1 Introduction

HMX is one of the most vigorous explosive materials in terms of heat output and gaseous products [1, 2]. HMX can offer large volume of gaseous products at low molecular weight [3,4,5,6]. Therefore, HMX has found wide applications in solid rocket propellant [7,8,9]. It was reported that HMX is insensitive to traditional catalysts [10]. The main approach that could affect thermolysis of HMX includes hydrogen atom abstraction with heterocyclic ring cleavage [11, 12].

Transition metal oxides were reported to have catalytic influence on HMX thermolysis with decrease in its onset decomposition temperature [7, 13]. Nanopowders, with increased surface areas, are promising materials for catalytic applications of different energetic systems [14]. High catalytic effect on HMX was reported through TiO2 NPs. HMX catalyzation includes a decrease in onset decomposition temperature, high reaction rate, and decrease in pressure exponent value [15,16,17]. These effects could be achieved using TiO2 NPs. Enhanced catalytic performance could be accomplished with particle size decrease [18,19,20].

Some metal oxide nanocomposites were prepared by different green methods which are used for the catalytic performance and other critical applications. Sol–gel, sol–gel-hydrothermal, and photo-deposition methods were used to synthesize pure TiO2, PdO/TiO2, and Pd/TiO2 nanostructures. Citric acid was used as a stabilizer, reducing, and capping agent, since it is green, available, clean, and nontoxic [21].

A study conducted by Safajou et al. [22] shows that TiO2 nanowire (NWs) was prepared by an alkaline hydrothermal process and the following formula Graphene/Pd/TiO2 NPs and Graphene/Pd/TiO2-NWs were synthesized by a combination of hydrothermal and photo-deposition methods. The synthesized nanocomposites were investigated for their enhanced photocatalytic degradation of dyes.

Also, photo-degradation of methylene blue was investigated by the utilization of the mixed metal oxides such as Fe2O3–TiO2 NPs, TiO2@SiO2 core/shell NPs and N-doped graphene quantum dot/TiO2 nanocomposite [23,24,25]. Finally, the photocatalytic degradation of azo dyes using TiO2 NPs supported Ag NPs which prepared by a green method was investigated by Rostami-Vartooni et al. [26].

Different chemical methods such as successive ion layer adsorption and reaction, chemical bath deposition, microwave, and hydrothermal served to deposition of CdS on the prepared TiO2 surface for use in different optoelectronic fields. TiO2/CdS nanocomposite was synthesized by hydrothermal method and was then deposited on the FTO surface to investigate their influences on the dye-sensitized solar cell performance [27]. Also, TiO2 NPs were prepared using tripodal tetra-amine ligands (complexing agent) by two-step sol–gel method for the application in dye-sensitized solar cells [28]. The effect of the ligand on the synthesis of different metal oxide NPs must be taken into consideration, and the size and optical properties of TiO2 NPs in a two-step sol–gel method was altered after the use of Schiff base ligands [29]. Also, the effect of tertiary amines on the synthesis and photovoltaic properties of TiO2 NPs in dye-sensitized solar cells was investigated [30]. Finally, a stable plasmonic-improved dye-sensitized solar cells was achieved by Ag NPs between TiO2 Layers [31].

It must be noted that, the as-fabricated mesoporous TiO2 fibers exhibit much higher photocatalytic activity and stability than both the conventional solid counterparts and the commercially-available P25. The abundant vapors released from the introduced foaming agents are responsible for the creation of pores with uniform spatial distribution in the spun precursor fibers [32]. In another study regarding nanomaterials-based TiO2 NPs, a novel and highly efficient visible-light-driven photocatalyst with robust stability made up of thoroughly mesoporous TiO2/WO3/g-C3N4 ternary hybrid nanofibers and TiO2/CuO/Cu had been fabricated through a foaming-assisted electro-spinning process followed by a solution-dipping process [33, 34]. Finally, a brilliant BiVO4@TiO2 core–shell hybrid mesoporous nanofiber was used for efficient visible-light-driven photocatalytic hydrogen production [35].

1.1 TiO2 catalyzation mechanism

It is widely established that nitramine decomposition can be catalyzed with ȮH radicals. TiO2 NPs are characterized with hydrous surface (surface-bound hydroxyl groups). The release of surface ȮH radicals can speed up HMX decomposition [36]. The required activation energy to liberate ȮH radicals from TiO2 surface is 65 kJ/mol; this value could decrease with the increase in the particle surface area [16]. Furthermore, NPs surface could absorb gaseous products offering high heat output [20]. TiO2 NPs could lower the required activation energy for HMX decomposition. Whereas HMX normal decomposition process include C-N bond breakage of heterocyclic ring, decomposition of catalyzed HMX with nanocatalyst includes dehydration of catalyst surface with the release of active ȮH radicals; these radicals would abstract H-atom from the heterocyclic ring [37].

Therefore, the catalytic decomposition process could take place at lower temperature and with lower activation energy [38]. The strength of surface-bounded –OH groups is a key parameter for the catalytic activity of oxides. Electronegativity of metal cation xi expresses the capability to withdraw electron pair (Eq. 1).

$$X_{i} = \, X_{o} \cdot \, \left( {1 \, - \, 2n} \right)$$
(1)

where xo and n are electronegativity of metal atom and the metal charge in the oxide state, respectively.

Metal oxide with high Xi have acid properties, whereas oxides with low Xi have base properties. Oxide point of zero charge (isoelectric point) describes the surface acidity; it is equal to the medium acidity in which oxide surface has no electric charge.

TiO2 NPs were verified to have superior efficiency compared with other oxides as well as microsize TiO2 [39, 40]. Reliable fabrication of nanoscopic TiO2 is an urgent demand. There is a vast benefit for synthesis technology that could offer fabrication of TiO2 NPs with constant product quality. Hydrothermal processing offered consistent fabrication of different oxide particles in dispersion [41].

1.2 Hydrothermal processing

Hydrothermal processing was reported to be a beneficial technology that could offer fabrication of highly crystalline oxides in dispersion [42, 43]. This technology includes direct mixing of metal salt feed with supercritical fluid (ScF),ScF can expose distinctive characteristics in terms of enhanced levels of OH− [18, 44,45,46,47,48]. Above critical conditions, phase boundary vanishes and a homogenous supercritical phase exists as displayed in Fig. 1 [49, 50].

Fig. 1
figure 1

Phase boundary of ScF with temperature and pressure [51]

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Oxide fabrication can be achieved via hydrolysis of metal salt with subsequent dehydration step (Eqs. 2 and 3) [52,53,54].

$${\text{Hydrolysis}}: {\text{ML}}_{x} + \, x{\text{OH}}^{ - } \to {\text{M}}\left( {{\text{OH}}} \right)_{x} + \, x \, L^{ - }$$
(2)
$${\text{Dehydration}}: {\text{M}}\left( {{\text{OH}}} \right)_{x} \to {\text{MO}}_{x/2} + \frac{x}{2} {\text{H}}_{2} {\text{O}}$$
(3)

Accordingly, this study reports on the consistent fabrication of TiO2 NPs of 5 nm particle size using hydrothermal synthesis. TiO2 NPs were developed in dispersion; consequently, colloidal TiO2 NPs were integrated into HMX particles. Uniform dispersion of TiO2 NPs into HMX was verified using SEM/mapping technique. The effectiveness of TiO2 NPs on HMX thermal decomposition was investigated using DSC and TGA. TiO2 NPs demonstrated superior catalytic efficiency. The endothermic phase change at 187 °C was decreased by 43.3%. The exothermic decomposition temperature was decreased by 10 °C with an increase in total heat release by 46.7%. This superior catalytic performance was accomplished at 1 wt % catalyst. TiO2 NPs catalyzing mechanism was correlated to the release of active surface ȮH radicals that could attack the heterocyclic ring with hydrogen atom abstraction with heterocyclic ring cleavage.

2 Experimental work

2.1 Hydrothermal synthesis of TiO2 NPs

The employed metal salt for TiO2 NP synthesis was titanium (IV) bis (ammonium lactato) dihydroxide ([CH3CH(O–)CO2NH4]2Ti(OH)2) (TIBALD) 50 wt% in H2O solution (CAS number 65104-06-5, Aldrich, Germany). ScW was employed at 400 °C, 240 bars (20 ml/min) (Flow A). 0.05 M solution of TIBALD in deionized water was employed at 25 °C, 240 bars (10 ml/min) (Flow B). TiO2 NPs were developed at the boundary of the flow (Fig. 2). Further details regarding the hydrothermal synthesis of TiO2 NPs can be observed in the following references [55,56,57,58].

Fig. 2
figure 2

Schematic for TiO2 NPs synthesis inside counter-current reactor

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2.2 Characterization of TiO2 NPs

Crystallinity and phase were investigated using X-ray diffraction (XRD) spectroscopy on a Brucker axis D8 diffractometer applying radiation of Cu Kα with (λ = 1.540598 Å), voltage of 40 kV, and current of 40 mA. The average nanostructure and the particle size determination of the synthesized TiO2 NPs were determined by applying a High-Resolution Transmission Electron Microscope (HRTEM, JEM2100, Jeol, Japan). The surface morphology and a specific appearance of the dry particles (pure TiO2 NPs) were examined with Scanning Electron Microscope (SEM, ZEISS, EVO-MA10, Germany). On the other hand, EDX technique (BRUKER, Nano GmbH, D-12489, 410-M, Germany) was applied to investigate the elemental configuration and the atomic percentage of the metals detected in the prepared samples. FTIR spectrometer Nicolet 380 by Thermo-electron Corporation was employed to investigate the nanoparticle chemical structure and their functional groups. Brunauer–Emmett–Teller (BET) method was used to describe the surface area and the measurements were carried out via the surface area analyzer (Nova 3200 Nitrogen Physisorption Apparatus USA) with liquid N2 as an adsorbate at − 196 °C. Finally, the mapping analysis after applied SEM/EDX technique was used to attain whole information about the clarity, distribution, and the position of the metals (pure TiO2 NPs) on the surface of HMX.

2.3 Integration of TiO2 NPs into HMX

All classical NP synthesis techniques include sintering and drying process which result in a dramatic decrease in NP surface area and reactivity. This is the first time ever to report on fabrication of colloidal TiO2 NPs and their integration into HMX crystalline structure. This approach could offer extensive surface area and reactivity; it could eliminate NP drying and the re-dispersion of dry aggregates.

TiO2 NPs were decanted from their synthesis medium and re-dispersed in acetone using ultrasonic probe homogenizer. HMX was dissolved in acetone colloid. The ratio of TiO2 NPs: HMX was 1: 99. TiO2 NPs were integrated into HMX using co-precipitation technique. The size and shape of TiO2/HMX hybrid was investigated using SEM/EDX mapping technique for giving further information regarding the simplicity, relationships, and the position of the TiO2 NPs incorporated with HMX.

2.4 Thermal behavior of catalyzed HMX

Thermal behavior of HMX catalyzed with TiO2 NPs was investigated using DSC Q20 by TA. Tested sample was heated from 50 to 500 °C. The heating rate was 5 °C/min, under N2 flow of 50 ml/min. The impact of TiO2 NPs on HMX weight loss was evaluated using TGA 55 by TA. The tested sample was heated from 50 to 500 °C. The heating rate was 5 °C/min under N2 flow at 25 ml/min.

3 Result and discussions

3.1 Characterization of the synthesized TiO2 NPs

TEM micrographs of the synthesized TiO2 NPs demonstrated mono-dispersed particles with uniform particle size and the particle size was found to be ranging from 3.0 to 10.0 nm with an average particle size recorded at 5.0 nm (Fig. 3a). HRTEM images provided a detailed investigation of structure, shape, and size of TiO2 NPs which demonstrated a high crystalline structure with spherical and cubic structures (Fig. 3b, c). This result was matched with the results described in previous publications [59,60,61].

Fig. 3
figure 3

TEM micrographs of TiO2 NPs a at 100 nm resolution, b at 20 nm, and c HRTEM at 5 nm

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Figure 4 confirmed high-quality mono-dispersed particles and high crystalline structure. The crystalline structure was investigated with X-ray diffraction (XRD). XRD pattern confirmed high-quality anatase crystalline structure (Fig. 4); this is the most common crystalline structure in catalyst applications. For data analysis in Fig. 4, sharp, strong, and intense peaks are observed in 2Ɵ = 25.1° (101), 28.1° (110), 37.5° (004), 48.9° (200), 54.4° (105), 55.6° (211), and 62.8° (204), while the main peak is located at 2Ɵ = 25.4°, these peaks are in a good matching with those of reference anatase TiO2 NPs (JCPDS 04-0477) [62]. This result was matched with the results described in previous publications [63,64,65,66]. The average crystallite size was calculated using the Debye–Scherrer Eq. (4) [67] and was found to be 10.12 nm:

Fig.4
figure 4

XRD diffractogram of the synthesized TiO2 NPs

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$$D=\frac{K\uplambda }{{\beta \cos}\theta }$$
(4)

where K = 0.9 and known as shape factor, λ is x-rays’ wavelength (1.54060 Ǻ for Cu-Kα), β is full width at half maximum (FWHM), and θ is the diffraction angle.

SEM image of the fabricated TiO2 NPs is presented in Fig. 5 a; the synthesized TiO2 layer appears as a uniform and bright layer, also, the corresponding EDX analysis (Fig. 5b) was similar in words of diffusion (Ti, O, and C atoms) over the grain lines. Also, the carbon atoms were due to the holder which is used in the imaging process [68]. This result was matched with the results described in previous publications [69,70,71,72].

Fig. 5
figure 5

SEM images of the synthesized TiO2 NPs (a) and the corresponding EDX elemental analysis (b)

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FTIR spectrum was a significant study that provides important data about the chemical functional groups represented in TiO2 NPs [73]. FTIR spectrum of developed TiO2 NPs confirmed the hydrous surface. The enhanced levels of IR absorption at 3500 cm−1 can be correlated to the O–H surface group stretch as shown in Fig. 6. The antatase Titania appears at the region from 800 to 400 cm−1 [74]. This result was matched with the results described in previous publications [75,76,77].

Fig. 6
figure 6

FTIR spectrum of the synthesized TiO2 NPs

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The electrical and chemical properties are dependent on the specific surface area and grain size, as the chemical and physical phenomena controlled by surface porosity and electrons conduction occur at TiO2 NP’s surface [78]. N2 adsorption–desorption isotherm of the prepared TiO2 NPs is shown in Fig. 7.

Fig. 7
figure 7

N2 Adsorption–desorption isotherm of the prepared TiO2 NPs

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According to the IUPAC classification, the obtained isotherm was of type (IV), indicating the presence of mesopores. The uptake of adsorbate was increased when pores became filled, and an inflection point occurred near the completion of the first monolayer [62, 77]. From Fig. 7, the calculated surface area of the prepared TiO2 NPs was 26.87 ± 0. 36 m2/g, a similar behavior was detected in the literature and matched our BET result [77, 79,80,81].

Morphology of TiO2-HMX nanocomposite was investigated with SEM, to verify the uniform integration of TiO2 NPs into HMX crystalline structure [82], while EDX examination was performed for its elemental analysis and purity estimation [83,84,85].

Dry agglomerates include drastic decrease in surface area and reactivity; therefore, the particles would act as micron rather than NPs [44, 54]. Consequently, integration of colloidal particles into HMX could maintain high surface area and reactivity.

Elemental mapping using SEM revealed uniform dispersion of TiO2 NPs into HMX as shown in Fig. 8. Co-precipitation technique offered uniform dispersion of TiO2 NPs into HMX. This approach could offer superior interfacial surface area (the calculated surface area of the prepared TiO2 NPs was 26.87 ± 0. 36 m2/g; Fig. 7) and catalytic performance. This result was matched with the results described in previous publications [66, 86,87,88,89].

Fig. 8
figure 8

Elemental mapping of TiO2 NPs integrated into HMX

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3.2 Catalytic activity of TiO2 NPs

TiO2 NPs demonstrated dramatic change in HMX thermal behavior. The endothermic phase change of HMX at 187 °C was decreased by 43.3%. The main outcome of this study is that temperature at maximum heat release rate was decreased by 10 °C with an increase in total heat release rate by 46.7% as exhibited in Fig. 9.

Fig. 9
figure 9

DSC thermogram of HMX catalyzed with TiO2 NPs to pure HMX

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The catalytic activity of TiO2 NPs was further evaluated with TGA. TGA thermogram confirmed DSC outcomes; temperature at total weight loss was decreased by 10 °C as displayed in Fig. 10a and b.

Fig. 10
figure 10

TGA thermogram of HMX (a) and HMX catalyzed with TiO2 NPs (b)

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At temperature higher than 150 °C, ȮH radicals would be evolved from TiO2 NPs surface. These active radicals will have high ability to abstract hydrogen from HMX structure [90]. After hydrogen abstraction, energy of N-NO2 bond would decrease significantly; this could lead to release of nitro group (NO2) [91]. The evolved NO2 group could abstract another H-atom from another HMX molecule. Adsorption of NO2 on the surface of TiO2 could increase the heat release in condensed phase as shown in Fig. 11 [7].

Fig. 11
figure 11

Catalytic mechanism of TiO2 NPs on HMX

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Whereas CH2O and N2O fragment will be evolved at low heating rate; HCN and NO2 will be evolved at high heating rate. Moreover, CH2O could be due to ȮH interaction with double bond. The main TiO2 NPs catalytic steps include de-hydroxylation of the metal oxide surface with the release of active ȮH radicals, nitramine decomposition through hydrogen abstraction with ȮH radicals, and adsorption of liberated NO2 on the surface of TiO2 NPs. At high decomposition temperature, the reaction of CH2O and NO2 would provide the main exothermic reaction.

4 Conclusion

Hydrothermal processing was reported to be a beneficial technology that could offer fabrication of highly-crystalline TiO2 NPs in dispersion. The particle size of the synthesized TiO2 NPs was found to be ranging from 3.0 to 10.0 nm with an average particle size recorded at 5.0 nm, and the calculated surface area of the prepared TiO2 NPs was found to be 26.87 ± 0.36 m2/g. The effective coating of TiO2 with HMX was conducted via co-precipitation technique. The synthesized TiO2 NPs demonstrated superior catalytic activity on HMX thermolysis. TiO2 NPs demonstrated dramatic change in HMX thermal behavior. The endothermic phase change of HMX at 187 °C was decreased by 43.3%. The main outcome of this study is that temperature at maximum heat release rate was decreased by 10 °C with an increase in total heat release rate by 46.7%. At temperature higher than 150 °C, ȮH radicals would be evolved from TiO2 NPs surface. These active radicals will have high ability to abstract hydrogen from HMX structure. TiO2 NPs catalytic mechanism includes the following: (1) Release of ȮH radicals initiating destruction of HMX molecule and (2) Adsorption of released NO2 to the NPs surface. Therefore, the total heat release would increase significantly. Integration of colloidal TiO2 NPs into HMX would secure high reactivity.