Single-Particle Imaging Photoinduced Charge Transfer of Ferroelectric Polarized Heterostructures for Photocatalysis.

Piezoelectric-assisted photocatalysis has a huge potential in solving the energy shortage and environmental pollution problems, and imaging their detailed charge-transfer process can provide in-depth understanding for the development of high-active piezo-photocatalysts; however, it is still challenging. Herein, topotactic heterostructures of TiO2@BaTiO3 (TO@BTO-S) were constructed by the epitaxial growth of ferroelectric BaTiO3 mesocrystals on TiO2-{001} facets, resulting in a ferroelectric photocatalyst with a polarization orientation on the surface. Notably, the photoinduced charge transfer in ferroelectric TiO2@BaTiO3 was accurately monitored and directly visualized at the single-particle level by the advanced photoluminescence (PL) imaging microscopy systems. The longer PL lifetime of TO@BTO-S demonstrated the efficient charge separation caused by a built-in electric field, which is constructed by the polarization orientation of BaTiO3 mesocrystals. Therefore, the TO@BTO-S heterostructure exhibits efficient piezoelectric-assisted photocatalytic pure water splitting, which is 290 times higher than photocatalysis. This work revealed time/spatial-resolved photoinduced charge transfer in piezoelectric assistance photocatalysts at the single-particle level and demonstrated the great role of polarization orientation in promoting charge transfer for photocatalysis.

Full-spectrum plasmonic semiconductors for photocatalysis.

Localized surface plasmon resonance (LSPR) of noble metal nanoparticles can focus surrounding light onto the particle surface to boost photochemical reactions and solar energy utilization. However, the rarity and high cost of noble metals limit their applications in plasmonic photocatalysis, forcing researchers to seek low-cost alternatives. Recently, some heavily doped semiconductors with high free carrier density have garnered attention due to their metal-like LSPR properties. However, plasmonic semiconductors have complex surface structures characterized by the presence of a depletion layer, which poses challenges for active site exposure and hot carrier transfer, resulting in low photocatalytic activity. In this review, we introduce the essential characteristics and types, synthesis methods, and characterization techniques of full-spectrum plasmonic semiconductors, elucidate the mechanism of full-spectrum nonmetallic plasmonic photocatalysis, including the local electromagnetic field, hot carrier generation and transfer, the photothermal effect, and the solutions for the surface depletion layer, and summarize the applications of plasmonic semiconductors in photocatalytic environmental remediation, CO2 reduction, H2 generation, and organic transformations. Finally, we provide a perspective on full-spectrum plasmonic photocatalysis, aiming to guide the design and development of plasmonic photocatalysts.

Constructing High-Active Surface of Plasmonic Tungsten Oxide for Photocatalytic Alcohol Dehydration.

Plasmonic semiconductors with broad spectral response hold significant promise for sustainable solar energy utilization. However, the surface inertness limits the photocatalytic activity. Herein, a novel approach is proposed to improve the body crystallinity and increase the surface oxygen vacancies of plasmonic tungsten oxide by the combination of hydrochloric acid (HCl) regulation and light irradiation, which can promote the adsorption of tert-butyl alcohol (TBA) on plasmonic tungsten oxide and overcome the hindrance of the surface depletion layer in photocatalytic alcohol dehydration. Additionally, this process can concentrate electrons for strong plasmonic electron oscillation on the near surface, facilitating rapid electron transfer within the adsorbed TBA molecules for C-O bond cleavage. As a result, the activation barrier for TBA dehydration is significantly reduced by 93% to 6.0 kJ mol-1, much lower than that of thermocatalysis (91 kJ mol-1). Therefore, an optimal isobutylene generation rate of 1.8 mol g-1 h-1 (selectivity of 99.9%) is achieved. A small flow reaction system is further constructed, which shows an isobutylene generation rate of 12 mmol h-1 under natural sunlight irradiation. This work highlights the potential of plasmonic semiconductors for efficient photocatalytic alcohol dehydration, thereby promoting the sustainable utilization of solar energy.

Non-metallic plasmon-assisted upconversion fluorescence for ultrasensitive hydrogen peroxide detection from nM to µM.

Precise monitoring and quantification of H2O2 is highly urgent and of great significance for biomedicine, food safety, environmental monitoring, etc. Herein, we proposed a facile near-infrared (NIR) excited fluorescent probe composed of upconversion nanoparticles (UCNPs) and non-metallic plasmonic WO3-x for ultrasensitive quantitative H2O2 detection. Plasmonic WO3-x with oxygen vacancy-induced LSPR achieved over 680-fold enhancement of upconversion fluorescence at 520 nm, and also acts as the sensitive recognition site for H2O2. H2O2 quenched the LSPR band of plasmonic WO3-x, further significantly influencing adjacent fluorescence signals depending on its concentration. The probe exhibits a good linear response to H2O2 with a low detection limit (10-9 M) and a wide concentration range (0-50 µM), and shows satisfactory application in the determination of H2O2 in blood and milk. This work may provide new ideas for the development of non-invasive fluorescent nanoprobes and plasmon-assisted biochemical detection methods.

Composite of formamidinium lead bromide perovskite FAPbBr3 with reduced graphene oxide (rGO) for efficient H2 evolution from HBr splitting.

Solar hydrobromic acid (HBr) splitting using perovskite photocatalysts provides an attractive avenue to store solar energy into hydrogen (H2) and bromine (Br2), while an efficient photocatalytic system is still demanded. As for the semiconductor photocatalyst, formamidinium perovskites show some superiorities in structural stability, light adsorption and charge dynamics compared to their methylammonium counterparts, which are fitter for the photocatalysis process. Herein, the composite of formamidinium lead bromide perovskite (FAPbBr3) with reduced graphene oxide (rGO) is prepared using a facile photoreduction method. Under simulated sunlight irradiation (AM1.5G, 100 mW cm-2), this FAPbBr3/rGO composite (100 mg) demonstrates a noteworthy enhancement in photocatalytic H2 evolution activity of 386.7 µmol h-1, and it exhibits a notable stability with no significant decrease after 50 h of repeated tests. The single particle PL (photoluminescence) microscope is employed to study the charge dynamics, revealing that rGO in the composite effectively promotes the carrier separation. This work provides a highly efficient and stable photocatalyst for HBr splitting, and offers an effective modification strategy on lead bromide perovskites.

Achieving Long-Lived Charge Separated State through Ultrafast Interfacial Hole Transfer in Redox Sites-Isolated CdS Nanorods for Enhanced Photocatalysis.

As opposed to natural photosynthesis, a significant challenge in a semiconductor-based photocatalyst is the limited hole extraction efficiency, which adversely affects solar-to-fuel efficiency. Recent studies have demonstrated that photocatalysts featuring spatially isolated dual catalytic oxidation/reduction sites can yield enhanced hole extraction efficiencies. However, the decay dynamics of excited states in such photocatalysts have not been explored. Here a ternary barbell-shaped CdS/MoS2/Cu2S heterostructure is prepared, comprising CdS nanorods (NRs) interfaced with MoS2 nanosheets at both ends and Cu2S nanoparticles on the sidewall. By using transient absorption (TA) spectra, highly efficient charge separation within the CdS/MoS2/Cu2S heterostructure are identified. This is achieved through directed electron transfer to the MoS2 tips at a rate constant of >8.3 × 109 s-1 and rapid hole transfer to the Cu2S nanoparticles on the sidewall at a rate of >6.1 × 1010 s-1, leading to an exceptional overall charge transfer constant of 2.3 × 1011 s-1 in CdS/MoS2/Cu2S. The enhanced hole transfer efficiency results in a remarkably prolonged charge-separated state, facilitating efficient electron accumulation within the MoS2 tips. Consequently, the ternary CdS/MoS2/Cu2S heterostructure demonstrates a 22-fold enhancement in visible-light-driven H2 generation compare to pure CdS nanorods. This work highlights the significance of efficient hole extraction in enhancing the solar-to-H2 performance of semiconductor-based heterostructure.

On-chip ultrasensitive and rapid hydrogen sensing based on plasmon-induced hot electron-molecule interaction.

Hydrogen energy is a zero-carbon replacement for fossil fuels. However, hydrogen is highly flammable and explosive hence timely sensitive leak detection is crucial. Existing optical sensing techniques rely on complex instruments, while electrical sensing techniques usually operate at high temperatures and biasing condition. In this paper an on-chip plasmonic-catalytic hydrogen sensing concept with a concentration detection limit down to 1 ppm is presented that is based on a metal-insulator-semiconductor (MIS) nanojunction operating at room temperature and zero bias. The sensing signal of the device was enhanced by three orders of magnitude at a one-order of magnitude higher response speed compared to alternative non-plasmonic devices. The excellent performance is attributed to the hydrogen induced interfacial dipole charge layer and the associated plasmonic hot electron modulated photoelectric response. Excellent agreements were achieved between experiment and theoretical calculations based on a quantum tunneling model. Such an on-chip combination of plasmonic optics, photoelectric detection and photocatalysis offers promising strategies for next-generation optical gas sensors that require high sensitivity, low time delay, low cost, high portability and flexibility.

Engineering a Copper Single-Atom Electron Bridge to Achieve Efficient Photocatalytic CO2 Conversion.

Developing highly efficient and stable photocatalysts for the CO2 reduction reaction (CO2 RR) remains a great challenge. We designed a Z-Scheme photocatalyst with N-Cu1 -S single-atom electron bridge (denoted as Cu-SAEB), which was used to mediate the CO2 RR. The production of CO and O2 over Cu-SAEB is as high as 236.0 and 120.1 µmol g-1 h-1 in the absence of sacrificial agents, respectively, outperforming most previously reported photocatalysts. Notably, the as-designed Cu-SAEB is highly stable throughout 30 reaction cycles, totaling 300 h, owing to the strengthened contact interface of Cu-SAEB, and mediated by the N-Cu1 -S atomic structure. Experimental and theoretical calculations indicated that the SAEB greatly promoted the Z-scheme interfacial charge-transport process, thus leading to great enhancement of the photocatalytic CO2 RR of Cu-SAEB. This work represents a promising platform for the development of highly efficient and stable photocatalysts that have potential in CO2 conversion applications.

Full-spectrum nonmetallic plasmonic carriers for efficient isopropanol dehydration.

Plasmonic hot carriers have the advantage of focusing, amplifying, and manipulating optical signals via electron oscillations which offers a feasible pathway to influence catalytic reactions. However, the contribution of nonmetallic hot carriers and thermal effects on the overall reactions are still unclear, and developing methods to enhance the efficiency of the catalysis is critical. Herein, we proposed a new strategy for flexibly modulating the hot electrons using a nonmetallic plasmonic heterostructure (named W18O49-nanowires/reduced-graphene-oxides) for isopropanol dehydration where the reaction rate was 180-fold greater than the corresponding thermocatalytic pathway. The key detail to this strategy lies in the synergetic utilization of ultraviolet light and visible-near-infrared light to enhance the hot electron generation and promote electron transfer for C-O bond cleavage during isopropanol dehydration reaction. This, in turn, results in a reduced reaction activation barrier down to 0.37 eV (compared to 1.0 eV of thermocatalysis) and a significantly improved conversion efficiency of 100% propylene from isopropanol. This work provides an additional strategy to modulate hot carrier of plasmonic semiconductors and helps guide the design of better catalytic materials and chemistries.

Assembly of gold nanorods functionalized by zirconium-based metal-organic frameworks for surface enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS) is a promising detection technique providing outstanding molecular fingerprint identification and high sensitivity of analytes. Developing sensitive and stable SERS substrates is highly desirable but remains a challenge. We herein report a wet-chemistry approach for the preparation of (Au nanorod core)@(Zr-based metal-organic framework shell) (Au nanorod@Zr-MOF) nanostructures with the Zr-MOF shell thickness ranging from 3 nm to 90 nm. The stacked Au nanorod@Zr-MOF composites exhibit remarkably improved SERS sensitivity because the MOF shell enriches the molecules to the abundant plasmonic hotspots between the Au nanorod cores. The optimized Au nanorod@Zr-MOF structures exhibit superior SERS activity for detecting 4'-mercaptobiphenylcarbonitrile molecules at a concentration as low as 2 × 10-10 M, with the SERS enhancement factor 2 and 8 times as high as that of ordered bare Au nanorod arrays and random stacking bare Au nanorods, respectively. This study enriches the library of hybrid nanostructures of plasmonic nanocrystals and MOFs, providing an integrated SERS platform with molecular enrichment capability for the realization of sensitive and quantitative analyte identification.

Light-induced thermal convection for collection and removal of carbon nanotubes.

Carbon nanotubes (CNTs) have exhibited immense potential for applications in biology and medicine, and once their intended purpose is fulfilled, the elimination of residual CNTs is essential to avoid negative effects. In this study, we demonstrated the effective collection and simple removal of CNTs dispersed in a suspension via thermal convection. First, a tapered fiber tip with a cone angle and end diameter of 10° and 3 µm, respectively, was fabricated via a heating and pulling method. Further, a laser beam with a power and wavelength of 100 mW and 1.55 µm, respectively, was launched into the tapered fiber tip, which was placed in a CNT suspension, resulting in the formation of a microbubble on the fiber tip. The temperature gradient on the microbubble and suspension surface induced thermal convection in the suspension, which resulted in the accumulation of CNTs on the fiber tip. The experimentally formed CNT cluster possessed a circular top surface with a diameter of 87 µm and an arched cross-section with a height of 19 µm. Furthermore, this CNT cluster was firmly attached to the fiber tip. Therefore, the removal of CNT clusters can be realized by simply removing the fiber tip from the suspension. Moreover, we simulated the thermal convection that caused CNT aggregation. The obtained results indicate that convection near the fiber tip flows toward it, which pushes the CNTs toward the fiber tip and enables their attachment to it. Further, the flow velocity is symmetrically distributed as a Gaussian function, which results in the formation of a circular top surface and arched cross-sectional profile for the CNT cluster. Our method may be applied in biomedicine for the collection and removal of nano-drug residues.

Molecularly imprinted photoelectrochemical sensor for detecting tetrabromobisphenol A in indoor dust and water.

The gradual emissions of tetrabromobisphenol A (TBBPA) from the primitive recycling of E-waste create human health threats, which urgently require to develop an efficient, rapid yet simple detection method. The present study conducts a highly sensitive molecularly imprinted photoelectrochemical sensor (MIPES) containing molecularly imprinted (MI)-TiO2, Au, and reduced graphene oxide for the trace detection of TBBPA in indoor dust and surface water from an E-waste recycling area. The photocurrent response is used to evaluate the sensing performance of the MIPES toward TBBPA detection. The working potential for amperometry is 0.48 V. The wavelength range for photoelectrochemical detection is 320-780 nm. The sensor shows a detection range of 1.68 to 100 nM with a low limit of detection of 0.51 nM (LOD = 3 sb/S) and a limit of quantification of 1.68 nM (LOQ = 3.3 LOD). In addition, the MIPES sensor exhibits rapid, excellent reproducibility, selectivity, and long-term stability toward TBBPA detection. The relative standard deviation of three measurements for real samples is less than 7.0%, and the recovery range is 90.0-115%. The surface of molecular imprinting contributes to the high charge separation and sensing photocurrent response of TBBPA, which is confirmed by single-particle photoluminescence spectroscopy. The present study provides a new facile sensor with highly sensitive yet rapid response to detect environmental pollutants in E-waste by using the MIPES.

Constructing Surface Plasmon Resonance on Bi2WO6 to Boost High-Selective CO2 Reduction for Methane.

Plasmonic Bi2WO6 with strong localized surface plasmon resonance (LSPR) around the 500-1400 region is successfully constructed by electron doping. Oxygen vacancies on W-O-W (V1) and Bi-O-Bi (V2) sites are precisely controlled to obtain Bi2WO6-V1 with LSPR and Bi2WO6-V2 with defect absorption. Density functional theory (DFT) calculation demonstrates that the V1-induced energy state facilitates photoelectron collection for a long lifetime, resulting in LSPR of Bi2WO6. Photoelectron trapping on V1 sites is demonstrated by a single-particle photoluminescence (PL) study, and 93% PL quenching efficiency is observed. With strong LSPR, plasmonic Bi2WO6-V1 exhibits highly selective methane generation with a rate of 9.95 µmol g-1 h-1 during the CO2 reduction reaction (CO2-RR), which is 26-fold higher than 0.37 µmol g-1 h-1 of BiWO3-V2 under UV-visible light irradiation. LSPR-dependent methane generation is confirmed by various photocatalytic results of plasmonic Bi2WO6 with tunable LSPR and different light excitations. Furthermore, the DFT-simulated pathway of CO2-RR and in situ Fourier transform infrared spectra on the surface of Bi2WO6 prove that V1 sites facilitate CH4 generation. Our work provides a strategy to obtain nonmetallic plasmonic materials by electron doping.

Light-Induced In Situ Formation of a Nonmetallic Plasmonic MoS2/MoO3-x Heterostructure with Efficient Charge Transfer for CO2 Reduction and SERS Detection.

Low-cost and abundant reserved nonmetallic plasmonic materials have been regarded as a promising substitute of noble metals for photocatalysis and surface-enhanced Raman scattering (SERS). In this paper, a MoS2/MoO3-x heterostructure was synthesized by light-induced in situ partial oxidation of MoS2 nanosheets, exhibiting strong surface plasmon resonance (SPR) in a vis-near-infrared (NIR) region. Continuously plasmon-induced hot electrons boost CO2 reduction to CO due to efficient photoelectron injection from MoS2 to MoO3-x. Under UV-vis-NIR irradiation, the CO generation rate reached 32.4 µmol g-1 h-1 with a selectivity of 94.1%, which was much higher than that of single MoS2 or MoO3-x. Furthermore, the plasmonic MoS2/MoO3-x heterostructure exhibits superior SERS performance for sensitive rhodamine 6G detection (10-9 M) with an enhancement factor of ∼106 because of the synergy between SPR and charge transfer effect. This work provides one novel mild synthetization of a plasmonic heterostructure and demonstrates its potential in plasmon-enhanced CO2 reduction and SERS detection.

Enhancement of exciton emission in WS2 based on the Kerker effect from the mode engineering of individual Si nanostripes.

Coupling between nanostructures and excitons has attracted great attention for potential applications in quantum information technology. Compared with plasmonic platforms, all-dielectric nanostructures with Mie resonances are more practical because of low-loss, low-cost and CMOS compatibility. However, weak field enhancements in single element dielectric nanostructures hinder their applications in both strong and weak coupling regimes. The Kerker effect arising from the far-field electro-magnetic interactions in dielectric nanostructures brings a new mechanism to realize effective coupling with excitons. Until now, it still remains unsolved whether effective Mie-exciton coupling can be realized based on pure far-field Kerker effect. Therefore, we proposed a silicon-on-insulator (SOI) integrated Mie resonator with a 135 nm top oxide layer to exclude the near-field coupling between excitons and silicon (Si) nanostripes. Through tuning the widths of Si nanostripes to obtain highly directional photoluminescence (PL) emission under Kerker conditions, strong PL enhancements can be observed, whose enhancement factors are comparable to the reported best performances of single all-dielectric or even plasmonic nanostructures coupling with 2D excitons. Our findings bring new strategies for strong light-matter interactions with near-zero heating loss and make it possible to construct 2D materials-silicon hybrid integration for future nanophotonic and optoelectronic devices.

Plasmonic hot electron transfer in anisotropic Pt-Au nanodisks boosts electrochemical reactions in the visible-NIR region.

Anisotropic plasmonic metals have attracted significant attention in enhancing the catalytic performance of catalysts due to their broad light-harnessing capabilities and active hot electrons; however, limited investigations have been dedicated towards improving their electrochemical reaction performance in the visible and near infrared (NIR) regions. Herein, anisotropic Pt-edged Au nanodisks (NDs) were synthesized by controlling the preferential loading of Pt and used as catalysts for plasmon-enhanced electrochemical methanol oxidation reactions (MORs) under visible-NIR light irradiation by, and the light-enhanced electric current over the Pt-edged Au NDs was found to be 3-fold higher than that under dark conditions. Wavelength-dependent electric current over the Pt-edged Au NDs for the MOR in the visible-NIR light region demonstrates that the light-induced enhancement of the electric current is due to surface plasmon resonance (SPR) of the Au NDs. Furthermore, plasmonic hot electron transfer was studied by the single-particle photoluminescence images and spectra of Au NDs and Pt-Au NDs, and the dipole surface plasmon resonance (DSPR) mode was proved to be the main channel for hot electron transfer. During the electrochemical reaction under visible-NIR light irradiation, a plasmonic hot electron is transferred to the electrode, and a "hot hole" is left on the surface, boosting the MOR.

Single-molecule and -particle probing crystal edge/corner as highly efficient photocatalytic sites on a single TiO2 particle.

The exposed active sites of semiconductor catalysts are essential to the photocatalytic energy conversion efficiency. However, it is difficult to directly observe such active sites and understand the photogenerated electron/hole pairs' dynamics on a single catalyst particle. Here, we applied a quasi-total internal reflection fluorescence microscopy and laser-scanning confocal microscopy to identify the photocatalytic active sites at a single-molecule level and visualized the photogenerated hole-electron pair dynamics on a single TiO2 particle, the most widely used photocatalyst. The experimental results and density functional theory calculations reveal that holes and electrons tend to reach and react at the same surface sites, i.e., crystal edge/corner, within a single anatase TiO2 particle owing to the highly exposed (001) and (101) facets. The observation provides solid proof for the existence of the surface junction "edge or corner" on single TiO2 particles. These findings also offer insights into the nature of the photocatalytic active sites and imply an activity-based strategy for rationally engineering catalysts for improved photocatalysis, which can be also applied for other catalytic materials.

Self-Z-scheme plasmonic tungsten oxide nanowires for boosting ethanol dehydrogenation under UV-visible light irradiation.

Nonstoichiometric tungsten oxides (WO3-x) with abundant oxygen vacancies were synthesized and used as nonmetallic plasmonic photocatalysts to promote ethanol dehydrogenation under UV-visible light irradiation. Plasmonic WO3-x have unique electronic structures that act as Z-scheme heterostructures. UV-excited photoelectrons were injected into the conduction band of WO3-x, stabilizing the free electron density and boosting plasmonic hot electron generation for ethanol dehydrogenation. The synergetic effect of UV and visible light excitations greatly enhances the aldehyde generation to 2696 µmol g-1 (3 hours) with a high selectivity of 91%, which is 74-fold and 12-fold higher than those obtained under only UV or visible light irradiation, respectively.

Two-Dimensional Au-Nanoprism/Reduced Graphene Oxide/Pt-Nanoframe as Plasmonic Photocatalysts with Multiplasmon Modes Boosting Hot Electron Transfer for Hydrogen Generation.

Two-dimensional Au-nanoprism/reduced graphene oxide (rGO)/Pt-nanoframe was synthesized as plasmonic photocatalyt, exhibiting activity of photocatalytic hydrogen generation greater than those of Au-nanorod/rGO/Pt-nanoframe and metallic plasmonic photocatalyst Pt-Au. The single-particle plasmonic photoluminescence study demonstrated that Au-nanorod has only a longitudinal plasmon resonance mode for hot electron transfer to rGO, while Au-nanoprism has in-plane dipole and multipole surface plasmon resonance modes for hot electron transfer, leading to highly efficient charge separation for hydrogen generation.

Facet-Dependent Photoreduction on Single ZnO Crystals.

Photocatalytic reactions occur at the crystal-solution interface, and hence specific crystal facet expression and surface defects can play an important role. Here we investigate the structure-related photoreduction at zinc oxide (ZnO) microparticles via integrated light and electron microscopy in combination with silver metal photodeposition. This enables a direct visualization of the photoreduction activity at specific crystallographic features. It is found that silver nanoparticle photodeposition on dumbbell-shaped crystals mainly takes place at the edges of O-terminated (0001̅) polar facets. In contrast, on ZnO microrods photodeposition is more homogeneously distributed with an increased activity at {101̅1̅} facets. Additional time-resolved measurements reveal a direct spatial link between the enhanced photoactivity and increased charge carrier lifetimes. These findings contradict previous observations based on indirect, bulk-scale experiments, assigning the highest photocatalytic activity to polar facets. The presented research demonstrates the need for advanced microscopy techniques to directly probe the location of photocatalytic activity.