Size and Morphology Controlled Synthesis of Boehmite Nanoplates

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Size and Morphology Controlled Synthesis of Boehmite Nanoplates and Crystal Growth Mechanisms Xin Zhang, Wenwen Cui, Katharine Page, Carolyn I. Pearce, Mark E. Bowden, Trent R. Graham, Zhizhang Shen, Ping Li, Zheming Wang, Sebastien Kerisit, Alpha T. N'Diaye, Sue B. Clark, and Kevin M. Rosso Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00394 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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Crystal Growth & Design

Size and Morphology Controlled Synthesis of Boehmite Nanoplates and Crystal Growth Mechanisms Xin Zhang1,*, Wenwen Cui1,2,3, Katharine L. Page4, Carolyn I. Pearce1, Mark E. Bowden1, Trent R. Graham5, Zhizhang Shen1, Ping Li2, Zheming Wang1, Sebastien Kerisit1, Alpha T. N’Diaye6, Sue B. Clark1,7, and Kevin M. Rosso1,* 1 – Pacific Northwest National Laboratory, Richland, WA, USA 2 – Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China 3 – University of Chinese Academy of Sciences, Beijing, China 4 – Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA 5 – The Voiland School of Chemical and Biological Engineering, Washington State University, Pullman, WA, USA 6 – Advanced Light Source, Berkeley, CA, USA 7 – Department of Chemistry, Washington State University, Pullman, WA, USA Abstract. The aluminum oxyhydroxide boehmite is an important crystalline phase in nature and industry. We report development of a flexible additive-free hydrothermal synthesis method to prepare high quality boehmite nanoplates with sizes ranging from under 20 nm to 5 µm via using hydrated alumina gels and amorphous powders as precursors. The size and morphology of the boehmite nanoplates was systematically varied between hexagonal and rhombic by adjusting precursor concentrations, pH, and the synthesis temperature, due to face-specific effects. The transformation mechanism is consistent with dissolution and reprecipitation, and involves

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transitory initial appearance of metastable gibbsite that is later consumed upon nucleation of boehmite. Detailed X-ray pair distribution characterization of the solids over time showed similarities in short-range order that suggest linkages in local chemistry and bonding topology between the precursors and product boehmite, yet also that precursor-specific differences in long-range order appear to manifest subtle changes in resulting boehmite characteristics, suggesting that the rate and extent of water release or differences in the resulting solubilized aluminate speciation leads to slightly different polymerization and condensation pathways. The findings suggest that during dissolution of the precursor that precursor-specific dehydration or solution speciation could be important aspects of the transformation impacting the molecularlevel details of boehmite nucleation and growth.

Introduction Boehmite (Aluminum oxyhydroxide, γ-AlOOH) is an important raw material that has a wide variety of applications including in adsorbents1-3, fire retardants4, coatings5, catalysts6-7, fuel cells8, and light-emitting diodes (LEDs)9. Boehmite is also an important precursor for the preparation of various alumina products, which, in turn, are used in multiple industries including catalysis, ceramics and metallurgy10-11. Furthermore, boehmite occurs in a variety of waste streams. For example, it is a major constituent in high-level nuclear waste stored in large quantities for decades at the Hanford Site, Washington, U.S.A., and at the Savannah River Site, South Carolina, U.S.A.12 Aluminum was a part of the original waste stream as a complexing agent for separations, as well as a dissolution product of co-disposed fuel cladding12-13. As one of the major resulting solid phases, boehmite is problematic in that it exhibits slow dissolution and must be dealt with in particulate form to process and vitrify the nuclear waste12, 14-15. In all the


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various application spaces for boehmite, fundamental research into its surface-specific interfacial chemistry enables a better understanding of its nanophase physical properties and reactivity. Moreover, understanding the solution conditions that dictate boehmite particle growth, size and morphology can lead to new synthesis protocols that tailor surface area and shape to exploit the reactive behavior of specific crystal facets. Boehmite particles tend to adopt a platelet morphology. Boehmite has an orthorhombic crystal structure (space group Cmcm), composed of octahedral double layers of edge-shared AlO4(OH)2 octahedra. The surface oxygens of these layers contain the hydroxyls and the resulting double layers aligned along the b-axis are weakly bound together by hydrogen bonding. This structure thus tends to feature a dominant (010) basal surface bound by relatively stable (100) and (101) edge facets and less stable (001) facets, as shown in Figure S1. Shape control of boehmite particles during synthesis can target a range of morphologies accessible from near-equilibrium growth conditions to kinetically-controlled conditions. Colloidal boehmite can be produced through either sol-gel precipitation of organic aluminum precursors16-17 or hydrothermal/solvothermal techniques. The precursor for hydrothermal synthesis can be aluminum salts18-20, gibbsite21-24, bayerite25 or amorphous aluminum hydroxide gels22,26. Generally, system pH exerts a dominant influence on boehmite morphology. In acidic conditions boehmite tends to form 1D nanorods or nanofibers, whereas in basic conditions 2D nanoplates or 3D near-nanocubes are observed27-30. Additives included in hydrothermal treatments appear to influence growth morphology by modifying boehmite growth kinetics17, 3133

. Some additives such as tartrate coordinate with solution phase Al3+ to form solvated

complexes leading to smaller boehmite nanoparticles by prolonging the nucleation phase and slowing crystal growth34. Various additives also can adsorb selectively on different crystal facets



of boehmite particles to adjust the crystal growth behavior; key facets include the (101), (001), (100) and (010) surfaces34-35. For example, the addition of alkyl carboxylic acids to boehmite in supercritical water was found to inhibit growth along the [001] and [010] axes relative to the [100] direction resulting in the formation of rodlike nanoparticles32. This reduction in crystal growth is attributed to a capping effect, that is hypothesized, for example, to disrupt the formation of bridging hydrogen bonds along the [010] axis36. Smaller ligands leading to greater surface coverage than equimolar amounts of larger additives appear to further perturb the aspect ratio of boehmite nanoparticles32. The addition of inorganic ions such as sulfate33, 37-38, chloride39 and nitrate under acidic hydrothermal conditions likewise adsorb to and inhibit the growth of the (010) and (001) facets, leading in this case to preferential growth along the [100] direction27, 37. Strategies to gain control of boehmite crystallite size include seeded growth, such as by introduction of fresh alumina gel during hydrothermal synthesis, which increases boehmite particle size40. Particle size also depends on the Al precursor. Boehmite grown from amorphous hydrated alumina gels tends to form smaller particles than that synthesized from equimolar amounts of gibbsite precursor, and, interestingly, the final size of boehmite particles shows a dependence on the particle size of the gibbsite precursor22. Furthermore, varying the temperature and reaction time of hydrothermal synthesis also impacts boehmite size and morphology28, 41. Lower temperatures generally yield smaller particle sizes. And growth transitions at various stages of synthesis can be exploited to tailor size and shape. For example, in alkaline conditions, boehmite initially grows mainly along the [101] direction and then later transitions to growth along the [010] direction; thus, thicker plate-like boehmite could be formed with longer reaction time41.


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Although many strategies have been developed to control the physical characteristics of boehmite nanocrystals, relatively few approaches are available that can be systematically tuned to achieve a broad range of possible outcomes. Furthermore, comparatively few approaches avoid the undesirable contamination complications from additives, and virtually no such studies attempt to relate growth outcomes to detailed interfacial chemistry. Here we report an additive-free synthesis method to produce well-defined boehmite nanoplates that can be easily tuned in size across two orders of magnitude from under 20 nm (diameter) to 5 µm using hydrated alumina gels and amorphous powders as precursors. The pH value, temperature of hydrothermal treatment, and nature of the aluminum precursor and its concentration are varied to adjust the particle morphology and size. The boehmite nanocrystal products are characterized in detail using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), synchrotron X-ray pair distribution function (PDF) analysis, and synchrotron X-ray absorption near-edge spectroscopy (XANES). Possible growth mechanisms at specific boehmite facets that yield particle size and shape outcomes are discussed. The boehmite crystal growth mechanism that we observe from either precursor appears to be dominated by dissolution and re-precipitation processes, which often proceeds through a transitory bar-shaped gibbsite intermediate phase that is the first crystalline product formed during boehmite crystallization. Experimental Methods Preparation of the Al(OH)3 gel precursor Typically, Al(NO3)3·9H2O (≥98%,

Sigma-Aldrich) was dissolved into deionized water

under stirring to form a homogeneous solution with a concentration of 0.25 M at room temperature, followed by addition of a 1 M NaOH (≥98%, Sigma-Aldrich) aqueous solution to



adjust the pH to ~10.0. After continuous stirring for 1 h, the solution was centrifuged to collect gel-like precipitates. The gel was washed with deionized water three times to remove all soluble salts. Synthesis of boehmite nanoplates Samples were grown using a hydrothermal method: Al(OH)3 gels or Al(OH)3 amorphous powders (dehydrated, Sigma-Aldrich) were dispersed into deionized water and then the pH was adjusted from 4~14 (HNO3 for acid and NaOH for base). The solution was transferred to a 20 ml Teflon container. The concentration of gels/amorphous powders (defined as the concentration of Al3+) ranged from 0.01 M to 1 M and the volume of the solution was 16 mL. The Teflon container was sealed into a steel vessel and then was heated in an electric oven at 120~200 oC for 2~48 h. The resulting white product was recovered by centrifuging and washing with deionized water three times. The solid sample obtained was dried at 80 oC overnight. Samples were characterized by various techniques including XRD, SEM, TEM, X-ray PDF, and XAS. X-ray diffraction XRD patterns of all samples were recorded on a Philips X’pert Multi-Purpose Diffractometer (MPD) (PANAlytical, Almelo, The Netherlands) equipped with a fixed Cu anode operating at 50 kV and 40 mA. XRD patterns were collected in the 5-80° 2θ-range. Phase identification was performed using JADE 9.5.1 from Materials Data Inc., and the 2012 PDF4+ database from International Center for Diffraction Data (ICDD) database. Crystallite sizes were determined by whole-pattern (Rietveld) fitting using Topas v5 (Bruker AXS). An anisotropic model was used with geometries based on microscopic observations42. For samples containing prismatic crystals a rectangular prism with two axes constrained to have equal length was used. For samples with plate-like crystals an elliptic cylinder with three independent axis lengths was


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used, a shape which has been found to be a good approximation to hexagonal platelets. Rietveld fitting was also used to estimate the relative fractions of compounds in samples containing a mixture of phases. Scanning electron microscopy The morphologies of all samples were examined by a Helios NanoLab 600i SEM (FEI, Hillsboro, OR). All samples were sputter coated with a thin layer of carbon prior to analysis (∼5 nm) to ensure good conductivity and imaging. Transmission electron microscopy As-prepared samples were dispersed in water by a sonicator for 5 min. Samples for TEM (FEI Titan TEM) observation were prepared by placing drops of solution onto the copper grid (Lacey Carbon, 300 mesh, Copper grid, Ted Pella, Inc.), which was then dried under ambient conditions prior to being introduced into the TEM chamber. The samples were imaged by using an acceleration voltage of 300 kV. SEM and TEM were also used to evaluate the size distribution of as-synthesized various boehmite samples. More than 25 particles were measured to calculate the average size of each particles. X-ray pair distribution function Data for use in X-ray PDF studies were collected for select samples series at the 11-ID-B beamline43 at the Advanced Photon Source, Argonne National Laboratory. Measurements were made at room temperature with samples loaded in polyimide capillaries, with a wavelength of 0.2112 Å (311 monochromator reflection, Photon Energy 58.62 keV) and a Perkin-Elmer amorphous silicon two dimensional (2D) image plate detector44. The detector was located at approximately 175 mm from the sample position. Data sets were acquired for either 3 minutes



(for nanocrystalline samples) or 5 minutes (for amorphous samples). The program Fit2D45 was used to integrate 2D scattering data in to 1D spectra, applying a mask and polarization correction during integration, and using a CeO2 powder standard for calibration. The normalized total scattering patterns, S(Q), were produced in the program PDFgetX246 by subtracting polyimide container scattering, utilizing the appropriate sample composition, and applying standard corrections for the area detector setup43. Pair distribution function patterns, G(r), were calculated via Fourier transformation of the total scattering data, utilizing a Q maximum of 21.5 Å-1. A TiO2 (anatase) dataset was fit in program PDFgui47 between 1 and 100 Å to calibrate the instrument effects on real-space data (Qdamp=0.034 Å-1 and Qbroad=0.011 Å-1). These parameters were held fixed during all subsequent data modeling. Al and O K-edge X-ray absorption spectroscopy A subset of synthesized boehmite nanoparticles covering a range of particles sizes from 10 nm to 1 µm were analyzed using synchrotron XANES at the Al K-edge, as well as O K-edge, on Beamline 6.3.1.1 at the Advanced Light Source, (Lawrence Berkeley National Laboratory, CA). Powdered samples were pressed into indium foil and mounted onto a copper sample probe using silver paint. The XANES signal was monitored at room temperature in total electron yield (TEY) mode at the Al K-edge. The energy was calibrated using the Si K-edge peak at 1846.3 eV48 for SiO2. XANES data were analyzed using the Athena interface to the IFEFFIT program49. Results and Discussion Effect of pH We first examined the effect of initial pH on the resulting boehmite nanocrystals using the Al gel precursor with a concentration of 0.5 M. For 200 oC and 48 h, we examined five different


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pH values, namely 4, 7, 10, 12, and 13.3, as shown in Table 1 (entries 1-5). XRD indicated all samples synthesized from these reactions were pure boehmite (Fig. 1A-1E); the diffraction pattern is in good agreement with reference data (ICDD PDF # 00-74-1895). The strong diffraction peak at the 2θ angle of 14.5 is assigned to (020) diffraction, based on the atomic structure along the basal plane normal (Fig.S1). All of the boehmite samples were highly crystalline and morphologically well-defined nanocrystals. XRD whole pattern fitting was used to estimate crystallite sizes along different crystallographic directions based on line broadening of the diffraction profiles from different crystallographic planes. To compare the size fitting from XRD, the SEM and TEM were also used to evaluate the size distribution of as-synthesized various boehmite samples. The sizes obtained from XRD fitting and microscopic observations from SEM and TEM were listed in Table S1 and S2, Figure S1 and S2.



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Table 1. Overview of boehmite synthesis conditions and resulting products. Entry Precursor

Concentration (M)

pH

Temp. (oC)

Time (h)

Product

Morphology

1

gel

0.5

4

200

48

boehmite

short nanoneedles

2

gel

0.5

7

200

48

boehmite

hexagonal nanoplates

3

gel

0.5

10

200

48

boehmite


4

gel

0.5

12

200

48

boehmite

rhombic nanoplates

5

gel

0.5

13.3

200

48

boehmite

rhombic nanoplates

6

gel

0.1

10

200

48

boehmite


7

gel

0.1

12

200

48

boehmite

rhombic nanoplates

8

gel

0.1

13.3

200

48

N/A

9

gel

0.01

10

200

48

boehmite

rhombic nanoplates

10

gel

0.01

12

200

48

boehmite

rhombic nanoplates

11

gel

0.01

13.3

200

48

N/A

12

gel

0.25

13.3

120

48

boehmite

rhombic nanoplates

13

gel

0.5

13.3

120

48

boehmite

rhombic nanoplates

14

gel

0.25

13.3

200

48

boehmite

rhombic nanoplates

15

powder

0.25

13.3

120

48

boehmite


16

powder

0.5

13.3

120

48

boehmite


17

powder

0.25

13.3

200

48

boehmite

rhombic nanoplates

18

powder

0.5

13.3

200

48

boehmite

rhombic nanoplates

19

gel

1

14

200

48

boehmite

irregular plate

20

powder

1

14

200

48

boehmite

rhombic nanoplates

N/A

N/A

21

gel

0.5

13.3

120

2

37% gibbsite 63% boehmite

22

gel

0.5

13.3

120

6

boehmite

bar-like gibbsite rhombic nanoplate boehmite rhombic nanoplates

23

gel

0.5

13.3

120

15

boehmite

rhombic nanoplates

24

gel

0.5

13.3

120

24

boehmite

rhombic nanoplates

25

powder

0.5

13.3

120

6

14% gibbsite 86% boehmite

26

powder

0.5

13.3

120

15

boehmite

bar-like gibbsite rhombic nanoplate boehmite rhombic nanoplates

27

powder

0.5

13.3

120

24

boehmite

rhombic nanoplates


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Figure 1. XRD patterns of boehmite synthesized at (A) 0.5 M, pH 4; (B) 0.5 M, pH 7; (C) 0.5 M, pH 10; (D) 0.5 M, pH 12; (E) 0.5 M, pH 13.3; (F) 0.1 M, pH 10; (G) 0.1 M, pH 12; (H) 0.01 M, pH 10; (I) 0.01 M, pH 12. The temperature and reaction time for all reactions were 200 oC and 48 h, respectively. The precursor was aluminum hydroxide gels.

Synthesis pH has a prominent influence on the resulting nanocrystal morphology, allowing some insight into the pH-dependence of the growth rate of specific crystal faces. SEM and TEM showed that the boehmite synthesized at pH 4 was comprised of short needle-shaped crystals with average lengths (along [001]) of 136.5 nm, widths (along [100]) of 32.9 nm, and heights (along [010]) of 25.8 nm (Fig. 2A, 3A and S2 and Table S2). However, samples synthesized from pH 7 to 13.3 maintained a nanoplate shape (Fig. 2B-2E and 3B-3E). The formation of plate-like boehmite nanocrystals when the pH was increased from 4 to 7 indicated a relatively high sensitivity of the relative growth rates of different faces in this pH regime, particularly for the [001] and [100] directions; the growth rate along the [001] direction appears to decrease whereas along the [100] direction it appears to increase, at higher pH. This suggests that pH controls not only the main transition from short needle-shape to platelet morphology, but within the pH regime yielding the platelet morphology smaller adjustments in pH can be used to tune the proportion of edge face types.




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Figure 2. SEM images of boehmite synthesized at (A) 0.5 M, pH 4; (B) 0.5 M, pH 7; (C) 0.5 M, pH 10; (D) 0.5 M, pH 12; (E) 0.5 M, pH 13.3; (F) 0.1 M, pH 10; (G) 0.1 M, pH 12; (H) 0.01 M, pH 10; (I) 0.01 M, pH 12. The temperature and reaction time for all reactions were 200 oC and 48 h, respectively. The precursor was aluminum hydroxide gels. The insert scale bar is 400 nm.



Figure 3. TEM images of boehmite synthesized at (A) 0.5 M, pH 4; (B) 0.5 M, pH 7; (C) 0.5 M, pH 10; (D) 0.5 M, pH 12; (E) 0.5 M, pH 13.3; (F) 0.1 M, pH 10; (G) 0.1 M, pH 12; (H) 0.01 M, pH 10; (I) 0.01 M, pH 12. The temperature and reaction time for all reactions were 200 oC and 48 h, respectively. The precursor was aluminum hydroxide gels. The insert scale bar is 200 nm.

For example, nanoplates were found to adopt hexagonal shapes in the pH 7 and 10 systems, whereas in the pH 12 and 13.3 systems the nanoplates exhibited rhombic shapes. Between pH 7 and 10, it was found that the nanoplate morphology can be tuned along specific axes. Compared with the pH 10 system, the product formed at pH 7 was longer along the [001] direction (94.8 nm


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at pH 7 versus 78.6 nm at pH 10), but shorter along the [100] direction (45.7 nm at pH 7 versus 60.1 nm at pH 10), and thinner along the [010] direction (10.3 nm at pH 7 versus 22.2 nm at pH 10) as shown in Table S2 and Figure S2. The increasingly fast growth rate along the [100] direction with increasing pH finally becomes fast enough at pH 12 to eliminate the (100) face altogether, yielding the rhombic nanoplate shape. Compared with the pH 12 system, the size of rhombic-shaped boehmite formed at pH 13.3 system is generally larger (79.6×79.6 nm2 at pH 12 versus 88.5 ×88.5 nm2 at pH 13.3) but also thinner along the [010] direction (25.3 nm at pH 12 versus 10.0 nm at pH 13.3) (Table S2 and Fig. S2). All of these shape and size trends with pH values at or above 7 can be summarized as follows: i) with increasing pH, growth along the [001] direction decreases while it increases along the [100] direction and ii) growth along the [010] direction increases from pH 7 to 12 and then decreases from pH 12 to 13.3. Scheme 1 illustrates the effect of pH on the crystal growth of boehmite nanocrystals.

Scheme 1. Illustration of the effect of pH on the growth of boehmite nanocrystals. The series progresses from “bar-shaped” to hexagonal platelets to rhombic platelets with increasing pH.

At the microscopic scale there are a number of possible pH effects on face stability. Alteration of solution pH changes the distribution of protonated and deprotonated amphoteric



sites in a fashion that is specific to each symmetry-unique face29, 50. This, in turn, changes the surface tension, modifying the relative proportion of each face in a manner consistent with achieving a thermodynamic minimum surface free energy at equilibrium34, 51-52. Varying the pH may also influence the ability of this system to form the hydrogen bonds that build up the structure along the [010] direction. Although the data in hand do not allow for conclusiveness, conceptually in acidic aqueous environments (like pH 4) proton binding to the (100) face could possibly saturate the surface in such a way that reduces the crystal growth rate along the [100] direction, while growth along [001] is less saturated, thus forming one dimensional, short needleshaped nanocrystals. Increasing the pH desaturates the (100) face allowing growth along the [100] direction to resume and subsequent formation of hexagonal (pH 7-10) and rhombic (pH 12-13.3) nanoplates. Effect of gel precursor concentration It was of interest to gain control of particle size independent of shape, i.e., at a constant pH. Given that particle size typically depends strongly on the initial concentration of precursor, we investigated the effect of precursor concentration on growth in the pH range from 10 to 13.3. As shown in Table 1 (entries 6-11), two concentrations of 0.1 M and 0.01 M and three pH values of 10, 12 and 13.3 were explored, using the same reaction time of 48 h and temperature of 200 oC as above. We chose concentrations lower than that used above (0.5 M) in an attempt to tune particle sizes to smaller average dimensions, while preserving pH-dependent shape. However, at pH 13.3 for both the 0.1 M and 0.01 M systems no product was formed. This reflects the importance of the Al/OH¯ concentration ratio for controlling the stability field of boehmite. In both cases, reaction of the Al(OH)3 gel with NaOH releases Al(OH)4¯ ions at concentrations insufficient to reach saturation with respect to boehmite.


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At the lower pH values between 10-12, boehmite was always produced. As was the case for 0.5 M precursor concentration, XRD indicated that samples produced from 0.01 M and 0.1 M gels were well-defined pure boehmite nanocrystals (Fig. 1F-1I). SEM and TEM showed all products had plate-like shapes (Fig. 2F-2I and 3F-3I). The boehmite from the 0.1 M and pH 10 conditions formed hexagonal shapes and the other three samples formed rhombic shapes. As was found for 0.5 M gel precursor, the size and the thickness of boehmite nanoplates increased with increasing pH from 10 to 12. And at a constant pH of 10 or 12, as hypothesized, the average particle size of boehmite decreased with decreasing precursor concentration. For example, at either pH values of 10 or 12, the size of boehmite nanoplates formed from 0.01 M gel was approximately three times smaller than the equivalent one from 0.5 M gel. Effects of precursor type and reaction temperature Finally, we explored the roles of precursor type and synthesis temperature on the size and shape of boehmite particles. As shown in Table 1 (entries 5, 12-18), two precursors of Al(OH)3 gel and amorphous powder were compared, at two concentrations of 0.25 M and 0.5 M, and at two synthesis temperatures of 120 and 200 oC, using a reaction time of 48 h and an initial pH of 13.3 in all cases. XRD indicated samples synthesized from all reactions were pure boehmite (Fig. 4). SEM and TEM showed all products maintained nanoplate shapes (Fig. 5 and 6). All boehmite samples from gel precursor exhibited a uniform rhombic shape, but boehmite samples from amorphous powders exhibited a hexagonal shape at 120 oC and a rhombic shape at 200 oC. In addition, the particle size of boehmite nanoplates from the gel precursor was smaller than that from the amorphous powder at the same conditions, as shown in Table S2, S3 and Figure 5, 6, S2, S3.



Figure 4. XRD patterns of boehmite synthesized at (A) 0.25 M gel, 120 oC; (B) 0.5 M, 120 oC; (C) 0.25 M, 200 oC; (D) 0.25 M powder, 120 oC; (E) 0.5 M powder, 120 oC; (F) 0.25 M powder, 200 oC; and (G) 0.5 M powder, 200 oC. The pH value and reaction time for all reactions were 13.3 and 48 h, respectively.

Figure 5. SEM images of boehmite synthesized at (A) 0.25 M gel, 120 oC; (B) 0.5 M, 120 oC; (C) 0.25 M, 200 oC; (D) 0.25 M powder, 120 oC; (E) 0.5 M powder, 120 oC; (F) 0.25 M powder, 200 oC; and (G) 0.5 M powder, 200 oC. The pH value and reaction time for all reactions were 13.3 and 48 h, respectively. The insert scale bar is 400 nm.


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Figure 6. TEM images of boehmite synthesized at (A) 0.25 M gel, 120 oC; (B) 0.5 M, 120 oC; (C) 0.25 M, 200 oC; (D) 0.25 M powder, 120 oC; (E) 0.5 M powder, 120 oC; (F) 0.25 M powder, 200 oC; and (G) 0.5 M powder, 200 oC. The pH value and reaction time for all reactions were 13.3 and 48 h, respectively. The insert scale bar is 100 nm.

To gain insight into possible differences between the two amorphous precursors, we compare their X-ray PDFs in Figure 7. A simulated PDF is provided for the boehmite product phase as a reference. It is immediately evident from their remarkably similar PDFs that the two precursors bear similar chemistry and structure as far as X-ray sensitive elements (Al and O) are concerned. The nearest neighbor pair correlations (corresponding to Al-O/OH in boehmite) are nearly identical, and other pair correlations are well matched in terms of peak location and shape. It is furthermore notable that the first three of four pair correlations, corresponding to the first few coordination shells, resemble those in the boehmite product (these are identified in Fig. S4 as Al-O/OH at ~1.9 Å, O/OH-O/OH between ~ 2.5 Å and 2.8 Å, Al-Al at ~2.9 Å, and next neighbor Al-O/OH at 3.4 Å). This indicates that all three phases share short-range structural similarities, which suggests that the dependence of boehmite characteristics on precursor type



may be sensitive to differences in long-range order of the precursors. Close examination reveals that the gel precursor is more ordered (it has more intense and more persistent correlations at larger real space distances) and its atomic structure is slightly expanded (shifted to larger realspace distances) relative to the powder precursor. Local atomic structure fits to the boehmite crystal structure are presented in Figure S5, and Table S3, and included later in Figure 11. It is difficult to imagine these slight structural distinctions could impose significant templating differences for the boehmite product. It thus seems more likely that another mechanism, such as the rate and extent of water release (or another kinetic driver), or differences in the resulting solubilized Al speciation, is the cause of the different reaction pathways and product characteristics. And for any given precursor, the concentration effect on particle size discussed above held true here as well; higher concentration produced larger boehmite particles in all cases except for the amorphous powder at 200 oC, which yielded slightly larger particles at 0.25 M than at 0.5 M.

Figure 7. X-ray PDF data showing the similar local atomic structures of the gel (black line) and powder (red dashed line) amorphous precursors. A calculated boehmite dataset is shown in light grey for reference.


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The reaction temperature also had a substantial effect on particle size.

The lower

temperature of 120 oC yielded much smaller particle size boehmite (more than 50% size decrease) than at our more standard temperature of 200 oC, for otherwise equivalent conditions. For example, the sizes of boehmite rhombic nanoplates synthesized at 120 oC using 0.25 M and 0.5 M gel concentrations were around 19.2×19.2×4.9 nm3 (XRD fitting: 15.7×15.7×4.1 nm3) and 20.1×20.1×5.1 nm3 (XRD fitting: 19.0×19.0×5.3 nm3), respectively. However, at 200 oC and the same concentrations the respective boehmite particles, which maintained a rhomobic nanoplate morphology, increased in size to around 37.5×37.5×7.5 nm3 (XRD fitting: 27.4×27.4×8.6 nm3) and 88.5×88.5×10.0 nm3 (XRD fitting: 39.5×39.5×16.1 nm3)(Table S2, S3 and Figure 5, 6, S2, S3). The collective insights gained into the effects of pH, precursor type and concentration, and temperature were used in some selective experiments to target synthesis of largest possible boehmite nanoplates via our general protocol. In an attempt to maximize nanoplate size we ran several hydrothermal reactions using high concentration precursors and high pH at 200 oC, from which we obtained two optimized conditions to prepare boehmite nanoplates with sizes around hundreds nanometers and several micrometers, as shown in Table 1 (entries 19 and 20). Boehmite nanoplates hundreds of nanometers in diameter were prepared using 1 M amorphous powder as the precursor, at pH 14 for 48 h (Fig. 8A and 8B). The shape of these platelets was rhombic. Using the same conditions except with gel precursor yielded microsized platelets of irregular shape up to 5 µm in diameter (Fig. 8C and 8D). The irregularity manifested as serrated edge structures transiting along the [100] direction terminated with {101} microfacets. XRD



indicated both of these two larger size samples were highly crystalline pure boehmite platelets (Fig. S4).

Figure 8. SEM and TEM images of boehmite synthesized at (A) and (B) 1.0 M amorphous powder precursor; and (C) and (D) 1.0 M gel precursor. The temperature and reaction time for all reactions were 200 oC and 48 h, respectively. The pH value of all reactions was 14.

Detailed characterization was performed to ensure that all boehmite particles produced, including these substantially larger ones, were highly crystalline materials even to their outermost surfaces. With the exception of perhaps high resolution TEM, most of the characterization techniques used provide information about the character of the bulk solid and only limited information about the structural integrity of the upper few Ångstroms of their surfaces. Synchrotron XANES at the Al and O K-edges, collected in total electron yield mode, for which the information depth is most sensitive to < 10 nm from the surface, on a subset of boehmite nanoplates of widely varying particle size (20 nm to several µm) showed spectral characteristics consistent boehmite independent of particle size (entries 13, 9, 19, 20, Fig. 6B,


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3H, 8C and 8D). The O K-edge XAS spectrum of boehmite is sensitive to the O bonding environment at the surface and arises from O 1s transition to O 2p unoccupied states mixed with the ligand s, p, and sp orbitals53. The two peaks present in the spectra for all boehmite samples at 535 and 540 eV can be assigned to σ* transitions of Al–O and O–H bonding. The spectra in Figure 9A closely resemble that for pseudoboehmite54-55, suggesting that the boehmite samples are hydrated with excess chemically bound water at the surface55. The Al K-edge XANES spectra for the boehmite samples (Fig. 9B) yield three edge maxima at 1568, 1570 and 1572 eV and a shoulder at higher energy (1575 eV)56. These spectral signatures are due to Al in boehmite having distorted octahedral symmetry57 which gives rise to a range of Al-O distances as discussed above. Some variability between the relative intensity of these three edge maxima occurs between the spectra for the boehmite nanoplatelets shown here (Fig. 9B) and between boehmite spectra in the literature (Fig. 9C). The O K-edge spectra (Fig. 9A) suggest the presence of chemisorbed water at the surface. XPS analysis in Huestis et al.58 confirmed the presence of adsorbed water (13.6 %) in the 20-80 nm boehmite sample. The variations in the Al K-edge spectra are thus likely due to variable water contents between the samples, the effect being accentuated by the high surface-to-volume ratio of the nanoplatelet samples. These XAS results suggest that the structural quality of the products in this study persist from bulk interiors to outermost surface planes of atoms. The finding helps reinforce the conclusion that the synthesis strategies developed in this study produce high quality boehmite nano-to-microparticles with controllable size and shape without sacrificing surface structural integrity.



Figure 9. (A) O K-edge spectra and (B) Al K-edge spectra for Table 1 entry 13 (blue), entry 9 (green), entry 19 (red) and entry 20 (black) boehmite particles. (C) Al K-edge spectra for boehmite/pseudoboehmite from Zhang at al., 2009 (blue)59, Hu et al., 2008 (green)54, Kato et al., 2001 (red)60 and Ildefonse et al., 1998 (black)56. Crystal growth mechanisms To begin to understand the mechanism of precursor transformation to boehmite, we examined the evolution of the solid phases over time (Table 1, entries 21-27). For the gel precursor at a premature reaction time of 2 h, XRD showed a mixture of 37% gibbsite and 63% boehmite (Fig. S7B). Although the gibbsite phase is thermodynamically unstable at our reaction conditions, it emerges as a metastable intermediate en route to the boehmite final product, consistent with Ostwald’s Law of Stages61. SEM and TEM characterization of this intermediate gibbsite phase showed nearly equant prismatic crystallites extended along the [001] direction with widths of around 100 nm (Fig. 10). Nascent boehmite nanoplates with subhedral rhombic shapes and sizes from 10 to 50 nm surrounded the gibbsite crystals (Fig. S8 and S9). These subhedral nanoplates ultimately grow into euhedral nanoplates at the expense of intermediate gibbsite at reaction times longer than 4 h (Fig. S7C-S7E). The gibbsite intermediate phase also emerges in the syntheses based on the amorphous powder precursor. In this case, for a 6 h reaction time XRD showed the solid phase product was a mixture of 14% gibbsite and 86% boehmite (Fig. S10B). Both of SEM and TEM


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characterization indicated that the gibbsite particles were prismatic in shape, again, but this time with a larger width than intermedium gibbsite formed from the gel precursor (Fig. 10). And, similarly, these gibbsite intermediates were surrounded by nascent boehmite nanoplates of subhedral morphology and with sizes from several nanometers up to 50 nm. This system evolves past the gibbsite intermediate at reaction times longer than 12 h (Fig. S10C-S10D), and by 24 h uniform hexagonally shaped boehmite nanoplates emerge (Fig. S11 and S12).

Figure 10. TEM images of sample synthesized using 0.25 M gel at 120 °C for 2 h. (A) low magnification, inset is a selected area electron diffraction (SAED) of the bar-shaped material;(B) high resolution TEM of the bar-shaped material; (C) small size particles attached on the barshaped material, inset is a SAED of the small size particles. The series of samples produced with different precursors and reaction times (Table 1, samples 21-27) were examined with X-ray PDF to further investigate the evolution of phases. Data were fit between 1 and 100 Å, refining a spherical particle form factor (as an approximation of average structural coherence length), lattice parameters, isotropic atomic displacement parameters, and fractional coordinates of the atoms. More details are given in SI. Figure 11 displays the results for the (A) gel and (B) powder precursor series. The PDF results support the observation of phase pure, high quality crystalline nanoplates that increase in crystallinity with the progression of time. It is evident from the quality of the fits (absence of low-r structural features in the difference curves) that there are no identifiable amorphous components in any of



the samples examined. The models used for the 2 h sample in (A) and the 6 h sample in (B) include both gibbsite and boehmite phases, while all other models are pure boehmite. The 2 h sample in (A) included 31% gibbsite and the 6 h sample in (b) included 12% gibbsite, which agree well with the XRD measurement (

Size and Morphology Controlled Synthesis of Boehmite Nanoplates

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