A review on plasmonic enhancement of activity and selectivity in electrocatalytic CO₂ reduction

Abstract

Utilizing plasmonic effects to assist electrochemical reactions exhibits a huge potential in tuning the reaction activities and product selectivity, which is most appealing especially in chemical reactions with multiple products, such as CO₂ reduction reaction (CO₂RR). However, a comprehensive review of the development and the underlying mechanisms in plasmon-assisted electrocatalytic CO₂RR remains few and far between. Herein, the fundamentals of localized surface plasmonic resonance (LSPR) excitation and the properties of typical plasmonic metals (including Au, Ag, and Cu) are retrospected. Subsequently, the potential mechanisms of plasmonic effects (such as hot carrier effects and photothermal effects) on the reaction performance in the field of plasmon-assisted electrocatalytic CO₂RR are summarized, which provides directions for the future development of this field. It is concluded that plasmonic catalysts exhibit potential capabilities in enhancing CO₂RR while more in situ techniques are essential to further clarify the inner mechanisms.

Graphical abstract

Keywords

localized surface plasmonic resonance (LSPR) effect / plasmonic metals / CO₂ reduction reaction (CO₂RR) / hot carrier effect / photothermal effect

1 Introduction

CO₂, the major source of greenhouse gases, is deteriorating the atmosphere. It is estimated that approximately 36.8 billion tons of CO₂ were emitted globally in 2023, with atmospheric concentrations reaching 423.25 parts per million (10⁻⁶ mg/kg), marking a significant increase of 2.82 × 10⁻⁶ mg/kg from the corresponding period in the previous year. Moreover, the global surface temperatures are projected to increase by more than 2 °C in the coming decades [1–4]. Worse still, global energy demands also continue to increase in the future, which aggravate the emission of CO₂. Under these conditions, the strategy of carbon neutrality has been proposed [5,6], wherein CO₂ is transformed into high value-added fuels and chemical products through chemical processes, offering a sustainable approach to storing renewable energy.

Among various chemical approaches, CO₂ reduction reaction (CO₂RR) has been proven to be a particularly promising process, during which CO₂ is efficiently captured and converted into single(C₁)/multi-carbon (C₂₊) products with a high economic value, such as carbon monoxide (CO), formate (HCOOH), methane (CH₄), ethylene (C₂H₄), ethanol (C₂H₅OH), n-propanol (n-C₃H₇OH), and other C₂₊ products (as shown in Fig.1) [7–9]. To date, substantial progress has been made in the field of electrochemical CO₂RR. As reported, there are various factors that influence the activity and selectivity of CO₂RR, including the surface structure, morphology, composition of the catalyst, the choice of electrolyte ions, pH, and the design of the electrochemical cell. Notably, the products using Ag and Au catalysts for catalyzing CO₂RR are CO, while the C₂₊ products are usually reported as the main products when using Cu catalysts [10–16]. However, significant challenges remain. The stable C=O double bonds are difficult to activate due to the non-polar linear symmetric structure of CO₂. Moreover, the CO₂RR process involves multi-electron transfer with sluggish kinetics. The first step of forming CO₂^·− is considered as the rate-determining step (RDS), which requires a negative overpotential of −1.9 V vs. standard hydrogen electrode (SHE) [2,17]. After being adsorbed on catalyst surfaces, the geometry of CO₂ is changed to a bent state, greatly reducing its activation barrier and improving its electron capture capability. However, the CO₂ activation process still requires a considerable overpotential to raise the Fermi energy (E_F) level of the catalyst above the reduction potential of CO₂, especially for producing multi-carbon products. The major product formation steps and their corresponding thermodynamic potentials are shown as [17]

\begin{array}{c}{\mathrm{C}\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O},\\ E^0=-0.53\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}),\end{array}

\begin{array}{c}{\mathrm{C}\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to \mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H},\\ E^0=-0.61\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}),\end{array}

\begin{array}{c}{\mathrm{C}\mathrm{O}}_2+8{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}\to {\mathrm{C}\mathrm{H}}_4+2{\mathrm{H}}_2\mathrm{O},\\ E^0=-0.24\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}),\end{array}

\begin{array}{c}2{\mathrm{C}\mathrm{O}}_2+12{\mathrm{H}}^{+}+12{\mathrm{e}}^{-}\to {\mathrm{C}}_2{\mathrm{H}}_4+4{\mathrm{H}}_2\mathrm{O},\\ E^0=-0.35\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}),\end{array}

\begin{array}{c}2{\mathrm{C}\mathrm{O}}_2+12{\mathrm{H}}^{+}+12{\mathrm{e}}^{-}\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}+3{\mathrm{H}}_2\mathrm{O},\\ E^0=-0.33\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}),\end{array}

\begin{array}{c}3{\mathrm{C}\mathrm{O}}_2+18{\mathrm{H}}^{+}+18{\mathrm{e}}^{-}\to {\text{n-C}}_3{\mathrm{H}}_7\mathrm{O}\mathrm{H}+5{\mathrm{H}}_2\mathrm{O},\\ E^0=-0.31\mathrm{V}(\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}).\end{array}

Nowadays, plasmon-assisted photocatalytic and electrocatalytic CO₂RRs have emerged as promising strategies in this field [18–23]. Among them, plasmon-assisted photocatalysis directly utilizes sunlight to drive CO₂ conversion into hydrocarbon fuels by employing plasmonic metal nanomaterials. In this process, photoexcitation serves as a means of generating charge carriers that can reduce the kinetic barrier of CO₂ activation. In comparison, plasmon-assisted electrocatalytic CO₂RR provides a powerful strategy to promote the separation of hot carriers via external circuits. Furthermore, benefiting from the light absorption characteristics, plasmon-assisted electrocatalysis has the potential to facilitate the activity and selectivity of CO₂RR compared to traditional electrocatalysis [24–26].

The topics of plasmonic catalysis have been extensively reviewed, which usually focus on synthetic methods, applications in catalysis, and mechanism studies [27,28]. For example, Nam’s group [29] reported on the design and synthetic approaches of multicomponent plasmonic nanoparticles (NPs). Jain’s group [30] detailed potential mechanisms for plasmon-assisted photocatalytic CO₂RR. A recent review by Dong et al. [31] emphasized the advantages of surface plasmons in regulating reaction selectivity in three aspects: adsorption, activation, and desorption. However, few reports have summarized the relationship between the intrinsic catalytic activities and the plasmonic effect in plasmon-assisted electrocatalytic CO₂RR, which is essential to provide guidelines for plasmonic catalyst design.

Herein, the fundamentals of localized surface plasmonic resonance (LSPR) and the optical properties of typical plasmonic metals, including Au, Ag, and Cu were presented in detail. The impacts of the hot carrier effect and photothermal effect were distinguished, emphasizing the mechanisms of the hot carrier effect and the multi-electron transfer process. Furthermore, the current developments in plasmon-assisted electrocatalytic CO₂RR classified by single plasmonic metal, bimetallic heterostructures, and plasmonic metal/semiconductor heterostructures were reviewed. In the end, the challenges and prospects of plasmon-assisted electrocatalytic CO₂RR were discussed.

2 Fundamentals of plasmon-assisted electrocatalytic CO₂RR

2.1 Basic concept of LSPR

LSPR is established when a beam of light irradiates metal nanostructures smaller than the wavelength of the incident light, causing electron resonance and collective oscillation (Fig.2). The LSPR excitation realizes efficient absorption of solar energy, which makes metal nanostructures as efficient solar energy carriers.

The excitation of LSPR exhibits a strong optical extinction on metal nanostructures. For a spherical nanoparticle, the relationship between the extinction cross section and the dielectric function of the metal is deduced using the Mie theory as [32]

(1){\sigma }_{\text{ext}}\sim \frac{{\epsilon }_2}{{[{\epsilon }_1+2{\epsilon }_{\text{m}}]}^2+{\epsilon }_2^2},

where {\sigma }_{\text{ext}} represents the extinction cross section, which reflects the ability of light absorption; {\epsilon }_1 and {\epsilon }_2 are the real and imaginary parts of the dielectric function of the metal, respectively, and {\epsilon }_{\text{m}} is the dielectric constant of the medium. {\epsilon }_1 is related to the polarizability of the metal, which is generally negative for most metals, while {\epsilon }_2 is dependent on the optical absorption of metal nanostructures. Notably, a maximum of {\sigma }_{\text{ext}} will be achieved when {\epsilon }_1 = −2{\epsilon }_{\text{m}}, and {\epsilon }_2 is as small as possible.

The values of {\epsilon }_1 and {\epsilon }_2 in different metals are shown in Fig.3(a) and Fig.3(b). Typical plasmonic metals Au, Ag, and Cu exhibit visible light-induced LSPR absorption due to their negative {\epsilon }_1 and relatively smaller {\epsilon }_2, and hence have gained great attention from researchers [32,34]. Fig.3(c) shows that the typical LSPR absorption peaks of Au, Ag, and Cu NPs are located at 520, 400, and 600 nm, respectively. Besides, the LSPR absorption peaks of these plasmonic metals are dramatically influenced by their shapes, sizes, and the directions of the incident light [35,36]. Fig.3(d) demonstrates that the LSPR absorption peaks display a red shift when the shape of the Ag nanostructures transforms from wires to particles or cubes [33]. The manipulable LSPR absorption indicates that efficient visible light utilization can be achieved through rational design of metal nanostructure morphologies.

The LSPR absorption induces electron transitions, which can be categorized into two mechanisms: interband and intraband transitions. Interband transition involves the excitation of electrons from the occupied d-band to the unoccupied s band, whereas intraband transition occurs when electrons are excited from the occupied s-band to the unoccupied s band, requiring an additional momentum. The electrons induced by interband transition exhibit longer lifetime (1 ps) than those induced by intraband transition (100 fs) [32]. The accessibility of the interband transition depends on the location of d-states relative to the E_F. Atwater’s group [37,38] theoretically predicted the threshold energy of interband transition in typical plasmonic metals (Fig.4(a) and 4(b)). The lowest energy of interband transition is located near X and L points, with a threshold energy of about 3.6 eV for Ag. Therefore, only ultraviolet (UV) illumination with a photon energy higher than 3.6 eV can excite the interband transition of Ag. In contrast, the d-band positions of Au and Cu are slightly lower than the E_F (approximately 2 eV), indicating that the interband transition can be excited by visible light [39,40].

The relationship between the size of Au nanospheres and the electron transitions is illustrated in Fig.4(c). The interband transition is independent of the geometry and size, and predominate when the incident photon energy is above the threshold energy. In contrast, the intraband transition is related to the morphology and particle sizes, and as particle size decreases, the intraband transition gradually dominates.

The photophysical processes of LSPR are generally illustrated in Fig.5. Initially, the LSPR excitation directly causes elevated electromagnetic (EM) fields. The EM fields are spatially heterogeneous on the surface of metal nanostructures, giving rise to localized plasmonic hotspots characterized by high field intensities [41]. Subsequently, the energy stored in EM fields is dissipated by either radiative photon emission or non-radiative plasmon decay. The latter produces energetic carriers within 100 fs, which further redistributes to hot carriers with a quasi-Fermi-Dirac distribution in the timescale of 100 fs to 10 ps. Eventually, the thermal dissipation occurs in the timescale of 100 ps to 10 ns, known as the photothermal effect [30,42].

2.2 Hot carrier or photothermal effects

Both the hot carrier and photothermal effects have dramatic influences on the chemical reactions, which causes the complicated separation of their respective contributions. Researchers have made many attempts to distinguish the hot carrier and photothermal effects [43–47].

The most direct approach to quantifying hot carrier and photothermal effects involve measuring the localized temperature induced by LSPR excitation, followed by a comparison of reaction rates under the measured temperature in the dark with those under illumination. Several studies have measured the surface temperature of plasmonic metals using an infrared camera or theoretical simulations [48–50]. However, because the temperature gradient, convections, and mass transport influence the surface temperature, it is difficult to precisely measure the real surface temperature. Willets et al. [46] used scanning electrochemical microscopy to distinguish the effect of thermal facilitated diffusion with hot carrier effects on a reversible redox reaction ({\text{Fe(CN)}}_6^{3-}/{\text{Fe(CN)}}_6^{4-}). However, they only clarified the thermal diffusion effects and are unsatisfied with disentangling the photothermal and hot carrier effects. Although theoretical simulation offers valuable insights for predicting the surface temperature of plasmonic metal during LSPR excitation, the techniques in experimentally separating the hot carrier and photothermal effects are essential to further validate their accuracy.

The hot carrier effect was effectively distinguished from the photothermal effect through the utilization of a photoelectrochemical (PEC) technique by Tian’s group [45], who divided the photocurrent curve into two regions: the rapid response current (RRC) region in the timescale of less than 0.5 s and the slow response current (SRC) region in the timescale of 10 s (Fig.6(a)). The RRC region was ascribed to hot carriers, whereas the SRC region was attributed to the photothermal effect. Lu et al. further validated the accuracy of distinguishing the nonthermal and photothermal effects by photocurrent curves [51,53]. Only the SRC region occurred when the electrode above the electrolyte was illuminated (Fig.6(b)). Since the hot carriers induced by illumination above the liquid were hardly transferred to the active sites in the electrolyte, the hot carrier effect was excluded, and the photothermal effects probably contributed to the SRC region.

The method of distinguishing hot carrier and photothermal effects by the PEC techniques is accessible and concise, especially in the field of plasmon-assisted electrocatalysis. For instance, it was reported the plasmon-assisted electrocatalytic CO₂RR on Cu nanowire [54]. The photo-response current mainly occurred in less than 0.5 s, indicating that the hot carrier effect was the dominant effect in plasmon-assisted electrocatalytic CO₂RR. Recently, Cai’s group [52] uncovered the hot carrier and photothermal effect in plasmon-assisted electrocatalytic CO₂RR on Ag electrodes. They divided the electrolysis current into the electrochemical current, the current induced by hot carrier, and the photothermal effect (Fig.6(c)). The current induced by the hot carrier effect was more predominant at lower cathodic potentials (Fig.6(d)). The results serve as a reminder of the fact that the contributions of the hot carrier and photothermal effects may be dependent on the applied potential and electrode materials. However, it is still confusing whether other factors, such as electrolyte surroundings and reaction types, affect the contributions of the hot carrier and photothermal effect and how these factors operate.

The hot carrier effect is frequently reported as the dominant factor in plasmon-assisted electrocatalytic CO₂RR. Herein, the interaction mechanisms of the hot carriers and the reactants are described. When reactants are adsorbed on the surface of plasmonic metals, the adsorbates are probably activated by hot carriers through two processes, indirect electron transfer and direct electron transfer [55–57]. In indirect electron transfer (Fig.7(a)), hot carriers are first generated in the metal nanostructures via light excitation and subsequently transferred to the adsorbates. When the reactants are strongly adsorbed on the metal surfaces, the hybrid orbital is formed between the adsorbates and the metal surfaces. In direct electron transfer (Fig.7(b)), hot carriers are directly excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the metal-adsorbate hybrid orbital [30,58].

The hot electron generated by the LSPR excitation transfers from metal nanostructures to the adsorbate and forms a transient negative ion (TNI) (Fig.7(c)). The short-lived TNI instantly loses the excited electron, but the energy stored in the excited electron is left on the adsorbate and thus activates the adsorbate, i.e., the TNI mechanism [49,59]. In the field of the plasmon-assisted electrocatalytic CO₂RR, hot electrons induced by the LSPR excitation can reduce the activation energy (E_a) or induce desorption of the intermediate via the TNI mechanism [60].

2.3 Characteristics of multi-electron transfer process

The complex multi-electron transfer process for CO₂RR exhibits different phenomena from the single-electron transfer reactions that are widely researched as model reactions in plasmon-assisted photocatalysis. For example, the multi-electron transfer reactions provide extra opportunities to regulate the product selectivity by tuning the electron numbers accumulated on the surface of the catalysts. Multi-carbon products are appealing in CO₂RR. Jain’s group [25,61] observed that multi-carbon products were obtained on Au NPs by manipulating the wavelengths and intensities of the incident light. The production of C₂H₆ occurred when the light intensity exceeded 300 mW/cm², while only CH₄ was observed below this threshold. They hypothesized that a higher light intensity (> 300 mW/cm²) resulted in an increased accumulation of hot carriers on the Au NPs, which facilitated the formation of CO₂⁻. The possibility of C−C coupling was increased with the increasing number of CO₂⁻ on Au NPs surfaces at a higher light intensity. Therefore, there was a shift in the CO₂RR products from CH₄ to C₂H₆ (Fig.8(a)).

They further attempted to establish a general framework for plasmonic catalysis of such multi-electron chemistry [62]. The reduction of Fe³⁺ to Fe²⁺ on Au NPs was chosen as the model reaction. In the absence of hole scavengers (Fig.8(b), left), the holes left on the Au NPs were frustrated to consume due to the sluggish water oxidation reaction. Worse still, more holes were accumulated on Au NPs with an increasing light intensity, which aggravated the recombination of the plasmon-excited hot carriers. Therefore, the reaction rates reached saturation when the light intensity was higher than 500 mW/cm². In the presence of ethanol (Fig.8(b), middle, and right), when the light intensity exceeded 500 mW/cm², the reaction rates sharply increased, which was different from that observed in the absence of hole scavengers. They demonstrated that the accumulated holes caused the oxidation of ethanol to acetaldehyde, accompanied by the reduction of two Fe³⁺ (a two-electron transfer process). By developing a kinetic model, they derived a fitting curve that aligned precisely with the experimental data (Fig.8(c)). This work demonstrated the feasibility of plasmon catalysis for the regulation of multi-electron transfer reactions, such as CO₂RR.

3 Plasmonic catalysts for CO₂RR

3.1 Single plasmonic metal

The binding energy of CO on the metal surface plays a crucial role in determining the activity and selectivity of CO₂RR process, as CO serves as an essential intermediate for further reducing to hydrocarbons or C₂₊ products [63–65]. Specifically, the stronger binding energy between the metal and *CO renders the metal surface toxic, leading primarily to hydrogen evolution reactions (HER), as seen on Pt and Ni. Weaker binding results in the release of CO before further reduction, ultimately yielding CO as the final product, as observed on Au and Ag. Cu has a moderate binding energy for CO in comparison, enabling the production of C₂₊ products (Fig.9) [64,66,67]. As a result, Cu is widely regarded as the optimal metal for CO₂RR. Factors such as incident photon energy [61], catalyst size, shape, and surface microenvironment also largely determine the mechanism and reaction path of CO₂RR [68–70]. Efforts have been made in recent years to comprehend the intricate CO₂RR process using single plasmonic metal nanomaterials, to adjust various factors that change the reaction pathway and achieve desired product selectivity. In this section, the research progress in single plasmonic metal primarily composed of Au, Ag, and Cu are focused on.

3.1.1 Plasmonic Au metal

Au NPs exhibit strong LSPR effects in the visible-light range, which has been demonstrated to generate sp-band electrons with lifetimes ranging from 100 fs to 10 ps via intraband (sp−sp) or interband (d−sp) excitations [38,71,72]. Au plasmon-assisted electrocatalytic CO₂RR takes full advantages of both light absorber and electrocatalyst, resulting in an enhanced activity and a controllable selectivity. Recently, Wei’s group [73] conducted an investigation on the current density and Faradaic efficiency (FE) as a function of the size of Au NPs during the CO₂RR process. The experiment was conducted in a three-electrode reactor with the Au NPs modified-glassy carbon (GC) electrode as the photocathode. Notably, Au NPs with a size of 20.2 nm exhibited a high absorption efficiency and low recombination rates of hot carriers, which demonstrated the optimal performance for converting CO₂ into methanol under an illumination of 520 nm. Compared to NPs, nanorods (NRs) exhibit a unique resonance absorption in both transverse and longitudinal resonances, which enhances their efficiency in utilizing solar energy [35]. Wu’s group [74] reported that Au NRs with a length of 60 nm and a diameter of 10 nm showed two absorption peaks at 520 nm (transverse resonance) and 800 nm (longitudinal resonance). Under an AM 1.5G (100 mW/cm²) illumination, the selectivity of CO was improved by approximately 20%, which was attributed to the reduction of the activation potential barrier of CO₂, as indicated by the Tafel slope analysis results (the Tafel slope decreased by 103.34 mV/dec).

3.1.2 Plasmonic Ag metal

Ag is another extensively reported plasmonic material used in plasmon-assisted electrocatalytic reactions. On the one hand, theoretical calculations suggest that only intraband transition (sp−sp) occurs in Ag NPs under illumination [38]. Intraband transition generates highly energetic electrons, which makes Ag NPs particularly favorable for reduction reactions of CO₂ [76]. On the other hand, it has been reported that the LUMO energy of CO₂ aligns well with the excitation energies of sp electrons in Ag₁₈ clusters (as a model of plasmonic Ag electrode), which thereby facilitates efficient transfer of the excited hot electrons to the LUMO orbital of CO₂ due to strong orbital coupling (Fig.10). Consequently, this process effectively reduces the energy barriers of C−O bond cleavage and promotes the formation of CO [75].

Using electron-beam evaporation, McCloskey’s group [49] fabricated an Ag thin film photocathode on a glass substrate, with Ti as an adhesion layer. They then performed an electrochemical treatment in a CO₂-saturated KHCO₃ electrolyte to finetune the LSPR absorption of Ag (Fig.11(a)). The results showed that the selectivity of CO was enhanced while the HER was suppressed from −0.6 to −0.9 V (vs. RHE). The FE of HCOOH and methanol also showed a slight increase under 365 nm LED illumination. Subsequently, in order to reveal the nature of this plasmonic enhancement, they conducted an in situ attenuated total reflectance-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) study of the Ag photocathode on a Ge crystal in CO₂-saturated 0.1 mol/L KHCO₃ solution [77]. The ATR-SEIRAS spectra showed that the CO peak (1983 cm⁻¹) emerged at a potential of −0.25 V (vs. RHE), both under illumination and in the dark conditions (Fig.11(b)). However, CO gas was detected at an onset potential of −0.3 V (vs. RHE) under illumination, while it was only detected at −0.5 V vs. RHE in the dark (Fig.11(c)). Based on this, they proposed the desorption induced by the electronic transitions (DIET) mechanism. The hot electrons excited by the plasmonic Ag are temporarily transferred to the unoccupied molecular orbital of CO. Subsequently, within a few femtoseconds, the electron decays back to the Fermi level of the metal and leaves energy in CO. This energy assists in overcoming the activation barrier of the CO absorption, resulting in the desorption of CO.

3.1.3 Plasmonic Cu metal

The earth-abundant, non-noble metal Cu exhibits an intense LSPR effect from UV-visible to the near-infrared region [78–80]. In addition, Cu has a significant catalytic performance for CO₂RR due to the moderate CO binding energy, and numerous Cu-based catalysts have been reported for electrocatalytic CO₂RR [81–83]. However, there are still few reports on plasmon-assisted electrocatalytic CO₂RR using Cu metal catalysts. Recently, Cu nanowire arrays (NAs) were reported as a plasmonic catalyst for CO₂RR under visible light illumination (Fig.12(a)), and the activity and selectivity of CO₂RR was improved due to the hot carrier effect. More importantly, the activation energy (E_a) experiments indicated that the transfer of hot electrons reduced the E_a of CO generation from 26.61 to 18.95 kJ mol⁻¹, without altering the E_a of HER (Fig.12(b) and Fig.12(c)), which exhibited the advantages of selectively tuning chemical reactions in plasmon-assisted electrocatalytic CO₂RR [54].

3.2 Bimetallic heterostructures

Constructing bimetallic heterostructure catalysts appear to be an effective approach that integrates light-harvesting plasmonic metals (including Cu, Ag, and Au) with catalytically active metals (such as Pd, Pt, and Rh) for tandem catalysis [2,8,84,85]. More importantly, the introduction of a second metal can facilitate the damping pathway of the plasmonic metal, where the decay of LSPR is expedited through the direct transfer of hot carriers to the second metal.

Gold−rhodium core/shell nanoflowers (Au@Rh NFs) with an average diameter of 23±3 nm were synthesized through a seed-mediated growth method, where Au NPs were utilized as the seed for Rh deposition (Fig.13(a)) [50]. In this bimetallic catalyst, Au plays the role of plasmonic metal to absorb visible light and generate hot electron-hole pairs. Rh acts as a catalytic active site to promote the direct transfer of electrons to the adsorbates. Au₈₂Rh₁₈ NFs exhibited a significantly enhanced CO production activity under illumination. Electrochemical impedance spectroscopy (EIS) analysis provided a potential mechanism of rates enhancement through LSPR excitation. The observed enhancement in CO₂RR was attributed to a lower energetic barrier of the reaction, as evidenced by a significant decrease in charge transfer resistance (R_ct) upon light illumination (Fig.13(b)). Additionally, the higher effective capacitance (C_eff) under illumination implied that plasmonic effects promoted rapid desorption of the product CO (Fig.13(c)) and thereby released more active sites.

It must be pointed out that the great challenge in CO₂RR lies in the low product selectivity of C₂₊ products. Cu displays an outstanding performance in catalyzing the formation of C₂₊ products. The McCloskey group designed a Cu−Ag photocathode, wherein Cu nanocorals (NCs) were electrochemically grown on the surface of Ag foil and subsequently coated with a 10 nm layer of Ag (Fig.13(d)) [86]. At low overpotentials (−0.6 to −0.8 V vs. RHE), CO production was enhanced, and H₂ was suppressed under illumination, which was attributed to the DIET mechanism. Further, the production of C₂H₄ was enhanced at high overpotentials (> −0.8 V vs. RHE), and the FE of C₂H₄ exhibited a slight increase under illumination (Fig.13(e)).

3.3 Plasmonic metal/semiconductor heterostructures

Under the resonance excitation of the metal nanostructure, the plasmon relaxes within tens of femtoseconds, which leads to most hot carriers recombining before migrating to surface active sites for redox reactions [42,87–90]. It is reported that plasmonic metal/semiconductor heterostructures can greatly accelerate catalytic reactions, which are ascribed to the fact that Schottky barriers formed at the interface of the heterostructures induce an internal electric field. The Schottky barriers effectively suppress hot carrier recombination and prolong its lifetime [91,92]. As the substrate, the semiconductor serves to immobilize and stabilize the plasmonic metal NPs, preventing aggregation and transformation of metal NPs during photoelectrocatalytic reactions and enhancing the overall stability [93,94]. Numerous studies demonstrated that plasmonic metal/semiconductor heterojunction catalysts exhibited superior CO₂RR activities. In this section, the application of metal/semiconductor heterostructures classified by wide band gap semiconductors (i.e., p-GaN, p-NiO) and narrow band gap semiconductors (p-Cu₂O) in plasmon-assisted electrocatalytic CO₂RR was summarized. It is worth noting that wide band gap semiconductors are only used to collect hot holes from the plasmonic metal because they cannot be excited by visible light illumination. In comparison, narrow band gap semiconductors can either produce carriers or collect hot holes under visible light illumination.

3.3.1 Plasmonic metal/wide band gap semiconductor heterostructures

Extracting hot holes from metal nanostructures is challenging because the mean-free path of hot holes (about 5−10 nm) is shorter than those of hot electrons (about 20 nm) [37,38,97]. Employing p-type semiconductors to collect hot holes provides an effective method for promoting the direct extraction of hot electrons by adsorbed molecules on the metal. DuChene et al. [95] prepared an Au/p-GaN photocathode, where p-GaN served as a wide band gap semiconductor support (an E_g of approximately 3.4 eV). The interaction between Au and p-GaN formed a downward band bending at the interface, thereby promoting the collection of hot holes (Fig.14(a)). PEC measurements showed that the Au/p-GaN photocathode exhibited an enhanced photocurrent (Fig.14(b)) and a positive shift of the open-circuit voltage (V_oc) (Fig.14(c)) under visible light illumination, which provided evidence for the hypothesis of p-GaN induced hole collection. They further constructed a multi-component photocatalyst Au-Cu/Al₂O₃/p-GaN by modifying the Au/p-GaN with an Al₂O₃ passivation layer and incorporating Cu active sites [69,98]. Under visible light illumination, the CO production rate was significantly enhanced (4.13 times higher than that of Au/p-GaN). Besides, Nam’s group [96] modified the Au/p-GaN photocathode with an organic molecule cis-dichloro-(4,4′-diphosphonato-Rubpy) (p-cymene) (RuCY) as a functional component for selectively reducing CO₂ to HCOOH with an FE of 96.8% (Fig.14(d) and Fig.14(e)). High binding energy is formed between Au and N-heterocyclic carbene (NHC), in which the filled sp² orbital of C_NHC is a strong σ-donor and the empty p orbital of C_NHC is an acceptor for π back-donation [99,100]. Therefore, the RuCY monolayer immobilized by NHC ligands played a crucial role in directly transferring electrons to CO₂.

In addition to the p-GaN semiconductor, p-NiO is also widely employed in various PEC devices due to their excellent chemical stability and the intrinsic nature of a large band gap (approximately 3.7 eV). Moreover, the synthesized methods of p-NiO are various, low-cost, and well applicable compared to other p-type semiconductors. For instance, Atwater’s group [101] deposited metallic Ni on FTO glass substrates by electron-beam vapor deposition. The samples were subsequently heated at 300 °C in ambient air to form the p-NiO film. The Cu/p-NiO photocathodes were obtained by directly depositing Cu NPs with a size of 8 nm on the p-NiO film with a thickness of 50 nm. The energy band diagram of Cu/p-NiO photocathodes upon plasmonic excitation was shown in Fig.15(a). The hot electron−hole transfer mechanism was consistent with that of the Au/p-GaN as discussed above. As a result, the FE of both CO and HCOOH increased by threefold compared to that observed in the dark, while the FE of H₂ decreased from 94% in the dark to approximately 58% under illumination at −0.7 V (vs. RHE) (Fig.15(b)). It is proposed that the plasmon-generated hot electrons on the Cu NPs preferentially activate CO₂, facilitate the rate-limiting formation of CO₂⁻, and thereby increase the FE of CO and HCOOH [102,103].

3.3.2 Plasmonic metal/narrow band gap semiconductor heterostructures

p-Cu₂O, an important p-type narrow band gap (approximately 2.2 eV) semiconductor, has been widely utilized in CO₂RR. It serves not only as an efficient hole collector but also as a visible light absorber for generating charge carriers. However, practical applications of Cu₂O are limited due to its instability in aqueous electrolytes and rapid carrier recombination. Combining Cu₂O with plasmonic metals can facilitate the separation of photogenerated carriers and improve the stability of Cu₂O. For instance, Li’s group [104] employed Ag-decorated Cu₂O nanowires for CO₂RR. The results showed that the production rate of CH₃COOH improved 5-fold at −0.7 V (vs. RHE) (from 44.4 µmol/(cm² h) in the dark to 212.7 µmol/(cm²·h) under illumination) (Fig.16(a) and Fig.16(b)). Several PEC measurements, including EIS and intensity-modulated photocurrent spectroscopy (IMPS), provided information on interface charge transfer in Cu₂O and Cu₂O/Ag. In detail, the equivalent circuit fitting resistances showed that the Cu₂O/Ag had a smaller value of the R_ct (5.11 Ω) than Cu₂O (15.16 Ω), reflecting the better charge transfer capability of the Cu₂O/Ag photocathode (Fig.16(c)). Additionally, the photogenerated charge transport time (τ_d) of the photocathodes was shortened from 0.79 ms (Cu₂O) to 0.26 ms (Cu₂O/Ag) (Fig.16(d)), confirming that LSPR excitation of Ag accelerates the charge transfer kinetics of Cu₂O/Ag and effectively suppresses interfacial recombination. Furthermore, the Cu₂O/Ag photocathode showed an excellent stability with a current density of 3 mA/cm² over 12 h.

In addition to PEC characterizations, in situ Raman spectroscopy also provided valuable insights into the chemical species formed during CO₂RR [22,106,107]. More recently, Landaeta et al. [105] utilized Raman techniques to observe the intermediates during CO₂RR on dendritic Cu₂O/Ag photocathodes (Fig.17(a)). The peak located at 2070 cm⁻¹ was assigned to CO, an important intermediate for the formation of C₂₊ products. The C−CO (798 cm⁻¹) and C−C (997, 1112 cm⁻¹) intermediates for the formation of CH₃COOH were also detected. To further explore the LSPR effect, the vibrational Stark shift of 4-mercaptobenzonitrile (MBN, C−N: approximately 2230 cm⁻¹) was probed under illumination and in the dark (Fig.17(b)). As the potential became more negative, the C−N stretch frequency was observed to decrease to lower Raman shifts with Stark tuning coefficients of 8.14 cm⁻¹ V⁻¹ in the dark and 2.89 cm⁻¹ V⁻¹ under light. The decreased Stark tuning coefficients represented the increase of negative charges on the electrode surface, which demonstrated the transfer of electrons from the plasmonic metal to the adsorbates.

4 Conclusions and outlook

Over the past century, the massive use of fossil fuels and excessive deforestation have led to excessive emissions of greenhouse gases (e.g., CO₂), progressively worsening the global climate crisis. To alleviate this issue, plasmonic metal nanomaterials have received great attention in the field of photocatalytic reduction of CO₂, and a series of promising results have been achieved. First, the thermodynamics and kinetics mechanisms of plasmon-assisted electrocatalytic CO₂RR were gradually revealed, including the process of hot carrier transfer, the mechanism of CO₂ adsorption and desorption, and the mechanism of multi-electron transfer in forming different reduction products (CO, HCOOH, CH₄, C₂H₆O, C₂H₄, C₂H₂, etc.) from CO₂. Next, a series of plasmonic catalysts have been developed to significantly enhance the selectivity and production rates in the CO₂RR process. In summary, plasmon-assisted electrocatalytic CO₂RR shows a great potential and offers a promising means to couple visible light with plasmonic metals.

As is summarized above, some progress has been made in plasmon-assisted electrocatalytic CO₂RR, but there is still a long way to go before the technology is industrialized. In the authors’ opinion, the following issues need to be addressed urgently:

First, the electron transfer and reaction mechanism of plasmon-assisted electrocatalytic CO₂RR need further investigation using in situ characterization techniques. During the CO₂RR process, the catalyst undergoes a series of changes, including catalyst surface reconstruction, morphological transformation, and active site evolution/annihilation, but this information cannot be monitored by the ex-situ methods. In situ characterization techniques can accurately capture changes in catalyst composition, adsorption intermediates, and reactant sources during the reaction. Although some progress has been made in situ X-ray absorption spectroscopy, infrared spectroscopy, and Raman spectroscopy, more work needs to be done to disclose the reaction mechanism by fully utilizing in situ characterization techniques.

Second, future catalysts should effectively absorb the entire solar spectrum and be low-cost. To date, most of the catalysts can only absorb some narrow bands of light, which greatly limits the solar energy conversion efficiency. Catalysts with full solar spectrum absorption will greatly improve the utilization efficiency of solar energy and enhance the performance of the catalyst during the CO₂RR process. Moreover, apart from noble metals Au and Ag, some low-cost metals such as Fe, Cu, Al, and their composite materials should be investigated to fabricate plasmonic catalysts for CO₂RR to reduce costs. Meanwhile, the structural design of Fe, Cu, and Al needs to be further optimized to improve their plasmonic absorption properties.

Third, C₂₊ products exhibit a higher economic value compared with C₁ products. Therefore, new catalysts for producing C₂₊ products should be developed. Morphology engineering is an effective strategy to manipulate the product selectivity. For example, Cu nanocavity is conducive to the formation of acetone, and graded Cu dendrites with superhydrophobic surfaces yield more C₂H₄. However, the relationship between the catalyst structure and the formation of C₂₊ products has not yet been established in plasmonic catalysis, which should be the focus of future research.

Finally, developing catalysts capable of producing C₂₊ products in neutral media, instead of alkaline electrolytes, is a crucial to advance the commercial application of CO₂RR. Alkaline electrolyte is widely used in CO₂RR due to its ability to inhibit HER. However, OH⁻ ions are prone to reacting with CO₂ to form carbonates/bicarbonates, which can block gas diffusion channels and hinder the activities of CO₂RR. Neutral electrolytes tend to be more chemically stable and friendly to environment compared to alkaline electrolytes. This stability is crucial for long-term operation and durability of the electrochemical system.

In conclusion, the field of plasmon-assisted electrocatalytic CO₂RR has made significant progress in recent years and still holds immense potential. The integration of plasmonic metal with electrocatalysis offers a promising avenue for enhancing the activity and selectivity of CO₂RR, thereby contributing to the development of sustainable energy. In prospect, further research in this area is expected to lead to the discovery of novel plasmonic materials and an improved understanding of the underlying mechanisms by combining in situ characterization techniques. These advancements are expected to pave the way for the practical application of plasmon-assisted electrocatalysis in industrial-scale CO₂RR, thus contributing to global efforts in mitigating climate change and promoting a more sustainable future.

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