| In recent years,plenty of efforts have been paid to the access to clean and renewable energy sources to reduce the pressure of energy scarcity and environmental pollution.Compared with wind energy,water energy,nuclear energy,and other new energy sources,solar energy has a series of advantages such as abundant resources,clean and safe,no pollution,on-site development and utilization,and small geographical restrictions.Therefore,solar energy is considered to be the most promising renewable energy source.Governments all over the world have included it in the important content of national development strategy planning,and introduced various policies to support the development and application of solar energy technology.Currently,the use of solar energy technology is mainly concentrated in three major strategies:photothermal conversion,photovoltaic conversion,and photo-chemical energy conversion.The researches and applications of solar energy in photothermal and photovoltaic have been extensive,and the field of photo-chemical energy conversion has recently become a research hotspot.Photo-chemical energy conversion is a process of converting solar energy into chemical energy after absorbing solar radiation.This field currently focuses on hydrogen production by photocatalytic water splitting,photo-chemical reduction of CO2,and photocatalytic chemical synthesis.Hydrogen has an extremely high energy density while being clean and free of contamination.Hydrogen is not only an ideal secondary energy source,but also used as an industrial raw material in a wide range of applications in petrochemical,electronic metallurgy,food processing,and other fields.At present,the hydrogen production technology using hydrocarbons,coal,methanol,etc.as raw materials are relatively mature in the industry,but this method of consuming non-renewable energy is difficult to sustain,and it is necessary to deal with emission reduction problems.As one of the raw materials for hydrogen production,water has a huge reserve on the earth,and it is clean and pollution-free,and has strong competitiveness in environmental protection and sustainability.The electrolyzed water hydrogen production industry is mature,safe and efficient,but it is expensive because it requires a large amount of electric energy during the preparation process.Photocatalytic water splitting to produce hydrogen,using abundant water as raw material,can directly convert solar energy into chemical energy,while cleaning and environmental protection,so it is subject to great attention and extensive research.The main factors affecting the photocatalytic water splitting of hydrogen production are as follows:(1)The light absorption properties of semiconductor materials.Only more solar energy absorbed can it provide more energy for the catalytic reaction.At present,the response range of photocatalytic materials to the spectrum has been expanded from ultraviolet light to visible light and near-infrared light.Specifically,the spectral response range of the material is broadened by methods such as doping,band engineering,and hetreostructure construction.The goal is to achieve a full spectrum response to light,and there have been many reports.(2)Photogenerated carriers separation property.Electron-hole pairs generated by semiconductor excitation are easily recombined,which limits the hydrogen production activity of the photocatalysts.The effective separation of photogenerated carriers is realized mainly by the construction of heterostructures based on p-n junctions,Schottky junctions,and band structure matching.Or use the external field to control the potential difference of the photo-generated carriers to control the transport direction.(3)Reaction rate of photogenerated electrons on H+ to H2.If there are not enough suitable hydrogen evolution reaction sites on the surface of the catalyst,the photogenerated electrons cannot be quickly consumed,and the electron-hole pairs are easily recombined on the surface of the photocatalyst,resulting in a lower catalytic reaction rate.The widely accepted view by researchers is that by surface modification,or by the construction of a co-catalyst,the semiconductor itself is only used as a medium for photo-electric energy conversion,and the supported co-catalyst functions to catalyze hydrogen production and oxygen evolution,thereby realizing the effective use of photogenerated carriers.This work focuses on enhancing the separation of photogenerated carriers and improving the catalytic activity.Gradually,the study begins from the structural regulation of the individual material to increase the catalytic active site,then to composite materials design to enhance photocarrier transport and to improve the utilization of carrier catalytic reaction.Finally,on the basis of doping electronic structure regulation,a small external electric field is used to further improve the separation rate and utilization rate of photogenerated carriers.Therefore,in order to achieve the design and preparation of high-efficiency water-splitting hydrogen-producing photocatalysts,this work aims to increase the catalytic active sites of materials through element doping and structural regulation;through heterostructure construction and co-catalyst,to realize efficient separation of photogenerated carriers and efficient catalytic reaction;directly regulates the transport of photogenerated carriers by means of external electric field,and utilizes element doping to form a highly catalytic active site distribution.The main research work are as follows:(1)For the photocatalytic activity regulation of individual photocatalyst,aiming at the requirements of traditional TiO2 photocatalytic materials in catalytic performance and composite structure regulation,the titanic acid(H2Ti3O7)nanobelts were used as template and titanium source,and the hydrolysis-corrosion-hydrolysis reaction process was carried out by simple self-template hydrothermal method.A multi-structure of TiO2 containing a high concentration of Ti3+ was obtained.The multi-structure of TiO2 nanocrystals is epitaxially grown on the surface of the H2Ti3O7 nanobelt template,so that the anatase phase TiO2 is directionally aggregated.This structure may facilitate the transfer of carriers along the length of the TiO2 nanocrystals and provide more active sites,while meeting the requirements of the solid/liquid separation based on the powder catalyst in practical applications.On the other hand,the presence of fluoride ions in the solution makes the corrosion process proceed rapidly,and at the same time,doping in the crystallization process of TiO2 induces the formation of Ti3+ defects.Especially the upward shift of the conduction band,giving the material a high catalytic water splitting for hydrogen production.In addition,this material can be prepared in batches.Therefore,the photocatalytic integrated water treatment equipment was designed and prototyped,and the preliminary exploration of sewage treatment was carried out,which showed the application prospect of considerable industrial wastewater treatment.(2)For the composite materials study,the typical visible light catalytic material CdS was chosen as the model photocatalyst,and the regulation of heterostructure construction and active site regulation on the photocatalytic water splitting for hydrogen production was studied.It is necessary to reduce the photogenerated electron-hole recombination rate and increase the catalytic reaction sites to achieve efficient transport and catalytic reactions of photogenerated carriers.In this work,CdS/Ni(OH)2 composites with different Ni(OH)2 loadings were prepared.The photocatalytic hydrogen production performance was studied.It was found that the photocatalytic properties of the composites increased with the increase of Ni(OH)2 loading.The hydrogen production performance first increases and then decreases and tends to be stable.The optimal value is 23.1%molar ratio(CdS:Ni(OH)2=10:3).Since Ni(OH)2 acts as a heterostructure to enhance photo-generated carrier separation,some photogenerated electrons will be enriched in Ni2+ due to transport limitations,and then Ni2+ in situ photoreduction will form Ni element.With the increase of Ni elemental mass,the photocatalytic hydrogen production performance is improved,and the hydrogen production performance is not significantly affected after the concentration is more than 8%,indicating that these Ni elements play an important role in the improvement of CdS photocatalytic hydrogen production performance.Compared with CdS/Ni(OH)2(23.1%)composites,the hydrogen production performance of CdS photocatalytic materials with Ni alone is much lower than that of CdS/Ni(OH)2(23.1%)composites,indicating the synergistic effect of Ni(OH)2 and Ni plays an important role in the improvement of CdS photocatalytic hydrogen production performance.In general,high-efficiency photocatalyst of photocatalytic water splitting for hydrogen production were constructed from the aspects of enhancing photo-generated carrier separation and increasing the activity of photogenerated carriers that have been transported to the catalyst surface to participate in the hydrogen production reaction.(3)For the electric enhanced photocatalysis,we chose the hematite(?-Fe2O3)as the target material.According to the transport direction of charged particles adjustion in external electric field and the mechanism of the HER and OER processes,the surface of the titanium sheet is hydrothermally deposited by in-situ FeOOH,calcined and phosphorus doped to obtain ?-Fe2O3 nanoarrays.The photo-electrochemical water splitting was studied.The results show that phosphorus doping can change the energy band structure of ?-Fe2O3,increase the electron density,and make VB move in the positive direction.It is beneficial to increase the photo-charge density and hole transport,thereby improving the OER performance.At the same time,the improvement of OER performance is beneficial to the catalytic reaction process of total water splitting and hydrogen production.On the Pt counter electrode,photogenerated electrons are the main factor affecting the performance of HER.In the PEC process,the high hydrogen production rate and the higher Faraday efficiency under bias voltage show the efficient photo-generated carrier separation and transport efficiency under the electric field,indicating that the electro-assisted photocatalysis is based on high-efficiency carrier separation.The enhanced photocatalytic performance achieved implies that this Z-scheme system structure has high application potential in the production of renewable energy. |
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