https://doi.org/10.1007/s41664-021-00184-x ORIGINAL PAPER
Study on the Photolysis Route of Nano 2,2ʹ,4,4ʹ,6,6ʹ–Hexanitrostillbene by Vibrational Spectroscopy
Huan Cheng1,2 · Shi‑Wei Yang2 · Dong‑Mei Wang2 · Bing Huang1 · Mei‑Kun Fan2 · Li‑Yuan Zhang1 · Tao Xu1 · Guang‑Cheng Yang1
Received: 25 January 2021 / Accepted: 2 April 2021 / Published online: 28 May 2021
© The Nonferrous Metals Society of China 2021
Abstract
The understanding of the photolysis process of 2,2ʹ,4,4ʹ,6,6ʹ–hexanitrostillbene (HNS, an insensitive high-energy explosive) is very important not only for enhancing the detonation performance but also for its lifetime prediction. In this work, UV–Vis light-induced photolysis of nano HNS was studied by different spectroscopic methods. Nano HNS was found to be sensitive to UV–Vis lights at 365 and 470 nm. The photolysis route of nano HNS was mostly the same as its bulk counterpart, which was likely to be the combination of the isomerization of –NO2 to –ONO and the breaking of the C–N bond in Ar–NO2 (Ar = Aro- matic ring). In addition, the possible mechanism of UV–Vis-induced visible color change was explored for the first time.
Keywords Photolysis · Raman · FTIR · Nano 2,2’,4,4’,6,6’-hexanitrostillbene · Color change
1 Introduction
Photolysis, unlike photocatalytic degradation, refers to the phenomenon that light energy is directly or indirectly trans- ferred to chemical bonds under the action of light. The mol- ecules are then transformed into excited states and split or transformed [1–6]. In recent years, the photolysis of organic pollutants has attracted widespread attention since it could help pollution control [5, 7–9]. For energetic materials, the exploration of the decomposition process is very impor- tant not only for enhancing the detonation performance but also for the lifetime prediction [10, 11]. High-energy materials (HEMs) generally have different decomposition pathways, which were related to different initiation sources,
such as light, heat, and spark [12, 13]. High-energy explo- sives (HEEs), a kind of special organic HEMs, have obvi- ous decomposition under UV and visible irradiation [14].
Past practices have demonstrated the photolysis mechanisms of HEEs via many techniques, such as time-of-flight mass spectrometry (TFMS) [15–17], electron paramagnetic reso- nance (EPR) spectroscopy [18, 19], time-resolved infrared and steady-state FTIR spectroscopy [12], and femtosecond laser pumping-probe techniques [20]. Among them, the Raman technique has also been widely utilized to study the photolysis of HEMs [21–24].
2,2ʹ,4,4ʹ,6,6ʹ–Hexanitrostillbene (HNS, C14H6N6O12, Fig. 1), an insensitive HEEs, is widely used in soft explo- sives, boosters, and heat-resistant explosives [25–28]. HNS is also considered as a typical photosensitive explosive [29].
So far, there have been preliminary reports on the photolysis mechanism and photolysis products of bulk HNS (powder).
The possible photodecomposition mechanism of HNS is the breaking of the C–N in Ar–NO2 (Ar = Aromatic ring) and the removal of NO after the isomerization of –NO2 to –ONO [29, 30]. Recently, nano HEEs such as nano HNS has gained popularity due to its extraordinary performance compared with its macro counterpart. For example, nano HNS has higher initiation sensitivity for a narrow pulse in applications [31, 32]. Since nano HNS has greater specific surface area, the potential reactivity of HNS is stronger (such as response to light irradiation). Therefore, it is important to determine
Huan Cheng and Shi-Wei Yang contributed equally to this work.
* Bing Huang
huangtingjun-111@163.com
* Mei-Kun Fan meikunfan@gmail.com
* Guang-Cheng Yang ygcheng@caep.cn
1 Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621999, China
2 Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 610031, China
whether the nano HNS follows the same photolysis pathway as the macro (powder form) HNS. Besides, the visible color change of nano HNS accompanying the photolysis process remains to be explored.
2 Materials and Methods
2.1 MaterialsNano HNS, with an average particle size of ~ 250 nm, is pro- vided by China Academy of Engineering Physics. Potassium bromide (KBr, 99.9%) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China).
2.2 Instruments
The UV–Vis light sources were purchased from Beijing Zhu- oli Hanguang Instrument Co., Ltd. (Beijing, China). UV–Vis (diffuse reflectance) absorption spectra of nano HNS before and after irradiation were performed with UV spectrometer (Maya2000PRO, Ocean Optics, USA). Raman spectra of nano HNS before and after irradiation were recorded by a customized Raman microscope equipped with Pixis-100BR CCD (Princeton Instrument, US) and Acton SP-2500i spec- trograph (Princeton Instrument, US). An excitation wave- length (633 nm) and a 50 × objective (N.A. = 0.60) were used. FTIR (Perkin Elmer, USA) spectra of nano HNS before and after irradiation were obtained by tableting method at room temperature, and the spectra were collected from 4000 to 400 cm−1 with a resolution of 4 cm−1. The EPR spectrom- eter (A300, Germany) was utilized to capture the free radi- cals during the photolysis process of nano HNS. Automatic specific surface area and aperture analyzer was utilized to acquire the specific surface area of nano and macro HNS.
2.3 Procedures
Firstly, the emision profiles of light sources with different wavelengths (290, 310, 365, 470, 560, and 660 nm) were scanned. Subsequently, to exclude the influence of the
Raman laser (633 nm, 12 mW) on the photolysis of nano HNS, the nano HNS was irradiated by laser light source, and the Raman spectrum of the nano HNS was collected at regu- lar intervals. Finally, the nano HNS was irradiated by light sources with different wavelengths (290, 310, 365, 470, 560, and 660 nm). For UV, FTIR, and Raman analysis, the spectra of the nano HNS before and after irradiation for different time under different incident wavelengths of UV–Vis lights were collected, andthe irradiation was conducted at room temperature in a dark environment. The EPR spectrometer (A300, Germany) was utilized to capture the free radicals during the photolysis process.
2.4 Theoretical Calculation
All quantum chemical calculations in this work were per- formed by Gaussian09 program [33], and the B3LYP density functional method with the basis set 6–311+G(d,p) was used for the simulation of the Raman spectrum of HNS.
3 Results and Discussion
3.1 Nano HNS Sensitivity to Different Light Sources Figure S1a showed the emision profiles of light sources with different wavelengths (290, 310, 365, 470, 560, and 660 nm).
Note that as shown in Fig. S1B, the laser source of Raman system (632.8 nm) has no effect on nano HNS. It is clear in Fig. 2a and Fig. S2 that nano HNS is more sensitive to the light located at 365 and 470 nm with no obvious Raman spec- tral changes for other wavelengths even after 8 h irradiation.
According to the UV–Vis spectrum of nano HNS (Fig. 2b) and the previous studies about HNS decomposition [29, 30], we suspect that the photolysis of HNS is not only related to the wavelength but also to the power of the beam. Table S1 showed the power of light sources with different wavelengths.
It is clear that for 290 and 310 nm, the power density of the light source is less than 1 mW. Thus, though the dif- fuse reflection spectrum of the nano HNS showed absorp- tion in these regions, almost no Raman spectra change was observed. Hence, 365 and 470 nm were chosen to explore the photolysis mechanism of nano HNS in this work.
3.2 Photolysis Mechanism I of Nano HNS–the Removal of NO After the Isomerization of –NO2 to –ONO
Harmonic vibrational wavenumbers have been calculated, as shown in Fig. 3a. The theoretical spectrum and the experi- mental spectrum of HNS are well-matched in 900–1400 cm−1, while there are some deviations (50–60 cm−1) above 1400 cm−1. The peaks at 1368 and 1560 cm−1 correspond to the symmetric
Fig. 1 The molecular structure of HNS
stretching vibration and asymmetric stretching vibration of -NO2, respectively. Figure 3b–e showed the Raman and FTIR spectra of nano HNS before and after irradiation with 365 and 470 nm, respectively. As shown in Fig. 3b and c, the nitro group of nano HNS has changed at 1368 cm−1 and 1560 cm−1 in the Raman spectra. A new peak attributed to C–O appeared at 1436 cm−1 in the Raman spectra, which was obtained from the theoretical spectrum of the photolysis product of HNS in Fig. 3a. Meanwhile, a new peak appeared at 1120 cm−1 (the stretching vibration of C–O) in the FTIR spectra (Fig. 4d and e). These results indicated that one of the photolysis mechanisms of nano HNS may be the removal of –NO– after the isomerization of –NO2 to –ONO (Pho- tolysis mechanism I in Fig. 5) [34].
3.3 Photolysis Mechanism II of Nano HNS–the Breaking of the C–N Bond
Figure 4a and b were the EPR spectra of nano HNS before and after irradiation at 365 and 470 nm, respectively. The results showed that nano HNS after the irradiation pro- duced stable free radical signals. Free radicals (•NO2) were identified during the photolysis process according to the previous report[35]. Combined with the change of –NO2 (Raman spectra, Fig. 3b and c), the breaking of the C–N bond (photolysis mechanism II in Fig. 5) was also believed to be one of the possible photolysis mechanisms of nano HNS[29].
Fig. 2 a From 1 to 7: Raman spectra of HNS before irradia- tion, and irradiation for 8 h with 290 nm, 8 h with 310 nm, 20 min with 365 nm, 8 h with 470 nm, 8 h with 560 nm, 8 h with 660 nm. The excita- tion power and the acquisition time were 12 mW and 20 s, respectively. b UV–Vis (diffuse reflectance) spectrum of nano HNS
Fig. 3 a The theoretical and the experimental spectra of HNS, and the theoretical spectrum of the photolysis product. Raman spectra of nano HNS before and after irradiation at b 365 nm and c 470 nm. FTIR spectra of nano HNS before and after irradiation at d 365 nm and e 470 nm
According to the previous study on the HNS powder, the photolysis mechanisms involve the removal of NO after the isomerization of –NO2 to –ONO[30] and the breaking of the C–N bond in Ar–NO2[29]. The same routes also apply to nano HNS as shown in Fig. 5.
3.4 The Mechanism of the UV–Vis‑Induced Color Change of Nano HNS
From SEM images (Fig. 6a and b), there is no apparent change between the morphology of nano HNS before and after the irradiation. Therefore, the morphology change was not the reason for the color transfer. On the other hand, the (diffuse reflectance) absorption peaks of photolyzed nano HNS at 420–440 nm (blue light) became stronger (Fig. 6c)
Fig. 4 The EPR spectra of nano HNS before and after the irra- diation at a 365 and b 470 nm
Fig. 5 The schematic diagram of the possible UV–Vis light-induced photolysis mechanism of nano HNS
Fig. 6 SEM images of nano HNS a before irradiation and b irradia- tion for 20 min at 365 nm in a dark environment. c UV absorption (diffuse reflectance) spectra of nano HNS with (in situ) irradiation at
365 nm for different time. The insets of c were the photos of the HNS before and after irradiation with different time
with the irradiation time longer (365 nm as an example).
It is known that the pale yellow light in the (white) day- light reflected into the eye changed to brownish-yellow light (the insets of Fig. 6c) after nano HNS absorbing more and more blue light. One of the reasons for the color change was very likely to be the formation of C–OH bond in the pho- tolyzed product of nano HNS from photolysis mechanism I.
The polarity of –OH is greater than –NO2, resulting in the enhancement of electron flow in the π-conjugated system, reducing the energy required for molecular activation[36]. In other words, it would enhance the absorption peak, leading to a darker color.
3.5 The Comparison of Nano and Macro HNS
The UV–Vis (diffuse reflectance) spectra of nano and macro HNS were shown in Fig. 2b and S3, respectively.
Since there was little difference between them, the sensitive UV–Vis light wavelengths were likely to be the same (365 and 470 nm). Besides, the photolysis efficiencies of nano HNS (77%, calculated from Fig. S4) after the irradiation for 8 h with 365 nm was bigger than macro one (54%), indicating a greater degree of photolysis for nano HNS under the same conditions. The reason is that nano HNS (9.96 m2/g) has a greater specific surface area than macro one (< 0.1 m2/g).
4 Conclusions
In conclusion, the UV–Vis light (365 and 470 nm)-induced photolysis mechanisms of nano HNS were explored by dif- ferent spectroscopic methods in this work. In general, nano HNS follows the same routes as HNS powder as well as the photolysis pathway of nano HNS was the combination of the removal of NO after the isomerization of –NO2 to –ONO and the breaking of the C–N bond in Ar–NO2. In addition, the visible color change induced by UV–Vis was caused by the generation of strongly polar groups.
Supplementary Information The online version contains supplemen- tary material available at https:// doi. org/ 10. 1007/ s41664- 021- 00184-x.
Acknowledgements The work was financially supported by National Natural Science Foundation of China (22006121, 21677117).
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