Cooperation of Hot Holes and Surface Adsorbates in Plasmon-Driven Anisotropic Growth of Gold Nanostars

Abstract

Light-driven synthesis of plasmonic metal nanostructures has garnered broad scientific interests. Although it has been widely accepted that surface plasmon resonance (SPR)-generated energetic electrons play an essential role in this photochemical process, the exact function of plasmon-generated hot holes in regulating the morphology of nanostructures has not been fully explored. Herein, we discover that those hot holes work with surface adsorbates collectively to control the anisotropic growth of gold (Au) nanostructures. Specifically, it is found that hot holes stabilized by surface adsorbed iodide enable the site-selective oxidative etching of Au⁰, which leads to nonuniform growths along different lateral directions to form six-pointed Au nanostars. Our studies establish a molecular-level understanding of the mechanism behind the plasmon-driven synthesis of Au nanostars and illustrate the importance of cooperation between charge carriers and surface adsorbates in regulating the morphology evolution of plasmonic nanostructures.

Plasmon-driven growth of noble metal nanostructures has garnered extensive scientific interests in the past two decades.^1–9 Previous studies have shown that the excitation of surface plasmon resonance (SPR) on seed nanoparticles produces energetic (or “hot”) electrons to drive the photochemical reduction of precursors and form various anisotropic noble metal nanostructures.^{6–7, 9–15} Nevertheless, it is noted that in reported processes of Au nanostructure synthesis, the potential of hot holes (the counterparts of hot electrons) for tuning the morphology evolution still remains elusive.

Herein, we found that those plasmon-generated hot holes worked cooperatively with surface adsorbates to control the anisotropic growth of Au nanostructures. Under visible-light irradiation, the addition of iodide into a growth solution containing planar-twinned Au seeds produced six-pointed Au nanostars instead of regular nanoprisms.⁷ Further studies showed that those Au nanostars were formed only under irradiation in the shorter-wavelength region (i.e., < 600 nm) of Au SPR that generated highly energetic interband holes, suggesting a hot-hole-driven process in photochemical reactions. Additionally, photoelectrochemical studies showed enhancements in both the anodic open-circuit potential (OCP) shift and the oxidative photocurrent on a Au nanocrystal electrode in the presence of iodide. These observations demonstrated that the coupling between surface-adsorbed iodide and hot holes facilitated the hot-hole-driven surface etching of Au. Moreover, the examination of morphology evolution of intermediate Au nanostructures during the growth confirmed that preferential hot-hole-driven etching occurred on edges exposing more high-index facets, which caused nonuniform lateral growths and eventually led to the formation of Au nanostars. Taken together, our results established a clear picture of the essential role of hole-adsorbate cooperation in governing photochemical processes and enabling the control of nanostructure morphology in plasmon-driven synthesis.

In a typical experiment, a growth solution was irradiated at 520 ± 10 nm (i.e., the SPR band of Au seeds)^{7, 9, 13} for 20 min. Then, potassium iodide (KI, 4 μM) was added and the mixture was irradiated continuously for another 60 min. Interestingly, six-pointed Au nanostars with tip-to-tip length around 437 ± 30 nm (Figures 1A and S2A) were formed instead of previously reported regular Au nanoprisms.^6–7 Au nanostars showed a broad absorption in the near-infrared region (Figure S2B).The selected area electron diffraction (SAED) pattern of a flat-lying Au nanostar (Figure S2C) was indexed to a [111] zone axis, suggesting that Au nanostars were single crystals bound by {111} facets on top and bottom faces. Further studies showed that similar Au nanostars were produced when replacing KI with sodium iodide (NaI, Figure S3A). Meanwhile, only regular Au nanoprisms were found in the absence of iodide (Figure S3B). Both results strongly suggested that iodide was necessary for forming Au nanostars.

Figure 1.

Scanning electron microscopy (SEM) images of Au nanostructures obtained from plasmon-driven growth of planar twinned Au seeds in the presence of iodide. (A) Six-pointed Au nanostars formed under 520 ± 10 nm irradiation (3.4 mW/cm²). (B) Regular Au nanoprisms formed under 640 ± 10 nm irradiation (3.4 mW/cm²). Scale bars in both (A) and (B) stand for 200 nm.

Iodide is commonly used to passivate the growth of Au {111} facets for controlling the morphology of Au nanostructures in wet synthesis.^16–21 However, the aforementioned growth solution only generated regular Au nanoprisms in the dark (Figure S3C), suggesting that the facet passivation by iodide should not govern the morphology evolution of Au nanostars. Thus, the formation of Au nanostars should arise from the participation of both iodide and the SPR excitation. It is noted that the SPR excitation generates three effects on Au nanoparticles: hot electron-hole pairs, enhanced local electromagnetic (EM) fields, and photothermal heating.^22–24 Under steady-state conditions, the high thermal conductivity of Au should create a homogeneous temperature distribution along Au seed nanoparticles,^25,26 making photothermal heating impossible to drive the anisotropic growth. Meanwhile, the symmetry of Au nanostars was found to be different from that of distribution of enhanced local EM fields excited by non-polarized light sources used in our study.^{7, 27} Therefore, the formation of Au nanostars must involve hot-carrier-driven processes associated with iodide.

Interestingly, the formation of Au nanostars only occurred when irradiating the growth solution with shorter wavelengths of Au SPR (< 600 nm, Figures S4A – S4E). Using longer wavelengths (> 600 nm) solely produced regular Au nanoprisms (Figures 1B, and S4F – S4H). It has been known that the SPR excitation of Au nanoparticles prompts both intraband (sp → sp) and interband (d → sp) transitions with interband transitions being dominant at shorter wavelengths.^28–34 Intraband transitions distribute a larger portion of photon energy to hot electrons and generate lukewarm holes, while interband transitions generate a larger population of highly energetic d-band holes coupled with relatively less energetic electrons.^28–34 Nevertheless, both types of plasmon-generated hot electrons carry energies above the Fermi level of Au nanoparticles (0.7 V vs. normal hydrogen electrode, NHE) that is well above the reduction potential of AuCl₄^- (1.002 V vs. NHE), making hot-electron-driven processes unlikely to have such an energy (i.e., wavelength) cut-off. Taken together, the coincidence of the observed wavelength dependence of nanostar formation and the interband transition threshold (ca. 2 eV, 620 nm)^{30, 32–33} implied that those hot holes generated via interband transitions should work together with iodide to determine the formation of Au nanostars.

Iodide has been known as a hole-mediator in dye-sensitized solar cells, and the hot-hole-driven oxidative etching has been reported for the dissolution of Au and Ag nanostructures.^35–42 Thus, it is very likely that iodide in the growth solution strongly adsorbed on the surface of Au seed nanoparticles to form Au-iodide species;^43–44 plasmon-generated hot holes were then trapped and stabilized on the adsorbed iodide and drove the oxidative etching of Au⁰. It is noted that iodide would also form stable complexes with Au⁺ and Au³⁺ ions to accelerate the oxidative etching of Au⁰.⁴⁵

The collective interaction between hot holes and iodide was verified by an anodic shift of OCP on a Au nanocrystal electrode pre-adsorbed with iodide under irradiation (Figure 2A). It is noted that the photo-driven OCP shift reflects the sign of electrolyte ions attached to the electrode.^46–47 Thus, plasmon-generated hot holes were trapped and localized by surface Au-iodide species, while hot electrons were accumulated on the Au electrode and attracted cations from the bulk electrolyte, leading to the anodic OCP shift. This is in contrast to the cathodic OCP shift observed on an Au electrode adsorbed with electron-trapping polyvinylpyrrolidone (PVP) (Figure 2A).^6–7 Furthermore, an oxidation peak at ca. 0.95 V vs. reversible hydrogen electrode (RHE) appeared in the linear sweep voltammetry (LSV) scan of an iodide-adsorbed Au nanocrystal electrode (Figure S5), corresponding to the one-electron oxidation of Au⁰ to AuI₂^-.⁴⁸ Moreover, chronoamperometry (I-t) measurements showed an enhanced steady-state anodic photocurrent in the presence of iodide (Figure 2B), confirming that the cooperative interaction between hot holes and iodide facilitated the oxidative etching of Au⁰.

Figure 2.

(A) OCP of Au nanocrystal electrode in the presence of 120 μM KI (red curve) and PVP (blue curve), respectively. The anodic change of an iodide-adsorbed Au electrode upon irradiation (shaded region) suggested that plasmon-generated hot holes were trapped and localized by surface Au-iodide species while hot electrons accumulated on the Au electrode and attracted cations from the bulk electrolyte. In contrast, the cathodic change on a PVP-adsorbed Au electrode suggested the attraction of anions from the bulk electrolyte by hot holes due to the trapping of hot electrons by PVP. (B) Chronoamperometry (I-t) of Au nanocrystal electrode at 1.03 V vs. RHE in the presence of 120 μM KI (red curve) and no KI (black curve). Shaded regions in both (A) and (B) represent the light irradiation using 530 ± 10 nm LED (240 mW/cm²).

The examination of shape evolution of Au seed nanoparticles revealed that the hot-hole-driven oxidative etching of Au⁰ assisted by iodide played an important role in the formation of Au nanostars. It should be noted that due to the strong affinity of iodide to Au,^43–44 iodide should adsorb across the whole surface of Au seed nanoparticles and facilitate the etching of Au⁰ at all exposed sites on growing Au nanostructures. However, the etching rate varied on different sites with different coordination numbers, and atoms with lower coordination numbers were less stable, thus exhibiting faster oxidative etching.^49–51 As shown in Figure 3A and Figure S6A, small hexagonal nanoprisms with edges mainly consisted of low-index Au {100} and Au {111} facets were formed in the initial 20-min iodide-free growth.⁴ Such a lateral growth was previously identified as a result of hot-electron-driven Au⁰ deposition assisted by PVP adsorbed along edges of planar-twinned Au seeds.⁷ When KI was added for a 30-min growth, the size of those nanoprisms kept increasing, but their corners became truncated, forming dodecagonal structures with six edges preserved from initial hexagonal nanoprisms and six new edges intersecting original ones (Figures 3B, 3D, S6B, and S6D). The overall increase in size came from the hot-electron-driven deposition of Au⁰, while the truncation of Au nanoprism corners resulted from the faster hot-hole-driven etching of Au⁰ due to their lower coordination numbers.

Figure 3.

Morphology evolution of Au nanostructures during the plasmon-driven growth. (A) A SEM image of Au hexagonal nanoprisms formed in the initial 20-min iodide-free growth; (B) A SEM image of Au dodecagonal structures formed after a further 30-min growth from (A) in the presence of KI; (C) A SEM image of Au nanostars formed after another 30-min growth from (B) in the presence of KI. Scale bars in all SEM images correspond to 100 nm. (D) A schematic showing the preferential etching (red arrows) at high-index edges that caused nonuniform growths (blue arrows) along different lateral directions to form Au nanostars. Nanostructures shown in (A), (B), and (C) were isolated from the same batch of synthesis at corresponding time points.

Furthermore, high-angle annular dark-field (HAADF) atomic-resolved scanning transmission electron microscopy (STEM) images identified that those Au dodecagonal nanostructures exhibited alternatively arranged low-index edges and high-index edges (Figures 4, S7, and S8, Table S1), in which low-index edges were preserved from initial Au nanoprisms (Figure 4A) and high-index edges corresponded to new intersecting edges formed from the truncated corners (Figures 4B and 4C). Those high-index facets with more low-coordinated sites were expected to serve as active sites for hot-hole-driven oxidative etching of Au⁰ and reduce the deposition rate of Au⁰. Thus, nonuniform deposition rates along different growth directions would lead low-index edges to evolve into tips (faster growth, Figure S9A) and high-index edges to trenches (slower growth) to form Au nanostars. Indeed, after another 30-min growth, we observed that those dodecagonal structures grew into Au nanostars (Figures 3C, 3D, S6C, S6D and S9B).

Figure 4.

Filtered HAADF-STEM images along two edges of a single dodecagonal intermediate nanostructure (see Figure S7 for more details). (A) A filtered HAADF-STEM image of one edge acquired along [011] zone axis. This edge is dominated by low-index facets as indicated in the figure. (B) and (C) Filtered HAADF-STEM images of another edge acquired along [112] zone axis. This edge is dominated by high-index facets. Green lines in (A), (B), and (C) denote the location of low-index facets, while red and orange lines denote the location of high-index facets. Red and orange lines were used in alternative for a clearer view. It should be noted that in (A), facets with same indices align along different directions because of the existence of twin boundaries (i.e., a mirror plane). Scale bars in (A), (B), and (C) all represent 2 nm.

In summary, we have demonstrated that the synergy between plasmon-generated hot carriers and surface adsorbates provided a unique control over the anisotropic growth of plasmonic nanostructures. More importantly, we discovered that iodide assisted the hot-hole-driven oxidative etching of Au seed nanoparticles preferentially at edges with high-index facets to counteract the hot-electron-driven Au⁰ deposition, leading to nonuniform growths along different lateral directions to form Au nanostars. Elucidating the essential function of plasmon-generated hot holes in our studies affirmed that both hot holes and hot electrons can be utilized in the light-driven synthesis of nanostructures. Moreover, it should be noted that the discovered cooperation of hot holes and iodide in our plasmonic system can be extended to general photo-excited processes. For instance, Au nanostars were also obtained by directly exciting the interband transition of planar-twinned seeds using blue light (470 ± 10 nm) in the presence of bromide (Figures S10 and S11). Taken together, our studies provide a comprehensive description of the interaction between hot carriers and surface adsorbates, and illustrate its great potential in regulating photochemical processes.