Synthesis of dehydrated zinc borates using the solid-state method: Characterization and investigation of the physical properties

Abstract

In this study, several dehydrated zinc borates are synthesised from ZnO and H₃BO₃ via the solid-state method. Five different ZnO:H₃BO₃ mole ratios (1:2–1:6) are prepared and used in the experiments. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and Raman spectroscopy techniques are used to characterize the products. Furthermore, scanning electron microscope (SEM) is used to investigate the surface morphologies. Additionally, the electrical resistivity and optical energy gaps of the dehydrated zinc borate samples synthesised at reaction temperatures were determined using the current/voltage characteristics and Ultraviolet-visible spectroscopy (UV-Vis) absorption measurements. Based on the experimental results, major phases of ZnB₄O₇ and Zn₄O(B₆O₁₂) compounds were synthesised at 800°C after a reaction time of 240 min and at 900°C at all reaction temperatures between 60 and 240 min. The dehydrated zinc borate particle sizes were found to be 395.53 nm–2.36 μm. The electrical bulk resistivity was determined to be 10⁷–10⁸ Ω.m, and the optical energy band gaps were determined to be approximately 3.26 eV and 2.21 eV.

Keywords

Electrical properties optical properties solid-state synthesis zinc borates

1 Introduction

Metal borates can be classified as metallic structures, such as sodium borates, calcium borates and magnesium borates. There are many types of synthesised metal borates, such as zinc borates, which have excellent thermal resistance. They are generally used in plastic, ceramic, paint, glass, electric insulation, wood, cement, medicine, construction, automotive and flame retardants. Moreover, zinc borates are used as a preservative in wood composites, as anticorrosive pigments in coatings and as polymer additives to promote char formation to suppress smoke and retard combustion [1 –10].

Metal borates can be produced via two different methods. These methods are known as hydrothermal and solid-phase synthesis methods. Generally, the synthesis procedure for zinc borates is based on hydrothermal synthesis. There are many studies on zinc borate synthesis at different reaction times and temperatures [11 –18]. Igarashi et al. [19] synthesised zinc borates in a two-step reaction. In the first step, zinc oxide and boric acid are combined and stirred at 60°C for 1.5 hours to achieve crystal formation. In the second step, the mixture is stirred continuously at 90°C for 4 hours, and seed crystals are added to the reaction mixture to enhance crystal growth. The results from many studies have indicated that the addition of seed crystals [20] can be used to improve the crystal yield and the purity of the desired product. Hydrophobic, nanostructured zinc borate (3ZnO·3B₂O₃·3.5H₂O) was synthesised by Tugrul et al. [21] using zinc carbonate and boric acid with different modifying agents and solvents to ensure the proper hydrophobicity and nanostructure. The solid-state method is based on the principle of reactions in a high temperature furnace with air and the aid of boron and the metal source. Del Longo et al. synthesised zinc borate glasses of 4ZnO·3B₂O₃ at a reaction temperature and time of 1250°C and 4 hours [22], respectively. Ivankov et al. prepared zinc borate glasses at 1200°C and studied the absorption, fluorescence and optical excitation spectra [23]. Speghini et al., synthesised compositions of 4PbO·2ZnO·5B₂O₃ and 2PbO·4ZnO·5B₂O₃ doped with Pr³⁺, Nd³⁺, Eu³⁺, Dy³⁺, Ho³⁺ and Er³⁺ at a reaction temperature and time of 900°C and 1 h [24], respectively.

Compared to the literature, the novelty of this study is the synthesis of anhydrous zinc borate compounds at reaction temperatures less than 1000°C. Different crystal structures were obtained with different morphologies with the change in reaction parameters such as; reaction temperatures, reaction times and mole ratios. The chemical and physical features of the samples were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM) and Ultraviolet-visible (UV-Vis) spectroscopy.

2 Experimental

2.1 Preparation of raw materials

Boric acid (H₃BO₃), which is used as the boron source during synthesis procedure, was obtained from Bandirma Boron Works (Eti Maden, Balikesir, Turkey) with a purity of 99.9%. The procured H₃BO₃ was ground and sieved through a 200 size mesh to increase the surface area available to the reaction medium. Zinc oxide (ZnO) was obtained from Sigma-Aldrich (Sigma-Aldrich St. Louis, USA) with a minimum purity of 99.0% (CAS Number 1314-13-2). Prior to the experimental studies, the raw materials were characterized using a Philips PANalytical Xpert Pro (PANalytical B.V., Almelo, The Netherlands) XRD at 45 kV and 40 mA at angles between of 7–90°.

2.2 Solid-state synthesis

For the solid state synthesis of zinc borates, the molar ratios of the reactants (ZnO:H₃BO₃) were varied between 1:2 and 1:6 with the aim of producing the most well-known anhydrous zinc borate compound, ZnB₄O₇ [11]. The homogenized raw material powders were mixed at the aforementioned ratios to investigate the effects of the various molar ratios on the products. The amount of ZnO is used for the experiments were 0.0070, 0.0054, 0.0044, 0.0037 and 0.0032 moles for the molar ratios of 1:2, 1:3, 1:4, 1:5 and 1:6, respectively. Likewise, the amount of H₃BO₃ was used for the experiments were 0.0140, 0.0162, 0.0175, 0.0184 and 0.0189 moles for the molar ratios of 1:2, 1:3, 1:4, 1:5 and 1:6, respectively. A Manfredi brand OL57 (Manfredi S.r.l., Torino, Italy) model hydraulic press apparatus was used to obtain pellets to ensure close contact between the reactants. A pressure of 100 bar was applied for 2 minutes. The expected reaction is given in Equation (1): $ZnO (s) + 4 H_{3} {BO}_{3} (s) \overset{heat}{\to} {ZnB}_{4} O_{7} (s) + 6 H_{2} O (g)$ (1)

Following the pelleting process, the pellets were placed in ceramic crucibles and placed in a high-temperature furnace (Protherm MOS 180/4, Ankara, Turkey) at temperatures between 500°C and 900°C for the reactions to take place. After synthesis, the products were ground in a ceramic mortar. The solid state synthesis procedure for zinc borates is given in Fig. 1.

2.3 Characterizations

Characterization studies of the synthesised products were performed using XRD, FT-IR and Raman spectroscopy methods. To identify the produced crystal structures, XRD analysis was applied to products over an angular range of 7–90°. The infrared spectra of the products were determined over a spectral range from 1800 cm^- 1 to 650 cm^- 1 by a PerkinElmer Spectrum One FT-IR (PerkinElmer, MA, USA). The Raman spectra of the samples were obtained over a range from 1800 cm^- 1 to 250 cm^- 1 using a PerkinElmer, Raman Station 300F (PerkinElmer, MA, USA). The spectral analyses were recorded at frequencies less than 1800 cm^- 1. This range was based on results from previous studies, which stated that the characteristic peaks of borates in infrared region are observed over a range from 500–1800 cm^- 1 [25, 26].

For B₂O₃ analysis, 1 g of product was dissolved in 5 ml of 37% HCl solution (Merck Millipore Darmstadt, Germany). A volume of 100 mL of pure water (18.3 mΩ.cm), which was obtained from a GFL 2004 (Gesellschaft für Labortechnik, Burgwedel, Germany) water purification system, was added. The reference compound was chosen as boric acid, and the same preparation procedure was applied to boric acid. The B₂O₃ contents of the samples were determined by acid-base titration, in which 0.1 M NaOH solution (Merck Millipore Darmstadt, Germany) was used as the titrant. The titration studies were carried out with a Metrohm 794 Basic Titrano brand titrator (Metrohm, Herisau, Switzerland).

The surface morphology and particle properties of the synthesised zinc borates were observed using a CamScan Apollo 300 Field-Emission SEM (CamScan, Oxford, United Kingdom) at 20 kV with a backscattering electron detector (BEI) and a magnification of 5000.

The dehydrated zinc borate samples synthesised at a reaction temperature of 800°C – 240 min and 900°C – 120 min were pressed under a 30 MPa pressure into pellets with diameters of 13 mm. Electrical resistivity measurements of the samples were carried out using standard current/voltage measurements at room temperature using a Keithley 6487 system in dark conditions with thermally evaporated gold contacts on both surfaces of the pellets. The bulk electrical resistivities of the samples were calculated using the following formula Equation (2): $ρ = R \frac{A}{l}$ (2) where R is the electrical resistance, A is the cross sectional area of the gold contacts and l is the thickness of the samples. The optical absorption spectra of the samples were recorded using a Perkin Elmer UV–vis spectrometer (PerkinElmer, MA, USA) at room temperature. For this measurement, the samples were dispersed in an HCl solution in a quartz tube (1 cm×1 cm).

3 Experimental results

3.1 Characterization of the raw materials

The zinc and boron sources used in these experiments were identified using XRD analyses. According to the XRD results, the raw materials were determined as “Zinc Oxide” (ZnO) with a powder diffraction file number (pdf no.) of 01-079-2205 and “Sassolite” (H₃BO₃) with a pdf no. of 01-073-2158.

3.2 XRD results

The XRD results and patterns of the synthesised dehydrated zinc borate compounds are given in Table 1. According to Table 1, six different types of dehydrated zinc borate compounds are obtained as the reaction parameters changed. The XRD patterns of the zinc borates synthesised at 800°C and 900°C are given in Fig. 2a and b, respectively.

From the XRD scores, for a 500°C reaction temperature, a ZnB₄O₇ (ZB¹) type of a compound (pdf no. 01-071-0634) formed with un-reacted raw material of ZnO (Z). For this temperature, the high XRD score of Z represents the major phase at this temperature, indicating that the formation of ZB¹ is beginning. At 600°C, the XRD scores of the Z phase decrease slightly and the XRD scores of the ZB¹ increase slightly, meaning that at that this temperature, there is less un-reacted Z, and ZB¹ formation is greater than at 500°C. Additionally, at 600°C, two other types of dehydrated zinc borate compounds are obtained: Zn₃(BO₃)₂ (ZB²) (pdf no. 01-071-2063) and ZnB₄O₇ (ZB³) (pdf no. 00-016-0283), in which ZB¹ and ZB³ exhibit different crystallographic properties even though they have the same structural formula. As the reaction temperature increases to 700°C, both the formations of ZB¹ and ZB² increased and the amount of un-reacted Z decreased. The highest ZB¹ and ZB² XRD scores of 59 and 39 are observed for a mole ratio of 1:4.

After a reaction temperature of 800°C, four different reaction times are studied. In the first 30 min, the formations of ZB¹, ZB², ZB³, and Zn₄O(BO₂)₆ (ZB⁴) (pdf no. 01-083-1424) and Zn₄O(BO₂)₆ (ZB⁶) (pdf no. 01-085-1778) are observed, in which ZB⁴ and ZB⁶ have different crystallographic properties, even though they have the same structural formula. The observed major phase is the ZB⁴. After 60 min of reaction time, the ZB⁴ phase transforms to other phases, and after 120 min of reaction time, the ZB⁴ phase is again observed as the major phase. However, the highest ZB⁴ XRD score of 57 is observed for a reaction time of 30 min and a mole ratio of 1:4. As the reaction time increases to 240 min, the ZB⁴ and ZB⁶ phases are transformed to ZB¹, ZB² and ZB³ phases. Among these phases, at an 800°C reaction temperature and 240 min of reaction time, the ZB¹ phase is the major phase and the highest XRD score of 73 is obtained for a mole ratio of 1:4.

In Fig. 2a, it is seen that ZB¹ has seven major characteristic peaks (h k l, d [Å]) at approximately 16.9° (2 1 1, 5.23 Å), 21.6° (1 0 2, 4.11 Å), 22.0° (0 2 0, 4.06 Å), 24.3° (0 2 1, 3.66 Å), 28.4° (3 0 2, 3.14 Å), 30.5° (3 1 2, 2.93 Å) and 31.3° (3 2 1, 2.86 Å).

To better explain the formation of the ZB¹ phase, a three dimensional surface plot is drawn using the Statistica 8.0 computer program (StatSoft Inc., Tulsa, USA) and shown in Fig. 3a. In the plot, the x-axis represents the reaction temperature (°C), the y-axis represents the XRD score and the z-axis represents the mole ratio. As seen in Fig. 3a, ZB¹ formation increases with the reaction temperature and the mole ratio from 1:2 (0.50 in the plot) to 1:4 (0.25 in the plot) and decreases from 1:4 to 1:6 (0.166 in the plot). The highest peak point represents the highest XRD score, which is observed for a 0.25 mole ratio and is equal to 1:4 and a reaction temperature of 800°C.

At a reaction temperature of 900°C and after reaction times of 30 and 60 min, three types of dehydrated zinc borate compounds are observed: ZB⁴, Zn₄O(B₆O₁₂) (ZB⁵) (pdf no. 01-076-0496) and ZB⁶. Like ZB⁴ and ZB⁶, ZB⁵ has the same structural formula, but different crystallographic properties. For a reaction time of 30 min, the ZB⁴ phase is the major phase and transforms to the ZB⁵ phase at a reaction time of 60 min. The highest ZB⁴ phase XRD score of 71 is obtained at a mole ratio of 1:6 after a reaction time of 30 min. The highest ZB⁵ phase XRD score of 75 is obtained at a mole ratio of 1:5 after a reaction time of 60 min. As the reaction time increased to 120 min, ZB² and ZB³ phases are observed at the 800°C reaction temperature. However, their XRD scores are very low, meaning that the formations are very weak and the major phase after a reaction time of 120 min reaction time is ZB⁵. The highest XRD score of 80 is obtained at a mole ratio of 1:6 after a reaction time of 120 min. As the reaction time increases to 240 min, the ZB5 XRD scores decreased slightly and the highest XRD score of 78 is observed at a mole ratio of 1:5. After a reaction time of 240 min, minor phases of ZB¹, ZB² and ZB³ are also obtained.

In Fig. 2b, we see that ZB⁵ has four major characteristic peaks (h k l, d [Å]) at approximately 29.2° (2 1 1, 3.05 Å), 38.0° (3 1 0, 2.36 Å), 45.3° (3 2 1, 2.00 Å) and 51.8° (4 1 1, 1.76 Å).

To better explain the formation of the ZB⁵ phase, a three dimensional surface plot is drawn and shown in Fig. 3b. In the plot, the x-axis represents the reaction time (min), the y-axis represents the XRD score and the z-axis represents the mole ratio. Because for a reaction time of 30 min ZB⁵ formation does not occur, the plot is drawn for reaction times of 60 min and 240 min.

As seen from Fig. 3b, ZB⁵ formation increases with the reaction time from 60 min to 120 min and decreases as the reaction time increases from 120 min to 240 min. As the mole ratio changed, the formation of ZB⁵ is slightly affected. The highest peak point, which represents the highest XRD score, is observed at a 0.15 mole ratio, which is equal to 1:6 after a reaction time of 120 min.

The best formations of ZB¹ are found for a reaction temperature of 800°C, a after a reaction time of 4 h and 1:4 and 1:5 mole ratios, which corresponded to XRD scores of 73 and 69, respectively. The best formations of ZB⁵ are found for a 900°C reaction temperature, after a reaction time of 2 h and 1:4 and 1:6 mole ratios, corresponding to XRD scores of 78 and 80, respectively.

The detailed crystallographic data for the synthesised major phases of the ZB¹ and ZB⁵ compounds are given in Table 2.

3.3 FT-IR and Raman spectral analysis results

Since the FT-IR and Raman spectra of the products obtained at 800 and 900°C are identical, so only the products synthesised at the reaction temperature of 800°C and a reaction time of 240 minutes are given in Fig. 4. In Fig. 4, it is seen that the asymmetric stretching of the three coordinate boron [ν_as(B₍₃₎-O)] was observed at approximately 1410 cm^- 1. The peak at the band value of 1190 cm^- 1 corresponded to the bending of the B-O-H [δ(B-O-H)]. The asymmetric stretching of the four coordinate boron [ν_as(B₍₄₎-O)] was observed between 991 and 986 cm^- 1. Symmetric stretching of the four coordinate boron [ν_s(B₍₄₎-O)] and bending of the three coordinate boron [δ(B₍₃₎-O)] were observed at approximately 910 cm^- 1 and 710 cm^- 1, respectively.

In the Raman spectrum of the products synthesised at a reaction temperature of 800°C and after a reaction time of 240 min, the band values of approximately 1375 cm^- 1 could be explained by the presence the pyro-borate groups.

Symmetric stretching of the three coordinate boron [ν_s(B₍₃₎-O)] and ν_s(B₍₄₎-O) band were observed at the peaks between 975 cm^- 1 and 806 cm^- 1, respectively. The peaks at approximately 499 cm^- 1 represent ν_p(B₅O₆(OH)₄]^-. The lower band values at 499 cm^- 1 can be explained by the presence of δ(B₍₄₎-O) (Fig. 5).

Both the obtained band values from the FT-IR and Raman spectra are in good agreement with the previous reports available in the literature [21 , 27].

3.4 B₂O₃ content of the synthesised zinc borate

The results of the B₂O₃ content analysis of the selected synthesised compounds are provided in Table 3. According to analysis results, the B₂O₃ contents were between 30.28% and 48.97% for different reaction temperatures and raw material molar ratios. At 500°C, the B₂O₃ percentage changed over a range from 44.72% and 48.52%. When the reaction temperature increased to 600°C, the B₂O₃ contents decreased to their lowest value of 30.28%. At 700°C, the B₂O₃ percentage varied between 35.71% and 43.32%. At 800°C, the B₂O₃ percentages varies between 39.96% and 48.97%. At 900°C, the B₂O₃ contents ranged from 42.54% to 45.34%. The difference in the B₂O₃ contents can be caused by the production of different types of zinc borates in the experiments. Nevertheless, the obtained results show similarity with previous work, which calculates the B₂O₃ percentages in zinc borates to be between 30% and 50% [28].

3.5 Surface morphology and particle size of the synthesised zinc borate

SEM analyses were conducted on the samples with the highest XRD scores for both the ZB¹ and ZB⁵. The surface morphologies of the synthesised zinc borates at reaction temperatures of 800°C and 900°C are presented in Fig. 6.

The obtained particles were not uniform in appearance. The particle size distribution of the ZB¹ synthesised at a reaction temperature of 800°C and after a reaction time of 240 min are smaller than the ZB⁵ products synthesised at a reaction temperature of 900°C and after a reaction time of 120 min. The particle sizes of the ZB¹ are between 395.53 nm and 1.88 μm and 650.06 nm and 2.36 μm at mole ratios of 1:4 and 1:5, respectively. The particle sizes for ZB⁵ are between 414.20 nm and 1.13 μm and 421.35 nm and 1.35 μm at mole ratios of 1:4 and 1:6, respectively. Agglomeration was observed in the ZB⁵ products.

3.6 The electrical and optical properties of the synthesised zinc borate

The current/voltage characteristics were measured using a standard technique for the samples synthesised at a reaction temperature of 800 and 900°C and for the same ZnO:H₃BO₃ mole ratios presented in Fig. 7. As seen in Fig. 7, the samples exhibit high resistance, which increased with synthesis temperatures for all same ZnO:H₃BO₃ mole ratios. The bulk resistivities of the samples were calculated using formula (1) and determined to be approximately 2.5×10⁷ and 1.2×10⁸ Ω.m, respectively.

The absorption spectra of the dehydrated zinc borate samples were measured over a wavelength range of 200–1100 nm at room temperature. Figure 8 shows the optical absorption spectra of compounds with the highest XRD scores synthesised at a reaction temperature of 800 and 900°C for the same ZnO:H₃BO₃ mole ratios.

The optical energy gaps were determined from extrapolation of the high energy part of absorption spectra to be approximately 3.26 eV and 2.21 eV, respectively and it showed similar properties with zinc borate glasses which were prepared by Altaf et al. [29]. In both, the samples sharply absorb photons at approximately 970 nm. The observations of sharp peaks in absorption spectra suggest the material could be used as an optical filter.

4 Conclusions

In this study, different types of dehydrated zinc borates were synthesised using a solid-state method with reactions of ZnO and H₃BO₃. ZnB₄O₇ (ZB¹) was obtained as a major phase at a reaction temperature of 800°C and after reactions times of 120 and 240 min. Higher XRD scores were obtained for a 240 min reaction time and 1:4 and 1:5 mole ratios. However, Zn₄O(B₆O₁₂) (ZB⁵) was obtained as a major phase at a reaction temperature of 900°C and after reactions times of 60, 120 and 240 min. Higher XRD scores were obtained after a 120 min reaction time and 1:4 and 1:6 mole ratios. FT-IR and Raman spectra and B₂O₃ contents of the dehydrated zinc borates were in agreement with the literature. The minimum particle sizes were found to be 395.53 nm–1.88 μm for a 1:4 ratio for ZB¹ and 414.20 nm–1.13 μm for a 1:4 ratio for ZB⁵. The bulk electrical resistivity of dehydrated zinc borate is determined to be in a range of 10⁷–10⁸ Ω.m, respectively. The high electrical resistivity of the borate sample means that it can be used in many industrial applications. The optical energy band gaps of the samples were determined to be approximately 3.26 eV and 2.21 eV. Furthermore, the material may potentially be used as an optical filter due to a sharp peak in the UV-Vis absorption spectrum at approximately 970 nm.

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