Plasmonic Metal Mediated Charge Transfer in Stacked Core-Shell Semiconductor Heterojunction for Significantly Enhanced CO2 Photoreduction

The paradox of thermal vs. non-thermal effects in plasmonic photocatalysis

The debate surrounding the roles of thermal and non-thermal pathways in plasmonic catalysis has captured the attention of researchers and sparked vibrant discussions within the scientific community. In this review, we embark on a thorough exploration of this intriguing discourse, starting from fundamental principles and culminating in a detailed understanding of the divergent viewpoints. We probe into the core of the debate by elucidating the behavior of excited charge carriers in illuminated plasmonic nanostructures, which serves as the foundation for the two opposing schools of thought. We present the key arguments and evidence put forth by proponents of both the non-thermal and thermal pathways, providing a perspective on their respective positions. Beyond the theoretical divide, we discussed the evolving methodologies used to unravel these mechanisms. We discuss the use of Arrhenius equations and their variations, shedding light on the ensuing debates about their applicability. Our review emphasizes the significance of localized surface plasmon resonance (LSPR), investigating its role in collective charge oscillations and the decay dynamics that influence catalytic processes. We also talked about the nuances of activation energy, exploring its relationship with the nonlinearity of temperature and light intensity dependence on reaction rates. Additionally, we address the intricacies of catalyst surface temperature measurements and their implications in understanding light-triggered reaction dynamics. The review further discusses wavelength-dependent reaction rates, kinetic isotope effects, and competitive electron transfer reactions, offering an all-inclusive view of the field. This review not only maps the current landscape of plasmonic photocatalysis but also facilitates future explorations and innovations to unlock the full potential of plasmon-mediated catalysis, where synergistic approaches could lead to different vistas in chemical transformations.

Core-Shell Semiconductor-Graphene Nanoarchitectures for Efficient Photocatalysis: State of the Art and Perspectives

Semiconductor photocatalysis holds great promise for renewable energy generation and environment remediation, but generally suffers from the serious drawbacks on light absorption, charge generation and transport, and structural stability that limit the performance. The core-shell semiconductor-graphene (CSSG) nanoarchitectures may address these issues due to their unique structures with exceptional physical and chemical properties. This review explores recent advances of the CSSG nanoarchitectures in the photocatalytic performance. It starts with the classification of the CSSG nanoarchitectures by the dimensionality. Then, the construction methods under internal and external driving forces were introduced and compared with each other. Afterward, the physicochemical properties and photocatalytic applications of these nanoarchitectures were discussed, with a focus on their role in photocatalysis. It ends with a summary and some perspectives on future development of the CSSG nanoarchitectures toward highly efficient photocatalysts with extensive application. By harnessing the synergistic capabilities of the CSSG architectures, we aim to address pressing environmental and energy challenges and drive scientific progress in these fields.

Plasmon-Mediated Organic Photoelectrochemistry Applied to Amination Reactions

Organic electrochemistry is currently experiencing an era of renaissance, which is closely related to the possibility of carrying out organic transformations under mild conditions, with high selectivity, high yields, and without the use of toxic solvents. Combination of organic electrochemistry with alternative approaches, such as photo-chemistry was found to have great potential due to induced synergy effects. In this work, we propose for the first time utilization of plasmon triggering of enhanced and regio-controlled organic chemical transformation performed in photoelectrochemical regime. The advantages of the proposed route is demonstrated in the model amination reaction with formation of C-N bond between pyrazole and substituted benzene derivatives. Amination was performed in photo-electrochemical mode on the surface of plasmon active Au@Pt electrode with attention focused on the impact of plasmon triggering on the reaction efficiency and regio-selectivity. The ability to enhance the reaction rate significantly and to tune products regio-selectivity is demonstrated. We also performed density functional theory calculations to inquire about the reaction mechanism and potentially explain the plasmon contribution to electrochemical reaction rate and regioselectivity.

Twin boundary defect engineering in Au cocatalyst to promote alcohol splitting for coproduction of H2 and fine chemicals

The microstructure of Au metal cocatalyst has been shown to significantly influence its optical and electronic properties. However, the impact of Au defect engineering on photocatalytic activity remains underexplored. In this study, we synthesize different Au-TiO2 composites by in-situ hybridizing face-centered cubic (F-Au) and twin boundary defect Au (T-Au) nanoparticles (NPs) onto the surface of TiO2. We find that T-Au NPs with twin defects serve as highly efficient cocatalysts for converting alcohols into their corresponding aldehydes while also generating H2. The optimized T-Au/TiO2 composite yields an H2 evolution rate of 6850 µmol h-1 g-1 and a BAD formation rate of 6830 µmol h-1 g-1, about 38 times higher than that of blank TiO2. Compared to F-Au/TiO2, the T-Au/TiO2 composite enhances charge separation, extends the lifetime of electrons, and provides more active sites for H2 reduction. The twin defect also improves alcohol reactant adsorption, boosting overall photocatalytic performance. This research paves the way for more studies on defect engineering in metal cocatalysts for enhanced catalytic activities in organic synthesis and H2 evolution.

Integration of ohmic junction and step-scheme heterojunction for enhanced photocatalysis

A novel and efficient photocatalyst, Cu2WS4/MoS2-Au plasmonic Step-scheme (S-scheme) heterojunction, was constructed for the first time and applied to remove environmental pollutants. Among all the prepared photocatalysts, the. Cu2WS4/MoS2-Au-5 exhibited the highest catalytic activity with an 89.1% reduction efficiency for Cr6+ and a 98.7% oxidation efficiency for Benzophenone-1 (BP-1) under visible light irradiation. The Cu2WS4/MoS2-Au photocatalyst exhibits stable performance and efficient photocatalytic activity due to effective charge separation, enhanced light absorption from localized surface plasmon resonance (LSPR) of gold nanoparticles, and the formation of an S-scheme heterojunction with strong oxidation-reduction capabilities. In addition, through analysis of experiments and theoretical calculations, it is speculated that the Cu2WS4/MoS2-Au follows a typical S-scheme photogenerated carrier transferring mechanism, which is verified by the finite difference time domain simulation, the free radical quenching experiments, the electron paramagnetic resonance analysis and the simulated charge density distribution. More importantly, the simulations of the work function and charge density distribution confirm the built-in electric field and the ohmic junction have been established at the interfaces between the Cu2WS4 and MoS2 (Cu2WS4/MoS2) as well as the interface between MoS2 and Au (MoS2-Au), respectively. The built-in electric field and ohmic junction enable efficient separation of photogenerated electrons and holes, ensuring the superior catalytic oxidation and reduction activities of the Cu2WS4/MoS2-Au photocatalyst. Finally, we propose a photocatalytic mechanism for the Cu2WS4/MoS2-Au plasmonic S-scheme heterojunction based on experimental results and simulated calculations. The research results of this study are significance for the development of the plasmonic S-scheme photocatalytic system.

Hollow Plasmonic P-Metal-N S-Scheme Heterojunction Photoreactor with Spatially Separated Dual Cocatalysts toward Artificial Photosynthesis

Semiconductor-based step-scheme (S-scheme) heterojunctions possess many merits toward mimicking natural photosynthesis. However, their applications for solar-to-chemical energy conversion are hindered by inefficient charge utilization and unsatisfactory surface reactivity. Herein, two synergistic protocols are demonstrated to overcome these limitations based on the construction of a hollow plasmonic p-metal-n S-scheme heterojunction photoreactor with spatially separated dual noble-metal-free cocatalysts. On one side, plasmonic Au, inserted into the heterointerfaces of CuS@ZnIn2 S4 core-shell nanoboxes, not only accelerates the transfer and recombination of useless charges, enabling a more thorough separation of useful ones for CO2 reduction and H2 O oxidation but also generates hot electrons and holes, respectively injects them into ZnIn2 S4 and CuS, further increasing the number of active carriers participating in redox reactions. On the other side, Fe(OH)x and Ti3 C2 cocatalysts, separately located on the CuS and ZnIn2 S4 surface, enrich the redox sites, adjust the reduction potential and pathway for selective CO2 -to-CH4 transformation, and balance the transfer and consumption of photocarriers. As expected, significantly enhanced activity and selectivity in CH4 production are achieved by the smart design along with nearly stoichiometric ratios of reduction and oxidation products. This study paves the way for optimizing artificial photosynthetic systems via rational interfacial channel introduction and surface cocatalyst modification.

Embedding Nano-Piezoelectrics into Heterointerfaces of S-Scheme Heterojunctions for Boosting Photocatalysis and Piezophotocatalysis

Step-scheme (S-scheme) heterojunctions have exhibited great potential in photocatalysis due to their extraordinary light harvesting and high redox capacities. However, inadequate S-scheme recombination of useless carriers in weak redox abilities increases the probability of their recombination with useful ones in strong redox capabilities. Herein, a versatile protocol is demonstrated to overcome this impediment based on the insertion of nano-piezoelectrics into the heterointerfaces of S-scheme heterojunctions. Under light excitation, the piezoelectric inserter promotes interfacial charge transfer and produces additional photocarriers to recombine with useless electrons and holes, ensuring a more thorough separation of powerful ones for CO2 reduction and H2 O oxidation. When introducing extra ultrasonic vibration, a piezoelectric polarization field is established, which allows efficient separation of charges generated by the embedded piezoelectrics and expedites their recombination with weak carriers, further increasing the number of strong ones participating in the redox reactions. Encouraged by the greatly improved charge utilization, significantly enhanced photocatalytic and piezophotocatalytic activities in CH4 , CO, and O2 production are achieved by the designed stacked catalyst. This work highlights the importance in strengthening the necessary charge recombination in S-scheme heterojunctions and presents an efficient and novel strategy to synergize photocatalysis and piezocatalysis for renewable fuels and value-added chemicals production.

Crystallization Engineering of CuNi2 S4 Ultra-Fine Nanocrystals with Optimized Band Structures for Efficient Photocatalytic Pollutant Degradation and Hydrogen Production

The mono-dispersed cubic siegenite CuNi2 S4 ultra-fine (≈5 nm) nanocrystals are fabricated through crystallization engineering under hot injection. The strong hydroxylation on mostly exposed CuNi2 S4 (220) surface leads to the formation of multi-valence (Cu+ , Cu2+ , Ni2+ , Ni3+ ) species with unsaturated hybridization and coordination micro-environments, which can induce rich redox reactions to optimize interfacial kinetics for the adsorbed reaction intermediates. The as-synthesized CuNi2 S4 nanocrystals with ultra-small particle size and the characteristics of being highly dispersed can increase specific surface area and hydroxylated active sites, which considerably contribute to the improvement of photocatalytic activities. Experimental and theoretical studies indicate that the CuNi2 S4 with unique surface condition can properly modulate the charge density distribution and the electronic band structure, thus achieving an optimal band gap for enhancing visible light absorption. Additionally, the strong hydroxylation on CuNi2 S4 (220) surface can not only make the photocatalytic process stable in alkaline environment but also bring about an impurity level between conduction and valence band, which facilitates the separation of photo-induced charge carriers by suppressing the rapid re-combination of exited electrons and holes. The optimization of band structure should be the intrinsic reason for the efficient photocatalytic pollutant degradation and hydrogen production under visible light illumination.

Universal Piezo-Photocatalytic Wastewater Treatment on Realistic Pollutant Feedstocks by Bi4TaO8Cl: Origin of High Efficiency and Adjustable Synergy

Clean water is a fundamental human right but millions struggle for it daily. Herein, we demonstrate a new piezo-photocatalyst with immense structural diversity for universal wastewater decontamination. Single-crystalline Bi₄TaO₈Cl nanoplates with exposed piezoelectric facets exhibit visible-light response, piezoelectric behavior with coercive voltages of ±5 V yielding 0.35% crystal deformation, and pressure-induced band-bending of >2.5 eV. Using five common contaminants of textile and pharmaceutical industries, we show that the nanoplates can mineralize them in all piezocatalytic, photocatalytic, and piezo-photocatalytic approaches with efficiencies higher than most catalysts developed for just one contaminant. Their efficiencies for feedstocks differing over 2 orders of magnitude in concentrations, the highest to date, are also demonstrated to simulate real-life situations. These extensive studies established that combining piezocatalytic and photocatalytic approaches can lead to a tremendous synergy exceeding >45%. The origin of synergy has been illustrated for the first time using band-bending models and improved charge transfer from valence and conduction band electronic surfaces. We further quantified synergy across reactants, concentrations, and ultrasonic frequency and power to demonstrate their versatility and unpredictability. Finally, seven parameters that contribute to synergy but create unpredictability have been identified for the rational design of piezo-photocatalysts for wastewater treatment.

Skin B/N-doped anatase TiO2 {001} nanoflakes for visible-light photocatalytic water oxidation

The limited visible-light-responsive photoactivities of most doped wide-bandgap photocatalysts with widened absorption range have long been the obstacles for the efficient conversion of solar energy to chemical energy by photocatalysis. The weak transport ability of visible-light-induced low-energy charge carriers, and numerous recombination centers arising from the energy-band modifiers along the transport path are two major factors responsible for such a mismatch. A potential solution is to shorten the transport path of photo-induced charges in well-modulated light absorbers with low-dimensional structure and the spatially concentrated dopants underneath their surfaces. As a proof of concept, skin B/N-doped red anatase TiO₂ {001} nanoflakes with the absorption edge up to 675 nm were synthesized in this study. Experimental results revealed that boron dopants in the TiO₂ nanoflakes from the hydrolysis of nanosized TiB₂ played a crucial role in controlling nitrogen doping in the surface layer of the nanoflakes. As visible light excitation occurs at the surface layer, the photons can be sufficiently absorbed by the formed energy levels at the surface layers, and the photogenerated charge carriers can effectively migrate to the surface, thus leading to efficient visible-light-responsive photocatalytic oxygen evolution activity from water oxidation.

Advanced Catalyst for CO2 Photo-Reduction: From Controllable Product Selectivity by Architecture Engineering to Improving Charge Transfer Using Stabilized Au Clusters

Despite enormous progress and improvement in photocatalytic CO₂ reduction reaction (CO₂ RR), the development of photocatalysts that suppress H₂ evolution reaction (HER), during CO₂ RR, remains still a challenge. Here, new insight is presented for controllable CO₂ RR selectivity by tuning the architecture of the photocatalyst. Au/carbon nitride with planar structure (p Au/CN) showed high activity for HER with 87% selectivity. In contrast, the same composition with a yolk@shell structure (Y@S Au@CN) exhibited high selectivity of carbon products by suppressing the HER to 26% under visible light irradiation. Further improvement for CO₂ RR activity was achieved by a surface decoration of the yolk@shell structure with Au₂₅ (PET)₁₈ clusters as favorable electron acceptors, resulting in longer charge separation in Au@CN/Au_c Y@S structure. Finally, by covering the structure with graphene layers, the designed catalyst maintained high photostability during light illumination and showed high photocatalytic efficiency. The optimized Au@CN/Au_c /G Y@S structure displays high photocatalytic CO₂ RR selectivity of 88%, where the CO and CH₄ generations during 8 h are 494 and 198 µmol/gcat., respectively. This approach combining architecture engineering and composition modification provides a new strategy with improved activity and controllable selectivity toward targeting applications in energy conversion catalysis.

Synergetic effect of Au nanoparticles and transition metal phosphides for enhanced hydrogen evolution from ammonia-borane

The hydrogen evolution from ammonia borane is intriguing but challenging due to its sluggish kinetics. In this regard, the gold nanoparticles amalgamation with metal phosphides is speculated to be more efficient catalysts. Here, the catalysts Au/Ni₂P and Au/CoP with the high synergetic effect of Au nanoparticles and metal phosphides were synthesized for ammonia borane hydrolysis. The activity of Au/Ni₂P increases 4.8-fold (i.e., 0.08 to 0.40 L∙h^-1) compared to pristine Ni₂P, and the activity of Au/CoP increases 1.7-fold (i.e., 0.74 to 1.27 L∙h^-1) compared to pristine CoP. This reveals that the synergetic effect of Au^δ+ and (Ni₂P) ^δ- is stronger than Au^δ+ and (CoP) ^δ- which is manifested by XPS analysis. The kinetics exposes that the activation energy of Au/Ni₂P (45.28 kJ∙mole^-1) is greater than Au/CoP (31.45 kJ∙mole^-1) and the TOF of Au/Ni₂P is less than Au/CoP. This research work presents an effective approach for producing active sites of Au^δ+ and (Ni₂P & CoP) ^δ- for ammonia borane hydrolysis to enhance the H₂ evolution rate.

Embedding Plasmonic Metal into Heterointerface of MOFs-Encapsulated Semiconductor Hollow Architecture for Boosting CO2 Photoreduction

Coupling hollow semiconductor with metal-organic frameworks (MOFs) holds great promise for constructing high-efficient CO₂ photoreduction systems. However, energy band mismatch between them makes it difficult to exert their advantages to maximize the overall photocatalytic efficiency, since that the blockage of desirable interfacial charge transfer gives rise to the enrichment of photoelectrons and CO₂ molecules on the different locations. Herein, an interfacial engineering is presented to overcome this impediment, based on the insertion of plasmonic metal into the heterointerfaces between them, forming a stacked semiconductor/metal@MOF photocatalyst. Experimental observations and theoretical simulations validate the critical roles of embedded Au in maneuvering the charge separation/transfer and surface reaction: (i) bridges the photoelectron transfer from hollow CdS (H-CdS) to ZIF-8; (ii) produces hot electrons and shifts them to ZIF-8; (iii) induces the formation of ZIF-8 defects in promoting the CO₂ adsorption/activation and transformation to CO with low energy barriers. Consequently, the as-prepared H-CdS/Au@ZIF-8 with optimal ZIF-8 thickness exhibits distinctly boosted activity and superb selectivity in CO production as compared with H-CdS@ZIF-8 and other counterparts. This work provides protocols to take full advantages of components involved for enhanced solar-to-chemical energy conversion efficiency of hybrid artificial photosynthetic systems through rationally harnessing the charge transfer between them.

Spatially Separated Redox Cocatalysts on Ferroelectric Nanoplates for Improved Piezophotocatalytic CO2 Reduction and H2O Oxidation

Utilizing solar and mechanical vibration energy for catalytic CO₂ reduction and H₂O oxidation is emerging as a promising way to simultaneously generate renewable energy and mitigate climate change, making it possible to integrate two energy resources into a reaction system for artificial piezophotosynthesis. However, the practical applications are hindered by undesirable charge recombination and sluggish surface reaction in the photocatalytic and piezocatalytic processes. This study proposes a dual cocatalyst strategy to overcome these obstacles and improve the piezophotocatalytic performance of ferroelectrics in overall redox reactions. With the photodeposition of AuCu reduction and MnO_x oxidation cocatalysts on oppositely poled facets of PbTiO₃ nanoplates, band bending occurs along with the formation of built-in electric fields on the semiconductor-cocatalyst interfaces, which, together with an intrinsic ferroelectric field, piezoelectric polarization field, and band tilting in the bulk of PbTiO₃, provide strong driving forces for the directional drift of piezo- and photogenerated electrons and holes toward AuCu and MnO_x, respectively. Besides, AuCu and MnO_x enrich the active sites for surface reactions, significantly reducing the rate-determining barrier for CO₂-to-CO and H₂O-to-O₂ transformation, respectively. Benefiting from these features, AuCu/PbTiO₃/MnO_x delivers remarkably improved charge separation efficiencies and significantly enhanced piezophotocatalytic activities in CO and O₂ generation. This strategy opens a door for the better coupling of photocatalysis and piezocatalysis to promote the conversion of CO₂ with H₂O.