Substantially Accelerated Response and Recovery in Pd-Decorated WO3 Nanorods Gasochromic Hydrogen Sensor.

The development of hydrogen (H2) gas sensors is essential for the safe and efficient adoption of H2 gas as a clean, renewable energy source in the challenges against climate change, given its flammability and associated safety risks. Among various H2 sensors, gasochromic sensors have attracted great interest due to their highly intuitive and low power operation, but slow kinetics, especially slow recovery rate limited its further practical application. This study introduces Pd-decorated amorphous WO3 nanorods (Pd-WO3 NRs) as an innovative gasochromic H2 sensor, demonstrating rapid and highly reversible color changes for H2 detection. In specific, the amorphous nanostructure exhibits notable porosity, enabling rapid detection and recovery by facilitating effective H2 gas interaction and efficient diffusion of hydrogen ions (H+) dissociated from the Pd nanoparticles (Pd NPs). The optimized Pd-WO3 NRs sensor achieves an impressive response time of 14 s and a recovery time of 1 s to 5% H2. The impressively fast recovery time of 1 s is observed under a wide range of H2 concentrations (0.2-5%), making this study a fundamental solution to the challenged slow recovery of gasochromic H2 sensors.

Reduced graphene oxide-incorporated calcium phosphate cements with pulsed electromagnetic fields for bone regeneration.

Natural calcium phosphate cements (CPCs) derived from sintered animal bone have been investigated to treat bone defects, but their low mechanical strength remains a critical limitation. Graphene improves the mechanical properties of scaffolds and promotes higher osteoinduction. To this end, reduced graphene oxide-incorporated natural calcium phosphate cements (RGO-CPCs) are fabricated for reinforcement of CPCs' characteristics. Pulsed electromagnetic fields (PEMFs) were additionally applied to RGO-CPCs to promote osteogenic differentiation ability. The fabricated RGO-CPCs show distinct surface properties and chemical properties according to the RGO concentration. The RGO-CPCs' mechanical properties are significantly increased compared to CPCs owing to chemical bonding between RGO and CPCs. In in vitro studies using a mouse osteoblast cell line and rat-derived adipose stem cells, RGO-CPCs are not severely toxic to either cell type. Cell migration study, western blotting, immunocytochemistry, and alizarin red staining assay reveal that osteoinductivity as well as osteoconductivity of RGO-CPCs was highly increased. In in vivo study, RGO-CPCs not only promoted bone ingrowth but also enhanced osteogenic differentiation of stem cells. Application of PEMFs enhanced the osteogenic differentiation of stem cells. RGO-CPCs with PEMFs can overcome the flaws of previously developed natural CPCs and are anticipated to open the gate to clinical application for bone repair and regeneration.

Controlled Band Offsets in Ultrathin Hematite for Enhancing the Photoelectrochemical Water Splitting Performance of Heterostructured Photoanodes.

Formation of type II heterojunctions is a promising strategy to enhance the photoelectrochemical performance of water-splitting photoanodes, which has been tremendously studied. However, there have been few studies focusing on the formation of type II heterojunctions depending on the thickness of the overlayer. Here, enhanced photoelectrochemical activities of a Fe2O3 film deposited-BiVO4/WO3 heterostructure with different thicknesses of the Fe2O3 layer have been investigated. The Fe2O3 (10 nm)/BiVO4/WO3 heterojunction photoanode shows a much higher photocurrent density compared to the Fe2O3 (100 nm)/BiVO4/WO3 photoanode. The Fe2O3 (10 nm)/BiVO4/WO3 trilayer heterojunction anodes have sequential type II junctions, while a thick Fe2O3 overlayer forms an inverse type II junction between Fe2O3 and BiVO4. Furthermore, the incident-photon-to-current efficiency measured under back-illumination is higher than those measured under front-illumination, demonstrating the importance of the illumination sequence for light absorption and charge transfer and transport. This study shows that the thickness of the oxide overlayer influences the energy band alignment and can be a strategy to improve solar water splitting performance. Based on our findings, we propose a photoanode design strategy for efficient photoelectrochemical water splitting.

Crystal Facet Engineering of TiO2 Nanostructures for Enhancing Photoelectrochemical Water Splitting with BiVO4 Nanodots.

Although bismuth vanadate (BiVO4) has been promising as photoanode material for photoelectrochemical water splitting, its charge recombination issue by short charge diffusion length has led to various studies about heterostructure photoanodes. As a hole blocking layer of BiVO4, titanium dioxide (TiO2) has been considered unsuitable because of its relatively positive valence band edge and low electrical conductivity. Herein, a crystal facet engineering of TiO2 nanostructures is proposed to control band structures for the hole blocking layer of BiVO4 nanodots. We design two types of TiO2 nanostructures, which are nanorods (NRs) and nanoflowers (NFs) with different (001) and (110) crystal facets, respectively, and fabricate BiVO4/TiO2 heterostructure photoanodes. The BiVO4/TiO2 NFs showed 4.8 times higher photocurrent density than the BiVO4/TiO2 NRs. Transient decay time analysis and time-resolved photoluminescence reveal the enhancement is attributed to the reduced charge recombination, which is originated from the formation of type II band alignment between BiVO4 nanodots and TiO2 NFs. This work provides not only new insights into the interplay between crystal facets and band structures but also important steps for the design of highly efficient photoelectrodes.

Optically Activated 3D Thin-Shell TiO2 for Super-Sensitive Chemoresistive Responses: Toward Visible Light Activation.

One of the well-known strategies for achieving high-performance light-activated gas sensors is to design a nanostructure for effective surface responses with its geometric advances. However, no study has gone beyond the benefits of the large surface area and provided fundamental strategies to offer a rational structure for increasing their optical and chemical performances. Here, a new class of UV-activated sensing nanoarchitecture made of highly periodic 3D TiO2, which facilitates 55 times enhanced light absorption by confining the incident light in the nanostructure, is prepared as an active gas channel. The key parameters, such as the total 3D TiO2 film and thin-shell thicknesses, are precisely optimized by finite element analysis. Collectively, this fundamental design leads to ultrahigh chemoresistive response to NO2 with a theoretical detection limit of ≈200 ppt. The demonstration of high responses with visible light illumination proposes a future perspective for light-activated gas sensors based on semiconducting oxides.

Template Engineering of CuBi2 O4 Single-Crystal Thin Film Photocathodes.

To develop strategies for efficient photo-electrochemical water-splitting, it is important to understand the fundamental properties of oxide photoelectrodes by synthesizing and investigating their single-crystal thin films. However, it is challenging to synthesize high-quality single-crystal thin films from copper-based oxide photoelectrodes due to the occurrence of significant defects such as copper or oxygen vacancies and grains. Here, the CuBi2 O4 (CBO) single-crystal thin film photocathode is achieved using a NiO template layer grown on single-crystal SrTiO3 (STO) (001) substrate via pulsed laser deposition. The NiO template layer plays a role as a buffer layer of large lattice mismatch between CBO and STO (001) substrate through domain-matching epitaxy, and forms a type-II band alignment with CBO, which prohibits the transfer of photogenerated electrons toward bottom electrode. The photocurrent densities of the CBO single-crystal thin film photocathode demonstrate -0.4 and -0.7 mA cm-2 at even 0 VRHE with no severe dark current under illumination in a 0.1 m potassium phosphate buffer solution without and with H2 O2 as an electron scavenger, respectively. The successful synthesis of high-quality CBO single-crystal thin film would be a cornerstone for the in-depth understanding of the fundamental properties of CBO toward efficient photo-electrochemical water-splitting.

Graphene Oxide-Assisted Promotion of Plant Growth and Stability.

The control and promotion of plant and crop growth are important challenges globally. In this study, we have developed a nanomaterial-assisted bionic strategy for accelerating plant growth. Although nanomaterials have been shown to be toxic to plants, we demonstrate herein that graphene oxide can be used as a regulator tool for enhancing plant growth and stability. Graphene oxide was added to the growth medium of Arabidopsis thaliana L. as well as injected into the stem of the watermelon plant. We showed that with an appropriate amount provided, graphene oxide had a positive effect on plant growth in terms of increasing the length of roots, the area of leaves, the number of leaves, and the formation of flower buds. In addition, graphene oxide affected the watermelon ripeness, increasing the perimeter and sugar content of the fruit. We believe that graphene oxide may be used as a strategy for enabling the acceleration of both plant growth and the fruit ripening process.

Ultrahigh-Mobility and Solution-Processed Inorganic P-Channel Thin-Film Transistors Based on a Transition-Metal Halide Semiconductor.

The development of p-channel devices with comparable electrical performances to their n-channel counterparts has been delayed due to the lack of p-type semiconductor materials and device optimization. In this present work, we successfully demonstrated p-channel inorganic thin-film transistors (TFTs) with high hole mobilities similar to the values of n-channel devices. To boost the device performance, the solution-processed copper iodide (CuI) semiconductor was gated by a solid polymer electrolyte. The electrolyte gating could realize electrical double layer (EDL) formation and a three-dimensional carrier transport channel and thus substantially increased charge accumulation in the channel region and realized a high mobility above 90 cm2/(V s) (45.12 ± 22.19 cm2/(V s) on average). In addition, due to the high-capacitance EDL formed by electrolyte gating, the CuI TFTs exhibited a low operation voltage below 0.5 V (Vth = -0.045 V) and a high ON current level of 0.7 mA with an ON/OFF ratio of 1.52 × 103. We also evaluated the operational stabilities of CuI TFTs and the devices showed 80% retention under electrical/mechanical stress. All the active layers of the transistors were fabricated by solution processes at low temperatures (<100 °C), indicating their potential use for flexible, wearable, and high-performance electronic applications.

In Situ Growth of Nanostructured BiVO4-Bi2O3 Mixed-Phase via Nonequilibrium Deposition Involving Metal Exsolution for Enhanced Photoelectrochemical Water Splitting.

Nonequilibrium deposition is a remarkable method for the in situ growth of unique nanostructures and phases for the functionalization of thin films. We introduce a distinctive structure of a mixed-phase, composed of BiVO4 and ß-Bi2O3, for photoelectrochemical water splitting. The mixed-phase is fabricated via nonequilibrium deposition by adjusted oxygen partial pressure. According to density functional theory calculations, we find that vanadium exsolution can be facilitated by introducing oxygen vacancies, enabling the fabrication of a nanostructured mixed-phase. These unique structures enhance charge migration by increasing the interfacial area and properly aligning the band offset between two crystalline phases. Consequently, the photocurrent density of the nanostructured mixed-phase thin films is about twice that of pristine BiVO4 thin films at 1.23 VRHE. Our work suggests that nonequilibrium deposition provides an innovative route for the structural engineering of photoelectrodes for the understanding of fundamental properties and improving the photocatalytic performance for solar water splitting.

Ionic-Activated Chemiresistive Gas Sensors for Room-Temperature Operation.

The development of high performance gas sensors that operate at room temperature has attracted considerable attention. Unfortunately, the conventional mechanism of chemiresistive sensors is restricted at room temperature by insufficient reaction energy with target molecules. Herein, novel strategy for room temperature gas sensors is reported using an ionic-activated sensing mechanism. The investigation reveals that a hydroxide layer is developed by the applied voltages on the SnO2 surface in the presence of humidity, leading to increased electrical conductivity. Surprisingly, the experimental results indicate ideal sensing behavior at room temperature for NO2 detection with sub-parts-per-trillion (132.3 ppt) detection and fast recovery (25.7 s) to 5 ppm NO2 under humid conditions. The ionic-activated sensing mechanism is proposed as a cascade process involving the formation of ionic conduction, reaction with a target gas, and demonstrates the novelty of the approach. It is believed that the results presented will open new pathways as a promising method for room temperature gas sensors.

SnS2 Nanograins on Porous SiO2 Nanorods Template for Highly Sensitive NO2 Sensor at Room Temperature with Excellent Recovery.

In order to develop high performance chemoresistive gas sensors for Internet of Everything applications, low power consumption should be achieved due to the limited battery capacity of portable devices. One of the most efficient ways to reduce power consumption is to lower the operating temperature to room temperature. Herein, we report superior gas sensing properties of SnS2 nanograins on SiO2 nanorods toward NO2 at room temperature. The gas response is as high as 701% for 10 ppm of NO2 with excellent recovery characteristics and the theoretical detection limit is evaluated to be 408.9 ppb at room temperature, which has not been reported for SnS2-based gas sensors to the best of our knowledge. The SnS2 nanograins on the template used in this study have excessive sulfur component (Sn:S = 1:2.33) and exhibit p-type conduction behavior. These results will provide a new perspective of nanostructured two-dimensional materials for gas sensor applications on demand.

Tungsten Trioxide Doped with CdSe Quantum Dots for Smart Windows.

Nanocrystal quantum dots (QDs) provide tunable optoelectronic properties on the basis of their dimension. CdSe QDs, which are size-dependent colloidal nanocrystals, are used for efficient electrochromic devices owing to their unique properties in modulating quantum confinement, resulting in enhanced electron insertion during the electrochromic process. Incorporating a well-known metal oxide electrochromic material such as WO3 into CdSe QDs enhances the redox process. Herein, we propose a facile method for producing and optimizing CdSe QDs doped in WO3. The fabrication of the electrochromic film involves a solution and annealing process. Moreover, the effect of the QD size to optimize the electrochromic layer is studied. As a result, the coloration efficiency of WO3 and optimized CdSe QD-WO3 are obtained as 68.6 and 112.3 cm2/C, respectively. Thus, size-tunable nanocrystal QDs combined with a metal oxide yield high-performance electrochromic devices and are promising candidates for producing smart windows.

Efficient Water Splitting Cascade Photoanodes with Ligand-Engineered MnO Cocatalysts.

The band edge positions of semiconductors determine functionality in solar water splitting. While ligand exchange is known to enable modification of the band structure, its crucial role in water splitting efficiency is not yet fully understood. Here, ligand-engineered manganese oxide cocatalyst nanoparticles (MnO NPs) on bismuth vanadate (BiVO4) anodes are first demonstrated, and a remarkably enhanced photocurrent density of 6.25 mA cm-2 is achieved. It is close to 85% of the theoretical photocurrent density (≈7.5 mA cm-2) of BiVO4. Improved photoactivity is closely related to the substantial shifts in band edge energies that originate from both the induced dipole at the ligand/MnO interface and the intrinsic dipole of the ligand. Combined spectroscopic analysis and electrochemical study reveal the clear relationship between the surface modification and the band edge positions for water oxidation. The proposed concept has considerable potential to explore new, efficient solar water splitting systems.

Thienoisoindigo-Based Semiconductor Nanowires Assembled with 2-Bromobenzaldehyde via Both Halogen and Chalcogen Bonding.

We fabricated nanowires of a conjugated oligomer and applied them to organic field-effect transistors (OFETs). The supramolecular assemblies of a thienoisoindigo-based small molecular organic semiconductor (TIIG-Bz) were prepared by co-precipitation with 2-bromobenzaldehyde (2-BBA) via a combination of halogen bonding (XB) between the bromide in 2-BBA and electron-donor groups in TIIG-Bz, and chalcogen bonding (CB) between the aldehyde in 2-BBA and sulfur in TIIG-Bz. It was found that 2-BBA could be incorporated into the conjugated planes of TIIG-Bz via XB and CB pairs, thereby increasing the π - π stacking area between the conjugated planes. As a result, the driving force for one-dimensional growth of the supramolecular assemblies via π - π stacking was significantly enhanced. TIIG-Bz/2-BBA nanowires were used to fabricate OFETs, showing significantly enhanced charge transfer mobility compared to OFETs based on pure TIIG-Bz thin films and nanowires, which demonstrates the benefit of nanowire fabrication using 2-BBA.

Synthesis of Numerous Edge Sites in MoS2 via SiO2 Nanorods Platform for Highly Sensitive Gas Sensor.

The utilization of edge sites in two-dimensional materials including transition-metal dichalcogenides (TMDs) is an effective strategy to realize high-performance gas sensors because of their high catalytic activity. Herein, we demonstrate a facile strategy to synthesize the numerous edge sites of vertically aligned MoS2 and larger surface area via SiO2 nanorod (NRs) platforms for highly sensitive NO2 gas sensor. The SiO2 NRs encapsulated by MoS2 film with numerous edge sites and partially vertical-aligned regions synthesized using simple thermolysis process of [(NH4)2MoS4]. Especially, the vertically aligned MoS2 prepared on 500 nm thick SiO2 NRs (500MoS2) shows approximately 90 times higher gas-sensing response to 50 ppm NO2 at room temperature than the MoS2 film prepared on flat SiO2, and the theoretical detection limit is as low as ∼2.3 ppb. Additionally, it shows reliable operation with reversible response to NO2 gas without degradation at an operating temperature of 100 °C. The use of the proposed facile approach to synthesize vertically aligned TMDs using nanostructured platform can be extended for various TMD-based devices including sensors, water splitting catalysts, and batteries.

Hierarchical carbon-silicon nanowire heterostructures for the hydrogen evolution reaction.

Silicon nanowires (SiNWs) opened up exciting possibilities in a variety of research fields due to their unique anisotropic morphologies, facile tuning capabilities, and accessible fabrication methods. The SiNW-based photoelectrochemical (PEC) conversion has recently been known to provide an efficiency superior to that of various photo-responsive semiconductor heterostructures. However, a challenge still remains in designing optimum structures to minimize photo-oxidation and photo-corrosion of the Si surface in a liquid electrolyte. Here, we report a simple method to synthesize hierarchically branched carbon nanowires (CNWs) on SiNWs utilizing copper vapor as the catalyst in a chemical vapor deposition (CVD) process, which exhibits outstanding photocatalytic activities for hydrogen generation along with excellent chemical stability against oxidation and corrosion. Thus, we believe that the CNW-SiNW photoelectrodes would provide a new route to developing high-performing cost-effective catalysts essential for advanced energy conversion and storage technologies.

Wet Pretreatment-Induced Modification of Cu(In,Ga)Se2/Cd-Free ZnTiO Buffer Interface.

We report a novel Cd-free ZnTiO buffer layer deposited by atomic layer deposition for Cu(In,Ga)Se2 (CIGS) solar cells. Wet pretreatments of the CIGS absorbers with NH4OH, H2O, and/or aqueous solution of Cd2+ ions were explored to improve the quality of the CIGS/ZnTiO interface, and their effects on the chemical state of the absorber and the final performance of Cd-free CIGS devices were investigated. X-ray photoelectron spectroscopy (XPS) analysis revealed that the aqueous solution etched away sodium compounds accumulated on the CIGS surface, which was found to be detrimental for solar cell operation. Wet treatment with NH4OH solution led to a reduced photocurrent, which was attributed to the thinning (or removal) of an ordered vacancy compound (OVC) layer on the CIGS surface as evidenced by an increased Cu XPS peak intensity after the NH4OH treatment. However, the addition of Cd2+ ions to the NH4OH aqueous solution suppressed the etching of the OVC by NH4OH, explaining why such a negative effect of NH4OH is not present in the conventional chemical bath deposition of CdS. The band alignment at the CIGS/ZnTiO interface was quantified using XPS depth profile measurements. A small cliff-like conduction band offset of -0.11 eV was identified at the interface, which indicates room for further improvement of efficiency of the CIGS/ZnTiO solar cells once the band alignment is altered to a slight spike by inserting a passivation layer with a higher conduction band edge than ZnTiO. Combination of the small cliff conduction band offset at the interface, removal of the Na compound via water, and surface doping by Cd ions allowed the application of ZnTiO buffer to CIGS treated with Cd solutions, exhibiting an efficiency of 80% compared to that of a reference CIGS solar cell treated with the CdS.

Hierarchical Ag nanostructures on Sn-doped indium oxide nano-branches: super-hydrophobic surface for surface-enhanced Raman scattering.

Herein, we fabricated a super-hydrophobic SERS substrate using Sn-doped indium oxide (Indium-tin-oxide: ITO) nano-branches as a template. ITO nano-branches with tens of nanometer diameter are first fabricated through the vapor-liquid-solid (VLS) growth to provide roughness of the substrate. 10 nm thickness of Ag thin film was deposited and then treated with the post-annealing process to create numerous air-pockets in the Ag film, forming a hierarchical Ag nanostructures. The resulting substrate obtained Cassie's wetting property with a water contact angle of 151°. Compared to the normal hydrophobic Ag nanoparticle substrate, increase of about 4.25-fold higher SERS signal was obtained for 7 µL of rhodamine 6G aqueous solutions.

Facile Solution Synthesis of Tungsten Trioxide Doped with Nanocrystalline Molybdenum Trioxide for Electrochromic Devices.

A facile, highly efficient approach to obtain molybdenum trioxide (MoO3)-doped tungsten trioxide (WO3) is reported. An annealing process was used to transform ammonium tetrathiotungstate [(NH4)2WS4] to WO3 in the presence of oxygen. Ammonium tetrathiomolybdate [(NH4)2MoS4] was used as a dopant to improve the film for use in an electrochromic (EC) cell. (NH4)2MoS4 at different concentrations (10, 20, 30, and 40 mM) was added to the (NH4)2WS4 precursor by sonication and the samples were annealed at 500 °C in air. Raman, X-ray diffraction, and X-ray photoelectron spectroscopy measurements confirmed that the (NH4)2WS4 precursor decomposed to WO3 and the (NH4)2MoS4-(NH4)2WS4 precursor was transformed to MoO3-doped WO3 after annealing at 500 °C. It is shown that the MoO3-doped WO3 film is more uniform and porous than pure WO3, confirming the doping quality and the privileges of the proposed method. The optimal MoO3-doped WO3 used as an EC layer exhibited a high coloration efficiency of 128.1 cm2/C, which is larger than that of pure WO3 (74.5 cm2/C). Therefore, MoO3-doped WO3 synthesized by the reported method is a promising candidate for high-efficiency and low-cost smart windows.

Polarized Light-Emitting Diodes Based on Patterned MoS2 Nanosheet Hole Transport Layer.

Here, this study successfully fabricates few-layer MoS2 nanosheets from (NH4 )2 MoS4 and applies them as the hole transport layer as well as the template for highly polarized organic light-emitting diodes (OLEDs). The obtained material consists of polycrystalline MoS2 nanosheets with thicknesses of 2 nm. The MoS2 nanosheets are patterned by rubbing/ion-beam treatment. The Raman spectra shows that {poly(9,9-dioctylfluorene-alt-benzothiadiazole), poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]} (F8BT) on patterned MoS2 exhibits distinctive polarization behavior. It is discovered that patterned MoS2 not only improves the device efficiency but also changes the polarization behavior of the devices owing to the alignment of F8BT. This work demonstrates a highly efficient polarized OLED with a polarization ratio of 62.5:1 in the emission spectrum (166.7:1 at the peak intensity of 540 nm), which meets the manufacturing requirement. In addition, the use of patterned MoS2 nanosheets not only tunes the polarization of the OLEDs but also dramatically improves the device performance as compared with that of devices using untreated MoS2 .