Solution-based oxide films for clean energy applications

The reduction of the energy consumption is currently one of the most critical global challenges, which applies to an extensive number of fields. Heating, ventilation, and air conditioning (HVAC) systems account for up to 20% of the global energy consumed by developed countries. Several HVAC systems rely on condensation heat exchangers, which harness the liquid-vapor phase change for the thermal transfer. Considering the air conditioning (AC) systems, the challenge posed by the systemic cooling poverty underscores the necessity of exploring new approaches with respect to conventional AC systems. An alternative and/or complementary solution is offered by thermochromic smart windows, which exhibit a passive modulation of the solar irradiation without requiring active energy inputs for activation. In addition to HVAC systems, the shortage of drinkable water poses an additional challenge affecting the global energy consumption. According to the FAO reports, nearly one-sixth of the world’s entire population are daily plagued from severe water scarcity. Among the various approaches, from solar-driven desalination plants to distillation systems for contaminated water, a promising solution is offered by the vapor collection from humid air. As emerges from these reports, even slight improvements in the efficiency of heat exchangers, water collectors, and smart windows could have a powerful impact on reducing the massive global energy consumption. The optimization of the latter three applications has been the focus of this doctoral research. Several oxide coatings were prepared following a chemical solution deposition (CSD) method, which will be introduced in Chapter 1. In the first experimental part (Chapter 2), the functional properties of hybrid silica (SiO2) coatings were tuned and exploited to control the efficiency of heat exchangers and moisture collectors. In the second part (Chapters 3-6), vanadium dioxide (VO2) thin films were employed for the fabrication of thermochromic smart windows. The temperature-dependent nature of the optical and electrical properties of VO2 provides a passive means to modulate the solar irradiation without relying on active energy inputs. The numerous parameters influencing the CSD method were optimized in Chapter 4, from which homogeneous and thermochromic VO2 thin films were obtained. Concerning the latter material, a currently unsolved challenge involves the reduction of the crystallization temperature below 400°C. In Chapter 5, an unconventional nanosecond pulsed laser annealing method was exploited to remarkably reduce the crystallization temperature of VO2 thin films. Additional challenges regarding VO2-based smart windows pertain to two essential features that must be ensured for real-world application, namely the transmission of visible light and the resistance against weatherability. Both challenges were addressed in Chapter 6, though the combination of VO2 with titanium dioxide (TiO2) and hybrid SiO2 coatings in bilayer configurations. In the final Chapter 7, the efficiency of VO2 thin films towards H2 gas detection was considered. The latter is becoming increasingly important in the field of decarbonization and implementation of low-impact renewable energy. Different morphologies and degrees of crystallinity were considered by tuning the laser-annealing and conventional furnace-annealing parameters, and the latter impact on the H2 gas sensing efficiency was investigated.