AQUEOUS EXFOLIATION OF TRANSITION METAL OXIDES FORENERGY STORAGE AND PHOTOCATALYSIS APPLICATIONS

Ahmed S. Etman

Aqueous Exfoliation ofTransition Metal Oxides forEnergy Storage andPhotocatalysis Applications

Vanadium Oxide and Molybdenum Oxide Nanosheets

Ahmed S. Etman

©Ahmed S. Etman, Stockholm University 2019 ISBN print 978-91-7797-514-4ISBN PDF 978-91-7797-515-1 Printed in Sweden by Universitetsservice US-AB, Stockholm 2018

To my family and myparents' spirit especiallymy Mom

Doctoral Thesis 2019Department of Materials and Environmental ChemistryStockholm University, Sweden

Faculty Opponent:Prof. Yury GogotsiDepartment of Materials Science and EngineeringDrexel University, USA

Evaluation committee:Prof. Ann CornellDepartment of Applied ElectrochemistryKTH Royal Institute of Technology, Sweden

Assoc. Prof. Fride Vullum-BruerDepartment of Materials Science and EngineeringFaculty of Natural Sciences, NTNU, Norway

Prof. Alexander DmitrievDepartment of PhysicsUniversity of Gothenburg, Sweden

Substitute:Assoc. Prof. Lars ErikssonDepartment of Materials and Environmental Chemistry (MMK)Stockholm University, Sweden

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Abstract

Two-dimensional (2D) transition metal oxides (TMOs) are a category of materials which have unique physical and chemical properties compared to their bulk counterparts. However, the synthesis of 2D TMOs commonly in-cludes the use of environmental threats such as organic solvents. In this thesis, we developed environmentally friendly strategies to fabricate TMO nanosheets from the commercially available bulk oxides. In particular, hy-drated vanadium pentoxide (V2O5·nH2O) nanosheets and oxygen deficient molybdenum trioxide (MoO3-x) nanosheets were prepared. The V2O5·nH2O nanosheets were drop-cast onto multi-walled carbon nanotube (MWCNT) pa-per and applied as a free-standing electrode (FSE) for a lithium battery. The accessible capacity of the FSE was dependent on the electrode thickness; the thickest electrode delivered the lowest accessible capacity. Alternatively, a composite material of V2O5·nH2O nanosheets with 10% MWCNT (VOx-CNT composite) was prepared and two types of electrodes, FSE and conventionally cast electrode (CCE), were employed as cathode materials for lithium batter-ies. A detailed comparison between these electrodes was presented. In addi-tion, the VOx-CNT composite was applied as a negative electrode for a so-dium-ion battery and showed a reversible capacity of about 140 mAh g−1. On the other hand, the MoO3-x nanosheets were employed as binder-free elec-trodes for supercapacitor application in an acidified Na2SO4 electrolyte. Fur-thermore, the MoO3-x nanosheets were used as photocatalysts for organic dye degradation. The simple eco-friendly synthesis methods coupled with the po-tential application of the TMO nanosheets reflect the significance of this thesis in both the synthesis and the energy-related applications of 2D materials.

Keywords: aqueous exfoliation, vanadium oxide nanosheets, molybdenum oxide nanosheets, energy storage, photocatalysis

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List of papers

I. A one-step water based strategy for synthesizing hydrated vanadium pentoxide nanosheets from VO2(B) as free-standing electrodes for lithium battery applications A. S. Etman, H. D. Asfaw, N. Yuan, J. Li, Z. Zhou, F. Peng, I. Persson, X. Zou, T. Gustafsson, K. Edström, J. Sun, J. Mater. Chem. A 4 (2016) 17988–18001. Contribution: planned and performed most of the experiments, wrote the major parts of the manuscript and participated in all discussions.

II. A Water Based Synthesis of Ultrathin Hydrated Vanadium Pentoxide Nanosheets for Lithium Battery Application: Free Standing Elec-trodes or Conventionally Casted Electrodes? A. S. Etman, A.K. Inge, X. Jiaru, R. Younesi, K. Edström, J. Sun, Electrochim. Acta 252 (2017) 254–260. Contribution: planned and performed most of the experiments, wrote the manuscript and participated in all discussions.

III. V2O5∙nH2O nanosheets and multi-walled carbon nanotube composite as a negative electrode for sodium-ion batteries A. S. Etman, J. Sun, R. Younesi J. Energy Chem. in press (2018) doi.org/10.1016/j.jechem.2018.04.011 Contribution: planned and performed most of the experiments, wrote the manuscript and participated in all discussions.

IV. Facile Water Based Strategy for Synthesizing MoO3-x Nanosheets: Ef-ficient Visible Light Photocatalysts for Dye Degradation A. S. Etman, H. N. Abdelhamid, Y. Yuan, L. Wang, X. Zou, J. Sun, ACS Omega 3 (2018) 2193–2201. Contribution: planned and performed most of the experiments, wrote the major parts of the manuscript and participated in all discussions.

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V. Molybdenum Oxide Nanosheets with Tunable Plasmonic Resonance: Aqueous Exfoliation Synthesis and Charge Storage Applications A. S. Etman, L. Wang, K. Edström, L. Nyholm, J. Sun, Adv. Funct. Mater. (2018) 1806699. Contribution: planned and performed most of the experiments, wrote the manuscript and participated in all discussions.

Publication by the author not included in the thesis:

VI. Synthesis of Vanadium Pentoxide Nanosheets

A. S. Etman, J. Sun, Y. Yuan, Patent, 2017, Patent No. WO/2018/013043.

VII. Toward full-cell non-aqueous zinc batteries: metallic Zn anode and Al cathode current collector A. S. Etman, M. Carboni, J. Sun, R. Younesi, in manuscript.

VIII. Insights into the Exfoliation Process of V2O5∙nH2O Nanosheets For-mation using Real-time 51V NMR A. S. Etman, A. Pell, P. Svedlindh, N. Hedin, X. Zou, J. Sun, D. Ber-nin, in manuscript.

IX. Synthesis and structure determination of large-pore zeolite SCM-14 Y. Luo, S. Smeets, F. Peng, A. S. Etman, Z. Wang, J. Sun, W. Yang, Chem. - A Eur. J. 23 (2017) 1–7.

X. Observation of Interpenetration Isomerism in Covalent Organic Frameworks T. Ma, J. Li, J. Niu, L. Zhang, A. S. Etman, C. Lin, D. Shi, P. Chen, L.-H. Li, X. Du, J. Sun, W. Wang, J. Am. Chem. Soc. 140 (2018) 6763–6766.

XI. Covalently linking CuInS2 quantum dots with a Re catalyst by click reaction for photocatalytic CO2 reduction J. Huang, M. G. Gatty, B. Xu, P. B. Pati, A. S. Etman, L. Tian, J. Sun, L. Hammarström, H. Tian, Dalt. Trans. 47 (2018) 10775–10783.

XII. Solution Processed Nanoporous NiO-Dye-ZnO Photocathodes: To-ward Efficient and Stable Solid-State p-Type Dye-Sensitized Solar Cells and Dye-Sensitized Photoelectrosynthesis Cells B. Xu, L. Tian, A. S. Etman, J. Sun, and H. Tian, Nano Energy, 55 (2019) 59–64.

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Contents

Abstract ............................................................................................................ i

List of papers ................................................................................................... ii

Contents ......................................................................................................... iv

Abbreviations ................................................................................................. vi

1. Introduction ................................................................................................. 1 1.1. Advances in 2D Materials and TMO Nanosheets ................................................... 1 1.2. Synthesis of TMO Nanosheets. .............................................................................. 1 1.3. Vanadium Pentoxide Nanosheets: Synthesis and Application in Lithium and Sodium-ion Batteries ..................................................................................................... 2 1.4. Molybdenum Oxide Nanosheets: Synthesis and Application in Dye Degradation and Supercapacitors ............................................................................................................ 5 1.5. Scope of the Thesis Study ..................................................................................... 7

2. Experimental Methods and Characterization Techniques .......................... 8 2.1. Exfoliation Methods ................................................................................................ 8

2.1.1. Exfoliation of Bulk Vanadium Oxides (VOx) .................................................... 8 2.1.2. Exfoliation of Bulk Molybdenum Oxides (MoOx) ............................................. 8

2.2. Characterization Techniques .................................................................................. 9 2.2.1. X-ray Diffraction (XRD) .................................................................................. 9 2.2.2. X-ray Photoelectron Spectroscopy (XPS) ...................................................... 9 2.2.3. Thermogravimetric Analysis (TGA) ................................................................ 9 2.2.4. Electron Microscopy..................................................................................... 10

2.3. Electrochemical Methods ..................................................................................... 10 2.3.1. Electrode Fabrication and Cell Assembly ..................................................... 10 2.3.2. Cyclic Voltammetry Technique ..................................................................... 11 2.3.3. Controlled-Current Technique ...................................................................... 12

2.4. Method of Photocatalytic Dye Degradation .......................................................... 12

3. Aqueous Exfoliation of Vanadium Oxides into V2O5·nH2O Nanosheets for Lithium and Sodium-ion Battery Applications ............................................... 13

3.1. Exfoliation of Vanadium (IV) Oxides into V2O5·nH2O Nanosheets ........................ 13 3.2. Exfoliation of Vanadium (V) Oxides into V2O5·nH2O Nanosheets ......................... 19 3.3. Fabrication of Free-Standing Electrodes of V2O5·nH2O Nanosheets@MWCNT Paper for Lithium Battery Applications ........................................................................ 22

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3.4. Fabrication of V2O5∙nH2O Nanosheets and MWCNT (VOx-CNT) Composite ........ 25 3.5. Electrochemical Performance of VOx-CNT Composite in Lithium Batteries: FSE vs. CCE ............................................................................................................................ 26 3.6. Electrochemical Performance of VOx-CNT Composite as a Negative Electrode for Sodium-ion Batteries ................................................................................................... 30

4. Water Based Fabrication of MoO3-x Nanosheets with Tunable Plasmonic Resonance: Photocatalysts for Dye Degradation and Electrode Material for Supercapacitors ............................................................................................ 34

4.1. Aqueous Exfoliation of MoO3: Structural, Morphological, and Optical Properties of MoO3-x Nanosheets ..................................................................................................... 34 4.2. Aqueous Exfoliation of MoO2/MoO3 Mixtures: Structural and Morphological Properties of MoO3-x Nanosheets ................................................................................ 38 4.3. Solar Light Irradiation of MoO3-x Nanosheets ....................................................... 40 4.4. Photocatalytic Activity of MoO3-x Nanosheets for Organic Dye Degradation ......... 43 4.5. Application of MoO3-x Nanosheets in Supercapacitors ......................................... 45

5. Conclusions ............................................................................................... 48

6. Future Outlook .......................................................................................... 50

7. Sammanfattning på Svenska .................................................................... 51

8. Acknowledgements ................................................................................... 52

9. References ................................................................................................ 54

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Abbreviations

AFM Atomic force microscopy CCE Conventionally cast electrode CV Cyclic voltammetry CVs Cyclic voltammograms ESI-MS Electrospray ionization mass spectrometry FTIR Fourier transform infrared FSE Free-standing electrode LIBs Lithium-ion batteries LSPR Localized surface plasmon resonance MB Methylene blue MWCNT Multi-walled carbon nanotube PVdF Polyvinylidene difluoride RhB Rhodamine B SAED Selected area electron diffraction SEM Scanning electron microscopy SEI Solid electrolyte interphase SIBs Sodium-ion batteries TEM Transmission electron microscopy TGA Thermogravimetric analysis TMOs Transition metal oxides VOx-CNT V2O5·nH2O nanosheets with 10% MWCNT XRD X-ray diffraction XPS X-ray photoelectron spectroscopy

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1. Introduction

1.1. Advances in 2D Materials and TMO Nanosheets Two-dimensional (2D) transition metal oxides (TMOs), chalcogenides and

carbides are promising materials for energy storage, catalysis, gas sensing, field-effect transistors, and biological sensing applications.[1–4] They have attracted a lot of research interest in the past ten years due to their unique physical, optical, electronic, and chemical properties as compared to their bulk counterparts.[5–7] The unique properties of the 2D materials can be attributed to: confinement of electrons in an ultrathin area, which facilitates their com-pelling electronic properties; strong in-plane covalent bonding which acquires them high mechanical strength; large lateral sizes with monolayer thickness, which provides them high surface area; and solution-based processability, which enables their fabrication into free-standing films.[1,3,4]

1.2. Synthesis of TMO Nanosheets. A variety of strategies have been reported for fabricating the TMO

nanosheets, and these strategies can be classified into two main categories: bottom-up approaches and top-down approaches. The bottom-up approach in-cludes a controlled growth of the 2D material from its precursors which can be layered or non-layered materials.[7] A few examples of bottom-up ap-proaches are hydrothermal synthesis,[8] graphene oxide template synthe-sis,[9,10] salt-template synthesis,[11] and chemical and physical vapor depo-sitions (CVD and PVD).[12,13] On the other hand, the top-down approach commonly involves the exfoliation of bulk layered precursors into 2D materi-als. Some examples of top-down approaches are mechanical milling,[14] liq-uid exfoliation, [5,6] chemical etching assisted exfoliation,[15] ion/molecule intercalation,[16] and protein induced exfoliation.[17,18]

Since the discovery of liquid exfoliation in 2008 by Coleman et al.,[19] it has become one of the most efficient techniques to synthesize TMO nanosheets. The method commonly involves sonicating the bulk material in a mixture of solvents, then centrifuging at high speed to separate the exfoliated nanosheets from the bulk materials. [5,6] Alternatively, exfoliation can be ac-complished via molecule or ion intercalation. Scheme 1 shows a schematic

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representation of the liquid exfoliation via ion/molecule intercalation. Liquid exfoliation can be applied for layered materials with weak Van der Waals in-teractions between the layers (e.g. MoO3, MoS2, and WS2),[20,21] or with co-valent bonds between the layers (e.g. VO2), [16,22] and even for non-layered materials (e.g. ZnSe).[23] However, the low yield of exfoliated material is one drawback of this method, and the use of environmentally unfriendly organic solvents is another. Therefore, a lot of research interest focuses on introducing a general exfoliation strategy that can offer high yields using eco-friendly sol-vents. [20,21]

Scheme 1.1. Schematic representation of the exfoliation of layered metal oxide by ion/molecule intercalation. The red balls represent the intercalating ions/molecules. Reproduced from paper V with permission from ©2018 Wiley–VCH.[24]

1.3. Vanadium Pentoxide Nanosheets: Synthesis and Application in Lithium and Sodium-ion Batteries

Vanadium is an earth-abundant and multivalent transition metal that can have oxidation states of +2, +3, +4, and +5. Vanadium oxides are semicon-ducting materials that possess outstanding physical and chemical properties such as multiple oxidation states, [25] different crystal structures, [26] and the ability to insert/deinsert a variety of ions or molecules. [16,27] Therefore, they are extensively used in many applications including lithium batteries,[28–34] sodium batteries,[35,36] thermochromic windows,[37,38] catalysis,[39] pseu-docapacitors,[40] magnetic devices, [41] and gas sensors. [42]

Vanadium pentoxide (V2O5) possesses a layered structure that facilitates the intercalation of alkali metal ions between the layers (e.g. Li+ and Na+). It

Layered Transition Metal Oxide Exfoliation

by Agitation or Sonication

Increasing the interlayer distance by intercalation (i.e. Weaken the bonds)

Transition Metal Oxide Nanosheets

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is considered to be a high capacity cathode material for lithium batteries be-cause it can host two equivalents of lithium ions per V2O5 unit (theoretical capacity is 294 mAh g−1). [43,44] However, the lack of lithium-ion in the pristineV2O5 electrodes requires pre-lithiation of the electrodes before assem-bling the full cell, which adds extra cost during the production process. Alter-natives, such as the use of lithium rich anode materials, e.g. the metallic lith-ium, are challenging due to safety concerns. Recent progress in polymer elec-trolytes has enabled the use of metallic lithium anodes, which paves the way for using cathode materials that initially lack lithium ions.[45,46] However, the V2O5 electrodes commonly exhibit a fast capacity fading in lithium batter-ies due to irreversible structural changes and dissolution in the electrolyte.[47–51] So far, electrochemical performance can be enhanced by using nanostruc-tured V2O5[52] or via forming composite electrodes;[53] however, these methods are commonly quite sophisticated and include the use of harmful re-agents.

Another interesting vanadium-based electrode material is hydrated vana-dium pentoxide (V2O5·nH2O). It possesses a bi-layered V2O5 structure with water molecules between the layers in the a-c plane (see Fig. 1.1). The inter-layer distance depends on the number of water molecules, and can vary be-tween 6.34 and 13.80 Å.[54] The large interlayer distance of V2O5·nH2O en-ables the insertion/deinsertion of many metal ions such as lithium,[55] so-dium,[27,56] magnesium,[57] and zinc ions.[58] V2O5·nH2O usually forms gels upon drying, and these can be classified into aerogels (high surface area gels obtained via supercritical-drying or freeze-drying) and xerogels (gels eva-porated traditionally in air). [59–62] The V2O5·nH2O gels can insert up to 4.0–5.8 equivalents of lithium ions per V2O5·nH2O gel unit, [63] and thus their theoretical gravimetric capacity is about 560–650 mAh g−1.[61]

Figure 1.1. Structure model for V2O5·nH2O viewed along the [010] direc-

tion.

c

a

d-spacing

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As mentioned earlier, the V2O5·nH2O electrode can also host sodium ions between its layers. A variety of nanostructured V2O5·nH2O materials were used as cathode for sodium-ion batteries (SIBs) in the high voltage region (1.0–3.5 V vs. Na+/Na). The electrochemical performance of V2O5·nH2O in SIBs depends on its morphology, slurry composition, electrolyte, and cycling potential window. [27,35,54,56,64,65] However, the use of sodium-ion free cathode materials requires the use of sodium rich anode material e.g. metallic sodium, which is associated with a lot of safety concerns. Recently, the low potential region (below 1 V vs. Na+/Na) of V2O5·nH2O has been explored, as this would enable its use as an anode material for SIBs.[66]

As a semiconducting transition metal oxide, V2O5·nH2O suffers from poor electronic conductivity[67] and low metal-ion diffusion rates.[68] These prob-lems can be addressed using nanostructured active materials instead of the bulk one, and by mixing active material with conductive carbon materials e.g. graphene or carbon nanotubes (CNT).[43,69] Generally speaking, vanadium oxide nanosheets could be prepared via the exfoliation of their bulk counter-parts or the chemical processing of solution precursors.[16] In particular, V2O5·nH2O nanosheets have been synthesized via refluxing, UV-irradiation, or the hydrothermal treatment of an aqueous solution of V2O5 powder mixed with H2O2;[70,71] or using sol-gel processing of vanadium (V) oxytripropox-ide precursor.[72] However, these synthetic approaches involve the use of harmful reducing reagents. Thus, there is a need to develop green methods for fabricating V2O5·nH2O nanosheets using commercially available precursors.

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1.4. Molybdenum Oxide Nanosheets: Synthesis and Application in Dye Degradation and Supercapacitors

Molybdenum oxides are versatile materials [73,74] that can be used in sev-eral applications such as lithium-ion batteries (LIBs), [75] sodium-ion batter-ies (SIBs),[76] supercapacitors [77], catalysis [17,78,79], gas sensors [80], and field-effect transistors.[81] They can exist in three different stoichi-ometries; the stoichiometric oxide MoO3 with a wide bandgap (~3 eV), the sub-stoichiometric form MoO3-x with a slightly smaller bandgap and the semi-metallic form MoO2 with smaller bandgap.[73] Molybdenum trioxide (MoO3) is a polymorph oxide that has four known phases (α-MoO3 with an orthorhombic unit cell,[82], β-MoO3 with a monoclinic unit cell,[83] h-MoO3 with a hexagonal unit cell,[84] and high pressure phase MoO3-II, with a mon-oclinic unit cell).[85] The commercially available phase is the α-MoO3, which has been extensively examined for decades due to its promising electrochem-ical and catalytic activities [86,87].

One significant advantage of α-MoO3 is that it is a semiconducting layered material (see Fig. 1.2) with a bandgap that varies depending on the ions/mol-ecules inserted between its layers.[88] The latter property allowed the use of MoO3 in photocatalysis applications such as organic dye degradation.[79,89–93] MoO3-x nanosheets (where x stands for oxygen vacancy) are even more efficient photocatalysts compared to their bulk counterparts because of their high aspect ratio, in addition to the presence of oxygen vacancies.[79,89–93]

Figure 1.2. Structure model for α-MoO3 viewed along the [010] direction.

The MoO3 has been used extensively in energy storage systems, especially in supercapacitors.[94,95] However, the electrochemical performance of MoO3 in supercapacitors is dependent on its morphology, the nature of the electrolyte, the thickness of the electrode, and the amount of oxygen vacancies in MoO3.[96–98] The thin film nanostructured MoO3 electrodes behave more superior than the conventionally cast bulk MoO3 especially at high rates due to diffusion limitations.[98] In addition, the presence of the oxygen vacancies improves the supercapacitive behavior of the MoO3. [86,99] Therefore, more

c

a

6

research interest goes to the nanostructured reduced MoO3-x electrodes with oxygen vacancies. [86,99]

To date, α-MoO3 has been prepared in a variety of nanostructures, includ-ing nanosheets,[77,100–102] nanobelts,[103,104] flower-likes hierarchical structure,[84] nanoflakes,[14,81,105–107] and nanoparticles.[108,109] In particular, MoO3 nanosheets have been fabricated using hydrothermal synthe-sis [100], salt-templated approach [11], chemical etching,[110] mechanical milling,[14] scotch-tap exfoliation,[18] protein induced exfoliation,[18] and liquid exfoliation of bulk α-MoO3.[17,81,105] However, most of these ap-proaches have low yields, involve several steps,[80] produce undesirable or-ganic by-products, and require high temperatures.[11]

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1.5. Scope of the Thesis Study The work described in this thesis introduced innovative and simple aqueous

exfoliation strategies for fabricating TMO nanosheets from their bulk precur-sors. The precursors were layered bulk TMOs with either covalent bonds (e.g. VO2(B) and V2O5) or Van der Waals interactions (e.g. MoO3) between the layers. These methods provided a scalable eco-friendly production of TMO nanosheets from commercially available precursors. The TMO nanosheets were tested for charge storage and photocatalysis applications.

Hydrated vanadium pentoxide (V2O5·nH2O) nanosheets were fabricated via two different approaches using vanadium dioxide and vanadium pentoxide precursors. The free-standing electrodes (FSE) of V2O5∙nH2O nanosheets@multi-walled carbon nanotube (MWCNT) paper were tested in lithium batteries. Furthermore, composite electrodes of freeze-dried V2O5∙nH2O nanosheets and MWCNT (hereafter labelled as VOx-CNT) were fabricated and explored as FSE and conventionally cast electrode (CCE) for lithium batteries. The composite material (VOx-CNT) was also examined as a negative electrode for sodium-ion batteries.

Molybdenum oxides (MoO3-x) nanosheets were fabricated by two different approaches using molybdenum dioxide and molybdenum trioxide precursors. The MoO3-x nanosheets offered a localized surface plasmon resonance (LSPR) that was tuned chemically and/or photochemically by solar light irradiation. The MoO3-x nanosheets were used as photocatalysts for organic dye degrada-tion. In addition, the MoO3-x nanosheets were tested as binder-free electrodes for supercapacitor applications, and the electrodes delivered a promising high rate capacitance up to 70 C g−1 at a scan rate of 1000 mV s−1.

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2. Experimental Methods and Characterization Techniques

2.1. Exfoliation Methods

2.1.1. Exfoliation of Bulk Vanadium Oxides (VOx) In a typical synthesis, an appropriate amount of the vanadium oxide pre-

cursor was refluxed in water (about 150 mg per 25 mL water) at 60–80 °C for 1–6 days. For VO2(B) and VO2(M) the exfoliation process required about six days; however, the 4:1 (weight ratio) mixture of V2O5 and VO2(M) was fully converted to nanosheets after only 24 h. The aqueous suspension of V2O5·nH2O nanosheets displayed a dark green color. At the end of the reflux, the aqueous suspension was placed on a glass substrate and dried at 80 °C for 5 h to form a free-standing film of V2O5·nH2O nanosheets.

2.1.2. Exfoliation of Bulk Molybdenum Oxides (MoOx)

The molybdenum oxide precursors were dispersed in water and refluxed at 80 °C for 5–7 days. We prepared three samples of MoO3-x nanosheets using different weight ratios of the MoO3 and MoO2 precursors. When pure MoO3 precursor was used the prepared nanosheets were assigned as MoO3-x-I; whereas MoO3-x-II was obtained using a mixture of 4MoO3:1MoO2 (weight ratio), and MoO3-x-III was fabricated using a mixture of 1MoO3:4MoO2. The exfoliated nanosheets were collected by centrifuging the formed suspension at high speed. The bulk materials precipitated, whereas the nanosheets re-mained dispersed in the mother liquor which could be dried at 80 °C for 5–8 hours to obtain the MoO3-x nanosheets as dried powders.

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2.2. Characterization Techniques

2.2.1. X-ray Diffraction (XRD) X-ray diffraction is a crucial technique for characterizing the material crys-

tal structure. Each material has its own XRD pattern that is a fingerprint of the material. When the X-ray beams encounter a material, they interact with the material and can be diffracted and/or absorbed by the matter, depending on the wavelength of the incident beams ( and the nature of the material. The constructive interference between the diffracted beams occurs only when Bragg’s law is fulfilled ( ).[111]

The XRD experiments in reflection mode were performed using an in house diffractometer (Panalytical) with a Cu Kα1 radiation source. The oper-ando XRD measurement was done using a synchrotron radiation facility with a wavelength (λ) of 0.20759 Å and a Perkin Elmer XRD1621 area detector (beamline P02.1, PETRA III, Hamburg, Germany). The reactants were added in a sealed 5 mL Pyrex tube which was then placed in a custom-made in situ reactor. The reactor allowed the reactants to be stirred and heated during data collection. The whole reactor was surrounded by aluminum as a safety pre-caution.

2.2.2. X-ray Photoelectron Spectroscopy (XPS) XPS is a surface characterization technique that includes bombarding the

surface with monochromatic X-ray radiation which results in the ejection of core electrons. The ejected electrons can be collected and their binding ener-gies can be calculated, and then used to identify the elements present and their oxidation states. [111] The XPS experiments were done using AXIS Ultra in-strument (Kratos Analytical Ltd.) equipped with monochromatic Al Kα radia-tion ( = 1486.7 eV). The binding energies were calibrated to the C 1s peak of (C−C) and (C−H) bonds (284.8 eV).

2.2.3. Thermogravimetric Analysis (TGA) The TGA involves heating up a sample with a defined mass between two

temperatures using a specific heating rate. The weight loss is monitored during the heating process. The TGA can also be equipped with a mass spectrometry unit to identify the species lost from the sample upon heating.[112] The water content of the vanadium pentoxide nanosheets was determined using TGA. The measurements were done using a Perkin-Elmer-TGA-7 instrument, and samples were heated in air from room temperature to 600 °C.

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2.2.4. Electron Microscopy Electron microscopy is an efficient technique to study the morphology and

structure of nanomaterials. It involves the use of accelerated electron beams to either scan the surface of the sample in scanning electron microscopy (SEM) or transmit through the specimen in transmission electron microscopy (TEM). The TEM can be operated in diffraction mode as well to get infor-mation about the sample crystal structure.[111] The TEM instruments used in examining different samples were equipped with either LaB6 filament (JEOL-JEM 2100) or field emission gun (TEM, JEOL-JEM-2100F) and the accelerating voltage was 200 kV. The SEM instrument model was a JEOL-JSM-7401F.

2.3. Electrochemical Methods

2.3.1. Electrode Fabrication and Cell Assembly The electrochemical tests of the vanadium oxide based electrodes for lith-

ium battery application were done in a two-electrode pouch cell with a lithium metal counter electrode. The working electrodes were either free-standing electrode (FSE) or conventionally cast electrode (CCE). Prior the electro-chemical measurements, the working electrodes were dried overnight under vacuum at 120 °C. The separator was glass fiber paper and the electrolyte was 1 M LiPF6 in 1:1 mixture of ethylene carbonate (EC) and diethyl car-bonate (DEC). The cells were prepared inside Ar-filled glove box (O2 and H2O levels ≤ 5 ppm).

The FSE of VOx@MWCNT paper (electrodes used in paper I) were fabri-cated by drop-casting the V2O5·nH2O suspension onto MWCNT paper. The mass loadings of the active materials for VO-45, VO-12, VO-7, and VO-4 electrodes were 5.2, 4.07, 1.14 and 0.56 mg, respectively.

For the VOx-CNT composite, FSE fabrication involved drop-casting the aqueous suspension of V2O5·nH2O with 10% MWCNT onto a piece of MWCNT paper and then freeze-drying the electrodes. The cast electrodes were prepared from a slurry with composition 80% of the active material (VOx-CNT composite), 10% carbon black (super C65 Imerys), and 10% of PVdF binder (Arkema). An aluminum foil with a thickness of 20 μm was used as the current collector. The active material mass loadings were 2.0 mg for FSE (circular electrode, diameter 13 mm) and 3.0 mg for CCE (circular elec-trode, diameter 20 mm). The mass loading calculations were based on the sum of masses of both the V2O5∙nH2O nanosheets and the 10% MWCNT.

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Likewise, the SIB experiments were done in a two-electrode pouch cell with a metallic sodium counter electrode. The separator was a piece of glass fiber paper and the electrolyte was 1 M NaPF6 in 1:1 mixture of ethylene car-bonate (EC) and diethyl carbonate (DEC). A few experiments were performed using 1 M NaPF6 in propylene carbonate (PC) with 0.5% fluoroethylene car-bonate (FEC) additive. The working electrodes were cast on a 20 μm thick aluminum foil (slurry composition 80% active material, 10% carbon black (super C65 Imerys), and 10% of PVdF binder (Arkema)). The mass loadings were about 1.10–1.25 mg and 3.00–3.20 mg for VOx-CNT circular electrodes of diameter of 13 mm and 20 mm, respectively. The mass loadings calcula-tions were based on the total masses of V2O5∙nH2O nanosheets and 10% MWCNT. The V2O5∙nH2O nanosheet electrodes (i.e. without MWCNT) mass loadings were about 1.75–1.94 mg for circular electrodes of diameter 13 mm.

The MoO3-x nanosheet electrodes were examined for use in supercapacitors using a three-electrode setup with a Pt-mesh counter electrode and a Ag/AgCl/3.5 M KCl reference electrode. The working electrodes were fabri-cated via drop casting the aqueous dispersion of MoO3-x nanosheets onto a piece of carbon paper (Quin-Tech, H23), to form binder-free electrodes. The electrodes mass loading varied between 6.25 and 50 μg cm−2. The electrolytes used in these experiments were a series of acidified 1 M Na2SO4 solutions with different pH values. The pH was controlled via the addition of 0.1 M H2SO4 to the 1 M Na2SO4 (Aldrich) solution.

2.3.2. Cyclic Voltammetry Technique Cyclic voltammetry is a fundamental technique to explore the electrochem-

ical behavior of electrode materials for battery or supercapacitor applications. In a standard cyclic voltammetry experiment, the potential is swept at a spe-cific scan rate between two vertex potentials and the electrode current is rec-orded as a function of the potential.[113] The cyclic voltammetry test of va-nadium oxide based electrodes for lithium battery application was performed in the potential window 1.7–3.9 V (vs. Li+/Li), whereas the potential window for application in the sodium-ion battery was 0.1–2.5 V (vs. Na+/Na).

The cyclic voltammetric experiments involving the MoO3-x nanosheet elec-trodes were done between 0.0 and 0.8 V (vs. Ag/AgCl/ 3.5 M KCl) applying scan rates between 5.0 and 1000 mV s−1. The charge/discharge electrode ca-pacity (C g−1), was determined by integrating the anodic/cathodic charge por-tions in the cyclic voltammograms using the relation:

Equation 2.1

where, is the charging/discharging current, is the mass of active ma-terials (g), and : is the scan rate (V s−1).

12

2.3.3. Controlled-Current Technique Controlled current measurements involve applying a constant charging/dis-

charging current using upper and lower cut-off potentials. The variation of the working electrode potential during the experiment is measured as a function of the experiment time.[113] Similar to cyclic voltammetry, the potential win-dows of galvanostatic charging/discharging for vanadium oxide based elec-trodes in lithium and sodium-ion batteries were 1.7–3.9 V (vs. Li+/Li) and 0.1–2.5 V (vs. Na+/Na), respectively. The theoretical and practical gravimetric electrode capacities were calculated using Faraday’s first law:

Equation 2.2

Equation 2.3

where, I: is the applied charging/discharging current, t: is the charging/dis-charging time, m: is the mass of active materials, n: number of moles of elec-trons, F: is Faraday’s constant, and M: is the molar mass of the active material.

2.4. Method of Photocatalytic Dye Degradation The photocatalytic activity of MoO3-x nanosheets toward the decolorization

of organic dye (methylene blue (MB) and rhodamine B (RhB)) was performed as follows: (1) 1 mL of dye solution of concentration 1 g/L was diluted to 20 mL, (2) 1 mL of the MoO3-x nanosheets suspension with a concentration of 8 g/L was added, (3) the mixture was stirred and irradiated with visible light (HALOLUX® CERM ECO, 150 W), and finally (4) at specific intervals sam-ples of 0.1 mL of the mixture were removed and diluted to 4 mL, and then its UV-vis spectrum was measured between wavelengths of 300–800 nm.

The removal efficiency of the dye was calculated using the following rela-tionship:

Removal efficiency (%) = ×100% Equation 2.4

where A0 is the initial absorbance of the pristine dye, and At is the absorb-ance of dye after reacting with the catalyst for a specific time t (min).

13

3. Aqueous Exfoliation of Vanadium Oxides into V2O5·nH2O Nanosheets for Lithium and Sodium-ion Battery Applications

3.1. Exfoliation of Vanadium (IV) Oxides into V2O5·nH2O Nanosheets

Vanadium dioxide is a polymorphic material with at least nine known crys-

tal phases,[22,26,114–120] such as the monoclinic phase (VO2(M)) and the bronze phase (VO2(B)). The latter phase possesses a layered structure with a monoclinic unit cell,[16,28,121,122] whereas the former has a nonlayered structure.[22] The VO2(M) used in this study was purchased from Aldrich, whereas the VO2(B) was prepared via a hydrothermal synthesis using ammo-nium metavanadate (NH4VO3) precursor.[16] The XRD pattern of the pristine VO2 (Fig. 3.1a) matched well with the standard pattern of VO2(B) (JCPDS No. 81-2392). Water molecules have a reasonable ability to intercalate into VO2(B) crystals via H-bonding along the c-axis.[16] The intensity of 00l re-flections in the VO2(B) XRD pattern is expected to increase upon water inter-calation because of the preferred orientation of the layered material (i.e. VO2(B)). Temperature has a crucial effect on the intercalation of water mole-cules into VO2(B); therefore, a variety of temperature ranges were examined, and the optimum condition (highest yield) was found to occur at 60 °C.

The refluxing of VO2(B) in water at 60 °C for three days resulted in ex-panding the spacing between VO2(B) layers as displayed by the emergence of a low angle peak at 2θ = 7.30° (see Fig. 3.1a). The continuation of the reflux for six days leads to complete conversion of VO2(B) to a new phase with broad diffraction peaks with 00l indices and d-spacing values of d/n (where n is an integer =1,2, 3, …., n). The XRD pattern of the newly formed phase matched well with the standard pattern of hydrated vanadium pentoxide (V2O5·nH2O, JCPDS No. 74-3093). Likewise, the V2O5·nH2O phase was obtained when the commercial vanadium dioxide (VO2(M), Aldrich) was used as the precursor, suggesting that the exfoliation process was not restricted by the structure of the VO2 precursor (see Fig. S1a in paper I).

XPS was employed to determine the vanadium oxidation states in the ex-foliated material (Fig. S1b paper I). The O 1s and V 2p3/2 peaks in the XPS

14

spectrum showed that the majority of vanadium had the oxidation state of +5 (V5+), and a minority had the oxidation state of +4 (V4+); this finding agrees well with previous XPS studies on V2O5∙nH2O.[123–125] The existence of vanadium (IV) can be attributed to the reduction of some of the vanadium (V) by the structural water molecules. The volumetric titration of V2O5·nH2O nanosheets dissolved in sulfuric acid against potassium permanganate (KMnO4) revealed the absolute ratio of V5+ to V4+ in the material is about 80% to 20%, which matches well with the XPS results. Consequently, the exfoli-ated nanosheet structural formula can be written as

[25,126,127]

Figure 3.1: (a) Powder XRD patterns for standard VO2(B) (JCPDS No. 81-

2392), bulk VO2(B), the materials formed when VO2(B) was refluxed for three, five, and six days at 60 C in water, and standard V2O5·nH2O (JCPDS No. 74-3093). The V2O5·nH2O and VO2(B) peaks are denoted by “*” and “0”, respectively. (b) Simple representation of the transformation of the bulk VO2(B) to V2O5·0.55H2O nanosheets using the structural models of both struc-tures viewed along the [010] direction (the vanadium octahedra and oxygen atoms are shown in grey and red, respectively). Reproduced from paper I with permission from ©2016 The Royal Society of Chemistry.[55]

15

Generally, the water molecules in V2O5·nH2O can be classified into two types: surface water and structural water. The former can be removed by heat-ing the V2O5·nH2O to the boiling point of water (i.e. 100 °C); however, the latter remains in the crystal structure and can be removed by heating to 350 °C to form V2O5. [128,129] The thermogravimetric analysis and in situ XRD were used to explore the water contents in our nanosheets. As we can see from the in situ XRD patterns in Fig 3.2a, the 00l reflections shift toward higher angles upon heating from room temperature to 100 oC (001 d-spacing changes from 1.37 nm at 25 oC to 1.01 nm at 100 °C), reflecting the removal of surface and structural water. The subsequent heating of the nanosheets leads to re-moval of the remaining structural water at ~360 °C, which forms V2O5.

Likewise, two stages of weight loss can be observed in the TGA plot (Fig 3.2b). The first stage presents between room temperature and 100 °C and in-cludes a weight loss of ~5.0 % which can be assigned to loss of surface and structural water. The second stage exists between 100–360 °C and involves an additional weight loss of 5% that can be attributed to the removal of the re-maining structural water to form V2O5. According to XPS, volumetric titra-tion, in situ XRD, and TGA, the structural formula of the exfoliated nanosheets can be written as at room temperature and over 100 °C.

Figure 3.2: (a) In situ XRD patterns of the V2O5·0.55H2O nanosheets

heated to temperatures between 25 and 360 °C; (b) TGA plot of the V2O5·0.55H2O nanosheets during heating from 25 to 600 °C. Reproduced from paper I with permission from ©2016 The Royal Society of Chemis-try.[55]

16

Based on the XRD and XPS, we can assume that the exfoliation of VO2(B) to V2O5 0.55H2O nanosheets includes two integrated steps as schematically represented in Fig 3.3. Firstly, the bulk VO2(B) crystals are dispersed via wa-ter insertion to form thin crystals. The thin crystal formation starts in the third day of refluxing and continues until the sixth day of reflux. Secondly, the ma-jority of the vanadium ions in the VO2(B) thin crystals are partially oxidized by O2 in the air, forming vanadium (V). Simultaneously, the water molecules are inserted between layers to form V2O5·0.55H2O nanosheets. The overall transformation reaction of VO2(B) to V2O5·0.55H2O nanosheets can be written as follow:

2VO2(B) + ½ O2 + H2O V2O5∙0.55H2O (Equation 3.1) Figure 3.3: Schematic representation for the mechanism of the water based

exfoliation of VO2(B) to V2O5·0.55H2O nanosheets (the red balls represent water molecules). Reproduced from paper I with permission from ©2016 The Royal Society of Chemistry.[55]

Reflux for 3 days at 60 °C

R

V2O5·nH2O nanosheets

Bulk VO2(B) (Aggregates of crystals)

V2O5·nH2O nanosheets + VO2(B) (thin crystals)

a b

c

17

Electron microscopy was used to examine the morphology of the bulk and exfoliated materials. As shown in Fig 3.4a the pristine VO2(B) precursor is composed of plate-like crystals, whereas the exfoliated V2O5·nH2O has an ul-trathin and transparent nanosheet morphology (Fig. 3.4b-e). In addition, the nanosheets have wrinkles with a thickness about 4.2 nm, as marked by arrows in Fig 3.4c. The AFM showed that the single nanosheet thickness is about 4.0–4.3 nm (Fig 3.4f). Based on the d-spacing of the 001 reflection (1.37 nm), the single nanosheet is composed of three layers. In addition, the lattice fringes of the 001 plane displayed in the HRTEM image (Fig. 3.4.e), confirmed that the nanosheet consists of 3–4 layers. The SAED of V2O5·nH2O displayed powder rings, which indicates the high degree of randomness of the nanosheets in the (a-b) plane (Fig 3.4d). Furthermore, the aqueous suspension of the V2O5·0.55H2O nanosheets exhibits the Tyndall effect, reflecting the colloidal nature and homogeneity of the nanosheets (see inset in Fig. 3.4c). The nanosheets are easy to handle and form a free-standing film upon drying on a glass substrate in air at 80 °C for 5 h (see inset Fig. 3.4b).

18

Figure 3.4: (a) and (b) SEM image of bulk VO2(B) and the exfoliated

V2O5·0.55H2O nanosheets, respectively; inset in (b) photograph of a free-standing film of the exfoliated V2O5·0.55H2O nanosheets. (c) TEM image of the exfoliated V2O5·0.55H2O nanosheets; inset in (c) photograph for Tyndall effect of the exfoliated V2O5·0.55H2O nanosheets dispersed in water. (d) SAED pattern obtained from the highlighted area in the inset TEM image. (e) HRTEM image for V2O5·0.55H2O nanosheets showing lattice fringes of 001 plane; and (f) AFM image of exfoliated V2O5·0.55H2O; inset in (f) height pro-file of the highlighted dashed line showing the nanosheet thickness is about 3–4 layers. Reproduced from paper I with permission from ©2016 The Royal Society of Chemistry.[55]

e

a

c

b

d

f

19

3.2. Exfoliation of Vanadium (V) Oxides into V2O5·nH2O Nanosheets

The synthesis of V2O5·nH2O nanosheets using vanadium IV precursors is not suitable for large scale production, because the precursors are expensive, and the process requires about a week to complete the exfoliation.[55] Plate-like nanosized V2O5 (prepared by annealing VO2(B) in air at 400 °C for 2 h) can be readily exfoliated to V2O5·nH2O nanosheets using the exfoliation method described in paper I.[55] However, the commercial V2O5 powder can-not be efficiently exfoliated using this method (see Figure S1 in paper II). Thus, the method needs to be optimized to exfoliate the commercial V2O5 powder.

The reflux of 4:1 (weight ratio) mixture of V2O5 and VO2(B) at 80 °C for 5 24 h resulted in a complete exfoliation of the oxide mixture to V2O5·nH2O nanosheets, as evinced by XRD patterns recorded after 5, 18, and 24h (Figure 3.5a). The XRD pattern after 24h agrees well with the standard pattern of V2O5∙H2O (JCPDS No. 74-3093). Replacing VO2(B) with VO2(M), did not affect the exfoliation process, indicating that the initial structure of the vana-dium (IV) precursor did not have a crucial role in the exfoliation process (see Figure S2b in paper II).

Operando XRD studies were performed to explore the structural changes during the exfoliation process. Figure 3.5.b displays a color map of the X-ray intensity as a function of reaction time at a given 2θ. Notably, after 90 min a new phase was revealed as indicated by the peak that emerged at 2θ = 0.65°, corresponding to a d-spacing of 18.0 Å (λ = 0.20759 Å). Figure 3.5c shows XRD patterns at time intervals of 0, 30, 60, 90, 120, 150, and 250 min; 2θ in this plot has been normalized to copper wavelength to compare directly with the in house XRD patterns. The intensities of the V2O5 reflections 200 (2θ = 15.38°, d-spacing = 5.76 Å) and 001 (2θ = 20.36°, d-spacing = 4.36 Å) dimin-ished with reaction time. Furthermore, after 90 min a new low-angle peak emerged at 2θ = 5.0° (d-spacing = 18.0 Å) and its intensity increased as the reaction proceeded. This peak can be assigned to the 001 reflection of V2O5∙H2O. In contrast, the ex situ XRD for the powder V2O5∙H2O displayed the 001 d-spacing of about 13.63 Å; this difference in d-spacing can be ex-plained by the presence of more water molecules between the V2O5∙H2O nanosheets when they are dispersed in water. Notably, after about 250 min the 002 reflection of V2O5∙H2O nanosheets can be resolved at 2θ = 10.0° (d-spac-ing = 9.0 Å). It is worth noting that the peak at 2θ = 2.5° probably comes from the setup or the background, as it is already present at the start of the reaction, and neither VO2(M) nor V2O5 possess peaks at this low 2θ value.

20

Figure 3.5. (a) Powder XRD of commercial V2O5 and a mixture of 80%

commercial V2O5 and 20% VO2 refluxed in water at 80 °C for 5, 18, and 24 h. The V2O5 and V2O5·0.5H2O phases are denoted by “°” and “*”, respec-tively. (b) Color map showing operando XRD during the synthesis of V2O5·0.5H2O nanosheets from a 1:4 mixture of VO2 and V2O5. (c) Selected XRD patterns measured at given intervals during the operando measurement. The V2O5 and the V2O5·0.5H2O phases are denoted by “°” and “*”, respec-tively. Reproduced from paper II with permission from ©2017 Elsevier Ltd.[130]

The chemical composition of the V2O5∙H2O nanosheets was determined us-ing X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). The detailed analysis is summarized in the supporting information of

21

paper II. The chemical formula after heating the nanosheets to 120 °C can be written as H0.2V1.8

5+ V0.24+ O5∙0.5H2O; [25,126,127] however, we will refer to it

as V2O5∙H2O for simplicity.

Exfoliated V2O5∙H2O existed as nanosheets, as seen in the SEM and TEM images in Fig. 3.6a and e. The single nanosheet thickness was about 4.0 nm, as determined using AFM (Fig 3.6 b and c). Based on the d-spacing of the 001 reflection (13.63 Å) the single nanosheet was composed of three layers; this is consistent with the HRTEM image in Fig. 3.6d, which shows the lattice fringes of the 001 plane. In addition, the selected area electron diffraction (SAED) in Figure 3.6f showed powder rings, indicating the random arrange-ment of the V2O5∙H2O nanosheets.

Figure 3.6. (a) and (b) SEM and AFM images of exfoliated V2O5∙0.5H2O

nanosheets, respectively. (c) Height profile of the highlighted dashed line in (b). (d) HRTEM image of V2O5∙0.5H2O nanosheets showing the thickness is about 3–4 layers. (e) and (f) TEM image of the exfoliated V2O5∙0.5H2O nanosheets and SAED pattern obtained from the highlighted area in (d), re-spectively. Reproduced from paper II with permission from ©2017 Elsevier Ltd.[130]

All in all, the morphology and chemical composition of V2O5∙H2O nanosheets, obtained using a vanadium (IV) precursor or a mixture of vana-dium (V) and vanadium (IV) precursors, were quite analogous. However, the exfoliation of a mixture of vanadium (V) and vanadium (IV) required only 24h, whereas that of vanadium (IV) needed about a week.

a b

d e f

c

3-4 layers

22

3.3. Fabrication of Free-Standing Electrodes of V2O5·nH2O Nanosheets@MWCNT Paper for Lithium Battery Applications

The conventional casting of lithium battery electrodes includes the use of an organic binder, which frequently causes a number of drawbacks such as, masking parts of the active material and decreasing the electronic conductiv-ity.[43,131] Therefore, research interest in binder-free electrodes keeps grow-ing. In this regard, we fabricated free-standing electrodes (FSEs) by drop-cast-ing the aqueous suspension of V2O5∙0.55H2O nanosheets onto MWCNT pa-per. The thickness of the active materials in the FSE was controlled by altering the concentration of the nanosheet in the suspensions. Four electrodes with active material thicknesses of 45, 12, 7, and 4 μm were synthesized. The elec-trodes were assigned as VO-45, VO-12, VO-7, and VO-4. Figure 3.7 displays a schematic representation of the FSEs and SEM images of their cross-sec-tions.

Figure 3.7. (a) Schematic representation of the FSE. (b), (c), (d), and (e)

SEM images of cross-sections of the four electrodes used in our study. Parts (b–e) Reproduced from paper I with permission from ©2016 The Royal Soci-ety of Chemistry.[55]

The electrochemical analysis of the FSEs was performed using cyclic volt-

ammetry and galvanostatic charge/discharge techniques in the potential win-dow of 1.7–3.9 V vs. Li+/Li. Figure 3.8a shows the CVs for the FSEs at a scan rate of 0.05 mV s−1. As seen from the CVs, the lithium ions insertion/deinser-tion involved two separate electron transfer processes as displayed by two pairs of redox peaks in all the CVs. The observed reversible reduction peaks can be assigned to V5+ being reduced to V4+ and V3+, respectively, and that the oxidation peaks consequently can be ascribed to an oxidation of V3+ to V4+ and V5+, respectively.[132] The overall electrochemical intercalation/deinter-calation of lithium ions can be written as:

b c

a

d e

23

V2O5∙0.55H2O + x Li+ + x e LixV2O5∙0.55H2O (Equation 3.2)

The crystallinity of V2O5 electrodes plays a significant role in their electro-chemical response. Crystalline V2O5 undergoes irreversible phase changes when cycled below 1.9 V vs. Li+/Li; these phase changes are commonly ob-served as a series of plateaus in the first discharge cycle. [49,50,133] On the other hand, amorphous V2O5 does not suffer from these structural changes due to its low crystallinity, and hence usually shows a sloping plateau in the first discharge cycle. [54,134,135] Accordingly, the galvanostatic curves of the first discharge cycles for different electrodes at an applied current density of 10 mA g−1, exhibited a sloping plateau (see Fig. 3.8b). In addition, the charge and discharge curves have almost the same shape, reflecting the absence of irreversible structural changes. TEM and SAED further confirmed that the morphology and the structure of the nanosheets were maintained after 20 elec-trochemical cycles (see Fig. S6, supporting information paper I).

Notably, the FSEs gravimetric capacities increased, as their thicknesses de-creased. For instance, at an applied current density of 10 mA g−1, the elec-trodes of thicknesses about 45, 12, 7 and 4 μm delivered capacities of 174, 267, 402 and 489 mAh g−1, respectively. This can be explained by the fact that, as the thickness of the V2O5∙0.55H2O nanosheets decreases, the role of the interfacial surface area becomes more important and the electrodes main-tain better contact with the electrolyte and the MWCNT paper.

For a deep understanding of the charge storage mechanism, we conducted further analysis on electrodes with different thickness, in particular, a thick electrode (VO-45) and a thin electrode (VO-4). The discharge profiles for the electrodes VO-45 and VO-4 are shown in Fig. 3.8c and d. For electrode VO-45, the first discharge disclosed a feature around 3.0 V, which did not occur in the subsequent cycles. This feature suggested that minor structural changes occurred after the first discharge. On the other hand, the electrode VO-4 showed little difference between the first discharge and subsequent cycles, in-dicating that the structural changes were less significant in a thinner electrode (see Fig. 3.8d).

The cyclic performance and coulombic efficiency of VO-45 and VO-4 are displayed in Fig. 3.8e and f. As a general trend, the performance of the thin electrode (VO-4) is superior compared to that of the thick electrode (VO-45). At current density 10 mA g−1, the thin electrode (VO-4) delivered a capacity of about 489 mAh g−1, which was roughly three times larger than that of the thicker electrode (174 mAh g−1). However, the thin electrode (VO-4) suffered from more rapid fading (especially at low rates) than the thick electrode (VO-45). The capacity fading can be attributed to the dissolution of the vanadium in the electrolyte,[136] in addition to the other parasitic reactions associated with the structural water which might form LiOH during electrochemical cy-cling.[137,138]

24

Figure 3.8. (a) Cyclic voltammograms for a scan rate of 0.05 mV s−1 of electrodes VO-45 (blue), VO-12 (orange), VO-7 (green) and VO-4 (red); and (b) the corresponding voltage capacity profiles at a current of 10 mA g−1. (c) and (d) Voltage capacity profiles of electrodes VO-45 and VO-4, respectively, cycled at 10 mA g−1. (e) Cyclic performance of electrodes VO-45 (blue dia-mond) and VO-4(red circle) at different current densities. (f) Coulombic effi-ciency of electrodes VO-45 (blue square) and VO-4 (red circle) at different current densities with their respective standard deviations. Reproduced from paper I with permission from ©2016 The Royal Society of Chemistry.[55]

b a

c d

e f

25

3.4. Fabrication of V2O5∙nH2O Nanosheets and MWCNT (VOx-CNT) Composite

Although V2O5∙nH2O nanosheets are a high capacity electrode material for

lithium and sodium ion batteries, diffusion limitations usually reduce their ac-cessible capacity. The electrochemical performance of the FSE presented in paper I was dependent on the film thickness.[55] As the electrode thickness increased, the accessible capacity decreased and the internal resistance of the electrode increased. The electrode material needs to be modified to overcome the aforementioned limitations.

We fabricated a composite material based on V2O5∙H2O nanosheets and MWCNT. Generally, the addition of a conducting material such as MWCNT to V2O5∙H2O nanosheets should give a composite with a higher conductivity compared to the pristine nanosheets. In addition, freeze-drying provides tun-nels between the nanosheets, which enhance the accessibility of the composite material to the intercalating metal ions (e.g. Li+ or Na+) compared to the pris-tine nanosheets.

After optimizing different factors affecting the synthesis of the composite material, we obtained the best morphology from a suspension of V2O5∙H2O with 10% MWCNT, diluted with water in the ratio 1(suspension):2(H2O), and then freeze-dried. As we can see from the SEM images in Fig. 3.9 a c, freeze-drying increases the number of wrinkles in the V2O5∙nH2O nanosheets, and the MWCNT are embedded inside the V2O5∙nH2O nanosheets matrix. The de-tails of this optimization are summarized in the supporting information of pa-per II). The composite material was referred to as (VOx-CNT).

VOx-CNT was fabricated into two types of electrodes as schematically rep-resented in Fig. 3.9d f: (1) free-standing electrodes (FSE) of VOx-CNT@MWCNT paper; and (2) conventionally cast electrodes (CCE). The FSE was synthesized by drop-casting a VOx-CNT suspension onto MWCNT paper, followed by freeze-drying. The CCE was prepared by mixing freeze-dried VOx-CNT composite (80%) with carbon black (10%) and PVdF binder (10%) to form a slurry, which was cast on to a 20 μm thick aluminum foil current collector.

26

Figure 3.9. (a), (b), and (c) SEM images of freeze-dried V2O5∙H2O

nanosheets mixed with 10% MWCNT (VOx-CNT composite). (d), (e), and (f) Schematic representation of VOx-CNT composite, the free-standing electrode (FSE), and the conventionally cast electrode (CCE), respectively. Parts (a c) Reproduced from paper II with permission from ©2017 Elsevier Ltd.[130] Part (d) Reproduced from paper III with permission from ©2018 Elsevier Ltd.[139]

3.5. Electrochemical Performance of VOx-CNT Composite in Lithium Batteries: FSE vs. CCE

The use of VOx-CNT composite material as electrodes (FSE and CCE) in lithium batteries was examined by cyclic voltammetry and galvanostatic charging/discharging, in the potential window of 1.7 to 3.9 V vs. Li+/Li.

The cyclic voltammograms (CVs) for the CCE and FSE at a scan rate of 0.05 mV s−1 are displayed in Figure 3.10a and b, respectively. The CV of the FSE shows one pair of redox peaks at potentials of 2.5 and 2.7 V for reduction and oxidation, respectively. However, the CV of the CCE has two pairs of redox peaks. The reduction peaks present at 2.4 and 2.8 V, whereas the oxida-tion peaks exist at 2.7 and 2.9 V. The shoulder and the constant current region

a

b

c

d

e

f

MW-CNT Paper

F.D. VOx _10% MW-CNT

fF.D. VOx _10% MW-CNT + Carbon black + PVDF

Aluminum Foil

27

between 2.9 and 3.7 V in the CV of the FSE suggest the presence of a distri-bution of redox potentials rather than the well-defined redox potential. It worth noting that the FSE has a lower electronic conductivity compared to CCE which contains a 10% of carbon black. This variation of CVs shape with the method of electrode preparation has been observed in previous reports about V2O5∙nH2O gels.[54,140–143]

The voltage capacity profiles for the CCE and FSE are displayed in Figure 3.10c and d, respectively. The sloping plateau during the first discharge sug-gests the formation of a solid solution instead of irreversible phase changes. This can be attributed to the low crystallinity of the V2O5∙nH2O nanosheets. The FSE showed higher capacity than CCE, suggesting that some parts of the active material in the CCE might be screened by the binder. In addition, the capacity fading for the FSE is more drastic than for the CCE, probably due to the different surface areas of the electrodes. [43] Generally, capacity fading in V2O5∙nH2O electrodes is due to the dissolution of vanadium in the electrolyte and the parasitic reactions with structural water in the V2O5∙nH2O.[137]

Figure 3.10e and f show the long-term cycling and rate capabilities of CCE and FSE, respectively. At applied current densities of 10, 50, 100, and 200 mA g−1, the CCE delivered capacities of 175, 130, 117, and 93 mAh g−1, re-spectively, whereas the FSE displayed capacities of 300, 200, 138 and 97 mAh g−1, respectively. Notably, the capacities of CCE and FSE were retained at low current density after cycling them at high current densities. In addition, the V2O5∙H2O nanosheet structure was maintained after cycling as confirmed by XRD (see Fig. S5 in supporting information of paper II). On the other hand, the coulombic efficiency of the CCE was close to 100% at different rates. Whereas the coulombic efficiency of the FSE was low in the initial cycles; however, it improved upon cycling and approached 100% at different rates.

All in all, the electrochemical performance of the FSE prepared from the VOx-CNT composite was outstanding compared to the FSE synthesized conventionally.[55] For example at current densities of 10 and 200 mAg−1, the FSE electrode (47 μm thick) fabricated using VOx-CNT displayed discharge capacities of 300 and 97 mAh g−1, respectively, whereas the FSE (45 μm thick-ness) prepared by the conventional method delivered capacities of 150 and 50 mAh g−1, respectively. Therefore, the modifications introduced on the elec-trode fabrication using VOx-CNT composite enhanced the accessibility of the electrodes toward lithium ions, and thus improved the accessible capacity. Ta-ble 3.1. summarizes the electrochemical performance of the hydrated vana-dium oxide electrodes reported in this thesis and previously reported literature data.

28

Figure 3.10. Comparison of the electrochemical behaviors of the CCE and

FSE. (a) and (b) Cyclic voltammograms of the first few cycles at a scan rate of 0.05 mV s−1 for the CCE and FSE, respectively. (c) and (d) Discharge and charge curves at an applied current of 10 mAg−1 for the CCE and FSE, respec-tively. The numbers on the plots refer to cycle number. (e) and (f) Cyclic per-formance and coulombic efficiency for the CCE and FSE, respectively. Re-produced from paper II with permission from ©2017 Elsevier Ltd.[130]

a b

c d

e f

29

Table 3.1. Comparison of the electrochemical performance in lithium bat-teries for the hydrated vanadium oxide electrodes reported in this thesis and in the literature. Reproduced from paper I by permission of ©2016 The Royal Society of Chemistry.[55]

Electrode material

description Morphology Capacity

(mAh g−1)

C-rate or current density

(mA g−1)

Potential window V (vs. Li+/Li)

Ref.

V2O5∙nH2O / MWCNT paper (FSE, 4 μm thick)

Ultrathin nanosheets

280 200 1.7–3.9 Paper I [55]

V2O5∙nH2O / MWCNT paper (FSE,45 μm thick)

Ultrathin nanosheets

50 200 1.7–3.9 Paper I [55]

VOx-CNT composite

(FSE, 47 μm thick)

Ultrathin nanosheets

97 200 1.7–3.9 Paper II [130]

VOx-CNT composite (CCE)

Ultrathin nanosheets

93 200 1.7–3.9 Paper II [130]

V2O5∙nH2O / CNT (FSE)

Nanobelts 241 200 2.0–4.0 [43]

V2O5∙nH2O xerogel /

graphene (CCE)

Ribbons 239 30 1.5–4.0 [69]

V2O5∙nH2O xerogel (CCE)

2D flakes 300 200 1.5–4.0 [27]

V2O5∙nH2O xerogel (CCE)

Network of long ribbons

250 0.1 C 1.5–3.6 [144]

V2O5∙nH2O xerogel / 2%

graphene (CCE)

Network of long ribbons

419 0.1 C 1.5–3.6 [144]

V2O5 aerogel (CCE)

Network of ribbons

291 0.25 C 1.5–4.0 [61]

30

3.6. Electrochemical Performance of VOx-CNT Composite as a Negative Electrode for Sodium-ion Batteries

The electrochemical behavior of VOx-CNT composite as a negative elec-trode for sodium-ion batteries was examined by cyclic voltammetry and gal-vanostatic charge/discharge techniques in 1 M NaPF6 dissolved in 1:1 mixture of EC and DEC. In all experiments, the lower cut-off was limited to 0.1 V (vs. Na+/Na); a variety of upper cut-off potentials were explored. The CVs indicated that lowering the upper cut-off voltage from 3 V to 2.5 V (vs. Na+/Na) increased the coulombic efficiency (see Fig. 2 in paper III).

Similar to the results obtained in lithium batteries, the galvanostatic dis-charge of the VOx-CNT composite between 0.1–2.5 V displayed a sloping plateau (Fig. 3.11a), indicating the low crystallinity of the composite mate-rial.[66,128] The first discharge cycle at a current density of 10 mA g−1 showed a capacity of 315 mAh g−1, whereas the following cycles delivered a reversible capacity of 140–150 mAh g−1. The irreversible capacity loss after the first discharge can be explained by the formation of the solid electrolyte interphase (SEI) layer and side reactions with structural water in V2O5·0.5H2O.[66] Interestingly, the contribution of MWCNT to the measured capacity was found to be minor, and thus the major contribution to the capacity came from the V2O5·nH2O nanosheets (see Figure 4a in paper III). Also, the MWCNT contributed to the irreversible capacity loss which occurred on the first discharge cycle. Notably, when the upper cut-off potential was limited to 2.0 V, the reversible capacity was lowered to 100 mAh g−1, whereas the cou-lombic efficiency did not show a significant change (Fig. 3.11c).

On the other hand, the voltage capacity profiles of V2O5·0.5H2O nanosheets (in absence of MWCNT) at a current density of 10 mA g−1, showed a discharge capacity of 235 mAh g−1 on the first cycle, whereas the subsequent cycles displayed a reversible capacity of 90–97 mAh g−1 (see Fig. 3.11b). Likewise, when the upper cut-off was limited to 2.0 V, the reversible capacity was lowered to 80 mAh g−1, and the coulombic efficiency also did not change much (see Fig. 3.11d). Therefore, in the subsequent electrochemical experi-ments, the cells were cycled between 0.1 and 2.5 V.

31

Figure 3.11. Comparing the electrochemical behavior of VOx-CNT com-

posite and V2O5·0.5H2O nanosheet anodes: (a) and (c) Discharge/charge pro-files of VOx-CNT composite at a current density of 10 mA g−1 and upper cut-off voltages of 2.5 and 2.0 V, respectively. (b) and (d) Discharge/charge pro-files of V2O5·0.5H2O electrodes at a current density of 10 mA g−1 and upper cut-off voltages of 2.5 and 2.0 V, respectively. (e) and (f) Cyclic performance and coulombic efficiency of VOx-CNT composite and of V2O5·0.5H2O elec-trodes, respectively, at different applied current densities (upper cut-off volt-age 2.5 V). Reproduced from paper III with permission of ©2018 Elsevier Ltd.[139]

c d

a

e f

b

32

The cyclic performance of VOx-CNT and V2O5·nH2O nanosheets are shown in Fig. 3.11e and f, respectively. At current densities of 20, 50, and 100 mA g−1, the VOx-CNT composite delivered capacities of 140, 95, and 45 mAh g−1, respectively. Notably, the capacity at low current density was retained after cycling at high rates (see Fig 4b in paper III). The capacity decay during electrochemical cycling was probably due to vanadium dissolution in the elec-trolyte [66], and could be decreased by using other electrolytes with low va-nadium dissolution rates.[145] Alternatively, the V2O5·0.5H2O electrodes showed capacities of 104, 57, and 30 mAh g−1 at applied current densities of 10, 40, and 100 mA g−1, respectively (see Fig. 3.11f). It worth noting that the first cycle for either VOx-CNT composite or V2O5·0.5H2O electrodes showed a low coulombic efficiency (about 40%); however, the efficiency improved upon cycling and approached 100% at different rates.

All in all, the electrochemical performance of the VOx-CNT composite an-ode was superior to that of V2O5·0.5H2O nanosheets without MWCNT, which reflects the significant role played by MWCNT. Probably, this improvement is due to the enhancement of the electrode’s electronic conductivity, in addi-tion to, the formation of more tunnels between the V2O5·0.5H2O nanosheets.

Generally, SIBs suffer from less stable SEI layers than LIBs, due to the higher solubility of sodium carbonate than lithium carbonate salts in the nonaqueous electrolytes.[146–148] Therefore, many studies have focused on developing electrode materials for SIBs with robust SEI layers. The stability of the SEI layer can be estimated using a “pause test” during galvanostatic charge/discharge. Typically, the cell was initially cycled for a few cycles, and then stopped at the fully sodiated (discharged or reduced) state for a given period (pause time). During this pause, no external current or potential was applied and the cell potential was recorded as a function of time. This process was repeated for different pause intervals (see Fig. 3.12a). The capacity loss during the pause was measured by subtracting the charge capacities before and after the pause.

Figure 3.12a displays the voltage time profiles for VOx-CNT anode during a pause test. As we can see from Fig 3.12b, the capacity and coulombic effi-ciency for the VOx-CNT anode decreased after the pause time, reflecting the presence of self-discharge in the cell under investigation. Notably, upon age-ing the VOx-CNT anode for 122 h (Fig. 3.12b and c), the capacity loss was less than 10% and the increase in the open circuit voltage was minor (dU~0.46 V). These results are similar to those reported for hard carbon,[148] and indi-cate that the SEI layer of VOx-CNT electrode is quite robust.

33

Figure 3.12. Pause test for VOx-CNT anode: (a) Charge/discharge cycling at a current density of 10 mAh g−1 accompanied by different “pause” times between 1 and 122 h. (b) Variation of discharge (black diamonds) and charge (red diamonds) capacities, and their corresponding coulombic efficiency (blue circles) with cycle number. (c) Charge capacity loss (red diamonds) and po-tential change (black circles) as a function of pause time. Reproduced from paper III with permission from ©2018 Elsevier Ltd.[139]

a

b c

34

4. Water Based Fabrication of MoO3-x Nanosheets with Tunable Plasmonic Resonance: Photocatalysts for Dye Degradation and Electrode Material for Supercapacitors

4.1. Aqueous Exfoliation of MoO3: Structural, Morphological, and Optical Properties of MoO3-x

Nanosheets Water molecules possess affinity to intercalate into layered metal oxides

such as α-MoO3.[149] Water intercalation usually weakens the bonds holding the layers together, allowing the layered material to be exfoliated by agitation or sonication. In a standard exfoliation experiment, the bulk precursor (α-MoO3) was dispersed in water and then refluxed at 80 °C for 5–7 days. The suspension color changed from white to yellow as a result of the formation of MoO3-x nanosheets. The exfoliated nanosheets were then separated from re-maining bulk precursor via centrifugation (speed 10,000 rpm). The yield ranged from 17 to 60% depending on the concentration of the starting suspen-sion and the reflux temperature. The yield could be increased by raising the reflux temperature and lowering the suspension concentration. When the nanosheets were dried in air at 80 °C for 2–5 h, they formed fine green strips (see Fig. S1c, supporting information, paper IV). Figure 4.1 shows a schematic representation of the exfoliation process.

The exfoliation of bulk MoO3 was followed by XRD. As can be seen from Fig. 4.2a, the exfoliated material can be assigned as a mixture of orthorhombic α-MoO3 and hydrated MoO3 (h-MoO3). Notably, when the exfoliation was carried out under dark conditions the exfoliation did not affect much, indicat-ing that visible light was not crucial to the exfoliation process. In the following sections, the MoO3-x nanosheets were fabricated via refluxing at 80 °C for seven days under ambient light, and the exfoliated product was referred to as MoO3-x-I.

35

Figure 4.1. Schematic representation of the aqueous exfoliation of bulk α-

MoO3 into MoO3-x nanosheets. Reproduced from paper IV with permission from ©2018 American Chemical Society.[150]

The oxidation states of Mo and the oxygen vacancies in the MoO3-x-I nanosheets were explored by XPS analysis. The Mo 3d spectrum showed a doublet at 233.0 and 236.1 eV which were assigned to Mo6+ 3d5/2 and Mo6+ 3d3/2, respectively (Fig. 4.2b). For the O1s spectrum, a broad asymmetric peak was observed. This peak could be deconvoluted into three peaks (Fig. 4.2 c). The red peak at 530.9 eV referred to O2−, and occurred at slightly lower en-ergy than that reported for commercial α-MoO3 (531.3 eV),[151] indicating that coordination environment between the O and Mo atoms in MoO3-x-I nanosheets differed from that in α-MoO3. Also, it has been reported earlier that the shift of the O 1s peak to a lower binding energy is due to the transfer of electrons to oxygen vacancies.[151,152] The blue peak at 531.7 eV repre-sented the surface adsorbed species (OH¯, O¯, or oxygen vacan-cies).[151,153,154] Lastly, the orange peak at 532.8 eV can be attributed to adsorbed water. [151,153,154] Raman and Fourier transform infrared (FTIR) spectra were also used to confirm that the chemical bonding and coordination environment in MoO3-x-I nanosheets were different from those in bulk α-MoO3 (see paper IV).

Bulk MoO3 BBulklk MM OoO

Centrifugation, Drying

Exfoliated MoO3-x

Unexfoliated MoO3 E f li d M O Unexfoliated

Reflux in water, 80 °C

7 days

36

Figure 4.2. Structure, chemical composition and optical properties of

MoO3-x-I nanosheets: (a) XRD patterns of exfoliated nanosheets using 0.45 (in the dark), 0.90, and 1.80 g of bulk α-MoO3 precursor, as well as standard pat-terns for h-MoO3 and α-MoO3. (b) and (c) Mo 3d and O 1s XPS spectra of MoO3-x-I nanosheets, respectively (d) and (e) UV−vis and NIR spectra, re-spectively, of MoO3-x-I nanosheets in aqueous solution under visible light ir-radiation for a given interval. Reproduced from paper IV with permission from ©2018 American Chemical Society.[150]

The optical properties of the MoO3-x-I nanosheets were explored using UV-vis-NIR spectroscopy. The nanosheets displayed a continuous absorption over the UV-vis region with a broad absorption peak at 320 nm (Fig. 4.2 d). The absorption in the visible region provided an explanation for the light green color of the MoO3-x-I nanosheets in aqueous suspension. This visible

c

b a

e d

37

absorption can be explained by the existence of d electrons coming from the intervalence charge-transfer between Mo5+ and Mo6+.[155]

The effect of visible light irradiation on the absorbance of MoO3-x-I nanosheets was examined by recording their UV-vis-NIR spectra under light irradiation (Fig. 4.2 d and e). The visible light absorption (500–800 nm) de-creased as the irradiation time increased. Notably, a couple of weak absorption peaks appeared at wavelengths of 954 and 1160 nm (Fig. 4.2 e). These peaks represented the localized surface plasmon resonance (LSPR) of MoO3-x-I

nanosheets [81,105–107] The intensity of the LSPR peaks, which can be at-tributed to the excitation of the free electrons by incident light, increased with irradiation time and reached a maximum after 10 min. [155] It is worth noting that, if the spectra backgrounds were subtracted, the intensity of the LSPR peaks remained unchanged after 10 min of light irradiation.

The pristine α-MoO3 had a plate morphology, as indicated by the SEM im-age in Fig. 4.3a. However, the exfoliated MoO3-x-I was composed of fine nanosheets with a few nanometer thickness (Fig. 4.3b–d). The AFM indicated that the MoO3-x-I nanosheet thickness was about 10 nm (Fig. 4.3e and f). Moreover, the aqueous suspension of MoO3-x-I displayed a Tyndall effect, in-dicating the formation of homogeneous and stable MoO3-x-I nanosheet sus-pension (inset Fig. 4.3 c).

Figure 4.3. Morphology of MoO3-x-I nanosheets: (a) SEM image of the bulk

α-MoO3. (b) and (c) SEM images of MoO3-x-I nanosheets at different magni-fications. Inset in (c) displays a photograph showing the Tyndall effect in an aqueous suspension of the MoO3-x-I nanosheets. (d) and (e) TEM and AFM images of the MoO3-x-I nanosheets. (f) Height profile of the highlighted dashed lines in (e). Reproduced from paper IV with permission from ©2018 American Chemical Society.[150]

1 μm

a b

d e

c

f

38

4.2. Aqueous Exfoliation of MoO2/MoO3 Mixtures: Structural and Morphological Properties of MoO3-x

Nanosheets Similar to exfoliation of bulk MoO3, we examined the exfoliation of a se-

ries of MoO3/MoO2 mixtures with different weight ratios. In particular, mix-tures of MoO3 and MoO2 in 4:1, 1:1, and 1:4 weight ratios were tested as pre-cursors for the exfoliation process. Among them, the most interesting samples were the 4:1 and 1:4 mixtures of MoO3 and MoO2; the exfoliated products obtained from them are referred as MoO3-x-II, and MoO3-x-III, respectively.

In a standard experiment, the molybdenum oxide mixture was dispersed in an appropriate amount of water and then refluxed at 80 °C for about 5–7 days. At the end of reflux, the unexfoliated materials were separated from the exfo-liated nanosheets by centrifugation at high speed (13,500 rpm) for 5–10 min. The unexfoliated materials were precipitated and the nanosheets remained dis-persed in the mother liquor, which had a deep blue color. The aqueous disper-sion of nanosheets was then dried at 80 °C for 5–8 h. The dried nanosheets were black–blue in color, and the yield was about 50–60%.

As displayed in Fig. 4.4a, the pristine MoO3-x-II was quite poorly crystal-line, with few resolved PXRD peaks, and thus it was difficult to distinguish its molybdenum oxide phase. However, the thermodynamically stable phase α-MoO3 was formed upon annealing MoO3-x-II at 350 °C, as shown by in situ XRD in Fig 4.4a. The TEM and SEM images in Fig. 4.4b–d, indicated that MoO3-x-II existed as nanosheets with a lateral size of 100–300 nm. The dif-fraction spots in the SAED image (Fig. 4.4e) approved that the nanosheets maintained an appropriate degree of ordered stacking.

Likewise, pristine MoO3-x-III had a very low crystallinity and was nearly amorphous, therefore, produced quite broad XRD peaks. The pristine phase was difficult to assign, but α-MoO3 was formed upon heating to 350 °C (see Fig. 4.4f). Electron microscopy confirmed that MoO3-x-III maintained a nanosheet morphology with lateral size in the range 150–350 nm (Fig. 4.4g–i). In addition, the SAED displayed that MoO3-x-III nanosheets had a lower degree of ordered stacking than MoO3-x-II (Fig. 4.4j).

39

Figure 4.4. Morphology and structure of exfoliated nanosheets: (a) XRD

patterns measured at temperatures of 25–600 °C, (b) and (d) SEM images (c) TEM image, and (e) SAED for sample MoO3-x-II (the inset shows the crystal from which the SAED is obtained). (f) XRD patterns measured at tempera-tures of 25–600 °C, (g) and (i) SEM images (h) TEM image, and (j) SAED for sample MoO3-x-III (the inset in (j) displays the crystal from which the SAED is collected). Reproduced from paper V with permission from ©2018 Wiley–VCH.[24]

2 μm

500 nm

b c

d e

a

2 μm

2 μm

g h

i j

f

40

4.3. Solar Light Irradiation of MoO3-x Nanosheets The pristine MoO3-x nanosheets prepared using different precursors showed

quite different UV-vis-NIR absorption spectra (Fig. 4.5 b–d, dashed lines). All samples displayed a main absorption peak at a wavelength of 425–450 nm, which explains the blue/green color of the samples. MoO3-x-II and MoO3-x-III nanosheets absorbed light in the vis-NIR regions, and these absorption peaks could be assigned as LSPR. In particular, the MoO3-x-III nanosheets showed a couple of well-defined LSPR peaks at wave lengths of 665 and 1000 nm. These can be explained by the presence of oxygen vacancies in MoO3-x-III compared to other samples, as confirmed by XPS analysis (see Fig. S2, sup-porting information of paper V).

Figure 4.5a shows the effect of solar light irradiation on the MoO3-x nanosheet suspensions dispersed in 1:1 water/ethanol targeting different peri-ods ranging from 10 min to 5 h. The suspension colors became deeper as the irradiation time increased. Notably, after 2 h of solar light irradiation, the UV-vis spectra of all MoO3-x nanosheet samples showed LSPR peaks (see dotted lines Fig. 4.5. b–d). The XPS spectra measured after 2 h of irradiation revealed that all samples contained a reasonable amount of Mo5+ which increases the amount of oxygen vacancies in the nanosheets, and consequently the LSPR response increases. It is worth noting that the LSPR peaks appeared at slightly different wavelengths for different samples (MoO3-x-I at 690 and 990 nm, whereas MoO3-x-II and MoO3-x-III at 620, 800, and 1000 nm), suggesting that the structure and the local environment of Mo and O atoms were different in each sample.

Interestingly, MoO3-x-I displayed the highest LSPR signal after 5 h of irra-diation compared to MoO3-x-II and MoO3-x-III (see solid lines in Fig. 4.5. b–d). Moreover, after aging the irradiated suspensions for six days in the dark, the LSPR signal for MoO3-x-I was clearly observed, whereas those of MoO3-x-II and MoO3-x-III decreased back to their initial states or even lower (see Fig. 3 in paper V).

41

The changes in the optical properties of MoO3-x nanosheets suggest that the nanosheets were transformed to hydrogen molybdenum bronze (HxMo1-x

6+ Mox5+O3) upon solar light irradiation.[81,107,156,157] The process can be summarized as follows: (1) when a photon whose energy exceeds the band gap energy of MoO3 is incident on MoO3, it generates holes and electrons (equation 4.1); (2) the holes ( ) react with the adsorbed water molecules to produce protons (H+) which intercalate in MoO3 to form HxMo1-x

6+ Mox5+O3 (equation 4.2 and 4.3); (3) the further irradiation of HxMo1-x

6+ Mox5+O3 gener-ates water molecules to form Mo1-x6+ Mox

5+O3-x 2 (equation 4.4));[81,107] (4) lastly the intervalence charge transfer between Mo5+ valence band and Mo6+ conduction bands causes visible changes in the suspension colors (equation 4.5).[156]

MoO3hν

MoO3*+ h++ e (Equation 4.1)

2h++ H2O→ 2H++ 12 O

2 (Equation 4.2)

MoO3 + xH++xe → HxMo1-x6+ Mox5+O3 (Equation 4.3)

HxMo1-x6+ Mox5+O3

hν Mo1-x6+ Mox

5+O3-x 2+ H

2O (Equation 4.4)

MoA6++ MoB

5+ hν

MoA5++ MoB

6+ (Equation 4.5)

42

Figure 4.5. Effect of solar light irradiation on the plasmonic resonance of

MoO3-x nanosheets: (a) Photographs of samples MoO3-x-I (left), MoO3-x-II (middle), and MoO3-x-III (right) dispersed in 1:1 H2O/ethanol (v/v ratio) and irradiated for 10 min – 5 h. (b), (c), and (d) UV-vis-NIR spectra for samples MoO3-x-I, MoO3-x-II, and MoO3-x-III, respectively, pristine (dashed) and after 2 h (dotted) and 5 h (solid) of solar light irradiation. Reproduced from paper V with permission from ©2018 Wiley–VCH.[24]

2 h

1 h

30 min

20 min

10 min

0 min

5 h

III I II a b

c

d

43

4.4. Photocatalytic Activity of MoO3-x Nanosheets for Organic Dye Degradation

Textile manufacturing includes the extensive use of organic dyes which are considered to be environmental threats in stream water.[158] Plenty of ap-proaches for treating dye-contaminated water have been reported , including photocatalysis[159], adsorption [160], and others.[161,162] For photocata-lytic degradation, a variety of materials have been used, such as zeolitic imid-azolate frameworks (ZIF-8) [163], and Degussa TiO2 (P25) and ZnO [164].

Herein, we examined the degradation of RhB and MB dyes under visible light irradiation in the presence of MoO3-x-I nanosheets. For comparison, bulk α-MoO3 was also explored. The UV-vis absorption spectrum of the pristine RhB solution displayed absorption peaks at 550 and 515 nm, whereas, the MB absorption peaks occurred at 665 and 605 nm (see black lines in Fig. 4.6a and b). These peaks refer to the monomeric (0–0 band) and dimeric (0–1 band) forms of the corresponding dye.

Interestingly, the addition of the MoO3-x-I nanosheets photocatalysts to the dye solution decreased the intensities of these absorption peaks, suggesting the photodegradation of the dye under investigation (Fig. 4.6a and b). Moreo-ver, the shape of absorption peaks changed instantaneously when nanosheets were added to the dye solution (see the red curves in Fig. 4.6a and b), indicat-ing that the reaction was very fast. The peak broadening suggests the modifi-cation of the dye chromophoric group upon photodegradation. When bulk MoO3 was used as the photocatalyst, the shape of the absorption peaks did not change much, though their intensity decreased with time (see Fig. 4.6c and d). The MoO3-x-I nanosheets removed the dye more efficiently than bulk MoO3, as shown in Fig. 4.6.e and f.

The photocatalytic dye degradation in this system can be attributed to the strong chemisorption between MoO3-x-I nanosheets and the dye, as well as the formation of reactive radical species such as OH• and O2

−•.[79,84] Giving that bulk MoO3 and MoO3-x-I nanosheets have low surface area (BET and Lang-muir surface areas, respectively are 4 and 6 m2/g for bulk MoO3, and 2 and 3 m2/g for MoO3-x-I), therefore the reactive radical species offer the key effect for the dye degradation. Furthermore, the dye degradation was confirmed us-ing electrospray ionization mass spectrometry (ESI-MS) and FTIR (see Fig.6 and S5 in paper IV).

44

Figure 4.6. Photocatalytic activity of MoO3-x-I nanosheets for dye degrada-

tion: (a) and (b) UV-vis spectra of RhB and MB, respectively, upon irradiation in the presence of MoO3-x-I nanosheets. (c) and (d) UV-vis spectra of RhB and MB, respectively, upon irradiation in the presence of bulk α-MoO3. (e) and (f) Removal efficiency of RhB and MB, respectively, using MoO3-x-I nanosheets (red) and bulk α-MoO3 (blue). Reproduced from paper IV with permission from ©2018 American Chemical Society.[150]

d

d d

a db

c d

de df

45

4.5. Application of MoO3-x Nanosheets in Supercapacitors

The electrochemical behavior of MoO3-x nanosheets as binder-free elec-trodes for supercapacitor applications was investigated in a three-electrode configuration using cyclic voltammetry. Platinum mesh and Ag/AgCl/3.5 M KCl were used as the counter and reference electrodes, respectively. Typi-cally, the binder-free electrodes were prepared by drop-casting the active ma-terial onto a piece of carbon paper (Quin-Tech, H23), and the electrode area was about 1 cm2. Previous reports have shown that, the method of electrode drop-casting, the electrolytes pH , and the potential window of cycling signif-icantly affect the electrochemical performance of the MoO3-x nanosheets. [96–98,165,166] After optimization, the MoO3-x nanosheet electrodes were cycled between 0.0 and 0.8 V (vs. Ag/AgCl/3.5 M KCl) in acidified 1M Na2SO4 with pH 1.62 (a detailed description is provided in paper V). [24]

Fig. 4.7a displays the CVs at scan rate of 20 mV s−1 for two electrodes with the same mass loading (25 μg cm−2) but, cast in the presence and absence of ethanol. Interestingly, the addition of ethanol during electrode casting in-creases the accessible capacity. This can be attributed to the moderation of the hydrophilic-hydrophobic interactions between the aqueous MoO3-x nanosheet suspension and the hydrophobic carbon paper. Notably, a couple of sharp well-defined redox peaks were observed in the CVs, previous studies have reported that these reversible cathodic peaks involve the reduction of Mo6+ down to Mo5+ and Mo4+, while the anodic peaks are due to the corresponding oxidations.[96,98] Since analogous CVs were obtained when 1 M H2SO4 was used as the electrolyte, protons (H+) most likely act as the counterions in the redox reactions of the MoO3-x nanosheets. The contribution from the carbon paper to the measured capacity was found to be very small compared to that of the nanosheets (see Fig. S4c in paper V).

As mentioned earlier in Chapter 3, binder-free electrodes have many ad-vantages such as the ability to be used as flexible electrodes.[55,131,167] However, the lack of conducting additive combined with the limited conduc-tivity of the active material, limit the thickness of active material on the elec-trodes. The electrochemical performance for electrodes of mass loadings rang-ing from 6.25 to 50.00 μg cm−2 at low (20 mV s−1) and high (1000 mV s−1) scan rates are presented in Fig 4.7b-d. As a general trend, the accessible ca-pacities at different rates decrease as the mass loadings increase from 6.25 to 25.00 μg cm−2. However, a significant drop in the capacity was observed when the loading was increased to 50.00 μg cm−2. As the electrode area was roughly equal for all of the electrodes, the active film thickness should increase with the mass loading. Consequently, the surface part of the deposited active ma-terial is only accessible to the electrolyte, and thus the gravimetric capacity of the thick electrode was lower than that of the thin electrode.

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Interestingly, long-term cycling of the electrodes with different mass loadings, displayed their stability and low capacity fading over 1000 cycles at a low rate (see Fig 4.7c). Likewise, the electrodes offered stable performance over 800 cycles when cycled at a high rate after 1000 cycles at a low rate (Fig. 4.7d).

Figure 4.7. Optimizing the casting method and electrode mass loading: (a) cyclic voltammograms measured at a scan rate of 20 mV s−1 for electrodes cast in the presence (black) and absence (red) of ethanol. (b) Variation of the discharge capacity of the tenth cycle with the electrodes mass loading at scan rates of 20 and 1000 mV s−1. (c) and (d) Variation of the discharge capacity with cycle number for electrodes of mass loadings of 6.25, 12.5, 25, and 50 μg cm−2, at scan rates of 20 and 1000 mV s−1, respectively. Reproduced from paper V with permission from ©2018 Wiley–VCH.[24]

The MoO3-x nanosheets showed a promising high rate performance as

shown in Figs. 4.8a-d. For instance, the electrode with a mass loading of 25 μg cm−2 provided discharge capacities of 116, 93, 80, and 62 C g−1 at scan rates of 5, 50, 200, and 1000 mV s−1, respectively. Moreover, the coulombic efficiency was close to 100% at different rates, with exception that at very low

a

c d

b

47

scan rate (5 mV s−1) the charge capacity was slightly lower than the discharge one. This observation suggests that an irreversible slow reaction might occur at low rates; this is most likely the irreversible formation of MoO2.[98] This irreversible reaction is not observable at moderate and high rates due to time limitations. Notably, the peak-to-peak separation did not expand to a high ex-tent when increasing the scan rate (<50mV for scan rate ≤200 mV s−1), reflect-ing the fast kinetics of the redox reaction of the MoO3-x nanosheets (see Fig 4.8d).

Figure 4.8. Rate capability test at different scan rates: (a) and (b) cyclic voltammograms at different scan rates. (c) Variation of charge-discharge ca-pacity with the scan rate, and the corresponding coulombic efficiency. (d) Var-iation of peak-to-peak separation (dU) with the scan rate. Reproduced from paper V with permission from ©2018 Wiley–VCH.[24]

a b

c

1

1ˋ

2

2ˋ

1

1ˋ

2

2ˋ

d

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5. Conclusions

To summarize, this thesis introduced novel water based exfoliation meth-ods to fabricate vanadium and molybdenum oxide nanosheets from commer-cially available precursors. These nanosheets were applied for energy storage and photocatalytic dye degradation applications.

The synthesis method involved refluxing the metal oxide precursor(s) in water at 60–80 °C for a few days. The water acted as the solvent and interca-lating molecules, which enabled the exfoliation of the metal oxide precursor. The method was not limited to the precursors with layered structure, but also those with non-layered structure could be exfoliated. The prepared nanosheets were explored by characterization techniques including SEM, TEM, ED, XPS, TGA, and XRD.

V2O5·0.5H2O nanosheets@MWCNT paper were used as FSEs for lithium battery applications. Their electrochemical performance were a function of the mass loadings and/or active material film thickness; thicker films had lower accessible capacities.

A composite material of VOx-CNT was prepared by freeze-drying a mix-ture of V2O5·0.5H2O nanosheets with 10% MWCNT. FSE and CCE of VOx-CNT composite were fabricated and tested for lithium battery applications. The FSE delivered higher gravimetric capacity compared to the CCE; how-ever, the coulombic efficiency of the CCE was better than that of the FSE, especially in the first few cycles. On the other hand, the FSE of VOx-CNT composite displayed a superior electrochemical performance compared to the traditionally prepared FSE of V2O5·0.5H2O nanosheets@MWCNT paper, in-dicating the freeze-drying and the addition of 10% MWCNT significantly af-fected the electrodes’ electrochemical performance.

The VOx-CNT composite was also used as a negative electrode material for a sodium-ion battery and showed a quite robust SEI layer which was compa-rable to hard carbon electrodes. The comparison between the VOx-CNT com-posite electrodes and the V2O5·0.5H2O nanosheet electrodes (i.e. without MWCNT) revealed that the MWCNTs enhanced the accessible capacity of V2O5·0.5H2O nanosheets and stabilized their electrochemical behavior.

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The MoO3-x nanosheets displayed interesting optical properties that could be tuned by solar light irradiation and/or the ratio of starting molybdenum ox-ide precursors. The MoO3-x nanosheets offered an outstanding localized sur-face plasmon resonance, especially after solar light irradiation. Moreover, the nanosheets could be used as photocatalysts for organic dye (MB and RhB) degradation. The degradation was completed within 10 min under visible light irradiation.

The MoO3-x nanosheets showed a promising high rate when employed as supercapacitor electrodes in an acidified Na2SO4 electrolyte. The electrode drop-casting method, electrolyte pH, and electrode mass loadings have crucial effects on the performance of the MoO3-x nanosheet binder-free electrodes. The optimum electrode mass loading was about 25 μg cm−2, and the optimum electrolyte pH was about 1.62. Under optimum conditions, the electrodes de-livered discharge capacities of 110 and 75 C g−1 at scan rates of 20 and 1000 mV s−1, respectively.

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6. Future Outlook

This thesis introduced efficient scalable methods to fabricate TMO nanosheets dispersed in water. However, all the electrochemical application of the FSE involved the use of hydrophobic carbon based substrates. In future work, the TMO suspension could be cast on a hydrophilic flexible substrate (e.g. cellulose with conducting additive). We would expect the TMOs to be cast more homogeneously over a hydrophilic substrate, and thus we could ob-tain a conformal coating over the cellulose nanofibers which could increase the volumetric capacity of the tested electrodes. Furthermore, the use of a solid-state electrolyte, to form an all-solid-state device, is another interesting topic to investigate for our FSEs. For the conventionally cast electrodes, the use of a water soluble binder is also an interesting point to be explored for these water based metal oxide nanosheet suspensions.

Another important issue is the use of these VOx nanosheets in the newly developed battery systems beyond lithium-ion batteries. We have introduced one example in which the TMO nanosheets were applied in SIB; however, other battery systems such as Zn-ion batteries, Mg-ion batteries, and K-ion batteries can be explored in the future.

Finally, the water based exfoliation technique could be modified and ap-plied to other transition metal oxides or chalcogenides such as manganese ox-ides and molybdenum disulfide.

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7. Sammanfattning på Svenska

Tvådimensionella (2D) övergångsmetalloxider (TMO) är nanotunna skikt som kan skapas av modermaterialet av samma förening. Nya unika fysikaliska och kemiska egenskaper erhålls jämfört med den tredimensionella moderför-eningen. Syntesen av 2D-TMO bygger traditionellt på användning av miljö-belastande organiska lösningsmedel. I denna avhandling utvecklas miljövän-liga strategier för att tillverka TMOs nanoskikt. I synnerhet har nanoskikt av hydratiserad vanadinpentoxid (V2O5∙nH2O) och molybdenoxid (MoO3-x) med defekter i syreinnehåll syntetiserats. Kolnanotubpapper (MWCNT) belades med nanoskikt av V2O5∙nH2O och studerades sedan som en fristående positiv elektrod (FSE) i ett litiumbatteri. Den tillgängliga kapaciteten hos en FSE be-ror på elektrodtjockleken; Den tjockaste elektroden levererade den lägsta till-gängliga kapaciteten. Som ett alternativt framställdes också ett komposit-material i form av nanoskikt av V2O5∙nH2O med 10% MWCNT (VOx-CNT-komposit) och två olika typer av elektroder skapades. En FSE och en kompo-sitelektrod (CCE) användes som katodmaterial för litiumbatterier. En detalje-rad jämförelse mellan dessa båda elektroder presenteras. Dessutom användes VOx-CNT-kompositen som en negativ elektrod för ett natriumjonbatteri och gav en reversibel kapacitet på ca 140 mAh g−1. I avhandlingen ingår också en studie av nanoskikten av MoO3-x i form av bindemedelsfria elektroder för su-perkondensatorer i en surgjord Na2SO4-elektrolyt. Vidare studerades också nanoskikten av MoO3-x som fotokatalysator för nedbrytning av organiska fär-gämnen. De enkla miljövänliga syntesmetoderna i kombination med egen-skaperna hos TMO-nanoskikten är de viktiga resultaten i denna avhandling som visar potentialen för 2D-materialsyntes och för deras olika energirelate-rade tillämpningar.

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8. Acknowledgements

I’d like to acknowledge my supervisor Junliang Sun for his guidance during my PhD study. Although, you are working part time at SU, I always got good and prompt communication with email. I want to acknowledge my co-super-visor Kristina Edström, and I was really lucky to be involved in your group at Ångström laboratory (the largest battery group in the Nordic Countries). You helped me a lot and created a lot of connections within the group. I want to thank Reza Younesi for his guidance through my PhD study; I learned a lot from you and gained experience in battery systems beyond lithium-ion, and I always felt that you are a friend more than a supervisor. I’d like to thank Leif Nyholm for his guidance especially in aqueous systems, supercapacitors and conversion materials; I learned a lot form your deep electrochemistry knowledge. Thanks to my second co-supervisor Xiaodong Zou; I always got valuable comments and suggestions from you during the group meetings.

Thanks to all friends and collaborators and colleagues at structural chemis-try Ångström laboratory; Habtom Asfaw, Torbjörn Gustafsson, Marco, Mario, Henrik, Fredrik, Zhaohui, Tim, Matt, Andoria, Ronnie, Yonas, Ruijun, Ste-ven, David, and Taha. I’d like to thank also our collaborators at Peking Uni-versity, Youyou, Ligang, and Tianqiong. Thanks to collaborators and co-au-thors, Hani Nasser, Diana Bernin, Jing Huang, Bo Xu, Lei Tian, and Haining Tian.

Many thanks to Tamara Church, Leif Nyholm and Reza Younesi for proof reading my thesis. I really appreciate your help.

My acknowledgement to all professors and senior researchers (former and current) at MMK for all courses training, and discussions: Cheuk-Wai Tai, German Salazar Alvarez, Niklas Hedin, Ulrich Häussermann, Jekabs Grins, Lars Eriksson, Sven Hovmöller, Mats Johnsson, Osamu Terasaki, Kjell Jans-son, Tom Willhammar, Andrew Kentaro Inge, Thomas Thersleff, Lennart Bergström, Mattias Eden, Arnold Maliniak, Josef Kowaleski, Anja-Verena Mudring, Aji Mathew, James Shen, Zoltan Bacsik, Tamara Church, Lynn McCusker, Feifei Gao, Wei Wan, Anna-Karin, and Hongyi Xu.

Thanks to the administration office, Tatiana Bulavina, Elisabeth Pernbom, Ann Loftsjö, Helmi Frejman, Daniel Emanuelsson, Rolf Eriksson, and Camila Berg. You always offered me kind help in administrative issues during my PhD study. I want to thank also, the technical help from the MMK workshop,

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Hans-Erik Ekström, and Per Jansson. Thanks to Anne Ertan, you guided me through the chemical inventory and lab safety courses.

Thanks to all my friends at MMK (former and current), Dickson, Yunchen, Yunxiang, Fei, Reji, Sumit, Kristina, Henrik, Yuan, Elina, Hong, Leifeng, Yifeng, Fabian, Yang, Trung-Dung Tran, Alexandra, Magda, Ning, Viktor, Mylad, Eleni, Stef, Yulia, Arnaud, Junzhong, Laura, Erik, Bin, Taimin, Jingjing, Jonas, Mirva, Przemyslaw, Chris, Inna, Istvan, Haoquan, Zhehao, Peng, Anne, Dariusz, Jie Su, Jie Liang, Naeem, Yi, and Jian.

I want to acknowledge Dr. Makarem Hussein for his continuous help and support. I have to say, I was lucky to work with you during my Master study. I have never felt that you were my boss during that period; you made the most significant role in developing my scientific career after graduation from Alex-andria University. You always provided me with self-confidence and opti-mism. Thanks to my Masters supervisors Philippe Vereecken (imec/KU-Leuven, Belgium) and Alex Radisic (imec, Belgium); I learned a lot from you and gained a lot of experience.

I want to thank my family for being a good support to me. I dedicate my thesis to the spirits of my parents. I was hoping to have my Mom beside me during the PhD graduation. All success in my life and carrier is sincerely from her support to me over the past 30 years. I really miss her, and no words can express all what she had done for me and my brother and sisters. I want to acknowledge my wife, Ghidaa, for being a good support to me especially in the most frustrating moments. Thanks to my little kids Omar and Lara. Thanks to my brother and sisters.

I want to thank my Egyptian friends in Sweden Moataz, Hani Nasser, Hani Mubarak, Kariem Ezzat, Ahmed Ramzy, Ahmed Fawzy, Mohamed, Rabea, and Hatem.

Thanks to my professors and colleagues at Alexandria University espe-cially Dr. Mahmoud Emara, Bishoy Morcos, Mohamed Ibrahim, Mohsen El-Tahway, Ahmed Mohsen, Hani Abdela-aal, prof. Adel Amer, prof. Taher Kassem, and prof. Mahmoud Mousa from Banha University.

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