Effects of Precursor Concentration and Reaction Time on Sonochemically Synthesized ZnO Nanoparticles

The sonochemical reaction between Zn(NO 3 ) 2 and hexamethylenetetramine (HMT) resulted in different products depending on amounts of Zn(II) cations in the aqueous solution. Crystalline phase pure ZnO nanoparticles were obtained by using 0.05 M Zn(NO 3 ) 2 -HMT. This single phase was formed by low power sonication for 20 min in an ultrasonic bath at ambient conditions and its morphology did not change for the next 60 min. Higher concentrations of precursors (0.1 and 0.5 M) led to a mixture of zinc compounds including plate-like Zn 5 (OH) 8 (NO 3 ) 2 (H 2 O). In contrast, a mixed phase ZnO/Zn 5 (OH) 6 (CO 3 ) 2 and a single phase Zn 5 (OH) 6 (CO 3 ) 2 were formed when the precursor concentrations were decreased to 0.01 and 0.005 M, respectively. The resulting infrared absorption and ultraviolet/visible emission were dependent on the phases and defects in these zinc compounds.


Introduction
Research and development in sonochemically synthesized zinc oxide (ZnO) has led to significant theoretical and technological progresses since 2004 1 .Theoretical studies suggest that the mechanism of ZnO formation involves acoustic cavitation upon ultrasonic irradiation.Jung et al. proposed the evolution of sonicated ZnO nuclei into spheres (isotropic growth), disks (suppression of 1-D growth), rods (1-D growth) and flower-like ZnO (multiple nanorods growth) 2 .In terms of technological advances, multifunctional ZnO with mechanical, physical, chemical and biological utility can now be synthesized using low-cost sonoreactors.Moreover, many commercial ultrasonic probes and baths are also available.A variety of ZnO morphologies ranging from spheres 2 , flower-like 2 , cauliflower-like 3 , needle-like 4 , sword-like 5 , octahedra 6 , nanosheets 7 , hexagonal nanotubes 2,8 to nanorods 1,2,[9][10][11] are obtained by variations in the sonochemical conditions.The morphologies are also dependent on the type of precursors and may result in other zinc compounds.In work by Gusatti et al. 12 , a mixed phase material consisting of Zn 5 (OH) 8 Cl 2 •H 2 O and ZnO was obtained by sonicating ZnCl 2 while under similar conditions the use of zinc nitrate hexahydrate (Zn(NO 3 ) 2 •6H 2 O) produced pure ZnO.Our previous work has shown that sonication of highly concentrated Zn(NO 3 ) 2 •6H 2 O and hexamethylenetetramine (HMT) solutions gives rise to zinc hydroxide nitrate hydrate (Zn 5 (OH) 8 (NO 3 ) 2 (H 2 O) 2 ) [6] .This zinc compound is an intermediate product which transforms into ZnO upon heat treatment.Since the concentration of the precursors clearly plays an important role in dictating the phase and morphology of the products from the sonochemical reaction, its effect and sonication time are investigated in this work.

Experimental Procedure
Reagent grade Zn(NO 3 ) 2 •6H 2 O and HMT powders (≥ 99.0% in purity) were used as precursors without any pretreatment.Aqueous solutions of the two starting materials were mixed under vigorous stirring (600 rpm) and heated to 50 °C.Once the solution turned into a stable viscous white colloid, the stirring was continued for another 10 min.The colloid was then transferred to a commercial ultrasonic bath (Kerry KC3) which, at a maximum electrical power of 75W, supplied a continuous 38 kHz ultrasonic wave under ambient conditions.After sonication, the white precipitate was collected by filtration and thoroughly rinsed with distilled water and ethanol.Finally, the precipitate was dried in an oven at 100 °C in an air atmosphere for 2 h.The effect of concentration was investigated using Zn(NO 3 ) 2 •6H 2 O and HMT concentrations of 0.005, 0.01, 0.05, 0.1 and 0.5 M while keeping the sonication time constant at 60 min.These samples are referred to as F1-F5 listed in Table 1.
To examine the effect of reaction time, four more samples (G1-G4) were prepared from 0.05 M Zn(NO 3 ) 2 -HMT using sonication times of 20, 40, 60 and 80 min.
Powder X-ray diffraction (XRD, Philips X' Pert MPD) of the synthesized products were carried out using CuKα radiation (λ = 1.54056Å).The morphologies were inspected by scanning electron microscopy (SEM, FEI Quanta 400) operating at 25 kV.The particle size of each nanoparticle was measured using the Image J program.The Fourier transform infrared (FTIR) spectra were obtained from pellets of the synthesized powder mixed with potassium bromide (KBr) by using a Perkin Elmer Spectrum One spectrophotometer.A milligram of each sample was mixed and crushed with about 0.2 g of dried KBr.The samples were formed into pellets 13 mm in diameter and 0.5 mm in thickness by mould pressing.The FTIR measurements were performed on the pellets placed in a window of IR radiation with measurements recorded between 350-4000 cm -1 at a resolution of 4 cm -1 .The room temperature photoluminescence (PL) spectrum was measured using a luminescence spectrometer (LS/55, PerkinElmer).

Effect of Zn(NO 3 ) 2 -HMT concentrations
During the reaction, the temperature of the colloid was gradually raised.The heating rate was decreased with the increase in concentration from 0.1675 °C/min (0.005 M) to 0.0578 °C/min (0.5 M).XRD patterns obtained from samples F1-F5 are shown in Figure 1 and the crystalline phases are summarized in Table 1.The concentration of Zn(NO 3 ) 2 and HMT were found to significantly impact the phase and purity of the Zn products.In Figure 1a, sample F1 prepared from the lowest concentration attempted in this study is identified as hydrozincite (Zn 5 (OH) 6 (CO 3 ) 2 ) (JCPDS card No. 01-072-1100) whose structure is monoclinic C2/m [13] .The hydrolysis of HMT releases NH 4 + cations and OH -anions as shown in Equations 1.1 and 1.2 and the dissolution of Zn(NO 3 ) 2 ⋅6H 2 O leads to Zn 2+ cations as indicated in Equation 2. In Equation 3, CO 3 2-is created by the reaction of the OH -anions and CO 2 from the air.The formation of Zn 5 (OH) 6 (CO 3 ) 2 is then produced by the sonochemical reaction shown in Equation 4. (1.1)

( ) 2 ( ) Zn NO H O Zn aq NO aq
(2) In the case of 0.01 M Zn(NO 3 ) 2 -HMT, the XRD pattern of sample F2 in Figure 1b indicates a mixture of hexagonal wurtzite ZnO with a space group of P63mc (JCPDF card No. 01-079-2205) and Zn 5 (OH) 6 (CO 3 ) 2 .The crystalline ZnO is formed by between a higher concentration of Zn(II) radicals and the hydroxide radicals in the sonochemical reactions according to Equation 5.1 and 5.2.(5.1) (5.2)  The peaks of Zn 5 (OH) 6 (CO 3 ) 2 are understandably lower than those of the single phase Zn 5 (OH) 6 (CO 3 ) 2 shown in Figure 1a.In both Figures 1a and b, samples F1 and F2 also contain an amorphous phase so lower and broader peaks contribute to a noisy background.
Since the major peak of Zn 5 (OH) 6 (CO 3 ) 2 at 33° is only just visible in Figure 1c, the XRD pattern of sample F3 (0.05 M Zn(NO 3 ) 2 -HMT) exhibits a single crystalline phase of hexagonal wurtzite ZnO.Compared to the XRD patterns in Figures 1a and b, the higher peak intensity and narrower peaks indicate high product crystallinity and result in the smooth pattern obtained due to its higher signal to noise ratio.This result is in good agreement with previous experimental findings using the same concentration in different sonoreactors 6 .The phase purity of this sample indicates that this concentration is recommended for the synthesis of high purity ZnO from Zn(NO 3 ) 2 -HMT.
By increasing the Zn(NO 3 ) 2 -HMT concentration to 0.1 M, ZnO is still the dominant phase in sample F4 but a second phase identified as monoclinic zinc hydroxide nitrate (Zn 3 (OH) 4 (NO 3 ) 2 ) with a space group C21/c is introduced (JCPDF card No.00-052-0627).As shown in Figure 1d, the diffraction peaks at 12.8° and 25.7° correspond to the (100) and (200) planes.Increasing the concentration of the precursors further to 0.5 M (sample F5) leads to another phase, making a total of three phases which may be indexed in the XRD pattern in Figure 1e.In addition to the characteristic ZnO and Zn 3 (OH) 4 (NO 3 ) 2 peaks, major peaks for zinc hydroxide nitrate hydrate (Zn 5 (OH) 8 (NO 3 ) 2 (H 2 O) 2 ) are found at 9.12° and 18.3° due to the (200) and (401) diffraction planes.Zn 5 (OH) 8 (NO 3 ) 2 (H 2 O) 2 has a monoclinic structure with a space group C2/m and tends to be the dominant product in the case of highly concentrated Zn(NO 3 ) 2 [6]   .The formations of Zn 3 (OH) 4 (NO 3 ) 2 and Zn 5 (OH) 8 (NO 3 ) 2 (H 2 O) 2 in sample F5 are respectively described by the sonochemical reactions in Equations 6 and 7. ))) The effect of the precursor concentration on the morphology of products is revealed by SEM images in Figure 2 and summarized in Table 1.By comparing single phase compounds in Figure 2a and Figure 2c, Zn 5 (OH) 6 (CO 3 ) 2 nanoparticles synthesized from 0.005 M Zn(NO 3 ) 2 -HMT (average diameter 87.8 ± 14.0 nm) are smaller than ZnO synthesized from 0.05 M Zn(NO 3 ) 2 -HMT (average diameter 109.0 ± 20.5 nm).With the sizes of single phase Zn 5 (OH) 6 (CO 3 ) 2 in the range of 60-128 nm and ZnO in the range of 80-146 nm, it follows that Figure 2b reveals a mixed phase of ZnO and Zn 5 (OH) 6 (CO 3 ) 2 spheroids of comparable diameter ranging from 48 to 108 nm with an average value of 81.3 ± 14.7 nm.ZnO and Zn 3 (OH) 4 (NO 3 ) 2 particles in Figure 2d have a larger average diameter of 141.0 ± 24.9 nm.By contrast, the morphology of mixed phases of ZnO/Zn 3 (OH) 4 (NO 3 ) 2 /Zn 5 (OH) 8 (NO 3 ) 2 (H 2 O) 2 in Figure 2e is best described as a microsheet with a size between 0.28 and 1.08 µm.This morphology is identical to a previous observation in Zn 5 (OH) 8 (NO 3 ) 2 (H 2 O) 2 [6]   .FTIR spectra of samples F1-F4 are compared in Figure 3. Predictably, the characteristic band of wurtzite ZnO [2,[14][15][16] in the range of 400-500 cm -1 is strongest in the case of single phase ZnO (sample F3) and weakest in the case of Zn 5 (OH) 6 (CO 3 ) 2 (sample F1).Other marked absorption bands in sample F3 correspond to O-H bending and stretching 17,18 .In the same range of vibrations by residual NO 3 -ions 16 , the carbonate stretches in samples F1 and F2 are observed at 1540 and 1480 cm -1 and similar to the previous report on sol-gel synthesis of hydrozincite 19 .The peaks at 1052 and 830 cm -1 are assigned to the ν 1 and ν 2 stretches of carbonate and are consistent with the carbonates   being bound to an octahedral and tetrahedral zinc centre as expected in the hydrozincite structure 20 .In addition, the band centered at 2345 cm -1 is present exclusively in samples with Zn 5 (OH) 6 (CO 3 ) 2 (samples F1 and F2) and is attributed to the absorption of atmospheric CO 2 on the metallic cation 21 .Interestingly, almost identical absorptions due to CO 2 are found in hydrozincite in the literature 19 .ZnO can be prepared from either Zn 5 (OH) 6 (CO 3 ) 2 [19,22] or Zn 5 (OH) 8 (NO 3 ) 2 (H 2 O) 2 [6] by pyrolysis.However, the aim of this research is to sonochemically synthesize ZnO powders of controlled structure and morphology in a single step in order to reduce the cost of production when this is used in the industry.Therefore, the heat treatment was not attempted in this instance.
In Figure 4, all samples exhibit UV emission peaks at 396 nm in their PL spectra corresponding to the near bandedge recombination of excitons in ZnO [1,[23][24][25][26] .Two peaks at both ends of the visible region from interstitial atoms and vacancies 25 are also observed.The narrow violet emission peak at 412 nm is due to the transition between a shallow defect level and the valence band.The broad red emission peak centered around 650 nm is related to nanostructured ZnO with different morphologies and deep defect levels well below the conduction band.The much higher intensity of this visible peak compared to that of the UV absorption is an indication of a high density of defects in the crystals of ZnO/Zn 3 (OH) 4 (NO 3 ) 2 .The inclusion of Zn 5 (OH) 6 (CO 3 ) 2 in sample F2 clearly shifts the emission peaks to a lower wavelength.

Effect of sonication time
Since 0.05 M Zn(NO 3 ) 2 -HMT in aqueous solutions gives rise to single phase ZnO, this concentration was selected for the study of reaction time.XRD spectra and SEM images of samples G1-G4 sonicated for 20-80 min are shown in Figures 5 and 6 and the results are summarized in Table 1.All samples possess the ZnO wurtzite structure.Without any other impurity peaks, characteristic ZnO peaks are clearly observed at 36.2°, 31.7°,34.3°, 47.5°, 56.5°, 59.6°, 62.8°, 66.3°, 67.8° and 69.0°, corresponding to the (101), (100), ( 002), ( 102), ( 110), ( 103), ( 200), ( 112), ( 201) and (004) planes, respectively.The XRD peak width shows little variation with the sonication time from 20 to 80 min.The crystallite size is calculated from the Scherrer formula as 34-43 nm which are comparable to the other works 22 but the crystallites tend to agglomerate as the sonication time is extended as shown by the SEM studies (Figure 6).Spheroidal nanoparticles are observed irrespective of sonication time.Thus, the phase and morphology formed during the initial 20 min of the sonochemical reaction do not change in the next 60 min.The short sonication time of 20 min is therefore sufficient for the synthesis of ZnO 2014; 17 (2) nanoparticles in this ultrasonic bath much shorter than that currently employed for commercial synthesis of ZnO.
Figures 7 and 8 do not indicate a large difference in IR absorption and UV-visible emission among samples G1-G4.Only ZnO and O-H stretching bands are pronounced in the FTIR spectra indicating the homogeneous ZnO products.UV, violet and red emission peaks resemble those previously observed in sample F3 with single phase ZnO.All these peaks occur at an identical position for samples G1-G4 with varying sonication times.These optical properties confirm the XRD and SEM results that the variation in sonication time from 20 to 80 min does not lead to significant changes in phase and morphology in the ZnO products.Nevertheless, the density of defects, compared by the ratio of UV to visible peak intensities 22,27 , is varied.

Conclusions
ZnO nanoparticles were grown after 20 min of sonication from a 0.05 M Zn(NO 3 ) 2 -HMT solution.
Changing the concentration of the precursors resulted in formation of other zinc compounds.Thus, a mixed phase of plate-like Zn 5 (OH) 8 (NO 3 ) 2 (H 2 O) 2 and ZnO formed when the concentration was 0.5 M. On the other hand, lower concentrations of 0.01 M gave rise to a ZnO/Zn 5 (OH) 6 (CO 3 ) 2 mixture while Zn 5 (OH) 6 (CO 3 ) 2 was the only phase found at 0.005 M. The phases and crystalline defects of these zinc compounds were confirmed by FTIR and PL spectra.Overall, the short reaction time and consistent morphology at 0.05 M suggest this is a promising technique for preparation of ZnO nanospheroids.