Boosting the photocatalytic nitrogen reduction to ammonia through adsorption-plasmonic synergistic effects

Ammonia is one of the most essential chemicals in the modern society but its production still heavily relies on energy-consuming Haber-Bosch processes. The photocatalytic reduction of nitrogen with water for ammonia production has attracted much attention recently due to its synthesis under mild conditions at room temperature and atmospheric pressure using sunlight. Herein, we report a high-performance Au/MIL-100(Cr) photocatalyst, comprising MIL-100(Cr) and Au nanoparticles in photocatalytic nitrogen reduction to ammonia at ambient conditions under visible light irradiation. The optimized photocatalyst (i.e., 0.10Au/MIL-100(Cr)) achieved the excellent ammonia production rate with 39.9 µg g_cat^?1 h^?1 compared with pure MIL-100(Cr) (2.73 µg g_cat^?1 h^?1), which was nearly 15 times that on pure MIL-100(Cr). The remarkable activity could be attributed to the adsorption-plasmonic synergistic effects in which the MIL-100(Cr) and Au are responsible to the strong trapping and adsorption of N₂ molecules and photo-induced plasmonic hot electrons activating and decomposing the N₂ molecules, respectively. This study might provide a new strategy for designing an efficient plasmonic photocatalyst to improve the photocatalytic performance of N₂ fixation under visible light irradiation.     

Dual-Plasmon Resonance Coupling Promoting Directional Photosynthesis of Nitrate from Air

Photocatalysis, particularly plasmon-mediated photocatalysis, offers a green and sustainable approach for direct nitrogen oxidation into nitrate under ambient conditions. However, the unsatisfactory photocatalytic efficiency caused by the limited localized electromagnetic field enhancement and short hot carrier lifetime of traditional plasmonic catalysts is a stumbling block to the large-scale application of plasmon photocatalytic technology. Herein, we design and demonstrate the dual-plasmonic heterojunction (Bi/Cs_xWO₃) achieves efficient and selective photocatalytic N₂ oxidation. The yield of NO₃⁻ over Bi/Cs_xWO₃ (694.32 μg g⁻¹ h⁻¹) are 2.4 times that over Cs_xWO₃ (292.12 μg g⁻¹ h⁻¹) under full-spectrum irradiation. The surface dual-plasmon resonance coupling effect generates a surge of localized electromagnetic field intensity to boost the formation efficiency and delay the self-thermalization of energetic hot carriers. Ultimately, electrons participate in the formation of ⋅O₂⁻, while holes involve in the generation of ⋅OH and the activation of N₂. The synergistic effect of multiple reactive oxygen species drives the direct photosynthesis of NO₃⁻, which achieves the overall-utilization of photoexcited electrons and holes in photocatalytic reaction. The concept that the dual-plasmon resonance coupling effect facilitates the directional overall-utilization of photoexcited carriers will pave a new way for the rational design of efficient photocatalytic systems.     

Plasmonic Switching of the Reaction Pathway: Visible‐Light Irradiation Varies the Reactant Concentration at the Solid–Solution Interface of a Gold–Cobalt Catalyst

Product selectivity of alkyne hydroamination over catalytic Au₂Co alloy nanoparticles (NPs) can be made switchable by a light‐on/light‐off process, yielding imine (cross‐coupling product of aniline and alkyne) under visible‐light irradiation, but 1,4‐diphenylbutadiyne in the dark. The low‐flux light irradiation concentrates aniline on the catalyst, accelerating the catalytic cross‐coupling by several orders of magnitude even at a very low overall aniline concentrations (1.0×10^?3 mol L^?1). A tentative mechanism is that Au₂Co NPs absorb light, generating an intense fringing electromagnetic field and hot electrons. The sharp field‐gradient (plasmonic optical force) can selectively enhance adsorption of light‐polarizable aniline molecules on the catalyst. The light irradiation thereby alters the aniline/alkyne ratio at the NPs surface, switching product selectivity. This represents a new paradigm to modify a catalysis process by light.     

Plasmonic Silver-Nanoparticle-Catalysed Hydrogen Abstraction from the C(sp3)−H Bond of the Benzylic Cα atom for Cleavage of Alkyl Aryl Ether Bonds

Selective activation of the C(sp³)−H bond is an important process in organic synthesis, where efficiently activating a specific C(sp³)−H bond without causing side reactions remains one of chemistry's great challenges. Here we report that illuminated plasmonic silver metal nanoparticles (NPs) can abstract hydrogen from the C(sp³)−H bond of the C_α atom of an alkyl aryl ether β-O-4 linkage. The intense electromagnetic near-field generated at the illuminated plasmonic NPs promotes chemisorption of the β-O-4 compound and the transfer of photo-generated hot electrons from the NPs to the adsorbed molecules leads to hydrogen abstraction and direct cleavage of the unreactive ether C_β−O bond under moderate reaction conditions (≈90 °C). The plasmon-driven process has certain exceptional features: enabling hydrogen abstraction from a specific C(sp³)−H bond, along with precise scission of the targeted C−O bond to form aromatic compounds containing unsaturated, substituted groups in excellent yields.     

Electrochemistry of triphenyltin acetate at a mercury-film glassy carbon electrode

The electrochemical behaviour of triphenyltin acetate was investigated by cyclic voltammetry, differential-pulse voltammetry and controlled potential electrolysis at a mercury-film glassy carbon electrode. Effects on the electrochemical response of the composition of supporting electrolytes, pH, electrode rotation speed and triphenyltin acetate concentration were determined. The electrochemical reduction of this compound was found to involve a preliminary adsorption process (E_peak ≈ ?0.7 V vs. SCE), the reduction of triphenyltin acetate to the triphenyltin radical (E_peak ≈ ?1.0 V) and reduction of the radical to the triphenyltin anion (E_peak ≈ ?1.4 V). A procedure for the determination of trace amounts of this compound by differential-pulse anodic stripping voltammetry in 50% (v/v) ethanol with 0.1 M acetic acid + 0.1 M ammonia solution was developed and applied to the analysis of a commercial powder formulation and water and fish samples. The limit of detection was 2.5 × 10^?9 M triphenyltin acetate.     

ESR spectra fo matrix-isolated bromobenzene cations formed by radiolysis

Exposure of dilute solution of bromobenzene and p-dibromobenzene in fluorotrichloromethane to ⁶⁰Co_γ-rays at 77 K gave the corresponding cations characterized by ESR spectra. Estimated spin densities on bromine of ≈ 30% and 23%, respectively, are greater than those predicted by comparison with neutral α-bromo radicals, R₂CBr (≈ 15%). Evidence for dimer cation formation in more concentrated solutions is presented.     

Photocatalytic N2 Fixation by Plasmonic Mo-Doped TiO2 Semiconductor

Photocatalytic N₂ fixation has attracted substantial attention in recent years, as it represents a green and sustainable development route toward efficiently converting N₂ to NH₃ for industrial applications. How to rationally design catalysts in this regard remains a challenge. Here we propose a strategy that uses plasmonic hot electrons in the highly doped TiO₂ to activate the inert N₂ molecules. The synthesized semiconductor catalyst Mo-doped TiO₂ shows a NH₃ production efficiency as high as 134 μmol·g^-1·h^-1 under ambient conditions, which is comparable to that achieved by the conventional plasmonic gold metal. By means of ultrafast spectroscopy we reveal that the plasmonic hot electrons in the system are responsible for the activation of N₂ molecules, enabling improvement the catalytic activity of TiO₂. This work opens a new avenue toward semiconductor plasmon-based photocatalytic N₂ fixation.     

In Situ Observation of Hot Carrier Transfer at Plasmonic Au/Metal-Organic Frameworks (MOFs) Interfaces

Constructing heterostructures have been demonstrated as an ideal strategy for boosting charge separation on plasmonic photocatalysts, but the detailed interface charge transfer mechanism remains elusive. Herein, that authors fabricate plasmonic Au and metal-organic frameworks (MOFs, NH₂−MIL-125 and MIL-125 used in this work) heterostructures and explore the interface charge transfer mechanism by in situ electron paramagnetic resonance (EPR) spectroscopy and electrochemical measurements. The plasmon-excited hot electrons on Au can transfer across the Au/MOF interface and be captured by the coordinatively unsaturated sites of secondary building units (Ti₈O₈(OH)₄ cluster) of the MOF structure, and the plasmon-excited hot holes on Au tend to transfer to and be trapped at the functionalized organic ligand (1,4-benzenedicarboxylate−NH₂). The spatially separated hot electrons and holes exhibit boosted the photocatalytic activity for chromium (VI) reduction and selective benzyl alcohol oxidation. This work illustrates the advantage of the versatile functionalization of MOF structures enabling molecular-level manipulation of interface charge transfer on plasmonic photocatalysts.     

Sunflower-Like Nanostructure with Built-In Hotspots for Alpha-Fetoprotein Detection

In the present study, a sunflower-like nanostructure array composed of a series of synaptic nanoparticles and nanospheres was manufactured through an efficient and low-cost colloidal lithography technique. The primary electromagnetic field contribution generated by the synaptic nanoparticles of the surface array structures was also determined by a finite-difference time-domain software to simulate the hotspots. This structure exhibited high repeatability and excellent sensitivity; hence, it was used as a surface-enhanced Raman spectroscopy (SERS) active substrate to achieve a rapid detection of ultra-low concentrations of Alpha-fetoprotein (AFP). This study demonstrates the design of a plasmonic structure with strong electromagnetic coupling, which can be used for the rapid detection of AFP concentration in clinical medicine.     

Surface Plasmon Resonance‐Enhanced Visible‐NIR‐Driven Photocatalytic and Photothermal Catalytic Performance by Ag/Mesoporous Black TiO2 Nanotube Heterojunctions

Ag/mesoporous black TiO₂ nanotubes heterojunctions (Ag‐MBTHs) were fabricated through a surface hydrogenation, wet‐impregnation and photoreduction strategy. The as‐prepared Ag‐MBTHs possess a relatively high specific surface area of ≈85 m² g^?1 and an average pore size of ≈13.2 nm. The Ag‐MBTHs with a narrow band gap of ≈2.63 eV extend the photoresponse from UV to the visible‐light and near‐infrared (NIR) region. They exhibit excellent visible‐NIR‐driven photothermal catalytic and photocatalytic performance for complete conversion of nitro aromatic compounds (100 %) and mineralization of highly toxic phenol (100 %). The enhancement can be attributed to the mesoporous hollow structures increasing the light multi‐refraction, the Ti³⁺ in frameworks and the surface plasmon resonance (SPR) effect of plasmonic Ag nanoparticles favoring light‐harvesting and spatial separation of photogenerated electron–hole pairs, which is confirmed by transient fluorescence. The fabrication of this SPR‐enhanced visible‐NIR‐driven Ag‐MBTHs catalyst may provide new insights for designing other high‐performance heterojunctions as photocatalytic and photothermal catalytic nanomaterials.     

Ab initio study of β-lactam antibiotics. I. Potential energy surface for the amidic CN bond breaking in the β-lactam + OH− reaction

The potential energy surface of the β-lactam + OH^? reaction, related to the mode of action of β-lactam antibiotics, was investigated using the ab initio Hartree—Fock method with the STO-3G basis set. Three possible reaction paths for the B_AC2 breaking of the amidic CN bond were obtained and discussed. The minimum-energy reaction path is characterized by the following processes: (1) the formation of a tetrahedral intermediate, ≈ 121 kcal mol^?1 more stable than the reagents; (2) a barrier, ≈ 15 kcal mol^?1 above the intermediate, which is mainly due to the partial breaking of the amidic bond; (3) the complete breaking of the amidic bond concerted with a proton transfer till the formation of the final product, ≈ 34 kcal mol^?1 more stable than the intermediate. The evolution of some molecular orbitals and of the electron population along the reaction path was also discussed.     

Role of Emissive and Non‐Emissive Complex Formations in Photoinduced Electron Transfer Reaction of CdTe Quantum Dots

Bimolecular photoinduced electron transfer (PET) from excited state CdTe quantum dot (QD*) to an electron deficient molecule 2,4‐dinitrotoluene (DNT) is studied in toluene. We observed two types of QD‐DNT complex formations; (i) non‐emissive complex, in which DNT is embedded deep inside the surface polymer layer of QD and (ii) emissive complex, in which DNT molecules are attached to QDs but approach to the QD core is shielded by polymer layer. Because of its non‐emissive nature, the lifetime of QD is not affected by dark complex formation, though the steady‐state emission is greatly quenched. However, emissive complex formation causes both, lifetime and steady‐state emission quenching. In our fitting model, consideration of Poisson distribution of the attached quencher (DNT) molecules at QD surface enables a comprehensive fitting to our time resolved data. QD‐DNT complex formation was confirmed by an isothermal titration calorimetry (ITC) study. Fitting to the time resolved data using a stochastic kinetic model shows moderate increase (0.05 ns^?1 to 0.072 ns^?1) of intrinsic quenching rate with increasing the QD particle size (from ≈3.2 nm to ≈5.2 nm). Our fitting also reveals that the number of DNT molecules attached to a single QD increases from ≈0.1–0.2 to ≈1.2–1.7, as the DNT concentration is increased from ≈1 mm to 17.5 mm . Complex formation at higher quencher concentration assures that the observed PET kinetics is a thermodynamically controlled process where solvent diffusion has no role on it.     

The density dependence of hot-electron thermalization in liquid argon,krypton and xenon

The time required for thermalization of hot electrons in liquid argon, krypton and xenon increases with decreasing density to a minimum value of ≈ 7 ns at a density of ≈ 1.2 × 10²² cm^?3. Previous data for the solids are found to lie an extrapolations of the liquid-phase density dependence.     

Towards Electrochemiluminescence Microscopy Exploration of Plasmonic-Mediated Phenomena at the Single-Nanoparticle Level

The rational design of functional plasmonic metasurfaces and metamaterials requires the development of high-throughput characterization techniques compatible with operando conditions and capable of addressing single-nanostructures. In their work, Wei et al. demonstrate the use of electrochemiluminescence microscopy to investigate the mechanism behind plasmon-enhanced luminescence induced by gold nanostructures. The use of gold plasmonic arrays was exploited to achieve the rapid spectroscopic evaluation of all the individual nanostructures, and the correlation of the results with high- resolution electron microscopy analysis, guaranteeing a strict one-to-one correspondence. The authors were able to identify two different mechanisms for the enhancement of [Ru(bpy)₃]²⁺-tri-n-propylamine electrochemiluminescence mediated by single gold nanoparticles and by small plasmonic clusters. In the future, the proposed characterization could be used for the rapid and in situ spectroscopic analysis of more complex plasmonic nanostructures and metasurfaces.     

Conductometric study of complexation reaction of 4′-nitrobenzo-15-crown-5 with Mn+2, Co+2, Y+3, and ZrO+2 cations in acetonitrile,methanol and their binary solvent solutions

The complexation reactions of Mn²⁺, Co²⁺, Y³⁺, and ZrO²⁺ cations with the macrocyclic ligand, 4′-nitrobenz-15-crown-5 (4′-NB15C5), in acetonitrile (AN), methanol and AN-MeOH binary mixtures have been studied at various temperatures using the conductometric method. The conductance data stand for the Me : L stoichiometric ratio 1 : 1. Values of the formation constants of the complexes were accumulated by plotting molar conductivity curves using the computer program, GENPLOT. The order of stability of the metal-ion complexes in pure AN at 15°C was found to be: (4′-NB15C5 · ZrO)²⁺ > (4′-NB15C5 · Mn)²⁺ ≈ (4′-NB15C5 · Co)²⁺ > (4′-NB15C5 · Y)³⁺. In the case of AN-MeOH binary solvent solutions with 50 and 75 mole percent of AN at the same temperature, the sequence of stability of the complexes was the following: (4′-NB15C5 · Mn)²⁺ > (4′-NB15C5 · ZrO)²⁺ ≈ (4′-NB15C5 · Co)²⁺ 〉 (4′-NB15C5 · Y)³⁺, and (4′-NB15C5 · Mn)²⁺ > (4′-NB15C5 · Y)³⁺ ≈ (4′-NB15C5 · Co)²⁺ > (4′-NB15C5 · ZrO)²⁺, respectively. The complexes formed are entropy stabilized in all cases.     

Hot‐Electron‐Transfer Enhancement for the Efficient Energy Conversion of Visible Light

Great strides have been made in enhancing solar energy conversion by utilizing plasmonic nanostructures in semiconductors. However, current generation with plasmonic nanostructures is still somewhat inefficient owing to the ultrafast decay of plasmon‐induced hot electrons. It is now shown that the ultrafast decay of hot electrons across Au nanoparticles can be significantly reduced by strong coupling with CdS quantum dots and by a Schottky junction with perovskite SrTiO₃ nanoparticles. The designed plasmonic nanostructure with three distinct components enables a hot‐electron‐assisted energy cascade for electron transfer, CdS→Au→SrTiO₃, as demonstrated by steady‐state and time‐resolved photoluminescence spectroscopy. Consequently, hot‐electron transfer enabled the efficient production of H₂ from water as well as significant electron harvesting under irradiation with visible light of various wavelengths. These findings provide a new approach for overcoming the low efficiency that is typically associated with plasmonic nanostructures.     

Collisional relaxation of transient vibrational energy distributions in a thermal unimolecular system. The variable encounter method

A method has been developed, called the Variable Encounter Method, for the study of the relaxation of an initial vibrationally cold ensemble of molecules into a vibrationally hot distribution by a known and variable number of successive collisions with a hot wall. The theory of the experiment is presented. The system studied was the isomerization of 1,1-cyclopropane-d₂ with a fused quartz wall temperature of 800 K to 1175 K, and average number of collisions from 2.3 to 22.3. Various modified gaussian and exponential models of energy transfer were found to give agreement with the data. The average down-step size was found to decline from ≤ 3500 cm^?1 at the lowest temperature to ≈ 2500 cm^? at the highest on the basis of a gaussian model. A mathematical analysis of the relation between mean first passage times and incubation times is given. Incubation times increase from ≈ 7 to ≈ 12 collisions with increasing temperature. Transient population distributions and the sequential reaction probabilities as a function of collision number are calculated.     

Interpretation of the optical dephasing time and lineshape function in the S0 → T1 exciton transitions of 1,4-dibromonaphthalene

The lineshape function for the S₀ → T₁ absorption in 1,4-dibromonaphthalene (DBN) is analyzed in terms of exchange theory. It is shown that the dominant optical dephasing mechanism for the electric dipole transition to the k = 0 state in the band results from the absorption and emission of a low energy optic phonon. This process dephases the optical absorption because of frequency differences of the phonon in the ground and excited state. In addition, it is shown how to extract the energy of the phonon responsible for dephasing, the phonon absorption rate, and the lifetime in the phonon promoted state from the data. The analysis of the data for DBN shows that very little dephasing of the optical transition occurs before ≈ 15 K but from 15 K to ≈ 40 K the singlet-triplet transitions to site I (20192 cm^?1) and site II (20245 cm^?1) are dephased by absorption and emission of an ≈ 38 cm^?1 and 45 cm^?1 phonon respectively. The phonon absorption rates by the k = 0 state in the exciton band are similar for both sites being 5 × 10⁶ s^?1 and 3 × 10⁵ s^?1 at 4 K and 7 × 10¹¹ s^?1 and 4 × 10¹¹ s^?1 at 30 K for site I and II respectively. Finally, the lifetimes in the phonon promoted state for sites I and II are 0.23 and 0.28 ps over the range 15–40 K.     

Porous gold nanodisks with multiple internal hot spots

For increasing the number of internal hot spots in the individual plasmonic nanoparticles, porous Au nanostructures were synthesized by a hybrid approach combining a physical process, which defined the overall shapes and dimensions of the nanostructures, and a chemical process, which incorporated nanopores inside the patterned nanostructures. This approach allows us to synthesize lithographically designed Au nanodisks containing numerous internal Raman hot spots in the form of nanopores. The increased number of hot spots successfully improved SERS intensity, and this experimental result was further elucidated by numerical electromagnetic simulations. The highly improved and homogeneous SERS intensities illustrate the great potential of the porous plasmonic nanodisks as a sensitive molecular imaging agent.     

Optical studies of the electronphonon interaction in the pyrene-PMDA complex

Low temperature phosphorescence spectra of pyrene-PMDA (pyromellitic acid dianhydride) imbedded in a naphthalene-PMDA host crystal are reported. The spectra exhibit resolved zero-phonon and multi-phonon structure which is significant since the phosphorescent state is characterized by ≈36% charge-transfer character. Several different phonons contribute to the structure with the dominant phonon having ground and excited state (from hot band analysis) frequencies of 25 and 15 cm^?1. The linear electronphonon coupling strength for the 25cm^?1 phonon is computed. This phonon is tentatively assigned to rotational motion of the rigid complex. Linewidth data yield a relaxation time of 0.4 ps at 2 K for the 25 cm^?1 phonon which is believed to be a pseudolocalized or resonant mode.