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A safe and efficient liquid-solid synthesis for copper azide films with excellent electrostatic stability
Nano Energy 66 (2019) 104135
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A safe and efficient liquid-solid synthesis for copper azide films with excellent electrostatic stability Chunpei Yu a, Wenchao Zhang a, **, Shiying Guo b, Bin Hu a, Zilong Zheng a, Jiahai Ye a, Shengli Zhang b, Junwu Zhu a, * a
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China Key Laboratory of Advanced Display Materials and Devices, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Electrosynthesis Nanoenergetic materials Primary explosives Copper azide Energy release
The traditional gas-solid synthesis of copper azide (CA) always requires rigorous conditions, such as the use of hazardous HN3 gas, and time-consuming preparation processes (>12 h). Both disadvantages impede its ecofriendly large-scale production and result in high safety risks for the practical synthesis. Here, a safe and effi cient (<40 min) liquid-solid strategy is developed to fabricate CA films. Employing CuO nanorods (NRs) arrays as precursors, a nest-like CA film with remarkable energetic performance and excellent electrostatic stability can be generated in situ in a NaN3 solution via an electro-assisted azidation method. The resulting CA film possesses an energy release of 1650 J g 1 and effective detonation performance. In particular, the electrostatic sensitivity (E50) value (0.81 mJ) is 16 times higher than that of the original CA powder (0.05 mJ), which is attributed to not only the unique nest-like structure of the CA film but also the uniform in situ incorporation of CuO. In addition, density functional theory (DFT) calculations are employed to provide insights into the detailed reaction path ways of the liquid-solid azidation process. Importantly, the constructed CA film can be directly integrated into a micro ignitor to realize nanoenergetics-on-a-chip due to the safe synthetic process as well as its high compati bility with microelectromechanical systems (MEMS) technology.
1. Introduction Energetic materials are one of ancient and extreme forms of energy storage and release. Polynitrogen compounds arouse great interest due to their large positive formation energies, high densities and fast energy release [1–11]. Among the reported polynitrogen compounds, transition-metal azides are one of the most important and classical types of primary explosives and have long-term chemical and thermal stabil ities, reliable and rapid initiation ability. Notably, as an energetic transition-metal azide with high brisance, copper azide (CA) is emerging as a replacement for the most used toxic lead-based primary explosives, such as lead azide and lead styphnate (LS) [12,13]. Copper is more eco-friendly and less toxic than lead, which reduces its risk to human health and the environment. In addition, CA has a more powerful initiation ability than that of lead azide and LS, which can minimize the amount of sensitive energetic materials needed and satisfy the devel opmental trend towards miniaturization [14,15].
Recently, the gas-solid azidation method has been commonly used for the preparation of CA materials by using copper-based materials and gaseous HN3 as precursors [14–22]. For example, Yang’s group utilized metal-organic framework (MOF)-derived porous carbon materials with a well-distributed copper source and HN3 gas for the synthesis of CA, which simultaneously led to low electrostatic sensitivity and high initiation ability [17]. Ye et al. adopted a gas-solid azidation technique to fabricate a porous CA film, which was integrated with a Ni–Cr micro initiator device to achieve successful ignition through joule heating [15]. However, the gas-solid azidation technique above (Fig. 1a) also requires highly toxic, readily self-ignitable and explosive HN3 gas, a long azidation time (>12 h), and a sophisticated and expensive setup [23]. The aforementioned obstacles not only put the synthetic process at risk but also impede its large-scale production. Furthermore, the utilization degree of the HN3 gas is very low, resulting in tremendous waste and post-processing problems. Hence, it is urgent to develop a facile, safe and yet scalable method to fabricate CA-based materials with
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Zhang), [email protected] (J. Zhu). https://doi.org/10.1016/j.nanoen.2019.104135 Received 16 July 2019; Received in revised form 30 August 2019; Accepted 24 September 2019 Available online 25 September 2019 2211-2855/© 2019 Elsevier Ltd. All rights reserved.
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satisfactory performances. Liquid-solid synthesis is always a magical route to fabricate func tional materials [24–27]. Herein, we present a safe and efficient liquid-solid strategy to produce CuO-derived CA films by employing CuO NRs arrays as precursors. The obtained CA film is highly exothermic and exhibits a lower electrostatic sensitivity compared with that of pristine CA or commonly used primary explosives. The improvement in electrostatic safety may be attributed to the nest-like structure of the film and the additional CuO doping, which can efficiently facilitate static charge transfer and reduce the friction-generated electrostatic energy. To the best of our knowledge, the electro-assisted azidation methodology in this work is the first demonstration of the direct and in situ growth of CA films in aqueous environments. More significantly, the azidation reaction time is greatly shortened (<40 min), which is bene ficial for its practical application and the scale-up of CA fabrication with low safety risks.
(OH)2 NRs. Then, the resultant films were washed with a substantial amount of deionized water and subsequently dried in an oven. For preparation of CuO, the Cu(OH)2 NRs films were heated at 200 � C for 3 h under the protection of N2. Finally, a dark brown layer comprising CuO NRs was formed on the surface of the Cu foil. 2.3. Preparation of CA films The preparation of the CA films was achieved by an electro-assisted azidation method. Briefly, the CA films were generated in situ on the surface of the CuO NRs arrays in an aqueous 0.02 M NaN3 solution at a constant current density of 1.0 mA cm 2 in a two-electrode system, where a Pt electrode was used as the auxiliary electrode. After a 10–40 min azidation reaction, the appearance of a reddish-brown film indicated the successful growth of a CA film on the Cu surface. Finally, the Cu foil with the product was removed from the reaction solution, rinsed with deionized water and anhydrous ethanol, and dried at room temperature before being stored in a vacuum desiccator.
2. Experimental section 2.1. Materials
2.4. Characterization
Cu foils (99.9%, 0.1 mm in thickness), hydrochloric acid (HCl, 36–38%), acetone (99.5%) and anhydrous ethanol (99.7%), and NaN3 (�99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. KOH (�85%) was obtained from Chengdu Kelong Chemical. Co., Ltd. All chemicals were of analytical grade and used as received without any further purification. Deionized water (Milli-Q) was used for all the experiments.
The phase structures were characterized by X-ray diffraction (XRD, Bruker D8 Advance) with monochromatized Cu Kα radiation (λ ¼ 0.15406 nm). FEI field-emission scanning electron microscopy (SEM, Quanta 250F), equipped with energy-dispersive X-ray spectros copy (EDX) capabilities, transmission electron microscopy (TEM, FEI Tecnai G2 20 LaB6) and high-resolution TEM (HRTEM, FEI Tecnai G2 F30 S-Twin), were used to examine the morphologies and compositions of the as-prepared samples. Fourier transform infrared (FT-IR) spectra of the samples were recorded on a Nicolet 750 FT-IR spectrophotometer. Additionally, the surface elemental compositions and chemical states of the samples were measured by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, Al-Kα, 1486.8 eV). Differential scanning calorimetry (DSC, Mettler Toledo) and thermogravimetry (TG, Mettler Toledo) were performed at a heating rate of 2.5 K min 1 over a tem perature range of 100–300 � C under a N2 flow of 30.0 mL min 1. The samples were scraped off the substrate for thermal analysis. For the laser ignition test, the CA film on Cu substrate is directly illuminated by the pulsed Nd:YAG laser. The laser beam was focused by means of lenses. The wavelength, pulse width and incident laser energy were set to 1064 nm, 6.5 ns and 30 mJ, respectively. The ignition process was recorded synchronously utilizing a high-speed camera (FASTCAM) capturing 25 000 frames per second. In addition, the laser ignition process of the CA film also recorded by a mobile phone.
2.2. Preparation of CuO NRs arrays films Cu foils samples 40 � 20 mm in size were ultrasonically cleaned in acetone, HCl solution, ethanol and deionized water for 10 min in each solution to remove surface impurities and the native oxide layer, fol lowed by desiccation under a N2 flow. Preparation of the CuO NRs arrays films was accomplished by electrochemical anodization and a subse quent annealing process [28]. Typically, anodization was performed at a constant current density of 4.0 mA cm 2 for 900 s at room temperature in a two-electrode system. The pre-treated Cu foil was used as working electrode (half of Cu foil was immersed in the electrolyte solution), Pt foil was used as the counter electrode, and an aqueous 3.0 M KOH so lution was used as the electrolyte. The distance between cathode and anode was set to 2.0 cm. After the anodization process, a pale blue film appeared on the surface of the Cu foil, indicating the formation of the Cu
Fig. 1. Schematic of (a) the traditional gas-solid reaction process for CA and (b) the liquid-solid process for the CA film in this work. 2
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For the electrostatic sensitivity test, the charge capacitance and electrode gap length were set to 500 pF and 0.12 mm, respectively. The test energy was given by the formula E ¼ 0.5CV2, where C is the capacitance of the capacitor in farads (F) and V is the charge voltage in volts (V). A 50% probability of ignition was calculated for the electro static sensitivity. Caution: Metal azide, which is very energetic and sensitive, can decompose explosively. It must be handled extremely carefully behind an explosion proof shield.
is a promising method for fabricating diverse energetic film materials, such as other metal azides, energetic MOFs, tetrazoles, and even pentazoles. The XRD patterns of the CuO NRs and CA film are shown in Fig. 2a. Clearly, the CuO NRs show two sets of diffraction peaks, the strong set is marked with purple triangles, which correspond to the Cu substrate, and the other set, labelled with black squares, is associated with the mono clinic phase of CuO (JCPDS Card 05–0661). After the azidation reaction, the CA film displays an additional three representative peaks at 11.8� , 27.9� and 31.9� , which can be indexed to the (110), (230) and (021) planes, respectively, of Cu(N3)2 (JCPDS card 21–0281). This observation implies the successful electrosynthesis of CA from a CuO precursor. In addition, FT-IR spectroscopy of the CA film (Fig. S4) depicts the typical asymmetric and symmetric vibrations of azides at 2134 and 2089 cm 1, and 1304 and 1266 cm 1, respectively, which further indicates the ex istence of N3 in the final product [29,30]. The surface composition and chemical states were analysed using XPS. In comparison with the wide scan of the CuO film (Fig. 2b), the appearance of additional signals at a binding energy of 400.0 eV for the CA surface can be assigned to the N 1s species [31]. Moreover, the N 1s peak can be resolved into two peaks at approximately 399.6 and 403.6 eV (Fig. 2c), which can be ascribed to N3 groups [32]. Based on the above results, it is strongly confirmed that the resulting CA film is composed of both Cu(N3)2 and CuO, indicating the successful in situ growth of a CA film in an aqueous environment for the first time. The optical images of Cu(OH)2, CuO and the corresponding CA film on Cu foils are presented in Fig. 2d. A faint-blue Cu(OH)2 film was first prepared via electrochemical anodization. Afterwards, black CuO was generated from Cu(OH)2 by annealing. Lastly, a reddish-brown CA film was formed on the Cu foil after the azidation reaction. The morphology
3. Results and discussion The electro-assisted azidation process for the CA film is schemati cally shown in Fig. 1b. Regular Cu(OH)2 NRs with ~20 μm in length and ~450 nm in diameter (Figs. S1 and S2), are first grown on the surface of Cu foil using electrochemical anodization in a KOH solution [28]. Af terwards, the Cu(OH)2 NRs are annealed at 200 � C to form CuO NRs by a dehydration reaction. The average length and diameter decrease to ~10 μm and ~350 nm, respectively (Fig. S3). The Cu foil-supported CuO NRs are able to serve as the working electrode in the following azidation reaction. Once a constant current of 1 mA cm 2 is applied, a CA film can be generated on the surface of the CuO NRs in an aqueous NaN3 solution with Pt as the counter electrode. In comparison to the traditional gas-solid strategy (Fig. 1a), this liquid-solid method enables the CA film to be fabricated at less risk in a substantially shortened time (<40 min) without hazardous HN3, all of which are beneficial for its large-scale practical production. In addition, the proposed liquid-solid approach is fully compatible with MEMS techniques. Thus, the prepared CA film can be successfully integrated with MEMS systems to construct func tional energetic devices. Moreover, this facile electrosynthesis approach
Fig. 2. (a) XRD patterns of the CuO NRs and CA film. (b) XPS survey scan spectra of the CuO NRs and CA film. (c) XPS spectrum of the N 1s regions for the CA film. (d) Optical images of the Cu(OH)2, CuO and CA films on the Cu foil. (e) Top view SEM images of the CA film at different magnifications. (f) Cross-sectional SEM image of the CA film. (g) TEM image of the CA film. (h,i) HRTEM images of the CA film. (j) EDX elemental mapping images of Cu, O, and N for the CA film. The azidation reaction time is set to 20 min. 3
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of the CA film can be observed in the typical SEM images. The CA film comprises several well-connected enclosed nest-like structures (Fig. 2e) with an average thickness of ~10 μm (Fig. 2f), and numerous ultrafine nanowires (NWs) with an average diameter of ~80 nm can be seen in side of the nests (inset of Fig. 2e). Furthermore, the corresponding EDX elemental mapping analysis reveals the uniform distribution of Cu, N, and O in the CA film (Fig. S11). Both observations evidently justify the formation of a uniform CA film on the surface of the Cu substrate through the liquid-solid azidation strategy. The structure of the NWs of the CA film was further characterized by TEM and HRTEM. Fig. 2g shows a typical TEM image of the NWs scraped from Cu foil. The average diameter of the NWs is estimated to be 80 nm, which is in accordance with the SEM observations. From the HRTEM images (Fig. 2h–i), the interplanar spacing is determined to be 0.40 nm, which corresponds to the (130) plane of Cu(N3)2. Furthermore, the uniform dispersion of Cu, O and N, as demonstrated by the EDX elemental mapping images for a single NW (Fig. 2j), verifies the ho mogeneous distribution of closely interconnected Cu(N3)2 and CuO. Fig. 3a shows a DSC thermogram for a CA film prepared with a 20 min azidation reaction. Only one sharp exothermic peak can be observed at a peak temperature of 195.5 � C with a heat output of 1650 J g 1, which is associated with the fast thermal decomposition of CA [22]. Due to the introduction of CuO, the energy release of the entire system decreases to an extent. The TG curve in Fig. 3b indicates that the mass loss of the CA film is ca. 28.8% when heated to 300 � C. Therefore, the Cu(N3)2 content in the CA film is calculated to be ca. 51 wt%. In addition, the E50 value of the CA film is ca. 0.81 mJ (Fig. 3c), which is much better than those of common primary explosives. For example, the E50 value of commercial LS is only 0.25 mJ, while the value of pristine Cu(N3)2 is 0.05 mJ. In addition, the E50 values of mercury fulminate (MF), a porous CA film and two graphene-modified LS composites (labelled GLS(I) and GLS(II)) are 0.51, 0.098, 0.4 and 0.5 mJ, respec tively [13,15,33]. Therefore, the prepared CA film exhibits superior electrostatic stability. The improvement in the electrostatic safety may be attributed to the nest-like structure and homogeneous distribution of CuO in the CA NWs, which facilitates static charge transfer and prevents
the accumulation of electric charge. Furthermore, we speculate that the CA confined in the NW structure can effectively reduce the electrostatic energy generated by friction. Interestingly, the resulting energetic film can be tailored into desired shape, as shown in Fig. 3d, revealing that the CA film is highly safe and capable of being shaped into the necessary form needed to meet the requirements of a functional device. A preliminary laser ignition test was used to qualitatively determine the explosive ability of the CA film. It can be observed that an obvious explosion occurs accompanied by a bright flame upon the stimulation from a laser (Movie S1). The whole reddish-brown CA film on the sur face of the Cu substrate is completely consumed (as shown in Fig. 3e). Interestingly, the explosion reaction of the CA film occurs too quickly to be captured by a high-speed camera (Fig. S6). The kinetics of the CA film were studied and compared with those of LS by non-isothermal DSC under different heating rates. The peak temperatures of the exothermic peaks are 195.5 � C, 207.3 � C, 211.6 � C, and 216.4 � C, corresponding to an increase in the heating rate (Fig. 3f). The apparent activation energy Ea (J⋅mol 1), which is defined as the minimum energy needed to initiate the reaction, can be calculated by the Kissinger method based on Eq. (1): ! � � βi AR Ea ln 2 ¼ ln (1) Ea RTpi Tpi where Tp is the peak temperature of the exothermic peak in the DSC curve (K), βi is the heating rate (K⋅min 1), and R is the universal gas constant (8.314 J mol 1 K 1). It follows that Ea can be estimated from the slope of a linear plot of ln(βi/T2pi) versus 1/Tpi (Fig. 3g). The acti vation energy for the CA film is calculated to be 119 kJ mol 1. Compared with the activation energy (184 kJ mol 1) estimated for LS (Fig. S7), the CA film has a lower potential barrier for the thermal decomposition reaction. Based on the above results, the CA film fabricated by the electro-assisted azidation approach is extremely favorable for practical applications with respect to its high safety and excellent energetic performance. Supplementary video related to this article can be found at https ://doi.org/10.1016/j.nanoen.2019.104135.
Fig. 3. (a) DSC curve of a CA film. (b) TG curve of a CA film. (c) Electrostatic sensitivities of LS, GLS(I), GLS(II), MF, porous CA, Cu(N3)2 and a CA film. (d) Photographs of shaped CA films. (e) Optical images of CA films before and after ignition. (f) DSC curves of CA films at various heating rates. (g) Linear fit for the plot of ln(βi/Tp2) against 1/Tp for the calculated activation energy. The azidation reaction time is set to 20 min. 4
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To determine the mechanism for the growth of the CA film, the azidation reaction was allowed to proceed for different amounts of time. The reaction time has a substantial effect on the compositions and properties of the CA films. The XRD patterns of the CA films with different reaction time are shown in Fig. S8. Obviously, the relative intensities of the characteristic diffraction peaks of Cu(N3)2 increase dramatically with increasing reaction time. Furthermore, a gradual decrease in the relative intensities of the CuO peaks indicates that an increasing amount of CuO has been converted into CA. In addition, when the reaction time is greater than 30 min, the characteristic peaks of CuN3 (JCPDS card 04–0622), in addition to the peaks of Cu(N3)2, can also be identified. The possible reason is that the Cu substrate is supposed to also be involved in the azidation reaction. To verify this proposal, a pristine Cu foil anode was directly utilized as a working electrode for the azidation process. A yellow film was grown on the Cu foil after the azidation reaction (Fig. S9a). As expected, only CuN3 is detected on the surface of the Cu foil (Fig. S9b), indicating that the CuN3 in CA film derives from the azidation of the Cu substrate. It should be pointed out that CuN3 presents a dense layer structure rather than nest-like structure (Figs. S9c–d). Typical SEM images of the CA films on Cu foil prepared with different reaction time are shown in Figs. S10–13, while their corresponding EDX and element mapping images are included. After a 10 min azidation reaction, the diameters of CuO NRs significantly decrease, and their surfaces are covered with filamentous CA NWs (Fig. S10b). In addition, a prototypical nest structure gradually forms from the interweaving NWs. As the reaction time increases past 20 min, the enclosed nest-like structures remain almost unchanged. CuO NRs are almost invisible on the surface of the CA film. From the EDX results (Figs. S10–13), the atomic percentage of N in the CA films increases substantially with increasing reaction time, thus resulting in an increase in the energy output and gas released (Fig. S14). In other words, a high N content in the CA film is accompanied by high brisance. However, an increase in the Cu(N3)2 or CuN3 content also increases the safety risk of the CA films. The E50 value is only 0.16 mJ for a CA film prepared with a 30 min azidation reaction. The E50 value is close to that of pristine Cu(N3)2 after a 40 min azidation reaction. Therefore, the properties of the CA films can be effectively tuned in this liquid-solid azidation strategy via the reac tion time. From the above results, the reaction process on the anodic surface can be summarized by the following formulas (Eqs. (2)–(4)): 2H2 O
4e → O2 þ 4Hþ
CuO þ 2Hþ þ 2N3 → CuðN3 Þ2 þ H2 O
Cu
e þ N3 → CuN3
(4)
As shown in Fig. 4a, once energized, water splitting occurs first to generate Hþ, which is helpful for releasing Cu2þ from the CuO NRs. Cu2þ subsequently combines with N3 to form Cu(N3)2, which is defined as the main azidation reaction. In this azidation process, the CuO NRs gradu ally dissolve, while CA NWs are continuously formed. The nest-like structure may be ascribed to the oxygen bubbles generated at the anode. The oxygen bubbles guide the growth of CA NWs. As the reaction time increases, the resulting CA NWs interweave to form the walls of the nest. Of course, as the reaction time goes on, some of the CuN3 derives from the oxidation of the Cu substrate and immediate combination with N3 . To confirm the proposed liquid-solid azidation process, DFT calcu lations are employed to provide further insights into the detailed reac tion pathways (see calculation details in Supporting Information). Fig. 4b displays the adsorption energy diagram of the initial reaction process on the surface of CuO. The reaction starts with the adsorption of two Hþ ions onto an O atom on the CuO surface. The adsorption energy of Hþ (ΔE1) is calculated to be 6.51 eV, which indicates that Hþ very easily adsorbs onto the CuO surface. In the next step of the reaction pathway, the energy barrier (ΔE2) that needs to be overcome for H2O to be released from CuO surface is only 0.37 eV, indicating that O atom readily separates from the original site to form a H2O molecule. The last step of the reaction is N3 adsorption onto the oxygen defect site, which has a large adsorption energy (ΔE3) of 3.20 eV and is beneficial to the phase change during the azidation reaction. Furthermore, the calculated energies (ΔE) show a clear exothermic driving force ( 11.18 eV) to wards the phase change, displacing the O2 in CuO with N3 to form Cu (N3)2 (Fig. S15). Overall, the current study further expands on the re action mechanism of the liquid-solid azidation process. The preparation of nanoenergetics-on-a-chip is a key step for sub stantially advancing microscale energy-consumption devices [34]. To demonstrate the feasibility of applying the as-prepared materials to nanoenergetics-on-a-chip, a CA film was integrated with a Pt/W wire micro igniter (labelled the Pt/W/CA micro igniter). Schematic diagrams and optical images of the electro-thermal ignition devices are shown in Fig. 5 and Fig. S16, respectively. After a Cu layer, with an average thickness of 1 μm, is deposited onto the Pt/W wire micro igniter (labelled the Pt/W/Cu micro igniter), the Cu layer is transformed into a CA layer through this liquid-solid azidation method. The ignition pro cesses, which is triggered by applying with a voltage of 50 V and recorded with a high-speed camera, were investigated for the Pt/W/Cu micro igniter and Pt/W/CA micro igniter. The time interval between
(2) (3)
Fig. 4. (a) A schematic of the growth mechanism of the CA film. (b) The calculated energy diagram and corresponding structures for the liquid-solid azidation reaction on the (111) surface of CuO. 5
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Fig. 5. Diagrams and corresponding ignition processes of (a, b) Pt/W/Cu micro igniter and (c, d) Pt/W/CA micro igniter.
adjacent frames was set to 20 μs. It can be seen (Fig. 5b) that only a small flame forms from heating the Pt/W/Cu wire. In stark contrast, an extremely intense flame erupts from the Pt/W/CA micro igniter (Fig. 5d). The flame, which results from the violent reaction of CA, is induced by joule heating. The maximum flame height, which is nearly 7 mm, is approximately twice that of the Pt/W/Cu micro igniter flame. Moreover, the rapid release of gas (N2) further supports the ignition effect. Therefore, it is obvious that a significant improvement has been realized in the energy output of the Pt/W/CA micro igniter. Excitingly, the experimental data demonstrate both a great progress in the suc cessful ignition properties and the promise for applications in MEMS devices.
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4. Conclusion To summarize, a unique nest-like structured CA film has been suc cessfully designed and fabricated by means of a facile electro-assisted azidation method. The prepared CA film exhibits excellent energetic performance and superior electrostatic stability, successfully responding to the tremendous challenges of CA materials. Moreover, in comparison with other reported azidation methods, this liquid-solid azidation method can be performed safely and efficiently under aqueous condi tions. Finally, the positive results in our work meets the demands for nanoenergetics-on-a-chip. We believe that these results will be of great importance to fundamentally understand detonation or ignition at the nanoscale, which will influence many related applications. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant 51576101, 51772152), the Fundamental Research Funds for the Central Universities (Grant 30918015102) and Qing Lan Project. The authors acknowledge Prof. Xufei Zhu (Nanjing University of Science and Technology) and Prof. Wansen Hua (Nanjing University of Science and Technology) for the fruitful discussion about the growth mechanism of the CA film, Prof. Fengli Bei (Nanjing Uni versity of Science and Technology) for HRTEM analysis and advices about the manuscript, Prof. Kefeng Ma (Nanjing University of Science and Technology) and Dr. Jingwen Sun (Nanjing University of Science and Technology) for carefully reading and reviewing the manuscript, Dr. Debin Ni (Shanxi Applied Physics and Chemistry Research Institute) for providing Pt/W/Cu micro igniters. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2019.104135.
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Nano Energy 66 (2019) 104135 Chunpei Yu studied chemistry at Nanjing University of Science and Technology and received his B.S. degree in 2015. He is currently a Ph.D. student in the group of Dr. Wenchao Zhang and Prof. Junwu Zhu at Nanjing University of Science and Technology. His main research interests are focused on the formation and application of nanostructured energetic materials.
Zilong Zheng is currently a Ph.D. student at Nanjing University of Science and Technology. His research interests are focused on micro/nano energetic materials, 3D printing technology and MEMS devices.
Wenchao Zhang received his Ph.D. degree from Nanjing Uni versity of Science and Technology in 2011. He is presently working as associate professor in the same university. His research interests are focused on nano energetic materials and MEMS ignition device.
Shengli Zhang received his Ph.D. degree from Beijing Uni versity of Chemical Technology in 2013. He then joined the Key Laboratory of Advanced Display Materials and Devices, Nanjing University of Science and Technology, where he is a professor in the Department of Materials Science and Engineering. His research interests are focused on electronic, optoelectronic and energy devices based on 2D materials.
Shiying Guo is currently a Ph.D. student at MIIT Key Labora tory of Advanced Display Materials and Devices. She received her B.S. in Materials Science and Engineering from Nanjing University of Science and Technology in 2016. Her current research interests are concentrated on exploring the 2D mate rial application in optoelectronics and energy devices.
Jiahai Ye received his Ph.D. degree from Nanjing University. He is currently an associate professor of Chemical Engineering at Nanjing University of Science and Technology. His research is focused on the synthesis and characterization of organic fluorescence materials and energy materials.
Bin Hu is pursuing his M.S. under the supervision of Dr. Wen chao Zhang. His research interests are concentrated on the preparation, characterization, and reactivity of nano energetic materials.
Junwu Zhu received his Ph.D. degree in Materials Chemistry from Nanjing University of Science and Technology in 2005. He is presently working as professor in the same university. His main research interests are focused on the preparation and application of functional materials based on carbon and nano structured composites.
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Full paper
A safe and efficient liquid-solid synthesis for copper azide films with excellent electrostatic stability Chunpei Yu a, Wenchao Zhang a, **, Shiying Guo b, Bin Hu a, Zilong Zheng a, Jiahai Ye a, Shengli Zhang b, Junwu Zhu a, * a
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China Key Laboratory of Advanced Display Materials and Devices, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Electrosynthesis Nanoenergetic materials Primary explosives Copper azide Energy release
The traditional gas-solid synthesis of copper azide (CA) always requires rigorous conditions, such as the use of hazardous HN3 gas, and time-consuming preparation processes (>12 h). Both disadvantages impede its ecofriendly large-scale production and result in high safety risks for the practical synthesis. Here, a safe and effi cient (<40 min) liquid-solid strategy is developed to fabricate CA films. Employing CuO nanorods (NRs) arrays as precursors, a nest-like CA film with remarkable energetic performance and excellent electrostatic stability can be generated in situ in a NaN3 solution via an electro-assisted azidation method. The resulting CA film possesses an energy release of 1650 J g 1 and effective detonation performance. In particular, the electrostatic sensitivity (E50) value (0.81 mJ) is 16 times higher than that of the original CA powder (0.05 mJ), which is attributed to not only the unique nest-like structure of the CA film but also the uniform in situ incorporation of CuO. In addition, density functional theory (DFT) calculations are employed to provide insights into the detailed reaction path ways of the liquid-solid azidation process. Importantly, the constructed CA film can be directly integrated into a micro ignitor to realize nanoenergetics-on-a-chip due to the safe synthetic process as well as its high compati bility with microelectromechanical systems (MEMS) technology.
1. Introduction Energetic materials are one of ancient and extreme forms of energy storage and release. Polynitrogen compounds arouse great interest due to their large positive formation energies, high densities and fast energy release [1–11]. Among the reported polynitrogen compounds, transition-metal azides are one of the most important and classical types of primary explosives and have long-term chemical and thermal stabil ities, reliable and rapid initiation ability. Notably, as an energetic transition-metal azide with high brisance, copper azide (CA) is emerging as a replacement for the most used toxic lead-based primary explosives, such as lead azide and lead styphnate (LS) [12,13]. Copper is more eco-friendly and less toxic than lead, which reduces its risk to human health and the environment. In addition, CA has a more powerful initiation ability than that of lead azide and LS, which can minimize the amount of sensitive energetic materials needed and satisfy the devel opmental trend towards miniaturization [14,15].
Recently, the gas-solid azidation method has been commonly used for the preparation of CA materials by using copper-based materials and gaseous HN3 as precursors [14–22]. For example, Yang’s group utilized metal-organic framework (MOF)-derived porous carbon materials with a well-distributed copper source and HN3 gas for the synthesis of CA, which simultaneously led to low electrostatic sensitivity and high initiation ability [17]. Ye et al. adopted a gas-solid azidation technique to fabricate a porous CA film, which was integrated with a Ni–Cr micro initiator device to achieve successful ignition through joule heating [15]. However, the gas-solid azidation technique above (Fig. 1a) also requires highly toxic, readily self-ignitable and explosive HN3 gas, a long azidation time (>12 h), and a sophisticated and expensive setup [23]. The aforementioned obstacles not only put the synthetic process at risk but also impede its large-scale production. Furthermore, the utilization degree of the HN3 gas is very low, resulting in tremendous waste and post-processing problems. Hence, it is urgent to develop a facile, safe and yet scalable method to fabricate CA-based materials with
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Zhang), [email protected] (J. Zhu). https://doi.org/10.1016/j.nanoen.2019.104135 Received 16 July 2019; Received in revised form 30 August 2019; Accepted 24 September 2019 Available online 25 September 2019 2211-2855/© 2019 Elsevier Ltd. All rights reserved.
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satisfactory performances. Liquid-solid synthesis is always a magical route to fabricate func tional materials [24–27]. Herein, we present a safe and efficient liquid-solid strategy to produce CuO-derived CA films by employing CuO NRs arrays as precursors. The obtained CA film is highly exothermic and exhibits a lower electrostatic sensitivity compared with that of pristine CA or commonly used primary explosives. The improvement in electrostatic safety may be attributed to the nest-like structure of the film and the additional CuO doping, which can efficiently facilitate static charge transfer and reduce the friction-generated electrostatic energy. To the best of our knowledge, the electro-assisted azidation methodology in this work is the first demonstration of the direct and in situ growth of CA films in aqueous environments. More significantly, the azidation reaction time is greatly shortened (<40 min), which is bene ficial for its practical application and the scale-up of CA fabrication with low safety risks.
(OH)2 NRs. Then, the resultant films were washed with a substantial amount of deionized water and subsequently dried in an oven. For preparation of CuO, the Cu(OH)2 NRs films were heated at 200 � C for 3 h under the protection of N2. Finally, a dark brown layer comprising CuO NRs was formed on the surface of the Cu foil. 2.3. Preparation of CA films The preparation of the CA films was achieved by an electro-assisted azidation method. Briefly, the CA films were generated in situ on the surface of the CuO NRs arrays in an aqueous 0.02 M NaN3 solution at a constant current density of 1.0 mA cm 2 in a two-electrode system, where a Pt electrode was used as the auxiliary electrode. After a 10–40 min azidation reaction, the appearance of a reddish-brown film indicated the successful growth of a CA film on the Cu surface. Finally, the Cu foil with the product was removed from the reaction solution, rinsed with deionized water and anhydrous ethanol, and dried at room temperature before being stored in a vacuum desiccator.
2. Experimental section 2.1. Materials
2.4. Characterization
Cu foils (99.9%, 0.1 mm in thickness), hydrochloric acid (HCl, 36–38%), acetone (99.5%) and anhydrous ethanol (99.7%), and NaN3 (�99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. KOH (�85%) was obtained from Chengdu Kelong Chemical. Co., Ltd. All chemicals were of analytical grade and used as received without any further purification. Deionized water (Milli-Q) was used for all the experiments.
The phase structures were characterized by X-ray diffraction (XRD, Bruker D8 Advance) with monochromatized Cu Kα radiation (λ ¼ 0.15406 nm). FEI field-emission scanning electron microscopy (SEM, Quanta 250F), equipped with energy-dispersive X-ray spectros copy (EDX) capabilities, transmission electron microscopy (TEM, FEI Tecnai G2 20 LaB6) and high-resolution TEM (HRTEM, FEI Tecnai G2 F30 S-Twin), were used to examine the morphologies and compositions of the as-prepared samples. Fourier transform infrared (FT-IR) spectra of the samples were recorded on a Nicolet 750 FT-IR spectrophotometer. Additionally, the surface elemental compositions and chemical states of the samples were measured by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, Al-Kα, 1486.8 eV). Differential scanning calorimetry (DSC, Mettler Toledo) and thermogravimetry (TG, Mettler Toledo) were performed at a heating rate of 2.5 K min 1 over a tem perature range of 100–300 � C under a N2 flow of 30.0 mL min 1. The samples were scraped off the substrate for thermal analysis. For the laser ignition test, the CA film on Cu substrate is directly illuminated by the pulsed Nd:YAG laser. The laser beam was focused by means of lenses. The wavelength, pulse width and incident laser energy were set to 1064 nm, 6.5 ns and 30 mJ, respectively. The ignition process was recorded synchronously utilizing a high-speed camera (FASTCAM) capturing 25 000 frames per second. In addition, the laser ignition process of the CA film also recorded by a mobile phone.
2.2. Preparation of CuO NRs arrays films Cu foils samples 40 � 20 mm in size were ultrasonically cleaned in acetone, HCl solution, ethanol and deionized water for 10 min in each solution to remove surface impurities and the native oxide layer, fol lowed by desiccation under a N2 flow. Preparation of the CuO NRs arrays films was accomplished by electrochemical anodization and a subse quent annealing process [28]. Typically, anodization was performed at a constant current density of 4.0 mA cm 2 for 900 s at room temperature in a two-electrode system. The pre-treated Cu foil was used as working electrode (half of Cu foil was immersed in the electrolyte solution), Pt foil was used as the counter electrode, and an aqueous 3.0 M KOH so lution was used as the electrolyte. The distance between cathode and anode was set to 2.0 cm. After the anodization process, a pale blue film appeared on the surface of the Cu foil, indicating the formation of the Cu
Fig. 1. Schematic of (a) the traditional gas-solid reaction process for CA and (b) the liquid-solid process for the CA film in this work. 2
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For the electrostatic sensitivity test, the charge capacitance and electrode gap length were set to 500 pF and 0.12 mm, respectively. The test energy was given by the formula E ¼ 0.5CV2, where C is the capacitance of the capacitor in farads (F) and V is the charge voltage in volts (V). A 50% probability of ignition was calculated for the electro static sensitivity. Caution: Metal azide, which is very energetic and sensitive, can decompose explosively. It must be handled extremely carefully behind an explosion proof shield.
is a promising method for fabricating diverse energetic film materials, such as other metal azides, energetic MOFs, tetrazoles, and even pentazoles. The XRD patterns of the CuO NRs and CA film are shown in Fig. 2a. Clearly, the CuO NRs show two sets of diffraction peaks, the strong set is marked with purple triangles, which correspond to the Cu substrate, and the other set, labelled with black squares, is associated with the mono clinic phase of CuO (JCPDS Card 05–0661). After the azidation reaction, the CA film displays an additional three representative peaks at 11.8� , 27.9� and 31.9� , which can be indexed to the (110), (230) and (021) planes, respectively, of Cu(N3)2 (JCPDS card 21–0281). This observation implies the successful electrosynthesis of CA from a CuO precursor. In addition, FT-IR spectroscopy of the CA film (Fig. S4) depicts the typical asymmetric and symmetric vibrations of azides at 2134 and 2089 cm 1, and 1304 and 1266 cm 1, respectively, which further indicates the ex istence of N3 in the final product [29,30]. The surface composition and chemical states were analysed using XPS. In comparison with the wide scan of the CuO film (Fig. 2b), the appearance of additional signals at a binding energy of 400.0 eV for the CA surface can be assigned to the N 1s species [31]. Moreover, the N 1s peak can be resolved into two peaks at approximately 399.6 and 403.6 eV (Fig. 2c), which can be ascribed to N3 groups [32]. Based on the above results, it is strongly confirmed that the resulting CA film is composed of both Cu(N3)2 and CuO, indicating the successful in situ growth of a CA film in an aqueous environment for the first time. The optical images of Cu(OH)2, CuO and the corresponding CA film on Cu foils are presented in Fig. 2d. A faint-blue Cu(OH)2 film was first prepared via electrochemical anodization. Afterwards, black CuO was generated from Cu(OH)2 by annealing. Lastly, a reddish-brown CA film was formed on the Cu foil after the azidation reaction. The morphology
3. Results and discussion The electro-assisted azidation process for the CA film is schemati cally shown in Fig. 1b. Regular Cu(OH)2 NRs with ~20 μm in length and ~450 nm in diameter (Figs. S1 and S2), are first grown on the surface of Cu foil using electrochemical anodization in a KOH solution [28]. Af terwards, the Cu(OH)2 NRs are annealed at 200 � C to form CuO NRs by a dehydration reaction. The average length and diameter decrease to ~10 μm and ~350 nm, respectively (Fig. S3). The Cu foil-supported CuO NRs are able to serve as the working electrode in the following azidation reaction. Once a constant current of 1 mA cm 2 is applied, a CA film can be generated on the surface of the CuO NRs in an aqueous NaN3 solution with Pt as the counter electrode. In comparison to the traditional gas-solid strategy (Fig. 1a), this liquid-solid method enables the CA film to be fabricated at less risk in a substantially shortened time (<40 min) without hazardous HN3, all of which are beneficial for its large-scale practical production. In addition, the proposed liquid-solid approach is fully compatible with MEMS techniques. Thus, the prepared CA film can be successfully integrated with MEMS systems to construct func tional energetic devices. Moreover, this facile electrosynthesis approach
Fig. 2. (a) XRD patterns of the CuO NRs and CA film. (b) XPS survey scan spectra of the CuO NRs and CA film. (c) XPS spectrum of the N 1s regions for the CA film. (d) Optical images of the Cu(OH)2, CuO and CA films on the Cu foil. (e) Top view SEM images of the CA film at different magnifications. (f) Cross-sectional SEM image of the CA film. (g) TEM image of the CA film. (h,i) HRTEM images of the CA film. (j) EDX elemental mapping images of Cu, O, and N for the CA film. The azidation reaction time is set to 20 min. 3
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of the CA film can be observed in the typical SEM images. The CA film comprises several well-connected enclosed nest-like structures (Fig. 2e) with an average thickness of ~10 μm (Fig. 2f), and numerous ultrafine nanowires (NWs) with an average diameter of ~80 nm can be seen in side of the nests (inset of Fig. 2e). Furthermore, the corresponding EDX elemental mapping analysis reveals the uniform distribution of Cu, N, and O in the CA film (Fig. S11). Both observations evidently justify the formation of a uniform CA film on the surface of the Cu substrate through the liquid-solid azidation strategy. The structure of the NWs of the CA film was further characterized by TEM and HRTEM. Fig. 2g shows a typical TEM image of the NWs scraped from Cu foil. The average diameter of the NWs is estimated to be 80 nm, which is in accordance with the SEM observations. From the HRTEM images (Fig. 2h–i), the interplanar spacing is determined to be 0.40 nm, which corresponds to the (130) plane of Cu(N3)2. Furthermore, the uniform dispersion of Cu, O and N, as demonstrated by the EDX elemental mapping images for a single NW (Fig. 2j), verifies the ho mogeneous distribution of closely interconnected Cu(N3)2 and CuO. Fig. 3a shows a DSC thermogram for a CA film prepared with a 20 min azidation reaction. Only one sharp exothermic peak can be observed at a peak temperature of 195.5 � C with a heat output of 1650 J g 1, which is associated with the fast thermal decomposition of CA [22]. Due to the introduction of CuO, the energy release of the entire system decreases to an extent. The TG curve in Fig. 3b indicates that the mass loss of the CA film is ca. 28.8% when heated to 300 � C. Therefore, the Cu(N3)2 content in the CA film is calculated to be ca. 51 wt%. In addition, the E50 value of the CA film is ca. 0.81 mJ (Fig. 3c), which is much better than those of common primary explosives. For example, the E50 value of commercial LS is only 0.25 mJ, while the value of pristine Cu(N3)2 is 0.05 mJ. In addition, the E50 values of mercury fulminate (MF), a porous CA film and two graphene-modified LS composites (labelled GLS(I) and GLS(II)) are 0.51, 0.098, 0.4 and 0.5 mJ, respec tively [13,15,33]. Therefore, the prepared CA film exhibits superior electrostatic stability. The improvement in the electrostatic safety may be attributed to the nest-like structure and homogeneous distribution of CuO in the CA NWs, which facilitates static charge transfer and prevents
the accumulation of electric charge. Furthermore, we speculate that the CA confined in the NW structure can effectively reduce the electrostatic energy generated by friction. Interestingly, the resulting energetic film can be tailored into desired shape, as shown in Fig. 3d, revealing that the CA film is highly safe and capable of being shaped into the necessary form needed to meet the requirements of a functional device. A preliminary laser ignition test was used to qualitatively determine the explosive ability of the CA film. It can be observed that an obvious explosion occurs accompanied by a bright flame upon the stimulation from a laser (Movie S1). The whole reddish-brown CA film on the sur face of the Cu substrate is completely consumed (as shown in Fig. 3e). Interestingly, the explosion reaction of the CA film occurs too quickly to be captured by a high-speed camera (Fig. S6). The kinetics of the CA film were studied and compared with those of LS by non-isothermal DSC under different heating rates. The peak temperatures of the exothermic peaks are 195.5 � C, 207.3 � C, 211.6 � C, and 216.4 � C, corresponding to an increase in the heating rate (Fig. 3f). The apparent activation energy Ea (J⋅mol 1), which is defined as the minimum energy needed to initiate the reaction, can be calculated by the Kissinger method based on Eq. (1): ! � � βi AR Ea ln 2 ¼ ln (1) Ea RTpi Tpi where Tp is the peak temperature of the exothermic peak in the DSC curve (K), βi is the heating rate (K⋅min 1), and R is the universal gas constant (8.314 J mol 1 K 1). It follows that Ea can be estimated from the slope of a linear plot of ln(βi/T2pi) versus 1/Tpi (Fig. 3g). The acti vation energy for the CA film is calculated to be 119 kJ mol 1. Compared with the activation energy (184 kJ mol 1) estimated for LS (Fig. S7), the CA film has a lower potential barrier for the thermal decomposition reaction. Based on the above results, the CA film fabricated by the electro-assisted azidation approach is extremely favorable for practical applications with respect to its high safety and excellent energetic performance. Supplementary video related to this article can be found at https ://doi.org/10.1016/j.nanoen.2019.104135.
Fig. 3. (a) DSC curve of a CA film. (b) TG curve of a CA film. (c) Electrostatic sensitivities of LS, GLS(I), GLS(II), MF, porous CA, Cu(N3)2 and a CA film. (d) Photographs of shaped CA films. (e) Optical images of CA films before and after ignition. (f) DSC curves of CA films at various heating rates. (g) Linear fit for the plot of ln(βi/Tp2) against 1/Tp for the calculated activation energy. The azidation reaction time is set to 20 min. 4
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To determine the mechanism for the growth of the CA film, the azidation reaction was allowed to proceed for different amounts of time. The reaction time has a substantial effect on the compositions and properties of the CA films. The XRD patterns of the CA films with different reaction time are shown in Fig. S8. Obviously, the relative intensities of the characteristic diffraction peaks of Cu(N3)2 increase dramatically with increasing reaction time. Furthermore, a gradual decrease in the relative intensities of the CuO peaks indicates that an increasing amount of CuO has been converted into CA. In addition, when the reaction time is greater than 30 min, the characteristic peaks of CuN3 (JCPDS card 04–0622), in addition to the peaks of Cu(N3)2, can also be identified. The possible reason is that the Cu substrate is supposed to also be involved in the azidation reaction. To verify this proposal, a pristine Cu foil anode was directly utilized as a working electrode for the azidation process. A yellow film was grown on the Cu foil after the azidation reaction (Fig. S9a). As expected, only CuN3 is detected on the surface of the Cu foil (Fig. S9b), indicating that the CuN3 in CA film derives from the azidation of the Cu substrate. It should be pointed out that CuN3 presents a dense layer structure rather than nest-like structure (Figs. S9c–d). Typical SEM images of the CA films on Cu foil prepared with different reaction time are shown in Figs. S10–13, while their corresponding EDX and element mapping images are included. After a 10 min azidation reaction, the diameters of CuO NRs significantly decrease, and their surfaces are covered with filamentous CA NWs (Fig. S10b). In addition, a prototypical nest structure gradually forms from the interweaving NWs. As the reaction time increases past 20 min, the enclosed nest-like structures remain almost unchanged. CuO NRs are almost invisible on the surface of the CA film. From the EDX results (Figs. S10–13), the atomic percentage of N in the CA films increases substantially with increasing reaction time, thus resulting in an increase in the energy output and gas released (Fig. S14). In other words, a high N content in the CA film is accompanied by high brisance. However, an increase in the Cu(N3)2 or CuN3 content also increases the safety risk of the CA films. The E50 value is only 0.16 mJ for a CA film prepared with a 30 min azidation reaction. The E50 value is close to that of pristine Cu(N3)2 after a 40 min azidation reaction. Therefore, the properties of the CA films can be effectively tuned in this liquid-solid azidation strategy via the reac tion time. From the above results, the reaction process on the anodic surface can be summarized by the following formulas (Eqs. (2)–(4)): 2H2 O
4e → O2 þ 4Hþ
CuO þ 2Hþ þ 2N3 → CuðN3 Þ2 þ H2 O
Cu
e þ N3 → CuN3
(4)
As shown in Fig. 4a, once energized, water splitting occurs first to generate Hþ, which is helpful for releasing Cu2þ from the CuO NRs. Cu2þ subsequently combines with N3 to form Cu(N3)2, which is defined as the main azidation reaction. In this azidation process, the CuO NRs gradu ally dissolve, while CA NWs are continuously formed. The nest-like structure may be ascribed to the oxygen bubbles generated at the anode. The oxygen bubbles guide the growth of CA NWs. As the reaction time increases, the resulting CA NWs interweave to form the walls of the nest. Of course, as the reaction time goes on, some of the CuN3 derives from the oxidation of the Cu substrate and immediate combination with N3 . To confirm the proposed liquid-solid azidation process, DFT calcu lations are employed to provide further insights into the detailed reac tion pathways (see calculation details in Supporting Information). Fig. 4b displays the adsorption energy diagram of the initial reaction process on the surface of CuO. The reaction starts with the adsorption of two Hþ ions onto an O atom on the CuO surface. The adsorption energy of Hþ (ΔE1) is calculated to be 6.51 eV, which indicates that Hþ very easily adsorbs onto the CuO surface. In the next step of the reaction pathway, the energy barrier (ΔE2) that needs to be overcome for H2O to be released from CuO surface is only 0.37 eV, indicating that O atom readily separates from the original site to form a H2O molecule. The last step of the reaction is N3 adsorption onto the oxygen defect site, which has a large adsorption energy (ΔE3) of 3.20 eV and is beneficial to the phase change during the azidation reaction. Furthermore, the calculated energies (ΔE) show a clear exothermic driving force ( 11.18 eV) to wards the phase change, displacing the O2 in CuO with N3 to form Cu (N3)2 (Fig. S15). Overall, the current study further expands on the re action mechanism of the liquid-solid azidation process. The preparation of nanoenergetics-on-a-chip is a key step for sub stantially advancing microscale energy-consumption devices [34]. To demonstrate the feasibility of applying the as-prepared materials to nanoenergetics-on-a-chip, a CA film was integrated with a Pt/W wire micro igniter (labelled the Pt/W/CA micro igniter). Schematic diagrams and optical images of the electro-thermal ignition devices are shown in Fig. 5 and Fig. S16, respectively. After a Cu layer, with an average thickness of 1 μm, is deposited onto the Pt/W wire micro igniter (labelled the Pt/W/Cu micro igniter), the Cu layer is transformed into a CA layer through this liquid-solid azidation method. The ignition pro cesses, which is triggered by applying with a voltage of 50 V and recorded with a high-speed camera, were investigated for the Pt/W/Cu micro igniter and Pt/W/CA micro igniter. The time interval between
(2) (3)
Fig. 4. (a) A schematic of the growth mechanism of the CA film. (b) The calculated energy diagram and corresponding structures for the liquid-solid azidation reaction on the (111) surface of CuO. 5
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Fig. 5. Diagrams and corresponding ignition processes of (a, b) Pt/W/Cu micro igniter and (c, d) Pt/W/CA micro igniter.
adjacent frames was set to 20 μs. It can be seen (Fig. 5b) that only a small flame forms from heating the Pt/W/Cu wire. In stark contrast, an extremely intense flame erupts from the Pt/W/CA micro igniter (Fig. 5d). The flame, which results from the violent reaction of CA, is induced by joule heating. The maximum flame height, which is nearly 7 mm, is approximately twice that of the Pt/W/Cu micro igniter flame. Moreover, the rapid release of gas (N2) further supports the ignition effect. Therefore, it is obvious that a significant improvement has been realized in the energy output of the Pt/W/CA micro igniter. Excitingly, the experimental data demonstrate both a great progress in the suc cessful ignition properties and the promise for applications in MEMS devices.
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4. Conclusion To summarize, a unique nest-like structured CA film has been suc cessfully designed and fabricated by means of a facile electro-assisted azidation method. The prepared CA film exhibits excellent energetic performance and superior electrostatic stability, successfully responding to the tremendous challenges of CA materials. Moreover, in comparison with other reported azidation methods, this liquid-solid azidation method can be performed safely and efficiently under aqueous condi tions. Finally, the positive results in our work meets the demands for nanoenergetics-on-a-chip. We believe that these results will be of great importance to fundamentally understand detonation or ignition at the nanoscale, which will influence many related applications. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant 51576101, 51772152), the Fundamental Research Funds for the Central Universities (Grant 30918015102) and Qing Lan Project. The authors acknowledge Prof. Xufei Zhu (Nanjing University of Science and Technology) and Prof. Wansen Hua (Nanjing University of Science and Technology) for the fruitful discussion about the growth mechanism of the CA film, Prof. Fengli Bei (Nanjing Uni versity of Science and Technology) for HRTEM analysis and advices about the manuscript, Prof. Kefeng Ma (Nanjing University of Science and Technology) and Dr. Jingwen Sun (Nanjing University of Science and Technology) for carefully reading and reviewing the manuscript, Dr. Debin Ni (Shanxi Applied Physics and Chemistry Research Institute) for providing Pt/W/Cu micro igniters. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2019.104135.
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Nano Energy 66 (2019) 104135 Chunpei Yu studied chemistry at Nanjing University of Science and Technology and received his B.S. degree in 2015. He is currently a Ph.D. student in the group of Dr. Wenchao Zhang and Prof. Junwu Zhu at Nanjing University of Science and Technology. His main research interests are focused on the formation and application of nanostructured energetic materials.
Zilong Zheng is currently a Ph.D. student at Nanjing University of Science and Technology. His research interests are focused on micro/nano energetic materials, 3D printing technology and MEMS devices.
Wenchao Zhang received his Ph.D. degree from Nanjing Uni versity of Science and Technology in 2011. He is presently working as associate professor in the same university. His research interests are focused on nano energetic materials and MEMS ignition device.
Shengli Zhang received his Ph.D. degree from Beijing Uni versity of Chemical Technology in 2013. He then joined the Key Laboratory of Advanced Display Materials and Devices, Nanjing University of Science and Technology, where he is a professor in the Department of Materials Science and Engineering. His research interests are focused on electronic, optoelectronic and energy devices based on 2D materials.
Shiying Guo is currently a Ph.D. student at MIIT Key Labora tory of Advanced Display Materials and Devices. She received her B.S. in Materials Science and Engineering from Nanjing University of Science and Technology in 2016. Her current research interests are concentrated on exploring the 2D mate rial application in optoelectronics and energy devices.
Jiahai Ye received his Ph.D. degree from Nanjing University. He is currently an associate professor of Chemical Engineering at Nanjing University of Science and Technology. His research is focused on the synthesis and characterization of organic fluorescence materials and energy materials.
Bin Hu is pursuing his M.S. under the supervision of Dr. Wen chao Zhang. His research interests are concentrated on the preparation, characterization, and reactivity of nano energetic materials.
Junwu Zhu received his Ph.D. degree in Materials Chemistry from Nanjing University of Science and Technology in 2005. He is presently working as professor in the same university. His main research interests are focused on the preparation and application of functional materials based on carbon and nano structured composites.
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