氨氛围热处理g-C3N4控制N空位浓度提高光催化制氢性能

热处理氧化石墨相氮化碳(g-C3N4)材料产生氮缺陷、提升其光催化制氢性能的研究备受关注,但其N空位浓度高且不可控、一定程度破坏g-C3N4晶体结构,降低g-C3N4的结晶度,导致光生电子-空穴对复合率高,致使其光催化制氢效率较低。基于上述问题,本研究以二氰二胺为前驱体制备了g-C3N4,与不同含量的尿素混合,在空气中加热快速热处理,通过X-射线衍射仪(XRD)、扫描电子显微镜(SEM)等测试手段,对其物相组成、微观形貌、光学吸收等进行了表征,在可见光条件下对样品进行了光催化制氢性能测试,研究了尿素的加入对热处理后g-C3N4材料的N空位浓度、结晶度及光催化制氢性能的影响。研究表明,尿素的加入降低了N空位的浓度,且提升了其结晶度。在优化的尿素添加量下,g-C3N4的可见光光催化制氢速率为6.5μmol·h^-1,是没有添加尿素处理的样品的3倍。该研究结果表明,利用尿素原位分解产生的NH3,可以抑制g-C3N4热处理过程中氮原子的氧化程度、实现调控N空位浓度,同时提高了结晶度,最终提升了其光催化制氢性能。

Engineering graphitic carbon nitride with expanded interlayer distance for boosting photocatalytic hydrogen evolution

Regulating interlayer distance is a crucial factor in the development of two‐dimensional(2D)nanomaterials.A 2D metal‐free photocatalyst,such as graphitic carbon nitride(g‐C3N4),exhibits morphology‐and microstructure‐dependent photocatalytic activity.Herein,we report a straightforward and facile route for the preparation of unique lamellar g‐C3N4,by co‐firing melamine and ammonium chloride via microwave‐assisted heating.Through the decomposition of NH4Cl,the evaporation of NH3 gas can effectively overcome van der Waals forces,expanding the interlayer distance of g‐C3N4,thereby creating a lamellar structure consisting of nanosheets.Compared with bulk g‐C3N4,the NH3‐derived lamellar g‐C3N4 exhibits a larger specific surface area and enhanced optical absorption capability,which increase photocatalytic hydrogen production because of the highly active structure,excellent utilization efficiency of photon energy,and low recombination efficiency of photogenerated charge carriers.This study provides a simple strategy for the regulation of the g‐C3N4 microstructure toward highly efficient photocatalytic applications.

Microwave awakening the n-π^(*) electronic transition in highly crystalline polymeric carbon nitride nanosheets for photocatalytic hydrogen generation

The n-π^(*) electronic transition in polymeric carbon nitride(PCN)can remarkably harvest visible light,which thus potentially promotes the photocatalytic hydrogen H2 generation.However,awaking the n-π^(*) lectronic transition has proven to be a grand challenge.Herein,we reported on the awakening of n-π^(*) electronic transition by microwave thermolysis of urea pellet,which yielded the PCN with absorption edge of 600 nm,near 140 nm red-shift from 460 nm of pristine PCN.The n-π^(*) electronic transition endows PCN with an increased photocata lytic H_(2) generation,with a highest H_(2) rate of 61.7μmol h^(-1) under visible light exposure,which is near 6 times higher than that by using the PCN from the thermolysis of urea pellets in an electric furnace(10.6μmol h^(-1)).Furthermore,the n-π^(*) transition in PCN leads to the longest wavelength of 535 nm that can initiate H2 generation,remarkably longer than the absorption edge of pristine PCN(460 nm).This work manifests the advantages of microwave sintering route to awaken the n-π^(*) electronic transition in PCN for an increased photocata lytic performance.

Optical ionization evolution effect on photocurrent produced from two-color femtosecond laser pulses

When laser intensity varies, the ionization induced by optical field can be described by multiphoton ionization （MPI） in low regime and tunneling ionization （TI） in high regime. An empirical formula was used to fit the ionization happened between these two limitation ionization processes. Based on this, ionization rate and photocurrent induced by two-color femtosecond laser pulses interaction with air-plasma were investigated numerically. It was found that they have different relations with the laser intensity.

Increasingπ-electron availability in benzene ring incorporated graphitic carbon nitride for increased photocatalytic hydrogen generation

Increasing the availability ofπ-electron in graphitic carbon nitride(g-C_(3)N_(4))can reduce the band gap and thus enhance the photocatalytic hydrogen(H_(2))generation activity upon exposure to visible light,However,such strategy has not yet been largely applied to increase the H_(2)generation of g-C_(3)N_(4).Herein,we succes s fully increased the amount ofπ-electron in g-C_(3)N_(4)by incorporatingπ-electron-rich benzene rings through copolymerization of melamine and trimesic acid in air.The incorporation of benzene rings not only extends the light absorption of g-C_(3)N_(4)to 650 nm,but also improves the electrical conductivity due to delocalization ofπelectrons in benzene rings.As a result,a 3.4 times enhancement of photocatalytic H_(2)generation was achieved from the g-C_(3)N_(4)with benzene ring incorporation in comparing with that of pristine g-C_(3)N_(4).More interestingly,H_(2)generation still occurs under irradiation of the light ofλ≥490 nm,above the absorption edge of pristine g-C_(3)N_(4)(~460 nm),illustrating the positive effectiveness of incorporated benzene rings on enhancing the H_(2)generation capacity of g-C_(3)N_(4).The present work manifests the advantages of increasingπ-conjugated electrons on designing highly active g-C_(3)N_(4)photocatalysts.

Spatially Isolated Noble-Metal-Free Redox Cocatalysts on CdS Nanorods for Increased Photocatalytic Hydrogen Generation

Spatially isolated oxidation and reduction cocatalysts on a semiconductor can realize efficient charge separation and thereby lead to increased photocatalytic hydrogen generation. However, the effective preparation of such photocatalysts has proven challenging.Herein, we report the facile synthesis of a novel noblemetal-free CdS/MoS/CoPi ternary photocatalyst via a visible light-induced synthesis route, in which MoSreduction cocatalysts were precisely grown on the two terminals of CdS nanorods, while CoPi oxidation cocatalysts were preferentially anchored onto the sidewalls of CdS nanorods. Such spatially isolated MoSand CoPi redox cocatalysts endow CdS nanorods with a rapid charge separation, which enhances their hydrogen generation activity. The CdS/MoS/CoPi photocatalyst with optimized CoPi amount achieves the highest Hgeneration rate of 206 μmol/h, which is 21 and 2 times higher than that achieved by using CdS alone(9.7 μmol/h) and CdS/MoS(105 μmol/h), respectively. The present work highlights the effectiveness of the spatial isolation of reduction and oxidation sites for efficient charge separation and thereby provides a promising strategy for the preparation of highly active photocatalysts.