates in ZnO are essential for application purposes [16]. With the property of special electron-shell structure, rare earth [RE] ions exhibit a better luminescence center by means of their special 4 f electron transition at different energy levels [17][18][19][20]. It is expected that Mn-doped ZnO nanostructures can be applicable to micro-and nano-optoelectronics. It is also well known in the literature that the physical and chemical properties of doped ZnO are heavily affected by the preparation conditions, dopant rate, and dopant elements such as Cr, Co, Fe, V, Mg and Ni [1].
The researches on ZnO were triggered by doping ZnO with transition metals (TM) [21,22]. Over the last decade, new steps were taken on revealing the useful magnetic properties convenient for industrial applications by doping ZnO with Fe, Co, and Mn elements [23][24][25].
Lately, an article was published by Naccarato et al [1] in 2019 to investigate how chemistry influences the refraction index and band gap and rationalize why certain classes of materials would execute better. Some of the refractive index calculation models were also given in Naccarato et al [1], namely, Ravindra et al [2], Moss [3], Hervé and Vandamme [4], Reddy and Anjayenulu [5], Kumar and Singh [6].
In this study, Zn 1−x Mn x O nanoparticles were synthesized using the hydrothermal method by varying the dopant ratio from x=0.01 to 0.05 with a 0.01 increment. The structural, optoelectronic, and photocatalytic properties of the samples were studied and the obtained results were correlated by the literature.

Experimental details
Mn-doped zinc oxide nanoparticles powders (Zn 1−x Mn x O: x=0.01, 0.02, 0.03, 0.04, and 0.05) were synthesized by hydrothermal method at different concentrations. Zinc nitrate hydrate, manganese acetate dihydrate, and hexamethylenetetramine were used as precursor materials. Deionized water was used as a solvent. After preparing the equimolar aqueous solution of the precursor materials, they were all mixed thoroughly with a magnetic stirrer for six hours at room temperature. Those solutions were transferred into the Teflon-lined autoclave. The reaction was conducted in an electric oven at 100°C for 12 h. After that, Mn-doped ZnO powders were separated from the solution by using a centrifuge. Separated powders were washed with deionized water and dried in air. The powders were calcined at 500°C to obtain Zn 1-x Mn x O nanoparticles.
The obtained Zn 1-xMnxO nanoparticles were characterized by the Rigaku Multiflex X-ray diffraction (XRD) instrument at room temperature, with monochromated CuKα (1.5418 Å) in the scan range of 2θ between 20°a nd 80°with a scan speed 3°/min and a step increment of 0.02. The surface morphologies of the samples were evaluated by scanning electron microscopy (SEM-EDX) Jeol-6390-LV. To detail the optical properties of the samples, double beam Shimadzu 2600 UV-Spectrometer was used with an integrating sphere in 300-700 nm wavelength range.
By the Perkin Elmer Spectrum Two FTIR-ATR spectrophotometer, the Fourier transform infrared (FTIR) spectra of all samples were recorded in the range of 4000-450 cm −1 .
The photocatalytic activity of ZnO and Mn-doped ZnO (Zn 1−x Mn x O) nanoparticles were investigated based on the degradation of the Methylene Blue (MeB) as model organic dye under ambient light. The concentration of nanoparticle dispersion and methylene blue solution was 1.0 mg ml −1 and 1.0×10 −5 M respectively. A 50 ml MeB solution was first prepared, then the appropriate amount of ZnO or Zn 1−x Mn x O nanoparticles were added. The dispersion was kept in magnetic stirring in dark for 30 min to ensure the adsorption/desorption equilibrium of methylene blue on the surface of nanoparticles. Aqueous dispersions of nanoparticles containing MeB were irradiated under visible light under continuous magnetic stirring. 1.75 ml aliquots were withdrawn at appropriate time intervals; suspended ZnO or Zn 1−x Mn x O nanoparticles were precipitated by centrifugation (4000 rpm for 3 min) and the absorption spectra of MeB were recorded using a Shimadzu UV mini 1240 UV-VIS spectrophotometer at a wavelength of 664 nm with deionized water as a reference. The rate of photocatalytic degradation for methylene blue was determined by the following equation [26]: where A o is the initial absorbance of MeB solution after adsorption equilibrium, A is the absorbance at the time (t), and k is the first order photocatalytic degradation rate constant.

Structural characterization
The structural behavior of all samples was determined by XRD analysis in the range of 20°2θ80°degrees. All peak positions were indexed and the second phase was not observed in all sample XRD patterns as shown in figure 1 Using XRD analysis of Zn 1−x Mn x O (x=0.01, 0.02, 0.03, 0.04, and 0.05) samples, the concentrationdependent average nanoparticle sizes, lattice parameters, volume of the unit cell, microstrain (ε), stress (σ), dislocation density (δ) (the amount of defect in the sample), the locality of the atoms and their displacement (u), and bond length L were determined and reported in table 1. The detailed calculations of ε, σ, δ, D, u, and L parameters were depicted in [1].
As for Mn-doped nanocrystals, the lattice parameter c was found in the range of 5.18 − 5.20 Å as shown in figure 2. There is a fluctuation in c/a lattice parameter ratio as shown in figure 3. Such fluctuations are true to be expected if Mn ions replace Zn ions in the lattice, as the Mn ions have smaller ionic radii (0.66 Å) than Zn ions (0.74 Å). The maximum microstrain ε occurred at x=0.04 and before that the ε values present a constant value of 0.0692 with increment Mn amount in the structure except at x=0.05. As seen in table 1, the negative signs of σ values depict compressive stress and these values exhibit a decrease until x=0.02 dopant rate and after that, it is constant for x=0.03 and 0.04, it reaches the minimum value of −4.357. The fluctuations in the strain and stress values may reveal many physical defects and dislocations in the host lattice structure as shown in figure 4. Therefore, dislocation density (δ) describing the amount of defect in the sample was also studied. As shown in  Table 1. Variation in concentration-dependent average nanoparticle sizes, lattice parameters, volume of the unit cell, microstrain (ε), stress (σ), dislocation density (δ) (the amount of defect in the sample), the locality of the atoms and their displacement (u), and bond length L and band-gap E g in Zn 1−x Mn x O nanoparticles.

FTIR analysis
Mn 2+ doped ZnO stretching vibrations were determined by a Perkin Elmer Spectrum Two FTIR-ATR spectrophotometer at room temperature in the range of 4000 to 450 cm −1 and the results are shown in figure 10.
As can be seen in detail in figure 10, the strong peak in 450-550 cm −1 range is owing to the Zn-O stretching mode, which slightly changes when Mn 2+ is doped in the ZnO structure and the values of absorption bands are found to be 465, 497, 504 and 517 cm −1 , respectively. The change in the peak position of the ZnO absorption bands exhibits that the Zn-O network is perturbed by the presence of Zn-Mn-O. Moreover, the small shift in the peak position in the doped sample may be attributed to the doping effect of metal ions. This behavior has also been reported for a study on Mn-doped ZnO [27,28]. The small absorption peak at 2900 cm −1 related to the -CH 2 stretching modes on the surface in all samples. Another very small absorption peaks appear 2350 cm −1 that indicates the characteristic band of CO 2 molecules on the surface of the sample taken from the air atmosphere during the measurements [29]. The band located at 850 cm −1 in all samples may be attributed to the bending vibration of nitrate (N-O); probably from the little residue of zinc nitrate used in the reaction [30].

Mechanism of photocatalyzation
Since methylene blue degradation predominantly occurs over the surface of photo catalytically active nanoparticles [31,32], the organic dye-nanoparticle suspension was kept in dark in the first 30 min to avoid photodegradation. After surface adsorption/desorption equilibrium of methylene blue was achieved, the dyenanoparticle suspension was irradiated with ambient light. Since some MeB molecules were adsorbed on the surface of nanoparticles, actual MeB concentration was considered as equal to the equilibrium concentration of this dye in the solution at the beginning of irradiation time [32].        Table 2 shows the photocatalytic degradation rate constants for ZnO and Zn 1-x Mn x O and degradation percentages of MeB in presence of these nanoparticles. Mn-doped ZnO nanoparticles exhibited lower degradation rate constants (k=0.0004-0.0009 min −1 ) than that of pure ZnO (k=0.0014 min −1 ) and the result was consistent with the relevant work for MeB degradation in presence of doped ZnO nanostructures [40,41]. Reduced photocatalytic activity of Zn 1-x Mn x O nanoparticles may be due to the change in the

Band gap calculation
The optical band gap E g can be determined using the following equation between the absorption coefficient (α) and the photon energy (hν): Figure 11. In equation (11), E g and k are the optical band gap and energy-independent constants, respectively. Since a F R ( ) is proportional to α and ZnO has direct allowed transitions, n is taken as 1/2. Thus, equation (11) can be transformed to:   These changes in the values of E g depend on several factors such as grain size, carrier concentration, lattice strain, the size effect of the dopant metals in ZnO lattice [45]. Koao et al [46] showed that the grain size and the luminescence intensity increased when an increase in the amount of Mn 3+ ions occurred. Our calculations also confirmed Koao's results except at x=0.05 doping ratio in terms of the grain sizes and band gap energies.

Refractive index calculation
There has been a great interest in high refractive index n and wide band gap E g because of their applications in optoelectronic and sensor industries. To increase the performance of optical interference filters, optical sensors (e.g. anti-reflection coatings), there is a need not only for the high refractive index but also for wide band gap materials. Lately, Naccarato et al [1] in 2019 published an article to relate the high refraction index and band gap, they evaluated more than 4000 semiconductors and investigated how the chemistry influences this inverse relationship between refraction index and band gap and rationalized why certain classes of materials would perform better. Naccarato et al [1] reviewed some of the refractive index calculation models, namely, Ravindra et al [2], Moss [3], Hervé and Vandamme [4], Reddy and Anjayenulu [5], Kumar and Singh [6].   As shown in figures 16(a), (b), the refractive index heavily depends on the Mn concentration. The highest refractive index values were obtained at %5 Mn doping and the minimum values were obtained at %3 Mn doping.
In all figures 16(a)-(e), all five models show the same pattern between the energy band gap and refractive index. Among all these five models, Moss and Kumar and Singh give similar results in the range of 2.29-2.33. In 2015, Tripathy [47] gave the following reliability ranges for the energy band gaps based on these five models and these values were listed in table 5.
As shown in table 4, our energy band gap values are in the range of 3.20 eV and 3.26 eV, thus refractive index values based on these models are valid.

The Urbach energy calculation
In figure 17, the logarithm of the absorption coefficient was plotted versus the photon energy eV ( n h ) for Zn 1−x Mn x O (x=0.01, 0.02, 0.03, 0.04, and 0.05) structures. The Urbach law states that the absorption coefficient (α) near band edges is having an exponential dependence on photon energy (hν), as given by the following relation: where E u is the Urbach energy indicating the effect of all possible defects and a 0 is a constant [48]. The Urbach energy E u values can be related to the width of the localized states in the band gap and are evaluated by using the following relation: where a is the absorption coefficient and is the photon energy [49], and the references therein.
However, since we only know the tabulated values of the absorption coefficient, there is a need to approximate the first derivative of a ln with respect to the photon energy ( n h ). This was done with two different finite difference methods, namely, 3-point central and 5-point formulas. The error in these approximations is of order 2 and 4, respectively. The Urbach energy values E u were evaluated at E g and plotted in figure 19       The grain sizes and the band gap energies fluctuated as the doping ratio increased. It was found that the refractive index strongly depends on the Mn concentration. At x=0.01 and x=0.05, the maximum and minimum refractive index   values were obtained, respectively. Mn-doped ZnO nanoparticles exhibited lower degradation rate constants (k=0.0004-0.0009 min −1 ) than that of pure ZnO (k=0.0014 min −1 ). The highest value of Urbach energy was approximately found in the range of 774 and 839 meV for 5% Mn .