Plasmonic Aerogels as a Three-Dimensional Nanoscale Platform for Solar Fuel Photocatalysis

Photoelectrochemical Enzyme Biosensor Based on TiO2 Nanorod/TiO2 Quantum Dot/Polydopamine/Glucose Oxidase Composites with Strong Visible-Light Response

Although photoelectrochemical (PEC) enzyme biosensors based on visible-light detection would have a high practical value, their development has been limited by the weak visible-light response of available photoactive substrates. Here, to enhance the visible-light response of a photoelectric substrate, a TiO₂ nanorods (NRs)/TiO₂ quantum dots (QDs)/polydopamine (PDA)/glucose oxidase nanocomposite was prepared via hydrothermal synthesis, followed by photopolymerization. TiO₂ QDs with strong light absorption and excellent photocatalytic activity were introduced between the TiO₂ NRs and the PDA. An efficient electron transport interface that formed as a result of the combination of the TiO₂ NRs, TiO₂ QDs, and the PDA could not only transfer electrons quickly and orderly, but also substantially improve the response of the TiO₂ NRs under visible light. Through a series glucose detection, a sensor based on the nanocomposite was found to exhibit superior sensing performance under visible light with a sensitivity of 4.63 μA mM^-1 cm^-2, a linear response over the concentration 0.1-4 mM, and a detection limit of 8.16 μM. This work proposes a biosensor that can detect under visible light, thereby expanding the application range of PEC enzyme biosensors.

3D Printed Scaffolds for Monolithic Aerogel Photocatalysts with Complex Geometries

Monolithic aerogels composed of crystalline nanoparticles enable photocatalysis in three dimensions, but they suffer from low mechanical stability and it is difficult to produce them with complex geometries. Here, an approach to control the geometry of the photocatalysts to optimize their photocatalytic performance by introducing carefully designed 3D printed polymeric scaffolds into the aerogel monoliths is reported. This allows to systematically study and improve fundamental parameters in gas phase photocatalysis, such as the gas flow through and the ultraviolet light penetration into the aerogel and to customize its geometric shape to a continuous gas flow reactor. Using photocatalytic methanol reforming as a model reaction, it is shown that the optimization of these parameters leads to an increase of the hydrogen production rate by a factor of three from 400 to 1200 µmol g^-1 h^-1 . The rigid scaffolds also enhance the mechanical stability of the aerogels, lowering the number of rejects during synthesis and facilitating handling during operation. The combination of nanoparticle-based aerogels with 3D printed polymeric scaffolds opens up new opportunities to tailor the geometry of the photocatalysts for the photocatalytic reaction and for the reactor to maximize overall performance without necessarily changing the material composition.

Photoenhanced Degradation of Sarin at Cu/TiO2 Composite Aerogels: Roles of Bandgap Excitation and Surface Plasmon Excitation

Multifunctional composites that couple high-capacity adsorbents with catalytic nanoparticles (NPs) offer a promising route toward the degradation of organophosphorus pollutants or chemical warfare agents (CWAs). We couple mesoporous TiO₂ aerogels with plasmonic Cu nanoparticles (Cu/TiO₂) and characterize the degradation of the organophosphorus CWA sarin under both dark and illuminated conditions. Cu/TiO₂ aerogels combine high dark degradation rates, which are facilitated by hydrolytically active sites at the Cu||TiO₂ interface, with photoenhanced degradation courtesy of semiconducting TiO₂ and the surface plasmon resonance (SPR) of the Cu nanoparticles. The TiO₂ aerogel provides a high surface area for sarin binding (155 m² g^-1), while the addition of Cu NPs increases the abundance of hydrolytically active OH sites. Degradation is accelerated on TiO₂ and Cu/TiO₂ aerogels with O₂. Under broadband illumination, which excites the TiO₂ bandgap and the Cu SPR, sarin degradation accelerates, and the products are more fully mineralized compared to those of the dark reaction. With O₂ and broadband illumination, oxidation products are observed on the Cu/TiO₂ aerogels as the hydrolysis products subsequently oxidize. In contrast, the photodegradation of sarin on TiO₂ is limited by its slow initial hydrolysis, which limits the subsequent photooxidation. Accelerated hydrolysis occurs on Cu/TiO₂ aerogels under visible illumination (>480 nm) that excites the Cu SPR but not the TiO₂ bandgap, confirming that the Cu SPR excitation contributes to the broadband-driven activity. The high hydrolytic activity of the Cu/TiO₂ aerogels combined with the photoactivity upon TiO₂ bandgap excitation and Cu SPR excitation is a potent combination of hydrolysis and oxidation that enables the substantial chemical degradation of organophorphorus compounds.

Plasmonic gold-cellulose nanofiber aerogels

Assembly of plasmonic nanomaterials into a low refractive index medium, such as an aerogel, holds a great promise for optical metamaterials, optical sensors, and photothermal energy converters. However, conventional plasmonic aerogels are opaque and optically isotropic composites, impeding them from being used as low-loss or polarization-dependent optical materials. Here we demonstrate a plasmonic-cellulose nanofiber composite aerogel that comprises of well-dispersed gold nanorods within a cellulose nanofiber network. The cellulose aerogel host is highly transparent owing to the small scattering cross-section of the nanofibers and forms a nematic liquid crystalline medium with strong optical birefringence. We find that the longitudinal surface plasmon resonance peak of gold nanorods shows a dramatic shift when probed for the cellulose aerogel compared with the wet gels. Simulations reveal the shift of surface plasmon resonance peak with gel drying can be attributed to the change of the effective refractive index of the gels. This composite material may provide a platform for three- dimensional plasmonic devices ranging from optical sensors to metamaterials.

Power of Aerogel Platforms to Explore Mesoscale Transport in Catalysis

We describe the opportunity to deploy aerogels-an ultraporous nanoarchitecture with co-continuous networks of meso/macropores and covalently bonded nanoparticulates-as a platform to address the nature of the electronic, ionic, and mass transport that underlies catalytic activity. As a test case, we fabricated Au||TiO₂ junctions in composite guest-host aerogels in which ∼5 nm Au nanoparticles are incorporated either directly into the anatase TiO₂ network (Au "in" TiO₂, Au_IN-TiO₂ aerogel) or deposited onto preformed TiO₂ aerogel (Au "on" TiO₂, Au_ON/TiO₂ aerogel). The metal-meets-oxide nanoscale interphase as visualized by electron tomography feature extended three-dimensional (3D) interfaces, but Au_IN-TiO₂ aerogels impose a greater degree of Au contact with TiO₂ particles than does the Au_ON/TiO₂ form. Both aerogel variants enable transport of electrons over micrometer-scale distances across the TiO₂ network to Au||TiO₂ junctions, as evidenced by electron paramagnetic resonance (EPR) and ultrafast visible pump-IR probe time-resolved absorption spectroscopy. The siting of gold nanoparticles in the TiO₂ network more effectively disperses trapped electrons. Density functional theory (DFT) calculations find that increased physical contact between Au and TiO₂, induced by oxygen vacancies, produces increased hybridization of midgap states and quenches unpaired trapped electrons. We assign the apparent differences in electron-transport capabilities to a combination of the relatively better-wired Au||TiO₂ junctions in Au_IN-TiO₂ aerogels, which have a greater capacity to dilute accumulated charge over a larger interfacial surface area, with an enhanced ability to discharge the accumulated electrons via catalytic reduction of adsorbed O₂ to O^2- at the interface. Solid-state ¹H nuclear magnetic resonance experiments show that proton spin-lattice relaxation times and possibly proton diffusion are strongly coupled to Au||TiO₂ interfacial design, likely through spin coupling of protons to unpaired electrons trapped at the TiO₂ network. Taken together, our results show that Au||TiO₂ interfacial design strongly impacts charge carrier (electron and proton) transport over mesoscale distances in catalytic aerogel architectures.

Universal Gelation of Metal Oxide Nanocrystals via Depletion Attractions

Nanocrystal gelation provides a powerful framework to translate nanoscale properties into bulk materials and to engineer emergent properties through the assembled microstructure. However, many established gelation strategies rely on chemical reactions and specific interactions, e.g., stabilizing ligands or ions on the nanocrystals' surfaces, and are therefore not easily transferable. Here, we report a general gelation strategy via nonspecific and purely entropic depletion attractions applied to three types of metal oxide nanocrystals. The gelation thresholds of two compositionally distinct spherical nanocrystals agree quantitatively, demonstrating the adaptability of the approach for different chemistries. Consistent with theoretical phase behavior predictions, nanocrystal cubes form gels at a lower polymer concentration than nanocrystal spheres, allowing shape to serve as a handle to control gelation. These results suggest that the fundamental underpinnings of depletion-driven assembly, traditionally associated with larger colloidal particles, are also applicable at the nanoscale.

Plasmon-Induced Charge Transfer: Challenges and Outlook

The decay of a surface plasmon, a collective electron oscillation at the surface of a metal, can generate hot charge carriers that may be transferred to an adjacent semiconductor. This plasmon-induced charge transfer process can be used to enhance photocatalysis, to create photodetectors, or to drive selective photochemistry. However, the charge transfer efficiency in many fabricated devices remains too low for practical applications, typically <1%. In this Perspective, I discuss critical aspects of designing plasmonic systems for improved performance and highlight important findings for maximizing the transfer efficiency. In particular, I draw attention to the article by Ma and Gao in this issue of ACS Nano that describes using real-time time-dependent density functional theory to give a detailed and informative look at the charge transfer dynamics at a TiO₂-Ag nanocluster interface.

In Situ Synthesis of Mesoporous TiO2 Nanofibers Surface-Decorated with AuAg Alloy Nanoparticles Anchored by Heterojunction Exhibiting Enhanced Solar Active Photocatalysis

We designed an electrospinning synthesis protocol to obtain in situ, the mesoporous TiO₂ nanofibers, which are surface-decorated with plasmonic AuAg nanoparticles (AuAg-mTNF-H). Such alloy nanoparticles are found to be partially exposed on the surface of the nanofibers. Characterization by HRTEM and EDS confirmed the formation of 1:1 AuAg alloy nanoparticles on the surface of TiO₂ nanofibers with heterojunction at the interfaces. On the basis of electron microscopic characterization, we proposed that, during the formation of the nanofibers, the incorporated metal ions with surface capping of negative charges migrated toward the outer surface of the nascent fibers under the influence of high positive voltage required for electrospinning. As a result, after the subsequent thermal treatment, the crystallization of TiO₂ nanofibers and the formation of alloy nanoparticles took place, leading to the formation of a deep heterojunction through partial embedment of the nanoparticles. The formation of AuAg alloy also restricted the oxidation of Ag, thus making the nanoparticles highly stable in ambient condition. Accordingly, such unique AuAg-mTNF-H photocatalyst shows strong light absorption property covering the entire range of visible wavelengths with stability. The solar light harvesting property of AuAg-mTNF-H was verified by monitoring solar light induced H₂ evolution via water splitting and photodecomposition of MB. In both the cases AuAg-mTNF-H showed excellent H₂ evolution and photodecomposition of dye.

Electroactive Area from Porous Oxide Films Loaded with Silver Nanoparticles: Electrochemical and Electron Tomography Observations

Electrochemical studies of nanomaterial-based electrodes have been widely developed for catalyst and energy-harvesting applications. The evolution of these electrodes over time and their efficiency have been extensively studied and analyzed in order to optimize their performance. However, the electrochemical responses of electrodes are rarely studied in terms of the position of the active species within these electrodes. In this paper, we highlight that the spatial location of silver nanoparticles (NPs) embedded inside semiconductive porous films, TiO₂ or Fe₂O₃, is crucial for the electrochemical response. In fact, by using cycling voltammetry and electron tomography experiments, we show the existence of an "electroactive area", corresponding to a reduced thickness of the sample in close vicinity to a fluorine-doped tin oxide substrate where most of the electrochemical responses originate. Our results demonstrate that, for a film thickness of several hundred nanometers, only less than 30 nm close to the substrate responds electrochemically. However, cyclic voltammetry empties the electroactive area of silver NPs. Therefore, application of chronoamperometry coupled to irradiation allowed regeneration of this area thanks to an increased diffusion of silver species. In this paper, we also show the significant diffusion of silver species within the film during electrochemical experiments, a phenomenon even increased by irradiation. These results are therefore an important step that shows the importance of the localization of active species within a porous film and help in understanding and increasing the durability of nanomaterial-based electrodes.

Chiral Plasmonic Nanocrystals for Generation of Hot Electrons: Toward Polarization-Sensitive Photochemistry

The use of biomaterials, with techniques such as DNA-directed assembly or biodirected synthesis, can surpass top-down fabrication techniques in creating plasmonic superstructures in terms of spatial resolution, range of functionality, and fabrication speed. In particular, by enabling a very precise placement of nanoparticles in a bioassembled complex or through the controlled biodirected shaping of single nanoparticles, plasmonic nanocrystals can show remarkably strong circular dichroism (CD) signals. We show that chiral bioplasmonic assemblies and single nanocrystals can enable polarization-sensitive photochemistry based on the generation of energetic (hot) electrons. It is now established that hot plasmonic electrons can induce surface photochemistry or even reshape plasmonic nanocrystals. We show that merging chiral plasmonic nanocrystal systems and the hot-election generation effect offers unique possibilities in photochemistry, such as polarization-sensitive photochemistry promoting nonchiral molecular reactions, chiral photoinduced growth of a colloid at the atomic level, and chiral photochemical destruction of chiral nanocrystals. In contrast, for chiral molecular systems, the equivalent of the described effects is challenging to observe because molecular species typically exhibit very small CD signals. Moreover, we compare our findings with traditional chiral photochemistry at the molecular level, identifying new, different regimes for chiral photochemistry with possibilities that are unique for plasmonic colloidal systems. In this study, we bring together the concept of hot-electron generation and the field of chiral colloidal plasmonics. Using chiral plasmonic nanorod complexes as a model system, we demonstrate remarkably strong CD in both optical extinction and generation rates of hot electrons. Studying the regime of steady-state excitation, we discuss the influence of geometrical and material parameters on the chiral effects involved in the generation of hot electrons. Optical chirality and the chiral hot-electron response in the nanorod dimers result from complex interparticle interactions, which can appear in the weak coupling regime or in the form of Rabi splitting. Regarding practical applications, our study suggests interesting opportunities in polarization-sensitive photochemistry, in chiral recognition or separation, and in promoting chiral crystal growth at the nanoscale.