Nanoarray Structures for Artificial Photosynthesis

Review on Microreactors for Photo-Electrocatalysis Artificial Photosynthesis Regeneration of Coenzymes

In recent years, with the outbreak of the global energy crisis, renewable solar energy has become a focal point of research. However, the utilization efficiency of natural photosynthesis (NPS) is only about 1%. Inspired by NPS, artificial photosynthesis (APS) was developed and utilized in applications such as the regeneration of coenzymes. APS for coenzyme regeneration can overcome the problem of high energy consumption in comparison to electrocatalytic methods. Microreactors represent a promising technology. Compared with the conventional system, it has the advantages of a large specific surface area, the fast diffusion of small molecules, and high efficiency. Introducing microreactors can lead to more efficient, economical, and environmentally friendly coenzyme regeneration in artificial photosynthesis. This review begins with a brief introduction of APS and microreactors, and then summarizes research on traditional electrocatalytic coenzyme regeneration, as well as photocatalytic and photo-electrocatalysis coenzyme regeneration by APS, all based on microreactors, and compares them with the corresponding conventional system. Finally, it looks forward to the promising prospects of this technology.

Particle-Based Photoelectrodes for PEC Water Splitting: Concepts and Perspectives

This comprehensive review delves into the intricacies of the photoelectrochemical (PEC) water splitting process, specifically focusing on the design, fabrication, and optimization of particle-based photoelectrodes for efficient green hydrogen production. These photoelectrodes, composed of semiconductor materials, potentially harness light energy and generate charge carriers, driving water oxidation and reduction reactions. The versatility of particle-based photoelectrodes as a platform for investigating and enhancing various semiconductor candidates is explored, particularly the emerging complex oxides with compelling charge transfer properties. However, the challenges presented by many factors influencing the performance and stability of these photoelectrodes, including particle size, shape, composition, morphology, surface modification, and electrode configuration, are highlighted. The review introduces the fundamental principles of semiconductor photoelectrodes for PEC water splitting, presents an exhaustive overview of different synthesis methods for semiconductor powders and their assembly into photoelectrodes, and discusses recent advances and challenges in photoelectrode material development. It concludes by offering promising strategies for improving photoelectrode performance and stability, such as the adoption of novel architectures and heterojunctions.

The Emergence of High-Performance Conjugated Polymer/Inorganic Semiconductor Hybrid Photoelectrodes for Solar-Driven Photoelectrochemical Water Splitting

Solar-driven photoelectrochemical (PEC) energy conversion holds great potential in converting solar energy into storable and transportable chemicals or fuels, providing a viable route toward a carbon-neutral society. Conjugated polymers are rapidly emerging as a new class of materials for PEC water splitting. They exhibit many intriguing properties including tunable electronic structures through molecular engineering, excellent light harvesting capability with high absorption coefficients, and facile fabrication of large-area thin films via solution processing. Recent advances have indicated that integrating rationally designed conjugated polymers with inorganic semiconductors is a promising strategy for fabricating efficient and stable hybrid photoelectrodes for high-efficiency PEC water splitting. This review introduces the history of developing conjugated polymers for PEC water splitting. Notable examples of utilizing conjugated polymers to broaden the light absorption range, improve stability, and enhance the charge separation efficiency of hybrid photoelectrodes are highlighted. Furthermore, key challenges and future research opportunities for further improvements are also presented. This review provides an up-to-date overview of fabricating stable and high-efficiency PEC devices by integrating conjugated polymers with state-of-the-art semiconductors and would have significant implications for the broad solar-to-chemical energy conversion research.

Photonic approach in stacked slabs having periodic holes for enhancing photocatalytic activities

Photonic approaches can improve the efficiencies of photo-electrochemical devices towards CO2 reduction and fossil fuel-free societies. In a system consisting of stacked dielectric slabs having periodic holes with each slab coated by photocatalyst layers at both sides, immersed in water, we show that an incident electromagnetic field is effectively confined in the photocatalyst layers, resulting in the enhancement of the photocatalytic activities. In addition, the antireflection effect was engineered by adjusting the distances between the photonic crystal slabs. Numerical results reveal an enhancement factor of 3 for the absorption of electromagnetic fields at the operation frequency in the 3rd band of the dispersion diagram, compared to the bulk photocatalyst. Our system has the feature of periodic holes allowing the movement of reaction products. An analytical model is developed using the revised plane wave method and perturbation theory, which captures the trends observed in numerical results.

Dual Heterojunctions and Nanobowl Morphology Engineered BiVO4 Photoanodes for Enhanced Solar Water Splitting

Photoelectrochemical (PEC) water splitting represents an attractive strategy to realize the conversion from solar energy to hydrogen energy, but severe charge recombination in photoanodes significantly limits the conversion efficiency. Herein, a unique BiVO4 (BVO) nanobowl (NB) heterojunction photoanode, which consists of [001]-oriented BiOCl underlayer and BVO nanobowls containing embedded BiOCl nanocrystals, is fabricated by nanosphere lithography followed by in situ transformation. Experimental characterizations and theoretical simulation prove that nanobowl morphology can effectively enhance light absorption while reducing carrier diffusion path. Density functional theory (DFT) calculations show the tendency of electron transfer from BVO to BiOCl. The [001]-oriented BiOCl underlayer forms a compact type II heterojunction with the BVO, favoring electron transfer from BVO through BiOCl to the substrate. Furthermore, the embedded BiOCl nanoparticles form a bulk heterojunction to facilitate bulk electron transfer. Consequently, the dual heterojunctions engineered BVO/BiOCl NB photoanode exhibits attractive PEC performance toward water oxidation with an excellent bulk charge separation efficiency of 95.5%, and a remarkable photocurrent density of 3.38 mA cm-2 at 1.23 V versus reversible hydrogen electrode, a fourfold enhancement compared to the flat BVO counterpart. This work highlights the great potential of integrating dual heterojunctions engineering and morphology engineering in fabricating high-performance photoelectrodes toward efficient solar conversion.

Discretely-supported transfer nanoimprint anti-reflection nanostructures on complex uneven surface of Fresnel lenses

Nanopatterning complex uneven surface of numerous functional devices to improve their performance is significantly appealing; however, it is extremely challenging. This study proposes a discretely-supported transfer nanoimprint technique to fabricate nanostructures on complex device surfaces containing multi-spatial frequencies. First, a discretely-supported nanoimprint template was designed based on the built energy criterion. A contact fidelity of over 99% was achieved between the designed template and the targeted complex uneven substrate surface. Next, the prefilled nanostructures on the template were transferred to the target surface after contact. By precisely controlling the amount of micro-droplet jetting on the template on-demand, the accumulation of the polymer in the micro-valley sites on the complex substrate was avoided, thus maintaining the morphology and generating function of the devices. Finally, high-quality Fresnel lenses with broadband wide-directional antireflection and excellent imaging performance were developed by imprinting subwavelength-tapered nanostructures on the relief surface.

Advances in photothermal regulation strategies: from efficient solar heating to daytime passive cooling

Photothermal regulation concerning solar harvesting and repelling has recently attracted significant interest due to the fast-growing research focus in the areas of solar heating for evaporation, photocatalysis, motion, and electricity generation, as well as passive cooling for cooling textiles and smart buildings. The parallel development of photothermal regulation strategies through both material and system designs has further improved the overall solar utilization efficiency for heating/cooling. In this review, we will review the latest progress in photothermal regulation, including solar heating and passive cooling, and their manipulating strategies. The underlying mechanisms and criteria of highly efficient photothermal regulation in terms of optical absorption/reflection, thermal conversion, transfer, and emission properties corresponding to the extensive catalog of nanostructured materials are discussed. The rational material and structural designs with spectral selectivity for improving the photothermal regulation performance are then highlighted. We finally present the recent significant developments of applications of photothermal regulation in clean energy and environmental areas and give a brief perspective on the current challenges and future development of controlled solar energy utilization.

A Pd-based plasmonic photocatalyst for nitrogen fixation through an antenna-reactor mechanism

Plasmonic metal nanocrystals (e.g., Au, Ag, and Cu) hold great promise for driving photocatalytic reactions, but little is known about the plasmonic properties of Pd nanocrystals. Herein, we constructed a plasmonic Pd/Ru antenna-reactor photocatalyst through the controllable growth of a Ru nanoarray 'reactor' on a Pd nano-octahedron 'antenna' and demonstrated a plasmonic Pd-driven N2 photofixation process. The plasmonic properties of Pd nano-octahedrons were verified using finite-difference time-domain (FDTD) simulations and refractive index sensitivity tests in water-glycerol mixtures. Notably, the constructed plasmonic antenna-reactor nanostructures exhibited superior photocatalytic activities during N2 photofixation, with a maximum ammonia production rate of 117.5 ± 15.0 μmol g-1 h-1 under visible and near-infrared (NIR) light illumination. The mechanism can be attributed to the ability of the plasmonic Pd nanoantennas to harvest light to generate abundant hot electrons and the Ru nanoreactors to provide active sites for adsorption and activation of N2. This work paves the way for the development of Pd-based plasmonic photocatalysts for efficient N2 photofixation and sheds new light on the optimal design and construction of antenna-reactor nanostructures.

Nanoporous Titanium Oxynitride Nanotube Metamaterials with Deep Subwavelength Heat Dissipation for Perfect Solar Absorption

We report a quasi-unitary broadband absorption over the ultraviolet-visible-near-infrared range in spaced high aspect ratio, nanoporous titanium oxynitride nanotubes, an ideal platform for several photothermal applications. We explain such an efficient light-heat conversion in terms of localized field distribution and heat dissipation within the nanopores, whose sparsity can be controlled during fabrication. The extremely large heat dissipation could not be explained in terms of effective medium theories, which are typically used to describe small geometrical features associated with relatively large optical structures. A fabrication-process-inspired numerical model was developed to describe a realistic space-dependent electric permittivity distribution within the nanotubes. The resulting abrupt optical discontinuities favor electromagnetic dissipation in the deep sub-wavelength domains generated and can explain the large broadband absorption measured in samples with different porosities. The potential application of porous titanium oxynitride nanotubes as solar absorbers was explored by photothermal experiments under moderately concentrated white light (1-12 Suns). These findings suggest potential interest in realizing solar-thermal devices based on such simple and scalable metamaterials.

A novel PEC and ECL bifunctional aptasensor based on V2CTx MXene-derived MOF embedded with silver nanoparticles for selectively aptasensing miRNA-126

A novel photoelectrochemical (PEC) and electrochemiluminescence (ECL) bifunctional aptasensor has been established for the detection of miRNA-126 using V2CTx MXene-derived porphyrin-based metal-organic framework embedded with Ag nanoparticles (Ag NPs) (denoted as AgNPs@V-PMOF) as a robust bioplatform. Due to the presence of V nodes in V2CTx MXene nanosheets, V-based MOF was prepared using tetrakis(4-carboxyphenyl)porphyrin as ligand, followed by the incorporation of Ag+ ions to form the AgNPs@V-PMOF Schottky heterojunction. Benefiting from the fast electron transfer of the V2CTx substrate and well-matched band-edge energy level of the photosensitive Ag NPs and V-PMOF, the constructed AgNPs@V-PMOF Schottky heterojunction exhibited the promoted transfer of the photogenerated carriers, showing superior PEC and ECL performances. Moreover, a large number of the complementary DNA strand of miRNA-126 can be immobilized over AgNPs@V-PMOF in view of the combined interaction of π-π stacking, van der Waals force, and Ag-N coordination between AgNPs@V-PMOF. Consequently, the developed AgNPs@V-PMOF-based aptasensor illustrated extremely low detection limits of 0.78 and 0.53 fM within a wide range from 1.0 fM to 1.0 nM of miRNA-126 detected by PEC and ECL techniques, respectively, superior to most reported miRNA aptasensors. Also, the provided bifunctional aptasensor demonstrated high selectivity, good stability, fine reproducibility, and acceptable regenerability, as well as promising potential for the analysis of miRNA-126 from living cancer cells. This work puts forward the development of aptasensors for the early and accurate diagnosis of cancer markers and extends the application of MOF in the biosensing field.

Out-of-Plane Electronics on Flexible Substrates Using Inorganic Nanowires Grown on High-Aspect-Ratio Printed Gold Micropillars

Out-of-plane or 3D electronics on flexible substrates are an interesting direction that can enable novel solutions such as efficient bioelectricity generation and artificial retina. However, the development of devices with such architectures is limited by the lack of suitable fabrication techniques. Additive manufacturing (AM) can but often fail to provide high-resolution, sub-micrometer 3D architectures. Herein, the optimization of a drop-on-demand (DoD), high-resolution electrohydrodynamic (EHD)-based jet printing method for generating 3D gold (Au) micropillars is reported. Libraries of Au micropillar electrode arrays (MEAs) reaching a maximum height of 196 µm and a maximum aspect ratio of 52 are printed. Further, by combining AM with the hydrothermal growth method, a seedless synthesis of zinc oxide (ZnO) nanowires (NWs) on the printed Au MEAs is demonstrated. The developed hybrid approach leads to hierarchical light-sensitive NW-connected networks exhibiting favorable ultraviolet (UV) sensing as demonstrated via fabricating flexible photodetectors (PDs). The 3D PDs exhibit an excellent omnidirectional light-absorption ability and thus, maintain high photocurrents over wide light incidence angles (±90°). Lastly, the PDs are tested under both concave and convex bending at 40 mm, showing excellent mechanical flexibility.

Directional editing of self-supported nanoarray electrode for adaptive paired-electrolysis

Anodic oxidation assisted hydrogen production under mild conditions powered by renewable electricity represents a sustainable approach to energy conversion systems. Here, we fabricated a versatile and universal self-supported nanoarray platform that can be intelligently edited to achieve adaptive electrocatalysis for alcohol oxidation reactions and hydrogen evolution reaction (HER). The obtained self-supported nanoarray electrocatalysts exhibit excellent catalytic activity due to the integration of multiple merits of rich nanointerface-reconstruction and self-supported hierarchical structures. Particularly, the membrane-free pair-electrolysis system coupling HER and ethylene glycol oxidation reaction (EGOR) required an applied voltage of only 1.25 V to drive the current density of 10 mA cm^-2, which is about 510 mV lower than that of the overall water splitting, showing the capability to simultaneously produce H₂ and formate with high Faradic efficiency and stability. This work demonstrates a catalytic self-supported nanoarray platform for energy-efficient production of high-purity H₂ and value-added chemicals.

Coemissive luminescent nanoparticles combining aggregation-induced emission and quenching dyes prepared in continuous flow

Achieving an ideal light-harvesting system at a low cost remains a challenge. Herein, we report the synthesis of a hybrid dye system based on tetraphenylene (TPE) encapsulated organic dyes in a continuous flow microreactor. The composite dye nanoparticles (NPs) are synthesized based on supramolecular self-assembly to achieve the co-emission of aggregation-induced emission dyes and aggregation-caused quenching dyes (CEAA). Numerical simulations and molecular spectroscopy were used to investigate the synthesis mechanism of the CEAA dyes. Nanoparticles of CEAA dyes provide a platform for efficient cascade Förster resonance energy transfer (FRET). Composite dye nanoparticles of TPE and Nile red (NiR) are synthesized for an ideal light-harvesting system using coumarin 6 (C-6) as an energy intermediate. The light-harvesting system has a considerable red-shift distance (~126 nm), high energy-transfer efficiency (Φ_ET) of 99.37%, and an antenna effect of 26.23. Finally, the versatility of the preparation method and the diversity of CEAA dyes are demonstrated.

Directional Growth of Conductive Metal-Organic Framework Nanoarrays along [001] on Metal Hydroxides for Aqueous Asymmetric Supercapacitors

Metal-organic frameworks (MOFs) are promising electrochemical materials that possess large specific surface areas, high porosities, good adjustability, and high activities. However, many conventional MOFs exhibit poor conductivity, which hinders their application in electrochemistry. In recent years, conductive MOFs (cMOFs) have attracted a considerable attention. As an important transition metal hydroxide, Ni(OH)₂ nanosheets exhibit a high theoretical specific capacitance and a high energy density but a poor electrical conductivity. In this study, we combined a typical cMOF(Ni-HHTP, HHTP = 2,3,6,7,10,11-hexahydroxybenzene) with Ni(OH)₂ nanosheets and synthesized a series of Ni-HHTP@Ni(OH)₂ nanoarrays. The composite materials exhibit a high electrical conductivity and ionic transfer efficiency and a good stability. Most importantly, our study reveals the chemical interaction between cMOFs and metal hydroxide composites and the relationship between facet exposure and the growth orientation of cMOFs. When Ni-HHTP@Ni(OH)₂-2 was assembled as a positive electrode material in an aqueous asymmetric supercapacitor, 98% of the initial capacitance was maintained after 5000 cycles at a high current density of 3 A g^-1. The findings of this study will provide meaningful insights into the design of cMOF composites combining other metal hydroxides.

DFT Modeling of CO2 Adsorption and HCOO• Group Conversion in Anatase Au-TiO2-Based Photocatalysis

Due to the merits of carbon circulation and hydrocarbon production, solar-assisted photocatalysis has been regarded as an ideal option for securing a sustainable future of energy and environment. In the photocatalytic carbon cycle process, surface reactions including the adsorption of CO₂ and the conversion of CO₂ into CH₄, CH₃OH, etc. are crucial to be examined ascribed to their significant influence on the performance of the photocatalysis. Because the conversion reaction starts from the formation of HCOO^•, the density functional theory (DFT) model was established in this study to investigate the micromechanism of CO₂ adsorption and the conversion of CO₂ to HCOO^• group in the anatase Au-TiO₂ photocatalytic system. The CO₂ adsorption bonding in six configurations was simulated, on which basis the effects of the proportion of water molecules and the lattice temperature increase due to the local surface plasmon resonance (LSPR) on the photocatalytic CO₂ adsorption and conversion were specifically analyzed. The results show that the experimental conditions that water molecules are released before CO₂ are favorable for the formation of the adsorption configuration in which HCOO^• tends to be produced without the need of reaction activation energy. This is reasonable since the intermediate C atoms do not participate in bonding under these conditions. Moreover, Au clusters have an insignificant influence on the adsorption behaviors of CO₂ including the adsorption sites and configurations on TiO₂ surfaces. As a result, the reaction rate is reduced due to the temperature increase caused by the LSPR effect. Nevertheless, the reaction maintains a very high rate. Interestingly, configurations that require activation energy are also possible to be resulted, which exerts a positive influence of temperature on the conversion rate of CO₂. It is found that the rate of the reaction can be improved by approximately 1-10 times with a temperature rise of 50 K above the ambient.

Self-Assembled Porphyrin Nanoleaves with Unique Crossed Transportation of Photogenerated Carriers to Enhance Photocatalytic Hydrogen Production

The preparation of self-assembled porphyrins with orderly stacked nanostructures for emulating natural photosynthesis has stimulated extensive efforts to optimize the energy conversion efficiency. However, the elucidation of how orderly stacked structures promote photocatalysis at the molecular level remains a great challenge. Here, unique porphyrin nanoleaves with designed and ordered structure are synthesized and show a hydrogen evolution rate higher than that of commercial powder. Photodeposition of cocatalysts and Kelvin probe force microscopy measurement suggest selective aggregation of photogenerated electrons and holes at different active sites. Combined with theoretical calculations, we find that the orderly packing changes molecular symmetry and induces a molecular dipole, which increases linearly along the π-π stacking direction and forms a strong built-in electric field. The built-in electric field drives photogenerated electrons and holes for the unique crossed transportation along different directions. These findings reveal how orderly stacked structures promote photocatalysis and provide a novel approach for highly efficient water splitting.

Boosting Unassisted Alkaline Solar Water Splitting Using Silicon Photocathode with TiO2 Nanorods Decorated by Edge-Rich MoS2 Nanoplates

To construct a highly efficient photoelectrochemical tandem device with silicon photocathode operating in alkaline conditions, it is desirable to develop stable and active catalysts which enable the photocathode to reliably perform under an alkaline environment. With nanostructured passivation layer and edge-exposed transition metal disulfides, silicon photocathode provides new opportunities for achieving unbiased alkaline solar water splitting. Here, the TiO₂ nanorod arrays decorated by edge-rich MoS₂ nanoplates are elaborately synthesized and deposited on p-Si. The vertically aligned TiO₂ nanorods fully stabilize the Si surface and improve anti-reflectance. Moreover, MoS₂ nanoplates with exposed edge sites provide catalytically active regions resulting in the kinetically favored hydrogen evolution under an alkaline environment. Interfacial energy band bending between p-Si and catalyst layers facilitates the transport of photogenerated electrons under steady-state illumination. Consequently, the MoS₂ nanoplates/TiO₂ nanorods/p-Si photocathode exhibits significantly improved photoelectrochemical-hydrogen evolution reaction (PEC-HER) performance in alkaline media with a high photocurrent density of 10 mA cm^-2 at 0 V versus RHE and high stability. By integrating rationally designed photocathode with earth-abundant Fe₆₀ (NiCo)₃₀ Cr₁₀ anode and perovskite/Si tandem photovoltaic cell, an unassisted alkaline solar water splitting is accomplished with a current density of 5.4 mA cm^-2 corresponding to 6.6% solar-to-hydrogen efficiency, which is the highest among p-Si photocathodes.