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Thermo-Stimuli Response of Doped MAu Clusters (n = 4 - 8, M = Si, Ge) at Discrete Temperatures: A BOMD Undertaking Krati Joshi, and Sailaja Krishnamurty J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05373 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on August 3, 2017
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The Journal of Physical Chemistry
Thermo-Stimuli Response of Doped MAun − Clusters (n = 4 - 8, M = Si, Ge) at Discrete Temperatures: A BOMD Undertaking. Krati Joshi* and Sailaja Krishnamurty Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi-630 006, India ABSTRACT Gold clusters are the emblematic members of nano functional materials with unique properties that are customizable by incorporating an impurity within them. One such contrivable property is their catalytic activity. Experimental investigations have demonstrated that on adding group-14 based atomic impurities such as Si, Ge and Sn, Au clusters demonstrate augmented catalytic behavior. The present work explores the structural response of such experimentally derived and catalytically active Si and Ge doped gold clusters (Aun M− (n = 4 - 8; M = Si, Ge) at finite temperatures. The study is carried out using Born-Oppenheimer Molecular Dynamics (BOMD) under the framework of Density Functional Theory (DFT). Dynamical simulations reveal that, the presence of Si impurity is seen to impart remarkably higher thermal stability in some cases as compared to Ge or even the parent Au cluster. In some other cases, both the dopant atoms are seen to reduce the thermal stability by 200-300 K with respect to the parent Au cluster. The enhanced or reduced dynamical stability of dopant clusters is explained on the basis of underlying structural arrangement in atoms and ensuing electronic properties. Amount of charge retained on the impurity atom and the contributions of the impurity atom towards the Frontier Molecular Orbitals (FMO) are seen to play a role in contriving the stability of a given cluster.
I.
INTRODUCTION.
W@Au12 − , envisaged using Density Functional Theory (DFT) and later confirmed using Photo-electron Spec-
Versatility of gold at the subnano cluster level has been
troscopy (PES).12,13 Likewise, Au12 − gives rise to an
debated over the past two decades using both experi-
endohedral geometry on doping with V, Nb and Ta.14
mental as well as theoretical methodologies.1–7 Owing to
Lievens et al15,16 have studied 3d-transition metal doped
their excellent catalytic potential, many attempts have
gold clusters using mass spectroscopy and shown that
been made to customize their properties in order to de-
dopant atoms act as electron donors. In this context,
sign a new genre of nano functional catalysts.8–11 Dop-
doping of gold clusters by group-14 atoms is interesting.
ing with an hetero atom is one of the viable alterna-
It is established in a number of reports that, group-14
tives available in order to orchestrate a wider range of
atoms can significantly re-mould the stability, structure
chemical properties with reference to its parent cluster.
and electronic properties of not only gold but also that
The first highly stable doped gold cluster was icosahedral
of the other transition metal based systems.17–19
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A noteworthy report in this context is an experimental
reduces the activation barriers towards the formation of
realization of Si, Ge and Sn doped anionic gold clusters
CO2 from CO and O2 . Despite a plethora of experi-
(MAux − , x = 5-8 and M = Si, Ge, Sn) by Pal et. al,20
mental measurements28,29 and theoretical reports,30–32
who identified their structures, using combined PES and
which have been devoted to examine the electronic and
theoretical methods. The study reveals that the struc-
structural properties of doped gold clusters (Aun M, M
tural deviation from the parent gold cluster is dependent
= dopant atom, n = 3-5), no studies have been applied
on both the dopant atom as well as the size of the clus-
to evaluate the response of the doped structure to finite
ter. For example, on Si doping, Au6 − cluster transforms
temperature. The thermal behavior determines the pos-
to a tetrahedral based 3D structure. On the other hand,
sibility of a particular conformation to exist in its catalyt-
GeAu5 − , SnAu5 − have quasi-planar structures. Parent
ically active state at realistic and working temperatures.
Au6 − is a trigonal planar conformation. For MAux − (M
Hence, addressing the question of thermal stability is im-
= Si, Ge and Sn, x = 6 and 7), all the three doped
portant, particularly it may help in designing new hybrid
systems have similar quasi-planar structures. SiAu8 −
Au-M nanostructures for potential applications such as
has a 3D structure with a dangling Au-Si bond while,
catalysis, micro-electronics etc.
GeAu8 − and SnAu8 − retain the quasi-planar structures. The largest experimentally studied group-14 atom doped
To some extent, issues related to thermal stability and
anionic gold cluster is Au16 − .21 The study reveals that
finite temperature behavior have been addressed for pris-
Au16 Si− is an exohedral structure with Si on the cage
tine or bare gold clusters.33–36 However, to the best of
wall and the overall cluster is decorated by the presence
our knowledge there is no systematic study which evalu-
of a dangling Au-Si bond. On the other hand, Ge and Sn
ates the finite temperature response of doped gold clus-
doped Au16 − exhibit exohedral structures without any
ters. Finite temperature studies on other doped sys-
dangling Au-Ge/Au-Sn bonds. In short, nature of the
tems such as carbon doping in silicon,37,38 Li doping in
group 14 dopant atom impacts the final conformation of
Aluminum39,40 , Sn doping in Lithium41,42 , C doping in
a gold cluster. In particular Si is seen to influence the
Al/Ga43 etc. have demonstrated that addition of im-
structure in a different manner as compared to Ge and
purity regulates the thermal stability of a system, while
Sn atoms.
imparting higher catalytic activity to the same. Hence, in the present work, we evaluate the thermal response of ex-
Accordingly, Si doping in gold clusters is most widely
perimentally reported Si/Ge doped Aun − (n = 4-8) clus-
studied using theoretical methodologies.22–26 Manzoor
ters and compare the same with those of undoped clus-
et. al.27 have recently studied the effect of doping with
ters. To fulfill this purpose, Born-Oppenheimer Molecu-
Si on the reactivity of Au7 0 cluster. They have shown
lar Dynamical simulations are performed on Aun M− (n =
that presence of a Si atom within Au cluster not only en-
4-8; M = Si/Ge/Au) clusters at finite temperatures. The
hances the O2 activation on Au7 0 but also significantly
results obtained are correlated with the interconnected
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using a linear combination of Gaussian orbitals as implemented in deMon 2.2.6 code.44 All the conforma(a) Au4 Si
(b) Au4 Ge
(c) Au5 -Bare
tions are optimized using the Perdew-Burke-Ernzerhof (PBE) exchange and correlation functional45 with 1997 Stuttguard-Dresden Relativistic Effective Core Potentials (RECPs)46,47 as the basis set for the valence electrons of gold. It is well established that, relativistic effects play an important role in describing the structure-
(d) Au5 Si
(e) Au5 Ge
(f ) Au6 -Bare
property correlation within gold clusters.1–4 5s, 5p, 5d and 6s electrons are considered to constitute the valence electrons for Au atoms. Double Zeta Valence Polarization (DZVP)48 basis set is used for Si and Ge atoms. No
(g) Au6 Si
(h) Au6 Ge
(i) Au7 -Bare
additional polarization functions are added. The GENA2 auxiliary functions are used to fit the charge density.49 The convergence of the geometries is based on gradient and displacement criteria with a threshold value of 10−5
(j) Au7 Si
(k) Au7 Ge
(l) Au8 -Bare
a.u and the criteria for convergence of an SCF cycle was set to 10−9 a.u. Figure 1 shows the conformations of bare Aux − (x = 5-9) and Aun M− clusters ( where n = 4 - 8; M = Si/Ge) considered in the study. The doped confor-
(m) Au8 Si
(n) Au8 Ge
(o) Au9 -Bare
Figure 1. Optimized conformations of experimentally reported Si (a, d, g, j, m) and Ge doped (b, e, h, k, n) Aun M− clusters (n = 4 - 8). Conformations reported for the corresponding Aux − (x = 5 - 9) clusters (c, f, i, l, o) are also shown.
mations are experimentally reported conformations20,50 while the undoped conformations correspond to the lowest energy conformations reported in the literature.51 All of the above conformations are initially optimized under the conditions mentioned above. The finite temperature
structural characteristics, building units and electronic properties.
behavior of the optimized conformation is then studied using Born-Oppenheimer Molecular Dynamics (BOMD) with the same exchange-correlation and RECPs stated
II.
COMPUTATIONAL DETAILS
above. The simulations are carried out between 300 K and 2000 K with an interval of 100 K.
All the calculations in the study are performed under the framework of Density Functional Theory (DFT),
At each temperature, the cluster is equilibrated for a
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time period of 10 ps followed by simulation time of 70
pyramidal structure. In contrast to doped clusters, Au5 −
ps. The temperature of the cluster is maintained using
has a planar ground state geometry. Interestingly, there
Berendeson’s thermostat (τ = 0.5 ps) in an NVT ensem-
are no Au-Au bonds in the Au4 Si− tetrahedra while
ble. It is well known that Noose-Hoover thermostat is
Au4 Ge− contains four Au-Au bonds. Relevant geometri-
superior then Berendeson’s thermostat. However, in one
cal parameters such as inter-atomic bond lengths, bond
of our earlier study,36 we have verified the molecular dy-
angles etc. are tabulated in the Figure 2. The Au-Si
namics results obtained by the two thermostats at lower
bond length is 2.33 ˚ A in Au4 Si− tetrahedra and Au-Si-
temperatures and found them to be consistent with each
Au angle is 109°. Typically, Au-Si bond lengths in var-
other. The nuclear positions are updated using a velocity
ious silicon doped Au clusters30 reported earlier range
Verlet algorithm with a time step of 1 fs. We hold the
between ∼2.34 - 2.6 ˚ A. Thus, the value noted in this case
total angular momentum of the cluster to zero, thereby
(Au4 Si− ) is at the lower end of the range. In an earlier
suppressing the cluster rotation.
work, Au4 Si− has been reported as an exceptionally sta-
The atomic positions and bond length fluctuations are
ble compound which can be synthesized in isolation50 on
analyzed using traditional parameter such as Root Mean
account of strong Au and Si interaction. The short Au-Si
Square Bond Length Fluctuations (δrms ) The δrms is de-
bond length, an ideal tetrahedral structure and the na-
fined as:
ture of the chemical bonding in SiAu4 has a one-to-one correspondence to that in SiH4 as suggested by Wang
δrms =
2 N (N − 1)
q 2 i − hR i2 X hRij t ij t i
