Synthesis of Hyperbranched Perovskite

Article pubs.acs.org/crystal

Synthesis of Hyperbranched Perovskite Nanostructures Ting Yang,† Zachary D. Gordon,‡ and Candace K. Chan*,† †

Materials Science and Engineering and ‡Chemical Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, 501 E Tyler Mall ECG 301, Tempe, Arizona 85287, United States S Supporting Information *

ABSTRACT: Complex functional perovskite oxides find use in a wide range of applications as dielectric, electronic, photocatalytic, optical, and ion-conducting materials. The ability to synthesize perovskites in hierarchical structures can allow for new properties or phenomena not observable in bulk morphologies. Herein we report on the solution-phase synthesis of cubic perovskite potassium lanthanum titanate into orthogonal nanostructures with hyperbranched and hexapod morphology through a facile hydrothermal synthesis without using a template, catalyst, substrate, or structure-directing agent. A systematic study of the reaction conditions and role of precursors was performed and a growth mechanism based on homogeneous dissolution−precipitation was determined.

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INTRODUCTION Metal-oxide materials can display unique electronic, structural, and optical properties when scaled down to nanometer dimensions.1,2 Perovskite materials in particular show a diverse and rich variety of functionalities and properties and find utility in many applications such as ferroelectrics and piezoelectrics,3,4 superconductors,5,6 dielectrics,7 heterogeneous catalysis,8 photocatalysis,9−12 and fuel cells13,14 and batteries.15−18 In many of these applications, nanostructured perovskite materials such as nanocrystals and nanowires have shown improved properties and revealed new phenomena compared with bulk morphologies.19−25 To this end, the ability to fabricate or synthesize functional oxides in a variety of nanostructured geometries is important. Recently there has been much interest in developing synthesis methods to obtain nanostructures with higher order geometries such as branching or hyperbranching. Such branched structures have been used in photovoltaic,26−28 photoelectrochemical,29−31 and optoelectronic32,33 applications. Thus far, these structures have been obtained using one of the following strategies: (1) catalyst-seeded growth in vapor deposition34−41 and solution-phase26 growth processes, (2) solution-phase growth with structure-directing agents such as polymers or surfactants,42−45 including colloidal branched polyhedra such as multipods,46 and (3) screw dislocation driven growth.28,36,47 A number of studies36−39,48,49 have reported the formation of orthogonal hyperbranched nanostructures using materials with a cubic crystal structure. In these cases, nanowire branches grow along the ⟨100⟩ directions and 90° to each other, with multiple levels of branches forming perpendicular to a previous generation of branches in an epitaxial manner. The best example of such hyperbranched nanostructures have been demonstrated in binary lead chalcogenide materials.36−39 To the best of our knowledge, however, there have not been any reports on the synthesis of analogous orthogonal © 2013 American Chemical Society

hyperbranched perovskite nanostructures, despite a similar cubic crystal structure. It is also unlikely that the vapor-phase growth strategies used for lead chalcogenides can be adapted for complex oxide materials due to their intrinsic differences in chemical properties. Herein we report the synthesis of orthogonal hyperbranched perovskite potassium lanthanum titanate (KLTO) nanostructures through a hydrothermal route, without using a template, catalyst, substrate, or structuredirecting agent. Solid solution titanates containing lanthanum and monovalent cations have been studied as dielectric50−52 and photocatalyst53 materials, as well as for their formation into layered structures.52,54,55 However, this is the first time such hierarchical morphologies have been observed in a cubic (A,B)TiO3 perovskite. The growth mechanism for the KLTO nanostructures was studied by varying the reaction time, reaction temperature, and type of precursors used. Detailed structural and morphology characterization was performed in order to better understand the growth mechanism.

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EXPERIMENTAL SECTION

All chemicals were purchased from commercial sources and were used without further purification. In a typical synthesis, 0.04 g of TiO2 nanopowder (25 nm anatase, Aldrich) and 0.14 g of La(NO3)3·6H2O (Fluka) were mixed in 12 mL of 4.5 M KOH (Sigma-Aldrich) solution. The mixture was heated at 200 °C in a 45 mL volume Teflon-lined stainless steel autoclave (Parr) for 12 h. The resulting products were filtered and washed with dilute hydrochloric acid to remove any La(OH)3 that may have precipitated in the synthesis. Deionized water was used to rinse the product until the filtrate pH became neutral, and the remaining white powder was dried overnight at 60 °C. The synthesis was also performed using K2Ti8 O17 nanowires56 or TiO2−B nanowires57 instead of anatase as the Ti Received: April 12, 2013 Revised: July 23, 2013 Published: August 1, 2013 3901

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source. In some experiments, 8 M LiOH (Sigma-Aldrich) or 8 M NaOH (Sigma-Aldrich) was used instead of KOH. Microwave hydrothermal synthesis was also performed using a CEM Explorer SP reactor (300 W, 2.56 GHz, 200 °C, 12 h), which had the capability for magnetic stirring during the reaction. X-ray diffraction (XRD) was used for phase identification and crystal structural determination on a PANalytical X’Pert Pro high resolution X-ray diffractometer with Cu Kα radiation. The sample morphology was examined with an XL30 Environmental FEG scanning electron microscope (SEM). Transmission electron microscopy (TEM) studies were performed on a Philips CM200-FEG and a JEOL 2010F microscope. All microscopes were equipped with energydispersive X-ray spectrometers (EDS) and were used for composition studies.

certain crystallographic directions. In the hyperbranched structures, new generations of nanowires were observed at a right angle to their prior generation. Nanoplates were also observed, but only made up about 10% of the sample. The XRD pattern of the typical product matched that for K0.469La0.531TiO3 (PDF 01-089-4903 in the ICDD database), which has a cubic perovskite structure of the form (A,B)TiO3 with lattice constant of 3.90 Å (Figure 1d). La(OH)3 was also observed as a byproduct in the XRD pattern but could be removed by washing the product with acid. High-resolution TEM (HRTEM) images (Figure 2) confirmed the observation that the side branches grew

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RESULTS AND DISCUSSION The SEM images of the products from a typical synthesis are shown in Figure 1a. Two types of orthogonal nanostructures

Figure 2. (a) TEM image of a branched nanowire. Insets show the FFT diffractograms of their corresponding region (boxed area). Arrows indicate growth directions along the {100} planes. (b) SAED pattern of a single nanowire. (c) HRTEM image of a branching region with lattice spacings labeled. (d) TEM image showing partially merged nanowire branches. (e) TEM image showing a branching interface with rounded corner.

Figure 1. (a) SEM image of the product distribution for a typical synthesis. (b) A hyperbranched octahedron nanowire cluster. (c) A hexapod nanowire cluster. (d) XRD patterns of a typical sample before (upper) and after (lower) acid washing. ●: La(OH)3.

were observed, in which clusters of nanowires were oriented into either a hyperbranched octahedron (Figure 1b), or a jacklike hexapod (Figure 1c). The diameter of the individual nanowires was about 50 nm or less, and the size of the clusters of nanowires ranged from 200 nm to 2 μm. The arrangement of nanowires was highly directional, with different branches of nanowires growing only along three directions perpendicular or parallel to each other, suggesting preferential growth along

perpendicular to the main branch. Lattice fringes were observed to extend continuously from the main branch to the side branches. Fast Fourier transform (FFT) of the HRTEM images at the branching interfaces (boxed areas in Figure 2a) further confirmed this by showing distinct spots and orthogonality along the growth directions. The selected-area electron diffraction (SAED) pattern was indexed as shown in Figure 3902

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2b and is consistent with the square lattice in the FFT diffractograms. The lattice spacing in the growth direction of the main branch was measured to be 1.95 Å, and the spacing perpendicular to the main branch was 3.89 Å (Figure 2c), which are in agreement with the 1.95 Å spacing for the (200) planes and 3.90 Å spacing for the (100) planes, respectively, for KLTO. These results are consistent with the cubic structure observed in XRD, although a small tetragonal distortion, which is commonly observed in perovskites,51 cannot be ruled out. This indicates that the main branches of the structure grew along one of the six ⟨100⟩ directions and the side branches grew along other ⟨100⟩ directions that are perpendicular to the direction of the main branch. It is worth noting that the surfaces of the nanowires were not smooth and there appeared to be defects near the surface of the main branch (dark streaks), which may be due to planar defects. Many structures were observed with adjacent nanowires partially merged, with incomplete crystal growth occurring in the gap separating them (Figure 2d). The interfacial junction between two branches was also often observed as a rounded corner (Figure 2e). These results suggest that the surfaces and corners of these structures are high energy sites that may promote additional crystal growth. EDS data were collected from multiple hyperbranched clusters during SEM examination, and the calculated result is shown in Table S1, Supporting Information. The average atomic weight percentages were normalized to Ti = 1, and the resulting composition was K0.36La0.83TiO2.72 with a much lower potassium concentration than expected based on the XRD pattern reference. EDS from the TEM gave a similar result with a K/La/Ti ratio of 0.32:0.71:1. Monovalent cation deficiency in (A,B)TiO3 materials has been observed for large diameter ions such as A = Rb+, K+, which have much larger ionic radii (1.52 and 1.33 Å, respectively) than La3+ (1.15 Å).50 The XRD reference pattern was obtained from KLTO synthesized using solid state reaction.50 It is possible that KLTO synthesized using hydrothermal reaction can have higher K+ deficiency due to differences between the two types of reactions, such as the hydration shells surrounding the cations. To better understand the growth mechanism for these orthogonal hyperbranched nanostructures, a series of experiments were conducted to determine the role of each parameter. The effect of reaction time (Figure S1, Supporting Information), reaction temperature (Figure 3), type of precursor (Figures S2 and S3, Supporting Information), and KOH concentration and type of alkaline precursor (Figure 4) were evaluated. The XRD results showed that for reaction times less than 2 h, the TiO2 precursor was the only solid in the solution (Figure S1a−c, Supporting Information), and no nanowire structures were observed. After 2 h of reaction, some hyperbranched structures were observed starting to form (Figure S1d, Supporting Information), which indicates that the rate of reaction is relatively fast. When the temperature was raised from 200 to 250 °C and all other conditions kept constant, the amount of previously mentioned nanoplate morphology significantly increased. Figure 3a shows an SEM image of a cluster of nanoplates observed mixed with the hyperbranched structures. XRD showed that the product was a mixture of KLTO and tetragonal H2La2Ti3O10 (HLTO) (Figure 3e). The comparable peak intensity of the HLTO peaks to that of KLTO is also an indication of the change in their relative amount. TEM

Figure 3. (a) SEM image of nanoplates. (b) TEM image at the corner of a nanoplate. Upper-left inset shows the FFT diffractogram of the nanoplate; bottom-left inset shows its HRTEM image with lattice spacing labeled. (c) SEM image of sample formed at 160 °C. (d) SEM image of sample formed at 150 °C. (e) XRD patterns of samples synthesized at different temperatures: ∗, KLTO; ▲, anatase TiO2; ■, H2La2Ti3O10.

examination revealed that the plates were single crystals and FFT showed a square pattern (Figure 3b). The d-spacing measured from the HRTEM image was 2.71 Å, which is close to the 2.70 Å spacing for the (110) planes of HLTO, and EDS analysis on the plate gave a La/Ti ratio of 1:1.21 without potassium present. This confirms that the nanoplates are HLTO and the edges of the plates are parallel to the ⟨100⟩ directions. In some of the nanoplates, nanowires were observed at the edges of the plates, as if the nanoplates may have initially started as separated nanowires, then formed into nanoplates due to lateral growth. When lowering the temperature to 160 °C, peaks from anatase and KLTO were observed in the XRD pattern (Figure 3e). SEM imaging revealed clusters showing octahedral morphologies but without fully formed nanowire structures (Figure 3c). At 150 °C, the product consisted of aggregates of nanoparticles (Figure 3d) and the XRD pattern indicated that they are the TiO2 precursor. These results show that the KLTO formation process needs reaction temperatures higher than 160 °C. Otherwise, the TiO2 precursor remained unreacted. Interestingly, although La(OH)3 is insoluble in basic solution, lanthanum ions played an important role in the synthesis of hyperbranched structures. Specifically, the KLTO composition was important for obtaining these structures. 3903

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the formation of K2Ti8O17 nanowires, which are readily formed in the absence of La(NO3)3 as mentioned previously. To further understand the growth mechanism, presynthesized K2Ti8O17 nanowires (Figure S2a, Supporting Information) and TiO2−B nanowires (Figure S2b, Supporting Information) were used as Ti sources. The composition and morphology of the Ti source did not seem to play a major role, because similar KLTO hyperbranched morphologies were observed (Figure S3a,b, Supporting Information) when using these nanowire precursors as when using the anatase nanopowder precursor (Figure 1). These results show that the Ti source is dissolved in the alkaline solution under hydrothermal treatment and reacts with the La3+ ions to form KLTO. The concentration of KOH was found to be a crucial factor for the formation of hyperbranched structures. When a 1.2 M KOH solution was used, the product only contained loose nanowires and nanoplates (Figure S4a, Supporting Information) and the XRD pattern (Figure S4b, Supporting Information) matched that of HLTO. The observation that HLTO can adopt a nanoplate morphology is consistent with the result from samples synthesized in 4.5 M KOH. When KOH concentration was increased from the typically used 4.5 to 8 M, a mixture of products was observed consisting of long nanowires and hyperbranched structures with short branches (Figure 4a). XRD analysis prior to acid washing showed that KLTO and La(OH)3 were the only two phases in these samples (Figure 4e), with the latter likely being the long nanowires observed in the SEM. Further increasing the KOH concentration to 11 M showed a similar result, except the KLTO hyperbranched structures had branches only a few tens of nanometers in length (Figure 4b). TEM imaging of these hyperbranched structures showed a spherical core in the center but with nanowires still growing in orthogonal directions (Figure 4b, inset). As shown in Figure 4b, some of the larger structures were already showing octahedral morphologies, while smaller ones were more spherical with textured surfaces representing nucleation sites for the nanowires. The presence of hyperbranched nanostructures with shorter branches can be explained by the lack of free La3+ ions in the solution due to the precipitation of La(OH)3 at higher pH. Therefore, 4.5 M was found to be the appropriate KOH concentration in order to maximize the yield of hyperbranched nanowires. Additionally, hyperbranched structures were not observed when other alkaline solutions were used in lieu of KOH. When 8 M LiOH was used, a mixture of microrods and nanoparticles was observed (Figure 4c). The XRD patterns (Figure 4e) showed the presence of La(OH)3 and anatase TiO2. After the product was treated with acid, only the nanoparticles remained (Figure 4c, inset), confirming that the rods were La(OH)3 and the nanoparticles were unreacted anatase. This is consistent with previous studies that found LiOH to be too weak of a base (compared with KOH or NaOH) to react with TiO2.60 When 8 M NaOH was used, La(OH)3 rods were also observed as well as some structures with a cube morphology (Figure 4d). EDS analysis indicated the presence of sodium in these materials, and the XRD patterns showed reflections that matched Na0.5La0.5TiO3, which has a cubic perovskite structure with lattice constant of 3.87 Å. Using these results and observations, a growth mechanism for these KLTO orthogonal hyperbranched structures and HLTO nanoplates is proposed as follows. In contrast to other

Figure 4. SEM image of synthesis using (a) 8 M KOH, (b) 11 M KOH (inset shows TEM image), (c) 8 M LiOH (inset shows postacid-wash product of TiO2 powders), and (d) 8 M NaOH. (e) XRD patterns of samples reacted with different bases prior to acid washing: ∗, KLTO; ●, La(OH)3; ▲, Na0.5La0.5TiO3.

When no La(NO3)3 was added as reagent, the product was potassium titanate (K2Ti8O17) nanowires (morphology shown in Figure S2a, Supporting Information), consistent with other reports using similar reaction conditions.56 In the absence of a Ti source, La(OH)3 nanowires and microrods were formed (Figure S2c, Supporting Information), also consistent with other studies.58,59 When first added into the alkaline solution, La3+ ions and hydroxide ions were observed to precipitate, forming bulk La(OH)3 at room temperature. With hydrothermal treatment, these precipitates underwent dissolution, nucleation, and growth to become nanowires or microrods. In the presence of a Ti source, the lanthanum ions would react to form KLTO hyperbranched structures. Using presynthesized La(OH)3 nanowires instead of La(NO3)3 as the lanthanum source did not result in formation of hyperbranched structures. Instead, the only product was La(OH)3 with the same nanowire and microrod morphology as the precursor, and there did not appear to be any other structures (Figure S3c, Supporting Information). The absence of any titanium-containing solid in the XRD pattern (Figure S3d, Supporting Information) suggests that the TiO2 precursor may have hydrolyzed but did not crystallize. This suggests that once crystallized into nanowires and microrods, the La(OH)3 would not dissolve and provide free La3+ ions in solution, leaving the Ti source unreacted. The La(OH)3 nanowires also appeared to suppress 3904

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Figure 5. (a) Schematic of growth steps for KLTO orthogonal nanostructures. (b) SEM image showing a new generation of nanowires grows perpendicular to its previous one. Arrows 1−4 indicate different branching generations. (c) Schematic of the HLTO nanoplate growth steps (d) TEM images of partially formed nanoplates. (e) SEM image of nearly completely formed nanoplates.

have sufficient alkalinity in order to hydrolyze and dissolve the Ti source. Once dissolved, the Ti4+, La3+, and K+ ions had to nucleate seeds from which the KLTO could grow. Under the typical growth conditions, these seeds could not be observed in the final product due to the high degree of branching. Under the conditions using high KOH concentration, reduction of free La3+ through the competing process of La(OH)3 crystallization (and perhaps also from the reduced mass transport rates due to the increased viscosity of the solution) resulted in shortened branching, and the seeds were clearly visible (Figure 4a,b). No KLTO seeds formed at all when La was absent from the solution, again supporting the dissolution−precipitation mechanism. The nanowires then formed at surface defects on the seeds along the six equivalent ⟨100⟩ directions. Some of the nanowires showed higher degrees of branching where side branches formed on main branches (Figure 5b), while those without side branching formed hexapod clusters. With regard to the formation of HLTO nanoplates, our proposed growth steps are shown in Figure 5c. Since HLTO was observed to adopt both nanowire and nanoplate morphologies (Figure S4, Supporting Information), it is possible that the nanoplates are formed from the lateral expansion of the nanowires. Ions from the solution could attach preferentially to the a and b directions, resulting in a step-like growing pattern as shown in Figure 5c−e. HLTO is a layered perovskite with Ruddlesden−Popper structure,74 whereby the ab plane adopts a perovskite structure, but some perovskite layers are sandwiched between rock salt type layers along the caxis. Therefore, growth is not equivalent along the three axes (a and b are the same, but different from c), resulting in planar growth parallel to the ab plane. This can promote the formation of 2D nanoplate morphologies. Additionally, it was observed that the majority of the nanoplates did not grow beyond certain dimensions (approximately 2 μm × 2 μm), suggesting the existence of a critical size.

solution-phase synthesis methods for growing perovskite nanowires61,62 and branched structures in materials with highly symmetric crystal structures,42,63 our system for obtaining KLTO hyperbranched structures does not contain catalysts or structure-directing agents. Furthermore, branching is not due to differences in growth rates in two or more crystal structures within the same material (e.g., polymorphism, which can lead to multipod structures)64,65 since only a cubic structure was observed for the KLTO hyperbranched structures. An in situ transformation mechanism uses the precursor as a template for topochemical growth of the product.66 BaTiO3, SrTiO3, and PbTiO3 have been synthesized in this manner from TiO2 or titanate precursors.56,66−72 The transformation mechanism puts constraints on the size of the product particles, which in general cannot be larger than their parent particles, and there is usually close crystallographic agreement between the parent and product compounds.66 In our study, the coexistence of the TiO2 precursor and KLTO hyperbranched product was observed at low temperatures and short reaction times. However, at the optimum reaction conditions, the hyperbranched product was observed no matter what the Ti source was. Moreover, the precursor morphology (e.g., nanowire or nanoparticle) did not seem to have any correlation to the shape of the final product. We also did not see any evidence of heterogeneous templating or indication that growth may occur through oriented attachment of small segments48,73 to form the hyperbranches. These observations suggest that at the appropriate temperature and reaction time, the Ti source can become completely dissolved, followed by reaction with free La3+ ions for KLTO nucleation and growth through a homogeneous dissolution−precipitation mechanism rather than a templating or transformation mechanism. Our proposed growth mechanism for the KLTO hyperbranches can be visualized by the schematic in Figure 5a. First, the precursors are dissolved and nucleate seeds, which then grow in size. Based on our observations, the pH of the reaction solution had to 3905

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Notes

Hierarchical and dendritic structures are often seen in electrodeposition, where the structure morphology can be controlled by tuning the growth rate and mass transport rate of reagents using the deposition voltage.75−77 Under hydrothermal conditions, reagents have higher solubility so supersaturation is usually low, resulting in particles with regular polyhedral faces if the growth is not limited by mass transport of precursor ions to the growing interface region. The equilibrium cube morphologies observed when NaOH was used (Figure 4d) instead of KOH may reflect mass transport and solubility differences in the two solutions. However, when the precursors are consumed faster than the mass transport rate, the growing crystals can show branching or dendritic structures. The observation of hexapod and hyperbranched morphologies in the typical synthesis conditions (Figure 1a) may be due to inhomogeneities in the system caused by convection and lack of stirring during the reaction. To test this, KLTO synthesis was performed in a microwave hydrothermal synthesis reactor, which allowed for magnetic stirring during the reaction. The predominant product was KLTO hyperbranched structures with a narrower size distribution than that observed in the conventional hydrothermal synthesis (Figure S5, Supporting Information). This can be explained by the improved temperature distribution and mass transport, which result in a single morphology.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the use of facilities within the LeRoy Eyring Center for Solid State Science at ASU and thank K. Weiss and Z. Liu for assistance with TEM. We also thank P.A. Crozier and K. Sieradzki for valuable discussions.

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CONCLUSION In summary, we synthesized cubic potassium lanthanum titanate nanowires with orthogonal hyperbranched and hexapod structures via a simple hydrothermal route without using a template, catalyst, structure-directing agent, or substrate. H2La2Ti3O10 nanoplates also formed as a significant side product at higher reaction temperature. The formation of these structures is through a homogeneous dissolution− precipitation reaction and is dependent on the mass transport rate of the reactants, which can be tuned with the reaction temperature and use of stirring. To our knowledge, this is the first demonstration of hierarchical nanostructures in a complex quaternary perovskite material. Generalization of this synthesis to other perovskite materials may lead to exciting new properties and physical phenomena for electronic, optical, catalysis, and energy storage applications.

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ASSOCIATED CONTENT

S Supporting Information *

EDS data obtained from SEM on KLTO hyperbranched structures, XRD and SEM of precursors used, and results for experiments with different reaction conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. C.K.C. and T.Y. conceived the experiments and wrote the paper. T.Y. and Z.D.G. performed the synthesis and characterization. Funding

This work was supported by the new faculty startup funds from the Fulton Schools of Engineering at ASU. Z.D.G. thanks the Fulton Undergraduate Research Initiative for funding. 3906

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dx.doi.org/10.1021/cg4005483 | Cryst. Growth Des. 2013, 13, 3901−3907