共价三嗪框架复合Co单活性位点光催化制合成气

合成气是费托合成的关键原料,目前其主要是通过天然气重整或煤气化来制备,但是该制备过程需要苛刻的合成条件.光催化技术利用太阳能将CO2还原制备合成气是未来能源开发的重要研究方向.到目前为止,已经开发了许多用于生产合成气的光催化系统.然而,大多数催化体系使用昂贵的金属如Re和Ru作为光敏剂和或催化剂,利用非贵金属催化剂光催化还原CO2制合成气是实现环境可持续发展的一种有效方法.共价三嗪框架(CTF)由于具有可调节的光吸收性质、在分子水平上可调节的电子结构、高表面积和富氮结构,因而在光催化体系中具有明显优势.在此,我们设计了一种基于分子内异质结的CTF光催化CO2还原生成合成气体系,通过傅里叶变换红外光谱(FT-IR)、核磁共振波谱(NMR)和X射线光电子能谱(XPS)等表征证明了CTF内的分子内异质结,采用X射线多晶衍射(XRD)、比表面积及孔隙度分析(BET)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和紫外-可见吸收光谱(UV-Vis DRS)等表征手段充分研究了CTF的组成、结构与光学性质.将所制备的CTF光催化剂应用于可见光催化还原CO2反应中,实验结果表明,基于分子内异质结的CTF在10 h内达到3303μmol g^-1(CO:H2=1.4:1)的合成气产率,相对于不具有分子内异质结的CTF催化体系,产率提高了3倍.时间分辨荧光发射衰减光谱、荧光光谱和瞬时光电流图谱等测试结果表明,分子内异质结极大提高了CTF内光生载流子的空间分离和转移效率.反应体系中自组装形成的吡啶Co配合物作为Co单活性位点可有效吸附和配位活化CO2分子.理论计算进一步证明含有分子内异质结结构的CTF能极大地促进电荷分离效率,从而实现催化性能的增强.这项工作不仅显示了CTF在光催化等领域潜在的应用前景,而且为合理设计基于CTF的光催化体系提供了新的见解.     

Porphyrin‐based covalent triazine frameworks: Porosity,adsorption performance,and drug delivery

Two porous porphyrin‐based covalent triazine frameworks (PCTFs), in which porphyrin is incorporated as building block, have been synthesized by the Friedel–Crafts reaction. The copolymer PCTFs show large Brunauer–Emmett–Teller specific surface area of up to 1089 m² g^?1, high CO₂ uptake capacity reaching 139.9 mg g^?1 at 273 K/1.0 bar, and good selectivity for CO₂/CH₄ adsorption attaining 6.1 at 273 K/1.0 bar. The resulting porous solids also can be used as matrices for drug delivery of ibuprofen in vitro. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 2594–2600     

Halogen modified two-dimensional covalent triazine frameworks as visible-light driven photocatalysts for overall water splitting

The covalent triazine framework CTF-1 as a member of the two-dimensional covalent organic frameworks(COFs) is a category of novel metal-free photocatalysts for water splitting. The large band gap severely restricts its energy conversion efficiency. By means of the first-principles calculations, we proposed the decoration of CTF-1 by anchoring halogen atoms onto benzene moieties for improving the solar-to-hydrogen(STH) efficiency. The electronic structures reveal that the halogen substitution successfully decreases the band gap of CTF-1. Meanwhile, the calculated free energy changes along the reaction pathway indicate that all these COFs can spontaneously drive overall water splitting under light irradiation in a specific acid-base environment. The time-dependent ab initio non-adiabatic molecular dynamics simulations suggest that the electron-hole recombination periods of these COFs fall in a few to tens of nanoseconds. Excitingly, CTF-1 modified by linking six iodine atoms onto the benzene ring in the para-position(CTF-1-6I) shows a quite low band gap of 2.81 eV, indicating that it is a visible-light driven COF for overall photocatalytic water splitting. Correspondingly, CTF-1-6I also exhibits an extraordinarily promising STH efficiency of 3.70%, which is an order magnitude higher than that of the pristine CTF-1.     

Crystalline covalent triazine frameworks manipulated by aliphatic amine modulator

Crystallization is an unsolved challenge in the chemistry of covalent triazine frameworks(CTFs) due to the poorly controlled simultaneous polymerization and crystallization processes. Herein, the synthesis of crystalline CTFs via the introduction of aliphatic amine as a dynamic modulator is reported. By optimizing the amount of aliphatic amine, the crystallization process can be controlled in an open system, resulting in the synthesis of crystalline CTFs. These crystalline CTFs exhibit much bett...     

Crystalline covalent organic frameworks with hydrazone linkages

Condensation of 2,5-diethoxyterephthalohydrazide with 1,3,5-triformylbenzene or 1,3,5-tris(4-formylphenyl)benzene yields two new covalent organic frameworks, COF-42 and COF-43, in which the organic building units are linked through hydrazone bonds to form extended two-dimensional porous frameworks. Both materials are highly crystalline, display excellent chemical and thermal stability, and are permanently porous. These new COFs expand the scope of possibilities for this emerging class of porous materials.     

Linker-activated covalent organic frameworks with azo bridges

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Semiconducting covalent organic frameworks: a type of two-dimensional conducting polymers

In recent years,as a new class of two-dimensional polymer,covalent organic frameworks(COFs) have attracted intensive attention and developed rapidly.This review provides an overview of a type of COFs which can be utilized as organic semiconductors.Carefully choosing monomers as the building blocks will bestow different types of semiconducting character on COFs.We summarize the p-type,n-type and ambipolar semiconducting COFs and highlight the effects of π-functional building blocks on the photoconductive behaviors of the semiconducting COFs.     

二维Ni-Salphen基共价有机骨架的制备及电容性能

利用2,3,6,7,10,11-六氨基三苯六盐酸盐（HATP）和4,6-二羟基-5-甲基间苯二甲醛（DMDB）为构筑基元,构筑了二维Ni-Salphen基共价有机骨架（COFs）电极材料（Ni-Salphen-COF）。通过一系列方法对Ni-Salphen-COF的结构、形貌和电化学性能进行了表征和测试。三电极系统测试结果表明,Ni-Salphen-COF具有优异的电化学性能,在1 A·g^-1时,比电容达到531 F·g^-1,并显示良好的循环稳定性（10 000次循环后电容保持率为89%）。同时,二电极系统测试结果显示,在1 A·g^-1时,Ni-Salphen-COF//AC （AC为活性炭）比电容达176 F·g^-1;在功率密度为900 W·kg^-1时,最大能量密度为55 Wh·kg^-1。良好的性能可能归因于Ni-Salphen结构提高了电极材料的电导率、氧化还原活性和电荷转移能力。     

An overview on covalent organic frameworks: synthetic reactions and miscellaneous applications

Covalent organic frameworks (COFs) are an emerging class of porous crystalline materials which are completely constructed from organic building blocks through robust covalent bonds. High surface areas, compositional and structural tunability, low density, and superior stability have rendered COF candidates in a variety of applications, such as adsorption and separation, catalysis, electronics, chemical sensing, optics, and so forth. To better understand the structures and properties of COFs as well as the design principles, it is of great significance to learn about the linkages formed during synthetic reactions that contribute to the high crystallinity and stability of COFs. In this review, we will first discuss various linkages that have been utilized for COF construction up to date, followed by an outline of their miscellaneous applications, providing a comprehensive and detailed overview in this file.     

Multiscale study on hydrogen storage based on covalent organic frameworks

In this paper, we performed a multiscale study on the hydrogen storage capacity of Li–Sc doped and Li-C₆₀ injected covalent organic frameworks (COFs)-based phthalocyanine, porphyrin and TBPS COFs. We combined the first-principles studies of hydrogen adsorption and grand canonical Monte Carlo (GCMC) simulations of hydrogen adsorption in nine designed COFs. The first-principles calculations revealed that the Li atoms can be doped on the surface of the Sc-doped COFs with binding energy from ?83.9 to ?160.2 kJ/mol. Each Li atom can bind three H₂ molecules with the adsorption energy between ?16.8 and ?20.0 kJ/mol. The GCMC simulations have predicted that all the nine designed COFs can reach the Department of Energy’s 2015 target (5.5 wt% and 40 g/L) at T = 77 K and P = 100 bar. The optimum conditions of hydrogen storage for Li-C₆₀@Li–Sc-PR-TBPS2, the promising materials, are T = 193 K (?80 °C) and P = 100 bar with a gravimetric H₂ density of 8.19 wt% and volumetric H₂ uptake of 42.6 g/L. Finally, we further convinced the importance of Sc in improving H₂ uptake in doped COFs.     

Solid phase microwave-assisted fabrication of Fe-doped ZIF-8 for single-atom Fe-N-C electrocatalysts on oxygen reduction

Fe-N-C endowed with inexpensiveness,high activity,and excellent anti-poisoning power have emerged as promising candidate catalysts for oxygen reduction reaction(ORR).Single-atom Fe-N-C electrocatalysts derived from Fe-doped ZIF-8 represent the top-level ORR performance.However,the current fabrication of Fe-doped ZIF-8 relies on heavy consumption of time,energy,cost and organic solvents.Herein,we develop a rapid and solvent-free method to produce Fe-doped ZIF-8 under microwave irradiation,which can be easily amplified in combination with ball-milling.After rational pyrolysis,Fe-N-C catalysts with atomic FeN4 sites well dispersed on the hierarchically porous carbon matrix are obtained,which exhibit exceptional ORR performance with a half-wave potential of 0.782 V(vs.reversible hydrogen electrode(RHE))and brilliant methanol tolerance.The assembled direct methanol fuel cells(DMFCs)endow a peak power density of 61 mW cm^-2 and extraordinary stability,highlighting the application perspective of this strategy.     

Phenothiazine-based covalent organic frameworks with low exciton binding energies for photocatalysis

Designing delocalized excitons with low binding energy (E_b) in organic semiconductors is urgently required for efficient photochemistry because the excitons in most organic materials are localized with a high E_b of >300 meV. In this work, we report the achievement of a low E_b of ∼50 meV by constructing phenothiazine-based covalent organic frameworks (COFs) with inherent crystallinity, porosity, chemical robustness, and feasibility of bandgap engineering. The low E_b facilitates effective exciton dissociation and thus promotes photocatalysis by using these COFs. As a demonstration, we subject these COFs to photocatalytic polymerization to synthesize polymers with remarkably high molecular weight without any requirement of the metal catalyst. Our results can facilitate the rational design of porous materials with low E_b for efficient photocatalysis.

We report the construction of phenothiazine-based covalent organic frameworks, which exhibited diverse structures, the feasibility of bandgap engineering, and unprecedented ultralow exciton binding energy of ∼50 meV for photocatalytic polymerization.     

Constructing energetic covalent organic frameworks with high stability and low sensitivity

Developing new functional explosives that display high stability, good energy performance, and low sensitivity are one of the key directions of energetic materials research. In this work, two-dimensional(2D)Schiff-based energetic covalent organic frameworks(COFs) are prepared based on triaminoguanidine salts with different anions as building blocks. Benefiting from the robust covalent bond in 2D extended polygons and strong π-π interactions in the eclipsed interlayers, the synthesized energetic ...     

Crystals as molecules: postsynthesis covalent functionalization of zeolitic imidazolate frameworks

A new crystalline zeolitic imidazolate framework, ZIF-90, was prepared from zinc(II) nitrate and imidazolate-2-carboxyaldehyde (ICA) and found to have the sodalite-type topology. Its 3D porous framework has an aperture of 3.5 A and a pore size of 11.2 A. The pores are decorated by the aldehyde functionality of ICA which has allowed its transformation to the alcohol functionality by reduction with NaBH4 and its conversion to imine functionality by reaction with ethanolamine to give ZIF-91 and ZIF-92, respectively. The N2 adsorption isotherm of ZIF-90 shows a highly porous material with calculated Langmuir and BET surface areas of 1320 and 1270 m2 g(-1). Both functionalized ZIFs retained high crystallinity and in addition ZIF-91 maintained permanent porosity (surface areas: 1070 and 1010 m2 g(-1)).     

Reversing electron transfer in a covalent triazine framework for efficient photocatalytic hydrogen evolution

Covalent triazine-based frameworks (CTFs) have emerged as some of the most important materials for photocatalytic water splitting. However, development of CTF-based photocatalytic systems with non-platinum cocatalysts for highly efficient hydrogen evolution still remains a challenge. Herein, we demonstrated, for the first time, a one-step phosphidation strategy for simultaneously achieving phosphorus atom bonding with the benzene rings of CTFs and the anchoring of well-defined dicobalt phosphide (Co₂P) nanocrystals (∼7 nm). The hydrogen evolution activities of CTFs were significantly enhanced under simulated solar-light (7.6 mmol h⁻¹ g⁻¹), more than 20 times higher than that of the CTF/Co₂P composite. Both comparative experiments and in situ X-ray photoelectron spectroscopy reveal that the strong interfacial P–C bonding and the anchoring of the Co₂P cocatalyst reverse the charge transfer direction from triazine to benzene rings, promote charge separation, and accelerate hydrogen evolution. Thus, the rational anchoring of transition-metal phosphides on conjugated polymers should be a promising approach for developing highly efficient photocatalysts for hydrogen evolution.

Reversing the electron transfer in a covalent triazine-based framework by Co₂P anchoring achieved highly efficient photocatalytic hydrogen evolution from water splitting.

Photocatalytic water splitting into hydrogen fuels has been considered as a promising technique for converting solar energy into chemical energy.^1–3 To achieve this target, it is necessary to design and construct photocatalysts with high solar-to-hydrogen (STH) conversion efficiency.^4,5 Recently, covalent triazine-based frameworks (CTFs),^6,7 as a new class of conjugated polymer materials, have attracted significant attention in the photocatalytic water splitting field owing to their visible-light response, organized architecture, adjustable pore-size, and controllable functionalization.^8–12 However, owing to the high charge recombination and fewer active sites for the hydrogen evolution reaction (HER), the photocatalytic activities of pristine CTFs are very low. To overcome this drawback, noble-metal platinum (Pt) is generally required in CTF-based photocatalytic systems as the HER cocatalyst for promoting charge separation and catalyzing hydrogen generation.^13–15 However, the high cost and scarcity of metallic Pt greatly limit the large-scale commercial applications. Thus, it is highly desirable to explore economical materials as noble-metal substitutes for achieving comparable or even superior hydrogen evolution activities.Recently, transition-metal phosphides (TMPs) have attracted intensive attention as HER cocatalysts for photocatalytic water splitting, owing to their unique structural and electronic properties.^16–20 Up to now, a variety of semiconductor materials, including metal oxides,^21,22 metal sulfides,^23,24 g-C₃N₄,^25,26 MOFs,^27,28etc., decorated with TMPs have been extensively investigated, and the hydrogen generation activity in some reports is even higher than that of Pt cocatalysts. However, related studies about the decoration of TMP cocatalysts on CTF photocatalysts for highly efficient hydrogen generation have not been reported so far. Taking into account the molecular structure of CTFs, the nitrogen sites in the triazine frameworks could easily coordinate with transition metal ions to form nitrogen–metal interactions.^29–31 In contrast, the coordination of TMPs with CTFs is relatively difficult owing to the high electronegativity of P sites which could draw electrons from metal atoms.¹⁷ Moreover, most reported TMPs were fabricated directly from precursor metal salts, oxides, etc.,^32–35 and the resultant large-dimensions usually lead to very limited contact-interface with CTFs, or even a physical mixture form. Accordingly, the charge transfer between TMPs and CTFs is significantly restrained. Thus, effective anchoring of TMPs on CTFs for achieving highly efficient photocatalytic hydrogen evolution still remains a great challenge.Herein, a one-step phosphidation strategy has been developed to achieve phosphorus bonding with the benzene rings of CTFs and the anchoring of well-defined dicobalt phosphide (Co₂P) nanocrystals (∼7 nm). The photocatalytic results clearly reveal that an excellent hydrogen evolution activity (7.6 mmol h⁻¹ g⁻¹) is achieved, which is much higher than that of the CTF/Co₂P composite (0.37 mmol h⁻¹ g⁻¹). More detailed studies confirm that the phosphidation strategy could effectively facilitate the interfacial bonding between CTFs and Co₂P nanocrystals. More importantly, the SI-XPS results clearly suggest that the charge transfer direction in CTFs is completely reversed, and the photo-generated electrons efficiently transferred from the triazine rings to P-bonded benzene rings, where the Co₂P cocatalyst attracted electrons through the interfacial P–C bonds for efficient hydrogen evolution. To our knowledge, this is the first report on incorporating TMPs on a CTF photocatalyst for enhancing the hydrogen evolution activity. Fig. 1A shows the basic procedures for the preparation of CTF polymer anchored Co₂P nanocrystals. Briefly, the CTFs with adsorbed cobalt ions were directly phosphatized by thermal decomposition of NaH₂PO₂ under an Ar atmosphere, which could simultaneously achieve P atom bonding with benzene rings and the anchoring of the Co₂P cocatalyst (marked as P-CTF-Co₂P). Fig. 1B shows the typical transmission electron microscopy (TEM) image of the obtained sample, clearly revealing that well-defined Co₂P nanocrystals with an average diameter of 5∼9 nm were uniformly dispersed on the CTF surface. Furthermore, the high-resolution TEM (HR-TEM) image (Fig. 1C) of the formed nanocrystals exhibited two lattice fringes with d-spacing values of 0.20 and 0.21 nm, respectively, which could be well indexed to the (211) and (121) planes of orthorhombic Co₂P crystals.^36,37Fig. 1D shows the X-ray diffraction (XRD) pattern of the obtained P-CTF-Co₂P sample. For comparison, the pristine CTF, and Co₂P nanocrystals have also been studied. It can be clearly seen that for the P-CTF-Co₂P sample, except for the two broad diffraction peaks at 7.3° and 26.1° attributed to the in-plane (100) facets and interlayer (001) stacking of CTFs,^38,39 the other XRD peaks well matched with those of the Co₂P crystals,^40,41 confirming the successful incorporation of the Co₂P cocatalyst in the CTFs. Furthermore, energy-dispersive X-ray (EDX) elemental mapping (Fig. 1E) clearly reveals the uniform distribution of C, N, Co and P elements in the whole detection region, further confirming the uniform dispersion of Co₂P on CTF polymers.Open in a separate windowFig. 1(A) Basic procedures and the ideal structure scheme for fabricating P-CTF-Co₂P catalysts; (B) TEM image and (C) HR-TEM image of the P-CTF-Co₂P catalyst; (D) XRD patterns of CTFs, Co₂P and P-CTF-Co₂P catalysts; (E) EDX elemental mapping images of the P-CTF-Co₂P catalyst.To further explore the P-bonding sites, high-resolution X-ray photoelectron spectroscopy (XPS) was performed on both P-CTF-Co₂P and pristine CTFs. As shown in Fig. 2A, for pristine CTF samples, the C 1s peak could be well fitted into two peaks located at 284.8 and 286.8 eV, which could be assigned to C–C C and N–C N bonds,^38,39,42 respectively. Notably, after the phosphidation treatment, a new peak at 285.3 eV attributed to P–C bonds was detected in the P-CTF-Co₂P sample,^43–45 and the C–C C to N–C N ratio was significantly decreased from 10.2 (pristine CTFs) to 7.1 (P-CTF-Co₂P). In contrast, compared with the N 1s spectrum of the pristine CTF sample, no evident change of peak shape and intensity could be detected in the P-CTF-Co₂P sample (Fig. 2B). These results clearly reveal that after the phosphidation treatment, P atoms should mainly bond with the carbon sites of the benzene rings in CTFs instead of the triazine rings.^46,47 To further confirm this inference, XPS studies on phosphatized CTFs without Co₂P nanocrystals (marked as P-CTFs) were also performed. It can be clearly seen from Fig. S9^† that similar changes for the C 1s and N 1s peaks of P-CTF-Co₂P (Fig. 2A and B) are observed in the P-CTF sample. Moreover, in both P-CTF-Co₂P and P-CTF samples, an evident P 2p peak corresponding to P–C bonds could be observed (Fig. 2C),⁴⁸ further indicating the bonding of P atoms with C atoms on the benzene rings in CTFs after the phosphidation treatment. However, note that the binding energy (BE) position of P–C peaks in P-CTF-Co₂P (133.6 eV) is slightly higher than that of P-CTF (133.2 eV), which should be attributed to the anchoring with Co₂P nanocrystals. In addition to the XPS investigations, solid-state cross-polarization magic angle spinning carbon-13 and phosphorus-31 nuclear magnetic resonance (¹³C-NMR and ³¹P-NMR) spectroscopy studies were further performed to explore the bonding sites of phosphorus atoms in the P-CTF (Fig. S10^†). The Fourier-transformed infrared (FTIR) spectrum was also employed (Fig. 2D). It can be clearly observed that the typical FTIR peaks of the triazine frameworks and the stretching vibrations of carbon–nitrogen (C–N) (1521 cm⁻¹ and 1354 cm⁻¹) in P-CTF-Co₂P are consistent with those of pristine CTFs.^38,39 However, the peaks at 1672 cm⁻¹ and 1014 cm⁻¹ corresponding to the stretching vibrations of C C and C–H in benzene rings were evidently decreased in P-CTF-Co₂P compared with those of pristine CTFs.^49,50 Combining the results of XPS, NMR, and FTIR, it can be concluded that in the obtained P-CTF-Co₂P sample, the carbon sites of the benzene rings in the CTFs were partially bonded with P-atoms, which should anchor Co₂P through P–Co bonding.Open in a separate windowFig. 2High-resolution XPS spectra of (A) C 1, (B) N 1s and (C) P 2p in CTF, P-CTF and P-CTF-Co₂P samples (the insets show the molecular structures); (D) Fourier transform infrared spectroscopy (FT-IR) of CTF and P-CTF-Co₂P samples.Furthermore, the hydrogen evolution activities of the P-CTF-Co₂P (2 wt% Co₂P) sample were evaluated under simulated solar light irradiation. For comparison, the CTF/Co₂P composite and pristine CTFs were also measured under the same conditions. As shown in Fig. 3A, the P-CTF-Co₂P sample exhibits a much higher H₂ evolution activity (15.2 mmol g⁻¹) than the CTF/Co₂P composite (0.7 mmol g⁻¹) at 2 h, while no evident hydrogen generation could be detected in the pristine CTF sample. To further explore the crucial roles of interfacial P-bonding in CTFs and the anchoring of the Co₂P cocatalyst, the hydrogen evolution rates of various samples were calculated and their comparison is shown in Fig. 3B. Obviously, compared with the excellent activity of P-CTF-Co₂P (7.6 mmol h⁻¹ g⁻¹), CTF/Co₂P and P-CTF/Co₂P only exhibit very low hydrogen evolution rates of 0.37 and 0.38 mmol h⁻¹ g⁻¹, respectively, while no evident activity could be detected for CTFs, Co₂P, and P-CTF samples. The above results clearly confirm that the significant enhancement of the hydrogen evolution activity of the P-CTF-Co₂P sample should be mainly attributed to the P-bonding and the anchoring of the Co₂P cocatalyst. Furthermore, the photocatalytic performances of CTFs decorated with Pt nanoparticles were also studied (the inset of Fig. 3B, Fig. S15^†), as Pt is generally recognized as the most active cocatalyst for hydrogen generation. Amazingly, the hydrogen production rates of the Pt cocatalyst at different amounts (1∼5 wt%) were all lower than that of the P-CTF-Co₂P sample, indicating that the rational bonding of the Co₂P cocatalyst on CTFs should be a promising strategy for achieving highly efficient photocatalytic hydrogen evolution.Open in a separate windowFig. 3(A) Photocatalytic H₂ evolution tests of the as-prepared photocatalysts; (B) comparative presentation of the hydrogen evolution rates; (C) ultraviolet-visible diffuse reflectance spectrum of the P-CTF-Co₂P photocatalyst (red solid line) and wavelength-dependent hydrogen production activities of the P-CTF-Co₂P photocatalyst within 2 h; light source: a 300 W Xe lamp equipped with various cut-off filters. (D) The cycling photocatalytic tests of P-CTF-Co₂P and the CTF/Co₂P composite; (E) I–t curves of various catalysts at −0.3 V (vs. SCE); (F) electrochemical impedance spectroscopy under light of the various catalysts. Measurements were conducted in 0.2 mol L⁻¹ Na₂SO₄ electrolyte solution under AM 1.5 G illumination.Furthermore, the relationship between wavelength and the hydrogen evolution activity of the P-CTF-Co₂P photocatalyst was studied and is shown in Fig. 3C. With increasing the wavelength from full-spectrum light irradiation (λ > 300) to 500 nm, the hydrogen evolution rate significantly decreased from 7.6 to 0.06 mmol h⁻¹ g⁻¹, which is generally consistent with their absorption spectrum. The highest value of the apparent quantum yields (AQYs) attained is 31.8% at 365 nm. Moreover, the photocatalytic stability and durability of P-CTF-Co₂P and CTF/Co₂P were examined by cycling experiments. As shown in Fig. 3D, the CTF/Co₂P composite demonstrated relatively poor stability, and there is no evident activity after only one cycling experiment, resulting from the incompact combination between CTFs and Co₂P (Fig. S5^†). In contrast, the P-CTF-Co₂P sample exhibits relatively high stability, and no evident inactivation was detected during the whole test. These results further confirm that in addition to enhancing activities, the interfacial P–C bonds and the anchoring of the Co₂P cocatalyst could also effectively promote the hydrogen evolution stability. To further confirm the efficient charge separation and electron transfer in P-CTF-Co₂P, photoelectrochemical (PEC) tests were performed. As shown in Fig. 3E, the amperometric I–t curves clearly reveal that P-CTF-Co₂P exhibits higher photocurrent density than pristine CTFs and CTF/Co₂P samples, confirming its more efficient charge separation capability. Moreover, electrochemical impedance spectroscopy (EIS) was also performed to explore the interfacial charge transfer process.^51,52 As shown in Fig. 3F, the P-CTF-Co₂P sample with the smallest arc radius revealed remarkably increased interfacial charge transport efficiency. These PEC results clearly demonstrate the highly efficient photogenerated charge separation and transfer in the P-CTF-Co₂P photocatalyst, which are highly consistent with the above photocatalytic hydrogen evolution results.The photo-induced charge separation and transfer in the excited state are crucial processes for determining the photocatalytic activity. Herein, we have demonstrated an in situ irradiation X-ray photoelectron spectroscopy (SI-XPS) technique for exploring the intrinsic charge separation and transfer mechanisms between Co₂P and CTFs. As shown in Fig. S20 and S21,^† no evident binding energy (BE) shift could be observed in both pristine CTF and Co₂P samples, confirming their relatively poor charge separation capability. Amazingly, for the P-CTF-Co₂P sample (Fig. 4), distinct variations for C 1s, N 1s, P 2p, and Co 2p peaks were detected in the excited state. More specifically, the C 1s and N 1s peaks in the CTFs shifted towards the high BE region by 0.3 eV accompanied by the broadening of peak shape (Fig. 4B and C), while the P 2p and Co 2p peaks of the Co₂P cocatalyst shifted towards the low BE direction by 0.3 and 0.4 eV (Fig. 4D and E), respectively.Interestingly, as shown in Fig. 4D, the P 2p peaks attributed to interfacial P–C bonds exhibited no evident BE change under dark and light irradiation. On the basis of the above SI-XPS results, it can be concluded that under light irradiation, the photo-excited electrons effectively transferred from CTFs to Co₂P through the P-bonding sites as well as the interfacial P–C bonds, leading to electron enrichment in Co₂P nanocrystals and hole enrichment in CTFs. To further confirm the crucial roles of interfacial P-bonding in CTFs and the anchoring of the Co₂P cocatalyst in promoting charge separation, SI-XPS studies for the CTF/Co₂P composite were also conducted. As shown in Fig. S22,^† no new peak or shape change of C 1s, N 1s, P 2p, and Co 2p peaks could be detected in CTF/Co₂P compared with pristine CTFs and Co₂P in the ground state, indicating no interfacial bonding in the CTF/Co₂P composite. Furthermore, in the excited state, no evident BE shifts and shape change could be observed, indicating the poor charge-separation capability of CTF/Co₂P, which is highly consistent with its fairly low hydrogen evolution activity. These SI-XPS results further confirm the crucial roles of interfacial P-bonding in CTFs and the anchoring of the Co₂P cocatalyst in efficiently promoting charge separation and enhancing the photocatalytic activity.Open in a separate windowFig. 4(A) Schematic illustration of the SI-XPS technique for direct observation of electron transfer in the excited state. The (B) C 1s, (C) N 1s, (D) P 2p and (E) Co 2p of SI-XPS spectra in the P-CTF-Co₂P sample tested under dark and light illumination.On the basis of the above results, it can be concluded that the P-bonding and Co₂P-cocatalyst anchoring should reverse the photo-induced electron transfer direction from the triazine to benzene rings in CTFs. To further confirm this speculation, the photocatalytic behavior of various CTF-based photocatalysts was studied and their electron transfer directions in excitation states have been proposed (Fig. 5A and B). First, when Co²⁺ ions were further bonded with the N sites of the triazine rings in the P-CTF-Co₂P sample (marked as P-CTF-Co₂P-Co), the hydrogen evolution activity remarkably increased from 7.6 up to 8.4 mmol h⁻¹ g⁻¹. This result clearly reveals that the Co–N bonding in P-CTF-Co₂P could further promote charge separation due to the effective hole trapping on Co sites. In contrast, when the Pt cocatalyst was decorated on the P-CTF-Co₂P sample (marked as P-CTF-Co₂P–Pt) to form Pt–N coordination, the photocatalytic activity significantly decreased to 4.6 mmol h⁻¹ g⁻¹, mainly resulting from the electron transfer competition between Pt and Co₂P. Furthermore, the decoration of the single Pt cocatalyst on CTFs promoted the photo-induced electron transfer from the benzene rings to triazine rings, and the photocatalytic activity achieved was up to 3.8 mmol h⁻¹ g⁻¹. Furthermore, in situ FTIR spectroscopy was performed for exploring the intermediate products in the photocatalytic process of the P-CTF-Co₂P sample with co-adsorption of H₂O vapor (Fig. S27^†), which also demonstrated the direction of electron transfer combined with the in situ XPS results. Accordingly, a possible mechanism has been proposed for clarifying the hydrogen evolution activities of the P-CTF-Co₂P photocatalyst (Fig. 5B). Owing to the P-bonding and the anchoring of Co₂P nanocrystals, the photo-generated electrons could effectively transfer from the triazine rings to benzene rings, where the electrons are trapped by the Co₂P cocatalyst through the interfacial P–C bonds for the hydrogen evolution reaction. Simultaneously, the holes left behind in the CTFs participated in oxidation reactions. More importantly, this mechanism may provide a new insight for understanding the crucial roles of interfacial P-bonding in CTFs and their bonding with Co₂P in promoting the charge separation for efficient hydrogen evolution.Open in a separate windowFig. 5(A) Photocatalytic hydrogen evolution activities of various samples and the scheme of electron-transfer direction in the excited state. (B) The ideal structural illustration of the interfacial bonding and charge transfer of the P-CTF-Co₂P photocatalyst for hydrogen evolution.     

MOFs fertilized transition-metallic single-atom electrocatalysts for highly-efficient oxygen reduction: Spreading the synthesis strategies and advanced identification

Metal-organic frameworks(MOFs) have been widely used in oxygen reduction reaction(ORR) of fuel cells and metal-air batteries, attributed to their unique structures and compositions. Recently, the preparation of transition-metallic single-atom electrocatalysts(TM-SACs) using MOFs as precursors or templates has made great progress. Herein, the development history of SACs prepared based on MOFs and their characterization are overviewed firstly, and then several strategies are summarized for prepari...     

Redistribution of Li-ions using covalent organic frameworks towards dendrite-free lithium anodes: a mechanism based on a Galton Board

Because of its high theoretical specific capacity and low reduction potential, Li metal is considered to be key to reaching high energy density in rechargeable batteries. In this context, most of the research has focused on suppressing dendrite formation during Li deposition to improve the cycling reversibility and safety of the batteries. Here, covalent organic framework(COF)film coating on a commercial polypropylene separator is applied as an ion redistributor to eliminate Li dendrites. The COF crystallites consist of ordered nanochannels that hinder the movement of anions while allowing Li-ions to transport across,leading to a high Li-ion transference number of 0.77±0.01. The transport of Li-ions across the COF film can be considered to be analogous to beads passing through a Galton Board, a model that demonstrates a statistical concept of a normal distribution.Thus, an even distribution of Li-ions is obtained at the COF/Li metal interface. The controlled Li-ion flux yields a smooth Li metal surface after 1,000 h(500 times) of cycling, leading to a significantly improved cycling stability and reversibility, as demonstrated by Cu||Li half cells, Li||Li symmetric cells, and Li Fe PO_4||Li full cells. These results suggest that, following the principle of a Galton Board, nanopore insulators such as COF-based materials are effective ion distributors for the different energy storage or conversion systems.     

Electroanalysis based on stand-alone matrices and electrode-modifying films with silica sol-gel frameworks: a review

Silica sol-gel matrices and its organically modified analogues that contain aqueous electrolytes, ionic liquids, or other ionic conductors constitute stand-alone solid-state electrochemical cells when hosting electrodes or serve as modifying films on working electrodes in conventional cells. These materials facilitate a wide variety of analytical applications and are employed in various designs of power sources. In this review, analytical applications are the focus. Solid-state cells that serve as gas sensors, including in chromatographic detectors of gas-phase analytes, are described. Sol-gel films that modify working electrodes to perform functions such as hosting electrochemical catalysts and acting as size-exclusion moieties that protect the electrode from passivation by adsorption of macromolecules are discussed with emphasis on pore size, structure, and orientation. Silica sol-gel chemistry has been studied extensively; thus, factors that control its general properties as frameworks for solid-state cells and for thin films on the working electrode are well characterized. Here, recent advances such as the use of dendrimers and of nanoscale beads in conjunction with electrochemically assisted deposition of silica to template pore size and distribution are emphasized. Related topics include replacing aqueous solutions as the internal electrolyte with room-temperature ionic liquids, using the sol-gel as an anchor for functional groups and modifying electrodes with silica-based composites.

     

Selective covalent conjugation of phosphorothioate DNA oligonucleotides with streptavidin

Protein-DNA conjugates have found numerous applications in the field of diagnostics and nanobiotechnology, however, their intrinsic susceptibility to DNA degradation by nucleases represents a major obstacle for many applications. We here report the selective covalent conjugation of the protein streptavidin (STV) with phosphorothioate oligonucleotides (psDNA) containing a terminal alkylthiolgroup as the chemically addressable linking unit, using a heterobifunctional NHS-/maleimide crosslinker. The psDNA-STV conjugates were synthesized in about 10% isolated yields. We demonstrate that the terminal alkylthiol group selectively reacts with the maleimide while the backbone sulfur atoms are not engaged in chemical conjugation. The novel psDNA-STV conjugates retain their binding capabilities for both biotinylated macromolecules and the complementary nucleic acid. Moreover, the psDNA-STV conjugate retained its binding capacity for complementary oligomers even after a nuclease digestion step, which effectively degrades deoxyribonucleotide oligomers and thus the binding capability of regular DNA-STV conjugates. The psDNA-STV therefore hold particular promise for applications e.g. in proteome research and novel biosensing devices, where interfering endogenous nucleic acids need to be removed from analytes by nuclease digestion.