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 Acknowledgements 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. i My Sincere thanks to Miss. Tay Qiuling, from Nanyang Technological University, 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. ii Table of Contents 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 Introduction 1.1 General properties of semiconductor nanomaterials . 1.1.1 Size dependent optical properties of semiconductor nanoparticles . 1.2 Current progress of semiconductor nanoparticles . 1.2.1 Core-shell semiconductor nanoparticles 1.2.2 Doping of semiconductor nanoparticles . 1.2.3 Composition of semiconductor nanoparticles 1.3 I-III-VI semiconductor nanoparticles 1.3.1 Methods to prepared I-III-VI nanoparticles 1.4 Design and applications 1.4.1 QDs as in vivo probes . 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 Experimental techniques .24 2.1 Materials . 24 2.2 Phase transfer of hydrophobic nanoparticles . 24 2.2.1 Phase Transfer of the hydrophobic CuInS2-ZnS nanocubes 24 2.2.2 Phase transfer of hydrophobic Zn-doped AgInS2 nanocrystals . 25 2.2.3 Preparation of water-soluble AgInS2-ZnS nanoparticles 25 2.3 Characterization 26 2.3.1 Chemical analysis . 26 iii 2.3.2 Morphological study . 27 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 CuInS2–ZnS Nanocubes with High Tunable Photoluminescence 33 3.1 Introduction 33 3.2 Synthesis of CuInS2-ZnS nanocrystals 34 3.3 Results and discussion . 35 3.3.1 Characterization of the structure of CuInS2-ZnS nanocubes . 35 3.3.2 Optical property of CuInS2-ZnS cube 41 3.3.3 Biological application of CuInS2-ZnS nanocubes 44 3.4 Summary 47 Chapter Zn doped AgInS2 Nanocrystals and Their Fluorescence Properties .51 4.1 Introduction . 51 4. Experiment procedures 53 4.2.1 Synthesis of AgInS2 and Zn-doped AgInS2 nanocrystals 53 4.3 Results and discussion . 54 4.3.1 Characterization of the structure of Zn-doped AgInS2 nanoparticles . 54 4.3.2 Optical property of Zn-doped AgInS2 nanocrystals 61 4.3.3 Biological application Zn-doped AgInS2 nanocrystals 66 4. Conclusions . 67 4.5 References 69 Chapter AgInS2-ZnS Heterodimers with Tunable Photoluminescence .71 5.1 Introduction . 71 5.2 Synthesis of AgInS2-ZnS nanocrystals . 72 5.3.1 Characterization of the structure of AgInS2-ZnS heterodimer 73 5.3.2 Optical properties of AgInS2-ZnS heterodimer . 83 5.3.3 Cell labeling using AgInS2-ZnS heterodimer . 88 Chapter Cu-In-Zn-S Nanoporous Spheres for Highly Efficient Hydrogen Evolution 96 iv 6.1 Introduction . 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 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 Conclusions and Future Work 134 8.1 Conclusions 134 8.2 Future Work 137 8.3 References 139 v Summary 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 vi Zn-doped nanoparticles showed strong PL emissions in the visible band from 520 to 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 vii graphene-based materials in the field of solar energy conversion. viii Chapter CuInZnS-Decorated Graphene Nanocomposites for Highly Efficient Hydrogen Production could also be proved by the TEM images (Figure 7.1E). From Table 7.1, it could be seen that the BET surface areas of the nanocomposites were gradually increased from 114.27 to 130.23 m2g-1 with increasing rGO amount, which were also higher than that of the pure CIZS nanospheres (81.84 m2g-1). This was because that the density of rGO was lower than CIZS nanospheres. The BET specific surface area of the nanocomposites would be effectively improved by increasing the amount of the light rGO component. The larger surface area of the nanocomposites could provide more surface areas for reactions and facilitate more effective charge carriers transportation, and thus enchancing their photocatalytic performance. 7.3.2 Hydrogen production using CuInZnS-Decorated Graphene nanocomposites as photocatalyst Figure 7.12 shows the normalized UV-vis absorption spectra of the samples pure CIZS, CG1, CG2 and CG5, together with that of the pure CIZS nanospheres. As compared to the pure CIZS nanospheres, the background in the visible light region was enhanced for the CIZS-rGO samples and the background became gradually stronger when the rGO amount was increased from to 5%. This was also shown by the changes in the color of the samples. As shown in the inset photograph, the pure CIZS nanospheres appeared yellow. Upon the adding of rGO, the nanocomposites became olive in color. The samples become darker with increasing the amount of rGO. At the same time, the band absorption of the nanocomposites was red-shifted with the increase of rGO which was probably due to the formation of COO-metal bond. Moreover, the strong interactions between 124 Chapter CuInZnS-Decorated Graphene Nanocomposites for Highly Efficient Hydrogen Production CIZS and rGO in the nanocomposites would change the absorption of the as-prepared composites. [28-30] 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. 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. The flatband potential was measured by impedance spectroscopy with Mott-Schottky plot. CuInZnS powder with weight of 15 mg was sonicated in dm-3 125 Chapter CuInZnS-Decorated Graphene Nanocomposites for Highly Efficient Hydrogen Production of ethanol to obtain a homogenous mixture. An appropriate amount of suspension was drop-casted on a conductive fluorine-tin oxide (FTO) glass with adhesive tapes which act as spacers attached on the sides of the substrate. The substrate was then heated at 80 oC for drying and the adhesive tape attached on the top side of the substrate was removed. Electrical contact was formed by applying silver paint on the top uncoated area of FTO and sticking copper tape on the silver paint. Three electrodes were used for the impedance measurements which include the working electrode (CuInZnS film), counter electrode (Pt plate) and reference electrode (Ag/AgCl, saturated KCl). 0.1 M of NaOH solution was used as the electrolyte. The measurements were carried out by Gamry electrochemical impedance spectroscopy and the potential was systemically varied between +0.2 to -1.4 V with frequency of 10, 50 and 100 Hz. Mott-Schottky graph was plotted by measuring the apparent capacitance as a function of potential under depletion condition at the semiconductor-electrolyte junction based on the following equation 1: ( ) (1) where Csc = capacitance of the space charge region, e = electron charge (1.602×10−19 C), ε = dielectric constant of the semiconductor, εo = permittivity of free space (8.85×10−14 Fcm-1), N = donor density (electron donor concentration for n-type semiconductor or hole acceptor concentration for p-type semiconductor), E = applied potential, Efb = flatband potential, k = Boltzmann constant (1.38×10−23 JK-1) and T is the absolute temperature. By extrapolating the (1/C2) vs. E graph to the potential axis, the flatband 126 Chapter CuInZnS-Decorated Graphene Nanocomposites for Highly Efficient Hydrogen Production potential can be determined. From Figure 7.14, the flatband potential of CuInZnS is -1.2 V vs. Ag/AgCl. To convert the potential to be against the normal hydrogen electrode (NHE), 0.197 V has to be added according to the following equation: E(NHE) = E(Ag/AgCl) + 0.197 V Hence, the flatband potential of CuInZnS is -1.003 V vs. NHE at pH 12.57 and to convert it to be at pH 7, the following equation is used: EF (pH) = EF + 0.059 (pH0 - pH) Thus, the flatband potential of CuInZnS is -0.67 V vs NHE at pH 7. 10 hz 50 hz 100 hz Mott-Schottky plot of CuInZnS 2.2 2.0 1.8 1.4 1.2 10 1/C x 10 (cm F ) 1.6 1.0 0.8 0.6 0.4 0.2 0.0 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 E (V) Vs Ag/AgCl 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. In this work, photocatalytic H2-production activities of the obtained CIZS-rGO composites were evaluated under visible-light irradiation using Na2SO3 and Na2S as sacrificial reagent and Pt as a co-catalyst. H2-production activities of the pure CIZS nanospheres and rGO were also measured. As shown in Figure 7.13, no hydrogen was detected when pure rGO was used as the photocatalyst with Pt as catalyst. In comparison, a low photocatalytic H2-production rate (2.1 mmol/h·g) was observed for 127 Chapter CuInZnS-Decorated Graphene Nanocomposites for Highly Efficient Hydrogen Production the pure CIZS nanospheres, which was expected due to the rapid recombination of conduction band (CB) electrons and valence band (VB) holes. The rGO exhibited a significant influence on the photocatalytic activity (Figure 7.13). Even with a small amount of graphene (0.5 wt.%), the H2 production rate was noticeably increased. For CIZS nanospheres, In the presence of a small amount of graphene (0.5%), the activity of sample Cu0.02In0.3ZnS1.47-rGO composites was slightly enhanced to 2.3 mmol/h·g, perhaps because the amount of graphene nanosheets was not large enough to efficiently disperse the Cu0.02In0.3ZnS1.47-rGO composites. The H2-production rate reached 3.3 mmol/h·g. when the ratio of graphene was increased to 1%. When the ratio was increased to 2%, the H2-production rate reached the highest value of 3.88 mmol/h·g. The H2-production of Cu0.02In0.3ZnS1.47-rGO composites with ratio 2% was 1.84 times to the pure Cu0.02In0.3ZnS1.47 nanospheres, and the rate was significantly greater than that of most semiconductor photocatalysts. This is attributed to two factors: 1st as compared to the pure Cu0.02In0.3ZnS1.47 nanospheres, the larger specific surface area of Cu0.02In0.3ZnS1.47-rGO composites (115.92 m2g-1), which could provide more active adsorption sites and photocatalytic reaction centres, therefore, enhancing the photocatalytic activity. 2nd, in the Cu0.02In0.3ZnS1.47-rGO composites system, graphene serves as an acceptor of the electron-hole pairs, leaving more charge carriers to form reactive species. However, a further increment of graphene ratio such as CG5 and CG20, the H2-production rate decreased to 3.6 mmol/h·g respectively. A similar result was also reported in the CdS-graphene system. [4] The possible reason for this was because of the introduction of excess black graphene which could shield the 128 Chapter CuInZnS-Decorated Graphene Nanocomposites for Highly Efficient Hydrogen Production active sites on the catalyst surface and decrease the intensity of light through the depth of reaction solution, which was termed the “shield effect”. [31] A tentative mechanism proposed for the high H2 production activity of the Cu0.02In0.3ZnS1.47-rGO composites sample is illustrated in Figure 7.15. The flatband potential was measured by impedance spectroscopy with Mott-Schottky plot, and the flatband potential of Cu0.02In0.3ZnS1.47-rGO composites was -0.67 V vs NHE at pH (Figure 7.14). Under visible light illumination, the VB electrons of Cu0.02In0.3ZnS1.47 nanospheres were excited to the CB, creating holes in the VB. The photogenerated electrons in the CB of Cu0.02In0.3ZnS1.47 nanospheres could be transferred to the Pt which deposited on the surface of Cu0.02In0.3ZnS1.47 nanosphere, and react with adsorbed H+ ions at the surface of Pt. In addition, some photogenerated electrons could also be transferred to the carbon atoms and Pt nanoparticles that were located on the graphene nanosheets to reduce H+ to H2. Therefore, Cu0.02In0.3ZnS1.47-rGO composites exhibited a higher H2 production activity than pure Cu0.02In0.3ZnS1.47 nanospheres, which was due to the introduction of graphene resulting in improvement on the separation of the photogenerated electron-hole pair, effectively prolonging the lifetime of charge carriers, increasing the reaction area and enhancing the H2 production. It could improve the separation of the photogenerated electron-hole pair, effectively prolonging the lifetime of charge carriers, enlarging the reaction area and enhance the H2 production. 129 Chapter CuInZnS-Decorated Graphene Nanocomposites for Highly Efficient Hydrogen Production 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. 7.4 Conclusions In summary, we successfully synthesized Cu 0.02In0.3ZnS1.47-rGO nanocomposite by a solvothermal method with high efficiency for the photocatalytic production of H2 from water splitting under visible-light. It was believed that the graphene nanosheets in the composite could efficiently enhance the photocatalytic activity. The best ratio between Cu0.02In0.3ZnS1.47 and rGO was found to be 2%, which could achieve a high visible-light photocatalytic H production rate of 3.8 mmol/h·g with the 0.5 wt% Pt as a co-catalyst. This result showed that the graphene nanosheet does not only act as a supporting layer for Cu0.02In0.3ZnS1.47 nanosphere but could also supress the charge recombination, improve interfacial charge transfer and provide a large number of active adsoprtion sites and photocatalytic reaction centers. This work demonstrated the potential of 130 Chapter CuInZnS-Decorated Graphene Nanocomposites for Highly Efficient Hydrogen Production graphene as a support for Cu 0.02In0.3ZnS1.47 nanospheres in photocatalytic hydrogen production under visible light, and it also provides a more general potential application application of graphene-based materials in the field of solar energy conversion. 131 Chapter CuInZnS-Decorated Graphene Nanocomposites for Highly Efficient Hydrogen Production 7.4 References [1] R. D. Cortright; R. R. Davda; J. A. Dumesic, Nature 2002, 418, 964 [2] A. Paracchino; V. Laporte; K. Sivula; M. Grätzel; E. Thimsen, Nat. Mater. 2011, 10, 456 [3] Y. D. Hou; B. L. Abrams; P. C. K. Vesborg; J. Rossmeisl; S. Dahl; J. Nørskov; I. Chorkendorff, Nat. Mater. 2011, 10, 434 [4] Q. Li; B. D. Guo; J. G. Yu; J. R. Ran; B. H. Zhang; H. J. Yan; J. R. Gong, J. Am. Chem. Soc. 2011, 133, 10878. [5] Z. G. Zou; J. H. Ye; K. Sayama; H. Arakawa; Nature 2001, 414, 625 [6] A. Hochbaum; P. D. 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Chem. Chem. Phys. 2011, 13, 3491 133 Chapter Conclusions and Future Work Chapter Conclusions and Future Work 8.1 Conclusions In this thesis, the research work focused on synthesizing I-III-VI nanoparticles and their applications. Firstly, we prepared Zinc doped AgInS and CuInS2 nanoparticles with different shapes including sphere and heterdimer structures. Corresponding PL properties of Zn-doped AgInS2 and CuInS2 nanoparticles were studied by lifetime measurment 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 nanoparticles and CuInZnS/graphene composites have been prepared through a template free hydrothemal method. The as-prepared Cu-In-Zn-S nanoparticles and composites with tuable absorption were used as a photocatalyst for hydrogen production under the illumination of visible light, both displayed high photocatalystic productivity. In Chapter of this thesis, CuInS2-ZnS alloyed nanocrystals were obtained by diffusing Zn ions into CuInS2 nanocrystal seeds. The as-obtained alloyed nanocrystals were highly crystalline and had unique cubic morphology with narrow size distribution. The Cu-In-Zn-S nanocubes showed tunable emissions in the visible band from 548 to 678 nm under UV excitation by adjusting the Zn content. Moreover, they also displayed high quality two-photon fluorescence. Based on the strong photoluminescene and two photon fluorsecence, the cell imagings excited by either 365 nm UV light or 800 nm infrared laser were successfully demonstrated. 134 Chapter Conclusions and Future Work Furthermore, 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. In Chapter 4, Zn-doped nanocrystals were synthesized through a facile solution method. The as-obtained Zn-doped AgInS2 nanocrystals were spherical and had narrow size distribution. The Zn content in the nanocrystals could be tuned by adjusting the reaction temperature, with tunable emissions in the visible band from 520 to 680 nm under UV excitation. As a result of doping Zn into AgInS2 nanocrystals, the non-radiative emission was suppressed, thus improving the quantum yield of the PL emission to as high as 41%. The corresponding mechanism for photoluminescene enhancment has been studied by transient absorption spectra. The nanocrystals with high quantum yield showed promising applications in cell imaging. Furthermore, the successful synthesis of the Zn-doped AgInS2 nanocrystals could provide the premise for future investigation of nanocrystal alloy systems arising from the I-III-VI2 group semiconductors. In Chapter 5, a facile hot injection method was developed to synthesize AgInS2– ZnS heterodimers. The diffusion of Zn into AgInS2 nanoparticles was well-controlled by adjusting the intermediate temperature from 90 oC to 180 oC. As a result, the PL emission colors of the obtained AgInS2–ZnS heterodimers could be finely tuned from green to red. In order to further study the mechanism of heterodimers structure, nm sized nanoparticles were obtained and the emission color could be further tuned to blue when the intermediate temperature was tuned down to 60 oC, which proved that 135 Chapter Conclusions and Future Work one part of the heterodimers was the Ag-In-Zn-S alloyed structure. Furthermore, the heterodimers exhibited well-defined TPF properties. Finally, the cell imaging using the obtained heterodimers excited by either UV or infrared light was successfully demonstrated. In Chapter 6, we successfully synthesized Cu-In-Zn-S nanospheres with high efficiency of the photocatalytic H production from water splitting under visible light 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. In addition, the costs of preparing the Cu-In-Zn-S alloy nanosphere structure could be lowered and the meothod for its preparation could be easily controlled. Moreover, the best photocatalytic activity of Cu-In-Zn-S alloy nanosphere structure can be as high as 2.35 mmol/h·g in the presence of mg catalyst. The high activity of the hydrogen production of the mesoporous structure could be attributed to the high surface area and the mesopores structure. This work demonstrates the potential of industrial hydrogen production with a low-cost method in the field of solar energy conversion. In Chapter 7, we successfully synthesized Cu-In-Zn-S/graphene nanocomposites by a solvothermal method with high efficiency of the photocatalytic H2 production from water splitting under visible light. It was believed that the graphene nanosheets in the composite could enhance the 136 Chapter Conclusions and Future Work photocatalytic H2 production activity. The optimum ratio between Cu-In-Zn-S microstructures and graphene was found to be 1:5, which could achieve a high visible light photocatalytic H production rate of 3.8 mmol/h·g with 0.5 wt% Pt as a co-catalyst. This result demonstrated that the graphene nanosheet does not only can act as supporting layer for Cu-In-Zn-S microstructure but also could supress the charge recombination, thus improving interfacial charge transfer and providing a large number of active adsoprtion sites as well as photocatalytic reaction centers. This work demostrated the potential of using graphene as a support for Cu-In-Zn-S microstructure in photocatalytic hydrogen production. Finally, it provides a potential application of graphene-based materials in the field of solar enery conversion. 8.2 Future Work This thesis presents several studies on Ag-In-Zn-S and Cu-In-Zn-S nanoparticles, microstructures for biological and energy conversion applications. However, more systematic studies on the alloyed structure and graphene based composites are still needed. In addition, how to improve the PL quantum yield is an important topic for other applications such as single nanoparticles imaging. Based on the current results and understanding, the following points are typically raised for future study: 1, Synthesis of Ag-In-Zn-S nanorods with high PL quantum yield and transfer to water-soluble by graphene oxide, which could be used for high resolution cell labeling. The AgInZnS nanoparticles could be successful attached on the 137 Chapter Conclusions and Future Work water-soluble graphene oxide and formed as nanocomposite; which would lead the AIZS-GO nanocomposite water-solube and still keep their good photoluminescene [1]. 2, Synergistic effect of molybdenum disulfide (MoS2) and graphene as co-catalysts for enhanced photocatalytic hydrogen production activity of CIZS nanoparticles. Recently, molybdenum disulfide with a layered structure has been extensively investigated as a promising electrocatalyst for H production. Its potential as a cocatalyst for photocatalytic H production has received much attention. MoS2-TiO2 system has been reported to show high photocatalytic performance for pollutant degradation [2],[3]. Therefore, it is promising for MoS 2-CuInZnS nanocomposites as the photocatalytic hydrogen production. 138 Chapter Conclusions and Future Work 8.3 References [1] Y. Sheng, X. Tang, E. Peng, J. Xue; J. Mater. Chem. B, 2013, 1, 512 [2] Y. Araki, K. Honna, H. Shimada, J. Catal. 2002, 207, 361 [3] Q. Xiang, J. Yu, M. Jaroniec, J. Am. Chem. Soc. 2012, 134, 6575 139 [...]... 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. .. 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... Centimeter μm Micrometer mm Millimeter nm Nanometer Å Angstrom ml Milliliter M Mol per liter, mol/L min Minute s Second ℃ Centigrade, which is the temperature unit eV Electron volt rpm Revolution per minute xviii List of Publications 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*... CdSe@CdS@ZnS nanoparticles [62] Their outermost ZnS 4 Chapter 1 Introduction 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... 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... Chapter 1 Introduction 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... 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... 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... 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... 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+) . 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. 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. 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