Synthesis of i III VI semiconductor nanoparticles and their applications

In this thesis, the research work focused on the fabrication of I-III-VI semiconductor nanoparticles and their applications on biological cell labeling and hydrogen production by water s

SYTHESIS OF I-III-VI SEMICONDUCTOR

NANOPARTICLES AND THEIR APPLICATIONS

TANG XIAOSHENG (Master of Engineering, University of Science and

Technology of China)

A THESIS SUBMITTED FOR THE DOCTOR OF PHILOSOPHY

DEPARTMENT OF MATERIALS SCIENCE &

ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE

2013

First and foremost, I would like to express my deepest and sincerest gratitude to

my supervisor, Dr Xue Junmin, for offering me this wonderful opportunity to pursue

my PhD degree His enthusiasm, integrity, and dedication for scientific research have been a major influence on me I have benefited tremendously from his immense knowledge, insightful intuition, patient guidance and encouragements throughout of years of my study

Secondly, I would like to thank my co-supervisor, Dr Gregory K.L Goh, from

Institute of Materials Research and Engineering He helped me so much on my research work

I will take this opportunity to appreciate the friendship and support from my group colleagues, Dr Eugene Choo Shi Guang, Sheng Yang, Yuan Jiaquan, Chen Yu, Erwin, Li Meng and Lee Wee Siang Vincent Thanks to Dr Eugene Choo Shi Guang,

he gave me some good advice and taught me how to operate equipments in our department

I wish to express my sincere gratitude to our department staffs, Mr Chen Qun,

Ms Lim Mui Keow Agnes, Dr Zhang Jixuan, Dr Yin Hong, Ms Yang Fengzhen, Mr Henche Kuan, Ms Chooi Kit Meng Serene, Ms He Jian, and Mr Chan Yew Weng for their support They have always been helpful, providing trainings for utilizing the technical facilities Without their support my research work would not have been possible to proceed

Miss Tan Hui Ru, from Institute of Materials Research and Engineering, and Dr Yu Kuai, from NUS Graduate School for Integrative Sciences and Engineering; they warmly helped and discussed with me in the photocatalystic testing and TEM characterization

I am grateful to my dear friends, Dr Wang Yu, Dr Yuan Du, Dr Zhang Xiaoxin,

Dr Ma Yuwei, Dr Wang Yinxiao, Mrs Ran Min, Mr Neo Chin Yong, … The joyful conversations with them and encouragement from them in the past a few years

Last but not least, I deeply owe to my dear parents for their unconditional love and

to my wife Liu Congrong for her endless support and loving care, and my son Tang Jixuan

Acknowledgements i

Table of Contents iii

Summary vi

List of Tables and Schemes ix

List of Figures x

List of Abbreviations xvi

List of Symbols xviii

Chapter 1 Introduction 1

1.1 General properties of semiconductor nanomaterials 1

1.1.1 Size dependent optical properties of semiconductor nanoparticles 1

1.2 Current progress of semiconductor nanoparticles 3

1.2.1 Core-shell semiconductor nanoparticles 3

1.2.2 Doping of semiconductor nanoparticles 5

1.2.3 Composition of semiconductor nanoparticles 5

1.3 I-III-VI semiconductor nanoparticles 7

1.3.1 Methods to prepared I-III-VI nanoparticles 7

1.4 Design and applications 9

1.4.1 QDs as in vivo probes 9

1.4.2 Two-photon cell labeling by QDs 11

1.4.3 Semiconductor nanomaterials for hydrogen production 12

1.5 Research objectives 13

Chapter 2 Experimental techniques 24

2.1 Materials 24

2.2 Phase transfer of hydrophobic nanoparticles 24

2.2.1 Phase Transfer of the hydrophobic CuInS 2 -ZnS nanocubes 24

2.2.2 Phase transfer of hydrophobic Zn-doped AgInS 2 nanocrystals 25

2.2.3 Preparation of water-soluble AgInS 2 -ZnS nanoparticles 25

2.3 Characterization 26

2.3.1 Chemical analysis 26

2.3.3 Optical properties 28

2.3.4 Cell viability assays 29

2.3.5 Cell labeling 30

2.3.6 Photocatalytic reactions 31

Chapter 3 CuInS 2 –ZnS Nanocubes with High Tunable Photoluminescence 33

3.1 Introduction 33

3.2 Synthesis of CuInS 2 -ZnS nanocrystals 34

3.3 Results and discussion 35

3.3.1 Characterization of the structure of CuInS 2 -ZnS nanocubes 35

3.3.2 Optical property of CuInS 2 -ZnS cube 41

3.3.3 Biological application of CuInS 2 -ZnS nanocubes 44

3.4 Summary 47

Chapter 4 Zn doped AgInS 2 Nanocrystals and Their Fluorescence Properties 51

4.1 Introduction 51

4 2 Experiment procedures 53

4.2.1 Synthesis of AgInS 2 and Zn-doped AgInS 2 nanocrystals 53

4.3 Results and discussion 54

4.3.1 Characterization of the structure of Zn-doped AgInS 2 nanoparticles 54

4.3.2 Optical property of Zn-doped AgInS 2 nanocrystals 61

4.3.3 Biological application Zn-doped AgInS 2 nanocrystals 66

4 4 Conclusions 67

4.5 References 69

Chapter 5 AgInS 2 -ZnS Heterodimers with Tunable Photoluminescence 71

5.1 Introduction 71

5.2 Synthesis of AgInS 2 -ZnS nanocrystals 72

5.3.1 Characterization of the structure of AgInS 2 -ZnS heterodimer 73

5.3.2 Optical properties of AgInS 2 -ZnS heterodimer 83

5.3.3 Cell labeling using AgInS 2 -ZnS heterodimer 88

Chapter 6 Cu-In-Zn-S Nanoporous Spheres for Highly Efficient Hydrogen Evolution 96

6.2 Preparation of CIZS nanoporous spheres 98

6.3 Results and discussions 98

6.3.1Characterization of the structure of CuInZnS spheres 98

6.3.2 Hydrogen production using CuInZnS spheres as photocatalyst 104

6.4 Conclusions 105

Chapter 7 CuInZnS-Decorated Graphene Nanocomposites for Highly Efficient Hydrogen Production 108

7.1 Introduction 108

7.2 Experimental 110

7.2.1 Preparation of CuInZnS nanospheres 110

7.2.2 Preparation of graphene oxides (GO) 110

7.2.3 Preparation of CIZS-rGO nanocomposites 111

7.3 Results and discussion 112

7.3.1Characterization of the structure of CuInZnS-Decorated Graphene nanocomposites 112

7.3.2 Hydrogen production using CuInZnS-Decorated Graphene nanocomposites as photocatalyst 124

7.4 Conclusions 130

7.4 References 132

Chapter 8 Conclusions and Future Work 134

8.1 Conclusions 134

8.2 Future Work 137

8.3 References 139

In this thesis, the research work focused on the fabrication of I-III-VI semiconductor nanoparticles and their applications on biological cell labeling and hydrogen production by water splitting Firstly, zinc-doped CuInS2 and AgInS2 nanoparticles with different shapes including the cube, sphere and heterodimer structures were prepared by hot-injection method Corresponding photoluminescent (PL) properties of Zn-doped AgInS2 and CuInS2 nanoparticles were studied by lifetime measurement and ultrafast laser The high quality two-photon PL was further used in the application of two-photon cell labeling Secondly, Cu-In-Zn-S nanospheres and CuInZnS microstructures-graphene composites were prepared through a template-free hydrothermal method The as-prepared product with tunable absorption was used as a photocatalyst for hydrogen production under the illumination

of visible light, which displayed high photocatalytic efficiency

For the I-III-VI alloy semiconductor nanoparticles, three studies were done The first study investigated CuInS2-ZnS alloyed nanocubes The CuInS2-ZnS nanocubes with tunable emissions from 548 to 678 nm were prepared by diffusing Zn ions into CuInS2 nanocrystal seeds They also displayed high quality two-photon fluorescence Based on the strong PL, the cell imagings excited by either 365 nm UV or 800 nm infrared lasers were demonstrated The successful synthesis of the CuInS2-ZnS alloyed nanocubes provided the premise for future investigation of alloyed systems arising from the I-III-VI2 group semiconductors The second study developed a facile solution method to synthesize Zn-doped AgInS2 nanoparticles The as-obtained

680 nm The nanocrystals with high quantum yield demonstrate promising applications in cell imaging The third study discussed AgInS2–ZnS heterodimer nanostructures PL emission of the AgInS2–ZnS heterodimers was finely tuned from green to red by the diffusion Zn into AgInS2 nanoparticles through adjusting the intermediate temperature from 90 oC to 180 oC Moreover, the heterodimers showed well defined two photon fluorescence (TPF) properties Finally, the cell imaging using AgInS2-ZnS excited by either UV or infrared light was successfully demonstrated For the I-III-VI semiconductor microstrues, two main studies were conducted In the first study, we synthesized Cu-In-Zn-S nanospheres by a template free and facile method The band gap of the Cu-In-Zn-S nanospheres could be tuned by the amount

of Cu doping Moreover, the mesopous nanostructure of the Cu-In-Zn-S nanospheres exhibited excellent photocatalytic activity for hydrogen production from water without any co-catalyst This work demonstrated the potential of industrial hydrogen production with a low-cost method in the field of solar energy conversion In the second study, we synthesized CuInZnS-rGO nanocomposites with high efficiency of the photocatalytic H2 from water splitting under visible light by a solvothermal method The CuInZnS-rGO nanocomposites displayed a high visible light photocatalytic H2 production rate of 3.8 mmol/h·g with 0.5 wt% Pt as a co-catalyst, which was the highest productivity for the Cu-In-Zn-S system Furthermore, this work demonstrated the use of graphene as a support for Cu-In-Zn-S microstructure in photocatalytic hydrogen production This provided a potential application of

Table 3 1 Elemental Analysis of the CuInS2-ZnS nanocubes by ICP-OES 37

Scheme 4 1 Schematic illustration showing the synthesis of Zn-doped AgInS2 nanocrystals at different diffusion temperatures 54

Scheme 5 1 Schematic illustration showing the synthesis of AgInS2-ZnS heterodimers using a hot injection method 73

Table 5 1 ICP analysis of chemical compositions of the heterodimers using

different intermediate temperatures 81

Table 5 2 EDX analysis of chemical compositions of the heterodimers obtained

using different intermediate temperatures 82

Table 6 1 BET specific surface area analysis of the spheres with different

amounts of Cu-doping 103

Table 7 1 BET surface areas of the obtained CIZS-rGO nanocomposites 123

Figure 1 1 A), Wide field HRTEM micrograph of Zn0.67Cd0.33Se nanocrystals B)

PL spectra with excitation of 365 nm for the ZnxCd1-xSe nanocrystals with

Zn mole fractions of (a) 0, (b) 0.28, (c) 0.44, (d) 0.55, (e) 0.67 [77] 7

Figure 1 2 A) HRTEM image of single ZnS-AgInS2 solid solution nanoparticle, B) photographs of UV-illuminated ZAIS nanoparticles solutions [36] 8

Figure 1 3 Photoluminescence properties of CuInS2/ZnS core-shell nanocrystals

[80] 9

Figure 1 4 Cross section of dual-labeled sample examined with a Bio-Rad 1024

MRC laser-scanning confocal microscope with a 40x oil 1.3 numerical aperture objective [73] 10

Figure 1 5 Two-photon fluorescence labeling of HeLa cancer cells after

uptaking water solube CdSe@AsS QDs for 2h [99] 11

Figure 1 6 Low and high magnification TEM images and XEDS maps (orange

= Zn, green = In and yellow = Ag) for ZnIn0.23Ag0.04S1.365 [117] 13

Figure 3 1 XRD pattern of CuInS2 seeds formed at 120 oC (Tetragonal CuInS2, JCPDS card No 85-1575) 36

Figure 3 2(A) XRD pattern of the obtained CuInS2-ZnS alloyed nanocubes with zinc mole fraction of 62% (B) TEM images of the as-obtained CuInS2-ZnS alloyed nanocubes Inset: Corresponding SAED pattern of the sample (C) The magnified TEM image of CuInS2-ZnS alloyed nanocubes Inset is the cubic model (D) EDX spectrum of the obtained CuInS2-ZnS nanocubes with zinc mole fraction of 62% Four elements, Cu, In, Zn S were detected (E) HRTEM of a single CuInS2-ZnS alloyed nanocube (F) The corresponding fast Fourier transform (FFT) image from area (E) 39

Figure 3 3 Histograms of size distributions of the CuInS2-ZnS alloyed nanocubes with different mole fractions of zinc, (A) 62%, (B) 52%,(C) 38%, respectively 39

Figure 3 4 TEM images of the CuInS2-ZnS alloyed nanocubes with zinc mole fractions of (A) 52% and (B) 38%, respectively 40

Figure 3 5 The absorption spectra of the CuInS2-ZnS alloyed nanocubes with different zinc mole fractions (A) 62%, (B) 52% and (C) 38%, respectively 41

Figure 3 6 (A) PL spectra of the CuInS2-ZnS alloyed nanocubes with different zinc mole fractions of 62%, 52% and 38%, respectively The measurements were under excitation of 365 nm UV The inset is the digital photograph of the samples in toluene under excitation of 365 nm (B) Upconversion spectra of the CuInS2-ZnS alloyed nanocubes with different zinc mole fractions of 62%, 52% and 38%, respectively The measurements were under excitation of 800 nm laser The inset is the digital photograph of the

with the standard rhodamine 6G ethanol solution (QY=95%) or rhodamine

101 ethanol solution (QY=100%) The quantum yields of the nanocubes were (A) 39%, (B) 37% and (C) 34%, respectively 42

Figure 3 8 PL relaxation of the nanocubes with zinc mole fraction of 38%, as

compared to that of the pure CuInS2 nanoparticles Inset is the photograph

of pure CIS nanoparticles under UV-365nm lamp 43

Figure 3 9 (A) Upconversion emission of the CuInS2-ZnS alloyed nanoparticles with zinc mole fraction of 52% with various input powers (B) The corresponding quadratic dependence of integrated fluorescence intensity with the input power of laser, showing that the upconversion mechanism is two- photon 44

Figure 3 10 (A) Digital photographs of the CuInS2-ZnS alloyed nanocubes dispersed in water under excitation of 365 nm UV (B) The digital photographs of the nanocubes with zinc mole fraction of 38% dispersed in water and toluene mixtures, demonstrating that the nanocubes could be dispersed in water completely after phase transfer (C) PL spectra of the CuInS2-ZnS nanocubes after phase transfer, in comparison with those of the nanocubes before phase transfer (D) The fluorescent image of the OCA17 cells labeled with the CuInS2-ZnS nanocubes with zinc mole fraction of 62% under 365 nm UV excitation (E) Multi-photon fluorescent image of NIH/3T3 cells labeled with the nanocubes with zinc mole fraction of 38% upon excitation of 800 nm laser 46

Figure 4 1 (A) XRD pattern of the AgInS2 nanocrystals obtained at 120 oC, (B) XRD pattern of the Zn-doped AgInS2 nanocrystals by diffusing Zn into the pre-formed AgInS2 at 120 oC, (C) EDX spectra of the pure AgInS2 and Zn-doped AgInS2 nanocrystals, (D) TEM image of the AgInS2 nanocrystals obtained at 120 oC (Inset: digital photograph showing the weak red emission

of the pure AgInS2 nanocrystals under UV excitation) and (E) TEM image showing the Zn-doped AgInS2 nanocrystals obtained at 120 oC (Inset: histogram showing the size distribution of the nanocrystals) 56

Figure 4 2 XPS spectra of (A) Ag 3d, (B) In 3d, (C) S 2p, (D) Zn 2p3 57 Figure 4 3 (A) ICP analysis showing the chemical compositions of the Zn-doped

AgInS2 nanocrystals prepared at different diffusion temperatures (B, C and D) TEM images of Zn-doped AgInS2 nanoparticles prepared at 150 oC, 180 o

C and 210 oC, respectively (Insets: histograms showing the size distribution of the nanocrystals) 59

Figure 4 4 (A) The absorption spectra (B) PL spectra of the Zn-doped AgInS2 nanocrystals prepared at 120 oC, 150 oC, 180 oC and 210 oC, respectively, (C) the corresponding digital photographs of the Zn-doped AgInS2nanocrystals dispersed in toluene under excitation of 365 nm UV 60

Figure 4 5 (A) PL emission spectra of the aqueous Zn doped AgInS2 alloy

nanoparticles at 180 oC versus the standard rhodamine 6G ethanol solution (QY=95%), (C) PL emission spectra of the Zn doped AgInS2 alloy nanoparticles at 150 oC, versus the standard rhodamine 101 ethanol solution (QY=100%) and (D) PL emission spectra of the Zn doped AgInS2 alloy nanoparticles at 120 oC, versus the standard rhodamine 101 ethanol solution (QY=100%) The quantum yields of the four alloy nanoparticles were 17%, 16%, 15% and 12%, respectively 62

Figure 4 6 (A) PL relaxations of the Zn-doped AgInS2 nanocrystals prepared at

120 oC, 150 oC, 180 oC and 210 oC, respectively and (B) Table showing the fit parameters of the relaxation plots The fit parameters are derived from

the equation: I(t) = A 1 exp(-t/1 ) + A 2 exp(-t/2 ) 64

Figure 4 7 Transient absorption spectra of Zn-doped AgInS2 nanocrystals at different time delays 64

Figure 4 8 Single-wavelength dynamics of Zn-doped AgInS2 nanocrystals Pump wavelength used is 400 nm, Probe wavelengths are indicated in the figure 65

Figure 4 9 (A) PL spectra of the Zn-doped AgInS2 nanocrystals in water under excitation of 365 nm UV, (B) the digital photograph showing the emissions

of the Zn-doped AgInS2 nanocrystals in water under 365 nm UV excitation and (C) The fluorescent image of the NIH/3T3 cells labeled with Zn-doped AgInS2 nanocrystals with green emission under 365 nm UV excitation (Inset: cell imaging with higher magnification) 67

was injected at 90 oC (C) HRTEM of an AgInS2-ZnS heterodimer and the inset is the schematic representation of the heterodimer (D) XRD patterns

of (a) pure AgInS2 nanoparticles and (b) AgInS2-ZnS heterodimers 74

Figure 5 2 The XRD pattern of the AgInS2-ZnS heterodimers synthesized using the intermediate temperature of 90 oC, in comparison with the standard XRD patterns of chalcopyrite AgInS2 and cubic ZnS 76

Figure 5 3 EDX spectrum of the AgInS2-ZnS heterodimers prepared at 210 oC for 2 h The zinc source was injected at 90 oC 76

Figure 5 4 (A) PL spectra of (a) the pure AgInS2 nanoparticles without visible emission observed, represented by the dashed line (b) the AgInS2nanoparticles at 210 oC for 30 s upon zinc injection (c) the AgInS2 nanoparticles synthesized at 210 oC for 30 mins upon zinc injection (d) the AgInS2-ZnS heterodimers synthesized at 210 oC for 2 hours upon zinc injection (B) The corresponding digital photographs of the four samples under excitations of day light and UV, respectively 78

Figure 5 5 A series of HRTEM images of a typical heterodimer at different

focus stages, presenting the structure evolution of the heterodimer with electron beam energy input 79

Figure 5 6 TEM images of AgInS2-ZnS heterodimers prepared using different intermediate temperatures (A) 120 oC, (B) 150 oC and (C) 180 oC respectively; Insets are HRTEM images of the single AgInS2-ZnS heterodimer circled by red dot line The scale bar is 2 nm (D) XRD patterns

of AgInS2-ZnS heterodimers prepared using different intermediate temperatures of 90 oC, 120 oC, 150 oC, 180 oC, respectively 83

Figure 5 7 The absorption spectra of AgInS2-ZnS heterodimers synthesized using the intermediate temperature of 90 oC, 120 oC, 150 oC and 180 oC, respectively 84

Figure 5 8 (A) Photography pictures and (B) PL spectra of the AgInS2-ZnS heterodimers with different emissions dispersed in toluene under excitation

of 365 nm 85

Figure 5 9 (A) PL emission spectra of the AgInS2-ZnS heterodimers at 90 oC versus the standard rhodamine 6G ethanol solution (QY=95%), (B) PL emission spectra of the AgInS2-ZnS heterodimers at 120 oC versus the standard rhodamine 6G ethanol solution (QY=95%), (C) PL emission spectra of the AgInS2-ZnS heterodimers at 150 oC versus the standard rhodamine 101 ethanol solution (QY=100%) and (D) PL emission spectra

of the AgInS2-ZnS heterodimers at 180 oC versus the standard rhodamine

101 ethanol solution (QY=100%) The quantum yields of the four heterodimers were 31%, 28%, 35% and 38%, respectively 86

Figure 5 10 TEM image of AgInS2 nanoparticles (blue emission) prepared using intermediate temperature of 60 oC 86

Figure 5 11 The upconversion fluorescence spectra of the obtained heterodimers

with various input powers (B) Quadratic dependence of integrated fluorescence intensity with the input power of laser 88

Figure 5 13 (A) Digital photographs of the obtained AgInS2-ZnS heterodimers with different emissions in water; (B) PL spectra of the AgInS2-ZnS heterodimers in water 90

Figure 5 14 (A) TEM image of obtained AgInS2-ZnS clusters (150 oC) after phase transfer; (B) A magnified TEM image of single copolymer coated AgInS2-ZnS heterodimers 90

Figure 5 15 Images of Hela cells labeled with the heterodimers with different

emission under UV excitation (A) green, (B) yellow, (C) red (D) Magnified Images of Hela cells labeled by heterodimers with green emission 91

Figure 5 16 (A) The bright microscopy picture of the heterodimers (dry powder

form) under excitation of 800 nm laser (B) The dark image without 800 nm laser excitation 92

Figure 5 17 (A) TPF image of NIH/3T3 cells labeled with the heterodimers with

red emission (B) The dark image without 800 nm laser excitation 92

Figure 6 1 XRD patterns of CIZS spheres with different amounts of Cu-doping

(a Cu0.01In0.25ZnS1.38, b Cu0.02In0.25ZnS1.385, c Cu0.04In0.25ZnS1.395, d

Cu0.08In0.25ZnS1.415) 99

Figure 6 2 (A) Low magnification FESEM image of Cu0.04In0.25ZnS1.395 spheres; (B) High resolution FESEM image of Cu0.04In0.25ZnS1.395 nanospheres; (C) Low-resolution TEM image of Cu0.04In0.25ZnS1.395 spheres; (D) High-resolution TEM image of Cu0.04In0.25ZnS1.395 spheres, and (E) HRTEM image of the single Cu0.04In0.25ZnS1.395 nanoparticles 99

Figure 6 3 EDX spectrum of the CIZS spheres 101 Figure 6 4 (A) High angle annular dark field (HAADF) image of an

individual Cu0.04In0.25ZnS1.395 sphere (B) Line profile analysis of S (yellow),

Cu (red), Zn (green) and In (blue-green), along axis of one single sphere; (C), (D) ,(E) and (F) are XEDS maps (orange=Cu, blue=In , green=Zn and yellow=S ) for one Cu0.04In0.25ZnS1.395 sphere 102

Figure 6 5 The XPS spectrum of Cu 2p of the CIZS spheres 102 Figure 6 6 Nitrogen adsorption-desorption isotherms of Cu0.04In0.25ZnS1.395 nanoporous spheres 103

Figure 6 7 (A) UV-Vis absorption spectra of the CIZS spheres with different

amounts of Cu (B) Hydrogen evolution from an aqueous solution containing 1.2 moldm-3 Na2SO3 and 0.7 moldm-3 Na2S catalyzed by CIZS spheres with different amount of doped Cu 104

Figure 7 1 (A) XRD patterns of CIZS-rGO nanocomposites with different

weight ratios of rGO to CIZS (B) Low magnification TEM image of CG2 nanocomposites (C) and (D) Magnified TEM image of CG2

nanospheres were porous and comprised of numerous nanoparticles (F) High-resolution TEM image of the CIZS nanospehres in the nanocomposites 114

Figure 7 2 EDS spectrum of Cu0.02In0.3ZnS1.47-rGO nanocomposites 115

Figure 7 3 (A) Line profile analysis of S (orange), Cu (red), Zn (green) and In

(blue), along axis of one single CIZS nanosphere in the CG2 nanocomposites (B), (C) and (D): XEDS maps (orange=Cu, blue=In and green=Zn) of CIZS nanospheres in the CG2 nanocomposites 115

Figure 7 4 XPS data from the surface of the CIZS nanospheres in CG2

nanocomposites: (A) Cu 2p core-level spectrum, (B) Zn 2p core-level spectrum; (C) In 3d core-level spectrum; (D) S 2p core-level spectrum 117

Figure 7 5 AFM image of graphene oxide nanosheet 118 Figure 7 6 XPS spectra of C 1s from (A) GO and (B) CG2 nanocomposites 118 Figure 7 7 Raman spectra of graphene oxide and CIZS-rGO 119 Figure 7 8 (A) Low magnification TEM image of CIZS nanospheres (Inset:

XRD pattern of CIZS nanospheres) (B) Magnified TEM image of pure CIZS nanospheres 120

Figure 7 9 TEM image of Cu0.02In0.3ZnS1.47 nanospheres 121

Figure 7 10 Nitrogen adsorption-desorption isotherms (A) and corresponding

pore size distribution curves (B) of samples Cu0.02In0.3ZnS1.47 solid powders 122

Figure 7 11 Nitrogen adsorption-desorption isotherms (A) and corresponding

pore size distribution curves (B) of samples Cu0.02In0.3ZnS1.47-rGO composites (CG2) solid powders 122

Figure 7 12 UV-Vis absorption spectra for the CG1, CG2 and CG5, as well as

the pure CIZS nanospheres Inset: the corresponding digital photographs of CG1, CG2, CG5 and pure CIZS nanospheres 125

Figure 7 13 Comparison of the visible-light photocatalytic activity of sample

rGO, CIZS, CG0.5, CG1, CG2, CG5 and CG20 for hydrogen production using 1.2 mol·L-1 Na2SO3 and 0.7 mol·L-1 Na2S solution as sacrificial reagent and 0.5 wt% Pt as a co-catalyst; 800 W Xe-Hg lamp was used as the light source 125

Figure 7 14 Mott-Schottky plot obtained at different frequencies for CuInZnS

film electrode with Ag/AgCl, saturated KCl reference electrode and Pt counter electrode immersed in 0.1 M NaOH electrolyte with pH 12.5 127

Figure 7 15 Schematic illustration of the charge separation and transfer in the

Cu0.02In0.3ZnS1.47-rGO composites system under visible light The photoexcited electrons transfer from the conduction band of the semiconductor Cu0.02In0.3ZnS1.47 not only to Pt, but also to the carbon atoms

on the graphene sheets, which is accessible to protons that could readily react to form H2 130

XRD X-ray diffractometer

EDS X-ray energy dispersive spectrometer

TEM Transmission electron microscopy

HRTEM High resolution transmission electron microscopy

SAED Selected area electron diffraction

UV-Vis-NIR Ultra violet-visible-near infrared spectroscopy

ICP-OES Inductively coupled plasma-optical emission spectrometry

XPS X-ray photoelectron spectroscopy

STEM Scanning transmission electron microscopy

PL Photoluminescence

BET Brunauer-Emmett-Teller

FTO Fluorine-Tin Oxide

NHE Normal Hydrogen Electrode

AIZS Silver indium zinc sulfide

CIZS Copper indium zinc sulfide

CIS Copper indium sulfide

QDs Quantum Dots

LEDs Light-Emitting Diodes

FFT Fast Fourier Transform

GO Graphene Oxide

TPF Two Photon Fluorescence

E g Band gap energy

1, Synthesis of ZnO Nanoparticles with Tunable Emission Colors and Their Cell

Labeling Applications

Xiaosheng Tang , Eugene Shi Guang Choo , Ling Li ,Jun Ding, Junmin Xue* Chemistry of Materials 2010, 22, 3383

2, Synthesis of CuInS2–ZnS alloyed nanocubes with high luminescence

Xiaosheng Tang , Wenli Cheng , Eugene Shi Guang Choo, Junmin Xue* Chemical

Communication，2011,47, 5217

3 Synthesis and characterization of AgInS2–ZnS heterodimers with tunable

photoluminescence

Xiaosheng Tang , Kuai Yu , Qinghua Xu , Eugene Shi Guan Choo , Gregory K L

Goh and Junmin Xue*, Journal of Materials Chemistry, 2011, 21, 11239

4 One-Pot Synthesis of Water-Stable ZnO Nanoparticles via a Polyol Hydrolysis

Route and Their Cell Labeling Applications

Xiaosheng Tang , Eugene Shi Guang Choo , Ling Li ,Jun Ding and Junmin Xue*,

Langmuir 2009, 25(9), 5271

5 Synthesis of Zn doped AgInS2 Nanocrystals and Their Fluorescence Properties

Xiaosheng Tang , Wenxi Bernice Ailsa Ho, Jun Min Xue*, The Journal of Physical

Chemistry C 2012, 116, 9769

6 Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production of

CuInZnS-Decorated Graphene Nanosheets

Xiaosheng Tang, Qiuling Tay, Zhong Chen, Yu Chen, Gregory K.L Goh,Junmin

7 A facile method to synthesize CuInZnS micostructure for hydrogen production by water splitting

Xiaosheng Tang, Qiuling Tay, Yu Chen, Zhong Chen,Gregory K.L Goh,Junmin

Xue* New Journal of Chemistry, under revision

8 Synthesis of AIZS@SiO2 core–shell nanoparticles for cellular imaging Applications

Sheng Yang, Tang Xiaosheng, Xue junmin*, Journal of Materials Chemistry 2012,

Chapter 1 Introduction

1.1 General properties of semiconductor nanomaterials

It is well-known that nanomaterials display lots of novel physical and chemical properties, as compared to their bulk materials, because of their reduced dimension and increased surface area [1-3] Therefore, nanomaterials have attracted the interests

of a large number of scientists during the past two decades [4-7] The size of nanomaterial ranges from a few nanometers to consisting of a few thousand atoms, in which the electron motion is confined by potential barriers in all dimensions Three effects can be observed due to this size effect For semiconductor nanoparticles, also called quantum dots (QDs) are normally highly fluorescent with a narrow emission bandwidth, which makes them highly attractive as fluorescent dyes in biological labeling applications [8, 9].Semiconductor nanoparticles, have been widely used in different areas including biological imaging, solar cell and photocatalyst because of their tunable optical properties [10-12] Hence, achieving tunable optical properties was the most pivotal step to determine the detailed application of QDs The following subsections provide a literature summary of QDs, which points out three major factors which would affect the optical properties of QDs including particles’ size, particles’ shape and the composition [13-16]

1.1.1 Size dependent optical properties of semiconductor nanoparticles

Semiconductor nanoparticles have an increasement in the band gap energy with decreasing particles size due to quantum size effect [14, 15, 17, 18] Therefore,

different photoluminescence emissions could be achieved by changing the particles size The detail mechanism can be described as follows When a semiconductor

nanoparticle is excited by the light which has a greater energy than Eg, the excited

electron leaves an orbital hole in the valence band [13, 19] The negative electron and positive charged hole forms an electron-hole pair Following this, radiative recombination accompanied with relaxation of the excited electron back to the valence band annihilates the exciton which will emit a relatively sharp emission band

at the band gap energy [13, 19] Based on the quantum confinement effect, when the size of nanoparticles was close to the Bohr radius, the states of free charge carrier in semiconductor NCs become quantized The movement of these electrons and holes is then determined by quantum mechanics In general, the exciton has a finite size within the crystal defined by the Bohr exciton diameter, which could range from 1 nm

to more than 100 nm If the size of semiconductor nanoparticles is smaller than the size of the exciton, the charge carriers will become spatially confined and increase in energy [13] Therefore, the different photoluminescence wavelength could be tuned

by changing the particles size

Furthermore, the photoluminescence is also sensitive to the surface of nanoparticles If the surface of semiconductor nanoparticles was passivated, the PL wavelength maximum will be close to the absorption band edge with little red-shift [20, 21] Otherwise, the surface traps would lead to red-shift and lower quantum yield

1.1.2 Shape dependent optical properties of semiconductor

nanoparticles

For semiconductor nanoparticles, the shape effect is also an important topic [22,

-24] Mona et al studied the shape dependent ultrafast relaxation dynamics of CdSe

nanodots and nanorods [22] In their study, they found the carrier relaxation dynamics

of the higher energy states in CdSe nanorods was faster than nanodots as the delay time increased [22] The reasoning was that lowering the symmetry from spheres to rods led to splitting of the energy level degeneracy Increasing the density of states along the long axis of rods would induce a speed up in the relaxation process which involves either electron-phonon or electron-hole coupling CdSe nanorods have a longer axis than nanodots, which results in a faster relaxation process [22]

1.2 Current progress of semiconductor nanoparticles

1.2.1 Core-shell semiconductor nanoparticles

In 1982, A Henglein published his work on the emitting semiconductor CdS nanocrystals (NCs), which he discovered by chance when he was studying the surface chemistry and catalytic processes of colloidal semiconductor particles [25, 26] In this experiment, he observed that the light was emitted upon the excitation at 390 nm [25, 27] Later, L Brus firstly presented quantum mechanical effect as the correct interpretation for blue-shift in the absorption spectrum of NCs [28, 29] Based on these pioneer scientists’ efforts, more and more research groups have begun to focus

on nanomaterials [30-33]

Over the past two decades, several approaches were developed to prepare

nanomaterials [9, 15, 34-40] Bawendi [15], Murray [41] and Norris [42] made use of hot injection method to prepare II-VI semiconductor NCs This method was based on the decomposition of organometallic reagents after injecting hot co-ordinating solvent, which was a milestone in the development of semiconductor nanoparticles [15, 41, 42] The II-VI semiconductor nanoparticles synthesized with this approach could produce highly crystalline nanoparticles with controllable size, which are two

important factors for adjusting photoluminescence property Peng et al developed the

hot injection method by using trioctylphosphine (TOP) and thiols as the surfactant to cap at the surface to protect the NCs and this became a popular approach to achieve high quality nanoparticles [43-45] Following these reported results, CdSe [43-46], CdTe [47, 48], HgTe [49, 50], ZnSe [51, 52], CdHgTe [53, 54], CdZnSe [55, 56], and CdSeTe [57, 58] nanoparticles were successfully synthesized

A lot of applications of semiconductor NCs require high intensity photoluminescence In order to improve the photoluminescence of semiconductor NCs, core-shell structures were developed For the core-shell structure, an electrically insulating material was designed to coat the surface of the semiconductor core In this novel core-shell structure, the exciton photogenerated in the core was prevented from spreading over the entire particle As a result, the exciton was forced to recombine while being spatially confined to the core, thus enhancing the photoluminescence [13, 19] The advanced core-shell structure was confirmed by the CdSe@ZnS and

CdSe@CdS core-shell strucutures [59-61] Subsequently, Weis et al studied the

core/shell/shell structures CdSe@CdS@ZnS nanoparticles [62] Their outermost ZnS

shell could prevent charge carrier penetration towards the surface of particles and also improve photostability

1.2.2 Doping of semiconductor nanoparticles

One effective method to alter the optical properties of semiconductors is by impurity doping It is well-known that the incorporation of impurities into semiconductor lattices can affect the electrical conductivity, as well as the magnetic and optical properties of the semiconductor For example, in terms of optical properties, luminescence activators such as Mn2+ or Er3+ make semiconductor nanocrystals interesting candidates for optical imaging applications due to their narrow emission lines and broad excitation profiles [63-67] Another study on ZnS nanocrystals doped with Mn2+ ions demonstrated orange emissions and opened up applications in electroluminescent displays, spintronics and biomedical labeling [68, 69] Optical dynamics in Mn2+-doped ZnS nanoparticles have been a hot topic In an earlier study of Mn2+-doped ZnS nanoparticles, lifetime shortening was observed and was interpreted based on the interaction of the s-p electron-hole of the host (ZnS) and the d-electrons of the impurity (Mn2+) [70] However, recent findings have indicated that the short lifetime component (in the nanosecond range) was not due to the Mn2+emission, but more likely due to the long-wavelength tail of ZnS emission of extremely weak intensity [71]

1.2.3 Composition of semiconductor nanoparticles

With the development of core-shell semiconductor nanoparticles, size tunable NCs have become a hallmark of the related nanostructures NCs have thus been

widely exploited for the potential applications including optoelectronics, quantum-dot lasers, biosensing and biological labeling [72-76] As some problems arise by the use

of size dependent particles in applications such as nanoelectronics, superlattice structures and biological labeling, it was proposed to instead develop alloyed semiconductor NCs to improve the photoluminescence with nearly the same size In

2003, Nie et al reported one single step method to synthesize a new class of alloyed

semiconductor CdSeTe NCs by varying the relative amounts of the precursors [72]

By this approach, the peak of the photoluminescence could be tuned by the composition of the as-obtained CdSe1-xTx NCs, and the quantum yield of up to 60% could be achieved [72] Based on changing the composition of anion in alloyed NCs,

Han et al proposed a similar process by changing the composition of cation between

the alloyed NCs [77, 78] They demonstrated two close systems of ZnxCd1-xSe and

ZnxCd1-xS nanocrystals, in which NCs showed a highly narrow luminescence spectral width with full width at half-maximum (FWHM) of 22-30 nm, and the quantum yield achieved 70% to 85% The alloyed QDs opened a new possibility in bandgap engineering [77, 78] From the Figure 1.1, it can be seen the Zn0.67Cd0.33Se nanocrystals with uniform size distribution and the PL could be adjusted by the Zn fraction

Figure 1 1 A), Wide field HRTEM micrograph of Zn0.67Cd0.33Se nanocrystals B)

PL spectra with excitation of 365 nm for the ZnxCd1-xSe nanocrystals with Zn mole fractions of (a) 0, (b) 0.28, (c) 0.44, (d) 0.55, (e) 0.67 [77]

1.3 I-III-VI semiconductor nanoparticles

1.3.1 Methods to prepared I-III-VI nanoparticles

I-III-VI bulk semiconductor crystals such as CuInSe2, CuInS2 and AgInS2 with direct band gap ranging from 1.05 eV to 1.5 eV, are important players in photovoltaic devices, even though such photovoltaic devices are known to be less expensive as compared to other types of solar cells fabricated through molecular beam epitaxy techniques [36, 79, 80] So far, CuInS2 and CuInSe2 nanocrystals have mainly been studied for their potential applications in solar energy conversion Because of their potential applications in solar cells, a large number of research groups started to focus

on this research area, and developed many approaches to prepare nanocrystals [11, 79, 81-83] These approaches include solvothermal synthesis, thermolysis, photochemical decomposition and the hot-injection method [11, 81-83] In 2004, Kudo’s group used

H2S gas as the precursor of sulfur to synthesize (AgIn)xZn2(1-x) S2 solid solution for photocatalysts This group then further reported the similar system of (CuIn)ZnS and

AgInS2-CuInS2 solid solutions photocatalytic system [84-88] However, the process required high temperature and toxic H2S gas Hence, this method may not be suitable for commercialization Xie’s group synthesized CuInS2 hierarchical microarchitectures by hydrothermal method, but the flower-like microstructure showed poor photocatalytic activity for hydrogen production [89]

Following the improvement of photocatalytic hydrogen with visible light irradiation, Kudo’s group firstly studied the relationship between bandgap and chemical composition of ZnS-AgInS2 and ZnS-CuInS2 solid solution system [84-88] Subsequently, they extended this research topic to the photoluminescent semiconductor nanoparticles, using a thermal decomposition method to prepare Zn-AgInS2 with tunable photoluminescence from green to red [36]

Figure 1 2 A) HRTEM image of single ZnS-AgInS2 solid solution nanoparticle, B) photographs of UV-illuminated ZAIS nanoparticles solutions [36]

It can be seen from Figure 1.2 that the Zn-AgInS2 nanoparticles was about 5 to 6

nm and digital picture ZAIS nanoparticles showed different colors including green, yellow, orange and red under UV-illumination Later, Peng’s group developed a simpler method by hot-injection for the synthesis of both CuInS2@ZnS and

AgInS2@ZnS nanoparticles (Figure 1.3) Their work isolated different factors which would lead to the emission peak and the intensity of the photoluminescence [80]

Recently, Pons et al successfully labeled two regional lymph nodes in vivo in mice,

and also proved the lower toxicity of CuInS2@ZnS quantum dots compared to

CdTeSe/CdZnS quantum dots [90] In 2011, Zhong et al exploited the photocatalytic

degradation RhB application by using ZnS-CuInS2 alloyed NCs with tunable emissions under the illuminated with UV-vis lamp[91]

Figure 1 3 Photoluminescence properties of CuInS2/ZnS core-shell nanocrystals

[80]

1.4 Design and applications

1.4.1 QDs as in vivo probes

Biological cell labeling using fluorescent organic dyes is a useful tool for single

or multiplex DNA detections However, organic fluorophores often suffer from photobleaching, low signal and exhibit broad emission bands [73, 74, 92, 93]

Recently, fluorescent semiconductor nanoparticles have attracted great interest and have been widely used for cellular labeling due to the advantages of high photoluminescence intensity, narrow band emission and stable fluorescence [73, 74]

In 1998, Alivisatos et al reported the biological application of CdSe@ZnS

nanoparticles with highly photoluminescence instead of the traditional fluorescent organic dyes [73] From the 3T3 fibroblasts labeling result showed in Figure 1.4, it was clearly demonstrated that the green and red labels were clearly spectrally resolved

to the eye and to a color Polaroid camera And the CdSe@ZnS nanoparticles based labeled samples also displayed very little photobleaching, this is far less than labeled with conventional dye molecules [73, 74]

Figure 1 4 Cross section of dual-labeled sample examined with a Bio-Rad 1024 MRC laser-scanning confocal microscope with a 40x oil 1.3 numerical aperture objective [73]

The development of semiconductor nanoparticles for biological cell labeling created new possibilities for further multicolor experiments and diagnostics

Moreover, the first successful cell labeling experiment by CdSe@ZnS QDs also established a new class of fluorescent probe instead of molecule dye [73] Lastly, because the tenability of the photoluminescence features of QDs, it could be used as direct sensitizers for traditional probes

1.4.2 Two-photon cell labeling by QDs

Recently, multiphoton bioimaging has been attracting much attention due to little near-infrared absorption from endogenous species and water [93-98] Multiphoton microscopy enables deep imaging of a variety of biological confocal microscopy, and

it has now become the primary sample with less overall photobleaching than with the

wide-field or fluorescence imaging technique in thick specimens Loh et al prepared

CdSe/AsS core-shell quantum dots and showed high two-photon fluorescence excited

at 800 nm, and the further successful application for two photon bioimaging, upconversion fluorescence labeling of HeLa cancer cells that had internalized the

CdSe/AsS QDs using IR laser (780 nm), as shown in Figure 1.5 [99]

Figure 1 5 Two-photon fluorescence labeling of HeLa cancer cells after uptaking

water solube CdSe@AsS QDs for 2h [99]

1.4.3 Semiconductor nanomaterials for hydrogen production

Since 2004, it has been reported that the annual global consumption of energy has increased by more than 50%, and most of the energy production was derived from the combustion of fuels, such as oil, natural gas and coal [100-108] As a consequence, CO2 gas which was the by-product due to burning the coal would result in a prominent greenhouse effect In order to mitigate such an environmental problem, finding a renewable energy sources is becoming more and more important Among the renewable energy sources of hydroelectric, solar, wind and biomass, hydrogen has been recognized as a potential energy carrier for its high energy capacity and environmental friendliness [102]

In 1972, Honda discovered the hydrogen could be produced through the photoelectrochemical splitting of water on n-type TiO2 electrodes [109] Hence, a large number of photocatalysts were reported for photocatalytic activities under UV irradiation such as La-doped NaTaO3, La2TiO7 and K2La2Ti3O10 over the past 40 years [110-112] With respect to the electromagnetic spectrum, the ultraviolet only covers 4% of solar radiation Therefore, a series of CdS-based materials including

ZnxCd1-xS, Mn doped CdS and Cu-doped ZnxCd1-xS were prepared for hydrogen

production by water splitting under visible light [113-116] Recently, Kudo et al

developed the MInS2 (M= Cu, Ag) sulfide solid solutions which showed high photocatalytic activities for hydrogen evolution under visible-light irradiation [105-108] However, using the method for synthesis of the sulfide solid solutions required a high temperature To improve on this, Chen and co-workers prepared two

Ag-In-Zn-S and Cu-In-Zn-S nanoporous solid solutions by a facile hydrothermal method [117,118]

Figure 1 6Low and high magnification TEM images and XEDS maps (orange = Zn, green = In and yellow = Ag) for ZnIn0.23Ag0.04S1.365 [117]

1.5 Research objectives

Traditional organic dyes and fluorescent proteins have been the major fluorescent probes used for biological and biomedical research However, these organic probes are highly susceptible to photobleaching, so they are not suitable for long-term cell imaging Therefore, QDs have been widely studied as a substitute based on their strong photoluminescence and long stability From this introduction,

we can see that the first essential step for the biological application of QDs is to synthesize high quality QDs with unique properties Recently, a wide variety of QDs

have been prepared for application on cell imaging as mentioned above Until now, most of the reported QDs have been based on II-VI semiconductor nanoparticles, such

as CdSe, CdTe and PbSe The release of heavy metal ions such as Cd2+ is frequently proposed to explain the toxicity of QDs, with the heavy ions being harmful to human health and pose a potential environment hazard, which prohibited the further application of II-VI semiconductor nanoparticles Based on the two drawbacks, I-III-VI alloyed nanoparticles were chosen as the substitute because of their unique strong photoluminescence, low toxicity and good biocompatibility

As discussed previously, alloyed semiconductor nanparticles including AgInZnS and CuInZnS, are a promising substitute for two-photon cell labeling instead of organic dyes Furthermore, the CuInZnS microstructure/graphene composites have an important photocatalytic application in hydrogen production by splitting water under irradiation of visible light Therefore, there are two interrelated applications for the I-III-VI semiconductor nanoparticles The first objective is to develop a facile approach to synthesize alloyed I-III-VI semiconductor and to test its two photon cell labeling application and the second one is to fabricate Cu-In-Zn-S nanoparticles and CuInZnS /graphene composites for hydrogen production More specifically, our aims are highlighted as follows

1 To synthesize high quality CuInS2–ZnS, Zn-Doped AgInS2 nanoparticles and AgInS2-ZnS heterodimer with tunable photoluminescence

2 To synthesize high quality Cu-In-Zn-S nanoparticles and CuInZnS /graphene composite with tunable band gap; and testing the hydrogen production under

visible-light illumination

3 To use QDs for two photon cell labeling

1.6 Outline

The seven chapters in the thesis are organized as follow:

Chapter 1, the present chapter briefly introduced the research, outlining its background and stating the research problem It has also set out its general objectives and research questions and explained a few terms used in the study Chapter 2 introduces the synthesis and characterization techniques used in this work Chapter 3 describes the synthesis process of CuInS2–ZnS alloyed nanocrystals by diffusing Zn ions into CuInS2 nanocrystal seeds Chapter 4 describes the method for synthesizing Zn-doped AgInS2 nanocrystals by diffusing Zn into the preformed AgInS2 seeds at high temperature in solution Chapter 5 describes AgInS2–ZnS nanoparticles with well-defined heterodimer structures synthesized using a hot injection method Chapter

6 and chapter 7 describe successfully synthesized Cu-In-Zn-S nanospheres and CuInZnS/RG nanocomposites with high photocatalytic efficiency for H2 production from water splitting under visible-light Chapter 8 summarizes the findings of this thesis

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