To confirm the formation of flower shaped nanostructures of ZnO, scanning electron micrography (SEM) is performed (Figure 1). It is clear that the ZnO/ZnMgO nano flower with 15:10 mM solution concentration and 4.50 hrs reaction time shows uniform nano structure formation on the sputter-deposited film (15% doped Mg:ZnO sample) (Figure 1(a)). Interestingly as doping concentration decreases, the uniformity of nano rods reduces (shown in Figure 1(b)). Also for the undoped sample, the uniformity and crystallinity is less than that of the sample with 5% and 15% Mg doped ZnO. This is evident from Figure 1(c), x-ray diffraction (XRD) and photo luminescence (PL) measurements. The crystallinity of all the samples is confirmed through x-ray diffraction (XRD) measurement.
Figure 2 shows the normalized XRD spectra of pure ZnO and Mg-doped ZnO nano flower at 5%, 15% doped and un-doped samples respectively. Figure 2 shows the High Resolution x-ray Diffraction (HRXRD) images of (a) pure ZnO (b) 15 mol% Mg-doped ZnO, (c) 5 mol% Mg-doped ZnO nano flower structures. the XRD pattern exhibit three major diffraction peaks which are assigned to (100), (002), and (101), respectively. These are consistent with the hexagonal wurtzite structure (JCPDS card No. 01-079-2205). It is observed from the XRD pattern that the position of XRD peaks shifted towards higher angle which indicates some lattice doping of the Mg atoms. With increasing Mg doping concentrations, shifts towards larger angles are observed; this is due to the smaller ionic radius of Mg2+ compared to Zn2+ [25-27].
In order to investigate the vibrational properties of the un-doped and 15% Mg doped ZnO nano flower structures, Raman spectroscopic analysis is carried out with the 514nm excitation wavelength. Figure 3 represents a Raman spectroscopic analysis of un-doped and 15% Mg doped sample. Raman peak shifts occur for three reasons; first due to phonon confinement effects, [28] second due to lattice strain, [29] and third due to oxygen vacancies [30].The 15% Mg doped ZnO nano flower shows the shift of the signals, when compared to the un-doped ZnO (which shows a rather weak Raman peak intensity). Also the Raman peak is broadened for the Mg doped ZnO. The peak at 587 cm−1 is shows maximum shift (assigned to the E1 (LO) mode) due to the formation of defects like oxygen vacancies [31].
Also to confirm the Mg doped in ZnO sample the Fourier Transform Infrared Spectra (FTIR) is carried out (as shown in Figure 4). FTIR data for ZnO and 15% Mg doped ZnO sample is recorded in the wavelength region 4000–300 cm-1. The band stretches at ~480 cm-1. This is due to the Zn–O stretching mode in the ZnO lattice [32]; the band at 1684 cm-1 is due to the first overtone of the crucial stretching mode of O-H [33-35]. The band stretches at ~3316 cm-1 due to Mg-O stretching mode. A wide absorption band in the region of 3316 cm-1 is due to the stretching vibration mode of a hydroxyl group [36].
Figure 5 shows the response of un-doped ZnO at 180 oC, and 15% Mg doped ZnO at 150 oC. Ethanol is introduced in various concentrations. Sensing is carried out on ethanol gas molecules based on adsorption and desorption. It is clearly visible that the linear increase in the response is due to availability of active surface sites for the adsorption of ethanol molecules. In this case, the surface doping of Mg into the ZnO is most likely responsible for the enhanced sensing performance. Interestingly Mg doped ZnO also has enhanced recovery characteristics when compared with un-doped ZnO based samples (as shown in Figure 6). This indicates that the Mg doped system (likely containing MgO on the surface), in fact has rather shallow defect states.
It is evident that sensing response gets better with increasing temperatures. Figure 6 (b) shows that 15% Mg doped ZnO senses better than un-doped ZnO at low temperature (140 oC). This is because of mainly two reasons. First, ZnO has larger (apparent) defect density when compared to ZnMgO. Furthermore, ZnMgO-ZnO results in p-n junction (heterojunction) diode and the built-in potential reduces the recombination at the defect states. Also, it is seen from Figure 6(b) that at low gas concentration, 15% Mg doped ZnO shows better performance than un-doped ZnO.