Nanoplasmon–Semiconductor Hybrid for Interface Catalysis

Abstract

We firstly, in this review, introduce the optical properties of plasmonic metals, and then focus on introducing the unique optical properties of the noble metal–metal-oxide hybrid system by revealing the physical mechanism of plasmon–exciton interaction, which was confirmed by theoretical calculations and experimental investigations. With this noble metal–metal-oxide hybrid system, plasmonic nanostructure–semiconductor exciton coupling interactions for interface catalysis has been analyzed in detail. This review can provide a deeper understanding of the physical mechanism of exciton–plasmon interactions in surface catalysis reactions.

1. Introduction

Surface plasmons (SPs) are coherent collective electrons oscillating along the interface where the signs of the real part of the dielectric function are different in the two sides [1]. Specifically, localized surface plasmons (LSPs) have been broadly applied in the fields of surface plasmon resonance sensors [2], surface-enhanced Raman scattering (SERS) [3], tip-enhanced Raman scattering (TERS) [4], and plasmonic photodetector [5].

Nowadays, the novel applications of SPs in surface catalysis, such as photocatalysis, have been extensively investigated [6,7,8,9,10,11,12,13,14,15,16,17,18]. To use solar energy in photocatalytic applications more efficiently, understanding the internal mechanism of SPs is of paramount importance. In most cases, with a properly designed nanostructure that is usually efficient in light-trapping [19,20], localized surface plasmon resonance (LSPR) can occur where the confined free electrons oscillate with the same frequency as the incident radiation and lead to a highly and intense localized electromagnetic field. Based on this phenomenon, SERS has been broadly studied, where the Raman signals can be enhanced over a large frequency range. After light absorption and LSPR excitation, the accumulated energy is transferred to electrons in the conduction band. Highly energetic electrons generated from plasmon decay are named as “hot electrons”, and these are a critical part in driving the surface catalysis; thus, not only is the energy to overcome the reaction barrier produced but also electrons for reduction catalysis are obtained. Further, by using propagating surface plasmon polaritons (PSPPs), the damages caused by direct incident lasers can also be avoided. Hence, plasmon-driven surface catalysis has several outstanding merits, e.g., the extremely high surface sensitivity and improved catalytic efficiency [21]. However, according to the catalysis dynamics investigated by ultrafast transient absorption spectroscopy, the lifetime of the hot electrons is relatively short and this is the major challenge for plasmon-induced hot-electron-transfer catalysis. Thermo-plasmonics has important implications for surface catalysis and chemical processes [22].

Metal oxides have emerged as potential candidates for solving these problems. For example, monolayer MoS₂, a two-dimensional material with a direct band gap of 1.8 eV [23,24], has a wide range of electronic, mechanical, thermal, optical, and chemical properties that have attracted a significant amount of attention [25]. Monolayer MoS₂ has a high transparency (>92%) in the visible light region, large surface-to-bulk ratio, quantum confinement effects, and high potential for promoting surface catalysis [23]. Furthermore, an MoS₂ monolayer can efficiently protect plasmonic metals (usually Ag) from rapid oxidation.

However, low yield of hot electrons and the large band gaps limit the applications of metal-oxides in catalysis. However, TiO₂ in particular has been attracting much interest in the photocatalytic field as an outstanding electron-accepting metal-oxide. The conduction band of TiO₂ has a high density of states. Because of its merits, TiO₂ has the ability to permit fast electron injection. Extensive studies have investigated the hybrid systems consisting of Au or Ag NPs with TiO₂ [26,27,28,29,30,31,32]. According to transient absorption spectroscopy [33,34,35], TiO₂ has many outstanding optical properties that make it superior to dye-sensitized semiconductors, such as SnO₂, ZnO [36,37], and In₂O₃. Many studies attempted to improve its photocatalytic efficiency [38,39,40], and the catalysis dynamics of TiO₂ photocatalytic reactivity have been investigated extensively [41,42,43,44,45].

Moreover, its thermal stability, photostability, low cost, and harmlessness make TiO₂ a highly robust competitor. However, TiO₂ has some drawbacks, including the large band gap of 3.3 eV, which limits the photo-absorption in the UV region of the solar spectroscopy [46,47,48,49]. Another crucial obstacle is the charge-charge recombination in metal-oxides, whose rate should be reduced to improve the catalysis efficiency [47,50]. The recombination results in an overall loss of the charge carriers before they reach the surface, and can be addressed by maximizing photon absorption with the help of plasmonic nanostructures [51].

The results of experiments in recent years can be seen from the

Table 1. The table demonstrates results of plasmon–exciton coupling for catalysis.

The Coupling of Plasmon-Exciton for Surface Catalysiss
	Materials	Graphene + Metal	TiO2 + Metal	MoS2 + Metal
Date
2015		Plasmon + graphene for catalysis	plasmon + TiO2
2016		Ultrafast Plasmon-Exciton Interaction of Ag Nanowire-Graphene	Ultrafast Plasmon on Ag/TiO2
2017		Plasmon–Exciton-Co-driven catalysis Reactions	Ag nanoparticles-TiO2 film hybrid for catalysis	MoS2-Ag nanoparticles hybrids for catalysis
2018		Electrically enhanced hot hole driven oxidation catalysis		Femtosecond dynamics of MoS2-Ag nanoparticles hybrid

.

2. Mechanisms

Cavity quantum electrodynamics (QED) is the proper mechanical description to study the quantum interactions between light and matter inside a microcavity. Taking an excited isolated atom for example, there is no effective mechanism, which leads to electron decay due to the two orthogonal eigenstates (ground and excited levels). However, Purcell discovered that the spontaneous emission is not only related to the emitter but also depends on environments, which can be confirmed by the case in which an atom is inside the cavity with perfectly reflecting walls. Based on this theory, we can consider the emitter and its environment as a whole system.

Although the connection between plasmonic nanostructures and a semiconductor forms a metal–semiconductor Schottky junction [20], combining the merits of noble metals and metal-oxides is a promising way to optimize photocatalytic devices by addressing several limitations, and to further improve the surface catalytic efficiency. The lifetime of hot electrons is obviously prolonged from the femtosecond scale to the picosecond scale, which is vital in driving the surface catalysis confirmed by Ding and coworkers with the fabricated graphene-Ag NPs system [52].

On the other hand, the band gaps of metal oxides are decreased and the density of states (DOS) is adjusted for improving the efficiency of hybrid structures. The localized SPR effect induced by plasmonic nanostructures can increase photon absorption due to the confined field enhancement. Understanding the internal mechanism and tenability of these hybrid systems, still experimentally and theoretically challenging subjects, is important for further investigations of the dynamics and various applications.

When excitons of metal-oxides strongly couple to the localized SPs, the novel hybridized energy states form plexcitons (also exciton-polaritons) as a type of polariton. Several studies have reported the optical advanced properties of plexcitonic nanostructures [53,54,55,56,57]. Importantly, the hybrid systems already have many applications such as in chemical sensors, pH meters [58], light harvesting [59], and optically active devices [60,61].

Hence, the mechanism of plasmon–exciton coupling interactions with the semi-classical theory and quantum theory will be introduced for deeper understanding of the unique properties of noble metal–metal-oxide hybrid systems. Further, the merits of the hybrid system will be confirmed experimentally and theoretically, by analyzing the ultrafast transient absorption and plasmon–exciton co-driven catalysis.

2.1. Free-Space Spontaneous Emission

To illustrate the basic physical mechanism underlying spontaneous emission, we first consider the electric dipole interaction between the single two-level system and the single mode of the electromagnetic field. The two-level system, which is formally analogous to a spin-1/2 system with two possible states, can be conveniently described by the Hamiltonian:

$$H_a=\hslash {\omega }_e|e\rangle \langle e|+\hslash {\omega }_g|g\rangle \langle g|$$

(1)

where $\hslash {\omega }_e$ and $\hslash {\omega }_g$ are the energies of the excited and the ground levels, respectively. The wave function of the two-level atom can be described as $|\psi (t)\rangle =C_e(t)|e\rangle +C_g(t)|g\rangle$, where $C_e$ and $C_g$ are the probability amplitudes of finding the atom in states $|e\rangle$ and $|g\rangle$ which represent the upper and lower level states of the atom, respectively.

In the absence of any interaction, an atom initially in its excited state $|e\rangle$ will remain there at all times. Transitions between the eigenstates of the atom result from the coupling of the atom to some other system. In the case of dipole coupling to a single electromagnetic field mode of frequency ${\omega }_c$, the total atom-field system can be described by the Jaynes–Cummings Hamiltonian:

$$H_s=H_a+H_f+H_{a-f}$$

(2)

where $H_f=\hslash {\omega }_c{a}^{†}a$ and the annihilation and creation operators $a$ and $a^{†}$ obey the boson commutation relation $[a,a^{†}]=1$. In the dipole and rotating wave approximation, the interaction Hamiltonian ($H_{a-f}$) between this field mode and the two-level atom can be described as follows:

$$H_{a-f}=-\widehat{d}\cdot \widehat{E}$$

(3)

where $\widehat{d}$ is the dipole moment operator of the transition, and $\widehat{E}$ is the electric field operator related to the polarization of the field mode. Hence, the total atom-field system in Equation (2) can be re-expressed as follows:

$$H=E_0+\hslash {\omega }_0{\sigma }_z+\hslash {\omega }_c{a}^{†}a+\hslash g(R)a^{†}{\sigma }_{-}+h.c$$

(4)

where ${\omega }_0\equiv {\omega }_e-{\omega }_g$ is the atomic transition frequency, and $g$ is half the so-called vacuum Rabi frequency. The vacuum Rabi frequency $2g(\widehat{R})$ normally depends on the location $\widehat{R}$ of the atom. The interaction is physically transparent: the atom can either absorb a photon and undergo a transition towards the excited state, or it can emit a photon when undergoing a transition from the excited to the ground state. Within the framework based on the Jaynes–Cummings model, the excitation number in the atom-field system is conserved. On the resonance (${\omega }_c={\omega }_0$), when the frequency of single electromagnetic field mode equals the frequency of atom transition, the time-dependent states of atom-field system are as follows:

$$|\psi \left(0\right)\rangle =\cos (\mathrm{gt})|e,0\rangle -i\sin (\mathrm{gt})|g,1\rangle$$

(5)

Therefore, the probability $P_g(t)$ for the atom to be in its ground electronic state $|g\rangle$ is as follows:

$$P_g(\mathrm{t})={\left|\langle g,1|\psi (\mathrm{t})\rangle \right|}^2={\sin }^2(\mathrm{gt})$$

(6)

The equation can simply demonstrate the form of spontaneous emission: the quantized field (fluctuation) in the vacuum induces transitions between the two states. Further, the puzzling oscillatory behavior of $P_g(t)$ at the “vacuum Rabi frequency” $2g$ can also be presented by the simple model, which results from the periodic exchange between the atom and cavity.

Based on Fermi’s golden rule, the transition rate of corresponding radiation from an excited state towards the lower energy level can be calculated as follows:

$$\gamma =\frac{2\pi }{{\hslash }^2}{\left|\langle f\left|\stackrel{\wedge }{\mu }\cdot \stackrel{\wedge }{E}\right|i\rangle \right|}^2\rho (\omega )$$

(7)

In the equation, $\gamma$ represents the transition decay rate from the initial (excited) state $|i\rangle$ to the final state $|f\rangle$. $\stackrel{\wedge }{\mu }$ and $\stackrel{\wedge }{E}$ are the electric dipole and vacuum-field operators, respectively. $\rho (\omega )$ represents the final photonic density of states, and the decay rate of the excited atomic state population in the free space can be represented as follows:

$${\gamma }_{free}=\frac{1}{{\tau }_{free}}=\frac{{\mu }_{if}{}^2{\omega }^3}{3\pi {\epsilon }_0\hslash c^3}$$

(8)

based on ${\left|\langle f|\widehat{\mu }·\widehat{E}|i\rangle \right|}^2=\frac{1}{3}{\mu }_{if}{}^2{E}_{vac}{}^2$, where ${\tau }_{free}$ is the radiative lifetime and $E_{vac}=\sqrt{(\hslash \omega )/2{\epsilon }_{0}V}$ [62,63,64,65,66].

2.2. Spontaneous Emission in Cavities

The Hamiltonian of a system that consists of an atom coupling to a single mode field can be described as $H_s=H_a+H_f+H_{a-f}$. The atom-cavity mode system has two dissipative processes: to the free space electromagnetic field background or to the outside world due to the mirror losses and diffraction. Further, the master equation of the atom-cavity mode can be described as follows:

$${\dot{\rho }}_s=-\frac{i}{\hslash }\left[H_{eff}\rho -\rho H_{{}_{eff}}^{†}\right]+\kappa a{\rho }_s{a}^{†}+\gamma \prime {\sigma }_{-}{\rho }_s{\sigma }_{+}$$

(9)

where $H_{eff}=H_s+H_{loss}$ and $H_{loss}=-i\hslash \frac{\gamma \prime }{2}{\sigma }_{+}{\sigma }_{-}-i\hslash \frac{\kappa }{2}{a}^{†}a$. According to the master Equation (8), the evolution of the state can be presented as follows:

$$i\hslash \frac{d|\psi (t)\rangle }{dt}=H_{eff}|\psi (\mathrm{t})\rangle$$

(10)

where the state can be introduced as $|\psi (\mathrm{t})\rangle =C_e(\mathrm{t})e^{\frac{1}{2}\delta t}|e,0\rangle +C_g(\mathrm{t})e^{\frac{1}{2}\delta t}|g,1\rangle$.

The coupling between LSPs and excitons results from the coherent dipole-dipole interaction [67]. Based on the brief introduction of basic concepts, we move forward to distinguishing the different regimes. The interactions could be divided into two different kinds (in the weak or strong coupling regime) by the lifetime of the LSPs and excitons. When the system is in the weak coupling regime, the lifetime of plasmonic resonance is relatively short compared with the spontaneous decay rate of the isolated emitter. In contrast, when the lifetime of LSPR is much longer, then it is in the limit of strong coupling.

In other ways, regimes can also be determined by comparing the three typical constants.

• The dipole coupling constant between the atom and the cavity mode $g$;
• The decay rate of the mode $\kappa$;
• The rate of spontaneous emission into electromagnetic field modes $\gamma \prime$.

When $\kappa$ and $\gamma \prime$ overwhelm the Hamiltonian dipole interaction represented by $g$($g\ll \kappa ,{\gamma }^{\prime }$), the system is in the weak coupling regime. In the limit regime, the role of the vacuum field is considered as a perturbation to the emitter, where the cavity and emitter can be separately treated. In contrast, the system is in the strong coupling regime if $g\gg \kappa ,{\gamma }^{\prime }$. In this limit regime, the emitter and cavity can only be treated as a system, and can only be descripted with quantum electrodynamics treatment.

2.2.1. Weak Coupling

The interaction between matter and light in the weak coupling regime, is not stronger than that outside the system, for example, the fast relaxation of LSPs [67,68]. LSPs can modify the absorption cross-section due to plasmonic nanostructures. The final density of states, for instance, is maximum at the plasmon resonance wavelength when the emitter is near metal NPs. When the cavity has only one mode of frequency ${\omega }_c$, $\rho (\omega )$ can be described as follows:

$$\rho (\omega )=\frac{2}{\pi }\frac{\Delta \omega }{4{(\omega -{\omega }_c)}^2+\Delta {\omega }^2}$$

(11)

which represents the density of states and can be influenced by the quality factor of cavity $Q={\omega }_c/\Delta \omega$. Further, according to Equation (11), it is obvious to find the local density of states (LDOS) maximum at $\omega ={\omega }_c$ [69]. In addition, the decay rate of an emitter placed within a plasmonic cavity can be calculated as follows:

$${\gamma }_{cav}={\beta }^2\frac{2{\mu }_{if}^2}{{\epsilon }_0\hslash }\frac{Q}{V}$$

(12)

Using the Purcell factor $F_P$ [69,70,71], the effect of cavity on the decay rate of the emitter can be described. The spontaneous decay rate is enhanced If $F_P>1$, otherwise, the cavity can inhibit the emission:

$$F_P=\frac{{\gamma }_{cav}}{{\gamma }_{free}}=\frac{{\tau }_{free}}{{\tau }_{cav}}=\frac{3}{4{\pi }^{{}_2}}Q(\frac{{\lambda }^3}{n^{3}V})$$

(13)

Moreover, the overall enhancement in the quantum yield ($\eta ={\gamma }_{rad}/({\gamma }_{rad}+{\gamma }_{nonrad})$) can be further determined by the competition between radiative and nonradiative rates, and the maximum can be achieved by optimizing the radiative rate enhancements and minimizing the nonradioactive losses [67,72].

2.2.2. Strong Coupling

The new quasiparticle (plexciton) in the strong coupling regime is formed with distinct properties possessed by neither original particle. The coherent coupling interaction between the atom and the cavity mode is so strong that when the photon emitted into the cavity is similar to be reabsorbed before escape (the light and matter exchange energy periodically), and results in two new mixed states separated energetically (named as Rabi splitting) [68,73,74]. Detailed information can be seen from Figure 1.

The perturbative analysis of coupling between the cavity mode and atom ceases to be justified. General solution for arbitrary $\mathrm{g}$, $\gamma \text{′}$, and $\kappa$ is of the form

$$C_e(\mathrm{t})=C_{e1}{e}^{{\alpha }_{1}t}+C_{e2}{e}^{\alpha 2t}$$

(14)

where

$${\alpha }_{1,2}=-\frac{1}{2}\left(\frac{\gamma \prime }{2}+\frac{\kappa }{2}+i\delta \right)\pm \frac{1}{2}{\left[{\left(\frac{\gamma \prime }{2}+\frac{\kappa }{2}+i\delta \right)}^2-4g^2\right]}^{1/2}$$

(15)

The constants $C_{e1}$ and $C_{e2}$ can be determined from initial conditions $C_e(0)=1$ and $C_g(0)=0$, respectively. In the strong coupling regime with $g\gg \kappa ,\gamma \prime$, and $\delta$, it can be adjusted to

$${\alpha }_{1,2}=-\frac{1}{2}\left(\frac{\gamma \prime }{2}+\frac{\kappa }{2}+i\delta \right)\pm ig$$

(16)

According to Equation (16), the evolution of the upper state population can be described, as well as the split at the vacuum Rabi frequency.

Based on the Jaynes–Cummings Hamiltonian, the system only has two states: $|e,n\rangle$ and $|g,n+1\rangle$. When the atom-field interaction is considered, the eigenenergies can be described as follows:

$$E_{2n}=\hslash \left(n+\frac{1}{2}\right){\omega }_c-\hslash R_n$$

(17)

$$E_{1n}=\hslash \left(n+\frac{1}{2}\right){\omega }_c+\hslash R_n$$

(18)

Here $R_n$ is the n-photon generalized Rabi frequency:

$$R_n=\frac{1}{2}\sqrt{{\delta }^2+4g^2\left(n+1\right)}$$

(19)

The energies of the state $|e,n\rangle$ and state $|g,n+1\rangle$ cross at $\delta =0$, but the atom-field interaction removes this degeneracy, causing the dressed states $|1,n\rangle$ and $|2,n\rangle$ to repel each other, or anticross. In other word, the anticrossing corresponds to the resonance condition ${\omega }_0={\omega }_c$ (a central peak at ${\omega }_c$ and two sidebands at ${\omega }_c\pm R_n$).

2.3. Femtosecond Absorption

Hot electrons can be generated from plasmon decay in plasmonic nanostructures, which can be treated as efficient light-trapping components and can significantly improve the photocatalytic efficiency of traditional semiconductor devices. The hybrid system of a noble metal and a metal oxide was once considered to have two separate components and electron transfer was considered to be difficult because the individual electrons were not able to get sufficient energy to overcome the Schottky barrier [76]. However, the electron transfer from gold to the TiO₂ electrode had been proved by the generated photocurrent under excitation of the plasmon band [77,78], which is relevant because the wide bandgap of TiO₂ limits the generation of photocurrents. The generated hot electrons will go through three steps: generation, injection, and regeneration. The reverse electron transfer from TiO₂ to gold, as the inverse process, is also proved in core-shell Ag-TiO₂ NPs [79]. Based on the investigations, the energy needed to overcome the reaction barrier in hybrid systems is relatively smaller than the bandgap of TiO₂. Hence, bringing the plasmonic nanostructures in contact with a semiconductor is a promising way to develop photocatalytic devices.

The electromagnetic decay takes places on a femtosecond timescale in plasmonic nanostructures, either radiatively through non-radiatively by transferring the energy to hot electrons or re-emitted photons [80,81,82,83,84].

To further reveal the internal mechanism of the prolonged lifetime of hot electrons in plasmon–exciton coupling interactions, the pump-probe ultrafast transient absorption spectroscopy (UTAS) is usually applied to investigate the timescale of the hot electron transfer process. Among the hot electrons transfer processes, the fast-electron injection into the metal oxide before recombination is crucial for improving the conversion efficiency. Direct evidence of electron transfer from Au NPs to TiO₂ can be obtained. Furube and coworkers revealed the internal mechanism of ultrafast plasmon-induced electron transfer from 10 nm Au NPs to TiO₂ NPs with femtosecond transient absorption spectroscopy [15,85,86], and revealed that the hot electron generation and injection were completed within 50 fs [42,45,87,88,89,90,91,92,93,94,95,96,97]. When the plasmons of gold NPs are excited, the electrons with a non-Fermi distribution undergo relaxation through the re-emission of photons or carrier multiplication within 100 fs due to the electron–electron interaction [52,98,99], electron–phonon interaction at the timescale of 1–10 ps, and phonon–phonon interactions around 100 ps [100,101]. Hence, researchers concluded that the hot electron injection resulted from electron–electron interaction.

The N₃/TiO₂ system, whose carrier injection efficiency is nearly 100%, is used for the comparison of the electron transfer yield [102]. According to the transient absorption intensity ratio between Au/TiO₂ and N₃/TiO₂ (~100% injection efficiency), the electron injection yield in Au/TiO₂ was evaluated to be about 40% under 550 nm excitation.

In addition, the Au/ZrO₂ system is set for monitoring the response of excited Au nanodots as a control group, where no electron is able to transfer from Au nanodots to ZrO₂, since the conduction band edge of ZrO₂ is located 0.9 V above that of TiO₂ [102]. The observed transient absorption of Au/TiO₂ can be attributed to the electron transfer into TiO₂, because there is no transient absorption for Au/ZrO₂ with a probe laser of 3500 nm. Further, the electron transfer was completed within 240 fs, as shown in Figure 2. The timescale of the regeneration process was also investigated by Tian and coworkers [103,104].

Similarly, an Ag NP–graphene hybrid has also been investigated widely as an appropriate candidate. With the unique properties of graphene, the graphene–Ag nanostructure hybrid systems have been widely applied in investigating plasmon–exciton co-driven surface catalysis, whose mechanism was not clearly elucidated. Ding and coworkers fabricated the graphene-Ag nanowire hybrid system (Figure 3a) to reveal the dynamic process of plasmon–exciton coupling interaction with UTAS, as shown in Figure 3b–e.

The fitted curve in Figure 3c indicates that the lifetime of plasmonic hot electrons is about 3.2 ± 0.8 ps, which is obviously prolonged compared with the situation for isolated Ag NWs (150 fs). Furthermore, graphene cannot only prolong lifetime of the hot electrons dramatically, but also results in the significant accumulations of the hot electrons.

Moreover, the mechanism of exciton–plasmon coupling can also be investigated based on hybrid system of MoS₂-Ag NP, according to the transmission spectra (Figure 4) and ultrafast absorption spectroscopy (Figure 4) results with a pump laser of 400 nm. It is revealed that the enhancement factors of excitonic states of MoS₂ are different; the excitonic state (637 nm) is enhanced significantly with plasmon–exciton coupling interaction, instead of the B excitonic state (595 nm). By comparing the lifetimes of the two excitonic states of MoS₂ (Figure 4), we find that the plasmon–exciton coupling interaction has a strong impact on the lifetime of excitonic states. According to the fitted transient absorption spectra of MoS₂-Ag NP (size of 21 nm) in Figure 4e–h, the lifetime of electron-electron interaction in the hybrid system is enhanced by about 8 times than that of MoS₂ alone. As for the electron–photon interaction, the lifetime is also increased significantly for excitonic states A and B. Hence, it is confirmed that the exciton–plasmon interaction improves the efficiency and probability of catalysis due to the long lifetime of carriers.

3. Applications on Photocatalysis

Since 2010, plasmonic hot electrons have been found to be critical in catalysis monitored by the SERS and TERS [12,106,107]. However, based on the investigations, the efficiency of surface plasmon-to-hot electron conversion is considered to be lower than 1% [108]. Hence, several studies attempted to achieve the goal of increasing the efficiency of plasmon-driven surface catalysis.

Before describing the specific application of the hybrid system, we experimentally confirm that the plasmon–exciton coupling degree can be well manipulated, for example, by changing the size of Ag NPs in the Ag NP–TiO₂ film hybrids.

Ding first synthesized a nanosized TiO₂ film on the quartz, with the thickness of ~208 nm and the absorption peak was centered at 524 nm. Above the TiO₂ film, Ag NPs with different sizes were synthesized, as shown in Figure 5. The optimal parameters of components can be studied by UV-Visible absorption spectroscopy, and the ultrafast transfer process of plasmonic hot electron from Ag NPs into TiO₂ film can be investigated by UTAS.

When the Ag NPs are generated under UV irradiation, in addition to the large size, the absorption intensity gradually increases and the strong absorbance peak is red shifted. The strong plasmon–exciton coupling interaction is formed only when the surface plasmon resonance peak of Ag NPs overlaps with the absorption peak of the TiO₂ film (524 nm).

According to the in situ real-time UV-Visible absorbance spectra of hybrid systems (Figure 5), the absorption intensity increases gradually when the UV irradiation time increases from 2 min to 15 min; further, the SPR peak of Ag NPs at 15 min is around 532 nm, while the growth is halted at 30 min and decreases when the time is up to 60 min.

According to the absorption peaks of the Ag NP–TiO₂ film where the Ag NPs were fabricated within 2 min (Figure 6a), there are two ultrafast absorption peaks around 532 nm and 475 nm. Focusing on the case of 532 nm, the electron-electron interaction is about 2 ps while the phonon- electron interaction can approach 71 ps, as shown in Figure 6b. According to the Ag NP size dependent, the ultrafast process of exciton–plasmon interaction of TiO₂ film-Ag NP hybrids in Figure 6c, when the size of Ag NPs increases, the intensity of absorption spectrum become stronger while the lifetime decreases. The shorter lifetime represents the stronger exciton–plasmon interaction in Ag NP–TiO₂ film hybrids. We can conclude that the degree of exciton–plasmon interaction can be well manipulated.

Figure 7a proves again that the superposition between the absorption peak of Ag NPs and TiO₂ film can help in monitoring the degree of the coupling interaction, due to the strongest SERS intensity of catalysis of Ag NP–TiO₂ film at 15 min.

The efficiency of the oxidation catalysis can be monitored by the ratio of the intensity at 1437 and 1071 cm⁻¹, where the former is attributed to the Ag mode of DMAB and the latter is attributed to the A1 mode of PATP. The excitation wavelength-dependent oxidation catalysis illustrates that, based on the match between the excitation laser wavelength and SPR peak of Ag NPs, we can obtain the highest yield of the product excited on 532 nm due to the plasmon–exciton coupling interaction. Further, the oxidation catalysis is efficient and stable on the Ag NP–TiO₂ film hybrids under different environments, including atmospheric, aqueous, and icy environments [110].

Thus, based on UV–Visible absorption spectroscopy, ultrafast transient absorption spectroscopy, and SERS, the plasmon–exciton coupling interaction in the Ag NP–TiO₂ film can be investigated in detail. To obtain the maximum catalytic activity for oxidation catalysis, the degree of coupling interaction can be adjusted by changing the size of Ag NPs and monitoring by the superposition of the SPR peak of Ag NPs and the absorption peak of the TiO₂ film.

Yang synthesized monolayer MoS₂–Ag NP hybrids where the size of Ag NPs was monitored for shifting LSPR peak to match the exciton energy of monolayer MoS₂ and, hence, to further monitor the degree of plasmon–exciton coupling interaction.

As shown in Figure 8, when the size increases, the LSPR peak of Ag NPs is red-shifted, as is the absorption peak of the hybrid system attributed to exciton–plasmon interaction. When they are coupled, LSPR can significantly enhance the excitation rate of the monolayer MoS₂ exciton through EM, and the generated collective states result in stronger optical absorption than the individual components.

On the monolayer MoS₂ substrate, there is no obvious phenomenon regarding occurring catalysis. With a low-intensity laser, the probability of the reduction catalysis on the Ag NPs substrate is much lower than on the MoS₂–Ag NP hybrid system. Further, the advantages of the exciton–plasmon coupling can be confirmed by investigating the Ag NP size-dependent exciton–plasmon co-driven surface catalysis, as shown in Figure 9. With the help of the strongest exciton–plasmon interaction near 532 nm, and highest probability and efficiency of reduction catalysis can be achieved.

In Figure 10, comparing the ratio between the Raman intensities of reactant (1338 cm⁻¹) and product (1432 cm⁻¹), we can conclude that the probability of the reduction catalysis for 4NBT adsorbed on Ag NPs directly is much lower than that on the MoS₂–Ag NP hybrid substrate, which supports the aforementioned conclusion.

Also, the physical mechanism of the exciton–plasmon interaction for catalysis has been interpreted theoretically, which revealed ultrafast charge transfer between plasmons and excitons in the hybrid system [112].

4. Conclusions

We reviewed the principle of plasmonic nanostructure–semiconductor exciton interaction and the applications of plasmonic nanostructure–semiconductor exciton interaction for chemical reactions. The exciton–plasmon interaction has greater potential for surface catalytic reactions than plasmon alone. This review can promote a better understanding of the physical mechanism of exciton–plasmon coupling for surface catalysis.