In-situ TEM and AFM investigation of morphological controls during

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In-situ TEM and AFM investigation of morphological controls during the growth of single crystal BaWO4 Lili Liu, Shuai zhang, Mark E. Bowden, Jharna Chaudhuri, and James J. De Yoreo Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01216 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017

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Crystal Growth & Design

In-situ TEM and AFM investigation of morphological controls during the growth of single crystal BaWO4 Lili Liu1, 2*, Shuai Zhang2*, Mark E Bowden2, Jharna Chaudhuri1# and James J De Yoreo2,3# 1

Department of Mechanical Engineering, Texas Tech University, Lubbock, TX 79409 Physcial Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352 3 Department of Materials Science and Engineering, University of Washington, Seattle, WA, 98185 2

*These authors contributed equally #Address correspondence to [email protected] and [email protected]

KEY WORDS Barium tungstate, crystal morphology, in-situ TEM, in-situ AFM ABSTRACT Barium tungstate (BaWO4) is a widely investigated inorganic optical material due to its attractive emission properties. Because those properties strongly depend on crystal structure and morphology, numerous approaches to controlling growth have been pursued. However an understanding of the growth mechanisms that lead to the wide range of morphologies obtained to date is largely lacking and most attempts to develop that understanding have been based on postgrowth analyses. Significantly, such analyses have led to the conclusion that certain BaWO4 crystal morphologies result from a non-classical growth process of oriented attachment. In this work, we systematically varied the morphology of BaWO4 crystals by adjusting the relative concentrations of solute, water, and ethanol. We then explored the growth mechanism leading to the observed range of morphologies through in-situ TEM and in-situ AFM. We find that even the most complex BaWO4 morphologies occur through purely classical growth mechanisms largely controlled by the content of solute and ethanol. The latter acts as an impurity to poison growth at low concentrations and low solute levels, but leads to development of growth instabilities and eventual dendritic growth at high alcohol and moderate solute concentrations by driving up the supersaturation. These findings also highlight the necessity of in-situ experiments to interpret ex situ observations of crystal growth and decipher the controlling mechanisms.

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INTRODUCTION The unique morphologies, properties, and functions of inorganic nanocrystals has stimulated intense interest in their synthesis and fabrication. The size and shape of particles strongly affect their properties, which impact practical applications of the materials.1-5 For example, the Morn transition temperature (TM) of α-Fe2O3 nanoparticles (0-D) is strongly dependent on the particle size; decreasing with it and tending to vanish below a diameter of ~ 8 nm, for spherical nanoparticles;4 Hexagonal plate-like nanocrystals of ZnO display much higher activity than rod-shaped crystals in the photocatalytic decomposition of methylene blue, showing that the polar (001) and (001) faces are more active than the nonpolar faces that are perpendicular to them.5 Among the various inorganic nanomaterials with the potential for optical applications, barium tungstate (BaWO4) has been investigated for use in all-solid-state lasers due to its attractive emission properties. 6-12 The scheelite form of BaWO4 is of particular interest for electro-optics due to its blue luminescence. 6, 13-15 As with other nanomaterials, the crystal properties of BaWO4 depend on the details of structure and morphology. Thus extensive research has been pursued to control BaWO4 synthesis, leading to growth of nanowires,16 spheres,17 whiskers,18 penniform nanostructures,19 and flower-like structures. 6, 20 Liu et al. employed a supramolecule template to synthesize a series of flower-like, spheroidal, fasciculus-like, and other morphologies of BaWO4 at mild conditions. 21 Subsequently, Zhao and coworkers fabricated various shaped BaWO4 hollow structures by a simple precipitation reaction between BaCl2 and Na2WO4 in the presence of poly (methacrylic acid) (PMAA) without any surfactant.22 These hollow structures included spheres, peanuts, and ellipsoids of different sizes. However, the above-mentioned approaches are relatively complex, involving high temperature or templates, and are likely to introduce impurities into the final products. In addition, the mechanisms underlying the various crystal morphologies have received little attention. Yin et al. addressed the complexity of shape-controlled synthesis of BaWO4 nanostructures by introducing a simple method based on adjustment of the volume ratio of ethanol to water. 14, 23 They also attempted to explain the evolution in crystal morphology generated by this attractive, low-cost process in terms of differences in the rates of nucleation, growth and oriented attachment (OA) of primary particles. Although they concluded that certain topologically complex morphologies were primarily the result of OA, this conclusion was based on post2


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growth analysis with no systematic evaluation of morphological evolution either in time or with changes in growth conditions. In this work, we synthesized the scheelite form of BaWO4 in an ethanol-water system and obtained a range of morphologies that could be controlled systematically through the choice of solution composition. We also explored the mechanisms through which these morphologies were formed by using SEM to examine morphologies over a wide range of conditions and timepoints, in situ TEM to real-time observe the nucleation and growth behavior, and in situ AFM to investigate the generation and propagation of growth sources on BaWO4 crystal surfaces. METHODS Materials and Methods. All chemicals were of analytical grade purity and were directly used without any treatment. Solutions of Na2WO4 (Sigma Aldrich, USA) and BaCl2 (Sigma Aldrich, USA) were prepared for a range of concentrations using a mixture of ethanol and water in varying ratios R (R=Vethanol/Vwater). In total, twelve different solution conditions were examined, summarized in Table 1, each of which generates its own crystal morphology. To make each growth solution, equal volumes of equi-molar solutions of each reagent were first prepared at room temperature by dissolving them in mixtures of distilled water and ethanol, which were in the volumetric ratios R given in Table 1. Vigorous stirring was necessary to ensure that all the reagents were completely dissolved in the solution. The BaCl2 solution was then added to the Na2WO4 solution under continuous stirring, to produce the final water-ethanol solution with equal Ba2+ and WO42- concentrations. In each case, a white precipitate appeared immediately, was collected by centrifugation, washed sequentially using deionized water and ethanol to remove the remaining impurities, and dried in atmosphere at room temperature.23 Characterization. The morphology and structure of all samples was determined using scanning electron microscopy (SEM, Helios NanoLab 600i, FEI, Hillsboro, OR), transmission electron microscopy (TEM, FEI Tecnai 20), and powder X-ray Diffraction. To prepare a TEM sample, we dispersed a small amount of the precipitate in ethanol with sonication for 10 min and then placed a drop of the resulting suspension onto a copper grid. Finally, we dried the samples at room temperature. X-ray diffraction data on large samples (e.g., 12.5mM, 1:50) were collected using a Rigaku Miniflex 600 Bragg-Brentano diffractometer, equipped with Cu Kα radiation and

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a post-diffraction monochromator. For small samples (e.g., 1.5mM, 1:40 ), the precipitate was loaded into a 0.5 mm diameter glass capillary and diffraction data were collected with a Rigaku Rapid II microbeam diffractometer. X-rays generated from a rotating Cr target were focused through a 300 micron diameter collimator and diffracted intensities were recorded on a 2D image plate covering 0 to 150 ° 2θ. In-situ TEM. In situ liquid phase TEM (LP-TEM) experiments in solution were carried out using a microfabricated fluid cell consisting of two silicon chips, each containing with an electron transparent silicon nitride window, separated by a 100nm spacer (Hummingbird Scientific, USA). The silicon chips were plasma-treated for 40 seconds (Harrick Plasma cleaner) to render them hydrophilic before assembling the liquid cell. The spacer-chip was firstly placed with membrane side up inside the well-cleaned fluid stage tip, and a 0.6 µL stock solution was pipetted onto its silicon nitride window. The upper chip was then placed membrane side down and align with the spacer-chip so that the two transparent windows could be observed in alignment under a stereo-microscope. Finally the liquid cell holder was placed in a pump station (Pfeiffer vacuum) to confirm it was vacuum-tight before loading it into the TEM (FEI Tecnai). Bright field images and movies were recorded with an Eagle CCD detector. Atomic Force Microscopy (AFM). All of the images were recorded in liquid using contact mode on a MultiMode VIII AFM (Bruker, CA). SNL-10 probes (Bruker, CA) were used in the experiment. The scan speed was varied between 1 Hz and 3 Hz, and the feedback control was carefully adjusted to minimize the possible disturbance to the crystal growth process by the AFM probe. The raw data were further analyzed using NanoScope Analysis V1.50 offline software (Bruker, CA).

RESULTS AND DISCUSSION Effect of the reaction conditions on the morphology and size of BaWO4 structures. BaWO4 samples were prepared using twelve different combinations of solute concentration and volume ratio of ethanol to water (R=Vethanol/Vwater) as shown in Table 1. The SEM results reveal a wide variety of crystal morphologies (Fig. 1) and HRTEM results show that all products were crystalline BaWO4, with the long direction of the crystal corresponding to

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the [001] direction (Fig. S1). The SEM images also reveal a strong and systematic dependence of morphology on the initial solute concentration and value of R, as shown in Fig. 1. Changes along the horizontal direction in Fig. 1 reveal the effect of salt concentration (1.5mM, 5mM, 12.5mM) and those seen along the vertical direction give the effect of R, with the alcohol content increasing from bottom to top. A summary of these dependencies is illustrated in Scheme 1, which shows a simplified morphological evolution with initial solute concentration and value of R. (Note the lowest values of R (1:40) for a solute concentration of 1.5 mM differed from that at higher solute concentrations (0 and 1:50), because, at 1.5 mM, no BaWO4 crystals were obtained for an R of either 0 or 1:50.) At the highest initial salt concentration (12.5mM), the products exhibited a smooth dipyramidal morphology in pure water (Fig. 1 L), but with increasing R, the shape gradually evolved to a shuttle-like morphology with 4-fold symmetry (Fig. 1 I, F, C,). The length-to-width increased, resulting in a sharper angle at each apex, the crystal surface became increasingly rough, and knot-like protrusions developed at the centerline of each crystallite. When R reached 1:5, the crystallites developed a periodic series of protrusions along each of the edges, indicating an evolution to 4-fold dendrites (Fig. 1C). Samples taken at early times, demonstrate that, indeed, each of these protrusions represents a separate arm of a dendritic structure. The aspect ratio also increased with R, reaching four times than one obtained in pure water at an R of 1:5 (Fig. 1 L). Figure S2 gives the average particle size and aspect ratio of the crystals as a function of initial concentration and R. The evolution from smooth dipyramids to 4-fold dendrites can be understood on the basis of well-known nucleation and crystal growth principles. 24, 25 Both the nucleation density and the crystal growth rate should increase with the value of R due to the decreasing solubility (increasing supersaturation) of BaWO4. These two effects should counter one another: higher nucleation density in a fixed solution volume should result in smaller size crystals, while higher growth rate should result in larger size crystals. In our experiments, the observed crystal size became greater with R indicating that the increase in growth rate had a larger impact on morphology than the increase in nucleation rate. Thus, because larger R also resulted in crystal lengthening, both the size and aspect ratio rose steadily with R (Fig. S2). The observed roughening with increasing supersaturation is typical of dendritic growth, which results from the competition between surface kinetics and mass transport. While the growth kinetics at the 5



interface rise with supersaturation, the limitation on mass transport remains fixed, leading to greater tendency towards morphological instability and an increasingly dendritic shape. 26, 27 The results for 5mM salt concentration and variable R, are summarized in Figs. 1B, E, H and K. In pure water, the crystallites exhibited a flower-like structure with high petal density and a diameter of approximately 10 µm. Increasing R to 1:50 produced a mix of dipyramids and flower-like structures, though with lower petal-density. As R was increased to 1:10 and then to 1:5, the percentage of flower-like crystals and petal density continued to decrease, while the dipyramids evolved into the shuttle-like structures described above. In some case, protruding crystals were also observed (Fig. S3). The flower-like structures suggest that, at these concentrations, nucleation centers are the source of multiple nuclei. The fact that these decrease in number as ethanol was added and, consequently, the supersaturation increased, suggest that they are the result of heterogeneous nucleation. Because heterogeneous sources have a lower free energy barrier to nucleation, they will dominate at low supersaturation.28 However, the number of potential sites of heterogeneous nucleation is limited by the number of such sources. As supersaturation rises, the probability of homogeneous nucleation becomes large enough to make such events possible on the timescale of an experiment. However, because these events can occur anywhere in solution, the number of potential sites of homogeneous nucleation greatly outnumbers the number of potential sites of heterogeneous nucleation. To test this hypothesis, we processed 5mM (1:10 and 1:50) solutions through a syringe filter to get rid of any particles in the starting solutions. The effect on morphology was dramatic. At an R of 1:50, the majority of the flower-like structures were eliminated in favor of simple dipyramidal crystals (Compare Fig. 2A to Fig. 1H). This demonstrates that, indeed, the flower-like structures are a result of heterogeneous nucleation events that dominate at low supersaturation. Moreover, when the solute level was held constant at 5mM, increasing the ethanol concentration to 1:10 — and hence increasing the supersaturation— drove formation of shuttle-like crystals with sizes reaching approximately 133 µm in length, which is around 13 times longer than achieved in unfiltered solutions (Fig. 2B). The impact of supersaturation on the competition between heterogeneous and homogeneous nucleation also provides a reasonable explanation for two other characteristics of the precipitates at 5mM salt concentration. First, in the absence of filtration, as homogeneous nucleation events

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begin to dominate, they decrease the solution supersaturation and thus reduce the likelihood of heterogeneous nucleation events, leading to a smaller number density of flower-like structures. Second, because growth rates are rapid enough to deplete the solution near the growth interface, as demonstrated by the evolution to dendrites at high supersaturation, petals that nucleate early deplete the surrounding solution and inhibit the nucleation of new petals, decreasing the final petal density. At the highest supersaturations, this effect decreases the number of petals to the point where even scissor-like crystals appear (Fig. 1B). When the initial salt concentration was decreased to 1.5mM, BaWO4 crystals did not form in pure water or at low R (1:50). Thus the smallest R at which BaWO4 crystals formed was 1:40. The SEM images show the products for an R of 1:40 or above (1:20, 1:10, and 1:5) were all flower-like structures (Fig. 1 A, D, G and J) where the major difference is the density and aspect ratio of the petals. At R = 1:40, the crystals were nearly spherical and exhibited the highest density of petals with the smallest aspect ratio. XRD shows that, despite the dramatic morphological difference, the crystal structure is still that of BaWO4 with the scheelite structure. (Figure S4 shows a comparison to crystals produced at 12.5mM salt and R=1:50). With increasing R, the number of the petals becomes smaller and the petal morphology evolves to the sharp pyramidal shape seen at 12.5mM salt. In-situ LP-TEM 29 was used to visualize the early stages of BaWO4 crystal formation in the low concentration regime. Figure 3A and 3B illustrate the growth behavior of crystals produced in 1.5 mM salt solution with R equal to 1:40 and 1:10, respectively. (See also Movies S1 and S2.) While the growth trajectories were similar for the two values of R within the time window of the experiment, Fig. 3 shows that, as in the bulk growth experiments (Fig. 1J), the crystals formed at R = 1:40 are nearly spherical while those generated at R = 1:10 grew at much higher rates and are more flower-like, exhibiting distinct petals that appear in many cases to be faceted, as observed in the bulk growth experiments (Fig. 1D). When R was increased to 1:5, the rapid growth rates made it unfeasible to capture the detailed growth trajectories, with the crystals reaching full size by the time the first image was collected (Fig. S5.) These observations imply that the morphologies seen in the SEM images (Fig. 1) are generated during the earliest stages of growth and that growth proceeds by purely classical mechanisms with no evidence for particle assembly and with the morphology controlled by supersaturation.

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To obtain a better understanding of the crystal growth process leading to the observed morphologies, in-situ AFM was used to observe growth on seed crystals for a range of solution conditions. As Fig.4 shows the BaWO4 crystal exhibited two morphologically distinct growth faces. One — denoted as (111) — was uniformly rough without any long-range features (Fig. 4B). The other — denoted as (110) — was also rough, but exhibited ridge-like features (Fig.4C). The reason for the distinct behavior is surely related to the difference in surface termination; the (110) face is terminated by Ba and O while the (111) face is terminated by W and O. However, the mechanistic reason cannot be stated a priori as there is no model that can definitively predict when a face will be rough, as in the case of the (111) face, or whether it is likely to form macrosteps, as on the (110) face. There are tendencies towards roughening or smoothing based on whether the coordination network in the plane of the face renders it an “F, S or K face”, as described by Hartman- Perdok of periodic bond chains,30 and there are tendencies towards step bunching depending on the step kinetic coefficient, elementary step spacing, diffusivity, impurity concentration and time constant for impurity adsorption.31 However, Hartman-Perdok theory is not definitive for any given system and the parameters associated with the tendency towards step bunching are not known for this system. For the purposes of this study, the salient point is that the two faces differ in growth mechanism, which we now show is reflected in the kinetics. In an alcohol-free aqueous solution of 1.5 mM Ba2WO4 and 1.5 mM Na2WO4, the (111) face initially exhibited many small particles, which either adsorbed to or nucleated on the surface (Fig. 5A). These increased in number to cover the surface (Fig. 5A-H) and, over time, expanded at a very slow growth rate into small flat islands that eventually merged (Fig. 5I-K). Compared to the (111) face, growth on the (110) face occurred at a much faster rate. Figure 5I-K shows the border between (111) and (110). Over time, the ridge-like structures became more prominent and the boarder migrated across the (111) face, while the islands on (111) only slightly increased in size. We found that the growth rates on (111) and (110) were 0.009 nm/s and 0.069 nm/s, respectively. These observations demonstrate that the growth kinetics on (110) are much greater on than on (111). We then examined growth in solutions with an R of 1:400 (Fig. 6A), 1:200 (Fig. 6B), and 1:50 (Fig. 6C). (See also movies S3, S4 and S5.) All of the data support the conclusion that the kinetics of growth on (110) greatly exceeds that on (111). For example, Figure 6A clearly shows the (111) face being overgrown by two bounding (110) faces. Moreover, the larger the value of 8


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R, the greater was the observed difference due to a strong dependence of the (110) growth rate on R; for an R of 1:400, 1:200 and 1:50, the measured growth speeds were 0.35 nm/s, 1.05 nm/s and 4.38 nm/s, respectively. Given the impact of the relative face-specific growth rates on crystal morphology, the observed effects explain why ethanol alters the morphology of BaWO4 crystals so strongly. Moreover, the emergence of the ridge-like morphology demonstrates the dominance of macrosteps on this face, points towards the development of growth instabilities that generate the dendritic morphology, which eventually dominates at high supersaturation. Furthermore, at the highest alcohol content (Fig. 6C), the development and rapid expansion of a rounded “hillock” suggests the development of the knot-like protrusions seen around the central plane of fully formed crystals grown at high supersaturation. Following imaging, the crystals were removed from the AFM liquid cell and examined by SEM (Figure S6 and S7). Figure S6 reveals that the highest region of the crystal, where AFM imaging was executed, corresponds to the central region, which consists of the (111) face along with many small regions of (110). Figure S7 shows that, as the value of R increases and the supersaturation gets larger, the (110) face of the pyramidal regions occupies increasingly less area relative to (111), as expected from the relative growth rates. Extrapolating to long growth times, the (110) faces will eventually become the four edges of each half of the bi-pyramid and, as the instability of the ridges grows, will expand outward as dendritic arms to produce the final four-fold dendritic morphology seen at the highest supersaturations. This evolution in growth habit is illustrated in Fig. 7.

Conclusion BaWO4 crystals with different morphologies were synthesized by simply adjusting the volume ratio of ethanol to water. The morphologies of the crystals varied systematically with both the concentration of the solute and the ethanol concentration. At high solute concentration, the morphology evolves from simple dipyramids to four-fold dendrites with dendrite arms along the dipyramidal edges and knot-like protrusions around the central plane. As the solute concentration is decreased, heterogeneous nucleation on sources that can be removed by filtration leads to generation of flower-like structures whose petal-density and percent occurrence decreases with 9



increasing supersaturation, consistent with the principles of classical nucleation theory. In situ AFM shows that all faces grow on rough surfaces at low supersaturation, even in pure water. However, there is a competition between growth on the large (111) faces and smaller (110) faces that exhibit a ridge-like morphology comprised of macrosteps. The introduction of increasing concentrations of alcohol leads to increasingly rapid growth on the (110) relative to the (111), increases in the roughness of the ridge-like features, and the development of rounded hillocks that presage the appearance of the knot-like protrusions. In situ LP-TEM confirms that the observed morphologies are generated absent any particle aggregation and assembly processes, starting at the earliest stages of growth. These results show that the complex BaWO4 morphologies previously attributed to oriented attachment actually occur through purely classical growth mechanisms influenced by ethanol as a modifier of nucleation, step propagation and the development of growth instabilities that are eventually manifest as the growth of dendritic single crystals. Thus these findings highlight the need for caution in deciphering growth mechanisms through interpretation of ex situ images collected long after growth is complete. Associated Content The Supporting Information is available free of charge on the ACS Publications website. TEM and SEM images; PXRD curves; in-situ TEM and AFM movies. Author information Corresponding Authors (J. J. D. Y.) Email: [email protected] (J.C.) Email: [email protected] ORCID Lili Liu: 0000-0002-9595-4303 Shuai Zhang: 0000-0003-0170-6470 Mark E Bowden: 0000-0003-3812-3340 Jharna Chaudhuri: 0000-0002-4822-8659 James J De Yoreo: 0000-0002-9541-733X

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Notes The authors declare no competing financial interest. Acknowledgements This research was supported by the U.S. Department of Energy, Basic Energy Science (BES), Division of Materials Science and Engineering, at PNNL. PNNL is a multi-program national laboratory operated for Department of Energy by Battelle under Contract No. DE-AC0576RL01830. L.L. Liu gratefully acknowledges the Doctoral Dissertation Completion Fellowship from Texas Tech University and wishes to express her many thanks to Dr. Dongsheng Li for training in in-situ TEM techniques. The authors also acknowledge Dr. Benjamin A Legg for helpful discussions about crystal growth theories. Reference (1) Anzlovar, A.; Kogej, K.; Orel, Z. C.; Žigon, M. Impact of Inorganic Hydroxides on ZnO Nanoparticle Formation and Morphology. Cryst. Growth Des. 2014, 14, 4262-4269. (2) Tao, F.; Wang, Z. J.; Yao, L. Z.; Cai, W. L.; Li, X. G. Shape-Controlled Synthesis and Characterization of YF3 Truncated Octahedral Nanocrystals. Cryst. Growth Des. 2007, 7, 854858. (3) Cao, S. W.; Zhu, Y. J. Hierarchically Nanostructured α-Fe2O3 Hollow Spheres: Preparation, Growth Mechanism, Photocatalytic Property, and Application in Water Treatment. J. Phys. Chem. C 2008, 112, 6253-6257. (4) Amin, N.; Arajs, S. Morin temperature of annealed submicronic α-F2O3 particles. Phys. Rev. B 1987, 35, 4810-4811. (5) Mclaren, A.; Valdes-Solis T.; Li, G.; Tsang, S. C. Shape and size effects of ZnO nanocrystals on photocatalytic activity. J.Am.Chem.Soc. 2009, 131, 12540-12541. (6) Wang, X. M.; Xu, H. Y.; Wang, H.; Yan, H. Morphology-controlled BaWO4 Powders via a Template-free Precipitation Technique. J. Cryst. Growth, 2005, 284, 254-261. (7) Fendler, J. H. Self-Assembled Nanostructured Materials. Chem. Mater. 1996, 8, 1616-1624. (8) Cerny, P.; Jelinkova, H. Near-quantum-limit Efficiency of Picosecond Stimulated Raman Scattering in BaWO4 Crystal. Opt. Lett. 2002, 27, 360-362.

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(21) Liu, J. K.; Wu, Q. S.; Ding, Y. P. Controlled Synthesis of Different Morphologies of BaWO4 Crystals through Biomembrane/Organic-Addition Supramolecule Templates. Cryst. Growth Des. 2005, 5, 445-449. (22) Zhao, X. F.; Li, T. K.; Xi, Y. Y.; Ng, D. H. L.; Yu, J. G. Synthesis of BaWO4 Hollow Structures. Cryst. Growth Des. 2006, 6, 2210-2213. (23) Zhang, L.; Dai, J. S.; Lian, L.; Liu, Y. Dumbbell-like BaWO4 microstructures: surfactantfree hydrothermal synthesis, growth mechanism and photoluminescence property. Superlattices Microstruct. 2013, 54, 87-95. (24) De Yore, J. J.; Vekilov. P. G. Principles of Crystal Nucleation and Growth. Rev. Mineral. Geochem. 2003, 54, 57-93. (25) Dove, P. M.; Han, N.; De Yoreo, J. J. Mechanisms of Classical Crystal Growth Theory Explain Quartz and Silicate Dissolution Behavior. PNAS 2005, 102, 15357-15362. (26) Malkin, A. J.; Kuznetsov, Y. G.; McPherson, A. In situ Atomic Force Microscopy Studies of Surface Morphology, Growth kinetics, Defect Structure and Dissolution in Macromolecular Crystallization. J. Cryst. Growth. 1999, 196, 471-488. (27) Chew, C. M.; Ristic, R. I.; Dennehy, R. D.; De Yoreo. J. J. Crystallization of Paracetamol under Oscillatory Flow Mixing Conditions. Cryst. Growth Des. 2004, 4, 1045-1052. (28) Vekilov, P. G. Nucleation. Cryst Growth Des. 2010, 10, 5007-5019. (29) De Yoreo, J. J.; Sommerdijk, N. A. J. M. Investigating Materials Formation with Liquidphase and Cryogenic TEM. Nat Rev Mater. 2016, 1, 16035. (30) Hartman, P., Perdok, W.G. On the relations between structure and morphology of crystals. I. Acta Crystallographica. 1955, 8, 49–52; On the relations between structure and morphology of crystals. II. Acta Crystallographica. 1955 8, 521–524; On the relations between structure and morphology of crystals. III. Acta Crystallographica. 1995, 8, 525–529. (31) Coriell, S. R., Chernov, A. A., Murray, B. T. and McFadden, G. B. Step bunching: generalized kinetics. J. Cryst. Growth, 1998, 183, 669– 682.

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Figure caption Table 1. BaWO4 crystallites prepared under different conditions Fig. 1. SEM images of representative BaWO4 morphologies as a function of the initial salt concentration and value of R. Scheme 1. Dependence of morphological evolution on initial salt concentration and value of R derived from the SEM images in Fig. 1. Fig. 2. SEM images of crystals grown from filtered solutions for salt concentrations of 5mM and values of R equal to (A) 1-10 and (B) 1-50. Fig. 3. Time-lapse in-situ TEM images showing the growth process of BaWO4 crystals. A) 1.5mM BaWO4 solution with R=1-40 at an approximate dose rate of 9.27 e/Å2s; B)1.5mM solution with R=1-10 at an approximate dose rate of 12.3 e/Å2s. Times are as indicated. Fig. 4. In situ AFM images showing the morphology of BaWO4 crystals in water. A) Low resolution image showing two distinct planes. B and C) High resolution images from (B) white dashed square in (A) showing topography of plane and (C) grey dashed rectangle in A showing 3D rendering of intersection between (111) and (110) faces as indicated. Fig. 5. Time lapse series of in-situ AFM images showing the growth of BaWO4 in 1.5 mM BaWO4 solution (R=0) A-H) on the (111) face and I-K) at the boundary between the (111) and (110) faces. The dashed lines indicates the border between the two faces. Times are as indicated. Fig. 6. Time lapse series of in-situ AFM deflection error images showing the growth of BaWO4 in 1.5 mM BaWO4 solution for R equal to A) 1:400, B) 1:200 and C) 1:50. Times are as indicated. Fig. 7. Cartoon summarizing the growth habit BaWO4 single crystals grown in the absence of alcohol at moderate supersaturation and the morphological evolution that results as supersaturation is increased either through increases in BaWO4 concentration or addition of alcohol as derived from the results of this study.

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Samples 1 2 3 4 5 6 7 8 9 10 11 12

Concentration of Na2WO4 (mM) 12.5 12.5 12.5 12.5 5 5 5 5 1.5 1.5 1.5 1.5

Concentration of BaCl2 (mM) 12.5 12.5 12.5 12.5 5 5 5 5 1.5 1.5 1.5 1.5

Table 1

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Ratios of ethanol to water (R ratio) 0 1:50 1:10 1:5 0 1:50 1:10 1:5 1:40 1:20 1:10 1:5


Fig.1

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Scheme 1

Fig. 2

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Fig. 3

Fig. 4

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Fig. 5

Fig. 6

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Fig. 7

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For Table of Contents Use Only

In-situ TEM and AFM investigation of morphological controls during the growth of single crystal BaWO4 Lili Liu1, 2*, Shuai Zhang2*, Mark E Bowden2, Jharna Chaudhuri1# and James J De Yoreo2,3# 1

Department of Mechanical Engineering, Texas Tech University, Lubbock, TX 79409 Physcial Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352 3 Department of Materials Science and Engineering, University of Washington, Seattle, WA, 98185 2

*These authors contributed equally #Address correspondence to [email protected] and [email protected]

The mechanism by which BaWO4 crystal morphology evolves as solute and ethanol concentrations are varied was investigated using in situ and ex situ AFM and electron microscopy. We find that the complex morphologies previously attributed to non-classical growth via oriented attachment are instead a consequence of classical growth processes including dendritic instabilities and heterogeneous nucleation on particulates removable by filtration

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In-situ TEM and AFM investigation of morphological controls during

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