Tuning the modal coupling in three-dimensional Au@Cu₂O@Au core–shell–satellite nanostructures for enhanced plasmonic photocatalysis

Abstract

The fascinating optical properties of coupled plasmonic nanostructures have attracted great attention and emerged as a promising platform for applications in catalysis, sensing, and photonics. When two resonant modes interact strongly, the coupled modes are formed with mixed properties inherited from the basic modes. However, limitations still exist in the understanding of the coupling between optical modes in individual structures. In addition, how the coupling in hybrid plasmonic structures correlated with their efficiencies in promoting photocatalysis remains an important and challenging question. Here we demonstrate that coupling in individual Au@Cu₂O@Au core–shell–satellite hybrid structures can be tuned through rationally designing the structural features for enhancing plasmonic photocatalysis. To further comprehend the optical phenomena of different coupled nanostructures, a model was developed based on the Mie theory, which provided a different perspective to analyze coupling qualitatively in individual nanoparticles. The insights gained from this work not only shed light on the underlying mechanisms of modal coupling in individual structures but also provide an important knowledge framework that guides the rational design of coupled plasmonic nanostructures for plasmonic photochemistry and photocatalysis.

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Introduction

Noble metal nanoparticles (NPs) are well known for their capacity to enhance the interactions between light and matter, exhibiting fascinating colors.¹ For an individual particle, the collective oscillation of its free electrons results in a localized surface plasmon resonance (LSPR) at a certain frequency determined by the size/shape of the NP and its dielectric environment. When two or more NPs are close enough to each other, the LSPR of the individual NPs interacts through their optical near fields, forming coupled oscillation modes and generating a significant change in the optical response.^2,3 Furthermore, coupling systems with more complex structures exhibit some strikingly different optical properties, such as plasmon hybridization,⁴ Fano resonance,^5–7 electromagnetically induced transparency (EIT),⁸ and plasmonic waveguiding,⁹ which are widely applied from sensing¹⁰ to energy harvesting,^11,12 and subwavelength optical imaging.¹³ Thus, plasmon coupling is the fundamental principle by which the optical resonances in NP assemblies are tuned. Moreover, plasmon coupling of the hybrid nanostructure can induce significant electromagnetic field enhancement, which is beneficial for plasmon-enhanced spectroscopy,^14,15 artificial photosynthesis,¹⁶ and plasmon-enhanced catalysis.^17–19

In the face of the increasingly serious energy crisis, one expects to maximize the conversion of solar energy into usable chemical energy through plasmon-enhanced photocatalysis.²⁰ Noble metal NPs combined with semiconductors were developed to form heterostructures where plasmonic metal acts as antennae, enhancing the light absorption and photogenerated carrier lifetime in semiconductors greatly.^21–24 Recent advances in tuning the surface/interfacial structures of the hybrid plasmonic nanocatalysts have shown great promise for their application in photocatalysis.^25–29 Notably, strong coupling can enhance the interaction between light and matter, which has been employed to modulate the catalytic activities and selectivity. For instance, the coupling between the LSPR and optical modes such as whispery gallery modes,³⁰ waveguide modes,^9,31 and Bloch waves³² has been demonstrated for broadband optical responses. In particular, Misawa and coworkers demonstrated that coupling between the LSPR mode and Fabry–Pérot (F–P) nanocavity mode has been shown to achieve optical responses over a broad range of wavelengths, which can promote water splitting.³³ In addition, the coupling strength can be tuned by varying the dielectric constant and the embedding depth of NPs.^34,35 However, limitations still exist in the understanding of the coupling between optical modes in individual 3D structures. The core–shell–satellite structures have been constructed in some previous studies for plasmon-enhanced nanocatalysis;^36,37 however, the quantitative correlations between structural features and modal coupling in hybrid structures remain a challenge yet to be addressed. Moreover, how the modal coupling in hybrid structures correlated with their activities in enhancing plasmonic photocatalysis remains unclear. Therefore, tuning the modal coupling in 3D hybrid plasmonic structures through precise control over geometric features and quantitative optical characterization studies is urgent for developing highly efficient plasmonic photocatalysts.

Herein, we present an Au@Cu₂O@Au (ACA) core–shell–satellite hybrid structure with tunable plasmon coupling for enhanced plasmon photocatalysis. In this structure, Au@Cu₂O NPs support a nanocavity mode in the visible spectral region. Furthermore, the growth of Au NPs as satellites with tailored sizes induces a modal splitting in the extinction spectra, suggesting the formation of a modal coupling. By deliberately controlling the thickness of the Cu₂O shell and the size of Au satellites, the coupling can be tuned and understood using a two-harmonic resonator model. The modal splitting in scattering spectra was also revealed through single-particle dark-field scattering (DFS) measurements. More importantly, the ACA NPs show much better catalytic activities than Cu₂O@Au NPs toward plasmon-driven chemical reactions revealed by in situ surface-enhanced Raman scattering (SERS) measurements. Ultrafast transient absorption (TA) spectroscopy and photoelectrochemical (PEC) measurements further show the superior performance of the ACA hybrid structure toward plasmonic photocatalysis.

Results and discussion

As schematically illustrated in Fig. 1A, we developed a two-step method for the construction of the ACA core–shell–satellite nanostructure (see experimental details in the ESI†). Au rhombicuboctahedral (RCO) NPs, which are single-crystalline in nature and ∼55 nm in size, were employed as the plasmonic core (Fig. 1B and S1†). Firstly, the Cu₂O shell was grown on the surfaces of the Au NPs to form the Au@Cu₂O structure with tunable shell thickness (Fig. 1C and S1, S2†). The range of Cu₂O shell thickness in Au@Cu₂O NPs can be tuned from ∼27 nm to ∼65 nm by simply changing the amount of Cu²⁺ precursors (Fig. S2†). Secondly, ACA NPs can be obtained by reacting the Cu₂O shell with HAuCl₄ through a galvanic replacement reaction (GRR) process (Fig. 1D and S3†). The core–shell–satellite structure can be then clearly visualized by HAADF-STEM imaging and corresponding EDS mapping of an individual ACA particle (Fig. 1E). X-ray diffraction (XRD) patterns further verify the composition of Au and Cu₂O in the ACA NPs and none of the Cu⁰ elements or CuAu alloy phase can be found (Fig. S4†). By deliberately controlling the amount of HAuCl₄, the size and density of the Au satellite in ACA NPs can be finely modulated, resulting in a different modal coupling mode that will be discussed later (Fig. S5†). The as-prepared ACA NPs exhibit excellent uniformity in both the overall size of ACA NPs and the density of the Au satellites, which will benefit the investigations of plasmon coupling in the ensemble extinction spectra (Fig. S6†). In the ACA structure, Cu₂O not only acts as a medium allowing light to spread but also as a spacer to control the distance between the cavity and Au satellites, which is similar to the spacer of NPs on thin film systems.³³ Moreover, the in situ GRR process could create close contacts between Au satellites and the Cu₂O shell (Fig. S5†), which could potentially enhance the plasmon coupling in contrast to the method that used long alkyl chain ligands as the linker to assemble the Au satellites.³⁸ Notably, the Au core could serve as an Au mirror in the ACA structure, which could improve the optical distance effectively when light undergoes repeated mirror reflection in the nanostructure. Therefore, it is envisioned that the ACA hybrid structure provides the superb capability to confine the electromagnetic field of light into a dielectric layer and enhance the coupling between light and matter vastly.

Fig. 1 Construction and characterization of Au@Cu₂O@Au (ACA) core–shell–satellite nanostructures with modal splitting in the extinction spectra. (A) Schematic illustration of the construction of the ACA nanostructure. (B) TEM image of Au NPs. (C) SEM image of Au@Cu₂O NPs. (D) SEM image of the ACA NPs with smaller Au satellites. (E) STEM image of ACA NPs and the corresponding EDS mapping images. The yellow, green, and blue colors depict Au, Cu, and O elements, respectively. (F) Extinction spectra of Au, Au@Cu₂O, and ACA NPs in aqueous solution. (G) Extinction spectra of ACA NPs fitted using a Gaussian function. The red and blue curves indicate the Gaussian fitting of the dual extinction bands. (H) Schematic illustration of the interaction between incident light and the ACA structure; an energy-level diagram of the modal coupling between the cavity mode in the Cu₂O layer and the LSPR mode of the Au satellites NPs. The ω_sp and ω_cavity are the resonant frequencies of plasmon and nanocavity modes. Scale bars: 100 nm.

The formation of a core–shell–satellite coupled structure introduced intriguing modifications to the optical properties. Fig. 1F shows the extinction spectra of Au, Au@Cu₂O, and ACA NPs in aqueous solution. Au NPs exhibit a dipole resonance peak localized at ∼530 nm, whereas Au@Cu₂O NPs show a dipole resonance peak located at ∼685 nm. Interestingly, the original dipole resonance peak of Au@Cu₂O splits into two peaks after the formation of Au satellites on the surface of Au@Cu₂O NPs. The Gaussian or Lorentzian fittings of the dual extinction bands of ACA NPs further show the peak splitting in the extinction spectrum of ACA NPs (Fig. 1G and S7†). The newly fitted peaks are labeled as ω⁺ and ω⁻, which are localized at ∼590 nm and ∼690 nm, respectively. Inspired by the modal strong coupling system developed by Misawa and coworkers,³³ we proposed that the observed peak splitting in our case is akin to the energy-level splitting into upper and lower modes generated by the modal coupling (Fig. 1H). Note that Fig. 1H shows the enhanced light reflection and absorption in the ACA structure but not a whispering gallery mode (WGM). Energy-level diagram of the coupling between the nanocavity mode in the Cu₂O layer and the LSPR mode of the Au satellite NPs was showed in the right panel of Fig. 1H. The ω_sp and ω_cavity are the resonant frequencies of plasmon and nanocavity modes. The LSPR mode of Au satellites is thought to be coupled with the nanocavity mode of the Au@Cu₂O layer in the ACA structure upon light excitation, leading to the peak splitting in the ensemble extinction spectrum. Although the nanocavity mode here is not a traditional F–P cavity mode as Misawa and coworkers demonstrated in their cases,^33,34 the inner Au@Cu₂O core in the ACA structure could be considered as a nanocavity mode when interacting with light and the Au satellites for enhanced light absorption. To satisfy the strong coupling regime between two oscillators, the frequency of the coupled system should be higher than the dephasing time of both oscillators.^39,40 However, it's very difficult for us to estimate the splitting energy on account of the thickness change of Cu₂O and the variation in Au satellite size during the process of loading Au satellites, so we couldn't obtain meaningful results such as the inverse cross curve of strong coupling.³³ Therefore, we use a modal coupling mechanism to account for the optical properties that we observed on the ACA structure and further propose another system for understanding the plasmon coupling in the three-dimensional ACA hybrid structures.

For the plasmonic NPs with a coupling system, it is well known that the size, permittivity, shape, and distance between NPs determine the optical properties of the coupled system.^41–45 Thus, we further tuned the structural features and plasmon coupling of ACA NPs by controlling the size and density of Au satellites (Fig. 2). By simply changing the amount of Au precursors involved in the GRR with the Cu₂O shell, ACA NPs with tunable size and density of Au satellites can be obtained (Fig. 2C–H and S8†). The thickness of the Cu₂O shell in ACA NPs is found to gradually decrease as the amount of HAuCl₄ increases (Fig. S8†). As shown in Fig. 2A, a gradual broadening of the dipole resonance peak of ACA NPs can be observed as the size and density of Au satellites first increase. A further increase in the size of Au satellites generates a shoulder peak (∼600 nm), which may indicate the formation of dual-extinction bands due to the peak splitting induced by modal coupling (Fig. S9†). The shoulder peak becomes stronger, so eventually, a broad extinction band was observed (Fig. 2A: F curve), suggesting the superposition of two peaks. As we proposed in Fig. 1H, these two peaks are classified as ω⁺ and ω⁻, indicating modal coupling between the nanocavity mode and LSPR mode of Au satellites. As the amount of HAuCl₄ further increases, the ω⁺ mode blueshifts and the ω⁻ mode redshifts, ultimately exhibiting two obvious individual peaks (Fig. 2A: G curve). Also, the extinction band of Cu₂O located at ∼470 nm decreases gradually during the GRR between the Cu₂O shell and HAuCl₄, suggesting the changes in the thickness of the Cu₂O shell in ACA NPs.

Fig. 2 Tuning the structural features and plasmon coupling of ACA NPs by controlling the size and density of Au satellites. (A) Experimental extinction spectra of Au NPs, Au@Cu₂O NPs (the dipole resonance peak located at 685 nm), and ACA NPs with distinct structures synthesized using different amounts of Au precursors. (B) Calculated extinction spectra of Au NPs, Au@Cu₂O NPs, and ACA NPs. The simulation models were constructed according to the TEM image of a single particle from C-i to H-i. (C–H) SEM and TEM images of ACA NPs with distinct structures, which correspond to the extinction spectra in panel A. Scale bars: 100 nm.

To gain more quantitative insights into the optical properties of coupled structures, we calculated the extinction spectra of ACA NPs with different densities and sizes of Au satellites using the finite-difference-time-domain (FDTD) method (Fig. 2B). The simulation models were constructed according to the TEM images of a single particle (Fig. 2C–H). The simulated results are in excellent agreement with our experimental observations, indicating that the coupling between the LSPR of Au satellites and the nanocavity mode of the Au@Cu₂O layer strongly affects their optical properties through peak splitting. Notably, the aggregated larger satellites could potentially affect the optical properties of ACA NPs. The plasmon–plasmon coupling between Au satellites cannot be completely excluded for sure. Our simulation models, as shown in Fig. 2, have already accounted for the aggregated states for larger Au satellites. However, it is very difficult to experimentally control the density of aggregated Au satellites for a quantitative comparison. Thus, to quantitatively show the impact of plasmon–plasmon coupling between Au satellites on the optical properties of ACA NPs, we have further carried out the FDTD simulations on ACA NPs with discrete and aggregated Au satellites, respectively. As shown in Fig. S10,† the aggregated Au satellites with a size of 8 nm have a negligible impact on the overall optical properties of ACA NPs. In contrast, for Au satellites with a size of 15 nm, broadening of the plasmon peaks of ACA NPs can be observed as the number of aggregated Au satellites increases. The dependence of optical properties on the coupling between Au satellites and the Au@Cu₂O layer can also be observed and tuned in ACA NPs using anisotropic Au NPs as initial seeds instead of spherical Au NPs (Fig. S11†).

We further investigated the impact of the Cu₂O shell on the modal splitting in the extinction spectra of ACA NPs by varying the thickness of the Cu₂O layer (Fig. 3). We constructed ACA NPs with 5 different Cu₂O thicknesses (the dipole resonance peak located at 630, 650, 667, 685, and 695 nm, respectively). As shown in Fig. 2A and S12,† distinct spectral evolution of extinction bands of ACA NPs with different Cu₂O thicknesses can be observed as a function of the amount of Au precursor. To further qualitatively investigate the spectral differences in different shell thicknesses, two Au@Cu₂O NPs with their dipole resonance peaks located at 630 nm and 667 nm were chosen for a detailed analysis of modal splitting (Fig. 3A and B). The red and blue curves indicate the Gaussian fitting of the dual extinction bands (ω⁻ and ω⁺), which clarifies the transient state from overlapping to the separation of dual-extinction bands. For the peak locations, ω⁺ blue shifts to a shorter wavelength while ω⁻ red shifts to a longer wavelength as the amount of Au precursor increases. As the amount of Au precursor increases, the dipole moment (proportional to the radius of the Au particle) of Au satellites becomes larger, which significantly enhances the modal coupling between the cavity mode and the LSPR mode. Thus, the enhanced modal coupling leads to the blueshift of ω⁺ and the redshift of ω⁻ with a larger modal splitting in extinction spectra. The amount of HAuCl₄ is found to play a crucial role in tuning the modal coupling in the ACA structure because it affects not only the dipole of the Au satellites but also the thickness of the Cu₂O shell in between. Therefore, an increase in the peak intensities of ω⁺ and ω⁻ modes was observed due to the enhanced modal coupling as the amount of HAuCl₄ increased at first with well-defined cavity mode coupling with the dipole modes of Au satellites. However, the peak intensity started to decrease when the amount of HAuCl₄ was in excess, leading to the overgrowth of Au satellites and the ill-defined Cu₂O shell, i.e., the less-defined nanocavity mode with weaker modal coupling (Fig. 3A and B). Therefore, the dual-extinction bands can be observed in Fig. 3B while not obvious in Fig. 3A because of the different degrees of separation between the two modes. However, the dual-extinction bands can't be observed in the case where the Cu₂O shell is very thick (Fig. S12D†). We further compared the full width at half maximum (FWHM) and the peak area changes in both cases (Fig. 3C). Both FWHM and peak area follow the trend of first increasing and then decreasing (Fig. 3C). To further understand the results of FWHM and peak area with the satellite variation, we compared the changes in FWHM and peak area for ACA NPs with 5 different Cu₂O thicknesses (Fig. S13†). All the samples showed a consistent variation trend with an increase first and a decrease later as the amount of HAuCl₄ increased. In particular, sample S13D with proper Cu₂O shell thickness represents the optimum condition for the modal coupling, which was preserved even with a higher amount of HAuCl₄. Therefore, our experimental observations suggested that the shell thickness of the Cu₂O layer plays a crucial role in modulating the modal splitting of optical extinction spectra.

Fig. 3 Tuning the modal splitting in the extinction spectra of ACA NPs with different Cu₂O thicknesses. (A and B) Extinction spectra of ACA NPs with different Cu₂O thicknesses (the dipole resonance peak located at 630 nm and 667 nm, respectively) as a function of the amount of Au precursor. The extinction spectra were fitted using the Gaussian function. The red and blue curves indicate the Gaussian fitting of the dual extinction bands. The concentration of HAuCl₄ varies from 0.3, 0.5, 1.0, 1.2, 1.5 to 1.7 mM (bottom to top) in panels (A and B). (C) Histogram of FWHM and peak area (extacted from the extinction spectra shown in panels (A and B)) as a function of the amount of Au precursor. (D) The G_factor of ACA NPs with different Cu₂O shell thicknesses as a function of the amount of Au precursor. The G_factor was color-coded with different shades and shown with corresponding values. The thickness of the Cu₂O increases from bottom to top.

To better understand the effect of Au satellites and Cu₂O shells on the optical properties of ACA NPs, the Mie theory was employed for a deeper analysis. We consider Au@Cu₂O NPs as the LSPR core coated with a dielectric shell, which is also in line with previous studies.^26,29,46,47 For the Au@Cu₂O NPs, an individual dipole resonance peak in the ensemble extinction spectra (Fig. 1F) was observed instead of a complicated multipolar optical mode,^48,49 which is also in excellent agreement with our simulation results (Fig. 2B). Also, it is difficult to simply consider it as a Mie resonator due to the presence of the Au core in Au@Cu₂O NPs which significantly changes its optical response.⁵⁰ For NPs much smaller than the wavelength of the absorbing light, only the dipole term is assumed to contribute to the absorption, while quadrupole resonances and other high-order modes are neglected.⁵¹ Thus, as depicted by Christian, both the cavity and the orbit of satellites independently form purely dipolar polarization states.⁵² It has been reported that a strong red shift and spectral peak broadening occur when the Au spherical monomer forms the dimer. Therefore, the spectral line shapes of extinction spectra reflect the interaction between two plasmonic metal NPs to a certain extent. The simplest description of this interaction, the coupling of two nearby oscillators, is in the coupling of two nearby dipoles. The interaction energy is given by⁵³

where P₁ and P₂ are the magnitudes of the dipole moments and r is the interparticle distance. The dipole moments of Au@Cu₂O NPs can be calculated from the Mie theory described by⁵⁴where ε_metal is the dielectric constant of the metal core, ε_dielectric is the dielectric constant of the dielectric shell, a is the metal NP radius and E₀ refers to the electromagnetic (EM) field composed of the incident radiation. Since the thickness of the dielectric Cu₂O shell in Au@Cu₂O NPs also plays a crucial role in affecting the optical properties, we further included this term in eqn (2), which is extensively discussed in the ESI† as the additional notes.

To simplify the ACA system, we make the following transformations analogous to self-consistent field theory in solving multi-electron atomic wave functions. If we separately consider the individual components in the coupled nanostructures, the EM field distribution and optical performance of one component will be influenced by the induced interaction of dipolar resonances with other nearby components. Under the incident light, the dipole moment of a metal NP is proportional to the size of the particle, while the vector sum of dipole moments of the cavity and other satellites can be described as follows:

where P_i is the dipole moment of satellite i. Thus, the model is simplified as the interaction between a dipole (one satellite in the system) and another dipole (system consisting of other satellites with cavity), and the interaction can be described using eqn (1), which is similar to the coupling in an asymmetric Au NP dimer.⁵⁵ The term r³ in eqn (1) shows that the interaction decays rapidly when moving away, explaining the experimental phenomenon well that there are few changes in the extinction spectrum of sample 5 (Fig. S12D†). This model qualitatively explains well why the size of the Au satellites and the thickness of the Cu₂O shell have an impact on the coupling interaction and account for the changes in the extinction spectra of coupled nanostructures with smaller satellites in previous work,³⁶ attributed to weak interaction energy. When we increase the amount of HAuCl₄, the diameter of Au satellites increases, which is reflected in the increase of the linewidth of extinction spectra, implying stronger interaction between coupled harmonic oscillators.

To further quantify the spectral change before and after loading Au satellites on ACA NPs, we defined an equation described by the ratio of the skewness (Δλ, Fig. S14†) of the peak to the full peak width as follows:

We compared the G_factor of ACA NPs with different Cu₂O shell thicknesses as a function of the amount of Au precursor. The G_factor was color-coded with different shades and shown with corresponding values. As shown in Fig. 3D, the value of the G_factor follows a volcano curve as the amount of HAuCl₄ increases for ACA NPs with thinner Cu₂O shells. In contrast, the G_factor value of ACA NPs with thicker Cu₂O shells gradually increases as the amount of Au precursors increases, which could be attributed to the dominance of the ω⁻ mode. Note that there are no specific physical meanings of the G value as we designed, so we are not able to claim whether the system satisfies the strong coupling regime or not based on the G values.

To more quantitatively understand the impact of plasmon coupling on the optical properties of the ACA hybrid structure, we further carried out the single-particle DFS measurements on the ACA NPs (Fig. 4). The scattering spectra of ∼30 randomly selected particles for each sample were measured using a hyperspectral detection system (Fig. S15,† see experimental details in the ESI†).⁵⁶ We used the Au@Cu₂O NPs whose scattering peak is located at 650 nm as a model system to investigate the spectral evolution of scattering spectra before and after forming Au satellites with different sizes and densities (Fig. 4A). The dipole scattering peak at ∼650 nm of Au@Cu₂O NPs shows a slight blueshift as the amount of Au precursors increases. More importantly, the scattering peak becomes significantly broadening along with increases in FWHM, as revealed by the statistical histogram of each dataset. As shown in Fig. 4B, we further correlated the resonance energy (E_res) of the scattering peak with the FWHM (Γ). Again, it is found that the E_res increases along with the increases in FWHM as the size of Au satellites increases, which is also in excellent agreement with the simulated results (Fig. 4C). Note that the thickness of the Cu₂O shell decreases as more Au precursors react with the shell through the GRR, which in turn increases the coupling strength that leads to the increase in FWHM. We also designed and fabricated Au@Cu₂O NPs with two different shell thicknesses to tune the plasmon coupling when forming ACA NPs with different thicknesses of Cu₂O cavities (Fig. S16 and S17†). We then classified them into coupled structures with different initial energies of 1.91 eV (blue), 1.85 eV (purple), and 1.79 eV (red), as shown in Fig. 4D. It is worth noting that as we added HAuCl₄ to the system, it is difficult to ensure that each particle undergoes the same changes. However, single-particle spectroscopy has an advantage over ensemble extinction spectroscopy, which allows one to count the changes in each particle. Thus, we counted all the samples with different HAuCl₄ added at the same initial Cu₂O thickness, and the trend of FWHM and peak position at that thickness can be obtained from the scatter plot (Fig. 4D). By comparing the scatter plots of Au@Cu₂O NPs with three different initial energies, we found that the resonance energy of Au@Cu₂O with higher initial energy varies faster with the FWHM. While the modal peak splitting cannot be observed in the single-particle scattering spectra, the dramatic broadening of scattering plasmonic modes also indicates the enhanced coupling between the plasmon mode of the Au satellite and the nanocavity mode of the inner Au@Cu₂O layer.

Fig. 4 Tuning the modal coupling in the scattering spectra of ACA NPs at the single-particle level. (A) Single-particle scattering spectra of ACA NPs loading Au satellites of different sizes and the corresponding statistical histogram of FWHM. The solid line shows the peak position of the scattering spectrum of Au@Cu₂O NPs. The insets show the corresponding geometric models of ACA NPs on the ITO substrate. (B) Γ vs. E_res of ACA NPs with different sizes of Au satellites. E_res is plotted as a function of Γ (FWHM) and color-coded by different samples (inset) from ∼30 individual particles, respectively. (C) Comparison of experimental and simulated changes in intensity and FWHM of scattering spectra with increasing dipole moment of Au satellites. (D) Histogram showing the correlation between Γ and E_res for the scattering spectra of ACA NPs with varying sizes of Au satellites. Each color-coded elliptical region represents the change in Γ with E_res of ACA NPs with varying sizes of Au satellites. (E) Schematic illustration of the plasmon damping of the ACA structure with varying sizes of dipole. (F) Schematic illustration of the energy transfer mechanism in the coupled structure of ACA NPs.

We further compared the FWHM of scattering spectra of ACA NPs with significantly different sizes of Au satellites (Fig. 4E). It is observed that the larger Au satellite induces significantly more plasmon damping, resulting in the broadening of the scattering peak and an increase in FWHM. The changes in FWHM could be attributed to the enhanced electron–phonon relaxation when surface adsorbates induce an electric dipole at the particle surfaces,⁵⁷ which has also been previously demonstrated using single-particle spectroscopy.⁵⁸ Recent studies have further demonstrated that energy transfer can cause a decrease in the electron temperature, which in turn decreases the electron–phonon relaxation time.⁵⁹ In the model of the ACA system, Au satellites serving as dipoles can accelerate the electron–phonon relaxation of the Au core, which accelerates the energy transfer process. The electron–phonon coupling time (τ_e–ph) is determined by the electronic temperature. The dependence of τ_e–ph can be described using the following equation,⁶⁰ according to the well-established two-temperature model for describing the electron dynamics of metal NPs.

where γ is the electron heat capacity coefficient of Au, g is the electron–phonon coupling constant, T_l is the lattice temperature, U is the initial absorbed energy after SPR excitation, and P_i is the fraction of energy injected into Cu₂O. Based on the results and discussion above, a possible mechanism for plasmon-induced energy transfer is proposed in Fig. 4F. Au satellites act as dipoles that accelerate the energy flow from the Au core to the Cu₂O shell. Note that we only consider the contribution of Au satellites as electric dipoles and ignore their contribution as catalytically active sites in this case. The Au satellites could also serve as catalytically active sites toward plasmon-driven photocatalytic reactions, which can be verified by following in situ SERS measurements.

To further explore the significance of the Au core on the enhanced coupled plasmonic properties of ACA NPs, we carried out the plasmon-driven dimerization of 4-nitrothiophenol (4-NTP) to 4,4′-dimercaptoazobenzene (DMAB) on ACA NPs; in this case, the Au satellites serve as the catalytically active sites (Fig. 5). Time-resolved SERS was used as a spectroscopic tool to monitor the molecular transformation from 4-NTP to DMAB upon laser excitation, in which the characteristic SERS peaks of 4-NTP and DMAB have been carefully demonstrated by previous studies (Fig. S18†).^61,62 Similar to plasmonic NPs on mirror systems, the Au core is expected to serve as a plasmonic mirror to confine the electromagnetic field of light into a dielectric layer and significantly enhance the plasmonic near-fields.^63–67 As illustrated in Fig. 5A, a CW excitation laser was focused on a 2 μm diameter focal spot by using a confocal Raman microscope. ACA NPs and Cu₂O@Au NPs with roughly similar sizes and densities of Au satellites were employed as the single-particle bifunctional substrates (Fig. S19†). Fig. 5B and C show the temporal evolution of SERS spectral features under isolated air at an excitation wavelength (λ_ex) of 785 nm on ACA NPs and Cu₂O NPs, respectively. Since the tests were performed in the presence of water, we excluded photothermal effects on the reaction and only considered carrier effects. At the initial stage, three SERS peaks located at 1078, 1338, and 1572 cm⁻¹ are observed in the Raman spectrum, corresponding to the characteristic C–S bond stretching (ν_C–S), symmetric nitro stretching (ν_NO2) and aromatic ring stretching (ν_ring) modes of 4-NTP.^68–70 After the laser excitation, four new SERS bands signifying the formation of DMAB emerged at 1143, 1181, 1444, and 1472 cm⁻¹, all of which became progressively more intense as the dimerization reaction proceeded.^70–72 The overall trend of 4-NTP on Cu₂O@Au follows the same law, despite some details different from ACA NPs, which is attributed to the different coupling rates (Fig. S19 and S20†). We further calculated the apparent fractions of DMAB and 4-NTP, denoted as θ_DMAB and θ_NTP, respectively, based on the relative intensities of the SERS peaks at 1438 cm⁻¹ (ν_N=N of DMAB) and 1338 cm⁻¹ (ν_NO2 of 4-NTP) with respect to the ν_ring mode at 1572 cm⁻¹ (see details in the ESI†). As shown in Fig. 5D and E, the temporal evolutions of θ_DMAB and θ_NTP both obey an apparent first-order rate law, which can be described using the following rate equations:

θ_DMAB(t) = θ_t=∞(t) × (1 − e^−kt) (6)

and

θ_NTP(t) = 1 − θ_t=∞ + θ_t=∞ × e^−kt (7)

where k is the apparent first-order rate constant and θ_t=∞ is the maximum yield of DMAB achievable at infinitely long reaction time. The three key parameters could be obtained by performing least-squares curve fitting to the experimental results. The rate constant for the 4-NTP coupling reaction on ACA NPs is significantly higher than that on Cu₂O@Au NPs. Moreover, the size and density of Au satellites in ACA NPs have also been optimized at an optimal amount of Au precursors for maximizing the rate constant (Fig. S21†). Either increasing or decreasing the amount of Au precursor results in a smaller rate constant in comparison to the results shown in Fig. 2A. These results suggested that the presence of the Au core in ACA NPs significantly enhanced the efficiency of plasmon-induced hot carriers for photochemical molecular transformations.

Fig. 5 (A) Schematic illustration of using SERS to monitor the plasmon-driven dimerization of 4-nitrothiophenol (4-NTP) on ACA NPs. (B and C) Comparison of time-resolved SERS spectra of 4-NTP on (B) ACA and (C) Cu₂O@Au NPs upon 785 nm laser excitation. (D and E) Temporal evolutions of θ_NTP, θ_DMAB, and θ_DMAB + θ_NTP on (D) ACA and (E) Cu₂O@Au NPs. The solid lines were obtained by performing least-squares curve fitting to the experimental results. (F) Cross-sectional views of calculated near-field enhancements of Au@Cu₂O, Cu₂O@Au, and ACA NPs (left to right) at 638 nm and 785 nm excitation, respectively. The field enhancements are plotted on a logarithmic scale (log|E/E₀|²). E: electric field; k: incident wave vector.

Strong electromagnetic field enhancement can promote hot carrier separation more efficiently in metal-semiconductor nanostructures for photocatalysis, which has been widely studied.^73–75 We propose that the different reaction behaviors between ACA NPs and Cu₂O@Au NPs in plasmon-driven dimerization of 4-NTP could be attributed to the different excitation strengths of plasmonic near-fields. To gain deep insights into the plasmonic near-field properties of different structures, we have further used FDTD to calculate the near-field enhancement of ACA, Cu₂O@Au, and Au@Cu₂O NPs (Fig. 5F). Fig. 5F shows the cross-sectional views of calculated near-field enhancements of ACA, Cu₂O@Au, and Au@Cu₂O NPs (|E/E₀|² plotted on a logarithmic scale) under 638 nm and 785 nm laser excitation, respectively. Near-field enhancement of Au@Cu₂O NPs is mainly concentrated at the interface between the core and shell layer while weak on the outer surface of NPs. For Cu₂O@Au NPs, the external electric field of NPs is excited, while the internal near-field enhancement is weak. The enhanced electric field in Fig. 5F (center: Cu₂O@Au NPs) is due to the presence of Au satellites that create inter-particle hotspots on the surface of Cu₂O NPs. In contrast, ACA NPs exhibit an enormous enhancement of both external and internal near-fields, and this synergistic effect greatly enhances the overall electromagnetic field for plasmonic photochemistry.

We further employed ultrafast TA spectroscopy to investigate the dynamics of plasmon decay of ACA hybrid structures in comparison to the corresponding Au@Cu₂O NPs (Fig. 6A). Both TA spectra of ACA and Au@Cu₂O NPs exhibit a bleaching signal at the plasmon resonance accompanied by two positive wings on both sides due to induced absorption (Fig. 6B and C). The electron–electron scattering occurred during the initial fast rise due to the high electron density of metals (Fig. 6D). In addition, a longer absorption rise can be observed for ACA NPs compared with Au and Au@Cu₂O NPs (Fig. S22–S24†), which is consistent with previous observations under coupling conditions.⁶⁶ The ultrafast decay arises from electron–phonon scattering on the order of a few picoseconds followed by the lattice cooling at a longer time scale. The fitted data revealed electron–phonon relaxation times (τ_e–ph) of 2.45 and 1.65 ps for Au@Cu₂O and ACA NPs (Table S1†), which is consistent with our inference that the Au satellites on the surface of the ACA NPs act as dipoles, accelerating the electron–phonon relaxation process of the Au core. In contrast, the Au NPs exhibited a long τ_e–ph (4.89 ps), which is in line with previous studies.

Fig. 6 (A) Schematic illustration of the ultrafast transient absorption (TA) spectroscopic measurements on ACA NPs. (B and C) TA spectra of (B) Au@Cu₂O and (C) ACA NPs, respectively, with various delay times under a 450 nm pump pulse. (D) Kinetics of the photoinduced bleach signal at 660 nm for Au@Cu₂O and ACA NPs. The results of the least-squares fitting are shown as solid curves. (E) Plots of photocurrent as a function of time for Au RCO, Au@Cu₂O, and ACA NPs according to light on/off at the same cathode applied potential (visible light illumination: λ > 400 nm). (F) Histogram of photocurrents of ACA NPs under different coupling conditions tuned by changing the amount of Au precursors.

To investigate the photocatalytic activities of the as-constructed ACA hybrid structures, we measured the photocurrents of ACA NPs in a three-electrode system using a Xe lamp as the light source (see details in the ESI†). Notably, charge recombination is significantly suppressed in PEC measurement with an applied bias voltage, allowing us to focus on how plasmonic charge density affects photocatalytic reactions. Fig. 6E shows the comparison of the photocurrent response among the Au, Au@Cu₂O, Cu₂O, Cu₂O@Au, and ACA NPs. No significant photocurrent response is observed in the Au NP, whereas a stronger photocurrent response occurs in Au@Cu₂O NPs. Remarkably, at an applied potential of −0.3 V, the photocurrent density of ACA NPs is 5-fold with respect to that of Au@Cu₂O NPs. Additionally, Cu₂O and Cu₂O@Au NPs show similar photocurrents to the Au@Cu₂O NPs, suggesting that the formation of ACA NPs plays a crucial role in the enhanced photocurrents. As shown in Fig. 6F, we further compared the photocurrent response of ACA NPs with different densities and sizes of Au satellites (samples as shown in Fig. 2A). First, all the ACA NPs exhibit significantly higher photocurrent responses compared to the Au@Cu₂O NPs. Moreover, the photocurrent responses of ACA follow the same trend of increasing and then decreasing (Fig. 2F and S25†), consistent with our analysis of changes in the line width and peak area (Fig. 3C) as well as previous work on photocatalytic hydrogenation.³⁷ While the correlation is very intriguing, the underlying mechanism could be much more complicated than we expected and require further scrutiny. Overall, our experimental observations strongly demonstrate the superior performance of the ACA hybrid structure toward plasmonic photocatalysis.

Finally, the method for constructing the core–shell–satellite structure that we developed here also shows versatility for creating unconventional hybrid plasmon structures for various optical applications (Fig. S26 and S27†). For instance, by using a chiral Au rhombic dodecahedron (RD) as the initial core,⁷⁶ we were able to demonstrate the plasmon coupling between the Au satellite and chiral nanocavity through the construction of chiral Au RD@Cu₂O@Au hybrid structures (Fig. S26†).⁷⁷ By simply controlling the amount of Au precursors that react with the Cu₂O shell through the GRR, the chiral ACA NPs with tunable chiral couplings and chiroptical activities can be obtained (Fig. S26 and S27†). Moreover, significantly enhanced electromagnetic fields were observed for chiral ACA structures because of the intense chiral plasmon coupling in the core–shell–satellite structure (Fig. S27†), and this phenomenon has also been observed in the mentioned achiral ACA NPs (Fig. 5F). FDTD simulations further revealed the chiral near-field distributions when excited using 824 nm left/right-handed circularly polarized (LCP/RCP) light (Fig. S27†). It is envisioned that the chiral ACA structure with significantly enhanced plasmon coupling and chiral near-fields could provide a promising platform for chirality-dependent plasmonic photocatalysis.^78–82

Conclusions

In summary, we have demonstrated a general approach for constructing core–shell–satellite coupled nanostructures with tunable optical properties for enhanced plasmon photocatalysis. By deliberately changing the size of Au satellites and the shell thickness of Cu₂O, we experimentally and theoretically demonstrated the modal splitting in the extinction spectra of the hybrid structures. We further employed single-particle scattering to show the spectral changes of plasmon bands on each particle. Moreover, the coupled nanostructures with tunable plasmon coupling exhibit excellent photocatalytic activities toward plasmon-driven chemical reactions, as revealed by in situ SERS measurements. TA spectroscopy and PEC measurements also show the superior performance of the hybrid structure toward plasmonic photocatalysis. Finally, this strategy can also be employed to construct chiral coupled nanostructures for applications in chirality-dependent plasmonic photocatalysis. It is envisioned that integrating plasmon coupling and excellent catalytic activity within the 3D coupled nanostructure represents a promising approach for applications in photonics and plasmonic photocatalysis.

Data availability

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Author contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22174104, 22472119, and 22405195). This work was also funded by the Basic Research Program of Jiangsu (Grant No. BK20240448 to L. S.). The authors also acknowledge the support of the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University and the Core Facility of Wuhan University. We also thank the Core Research Facility of the College of Chemistry and Molecular Sciences at Wuhan University for SEM and TEM characterization studies.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5sc00610d

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