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Preparation and characterization of nanocrystalline and mesoporous strontium titanate thin films at room temperature.

Abstract The low temperature perovskite-type strontium titanate ([SrTiO.sub.3]) thin films and powders with nanocrystalline and mesoporous structure were prepared by a straightforward particulate sol-gel route. The prepared sol had a narrow particle size distribution with hydrodynamic diameter of about 17 nm. X-ray diffraction (XRD) revealed that the synthesized powders had a perovskite-[SrTiO.sub.3] structure with preferable orientation growth along the (1 0 0) direction. TEM images showed that the average crystallite size of the powders annealed in the range 300-800[degree]C was around 8 nm. FE-SEM analysis and AFM images revealed that the deposited thin films had mesoporous and nanocrystalline structure with the average grain size of 25 nm at 600[degree]C. Based on Brunauer-Emmett-Taylor (BET) analysis, the synthesized powders showed mesoporous structure with BET surface area in the range 92-75 [m.sup.2]/g at 400-600[degree]C. One of the smallest crystallite sizes and one of the highest surface areas reported in the literature were obtained, which can be used in many applications, such as photocatalysts.

Keywords Strontium titanate, Room temperature, Sol-gel, Nanocrystalline

Introduction

Nanocrystalline titanium dioxide has a wide range of applications, such as ultraviolet filters for optics and packing materials, (1) antireflection coatings for photovoltaic cells and passive solar collectors, (2) photocatalysts for purification and treatment of water and air, (3) anodes for lithium-ion batteries, (4) electrochromic displays, (5) transparent conductors, (6) self-cleaning coatings of windows and tiles, (7) humidity sensors, (8) and gas sensors. (9) Recently, interest in titanium-based oxides with the perovskite crystalline phase of the form of ABO3 for photocatalytic applications has increased. (10-12) These oxides have been regarded as functional materials because of their excellent dielectrical, ferroelectrical, pyroelectrical, piezo-electrical, photorestrictive, maenetorestrictive, and electro-optical characteristics. (13), (14) Strontium titanate ([SrTi0.sub.3]), being one member of this well-known family, has a broad range of the above-mentioned properties and can be used as a photocatalyst material (15). For photocatalytic applications many efforts have been aimed at improving the photocatalytic activity of [SrTi0.sub.3] by controlling the microstructure with porous, high specific surface area films. (16), (17) In this paper we present, for the first time, [SrTi0.sub.3] with nanocrystalline structure by sol-gel process at room temperature. In addition, further studies based on the physical and chemical characteristics of produced [SrTi0.sub.3] films and powders (such as phase structure, crystallite size, phase composition, microstructure, and specific surface area) are needed in order to predict and optimize photocatalytic activity of the prepared material.

[SrTi0.sub.3] can be obtained by different deposition techniques, such as the solid state reaction technique, (18) the sol-gel method, (19-21) the molten salt synthesis, (22) the polymerized complex technique (23) and the hydrother-mal technique. (24) Among chemical routes, sol-gel techniques offer important advantages over other methods because of their low-cost simple synthetic route, excellent compositional control, high homogeneity at the molecular level, lower crystallization temperature, and the feasibility of producing thin films on complex shapes when dip coating is used.

[SrTi0.sub.3] sol was prepared using strontium ethyl hexanoate and titanium isopropoxide by Thomas et al. (25) Films with good crystallinity were obtained at an annealing temperature of 650[degree]C. Zhang et al. (26) prepared nanocrystalline [SrTi0.sub.3] by a stearic acid sol-gel technique with crystallite size of 26-120 nm at 500-1000[degree]C. Also using a sol-gel route, Xuewen prepared [SrTi0.sub.3] nanopowder from [Sr([NO.sub.3]).sub.2] and [Ti([C.sub.4][H.sub.9]0).sub.4] precursors. (27)The crystallization temperature of the sample was determined to be 700[degree]C. In addition, the crystallite size of the sample was calculated in the range 12-25 nm. Bao et al. (28) produced sol-gel-derived [SrTi0.sub.3] thin films using semihydrate strontium acetate and titanium tetra-n-butoxide as starting materials, with acetic acid and 2-methoxycthanol as solvents. In order to crystallize the amorphous phase, the films were annealed at 750[degree]C for 1 h. Zhu et al. (29) reported [SrTi0.sub.3] (STO) buffer layers with different STO seed layers on Ni (200) substrates using titanium isopropoxide, strontium acetate, acetylacetone, and trifluoroacc-tic acid. The crystallized [SrTi0.sub.3] at 300[degree]C had grain size of about 30-50 nm. They found that the seed layer solution concentration remarkably influenced the orientations of the subsequent STO precursor layers. A sol-gel-hydrothermal method was reported to synthesize [SrTi0.sub.3] crystallites by Xu et al. (30) They used strontium acetate and tctrabutyl titanate as precursors with acetic acid and ethanol. The crystallized [SrTi0.sub.3] at 200[degree]C showed particle size of 400 nm with integrated multipod-likc star morphology. They concluded that as the reaction duration continued, the crystal shape gradually varied from unfeatured conglomeration through star shape and then to cubic. Cui et al.31 synthesized perovskite-type [SrTi0.sub.3] nanoparticles with size of 50-100 nm by a sol-gel method using chloride precursors and propylene oxide as a gelation agent. Puangpetch et al. (16) synthesized mesoporous-assemblcd [SrTi0.sub.3] nanocrystals using strontium nitrate and tctra-isopropyl orthotitanate as precursor, anhydrous ethanol and ethylene glycol as solvent, and laurylamine hydrochloride, cetyltrimethylammonium bromide and cetyl-trimethyl ammonium chloride as structure-directing surfactants. The prepared [SrTi0.sub.3] sample had average crystallite sizes in the range of 20-40 nm and Brunaucr-Emmett-Taylor (BET) surface areas in the range of 9-28 [m.sup.2]/g at 700[degree]C. Liu et al. (32) prepared [SrTi0.sub.3] nanopowders in the range of 20-30 nm by the sol-gel combustion method using citric acid as a reductant / fuel and nitrate as an oxidant at a low temperature of 400[degree]C.

The aim of the present work is to synthesize nanocrystalline [SrTi0.sub.3] materials at a low temperature and with minimal heat treatment by employing a suitable aqueous particulate sol-gel route rather than the polymeric sol-gel methods reported previously. Therefore, less energy is consumed during the production process, which has good potential for industrial applications. This process can be defined as environmentally friendly processing as it uses an aqueous solution. Another of the advantages is using an alternative to acetate (i.e., strontium chloride) as a strontium source to produce a low-cost product. Besides controlling the phase structure, composition homogeneity, crystallite size, monodispersity, and microstructure, the cost of the product is also an important concern. Therefore, starting with a low-cost precursor such as strontium chloride rather than strontium acetate may reduce the total cost of the production. Since the pores in particulate sol-gel processes are much larger than those found in the polymeric sol-gel route, the capillary stress developed during the drying process is lower and less shrinkage occurs. (33) Therefore, it is possible to produce crack-free thin films with mesoporous structure by the particulate sol-gel processes. The present method is thus a simple and cheap process with the capability of producing [SrTi0.sub.3] in the forms of thin films and powders with a faster production rate in comparison to the previous polymeric sol-gel methods reported in the literature.

Experimental

Preparation of the strontium titanate sol

Titanium letraisopropoxide (TTIP) with a purity of 97% (Aldrich, UK) and strontium chloride hexahy drate ([SrCl.sub.2]-[6H.sub.2]O) with a purity of 99% (Aldrich, UK) were used as titanium and strontium precursors, respectively. Analytical grade hydrochloric acid (HQ) 37% (Fisher, UK) was used as a catalyst for the peptisation, and deionised water was used as a dispersing medium. Hydroxypropyl cellulose (HPC) with an average molecular weight of 100,000 g/mol (Acros, USA) was used as a polymeric fugitive agent (PFA).

The [SrTi0.sub.3] system was prepared by a particulate sol-gel method. The first step was the preparation of Ti02 sol, based on our previous study.3 In a separate beaker, the stoichiometric amounts of SrCl2-6H20 and HPC were dissolved in deionised water at room temperature and stirred for 30 min. HPC concentration (i.e., 5 g/L) was defined according to previous study specifications,35 which induced the highest surface area. This solution was then mixed with Ti02 sol, during 2 h of stirring.

The sol was stable and no gelation occurred during preparation. Sol was characterized in particle size by the dynamic light-scattering technique (DLS) using a Malvern ZetaSizcr 3000HS at 20[degree]C with a 10 mW He-Ne laser, 633 nm wavelength, and 90[degree] fixed scattering angle. The stability of prepared precursor was also determined with zeta potential using the same instrument.

Preparation of strontium titanate thin films

Films were deposited onto 10x5x1 mm quartz substrates, in order to avoid the Ti02 X-ray diffraction (XRD) peak overlapping with the peaks of the most commonly used Si and Al substrates. Before deposition, substrates were cleaned using a high power sonic probe consecutively in water, ethanol, and acetone, and dried at 70[degree]C for 15 min. One layer of film was deposited by dip coating. The substrates were immersed in the precursor and kept there for a few minutes, followed by a withdrawing speed of 0.6 cm/ min. The subsequent heat treatment was optimized as follows. The films were dried at 150[degree]C for 1 h, annealed at a rate of 5[degree]C/min up to different temperatures (300, 400, 500, 600, and 800[degree]C) and held at these temperatures for 1 h in air. The microstructure of the films was characterized using an FE-SEM JEOL 6340 scanning electron microscope and the topography using an AFM Nanoscope III (Digital Instruments, Inc.). The average grain size of the films was determined based on FE-SEM and AFM micrographs.

Synthesis of strontium titanate powders

Powders were prepared by drying each sol at room temperature for 72 h. Powders were thermally processed in the same way as the films. The phase composition and crystallite size of the powders were characterized using an XRD diffraclometer Philips X'perl PW3020, Cu Ka and TEM JEOL 200CX. The crystallite size of the [SrTi0.sub.3] phase was calculated from their reflections using the Debye-Scherrer equation (36):

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

d = k[lambda]/B cos [theta], (1)

where d is the crystallite size, k a constant of 0.9, [lambda]. the X-ray wavelength of Cu which is 1.5406 A[degree], [theta] the Bragg angle in degrees, and B the full width at half maximum (FWHM) of the peak. A cerium oxide sample was used as an instrument standard. This is a perfect sample with nearly no size or strain broadening.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Powders were also characterized with regard to thermal behavior using simultaneous differential thermal analysis (TA-SDTQ600), with a heating rate of 5[degree]C/min in air up to 1000[degree]C, and specific surface area and pore volume by nitrogen absorption, from BET and Barrett-Joyner-Helenda (BJH) equations, respectively, at 77.3 K using a Micromeritics Tristar 3000 analyzer. Prior to BET measurement, powders were degassed for 24 h at 40[degree]C with pressure of 0.1 Pa. To prevent any possible crystallization during outgassing, a higher drying temperature was avoided.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Results and discussion

Particle size

Figure 1 shows the mean size of the particles for the prepared sol. It can be observed that the sol had a narrow particle size distribution with a hydrodynamic diameter of about 17.2 nm. The particle size of the [TiO.sub.2] sol reported in our previous study was [17.0 nm.sup.37] Therefore, no significant increase in the mean size of the particles was observed for [SrTi0.sub.3] sol, which confirms that the stability of the sol is maintained when a solution of strontium chloride is added into [TiO.sub.2] sol.

The zeta potential of the particles is shown in Fig. 2. The stability of the sol was achieved by both electrostatic stabilization and steric mechanisms. The electrostatic stabilization mechanism within the sol had an effect on particle interaction because of the distribution of charged species, whereas the steric repulsion mechanism involves PFA added to the system adsorbing onto the particle surface and preventing the particle surfaces from coming into close contact. It has been reported that usually the range of zeta potential in unstable sols is from -30 to +30 mV. (38) The average zeta potential of particles in the sol was 45.1 mV. This sol was found to be stable over 6 months since its zeta potential was constant during this period.

Crystal characterization

XRD analysis

Figure 3 shows the XRD patterns of as-synthesized and annealed powders in the range 300-800[degree]C for 1 h.

It is evident that all powders had crystalline structures containing a [SrTi0.sub.3] phase at all temperatures because of sharp peaks. Moreover, some traces of titania phases in the forms of anatase, brookite, and rutile structures were also observed. The anatase and brookite phases were found with the strongest peaks at 2[theta] = 25.3[degree] (1 0 1) and 2[theta] = 41.0[degree] (0 2 2), respectively, whereas the rutile phase was obtained with the strongest peak at 2[theta] = 21.4[degree] (1 1 0). Furthermore, the preferable orientation of [SrTi0.sub.3] was detected along the 2[theta] = 22.7[degree] (1 0 0) direction. It is evident that we succeeded in producing crystalline [SrTi0.sub.3] at the low temperature by a straightforward particulate sol-gel process. Therefore, this process has good potential for industrial applications because less energy is consumed during the production process.
Table 1: Chemical composition of 316L stainless steel (wt%)

Stages                                          Peak  Weight
                                            position  change
                                         ([degree]C)     (%)

Water evaporation                                 63   17.84

Crystallization of SrTi03-further water           72    6.45

evaporation                                      102    4.84
                                                 139    1.86

HPC decomposition                                289   12.10

Glass transition temperature                     868   10.19
of SfTi03-decomposition
of residual inorganic components


[FIGURE 7 OMITTED]

The effect of annealing temperature on average crystallite size of [SrTi0.sub.3]powders is shown in Fig. 4. The average crystallite size of all powders annealed in the range 300-400[degree]C was around 8.5 nm, and a gradual decrease down to 7 nm was observed after annealing at 800[degree]C. It is usually reported that the crystallite size of a material increases with an increase of annealing temperature because of crystal growth. (39) The observed inconsistent phenomenon in the present work can be explained by an increase of annealing temperature higher than the glass transition temperature ([T.sub.g]) of [SrTi0.sub.3]. It has been reported that the [T.sub.g] temperature of [SrTi0.sub.3] is around 730[degree]C, (40), (41) but based on the SDT result ("Thermal analysis" section), this temperature was determined to be in the range of 750-900[degree]C in this work. In addition, owing to the fact that the driving force of crystal growth is not provided at annealing temperatures below 400[degree]C, a constant crystallite size was obtained in this range of temperatures. Consequently, in the present work not only was [SrTi0.sub.3] material synthesized at the low temperature, but also one of the smallest crystallite sizes reported in the literature was obtained.

[FIGURE 8 OMITTED]

TEM analysis

Figure 5 highlights a TEM bright-field image of as-synthesized [SrTi0.sub.3] powder. As seen, the powder exhibits uniform morphology in particle size and shape. Furthermore, the relative electron diffraction pattern (inset of Fig. 5) indicates a random orientation for the powder with amorphous structure. The average crystallite size of the powders is around 8 nm, which is in good agreement with that obtained by XRD analysis.

Thermal analysis

Simultaneous differential thermal analysis (SDT) of [SrTi0.sub.3] powder is shown in Fig. 6. In addition, the description for the peak position and weight loss of the powders is summarized in Table 1. The powder undergoes a dehydration process as located by an endothermic peak al 63[degree]C. Crystallization of [SrTi0.sub.3] is observed in three stages by the exothermic peaks localized at 75, 102, and 139[degree]C. The addition of HPC into the sol influences the process of organic decomposition, as observed by an exothermic peak localized at 289[degree]C. This is consistent with the result reported previously for the composite [TiO.sub.2]-HPC powder. (9) It can be observed that the [T.sub.g] temperature of [SrTi0.sub.3] starts al 750[degree]C and continues up to 900[degree]C. Therefore, an endothermic peak located at 868[degree]C can be attributed to the [T.sub.g] temperature of [SrTi0.sub.3] material.

The weight change of the powder occurs at six stages, namely, below 63[degree]C, between 63 and 86[degree]C, between 86 and 123[degree]C, between 123 and 200[degree]C, between 200 and 840[degree]C, and above 840[degree]C. In the first stage (below 63[degree]C), the weight loss is a result of the evaporation of water. In the second, third, and fourth stages the weight losses are ascribed to the evaporation of structural water. Decomposition of HPC occurs in the fifth stage from 200 to 840[degree]C. In the last stage (above 840[degree]C) the weight loss can probably be ascribed to the decomposition of residual inorganic components.

Microstructure.

FE-SEM analysis.

Figure 7 shows surface micrographs of sol-gel prepared thin film annealed at 600[degree]C for 1 h. It can be observed that the deposited film had crystalline structure, which is in good agreement with XRD results. Furthermore, relatively dense, homogeneous, nanograms and crack-free film were obtained. This is because of the low evaporation rate of water as the dispersant medium, which has kept the films crack-free. In addition, the interstices between the particles caused by HPC are noticeable, resulting in a porous structure with irregular pore shape. It is evident that the average grain size of the film annealed at 600[degree]C is around 25 nm.

AFM analysis

Figures 8 and 9 illustrate 2D and 3D topographies of the thin films sintered at 600 and 800[degree]C for 1 h, respectively. As can be observed in the 2D images, all deposited films show that they are homogeneous, rough, and uniform, with nanosized grains. Moreover, the sol-gcl-deposited [SrTi0.sub.3] films have a nanostruc-tured and porous topography. Based on 3D images, it can be concluded that the film annealed at 600[degree]C has a hill-valley like morphology made up of small grains, whereas it shows a columnar like morphology after annealing at 800[degree]C. The average grain size of the film annealed at 600[degree]C is around 22 nm, and a gradual increase up to 32 nm occurred by annealing at 800[degree]C. This result is in good agreement with that obtained from FE-SEM images.

[FIGURE 9 OMITTED]

Specific surface area

Figure 10 illustrates the [N.sub.2] adsorption-desorption isotherms of the powders annealed at 400 and 600[degree]C for 1 h. The isotherm corresponding to these powders represents a combination of types II and IV corresponding to mesoporous materials (pore diameter >; 2 nm), with an upper size restriction because the adsorption and desorption branches are closed in the pressure region near saturation. (42) The BET-specific surface area of [SrTi0.sub.3] annealed at 400[degree]C was 92 [m.sup.2]/g, while it decreased down to 75 [m.sup.2]/g after annealing at 600[degree]C. In addition, the pore size of the powder was in the range 5-7 nm, corresponding to a mesoporous structure.

[FIGURE 10 OMITTED]

Conclusions

Nanostructured and mesoporous [SrTi0.sub.3] thin films and powders have been successfully prepared via a particulate sol-gel route. Titanium isopropoxide and strontium chloride were used as titanium and strontium precursors, whereas HPC was used as a PFA. The prepared sol was stable over 6 months, confirmed by zeta potential analysis. XRD analysis confirmed that as-synthesized powder had a crystalline structure. FE-SEM images showed that in all cases relatively dense, homogeneous, nanograins and crack-free films were obtained. AFM analysis confirmed that the deposited [SrTi0.sub.3] films had a nanocrystalline and porous structure. BET analysis revealed that the isotherm corresponding to [SrTi0.sub.3] powders represented a combination of types II and IV, corresponding to mesoporous materials.

Acknowledgments The authors wish to acknowledge Mr. David Nicol for his help with TEM analysis. M.R. Mohammadi would like to thank the financial support from Iran Nanotechnology Initiative Council.

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M. R. Mohammadi (El)

Department of Materials Science & Engineering, Sharif University of Technology, Azadi Street, Tehran, Iran e-mail: mrm41@cam.ac.uk; mohammadi@sharif.edu

D. J. Fray

Department of Materials Science & Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK

DOI 10.1007/sl 1998-011 -9347-9
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Author:Mohammadi, M.R.; Fray, D.J.
Publication:JCT Research
Date:Oct 1, 2011
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