Abstract: We introduce a rapid method to test the photocatalytic activity of singlet‐oxygen‐producing photosensitizers using a batch cell, a LED laser and a conventional potentiostat. The strategy is based on coupling of photo‐oxidation of hydroquinone and simultaneous electrochemical reduction of its oxidized form at a carbon electrode in an organic solvent (methanol). This scheme gives an immediate response and avoids complications related to long‐term experiments such as oxidative photo‐degradation of photosensitizers and singlet oxygen traps by reactive oxygen species (ROS). Among the tested compounds, a fluoro‐substituted subphthalocyanine showed the highest photocurrent and singlet oxygen quantum yield (ΦΔ) in comparison to phenoxy‐ and tert‐butyl‐substituted analogues, whereas the lowest photocurrents and yields were observed for aggregated and dimeric phthalocyanine complexes. The method is useful for fast screening of the photosensitizing activity and represents the first example of one‐pot coupling of electrochemical and photocatalytic reactions in organic media.

Abstract: (Sun)Light is an abundantly available sustainable source of energy that has been used in catalyzing chemical reactions for several decades now. In particular, studies related to the interaction of light with plasmonic nanostructures have been receiving increased attention. These structures display the unique property of localized surface plasmon resonance, which converts light of a specific wavelength range into hot charge carriers, along with strong local electromagnetic fields, and/or heat, which may all enhance the reaction efficiency in their own way. These unique properties of plasmonic nanoparticles can be conveniently tuned by varying the metal type, size, shape, and dielectric environment, thus prompting a research focus on rationally designed plasmonic hybrid nanostructures. In this review, the term “hybrid” implies nanomaterials that consist of multiple plasmonic or non-plasmonic materials, forming complex configurations in the geometry and/or at the atomic level. We discuss the synthetic techniques and evolution of such hybrid plasmonic nanostructures giving rise to a wide variety of material and geometric configurations. Bimetallic alloys, which result in a new set of opto-physical parameters, are compared with core–shell configurations. For the latter, the use of metal, semiconductor, and polymer shells is reviewed. Also, more complex structures such as Janus and antenna reactor composites are discussed. This review further summarizes the studies exploiting plasmonic hybrids to elucidate the plasmonic-photocatalytic mechanism. Finally, we review the implementation of these plasmonic hybrids in different photocatalytic application domains such as H2 generation, CO2 reduction, water purification, air purification, and disinfection.

Abstract: Methane (CH4) clathrate hydrates have gained much attention in the ever-growing search for novel energy storage methods; however, they are currently limited due to their poor water-to-hydrate conversions and slow formation kinetics. To surmount these bottlenecks, significant research has been centered on the design of novel methods (porous media). In this vein, the present work explores two hydrophobic mesoporous solids, an alkyl-grafted mesoporous silica (SBA-15 C8) and a periodic mesoporous organosilica (Ring-PMO), in their ability to promote CH4 clathrates. Both materials have shown to facilitate CH4 clathrate formation at mild operating conditions (6 MPa and 269–276 K). The study revealed that the maximal CH4 storage capacities are strongly linked to the critical/optimal quantity of water in the system which was determined to be at 130% and 200% of the pore volume for SBA-15 C8 and Ring-PMO, respectively. Up to 90% and 95% of the maximum water-to-hydrate conversions were achieved in 90 min at the lowest experimental temperature and critical water content for SBA-15 C8 and Ring-PMO, respectively. At these conditions, SBA-15 C8 and Ring-PMO showed a maximum gas uptake of 98.2 and 101.2 mmol CH4/mol H2O, respectively. Both the materials exhibited no chemical or morphological changes post-clathrate formations (characterized using FT-IR, N2 sorption, XRD, and TEM), inferring their viability as clathrate promoters for multiple cycles. An integrated multistep model was considered adequate for representing the hydrate crystallization kinetics and fits well with the experimental kinetic data with a low average absolute deviation in water-to-hydrate conversions among the three distinct kinetic models analyzed. Overall, the results from this study demonstrate hydrophobic porous materials as effective promoters of CH4 clathrates, which could make clathrate-based CH4 storage and transport technology industrially viable.

Abstract: In the recent advances towards more sustainable global energy supply, H2 is a possible alternative for large scale energy storage. In this view, Liquid Organic Hydrogen Carriers (LOHC) are a class of molecules that allow for easier long term energy storage compared to conventional H2 technologies. CFD simulations were used to showcase the hydrodynamics of the dehydrogenation of a LOHC in a new reactor unit, via a cold flow mock-up study. This reactor was designed to allow for a swirling motion of the liquid carrier material, favouring the removal of H2 gas from the flow and forcing the equilibrium of the reaction towards dehydrogenation, as well as to keep the catalyst particles in motion. The CFD simulations were validated qualitatively with experimental operation of the reactor, in a system with identical dimensionless numbers (Reynolds and Stokes), in order to use less costly products during the prototyping phase.

Abstract: Hydrogen (H2) storage, in the form of clathrate hydrates, has emerged as an attractive alternative to classical storage methods like compression or liquefaction. Nevertheless, the sluggish enclathration kinetics along with low gas storage capacities in bulk systems is currently impeding the progress of this technology. To this end, unstirred systems coupled with porous materials have been shown to tackle the aforementioned drawbacks. In line with this approach, the present study explores the use of hydrophobic periodic organosilica nanoparticles, later denoted as hollow ring-PMO (HRPMO), for H2 storage as clathrate hydrates under mild operating conditions (5.56 mol% THF, 7 MPa, and 265–273 K). The surface of the HRPMO nanoparticles was carefully decorated/functionalized with THF-like moieties, which are well-known promoter agents in clathrate formation when applied in classical, homogeneous systems. The study showed that, while the non-functionalized HRPMO can facilitate the formation of binary H2-THF clathrates, the incorporation of surface-bound promotor structures enhances this process. More intriguingly, tuning the concentration of these surface-bound promotor agents on the HRPMO led to a notable effect on solid-state H2 storage capacities. An increase of 3% in H2 storage capacity, equivalent to 0.26 wt%, along with a substantial increase of up to 28% in clathrate growth kinetics, was observed when an optimal loading of 0.14 mmol g−1 of promoter agent was integrated into the HRPMO framework. Overall, the findings from this study highlight that such tuning effects in the solid-state have the potential to significantly boost hydrate formation/growth kinetics and H2 storage capacities, thereby opening new avenues for the ongoing development of H2 clathrates in industrial applications.

Abstract: Photocathodes play a vital role in photoelectrocatalytic water splitting by acting as catalysts for reducing protons to hydrogen gas when exposed to light. Recent advancements in photocathodes have focused on addressing the limitations of noble metal-based materials. These noble metal-based photocathodes rely on expensive and scarce metals such as platinum and gold as cocatalysts or ohmic back contacts, respectively, rendering the final system less sustainable and costly when applied at scale. This mini-review summarizes the important recent progress in the development of non-noble metal-based photocathodes and their performance in the hydrogen evolution reaction during photoelectrochemical (PEC) water splitting. These advancements bring non-noble metal-based photocathodes closer to their noble metal-based counterparts in terms of performance, thereby paving the way forward toward industrial-scale photoelectrolyzers or PEC cells for green hydrogen production.

Abstract: Introducing defects in TiO2-based photocatalytic materials is a promising strategy for improving light-driven CO2 reduction. However, defects such as oxygen vacancies are generally unstable. As a solution and to further enhance the photocatalytic activity, metal doping has been applied. This mini review aims to summarize recent progress in this particular field. Herein, we have classified metal-doped architectures into three different categories: single metal doping, alloy- and co-doping, and doping of morphologically nanoengineered TiO2−x substrates. The direct relationship between specific metals and product selectivity remains complex, as selectivity can vary significantly among seemingly similar materials. However, numerous methods do show promise in fine-tuning selectivity towards either CO or CH4. In terms of photocatalytic turnover, remarkable yields have been reported in isolated reports, but insufficient experimental data and divergent reaction conditions hamper a true comparison. This puts an emphasis on the need for standardized activity testing.

Abstract: Direct photocatalytic reduction of CO2 has become an highly active field of research. It is thus of utmost importance to maintain an overview of the various materials used to sustain this process, find common trends, and, in this way, eventually improve the current conversions and selectivities. In particular, CO2 photoreduction using plasmonic photocatalysts under solar light has gained tremendous attention, and a wide variety of materials has been developed to reduce CO2 towards more practical gases or liquid fuels (CH4, CO, CH3OH/CH3CH2OH) in this manner. This Review therefore aims at providing insights in current developments of photocatalysts consisting of only plasmonic nanoparticles and semiconductor materials. By classifying recent studies based on product selectivity, this Review aims to unravel common trends that can provide effective information on ways to improve the photoreduction yield or possible means to shift the selectivity towards desired products, thus generating new ideas for the way forward.

Abstract: Cluster beam deposition is employed for fabricating well-defined bimetallic plasmonic photocatalysts to enhance their activity while facilitating a more fundamental understanding of their properties. AuxAg1-x clusters with compositions (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1) spanning the metals' miscibility range were produced in the gas-phase and soft-landed on TiO2 P25-coated silicon wafers with an optimal coverage of 4 atomic monolayer equivalents. Electron microscopy images show that at this coverage most clusters remain well dispersed whereas EXAFS data are in agreement with the finding that the deposited clusters have an average size of ca. 5 nm and feature the same composition as the ablated alloy targets. A composition-dependant electron transfer from Au to Ag that is likely to impart chemical stability to the bimetallic clusters and protect Ag atoms against oxidation is additionally evidenced by XPS and XANES. Under simulated solar light, AuxAg1-x clusters show a remarkable composition-dependent volcano-type enhancement of their photocatalytic activity towards degradation of stearic acid, a model compound for organic fouling on surfaces. The Formal Quantum Efficiency (FQE) is peaking at the Au0.3Ag0.7 composition with a value that is twice as high as that of the pristine TiO2 P25 under solar simulator. Under UV the FQE of all compositions remains similar to that of pristine TiO2. A classical electromagnetic simulation study confirms that among all compositions Au0.3Ag0.7 features the largest near-field enhancement in the wavelength range of maximal solar light intensity, as well as sufficient individual photon energy resulting in a better photocatalytic self-cleaning activity. This allows ascribing the mechanism for photocatalysis mostly to the plasmonic effect of the bimetallic clusters through direct electron injection and near-field enhancement from the resonant cluster towards the conduction band of TiO2. These results not only demonstrate the added value of using well-defined bimetallic nanocatalysts to enhance their photocatalytic activity but also highlights the potential of the cluster beam deposition to design tailored noble metal modified photocatalytic surfaces with controlled compositions and sizes without involving potentially hazardous chemical agents.

Abstract: The major mechanism responsible for plasmonic enhancement of titanium dioxide photocatalysis using gold nanoparticles is still under contention. This work introduces an experimental strategy to disentangle the significance of the charge transfer and near-field mechanisms in plasmonic photocatalysis. By controlling the thickness and conductive nature of a nanoparticle shell that acts as a spacer layer separating the plasmonic metal core from the TiO2 surface, field enhancement or charge transfer effects can be selectively repressed or evoked. Layer-by-layer and in situ polymerization methods are used to synthesize gold core–polymer shell nanoparticles with shell thickness control up to the sub-nanometer level. Detailed optical and electrical characterization supported by near-field simulation models corroborate the trends in photocatalytic activity of the different systems. This approach mainly points at an important contribution of the enhanced near field.

Abstract: Supported carbon nanosheets and carbon nanotubes served as sacrificial templates for preparing spacious TiO2 photocatalytic thin films. Amorphous TiO2 was deposited conformally on the carbonaceous template material by atomic layer deposition (ALD). Upon calcination at 550 °C, the carbon template was oxidatively removed and the as-deposited continuous amorphous TiO2 layers transformed into interlinked anatase nanoparticles with an overall morphology commensurate to the original template structure. The effect of type of template, number of ALD cycles and gas residence time of pollutant on the photocatalytic activity, as well as the stability of the photocatalytic performance of these thin films was investigated. The TiO2 films exhibited excellent photocatalytic activity toward photocatalytic degradation of acetaldehyde in air as a model reaction for photocatalytic indoor air pollution abatement. Optimized films outperformed a reference film of commercial PC500.

Abstract: We propose the concept of a ‘rainbow’ photocatalyst that consists of TiO2 modified with gold-silver alloy nanoparticles of various sizes and compositions, resulting in a broad plasmon absorption band that covers the entire UV–vis range of the solar spectrum. It is demonstrated that this plasmonic ‘rainbow’ photocatalyst is 16% more effective than TiO2 P25 under both simulated and real solar light for pollutant degradation at the solid-gas interface. With this we provide a promising strategy to maximize the spectral response for solar to chemical energy conversion.

Abstract: Transparent photocatalytic TiO2 thin films hold great potential in the development of self-cleaning glass sur-
faces, but suffer from a poor visible light response that hinders the application under actual sunlight. To alleviate this problem, the photocatalytic film can be modified with plasmonic nanoparticles that interact very effectively with visible light. Since the plasmonic effect is strongly concentrated in the near surroundings of the nano- particle surface, an approach is presented to embed the plasmonic nanostructures in the TiO2 matrix itself, rather than deposit them loosely on the surface. This way the interaction interface is maximised and the plasmonic effect can be fully exploited. In this study, pre-fabricated gold nanoparticles are made compatible with the organic medium of a TiO2 sol-gel coating suspension, resulting in a one-pot coating suspension. After spin coating, homogeneous, smooth, highly transparent and photoactive gold-embedded anatase thin films are ob- tained.

Abstract: Research has shown the potential of photofuel cells (PFCs) for waste water treatment, enabling the (partial) recovery of the energy released from the degraded compounds as electricity. Literature on PFCs targeting air pollution on the other hand is extremely scarce. In this work an autonomously operating air purification device targeting sustainable electricity generation is presented. Knowledge on gas phase operation of PFCs was gathered by combining photocatalytic and photoelectrochemical measurements, both for TiO2 and WO3-based photocatalysts. While TiO2-based photocatalysts performed better in direct photocatalytic experiments, they were outperformed by WO3-based photoanodes in all-gas-phase PFC operation. Not only do WO3-based photocatalysts generate the highest steady state photocurrent, they also achieved the highest fuel-to-electricity conversion (>65 %). The discrepancies between gas phase photocatalytic and photoelectrochemical processes highlight the difference in driving material properties. This study serves as a proof-of-concept towards development of an autonomous, low-cost and widely applicable waste gas-to-electricity PFC device.