Nitrogen-rich carbon nitride network materials via the thermal decomposition of 2,5,8-triazido-s-heptazine

This report describes the rapid and slow thermal decomposition of an energetically unstable polycyclic and heterocyclic azide, triazido-s-heptazine (C6N16), to produce nitrogen-rich CNx materials (x > 1.2). An analysis of gaseous byproducts shows that this large heterocyclic precursor releases primarily N2 gas during its decomposition. The product composition and its morphology are dependent on the rapidity of the TAH decomposition. The CNx products are thermally stable to 500 degrees C and exhibit variations in H and O content dependent on precursor preparation and atmospheric exposure. The rapid decomposition of TAH leads to visibly porous powders, while slow decomposition yields smooth monoliths that are reminiscent of the morphology of the starting polycrystalline powder. IR and NMR spectral similarities between the amorphous CNx products and several previously reported heptazine molecules and extended heptazine networks supports significant retention of heptazine motif in these amorphous carbon nitride extended materials.     

From Heptazines to Triazines – On the Formation of Poly(triazine imide)

Poly(triazine imide), a 2D extended carbon nitride network compound that is obtained from ionothermal synthesis in LiCl/KCl or LiBr/KBr salt melt has been known for over a decade. We now have investigated the formation process of this material starting from various triazine‐ and heptazine‐based precursors as well as the differences between ionothermal and conventional synthesis via thermal condensation. Independent of chosen starting material, melem (triamino‐s‐heptazine) is initially formed from the starting material as the imminent precursor to poly(triazine imide). We elucidate the impact of various different carbon nitride precursor compounds on the formation process, propose a mechanism for the back reaction of heptazines to triazines, and rationalize the occurring processes.     

Novel Nanoscroll Structures from Carbon Nitride Layers

Nanoscrolls (papyrus‐like nanostructures) are very attractive structures for a variety of applications, owing to their tunable diameter and large accessible surface area. They have been successfully synthesized from different materials. In this work, we investigate, through fully atomistic molecular dynamics simulations, the dynamics of scroll formation for a series of graphene‐like carbon nitride (CN) two‐dimensional systems: g‐CN, triazine‐based g‐C₃N₄, and heptazine‐based g‐C₃N₄. Our results show that stable nanoscrolls can be formed for each of these structures. Possible synthetic routes to produce these nanostructures are also addressed.     

Crystalline Intermediates during Polycondensation Reactions in the C–N–Cl System – The Paddle‐Wheel Molecule N(C6N7Cl2)3

Solid state metathesis reactions between cyanuric chloride and C–N–H or alkali metal–(B–)C–N compounds, respectively, were carried out in the temperature range between 150 °C to 500 °C, studying intermediate stages of reactions and targeting the formation of carbon nitride materials by elimination of HCl or alkali metal chlorides. Although cyanuric chloride was reacted with quite a number of different reaction partners such as melamine, cyanamide, lithium nitride, lithium or sodium carbodiimide, lithium nitridoborate or sodium dicyandiamide, always the same intermediate compounds appeared in the reactions mixtures. Colorless, needle‐shaped crystals of the tertiary amine N(C₃N₃Cl₂)₃ ( 1 ) were obtained at temperatures around 200–250 °C. Temperatures as high as 400 °C yielded yellow, plate‐like crystals of the heptazine compound C₆N₇Cl₃ ( 2 ). At even higher temperatures, the reaction products were of poorer crystallinity, but evidence of the formation of another crystalline intermediate was given by X‐ray powder diffraction and electron diffraction experiments. This third intermediate is assumed to be a tertiary amine, quite similar to 1 , however, having heptazine ligands instead of triazine ligands and is assigned with the formula N(C₆N₇Cl₂)₃ ( 3 ). Theoretical calculations were performed for the structures and the vibrational spectra of 1 and 3 . Theoretical calculations and a structure refinement based of X‐ray powder diffraction data yielded a plausible structural model for compound 3 .     

Measurements of density and sticking probability of CN(X(2)Sigma(+)) radicals by laser-induced fluorescence spectroscopy

CN(X(2)Sigma(+)) radicals were produced by the decomposition of BrCN with the microwave discharge flow of Ar under the conditions of Ar pressure in the range of 0.40-0.70 Torr. The laser-induced fluorescence (LIF) spectra of the CN(A(2)Pi(i)-X(2)Sigma(+)), 4-0, 5-1, and 7-2 bands were observed, and their intensities were calibrated against Rayleigh-scattering intensity by Ar atoms, from which the CN(X(2)Sigma(+)) radical density (n(CN(X))) was determined as (0.67+/-0.25) x 10(18) to (4.42+/-0.83) x 10(18) m(-3). Hydrogenated amorphous carbon nitride (a-CN(x):H) films were formed by depositing the CN(X(2)Sigma(+)) radicals on Si substrates in the same reaction system as LIF. The sticking probability (s) of the CN(X(2)Sigma(+)) radicals onto the a-CN(x):H films was determined by using n(CN(X)), the flow speed, and the weight (w) of a-CN(x):H. The s value was determined as (6.4+/-6.4) x 10(-2) to (2.5+/-1.2) x 10(-2), where the errors are predominantly determined by those in n(CN(X)) and w. The procedure described in the present study will provide a methodology to determine the sticking probability of the precursor radicals of the film formation based on the gas-phase LIF spectroscopy.     

一种新BCN化合物先驱体的合成及其表征

以三聚氰胺和硼酸为原料在水溶液中反应合成了一种新的BCN化合物先驱体, 通过元素分析、XRD、FT-IR、电喷雾质谱及单晶X射线衍射对其进行了表征. 结构分析表明该化合物属单斜晶系, 化学式为C3H6N6(H3BO3)2, 是由C3H6N6分子和H3BO3分子通过氢键加合组装成的具有三维超分子结构的化合物. 将该先驱体在1900 ℃氮气气氛下热解, 对产物进行XRD和XPS表征, 结果表明得到了含碳量较低的具有乱层石墨结构的新型B3CN3化合物.     

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。     

热聚合硫氰酸铵制备多孔 g-C3N4纳米片及其可见光催化分解水制氢性能

二维层状半导体材料与其体相堆积结构相比表现出独特的性质,有望在纳米材料科学领域取得新的突破.基于对太阳能利用的研究,二维半导体光催化材料引起了研究者的广泛关注.诸多半导体材料已被设计合成二维纳米片结构应用于光催化领域,如 MoS2, WS2, SnS2和TiO2等.石墨相氮化碳(g-C3N4)是一种典型的非金属二维聚合物半导体.二维层状结构的组成使得 g-C3N4纳米片能够表现出优异的光电性质.然而,其合成目前仍然存在很大困难.目前已报道的单层或多层 g-C3N4的制备主要有超声辅助溶剂剥离法、热处理法、插层法和电化学合成法等.但这些方法存在合成复杂和引入结构缺陷等不足.另外,在体相组成中插入孔结构也能够提高 g-C3N4的光催化活性.目前常用的方法主要是模板法.然而,在这些生孔过程中往往引起聚合度降低,增加长程无序度,不利于光生载流子的传输.因此,如果将多孔结构引入 g-C3N4纳米片,同时提高其聚合度结构,将在很大程度上提高其光催化性能.本文利用直接氨气热聚合的方法,将硫氰酸铵进行高温热处理,一步法合成出较高聚合度的多孔 g-C3N4纳米片,在可见光照射下表现出较高的产氢活性和稳定性.采用 X射线衍射(XRD)、红外光谱(FTIR)、荧光光谱(PL)和电子顺磁共振(EPR)等方法对多孔 g-C3N4纳米片结构进行了详细表征.在助催化剂 Pt存在下,采用可见光照射(>420 nm)分解水产氢的方法评价了其光催化性能.结果表明,热处理温度对产物结构及性能具有较大影响. XRD结果表明,在450oC热处理,硫氰酸铵未完全聚合,与前期氮气热处理的结论不同.当热聚合温度上升至500oC,石墨相结构形成.至600oC时,石墨相的层间距缩小,且聚合度没有明显下降.这表明氨气气氛抑制了原料分解,提高了分解聚合温度,同时增加了产物的聚合度. FTIR结果表明,热聚合温度对产物 C–N共轭结构改变不大,但在810 cm–1处的峰位向长波数移动,表明七嗪环单元含量增加,再次证明高的热聚合温度没有造成明显的结构分解,反而促进了聚合结构的形成.扫描电镜与氮气吸脱附分析表明,随着聚合温度升高,产物粒子尺寸变小,形貌呈现层状分布,并伴随多孔状的产生,因此比表面积和孔体积显著增大,吸收带边发生蓝移. PL和 EPR结果表明,聚合温度从500增至600oC,样品光生载流子的复合速率下降,导带离域电子密度增加,从而有利于光催化性能的提高.光解水产氢性能测试表明,聚合温度升高有利于催化剂产氢速率提高;600oC所得样品的产氢速率达340μmol/h.进一步分析表明,产氢速率与比表面积基本成正相关关系,说明层状多孔结构的形成是影响产氢性能的重要因素.经过多轮循环测试,其产氢性能保持稳定而没有显著下降,表明其活性稳定性良好.     

Formation of melamium adducts by pyrolysis of thiourea or melamine/NH4 Cl mixtures

Pyrolysis of prominent precursor compounds for the synthesis of carbon nitride type materials (e.g., melamine, thiourea) have been studied in detail. Molecular adducts containing monoprotonated melamium C(6)N(11)H(10)(+) and melaminium HC(3)N(3)(NH(2))(3)(+) ions, respectively, have been identified as intermediates. The adduct C(6)N(11)H(10)Cl·0.5NH(4)Cl was obtained by the reaction of melamine C(3)N(3)(NH(2))(3) with NH(4)Cl at 450 °C. During the pyrolysis of thiourea, guanidinium thiocyanate was initially formed and subsequently the melamium thiocyanate melamine adduct C(6)N(11)H(10)SCN·2C(3)N(3)(NH(2))(3) was isolated at 300 °C. A second melaminium thiocyanate melamine adduct with the formula HC(3)N(3)(NH(2))(3)SCN·2C(3)N(3)(NH(2))(3) represents an intermediary reaction product that is best accessible at low pressures. The crystal structures of the compounds were solved by single-crystal XRD. Unequivocal proton localization at the C(6)N(11)H(10)(+) ion was established. A typical intramolecular and interannular hydrogen bridge and other characteristic hydrogen-bonding motifs were identified. Additionally, the adducts were investigated by solid-state NMR spectroscopy. Our study provides detailed insight into the thermal condensation of thiourea by identifying and characterizing key intermediates involved in the condensation process leading to carbon nitride type materials. Furthermore, factors promoting the formation of melamium adduct phases over melem are discussed.     

Photophysics and Photocatalysis of Melem: A Spectroscopic Reinvestigation

Graphitic carbon nitride (g‐CN) is one potential metal‐free photocatalyst. The photocatalytic mechanism of g‐CN is related to the heptazine ring building unit. Melem is the simplest heptazine‐based compound and g‐CN is its polymeric product. Thus, studies on the photophysical properties of melem will help to understand the photocatalytic mechanism of heptazine‐based materials. Herein, the spectroscopic features of melem were systematically explored through measuring its absorption spectrum, fluorescence spectrum, and fluorescence decay. Both fluorescence spectroscopy and fluorescence decay measurements show that the condensation of melamine to melem causes stronger photoluminescence, whereas the condensation of melem to g‐CN causes weaker photoluminescence. In addition, all observations reveal that a mixture of monomer melem and its higher condensates is more easily obtained during the preparation of melem, and that the higher condensates of melem affect the photophysical properties of melem dominantly. The photocatalytic hydrogen evolution of melem has also been measured and the monomer melem has negligible photoinduced water‐splitting activity.     

Ionothermal Synthesis of Triazine–Heptazine‐Based Copolymers with Apparent Quantum Yields of 60 % at 420 nm for Solar Hydrogen Production from “Sea Water”

Polymeric carbon nitride (PCN), in either triazine or heptazine form, has been regarded as a promising metal‐free, environmentally benign, and sustainable photocatalyst for solar hydrogen production. However, PCN in most cases only exhibits moderate activity owing to its inherent properties, such as rapid charge carrier recombination. Herein we present a triazine–heptazine copolymer synthesized by simple post‐calcination of PCN in eutectic salts, that is, NaCl/KCl, to modulate the polymerization process and optimize the structure. The construction of an internal triazine–heptazine donor–acceptor (D‐A) heterostructure was affirmed to significantly accelerate interface charge transfer (CT) and thus boost the photocatalytic activity (AQY=60 % at 420 nm). This study highlights the construction of intermolecular D‐A copolymers in NaCl/KCl molten salts with higher melting points but in the absence of lithium to modulate the chemical structure and properties of PCN.     

Synthesis and structure of 2,5,8-triazido-s-heptazine: an energetic and luminescent precursor to nitrogen-rich carbon nitrides

Derivatized s-triazine (C3N3) precursors have seen significant recent use in the production of carbon nitride materials. Larger polycyclic molecular precursors, such as those containing an s-heptazine core (C6N7 or tri-s-triazine), may improve stability and order in carbon nitride products. In this Communication, we describe the synthesis and crystal structure of 2,5,8-triazido-s-heptazine (2). Synthesis of 2 was achieved from melon, an oligomeric s-heptazine synthesized by the pyrolysis of NH4SCN. Melon was converted to molecular 2,5,8-trichloro-s-heptazine, which was then transformed to the triazide upon reaction with (CH3)3SiN3. The crystal structure of 2 verifies that the s-heptazine is planar and the azides adopt a pinwheel-like C3h arrangement around the periphery. The s-heptazine core shows pi delocalization in the C-N bonds around the periphery (av. 1.33 A), while the internal planar C-N bonds are longer (1.40 A). The heptazine units pack into parallel, but offset, layered sheets in the crystal. The triazide 2 exhibits photoluminescence at 430 nm and rapidly and exothermically decomposes upon heating at 185 degrees C to produce a tan thermally stable carbon nitride powder with a formula near C3N4.     

Computation of aromatic C3N4 networks and synthesis of the molecular precursor N(C3N3)3Cl6

The successful synthesis and structural characterization of molecules that represent segments of extended solids is a valuable strategy for learning metric and stereochemical characteristics of those solids. This approach has been useful in cases in which the solids are particularly difficult to crystallize and thus their atomic connectivity and overall structures become difficult to deduce with X-ray diffraction techniques. One such class of materials is the covalently linked C(x)N(y) extended solids, where molecular analogues remain largely absent. In particular, structures of C(3)N(4) solids are controversial. This report illustrates the utility of a simple molecule, N(C(3)N(3))(3)Cl(6), in answering the question of whether triazine based C(3)N(4) phases are layered or instead they adopt 3D structures. Here, we present density functional calculations that clearly demonstrate the lower stability of graphitic C(3)N(4) relative to 3D analogues.     

Size Effects of the Anions in the Ionothermal Synthesis of Carbon Nitride Materials

Semiconducting carbon nitride polymers are used in metal-free photocatalysts and in opto-electronic devices. Conventionally, they are obtained using thermal and ionothermal syntheses in inscrutable, closed systems and therefore, their condensation behavior is poorly understood. Here, the synthetic protocols and properties are compared for two types of carbon nitride materials – 2D layered poly(triazine imide) (PTI) and hydrogen-bonded melem hydrate – obtained from three low-melting salt eutectics taken from the systematic series of the alkali metal halides: LiCl/KCl, LiBr/KBr, and LiI/KI. The size of the anion plays a significant role in the formation process of the condensed carbon nitride polymers, and it suggests a strong templating effect. The smaller anions (chloride and bromide) become incorporated into triazine (C₃N₃)-based PTI frameworks. The larger iodide does not stabilize the formation of a triazine-based polymer, but instead it leads to the formation of the heptazine (C₆N₇)-based hydrogen-bonded melem hydrate as the main crystalline phase. Melem hydrate, obtained as single-crystalline powders, was compared with PTI in photocatalytic hydrogen evolution from water and in an OLED device. Further, the emergence of each carbon nitride species from its corresponding salt eutectic was rationalized via density functional theory calculations. This study highlights the possibilities to further tailor the properties of eutectic salt melts for ionothermal synthesis of organic functional materials.     

Poly(triazine imide) with intercalation of lithium and chloride ions [(C3N3)2(NH(x)Li(1-x))3⋅LiCl]: a crystalline 2D carbon nitride network

Poly(triazine imide) with intercalation of lithium and chloride ions (PTI/Li⁺Cl^?) was synthesized by temperature‐induced condensation of dicyandiamide in a eutectic mixture of lithium chloride and potassium chloride as solvent. By using this ionothermal approach the well‐known problem of insufficient crystallinity of carbon nitride (CN) condensation products could be overcome. The structural characterization of PTI/Li⁺Cl^? resulted from a complementary approach using spectroscopic methods as well as different diffraction techniques. Due to the high crystallinity of PTI/Li⁺Cl^? a structure solution from both powder X‐ray and electron diffraction patterns using direct methods was possible; this yielded a triazine‐based structure model, in contrast to the proposed fully condensed heptazine‐based structure that has been reported recently. Further information from solid‐state NMR and FTIR spectroscopy as well as high‐resolution TEM investigations was used for Rietveld refinement with a goodness‐of‐fit (χ²) of 5.035 and wRp=0.05937. PTI/Li⁺Cl^? (P6₃cm (no. 185); a=846.82(10), c=675.02(9) pm) is a 2D network composed of essentially planar layers made up from imide‐bridged triazine units. Voids in these layers are stacked upon each other forming channels running parallel to [001], filled with Li⁺ and Cl^? ions. The presence of salt ions in the nanocrystallites as well as the existence of sp²‐hybridized carbon and nitrogen atoms typical of graphitic structures was confirmed by electron energy‐loss spectroscopy (EELS) measurements. Solid‐state NMR spectroscopy investigations using ¹⁵N‐labeled PTI/Li⁺Cl^? proved the absence of heptazine building blocks and NH₂ groups and corroborated the highly condensed, triazine‐based structure model.     

Synthesis,Crystal Structure,and Selected Properties of [Au(S2CNH2)2]SCN: A Precursor for Gold Macro-Needles Consisting of Gold Nanoparticles Glued by Graphitic Carbon Nitride

A new preparation route is developed for the synthesis of needle-like crystals of [Au(S₂CNH₂)₂]SCN, which avoids disproportionation of the Au^I salt used as a starting material. In the crystal structure, the two crystallographically independent Au^III centers are in a square-planar environment of two S₂CNH₂ ligands. The Hirshfeld surface analysis reveals the presence of noncovalent intermolecular S⋅⋅⋅S interactions, which are essential for the spatial arrangement of the molecules. Density functional theory (DFT) calculations including dispersion and damping corrections result in a unit cell volume very close to the value determined experimentally. Thermal decomposition in an inert atmosphere generates black needles with lengths of up to 500 μm. X-ray powder diffraction and pair distribution function analyses demonstrate that the needles are composed of nanosized crystals with a volume-weighted average domain size of 20(1) nm. According to results of X-ray photoemission experiments, the black needles are covered by a nitrogen-rich carbon nitride with composition near (CN)₂N. ¹³C solid-state NMR investigations indicate that two different carbon species are present, with signals corresponding well to heptazine units as in melon and triazine units as in poly(triazin imide) type compounds. Scanning transmission electron microscopy tomography evidences that the needles are composed of slightly elongated nanoparticles.     

Paracyanogen reexamined

Samples of paracyanogen were prepared by the pyrolysis of silver cyanide, the photolysis of cyanogen, and by sputtering a carbon electrode in a nitrogen plasma. This work was undertaken to establish whether these materials are alike and to derive structural information on the basis of FT-IR, ¹³C-MAS—NMR, and through temperature-programmed thermal decomposition. Paracyanogen derived from the above sources is indeed alike spectrally and thermally. Paracyanogen is a polymer that incorporates carbon, mostly sp², and nitrogen in a disordered structure. Cyanogen is the main volatile component released in the thermal decomposition of paracyanogen. The cyanogen released was identified by FT-IR and MS. The rate of release reaches a maximum at ca. 700°C. Paracyanogen is not an appropriate carbon nitride precursor because of its limited thermal stability. © 1993 John Wiley & Sons, Inc.     

Embedding Carbon Nitride into a Covalent Organic Framework with Enhanced Photocatalysis Performance

We report a new strategy to construct porous carbon nitride (PCN) by embedding a heptazine unit–the primary building block of carbon nitride–into the backbone of a covalent organic framework (COF). The strategy results in a new type of PCN which bears a fibrous morphology, high surface area and wide visible absorption. The photocatalytic performance was evaluated by photodegradation of an organic dye. We found that the introduction of the heptazine unit has a prominent effect on the catalytic activity, which demonstrates an effective strategy to prepare carbon nitride materials. This work opens up a new way for the preparation of carbon nitride for photocatalysis applications.     

Low-temperature approach to synthesize iron nitride from amorphous iron

Iron nitride was prepared by a nitridation reaction in NH 3 using amorphous iron as precursor. The precursor was prepared at ambient temperature through the process of reducing ferrous sulfate by potassium borohydride, followed by the nitridation at different temperatures. The nitridation reaction occurred at 548 K, and -Fe 2-3N was formed at 573 K. The reaction temperature was much lower than that using crystallized iron because of the characteristics of the amorphous materials. The existence of a small quantity of boron (1.6 wt.%) improved the stability of the amorphous precursor, which guaranteed an amorphous iron precursor at nitriding temperatures in excess of 548 K.     

The Reactivity of Cyameluric Chloride C6N7Cl3 towards Phosphines and Phosphine Oxides

Organophosphines (R₂PH) and phosphineoxides (R₂OPH) show a very high reactivity towards cyameluric chloride C₆N₇Cl₃. For example, 2,4,6‐trisdiphenylphosphino‐tri‐s‐triazine ( 1 ) forms quantitatively within a few seconds. Tris‐diphenylphosphinsulfide‐s‐heptazine ( 2 ) was obtained by reaction of 1 with sulfur. These compounds represent a new class of s‐heptazine derivatives which tend, unlike their s‐triazine analogues, to decompose in solution. 1 forms crystals with nitromethane, which were analysed by single‐crystal X‐ray diffraction. The nitromethane molecules fill the gaps in the crystal lattice supported by hydrogen bonds, C–H ··· π ring, and N–O ··· π ring interactions. All compounds were characterized by ¹H, ¹³C and ³¹P NMR and vibrational (FT‐IR, Raman) spectroscopy. The thermal stability of selected derivatives was measured by TG, indicating surprisingly low thermal decomposition temperatures.