Synthesis and optimization of boron nitride nanotubes for stable aqueous dispersions
Köken, Deniz (2016) Synthesis and optimization of boron nitride nanotubes for stable aqueous dispersions. [Thesis]
As structural analogues of carbon nanotubes (CNT), boron nitride nanotubes (BNNT) possesses extraordinary mechanical, electrical, thermal and optical properties. These unique properties, makes them promising materials applications in composite, hydrogen storage, radiation shielding, biomaterials. However, difficulties in high yield BNNT synthesis and obtaining stable dispersions of BNNTs in aqueous media remain as main challenges. This thesis research focuses on the BNNT synthesis and optimization thereof in addition to preparation of stable aqueous dispersions of BNNTs. High yield synthesis of BNNTs on Si wafers and BNNFs as well as floating BNNT form by modified growth vapor trapping-BOCVD method in conventional tube furnace at 1200 °C were accomplished in this research. Synthesis of BNNTs were optimized in terms of temperature, catalyst ratio, catalyst amount, ammonia flow, reaction time and system parameters which allowed high yield synthesis of good quality BNNTs with vacuum free, low cost, novel growth vapor trapping-BOCVD route. As-synthesized BNNTs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, Fourier Transform Infra-Red spectroscopy (FTIR) and electron energy loss spectroscopy (EELS). Following the successful synthesis of BNNTs, BNNT synthesis from boron minerals were investigated. Boron minerals Ulexite and Etidot-67, gifted from ETİ Maden İşletmeleri, were used as boron precursors for the synthesis of floating BNNTs in this research. Assynthesized BNNTs were characterized by SEM and RAMAN spectroscopy. Two modification approaches were utilized for the surface modifications of BNNTs: Covalent functionalization and non-covalent functionalization. In covalent functionalization approach, BNNT’s surfaces were hydroxylated by nitric acid treatment and ozone treatment, which reduces the van der Waals forces between nanotubes. Moreover, hydroxyl groups on the surface of BNNTs can be used as starting spots for the further functionalization. In-non covalent functionalization approach, BNNTs were wrapped with ionic surfactants and polymers (b-PEI (branched polyethyleneimine) and poly(allylamine hydrochloride) (PAH) via π-π interaction to prevent agglomeration of BNNTs and dispersed in aqueous media. Dispersions of as-functionalized BNNTs characterized by Fourier Transform Infra-Red spectroscopy (FTIR) and dynamic light scattering spectroscopy (DLS). BNNT thin film production by dip LbL method was investigated. LbL method allows high control over the architecture of thin film in addition to film thickness. BNNT dispersions were used in combination with b-PEI and PSS Poly(styrenesulfonate) for the production of BNNT thin films on glass substrates. Different pH values of as-prepared dispersions were tested in order to achieve 5 bl (bilayer) and 10 bl thick BNNT thin films by dip LbL method. Synthesis parameters of BNNTs were successfully optimized and synthesis of BNNTs were achieved. As-synthesized BNNT’s surfaces were modified with covalent and noncovalent functionalization methods and stable aqueous dispersions of BNNTs were achieved. We found that, non-covalent functionalization of BNNTs allows BNNTs to be dispersed in aqueous media with good dispersion stability.
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