Quantum chemical studies of niobium and vanadium doped gold clusters

Doctoraatsproefschrift aan de Faculteit Wetenschappen van de KU Leuven Quantum Chemical Studies of Niobium and Vanadium-doped Gold Clusters PHAM VU NHAT Promotor: Prof Minh Tho Nguyen Members of the Jury: Prof Luc Van Meervelt Prof Marc Hendrickx Prof Tatjana Vogt Prof Frank De Proft Prof Ewald Janssens Dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Science in Chemistry December 2012 © 2012 Faculteit Wetenschappen, Geel Huis, Arenberg Doctoraatsschool, Kasteelpark Arenberg 11, B-3001 Heverlee, België Alle rechten voorbehouden Niets uit deze uitgave mag worden vermenigvuldigd en/of openbaar gemaakt worden door middel van druk, fotokopie, microfilm, elektronisch of op welke andere wijze ook zonder voorafgaandelijke schriftelijke toestemming van de uitgever All rights reserved No part of the publication may be reproduced in any form by print, photoprint, microfilm, electronic or any other means without written permission from the publisher ISBN 978-90-8649-576-4 D/2012/10.705/93 to my parents, my wife, and my daughter Buồn trông phong cảnh quê người, đầu cành quyên nhặt, cuối trời nhạn thưa Strange landscapes met the mournful eyes, on trees cuckoos galore, at heaven's edge some geese Kieu Story, Nguyen Du i ii Members of the Jury Prof Dr Luc Van Meervelt (Chair) Department of Chemistry, KU Leuven Prof Dr Marc Hendrickx Department of Chemistry, KU Leuven Prof Dr Tatjana Vogt Department of Chemistry, KU Leuven Prof Dr Frank De Proft Faculty of Sciences, Vrije Universiteit Brussel (VUB) Prof Dr Ewald Janssens Department of Physics and Astronomy, KU Leuven Prof Dr Minh Tho Nguyen (Promoter) Department of Chemistry, KU Leuven iii iv Acknowledgments This work has been carried out during the period of January 2009 – December 2012 at the Department of Chemistry, University of Leuven, Belgium My journey to this point turned out to be much less difficult than I expected as it has been accompanied and supported by many people Now is a pleasant opportunity to express my gratitude for all of them First and foremost, I would like to thank my supervisor Prof Minh Tho Nguyen for his endless patience and enthusiasm, along with his understanding and motivating guidance during my Ph.D studies I have really been fortunate that I have come to know him in my life I also would like to send special thanks to Prof Arnout Ceulemans, Prof Marc Hendrickx, Prof Paul Geerlings (VUB) and Prof Frank De Proft (VUB) I did benefit much from their excellent knowledge and teaching on quantum chemistry Besides, I am grateful to all members of the Jury for their valuable comments, suggestions and corrections that greatly enrich my work I in addition appreciate the timely helps from Mrs Rita Jungbluth and Dr Hans Vansweevelt I always got the right answers whenever knocking their doors I thank also Prof Marc Hendrickx and Drs Pieter Kelchtermans for the translation of the “Samenvatting” I am also indebted to Profs Peter Lievens and Ewald Janssens for valuable discussion on clusters, Dr Andre Fielicke from the Fritz-Haber Institute in Berlin, Germany, for providing me the original plots of his experimental IR spectra on Nb clusters, and Prof Jerzy Leszczynski for his support during my visit to the Jackson State University, Mississippi, USA Financial support from the KU Leuven Research Council and the Can Tho University is gratefully acknowledged How sad and tough would it be to live far from the family and home without friends Thus, I am truly thankful to all my colleagues in the Division of Quantum Chemistry for the friendly working atmosphere, and to all my Vietnamese friends in Leuven for the numerous nice non-scientific meetings Finally, the greatest thanks go to my parents, my wife and my daughter They stand beside me all the time and bring the sunshine to me in the most difficult moments I dedicate this thesis to them v vi This thesis is based on the following publications: 1) A New Look at the Structure and Vibrational Spectra of Small Niobium Clusters and Their Ions Pham Vu Nhat, Vu Thi Ngan and Minh Tho Nguyen, J Phys Chem C, 2010, 114, 13210–13218 2) Electronic Structures, Vibrational and Thermochemical Properties of Neutral and Charged Niobium Clusters Nbn, n = 7−12 Pham Vu Nhat, Vu Thi Ngan, Truong Ba Tai, and Minh Tho Nguyen, J Phys Chem A, 2011, 115, 3523–3535 3) Reply to “Comment on „Electronic Structures, Vibrational and Thermochemical Properties of Neutral and Charged Niobium Clusters Nb n, n = 7–12.‟” Pham Vu Nhat, Vu Thi Ngan, Truong Ba Tai, and Minh Tho Nguyen, J Phys Chem A, 2011, 115, 14127–14128 4) Structures, Spectra, and Energies of Niobium Clusters from Nb 13 to Nb20 Pham Vu Nhat and Minh Tho Nguyen, J Phys Chem A, 2012, 116, 7405–7418 5) Trends in Structural, Electronic and Energetic Properties of Bimetallic Vanadium– gold Clusters AunV with n = 1–14 Pham Vu Nhat and Minh Tho Nguyen, Phys Chem Chem Phys., 2011, 13, 16254– 16264 6) Theoretical Study of AunV-CO, n = – 14: The Dopant Vanadium Enhances CO Adsorption on Gold Clusters Pham Vu Nhat, Truong Ba Tai and Minh Tho Nguyen, J Chem Phys., 2012, 137, 164312–164324 7) The Icosahedral Evolution in Bimetallic Clusters AunV, with n = 12 – 20 Pham Vu Nhat and Minh Tho Nguyen, J Chem Phys., 2012, (submitted) vii 8) Structural Evolution, Vibrational Signatures and Energetics of Small Niobium Clusters Pham Vu Nhat and Minh Tho Nguyen, Coord Chem Rev., 2012, (to be submitted) Other publications: 9) Structure and Stability of Aluminum-doped Lithium Clusters (LinAl0/+, n = 1–8): A Case of The Phenomenological Shell Model Truong Ba Tai, Pham Vu Nhat and Minh Tho Nguyen, Phys Chem Chem Phys., 2010, 12, 11477–11486 10) Electronic Structure and Thermochemical Properties of Small Neutral and Cationic Lithium Clusters and Boron-Doped Lithium Clusters: Lin0/+ and LinB0/+ (n = 1–8) Truong Ba Tai, Pham Vu Nhat, Minh Tho Nguyen, Shenggang Li, and David A Dixon, J Phys Chem A, 2011, 115, 7673–7686 11) Theoretical 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2002, 66, 035418 101 P Schwerdtfeger, M Dolg, W H E.Schwarz, G A.Bowmaker, and P D W Boyd, J Chem Phys., 1989, 91, 1762 102 P Schwerdtfeger and M Dolg, Phys Rev A, 1991, 43, 1644 103 D Die, X Y Kuang, J J Guo and B X Zheng, J Mol Struct Theochem, 2009, 902, 54 104 D Die, X Y Kuang, J J Guo and B X Zheng, J Phys Chem Solids, 2010, 71, 770 163 Chapter 105 H Tanaka, S Neukermans, E Janssens and R E Silverans, J Chem Phys., 2003, 119, 14 106 D Die, X Y Kuang, J J Guo and B X Zheng, Physica A, 2010, 389, 5216 107 T P Martin, Phys Rep., 1996, 273, 199 108 F Baletto and R Ferrando, Rev Mod Phys., 2005, 77, 371 109 M B Knickelbein, Annu Rev Phys Chem., 1999, 50, 79 110 A H Pakiari and Z Jamshidi, J Phys Chem A, 2007, 111, 4391 111 K E Shafer-Peltier, C L Haynes, M R Glucksberg and R P Van Duyne, J Am Chem Soc., 2003, 125, 588 164 Chapter General Conclusions and Outlook About the work done in this thesis and future promising studies Chapter 5.1 General conclusions Our main interest in this doctoral study was to cope with the microscopic details of metallic structures having dimension of nanometers by using quantum chemical methods We mainly concentrated on the structural, electronic and energetic properties of two transition metal clusters, which would provide a fundamental understanding for their possible future technological applications We also investigated the effect of doping to certain properties of cluster systems Most of the calculations were performed in the framework of density function theory in conjunction with the pseudo-potential atomic basis sets In the following section we describe briefly how the thesis has been evolved Chapter is a concise overview on the metal clusters studied In this chapter we introduced the importance of such nano-particles in terms of both scientific and technological viewpoints Recent achievements in the study of transition metal clusters have also been discussed We in addition mentioned the general trends resulting from the presence of the d electrons, the lanthanoid contraction, and the relativistic effects, which have strong effects on many properties of heavy d-element clusters More significantly, this emphasized our motivation to tackle some rather difficult targets for theoretical investigations Indeed, while experimental observations become more and more available, relatively little quantitative theoretical studies have been carried out to understand and interpret the observed findings Chapter is a critical analysis about the performance of current DFT methods and suitable applications of various available functionals in transition metal chemistry Albeit DFT calculations have become an integral part for analyzing electronic structures and related properties of transition metal compounds, each functional naturally has its own advantages and shortcomings for specific systems This analysis has been served as a preliminary but useful guidance in choosing functionals suitable for certain parameters of a given system To some extent, functionals with low amount of HF exchange usually perform better than fully hybrid methods in calculating the molecular properties of compounds containing only transition metal elements However, we recognized that no functional is actually 166 Conclusion and outlook reliable for all structural and energetic parameters Hence, it is crucial to examine carefully the applicability of different computational options for every new type of compounds in the studies by a systematic comparison of the corresponding computed results to either experimental data or high-level MO methods In Chapter we systematically investigated the structural evolution, vibrational signatures and energetics of Nbn clusters with n = − 20, in both neutral and charged states, using the BPW91 functional and the pseudo-potential correlationconsistent cc-pVaZ-PP basis sets (a = D and T) The M06 functional was employed to evaluate some basic thermo-chemical parameters such as atomization and dissociation energies Coupled-cluster theory CCSD(T) single point calculations were also carried out for iso-energetic states of several systems smaller than Nb12 The calculations were calibrated by available experimental data Several basic energetics of clusters including electron affinities, ionization energies, binding energies per atom, and stepwise dissociation energies were computed and compared with the measured values In particular, the vibrational (IR) spectra of systems smaller than Nb13 were investigated to interpret the observed farIR features Although the experimental IR spectra for larger clusters have not been published so far, they are also presented as predictions that may allow the optimal structures to be assigned when the relevant spectroscopic information is available Based on these theoretical and experimental results, some general and important conclusions for Nbn clusters with n ranging from to 20 were drawn From a growth mechanism point of view, we found that the optimal structure of the cluster at a certain size is not simply generated from that of the smaller one by adding an atom randomly Instead, Nbn clusters prefer a close-packed growth behavior Systems larger than 12 atoms tend to form a compact structure with one Nb atom encapsulated by a cage constituted from five and six triangles Unlike many 3d transition metals, whose volumes are smaller, the clusters containing 13 and 19 Nb atoms not exist as icosahedra and double-icosahedra A distinct case is Nb15 as it bears a slightly distorted solid bcc (body-center cubic) structure 167 Chapter An interesting observation from the vibrational signatures is that while the spectra of smaller systems are found to be strongly dependent on the addition or removal of an electron from the neutral, those of the larger size clusters are likely to be independent on the charged state The neutrals and their corresponding ions usually have quite similar IR patterns, due to the geometrical similarity This also shows that the electronic configurations become less important in medium and large size clusters The geometric effect, or atomic arrangement, plays a more significant role than the electronic distribution in determining the stability of larger systems Concerning the energetic properties, both functionals BPW91 and M06 are found to be reliable in predicting the EA and IE values, as compared to experiment However, the former is likely deficient in predicting the binding energy per atom and dissociation energies of niobium clusters, where the latter performs much better The average absolute errors relative to experiment of the BPW91 method are ~0.6 eV for De and 0.5 for BE as compared to ~0.2 eV and < 0.1 eV using the M06 functional Since neither dissociation energies nor binding energies are experimentally reported for clusters containing more than 11 Nb atoms, we would suggest to adopt the De values predicted by the M06 functional for Nbn in the range of n = 12 – 20 Of the clusters considered, Nb2, Nb4, Nb8 and Nb10 were found to be magic as they hold the number of valence electrons corresponding to the closed shell in the sequence (1S/1P/2S/1D/1F….) Owing to the valence electrons that satisfy the empirical 2(N+1)2 rule, Nb10 is observed to have a spherically aromatic character, high chemical hardness and large HOMO-LUMO gap The open-shell Nb15 system is also particularly stable, and it can form a highly symmetric structure in all charged states In addition, for species with an encapsulated Nb atom, an electron density flow takes place from the outside skeleton to the central atom, and the greater the charge involved the more stabilized the cluster is Another project of this doctoral study was a comprehensive investigation on the growth behavior, energetics and CO affinity of vanadium-doped gold clusters AunV with n = – 20 In Chapter 4, we examined in detail how the gold systems are 168 Conclusion and outlook perturbed due to the presence of a dopant Most of the calculations were carried out using the BB95 and B3LYP functionals, in conjunction with the pseudo-potential ccpVDZ-PP basis set for metals and the full-electron cc-pVTZ basis set for non-metals Concerning the structural features of global minimum, we observed a substantial atomic rearrangement when one V atom is introduced Except for Au2V and Au4V, the doped AunV and the pure Aun+1 clusters bear a completely different shape in their ground state In addition, contrary to the tendency of pure gold clusters to exhibit planar structures, a crossover from planarity to non-planarity occurs quite early in AunV species, likely to initiate at Au6V Another important finding is that during the growth, vanadium is always endohedrally doped as a way of maximizing its coordination numbers and augmenting the charge transfer Analyses of energetic properties including the bond energy (BE), fragmentation energy (Ef), and second-order energy difference (∆2E) demonstrated that the presence of V atom enhances considerably the thermodynamic stability of odd-numbered Au clusters but tends to reduce that of even-numbered counterparts The atomic shapes induce an apparently more important effect on the clusters stability than their electronic structure Even though Au2V, Au6V, Au12V, Au14V possess an odd number of electrons, they are rather stable due to their ability to form compact symmetric structures Especially, if both molecular shape and electronic distribution are satisfied, the resulting cluster becomes particularly stable such as the anion Au12V–, which can thus combine with the cation Au+ to form the superatomic molecule of the type [Au12V]Au The calculated properties of the clusters with n = – 12 appear to be extremely sensitive with the functionals employed The hybrid functional B3LYP is significantly biased towards 2D conformations for metal frameworks Moreover, while the meta-GGA BB95 predicts an icosahedral growth pattern in these species, the former yields a cuboctahedral evolution In addition, for some systems, several lower-lying isomers are found to be very close in energy that DFT computations cannot clearly establish their ground electronic states In this context, further 169 Chapter experimental results, for example the far-IR spectra of binary metallic complexes, is necessary for more definitive assignment of their equilibrium structures For the adsorption of CO on these binary clusters, we found that when both Au and V sites are exposed, CO adsorption on V atom is energetically favorable, because with partially filling d orbitals vanadium is more willing to undergo an interaction with either empty or fulfilled orbitals of CO However, for larger systems, when vanadium is completely doped inside the gold cage, low-coordinated Au atoms become, as in the pure clusters, the preferred sites The binding energies of AunVCO adducts were also computed and compared with those of Aun+1CO species in order to evaluate the effects of V on the CO adsorption affinity In the smallest systems such as Au2V, Au3V and Au4V, the CO interaction in V-doped clusters is weaker than those of the pure hosts For the larger systems, the presence of V invariably results in a reinforcement of CO adsorption Among the doped clusters considered, AuV has the highest CO adsorption affinity as it can form a typical π-back donation bond between V and CO Further examination of densities of states clearly provided support for the view that vanadium-carbonyl complexes have larger CO binding energies and larger CO frequency shifts than their isoatomic gold-carbonyl counterparts 5.2 Outlook Deeper insights into the electronic, geometric and energetic properties of transition metal clusters are very important for the continuing development of relevant nanostructure materials and technologies That renders evident the need for detailed theoretical understanding of such systems The results reported in the present work could thus be helpful for future experimental and theoretical studies of metal clusters However, several open issues still remain unresolved and some challenging perspectives would motivate further investigations 170 Conclusion and outlook Although DFT methods provide us with reasonable information on the ground state properties of these species, they still demand a substantial computational cost Hence, up to now, the study is limited to systems with a small number of atoms, and few calculations of excited state properties could be performed In the coming years, we hope to see breakthrough developments both in terms of methodology and computational facilities in such a way that theoretical investigations on larger clusters of metals and semiconductors will become less challenging and more reliable We are also looking forward to the appearance of spectroscopic data from various types of experiment in order to confirm our predictions Then further studies on larger systems could be carried out in concert The applications of transition metal clusters in colloidal chemistry, catalysis, medical science, electronic industries… are also interesting, challenging and promising perspectives Therefore, theoretical investigations on optoelectronic and magnetic behaviors, aggregate properties and interactions of these species with industrial gases (H2, CO, NO, CH4, ) and with organic and biomolecular compounds (from simple alcohols, amino acids, to DNA bases, ) … are truly necessary and could open a wide avenue for quantum chemical theory to play its essential role, to predict with confidence the chemical phenomena Some extended research projects can be implemented on the basis of this doctoral dissertation, namely: (1) One of the important aspects of metal cluster research is the study of the trends in reaction properties as a function of cluster size Although both gold and niobium clusters have been used as the prototypical systems for such purpose, insights into the interaction mechanism, chemical bonding and energetic are still very limited Thus a promising project is to extensively probe the reactivities of these species with industrial gases such as small hydrocarbons, COx, NxO The results could provide some perspectives for an actual use of these transition metal clusters as efficient catalysts for reactions in the gas phase (2) Due to the good biocompatibility, facile synthesis and high affinities toward various biomolecular moieties, gold nanoparticles are widely 171 Chapter used in modern biochemical sensors, protein engineering and gene technology However, relatively little is known about the mechanisms of the corresponding molecular interactions Therefore, the studies on intermolecular interactions between gold clusters, in either pure and doped forms and in different charged states, with amino acids and DNA bases could be an excellent framework to gain deeper insights into the biomolecular adsorption on the well-controlled gold surfaces 172 [...]... structures of Nb clusters is reported in the literature so far and the identity of several Nbn species remains a matter of debate Similarly, our knowledge about structural and electronic properties of bimetallic gold -vanadium systems is still very limited, in contrast to the welldeveloped understanding of pure gold clusters Hence, the purpose of the current work is twofold The first part is an assignment of. .. niobium (Nb) and gold (Au), in both pure and doped forms These are two representative elements for which experimental results are available Both niobium and gold clusters have been the subject of numerous experimental investigations as the early studies on clusters were carried out on the dissociation energy of the dimers Au 2 and Nb2.21,22 However, relatively little information on the geometric and electronic... quantitative definitions of chemical notions including electronegativity, softness, hardness, and the electronic origins of chemical bonding and reactivity… could be defined and understood more clearly within the framework of density functional theory.43 In Chapter 2, a concise description of DFT methods and their performance in transition metal clusters will be presented Then the applications of these methodologies... combination of a GGA exchange functional in DFT with the HF exchange integral by using a standard error function.9,10 Furthermore, we can make use of DFT for quantitative definitions of chemical parameters including the electronegativity, softness, hardness, and for the interpretations of the electronic origins of chemical bonding and reactivity.2,11 2.2 Functional classification The wave function and the... of nanometers In this field, the knowledge on atomic and electronic structures of elemental clusters provides fundamental information for the design of new nanomaterials implemented in the modern and future technologies In a recent paper, Loth and co-workers4 revealed that anti-ferromagnetic nanostructures, composed of just two rows of six iron atoms, might become a new generation of memory chips and. .. complex and subtle relationships between structure, both electronic and atomic, with stability and reactivity Generally, one may only expect that the physical and chemical properties of small and medium-sized clusters, containing no more than a few hundred atoms (diameters of 1–3 nm),6 strongly depend on either their sizes or shapes, and differ considerably from both individual atoms or molecules and bulk... ratio, (ii) availability of active absorption and reactions sites, (iii) 2 Introduction wide range of coordination number, and (iv) easy migration and atomic rearrangements facilitating the chemical bond breaking and forming Until recently, the structural assignment for metal clusters, particularly for those of transition elements, constituted great challenges for both experimental and theoretical approaches... which direct and substantial metal-metal bonding is present, without the benefit of stabilizing ligands The study in metal clusters has grown exponentially since late 1970’s, motivated by both academic and industrial concerns 2,3 In the age of miniaturization of electronic devices, increasing interest has been placed, among other things, in the evolution of structures and related properties of materials... various classical and modern analytical techniques, a large numbers of properties on size-selected clusters, especially for those of transition elements, have been investigated and reported To obtain insights into the stability of 6 Introduction a certain cluster, mass spectroscopy and laser spectroscopy approaches are often employed, while atomic and molecular methods are used to probe their chemical reactivities... pseudo-potential composed of the inner electrons along 3 Chapter 1 with the nuclei This highly delocalized behavior of valence electrons brings about the main characteristics of simple metal clusters: the formation of electronic shells and the occurrence of shell closing effects are somewhat similar to those in free atoms In other words, the electronic structure of these clusters appears to reflect that of a spherical