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Room-temperature optoelectronic gas sensing presents a viable solution for efficient and low-cost NO2 gas monitoring. Two-dimensional (2D) metal oxychalcogenides, induced by the partial sulfur replacement with oxygen in metal chalcogenide, have been considered to be suitable sensitive materials owing to their excellent long-term stability and complete reversibility. However, the correlation of oxygen-to-sulfur content with gas sensing performances has yet to be known, hindering the optimization process of such an emerging NO2 gas sensing approach. In this work, we realize 2D ultrathin indium oxysulfide by combining the annealing of bulk In2S3 crystals and subsequently liquid phase exfoliation, while the degree of sulfur replacement with oxygen is controlled by the initial annealing temperature. At an optimum annealing temperature of 800 °C, the sample exhibits an optical bandgap of 2.19 eV and a photoexcited carrier lifetime of 272.82 ns. As a result, it demonstrates a response factor of 7.26 for 126 ppb of NO2 under blue light illumination at room temperature, while complete reversibility, high selectivity, and excellent long-term stability are also achieved. An extremely low limit-of-detection of 0.0342 ppb is shown, far beyond that of reported room-temperature optoelectronic NO2 sensors. This work presents the optimization of the implementation of 2D metal oxychalcogenides in NO2 gas sensing, paving the way to develop next-generation NO2 monitoring systems. [ABSTRACT FROM AUTHOR]

Layered double hydroxides (LDHs) have attracted numerous interests in developing high-performance electrochemical supercapacitors (SCs). However, the anchored conductive substrates are critical in the high-dense growth of LDH with favorable crystal orientations for maximizing performances, in which the conventional amorphous carbon-based materials and microporous nickel foams cannot be fitted. Herein, we utilize the electrodeposited 3D dendritic copper as the conductive substrate for the vertical growth of CoNi-LDH nanosheets. The composite possesses excellent electrical conductivity, a high degree of crystallinity with (003) orientation, and a large surface area, resulting in an impressive specific capacitance of 5743 mF cm–2 at 1 mA cm–2 with 56.6% rate capability at 20 mA cm–2 and excellent cycling lifespan. In comparison, pure CoNi-LDH shows a smaller specific capacitance of 1798.18 mF cm–2 at 1 mA cm–2 with 46.1% rate capability at 20 mA cm–2. Such improved electrochemical performances of the composite electrode are possibly due to the sufficient exposure of the active centers, the shortened diffusion distance of electrolyte ions, and the synergistic interaction of the copper core and the LDH shell. The assembled asymmetric supercapacitor device based on the as-prepared composite anode and the activated carbon cathode exhibits an energy density of 0.33 mW h cm–2 at a large power density of 1.5 mW cm–2. This work reveals a facile and feasible strategy for improving the performances of LDH-based SC devices through the engineering of conductive substrates. [ABSTRACT FROM AUTHOR]

The exhibition of plasmon resonancesin two-dimensional (2D) semiconductorcompounds is desirable for many applications. Here, by electrochemicallyintercalating lithium into 2D molybdenum disulfide (MoS2) nanoflakes, plasmon resonances in the visible and near UV wavelengthranges are achieved. These plasmon resonances are controlled by thehigh doping level of the nanoflakes after the intercalation, producingtwo distinct resonance peak areas based on the crystal arrangements.The system is also benchmarked for biosensing using bovine serum albumin.This work provides a foundation for developing future 2D MoS2based biological and optical units. [ABSTRACT FROM AUTHOR]

Quasi-two-dimensional(quasi-2D) molybdenum disulfide (MoS2) is a photoluminescence(PL) material with unique properties.The recent demonstration of its PL, controlled by the intercalationof positive ions, can lead to many opportunities for employing thisquasi-2D material in ion-related biological applications. Here, wepresent two representative models of biological systems that incorporatethe ion-controlled PL of quasi-2D MoS2nanoflakes. Theion exchange behaviors of these two models are investigated to revealenzymatic activities and cell viabilities. While the ion intercalationof MoS2in enzymatic activities is enabled via an externalapplied voltage, the intercalation of ions in cell viability investigationsoccurs in the presence of the intrinsic cell membrane potential. [ABSTRACT FROM AUTHOR]

α- and β-Phase MoO3are synthesizedusing an electrodeposition method on fluorine-doped tin oxide (FTO)glass substrates from sodium-molybdate (Na2MoO4) solutions. We show that it is possible to obtain both α-and β-MoO3by manipulating the cyclic voltammetry(CV) parameters during electrodeposition. Raman spectroscopy, X-raydiffraction, and scanning electron microscopy indicate that the appliedpotential range and sweep rate are strongly influential on the phaseobtained and the surface morphology of the electrodeposited thin films.Gasochromic measurements were carried out on the annealed samplesby exposing them to H2gas. It was revealed that α-MoO3thin films provided better response to H2interactionthan β-MoO3films did. Additionally, porous filmsprovided significantly larger responses than smooth films. [ABSTRACT FROM AUTHOR]

The construction of two-dimensional (2D) van der Waals (vdW) heterostructures over black phosphorus (BP) has been attracting significant attention to better utilize its inherent properties. The sandwich of zero-dimensional (0D) noble metals within BP-based vdW heterostructures can provide efficient catalytic channels, modulating their surface redox potentials and therefore inducing versatile functionalities. Herein, we realize a 2D WS 2 -Au-BP heterostructure, in which Au nanoparticles are connected between BP and WS 2 via ionic bonds. The ultralow conduction band minimum position, the reduced adsorption energies of O 2 , and the increased dissociation barrier energy of O 2 - into 2O contribute greatly to improving the long-term stability of BP in the air. The formation of heterostructures can reduce the potential barrier energy in target gas molecules, thus enhancing the absorption energy and charge transfer. Taking the paramagnetic NO 2 gas molecules as a representative, a stable response magnitude of 2.11 to 100 ppb NO 2 is achieved for 80 days, which is far larger than the initial responses of most BP-based materials. A practical gas sensing system is also developed to demonstrate its real-world implementation. This work provides a promising demonstration of 0D noble metal within 2D BP-based vdW heterostructure for simultaneously improving the long-term stability and room-temperature reversible gas sensing.

The emergence of piezoelectric properties in two-dimensional (2D) layered transition metal dichalcogenides (TMDs) has triggered the intensive research on using low dimensional materials for conversion of mechanical stimuli into electrical signals or vice versa. While the bulk intrinsically presents no piezoelectric property, the origin of the piezoelectric responses in their 2D thin planes is ascribed to the loss of centrosymmetry. There are also other categories of 2D layered materials such as post-transition metal dichalcogenides (PTMDs) that might be of interests, which have been confirmed theoretically and are yet to be fully explored experimentally. In this work, we investigate the thickness-dependent piezoelectric responses of 2D tin disulfide (SnS 2 ) nanosheets as a representative of layered PTMDs. The results indicate that the 2D SnS 2 nanosheets with a thickness of ∼4 nm present an effective out-of-plane piezoelectric response of 2 ± 0.22 pm/V. Furthermore, the thickness dependence of the piezoelectric behavior at a resonant frequency shows that the piezoelectric coefficient decreases with increasing the thickness of 2D SnS 2 nanosheets. Additionally, in reference to periodically poled lithium niobate piezoelectric crystal, the measured effective lateral piezoelectric coefficients at different voltages range from 0.61 to 1.55 pm/V with the average value at ∼1 pm/V. This study expands candidates for new piezoelectric materials in the 2D domain with comparable vertical and lateral coefficients, potentially opening a broader horizon for integration into sensors, actuators, and micro- and nanoelectromechanical systems.

Sulfur-rich molybdenum sulfides are an emerging class of inorganic coordination polymers that are predominantly utilized for their superior catalytic properties. Here we investigate surface water dependent properties of sulfur-rich MoS x (x = 3 2 / 3 ) and its interaction with water vapor. We report that MoS x is a highly hygroscopic semiconductor, which can reversibly bind up to 0.9 H 2 O molecule per Mo. The presence of surface water is found to have a profound influence on the semiconductor's properties, modulating the material's photoluminescence by over 1 order of magnitude, in transition from dry to moist ambient. Furthermore, the conductivity of a MoS x -based moisture sensor is modulated in excess of 2 orders of magnitude for 30% increase in humidity. As the core application, we utilize the discovered properties of MoS x to develop an electrolyteless water splitting photocatalyst that relies entirely on the hygroscopic nature of MoS x as the water source. The catalyst is formulated as an ink that can be coated onto insulating substrates, such as glass, leading to efficient hydrogen and oxygen evolution from water vapor. The concept has the potential to be widely adopted for future solar fuel production.

Ingestible sensing capsules are fast emerging as a critical technology that has the ability to greatly impact health, nutrition, and clinical areas. These ingestible devices are noninvasive and hence are very attractive for customers. With widespread access to smart phones connected to the Internet, the data produced by this technology can be readily seen and reviewed online, and accessed by both users and physicians. The outputs provide invaluable information to reveal the state of gut health and disorders as well as the impact of food, medical supplements, and environmental changes on the gastrointestinal tract. One unique feature of such ingestible sensors is that their passage through the gut lumen gives them access to each individual organ of the gastrointestinal tract. Therefore, ingestible sensors offer the ability to gather images and monitor luminal fluid and the contents of each gut segment including electrolytes, enzymes, metabolites, hormones, and the microbial communities. As such, an incredible wealth of knowledge regarding the functionality and state of health of individuals through key gut biomarkers can be obtained. This Review presents an overview of the gut structure and discusses current and emerging digestible technologies. The text is an effort to provide a comprehensive overview of ingestible sensing capsules, from both a body physiology point of view as well as a technological view, and to detail the potential information that they can generate.

Few-layer two-dimensional (2D) molybdenum oxide nanoflakes are exfoliated using a grinding assisted liquid phase sonication exfoliation method. The sonication process is carried out in five different mixtures of water with both aprotic and protic solvents. We found that surface energy and solubility of mixtures play important roles in changing the thickness, lateral dimension, and synthetic yield of the nanoflakes. We demonstrate an increase in proton intercalation in 2D nanoflakes upon simulated solar light exposure. This results in substoichiometric flakes and a subsequent enhancement in free electron concentrations, producing plasmon resonances. Two plasmon resonance peaks associated with the thickness and the lateral dimension axes are observable in the samples, in which the plasmonic peak positions could be tuned by the choice of the solvent in exfoliating 2D molybdenum oxide. The extinction coefficients of the plasmonic absorption bands of 2D molybdenum oxide nanoflakes in all samples are found to be high (ε > 10(9) L mol(-1) cm(-1)). It is expected that the tunable plasmon resonances of 2D molybdenum oxide nanoflakes presented in this work can be used in future electronic, optical, and sensing devices.

Nitrogen dioxide (NO2) is a gas species that plays an important role in certain industrial, farming, and healthcare sectors. However, there are still significant challenges for NO2 sensing at low detection limits, especially in the presence of other interfering gases. The NO2 selectivity of current gas-sensing technologies is significantly traded-off with their sensitivity and reversibility as well as fabrication and operating costs. In this work, we present an important progress for selective and reversible NO2 sensing by demonstrating an economical sensing platform based on the charge transfer between physisorbed NO2 gas molecules and two-dimensional (2D) tin disulfide (SnS2) flakes at low operating temperatures. The device shows high sensitivity and superior selectivity to NO2 at operating temperatures of less than 160 °C, which are well below those of chemisorptive and ion conductive NO2 sensors with much poorer selectivity. At the same time, excellent reversibility of the sensor is demonstrated, which has rarely been observed in other 2D material counterparts. Such impressive features originate from the planar morphology of 2D SnS2 as well as unique physical affinity and favorable electronic band positions of this material that facilitate the NO2 physisorption and charge transfer at parts per billion levels. The 2D SnS2-based sensor provides a real solution for low-cost and selective NO2 gas sensing.

Nanoporous Nb2O5 has been previously demonstrated to be a viable electrochromic material with strong intercalation characteristics. Despite showing such promising properties, its potential for optical gas sensing applications, which involves the production of ionic species such as H(+), has yet to be explored. Nanoporous Nb2O5 can accommodate a large amount of H(+) ions in a process that results in an energy bandgap change of the material which induces an optical response. Here, we demonstrate the optical hydrogen gas (H2) sensing capability of nanoporous anodic Nb2O5 with a large surface-to-volume ratio prepared via a high temperature anodization method. The large active surface area of the film provides enhanced pathways for efficient hydrogen adsorption and dissociation, which are facilitated by a thin layer of Pt catalyst. We show that the process of H2 sensing causes optical modulations that are investigated in terms of response magnitudes and dynamics. The optical modulations induced by the intercalation process and sensing properties of nanoporous anodic Nb2O5 shown in this work can potentially be used for future optical gas sensing systems.

Solvothermally synthesized Ga2O3 nanoparticles are incorporated into liquid metal/metal oxide (LM/MO) frameworks in order to form enhanced photocatalytic systems. The LM/MO frameworks, both with and without incorporated Ga2O3 nanoparticles, show photocatalytic activity due to a plasmonic effect where performance is related to the loading of Ga2O3 nanoparticles. Optimum photocatalytic efficiency is obtained with 1 wt % incorporation of Ga2O3 nanoparticles. This can be attributed to the sub-bandgap states of LM/MO frameworks, contributing to pseudo-ohmic contacts which reduce the free carrier injection barrier to Ga2O3.

Electrical-based biosensing platforms offer ease of fabrication and simple sensing solutions. Recently, two-dimensional (2D) semiconductors have been proven to be excellent for the fabrication of field effect transistors (FETs) due to their large transconductance, which can be efficiently used for developing sensitive bioplatforms. We present a 2D molybdenum trioxide (MoO3) FET based biosensing platform, using bovine serum albumin as a model protein. The conduction channel is a nanostructured film made of 2D α-MoO3 nanoflakes, with the majority of nanoflake thicknesses being equal to or less than 2.8 nm. The response time is impressively low (less than 10 s), which is due to the high permittivity of the 2D α-MoO3 nanoflakes. The system offers a competitive solution for future biosensing applications.

Two-dimensional (2D) transition metal dichalcogenide semiconductors offer unique electronic and optical properties, which are significantly different from their bulk counterparts. It is known that the electronic structure of 2D MoS2, which is the most popular member of the family, depends on the number of layers. Its electronic structure alters dramatically at near atomically thin morphologies, producing strong photoluminescence (PL). Developing processes for controlling the 2D MoS2 PL is essential to efficiently harness many of its optical capabilities. So far, it has been shown that this PL can be electrically or mechanically gated. Here, we introduce an electrochemical approach to actively control the PL of liquid-phase-exfoliated 2D MoS2 nanoflakes by manipulating the amount of intercalated ions including Li(+), Na(+), and K(+) into and out of the 2D crystal structure. These ions are selected as they are crucial components in many bioprocesses. We show that this controlled intercalation allows for large PL modulations. The introduced electrochemically controlled PL will find significant applications in future chemical and bio-optical sensors as well as optical modulators/switches.

Here, we demonstrate that niobium pentoxide (Nb(2)O(5)) is an ideal candidate for increasing the efficiencies of dye-sensitized solar cells (DSSCs). The key lies in developing a Nb(2)O(5) crisscross nanoporous network, using our unique elevated temperature anodization process. For the same thicknesses of ∼4 μm, the DSSC based on the Nb(2)O(5) layer has a significantly higher efficiency (∼4.1%) when compared to that which incorporates a titanium dioxide nanotubular layer (∼2.7%). This is the highest efficiency among all of the reported photoanodes for such a thickness when utilizing back-side illumination. We ascribe this to a combination of reduced electron scattering, greater surface area, wider band gap, and higher conduction band edge, as well as longer effective electron lifetimes.