Title Page

Contents

Abstract 19

Chapter Ⅰ. INTRODUCTION 21

1.1. Overview 21

1.2. Background and Motivation 23

1.3. Scope of dissertation 29

Chapter Ⅱ. Non-oxidized bare copper nanoparticles with surface excess electrons in the air 33

2.1. Introduction 33

2.2. Experimental section 35

2.2.1. Synthesis of [Gd₂C]²⁺·2e¯ electride 35

2.2.2. Preparation of CuNPs 35

2.2.3. Characterizations 35

2.3. Results and discussion 36

2.3.1. Non-oxidized surface of bare CuNPs 36

2.3.2. Negatively charged surface state of CuNPs 38

2.3.3. Long-term air stability of non-oxidized bare CuNPs 40

2.3.4. Atomic surface structure confirms the absence of monolayer oxide 41

2.3.5. Surface-accumulated excess electrons of bare CuNPs 43

2.3.6. Energetics of oxidation process: Endothermic reaction of negatively charged Cu surface 43

2.4. Conclusion 46

Chapter Ⅲ. Rational synthesis of non-oxidized bare metal nanoparticles in the air 47

3.1. Introduction 47

3.2. Experimental section 48

3.2.1. Synthesis of [Gd₂C]²⁺·2e¯ electride 48

3.2.2. Synthesis of CuNPs 49

3.2.3. Synthesis of AgNPs 49

3.2.4. Synthesis of SnNPs 49

3.2.5. Characterizations 49

3.3. Results and discussion 50

3.3.1. Synthesis of non-oxidized bare MeNPs by the wet chemical process 50

3.3.2. Atomic-scale chemical and surface analysis of non-oxidized CuNPs 53

3.3.3. Air stability of bare CuNPs in air 56

3.3.4. Reusability of [Gd₂C]²⁺·2e¯ electride flakes 59

3.3.5. Fabrication of electrodes using CuNP ink 60

3.3.6. Rational synthesis strategy for non-oxidized bare AgNPs and SnNPs 62

3.4. Conclusion 65

Chapter Ⅳ. Boosted heterogeneous catalysis by surface-accumulated excess electrons of non-oxidized bare copper nanoparticles on the electride support 66

4.1. Introduction 66

4.2. Experimental section 68

4.2.1. Synthesis of [Ca₂N]⁺·e¯ electride 68

4.2.2. Preparation of CuNPs by thermal treatment and wet chemical solution process 69

4.2.3. General experimental procedure for sulfenylation of (aza)indole derivatives 69

4.2.3. Characterizations 69

4.3. Results and Discussion 70

4.3.1. Charge Transfer for Strong Metal-Support Interactions 70

4.3.2. Catalytic Sulfenylation of Indoles 72

4.3.3. Negatively Charged and Non-Oxidized Surface of CuNPs 75

4.3.4. Proposed Mechanism for the Indole Sulfenylation 80

4.3.5. Scope of Sulfenylation Using Various Disulfide and Indole Derivatives 82

4.4. Conclusion 84

Chapter Ⅴ. Non-oxidized nano SAC alloy particles via wet chemical solution process 86

5.1. Introduction 86

5.2. Experimental section 87

5.2.1. Synthesis of [Gd₂C]²⁺·2e¯ electride 87

5.2.2. Synthesis of nano solder alloy 87

5.2.2. Characterizations 87

5.3. Results and discussion 88

5.4. Conclusion 92

Chapter Ⅵ. SUMMARY AND FUTURE OUTLOOK 93

References 94

Table 3-1. Optimization of synthesis of various MeNPs by wet chemical solution process. 53

Table 3-2. Comparison of electrical properties of the electrodes prepared using inks of negatively charged Cu NPs and commercial Cu NPs. 61

Table 4-1. Results obtained from the catalytic sulfenylation of indole (1a) with various catalysts. 73

Table 4-2. Optimization of sulfenylation of 1a using CuNPs/[Ca₂N]⁺·e¯ catalyst. 74

Table 4-3. Results obtained from the catalytic sulfenylation of indole (1a) with negatively charged CuNPs separated from the electride and heterogeneous system of CuNPs grown on the electride. 75

Table 4-4. The substrate scope of the sulfenylation of indole derivatives. [a] All reactions were carried out at 0.2 mmol scales (1: 0.2 mmol, 2: 0.15 mmol). [b] Isolated yields. 83

Table 4-5. The substrate scope of the sulfenylation of 7-azaindole. [a] All reactions were carried out at 0.2 mmol scales (1: 0.2 mmol, 2: 0.15 mmol). [b] Isolated yields. 84

Table 5-1. ICP-OES analysis of nSAC alloy particles. 90

Fig.1-1. Schematic illustration of various applications of MeNPs. 21

Fig.1-2. Major synthesis approach for the preparation of MeNPs by various methods. 22

Fig.1-3. Graphical depiction of oxidation of MeNPs. 23

Fig.1-4. Schematic of cathodic protection technique for the prevention of the corrosion of metals. 24

Fig.1-5. Schematic illustration and a digital photograph of sodium metal in liquid ammonia generating solvated electrons. 25

Fig.1-6. Schematic illustration of Cs⁺(15-crown-5)₂.e¯ complex and [Ca₂₄Al₂₈O₆₄]⁴⁺.4e¯. 25

Fig.1-7. Schematic image of the crystal structure of [Ca₂N]⁺·e¯, [Y₂C]²⁺·2e¯, and [Gd₂C]²⁺·2e¯. 26

Fig.1-8. Schematic illustration depicting various applications of electrides. 27

Fig.1-9. Schematic representation of an arbitrarily shaped conductor with a Gaussian surface infinitesimally close to the surface. 27

Fig.1-10. Electron density change induced by charging for Na (a and c) and Ag clusters (b and d). a, b) Change of number of electrons at the surface of the sphere of radius r. c, d) Change of the total number of... 29

Fig.1-11. Schematic overview of the thesis. 30

Fig.2-1. a) Schematic illustration of CuNP growth on two-dimensional [Gd₂C]²⁺·2e¯ electride. b) SEM image of CuNPs on the electride. 37

Fig.2-2. a) TEM image of as-prepared CuNPs on the oxidized electride (inset: Fast Fourier transformation(FFT) pattern) and enlarged TEM image showing an atomic structure of fcc Cu(110). b) EEL spectra for... 38

Fig.2-3. a) TEM image of conventional CuNP, showing that the NP was oxidized to Cu₂O with an interplanar distance of 0.30 nm from the boxed position. b) EEL spectra for positions (1-3) at the surface and (4) in... 38

Fig.2-4. XPS spectra of CuNPs, copper (II) acetate and Cu foil. Two satellites (940.5 and 944 eV) for copper (II) acetate are characteristics of Cu²⁺. 39

Fig.2-5. Work function (φ) values of CuNPs and [Gd₂C]²⁺·2e¯ electride obtained by KPFM measurements. Peaks of solid lines represent the average values (φav) for CuNPs (~3.2 eV, orange) and [Gd₂C]²⁺·2e¯...[이미지참조] 39

Fig.2-6. HR-TEM image of the 9R phase in the CuNPs. The twin boundary (TB, white lines) and atomic structures (green balls) correspond to the 9R phase. 40

Fig.2-7. a,b) TEM images of CuNPs as a function of exposure time in air. c) EEL spectra (1-4) for a and each surface for b. The separated NP from the oxidized electride (4) is also metallic copper, indicating that... 41

Fig.2-8. Cu L₃M₄₅M₄₅ Auger spectra of the CuNPs (top), Cu foil with the oxidized surface of 1 nm thickness (middle), and Cu₂O powder (bottom). Strong peak at 916.8 eV of Cu foil comes from the surface oxide. In... 42

Fig.2-9. Relative energy profiles upon the adsorption of O atom onto the neutral (Q＝0 e) (blue circle: flat surface, cyan circle: step edge) and charged (Q＝−2.5 e) fcc Cu(111) facets (red circle: flat surface, orange... 44

Fig.2-10. Schematic illustration of the model for the oxidation process of conventional CuNPs (top) and the non-oxidized CuNPs with the surface-accumulated excess electrons (bottom). 46

Fig.3-1. a) Schematic illustration for the growth of MeNPs in alcoholic solutions containing the dissolved precursor compounds. Green and pink spheres represent metal cations and counter anions, respectively.... 52

Fig.3-2. a) Photograph of CuNP powder synthesized by wet chemical solution process. b) SEM image of as-prepared CuNPs. c) TEM image of as-prepared CuNPs. 54

Fig.3-3. a) ADF STEM image of as-prepared CuNP. b) EELS elemental mapping of a. c) EEL spectra obtained from marked points 1−3 at the surface of CuNP in d show metallic L₃,₂ edges. White lines of Cu... 54

Fig.3-4. a) Cu L₃M₄₅M₄₅ AES of as-prepared CuNPs. b) Work function histogram obtained from as-prepared CuNPs obtained from KPFM measurement. 56

Fig.3-5. a) ADF STEM image of an air-exposed CuNP for 10 days. b) Enlarged HR-STEM image of boxed region in a. c) Interplanar distance profile from marked regions 1−3 in b. The interplanar distance of 0.21... 56

Fig.3-6. EELS data obtained from surface points 1−9 in the STEM images shown in Fig.3-5 of air-exposed CuNPs. 57

Fig.3-7. a−f) EELS mapping distribution of air-exposed CuNPs for 10, 20, and 30 days respectively. Cu L edge (red), C K edge (yellow) Gd M edge (green), O K edge (blue). Scale bars correspond to 2nm. 58

Fig.3-8. XPS as-prepared and air-exposed CuNPs (15 days). Insets show the negative shift in binding energy of wet-chemically synthesized CuNPs from metallic Cu (green dotted line, 932.6 eV). 58

Fig.3-9. XRD patterns analysis of commercial CuNPs, as-prepared CuNPs, and air-exposed CuNPs for 10 days intervals (bottom to top). 59

Fig.3-10. a) Optical image of [Gd₂C]²⁺·2e¯ electride chips used for wet chemical synthesis of CuNPs; before (left) and after (right) reaction. b) Powder XRD patterns of [Gd₂C]²⁺·2e¯ electride chips; as-prepared for a... 60

Fig.3-11. a) Photograph of CuNP powder synthesized by the reused [Gd₂C]²⁺·2e¯ electride chips. b) TEM image of CuNPs synthesized using the reused [Gd₂C]²⁺·2e¯ electride. c) STEM image of CuNPs synthesized... 60

Fig.3-12. a-b) Cross-sectional SEM images of the electrodes by using wet chemically synthesized Cu NPs and commercial Cu NPs. Insets: photograph of the Cu NP inks prepared with wet chemically synthesized... 61

Fig.3-13. a) ADF STEM image of an as-prepared AgNP. b) EELS mapping image of a. Ag M edge (pink), C K edge (yellow) Gd M edge (green), O K edge (blue). c) ADF STEM image of an as-prepared SnNP. d)... 62

Fig.3-14. a,b) EELS data obtained from surface points 1−3 in the STEM images (Fig.3-14b and d) of as-prepared Ag and Sn NPs respectively. 63

Fig.3-15. a) Histogram of AgNPs measured by KPFM. b) Ag 3d XPS spectra of AgNPs. c) Histogram of SnNPs measured by KPFM. d) Sn 3d XPS spectra of SnNPs. 64

Fig.3-16. a) ADF STEM image of air-exposed AgNP. b) EELS mapping image of a. Ag M edge (pink), C K edge (yellow) Gd M edge (green), O K edge (blue). c) EEL spectra obtained from marked points 1-4 at the... 64

Fig.4-1. a−c) CuNPs on TiO₂₋ₓ (a), Zn dust (b), and [Ca₂N]⁺·e¯ electride (c). Oxygen vacancies (Vₒ) in the reduced titanium dioxide provide additional electrons to CuNPs (red arrows in a). The inherent anionic... 71

Fig.4-2. a) SEM image of CuNPs on the surface of [Ca₂N]⁺·e¯ electride. b) Histogram of work function values for the CuNPs and [Ca₂N]⁺·e¯ measured by KPFM. c) XPS spectra of Cu 2p for the CuNPs on Zn... 76

Fig.4-3. XPS measurement results of entry 11 ([Ca₂N]⁺·e¯ + CuI), entry 12 ([Ca₂N]⁺·e¯ + CuBr₂), and after reaction of entry 3 (CuNPs/[Ca₂N]⁺·e¯) in Table 1. 77

Fig.4-4. Auger spectra of Cu L₃M₄₅M₄₅ of CuNPs on the Zn dust (blue) and [Ca₂N]⁺·e¯ electride (red). 77

Fig.4-5. a, b) High-resolution STEM images of a CuNP on the [Ca₂N]⁺·e¯ electride (a), showing the non-oxidized surface structure as confirmed by the interplanar distance of 0.18 nm of fcc Cu plane at the... 78

Fig.4-6. a) ADF-STEM image of commercial CuNPs. b) High-Resolution ADF-STEM image of commercial CuNPs. c) EEL spectra from the marked positions 1−3 in a. 79

Fig.4-7. a) High resolution ADF-STEM image of the Cu/Zn dust from boxed region in the top-left inset. b) High resolution ABF-STEM image of a. c-d) STEM image and EEL spectra from the marked positions 1−3... 80

Fig.4-8. a) Reaction between 1a and 2a in the presence of a radical scavenger, TEMPO. b) Reaction between 2a and 3-methyl indole (4). c) Reactions of 2a and N-substituted indoles 6a and 6b. 81

Fig.4-9. Proposed mechanism for the sulfenylation of indoles, using the depicted molecules 1a and 2a as examples. The activated species, iminium ion intermediate, and sulfenylated indole product with thiophenol... 81

Fig.5-1. a) SEM image SAC alloy particles synthesized at 25 ℃. b) Particle size distribution of a. 88

Fig.5-2. DSC of SAC alloy particles synthesized at 25 ℃. 89

Fig.5-3. a) SEM image nSAC alloy particles synthesized at 50 ℃. b) Particle size distribution of a. 89

Fig.5-4. DSC of nSAC alloy particles synthesized at 50 ℃. 90

Fig.5-5. XRD pattern of nSAC alloy particles, commercial SAC particles, SnO₂, and SnO. 91

Fig.5-6. Work function histogram of nSAC alloy particles obtained from KPFM measurements. 92

 Metal nanoparticles (MeNPs) have been employed in diverse fields due to their distinctive physicochemical properties in comparison to their bulk counterparts. However, their vast range of applications is restricted by the inherent susceptibility to oxidation in the air. In general, most metals are inevitably oxidized when exposed to air generating thermodynamically stable metal oxides on their surface. Indeed, various strategies including surface passivation techniques and post-treatments have been proposed to prevent the oxidation of MeNPs. Nevertheless, there is no practical way to autonomously prevent the oxidation of bare MeNPs in the air.

Electrides are stoichiometric ionic crystals in which electrons act as anions in a structural real space. Here, we utilized two-dimensional inorganic electrides such as [Gd2C]2+·2e- and [Ca2N]+·e- electrides as excellent electron-donating agents for the synthesis of non-oxidized MeNPs. A solidstate thermal treatment was employed to synthesize CuNPs on [Gd2C]2+·2e- electride support, exhibiting an ultra-high oxidation resistance over seven months in the air. Spontaneous charge transfer during nucleation and growth resulted in surface accumulated excess electrons on the surface of CuNPs which was explicitly demonstrated by the reduced work function (~3.3 eV) and lowered binding energy value (~931.8 eV) than the metallic Cu. Theoretical investigations demonstrated that the surface-accumulated excess electrons impede the oxidation process provoking an energetically unfavorable endothermic reaction. Surface accumulated excess electrons on the CuNPs act as a natural surface passivation layer preventing oxidation in the air.

A wet-chemical solution process approach was implemented for the large-scale synthesis of CuNPs separated from the [Gd2C]2+·2e- electride support. Metal precursors were dissolved in anhydrous alcoholic solvents and [Gd2C]2+·2e- electride acts as a reducing agent by transferring its interstitial anionic electrons resulting in negatively charged MeNPs. CuNPs exhibited lowered work function (~4.2 eV) and binding energy value (~932.2 eV) than the metallic Cu. Additionally, we extended our protocol for the synthesis of non-oxidized Ag and Sn NPs with surface excess electrons. We could successfully put forward a rational strategy for the synthesis of non-oxidized and non-passivated MeNPs in the air.

A heterogeneous catalytic sulfenylation reaction was carried out using CuNPs grown on [Ca2N]+·e- electride support. Substantial electron transfer from the two-dimensional [Ca2N]+·e- electride to CuNPs through metal-support interactions resulted in the surface accumulation of excess electrons hindering oxidation and providing a negatively charged state. The unrestricted actives on the surface of CuNPs activate indoles and disulfides generating 3-arylthioindole under mild conditions without any additives. This study defines the role of excess electrons on the nanoparticle-based heterogenous catalyst that can be rationalized in versatile systems.

Furthermore, a wet chemical synthesis route for the synthesis of non-oxidized solder alloy particles at the nanoscale is also discussed using [Gd2C]2+·2e- electride as an electron donating agent in an alcoholic medium.

In short, a novel synthesis pathway for non-oxidized MeNPs with surface excess electrons in the air is depicted by employing two-dimensional electrides by solid-state and wet-chemical synthesis routes which can be implemented in numerous applications.