Characterization of molybdenum oxide nanostructures by advanced pair distribution function modelling

Troels Lindahl Christiansen

Characterization of molybdenum oxide nanostructures by advanced pair distribution function modelling

Troels Lindahl Christiansen

Abstract

Increasing global energy consumption necessitate the discovery of new materials for sustainable solutions to energy conversion, energy storage, and catalysis. Development of new materials has paralleled the ability to characterize materials structure on an atomic scale, because the ability to characterize materials enables material design through the understanding of the relationship between structure and properties. Nanotechnology and nanomaterials have the potential to meet these challenges, however the small length scale and disordered structures complicates the use of conventional crystallographic techniques. Total X-ray scattering in combination with atomic pair distribution function (PDF) analysis offer an alternative to these techniques, by allowing a detailed view of the atomic structure without the requirement of long-range order. Here, we apply X-ray total scattering and PDF to various structural problems with the aim of obtaining new knowledge of nanostructures as well as furthering modelling methods within PDF analysis. The dissertation contains three main projects all related to molybdenum oxides, specifically focusing on the changes in structure occurring at the nanoscale Project one is centered on MoO2, which has applications in catalysis, as well as being a new candidate material for highcapacity anode for Li-ion batteries. When closely examining the powder X-ray diffraction (PXRD) data published in different MoO2 studies, it becomes evident that the scattering pattern changes fundamentally as the MoO2 particle size reach the nanoscale. These studies, as well as our own initial investigations, show that nanostructured MoO2 is not readily characterized using conventional crystallographic techniques. In chapter two of this thesis, the novel nanostructure of MoO2 was elucidated through a combination of PDF analysis and high-resolution transmission electron microscopy (HR-TEM). A model for the average atomic arrangement was found by considering the structural trends in molybdenum oxides and other compounds with structures related to the distorted rutile structure of bulk MoO2. The structural change was linked to defects in connection of the [MoO6] octahedras making up the structure. The new structural model was fully supported by the HR-TEM images, thus strengthening the results by combing the excellent statistical sampling of the PDF method with the s patial sensitivity and strong visual nature of HR-STEM.Project two determines how the formation mechanism of nanostructured MoO2 differs from that of the usual distorted rutile MoO2. Knowledge about formation mechanisms of solid materials is essential in achieving control of the synthesis, andthus the ability to design new materials with improved properties. The acid-assisted hydrothermal synthesis of MoO2, using ethylene glycol in a dual role as reducing agent and surfactant, was investigated using in situ total X-ray scattering. Structural transformation, led by defect formation, was found to depend on the pH of the precursor solution. Thus, nanostructured and defect-free MoO2 could be synthesized at pH = 5 and pH = 1, respectively. The formation mechanisms were found to be significantly different at different pH values. Additionally, the effects of ethylene glycol concentration and temperature was investigated, and both were shown to be highly influential in determining the structure of the synthesis product. High ethylene glycol concentration resulted in a tendency to form nanostructured MoO2, while high temperature could be used to avoid the structural defects completely. Lastly, the structure of MoOx supported on γ-Al2O3 and zeolite particles was investigated using differential PDF (d-PDF) and Raman spectroscopy. Conventional real space Rietveld refinements of the d-PDF data revealed that the structure was highly disordered and polydisperse, but with similarities to known molybdenum oxide polyoxometalate structures occurring in solution. A new approach to characterizing structure through PDFs, where a computer algorithm is used to generate new MoOx structures, was developed and applied. The principle of the approach is to iteratively alter known molybdenum oxide structures to automatically generate new clusters that can be fitted to the d-PDFs. From the refinement results of thousands of these computer-generated clusters, information on the average structure of the supported MoOx structure could be extracted. A unit of three edge-sharing octahedras, referred to as a triad, was found as a reoccurring motif in all the d-PDF data, regardless of loading percentage and nature of the support.

Originalsprog	Engelsk

Forlag	Department of Chemistry, Faculty of Science, University of Copenhagen
Status	Udgivet - 2019

Adgang til dokumentet

PHD- Troels Lindahl ChristiansenForlagets udgivne version, 86,5 MB

https://soeg.kb.dk/permalink/45KBDK_KGL/1ed7rpq/alma99123213470305763

Andre filer og links

Sign in to request a library copy

Citationsformater

@phdthesis{dfc6778612474808954cebb77e703fae,

title = "Characterization of molybdenum oxide nanostructures by advanced pair distribution function modelling",

abstract = "Increasing global energy consumption necessitate the discovery of new materials for sustainable solutions to energy conversion, energy storage, and catalysis. Development of new materials has paralleled the ability to characterize materials structure on an atomic scale, because the ability to characterize materials enables material design through the understanding of the relationship between structure and properties. Nanotechnology and nanomaterials have the potential to meet these challenges, however the small length scale and disordered structures complicates the use of conventional crystallographic techniques. Total X-ray scattering in combination with atomic pair distribution function (PDF) analysis offer an alternative to these techniques, by allowing a detailed view of the atomic structure without the requirement of long-range order. Here, we apply X-ray total scattering and PDF to various structural problems with the aim of obtaining new knowledge of nanostructures as well as furthering modelling methods within PDF analysis. The dissertation contains three main projects all related to molybdenum oxides, specifically focusing on the changes in structure occurring at the nanoscale Project one is centered on MoO2, which has applications in catalysis, as well as being a new candidate material for highcapacity anode for Li-ion batteries. When closely examining the powder X-ray diffraction (PXRD) data published in different MoO2 studies, it becomes evident that the scattering pattern changes fundamentally as the MoO2 particle size reach the nanoscale. These studies, as well as our own initial investigations, show that nanostructured MoO2 is not readily characterized using conventional crystallographic techniques. In chapter two of this thesis, the novel nanostructure of MoO2 was elucidated through a combination of PDF analysis and high-resolution transmission electron microscopy (HR-TEM). A model for the average atomic arrangement was found by considering the structural trends in molybdenum oxides and other compounds with structures related to the distorted rutile structure of bulk MoO2. The structural change was linked to defects in connection of the [MoO6] octahedras making up the structure. The new structural model was fully supported by the HR-TEM images, thus strengthening the results by combing the excellent statistical sampling of the PDF method with the s patial sensitivity and strong visual nature of HR-STEM.Project two determines how the formation mechanism of nanostructured MoO2 differs from that of the usual distorted rutile MoO2. Knowledge about formation mechanisms of solid materials is essential in achieving control of the synthesis, andthus the ability to design new materials with improved properties. The acid-assisted hydrothermal synthesis of MoO2, using ethylene glycol in a dual role as reducing agent and surfactant, was investigated using in situ total X-ray scattering. Structural transformation, led by defect formation, was found to depend on the pH of the precursor solution. Thus, nanostructured and defect-free MoO2 could be synthesized at pH = 5 and pH = 1, respectively. The formation mechanisms were found to be significantly different at different pH values. Additionally, the effects of ethylene glycol concentration and temperature was investigated, and both were shown to be highly influential in determining the structure of the synthesis product. High ethylene glycol concentration resulted in a tendency to form nanostructured MoO2, while high temperature could be used to avoid the structural defects completely. Lastly, the structure of MoOx supported on γ-Al2O3 and zeolite particles was investigated using differential PDF (d-PDF) and Raman spectroscopy. Conventional real space Rietveld refinements of the d-PDF data revealed that the structure was highly disordered and polydisperse, but with similarities to known molybdenum oxide polyoxometalate structures occurring in solution. A new approach to characterizing structure through PDFs, where a computer algorithm is used to generate new MoOx structures, was developed and applied. The principle of the approach is to iteratively alter known molybdenum oxide structures to automatically generate new clusters that can be fitted to the d-PDFs. From the refinement results of thousands of these computer-generated clusters, information on the average structure of the supported MoOx structure could be extracted. A unit of three edge-sharing octahedras, referred to as a triad, was found as a reoccurring motif in all the d-PDF data, regardless of loading percentage and nature of the support. ",

author = "Christiansen, {Troels Lindahl}",

year = "2019",

language = "English",

publisher = "Department of Chemistry, Faculty of Science, University of Copenhagen",

}

TY - BOOK

T1 - Characterization of molybdenum oxide nanostructures by advanced pair distribution function modelling

AU - Christiansen, Troels Lindahl

PY - 2019

Y1 - 2019

N2 - Increasing global energy consumption necessitate the discovery of new materials for sustainable solutions to energy conversion, energy storage, and catalysis. Development of new materials has paralleled the ability to characterize materials structure on an atomic scale, because the ability to characterize materials enables material design through the understanding of the relationship between structure and properties. Nanotechnology and nanomaterials have the potential to meet these challenges, however the small length scale and disordered structures complicates the use of conventional crystallographic techniques. Total X-ray scattering in combination with atomic pair distribution function (PDF) analysis offer an alternative to these techniques, by allowing a detailed view of the atomic structure without the requirement of long-range order. Here, we apply X-ray total scattering and PDF to various structural problems with the aim of obtaining new knowledge of nanostructures as well as furthering modelling methods within PDF analysis. The dissertation contains three main projects all related to molybdenum oxides, specifically focusing on the changes in structure occurring at the nanoscale Project one is centered on MoO2, which has applications in catalysis, as well as being a new candidate material for highcapacity anode for Li-ion batteries. When closely examining the powder X-ray diffraction (PXRD) data published in different MoO2 studies, it becomes evident that the scattering pattern changes fundamentally as the MoO2 particle size reach the nanoscale. These studies, as well as our own initial investigations, show that nanostructured MoO2 is not readily characterized using conventional crystallographic techniques. In chapter two of this thesis, the novel nanostructure of MoO2 was elucidated through a combination of PDF analysis and high-resolution transmission electron microscopy (HR-TEM). A model for the average atomic arrangement was found by considering the structural trends in molybdenum oxides and other compounds with structures related to the distorted rutile structure of bulk MoO2. The structural change was linked to defects in connection of the [MoO6] octahedras making up the structure. The new structural model was fully supported by the HR-TEM images, thus strengthening the results by combing the excellent statistical sampling of the PDF method with the s patial sensitivity and strong visual nature of HR-STEM.Project two determines how the formation mechanism of nanostructured MoO2 differs from that of the usual distorted rutile MoO2. Knowledge about formation mechanisms of solid materials is essential in achieving control of the synthesis, andthus the ability to design new materials with improved properties. The acid-assisted hydrothermal synthesis of MoO2, using ethylene glycol in a dual role as reducing agent and surfactant, was investigated using in situ total X-ray scattering. Structural transformation, led by defect formation, was found to depend on the pH of the precursor solution. Thus, nanostructured and defect-free MoO2 could be synthesized at pH = 5 and pH = 1, respectively. The formation mechanisms were found to be significantly different at different pH values. Additionally, the effects of ethylene glycol concentration and temperature was investigated, and both were shown to be highly influential in determining the structure of the synthesis product. High ethylene glycol concentration resulted in a tendency to form nanostructured MoO2, while high temperature could be used to avoid the structural defects completely. Lastly, the structure of MoOx supported on γ-Al2O3 and zeolite particles was investigated using differential PDF (d-PDF) and Raman spectroscopy. Conventional real space Rietveld refinements of the d-PDF data revealed that the structure was highly disordered and polydisperse, but with similarities to known molybdenum oxide polyoxometalate structures occurring in solution. A new approach to characterizing structure through PDFs, where a computer algorithm is used to generate new MoOx structures, was developed and applied. The principle of the approach is to iteratively alter known molybdenum oxide structures to automatically generate new clusters that can be fitted to the d-PDFs. From the refinement results of thousands of these computer-generated clusters, information on the average structure of the supported MoOx structure could be extracted. A unit of three edge-sharing octahedras, referred to as a triad, was found as a reoccurring motif in all the d-PDF data, regardless of loading percentage and nature of the support.

AB - Increasing global energy consumption necessitate the discovery of new materials for sustainable solutions to energy conversion, energy storage, and catalysis. Development of new materials has paralleled the ability to characterize materials structure on an atomic scale, because the ability to characterize materials enables material design through the understanding of the relationship between structure and properties. Nanotechnology and nanomaterials have the potential to meet these challenges, however the small length scale and disordered structures complicates the use of conventional crystallographic techniques. Total X-ray scattering in combination with atomic pair distribution function (PDF) analysis offer an alternative to these techniques, by allowing a detailed view of the atomic structure without the requirement of long-range order. Here, we apply X-ray total scattering and PDF to various structural problems with the aim of obtaining new knowledge of nanostructures as well as furthering modelling methods within PDF analysis. The dissertation contains three main projects all related to molybdenum oxides, specifically focusing on the changes in structure occurring at the nanoscale Project one is centered on MoO2, which has applications in catalysis, as well as being a new candidate material for highcapacity anode for Li-ion batteries. When closely examining the powder X-ray diffraction (PXRD) data published in different MoO2 studies, it becomes evident that the scattering pattern changes fundamentally as the MoO2 particle size reach the nanoscale. These studies, as well as our own initial investigations, show that nanostructured MoO2 is not readily characterized using conventional crystallographic techniques. In chapter two of this thesis, the novel nanostructure of MoO2 was elucidated through a combination of PDF analysis and high-resolution transmission electron microscopy (HR-TEM). A model for the average atomic arrangement was found by considering the structural trends in molybdenum oxides and other compounds with structures related to the distorted rutile structure of bulk MoO2. The structural change was linked to defects in connection of the [MoO6] octahedras making up the structure. The new structural model was fully supported by the HR-TEM images, thus strengthening the results by combing the excellent statistical sampling of the PDF method with the s patial sensitivity and strong visual nature of HR-STEM.Project two determines how the formation mechanism of nanostructured MoO2 differs from that of the usual distorted rutile MoO2. Knowledge about formation mechanisms of solid materials is essential in achieving control of the synthesis, andthus the ability to design new materials with improved properties. The acid-assisted hydrothermal synthesis of MoO2, using ethylene glycol in a dual role as reducing agent and surfactant, was investigated using in situ total X-ray scattering. Structural transformation, led by defect formation, was found to depend on the pH of the precursor solution. Thus, nanostructured and defect-free MoO2 could be synthesized at pH = 5 and pH = 1, respectively. The formation mechanisms were found to be significantly different at different pH values. Additionally, the effects of ethylene glycol concentration and temperature was investigated, and both were shown to be highly influential in determining the structure of the synthesis product. High ethylene glycol concentration resulted in a tendency to form nanostructured MoO2, while high temperature could be used to avoid the structural defects completely. Lastly, the structure of MoOx supported on γ-Al2O3 and zeolite particles was investigated using differential PDF (d-PDF) and Raman spectroscopy. Conventional real space Rietveld refinements of the d-PDF data revealed that the structure was highly disordered and polydisperse, but with similarities to known molybdenum oxide polyoxometalate structures occurring in solution. A new approach to characterizing structure through PDFs, where a computer algorithm is used to generate new MoOx structures, was developed and applied. The principle of the approach is to iteratively alter known molybdenum oxide structures to automatically generate new clusters that can be fitted to the d-PDFs. From the refinement results of thousands of these computer-generated clusters, information on the average structure of the supported MoOx structure could be extracted. A unit of three edge-sharing octahedras, referred to as a triad, was found as a reoccurring motif in all the d-PDF data, regardless of loading percentage and nature of the support.

UR - https://soeg.kb.dk/permalink/45KBDK_KGL/1ed7rpq/alma99123213470305763

M3 - Ph.D. thesis

BT - Characterization of molybdenum oxide nanostructures by advanced pair distribution function modelling

PB - Department of Chemistry, Faculty of Science, University of Copenhagen

ER -