First Principle Study: Adsorption of Molecular Hydrogen Sulphide on Gold Clusters

ABSTRACT

We present theoretical results of the study of H2S adsorption on gold cluster Aun(n = 1; 5) using density functional theory with Perdew-Burke-Ernzerhof (PBE) exchange-correlation energy functional. Minimum energy structures of the gold cluster along with their isomers are considered in the optimization process. H2S molecule is observed to adsorb on to the gold cluster. However, the adsorption energy decreases with increasing cluster size. The structure of the gold clusters are similar before and after adsorption of H2S molecule. The structure of gold cluster remain planar. The adsorbed molecule get adjusted in a way that their center of mass lie on the plane of the gold cluster.

The adsorbed molecules get attached to a single gold atom and there is no preference to get adsorbed in between the gold. H2S dissociation is not favoured on the Au clusters.

TABLE OF CONTENTS

1 Introduction 7
2 Density functional theory 12
2.1 Validity of non-local potentials . . . . . . . . . . . . . . . . . 15
2.2 Validity of frozen core approximation in total energies studies 16
2.3 Local Density Approximation . . . . . . . . . . . . . . . . . . 19
2.3.1 Plane-Wave Pseudo potential Method . . . . . . . . . 20
2.3.2 Pseudo potential approximation . . . . . . . . . . . . 21
2.3.3 Ultra-Soft (Vanderbilt) pseudo potentials . . . . . . . 22
2.3.4 Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4 Fermi Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.5 Density of states . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.6 Computational Details . . . . . . . . . . . . . . . . . . . . . . 25
3 General consideration 28
3.1 Au clusters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.1.1 Electronic properties DOS . . . . . . . . . . . . . . . . 32
3.1.2 Projected Density of state . . . . . . . . . . . . . . . . 33
3.2 Molecular Hydrogen and Hydrogen Sulphide . . . . . . . . . . 34
3.2.1 Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.2.2 Hydrogen Sulphide . . . . . . . . . . . . . . . . . . . . 34
3.2.3 Electronic properties:DOS and Proj-DOS . . . . . . . . 35
3.3 H2S attached to Au clusters . . . . . . . . . . . . . . . . . . . 36
3.3.1 Electronic properties: DOS and Proj-DOS . . . . . . . 40
3.3.2 Dissociation of H2S on Aun clusters . . . . . . . . . . 42
4 Conclusion and Summary 44

CHAPTER ONE

1 Introduction

Finite gold clusters and gold-containing nanometer-sized structures are currently being investigated because of their applications in a wide variety of areas, including medical science[1], molecular electronic devices[2-4], catalysis[5-7] and chirality of nano structures[8,9]. The structural and electronic properties of gold clusters and their compounds have been studied using various theoretical and experimental methods. It is also noted that nano size noble metal clusters and compounds have been attracting attention not only in areas of medicine, catalysis, but also for fabrication of nano devices and other applications due to their unique physical and chemical properties which de-
pend strongly on cluster size[10,11].

Nano sized gold clusters have been studied as a new kind of catalyst to reduce air pollution[12,13]. The free and supported gold clusters can efficiently catalyze the CO oxidation reaction at low temperatures. Au8 is found to be the smallest catalytically active cluster. The adsorption properties of NO molecule on small gold clusters have been studied theoretically as well as experimentally in order to understand the mechanism of catalysis. Hakkinen et al.[10] investigated the atomic and electronic structures of the neutral and anionic Au2􀀀10 clusters using density functional theory (DFT) with the generalized gradient approximation GGA. They found two-dimensional (2D) structures up to seven atom for neutral clusters and six atom for anionic clusters.

There are several literatures available on Au surfaces and cluster. Wang et al [14], have reported local density approximation LDA based DFT studies of neutral Aun(n = 2 􀀀 20) clusters and found two dimensional structures up to six gold atoms. On the other hand, two dimensional structures are found by Fernandez et al.[12] up to 12 atoms in the anionic, 11 atoms in the neutral, and 7 atoms in the cationic clusters in the study of the electronic and structural properties of the noble metal clusters Cu, Ag, and Au in their neutral and charged states. DFT study predicts that all Aun(n= 1-6) clusters with different charged states adsorb NO molecule where Au􀀀 3 cluster shows least stability. Laser-ablated gold clusters react with the NO molecule, in excess argon and neon, yielding the neutral nitrosyl complexes AuNO and AuNO2 as the main product. Varganov et al [15], have studied the reaction of molecular hydrogen with the dimer and trimer gold clusters using DFT, second order perturbation theory MP2, and coupled cluster (CCSDT) methods. They report that up to two H2 molecules easily bind to neutral Au2 and Au3 clusters but molecular hydrogen does not form stable complexes with Au􀀀

2 and Au􀀀

3 clusters. The preference of two dimensional structures in Au clusters has been attributed to the existence of strong relativistic effects, which enhances the s-d hybridization by shrinking the size of the 6s orbitals. Pyykko[16] has explained the large relativistic effects for high Z elements due to interacting relativistic and shell structure effects, higher shells being orthogonal to the lower ones with same “l” value. Study of ion mobility measurements[17] on gold cluster cations Au+ for n > 14 show planar structure up to the seven atom gold cluster.

Calculations on the interaction of sulfur containing molecules with gold clusters have been carried out to understand the chemistry at the gold-sulfur interface. To investigate the chemisorption of alkane thiolates on gold surfaces, Sellers et al.[18] studied Au16SCH3 cluster to be used as a model for the thiolate-Au (111) surface. In their work the geometry was optimized by varying the AuS distances and the AuSC angle only. A similar model calculation
was also performed by Beard-more et al.[19] for Au17SCH3 and Majumder et al.[20] for Au24SCH3. In both cases, the emphasis was to understand the chemical interactions at the gold-sulfur interfaces. Thiol-terminated molecular diodes interacting with the noble metal clusters (Cu, Ag, and Au) have been investigated[17] using DFT, which have particular implications in the held of nano-electronics[21-22].

Heavy metal chalcogenide clusters are widely used in catalysis, photo-catalysis, and micro-electronics. They also play an important role in some luminescent properties, for example, as photosensitive and nonlinear optical materials.

Gold sulde, in bulk, is a semiconductor with intermediate band gap and has mixed ionic and covalent bonding character[23].

To utilize the properties of gold clusters, it is necessary to avoid their coalescence. Molecules containing sulfur atoms are often used as surfactants, since they form particularly stable gold nano-clusters due to the strength of
the gold- sulfur bond[24]. The interaction of H2S with gold and silver can also serve as a simple model system for understanding the self-assembly of alkane thiols (RSH) on noble metal surfaces[25]. The adsorption of H2S on a number of metal surfaces, including Ni(100)[26], Cu(100)[27], Au(110)[28], Au(100)[29], and Ag(111)[30] have been reported. A variety of techniques, including low energy electron diraction, temperature programed desorption, high resolution electron energy loss spectroscopy, x-ray photo-emission spectroscopy, ultraviolet photo-emission spectroscopy, near-edge x-ray adsorption one structure, and scanning tunneling microscopy have been used to characterize such surfaces. In most cases, dissociation of H2S and formation of a chemisorbed sulfhydryl SH species is observed at low coverage and low temperature below 170 K; the major exception being gold and silver surfaces.

In order to design new molecular devices it is extremely important to understand the chemical interactions of these molecular systems at the junctions.

Several theoretical investigations, describing the adsorption of alkane thiols on gold surfaces, represented by cluster models, have been reported. Pioneering calculations on adsorption of SCH3 on Au16, Au17[31] and Au24[24] are
carried out with an emphasis to understand the chemistry at the gold-sulfur interface. It is found that the geometries of the atoms beyond the terminal sulfur are virtually enacted even after strong adsorption. Remarkably little work has been reported for H2S with gold clusters. In this work, we report a theoretical study of the adsorption of H2S molecules on small neutral gold clusters, Aun(n = 1 􀀀 5) and compare our calculations with available results. The adsorption of molecular hydrogen sulphide on gold cluster was performed using DFT calculation, in which the minimum energy for adsorption of molecular hydrogen sulphide on the various cluster was reported. We analyzed our results with the density of state to have a deep insight of the electronic states.

This work is organized as follows. In chapter 2 we review the fundamental theoretical tools used in the calculation and the computational details of our work. In chapter 3 we present and discuss the results of this work and additionally, in chapter 4, we present a brief summary of our results and conclusion.