Highly Thermostable and Insensitive Energetic Hybrid Coordination

Article pubs.acs.org/cm

Highly Thermostable and Insensitive Energetic Hybrid Coordination Polymers Based on Graphene Oxide−Cu(II) Complex Adva Cohen,†,∥ Yuzhang Yang,§,∥ Qi-Long Yan,*,† Avital Shlomovich,† Natan Petrutik,† Larisa Burstein,‡ Si-Ping Pang,*,§ and Michael Gozin*,† †

School of Chemistry, Faculty of Exact Science, Tel Aviv University, Tel Aviv, 69978, Israel Wolfson Applied Materials Research Center, Tel Aviv University, Tel Aviv, 69978, Israel § School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China ‡

S Supporting Information *

ABSTRACT: New highly energetic coordination polymers (ECPs), based on the graphene oxide (GO)-copper(II) complex, have been synthesized using 5,5′-azo-1,2,3,4-tetrazole (TEZ) and 4,4′-azo-1,2,4-triazole (ATRZ), as linking ligands between GO-Cu layers. The molecular structures, sensitivity, and detonation performances of these ECPs were determined. It was shown that these energetic nanomaterials are insensitive and highly thermostable, due to high heat and impact dissipation capacity of GO sheets. In particular, the GOTEZ-Cu(II) ECP shows low sensitivity to impact and electrostatic discharge (Im = 21 J; ESD of 1995 mJ) and has a comparable detonation performance to RDX. Also, our novel GO/Cu(II)/ATRZ hybrid ECP GO-Cu(II)-ATRZ ECP exhibits high density (2.85 g·cm−3), remarkably high thermostability (Tp = 456 °C), and low sensitivity (Im > 98 J; ESD of 1000 mJ). The latter material has a calculated detonation velocity of 7082 m·s−1, which is slightly higher than that of energetic ATRZ-Cu(II) 3D MOF and higher than one of the top thermostable explosives HNS (Tp = 316 °C; 7000 m s−1).

1. INTRODUCTION As an active research topic, the development of highly thermostable insensitive energetic materials (EMs) is becoming increasingly important. Civil (deep-well oil and gas extraction) and defense (modern insensitive munition) industries are seeking for such materials for high-performance perforator and low-vulnerability warhead applications.1−3 The currently used EMs for these applications,4−6 such as 1,3,5,7-tetranitro-1,3,5,7tetrazocine (HMX)4 with a decomposition temperature (Tm) of 291 °C, hexanitrohexaazaisowurtzitane (CL-20, Tm = 232 °C),5 and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB, Tm = 350 °C),6 have some limitations due to either their high sensitivity (CL-20, HMX) or relatively low energy content (TATB). In terms of highly thermostable insensitive energetic compounds, there are several strategies for their molecular design, including the formation of salts, introduction of amine functional groups into aromatic moieties of energetic molecules,7,8 formation of conjugated molecular structures (e.g., HNS),9,10 and condensation with a triazole ring/s.11,12 In addition to molecular level designs, modern nanotechnology and material science provide new methodologies for the development of highly thermo-stable and insensitive, yet powerful, EMs.13−16 These approaches include preparation of energetic coordination polymers (ECPs), based on carbon nanomaterials (CNMs),13 energetic metal organic frameworks (MOFs), 14,15 and cocrystals.16 The three-dimensional (3D) energetic MOFs, © 2016 American Chemical Society

such as [Cu(ATRZ)3(NO3)2]n and [Ag(ATRZ)1.5(NO3)]n, in which 4,4′-azo-1,2,4-triazole (ATRZ) was used as the ligand, were found to have better impact sensitivity (22.5 and 30 J) and energetic performances in comparison with their 1D and 2D counterparts. 14 Yet, it was reported that both [Cu(ATRZ)3(NO3)2]n and [Ag(ATRZ)1.5(NO3)]n 3D MOFs were less thermostable (Tp < 270 °C) than their parent ATRZ ligand (Tp = 313 °C). A recent study showed that the thermostability of this type of 3D MOFs could be improved by using a 3-(1H-tetrazol-5-yl)-1H-triazole (H2tztr) ligand, leading to decomposition temperatures as high as 355 °C.15 Unfortunately, this strategy did not significantly improve the sensitivity to the mechanical stimuli (impact and friction) and to the electrostatic discharge (ESD) performance of energetic MOFs, which was also observed in many cases of energetic cocrystals, due to intrinsic sensitivity limitations of the building ligands or cocrystallized components.17−19 An integration of the CNMs with the MOFs could be a better approach to largely improve both thermostability and sensitivity of the energetic MOFs.13 In particular, it was recently reported that graphene oxide (GO) could stabilize and desensitize HMX and other high EMs via facile mechanical Received: May 5, 2016 Revised: August 12, 2016 Published: August 14, 2016 6118

DOI: 10.1021/acs.chemmater.6b01822 Chem. Mater. 2016, 28, 6118−6126

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Chemistry of Materials

Figure 1. Preparation process of GO-Cu(II)-ATRZ ECP.

nano EMs, since these metal ions would function as combustion catalytic centers. In this work, several 3D ECPs were designed and synthesized, using 5,5′-azo-1,2,3,4-tetrazole (TEZ)34 and ATRZ, as bridging ligands (Scheme S1, Supporting Information) of the GO-Cu(II) complex. The layers of GO-Cu(II) complex were prepared by the reaction of GO with Cu(NO3)2 in aqueous solution (Supporting Information). Four typical nano EMs were synthesized: (a) GO-Cu(II)-ATRZ and GO-Cu(II)-ATRZ ECPs were prepared by reactions of the GO-Cu(II) complex with the corresponding ligands (Figure 1). (b) GO/Cu(II)/TEZ and GO/Cu(II)/TEZ hybrid ECPs were prepared by complexation of corresponding ligands with Cu(II) ions, in the presence of the dispersed GO-Cu(II) complex (Scheme S1). Our new nano EMs were comprehensively characterized, and their energetic performance was evaluated and compared with the related energetic ATRZ-Cu(II) MOF and TEZ-Cu(II) complex. For comparison purposes, simple mechanical mixtures of these two energetic materials with GO were also prepared with the solution sonication method. Both of these new energetic nano EMs were also found to be very thermally stable and insensitive.

mixing, resulting in an improved mechanical strength of the obtained material, as well.20 In addition to mechanical mixtures, the GO can also be chemically functionalized and converted into novel EMs, as GO itself is exothermic13 and can readily undergo violent decomposition, due to the abundant oxygencontaining functional groups on its basal sheet (phenol, hydroxyl, and epoxide groups) and at its edges (carboxylic groups).21 These functional groups make GO an excellent candidate for further chemical functionalization.22−24 Since GO is also highly dispersible in water and many organic solvents, different compounds can react with its oxygen-bearing functional groups.25 For example, functionalization of GO with amines results in the formation of nitrogen-doped nanomaterials26 and was achieved by using coupling agents or by a direct reaction of amines with GO’s epoxy groups.27,28 The metal ions can also be bound to GO sheets, forming crosslinked structures, with greatly improved mechanical strength,29 and many other functional nanomaterials, with promising electronic properties.30,31 By utilizing similar functionalization strategies, various energetic complexes and MOFs could be integrated and attached to GO layers, where transition metal ions are used as cross-linking/bridging units.32,33 From our perspective, the conjugation of energetic MOFs or complexes with GO should significantly improve the thermostability and insensitivity of the resulted hybrid nanomaterials, due to the capability of GO to very efficiently dissipate mechanical shock, heat, and electrostatic discharge on a molecular and a macromolecular level. Furthermore, the involved transition metal ions could further improve final combustion performance of these next-generation

2. EXPERIMENTAL SECTION 2.1. Materials. The commercially available graphene oxide (GO, 1−5 layers) was used as received from market with an oxygen content over 42%. Guanidine (99%), diaminoguanidine (98%), copper nitrate trihydrate (99%), nickel nitrate (99%), cuprous chloride (98%), cobalt 6119


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Figure 2. Nonisothermal DSC thermograms of (a) GO-Cu(II)-TEZ ECP, (b) TEZ-Cu(II) complex, (c) pristine GO, (d) GO/Cu(II)/TEZ hybrid ECP, (e) GO-Cu(II)-ATRZ ECP, (f) ATRZ-Cu(II) MOF, and (g) GO/Cu(II)/ATRZ hybrid ECP. All thermograms were measured at a heating rate of 10 °C·min−1.

agreement with the literature.26 The copper atoms of the GOCu(II) complex can further bind ATRZ or TEZ ligands, forming 3D layered ECPs. It has been reported that the multifunctionality of GO allows it to act as a directing agent in macromolecular assemblies, leading to the formation of GO intercalation into MOF structures and to the formation of GO/ MOF hybrids.36−38 From our perspective, it is exactly the case for the reported in this work new ECPs, in which nitrogen-rich energetic ligands were used for the first time. There are at least three major advantages of our novel ECPs over many other energetic nanomaterials: (a) The safety of our GO-based ECPs has been greatly improved. (b) The density was increased, due to a packing (compacting) of molecules between the functionalized GO layers. (c) The thermostability of our GO-based ECPs is enhanced versus pristine GO and non-GO-containing copper complexes (DSC analysis of thermal decomposition behavior; Figure 2), due to superior heat dissipation of conjugated GO sheets. 3.2. Thermal Stability. The [Cu(ATRZ)3(NO3)2]n 3D MOF was reported to decompose with a peak temperature (Tp) of 267 °C (5 °C·min−1).14 We found that when this MOF was integrated into GO-Cu(II) complex layers via coordination, a novel hybrid ECP was formed. The latter hybrid ECP exhibited much better thermostability (Figure 2d and g), since highly efficient heat dissipation by GO sheets39 prevents the formation of localized hot spots inside this new hybrid ECP. In comparison, the pristine GO decomposes exothermically at a Tp of 204.8 °C (Figure 2c). Our GO/Cu(II)/ATRZ hybrid ECP, with a nitrogen content of 28.95%, has a decomposition heat release (ΔHd) as high as 8670 J·g−1 and a Tp of 455 °C (Figure 2g), which are substantially higher than the parent [Cu(ATRZ)3(NO3)2]n 3D MOF. TGA analyses for the examined materials in this study (Figure S8; Supporting Information) show similar results, and the exothermic peak corresponds to the second mass loss of the ECP materials and hybrid ECPs, where the first mass loss processes were due to a loss of some oxygen-containing functional groups and bound

nitrate (98%), iron(II) chloride tetrahydrate (99%), and hydrazine monohydrate (98%, N2H4 64−65%) were obtained from SigmaAldrich and stored under nitrogen to discourage oxidation. Ultrapure deionized water (resistivity >18 MΩ) was obtained from a Mili-Q Biocel system. Whatman Anodisc membranes (0.2 μm pore size, 47 mm diameter) were used during filtration for support of fabricated papers. 2.2. Preparation Methods for the ECPs. GO-Cu(II)-ATRZ ECP and GO-Cu(II)-ATRZ hybrid ECPs were prepared in a two-step procedure. In the first step, to the GO dispersion in water was added Cu(NO3)2, and the reaction mixture was heated to 80 °C for 3 h. A reaction temperature of above 60 °C was found to be essential for efficient preparation of the GO-Cu(II) complex, which was formed as a precipitate. FTIR and Raman spectroscopy analyses of GO, which was collected from pristine GO dispersion in water (heated at 65 °C for 48 h), unambiguously showed no changes in GO’s functional groups (Figure S4 and Table S2; Supporting Information). The resulting GO-Cu(II) complex was collected by filtration or centrifugation, washed with water (to remove traces of unbound copper ions), and redispersed in water for the nitrogen-rich ligand coordination reaction. The following procedures are shown in the Experimental Part of the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. The Effect of Reaction Solvents on Quality of the GO-Cu(II) Complex. In addition to water, many other solvents were also evaluated for GO-Cu(II) complex preparation. We found that sulfolane, DMSO, and DMF were also suitable for this purpose (Figure S1; Supporting Information), due to a good dispersity of GO in these solvents.35 Among all examined solvents, the DSC analyses showed that the GO-Cu(II) complex that was prepared in water was the most thermostable one (Figure S2; Supporting Information). Other solvents and solutions, including ethylenediamine (EDA), pyridine, aqueous solutions of guanidine, or urea, functioned as ligands, producing other ECPs (as measured by FTIR and DSC; Figures S2, S3 and Table S1; Supporting Information), which exhibited relatively low energy content and thermostability below 270 °C. We think that he GO-Cu(II) complex is formed by coordination of Cu(II) ions to GO, mostly via hydroxyl and carboxylic functional groups (Figure 1), which is in a good 6120


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Figure 3. Raman spectra with D, G, and 2D band peaks, where the decreased D band peak intensity of GO is compared to its ECP analogues, indicating more structural defects in GO, after complexation with Cu(II) ions and further coordination with nitrogen-rich ligands.

Figure 4. XPS binding energy spectra of (a) GO-Cu(II)-ATRZ ECP, (b) GO-Cu(II)-TEZ ECP, and (c) pristine GO; the fitted C 1s peak curves for (d) pristine GO; (e) GO-Cu(II)-TEZ ECP and (f) GO-Cu(II)-ATRZ ECP; the fitted N 1s peaks for (h) GO-Cu(II)-TEZ ECP and (i) GO-Cu(II)ATRZ ECP; and the Cu 2p spectrum of (g) GO-Cu(II)-ATRZ ECP and GO-Cu(II)-TEZ ECP, without peak fitting.

water molecules. The final residue masses of these materials were in a range of 32−37%, except for the GO/Cu(II)/ATRZ hybrid ECP, which had only 19.6% residue. The latter hybrid ECP had a more complete decomposition reaction and the largest heat release. Moreover, the density of GO/Cu(II)/ ATRZ hybrid ECP, measured by helium gas pycnometry, was about 2.8 g·cm−3 (Cu content of below 30 wt %), which is even higher than the density of the metallic Al. Notably, GO-Cu(II)ATRZ ECP has a much lower nitrogen content (18.0%), due to a smaller amount of ligands, present in this energetic nanomaterial. It has a sharper exothermic peak at 311 °C, which is still higher than the decomposition temperature of the parent [Cu(ATRZ)3(NO3)2]n 3D MOF. This observation indicates that a higher degree of ligand-Cu(II) cross-links in the structure of layered nanomaterials results in their better thermostability and higher energy output. Similar observations were made in cases of TEZ-containing ECPs. It was found that the GO-Cu(II)-TEZ ECP decomposes at a Tp of 195 °C, covered by a ΔHd of 1749 J·g−1 (Figure 2a), while the GO/Cu(II)/TEZ hybrid ECP has a Tp of 272 °C (Figure 2d) and lower ΔHd (1369 J·g−1). Both of these ECPs are much more stable than the corresponding TEZ-Cu(II) complex, where the latter is very sensitive to heat and undergoes fast decomposition below 150 °C (Figure 2b) and

even runaway chemical reaction, when the sample mass is over 0.2 mg (Figure S5; Supporting Information). 3.3. Chemical Bonding and Morphology. In order to study the chemical bonding and morphologies of our ECPs, the Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), FTIR, and scanning electron microscopy (SEM) techniques were used. Figure 3 and Figure S4 (Supporting Information) show the Raman spectra of all our materials, with summarized peak parameters in Table S3 (Supporting Information). It was reported that in the case of graphene, the D band at 1344 cm−1 corresponds to the defect of the activated band in sp2 hybridized carbon atoms, while the G band could be attributed to the first-order band of sp2 carbon atoms.40 After graphene oxidation, the intensity of the D band in GO is enhanced, due to the increase of disorder, while the G band becomes much broader, owing to the presence of isolated double bonds. The G band of these bonds in GO usually resonates at higher frequencies than that of graphite, due to interactions between the stacked graphene layers.41 These two bands could be observed in all our GO-containing samples, with a different but comparable D/G intensity ratio, indicating that the basic structure of GO remains in these materials after functionalization. A general observation is that a higher disorder in graphite leads to a broader G band, as well as to a broader D band, with 6121


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Figure 5. SEM images of (a) GO with 1−5 layers, (b) GO-Cu(II)-ATRZ ECP, (c) GO-Cu(II)-TEZ ECP, (f) GO/Cu(II)/TEZ hybrid ECP, and (g) GO/Cu(II)/ATRZ hybrid ECP. EDS spectra are showing elemental analysis on the surface of these samples: (d) GO-Cu(II)-ATRZ ECP and (e) GO-Cu(II)-TEZ ECP, mechanical mixture of GO with Cu(II)-TEZ complex (h), mechanical mixture of GO with [Cu(ATRZ)3(NO3)2]n 3D MOF (i), pure [Cu(ATRZ)3(NO3)2]n 3D MOF crystals (j).

oxidation, showing the presence of different oxygen-containing functional groups.44 Two of our ECPs showed new XPS peaks in C 1s curves, which were attributed to the presence of a C− O−Cu bond (287.5 eV) in these nanomaterials.45 This observation strongly supports our initial postulation that Cu(II) ions are bound to the GO surface via oxygen atoms, and it could be further proved by the concentration decrease of C−O bonds in these ECPs. The N 1s peaks at 398.9 eV ( N−) and at 399.9 eV (−NH/−N−N−) are originated from ATRZ (Figure 4i) and TEZ (Figure 4j) ligands. Since these ligands are water-soluble and the final products were thoroughly washed by water, the presence of these bonds confirms that a coordination reaction between these ligands and Cu(II) ions in the GO-Cu(II) complex indeed took place. In addition, the N1 peak at 401.8 eV indicates the presence of positively charged N atoms, as the result of the bonding with the Cu(II) ions.46,47 For each ECP, two peaks were also shown in Cu 2P3/2 curves at 932.9 and 394.4 eV (Figure S6). Those peaks can be assigned to Cu−N and Cu−O bonds, respectively. A trace amount of NO3− ions was also observed in GO-Cu(II)ATRZ ECP, indicating the presence of [Cu(ATRZ)3(NO3)2]n 3D MOF. It indicates that a little amount of GO/Cu(II)/ ATRZ hybrid ECP (Scheme S2; Supporting Information) was formed as an impurity in this ECP. These results also provide a firm support to the proposed structures of the GO-Cu(II)ARTZ and GO-Cu(II)-TEZ ECPs (Figure 1). The FTIR analyses further confirmed the above-mentioned chemical bonding analysis. As shown in Figure S7 (Supporting Information) and Table S5 (Supporting Information), several types of oxygen-containing functional groups in GO were assigned as follows: −OH at ν = 3440 cm−1, carboxyl −CO and −C−O at ν = 1741 and 1398 cm−1, respectively, aromatic

higher relative intensity than the G band, which was also the case in all of our ECPs.42 The peaks at 288, 295, 289, and 306 cm−1 correspond to the Ag mode of Cu(II)-O bonds (Figure 3a, b, d, and e). The decreased intensity of the D peak in the spectrum of GO-Cu(II)-TEZ ECP, as compared to GO/ Cu(II)/TEZ hybrid ECP, clearly indicates the presence of a greater amount of structural defects on functionalized GO, after complexation with Cu(II) ions. Similar observations were made for GO-Cu(II)-ATRZ ECP and GO/Cu(II)/ATRZ hybrid ECP. The shoulder peak at 1047 cm−1 in the spectrum of GO-Cu(II)-ATRZ hybrid ECP corresponds to stretching of the N−O bond, indicating a high content of NO3− in this hybrid ECP.43 However, in the case of GO/Cu(II)/TEZ hybrid ECP, water molecules are most probably bound to the copper metal center, instead of NO3−, since TEZ could be negatively charged in the TEZ-Cu(II) complex. This conclusion was further supported by elemental analysis, showing a higher nitrogen content for the GO/ Cu(II)/ATRZ hybrid ECP. In comparison, based on mechanical mixing, the crystals of the [Cu(ATRZ)3(NO3)2]n 3D MOF and TEZ-Cu(II) complex could also be bound to the GO surface, leading to aggregation of GO sheets into stacked arrays, either by van der Waals or by electrostatic interactions. This type of stacked array can easily undergo disassembly, as was observed by SEM analysis (Figure S8, Supporting Information). To further assess the bonding chemistry of nitrogen-rich ligands, Cu(II) ions, and functional groups on GO sheets, the X-ray photoelectron spectroscopy (XPS, Figures 4 and S6, Table S4; Supporting Information) and FTIR (Figure S7; Supporting Information) techniques were used. The fitted C 1s XPS spectra of GO clearly indicate a considerable degree of 6122


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Chemistry of Materials −CC at ν = 1623 cm−1, and epoxy −C−O at ν = 1288 cm−1 and ν = 1076 cm−1. These are in full agreement with the literature reports.45 In comparison, the GO-Cu(II)-TEZ and GO-Cu(II)-ATRZ ECPs showed relatively lower intensity of the O−H stretching at 3240 cm−1, indicating that O−H groups are involved in binding of the Cu atoms in these ECPs. For pristine GO, the presence of a low intensity of CO stretching vibrations at 1740 cm−1 in all of our ECPs indicates that the carbonyl group partially remained after the formation of these ECPs. The peak of CO was detected in all ECP materials, except GO-Cu(II)-ATRZ ECP, possibly due to a reduction of GO by Cu(II). It results in a higher copper content and the presence of Cu(I) in GO-Cu(II)-ATRZ ECP (932.1 eV for Cu 2p in Figure S6; Supporting Information). The vibration of C−O(H) in the carboxyl group at 1370 cm−1 is also partially decreased. The vibration peaks, corresponding to the epoxy group at 1076 cm−1 and hydroxyl group at 1370 cm−1, have very weak intensities in both ECPs. Notably, in the hybrid ECPs, the intensity of these groups is as high as in the pristine GO. These results suggest that hydroxyl groups in GO are mostly responsible for the binding of the Cu(II) ions, which are then coordinated with the ligands for formation of the ECPs. The above-mentioned ECPs, hybrid ECPs, and the GOcontaining mechanical mixtures were found to be very different in their morphologies. Figure 5 shows SEM images and EDS surface elemental analyses of studied materials and mixtures. The stacked GO sheets, with a thickness of about 5−10 nm (Figure 5a), are well separated. The GO-Cu(II)-ATRZ and GO-Cu(II)-TEZ ECPs (Figures 5b and c) exhibit a rough bulky surface, probably due to the complexation with Cu(II) and coordination with the ligands. In comparison, the GO/ Cu(II)/ATRZ and GO/Cu(II)/TEZ hybrid ECPs (Figure 5f and g) show “plowed”-like surfaces with irregular shapes, indicating the presence of hybrid crystalline structures. However, in the case of the mechanical mixtures of GO with the complex and MOF crystals, those crystals could either be fully attached to the surface of GO or detached from the GO (Figures 5h and i). A typical crystal size of [Cu(ATRZ)3(NO3)2]n 3D MOF was too large (>50 μm) to be fully covered by GO sheets that were used in this work (Figure 5j), resulting in higher mechanical sensitivity and lower thermal stability, in contrast to our corresponding ECPs. The EDS surface elemental analyses (Figures 5d and e) further proved the presences of Cu and N atoms, indicating that the GO is bound to copper, which in turn is coordinated to the energetic ligands. 3.4. Elemental Compositions and Nanoscale Structures. The compositions and the atomic content on the surface of the studied nano EMs were determined by elemental analysis, XPS, and EDS (Table S6; Supporting Information). The formulas of our materials were determined as GO (C50H27O25), GO-Cu(II)-ATRZ ECP (C22H16O21N15Cu3), GO-Cu(II)-TEZ ECP (C27H21N7O33Cu3), and their hybrid ECPs, as (C27H22O49N36 Cu6) and (C21H26O49N42Cu6), respectively. In order to show the interface structure of the GO layers with the crystals in the hybrid ECPs, the TEM images were obtained (Figure 6). It was shown that the GO layered structure was maintained in cases of the GO/Cu(II)/ATRZ and GO/ Cu(II)/TEZ hybrid ECPs, which further supported and proved our conclusions based on the above-mentioned Raman and FTIR spectroscopy findings. TEM images showed some

Figure 6. TEM images of GO/Cu(II)/ATRZ hybrid ECP (a, b) and GO/Cu(II)/TEZ hybrid ECP (c, d), deposited on copper substrates. A crystal of [Cu(ATRZ)3(NO3)2]n 3D MOF was chemically attached to the GO surface (a); layered assemblies could be grown, by using the of GO sheets as templates. The GO sheets are embedded into the TEZ/Cu(II) crystals (c), which grow from tiny nucleation sites (d).

differences between these two hybrid ECPs, in terms of their morphology. In the case of the GO-Cu(II)-ATRZ ECP and GO-Cu(II)-TEZ ECP, no crystals could be observed on the surface layers of the GO-Cu(II) complex. Relatively large crystals of [Cu(ATRZ)3(NO3)2]n 3D MOF were chemically bound to the GO surfaces (Figure 6a), and the layered crystals were grown on top of the GO sheets. In comparison, the GO layers were embedded into Cu(II)-TEZ crystals (Figure 6c), which grow from tiny nucleation sites, where the Cu(II) ions were bound to the GO surface. We could summarize that the main differences between the ECPs and corresponding hybrid ECPs are as follows: (a) ECPs have amorphous structures, with less nitrogen content, while the hybrid ECPs are crystalline materials, organized between the GO sheets. (b) ECPs are generally GO-based homogeneous layered materials, with smaller thickness in comparison to hybrid ECPs. (c) The hybrid ECPs are GO-intercalated crystals of energetic complexes or MOFs. 3.5. Detonation Performances and Mechanical Sensitivity. In order to predict the detonation performances of our GO-based ECPs, the EXPLO-5 software was used, based on measured heat of formation (ΔHf) and density (ρ) data. This software utilizes an algorithm based on the Becker− Kistiakowsky−Wilson equation of state (BKW-EOS) for gaseous detonation products. The experimental density, heat of formation, detonation parameters, and sensitivity results of the explored EMs are summarized in Table 1. We found that most of our ECPs have a lower velocity of detonation (VDet) than RDX (8795 m·s−1 at a density of 1.76 g·cm−3), due to ECPs’ lower heat of formation. Although the pristine GO is not detonable, it was found to be energetic in our experiments. Yet, EXPLO-5 software could not calculate its detonation parameters, due to a too low detonation heat. As an oxygen 6123


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Table 1. Physical Properties, Mechanical Sensitivity, and Detonation Performances of ATRZ-Cu(II) and TEZ-Cu(II) Energetic ECPs Based on GO, in Comparison with TNT and RDX parameters

GO

MOF−1

ECP−1

ECP−2

hybrid ECP−1

hybrid ECP−2

TNT

RDX

Mw [g mol−1] %N [wt %]a Tp [°C]b ρ [g cm−1]c Ωco2 [%]d ESD (mJ) ΔHd [J g−1]e ΔUf° [kJ mol−1]f impact [J] friction [N] PC‑J [kbar]i VDet [m s−1]j

1027.7 0.91% 204.8 1.91 −137.8% > 2000 2097 −1869 > 98 > 360 375l 3836l

679.9 53.35% 243.0 1.64k −58.8%

1017.1 20.66% 311.1 2.25 −55.7% 250 811 −1928 28 > 360 230 7009

1162.1 8.43% 195.1 2.15 −47.4% 1995 1743 +3915 21 > 360 412 8494

2015.3 25.01% 455.9 2.85 −19.9% 1000 8670 −9344 > 98 > 360 209 7082

2032.0 28.95% 272.3 1.85 −9.5% 1000 1369 −5542 40 > 360 149 6017

227.1 18.50% 295.0 1.65 −74%

222.1 37.84% 241.2 1.80 −21.6%

103 −54.4 15 > 360 213 7304

2827 +83.8 7.5 120 349 8795

1450 +1651 16k > 360 340k 6860k

Nitrogen content. bDecomposition peak temperature from differential scanning calorimetry (DSC) curve obtained at heating rate of 10 °C min−1. Density measured by helium gas pycnometry, at 25 °C. dOxygen balance (for CaHbNcOd, Ω = (a − 2b − 0.5d) × 1600/Mw). eHeat release from nonisothermal decomposition. fCalculated energy of formation; g, heat of detonation. iDetonation pressure. jDetonation velocity. kThis value is taken from the updated results from the literature6 published by the same group.6 lThese values were calculated for a mixture of 80% of GO with 20% of ammonium perchlorate, as the oxidant. MOF−1, with a formula of [Cu(ATRZ)3(NO3)2]n was taken from the literature.6 Hybrid ECPs−1 and hybrid ECPs−2 represent GO/Cu(II)/ATRZ and GO/Cu(II)/TEZ hybrid ECPs. ECP−1 and ECP−2 represent GO-Cu(II)-ATRZ ECP and GOCu(II)-TEZ ECP, respectively. a c

455.9 °C and detonation velocity of 7082 m·s−1. Our new methodology provides a novel platform for preparation of the next generation of nano EMs with improved thermostability and sensitivity.

lean fuel, the combination of GO with ammonium perchlorate (AP) could become detonable (VDet of 3836 m·s−1). The GO/ Cu(II)/ATRZ hybrid ECP, with density of 2.85 g·cm−3, was calculated to have a VDet. of 7082 m·s−1, which is slightly higher than that of [Cu(ATRZ)3(NO3)2]n 3D MOF (VDet of 6860 m· s−1). Remarkably, as a much more stable EM, the GO/Cu(II)/ ATRZ hybrid ECP has slightly higher VDet than hexanitrostilbene (HNS, VDet = 7000 m·s−1) and comparable VDet to widely used insensitive thermostable explosive TATB (VDet of 7350 m·s−1). In terms of their mechanical sensitivity and sensitivity to the ESD, the explored in this study materials were found to have a range of sensitivity. The most insensitive material was found to be GO/Cu(II)/ATRZ hybrid ECP (Table 1), which is even less sensitive than TATB. The ESD results measured for our ECPs were found to be higher than 205 mJ, showing insensitivity to electrical discharge, due to electron conductivity of GO sheets, where the GO-Cu(II)-TEZ ECP achieved a very impressive 1995 mJ value. According to the hot-spot theory for impact initiation, a variety of mechanisms has been proposed. It includes adiabatic compression of trapped gas in voids, friction involving sliding or impacting surfaces, shear band formation, and other mechanisms.48,49 The GO/Cu(II)/ATRZ hybrid ECP has much higher thermal stability than other materials, indicating that the critical size of hot-spot should be large enough for initiation of this material. GO sheets are highly heat conductive, leading to a fast dissipation of formed hot spots. We believe that this is the major mechanism by which the GO could improve the impact performance of studied EMs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01822. Descriptions of the experimental details, characterization techniques, figures on preparation paths to GO/Cu(II)/ ATRZ and GO/Cu(II)/TEZ hybrid ECPs, dispersion of GO-Cu(II) in different solvents, XPS spectra and the curves for Cu 2p peaks, corresponding EDS results, Raman and FTIR of the explored materials, DSC curves of GO-Cu(II) complexes from different solvents, SEM images of GO based mixtures and hybrid ECPs, thermogravimetric analysis (TGA) curves of involved materials, tables on summary of Raman peak parameters, elemental analysis, FTIR and peak assignments, and XPS bonding information (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M. Gozin). *E-mail: [email protected] (Q-L. Yan). *E-mail: [email protected] (S.-P. Pang). Author Contributions ∥

4. CONCLUSIONS In conclusion, the GO can readily undergo complexation with Cu(II) ions at elevated temperature, and the resulting GOCu(II) complex could be further coordinated with nitrogendonor energetic ligands, forming thermostable and insensitive ECPs or hybrid ECPs, depending on the reaction conditions. In particular, a highly insensitive and thermostable GO-Cu(II)ATRZ hybrid ECP has an exothermic decomposition Tp of

These authors contribute equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support provided by the Tel Aviv University and Momentum Fund (Ramot). We are thankful to Dr. Artium Khatchtouriants, Dr. 6124


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Zehava Barkay, and Mr. Tal Chen and Mrs. Sivan Fishman for their valuable contributions.

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Highly Thermostable and Insensitive Energetic Hybrid Coordination

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