Synthesis and study of structural properties of Sn doped ZnO nanoparticles

Sn doped ZnO nanoparticles have been synthesized using a simple chemical solution method. The stock solutions of starting materials 50 mM zinc nitrate Zn(NO₃)₂-2H_2O, 50 mM stannic chloride SnCl₄ 5H_2O and 100 mM sodium hydrate NaOH were prepared using deionized water as a solvent for each sample. The solution of NaOH was dropped into the solution of Zn(NO₃)₂-2H₂O under continuous magnetic stirring till the formation of white precipitates of Zn(OH)₂. The precipitates were filtered off and washed with distilled water for five times. The precipitates of Zn(OH)₂ were first dried in air at room temperature and then heated at 250 °C for 24 hours to attain fine crystallinity. For the preparation of 5 wt.%, 10 wt.%, 15 wt.% and 20 wt.% Sn doped zinc oxide, the stock solutions of 50 mM Zn(NO₃)₂-2H₂O and 50 mM SnCl₄. 5H₂O were mixed accordingly to each composition and the same procedure as used for the preparation of ZnO was adopted for each case.

The synthesized samples were characterized using a Rigaku XRD instrument with monochromatic CuKa radiation source having wavelength of 1.5418Å. The morphology studies and compositional analysis of nanoparticles were carried out by FESEM integrated with EDX (TIMA3 LM TES-CAN). The samples were coated with carbon using low voltage sputtering and then analyzed under SEM operating at a voltage of 20 kV.

Fig. 1a shows the XRD patterns of ZnO with 0 % Sn concentration. The X-ray diffraction patterns of samples a, b and c are consistent with the reference codes 01-005-0664 and 01-075-0576. The peak with (h k l) value (1 0 1) positioned at 36.36° has maximum intensity. The (1 0 1) major plane along with some other minor planes indicate the polycrystalline nature of the prepared material having hexagonal geometry [21,22].

When the Sn doping of ZnO material increased above 10 wt.%, the intensity of the major peak has increased showing the improvement in the crystallinity [23]. Beyond 10 wt.% Sn doping, the decrease in intensity of the major peak corresponding to (0 1 5) plane has been observed what indicates poor crystallinity. Sn doping has improved the crystallinity up to the concentration level of 10 wt.% but ZnO crystal structure showed poor crystallinity above the Sn concentration level of 10 wt.%.

The grain sizes of the prepared samples were calculated using Debye-Scherrer’s formula [24]: d=Kλ<!-- ? -->β<!-- ? -->cos⁡<!-- ? -->θ<!-- ? --> $$d = {\text{ }}\frac{{K\lambda }}{{\beta \cos \theta }}$$ (1)

where d is the grain size, k is a constant taken to be 0.89 for the hexagonal structure, λis the wavelength of X-ray used (1.5418Å), β is full width at half maximum and 0 is the Bragg’s angle. Equation 1 is multiplied by a factor n/180 to change the value of W from degree to radian:

The calculated average grain size of pure ZnO is 21 nm and it decreases up to 17 nm for 20 wt.% Sn-doped sample. The variation in grain size with the increase in Sn concentration is shown in Fig. 2 and Table 1. The grain size decreases with Sn content which is likely due to the substitution of Sn metal into zinc oxide lattice [25,26].

The strain produced in the samples due to Sn doping has been calculated using the relation [27]: ∈<!-- ? --> = β<!-- ? -->cos⁡<!-- ? -->θ<!-- ? -->4 $$\in {\text{ = }}\frac{{\beta \cos \theta }}{4}$$ (3)

where C is strain and θ is the Bragg’s angle in degrees.

Dislocation density was calculated by [28]: δ<!-- d -->=1d2 $$\delta = \frac{1}{d^2}$$ (4)

The dislocation density (δ), defined as the length of dislocations lines per unit volume has been estimated using equation 4.

Calculated values of strain and dislocation density are presented in Table 2.

The effect of strain in ZnO nanoparticles due to Sn dopant is demonstrated by the grain size (d), strain (∈), and dislocation density (δ) given in Table 2. For the samples with Sn-doping concentration of 5 wt.%, the grains tend to increase in size because of the small strain in the sample that influences the normal growth of ZnO. However, when Sn-doping concentration is 10 wt.%, the grain size slightly decreases because of higher strain. This shows again that the best crystalline structure is achieved only up to 10 wt.% Sn doping as previously followed from XRD analysis. At higher doping up to 10 wt.%, the grain size decreases again as a result of greater stress leading to poor crystallinity. It should be mentioned that the strain value (∈) depends on both |3 and cos0 as shown in equation 3. Dislocation density (δ) may also be considered as a measure of crystallinity. Among the different doping concentrations 5 wt.% and 10 wt.% doping give the smallest (δ) which shows the best crystalline structure compared to higher or lower doped ones [29].

The microstructure and morphology of nanoparticles were analyzed using FESEM. Fig. 2 shows the micrographs of the samples of Sn doped ZnO nanoparticles. The images show clearly the morphological changes due to the different doping levels of Sn in ZnO. The Sn doped ZnO nanoparticles are inhomogeneous in nature. Interestingly, when we increase the doping rate, the morphological changes of the microstructure show the mechanism of nucleation and/or wrinkles growth [30]. These variations are produced by Sn atoms which promote the growth of network-winkles destressing the microstructure and reducing energy of the system. Similar result was presented by Bahsi et al. [31] in the study of the effect of Mn and Cu doping on microstructure of ZnO. Furthermore, the surface micrographs showed that the doping of Sn plays an important role in changing and improving the structure of the ZnO nanoparticles. The ratio of Sn is the main factor that controls the morphology and structure of the final product. This may be due to the defects created by Sn doping. The size of these crystalline structures varies in the range of ∼54 nm to 111 nm in Fig. 3A and Fig. 3B. When the concentration of Sn dopant increased from 5 wt.% to 10 wt.%, the Sn-doped ZnO nanoparticles showed noticeable variations in the surface morphology [32]. Anisotropic growth of crystal results from the driving force of the dopant on ZnO lattice, hence, Sn-doped ZnO nanostructures have a granular shape as shown in Fig. 3A to Fig. 3C. Furthermore, ZnO nanoparticles at higher Sn concentrations (above 10 wt.%), (Fig. 3D and Fig. 3E) tend to agglomerate and show irregular flake structure [32].

EDX analysis was carried out to study the quality and composition of Sn-doped ZnO nanostructured samples [33]. EDX analysis confirmed the presence of Zn, O and Sn elements in the synthesized materials. Fig. 4a shows the spectrum of pure ZnO with two to three high intense peaks which are associated with Zn and O atoms.

The measured atomic percentage of these elements is about 42.04 and 57.96, respectively. Fig. 4b and Fig. 4d exhibit the results of EDX analysis of Sn-doped ZnO nanoparticles; the measured Sn contents are 0.52, 3.27, 8.03 and 9.68 atomic percentage, respectively, for the fournominal compositions of 5 wt.%, 10 wt.%, 15 wt.% and 20 wt.%, indicating that Sn percentage in the ZnO nanoparticles increases according the nominal loading of Sn [33, 34]. Results of elemental analysis of ZnO doped by different concentrations of Zn are compared in Table 3.