Investigation On Tunability Of Solar Absorption Of Core-Shell Ag@TiO₂ Nanoparticles And Their Applications In Photothermal/Voltaic Conversions

As the energy source of the earth,solar energy will become the leading energy source to support human activities in the near future due to its abundant reserve,flexible exploitation and pollution-free transportation processes.The utilization efficiency of solar energy is still dissatisfactory at present.Therefore,how to achieve the high-efficiency collection and conversion of solar energy using the novel devices has gradually become a bottleneck for the development of solar energy technologies.The unique surface plasmon resonance effect of nano-scale metals have attracted much attention in the area of near-field optics.The absorption bandwidths of regular plasmon-generating nanoparticles,however,are often limited to a narrow waveband,and cannot be well matched with the spectral requirements in the specific applications.Therefore,it is necessary to obtain plasmon-generating nanoparticles with special structures through reasonable structural design,which enables to realize the broadband absorption of plasmas together with continuously tunable absorption peaks.In this paper,the absorption spectra of Ag@TiO₂ nanoparticles were tuned and controlled.After that,performance of the devices in the application areas such as photothermal conversion,photovoltaic conversion,and photovoltaic/thermal conversion basing on surface plasmon resonance and/or spectrally selective absorption of core-shell nanoparticles were investigated.Firstly,Ag@TiO₂ core-shell nanoparticles were successfully synthesized using a solvothermal synthesis method under normal temperature and low toxicity conditions.We prepared the nanoparticles by rapidly reducing AgNO3 and simultaneously controlling the slow hydrolysis process of titanium butoxide using N,N-dimethylformamide.The broadband absorption spectrum and continuously tunable absorption peak positions of nanoparticles were achieved by changing the molar ratio of precursors,inducer concentration,reaction temperature,and reaction time.The growth mechanism of the nanoparticles was discovered based on the morphology change and optical absorption performance of the synthesized particles.The design principles for controlling the optical properties of core-shell Ag@TiO₂ nanoparticles were obtained.Finally,Ag@TiO₂ nanoparticles with sunlight-matched absorption spectrum were successfully prepared.For the application of the plasmon-generating nanoparticles in photothermal conversion,the Ag@TiO₂ nanoparticles were dipersed in water to form the stable nanofluids.Moreover,the simultaneous water evaporation and heat collection performance of nanofluid-based evaporators driven by solar heating were studied.The effects of both nanoparticle concentration and optical concentration were considered.It was found that the evaporation efficiency reaches 53.9% when the particle concentration is only 200 ppm.The enhanced evaporation of water is mainly resulted from the strong absorptivity of particles,rather than the increase of bulk water temperature.The energy conversion and transportation mechanisms in nanofluid evaporator were also analyzed under the assumption that the nanofluid serves as a participating medium.Additionally,the nanoparticles were loaded on flexible membranes by vacuum filtration or electrostatic adsorption to obtain nanoparticles loaded membranes.The steam generation performance of the nanoparticle membranes driven by interficial solar heating was studied.The effect of the areal density of nanoparticles on evaporation performance was analyzed,and the optimal areal density was obtained.Moreover,the effect of optical concentration on evaporation performance was studied under the optimal areal density.Finally,both the energy-transmitted rule and the mechanism of the photothermal conversion enhancements,at the interface of solar-heated nanoparticle membrabe as well as under the optimal areal density,were analyzed.For the application of the plasmon-generating nanoparticles in photovoltaic conversion,we developed a new stratage to improve the light harvesting and reduce the charge recombination,which employed a bilayer photoanode film consisting of a core–shell plasmon-generating nanoparticles mixed absorbing layer and a mesoporous TiO₂ particles based scattering layer.We mixed a small amount of core-shell structured Ag@TiO₂ nanoparticles with commercial available P25 particles to analyzed the energy transformation and electron transfer dynamics in plasmonic nanoparticle-doped devices.The concentration of loaded nanoparticles was optimized.Additionally,mesoporous TiO₂ was employed to generate a bilayer photoanode to further improve the performance of the cells.The optimized bilayer film device has higher short circuit current（19.22 mA/cm2）,higher open circuit voltage（0.76 V）and appreciable filling factor（68%）.The photovoltaic coversion efficiency of the optimized device receaches 10.02%,which is about 13% higher than that of monolayer photoanode film based device.For the application of the plasmon-generating nanoparticles in the combined heat and power generation system,a theoretical model to evaluate the performance of solar beam splitters basing on the optical as well as the thermal properties of Ag@TiO₂ nanofluids was established.To improve the work function of nanofluid based beam splitters,the effect of nanoparticle concentration on both the photothermal and photovoltaic performances was considered.Tunable nanoparticleconcentrations were adopted to obtain high-efficiency solar energy utilization.In this way,differential distribution between electric and thermal energy demands can be realized to match the variable requirements of electric energy and thermal energy across geographical differences.It was found that the increased thermal efficiency resulted from the strong absorption of Ag@TiO₂ nanoparticles is more than the required compensates for the electrical efficiency drop.The merit functions of 200 ppm ethylene glycol aqueous solution-based and water-based nanofluid beam splitters are 2.05 and 1.45 times of that independently operated solar cells,respectively.