"If the path be beautiful, let us not ask where it leads."
-- Anotale France
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.
Understanding Heterogeneous Nucleation in Nanocrystal Growth with Molecular Probes
Despite the successful synthesis of various bimetallic nanocrystals through seeded growth, it has been extremely challenging to resolve the second metal deposited on the surface of a seed, in particular, when the deposited amount is below one monolayer. 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 can induce unexpected reactions, making it difficult to elucidate the mechanistic details. It is also challenging to resolve the elemental composition during the course of a solution-phase synthesis by these imaging- or diffraction-based techniques. Built upon the success of a prior NSF project (CHE-1412006), Qin’s renewal proposal–“Understanding Heterogeneous Nucleation in Nanocrystal Growth with Molecular Probes”–has been funded for three years by the MSN Program (CHE-1708300, 2017–2020). Dr. Qin proposed to turn the synthesis of core-frame nanocubes into a model system for investigating the heterogeneous nucleation and growth of a second noble metal on the surface of Ag nanocrystals in solution using an isocyanide-based SERS probe (R-NC). With a starting date of August 1, 2017, Dr. Qin seeks to fully develop the in-situ SERS technology, with a remarkable capability for metal differentiation and detection sensitivity down to the sub-monolayer level, to elucidate the mechanistic details involved in the seeded growth of a catalytically significant noble metal on the surface of a Ag or Au nanocrystal seed.
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.
Bimetallic Nanocrystals for Catalyzing and Probing Stepwise Reduction and Oxidation Reactions
Aromatic azo compounds are high-value chemicals extensively utilized as pigments, drugs, and food additives, but their production typically requires stoichiometric amounts of environmentally unfriendly metals or nitrites. There is an urgent need to develop a dual catalytic system capable of reducing nitroaromatics to aromatic amines and then oxidizing the amines to azo compounds. In 2016, Dr. Qin demonstrated, for the first time, the formation of trans-4,4’-dimercaptoazobezene (trans-DMAB) during the reduction of 4-nitrothiophenol (4-NTP) by NaBH4 under ambient conditions when Ag@Pd-Ag core-frame nanocubes were employed as a dual catalyst and an active SERS substrate. Remarkably, the time-dependent SERS spectra reveal three sequential processes that include the Pd-catalyzed reduction of 4-NTP to 4-aminethiophenol (4-ATP) by hydrogen, a period during which the 4-ATP remained until all the hydrogen had been depleted, and the Ag-catalyzed oxidation of 4-ATP to trans-DMAB by the O2 from air. This research promises a sustainable approach to the production of aromatic azo compounds.
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.
Shape Stability of Colloidal Ag Nanocrystals
Noble-metal nanocrystals are of critical importance to an array of fundamental studies and a broad spectrum of applications in photonics, sensing, imaging, and catalysis. It has been established that the properties of these nanocrystals and their performance in an application are determined by a set of parameters, including size, shape, composition, and structure. Among these parameters, shape has received the greatest interest owning to strong correlations between the shape of a nanocrystal and its properties. Despite the remarkable success in synthesizing noble-metal nanocrystals with many diversified shapes, it remains a grand challenge to preserve the shapes with sharp corners and concave faces. In fact, there is an ever-increasing gap between the ability to synthesize noble-metal nanocrystals with different shapes and the competence to preserve these shapes. We are filling this gap by systematically investigating the physical and chemical changes experienced by noble-metal nanocrystals in a colloidal suspension, and thereby identify effective strategies for the preservation of their shapes. I focus on Ag nanocrystals because Ag is the most reactive noble metal and it has been most challenging to preserve the sharp corners on Ag nanocrystals.