An accelerated and effective synthesis of zinc borate from zinc sulfate using sonochemistry

Recently, sonochemistry has been used for the synthesis of inorganic compounds, such as zinc borates. In this study using zinc sulphate heptahydrate (ZnSO4·7H2O) and boric acid (H3BO3) as starting materials, a zinc borate compound in the form of Zn3B6O12·3.5H2O was synthesized using an ultrasonic probe. Product’s characterization was carried out with using X-ray diffraction (XRD), Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR) and Raman spectroscopy. Zinc borate compound’s chemical bond structure was observed with Raman and FTIR. From the XRD results it was seen that Zn3B6O12·3.5H2O can be quickly synthesized upon heating at 80°C and 85°C (55 min) or 90°C (45 min) in very high yield (>90%). The minimum particle size obtained was ~143 μm from the SEM results. Zinc borate compound was synthesized at a lower temperature in less time than other synthesized zinc metal compound in literature.

. Zinc borates can be used as non-flammable and fire retardant additives in PVC, halogenated polyester and nylon, as combustion retardants and corrosive inhibitors, in the manufacture of refractory plastic materials, in electric/electronic materials, in the textile and paper industries and as a fungicide/insecticide in wood accessories (Gürhan et al., 2009;Schubert et al. 2002).
Several studies have been carried out towards the synthesis of zinc borate using a hydrothermal method. Gürhan et al. (2009) carried out 3.5 mol hydrated zinc borate' synthesis and examined synthesis parameters' effects. Nies and Campbell (1980) carried out the synthesis of zinc borate using sodium potasium tetraborate (K 2 B 4 O 7 ) and zinc chloride, bromide, sulfate and nitrate as the starting materials at 95°C for 5.5, 6 and 24 h., respectively. Yumei et al. (2006) obtained crystal and hydrophobic zinc borate nanodiscs using borax tetraborate decahydrate (Na 2 B 4 O 7 ·10H 2 O) and zinc sulfate heptahydrate (ZnSO 4 ·7H 2 O) via a hydrothermal method. Oleic acid was used as a modifying agent. The reaction was realized at 70°C for 6.5 h. Igarashi et al. (2004) have synthesized zinc borate using a two-step reaction. This involved heating boric acid (H 3 BO 3 ) and zinc oxide (ZnO) at 60°C for 1.5 h. to form the initial crystals of zinc borate, which were then further heated at 90°C for 4 h. Gao and Liu (2009) studied the synthesis of zinc borate hydrated minerals under reflux conditions with 11 h. Ata et al. (2011) have studied the synthesis of zinc borates using zinc oxide (ZnO) and boric acid (H 3 BO 3 ) at 68-120°C, a rotating speed of 333-767 rpm over 60-200 min. At the end of their study, the optimum yield was obtained at 85°C, 766 rpm and 183 min with the conversion of zinc oxide found to be 99%. Chen et al. (2009) Acarali et al. (2013) examined different modification agents' effects used to procure hydrophobic zinc borate product at between 50-120°C over 1-5 h. Kipcak et al. (2016) obtained zinc borate from boric acid and zinc oxide 1-5 h and at 95°C. Sonochemical systems create multi-bubble environments in which chemical reactions occur with the strong and rapid connection of the reactants. In a sonochemical synthesis, the chemical reactions are carried out using the kinetic energy supplied from acoustic cavitation. The acoustic cavitation ensures high pressure, high temperature and high energy levels to the reactants for a very short period of time and accelerates the reactions with high reaction efficiencies (Lupacchini et al., 2017;Mettin et al., 2015;Timothy et al., 2017).
The hydrothermal synthesis of zinc borates has been widely studied and reported in the literature. In our research group, we have studied the ultrasonic synthesis of zinc borates using ZnO and Zn 5 (CO 3 ) 2 ·(OH) 6 as the starting materials (Ersan et al., 2016). Due to the high cost of the starting materials used these two studies with respect to ZnSO 4 ·7H 2 O, this study's novelty is the cheap and rapid synthesis's of zinc borate by using the method of sonochemistry.

The as-synthesized zinc borate compounds' XRD results and their reaction yields
According to the XRD results, the as-synthesized compound was defined as "zinc oxide borate hydrate (Zn 3 B 6 O 12 ·3.5H 2 O)" with the pdf No. "00-035-0433" and was identical to that previously reported in the literature (Bardakci et al., 2013;Ersan et al., 2016;Kipcak et al., 2014Kipcak et al., , 2015Kipcak et al., , 2016Vardar et al., 2017). The as-synthesized zinc borate compound's XRD results are given in Figure 1.
According to the XRD results, the minimum required reaction time to complete formation of zinc borate was 55 min at 80°C and 85°C and 45 min at 90°C in SET-1; 55 min at 85°C and 45 min at 90°C in SET-2 and 55 min at 80°C, 50 min at 85°C and 45 min at 90°C in SET-3. In SET-1, at 80°C and 50 min the zinc borate was partly synthesized. The lowest XRD score was obtained in SET-1 and the highest XRD score was procured in SET-2. The XRD score is a similarity measure of the as-synthesized compound with the reference pdf card. Meaning that all of the peak intensities (%) and peak locations matched perfectly with the pdf card number of reference mineral, the XRD score of analyzed mineral is equal to 100 .
The compounds' XRD patterns with the highest XRD scores are given in Figure 2. The characteristic peaks are in mutual agreement with the studies reported in the literature (Ersan et al., 2016;Kipcak et al., 2014Kipcak et al., , 2015Kipcak et al., , 2016Vardar et al., 2017). In addition, the as-obtained XRD scores were slightly higher than those previously reported in the literature ( Table 1).
The reaction yields are given in Figure 3, which shows that the reaction yields were increased by the increase in reaction time and temperature. In addition, the highest and the lowest reaction yields were obtained in SET-3, and SET-1, respectively. The highest reaction yields were procured at 90°C over 55 min for all of the sets, which were calculated to be 98.3 ± 0.2%, 98.4 ± 0.2% and 99.2 ± 0.3% for SET-1, SET-2 and SET-3, respectively. In accordance with the XRD scores, the reaction yields were found to be higher than those previously reported in the literature and significantly higher than those reported by Ersan et al. (2016) and Vardar et al. (2017).

The as-synthesized zinc borate compounds' FT-IR and Raman spectroscopy
The optimum zinc borate compounds' recorded IR spectra from chosen reactions are given in

Synthesized zinc borate compounds SEM results
SEM images of the selected samples of zinc borate obtained from each reaction set are displayed in Figure 6.
It can be observed that for all the starting materials, the as-obtained zinc borate compounds were agglomerates of sub-microscale elliptic particles. When the morphology of the as-synthesized zinc borate particles was compared with that of a previous study, in which ZnSO 4 ·7H 2 O was used as the zinc source, it was observed that the use of an ultrasonic synthesis method changed the shape of the as-obtained particles given in Kipcak et al. (2016). For SET-1, products' particle size was observed to be between 942.52 nm and 179.11 nm. On the other hand, when sodium borate was used as the boron source, products' particle size was reduced to 143.41 nm (SET-2). For SET-3, the particle products' particle size was observed to be between 596.63 nm and 156.03 nm.

Conclusion
In this study, zinc borate in the form of Zn 3 B 6 O 12 ·3.5H 2 O was synthesized from various boron sources and ZnSO 4 ·7H 2 O via a sonochemical synthesis method. Based on the results, the following conclusions can be drawn: Ultrasonification reduced the synthesis time of zinc borate to 45 min at 90°C and the reaction yield using these synthesis parameters was 98.4 ± 0.3%. The spectroscopic results showed that the products had the characteristic band vibrations in both the IR and visible-light regions.
The SEM images of the zinc borate compounds proved that the synthesis method influenced the particle morphology and size. Being an energy saving, rapid and effective method makes this sonochemical synthesis an alternative pathway for the green synthesis of zinc borate.   For the sonochemical synthesis, a temperature controlled batch type glass reactor (100 mL capacity) equipped with a cooling jacket was used. Pure water obtain from a GFL 2004 instrument (Gesellschaft für Labortechnik, Burgwedel, Germany) was used as the liquid medium. In the synthesis, the appropriate amount of H 3 BO 3 was dissolved in 25 mL of pure water and heated to the target temperature. ZnSO 4 ·7H 2 O, NaOH and commercial zinc borate seed (1% w/w as H 3 BO 3 ) were then added for SET-1. Na 2 B 4 O 7 ·5H 2 O and Na 2 B 4 O 7 ·10H 2 O were used instead of NaOH for SET-2 and SET-3, respectively. The amounts of the starting materials used are given in Table 2.
A Bandelin Sonopuls HD 2070 (20 kHz) model ultrasonic homogenizer (Bandelin electronic GmbH & Co. KG, Berlin, Germany) was used for the synthesis. After the determined reaction time, the slurry was filtered and washed with purified water (70-80°C) to remove the unreacted H 3 BO 3 and Na 2 SO 4 from the synthesized zinc borate compound. Then, the resulting solid was dried using an EcoCELL 111 model incubator (MMM Medcenter Einrichtungen GmbH, Planegg, Germany) at 105°C to remove the excess water. The reaction procedure is identical to that reported to Ersan et al. (2016). For examine reaction parameters' effects on the as-synthesized compounds, the synthesis procedure was repeated at 80, 85 and 90°C and with reaction times of 40, 45, 50 and 55 min, respectively.

Zinc borate compounds' characterization
The as-synthesized zinc borate compounds were characterized by XRD analysis, Raman spectroscopy and FTIR. The crystalline structure of the products was analyzed using XRD. The reaction efficiency calculations were carried out using ZnSO 4 ·7H 2 O as the limiting reactant. The reaction efficiency (Y D ) was calculated using Eq. 4 (Fogler, 1999): where N D presents consumed key reactant's number of moles. N A0 and N A present key reactant's the initial and final moles. The FT-IR spectra were recorded on a PerkinElmer Spectrum One model FT-IR spectrometer (PerkinElmer, MA, USA) equipped with a universal attenuation total reflectance (ATR) sampling accessory in the spectral range between 1800-650 cm -1 . The as-synthesized zinc borate samples' Raman spectra were measured on a Perkin Elmer Raman Station 400F (PerkinElmer, CT, USA) Raman spectrometer's range between 1600-250 cm -1 . These spectral ranges were selected due to the characteristic vibration range of boron minerals. The morphology and surface properties of the zinc borate particles were observed on a CamScan brand Apollo 300 field-emission SEM (CamScan, Oxford, UK) equipped with a back-scattering electron (BSE) detector at 15 kV. The magnification was set to 10000.