NANOSTRUCTURED SEMICONDUCTOR FILMS: SYNTHESIS, SURFACE FUNCTIONALIZATION AND INNOVATIVE APPLICATIONS

In recent years, photoactive semiconductors have received ever growing interest, as testified by the remarkable number of related publications, thanks to their promising applications in manifold fields such as environmental remediation and photovoltaics. Among the photoactive semiconductors, titanium dioxide has been by far the most investigated owing to its cheapness, non-toxicity and stability to photocorrosion. Titanium dioxide can be successfully applied to the photocatalytic remediation of air and water pollutants, H2 production from water splitting, and in solar light harvesting using second generation solar cells; it is a biocompatible material, and it can be employed to obtain self-cleaning surfaces. Although a few commercial applications employing nanometric TiO2 are already on the market, many issues still remain to be addressed to obtain efficient, reliable and durable materials. The present thesis work focuses onto the synthesis and the study of the physicochemical properties of nanometric TiO2. My research activity has focused on two main subjects, one more applicative and the other more fundamental. The first part was devoted to the photocatalytic applications of TiO2. Photocatalytic oxidation of pollutants is one of the most promising technologies in environmental protection and remediation, especially for the removal of low concentration pollutants in slightly contaminated enclosed atmospheres. Nanometric titania has been successfully applied to the photo-oxidation/reduction of numerous organic and inorganic pollutants, both in gaseous phase and in solution. Several concretes and paintings containing nanometric titania that photo-oxidize pollutants are already on the market, but many disadvantages remain to be overcome in order to obtain commercially successful products. Hence, the first part of my research was directed towards the improvement of the photocatalytic activity of TiO2 to obtain more efficient photocatalysts for the degradation of environmental pollutants. The photocatalytic activity of titania is strongly affected by its particles’ physicochemical features, which, in their turn, are imposed by the synthetic path adopted for the material preparation. Therefore, it is essential to tailor the physicochemical characteristics of titania particles using an appropriate synthetic procedure in order to obtain highly active samples. A considerable part of my PhD project was devoted to the optimization of several synthetic procedures in order to produce TiO2 powders and films with tailored optical, morphological and electronic features. One of the main disadvantages of TiO2 is its large band gap (3.2 eV for anatase, 3.0 eV for rutile), which corresponds to a light absorption in the UV region. Thus, currently TiO2 based materials require UV irradiation in order to activate the photocatalytic process. As only 5% of solar light is in the UV region, a shift towards visible absorption is required to improve the photocatalytic activity of TiO2 under solar irradiation. The introduction of non-metal ions in the TiO2 lattice represents one of the most promising approaches to induce a bathochromic shift, i.e., a shift of the absorption edge of TiO2 to longer wavelengths, and consequently increase the photocatalytic response of doped samples into the visible region. Therefore, during my thesis, I synthesized several doped samples with non-metals such as N, in order to assess if a bathochromic shift effectively leads to a higher photocatalytic activity in the visible region and, more important, under solar irradiation. N-doped TiO2 samples were obtained from different titania precursors (Ti(Oi-Pr)4, TiCl3) and adopting different N-sources (ammonia, triethylamine, tea). All obtained samples were exhaustively characterized, in order to obtain a complete picture of the modifications induced in the titania structure and surface features by the modifications of the synthetic pathway. Samples were characterized from the structural, morphological, electrochemical, optical and compositional point of view. Moreover, other features, such as magnetic properties, were determined and ab initio calculations of the electronic properties of the doped samples were performed. All N-doped samples showed a broad absorption in the visible region which was traced back, on the grounds of first principles calculations, to the formation of localized intragap electronic levels. Sample thin films were tested for their photocatalytic activity, under UV, visible and simulated solar irradiation, towards the degradation of gas phase ethanol and acetaldehyde. The most active N-doped sample, both under UV and solar irradiation, was the oxide showing the largest amount of paramagnetic N_b^• species. Under visible irradiation instead, the sample with the largest activity was the one showing the narrowest apparent band gap and the concomitant presence of anatase and brookite polymorphs, which might hinder charge recombination processes. The structure of N-doped samples was elucidated not only by ordinary powder diffraction, but also by means of synchrotron radiation, using Extended X-ray Absorption Fine Structure (EXAFS) to understand the position of dopant ions inside the TiO2 crystal lattice. These data were obtained during a short research stay at the European Synchrotron Radiation Facility (ESRF) in Grenoble. Average Ti nearest neighbors distances were obtained from EXAFS experiments and compared with Density Functional Theory (DFT) calculations, showing that N substitutes oxygen at low levels of doping, whereas oxygen vacancy creation is observed at higher dopant concentrations. Another strategy to improve the photocatalytic activity of TiO2 involves the enhancement of the adsorption and diffusion of pollutants into TiO2. In this respect, I investigated the effect of the modification of TiO2 morphology to obtain mesoporosity via different template syntheses. Mesoporous materials have been consistently proposed to produce better performing catalysts in many fields of catalysis. Here, the morphologic features of titania particles were tailored by using soft templates, in order to obtain materials with a high degree of porosity in the mesoporosity range. Two classes of soft templates were investigated: alkylpyridinium surfactants and block copolymers of the Pluronic family. As for the first class, both monomeric (dodecylpyridinium chloride, DPC) and dimeric gemini-like surfactants (gemini spacer 3, GS3) were employed. Mesoporous TiO2 samples were synthesized by a classical sol-gel route followed by an hydrothermal growth in the presence of one of the structure directed agents. The surfactant/oxide interactions at the solid/liquid interface were evaluated by adsorption isotherms, showing marked differences between the two surfactants. While DPC exhibited weak adsorbate/adsorbent interactions and weak self-aggregation tendency, resulting in the formation of very small, globular micelles, GS3 instead showed strong interactions with the TiO2 surface and the formation of elongated rods and further hexagonal arrangements could be proposed. Such different behaviors lead to significant diversities in the porous structure of the TiO2 samples. The small pores generated by the DPC micelle tend to collapse because of the heat of combustion generated during the surfactant removal step at 600 °C. On the contrary, GS3 leads to a significant fraction of pores in the mesoporosity range. However, the use of cationic surfactants has an intrinsic limitation: high calcination temperatures are required to remove entirely the template. Such harsh conditions markedly reduce the surface area of the oxide due to particle sintering and crystal growth. Non-ionic structure directing agents, such as amphiphilic block copolymers, can be instead completely removed at much lower temperatures. Three block copolymers of the Pluronic family, characterized by different micelle size in water as determined by light scattering analysis, were employed to induce mesoporosity in nano-TiO2. The surfactants were removed by combining UV and thermal treatments in order to avoid pore collapse while obtaining a good oxide crystallinity. Obtained samples presented a high surface area and significant fraction of pores in the mesoporosity range. A good correlation was observed between the sequence of average pore size in mesoporous TiO2 and the micelle size of the used copolymer. A fine modulation of pore size and total volume was obtained by changing polymer type and concentration, effectively enhancing the photocatalytic properties of the oxide towards the degradation of methylene blue. The mesoporous oxides were also used as scaffolds to obtain Bi-promoted TiO2, resulting in a further increase of the photocatalytic performance (see below). Another limitation of TiO2 as photocatalyst is its low quantum yield. Among the factors that concur to reduce the titania photocatalytic efficiency, the recombination of photogenerated electrons and holes plays a leading role by competing with the transfer of photogenerated charges to species adsorbed at the photocatalyst surface. Quantum yields could thus be improved by slowing down such recombination processes. The use of metal particles or mixed oxides with a suitable band structure has been proposed to slow down the recombination process. In fact, if the metal/second oxide has an available electronic level just below the conduction band of TiO2, electrons photogenerated on TiO2 are prompted to migrate to the metal/second oxide, thus enhancing the charge separation and slowing down the recombination process. Noble metals, such as Pt, have been extensively studied in the literature for this purpose and they have proven to be highly effective in enhancing the TiO2 photocatalytic activity. In my work, Bi2O3 was investigated as a cheaper alternative to noble metals to enhance the photocatalytic performances of TiO2. Bi2O3 is non-toxic and environmentally friendly material which, thanks to its band structure, could trap photo-generated electrons, and thus improve the overall quantum efficiency of the material. Theoretical calculations have shown that the specific band structure of Bi2O3-TiO2 could significantly improve the oxide photocatalytic efficiency. In my study, Bi2O3 was allowed to form into the mesoporous network of TiO2 samples obtained by surfactant template synthesis. The obtained materials were characterized by X-ray diffraction (XRD), N2 adsorption at subcritical temperatures (BET), high resolution transmission microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, and zeta potential determinations, providing an insight into the composite structure and into the specificity of the Bi2O3-TiO2 composites with respect to traditional sol-gel TiO2 nanomaterials. All samples were tested for the photocatalytic degradation of methylene blue stains and of formic acid under dry and wet conditions, respectively. The presence of Bi promotes the photocatalytic activity of the final samples in both tested reactions. Photocurrent measurements of Bi2O3-TiO2 composites were performed in order to assess any effect of the Bi addition on the fate of the photogenerated electron-hole pair. The obtained results agree with the observed marked enhancement in photocatalytic activity of the Bi2O3-TiO2 samples, showing an increased recombination time of photogenerated charges in Bi2O3-TiO2 composites. This effect may be related to the finely dispersed nature of Bi2O3 within the mesoporous network of the TiO2 scaffold. A crucial aspect that needs to be addressed for the commercial application of TiO2 materials is their reusability, which is strictly connected to their efficiency in removing recalcitrant compounds. Real life effluents often contain a mixture of pollutants, some of which can be highly recalcitrant compounds. It has been observed that such recalcitrant pollutants or their degradation intermediates can strongly adsorb onto the TiO2 surface, irreversibly poisoning the photocatalyst. The deposition of titania particles in a thin layer is essential for the material applications because it simplifies the separation of the photocatalyst from the effluents and optimizes photon absorption. However, by reducing the available surface area, the deposition in films markedly increases the poisoning effects. A possible strategy to tackle this issue is the combination of photocatalysis with other oxidation techniques, in particular advanced oxidation techniques. In this thesis work, a combination of photocatalysis by TiO2 films and ozonation treatments was studied to achieve the complete oxidation of highly recalcitrant pollutants such as bisphenol A and cumylphenol. A specific deposition procedure of the TiO2 film onto a rough Al support was developed in order to obtain photocatalytic films with high surface area and good mechanical stability. Photocatalytic ozonation was compared to the separate photolytic, photocatalytic, and ozonation techniques to investigate the synergistic processes taking place in the combined treatment. The combination of the two treatments leads to synergistic effects that dramatically enhance the final mineralization of the pollutants. Moreover, the degradation pathway taking place during the photocatalytic ozonation of bisphenol A and 4-cumylphenol was studied by combining HPLC–MS determinations and FTIR analyses of the used catalyst. The knowhow gained in the field of oxide synthesis and photocatalysis was then exploited in the development of oxide-based materials with tailored surface properties by means of surface functionalization with siloxanes. In recent years, hydrophobic modification of oxide surfaces has attracted growing attention because of its vast technological relevance. Siloxanes, compounds with the general formula R-(CH2)n-Si-(OR’)3, are among the functionalizing agents employed to modulate the surface energy, wettability and adhesion properties of oxides, thanks to their ability to form durable bonds with inorganic compounds, upon hydrolysis of labile –OR’ groups. Furthermore, siloxanes may serve as robust coupling agents between organic materials and the oxide for the preparation of a new class of hybrid nanocomposites showing interesting photophysical properties and applications. Firstly, the role played by the structure of the siloxane molecule onto the wetting features of a smooth surface was investigated. The surface energy of different hydrophobing molecules, both fluorinated and unfluorinated, deposited in smooth layers over an inert substrate, was determined by analyzing contact angle values with literature models. The obtained values were compared with dipole moments determined by theoretical calculations employing semiempirical Hamiltonians, finding a close correlation between the calculated dipole moments and the polar components of the surface energy. Siloxanes were then employed to functionalize TiO2 nanoparticles, in order to obtain rough composite films. The functionalization of nanometric TiO2 with siloxanes is even more promising as it has lead to a series of applications uniquely related to the peculiar features of this oxide. For instance, the photocatalytic activity of TiO2 can be exploited to create hydrophobic/hydrophilic patterns by irradiating a siloxane-TiO2 film with UV light through a suitable photomask, a procedure known as photocatalytic lithography. The siloxane is photocatalytically degraded in the areas exposed to UV light, while the siloxane monolayer remains intact in the areas covered by the photomask. The resulting hydrophobic/hydrophilic pattern can be exploited in numerous applicative fields, for example to promote the site selective condensation of water from the gas phase or the site specific adsorption of hydrophilic/hydrophobic molecules. In this study, the TiO2 surfaces functionalized by different siloxanes were tested in self-cleaning experiments. Further, patterned structures with tunable hydrophobic and oleophobic patches were obtained by exploiting the photocatalytic activity of TiO2 films. The resulting wetting contrast was exploited to obtain a site selective adsorption of a dye molecule, with a procedure that can be adapted to the site selective deposition or growth of a large variety materials, such as semiconductor quantum dots, polymers or biological molecules. Notwithstanding the great interest and the manifold applications of these composite materials, the attachment of hydrophobizing molecules at TiO2 surfaces still remains poorly understood at the molecular level and hardly discussed in the literature. My research activity was aimed at filling the gap by investigating the fundamental features of bonding and structure of the siloxane layers onto TiO2 nanoparticle films. The influence of the siloxane amounts on the wettability and self-cleaning properties of TiO2 was studied, together with the role played by the hydrophobing molecule structure (aliphatic vs. aromatic side-chain, linear vs. branched, length of the side-chain, fluorinated vs. un-fluorinated molecules). The studied siloxanes were both commercial and laboratory-made, the latter synthesized by the research group of Prof. Benaglia (Dipartimento di Chimica, Università di Milano). The modes of attachment of siloxane molecules at the TiO2 surface were investigated by combining data of CP/MAS NMR with ATR-FTIR and XPS analyses, giving a detailed picture of the siloxane layer structure and interaction with the oxide. It appears that the attachment modes of silicon, besides changing with the siloxane content of the surface, are markedly affected by the siloxane structure. For instance, alkyl trifunctional siloxanes give rise, starting for low oxide coverage (9% w/w), to continuous functionalized layers in which silicon atoms are progressively bound by one, two, or three groups, these being either – O–Ti or –O–Si. These films are uniform and highly hydrophobic showing excellent self-cleaning properties at low contents; they present a Cassie-Baxter wetting behavior in which water drops float over a composite solid-gas carpet. The substitution of the alkyl chain with aromatic end groups favors localization versus spreading for the siloxanes, due to π-π stacking interactions. In these cases, the films, which are locally ordered, are less uniform on the whole. The bifunctional biaryl compound gives rise to layers which are initially, i.e., at low coverage, hydrophilic and end up to be hydrophobic at higher coverage. These are characterized by patch-wise localizations producing a wettability in which the water drops spread following the surface rough profile. Therefore, the structure of the siloxane appears to be a key parameter tuning the features of wettability of the surface by water. Siloxanes are employed not only to modulate the wettability of oxides, but they can be exploited as linkers to attach new functionalities, such as dyes, biological molecules, and nanoparticles, to the oxide surface. By patterning the siloxane monolayer, a site-selective functionalization of the oxide surface can be obtained. Among the available patterning techniques, probe-based electro-oxidative lithography offers one of the best lateral resolution available (line width as narrow as 30 nm). So far, this technique has been applied almost exclusively to Si substrates. In order to fully exploit this technique, its application to other technologically relevant substrates is required. In the present thesis, probe-based electrooxidative lithography of octadecyltrichlorosilane (OTS) monolayers adsorbed on TiO2 and indium tin oxide (ITO) are reported for the first time. The conductivity of the layer and the environmental humidity are critical parameters, affecting the stability of the water meniscus between the probe and the substrate and thus the electro-oxidation process. The resulting surface functionalization was exploited to obtain the site selective growth of metal nanoparticles. The electro-oxidation mechanism was studied by advanced characterization techniques such as Scanning Kelvin Probe Microscopy (SKPM), and the oxidation processes taking place on Si, ITO and TiO2 were compared. For instance, in the case of OTS-ITO, a local overoxidation of the ITO substrate occurs simultaneously to the monolayer oxidation, whereas in OTS-TiO2, no overoxidation of the oxide substrate takes place. This latter part of the work was carried out as collaboration between the group I belong to (Prof. Ardizzone’s group of the Università degli Studi di Milano) and the group of Prof. Schubert of the Friedrich-Schiller Universität, Jena (Germany), where I spent a 5-month research period followed by several short stays.