Synthesis and processing technique to reduce the particle size of ZnO pellet by hybrid vibrational annealing and dry quenching set up


 The reduction in particle size of ZnO pellet in the nanoscale range was achieved using a hybrid thermo-vibrational annealing and dry-quenching set-up, which was designed and fabricated in-house and is simple and low-cost to operate. Initially, ZnO pellets were synthesized through pyrophoric method and then annealed at 8000C in this processing device. To confine grain growth during the recrystallization stage of annealing, vibrational energy was applied during annealing and dry-quenching to cool. This step-by-step post-synthesis technique involves the simultaneous application of thermal and mechanical vibrational energy to the ZnO pellet for 4 hours, followed by vibrational-dry quenching. Both annealed pellets, i.e., ZnO annealed without vibration and ZnO annealed with vibration, were studied using XRD, SEM, HRTEM, FTIR, UV-Visible, and Raman spectroscopy to conduct a comparative investigation of various structural, microstructural, and optical properties. Although both ZnO pellets had polycrystalline hexagonal wurtzite structures without any secondary phases, the average crystallite sizes of ZnO pellets with vibration were smaller than those without vibration, which were 36.566nm and 25.308nm, respectively, according to the XRD data. Analysis using SEM and HRTEM yielded similar confirmatory results. FTIR and Raman examinations revealed the presence of various functional groups and vibrational modes, which were confirmed by experimental results. The greater optical bandgap of 3.36 eV seen in the UV-Visible spectra of the ZnO with vibration sample indicates that it is a promising material for the creation of enhanced electrical, optoelectronic, and sensor devices based on ZnO nanoparticles, among other applications.

One of the most important properties of ZnO is an n-type II-VI compound semiconductor material with a wide and direct bandgap of 3.37eV [16] [16] and its high 60meV exciton binding energy at 300K [17]. For photonic devices that can be operated at room temperature (RT), an exciton binding energy high enough to enable near-band-edge exciton emission even at room temperature or high temperature is an attractive option [18]. In addition, excellent photocatalytic nature of ZnO makes it a suitable candidate for fossil fuels [19]. Research is ongoing to further enhance its usefulness in a variety of scientific domains by manipulating its size in the nanoscale region, which has numerous advantages (1-100nm). It's because ZnO's physical, optical, and magnetic properties in this size range are superior to those in its broader size range, and this is why.
In order to reduce the particle size of ZnO pellet to the nanoscale, many energy-saving methods have been described, which are divided into three phases, such as solid, liquid, and vapour phase, which are carried out via different routes (like physical, chemical and biological) [20].
For example, various widely used methods are co-precipitation method [21], hydrothermal method [22][23], spray pyrolysis [24], microemulsion process [25], sol-gel [26][27], and solution combustion synthesis (SCS) technique [28]. Because it is less expensive, simpler, greener, and uses less energy than other approaches, the combustion process has become increasingly popular for synthesising ZnO powders. In the combustion method, well-crystalline nano-powders with high purity are formed in a shorter time without the use of expensive instruments. Recent studies have shown that the particle size of ZnO powders ranged between 20nm-50nm using the SCS method [29][30][31][32]. In addition, doping with transition metals (Co, Ni, etc.) and lowering the annealing temperature have both been found to reduce particle size [33].
However, the synthesis of ZnO powder, which is not suited for many device applications, such as gas sensors, solar cells, varistors, and spintronics, is a significant shortcoming of such approaches. As a result, pellets or thin films of material are frequently required.Although some researchers have reported the production of ZnO pellets with a particle size in the nanoscale [34][35][36][37], the big question arises whether it is possible to reduce the grain and particle size of MOS materials during annealing, which generally increases during the recrystallization stage. To the best of our knowledge, studies on the concept of reduction of grain or particle size of oxide semiconductor pellets during heat treatment are yet to be available.
To address the current gaps in knowledge, we therefore made ZnO pellets by applying mechanical vibrational energy during annealing followed by vibrational dry quenching using hybrid thermo-vibrational annealing and dry-quenching setup. This was done to obtain nanograins and particles after synthesizing ZnO pellets by the pyrophoric method. The entire assembly was made inexpensively and with less instrumentation. A comparative analysis was carried out between ZnO without vibration (ZWOV) and ZnO with vibration (ZWV) pellets through a systematic study of their structural, microstructural, morphological, and optical properties using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) spectroscopy, High-Resolution Transmission Electron Microscopy (HRTEM), Fourier Transform Infrared (FTIR) spectroscopy, UV-Visible, and Raman spectroscopy.  3], and nitric acid (HNO3). All the chemicals are brought from Merck company, maintaining 99.9% purity. Throughout the experiment, distilled water was used as per the requirements. To obtain our desired sample of a particular amount, zinc acetate dihydrate, ethylene glycol, and glycerol were added in a beaker with proper measurements followed by the addition of distilled water as per the requirements. Then the mixture was continuously stirred using a glass rod until the formation of a transparent solution. After that, 2-propanol, and tri-ethylene amine (TEA), were added with stirring slowly for a few minutes, then filtrated to obtain a dust particle-free clear solution. For the auto-combustion process, 20ml of TEA,10ml of nitric acid, and some distilled water were added to the solution with proper mixing, which was then allowed to heat at ~200 0 C, keeping it on a hot plate. After nearly 15-20 minutes, combustion of the solution occurred, and the solution turned into a black color powder. The obtained powders are collected in an alumina crucible and calcinated in a hightemperature muffle furnace at 700 0 C for 5hrs to remove carbon and other impurities if present.
To get more fine powders of ZnO, it was grinded several times in an agate mortar. Then by adding 4-5 drops of polyvinyl alcohol (acts as a binder) and applying 6 tons of pressure using a hydraulic press, the fine powders were pelletized of diameter 15mm and thickness 3mm. For the removal of internal stress and lattice defects, the pellets were annealed at 800 0 C for 4 hours by using a "hybrid thermo-vibrational annealing and dry-quenching setup". Detail description of this setup and its principle is discussed below.

2.2Detail description of the Experimental setup
Scheme 1represents a perspective view of our designed nano-crytalline material processing device having the facility of thermo-vibrational annealing as well as vibrational dry quenching.
Here, vibrational energy is applied to the synthesized ZnO pellets during annealing and dry quenching to reduce its grain size during the recrystallization stage of annealing as well as quenching time.
The whole setup comprises two chambers, one over the other connected with a mechanical vibrator. The first chamber is used for annealing, and the second is for dry-quenching of the pellet, where the platform is associated with a huge vibrator providing vibrational energy during annealing as well as dry-quenching.
The first chamber consists of a horizontal tube furnace, an alumina crucible, sample, thermocouple, inert gas inlet, and outlet port. The maximum temperature range of this tube furnace is limited upto 1000 0 C, which is measured by the thermocouple associated with it.
Further, the two ports (i.e., gas inlet and outlet) which are attached to the walls of this chamber are used for providing an inert atmosphere within it. Here, we have used argon gas that is inserted through the inlet port and exits through the outlet port to prevent contamination from other impurities present in the surroundings.
The second chamber consists of primary, secondary, and ternary containers, a lid cover, gas inlet, outlet port, ice cubes, multiple thermo-cool sheets, silica aerogel, and an exhaust fan. The annealed pellet is subjected to dry-cooling in the second chamber to obtain defects-free nanocrystalline material. The primary container is used for placing the annealed sample for dry-quenching purposes. The gap between primary and secondary containers is filled with many small pieceof ice for sudden cooling of the annealed pellet. Further, multiple thermocool sheets and silica aerogel, which are good thermal insulators, are used inside the gap between inner and outer metallic containers, as shown in Scheme 1, to prevent the radiations from the surroundings enter the chamber. For the removal of water vapours if present inside the walls of the primary container, an exhaust fan is also connected.
Scheme 1Schematic diagram of thermo-vibrationalannealing attached with vibrational dry quenching setup

2.3Working
First, annealing of the cold-worked pellet was done at 800 0 C for 4 hours, keeping it at the middle of the horizontal tube furnace. Here, a small rectangular shape of alumina crucible was used as the sample holder. To avoid heat radiation, both ends of the tube were closed with two thick iron plates. To prevent contamination from the surroundings, an inert gas environment was created in the furnace by a continuous flow of Argon gas through two narrow pipes (gas inlet and outlet), which were inserted through two small holes of the plates, as shown in Scheme 1. Temperature measurement was done by using the thermocouple during the annealing process. After annealing, the sample was suddenly put into the primary container of the second chamber by opening the lid cover for a quick cooling process. To avoid grain regrowth, vibrational energy is also applied during quenching. Some ice pieces were taken inside the inner metallic container for dry quenching.The exhaust fan was also used to remove any form of moisture that could occur during this process.

2.4Sample Characterization
The analysis of crystal structure, phase identification, and average crystallite size of both the prepared ZnO pellets ZWOV and ZWV were determined by X-ray diffractometer (model-H12, Rigaku Ultima IV, Japan)using CuKα radiation( =1.54A 0 ) in the 2θ range of 20 0 -90 0 with the scan rate of 0.01 θ/s. Surface morphological including the shape and size of grains were studied through Scanning Electron Microscopy(SEM) by using Hitachi-S3400N with the operating potential of 15kV.High Resolution Transmission Electron Microscopy(HRTEM) of both samples were carried out to accurately study their particle size and structure by using JEOL F200, operating at 200kV. Using image-J software, average grain size and particle size were calculated. The presence of several functional groups was investigated by Fourier Transform Infrared Spectroscopy (FTIR) using Spectrum Two-104789in the wavenumber range of 400-4000cm -1 . The absorption spectra were recorded using a UV-Visible Spectrometer with Shimadzu UV-2450 in the wavelength range of 200-800nm at room temperature to determine the sample bandgap. Finally, Raman spectra of both the annealed ZnO pellets were observed using a Raman spectrometer with Diode LASER ( =437nm).

Scherrer method
It was observed that the value of ' ' also decreases withtheapplication of mechanical vibration to the sample. For ZnO without vibration (ZWOV) and ZnO with vibration (ZWV), ' ' was found to be 47.791A 03 and 46.784A 03 , respectively.
The bond length(L) of Zn-O was calculated by using the following equation [39], Where, is the position parameter which is given by, Table 3 represents a comparative calculated value of ⁄ ratio, unit cell volume( ), bond length( ) of ZWOV,and ZWVpellets. The reason forthe slight decrease of unit cell volume and bondlength in ZWV sample is due to the reduction of lattice parameters. As there is no change in the ⁄ ratio(~1.6) as per the hexagonal structure, we can say that there is no impact of vibrational energy on the crystal structure of ZnO lattice.

3.1.2Uniform Deformation Model(UDM)
The Similarly, = 4 Now putting the values of Eq. (10) and (11) in Eq. (9), we have = + 4 => = + 4 (12) Eq. 12 represents a straight line equation, whereby plotting a graph between (in Yaxis) and 4 (in X-axis), both crystallite size( ) and microstrain( ) can be determined from the Y-intercept and slope of the graphrespectively, which is given in Table3. Fig.2 represents the W-H plot of both ZWOV and ZWV pellets.A decrease of lattice strain in ZWV pellet was observed, which may be due to the reduction of lattice defects like voids present in the sample due to vibrational annealing.   annealed 'without' vibrations. The formation of smaller grains of ZWV sample was confirmed from its histogram graphs, as shown in Fig.3. This proves that with applying mechanical vibrational energy to the sample during annealing, the surface-to-volume ratio increases with a reduction in grain size. This little porosity with smaller grains of ZWV sample makes this material more promising to be used for gas sensing applications. Agglomeration of grains at some places in ZWV reveals the existence of more attractive forces between nanoparticles due to the high surface energy of the sample [41]. We go forward for HRTEM analysis to further calculate average particle size.

Energy-dispersive X-ray (EDX) spectra analysis
EDX spectra of both the annealed pellets are shown in Fig.4a, b. These spectra reveal the high purity of the annealed pellets as they don't contain any other impurity phases except 'Zn' and 'O' elements due to high annealing temperatures (i.e., at 800 0 C). The peaks obtained at 1keV, 8.6keV, and 9.6keV represents 'Zn,' and that observed at 0.5keV represents 'O' atom. This is in good agreement with the XRD result, which confirms the successful formation of a singlephase ZnO lattice. The weight and atomic percentages of Zn and O of the two samples are shown in Table 4.

High Resolution Transmission Electron Microscopy (HRTEM) analysis
A deep understanding of the size, shape, and arrangement of particles can be obtained from a High Resolution Transmission Electron Microscopy (HRTEM) image. Fig.5a, b represents HRTEM images of synthesized ZnO pellets annealed in the presence and absence of mechanical vibrational energy, i.e., ZWOV and ZWV. From these images, it can be observed that our synthesized ZnO pellets are nearly spherical in shape with little porous structure, which is distributed randomly inside the material. But, as compared to the ZWOV sample, the particle size of the ZWV sample is quite small. The calculated average particle size obtained from the histogram of these corresponding images of ZWOV and ZWV samples are 36.185nm and 25.221nm, respectively, which are in good agreement with XRD results.

3.5Fourier Transform Infrared Spectroscopy (FTIR) Studies
Fourier transform infrared (FTIR) spectroscopy of bothZWOV and ZWV samples were carried out to detect several functional groups and chemical bonds associated with the samples. Fig.6 represents the FTIR spectrum of both the prepared samples, recorded in the wavenumber range of 400-4000cm -1 . Different absorption peaks observed from these spectra are 698.583cm -1 , 885cm -1 , 1033.217cm -1 , 3375cm -1 and 698.701cm -1 ,885cm -1 , 1033.347cm -1 , 3375cm -1 for ZWOV and ZWV samples respectively. The absorption peak found at 698.583cm -1 and 698.701cm -1 corresponds to the C-H out-of-plane bending of alkene groups. The absorption peaks observed at 885cm -1 are ascribed by the presence of some carbonated impurities in the samples [42][43]. The peaks in the range of 1000-1250cm -1 correspond to the vibrational stretching mode of C-N bond, which may result from the reaction of the primary amine group with zinc acetate dihydrate [44][45]. The broad peak obtained at 3375cm -1 is attributed to the vibrational mode of the O-H bond [46]. A slight shifting of peak position is observed in these spectra, which may be due to the effect of particle size as reported in different studies [47][48]. The optical bandgap of both the samples was investigated by using the following Tauc's equation [49], where α is the absorption coefficient, hν is the energy of the photon; A is a constant, and Eg is the bandgap to be evaluated. The inset of Fig.7 From Tauc's plot, the calculated bandgaps of ZWOV and ZWV samples are 3.33eV and 3.36eV, respectively. Increase in bandgap of ZWV sample implies a decrease of particle size obtained by applying mechanical vibrational energy during annealing.
Due to this higher bandgap, our synthesized ZWV material can be selected as a suitable candidate for high performance electronic and optoelectronic devices.

Raman Spectroscopy Studies
In order to identify several vibrational modes present in the crystal, lattice defects or any structural disorders, Raman spectroscopy has been used as the most versatile tool. Fig.8 represents room temperature Raman spectra of both the annealed ZnO pellets i.e., ZWOV and ZWV recorded in the range of 200-1200cm -1 . As per the group theory, total phonon normal modes observed at the gamma point of Brillouin zone of a hexagonal wurtzite structured ZnO lattice is 2A1+2E1+2B1+2E2. Out of these eight modes, one set of A1 and E1 i.e., (A1, E1) are acoustic mode, whereas the remaining set of six modes i.e., (A1, E1,2B1,2E2) are optical modes.
Two polar optical modes A1 and E1 are infrared active and are divided into transverse and longitudinal modes denoted as (A1(TO), A1(LO)) and (E1(TO), E1(LO)). Similarly, the non-polar phonon mode E2 is categorized into high and low frequency phonon modes i.e., E2H and E2L [50][51][52]. These E2H and E2L are associated with oxygen atoms and Zn sublattice respectively [57]. As shown in Fig.8 [54]. Similarly, high frequency E2H vibrational mode are assigned to the highly intense peaks at 435.728 and 435.159cm -1 for ZWOV and ZWV samples respectively [55]. As we can see from Fig.8, the peak intensity of the highly intense peak obtained for E2H mode of ZnO pellet decreases with peak broadening on the application of vibrational energy during annealing as well as dryquenching, indicating the decrease in particle size. This result matches well with the XRD result, and this band is called as the characteristics band of wurtzite structure. The peaks centered at 579.729 and 576.885cm -1 are assigned to E1(LO) modes, which may be due to oxygen vacancies, free carriers and zinc interstitial [56]. The broad peaks around 1153.732 and 1150.443cm -1 are due to the optical overtone [57]. A narrow peak centered at 1184.635cm -1 of ZWOV sample may exist due to the second order vibration.

Fig.8 Raman spectrum of ZWOV and ZWV pellets 4Conclusions
A successful synthesis of ZnO pellets were performed through the pyrophoric method followed by annealing at 800 0 C for 4 hours using "vibrational annealing and dry quenching" setup. To observe variation in different properties like structure, morphology, microstructure, particle size, composition, optical bandgap and several vibrational modes, a comparative analysis between ZWOV and ZWV samples was studied through several characterizations such as XRD, SEM, EDX, HRTEM, FTIR, UV-Visible and Raman spectroscopy. XRD results reveal a hexagonal wurtzite structure with no impure phases in both the samples. Also, a decrease in average crystallite size of ZnO pellets, annealed in presence of mechanical vibrational energy was observed as compared to that of pellets annealed without vibration, which are found to be 36.566nm and 25.308nm respectively. SEM images signify slight porous arrangement of grains in ZWVwith mean grain sizecalculated from its corresponding histogram, was found to be 68nm, whereas it was nearly 130nm in ZWOV sample. EDX spectra confirm the high purity of both the annealed samples. HRTEM images clarifythe decrease in particle size of ZWV samples in comparison to ZWOV samples. FTIR shows the very slight shifting of most of the absorbance peaks towards higher wavenumber sites, which may result due to formation of smaller particle size. The optical bandgaps of both the samples were compared through UV-Visible spectroscopy. From UV-Visible analysis, the increase in bandgap energy (Eg) from 3.33eV to 3.36eV of ZWV samples confirmed the lowering of particle size. From Raman spectroscopy, in addition of the presence of several vibrational modes, a decrease in peak intensity with little broadening was observed in ZWV sample, which corresponds to the XRD results. This works gives a new idea to the scientific community for the synthesizing ZnO pellets of decreased particle size within less instrumentation and low cost, which enhances its ability to be used in different area of nano-fields such as gas sensors, optoelectronics, photocatalysis and several high-energy storage electronic devices.