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Sol–Gel Derived 2D Nanostructures of Aluminum Hydroxide Acetate: Toward the Understanding of Nanostructure Formation Ruohong Sui, John M.H. Lo, Christopher Brian Lavery, Connor E Deering, Kyle G. Wynnyk, Nancy Chou, and Robert A Marriott J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12490 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Sol–Gel Derived 2D Nanostructures of Aluminum Hydroxide Acetate: Toward the Understanding of Nanostructure Formation Ruohong Sui, John M. H. Lo, Christopher B. Lavery, Connor E. Deering, Kyle G. Wynnyk, Nancy Chou, and Robert A. Marriott* Chemistry Department, University of Calgary, 2500 University Drive, Northwest, Calgary, Alberta, Canada, T2L 4N1 *

To whom correspondence should be addressed, [email protected].

Department of Chemistry, University of Calgary 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4

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Abstract. Two-dimensional (2D) metal oxide nanostructures have generated a great deal of attention in material science for their prospective wide-ranging applications; therefore, a scalable and economical method for producing these structures is an asset. In this research, a simple procedure for the preparation of 2D aluminum hydroxide acetate macromolecules ([Al(OH)(OAc)2]m) has been developed via a nonaqueous sol─gel route at a mild reaction temperature and ambient pressure. To gain a greater understanding of the mechanism for how the self–assembly of these 2D structures occurs, a combination of in-situ Fourier transform infrared (FTIR) measurements and density functional theory (DFT) calculations were utilized. It was found that the bridging OH−1 and coordination modes of the organic ligands guide the assembly of the planar nanostructures. The theoretical calculation results show that the structures of the [Al(OH)(OAc)2]8 oligomer can either be a linear or planar structure, and the latter is more thermodynamically favorable than its linear counterpart. The simple synthesis method described herein could possibly open a new avenue for designing 2D nanostructures via ligand directed anisotropic condensation reactions.


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Introduction. For improved functionality, metal oxides and metalorganics with well-defined nanostructures and uniform compositions have found important applications in catalysis, electronics, medicine, separation, environmental engineering, and energy conversions.1-7 In this context, Al2O3 based materials have been widely used in the energy industry, for example, Claus catalysts for conversion of sulfides from natural gas and petroleum production.8-11 However, in order to meet increasingly stringent environmental regulations, it is necessary to improve current technologies and to develop new generation catalysts.. Notwithstanding the synthetic utility of conventional precipitation and aqueous sol‒gel methodology, such procedures are not suitable for the preparation of finely tuned nanostructures because in an aqueous media, uncontrollable rapid hydrolysis and condensation of metal precursors, such as metal/transition metal alkoxides and chlorides, leads to products with random shapes, sizes, and compositions. Much research has been focused on synthesizing desired nanostructures, such as templating, reverse micelle sol‒gel, and sol‒gel combustion process.12-14 Among these attempts, non-aqueous sol‒gel methods have recently generated a great deal of attention for synthesizing a number of metal oxides, hybrid oxides, and metalorganic materials, owing to (i) the enhanced ability for controlled morphology and homogeneity of the final products and (ii) the convenience for scale-up.15-18 Indeed, non-aqueous sol‒gel methodology has emerged as a feasible and scalable technology for synthesizing tailored nanomaterials, including 0D (nanospheres), 1D (nanofibers/tubes), and 3D (porous monoliths) architectures.17,19-22 From a materials science perspective, understanding the reaction mechanism and studying how small molecules selfassemble into a macromolecule with a specific geometry is the first step for nanostructure design and bottom-up synthesis of desired morphologies. Two strategies for the non-aqueous approach have been attempted using (i) relatively mild reactions such as elimination of alkyl halides, ethers and esters at elevated temperatures,16 or (ii) in–situ generated water via aldol-like condensation (eq. 1)15 or esterification (eq. 2)23 to control the net sol─gel reaction rate. 2CH3COCH3 → CH3COCH=C(CH3)2 + H2O ACS Paragon Plus Environment

(eq. 1) 3


ROH + R'COOH → R'COOR + H2O

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(eq. 2)

The generated water in eq. 1 has triggered synthesis of titanium oxo clusters and highly crystalline titania nanoparticles.24,25 Similarly, the water produced by esterification reaction in eq. 2 also has been used for making different morphologies of metal‒oxo‒acetates that could be calcined into metal oxides.26,27 For example, metal isopropoxide has been reacted with excess acetic acid (HOAc) to generate 1D nanostructures of [Ti6O9(OAc)6]n and [Al(OH)(OAc)2]n in supercritical CO2 (scCO2).28-30 The mechanism of 1D Ti(IV)-based nanostructure formation was proposed as anisotropic condensation and formation of the linear macromolecules structure. However, the reaction mechanism and kinetics of aluminum isopropoxide with HOAc is not well understood. In a separate study, aluminum trichloride (AlCl3) was used to react with aqueous potassium acetate to prepare fibrous Al(OH)(OAc)2 in an ionic liquid.31 Despite numerous studies on 0D, 1D and 3D nanostructure formation, reports on sol-gel derived 2D architectures (nanosheets or nanoribbons) are scarce, even with the many potential important applications in sensors, electronics, and energy conversions.32-35 In this account, we describe a simple procedure for synthesis of 2D [Al(OH)(OAc)2]n, which can be easily transformed to 2D alumina by heat treatment. Also, we try to answer some basic questions regarding the synthesis process pertaining to reaction mechanisms in addition to how the starting molecules assemble into 2D structures. Methods. Synthesis and online IR monitoring. A 500 mL three-neck flask equipped with a magnetic stirrer and a heating mantle was used as the sol–gel reactor. The three necks of the flask were separately connected to an attenuated total reflection-Fourier transform infrared spectroscopy (ATR–FTIR) DiPost probe (Mettler Toledo), a reflux condenser and a thermocouple, respectively. The thermocouple and heating mantle were connected to a temperature controller (Omega) for tuning the flask reactor temperature (see Figure S1 in supporting information for the experimental setup). In a typical experiment, 100 mL of n-heptane (anhydrous, 99%, Sigma-Aldrich) and 30.00 g of acetic acid (HOAc) (99.7%, Sigma-Aldrich, dehydrated with 5A molecular sieve for 24 hrs before use) were added ACS Paragon Plus Environment

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sequentially to the flask and the mixture was heated to the desired reaction temperature (i.e., T = 323, 333, 343, or 353 ±1 K) with stirring. After the temperature of the reaction mixture stabilized, 10.21 g of aluminum isopropoxide (98%, Sigma–Aldrich) was added to the flask, then immediately in-situ FTIR spectra were collected automatically in the range of 600–4000 cm-1 with a resolution of 2 cm−1, using a ReactIR ic10 system (Mettler Toledo). The IR monitoring was conducted continuously until the sol–gel process was complete. The IR collection intervals varied from one minute (at the beginning of the reaction) to 30 minutes (after 7.5 hours of reaction when the reaction kinetics slowed down). A colloidal suspension was formed first, followed by a slow gelation that increased the viscosity of the fluid. When the gelation stopped the magnetic stirring from spinning and no sign of moving fluid, the gel was aged for two days at the reaction temperature, and the resulting material was dried under vacuum at T = 353 K for 12 h. For comparison, some of the product samples were calcined at 873 and 1073 K for two hours; each sample was brought to the respective calcination temperature and cooled to room temperature at a ramp rate of 5 K min-1 in air. Characterization. Transmission electron microscopy (TEM) images were obtained using a Phillips Tecnai F20 operated at 200 kV. Scanning electron microscopy (SEM) images were recorded using a FEI Philips XL30 at 20 kV with gold coating to prevent specimen charging. The elemental analysis was performed on an X–ray photoelectron spectrometer (XPS) (Axis Ultra, Kratos Analytical). XRD data was obtained using a Rigaku Multiflex diffractometer with a copper target at a speed of 2° / min with a step size of 0.02°. The carbon concentrations in the sol-gel products were corrected by subtracting the portion contributed by the hydrocarbon contaminations,36 by using the high resolution C1s XPS data and taking into account the 1:1 ratio of C=O to C-H in the acetate ligands. For final product FTIR analysis, a Thermo Scientific Nicolet iS10 instrument was used to examine the KBr pellets with ca. 2 % powder sample included. To measure the mass loss at an elevated temperature, a thermal gravimetric analysis (TGA) was conducted on a SETARAM Labsys EVO system. The samples were heated from 298 to 873 K at a scanning rate of δT = 5 K · min−1. The dynamic weighing range used in this work is m = 1.000 g. In this weighing range the instrument resolution is 2 · 10−7 g with a weighing precision of ± 0.01 %. The ACS Paragon Plus Environment

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instrument was swept with 99.999 % helium at a rate of 25 mL · min−1 to ensure a stable and inert atmosphere over the course of the experiments. For headspace gas analysis during the sol-gel reactions, a Varian CP-3800 gas chromatography instrument was used, which is equipped with Rt-U-Bond and Molsieve 5A columns and high-sensitivity Gowmac TCD detectors. For Raman analysis, the sample powder was packed into aluminum disks before being loaded into a Bruker MultiRam spectrometer equipped with a 1000 mW, 9396 cm-1 (1064.28 nm) HeNe laser and a liquid-nitrogen-cooled germanium diode detector. The samples were analyzed over 5000 scans at a laser power of 50 mW. DFT Calculations. The geometry optimizations of the linear and square forms of [Al(OH)(OAc)2]8 were performed at the DFT/B3LYP level of theory37,38 with Pople’s 6-31G(d) basis set39,40 using the Gaussian 09 suite program.41 An ultrafine integration grid and tight convergence criterion were employed to improve the accuracy of computation. The optimized structures were further characterized by normal mode analysis to ensure they correspond to local minima on the potential energy hypersurface. Results and Discussion. In the initial stages of nanostructure synthesis, aluminum isopropoxide powder was found to be dissolved into the acetic acid solution. The clear solution became cloudy (a sign of sol accumulation) after some reaction time, depending on the reaction temperature. Following the formation of the sol, the viscosity of the fluid increased to the point of obstructing the magnetic stirring bar. During sol and gel formation, occasional bubbles were observed escaping from the liquid phase. GC analysis of the headspace gas revealed that the effervescing gas was propene. Electron Microscopy. In this study, different reaction temperatures were examined to probe the effect on the resulting products; all other reaction parameters were kept constant. Changing reaction temperature was found to effect the product morphologies significantly. The lower temperatures of 323 and 333 K resulted in rod-like materials. The rod diameters were in the range of 50-150 nm, while the length was up to 1 µm (Figure 1a). When the reaction temperature was elevated to 343 K, the product morphologies became irregular, and the dominant shape was thick plates with different sizes (Figure 1b). ACS Paragon Plus Environment

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At a reaction temperature of 353 K, the product was well-defined ribbons with a width ca. 80 nm and a length up to 2 µm (Figure 1c). These thin 2D structures formed at 353 K were further examined using TEM (Figure 2) to reveal more structural detail. The nanoribbons were prevalent and often arranged in a way similar to flower petals (Figure 2a), while larger 2D structures were also found having a width of a few hundred nanometers (Figure 2b). Some larger sized 2D structures were curled, likely to reduce the interfacial energy (Figure 2c and d). These curled nanostructures may inspire future exploration in nanotube synthesis, similar to TiO2 nanotube formation through a ribbon rolling mechanism under hydrothermal synthetic conditions.42 The sol-gel product morphologies in this research were quite different from the previously reported nanofiber formation, where similar synthetic methodology had been used except that a supercritical fluid was the reaction media.30 Solvent effect on the final product shapes will be discussed later.

Figure 1. SEM images of the sol-gel products prepared at a reaction temperature of 323 (a), 343 (b), and 353 K (c). Notes: The white scale bar = 2 µm. The morphologies of the products produced at 323 K and 323 K are very similar.


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Figure 2. TEM images of the sol−gel products prepared at a reaction temperature of 353 K: the agglomerated nanoribbons which are dominant in the product (a), the larger-sized 2D flat structures (b), the 2D structures curled along the axial direction (c), and the 2D structures curled perpendicularly to the axial direction (d).

XRD. The XRD patterns of the products prepared at different reaction temperatures are shown in Figure 3. Apparent from the XRD patterns, Al(OAc)3 was prevalent in the samples prepared at a reaction temperature range of 323─343 K, and a lesser amount of Al(OH)(OAc)2 was also found in those product mixtures. At elevated reaction temperatures both Al(OH)(OAc)2 and Al(OAc)3 crystalline phases progressively increased in the products with the former being produced relatively more. However, at 353 K, Al(OAc)3 disappeared and Al(OH)(OAc)2 was the only crystalline phase in the solid products. In conjunction with the electron microscopy results, it can be proposed that the 2D structures are of the Al(OH)(OAc)2 crystalline phase, while the rods are more likely to be Al(OAc)3. Note that the XRD pattern shown in yellow in Figure 3 is different from that of the 1D structures synthesized in the supercritical fluid by Charpentier and co-workers, despite the fact that both products were produced at ACS Paragon Plus Environment

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the same temperature of 353 K.30 It is not surprising to see that the shapes are varied when the crystalline phases are different.

d

d

Intensity (a. u.)

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t

d

d

d t

t t

0

5

10

15

20

25

30

35

40

45

50

2 θ (° )

Figure 3. XRD patterns of the materials synthesized at different reaction temperatures at 323 K (brown), 333 K (green), 343 K (black), and 353 K (ochre). The peaks labeled “t” and “d” are assigned to Al(OAc)3 and Al(OH)(OAc)2, respectively (PDF#014-0774 and PDF#013-0883).

The XRD pattern trend in Figure 3 can be explained by considering the reactions between aluminum isopropoxide and acetic acid: Al(OiPr)3 + 3HOAc ⇌ Al(OAc)3 + 3CH3-CHOH-CH3

(eq. 3)

CH3-CHOH-CH3 ⇌ CH3-CH=CH2 + H2O

(eq. 4)

Al(OiPr)(OAc)2 + H2O ⇌ Al(OH)(OAc)2 + PriOH

(eq. 5)

The reaction in eq. 3 does not seem difficult because of the excess acetic acid (10 equi.), and it appears that this reaction is more facile at higher reaction temperatures up to 343 K. The reaction in eq. 4 is supported by the formation of propene, which was confirmed by GC analysis. It is noted that eq. 4, which produces water for the proceeding of eq. 5, is also favored at higher temperatures and the ACS Paragon Plus Environment

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presence of a Lewis acid.43 This is in line with the observation that formation of Al(OH)(OAc)2 was promoted by a higher reaction temperature (Figure 3). Despite the solid XRD results, it was not clear what the driving force was for the formation of different morphologies and if there were any other amorphous materials formed that were not detectable by XRD. Hence, further investigations were carried out utilizing XPS, Raman, and FTIR techniques. XPS. XPS is a surface characterization technique that detects a few layers of atoms below the surface of both crystalline and amorphous materials. As described in the XRD section, two different products, Al(OAc)3 and Al(OH)(OAc)2, were prepared, and this information was useful for interpretation of the XPS data. Indeed, it was revealed that the major product formed at higher temperature (353 K) exhibited an Al:C:O molar ratio of 10.05:40.22:49.73, close to the formula of Al(OH)(OAc)2 (Table 1). At a lower temperature (323 K), the resulting element concentrations are comparable with the formula Al(OAc)3. The deviation in Al and C concentrations between the experimental and the calculated values are in agreement with the XRD results, indicating a product mixture of Al(OAc)3 and Al(OH)(OAc)2, and it could be written as Al(OH)x(OAc)3-x, where x is the molar fraction of Al(OH)(OAc)2.

Table 1. Normalized XPS analysis results of the as−prepared sol−gel products Al2p

C1sc

O1s

Exp.

9.67

45.86

44.47

Cal.

7.69

46.15

46.15

Exp.

10.05

40.22

49.73

Cal.

10.00

40.00

50.00

Product Al(OAc)3 a

Al(OH)(OAc)2 b

Note: a prepared at the reaction temperature 323 K, b prepared at 353 K, and c the carbon concentrations were corrected due to the adsorbed carbon in the samples (see method and Figures S2 and S3 in supporting information for details).


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Powder vibrational spectroscopy. Further structural information about the sol-gel products was revealed by powder FTIR spectroscopy. As shown in Figure 4, the reaction precursor aluminum isopropoxide exhibits C-H bands at ~2965 cm−1 and a C-C band at 1387 cm−1. Different from the starting material, all sol-gel products show peaks of bridging acetate bidentate; the bands at 1582 and 1477 cm−1 are assigned to bidentate COO−1 νasym and νsym vibrations, respectively. The band at 1655 cm−1 is attributed to the monodentate COO−1 νasym vibration.44,45 The νsym monodentate COO−1 vibration is likely hidden in the right shoulder of the peak at 1477 cm−1. The bond structures of monodentate, bridging, and chelating bidentates are shown in scheme 1. If the spectra of b, c, and d in Figure 4 are compared, it can be observed that the monodentate peak at 1655 cm−1 gradually decreases with an elevated reaction temperature, showing a sign of further condensation at a higher temperature into the bidentate or substitution by an -OH group. It should be pointed out that formation of a bidentate bridging mode is crucial for the formation of macromolecules and consequently the sol-gel nanostructures which will be discussed in more detail later. In Figure 4 it can also be observed that there is a difference between the products prepared at different reaction temperatures: the high-temperature products show a strong peak belonging to the bridging –OH peak at 977 cm−1 (this is confirmed by IR analysis of the product after deuterium replacement, see Figure S4 in supporting information),44 but this peak is absent in the counterpart produced at 323 K. The formation of this bridging –OH is considered a fundamental step for the nanostructure self-assembly and will also be further discussed later. It is noted that the non-bridging –OH is present in the products as a broad band at ~3446 cm−1, which is assigned to the small portion of Al(OH)(OAc)2 component. In addition, the sharp peak at 3699 cm−1 is assigned to the non-hydrogen-bonded –OH group.


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Figure 4. The powder FTIR spectra of the precursor aluminum isopropoxide (a), and sol–gel products prepared at a reaction temperature of 323 K (b), 333 K (c), and 343 K (d).

Scheme 1. The monodentate, chelating, and bridging bidentate binding modes of acetate. Raman spectroscopy was also used for analysis of all the products produced at different temperatures, in which the functional groups of CH, CO, CC, and COO were identified (see Figure S5 in supporting information). However, the variation of the spectra between the samples is quite small. In situ FTIR spectra. ATR-FTIR has proven to be a powerful tool for online monitoring of reaction intermediates and kinetics.29 In this study, the technique was crucial in providing information for a better understanding of the sol-gel reaction mechanism and kinetics. The in-situ FTIR spectra, collected over a 32 h period when Al(OiPr)3 was reacted with 10 equivalents of HOAc at a temperature of 353 K, is presented in Figure 5a. The bands in the region of 700–2250 cm−1 are of interest and thus are presented in Figure 5b for a more clarified observation. The trends of changing band intensities are marked by the arrows. On the basis of this figure, the intensity of some bands did not change significantly throughout ACS Paragon Plus Environment

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the sol-gel reactions, e.g., bands of C–H (~2800–3000 cm−1) and bands of non-hydrogen bonded OH stretching (~3600-3800 cm−1, with some noise peaks); in contrast, the bands of acetate vibration (1470– 1726 cm−1) and bridging OH bending (977 cm−1) changed significantly throughout the course of the reactions. The carbonyl (C=O) band of HOAc (1715–1726 cm−1) decreased in the first few minutes only and was stable throughout the rest of the reactions. For convenience, the IR peak assignments are listed in Table 2.

Figure 5. (a) 3D view of in situ FTIR spectra over a 32 h time period at 353 K. (b) expanded region highlighting the νcoo- and C-C bands as a function of time (arrows indicate progressive trends of absorbance with reaction time).


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Table 2. Assignment of major IR peaks IR peaks (cm−1)

Assignment

1715–1726

HOAc: carbonyl

1650–1681

νasym: Al–OAc

acetate monodentate

1580–1592

νasym: Al–OCH(CH3)O–Al

acetate bridging bidentate

1470–1480

νsym: Al–OCH(CH3)O–Al

acetate bridging bidentate

1288–1290

C–C: HOAc:

1037–1055

C–C

acetate monodentate

977

OH

bridging OH bending

950

HO–iPr

936–945

HOAc:

Note

Consideration of all of the possible chemical reactions is the first step for interpretation of these in situ FTIR data. In Figure 5, the spectrum of starting material Al(OiPr)3 is not observed, and the acetic acid peaks at 1726 and 1288 cm-1 are quite stable with the exception of the very beginning of the reaction. This indicates that the initial reactions between Al(OiPr)3 and HOAc was fast. Considering the very high chemical activity of Al(OiPr)3, it is not surprising to see that the reaction rate of ligand exchange, OiPr being substituted by OAc ligands, was rapid. It is noted that the ligand exchange may result in shortlived intermediates of Al(OiPr)2(OAc) and Al(OiPr)(OAc)2, which eventually transforms to Al(OAc)3 (eq. 3). One of the important steps in this type of sol-gel process is formation of water (eq. 4), which consequently induces hydrolysis (eq. 5) followed by condensation (eq. 6). mAl(OH)(OAc)2 ⇌ [Al(OH)(OAc)2]m

(eq. 6)

The macromolecule solubility decreases during growth, and eventually evolves into sol and gel (the solid network with liquid in the pores).5 According to Figure 5, the generated water was quickly consumed as shown by the absence of water peaks in the whole process. In seeking a species that can be used for monitoring the sol-gel process, the bridging Al–OH–Al peak at 977 cm-1 proved useful.46 By


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plotting the 977 cm-1 peak height vs. reaction time (Figure 6, the blue curve), a curve with a threshold at 71 minutes is obtained, which shows the kinetics of macromolecule [Al(OH)(OAc)2]m formation. This kinetic data is similar to that collected for titanium isopropoxide reaction with acetic acid.47 The dormant time before the threshold in Figure 6 is anticipated because of the slow reaction in eq. 4 and accumulation of monomers, Al(OH)(OAc)2, for the condensation reactions to start. For these macromolecules, the bridging ligands of –OH and –OAc link the monomers together. Parallel to the bridging –OH bond formation, the acetate bridging bidentate peaks at 1592 and 1480 cm-1 are shifted to lower wavenumbers (red shift), 1580 and 1470 cm-1, respectively (Figure 5). This indicates that the bond length of the bridging acetate becomes shorter while the reaction proceeds, due to bridging –OH formation. It is noted that further hydrolysis of Al(OH)(OAc)2 to form Al(OH)2(OAc) and Al(OH)3 did not occur as the metal complexes are stabilized by the carboxylate bidentate ligands.48

0.6 0.5

Peak Height (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.3 0.2 0.1 0 1

10

100

1000

10000

Time (min.)

Figure 6. The plot of 977 cm-1 peak height vs. reaction time at a reaction temperature of 353 (blue), 343 (red), and 333 K (green). The corresponding threshold of the curves is at 71, 1080, and 2100 min, respectively.

For comparison, Figure 7 shows the in situ FTIR spectra at a lower reaction temperature of 323 K. In contrast to Figure 5, there is no 977 cm-1 peak, indicating that the bridging –OH bond is not formed. Even though the XRD results suggested there was at least some Al(OH)(OAc)2 formed, the product IR spectra do not show bridging –OH bonds. Instead, a non-bridging –OH band at ca. 3446 cm-1 appears ACS Paragon Plus Environment

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(Figure 4), suggesting Al(OH)(OAc)2 exists in the form of monomer. This indicates that an activation energy has to be overcome to form the bridging –OH bonds in this system and a temperature of 323 K is insufficient to do so. Secondly, the acetate bridging bidentate peaks show a blue shift through the reaction process; the COO−1 νasym vibration shifts from 1572 to 1588 cm-1, while the νsym vibration shifts from 1466 to 1477 cm-1. This phenomenon is similar to the carbonyl C=O stretch where carboxylic acid exhibits a higher IR frequency than an ester. It can be explained by weakening of C=O bond by connection to a more electronegative group –OH rather than less electronegative ─OAc. Thirdly, the total acetate monodentate band intensity in the range of 1650-1673 cm-1 in Figure 7 is noticeably larger than those in Figure 5. In other words, the change in binding mode of acetate from monodentate to bidentate is more complete at a higher temperature. This is in line with the nanostructure observations, well-defined 2D structures at 353 K and not-well-defined rod-like structures formed at lower temperatures. These results indicate that a bidentate acetate binding mode, in addition to bridging –OH, is important for constructing 2D nanostructures.

Figure 7. In situ FTIR spectra over a 49.5 h time period at reaction temperature 323 K: expanded region highlighting the νcoo- and C-C bands as a function of time (arrows indicate progressive trends of absorbance with time).


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In-situ FTIR spectra collected at 333 K and 343 K are similar to Figure 5 (collected at 353 K), except that the bridging –OH peak at 977 cm-1 is significantly less pronounced (see Figures S6 and S7 in supporting information). This again agrees with XRD and SEM results. According to the XRD results in Figure 3, there is a certain amount of Al(OH)2(OAc) in the products. Referring to the SEM images in Figure 1, there is a small portion of planar structure formation. In Figure 6, it can be noticed that the thresholds are a function of reaction temperature and this is because the reaction that generates water (eq. 4) is favored at higher temperatures. It should be mentioned that in our IR spectra, chelating bidentate acetate peaks were not distinguishable, if there was any.

Nanoribbon formation mechanism.

As described earlier, our FTIR analysis results, both powder and in-situ, have revealed the formation of bridging –OAc and –OH bonds. In this step, the monomer can either form planar units which eventually may form a 2D nanostructure (Figure 8a–b), or form linear units that can generate a 1D structure (Figure 8c). In order to compare the thermodynamic properties of the linear and planar structures, we used simple octomers as models. Our DFT calculations show that the square octomer has a lower free energy than the linear octomer (Table 3). This suggests that formation of a planar structure is more thermodynamically favorable. However, it is noted that the surface tension energy of the 1D and 2D structures in solution was not considered. Additionally, it is also noted that the high surface energy may result in formation of multi-layer stacking in the final products as was observed under an electron microscope.


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Figure 8. DFT calculated structures of the planar octomer in top view (a) and side view (b), and linear octomer (c). Note: aluminum in pink, carbon in dark grey, oxygen in red, and hydrogen in light grey.

Table 3. Computed thermodynamic quantities Conformer

Electronic + ZPE

Enthalpy (298 K)

Free Energy (298 K)

Square

-6202.952553 au

-6202.852237 au

-6203.094355 au

Linear

-6202.917181 au

-6202.817906 au

-6203.066393 au

∆E(linear → square)

-92.9 kJ mol-1

-90.14 kJ mol-1

-73.4 kJ mol-1

Note: Negative value of ∆E implies a higher thermodynamic stability of the square conformer.

As alluded to previously, in a separate study, a 1D structure of [Al(OH)(OAc)2]m was formed in scCO2,30 even with the same reaction temperature and molar ratio of the reactants as in this research. We attribute this to the low viscosity of the supercritical fluid and also the cluster effect,17,49 where the reaction kinetics play an important role for linear macromolecule formation. It is assumed that the linear ACS Paragon Plus Environment

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structure is kinetically favorable, and the planar structure is thermodynamically favorable, because the linear molecules appear easier to form. To form the planar structure from linear molecules, the bridging acetate bonds need to open and attack the aluminum centers in other linear molecules, hence the polarity of the solvent may play a role in these reactions; a polar solvent would make the aluminum centers less nucleophilic. Even though pure scCO2 and heptane are both non-polar, dissolved HOAc in these solvent makes polar solutions. HOAc is also known to have a tendency to form dimers through hydrogen bonds.50 The previous study by Han et al. using FTIR shows that there is more HOAc monomer in scCO2 than in heptane,51 which makes the HOAc-scCO2 mixture more polar. Therefore, the higher polarity environment of HOAc-scCO2 favors linear structure formation; on the other hand, the lower polarity environment of HOAc-heptane leads to planar structure formation. It should be mentioned that a macromolecule is not necessarily a nanoparticle/nanofiber/nanoribbon, and the nanostructures are often a collection of the macromolecules.5 Thermal stability and calcination. Results from thermal gravimetric analysis (TGA) of Al(OAc)3 and Al(OH)(OAc)2 are shown in Figure 9. After heating up to 873 K, the total weight loss of Al(OH)(OAc)2 was 64.4%, also comparable to the calculated value of 68.5% (Al(OH)(OAc)2 → Al2O3). Similarly, the weight loss of Al(OAc)3 is 71.8%, comparing to the calculated 75.0% (Al(OAc)3 → Al2O3). The lower experimental weight loss than the theoretical weight loss is attributed to coke formation on the sample surface, supported by the fact that the samples became brown after TGA analysis. If the two figures in Figure 9 are compared, Al(OH)(OAc)2 shows a higher thermal stability than Al(OAc)3. At the peak at ca. 584 K, two acetate ligands were thermally removed from Al(OH)(OAc)2. In contrast, Al(OAc)3 starts to decompose significantly at a temperature as low as ca. 440 K. At the peak at ca. 533 K, about 1/3 of total weight loss occurred, likely due to the loss of one acetate ligand.


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T (K)

T (K) 700

300 4.5 4

-20

3.5 3

-30 2.5 -40 2 -50 1.5 -60

0.5

-80

0

700 2.5

-10 2 -20 -30

1.5

-40

1

-50

1

-70

500

dTG (mg/min)

-10

0

TG (%)

0

500

dTG (mg/min)

300

TG (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5 -60 -70

0

Figure 9. TGA results of the sol-gel products obtained at the reaction temperature 323 K (Left) and 353 K (right). Orange curve shows the weight loss percentage and the black line shows the differential value of dw/dt, where w= weight (mg), t = time (min). The heating rate = 5 K/min.

Al(OAc)3 and Al(OH)(OAc)2 can be heated to obtain Al2O3 nanostructures, and the latter is widely used as an industrial catalyst and/or catalyst support. For instance, Al2O3 generated from Al(OH)(OAc)2 has found its application as a cracking catalyst due to its large pore structures that are accessible for long chain hydrocarbons.52 Our SEM analysis shows that the planar structure survived after heating Al(OH)(OAc)2 at 873 and 1073 K for two hours (see Figures S8 and S9 in supporting information). The XRD patterns show that these calcined materials are amorphous and γ-Al2O3, respectively (see Figure S10 in supporting information), this agrees with the properties of Al2O3 nanofibers produced in scCO2.30 Conclusions. Reaction of aluminum isopropoxide with acetic acid in heptanes was found to produce either 2D Al(OH)(OAc)2 at a reaction temperature 353 K or Al(OAc)3 nano-rods at a lower temperature. The in-situ ATR-FTIR results have shown that a higher reaction temperature favors formation of bridging –OH and acetate bidentate ligands, and these ligands play an important role for the directing nanostructure formation. In addition, in order to explain the self-assembly of the nanoribbons in heptane, density functional theory (DFT) calculations were employed to simulate the planar oligomer and its free energy was compared to that of the linear structure. While the planar structure is more thermodynamically favorable than its linear counterpart, the linear structure is more kinetically


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favorable. This result explains why the linear structures were favorable in supercritical fluid and planar structures were made in organic solvent. Future work will include: (i) demonstrating industrial applications for the 2D aluminum-based nanostructures in separation and catalysis; (ii) introducing a secondary metal to the 2D alumina nanostructures to reduce fouling on the catalyst surface for sub dew point Claus reactions in an effort to eliminate side products, e.g., COS and CS2; and (iii) using this scalable sol-gel method for making other 2D metal oxides semiconductors (e.g., Ga2O3 and even TiO2) for electronic, solar energy, and medical applications. Acknowledgement. This research has been funded through the Natural Science and Engineering Research Council of Canada (NSERC) and Alberta Sulphur Research Ltd. (ASRL) Industrial Research Chair program in Applied Sulfur Chemistry. The authors are grateful to NSERC and supporting member companies of ASRL. We thank Drs. Tobias Fürstenhaupt and Michael Schoel of Microscopy and Imaging Facility, University of Calgary, for the transmission electron microscopy imaging. We also thank Dr. Dimitre Karpuzov of Alberta Centre for Surface Engineering and Science, for the XPS analysis. This work was supported by equipment and infrastructure grant from the Canadian Foundation for Innovation (CFI) and the Alberta Science and Research Authority. Supporting Information available: The schematic experimental setup, XPS spectra of C1s, FTIR spectra of the deuterium substituted samples, Raman spectra of the samples prepared at different temperatures, in-situ FTIR collected at 333 and 343 K, XRD patterns and SEM images of Al(OH)(OAc)2 after calcination at 873 and 1073 K. This material is available free of charge via the internet at http://pubs.acs.org.

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SolâGel-Derived 2D Nanostructures of Aluminum ... - ACS Publications

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