"If the path be beautiful, let us not ask where it leads."
-- Anotale France
The Qin Lab has pioneered a set of in situ techniques for the characterization of atomic/molecular events on the surface of noble-metal nanocrystals in a liquid phase and under operando conditions. We established the use of surface-enhanced Raman Scattering (SERS) in probing the heterogeneous nucleation and growth of colloidal nanocrystals, as well as catalytic reactions on the surface of designer nanocrystals. We are widely recognized for the original contributions to the rational synthesis of metal nanocrystals with desired properties.
Metal-Sensitive Functionalization and Self-Assembly of Bimetallic Nanocrystals
Building upon prior success, we are embarking new adventure in developing new design principles and methods to fabricate a novel class of bifunctional nanoreactors from Ag@M (M: Pt, Pd, and Rh) core-frame nanocubes via metal-selective surface functionalization and self-assembly. We hypothesize that molecules bearing isocyanide groups at the two ends are used to selectively bind to the M atoms on the edges of the core-frame nanocubes, serving as “clips” to bring two nanocubes face to face for the generation of a dimer. The gap region between the two nanocubes naturally presents a well-controlled nanoreactor, in which the side faces can be orthogonally functionalized with the reactants while the M atoms not coordinated by isocyanide will serve as the catalyst. Due to a strong plasmonic coupling between the two Ag nanocubes separated by a gap of only a few nanometers, the nanoreactor offers a unique capability to monitor various types of important catalytic reactions by in situ SERS. The model reactions include the hydrogenation of nitroaromatics for the production of thermodynamically unfavorable products such as hydroxylamine and azo compounds, as well as the bond-selective hydrogenation of cinnamaldehyde, a catalytic reaction pivotal to the production of fragrance, agrochemical, and pharmaceutical compounds.
This research has been supported by the National Science Foundation (NSF) through the Macromolecular, Supramolecular and Nanochemistry (MSN) Program (CHE-2002653, 1/1/2021–12/3/2024).
Understanding Heterogeneous Nucleation in Nanocrystal Growth with Molecular Probes
Bimetallic nanocrystals have received ever-growing interest owing to their properties that are often superior to the monometallic counterparts. Although site-selective deposition has been successfully established for a set of bimetallic systems, it is still impossible to detect and quantify the second metal being deposited on the surface of nanocrystal-based seeds while they are still suspended in the reaction solution. In recent years, in situ tools based on liquid-cell transmission electron microscopy and small or wide-angle X-ray scattering have been used to address this issue. Unfortunately, both electrons and X-rays induce unexpected reactions, making it difficult to elucidate the mechanistic details. It is a major challenge to resolve the surface atomic composition during the course of a solution-phase synthesis by all those imaging- or diffraction-based techniques. Dr. Qin addressed this challenge by forging a paradigm shift through the development of isocyanide-based probes for investigating the heterogeneous nucleation and growth of a second metal such as Pt, Pd, Rh, Ir or Ru on Ag nanocrystals. In the proof-of-concept demonstration published in ACS Nano (2017), Dr. Qin established the use of 2,6-dimethylphenyl isocyanide (2,6-DMPI) as a SERS probe to investigate the heterogeneous nucleation of Pt on Ag nanocubes in the original growth solution. Because the hot spots for SERS and the sites for nucleation coincide at the edges of a Ag nanocube, there is unprecedented sensitivity. She demonstrated that in situ SERS had the sensitivity to detect as few as 27 Pt atoms deposited on the edge of a 39-nm Ag nanocube. The JACS work focused on the nucleation of Pd on Ag nanocubes. Dr. Qin argued that 2,6-DMPI could bind to Pd atoms in three distinct configurations to provide a more detailed picture about the early stage of a nucleation process. We further elucidated the use of 2,6-DMPI as a spectroscopic probe to elucidate the states of Pd atoms on the edges of a Ag nanocube, as well as the diffusion of Pd adatoms to corners and side faces. The isocyanide group of 2,6-DMPI binds to the Ag and Pd via sigma-donation and pi-back-donation, respectively, leading to distinct positions for the vibrational bands of NC(Ag) and NC(pd). In addition, isocyanide can bind to one, two, and three adjacent Pd atoms for the generation of atop, bridge, and hollow configurations, respectively, giving rise to bands pf NC(Pd)-atop, NC(Pd)-bridge, and NC(Pd)-hollow with down-shifted frequencies. By monitoring the evolution of the -NC vibrational bands as a function of time, we were able to track the heterogeneous nucleation and early-stage deposition of Pd on Ag nanocubes. Most importantly, this in situ technique opens up the opportunity for investigating the explicit roles played by the reaction temperature and type of Pd(II) precursor in affecting the heterogeneous nucleation and growth of Pd-Ag bimetallic nanocrystals. This research greatly advances our understanding of the nucleation and growth of nanocrystals, paving the way for rational and deterministic synthesis of nanomaterials with controlled properties
This research has been supported by the National Science Foundation (NSF) through the Macromolecular, Supramolecular and Nanochemistry (MSN) Program (CHE-1708300, 8/1/2017–7/3/2021). We have published 11 peer-reviewed papers, including an invited review article to Chemical Reviews.
2. Putting Nanomaterials to Work for Biomedical Research
Because of their small sizes and unique properties, nanocrystals are finding widespread use in studying complex biological systems. This work aims to advance biomedical research by developing new tools and methods based on functional nanocrystals. Our current efforts include development of gold nanocages as contrast agents for optical imaging modalities (e.g., optical coherence tomography, photoacoustic tomography, and multi-photon luminescence), and as photothermal agents for therapeutic treatment. We are exploring the use of gold nanocages and other metal nanocrystals as substrates for SERS- and LSPR-based detection. We are developing nanoscale capsules by integrating gold nanocages with smart polymers and/or phase-change materials for targeted delivery and controlled release with superb spatial/temporal resolutions. We are applying electrospun nanofibers to neural tissue engineering, drug delivery, stem cell research, and repair of tendon-to-bone insertion. In addition, we are designing new colloidal particles with superparamagnetic features for separation, detection, manipulation, and tracking of biological species and cellular events. All these research activities will contribute to the emerging fields of nanomedicine and regenerative medicine.
Revitalizing Silver Nanocrystals as a Redox Catalyst by Modifying Their Surface with an Isocyanide-Based Compound
Silver nanocrystals have found widespread use as catalysts toward oxidation reactions such as ethylene epoxidation. In contrast, Ag shows limited catalytic activities toward reduction reactions when compared to other platinum-group metals (PGMs). One approach to overcome this limitation is to decorate the surface of Ag nanocrystals with a different PGM for the fabrication of Ag-based bimetallic nanocrystals. To this end, we pioneered the synthesis of Ag@M (M: Pt, Pd, Au or Rh) nanocrystals with a core-frame, core-shell, or core-satellite structure. The results are summarized in an invited review in Accounts for Chemical Research (2017). The successful development of such bimetallic nanocrystals offers a probe with integrated plasmonic and catalytic activities for catalyzing a reaction while reporting on the chemical species involved through in situ SERS. An early communication was published in Journal of the American Chemical Society (2015). An invited review was recently published in Angewandte Chemie International Edition (2020). Another avenue to expand the catalytic capacity of Ag nanocrystals is to modify their surface by introducing an organic ligand, but it has been known that the organic ligands sticking to the surface could relentlessly “poison” the catalytic particles by simply blocking the active sites. In the work published in Chemical Science (2020)., we discovered that isocyanide compounds could adsorb on the surface of Ag nanocubes to enable Ag-catalyzed redox reactions between isocyanide and nitroaromatic molecules toward the production of an aromatic azo compound.
This research has been supported by the American Chemical Society Petroleum Research Fund (PRF# 59664-ND10, 9/1/2019-8/3/2022).
3. Facet-controlled Nanocrystals for Catalysis
Nanocrystals hold the key to the continuous progress towards a cleaner environment and sustainability. We aim to demonstrate the fabrication of facet-controlled nanocrystals with optimized properties for a range of related applications. For example, we are exploring the use of facet-controlled nanocrystals for improving the performance of fuel cells and catalytic converters. This research requires a superb understanding of structure-property relationships that guide the design and synthesis of novel nanocrystals with well-controlled sizes and shapes (or facets) for specific applications.
Replacement-Free Synthesis of Bimetallic Nanocrystals
Bimetallic nanocrystals have received ever-growing interest owing to their properties often superior to the monometallic counterparts. Among various methods, seeded growth represents a powerful route to bimetallic nanocrystals. This approach is built on the concept that preformed nanocrystals with uniform and well-controlled size, shape, and lattice/twin structure can serve as seeds to template and direct the deposition of a different metal. The capability of this approach is, however, critically limited by galvanic replacement reaction when the metal to be deposited is less reactive than the seed (e.g., when Ag nanocrystals are used as seeds for the deposition of Au, Pd, or Pt). The involvement of galvanic replacement not only makes it difficult to control the outcome of a synthesis but also causes degradation to structures and properties. Although a number of groups had attempted to suppress the galvanic reaction, it remained a challenge until a few years ago when Dr. Qin successfully established the scientific basis and rules for achieving galvanic replacement-free growth of a metal on the seeds made of a more reactive metal. Her strategy relies on the introduction of a faster parallel reaction to compete with and thereby suppress the galvanic reaction. When a salt precursor is titrated into a suspension of seeds in the presence of an additional reducing agent, the precursor can be reduced by both the reducing agent (via chemical reduction at a rate of Rred) and the seeds (via galvanic replacement at a rate of Rgal). Under the condition of Rred > Rgal, the precursor will be primarily reduced by the reducing agent instead of participating in the galvanic reaction. When self-nucleation is eliminated by introducing the precursor dropwise, the metal atoms derived from the chemical reduction can be directed to nucleate from the surface of the seeds, generating bimetallic nanocrystals with a core-frame or core-shell structure. This research has been supported by the National Science Foundation (NSF) through the Macromolecular, Supramolecular and Nanochemistry (MSN) Program (“Replacement-Free Growth of Au on Ag Nanocrystal Seeds”, CHE-1412006, 2014–2017). During this project, Dr. Qin has already published 10 peer-reviewed papers, including an invited review article to Accounts for Chemical Research.