Morphology-Controlled Synthesis of Nanocrystalline η-Al2O3 Thin ...
Morphology-Controlled Synthesis of Nanocrystalline η-Al2O3 Thin Films, Powders, Microbeads, and Nanofibers with Tunable Pore Sizes from Preformed Oligomeric Oxo-Hydroxo Building Blocks Christoph Weidmann, Kirstin Brezesinski, Christian Suchomski, Kristin Tropp, Natascha Grosser, Jan Haetge, Bernd M. Smarsly*, and Torsten Brezesinski* Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff Ring 58, Giessen 35392, Germany.
Figure S1. (a) Dynamic light scattering (DLS) spectrum of aluminum-oxo-hydroxo species in MeOH. The average hydrodynamic radius is 1 nm. (b,c) Thermogravimetric analysis-mass spectrometry (TGA-MS) and differential scanning calorimetry (DSC) studies of aluminum-oxohydroxo species in the temperature range between 25 °C and 1000 °C in flowing synthetic air. The heating rate was 5 °C/min. The dashed line in parts (b) and (c) is the TGA curve, while the solid line in part (b) is the DSC curve. The MS analysis in part (c) shows H2O (m/e = 18) in red, HCl (m/e = 36) in blue, CO2 (m/e = 44) in purple, and C7H7+ (m/e = 91) in green.
The TGA curve in Figure 1b reveals a mass loss of ~65 % of the air-dried aluminum-oxohydroxo species by 1000 °C. The first endothermic peak at ~140 °C in the DSC curve in Figure 1b is characteristic of a desorption process. This process is associated with a mass loss of ~20 % due to adsorbed H2O, as confirmed by MS (see Figure 1c). The major mass loss of ~40 % occurs in the temperature range between 150 °C and 500 °C and can be attributed to elimination of mostly hydroxyl groups (see second endothermic peak at ~250 °C in the DSC curve).1 In addition, we find minor amounts of both C7H7+ and HCl (see Figure 1c) due to adsorbed benzyl alcohol and/or benzyl chloride. In this regard, we note that the HCl signal might also be related to the presence of Al−Cl groups in the aluminum-oxo-hydroxo material. The crystallization of the initially amorphous material begins at ~680 °C, as indicated by the first exothermic peak in the DSC curve. The second exothermic peak at ~850 °C presumably denotes the solid-solid conversion of η-Al2O3 to α-Al2O3. This result is in fair agreement with WAXD data for samples prepared with no polymer template (see Figure S5). The slight deviation (850 °C vs. 900 °C) is likely due to the different heat treatment conditions used in both
experiments. Lastly, it can be seen from the data in Figure 1c that the minor mass loss between 600 °C and 1000 °C is associated with the formation of CO2. We believe that some of the organic constituents partially carbonize during the course of thermal treatment and then react slowly with oxygen from synthetic air at high temperatues to produce CO2.
[1]
Frost, R. L.; Kloprogge, J. T.; Russell, S. C.; Szetu, J. Appl. Spectrosc. 1999, 53, 572-582.
Figure S2. FTIR spectrum of air-dried aluminum-oxo-hydroxo material.
The presence of bridging hydroxyl groups in the oligomeric material is also confirmed by Fourier transform infrared spectroscopy (FTIR). The spectrum in Figure S2 shows several strong overlapping bands in the high-frequency range between 3600 cm−1 and 3000 cm−1, which can be
assigned to stretching vibrations (ν(OH)) from both bridging and surface hydroxyl groups1-3 and adsorbed H2O. We also find six very weak bands (not indexed in Figure S2) at 2959 cm−1 (νas(CH2)), 2905 cm−1 (νs(CH2)), 1495 cm−1 (ν(C=C)), 1454 cm−1 (δas(CH2)), 1384 cm−1 (ν(C−C)), and 1209 cm−1 (ρ(C–CH2)) due to adsorbed benzyl alcohol and/or benzyl chloride. The strong band at 1631 cm-1 stems from adsorbed H2O. In the middle-frequency range, we find two bending vibrations (δ(OH)) at 1054 cm−1 and 956 cm−1.4 These bands are also related to bridging hydroxyl groups. The broad band between 1100 cm−1 and 950 cm−1 can be attributed to a stretching ν(HO−Al=O) vibration.1 Moreover, there is one more band in the low-frequency range at ~745 cm−1, which can be assigned to out-of-plane vibrations (γ(OH)) of Al−OH groups.4 Lastly, we note that vibrations from Al−Cl groups can only be observed in the low-frequency region, i.e., at wavenumbers less than 500 cm−1.
[1]
Ram, S.; Rana, S. Mater. Lett. 2000, 42, 52-60.
[2]
Kubicki, J. D.; Apitz, S. E. Am. Mineral. 1998, 83, 1054-1066.
[3]
Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497-532.
[4]
Frost, R. L.; Kloprogge, J. T.; Russell, S. C.; Szetu, J. Appl. Spectrosc. 1999, 53, 572-582.
Figure S3. (a) 1H NMR spectrum of the reaction mixture after 12 h at 80 °C. (a) The chemical shift, δ, in the range between 5 ppm and 4.4 ppm is shown in the inset; four singlets along with integrated peak area values can be seen. These peaks represent (from left to right) the distribution of benzyl alcohol, benzyl chloride (BnCl), dibenzyl ether (Bn2O), and benzyl ethyl ether (BnOEt). A distinct assignment of all 1H NMR peaks is shown in Table S1. (b) Gas chromatogram of the same reaction mixture. Table S2 summarizes the GC-MS results.
Table S1. Assignment of 1H NMR peaks. Chemical shift, δ
Assignment
Multiplicity
(ppm)
Coupling constant , J
Compound
(Hz)
7.4-7.25
C6H5CH2−X
m
-
Benzyl alcohol, Benzyl chloride, Dibenzyl ether, Benzyl ethyl ether
4.65
C6H5CH2OH
s
-
Benzyl alcohol
4.58
C6H5CH2Cl
s
-
Benzyl chloride
4.55
(C6H5CH2)2O
s
-
Dibenzyl ether
4.49
C6H5CH2OCH2CH3
s
-
Benzyl ethyl ether
3.67
CH3CH2OH
q
7.0
Ethanol
3.53
C6H5CH2OCH2CH3
q
7.0
Benzyl ethyl ether
2.08
R−OH
Benzyl alcohol, Ethanol
1.24
C6H5CH2OCH2CH3
t
7.0
Benzyl ethyl ether
1.22
CH3CH2OH
t
7.0
Ethanol
0
Si(CH3)4
s
-
Tetramethylsilane
R = C2H5, C6H5CH2; X = C2H5O, C6H5CH2O, HO, Cl
Table S2. GC-MS analysis. Compound
m/z [MP]
m/z [BP]
m/z [FP]
1.71
Ethanol
46
31
45,29,27
5.84
Benzaldehyde
106
105
77,51,50
6.59
Benzyl chloride
126
91
92,65,39
6.84
Benzyl alcohol
108
107
79,77,51
7.16
Benzyl ethyl ether
136
91
92,79,77
13.36
Dibenzyl ether
198
92
91,79,65
Retention time (min)
[MP] = molecule peak, [BP] = base peak, [FP] = fragment peak, m/z = mass-to-charge ratio
The synthesis route to dispersable aluminum-oxo-hydroxo building blocks described in this work is based on the hydroxylation of anhydrous AlCl3. In the synthesis, AlCl3 first reacts with EtOH
to
produce
a
complex
compound
with
the
probable
composition
of
AlCl2(OEt)·2AlCl3·10EtOH.1 The hydroxylation process itself begins when benzyl alcohol is added (i.e., aluminum is attacked by the nucleophilic hydroxyl group of benzyl alcohol) and leads to the formation of both Al−OH groups and benzyl chloride. The heterolytic C−O bond cleavage is facilitated by the fact that the phenyl ring is capable of delocalizing the positive charge via resonance. Other oxygen donors, including tert-amyl alcohol and 1-phenylethanol, also lend themselves to the synthesis of oligomeric aluminum-oxo-hydroxo species as they are capable of forming stable carbocations (i.e., they readily transfer the hydroxyl group to aluminum chloride derivatives). Apart from benzyl chloride, we also find small amounts of benzyl ethyl ether and dibenzyl ether, which are formed during synthesis due to etherification
reactions. It is well known that alkyl chlorides react readily with both primary and secondary alcohols through nucleophilic substitution reactions to form symmetric and asymmetric ethers, respectively. The observed ratio of approximately 4:1 Bn2O:BnOEt (see Figure S3a) is consistent with the BnOH:EtOH ratio used in the synthesis.
[1]
Mehrotra, R. K.; Mehrotra, R. C. Z. Anorg. Allg. Chem. 1961, 311, 198-202.
Figure S4. Morphology of KLE-templated (a,b) and PIB53-b-PEO45-templated (c,d) η-Al2O3 thin films. (a,c) Low-magnification top view SEM images showing highly ordered cubic networks of open pores averaging 25 nm and 20 nm in diameter, respectively. It can be clearly seen that the thin film materials studied in this work are crack-free on the micrometer level after annealing at 900 °C. The presence of major structural defects can be also ruled out; only few smaller domains with square symmetry are observed in part (a). (b,d) Bright-field TEM images obtained from the same thin films shown in parts (a) and (c). These data provide ample evidence that the cubic pore structure observed at the top-surface of both materials persists throughout the bulk of the films.
Figure S5. (a) WAXD patterns obtained on hierarchically porous η-Al2O3 microbeads heated to different annealing temperatures of 800 °C (A), 900 °C (B), and 1000 °C (C) in air. Crystalline domain sizes start at ~3 nm and reach ~6 nm at 1000 °C. This slow domain growth provides a hypothesis for understanding why the nanoscale structure is retained in the different η-Al2O3 materials after annealing in air at temperatures as high as 1000 °C. (b) WAXD patterns obtained on both non-templated (A) and PIB53-b-PEO45-templated (B) powder materials heated to 900 °C and 1150 °C, respectively. The stick pattern shows JCPDS reference card no. 42-1468 for αAl2O3. The fact that the solid-solid conversion of η-Al2O3 to α-Al2O3 (thermodynamically stable
bulk phase of Al2O3) is observed at considerably lower annealing temperatures in samples prepared with no polymer template underlines the profound effect that nanoconfinement has on the thermal stability of the eta-alumina phase. This conversion leads to crystallites with 40 nm average diameter and is further accompanied by the loss of nanoscale periodicity/porosity in polymer-templated thin film and powder materials.
Figure S1. (a) Dynamic light scattering (DLS) spectrum of aluminum-oxo-hydroxo species in MeOH. The average hydrodynamic radius is 1 nm. (b,c) Thermogravimetric analysis-mass spectrometry (TGA-MS) and differential scanning calorimetry (DSC) studies of aluminum-oxohydroxo species in the temperature range between 25 °C and 1000 °C in flowing synthetic air. The heating rate was 5 °C/min. The dashed line in parts (b) and (c) is the TGA curve, while the solid line in part (b) is the DSC curve. The MS analysis in part (c) shows H2O (m/e = 18) in red, HCl (m/e = 36) in blue, CO2 (m/e = 44) in purple, and C7H7+ (m/e = 91) in green.
The TGA curve in Figure 1b reveals a mass loss of ~65 % of the air-dried aluminum-oxohydroxo species by 1000 °C. The first endothermic peak at ~140 °C in the DSC curve in Figure 1b is characteristic of a desorption process. This process is associated with a mass loss of ~20 % due to adsorbed H2O, as confirmed by MS (see Figure 1c). The major mass loss of ~40 % occurs in the temperature range between 150 °C and 500 °C and can be attributed to elimination of mostly hydroxyl groups (see second endothermic peak at ~250 °C in the DSC curve).1 In addition, we find minor amounts of both C7H7+ and HCl (see Figure 1c) due to adsorbed benzyl alcohol and/or benzyl chloride. In this regard, we note that the HCl signal might also be related to the presence of Al−Cl groups in the aluminum-oxo-hydroxo material. The crystallization of the initially amorphous material begins at ~680 °C, as indicated by the first exothermic peak in the DSC curve. The second exothermic peak at ~850 °C presumably denotes the solid-solid conversion of η-Al2O3 to α-Al2O3. This result is in fair agreement with WAXD data for samples prepared with no polymer template (see Figure S5). The slight deviation (850 °C vs. 900 °C) is likely due to the different heat treatment conditions used in both
experiments. Lastly, it can be seen from the data in Figure 1c that the minor mass loss between 600 °C and 1000 °C is associated with the formation of CO2. We believe that some of the organic constituents partially carbonize during the course of thermal treatment and then react slowly with oxygen from synthetic air at high temperatues to produce CO2.
[1]
Frost, R. L.; Kloprogge, J. T.; Russell, S. C.; Szetu, J. Appl. Spectrosc. 1999, 53, 572-582.
Figure S2. FTIR spectrum of air-dried aluminum-oxo-hydroxo material.
The presence of bridging hydroxyl groups in the oligomeric material is also confirmed by Fourier transform infrared spectroscopy (FTIR). The spectrum in Figure S2 shows several strong overlapping bands in the high-frequency range between 3600 cm−1 and 3000 cm−1, which can be
assigned to stretching vibrations (ν(OH)) from both bridging and surface hydroxyl groups1-3 and adsorbed H2O. We also find six very weak bands (not indexed in Figure S2) at 2959 cm−1 (νas(CH2)), 2905 cm−1 (νs(CH2)), 1495 cm−1 (ν(C=C)), 1454 cm−1 (δas(CH2)), 1384 cm−1 (ν(C−C)), and 1209 cm−1 (ρ(C–CH2)) due to adsorbed benzyl alcohol and/or benzyl chloride. The strong band at 1631 cm-1 stems from adsorbed H2O. In the middle-frequency range, we find two bending vibrations (δ(OH)) at 1054 cm−1 and 956 cm−1.4 These bands are also related to bridging hydroxyl groups. The broad band between 1100 cm−1 and 950 cm−1 can be attributed to a stretching ν(HO−Al=O) vibration.1 Moreover, there is one more band in the low-frequency range at ~745 cm−1, which can be assigned to out-of-plane vibrations (γ(OH)) of Al−OH groups.4 Lastly, we note that vibrations from Al−Cl groups can only be observed in the low-frequency region, i.e., at wavenumbers less than 500 cm−1.
[1]
Ram, S.; Rana, S. Mater. Lett. 2000, 42, 52-60.
[2]
Kubicki, J. D.; Apitz, S. E. Am. Mineral. 1998, 83, 1054-1066.
[3]
Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497-532.
[4]
Frost, R. L.; Kloprogge, J. T.; Russell, S. C.; Szetu, J. Appl. Spectrosc. 1999, 53, 572-582.
Figure S3. (a) 1H NMR spectrum of the reaction mixture after 12 h at 80 °C. (a) The chemical shift, δ, in the range between 5 ppm and 4.4 ppm is shown in the inset; four singlets along with integrated peak area values can be seen. These peaks represent (from left to right) the distribution of benzyl alcohol, benzyl chloride (BnCl), dibenzyl ether (Bn2O), and benzyl ethyl ether (BnOEt). A distinct assignment of all 1H NMR peaks is shown in Table S1. (b) Gas chromatogram of the same reaction mixture. Table S2 summarizes the GC-MS results.
Table S1. Assignment of 1H NMR peaks. Chemical shift, δ
Assignment
Multiplicity
(ppm)
Coupling constant , J
Compound
(Hz)
7.4-7.25
C6H5CH2−X
m
-
Benzyl alcohol, Benzyl chloride, Dibenzyl ether, Benzyl ethyl ether
4.65
C6H5CH2OH
s
-
Benzyl alcohol
4.58
C6H5CH2Cl
s
-
Benzyl chloride
4.55
(C6H5CH2)2O
s
-
Dibenzyl ether
4.49
C6H5CH2OCH2CH3
s
-
Benzyl ethyl ether
3.67
CH3CH2OH
q
7.0
Ethanol
3.53
C6H5CH2OCH2CH3
q
7.0
Benzyl ethyl ether
2.08
R−OH
Benzyl alcohol, Ethanol
1.24
C6H5CH2OCH2CH3
t
7.0
Benzyl ethyl ether
1.22
CH3CH2OH
t
7.0
Ethanol
0
Si(CH3)4
s
-
Tetramethylsilane
R = C2H5, C6H5CH2; X = C2H5O, C6H5CH2O, HO, Cl
Table S2. GC-MS analysis. Compound
m/z [MP]
m/z [BP]
m/z [FP]
1.71
Ethanol
46
31
45,29,27
5.84
Benzaldehyde
106
105
77,51,50
6.59
Benzyl chloride
126
91
92,65,39
6.84
Benzyl alcohol
108
107
79,77,51
7.16
Benzyl ethyl ether
136
91
92,79,77
13.36
Dibenzyl ether
198
92
91,79,65
Retention time (min)
[MP] = molecule peak, [BP] = base peak, [FP] = fragment peak, m/z = mass-to-charge ratio
The synthesis route to dispersable aluminum-oxo-hydroxo building blocks described in this work is based on the hydroxylation of anhydrous AlCl3. In the synthesis, AlCl3 first reacts with EtOH
to
produce
a
complex
compound
with
the
probable
composition
of
AlCl2(OEt)·2AlCl3·10EtOH.1 The hydroxylation process itself begins when benzyl alcohol is added (i.e., aluminum is attacked by the nucleophilic hydroxyl group of benzyl alcohol) and leads to the formation of both Al−OH groups and benzyl chloride. The heterolytic C−O bond cleavage is facilitated by the fact that the phenyl ring is capable of delocalizing the positive charge via resonance. Other oxygen donors, including tert-amyl alcohol and 1-phenylethanol, also lend themselves to the synthesis of oligomeric aluminum-oxo-hydroxo species as they are capable of forming stable carbocations (i.e., they readily transfer the hydroxyl group to aluminum chloride derivatives). Apart from benzyl chloride, we also find small amounts of benzyl ethyl ether and dibenzyl ether, which are formed during synthesis due to etherification
reactions. It is well known that alkyl chlorides react readily with both primary and secondary alcohols through nucleophilic substitution reactions to form symmetric and asymmetric ethers, respectively. The observed ratio of approximately 4:1 Bn2O:BnOEt (see Figure S3a) is consistent with the BnOH:EtOH ratio used in the synthesis.
[1]
Mehrotra, R. K.; Mehrotra, R. C. Z. Anorg. Allg. Chem. 1961, 311, 198-202.
Figure S4. Morphology of KLE-templated (a,b) and PIB53-b-PEO45-templated (c,d) η-Al2O3 thin films. (a,c) Low-magnification top view SEM images showing highly ordered cubic networks of open pores averaging 25 nm and 20 nm in diameter, respectively. It can be clearly seen that the thin film materials studied in this work are crack-free on the micrometer level after annealing at 900 °C. The presence of major structural defects can be also ruled out; only few smaller domains with square symmetry are observed in part (a). (b,d) Bright-field TEM images obtained from the same thin films shown in parts (a) and (c). These data provide ample evidence that the cubic pore structure observed at the top-surface of both materials persists throughout the bulk of the films.
Figure S5. (a) WAXD patterns obtained on hierarchically porous η-Al2O3 microbeads heated to different annealing temperatures of 800 °C (A), 900 °C (B), and 1000 °C (C) in air. Crystalline domain sizes start at ~3 nm and reach ~6 nm at 1000 °C. This slow domain growth provides a hypothesis for understanding why the nanoscale structure is retained in the different η-Al2O3 materials after annealing in air at temperatures as high as 1000 °C. (b) WAXD patterns obtained on both non-templated (A) and PIB53-b-PEO45-templated (B) powder materials heated to 900 °C and 1150 °C, respectively. The stick pattern shows JCPDS reference card no. 42-1468 for αAl2O3. The fact that the solid-solid conversion of η-Al2O3 to α-Al2O3 (thermodynamically stable
bulk phase of Al2O3) is observed at considerably lower annealing temperatures in samples prepared with no polymer template underlines the profound effect that nanoconfinement has on the thermal stability of the eta-alumina phase. This conversion leads to crystallites with 40 nm average diameter and is further accompanied by the loss of nanoscale periodicity/porosity in polymer-templated thin film and powder materials.
