Controlling the conditions for synthesis of strontium

Published by Maney Publishing (c) IOM Communications Ltd and the Australasian Institute of Mining and Metallurgy

Controlling the conditions for synthesis of strontium titanate nanopowders from celestite ore M. M. Rashad*1, R. Roshdi1, K. El-Barawy1 and A. T. Kandil2 Strontium titanate SrTiO3 nanopowders have been successfully prepared through oxalate precursor route using Egyptian celestite ore. Celestite ore was reduced with the carbon at the annealed temperature of 1100uC for 3 h to obtain water soluble strontium sulphide that treated with a dilute hydrochloric acid to produce strontium chloride. The formed strontium chloride and titanium dioxide were mixed with a certain amount of oxalic acid to form strontium titanium oxalate complexing precursors. The effect of oxalic acid molar ratio, annealing temperature, time and hydrogen peroxide additive on the crystal structure, crystallite size and microstructure was systematically studied. The results revealed that a well crystallite single phase of SrTiO3 nanopowders was achieved in the presence of hydrogen peroxide using 1?5 molar ratio of oxalic acid at the annealing temperature of 1000uC for 1 h. The crystallite size of the formed powders increased with increasing annealing temperature and time. The crystallite size was in the range between 50 and 80 nm. The SEM images of the formed SrTiO3 particles appeared as cube-like structure. The band gap energy and the direct current resistivity of the produced powders were y3?6 eV and 4?4036104 V m respectively for the sample in the presence of 10% hydrogen peroxide annealed at 1000uC for 2 h. Keywords: Strontium titanate, Ores, Wet chemicals methods, Nanomaterials, Optical and electrical properties

Introduction Strontium titanate SrTiO3 is a paraelectric cubic structured perovskite material at room temperature. It exhibits a large dielectric constant of y300 of the sintered ceramics. At temperatures ,105 K, its cubic structure transforms to tetragonal ferroelectric phase. Furthermore, SrTiO3 has various physical properties because of its ferroelectricity, thermoelectricity with a thermal conductivity of 12 W m21 K21, photocatalysis and superconductivity at temperatures ,20 K. In addition, it has good mechanical strength with the Mohs hardness of 5?5, high thermal and chemical stability, low dielectric loss, a low coefficient of thermal expansion of 9?461026 uC21, a high melting temperature of 2080uC, a refractive index of 2?31–2?38 nearly identical to that of the diamond and semiconductor with a band gap of y3?2 eV (Brankovic´ et al., 2004; George et al., 2009; Ianculescu et al., 2007). However, the characterisation and properties of strontium titanate are responsible for their widespread applications in the manufacture of thermistors, multilayer ceramic 1

Central Metallurgical Research and Development Institute, PO Box 87, Helwan, Egypt Faculty of Science, Helwan University, Helwan, Egypt

2

*Corresponding author, email [email protected]

capacitors, electro-optical devices, electromechanical devices, dynamic random access memory, infrared detectors, oxygen sensor, nitrogen oxides NOx photo degradation internal combustion engine and solar cells. Moreover, superconducting quantum interference device is fabricated using superconductor thin films developed on SrTiO3 (Brankovic´ et al., 2004; Chen et al., 2009; George et al., 2009; Ianculescu et al., 2007). Recently, photoluminescence of blue light emission from the Arzirradiated SrTiO3 and electron doped SrTiO3 single crystals at room temperature has been reported (George et al., 2009). In addition, it is reported that strontium titanate shows considerable activities for the water splitting into H2 and O2 in stoichiometric ratio under UV irradiation (Puangpetch et al., 2010). There are various synthesis methods applied to prepare SrTiO3 powders including conventional solid state reaction (Taibi and Kermoun, 1999), coprecipitation (Balaya et al., 2006; Roy and Bera, 2005), hydrothermal synthesis (Jitputti et al., 2007; Zhang et al., 2004), solvothermal synthesis (Yang et al., 2009), combustion synthesis including oxalate precursor route (Chen et al., 2009; George et al., 2009; Ishikawa et al., 2008; Liu et al., 2008), mechanochemical reaction (Hungra et al., 2004; Wang et al., 2007), molten salt method (Puangpetch et al., 2008) and sol–gel (Vaidyanathan et al., 2006; Wang et al., 2001). The oxalate precursor route is the

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Published by Maney on behalf of the Institute and The AusIMM Received 12 January 2011; accepted 16 May 2011 DOI 10.1179/1743285511Y.0000000015

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most intensely examined route for generating SrTiO3 from complex precursor. The use of an inexpensive inorganic salt precursor improves the cost effectiveness of the powder production which facilitates the synthesis of the crystallised powder with ultrafine particle size and high purity (Rashad et al., 2007). On the other hand, celestite ore SrSO4 is the main source for the production of most of the strontium compounds. Over 95% of the world production is consumed by the chemical industry for conversion to various strontium compounds. The most common commercial process for producing strontium compounds from celestite ore is the ‘black ash’ process (Hessien et al., 2009). From our knowledge, in a published work, the process of the synthesis of strontium titanate using celestite ore with titanium hydroxide gel by hydrothermal method at 250uC in a 5M KOH solution was found to be completed after 96 h of treatment (Hernandez et al., 2009). No work has been published on the synthesis of strontium titanate from celestite ore using oxalate precursor method. The present study aims at synthesising strontium titanate nanopowders using oxalate precursor route from Egyptian celestite ore and pure rutile. The effect of annealing temperature, annealing time, oxalic acid molar ratio and hydrogen peroxide concentration on the crystal structure, crystallite size and microstructure was systematically studied. The change in the optical and electrical properties was determined.

Experimental Materials Celestite ore SrSO4 is present in Egypt mainly in, namely, Wadi Essel with reserves of 2?3 million tons. The elemental analysis of the sample was performed using X-ray fluorescence spectrometry (XRF; Philips 1410). Rutile TiO2 (99?8%) is produced from Egyptian ilmenite ore via smelting and sulphate leaching of titania rich slag. Pure oxalic acid and hydrogen peroxide were used for organic acid precursor processing.

Synthesis of strontium titanate nanopowders from celestite ore

ions according to the following equations (Rashad et al., 2007, 2008; Tang and Chen, 2007) SrCl2 ?Sr2z z2Cl{

(1)

TiO2 zH2 C2 O4 ?TiOC2 O4 zH2 O

(2)

2Sr2z z2TiOC2 O4 z2H2 O2 ? Sr2 Ti2 O4 ðC2 O4 Þ2 z2H2 O

(3)

Sr2 Ti2 O4 ðC2 O4 Þ2 2H2 Oz2C2 O2{ 4 ? Sr2 Ti2 O2 ðC2 O4 Þ4 z2O2 z2H2 O

(4)

Sr2 Ti2 O2 ðC2 O4 Þ4 ?2SrTiO3 z4COz4CO2

(5)

As the titanyl oxalate was dissolved in hydrogen peroxide, peroxide groups (O–O) were inset to form strontium peroxotitanate oxalate complex which was easily decomposed by heating to strontium titanate and carbon oxides gases. The solution was stirred and gently evaporated at 80uC till a clear, viscous resin was obtained, and then it was dried at 110uC for 24 h. The dry precursors were heated (calcined) at a rate of 10uC min21 in static air atmosphere up to required different temperatures from 900 to 1100uC at different calcined times of 1–3 h.

Characterisation The final products were characterised using X-ray diffraction (XRD) on a Bruker axis D8 diffractometer ˚ ). The using Cu Ka radiation (wavelength l51?5406 A average crystallite size of the powders was estimated automatically from corresponding XRD data using Xray line broadening technique employing the classical Debye–Scherrer formula for the most intense peak (110) plane determined from the XRD data. The micrographs of SrTiO3 samples were examined by direct observation via scanning electron microscopy (SEM; JSM-5400). The optical properties were measured using UV–visible– near IR spectrophotometer (Jasco-V570). The direct current (DC) resistivity was measured using a 4339B Agilent high resistance meter.

Procedure Ore sample was ground to 2200 mesh (275 mm) and treated with hydrochloric acid to remove the calcium carbonate and the other acid soluble salts. After filtrating, cleaning and drying the insoluble SrSO4 salt, coke was used as the reducing agent with a carbon/ SrSO4 mole ratio of 2 : 1. The produced mixture was subjected to reduction treatment at 1100uC for 3 h in the presence of nitrogen gas to reduce the insoluble SrSO4 to the soluble strontium sulphide SrS. Then, the produced SrS was dissolved in deionised water and hydrochloric acid. The formed SrCl2 solution was filtered to remove unreacted coke and silica. The strontium titanate powders were prepared using organic acid precursor method using a solution of the dissolved SrCl2 from celestite ore and titanium dioxide TiO2 (99?8% purity) in the presence of stoichiometric amount of organic acid (oxalic 99?5% purity) with the addition of different amounts of hydrogen peroxide (5–10%). The molar ratios of Sr2z/Ti4z ions were stable at 1. The molar ratio of the oxalic acid was added according to the reaction of 1 mole of the acid with both Sr2z and Ti4z

Results and discussion Characterisation of celestite ore Table 1 shows the chemical analysis of celestite ore (Wadi Essel Area, Red Sea Coast, Egypt) using XRF. The celestite ore contains mainly 45?12%SrO and 34?87%SO3 in addition to 10?23%CaO. The reduction of celestite ore with coke in the presence of nitrogen gas led to the formation of soluble strontium sulphide SrS. The complete reduction of the strontium sulphate to strontium sulphide was observed Table 1 Chemical composition of Egyptian celestite ore Oxides

wt-%

Oxides

wt-%

SrO SO3 SiO2 Al2O3 CaO

45.12 34.87 0.89 0.29 10.23

MgO Na2O BaO Fe2O3 LOI*, 1000uC

1.08 1.67 0.31 0.04 6.00

*LOI5loss on ignition.


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1 X-ray diffraction pattern of produced strontium titanium oxide powders in absence of hydrogen peroxide with oxalic acid molar ratio of 1?0 annealed at 1000uC for 2h

for the sample reduced at 1100uC for 3 h as mentioned in our previous work (Hessien et al., 2009).

Synthesis of strontium titanate The XRD pattern of the produced powder in the presence of 1?0 molar ratio of oxalic acid and in the absence of hydrogen peroxide annealed at 1000uC for 2 h is shown in Fig. 1. It can be noticed that four different phases of cubic strontium titanate (JCPDS no. 84-0444), tetragonal Sr2TiO4 (JCPDS no. 72-2041), titanium oxide Ti4O7 (JCPDS no. 50-0787) and rutile TiO2 (JCPDS no. 87-0710) were formed. The XRD patterns of the produced strontium titanate powders prepared at different oxalic acid molar ratios from 1?1 to 1?5 in the presence of 10% hydrogen peroxide annealed at 1000uC for 2 h are illustrated in Fig. 2. The results showed that the increase in the oxalic acid molar ratio led to the formation of a well crystalline strontium titanate SrTiO3 phase. With the oxalic acid molar ratio of 1?1, strontium titanate SrTiO3 powders were formed with traces of the TiO2 phase. However, at a higher

2 X-ray diffraction patterns of produced strontium titanium oxide powders in presence of 10% hydrogen peroxide at different oxalic acid molar ratios annealed at 1000uC for 2 h

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3 X-ray diffraction patterns of produced strontium titanium oxide powders in presence of 10% hydrogen peroxide annealed at different temperatures for 2 h at oxalic acid molar ratio of 1?5

oxalic acid molar ratio of 1?5, a well crystalline single phase of SrTiO3 was obtained. The peaks related to SrTiO3 phase at 2h of 22?75, 32?39, 39?94, 46?47, 57?77, 67?80 and 77?18u were present. These peaks are ascribed to (100), (110), (111), (200), (211), (220) and (310) diffraction planes of cubic strontium titanate phase (SrTiO3) (JCPDS card no. 84-0444). The crystallite sizes of the obtained powders from the most intense peak (110) were 80 and 60 nm at oxalic acid molar ratios of 1?1 and 1?5 respectively. The role of hydrogen peroxide exhibits some important advantages such as significantly lower temperatures of synthesis and shorter reaction time, avoiding the milling and homogenising of raw materials and of final product. As a result, the metal titanates obtained are of higher purity, with fine crystalline structure and homogeneous grain size composition (Tang and Chen, 2007). Figure 3 shows the XRD patterns of the obtained strontium titanate in the presence of 10% hydrogen peroxide using the oxalic acid molar ratio of 1?5 at different calcination temperatures from 900 to 1100uC for 2 h. The results indicated that tetragonal Sr2TiO4 (JCPDS no. 72-2041) was formed as the impurity phase with cubic strontium titanate SrTiO3 at 900uC.

4 X-ray diffraction patterns of produced strontium titanium oxide annealed at 1000uC for 1 h with 5 and 10% hydrogen peroxide with oxalic acid molar ratio of 1?5


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5 X-ray diffraction patterns of produced strontium titanium oxide in presence of 10% hydrogen peroxide with oxalic acid molar ratio of 1?5 annealed at 1000uC for different times

Increasing the annealing temperature up to 1000 and 1100uC led to the formation of the single phase of strontium titanate phase. The crystallite size of the produced powders was increased by increasing the calcination temperature. It increased from 50 to 60 then to 70 nm at temperatures of 900, 1000 and 1100uC respectively. The XRD patterns of the produced strontium titanate at different hydrogen peroxide concentrations of 5 and


10% using the oxalic acid molar ratio of 1?5 annealed at 1000uC for 2 h are given in Fig. 4. It can be observed that at the low hydrogen peroxide concentration of 5%, traces of monoclinic titanium oxide Ti3O5 phase was formed with the main strontium titanate phase. With the hydrogen peroxide concentration of 10%, a well crystalline single phase of strontium titanate nanopowders was observed. The crystallite size of the formed powders at the low hydrogen peroxide concentration (5%) was 70 nm compared with 60 nm at the H2O2 concentration of 10%. Figure 5 shows the XRD patterns of the produced strontium titanate powders annealed at 1000uC for different times from 1 to 3 h in the presence of 10% hydrogen peroxide. It can be seen that a pure single phase of the produced strontium titanate was obtained at different studied annealing times. The crystallite size of the obtained powders was slightly changed at different annealing times. The SEM images of the strontium titanate powders produced using different molar ratios of oxalic acid, different calcination temperatures of 900 and 1000uC and different hydrogen peroxide concentrations of 5 and 10% were given in Fig. 6. The micrographs showed that the microstructures of the strontium titanate particles were cubes and homogenous and the grains were very fine. However, the only change that appeared is that the particle size at the oxalic acid molar ratio of 1?3 (Fig. 6a) is higher than that at the oxalic acid molar ratio of 1?5 (Fig. 6b). Furthermore, the image of the strontium titanate samples synthesised at 900uC (Fig. 6c) showed

6 Images (SEM) of produced strontium titnate annealed at a 1000uC, 10%H2O2 and oxalic acid ratio of 1?3, b 1000uC, 10%H2O2 and oxalic acid ratio of 1?5, c 900uC, 10%H2O2 and oxalic acid ratio of 1?5 and d 1000uC, 5%H2O2 and oxalic acid ratio of 1?5


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7 Ultraviolet–visible transmittance spectra of formed SrTiO3 in presence of 10% hydrogen peroxide with oxalic acid ratio of 1?5 annealed at 1000uC for 1 h

that the particles had a cube-like structure with nearly a uniform size distribution and also contained some agglomeration. The grain size of the formed powders for the annealed sample at 900uC is lower than that of the sample annealed at 1000uC (Fig. 6b) which agrees with the data given by the XRD patterns applying Debye–Scherrer equation. Figure 6d shows the SEM image of the obtained powders in the presence of 5% of hydrogen peroxide. It can be observed that the cube-like structure of the obtained powders was formed compared with the pure cube-like shape obtained at high hydrogen peroxide. The optical transmittance spectrum in the wavelength range of 200–1000 nm for the produced pure strontium titanate annealed at 1000uC for 1 h in the presence of 10% hydrogen peroxide with the oxalic acid molar ratio of 1?5 is shown in Fig. 7. The results revealed that with increasing wavelength longer than 240 nm, the increase in the transmittance absorbance was observed and the transmission dropped rapidly at 313 nm. The transparency of the samples exhibited a sharp decrease in the UV region, as viewed from the transmittance spectra. This decrease was caused by the fundamental absorption of light. Considering the high absorption region, the transmittance T with the absorption coefficient a follows the simple relation (Tang and Tang, 2003) T~Ae{ad

(6)

where T is the transmittance, A is nearly equal to unity at absorption edge and d is the thickness of the films (1 mm). Analysis of optical absorption spectra is one of the most productive tools for determining the optical band gap of the film. From these spectral data, the absorption coefficient a was calculated using the relationship (Caglar et al., 2010) Table 2 Direct current resistivity of formed titanate powders annealed at temperatures

160

Temperature/uC

DC resistivity, V m

900 1000

4.0376104 4.4036104

strontium different

8 Plot of [a(hn)]1/3 versus photon energy hn of formed SrTiO3 in presence of 10% hydrogen peroxide with oxalic acid ratio of 1?5 annealed at 1000uC for 1 h

ðahnÞ~A hn{Eg

m

(7)

where a is the absorption coefficient which was calculated from the transmittance data, A is an energy independent constant, m is a constant which determines the type of the optical transition (m51/3 for indirect forbidden transition, m51/2 for indirect allowed transition, m52/3 for direct forbidden transition and m52 for direct allowed transition) and Eg is the optical band gap. It is evaluated that the optical band gap of the SrTiO3 particles has a direct optical transition between valence and conduction bands. The plot of [a(hn)]1/3 versus the photon energy hn as shown in Fig. 8 yields in the sharp absorption edge for the high quality particles by a linear fit. However, the band gap energy obtained was 3?6 eV which was higher than that of the bulk 3?2 eV. The results may be attributed to the change of the particle size and the presence of some impurities of soluble inorganic species, with the formed strontium titanate. Furthermore, the shift in the band gap is due to the quantum confinement effect (Caglar et al., 2010; Tang and Chen, 2007; Tang and Tang, 2003). It is known that the size quantisation is due to the localisation of electrons and holes in the semiconductor nanocrystal˚ ) which causes a lites (nanosize in the range of 30–110 A change in electronic band structure and hence an optical band gap larger than that of the bulk (Singh et al., 2008). The DC resistivity of the produced strontium titnate powders annealed at 900 and 1000uC for 1 h is shown in Table 2. It can be observed that the DC resistivity was increased by increasing the annealing temperature due to the formation of a well crystalline strontium titnate phase.

Conclusion Strontium titanate nanoparticles have been successfully synthesised from Egyptian celestite ore and pure rutile via oxalate precursor route. The celestite ore was reduced with carbon to form water soluble SrS which was treated with hydrochloric acid to form strontium chloride. The formed strontium chloride and titanium


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dioxide were mixed with the 1?0–1?5 molar ratios of oxalic acid to form strontium titanium peroxo-oxalate complexing precursors which were annealed at different annealing temperatures from 900 to 1100uC for different times from 1 to 3 h in the presence of 5 and 10% hydrogen peroxide. The results indicated that single phase of strontium titanate nanopowders was obtained at annealing temperatures from 1000 to 1100uC for different times from 1 to 3 h in the presence of 10% hydrogen peroxide using 1?5 molar ratio of oxalic acid. The crystallite size of the obtained powders was increased with increasing annealing temperature and time. The crystallite size of the produced powders was in the range between 50 and 80 nm. The microstructures of the produced SrTiO3 particles appeared as cube-like structure. The band gap of the formed SrTiO3 was 3?6 eV. A higher DC resistivity of 4?40376104 V m was achieved for pure SrTiO3 calcined at 1000uC for 1 h.

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