Jinshui Zhang

Exfoliated Graphitic Carbon Nitride Nanosheets as Efficient Catalysts for Hydrogen Evolution Under Visible Light

Graphitic carbon nitride nanosheets are extracted, produced via simple liquid-phase exfoliation of a layered bulk material, g-C3N4. The resulting nanosheets, having ≈2 nm thickness and N/C atomic ratio of 1.31, show an optical bandgap of 2.65 eV. The carbon nitride nanosheets are demonstrated to exhibit excellent photocatalytic activity for hydrogen evolution under visible light.

Synthesis of a Carbon Nitride Structure for Visible‐Light Catalysis by Copolymerization

and nonmetallic elements (N, C, B) creates localized/ delocalized states in the band gap and thus extends its optical absorption to the visible region, but doping usually comes with accelerated charge recombination and lower stability of the doped materials. Meanwhile, various other inorganic, non-TiO2-based, visible-light catalysts have been developed (e.g., metal oxides, nitrides, sulfides, phosphides, and their mixed solid solutions), whereby Ga, Ge, In, Ta, Nb, and W are the main metal constituents. However, sustained utilization of solar energy calls for the development of more abundant and stable catalysts working with visible light, and this has remained challenging so far. Recently, a polymeric semiconductor on the basis of a defecteous graphitic carbon nitride (g-C3N4), was introduced as a metal-free photocatalyst which fulfills the basic requirements for a water-splitting catalyst, including being abundant, stable, and responsive to visible light. In the following, we use the notation “g-C3N4” to describe this class of materials rather than the idealized structure. The most active system is in fact presumably an N-bridged “poly(tri-s-triazine)”, already described by Liebig as “melon”. A semiconductor structure with band edges straddling the water redox potential was revealed for melon by DFT calculations, albeit electrochemical analysis is still awaited. g-C3N4 is considered to be the most stable phase of covalent carbon nitride, and facile synthesis of the melon substructure from simple liquid precursors and monomers allows easy engineering of carbon nitride materials to achieve the desired nanostructures via soft-chemical processing routes and methods. For instance, a high surface area (67–400 mg ) can be imparted on g-C3N4 materials by polymerization of cyanamide on a silica template, which results in photocatalytically more active g-C3N4 nanostructures. [8] Metal-doped gC3N4 can also be conveniently obtained by polymerization of dicyandiamine in the presence of metal salts, and thus multifunctionalization of such materials for a variety of applications can be achieved. Most importantly, the electronic and optical properties of carbon nitride, regarded as a polymer semiconductor, are in principle adjustable by organic protocols. Such organic protocols have been widely used to control the performance of traditional p-conjugated polymers, for example, to improve solar-cell efficiencies by constructing copolymerized donor–acceptor structures, or to modify electronic properties by co-blending with p/n-type organic dopants. Our aim was to use such organic modifications to extend the insufficient light absorption of g-C3N4 (a result of its large band gap of 2.7 eV, which corresponds to wavelengths shorter than 460 nm) towards the maximum of the solar spectrum. Here we demonstrate that the optical absorption of carbon nitride semiconductor materials is extendable into the visible region up to about 750 nm by simple copolymerization with organic monomers like barbituric acid (BA). The electronic and photoelectric properties of the modified carbon nitrides were then investigated to elucidate their enhanced activity for hydrogen production from water containing an appropriate sacrificial reagent with visible light. In principle, BA can be directly incorporated into the classical carbon nitride condensation scheme (Scheme 1). New carbon nitride structures were therefore synthesized by dissolving dicyandiamide with different amounts of BA in water, followed by thermally induced copolymerization at 823 K. For simplicity, the resulting samples are denoted CNBx, where x (0.05, 0.1, 0.2, 0.5, 1, 2) refers to the weighedin amount of BA. The structure, texture, and electrochemical properties of these materials were characterized, and their photochemical performance analyzed. Their XRD patterns (Figure S1, Supporting Information) are dominated by the characteristic (002) peak at 27.48 of a graphitic, layered structure with an interlayer distance of d = [*] J. Zhang, X. Chen , Prof. X. Fu, Prof. X. Wang State Key Laboratory Breeding Base of Photocatalysis Fuzhou University, Fuzhou 350002 (China) E-mail: xcwang@fzu.edu.cn

Fe-g-C3N4-Catalyzed Oxidation of Benzene to Phenol Using Hydrogen Peroxide and Visible Light

A bioinspired iron-based catalyst with semiconductor photocatalytic functions in combination with a high surface area holds promise for synthetic chemistry via combining photocatalysis with organosynthesis. Here exemplified for phenol synthesis, Fe-g-C(3)N(4)/SBA-15 is able to oxidize benzene to phenol with H(2)O(2) even without the aid of strong acids or alkaline promoters. By taking advantage of both catalysis and photocatalysis functions of g-C(3)N(4) nanoparticles, the yield of the phenol can be markedly promoted.

Layered Nanojunctions for Hydrogen‐Evolution Catalysis

The production of chemical fuels by using sunlight is an attractive and sustainable solution to the global energy and environmental problems. Photocatalytic water splitting is a promising route to capture, convert, and store solar energy in the simplest chemical compound (H2). [1] Since the initial report of a photoelectrochemical cell using Pt-TiO2 electrodes for hydrogen evolution by Fujishima and Honda in 1972, considerable studies have been focused on the development of highly efficient and stable photocatalyst powder systems, and especially on using earth-abundant semiconductors and co-factors for water splitting. In practice, the achievement of the conversion of solar energy into hydrogen necessitates the spatial integration of semiconductors and co-catalysts to form surface junctions, so as to optimize the capture of light and to promote charge separation and surface catalytic kinetics. The construction of effective surface junctions is therefore of vital importance, and not only strongly depends on the properties, such as crystal structure, band structure, and electron affinity, of both semiconductors and catalysts but also on the interface between the two materials. In photocatalysis, an ohmic contact between photocatalysts and cocatalysts can allow the prompt migration of light-induced charge, thus resulting in an efficient photocatalytic reaction. Recently, we found that graphitic carbon nitride (g-CN), a polymeric melon semiconductor with a layered structure analogous to graphite, meets the essential requirements as a sustainable solar energy transducer for water redox catalysis; these requirements include being abundant, highly-stable, and responsive to visible light. g-CN is indeed a new type of visible-light photocatalyst that contains no metals, and has a suitable electronic structure (Eg = 2.7 eV, conduction band at 0.8 V and valence band at 1.9 V vs. RHE) covering the water-splitting potentials. An improvement in the efficiency of H2 production has been demonstrated by the introduction of nanohierarchical structures into g-CN. It is noted that, like many other photocatalysts, g-CN alone shows very poor electrocatalytic activities for water splitting and relies on surface co-catalysts to activate its photocatalytic functions. The co-catalyst cooperates with the light harvester to facilitate the charge separation and increases the lifetime of the photogenerated electron/hole pair, while lowering activation barriers for H2 or O2 evolution. Thus, the use of a co-catalyst leads to an increase in overall photocatalytic performance, including activity, selectivity, and stability. Generally, the efficiency of a given photocatalytic system is dependent on the ability of the co-catalysts to support reductive and/or oxidative catalysis. In particular, the structural characteristics and intrinsic catalytic properties of a co-catalyst are important. However, the study of the structural and electronic compatibility between g-CN and co-catalysts has been limited so far. The co-catalysts used are mainly platinum group metals or their oxides, which are scarce and expensive. Photocatalytic/catalytic systems based on abundantly available materials are certainly desirable for large-scale hydrogen production for future energy production based on water and sunlight. Among various hydrogen-evolution reaction (HER) catalysts, molybdenum sulfur complexes have received a lot of attention. MoS2 was found to be a good electrocatalyst for H2 evolution, and the HER activity stemmed from the sulfur edges of the MoS2 crystal layers. [10] When grown on graphene sheets, nanostructured MoS2 exhibited excellent HER activity owing to the high exposure of the edges and the strong electronic coupling to the underlying planar support. Incomplete cubane [Mo3S4] + clusters and amorphous MoS2 are also proven HER catalysts. Some of these HER catalysts have been used in photocatalytic H2 production and they exhibited a remarkable promoting effect. MoS2 has a similar structure to graphite; it has a layered crystal structure consisting of S Mo S “sandwiches” held together by van der Waals force. The fact that g-CN and MoS2 have analogous layered structures should minimize the lattice mismatch and facilitate the planar growth of MoS2 slabs over the g-CN surface, thus constructing an organic–inorganic hybrid with graphene-like thin layered heterojunctions (Scheme 1a). Such a distinct nanoscale structure has some advantages. It can increase the accessible area around the planar interface of the MoS2 and g-CN layers and diminish the barriers for electron transport through the co-catalyst, thus facilitating fast electron transfer across the interface by the electron tunneling effect. Also, thin layers can lessen the light blocking effect of the co-catalyst, thus improving the light utilization by g-CN. Importantly, the intrinsic band structures [*] Y. Hou, J. Zhang, G. Zhang, Y. Zhu, Prof. X. Wang Research Institute of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University Fuzhou 350002 (China) E-mail: xcwang@fzu.edu.cn

Boron- and Fluorine-Containing Mesoporous Carbon Nitride Polymers: Metal-Free Catalysts for Cyclohexane Oxidation

Recently, various lightweight materials with diverse nanomorphologies that contain heteroatoms such as nitrogen, boron, or fluorine have been actively pursued because of their unusual properties, such as in catalytic applications or as semiconductors. For example, ordered and disordered modifications of carbon nitride (C3N4) extend the property profile of carbon nanostructures and have numerous potential areas of applications ranging from semiconductors to fuel cells. A large number of reports deals with the synthesis of different modifications of bulk CxNy materials. [2] The synthesis of these nitrogen-rich species generally includes thermal condensation of nitrogen-rich precursors, often from molecules containing or generating triazine rings. For example, through a solid-state reaction of 2,4,6-triamino-1,3,5-triazine with 2,4,6-trichloro-1,3,5-triazine at high pressure and high temperature, Wolf and co-workers obtained a well-characterized and highly crystalline graphitic carbon nitride derivative. However, it was shown that the more ideal bulk carbon nitride solids perform rather weakly in some catalytic processes, while more disordered, polymeric versions showed nice activity, as structural defects or surface terminations seemed to play a key role for the catalytic activation. To enhance the performance of carbon nitride both as a support and as a catalyst, the specific surface had to be enhanced, and nanocasting with hard templates using porous silicas has been explored recently for the replication of porous carbon nitride materials with controlled mesopore structures. This nanocasting method is good for the proof of principle, but it is hardly transferable to a practical process, as the templates have to be removed in an extra step involving aqueous ammonium bifluoride (NH4HF2) or hydrogen fluoride (HF), which are hazardous and not environmentally friendly and also prohibit further functionalization. A general and robust method for the synthesis of porous polymeric carbon nitrides without the extra step of removing hard templating silica structures has yet to be developed. Ionic liquids are an especially promising choice to achieve this aim. Ionic liquids are generally defined as organic salts with a low melting point, usually below 100 8C. They inherit many features of inorganic molten salts, such as excellent chemical and thermal stability (in some cases in excess of 400 8C) and negligibly small vapor pressure, with the convenience of being liquid under ambient conditions. They have found application in numerous fields, for example as solvents or catalysts in organic synthesis, as electrolytes, or in chemical separations. Recently, the advantages of ionic liquids in inorganic synthesis have been gradually realized and have received more and more attention. For instance, they turned out to be interesting solvents in the synthesis of nanoparticles owing to their special solvent structure that derives from the ion–ion interaction and extended hydrogen-bonding motifs. From the many different available ionic liquids, we chose a class with BF4 counterions, as the anion might enter the C N condensation scheme, while boron and fluorine doping could improve the catalytic activity of the discussed systems and contribute to the performance of such materials. Herein we demonstrate that the simple, commercially available room-temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) is a unique soft template for the easy synthesis of boronand fluorineenriched mesoporous polymeric carbon nitride, in which an organic precursor, for example dicyandiamide (DCDA), selfcondensed into carbon nitride solids in the presence of BmimBF4. We show that the resulting materials possess high nitrogen, boron, and fluorine contents, high surface area, local graphitic order, and an excellent photoconductivity under visible light. We also demonstrate the catalytic properties of these CNBF materials in the heterogeneous, metal-free oxidation of cyclohexane. The CNBF materials show nice performance in the oxidation of cyclohexane with good conversion and excellent cyclohexanone selectivity. The structures of these hybrid materials were investigated by powder X-ray diffraction (XRD), nitrogen gas adsorption, and transmission electron microscopy (TEM) measurements. The XRD pattern, for example of CNBF-0.3 (r = 0.3, r denotes the mass ratio of BmimBF4 to DCDA), is dominated by the (002) interlayer-stacking peak of carbon nitrides, [*] Dr. Y. Wang, Prof. Dr. X. Wang, Prof. Dr. M. Antonietti Department of Colloid Chemistry Max-Planck Institute of Colloids and Interfaces Research Campus Golm, 14424 Potsdam (Germany) Fax: (+ 49)331-567-9502 E-mail: yong.wang@mpikg.mpg.de

Sulfur-mediated synthesis of carbon nitride: Band-gap engineering and improved functions for photocatalysis

Sulfur-mediated synthesis has been developed to modify the texture, optical and electronic properties, as well as the photocatalytic functions of a carbon nitride semiconductor. The water oxidation reaction has been achieved at a moderate rate with only photocatalysts without the aid of co-factors.

Two-dimensional covalent carbon nitride nanosheets: synthesis, functionalization, and applications

The development of new layered materials has experienced an evolution from graphene to metal oxide and metal chalcogenide nanosheets, and more recently to two-dimensional (2D) covalent organic frameworks, such as conjugated carbon nitride nanosheets (CNNs) with a spectral gap in the band structure. The anisotropic 2D geometric morphology, together with the aromatic π-conjugated framework, endows polymeric CNNs with unique properties, such as an enlarged surface area with a highly opened-up flat structure, reduced thickness with enhanced electron mobility and with intrinsic semiconductive features, which support their attractive bandgap- and surface-engineered applications ranging from energy-related topics to other new emerging fields. In this review, recent research advances in the establishment of two synthetic strategies for CNNs are firstly overviewed, namely, the top-down delamination of graphitic carbon nitride (CN) solids and the bottom-up assembly of molecular building blocks in a 2D manner. Efficient approaches aimed at advancing CNNs for target-specific applications, including hybridizing, doping, sensitization, copolymerization and nanorefinement, are also described as possible solutions.

Nanospherical Carbon Nitride Frameworks with Sharp Edges Accelerating Charge Collection and Separation at a Soft Photocatalytic Interface

Shape engineering of a g-C3 N4 framework, with interconnecting nanosheets and highly open-up spherical surfaces with sharp edges, can easily accelerate charge separation and promote mass transfer for photoredox catalysis, achieving an apparent quantum yield of 9.6% at 420 nm in an assay of the photocatalytic hydrogen evolution reaction.

Sol Processing of Conjugated Carbon Nitride Powders for Thin-Film Fabrication†

The chemical protonation of graphitic carbon nitride (CN) solids with strong oxidizing acids, for example HNO3, is demonstrated as an efficient pathway for the sol processing of a stable CN colloidal suspension, which can be translated into thin films by dip/disperse-coating techniques. The unique features of CN colloids, such as the polymeric matrix and the reversible hydrogen bonding, result in the thin-film electrodes derived from the sol solution exhibiting a high mechanical stability with improved conductivity for charge transport, and thus show a remarkably enhanced photo-electrochemical performance. The polymer system can in principle be broadly tuned by hybridization with desired functionalities, thus paving the way for the application of CN for specific tasks, as exemplified here by coupling with carbon nanotubes.

Molecular and textural engineering of conjugated carbon nitride catalysts for selective oxidation of alcohols with visible light

A thiophene motif has been integrated into mesoporous carbon nitride (MCN) to improve the photoactivation of molecular oxygen towards the selective oxidation of alcohols. This modification also extends the photoactive part of the spectrum of MCN by electronic modification of the conjugated system.