A Thesis

Entitled

Autohydration of Nanosized Cubic Zirconium Tungstate

by

Nathan A. Banek

Submitted to the Graduate Faculty as partial fulfillment of the

requirements for the Master of Science in Chemistry

______Dr. Cora Lind, Committee Chair

______Dr. Jared Anderson, Committee Member

______Dr. Jon Kirchhoff, Committee Member

______Dr. Patricia Komuniecki Dean of College of Graduate Studies

The University of Toledo

August 2011

An Abstract of

Autohydration of Nanosized Cubic Zirconium Tungstate

by

Nathan A. Banek

Submitted in partial fulfillment of the requirements for The Master of Science in Chemistry

The University of Toledo August 2011

In recent years, negative thermal expansion (NTE) materials have become of increasing interest. These materials contract upon heating, and have potential for achieving better control of thermal expansion in composite materials. By using an NTE compound as a filler material into these composites, it is possible to offset the positive thermal expansion of other components in the composite. As a result, these NTE materials can find use in a wide range of applications such as optics, polymers, electronics, tooth fillings and any other area where exact positioning of parts over a wide range of temperatures is crucial.

One of the most popular NTE materials is cubic ZrW2O8. Thermodynamically stable zirconium tungstate was first synthesized in the 1950’s through traditional solid state methods. It was only recent that the metastable phases could be achieved through low temperature methods, that involves conversion of a precursor material

ZrW2O7(OH)2·2H2O to cubic ZrW2O8. Through hydrothermal synthesis, previous work on ZrW2O7(OH)2·2H2O showed exceptional particle and morphology control with use of

iii

alcohols/HCl, which is desirable for optimal composite integration. It was recently discovered that ZrW2O8 particles obtained through this synthesis route had reduced stability in atmosphere. The instability was linked to autohydration that changed the properties of the material resulting in weak positive thermal expansion. Interestingly, nanosized ZrW2O8 obtained hydrothermally in perchlorate/NaCl with the absence of alcohols show very limited autohydration; however this is a non-preferred synthesis route due to high agglomeration levels. Reported autohydration on mixed ZrMoxWx-1O8 solid solutions by Sleight et al. provided a logical defect driven explanation for the cubic

ZrW2O8 nanoparticles.

Detailed investigation was performed on cubic ZrW2O8 hydrothermally obtained by the alcohol/HCl synthesis pathway. An understanding of what is causing autohydration was discovered through the used of powder X-ray diffraction, scanning transmission electron microscopy, thermogravimetric analysis and Brunauer-Emmett-

Teller surface analysis.

iv

Acknowledgements

It has been a great journey to arrive at this point in my life and I could not have done it by myself. I would like to recognize those who have provided significant help along the way.

I would like to thank my parents and my family for their continued support throughout my life; it would have been very hard to succeed without them. I would like to thank my advisor Dr. Cora Lind for her continued support and advice on this project.

Dr. Lind has taught me a significant amount of details that I am very grateful for. I would like to thank my committee members, Dr. Jon Kirchhoff and Dr. Jared Anderson for their advice and support. I would like to thank my labmates, past and present, for their support and friendship. Many thanks to Pannee Burckel for her assistance with X-ray diffraction and electron microscopy experiments; Stacy Gates for various instrumentation help; Dr.

Wendell Griffith and Tamam Baiz for providing excellent advice when I needed it; and

Pam Samples and Charlene Hanson for their administrative help. I would also like to thank the University of Toledo for providing me with the opportunity to further my education.

This material is based upon work supported by the National Science Foundation under Grant No. 0840474 and Grant No. 0545517.

v

Contents

Abstract ...... iii

Acknowledgements ...... v

Contents ...... vi

List of Tables ...... viii

List of Figures ...... x

1. Introduction ...... 1 1.1 Thermal Expansion ...... 1 1.2 Negative Thermal Expansion Materials ...... 2 1.2.1. Scandium Tungstate Family ...... 3 1.2.2. Zirconium Vanadate Family ...... 4 1.2.3 ZrW2O8 Family ...... 4 1.3. Nanoparticles ...... 6 1.4. ZrW2O8 Composites...... 7 1.5. Hydration of ZrW2O8 ...... 8 1.6. Project Goals ...... 9

2. Characterization ...... 10 2.1. Powder X-ray Diffraction ...... 10 2.1.1. Diffraction Experiments...... 11 2.1.2 Crystallite Size Estimates ...... 12 2.2. Electron Microscopy ...... 13 2.2.1 Field Emission Scanning Electron Microscopy ...... 14 2.2.2 Scanning Transmission Electron Microscopy ...... 15 2.3. Thermogravimetric analysis...... 15 2.4. Brunauer-Emmett-Teller Surface Analysis ...... 16

3. Synthetic Variables in the Preparation of ZrW2O7(OH)2·2H2O ...... 17 3.1. Introduction ...... 17 3.2. Zirconium Tungstate Synthesis...... 18 3.2.1 Hydrothermal Synthesis ...... 18 3.2.2 Micron-sized ZrW2O8 Particles ...... 18 3.2.3 Nanosized ZrW2O8 Particles ...... 19

vi

3.3. General Synthesis Protocols ...... 19 3.4. Results and Discussion ...... 25 3.4.1 Effect of Temperature ...... 25 3.4.2 Effect of Reaction Time ...... 29 3.4.3 Effect of Acid Concentration ...... 33 3.4.4 Influence of Solvent Type on ZrW2O7(OH)2·2H2O Morphology ...... 38 3.4.5. Effect of Solvent Concentration...... 44 3.5. Conclusions ...... 46

4. Autohydration of Cubic ZrW2O8 ...... 49 4.1. Introduction ...... 49 4.2. Experimental ...... 51 4.2.1 Preparation of Cubic ZrW2O8 from Nanosized ZrW2O7(OH)2·2H2O ..... 51 4.2.2. Study of Autohydration Kinetics ...... 51 4.3. Results and Discussion ...... 53 4.3.1 Effect of Temperature ...... 54 4.3.2 Effect of Heating Time ...... 61 4.3.3 Effect of Acid Concentration ...... 63 4.3.4 Effect of Solvent Concentration...... 65 4.3.5 Effect of Solvent Type ...... 72 4.4. Conclusions ...... 74

5. Causes of ZrW2O8 Autohydration ...... 75 5.1. Introduction ...... 75 5.2. STEM Results ...... 75 5.3. Formation of ZrW2O8 at Higher Temperatures ...... 81 5.4. BET Surface Area Analysis ...... 89 5.5. Conclusions ...... 90

6. Summary and Future Work ...... 93

References ...... 96

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List of Tables

Table 3-1: Synthetic variables for all ZrW2O7(OH)2·2H2O samples ...... 21

Table 3-2: Variation of crystallite size as a function of temperature for different subsets of samples ...... 25

Table 3-3: Variation of crystallite size as a function of reaction time for different subsets of samples...... 29

Table 3-4: Differences in crystallite size for different acid concentrations for samples prepared at 130 ºC...... 34

Table 3-5: Differences in crystallite size based with acid concentration for samples prepared at 210 ºC...... 35

Table 3-6: Variance in crystallite size for samples prepared with different alcohols...... 38

Table 4-1:Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared at 130 ºC. (NBA68C) ...... 58

Table 4-2: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared at 170 ºC. (NBA70C) ...... 59

Table 4-3: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared in 2.5 mL 1-propanol at 130 ºC (NBA69C)...... 67

Table 4-4: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared in 2.5 mL 1-propanol at 170ºC. (NBA71C) ...... 68

Table 4-5: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared in 0.5 mL 1-propanol at 130ºC. (NBA149C) ...... 70

Table 4-6: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared in 0.5 mL 1-propanol at 170ºC. (NBA148C) ...... 71

Table 5-1: Crystallite sizes for 130 ºC hydroxide hydrate precursors and the corresponding cubic samples converted at 600 and 650 ºC...... 85

viii

Table 5-2: Crystallite sizes for 170ºC hydroxide hydrate precursors and the corresponding cubic samples converted at 600 and 650 ºC...... 85

Table 5-3: Crystallite sizes for 210 ºC hydroxide hydrate precursors and the corresponding cubic samples converted at 600 and 650 ºC...... 87

Table 5-4: BET data for cubic ZrW2O8 samples...... 90

ix

List of Figures

Figure 1-1: (a) Transverse vibrations of M-O-M linkage upon heating (b) Polyhedra showing reduction in metal to metal distances through rocking motions...... 2

Figure 1-2: Cubic zirconium tungstate unit cell with (dark) WO4 tetrahedra and (bright) ZrO6 octahedra...... 6

Figure 2-1: Diffraction of X-rays within crystals...... 11

Figure 3-1: TEM and SEM images of hydroxide hydrate samples prepared at (a) 130 ºC, (b) 170 ºC and (c) 210 ºC...... 28

Figure 3-2: TEM images of hydroxide hydrate samples heated at 130 ºC for (a) 24 h, (b) 72 h and (c) 144 h...... 31

Figure 3-3: TEM images of hydroxide hydrate samples heated at 210 ºC for (a) 24 h and (b) 72 h...... 32

Figure 3-4: XRD of hydroxide hydrate heated at 210 ºC for 2 h in (a) 2.0 mL 1-propanol, 6.0 mL HCl and (b) 3.0 mL 1-propanol, 5.0 mL HCl...... 33

Figure 3-5: PXRD patterns for samples prepared in 2.5 mL HCl and heated for (a) 7 d and (b) 1 d...... 34

Figure 3-6: TEM images of hydroxide hydrate samples prepared with (a) 4 mL HCl, (b) 5 mL HCl and (c) 5.8 mL HCl...... 37

Figure 3-7: TEM images of hydroxide hydrate prepared in 2.5 mL ethanol, heated at 130 ºC for 24 h...... 39

Figure 3-8: TEM images of hydroxide hydrate prepared in 2.5 mL 1-propanol, heated at 130 ºC for 24 h...... 40

Figure 3-9: TEM images of hydroxide hydrate prepared in 2.5 mL 1-butanol, heated at 130 ºC for 24 h...... 40

Figure 3-10: TEM images of hydroxide hydrate prepared in 2.5 mL 1-pentanol, heated at 130 ºC for 24 h...... 41

x

Figure 3-11: TEM images of hydroxide hydrate prepared in 2.5 mL 1-hexanol, heated at 130 ºC for 24 h...... 41

Figure 3-12: SEM images of hydroxide hydrate prepared in 2.5 mL 1-heptanol, heated at 130 ºC for 24 h...... 42

Figure 3-13: TEM images of NBA70 hydroxide hydrate particles prepared in 0.5 mL 1- propanol...... 45

Figure 3-14: TEM images of NBA71 hydroxide hydrate particles prepared in 2.5 mL 1- propanol...... 46

Figure 4-1: Powder X-ray diffraction patterns of ZrW2O8 (a) as prepared (no hydration), (b) after a few days (partially hydrated), and (c) after one year of expose to atmosphere (close to fully hydrated)...... 50

Figure 4-2: TGA plot of partially hydrated ZrW2O8 showing weight loss due to dehydration...... 50

Figure 4-3: PXRD scans of cubic ZrW2O8 prepared in (a) alcohol/HCl after one month of atmosphere exposure, and (b) HClO4/NaCl after six months of atmosphere exposure. ... 54

Figure 4-4: PXRD patterns of samples prepared at (blue) 130 ºC and (red) 210 ºC after 7 d of autohydration...... 55

Figure 4-5: PXRD scans of cubic ZrW2O8 samples from precursors at (red) 130 ºC, (blue) 170 ºC, and (black) 210 ºC with 0.5 mL 1-propanol after 7 d autohydration...... 56

Figure 4-6: PXRD scan after 22 d of autohydration for cubic ZrW2O8 samples prepared in 0.5 mL 1-propanol and heated at (red) 130 ºC, (blue) 170 ºC, and (black) 210 ºC...... 57

Figure 4-7: Lattice constant values for a 130 ºC sample (♦) and a 170 ºC (■) sample after different periods of autohydration. The error bars included represent three estimated standard deviations...... 59

Figure 4-8: PXRD scans of sample NBA81C after (red) 0 d and (blue) 29 d of exposure to atmosphere...... 62

Figure 4-9: PXRD scans of sample NBA79C after (red) 0 d and (blue) 22 d of exposure to atmosphere...... 62

Figure 4-11: PXRD scans of samples prepared in various acid concentrations resulting in identical hydration behavior...... 64

Figure 4-12: PXRD scan after 22 d of hydration for samples prepared in (a) 0.5 mL 1- propanol and (b) 2.5 mL 1-propanol...... 66

xi

Figure 4-13: Lattice constants for a sample prepared at 130 ºC in (■) 2.5 mL of alcohol and (♦) 0.5 mL of alcohol sample as a function of hydration time. The error bars represent 3 estimated standard deviations...... 67

Figure 4-14: Lattice constants for a sample prepared at 170 ºC in (■) 2.5 mL of alcohol and (♦) 0.5 mL of alcohol sample as a function of hydration time. The error bars represent 3 estimated standard deviations...... 69

Figure 4-15: Lattice constants for two samples prepared at 130 ºC under identical conditions as a function of hydration time. The error bars represent 3 estimated standard deviations...... 71

Figure 4-16: Lattice constants for two samples prepared at 170 ºC under identical conditions as a function of hydration time. The error bars represent 3 estimated standard deviations...... 72

Figure 4-17: PXRD scan for samples prepared 2.5 mL of diethyl ether at 220 ºC for 1.5 h after (a) 0 d hydration and (b) 20 d of hydration...... 73

Figure 4-18: TEM image of hydroxide hydrate prepared in diethyl ether...... 73

Figure 5-1: TEM images comparing morphologies in the (a) hydroxide hydrate and (b) cubic phase...... 76

Figure 5-2: TEM image of cubic zirconium tungstate converted from a hydroxide hydrate precursor prepared at 130 ºC (1) Highly crystalline tip, (2) amorphous region and (3) discontinuous crystalline region...... 77

Figure 5-3: Cubic zirconium tungstate converted from a hydroxide hydrate precursor prepared at 170 ºC viewed with (a) SEM and (b) TEM...... 77

Figure 5-4: TEM image of cubic zirconium tungstate converted from a hydroxide hydrate precursor prepared at 170 ºC showing polycrystalline structure...... 78

Figure 5-5: TEM image of cubic zirconium tungstate converted from a hydroxide hydrate precursor prepared at 170 ºC showing discontinuities in crystal lattice direction...... 79

Figure 5-6: SEM images of (a) hydroxide hydrate and (b) cubic ZrW2O8, showing no changes in particle morphology...... 79

Figure 5-7: TEM image of crystal lattice fringes in NBA72C converted from a hydroxide hydrate precursor prepared at 210 ºC...... 80

Figure 5-8: TEM images of NBA72C converted from a hydroxide hydrate precursor prepared at 210 ºC showing minor crystal defects...... 80

xii

Figure 5-9: PXRD patterns after 7 d of autohydration of cubic ZrW2O8 obtained from heat treatment of a 130º C synthesized hydroxide hydrate precursor after conversion at (a) 600 ºC for 30 min and (b) 650 ºC for 30 min...... 83

Figure 5-10: TEM image of cubic ZrW2O8 from a hydroxide hydrate precursor prepared at 130 ºC and converted at (a) 600 ºC and (b) 650 ºC...... 84

Figure 5-11: High magnification TEM image of cubic ZrW2O8 from a hydroxide hydrate precursor prepared at 130 ºC and converted at 650 ºC...... 84

Figure 5-12: PXRD patterns after 7 d of autohydration of cubic ZrW2O8 that was obtained from heat treatment of a 130º C synthesized hydroxide hydrate at (a) 600 ºC for 30 min and (b) 650 ºC for 30 min...... 86

Figure 5-13: TEM image of cubic ZrW2O8 sample from hydroxide hydrate precursor prepared at 170 °C and converted at 650 ºC...... 86

Figure 5-14: PXRD patterns after 7 d of autohydration of cubic ZrW2O8 that was obtained from heat treatment of a 130º C synthesized hydroxide hydrate at (a) 650 ºC for 30 min and (b) 600 ºC for 30 min...... 88

Figure 5-15: TEM image of cubic ZrW2O8 converted at 650 ºC from a hydroxide hydrate precursor prepared at 210 °C...... 88

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Chapter 1

1. Introduction

1.1 Thermal Expansion

Thermal expansion can be defined by the increase of length, area, or volume of matter with an increase in temperature. This unique property varies for every material, and is quantified by the thermal expansion coefficient (α), given by Equation 1-1.

dV 1  V (Equation 1-1) dT V

In most materials, the bond length increase due to thermal energy results in an increase of the bulk material volume with temperature. In non-NTE metal oxides this volume increase occurs by linear expansion of the metal-oxygen-metal (M-O-M) bonds.

Expansion can prove to be problematic when two objects with different thermal expansion coefficients are in contact with one another, such as a polymer coating on a metal. When heated, the bonds of each material expand at different rates, and the contact interactions can break, which will lead to cracks or delamination. Thermal expansion problems such as these can be better controlled through the use of composites and coatings containing an NTE material.

1

1.2 Negative Thermal Expansion Materials

Negative thermal expansion (NTE) materials are a class of materials that contract in volume or along some crystallographic axes when heated.1-5 According to Equation I-

1, the resulting coefficient (α) is negative. Several different specific classes of NTE materials exist, some occur naturally such as zeolites and ice crystals, while others can be synthesized through various solid state reactions.6, 7 Examples of synthesized materials

2, 8-10 with related atomic network structures are ZrW2O8, ZrV2O7, and Sc2W3O12. A common characteristic of these NTE materials is that linear M-O-M linkages are present, and that the MOx polyhedra form a corner sharing network. In non-NTE materials, absorbed thermal energy causes longitudinal vibrations of M-O-M linkages, resulting in an increase in overall volume (Figure 1-1). In NTE materials, however, when thermal energy is absorbed, the open framework allows for transverse vibrations of the corner

2 sharing oxygen atoms. This causes a rocking motion of the MOx polyhedra, reducing the bond angle and second nearest neighbor distances, thus reducing the overall volume.2

Figure 1-1: (a) Transverse vibrations of M-O-M linkage upon heating (b) Polyhedra showing reduction in metal to metal distances through rocking motions.

2

1.2.1. Scandium Tungstate Family

The scandium tungstate family has the general formula A2M3O12, where A can be a number of trivalent cations, and M can be either Mo or W. The scandium tungstate

2, structure was first described in 1966, it consists of ScO6 octahedra and WO4 tetrahedra.

11 Each octahedron shares corners with six tetrahedra, and each tetrahedron shares corners with four octahedra, resulting in a low density open framework. At high temperatures, materials adopt an orthorhombic structure in space group Pnca and exhibit anisotropic NTE.2 These materials can also transform to a monoclinic phase at low temperatures, which shows similar connectivity. In this phase, the M-O-M bond angles are not equal to 180 º, therefore the materials exhibit positive thermal expansion.2

Properties of compounds in the scandium tungstate family are highly dependent on the identity of the trivalent cation.7, 12-14 The electronegativity of the A site cation affects the oxygen interactions within the framework.15 Lower electronegativity results in a larger partial negative charge on the oxygen atoms, which favors the less dense orthorhombic phase. Higher electronegativity leads to a preference for the denser monoclinic phase, and requires high temperatures for conversion to the orthorhombic structure. Examples of this are ScW3O12 and Al2W3O12. Scandium has an electronegativity of 1.2, and ScW3O12a undergoes the phase transition at 9 ºC. Al2W3O12 transforms to the orthorhombic phase at 200 ºC, and Al has an electronegativity of 1.47.15

While this hold true for a many compounds with a single atom type on the A site, mixing different cations on this site changes the phase transition behavior.16

The magnitude of negative thermal expansion was found to depend on the cation size for the Sc2W3O12 family. Smaller cations form smaller polyhedra, which have less

3

ability to distort compared to a larger one. The compound Sc2W3O12 has a thermal

-1 17 expansion coefficients of αl= -2.2 ×10-6 °C .

1.2.2. Zirconium Vanadate Family

Compounds belonging to the zirconium vanadate family have a general formula

18 of AM2O7. When M = V, the A cation can be Zr and Hf. When M = As, the A cation can be Zr or Th.19 When M = P, the A cation can be Zr, Hf, Ti, U, Th, Pu, Ce, Mo, W,

Re, Pb, Sn, Ge, or Si.18 The vanadate and phosphate compounds exist in a cubic structure in space group of Pa3¯. These compounds adopt a NaCl structure made up of AO6

4, 20 octahedra and M2O7 units.

Below 350 K, zirconium vanadate adopts a 3 x 3 x 3 superstructure that exhibits positive thermal expansion.21 In this phase, the M-O-M bond angles between polyhedra are between 130 and 165°. From 350 to 373 K and incommensurate superstructure forms with lower symmetry.21, 22 Above 373 K, the simple Pa3¯ cell is preferred, in which polyhedral are connected by linear M-O-M linkages. From 373 to 850 K, cubic ZrV2O7

-6 -1 4 has a negative thermal expansion with a coefficient of α = -7.1  10 K . From 950 to

1050 K a slope change occurs to ~ -1.0 10-6 K-1.4 The NTE in this family is due to transverse vibrations of the oxygen atoms; however, rotations within the structure are not possible without some distortion of the octahedra and tetrahedra.

1.2.3 ZrW2O8 Family

The zirconium tungstate family (ZrW2O8) has the general formula AM2O8, where

A = Zr or Hf, and M = Mo or W. ZrW2O8 was first prepared in 1959 by Graham through sintering of binary oxides, however, its structure was not well understood until the

4

23-25 1990’s. The structure consists of ZrO6 octahedra and WO4 tetrahedra, and the metal atoms are connected through corner sharing oxygen atoms. ZrO6 octahedra share corners with six WO4 tetrahedra, while each WO4 tetrahedron shares corners with three ZrO6 octahedra, leaving one oxygen unbound (Figure 1-2).1 At 430 K, an order-disorder phase transition is observed, which results in an orientational rearrangement of the WO4

25 tetrahedra. Both - and -ZrW2O8 adopt cubic structures.

Similar to orthorhombic Sc2W3O12, a temperature increase causes a rocking motion of the polyhedra through transverse vibrations of the corner sharing oxygen atoms in the linear M-O-M linkages. Zirconium tungstate is of high interest as a NTE material because it possesses a number of desirable properties, including a wide temperature range of isotropic expansion with  values of -8.8  10-6 K-1 from 0.3 to 430 K and -4.9  10-6

K-1 from 430 to 1050 K.25

A high pressure phase of zirconium tungstate exists as well. This phase is referred to as the γ-phase, and it displays positive thermal expansion.26 The phase transition occurs at 210 MPa hydrostatic pressure, and is irreversible upon decompression. Heating to 393 K is necessary to convert the material back to α-phase.

5

Figure 1-2: Cubic zirconium tungstate unit cell with (dark) WO4 tetrahedra and (bright) ZrO6 octahedra.

Another member of the ZrW2O8 family is ZrMo2O8. Zirconium molybdate can exist in monoclinic, trigonal, and cubic structures.4, 9 The monoclinic phase is thermodynamically stable at room temperature but its formation is kinetically unfavored, and its preparation requires long periods of heating at 873 K. The trigonal phase is only thermodynamically stable between 952 and ~1100 K, however it can easily be prepared at lower temperatures due to the slow formation of the monoclinic phase. The cubic phase can be prepared through the dehydration of a ZrMo2O7(OH)2·2H2O, it displays

-6 -1 -6 -1 NTE with α-values of -6.9  10 K from 2 to 200 K and -5.0  10 K from 200 to 573

K.9

1.3. Nanoparticles

The doors to an entire new field of research were opened when Feynman wrote,

“There’s Plenty of Room at the Bottom.”27 This statement pointed out that manipulation at an atomic scale should be possible. He was particularly interested in atomic level manipulations that could still be characterized. This of course was not possible with the

6

technology available during his lifetime. Later, many characterization methods for nano- materials were developed and optimized, such as electron microscopes, atomic force microscopes and scanning tunneling microscopes. Feynman recognized that forces such as surface tension and van der Waals would become increasingly dominant as an object’s size approached the nanoscale.

The term nanoparticle is a general classification pertaining to particles with size dimensions ranging from 1 to 100 nm. There are many different shapes of nanoparticles including rods, tubes, cubes, or sphere-like.28, 29 It is understood that nanoparticles show size-dependent properties that differ from the bulk material. This was known empirically since the Middle Ages for pottery with glazes and glasses that serendipitously contained metal nanoparticles. It was also discovered that the red stained glass windows in medieval church cathedrals contained gold nanoparticles, more recently, the red color could be explained by the fact that these gold nanoparticles display size-dependent changes in surface plasmon resonance.30

An interesting feature of nanomaterials compared to bulk materials is their high surface areas per mass. This is exceptionally beneficial for applications such as hydrogen storage, as gases easily adhere to the large surfaces under high pressures.31 High surface areas also yield better ratios of surface functionalization per mass for applications such as drug delivery or composite formation.32

1.4. ZrW2O8 Composites

ZrW2O8 has been successfully integrated into various materials with the goal of reducing the coefficient of thermal expansion of the resulting composite. Ceramic composites are formed at pressures well above the γ-phase transformation. In one report,

7

33 a ZrW2O8/ZrO2 composite was formed at 750 MPa. Heating to 393 K only partially converted γ-ZrW2O8 back to α-ZrW2O8. This is because the particles, when heated, formed an outer shell of α-ZrW2O8. The α-phase is less dense, leading to a sudden increase in volume. The limited space in the particles caused increased pressure on the inner parts of the ZrW2O8 particles, which always retained some γ-ZrW2O8.

In contrast to ceramic composite formation, a more practical use of ZrW2O8 is in polymer composites.34 These composites are formed through mixing processes at atmospheric pressure. In order to make best use of ZrW2O8 in a controlled thermal expansion coating, it is desirable to have a homogeneous mixture. It is also desirable to have a strong interaction between the ZrW2O8 particles and the polymer matrix; this is possible through surface functionalization. Both of these aims can be better achieved through the use of unagglomerated nanoparticles for even distribution and high functionalization yields.

1.5. Hydration of ZrW2O8

In the 1990’s, Sleight’s group observed that cubic zirconium tungstate molybdate solid solutions with 30 to 90% tungsten content incorporated atmospheric moisture into the framework.35 This hydration results in an increase in the tungsten coordination number from 4 to 5, and a change in space group from P213 to Pa¯3. This higher coordination number leads to rotations of the WO5 and ZrO6 units, causing a contraction of the unit cell, and resulting in a denser framework with weak positive thermal expansion. This was alarming, because if this hydration could occur in ZrW2O8 as well, it would be problematic for any proposed uses. Hydrated cubic ZrW2O8·xH2O was first prepared by Duan et al. by hydrothermal treatment in water at temperatures above 100

8

°C.35 Full hydration of zirconium tungstate results in a linear decrease in lattice constant from 9.14 Å for ZrW2O8 to 8.84 Å for ZrW2O8·1H2O. It was concluded that while

ZrW2O8 can in fact hydrate, the harsh conditions required to do so are unlikely to be encountered during use of the material as an NTE material. However, our group found recently that nanosized cubic ZrW2O8 hydrates much more readily than micron sized particles, and that autohydration in atmosphere is rapid enough to interfere with its use in controlled thermal expansion composites.36

1.6. Project Goals

The thesis work presented here had two goals. The first goal was to synthesize a number of nanoparticulate ZrW2O8 samples in order to study the hydration behavior as a function of particle size and morphology (chapter 3). Results of the study of hydration kinetics are outlined in chapter 4.

The second goal of this project was to investigate why the samples were autohydrating (chapter 5). It was hypothesized that this behavior could be related to the number of defects present in the particles. A study of defects as a function of synthesis conditions could lead to optimizations that might allow future use of nanoparticulate

ZrW2O8.

9

Chapter 2

2. Characterization

The materials discussed in this project were characterized with a variety of analytical techniques including powder X-ray diffraction (PXRD), scanning transmission electron microscopy (STEM), thermogravimetric analysis (TGA), and Brunauer-Emmett-

Teller (BET) surface analysis.

2.1. Powder X-ray Diffraction

Powder X-ray diffraction was used in this project for phase identification, crystallite size estimates, and sample hydration analysis. Crystals are made up of an ordered repeating structure of atoms, and produce diffraction patterns with well-defined peaks when exposed to X-rays. The peak positions are characteristic of the material being analyzed. If the sample is amorphous, or has no long range order, distinct peaks will not be observed.

In a lab diffractometer, X-rays are generated by stripping electrons from a tungsten filament. A high voltage is used to direct the electrons at a metal target, where they cause ejection of core electrons from the atoms in the metal target. To fill the hole created by this process, outer shell electrons drop to the lower energy levels. The

10

difference in energy is released by emission of an X-ray characteristic of the metal. For example, the Kα radiation used in this research occurs from a transition from the L to the

K level. In this research, Cu-Kα radiation was used, which consists of Kα1 and Kα2 contributions with wavelengths of 1.54056 Å, and λ = 1.54439 Å, respectively. The Kα2 component has half the intensity of the Kα1 contribution, leading to doublet peaks. The instrument used for this research was a PANalytical X’pert Pro diffractometer in Bragg-

Brentano configuration, with an X’Celerator detector.

2.1.1. Diffraction Experiments

Crystal structures are made up of repeating units, called unit cells. When X-rays pass through crystalline samples, they are diffracted. When a phase difference of nλ occurs, constructive interference leads to fulfillment of Bragg’s law: nλ = 2d·sinθ (Figure

2-1).37

Figure 2-1: Diffraction of X-rays within crystals.

where n is an integer > 1, λ is the wavelength of the incident beam, d represents the d- spacing, which is the distance between parallel crystal planes, and θ represents the angle between the plane and the beam.

11

Powder X-ray diffraction can be used for phase identification, as the d-spacings fulfilling Bragg’s law are characteristic of each crystal structure.

2.1.2 Crystallite Size Estimates

The widths of the peaks in a diffraction pattern contain information about the sample. The full width at half max (FWHM) is inversely proportional to the size of the crystals, as described by the Scherrer equation, Equation 2-1,

B = 0.9 λ/t·cosθ (Equation 2-1)

where B is the FWHM in radians, λ is the X-ray wavelength, t is the crystallite size, and θ is the angle of the incident beam. Solving for t, the crystallite size can be calculated. This equation is only valid for broadening due to crystallite size. The experimental FWHM needs to be corrected for the instrument’s contribution to peak broadening using Equation

2-2. To determine the instrument broadening, a highly crystalline silicon sample was scanned on the PXRD. The observed FWHM of this pattern is solely due to the instrument broadening, and the results were used to correct all sample data presented in this thesis.

2 2 2 B crystallite= B measured - B instrument (Equation 2-2)

Crystallite size is related to particle size; however they are not always same, as multiple crystals can be connected to form a larger particle. The particle size is best determined by electron microscopy.

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2.2. Electron Microscopy

When working with nanoparticles, both particle size and agglomeration need to be characterized, which is best achieved by electron microscopy. Light microscopy has limits to resolution given by the Rayleigh criterion (Equation 2-3).

R = 0.61λ / NA (Equation 2-3)

where R is the output resolution, λ is the wavelength of light used, and NA is the numerical aperture of the microscope. Using an oil immersion lens, the approximate minimal resolution for visible light is 200 nm. Electrons have much smaller wavelengths, which allows for resolving features down to around 1 nm.

The use of electron microscopy during this project is beneficial for observing particle size, particle morphology, and particle agglomeration. X-ray diffraction gives an estimated average size of the crystals present in a sample, but only electron microscopy can give detailed information about each particle. Data obtained from transmission of electrons through the samples, known as transmission electron microscopy (TEM), provides crystal lattice fringes, which can reveal any structural anomalies.

Charging may occur when non-conductive samples are exposed to high energy electron beams. This can interfere with detection due to the build up of excess electrons on the surface of the sample. In order to prevent this from occurring, samples can be coated with a conductive layer, which is often applied by sputtering. Some examples of conductive coatings used are gold and carbon. For this research, gold sputtering was

13

used on samples when viewing on standard SEM sample holders, no sputtering was required for copper grids.

2.2.1 Field Emission Scanning Electron Microscopy

One of the microscopes used for this research was a JEOL 7500F field emission scanning electron microscope (FE-SEM). The field emission process is beneficial because it uses a much lower acceleration voltage compared to other SEM. FE-SEM also provides much higher resolution.

Samples generally do not need a conductive coating due to the lower acceleration voltage; however coating requirements will depend on the desired resolution. Coating of samples for this research, when required, was performed with a gold sputter. For this research, samples were coated with gold when necessary.

In an SEM, electrons are generated by passing a current through a tungsten filament and applying a large potential. This generates an electron beam that can be focused with electromagnets and directed at the sample. Samples, when hit with electrons, eject secondary electrons that are detected, and the data is output to a monitor for visual representation. Holey carbon copper TEM grids, purchased from Ted Pella and

SPI, were used to view the samples. Samples were dispersed in methanol solution by sonication, and then a drop of the mixture was added to the lighter side of the TEM grid.

The JEOL JSM-7500F is also equipped with a transmission electron detector that was used, however the resolving power was not adequate for smaller particles.

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2.2.2 Scanning Transmission Electron Microscopy

TEM sample analysis was performed in a Hitachi 2300-A STEM at 200 kV acceleration voltage. Transmission electron microscopy differs from an SEM in how it detects and outputs the data. The sample is bombarded with much higher energy electrons, and instead of the majority of the electrons producing secondary electrons, they pass through the sample, scattering based on atomic locations. The transmitted electrons reach a detector, and the data is output to a monitor for visual representation.

Transmission of the electrons is limited by the sample. If the sample is too thick, electrons cannot transmit and will appear as an absence of intensity, due to no electrons hitting the detector. Therefore the sample needs to be thin enough for high resolution details of a lattice framework to be visible. Depending on composition, sample thicknesses of 1 μm or less can be imaged.

2.3. Thermogravimetric analysis

Thermogravimetric analysis (TGA) instruments allow detection of mass changes as a function of temperature. A known weight of the sample is placed on a sensitive balance lever, and heated to a temperature using a continuous ramp. Samples are heated in the presence of air or a purge gas. The mass loss or gain is continuously recorded as the temperature is increased. For this project, TGA was used to determine the quantity of water that had diffused into the ZrW2O8 framework. For this thesis, a TA Instruments

SDT 2960 Simultaneous TGA-DTA instrument was used.

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2.4. Brunauer-Emmett-Teller Surface Analysis

Brunauer-Emmett-Teller (BET) surface analysis is a technique for measuring

38 surface area and porosity. Sorption of gases (He, N2, or Ar) onto the surface of a known mass of material is measured as a function of relative pressure. A sample is degassed under heat and vacuum, after which it is cooled to liquid N2 temperatures and an inert gas is added in controlled amounts. The gas adsorbs on the surface to form one monolayer as pressure builds up in the sample chamber. The monolayer of gases forms a dipole and allows for a second layer to build up on it. Further layers are attributed to condensation forces. Pressures are increased and decreased to measure adsorption and desorption amounts. The monolayer of gas coverage can be determined with equation 2-4,

p11 c p  (Equation 2-4) v() p00 p vmm c v c p

where (p) is the adsorption pressure, (v) is the volume of a gas adsorbed at that pressure

(p0) is the saturation vapor pressure, (vm) is the volume of a gas adsorbed when 1 monolayer is present, (c) is a constant. The surface area can be calculated with equation

2-5 and the known mass and is expressed in units of m2/g.

vmc NA Stotal  (Equation 2-5) MV

where (Stotal) is the total surface area covered, (vm) is the monolayer gas volume, (N) is

Avogadro’s number, (Ac) is the adsorption cross section, and (MV) is the molar volume of adsorbing gas.

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Chapter 3

3. Synthetic Variables in the Preparation of ZrW2O7(OH)2·2H2O

3.1. Introduction

Research carried out on zirconium tungstate nanoparticle synthesis prior to this thesis project involved optimizing particle size while lowering agglomeration. The synthesis involved a low temperature method that made it possible to convert a precursor,

ZrW2O7(OH)2·2H2O, to cubic ZrW2O8 by heat treatments at 600 to 650 ºC. It was initially discovered that nanoparticles instead of micron sized particles of the precursor material could be obtained by hydrothermal synthesis in perchloric acid/NaCl instead of HCl.39

However, significant agglomeration was observed, which could lead to inhomogeneous filler distribution in polymer composites. It was shown that alcohol/HCl mixtures could reduce the agglomeration problem while still maintaining the desired particle size.

However, recent observations suggest that these solvothermally prepared nanoparticles are prone to autohydration.

It is important to distinguish the zirconium tungstate hydroxide hydrate precursor, which has been referred to as “hydrated precursor” in the literature, from the hydrated zirconium tungstate that is the focus of this thesis. To avoid confusion in terminology,

17

ZrW2O7(OH)2·2H2O will be referred to as hydroxide hydrate, while ZrW2O8·xH2O will be referred to as hydrated zirconium tungstate for the remainder of this thesis.

3.2. Zirconium Tungstate Synthesis

3.2.1 Hydrothermal Synthesis

Hydrothermal synthesis is a soft chemistry method used in this research for the formation of ZrW2O7(OH)2·2H2O, a precursor to cubic ZrW2O8. The word

“hydrothermal” implies that the reaction takes place in an aqueous environment at high temperatures contained within a Parr bomb, or autoclave, lined with a Teflon vessel. The

Parr bomb must be sealed to prevent vapors from escaping. Hydrothermal methods can be used for crystal growth, digestion, hydrolysis, and oxidation. It is a common technique for the preparation of metal oxide powders; some examples include zirconia, alumina, and titania. More complex setups exist that include stirring mechanisms, as well as ultrasonic and microwave compatible reaction vessels. Non-aqueous solvents may be added to the reaction vessel in place of water, this is referred to as a solvothermal reaction.

3.2.2 Micron-sized ZrW2O8 Particles

Zirconium tungstate is an ideal candidate for a filler material in controlled thermal expansion composites. It was first discovered in 1959 by Graham, who obtained it by mixing the binary oxides ZrO2 and WO3 and heating to 1400 K. Zirconium tungstate is thermodynamically stable from 1378 to 1503 K, and metastable up to 1050 K, where it decomposes into the binary oxides. A low temperature synthesis from a precursor

18

prepared by the hydrothermal route in acidic solution was first reported in the 1970’s.6

The precursor formed is zirconium tungstate hydroxide hydrate (ZrW2O7 (OH)2·2H2O).

This precursor can be heated at 873 K for a topotactic recrystallization into cubic

ZrW2O8. The hydroxide hydrate precursor was originally obtained in the presence of

HCl. It was later found that the synthesis can also be carried out in other acids, however, the crystallization kinetics depend strongly on the hydronium and chloride ion concentrations. 40

3.2.3 Nanosized ZrW2O8 Particles

When nanoparticles of ZrW2O8 were first synthesized in perchloric acid/NaCl mixtures, significant agglomeration was observed. An alcohol/HCl route was used to overcome the agglomeration problems. Optimized sample conditions were determined to be 2.7 M 1-butanol/7 M HCl mixtures, resulting in rod shaped particles that were 15 to 50 nm wide by 200 to 500 nm long, and formed agglomerates of 50 to 100 nm by 300 to 600 nm.39

3.3. General Synthesis Protocols

All syntheses of nanoparticulate ZrW2O8 from its precursor ZrW2O7(OH)2·2H2O were carried out under solvothermal conditions. A general synthesis is given below, and details for each sample are summarized in (Table 3-1).

The starting materials, 0.450 g (1.5 mmol) of ZrOCl2·xH2O and 0.660 g (2 mmol) of NaWO4·2H2O, were each dissolved in 1.25 mL of distilled H2O. A 23 mL Teflon lined

Parr bomb insert equipped with a magnetic stirring rod was placed on a stirring plate, and

2.5 mL of alcohol were added. The dissolved starting compounds were simultaneously

19

poured into the Teflon container, resulting in formation of a white precipitate. After stirring for a few minutes, 5 mL of concentrated HCl were added, bringing the total volume to 10 mL. After stirring for an additional few minutes, the stir bar was retrieved, the autoclave was closed, placed into a cool oven and heated to 130 °C for 24 h.

The Parr bomb was allowed to cool to room temperature, and the white powder was recovered by centrifugation. It was washed and centrifuged until the pH of the supernatant was neutral. The powder was dried at 60 °C and characterized by PXRD and electron microscopy.

All samples prepared in this research and the conditions under which they were prepared are listed in (Table 3-1). Several variables were investigated to determine factors influencing particle size and morphology and their effect on hydration. These included type of organic solvent added, volume of solvent, acid concentration, temperature, and heating time. Samples are sorted by solvent type, and within each solvent type they are sorted by temperature. The majority of samples were prepared in a

23 mL Parr bomb, a few samples were prepared in a 125 mL Parr bomb, and these appear at the end of the table. These samples were prepared using the same amounts of

ZrOCl2·xH2O and NaWO4·2H2O, leading to a lower concentration of the starting materials. The PXRD result column has scan results that are represented as hydroxide hydrate (HH), amorphous (A), amorphous plus a minor hydroxide hydrate phase (A +

HH), did not recover sample (DNR), impurities (I), and tungsten oxide (WO3). Crystallite size estimates are listed in the size column; samples that are listed as 100+ nm in size had peak widths equivalent to the silicon. Some samples were reheated in another solvent environment after the initial recovery, these are labeled with (subs).

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Table 3-1: Synthetic variables for all ZrW2O7(OH)2·2H2O samples

Sample Vacid Valcohol Vwater T Time PXRD Size (mL) (mL) (mL) (°C) (h) result (nm) NBA1 5 - 5 170 16 HH 100+ NBA15 5 1.0 MeOH 4 130 72 HH 23.2 ± 1.5 NBA12 5 2.5 MeOH 2.5 130 24 A+HH -- NBA150 5 2.5 MeOH 2.5 130 24 A+HH -- NBA47 5 1.0 MeOH 4 130 24 HH 19.8 ± 1.7 NBA95 5 0.5 MeOH 4.5 210 24 HH 50.2 ± 4.8 NBA44 5 2.5 EtOH 2.5 130 24 HH 19.6 ± 2.3 NBA2 5 2.5 EtOH 2.5 170 16 HH 100+ NBA3 5 2.5 EtOH 2.5 170 84 HH 100+ NBA96 5 0.5 EtOH 4.5 210 24 HH 48 ± 2.9 NBA4 5 2.5 EtOH 2.5 170 - DNR -- NBA68 5 0.5 1-PrOH 4.5 130 24 HH 25.9 ± 2.0 NBA69 5 2.5 1-PrOH 2.5 130 24 HH 21 ± 2.4 NBA76 4 0.5 1-PrOH 5.5 130 72 HH 32.1 ± 2.9 NBA77 4 2.5 1-PrOH 3.5 130 72 HH 29.2 ± 2.0 NBA14 5 2.5 1-PrOH 2.5 130 24 HH 37.8 ± 2.5 NBA16 2.5 2.5 1-PrOH 5 130 168 A+HH -- NBA17 2.5 2.5 1-PrOH 5 130 24 A -- NBA80 4 0.5 1-PrOH 5.5 130 24 HH 28.4 ± 2.4 NBA83 4 2.5 1-PrOH 3.5 130 24 HH 16.4 ± 2.1 NBA84 5.8 0.5 1-PrOH 3.7 130 24 HH 22.7 ± 1.9 NBA149 5 0.5 1-PrOH 2.5 130 24 HH 22.9 ± 2.1 NBA86 5.8 2.5 1-PrOH 1.7 130 24 HH 27.0 ± 2.7 NBA92 4 4.0 1-PrOH 2 130 24 HH 20.8 ± 2.3 NBA93 4 4.0 1-PrOH 2 150 24 HH 28.1 ± 3.0 NBA70 5 0.5 1-PrOH 4.5 170 24 HH 35.1 ± 1.4 NBA148 5 0.5 1-PrOH 4.5 170 24 HH 39.3 ± 2.5 NBA153 5 2.5 1-PrOH 2.5 170 24 HH 35.5 ± 2.6 NBA71 5 2.5 1-PrOH 2.5 170 24 HH 33.9 ± 2.3 NBA90 5 0.5 1-PrOH 4.5 190 24 HH 40.7 ± 3.3 NBA91 5 2.5 1-PrOH 2.5 190 24 HH 40.9 ± 2.8 NBA72 5 0.5 1-PrOH 4.5 210 24 HH 63.6 ± 4.1 NBA74 5 2.5 1-PrOH 2.5 210 24 HH 48.9 ± 4.3

NBA78 4 0.5 1-PrOH 5.5 210 72 HH 100+ NBA79 4 2.5 1-PrOH 3.5 210 72 HH 70 ± 5.3 NBA81 4 2.5 1-PrOH 3.5 210 24 HH 60.2 ± 2.9 NBA82 4 0.5 1-PrOH 5.5 210 24 HH 59.8 ± 3.0 NBA85 5.8 0.5 1-PrOH 3.7 210 24 HH 62.2 ± 5.4 NBA87 5.8 2.5 1-PrOH 1.7 210 24 HH+I 69.2 ± 7.1 NBA94 4 4.0 1-PrOH 2 210 24 HH 48.3 ± 1.7 NBA102 3.5 4.0 1-PrOH 2.5 210 24 DNR -- NBA121 6.5 1.5 1-PrOH 2 210 2 HH 43.8 ± 7.0 NBA122 6.5 1.0 1-PrOH 2.5 210 3 HH+I 43.2 ± 3.8 NBA147 5 0.5 1-PrOH 4.5 210 24 HH 53.2 ± 4.2 NBA103 3.5 4.0 1-PrOH 2.5 220 24 HH 61.7 ± 1.5 NBA104 3.5 0.5 1-PrOH 6 220 24 HH 100+ NBA105 3 5.0 1-PrOH 2 220 24 HH+I --

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NBA106 2.5 5.0 1-PrOH 2.5 220 24 HH+I -- NBA107 4 4.0 1-PrOH 2 220 15 HH 39.7 ± 3.0 NBA108 4 4.0 1-PrOH 2 220 17 HH 60.7 ± 3.4 NBA109 4 4.0 1-PrOH 2 220 10 HH+I 36.0 ± 2.6 NBA110 4 4.0 1-PrOH 2 220 12 HH+I 74.6 ± 6.1 NBA111 4 4.0 1-PrOH 2 220 7 HH+I 44.2 ± 6.4 NBA112 4 4.0 1-PrOH 2 220 3 HH+I 56.3 ± 5.5 NBA113 4 4.0 1-PrOH 2 220 2 HH+I 47.0 ± 5.5 NBA114 6 2.0 1-PrOH 2 220 2 HH 40.6 ± 4.0 NBA115 5 3.0 1-PrOH 2 220 2 HH 43.7 ± 4.0

NBA116 7 1.0 1-PrOH 2 220 2 WO3 -- NBA117 7.5 0.5 1-PrOH 2 220 2 WO3 -- NBA118 6.5 1.5 1-PrOH 2 220 2 HH 43.6 ± 4.9 NBA119 6.5 1.0 1-PrOH 2.5 220 2 A+HH -- NBA120 6.5 0.5 1-PrOH 3 220 2 A+HH -- NBA123 6 2.0 1-PrOH 2 220 2 HH+I 52.2 ± 4.6 NBA124 6 2.0 1-PrOH 2 220 1.5 HH 43.2 ± 3.8 NBA125 6 2.0 1-PrOH 2 220 4 HH+I 59.3 ± 7.7 NBA126 6 2.0 1-PrOH 2 220 6 HH+I 52.7 ± 4.5 NBA127 6 2.0 1-PrOH 2 220 1.5 HH 34.7 ± 3.5 NBA128 6 1.0 1-PrOH 3 220 1.5 HH 41.5 ± 3.3 NBA129 6 2.0 1-PrOH 2 220 1 (p) HH 35.3 ± 3.6 NBA104 3.5 0.5 1-PrOH 6 220 24 HH 100+ NBA130 6 2.0 1-PrOH 2 230 0.5 A+HH -- NBA131 6 2.0 1-PrOH 2 230 1 HH 39.2 ± 3.6

NBA73 5 0.5 1-PrOH 4.5 240 24 WO3 -- NBA75 5 2.5 1-PrOH 2.5 240 24 WO3 -- NBA101 5 0.5 2-PrOH 4.5 210 24 HH 50.0 ± 2.9 NBA151 5 2.5 1-BtOH 2.5 130 24 HH 20.1 ± 1.5 NBA98 5 0.5 1-BtOH 4.5 210 24 HH 55.4 ± 4.8 NBA99 5 0.5 1-PnOH 4.5 210 24 HH 55.0 ± 3.6 NBA45 5 2.5 1-PnOH 2.5 130 24 HH 31.2 ± 3.5 NBA21 5 2.5 cycloPnOH 2.5 130 24 HH 26.3 ± 3.0 NBA46 5 2.5 1-HxOH 2.5 130 24 HH 40.9 ± 3.0 NBA100 5 0.5 1-HxOH 4.5 210 24 HH 59.8 ± 4.4 NBA5 5 2.5 1-HpOH 2.5 130 24 HH 36.9 ± 2.7 NBA6 5 2.5 1-HpOH 2.5 130 48 HH 33.1 ± 4.5 NBA7 2.5 3.3 1-HpOH 4.2 130 24 HH 93.9 ± 9.0 NBA8 2.5 3.3 1-HpOH 4.2 130 48 HH 55.4 ± 4.8 NBA9 2.5 3.3 1-HpOH 4.2 130 96 HH 43.6 ± 6.6 NBA10 5 3.3 1-HpOH 1.7 130 24 HH 36.3 ± 3.6 NBA11 5 4.3 1-HpOH 0.7 130 24 HH 41.1 ± 2.8 NBA13 2.5 3.3 1-HpOH 4.2 130 168 HH 43.8 ± 3.1 NBA38 5 2.5 1-HpOH 2.5 210 HH 56.5 ± 6.8 NBA39 5 2.5 1-HpOH 2.5 210 HH 55.3 ± 4.5 NBA132 6 2.0 1-HpOH 2 230 1 A+HH -- NBA51 5 2.5 1-OcOH 2.5 130 24 HH 34.0 ± 3.4 NBA59 5 2.5 ethylene gly. 2.5 110 72 A+HH --

NBA56 5 2.5 ethylene gly. 2.5 120 24 WO3 -- NBA55 5 2.5 ethylene gly. 2.5 130 24 A+HH --

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NBA135 6 2.0 ethyl ether 2 220 1.5 (p) DNR -- NBA136 6 2.0 ethyl ether 2 220 1.5 (p) DNR -- NBA137 5 2.5 ethyl ether 2.5 220 1.5 subs 5 5 130 20 HH 43.3 ± 4.2 NBA138 5 2.5 ethyl ether 2.5 220 1.5 subs 5 -- 5 130 20 HH 37.7 ± 4.2 NBA139 5 2.5 ethyl ether 2.5 220 1.5 subs 2.5 7.5 220 20 HH 54.2 ± 4.2 NBA140 5 2.5 ethyl ether 2.5 220 1.5

subs 6.5 -- 3 220 20 HH+WO3 57.4 ± 6.9 NBA141 5 2.5 ethyl ether 2.5 220 1.5 HH 40.5 ± 5.0 NBA142 5 2.5 ethyl ether 2.5 220 1.5 HH 41.3 ± 3.7 NBA143 5 2.5 ethyl ether 2.5 220 1 subs 5 5 130 20 HH 40.5 ± 4.3 NBA144 5 2.5 ethyl ether 2.5 220 1.5 5.0 subs P/NaCl -- 5 130 20 HH 38.3 ± 4.4 NBA146 5 2.5 ethyl ether 2.5 220 0.5 HH 40.5 ± 4.3 NBA133 6 3.0 diisopropyl ether 1 130 24 HH 44.8 ± 4.2 100.2 ± NBA134 6 3.0 diisopropyl ether 1 170 24 HH 10 NBA145 5 2.5 1-methoxypropane 2.5 220 1.5 HH 40.5 ± 3.4 1.0 MeOH/1.5 1- NBA88 5 PrOH 2.5 130 24 HH 18.4 ± 1.8 1.0 MeOH/1.5 1- NBA89 5 PrOH 2.5 210 24 HH 49.9 ± 4.0 0.3 MeOH & 2.2 1- NBA35 5 HpOH 2.5 125 24 HH 28.0 ± 2.5 0.3 MeOH & 3.7 1- NBA36 5 HpOH 1 130 24 HH 25.2 ± 2.2 0.3 MeOH & 2.2 1- NBA37 5 HpOH 2.5 120 24 HH 61.2 ± 5.3 0.5 MeOH & 2.0 1- NBA28 5 HpOH 2.5 130 24 HH 27.1 ± 3.8 0.5 MeOH & 2.8 1- NBA29 5 HpOH 1.7 130 24 HH 26.3 ± 2.7 1.0 MeOH & 1.5 1- NBA18 5 HpOH 2.5 130 24 HH 21.5 ± 4.1 1.5 MeOH & 1.0 1- NBA19 5 HpOH 2.5 130 24 HH 22.0 ± 3.8 0.5 MeOH & 3.8 1- NBA31 5 HpOH 0.75 130 24 HH 39.2 ± 3.2 0.3 MeOH & 2.2 1- NBA32 5 HpOH 2.5 130 24 HH 39.2 ± 3.2 0.2 MeOH & 2.3 1- NBA33 5 HpOH 2.5 130 24 HH 26.3 ± 2.9 0.3 MeOH & 2.2 1- NBA34 5 HpOH 2.5 130 17 HH 26.9 ± 2.3 1.5 1-EtOH & 1.0 1- NBA24 5 HpOH 2.5 130 24 HH 25.2 ± 2.7 1.5 1-PrOH & 1.0 1- NBA20 5 heptOH 2.5 130 24 HH 19.2 ± 3.8 1.5 1-BuOH & 1.0 1- NBA23 5 HpOH 2.5 130 24 HH 21.5 ± 2.7

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1.5 1-PnOHl & 1.0 1- NBA25 5 HpOH 2.5 130 24 HH 19.2 ± 2.6 1.5 2-PnOHl & 1.0 1- NBA26 5 HpOH 2.5 130 24 HH 28.8 ± 2.5 1.5 cycloPnOH & 1. 1- NBA27 5 HpOH 2.5 130 24 HH 28.0 ± 2.7 1.51-HxOH & 1.0 1- NBA30 5 HpOH 2.5 130 24 HH 26.8 ± 2.4 NBA62 5 2.5 THF 2.5 130 24 HH 25.2 ± 4.4 NBA63 5 2.5 THF 2.5 130 65 HH 25.3 ± 2.5 NBA64 5 3.4 THF 2.5 130 65 DNR -- NBA65 5 3.5 THF 2.5 130 65.5 HH 32.2 ± 3.6 NBA67 5 0.5 THF 4.5 130 24 HH 24.6 ± 1.4 5 H2SO4 NBA60 /NaCl 1.5 1-BtOH/1.0 THF 2.5 130 24 WO3 -- NBA61 5 1.5 1-BtOH/1.0 THF 2.5 130 24 HH 27.5 ± 2.7 1.5 1-BtOH & 1.0 NBA57 5 DMSO 2.5 130 24 HH 26.7 ± 3.3 1.5 1-PrOH & 1.0 NBA58 5 DMSO 2.5 130 24 HH 24.7 ± 2.7 102.7 ± NBA97 5 0.5 DMSO 4.5 210 24 HH 12.3 NBA66 5 2.5 methylene Cl 2.5 130 24 HH 36.5 ± 3.0

NBA22 5 4.3 n-heptane 0.7 130 24 WO3 -- 125ml autoclave NBA54 0 47.5 MeOH 2.5 130 336 A+HH -- NBA49 25 22.5 1-PrOH 2.5 130 24 HH 52.2 ± 6.5 NBA40 12.5 35 1-BtOH 2.5 130 92 A+HH -- NBA41 25 22.5 1-BtOH 2.5 130 24 HH 20.3 ± 2.4 NBA42 25 22.5 1-BtOH 2.5 120 24 HH 63.2 ± 8.4 NBA43 25 22.5 1-BtOH 2.5 210 24 HH 26.5 ± 3.1 NBA52 25 22.5 1-HpOH 2.5 130 24 HH 67.3 ± 1.7 NBA53 30 17.5 1-HpOH 2.5 130 24 A+HH -- 69.5 ± NBA48 25 22.5 1-HpOH 2.5 130 24 HH 12.0 73.5 ± NBA50 25 22.5 1-OctOH 2.5 130 24 HH 45.5

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3.4. Results and Discussion

3.4.1 Effect of Temperature

Samples were prepared at different temperatures under otherwise identical conditions (Table 3-2). Different subsets used specific conditions for alcohol concentration, acid concentration, and time, however this section focuses on crystallite size and morphology as a function of temperature. The procedure for crystallite size estimates was described in detail in section 2.1.2. For samples reproduced under identical conditions, size estimates can vary up to 11%, which is similar to the estimated standard deviation. Any synthesis variable resulting in changes in crystallite size above 10% has a significant influence.

Table 3-2: Variation of crystallite size as a function of temperature for different subsets of samples

Sample T/ºC Crystallite size (nm) ESD (nm) Subset 1 NBA68 130 26 2.0 NBA70 170 35 1.4 NBA72 210 64 4.1 Subset 2 NBA69 130 21 2.4 NBA71 170 34 2.3 NBA74 210 49 4.3 Subset 3 NBA80 130 28 2.4 NBA81 210 60 2.9 Subset 4 NBA83 130 16 2 NBA82 210 60 3 Subset 5 NBA92 130 21 2.3 NBA94 210 48 1.7

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Samples NBA68, NBA70, and NBA72 were prepared at 130 ºC, 170 ºC and 210

ºC, respectively. Crystallite size estimates were 26 ± 2.0 nm, 35 ± 1.4 nm, and 64 ± 4.1 nm. Samples NBA69, NBA71, and NBA74 were obtained at temperatures of 130 ºC, 170

ºC, and 210 ºC. Crystallite size estimates for these samples gave 21 ± 2.4 nm, 34 ± 2.3 nm and 49 ± 4.3 nm, respectively. Samples NBA80 and NBA81, were prepared at 130 ºC and 210 ºC, giving crystallite size estimates of 28 ± 2.4 nm, and 60 ± 2.9 nm. Samples

NBA83 and NBA82 were also prepared at temperatures of 130 ºC and 210 ºC. The crystallite size estimates were 16 ± 2 nm, and 60 ± 3 nm. Samples NBA92, prepared at

130 ºC, and NBA94, prepared at 210 ºC, had crystallite size estimates of 21 ± 2.3 nm and

48 ± 1.7 nm, respectively.

Two samples (NBA73 and NBA75) were prepared at 240 ºC to further investigate crystal growth at higher temperatures. X-ray diffraction analysis of these samples showed a minor hydroxide hydrate phase, with the majority of the sample crystallizing as tungsten oxide.

Particle size analysis was performed with an FE-SEM and STEM on samples

NBA68, NBA70, and NBA72. Analysis revealed an increase in particle size for samples prepared at higher temperatures. NBA68, prepared at 130 ºC, was composed of rod-like structures that were 15 to 30 nm wide and 100 to 500 nm in length (Figure 3-1a).

Agglomerated particles of about 4 to 6 rods were common, however, in some cases well over 10 particles were joined together. Sample NBA70, prepared at 170 ºC, showed more defined rod-like particles with distinguished crystal facets at the tip of the particles. Sizes ranged from 15 to 50 nm in width and 350 to 650 nm in length (Figure 3-1b). The sample showed less agglomeration than NBA68, generally 2 to 3 rods were joined together, and

26

some single rods were observed as well. NBA72, prepared at 210 ºC, gave a larger particle size. Particles ranged from 40 to 120 nm in width, and 250 to 800 nm in length

(Figure 3-1c). Very distinct facets were observed, and particles were not agglomerated.

Analysis of crystallite size as a function of temperature reveals an increasing trend. Each 40 ºC temperature change showed a size increase of more than 11%, which had previously been established as the error margin of samples prepared under identical conditions. Therefore, temperature is an important factor influencing crystallite size estimates. This is not surprising, as crystal growth is accelerated at higher temperatures, leading to increased size over identical duration of reaction.

Data from crystallite size estimates alone suggest that higher temperatures were unfavorable for nanoparticle formation; however, while individual particles were larger, agglomeration was reduced significantly at higher temperatures. Particle size distributions were narrower at 130 and 170 ºC than for sample prepared at 210 ºC, however, reaction time could be used to control particle size distribution. Samples prepared at too high temperatures (240 ºC) can cause problems with complete hydroxide hydrate formation. At this temperature the solvent becomes unstable, and decomposes with formation of a black compound.

27

Figure 3-1: TEM and SEM images of hydroxide hydrate samples prepared at (a) 130 ºC, (b) 170 ºC and (c) 210 ºC.

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3.4.2 Effect of Reaction Time

To investigate the effect of reaction time on crystallite and particle size, subsets of samples prepared under identical conditions but heated for different amounts of time were compared (Table 3-2). The minimum time required for complete crystallization was also investigated at higher temperatures.

Table 3-3: Variation of crystallite size as a function of reaction time for different subsets of samples. Sample Time (h) Crystallite Size (nm) ESD (nm) Subset 1 NBA80 24 28 2.4 NBA76 72 32 2.9 NBA152 144 39 2.9 Subset 2 NBA81 24 60 2.9 NBA79 72 70 5.3 Subset 3 NBA62 24 25 4.4 NBA63 65 25 2.5 Subset 4 NBA47 24 20 1.7 NBA15 72 23 1.5

Crystallite size estimates for similarly prepared samples (4 mL HCl, 0.5 mL 1- propanol, 130 ºC) were heated for 24 h (NBA80), 72 h (NBA76) and 144 h (NBA152).

Crystallite size, were 28 ± 2.4 nm, 32 ± 2.9 nm, and 39 ± 2.9 nm, respectively.

Another set of samples (4 mL HCl, 2.5 mL 1-propanol, 210 ºC) reacted for 24h

(NBA81) and 72 h (NBA79) prepared under conditions identical to one another gave crystallite size estimates of 60 ± 2.9 nm, and 70 ± 5.3 nm.

29

Samples NBA62 and NBA63 were prepared under similar conditions (5 mL HCl,

2.5 mL THF, 130 ºC) and heated for 24 h and 65 h, respectively. The crystallite size estimates were 25 ± 4.4 nm (NBA62) and 25 ± 2.5 nm (NBA63).

Samples NBA47 and NBA15 were prepared under similar conditions (5mL HCl,

1.0 mL methanol, 130 ºC) and heated for 24 h and 72 h, respectively. The crystallite size estimates were 20 ± 1.7 nm, and 23 ± 1.5 nm.

Small but notable crystallite size increases were seen for samples NBA80 and

NBA152, these samples were heated at 130 ºC for 24 h and 144 h (4 mL HCl and 0.5 mL

1-propanol). Around a 10 nm increase in crystallite size occurred, this can be explained by Ostwald ripening. Ostwald ripening occurs when smaller crystals re-dissolve back into solution over time and cause further growth of larger particles, increasing the average size. A similar trend was seen for samples NBA81 and NBA79, which were heated at

210 ºC for 24 h and 72 h, respectively. The crystallite size average for the sample heated for 72 h showed an increase of ~10 nm. In contrast, samples prepared at lower temperatures (130 ºC) showed no increase in crystallite size for similar reaction times.

Smaller crystals are more stable and less soluble at lower temperatures, resulting in slower Ostwald ripening.

While the samples heated for 24 h (NBA80) and 72 h (NBA76) at 130 ºC showed no increase in average crystallite size with time, significant differences were observed by

SEM. Surprisingly, the 24 h duration sample had a much better particle size distribution, with particle sizes ranging from 20 to 40 nm wide and 400 to 600 nm in length. Particles were mostly agglomerated into groups of 4 to 5 rods, however some rods were separate

(Figure 3-2a).

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Figure 3-2: TEM images of hydroxide hydrate samples heated at 130 ºC for (a) 24 h, (b) 72 h and (c) 144 h.

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The sample heated for 72 h gave a much broader particle size distribution, with particle widths ranging from 10 to 40 nm and overall length differences up to one order of magnitude (Figure 3-2b). The particle distribution of a sample heated for 144 h was more similar to the 24 h sample; however the particle sizes were larger as expected from continued Ostwald ripening. Particle widths ranged from 20 to 60 nm and lengths ranged from 500 to 1000 nm (Figure 3-2c).

Samples NBA79 and NBA 81 (prepared at 210 ºC) differed in crystallite size by ~

10 nm, and also showed effects of Ostwald ripening. The sample reacted for a duration of

72 h had a more homogenous particle size distribution, however, a larger average particle size of 60 to 100 nm in width and up to 1 micron in length was seen (Figure 3-3). Since the reaction took place at 210 ºC, this most likely accelerated the ripening process, and can explain the difference in crystallite size estimates.

Figure 3-3: TEM images of hydroxide hydrate samples heated at 210 ºC for (a) 24 h and (b) 72 h.

Several samples were prepared to investigate the minimum heating time required for complete crystallization. High temperatures gave better particle morphology, and short reaction times at high temperatures could result in small, well crystallized particles

32

while avoiding Ostwald ripening. Samples NBA114 and NBA115 were both reacted for 2 h at 220 ºC under slightly different conditions to determine if complete crystallization was possible. Both samples showed phase pure hydroxide hydrate in the XRD pattern

(Figure 3-4).

Figure 3-4: XRD of hydroxide hydrate heated at 210 ºC for 2 h in (a) 2.0 mL 1-propanol, 6.0 mL HCl and (b) 3.0 mL 1-propanol, 5.0 mL HCl.

Crystallization was shown to be dependent on heating times, however after full crystallization had been achieved particle size changes occurred through Ostwald ripening. Reactions using the minimal crystallization time would be most ideal for the smallest and most homogenous particles, as nucleation instead of growth dominates.

3.4.3 Effect of Acid Concentration

Two series of samples were investigated to determine the effect of acid volume present in the autoclave during the reaction. Crystallization kinetics are dependent on hydronium and chloride ion concentrations, therefore a difference in HCl concentration

33

was worth investigating. Only two temperatures were chosen for this set of reactions, 130

ºC and 210 ºC.

Samples NBA17, NBA83, NBA69, and NBA86 were prepared under similar conditions except with acid volumes of 2.5, 4, 5, and 5.8 mL, respectively. The sample prepared in 2.5 mL of HCl was amorphous upon recovery, whereas crystallite size estimates for NBA83, NBA69, and NBA 86 were 16 ± 2.1 nm, 21 ± 2.4 nm, and 27 ± 2.7 nm, respectively.

Table 3-4: Differences in crystallite size for different acid concentrations for samples prepared at 130 ºC. Sample Vacid (mL) Crystallite size (nm) ESD (nm) NBA83 4.0 16 2.1 NBA69 5 21 2.4 NBA86 5.8 27 2.7

Figure 3-5: PXRD patterns for samples prepared in 2.5 mL HCl and heated for (a) 7 d and (b) 1 d.

Since NBA17 showed little signs of crystallization, a sample was prepared with

2.5 mL HCl, and was reacted for 1 week to investigate the amount of time required to obtain the hydroxide hydrate (Figure 3-5). Hydroxide hydrate peaks were observed,

34

although a major amorphous phase still persisted. The crystallization time for this acid concentration is unreasonably long for applications.

NBA82, NBA72 and NBA85 were prepared under similar conditions with using acid volumes of 4, 5, and 5.8 mL, respectively. Crystallite size estimates for these samples were, 60 ± 3.0 nm, 64 ± 4.1 nm, and 62 ± 5.4 nm, respectively.

Table 3-5: Differences in crystallite size based with acid concentration for samples prepared at 210 ºC. Sample Vacid Crystallite size (nm) ESD (nm) NBA82 4 60 3.0 NBA72 5 64 4.1 NBA85 5.8 62 5.4

The data for samples prepared at 130 ºC shows an increasing crystallite size with acid concentration. Hydronium and chloride ions play an important role in determining the crystallization rate; therefore these results are not surprising. Lower HCl concentration results in slower crystallization, as can be seen for the sample prepared with 2.5 mL HCl. The faster initial crystallization at higher acid concentration is equivalent to giving samples more time to undergo Ostwald ripening at lower temperatures. At high temperatures, crystallization is fast enough that neither time nor acid concentration have a significant effect on particle size.

Scanning electron microscope analysis for samples NBA82, NBA72, and NBA85 revealed a slight difference in morphology. No significant differences in particle width between samples were observed, however, lengths differed between the samples. The sample with the lowest acid concentration (4 mL HCl) was composed of rod-like particles of varying aspect ratios. The majority of particles were 60 to 80 nm in width, and 100 to 300 nm in length (Figure 3-6a). The sample with intermediate acid

35

concentration (5 ml HCl) had particles mostly 60 to 100 nm wide and 400 to 600 nm in length (Figure 3-6b). The sample with the most acid present (5.8 mL HCl) gave particles mostly 60 to 100 nm wide and 700 to 1000 nm long (Figure 3-6c).

The crystallite size data showed increased size with increasing acid concentration for samples heated at 130 ºC. Samples will crystallize at a slower rate with lower acid concentration as seen with the sample prepared in 2.5 mL HCl, where it took nearly 1 week to achieve some crystallinity. Temperature influences crystal growth far more than acid concentration as observed with the samples prepared at 210 ºC. When observing the

210 ºC samples under an SEM, it was evident that samples prepared with lower acid concentration had much smaller particle lengths. While crystallite sizes were in the same range for all samples, the difference in aspect ratios suggests that lower acid concentrations favor nucleation over growth.

36

Figure 3-6: TEM images of hydroxide hydrate samples prepared with (a) 4 mL HCl, (b) 5 mL HCl and (c) 5.8 mL HCl.

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3.4.4 Influence of Solvent Type on ZrW2O7(OH)2·2H2O Morphology

3.4.4.1 Primary Alcohol Type

To obtain a better understanding of the effect of the organic solvent added on particle size and morphology, a series of samples were prepared with 2.5 mL of single chain primary alcohols ranging from methanol to 1-heptanol.

Primary alcohol comparison samples were prepared in 5.0 mL of HCl, and reacted at 130 ºC for 24 h. The samples were NBA12 (methanol), NBA44 (ethanol), NBA69 (1- propanol), NBA151 (1-butanol), NBA45 (1-pentanol), NBA46 (1-hexanol), and NBA5

(1-heptanol). All samples crystallized as phase pure hydroxide hydrate with the exception of NBA12 prepared in methanol. This behavior was reproducible.

Table 3-6: Variance in crystallite size for samples prepared with different alcohols. Alcohol Crystallite size (nm) ESD (nm) ethanol 20 2.3 1-propanol 21 2.4 1-butanol 20 1.5 1-pentanol 31 3.5 1-hexanol 41 3

1-heptanol 37 2.7

Crystallite sizes were calculated for fully crystalline samples. Samples prepared in ethanol, 1-propanol and 1-butanol all gave similar crystallite sizes ( 20 ± 2.3 nm, 21 ± 2.4 nm, 20 ± 1.5 nm). The sample prepared in 1-pentanol showed a near 50% increase in crystallite size (31 ± 3.5 nm), and 1-hexanol caused doubling in crystallite size (41 ± 3 nm). The miscibility of primary alcohols in water is limited to methanol through 1- butanol. If no alcohol is present in the aqueous phase, the reaction effectively occurs in a

38

pure aqueous phase with higher acid and starting material concentrations, resulting in larger crystallites.

Particle size and morphology were similar for the samples prepared in ethanol and

1-propanol. Agglomeration was severe in both samples; the sample prepared in ethanol gave agglomerates of about 200 nm in width and up to 1 μm in length. Each set of agglomerated particles consisted of individual particles 15 to 20 nm in width (Figure

3-7).

Figure 3-7: TEM images of hydroxide hydrate prepared in 2.5 mL ethanol, heated at 130 ºC for 24 h.

Similar agglomeration was observed for the sample prepared in 1-propanol, however it was less severe. Agglomerated units were generally about 100 nm in width, and 300 to 500 nm in length. Individual particles within the agglomerate were 12 to 20 nm in width (Figure 3-8). The morphology of the rods was similar to the sample prepared in ethanol, with high aspect ratio rods with round ends instead of flat faces.

39

Figure 3-8: TEM images of hydroxide hydrate prepared in 2.5 mL 1-propanol, heated at 130 ºC for 24 h.

The sample prepared in 1-butanol showed much less agglomeration. While agglomerates existed, other particles existed as single separate crystals. The agglomerates were generally about 60 to 100 nm in width and 200 to 300 nm in length (Figure 3-9).

The morphology of the particles was rod-like, and the majority of the rods formed flat ends.

Figure 3-9: TEM images of hydroxide hydrate prepared in 2.5 mL 1-butanol, heated at 130 ºC for 24 h.

The sample prepared in 1-pentanol gave rod-like particles of varied agglomeration. Agglomerates consisted of 2 to 5 rods; however, some sets had up to 10

40

smaller rods joined together. The particle size distribution was very broad, mostly the widths were 20 to 50 nm, and lengths varied from 100 to 800 nm (Figure 3-10).

Figure 3-10: TEM images of hydroxide hydrate prepared in 2.5 mL 1-pentanol, heated at 130 ºC for 24 h.

The sample prepared in 1-hexanol was very similar to the sample prepared in 1- pentanol. A broad particle size distribution was observed with particles ranging from 20 to 50 nm wide. The length of the particles was irregular, some particles were around 100 nm long, while others were around 300 to 500 nm long, and some particles even gave lengths up to 1 μm (Figure 3-11).

Figure 3-11: TEM images of hydroxide hydrate prepared in 2.5 mL 1-hexanol, heated at 130 ºC for 24 h.

41

The sample prepared in 1-heptanol was very similar to the sample prepared in 1- pentanol and 1-hexanol. A broad particle size distribution was observed with particles ranging from 30 to 80 nm wide. The length of the particles varied from 100 nm to 1 μm

(Figure 3-12).

Figure 3-12: SEM images of hydroxide hydrate prepared in 2.5 mL 1-heptanol, heated at 130 ºC for 24 h.

Samples prepared in ethanol and 1-propanol were very similar in both agglomeration and morphology. Both samples had pointed tips with a gradual change in slope. This suggests that growth along the a-axis for the crystals is less favored than along the c-axis. This explains the very large aspect ratio. The sample prepared in 1- butanol was visibly different, the rods had flat ends. Differences in crystallization time based on alcohol type are corroborated by the sample prepared in 2.5 mL methanol that did not fully crystallize in a 24 h period. A sample containing 1.0 mL of methanol was heated for the same amount of time. Full crystallization of hydroxide hydrate was achieved. All data suggest that the shorter chain alcohols may coordinate to the (hk0) crystal faces, leading to slower growth. The samples prepared in 1-pentanol, 1-hexanol,

42

and 1-heptanol contained a variety of particles, crystallite sizes increased as well. The previous hypothesis on the non-miscibility of these solvents is supported by this particle diversity. It would appear that a very minute amount of solvent is present within the aqueous layer, causing some growth restraints, however there is not enough alcohol to maintain control over all particles.

3.4.4.2 Non-primary Alcohols

Other organic solvents, such as ethylene glycol, di-ethyl ether, and tetrahydrofuran, were explored to investigate whether similar effects as with alcohols would be seen.

Sample NBA55 was prepared in 2.5 mL ethylene glycol and 5 mL HCl at 130 ºC for 24 h. The XRD scan revealed formation of hydroxide hydrate, however, a significant amorphous phase was present as well. In addition, the acid and high temperatures destroyed the glycol, turning the solution black. Samples NBA56 and NBA59 were prepared with ethylene glycol at lower temperatures, however, full crystallization could not be achieved.

Another set of samples were successfully crystallized in THF. As THF is fully miscible in water, this was not surprising. NBA62 was prepared in 2.5 mL THF, 5 mL

HCl and reacted at 130 ºC for 24 h. It had an average crystallite size of 25 ± 2.5 nm.

Another sample prepared with a lower of THF (0.5 mL) gave similar numbers of 25 ± 1.4 nm.

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3.4.5. Effect of Solvent Concentration

Solvent concentration was varied in samples to determine its effect on crystallization and morphology. Alcohols interact strongly with the forming crystals, thus their concentration is likely to lead to different morphologies.

It was discussed in chapter 3.4.4 that sample NBA12 (prepared in 2.5 mL methanol and 5.0 mL HCl at 130 ºC for 24 h) did not fully crystallize as hydroxide hydrate. This sample was reproduced, and again, an amorphous phase was present. In another sample, the methanol volume was reduced to 1.0 mL while all other conditions were kept the same. This sample crystallized as phase pure hydroxide hydrate. This was one of the only cases where the solvent concentration had a significant impact on crystallization kinetics.

Samples NBA68 and NBA69 prepared in 0.5 mL and 2.5 mL 1-propanol at 130

ºC, respectively, and gave crystallite size estimates of 26 ± 2.0 and 21 ± 2.4 nm. Samples

NBA70 and NBA71 prepared in 0.5 mL and 2.5 mL 1-propanol at 170 ºC, respectively, gave crystallite size estimates of 35 ± 1.4 and 34 ± 2.3 nm. Samples NBA72 and NBA74 prepared in 0.5 mL and 2.5 mL 1-propanol (210 ºC), respectively, gave crystallite size estimates of 62 ± 2.6 and 51 ± 3.4 nm.

Samples were reproduced to test consistency of the synthesis, NBA149 (130 ºC),

NBA148 (170 ºC), and NBA147 (210 ºC) in 0.5 mL 1-propanol the crystallite size estimates were 23 ± 2.1 nm , 36 ± 2.3 nm, and 55 ± 2.6 nm.

Samples originally prepared with 2.5 mL were reproduced to test consistency of the synthesis, LY2 was 21 ± 2.0 nm (130 ºC), NBA153 was 35 ± 2.6 nm (170 ºC), and

LY4 was 56 ± 1.1(210 ºC).

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Crystallite size measurements were reproducible for identical synthetic conditions within the margin of error previously established. Solvent concentration had minor variations on crystallite size, however there was no difference in size within 3 estimated standard deviations. Solvent concentration should not be considered an influencing factor on control of crystallite size.

3.4.5.1 Particle Size and Morphology

NBA70 was observed as rod-like structures with sizes ranging from 30 to 40 nm in width and 400 to 500 nm in length (Figure 3-13). The particle formed distinct rods with crystal facets visible on the ends of the majority of rods. Agglomeration was very low, with about 3 to 5 particles stuck together, while some particles were completely separate from others.

Figure 3-13: TEM images of NBA70 hydroxide hydrate particles prepared in 0.5 mL 1-propanol.

NBA71 was prepared under the same conditions as NBA70 except that 2.5 mL of

1-propanol were used. The particle size differed significantly from NBA70, with most particles having a width of around 35 nm, however, lengths ranged from 100 to 300 nm

45

(Figure 3-14). The particles were fairly agglomerated, with agglomerates consisting of 5 to 10 particles.

Figure 3-14: TEM images of NBA71 hydroxide hydrate particles prepared in 2.5 mL 1-propanol.

Larger alcohol volumes had a significant effect on particle aspect ratios. Kozy et al. previously reported that alcohol additions limit the particle widths, and it appears that there is a similar effect on the lengths.4 Alcohol may slow down the tetragonal c-axis growth. By using less alcohol, large aspect ratios can be obtained as observed in NBA70.

Notably, the morphologies of the particles also varied with alcohol volume. Larger alcohol quantities lead to more rounded edges, this is likely because alcohols coordinated to all faces preventing preferential growth in the a/b plane, limiting crystal facet appearances. Smaller alcohol quantities, such as 0.5 mL, gave rods, and still had a significant effect on the crystallite size when compared to a sample with no alcohols.

3.5. Conclusions

The crystallite size, morphology, and particle size of hydroxide hydrate can be controlled by proper choice of reaction conditions. The width of the nanoparticles is

46

closely related to the estimated crystallite size. The most influential variable for crystallite size control was temperature. Temperature also had a large effect on how agglomerated the particles were. Small crystals with little agglomeration are desirable, however, based on temperature alone, they could not be obtained. Based on the samples analyzed, it would appear that a slightly larger crystallite size is a good tradeoff for the significant reduction in particle agglomeration.

Reaction time had an effect on both crystallite size and particle size. Through longer heating times, smaller crystals can re-dissolve and cause further growth of the larger crystals. Agglomeration appeared to remain the same for all heating times, and there was no visible morphology improvement. It can be concluded that heating longer than the minimum required crystallization time is not beneficial for zirconium tungstate hydroxide hydrate nanoparticles.

Acid concentration effects crystallization times. At 130 ºC, crystallite size was affected as well. When the temperature was increased to 210 ºC, there was no difference in crystallite sizes as a function of acid concentration. Electron microscopy revealed notable differences in the lengths of the particles. Increased acid concentration accelerated c-axis growth, and thus increased the length of the particles. This makes acid concentration a useful variable for obtaining an ideal particle aspect ratio, especially as it appeared to have little influence on the particle widths.

Solvent additives must be miscible with water to affect the particles. Alcohols with more than four carbon atom show poor miscibility problems at 130 ºC. For ethanol,

1-propanol and 1-butanol, there was no difference in the estimated crystallite size.

Methanol strongly affected the crystallization time of the hydroxide hydrate, as full

47

crystallization could not be achieved for a 24 h heating duration. Alcohol type tends to have the largest influence on agglomeration. As alcohol chain length increased, the agglomeration decreased, however, higher chain alcohols led to a wide particle size distribution. The alcohol concentration strongly influenced particle size and morphology, but it was determined that it did not have a large effect on crystallite size. The use of less alcohol led to more defined tetragonal rod-like structures, while higher alcohol concentration gave rounded edges. Much like with acid concentration, the variation in solvent concentration can be advantageous to obtain distinct morphologies for a desired application.

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Chapter 4

4. Autohydration of Cubic ZrW2O8

4.1. Introduction

The autohydration behavior of nanoparticulate cubic ZrW2O8 was originally discovered through powder X-ray diffraction phase analysis. A sample was converted to cubic ZrW2O8, and X-rayed 11 days after heat treatment. When the PXRD pattern was collected, it revealed peak splitting that could be attributed to the presence of two cubic phases with lattice constants of approximately 9.12 and 8.95 Å.36 Several reflections were absent in the smaller cell. The lattice constant of cubic ZrW2O8 has been reported as 9.14

Å, which is similar to the larger cell. The sample was monitored over a period of several months. During this time, the peaks continued to shift to higher angles, and ultimately formed a single cubic phase with a lattice constant of 8.95 Å. Thermogravimetric analysis of this sample showed a weight loss of 2.2% (Figure 4-1 and 4-2). The reduced lattice constant and the absence of certain reflections in the pattern resembled Duan’s report of a

35 partially hydrated ZrW2O8·xH2O phase. To investigate whether the peak shifts were a result of hydration, the sample was heated to 200 ºC to remove any water present in the framework, and XRD data were collected immediately. The pattern showed sharp, single peaks corresponding to the typical lattice constant of unhydrated ZrW2O8. The hydration

49

state of the sample that had been exposed to atmosphere for one year was

ZrW2O8·0.75H2O by PXRD, and ZrW2O8·0.73H2O by TGA.

Figure 4-1: Powder X-ray diffraction patterns of ZrW2O8 (a) as prepared (no hydration), (b) after a few days (partially hydrated), and (c) after one year of expose to atmosphere (close to fully hydrated).

Figure 4-2: TGA plot of partially hydrated ZrW2O8 showing weight loss due to dehydration.

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4.2. Experimental

4.2.1 Preparation of Cubic ZrW2O8 from Nanosized ZrW2O7(OH)2·2H2O

Cubic ZrW2O8 was prepared by heat treating the hydroxide hydrate precursors.

Around 200 mg of each sample were heated from 25 to 600 ºC over a period of 2 h. The temperature was held at 600 ºC for 30 min, after which the samples were removed and quenched in air. All samples showed ~10% mass loss, which corresponds to the evaporation of three water molecules. The samples were immediately scanned on the

PXRD to prevent autohydration. Hydroxide hydrate samples that have been converted to the cubic phase are referred to by the same sample numbering scheme as in chapter 3, but a “C” is added to the name denoting cubic ZrW2O8; e.g., NBA68C is the cubic ZrW2O8 phase formed by heat treatment of hydroxide hydrate NBA68. As the cubic samples discussed in this chapter were obtained at 600 °C, any references to temperature and time refer to synthesis conditions of the hydroxide hydrate precursor unless specifically stated otherwise.

4.2.2. Study of Autohydration Kinetics

Powder X-ray diffraction is a useful method to study hydration as a function of time for nanoparticulate ZrW2O8. Samples were scanned at fixed time intervals in sequence. Peak shifts corresponding to lattice constant changes can be used to determine hydration rates.

For this study, zirconium tungstate samples were prepared and dehydrated in an oven at 200 ºC for 30 min to ensure that all water had been removed from the samples.

51

The sample vials were removed from the oven, and immediately capped to prevent water from entering until scans could be performed. Five minute intervals between samples were chosen for simplicity, and a 2.5 min scan was selected to allow enough time to prepare the next sample. When the first sample was uncapped, this was designated as day

0. After the scan on each sample was complete, it was placed back into the vial but left uncapped. The next sample was uncapped and prepared for a 2.5 min scan; this process was repeated until all samples had been scanned. Each sample’s day 0 was staggered by 5 min, which ensured that each sample was exposed to ambient moisture for equal amounts of time.

The PXRD data were analyzed in JADE 8.0, peak profiles were fitted and the positions were extracted. Each peak position corresponds to a d-spacing based on Bragg’s law as shown in chapter 2.1.1. Using the d-spacing (dhkl) and Miller indices of each peak

(hkl), the lattice constant (a) was calculated based on the formula for a cubic crystal system (Equation 4-1).

a dhkl  (Equation 4-1) h2 k 2 l 2

Lattice constants for each extracted peak position were determined and averages and estimated standard deviation calculated. At least three strong peaks between angles of 16 and 40º 2θ were used for averaging; however, when more peaks were usable, they were included to improve standard deviations whenever possible. This technique is only reliable for samples that hydrated at a homogenous rate. It is important to note that it

52

relies on average peak positions, therefore it is not representative of every particle within the system, but it provides an average lattice constant for the entire sample.

4.3. Results and Discussion

In this section, only selected samples of cubic ZrW2O8 were studied. Many samples in chapter III were prepared under exploratory conditions and are not listed in this section, but all organic solvent prepared samples with particle widths in the nanometer size range showed signs of autohydration. Samples prepared in perchlorate/NaCl mixtures by previous group members had comparable crystallite sizes to the alcohol samples. The perchlorate samples showed slower and less complete hydration even after a six month period, compared to samples prepared in alcohol/HCl after a one month period (Figure 4-3). This suggests that crystallite size is not the only factor affecting hydration behavior. Another possible cause could be that the alcohol/HCl prepared samples may have considerably more defects present due to differences in synthesis. However, samples prepared in the absence of organic solvents show severe agglomeration, thus it is necessary to find synthetic conditions that reduce the number of defects while avoiding agglomeration. Variations in the synthetic conditions of solvothermally prepared samples were explored in chapter 3, and this chapter presents in- depth studies of samples selected to encompass a wide range of hydration rates and distinct synthesis conditions. The goal of the studies was to correlate reaction conditions and the resulting particle morphology and crystallinity with hydration behavior.

53

Figure 4-3: PXRD scans of cubic ZrW2O8 prepared in (a) alcohol/HCl after one month of atmosphere exposure, and (b) HClO4/NaCl after six months of atmosphere exposure.

4.3.1 Effect of Temperature

The temperature used for preparing the hydroxide hydrate was found to significantly influence crystallite size and particle agglomeration. Several samples prepared under identical conditions except for temperature were converted to the cubic phase and analyzed by PXRD for peak shifts to correlate temperature variation with autohydration.

Samples NBA83 and NBA81 were prepared at 130 ºC and 210 ºC, respectively.

The corresponding cubic samples, NBA83C and NBA81C, were scanned after seven days of autohydration (Figure 4-4). A significant difference in hydration rates was observed. Notably, the peak shapes were very different. The sample prepared at 130 ºC hydrated more rapidly with significant peak broadening, while the sample prepared at

210 ºC retained a sharper peak shape and only showed tailing at higher angles. Recalling

54

from chapter 3, these samples had significant differences in crystallite sizes of 17 ± 2.4 nm (NBA83) and 60 ± 2.9 nm (NBA81) in the hydroxide hydrate phase, their cubic phase crystallite sizes were 28 ± 3.7 nm (NBA83C) and 49 ± 2.4 nm (NBA81C). This suggests that smaller particles hydrate more rapidly.

Figure 4-4: PXRD patterns of samples prepared at (blue) 130 ºC and (red) 210 ºC after 7 d of autohydration.

Samples NBA68C, NBA70C, and NBA147C were obtained by heat treatment of their precursors, which were prepared at 130 ºC, 170 ºC, and 210 ºC, respectively. X-ray diffraction patterns were collected after seven days of ambient autohydration (Figure

4-5). Significant peak shifts and broadening, accompanied by losses in peak intensity, were observed for the sample originally prepared at 130 ºC. Notably, the sample prepared at 210 ºC showed considerably less hydration with minimal loss in intensities. The sample prepared at 170 ºC was a clear intermediate between the other two preparation temperatures. Their differences in crystallite sizes were 26 ± 2.0 nm (NBA68C), 35 ± 1.4 nm (NBA70C), and 55 ± 2.6 nm (NBA147C) in the hydroxide hydrate phase and 30 ±

3.0 nm (NBA69C), 38 ± 3.4 nm (NBA71C), and 46 ± 2.8 nm (NBA74C) in the cubic

55

phase. The crystallite size for the 170 and 210 °C samples were identical for the hydroxide hydrate and cubic phases, while the 130 °C sample, NBA69C, increased in crystallite size by about 50%.

Figure 4-5: PXRD scans of cubic ZrW2O8 samples from precursors at (red) 130 ºC, (blue) 170 ºC, and (black) 210 ºC with 0.5 mL 1-propanol after 7 d autohydration.

The three samples were monitored over a period of time to gain a better understanding of their hydration behavior. Autohydration after 22 d revealed significant hydration for the samples prepared at 130 ºC and 170 ºC (Figure 4-6). The sample prepared at 210 ºC showed continued hydration as well, but the peak shape differed from the other two, with peak tailing to high angles and intensity still present for the (111) reflection. Samples prepared at 130 ºC and 170 ºC showed a significant loss in (111) peak intensity, their peaks broadened, and shifted to a higher angle. The sample prepared at

130 ºC had slightly narrower peaks in comparison to the sample prepared at 170 ºC, but both samples had much larger peak shifts than the sample prepared at 210 ºC.

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Figure 4-6: PXRD scan after 22 d of autohydration for cubic ZrW2O8 samples prepared in 0.5 mL 1-propanol and heated at (red) 130 ºC, (blue) 170 ºC, and (black) 210 ºC.

The sample prepared at 130 ºC had an average lattice constant of 8.993 ± 0.005 Å after 22 d of hydration. Based on the lattice constant reported by Duan for fully hydrated

35 ZrW2O8·1H2O, linear interpolation was performed, and a composition of ZrW2O8·(0.49

± 0.02)H2O was calculated. The sample prepared at 170 ºC had an average lattice constant of 9.024 ± 0.003 Å after 22 d of hydration. This corresponded to an estimated formula of ZrW2O8·(0.39 ± 0.01) H2O. The sample prepared at 210 ºC had an average lattice constant of 9.11 ± 0.007 Å, which corresponded to a composition of ZrW2O8·(0.10

± 0.02) H2O. It should be noted that fitting of peaks in Jade with an asymmetry variable will give lattice constants that are dominated by the peak maxima, which explains the very small change for the 210 °C sample.

The hydration rate for the samples prepared at 130 ºC and 170 ºC was obtained from lattice constant data from PXRD scans over a period of 22 d (Table 4-1 and 4-2;

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Figure 4-7). Linear trends were observed in the plots of lattice constant average as a function of autohydration time. Clear outliers were excluded from the fits.

For the sample prepared at 130 ºC, two hydration rates were determined from the change in slope in the plot of NBA68C (Figure 4-7). Linear trends were determined and the sample had a hydration rate of (0.044 ± 0.004)H2O/day for times up to nine days.

Then a slower hydration rate of (0.013 ± 0.001)H2O/day was determined for days 9 to 22.

The combined rates for their respective time period gave a composition of ZrW2O8·(0.526

± 0.03) H2O after 22 d hydration.

For the sample prepared at 170 ºC, days 1 and 3 were outliers and not included. A linear trend was observed for the 22 d time period (Figure 4-7). The hydration rate was determined to be (0.018 ± 0.02)H2O per day. This rate over the time period of 22 d gave a composition of ZrW2O8·(0.396 ± 0.02) H2O after 22 d hydration.

Table 4-1: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared at 130 ºC (NBA68C). Time (d) Lattice constant (Å) ESD (Å) 0 9.155 0.003 1 9.121 0.004 3 9.105 0.004 5 9.069 0.007 7 9.048 0.002 9 9.031 0.007 11 9.029 0.005 13 9.021 0.006 15 9.017 0.005 22 8.993 0.005

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Table 4-2: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared at 170 ºC (NBA70C). Time (d) Lattice constant (Å) ESD (Å) 0 9.144 0.001 1 9.145 0.001 3 9.138 0.003 5 9.107 0.002 7 9.098 0.006 9 9.082 0.004 11 9.069 0.007 13 9.055 0.007 15 9.052 0.003 22 9.024 0.008

9.16 9.14 9.12 9.1 9.08 9.06 9.04 9.02

Lattice constant (Å) Latticeconstant 9 8.98 8.96 0 5 10 15 20 25 2 theta (degrees)

Figure 4-7: Lattice constant values for a 130 ºC sample (♦) and a 170 ºC (■) sample after different periods of autohydration. The error bars included represent three estimated standard deviations.

Based on the data collected for samples prepared at different temperatures, it is evident that increased heating beyond a threshold temperature (>170 ºC) during the preparation of the hydroxide hydrate precursor had a significant effect on ZrW2O8 hydration rates. For low temperature samples, crystallite size changed during conversion to the cubic phase, most notably with samples prepared at 130 ºC, which have a 50% increase in size. The samples prepared at lower temperatures showed broad peaks and

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shifts in the peak maximum, however they were more similar to one another than the sample prepared at 210 ºC. The peak shape suggests that all particles in the sample were hydrating, with some distribution in rates, resulting in broad, symmetric peaks and a continuous shift in peak maxima in the PXRD scans. While hydration was slowed for the sample prepared at 210 ºC, it was not eliminated. The tailing peak shape of this sample suggests that a small portion of the sample hydrates rapidly, while the majority of particles show very slow hydration. Electron microscopy on the 210 ºC sample revealed a narrow particle width distribution of around 50 nm, and a length distribution from 100 to

300 nm. The sample prepared at 170 ºC had a broader width distribution. Most particles were 20 to 40 nm wide, while some particles were up to 50 nm wide. A similar length distribution to the 210 ºC sample was observed, as well as comparable agglomeration levels. The sample that hydrated at the fastest rate was observed as highly agglomerates

50 to 100 nm wide and 300 to 800 nm long. Individual particles were 20 to 30 nm wide but lengths could not be determined because agglomeration. A strong dependence on length is unlikely because samples prepared at 170 and 210 ºC were observed to have similar particle lengths, yet hydrated at different rates. These data suggest that the hydration is likely to be dependent on the crystallite size and particle widths.

Increased temperatures during the preparation of the hydroxide hydrate precursor lead to improved crystallinity with defined faces. Electron microscopy suggests that the lower temperature samples may contain more defects. This would allow for more rapid water absorption if defects contribute to facile hydration. Micron sized ZrW2O8 particles with low defect concentrations do not react with atmospheric moisture at all, while autohydration has been reported for micron sized mixed cation ZrW2-xMoxO8

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compositions, which are likely to contain more defects than pure ZrW2O8 or ZrMo2O8.

The slower hydration of the 210 ºC sample, which shows well defined crystallites in the electron microscope, suggests that the presence of a smaller number of defects slowed down the autohydration. Defect concentration could be further reduced with changes in other synthesis variables or heating to higher temperatures, although the stability of the solvent at temperatures of 240 ºC and above limits the temperatures suitable for synthesis.

4.3.2 Effect of Heating Time

Heating time had different effects on the hydroxide hydrate samples depending on other synthesis conditions. Generally, it increased crystallinity as well as crystallite size estimates, and vastly changed particle size distribution through Ostwald ripening. The effects were largest at high temperature, thus samples prepared at 210 ºC were analyzed in this section.

Samples NBA81C and NBA79C were converted from precursors that were heated at 210 ºC for 24 h and 72 h, respectively. Samples were allowed to autohydrate, and scanned by PXRD seven days later. The sample heated for 24 h showed a slight intensity loss, therefore the data were scaled to facilitate comparison of the peak shifts. The peaks mostly overlapped, however, there was slightly more hydration for the sample heated for

24 h. The samples were scanned again after 29 d of autohydration, when significant peak tailing was observed. It is not possible to determine a meaningful hydration rate for these samples, because their hydration process is highly heterogeneous. Comparing each hydrated sample with their 0 day scan, it is clear that the samples are hydrating, but at a very slow rate (Figure 4-8 and 4-9).

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Figure 4-8: PXRD scans of sample NBA81C after (red) 0 d and (blue) 29 d of exposure to atmosphere.

Figure 4-9: PXRD scans of sample NBA79C after (red) 0 d and (blue) 22 d of exposure to atmosphere.

There is little change in the position of the peak maxima, indicating that a large portion of these samples shows very limited hydration. However, the significant increase in intensity at higher angles is evidence that part of the samples hydrates at varying rates.

The sample prepared from the precursor heated for 24 h showed more hydration. It is

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important to note that the crystallite sizes of the hydroxide hydrate precursors of these samples differed by around 10 nm. The crystallize size averages were 70 ± 5.3 nm

(NBA79) and 60 ± 2.9 nm (NBA81) and this was explained by Ostwald ripening in chapter III-4.2. The estimated crystallite sizes for the cubic phase were 52 ± 2.6 nm

(NBA79C) and 49 ± 2.4 nm (NBA81C). The particle size distribution for the sample heated for 24 h was very broad. The majority of widths were 40 to 80 nm and lengths ranged from 500 to 1200 nm. The sample heated for 72 h had a higher crystallite size and a more narrow size distribution. Particle sizes were generally 80 to 100 nm wide and

1000 nm long. The heterogeneous hydration for the samples with broad particle size distribution suggests that hydration rates could be dependent particle width, and length.

The fact that both samples contain significant portions that show limited hydration suggests that there could be a cut-off in particle width or length above which hydration is very slow, as both samples contain larger particles of similar length and width. This is supported by the observation that the sample with smaller particles showed more hydration.

4.3.3 Effect of Acid Concentration

Acid concentration was observed to have a strong effect on crystallization rates in the hydroxide hydrate samples. It also changed the particle morphology; less acid resulted in a decrease in particle length. Hydration rates should differ if particle length has an effect on hydration.

Samples NBA82C, NBA72C, and NBA85C were all obtained from their respective hydroxide hydrate precursors by heating at 600 ºC for 30 min. The samples were scanned 29 d after exposure to atmosphere (Figure 4-10). The scan revealed that

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there was no difference in hydration rates between the three samples. All samples had asymmetric peak shapes with significant tailing present.

Figure 4-10: PXRD scans of samples prepared in various acid concentrations resulting in identical hydration behavior.

Since there was no variation in hydration rate for samples prepared with different acid concentrations, it can be concluded that the differences in crystallization kinetics did not influence the stability of the cubic ZrW2O8 particles with respect to hydration. These samples all showed a similar hydration rate, with peak tailing and slow shifts in the peak maxima. All three samples had similar crystallite sizes (60 ± 3.0 nm, 64 ± 4.1 nm, and 62

± 5.4 nm) in the hydroxide hydrate phase and the cubic phase (52 ± 3.5 nm for NBA82C,

55 ± 5.1 nm for NBA72C, and 54 ± 4.2 nm for NBA85C, respectively). The particle widths for each sample were similar (~60 to 100 nm). Since the particle lengths differed

(100 to 300 nm, 400 to 600 nm, and 700 to 1000 nm) due to the different acid concentrations, hydration appears to show limited dependence on particle length. It can be concluded that particles with similar width distributions will hydrate similarly, as the majority of the surface area is comprised of the (hk0) faces. As found for the time study

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in 4.2.2, it appears that samples with larger particle widths and crystallite sizes show limited hydration.

4.3.4 Effect of Solvent Concentration

Solvent concentration was shown to have an effect on crystallization rates, and also resulted in significant differences in particle size and morphology as seen in chapter

3.

Samples NBA68C and NBA69C were obtained from their respective hydroxide hydrate precursors, which were prepared at 130 ºC with 0.5 and 2.5 mL of 1-propanol.

Both hydroxide hydrate samples were fully crystalline with nearly equivalent crystallite sizes of 25.9 ± 2.0 nm (NBA68) and 21 ± 2.4 nm (NBA69), in the cubic phase their sizes were 33 ± 4.2 nm (NBA68C) and 30 ± 3.1 nm (NBA69C). Their particle sizes differed only in length (400 to 500 nm, and 100 to 300 nm respectively).

The samples were analyzed after 7 d of autohydration, and revealed no significant difference in hydration rates. After 22 d of autohydration, a slight difference in peak shapes was apparent. The sample prepared in 0.5 mL alcohol had slightly broader peaks compared to the sample prepared in 2.5 mL alcohol however, their average lattice constant for 22 d of hydration was identical within estimated standard deviations.

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Figure 4-11: PXRD scan after 22 d of hydration for samples prepared in (a) 0.5 mL 1-propanol and (b) 2.5 mL 1-propanol.

Both samples showed homogenous hydration; therefore it was possible to plot the lattice constant as a function of hydration time (Table 4-1 and 4-3; Figure 4-12). Overall, there was a minor difference in hydration rate for these samples; however, when considering 3 estimated standard deviations as included in the plot, the difference is within error. The hydration rate for NBA68C, previously reported, resulted in a composition of ZrW2O8·(0.53 ± 0.03)H2O after 22 d. The calculated hydration rates for

NBA69C were (0.037 ± 0.003)H2O/day from 0 to 5 d, and (0.013 ± 0.001)H2O/day from

5 to 22 d. The combined rates for their respective time period gave a composition of

ZrW2O8·(0.41 ± 0.02) H2O after 22 d hydration. A composition of ZrW2O8·(0.41 ±

0.01)H2O was calculated from the estimated lattice constant after 22 d of autohydration.

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Table 4-3: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared in 2.5 mL 1-propanol at 130 ºC (NBA69C). Time (d) Lattice constant (Å) ESD (Å) 0 9.135 0.001 1 9.130 0.002 3 9.101 0.004 5 9.083 0.003 7 9.073 0.004 9 9.064 0.004 11 9.057 0.005 13 9.043 0.005 15 9.038 0.002 22 9.018 0.003

9.16 9.14 9.12 9.1 9.08 9.06 9.04 9.02

Latticeconstant (Å) 9 8.98 8.96 0 5 10 15 20 25 2 theta (degrees)

Figure 4-12: Lattice constants for a sample prepared at 130 ºC in (■) 2.5 mL of alcohol and (♦) 0.5 mL of alcohol sample as a function of hydration time. The error bars represent 3 estimated standard deviations.

Analysis of two samples prepared at 170 ºC showed similarly reproducible results when comparing these two solvent concentrations. The sample prepared in 0.5 mL of 1- propanol initially showed slower hydration; however, after a few days the hydration rate increased, and eventually of the hydration state followed the same trend as for the sample

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prepared in 2.5 mL of 1-propanol. The hydration rate for NBA70C, previously reported, resulted in a hydration state of ZrW2O8·(0.40 ± 0.02)H2O after 22 d of autohydration.

Sample NBA71C had a rate of hydration of (0.026 ± .002)H2O/day from 0 to 11 d (Table

IV-4). The second rate of hydration was (0.008 ± 0.002)H2O/day from 11 to 22 d. The combined rates for their respective time period gave a final composition of ZrW2O8·(0.38

± 0.02)H2O (Figure 4-13). A composition of ZrW2O8·(0.39 ± 0.02)H2O was calculated from the estimated lattice constant after 22 d of autohydration.

When comparing subsets of samples prepared with only one variable changed, it can be concluded that solvent concentration has no significant effect on hydration within three estimated standard deviations. Notably, any minor differences during hydration studies are insignificant in comparison to the lattice constant differences observed in the day 0 scans. Synthesis optimizations based on changes in solvent concentration should not be considered for attempts to reduce hydration, but allow for optimization of particle size and agglomeration for given applications without deteriorating stability to atmospheric moisture.

Table 4-4: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared in 2.5 mL 1-propanol at 170 ºC. (NBA71C) Time (d) Lattice constant (Å) ESD (Å) 0 9.130 0.002 1 9.131 0.002 3 9.117 0.001 5 9.102 0.003 7 9.087 0.004 9 9.065 0.004 11 9.049 0.004 13 9.044 0.004 15 9.040 0.005 22 9.022 0.007

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9.16 9.14 9.12 9.1 9.08 9.06 9.04 9.02 Lattice constant (Å) constant Lattice 9 8.98 8.96 0 5 10 15 20 25 2 theta (degrees)

Figure 4-13: Lattice constants for a sample prepared at 170 ºC in (■) 2.5 mL of alcohol and (♦) 0.5 mL of alcohol sample as a function of hydration time. The error bars represent 3 estimated standard deviations.

Samples prepared under identical conditions were compared to get a more realistic error estimate for the PXRD analysis. For the samples prepared at 130 ºC, all data points matched within three estimated standard deviation except for day 0. The hydration rate of NBA68C was previously established and a composition of

ZrW2O8·(0.53 ± 0.03)H2O after 22 d of autohydration was observed (Table 4-1). The plot of a reproduced sample, NBA149C had two linear trends that were observed (Table 4-5 and Figure 4-14). From days 0 to 5 a hydration rate of (0.047 ± 0.003)H2O/day was determined. From days 5 to 22 a hydration rate of (0.013 ± 0.002)H2O/day was determined. A composition of ZrW2O8·(0.46 ± 0.02)H2O was calculated for 22 d of autohydration. A composition of ZrW2O8·(0.49 ± 0.03)H2O was calculated from the estimated lattice constant 22 d of autohydration. Crystallite sizes for these samples were

26 ± 2.0 nm (NBA68) and 24 ± 2.1 nm (NBA149) in the hydroxide hydrate phase, in the cubic phase sizes were 32 ± 4.2 nm (NBA68C) and 33 ± 1.8 nm (NBA149C).

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For samples prepared at 170 ºC, all data points were within 3 estimated standard deviations. Rates were determined from the slope of the lines. NBA70C was previously reported with a composition of ZrW2O8·(0.40 ± 0.02)H2O after 22 d of autohydration(Table IV-2). Similarly prepared sample NBA148C had a hydration rate of

(0.016 ± 0.001)H2O/day over the period of 22 d, which corresponded to a final composition of ZrW2O8·(0.36 ± 0.03)H2O (Table 4-6 and Figure 4-15). Crystallite sizes for these samples were 35 ± 1.4 nm (NBA70) and 39 ± 2.5 nm (NBA148) in the hydroxide hydrate phase, and 43 ± 1.0 nm (NBA70C) and 38 ± 2.8 nm (NBA148C) in the cubic phase.

This autohydration data supports the reproducibility of these synthesis procedures previously reported for crystallite size estimates, and the validity of the hydration rate studies by PXRD analysis.

Table 4-5: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared in 0.5 mL 1-propanol at 130 ºC (NBA149C). Time (d) Lattice constant Ǻ ESD 0 9.131 0.004 1 9.122 0.004 3 9.094 0.002 5 9.061 0.002 7 9.047 0.005 9 9.035 0.004 11 9.023 0.004 13 9.018 0.003 15 9.004 0.007 22 8.994 0.009

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9.16 9.14 9.12 9.1 9.08 9.06 9.04 9.02

Lattice constant (Å) Latticeconstant 9 8.98 8.96 0 5 10 15 20 25 2 theta (degrees)

Figure 4-14: Lattice constants for two samples prepared at 130 ºC under identical conditions as a function of hydration time. The error bars represent 3 estimated standard deviations.

Table 4-6: Hydration of cubic ZrW2O8 obtained from a hydroxide hydrate precursor prepared in 0.5 mL 1-propanol at 170 ºC (NBA148C). Time (d) Lattice constant Ǻ ESD 0 9.135 0.002 1 9.136 0.002 3 9.114 0.004 5 9.111 0.006 7 9.102 0.008 9 9.088 0.002 11 9.076 0.006 13 9.058 0.005 15 9.056 0.007 22 9.038 0.005

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9.16 9.14 9.12 9.1 9.08 9.06 9.04 9.02

Lattice constant (Å) Latticeconstant 9 8.98 8.96 0 5 10 15 20 25 2 theta (degrees)

Figure 4-15: Lattice constants for two samples prepared at 170 ºC under identical conditions as a function of hydration time. The error bars represent 3 estimated standard deviations.

4.3.5 Effect of Solvent Type

Solvent types were shown to have a large influence on particle size, morphology, crystallite size and crystallization. This makes effects of solvent types on hydration more difficult to determine, and direct comparisons could be misleading. Diethyl ether was used to determine if hydration could be reduced with a non-alcohol solvent. The scan after 20 d of autohydration revealed significant hydration with flattened peak maxima, indicating the presence of particles with varying hydration states (Figure 4-16).

The estimated crystallite size for this sample was 41 ± 5.0 nm in the hydroxide hydrate phase and 39 ± 2.3 nm in the cubic phase. The particle size distribution was broad, ranging from 20 to 80 nm in width and 100 to 500 nm in length. The particles showed no significant agglomeration and were observed as individual rod (Figure 4-17).

The broad particle size distribution suggests miscibility problems had occurred during synthesis. The very broad X-ray peaks after 20 d of hydration agree with the very wide

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particle width distribution, corroborating that particle width is strongly correlated with hydration behavior.

Figure 4-16: PXRD scan for samples prepared 2.5 mL of diethyl ether at 220 ºC for 1.5 h after (a) 0 d hydration and (b) 20 d of hydration.

Figure 4-17: TEM image of hydroxide hydrate prepared in diethyl ether.

Other organic solvents will work in the standard synthesis method; however hydration cannot be easily related to solvent type. Synthetic optimizations should most

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likely be performed in the future to obtain the most homogenous size distribution possible. Miscibility issues with higher chain alcohols and many organic solvents limit their suitability in the synthesis.

4.4. Conclusions

Powder X-ray diffraction was a useful characterization method to determine rates of hydration for samples prepared under different conditions. Of all the synthesis variables tested for effects on hydration behavior, temperature had the most significant impact on hydration rate. There appears to be a threshold temperature (above 170 ºC) above which, hydration is significantly slower. Differences in hydration rates were related to crystallite size estimates. Smaller crystallites (20 to 35 nm) hydrated more rapidly than samples with larger crystallites (50 to 80 nm). Samples with large crystallite size still showed slow hydration, which will eventually render the NTE material useless unless dehydrated at periodic intervals. Samples heated at the same temperature for different amounts of time showed that the resulting particle size distribution also affects hydration behavior. Narrow size distributions lead to homogeneous hydration, while wide size distributions give a variety of hydration states. Very large particles show almost negligible hydration over periods of 3 to 4 weeks.

Agglomeration showed little effect on hydration rate, suggesting that particle widths and the corresponding crystallite size are the most important factor in determining hydration rates. However, samples prepared in the absence of alcohols showed much slower hydration than alcohol/HCl samples of similar particle and crystallite size. This suggests that defects influence hydration, and that a balance between size, agglomeration, and defect concentration must be found for optimized synthesis conditions.

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Chapter 5

5. Causes of ZrW2O8 Autohydration

5.1. Introduction

Autohydration occurred in all nanoparticle samples prepared with organic solvent/HCl mixtures. Temperature of hydroxide hydrate preparation had a large effect on hydration, which suggested a correlation with crystallite size. However, the observation of much slower hydration in samples with similar crystallite sizes, but prepared in NaCl/perchloric acid mixtures, suggested that defects play an important role.

Defects in the precursor can persist during conversion to the cubic phase. To investigate the defects present, high magnification STEM imaging was used to analyze the structure of the particles. Surface area analysis was performed by BET to determine the relationship between surface area and hydration rate.

5.2. STEM Results

Valuable information is likely to come from comparison of samples that showed differences in hydration rates. For sample NBA69C, prepared at 130 ºC, scanning transmission electron microscopy analysis revealed that large deformations occurred during conversion to cubic ZrW2O8 for samples prepared at 130 ºC. Agglomerated but

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distinct rods were present in the hydroxide hydrate sample (Figure 5-1a). In contrast, the cubic phase no longer contained distinguishable rods, instead, globules were observed.

One region of defects was visible as dark lines in the particle (circled in Figure 5-1b).

Figure 5-1: TEM images comparing morphologies in the (a) hydroxide hydrate and (b) cubic phase.

Similar observations were made for sample NBA68C, which was also prepared at

130 ºC. Clear separation of the rods within the agglomerates was lost, the particles in each agglomerate appeared to have fused together as one unit.

Closer analysis of this sample at high magnification detected localized crystalline regions within the particle, as well as an amorphous component (Figure 5-2). There was no correlation in the orientation of the observed lattice fringes from different areas of the same particle.

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Figure 5-2: TEM image of cubic zirconium tungstate converted from a hydroxide hydrate precursor prepared at 130 ºC (1) Highly crystalline tip, (2) amorphous region and (3) discontinuous crystalline region.

Samples NBA70C and NBA71C were prepared at 170 ºC and showed a slightly slower hydration rate in comparison to samples prepared at 130 ºC. Electron microscopy revealed a minor degree of particle fusing, but the majority of the particles were clearly distinguishable as rods (Figure 5-3).

Figure 5-3: Cubic zirconium tungstate converted from a hydroxide hydrate precursor prepared at 170 ºC viewed with (a) SEM and (b) TEM.

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Higher magnification images of NBA70C revealed improved crystallinity when compared to the sample prepared at 130 ºC. However, this sample showed changes in crystal lattice orientation as well. Visible lattice fringes at various angles to one another suggest that the particle is polycrystalline, and the gradual decrease in overall intensity from the edge to the center gives evidence that this particle is round in shape (Figure

5-4).

Figure 5-4: TEM image of cubic zirconium tungstate converted from a hydroxide hydrate precursor prepared at 170 ºC showing polycrystalline structure.

Further analysis of the same sample at higher magnification showed regions of crystallinity as well as amorphous regions. A circular crystalline area was visible suggesting a seed crystal had formed and did not grow in similar orientation as another region. Two non-oriented crystalline regions next to one another are likely to result in grain boundary defects (Figure 5-5).

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Figure 5-5: TEM image of cubic zirconium tungstate converted from a hydroxide hydrate precursor prepared at 170 ºC showing discontinuities in crystal lattice direction.

Sample NBA72C was prepared at 210 ºC and showed reduced hydration in comparison to samples prepared at lower temperatures. The sample was observed as rod- like particles with facets on the tips. A clean conversion from hydroxide hydrate to cubic phase with no detectable changes in particle morphology was observed as well (Figure

5-6)

Figure 5-6: SEM images of (a) hydroxide hydrate and (b) cubic ZrW2O8, showing no changes in particle morphology.

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Higher magnification revealed good crystallinity within the particle. The particles grew as single crystals, as opposed to polycrystalline particles as observed with samples prepared at 130 and 170 ºC. An amorphous surface region was observed, as well as defects near the edge of the particle (Figure 5-7 and 5-8).

Figure 5-7: TEM image of crystal lattice fringes in NBA72C converted from a hydroxide hydrate precursor prepared at 210 ºC.

Figure 5-8: TEM images of NBA72C converted from a hydroxide hydrate precursor prepared at 210 ºC showing minor crystal defects.

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Another sample (NBA85C) originally prepared at 210 ºC showed improved crystallinity, existing as a single crystal with no noticeable defects present. As with all samples prepared at 210 ºC, this sample showed a significantly reduced hydration rate.

Scanning transmission electron microscopy proved to be a highly beneficial characterization tool for samples originally prepared at 130, 170 and 210 ºC. The similarity in hydration rates for the samples prepared at 130 ºC and 170 ºC, in chapter 4, suggested that a threshold temperature was required for heating in order to significantly reduce the rate of hydration. Electron microscopy confirmed this hypothesis, revealing changes in crystal lattice orientation within particles for samples prepared at 130 and 170

ºC. These samples appear to be polycrystalline as opposed to the single crystals observed for the sample prepared at 210 ºC. Because the samples contain polycrystalline particles, crystallite size estimates for samples prepared at lower temperatures may not always be an accurate representation for particle width as previously thought. Slow hydration still occurred for the sample prepared at 210 ºC, and some visible defects were present. It is possible that the defect concentration and resulting hydration can be further reduced through synthetic conditions.

5.3. Formation of ZrW2O8 at Higher Temperatures

Numerous samples prepared at 130 ºC showed particle fusing and loss of distinct crystal facets after heat treatments. This behavior was less pronounced in a sample prepared at 170 ºC, and even less detectable in samples prepared at 210 ºC. Temperature can obviously influence the number of defects present and thus the stability of the crystal morphology. This raises the question whether the temperature chosen to convert the

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hydroxide hydrate to the cubic phase could also affect the formation or annealing of defects and thus the particle morphology and hydration rate.

Several hydroxide hydrate samples that were studied as cubic ZrW2O8 in chapter

IV were converted to the cubic phase with a higher heat treatment temperature. Around

150 mg of each sample were weighed out and heated in a furnace to 650 ºC over 2 h. The samples were held at this temperature for another 30 min and quenched in air. All samples were successfully converted to the cubic phase with this method, although a small amount of tungsten oxide was detected in two samples. These samples were allowed to autohydrate for seven days to compare results with the corresponding cubic phase samples heat treated at 600 ºC.

Crystallite size can change during conversion to the cubic phase, and is likely to depend on conversion temperature. Table 5-1 summarized the crystallite size estimates of the hydroxide hydrate, 600 °C cubic phases, and 650 °C cubic phases. Sample NBA69

(130ºC) converted at 600 ºC gave a crystallite size estimate for the cubic phase of 30 ±

1.8 nm. This was a noticeable increase from the hydroxide hydrate (23 ± 1.9 nm), however, research by previous group members found that a size increase of this magnitude upon conversion to the cubic phase was normal. The sample converted at 650

ºC had a crystallite size of 44 ± 2.2 nm, a significant increase from the original crystallite size.

The two cubic samples converted at 600 and 650 °C showed a significant difference in hydration rates after 7 d (Figure 5-9). Broad peaks with significant shifts in the maxima were observed for the sample converted at 600 ºC. In contrast, the sample

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converted at 650 ºC showed minor peak tailing with peak maxima close to the original peak positions for unhydrated material.

Figure 5-9: PXRD patterns after 7 d of autohydration of cubic ZrW2O8 obtained from heat treatment of a 130º C synthesized hydroxide hydrate precursor after conversion at (red) 600 ºC for 30 min and (blue) 650 ºC for 30 min.

Electron microscopy revealed that samples prepared at 130 ºC and heat treated at

650 ºC still formed similar sized fused globules leaving no indication that the sample was once composed of rod-like particles (Figure 5-10). Within the globules, the sample converted at 600 ºC had visible contrast lines down the c-axis that indicate a rod was once present.

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Figure 5-10: TEM image of cubic ZrW2O8 from a hydroxide hydrate precursor prepared at 130 ºC and converted at (a) 600 ºC and (b) 650 ºC.

Higher magnification revealed the presence of distinct nuclei that could act as seed crystals within the sample, about 30-50 nm in diameter, which is in agreement with the large increase in crystallite size (Figure 5-11).

Figure 5-11: High magnification TEM image of cubic ZrW2O8 from a hydroxide hydrate precursor prepared at 130 ºC and converted at 650 ºC.

Comparing other 130 ºC samples converted to the cubic phase, an interesting trend was observed when comparing the crystallite size increases as a function of conversion temperature for the different cubic materials (Table 5-1). Samples originally

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prepared at 130 ºC (NBA44, NBA69, NBA149) all showed an increase in crystallite size of ~50% when converting to the cubic phase at 600 ºC, however, when converting to the cubic phase at 650 ºC the same samples had an increase in crystallite size of ~100%.

Table 5-1: Crystallite sizes for 130 ºC hydroxide hydrate precursors and the corresponding cubic samples converted at 600 and 650 ºC. HH Sample Crystallite 600 ºC Crystallite 650 ºC Crystallite Size (nm) sample Size (nm) sample Size (nm) NBA44 20 ± 2.3 NBA44C 29 ± 1.4 NBA44C 37 ± 2.8 NBA69 21 ± 2.4 NBA69C 30 ± 3.2 NBA69C 44 ± 2.2 NBA149 23 ± 2.1 NBA149C 33 ± 1.8 NBA149C 47 ± 2.5

Samples originally prepared at 170 ºC (NBA71, NBA148) showed no increase in size when converting to the cubic phase at 600 ºC, however, when converting to the cubic phase at 650 ºC there was a 20 to 25% increase in size (Table 5-2).

Table 5-2: Crystallite sizes for 170ºC hydroxide hydrate precursors and the corresponding cubic samples converted at 600 and 650 ºC. HH Sample Crystallite 600 ºC Crystallite 650 ºC Crystallite Size (nm) sample Size (nm) sample Size (nm) NBA71 34 ± 2.3 NBA71C 38 ± 3.4 NBA71C 45 ± 3.2 NBA148 39 ± 2.5 NBA148C 38 ± 2.9 NBA148C 49 ± 3.9

Sample NBA71 was originally prepared at 170 ºC, and the two cubic phase samples heat treated showed differences in hydration rates after 7 d. The crystallite sizes for these heat treatments were 38 ± 3.4 (600 ºC) and 45 ± 3.2 (650 ºC). Electron microscopy revealed that these particles showed some signs of fusing, however many particles were observable as rods 30 to 60 nm in width and 100-300 nm in length (Figure

5-13).

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Figure 5-12: PXRD patterns after 7 d of autohydration of cubic ZrW2O8 that was obtained from heat treatment of a 170º C synthesized hydroxide hydrate at (red) 600 ºC for 30 min and (blue) 650 ºC for 30 min.

Figure 5-13: TEM image of cubic ZrW2O8 sample from hydroxide hydrate precursor prepared at 170 °C and converted at 650 ºC.

Samples prepared at 210 ºC (NBA74, NBA79, NBA81, NBA82, NBA85,

NBA147) had no significant size change when converting to the cubic phase at 650 °C

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within estimated standard deviations (Table V-3). However, for several samples, a slight reduction in size was observed when the cubic phase was crystallized at 600 °C.

Table 5-3: Crystallite sizes for 210 ºC hydroxide hydrate precursors and the corresponding cubic samples converted at 600 and 650 ºC. HH Crystallite 600 ºC Crystallite Size 650 ºC sample Crystallite Size Sample Size (nm) sample (nm) (nm) NBA74 49 ± 4.3 NBA74C 44 ± 3.6 NBA74C 52 ± 3.9 NBA79 70 ± 5.3 NBA79C 56 ± 4.6 NBA79C 63 ± 4.7 NBA81 60 ± 2.9 NBA81C 49 ± 2.4 NBA81C 60 ± 4.0 NBA82 60 ± 3.0 NBA82C 55 ± 1.0 NBA82C 63 ± 5.5 NBA85 62 ± 5.4 NBA85C 54 ± 4.0 NBA85C 59 ± 1.5 NBA147 53 ± 4.2 NBA147C 42 ± 2.0 NBA147C 46 ± 2.0

Sample NBA79C (650 ºC heat treatment), obtained from a hydroxide hydrate precursor prepared at 210 ºC, showed no difference in hydration rate when compared to the cubic sample converted at 600 ºC (Figure 5-14). Electron microscopy revealed particles as well defined rods (Figure 5-15). Similar morphology was also observed for the hydroxide hydrate and the cubic phase heat treated at 600 ºC. The crystallite sizes for this sample were 56 ± 4.6 nm (600 ºC) and 63 ± 4.7 nm (650 ºC).

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Figure 5-14: PXRD patterns after 7 d of autohydration of cubic ZrW2O8 that was obtained from heat treatment of a 130º C synthesized hydroxide hydrate at (red) 650 ºC for 30 min and (blue) 600 ºC for 30 min.

Figure 5-15: TEM image of cubic ZrW2O8 converted at 650 ºC from a hydroxide hydrate precursor prepared at 210 °C.

Data from the crystallite size increases and the visible fusing in TEM images suggest that a threshold temperature exists for preparing high quality hydroxide hydrate samples solvothermally in HCl. Hydroxide hydrate formation at temperatures below this

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minimum temperature results in particles with incomplete growth, made up of polycrystalline material. The threshold temperature is between 170 and 210 ºC as seen from the appearance of single crystal particles in the 210 °C sample converted at 600 ºC, and the minimal size change for both heat treatment temperatures. Grain boundary defects are likely present in samples solvothermally prepared below this temperature as shown by TEM, facilitating hydration. The size of the crystallites also determines hydration rates, while the defects determine accessibility of particles to water. Further synthesis optimizations for usable cubic ZrW2O8 would benefit most from preparation temperatures of ~210 ºC and above, or other approaches that result in single crystalline particles with low defect concentrations.

5.4. BET Surface Area Analysis

Surface area analysis was carried out on selected samples to investigate whether there is a relationship between surface area and hydration rates. Higher surface areas result in more sites for water to enter the framework, rapidly changing its composition.

Samples from precursors prepared at different temperatures were chosen, as temperature for hydroxide hydrate preparation showed the most significant impact on hydration, particle size, and morphology. Samples NBA149C, NBA148C and NBA147C prepared at 130 ºC, 170 ºC, and 210 ºC were compared to a sample composed of micron size particles that showed no hydration, as well as to a perchlorate/NaCl sample with small crystallite size that showed significantly reduced hydration. Another sample, NBA81C was analyzed as well, this sample showed very slow hydration. Results are summarized in Table 5-4, and samples are ordered by increasing rates of hydration, e.g. LK40b had less observed hydration than the samples listed below it.

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Table 5-4: BET data for cubic ZrW2O8 samples. Sample Crystallite size (nm) BET (m2/g) NBA1C 100+ 4.75 LK40b 37 ± 2.0 6.46 NBA147C 42 ± 2.0 13.82 NBA148C 38 ± 2.9 18.13 NBA149C 33 ± 1.8 23.38

When comparing the samples prepared in alcohol/HCl mixtures, crystallite size decreased with an increase in surface area. These results seem obvious, however, the perchloric acid/NaCl sample (LK40b) has the smallest crystallite size and a surface area only slightly larger than the micron sized particle. This sample hydrated much more slowly than any solvothermally prepared samples. A strong correlation exists between sample hydration rates and BET surface area. Larger surface areas are equivalent to more atmospheric moisture exposure over similar time periods. The BET data suggest that defects play a more significant role than crystallite size, with defects creating large amounts of surface area. Grain boundaries are most likely the cause of the increased surface area, which is supported by TEM analysis.

5.5. Conclusions

Hydroxide hydrate samples prepared solvothermally in HCl showed problems with crystallinity when heated at temperatures below 210 ºC. This was more observable for samples prepared at 130 ºC where notable fusing between rods occurred when samples were converted to the cubic phase. Additionally, high magnification TEM imaging revealed localized regions of crystallinity and seed crystals present within the particle globules, as well as amorphous regions. Increasing the synthesis temperature to

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170 ºC alleviated a majority of the particle fusing, and increased crystalline content within the particle. Hydration rates were only slightly reduced, and TEM images revealed that these particles were highly polycrystalline. There were many areas with different crystal lattice orientations, suggesting formation of a similar number of defects as in samples prepared at 130 ºC. These samples showed more similar hydration behavior than the samples prepared at 210 ºC, which were observed as larger single crystal particles.

Hydration depends on crystallite size, but not exclusively. A low temperature solvothermally prepared sample, with a small crystallite size, will contain a large number of defects that facilitate autohydration. However, these particles showed an increase in crystallite size when heat treated at 650 ºC, which reduced hydration rates in comparison the 600 ºC heat treated samples. A sample that showed no improvement in hydration, similarly showed no change in crystallite size. This suggests that increases in crystallite size for solvothermally prepared samples simultaneously leads to a reduction in the number of defects. For solvothermally prepared samples crystallite size appears to be correlated to the number of defects. In contrast, for perchlorate/NaCl samples a small crystallite size does not always correspond to a large number of defects. The number of defects is determined by internal crystallographic order or disorder within each particle.

Grain boundaries are likely the most common defect present, and would provide a good explanation for the large difference in BET surface area. Studies have shown that O2 will diffuse into ZrO2 polycrystalline materials through grain boundaries at considerably

41 faster rates than ZrO2 samples that were annealed at high temperatures. Water has a smaller molecular volume than O2, supporting the hypothesis that grain boundary defects can facilitate autohydration. This is further corroborated by the use of N2 in BET surface

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area analysis that shows a decreasing hydration rate trend with decreasing surface areas.

This suggests that synthetic variables for solvothermally prepared samples need to be further explored at higher temperatures in order to overcome the residual hydration seen with samples prepared at 210 ºC. Small single crystalline particles without defects would be most desirable for applications.

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Chapter 6

6. Summary and Future Work

A broad range of samples were successfully synthesized under varying synthetic conditions. This allowed control of morphology for the hydroxide hydrate. The type of alcohol used had a large influence on the crystallization times, particle size, and morphology. With increased heating temperatures, agglomeration was reduced, however, increased crystallite size resulted. Samples prepared at higher temperatures also showed better crystal morphology and crystallinity. Other non-alcohol solvents successfully limited the particle growth; however miscibility issues limit the solvent type suitable for use. Ostwald ripening was observed for extended heating times at high temperatures, while solubility of particles appeared to be limited at low temperatures. Both solvent concentration and acid concentration were shown to have a large influence on the lengths of the particles, but not the widths or crystallite size.

The wide variety of hydroxide hydrate samples synthesized provided for a large difference in hydration rates of the nanosized cubic ZrW2O8 samples. The synthetic variable that showed the most influence on autohydration rates was temperature. Samples prepared at 210 ºC hydrated significantly less than samples prepared at lower temperatures. Rates were determined for faster hydrating samples, and it was found that

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acid concentration and solvent concentration did not have an influence on the rates of autohydration. Samples prepared at 130 ºC and 170 ºC displayed only slightly different autohydration rates to one another in comparison to the samples prepared at 210 ºC.

Interestingly, a significant portion of the 210 ºC showed very limited hydration.

Crystallite sizes increased during conversion to the cubic phase for samples prepared at 130 and 170 ºC. High magnification views of these samples suggested that each particle was polycrystalline in nature. The well defined rod shape of the hydroxide hydrate precursors was lost and globules were observed instead. The morphology was more distorted for higher temperature conversions to the cubic phase. No changes or morphology were seen during conversion of 210 ºC hydroxide hydrate samples to the cubic phase. These samples contained single crystalline particles

While hydration rates show correlation to crystallite size and particle width for solvothermal samples prepared under similar conditions comparison with samples prepared in the absence of organic solvent indicates that other factors dominate.

Hydration rates are strongly correlated with BET surface area for all samples regardless of synthetic conditions. Samples with similar crystallite and particle sizes showed very different surface areas and resulting hydration rates. This suggests that defects contribute significantly to the surface area of the particles. Samples with high surface areas showed a large number of grain boundaries for disordered regions in the TEM.

Further improvements must be directed at formation of single crystalline defect free small particles it seems likely that well crystallized particles with widths of 50 to 60 nm will show very slow hydration. Synthetic conditions must be further optimized to produce such particles. Further increasing heating temperature for the solvothermally

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prepared samples is limited to the stability of the organic solvent. Solvent stability was not explored and a more stable organic solvent may exist for temperatures above 240 ºC, however it must be miscible for use. A stirring setup may enhance the effect of the solvent, even if full miscibility is not achieved. A microwave capable autoclave may be a worthwhile investment for future work on this project. Microwave synthesis can reach high temperatures in short times, accelerating crystallization which could result in smaller single crystal particles with fewer defects. Variations in heat treatments during conversion to the cubic phase should be explored. Proper heat treatment can anneal samples and reduce the number of defects; long heat treatments may cause decomposition into binary oxides.

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