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In Situ Direct Observation of Adsorption and Desorption on a Single Crystal of 2,4,6-Trinitrophenol (TNP) Alexander Kovalev* and Heinz Sturm Division “Nanotribology and Nanostructuring”, BAM − Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12205, Berlin, Germany S Supporting Information *

ABSTRACT: For the first time, we report the direct observation of the nanoscale adsorption and desorption of 2,4,6-trinitrophenol (TNP) molecules on the crystal surface during atomic force microscopy scanning. Our results reveal the position instability of TNP molecules at the border of molecular layers under normal ambient conditions. The observed phenomena are of great interest for exploring the origin of hot-spot generation on the surface of energetic materials. On the basis of the explored properties of TNP crystal, a plausible mechanism of hot-spot formation upon weak initiation at the nanoscale is proposed.

using atomic force microscopy (AFM).13 AFM monitoring of the solid−solid phase transition of HMX has also been reported.14 However, the investigation of evaluation of surface morphology with regard to the molecular structure of an EM on the nanoscale is not described in the literature. For measuring the surface structure, probing the surface forces, or the handling of objects,15 the AFM techniques could be used to provide new insight into exploring the fundamental properties of an EM on the nanoscale. From the macroscopic point of view, the explosive event is divided into four progressive stages: ignition, burning, detonation transition, and propagation of detonation.3,16 The recognized idea of ignition initiation is that ignition begins within a very small area or volume, called a “hot-spot”.17 It is postulated that the localized “hot-spot” is the source for the initiation of molecular decomposition and the corresponding thermomechanical destruction of material. Further, it is accepted that the rate of chemical decomposition and energy released during the process are sufficient to establish the detonation.18 However, the origin of hot-spot formation and the associated initiation process are as yet poorly understood.19 It is widely believed that micro- and nanoscale imperfections in EMs, such as voids, cracks, polycrystallinity, pores, dislocations, impurities, vacancies, and other defects, play a key role in the formation of a hot-spot and thus lead to deflagration or detonation, because they are always present in a real EM.3,4,20−22 Mechanisms particular to the initial decomposition of EM molecules were studied using theoretical and molecular

1. INTRODUCTION Nanoscale investigation of the behavior of energetic materials (EM) promises new insight into the fundamental mechanism and origin of explosions. The solid explosives nitrophenol and nitramine (trinitrophenol − TNP, cyclotrimethylene-trinitramine − RDX, trinitrotoluene − TNT, cyclotetramethylenetetranitramine − HMX, and others) have been well-known for a long time, but scientific investigation of their properties on the nanoscale has been undertaken only in the past decade, and open questions about new significant features of the origin and nature of the explosion remain to be answered. The main performance of an EM as concerns detonation energy is based on intrinsic features related to its molecular structure, which is made up of the juxtaposition of oxidant and reducer. For these EMs almost all physical parameters (such as the energy of formation, solidification point, heat of fusion, detonation velocity, deflagration point, impact sensitivity, etc.) that are necessary to calculate and predict the power of detonation or explosion are well-known, and the fundamental chemical reactions are also well understood.1−4 However, the origin of the initiation of an explosion on the nanoscale is completely unknown, even though an unexpected initiation of an EM is an issue relevant to safety assessments of the handling of such materials. Using the molecular dynamics simulation method allows calculation of the internal compression, shear stress, propagation rate, molecular collision, and conformation.5 The heat of sublimation, vapor pressure, and sublimation rate on the surface of an EM have been studied theoretically and compared to the experimental data.6−11 The rate of molecular adsorption and desorption on a surface has been investigated as a function of temperature as well,12 and the dependence between the roughness and thickness of a thin EM layer has been evaluated © 2012 American Chemical Society

Received: March 19, 2012 Revised: May 21, 2012 Published: June 6, 2012 3557

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Figure 1. A series of AFM images showing the evolution of the surface morphology during continuous AFM scanning of the TNP surface. (A) Crystal surface after 17 scans. The region undergoing the evident sublimation is marked by a blue rectangle. (B) The same surface of the crystal after 10 additional scans over about 1 h. On the right, a series of five successive images is shown, each denoted by the corresponding time and frame number.

nanoparticles.26 Our previous results point to an initially inexplicable occurrence of objects outside the stimulated region, a still unrecognized chemical composition of the spherical-like nanoparticles, and an undetected source of matter for the objects formed. We suggested that the observed nanoparticles are formed by partially decomposed TNP molecules. However, a reasonable explanation of the unpredictable location and plausible source of matter for these objects remained unclear. Study of the electrochemical oxidation of TNP27 showed that the nature of degradation products of TNP is governed by the oxidation reaction occurring through the release of nitro groups and by the addition of hydroxy to the benzene ring, leading to a series of oxidation and denitration processes. Further degradation can entail oxidative opening of the aromatic ring of the TNP molecule. Three major modes of a partial

simulation techniques at high temperatures 1500 and 2500 K.22,23 Nanoscale testing of the thermo−mechano−chemical response of pentaerythritol tetranitrate (PETN) by means of a heated AFM tip was also performed at temperatures between 50 and 500 °C.24 We must note that the uncontrolled violent decomposition of an EM was not observed during these heated experiments. It is known and proved that the reaction of an EM depends on the temperature or heat flow. The surface restructuring of the single crystal PETN was observed by using AFM at ambient pressure in an open environment, revealing surface rearrangement and possibly evaporation of PETN molecules at temperatures as low as 30 °C.25 Exploring the ability of nanosized TNP crystals to start decomposition on the nanoscale by means of external nanomechanical stimulus, spontaneously generated objects were observed, appearing as agglomerated spherical-like 3558

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Figure 2 shows the plots of the area and perimeter of a sublimated “island” over time. Since the observed phenomenon

decomposition of nitramine and nitrophenol molecules are known: (1) homolysis of the C−OH and C−NO2 bonds for nitrophenols or N−NO2 bonds for nitramines leading to radical intermediates; (2) inter- or intramolecular hydrogen transfer, resulting in HONO elimination; (3) nitro/nitride isomerization phenomenon or CONO rearrangement for nitrophenol.19,22,28,29 In this paper we report for the first time the observation of adsorption and desorption of TNP molecules on the surface of a TNP crystal excited by the tapping AFM tip under normal ambient conditions. The observed phenomena led to the nanoscale restructuring of the surface and revealed molecular mobility on a surface that has never before been observed or described for TNP crystal. We are convinced that precisely this explored property of EM is directly related to nanoparticle production, as well as to hot-spot formation upon weak initiation of TNP on the nanoscale.

2. EXPERIMENTAL SECTION 2.1. Material and Crystal Growth. We used microsized crystals of TNP, obtained from an aqueous saturated solution (∼5 × 10−2 mol/L at 22 °C) by depositing various ∼100 μL droplets on a substrate at room temperature. The dry, yellow TNP crystals were ready to use after 1−2 days. The chemical formula of the TNP molecule is C6H3N3O7. The TNP crystal has an orthorhombic crystal system, and the point group is mm2/C2v. The unit cell of TNP crystal consists of eight single molecules and has the dimensions a = 9.25 Å, b = 19.13 Å, c = 9.7 Å, v = 1720 Å3.30−32 2.2. Experimental Method. AFM studies in the intermittent contact mode, also known as tapping mode, were performed with an AFM (Nanotec Electronica SL, Madrid, Spain). Silicon cantilevers (Mikromasch Co.) coated by silicon nitride (Si3N4) with a resonant frequency of 312 kHz, a Q factor of 602, and a spring constant of 31.2 N/m were used. All AFM measurements were carried out under ambient conditions. The dynamic restructuring of the surface was discovered using the continuous scanning mode, which allows the recording of a whole series of successive AFM images of a selected surface area. Over about 3 h, 32 successive AFM images were recorded at an identical lateral position. The series of images consists of 16 downward directed scans and 16 upward directed scans arranged in chronological order. The time of a single scan is 5 min 43 s.

Figure 2. (a) The dependence of the decreasing area (blue circles) of the TNP molecular layer on time. The good fit (dashed line) with experimental data was achieved by applying the quadratic power law function shown in the upper right corner of the plot. (b) The dependence of the molecular layer’s perimeter (length of border) on time. The perimeter reduction is directly proportional to time.

is dynamic, the time stamp is chosen from that scan line which passes approximately through the center of the sublimation region. For the scan lines above and underneath, the known parameters of scan direction, image line index, and scan rate are used to estimate the correct time used in Figures 2 and 4. The areas of terraces undergoing sublimation were analyzed using the watershed-based segmentation technique implemented in the free image analysis software Gwyddion (http://gwyddion. net/) and plotted over time in Figure 2a. The appearance of a molecular dislocation in the form of a hole within the area was not recognized after watershed segmentation. The area of the island decreases nonlinearly over time and can be fitted well to a quadratic function. The corresponding equation is presented within the plot. It should be noted that a terrace undergoing sublimation is shapeless in a plane. No correlation was detected between the shape of considered terrace and the crystallographic orientation of lattice structure. Nevertheless, the decreasing area strongly follows a quadratic power law. Since the decreasing area follows a quadratic power law, the perimeter must be a linear function of time. Hence, the perimeter of the sublimated molecular layer was independently calculated and plotted in Figure 2b. This shows clearly that neither the shape of the island nor the curvature of the borderline influences the desorption rate. The similar linear behavior of island shrinkage has been observed previously on the surface of the PETN crystal.25 As clearly seen in Figure 2b, the perimeter reduction of the sublimated area is directly proportional to the time. Combined with the fact that no holes appeared within the selected area,

3. RESULTS AND DISCUSSION 3.1. Desorption of TNP Molecules on the Crystal Surface (Sublimation). The successive AFM imaging revealed the nanoscale dynamical restructuring of the TNP surface (Figure 1). In Figure 1A the initial state of a TNP crystal surface is shown. This crystal was selected because its upper surface is nearly parallel to the substrate. The surface structure appears with flat terraced steps, as expected. Figure 1B shows the same position as Figure 1A after continuously scanning for about 1 h. One can easily observe that the terraces underwent some transformation. A special region of interest in Figure 1A is marked by a blue square. On the right side of Figure 1 the five zoomed AFM images of this area are shown in chronological order and denoted by the corresponding time. This image series reveals the as-yet undetected feature of a TNP surface to extensively desorb molecules from a native surface at room temperature, normal pressure, and humidity through scanning in the intermittent AFM mode. On the first zoomed image the top terrace appears as an “island,” while 5 min before it was still a “peninsula”. This means that the bridge disappeared during scanning. The persistent decrease in the island area is clearly observable in the images of a series. 3559

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Figure 3. AFM images of the TNP surface showing the adsorption phenomenon. (A) The initial state of the TNP surface corresponding to the first frame of the series. The region undergoing desublimation is marked by a blue square. (B) The state of the TNP surface after 26 scan passes for 2 h 22 min 54 s. The set of cutoff frames corresponding to the desublimation region around images (A) and (B) is shown. Each image is denoted by the corresponding scan time and frame number. The chronological order is clockwise.

this means that TNP molecules leave its molecular layer only at the border. The calculated lateral desorption rate of the molecular layer is 0.2 nm/s, as indicated by the slope of the fitted line. Taking into account that we used the intermittent contact mode for imaging, it could be suggested that TNP molecules are “knocked out” by the tapping AFM tip. However in this case, the dissipated energy of oscillated tip must be sufficient to overcome the intermolecular binding of the TNP molecule situated at the border of the crystalline molecular

layer. It is well established that TNP molecules interact through ionic, hydrogen bonding, and π−π interaction.33−35 But the energy of these interactions is higher than the mechanical energy dissipated through the contact of the AFM tip. It should be pointed out that the energy per intermolecular binding site can be significantly reduced in the case of lattice defects in the crystal such as vacancies, voids, edge and screw dislocations.36,37 However, for the present situation of a crystalline 3560

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readily achieved under ambient conditions than from the vapor phase onto a crystal surface. To explain the sublimation we tried to apply the simple approach that molecules at the border of a molecular layer are “knocked out” by means of the tapping AFM tip, but found it implausible. The application of a similar mechanical approach to explain crystallization seems improbable on the basis that the AFM tip cannot be a source of TNP molecules, and the AFM tip is not able to “insert” molecules exactly into the border of molecular layer with correct orientation. Thus we note that the observed adsorption and desorption phenomena cannot be described by the suitable mechanical analog of a tapping tip and accompanied by mechanical energy transfer into the surface. This deposition region also exhibited the epitaxial growth of neighboring molecular layers. The growth evolution of molecular layers can be easily distinguished in frames 1 through 26, which are shown in Figure 3. The moment of interest is the coalescence of molecular layers, which is marked and denoted in the frame series of Figure 3. Summarizing the description of crystallization on the TNP surface, we would like emphasize that the intensive adsorption of TNP molecules is located only within the region marked by a blue square in Figure 3A,B. 3.3. Surface Effects of TNP Crystal. We point out that the sublimation and crystallization on the TNP surface probably depends on the local surface curvature of a crystal: The sublimation appeared at a surface region with positive curvature (convex surface), and crystallization took place at a region with negative curvature (concave surface). However, the curvature of the surface within these areas is extremely low in both cases. Moreover, the location of sublimation does not correspond to the highest point of the convex surface, nor does the location of crystallization correspond to the lowest height of the concave surface within the scanned area of the TNP crystal. Computing any of the physical quantities using the presented data should be done very carefully, and two important peculiarities are mentioned below. First, as it is known, the equation describing the probability of molecule to desorb from the surface at the given temperature T is φ = νeffe−ΔE/kBT, where kB is the Boltzmann constant, νeff is an effective vibration frequency, and ΔE is the energy barrier for a molecule to escape from the surface.8,12,38 From the molecular kinetic theory it is known that at the equilibrium state ΔE is the difference in the molecular potential energy between the vapor phase and solid phase. But in our case, the additional dissipated energy introduced by the tapping AFM tip and the internal degrees of freedom of the TNP molecule11 should also be taken into account. The second peculiarity concerns the vapor pressure and sublimation rate on the surface of the TNP crystal. It is evident that the observed desorption of TNP molecules should lead to an increase of vapor pressure within the region of sublimation (see the series of images in Figure 1). At first sight, for the calculation of vapor pressure we can use the equation p = p0e−ΔH/kBT (where ΔH is the enthalpy or the heat of sublimation), which is based on the well-known Clausius− Clapeyron relation. But our experiments were conducted at constant room temperature, and we also suggest that the tapping AFM tip cannot increase the temperature within the selected region of the TNP crystal (see the series of images in Figure 1), while at the same time decreasing the temperature in the other region (see the series of images in Figure 3). Because of the constancy of temperature, the equation based on the Clausius−Clapeyron relation therefore cannot be applied

monolayer on top of a crystal without visible imperfections, this assumption is not reasonable. 3.2. Adsorption of TNP Molecules on the Crystal Surface (Crystallization). A second region of interest was detected on the same TNP crystal surface facet at a distance of about 1 μm (see Figure 3). Then our attention was drawn to the small “hole”, a dislocation that can be distinguished on the left border within the blue square in Figure 3A. The observed dislocation seems to be the edge dislocation in a molecular layer. In the series of frames shown in Figure 3 in clockwise order, this dislocation becomes smaller in each successive frame and completely vanishes in frame 9 after 45 min 44 s. We suggest that at this place we observed the absorption of TNP molecules on the surface of the crystal. We prefer the term “absorption” to “adsorption” because the TNP molecules are incorporated into a molecular layer of crystal. It should be emphasized that the observed adsorption took place exactly at the border of a molecular layer, resulting in the growth of this layer within the plane. Using the watershed method again, the area of the desublimation region was evaluated and plotted in Figure 4a as a function of the scan time. The time was calculated in the way explained above. The dependence between the crystallization area and time follows a quadratic function. The perimeter reduction of the crystallization area was also evaluated (Figure 4b). The “healing” of dislocation in

Figure 4. (a) The dependence of the decreasing area (blue circles) of the hollow in the TNP molecular layer on time. The good fit (dashed line) with experimental data was achieved by applying the quadratic function shown in the upper right inset. (b) The dependence of the perimeter (length of hollow border) on time. The perimeter reduction is directly proportional to the scan time.

the molecular layer is directly proportional to time, just as the sublimation described above. However, the crystallization rate is 0.06 nm/s, which is more than three times slower than the sublimation rate. Here we have to conclude that transfer of molecules from the TNP crystal into the vapor phase is more 3561

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directly to estimate the local vapor pressure, because the pressure should be a constant at the constant temperature. The local enthalpy of sublimation/vaporization should depend on the surface curvature, because the free surface energy is different for the concave and convex parts of a surface, an effect which is governed by surface tension. Thus, the energies introduced by an AFM tip to the surface regions, which are marked by blue squares in Figures 1 and 3, are equal. Why was the probability of molecules to desorb from the surface within the one part of the surface increased, but an influx of molecules initiated on the other part of the surface? From our point of view, the real physical origin of the adsorption/desorption phenomena on the surface of the TNP crystal is more complex and cannot be reasonably described by means of molecular kinetic theory alone. Perhaps the best way to estimate the free energy change across a solid/vapor interface is to use the molecular simulation method proposed for a solid/liquid interface.39 It is important to emphasize again that two different and reversible phenomena took place within the small area of TNP surface at the same time. These observations provide further evidence that TNP molecules, prior to their adsorption on a crystal surface, must be present in a vapor (or liquid) phase above the surface. Thus, the surface of a TNP crystal on the nanoscale is complex and thermodynamically unstable with regard to phase transformations at ambient conditions: The TNP molecules can leave a surface, they are mobile on (or above) the surface and they can adsorb into the surface, building up new molecular layers, and they definitely do so without any stimulation by the tapping tip. Adapting a model for the molecular flow of trinitrotoluene molecules (TNT) near a TNT particle,10 we suggest a similar scenario: Close to the borderline of a layer, three different fluxes can be associated with the phenomenon, namely, molecular desorption, adsorption, and diffusion away from the original site. The mobile molecules can form the “molecular TNP vapor phase” on the surface of the crystal. Assuming an unknown but most likely high TNP concentration, perhaps even close to the saturation point, this “vapor phase” acts as the source of matter for the adsorption on the surface of crystal. Additionally, TNP molecules from the vapor phase can be adsorbed on any site of the surface. This model then allows us to explain the instantaneous formation of nanoparticles observed at the unpredictable sites, as published in our previous work.26 Taking into account the diffusion of TNP molecules in the vapor phase, the location of nanoparticles generated outside of the DPL test region can be accepted. We also suggest that this vapor phase with mobile molecules may be a natural feature of some EMs and that the vapor phase is present on all crystal facets of a TNP crystal. So, the vapor TNP phase is the main source of free EM molecules that can be involved in the formation of a hot-spot on any location of an EM surface. In Figure 5a the illustration of the TNP molecules’ packing in the crystal is shown with respect to the symmetry of packing. The layers with different symmetry are colored differently. The molecular structure of a TNP crystal appears as alternating molecular layers of different symmetry parallel to the (010) plane. The distance between identical layers was calculated to 1.9 nm using the free software Mercury 2.3, so the thickness of a single molecular layer is 0.85 nm. This value is in accordance with the averaged height steps measured using several crosssectional profiles collected at different positions, yielding a

Figure 5. (a) View of the layered molecular structure of the TNP crystal. The low-indexed crystallographic planes (001), (010), (100) are depicted and denoted. The molecular layers with the same crystallographic symmetry of molecular packing are highlighted in the same color. The double-layered structure parallel to the (010) plane is distinguished. The thickness of the double layer is 1.9 nm according to the calculation of the Mercury 2.3 software. (b) The cutoff frame of the AFM image with the TNP crystal used for study. The blurring is due to the digital zoom. The height range (from dark to bright) is 1.3 μm. (c) TNP crystal after examination. The white dotted square shows the region of successive AFM scanning for 3 h 8 min 30 s.

value of 0.94 nm ± 0.15 nm.25 For TNP crystal, we ensured that the observed adsorption and desorption phenomena appeared on the (010) plane. 3.4. Plausible Mechanism of a Hot-Spot Formation upon Weak Initiation of an Explosive Molecular Crystal. Summarizing all above-mentioned facts of the study presented and the results of our previous work, the investigation of a TNP surface by means of AFM,26 and also the well established fact that the initiation of the explosion can begin in the vapor phase,17,28,29 it seems plausible to propose the following mechanism of hot-spot formation on TNP molecular crystal in 3562

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decomposition of EM surface molecules. Once this hot-spot is formed, an exothermic chain reaction starts and ignites the solid explosive. Major efforts are currently underway to detect and analyze minute amounts of explosive materials to increase security for such applications as public transportation. All such methods need to measure very low concentrations of molecules or nanoparticles consisting of EM. So finally, our study has shown that AFM-based nanoscale investigations are crucial for a fundamental investigation of the physical chemistry of energetic materials subliming from and adhering at surfaces. The additional benefit of the AFM technique is that it can work with very low quantities during preparation. The analysis of such dangerous substances should not be underestimated.

response to weak initiation stimuli: We propose that, due to external stimuli of an EM surface (such as an impact, heating, friction or others), an extensive desorption of EM molecules takes place in a first step. But at the same time local adsorption can occur near the site of desorption, as was observed in our experiments (see Figure 3). It is well-known that adsorption releases energy. The released energy is absorbed as latent heat by the crystal surface. This increases both the concentration and the desorption rate of TNP molecules in the metastable vapor phase. Within the vapor phase the local concentration of TNP molecules can reach a point of saturation, and the probability of their collisions and interactions is significantly high. In this case the first chemical reaction of molecules occurs in the vapor phase, leading to the partial low-temperature decomposition of TNP molecules, which may be accompanied by the production of gaseous products with high kinetic energy and by the solidified products observed in the form of nanoparticles.26 The free and moveable molecules of an EM can be involved in spontaneous, low-temperature chemical decomposition. This process repeats itself, increasing the local heat transfer and temperature around the site. The heat released is expended in heating the location on the surface and promoting further violent molecular decomposition, thus forming a hot-spot that can lead to the ignition and detonation of the EM. If the energy exchange processes can be spatially delimited, including the build-up of the gaseous phase, the temperature production is accompanied by increasing pressure, and, as a result, detonation becomes probable. We believe that this mechanism applies to EMs such as RDX, HMX, and PETN molecular crystals, because they have similar molecular packing and packing symmetry.

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ASSOCIATED CONTENT

S Supporting Information *

AFM movie showing the surface restructuring of TNP crystal. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: ++49 - (0)30 - 8104 - 4541. Fax: ++49 - (0)30 - 8104 1817. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “innovation offensive program” of BAM - Federal Institute for Materials Research and Testing. The authors wish to thank Dr. Thomas Lehmann (BAM division 2.3, “Explosives”) for his scientific support and fruitful discussions and for his tips concerning the handling of EM.

4. CONCLUSIONS Molecular mobility on a surface of a TNP crystal was revealed for the first time. Surface restructuring on the nanoscale was observed directly under ambient conditions. Within a micrometer square, two different regions are recognized, simultaneously undergoing sublimation and desublimation. The desorption and adsorption of TNP molecules appeared only on the border of molecular layers. A transport model for TNT nanoparticles known from the literature was extended to describe the transport processes between the solid crystal phase and the gaseous phase. Our findings demonstrate some unstable behavior of TNP, explaining the spontaneous generation of nanoparticle products during AFM investigation of a TNP surface. On the basis of the revealed nanoscale properties of TNP crystal, the plausible mechanism of hot-spot formation upon weak initiation is proposed. This result contributes to a deeper understanding of the conditions required to start hot-spot formation for ignition. Molecular instability on the crystal surface apparently has no effect on the physical properties of EM on the macroscale, but it is an important factor of the thermodynamic behavior of the surface on the nanoscale. The model contains liquids and gases close to the solid surface, for which the equilibrium state depends on temperature and pressure. The small excess energy inserted by the tapping tip acts mainly in the mobile phase, increasing the kinetic energy of TNP molecules and the temperature as well. Furthermore, condensation and crystallization are accompanied by energy release, which is an additional source for the possible low-temperature decomposition and likewise contributes to the violent chemical

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