Plasmon-induced Charge Transfer Research Articles

(Invited) Fundamentals of Plasmon-Induced Charge Transfer in Semiconducting Materials: Showcasing OER Catalysis

Using Localized Surface Plasmon Resonance Effect for Enhancing Electrochemical reactions has been reported earlier both in terms of direct electron injection/transfer (DET) in outer-sphere reductive processes as well as charge injection into semiconductors via plasmon-induced resonant electron transfer processes (PIRE). While the former (DET) is mainly influenced by the lifetimes of the ejected hot electrons and their rapid cooling at the interface. For inner sphere reductive processes via the LSPR phenomenon, direct charge injection into the LUMO states of the adsorbed species is required. In this regard, it is imperative to distinguish between the inter-band charge transfer of the adsorbed species because of exposure to photons.In contrast to these charge injections into semiconductors via plasmon-induced resonant electron transfer processes (PIRE), they must be more understood and complex. In this presentation, we will use the well-known endothermic anodic oxygen evolution reaction (OER) in alkaline pH to showcase the fundamental aspects of charge injection and its effect on the hole-driven OER mechanism. For this well-known OER catalyst, layered double hydroxides of Ni with Fe and Co dopants will be used. The electrochemical enhancement of OER will be explained based on detailed structural motifs of the semiconductors, its band structure, and the resonance effect of Au induced by the LSPR effect. Direct and indirect bandgaps will be discussed in the context of three possible mechanisms: (i) Charge carriers are directly injected into the semiconductor via LSPR. The semiconductor's conduction band is usually (-0.1 to 0.0 eV vs. NHE), and the valence band is between (2.00 and 3.5 eV), corresponding to electron energy between -2.00 to 3.5 eV vs. NHE. For LSPR nanoparticles, SPR energy is between 1.0 and 4.0 eV. Also, the Fermi energy of LSPR is usually 0.0 vs. NHE. Hence, LSPR only enables energetic electrons to be transferred from metals to semiconductors. It is the interface between LSPR and the semiconductor which is important. Charge injection is more prevalent when metal LSPR is of lower energy than the semiconductor—direct electron transfer (DET). (ii) Transfer does not involve direct electron injection but via (a) near-field electromagnetic and (b) resonant photon scattering mechanism. This has been shown to work by adding a thin dielectric material between the LSPR and the semiconductor. This would be most likely in the case of OER, where surface-localized plasmons will be the most important determinant instead of recombination events in the bulk of the semiconductor. This is most important for OER as it depends on the surface hole concentration. It should be noted that larger nanoparticles (>50 nm) have increased resonant photon scattering. Such a mechanism is more prevalent when we overlap metal LSPR and semiconductor bands, leading to plasmon-induced resonant electron transfer (PIRET). (iii) When metal LSPR is in direct contact with the semiconductor, all three phenomena could be active.

The role of the plasmon in interfacial charge transfer.

The lack of a detailed mechanistic understanding for plasmon-mediated charge transfer at metal-semiconductor interfaces severely limits the design of efficient photovoltaic and photocatalytic devices. A major remaining question is the relative contribution from indirect transfer of hot electrons generated by plasmon decay in the metal to the semiconductor compared to direct metal-to-semiconductor interfacial charge transfer. Here, we demonstrate an overall electron transfer efficiency of 44 ± 3% from gold nanorods to titanium oxide shells when excited on resonance. We prove that half of it originates from direct interfacial charge transfer mediated specifically by exciting the plasmon. We are able to distinguish between direct and indirect pathways through multimodal frequency-resolved approach measuring the homogeneous plasmon linewidth by single-particle scattering spectroscopy and time-resolved transient absorption spectroscopy with variable pump wavelengths. Our results signify that the direct plasmon-induced charge transfer pathway is a promising way to improve hot carrier extraction efficiency by circumventing metal intrinsic decay that results mainly in nonspecific heating.

Enhancement of visible light response of TiO2 photocatalyst by 3D-deposited Ag nanowires and its charge separation mechanism

In 3D-deposited AgNW/TiO2, which is prepared by spray-applying titanium dioxide suspension to deposited silver nanowire sheets, the synergistic effects of increased crossing points of AgNWs to enhance localized surface plasmon resonance excitation and longer-lived electrons in the conduction band of TiO2 generated by plasmon-induced charge transfer have successfully resulted in photocatalytic activity in the visible light range. We have developed photocatalytic sheets in which TiO2 particles are uniformly attached to 3D-deposited AgNWs. Regarding the prepared sheet, it was confirmed that TiO2 was indeed well adhered to the AgNWs, and electron transfer was efficient at the interface. This sheet solves the problem that the response wavelength range of the photocatalytic reaction using TiO2 is only in the ultraviolet region and exhibits sufficient photocatalytic effect in the visible light region. Transient absorption spectroscopy measurements in the diffuse reflectance configuration confirmed that the electrons of AgNWs actually move into the conduction band of TiO2 under visible light and that the interaction is independent of the excitation light intensity, thereby extending the lifetime of the electrons.

Plasmon-Induced Ultrafast Hot Hole Transfer in Nonstoichiometric CuxInyS/CdS Heteronanocrystals.

Plasmonic semiconductors are promising candidates for developing energy conversion devices due to their tunable band gap, cost-effectiveness, and nontoxicity. Such materials exhibit remarkable capabilities for harvesting infrared photons, which constitute half of the solar energy spectrum. Herein, we have synthesized near-infrared (NIR) active CuxInyS nanocrystals and CuxInyS/CdS heterostructure nanocrystals (HNCs) to investigate plasmon-induced charge transfer dynamics on an ultrafast time scale. Employing femtosecond transient absorption spectroscopy, we demonstrate that upon exciting the HNCs with sub-band gap NIR photons (λ = 840 nm), the hot holes are generated in the valence band of plasmonic CuxInyS and transferred to the adjacent semiconductor. The decreased signal intensity and accelerated hole phonon relaxation dynamics for HNCs reveal efficient transfer of plasmon-induced hot carriers from CuxInyS to CdS under both 840 and 350 nm laser excitations, providing a pathway for enhanced carrier utilization. These findings shed light on the potential of ternary chalcogenides in plasmonic applications, highlighting efficient hot carrier extraction to adjacent semiconductors.

Enhanced visible-light photocatalytic activity of Fe3O4@MoS2@Au nanocomposites for methylene blue degradation through Plasmon-Induced charge transfer

In this study, we synthesized Fe3O4@MoS2@Au nanoparticles as a photocatalyst for the degradation of methylene blue (MB). The presence of gold nanoparticles induced Localized Surface Plasmon Resonance (LSPR), extending the absorption range into the visible light spectrum. Under green light exposure (540 nm, 8 W), the Fe3O4@MoS2@Au photocatalyst exhibited remarkable performance, achieving a degradation efficiency of 98.95%, outperforming Fe3O4@MoS2, which reached 72.46%. The pseudo-first-order reaction rate constant for Fe3O4@MoS2@Au was 3.8 × 10−3 min−1, surpassing Fe3O4@MoS2 by 2.7 times. Additionally, Fe3O4@MoS2@Au demonstrated superior degradation efficiency under natural light, reaching 78% after 3 h compared to 70.2% for Fe3O4@MoS2. To elucidate the degradation mechanism, density functional theory (DFT) based computational simulations were employed to analyze the electron charge density at each step of the degradation process. The density of state simulation revealed a shift in electron energy levels towards higher energies in Fe3O4@MoS2@Au compared to Fe3O4@MoS2, thereby promoting electron transfer and enhancing the efficiency of photodegradation.

Understanding Metal-Semiconductor Plasmonic Resonance Coupling through Surface-Enhanced Raman Scattering.

Although there has been intense research on plasmon-induced charge transfer within metal/semiconductor heterostructures, previous studies have all focused on the surface plasmonic resonance (SPR) of only noble metals. Herein and for the first time, we observe and take into account the plasmonic coupling between SPR of both noble-metal and semiconductor nanostructures. A W18O49/Ag heterostructure composed of metallic Ag nanoparticles (Ag NPs) and semiconducting W18O49 nanowires (W18O49 NWs) is designed and fabricated, which exhibits a broad and strong SPR absorption in the visible wavelength range. This SPR band is attributed to the SPR coupling between the SPR of both Ag NPs and W18O49 NWs. Surface-enhanced Raman scattering (SERS) is then used to reveal the interactions between the metal SPR, semiconductor SPR, and the heterostructure's charge transfer (CT) process, demonstrating that such coupled SPR enhanced the heterostructure's internal CT and SERS signals. Finally, we proposed a new coupled-plasmon-induced charge transfer mechanism to interpret the improved CT efficiency between the SERS substrate and molecules. Our work provides insight for further studies on plasmonic effects and interfacial charge transfer in metal/semiconductor heterostructures.

Schottky barrier effect on plasmon-induced charge transfer.

Plasmon-induced charge transfer causes electron-hole spatial separation at the metal-semiconductor interface, which plays a key role in photocatalytic and photovoltaic applications. The Schottky barrier formed at the metal-semiconductor interface can modify the hot carrier dynamics. Taking the Ag-TiO2 system as an example, we have investigated plasmon-induced charge transfer at the Schottky junction using quantum mechanical simulations. We find that the Schottky barrier induced by n-type doping enhances the electron transfer and that induced by p-type doping enhances the hole transfer, which is attributed to the shift of the Fermi energy and the band bending of the Schottky junction at the interface. The Schottky barrier also modifies the layer distribution of hot carriers. In particular, for the system with a large band bending, there exists electron-hole spatial separation inside the TiO2 substrate. Our results reveal the mechanism and dynamics of charge transfer at the Schottky junction, and pave the way for manipulating plasmon-assisted photocatalytic and photovoltaic applications.

Evidence of direct charge transfer in plasmon-mediated photocatalytic water splitting: A time-dependent density functional theory study

Photocatalytic water splitting is a promising route for hydrogen production and solar energy storage. Plasmon-mediated water splitting has the potential to harvest photons with longer wavelengths compared with semiconductor-based photocatalysis. However, the mechanism of plasmon-induced charge transfer, the determining step of photochemistry, is not well understood. Here, we studied plasmon-mediated water splitting at atomic length scale and femtosecond timescale. Linear-response time-dependent density functional theory calculations and Ehrenfest dynamics simulations were performed for a realistic H2O@Au6 model excited by the femtosecond laser. Wavelength-dependent charge transfer mechanisms were demonstrated. Especially, for the excitation of 2.25 eV that falls into the visible spectrum, evidence was presented for the dominant direct transfer of d-orbital electrons from the gold cluster to the adsorbed water molecule. In this mechanism, the charge transfer leapfrogs the processes of excitation and thermalization within gold described in the classical theory. The results can assist the design of more energy-efficient solar water splitting.

Plasmon-induced charge transfer desorption/ionization for mass spectrometric analysis of small biomolecules

Laser desorption/ionization (LDI) has been an essential ionization means for biological mass spectrometry (MS). It relies on an energy transfer medium to adsorb the light energy and convert it into other forms of energy to desorb and ionize analyte molecules. Noble metal nanomaterials, which are well-known as plasmonic nanomaterials and exhibit unique plasmonic effects, have been increasingly used in LDI. Various mechanisms were proposed; however, the mechanisms remain to be elucidated and verified, especially for biomolecules. Herein, we proposed a plasmon-induced charge transfer desorption/ionization (PICTDI) mechanism. Upon being shined with a laser beam, plasmonic nanoparticles, such as gold nanoparticles (AuNPs), silver nanoparticles (AgNPs) or their mixture, transfer charges (electrons, protons or other cations) from the nanoparticles to surrounding analytes through surface plasmon, leading to analyte desorption and ionization. Hybrid plasmon-excited LDI surpassed single plasmon-excited mode, which can be proved in both positive and negative ion mode and confirmed by a variety of biomolecules, from nucleosides to saccharides to short peptides. This mechanism provides a sound understanding and a useful guideline for rational design of new plasmonic charge transfer nanomedia.

Basic aspects of gold nanoparticle photo-functionalization using oxides and 2D materials: Control of light confinement, heat-generation, and charge separation in nanospace.

Although the optical properties of localized surface plasmon resonance and the relaxation processes of excited hot electrons in gold nanoparticles (AuNPs) have been well understood, the phenomena that occur when AuNPs relax on solid surfaces of semiconductors or insulators remain largely unknown. Thermal energy diffusion and electron transfer are relatively simple physical processes, but the phenomena they induce are interesting because of a variety of new application developments. In this Perspective, we introduce the fundamental aspects as well as advanced applications of several new physical phenomena induced by AuNPs-based hybrid materials with oxides or 2D materials. Localized heat can induce a great force on the surrounding medium to control mass transport, and plasmon-induced charge transfer reactions are expected to have applications in photocatalysis and solar cells. We also review increasing reports on the development of nano-optical sensors, transistors, and nano-light sources based on precisely controlled device structures utilizing AuNPs.

Effect of light polarization on plasmon-induced charge transfer.

Plasmonic nanoclusters can strongly absorb light energy and generate hot carriers, which have great potentials in photovoltaic and photocatalytic applications. A vital step for those plasmonic applications is the charge transfer at the metal-semiconductor interface. The effect of the light polarization on the charge transfer has not been theoretically investigated so far. Here, we take the Ag-TiO2 system as a model system to study the polarization effect using time-dependent density functional theory simulations. We find that the charge transfer is sensitive to the light polarization, which has its origin in the polarization-dependent hot carrier distributions. For the linearly polarized light, it shows a sine-square dependence on the polar angle, indicating that the charge transfer response to the linear polarization can be decomposed into components perpendicular and parallel to the interface. We also find that there exists directional charge transfer with a circular light polarization. Our results demonstrate that the light polarization can significantly affect the charge transfer behavior and, thus, offer a new degree of freedom to manipulate the plasmonic applications.

Spectroscopic signatures of plasmon-induced charge transfer in gold nanorods.

Plasmon-induced charge transfer has been studied for the development of plasmonic photodiodes and solar cells. There are two mechanisms by which a plasmonic nanoparticle can transfer charge to an adjacent material: indirect transfer following plasmon decay and direct transfer as a way of plasmon decay. Using single-particle dark-field scattering and photoluminescence imaging and spectroscopy of gold nanorods on various substrates, we identify linewidth broadening and photoluminescence quantum yield quenching as key spectroscopic signatures that are quantitatively related to plasmon-induced interfacial charge transfer. We find that dark-field scattering linewidth broadening is due to chemical interface damping through direct charge injection via plasmon decay. The photoluminescence quantum yield quenching reveals additional mechanistic insight into electron-hole recombination as well as plasmon generation and decay within the gold nanorods. Through these two spectroscopic signatures, we identify charge transfer mechanisms at TiO2 and indium doped tin oxide interfaces and uncover material parameters contributing to plasmon-induced charge transfer efficiency, such as barrier height and resonance energy.

Manipulating Hot-Electron Injection in Metal Oxide Heterojunction Array for Ultrasensitive Surface-Enhanced Raman Scattering.

Efficient photoinduced charge transfer (PICT) resonance is crucial to the surface-enhanced Raman scattering (SERS) performance of metal oxide substrates. Herein, we venture into the hot-electron injection strategy to achieve unprecedented enhanced PICT efficiency between substrates and molecules. A heterojunction array composed of plasmonic MoO2 and semiconducting WO3-x is designed to prove the concept. The plasmonic MoO2 generates intense localized surface plasmon resonance under illumination, which can generate near-field Raman enhancement as well as accompanied plasmon-induced hot-electrons. The hot-electron injection in direct interfacial charge transfer and plasmon-induced charge transfer process can effectively promote the PICT efficiency between substrates and molecules, achieving a record Raman enhancement factor among metal oxide substrates (2.12 × 108) and the ultrasensitive detection of target molecule down to 10-11 M. This work demonstrates the possibility of hot-electron manipulation to realize unprecedented Raman enhancement in metal oxides, offering a cutting-edge strategy to design high-performance SERS substrates.

Patterning of transition metal dichalcogenides catalyzed by surface plasmons with atomic precision

Summary Plasmon-induced charge transfer has drawn considerable interests and spurred rapid developments of photovoltaics and photocatalysis. Various plasmonic metal/transition metal dichalcogenide (TMD) heterostructures have been developed to promote charge separation. For plasmon-induced catalysis, the transformation of TMDs is rare, and the erosion of TMDs has not yet been reported. Here, we report the etching of two-dimensional TMDs by planar surface plasmon polaritons (SPPs). We found that SPPs can etch various TMDs into desired layers and lateral size through controlling the light power and incident direction. In aqueous media, the strong plasmonic coupling at Au/MoS2 interface generates holes in the valence band of MoS2 that can weaken its interlayer interaction. As a synergistic effect, the plasmonic hot electrons produce oxidizing H2O2 to dissolve MoS2 from the top layers. Our results provide a new perspective for understanding the plasmonic coupling at metal-TMD interfaces and a reliable route toward fabricating well-defined TMDs.

Plasmon induced charge transfer mechanism in gold-TiO2 nanoparticle systems: The size effect of gold nanoparticle

The application of surface plasmon in the solar-cell design has become a hot topic in the field of photovoltaic research. The enhancement of the photoelectric conversion efficiency is due to charge transfer caused by photoinduced injection of electrons from the metal to the corresponding acceptors. Revealing the basic physical mechanism further is of very important practical significance. We used the femtosecond time-resolved IR ultrafast spectroscopy technology and chose to excite the plasmon band of gold while changing the size of the gold nanoparticle to regulate the complex nanoprocess of the separation and recombination of photogenerated electrons in gold assembled with TiO2 systems. Behavior of hot holes in gold was also considered. We found that larger gold particles resulted in longer charge recombination times. The mechanism is discussed in detail in terms of restricted carrier diffusion in the nanospace.

3D Ag/ZnO microsphere SERS substrate with ultra-sensitive, recyclable and self-cleaning performances: application for rapid in site monitoring catalytic dye degradation and insight into the mechanism

A highly sensitive, recyclable and self-cleaning surface-enhanced Raman scattering (SERS) substrate for rapid in situ monitoring photocatalytic degradation of dyes has been fabricated by growing Ag nanoparticles (NPs) on three-dimensional (3D) nanosheet-built ZnO microspheres (MSs). Used as a SERS substrate for the detection of Rhodamine 6 G (R6 G) molecules, the fabricated 3D Ag/ZnO MSs display high sensitivity and reproducibility as well as outstanding self-cleaning ability. The 3D Ag/ZnO MSs can detect the SERS signals of R6 G molecules even at very low concentration of 10-9 M. In the light of these outstanding performances, the 3D Ag/ZnO MSs were employed as a SERS-active substrate for rapid in situ monitoring photocatalytic degradation of R6 G molecules. The experimental results distinctively demonstrate that the enhanced photocatalytic degradation of R6 G molecules is attributed to the efficient plasmon-induced charge transfer at the interface of the Ag NPs, ZnO MSs and R6 G molecules. The as-prepared 3D Ag/ZnO MSs display potential applications for as a multifunctional platform to rapid in situ monitoring photocatalytic degradation of organic pollutants in polluted water.

Potential energy shift of the Fermi level at plasmonic structures for light-energy conversion determined by graphene-based Raman measurements.

Single layer graphene was used to determine the electrochemical potential of plasmonic nano-structures for photoelectrochemical energy conversions. From electrochemical Raman measurements of the graphene layer under near-infrared light, illumination has revealed the relationship between the photoenergy conversion ability and the Fermi level of the plasmonic structure. The determination is based on in situ monitoring of G and 2D Raman bands of the graphene layer on plasmonic structures. The correlation plots of G and 2D bands show the dependence on the photoconversion ability. The present electrochemical Raman measurements provide detailed understanding of the plasmon-induced charge transfer process for further developments on the ability.

Plasmon-Induced Charge Transfer: Challenges and Outlook

The decay of a surface plasmon, a collective electron oscillation at the surface of a metal, can generate hot charge carriers that may be transferred to an adjacent semiconductor. This plasmon-induced chargetransfer process can be used to enhance photocatalysis, to create photodetectors, or to drive selective photochemistry. However, the charge transfer efficiency in many fabricated devices remains too low for practical applications, typically <1%. In this Perspective, I discuss critical aspects of designing plasmonic systems for improved performance and highlight important findings for maximizing the transfer efficiency. In particular, I draw attention to the article by Ma and Gao in this issue ofACS Nano that describes using real-time time-dependent density functional theory to give a detailed and informative look at the charge transfer dynamics at a TiO2-Ag nanocluster interface.

Gold/Ceria Nanostructures for Plasmon-Enhanced Catalytic Reactions Under Visible Light

Driving catalytic reactions with sunlight is an excellent example of sustainable chemistry. Gold nanocrystals have proven promising in harvesting solar energy for chemical reactions due to their extraordinary and tailorable localized surface plasmon resonance (LSPR) properties. Gold/semiconductor hybrid nanostructures have recently attracted much attention because of LSPR-induced light focusing in the vicinity of Au nanocrystals and/or plasmon induced charge transfer across the interface. CeO2 has shown great potential in catalysis due to its attractive chemical and physical properties, including its unique 4f electron configuration and oxygen ion conductivity. Intimate integration of CeO2 with Au nanocrystals at nanoscale is therefore expected to enable Au/CeO2 to function as an excellent photocatalyst for desired catalytic reactions. However, the rational preparation of plasmonic Au/CeO2 nanostructures has yet remained challenging. In addition, to fully improve the catalytic activity of Au/CeO2 requires a thorough fundamental understanding of the underlying plasmon-enhanced mechanisms. We have recently performed systematic studies on the design of Au/CeO2 with enhanced activities for the oxidation of alcohols and carried out investigations on the plasmon-enhanced mechanisms. First, we developed a versatile approach for coating CeO2 on monometallic Au and bimetallic Au@Pt, Au@Pd nanocrystals to produce multifunctional core@shell nanostructures with plasmon resonance wavelengths tunable from the visible to near-infrared region. The calcined (Au core)@(CeO2 shell) nanostructures display high photocatalytic activities toward the oxidation of benzyl alcohol to benzaldehyde under the illumination of visible light. The enhanced photocatalytic activities are attributed to the synergistic effect between the Au nanocrystal core acting as a plasmonic component for efficient light harvesting and the CeO2 shell providing catalytically active sites for the oxidation reaction. Second, for core@shell structures, the effective accessibility of the active metal core by reactants is of particular importance. Dense shells might hinder hot holes in the Au core from being accessed by reactant molecules, which leads to the recombination of electrons and holes and therefore the reduction of photocatalytic activities. In this regard, mesoporous shell is of great promise since it offers a large number of continuous channels, high specific surface areas, and versatile pore structures. We successfully prepared mesoporous Au/CeO2 microsphere photocatalysts with different Au loading amounts using an aerosol spray method. The mesoporous nature of CeO2 endows the Au/CeO2 sample with drastically boosted activities that are about 23 times larger than those of (Au nanosphere core)@(CeO2 dense shell) nanostructures. [1] B. X. Li, T. Gu, T. Ming, J. X. Wang, P. Wang, and J. F. Wang,* J. C. Yu, ACS Nano, 8 (2014), 8152-8162. [2] H. L. Jia, X. M. Zhu, R. B. Jiang, and J. F. Wang,* ACS Appl. Mater. Interfaces (2017), 9, 2560-2571.

Anomalous photoluminescence enhancement due to hot electron transfer in core–shell Au–CdS nanocrystals

At room temperature, an enhanced photoluminescence (PL) intensity of core–shell Au–CdS nanocrystals (NCs) with respect to that of CdS NCs is observed; that is weakly affected at low temperature. PL intensity as a function of temperature depicts three different regions in CdS and Au–CdS NCs; the difference in the nature of curves implicates hot electron transfer from Au to CdS. Even though, the Fermi level of Au is at lower energy than conduction level of CdS NCs, plasmon induced charge transfer from Au to CdS overcomes the potential barrier that enhances band edge emission intensity at room temperature. Moreover, at lower temperature, PL emission splits in narrow and sharp distinct features from free exciton and bound exciton levels along with the first and second order phonon replica. In case of core–shell Au–CdS NCs, emission from free exciton and bound exciton remains unaffected, while phonon replica are perturbed; measured energy values of narrow lines fit well with the empirical Varshni equation revealing an intrinsic nature of CdS. The present work demonstrates decay of plasmon by generating electron in semiconductor; the concept with great potential for energy harvesting.