Abstract
In this study, Zn3B6O12·3.5H2O, a type of a zinc borate hydrate, was synthesized from the sodium borate mineral Na2B4O7·5H2O. Two different zinc sources, i.e. ZnSO4·7H2O and ZnCl2, were used in the hydrothermal synthesis. Products were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. Product morphologies were studied using scanning electron microscopy (SEM). Then optical absorption characteristics and electrical properties were investigated. Based on these results, Zn3B6O12·3.5H2O was obtained under many synthetic conditions as a single phase with high reaction efficiencies and sub-micrometer (100 nm to 1 μm) particle sizes. The electrical resistivity and optical energy gap were found as 8.8×1010 Ω cm and 4.13 eV, respectively. The novelty obtained in this study is the synthesis of zinc borate hydrate compound with high crystallinity without using any modification agent or organic solvent.
Introduction
Zinc borates are well-known members of the metal borate family because of their high heat resistance properties, which enable them to be used as fire retardants in the plastic and rubber industries. They also have applications as char promoters, as anti-arcing agents, in the preservation of wood composites, as modifiers of electrical-optical properties and as after-glow suppressant additives (Shen and Ferm, 1996; Ivankov et al., 2001; Tian et al., 2006; Shi et al., 2008; Kilinc et al., 2010). One of the most common commercial forms of zinc borate is 2ZnO·3B2O3·3.5H2O (Shen and Ferm, 1996). Particles of this material must be small in size to obtain an effective and uniform distribution. Additionally, there are other forms of zinc borate hydrates such as 2ZnO·2B2O3·3H2O, 4ZnO·B2O3·H2O, Zn2B6O11·3H2O, Zn2B6O11·7H2O and Zn3B10O18·14H2O (Shi et al., 2008; Tian et al., 2008; Gao and Liu, 2009; Gurhan et al., 2009; Zheng et al., 2009; Bardakci et al., 2013; Gao et al., 2013). Hydrothermal synthesis is commonly preferred for the preparation of zinc borates. In the hydrothermal procedure, the zinc and boron sources are dissolved in a liquid medium. They react due to the temperature increase and use of modification agents (Gurhan et al., 2009; Kilinc et al., 2010; Li et al., 2010; Tugrul and Acarali, 2011; Gao et al., 2013). The characterization of synthesized compounds plays a significant role in scientific studies and industrial applications. The characterization of hydrated zinc borate minerals is commonly based on the determination of the hydrophobicity and thermal properties based on the thermodynamic and dehydration behavior (Speghinia et al., 2000; Gurhan et al., 2009; Kilinc et al., 2010; Li et al., 2010; Tugrul and Acarali, 2011; Gao et al., 2013).
Gao and Liu (2009) synthesized zinc borate hydrated minerals at the boiling point of the solvent and with a reaction time of 11 h. Bardakci et al. (2013) synthesized zinc borate hydrate using the raw materials zinc oxide (ZnO) and boric acid (H3BO3) at 95°C for 2 h. Tugrul and Acarali (2011) studied the effects of different modification agents to obtain hydrophobic zinc borate hydrate minerals. Zheng et al. (2009) prepared 4ZnO·B2O3·H2O after 7 h using phosphate esters as a modification agent. The mineral 2ZnO·3B2O3·3.5H2O was produced using oleic acid as a modification agent at 95°C and 4 h (Li et al., 2010). Gao et al. (2013) investigated the effects of different modification agents on the morphological properties of synthesized zinc borate hydrates. Mergen et al. (2012) studied the effects of zinc borate addition to PVC to strengthen the flame retardant features. Additionally, the electrical and optical characterizations of the zinc borate compounds have only been studied in the dehydrated forms (Longo et al., 1998; Speghinia et al., 2000; Ivankov et al., 2001).
As observed in the literature, the synthesis of zinc borate particles with higher crystallinities requires high reaction temperatures (≥95°C), long reaction times (≥2 h) and modification agents (i.e. oleic acid) to obtain homogeneous particle size distributions (Longo et al., 1998; Speghinia et al., 2000; Ivankov et al., 2001; Shete et al., 2004; Tian et al., 2008; Gao and Liu, 2009; Gurhan et al., 2009; Zheng et al., 2009; Kilinc et al., 2010; Li et al., 2010; Tugrul and Acarali, 2011; Mergen et al., 2012; Bardakci et al., 2013; Gao et al., 2013). There are two novel contributions of this study. The first contribution is the preparation of zinc borate hydrate minerals with high crystallinities under moderate conditions without using any type of organic solvent or modification agent. Second, the effects of the reaction temperature and reaction time on the morphologies of the synthesized minerals were investigated. Hence the electrical and optical characterizations of zinc borate compounds have been studied only in the dehydrated form, the investigation of the electrical and optical properties of hydrated zinc borate compounds is the second novel contribution of the study.
Results and discussion
Results of the raw material characterization
Based on the X-ray diffraction (XRD) results of the starting materials, the zinc source of zinc sulfate heptahydrate (ZnSO4·7H2O) was identified as a mixture of Bianchite (ZnSO4·7H2O, powder diffraction file (pdf) number 01-075-0949) and Goslarite (ZnSO4·6H2O, pdf: 00-009-0395). The other zinc source was determined to be a mixture of two different zinc chlorides (ZnCl2) with pdf numbers ‘01-074-0519’ and ‘01-070-1284’. The boron sources were identified as sassolite (H3BO3, pdf: 01-073-2158) and tincalconite (Na2B4O7·5H2O, pdf: 00-007-0277).
XRD results of the synthesized zinc borates
The XRD scores of the zinc borates, which were synthesized at different temperatures and different reaction times are given in Table 1 [XRD scores can be explained as; when 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 (İbroşka et al., 2015)]. The type of synthesized zinc borate was found to be zinc oxide borate hydrate (Zn3B6O12·3.5H2O), with pdf number 00-035-0433. The same zinc borate compound was synthesized in the studies of Bardakci et al. (2013), Kipcak et al. (2014a,b) and Kipcak et al. (2015). The XRD scores obtained from the reactions of ZnSO4·7H2O, Na2B4O7·5H2O, H3BO3 (ZS-T) and ZnCl2, Na2B4O7·5H2O, H3BO3 (ZC-T) were plotted against the reaction time and temperature using StatSoft Statistica software, where the y-axis shows the XRD score, the z-axis shows the reaction temperature and the x-axis shows the reaction time; this plot is given in Figure 1. The results shown in Figure 1 indicate that the XRD scores of the zinc borates increased with increasing time and temperature.
XRD scores of the synthesized zinc borates.
| Temperature (°C) | Time (h) | Set 1 (ZS-T) | Set 2 (ZC-T) |
|---|---|---|---|
| 90 | 1 | 6a | – |
| 2 | 63a | 72 | |
| 3 | 72 | 76 | |
| 4 | 77 | 80 | |
| 80 | 1 | 6a | – |
| 2 | 27a | 70 | |
| 3 | 68a | 75 | |
| 4 | 72 | 77 | |
| 70 | 1 | 14a | 23a |
| 2 | 26a | 32a | |
| 3 | 40a | 48a | |
| 4 | 69 | 68a |
aCrystal formation was not completed.
XRD score graph of the synthesized zinc borates according to the reaction time and reaction temperature.
In the ZS-T experiments, zinc borate was formed using a reaction temperature of 70°C and a reaction time of 4 h. In contrast, at the same temperature, the formation of zinc borate did not occur in the ZC-T experiments. The zinc borates were coded in the format ‘set code-reaction temperature-reaction time’, where the set codes were ‘ZS-T’ and ‘ZC-T’. For example, the product synthesized at 80°C and 4 h using ‘ZnSO7·7H2O-Na2B4O7·5H2O-H3BO3’ was coded as ‘ZS-T-80-4’. At a reaction temperature of 80°C, the zinc borate formation started after 1 h of reaction, but complete formation was only achieved after 4 h of reaction in the ZS-T set. In contrast, in the ZC-T set after 1 h of reaction, amorphous zinc borate was obtained. Crystalline zinc borate formed after 2 h of reaction, and better XRD scores were obtained after 3 and 4 h of reaction. At reaction temperatures of 80°C and 90°C in the ZS-T set, the formation of zinc borate began after 1 h of reaction. Complete formation was achieved after 3 h of reaction, and the XRD score increased for 4 h. The increase in the reaction temperature decreased the reaction time by 1 h for the ZS-T set. At the same temperature in the ZC-T set, amorphous zinc borate was again obtained after 1 h of reaction. Crystalline zinc borate was formed after 2 h of reaction, and better XRD scores were obtained as the reaction time increased to 3 and 4 h. The XRD scores of the ZC-T set were found to be higher than the ZC-T set. Some selected zinc borate XRD patterns are shown in Figure 2. In optimal zinc borate, formations were given as 95°C after 2 h, 100°C after 2 h and 90°C after 2 h in the studies of Bardakci et al. (2013), Kipcak et al. (2014a,b) and Kipcak et al. (2015), respectively. It is seen that the XRD results were in agreement with the literature.
XRD patterns of selected zinc borates.
According to Figure 2, the characteristic peaks and respective miller indices (h k l) and d spacing [Å] of synthesized zinc borates were determined to be 17.7° (1 1 -1) [5.88 Å]; 18.1° (0 2 0) [4.90 Å]; 20.6° (1 0 1) [4.30 Å]; 21.8° (1 2 0) [4.08 Å]; 23.7° (1 2 -1) [3.74 Å]; 25.8° (2 1 0) [3.44 Å]; 28.8° (0 1 2) [3.10 Å]; 30.2° (2 2 1) [2.95 Å]; and 36.8° (2 3 0) [2.44 Å].
FT-IR and Raman spectral analysis results for the synthesized products
Fourier transform infrared spectroscopy (FT-IR) spectra of selected zinc borates are shown in Figure 3. In the FT-IR spectra, the asymmetrical [νas(B(3)-O)] and symmetrical [νs(B(3)-O)] stretching bands of three-coordinate boron to oxygen were observed in the wavenumbers of 1407–1251 and 919–916 cm-1, respectively. The IR peaks in the range of 1062–1059 cm-1 corresponded to the asymmetrical stretching of the four-coordinate boron to oxygen bands [νas(B(4)-O)]. The symmetrical stretching bands of four-coordinate boron to oxygen [νs(B(4)-O)] were observed between 856 and 791 cm-1. The bending mode of boron-oxygen-hydrogen [δ(B-O-H)] was observed between 1188 and 1107 cm-1. The peaks at approximately 750 cm-1 were attributed to the [νp(B(OH)4-)] band, while the bending mode of the three-coordinate boron to oxygen bond [γ(B(3)-O)] was observed at approximately 656 cm-1.
FT-IR spectra of selected zinc borates.
In Figure 4, the Raman spectra of selected zinc borates are given. In Raman spectra, the bending of δ(B-O-H) was observed in the range of 1191–1186 cm-1. νas(B(4)-O) and νs(B(4)-O) bands were observed in the ranges of 1050–1048 cm-1 and 851–848 cm-1, respectively. The peaks in the range of 932–924 cm-1 corresponded to the bending of νs(B(3)-O), and vibrations of the νp(B(OH)4-) bonds were observed at approximately 755 cm-1. The peaks at approximately 664 cm-1 coincided with γ(B(3)-O), and the peaks at 578 cm-1 belonged to the δ(B(3)-O)/δ(B(4)-O) bonds. In the range of 437–298 cm-1, the observed peaks were due to δ(B(4)-O) bonds.
Raman spectra of selected zinc borates.
Both FT-IR and Raman spectra of synthesized zinc borates are in agreement with the previous studies and literature data (Yongzhong et al., 2000; Bardakci et al., 2013; Kipcak et al., 2014a,b; Kipcak et al., 2015).
Surface morphologies and particle sizes of the synthesized zinc borates
The morphologies of the synthesized zinc borate minerals were investigated using scanning electron microscopy (SEM) analysis. The SEM results are shown in Figure 5. The surface morphologies differed due to the changes in the reaction temperature and time for both ZS-T and ZC-T. In both sets, the particle sizes of the synthesized minerals decreased with increasing reaction temperature and time.
SEM surface morphology and particle sizes of the zinc borate hydrate compounds.
In the ZS-T experiments, smooth-edged crystal formation was determined on the sub-microscale. The shapes of the resulting particles were not uniform at 80°C, and the particle sizes were in the range of 525 nm to 2.00 μm. More uniform particle sizes and crystal shapes were observed as the temperature increased. At the reaction temperature of 90°C, the particle sizes were in the range of 380 nm to 1.77 μm after a reaction time of 3 h and 332 nm to 1.70 μm after a reaction time of 4 h.
In the ZC-T experiments, sharp-edged crystal formation was determined on the sub-microscale. The mineral synthesized at 80°C was a mixture of different crystal sizes. At this reaction temperature, the particle size was in the range of 327 nm to 1.38 μm. Smaller particle formation may have occurred due to cracks in the crystals. As the temperature increased, the particle sizes increased to 485 nm to 2.50 μm. The best particle formation was observed in the experiment at 90°C with a reaction time of 4 h.
Yield calculation of synthesized zinc borate hydrates
Figure 6 shows the reaction yields of the synthesized zinc borate hydrates; the reaction yields increased with increasing temperature and reaction times for both sets. In the ZS-T set, higher reaction yields were observed at 90°C and lower reaction yields were observed at 80°C and 70°C. The reaction yields were calculated to be at 86.5–99.7% and 86.2–99.6% for the ZS-T and ZC-T sets, respectively. The highest reaction yields were calculated for the reaction temperature of 90°C and reaction time of 4 h as 99.7±0.2% and 99.6±0.2% for the ZS-T and ZC-T sets, respectively. The yields of the ZS-T sets were higher than those of the ZC-T sets. Reaction yields were higher in this study than in the studies given in the literature obtained; the reaction yields in the optimum formations were found to be 86.78% in Bardakci et al. (2013), 95.7% in Kipcak et al. (2014a,b) and between 96.1 and 97.8% for different raw materials in Kipcak et al. (2015).
Reaction yields of the synthesized zinc borates from a. ZS-T, b. ZC-T.
Physical properties of the synthesized zinc borates
The UV-vis spectra of selected zinc borates are shown in Figure 7 (i.e. ZC-T-90-4 h and ZS-T-90-4 h). They have the highest reaction yields at the reaction temperature of 90°C and reaction time of 4 h. The spectra were measured in the wavelength range of 200–1100 nm at room temperature. The optical energy gaps of the zinc borate compounds, i.e. ZC-T-90-4 h and ZS-T-90-4 h, were determined by extrapolating the high-energy portions of the absorption spectra and were about the same at 4.13 eV.
The optical absorption spectra of ZC-T-90-4 h and ZS-T-90-4 h.
Figure 8 shows the current voltage characteristics of the ZC-T and ZS-T zinc borate minerals synthesized with a reaction temperature of 90°C and a reaction time of 4 h. The resistivities of ZC-T-90-4 h and ZS-90-4 h were determined to be about the same at 8.8×1010 Ω cm. These data were obtained from the current voltage curves.
The current voltage characteristics of ZC-T-90-4 h and ZS-T-90-4 h.
Conclusion
The present study was evaluated and showed that Zn3B6O12·3.5H2O type zinc borates were synthesized by varying several reaction parameters without using any type of agent or solvent. Based on the results, the formation of crystalline zinc borate increased with increasing reaction temperature and reaction time in both ZS-T and ZC-T. The XRD scores of the ZC-T set were found to be higher than those of the ZS-T set. The XRD results showed that the zinc borates could be synthesized via ZS-T reactions at 70°C for 4 h, at 80°C for 4 h and at 90°C for 3 h. Similarly, zinc borates could be synthesized via ZC-T reactions at 80°C for 2 h and at 90°C for 2 h. The SEM results showed that the smoothness and homogeneity of the products increased with increasing temperature. The highest yields were obtained at 90°C and 4 h for both ZS-T and ZC-T with yields of 99.7% and 99.6%, respectively. The electrical resistivities and optical energy gaps of the minerals synthesized at 90°C and 4 h for both ZS-T and ZC-T were found to be approximately 8.8×1010 Ω cm and 4.13 eV, respectively.
Experimental section
Characterization of the raw materials
The synthesis of zinc borate was performed using ZnSO4·7H2O and ZnCl2 as the zinc sources, which were obtained from Sigma Aldrich Reagent Plus®, Taufkirchen, Germany (≥99.0% purity). Na2B4O7·5H2O and H3BO3 were supplied by Bandırma Boron Works (Eti Maden, Balıkesir, Turkey) with a purity of 99.9%. Boric acid was sieved through a Fritsch analysette 3 Spartan pulverisette 0 vibratory sieve-shaker (Fritsch, Idar-Oberstein, Germany) (particle size, <70 μm) after being subjected to crushing and grinding by Retsch RM 100 (Retsch GmbH & Co KG, Haan, Germany) when the other raw materials were used without any pretreatment. The identification studies of raw materials were performed using a Philips PANalytical Xpert Pro (PANalytical B.V., Almelo, The Netherlands) XRD with the working parameters of 45 kV and 40 mA (Cu Kα radiation).
Zinc borate synthesis
To determine the optimum synthesis parameters, some preliminary experiments were conducted using different ZS-T and ZC-T molar ratios. ZS-T and ZC-T molar ratios of 1:1:3 (Kipcak et al., 2014b) and 1:1:2, respectively, were selected for further experiments.
The expected reactions are shown in Eqs. (1) and (2):
To determine the effects of the reaction temperature and reaction time on the synthesized product, the temperatures and reaction times were varied between 70°C and 90°C and from 1 to 4 h, respectively. For experiments with ZS-T, 0.0189 mol Na2B4O7·5H2O and 0.0566 mol H3BO3 were dissolved in 25 mL of pure water obtained from GFL 2004 (Gesellschaft für Labortechnik, Burgwedel, Germany), and 0.0189 mol ZnSO4·7H2O was then added to the reactor. For better crystallization, a commercial zinc borate (Zn3B6O12·3.5H2O), which was retrieved from a local supplier (Melos A.Ş., Istanbul, Turkey), was also added to the reactor (0.5% w/w as Na2B4O7·5H2O+H3BO3). In the ZC-T experiments, the amounts of ZnCl2, Na2B4O7·5H2O and H3BO3 were 0.0220, 0.0220 and 0.0439 mol, respectively. The raw materials were reacted in closed temperature-controlled vessels. After the determined reaction time, the solution was filtered through a blue ribbon filter paper (Chmlab, Barcelona, Spain), and the crystallized products on the filter paper were washed three times with pure water (approximately 1000 mL) at 50–60°C to remove unreacted reagents and Na2SO4 and NaCl by-products. Then the washed products were dried in an Ecocell LSIS-B2V/EC55 model incubator (MMM Medcenter Einrichtungen GmbH, Planegg, Germany) at 105°C for 24 h. The experimental method is shown schematically in Figure 9.
Schematic display of the experimental method.
Zinc borate characterization studies
To identify and determine the characteristics of the produced samples, XRD, FT-IR and Raman spectroscopy techniques were performed. The analysis parameters for XRD were arranged as specified in the Characterization of the raw materials section. The XRD data were collected from 10°<2θ<70° with a scan step size of 0.03°.
For FT-IR analyses, PerkinElmer Spectrum One FT-IR (PerkinElmer, MA, USA) was used with universal attenuation total reflectance (ATR) sampling accessory (Diamond/ZnSe crystal). For Raman analyses, Perkin Elmer Raman Station 400F (PerkinElmer, CT, USA) was used. The scan ranges were selected at 650–1800 cm-1 and 250–1800 cm-1 for FT-IR and Raman, respectively. Based on the literature, for FT-IR and Raman analyses, the spectral ranges were set between 600 cm-1 and 1800 cm-1 (Kipcak et al., 2014a,b).
The morphological structures and particle sizes of the synthesized zinc borate minerals were investigated using a CamScan Apollo 300 Field-Emission SEM (CamScan, Oxford, UK). The zinc borate samples were coated with platinum-gold (Pt-Au) to improve the conductivity.
Determination of the physical properties of zinc borate
The synthesized zinc borate minerals were pressed under a pressure of 20 MPa into pellets with 13-mm diameters and approximately 0.4- to 0.5-mm thicknesses. The electrical resistivity measurements of the synthesized zinc borate minerals with the highest crystalline yields were performed using standard current voltage measurement at room temperature on a Keithley 2400 in the dark with thermally evaporated silver contacts on two surfaces of the pellets. The ultraviolet-visible (UV-vis) absorption spectra of the zinc borate minerals were measured using a Perkin Elmer Lambda 35 UV-vis spectrophotometer in the wavelength range of 200–1100 nm at room temperature. In these measurements, the zinc borate powders were dispersed in HCl (5% v/v) solutions at the same concentration in quartz tubes (1 cm×1 cm).
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