Optimization Of Photoinduced Charge Separation And Transfer In Potassium Poly(Heptazine Imide) For Boosted Photocatalytic Hydrogen Evolution

Semiconductor photocatalysis is an extremely promising technology for solar energy utilization,the essence of which is to convert low-density solar energy into high-density chemical energy for storage through a series of reactions,and the development of efficient and inexpensive photocatalysts is the core task of this technology.Since 2009,melon based carbon nitride has been widely used as a photocatalyst with suitable energy band and high stability for photocatalytic H₂ evolution.However,the intrinsic structure of melon-based carbon nitride contains a large number of terminal amino groups,and the formation of hydrogen bonds between the amino groups severely inhibits the photocatalytic H₂ evolution activity.In recent years,potassium induced poly（heptazine imide）（abbreviated as KPHI）,as a crystalline carbon nitride material,has received wide attention in the field of photolytic H₂ evolution benefit from less in-plane hydrogen bonding and more favorable to photocatalytic reactions.However,the current research on KPHI still suffers from a series of problems such as difficult morphology optimization,low in-plane crystallinity,which are not conducive to photogenerated charge separation and transfer,resulting in low photocatalytic H₂ evolution activity.In view of this situation,the present study initially solved the above problems through morphology regulation,crystallinity regulation,electron storage performance regulation and photocatalytic reaction environment regulation,significantly optimized the performance of photogenerated charge separation and transfer,and enhanced the photocatalytic H₂evolution activity.The details are as follows:（1）Morphology regulation is an important strategy to enhance the photocatalytic H₂ evolution activity.However,the current synthesis process of KPHI is complicated and morphology optimization is difficult.In this study,we developed a vapor deposition process to synthesize KPHI micron box,achieving KPHI synthesis and morphology optimization in one step,which promoted the separation and transfer of photogenerated charges and enhanced the photocatalytic H₂evolution activity.The result shows that the size of KPHI micron box is about 5-20μm,and the inner wall size of KPHI micron box is smaller than that of current bulk KPHI synthesized by direct mixing and calcinating.Based on the above optimization,the photocatalytic H₂ evolution activity of KPHI micron box reaches to 1948μmol h^-1 under visible light irradiation at wavelengths greater than 420 nm,which is about 2.9 times higher than that of bulk KPHI(664μmol h^-1).The main reason is that the KPHI micron box has a more regular shape and smaller inner wall size compared with the bulk sample,which makes the photogenerated electrons easier to transfer to the surface to participate in the photocatalytic H₂ evolution reaction and reduces the charge carrier combination.Additionally,the micron box structure is more favorable for light absorption.These synergistically increase the photocatalytic H₂ evolution activity of KPHI micron box.（2）Although the micron box morphology effectively enhances the photocatalytic H₂ evolution activity,the size is still at the micrometer level.Additionally,the current morphology nanomodulation process is complicated and energy consumption is high.To solve this problem,this study developed a steam-assisted method to synthesize KPHI nanoparticles,which achieved nanostructured KPHI in one step,significantly promoted the photogenerated charge separation and transfer,and enhanced the photocatalytic H₂ evolution activity.The result shows that the size of KPHI nanoparticles is significantly reduced compared with the directly calcined bulk KPHI,and the specific surface area increases from 8.6 to 33.8 m²/g.The H₂ evolution test exhibts that the photocatalytic H₂ evolution activity of KPHI nanoparticles reaches to 2161μmol h^-1 under visible light irradiation at wavelengths greater than 420 nm,which is about 3.4 times higher than that of the bulk KPHI(618μmol h^-1).The mechanism analysis indicates that the increased photocatalytic H₂evolution activity of KPHI nanoparticles is mainly due to the smaller size of nanoparticles,which enhances the photogenerated charge separation and transfer towards the material surface,reducing the bulk phase complexation.Furthermore,KPHI nanoparticles have larger specific surface area,which can provide more active sites for H₂ evolution reaction.In this study,the KPHI synthesis and naostructured morphology optimization was achieved in one step,which solved the current problems of complication and high energy consumption in KPHI morphology nanomodulation.（3）Compared with morphology,crystallinity has a greater influence on the charge separation and transfer of KPHI.However,the deamination degree of KPHI synthesized by existing methods is insufficient,resulting in low crystallinity,and this issue is especially serious when prepared in large quantities.To address this problem,this study proposed an alkali-assisted strategy to achieve deep deamidation in synthesis of KPHI materials with high in-plane crystallinity,which promoted the separation and transfer of photogenerated charges and enhanced the photocatalytic H₂ evolution activity.The result shows that the in-plane crystallinity of the highly crystalline KPHI is greatly optimized and the performance of photogenerated charge separation and transfer is significantly enhanced compared with that of the low-crystalline KPHI.The photocatalytic H₂ evolution test exhibits that the average H₂ evolution activity of highly crystalline KPHI reaches to 2223μmol h^-1 under visible light irradiation at wavelengths greater than 420 nm,which is approximately 18 times higher than that of low-crystalline KPHI(123.5μmol h^-1).Mechanism analysis and theoretical simulations indicate that the enhanced photocatalytic H₂ evolution activity is mainly attributed to the in-plane structure and stronger structural conjugation of high-crystalline KPHI,which is conducive to the separation of photogenerated charges in the material bulk phase,especially the transfer to the surface.In this study,the crystallinity of KPHI was effectively enhanced by using the alkali-assisted strategy,which solved the problem of incomplete deamidation and low crystallinity in the current studies.（4）Crystallinity optimization can mainly enhance charge transfer to the material surface,but the intrinsic key to the photogenerated charge separation of KPHI is its unique electron storage effect.Unfortunately,the relevant influence law is not yet clear.To solve this issue,this study quantitatively regulated the electron storage capacity by adjusting the K⁺content in KPHI,revealed the relationship between the electron storage performance and charge separation of KPHI,and accordingly optimized the charge separation and transport of KPHI to enhance the photolytic hydrogen precipitation activity.The result shows that the photogenerated electron storage capacity increases and then decreases as the K⁺content reduces,and the electron storage capacity reaches 198μmol g^-1 when the K⁺content was 2.13%.In contrast,the photogenerated charge separation performance of KPHI gradually decreases as the K⁺content reduces.This result indicates that the K⁺content is more determinant of the charge separation and transfer performance of KPHI than the electron storage,and a higher K⁺is more favorable for photogenerated charge separation and transport.Based on this conclusion,this work further regulated the K⁺content of KPHI,and synthesized KPHI with high K⁺content and tested its photocatalytic H₂ evolution activity.The results exhibit that the photocatalytic H₂ evolution activity of KPHI under visible light irradiation reaches to 2467μmol h^-1 when the K⁺content is 10.56%,which is 6.48 times higher than that of the original KPHI(380.5μmol h^-1).The increase of photocatalytic H₂ evolution activity is mainly attributed to that the higher K⁺content promoted the separation and transfer of photogenerated charges.This study controllably regulated the electron storage capacity of KPHI,and revealed the correlation between electron storage performance and charge separation and transfer.（5）The regulation of charge separation and transport,such as morphology,crystallinity and electron storage properties,is mostly focused on the KPHI material itself,but the regulation of the hydrogen evolution reaction environment is currently lacking.Therefore,this study achieved the internal-external co-optimization of photogenerated charge separation and transfer by regulating the H₂ evolution reaction environment of KPHI,and enhanced the photocatalytic H₂ evolution activity.The result shows that adding cations to the KPHI reaction system can promote the photogenerated charge separation in the bulk phase,and different types of cations have different effects on the promotion of photogenerated charge separation and transfer,among which alkali metal K⁺shows the best effect.The photogenerated charge separation and transfer performance of KPHI show a trend of increasing and then decreasing with the increase of K⁺concentration.The H₂ evolution test exhibits that the photocatalytic H₂evolution activity of KPHI with different anionic potassium salts is above 2400μmol h^-1,and it increases and then stabilizes with increasing K⁺concentration in the solution.Furthermore,the highest photocatalytic H₂ evolution activity of KPHI reaches to 2620μmol h^-1 when the K⁺concentration is 0.2 M,which is about 4.28 times higher than that in pure water(612μmol h^-1).The K⁺adsorbed on the surface of KPHI is similar to the electron capture site,which has an attractive effect on the material bulk phase photogenerated electrons and can promote it transfer from bulk phase to material surface,and enhance the separation and transfer efficiency of photogenerated charges.In this study,the separation and transfer efficiency of KPHI was improved by regulating the H₂evolution reaction environment,which can provide a reference for the optimization of the H₂ evolution activity of other photocatalysis materials.