Impact of annealing on the structural and optical properties of ZnO nanoparticles and tracing the formation of clusters via DFT calculation

2.1 Syntheses

ZnO NPs were synthesized through auto-combustion wet chemical route (Aljawfi et al., 2016) and annealed at different temperatures. The raw materials of zinc nitrate dehydrate [Zn (NO₃)·6H₂O] and citric acid were used without purification treatment. Nitrates and citric acid were stirred separately in 40 ml distilled water on hot plate (50–80 °C) using magnetic stirrer. The molarity ratio of citric acid to nitrate was 1:1. Then the solutions of nitrates and citric acid were mixed together and continually stirred at 50–80 °C, the measured pH was less than 4. Few drops of ethylene glycol [CH₂(OH)CH₂OH] were added as a fuel. The solution was continuously stirred for about 3 h until the excess water evaporated completely and appearance of gel form. The product gel was dried in a furnace at 150 °C for 4 h followed by grinding for 1 h. Final annealing of the dried soft powder was achieved by using a muffle programmable furnace that warrants the homogeneity of space heating crucially. The rate of arising temperature adjusted to be slow and hold at 200 °C for 6 h. Then the furnace switched off and led for spontaneous cooling and decreasing the temperature to lower than 50 °C. Same steps have been repeated for other sample but with different annealing temperature 400, 600 and 800 °C. All the obtained powder samples have white color, except the powder annealed at 200 °C that was in bright brown color, which may be due to residue carbon ions.

2.2 Characterization techniques

The crystal structural and purity phase of the studied samples have been carried out using X-ray diffraction (XRD), Bruker D8 ADVANCE, with Cu K_α radiation (λ = 1.54056 Å) in the range of 30° ≤ 2θ ≤ 70°. FTIR spectra of the samples were obtained by Perkin Elmer BX2 FTIR spectrometer; the powder was mixed with KBr and pressed as pellets. The TEM images are obtained by a field emission transmission electron microscope (FETEM-Jeol 2010). Photoluminescence (PL) spectrum of the ZnO NPs samples were recorded using fluorescence spectrometer (PerkinElmer LS), the excitation wavelength was 325 nm using Ar ion laser with a power of 12 MW.

2.3 Computational details

In the present theoretical calculation, time-independent and time-dependent density functional theory (DFT/TD-DFT) were used along with 6-31G(d,p) basis set as implemented in Gaussian 09 software (Frisch, et al., 2009). The Becke's three parameter hybrid functional (B3) (Becke, 1988) for the exchange part and the Lee–Yang–Parr (LYP) correlation functional (Lee et al., 1988; Dennington et al., 2009) were used within DFT/TD-DFT framework. Furthermore, UV/Vis spectrum has been also simulated at TD-B3LYP/6-31G (d,p) level of theory.

3.1 X-ray diffraction analysis

Fig. 1 shows the XRD patterns of pure ZnO NPs annealed at different temperature 400, 600 and 800 °C. The reflections are consistent with that of hexagonal ZnO wurtzite structure with P6₃ mc space group (JCPDS, No. 36-1451). Within the limitation of the used XRD technique, there is no additional peak corresponding to any secondary/impurity phase, and the absence of foreign peaks warrants the single phase nature of ZnO NPs and the chemical reaction by auto-combustion route have been performed successfully and the starting organic precursors might have been decomposed at high annealing temperature. However, the XRD peaks of the sample annealed at 200 °C (inset Fig. 1 shows weak and boarder peaks that indicates to low crystal quality and the possibility of formation of other kinds of precipitation or clusters. The sequence annealing temperature optimizes the crystallinity and increases the crystallite size. The mean crystallite sizes (L) have been calculated using Debye–Scherer equation (Scherrer, 1918):

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Fig. 1

X-ray diffraction (XRD) patterns for ZnO NPs annealed at 400 °C, 600 °C and 800 °C, inset XRD pattern of the sample annealed at 200 °C.

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3.2 FT-IR spectra

Fig. 2 displays the FTIR spectra of ZnO NPs annealed thermally at 400, 600 and 800 °C. The peak at ∼3400 cm⁻¹ represents the fundamental stretching vibration of O—H bond of hydroxide group or water moisture adsorbed at the surfaces. The H⁺ in the environment always absorb strongly by the O²⁻ anions of ZnO hexagonal to form O—H bond, whereas the hydroxyl (OH⁻) group residual in raw material tends to be absorbed by the cations Zn²⁺ due to electrostatics potential that leads to the formation of O—H functional group, therefore O—H band exists strongly in ZnO and other metal oxides NPs (Lenartowicz and Marek, 2017). The peaks at the range of 415–520 cm⁻¹ are originated to the stretching vibration modes of Zn-O-Zn indicating to the complete transformation from zinc nitrate to zinc oxide at 400 °C and that analogous to XRD result. Absorption bands at 2900 cm⁻¹ are attributed to C—H bond, but in the present samples that peak is very weak. It is known that the IR bands of H—H, C—H and O—H possess partially anharmonic vibration due to the light mass of the H atom. The bands at 1300 and 1600 cm⁻¹ is assigned to the asymmetric and symmetric C⚌O modes respectively, which are residual from the used raw materials like nitrate or citric acid. The detected bands decrease with increase the annealing temperature but do not vanish even with high annealing temperature (800 °C), which indicates to the instability of the surface structure of NPs and the tendency of the dangling bonds at the surfaces to bind with some functional group.

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Fig. 2

FTIR spectra of the samples annealed at different annealing temperatures 400 °C, 600 °C and 800 °C.

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3.3 TEM and SEM

The impact of annealing temperature on the grains size, grains boundary and the interfaces between aggregated grains can been be seen in TEM images. Fig. 3(a, b) shows the TEM images of the extreme samples that annealed at 400 °C and 800 °C. The grown nanoparticles possess remarkable difference in sizes and shapes, mostly semi-sphere and nearly hexagonal. The average sizes (diameters) increases from 18 nm for ZnO NPs annealed at 400 °C to 37 nm for the sample annealed at 800 °C, other values are summarized in Table 1. The selected area electron diffraction (SAED) is illustrated in inset Fig. 3(a), in which six circular patterns represent the hexagonal planes of ZnO (1 0 0), (0 0 2), (1 0 1), (1 0 2) (1 1 0) and (1 0 3), these patterns are in analogous with that of XRD patterns. The inset in Fig. 3(b) shows the high-resolution TEM (HRTEM) image for the sample annealed at 800 °C, where the distance between two adjacent planes is about 0.26 nm which corresponds with the d-spacing of ZnO (0 0 2) plane perpendicular to the c-orientation [0 0 0 1]. The grains, perturbation in the lattice texture at the grain boundary and the interface of coalescence grains due to thermal annealing can be seen in Fig. 3(c).

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Fig. 3

TEM images, (a) grain size of ZnO NPs annealed at 400 °C, (b) grain size of ZnO NPs annealed at 800 °C and (c) show the dislocation of lattice texture at the grain boundary, the aggregated grains, show the interfaces of adjusted grains and the thickness of coalescence region.

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Fig. 4(a–d) illustrates SEM images of ZnO NPs annealed at different temperatures, it can be seen an appreciable change in the size of NPs. The particles and morphology of ZnO NPs annealed at 200 °C are undistinguishable and agglomerated with each other without clear boundaries, which give an indication to that, the annealing temperature of ZnO NPs at or lower than 200 °C was insufficient for good crystallization Fig. 4(a)). The scheme in inset, Fig. 4(a) represents a single particle contains number of grains. The SEM images of the samples annealed at 400 °C, 600 °C and 800 °C show segregated NPs with clear boundaries and quasi-spherical shape. The sizes of NPs have been calculated statistically and the size distributions have been represented by histograms as shown in the inset, Fig. 4(b–d). The average size of NPs was extracted by fitting the histogram with Log-Normal equation,

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Fig. 4

SEM images (a–d) of the samples annealed at different temperatures. The inset in (a) represents the scheme of the NPs containing number of grains, The Inset (b–d) shows the histogram for the size distribution of ZnO NPs, the Log-normal fit is represented by a solid and white line.

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It is noteworthy to mention that, the calculated values of the crystallite (grain) size estimated via XRD and TEM images are almost same and smaller than size of NPs calculated from SEM images, where the particle (in nano-scale) involves a number of grains (inset Fig. 4(a)). Under the effect of thermal annealing, the grains (small crystals) are aggregated and grouped to form nanoparticles, TEM images gives a cross-section image of these grains with clear boundaries of the grains. The single grain has almost same crystal lattice orientation (crystallite). SEM images obtain the external micrograph and dimensions of the NPs. The particle size increased rapidly as the annealing temperature increase, where the thermal annealing possesses strong effect on nucleation, crystal growth and on the aggregating of small grains.

The local atomic defects at the surfaces favor the merging process by stimulating the coalescence of more grains during annealing (Dillon and Harmer, 2007), where, at the low annealing temperature the grains are not completely formed and the ions assembled as small cluster, which required high energy to start crystal growth. With high annealing temperature the small grains would be aggregated (or re-crystalline) together to form big size NPs this process is associated with the changing in the Gibbs free energy (G) (Zhang et al., 2014). At high temperature, the entropy is high and the surfaces are unstable and the ions would be able to transfer between the grains boundaries and rearrange themselves at lowest energy, the grain or small particles going to be more stable chemically as the grain size increase.

3.4 Micro-Raman scattering

ZnO wurtzite crystal structure possesses P6₃mc space group. The optical Raman phonons of ZnO at the zone center can be represented by the irreducible representation: Γ = 1A₁ + 1E₁ + 2E₂ + 2B₁, where 1A₁ and 1E₁ both represent Raman and infrared active modes and polar branches, which spilt into longitudinal optical (LO) and transversal optical (TO) phonon associated with different wavelength. The 2E₂ is non-polar Raman active modes consist of low-frequency mode (E_2L) associated with vibration of Zn heavy ions and high-frequency mode (E_2H) associated with O vibration (Bundesmann et al., 2003; Damen et al., 1966). The 2B₁ are two silent modes (inactive in Raman spectra). Fig. 5(a) shows typical Raman spectra of pure ZnO NPs annealed at different temperature 200, 400 °C, 600 °C and 800 °C, in the range of 200–700 cm⁻¹. Four Raman peaks can be seen clearly at about 330, 378, 435 and 580 cm⁻¹. These peaks are resemble to that of ZnO hexagonal, where, the peak located at 330 cm⁻¹ is assigned to the difference between the high frequency and low frequency E_2H–E_2L (Cusco, 2007). The peak at 378 cm⁻¹ is attributed to the A₁ (TO) polar branch. The highest peak at about 438 cm⁻¹ is ascribed to the high-frequency branch of E₂ high mode (E_2H) and characteristic of ZnO wurtzite structure (Cusco, 2007); this mode involves the oxygen vibration with respect to the stationary Zn ions and the increasing in the intensity of this mode as the annealing temperature increase indicates decreasing in the oxygen vacancy owing to the annealing in air. Usually, the shifting and broadening in Raman modes associated with thermal annealing treatment is related to either intrinsic defect like oxygen vacancy or due to the quantum size effect (Richter et al., 1981), but in present work, there is no clear shifting or boarding in the Raman mode E_2H (438 cm⁻¹), which have been observed in the magnified version of this mode as shown in Fig. 5(b), where the Raman lines shape (E_2H) of 600 °C and 800 °C annealed samples are same and there is no broadening or shifting could be detected, which indicates to that the crystallite dimensions are large (measured by XRD) and have no quantum size effect accompanied with the high annealing temperatures. The samples annealed at 200 °C and 400 °C show decline, asymmetric and broadening peaks but no shifting in the peaks position that may be due to the existence of oxygen vacancy (Parker and Siegel, 1990), accompanied with the low annealing temperatures, where, the oxygen loss contribute on the observed slightly broadening in Raman modes. The nature of defects and their contribution on the broadening and shifting of Raman mode is tentatively attributed to oxygen related non-stoichiometry defects that have been studied in TiO₂ NPs by Parker and Siegel (1990). The Raman mode at 580 cm⁻¹ is attributed to A₁ (LO) phonon mode, which is commonly assigned to oxygen vacancy (Vo) and zinc interstitial (Zn_i) (Das et al., 2011), and it is weak peak in the 200 °C annealed sample and not observed clearly in the Raman spectrum of ZnO annealed at 800 °C, that may due to re-conformation happens to form high-quality crystal and recovery the dislocated atoms at the grain boundaries. Hence, the concentration of the V_O is decreased as the annealing temperature increase. The intensity of the main peaks increases with increase in the annealing temperature, which indicates to improvement and optimization in the crystallinity by the heating treatment.

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Fig. 5

(a) Raman spectroscopy for the ZnO NPs annealed at different temperatures. (b) A magnification version of E_2H mode.

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3.5 Photoluminescence studies

Fig. 6(a–d) shows room temperature PL spectra of pure ZnO NPs annealed at different temperature, there exits two main UV spectral bands, UV-PL band and visible band. The UV emission band is ascribed to the electronic decay from conduction band (CB) to valence band (VB) (Yan et al., 2008). At the near band edge, the UV-PL emission bands are centered at about 396 nm (3.13 eV), 393 nm (3.16 eV) and 388 nm (3.20 eV) for the samples annealed at 400 °C, 600 °C and 800 °C respectively, the energies of UV-PL emission band slightly increase as thermal annealing increase and are summarized in Table 1. The FWHM values of the UV peaks estimated by fitting the UV peaks with Gaussian function found to be 76.16¸ 28.99 and 14.41 nm for the samples annealed at 400 °C, 600 °C and 800 °C respectively, the gradual decreasing in the FWHM is due to the improved crystallinity and reduce native point defects with further annealing treatment. Low intensity and asymmetric in the UV peak is due to electronic decay from CB to energy deep defects band (Giri, 2007), where the presence of local defects in a semiconductor is known to produce additional energy bands within the band gap region. Thus, the increasing in the intensity and symmetric trend in the UV-PL bands in sequence with annealing temperature reveals the optimization in the crystallinity and recovery the dislocated atoms at the grain boundaries and reduces the Vo²⁺ point defect (Aljawfi et al., 2016). The UV-PL band of ZnO annealed at 200 °C showed anomalous behavior where the UV-PL line shape is diminished clearly and appeared as small plateau in the range $\sim$ 272 to 320 cm⁻¹ (blue shift to lower wavelength) with high band gap energy 4.5–3.8 eV, which is very high value comparing to that of ZnO, that is indicative of low quality in the crystallinity and may be due to formation of clusters accompanied by low annealing temperature.

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Fig. 6

(a–d) UV-PL spectra for ZnO NPs annealed at different temperature with Gauss multi Fit. (e) The scheme of band gap energy with the main energy deep level within the band gap region.

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The visible emission bands (Fig. 6(a–d)) are broad and convoluted due to the superposition of many overlapped bands; these composed bands are attributed to the existence of different deep energy bands, Gaussian fit has been used to differentiate between the closed bands. The minimum fitting shows two bands green lines and one blue. The locations of these visible bands are summarized in Table 1. For complete comprehension of the nature of VIS-PL emission band, it can be recalled that ZnO possesses tetrahedral coordinate structure, the ion of Zn²⁺ in the tetrahedral local coordinate symmetry is surrounded by four ions of antion O^2– as first shell coordination number, similarly the O ion is surrounded by four ions of Zn. Zn²⁺ and O^2– atoms are stacked in different planes normal on the [0 0 0 1] orientation, the O has partial charge of O^2– (base) and the Zn metal has an oxidation state of Zn²⁺ (acid). So, the zinc vacancy (V_Zn) as local atomic defect leaves four coordination number of O ions, which behave like holes (V_Zn²⁻) and tend create an acceptor energy band at $\sim$ 0.85 eV for the 800 °C sample above VB maximum. The decay of electrons from CB to the V_Zn acceptor band is associated with PL blue emission band. The wavelength of the blue emission was 478 nm (2.59) and 506 (2.45 eV) nm for the extreme sample annealed at 200 and 800 °C respectively, other values are summarized in Table 1. Also, oxygen interstitial (O_i) can be considered as acceptor-like (Amish, 2010). The V_O leaves four ions of Zn with weak dangling bonds and free electrons; these electrons make an energy band close and lower than CB as donor band, located at 1.1 eV above VB maximum for the ZnO NPs annealed at 800 °C, point defects of V_O as well as Zn_i lead to increase the free electron carrier and can be considered donor-like, and the decay of electron from CB to the created band V_O is associated with emitting PL green photons as visible band. The green emission at 525 nm (2.3 eV) and 542 nm (2.2 eV) for the samples annealed at 200 and 800 °C respectively. Usually the green line is ascribed to local defect of oxygen vacancy (V_O) (Lin et al., 2001). It is predicted that, the surfaces of ZnO NPs annealed at 200 °C would contain rich V_O, and the oxygen non-stoichiometry (O/Zn  $<$  1) would be reduced in sequence with annealing in air (O/Zn  $\approx$  1). The other green line emission located at about 528 nm (2.35 eV) and at 542 nm (2.29 eV) for ZnO NPs annealed at 200 and 800 °C respectively is attributed to Zn_i defects, but the Zni bands are unstable at room temperature because it is so close to CB (Aljawfi et al., 2016). Therefore, ZnO is considered as N-type semiconductors even with the absence of the extrinsic defects. The scheme of DELs bands for the sample annealed at 800 °C is shown in Fig. 6(e). With low minimum annealing temperature, the possibility of vacancies like V_O, V_Zn is more than interstitial defects like Zn_i or O_i and at a high annealing temperature in air; the possibility of interstitial defects like O_i would be more as (Lin, 2001):

4.1 Geometry structure

Fig. 7 shows the optimized structure of Zn_nO_n (n = 3, 5, 7, 10) clusters. The minimum energy values were –14605901.19 and −48718642 (kJ/mol) and the molecular weights were 244.138 and 813.79 g/mol for Zn₃O₃ and Zn₁₀O₁₀ respectively other values are summarized in Table 2.

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Fig. 7

The geometry structure of Zn_nO_n (n = 3, 5, 7 and 10) cluster.

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4.2 Electronic structure

The small cluster or molecule has bound higher energy excited electronic state (conductivity state) in addition to the ground electronic state (valence state). The ground state and excited state are known as high occupied molecular state (HOMO) and low unoccupied molecular orbital (LUMO) respectively. The UV–VIS absorption spectra are arising when an electron excited from HOMO to a higher-energy orbital LUMO that can be occurred when a photon with the energy equal to the difference between the states (E_HOMO–E_LUMO) is absorbed by an electron in ground state and promote to a vacant higher energy orbital of the excited state (Tovar et al., 2008), while The UV–VIS emission spectra are arising when an electron at the excited state interact with incident photon leading to decay the electron from excited state to the ground state. So, the band gap energy (E_HOMO–E_LOMO) evaluated from absorption or emission spectra are logically same.

Fig. 8 shows the density of electronic states, HOMO and LUMO orbital and the Fermi level (E_f = 0 eV) for Zn₃O₃ and Zn₁₀O₁₀ clusters. The occupied orbital (HOMO) are represented by green lines and the unoccupied orbital (HOMO) are represented by blue color lines, the width between them represents the band gap energy. The optical band gap energy can be evaluated directly from the difference energy between HOMO and LUMO, i. e. the calculated E_HOMO and E_LUMO of Zn₃O₃ cluster and the band gap between them were HOMO energy = −0.16624 a.u., LUMO energy = −0.33371 a.u. and the HOMO-LUMO energy gap (singlet) = 0.167 a.u. (0.167  $\times$  27.211 = 4.5 eV).

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Fig. 8

The density of electronic structure (DOS) and HUMO-LUMO gap.

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The negative orbital energies of HOMO and LUMO lower than Fermi level gives the 1st ionization energy and electron affinity of a molecular system respectively. The values of band gap energy for other clusters are shown in Table 2. HOMO and LOMO and the band gap energy further illustrated in the 3D frontier molecular orbital (FMOs) plot and depicted in Fig. 9. The red and green colors represent the positive and negative phases respectively. The frontier orbital, HOMO and LUMO are very important parameters to evaluate the interaction way of small cluster or molecules with other ions or cluster, where, HOMO is the outermost orbital containing electrons and has a tendency to release electrons as an electron donor, while, LUMO orbital has free space to accept electrons as electron acceptor. The obtained values of band gap energy of clusters were 4.6, 3.2, 4.6 and 3.7 eV for the Z₃nO₃ (ring), Zn₅O₅ (un-uniform)_, Z₇nO₇ (ring) and Z₁₀nO₁₀ (elliptic) respectively. The band gap energies of the clusters are higher than that of ZnO NPs, which implies that the clusters are more stable and required higher energy for exciting an electron from HOMO to LUMO state. Furthermore, the high band gap of the simulated clusters are located in the range (3.2–4.6 eV) corresponding with the band gap energy of ZnO NPs annealed at low temperature (200 °C) evaluated from the decline and plateau line ship of UV-PL which lies in the range of 380–270 nm. This agreement indicates to that the ZnO NPs itself contain clusters and the existence of clusters in 200 °C sample is the responsible for the high band gap, blue shift and lowering UV-PL band. It can be seen that the band gap energies of the clusters are higher than that of ZnO NPs annealed at 400 °C, 600 °C and at 800 °C indicating to the absence of clusters in these samples.

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Fig. 9

the 3D frontier molecular orbital (FMOs) plot of HOMO-LUMO represented by color with band gap energy.

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4.3 UV–VIS absorption spectra

Fig. 10 highlights the simulated UV–VIS absorption spectra of Zn₃O₃ and Zn₁₀O₁₀ clusters computed by TD-B3LYP/6-31G(d,p) level of theory. The two UV absorption peaks centered at 323 (3.8 eV) and 272 (4.56 eV) nm for Zn₃O₃ cluster and at 398 nm (3.2 eV) for Zn₁₀O₁₀ cluster, which are quit agreement with the location of the UV-PL emission band of the ZnO NPs annealed at low temperature Fig. 6(a)), which confirms the existence of clusters in the sample annealed at 200 °C. The other values of the band gap energy calculated from UV–VIS absorption spectra are summarized in Table 2. There is no indication to presence the cluster in the samples that annealed at 400 °C, 600 °C and 800 °C. So, for avoid the formation of clusters in the metal oxide like ZnO NPs synthesized through wet chemical process, the minimum annealing temperature for complete decomposed should be higher than 400 °C.

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Fig. 10

UV–VIS spectra of the cluster calculated by TD-DFT.

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4.4 Molecular electrostatic potential map (MEP)

For complete comprehension of the chemical reaction of the clusters with other ions present in the precursor or in air, we used the molecular electrostatic potential (MEP) maps for the clusters calculated at B3LYP/6-311G(d, p) level. Where, the electrostatic potential of nuclei and electrons of cluster is represented by colors code. There exists three main colors; blue, red and green. The surfaces with blue and red colors represent the positive and negative values of the potential respectively. The surfaces with green colors indicate zero potential. The MEP map for the clusters of theory is shown in Fig. 11 for the clusters. In case of Zn₃O₃ the color ranges from −5.810  $\times 10$ ⁻² (deepest red) to +5.810  $\times 10$ ⁻² (deepest blue). The red color surfaces with negative MEP belong to high electron density, indicating a strong attraction between oxygen and the metal atoms with positive partial charge like Zn²⁺ or H⁺ atoms or carbon to form C⚌O₂ (assessed by IR). The blue color surfaces correspond to areas of lowest electron density. Which tend to attract an O^2–, hydroxide group (O—H)⁻ or Nitrogen atoms N⁻. The green color region means the surface is stable, yellow colure indicating to weak attractive area. That is showing how the Zn atoms fever bind with O atoms in precursors with high annealing during the coalescence of grains and vice versa. Many works demonstrate the importance of annealing and environment conditions on the structural properties of metal oxide (Aljawfi et al., 2018), the present work affirms that the annealing temperature is very important for well crystallization, avoiding the formation of clusters and enhances the UV emission spectra that is be govern for the technical application (Kulyk et al., 2009).

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Fig. 11

Molecular electrostatic potential map (MEP).

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