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Controlled Synthesis of Carbon Nanocoils and Their Application in SERS

Author: LiDaWei
Tutor: PanLuJun
School: Dalian University of Technology
Course: Optical Engineering
Keywords: Carbon Nanocoils Morphological Control Growth Mechanism SilverNanoparticles Surface Enhanced Raman Scattering
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Type: PhD thesis
Year: 2013
Downloads: 20
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Abstract


With the development of nanotechnology research, different kinds of new low-dimension nanostructure materials emerge one after another and exhibit novel physical and chemical properties, which greatly promote the development of nanoscience. In nanoscience research field, carbon nanomaterials (carbon nanotubes. graphene. carbon nanocoils. etc.) have recently caused people’s extentive attention. Compared with other low-dimension carbon nanostructures, carbon nanocoils (CNCs) exhibit different novel physical and chemical properties from the others due to their peculiar helical morphologies. These properties, as well as their helical shapes, make CNCs suitable for wide applications. So far, many research groups have efficiently synthesized CNCs with large coil diameter (greater than100nm). However, there still has no breakthrough in the controlled preparation techniques for growing CNCs with different coil diameters, especially for CNCs with small coil diameter (less than100nm). In this thesis, we focus our research on controlled preparation and growth mechanism of CNCs with different coil diameter, tried to improve their controllability and production rate, to explore their formation mechanism, which would lay a solid foundation for the targeted growth, morphological control, characteristic study and practical application. In addition, we have explored the application of CNCs as a novel three-dimension template in surface enhanced Raman scattering (SERS). Around the above content, this paper has carried out mainly the following aspects of work:(1) Controlled preparation of CNCs with different coil diameter using different methods:Firstly, the mixture of carbon micro and nanocoils (CMCs/CNCs) was synthesized by catalytic pyrolysis of acetylene at650℃using NiSO4as the catalyst precursor. Experimental results indicate that generally the CMCs are in double-helical form and have a coil diameter ranging from4to10um; while most of the grown CNCs are in twisted single-helical form and have a coil diameter of300to400nm. respectively. Raman spectra and XRD analysis indicate that the real active catalyst precursor for growing CMCs/CNCs is NiSO4, while NiO is not an effective catalyst precursor for synthesizing CMCs or CNCs.Secondly, CNCs with diameter from100to150nm have been synthesized by catalytic decomposition of acetylene at700℃using Fe-Sn-O catalyst film prepared by a spin-coating method. It is found that catalyst films with different morphologies are obtained by changing the spin-coating times, which lead to the formation of different multi-layer carbon nanostructures, including CNCs/carbon layer/vertically aligned carbon nanotubes sandwich-like structures and CNCs/carbon double-layer structures.Thirdly, CNCs with small coil diameters have been synthesized on SiO2substrates by thermal chemical vapor deposition using Fe catalyst films prepared by ion sputtering. It is found that multiwall CNCs (MWCNCs) with coil diameters less than100nm and filament diameters less than30nm are grown, which are much thinner than for conventional CNCs. The yield of MWCNCs is decreased rapidly with a lower Fe film thickness. It is also found that large particles with irregular shapes lead to the growth of helical MWCNCs, while large particles with regular circular shapes tend to grow straight CNTs. Based on the experimental results, a growth model for MWCNCs is proposed.(2) CNCs with controlled shape, coil diameter and coil pitch have been synthesized in a CVD system by changing the reaction temperature and acetylene flow rate. It is found that CNC-CNF hybrid structures are produced by increasing growth temperature from750to810℃during a single synthesis run, while CNF-CNC hybrid structures are produced by decreasing the temperature from810to750℃. Similarly, by changing growth temperature from750to810℃and then back to750℃during a single run, CNC-CNF-CNC complex hybrid structures can be obtained. During the CVD process, the pulsing of acetylene and the changing of acetylene flow rate periodically are also found to be effective in controlling the structure of CNCs, through which CNCs with periodic helical structures can be produced. It is found that the higher the flow rate of acetylene, the smaller the coil pitch and diameter of the grown CNCs.(3) Ag@C core-shell silver nanoparticles (NPs) with diameters in the range of40-80nm have been synthesized by hydrothermal route&low temperature heating-stirring method. Experimental of SERS results show that Ag@C NPs with a thick shell (more than3nm) have a very low SERS activity, while those with an ultra-thin film (less than1nm) have a high SERS activity, indicating that carbon shell thickness is a key factor affecting the SERS, which has also been evaluated by finite-difference time-domain simulation (FDTD). The carbon layer is proven to have three functions of protecting the AgNPs against oxidation, controlling the spacing of AgNPs, and reducing the surface electric property of AgNPs. Therefore, Ag@C NP aggregates are easily formed which may produce the higher hot spots than the bare AgNPs.(4) An uniform AgNP film with controlled nanoparticle size, shape and density have been prepared on TiO2film by photocatalysis reduction method. The influences of TiO2film thickness and morphology on the AgNP’s growth have been systemically studied. Experimental SERS results show that the AgNP film prepared with different TiO2films and ultraviolet light irradiation time exhibit different SERS signals. Atomic force microscope and scanning electron microscope analysis reveal that the size effect is a key factor attecting the SERS of our prepared AgNP films, which has also been evaluated by finite-difference time-domain simulation. It is found that the larger the average AgNP’s size (≤λ/4) and roughness, the higher the Raman enhancement. The SERS mapping images exhibit a good uniform Raman enhanced results in our prepared TiO2-catalyzed AgNP film with a low relative standard deviation of approximately10%.(5) A novel SERS substrate was produced by combining AgNPs and CNCs. Three different methods were developed for loading AgNPs on CNCs, which include (1) direct deposition of AgNPs on CNCs by radio-frequency magnetron sputtering (RFMS) to form Ag-CNC hybrid,(2) deposition of a TiO2film on CNCs by RFMS, following by photoinduced growth of AgNPs to form Ag-TiO2-CNC hybrid (called A-substrate), and (3) deposition of a TiO2film on CNCs by spin-coating and then photoinduced growth of AgNPs to form Ag-TiO2-CNC hybrid (called B-substrate). Experimental SERS results showed that B-substrates exhibited the highest SERS enhancement. The as-prepared Ag-TiO2-CNC substrates also showed much higher Raman signal enhancement than ordinary planar SERS substrates in our system. This was mainly due to the unique three-dimensional structure where the large surface area was available for loading more densely packed AgNPs which contributes abundant Raman hot spots.

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