Introduction

Solid composite propellants are usually composed of binders, energetic fuel additives, oxidizers, and combustion catalysts. Its combustion is a complex transfer process of mass and energy (Kapoor et al. 2009; Singh et al. 2000; Lan et al. 2015). Cyclotrimethylene trinitramine (RDX) is a common oxidizer in solid composite propellants, and its thermal decomposition performance greatly influences the burning behavior of propellants: reducing the thermal decomposition temperature and the apparent activation energy, shortening the ignition delay time and increasing the combustion rate (Bu et al. 2018; Ebrahim et al. 2012; Zhang et al. 2015). It is a fact that the lower the decomposition temperature is, the higher the burning rate is (Li et al. 2013; Fitzgerald and Brewster 2004). The performance of solid composite propellants can be significantly improved by adding a small quantity of combustion catalysts in propellants, especially nano-sized catalysts due to their smaller size, larger specific surface area, and higher catalytic activity, including nano-sized metals, nano-sized metal oxides, and their composites (Armstrong et al. 2003; Yuan et al. 2014; IIhan et al. 2015).

Spinel ferrites are fascinating materials in the world of nano-regime with a general formula MFe2O4, where M represents a class of divalent metal cations such as Cu, Mg, and Co. Copper ferrite (CuFe2O4) is a kind of spinel ferrite materials with tetragonal structure or cubic structure depending on the preparation conditions (Rashad et al. 2012; Sultan and Singh 2009; Phuruangrat et al. 2016). In certain technological conditions, the tetragonal structure of CuFe2O4 could be converted into cubic phase, such as at temperature below 300 °C (Nedkov et al. 2006) or rapid cooling from high temperature (above 760 °C) (Kim et al. 2004). Cation distribution affects the properties of the spinel ferrite materials to some extent. The copper ferrite (CuFe2O4) as a magnetic p-type semiconductor with narrow band gap has attracted great attention because of its excellent magnetic properties, high electric conductivity, and high thermal stability and catalytic activity. It has been widely investigated and used in electrode materials (Yeh and Shen 2008), high density magnetic optic recording devices (Kim et al. 2004; Kurian and Mathew 2017), magnetic resonance imaging (Kurian and Mathew 2017), drug-loading materials (Arias et al. 2008), and catalytic uses (Rashad et al. 2012). In this work, the copper ferrite (CuFe2O4) with the cubic spinel structure was successfully prepared by a one-step solvothermal method.

Nanoparticles with high density of surface-active sites and excellent catalytic performance usually tend to agglomerate and thus reducing its catalytic activity. The self-assembled method is widely used in the preparation of carbon-based nanocomposite materials due to its simple and efficient properties (Tao et al. 2017; Thiruvengadathan et al. 2014; Zhang et al. 2016a, b). Graphene oxide (GO) is a derivative of graphene and was frequently used as substrate for composite materials with better performance in recent years (Zhu et al. 2010a, b). In this work, CuFe2O4/GO nanocomposite combustion catalyst was fabricated by a versatile self-assembly method. GO in the nanocomposite can not only inhibit the agglomeration of nanoparticles, but also accelerate the charge transportation and enhance the catalytic performance of the nanocomposite owing to its excellent properties (Chen et al. 2016; Li et al. 2012).

Experimental section

Reagents and materials

All the AR-chemical reagents were commercially available and used as received without further purification. Graphene oxide (GO) was prepared from natural graphite basing on a modified Hummer’s method (Marcano et al. 2010; Li et al. 2018). RDX was provided by the Xi’an Modern Chemistry Research Institute.

Materials synthesis

The CuFe2O4 nanoparticles were synthesized according to a one-pot solvothermal method previously reported by our group (Zhang et al. 2015). Briefly, 4 mmol of CuCl2·2H2O, 8 mmol of FeCl3·6H2O, 12 mmol of NaAc, and 1.0 g of PVP (the molar ratio of Cu:Fe is 1:2) were dissolved in 70 ml of ethylene glycol and stirred vigorously for 1 h. The mixture solution was sealed in a 100 mL Teflon-lined stainless steel autoclave, held at 180 °C for 12 h and allowed to cool to room temperature. Finally, the as-obtained black precipitate was centrifuged, washed with ethanol and deionized water several times, and dried at 60 °C in a vacuum oven for 24 h.

The CuFe2O4/GO nanocomposite was prepared by a versatile self-assembly approach (Thiruvengadathan et al. 2014). The detailed preparation processes were as follows: 30 mg of GO powder was dispersed in 30 mL deionized water and sonicated for 2 h to form a brown GO aqueous solution with a concentration of about 1 mg mL−1 for later use. 0.3 g of as-prepared CuFe2O4 nanoparticles was dispersed in deionized water with sonication for 1 h to be a homogeneously dispersed suspension, and the pH value of this suspension was adjusted to four using 0.1 mol L−1 HCl solution. Subsequently, the resulting GO aqueous solution was then added to the above suspension dropwise and the mixed solution was sonicated for another 2 h. Finally, the obtained CuFe2O4/GO nanocomposite was centrifuged, washed, and dried.

Materials characterization

The crystal structure, phase composition, and morphological and structural characterization of samples were determined by XRD, XPS, BET, SEM, and TEM. The powder X-ray diffraction (XRD) patterns in the 2θ range of 5°–80° were recorded on a Rigaku Mini Flex 600 X-ray diffractometer using Cu Kα radiation (λ = 0.1541 nm). X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Thermo Scientific ESCALAB 250 Xi X-ray photoelectron spectrometer. The Brunauer-Emmett-Teller (BET) specific surface area and pore volume of as-prepared samples were performed by an Autosorb-iQ automated gas sorption system. Scanning electron microscope (SEM) images were carried out using a Carl Zeiss SIGMA field emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were measured on a FEI Tecnai G2 microscope operated at 200 kV.

Catalytic activity measurements

RDX was selected as target materials for investigating catalytic activities of the as-prepared samples. The as-prepared samples and RDX were fully mixed in the mass ratio of 1∶4 by rubbing method. Thermal decomposition analysis of the resulting mixture was performed using a 200F3 differential scanning calorimeter (DSC) (Netzsch, Germany) at a heating rate of 10 °C min−1 in an N2 atmosphere at a flow rate of 40 mL min−1 under ambient atmosphere pressure.

Results and discussion

Materials characterization

The powder XRD diffraction patterns of the resulting GO, CuFe2O4, and CuFe2O4/GO nanocomposite are shown in Fig. 1. The characteristic diffraction peak of GO centered at 2θ = 12.5° can be assigned to the (001) reflection. The reflection peaks of as-prepared CuFe2O4 nanoparticles correspond well with the standard pattern of spinel phase of CuFe2O4 (JCPDS No. 77-0010), indicating the formation of pure spinel phase CuFe2O4. All characteristic diffraction peaks of CuFe2O4/GO nanocomposite are consistent with the pure CuFe2O4 nanoparticles. The characteristic diffraction peak of GO is not detected in the CuFe2O4/GO nanocomposite, which may be because the amount of GO is too small to detect by XRD (Noor-ul et al. 2016). The presence of GO in the nanocomposite is further demonstrated by SEM, TEM, and FT-IR that followed.

Fig. 1
figure 1

Powder XRD patterns of GO, CuFe2O4, and CuFe2O4/GO nanocomposite

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FT-IR spectra of the pure GO, the CuFe2O4 nanoparticles, and the CuFe2O4/GO nanocomposite are shown in Fig. 2. In the FT-IR spectrum of the pure GO, the characteristic peaks of oxygen-containing functional groups C=O, C-OH, and C-O-C are at 1725 cm−1, 1398 cm−1, and 1058 cm−1, respectively (Zhu et al. 2010a, b). The peak appeared at 1623 cm−1 is attributed to the skeletal vibration of aromatic domains (Meidanchi and Akhavan 2014). The broad peak located at 3422 cm−1 is assigned to the stretching vibration mode of O-H in adsorbed water molecules and the oxygen-containing functional groups on the surface of GO sheets. In the CuFe2O4 nanoparticles, the peak observed at about 438 cm−1 can be attributed to octahedral Cu2+ (Cu-O mode) stretching vibration, and the peak at about 620 cm−1 can be assigned to tetrahedral Fe3+ (Fe-O mode) stretching vibration (Phuruangrat et al. 2016). In the CuFe2O4/GO nanocomposite, it can be clearly observed that there exist the characteristic peaks of GO and CuFe2O4 nanoparticles. The results of both XRD and FT-IR indicate that the CuFe2O4/GO nanocomposite was successfully prepared via a versatile self-assembly approach.

Fig. 2
figure 2

FT-IR spectra of GO, CuFe2O4, and CuFe2O4/GO nanocomposite

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The morphology and the structure of the CuFe2O4 nanoparticles and the assembled CuFe2O4/GO nanocomposite were characterized by SEM (Fig. 3). From Fig. 3(a), it can be seen that the as-prepared CuFe2O4 nanoparticles have a well-defined spherical structure and a small size distribution with an average particle size of 200 nm, and exhibit rough surfaces, which is beneficial to enhancing the interaction between GO sheets and CuFe2O4 nanoparticles. In the CuFe2O4/GO nanocomposite, it can be clearly seen that the GO sheets are tightly wrapped on the surfaces of CuFe2O4 nanoparticles like a layer of thin gauze clothing, forming a three-dimensional (3D) interconnected network structure between particles. The self-assembly process did not destroy the composition and morphology of the spinel-structured CuFe2O4 particles. The most important point is that GO sheets and CuFe2O4 nanoparticles touch each other a lot, and little separation could be observed in the CuFe2O4/GO nanocomposite. This is beneficial to preventing CuFe2O4 nanoparticles from agglomeration and increasing the surface-active sites of CuFe2O4 nanoparticles, thereby improving the catalytic performance of the nanocomposite. The Brunauer-Emmett-Teller (BET) specific surface area and pore volume of the samples are shown in Table 1. The BET surface area and the total pore volume of CuFe2O4/GO nanocomposite are found to be 45.27 m2 g−1 and 0.106 cm3 g−1, respectively, which are higher than those of CuFe2O4. This is consistent with the analysis of SEM.

Fig. 3
figure 3

SEM images of CuFe2O4/GO nanocomposite

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Table 1 Textural properties of CuFe2O4 and CuFe2O4/GO nanocomposite
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The crystal structure of the CuFe2O4/GO nanocomposite and visualized effects of self-assembly were further investigated by TEM (Fig. 4). The same morphology and structure could be observed as SEM images. The TEM images [Fig. 4(a, b)] showed that the CuFe2O4 particles with an average size around 200 nm consist of lots of smaller primary nanocrystals. The middle of the CuFe2O4 particle is lighter relative to its edge, implying its hollow nano-spherical structure feature. The CuFe2O4 particles are wrapped tightly by transparent GO sheets, forming a three-dimensional (3D) network structures. It is clear that the transparent GO sheets with lots of wrinkles exist on the edge of CuFe2O4/GO nanocomposite. The unique structure feature of the CuFe2O4/GO nanocomposite could effectively prevent from combining of electrons and holes in the catalytic decomposition process of RDX, which provides an excellent pathway of charge transfer and accelerate the transfer of electrons, so the structure feature of CuFe2O4/GO nanocomposite is beneficial to accelerating the catalysis process and enhancing its catalytic performance (Li et al. 2013; Zu et al. 2016). In the HRTEM images [Fig. 4(c, d)], the interplanar spacings of 0.251 and 0.210 nm correspond to the lattice fringe spacings of the (311) and (400) diffraction planes for the spinel CuFe2O4 (JCPDS No. 77-0010), respectively, which suggests that CuFe2O4 particles in the CuFe2O4/GO nanocomposite have a well-defined crystallinity. The TEM and HRTEM results further confirm that the CuFe2O4/GO nanocomposite was prepared by a versatile self-assembly approach.

Fig. 4
figure 4

TEM and HRTEM images of CuFe2O4/GO nanocomposite

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To reveal the surface elementary composition and the chemical states of elements of the CuFe2O4/GO nanocomposite, X-ray photoelectron spectroscopy (XPS) measurement was taken. The survey spectrum, the copper (Cu 2p), iron (Fe 2p), oxygen (O 1 s), and carbon (C 1 s) spectra were analyzed (Fig. 5). In the survey spectrum, it can be seen that there are four elements Cu, Fe, O, and C in the CuFe2O4/GO nanocomposite [Fig. 5(a)]. In Fig. 5(b), the two binding energy peaks at 933.2 eV for Cu 2p3/2 and 953.5 eV for Cu 2p1/2 with a shake-up satellite peak at about 950 eV indicate the presence of the Cu(II) oxidation state in the composite (Nedkov et al. 2006; Tang et al. 2016). For Fe 2p spectrum [Fig. 5(c)], two prominent peaks with binding energy value of 710.9 ev and 724.6 eV can be assigned to the spectra of Fe 2p3/2 and Fe 2p1/2, respectively, indicating the Fe (III) oxidation state of CuFe2O4 in the CuFe2O4/GO nanocomposite (Zu et al. 2016; Shu et al. 2018). The high-resolution O 1s spectrum in the nanocomposite can be divided into three main peaks [Fig. 5(d)]. The peak centered at 530.6 eV is assigned to the lattice oxygen in the crystal lattice of CuFe2O4 particle (Zhou et al. 2015). The peaks located at 532.3 eV and 533.6 eV are attributed to the C-O and C-O-C, respectively, in GO (Krishnamoorthy et al. 2013; Ding et al. 2018). The high-resolution C 1s spectrum of CuFe2O4/GO nanocomposite shown in Fig. 5(e) consists of three main components: C-C/C=C (284.8 eV) in the aromatic rings, C-O (286.6 eV) of epoxy and alkoxy, and C=O (289.3 eV) groups (Zhang et al. 2016a, b). The above results further confirm that GO and CuFe2O4 nanoparticles were successfully assembled together, and the CuFe2O4/GO nanocomposite formed.

Fig. 5
figure 5

X-ray Photoelectron spectroscopy of CuFe2O4/GO nanocomposite: (a) survey spectra, (b) Cu 2p, (c) Fe 2p, (d) O 1 s, and (e) C 1 s

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The influence on the thermal decomposition of RDX

Cyclotrimethylene trinitramine (RDX) as a frequently used high-energy explosive is one of the major ingredients in composite solid propellants (Zhang et al.2015), and its thermal decomposition properties directly influence the combustion behavior of the propellants. In order to further investigate the catalytic decomposition performance of the CuFe2O4/GO nanocomposite catalyst for RDX, the as-prepared nanomaterials were explored as a potential burning additive of the thermal decomposition of RDX. The catalytic performance of GO, pure CuFe2O4, and self-assembled CuFe2O4/GO nanocomposite was investigated by DSC analysis at a heating rate of 10 °C min−1 from room temperature to 300 °C in an N2 atmosphere at a flow rate of 40 ml min−1 under ambient atmosphere pressure. The apparent activation energy of the catalytic decomposition process was calculated by Kissinger method and Ozawa method with DSC data at different heating rates (5.0, 10.0, 15.0, and 20.0 °C min−1) (Kissinger 1957; Ozawa 1965). The samples for DSC analysis were prepared as follows: the as-synthesized GO, CuFe2O4, and CuFe2O4/GO nanocomposite were fully mixed with RDX in the mass ratio of 1∶4, respectively, and ground gently in an agate mortar to the good uniformity; sample mass, approximately 0.5 mg.

The DSC curves of pure RDX and the mixtures of as-obtained powders with RDX are shown in Fig. 6. The exothermic peak temperatures of the decomposition processes are used to evaluate the catalytic effect of pure GO, CuFe2O4, and self-assembled CuFe2O4/GO nanocomposite on RDX. The peak temperatures of RDX, RDX+GO, RDX+CuFe2O4, and RDX+CuFe2O4/GO nanocomposite are 241.27, 225.77, 221.99, and 220.34 °C, respectively. Compared with the pure RDX, the peak temperatures of RDX+GO, RDX+CuFe2O4, and RDX+CuFe2O4/GO nanocomposite reduce by 15.50, 19.28, and 20.93 °C, respectively, indicating that pure GO, CuFe2O4, and self-assembled CuFe2O4/GO nanocomposite have catalytic effect on the thermal decomposition of RDX, and the CuFe2O4/GO nanocomposite has the best catalytic effect among them. An exothermic peak located at 205 °C is attributed to the melting of RDX, with no significant change in all DSC curves. In addition, the apparent activation energy of the catalytic thermal decomposition for CuFe2O4/GO nanocomposite calculated by Kissinger method and Ozawa method is 142.56 and 143.32 kJ mol−1, respectively, which is approximately equal. Compared with the value of pure RDX (172.6 kJ mol−1) reported in the previous literature (Zhang et al. 2015), it is significantly reduced by about 30 kJ mol−1. The above results indicate that the as-prepared self-assembled CuFe2O4/GO nanocomposite has excellent catalytic performance, which can be attributed to the excellent conductivity of GO in the CuFe2O4/GO nanocomposite and the “positive synergistic effect” between GO sheets and CuFe2O4 nanoparticles in the CuFe2O4/GO nanocomposite.

Fig. 6
figure 6

DSC curves of pure RDX and RDX mixed with GO, CuFe2O4, and CuFe2O4/GO

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Figure 7 shows the DSC curves of RDX mixed with CuFe2O4/GO nanocomposite in different mass ratios at a heating rate of 10 °C. The exothermic peak temperatures of catalytic thermal decomposition for CuFe2O4/GO + RDX (1:3), CuFe2O4/GO + RDX (1:4), and CuFe2O4/GO + RDX (1:5) are 220.26, 220.34, and 223.55 °C, respectively, all decreasing by 21.01, 20.93, and 17.72 °C compared with the pure RDX (241.27 °C). Among them, the decomposition peak temperatures of the CuFe2O4/GO + RDX (1:3) and CuFe2O4/GO + RDX (1:4) systems are almost equal. Therefore, we use RDX + CuFe2O4/GO (1:4) as the best mixing ratio in this work.

Fig. 7
figure 7

DSC curves of RDX mixed with CuFe2O4/GO in different mass ratios

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Conclusion

In summary, we successfully prepared CuFe2O4/GO nanocomposite by a versatile self-assembly approach and studied its catalytic decomposition performance for RDX. CuFe2O4 nanoparticles modified by H+ ions were tightly coated by GO sheets, which could effectively prevent CuFe2O4 nanoparticles from aggregating. The self-assembly process did not destroy the composition and morphology of the spinel-structured CuFe2O4 particles. The excellent conductivity of GO in the CuFe2O4/GO nanocomposite could also accelerate the transfer of electrons during the catalytic thermal decomposition process of RDX, which further enhances its catalytic performance. The DSC results show that the self-assembled CuFe2O4/GO nanocomposite can significantly reduce the exothermic peak temperature and the apparent activation energy of RDX from 241.27 to 220.34 °C and from 172.6 to 142.56 kJ mol−1, respectively. The CuFe2O4/GO nanocomposite exhibits excellent catalytic decomposition activity and good application performance in composite solid propellant, and the catalytic effect of CuFe2O4/GO nanocomposite on the thermal decomposition of RDX is higher than those of single-CuFe2O4 nanoparticles and GO sheets. In addition, this effective, scalable, and environmentally benign route for preparing CuFe2O4/GO nanocomposite can also be expanded to prepare other GO-based functional nanomaterials used as catalysts, superior anode materials for lithium-ion batteries, and other fields.