Sunlight Absorption Properties And Control Method Of Nanoparticles And Their Suspensions

With the rapid development of technology as well as the growth of world population, the dependence on energy is increasing. Additionally, the traditional energies will pollute the environment in combustion process. Therefore, it is urgent to develop new and renewable energies. Solar radiation is the main source of energy on earth, which provides an effective approach to ease the energy problem. The solar energy can be converted into electricity, thermal energy and chemical energy. To improve the conversion efficiency, light absorption has to be enhanced, which is the primary step in solar energy conversion. For different conversion forms, the effective absorption wave band is also different. Therefore, it needs to tune the absorption spectrum effectively to achieve selectively enhancement of light absorption. Based on solar energy utilization, nanostructures as research object (including single nanoparticle and their suspensions), the studies on tuning the absorption spectra of nanostructures are performed as well as the performance test. The main contents are listed as follows.1. Theories on simulating optical properties of nanostructuresFor the physical process of photo-thermal and photo-chemistry conversion, the simulation models of nanostructures are established based on electromagnetic theory. The interactions between light and nanostructures are described by Maxwell’s equations. The physical problems are modeled by Mie theory, discrete dipole approximation (DDA) and finite difference time domain (FDTD) method.2. Optical properties of nanoparticle with single componentThe band-gap properties of semiconductors are analyzed to obtain the control objective. Based on nanostructure with single component, the theoretical model is established to simulate the optical properties. The effects of material, size, shape, aspect ratio and surrounding medium on absorption properties are discussed thoroughly. According to the simulation results, the control mechanism of spectral absorption characteristics can be revealed. Optical absorption is dominant when the particle size is small. As the size increases, absorption is gradually weakened, while scattering is enhanced. The light absorption is enhanced as the particle symmetry decreases. An increase in refractive index of surrounding can lead to the resonance peaks shift to longer wavelength.3. Optical properties of plasmonic composite nanoparticle and optimizationThe adjustable range of spectrum is limited for nanostructure with single component. To solve this problem, a core/shell composite nanostructure is designed. Utilizing the localized surface plasmon resonance (LSPR) effect excited on metal surface, the spectral characteristics can be tuned successively. The theoretical model for core/shell nanostructure composing of metal and semiconductor is established. Spectral modulation and control process can be revealed by analyzing the effects of core material, shell material, core/shell relative thickness, and stacking order of core and shell materials. The absorption properties of3-layer nanoshell composing of metal/dielectric/semiconductor are also studied. The structure is optimized by discussing the effects of relative sizes on overall absorption. The electric field around the composite nanostructure is simulated. And the modulation and control mechanism by LSPR effect is explained. The resonance peak can be tuned in a wide range of wavelength by controlling the thickness of each layer. The spectrum of composite nanostructure with three layers has a higher adjustability.4. Optical properties of plasmonic nanoparticle suspensionsBased on FDTD method, the models for Al/CdS nanofluid used in photo-chemistry area and TiO2/Ag nanofluid used in photo-thermal area are established respectively to simulate the propagation of electromagnetic wave within nanofluid. The effects of relative sizes of core and shell layers and interactions among nanoparticles on optical absorption are discussed in detail. The spectrum characteristics can be tuned selectively by controlling particle size and concentration. Reducing the particle size and increasing the concentration of the suspension system can obtain an enhanced absorption of light.5. Performance test of plasmonic nanofluidThe plasmonic nanoparticles are synthesized by photo-induced reduction method. By dispersing them in water, the plasmonic nanofluid can be prepared. The photo-thermal performance of nanofluid is examined with direct exposure to sunlight. Omega10K3MCD1thermistors are used as temperature sensors to measure the temperatures within nanofluid. The resistances are collected by Keithly2700multifunctional data acquisition. Hoge-3calibration equation is used to calibrate the measured data. The photo-thermal performances of different nanofluids are studied under the same solar irradiance, and the main causes for the discrepancies are analyzed. The effect of concentration on photo-thermal performance is also discussed. TiO2/Ag plasmonic nanofluid exhibits a higher temperature compared with that of TiO2common nanofluid.