Synthesis of carbon nitride hollow microspheres with highly hierarchical porosity templated by poly (ionic liquid) for photocatalytic hydrogen evolution

Hydrogen, as a sustainable and clean energy, has been considered as a promising candidate to replace fossil fuels. And it is meaningful to fabricate the photocatalysts to drive photocatalytic water splitting leading to hydrogen production. Herein, a facile approach was developed by the means of the template effect of poly (ionic liquid) and self-assembly of cyanuric acid and melamine through hydrogen bonds, to obtain carbon nitride hollow microspheres with highly hierarchical porosity. The influence of poly (ionic liquid) concentration on the structure and photocatalytic activity of as-prepared carbon nitride was investigated. The optimized carbon nitride hollow microspheres possessed the multiple porous channels and improved surface area (71 m²/g) due to the decomposition of poly (ionic liquid) and cyanuric acid-melamine supramolecular aggregates. Moreover, the as-prepared carbon nitride hollow microspheres exhibited a remarkable catalytic activity in the photocatalytic hydrogen evolution reaction under visible light irradiation. Especially, the sample CN-0.02 exhibits the highest hydrogen evolution rate (90.1 μmol h⁻¹). The outstanding photocatalytic activity is attributed to the high specific surface area, broad light absorption range and fast separation rate of photogenerated electron–hole pairs. This novel method opens up a new way toward the development of highly-active photocatalysts for water splitting.     

设计p-n同质结构实现碳氮光催化剂的全解水性能

非金属氮化碳(CN)因其独特的光催化性能而备受关注.本文利用水热处理、高温烧结、高能球磨和烧结的方法成功制得一种具有混合结构的CN光催化剂.先以三聚氰胺为原料进行水热处理(180℃,24 h),过滤干燥后,转移到高纯氩气保护下的管式炉中,于550℃处理1 h得到CN材料.然后将CN用三聚氰胺和氟化铵水热180℃处理24 h,过滤、干燥、煅烧(550℃,1 h)得到第二种材料.最后将其与CN材料按等比例混合,经高能球磨研磨,再于管式炉中在气氛保护下淬火,得到最终催化剂样品.由于这种结构的界面作用,使CN光催化剂显示出了高的光催化活性.它的产氢效果可以高达17028.82μmol h^–1g^–1,在420 nm光照下,其光量子效率也达到11.2%.随后,采用纳秒级别的时间分辨萤光(PL)光谱测得其荧光寿命为9.9 ns.有助于电子与空穴参与反应更有趣的是,在不加牺牲剂时,该光催化剂具有高效的全解水效果,其产氢效率为270.95μmol h^–1g^–1,产氧效率为115.21μmol h^–1g^–1,有望实际用于全解水反应中.另外,通过紫外可见漫反射光谱,PL光谱和材料的比表面等测试来考察该CN光催化剂效果好的原因.发现该材料具有更高比表面积有更多活性点参与反应.同时,通过电化学测试获得了肖特基曲线和电流-电压曲线,发现该光催化剂里含有少量的pn结构,这种结构使材料在弱光下也会产生光生载流子,实际上它是起到光生载流子的激发作用,即在相同光照下,就会产生更多的光生载流子数量,从而进一步提高了其催化效果.因此,本工作对优化碳氮光催化剂的催化效果有很好的指导意义.     

原位构筑质子化氮化碳/碳化钛光催化制氢肖特基异质结

氢气因其具有高燃烧热、可再生性以及燃烧产物无污染等优势被认为是一种绿色可再生能源,是取代化石燃料的候选能源之一.然而,如何利用自然界中丰富的太阳能和水资源实现光分解水制氢的关键在于开发高效的光催化剂.在尺寸明确、能级带隙匹配的纳米材料间进行完美的界面复合(异质结构筑)是实现高效太阳能-氢能转换的最佳途径.石墨相氮化碳(CN)材料因其电子结构可调和化学性能稳定等特性被光催化界所关注.然而,氮化碳材料较弱的电学性能如电荷传输能力差及电子-空穴对复合率高导致其表现出较低的光催化制氢效率.基于此,我们用盐酸对氮化碳进行质子化处理,使材料表面电荷发生改变,从而实现氮化碳的电子带隙调节和电导率提升.在此基础上,将二维碳化钛原位负载于质子化的氮化碳(PCN)纳米片表面构筑肖特基结.PCN纳米片与碳化钛纳米片间的良好界面接触促进了电荷在材料界面上传输,进而加速了氮化碳材料的电荷分离,实现了氮化碳光催化剂活性的提升.Zeta电位测试结果显示,CN和PCN的表面电位分别为?9.5和27.3 mV,表明质子化处理可以有效改变材料表面电荷,并促其与碳化钛纳米片进行静电组装.该结果进一步得到了扫描电子显微镜(SEM)和原子力显微镜(AFM)的证实.改变表面电荷使氮化碳材料的能带宽度由2.53 eV(CN)减小到2.41 eV(PCN),增强了可见光区吸收.同时,PCN的光电流密度提升了约4倍,电子阻抗和激发态电子的辐射复合都显著降低.将PCN与碳化钛复合制得复合材料(PCN-x,x=10,20,40),实验结果表明5 g的PDN最佳负载碳化钛的量为20 mg(PCN-20).在标准太阳模拟器的可见光区(>420 nm),复合材料PCN-20的光催化水分解产氢量可达2181μmol·g-1,是CN催化剂的约5.5倍,PCN的2.7倍,并且经过5次产氢循环后PCN-20仍具有稳定的氢气释放速率.以上结果表明,氮化碳材料可以通过质子化处理以及与适量的碳化钛复合实现光催化产氢性能的提升,其中碳化钛在体系中起助催化剂的作用.该研究结果可为其他半导体光催化剂的性能优化以及非贵金属助催化剂的研究提供新思路.     

Hydrogen Production on a Hybrid Photocatalytic System Composed of Ultrathin CdS Nanosheets and a Molecular Nickel Complex

The production of clean and renewable hydrogen through water splitting by using solar energy has received much attention due to the increasing global energy demand. We report an economic and artificial photosynthetic system free of noble metals, consisting of ultrathin CdS nanosheets as a photosensitizer and nickel‐based complex as a molecular catalyst. Emission quenching and flash photolysis studies reveal that this hybrid system allows for effective electron transfer from the excited CdS nanosheets to the nickel‐based complex to generate reduced intermediate species for efficient hydrogen evolution. Notably, the unique morphological and structural features of the ultrathin CdS nanosheets contribute to the highly efficient photocatalytic performance. As a consequence, the resulting system shows exceptional activity and stability for photocatalytic hydrogen evolution in aqueous solution with a turnover number (TON) of about 28 000 versus catalyst and a lifetime of over 90 h under visible light irradiation.     

一维硫化镉/二维二硫化钨纳米异质结用于超高活性光催化制氢

基于半导体的太阳能光催化分解水制氢技术是一种环境友好、潜力巨大的绿色氢能制造方案.常用的块体半导体材料一般具有较弱的可见光吸收、快速的光生载流子复合以及较低的光催化制氢效率等缺点.因此,设计开发具有宽光谱光吸收、稳定性好、催化活性高的太阳能光催化材料是促进光催化制氢发展的关键,也是该研究方向的挑战之一.硫化镉纳米材料是...     

Carbon-based H2-production photocatalytic materials

Photocatalytic hydrogen production from water splitting is of promising potential to resolve the energy shortage and environmental concerns. During the past decade, carbon materials have shown great ability to enhance the photocatalytic hydrogen-production performance of semiconductor photocatalysts. This review provides a comprehensive overview of carbon materials such as CNTs, graphene, C₆₀, carbon quantum dots, carbon fibers, activated carbon, carbon black, etc. in enhancing the performance of semiconductor photocatalysts for H₂ production from photocatalytic water splitting. The roles of carbon materials including supporting material, increasing adsorption and active sites, electron acceptor and transport channel, cocatalyst, photosensitization, photocatalyst, band gap narrowing effect are explicated in detail. Also, strategies for improving the photocatalytic hydrogen-production efficiency of carbon-based photocatalytic materials are discussed in terms of surface chemical functionalization of the carbon materials, doping effect of the carbon materials and interface engineering between semiconductors and carbon materials. Finally, the concluding remarks and the current challenges are highlighted with some perspectives for the future development of carbon-based photocatalytic materials.     

Polymeric Carbon Nitride/Reduced Graphene Oxide/Fe2O3: All‐Solid‐State Z‐Scheme System for Photocatalytic Overall Water Splitting

The charge transfer between hydrogen evolution photocatalysts (HEPs) and oxygen evolution photocatalysts (OEPs) is the rate‐determining step that controls the overall performance of a Z‐scheme water‐splitting system. Here, we carefully design reduced graphene oxide (RGO) nanosheets for use as solid‐state mediators to accelerate the charge carrier transfer between HEPs (e.g., polymeric carbon nitride (PCN)) and OEPs (e.g., Fe₂O₃), thus achieving efficient overall water splitting. The important role of RGO could also be further proven in other PCN‐based Z‐systems (BiVO₄/RGO/PCN and WO₃/RGO/PCN), illustrating the universality of this strategy.     

A Hydrogen Farm Strategy for Scalable Solar Hydrogen Production with Particulate Photocatalysts

Scalable solar hydrogen production by water splitting using particulate photocatalysts is promising for renewable energy utilization. However, photocatalytic overall water splitting is challenging owing to slow water oxidation kinetics, severe reverse reaction, and H₂/O₂ gas separation. Herein, mimicking nature photosynthesis, a practically feasible approach named Hydrogen Farm Project (HFP) is presented, which is composed of solar energy capturing and hydrogen production subsystems integrated by a shuttle ion loop, Fe³⁺/Fe²⁺. Well‐defined BiVO₄ crystals with precisely tuned {110}/{010} facets are ideal photocatalysts to realize the HFP, giving up to 71 % quantum efficiency for photocatalytic water oxidation and full forward reaction with nearly no reverse reaction. An overall solar‐to‐chemical efficiency over 1.9 % and a solar‐to‐hydrogen efficiency exceeding 1.8 % could be achieved. Furthermore, a scalable HFP panel for solar energy storage was demonstrated under sunlight outdoors.     

气-固相光催化分解硫化氢制氢

利用气-固相反应初步考察了几种常用半导体光催化剂在无氧条件下分解硫化氢产氢的活性.在研究的TiO2,CdS,ZnS,ZnO和ZnIn2S4等催化剂中,ZnS具有较高的光催化产氢活性.在ZnS上担载贵金属Ir可明显提高产氢速率,在ZnS制     

离子热处理合成氮缺陷石墨相氮化碳及其光解水制氢性能

以尿素和二氰二胺为原料热聚合得到石墨相氮化碳,分别采用直接二次煅烧和熔盐离子热后热处理在不同温度下对产物进行后热处理,得到氮缺陷氮化碳CN和CNS。利用X射线衍射(XRD)、紫外-可见漫反射光谱(UV-Vis DRS)、透射电镜(TEM)等手段对所制备样品进行表征和分析,探讨了不同热处理温度和加热方法对催化剂微观聚合结构的影响;同时以光解水制氢为测试方法,考察了催化剂的可见光催化性能。结果表明,熔盐离子热更有利于氮化碳的层间聚合,得到高结晶度材料;在面内七嗪聚合单元中引入氮缺陷,产生末端氰基,优化电荷密度分布,增强电荷流动性;克服粒子尺寸效应,扩展催化剂的光吸收范围;当后热处理温度为500℃时,制备的CNS-500表现出优异的光解水制氢活性,是同温度下直接热处理得到的催化剂的3.84倍。     

Hydrophilic Conjugated Materials for Photocatalytic Hydrogen Evolution

Photocatalytic hydrogen evolution is viewed as a promising green strategy to utilize the inexhaustible solar energy and provide clean hydrogen fuels with zero‐emission characteristic. The nature of semiconductor‐based photocatalysts is the key point to achieve efficient photocatalytic hydrogen evolution. Conjugated materials have been recently emerging as a novel class of photocatalysts for hydrogen evolution and photocatalytic reactions due to their electronic properties can be well controlled via tailor‐made chemical structures. Hydrophilic conjugated materials, a subgroup of conjugated materials, possess multiple advantages for photocatalytic applications, thus spurring remarkable progress on both material realm and photocatalytic applications. This minireview aims to provide a brief review of the recent developments of hydrophilic conjugated polymers/small molecules for photocatalytic applications, and special concern on the rational molecular design and their impact on photocatalytic performance will be reviewed. Perspectives on the hydrophilic conjugated materials and challenges to their applications in the photocatalytic field are also presented.     

Black Tungsten Nitride as a Metallic Photocatalyst for Overall Water Splitting Operable at up to 765 nm

Semiconductor photocatalysts are hardly employed for overall water splitting beyond 700 nm, which is due to both thermodynamic aspects and activation barriers. Metallic materials as photocatalysts are known to overcome this limitation through interband transitions for creating electron–hole pairs; however, the application of metallic photocatalysts for overall water splitting has never been fulfilled. Black tungsten nitride is now employed as a metallic photocatalyst for overall water splitting at wavelengths of up to 765 nm. Experimental and theoretical results together confirm that metallic properties play a substantial role in exhibiting photocatalytic activity under red-light irradiation for tungsten nitride. This work represents the first red-light responsive photocatalyst for overall water splitting, and may open a promising venue in searching of metallic materials as efficient photocatalysts for solar energy utilization.     

石墨相氮化碳纳米管的合成及光催化产氢性能

通过硬模板法,采用氰胺前驱物和二氧化硅纳米管(SiO₂-NTs)模板,合成石墨相氮化碳纳米管(CN-NTs)光催化剂。采用扫描电镜(SEM)、透射电镜(TEM)、X射线粉末衍射(XRD)、傅立叶变换红外光谱(FT-IR)、氮气吸附/脱附测试、紫外可见漫反射光谱(UV-Vis DRS)、荧光光谱、热重分析(TGA)等手段对CN-NTs催化剂的结构与性能进行表征。结果表明,CN-NTs的化学组成是石墨相氮化碳(g-C₃N₄),形貌为均匀的纳米管,且是介孔材料。与体相氮化碳(B-CN)和介孔石墨相氮化碳(mpg-CN)相比,CN-NTs的光吸收带边蓝移到440 nm,荧光发射谱的峰强减弱。在可见光(λ>420 nm)照射下,CN-NTs具有较高的光催化分解水活性,产氢速率为58 μmol/h,且表现出良好的光催化活性稳定性和化学结构稳定性。研究结果表明纳米管状结构能有效促进g-C₃N₄半导体激子解离,提高光生电子-空穴的分离效率,进而显著优化g-C₃N₄的光催化产氢性能。     

Black Tungsten Nitride as a Metallic Photocatalyst for Overall Water Splitting Operable at up to 765 nm

Semiconductor photocatalysts are hardly employed for overall water splitting beyond 700 nm, which is due to both thermodynamic aspects and activation barriers. Metallic materials as photocatalysts are known to overcome this limitation through interband transitions for creating electron–hole pairs; however, the application of metallic photocatalysts for overall water splitting has never been fulfilled. Black tungsten nitride is now employed as a metallic photocatalyst for overall water splitting at wavelengths of up to 765 nm. Experimental and theoretical results together confirm that metallic properties play a substantial role in exhibiting photocatalytic activity under red‐light irradiation for tungsten nitride. This work represents the first red‐light responsive photocatalyst for overall water splitting, and may open a promising venue in searching of metallic materials as efficient photocatalysts for solar energy utilization.     

光催化全分解水制氢中助催化剂表面氢氧复合反应的抑制

在光催化全分解水产氢的过程中, Pt等助催化剂在催化产生氢的同时也会诱导催化氢气和氧气重新复合为水的逆反应,严重降低了悬浮体系光催化全分解水产氢的效率.我们综述了近年来在逆反应抑制方面的研究进展,总结和对比分析了各种抑制逆反应策略的特点,并对将这些方法应用于悬浮体系光催化全分解水制氢的前景进行了展望.     

Highly Efficient Photocatalytic H2 Evolution from Water using Visible Light and Structure‐Controlled Graphitic Carbon Nitride

The major challenge of photocatalytic water splitting, the prototypical reaction for the direct production of hydrogen by using solar energy, is to develop low‐cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g‐C₃N₄) from a low‐cost precursor, urea, is reported. The g‐C₃N₄ exhibits an extraordinary hydrogen‐evolution rate (ca. 20 000 μmol h^?1 g^?1 under full arc), which leads to a high turnover number (TON) of over 641 after 6 h. The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5 % under visible light, which is nearly an order of magnitude higher than that observed for any other existing g‐C₃N₄ photocatalysts. Furthermore, it was found by experimental analysis and DFT calculations that as the degree of polymerization increases and the proton concentration decreases, the hydrogen‐evolution rate is significantly enhanced.     

Strategy of Graphdiyne (g−CnH2n-2 ) Preparation Coupling with the Flower-Like NiAl-LDH Heterojunctions for Efficient Photocatalytic Hydrogen Evolution**

Graphdiyne (g−C_nH_2n-2), a novel two-dimension carbon allotrope material composed of a sp- and sp²-hybrid carbon network, has been widely explored since it was synthesized for the first time by Li's group in 2010. A series distinct and excellent properties bestow graphdiyne excellent performance in many fields. Here, an innovative progress for preparing graphdiyne by using Cu⁺ contained material as catalyst is reported and the composite CuI-GD is coupled with flower-like NiAl-LDH to produce H₂ from photocatalytic water splitting. The results of FTIR and Raman spectroscopy together reveal that graphdiyne nanosheets are synthesized successfully by employing a cross-coupling method. Photocatalytic hydrogen evolution performance shows that the hydrogen production activity of CuI-GD/NiAl-LDH has a 15- and 216-fold enhancement compared with CuI-GD and NiAl-LDH, respectively. A series of characterizations are carried out to expound the underlying reasons in the enhancement of the photocatalytic hydrogen production performance of CuI-GD/NiAl-LDH. Meanwhile, a possible mechanism for the photocatalytic hydrogen evolution process was proposed to understand the interaction among these materials.     

Cobalt‐Embedded Nitrogen‐Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values

Despite being technically possible, splitting water to generate hydrogen is still practically unfeasible due mainly to the lack of sustainable and efficient catalysts for the half reactions involved. Herein we report the synthesis of cobalt‐embedded nitrogen‐rich carbon nanotubes (NRCNTs) that 1) can efficiently electrocatalyze the hydrogen evolution reaction (HER) with activities close to that of Pt and 2) function well under acidic, neutral or basic media alike, allowing them to be coupled with the best available oxygen‐evolving catalysts—which also play crucial roles in the overall water‐splitting reaction. The materials are synthesized by a simple, easily scalable synthetic route involving thermal treatment of Co²⁺‐embedded graphitic carbon nitride derived from inexpensive starting materials (dicyandiamide and CoCl₂). The materials’ efficient catalytic activity is mainly attributed to their nitrogen dopants and concomitant structural defects.     

Revisiting the Limiting Factors for Overall Water‐Splitting on Organic Photocatalysts

In pursuit of inexpensive and earth abundant photocatalysts for solar hydrogen production from water, conjugated polymers have shown potential to be a viable alternative to widely used inorganic counterparts. The photocatalytic performance of polymeric photocatalysts, however, is very poor in comparison to that of inorganic photocatalysts. Most of the organic photocatalysts are active in hydrogen production only when a sacrificial electron donor (SED) is added into the solution, and their high performances often rely on presence of noble metal co‐catalyst (e.g. Pt). For pursuing a carbon neutral and cost‐effective green hydrogen production, unassisted hydrogen production solely from water is one of the critical requirements to translate a mere bench‐top research interest into the real world applications. Although this is a generic problem for both inorganic and organic types of photocatalysts, organic photocatalysts are mostly investigated in the half‐reaction, and have so far shown limited success in hydrogen production from overall water‐splitting. To make progress, this article exclusively discusses critical factors that are limiting the overall water‐splitting in organic photocatalysts. Additionally, we also have extended the discussion to issues related to stability, accurate reporting of the hydrogen production as well as challenges to be resolved to reach 10 % STH (solar‐to‐hydrogen) conversion efficiency.     

ZnCr Layered Double Hydroxide (LDH) Nanosheets Assisted Formation of Hierarchical Flower‐Like CdZnS@LDH Microstructures with Improved Visible‐Light‐Driven H2 Production

The development of new semiconductor photocatalysts toward splitting water has supplied a promising way to obtain sustainable and clean hydrogen energy. Herein, CdZnS@layered double hydroxide (LDH) composites with a hierarchical flower‐like microstructure have been fabricated with the aid of ZnCr–LDH nanosheets as templates. XRD, SEM and HRTEM show that the ZnCr–LDH nanosheets are uniformly dispersed within the composites. The surface of the hierarchical structures is rough and composed of numerous nanocrystals of CdZnS. The HRTEM images indicate that the surface of CdZnS nanocrystals is mainly composed of the (111) plane. Moreover, the visible‐light‐driven H₂ production performance of the CdZnS in the presence and absence of ZnCr–LDH nanosheets has been measured. The results show that ZnCr–LDH nanosheets play an important role in the hierarchical morphology and photocatalytic activity of the as‐prepared samples. In the water‐splitting process, the visible‐light‐driven H₂‐production rate of hierarchical flower‐like CdZnS@LDH is 4.03 times and nearly 10 times higher than that of pristine CdZnS microsphere and pure commercial CdS, respectively. Therefore, this work not only achieves enhanced catalytic performance of the CdZnS by the introduction of ZnCr–LDH nanosheets, but also supplies an insight into the relationship between the hierarchical morphology and the semiconductor photocatalytic activity.