Dissertation
Monodisperse ZnO Micro and NanoparticlesObtained by Micro Segmented Flow SynthesisDissertationSubmitted toFaculty of Mathematics and Natural ScienceTechnical University of IlmenauTo obtain the academic degreedoctor rerum naturalium(Dr. rer. nat.)byShuning LiSept. 2012urn:nbn:de:gbv:ilm1-2012000267
Doctoral CommitteeChairman:Prof. Dr. rer. nat. habil. Michael Köhler(Department of Physical Chemistry and Microreaction Technology, TUIlmenau)Referees:Prof. Dr. rer. nat. habil. Michael Köhler(Department of Physical Chemistry and Microreaction Technology, TUIlmenau)Prof. Dr. rer. nat. habil. Uwe Ritter(Institute for Chemistry and Biotechnology, TU Ilmenau)Dr. Challa Kumar(Center for Advanced Microstructures and Devices, Louisiana StateUniversity)Members:Prof. Dr. rer. nat. habil. Andreas Bund(Department of Electrochemistry and Galvano Technique, TU Ilmenau)Dr. rer. nat. Alexander Groß(Department of Physical Chemistry and Microreaction Technology, TUIlmenau)Date for the viva voce: 03. 09. 2012Date for the defense: 03. 09. 2012
To my familyfor their endless love and support
AbstractAbstractZnO micro and nanoparticles have attracted considerable interest because of theirremarkable performance in electronics, optics and photonics. As a wide band gapsemiconductor material, ZnO is also a potential candidate for various applicationsincluding gas sensing, light emitting devices and solar cells. Although sometechnologies have been developed to produce well-defined ZnO particles of differentshapes and sizes, ZnO particles prepared by micro segmented flow synthesis have beenrarely reported. The aim of this work was to develop a microfluidic system based on themicro segmented flow method and to test whether the microfluidic components aresuitable for the generation and investigation of ZnO particles with improvedhomogeneity.In order to optimize the experimental conditions, ZnO particles were first synthesized inbatch. The optimized batch conditions were then adapted to two microfluidicarrangements for continuous synthesis of ZnO particles below 100 C. The set-upsincluded computer-controlled syringe pumps, T-injectors, PTFE tubings and PTFE knotmixers in a thermostat water bath. The ZnO particles were obtained under strongalkaline conditions at elevated temperature in aqueous solution and DMSO solution.Needle-like, flower-like and compact ZnO particles were obtained. In nearly all cases, astrong effect of the flow conditions on the homogeneity of the formed particles wasobserved. The higher quality of the particles can be attributed to the fast mixing andenhanced heat transfer caused by segment-internal convection.In addition, two other microfluidic set-ups were developed to control the ZnO formationreaction at temperature up to 150 C. A static micromixer was used for mixing thereactants at room temperature. The formation of segmented flow was realized byinjection of the reaction mixture into a carrier stream. The particle growth took place inPTFE tube coils inside a thermostat, which allowed to heat up to 150 C. By using thisset-up, flower-like, star-like, and spherical ZnO particles were successfully synthesized.The shape and size of the formed particles were strongly dependent on the reactantconcentration and the molar ratio of NaOH/Zn(Ac)2. The total residence time forpreparation of these particles was only 9.3 s, which is very short compared to the mostVII
Abstractconventional methods.The effect of the solvent on the formation of ZnO particle has also been investigatedusing this microfluidic set-up. Two different experimental conditions were applied toprepare ZnO particles, where Zn(Ac)2 and NaOH in ethylene glycol (EG) were mixedwith water or water/EG mixing solvent to achieve different water contents in the finalmixture solution. The formation of homogeneous particles was characterized by SEMand TEM. A stronger dependence of the particle size and shape on the water content wasobserved. Furthermore, the water content can be used for tuning the optical absorptionspectra of the formed ZnO particles.Besides the ZnO microparticles, ZnO nanoparticles with an average diameter around 4-5nm have been synthesized using Zn(Ac)2 and LiOH in ethanol. The preparednanoparticles exhibited green and blue emission under excitation at 325 nm. In order tounderstand the size-dependent optical properties of ZnO nanoparticles, extended X-rayabsorption fine structure (EXAFS) spectroscopy was applied to study their localstructure properties and compared with that of ZnO flower-like microparticles. TheEXAFS measurements revealed higher vacancies and a higher degree of structuraldisorders in the nanoparticles than the microparticles. These disorders and vacanciescould contribute to the blue shift of the visible emission from ZnO nanopartilces.Due to the potential applications of semiconductor-metal composite particles in diverseareas, the flower-like ZnO microparticles obtained by micro segmented flow synthesiswere used to fabricate ZnO/4-MBA/Au composite particles using a simple strategy. Theformed composite particles were very homogeneous in shape and size. The surfacecoverage of Au nanoparticles on ZnO/4-MBA particles can be adjusted by changing themolar ratio of ZnO/4-MBA to Au. In order to study the interaction of 4-MBA moleculeswith ZnO and Au particles, Raman spectra of ZnO/4-MBA and ZnO/4-MBA/Auparticles were analysed.In summary, the segmented flow technique is suitable to generate ZnO particles withcontrolled size and morphology. Compared to most conventional methods, thistechnique offers several advantages, and it provides a new insight into material synthesisunder environmentally friendly conditions.VIII
AbstractAbstractMikro- und Nanopartikel aus Zinkoxyd (ZnO) besitzen bemerkenswerte Eigenschaftenfür Applikationen im Bereich der Elektronik, Optik und Photonic. Als einHalbleitermaterial mit großer Bandlücke ist ZnO ebenfalls für die Entwicklung vonSensoren, Light Emitting Diodes (LEDs) und Solarzellen von hohem Interesse. DieHerstellung definierter Materialien mit einheitlicher Morphologie und enger PartikelGrößenverteilung ist hierzu eine wichtige Voraussetzung. Verschiedene Verfahren zurHerstellung entsprechender Partikel sind in der Vergangenheit untersucht worden. Dietropfenbasierte Mikrofluidik bietet die Möglichkeit einer exzellenten Reaktionskontrolledurch die Verwendung eines Tropfens als Reaktionsgefäß. Kurze Mischzeiten, ermöglichensonebenstöchiometrischen Parametern eine exakte Reaktionsführung. Ziel der hier vorliegendenDissertationsschrift ist die Untersuchung der ZnO-Präzipitation in entsprechendenmikrofluidischen Systemen sowie die Charakterisierung der hergestellten Materialien.IX
AbstractX
List of PublicationsList of PublicationsJournal Papers1. Y. Li, D. G. Yamane, S. Li, R. Reddy, J. S. Goettert, K. Nandakumar, C. S. S. R.Kumar, “Geometric Optimization of Liquid-Liquid Slug Flow in Flow-FocusingMillifluidic Devices”, 2012, submitted2. S. Li, A. Knauer, K. Risch, U. Ritter, J. M. Köhler, “Synthesis and Characterization ofZnO/4-Mercaptobenzoic Acid/Au Composite Particles”, Materials Letters, 2012,revised3. S. Li, A. Roy, H. Lichtenberg, G. Merchan, C. S. S. R. Kumar, J. M. Köhler, “LocalStructure of ZnO Micro Flowers and Nanoparticles Obtained by Micro SegmentedFlow Synthesis”, ChemPhysChem, 2012, 13, 1557-15614. S. Li, G. A. Groß, P. M. Günther, J. M. Köhler, “Hydrothermal Micro ContinuousFlow Synthesis of Spherical, Cylinder-, Star- and Flower-like ZnO Microparticles”,Chemical Engineering Journal, 2011, 167, 681-6875. Knauer, A. Thete, S. Li, H. Romanus, A. Csáki, W. Fritzsche, J. M. Köhler,“Au/Ag/Au Double Shell Nanoparticles with Narrow Size Distribution Obtained byContinuous Micro Segmented Flow Synthesis”, Chemical Engineering Journal, 2011,166, 1164-11696. S. Li, S. Meierott, J. M. Köhler, “Effect of Water Content on Growth and OpticalProperties of ZnO nanoparticles Generated in Binary Solvent Mixtures by MicroContinuous Flow Synthesis”, Chemical Engineering Journal, 2010, 165, 958-9657. Z. Chang, C. A. Serra, M. Bouquey, I. Kraus, S. Li, J. M. Köhler, “MultiscaleMaterials from Microcontinuous-Flow Synthesis: ZnO and Au Nanoparticle-FilledUniform and Homogeneous Polymer Microbeads”, Nanotechnology, 2010, 21,0156058. S. Li, P. M. Günther, J. M. Köhler, “Micro Segmented-Flow Technique forContinuous Synthesis of Different Kinds of ZnO Nanoparticles in Aqueous and inXI
List of PublicationsDMSO Solution”, Journal of Chemical Engineering of Japan, 2009, 42, 338-345Conference Proceedings1. Z. Chang, S. Li, M. Bouquey, I. Kraus, C. A. Serra, J. M. Köhler, lsBasedonPolymerMicroparticles/Inorganic Nanoparticles Composites”, Proceedings of the SecondWSEAS International Conference on Nanotechnology, 20102. S. Li, P. M. Günther, J. M. Köhler, “Micro Segmented-Flow Technique forContinuous Synthesis of Fluorescent ZnO Micro- and Nanoparticles”, Micro SystemTechnique Congress, 2009XII
Table of ContentsTable of Contents1Introduction -------------------------------11.11.21.32ZnO crystals ----------------------------11.1.1Crystal structure --------------31.1.2Physical properties and device applications --------------------------------41.1.3Synthesis methods ------------9Micro segmented flow -------------- 131.2.1Segment formation and manipulation ------------------------------------- 141.2.2Unique features of micro segmented flow -------------------------------- 151.2.3Applications of micro segmented flow ------------------------------------ 18Aim and objective ------------------- 21Materials and methods ---------------- 222.1Microfluidic systems for synthesis of ZnO particles ----------------------------- 222.2Particle synthesis -------------------- 282.2.1ZnO micro and nanoparticles ----------------------------------------------- 282.2.2Au nanoparticles ------------ 282.2.3ZnO/Au composite particles ------------------------------------------------ 292.2.4ZnO/4-MBA/Au composite particles -------------------------------------- ------------------------------------------- 302.4Characterization --------------------- 322.4.1pH meter --------------------- 322.4.2UV/Vis spectroscopy ------ 322.4.3Photoluminescence (PL) spectroscopy ------------------------------------ 332.4.4Scanning electron microscopy (SEM) ------------------------------------- 332.4.5Transmission electron microscopy (TEM) ------------------------------- 34XIII
Table of Contents32.4.6Differential centrifugal sedimentation (DCS) ---------------------------- 352.4.7Small angle X-ray scattering (SAXS) ------------------------------------- 362.4.8X-ray diffraction -------- 372.4.9X-ray absorption spectroscopy (XAS) ------------------------------------ 382.4.10Thermogravimetric analysis (TGA) --------------------------------------- 402.4.11Raman spectrosopy -------- 41Results and discussion ----------------- 423.1Continuous synthesis of ZnO particles by micro segmented flowtechnique in aqueous and dimethylsulfoxide (DMSO) solutions --------------- 423.1.1Batch synthesis of ZnO particles ------------------------------------------- 423.1.2Micro segmented flow synthesis of ZnO particles in -------------------------------------- 433.1.3Micro segmented flow synthesis of ZnO particles in ----------------------------------- 463.2Hydrothermal micro segmented flow synthesis of ZnO microparticleswith different morphologies ------- 503.3Effect of the water content on growth and optical properties of ZnOparticles generated by micro segmented flow synthesis ------------------------- 603.4Local structure studies of ZnO micro flowers and nanoparticlesobtained by micro segmented flow synthesis ------------------------------------- 713.5Synthesis and characterization of ZnO composite particles --------------------- 793.5.1Solution-based method ---- 793.5.2Assembly method ---------- 824Conclusion and outlook --------------- 905References -------------------------------- 926Appendix --------------------------------109XIV
Table of Contents6.1Abbreviations -----------------------1096.2Synthesis of water core/polymer shell particles by co-axial capillariesmicrofluidic device -----------------1106.36.46.2.1Experimental methods ----1116.2.2Results and discussion ----112Generation of monodisperse slugs by a flow-focusing millifluidic chip -----1216.3.1Experimental methods ----1216.3.2Results and discussion ----122Preparation of Co-doped ZnO nanoparticles by micro segmented flowsynthesis -----------------------------1256.5Batch synthesis of Ag and Au nanoparticles fic publications -------------1296.7Curriculum vitae --------------------1326.8Acknowledgements ----------------134XV
1 Introduction11.1IntroductionZnO crystalsIn recent years, significant interest has emerged in wide band gap semiconductor materials due toan increasing need for short-wavelength photonic devices and high-power, high-frequencyelectronic devices [1]. Zinc oxide, as a typical II-VI semiconductor material, has received muchattention lately because of its direct wide band gap ( 3.3 ev at 300 K) and high exciton bindingenergy (60 meV). It allows high efficient excitonic emission at room temperature and makesZnO a promising material for UV optoelectronic devices such as UV light emitting diodes andphotodetectors. The lack of a center of symmetry in wurtzite, combined with largeelectromechanical coupling, results in strong piezoelectric and pyroelectric properties and theconsequent use of ZnO in mechanical actuators and piezoelectric sensors [2]. Moreover, underhigh pressure, the melting point of ZnO is 1975 C, which determines its high thermal stability.Transparency to visible light provides opportunities to develop transparent electronics and UVoptoelectronics. Due to its attractive physical properties, ZnO has turned into a new hot focus inthe field of optics [3], optoelectronics [4], and sensors [5-6] during the past decade.ZnO can be considered as an “old” material and has been studied for several decades. In terms ofcharacterization, the lattice parameter studies date back to 1929 by Fuller [7] and 1935 by Bunn[8], detailed optical properties were investigated by Mollwo in 1954 [9] and vibrationalproperties were studied by Raman scattering by Damen et al. in 1966 [10]. The research peakedat the end of 1970s and beginning of 1980s, driven by the availability of good bulk singlecrystals and first epitaxial layers [11]. Then the interest faded away. The main obstacle to thedevelopment of ZnO has been the lack of reproducible and low-resistivity p-type ZnO [12-13].The present renaissance in ZnO research started in the middle 1990s. It has been documented bynumerous conferences, workshops, symposia and more than 1000 ZnO-related papers per yearcompared to slightly beyond 100 in 1970 [14-15]. More reviews and books on ZnO have beenpublished recently [2, 11, 14, 16-19].It is necessary to know that ZnO occupies already an enviable place in the industrial market.ZnO is industrially produced at levels of 105 tons each year. It is widely used in the rubber1
1 Introductionmanufacturing ( 36 %), in the industry of ceramics as a flux ( 26 %), in the chemical industry( 20 %), in the animal food as trace elements ( 12 %) and in paints ( 3 %; 50 % in 1961). Thelast 3 % are used for different applications, for example in electronics (ferrites, varistors), ends ofmatches and pharmaceutical industry [20].Fig. 1.1 Different morphologies of ZnO nanostructures [21].Nanostructured ZnO materials have also received considerable interest because of theirremarkable performance in electronics, optics and photonics. Different nanostructuremorphologies of ZnO have been reported including nanowires [22], nanorods [23], nanocombs[24], nanoflowers [25], nanorings [26], nanohelices [27-28] and nanocages [29] (Fig. 1.1). Thesenanostructures can be obtained by different fabrication methods, such as vapor-phase transport[30], chemical vapor deposition [31-33] and hydrothermal synthesis [17]. With reduction in size,ZnO nanostructures possess several unique advantages such as high specific surface area,chemical stability, electrochemical activity and high electron communication features, whichmainly result from the quantum confinement effect [2]. Such properties indicate a wide range ofnovel applications in photodetectors, sensors, light emitting diodes and varistors. They are also2
1 Introductionattractive for biomedical applications due to their bio-safety and large surface area [34].This section gives an in-depth description of the crystal structure, physical properties,applications and synthesis techniques of ZnO.1.1.1 Crystal structureZinc oxide crystallizes in three forms: hexagonal wurtzite, cubic zincblende, and the rarelyobserved cubic (rocksalt) structure. At ambient temperature and pressure, the wurtzite structureis most stable and thus most common. The zincblende form can be stabilized by growing ZnO onsubstrates with cubic lattice structure. In both cases, the zinc and oxide centers are tetrahedral.The rocksalt (NaCl) structure is only observed at relatively high pressures (about 10 GPa) [35].The cubic zincblende and rocksalt structures are illustrated in Fig. 1.2 [19].Fig. 1.2 The rockslat (left) and zincblende (right) phases of ZnO. Oxygen atoms are shown as white spheres, zincatoms as black spheres. Only one unit cell is illustrated for clarity [19].Wurtzite ZnO has a hexagonal structure with lattice parameters a 0.32495 nm and c 0.52069nm; their ratio c/a 1.60 is close to the ideal value for hexagonal cell c/a 1.633. As in mostgroup II-VI materials, the bonding in ZnO is largely ionic. The structure of wurtzite ZnO can bedescribed as a number of alternating planes composed of tetrahedrally coordinated O 2- and Zn2 stacked alternately along the c-axis, as shown in Fig. 1.3 [2]. The tetrahedral coordination inZnO results in polar symmetry along the hexagonal axis. This polarity is responsible for some of3
1 Introductionthe properties of ZnO, including its piezoelectricity and spontaneous polarization. It is also a keyfactor in crystal growth, etching
WSEAS International Conference on Nanotechnology, 2010 2. S. Li, P. M. Günther, J. M. Köhler, “Micro Segmented-Flow Technique for Continuous Synthesis of Fluorescent ZnO Micro- and Nanoparticles”, Micro System Technique Congress, 2009
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FINAL DISSERTATION: BA OCCUPATIONAL HEALTH AND SAFETY MANAGEMENT MODULE CODE M3N211744-13-C How can working in an offshore environment in the UK affect the health and safety, wellbeing and family life of an offshore worker? 15 th August 2014 “This dissertation is my own original work and has not been submitted elsewhere in fulfilment of the requirements of this or any other award” 43 pages .
