Preparation Of Silicon-Based Composite Materials By Recycling Retired Photovoltaic Silicon Wafers And Their Photocatalytic Activity

In light of the global push towards carbon neutrality,the photovoltaic（PV）industry has garnered significant attention as a clean energy source worldwide.As PV industry rapidly expands,the issue of recycling retired PV panels has gained prominence.Efficiently recycling and utilizing these panels has become a crucial topic within the realm of environmental science and engineering.During the recycling process of photovoltaic modules,the separation and analysis of multi-component recycled materials are pivotal in achieving optimal resource utilization.Specifically,when it comes to recycling crystalline silicon wafers from retired photovoltaic panels,the primary focus is on separating the polyethylene-polyvinyl acetate copolymer（Ethylene Vinyl Acetate,EVA）adhesive to enable resource utilization.Given that silicon wafers are the essential components of photovoltaic modules,their recycling holds particular significance.The removal of silver and other metal impurities from silicon wafers is a key step in recycling high-purity silicon wafers.This high-purity silicon can then be utilized to produce high-performance semiconductor materials like nano-silicon or nano-silicon carbide,which have demonstrated efficacy as photocatalysts for degrading organic matter.Leveraging high-purity silicon from retired photovoltaic modules for the production of advanced semiconductor materials represents a promising avenue for achieving resource recycling.Preparing heterostructure materials by combining two or more materials with different properties is essential for achieving material functionalization.Selecting high-quality active materials,such as titanium dioxide（Ti O₂）or silver orthophosphate（Ag₃PO₄）,and other modified versions,is crucial for enhancing photocatalytic efficiency.However,establishing a stable and effective interaction between active materials and silicon or silicon carbide substrates remains a challenge in current research.The stability of heterojunction composites,efficient separation of photogenerated carriers,and controllability of photocatalytic activity are key issues that require urgent attention.The stability of heterojunction composites,efficient separation of photogenerated carriers,and controllability of photocatalytic activity are key issues that require urgent attention.This paper focuses on efficiently recycling silicon materials from decommissioned PV modules and converting them into high-performance photocatalysts.The study investigated the EVA thermal separation mechanism,green leach for silver,and impurity removal process.High-purity silicon wafers are obtained through mechanical ball milling and carbothermal reduction.The method to prepare nano-silicon carbide wires and explore the photocatalytic mechanism of recycling high-purity silicon and preparing nano-silicon carbide is discussed.Additionally,the paper discusses the use of hydrothermal and chemical co-precipitation methods to load Ti O₂and Ag₃PO₄on nano-silicon and nano-silicon carbide for controllably preparing silicon-based heterojunction composite materials.Various characterization methods are used to analyze the micromorphology,crystal structure,and photoelectric properties of the composite photocatalytic materials,elucidating the enhancement mechanism of photodegradation performance of organic pollutants by silicon-based composite materials.The research aims to achieve direct regeneration and resource utilization of silicon wafers from decommissioned photovoltaic modules,providing new solutions for environmental governance and promoting sustainable development.The main research results are summarized as follows:（A）Study on the two-stage pyrolysis separation mechanism of decommissioned photovoltaic panels,the recovery of silicon wafers for silver extraction and impurity removal,as well as the photocatalytic mechanism of silicon and silicon carbide.A two-stage step-by-temperature pyrolysis experiment was conducted,with thermogravimetric（TG-DTG）analysis performed on the EVA in decommissioned photovoltaic panels.The study combined experimental data and theoretical calculations to investigate the pyrolysis kinetics and mechanism.The use of methylsulfonic acid and hydrogen peroxide leaching systems instead of strong oxidizing acid leaching for silver impurity removal was discussed,along with the effects of different reaction conditions on silver leaching efficiency.Results indicated that increasing the heating rate shifted the thermal weight loss curve of EVA to higher temperatures,with a heating rate of 20℃·min^-1requiring less energy for thermal decomposition.Pyrolysis kinetic parameters were calculated using the Kissinger method,and optimal process conditions for silver leaching in the methylsulfonic acid-hydrogen peroxide system were identified.The concentration of methylsulfonic acid was found to have the most significant impact on the silver leaching rate,followed by reaction time,reaction temperature,and hydrogen peroxide concentration.Furthermore,impurities were removed from the recovered silicon to obtain high-purity silicon powder.Nano-silicon carbide was synthesized by adjusting different temperatures and carbon-to-silicon ratios of recycled high-silicon material.Results showed that r-Si C with a carbon-to-silicon ratio of 1:1 and prepared at 1500°C exhibited a more uniform structure and crystal form.Initial investigations into the photocatalytic mechanism revealed that both r-Si and r-Si C demonstrated some photoelectric transfer efficiency and degradation ability,although further enhancements are needed to improve their photocatalytic activity.（B）Study on the performance and mechanism of a silver phosphate/titanium dioxide/photovoltaic silicon composite photocatalyst（APO/TIO/r-Si）directly prepared from silicon wafers（r-Si）recycled from decommissioned photovoltaic modules as the substrate.The high-purity silicon wafers were recycled to remove impurities and then ball-milled to obtain recycled silicon powder（r-Si）.Ternary heterojunction composite materials of Ag₃PO₄/Ti O₂/r-Si（APO/TIO/r-Si）were prepared using a hydrothermal method and co-precipitation method.The photocatalytic performance of Ag₃PO₄doped materials with different concentrations was investigated using the Rhodamine B（Rh B）degradation rate as the performance index.The 40%APO/TIO/r-Si heterojunction exhibited excellent photodegradation activity under visible light irradiation,achieving a photocatalytic degradation efficiency of 93.0%within 30 minutes and a removal rate of 82.8%after5 cycles.Studies identified superoxide radicals（^·O₂^-）and hydroxyl radicals（^·OH）as the main factors in the photodecomposition of Rh B.High-performance liquid chromatography-mass spectrometry（HPLC-MS）analysis revealed that N-deethylation was the main conversion path for Rh B dye.The study suggests that the composite material has a direct Z-type heterojunction electron transfer pathway with r-Si serving as the charge transfer center.The formation of the heterojunction not only effectively reduces interface resistance but also enhances the separation of photocarriers,thereby improving photooxidation capability and enhancing the stability of the composite system.（C）Study focuses on the preparation and performance evaluation of a composite photocatalyst（APO/TIO/r-Si C）derived from regenerated silicon wafers（r-Si）obtained from decommissioned photovoltaic modules.High-purity silicon（r-Si）sourced from retired photovoltaic panels was used to create silicon carbide nanowires（r-Si C）through mechanical ball milling and carbothermal reduction methods.Subsequently,these nanowires were modified using hydrothermal synthesis and co-precipitation techniques to produce the APO/TIO/r-Si C composite material.Experimental findings indicate that the photocatalytic efficiency of the r-Si C nanowires exceeds that of commercial Si C by 2.5 times.Immobilizing Ti O₂and Ag₃PO₄on the r-Si C nanowires enhances the specific surface area of the composite,facilitating exposure of active sites,promoting interfacial electron transfer,and enhancing the generation of radicals such as^·O₂^-.The composite exhibited exceptional degradation rates,with over 90%degradation of 6 organic dyes achieved within 15minutes and 87.9%degradation of tetracycline（TC）.The material maintained high degradation efficiency even after 4 cycles,highlighting the stability and effectiveness of the ternary photocatalyst,which forms Z-type and II-type double heterojunctions.