Photocatalytic Surface Restructuring in Individual Silver Nanoparticles

Photocatalytic Degradation, Anticancer, and Antibacterial Studies of Lysinibacillus sphaericus Biosynthesized Hybrid Metal/Semiconductor Nanocomposites

The biological synthesis of nanocomposites has become cost-effective and environmentally friendly and can achieve sustainability with high efficiency. Recently, the biological synthesis of semiconductor and metal-doped semiconductor nanocomposites with enhanced photocatalytic degradation efficiency, anticancer, and antibacterial properties has attracted considerable attention. To this end, for the first time, we biosynthesized zinc oxide (ZnO) and silver/ZnO nanocomposites (Ag/ZnO NCs) as semiconductor and metal-doped semiconductor nanocomposites, respectively, using the cell-free filtrate (CFF) of the bacterium Lysinibacillus sphaericus. The biosynthesized ZnO and Ag/ZnO NCs were characterized by various techniques, such as ultraviolet-visible spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, and photoluminescence spectroscopy. The photocatalytic degradation potential of these semiconductor NPs and metal-semiconductor NCs was evaluated against thiazine dye, methylene blue (MB) degradation, under simulated solar irradiation. Ag/ZnO showed 90.4 ± 0.46% photocatalytic degradation of MB, compared to 38.18 ± 0.15% by ZnO in 120 min. The cytotoxicity of ZnO and Ag/ZnO on human cervical HeLa cancer cells was determined using an MTT assay. Both nanomaterials exhibited cytotoxicity in a concentration- and time-dependent manner on HeLa cells. The antibacterial activity was also determined against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus). Compared to ZnO, Ag/ZnO NCs showed higher antibacterial activity. Hence, the biosynthesis of semiconductor nanoparticles could be a promising strategy for developing hybrid metal/semiconductor nanomaterials for different biomedical and environmental applications.

Potential-Dependent Temporal Dynamics of CO Surface Concentration in Electrocatalytic CO2 Reduction

Beyond the identity and structure of an intermediate, changes in its concentration on and near the electrode surface with time are a critical component to understand and improve selectivity and reactivity in electrochemical transformations. We apply pulsed-potential electrochemical Raman scattering microscopy to measure the potential-dependent temporal evolution of CO formed during electrocatalytic CO₂ reduction in acetonitrile on Ag electrodes. At driving potentials positive of the onset potential as determined by cyclic voltammetry, CO accumulates on the electrode surface at time scales longer than 1 s. Near the ensemble onset potential, CO resides on the electrode surface for approximately 100 ms. At potentials known to evolve CO from the electrode surface, CO remains adsorbed on the electrode for less than 10 ms. The time scales accessible in our strategy are nearly 3 orders of magnitude faster than transient Raman or infrared measurements, allowing direct measurement of the temporal evolution of intermediates.

In situ and Operando Spectroscopies in Photocatalysis: Powerful Techniques for a Better Understanding of the Performance and the Reaction Mechanism

In photocatalysis, a set of elemental steps are involved together at different timescales to govern the overall efficiency of the process. These steps are divided as follow: (1) photon absorption and excitation (in femtoseconds), (2) charge separation (femto- to picoseconds), (3) charge carrier diffusion/transport (nano- to microseconds), and (4 and 5) reactant activation/conversion and mass transfer (micro- to milliseconds). The identification and quantification of these steps, using the appropriate tool/technique, can provide the guidelines to emphasize the most influential key parameter that improve the overall efficiency and to develop the "photocatalyst by design" concept. In this review, the identification/quantification of reactant activation/conversion and mass transfer (steps 4 and 5) is discussed in details using the in situ/operando techniques, especially the infrared (IR), Raman, and X-ray absorption spectroscopy (XAS). The use of these techniques in photocatalysis was highlighted by the most recent and conclusive case studies which allow a better characterization of the active site and reveal the reaction pathways in order to establish a structure-performance relationship. In each case study, the reaction conditions and the reactor design for photocatalysis (pressure, temperature, concentration, etc.) were thoroughly discussed. In the last part, some examples in the use of time-resolved techniques (time-resolved FTIR, photoluminescence, and transient absorption) are also presented as an author's guideline to study the elemental steps in photocatalysis at shorter timescale (ps, ns, and µs).

Complete surface reconstruction of nanoporous gold during CH4 pyrolysis

The catalytic activity and selectivity of metallic nanocatalysts can be controlled using physical and chemical methods to tune the exposed crystal facets. Nanoporous metals (NPMs) have unique bicontinuous structures, large specific surface areas, and high catalytic activities, and are widely used in the field of heterogeneous catalysis. However, owing to the complex surface topography of NPMs, it is difficult to regulate their exposed crystal facets over a large area. In this study, nanoporous gold (NPG) is successfully prepared with a complete regular surface that exposes the Au {111} and {100} facets through a methane pyrolysis reaction. The results of high-spatial and -temporal resolution in situ experiments and theoretical calculations indicate that C species significantly weaken the interaction between surface Au atoms with low coordination numbers and their surrounding atoms, which results in the migration and recombination of surface atoms. This research fundamentally clarifies the reconstruction mechanism of porous materials during methane pyrolysis and provides a theoretical basis for the targeted regulation of exposed NPM surfaces.

Accelerating photo-thermal CO2 reduction to CO, CH4 or methanol over metal/oxide semiconductor catalysts

Photo-thermal reduction of atmospheric carbon dioxide into methane, methanol, and carbon monoxide under mild conditions over suitable (photo)catalysts is a feasible pathway for the production of fuels and platform chemicals with minimal involvement of fossil fuels. In this perspective, we showcase transition metal nanoparticles (Ni, Cu, and Ru) dispersed over oxide semiconductors and their ability to act as photo catalysts in reverse water gas shift reaction (RWGS), methane dry reforming, methanol synthesis, and Sabatier reactions. By using a combination of light and thermal energy for activation, reactions can be sustained at much lower temperatures compared to thermally driven reactions and light can be used to leverage reaction selectivity between methanol, methane, and CO. In addition to influencing the reaction mechanism and decreasing the apparent activation energies, accelerating reaction rates and boosting selectivity beyond thermodynamic limitations is possible. We also provide future directions for research to advance the current state of the art in photo-thermal CO₂ conversion.

Synthesis of silver and gold nanoparticles-enzyme-polymer conjugate hybrids as dual-activity catalysts for chemoenzymatic cascade reactions

Novel hybrids containing silver or gold nanoparticles have been synthesized in aqueous media and at room temperature using enzymes or tailor-made enzyme-polymer conjugates, which directly induced the formation of inorganic silver or gold species. The choice of pH, protein, or bioconjugate strongly affected the final metallic nanoparticles hybrid formation. Using Candida antarctica lipase (CALB) in a solution, nanobiohybrids containing Ag₂O nanoparticles of 9 nm average diameter were obtained. The use of tailor-made bioconjugates, for example, the CALB modified with dextran-aspartic acid polymer (Dext6kDa), resulted in a nanobiohybrid containing smaller Ag(0)/Ag₂O nanoparticles. In the case of nanobiohybrids based on gold, Au(0) species were found in all cases. The Au-CALB hybrid contained spherical nanoparticles with 18 nm average diameter size, with a minor range of larger ones (>100 nm) while the AuNPs-CALB-Dext6kDa hybrid was formed by much smaller nanoparticles (9 nm, minor range of 22 nm), and also nanorods of 20-30/40-50 nm length. Using Thermomyces lanuginosus lipase (TLL), apart from the nanoparticle formation, nanoflowers with a diameter range of 100-200 nm were obtained. All nanobiohybrids maintained (dual) enzymatic and metallic activities. For instance, these nanobiohybrids exhibited exquisite dual-activity for hydrolysis/cycloisomerization cascades starting from allenic acetates. By merging the transition metal reactivity with the inherent lipase catalysis, allenic acetates directly converted to the corresponding O-heterocycles in enantiopure form catalysed by AgNPs-CALB-Dext6kDa, taking advantage of a kinetic resolution/cyclization pathway. These results showed the high applicability of these novel hybrids, offering new opportunities for the design of novel reaction cascades.

Silver nanomaterials: synthesis and (electro/photo) catalytic applications

In view of their unique characteristics and properties, silver nanomaterials (Ag NMs) have been used not only in the field of nanomedicine but also for diverse advanced catalytic technologies. In this comprehensive review, light is shed on general synthetic approaches encompassing chemical reduction, sonochemical, microwave, and thermal treatment among the preparative methods for the syntheses of Ag-based NMs and their catalytic applications. Additionally, some of the latest innovative approaches such as continuous flow integrated with MW and other benign approaches have been emphasized that ultimately pave the way for sustainability. Moreover, the potential applications of emerging Ag NMs, including sub nanomaterials and single atoms, in the field of liquid-phase catalysis, photocatalysis, and electrocatalysis as well as a positive role of Ag NMs in catalytic reactions are meticulously summarized. The scientific interest in the synthesis and applications of Ag NMs lies in the integrated benefits of their catalytic activity, selectivity, stability, and recovery. Therefore, the rise and journey of Ag NM-based catalysts will inspire a new generation of chemists to tailor and design robust catalysts that can effectively tackle major environmental challenges and help to replace noble metals in advanced catalytic applications. This overview concludes by providing future perspectives on the research into Ag NMs in the arena of electrocatalysis and photocatalysis.

Decoding Chemical and Physical Processes Driving Plasmonic Photocatalysis Using Surface-Enhanced Raman Spectroscopies

In order to mitigate the advancing effects of environmental pollution and climate change, immediate action is needed on social, political, and industrial fronts. One segment of industry that contributes significantly to this current crisis is bulk chemical production, where fossil fuels are primarily used to drive reactions at high temperatures and pressures. Toward mitigating the environmental impact of these processes, solar energy has shown promise as a clean and renewable alternative for the photocatalytic synthesis of chemicals. In recent decades, plasmonic materials have emerged as candidates for making this a reality. Because of their unique and tunable interactions with light, plasmonic materials can be used to create energy-rich nanoscale environments. In fact, there is a growing library of chemical reactions that can utilize this plasmonic energy to drive industrially relevant chemistries under standard ambient conditions. However, the efficiency of these reactions is typically low, and a lack of mechanistic understanding of how energy is transferred from plasmons to molecules hinders reaction optimization for use on large scales.To decode the complex chemical and physical processes involved in plasmon-driven photocatalytic reactions, we use surface-enhanced Raman spectroscopy (SERS). In this Account, we detail SERS techniques that we have used and are developing to study molecular transformations, charge transfer, and plasmonic heating in dynamic plasmon-molecule systems on time scales ranging from seconds to femtoseconds. SERS is an ideal analytical tool for understanding plasmon-molecule interactions, as it gives highly specific information about molecular vibrations with high sensitivity, down to the single-molecule level. Importantly, SERS allows for simultaneous pumping of a plasmonic resonance and probing of the enhanced Raman signal from nearby molecules. We have already used these techniques to study a plasmon-driven methyl migration with nanoscale spatial specificity and to understand the charge transfer mechanism and role of heating in the plasmon-mediated dimerization of 4-nitrobenzenethiol. Importantly, from this work we conclude that direct charge transfer, not heating, may play a significant role in driving many plasmon-driven reactions. Despite these recent insights, more work is needed in order to obtain a comprehensive understanding of the broad range of chemistries accessible in plasmon-molecule systems. In the future, our continued development of these SERS-based techniques shows promise in answering questions regarding direct charge transfer, resonance energy transfer, and excitation conditions in plasmon-mediated chemistries.