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- Chaudhary, Shilpi
(author)
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Modification of Oxide Surfaces with Functional Organic Molecules, Nanoparticles, and Hetero-Oxide Layers
- 2015
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Doctoral thesis (other academic/artistic)abstract
- The research work described in this thesis is concerned with the modification of oxide surfaces, as reflected by its title. The surfaces and their modification have been studied using a range of experimental surface characterization tools, in particular x-ray photoelectron spectroscopy (XPS), fluorescence microscopy, scanning electron microscopy, atomic force microscopy, and scanning tunneling microscopy. A large part of the thesis is related to the modification of oxide or metal surfaces with nanoparticles. In particular, three different immobilization schemes for the coupling of molecularly imprinted polymer (MIP) nanoparticles to silicon oxide (SiO2) and gold surfaces were designed and characterized at every step. The first method reports the immobilization of MIPs using a photo-coupling agent in combination with an aminosilane compound. The second method explores an epoxysilane-based coupling agent to directly anchor the nucleophilic core-shell MIP nanoparticle to the surface. Both methods were proven to be non-destructive towards the specific binding sites of the MIP nanoparticles. The third scheme offers the immobilization of nucleophilic core-shell nanoparticles on model gold surfaces using self-assembled monolayers of 11-mercaptoundecanoic acid activated by carbodiimide/N-hydroxysuccinimide. All three coupling methods are quite versatile and can be used in biosensors to couple functional nano-objects with transducer surfaces. In addition to these investigation directly aimed at the immobilization of nanopartciles, more fundamentally oriented studies were carried out on the modification of the rutile TiO2(110) surface with silane molecules to obtain a detailed understanding of adsorption mechanism and geometry of these silanes. The deposition of a different type of nanopartciles, block copolymer reverse micelles loaded gold nanoparticles, on a titanium dioxide surface was tested using electrospray deposition. The study demonstrates that electrospray deposition is a viable method for depositing metal single-size metal nanoparticles onto a surface in vacuum, thereby retaining the clean vacuum conditions. Furthermore, it was shown that the removal of the block copolymer shell after deposition can be achieved both by atomic oxygen and an oxygen plasma, with the atomic oxygen being somewhat more efficient. Overall, it was demonstrated that a TiO2 surface decorated with narrow sized gold nanoparticles could be created, a result of importance in the catalysis domain. The last part of the thesis is concerned with the true in-situ investigation of growth of hetero-oxide layers on oxide surfaces from metal precursors. Tetraethyl orthosilicate (TEOS) was used as precursor for the chemical vapor deposition of silicon oxide on rutile TiO2(110). The growth was monitored in real time using ambient pressure XPS (APXPS), which revealed the dissociative adsorption with the formation of new species in the presence of a TEOS gas phase reservoir. Annealing results in the formation of SiO2 and of a mixed titanium/silicon oxide. Furthermore, tetrakis(dimethylamino)titanium was employed in the atomic layer deposition (ALD) of TiO2 on RuO2. The APXPS results showed evidence was for side reactions beyond the idealized scheme of ALD.
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| 2. |
- Chaudhary, Shilpi, et al.
(author)
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Real-Time Study of CVD Growth of Silicon Oxide on Rutile TiO2(110) Using Tetraethyl Orthosilicate
- 2015
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:33, s. 19149-19161
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Journal article (peer-reviewed)abstract
- The interaction of the ruffle TiO2(110) surface with tetraethyl orthosilicate (TEOS) in the pressure range from UHV to 1 mbar as well as the TEOS-based chemical vapor deposition of SiO2 on the TiO2(110) surface were monitored in real time using near-ambient pressure X-ray photoelectron spectroscopy. The experimental data and density functional theory calculations confirm the dissociative adsorption of TEOS on the surface already at room temperature. At elevated pressure, the ethoxy species formed in the adsorption process undergoes further surface reactions toward a carboxyl species not observed in the absence of a TEOS gas phase reservoir. Annealing of the adsorption layer leads to the formation of SiO2, and an intermediate oxygen species assigned to a mixed titanium/silicon oxide is identified. Atomic force microscopy confirms the morphological changes after silicon oxide formation.
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| 3. |
- Chaudhary, Shilpi, et al.
(author)
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X-ray photoelectron spectroscopy study of adsorption of (3-mercaptopropyl)trimethoxysilane and n-propyltriethoxysilane on a rutile TiO2(110) surface
- 2015
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In: Advanced Materials Letters. - 0976-3961. ; 6:4, s. 279-283
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Journal article (peer-reviewed)abstract
- We have studied the adsorption of two silane compounds, (3-mercaptopropyl) trimethoxysilane (MPTMS) and n-propyltriethoxysilane (PTES), on a rutile TiO2(110) surface using angle dependent X-ray photoelectron spectroscopy. The observation of the S 2p line, in the case of MPTMS, and the C 1s line for both MPTMS and PTES confirms the adsorption of the molecules. For a dose of 122 Langmuirs of MPTMS we find room temperature coverage of 0.55 monolayers, while for a 60 Langmuir dose of PTES the coverage is found to be 0.89 monolayers. Thus, MPTMS has a considerably lower sticking coefficient on the rutile TiO2(110) surface than PTES. Both PTES and MPTES are found to bind dissociatively to the surface. An analysis of angle dependent data further suggests that for MPTMS the thiol group and thus alkyl chain points away from the surface, while for a 0.5 monolayer coverage of PTES the alkyl chain is oriented towards the surface. A higher coverage, ~1 monolayer, the behavior seems to be reversed for at least a fraction of all molecules. Temperature programed XPS measurements suggest that the oxy groups of both molecules desorb from the surface at 550 K, which is in accordance with literature. The present study thus provides information on how these silane coupling agents bind to titanium oxide and what their molecular orientation is on the surface.
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| 4. |
- Kamra, Tripta, et al.
(author)
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Covalent immobilization of molecularly imprinted polymer nanoparticles using an epoxy silane.
- 2015
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In: Journal of Colloid and Interface Science. - : Elsevier BV. - 1095-7103 .- 0021-9797. ; 445, s. 277-284
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Journal article (peer-reviewed)abstract
- Molecularly imprinted polymers (MIPs) can be used as antibody mimics to develop robust chemical sensors. One challenging problem in using MIPs for sensor development is the lack of reliable conjugation chemistry that allows MIPs to be fixed on transducer surface. In this work, we study the use of epoxy silane to immobilize MIP nanoparticles on model transducer surfaces without impairing the function of the immobilized nanoparticles. The MIP nanoparticles with a core-shell structure have selective molecular binding sites in the core and multiple amino groups in the shell. The model transducer surface is functionalized with a self-assembled monolayer of epoxy silane, which reacts with the core-shell MIP particles to enable straightforward immobilization. The whole process is characterized by studying the treated surfaces after each preparation step using atomic force microscopy, scanning electron microscopy, fluorescence microscopy, contact angle measurements and X-ray photoelectron spectroscopy. The microscopy results show that the MIP particles are immobilized uniformly on surface. The photoelectron spectroscopy results further confirm the action of each functionalization step. The molecular selectivity of the MIP-functionalized surface is verified by radioligand binding analysis. The particle immobilization approach described here has a general applicability for constructing selective chemical sensors in different formats.
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