1 Introduction

Up to now, a considerable attention is being paid to transparent n-type semiconductor ZnO with a wide band gap of 3.37 eV, a large exciton binding energy of 60 meV, the ultraviolet lasing at room temperature, and the availability of large-area single-crystalline bulk substrate [1–4]. Moreover, the wide band-gap metal oxide semiconductors and its compositions have a huge spectrum of technological applications such as conducting electrodes in solar cells; liquid crystal panel displays; surface acoustic wave devices; ultraviolet light emission; heat mirrors; lasing blue light-emitting diodes; short-wavelength laser diodes; and commercialization of group III nitride-based blue lasers, transparent electronic devices, and so forth [5–10]. Currently, the advance of this type of materials paves the way for the commercialization of these materials for heterostructure device design, which may eventually compete with the group III nitrides and photonic devices. Furthermore, emission and absorption of metal oxide semiconductors can be tuned by means of alloying of different materials and by building quantum wells [11–15]. Therefore, synthesis and characterization of the alloy system Mg x Zn1−x O nanostructures is important from the viewpoint of band-gap engineering and may provide a new optically tunable family of wide-band-gap materials that can be used in UV-luminescent applications as well as a potential conjugate material for the group III nitrides [16, 17]. In fact, the modulation of the band gap of ZnO can be controlled via dope divalent (n-type and/or p-type) substitution on the cation site and/or other ions into the lattice with keeping the lattice constants close to each other [18–22]. The electrical, magnetic, and optical properties of ZnO have been improved by metal ions doping [23–34].

Moreover, it is imperative to choose a material that can control the mobility carrier’s concentration down to intrinsic level and to develop a route to fabricate a high-quality single-crystalline thin film.

Currently, Mg-doped ZnO has great attracted scientific community because of their unique UV-luminescent properties based on the radiative recombination of the electron–hole pairs [21, 22]. By far, many methods, such as sol–gel process, co-precipitation, microwave, spray pyrolysis, gas phase, were developed for the synthesis of Mg-doped ZnO nanoparticles [22]. In most of these methods, various capping agents and surfactants are used to prevent the growth of particles with time. Therefore, it is important in photoluminescent application to synthesize chemical homogeneous and fine powders of Mg-doped ZnO. In this regard, hydrothermal technique is relatively low-cost, effective, pure, homogenous, useful, and simple processing technique widely applied to synthesize a variety of nanocrystalline materials and reactivity over standard ceramic processing [4, 5].

However, to the best of our knowledge, there is no example in the literature for the synthesis of MgZnO nanoflowers composed of nanosheets by using hydrothermal technique. Herein, we developed a novel procedure to synthesize Mg-doped ZnO nanoflowers at various Mg concentrations by hydrothermal technique. The structural and optical properties of the Mg x Zn1−x O nanoflowers with varying Mg compositions have been examined in detail.

2 Experimental details

2.1 Synthesis of Mg x Zn1−x O nanoflowers

All reagents are used without further purification. The process of preparing Mg x Zn1−x O nanoflowers is described as follows: the molar ratio of Mg 2+/Zn 2+ was adjusted to x = 0, 0.10, 0.15, 0.20, and 0.25 at %, zinc acetate dehydrate Zn(CH3COO)2·2H2O, magnesium acetate tetrahydrate Mg(CH3OO)2·4H2O. The nominal moles of zinc acetate and magnesium acetate were dissolved into 40 ml deionized water, and 2 g urea was added to this solution under 15 min stirring. Then, the obtained aqueous solution was transferred to a 100-mL Teflon-lined stainless autoclave and baked at 200 °C and autogenous pressure (15 bars, the water vapor pressure at this temperature) for 3 h. After the hydrothermal reaction, the autoclave was allowed to cool down naturally to room temperature. Finally, the products were decanted and rinsed with water to eliminate the remaining impurities and dried at 80 °C overnight in an oven.

2.2 Characterization techniques

The crystalline structure of the obtained products was examined by X-ray diffraction technique, using an X’Pert MPD Philips instrument, with Cukα incident radiation operating at 5 kV and 30 mA. The diffraction patterns were collected at room temperature over the 2θ range of 10–70°, with an acquisition time of 10.0 s at 0.01° step size. The X-ray patterns were analyzed by matching the observed peaks with the standard pattern provided by JCPDS file. The microstructural and morphological features of as-prepared samples were analyzed by a field-emission scanning electron microscopy (FE-SEM) on a JEOL 6400 microscope equipped with an energy-dispersive X-ray spectroscopy (EDS) as well as transmission electron microscope (TEM) using a JEOL 3010 operated at 300 kV. Samples for TEM were prepared by air-drying a drop of a sonicated ethanol suspension of particles onto a carbon-coated copper mesh. The optical transmittance of the thick films of thickness 2 μm was determined by UV–Visible spectroscopy UV–VIS–NIR spectrophotometer (Shimadzu UV-3600). Photoluminescence (PL) spectra of the pellet samples having 2 μm thickness were measured at room temperature by fluorescence spectrophotometer (Jobin–Yvon FL3–21) using a Xe lamp (500 W) as a light source excited at 325 nm.

3 Results and discussion

3.1 Structural observation

The crystal structure and phase purity of the prepared Mg x Zn1−x O nanoflowers was assessed using X-ray diffraction. The typical X-ray diffractions of the Mg x Zn1−x O nanoflowers with varying Mg doping rates (x = 0.0, 0.1, 0.15, 0.20, and 0.25 mol) are presented in Fig. 1a. Indeed, X-ray pattern presented in Fig. 1a exhibited a preferred orientation along the (002) plane, showing the crystallographic hexagonal wurtzite with three typical peaks corresponding to the (100), (002), and (101) planes of the pure hexagonal ZnO phase. The lattice parameters of ZnO, as evaluated by Rietveld refinement, are a = 0.3254 nm and c = 0.5211 nm. These values are very close to true values (compared with JCPDS Card No. 80‐0075). It should be noted that no diffraction peaks from impurities have been found, revealing the high-purity crystal phase of ZnO.

Fig. 1
figure 1

a X-ray diffraction patterns of Mg x Zn1−x O nanoflowers, b crystallite size (D), lattice strain, and c-axis length calculated using Williamson–Hall equation, c–f EDS spectrum of the Mg x Zn1−x O nanoflowers, c 0.1 Mg-doped ZnO, d 0.2 Mg-doped ZnO, e 0.3 Mg-doped ZnO, and f 0.4 Mg-doped ZnO

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As shown in Fig. 1a, Mg-doped ZnO (x = 0, 0.10, 0.15, 0.20, and 0.25 mol) showed a single peak corresponding to the (002) plane of ZnO and the spectrum retains their wurtzite structure, indicating that a pure hexagonal phase Mg x Zn1−x O was prepared. It is worth noting that the significant decrease in peak intensity of (002) reflection is clearly detected in Mg-doped ZnO, indicating that the crystal quality decreases with increasing Mg compositions.

To determine the crystallite size (D) and lattice stain (η) of the Mg x Zn1−x O nanoflowers from the width of lines in the X-ray spectrum, we used Williamson–Hall equation [18]. In this method, the broadening of diffraction peaks is due to crystallite sizes and inhomogeneous strain [21, 22].

$$\frac{\beta \,\cos \theta }{\lambda } = \frac{k}{D} + \frac{\eta \,\sin \theta }{\lambda }$$
(1)

where k is a constant (with a typical value of about 0.9), which depends on the peak shapes, crystallite habit, and particle shape; λ is the radiation X-ray wavelength used; β is the broadening of the diffraction line measured at half its maximum intensity in radians; and θ is the Bragg’s diffraction angle.

The crystallite size was determined from the three intense peaks of Mg x Zn1−x O spectrum that are (101), (002), and (100). The crystallite size, lattice strain, and c-axis lattice constant using Williamson–Hall equation versus Mg compositions are presented in Fig. 1b. As shown in Fig. 1b, the particle sizes, lattice strain, and c-axis length decreased as the Mg mol fraction increased from zero up to 0.25 mol of Mg x Zn1−x O nanoflowers. The crystallite size and lattice strain are gradually decreased with increasing Mg content. This is attributed to the replacement of Mg with Zn in the crystal lattice. Moreover, the decrease in c-axis parameter with increasing Mg mole fraction is due to Mg ions occupying the interstitial sites. Such a change is expected, because the Mg2+ ion has smaller ionic radius of 65 pm compared to Zn2+ (74 pm) ion. We concluded that the pure phase of the samples and the decrease in c-axis length imply that the Mg atoms occupy ZnO lattices effectively. The Mg incorporation and elemental compositions of the prepared Mg x Zn1−x O (x = 0.1, 0.2, 0.3, and 0.4 wt%) nanoflowers were evaluated by energy-dispersive X-ray spectroscopy (EDS), as shown in Fig. 1c–f, respectively. The EDS spectrum reveals that the sample contains only zinc, magnesium, and oxygen. No other impurities were detected, confirming the high purity of the product. The obtained EDS results confirm the results obtained from X-ray study in Fig. 1a.

The morphology and detailed structural information were determined by FE-SEM. Figure 2a–d, shows the FE-SEM micrographs with different magnification of Mg0.1Zn0.9O nanoflowers. As can be seen from Fig. 2a, b the product consists of a large scale and with high uniformity of flower-like hierarchical structure. A high-magnification FE-SEM image is presented in Fig. 2c, d. Each flower unit is composed of several nanosheets and/or petals with the length of about several micrometers and the thickness of about 21 nm. All the petals are joined to each other through their bases in such a manner that the flower exhibits a spherical shape. The full array of the flower-shaped structure is about 4 μm while the width ranges around 20 nm.

Fig. 2
figure 2

a–d FE-SEM micrographs with different magnifications of Mg x Zn1−x O (x = 0.1) nanoflowers

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The morphology, structure, and composition of the synthesized products have been characterized in detail using TEM, HRTEM, and selected area electron diffraction (SAED). The typical TEM images of Mg0.1Zn0.9O nanoflowers are presented in Fig. 3a. A typical TEM image of Mg0.1Zn0.9O nanoflowers reveals that their geometrical shapes are flower-like and are consisted of flake-like nanostructures. Each flower unit is composed of several nanosheets with the length of about several micrometers and the thickness of about 21 nm. Furthermore, the corresponding HRTEM image and SAED pattern in Fig. 3b reveal that the Mg0.1Zn0.9O nanoflowers are structurally uniform and single crystal. The interplanar distance is measured to be about 0.49 nm, which matches well with the (002) plane value of hexagonal ZnO. The clearly visible lattice fringes are indicative of the high crystallinity of the product. The corresponding SAED pattern, inset in Fig. 3b, proves the single-crystalline nature of Mg0.1Zn0.9O nanostructures, growing along (1 0 0) direction without defect or dislocation.

Fig. 3
figure 3

a TEM micrographs, b HRTEM image and the inset is the EDXS image of Mg x Zn1−x O (x = 0.1) nanoflowers, c mechanism of crystallite growth of Mg-doped ZnO nanoflowers

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On the basis of the FE-SEM and TEM images (Figs. 2, 3), the possible crystallite growth mechanism of the prepared Mg-doped ZnO nanoflowers was proposed, as illustrated in Fig. 3c. The growth of the nanoflowers follows two-step processes: nucleation and growth. In the first step, during the hydrothermal treatment, the nucleation occurs while the growth started in the second step with increasing incubation time. As the time increased up to 3 h, this provided a sufficient environment for the growth of Mg-doped ZnO nuclei throughout the hydrothermal reactor, leading to the formation of Mg-doped ZnO nanoflowers within hydrothermal reactor in a (0001) direction.

3.2 Optical properties studies

The optical absorption spectra of the as-prepared Mg x Zn1−x nanoflowers were examined to evaluate the optical band energy gap. Assuming the band gap to be parabolic in nature and for the direct band-gap materials, the absorption coefficient (α) is related to the band gap (E g) by the relation [6, 7]:

$$\left( {\alpha h\upsilon } \right)^{2} = B\left( {h\upsilon - E_{\text{g}} } \right)^{n}$$
(2)

where h is the planks constant, υ is the photo frequency, B is a constant, and n is the transition type. The absorption coefficient (α) can be extracted from

$$I = I_{ 0} \,e^{ - \alpha t}$$
(3)

where I and I 0 are the intensities of the transmitted light and incident light, respectively, and t is the thickness of the sample. The optical band gap (E g) of the as-prepared Mg x Zn1−x O nanoflowers was estimated from the plot of (αhυ)2 versus hυ as shown in Fig. 4a. The linear regime of a plot of (αhυ)2 on the y-axis versus hυ on the x-axis is interpolated to cut x-axis corresponding to y = 0. The intercept on the x-axis is a measure of the E g of the tested samples. It is well known that the E g value depended upon many key factors such as particle shape and size and defect concentration [1, 18]. As shown in Fig. 4b, the optical band energy gap increased with increasing Mg concentration of Mg x Zn1−x O nanoflowers. The increase in E g with increasing Mg composition may be evoked for two reasons. First, the solubility of Mg ions throughout ZnO lattice planes leads to open a new path of charge carriers and, in turn, an increase in the band energy gap. Second, the defect concentration throughout the Mg x Zn1−x O system results in a reduction in E g value [14–18]. The optical transmittance is an essential optical parameter for transparent semiconductors metal oxides [10, 13]. As expected, the Mg inclusion well into the crystallites of the ZnO reflected a systematic variation in the optical properties of Mg x Zn1−x O nanoflowers with increasing Mg composition. The transmittance versus wavelength at room temperature of the as-prepared Mg x Zn1−x O is presented in Fig. 5. Obviously, the optical transmittance is increased with the addition of the Mg ions into Mg x Zn1−x O nanoflowers. The transmission edge is found to shift to lower wavelengths, i.e., UV region with increasing Mg content in the samples. The shift of transmission edge to lower wavelength with increasing Mg content is due to the size decreasing in the samples with increasing Mg content [7]. As a result of the incorporation of Mg, a gradual shift of the transmission edge toward shorter wavelength is noticed. All the tested samples exhibited an optical transparency, which was more than 86 %. It may be noted that no significant variation in transmittance was observed in the visible range with increasing Mg concentration in the MgZnO samples. This suggests that scattering defect centers remain unaffected with increasing Mg concentration [17].

Fig. 4
figure 4

a Plots of (αhυ)2 versus hυ for Mg x Zn1−x O nanoflowers, b plot of Eg versus Mg compositions

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Fig. 5
figure 5

Transmittance spectra of Mg x Zn1−x O nanoflowers at room temperature

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The (PL) spectrum at room temperature of as-prepared Mg x Zn1−x O nanoflowers with different Mg compositions is presented in Fig. 6. It is clear that the PL peak position shifted from 3.33 to 4.53 eV for Mg mol fraction from x = 0.0 to 0.25. The luminescence band of the undoped ZnO was observed at 380 nm, and in the case of the Mg2+-doped samples, the band is shifted in turn to 373, 361, 354, and 345 nm at Mg mol fraction of 0.1, 0.15, 0.20, and 0.25, respectively. In all the samples, near-band-edge emission was obtained, and no peaks corresponding to deep-level emission were detected. It is found that the near-band emission, the peaks of Mg x Zn1−x O samples, showed a blue-shifting to higher energies with increasing Mg content. The PL peak corresponding to Mg x Zn1−x O nanoflowers is wide. The broadening of the luminescence peaks is normally observed in an alloy semiconductor [18], where charge carriers observe different potentials depending on the local concentration and also on the arrangements of the substituting elements. However, the broad visible emission bands are attributed to the high-defect concentration-related deep-level emission throughout Mg x Zn1−x O nanoflowers system, and the decrease in the non-radiative recombination defects and the increase in the non-radiative recombination lifetime [10, 19]. It is worth mentioning that the PL intensity became weaker due to the degradation of the Mg x Zn1−x O nanoflowers crystallinity as confirmed above by X-ray. Furthermore, the inclusion of MgO into ZnO enhances the UV emission of the Mg x Zn1−x O nanoflowers.

Fig. 6
figure 6

Photoluminescence spectra for the Mg x Zn1−x O nanoflowers at room temperature with different Mg compositions

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4 Conclusions

We describe a novel simple method to synthesize Mg x Zn1−x O nanoflowers on a large scale at comparatively low temperature using hydrothermal technique. X-ray spectra revealed that Mg x Zn1−x O nanoflowers have a pure hexagonal phase. The incorporation of Mg in ZnO was confirmed through energy-dispersive X-ray (EDS) spectrum. The optical energy gap of the samples increases with increasing Mg contents from 3.34 to 4.67 eV. We believe that this study provides a promising and simple method for the fabrication of structure-tunable metal oxide nanocomposites that can potentially be used in optoelectronics nanodevices.