Vibrational Order , Structural Properties , and Optical Gap of ZnO Nanostructures Sintered through Thermal Decomposition

The sintering of different ZnO nanostructures by the thermal decomposition of zinc acetate is reported. Morphological changes from nanorods to nanoparticles are exhibited with the increase of the decomposition temperature from 300 to 500C.Thematerial showed a loss in the crystalline order with the increase in the temperature, which is correlated to the loss of oxygen due to the low heating rate used. Nanoparticles have a greater vibrational freedom than nanorods which is demonstrated in the rise of the main Ramanmode E


Introduction
Zinc oxide is one of the most promising semiconductors of group II-VI for manufacturing technological devices on a nanometric scale.Since the surface/volume ratio in nanostructures is higher than in bulk materials, the quantum effects dominate the properties.The precise control between morphology and the desired properties has been used in the study and fabrication of gas nanosensors [1][2][3], light emitting diodes [4], solar cells [5][6][7], field effect transistors [8], and in photocatalysts [9][10][11] among others [12].Throughout the pursuit of these objectives, a wide diversity of sintering techniques have been implemented.Lower cost methodologies, such as thermal decomposition, are the ones positioned in the first place.The thermal decomposition method is an endothermic process where heat is used to break the links within the initial precursor in order to obtain one or more final components.It has been found that this technique allows the growth of nanoparticles [13][14][15][16], nanowires, and nanorods [17][18][19] among others [20].Zinc acetate decomposes into zinc oxide when being subjected to thermal treatments, generally above 200 ∘ C.This methodology provides an advantage compared to other methods due to the simplicity and the fact that it does not require reducing or additive agents.To obtain zinc oxide through thermal decomposition, some methodologies have reported results as a function of the decomposition temperature, time, and the aging [16,19,[21][22][23][24]. However the relationship between morphology, structural, vibrational, and optical properties and the decomposition process is still not clear.In this work, ZnO nanostructures have been sintered from the thermal decomposition of zinc acetate at three different temperatures (300, 400, and 500 ∘ C) for a low heating rate and a long annealing time.The morphological, structural, vibrational, and material optical gap variations are being studied.

Experimental Details
The sinterization of ZnO nanostructures is as follows: 0.5 g of zinc acetate dihydrate (Zn(CH 3 COO) 2 ⋅ 2H 2 O Sigma Aldrich 98+%) is ground in a mortar for 5 minutes; the powdered material is placed in a crucible, which is inserted in a programmable furnace and heated at a rate of 100 ∘ C/h to a temperature of 300, 400, and 500 ∘ C for a period of 24 hours.The final product is approximately 0.15 g of ZnO, which is stored within a controlled environment for its respective characterization.
The structural characterization was carried out by using a Bruker D8 Advance X-Ray diffractometer (radiation of CuK  ,  = 1,5406 Å); the vibrational order was assessed through Raman microscopy by using -Raman Confocal LabRam HR Horiba Jobin Yvon equipment with a monochromatic laser source of 473 nm.The optical gap was established from absorption by using a UV/Vis Perkin Elmer Lambda 20 spectrophotometer.The morphology was identified by the use of a Transmission Electron Microscope (TEM) JEOL 2010-F working at 200 kV and a Scanning Electron Microscope (SEM) Hitachi 5500 at 30 kV.
The change of the morphology of ZnO sintered by thermal decomposition of zinc acetate has already been reported [22][23][24].Farbod and Jafarpoor [22] report an increase in oxygen deficiency with the annealing time and the final color of the sample.Each increment of the temperature leads to an increase in the total sinter time; for 500 ∘ C the long heating time (5 hours) may be responsible for a oxygen deficiency in the final sample.This is also related to the color of the powder which changes from white at 300 ∘ C to pale yellow at 500 ∘ C.

Results and Discussion
The thermodynamic mechanism of the zinc acetate decomposition into ZnO is well known.Initially, the loss of 2 water molecules is observed.The anhydrous acetate reacts with water in the environment to form a basic zinc acetate and acetic acid CH 3 COOH.In the decomposition of zinc complex, 4ZnO molecules, acetate CH 3 COCH 3 , and carbon dioxide CO 2 are produced [18,25].The reaction is as follows: The variation in the properties of the ZnO during this process must be correlated to the combination of decomposition temperature, heating rate, and time used for the experiment.For our case of study, Figure 1 shows TEM and SEM images of the material obtained at a fixed heating rate at a temperature of (a-d) 300, (e-h) 400, and (i-l) 500 ∘ C. The sample sintered at 300 ∘ C is mainly formed by nanorod structures with an average diameter of 35 ± 10 nm and an average length of 400 ± 196 nm.The preferential growth of the nanorods is in the (0001) direction.A high dispersion in the size of the nanorods is observed (inset Figures 1(a) and 1(b)).A low heating rate produces a more achievable loss of water molecules, which are essential in the decomposition process of the zinc acetate [22].During the formation of hexagonal ZnO, the polar faces (0001) are unstable and have a faster growth rate than the nonpolar faces.By the time the temperature has reached the 300 ∘ C there is a lack of nucleation sites, and the long period of time of the annealing promotes then the growth of rod-like structures orientated along the  axis.
For the sample collected at 400 ∘ C a change in the morphology is observed.The resulting nanostructures are a combination of nanorods and nanoparticles.The average sizes of the nanoparticles are 48 ± 13 nm (inset Figure 1(g)) whereas the nanorods have an average diameter of 34 ± 9 nm with an average length of 332±88 nm (inset Figures 1(e)-1(f)).
Once the temperature has increased to 500 ∘ C, hexagonal-like nanoparticles of 104 ± 22 nm are observed (inset Figure 1(i)).A higher temperature creates more nucleation sites of which nanoparticles can start to grow, producing a higher consumption of the precursor, favoring a homogeneous growth in all the directions of the hexagonal structure and lead the transformation from nanorods to nanoparticles.
3.1.Structure.Figure 2 shows the XRD diffractograms for the treated material at 300, 400, and 500 ∘ C, respectively.All the diffraction peaks correspond to the hexagonal phase of the ZnO according to JCPDS card number 36-1451; no impurity peaks were detected.The lattice parameters were established through the Cohen method and the mean crystallite size () was determined by the use of Scherrer's equation: where  is the wavelength of the X-ray radiation and  is the full width at half maximum (FWHM) of each diffraction peak.The different values for  and the lattice parameters are presented in Table 1.
It is observed that the temperature has an influence on crystallite domain size.Narrow peaks are present in the 300 ∘ C sample in which the crystallite size is comparable to the diameter of the nanorods.This may imply that the nanorods are formed by groups of crystallite pilled-up along the growth axis .An increment in the FWHM value and a decrease in the relative intensity of the peaks are observed for the higher temperature of 500 ∘ C. At this temperature there are more nucleation sites that allow the growth of nanoparticles formed by random arrangements of smaller crystallites.Increasing the temperature does not promote grain boundary diffusion; as a result a loss in the crystalline order is observed.ZnO has a tendency to lose oxygen [26], during the heating time before reaching the annealing temperature of 500 ∘ C in which more water escapes from the furnace and it is more likely to obtain a nonstoichiometric material.
the nanostructured material there are no constraints for the longitudinal optical modes due to the fact that the structures are randomly organized.The bands located between the interval of 327 to 332 cm −1 and 528 to 538 cm −1 correspond to the multiphononic modes of second order, which are expressed with [ 2 (high) −  2 (low)] and [ 2 (high) +  2 (low)], respectively and do not differ from other reports.
The nanorod morphology constrains the vibrational degrees of freedom of the molecules.The nanoparticle morphology allows a greater freedom in the vibration of the characteristic mode  2 (high).The  1 longitudinal and transversal modes are associated to parallel displacement of oxygen atoms on the  axis and are more intense for the 300 ∘ C sample since the structure is preferentially oriented in this direction.As a result of the increase in the temperature, the  1 modes decrease due to the oxygen deficiency.In addition, the presence of the mode  1 (LO) is more prominent with the increase in the temperature which is related to an oxygen deficiency [29][30][31].
3.3.Optical Absorption. Figure 4(a) shows the absorbance spectrum as a function of the wavelength for the sintered materials at 300, 400, and 500 ∘ C. From these results, the forbidden energy band   is established according to Tauc's expression: where  is the absorption,  is a constant, and  is taken as 1/2 for materials.From the dependence of (ℎ]) 2 with the energy (Figure 4 Ref.  is obtained.The energies determined for the materials were 3.21 ± 0.04, 3.20 ± 0.07, and 3.17 ± 0.03 eV for 300, 400, and 500 ∘ C, respectively.
A small decrease in the   value with the increase in the temperature is observed.The values hereby reported are lower than the one for the bulk material of 3.37 eV.This is opposite to the expected outcome for nanostructures and it may be due to the presence of defects and impurity levels inside the band gap and is in good agreement with other reports [16,21,22,32].

Conclusions
The thermal decomposition of zinc acetate allows the production of ZnO nanorods at 300 ∘ C and hexagonal-like nanoparticles at 500 ∘ C. Increasing temperatures favor the homogeneous crystallization among the different polar and nonpolar planes, as well as promoting the morphological transformation from nanorods to nanoparticles.A higher temperature creates more nucleation sites in which nanoparticles can start to grow.The slow heating rate is responsible for the loss of water and the oxygen deficiency rise.The average diameter of the nanorods is comparable to the crystallite size.Increments in the temperature reduce the size of the crystalline domains.The sintered material at 300 ∘ C shows a better crystalline organization in comparison to the obtained material at 400 and 500 ∘ C. The rise of the temperature increases the presence of structural defects, which are demonstrated with the increase of Raman mode  1 (LO) and the reduction of the  1 modes, related to the presence of oxygen vacancies.The energy band gap decreases with the temperature and in all the cases is lower than the bulk value.

Figure 3
shows the micro-Raman spectra results of the obtained materials as well as the ZnO reference.

Figure 1 :
Figure 1: TEM and SEM images of ZnO samples fabricated at (a-d) 300 ∘ C, (e-h) 400 ∘ C, and (i-l) 500 ∘ C. The insets on (a, b, e-g, and i) represent the size distribution of the structures.The arrow in (j) shows a hexagonal-like crystal present in the sample at 500 ∘ C.

Figure 3 :
Figure 3: (a) Raman spectra of ZnO samples fabricated at 300, 400, and 500 ∘ C, respectively, and reference.(b) Raman spectra at room temperature of ZnO samples fabricated at 300, 400, and 500 ∘ C, respectively.ZnO phonon modes as well as molecular vibrations are observed for comparison, and the spectrum of ZnO reference is shown.

Table 1 :
Lattices parameters and crystallite size.

Table 2 :
ZnO vibrational modes and relative intensity of the bands.

Table 2
Figure 2: XRD diffractogram of ZnO nanostructure samples fabricated at 300, 400, and 500 ∘ C, respectively.This growth can be observed from the presence of the peaks at 31.76, 34.32, 36.22,47.56, 56.58, 62.84, 66.44, 68.04, 69.12, 72.7, and 76.94 which correspond to (100), ( presents the vibrational peaks and the respective percentage of relative intensity (  / 0 ), where  0 is the intensity of the most intense and characteristic band  2 (high) antisymmetric, which is related to oxygen attached to zinc atoms in the tetrahedral configuration.Longitudinal optical modes  1 (LO) and  1 (LO) are not evident in the obtained material, since they are only visible when the axis  of the wurtzite structure is parallel to the surface.For