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Highly dispersed SrTiO3 nanocubes from a rapid sol-precipitation method Yanan Hao, Xiaohui Wang* and Longtu Li SrTiO3 nanocubes and their hyperstable nanocrystalline sols were synthesized by a rapid sol-precipitation method under atmospheric pressure. Using triethylene glycol (TEG) to control the hydrolysis rate of tetrabutyl titanate, the SrTiO3 nanocrystalline sol was obtained in as little time as 2 h. The formation kinetics of the SrTiO3 nanocubes indicated that controlled hydrolysis is critical to the generation of a well defined cubic shape. The Fourier transform infrared (FT-IR) spectrum confirms the existence of TEG molecules on the surface of the particles and explains the high dispersion of the nanocubes in polar

Received 10th January 2014 Accepted 30th April 2014 DOI: 10.1039/c4nr00171k www.rsc.org/nanoscale

solvents. Owing to the large specific surface area (99.065 m2 g1), cubic SrTiO3 nanocrystals showed enhanced photocatalytic activity. A high-quality SrTiO3 nanocrystal film was prepared by spin-coating of the hyperstable sol at 100–160  C, providing a new low-temperature route for the fabrication of perovskite thin films.

Introduction Perovskite oxides have been extensively studied due to their widespread applications in electronics, sensing, catalysts, and energy storage.1 SrTiO3 is a typical perovskite-type oxide with good electronic performance, high photocatalytic activity and tunable chemical and physical properties that has been widely used in solar cells, H2 generation, semiconductors and highdensity dynamic random access memory.2 It is well known that the properties of perovskite nanoparticles are highly dependent on morphology and crystallite size.3 Especially, the cubic morphology has attracted extensive attention because it can provide larger specic surface areas, interfaces and abundant active sites for reactions and transportations. Kuang et al. observed enhanced photocatalytic activity in hydrogen evolution from water splitting with porous SrTiO3 nanocubes.4 Moreover, nanocubes are the best candidates for the formation of close-packed structures. Minura et al. reported that highly ordered assemblies consisted of BaTiO3, SrTiO3 and BaTiO3– SrTiO3 mixture nanocube single crystals showing a non-linear piezoresponse curve.2g Consequently, morphology control is becoming increasingly important in the synthesis of nanocrystals. Nanosized SrTiO3 particles are either synthesized by solidstate reaction, sol–gel processes, hydrothermal/solvothermal reaction, molten salt synthesis, combustion or by template synthesis.5 The control of the particle size and/or design of the morphological properties can be realized by modifying the

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. E-mail: wxh@mail. tsinghua.edu.cn; Tel: +86 10 62784579

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synthesis parameters, such as the raw material ratios, reaction time, temperature, etc.,6 or via introduction of dopant atoms or surfactants.7 Da silva et al. found that the calcination atmosphere in a polymeric precursor method played an important role in both crystal size and photoluminescent behavior of the SrTiO3 nanocrystallites.6b In Alfredsson et al.'s model, the ionic radius of the doping ions greatly affected the crystal morphologies. Fujinami et al. found that oleic acid can promote the formation of sub-10 SrTiO3 nanocubes and enhance their dispersibility in non-polar organic solvents.3 Different synthetic routes can also lead to the diverse particle morphologies.8 Hu et al. reported a microemulsion method to synthesize sub-20 nm SrTiO3 nanocuboids with a lamellar structure.9 Dang et al. synthesized monodispersed SrTiO3 nanocubes by a hydrothermal method.10 Conventional sol–gel processes or solid-state reactions cannot acquire ultrane SrTiO3 particles based on the high-temperature calcining step. Besides, spherical or irregular morphology was always obtained by the sol–gel processes because of the rapid formation of a gel structure.5c,11 In this study, we use a rapid sol-precipitation method to synthesize highly dispersible SrTiO3 nanocubes and their hyperstable sols. Similar to the sol preparation in the conventional sol–gel process, the reaction was conducted under mild conditions (160  C) without the aid of high pressure or a protective atmosphere. The crystallization and grain growth rate were rather low as conrmed by the XRD and TEM analyses. Here, we rst proposed that the controlled hydrolysis is a determinant for the formation of a well-dened cubic shape. The spherical or irregular shape of SrTiO3 particles derived from the liquid phase reactions was mainly caused by severe hydrolysis at the initial reaction stage. The SrTiO3 nanocubes have relatively large specic surface areas (99.065 m2 g1) and thus

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they exhibit an enhanced photocatalytic activity in the decomposition reaction of Rh B under UV irradiation. The FT-IR spectrum revealed that the surface of the SrTiO3 nanocubes was capped with a TEG layer, which makes the SrTiO3 nanoparticles soluble in polar organic solvent and form a hyperstable nanocrystal sol. Owing to the high stability and good dispersibility, this sol can suit a variable coating (spin-coating, dip-coating, and cast-coating), spraying, and printing techniques. We successfully prepared highly crystallized SrTiO3 thin lms by spin-coating the nanocrystal sol at 100–160  C with no annealing step. This low-temperature route for highly crystallized thin lms is signicant for future device fabrications.

Experimental Hyperstable SrTiO3 sol and nanocubes were synthesized via the controlled hydrolysis of tetrabutyl titanate [Ti(OBu)4; 98%] in triethylene glycol (TEG; 99%) solvent. The synthetic route is quite simple, which can be readily extended to large-scale fabrication. Firstly, 16 mmol Sr(OH)2$8H2O (99%), 16 mmol Ti(OBu)4, 8 mL NH4OH (25–28%) and 0.74 g PVP (K30) were added to 40 mL TEG solvents. All reagents were obtained from the Sinopharm Chemical Reagent Company (Beijing, China). Then the mixture was gradually heated to 160  C for 2 h with stirring by a magnetic agitator. Aer cooling, a yellow transparent hyperstable sol was formed. Deionized water (volume ratio: H2O/organic solvent z30) was employed to precipitate the sols. Aer separation, the as-prepared SrTiO3 nanocubes were washed with ethanol and deionized water and then dried at 80  C for 12 hours. A SrTiO3 nanocrystal thin lm was deposited on Pt-coated SiO2/Si by spin-coating of the hyperstable SrTiO3 nanocrystal sol at 3000 rpm for 40 s, followed by solvent evaporation at 160  C for 5 min. Multiple spin-coatings were applied to achieve various lm thicknesses. Top electrodes of platinum, 0.4 mm in diameter, were ion-sputtered onto the surface of the SrTiO3 nanocrystal thin lm for measurements of the electrical properties. Dynamic light scattering (DLS) analysis of the SrTiO3 sol was carried out by a Nanoparticle Analyzer (SZ-100, Horiba Ltd., Kyoto, Japan). The phase structures of the SrTiO3 nanoparticles were determined by powder X-ray diffraction (XRD; D/Max-2500, ˚ Rigaku Co., Tokyo, Japan) using CuKa radiation (l ¼ 1.5418 A)

in step scan mode in a 2q range from 20 to 80 with a step of 0.02 and a retention time of 1 s. The average crystallite size was estimated by Scherrer's equation using the soware “THCLXPD” with the Fourier integral method. The prole of the (200) peak was also step scanned in a 2q range from 42 to 50 with a step of 0.02 and a retention time of 1 s. The instrumental broadening was deducted by deconvolution before the calculation. The high resolution transmission electron microscopy (HR-TEM) images and selected area electron diffraction (SAED) patterns were obtained using a high-resolution TEM (JEM-2010, JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 200 kV and TEM (HT-7700, Hitachi Ltd., Tokyo, Japan) at an accelerating voltage of 100 kV. The particle size distribution histogram of the SrTiO3 nanocubes derived from the TEM micrograph was

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obtained from 640 nanoparticles. The Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on an FT-IR spectrometer (Vertex 70v, Bruker Ltd., Germany) by mixing the sample with KBr in a mass ratio of SrTiO3–KBr ¼ 1 : 100. And the spectrum was measured in a range from 600 cm1 to 2000 cm1 in transmission mode with 128 time measurements and a resolution of 4 wavenumbers. The specic surface area of the powders was obtained using the Brunauer–Emmett–Teller (BET) method (Quadrasorb SI-MP, Quantachrome, Boynton Beach, FL, USA). The surface morphology and lm thickness were observed by a scanning electron microscope (SEM; JSM7001F, JEOL Ltd., Tokyo, Japan) operated at 20 kV. The photocatalytic activity of the samples was estimated by analyzing the degradation of the Rhodamine B dye (Rh B; 2.5 mg L1) in a 100 mL aqueous solution with the same amount of each sample (0.1 g) under UV irradiation. An iodine– gallium lamp (Shanghai Hualun Lamp Company, 400 W and l: 350–450 nm) was used as the light source. Before UV irradiation, the suspension was le in the dark for 120 min to reach an adsorption–desorption equilibrium state. Photocatalytic decomposition of Rh B was monitored by changes in absorbance at 462 nm of the UV-Vis spectra (Hitachi U-4100 spectrophotometer).

Results and discussion Sub-10 nm SrTiO3 nanocubes and their hyperstable nanocrystal sols were obtained by this method. The TEM image (Fig. 1a) shows that the nanocubes are quite uniform both in size and in morphology. The inserted SAED pattern reveals typical diffraction rings of the SrTiO3 perovskite structure, which conrms the

Fig. 1 (a) TEM image, (b) HR-TEM image, (c) particle size distribution of the SrTiO3 nanocubes, and (d) DLS analysis of the SrTiO3 nanocrystal sol prepared by the rapid sol-precipitation method at 160  C for 2 h.

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high crystallinity of the particles. Compared with the reported hydrothermal preparations, the SrTiO3 nanoparticles prepared by this method have a more regular cubic shape, smaller particle size and higher monodispersity due to the surface coating of the PVP and TEG organic layer.3,9,10 The HR-TEM image (Fig. 1b) shows well-ordered lattice fringes of 0.278 nm from the {110} planes and conrms the single-crystallinity of a separated SrTiO3 nanocube. The inserted SAED pattern of an individual SrTiO3 nanocube is also present in a single set of diffraction spots in a square arrangement. From the TEM observation, the average particle size of the nanocubes is as small as 8.60 nm (Fig. 1c). A nearly transparent SrTiO3 nanocrystal sol was formed during the synthetic procedure. It is worth noting that this sol cannot be precipitated even at 10 000 rpm centrifugation. Furthermore, they were stable for several months without any precipitation, aggregation or gelation. Fig. 1d shows that the DLS analysis of the hyperstable sol is slightly larger than that measured by TEM. As no obvious aggregation with a diameter of 25–30 nm was observed from numbers of the TEM images, this can be explained by the spontaneous connection of the nanocubes which have the tendency to form iso-oriented aggregations in TEG solvent. The DLS result conrms that the hyperstable sol consists of welldispersed SrTiO3 nanoparticles instead of the mixture of the reagents. Correspondingly, a typical Tyndall effect is observed for this transparent sol (0.2 M), shown in the inserted photographic image. Derived from the FT-IR spectrum in Fig. 2, there is an organic surface layer on the SrTiO3 nanocubes, which is mainly composed of TEG molecules and PVP long chains. The spectrum for TEG shows two obvious bands at 1118 and 1057 cm1, which can be assigned to the stretching vibrations of C–O and O–H bonds. Correspondingly, the SrTiO3 nanoparticles that dried directly from the sol also show a strong band at around

1118–1057 cm1, indicating that a large amount of TEG molecules were attached on SrTiO3 nanoparticles before the precipitation. This TEG surface layer promotes the formation of SrTiO3 nanocrystal sol and makes it miscible with other polar solvents, such as alcohol, acetone, and even low volume fraction of water (

Highly dispersed SrTiO₃ nanocubes from a rapid sol-precipitation method.

SrTiO₃ nanocubes and their hyperstable nanocrystalline sols were synthesized by a rapid sol-precipitation method under atmospheric pressure. Using tri...
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