Gold surfaces and nanoparticles are protected by Au(0)–thiyl species and are destroyed when Au(I)–thiolates form

Abstract

The synthetic chemistry and spectroscopy of sulfur-protected gold surfaces and nanoparticles is analyzed, indicating that the electronic structure of the interface is Au(0)-thiyl, with Au(I)-thiolates identified as high-energy excited surface states. Density-functional theory indicates that it is the noble character of gold and nanoparticle surfaces that destabilizes Au(I)-thiolates. Bonding results from large van der Waals forces, influenced by covalent bonding induced through s-d hybridization and charge polarization effects that perturbatively mix in some Au(I)-thiolate character. A simple method for quantifying these contributions is presented, revealing that a driving force for nanoparticle growth is nobleization, minimizing Au(I)-thiolate involvement. Predictions that Brust-Schiffrin reactions involve thiolate anion intermediates are verified spectroscopically, establishing a key feature needed to understand nanoparticle growth. Mixing of preprepared Au(I) and thiolate reactants always produces Au(I)-thiolate thin films or compounds rather than monolayers. Smooth links to O, Se, Te, C, and N linker chemistry are established.gold-sulfur bonding | synthesis | mechanism | electronic structure | nanoparticle G old self-assembled monolayers (SAMs) and monolayerprotected gold nanoparticles form important classes of systems relevant for modern nanotechnological and sensing applications (1-5). Understanding the chemical nature of these interfaces is critical to the design of new synthesis techniques, the design of new spectroscopic methods to investigate them, and to developing system properties or device applications. Over the last 10 y, great progress has been made in understanding the atomic structures of gold-sulfur interfaces (6). Most discussion (7) has focused on the identification of adatom-bound motifs of the form RS-Au-SR (where R is typically a linear alkyl chain or phenyl group) sitting above a regular Au(111) surface (8-10) or on top of a nanoparticle core of regular geometry (11), Other variant structures have also been either observed, such as polymeric chains such as the trimer RS-Au-SR-Au-SR) (10, 11), or proposed (8,12,13). By considering the four isomers of butanethiol (14), we have shown that alternative structures can also be produced in which RS groups bind directly to an Au(111) surface without gold adatoms. This occurs whenever steric interactions across the adatoms are too strong or steric intermolecular packing forces allow for very high surface coverages if both adatom and directly bound motifs coexist in the same regular SAM (15). The cross-adatom steric effect has also been demonstrated for gold nanoparticles (16), and we have shown that Coulombic interactions between charged tail groups can also inhibit adatom formation (17). SAMs involving adatoms have poor longrange order owing to the surface pitting that is required to deliver gold adatoms, while directly bound motifs lead to regular surfaces (18).There is clearly a delicate balance between the forces that direct these different in...

Putting David Craig’s Legacy to Work in Nanotechnology and Biotechnology

“…[395] Many properties of the two head-group arrangements are very similar despite the significant change in coordination, suggesting that the binding mechanism is similar in both cases. Also the binding strengths of the chemisorbed forms are not that much greater than those of the physisorbed forms [382,396] and are consistent with the binding of nitrogen and phosphorous bases to the surface, processes known to be controlled by the dispersion interaction. The postulate that both the 'physisorption' and 'chemisorption' processes depicted in the above reactions involve primarily dispersion forces and the Au 0 -thiyl valence state provides a consistent analysis of all of these data, including that for other chemical variations such as the replacement of Au with Ag or Cu and the replacement of S with O, Se, or Te.…”

“…While such ideas have always been recognized in Pearson's concept of hard/soft acids and bases, [378][379][380][381] that the dispersion force can be large enough to compete with covalent and ionic binding is just coming to be understood. [382,383] [383] and that Au 0 -thiyls are the species that actually protect gold surfaces and nanoparticles while Au I -thiolate production etches surfaces and inhibits nanoparticle formation. [382] The competition between covalent/ionic and van der Waals bonding is critical in each case, and Pearson's ideas predict the main results as S and Au are soft while Fe can be either hard or soft.…”

“…[382,383] [383] and that Au 0 -thiyls are the species that actually protect gold surfaces and nanoparticles while Au I -thiolate production etches surfaces and inhibits nanoparticle formation. [382] The competition between covalent/ionic and van der Waals bonding is critical in each case, and Pearson's ideas predict the main results as S and Au are soft while Fe can be either hard or soft. The Au-Au bond has always been regarded as anomalous, with the concept of 'aurophilicity' used to describe it, [384,385] but this is now known to be just the strong dispersion force acting on top of strong s-s interactions that make the gold atoms in a gold dimer more like those in gold metal than like isolated gold atoms.…”

“…[386,387] Understanding whether or not processes occur on the Au 0 -thiyl or Au I -thiolate potential-energy surfaces is essential to rationalizing the known synthetic chemistry and spectroscopy of gold surfaces and nanoparticles. [382] Work with Noel Hush (Sydney University) and Jens Ulstrup and Jingdong Zhang (Danish Technical University) has shown that gold (111) surfaces react with thiol or disulfide reagents to form initially 'physisorbed' species and then 'chemisorbed' ones (Scheme 1):…”

“…The postulate that both the 'physisorption' and 'chemisorption' processes depicted in the above reactions involve primarily dispersion forces and the Au 0 -thiyl valence state provides a consistent analysis of all of these data, including that for other chemical variations such as the replacement of Au with Ag or Cu and the replacement of S with O, Se, or Te. [382] However, gold does not react with solutions containing thiolate anions RS À , but in an electrochemical cell in which Au I is produced at the anode and RS À at the cathode, the two species combine in solution to form Au I -thiolate compounds and thin films that then precipitate onto the surface (Scheme 2). [397] Au I and thiolate ions are present as intermediates and their reaction produces an end product completely different to the protected SAM-covered surface produced by reactions with thiols and disulfides.…”