Plate-Like Colloidal Metal Nanoparticles

2.6.1. General Processes in Postsynthetic Modification of Metal Nanoplates

To contextualize the intrinsic and extrinsic factors that drive synthetic outcomes in postsynthetic modification of metal nanoplates, we briefly review common metal redox processes and metal deposition on metal thin films. As we have already seen in the initial synthesis of nanoplates, interactions between metal ions and reduced metal species can dramatically impact final particle outcomes. Two processes that are widely used for nanoplate modification are GRR and UPD. GRRs involve the oxidation of a zerovalent metal particle or substrate by an incoming metal ion of higher reduction potential.⁹⁶⁻¹⁰¹ UPD refers to the deposition of metal monolayer(s) on another metal substrate at a potential that is less negative than the equilibrium reduction potential of the depositing metal, and it is typically attributed to strong electronic interactions between substrate and depositing metal, which alter the reduction potential of the depositing metal from its bulk value.^102−104

In addition to these redox processes, principles from thin film metal deposition can also predict outcomes in nanoplate modification. Factors such as lattice mismatch and bond dissociation energy direct metal-on-metal deposition outcomes and have been classified into three categories, depending on the final morphology of the deposited metal: island, layered, and layer + island.^105,106 For island growth modes, a portion of the underlying substrate remains exposed after metal deposition (provided there is no postsynthetic fusion or ripening processes). This growth mode, also called Volmer–Weber (VW) growth, leads to islands of the depositing metal, even at surface coverages equivalent to greater than one monolayer, and is typically attributed to the incoming metal having more favorable enthalpic interactions with itself than with the substrate. Layered growth, also referred to as Frank–van der Merwe (FM) deposition, proceeds by the successive formation of continuous monolayers of the depositing metal.¹⁰⁷ Here, a complete monolayer of the secondary metal is deposited prior to the formation of additional atomic layers. A combination of layer + island growth may also be observed and is called Stranski–Krastanov (SK) growth. In SK growth, layered growth transitions into an island growth pathway after reaching a critical surface coverage threshold.¹⁰⁸

2.6.2. Nanoislands on Nanoplates

Like in the case of thin films, island deposition on nanoplates may be driven by metal–metal interactions such as bond enthalpies, lattice mismatch, and redox interactions. However, in the case of nanoplates, the secondary metal deposition may also be guided by surface ligand modifications. For nanoisland formation on nanoplates, the most frequently studied metal combination has been Pt deposition on Au, where Pt is known to deposit on Au substrates via VW growth modes due to the enthalpic driving forces of Pt–Pt bond formation vs Pt–Au, and this deposition motif has been observed on other Au nanoparticle substrates as well.¹⁰⁹

Synthesis of nanoisland morphologies on nanoplates adapt the same seed-mediated approach used for the preparation of metal nanoplates, where the underlying metal nanoplate is synthesized separately and then introduced to a new chemical environment that facilitates the growth of island deposits. Studies have shown that reaction kinetics are important in determining the driving forces that dominate the observed deposition morphologies.¹¹⁰ In the case of Pt islands on Au, using ¹⁹⁵Pt-NMR studies of Pt(IV) speciation in water, the populations of various Pt(IV) complexes could be identified, and showed that Pt(IV) precursor species that were more aggressive oxidants led to competitive GRRs in solution, ultimately creating heterogeneous Pt–Au alloyed triangular frames. However, when speciation of Pt(IV) produced a less aggressive oxidant, the surface chemistry of the underlying Au platelet, combined with Pt–Pt and Pt–Au bond enthalpies, explained Pt deposition patterns that arose in regular arrays of Pt islands (Figure 7). A tilt series of high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were used to study the 3D structure of Pt islands via tomographic reconstruction (Figure 7).¹¹¹ Similar to Pt deposition on films of Au(100) and Au(111),¹¹² the Pt islands were found to exhibit irregular “tree-like” morphologies, where islands branch outward as they grow longer, consistent with a dendritic growth pathway often observed for larger Pt nanoparticles.¹¹³

When considering the driving forces that determine the arrangement and size of nanoislands, as well as their development vs other redox-mediated seed modifications, it may be difficult to decouple the influence of surface crystallinity and surface chemistry for a specific pathway of metal deposition. In 2010, Han and co-workers investigated the impact of substrate crystallinity on the observed mode of island deposition.¹¹⁴ Here, Au nanoparticles bound by low index facets (e.g., Au(100), Au(110), and Au(111)) were used as substrates for Pt island deposition. Epitaxial deposition of Pt islands was observed on all facets, suggesting no crystallographic preference for Pt island nucleation and growth. However, when there is a large difference in crystal facet reactivity, due to either the surface energy or facet–ligand interactions, facet-selective island deposition is observed. For example, Millstone et al. have shown the ability to site selectively deposit Pt and Pd metals onto Au nanoplates using surface ligands to selectively block growth on well-passivated sites.^110,111,115

Redox processes such as UPD and GRRs can also strongly influence the mode of island/secondary metal deposition on nanoparticle substrates, leading to hybrid and ultimately frame-like structures. For example, Park and co-workers synthesized Au@Pt nanoplates. Here, a Ag UPD layer was precoated on the surface of Au nanodisks and then Pt atoms were selectively grown on the periphery of Au nanodisks via a GRR between the Ag UPD layers and Pt (IV) species, leading to the formation of Au@Pt nanodisks (Figure 8).^116,117

2.6.3. Nanoframes

In addition to producing island-like hybrid structures, processes such as GRR and UPD can build from nanoplate scaffolds to yield a wide array of both compositionally and architecturally complex nanostructures. One of the earliest demonstrations of these plate-to-frame transformations used Ag nanoplates as a starting template. Here, Au/Ag triangular and circular nanoframes were obtained via GRR between Ag nanoplates and Au(III) precursors.^118,119 Au(III) reacted with (111) facets of the Ag nanoplates, and led to the formation of central voids while retaining the triangular shape (Figure 9).

Over the past two decades, GRRs have been used to synthesize progressively more complex systems via further refinement of reaction conditions. For example, Han and co-workers synthesized particle-in-a-frame (PIAF) nanostructures by conducting the reaction at low temperature (0 °C). The lower temperature allowed site-selective GRR on the edge regions of Ag nanoplates (due to higher surface energy of the nanoplate edges relative to their terrace regions). This process first creates isolated Au deposits which then grow and ultimately fuse over the course of the reaction yielding Au PIAF nanostructures (Figure 10).¹²⁰

In addition, multistep chemical reactions including successive and selective deposition and etching steps have been shown to create framed structures from nanoplate starting materials.^121,122 For example, Park and co-workers have developed a versatile synthetic approach that includes successive redox-mediated reactions such as metal deposition of Pt via GRR between Ag UPD layers and Pt(IV) ions, and selective etching of the Au domain via the comproportionation reaction of Au ions. Thus, Au nanodisks could be transformed into PtAu nanorings with various shapes (e.g., circular, triangular and hexagonal shapes) could be synthesized by applying this synthetic strategy to other shapes of Au nanoplates (Figure 11).¹²³

2.6.4. Epitaxial Growth

Epitaxial growth of secondary metals on metal nanoplates has also produced a wide range of nanoplate-based heterostructures. In nanoisland deposition, VW growth modes dominate. In the following examples, secondary metal deposition proceeds either by FM or SK growth modes and produces additive rather than hollow nanostructures, with fundamental symmetries imposed by the original plate-like particle shapes. For epitaxial growth of secondary metals on preformed nanoparticles, a commonly quoted rule of thumb is that a lattice mismatch between seeds and secondary metals must be smaller than ∼5%.¹²⁴

Epitaxial secondary metal growth has been shown for both homogeneous and heterogeneous metal combinations. In the homogeneous case, the same metal as the starting platelet is added to the nanoplate solution and used to control nanoplate edge length, thickness, or shape. In one example, Mirkin and co-workers investigated the mechanism of selective growth of Au on the edge regions of Au nanoplates with respect to surface crystallinity and suggested a synthetic strategy for controlling the lateral size of Au nanoplates via multistep overgrowth of Au (Figure 12A).¹²⁵ Yin and co-workers investigated how the reduction rate of Ag and the presence of citrate ions as a facet-blocking agent impact the lateral size of resulting Ag nanoplates (Figure 12B,C).²⁸ A similar effect was reported by Kuttner et al.¹²⁶

Shape and thickness of nanoplates can also be controlled through epitaxial growth of secondary metals on nanoplates. In one example, Park and co-workers controlled the shape of nanoplates from circular to hexagonal via deposition of Au in the presence of cetyltrimethylammonium bromide (CTAB) and I^– (which can preferentially block Au (111) facets).¹²⁷ Wang and co-workers modulated the thickness of Au hexagonal nanoplates by controlling the amount of Au added onto the Au nanodisks and the concentration of CTAB in the absence of iodide ions.¹²⁸

Like halide ions in the previous examples, different ligand types have been used to modulate epitaxial growth. For example, the growth mode of Ag on Ag nanoplates (e.g., edge- vs face-favored growth) could be modulated by considering different affinities of polyvinylpyrrolidone (PVP) and citrate ions toward (111) and (100) facets of Ag nanoplates, respectively.¹²⁹ Iodide ions again play a crucial role in the growth of Ag on Au nanodisks. In the presence of iodide ions, Ag atoms could be deposited on both the top and bottom faces of Au nanodisks because iodide ions tend to preferentially adsorb on Au (111) facets and Ag ions readily react with iodide ions due to their high affinity for AgI (K_sp(AgI) = 8.3 × 10^–17), and these processes ultimately lead to 3D Au@Ag dodecahedra. On the other hand, in the absence of iodide ions, Ag atoms were uniformly deposited on the entire surface of Au nanodisks resulting in Au@Ag core–shell nanoplates (Figure 13).¹³⁰

Interestingly, ligands have even been used to elicit chiral growth on Au nanoplates. In work by Ma et al., chiral ligands such as l-cysteine and l-glutathione were postsynthetically added to cetyltrimethylammonium chloride (CTAC)-coated Au nanoplates in the presence of ascorbic acid and HAuCl₄, to yield propeller like structures with well-defined chiral geometries, as evidenced by circular dichroism analysis. Moreover, the degree of chirality could be tuned via the proportion of chiral ligands added during synthesis.

Finally, it should be noted that all of the above-discussed processes: island growth, redox-mediated frame formation, and epitaxial growth, can be observed within a single system with only subtle changes in reaction conditions. For example, morphology evolution from Au nanoplates into Au tripods was observed by exploiting the different redox potentials of triangular nanoplate edges and tips.¹³¹ Corner- and edge-selective growth modes of Au on Au nanoplates could be controlled by considering the chemical potential of Au atoms in solution and the accessibility of solutes to different sites (e.g., edge, corner, or face) (Figure 14).¹³²

3.1.1. Copper

Cu has been considered the minor player of the coinage triad, due to its tendency to oxidize rapidly in the presence of oxygen and its lower plasmonic efficiency in the visible part of the electromagnetic spectrum.^143,144 For these reasons, the colloidal chemistry of Cu is often focused on the use of Cu oxides and sulfides for the preparation of semiconductor nanoparticles.¹⁴⁵ Cu sulfide can also serve as the starting material for cation-exchange processes in postsynthetic modification of colloids, exploiting solvation energies and Lewis acid–base interactions.¹⁴⁶ More recently, Cu metallic nanoparticles received a renewed interest as earth-abundant element for sustainable plasmon-driven catalysis,¹⁴⁷ as well as high quality plasmonic nanoparticles in the mid-infrared.^148−150 The synthesis of 2D metal Cu colloids was first reported by Pileni’s group, using an excess of hydrazine to reduce Cu bis(2-ethylhexyl)sulfosuccinate in a reverse micellar system.^151,152 The prepared nanodisks presented an average diameter of 27 nm, and high resolution (HR) TEM and X-ray diffraction (XRD) analysis confirmed they were fcc crystals with extended parallel {111} facets, developed through the formation of stacking fault defects. The use of PVP and hydrazine in dimethylformamide (DMF) was proposed by Pastoriza-Santos et al. a few years later, successfully improving the overall shape yield (>95%) and increasing the aspect ratio of the synthesized platelets, which present an edge length of 50 nm and a thickness of 20 nm.¹⁵³ This geometrical modification results in a red-shift of the colloids’ localized surface plasmon resonance (LSPR) away from Cu interband transitions, thereby reducing damping and enabling the formation of an intense and sharp extinction peak around 600 nm. Moreover, the use of DMF as solvent and PVP as stabilizing agent enabled the authors to perform the whole synthesis under ambient atmosphere without significant oxidation. The same polymer was used in other synthetic setups to increase the aspect ratio of Cu platelets even further (200 nm edge length, only 8 nm in thickness), improving their catalytic performance and their stability under the reaction conditions. For example, Sun et al. exploited potassium sodium tartrate to lower the reduction rate of the Cu precursor and kinetically induce the formation of large platelets.¹⁵⁴ Cu nanoplates were also synthesized in hydrothermal approaches using milder reducing agents and basic pH.^155−160 These methodologies yield Cu nanoplates with extremely high aspect ratios (up to 30 μm in edge length for a thickness of ∼100 nm);¹⁵⁹ however, this comes at the expense of overall quality and size dispersity in the colloid. Two recent publications demonstrate the possibility of synthesizing Cu 2D nanoparticles under similar conditions as those used for the other coinage metals. Luc et al. used Cu nitrate, ascorbic acid, and CTAB for the preparation of triangular Cu nanosheets showing high performance for the electrochemical production of acetate from carbon monoxide.¹⁶¹ In 2022, Wu et al. proposed to establish a balance between oxidative oxygen gas and reductive glucose to control the formation of Cu nanodisks. The reaction takes up to 3 days to complete, offering a better control over size dispersity and shape-yield (an average of 80 nm in edge length and 7 nm in thickness,Figure 16A,B).¹⁶² The obtained Cu nanodisks show promising catalytic activity for the electrochemical reduction of nitrate to ammonia, a zero CO₂ emission process alternative to the standard Haber-Bosch process.^163−166

3.1.2. Palladium

Pd is a key component for several catalytic processes, including CO₂ reduction,¹⁶⁷ hydrogen production (and storage),¹⁶⁸ as well as important carbon–carbon coupling reactions.¹⁶⁹ The activity and efficiency of these catalysts varies greatly, depending on the specific crystallographic facet that is involved in the reaction, and would therefore greatly benefit from the possibility to rationally design Pd surfaces. Control over the growth and geometry of Pd nanocrystals was first attempted by Xia and co-workers in the early 2000s, using a solvothermal/polyol method. In this approach, PVP was used as both capping and reducing agent. Despite the presence of extended low-energy {111} facets, the overall surface energy of nanoplates is higher compared to isotropic nanoparticles due to the presence of parallel stacking faults and high curvature areas at the tips, which are composed of partially uncoordinated metal atoms.³⁵ Consequently, a key aspect to favor nanoplate growth was identified in the use of kinetically controlled processes, deviating from thermodynamic control and inducing the formation of seeds with stacking faults that could grow into the nanoplate shape.¹⁷⁰ This was achieved by reducing the deposition rate of the metal precursor,¹⁷¹ varying PVP molecular weight,¹⁷² reaction temperature and pH, as well as through the use of oxidative etching to ensure a tighter control over the growth rate,^68,171 and mild reducing agents such as hydroxylamine.⁸⁰

Trinh et al. were able to significantly improve the outcome of the solvothermal/PVP approach by adding formaldehyde to the growth mixture.¹⁷³ Formaldehyde was proposed to adsorb selectively onto Pd {111} facets, thereby suppressing the elimination of {111} side surfaces,¹⁷⁴ slow down the growth kinetics (improving control over size dispersity), and resulting in the preferential formation of Pd hexagonal nanoplates over triangular ones (Figure 16C). The higher level of control over the formation of nanoplates enabled the extensive characterization of their catalytic effect on Suzuki coupling reactions.¹⁷³ Despite these improvements, the resulting colloids still present a rather high size polydispersity (∼30%). Moreover, the presence of high concentrations of PVP, which can drastically passivate the surface, represents a major drawback for the use of the synthesized colloids as heterogeneous catalysts. These observations, together with the progress achieved in the understanding of the role of halides in the synthesis of Au and Ag nanoparticles, shifted the focus of the community toward the use of small molecule adsorbates.³²

The role of halides has been investigated by various groups, who found them to play a similar role to the one described for Au and Ag: bromide stabilizes {100} facets,^34,175,176 whereas iodide induces the formation of penta-twinned structures such as nanowires, decahedra, and nanorods, probably through selective binding onto {111} facets.^177,178 However, the higher energy difference for the facets in Pd (and Pt) compared to coinage metals, prevents iodide from inducing the formation of platelets or triangular structures.¹⁷⁹ Interestingly, the quest for stronger and more selective surface-blocking agents was supported by the vast scientific literature dedicated to the catalytic activity of Pd.¹⁸⁰ In this direction, N. Zheng and co-workers successfully used carbon monoxide (CO, known poisonous species for Pd-based catalysts)¹⁸¹ to selectively promote the growth of structures with extended {111} planes, leading to the formation of ultrathin (1.8 nm thick) nanosheets.⁸⁸ In 2013, the same group was able to completely eliminate the use of PVP as a capping agent by implementing a carbonyl Pd complex ([Pd₂(μ-CO)₂Cl₄]^2–)) as the metal precursor in aqueous media, in the presence of CO.¹⁸¹

Li et al. were able to improve on this synthesis and gain control over the edge dimension of the nanosheets within a wider size range, from 6 to 60 nm, with standard deviation <20%.¹⁸² These results were achieved using tungsten hexacarbonyl (W(CO)₆), citric acid, and CTAB. The role of W(CO)₆ is 2-fold: on one side it enables a tighter control over the concentration of CO, while on the other it introduces high-oxidation state tungsten species that can promote the formation of Pd nuclei. Citric acid helps the degradation of W(CO)₆, while regulating the extent of nucleation at the first stages of the growth; moreover, citrate was shown to selectively bind on {111} facets, further promoting the stabilization of the platelet shape.^34,183 By including oxidative etching from I^–/O₂ pairs, Huan et al. recently demonstrated the synthesis and self-assembly of ultrathin nanodisks and nanorings with an impressively narrow size dispersion (Figure 16D).¹⁸⁴

Interestingly, even though seeded-growth protocols have been explored, they normally involve the use of already “shape-developed” seeds. In other words, despite some efforts in controlling the crystallography of the nucleated seeds, there has not been a consistent effort in the production of stable, rationally designed Pd seeds. This aspect certainly holds interesting potential to improve the yield and size dispersity of the prepared Pd colloids. It is worth mentioning that the exploration of Pd nanoparticle synthesis has also led to the preparation of exotic 2D shapes that have not been reported for Au. In a first example, Yin et al. demonstrated how, by taking advantage of the rich complexation chemistry of the d⁸ group to modulate the redox potential of the Pd precursor, it is possible to induce twinning in the [211] direction, leading to Pd hexagonal nanosheets presenting a thicker frame around their edges.¹⁸⁵ In an opposite direction, Li et al. exploited the combination of overgrowth and oxidative etching mediated by the presence of bromide and oxygen, to induce the transformation of ultrathin Pd hexagonal nanosheets into nanorings with remarkable catalytic performance.¹⁸⁶ In a third example, Yin et al. reported the synthesis of Hanoi Tower-like nanostructures composed of multilayered 2D nanosheets. The synthesis of this interesting geometry was obtained by exploiting the competitive adsorption of CO and Pd₄(CO)₄(OAc)₄ on Pd low-index facets. Finally, propeller-like Pd nanostructures have been prepared by favoring oxidative etching conditions during the formation of ultrathin 2D nanosheets.^187,188

3.1.3. Platinum

Similar to Pd, crystallographic control over Pt nanoparticle synthesis has radical implications toward their application as catalysts for several organic reactions, including hydrogenation and dehydrogenation, artificial photosynthesis, and fuel-cell reactions.^{149,189−192} The synthesis of Pt nanoparticles typically involves the reduction of Pt(II) or Pt(IV) precursors, which can take place both in aqueous and organic media, and can be performed using different reducing agents.^193−195 The exploration of shape control in Pt nanoparticle synthesis began in the late 1990s, together with the other fcc metals. Early work by El-Sayed and co-workers involving the use of hydrogen gas as reducing agent immediately identified reduction kinetics as a key aspect in controlling the final crystallography of the product.^196,197 Several years later, Xia and co-workers identified a similar trend in polyol-PVP solvothermal syntheses.¹⁹⁸ In one example, they took advantage of the slower reduction rate of nitroplatinate complexes, compared to chloroplatinate salts, to control the ratio of {111} and {110} facets, by adding sodium nitrate to the growth solution.¹⁹⁴ Moreover, the extension of thermolytic reduction to other high-boiling-point solvents (oleylamine, hexadecylamine, or oleyl alcohol) led to the preparation of branched structures including multipods and stars.¹⁹⁹ Finally, the use of halides as shape-directing agents was also explored in several studies for the preparation of cubic, octahedral, and flower-like Pt nanoparticles.^90,200−202 However, the controlled preparation of high quality Pt nanoplates remains elusive, with Pt epitaxial growth on preformed platelet structures and 2D template-confined growth as the only routes to access this type of structure.^203−206 The reason for this observation is likely found in the uniquely high internal strain energy associated with the formation of twin defects and stacking faults in Pt nanocrystals.^207,208

Notwithstanding, twinned Pt crystals have been synthesized in the form of planar nanomultipods, where the formation of transient triangular monotwinned seeds was proposed as the first necessary stage of the growth mechanism.^209,210 The first report of this type of structure dates back to 2006, when Maksimuk et al. reported the synthesis of planar tripods using a combination of Pt acetylacetonate, 1,2-hexadecanediol (the reducing agent), adamantanecarboxylic acid and hexadecylamine (forming a double capping agent system), and diphenyl ether (the solvent). By reducing the temperature of the synthesis to around 160 °C, the formation of planar tripods was maximized above 25%, with the major byproducts consisting of monopods and bipods and only a small fraction of Pt nanocubes (Figure 16F,G).²⁰⁹ The mechanism was proposed to involve the formation of crystal twinning in the {111} plane of Pt seeds (identified by the forbidden 1/3 {422} diffraction spot in the SAED pattern). This crystal defect leads to the formation of three re-entrant troughs along the same twin plane. Selective crystal growth along the three equivalent ⟨211⟩ directions induced the transformation of the troughs into ridges and then again into troughs. The alternation of these two features ultimately resulted in the formation of three coplanar branches in the[21̅1̅], [1̅21̅], and [1̅1̅2] directions.²¹¹ Interestingly, the aspect ratio of the prepared nanoparticles could be tuned between 7 and 20 by varying the reaction time.^209,211 The same growth mechanism was applied to penta-twinned structures by Lacroix et al., leading to the formation of 5-fold Pt nanostars using oleylamine and hydrogen gas as solvent/stabilizer and reducing agent, respectively. By carefully controlling the reduction rate, single and penta-twinned Pt seeds were stabilized and subsequently grown into branched structures (Figure 16E), with yields around 20%.²¹²

In a recent communication, Gu and co-workers were able to combine ammonium fluoride and hydrogen gas to induce the formation of Pt tribranched nanoribbons (Figure 16H,I).²¹³ This seems to evolve from a similar Pt seed structure as for the planar tripod. However, the presence of both fluorine atoms and ammonium cations enables the formation of extended zig-zagged monocrystalline planar surfaces with 500 nm in length and a width of 40 nm. This morphology was prepared in a relatively high yield and certainly represents an important step toward the synthesis of thin planar Pt nanosurfaces, which hold high potential for several catalytic processes.

It is interesting to point out that the formation of planar multipods seems to be the preferential growth pattern for Pt crystals. This can be explained by looking at the reduction and growth driving forces for different noble metals, as suggested by Ravishankar and co-workers in their seminal work.^214−216 However, both shape yield and size polydispersity should be considerably improved to reach the state-of-the art quality of other metal colloids. Because all of these structures require the formation and stabilization of Pt twinned crystals, their preparation would significantly benefit from the controlled preparation of single-twinned, planar-twinned, or multiply twinned Pt seeds in high-yield. In this direction, Ruan et al. proposed the use of a specifically designed Pt binding peptide to induce the formation of monotwinned Pt seeds.^217,218 More recently, Chen et al. achieved similar results using a continuous flow reactor to control the nucleation step.²¹⁹ In both examples, the prepared monotwinned crystals were used for the high-yield preparation of Pt right bipyramids; however, under appropriate growth conditions, these could potentially lead to a more controlled preparation of Pt planar twinned structures such as nanotriangles.

3.1.4. Nickel

Ni nanoparticles display interesting catalytic and magnetic properties,^220−223 both as ferromagnetic and superparamagnetic nanocrystals.^224,225 Controlling Ni nanoparticle size and shape has important consequences on the material’s magnetic anisotropy, coercivity, and blocking temperature.²²⁶ When synthetizing Ni colloids, it is important to remember that Ni is not a noble metal. Consequently, reaction kinetics are generally harder to control, due to the need of a strong reducing agent to reach the Ni metallic state. Moreover, the formation of oxide species is another concern that needs to be addressed during synthesis, because it may have important consequences on the catalytic performance of the synthesized colloids.^227,228 For this reason, there are very few reports on aqueous syntheses of Ni particles. Nonetheless, the growth of highly homogeneous nanospheres, nanocubes and nanorods has been reported by means of thermal decomposition of organometallic precursors, as well as hydrothermal and polyol syntheses.^229−233 Interestingly, Ni nanoparticles can adopt both fcc and hcp crystal structures, depending on the synthetic conditions.^234,235 Despite this possibility, Ni nanoplates have only been reported as fcc crystals. The first report on triangular Ni nanoparticles dates back to the year 2000, when Bradley et al. reduced bis(cyclooctadiene)nickel (or Ni(COD)₂) in the presence of tetra-n-octylammonium glycolate, using hydrogen gas as reducing agent and THF as the solvent.²³⁶ Electron microcopy showed the presence of both smaller spheres and nanotriangles, with a low yield of anisotropic structures, which made it impossible to characterize their magnetic properties.

In 2006, Leng et al. were able to control the formation of twins and thereby encourage the growth kinetics toward the formation of triangular and hexagonal nanoplates in high yield. Specifically, iron pentacarbonyl was used to induce Ni nanoparticle nucleation in a thermal decomposition setup, using Ni formate as precursor, 1-octadecene as solvent, and a combination of oleic acid and oleylamine as capping agents.^237,238 The edge length could be tuned, from 19 up to several hundreds of nanometers, by varying the nucleation density. In the same publications, Ni nanoplates showed improved and tunable magnetic properties compared to both bulk and isotropic colloids. The authors attributed the increased blocking coercive force to the magnetic shape anisotropy derived from the 2D structure.²³⁷ Another important consequence of the magnetic anisotropy is the possibility to tune the magnetic properties of the particles by varying their alignment with respect to the applied external field.²²⁴ Optical characterization confirmed the presence of a plasmon band in the UV–visible range.²³⁸ In a follow-up study, the size polydispersity and shape yield were improved further, achieving Ni nanotriangles with an average edge length of 15 ± 2 nm and a thickness of 6 nm (Figure 17A). Interestingly, SAED analysis did not show any 1/3{422} diffraction spot, suggesting that the triangles were either single-crystals or monotwinned (with a twin parallel to the probing electron beam).²³⁹ The first hydrothermal approach for the synthesis of Ni nanoplates was reported by Xu et al. in 2007.²²⁹ In their synthesis, Ni chloride was used as the metal precursor, sodium dodecyl sulfate as surfactant, and sodium hypophosphite as reducing agent when operating in basic conditions. In order to push the reduction of Ni to its metallic state, an autoclave system was used to maintain a temperature of 110 °C for 14 h. Interestingly, the aqueous environment imposed a completely different growth mechanism, with the initial formation of Ni hydroxide nanoplates that were reduced into metallic Ni in a second stage of the hydrothermal process. Recently, Peng and co-workers successfully controlled the growth of Ni trigonal and hexagonal nanoplates, assisted by tungsten hexacarbonyl W(CO)₆.²²⁶ The main advantage of this strategy is a reduced nucleation temperature (150 °C) and the possibility to perform the synthesis without the use of hydrogen gas. It should be noted that the mechanistic role of W(CO)₆ is different in the case of Ni, compared to analogous Pd and Pt syntheses, because W⁶⁺ cannot reduce Ni atoms into the metallic state. Interestingly, HRTEM and SAED indicate the presence of 1/3{422} forbidden diffraction spots, indicative of the presence of stacking faults for the highest aspect ratio particles, whereas triangular platelets seemed to be single crystalline. However, the presence of a single twin defect parallel to the {111} facets was not ruled out by either tilt SAED analysis or tomography reconstruction. Finally, it is worth mentioning the use of Ni in several alloy formulations. For example, NiCu nanoplates were reported in 2012, showing a similar growth mechanism and identical crystallography to both Ni and Cu nanoplates.²⁴⁰ In another example, Ni@Ru and NiCo@Ru hexagonal nanosandwiches were obtained, with superior electrocatalytic activity for the oxygen evolution reaction.²⁴¹

3.1.5. Rhodium

Rh is one of the rarest and most precious metals on Earth. It shows excellent catalytic activity for several important industrial processes, including hydrogenation, hydroformylation, hydrocarbonylation, and aryl halide coupling.^242−248 Commercially, it is one of the main components of catalytic converters.²⁴⁹ Such a superior catalytic profile has been a strong motivation for the preparation of Rh nanoparticles with improved activity by optimization of their crystallographic structure. However, compared to other fcc metals, the development of synthetic protocols to control size and shapes of Rh nanoparticles has been more challenging because of the high surface free energy of this metal, which is three and two times higher than those of Au and Pt, respectively.²⁵⁰ Notwithstanding, early studies quickly demonstrated the possibility to control both the efficiency and selectivity of Rh catalysts, by controlling the synthesis of nanospheres, nanocubes, and nanotetrahedra.^251,252 These works showed the tendency of Rh to yield polygonal and multipod structures.^253−255 The preparation of Rh nanotriangles was reported in 2011 by Biacchi and Schaak, applying a kinetic growth control using RhCl₃ as metal precursor and triethyleneglycol as the solvent and reducing agent, in a polyol synthetic scheme.²⁵⁶ High resolution TEM analysis confirmed the {111} nature of the extended facets, but the lack of a complete 3D analysis prevent the identification of the presence of twin defects in the nanocrystals. The optimization of colloidal synthetic strategies in subsequent years focused mainly on the preparation of ultrathin nanosheets and high-index faceted nanocrystals, to maximize the catalytic performance. Son and co-workers reported the preparation of ultrathin Rh nanoplates, exploiting a weak Rh(I)–Rh(I) interaction that is enhanced by van der Waals interactions when the metal precursor is coordinated by oleylamine molecules.²⁵⁷ The resulting Rh nanoplates present a thickness of only 1.3 ± 0.2 nm and a triangular or trapezoidal profile, with angles of 60 and 120°. Side-view HRTEM analysis confirmed the presence of stacking faults along the [111] direction.²⁵⁷ More recently, Huang et al. re-evaluated this oleylamine-mediated synthetic route.²⁵⁸ Specifically, Rh(ac)₃, glucose, and oleylamine were mixed in N,N-dimethylformamide to yield Rh tetrahedral, concave tetrahedral, and hexagonal nanoplates. Shape selectivity was achieved by reducing the reduction rate, thus inducing the formation of kinetic products. The obtained 2D particles showed improved performance for both hydrogen evolution and oxygen evolution reactions.²⁵⁸ In a different approach, Zheng and co-workers adapted a Rh precursor to the synthetic methodology previously developed for the preparation of Pd ultrathin nanosheets.¹⁸⁰ Specifically, Rh platelets were obtained in N,N-dimethylformamide, using PVP as capping agent and carbon monoxide as shape inducing agent (Figure 17B).²⁵⁹ The platelets had an average thickness of 0.9 ± 0.4 nm, corresponding to 3–5 atomic layers, whereas the edge length could be varied between 120 and 1300 nm.

Several solvothermal approaches have also been explored for the preparation of ultrathin Rh nanosheets.²⁶⁰ In a recent publication, Zhang et al. reported the preparation of hyperbranched triangular Rh nanoplates by heating a mixture of RhCl₃ and 1-octadecylamine in a Teflon autoclave, achieving a final thickness of 4 nm and edge length around 200 nm.²⁶¹ In another example, Duan et al. reported the solvothermal preparation of monoatomically thin Rh sheets combining Rh(acac)₃, benzyl alcohol, formaldehyde, and PVP as capping agent.²⁶² The authors reported a thickness <4 Å and an edge length of 500 nm, with a structure composed of 100% surface atoms (Figure 17C). The possibility to stabilize such a configuration arises from the combination of a δ-bonding framework composed of the hybrid orbitals in Rh atoms and the stabilizing role of PVP molecules. Interestingly, HRTEM analysis showed a hexagonal atomic organization, but electron diffraction indicated an hcp structure instead of the expected fcc for Rh. This is likely a direct consequence of the monatomic thickness of these nanosheets.

3.1.6. Aluminum

Al is the most abundant metal composing the Earth’s crust, thereby holding a great potential as a low-cost plasmonic material for optical applications.²⁶³ However, similarly to other highly reductive materials, the wet-chemical synthesis of well-defined anisotropic structures has proven to be a difficult challenge. Several research groups have made significant progress, managing to tune the syntheses of spherical, cubic, bipyramidal, and even rod-shaped particles.^264−268 In all cases, the key and unique feature of Al nanocrystal growth is the formation of a 2–3 nm oxide layer surrounding the nanoparticles.^269−271 For example, the selective oxidation of different crystallographic facets in growing Al nanocrystals was exploited by Luo et al. for the preparation of sub-2 nm Al nanosheets, which remains the only report to date.⁸⁹ The authors injected a mixture of oxygen and nitrogen gas in the growth solution to regulate the concentration of oxygen during the reduction of AlCl₃ precursor by LiAlH₄ at 140 °C. When the oxygen content was increased from 15 to 50 vol%, the thickness of the obtained Al nanosheets decreased from 18 to 1.5 nm (Figure 17D). Interestingly, the thickness of the oxide layer was estimated to be <0.5 nm, significantly less than that reported in other studies. A similar approach could potentially be applied to other highly reductive metals such as Ni or Cu.

3.1.7. Iridium

Ir is one of the most promising catalysts within the platinum group. In its metallic state, Ir presents 7 electrons distributed in its 5d orbitals. This configuration enables the adsorption of small molecules (e.g., carbon monoxide, oxygen, hydrogen, ethylene, acetylene) to form reaction intermediates that ultimately enhance the catalytic performance of the material.²⁷² Ir nanoparticles outperform state-of-the-art catalysts in several organic reactions, including hydrogen and oxygen evolution, (de)hydrogenation, cyclization, and (cyclo)isomerization reactions.^273−277 On top of this outstanding catalytic performance toward organic reactions, Ir nanoparticles were reported to exhibit enzyme-like activity.^278−282 Moreover, Ir nanoparticles were also used for the surface-enhanced Raman scattering (SERS) detection of various analytes.^283−285 This capability is particularly interesting when combined with the superior catalytic activity of Ir nanocrystals, as it may open up the possibility to simultaneously perform and monitor specific chemical reactions within a single material. Finally, Ir colloids display interesting luminescence properties.^272,286,287 Despite the versatile nature of this element, little attention has been dedicated so far to the preparation of Ir nanoparticles, which have mainly focused on small spherical colloids.^278,288 The first characterization of anisotropic Ir nanostructures was reported by Kundu and Liang in 2011.²⁸⁹ In this work, the photochemical reduction of IrCl₃ was performed under 4 h of continued UV light irradiation of a basic 1,2-dihydroxynaphthalene solution in the presence of CTAB. By reducing the surfactant concentration, the authors were able to direct the synthesis toward the formation of spherical nanoparticles, nanochains, nanoflakes (Figure 17E), and nanoneedles.²⁸⁹ Most recently, the preparation of two-dimensional Ir nanostructures has acquired increased attention when Cheng et al. reported the preparation of ultrathin Ir nanosheets.²⁹⁰ Carbon monoxide was identified as the shape directing agent that stabilizes {111} facets during growth, under similar conditions reported for other fcc metals of the platinum group. Additionally, some theoretical works have recently analyzed the adsorption of carbon monoxide on the different crystallographic facets of an Ir crystal, suggesting the possibility of achieving selective growth of both platonic shapes and anisotropic structures, such as rods or plates.^291−293 This certainly represents an exciting opportunity for future research endeavors toward the development of new catalysts.

3.1.8. Lead

Despite its toxicity, Pb nanostructures are attractive candidates for the study of quantum effects on superconductivity, with applications in electronics, spintronics, and batteries.^294−296 From a synthetic perspective, the low melting point of Pb represents a unique feature within the fcc metal family. This feature enables a new mechanism, called solution-liquid–solid mechanism, where, upon reduction of the precursor, metallic Pb nanoparticles would exist either in solid or liquid state depending on their size, as the higher surface energy of the smaller colloids would reduce the melting point below the temperature used for growth.^297,298 Two-dimensional Pb nanoparticles where first documented in 2004 by Wang et al., as a byproduct of the synthesis of high aspect ratio single-crystal nanowires.²⁹⁷ In this work, a polyol growth approach in the presence of PVP was applied for the reduction of Pb acetate. A few years later, a kinetically controlled polyol approach was optimized by Jaeger and co-workers, where PVP was substituted by trioctylphosphine and oleic acid. Optimizing the temperature and reaction time, Pb nanoplates with thicknesses ranging between 20 and 100 nm, and a lateral size of several micrometers were obtained as the main product.²⁹⁹ Extensive SAED analysis confirmed the {111} nature of the two parallel facets, as well as the presence of the forbidden 1/3{422} diffraction signal. A different approach was proposed in the same year by Lu et al., in which the kinetic formation of platelets and hemispherical nanoparticles was achieved by combining PVP and CTAB in certain molar ratios inside the growth mixture (Figure 17F).²⁹⁸ Magnetic measurements of isotropic, anisotropic, and 2D Pb nanostructures confirmed a superconducting behavior below a critical temperature of ∼7 K, with a Meissner effect 1 order of magnitude smaller compared to the bulk material.²⁹⁵

3.1.9. Ytterbium

As with other rare earth elements belonging to the lanthanide group, Yb finds application as a photon upconverting material for the preparation of contrast agents for in vivo imaging.^300−303 Some research groups have explored the preparation of ytterbium oxide nanodisks.³⁰⁴ However, the preparation and characterization of Yb metal nanoplates/disk/sheets remain undiscovered territory.

3.2.1. Magnesium

Mg holds great promise for several industrially relevant fields, including hydrogen storage and rechargeable batteries.^319−324 Its abundance and low cost are also important contributing factors that motivate deeper investigation on the fabrication of magnesium nanostructures. Top-down fabrication and gas-phase deposition were initially investigated,^322,325 showing sufficient size and shape control to yield the first preparation of Mg nanoplates.^326,327 On the other hand, the chemical synthesis of Mg colloids remained vastly unexplored until very recently, when Mg nanoparticles emerged as alternative plasmonic materials to the more expensive coinage metals.^328,329 In 2012, Viyannalage et al. reported the first wet-chemical synthesis of Mg nanoplates, using Li naphthalide as the reducing agent.³³⁰ The synthesis was later optimized by Ringe’s group (Figure 19A,B) in a series of seminal papers demonstrating the possibility of Mg plasmonics to target intense optical activity throughout the whole UV–vis–IR spectral range.^324,331 The particles ranged between 100 and 300 nm in size, with three main plasmon resonances, two of them in the UV range and one occupying the vis-NIR, respectively.^329,331,332 The hcp structure of metallic Mg was confirmed for the nanoplates, which are single crystalline and expose low index facets belonging to the {0001} (top and bottom) and {1010} (six sides) families of planes. The wet-chemical synthesis of Mg colloids is still in its infancy, and much is left to uncover prior to shape and size control.³³³ Similar to Al, Mg nanoparticles also form a thin self-protecting oxide layer which keeps them stable in low-oxidative environments such as air (few hours) or alcoholic suspensions (weeks).^331,334 This might be improved in the future through surface chemistry and other protective layers.³³⁵

3.2.2. Cobalt

The preparation of anisotropic Co nanoparticles can find potential applications as ferrofluids and high density magnetic memory devices.³³⁶ The rational synthesis of Co colloids is particularly challenging due to its rich crystallography, presenting fcc, hcp, and epsilon (ε) crystal habits.³³⁷ At the nanometer scale, the three crystal phases are readily interchangeable, with ε-Co becoming the most stable form above 200 °C.³³⁸ The first preparation of Co nanodisks in pure hcp crystal phase was reported in 2002 by Puntes et al., exploiting the thermal decomposition of Co carbonyl organometallic compounds in the presence of linear amines and trioctylphosphine oxide (Figure 19C).³³⁹ The obtained nanodisks showed a narrow size distribution and high tendency to assemble into chain like structures.^337,339,340 Several years later, multiple hydrothermal approaches were reported, achieving higher aspect-ratio Co nanoplates with improved magnetic properties (Figure 19D).^229,341,342 Interestingly, the formation of Co nanoplates has also been explored in the presence of external electromagnetic fields, which would be particularly interesting when nanostructures are grown directly on substrates.³⁴³

3.2.3. Zinc

By far the most abundant use of Zn in colloidal systems is the production of ZnO particles,³⁴⁴ a wide-bandgap semiconductor with an energy gap of 3.37 eV that finds numerous applications in catalysis, sunscreen products, and food packaging.³⁴⁵ However, the wet-chemical synthesis of metallic Zn nanoparticles was also investigated by several groups.^346,347 In 2014, the group of M. V. Kovalenko reported the only synthetic protocol for the preparation of hexagonal metallic Zn nanoplates to date.³⁴⁸ Specifically, the synthesis is based on the use of LiN(SiMe₃)₂ for the transient formation of metallic amide complexes that are thermally decomposed in situ. The growth of hexagonal nanoplates is induced by combining oleylamide complexes and a noncoordinating solvent such as octadecene.³⁴⁸ The prepared colloids present a single crystal hcp structure and a narrow size distribution between 40 and 200 nm (Figure 19E,F).

3.2.4. Ruthenium

Ru nanocrystals are effective catalysts for several heterogeneous and electrocatalytic reactions, including Fischer–Tropsch, hydrogenation, CO oxidation and methanation, and hydrogen production through amine borane dehydrogenation, or water-splitting reactions.^349,350 Similar to Co, Ru can also generate stable crystal structures with both fcc and hcp atom arrangements, which has significant implications for their catalytic activity.^351−353 The relatively low oxidation and reduction potentials represent the main challenge when controlling the growth of Ru nanoparticles. Nonetheless, the rational synthesis of metallic Ru nanocrystals has been optimized for the preparation of various unique morphologies, including capped columns and hourglasses.^354,355 For the specific case of 2D nanocrystals, the first report of a nanoplate synthesis was achieved by means of a polyol approach.³⁵⁶ A few years later, the hydrothermal decomposition of RuCl₃ in the presence of formaldehyde led to the synthesis of triangular but irregular Ru nanoplates, with an average edge length of 24 nm and a thickness of less than 4 nm.³⁵⁴ Interestingly, these triangular platelets presented a large number of stacking faults in their XRD analysis despite showing an hcp crystal habit. The formation of defects is probably related to the high reaction temperature and growth rate and is necessary to achieve a triangular shape instead of the hexagonal profile predicted by Wulff construction. In the same work, the authors also explored the use of sodium oxalate as shape directing agent for the preparation of Ru-capped columns. Specifically, oxalate species can selectively adsorb on Ru (101̅0) during the initial growth stage, inducing the formation of the column trunk; over the course of the reaction, the same oxalate species undergo thermolysis, leading to the overexpression of Ru (0001) facets in their predicted hexagonal profile (Figure 19G).^354,357 The preparation of even thinner 2D Ru nanostructures was reported by Kong et al., who optimized the solvothermal decomposition of Ru(acac)₃ in the presence of 2-propanol (contributing to anisotropic growth), and urea (preventing aggregation). The prepared free-standing Ru platelets are several hundreds of nm in size, with a thickness around 1 nm. The huge surface-to-volume ratio of these nanostructures translated into a remarkable catalytic activity toward both hydrogen and oxygen evolution reactions.³⁵⁸ In a recent report, Ramamoorthy et al. explored an organometallic route for growing Ru icosahedral, triangular, and 3-fold nanostars (Figure 19H, I).³⁵⁹ The proposed approach revolves around the hydrogenation of a [Ru(COD)(COT)] complex in the presence of hexadecylamine and lauric acid. The authors identified the CO generated in situ from the catalyzed decomposition of lauric acid as the key shape directing agent, stabilizing the icosahedral crystals at high concentration, and the 2D growth of the hcp crystals at lower concentration. The prepared 3-fold Ru nanostars have a dimension around 10 nm and a thickness of only 0.8 nm (corresponding to 5 or 6 atomic layers) and a standard deviation around 15%.³⁵⁹

3.3.1. Lithium

Li is the lightest and simplest of all metals, as well as the element that can best approximate a quasi-free-electron model, representing a unique opportunity for the fundamental understanding of size-dependent physical properties.^369,370 In a recent publication, Sun et al. reported the electrochemical growth of hexagonal ultrathin Li nanoplates using a solid-state open-cell configuration inside a TEM.³⁶⁹ The high-vacuum conditions enable the full characterization of such a highly reactive structure, showing an edge length of few hundreds of nm against a thickness <10 nm (Figure 21A,B). The authors were able to confirm the bcc nature of the nanoplates, with the top and bottom facets identified as (111) planes, and the 6 side facets assigned to the {110} family.³⁶⁹ The proposed mechanism involves the preferential oxidation of the basal (111) facets to break the symmetry of the growing crystal, ultimately preventing any growth along the [111] direction. The presence of an extremely low concentration of oxygen within the TEM column did not prevent the oxidation of the Li nanoplates indefinitely, with a complete transformation into Li₂O within about 3 h. Nonetheless, the authors were able to confirm the plasmonic nature of the grown structures by cathodoluminescence (CL) and electron energy loss spectroscopy (EELS). A similar setup was also applied for the growth of Na nanoplates, but the synthesis was found to be less reproducible.

3.3.2. Iron

Magnetic Fe nanoparticles have been extensively investigated in several oxidation states and crystallographic habits. In 2012, Guan et al. reported the external field-assisted preparation of 2D Fe nanoplates by NaBH₄ reduction of a concentrated FeSO₄ solution. Interestingly, the prepared 2D nanostructures featured an amorphous phase (Figure 21C,D), with significantly enhanced coercivity and decreased saturation magnetization.³⁷¹ The authors suggested that the obtained nanoplates are a kinetic product generated by the formation of a large number of Fe nuclei via reduction, which are subsequently assembled by the external magnetic field.

3.4.1. Indium

In is a low melting point metal (157 °C) that has found applications in electronics, catalysis, and as a component of several alloys. Moreover, metallic In nanoparticles are attractive for their plasmonic properties and superconductivity.^382,383 The first synthesis of In nanotriangles was achieved through a hydride reduction in alkylamine (used as solvent); specifically, Li borohydride was used to reduce In trichloride (InCl₃) in N,N-diethylaniline, resulting in the growth of polyhedral nanocrystals, with significant size and shape polydispersity.³⁸⁴ A polyol method has been optimized for the room temperature growth of In nanotriangles and nanowires displaying a high level of control over morphology and size.³⁸⁵ In this protocol, NaBH₄ tetraethylene glycol solution was added dropwise into an InCl₃ and PVP ethylene glycol solution after nitrogen purging. Interestingly, the formation of wires or triangular nanoplates was controlled by varying the rate of addition of the reducing agent, pointing toward a kinetic control of the growth. The In nanotriangles present an edge length of 60 ± 8 nm and a shape-yield >50%, with SAED analysis confirming the expected tetragonal crystal structure (Figure 22A–C). Although control over the growth kinetics can certainly be improved, the authors were able to study the plasmonic properties of the prepared colloids, and demonstrated their use for the SERS detection of tryptophan.³⁸⁵ Finally, the study of In nanoparticle synthesis led to the preparation of one-dimensional hierarchical multilayered nanostructures.³⁸⁶ This exotic shape originates from the initial incorporation of Zn atoms within In nanorods, which are later removed from the structure (following a chemical gradient), inducing a ripening process of the remaining architecture.

3.4.2. Bismuth

As a bulk material, Bi is a metalloid with a small band overlap, and the element presenting the smallest electron effective mass. The size reduction of Bi crystals into the nanometer scale yields an unusual metalloid-to-semiconductor transition, which finds application in optoelectronics and thermoelectronics applications, as well as in the study of spin–orbit interactions and topological insulators.^387−390 Moreover, Bi nanoparticles are one of the best catalysts for the solid–liquid–solid growth of semiconductor nanorods and nanowires.^391,392 These characteristics attract considerable attention toward the synthesis of Bi colloids under different conditions.^393−396 In one of the first reports, the preparation of Bi nanoplates was achieved using PVP as reducing agent and stabilizer in a polyol method: the ratio between PVP and NaBiO₃ could be tuned for the preparation of Bi nanocubes (PVP:Bi = 1.6) or Bi nanotriangles (PVP:Bi = 0.8), with a shape yield around 30%.³⁹⁷ Moreover, trace amounts of Fe³⁺ induced the formation of thin Bi nanobelts of several micrometers in length and up to 600 nm in width. By changing the Bi precursor to Bi[N(SiMe₃)₂]₃, Wang et al. reported the preparation of Bi nanoplates with a hexagonal profile and significantly smaller dimensions (50–100 nm) at a shape-yield around 60%.³⁹⁸ Taking advantage of the relatively low melting point of Bi (271.4 °C in the bulk), Wang et al. reported the preparation of Bi hexagonal nanodisks in a solvent-free thermolysis process.³⁹⁹ The formation of disks was induced by the selective adsorption of 1-dodecanethiol on the different facets of rhombohedral Bi nanocrystals, while the same molecule was also responsible for initiating the growth through a radical process, leading to the reduction of Bi thiolate (Figure 22D). The authors were also able to roughly control the edge length of the grown disks by varying the reaction temperature, the reaction time, and the concentration of the capping agent. After optimization, Bi nanodisks showed an edge length of 100–260 nm and an average thickness of 17 nm, with a size polydispersity around 15%. The single-crystal rhombohedral structure was confirmed by SAED, with flat faces perpendicular to the c-axis (Figure 22E).

More recently, Bi emerged as an interesting material for biological applications, with Bi nanoparticles showing low toxicity levels and the ability to stimulate the generation of reactive oxygen species (ROS).^400,401 In this context, Song et al. synthesized Bi nanotriangles of around 50 nm edge length and functionalized them with Zn protoporphyrin IX and human serum albumin for the sonodynamic treatment of rheumatoid arthritis.⁴⁰² The Bi nanotriangles were synthesized through an optimized hydrothermal process in alkaline conditions using ascorbic acid as the reducing agent and Na₂EDTA to complex Bi(NO₃)₃ and regulate the release of Bi³⁺ during growth (Figure 22F), while a carboxylic acid-terminated PVP was used as surfactant.^402,403

5.1.1. LSPR Modes and Characterization

Localized surface plasmon resonances bring about light scattering and absorption, electromagnetic field enhancement at the nanoscale, photothermal effects, production of hot electrons, and photocatalytic activity. Understanding the LSPR modes is therefore essential for the development of plasmon-based applications.

It is well-established that LSPR frequencies strongly depend on the nanoparticle material and geometry. Au and Ag are the most widely studied plasmonic materials due to their highly tunable LSPR wavelength in the visible (vis) and near-infrared (NIR) or even mid-infrared region (MIR), and well-developed wet-chemical synthesis.²⁶ To date, other 2D plasmonic nanoparticles have been reported to support LSPR in the vis-NIR region (see section 3 of this review).^{88,153,182,331,418,433−435} Langhammer et al. compared the LSPR peak position and line width of Au, Pt, and Al nanodisks, as a function of nanodisk diameter, both experimentally and through theoretical simulations (Figure 24A).^418,435 The observed pronounced differences in LSPR peak position and line width can be attributed to the bulk dielectric functions of different metals and the damping produced by different mechanisms (intraband damping, interband damping, radiation damping, and radiation quenching).⁴¹⁸ Notwithstanding, all three types of nanodisks exhibit a major LSPR peak that follows the well-known spectral red-shift with increasing nanodisk diameter (or aspect ratio), indicating that charge oscillations in different 2D metallic nanoparticles have a general commonality. Simulations have been performed to reveal the LSPR modes supported by 2D plasmonic nanoparticles and the impact of different geometrical parameters on the LSPR frequency, typically using Ag nanoplates as a model system.^436,437 Briefly, in-plane and out-of-plane LSPR modes are characteristic for 2D metallic nanoparticles due to their platelike morphology. The major LSPR peaks shown in Figure 24A(iv–vi) are associated with the in-plane dipolar mode, the corresponding charge oscillation being illustrated in Figure 24B(i).⁴³⁶ The LSPR peak of the in-plane dipole mode is sensitive to several geometrical parameters, including edge length (or diameter), thickness, shape of the basal plane, and sharpness of the tips. Taking triangular Ag nanoplates as an example, the peak red-shifts with increasing edge length and blue-shifts with increasing truncation (Figure 24C).⁴³⁷ Further experimental works, both on Au and Ag nanoplates, also reported a blue-shift of the dipole LSPR peak with increasing thickness^128,438 and rounded shape transformation (Figure 24D).^439,440 Notably, some spectra in Figure 24C,D display other LSPR resonances on the shorter-wavelength side, with weaker intensities than that of the in-plane dipolar mode. According to simulations, the peak at around 340 nm represents an out-of-plane quadrupolar mode (Figure 24B(iv)), whereas the out-of-plane dipolar mode (Figure 24B(ii)) and the in-plane quadrupolar mode (Figure 24B(iii)) both overlap, contributing to the broad shoulder at 400–500 nm.^436,437,441 Note that when nanoplates are sufficiently thin and high aspect ratio, out-of-plane modes exhibit extinction coefficients so low as to not be observed experimentally.^442,443

Different techniques have been applied for the characterization of LSPR modes and understanding their near-field and far-field properties. Wang et al. carried out a series of experiments based on single-particle dark-field scattering to investigate the far-field scattering properties of individual Au nanoplates and oligomers,^439,441,444 as well as Fano resonances⁴⁴⁵ and plasmon coupling based on nanoparticle-on-mirror (NPoM) systems.^446−449 In addition to the commonly seen horizontally oriented nanoplates, they successfully measured vertically oriented hexagonal Au nanoplates, where the quadrupolar LSPR mode could be excited and measured more efficiently than on the horizontally oriented ones (Figure 25A).⁴⁵⁰ Because the higher-order LSPR modes are nonradiative (also known as “dark modes”), the accessibility of far-field excitation is very limited. On the contrary, near-field microscopic techniques, such as scanning near-field optical microscopy (SNOM),^451−453 scanning transmission electron microscopy with EELS,^454,455 and CL spectroscopy, can excite the LSPR modes locally and therefore can be used to characterize higher-order LSPR modes. The pioneering work by Stéphan et al. opened the possibility of LSPR visualization at high spatial resolution by STEM-EELS measurements, where different LSPR modes of a triangular Ag nanoplate can be selectively excited by a subnanometer electron beam (Figure 25B).⁴⁵⁴ Imura et al. demonstrated by 3D SNOM measurements that the in-plane and out-of-plane LSPR modes can be selectively visualized by varying the distance between the probe tip and the Au nanoplate, even though the two LSPR modes were spectrally and spatially overlapping (Figure 25C).^451,456 However, the near-field distributions of plasmonic nanoplates measured by different groups or with different techniques are always slightly different, especially in terms of the higher-order modes. It is therefore challenging to accurately compare the results among different literature reports due to the difficulty in estimating the effect of experimental details on the results. In this regard, Kociak et al. combined EELS and CL measurements to investigate the exact same individual Au nanoplates to provide a fair comparison of the two techniques.⁴⁵⁷ They thereby demonstrated that both techniques can detect the dipolar mode with good agreement, but the higher-order modes are accessible only by EELS (Figure 25D). Likewise, rigorous comparison between other near-field and far-field techniques is highly desired for better interpretation of the experimental results obtained from different techniques.

Besides LSPR modes, micrometer-scale plasmonic platelets can support propagating surface plasmon polaritons (SPP), resembling those supported by plasmonic thin films. Imaging of SPPs on Au platelets was recently reported using time-resolved two-photon photoemission electron microscopy (TR-2PPE-PEEM), where electron emission can be triggered optically.^458−461 Meyer zu Heringdorf and Giessen et al. reported the detection of short-range and long-range SPPs (SR-SPP and LR-SPP) at the Si–Au interface and Au–vacuum interface for Au platelets deposited on Si substrates (Figure 26A,B).⁴⁵⁸ SR-SPPs exhibit significantly shorter wavelength than LR-SPPs and decay quickly with propagation (Figure 26C). However, when the Au platelets are thin enough (∼22 nm) and patterned with a concentric grating (inset of Figure 26D), the SR-SPPs can couple into a central disk, leading to localized two-photon ultrafast electron emission from a 60 nm spot (Figure 26E). They further applied this technique to image the plasmonic skyrmion lattice (Figure 26F), where the SPPs were detected by interference with a probe pulse.⁴⁶⁰ These intriguing plasmonic properties will create new opportunities in nanophotonic applications.

5.1.2. Spectroscopies

The unique plasmonic features of 2D metallic nanoparticles have been exploited in various light–matter interactions including the use of plasmonic nanoplates as individual optical nanocavities,^462−465 atomically flat substrates,^447,466,467 or components of bowtie nanostructures in a tip-to-tip configuration.^{423,468−471} Plasmon–exciton coupling has been demonstrated by several groups using Au or Ag nanoplates as the plasmonic platform and other 2D fluorescent materials, such as J-aggregate sheets and 2D transition metal dichalcogenides (TMDCs), as the exciton provider.^462−465 The flat surface allows the plasmonic nanoparticles to adhere to these 2D fluorescent materials over a large contact area and thereby interact with them efficiently. Under a relatively low coupling strength, the plasmon–exciton system exhibits Fano resonances, where the eigenenergies of the system are actually degenerate.⁴⁶⁵ By varying the dimension and composition of the nanoplates, the lower plasmon damping and concomitant narrower plasmon line width can drive the system into the strong coupling regime, as evidenced by the splitting in the eigen energies known as Rabi splitting (Figure 27A).^462−464 In general, plasmonic nanoplates with small mode volumes are favorable for achieving strong coupling.

Bowtie nanostructures consisting of two tip-to-tip plasmonic nanotriangles with a gap size less than tens of nanometers, as first proposed by Moerner et al.,⁴⁷¹ are an ideal platform to concentrate large electric fields into an ultrasmall volume. Taking advantage of the ultrastrong plasmonic hot spots, this design has been widely applied to facilitate the development of sensors,⁴²⁰ optical tweezers,^470,472 high-harmonic generation,^473,474 surface-enhanced Raman scattering,⁴⁷⁵ and nanophotonic devices.^425,476 Odom et al. further proposed a 3D-bowtie nanostructure, in which the Au nanoplates were folded along the dimer axis, giving rise to a higher excitation efficiency of the quadrupolar LSPR mode, compared to that of the planar 2D-bowtie nanostructure. They successfully developed room-temperature directional nanolasers by fabricating these 3D-bowtie arrays on dye-doped polymer slabs (Figure 27B).^423,469 In the aforementioned works, the bowtie nanostructures were all made by top-down nanofabrication techniques, such as electron beam lithography (EBL), due to the need to precisely control the orientation and position of the plasmonic triangles, which seems less relevant to the chemically prepared nanoparticles discussed in this review. However, such bowtie nanostructures and arrays can also be fabricated by applying focused ion beam (FIB) milling on large chemically grown Au nanoplates, as proved by Hecht et al. (Figure 27C).⁴⁶⁶ Compared to the EBL-fabricated Au bowtie nanostructures, those fabricated by FIB possess an ultrasmooth surface and well-defined crystalline structure, leading to superior optical performance as evidenced by the much brighter two-photon excited photoluminescence (TPPL) signals (Figure 27C(v)). This technique can be extended to other complex nanostructures with improved optical performance compared with the EBL-fabricated ones, suggesting great potential for chemically prepared 2D plasmonic nanoparticles to be used in plasmonic nanocircuits.

NPoM systems are another popular configuration for studying light–matter interactions, where the nanoplates can either serve as the “nanoparticle” or the “mirror”, leading to a highly confined flat gap, i.e., an ideal optical nanocavity.^477−479 When serving as the “nanoparticle”, the plasmonic nanoplates are usually deposited on an evaporated metal thin film with other optical species introduced in the flat nanogap, where plasmons can modify the spontaneous emission strength, fluorescence spectrum, decay rate, and radiation pattern of these optical species.⁴⁸⁰ This configuration is specifically beneficial for achieving strong coupling between plasmonic nanoparticles and TMDCs, due to the ultrastrong field enhancement in the nanogap and the overall small mode volume. Qiu et al. demonstrated strong coupling by a sandwiched Au–WS₂–Au system composed by chemically prapared Au nanoprisms and WS₂ monolayers supported by Au thin films, giving rise to strong coupling approaching the single-exciton level with a large vacuum Rabi splitting of up to 163 meV.⁴⁸¹ Through precise control over the spacing between the two Au surfaces, different orders of Fabry–Pérot resonances can be supported within the optical cavity, offering more optical modes for modulating photoluminescence of the sandwiched WS₂.⁴⁸² Furthermore, the plasmonic nanoplates can also serve as an atomically flat “mirror” of NPoM systems. For example, Wang et al. used Au nanoplates as the basis of a NPoM system to demonstrate different charge tunneling behaviors of insulating and conductive molecular junctions, as evidenced through the single-particle scattering spectra of the plasmonic heterodimers.⁴⁶⁷ In another recent example, Xu et al. embedded single up-conversion nanoparticles in the nanogap of a tilted-nanocavity-coupled system using Ag microplates as substrates (Figure 27D), which enabled sub-50 ns ultrafast upconversion luminescence with directionality and chirality.⁴⁸³ Because these systems are extremely sensitive to the nanometer gaps, atomically flat plasmonic nano- and microplates are highly desired for eliminating the perturbative effects from the rough surfaces of conventional metal thin films. Similar systems can be further extended to study photoluminescence, photocatalysis, SERS, and coherent nonlinear processes.⁴⁸⁴

5.1.3. Sensing

Plasmonic nanoplates have been widely used for sensing through refractive index changes,^{128,485−488} molecular sensing based on surface-enhanced spectroscopies,^420,489,490 and colorimetric sensing based on etching.^491,492 Plasmonic nanoplates may play the role of optical nanoantenna, substrate, or colorimetric reagent, depending on the detection strategy.

LSPR is strongly responsive to changes in the refractive index of the surrounding medium, including solvent and surface binding molecules. The refractive index sensitivity is dependent on the size, shape, composition, and specific LSPR mode of the plasmonic nanoparticles.⁴⁹³ A few works from different groups have shown that the in-plane dipolar plasmon mode of Au and Ag nanoplates exhibits excellent refractive index sensitivities 400–1000 nm/RIU (refractive index unit), with figures of merit above 3 at different wavelengths in the vis-NIR region.^{128,485−488} These values are on average higher than those for other plasmonic nanoparticles (Figure 28A).^128,494 The high refractive index sensitivity can be attributed to the 2D geometry with a high aspect ratio and therefore a high ratio of surface area to bulk atoms of nanoplates, resulting in a larger impact of the dielectric environment on the charge oscillation of LSPR.^487,493

Although surface ligand detection is also possible through refractive index sensing,⁴⁸⁶ plasmonic nanoplates are much more commonly used for ultrasensitive molecular detection through surface-enhanced spectroscopies, such as plasmon-enhanced fluorescence,⁴²⁰ SERS,^409,475,490 tip-enhanced Raman spectroscopy (TERS),⁴⁸⁹ and surface-enhanced infrared absorption (SEIRA).⁴⁹⁵ In this sort of plasmonic sensor, triangular nanoplates are beneficial due to their sharp tips and intense local field enhancement.⁴⁹³ In the pioneering work by Moerner et al., the authors utilized lithographically fabricated Au bowtie nanoantennas to achieve large single-molecule fluorescence enhancements (Figure 28B). Under excitation light polarized parallel to the long axis of the bowtie antenna, the electric field enhancement at the gap area could reach more than 100-fold and thereby the fluorescence brightness would be enhanced up to 1340-fold.⁴²⁰ Similarly, lithographically fabricated bowtie nanostructures were demonstrated for SERS with enhancement factors exceeding 10¹¹, and for SEIRA with enhancement factors higher than 10⁷.^475,495 Ding et al. constructed Au bowties by DNA-origami-directed assembly (see section 4) of chemically prepared triangular nanoplates and demonstrated single-molecule SERS of individual nanostructures.⁴⁰⁹ Apart from the well-established bowtie nanostructures, hot spots can be generated in other configurations for molecular detection. Richard-Lacroix and Deckert covered atomically flat plasmonic nanoplates with a self-assembled monolayer of 16-mercaptohexadodecanoic acid and used them as a platform for TERS (Figure 28C). Taking advantage of the tightly confined light between the Ag-coated TERS tip and Au nanoplates, only the molecules located directly under the TERS tip can actually contribute to the anti-Stokes/Stokes Raman signals, allowing for an accurate evaluation of the local heating of the molecules.⁴⁸⁹ In work by Zheng et al., hot spots were generated for the SERS detection of rhodamine 6G by light-directed reversible assembly of Au nanoplates based on plasmon-enhanced thermophoresis (Figure 28D).⁴⁹⁰ Other self-assembly strategies, as mentioned in section 4, can be adopted for generating compact assemblies and improving the sensing performance of plasmonic nanoplates.

Colorimetric sensing based on etching is another popular sensing approach based on plasmonic nanoplates, especially for their application of biosensing.^491,492,496 It is well-known that Au and Ag nanoplates can be directly etched by highly oxidative analytes, such as H₂O₂, As⁵⁺, Cr⁶⁺, ClO^–, and NO^2–,⁴⁹⁷ giving rise to morphological changes and thereby color changes readable by the naked eye without any expensive or sophisticated instrumentation. However, using H₂O₂ as an example, it typically requires high concentration (∼mM) and high reaction temperature (∼60 °C) to rapidly etch Au nanoparticles, which limits the applicability for biosensing purposes. An alternative strategy is to introduce another species to control the etching process, which may allow for more sensitive detection. Figure 28E shows a typical reaction pathway for the detection of trace H₂O₂ in live cells using triangular Au nanoplates functionalized by horseradish peroxidase (HRP), which can etch the Au nanoplates under mild conditions by the strong coordination of Br^– generated by a biocatalytic enzymatic reaction.^491,498 On the contrary, it was found that a small amount of uric acid binding to the Ag nanoplates can prevent them from etching, resulting in a high detection limit down to only 10 nM.⁴⁹⁶ Thanks to their relatively low stability and outstanding colorimetric properties, Au and Ag nanoplates have been used in a variety of colorimetric sensing protocols.^497,499