Three novel organic-inorganic hybrid materials based on decaoxovanadates obtained from a new liquid phase reaction

Abstract

Three organic-inorganic hybrid materials based on decavanadates: H₄V₁₀O₂₈·(HMTA-CH₂OH)₂·6H₂O (1), H₄V₁₀O₂₈·(HMTA-CH₂OH)₂·4H₂O (2) and H₂V₁₀O₂₈·(HMTA)₂·[Co(H₂O)₆]₂·6H₂O (3) (HMTA = hexamethylenetetramine) have been crystallized from a new liquid phase reaction. These complexes display 3D supramolecular networks through various hydrogen bonds and generate typical binodal topologies. Complex 1 was further used as a functional precursor to prepare V, N-containing mesoporous carbon foams (V, N-MCFs), which exhibit highly ordered mesostructures with specific surface areas of 653 m² g⁻¹ and uniform pore sizes of 2.9 nm. Cyclic voltammetry and charge-discharge tests indicate that V, N-MCFs possess much higher capacitance in comparison with pure mesoporous carbon foams (MCFs).

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Introduction

Exploratory synthesis and research on organic–inorganic hybrid materials has been an attractive field due to their potential applications as functional materials, as well as their intriguing variety of architectures and topologies.¹ Polyoxovanadates are one kind of significant metal-oxo clusters with nanosizes, abundant networks, interesting electronic and structural properties which have considerable importance in the area of catalysis, biology, geochemistry and material sciences,² and they have been employed as inorganic building blocks for constructing supramolecular arrays with various organic ligands recently.³ Among the polyoxovanadates, decavanadates are one of the most common polyanions in the presence of a number of organic bases. Experimental studies have shown that whenever the pH of the initial solution is in the range of 4 to 6, decavanadates containing salts are predominantly isolated⁴ whereas the decavanadate polyanions are fully oxidized and frequently protonated, and the available nucleophilic surface of oxygen atoms on decavanadates interacts with organic ligands to form multiple hydrogen bonds for constructing a high-dimensional supramolecular network.

Herein, we report the synthesis and structural characterization of three organic-inorganic hybrid materials based on decavanadates: H₄V₁₀O₂₈·(HMTA-CH₂OH)₂·6H₂O (1), H₄V₁₀O₂₈·(HMTA-CH₂OH)₂·4H₂O (2), H₂V₁₀O₂₈·(HMTA)₂·[Co(H₂O)₆]₂·6H₂O (3) (HMTA = hexamethylenetetramine). All of them were obtained from a novel liquid phase reaction (Scheme 1).

Scheme 1 Syntheses of 1–3.

Porous carbon materials, which possess excellent chemical and physical stability, are very interesting materials for sensors, membranes, separation, catalyst supports, energy conversion and storage.⁵ Especially, mesoporous carbon foams (MCFs) attract broad attention as supercapacitor electrode materials due to their high specific surface area, abundant mesoporous structure and appropriately porous size for quick mass transfer and ion diffusion.⁶ However, due to their low electrical conductivity and relatively inert surface, the specific capacitance of MCFs is still limited.⁷ To improve the electrochemical behaviors, various compounds were introduced into the MCFs.⁸ For instance, it was reported that the incorporation of vanadium oxides or vanadate salts into MCFs could enhance their electrochemical properties,^8h–8j while introducing compounds containing nitrogen^8k–8n into MCFs could increase their graphitization degree and consequent conductivity. Complex 1 is easy to prepare and can be dissolved in water, therefore, we employed complex 1 as a functional precursor to prepare V, N-containing mesoporous carbon foams (V, N-MCFs) through an evaporation induced self-assembly approach, and evaluated their electrochemical behaviors through cyclic voltammetry and galvanostatic charge-discharge analysis.

Experimental

Materials and measurements

All the starting reagents were purchased commercially and used without further purification. Elemental analyses were performed on a Perkin-Elmer 2400II elemental analyzer. IR spectra were measured in KBr pellets on a Nicolet 5DX FT-IR spectrometer. Transmission electron microscopy (TEM) images were obtained by using a JEOL JEM-1230. Nitrogen sorption analyses were performed on Micromeritics Tristar 3000, with samples being heated to 200 °C under vacuum (10⁻⁵ Torr) for 2 h. Nitrogen adsorption data were taken at the liquid nitrogen temperature using a Micromeritics Tristar 3000 analyzer to calculate the specific surface area by the Brunauer-Emmett-Teller (BET) model. Thermal gravimetric analyses (TGA) were carried out using a Netzsch STA 449C thermoanalysis instrument in the temperature range 30–800 °C. Powder X-ray diffraction (XRD) experiments were taken on a Bruker D8 Advance diffractometer with Cu-Kα 1 (λ = 1.54056 Å) radiation. Raman spectra were obtained by using a Renishaw Invia. system, under λ_exc = 514 nm laser excitation.

Synthesis of H₄V₁₀O₂₈·(HMTA-CH₂OH)₂·6H₂O (1) and H₄V₁₀O₂₈·(HMTA-CH₂OH)₂·4H₂O (2)

0.466 g NH₄VO₃ (4.00 mmol), 40 ml of CH₃OH (methanol) and 15 ml of 30% H₂O₂ were mixed and stirred at 60 °C for 4 h, the resulting orange solution (pH between 3.5 and 4.5) was kept at 20 °C. Orange block crystals of 1 suitable for X-ray crystallographic study were crystallized via slow evaporation in 2 weeks. Yield: 86% based on V. IR (KBr pellets, ν/cm⁻¹): 3428(s), 3209(s), 3001(m), 1631(m), 1407(m), 1385(m), 1261(m), 1230(w), 1106(m), 1072(m), 1037(m), 999(m), 833(s), 744(s), 663(m), 594(m), 528(m), 501(m) and 447(s). Anal. Calcd. for C₁₄H₄₆N₈O₃₆V₁₀: C, 11.91%; H, 3.28%; N, 7.93. Found: C, 11.85%; H, 3.24%; N, 8.01. If the resulting orange solution was evaporated at 50 °C, brown block crystals of 2 could be obtained in 2 days. Yield: 68% based on V. IR (KBr pellets, ν/cm⁻¹): 3425(s), 2923(s), 2854(m), 1639(s), 1403(m), 1303(w), 1257(m), 1218(w), 1022(m), 983(m), 960(s), 744(m), 586(m) and 509(m). Anal. Calcd. for C₁₄H₄₂N₈O₃₄V₁₀: C, 12.22%; H, 3.08%; N, 8.14. Found: C, 12.15%; H, 3.04%; N, 8.21.

Synthesis of H₂V₁₀O₂₈·(HMTA)₂·[Co(H₂O)₆]₂·6H₂O (3)

0.466 g NH₄VO₃ (4.00 mmol), 0.125 g Co(CH₃COO)₂·4H₂O (0.50 mmol), 30 ml CH₃OH, 15 ml H₂O₂ and 10 ml of distilled water were mixed and stirred at 60 °C for 4 h, the resulting green solution (pH between 3.5 and 4.5) was kept at 20 °C. Yellow block crystals suitable for X-ray crystallographic study were separated via slow evaporation in 2 weeks. Yield: 16% based on V. IR (KBr pellets, ν/cm⁻¹): 3428(s), 3228(m), 2957(m), 1601(m), 1487(m), 1344(m), 1305(w), 1272(w), 1207(m), 1126(m), 1010(s), 985(m), 959(s), 855(m), 732(w), 689(w) and 655(s). Anal. Calcd. for C₁₂H₆₂Co₂N₈O₄₆V₁₀: C, 8.57%; H, 3.72%; N, 6.66. Found: C, 8.50%; H, 3.63%; N, 6.71.

Preparation of V, N-MCFs and MCFs

V, N-MCFs were prepared by using resol as an organic precursor, complex 1 as a functional precursor and triblock copolymers Pluronic P123 (Mw = 5800, EO₂₀–PO₇₀–EO₂₀) as a template. The resol was prepared according to the procedure described in the literature.^5b Typically, 1.00g of P123 and 0.03g complex 1 was dissolved in 25 g of 4 : 1 weight ratio of ethanol and water mixture. Then 5.00 g of resol precursors in ethanol solution containing 0.61 g of phenol and 0.39 g of formaldehyde was added into above mixture. A homogeneous solution was obtained after stirring for 20 min. The solution was coated as a thin polymer membrane on the surface of a dish and the membrane was kept at 25 °C for 8 h. Thermal polymerization of the membranes was carried out at 100 °C for 12 h. The thin membrane was crushed into powders and carbonized at 800 °C in a purified nitrogen flow with a heat rate of 1 °C min⁻¹ to decompose the surfactant template and to obtain V, N-MCFs. As a comparison, we synthesized MCFs by a similar approach without a functional precursor.

X-ray crystallography

X-ray single crystal data were collected on a Bruker APEX II diffractometer equipped with a graphite–monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 296(2) K. Intensity data were corrected by Lorentz-polarization factors and empirical absorption. The structures were solved by direct methods and expanded with difference Fourier techniques. All non-hydrogen atoms were refined anisotropically. Except for the hydrogen atoms on oxygen atoms which were located from the difference Fourier maps, the other hydrogen atoms were generated geometrically. All calculations were performed using SHELXS-97 and SHELXL-97.⁹ Further details for structural analyses are summarized in Table 1. CCDC-685560 (1), 746415 (2) and 745997 (3) contain the crystallographic data in CIF format.

Table 1 Crystal data and structure refinements for 1–3

Complex	1	2	3
Empirical formula	C14H46N8O36V10	C14H42N8O34V10	C12H62Co2N8O46V10
Formula weight	1411.99	1375.96	1681.96
Crystal system	Triclinic	Monoclinic	Monoclinic
Space group	P 1̄	P21/c	C2/c
a/Å	9.4462(4)	12.0418(3)	22.0808(15)
b/Å	10.6899(4)	15.7976(3)	9.7061(5)
c/Å	10.9170(4)	11.0989(3)	24.3647(15)
α (°)	103.250(2)	90	90
β (°)	98.612(2)	107.98(0)	109.045(8)
γ (°)	102.932(2)	90	90
V/Å^3	1022.07(7)	2008.25(7)	4936.0(5)
Z	1	2	4
D c	2.294	2.275	2.263
μ/mm^−1	2.301	2.335	2.581
F(000)	704	1368	3368
θ min, θmax (°)	1.96, 27.52	1.78, 27.72	2.59, 27.57
Reflections collected	17412	31667	36884
Unique reflections (Rint)	4686 (0.0268)	4693 (0.0500)	5695 (0.0833)
Observed reflections
(I > 2σ(I))	3889	3797	4079
Parameters refined	334	319	416
S on F^2	1.013	1.001	1.010
R, wR (I > 2σ(I))	0.0295, 0.0792	0.0352, 0.0932	0.0490, 0.1322
R, wR (all data)	0.0387, 0.0859	0.0469, 0.1004	0.0750, 0.1499

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Electrochemical evaluation

The electrochemical measurements were done in a three-electrode experimental set-up. A saturated calomel electrode (SCE) was used as the reference electrode, and nickel foil as the counter-electrode. The working electrode was prepared as follows: V, N-MCFs or MCFs powders were mixed with poly(tetrafluoroethylene) and graphite with a mass ratio of 8.5 : 1 : 0.5. The mixture was pressed between two pieces of nickel foam under 30 MPa. Thereafter, the electrode was dried overnight at 100 °C. Cyclic voltammograms and galvanostatic charge-discharge behavior were tested in the electrolyte of 1.0 mol L⁻¹Na₂SO₄ aqueous solution using a CHI 660D electrochemical workstation, and the potential window was chosen in the range of 0–0.6 V versusSCE. The specific capacitance C (F/g) is calculated from the gravimetric discharge process according to the equation: C = (i × Δt_d)/(m × ΔV) where i is the constant current (A), m is the mass of the activated substance (g), Δt_d is the discharging time (s), and ΔV is the potential change (V). The potential range we used to estimate was 0.2 to 0.4 V.

Results and discussion

Synthesis

Our synthetic strategy for complexes 1, 2 and 3 are depicted in Scheme 1. These three hybrid compounds were obtained from similar liquid phase reactions, in which methanol and NH₄⁺ should be oxidized to [HMTA-CH₂OH]⁺ or HMTA by H₂O₂ with the existence of vanadate. There are more than four hundred organometallic polymers containing HMTA groups in the Cambridge Structural Database,¹⁰ most of the HMTA groups in these structures came from original organic ligands, and three of which are hybrid materials based on polyoxovanadates,.^4a,4j It is notable that the [HMTA-CH₂OH]⁺ or HMTA groups in complexes 1–3 came from in situ oxidization of methanol and NH₄⁺. Besides, if sufficient NH₃·H₂O was added into the resulting solution subsequently (Scheme 2), rhombus colorless crystals of HMTA can be further separated.¹¹HMTA is an important chemical product which is widely used in the textile and plastics industry, organic synthesis as well as the pharmaceutical industry. In general, HMTA can be generated by condensation of formaldehyde and ammonia in alkaline media and will be hydrolyzed in an acid medium.¹² However; it is much harder to obtain HMTA from methanol by a one step reaction,¹³ to our best knowledge, there is no reported case about these unusual reactions which can be carried out smoothly under mild conditions with the advantages of low cost, quick action and simple steps, this new liquid phase reaction may have potential industrial applications.¹⁴

Scheme 2 Syntheses of HMTA.

Crystal structure

H₄V₁₀O₂₈·(HMTA-CH₂OH)₂·6H₂O (1). Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the triclinic space groupP1̄. The crystallographically independent unit contains a half protonized [H₄O₂₈V₁₀]²⁻ anion, one [HMTA-CH₂OH]⁺ cation as well as three lattice water molecules. As shown in Fig. 1, the centrosymmetric [H₄O₂₈V₁₀]²⁻cluster anion lying about an inversion centre contains ten [VO₆] octahedral combined via shared edges and shared corners, and the V–O bond distances fall in the range of 1.599(7)–2.345(3) Å which are comparable to those in [H_nV₁₀O₂₈](6−n⁾⁻ reported in the literature.⁴ The [HMTA-CH₂OH]⁺ cations, which are counter-ions of the [H₄O₂₈V₁₀]²⁻ anions, are located in the gaps with lattice water molecules and provide various (O–H⋯O, N–H⋯O and C–H⋯O) hydrogen bonds (Table 3) to link [H₄O₂₈V₁₀]²⁻ anions and generate the novel 3D supramolecular network. (Fig. 2)

Fig. 1 Structure of 1 showing the atom labeling, thermal ellipsoids are shown at the 30% probability level. All the hydrogen atoms are omitted for clarity. Symmetry code for the generated atoms: (#1) 1 − x, 1 − y, 1 − z.

Fig. 2 View down the a axis of the 3-D supramolecular network of 1, showing the multiple hydrogen bonding interactions (broken lines), all H atoms are omitted for clarity.

In the topological view, the most interesting structural feature of 1 is the unusual 3D network assembled from [HMTA-CH₂OH]⁺ cations and decavanadate [H₄O₂₈V₁₀]²⁻ anions through various hydrogen bonds. As shown in Fig. 3, each [HMTA-CH₂OH]⁺ cation is linked to four [H₄O₂₈V₁₀]²⁻ anions and one neighboring [HMTA-CH₂OH]⁺ cation to represent a 5-connected node (Fig. 3a), meanwhile each [H₄O₂₈V₁₀]²⁻ anion serves for a 8-connected node by combining eight nearby [HMTA-CH₂OH]⁺ cations (Fig. 3b). In this way, a non-interpenetrating (5,8)-connected network with Schläfli symbol of (3².4¹⁰.5⁸.6⁴.7⁴)(3².4⁶.5²)₂ is formed (Fig. 3c). The extended point symbol calculation using TOPOS 4.0^[9] gives (3. 3. 4. 4. 4. 4. 4. 4. 5₂. 5₂) for the 5-connected node and (3. 3. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 5. 5. 5. 5. 5. 5. 5. 5. 6₄. 6₄. 6₅. 6₅. 7₂. 7₂. 7₈. 7₈) for the 8-connected node. This resulting binodal (5,8)-connected topology is known as the rare case of the topological type 5,8T2. To date, only 3 metal organic frameworks¹⁵ showing 5,8T2 topology are collected on the TOPOS¹⁶ databases, all of them are composed by coordination bonds, complex 1 is the first example of 5,8T2 topology dominated by hydrogen bonds. Besides, it is noteworthy that this network possesses 1D quadrangle channels running parallel to the c axis, which are occupied by the lattice waters.

Fig. 3 (a) 5-connected [HMTA-CH₂OH]⁺ cation linked with four [H₄O₂₈V₁₀]²⁻ anions and one neighboring [HMTA-CH₂OH]⁺ cation; (b) 8-connected [H₄O₂₈V₁₀]²⁻ anion linked with eight [HMTA-CH₂OH]⁺ cations. (c) Schematic representation of the (5,8)-connected network of 1.

H₄V₁₀O₂₈·(HMTA-CH₂OH)₂·4H₂O (2). Complex 2 crystallizes in the monoclinic P2₁/c space group. As shown in Fig. 4, each crystallographically independent unit contains a half protonized [H₄O₂₈V₁₀]²⁻ anion, one [HMTA-CH₂OH]⁺ cation and two lattice water molecules. The [H₄O₂₈V₁₀]²⁻ anion also lies about an inversion centre in this structure, and both bond lengths and angles of the centrosymmetric [H₄V₁₀O₂₈]²⁻ anion are similar to ones in complex 1. The distance between neighboring two [H₄V₁₀O₂₈]²⁻ anions is closer than that of complex 1, therefore there are weak hydrogen bonding interactions (O13–H13⋯O14#6) between these cluster anions. [HMTA-CH₂OH]⁺ cations and lattice water molecules are located in the narrower gaps providing multiple hydrogen bonds and further stabilize the supramolecular structure (Fig. 4, Fig. 5).

Fig. 4 Structure of 2 showing the atom labeling, thermal ellipsoids are shown at the 30% probability level. All the hydrogen atoms and lattice water molecules are omitted for clarity. Symmetry code for the generated atoms: (#1) 1 − x, 1 − y, 1 − z.

Fig. 5 View down the a axis of the 3D supramolecular network of 2, showing the multiple hydrogen bonding interactions (broken lines), all H atoms are omitted for clarity.

Compared with complex 1, the network of 2 is more complicated. As shown in Fig. 6, each [HMTA-CH₂OH]⁺ cation is linked to four [H₄O₂₈V₁₀]²⁻ anions and two neighboring [HMTA-CH₂OH]⁺ cations to represent a 6-connected node; each [H₄O₂₈V₁₀]²⁻ anion serves for a 12-connected node by combining eight nearby [HMTA-CH₂OH]⁺ cations and four neighboring [H₄O₂₈V₁₀]²⁻ anions through hydrogen bonds. The topology analysis shows that the [HMTA-CH₂OH]⁺ cation and [H₄O₂₈V₁₀]²⁻ anion have the Schläfli symbol (3⁶.4⁸.5)₂ and (3¹².4²⁴.5²⁰.6⁹.7) [extended point symbol (3. 3. 3. 3. 3. 3. 4. 4. 4. 4. 4. 4. 4. 4₃. 5₆) and (3. 3. 3. 3. 3. 3. 3. 3. 3. 3. 3. 3. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4. 4(₃). 4(₃). 5. 5. 5. 5. 5. 5. 5. 5. 5₂. 5₂. 5₂. 5₂. 5₂. 5₂. 5₂. 5₂. 5₂. 5₂. 5₄. 5₄. 6₄. 6₄. 6₈. 6₈. 6₈. 6₉. 6₉. 6₉. 6₉. 7₄)], respectively, to give rise to a novel 3D (6,12)-connected net (Fig. 6c), which has been deposited in the TOPOS database (rcsr.anu.edu.au) as fly1. In this topological structure, all the water molecules are located at the 1D quadrangle channels running parallel to the c axis.

Fig. 6 (a) 6-connected [HMTA-CH₂OH]⁺ cation linked with four [H₄O₂₈V₁₀]²⁻ anions and two neighboring [HMTA-CH₂OH]⁺ cations; (b) 12-connected [H₄O₂₈V₁₀]²⁻ anion linked with eight [HMTA-CH₂OH]⁺ cations and four neighboring [H₄O₂₈V₁₀]²⁻ anions; (c) Schematic representation of the (6, 12)-connected network of 2.

To the best of our knowledge, the construction of binodal nets with high coordination numbers is still a challenging issue in coordination chemistry.¹⁷ The RCSR database contains only 17 high-connected binodal nets that are not edge transitive and have a node degree larger than eight. In 2009, the analysis by Blatov and Proserpio revealed that only 18 binodal nets (0.39%) containing two 6–12-coordinated nodes are found (one alb and 17 nia nets) among the 2131 three-dimensional metal–organic polymeric frameworks and 2473 hydrogen-bonded organic and metal–organic frameworks.^17a Thus, the discovery of this new topology fly1 is fundamental in the crystal engineering of supramolecular networks.

H₂V₁₀O₂₈·(HMTA)₂·[Co(H₂O)₆]₂·6H₂O (3). Complex 3 crystallizes in the monoclinic C2/c space group. As shown in Fig. 7, each crystallographically independent unit contains a half protonized [H₂O₂₈V₁₀]⁴⁻ anion, one [Co(H₂O)₆]²⁺ cation, one HMTA molecule and three lattice water molecules. In this structure, the [H₂O₂₈V₁₀]⁴⁻ anion lies about a twofold axis. The Co–O bond distances fall in the range of 2.049(4)–2.124(3) Å and the values of cis –O–Co–O angles are between 88.10(14) and 92.68(18)°. As shown in Fig. 8, HMTA and water molecules connect [H₂O₂₈V₁₀]⁴⁻ anions and [Co(H₂O)₆]²⁺ cations and further generate the supramolecular network through multiple hydrogen bonds.

Fig. 7 Structure of 4 showing the atom labeling, thermal ellipsoids are shown at the 30% probability level. All the hydrogen atoms and lattice water molecules are omitted for clarity. Symmetry code for the generated atoms: (#1) 1 − x, y, 0.5 − z.

Fig. 8 View down the b axis of the 3-D supramolecular network of 4, showing the multiple hydrogen bonding interactions (broken lines), all H atoms are omitted for clarity.

As shown in Fig. 9, each HMTA molecule is linked to four [H₂O₂₈V₁₀]⁴⁻ anions to represent a 4-connected node; each [H₂O₂₈V₁₀]⁴⁻ anion serves for a 8-connected node by combining eight nearby HMTA molecules through hydrogen bonds; thus, a well-known binodal (4, 8)-connected topology of fluorite (flu) network with Schläfli symbol of (4¹².6¹².8⁴)(4⁶)₂ is formed. Additionally, it is worth mentioning that this network possesses three types of 1D quadrangle channels running parallel to the b axis, the smallest channels are empty, the medium-sized channels are filled with water molecules, while the largest ones are occupied by the hydrated [Co(H₂O)₆]²⁺ cations and lattice water molecules (Fig. 9c)

Fig. 9 (a) 4-connected HMTA molecule linked with four [H₄O₂₈V₁₀]²⁻ anions; (b) 8-connected [H₄O₂₈V₁₀]²⁻ anion linked with eight HMTA molecules; (c) Schematic representation of the (4, 8)-connected network of 3.

Structure of mesoporous carbon foam

Fig. 10 shows the nitrogen adsorption and desorption isotherms and pore size distribution curves of V, N-MCFs and MCFs. The isotherms of V, N-MCFs show representative type IV curves¹⁸ which exhibit a sharp capillary condensation step at relative pressures of 0.4–0.6, corresponding to the narrow pore size distribution. The V, N-MCFs possess the Brunauer-Emmett-Teller (BET) surface area of 653 m² g⁻¹. The pore size distribution of V, N-MCFs, which is calculated from the desorption branch of the isotherm using the Barrett-Joyner-Halenda method, exhibits single pores with the most probable pore size of 2.9 nm. For comparison, MCFs possess the BET area of 725 m² g⁻¹ and the most probable pore size of 3.2 nm. These results reveal that V, N-MCFs have mesoporous structure and possess uniform pore size, and the functionalization has no obvious influence on the framework of V, N-MCFs. Fig. 11 shows the transmission electron microscopy (TEM) image of V, N-MCFs. It indicates that V, N-MCFs have a highly ordered mesostructure with uniform pores and the individual pore size is around 3 nm, which matches the results of nitrogen adsorption and desorption analyses. As shown in Fig. S1†, the TG curve of complex 1 under air atmosphere indicates that complex 1 was decomposed and reduced up to 50 °C, and the final residuals may be V₂O₅, the observed total weight loss of 35.19% is close to the calculated one of 35.60%. As shown in Fig. S2†, the TG curve of V, N-MCFs under air atmosphere indicates that the amount of vanadium in the V, N-MCFs was around 2.4 wt.%. The mass ratio of C (83 wt%) H (2.8 wt%) and N (1.1 wt%) in V, N-MCFs were detected by CHN elemental analysis. As shown in Fig. S3†, the wide-angle XRD patterns of V, N-MCFs and MCFs are similar. Two broad diffraction peaks located around 24° and 44° are observed in both V, N-MCFs and MCFs which can be assigned to the (002) and (100) lattice planes of the graphite structure. There is no other significant peak in the case of V, N-MCFs, which is due to the limited content of impurities. The Raman spectra of V, N-MCFs and MCFs were also analyzed. As shown in Fig. S4†, both of the two spectra show a distinct pair of broad bands near 1580 cm⁻¹ (G band) and 1337 cm⁻¹ (D band). The G band and D band are assigned to the hexagonal carbon plane and crystal defects or imperfections, respectively. The ratio of the relative intensity of these two bands (I_D/I_G) is proportional to the number of defect sites in the graphite carbon. The lower the ratio is, the higher the graphitization is. It can be calculated that the I_D/I_G ratio declines from 1.06 (MCFs) to 0.93 (V, N-MCFs), which shows that the graphitization degree of V, N-MCFs is increased in comparison with MCFs. These results indicate that the electrical conductivity of V, N-MCFs was consequently improved.^8h

Fig. 10 Nitrogen adsorption and desorption isotherms and the pore size distributions (insert) of V, N-MCFs and MCFs.

Fig. 11 TEM photograph of V, N-MCFs.

Electrochemical properties

The electrochemical performances of the prepared V, N-MCFs and MCFs were investigated by cyclic voltammetry and galvanostatic charge-discharge tests. Fig. 12 shows the cyclic voltammetry (CV) curves of V, N-MCFs and MCFs electrodes, and the potential window was chosen in the range of 0–0.6 V versusSCE. It is well-known that the specific capacitance is an important parameter to evaluate the performance of electrochemical supercapacitors and the capacitance of material is in proportion to the areas of its CV curves, it can be observed that the capacitance of the V, N-MCFs (Fig. 12a) is much larger than that of MCFs (Fig. 12b). The CV curves of V, N-MCFs were characterized with rectangle-like shape at sweep rates of 2 and 5 mV s⁻¹, which indicates an ideal capacitance behavior;¹⁹ however, they veered away much from rectangle-shape at scanning rates of 20 mV s⁻¹ and 50mV s⁻¹, suggesting resistance-like electrochemical behavior.²⁰

Fig. 12 Cyclic voltammetry curves of V, N-MCFs (a) and MCFs (b) electrodes.

The galvanostatic charge-discharge behaviors reflect substantial electrical energy storage; based on the results of charge-discharge cycling, the specific discharge capacitance of a single electrode in the capacitors can be calculated. As shown in Fig. 13, both V, N-MCFs and MCFs electrodes present linear galvanostatic charge-discharge curves at the loading current density of 200 mA g⁻¹. The calculated specific capacitance of the V, N-MCFs reaches 113 F g⁻¹, much higher than that of the MCFs (72 F g⁻¹). These results proved that the specific capacitance of V, N-MCFs was significantly improved compared with MCFs.

Fig. 13 Galvanostatic charge-discharge curves of the V, N-MCFs and MCFs electrodes at the loading current density of 200 mA g⁻¹.

Conclusion

In summary, a novel liquid phase reaction resulted in the formation of three organic-inorganic hybrid materials based on decavanadates: H₄V₁₀O₂₈·(HMTA-CH₂OH)₂·6H₂O (1), H₄V₁₀O₂₈·(HMTA-CH₂OH)₂·4H₂O (2), H₂V₁₀O₂₈·(HMTA)₂·[Co(H₂O)₆]₂·6H₂O (3). Complexes 1, 2 and 3 display 3D supramolecular networks with three different binodal topologies through various hydrogen bonds. Complex 1 was further used as a functional precursor to obtain V, N-MCFs, which exhibit highly ordered mesostructure with specific surface areas of 653 m² g⁻¹ and uniform pore sizes of 2.9 nm. Compared with MCFs, V, N-MCFs possess much higher specific discharge capacitance. It can be expected that V, N-MCFs have a promising prospect for application in supercapacitors.

Acknowledgements

The project was supported by the Science and Technology Department of Zhejiang Province for financial support of the project (2007C21174), the National Natural Science Foundation of China (20973127) and Shanghai Nanotechnology Promotion Center (0952nm00800). The authors are also grateful to Prof. V. A. Blatov for the help about topological analysis.

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Footnote

† Electronic supplementary information (ESI) available. CCDC reference numbers 685560, 745997, 746415. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ce05605k

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