PHOTO-RESPONSIVE OXIDES AS PLATFORMS FOR ENVIRONMENTAL REMEDIATION, HYBRID MATERIALS SYNTHESIS AND SMART SYSTEMS

Photoactive semiconductors are a hot topic of research due to their applications in environmental remediation, photovoltaics, smart devices, light-activated synthesis and self-cleaning surfaces. Among them, oxide semiconductors play a main role thanks to their wide availability, stability, ease of preparation and tunable surface properties. In this context, my Ph.D. focused on the application of oxide semiconductors for three main purposes: pollutant remediation, photocatalytic synthesis of hybrid materials, and smart systems for the controlled release of active substances. Alongside this main research project, I developed two original hands-on activities based on oxide systems for public engagement. Oxide semiconductors for environmental remediation. Photocatalysis is an advanced oxidation process that can achieve the complete degradation of contaminants without the addition of reagents. However, its real-life application has been hindered by several limitations, such as the use of nanosized powder, costs of light irradiation, possible accumulation of toxic reaction intermediates and the sensitivity to complex water matrices. During my Ph.D., I investigated the deposition of photocatalyst powder on macroscopic devices based on aluminum plates for air purification. Aluminum is a cheap and technologically-relevant substrate, but its application as substrate for photocatalyst immobilization has been hampered by adhesion issues and metal ion diffusion within the photocatalytic layer that increases recombination of photogenerated carriers. Thus, I investigated the use of silica interlayers to promote adhesion, efficiency and reusability of TiO2 films on aluminum plates. Films were prepared from stable titania sols and deposited on aluminum substrates with different surface morphology and with silica interlayers of different thickness. The study of the coating structure, morphology, optical properties, adhesion and hardness showed that the nature of the substrate and its surface roughness determined the optimal number of silica interlayers. When the silica interlayer was too thin, moderate cracking was still observed, whereas a too thick silica interlayer led to peeling off of the film. The use of rougher surfaces, as in the case of sand-papered aluminum, required a higher number of silica layers to promote a more homogeneous surface where the titania coating could effectively adhere. However, the addition of a thicker silica layer did not erase the effect of the sand-paper pre-treatment on surface roughness. Films on sand-papered substrates showed promoted photocatalytic activity with respect to the smoother counterparts, possibly due to their larger exposed contact area. The prepared films exhibited excellent light-induced superhydrophilicity and self-cleaning properties towards fouling agents (alkylsilanes). Photocatalytic degradation tests were carried out using both a model volatile organic compound (ethanol) and NOx. The silica interlayer proved crucial to promote the film robustness, effectively increasing the mechanical stability and reusability when a thicker interlayer was adopted on sand-papered aluminum plates. In order to cut the costs associated with lamp irradiation, the visible-light promotion of large band gap photocatalysts is a widely investigated approach. In this regard, I studied the modification of TiO2 with Sn and N species aiming to improve the photocatalyst visible-light absorption for the solar-light photocatalityc degradation of emerging pollutants. Three different synthetic routes were investigated: a bulk synthesis, where Ti and Sn precursors were both added in the sol-gel synthesis, a seeded procedure, where pre-formed SnO2 crystals were added to TiO2 synthesis, and a mechanical mixture, where the oxides were mixed together then calcined. Marked differences were observed in the final composites’ structural, morphological and optical properties, leading to notable changes in the photocatalytic performance. Interestingly, bulk and seeded samples showed notable photochromic properties under UV light, which varied based on the doping level: this is the first time photochromic effects have been observed in Sn-promoted TiO2. These findings can be related to the different nature of the defects introduced in the oxide lattices depending on the synthetic route, which reflect in the photocatalytic performances of the modified semiconductors. The photocatalytic degradation of wastewater pollutants in complex matrices requires a close scrutiny of the generated byproducts to avoid possible accumulation of intermediates even more toxic than the parent compound. In this respect, I determined that the degradation of tetracycline, a widely used antibiotic, by a benchmark TiO2 sample, despite the fast pollutant disappearance, leads to poor mineralization and byproduct accumulation, especially in the presence of common electrolytes, such as HCO3-. Conversely, the use of commercial ZnO samples with the same surface area resulted in a faster tetracycline degradation kinetics and a much higher mineralization degree compared to TiO2 in all the investigated water matrices. These results can be attributed to different photo-degradation pathways followed by the two oxides, as shown by tests with radical scavengers and by-product analyses. While TiO2 degradation pathways are strongly dependent on both hydroxyl radicals and holes, ZnO mineralization activity is mostly related to holes, which limits the interference of •OH-scavenger species such as bicarbonates. Photo-induced synthesis of oxide-polyaniline composites for environmental remediation. To date, photocatalysis remains a comparatively slower and costlier wastewater treatment compared to adsorption. For this reason, during my Ph.D I also investigated new generation adsorbents characterized by easier regeneration and ability to perform a controlled release of the adsorbed species to be further treated or reused. To this aim, I investigated polyaniline (PANI) composites prepared via an innovative photocatalytically-induced synthesis. PANI materials have been recently adopted as sorbents for environmental remediation due to their stability, redox properties and acid-base characteristics. However, PANI traditional oxidative synthesis (here labeled as PANI-aniline) adopts noxious and toxic reagents (aniline and (NH4)2S2O8) and leads to carcinogenic by-products and large amounts of waste. The alternative photocatalytic approach I developed is a two-step synthesis starting from aniline dimer (N-(4-aminophenyl)aniline) and exploiting TiO2 photocatalyst to initiate the oligomerization, and a greener oxidant (H2O2) in the polymerization step. The resulting PANI-TiO2 nanocomposites showed very different structural, morphological and surface properties with respect to PANI-aniline, resulting in fast and efficient removal of water pollutants. To better understand the reaction pathway and tailor the material properties, the relative roles played by TiO2 and H2O2 in the synthetic procedure were investigated in depth. UV-irradiated TiO2 was found to promote PANI crystallinity and polymer-oxide interactions. The amount of added H2O2 has a crucial role on the composite properties by promoting either surface growth of PANI chains or polymerization in the liquid bulk. High H2O2 amounts seem to promote a homogenous polymer formation mechanism, leading to nanocomposites with high PANI content and thermal stability, but low crystallinity degree and surface area. Low H2O2 quantities give rise to highly porous, large surface area nanocomposites with good crystallinity but low PANI content. The latter samples exhibited the best performance in pollutant sorption tests, achieving a fast and complete removal of dyes and heavy metals also in the presence of electrolytes. These samples also showed reusability in consecutive stress tests and could be regenerated simply by treatment with alkaline aqueous solution at room temperature. The next step was to investigate the role of the nature and morphological features of the semiconductor: commercial TiO2 photocatalysts with either 50 m2g-1 (labeled TiO2-P25) and 12 m2g-1 (TiO2-Kronos) were compared with WO3 either lab-synthesized (3.5 m2g-1, named WO3-Synt) or commercial (6.1 m2g-1, WO3-Comm). The composites showed a nanorod / nano-wire morphology: the length of the polymeric rods and the embedding of the oxide particles within the polymer network strongly depended on the nature and morphology of the photocatalyst. Furthermore, while > 80% total dye removal capacity was observed for all samples (with the exception of PANI-WO3-Comm), notable differences were observed in terms of released tests. In particular, PANI-oxide composites consistently showed dye-release capacities far higher than PANI-aniline. The ease of desorption opened the door to the facile regeneration of the adsorbent and to the adsorbate recovery for its recycle in a circular economy perspective. Therefore, I investigated an adsorption-photocatalysis coupled system which exploited the reversibility of the pollutant removal process. In particular, after consecutive dye adsorption cycles, the contaminant was released by the PANI-oxide adsorbent and subsequently mineralized by a ZnO driven photocatalytic process. The nature of the adsorption process was deeply investigated and selectivity tests with cationic and anionic dye mixtures proved the preferential adsorption of PANI-oxide adsorbents towards anionic dyes. In the end, the promising and reversible adsorption capability of PANI composites prompted me to investigate their possible application in CO2 capture systems. Thus I have worked on reviewing the literature works on the topic, comparing the performances of different PANI materials towards CO2 removal. Smart systems based on light-responsive oxides. The intrinsic characteristics of semiconductor oxides, such as their photocatalytic and surface properties, can be exploited in the design of smart systems for the controlled release of unstable active substances, such as essential oils. Among them, cinnamaldehyde (CIN) is a low-cost natural compound endowed with antibacterial, anti-cancer, antifungal, and anti-inflammatory properties. However, CIN has poor water solubility, high volatility and very poor stability in environmental conditions, undergoing degradation when exposed to heat, light or even oxygen. These issues hinder CIN applicability, thus smart systems able to store this active substance and to safely release it at will, are of extreme interest for the scientific community. In this context, during my Ph.D. I developed oxide-based hybrid systems for the release of CIN catalyzed by acidic pH. The smart system was obtained by a grafting method based on amino-silane linkers and imine chemistry: (3-aminopropyl)triethoxysilane (APTES) was adopted for the functionalization of the oxide surface. The terminal amine group of the silane (-NH2) was used for a condensation reaction with the aldehydic group of CIN (-HC=O), yielding an imine bond (-HC=N-) between APTES and CIN and a loading of ca. 5 molecules/nm2, determined with CHN and TG analyses. The covalent grafting of cinnamaldehyde, showed by FTIR spectra, preserved the molecule stability, simplifying storage. Release tests were performed at pH values between 5.0 and 7.4: thanks to the pH-sensitivity of imine bonds, a fast CIN release was observed at pH 5.0. The grafting procedure was also performed on a porous semiconductor film, demonstrating the versatility of this method. Exploiting the oxide photoactivity, the fouled film was regenerated upon 1h UV irradiation, opening the door to reusable devices for CIN controlled release. Besides the conventional approach of loading bioactive compounds on solid drug carriers, smart systems based on particle-stabilized emulsions (i.e., Pickering emulsions) are receiving increasing attention from the scientific community. In this regard, during my last year of Ph.D. I investigated oil-in-water Pickering emulsions prepared with food-grade vegetable oils and stabilized with bare ZnO particles. FTIR studies highlighted that, during emulsification, ZnO particles undergo an in situ functionalization by fatty acids present in the vegetable oil. This procedure gives rise to very stable and homogeneous emulsions (mean droplet size ca. 1 μm). Confocal microscopy images demonstrated the high stability of the system towards long time storage (more than 9 months), temperature variations, mechanical stress and increased ionic strength. ZnO-Pickering emulsions were loaded with CIN in the oil phase, in order to store the active molecule and release it at will by the application of five different stimuli. In particular, thanks to the semiconductor and amphoteric properties of ZnO, the developed smart system was able to release CIN by switching to a water-in-oil Pickering emulsion when subjected to acidification, UV and solar light irradiation, CO2 bubbling and the addition of bi/trivalent cations. This is the first report of an emulsion system responsive to five different stimuli. Depending on the type of stimulus, either a burst release or a controlled release over the course of several hours could be achieved. The emulsion switching can be attributed to the oxide surface charge: when ZnO is negatively or slightly positively charged, the oil-in-water emulsion is stable, while, when the oxide surface has high positive charge, the oil droplets’ intrinsic negative charge is neutralized and coalescence phenomena occur. A more positive ZnO surface charge can be achieved through the addition of acidic species (such as H+ and H2CO3 via CO2 bubbling), multivalent cations, which give specific adsorption on ZnO surface, and through light irradiation, which activates the photocatalyst and generates acidic species. The starting oil-in-water emulsion could be reobtained by basification, N2 bubbling and storage in the dark. The ZnO Pickering emulsions were able to safely store and release CIN molecules, which did not undergo any degradation neither during storage, nor after release in water solution. In the end, I have contributed to a work on near infrared (NIR)-emitting GdVO4:Nd systems. This composite material proved promising for bioimaging applications, thus, I exploited my experience oxide synthesis to investigate the role of the synthetic procedure on the material properties and NIR-emitting activity. Moreover, test on GdVO4:Nd functionalization with silane molecules (octylsilane and APTES) were carried out. The modification of the material surface with organic compounds can led to a possible increase in the material biocompatibility, as well as to the possible grafting of active molecules, such as cinnamaldehyde, for application in theragnostic. Chemistry dissemination activities. During my PhD, I was involved in chemistry dissemination activities in the framework of the “Piano Lauree Scientifiche, PLS”. In this context, I helped to develop two laboratory activities for high school teachers and students. The first one, aimed at teaching the basic concepts of surface science, focused on the preparation of superhydrophobic coatings based on films of surface functionalized oxide particles. The film’s superhydrophobicity was tested for different applications (anti-stain, self-cleaning, liquid transportation) and compared with model hydrophobic, hydrophilic, and superhydrophilic surfaces. The second activity mimicked the chemistry of stained glass, introducing basic concepts of redox reactions, chemistry of color, and plasmonic nanoparticles. Stained glass colors were copied through the deposition, on glass slides, of silica coatings colored by metal ions and nanoparticles. A silica sol was used as matrix to embed metal ions, which were reduced in situ by thermal treatment on a hot plate. The formation of metal nanoparticles by this procedure induces plasmonic colors in the glass coating, thus “mimicking” the ancient procedure of stained-glass fabrication. These works led to two publications on the Journal of Chemical Education.