Photocatalytic Water Splitting: How Far Away Are We from Being Able to Industrially Produce Solar Hydrogen?

Solar water splitting (SWS) has been researched for about five decades, but despite successes there has not been a big breakthrough advancement. While the three fundamental steps, light absorption, charge carrier separation and diffusion, and charge utilization at redox sites are given a great deal of attention either separately or simultaneously, practical considerations that can help to increase efficiency are rarely discussed or put into practice. Nevertheless, it is possible to increase the generation of solar hydrogen by making a few little but important adjustments. In this review, we talk about various methods for photocatalytic water splitting that have been documented in the literature and importance of the thin film approach to move closer to the large-scale photocatalytic hydrogen production. For instance, when comparing the film form of the identical catalyst to the particulate form, it was found that the solar hydrogen production increased by up to two orders of magnitude. The major topic of this review with thin-film forms is, discussion on several methods of increased hydrogen generation under direct solar and one-sun circumstances. The advantages and disadvantages of thin film and particle technologies are extensively discussed. In the current assessment, potential approaches and scalable success factors are also covered. As demonstrated by a film-based approach, the local charge utilization at a zero applied potential is an appealing characteristic for SWS. Furthermore, we compare the PEC-WS and SWS for solar hydrogen generation and discuss how far we are from producing solar hydrogen on an industrial scale. We believe that the currently employed variety of attempts may be condensed to fewer strategies such as film-based evaluation, which will create a path to address the SWS issue and achieve sustainable solar hydrogen generation.

Enhanced Gas Sensing Performance of ZnO/Ti3C2Tx MXene Nanocomposite.

A representative of titanium carbide MXene, Ti3C2Tx is a promising candidate for high performance gas sensing and has attracted significant attention. However, MXene naturally has a multilayer structure with low porosity, which prevents its gas-sensing activity. Zinc oxide (ZnO) has long been utilized as a gas detector. Despite its good response to multiple gases, high operation temperature has limited its widespread use as a gas-sensing material. In this study, a room-temperature toxic gas sensor was prepared from ZnO/Ti3C2Tx MXene nanocomposite consisting of 2D few-layered MXene and 1D ZnO nanoparticles. A simple technique for synthesizing the nanocomposite was established. The physicochemical properties of the nanocomposite were fine-controlled with more active sites and higher porosity. The sensitivity and gas-selectivity of the sensing material were closely examined. The nanocomposite showed enhanced response and recovery behaviors to toxic gases, which outperformed pure Ti3C2Tx MXene and pure ZnO. This study offers a practical strategy by which to increase the gas-sensing performance of Ti3C2Tx MXene, and expands comprehensive understanding of the gas-sensing process of ZnO/Ti3C2Tx p-n heterostructure.

Role of p-n junction initiated mixed-dimensional 0D/2D, 1D/2D, and 2D/2D BiOX (X = Cl, Br, and I)/TiO2 nanocomposite interfaces for environmental remediation applications: A review.

Nowadays, we are critically facing various environmental issues. Among these, water contamination is the foremost issue, which worsens our health and living organisms in the water. Thus, it is necessary to provide an avenue to minimize the toxic matter through the development of facile technique and harmless photocatalyst. In this review, we intended to uncover the findings associated with various 0D, 1D, and 2D nanostructures featured photocatalysts for advancements in interfacial characteristics and toxic matter degradation. In this context, we evaluated the promising mixed-dimensional 0D/2D, 1D/2D, and 2D/2D bismuth oxyhalides BiOX (X = Cl, Br, and I) integrated TiO2 nanostructure interfaces. Tunable mixed-dimensional interfaces highlighted with higher surface area, more heterojunctions, variation in the conduction and valence band potential, narrowed band gap, and built-in electric field formation between BiOX and TiO2, which exhibits remarkable toxic dye, heavy metals, and antibiotics degradation. Further, this review further examines insights into the charge carrier generation, separation, and shortened charge transfer path at reduced recombination. Considering the advantages of type-II, S-scheme, and Z-scheme charge transfer mechanisms in the BiOX/TiO2, we heightened the combination of various reactive species generation. In a word, the concept of mixed-dimensional BiOX/TiO2 heterojunction interface endows toxic matter adsorption and decomposition into useful products. Challenges and future perspectives are also provided.

Palladium nanoparticle-decorated multi-layer Ti3C2T x dual-functioning as a highly sensitive hydrogen gas sensor and hydrogen storage.

In this work, palladium nanoparticle (PdNP)-decorated Ti3C2T x MXene (Pd-Ti3C2T x ) was synthesized by a simple two-step process. For this, multilayer Ti3C2T x MXene (ML-Ti3C2T x ) was first prepared by a selective HF etching technique, and PdNPs were directly grown on the surface of ML-Ti3C2T x flakes using a polyol method. The relative weight fraction of PdNPs to ML-Ti3C2T x was elaborately controlled to derive the optimal size and distribution of PdNPs, thereby to maximize its performance as a hydrogen sensor. The optimized Pd-Ti3C2T x nanocomposite showed superb hydrogen-sensing capability even at room temperature with sharp, large, reproducible, concentration-dependent, and hydrogen-selective responses. Furthermore, the nanocomposite also unveiled some extent of hydrogen storage capability at room temperature and 77 K, raising a possibility that it can dual-function as a hydrogen sensor and hydrogen storage.

Highly Deformable Fabric Gas Sensors Integrating Multidimensional Functional Nanostructures.

Highly strain-endurable gas sensors were implemented on fabric, which was taken from a real T-shirt, employing a sequential coating method. Multidimensional, functional nanostructures such as reduced graphene oxide, ZnO nanorods, palladium nanoparticles, and silver nanowires were integrated for their realization. It was revealed that the fabric gas sensors could detect both oxidizing and reducing gases at room temperature with differing signs and magnitudes of responses. Noticeably, the fabric gas sensors could normally work even under large strains up to 100%, which represents the highest strain tolerance in the gas sensor field. Furthermore, the fabric gas sensors turned out to bear harsh bending and twisting stresses. It was also demonstrated that the sequential coating method is an effective and facile way to control the size of the fabric gas sensor.

An enhanced recyclable 3D adsorbent for diverse bio-applications using biocompatible magnetic nanomulberry and cucurbituril composites.

Herein, we describe the synthesis of highly water-dispersible and biocompatible 3D adsorbents via a rapid two-step strategy employing a mesoporous magnetic nanomulberry-shaped Fe3O4 (MNM) on diatomaceous earth (DE) and cucurbituril (CB; MNM-DE-CB). Coating of CB on the surface of MNM-DE via hydrogen bonds not only enhanced the dispersibility of CB, but also improved the stability of MNM-DE. The ability of the adsorbent to remove dyes from water was investigated as a function of metal ions, solution pH, temperature, and concentration to determine optimum reaction conditions. Unlike MNM-DE, MNM-DE-CB exhibited highly efficient, rapid dye removal and recyclability in aqueous solution, and low cytotoxicity toward cancer cells in drug delivery tests. MNM-DE-CB is a promising green adsorbent with potential for diverse applications including water remediation, interface catalysis, bio-sample preparation, and drug delivery.

Photon-Induced Superior Antibacterial Activity of Palladium-Decorated, Magnetically Separable Fe3O4/Pd/mpg-C3N4 Nanocomposites.

Three-component nanocomposites (Fe3O4/Pd/mpg-C3N4) have been systematically synthesized using a three-step solution method for the photocatalytic bacterial decontamination. The mesoporous g-C3N4 nanosheets (mpg-C3N4), which were prepared by the acid treatment, showed a great improvement in photocatalytic performance. The photoluminescence intensity of the mpg-C3N4 nanosheets was disclosed to drop about 60% from the value of normal g-C3N4 nanosheets. Decoration of mpg-C3N4 with palladium (Pd) nanoparticles led to the effective suppression of carrier recombination and the carrier migration to Fe3O4 nanoparticles. It was revealed that the three-component nanocomposites degraded 99.9% of E. coli and 99.8% of S. aureus bacterial strains within 2 h of solar light irradiation at a 100 µg/mL concentration, demonstrating their superb photocatalytic antibacterial activity. In addition, the nanocomposites could be easily separated from the bacterial cells and repeatedly used for photocatalytic bacterial degradation with good recyclability. The strong photon-induced antibacterial activity and good recyclability of the three-component nanocomposites may represent their potential as a promising antibacterial photocatalyst.

The Synergistic Effect of Zinc Ferrite Nanoparticles Uniformly Deposited on Silver Nanowires for the Biofilm Inhibition of Candida albicans.

Near-monodisperse zinc ferrite nanoparticles (ZnFe2O4 NPs) are synthesized by a co-precipitation method and deposited on the surface of silver nanowires (AgNWs), employing a stepwise solution method. The resulting hybrid nanostructures (ZnFe2O4@AgNWs) show a thin and uniform layer of ZnFe2O4 NPs at an optimum weight ratio of 1:6 between the two component nanostructures. The hybrid nanostructures retain the high crystal quality and phase purity of their constituents. It is demonstrated that the ZnFe2O4@AgNWs hybrid nanostructures are effective at inhibiting the biofilm formation of Candida albicans cells. The biofilm inhibition activity of the hybrid nanostructures is estimated to be more than 50% at a low concentration of 100 µg/mL from both crystal violet assay and XTT assay, which are more than 8-fold higher than those of pure AgNWs and ZnFe2O4 NPs. This greatly enhanced biofilm inhibition activity is attributed to the ZnFe2O4 NPs-carrying membrane penetration by AgNWs and the subsequent interaction between Candida cells and ZnFe2O4 NPs. These results indicate that the ZnFe2O4@AgNWs hybrid nanostructures have great potential as a new type of novel antibiofilm agent.

Diffusion-Driven Al-Doping of ZnO Nanorods and Stretchable Gas Sensors Made of Doped ZnO Nanorods/Ag Nanowires Bilayers.

A crystal-damage-free nanodoping method, which utilized the vacuum drive-in diffusion of Al into ZnO nanorods, was developed. In this method, vertical ZnO nanorod arrays that were grown by chemical bath deposition beforehand were deposited with Al thin film and subsequently heat-treated under a high vacuum. At an optimum condition, the surface Al atoms were completely diffused into ZnO nanorods, resulting in Al-doped ZnO nanorods. Stretchable gas sensors were fabricated by sequentially drop-casting the Al-doped ZnO nanorods and silver nanowires on polydimethylsiloxane substrate. The resistance and response of the sensor could be optimized through the elaborate control of relative densities of Al-doped ZnO nanorods and silver nanowires. The sensor showed a high response of 32.3% to 10 ppm of NO2 gas at room temperature, even under a large strain of 30%. The NO2-sensing mechanism of Al-doped ZnO nanorod/silver nanowire bilayer sensors is discussed on the basis of a synergistic interplay of Al-doped ZnO nanorods and silver nanowires.

Ag nanowire/ZnO nanobush hybrid structures for improved photocatalytic activity.

Reverse-engineered Ag nanowire/ZnO nanobush hybrid structures have been synthesized for the first time by a combination of polyol method and low-temperature solution method. Through the elaborate control of Ag-to-ZnO weight ratio, vertically aligned ZnO nanobushes grew on the surface of well-faceted Ag nanowires. The Ag/ZnO hybrid nanostructures showed the crystal features of both Ag nanowires and wurtzite ZnO nanostructures. They exhibited strong UV absorption, whereas their photoluminescence spectra were much weaker than pure ZnO nanostructures due to the inhibited carrier recombination. The photocatalytic activity of Ag/ZnO hybrid nanostructures was greatly improved compared to pure ZnO nanostructures. Furthermore, they showed good cyclic performance and easy recovery from the test solution, demonstrating the possibility of their practical use.

Conductive Elastomers for Stretchable Electronics, Sensors and Energy Harvesters.

There have been a wide variety of efforts to develop conductive elastomers that satisfy both mechanical stretchability and electrical conductivity, as a response to growing demands on stretchable and wearable devices. This article reviews the important progress in conductive elastomers made in three application fields of stretchable technology: stretchable electronics, stretchable sensors, and stretchable energy harvesters. Diverse combinations of insulating elastomers and non-stretchable conductive materials have been studied to realize optimal conductive elastomers. It is noted that similar material combinations and similar structures have often been employed in different fields of application. In terms of stretchability, cyclic operation, and overall performance, fields such as stretchable conductors and stretchable strain/pressure sensors have achieved great advancement, whereas other fields like stretchable memories and stretchable thermoelectric energy harvesting are in their infancy. It is worth mentioning that there are still obstacles to overcome for the further progress of stretchable technology in the respective fields, which include the simplification of material combination and device structure, securement of reproducibility and reliability, and the establishment of easy fabrication techniques. Through this review article, both the progress and obstacles associated with the respective stretchable technologies will be understood more clearly.

Cobalt-assisted in situ synthesis of crystalline bismuth nanoparticle arrays.

Almost monodisperse, crystalline Bi nanoparticle arrays were synthesized using a newly developed method, magnetically assisted growth of Bi nanoparticles (MAGBINs). The MAGBIN utilizes co-sputtering from Bi and Co targets at an elevated temperature. Crystalline Bi nanoparticles with hexagonal morphology were formed in situ on a Si substrate with a thin surface oxide during this process. The size and density of Bi nanoparticles could be controlled by adjusting the relative powers applied to Bi and Co targets, and they showed opposite trends against the relative powers. Several physical processes such as Co agglomeration, element-selective growth, and Ostwald ripening were proposed to be involved in this Bi nanoparticle growth. The MAGBIN is a facile method to synthesize crystalline Bi nanoparticle arrays, which does not need any chemical agents, complex process, or lithography.

Aluminum silicide microparticles transformed from aluminum thin films by hypoeutectic interdiffusion.

Aluminum silicide microparticles with oxidized rough surfaces were formed on Si substrates through a spontaneous granulation process of Al films. This microparticle formation was caused by interdiffusion of Al and Si atoms at hypoeutectic temperatures of Al-Si systems, which was driven by compressive stress stored in Al films. The size, density, and the composition of the microparticles could be controlled by adjusting the annealing temperature, time, and the film thickness. High-density microparticles of a size around 10 µm and with an atomic ratio of Si/Al of approximately 0.8 were obtained when a 90-nm-thick Al film on Si substrate was annealed for 9 h at 550°C. The microparticle formation resulted in a rapid increase of the sheet resistance, which is a consequence of substantial consumption of Al film. This simple route to size- and composition-controllable microparticle formation may lay a foundation stone for the thermoelectric study on Al-Si alloy-based heterogeneous systems.

Cracked titanium film on an elastomeric substrate for highly flexible, transparent, and low-power strain sensors.

Strain-dependent cracking behaviors in thin titanium (Ti) films on polydimethylsiloxane (PDMS) substrates were systematically investigated for their application to sensitive, flexible, transparent, and portable strain sensors. When uniaxially elongated, vertical cracks were developed in the low-strain range, and beyond a critical strain, tilted cracks appeared to intersect the vertical cracks. The cracking behaviors were also dependent on Ti film thickness. The varying strain-dependent crack patterns produced a significant resistance change in response to the applied strain, particularly, in the high- and broad-strain range. For a 180-nm-thick Ti film on PDMS substrate, a gauge factor of 2 was achieved in the range of 30% to 50% strain. The operation power was extremely low. All the Ti films on PDMS substrates were transparent, highly flexible, and very easy to fabricate. These results suggest that cracked Ti films on PDMS substrates could be a viable candidate for realizing a low-cost, flexible, transparent, and portable strain sensor.

Design rules for nanogap-based hydrogen gas sensors.

Nanoscale gaps, which enable many research applications in fields such as chemical sensors, single-electron transistors, and molecular switching devices, have been extensively investigated over the past decade and have witnessed the evolution of related technologies. Importantly, nanoscale gaps employed in hydrogen-gas (H(2)) sensors have been used to reversibly detect H(2) in an On-Off manner, and function as platforms for enhancing sensing performance. Herein, we review recent advances in nanogap design for H(2) sensors and deal with various strategies to create these gaps, including fracture generation by H(2) exposure, deposition onto prestructured patterns, island formation on a surface, artificial manipulation methods, methods using hybrid materials, and recent approaches using elastomeric substrates. Furthermore, this review discusses a new nanogap design that advances sensing capabilities in order to meet the diverse needs of academia and industry.

Co nanoparticle hybridization with single-crystalline Bi nanowires.

Crystalline Co nanoparticles were hybridized with single-crystalline Bi nanowires simply by annealing Co-coated Bi nanowires at elevated temperatures. An initially near-amorphous Co film of 2-7 nm in thickness began to disrupt its morphology and to be locally transformed into crystallites in the early stage of annealing. The Co film became discontinuous after prolonged annealing, finally leading to isolated, crystalline Co nanoparticles of 8-27 nm in size. This process spontaneously proceeds to reduce the high surface tension and total energy of Co film. The annealing time required for Co nanoparticle formation decreased as annealing temperature increased, reflecting that this transformation occurs by the diffusional flow of Co atoms. The Co nanoparticle formation process was explained by a hole agglomeration and growth mechanism, which is similar to the model suggested by Brandon and Bradshaw, followed by the nanoparticle refinement.

Structure-dependent growth control in nanowire synthesis via on-film formation of nanowires.

On-film formation of nanowires, termed OFF-ON, is a novel synthetic approach that produces high-quality, single-crystalline nanowires of interest. This versatile method utilizes stress-induced atomic mass flow along grain boundaries in the polycrystalline film to form nanowires. Consequently, controlling the magnitude of the stress induced in the films and the microstructure of the films is important in OFF-ON. In this study, we investigated various experimental growth parameters such as deposition rate, deposition area, and substrate structure which modulate the microstructure and the magnitude of stress in the films, and thus significantly affect the nanowire density. We found that Bi nanowire growth is favored in thermodynamically unstable films that facilitate atomic mass flow during annealing. A large film area and a large thermal expansion coefficient mismatch between the film and the substrate were found to be critical for inducing large compressive stress in a film, which promotes Bi nanowire growth. The OFF-ON method can be routinely used to grow nanowires from a variety of materials by tuning the material-dependent growth parameters.

Simple two-step fabrication method of Bi2Te3 nanowires.

Bismuth telluride (Bi2Te3) is an attractive material for both thermoelectric and topological insulator applications. Its performance is expected to be greatly improved when the material takes nanowire structures. However, it is very difficult to grow high-quality Bi2Te3 nanowires. In this study, a simple and reliable method for the growth of Bi2Te3 nanowires is reported, which uses post-sputtering and annealing in combination with the conventional method involving on-film formation of nanowires. Transmission electron microscopy study shows that Bi2Te3 nanowires grown by our technique are highly single-crystalline and oriented along [110] direction.