A. Fortunelli
CNR-ICCOM, via G. Moruzzi 1, Pisa, 56124, Italy; alessandro.fortunelli@cnr.it, afloer@caltech.edu
I will present some results of our efforts exploring a predictive computational nanomaterials science – modeling materials with nanoscale structural features and searching for associated novel phenomena and properties, with particular attention to nano-architectured metal systems and their kinetics and dynamic phenomena, such as structural and electron dynamics. Our strategy starts from fundamental methods and concepts drawn from quantum mechanical approaches and aims at achieving accurate predictions under realistic conditions and environment.
First, the compositional and structural freedom of nanostructured systems calls for developing and implementing exhaustive search algorithms able to investigate the novel structural motifs, paths and mechanisms that such systems exhibit (“nanostructure problem”). In this context, I will briefly discuss a Reactive Glocal Optimization (RGO) approach as a computational protocol able to explore the reactive phase space of these systems in the presence of reactant molecules1, with examples of application to the thermodynamics and kinetics of elemental diffusion in alloyed systems2.
The natural deployment of such techniques is in the field of catalysis. Starting from the smallest systems, sub-nanometer (or “ultranano”) supported metal clusters (containing up to 10-20 metal atoms) represent a new class of materials which have been shown in several cases to exhibit superior catalytic properties in efficiency and selectivity with respect to traditional catalytic systems in addition to optimizing atom-economy efficiency. I will show how the application of systematic sampling under realistic conditions naturally leads to the concept of ligand/surface catalytic complex. The simultaneous presence on the metal catalyst of ligands and chemical species at various stages of the reaction and the non-linear interactions among them and with the support as a norm translates into the formation in-situ and under reaction conditions of a complex aggregate which acts as the real catalytically active species. This idea will be discussed and illustrated with a few selected examples3, highlighting its relationship to methods and results developed in the field of homogeneous catalysis4.
Moving to larger systems, results will be first reported on particle restructuring under realistic conditions and its relevance to carbon nanotube growth, (selective) oxidation reaction, and ammonia synthesis. Going to the extreme, it will be then shown how harsh reactive ligands can lead to phase transformations from nanocrystals to nanoporous particles via dealloying, as for Ni-Pt nanostructures under electrochemical oxygen reduction reaction (ORR) conditions5. Such “exotic” nanostructures obtained by electrochemical leaching of more electropositive metals (e.g., Ni) from nano-architectured alloys with another metal (e.g., Pt) typically exhibit continuously connected cavities (pores) of nanoscale size, and have attracted explosive interest in the last 15 years due to their unique properties in catalysis, sensors, and opto-electronic devices. However, lack of fundamental knowledge still remains on these systems concerning their geometric features and the relationships with the corresponding properties. A relationship will here be proposed between surface coordination environment and catalytic function and the origin of enhanced catalytic performance of these systems in the ORR, which is the rate determining step in low-temperature hydrogen fuel cells for sustainable and energy-efficient electrical power, with the final goal of designing Pt metallic systems with optimal catalytic activity.
Moving to electron dynamics, the absorption spectra of metal nanostructures will be explored using time-dependent density-functional-theory (TDDFT) methods. The sensitivity of Surface Plasmon Resonances (SPR)6 to the nanostructure environment and the possible damping of the absorption intensity in the visible region due to adsorbed species is a first issue which limits applications in molecule detection via Raman spectroscopy, enhanced plasmonic phenomena in metal nanogaps, biosensing, etc. Two different strategies will be investigated to overcome this issue and achieve intense SPR peaks in the whole near-IR/vis region. The first strategy is based on molecule/nanostructure resonance coupled with plamon/plamon interactions due to proximity effects. The coupling between the plasmonic modes of Au nanowires at close distances and those involving the interaction of the wires with adsorbed ligand species will be explored, and the search for synergic interactions in the optical response (‘hotspot’ enhancement of response fields) will be discussed7. The second strategy is based on fine tuning the chemical features of the ligands8,9.
Focusing on thiolated AuN(SR)M nanoclusters of well-defined atomistic structure, I will show how large enhancements in absorption in the optical region can be achieved by tuning the steric and electronic properties of the SR ligands. Charge decompression via steric hindrance and delocalization via -conjugation to achieve optimal band alignment will be used as guiding principles leading to a resonance phenomenon in which many excitations, of composite Au/S/R character and involving the nanocluster as a whole, crowd in a narrow energy interval. This strategy allows one to circumvent the issue of SPR damping by the environment (SPR “re-birth”), and represents a step forward toward the goal of an in-silico design of nanocrystals with desired optical properties.
If time will allow, dielectric (oxide) ultrathin films grown on metal or semiconducting surfaces are examples of so-called 2D materials that have attracted an explosive interest in view of many applications, including transport and electronic devices. I will show how an interplay of theory and experiment working in close synergy is mandatory to arrive at a correct elucidation of the structural properties of such complex materials, and how such a detailed information opens the way to a full understanding and possibly control of the response properties of such systems10. In particular, an in-depth analysis of the charge and electrostatic potential at the metal/semiconductor junction to single out physical quantities such the work function, band bending and charge transfer at the interface allows one to shed light on electronic transport interfacial phenomena via quantum approaches11 and provides a link between the knowledge accumulated on model surface science systems and technological electronic devices based on 2D oxide phases.
References
1. F. R. Negreiros, et al., Nanoscale 4, 1208 (2012); ACS Cat. 2, 1860 (2012).
2. M. Asgari et al., J. Chem. Phys. 141, 041108 (2014); L. Sementa et al. Phys. Chem. Chem.
Phys. 16, 24256-24265 (2014).
3. F. R. Negreiros, et al., Comptes Rendue Chim. 17, 625–633 (2014); L. Sementa, et al., Phys.
Chem. Chem. Phys. 16, 26570-7 (2014); Y. Wang et al. Chem. Eur. J. 19, 406-413 (2013).
4. L. Sementa, et al., Inorg. Chim. Acta 431, 150–155 (2015).
5. Fortunelli et al. Chem. Sci. 6, 3915-3925 (2015).
6. N. Durante et al., J. Phys. Chem. C 115, 6277 (2011); G. Barcaro et al., J. Phys. Chem. C
115, 24085 (2011). G. Barcaro, et al. J. Phys. Chem. C, 118, 12450–12458 (2014).
7. L. Sementa et al., ACS Photon. 1, 315–322 (2014).
8. P. R. Nimmala et al. J. Chem. Phys. Lett. 6, 2134−2139 (2015).
9. L. Sementa et al. Chem. Comm. 51, 7935–7938 (2015).
10. M. Denk et al. ACS Nano, 8, 3947–3954 (2014).
11. Katagiri et al. Nano Lett. 16, 3788-94 (2016).