Visible Light Photocatalytic Activity Of Silicon Nanoporous Array And Cu₂O/Si-NPA Nanocomposites

Recently,the environmental pollution and energy shortages are two major crises faced with people.Utilizing photocatalysis to degrade organic pollutants was considered to be an effective method to solve the above problems.With the development of society,the inappropriate treatment of industrial wastewater are worsening the water quality.And the pollutants have the characteristics of wide range of contaminants,complicated component,difficult to degrade,making the remediation far from satisfactory.So,how to technical y realize the low energy consumption of organic pollutants treatment in water at low cost is one of the important problems faced with people in water pollution treatment.In the past few years,utilizing photocatalysis to degrade the pollutants under the sunlight irradiation has been attracting more and more people's attention.What concerns people most is how to prepare the high efficiency photocatalysts with high stability and easy to recycle.In the previous studies,Titanium dioxide?³.2ev?has been extensively studied owing to its high catalytic activity,good chemical/thermal stability and resistance of corrosion,which was considered to be the most promising semiconductor photocatalytic material.However,the active holes needed for photocatalysis can only be generated under ultraviolet il umination,which severely limit the utilization of sunlight to only ⁵% of its total energy.Though doping/codoping,dye sensitization and surface modification can expend the usable region of the sunlight,the utilization region of the sunlight and the stability of the modified TiO2 are far from satisfactory.Meanwhile,there is also stability problem in modified TiO 2.Therefore,how to prepare high efficiency photocatalysts or nanocomposites photocatalysts is of great value.The crystal silicon?Si?has been widely applied in the information industry as the fundamental material of modern industry.Just from the band structure speaking,bulk silicon material is indirect bandgap semiconductor with the bandgap of 1.12 ev,which gives very low infrared luminous efficiency at room temperature.However,as the decrease of the feature size,the bandgap can be changed from indirect to direct/quasidirect one due to the quantum confinement effect,which can increase the efficiency of luminous,laying a physical foundation for its application in the field of optoelectronics.Meanwhile,Cu₂ O is a P-type semiconductor material with the direct bandgap of 2.17 eV,which can directly utilize the visible light accounting for 45% of the solar energy.Obviously,Cu₂ O can directly exploit the solar energy,which is likely to be applied in the optoelectronic fields.So,We may obtain a high efficiency photocatalyst if we combine the silicon nanomaterials with cuprous oxide with the optimized conditions.Based on the above analysis,a hierarchical structure,silicon nanoporous array?SiNPA?,was prepared by hydrothermal method and the photocatalytic activity was investigated by selecting the methyl orange solution as the targeted pollutants.Selecting Si-NPA as the functional substrate,nanocomposites of Cu₂O/Si-NPA,Cu₂O:Cu/Si-NPA?with Cu₂ O as the main component?,Cu:Cu₂O/Si-NPA?with Cu as the main component?and Cu/Si-NPA were prepared by an immersion-plating method,and their surface morphologies,microstructures and visible photocatalytic degradation performances to methyl orange were characterized.And we analyzed the effect of Cudoping in the Cu₂ O.The main results are as follows:???The photocatalytic properties of Si-NPA under the visible irradiation?1?Si-NPA samples exhibit a broad,high absorptivity which is up to ⁹6% in the range of 400-800 nm,which attributes to the change of the band structure causing by the silicon nanocrystals,the size distribution of silicon nanocrystals and the unique structure of and array structure of Si-NPA samples.Obviously,the excel ent broadspectrum absorption property of Si-NPA are beneficial to promote the yield of photogenerated carriers,which provides an important basis for obtaining a high efficiency photocatalyst under the visible irradiation.?2?The dependence between the degradation ratio and initial MO concentration using the fresh Si-NPA samples as photocatalysts and 10 mg/l,20 mg/l,40 mg/l of MO solution as target pollutants,the photocatalytic activities of Si-NPA were investigated.It was shown that the optimal degradation concentration was 20mg/l with the photodegradation efficiency of 97.7%.The relevant mechanism was studied and analyzed,which provided the guidance for the selection of solution concentration in the following experiments.?3?The photocatalytic activity of Si-NPA decrease with the aging time.We performed the experiments by taking the Si-NPA samples aged a day,a week,and a month as photocatalysts with the 20mg/l MO solution as the objective pollutants.It disclosed that the surface of silicon oxygen non-radiative recombination center formed with the aging time accelerates the capture and recombination of the photo-generated carriers.In addition,the hydrophobicity caused by the surface oxidation of Si-NPA is another possible reason for the decrease of the photocatalytic performance.?4?The recovery of photocatalytic activity of the aged Si-NPA samples.Aiming at the possible reasons,two approaches were proposed.One is that treating the aged samples with a certain concentration of HF solution for a suitable period of time to remove the surface oxidation of its surface;the other way is that treating the samples in ethanol before the photocatalytic experiments to improve the hydrophobicity of the surface.It was shown that there was difference between the above methods.?5?The recycle of Si-NPA samples.It disclosed that photo-degradation efficiency gradually decreases with the increase of the cycling cycles.Apart from the oxidation in the experiment,the gradual loss of silicon nanocrystals in Si-NPA may be another important reason,we found that the photocatalytic activity can be recovered after treating the samples in the original solution?Fe?NO3?3+ HF solution?for a proper time.???The controlled-preparation and photocatalytic activity of Cu₂O/Si-NPA StructuresThe preparation of Cu₂O/Si-NPA nanocomposites and Cu-doped Cu₂O/Si-NPA structure.The morphology,microstructure and chemical composition of semiconductor materials have an obvious effect on the photocatalytic activity.Utilizing silicon nanoporous pillar array?Si-NPA?as substrates,and through changing the preparing parameters,nanocomposites of Cu₂O/Si-NPA,Cu₂O: Cu/Si-NPA?with Cu₂ O as the main component?,Cu: Cu₂O/Si-NPA?with Cu as the main component?and Cu/Si-NPA were prepared by an immersion-plating method,and their surface morphologies,microstructures properties were characterized.The photocatalytic activity and mechanism of Cu₂O/Si-NPA nanocomposites.It was shown that Cu₂O: Cu/Si-NPA structure exhibited the highest photocatalytic activity,which is about 20% higher than Cu₂O/Si-NPA nanostructure.It disclosed that the Cudoping in Cu₂ O can promote the production of photo-generated carriers,reducing the recombination probability of carriers and enhancing the photocatalytic efficiency.Meanwhile,there was seldom photocatalytic activity in Cu:Cu₂O/Si-NPA and Cu/SiNPA nanocomposites,indicating that little photo-generated carriers was produced under the visible light irradiation,together with the recombination of photogenerated carriers from the inside to the surface of the material,leading to the reduction of the active sites.So,the photo-degradation efficiency was seriously reduced.The improvement of photo-degradation efficiency of the metal oxide semiconductors by homogeneous metal doping might be meaningful for the design and preparation of related materials.