Biopolymer-Mediated Inorganic Crystal Growth at Reduced

Mar 20, 2014 - We describe and develop an example of the successful application of a broadly applicable biomimetic growth process to the crystal growt...
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Biopolymer-Mediated Inorganic Crystal Growth at Reduced Temperature with Enhanced Kinetics and Morphological Direction Zili Zhang,†,‡ Hongli Suo,† Lin Ma,† Ahmed Kursumovic,‡ Yan Xu,† Min Liu,† Judith L. MacManus-Driscoll,‡ and Stuart C. Wimbush*,‡,§ †

Key Laboratory of Advanced Functional Materials, Ministry of Education, College of Materials Science and Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing 100124, China ‡ Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom § The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand S Supporting Information *

ABSTRACT: We describe and develop an example of the successful application of a broadly applicable biomimetic growth process to the crystal growth of the complex inorganic quaternary high-temperature superconducting ceramic oxide YBa2Cu3O7−δ. By combining effective biopolymer mediation with established synthetic single crystal growth processes, we leverage a reduction in the required growth temperature from 1045 to 920 °C to enable the successful use of a molten salt synthesis technique in place of the high-temperature solution method commonly employed. Further, the influence of the biopolymer on the seed crystal nucleation provides for morphological control of the synthesis product, offering the potential for designed macromorphologies through biopolymer selection or customization. The method described is directly applicable to other difficult-to-synthesize materials that presently rely on similar conventional techniques, or for which such approaches have been found to fail, thereby providing a novel route to the successful bulk synthesis of complex refractory materials.

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ubiquitous throughout the field of functional materials, enabling properties as diverse as ferro- and piezoelectricity,7 ferromagnetism8 and multiferroicity,9 giant10 and colossal magnetoresistivity,11 high-κ dielectricity,12 and transparent conductivity.13 This multitude of functionality provides for technological applications across the gamut in the fields of energy, optical, electronic, and spintronic materials, as well as in nanoscale heterostructures14 and interfaces.15 However, single crystal growth of perovskite materials is universally difficult in spite of their wideranging technological applicability, as a result of component volatility,16 incongruent melting,17 low growth rates,18 or abnormal grain growth,19 consequently requiring complex and unwieldy growth processes. In the case of YBCO, the quaternary composition of the material and the necessity of cation ordering across the perovskite-like units, the variable oxygen stoichiometry and the requirement for adequate oxygenation in order to realize the optimal superconducting properties, and the added complication of the distorted orthorhombic structure and resulting twinning tendency all

INTRODUCTION Artificial approaches to biomineralization seek to replicate the processes used in nature to synthesize complex nanostructured inorganic materials under benign environmental conditions.1 Evolutionary intelligence has resulted in methods able to effectively reduce the processing temperature,2 neutralize the pH,3 speed the process,4 and control the resulting material morphology5 to levels beyond those attainable with conventional synthetic chemistry techniques. Here, we apply a biopolymer-mediated synthesis technique to the quaternary compound YBa2Cu3O7−δ (YBCO), demonstrating a combination of three of the above effects in an artificial biomimetic process. The technique is directly applicable to other materials systems and, in the case of the present material, could find application in the melt-texturing of YBCO bulks,6 the primary method of production of technological bulk superconductors, where the reduced synthesis temperature as well as the fine dispersion of nanoscopic seed crystallites may offer significant benefits in terms of material homogeneity and speed and ease of production. Conventional Materials Synthesis Challenges. The crystallographic structure of the YBCO unit cell can be viewed as a stacking of three oxygen-deficient and, consequently, distorted perovskite units. The perovskite structure is © 2014 American Chemical Society

Received: December 20, 2013 Revised: March 3, 2014 Published: March 20, 2014 2296

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detector. Microstructures were observed by scanning electron microscopy (SEM, JEOL JSM-5800 LV, 20 kV). The reaction process was studied by differential thermal analysis (DTA, TA Instruments SDT Q600) conducted under the same conditions as the synthesis. Superconducting properties were measured in a cryogenic vibrating sample magnetometer (Cryogenic Ltd.) using a critical state model to estimate the critical current density from the magnetic moment.

conspire to make single crystal growth of the material extremely challenging. One method that has been applied successfully to the single crystal growth of various perovskites and other functional oxides is the molten salt synthesis technique first developed for (Ba/Sr)Fe12O19.20 The method consists of reacting the constituent oxides or their precursors within a suitable salt system at a temperature at which the salts are molten, with the molten salts acting as a transport medium for diffusion of the solid oxides.21 It has been successfully applied to the synthesis of anisotropic granular materials ranging from BaTiO322 to PbBi2Nb2O923 and Bi2WO624,25 to Ni1−xZnxFe2O4.26 Only a few salts can be used, either chlorides22−26 (NaCl, KCl, and the NaCl−KCl eutectic) or sulphates24,26 (Li2SO4, Na2SO4, and the Li2SO4−Na2SO4 eutectic), with the NaCl−KCl eutectic being most widely employed. The superiority of the eutectic mixtures over the individual salts lies in their reduced melting points, facilitating a lower growth temperature. The molten salt synthesis technique was naturally utilized in the attempt to produce single crystals of YBCO immediately following its discovery, given its similarity in composition and structure to many of the other successful synthesis products achieved via the method. However, the attempt on YBCO using NaCl−KCl was unsuccessful;27 YBCO found to be unstable in the NaCl−KCl system, tended to undergo the following decomposition reaction at the elevated synthesis temperature:28

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RESULTS AND DISCUSSION Reduced-Temperature Crystal Growth by Molten Salt Synthesis: Heat Treatment Stage. Samples were prepared via dextran-mediated synthesis from a molten salt,29 in which the salt was varied from NaCl to KCl, in addition to using a NaCl−KCl eutectic mixture and also preparing a control sample for which no salt was used. All the samples thus obtained were observed by X-ray diffraction (Figure S1 of the Supporting Information) to comprise almost phase-pure YBCO of a similar degree of crystallinity and phase purity. No significant differences in the X-ray scans of the different samples were observed, indicating that in all cases, the synthesis of the YBCO phase itself is unaffected by the presence or absence of the different salts. No residual salt content is observed in the Xray measurements, nor can any salt constituents be detected in the SEM. We attribute this to complete evaporation of the low salt content during the elevated temperature phase of the synthesis. Such a propensity for evaporation has been confirmed by thermogravimetric measurements of the individual salts (not shown). It is apparent that the combination of lowered synthesis temperature, shorter synthesis time, and reduced salt content enabled by the biopolymer-mediated nucleation effectively inhibits the decomposition reaction shown above. As we show below, increasing the synthesis temperature does result in decomposition of the YBCO, as previously observed, as does increasing the amount of salt present to the levels typically used. The successful adoption of the molten salt synthesis technique does, however, enable the various salt additions to exert a significant influence on the morphology of the synthesis product. As shown in Figure 1, the control sample prepared without salt addition appears porous and spongelike, as has commonly been reported for similar biopolymer-mediated syntheses.30,31 In contrast, upon addition

YBa 2Cu3O7 − δ + 4(Na, K)Cl + 1/2(δ − 1/2)O2 → 1/2Cu 2Y2O5 + 2CuO + 2BaCl 2 + 2(Na, K)2 O

In this work, we demonstrate effective biopolymer mediation of seed nucleation acting to reduce the temperature required for the molten salt synthesis of YBCO below that which favors the above reaction, while also promoting the formation of nanoscopic seed crystallites that reduce the synthesis time and the amount of salt required to effect the process. We combine understanding of the mechanism of molten salt synthesis as applied to YBCO crystal growth with that of the biopolymermediated seed nucleation and demonstrate that appropriate choices of biopolymers are able to dramatically influence the final morphology of the synthesis product, providing opportunities to tailor the form of individual crystallites or the macroscopic assembly.

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EXPERIMENTAL SECTION

Biopolymer-Mediated Molten Salt Synthesis. YBCO crystalline aggregates were synthesized by a two-stage biomimetic method: (1) in the room temperature stage, a biopolymer (5.0 g dextran Mr = 70000 or medium molecular weight chitosan) was mixed with a precursor solution (2.5 mL) prepared by dissolving Y(NO3)3·6H2O (1.915 g, 0.05 M), Ba(NO3)2 (2.613 g, 0.10 M), and Cu(NO3)2· 2.5H2O (3.489 g, 0.15 M) in distilled water (100 mL). The biopolymer-precursor mixture was left for one day at room temperature to harden, during which time seed nucleation occurs. Subsequently, an ionic salt (0.025 g NaCl, KCl, or 1:1 molar ratio NaCl−KCl eutectic) was sprinkled uniformly across the surface of the hardened mixture. This is approximately a 3:1 molar ratio of salt:YBCO. (2) In the heat treatment stage, the mixture was heated in a box furnace to 920 °C at 10 °C min−1 and sintered for 2 h in air then furnace-cooled to room temperature at 2 °C min−1. A comparison sample of commercial YBCO powder (Aldrich) with the NaCl addition was also sintered by the same process. Sample Characterization. The phase composition of the samples was characterized by the Cu Kα X-ray diffraction using a Bruker D8 Advance diffractometer equipped with a LynxEye one-dimensional

Figure 1. Electron microscopy images of YBCO samples synthesized by the biopolymer-mediated method using dextran (a) without a salt additive, or with the addition of (b) NaCl, (c) KCl, and (d) an NaCl− KCl eutectic mixture. The effect of the salt addition is to transform the spongelike morphology of the resulting material into an interconnected aggregate of crystalline platelets. 2297

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of the salt, all the samples instead self-assemble into dense physically and electrically29 interconnected aggregates of distinctly crystalline platelets. Figure 2 shows the microstructural result of the same molten salt synthesis process applied to a sample of commercial

Figure 2. (a) A localized region of platelet grains formed from a commercial YBCO powder having undergone the molten salt processing in the presence of NaCl contrasted with (b) the same commercial YBCO powder sintered in the absence of a salt additive, showing no such platelets.

Figure 3. High-temperature DTA traces of various combinations of reactants and products together with NaCl to elucidate the reaction mechanism.

preformed YBCO powder (without the use of a biopolymer). Close examination enables a small number of similarly shaped platelets to be identified in localized regions located at the surface of the otherwise powderlike morphology. This indicates that even at this reduced temperature, insufficient to form YBCO in the absence of the biopolymer, the morphology of pre-existing YBCO can be modified. One possibility is that the molten salt is acting instead as a flux, promoting crystal growth in a similar manner to BaO−CuO in the high-temperature solution method of single crystal growth.32 The function of a flux in this process is to lower the melting point of the product such that it melts and then recrystallizes from the supersaturated salt solution. The consequently lowered temperature of the melt contributes to a slower growth of larger crystals. To confirm whether this is the case here, we conducted DTA measurements (Figure S2 of the Supporting Information) that revealed that indeed the melting temperature of YBCO in the presence of the various salts decreased from its bulk value of ∼1020 °C to 953 °C (NaCl), 992 °C (KCl), and 942 °C (NaCl−KCl eutectic). However, in all cases, the reduced melting temperature remains higher than our synthesis temperature of 920 °C, meaning that the salt cannot act as a flux in our synthesis. The superconducting properties of the materials prepared by this method are also of interest to confirm the successful synthesis as well as to indicate possible added benefits of the method. Samples prepared with each of the salt additives exhibit a narrow range of superconducting transition temperatures in excess of 88 K (Figure S3 of the Supporting Information), as expected for close to optimally doped YBCO. This result is of note since these samples have been synthesized in air, without the benefit of an oxygen atmosphere to ensure the degree of oxygenation required for an optimal transition temperature. The intragranular critical current densities of the materials are found to be up to an order of magnitude greater than those of conventionally processed YBCO powders, in the best case even surpassing those of high-performance melttextured material (Figure S4 of the Supporting Information). To further explore the effects of the salt addition, a detailed study of the DTA traces (Figure 3) of various combinations of reactants and products with NaCl was performed. Where present, NaCl itself is observed to melt around 800 °C as expected, indicating that up to that temperature it has remained

unreacted with the other constituents. When the temperature reaches 885 °C, a broad peak occurs only in the mixture of reactants and NaCl, indicating the dissolution of the reactants into the molten salt. This is the procedure that enables the molten salt synthesis, and YBCO begins to form. At around 930 °C, above the synthesis temperature, peaks are observed in the traces of both the reactant/salt mixture and the preformed YBCO/salt mixture, which is the YBCO dissolving into the molten salt. At temperatures beyond 940 °C, two further peaks are observed that correspond to the melting and subsequent decomposition of YBCO as seen previously. These peaks vary in the temperatures at which they occur; however, comparing the trace without salt addition to the others, it is clear that both the melting and decomposition temperatures have been lowered by the salt addition. The different temperature regimes evidenced in the DTA measurements highlight the difference between the molten salt synthesis process working from precursor nitrates and from preformed YBCO. In the former case, the salt acts as a solvent for the precursors with dissolution occurring at 885 °C, similar to the standard molten salt synthesis. In the latter case, this dissolution is delayed to 930 °C, above the synthesis temperature employed in this work. Consequently, the molten salt acts instead as a flux for the preformed YBCO, being effective only in its immediate vicinity. This also explains why previous attempts at the molten salt synthesis of YBCO have failed. In order to fully dissolve all the material present, a large amount of salt must be added to the mixture (typically a 1:1 weight ratio).27 However, we have previously shown33 that on increasing the amount of NaCl present, the YBCO decomposes, with the optimal addition amount being the 0.025 g used in this work. This is a 0.33:1 NaCl:YBCO weight ratio. The key to being able to use less salt addition is the particle size difference. In the molten salt synthesis using solid oxide precursors, the precursor grain sizes are of the order of hundreds of micrometers, similar to the preformed YBCO powder shown in Figure 2. In the biopolymer-mediated process, the nitrate precursors become sequestered within the biopolymer network formed on hardening of the biopolymerprecursor mixture at room temperature (the “egg-box” model), producing finely dispersed nanosized seed crystallites of YBCO within the nanoreactor cages of the biopolymer.29 Although the biopolymer clearly decomposes partway through the subse2298

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quent heat treatment as the temperature is raised to 920 °C, the nanosized YBCO crystallites remain to become the seeds for the later growth process. A further result of our study affecting the possibility of a successful synthesis is the temperature window over which the synthesis can take place. We have shown that there is only a narrow window from 910 to 925 °C, in which a pure product is obtained; below this, YBCO is unable to form, and above this, the YBCO itself begins to dissolve into the molten salt. Careful control of the synthesis temperature is therefore crucial. Morphological Direction by Biopolymer Selection: Room Temperature Stage. The additional advantage offered by the biopolymer-mediated synthesis technique over the standard molten salt synthesis, beyond enabling it to work below the decomposition temperature of YBCO, lies in the morphological control it offers. In a standard molten salt synthesis, the dimensions of the product are determined by the growth anisotropy of the crystal itself, while the crystallite orientation is random, determined by spontaneous homogeneous nucleation from the melt. Here, we have demonstrated controlled heterogeneous nucleation of the crystallites on networks of YBCO seeds formed at room temperature through the biosynthesis process, enabling constraints to be imposed on both the size/shape and the arrangement of individual crystallites. Since these constraints originate from the specific biomaterial used, the nature of the nucleation can be influenced through the choice of a different organic matrix. As shown in Figure 4, the use of a different biopolymer, in this case chitosan,

a sufficiently high concentration of cations in the precursor solution that nucleation tends to occur. This can be viewed either as an increase in the cation supersaturation at the interface or as a lowering of the activation energy for nucleation. The topography and geometrical structure of the biopolymer functional groups at the interface are important factors influencing the shape and alignment of the YBCO seeds. The effects are illustrated schematically in Figure 5. In the case of

Figure 5. Structural accommodation of YBCO seed crystallites by hydroxyl functional groups in (a) dextran and (b) chitosan, leading to localized orientational alignment and increasingly anisotropic growth, respectively. Broken lines at the interface contrast the planar topography of the exposed functional groups of the dextran with the concave topography of the chitosan.

dextran, half of the −OH functional groups form hydrogen bonds with each other and are consequently not exposed to the nucleating crystal. The exposed functional groups form a planar topography as indicated. In contrast, the exposed functional groups of the chitosan form a concave topography as also indicated. This concave topography increases the spatial charge density, resulting in a three-dimensional clustering of crystallites, while the planar topography exhibits periodic arrays of functional groups distributed across the interface which can be used to template a periodic, two-dimensional nucleation. Geometrical structural matching at the interface between the crystal lattice and the organic matrix is a key concept for nucleation; the distances between the regularly spaced organic functional groups along the interface are commensurate only with certain lattice spacings of the YBCO. For example, the exposed −OH functional groups in dextran exist periodically along the polymer backbone with a spacing of 0.778 nm,35 exactly twice the b lattice parameter of YBCO (0.389 nm), providing nucleation sites for oriented crystallite formation to either side of the backbone. The other −OH functional groups are engaged in hydrogen bonding with each other, which lends the polymer a structural rigidity along its length, allowing the persistence of this orientational alignment and consequent platelet connectivity over significant distances. At the same time, the spacing between neighboring −OH functional groups is such that only the smallest Cu2+ cation can be accommodated between them, which may serve to encourage the initial low-

Figure 4. YBCO platelets formed using (a) dextran, contrasted with (b) chitosan. The average platelet dimensions are 5.5 × 0.9 μm (ratio 6:1) with dextran and 6.4 × 0.8 μm (ratio 8:1) with chitosan, while the local orientational alignment and connectivity obtained with dextran is lost on moving to chitosan.

results in more anisotropic platelet formation (improved nucleation control) but in random orientations (reduced crystalline arrangement). To influence the shape of the crystallite, different types of interactions at the interface between the ions of the YBCO phase and the functional groups on the organic matrix surface, including lattice geometry, electrostatic potential, polarity, stereochemistry, space symmetry, and topography,34 all play a role. Through these interfacial interactions, the matching of charge based on the lattice geometry of the YBCO, the polarity of the organic surface, and its stereochemistry can give rise to a change in the activation energy for nucleation of a particular crystal plane making the preferred growth direction different from that which arises naturally and thereby controlling the anisotropy of the crystal. The electrostatic interaction at the interface between the YBCO cations and the organic functional groups is the key to initiating the nucleation of YBCO seeds. Some parts of the organic surface feature more functional groups than others, which results in localized regions of high spatial charge density at the interface. These regions will attract 2299

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Notes

temperature nucleation of the crystallite on the dextran template. In contrast, in chitosan the functional group repeat spacing is 1.043 nm,36 close to the c lattice parameter of YBCO (1.168 nm), with the preferred nucleation of this plane being aided by the affinity of the nucelophilic −NH2 group for the Y3+ ion. In this case, no hydrogen bonding is present to restrict the flexibility of the loosely coupled units of the polymer chain, which can be expected to twist along its length, eliminating any orientational alignment between neighboring crystallites and disrupting their connectivity. However, by constraining the growth of the seed crystal in the c direction, chitosan mediation further favors a−b directed growth thereby enhancing the anisotropy of the resulting crystallites. Thus we see that if we can calculate and synthesize an organic matrix of appropriate functional geometry, we could then exactly control the shape and arrangement of the seed crystals and the ultimate morphology of the synthesis product. This preseeded nucleation makes the biopolymer-mediated synthesis process similar to the high-temperature solution method (albeit at a far lower temperature as a result of not needing to melt the YBCO). The essential difference is that while the solution method relies on supersaturation to provide the driving force for crystal growth, the lower temperature employed in the biopolymer-mediated process means that the YBCO remains in the solid phase and is therefore able to retain the memory of its biotemplated seeds throughout the synthesis process. Newly formed YBCO immediately precipitates from the salt solution and grows the nearby seeds in their predetermined orientation, ultimately causing them to fuse together as they meet.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.Z. and H.S. are financially supported by the National Natural Science Foundation of China (Grant 51171002), by Beijing Municipal Natural Science Foundation (Grants 2132011 and KZ201310005003), by the Doctoral Program of Higher Education of Special Research Fund (Grant 20121103110012), and by the Project of Construction of Innovative Teams and Teacher Career Development for Universities and Colleges Under Beijing Municipality (Grant IDHT20130510). Research consumables and part of the time spent by J.L.M.-D. were funded by a European Research Council Advanced Investigator Grant (ERC-2009-AdG247276-NOVOX). S.C.W. was supported in the work in Cambridge by The Leverhulme Trust and The Isaac Newton Trust.

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CONCLUSION In summary, we have demonstrated the successful growth of YBCO crystallites using a molten salt synthesis under the assistance of biopolymer mediation. Through the influence of the biopolymer on seed nucleation, the synthesis temperature, time, and the amount of salt required can be reduced to a level that inhibits the decomposition of the YBCO that would otherwise occur. Additionally, the templating effect of the biopolymer causes the self-assembly of YBCO platelets to form a dense physically and electrically interconnected cluster with localized orientational alignment. We further demonstrate the prominent effect of the choice of biopolymer on the macromorphology of the synthesis product, raising the prospect of designed morphologies to meet particular functional goals.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray diffraction patterns of YBCO samples synthesized by the biopolymer-mediated method with different salt additions, DTA traces of commercial YBCO powder mixed with different salts, diamagnetic susceptibility of biopolymer-synthesized YBCO samples, and critical current densities of biopolymersynthesized YBCO samples compared with a commercial YBCO powder and a high-performance melt-textured YBCO pellet. 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]. Tel: +64 (0)4 931 3225. Fax: +64 (0)4 931 3117. 2300

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