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ItemSeed-Mediate Synthesis of Gold Nanocrystals: The Effects of Lattice Mismatch on Growth Patterns(Georgia Institute of Technology, 2023-12-08) Pawlik, Veronica DanaNanomaterials have long fascinated both the casual observer and scientific mind alike. The utility of Au nanocrystals in particular has inspired applications ranging from plasmonics to catalysis. Over time, the ability to finely tune both shape and size has greatly improved their merits for these applications. To further expand and enhance the properties of Au nanocrystals, bimetallic compositions were introduced. Of the various atomic arrangements possible for bimetallic nanocrystals, the core-shell structure is most commonly utilized. This morphology is typically synthesized through a seed-mediated process. Growing one metal on another can introduce challenges. In this dissertation, I explore the effects that increasing lattice mismatch has on the seed-mediated growth of noble-metal nanocrystals. First, the case of no lattice mismatch was investigated during the growth of Au on Au spherical seeds to generate AuRD fully enclosed by {110} facets. The lack of lattice mismatch led to layer-by-layer growth. The kinetics of the synthesis could easily be tuned to favor either deposition or diffusion to achieve concave RD, trisoctahedra, or octahedra. These AuRD were then utilized in another seed-mediated growth to improve the thermal stability of the AuRD. Specifically, 1 ML of Pt was added and this ultrathin layer of Pt was able to improve the thermal stability of the high energy {110} facets from degrading at 100 °C to persisting at 450 °C. Computational studies revealed that the thermal stability of the Au-supported Pt skin was even greater than that expected for pure Pt. This effect was attributed to the strain induced by the formation of a 3.8% lattice-mismatched Pt overlayer on Au. Finally, single-crystal Rh@Au truncated octahedra were synthesized at a lattice mismatch of 7.2%. The large mismatch led to an island growth mode, which, could be tuned through the use of gentle kinetic knobs. The addition of NaOH indirectly increased the reduction rate to help modulate the number of Au islands formed on the Rh seeds. Conversely, the addition of KBr slowed down the reduction, allowing the Au adatoms to diffuse across the Rh seed. This work provides insight into the effects of lattice mismatch on the growth mode of nanocrystals, moving one step closer to the rational synthesis of novel nanomaterials with desired characteristics.
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ItemKinetically-Controlled Synthesis Of Mono-, Bi-, And Multi-Metallic Nanocrystals(Georgia Institute of Technology, 2023-07-24) Wang, ChenxiaoCrucial in a myriad of applications ranging from catalysis to biomedicine, noble-metal nanocrystals exhibit physicochemical properties strongly governed by their size, morphology, and composition. The strong correlations offer opportunities to optimize their figures of merit, thereby augmenting their overall effectiveness. As research advances from simple mono-metallic nanocrystals to multi-metallic and hybrid nanostructures with diverse architectures and atomic distributions, the escalating complexity presents synthetic chemists with ever-increasing challenges. In this dissertation, I develop two general strategies, namely template-mediated growth and dropwise injection of precursor, aiming to control the structural characteristics of mono-, bi-, and multi-metallic nanocrystals, while exploring their potential applications in catalysis and biomedicine. First, amorphous Se nanospheres were employed as templates to mediate the nucleation and growth of Au nanoparticles through a galvanic replacement reaction. By leveraging the reducing power of Se and the pH-sensitive reaction kinetics, precise control over the size and number of Au particles on each Se sphere was achieved, resulting in hybrid nanoparticles with diverse morphologies. The presence of Au patches on these hybrid nanoparticles provides an experimental handle to optimize the ligand distribution, significantly augmenting cellular uptake and cytotoxicity for the Se nanospheres. Shifting focus to a bi-metallic system, I employed Pd cubic nanocrystals as templates to direct the surface deposition of Rh in a layer-by-layer manner. With rigorous regulation of the reaction kinetics, I successfully synthesized Pd@Rh nanocrystals featuring smooth, well-defined {100} facets and large sizes. The strong Rh−Rh binding within the shell imparted exceptional thermal stability to the core–shell nanocubes. Afterwards, chemical wet etching was employed to fabricate Rh nanocages with well-defined {100} surface and ultrathin walls from the core–shell nanocubes. Building upon these insights, I extended the two strategies to control the composition of complex alloys. By utilizing well-defined and highly stable Rh cubic nanocrystals as templates, together with a tight control over the reduction kinetics through dropwise injection of the precursor mixture, cubic-shaped nanocrystals featuring a nearly equimolar RuRhPdPt alloy surface were obtained. These alloy nanocubes demonstrated superior thermal stability in terms of both shape and composition, along with enhanced catalytic performance toward ethanol oxidation.
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ItemPhase-Controlled Synthesis of Ruthenium and Palladium Nanocrystals(Georgia Institute of Technology, 2022-06-21) Janssen, Annemieke LouiseThe utility of noble-metal nanocrystals in applications ranging from catalysis to biomedical applications has increased with the ability to finely tune their shapes and sizes. In particular, the catalytic activity of the nanocrystals is strongly affected by their shape, as this parameter is directly related to the atomic arrangement on the surface. Another way to alter the surface atomic arrangement is through changing the crystal structure (or phase) of the nanocrystal, a property known as polymorphism. A powerful method to achieve such control over the crystal structure is through template-directed growth to obtain metastable core-shell nanocrystals. In this dissertation, I present a number of studies delving into the mechanistic details behind template-directed phase control for Pd and Ru, alongside an evaluation of their catalytic properties. First, the importance of particle size on successful template-directed deposition was demonstrated through deposition of Ru on 12-, 18-, 22-, and 26-nm Pd nanoplates, where small nanoplates resulted in fcc-Ru shells, while the larger ones gave hcp-Ru overgrowth. The size dependence was ascribed to a trade-off between the bulk and surface energies that changed with particle size. On small nanoplates, the high proportion of total surface area coming from the side faces makes it favorable to grow fcc-Ru, which deposits smoothly on the side facets (low surface energy) at the expense of forming a metastable phase (high bulk energy). For large nanoplates, only a small proportion of the surface area comes from the side, promoting the growth of hcp-Ru as the resulting jagged side faces (high surface energy) could be compensated by the formation of a thermodynamically stable phase (low bulk energy). To further elucidate the mechanistic details involved in phase-controlled synthesis, the influence of the template’s shape was investigated next. When Ru was deposited on 8-25 nm Pd cubic nanocrystals, the Ru shell took an fcc phase, but on 14-26 nm Pd octahedral nanocrystals, the Ru was deposited as fcc on the small templates before reverting to hcp on the larger ones. The {100} facets displayed on cubic templates forced the Ru to take the fcc phase due to a symmetry mismatch between the facets of the fcc¬-Pd template and hcp-Ru, while on octahedral templates, the displayed {111} facets could be symmetrically aligned with either hcp- or fcc-Ru, allowing for the overgrowth of either crystal structure. Thus, on octahedral templates, the crystal structure depends on particle size and it is determined by the balance of surface and bulk energies. With an improved understanding of template-directed phase control, this method could be extended to obtain hcp-Pd deposition on an hcp¬-Ru template. Under careful control of the reaction conditions, Pd could be deposited on an hcp-Ru template in either the standard fcc phase, or in a novel, metastable hcp phase. It was essential to slow down the reduction rate of the Pd precursor in order to obtain phase-controlled Pd. The ability to control the crystal structure of noble-metal nanocrystals, coupled with a mechanistic understanding of this process, will enable the development of nanostructured materials with unique properties through rational and deterministic syntheses.
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ItemPlatinum-Based Nanocrystals and Their Use as Electrocatalysts toward Oxygen Reduction(Georgia Institute of Technology, 2021-10-01) Xie, MinghaoPlatinum (Pt) is an intriguing catalytic material for a variety of reactions. One of the most important applications of Pt is in catalyzing the oxygen reduction reaction (ORR), a process key to the operation of fuel cells and metal-air batteries. However, its extremely low abundance in the earth crust and ever-increasing price have created a barrier to the large-scale use of this metal. One solution to this issue is to increase the atomic utilization efficiency (AUE) of Pt. This dissertation is focused on the development of synthetic strategies for the facile synthesis of Pt-based nanocrystals with well-controlled surface structures, compositions, and crystal structures, in an effort to optimize their catalytic performance toward ORR. Based on an oil-phase synthesis, I studied the nucleation and growth of Pt nanocrystals, revealing the roles played by oleic acid at different stages of the synthesis. Afterwards, my coworker and I designed a facile route to the production of Pt-Co truncated octahedral nanocrystals with tunable sizes and compositions. The nanocrystals delivered enhanced mass activity compared to those of commercial catalysts in both liquid half-cell and fuel cell tests. I also scaled up the production by in situ growth of Pt-Co nanocrystals on the surface of carbon supports. The effect of carbon supports on both synthetic and catalytic process were analyzed by comparing the electrochemical performance of the nanocrystals grown on different types of carbon supports. In the last project, I developed a strategy for the synthesis of Pt-Co@Pt octahedral nanocrystals featuring an intermetallic, face-centered tetragonal Pt-Co core and an ultrathin Pt shell, together with the dominance of {111} facets on the surface. When evaluated as a catalyst toward ORR, the nanocrystals delivered a mass activity which was 13.4 times higher than that of a commercial Pt/C catalyst. More significantly, the mass activity of the nanocrystals only dropped by 21% after 30,000 potential cycles, promising an outstanding catalyst with optimal performance for ORR.
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ItemControlling the Synthesis of Palladium and Platinum Nanocrystals for Electrocatalytic Applications(Georgia Institute of Technology, 2021-07-27) Chen, RuhuiNoble-metal nanocrystals with well-controlled attributes have found use in a variety of applications, including those related to electrocatalysis and energy conversion. Since the electrocatalytic performance of noble-metal nanocrystals critically depends on their sizes, shapes, and internal structures, gaining deep insights into their nucleation and growth is essential to achieving an ultimate control over their colloidal synthesis. In this dissertation, I present a number of methods for controlling the synthesis of Pd and Pt nanocrystals to improve their purity and uniformity in terms of size, shape, and structure, together with evaluation of their electrocatalytic performance toward oxygen reduction. First, the synthesis of Pt nanobars with aspect ratios tunable up to 2.1 was demonstrated by simply heating a Pt(IV) precursor in N,N-dimethylformamide in the presence of poly(vinyl pyrrolidone). A mechanistic study revealed that both particle coalescence and localized oxidative etching followed by preferential growth contributed to the anisotropic growth vital to the formation of nanobars. Recognizing the challenge in mass production of metal nanocrystals without compromising the product quality, a continuous and scalable route to Pt multipods was then developed by switching from a batch to a continuous flow reactor. Investigation on the morphological evolution of Pt multipods indicated that the anisotropic growth arose from a combination of fast autocatalytic surface reduction and attachment of smaller particles. When supported on carbon, the Pt multipods exhibited enhanced activity toward oxygen reduction relative to the commercial Pt/C catalyst. In addition to scaling up the production volume, the tubular flow reactor was further demonstrated as a powerful tool to tightly control the nucleation step of a synthesis. With Pd nanocrystals as an example, the nucleation and growth could be decoupled from each other using a flow reactor to trigger the nucleation, generating nanocubes highly uniform in both size and shape. Both the temperature and duration for nucleation were found to significantly impact the seed diversity and thus the quality of resultant nanocrystals. This methodology was also successfully extended to the preparation of uniform, sub-5 nm Pt nanocubes. Utilizing this strategy, Pt right bipyramids with a single twin plane and covered by {100} facets were prepared in high quality. The Br- ions involved in the synthesis, as well as the pair of temperatures used for the nucleation and growth steps, played critical roles in mediating the formation of singly-twinned seeds and directing their evolution into right bipyramids. The ability to control the nucleation and growth, coupled with in-depth mechanistic understanding of these processes, will contribute to the rational synthesis of noble-metal nanocrystals with desired features for electrocatalytic and other applications.
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ItemCopper-Based Nanocrystals and Their Use as Catalysts for the Electrochemical Reduction of Carbon Dioxide(Georgia Institute of Technology, 2020-12-06) Lyu, ZhihengBenefiting from the high abundance, low price, and fascinating properties of copper (Cu), Cu and Cu-based nanocrystals have found wide-spread use in many applications. In recent years, they received increasing attention as catalysts due to their uniqueness in generating substantial amounts of hydrocarbons and oxygenates in electrochemical CO2 reduction. Both experimental and computational studies suggest that the selectivity and activity of Cu-based catalysts are highly dependent on their surface structures, emphasizing the importance of shape-controlled synthesis of Cu nanocrystals. In this dissertation, I will introduce different strategies of synthesizing Cu nanocrystals with well-defined shapes, together with their catalytic performance in electrochemical CO2 reduction. By leveraging the reduction potential difference, a trace amount of Pd(II) was introduced to the Cu(II) precursor to induce the formation of Pd seeds, onto which Cu atoms deposited and grew into a right bipyramidal shape enclosed by {100} facets and twin boundaries. The coordination of hexadecylamine to metal ions was also revealed, which significantly slowed down their reduction rates and contributed to the generation of multiple parallel planar defects in a seed. Switching from one-pot synthesis to seed-mediated growth, Au@Cu core-shell nanocubes with sizes below 30 nm were produced, taking advantage of the small size of 5-nm Au seeds. Due to the large lattice mismatch (12%) between Au and Cu, an island growth mode was observed for Cu, resulting in a random location of Au core inside the Cu shell. When applying Pd icosahedra with a relatively larger size (13 nm) as seeds, Pd-Cu Janus nanocrystals with different shapes and twinned structures were obtained. By varying the reduction rate of Cu(II) precursor from slow to fast, Cu atoms selectively grew from either the vertex or the edge of an icosahedral seed, leading to the formation of penta-twinned or singly-twinned nanostructures. The presence of twin boundaries, exposure of Pd as a CO generator, and phase-segregated morphology of Pd and Cu all made Pd-Cu nanocrystals promising catalysts for electrochemical CO2 reduction. In addition to manipulating the twin defects and composition, I also demonstrated that both C2+ selectivity and catalytic stability of Cu nanocrystals could be improved by introducing surface oxidation and controlling the oxidation pathway. Compared to Cu nanowires oxidized by O2, which possessed a rough surface and non-uniform oxide layer, those oxidized by H2O2 showed a much smoother surface covered by oxide sheath with even thickness. The uniformity of the sheath greatly mitigated the fragmentation of nanowires, contributing to their superior stability in CO2 reduction.