Growth of 2D Plate-Like HMX Crystals on Hydrophilic Substrate

Article pubs.acs.org/crystal

Growth of 2D Plate-Like HMX Crystals on Hydrophilic Substrate Yinlu Jiang,†,‡ Jinjiang Xu,† Haobin Zhang,† Yu Liu,† Liu Pu,†,‡ Haibo Li,† Xiaofeng Liu,† and Jie Sun*,† †

Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, Sichuan, People’s Republic of China School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Two-dimensional (2D) plate-like HMX crystals have been grown first on hydrophilic substrate using an evaporation/solvent−nonsolvent crystallization technique. As-grown crystals have been investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, scanning electron microscopy (SEM), confocal laser scanning microscope (CLSM), and atomic force microscopy (AFM). The results unambiguously indicate that the plate-like crystals with large (011) faces are βHMX, and the fluctuations in the smooth area of (011) face are monomolecular or bimolecular HMX, which suggests the mechanism of monomolecular stacking pattern and layer-by-layer growth. Furthermore, the distinct recess consisting of hexagons parallel to each other is observed on the center of the (011) face. The special growth morphology, which is markedly different from that by the classical spiral growth, is attributed mainly to the negative concentration gradient in the constrained condition. forms of HMX, β-HMX is the thermodynamically stable form at room temperature and atmospheric pressure,13 which markedly shows anisotropy in many aspects. The strength and elastic precursor shock decay in HMX crystals is anisotropic, which affects the shock initiation energy.14 In addition, the specific surface properties can also play an important role in the interactions between HMX crystals and polymer binder, and consequently affect the mechanical strength and safety of PBXs.15 To date, the anisotropic knowledge of HMX crystal surfaces, such as mechanical strength and initial detonation, has been obtained primarily in light of the treatment of a big single crystal14,16 and the theoretical models,17 which are time-consuming and high-cost. Based on the relationship between anisotropy and morphology, a large number of experimental studies have been focused on the HMX crystal morphology, such as the prismatic18,19 and spherical20 crystal at present. However, it is proposed that preparation of 2D plate-like HMX crystal with different dominant faces would be the best way to study the anisotropy of crystal faces.

1. INTRODUCTION Anisotropy is the property of orientation dependence, and is a critical factor in the physical or mechanical properties in crystalline materials, such as mechanical properties,1 magnetic properties,2 and electrical and optical performance.3 As for explosive crystal, the mechanical properties and thermal expansion are dramatically different with diverse crystal faces.4−6 The shock sensitivity of pentaerythritol tetranitrate (PETN) depends strongly on the crystal orientation: highly sensitive along the (110) direction and highly insensitive along the (100) direction. 5 Furthermore, the sensitivity and detonation properties of explosives are influenced largely by the morphological diversity, which is closely related to the growth rate anisotropy of the crystal face.7,8 Compared with the multiangular explosive crystal, the cubic or spherical crystal has better stability to thermal and impact stimulation.9,10 The spheroidization of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) can control or diminish preferred orientation and anisotropic expansion of polymer-bonded explosives (PBXs).11 Therefore, understanding the morphological properties of crystal materials and the response of exposed surfaces to external factors has become increasingly important in crystal application.12 HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) is widely used in many PBXs and propellants due to its high energy density and good thermal stability. In all polymorphic © 2014 American Chemical Society

Received: November 7, 2013 Revised: March 14, 2014 Published: April 17, 2014 2172

dx.doi.org/10.1021/cg401669w | Cryst. Growth Des. 2014, 14, 2172−2178

Crystal Growth & Design

Article

crystal habit of HMX in solution is predicted on the basis of the occupancy model.27 The chosen solvent is DMSO with the density of 1.095 g/cm3,28 and the periods are 1 ns and 500 ps for the equilibration and production stage, respectively. The table document with basic information on important crystal habit faces of HMX is created including the calculated attachment energies (Eatt), corrected attachment energies (EDMSO att), the interplanar spacing (dhkl), and the total surface percentages (ηvacuum and ηDMSO).

Crystallization in a constrained condition is defined as crystallization in a restricted space, which is broadly used in polymer crystallization.21,22 The 2D plate-like explosive crystal could also be acquired in two-dimensional constrained condition, and there are many papers published on the explosive crystal growth in the restricted system. Zhang reported the influence of the speed of evaporation on the morphology of PETN on the silicon substrate.23 Duan found dendrite growth of RDX on the glass substrate.24 Yang prepared the one-molecule-thick single-crystalline nanosheets of LLM-105 on highly oriented pyrolytic graphite (HOPG) substrate.25 However, the growth of plate-like HMX crystal has not been reported yet. In this work, crystallization in the two-dimensional constrained condition is used in HMX for the first time. The 2D plate-like HMX has been prepared by an evaporation/solvent− nonsolvent crystallization method on the hydrophilic substrate, and the morphology of as-grown crystals was investigated in detail.

3. RESULTS AND DISCUSSION 3.1. Phase Analysis of HMX. The phase of HMX-1 and HMX-2 were identified by combined use of XRD and FT-IR. Figure 1 showed the XRD patterns of two samples. Obviously,

2. EXPERIMENTAL AND COMPUTATIONAL SECTION 2.1. Experiment. 2.1.1. Materials and Reagents. HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) with its purity above 99.5% was purchased from Gansu Yinguang Chemical Industry Group Co., Ltd. of China. Dimethyl sulfoxide (DMSO) was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. of China. H2O2, H2SO4, acetone, and ethanol were purchased from Chengdu Union Chemical Industry Reagent Research Institute of China. Deionized water was supplied by the Institute of Electronic Engineering, China Academy of Engineering Physics. All reagents were of analytical grade and used as received without further purification. 2.1.2. Crystallization of HMX. The glass substrates were cleaned with a sequence of acetone, ethanol, Piranha etching solution composed of 98% H2SO4 and 30% H2O2 in volume ratios of 3:1 at room temperature (Caution: Piranha solution is highly acidic and corrosive), and deionized water. Subsequently, the substrates were blown dry with air, and then the hydrophilic glass substrates were kept in the oven at 50 °C for 30 min. 0.3 g HMX raw (HMX-1) was dissolved in 1 g DMSO, and heated to 50 °C under stirring. The HMX solutions were spread on the substrates by the drop-casting method, and the substrates were kept in the oven at 50 °C for 0.5−2 h. With the evaporation of the drop from the substrate, the boundary of the substrate mainly consisted of fractal; the remainder were the plate crystals, including the hexagon and quadrangle. In this work, the hexagonal HMX (HMX-2) was investigated emphatically due to the large proportion of it. 2.1.3. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance with a Cu Kα radiation (λ = 1.544 39 Å); the voltage and current applied were 40 kV and 40 mA, respectively. The data were collected from 5° to 50° in 2θ, with an increment of 0.02° and a scan speed of 0.2 s per step. Fourier transform infrared (FT-IR) spectra were taken with KBr pellets in Nicolet model 6700 spectrograph. The resolution was 4 cm−1, and the scan range was 400−4000 cm−1. Scanning electron microscopy (SEM) images were obtained with a Hitachi TM-1000 microscope operated at an acceleration voltage of 2 kV. Confocal laser scanning microscope (CLSM) images were measured by Olympus OLS-3000 microscope with the optical pumping at 408 nm of a semiconductor laser, and the longitudinal sampling spacing was 0.05 μm. Atomic force microscope (AFM) images were examined using a Nanoscope IIa Multimode AFM, operated in contact mode using the cantilever (μ mash-NSC15/ AIBS) at 0.1−0.3 N/m and 343 kHz at room temperature. All images were performed at the scan rate of 1 Hz. The contact angle for DMSO on the hydrophilic glass substrate was measured by the contact angle measuring system G2 (Germany Kruss). 2.2. Computation. All the calculations are performed with the molecular modeling software package Materials Studio 5.5.26 The

Figure 1. XRD patterns of Raw HMX-1 (a) and HMX-2 (b).

the XRD pattern of HMX-1 was consistent with the criterion pattern of β-HMX. In contrast with the pattern of HMX-1 and the criterion pattern of β-HMX, that of HMX-2 showed a striking difference due to its preferential orientation, which composed with only the three peaks (011), (022), and (033). Fourier transform infrared spectra (FT-IR) was employed to further understand the phases of two samples of HMX.29 Figure 2 showed FT-IR spectra of HMX-1 and HMX-2, both of which had the same spectrogram. In addition, there was no

Figure 2. FT-IR spectra of HMX-1 (a) and HMX-2 (b). 2173



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Figure 3. SEM images of HMX-1(a) and HMX-2 (b).

on a scan area 10 × 10 μm2. It was worth noting especially that the step height was 0.6 ± 0.1 nm, as shown by the line profile in Figure 5d, surprisingly matching with d011 = 0.60177 nm and the thickness of dimolecular HMX. The results above suggested the layer-by-layer growth mechanism by the monomolecular stacking on the (011) face of HMX-2.31 The limitation of AFM is the relatively low measuring height in the z-direction, typically only several hundred nanometers. In order to further confirm the recess, the confocal laser scanning microscope (CLSM), which allows nondestructive optical observation and reaches the micrometer-sized resolution in zdirection, was used in the region of Figure 4b to reveal the special surface morphology in more detail. It was remarkable that the regular recess was clearly discerned in Figure 6. Figure 6a showed the regular steps, which corresponded with Figure 5b, and two heights of 7.90 and 11.10 μm could be observed along the yellow and white arrows, respectively, in Figure 6b. It revealed the fact that the visual layers, shown in Figure 4b, experienced the growth process from the monomolecular to microsize layer. According to the observations above, the morphology of HMX-2 could be described visually by the schematic of Figure 7, in which HMX is the hexagonal plate as a whole with the regular recess in the center. The x-axis, y-axis, and z-axis are the space directions in Figure 7. 3.3. Growth Mechanism of 2D Plate-Like HMX. 3.3.1. Morphological Importance of (011) Face. According to XRD pattern of HMX-2, the (011) face has the striking preferred orientation, which suggests the dominant (011) face. It is well-known that the morphological importance of crystal faces is inversely proportional to their growth rates,32 which could be affected not only by the external factors, but also by the internal factors.33 On one hand, the interplanar spacing of a face is greater; its growth rate will be slower on the basis of BFDH model.34−36 In other words, the greater interplanar spacing of a face suggests its more morphological importance. The molecular model of (011) face and the crystal habit of HMX via BFDH model in vacuum are shown as Figure S4 and S5 in Supporting Information for details. On the other hand, the growth rate of the crystal face is proportional to its attachment energy.37,38 The face with lower attachment energy is predicted to be a slower-growing face and hence to have higher morphological importance, which has been demonstrated on HMX by the theoretical models.27,39 According to the simulation in DMSO, the (011) face has the lowest attachment energy in the dominant crystal habit faces of HMX,

transmittance band in the wavenumber range between 700 and 750 cm−1 in the experimental FT-IR, which was different from the spectra of α, γ, and δ-HMX. Based on the XRD curves and FT-IR spectra, it was easy to confirm that these two HMX samples were both β-HMX. Besides, the preferred orientation of HMX-2 was striking, which suggested the appearance of the (011) face.30 3.2. Morphology Analysis of 2D Plate-Like HMX. Figure 3 showed the representative SEM images of HMX-1 and HMX-2, which were different from each other. The morphology of HMX-1 was primarily a prismatic type with the size of 20 μm approximately (Figure 3a), whereas that of HMX2 exhibited the hexagonal plate type with the size of 300 μm × 100 μm roughly, and the thickness was different from 10 to 100 μm due to the difference of evaporation rate and growth time at different crystallization sites, shown in Figure 3b, Figure 4b, and Figure S1 in Supporting Information. However, the larger hexagonal plate-like crystals were observed in the center of the substrate, which was attributed mainly to the longer crystallization time compared with that at the edge of the substrate. Interestingly, a special morphology for HMX-2 with hexagonal steps was observed, as depicted in Figure 4a. Figure 4b, c, d, and e were the amplified images of the b, c, d, and e regions in Figure 4a, respectively. The evident recess, which was composed of the inerratic hexagons parallel to each other, was observed in the center of the dominant face, shown in Figure 4b, but there was a slope surface shoaling along the red arrow in the bottom of the recess, shown in Figure 4c. The edge of the crystal also showed the step pattern, but presented the rising tendency, shown in Figure 4d and e. (The morphology could also be observed on the surface of thin HMX crystals in Figure S1b in Supporting Information.) Atomic force microscopy (AFM) has been widely applied in investigating the microtopography of materials, which can provide a clear picture of surface morphology on the nanometer scale. Figure 5 showed the smoother areas of the HMX-2 surface, the f region and g region in Figure 4c. Figure 5a (3D image of Figure 5a was shown as Figure S2 in Supporting Information), which was the bottom of the recess, was acquired on an area of 1.5 × 1.5 μm2. The surface was composed of a layer structure with the height of 0.3 ± 0.05 nm, which matches with the thickness of monomolecular HMX, as shown by the line profile in Figure 5b. Besides, Figure 5c (3D image of Figure 5c was shown as Figure S3 in Supporting Information), which was the flat region close to the bottom of the recess, was a typical AFM image of the layer-by-layer morphology acquired 2174



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Figure 4. SEM images of HMX-2 in the center region of the substrate: (a) surface morphology of HMX-2; (b), (c), (d), and (e) morphologies of b, c, d, and e region in part a, respectively.

direction perpendicular to the (011) face for β-HMX, which will lead to the elongation of the majority of crystals along the [100] direction.40 It will strengthen further the dominance of the (011) face. As the HMX crystal grows, the (011) face will tile on the substrate due to its morphological importance. Finally, compared with the growth in three-dimensional space, the crystal growth on the two-dimensional substrate will make a switch of growth mode from longitudinal thickening to lateral growth. Based on the reasons above, the (011) face will become the most dominant face in HMX. 3.3.2. Growth Mechanisms of Plate-Like HMX. In view of the area occurrence of the (011) face in all important faces, HMX crystal will not grow into a plate. However, the morphology of HMX has a strong dependence on the thickness

and will become exposed gradually in the crystal growth process. Table 1 summarizes the results of simulations and experimentally observed crystal habits for HMX growth from the vacuum and DMSO solution. Although the area occurrence of the (011) face in DMSO (54.03%) is lower than that in vacuum (60.32%), the (011) face is still the most morphologically important face in HMX. The simulation result in DMSO is similar to that in other solvents.27 The crystal habit of HMX via occupancy model in DMSO is shown in Figure S6 in Supporting Information for details, and the hexagon is the inherent shape of the (011) face based on the morphologies in Figure S5 and S6. Furthermore, the (011) face is a known cleavage plane of β-HMX because the intermolecular forces of the bc* plane are weaker than the ac* or ab* plane. The intermolecular interactions are stronger along the [100] 2175



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Figure 6. CLSM images of the region of Figure 4b: (a) the 3D image, (b) the 2D image and the depth in the z-direction.

Figure 7. Schematic of HMX-2 growth: (a) lateral view and (b) cutaway view.

Figure 5. AFM images of (011) face: (a) 2D image of the f region in Figure 4c; (b) cross-sectional profile along the line in part a; (c) 2D image of the g region in Figure 4c; (d) cross-sectional profile along the line in part c.

Table 1. Calculated Attachment Energies (Eatt, kcal/mol) and Corrected Attachment Energies (EDMSO att, kcal/mol), Interplanar Spacing (dhkl, Å), Relative Occupancy (k), and the Area Occurrences (ηvacuum and ηDMSO, %) of Important Crystal Habit Faces

of the liquid layer in our study. For example, HMX crystal will grow into the prismatic shape on common glass, shown in Figure S7. In the constrained condition, the crystal morphology would be engineered because of the growth space restriction in some directions but free in others. As the thickness of the solution layer decreases, the crystal growth will be restricted in the z-axis direction perpendicular to the substrate. In order to obtain the thin solution layer, the hydrophilic glass substrate was used as the crystallization substrate, since the contact angle for DMSO on the substrate becomes smaller from 34.84° to 11.43° after activating. It is beneficial to the growth of the perfect plate-like crystal in the constrained condition. In light of results from SEM, CLSM, and AFM images of plate-like HMX-2 on hydrophilic substrate, it can be concluded that the crystal growth occurs on monomolecular steps, which are generated by layer-by-layer expansion in the exposed face. When supersaturation of the solution reaches the critical point, the nuclei will form randomly and further grow into crystals

face

dhkl

Eatt

ηvacuum

k

EDMSO att

ηDMSO

(011) (11−1) (020) (100) (10−2)

6.025 5.523 5.525 5.403 4.317

−26.85 −40.65 −38.76 −52.64 −45.04

60.32 30.33 6.37 1.91 1.07

0.81 0.71 0.69 0.83 0.79

−21.75 −28.86 −26.74 −43.69 −35.58

54.03 34.94 10.24 0.44 0.35

with different sizes on the substrate. According to the Ostwald ripening,41 the small crystals will dissolve and redeposit onto larger crystals in the crystal growth process because larger particles are more energetically favored than smaller particles.42 With increase of the supersaturation by slow evaporation, the new HMX molecules will attach to the stable sites on the larger crystal surface and form the growth step, and then, the crystal grows circularly by the Kossel model to form the unimolecular 2176



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Figure 8. Growth schematic of the recess on the (011) face of HMX-2: (a), (b), and (c) are the elevation views of crystal in the growth process, (d) is the elevation view of the as-grown crystal. ha, hb, and hc are the heights of solution layers, hd is the crystal thickness, and ha, hb, hc, and hd become successively smaller.

layer.43 Due to the restriction on the direction vertical to the substrate, the crystal will grow along the direction parallel to the substrate and form the plate-like morphology. 3.3.3. Growth Mechanisms of the Recess on the (011) Face. To the best of our knowledge, the well-regulated recess on the surface of the HMX crystal has not been reported before. The negative concentration gradient caused by solute transportation is considered to play a crucial role in formation of the recess, and the growth schematic of the recess is shown in Figure 8, which are the elevation views of the crystal in the growth process. With crystal growth and solution level reduction, the top surface of the plate-like crystal is gradually closer to the top surface of the solution, and the solute for crystal growth will be obtained via solute transportation from the periphery (Figure 8a). Because of the absorption in the transportation process, the solute will be gradually lost. As it gets closer to the center region, the solute will become reduced (Figure 8b and c). The negative concentration gradient is generated from edge to center. Growth of the crystal will result in formation of the well-regulated recess (Figure 8d). In addition, the recess morphology is evidently different from that by classical spiral growth, in which screw dislocations provide a continuous source of new steps,44 such as calcite growth on the cleavage surface,45 tetragonal lysozyme growth in (110) faces,46 and Co/Zn-MOF-5 crystal growth with edges parallel to the [100] directions.47 The other interesting fact is the recess with different depth shown in Figures 4c and 6b, which may be related to the residual solution in the recess.

the negative concentration gradient from the edge to the center, and distinguished from the classical spiral growth. In addition, formation of the 2D plate-like morphology of HMX should be investigated further, especially regarding the formation of quadrangular plate-like HMX and the interaction between the crystal face and the substrate. The most interesting and challenging task is to prepare the 2D plate-like HMX crystal with different exposed faces. Even so, the successful growth of hexagonal plate-like HMX may provide a stringent baseline for understanding the anisotropy of the HMX crystal. In addition, the method is simple for modifying the morphology of the HMX crystal, and can also be extended to other crystal materials, opening the door for fundamental studies of crystal morphology.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of thin HMX crystals, 3D AFM images of Figure 4a and c, group distribution of (011) faces, crystal habit of HMX via BFDH model in vacuum, crystal habit of HMX via the occupancy model in DMSO. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Fax: +86 816 2495856, Tel: +86 816 2482002. Author Contributions

Yinlu Jiang and Jinjiang Xu contributed equally to this work.

4. CONCLUSION The well-formed 2D plate-like HMX was obtained by evaporation/solvent−nonsolvent crystallization on the hydrophilic glass substrate. Different experimental techniques such as XRD, FT-IR, SEM, CLSM, and AFM were used to gain insight into the polymorphism and morphology of HMX. Compared with the prismatic and spherical HMX growing in the conventional system, as-grown β-HMX indicated the special hexagonal plate-like morphology with the exposed (011) face, which coincides with the growth theory and simulation, and the hexagonal steps can be observed in the exposed face. The microtopography studies revealed the mechanism of monomolecular stacking and layer-by-layer stacking growth on the dominant face, which could be better explained by the Kossel model. Moreover, the special recess consisting of the inerratic hexagon parallel to each other was observed in the center surface of the dominant face, which was strongly dependent on

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Science Foundation of China Academy of Engineering Physics (NO. 2012A0302013 and NO. 2012A0201007) and the National Science Foundation (NO. 11372290). The authors gratefully acknowledge Prof. Chaoyang Zhang, associate professor Kemei Cheng, associate professor Mingshui Zhu, and Miss Hui Wang for their help in HMX simulation, AFM, CLSM and SEM characterization, respectively.

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REFERENCES

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Growth of 2D Plate-Like HMX Crystals on Hydrophilic Substrate

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