Hot Electron Transport on Three-Dimensional Pt/Mesoporous TiO2 Schottky Nanodiodes

Effects of Surface Plasmon and Catalytic Reactions on Capacitance in Metal-Semiconductor Schottky Diodes

Capacitors, renowned for their high power and energy density attributed to their rapid discharge capabilities, hold significant promise as energy storage devices. While capacitors with various junctions have been the subject of extensive research, there remains a significant knowledge gap concerning the fundamental photoelectron dynamics within metal-semiconductor (MS) junction capacitance, which is crucial for the development of photodevices. Here, we demonstrate the effects of localized surface plasmon resonance (LSPR) on the capacitance behavior of Au/TiO2 Schottky diodes, one of the MS junction diode systems. We have confirmed that the plasmonic Au/TiO2 shows substantial photoresponsiveness, with an increase in net total capacitance under stronger plasmonic effects, as confirmed by wavelength- and power-dependent studies. To enhance our understanding of the electron transfer process occurring at the MS junction, we expand our investigation of capacitance within a catalytic reaction environment, specifically focusing on Pt/TiO2 Schottky diodes. We show the catalytic environment changes capacitance values of the Pt/TiO2, and more active catalytic reactions decrease net total capacitance due to catalytic electrons interfering with diffusion electrons. The LSPR effect and catalytic reaction significantly impact the net total capacitance with a predominant contribution associated with hot electrons, catalytic electrons, and diffusion effects. These findings provide valuable insights for designing and optimizing efficient optical and catalytic devices across diverse applications.

Unraveling oxygen vacancy-driven catalytic selectivity and hot electron generation on heterointerfaces using nanostructured platform

Modulating the physicochemical properties of oxides is crucial to achieve efficient and desirable reactions in heterogeneous catalysis. However, their catalytic role is not clearly identified because unevenly distributed interfaces and close conjugation with metal catalysts may hinder distinguishing their contribution in complex random structures. Here, we demonstrate a model platform composed of well-aligned CeOx nanowire arrays on Pt catalysts to observe their catalytic role systematically. Independently modulating the crystallinity and oxygen vacancy concentration of oxide nanowires, while preserving heterogeneous interfaces, enables quantitative analysis of their individual effects on partial oxidation selectivity, resulting in hot electron generation during methanol oxidation reactions. CeOx treated with vacuum annealing on Pt exhibits 1.47- and 2.12-times higher selectivity to methyl formate and chemicurrent yield than CeOx without annealing on Pt. Density-functional theory calculations reveal that the promoted charge transfer from the electron-accumulated interface driven by oxygen vacancy acts as a key parameter in enhancing selectivity.

Recent Progress on the Catalytic Application of Bimetallic PdCu Nanoparticles

Bimetallic PdCu nanoparticles with different Pd:Cu ratios and morphologies can be synthesized and immobilized on a variety of support materials. Accordingly, PdCu nanoparticles can be efficiently applied as heterogeneous catalysts in a large number of organic transformations including C-C coupling and cross-coupling reactions. As related to their favorable electronic and structural interactions, the catalytic performances of PdCu bimetallic nanoparticles may be superior to monometallic species. The heterogeneous catalysts can be recovered and reused, and the presence of copper tends to reduce the cost of the expensive Pd catalyst, which is beneficial for industrial applications.

Enhanced Hot Electron Flow and Catalytic Synergy by Engineering Core-Shell Structures on Au-Pd Nanocatalysts

The synergistic catalytic performances of bimetallic catalysts are often attributed to the reaction mechanism associated with the alloying process of the catalytic metals. Chemically induced hot electron flux is strongly correlated with catalytic activity, and the interference between two metals at the atomic level can have a huge impact on the hot electron generation on the bimetallic catalysts. In this study, we investigate the correlation between catalytic synergy and hot electron chemistry driven by the electron coupling effect using a model system of Au-Pd bimetallic nanoparticles. We show that the bimetallic nanocatalysts exhibit enhanced catalytic activity under the hydrogen oxidation reaction compared with that of monometallic Pd nanocatalysts. Analysis of the hot electron flux generated in each system revealed the formation of Au/PdOx interfaces, resulting in high reactivity on the bimetallic catalyst. In further experiments with engineering the Au@Pd core-shell structures, we reveal that the hot electron flux, when the topmost surface Pd atoms were less affected by inner Au, due to the concrete shell, was smaller than the alloyed one. The alloyed bimetallic catalyst forming the metal-oxide interfaces has a more direct effect on the hot electron chemistry, as well as on the catalytic reactivity. The great significance of this study is in the confirmation that the change in the hot electron formation rate with the metal-oxide interfaces can be observed by shell engineering of nanocatalysts.

How Hot Electron Generation at the Solid-Liquid Interface Is Different from the Solid-Gas Interface

Excitation of hot electrons by energy dissipation under exothermic chemical reactions on metal catalyst surfaces occurs at both solid-gas and solid-liquid interfaces. Despite extensive studies, a comparative operando study directly comparing electronic excitation by electronically nonadiabatic interactions at solid-gas and solid-liquid interfaces has not been reported. Herein, on the basis of our in situ techniques for monitoring energy dissipation as a chemicurrent using a Pt/n-Si nanodiode sensor, we observed the generation of hot electrons in both gas and liquid phases during H₂O₂ decomposition. As a result of comparing the current signal and oxygen evolution rate in the two phases, surprisingly, the efficiency of reaction-induced excitation of hot electrons increased by ∼100 times at the solid-liquid interface compared to the solid-gas interface. The boost of hot electron excitation in the liquid phase is due to the presence of an ionic layer lowering the potential barrier at the junction for transferring hot electrons.

Hot Electron Phenomena at Solid-Liquid Interfaces

Understanding the role of energy dissipation and charge transfer under exothermic chemical reactions on metal catalyst surfaces is important for elucidating the fundamental phenomena at solid-gas and solid-liquid interfaces. Recently, many surface chemistry studies have been conducted on the solid-liquid interface, so correlating electronic excitation in the liquid-phase with the reaction mechanism plays a crucial role in heterogeneous catalysis. In this review, we introduce the detection principle of electron transfer at the solid-liquid interface by developing cutting-edge technologies with metal-semiconductor Schottky nanodiodes. The kinetics of hot electron excitation are well correlated with the reaction rates, demonstrating that the operando method for understanding nonadiabatic interactions is helpful in studying the reaction mechanism of surface molecular processes. In addition to the detection of hot electrons excited by a catalytic reaction, we highlight recent results on how the transfer of the hot electrons influences surface chemical and photoelectrochemical reactions.

Manipulation of hot electron flow on plasmonic nanodiodes fabricated by nanosphere lithography

Energy conversion to generate hot electrons through the excitation of localized surface plasmon resonance (LSPR) in metallic nanostructures is an emerging strategy in photovoltaics and photocatalytic devices. Important factors for surface plasmon and hot electron generation are the size, shape, and materials of plasmonic metal nanostructures, which affect LSPR excitation, absorbance, and hot electron collection. Here, we fabricated the ordered structure of metal-semiconductor plasmonic nanodiodes using nanosphere lithography and reactive ion etching. Two types of hole-shaped plasmonic nanostructures with the hole diameter of 280 and 115 nm were fabricated on Au/TiO₂Schottky diodes. We show that hot electron flow can be manipulated by changing the size of plasmonic nanostructures on the Schottky diode. We show that the short-circuit photocurrent changes and the incident photon-to-electron conversion efficiency results exhibit the peak shift depending on the structures. These phenomena are explicitly observed with finite difference time domain simulations. The capability of tuning the morphology of plasmonic nanostructure on the Schottky diode can give rise to new possibilities in controlling hot electron generation and developing novel hot-electron-based energy conversion devices.

Electronic Control of Hot Electron Transport Using Modified Schottky Barriers in Metal-Semiconductor Nanodiodes

Hot electron flux, generated by both incident light energy and the heat of the catalytic reaction, is a major element for energy conversion at the surface. Controlling hot electron flux in a reversible manner is extremely important for achieving high energy conversion efficiency. Here we demonstrate that hot electron flux can be controlled by tuning the Schottky barrier height. This phenomenon was monitored by using a Schottky nanodiode composed of a metal-semiconductor. The formation of a Schottky barrier at a nanometer scale inevitably accompanies an intrinsic image potential between the metal-semiconductor junction, which lowers the effective Schottky barrier height. When a reverse bias is applied to the nanodiode, an additional image potential participates in a secondary barrier lowering, leading to the increased hot electron flow. Besides, a decrease of tunneling width results in facile electron transport through the barrier. The increased hot electron flux by the chemical reaction (chemicurrent) and by the photon absorption (photocurrent) indicates hot electrons are captured more effectively by modifying the Schottky barrier. This study can shed light on a quantitative understanding and application of charge behavior at metal-semiconductor interfaces, in solar energy conversion, or in a catalytic reaction.

Phase change material based hot electron photodetection

We introduce a phase change material (PCM) based metal-dielectric-metal (MDM) cavity of gold (Au)-antimony trisulfide (Sb2S3)-Au as a hot electron photodetector (HEPD). Sb2S3 shows significant contrast in the bandgap (Eg) upon phase transition from the crystalline (Cry) (Eg = 2.01 eV) to the amorphous (Amp) (Eg = 1.72 eV) phase and forms the lowest Schottky barrier with Au in its Amp phase compared to conventional semiconductors such as Si, MoS2, and TiO2. The proposed HEPD is tunable for absorption and responsivity in the spectral range of 720 nm < λ < 1250 nm for the Cry phase and 604 nm < λ < 3542 nm for the Amp phase. The single resonance cavity and thus the sensitivity of the designed HEPD device can be changed to the double resonance cavity via the Cry to Amp phase transition. The maximum predicted responsivities for the single and double cavities are 20 and 24 mA W-1, respectively, at 950 nm and 1050 nm wavelengths which is the highest among all previously proposed planar HEPD devices. An anti-symmetric resonance mode at a higher wavelength is observed in the double cavity with 100% absorption. Owing to a high index of Sb2S3, an ultrathin ∼40 nm (∼λ/15) MDM cavity supports a critical light coupling to achieve high-efficiency HEPDs. Furthermore, a reversible and ultrafast (∼70 ns) Cry to Amp phase transition of Sb2S3 makes it suitable for many tunable photonics applications ranging from the visible to near-infrared region. Finally, we have introduced a novel scheme to switch between the single and double cavity by exploiting a semiconductor to metal phase transition in a PCM called VO2. The integration of VO2 as a coupling medium in the double cavity has increased the responsivity up to 50% upon phase transition to the metal phase. The proposed design can be used in optical filters, optical switches, ultrathin broad or narrow band solar absorbers, and other energy applications such as water splitting.

Controlling hot electron flux and catalytic selectivity with nanoscale metal-oxide interfaces

Interaction between metal and oxides is an important molecular-level factor that influences the selectivity of a desirable reaction. Therefore, designing a heterogeneous catalyst where metal-oxide interfaces are well-formed is important for understanding selectivity and surface electronic excitation at the interface. Here, we utilized a nanoscale catalytic Schottky diode from Pt nanowire arrays on TiO2 that forms a nanoscale Pt-TiO2 interface to determine the influence of the metal-oxide interface on catalytic selectivity, thereby affecting hot electron excitation; this demonstrated the real-time detection of hot electron flow generated under an exothermic methanol oxidation reaction. The selectivity to methyl formate and hot electron generation was obtained on nanoscale Pt nanowires/TiO2, which exhibited ~2 times higher partial oxidation selectivity and ~3 times higher chemicurrent yield compared to a diode based on Pt film. By utilizing various Pt/TiO2 nanostructures, we found that the ratio of interface to metal sites significantly affects the selectivity, thereby enhancing chemicurrent yield in methanol oxidation. Density function theory (DFT) calculations show that formation of the Pt-TiO2 interface showed that selectivity to methyl formate formation was much larger in Pt nanowire arrays than in Pt films because of the different reaction mechanism.

Enhanced flux of chemically induced hot electrons on a Pt nanowire/Si nanodiode during decomposition of hydrogen peroxide

Identifying the charge transfer at metal-semiconductor interfaces by detecting hot electrons is crucial for understanding the mechanism of catalytic reactions and the development of an engineered catalyst structure. Over the last two decades, the development of catalytic nanodiodes has enabled us to directly measure chemically induced hot electron flux and relate it to catalytic activity. A crucial question is the role of interfacial sites at metal-oxide interfaces in determining catalytic activity and hot electron flux. To address this issue, a new design of catalytic nanodiodes employs nanoscale Pt wires and a semiconducting substrate. Here, we fabricated a novel Schottky nanodiode, a platinum nanowire (Pt NW) deposited Si catalytic nanodiode (Pt NW/Si) that exhibits an increased number of metal-semiconductor interfacial sites (Pt/Si) compared with a Pt film-based Si nanodiode (Pt film/Si). Two types of Pt/Si catalytic nanodiodes were utilized to investigate the electronic properties of the Pt/Si interface by detecting hot electrons and observing reactivity during the H₂O₂ decomposition reaction in the liquid-solid system. We show that the Pt NWs had higher catalytic activity because of the surface defect sites on the Pt NW surface. We observed a higher chemicurrent yield on the Pt NW/Si nanodiode compared with the Pt film/Si nanodiode, which is associated with the shortened travel length for the hot electrons at the edge of the Pt nanowires and results in a higher transmission probability for hot electron transport through metal-oxide interfaces.