Photonic manipulation and photothermal conversion

Micro/nanoscale photonic manipulation is gaining popularity to benefit the design of surfaces or configurations with spectral selectivity for the spectrally selective solar absorbers (SSAs)/thermal emitter and radiative cooling materials. It shows great promise of applications on thermophotovoltaics, concentrated solar thermal plants, solar thermoelectric generators, industrial solar heating, and passive/self-adaptive daytime radiative cooling. Photothermal conversion enabled by micro/nanomaterials can effectively capture light and efficiently convert light energy to heat, which can be utilized for diverse kinds of applications, such as photothermal therapy, solar electricity generation, clean water generation. 1. The near-infrared optical properties of silica (SiO2) thin film embedded with tungsten (W) nanoparticles at varying volume fractions are experimentally investigated. The samples are prepared by using the technique of magnetron sputtering. The formation and distribution of W nanoparticles are characterized using transmission electron microscopy, and the volume fraction of W nanoparticles is validated by Auger electron spectroscopy. Near and mid-infrared diffuse reflectance measurements are conducted using Fourier transform infrared spectroscopy. The samples exhibit wavelength selective optical response in the near-infrared region and are suitable for applications involving selective thermal emitters/absorbers. Measured reflectance data is utilized to estimate the effective dielectric function of the nano-composites. Calculated reflectance spectra in different samples are compared to the measured spectra using the experimentally measured dielectric function of these samples in the near-infrared region. Reflectance spectra after thermal annealing at different temperatures are compared to show how the thermal treatment affects the optical properties of samples. Optimized structures are proposed for thermal emitters and absorbers with different volume fractions of W nanoparticles. 2. Wavelength selective thermal devices have great applications in concentrating solar power systems, high-temperature thermoelectric systems, and solar thermophotovoltaics (STPVs). Lack of high-temperature stability and spectrally selective emissivity in different wavelength regions limit their efficiency. A one-dimensional HfO2/Al2O3-W nanocomposites/W/Al2O3/W multilayered photonic structure is proposed as a potential wavelength-selective thermal device, and theoretically investigate the emission properties of the proposed mie-resonance metamaterials from visible to mid-infrared region. HfO2 thin layer is introduced to serve as an anti-reflection coating film and the W layer acts as an IR reflection layer that enhances the absorptivity/emissivity in the visible and near-infrared region while reducing the mid-infrared emission simultaneously. Effects of geometric parameters are discussed, such as different radii and volume fractions of W nanoparticles, the thickness of Al2O3-W nanocomposites, and HfO2 thin film. The proposed thermal absorber and emitter exhibit nearly unity absorptance in both visible and near-infrared regions, while the emittance approaches zero in the mid-infrared region. The selective absorption/emission window is tunable by varying geometric parameters. The proposed solar thermal devices have great potentials in engineering applications such as STPVs and solar thermoelectric generators (STEG) due to the flexibility of geometric parameters and ease of fabrication. 3. Spectrally SSAs, which harvest heat from sunlight, are the key to concentrated solar thermal systems. An ideal SSA must have an absorptivity of unity in the solar irradiance wavelength region (0.3 um - 2.5 um), and its infrared thermal emissivity must be zero to depress spontaneous blackbody irradiation (2.5 um - 25 um). Current SSA designs which utilize photonic crystals, metamaterials, or cermets are either cost-inefficient due to the complexity of the required nanofabrication methods or have limited applicability due to poor thermal stability at high temperatures. We conceptually present blackbody-cavity solar absorber designs with nearly ideal spectrally selective properties, capable of being manufactured at scale. The theoretical analyses show that the unity solar absorptivity of the blackbody cavity and nearly zero infrared emissivity of the outer surface of SSAs allow for a stagnation temperature of 880°C under 10 suns. The performance surpasses state-of-the-art SSAs manufactured using nanofabrication methods. This design relies only on traditional fabrication methods, such as machining, casting, and polishing. This makes it suitable for large-scale industrial applications, and the "blackbody cavity" feature enables easy integration with existing concentrated solar thermal systems using the parabolic reflector and Fresnel lens as optical concentrators. 4. Spectrally SSAs are widely employed in solar thermal energy systems. Thermal radiative properties of metamaterials consisting of 1-D and 2-D grating-mie-metamaterials (W nanoparticles embedded in Al2O3) on top of multilayered refractory materials (W-Si3N4-W) are theoretically investigated as a promising SSA. The proposed metamaterial shows high absorptance from the ultraviolet to near-infrared lights, while exhibiting low emittance in the mid-infrared regime owing to mie-resonances, surface plasmon polaritons, and metal-dielectric-metal resonance. The optical properties of designed metamaterial solar absorbers are angular independence of up to 75° and polarization insensitive. The total absorptance of 1-D and 2-D grating-mie-metamaterials are 90.59% and 94.11%, respectively, while the total emittance is 2.89% and 3.2%, respectively. The photon-to-heat conversion efficiency is theoretically investigated under various operational temperatures and concentration factors. The thermal performance of grating-mie-metamaterials is greatly enhanced within a one-day cycle, and the stagnation temperature under different concentration factors manifests the potential feasibility in mid and high-temperature solar thermal engineering. 5. SSAs with high performance is the key to concentrated solar power systems. Optical metamaterials are emerging as a promising strategy to enhance selective photon absorption, however, the high-temperature stability (> 500°C) remains one of the main challenges for practical applications. In this study, a multilayered metamaterial system (Al2O3/W/SiO2/W) based on metal-insulator-metal Fabry-Pérot resonance effect has been demonstrated with high solar absorptance over 92%, low thermal emittance loss below 6% (100°C blackbody), and significant high-temperature thermal stability: it has been proved that the optical performance remains 94% after 1-h thermal annealing under ambient environment up to 500°C, and 94% after 96-h thermal cycle test at 400°C. Outdoor tests demonstrate that a peak temperature rise (193.5°C) can be achieved with unconcentrated solar irradiance and surface abrasion resistance test yields that SSAs have a robust resistance to abrasion attack for engineering applications. 6. Photon-to-cooling phenomenon relies on the atmospheric transparency window to dissipate heat from the earth into outer space, which is an energy-saving cooling technique. A highly effective aluminized polymethylpentene (PMP) thin-film thermal structure is theoretically and experimentally demonstrated. The emissivity of aluminized PMP thin films matches well with the atmospheric transparency window to minimize parasitic heat losses. This photon-to-cooling structure yields a temperature drop of 8.5 K in comparison to the ambient temperature and a corresponding radiative cooling power of 193 W m-2 during a one-day cycle. The easy-to-manufacture feature of an aluminized PMP thin film makes it a practically scalable radiative cooling method. 7. Traditional building materials such as wood and concrete cannot effectively regulate the heat flux of buildings. Compressor-based cooling systems are used to provide comfortable interior environments for humans, contributing significantly to global energy consumption. It has recently been demonstrated that sub-ambient passive daytime radiative cooling has been obtained by efficiently radiating thermal energy to the cold outer space through the atmospheric transparent window while reflecting most of the solar irradiance. A high-performance daytime radiative cooling material processed by hydraulic pressing melamine-formaldehyde (MF) particles and thermally annealing them into a cross-linked photonic cooling bulk as an efficient solar reflector and infrared thermal emitter. It reaches a sub-ambient stagnation temperature of 3.6°C under direct sun irradiance (750 W m-2), which is 12°C and 5°C below the concrete and the wood as control group temperatures, respectively. The two-step fabrication process is straightforward and can be easily scaled up for industrial manufacturing. The as-prepared MF cooling material shows highly desirable fire-retardant properties and is self-extinguishing, making it an excellent material for building safety. The material is durable and spectrally robust in harsh environments, such as long exposure to acidic and alkaline solutions. 8. Passive daytime radiative cooling (PDRC) cools an object down by simultaneously reflecting sunlight and thermally radiating heat to the cold outer space through the Earth's atmospheric window. However, for practical applications, current PDRC materials are facing unprecedented challenges such as complicated and expensive fabrication approaches and performance degradation arising from surface contamination. In this study, we develop scalable cellulose-fiber-based composites with excellent self-cleaning and self-cooling capabilities, through air-spraying ethanolic polytetrafluoroethylene (PTFE) microparticles suspensions embedded partially within the micro-sized pores of the cellulose fiber to form a dual-layer structure with PTFE particles atop the paper. The formed superhydrophobic PTFE coating not only protects the cellulose-fiber-based paper from water wetting and dust contamination for real-life applications but also reinforces its solar reflectivity by sunlight backscattering. It results in a sub-ambient cooling performance of 5°C under a solar irradiance of 834 W m-2 and a radiative cooling power of 104 W m-2 under a solar intensity of 671 W m-2. The self-cleaning surface of composites keeps its good cooling performance for outdoor applications and the recyclability of the composites extends its lifespan after one life cycle. Additionally, dyed cellulose-fiber-based paper can absorb appropriate visible wavelengths to display specific colors and effectively reflect near-infrared lights to reduce solar heating, which synchronously achieves effective radiative cooling and aesthetic varieties. 9. Subambient radiative cooling is an emerging passive cooling strategy that simultaneously reflects the incident solar irradiance to depress the heat gain and radiates heat from objects to enhance the heat loss without any electricity consumption from compressor-based air-conditioning. Although numerous efforts have been dedicated to developing materials, such as complicated photonic crystals and metamaterials or expensive polymer composites with both high solar reflectance and infrared emittance, the gap still exists between efficient radiative cooling performance and an affordable radiative cooling device. In this study, a facile, low-cost, and home-built approach to achieve efficient subambient radiative cooling, which employs commercially available materials of the polyethylene (PE) bubble wrap and aluminum foils, is reported for scalable industrial and domestic applications. The aluminum foils are infrared-reflective and sunlight-opaque, acting as a solar shield and infrared waveguide, which can both block the solar irradiation and guide infrared thermal radiation to the cold outer space. PE thin film is infrared-transparent with a high mid-infrared transmittance that allows mid-infrared thermal radiation to pass through. It is thermally insulating with a low thermal conductivity of 0.038 W m-1 K-1 after being fabricated into air-filled bubbles to minimize parasitic non-radiative heat transfer. An average subambient temperature reduction of 4.0°C has been achieved during the noontime in summer. These commercially available materials make this design a practical technique for people to realize an affordable and comfortable interior environment during the summer in a cost-effective manner without any professional constructions. The nature of low-cost, home-built, and easily integrated into buildings renders it attractive for everyone, especially for those in developing regions. 10. While solar power systems have offered a wide variety of electricity generation approaches including photovoltaics, solar thermal power systems, and solar thermoelectric generators, the ability to generate electricity at both the daytime and nighttime with no necessity of energy storage remains challenging. In this study, we propose and verify an environment-friendly, sustainable, and cost-effective strategy of harvesting solar energy by solar heating during the daytime and harnessing the coldness of the outer space through radiative cooling to produce electricity at night using a commercial thermoelectric module. It enables electricity generation for 24 h a day. We experimentally demonstrate a peak power density of 37 mW m-2 at night and a peak value of 723 mW m-2 during the daytime. A theoretical model that accurately predicts the performance of the device is developed and validated. The feature of 24-h electricity generation shows great potential energy applications of off-grid and battery-free lighting and sensing. 11. Interfacial solar steam generation is emerging as a promising technique for efficient desalination. Although increasing efforts have been dedicated, challenges exist for achieving a balance among a plethora of performance, e.g., rapid evaporation, durability, low-cost deployment, and salt rejection. In this study, we demonstrate that the carbonized manure can convert 98% of the sunlight into heat and the strong capillarity of porous carbon fibers networks, pumps sufficient water to evaporation interfaces. Salt diffusion within microchannels enables quick salt drainage to the bulk seawater to prevent salt accumulation. With these advantages, this biomass-derived evaporator is demonstrated to feature a high evaporation rate of 2.81 kg m-2 h-1 under 1 sun with broad robustness to acidity and alkalinity. These advantages together with facial deployment offer an approach for converting farm waste to energy with high efficiency and easy implementation, particularly well-suited for developing regions. 12. Interfacial solar desalination shows great promise to become an alternative technique for the freshwater generation. However, the unavoidable fouling undermines continuous evaporation because the accumulated salt crystals weaken sunlight absorption and salt drainage, especially in high-salinity brines. Therefore, developing a photothermal evaporator with excellent salt rejection capability is urgent for high-salinity solar desalination. In this study, a carbonized cattle manure-based photothermal evaporator with hierarchically bimodal pores is validated as an efficient desalinator for high-salinity brine (> 15 wt%). Taking advantage of the bimodally porous structure and interconnected microchannels formed by carbon fiber networks, it realizes rapid water transportation and quickly replenishes the surface-desalinated brine to prevent salt accumulation and enable stable freshwater generation. This carbonized manure evaporation device demonstrates a rapid evaporation rate of 2.25 kg m-2 h-1 under 1 sun irradiance (1 kW m-2) using a 15 wt% NaCl solution, as well as excellent long-term stability for the high-salinity desalination process. The nano-sized channels offer efficient nanoscale light trapping through multiple reflections and scatterations that occur within the channel walls. Furthermore, the abundance of raw materials provides a desirable and efficient approach for converting farm waste to energy, making it particularly well-suited for developing regions. 13. Interfacial evaporation is gaining popularity as a facile and effective method for harvesting solar energy and yielding freshwater from sewage and seawater. However, challenges exist for achieving a balance among a plethora of performance metrics, e.g., low cost, high photothermal efficiency, off-grid deployment, and negligible environmental impact. In this study, a hydrogel evaporator is prepared by combining agar, naturally abundant ocean biomass, with titanium nitride nanoparticles. This evaporator has vertically aligned water channels and is fabricated by an ice template-induced self-assembly method, enabling the formation of a biomimetic wood structure. The rapid water transport and salt drainage within its aligned channels, effective water activation in hydrogel molecular meshes, and efficient heat localization allow this hydrogel evaporator to achieve an evaporation rate of 5.15 kg m-2 h-1 under the irradiance of one sun (1 kW m-2). Moreover, this hydrogel evaporator is easy to be recycled without performance reduction to achieve an extended lifespan with the advantage of facile thermal recyclability after long-term utilization. The freeze-thawing fabrication of this hydrogel evaporator is feasible for scalable deployment. This work offers new possibilities for high-quality freshwater yields with cost-effective raw materials and deployable solar desalination systems for industrial implementations.--Author's abstract