Plane-wave density functional theory investigation of adsorption of 2,4,6-trinitrotoluene on al-hydroxylated (0001) surface of (4 × 4) α-alumina

Plane-Wave Density Functional Theory Investigation ofAdsorption of 2,4,6-Trinitrotoluene on Al-Hydroxylated(0001) Surface of (4 3 4) a-Alumina

Manoj K. Shukla,* and Frances Hill

This article reports the results of the theoretical investigation of

adsorption of 2,4,6-trinitrotoluene (TNT) on Al-hydroxylated

(0001) surface of (4 3 4) a-alumina (a-Al2O3) using plane-wave

Density Functional Theory. Sixteen water molecules were used

to hydroxylate the alumina surface. The Perdew–Burke–Ernzer-

hof functional and the recently developed van der Waals func-

tional (vdW-DF2) were used. The interaction of electron with

core was accounted using the Vanderbilt ultrasoft pseudopoten-

tials. It was found that hydroxylation has significant influence

on the geometry of alumina and such changes are prominent

up to few layers from the surface. Particularly, due to the Al-

hydroxylation the oxygen layers are decomposed into sublayers

and such partitioning becomes progressively weaker for interior

layers. Moreover, the nature of TNT adsorption interaction is

changed from covalent type on the pristine alumina surface to

hydrogen-bonding interaction on the Al-hydroxylated alumina

surface. TNT in parallel orientation forms several hydrogen

bonds compared to that in the perpendicular orientation with

hydroxyl groups of the Al-hydroxylated alumina surface. There-

fore, the parallel orientation will be present in the adsorption of

TNT on Al-hydroxylated (0001) surface of a-alumina. Further, the

vdW-DF2 van der Waals functional was found to be most suita-

ble and should be used for such surface adsorption investiga-

tion. VC 2014 Wiley Periodicals, Inc.

DOI: 10.1002/jcc.23712

Introduction

Alumina is one of the most important materials in earth’s sur-

face while aluminum is one of the most abundant elements in

the earth’s crust.[1] Alumina has high insulating property and

structural stability over a large temperature range and has

wide applications in various areas of science and technol-

ogy.[1–4] It has different phases and the a-phase called as

a-alumina (a-Al2O3) is the most stable and exists naturally in

nature.[5,6] It is well known that in the presence of water, the

Al-terminated (0001) surface of a-alumina undergoes hydroxy-

lation and such hydroxylation plays an important role in the

chemistry of alumina surface. Therefore, it is not surprising

that the hydration of alumina surface has been an active area

of research using both the experimental and theoretical meth-

ods.[7–13] The Al-terminated alumina surface behaves as a

strong Lewis acid and such property originates from the low

coordination of surface Al atoms on clean a-Al2O3. This is

because the surface Al atoms are three-coordinated while the

interior Al atoms are six-coordinated with oxygen atoms. Con-

sequently, a water molecule in extremely low coverage regime

dissociates into H1 and OH2 ions and the resulting hydroxyl

ion binds with surface Al-atom and H1 bind with nearby sur-

face oxygen atom from the second layer of alumina.

Water binding on the metal-oxide surfaces is a complex

phenomenon and is affected by several factors such as surface

structure and acidity and basicity of surface metal and oxygen

sites, respectively.[8] For example, the dissociative nature of

water adsorption is more favorable on the a-Al2O3 (0001) sur-

face than on the MgO (100) surface and such distinct adsorp-

tion property is related to the fact that the surface Al atom is

tri-coordinated, and thus, shows much stronger Lewis acid

property than the surface Mg atom which is penta-coordi-

nated.[14] However, it should be noted that chemistry of the

Mg atom is also quite different than that of the Al atom.

Wang et al.[15] have performed detailed investigation of

adsorption and dissociation mechanisms of water on the Al-

terminated (0001) surface of a-alumina using the BLYP func-

tional with all-electron triple numerical basis sets and employ-

ing the unit-cell and (2 3 2) supercell slab model. It was

found that once the surface Al atom is hydroxylated by one

water molecule, further hydroxylation of this surface Al atom

would be very difficult. These authors concluded that a fully

hydroxylated surface of Al-terminated a-Al2O3 exhibits rela-

tively inert behavior for further hydroxylation. It appears that

by fully hydroxylated surface in this investigation,[15] the

authors mean surface Al-hydroxylation as reported in an earlier

investigation by Ranea et al.,[16] where the surface Al-atoms

are hydroxylated and nearby surface oxygen atoms are proto-

nated. Ranea et al.[16] have described fully hydroxylated Al-

terminated a-alumina surface in which each of the surface Al-

atoms are triply hydroxylated and all the surface oxygen

atoms are protonated. However, both of these investiga-

tions[15,16] are in agreement that the 1,2-dissociation path is

thermodynamically more preferable than the 1,4-dissociation

mechanism.[17] Al-Abadleh and Grassian[10] have studied the

adsorption of water on the a-alumina surface using the FTIR

spectroscopic technique and found that relative humidity has

M. K. Shukla, F. Hill

Environmental Laboratory, US Army Engineer Research and Development

Center, 3909 Halls Ferry Road, Vicksburg, MS 39180

E-mail: [email protected]

VC 2014 Wiley Periodicals, Inc.

Journal of Computational Chemistry 2014, DOI: 10.1002/jcc.23712 1

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significant influence on the FTIR spectra. For example, at low

relative humidity, which is less than 10%, water adsorption on

the alumina surface is in orderly fashion through the formation

of a stable hydroxide layer. But water molecules are adsorbed

nondissociatively in a structured overlayer within the interme-

diate relative humidity range (10–70%) while disordered water

adsorption is revealed at the high relative humidity.

We have recently performed, to the best of our knowledge,

the first plane-wave density functional theory (DFT) level of

investigation on adsorption of 2,4,6-trinitrotoluene (TNT) on

Al-terminated (0001) surface of (4 3 4) a-alumina.[18] We have

found that TNT is adsorbed strongly on the alumina surface

and consequent to adsorption the interacting surface Al-atoms

are pulled up toward the adsorbate. In the adsorbed complex,

TNT will orient in parallel to (0001) surface of a-alumina. Fur-

ther, based on the charge density difference analysis, the TNT

adsorption on alumina was found to have significant covalent

interaction. The surface of a-alumina in the presence of water

is hydroxylated and such hydroxylation depends on the per-

centage relative humidity present in the environment.[10,15–17]

Therefore, it is imperative to study the adsorption of munitions

compounds on the hydroxylated alumina surface. Therefore, in

this article, we have investigated the adsorption of TNT on Al-

hydroxylated (0001) surface of a-alumina using the plane-wave

DFT approach using Perdew–Burke–Ernzerhof (PBE) and

recently developed van der Waals vdW-DF2 functional. We

found that hydroxylation has significant influence on the

geometry of alumina. For example, due to the hydroxylation

the top most oxygen layer gets divided into three sublayers.

Further, it was also revealed that on the hydroxylated surface

the TNT adsorption is much weaker than on the pristine

(unhydroxylated) alumina surface. Similarity and differences

between the pristine and hydroxylated surfaces with respect

to the TNT adsorption is discussed.

Computational Details

All calculations were performed in three-dimensional periodic

boundary conditions under the generalized-gradient approxi-

mations (GGA) using the PBE functional[19] and a recently

developed high accuracy van der Waals density functional

(vdW-DF2).[20–23]

The vdW-DF2 functional is second version of van der Waals

density functional vdW-DF.[20–24] These functionals add long

range (nonlocal) correlations to local or semilocal correlation

functionals, and thus called nonlocal correlation functionals.

The vdW-DF2 uses a more accurate semilocal exchange func-

tional compared to the vdW-DF functional. In both functionals,

the nonlocal correlation energy Enlc is calculated using the

formula:

Enlc 5

ð ðdradrbnðraÞ/ðra; rbÞnðrbÞ (1)

In this equation, n(r) is electron density and /ðra; rbÞ is some

integration kernel. The exchange-correlation energy Exc in

these approaches is computed using the formula:

Exc5EGGAx 1ELDA

c 1Enlc (2)

The right hand side terms in the above equation are the

GGA exchange energy, the local density approximation correla-

tion energy and nonlocal correlation energy term, respectively.

In the vdW-DF functional, the EGGAx term is computed in the

revPBE approximation.[25] The main difference between PBE

and revPBE is the value of empirical coefficient (j) used in the

local exchange enhancement factor; the j being 0.804 in

PBE[19] and 1.245 in revPBE functional.[25] The revPBE provides

improved atomization energies and chemisorption energies

than the PBE functional.[25,26] The choice of revPBE exchange

in the vdW-DF functional was to ensure that van der Waals

binding effect comes from the correlation term in the approxi-

mation scheme.[20] However, vdW-DF functional overestimates

the long-range dispersion interaction and underestimates

hydrogen-bond strength.[23,24] These problems have been

addressed in the vdW-DF2 functional in which the revPBE

exchange is replaced with PW86 exchange, since revPBE is

generally too repulsive near the equilibrium separation while

PW88 is less repulsive.[23,24] Moreover, the screened exchange

(Zab) used in the vdW-DF functional was 20.8491 while

21.887 is used in the vdW-DF2 functional.[20,23]

All calculations were performed using Quantum Espresso

program, which is based on the plane-wave DFT.[27] Interaction

of electron-core was approximated by the ultrasoft pseudopo-

tentials available in the Quantum Espresso package. Moreover,

the ultrasoft pseudopotentials available for PBE functional

were also used for the vdW-DF2 functional. The wave function

cutoff of 25 Ry and the kinetic energy cutoff for charge den-

sity and potential as 250 Ry were used. Geometries of the bulk

systems were completely relaxed. However, during the surface

relaxation of hydroxylated alumina and for the complex of

TNT adsorbed on the hydroxylated alumina surface, the bot-

tom eight layers of the alumina were kept frozen to the

relaxed bulk geometry. Moreover, the full geometry optimiza-

tion of such systems was also performed. Further, since the

considered system was large, containing 549 atoms (among

them 512 are heavy atoms), geometry optimizations were per-

formed only at the gamma point.

We considered Al-terminated (0001) surface of (4 3 4) a-

Al2O3 in the present investigation. The considered super cell

contained 480 atoms with the top layer consisting of 16 Al

atoms. The super cell was constructed from the X-ray crystallo-

graphic geometry of a-Al2O3, which shows Al-terminated

(0001) surface where Al and oxygen atoms are arranged in the

AlOAlAlOAlAlO. . . type of layers totaling 18 layers stacked in

the c-direction; the lattice parameters of the hexagonal (1 3

1) unit cell were experimentally determined to be a 5 4.7602

A and c 5 12.9933 A.[28] The consideration of such a large (4 3

4) unit cell in the present calculation was necessary to avoid

self-interaction of adsorbate (TNT) with its periodic image

along the “a” and “b” directions. The nearest neighbor dis-

tance of TNT with its periodic image was found to more than

12 A in the present calculation. The adsorption energy or

binding (DEad) energy of TNT on Al-hydroxylated (0001) surface

of a-alumina was computed using the formula:

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2 Journal of Computational Chemistry 2014, DOI: 10.1002/jcc.23712 WWW.CHEMISTRYVIEWS.COM

DEad52ðEcomplex2ETNT2EhydroxylatedaluminaÞ

where Ecomplex represents the total energy of the complex and

ETNT and Ehydroxylatedalumina represent the total energy of the

TNT and Al-hydroxylated (4 3 4) a-Al2O3, respectively, within

the complex geometry.

Results and Discussion

Geometries of bulk and surface relaxed (4 3 4) Al-terminated

a-Al2O3(0001) obtained at the plane-wave DFT using the PBE

and vdW-DF2 functionals and ultrasoft pseudopotentials with

25 Ry of wave function cutoff have been discussed in our pre-

vious publications[18,29] and therefore, will be only very briefly

reported here. For the surface relaxation calculation, 20 A of

vacuum space was added along the c-direction. For the sur-

face relaxed structure, surfaces showed significant inward

relaxation and such relaxations were found to be limited to

only few layers. The maximum relaxation was revealed for the

top layer and consequently after surface relaxation this layer

(Al-containing plane) became very close to the second layer

(oxygen-containing layer). For example, using the PBE (vdW-

DF2) functional the second layer was computed to be 0.84

(0.83) A below the top layer in the bulk structure while such

distance was decreased to 0.12 (0.24) A in the surface relaxed

structure of (4 3 4) Al-terminated a-Al2O3 (0001).[18,29] Further,

in our earlier investigation, we have also shown that plane-

wave DFT is able to provide reliable molecular geometry of

TNT.[18]

Structure of Al-hydroxylated (0001) surface

of (4 3 4) a-Al2O3

The structure of the Al-hydroxylated surface was obtained by

hydroxylating each of the surface Al-sites with hydroxyl group

and protonating nearby oxygen sites of (4 3 4) a-alumina so

that the surface (and complete structure) remained neutral.

Thus, 16 water molecules were used in this process. Evidently,

all surface oxygen sites (from the top most oxygen layer) of

alumina are not protonated. The optimized geometry of Al-

hydroxylated (4 3 4) a-alumina is shown in the Figure 1. The

selection of oxygen sites for protonation is justified since ear-

lier investigations have shown that the 1,2-dissociation path

for hydration of alumina surface is thermodynamically most

preferable.[15,16] Further, to shed light on how the geometry of

alumina is modified due to surface hydroxylation, the top view

of the surface relaxed and Al-hydroxylated surface of alumina

containing only four top layers of alumina (AlOAlAl layers) are

shown in the Figure 2. It is evident from the Figure 2(a) that

for the unhydroxylated surface relaxed alumina each of the

surface Al atom is surrounded by three equidistant oxygen

atoms (O1, O2, and O3) from the second layer with AlAO dis-

tance of about 1.693 (1.696) A using the PBE (vdW-DF2) func-

tional. Thus, these bonded oxygen atoms form an equilateral

triangle and the position of Al atom can be projected at the

center of the triangle. Conversely, each of the oxygen atoms

(from the outer most oxygen layer) is bonded with three near-

est Al atoms each from the first (Al1), third (Al3), and fourth

(Al4) layers with bond distances of 1.693 (1.696), 1.811 (1.820),

and 1.900 (1.924) A, respectively, at the PBE (vdW-DF2) level

(Figure 2a). However, the corresponding distance with the

application of PW91 functional under plane-wave approxima-

tion using projector augmented wave approach and plane-

wave cutoff of 400 eV were found to be 1.703, 1.820, and

1.905 A, respectively.[16] Due to surface relaxation, the Al-

terminated layer comes very close to the oxygen-containing

second layer; therefore, it is not surprising that Al1AO2 bond

distance is the smallest and Al4AO2 bond distance is the larg-

est, where O2 is the oxygen atom from the second layer (Fig-

ure 2a). Moreover, it should be noted that for interior layers,

each oxygen atom is four-coordinated to Al atoms while each

Al atom is six-coordinated to oxygen atoms. Thus, each of the

surface oxygen atoms (in unhydrated alumina) are also under

coordinated compared to those belonging to interior layers.

We found that hydroxylation leads to a significant change in

the surface geometry of alumina and subsequently bond dis-

tances are substantially modified. Thus, in the Al-hydroxylated

alumina, each of the surface Al-atoms (in the hydroxylated

form) is bonded with three oxygen atoms from the second

layer of alumina, where one of the oxygen atoms (O1) is pro-

tonated (Figures 1 and 2). The Al1AO1(H) bond distance has

been predicted to be 1.859 (1.861) A and Al1AO2 and Al1AO3

bond distances are predicted to be 1.747 (1.752) and 1.758

Figure 1. Optimized geometry of Al-hydroxylated (4 3 4) a-alumina whose

bottom eight layers were fixed to the bulk alumina geometry. Inset shows

how the surface Al atom is hydroxylated and the nearby surface oxygen

atom is protonated. Ow represents water oxygen and O1, O2. . .. show how

various oxygen layers are decomposed into sublayers due to the surface

Al-hydroxylation. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]



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(1.764) A, respectively, using the PBE (vdW-DF2) functional

(Figure 2b). However, in the pristine surface the Al1AO (or

AlAO) bond distance was found to be 1.693 (1.696) A at the

PBE (vdW-DF2) functional. Thus, Al-hydroxylation leads to

asymmetric change among the AlAO surface bonds.

In Al-hydroxylated a-Al2O3 system, the first three layers cor-

respond to hydrogen, oxygen, and hydrogen layers and they

appear due to the hydroxylation of the surface Al-sites and

the protonation of the selected surface oxygen sites. The inter-

layer spacing between consecutive layers is shown in the Table

1. Next is the Al-layer, which is in the hydroxylated form and

belongs to the top layer of pristine alumina. After this layer is

the oxygen layer, which can be partitioned into three sub-

layers due to the surface Al-hydroxylation. The top one among

these sublayers contains O1 oxygen atom and equivalent sites

(O1 sublayer), which are protonated due to hydroxylation reac-

tion. This layer is about 0.56 A below the Al-layer at the PBE

level (Table 1). It should be noted that for the pristine surface-

relaxed alumina such interlayer distance was computed to be

only 0.12 A.[18,29] Such a large increase in the vertical interlayer

spacing by 0.44 A is due to the fact that surface Al-atoms are

pulled-up due to the hydroxylation and the protonation of

oxygen sites. Moreover, the coordination of surface Al atoms is

also increased due to the hydroxylation. Further, although the

O1 and equivalent sites are also protonated and pulled up ver-

tically, the weakening of the Al1AO1(H) bond would also aid

upward shift of surface Al atoms. The second sublayer con-

tains O2 oxygen atom and equivalent sites and this sublayer

(O2 sublayer) is about 0.17 A below the O1 sublayer. Con-

versely, the O3 oxygen atom and equivalent sites form the

Figure 2. Top view of fully optimized geometry of (a) surface relaxed, and (b) Al-hydroxylated (4 3 4) a-alumina obtained using the PBE functional. Only

top four layers of alumina are shown. Further, in (a) only a portion of surface area is shown. Bond distances are in A. Small insert at the right top corner in

(b) shows the side view of a portion of a-alumina. For clarity, atoms with golden, gray, and blue colors are Aluminum belonging to the 1st, 3rd, and 4th

layer of pristine a-alumina, respectively. Spheres with red color represent oxygen and small spheres in the light gray color represent hydrogen atoms. Fur-

ther, Al1, Al3, and Al4 show atoms from the 1st, 3rd, and 4th layers, respectively. Conversely, O1, O2, and O3 all belong to the 2nd layer only.



third oxygen sublayer (O3 sublayer) and this layer is very close

to the second sublayer (O2 sublayer) with a vertical distance

of only about 0.03 A (Figure 1 and Table 1). The next two

layers are Al-containing layers, and they are separated by

about 0.33 A, the corresponding interlayer spacing for unhy-

droxylated alumina was found to be 0.27 A at the similar level

of the theory (PBE functional). The next oxygen layer can again

be divided into three sublayers, but interlayer distances have

been significantly reduced indicating that surface hydroxyla-

tion has a significant influence on only up to few layers from

the top, and only limited influence has been revealed for fur-

ther interior layers from the surface (Table 1 and Figure 2).

Similar results were also revealed when recently developed

van der Waals density functional (vdW-DF2) theory was used

in the calculation (Table 1).

Adsorption of TNT on Al-hydroxylated (0001) surface of (4 3

4) a-Al2O3

In the present work, we have considered both the vertical and

parallel orientations of TNT with respect to the (0001) surface of

hydroxylated alumina. The optimized geometry of TNT

adsorbed on Al-hydroxylated alumina surface in vertical orienta-

tion obtained using the vdW-DF2 functional is shown in the Fig-

ure 3. It is clear from this figure that both oxygen atoms of the

nitro group at the C4 site of TNT are involved in the adsorption

interaction with hydroxyl groups of hydroxylated alumina sur-

face and these hydroxyl groups are rotated to maximize interac-

tion with the adsorbate (TNT). Selected geometrical parameters

of the complex obtained using PBE and vdW-DF2 functionals

are shown in the Table 2. The computed adsorption distances,

O41� � �HOw1 and O42� � �HOw2, are predicted to be 2.054 and

2.094 A, respectively, using the PBE functional and 2.021 and

2.052 A, respectively, using the vdW-DF2 functional (Table 2

and Figure 3). Further, we have also compared the geometrical

parameters of TNT before and after adsorption on the hydroxy-

lated alumina surface, but a significant change was not

revealed. Moreover, the comparison of the geometrical parame-

ters of hydroxylated alumina surface with and without TNT

adsorption also did not reveal significant change, except the

rotation of surface hydroxyl groups involved directly in the

adsorption interaction (Table 2 and Figure 3). The computed

adsorption energy of TNT on the Al-hydroxylated alumina sur-

face using the PBE functional was predicted to be 5.0 kcal/mol,

while using the vdW-DF2 functional, it was found to be 8.3

kcal/mol. However, for complexes whose eight bottom alumina

layers were kept frozen to the bulk geometry in the geometry

optimization, the computed adsorption energy was found to be

4.2 and 7.8 kcal/mol using the PBE and vdW-DF2 functionals,

respectively. In our earlier investigation, we found that for the

same orientation of TNT on the pristine (4 3 4) Al-terminated

a-alumina surface would have adsorption energy of about 25.2

kcal/mol using the PBE functional.[18] Evidently, the hydroxyla-

tion provides screening effect and TNT is only weakly adsorbed

on Al-hydroxylated alumina surface. However, such a weak

adsorption of TNT in vertical orientation on hydroxylated sur-

face of alumina is understandable since adsorption did not

cause significant change on either of the geometries of the

adsorbate and the adsorbent, except that hydroxyl groups of

surface involved in direct interaction are rotated.

Table 1. Interlayer spacing (A) of Al-hydroxylated (4 3 4) a-alumina before and after adsorption of TNT.

PBE vdW-DF2

Partial optimized Full optimized Full optimized

Layer Hydrox 1TNTV Isolated Hydrox 1TNTV Isolated Hydrox 1TNTV 1TNTP

H – – – – – – – – –

O 0.53 0.54 – 0.55 0.55 – 0.56 0.57 0.62

H 1.49 1.48 – 1.48 1.48 – 1.49 1.48 1.45

Al 0.23 0.23 – 0.23 0.23 – 0.23 0.23 0.25

O1 0.56 0.56 0.12 0.56 0.56 0.24 0.57 0.56 0.55

O2 0.16 0.16 – 0.17 0.17 – 0.19 0.20 0.21

O3 0.02 0.03 – 0.03 0.03 – 0.03 0.03 0.03

Al 0.85 0.84 0.89 0.84 0.84 0.90 0.84 0.84 0.83

Al 0.34 0.34 0.27 0.33 0.33 0.28 0.35 0.35 0.35

O4 0.92 0.93 1.02 0.93 0.93 1.02 0.93 0.93 0.94

O5 0.02 0.02 – 0.02 0.02 – 0.02 0.02 0.01

O6 0.01 0.01 – 0.01 – – – –

Al 0.85 0.85 0.90 0.85 0.85 0.89 0.86 0.86 0.86

Al 0.50 0.50 0.45 0.50 0.50 0.47 0.52 0.52 0.52

O7 0.84 0.84 0.87 0.84 0.84 0.87 0.85 0.85 0.84

O8 0.02 0.02 – 0.02 0.02 – 0.02 0.02 0.02

Al 0.84 0.84 0.84 0.83 0.83 0.85 0.84 0.84 0.84

Al 0.49 0.49 0.52 0.50 0.50 0.53 0.52 0.52 0.52

For comparison, the interlayer spacing of surface relaxed (4 3 4) a-alumina is also given. For explanation of layers, see Figure 1. In partial optimized

geometry, the bottom eight layers of alumina were kept frozen to its optimized bulk geometry. Hydrox refers to Al-hydroxylated alumina, 1TNTV and

1TNTP refer to the complex of adsorbed TNT on Al-hydroxylated a-alumina in vertical and parallel orientations, respectively, and Isolated refers to sur-

face relaxed a-alumina. Further, for isolated alumina, the O1, O2, and O3 belong to second layer, O4, O5, and O6 belong to 5th layer, O7 and O8 belong

to 8th layer.




Figure 4 shows the optimized geometry of adsorbed com-

plex of TNT in parallel orientation on the Al-hydroxylated a-

Al2O3 surface using the vdW-DF2 functional. It is clear that

complex geometry is stabilized by the presence of several

hydrogen bonds with hydrogen bond distance ranging

between 2.039 and 2.989 A. Notably, nitro groups at the C2

and C6 sites of TNT and some of the hydroxyl groups of the

surface are rotated to form these hydrogen bonds. Moreover,

a methyl hydrogen and C3H site also form weak hydrogen

bonds with OW2 and OW4 sites, respectively, with respective

hydrogen bond distance of 2.393 and 2.989 A. The computed

adsorption energy of TNT in parallel orientation on Al-

hydroxylated a-alumina surface was found to be 27.2 kcal/mol,

and this energy is over three times larger than the complex in

which TNT is adsorbed in the vertical orientation. The adsorp-

tion energy of TNT in parallel orientation on (0001) surface of

pristine (4 3 4) a-alumina at the vdW-DF2 functional level was

found to be about 68.3 kcal/mol. Thus, computed adsorption

energy of TNT on Al-hydroxylated surface is significantly

smaller than on the pristine alumina surface. However, com-

puted adsorption energy of 27.2 kcal/mol is still substantial

and originates due to the existence of multiple hydrogen

bonds.

To evaluate the influence of van der Waals corrected DFT

functional, the geometry of the complex of TNT in parallel ori-

entation on the Al-hydroxylated alumina surface was also opti-

mized using the PBE functional. For this optimization, the

vdW-DF2 optimized geometry was used. The optimized geom-

etry of the complex at the PBE level was found similar to that

obtained at the vdW-DF2 functional as shown in the Figure 4.

The hydrogen bond distances in the complex as revealed by

the both functionals are shown in the Table 3. It is clear from

the data shown in the Table 3 that computed hydrogen bond

distances at the vdW-DF2 level are consistently shorter than

the corresponding distance obtained using the PBE functional,

except the methyl hydrogen interacting with the Ow2 site,

Table 2. Selected bond distances (A) of complex formed by TNT adsorp-

tion on the Al-hydroxylated (0001) surface of (4 3 4) a-alumina in vertical

orientation.[a]

Bond

PBE vdW-DF2

Partial

optimized

Full

optimized

Partial

optimized

Full

optimized

O41� � �HOW1 2.065 2.054 2.023 2.021

O42� � �HOW2 2.111 2.094 2.071 2.052

C4AN4 1.479 1.477 1.490 1.488

N4AO41 1.242 1.243 1.256 1.257

N4AO42 1.242 1.244 1.256 1.257

OW1AH 0.969 0.971 0.969 0.969

Al1AOW1 1.709 1.706 1.717 1.713

Al1AO1 1.859 1.860 1.860 1.862

Al1AO2 1.751 1.751 1.756 1.756

Al1AO3 1.754 1.755 1.759 1.760

OW2AH 0.972 0.972 0.972 0.972

Al2AOW2 1.708 1.705 1.716 1.712

Al2AO4 1.859 1.861 1.862 1.863

Al2AO5 1.747 1.748 1.751 1.751

Al2AO6 1.762 1.762 1.767 1.767

[a] For explanation of bond, please see Figure 3.

Figure 3. Adsorbed complex of TNT on Al-hydroxylated (4 3 4) a-alumina surface in vertical orientation obtained through full geometry optimization. The

TNT� � �a-alumina adsorption distances are in A and correspond to vdW-DF2 (PBE) functional level. Values of different bond distances are presented in the

Table 2. The periodic boundaries are not shown in the Figure. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]




which was found to be about 0.006 A longer. The adsorption

energy at the PBE functional was computed to be 12.3 kcal/

mol and it is about 15 kcal/mol smaller than that obtained

using the vdW-DF2 level. Thus, the computed larger stability

of the complex at the vdW-DF2 functional than the PBE func-

tional is due to the inclusion of van der Waals correction and

the presence of less repulsive exchange function in the former

functional (vdW-DF2)[20–24] which consistently predicts shorter

hydrogen bond distance than obtained using the former (PBE)

functional.

It should be noted that all computations have been per-

formed at 0 K. Further, due to the large size of the considered

systems vibrational frequency analysis cannot be performed to

account for the temperature effect. However, experimentally

determined adsorption entropy term (TDS) for adsorption on

mineral surfaces is around 10 kcal/mol at room tempera-

ture.[30–34] Correction of computed adsorption energies for both

orientations of TNT on Al-hydroxylated alumina surface with

entropy term would lead to destabilized adsorption for vertical

orientation and about 17 kcal/mol of stabilized adsorption for

parallel orientation of TNT using the vdW-DF2 functional. More-

over, the inclusion of such correction on adsorption energy

obtained using the PBE functional would yield stabilized com-

plex of TNT in the parallel orientation by about 2 kcal/mol. This

indicates that only the parallel orientation of TNT on Al-

hydroxylated surface of alumina would be stable at the room

temperature and van der Waals corrected DFT functional such

as vdW-DF2 should be used for such surface adsorption investi-

gation. Thus, TNT will be adsorbed on the Al-hydroxylated a-alu-

mina surface in parallel orientation. Further, the comparison of

geometrical parameters of TNT and hydroxylated alumina in the

complex geometry to the corresponding isolated forms did not

reveal significant change, except of some rotation of nitro

groups of TNT and hydroxyl group of surface and slight change

among no bonds of nitro groups due to adsorption. Thus, in

parallel adsorption of TNT also the geometries of adsorbate and

adsorbent will not show significant change.

Charge density difference map and the nature of adsorption

The electron density difference maps due to the adsorption of

TNT on Al-hydroxylated (4 3 4) a-Al2O3 surface generated

using the VESTA program[35] is shown in the Figure 5. These

maps were obtained by subtracting the individual charge den-

sities of TNT and Al-hydroxylated alumina within the adsorbed

complex geometry from the charge density of the adsorbed

complex using the vdW-DF2 functional. Moreover, in these

plots, the isosurface with yellow (green) color indicates the

region of electron gain (loss) due to the adsorption.

Figure 5a shows that there is a small amount of electron

gain near the both oxygen atoms (O41 and O42) of nitro

Figure 4. Adsorbed complex of TNT on Al-hydroxylated (4 3 4) a-alumina surface in parallel orientation obtained through full geometry optimization. The

TNT� � �a-alumina adsorption distances are in A and correspond to vdW-DF2 functional level. The periodic boundaries are not shown in the Figure. [Color

figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Table 3. Hydrogen bond distance (A) in the complex of TNT adsorbed in

parallel orientation on the Al-hydroxylated (0001) surface of (4 3 4) a-

alumina obtained through full geometry optimization using PBE and

vdW-DF2 methods.[a]

Bond vdW-DF2 PBE

O61� � �HOw1 2.061 2.109

H(CH3)� � �Ow2 2.393 2.387

O21� � �HOw2 2.039 2.064

O21� � �HOw3 2.300 2.359

C3H� � �Ow4 2.989 3.105

O41� � �HOw4 2.523 2.634

O42� � �HOw6 2.897 2.923

O42� � �HOw5 2.726 2.970

[a] For explanation of bonds, please see Figures 3 and 4.





group of TNT involved in the adsorption and such buildup of

electron density is in the bonding (adsorption) direction. Con-

versely, there is some electron loss in the bonding direction of

hydroxyl hydrogen atoms involved in the adsorption of TNT in

the vertical orientation. Thus, there are buildup of small

amount of negative charge near nitro group oxygen atoms

(O41 and O42) and small amount of positive charge near the

hydrogen atoms of hydroxyl groups (directly involved in the

adsorption) and these charge buildups are in the bonding

direction. Similarly, for adsorbed complex formed due to the

parallel orientation of TNT there is small buildup (gain) of neg-

ative charge at oxygen sites of TNT directly involved in the

adsorption interaction and a small buildup of positive charge

at corresponding hydroxyl hydrogen sites, and these buildups

are in the bonding direction. Similarly, there are small buildup

of positive charge at a methyl hydrogen and C3H site while

small buildup of negative charge at the corresponding

hydroxyl oxygen sites (Figure 5b). Buildup of such charges at

the bonding centers in the direction of bond is characteristic

feature of hydrogen bond. Thus, the adsorption of TNT on Al-

hydroxylated (4 3 4) a-Al2O3 surface is characterized by the

hydrogen bonding interaction between relevant complemen-

tary sites of adsorbate and adsorbent. The hydrogen bonding

nature of adsorption interaction is also consistent with the

computed adsorption energy, which has been predicted to be

significantly smaller than covalent adsorption on the pristine

alumina surface.

Conclusions

We have studied the adsorption of TNT on Al-hydroxylated (4

3 4) a-Al2O3 surface using PBE and vdW-DF2 functionals and

latter functional was found to be most suitable for such inves-

tigation. We found that hydroxylation has significant influence

on the geometry of alumina and such influences are

prominent only on a few layers from the surface. Due to the

Al-hydroxylation the second layer (oxygen atoms containing

layer) of pristine alumina surface has been found to be

decomposed into three sublayers, namely O1, O2, and O3; the

vertical distance between O2 and O3 layers was predicted to

be much smaller than that between the O1 and O2 sublayers.

Such a partition of oxygen layers due to surface hydroxylation

of alumina becomes progressively weaker as going away from

the surface. TNT will be adsorbed in parallel orientation on the

Al-hydroxylated a-alumina surface. Adsorption of TNT on the

Al-hydroxylated alumina surface is much weaker than on the

pristine alumina surface and such adsorption (on the hydroxy-

lated surface) is characterized by the hydrogen bonding inter-

action between relevant complementary sites of TNT and

hydroxyl groups of Al-hydroxylated a-alumina. The nature of

such interaction was validated from the charge density differ-

ence map, which showed the buildup of small amount of

opposite charges at the bonding centers in the direction of

bond. Further, due to the adsorption, a noticeable change

among the geometries of adsorbate and adsorbent, except

the orientation of surface hydroxyl group and rotation of nitro

groups of TNT, was not revealed.

Acknowledgments

The use of trade, product, or firm names in this report is for descrip-

tive purposes only and does not imply endorsement by the U.S. Gov-

ernment. The tests described and the resulting data presented

herein, unless otherwise noted, were obtained from research con-

ducted under the Environmental Quality Technology Program of

the United States Army Corps of Engineers. Permission was granted

by the Chief of Engineers to publish this information. The findings of

this report are not to be construed as an official Department of the

Army position unless so designated by other authorized documents.

The authors thank Dr. Michael Cuddy of USACE and Dr. Leonid Gorb

of BTS, LLC for their editorial comments.

Figure 5. Isosurface depicting change in electron density after adsorption of TNT on Al-hydroxylated surface of (4 3 4) a-Al2O3 (0001) using the vdW-DF2

functional in (a) vertical and (b) parallel orientations. Yellow region shows isosurface for gain and green region shows the isosurface for the loss of charge

density. Isosurface corresponds to 0.001 e/A3. The (0001) surface boundaries have been removed in this figure.



Keywords: TNT adsorption � a-alumina � surface relaxation �plane-wave DFT � PBE � vdW-DF2 � ultrasoft pseudopotential �Al-hydroxylation

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Received: 19 March 2014Revised: 12 June 2014Accepted: 3 August 2014Published online on 00 Month 2014



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