Developing synthetic strategies for ligand free noble metal nanostructures

... values 82 6.3.3 Proposed mechanism for the formation of ligand- free nanorattles 84 6.3.4 Effect of nanocavity of mHSS on the formation of ligand- free nanorattles 86 6.4 Conclusion ... developed for tuning the size of ligand- free Au nanoparticles In this method, mesoporous hollow silica shells were employed as nanoreactors for tuning size and shape of noble metal nanostructures. .. rate inside the cavity of mHSS for the formation of yolk-shell nanoparticles The formation of ligand- free YSNs can be tuned simply by varying the pH of the noble metal precursor aqueous solution

LIGAND-FREE NOBLE METAL NANOSTRUCTURES

SHAIK FIRDOZ

(M.Tech, IIT-Roorkee, India)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2014

I hereby declare that this thesis is my original work and it has been written by me in its entirety I have duly acknowledged all the sources of information which have been used in the thesis

This thesis has also not been submitted for any degree in any university previously

SHAIK FIRDOZ

19 Dec 2014

i

Acknowledgement

Firstly, I would like to express my sincere heartfelt gratitude to my supervisor, Asst Prof Lu Xianmao, for his suggestions, invaluable guidance and continuous support given to

me during my entire PhD candidature

In the same, I would like to extend my sincere thanks to the Department of Chemical and Biomolecular Engineering for offering me a NUS research scholarship, which helps me a lot

to complete my PhD studies successfully without any financial hurdles during my PhD candidature

I would like to express my thanks to my lab mates Dr Zhang Weiqing, Dr Niu Wenxin,

Dr Sun Zhipeng, Dr Chen Ningping, Dr Han Hui, Dr Guo Chunxian, Dr Chen Shaofeng,

Mr Ton Tran, Ms Zhao Dieling, Mr Zhao Qipeng, Mr Chen Shucheng and Ms Gamze Yilmaz for their valuable discussions and the good fun we shared in the lab

I would like to thank Mr Chia Phai Ann, Mr Liu Zhicheng, Mr Mao Ning, Dr Yuan Zeliang, Dr Yang Liming, Mr Tan Evan Stephen, Mr Ang Wee Siong, Ms Li Fengmei, Ms

Li Xiang and Ms Samantha for their kind support and help during my PhD candidature

I would like to thank specially my friends Mr Karthik, Mr Akshay, Mr Dara Nuthan and Mr Sai Naresh reddy for their kind help, suggestions and tons of fun given to me and make my stay very pleasant and happy

Finally, I would like to express my deepest love to my beloved parents, parent-in-law and brother-in-law Mr Shaik Abid, for their kind support Last but not least, my sincere and special thanks given to my wife Ms Shaik Asma for her un-conditional support and help

ii

during my study It is not exaggeration to say that I could not able to complete my PhD work without her support and love I would like to extent my sincere thanks to all my friends and well-wishers who directly or indirectly help me to finish my PhD work

iii

Table of Contents

Acknowledgement i

Summary viii

List of Abbreviations xi

List of Figures xiii

List of Tables xxi

Chapter 1 Introduction 1

1.1 Background 1

1.2 Objectives 2

1.3 Organization of thesis 3

Chapter 2 Literature Review 4

2.1 Core-shell nanoparticles 4

2.2 Rattle–type hollow structures (Yolk-shell / Nanorattles) 5

2.3 Methodologies for the fabrication of noble metal (M@SiO2) YSNs 6

2.3.1 Synthetic approaches 6

2.3.2 Selective etching or dissolution method 7

2.3.3 Pre-shell method or Ship-in-bottle method 14

2.3.4 Template free methods 18

2.3.5 Galvanic replacement method 19

2.3.6 One pot method 19

2.4 Applications of Yolk-shell noble metal nanoparticles 22

iv

2.4.1 Yolk-shell nanoparticles as nanoreactors 22

2.4.2 Yolk-shell nanoparticles as drug delivery carriers 26

2.4.3 Yolk-shell nanoparticles for lithium-ion batteries 28

2.5 Our Proposed Method 29

Chapter 3 Experimental Section 31

3.1 Method and materials 31

3.2 Solution preparation 32

3.3 Procedures 32

3.3.1 Synthesis of mesoporous hollow silica shells of size 100 nm and 230 nm (mHSS-100 & mHSS-230) 32

3.3.2 Synthesis of silica spheres (SiO2) 33

3.3.3 Synthesis of ligand-free Au@SiO2 nanorattles by thermal method 34

3.3.4 Catalytic reduction of 4-nitrophenol by Au@SiO2 nanorattles 35

3.4 Synthesis of ligand-free Au nanoplates by photochemical reduction method 36

3.4.1 Synthesis of spherical gold yolk-shell nanoparticles in absence of Ag+ ions (Au@mHSS) 36

3.4.2 Synthesis of Au triangular nanoplates in the presence of Ag+ ions 37

3.4.3 Etching of Au triangular yolk-shell nanoplates with HF 37

3.5 Synthesis of ligand-free M@SiO2 (M= Au, Ag, Pt and Pd) nanorattles by using mHSS as smart nanoreactors 38

3.5.1 Synthesis of Ag@mHSS yolk-shell nanoparticles 38

3.5.2 Synthesis of Ag@mHSS yolk-shell nanoparticles with different pH values 38

3.5.3 Synthesis of Au@mHSS yolk-shell nanoparticles 38

v

3.5.4 Synthesis of Pd@mHSS yolk-shell nanoparticles 39

3.5.5 Synthesis of Pt@mHSS yolk-shell nanoparticles 39

3.6 Characterization Methods 39

3.6.1 Ultraviolet-visible spectrophotometer (UV-Vis) 39

3.6.2 X-ray photoelectron spectroscopy (XPS) 39

3.6.3 Inductively coupled plasma mass spectrometry (ICP-MS) 40

3.6.4 Brunauer-Emmett-Teller (BET) measurements for mHSS-100 and mHSS-230 40

3.6.5 Zeta-Potential measurements 40

3.6.6 FT-IR measurement for mHSS-100 41

3.6.7 Mass spectrum analysis of different noble metal precursor’s solutions 41

3.6.8 Scanning electron microscopy (SEM) 41

3.6.9 Transmission electron microscopy (TEM) 41

Chapter 4 Volume-confined Synthesis of Ligand-free Gold Nanoparticles with Tailored Sizes for Enhanced Catalytic Activity .42

4.1 Introduction 42

4.2 Results and discussion 45

4.2.1 Synthesis of mesoporous hollow silica shells (mHSS) 45

4.2.2 Synthesis of Au@SiO2 nanorattles using mHSS-230 46

4.2.3 Characterizations of Au@mHSS nanorattles 49

4.2.4 Synthesis of Au@SiO2 nanorattles using mHSS-100 52

4.2.5 Catalytic activities of Au@SiO2 nanorattles for reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in presence of excess NaBH4 54

4.3 Conclusion 60

vi

Chapter 5 Synthesis of Ligand-free Au Triangular Nanoplates.61

5.1 Introduction 61

5.2 Results and discussion 64

5.2.1 Synthesis of Au triangular nanoplate inside mHSS 64

5.2.2 Characterizations of Au triangular nanoplates 65

5.2.3 Synthesis of spherical gold nanoparticles inside mHSS in the absence of Ag+ ions 68

5.2.4 Effect of chloride ions on the formation of Au triangular nanoplates 70

5.2.5 Proposed mechanism for the growth of Au triangular nanoplate inside mHSS 70

5.3 Conclusion 73

Chapter 6 Synthesis of M@SiO2 (M = Ag, Au, Pd, Pt) Yolk-Shell Nanoparticles 74

6.1 Introduction 74

6.2 Results and discussion 76

6.2.1 Synthesis of ligand-free M@SiO2 nanorattles (M = Ag, Au, Pd & Pt) using mHSS 76 6.2.2 Characterizations of ligand-free M@SiO2 nanorattles (M = Ag, Au, Pd & Pt) 78

6.3 Growth mechanism for the formation of ligand-free nanorattles 79

6.3.1 Synthesis of Ag@mHSS nanorattles at different time periods 79

6.3.2 Synthesis of Ag@mHSS nanorattles at different pH values 82

6.3.3 Proposed mechanism for the formation of ligand-free nanorattles 84

6.3.4 Effect of nanocavity of mHSS on the formation of ligand-free nanorattles 86

6.4 Conclusion 87

Chapter 7 Conclusions and Future Work 89

7.1 Conclusions 89

vii

7.2 Future Work 93

References .96

Annexure 115

List of Publications 115

viii

Summary

As an important class of nanomaterials, noble metal nanostructures (NMNs) have attracted significant attention because of their wide variety of applications in catalysis, photonics, sensing and medicine It is well known that the catalytic and optical properties of NMNs can be effectively tailored by tuning their size and shape Therefore, a number of synthetic routes have been developed for morphology-controlled synthesis of NMNs Among these synthetic methods, wet chemical synthesis is probably the most powerful approach to the preparation of noble metal nanostructures in large quantity with controlled shapes, sizes, and compositions To achieve control over shape and size during synthesis, capping ligands such as polymers, surfactants, dendrimers, or small organic molecules are generally employed However, the presence of ligands on the surface of NMNs may drastically affect their catalytic activity, stability, and suitability for biological applications Especially for catalysis, bare NMNs free of capping ligands are highly desirable But without capping ligands, it is difficult to tailor the size and morphology of the nanoparticles since they would all grow into the most thermodynamically stable form In addition, ligand-free NMNs are highly unstable and easily aggregate in solution

Therefore, the objective of the current research is to design synthetic methods for free NMNs with controllable size, shape, composition and exposed facets Firstly, we developed a volume-confined method to tune the size of gold nanoparticles inside the cavity

ligand-of mesoporous hollow silica shells (mHSS) without using any organic capping ligands In this method, mHSS with an average sizes of 100 ± 2.3 nm (mHSS-100, pore volume: 1.321

cm3/g) and 230 ± 4.2 nm (mHSS-230, pore volume: 0.215 cm3/g) were synthesized by using

of mHSS, or by using mHSS with different cavity volumes The resulting ligand-free Au nanoparticles exhibited much enhanced catalytic activity towards the reduction of 4-nitrophenol to 4-aminophenol with NaBH4 compared to citric acid-capped Au nanoparticles

of the same size

Next, a photochemical reduction method together with the introduction of Ag+ ions was

developed to grow ligand-free Au triangular nanoplates inside the cavity of mHSS In this case, mHSS were soaked in a mixture of HAuCl4, ethanol and AgNO3 solution After

centrifugation, the samples were exposed to UV light The ethanol molecules trapped inside the mHSS generated radicals which acted as reducing agent to reduce AuCl4- The resulting

Au atoms nucleated inside the mHSS and grew into nanoparticles The shape of the gold nanoparticles was successfully tuned from spheres to triangular nanoplates by varying the ratio of [AuCl4-]:[Ag+] It is believed that the presence of Ag+ ions can alter the reduction

kinetics of AuCl4- and facilitate the development of twinned seeds, which gradually develop

into Au triangular nanoplates inside the nanocavity of mHSS The effect of Ag+ and ethanol

for the formation of Au triangular nanoplates was examined

Finally, we extended the synthesis of ligand-free nanoparticles to other noble metals including Ag, Pd, Pt which cannot be obtained inside of mHSS by using simple heating methods In this study, the precursor solutions of different noble metals were loaded inside

<4, nanoparticles were not formed This is because at pH > 4, deprotonated species (≡Si-O-)

present on the mHSS surface can act as reducing agent for the conversion of noble metal precursor ions into metal atoms The nanocavity of mHSS also plays a critical role to lower the critical radius of nucleation for the growth of nanoparticles inside the cavity of mHSS

xi

List of Abbreviations

YSNs Yolk-shell nanoparticles

mHSS Mesoporous hollow silica shells

NMNs Noble metal nanostructures

SiO2 Silicon dioxide

M@SiO2 M = Ag, Au, Pd, Pt

AgNO3 Silver nitrate

H2PtCl6.xH2O Chloroplatinic acid hydrate

HAuCl4∙3H2O Gold(III) chloride trihydrate

Na2PdCl4 Sodium tetrachloropalladate

NaOH Sodium hydroxide

DI H2O Deionized water

NH3.H2O Ammonia solution

VTMS Vinyltrimethoxysilane

PBzMA Poly(benzyl methacrylate)

PEMs Polyelectrolyte multilayers

TEOS Tetra ethyl orthosilicate

TSD N-3-(trimethoxysilyl) propyl ethylenediamine

xii

PVP Poly(N-vinyl-2-pyrrolidone)

SPR Surface plasmon resonance

CTAB Hexadecyltrimethylammonium bromide

SERS Surface enhanced Raman scattering

PS-co-PMAA Poly(styrene-co-methylacrylic acid)

TOF Turn over frequency

PEG Polyethylene glycol

EDX Energy dispersive X-ray spectroscopy

FESEM Field Emission Scanning Electron Microscopy

TEM Transmission electron microscopy

HRTEM High-Resolution Transmission Electron Microscopy

UV-Vis Ultraviolet-Visible

XPS X-ray Photoelectron Spectroscopy

ICP-MS Inductively coupled plasma mass spectrometry

ECSAs Electrochemically activated surface areas

BET Brunauer-Emmett-Teller

SAED Selected Area Electron Diffraction

FT-IR Fourier transform infrared spectroscopy

xiii

List of Figures

Figure 2.1 Different core/shell nanoparticles: (a) spherical core/shell nanoparticles; (b)

hexagonal core/shell nanoparticles; (c) multiple small core materials coated by single shell material; (d) nanomatryushka material; (e) movable core within hollow shell material Reprinted with permission from reference.7 5

Figure 2.2 Schematic illustration of etching strategies for preparing YSNs Reprinted with

permission from reference.20 6

Figure 2.3 (A, B) Backscattering SEM and (C, D) TEM images of Au@SiO2@PBzMA

particles before (A, C) and after (B, D) HF etching (E, F) TEM images of Au@SiO2@PBzMA synthesized at different time periods of polymerization: (E) 3 h and (F)

6 h Reprinted with permission from reference.22 8

Figure 2.4 TEM images of (A) PS-co-P4VP microspheres, (B) Au/PS-co-P4VP

microspheres, (C, D) Au/PS-co-P4VP@HMSM microspheres, and (E, F) Au@HMSM shell microspheres Reprinted with permission from reference.23 9

yolk-Figure 2.5 TEM images of (A) Au-decorated PS nanospheres, (B) PS@Au@silica colloids,

and (C) hollow silica spheres with multi Au nanoparticles after burning off PS templates (D) Backscattering SEM (inset, normal SEM) image of multicore hollow silica spheres Reprinted with permission from reference.24 10

Figure 2.6 (a) Schematic illustration for the synthesis of Au@mSiO2 and Fe2O3@mSiO2

YSNs; TEM images of (b) Au@SiO2@mSiO2, (c) rattle-type Au@mSiO2, (d and inset)

xiv

ellipsoidal Fe2O3@SiO2@mSiO2, (e, inset: SEM image of deliberately selected broken

ellipsoids) rattle-type Fe2O3@mSiO2 Reprinted with permission from reference.28 12

Figure 2.7 (a-f) TEM images of Au@SiO2 nanoreactors and (g, h) silica hollow shells (a, b)

Gold core diameters are 104±9 nm, (c, d) 67±8 nm, and (e, f) 43±7 nm The scale bars represent 200 nm (a, c, e, and g) and 100 nm (b, d, f, h) Reprinted with permission from reference.18 13

Figure 2.8 Schematic procedure for the preparation of yolk/shell nanoparticles through the

ship-in-bottle method Reprinted with permission from reference.20 14

Figure 2.9 TEM images of hollow silica particles (a) and Cu core rattle-type silica particles

(b: 1 cycle; c: 2 cycles; d: 3 cycles) Reprinted with permission from reference.31 15

Figure 2.10 Schematic procedure for the preparation of PPy-CS hollow nanospheres

containing movable Ag Cores (Ag@PPy-CS) Reprinted with permission from reference.33 16

Figure 2.11 TEM images of (a) silica nanorattles of 110 nm, (b) SRG-1, (c) SRG-2, and (d)

SRG-3 Reprinted with permission from reference.34 17

Figure 2.12 Schematic formation of SnO2 hollow spheres inside mesoporous silica

nanoreactors Reprinted with permission from reference.35 17

Figure 2.13 TEM images showing the formation of hollow CoSe nanocrystals, from top-left

to bottom-right: 0 s, 10 s, 20 s, 1 min, 2 min, and 30 min The Co/Se molar ratio was 1:1 Reprinted with permission from reference.36 18

xv

Figure 2.14 TEM images of Au@HSNs synthesized using 1 ml of HAuCl4 solution: (a-b)

before calcination and after being washed several times with water; scale bars: 50 and 20 nm, respectively; (c-d) after calcination, the small clusters of Au aggregate to give larger Au nanoparticles (indicated by white arrows); scale bars: 100 and 20 nm, respectively Reprinted with permission from reference.44 20

Figure 2.15 (A) Structural illustration of an Au@oxide composite nanoreactor (B) TEM

image of Au@oxide composite nanoreactors (C) Typical reactions tested for Au@oxide composite nanoreactors Reprinted with permission from reference.20 23

Figure 2.16 (A) TEM images of Au@SiO2 yolk-shell nanoreactors, and (B) Time-dependent

UV-vis spectral changes of Au@SiO2 yolk/shell nanoreactors used for catalytic studies

Reprinted with permission from reference.46 25

Figure 2.17 (A) TEM images of (a) Au@SiO2, (b) Au@SiO2@ZrO2, (c) Au@hm-ZrO2, (d)

Au@SiO2@TiO2, and (e) Au@hm-TiO2 (B) A comparison of catalytic activity for CO

oxidation by Au@hm-TiO2 (squares) and Au@hm-ZrO2 (circles) nanoreactors calcinated at

100 °C and 300 °C respectively Reprinted with permission from reference.49

25

Figure 2.18 (A) Schematic illustration for the synthesis of Pd@mesoporous silica composite

nanoreactors (B) TEM images of Pd@mesoporous silica composite nanoreactors (C) Typical Suzuki reaction Reprinted with permission from reference.20 26

Figure 2.19 (A) Schematic illustration for the synthesis of YSNs-PEG/FA (B) A possible

mechanism accounting for killing of MCF-7 cells by DOX-YSNs Reprinted with permission from reference.20 27

xvi

Figure 2.20 TEM images of (a) Co@Au nanospheres and (b) cells exposed to Co@Au

nanospheres Reprinted with permission from reference.56 28

Figure 4.1 Schematic illustration for the synthesis of ligand-free Au nanoparticles using

hollow mesoporous silica shells 44

Figure 4.2 TEM images of (a) PS-266@SiO2 core-shell particles, (b) 230-nm SiO2 hollow

shells, and (c) SEM image of 230-nm SiO2 hollow shells 45

Figure 4.3 N2 adsorption-desorption isotherms of (a) mHSS-100 and (c) mHSS-230; and

pore size distributions of (b) mHSS-100 and (d) mHSS-230 46

Figure 4.4 TEM images of (a) 230-nm SiO2 hollow shells and (b-f) Au nanoparticles with

diameters of 7 (Au@SiO2-7), 10 (Au@SiO2-10), 26 (Au@SiO2-26), 36 (Au@SiO2-36) and

42 nm (Au@SiO2-42) synthesized inside of the hollow shells using HAuCl4 solutions of

0.005, 0.01, 0.1, 0.25 and 0.5 M, respectively 48

Figure 4 5 Plot of measured and calculated particle diameters vs [HAuCl4]1/3 49

Figure 4.6 (a) HRTEM image of a single Au nanoparticle in Au@SiO2-26 (b) UV-vis

extinction spectra of Au@SiO2-10, Au@SiO2-26, and Au@SiO2-36 (c, d) EDX line spectrum

of a single gold nanoparticle of Au@SiO2-26 50

Figure 4.7 EDX spectrum of Au@SiO2-26 51

Figure 4.8 XPS spectra of as-synthesized Au@SiO2-26: (a) Au 4f and (b) survey scan 51

xvii

Figure 4.9 (a) SEM and (b) TEM images of 100-nm SiO2 hollow shells (mHSS-100) (c, e, g)

TEM images of Au nanoparticles with diameters of 6 (Au@SiO2-6), 14 (Au@SiO2-14), 18

nm (Au@SiO2-18), synthesized inside of mHSS-100 by impregnating HAuCl4 solutions of

0.01, 0.25 and 0.5 M, respectively; (d, f, h) corresponding histograms of Au nanoparticle sizes for Au@SiO2-6, Au@SiO2-14, and Au@SiO2-18 respectively; (i) a low magnification

53

Figure 4.10 Time-dependent UV-Vis absorption spectra for the reduction of 4-nitrophenol

catalyzed by (a) Au@SiO2-26 and (c) Au@SC-26, respectively (b) ln(Ct/C0) vs time for the

reduction of 4-nitrophenol catalyzed with Au@SiO2 and Au@SC nanoparticles of different

sizes in the presence of excess NaBH4 (d) Conversion of 4-nitrophenol vs time (Inset: TOF

bar chart) 55

Figure 4.11 Time-dependent UV-Vis absorption spectra for the reduction of 4-nitrophenol

catalyzed by (a) Au@SC-10, (b) Au@SiO2-10, and (c) Au@SiO2-36, respectively 56

Figure 4.12 TEM images of sodium citrate-capped Au nanoparticles: (a) Au@SC-10, (c)

Au@SC-26 and (b, d) the corresponding histograms of Au nanoparticle sizes, respectively 59

Figure 5.1 (a) Schematic illustration for the growth of ligand-free Au triangular nanoplates

and nanoparticles inside silica shells under UV irradiation (b-d) TEM images of Au nanocrystals synthesized under UV irradiation for 24 hours at different ratios of [AuCl4-]:

[Ag+] (b) 30:1; (c) 50:1; and (d) without Ag+ (e) UV-vis absorption spectra of the Au

nanocrystals 63

xviii

Figure 5.2 TEM images of (a) 124 nm PS beads, (b) PS-124@vinyl-SiO2, (c) mHSS, and (d)

FESEM image of mHSS 64

Figure 5.3 EDX elemental mappings of Au (red) and Ag (green) and the spectrum for the Au

nanoplates synthesized at Au:Ag = 30:1 All scale bars are 50 nm 65

Figure 5.4 (a) TEM image of the Au triangular nanoplates after the removal of silica shells

(b) SAED pattern of a Au triangular nanoplate (c) HRTEM image of a Au triangular nanoplate (d) TEM side view of a Au triangular nanoplate 67

Figure 5.5 TEM images of Au triangular nanoplates synthesized in the presence of Ag+ ions

under UV irradiation at different times: (a) 1, (b) 6, (c) 12 and (d) 24 hrs (Scale bars: 50 nm) The insets are the corresponding HRTEM images (Scale bars: 5 nm) 68

Figure 5.6 TEM images of Au@mHSS synthesized without Ag+ ions under UV light for

different time periods: (a) 1 hour, (b) 6 hour, (c) 12 hour and (d) 24 hour (All scale bars are

50 nm) 69

Figure 5.7 TEM images of the samples obtained by soaking mHSS in 0.1M aqueous HAuCl4

solution for a period of 24 hours (a) with ethanol but no UV light irradiation and (b) with UV light but without ethanol (c) TEM image of Au@mHSS synthesized by soaking mHSS in a mixture of 0.1 M HAuCl4 (prepared with saturated NaCl solution) and ethanol but without

Ag+ ions 70

Figure 5.8 HRTEM images of Au seeds obtained in the presence and absence of Ag+ ions:

(a) twinned and (b) single crystalline seeds respectively 72

xix

Figure 6.1 Schematic illustration for the synthesis of ligand-free yolk-shell nanoparticles

M@mHSS (M=Ag, Au, Pd & Pt) 76

Figure 6.2 TEM images of ligand-free M@mHSS (M = Ag, Au, Pd & Pt) synthesized by

soaking mHSS in respective metal precursor solutions: (a) Ag@mHSS, (b) Au@mHSS, (c) Pd@mHSS, and (d) Pt@mHSS 77

Figure 6.3 Low magnification TEM images of (a) Ag@mHSS, (b) Au@mHSS, (c)

Pd@mHSS, and (d) Pt@mHSS 78

Figure 6.4 (a, d, g, j) HRTEM images of Ag, Au, Pd and Pt core inside of mHSS (b, e, h, k)

TEM images of M@mHSS (M= Ag, Au, Pd & Pt) and (c, f, i, l) the corresponding EDX elemental mappings of Ag, Au, Pd, Pt (red), O (blue) and Si (green), respectively (All unmarked scale bars are 50 nm) 79

Figure 6.5 TEM images of Ag@mHSS synthesized by soaking mHSS in 0.1M AgNO3

solution for different time periods: (a) mHSS, (b) 1 min, (c) 1 hour, (d) 6 hour, (e) 12 hour and (f) 24 hour 81

Figure 6.6 UV-vis absorption spectra of mHSS, AgNO3 solution aged for 24 hours, and

mHSS in AgNO3 solution aged for 24 hours 82

Figure 6.7 TEM images of Ag@mHSS synthesized by soaking mHSS in 0.1M AgNO3

solution for a period of 1 hour at different pH values: (a) 1.6, (b) 2.3, (c) 3.1, (d) 4.2, (e) 5.3 and (f) 9.3 83

Figure 6.8 Schematic illustration of the formation of metal core nanoparticles inside mHSS.

84

xx

Figure 6.9 FT-IR spectrum of mHSS 85

Figure 6.10 TEM images of (a, c) Au and (b, d) Ag nanoparticles formed on the surface of

SiO2 spheres 87

xxi

List of Tables

Table 3.1 Chemicals and materials 31

Table 3.2 Concentration of Au present in different YSNs measured by ICP-MS 36

Table 4.1 Comparison of the catalytic activities of Au@SiO2 nanorattles and sodium

citrate-capped Au nanoparticles for the reduction of 4- nitrophenol 58

Table 6.1 Zeta-potentials of mHSS at different pH values 85

In typical wet chemical syntheses, capping agents are employed to achieve arrested growth and control over the morphology of the resulting nanostructures The functions of capping agents are to prevent aggregation, increase stability, and direct the shape of the nanoparticles Using various capping agents, noble metal nanoparticles with diverse shapes such as spheres, rods, cubes, disks, wires, tubes, branched, triangular prisms and tetrahedral nanoparticles have been produced.1 But the presence of surfactants on the surface of

nanoparticles may drastically affect their catalytic activity, stability in harsh conditions and usage in biological applications.2 For example, the activity of gold catalysts is detrimentally

affected when strong covalent capping agents (e.g., alkanethiol molecules and phosphine

complexes) are present even in minute amounts.3

Methods like laser ablation4 and bio-based approaches5-6 are currently available to

synthesize ligand-free noble metal nanoparticles However, besides tedious procedures, the difficulties in scaling up the synthesis and controlling the growth of nanoparticles have

ligand-A facile volume confined synthesis was developed for tuning the size of ligand-free ligand-Au nanoparticles In this method, mesoporous hollow silica shells were employed as nanoreactors for tuning size and shape of noble metal nanostructures mHSS served as nano-containers for the impregnation of HAuCl4 solution before they were separated from the bulk

solution With a simple heating process, the Au precursor confined within the cavity of the isolated hollow shells was converted into ligand-free Au nanoparticles The size of the Au nanoparticles can be tuned precisely by loading HAuCl4 solution of different concentrations,

or by using mHSSs with different cavity volumes With the reduction of 4-nitrophenol in presence of NaBH4 as a model reaction, we further assessed the catalytic activity of the

ligand-free Au nanoparticles and found much improved performance compared to sodium citrate capped Au nanoparticles

A photochemical reduction method was introduced to tune the shapes of ligand-free gold nanoparticles inside the nanocavity of mHSS in the presence of Ag+ ions We found that, by

varying the molar ratio of [AuCl4-]: [Ag+] from 50:1 to 30:1 in the reaction, the shape of the

gold nanoparticles can be tailored from spheres to triangular plates

) on mHSS surface at higher pH > 4 can act

as reducing agents for the conversion of noble metal ions to nanoparticle

1.3 Organization of thesis

The thesis is composed of seven chapters Chapter 1 introduces the general background

of noble metal nanoparticles and research objectives Chapter 2 provides a comprehensive literature review on the synthesis of yolk-shell nanoparticles and applications In Chapter 3, detailed experimental procedures and characterization techniques employed in this work are presented In Chapter 4, a volume-confined synthesis of ligand-free gold nanoparticles with

tailored sizes for enhanced catalytic activity is discussed The synthesis of ligand-free Au

triangular nanoplate inside a nanocavity of mHSS using photochemical reduction method is introduced in Chapter 5 The following chapter presents the synthesis of ligand-free noble metal M@SiO2 (M=Ag, Au, Pd, Pt) yolk-shell nanoparticles and the growth mechanism

Chapter 7 concludes the core findings of this thesis along with future work

4

Chapter 2 Literature Review

The syntheses of ligand-free noble metal nanoparticles in this thesis work are achieved

by using hollow silica shells as the templates, which lead to a core-shell or yolk-shell structure Therefore, the current progress for different synthetic strategies of core-shell and yolk-shell nanoparticles (YSNs) as well as their applications is discussed

2.1 Core-shell nanoparticles

Core-shell nanoparticles constitute a special class of nanocomposites materials, in which particles of one material are coated with a thin layer of another material The coating on the

core particles may provide many advantages, viz an opportunity for surface modification,

stability against aggregation, solvent compatibility, controlled release of core, reduction in precious materials and so on.7 Core-shell nanostructures are widely used in different

applications such as electronics,8-10 biomedicine,11 pharmaceutical,12 optics,13 and catalysis.

14-15

Core-shell nanoparticles are broadly divided into five categories as shown in Figure 2.1 Each type of core-shell nanoparticles has its own importance and applications Based on their material properties, the core-shell nanoparticles are further classified into four different groups: (1) inorganic/inorganic, (2) inorganic/organic, (3) organic/organic and (4) organic/inorganic Among these, inorganic/inorganic core/shell nanoparticles are the most important class of core-shell nanoparticles, since these types of materials are commonly used

as ideal candidates for catalysis, optoelectronics, and bioimaging applications

5

Figure 2.1 Different core/shell nanoparticles: (a) spherical core/shell nanoparticles; (b)

hexagonal core/shell nanoparticles; (c) multiple small core materials coated by single shell material; (d) nanomatryushka material; (e) movable core within hollow shell material Reprinted with permission from reference.7

2.2 Rattle–type hollow structures (Yolk-shell / Nanorattles)

Rattle–type hollow structures represent a new class of special core-shell nanoparticles and generally referred to as hollow shells with a void space between the solid particle core and the shell, where the core can move freely inside the shell These are also called yolk-shell nanoparticles (YSNs).16 Recently, researchers have paid much attention towards the synthesis

of YSNs, owing to their unique optical and electrical properties and great potential in biosensors, lithium-ion batteries, biomedicine, surface-enhanced Raman scattering, imaging, and catalysis applications.17-19 In particular, they can act as nanoreactors that can provide

small spaces for the controlled synthesis of new nanomaterial or for catalytic reactions to

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occur It has been demonstrated that YSNs are ideal candidates for catalytic reactions compared to core-shell nanoparticles In this section we mainly emphasize on the fabrication

of YSNs with metal core of Au, Ag, Pt and Pd and silica shell (SiO2)

Figure 2.2 Schematic illustration of etching strategies for preparing YSNs Reprinted with

permission from reference.20

2.3 Methodologies for the fabrication of noble metal (M@SiO2) YSNs

2.3.1 Synthetic approaches

Till date, several methods such as (1) selective etching or dissolution method; (2) soft templating method; (3) template free method; (4) galvanic replacement method; (5) pre-shell / ship-in-bottle method, and (6) one-pot method have been employed for the synthesis of YSNs Among them, selective etching or dissolution methods, pre-shell/post-core, template

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free method and one-pot method are noteworthy for the fabrication of different types of YSNs The combinations of the above methods can also allow to fabricate complex nanorattles with unique properties.20

2.3.2 Selective etching or dissolution method

Selective etching methods are commonly employed for the fabrication of YSNs It is a multi-step process in which the pre-synthesized core materials are first coated with one or two layers of different materials to form sandwich structures followed by a selective etching

of one of the coated layers or metal core either by dissolution using a solvent or calcination (Figure 2.2) This method is also employed for the fabrication of YSNs with non-spherical structures such as ellipsoids, sword-in-sheath and cocoons.16

Kim and coworkers21 first applied this approach for the synthesis of Au@carbon

nanorattles Later, Xia and coworkers have successfully synthesized hollow Au@polymer nanoratlle structures In their work, Au@silica nanoparticles were synthesized using sol-gel method followed by encapsulation with a polymer poly(benzyl methacrylate) (PBzMA) to form Au@SiO2@PBzMA hybrid particles The silica layer was selectively etched with HF to

form hollow polymer beads containing movable gold cores (Figure 2.3).22

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Figure 2.3 (A, B) Backscattering SEM and (C, D) TEM images of Au@SiO2@PBzMA

particles before (A, C) and after (B, D) HF etching (E, F) TEM images of Au@SiO2@PBzMA synthesized at different time periods of polymerization: (E) 3 h and (F)

6 h Reprinted with permission from reference.22

Zhang and coworkers used a similar approach for the synthesis of Au nanorattles In their method, PS-co-P4VP microsphere polymers were employed as the template AuCl4- ions

were immobilized on the microspheres through coordination affinity of metal precursor ions with microsphere shell, followed by reduction with NaBH4 to form gold nanoparticles on the

microspheres Silica shell was deposited on the spheres through sol-gel process Finally these hybrid particles were calcinated at high temperature (550 ºC) to remove microsphere templates and eventually Au@microsphere silica shells were obtained (Figure 2.4).23

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Figure 2.4 TEM images of (A) PS-co-P4VP microspheres, (B) Au/PS-co-P4VP

microspheres, (C, D) Au/PS-co-P4VP@HMSM microspheres, and (E, F) Au@HMSM shell microspheres Reprinted with permission from reference.23

yolk-A.Archer and coworkers24 have also successfully synthesized nanorattles with multicore

nanoparticles (Figure 2.5) In this method, Au nanoparticles were grown on the surface of amino-group-functionalized polystyrene (PS) spheres followed by direct coating of silica shell and removal of the PS spheres by calcination at 450 °C

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Figure 2.5 TEM images of (A) Au-decorated PS nanospheres, (B) PS@Au@silica colloids,

and (C) hollow silica spheres with multi Au nanoparticles after burning off PS templates (D) Backscattering SEM (inset, normal SEM) image of multicore hollow silica spheres Reprinted with permission from reference.24

Kim and coworkers25 used the same strategy to fabricate α-Fe2O3 capsules with Au, Pt,

Ag or bimetallic (AuPt) cores In their method, polyelectrolyte multilayer’s (PEMs) were coated on melamine-formaldehyde (MF) templates by LBL technique and metal nanoparticles were consecutively synthesized on the templates After loading metallic nanoparticles, the PEMs particles were encapsulated with a layer of α-FeOOH followed by calcination at 700 °C which eventually gave Au@-Fe2O3 nanorattles This calcination step

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not only removed the templates and organic components but also led to the formation of a single large metallic core through thermal agglomeration

Recently, Tang and coworkers26 employed a different approach for the synthesis of

YSNs In their method, a three-layer sandwich structure of organic-inorganic hybrid solid silica spheres (HSSs) was prepared by hydrolysis and condensation of TEOS (tetra ethyl orthosilicate) and N-3-(trimethoxysilyl) propyl ethylenediamine (TSD) The YSNs were fabricated from HSSs by selectively etching the middle layer (organosilica layer) with an appropriate amount of aqueous hydrofluoric acid (HF)

Partial removal of the core from the core-shell was demonstrated by Fuertes and coworkers In their approach, solid core/mesoporous shell silica microspheres were first synthesized followed by loading with a carbon precursor that was sequentially carbonized Later, controlled dissolution of silica was done with NaOH to produce silica@carbon YSNs with various particle sizes by varying the etching time.27 Shi and coworkers followed a

similar strategy for the formation of Au@mSiO2 and Fe2O3@mSiO2 YSNs based on the

selective etching of structurally different silica shells In their method, pre-synthesized Au nanoparticles were coated with a thick silica shell and then coated with a mesoporous silica shell (Figure 2.6).28 The inner thicker silica was selectively removed after treating with

Na2CO3 aqueous solution, leaving Au@mSiO2 YSNs

Song and coworkers fabricated Au@SiO2 yolk-shell nanoparticles based on selective

etching of Au cores In their method, presynthesized gold nanoparticles were coated with a silica shell using a modified Stober’s method followed by selective etching of Au cores with KCN solution Gold cores with different sizes were obtained (Figure 2.7).18 Based on the

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similar strategy, a series of rattle type nanostructures such as Pt@carbon,17 Au@ZrO2,29 and

Au@SnO230 were fabricated

Figure 2.6 (a) Schematic illustration for the synthesis of Au@mSiO2 and Fe2O3@mSiO2

YSNs; TEM images of (b) Au@SiO2@mSiO2, (c) rattle-type Au@mSiO2, (d and inset)

ellipsoidal Fe2O3@SiO2@mSiO2, (e, inset: SEM image of deliberately selected broken

ellipsoids) rattle-type Fe2O3@mSiO2 Reprinted with permission from reference.28

Though a number of YSNs were fabricated through this method, the involvement of tedious multi-step process, usage of toxic agents such as HF, KCN for etching and difficulty

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in controlling the size, shape and composition of metal core have limited its usage for the fabrication of YSNs

Figure 2.7 (a-f) TEM images of Au@SiO2 nanoreactors and (g, h) silica hollow shells (a, b)

Gold core diameters are 104±9 nm, (c, d) 67±8 nm, and (e, f) 43±7 nm The scale bars represent 200 nm (a, c, e, and g) and 100 nm (b, d, f, h) Reprinted with permission from reference.18

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2.3.3 Pre-shell method or Ship-in-bottle method

An alternative approach for the synthesis of nanorattles is a preshell/postcore method, using mesoporous shell themselves as a nanoreactor for the production of tunable sizes of nanoparticles inside the nanocavity of mesoporous shells The schematic process for the formation of YSNs is shown in Figure 2.8 Briefly, shells were synthesized first and followed

by diffusion of two types of reactants such as precursors for the core metal and reducing agents into the nanocavity of the shells A large number of different types of metal cores are formed inside the shell through self-assembly and chemical reaction

Figure 2.8 Schematic procedure for the preparation of yolk/shell nanoparticles through the

ship-in-bottle method Reprinted with permission from reference.20

By using this approach, Hah and coworkers successfully synthesized Cu@silica rattles Presynthesized silica shells were soaked in a mixture of copper nitrate and hydrazine monohydrate They claimed that Cu2+ ions and hydrazine molecules were diffused inside the

nanocavity of silica shell and Cu2+ ions were reduced by hydrazine molecules and eventually

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Cu@SiO2 nanorattles were formed The size of copper nanoparticle can be tuned by repeating

the soaking-reduction-separation cycle as shown in Figure 2.9.31

Figure 2.9 TEM images of hollow silica particles (a) and Cu core rattle-type silica particles

(b: 1 cycle; c: 2 cycles; d: 3 cycles) Reprinted with permission from reference.31

A similar approach was followed by Sara and coworkers for the synthesis of Au@silica nanocomposite in which a gold precursor and NaBH4 were diffused inside the hollow silica

shells to form gold nanoparticles inside the hollow silica shell.32 In all the reported methods,

normally two types of reactants diffuse sequentially into the cavity and react to form metal nanoparticles Hence, these approaches have inherent difficulty to ensure that the reaction takes place exclusively inside the shell.16 Cheng and coworkers developed a novel method for

the production of various sized Ag nanoparticles inside the polypyrrole-chitosan hollow nanospheres by treating with ultraviolet rays Under ultraviolet rays the photolysis of chitosan takes place and hydroxymethyl radicals were generated and utilized to reduce the silver ions

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to metallic silver in the hollow nanospheres Moreover, the amount of AgNO3 loaded into the

nanoreactors can be easily controlled by adjusting the pH sensitive permeability of the polymer shells This is schematically represented in Figure 2.10.33

Figure 2.10 Schematic procedure for the preparation of PPy-CS hollow nanospheres

containing movable Ag Cores (Ag@PPy-CS) Reprinted with permission from reference.33

Tang and coworkers recently developed a novel nanoreactor for the preparation of tunable gold cores inside the silica nanorattles They constructed a three-layer sandwich structure in which the middle layer is selectively etched and leaving a plenty of alkyamino groups that act as in situ reducing agent and stabilizer for the growth of metal core The size

of the gold nanoparticles can be tuned by soaking the silica shells in different concentrations

of gold precursors TEM images of silica nanorattles are shown in Figure 2.11.34 But the

method requires silica shells with special functional groups

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Figure 2.11 TEM images of (a) silica nanorattles of 110 nm, (b) SRG-1, (c) SRG-2, and (d)

SRG-3 Reprinted with permission from reference.34

In recent studies, Lou and coworkers successfully synthesized hollow SnO2 nanoparticles

inside the mesoporous silica shell In their method, mesoporous silica shells (mHSS) were synthesized first and soaked in a molten metal salt hydrate (highly concentrated) with a melting temperature below 100 ºC The infiltrated mHSS were isolated and calcinated at high

a temperature of 700 ºC.35 The infiltrated precursor underwent oxidation process and led to

the formation of SnO2 nanoparticle Finally, mHSS was etched with HF The schematic

process for this method is shown in Figure 2.12

Figure 2.12 Schematic formation of SnO2 hollow spheres inside mesoporous silica

nanoreactors Reprinted with permission from reference.35