SYNTHESIS AND CHARACTERIZATION OF MESOPOROUS ZINC OXIDE NANOPARTICLES

In this investigation, highly crystalline and mesoporous Zinc oxide (ZnO) nanoparticles with the large surface area were synthesized without calcination. Furthermore, the effects of different pH values on structural, physicochemical and textural properties of ZnO nanoparticles were comprehensively investigated. Rietveld refinement implied that the pH variation had significant effects on the crystal structure of ZnO nanoparticles. The phase, molecular and elemental structures confirmed the formation of ZnO as a major phase in all nanopowders. The morphology of ZnO nanoparticles was irregular with an average size of 45± 9 nm. Both phase and atomic structures confirmed the polycrystalline arrangement of ZnO nanoparticles. Moreover, isotherms confirmed the mesoporous structure of all ZnO nanoparticles with superior specific surface area and porosity volume. Thus, owing to the concoction of high crystallinity, superior surface area and porosity volume, resultant ZnO nanoparticles can be effectively employed for diverse multifunctional therapeutic applications.

Furthermore, hydrothermal synthesis of ZnO nanoparticles critically depends on varied synthesis parameters such as molar concentration [36,38] and pH value [30] of precursors, hydrothermal temperature [37] and calcination temperature [29] etc. For instance, an increase in molar concentration of precursors increased to both degree of crystallinity and crystal size of ZnO nanoparticles [36]. The homogenous growth of ZnO particles was observed at a threshold pH value [30]. Similarly, the size of ZnO particles increased with increase in hydrothermal temperature [37], 3 whereas, decreased with precursor concentration [38]. Furthermore, calcination temperature removed the organic compounds, thus produced the pure crystalline structure of ZnO nanoparticles [29]. Baruah et al. [39] investigated the influence of pH variation on the dimensions and morphology of ZnO nanoparticles prepared using hydrothermal process. The investigation reported that the growth of ZnO nanorods-like particles was rapid in alkaline conditions, whereas, growth of ZnO particles eroded in acidic conditions [39]. Similarly, calcination temperature had a significant effect on the morphology of ZnO nanoparticles. Kumar et al. [40] reported that the morphology and size of ZnO nanoparticles transformed from rods-like to short prisms-like with the increase in the calcination temperature. Wang et al. [41] reported that the ageing temperature had a critical effect on the morphology of ZnO nanoparticles, and the different particle morphologies exhibited different electrical conductivity. Koutu et al. [2] investigated the effects Herein, with this motivation, ZnO nanoparticles were hydrothermally synthesized using solid solutions with different pH values. The structural, physicochemical and textural properties of nanoparticles were comprehensively studied using XRD, FTIR, FESEM, HRTEM, EDX, and BET techniques. The crystal structure of nanoparticles was determined using Rietveld refinement. 4 The structure-property correlations were comprehensively discussed in the light of published results.

Materials and method
High purity (99%) chemical reagents were employed without further purification. Zinc nitrate hexahydrate (ZNH, Merck) and sodium hydroxide (SH, Merck) were used. The hydrothermal assisted wet precipitation route was employed for synthesis purposes. ZNH was used as Zn 2+ ion source, and SH was used to control the pH value of the precursor. The synthesis methodology has been schematically shown in Fig 1. Typically, 1 M hydrous precursors of ZNH and SH were separately prepared at room temperature (27℃). Later, SH precursor was drop-wise added into ZNH precursor under vigorous stirring. The pH value of the solid solution was continuously monitored using a calibrated digital pH meter. The five solid solutions of Zn 2+ ions were separately prepared with different pH values. After homogeneous mixing, the milky-white suspension was put in Teflon bottle, sealed in a metallic autoclave and heated at 110± 3℃ in an electric furnace for 24 hours. Upon annealing, the resulting crystals were repeatedly centrifuged and washed with distilled water for five times. Later, the obtained products were dried at 100± 3℃ for 24 hours in an air oven and then grounded into a fine powder. The nomenclatures of synthesized products categorized based on the pH value of their solid solutions have been given in Table 1.
The crystallite size (Xs) was estimated using the Debye Scherrer formula (Eq. 1): Where λ is the wavelength of radiations, β is full width at half maximum (FWHM), and θ is the diffraction angle. In order to determine the size of ZnO crystals, atomic planes (100), (021), was calculated using the peak-area method.
βhklcos (θhkl) = 0.9 + 4 sin( ℎ ) Eq. (2) Where ε is the lattice strain in ZnOcrystals. The Bruker Tensor 27 spectrometer was used to record the FTIR spectra using KBr pellets. The morphology and atomic structure of nanoparticles were studied using FESEM (JEOL JSM-7610F Plus) and HRTEM (FEI Tecnai) electron microscope. The Image-J program was used to measure the dimensions of nanoparticles.
The elemental composition of nanoparticles was ascertained usingBrukerXFlash 4010 EDX tool.
The Autosorb-1-C Quantachrome was employed to measure the specific surface area and porosity of nanoparticles.

Crystal structure
The fitted XRD profiles obtained after Rietveld refinement have been shown in Fig 3. The goodness of fit parameters (σ, Rwp, Rb) and crystal structure parameters of each nanopowder has been given in Table 3. The value of σ for all nanopowders was significantly less than four and was acceptable [51]. With the increase in pH value from 7 to 10, the concentration of stoichiometric ZnOphase increased, whereas, the concentration of ZNH phase decreased. It had been clear that the pH values of nine and 10 were optimal to produce the maximum concentration of ZnO crystals.
Although some disparity in the standard (a= 3.250 Å and c= 5.206 Å) and experimental lattice parameters of ZnO crystals were observed, interestingly, the distortion ratio (c/a) of experimental lattice parameters was close to the standard value of 1.60. The ZnO9 nanopowder exhibited stoichiometric lattice parameters of a= 3.250 Å and c= 5.206 Å, as mentioned in Table 3. The size of ZnO crystals increased with the increase in pH value up to 10 as calculated using various models (Table 3). Apart from it, all models confirmed the nanodimensional regime of ZnO crystals. The growth pattern of ZnO crystals, as suggested by various crystallographic models (Rietveld, Scherrer, and WH-ISM) agreed with the change in crystal size, as suggested by FWHM interpretation. The ZnO crystals in all nanopowders were subjected to tensile strain. Furthermore, the degree of crystallinity increased with the increase in pH value up to 10, as given in Table 3.
The ZnO9 and ZnO10 nanopowders exhibited maximum crystallinity of 98%. Thus, the overall analysis concluded that variation in pH value had a significant influence on the crystal structure of ZnO crystals. The solid solution of nine pH value produced a maximum concentration of ZnO crystals with stoichiometric lattice parameters, nanodimensional size and maximum crystallinity.

Molecular structure
The FTIR spectra of all nanopowders have been shown in

Morphological and elemental structure
The FESEM micrograph of ZnO9 nanoparticles has been shown in Fig 5(a). Particles were irregular in shape and agglomerated. Particles of vivid shapes and size were present in ZnO9 nanopowder. Bigger particles were an agglomeration of numerous smaller particles.
Agglomeration of nanoparticles has been a common feature owing to their high surface energy.
The average size of individual nanoparticles was 45± 9 nm, which was in agreement with the crystal sizes calculated using different models as given in Table 3.
The EDX micrograph of ZnO9 nanoparticles has been shown in Fig 5( Fig 5(b). The weight and atomic ratio of Zn:O were 4.69 and 1.14, respectively, which had been in agreement with the similar results reported elsewhere [55][56].
The HRTEM micrographs showing particle morphology and atomic structure of ZnO9 nanoparticles have been shown in Fig 5(c-d). Particles were confirmed to be irregular in shape and agglomerated [57]. The average size of particles was 47± 9 nm. Furthermore, the SAED pattern (Fig 5d) also supported the polycrystalline structure of ZnO crystals in agreement with the XRD phase analysis (Fig 2) [58]. The hexagonal pattern of spots indicated the highly crystalline composition of the particles.

Mesoporous structure
The isotherms and pore size distribution curves (inset) of all nanopowders have been shown in Fig 6. The isotherms of all nanopowders were of type IV with H3 hysteresis loop, suggested the mesoporous structure of nanomaterials. Furthermore, isotherms depicted that the gas adsorption capacity of nanoparticles in increasing order was ZnO10< ZnO12< ZnO9< ZnO8< ZnO7. Moreover, the BJH curves (inset) suggested the range of diameter of pores between 5-40 nm, which further confirmed the mesoporous structure of all nanoparticles. The average values of specific surface area and pore volume of nanoparticles have been given in Table 4. Except for ZnO12, the specific surface area of nanoparticles in increasing order was ZnO10<ZnO9< ZnO8< ZnO7. It had already been attributed to the fact that the smaller the particle size (Table 3), greater is their surface area. Interestingly, the surface area of synthesized 15 nanoparticles was greater than the results reported elsewhere [57][58][59]. Notably, particles with large particle surface area and porosity volume favour the drug delivery and other similar therapeutic applications requiring superior adsorption capacity.

Conclusion
The ZnO nanoparticles were successfully derived from ZnO solid solutions with different pH values. A facile hydrothermal rote was employed to prepare the ZnO nanoparticles. The pH variation had little effect on phase and molecular structures of ZnO nanoparticles. On the other hand, pH variation had a significant effect on crystal structure parameters like weight fraction of constituent phases, lattice parameters, crystal size, lattice strain and crystallinity of ZnO nanoparticles. Results implied that the pH value of nine produced the highest concentration of ZnO phase with stoichiometric lattice parameters, nanodimensional crystal size and highest crystallinity. Moreover, ZnO nanoparticles were irregular in shape with an average size of 45± 9 nm. The particle sizes calculated using Rietveld, Scherrer, WH-ISM, FESEM and HRTEM models were close to each other. All nanoparticles exhibited a mesoporous structure with superior surface area and porosity. The variation of pH had a significant effect on particle size, and therefore, particle surface area attribute was also dependent on their pH condition during synthesis. Interestingly, the particle surface area of synthesized nanoparticles was superior to the results reported in the literature.
Thus, due to the amalgamation of high crystallinity, mesoporous structure, and large particle surface area, the resultant nanoparticles can be employed for several multifunctional therapeutic applications, including drug delivery agents.

Disclosure statement:
There is no conflict of interest related to the present work.
Funding information: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.