NANOSTRUCTURED OXIDES AS LEADING ACTORS FOR ENVIRONMENTAL REMEDIATION, SMART SURFACES AND ENERGY APPLICATIONS

In the last decades, metal oxides have been widely employed in the field of nanotechnology, thanks to their physicochemical characteristics. In fact, oxidic compounds can be exploited for manifold applications due to their chemical, thermal and mechanical stability, the low-cost, the low- or non-toxicity. Moreover, the possibility to easily obtain metal oxides as nano or nanostructured powders together with their high reactivity, mainly due to the presence of polar hydroxyl groups populating the surface, have always attracted the scientific community interested in the field connected to catalysis. Among the most adopted metal oxide in materials science, titanium dioxide (TiO2), tungsten oxide (WO3), silicon dioxide (SiO2) and aluminium oxide (Al2O3) can be mentioned. Actually, the properties of the metal oxides strictly depend on their electronic structure. Some of them are semiconductors (e.g. TiO2 and WO3), while others are insulators (e.g. SiO2 and Al2O3), in dependence on the width of the energy gap between the valence and the conduction bands. Semiconductors, thanks to the relatively small band gap with respect to insulators, can be exploited for a wide number of recently developed applications in chemistry, physics and materials science. One of these applications exploited light with suitable wavelength for the promotion of electrons from the valence band to the conduction band, in order to promote reduction or oxidation reactions. This field of the physical chemistry is called photocatalysis and is the main focus of this Thesis. Photocatalysis, especially if titanium dioxide-based, can be useful for promoting a wide range of chemical reactions, e.g. hydrogen and fuels production, CO2 reduction, metal reduction/oxidation. In the last decades, photocatalysis was also proposed as an innovative and effective environmental remediation technique for the oxidative degradation of organic compounds constituting atmospheric or water pollution. In fact, photocatalysis can be able, differently from traditional remediation techniques (e.g. biological treatments, adsorption by activated carbon, …), to completely degrade the pollutants and their oxidation intermediates and by-products to harmless compounds (water, CO2, and, in case, inorganic salts). Nonetheless, photocatalysis suffers from critical issues which, nowadays, still prevent its use on a large utility-scale. Among these, the low quantum yields and, thus, efficiency of the photocatalytic process with respect to the light irradiation adopted to activate the photocatalyst, the need of UV light to activate large band gap semiconductors, such as TiO2, and the high costs for the removal of the photocatalyst from the polluted effluent once the remediation treatment has been performed. This Thesis is divided into three different parts, on the grounds of the materials studied and their final application. In Part I (Chapters 1–7), TiO2-based materials for photocatalysis, variably modified and engineered by adopting different strategies for the promotion of specific properties, are reported. In Part II (Chapters 8–11), the preparation and the characterization (also by means of electrochemical techniques) of silica and titania films with tailored porosity and surface properties are presented. Eventually, Part III (Chapters 12–13) reports the study of active materials for heterogeneous catalysis and photo-electrocatalysis in the framework of the research of new energy sources and novel materials with energy conversion applications. More in detail, Part I of this Thesis (Chapters 1–7) was devoted to the study of TiO2-based photocatalysts for application in the field of pollutants degradation (both in the gas and the liquid phase) and to the effort to overcome the typical issues of photocatalysis by adopting different approaches (e.g. doping, co-doping, coupling with smaller band gap semiconductors, development of floating device). Moreover, photocatalysis was also exploited for surface modification / lithographic purposes and for photoelectrochemical applications, which will be described in Chapters 11 and 13, respectively. Firstly, the characterization and the photocatalytic activity tests of differently prepared TiO2 nanostructured samples were performed. By varying the calcination temperature (from 300 to 600 °C) different phase composition and morphologies were obtained and the efficiency towards tetracycline degradation was thus optimized for the laboratory synthesized TiO2 samples. Tetracycline is an antibiotic compound of recent interest as a micropollutant of wastewaters and surface waters, due to its wide use for both human and animal treatments. In Chapter 1, the development of two different low-cost TiO2 immobilized systems is also reported: alumina macroscopic supports were adopted together with a prepared titania sol and an easy deposition technique, revealing good activity, as well as resistance, towards tetracycline degradation. In order to develop accessible active devices through the photocatalyst immobilization on a support, titanium meshes were also employed (Chapter 2). However, differently from the work reported in Chapter 1, in Chapter 2, the focus of the work was addressed to the study of the photocatalytic environment, in terms of water medium composition. The photocatalytic degradation of four different organic compounds (tetracycline, paracetamol, caffeine and atenolol) belonging to the class of pharmaceutics and personal care products (PPCPs) and classified as emerging contaminants was tested both for the single pollutants and for their mixture. The role of the solution composition and especially the role of electrolytes, were also studied by performing photocatalytic tests in commercial and simulated drinking waters. This work falls within the framework of the necessity to comprehend the potential of photocatalysis even in the case of water effluent constituted by complex matrix in terms of solvent composition and presence of many different pollutants at low concentration ranges. In fact, the presence of several organic compounds to be degraded, as well as the electrolytes present in the water matrix and its composition and physicochemical properties can deeply affect the efficiency of the photocatalytic process, the adsorption and the degradation mechanisms. Then, in the following three Chapters (3–5), the effects of doping and co-doping of titanium dioxide with metal and non-metal species, mainly in order to promote the activity of TiO2 under visible light, but also to enhance its efficiency were explored. In-depth characterizations were performed on N,Ta/Nb titania samples from the structural, morphological, spectroscopic and electronic point of view (by XRPD, surface area and porosity, XPS, EPR and DRS analyses) to comprehend the modifications provided by the guest species on the photocatalysts and how this could reflect on the photocatalytic performance. Moreover, a combined experimental / theoretical approach was followed for a better comprehension, by means of DFT simulations. The samples were finally tested towards the photocatalytic degradation of ethanol in the gas phase. The sole Ta doping on differently synthesized TiO2 samples and for paracetamol degradation was also studied, with particular emphasis on the photocatalyst surface acidity (performing titrations of the photocatalyst acid sites by phenylethylamine adsorption in liquid-solid phase), its stability in suspension and the recognition of degradation intermediates (by gas chromatography – mass spectroscopy analyses) possibly responsible for the photocatalytic performance among the different samples. This permitted to evaluate parameters which are often disregarded but become pivotal when photocatalytic degradations in the aqueous phase are considered. Sn and Zn were also investigated as elements able to strongly modify the structural characteristics of TiO2 samples, mainly in terms of phase composition and surface defectivity (recognized by HR-TEM, XPS and electrophoretic analyses). The adopted amount of metal modifiers during synthesis also allowed the possible formation of composite oxidic materials to be observed and its effects to be studied. Moreover, the concomitant use of N as a guest species able to promote the visible light absorption, allowed superior effects in the visible light harvesting to be recognized and enhanced activity under visible irradiation to be proved. The effect of the guest species on the reaction mechanism was also investigated by mass spectroscopy analyses, proving significant variations in the case of Zn promoted samples, with respect to pristine, N- and Sn- modified photocatalysts. This work paved the way for the last two Chapters of Part I of the Thesis, regarding composite photocatalytic materials, both in the case of oxidic composites (Chapter 6) and inorganic-organic ones (Chapter 7). In the former case, WO3 was selected as a minority photocatalyst together with TiO2 in order to comprehend the effects provided by a sort of co-photocatalyst in the degradation of tetracycline (in the liquid phase) and ethanol (in the gas phase). Also in this case, the differences pertaining the degradation mechanisms reported for both reactions were reconnected to the specific modifications of the bulk and the surface properties of TiO2 by WO3 species. In the latter case, instead, a novel methyl methacrylate-based ter-polymer, with specific dual wetting properties (thanks to the use of a fluorinated co-monomer) when reduced in foils, was adopted to assemble a floating, transparent and photocatalytically active device. For this purpose, the physicochemical characteristics of the ter-polymer were finely tailored and, successively, SiO2 (with adhesion and protection tasks) and TiO2 (photoactive) layers were deposited from home-made colloidal solutions by spray-coating technique. The photocatalytic activity of this organic-inorganic multilayer device was successfully tested towards the degradation of both atmosphere and water contaminants, in the context of its possible use for remediation from organic substances and their vapours from polluted large water basins. In these cases, in fact, the use of nanostructured powders, even as immobilized systems, should be avoided for issues related to photocatalyst removal, while the floatability promotes light harvesting. Throughout Part II of the Thesis, instead, the wide range of applicability of oxide nanomaterials obtained as films is reported. In fact, oxidic compounds such as SiO2 and TiO2 can be easily obtained as nanometric or micrometric films deposited on supports (e.g. glass slide, silicon wafer, conductive glass) from colloidal solutions or powder suspensions. In this regard, in this Thesis, SiO2 films were exploited for understanding the effects provided by surface or morphology modifications from a fundamental point of view. For this purpose, an intense use of electrochemical characterization techniques (cyclic voltammetry and electrochemical impedance spectroscopy) and surface wettability analyses by contact angle measurements was done. In Chapter 8 the effects provided by the deposition of an insulating layers of SiO2 onto a conductive glass were studied in terms of mass transport and charge transfer phenomena. By varying the thickness of these devices, which actually can be described as modified electrodes, different behaviours in the voltammetric signal were recorded, despite their strong insulating character. The voltammogram shape variations were interpreted in the light of previous theoretical studies reporting simulations of cyclic voltammetry analyses for electrodes modified by deposition of electroinactive layers. For the first time, the outcomes only reported on the basis of theoretical simulations were experimentally observed. The work presented in the Chapter 8 can be also seen as introductory to the following one. In fact, on the basis of the results obtained by electroinactive layers modified electrodes, especially regarding the variation of the diffusion and transport phenomena to the electroactive surface, porous SiO2-based electrodes with peculiar properties were developed. These electrodes, characterized by controlled mesoporosity due to the preparation involving nanometric polystyrene latex beads as solid templating agents, were deeply characterized under the physicochemical point of view (by SEM, FE-SEM, AFM, UV-vis spectroscopy, water contact angle measurements) and exploited for sensing applications. In particular, thanks to its morphological, surface charge and wetting properties, the modified electrode proved effective in the selective detection of dopamine in the presence of mucin as interfering agent. Dopamine is a neurotransmitter whose quantification in biofluids is pivotal for the diagnosis of the Parkinson’s disease, while mucin is a large dimension protein which can interfere in dopamine detection, as actually proved in the case of a bare electrode. The development of these tailored morphologies is to be seen in the context of the research devoted to the preparation of high selective and antifouling sensors. High selectivity and antifouling (i.e. reliable quantifications together with long electrode durability) are considered major issues nowadays in the field of electroanalysis. In the following two Chapters, instead, the behaviour of organic-inorganic hybrids is reported. In Chapter 10, SiO2 substrates were adopted for surface modification by alkylsilanes, molecules easily reacting with oxidic surfaces thus modifying their wettability, even for obtaining superhydrophobic surfaces. A vast number of characterizations (solid state NMR, surface free energy analyses, FTIR, electrochemical techniques) allowed the behaviour from a molecular point of view of alkylsilanes chemisorbed on a silica surface to be understood. In Chapter 11, in the framework of a hands-on activity for students and with both research and didactic purposes, the wetting modifications of TiO2 films from suprhydrophilicity to superhydrophobicity were investigated by means of functionalization by alkylsilanes as well as of photocatalysis. In this case, the formation of organic-inorganic hybrids as films was exploited not only to obtain superhydrophobic surfaces but also for patterning applications thanks to photocatalysis. In fact, by irradiating such a modified TiO2 surface by UV light, the photocatalytic process can be activated for selectively degrade the functionalizing molecule and create patterned surfaces characterized by wetting contrasts. This approach is generally called photocatalytic lithography and falls in the framework of surface wettability modifications by functionalizing oxidic compounds with organic amphiphilic molecules. In the last decades, these approaches have been receiving great interest for their everyday-life impacting aspects: self-cleaning, anti-stain and anti-corrosion surfaces are just few examples of the applicability of these complex materials which can be used for new technologies such as smart glasses, pollution remediating buildings, long-lasting ship hulls. Eventually, in the last part of the Thesis, studies regarding the preparation on composite materials for energy-related applications are presented. In Chapter 12, SiO2/Al2O3 powders with different relative composition were adopted as catalytic supports for the hydrodeoxygenation reaction of guaiacol. Ni was selected as active catalyst the effects of its concentration onto the support were explored. The guaiacol hydrodeoxygenation reaction is often taken into account as a model reaction for studying the upgrading of bio-oils. Bio-oils are the product of the conversion of lignocellulosic biomass, a renewable source of energy which received great attention in the last few years, in the context of the progressive depletion of fossil fuels and the incumbent need to find new and renewable energy sources. In the last Chapter, instead, colloidal synthesis approaches were adopted to obtain oxidic heterojunctions for photoelectrochemical applications. The two components of these heterojunctions were WO3 and copper vanadate (Cu2V2O7), in different ratios. Cu2V2O7 is a small band gap semiconductor which enables the heterojunction to be active under visible light and to promote charge transfer phenomena, favourable for enhancing the photocatalytic efficiency with respect to bare WO3. A vast characterization was performed adopting HR-TEM, in-situ XRD, photoluminescence and nanosecond transient absorption spectroscopies, in order to understand the physicochemical phenomena occurring at the interface between the two oxides and to optimize the heterojunction composition for the final application.