PATTERNING AND MODULATION OF OXIDE SURFACE PROPERTIES
Creators: name: SOLIVERI, GUIDO
Publisher: name: Universit� degli Studi di Milano
Year: 2015

description: Most of the topics dealt with in this thesis belong to surface science. The starting point was the fundamental understanding of phenomena at the oxide-gas interface and the effect of its modification. Such knowhow was then used to solve (or, at least, to attempt to solve) issues of critical impact in everyday life: the increasing lifetime of building materials employed in low-impact smart houses; the fouling prevention in electroanalytical sensors for neurotransmitter detection; the unspecialized laboratories accessibility to microlithography, critical to device miniaturization. These challenges might seem not related, but they actually share deep scientific and technological foundations. The physicochemical modification of oxide surfaces, the creation of organic/inorganic hybrids and the exploiting / the enhancing of semiconductor peculiar properties allowed us, starting from the foundation, the realization of proof-of-concept protocols and devices, ready for the pre-commercial development. The Leitmotif of my research was the synthesis and the modification of titanium dioxide surfaces. TiO2 has been the main character in physico, physicochemical and material science researches of the last 50 years. Biocompatibility and low cost make it engaging for many applications. Its (near-UV active) semiconductor features, well known and abundantly investigated by the scientific community, are acquiring central interest also in many markets with the development of self cleaning coatings, windows and asphalts, anti-fogging mirrors and self-sterilizing surgery rooms and instrumentations. New generation batteries and solar cells are going to be developed as commercial prototypes. One of the biggest challenges in the titania fundamental research is the enhancement of activity in the solar spectrum. First, the most recent aspects in titania doping and promotion were touched. While, in the last twenty years, great effort has been made in the mono-atomic doping of titania and in the understanding of the influence of the dopant position in the titania lattice and its electronic behavior, the most recent literature describes the co-promotion of the material by two (or more) atoms doping. The metal/non-metal codoping seems especially promising; the synergetic effect of the two atoms in the TiO2 lattice was both theoretically and experimentally proved. In this contest, the N/Nb codoping was analyzed, investigating the effect of the atoms in the lattice from morphological (surface area, porosity and crystallographic structure) and electronic point of view (EXAFS, UV-Vis absorption and EPR analyses). N/Nb codoping was compared with N/Ta co-doped samples, synthesized by two different procedures. The photoactivity of the two sample families was tested by a model reaction (the degradation of ethanol, throughout acetaldehyde intermediate) both under UV and solar simulated irradiation. Then, a different approach in the modification of surfaces was tested. The assembly of organic/inorganic hybrids was tested; thanks to the formation of organic mono- or multi-layers at the surface,they can tune the chemistry, the polarity and the adhesion properties of the interface. Siloxanes were used as active agents, thanks to their compatibility with oxide materials and, especially, for the ability to self-assemble at the surface to form a monolayer. Siloxanes are able to react with the -OH groups at the surface, chemisorbing and polymerizing at the interface in such a way to form a monolayer with tunable functionalities. Many different silanes were tested and their dipole momenta were related to their wettability properties. Such siloxanes chemisorb strongly both from the gas phase and the liquid phase. Their reactivity, both on smooth and rough surfaces, was tested vs the temperature of functionalization in gas phase. Many characterization techniques were adopted to understand the behavior of such molecules from a molecular point of view: magnetic (solid state NMR), microscopic (SEM, TEM, AFM), optical and electrochemical (CV and EIS). The science of adhesion and wettability was also adopted for the development of superhydrophobic coatings. Titanium dioxide particles with engineered morphology were used as the best candidate to create superhydrophobic/superhydrophilic patch-wise surfaces, exploiting their photoactivity (photolithograpy). The core of the thesis was the synthesis, modification and application of transparent photoactive thin films. A procedure for the synthesis of smooth, transparent and photoactive TiO2 thin layers was developed, and used to produce highly applicative devices and protocols. Such synthetic strategy is highly tunable and reproducible; the obtained films are robust and active and, most of all, require simple instrumentation (sol-gel procedure), which is highly appealing for the market. The films were properly characterized both form the morphological/mechanical and photochemical point of view. Apart their transparency and their thickness, the films were highly crystalline (pure anatase phase). Such procedure was firstly designed as a proof-of-concept for self-cleaning windows, but, thanks to its versatility and the high activity of the films, it leads the path towards highly applicative procedures and devices. The smoothness and the photoactivity brought me to the field of photolithograpy, especially in the direction of microlithography. The high activity of the titania allowed the use of safe and low-energetic lamps. No collimation was required to obtain a resolution lower than 5 �m. First of all, I tested the lithography on siloxane monolayer films, as a proof-of-concept of resolution and efficiency. But siloxanes, as many other self-assembled monolayer molecules, can be the pillars for 3D fabrication. Such monolayers were used as polymerization initiators for polymer brushes. If the initiators of polymerization are patterned, patterned polymer brushes will be obtained. That was the first report of polymer-brushes lithography exploiting the photoactivity of TiO2. Remote photocatalytic lithography makes this procedure extremely versatile. Exploiting the remote photocatalysis, in principle, any material can be used as a support for patterned polymer brushes growth (provided that the initiator are able to graft the surface). The developed protocol for the synthesis of TiO2 thin films was also used to design and engineer complex electrodes for cyclovoltammetric analyses of biological samples. Electrochemistry seems to be the best candidate for the development of an analytical option with sensitivity comparable with present analytical procedures but reduced time-per-analysis and cost. Unfortunately, catecholamines chemisorb and polymerize on metal and oxide electrodes quickly, making the device useless. Covering the electrode by a homogeneous, nano-porous thin layer of titania makes the surface photoactive. That is the first example in literature of self-cleaning nano-engineered electrodes for cyclic voltammetry. After the detection, also in simulated human serum and liquor, a fast and simple irradiation of the device, under non-hazardous UV-A lamp, degrades all the fouling on the surface without altering its features. The sensor, after each UV treatment, recovers its pristine performances, with full recovery in terms of selectivity and sensitivity. Irradiation trials were also performed directly in the analytical mixture, as a proof of concept for on-site application. Modern era requires flexible and light materials for the building industry. Polymers are acquiring more and more interest thanks to their increasing performances and their smart properties. The drawbacks of such materials are connected to the low resistance to the UV light, the softness and the difficulties in cleaning procedure. The use of organic/inorganic hybrid, or better the coverage of plastic materials with an oxidic thin layer, can solve many of these problems, lengthening the lifetime of such materials. If the covering oxide is also photoactive, the material can be self-cleaned when exposed to solar light. That is a big chemical challenge, because of many synthetic problems. Two different approaches were tested to solve this relevant issue. On one side, the hydrophobicity of ionic liquid modified SPES (sulfonated polyether sulfone) was combined with designed morphological features to confer superhydrophobicity. On the other side, the polymeric surface was covered with a transparent titania layer active in the near UV-region, able to mineralize organic molecules chemisorbed at the surface. Eventually, a different approach to modify oxidic (and not only) surfaces is the creation of a homogeneous layer of Ag nanoparticles by an innovative microwave procedure. That simple and accessible strategy allowed us to produce plasmonic surfaces (thanks to the dimension and the homogeneity of the Ag particles) with countless applications. The layer was shown to be a very active substrate for surface enhancement Raman spectroscopy (SERS). Thanks to the versatility of the synthetic method, all shapes and dimensions can be covered. That makes it a perfect candidate for the production of new generation of SERS sensors. The sensitivity towards molecules of environmental and biomedical interest was proved.