Effect of NaOH concentration on optical properties of zinc oxide nanoparticles

Fig. 1a shows typical XRD patterns of the as-prepared samples obtained at different NaOH concentrations by wet-chemical method. All the peaks in the obtained patterns are well matched with the JCPDS file #79-0206 of the standard ZnO which can be indexed as corresponding to the reflection ofhexagonal wurtzite ZnO. No characteristic peaks of impurity phases were observed. Cell parameters were calculated from the diffraction pattern and the obtained values: a = 3.154 Å and c = 5.023 Å were slightly smaller than those of the standard JCPDS #79-0206 cell parameters, a = 3.249 Å and c = 5.206 Å.

Field emission scanning electron microscopy (FE-SEM) images, shown in Fig. 2, show the morphology of the as-prepared ZnO nanoparticles (C1-C4). All the samples were analyzed at 10 kV accelerating voltage.

Optical emission of the above samples was studied by photoluminescence spectroscopy. The samples were excited at 320 nm excitation wavelength and the emission spectrum of each sample was plotted, as shown in Fig. 3.

In order to study the effect of reaction temperature on the vibrational properties of as-prepared ZnO samples, their room temperature Raman spectra were measured using 633 nm laser source at 10 mW power. Fig. 4 shows the Raman spectra of all the as-prepared ZnO samples.

Comparing the sample diffraction data ( Fig. 1a) with the standard JCPDS #79-0206, slight shift towards lower Bragg angles is observed which represents the presence of uniform strain in the crystallites of the as-prepared samples [24]. The diffraction peaks are also broadened which represents smaller crystallite size of the material. Small variation in the intensity and broadening of peaks represents the rearrangement of crystal plane according to synthesis conditions [23]. Also reduction of the values of cell parameters: a and c indicates the presence of some stress or strain in the crystal lattice of the samples [22, 23]. The strain observed in the samples can be explained by the Uniform Deformation Model (UDM) using Williamson-Hall (W-H) plot [22, 24] which claims that the diffraction line broadening is due to strain (є) effect and reduction in crystallite size. The UDM equation was calculated by using the diffraction peak broadening and is given by [22, 24]:

This equation represents the Uniform Deformation Model (UDM) which assumes that the strain is uniform in all crystallographic directions [22, 24]. The amount of strain was calculated by plotting a graph β cos θ versus 4sin θ shown in Fig. 1b. The two main properties extracted from the plot are the crystallite size and the associated lattice strain.

From the linear fit of the data (Fig. 1b), the crystallite size (d) is calculated from the y-intercept, and the microstrain (є) is obtained from the slope of the linear fit [22, 24]. The strain may be either compressive or tensile. The strain (є_zz) in the samples along c-axis can be calculated using equation [25] given by:

where c_o is the unstrained lattice parameter and c is the lattice parameter calculated from the XRD data. The value of the strain calculated from equation 2 was -3.515. Negative value accounts for the compressive strain [25] and it is also confirmed by the values of microstrain є calculated from the W-H plot of the samples (Table 2). The values of average crystallite size calculated by both UDM method and Scherrer method, and strain for all the samples are listed in Table 2.

From Table 2, it is observed that the crystallite size of the as-prepared ZnO samples is reduced with the increase in the NaOH concentration, which is well supported by the increase in the values of strain in the samples with respect to the NaOH concentration. Various researchers [22-24] have reported that the strain in the samples is inversely proportional to the crystallite size, and this is clearly observed in the above results also. The strain may be caused by the change in the bond-length due to a change in the values of lattice constants (a and c) [23] and due to charge-carrier gradient within the crystal structure, resulting in overall shrinkage of the crystallite size.

Fig. 1c shows the variation in the crystallite size of the as-prepared ZnO nanoparticles (obtained by Scherrer analysis and W-H analysis) with respect to the NaOH concentration.

From the crystallite size vs. molar ratio plot (Fig. 1c), it is observed that there is a small variation in the values of the crystallite size obtained from both the methods. This variation is caused by the difference in averaging the particle size distribution [22]. The curves show a linear and systematic reduction in the crystallite size with an increase in NaOH concentration from 0.1 M (sample C1) to 0.2 M (sample C2) and a sudden drop in the crystallite size on further increasing NaOH concentration to 0.3 M (sample C3) and 0.4 M (sample C4). There may be two possible phenomena responsible for obtaining smaller crystallite size: (a) supersaturation, (b) NaOH reaction within the solution mixture.

Supersaturation is critical because it is a driving force for crystal nucleation and growth. At low supersaturation, crystals can grow faster than they nucleate, which results in a larger crystal size while at higher supersaturation, crystal nucleation dominates crystal growth, ultimately resulting in smaller crystals [26]. The obtained results are in fair agreement with the observation that by increasing the level of supersaturation, the crystallite size is reduced.

During the synthesis, gradual adding of NaOH to the zinc acetate solution may have caused sudden temperature gradient resulting in fast nucleation. It has been suggested that Na⁺ is attracted by the OH^- around the nanocrystal and forms a virtual capping layer, thus, inhibiting the nanocrystal growth [2]. This is explained by the help of following equations:

In equation 4, the product is not necessarily ZnOH42−${\rm{Zn}}\left( {{\rm{OH}}} \right)_4^{2 - }$, but could also be in the form of Zn(OH)⁺, Zn(OH)₂, or Zn(OH)^3-, depending on the parameters, such as the concentration of Zn²⁺ and OH^— ions [2]. Depending upon concentrations of Na⁺ and OH^- ions, the size of nanocrystals varies which could be observed in the obtained results for the samples. This reduction in size may have induced development of strain in the crystal lattice which would have led to an increase in the number of intrinsic and/or extrinsic defects in the particles. These observations could be easily seen in the FE-SEM images of the as-prepared samples also.

FESEM images show the presence of homogenous, well-dispersed particles having well-defined grain boundaries. Particle growth in a wet-chemical process could be explained by the phenomenon of nucleation and then aggregation [2]. Actually, the rate of particle growth is governed by the concentration of the precursors and their reactivity. In the present work, crystal nucleation is dominant because of higher supersaturation of NaOH, resulting in rapid conversion of Zn²⁺ ions into [Zn(OH)4]^2- complexes with relatively low aggregation, which leads to smaller particle size with spherical shape [2, 26] having the average diameter of 60 nm for sample C1 and 50 nm for sample C4. The FESEM images show reduction in the diameter of the ZnO nanoparticles which is in accordance with the XRD peak width (FWHM) analysis and the crystallite size calculations of the samples (Table 2). Table 3 shows the average diameter of as-prepared samples obtained from the field emission scanning electron microscope images.

Fig. 3 shows emission spectra of the as-prepared ZnO nanoparticles excited at 320 nm. Various researchers [3, 21, 22] suggest that the PL spectrum of ZnO generally consists of two emission bands: one in the UV region (near 370 nm to 400 nm) and a broad band in the visible region (400 nm to 700 nm) of the spectrum. Depending upon the intensity and width of the peaks in the corresponding band, one can infer the structure of the material. For instance, in case of nanorods and nanotubes, the PL spectrum mainly consists of a strong and sharp UV emission band and a relatively weak defect (visible) emission peak [2, 21, 27], while in case of quantum dots and spherical nanoparticles, PL spectrum shows a weak UV emission band and a broad visible emission band covering almost entire visible region [3, 22]. These studies and the emission spectra of the as-prepared ZnO samples clearly show that the as-prepared ZnO samples are spherical in shape having nanodimensions which is also confirmed by the FESEM images of the samples.

According to the available reports [3, 21, 22, 27], a defect free perfect crystal of ZnO has a single emission peak in the visible region while an increase in the number of emission peaks indicates the presence of defect levels in the material. The position of the peaks provides information about the depth and type of defects in the material [3].

In the emission spectra ( Fig. 3), a small peak centred between 380 nm to 390 nm (UV region) is observed for all the samples while multiple peaks are observed from violet to green region of the spectrum. The major emission peaks in the visible region are centred between 400 nm to 445 nm (violet region), 445 nm to 495 nm (blue region) and 495 nm to 565 nm (green region). The study of the PL graph also shows that with the increase in NaOH concentration emission peaks around 568 nm (yellow region) and 592 nm (orange-red region) appear for samples C3 and C4 but are absent in samples C1 and C2. Moreover, most of the prominent emission peaks show a shift towards higher energy (blue shift) with an increase in NaOH concentration.

These yellow and orange-red emissions are caused by oxygen interstitials and oxygen interstitials-zinc vacancies complexes respectively [28]. It may be inferred that high O^-/O^2- ion concentration increases intrinsic strain in the lattice of the sample due to trapping of oxygen ions from the reaction solution into the interstitials which causes transitions from conduction bottom to the oxygen interstitial site emitting yellow emissions around 2.2 eV [22, 28]. Moreover, it could have also caused zinc vacancies in the crystal thus resulting in the formation of zinc vacancies complexes causing transitions from vacant zinc sites to interstitial oxygen sites [28]. Most prominent emission bands observed from the emission spectra are in the blue and green region which may be due to lattice defects developed as a result of compressive strain produced at the intrinsic crystal lattice [21, 28]. This leads to the formation of oxygen and zinc vacancies and interstitials simultaneously, which causes transition of energy from conduction band or zinc interstitials to the vacant zinc sites [21, 22, 28] resulting in blue emission, while the same originating from deep levels causes green emissions. The UV, violet and green emission bands, also visible in the spectra, are caused by their corresponding defect sites as reported by various researchers [3, 21, 22, 27].

Further, it is observed that the prominent emission peaks show blue shift with the increase in NaOH concentration. This peak shift may be attributed to the development of strain in the crystal structure which can be easily observed from the Uniform Deformation Model (UDM) explained in Fig. 1b. Comparing the values of strain from UDM analysis, it can be inferred that increasing the supersaturation (NaOH concentration) results in increment in the values of compressive strain, which in turn increases the intrinsic defects in the material as it is clearly observed in the corresponding emission spectra. This strain may have affected the bond length as well as the bond angle of the lattice [22] which could have resulted in the development of various defects in the crystal structure like oxygen vacancy (V_o), zinc vacancy (V_Zn), oxygen interstitial (Oi), zinc interstitial (Zni), oxygen anti-site (O_Zn), and zinc anti-site (Zn_o) [3, 21, 22, 27]. The position of the emission peaks varies with the NaOH concentration and could be related to the depth of the level of defects in the crystal structure of the sample causing Deep Level Emissions (DLE) [28]. Although the exact mechanism of the deep level emissions is still unclear, it could be inferred that these deep levels create intermediate states in the band gap of the material [3].

The PL spectra show that for UV exciton wavelengths, the ZnO samples show high emission intensity in broad visible region, hence ZnO becomes a promising material for applications in the field of UV detectors and sensors as well as photoelectric devices.

Fig. 4 shows room temperature Raman spectra of the as-prepared ZnO samples measured using 633 nm laser source at 10 mW power. Various researchers [30-32] reported that wurtzite ZnO belongs to the space group C6ν4${\rm{C}}_{6\nu }^4$ with two formula units per primitive cell where all atoms occupy C_3ν sites. Also, it is well known that ZnO has eight sets of characteristic optical phonon modes at the center of Brillouin zone (Γ-point), which according to group theory [21, 27, 29] is predicted as:

where A₁ and E₁ are two polar phonon modes, each comprising of longitudinal optical (LO) and transverse optical (TO) components, which are both Raman and IR active; E₂ is non-polar and only Raman active with two frequency modes: E₂ low (E_2L) for Zn sublattice and E₂ high (E_2H) for oxygen sublattice while B1 modes are Raman inactive [30-32]. Strong E_2H mode is characteristic of wurtzite lattice and shows good crystallinity [30].

From Fig. 4, one can find that with an increase in NaOH concentration there is a significant red shift (lower energy) in the peaks centered around 435 cm^-1 for E_2H mode, 531 cm^-1 for 2LA {E_2H+E_2L} mode, 560 cm^-1 for A₁(L0) mode and 765 cm^-1 and 926 cm^-1 for multiphoton scattering [29, 30, 32] as well as blue shift towards higher energy in the Raman peaks centered around 334 cm^-1 for 2E₂(M) {E_2H-E_2l} mode, 387 cm^-1 for A₁ (TO) mode, 480 cm^-1 for 2LA mode, 603 cm^-1 for E₁(LO) mode, 678 cm^-1 for acoustic overtone vibration mode. It also reveals that the samples have nanodimensions [30, 32]. The peaks around 234 cm^-1 may be attributed to the 2E_2L second order phonon mode which may have arose due to presence of zinc sublattice [29, 32].

According to the available reports [29-32] for bulk ZnO, A₁ (LO) phonon mode is commonly assigned to the oxygen vacancies, zinc interstitials or defect complexes containing oxygen vacancy and zinc interstitial in ZnO. Strong multiphonon scattering reveals quantum confinement effect in the material and the optical phonon confinement resulting in phonon frequency shift of few cm^-1 in the sample [32]. These two phenomena are observed in the Raman spectra of the as-prepared nanoparticles also.

Out of the four fundamental characteristic peaks for wurtzite hexagonal ZnO, two dominant peaks at 435 cm^-1 (corresponding to E_2H phonon modes) and other at 560 cm^-1 (for A₁(LO) mode) are red shifted as compared to the corresponding peaks of standard ZnO. Hence, the effect of blue phonon shift could be ignored. Moreover, there is asymmetric broadening in the E₂H mode ∼435 cm^-1) of the samples. This broadening of the spectra, asymmetric line shape and strong red shift of E_2H mode towards lower energy may be attributed to the optical phonon confinement [27, 30-32]. Spatial confinement by the nanostructure boundaries and phonon localization of defects [32] may be responsible for the observed peak shifts in the corresponding Raman spectra with the rise in NaOH concentration. It may also be attributed to the presence of surface defects [32] or deep level defects [29, 31] in the samples. Comparing with the emission spectra ( Fig. 3), one can infer that the deep level defects may have caused the red shift in the Raman peaks [29, 31]. From the study of X-ray diffraction patterns and emission properties studies, it has been shown that these defects are attributed to the compressive strain in the crystals which was found dependent on the NaOH concentration. The obtained experimental results are in fair agreement with the Fonoberov and Balandin theory of optical phonons [31, 32], according to which the asymmetry of wurtzite crystal lattice leads to the quantum dot shape-dependent splitting of the frequencies of polar optical phonons in a series of discrete frequencies.

The obtained results agree with the phenomenon of optical phonons in as-prepared ZnO nanoparticles and could be well-harnessed in the applications of optoelectronic and photoelectronic devices.