Can a Photosensitive Oxide Catalyze Decomposition of Energetic

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Can a Photo-Sensitive Oxide Catalyze Decomposition of Energetic Materials? Fenggong Wang, Roman V. Tsyshevsky, Anton S Zverev, Anatoly Y. Mitrofanov, and Maija M. Kuklja J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10127 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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The Journal of Physical Chemistry

Can a Photo-sensitive Oxide Catalyze Decomposition of Energetic Materials? Fenggong Wang1, Roman Tsyshevsky1, Anton Zverev2,3, Anatoly Mitrofanov2, Maija M. Kuklja1,* 1

Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA 2 Department of Organic and Physical Chemistry, Kemerovo State University, Kemerovo 650043, Russia 3 Yurga Institute of Technology, National Research Tomsk Polytechnic University, Yurga 652057, Russia

Abstract Organic-inorganic interfaces provide both intrigues and opportunities for designing systems that possess properties and functionalities inaccessible by each individual component. In particular, the electronic, catalytic, and defect properties of inorganic surfaces can significantly affect the adsorption, decomposition, and photoresponse of organic molecules. Here, we choose the formulation of TiO2 and trinitrotoluene (TNT), a highly catalytic oxide and a prominent explosive, as a prototypical example to explore the effect of a catalytic oxide additive on the photosensitivity of energetic materials. We show that, whether or not a catalytic oxide additive can help molecular decompositions under light illumination depends largely on the band alignment between the oxide surface and the energetic molecule. For the composite of TiO2 and TNT, the lowest unoccupied molecular orbitals (LUMOs) of TNT merge within the conduction band (CB) of TiO2. As such, no optical transition corresponding to available laser energies is observed. However, oxygen vacancy can lead to electron density transfer from the surface to the energetic molecules, causing an enhancement of the bonding between molecules and surface and a reduction of the molecular decomposition activation barriers. Therefore, when other (than optical) forms of energy (shock, heat, etc.) flow into molecules, molecular decompositions may be triggered more easily.

*

Corresponding author email: [email protected] with cc [email protected], tel: 703-292-4940

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1. Introduction Interactions of energetic materials with laser irradiation are poorly understood and typically perceived of the thermal nature.1 Even less is known about routes to tailor the physical and chemical properties of energetic materials by dopants or by mixing with light-responsive additives.1 Studies of energetic materials continue attracting extensive attention due to the desire to integrate these materials more fully into modern technological, environmental, pharmaceutical, and defense applications, including, but not limited to, rocket engine fuels and propellants, construction and coal mining industries, medicine, explosive sensing and detection, all of which require highly controllable handling of these materials. 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 Ever increasing challenges of sustainability in global energy and environment also require a better understanding of the energy absorption and release processes of energetic materials, of how they interact with environment and respond to mild external stimuli, and of ways to remedy their toxicities.10,11,12 Light-matter interactions have been shown to substantially affect materials' structural, electronic, ferroelectric, magnetic, photovoltaic, photocatalytic, and other photoinduced properties.13,14,15,16 It is thus appealing to explore the photosensitivity of energetic materials, which will potentially enrich their current applications, for example, to promote degradation of toxic pollutants generated during the preparation and use of energetic materials, or to suggest fundamentally new architectures of devices for energy storage and conversion. In particular, laser-induced energy absorption at a metal oxide–energetic material interface has been proposed as a way to initiate chemical decomposition and to control the energy release process in energetic materials.17 The electronic, catalytic, and defect-induced properties of organic-inorganic interfaces can significantly alter the adsorption, chemical stability, decomposition, and photo-response of organic energetic molecules.18,19 Most current energetic materials have wide band gaps and can only absorb violet or ultraviolet light, prohibiting their absorption of the vast proportion of solar light and limiting the frequency range of useful laser light. Recent experiments and our previous study of the PETN (pentaerythritol tetranitrate) and MgO formulation demonstrated that it is feasible to alter the photoresponsive properties of energetic materials by mixing them with oxide additives, because oxide defects at interfaces induced local electronic states in the band gap, and as a result, the decomposition chemistry may be initiated by laser light with an unusually low photon energy (1.17 eV).17,20,21,22 While the opportunity to skillfully design interfaces with desired features by combining an explosive with an appropriate oxide is compelling, the clear recipe on how to achieve this and the fundamental understanding of the basic interface properties are still lacking. Our research aims at making inroads into this area. So, an ultimate question is: can a highly catalytic oxide additive increase the photosensitivity of an explosive? Here, we deliberately select the formulation of a widely used catalytic oxide TiO2 and a prototypical energetic material TNT (trinitrotoluene, C7H5N3O). Photodegradation of TNT in TiO2 suspension was previously investigated; 23, 24 ,25 however, the specific mechanism, in particular, the interaction at the interface between TNT and TiO2 is still unclear. We mimic the interface between TiO2 and TNT using a slab model with TNT molecules adsorbed on the most stable rutile TiO2 surface [(110) surface)]. Compared to MgO (7.8 eV),26,27 TiO2 has a much lower band gap (3.0 eV) at the visible-light edge. 28,29,30 Further, it can crystalize in several phases and accommodate various defect configurations, surface terminations and reconstructions, 2 ACS Paragon Plus Environment

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potentially featuring a great tunability. Thus, it serves as a good example to study the interplay between an energetic material and a catalytic oxide additive. We show that the TNT-TiO2 (110) interface does not induce localized gap states and hence no direct optical transitions corresponding to available laser energies are observed. However, the presence of surface oxygen vacancies, always present in oxides, leads to the electron density transfer from TiO2 to TNT, and facilitates molecular decompositions. 2. Methods 2.1 Computational methods The VASP code was used to perform density functional theory (DFT) calculations with the PBE generalized gradient approximation (GGA) functional. 31 , 32 , 33 , 34 , 35 All chemical elements were represented by the projector augmented wave (PAW) pseudopotentials36 while the energy cutoff for the plane wave basis set was 450 eV. The Monkhorst-Pack method was used to sample the Brillouin zone,37 where the Γ-only k point was used for structural optimizations while the 2 2 1 k mesh was used for electronic structure calculations. Full structural optimizations were performed for bulk crystals while only internal atomic coordinates were relaxed for slabs and molecules. The atomic relaxation was terminated when the atomic forces acting on each ion were less than 0.02 eV/. To refine the calculated band gaps, we adopted the HSE06 hybrid functional38 at Γ k point based on structures optimized with the PBE functional. For systems containing oxygen vacancies, structural optimizations were performed by the PBE+U method with the Hubbard U for Ti d electrons U=4.5 eV because it was shown that this method can well localize the polaronic effect of the vacancy while the PBE method fails to do that.39 The absorption spectrum of TNT molecules was calculated by the time-dependent DFT (TDDFT) approach with the HSE06 functional, as implemented in Gaussian 09.40,41 The 6-31+G(d,p) basis set was used for all molecular calculations. In our models, five TiO2 trilayers were included, with symmetric surfaces at both sides except that molecules were only adsorbed on one side, so that the dipole-dipole interaction is minimized.42,43 A vacuum layer of at least 18 was used to avoid spurious interactions between periodically repeated supercells. The vdW-DF method was also used to test the effect of dispersion on geometry configurations and adsorption energies.44 2.2 Experimental details TNT (b)

TiO2 (a)

20 μm

0.2 mm

FIG. 1 The microphotographs of (a) TiO2 and (b) TNT samples.



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We prepared a TiO2 powder sample with 99.5% purity of the rutile phase, as determined by the diffractometer DR-02 RADIAN (NTC Expert center, Moscow, Russia) with a wavelength of λ = 1.541874 Å. The TiO2-TNT composite samples were prepared according to the following procedure. First, the oxide powder was manually ground in an agate mortar until the powder grain size reaching ~1 µm (Fig. 1). Then, the ground TiO2 powder was added to the mortar filled with TNT and stirred for 5 minutes. The mixture of TNT and TiO2 was subsequently heated in the drying oven for 10 minutes at a constant temperature of 87°C to melt TNT, so that the TiO2 particles are uniformly covered by TNT. This temperature guarantees that the TNT grains can be completely melted (3.2 g, 10 minutes), as the melting point of TNT is only 80.7°C.45 In our experiment, the weight concentration of TNT in the TiO2-TNT composite is 0.5%. The optical reflectance spectra of the obtained composite were subsequently measured and recorded in the range of 190 to 1200 nm (1.03~6.52 eV) using the Shimadzu UV-3600 spectrophotometer (Shimadzu Corporation, Kyoto, Japan) with UV-VIS-NIR integrating sphere attachment ISR-3100. The optical reflectance spectra of pure TiO2, pure TNT, and the TiO2-TNT composite were registered with respect to the barium sulfate powder. The obtained dependencies were transformed according to the KubelkaMunk formula:

(1) where R is the reflectance. Kubelka-Munk function F(R) gives a ratio between the absorption coefficient and the scattering coefficient (K/S).46

3. Results and discussion 3.1 Structural and electronic properties of TiO2

FIG. 2. The schematic atomic structure representations of rutile (a) bulk TiO2 and (b) (110) TiO2 surface with five atomic trilayers. The surface is symmetric and under-coordinated bridging O atoms are present at the outmost positions. Larger spheres represent Ti atoms and smaller spheres represent O atoms. (c) The relative energy position of the O vacancy state (F0 center) versus the valence band maximum (VBM) and the conduction band minimum (CBM), calculated by the HSE06 functional within the single-particle DFT framework.


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Simple test calculations of bulk rutile TiO2 (Fig. 2) by the PBE method yield equilibrium lattice constants of a=b=4.633 , c=2.982 , comparable to the experimental values of a=b=4.594 , c=2.959 . The band gaps of bulk TiO2 calculated with the PBE and HSE06 functionals are 1.65 and 3.05 eV, respectively. Note that the band gap calculated by the HSE06 method is in good agreement with experiment (~3 eV),47 suggesting that the HSE06 functional can indeed predict correctly the band gap of pure TiO2 while the standard PBE functional underestimates it, as anticipated. We choose the rutile (110) surface (Fig. 2) because it is the most stable surface of the rutile TiO2. Our test calculations show that five trilayers are thick enough to converge the band gaps. The band gaps of the rutile TiO2 (110) surface calculated with the PBE, PBE+U, and HSE06 functionals are 1.46, 2.09, and 2.95 eV, which are slightly smaller than their corresponding bulk values, as expected. The major electronic compositions of the valence and conduction bands (VB and CB) of TiO2 surfaces are O 2p and Ti 3d states, respectively. All these features agree well with literature.47 An oxygen vacancy is a defect that is hard to avoid in semiconductors like TiO2. It can significantly affect the electrical conductivity and catalytic activity of TiO2. For example, it has been shown that the O vacancy state in the rutile TiO2 (110) surface plays a predominant role in the bonding and surface chemistry of NO2.48 Previous formation energy calculations based on the HSE06 and GW methods indicated that the O vacancy is a shallow donor in bulk TiO2, with the transition energy level between the doubly positively charged (+2) and neutral (0) states (referred to as +2/0) (the +1 charge state is unstable) close to or merging into the CB. Nevertheless, the energy position of the O vacancy state in TiO2 surface is still under debate. Standard DFT calculations fail to predict the correct nature of the O vacancy while hybrid functionals are able to reproduce the experimental energy of the O vacancy state on the rutile TiO2 (110) surface, i.e., around 0.7 eV below the conduction band minimum (CBM).49 It was suggested that structural optimizations should be performed with hybrid functionals in order to get the correct localization nature of the O vacancy.42 However, this is beyond our computational capabilities. We therefore employed the DFT+U method for structural optimizations of TiO2 surfaces containing vacancies and then used the HSE06 functional for the electronic structure calculations of the DFT+U optimized configurations. This is reasonable, because unlike the standard DFT, the DFT+U approach was suggested to be capable of localizing the polaronic O vacancy on the TiO2 surface.50 We adopted the slab model with one surface bridging O atom (outmost O atom) removed from both sides of the slab so that the dipole interaction is minimized. We note that it is less likely to form O vacancy in plane or at subsurface. Not surprisingly, our DFT+U calculations give rise to an O vacancy state at 0.9 eV below the CB, in agreement with previous calculations.50 Thus, performing the HSE06 electronic structure calculations on the DFT+U optimized structures does not diminish the validity of our prediction, as the polaronic effect of the O vacancy is well preserved in the DFT+U optimized structures.



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3.2 Electronic and optical properties of the TNT molecule

FIG. 3. (a) The absorption spectrum of the isolated TNT molecule was calculated by the time-dependent DFT (TDDFT) with the HSE06 functional and the intensity f is depicted as a function of energy. The inset shows the nature of the wavefunctions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. (b) Comparison of the HOMO-LUMO gap calculated by the DFTHSE06 method and the optical transitions of the TNT molecule calculated by the TDDFT-HSE06 method.

Further, we investigate the electronic and optical properties of isolated TNT molecules. The electronic wavefunctions (Fig. 3) of both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) show that, while both the HOMO and LUMO arise from the 2pz states of C and O, there is also an additional substantial contribution of the N 2p states of the nitro groups to the LUMO states; a typical feature for the nitro compounds.1 To include the excitonic effect for the absorption spectrum of TNT molecules, 51 we adopted the time-dependent DFT (TDDFT) method with the HSE06 functional (TDDFT-HSE06). As shown in Fig. 3, it is evident that the excitonic absorption peak at 3.85 eV corresponds to the lowest singlet-singlet transition. This agrees well with previous experimental observation of a very low intensity peak at 326 nm (3.80 eV).52 The lowest singlet-triplet transition energy calculated by the TDDFT-HSE06 approach is 2.75 eV. All other transitions occur at higher energies, as shown by the broad absorption band between 4.5 and 6 eV. The HOMO-LUMO gap calculated with the HSE06 functional within the DFT framework (DFT-HSE06) is 4.63 eV, which is 0.78 eV larger than the excitonic absorption energy by the TDDFT-HSE06 approach. This strongly suggests the presence of tightly bound singlet excitons in TNT molecules. Actually, in organic molecular crystals, the intermolecular interactions are very weak and the dielectric constant is very small, resulting in large exciton binding energies.53,54 The dielectric constant of the TNT molecule is close to that of the PETN molecule (~ 2), and therefore they are both expected to have large singlet exciton binding energies,1 characteristic for molecular crystals. On one hand, the presence of excitons in TNT molecules suggests that, as a nitro-compound, the required light energies to decompose TNT may be lowered, corresponding to the excitonic mechanism of detonation 6 ACS Paragon Plus Environment

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initiation.55 On the other hand, since the oxide additive has a much larger dielectric constant (86-173 for TiO2), the electrostatic Coulomb force between electron and hole is largely screened. As a result, the excitonic effect may play a less significant role at the interface in the combined explosive-oxide formulation. 3.3 TNT molecule adsorbed on TiO2 surface

FIG. 4. The schematic atomic representation of the periodic supercell with one TNT molecule adsorbed on the (110) rutile TiO2 surface. Two O atoms of the TNT molecule sit atop the under-coordinated surface Ti atoms, leading to the TNT molecule positioned almost perpendicular to the surface.

To further investigate the properties of the interface between TNT and TiO2, we adopted a slab model containing TNT molecules adsorbed on the rutile TiO2 (110) surface. All probed configurations were fully optimized and the most favorable model supercell structure was identified based on energetic considerations. The in-plane repetition of the adopted supercell is 6 3 (17.8 19.5) and the TiO2 surface contains five atomic trilayers. In this combined system, TNT molecules are connected with the TiO2 surface in such a way that two molecular O atoms sit on top of two under-coordinated surface Ti atoms (Fig. 4), and the TNT molecule is positioned nearly perpendicular to the TiO2 surface. We also explored the configurations with TNT molecules parallel to the surface. However, since under-coordinated O atoms are present at the outmost positions of the surface, they repel strongly the N and O atoms of TNT molecules, leading to the parallel adsorbed configuration being unstable. Indeed, our attempts to obtain a reasonable converged “parallel” adsorption configuration have failed. In the perpendicularly adsorbed configuration, the optimized distances between the surface Ti atoms and molecular O atoms are 2.36 and 2.42 , which are longer than the regular Ti-O bond lengths in bulk TiO2 (1.94 and 2.01 ). The binding energies Eb calculated by the PBE and vdW-DF methods are 1.66 and 1.67 eV, respectively, where the binding energy Eb is defined by Eb=Etot(TiO2)+Etot(TNT) - Etot(TiO2+TNT), and Etot(TiO2), Etot(TNT), and Etot(TiO2+TNT) are the total energies of the plain TiO2 surface, the isolated TNT molecule, and the combined adsorption system, respectively. The strong binding between TNT and TiO2 suggests that the adsorption is of the chemisorption character, in which the covalent bonding interaction plays a major role while the van der Waals interaction plays 7 ACS Paragon Plus Environment


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only a minor role. This observation offers yet another explanation on why the parallel adsorbed configuration is unstable, that is, in this configuration, covalent bonds are unlikely to form. Note that the large binding energy is also consistent with the fact that TiO2 is a good catalyst, as the first step to catalyze a chemical reaction is to adsorb the molecule.

FIG. 5. (a) The total and projected density of states (TDOS and PDOS) and (b) the charge density for the conduction band minimum (CBM) and (c) a higher conduction band (CBM+3) of the system with TNT molecules adsorbed on the perfect rutile TiO2 (110) surface calculated by the DFT-HSE06 method. It is evident that no local electronic states are present in the band gap and the bottom of the CB is mainly residing on the TiO2 surface while the electronic states of the TNT molecules are residing on the conduction band at higher energies.

When molecules are chemically adsorbed on a catalytic surface, their optical and decomposition properties can be significantly influenced by the interaction between molecules and the surface, such as charge transfer or band alignment. Specifically, when TNT molecules are adsorbed on the TiO2 surface, their electronic orbitals are aligned with those of the TiO2 surface as a consequence of covalent bonding. The precise band alignment requires a detailed analysis of the accuracy of the adopted method in describing the electronic properties of both TiO2 and TNT. The DFT-HSE06 method predicts correctly the band gap of TiO2 and overestimates the HOMO-LUMO gap (4.63 eV) of TNT compared to the exciton absorption energy (3.85 eV). However, we point out that the tightly bound exciton will essentially be eliminated in the combined TNT-TiO2 system due to the pronounced screening effect of the strongly dielectric TiO2 additive. As such, the light absorption of the combined TNT-TiO2 system will not be affected by the conduction states corresponding to the lowest singletsinglet transition (the exciton states) of the isolated TNT molecules. In this regard, the position of the conduction states corresponding to the second singlet-singlet transitions becomes relevant when considering the band alignment. Since the HOMO-LUMO gap of TNT calculated by the DFT-HSE06 method is very close to the second singlet-singlet transition energy of isolated TNT molecules, there is no need to further correct the band alignment result of the combined TNT-TiO2 system calculated by the DFT-HSE06 method. 8 ACS Paragon Plus Environment

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FIG. 6. The schematic energy diagram of the plain rutile TiO2 bulk and (110) surface, the TNT molecules adsorbed on the oxide surface, pure surface with F0 center, and surface containing both O vacancies and TNT molecules. Orbitals are aligned with the VBM from the HSE06 calculations. VBM (valence band maximum), CBM (conduction band minimum), HOMO (highest occupied molecular orbital), and LUMO (lowest unoccupied molecular orbital).

Figure 5 shows the electronic structure of the system with TNT molecules adsorbed on the TiO2 surface calculated by the DFT-HSE06 method. Unlike the formulation of PETN and MgO1, no local electronic states associated with TNT orbitals are present in the band gap of the TNT - TiO2 composite. The LUMO states of the TNT molecules composed of the 2p orbitals of O, N, and C merge into the TiO2 CB formed by the surface O 2p and Ti 3d orbitals. As a result, red-shift of the light absorption edge in the combined system relative to that in the pure TiO2 is not observed. This means that even under irradiation by light with sufficient photon energies, electrons can only be excited and relaxed to the CB residing on the TiO2 surface. Thus, no significant electron density is transferred to the TNT molecules, nor does the irradiation generate excited states of the TNT molecules, which would be beneficial for prompting molecular decompositions similar to those observed in PETN. 56 Absence of obvious optical transitions from the surface VBM to the LUMO states of TNT molecules (Fig. 6), corresponding to the first or second harmonic energy (1.17 and 2.33 eV) of available lasers, suggests that the adsorption on a perfect TiO2 surface can hardly enhance the light absorption and photosensitivity of the TNT explosive molecule at lower energies. Indeed, this is verified by our experimental measurements of the absorption spectra. As shown in Fig. 7, the absorption spectrum of the TiO2-TNT sample is almost identical to that of the pure TiO2 sample. Also, the absorption intensity of the combined TiO2-TNT sample is even lower than that of the pure TNT at photon energies of ~3 eV. We emphasize that our absorption measurements of pure TiO2 and pure TNT agree well with our calculations (the band gap energy Eg of TiO2 at ~3 eV, and singlet-singlet transition S0→S1 energy of TNT at ~3.8 eV). 9 ACS Paragon Plus Environment


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FIG. 7. The experimentally measured absorption spectra of pure TNT molecules, TiO2 bulk, and the mixture of TNT and TiO2. Apparently, mixing TNT with TiO2 does not red-shift the absorption edge of TNT.

3.4 Molecules adsorbed on TiO2 surface containing vacancies In conventional oxide semiconductors like TiO2, defects, especially O vacancies, are always found even in high quality materials. The presence of defects can significantly affect the structural, electronic, optical, and catalytic properties of bulk, surface, and interface systems. For example, surface O vacancies may drive abrupt structural distortions at the surface, induce charge transfer from surface to molecules, and alter the adsorption and decomposition of molecules on surfaces. 57 , 58 Previously, it was found that O vacancies in MgO induce gap states and promote optical transitions from O vacancy states to molecular (PETN) LUMO states1,22. In bulk TiO2, the O vacancy (F0 center) generates a doubly occupied state at 2.3 eV above the VBM, as confirmed by our HSE06 calculations (Fig 6). Therefore, we investigated the role of O vacancies in the TNT-TiO2 composites. We adopted the adsorption configuration with two O atoms of the TNT molecule sitting atop the surface Ti atoms. Apparently, the presence of surface oxygen vacancies significantly strengthens the binding between the TNT molecules and the TiO2 surface; the adsorption energy obtained by PBE is 2.66 eV. The distance between the molecular O atom and the corresponding surface Ti atom contracts from 2.39 to 2.06 (Fig. 8). This is not surprising, as the extra positive charge of surface Ti4+ ions tends to be compensated by the enhanced bonding with molecular oxygen atoms (due to Coulomb interaction), drawing molecules closer to the surface. Bader charge density analysis shows that molecules become negatively charged. As shown by the differential charge density of the TiO2 with surface O vacancies before and after molecular adsorption, 0.62 e is transferred from the surface to the molecules (Fig. 8). The amount of transferred electron density is not as big as that in the PETN-MgO system, where PETN molecules withdraw almost two electrons from the O vacancy22. Its effect on 10 ACS Paragon Plus Environment

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making TNT molecules negatively charged is nevertheless notable. The smaller electron density transfer occurs because the Ti4+ ion itself is able to accommodate most of the electron density of the vacancy by lowering the valence state from 4+ to 3+.

FIG. 8. (a) The atomic structure representation of the system with TNT molecules adsorbed on the TiO2 surface containing O vacancies. The presence of O vacancies enhances the adsorption by shortening the distance between the molecular O atom and the corresponding surface Ti atom. (b) The differential charge density [(TiO2, adsorbed)- (TiO2)- (TNT)] of the TiO2 surface with vacancies before and after adsorption of the TNT molecules. It is apparent that electron density (yellow isosurface) accumulates on the molecular O atoms bonding with the surface Ti atoms, corresponding to electron density transfer from surface to molecules.

When oxygen vacancies are introduced in the composite of TiO2 and TNT, the mixing states of O vacancy and TNT molecule locate close to the CB. As a result, some electron density originated from the vacancy is transferred to the TNT molecules. There is no obvious optical transition corresponding to the available laser light (1.17 eV or 2.33 eV). The only possible optical transitions will excite the electrons from the mixed states of O vacancy and TNT to energies higher than the bottom of the CB. However, these hot electrons can itinerate all over the TiO2 surface and then relax to the bottom of the CB through a fast electron-phonon scattering process accompanied by heat release. In this case, excited electrons are not localized on the TNT molecules, nor is there any significant energy flowing into TNT molecules so as to initiate decomposition. In other words, even though O vacancies induce electron density transfer from the surface to the TNT molecules, molecular decomposition through optical channels (i.e., via the electronically excited state potential energy surface) at a large scale are unfavored. Therefore, the effects of oxygen vacancies on molecular decompositions will be mainly exerted through the enhancement of adsorption stability and electron density transfer from the surface to the molecules, rather than through optical transitions. Since the bonding between the TNT molecules and the TiO2 surface is significantly enhanced due to the electron density transfer when TNT molecules are adsorbed on the TiO2 surface with O vacancies, they are more opt to decompose if there exist some other forms of stimuli such as impact, shock, or heat. 3.5 Decomposition of energetic molecules adsorbed on TiO2 surface containing vacancies To look further into the effect of the TiO2 surface on the thermal stability of the TNT molecule, we investigate the most common decomposition pathway of nitro compounds. We explicitly simulated the 11 ACS Paragon Plus Environment


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predominant decomposition pathway of ground state TNT at high temperatures, 59,60,61 the C-NO2 homolysis reaction, in the isolated TNT molecule (Eq. 1) and in the TNT molecule adsorbed on the ideal TiO2 surface (Eq. 2) and the surface containing vacancies (Eq. 3): C7H5N3O6 → C7H5N2O4● + ●NO2 TiO2-C7H5N3O6 → C7H5N2O4● + TiO2-NO2● TiO2-Ovac-C7H5N3O6 → C7H5N2O4● + TiO2-Ovac-NO2●

(2) (3) (4)

The molecular decomposition path through the C-N bond cleavage is known to be a barrierless reaction59. For such a reaction, the reaction energy is equal to the reaction activation barrier. Our PBE calculations show that TNT adsorption on the perfect TiO2 surface slightly stabilizes the TNT molecule, increasing the C-N bond cleavage energy from 70 kcal/mol to 76 kcal/mol. However, when TNT molecules are adsorbed on the TiO2 surface containing vacancies, the C-N bond cleavage energy decreases significantly to only 52 kcal/mol (Table 1). We attribute this substantial reduction of the C-N bond cleavage energy to the enhanced bonding interaction between the TNT molecule and the TiO2 surface and to the electron density transfer from the surface to the TNT molecules. Note that when extra electron is introduced in the isolated molecule, it tends to delocalize all over the molecule and the effect on reducing the C-N bond cleavage energy is only minor. Finally, we would like to point out that even though the C-N bond fission energy in the presence of the surface vacancy is close to the second harmonic energy of our available laser (2.33 eV), light absorption in this case does not induce electron excitations attributed to the TNT molecules. Therefore, the efficacy of laser excitation on the molecular decomposition appears limited. 4. Conclusions In summary, we have chosen a highly catalytic oxide with near visible-light-edge band gap (TiO2), in which the nature of the O vacancy is different from that in an insulator like MgO, to explore the effects of the oxide additive on the adsorption, light response, and decomposition of energetic materials. We demonstrate that, contrary to the naive (but tempting) expectation that a highly catalytic oxide is always helpful for promoting optical molecular decompositions, the electronic orbital alignment between TiO2 and TNT does not introduce any electronic band gap states, and therefore no optical transitions corresponding to the first or second harmonic energy of available lasers are feasible. Nevertheless, the surface O vacancy strengthens the molecular adsorption of energetic molecules on the oxide surface and leads to electron density transferred from the surface to the molecules. This reduces the decomposition barrier of energetic molecules and prompts their decomposition, most likely, through thermal excitation. At the same time, there is no any obvious energy flowing into molecules by optical transitions upon light absorption. In other words, the O vacancies in the TNTTiO2 mixtures, while facilitating the reduction of the decomposition activation barrier, the detailed possible optical transitions have to be carefully analyzed, as the newly generated electronic states may or may not be consistent with realistic optical transitions and breaking the bonds through optical channels. 12 ACS Paragon Plus Environment

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Finally, we would like to emphasize that the present study has paramount physical implications regarding the applications of energetic materials. In the area of energetic materials and explosives, achieving controllable surface chemistry is an ambitious compelling yet challenging goal. This means that tunable sensitivity to initiate decomposition (and further possible detonation) of energetic materials can be realized (most promisingly) through modifying the ground state, excited or charged state potential surface. Usually, an extra charge on molecules may facilitate decomposition; however, our results suggest that while the localization of the extra charge is imperative, it can be completely negated by a delocalized charge distribution. An excited state can also facilitate decomposition; it nevertheless is difficult (or energetically costly) to prepare. In this regard, our study finds a route to either introduce localized charge density into molecules so as to reduce molecular decomposition barriers, or to localize the molecular states in the band gap of the oxide additive. While the former is spontaneously achieved when molecules are adsorbed on a surface with vacancies, the latter requires delicate band alignments of individual components at the interfaces, which might be achieved by searching oxide candidates with high conduction band positions, and, in turn, more ionic oxide candidates with their metal elements having small electronegativity. In any case, a high throughput searching approach can always be helpful. 5. Acknowledgments This research is supported by the US ONR (Grants N00014-12-1-0529 and N00014-16-1-2069) and NSF. Computational resources are supported by the NSF XSEDE (Grants TG-DMR150074 and TG-DMR-130077), DOE NERSC (Contract DE-AC02-05CH11231), and the Maryland Advanced Research Computing Center. MMK is grateful to the Office of the Director of National Science Foundation for support under the IRD program. Any appearance of findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF and other funding agencies.



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TOC Graphic:


Can a Photosensitive Oxide Catalyze Decomposition of Energetic

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