Enabling metallic behaviour in two-dimensional superlattice of semiconductor colloidal quantum dots.

Semiconducting colloidal quantum dots and their assemblies exhibit superior optical properties owing to the quantum confinement effect. Thus, they are attracting tremendous interest from fundamental research to commercial applications. However, the electrical conducting properties remain detrimental predominantly due to the orientational disorder of quantum dots in the assembly. Here we report high conductivity and the consequent metallic behaviour of semiconducting colloidal quantum dots of lead sulphide. Precise facet orientation control to forming highly-ordered quasi-2-dimensional epitaxially-connected quantum dot superlattices is vital for high conductivity. The intrinsically high mobility over 10 cm2 V-1 s-1 and temperature-independent behaviour proved the high potential of semiconductor quantum dots for electrical conducting properties. Furthermore, the continuously tunable subband filling will enable quantum dot superlattices to be a future platform for emerging physical properties investigations, such as strongly correlated and topological states, as demonstrated in the moiré superlattices of twisted bilayer graphene.

Novel Bending Sensor Based on a Solution-Processed Cu2O Film with High Resolution Covering a Wide Curvature Range.

A Cu2O film is prepared on a flexible polyethylene terephthalate substrate for a bending sensor using the spin-spray method, a facile and low-environmental-load solution process. The Cu2O bending sensor shows high sensitivity and high resolution not only over a wide range of curvatures (0 < κ < 0.21 mm-1) but also for very small curvature changes (Δκ = ∼ 0.03 mm-1). The bending response of the sensor exhibited a curvature change of high linearity with a good gauge factor (18.2) owing to the grain-boundary resistance and piezoresistive effects of the fabricated Cu2O film. In addition, the sensor possesses good repeatability, stability, and long-term (>30 days) and mechanical fatigue durability (1000 bending-release cycles). The sensor is capable of detailed monitoring of large- and small-scale human motions, such as finger bending, wrist bending, nodding, mouth opening/closing, and swallowing. In addition, excellent stability and repeatability of the monitoring performance is observed over a wide range of motion angles and speeds. All of these results demonstrate the potential of the flexible bending sensor based on the Cu2O film as a candidate for healthcare monitoring and wearable electronics.

Exclusive Electron Transport in Core@Shell PbTe@PbS Colloidal Semiconductor Nanocrystal Assemblies.

Assemblies of colloidal semiconductor nanocrystals (NCs) in the form of thin solid films leverage the size-dependent quantum confinement properties and the wet chemical methods vital for the development of the emerging solution-processable electronics, photonics, and optoelectronics technologies. The ability to control the charge carrier transport in the colloidal NC assemblies is fundamental for altering their electronic and optical properties for the desired applications. Here we demonstrate a strategy to render the solids of narrow-bandgap NC assemblies exclusively electron-transporting by creating a type-II heterojunction via shelling. Electronic transport of molecularly cross-linked PbTe@PbS core@shell NC assemblies is measured using both a conventional solid gate transistor and an electric-double-layer transistor, as well as compared with those of core-only PbTe NCs. In contrast to the ambipolar characteristics demonstrated by many narrow-bandgap NCs, the core@shell NCs exhibit exclusive n-type transport, i.e., drastically suppressed contribution of holes to the overall transport. The PbS shell that forms a type-II heterojunction assists the selective carrier transport by heavy doping of electrons into the PbTe-core conduction level and simultaneously strongly localizes the holes within the NC core valence level. This strongly enhanced n-type transport makes these core@shell NCs suitable for applications where ambipolar characteristics should be actively suppressed, in particular, for thermoelectric and electron-transporting layers in photovoltaic devices.

Hydrogen Production System by Light-Induced α-FeOOH Coupled with Photoreduction.

Invited for the cover of this issue is Tetsuya Yamada, Ken-ichi Katsumata and co-workers at Tokyo Institute of Technology and Tokyo University of Science. The image depicts rust producing hydrogen and purifying the pollutants at the same time by photocatalytic reaction. Read the full text of the article at 10.1002/chem.201903642.

Solution-mediated nanometric growth of α-Fe2O3 with electrocatalytic activity for water oxidation.

This paper describes a simple, low-temperature, and environmentally friendly aqueous route for the layer-by-layer nanometric growth of crystalline α-Fe2O3. The formation mechanism involves alternative sequences of the electrostatic adsorption of Fe2+ ions on the surface and the subsequent onsite oxidation to Fe3+. A combination analysis of X-ray diffraction, scanning electron microscopy, UV-Vis spectroscopy, and X-ray photoelectron spectroscopy revealed that α-Fe2O3 is directly formed without post-growth annealing via designed chemical reactions with a growth rate of ca. 1.7 nm per deposition cycle. The obtained α-Fe2O3 layer exhibits electrocatalytic activity for water oxidation and, at the same time, insignificant photo-electrocatalytic response, indicating its defective nature. The electrocatalytic activity was tailored by annealing up to 500 °C in air, where thermal diffusion of Sn4+ into the α-Fe2O3 lattice from the substrate probably provides an increased electrical conductivity. The subsequent surface-modification with Ni(OH)2 lowers the overpotential (250 mV at 0.5 mA cm-2) in a 1 M KOH solution. These findings open direct growth pathways to functional metal oxide nanolayers via liquid phase atomic layer deposition.

Hydrogen Production System by Light-Induced α-FeOOH Coupled with Photoreduction.

Solar-driven catalysts on semiconductors to produce hydrogen are considered as a means to solve environmental issues. In this study, H2 production coupling with oxygen consumption by noble metal-free α-FeOOH was demonstrated even though the conduction band edge was lower than the reduction potential of H+ to H2 . For activation of α-FeOOH, an electron donor, Hg-Xe irradiation, and low pH (ca. 5) were indispensable factors. The H2 production from H2 O was confirmed by GC-MS using isotope-labeled water (D2 O) and deuterated methanol. The α-FeOOH synthesized by coprecipitation method showed 25 times more active than TiO2 . The photocatalytic activity was stable for over 400 h. Our study suggests that α-FeOOH known as rust can produce H2 by light induction.

Compositional redistribution in CaO-Al2O3-SiO2 glass induced by the migration of a steel microsphere due to continuous-wave-laser irradiation.

A high-power continuous-wave (CW) laser was used to move a steel microsphere through a CaO-Al2O3-SiO2 glass block at room temperature along a trajectory toward the laser source. A compositional analysis revealed that the CaO concentration in the glass decreased at the center of the microsphere's trajectory but increased in the area adjacent to it; the SiO2 concentration showed an opposite trend while the Al2O3 concentration did not change. Further, the compositional difference between the center and the area adjacent to the microsphere trajectory depends on the velocity of the microsphere, which is controllable by tuning the laser power.

Fully Depleted Ti-Nb-Ta-Zr-O Nanotubes: Interfacial Charge Dynamics and Solar Hydrogen Production.

Poor kinetics of hole transportation at the electrode/electrolyte interface is regarded as a primary cause for the mediocre performance of n-type TiO2 photoelectrodes. By adopting nanotubes as the electrode backbone, light absorption and carrier collection can be spatially decoupled, allowing n-type TiO2, with its short hole diffusion length, to maximize the use of the available photoexcited charge carriers during operation in photoelectrochemical (PEC) water splitting. Here, we presented a delicate electrochemical anodization process for the preparation of quaternary Ti-Nb-Ta-Zr-O mixed-oxide (denoted as TNTZO) nanotube arrays and demonstrated their utility in PEC water splitting. The charge-transfer dynamics for the electrodes was investigated using time-resolved photoluminescence, electrochemical impedance spectroscopy, and the decay of open-circuit voltage analysis. Data reveal that the superior photoactivity of TNTZO over pristine TiO2 originated from the introduction of Nd, Ta, and Zr elements, which enhanced the amount of accessible charge carriers, modified the electronic structure, and improved the hole injection kinetics for expediting water splitting. By modulating the water content of the electrolyte employed in the anodization process, the wall thickness of the grown TNTZO nanotubes can be reduced to a size smaller than that of the depletion layer thickness, realizing a fully depleted state for charge carriers to further advance the PEC performance. Hydrogen evolution tests demonstrate the practical efficacy of TNTZO for realizing solar hydrogen production. Furthermore, with the composition complexity and fully depleted band structure, the present TNTZO nanotube arrays may offer a feasible and universal platform for the loading of other semiconductors to construct a sophisticated heterostructure photoelectrode paradigm, in which the photoexcited charge carriers can be entirely utilized for efficient solar-to-fuel conversion.

Anodic and Cathodic Extracellular Electron Transfer by the Filamentous Bacterium Ardenticatena maritima 110S.

Ardenticatena maritima strain 110S is a filamentous bacterium isolated from an iron-rich coastal hydrothermal field, and it is a unique isolate capable of dissimilatory iron or nitrate reduction among the members of the bacterial phylum Chloroflexi. Here, we report the ability of A. maritima strain 110S to utilize electrodes as a sole electron acceptor and donor when coupled with the oxidation of organic compounds and nitrate reduction, respectively. In addition, multicellular filaments with hundreds of cells arranged end-to-end increased the extracellular electron transfer (EET) ability to electrodes by organizing filaments into bundled structures, with the aid of microbially reduced iron oxide minerals on the cell surface of strain 110S. Based on these findings, together with the attempt to detect surface-localized cytochromes in the genome sequence and the demonstration of redox-dependent staining and immunostaining of the cell surface, we propose a model of bidirectional electron transport by A. maritima strain 110S, in which surface-localized multiheme cytochromes and surface-associated iron minerals serve as a conduit of bidirectional EET in multicellular filaments.

In vivo osteoconductivity of surface modified Ti-29Nb-13Ta-4.6Zr alloy with low dissolution of toxic trace elements.

Simulated Body Fluid (SBF) has served as a useful standard to check the bioactivity of implant materials for years. However, it is not perfectly able to imitate human serum; sometimes disparities between the SBF test and animal test were confirmed. Therefore, to ensure the reliability of the results of the SBF test obtained from our previous study, an animal study was performed to check osteoconductivity of surface modified implant materials. Three types of solution processes, hydrothermal (H), electrochemical (E), and hydrothermal-electrochemical (HE), were performed on the Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) to improve its bioactivity, and their bioactivities were measured in vivo using bone-implant contacts (BICs). BICs of the HE- and H-treated samples were significantly higher than that of the control. Metal ion diffusion towards the bone was also evaluated to examine the adverse effect of metal ions. No metal ion diffusion was observed, indicating the safety of our solution processed implant materials.

Anisotropic growth of gas-liquid precipitated ceria mesocrystals to wires several micrometers in length.

Ceria (CeO2) wires with lengths of 6 µm and diameters of tens of nanometers are fabricated through the anisotropic growth of mesocrystals. In the gas-liquid precipitation process, an aqueous Ce(NO3)3 solution is used as a starting material and NH3 gas is used to induce CeO2 precipitation at the gas-liquid interface. CeO2 mesocrystals, formed by this process at 60 °C, grow in the direction of 〈011〉 into micrometer length wires exposing {001} and {011} on their side walls. It is shown that the initial pH of the starting material solution is a key parameter to attain anisotropic growth of the CeO2 mesocrystals. We thus propose the formation mechanism of micrometer length-CeO2 wires from mesocrystals.

Fabrication of nitrogen-doped ZnO nanorod arrays by hydrothermal synthesis and ambient annealing.

Nitrogen-doped ZnO nanorod arrays (N:ZnO NRAs) were fabricated by hydrothermal synthesis using a zinc-ammine complex solution, followed by annealing at elevated temperatures under ambient conditions. After annealing at 400 °C for 1 h, Raman spectra indicated that nitrogen was incorporated in the ZnO crystal structure. NH3-ligands in the zinc-ammine complex precursor were incorporated in ZnO crystals during hydrothermal crystal growth and were then ruptured during annealing. Photoluminescence spectra indicated that during post-annealing, the nitrogen was incorporated into the oxygen site, which created accompanying defects such as oxygen vacancies and/or interstitial oxygen. The absorption edge in diffuse-reflectance UV-visible spectra revealed visible absorption after post-annealing. In addition, the N:ZnO NRAs generated strong visible-light-induced photocurrents. Nitrogen doping caused a decline in carrier density, as confirmed by an electrochemical Mott-Schottky plot. These results suggest that this cost-effective fabrication has many potential applications such as solar-induced water splitting.

A solution-processed tin dioxide film applicable as a transparent and flexible humidity sensor.

An all-solution-processed transparent tin oxide (SnO2)-based humidity sensor was directly prepared on borosilicate glass (SnO2-G) and a flexible polyethylene terephthalate (SnO2-PET) substrate without using a template. The entire process included film deposition by a spin-spray process at 90 °C and subsequent hot water treatment (HWT) at 100 °C. The resistivity of the films dramatically decreased and had semiconductor characteristics after the HWT, even though the as-prepared SnO2-G and SnO2-PET samples were insulators. Based on the results, the variation of the resistivity could be attributed to the formation of a hydroxyl layer on the crystallized SnO2 surface. With the help of the HWT on the SnO2 films, the formation of tin hydroxyl derivatives provided mobile protons, which led to the variation of the electrical properties of SnO2 at ambient conditions with different humidities. The sensitivity of the SnO2-G-HWT and SnO2-PET-HWT at 95% relative humidity (RH) was 35.2 and 3.5 times higher, respectively, than that at 5% RH. Both the sensitivity of the SnO2-G-HWT and SnO2-PET-HWT samples showed a good uptrend corresponding to the increase of RH at 20 ± 1 °C, and the response/recovery time of SnO2-G-HWT and SnO2-PET-HWT was 51/38 s and 69/47 s in the % RH range of 30-70% at 20 ± 1 °C, respectively.

Influence of Surface Morphology and Conductivity on Photocatalytic Performance of Solution-Processed Zinc Oxide Film.

Solution-processed ZnO films with various structures have been fabricated and the influence of surface morphology and conductivity on photocatalytic performance, including improvement of conductivity, hydrogen production, and photodegradation of rhodamine B, has been investigated. Surface morphology was controlled by solution conditions, and as-fabricated films had rod, dense, and flower-like structures. Improved conductivity was revealed by the photocatalytic activity of ZnO, the influence of surface morphology on the photodegradation of rhodamine B, and the relationship with conductivity and hydrogen production.

Study on the Effect of Pt Intercalation into Layered Niobate Perovskite for Photocatalytic Behavior.

A novel photocatalyst consisting of an intercalated perovskite H(1-2x)Pt(x)LaNb2O7 was fabricated by ion exchange. Synchrotron X-ray diffraction and X-ray photoelectron spectroscopy results confirmed that Pt(2+) exists within the interlayer space of the layered perovskite. H(1-2x)Pt(x)LaNb2O7 composed of layered niobate perovskite and intercalated Pt(2+) completely degraded a 20 ppm phenol solution in 3 h under irradiation by Xe light, which exhibits photocatalytic activity superior to those of RbLaNb2O7, Pt-deposited RbLaNb2O7, and HLaNb2O7. From first-principles density functional theory simulation, high photocatalytic activity of H(1-2x)Pt(x)LaNb2O7 is attributed to the emergence of a new O 2p-Pt 5d hybridized band on top of the valence band.

Porous ZrO2 sheets synthesized using an ionothermal method and their absorption properties.

In this study, porous ZrO2 sheets were synthesized using an ionothermal method combined with heat treatments at 400, 600 and 800 °C. Following ionothermal synthesis, NH4Zr2F9 with a sheet-like structure was obtained. After heat treatment, the NH4Zr2F9 was transformed into monoclinic ZrO2 with a porous sheet structure. The ZrO2 was characterized using XRD, FT-IR, FE-SEM TEM, TG-DTA, BET, and zeta potential analysis. The specific surface area of the samples increased with heat treatment temperature, being 12, 17, and 19 m(2) g(-1) for 400, 600, and 800 °C, respectively. In addition, measurement of the zeta potential of the samples in KCl solution showed that all samples were negatively charged at pH 7, and had different isoelectric points. Adsorption was evaluated using methylene blue and methyl orange and the results indicated that samples heated at different temperatures possessed different selectivities for cationic and anionic dyes.

Preparation of iron oxide-impregnated spherical granular activated carbon-carbon composite and its photocatalytic removal of methylene blue in the presence of oxalic acid.

The spherical granular activated carbon-carbon composites (GAC-Fe) with different iron oxide contents (Fe mass% = 0.6-10) were prepared by a pore volume impregnation method. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and N2-adsorption results confirm the presence of amorphous iron oxide, pyrolytic carbon, and graphitized globular carbon nanoparticles covered with amorphous carbon in the CAG-Fe. The rate of photodegradation of methylene blue (MB) in aqueous solution under UV light in the presence of oxalic acid correlates with porosity of the prepared materials. The total MB removal includes the combination of adsorption and photodegradation without the addition of H2O2. The results of total organic carbon (TOC) analysis reveal that the decolorization of MB in aqueous solution containing oxalic acid corresponds to the decomposition of organic compounds to CO2 and H2O.

Photodegradation of gaseous acetaldehyde and methylene blue in aqueous solution with titanium dioxide-loaded activated carbon fiber polymer materials and aquatic plant ecotoxicity tests.

TiO2-supported activated carbon felts (TiO2-ACFTs) were prepared by dip coating of felts composed of activated carbon fibers (ACFs) with either polyester fibers (PS-A20) and/or a polyethylene pulp (PE-W15) in a TiO2 aqueous suspension followed by calcination at 250 °C for 1 h. The as-prepared TiO2-ACFTs with 29-35 wt.% TiO2 were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and N2 adsorption. The TiO2-ACFT(PS-A20) samples with 0 and 29 wt.% TiO2 were microporous with specific surface areas (S BET) of 996 and 738 m(2)/g, respectively, whereas the TiO2-ACFT(PE-W15) samples with 0 and 35 wt.% TiO2 were mesoporous with S BET of 826 and 586 m(2)/g, respectively. Adsorption and photocatalytic activity of the as-prepared samples were evaluated by measuring adsorption in the dark and photodegradation of gaseous acetaldehyde (AcH) and methylene blue (MB) in aqueous solution under UV light. The TiO2 loading caused a considerable decrease in the S BET and MB adsorption capacity along with an increase in MB photodegradation and AcH mineralization. Lemna minor was chosen as a representative aquatic plant for ecotoxicity tests measuring detoxification of water obtained from the MB photodegradation reaction with the TiO2-ACFT samples under UV light.

Porous fluorine-doped tin oxide as a promising substrate for electrochemical biosensors-demonstration in hydrogen peroxide sensing.

Conducting porous substrates of high hydrophilicity are advantageous in applications of electrochemical biosensors as host electrodes, offering not only fast charge transport and large sensing surface areas but also necessary wettability in aqueous analytes. In this study, inexpensive, highly hydrophilic (contact angle < 5°), conducting (sheet resistance of 17 Ω□-1) porous fluorine-doped tin oxide (FTO) glass is fabricated from commercial FTO glass with a novel, simple, one-step Sn4+-based anodic treatment process, and used as a host electrode for electrochemical biosensors. We demonstrate its superior performance with hydrogen peroxide sensing. The hydrogen peroxide sensor is fabricated by simply depositing Pt nanoparticles onto the surface of the porous FTO substrate (PFTO) with a polyol process. Pt-decorated commercial FTO (CFTO) is also investigated as a control. The sensitivity achieved with the Pt-decorated PFTO is almost one order of magnitude higher than that of the Pt-decorated CFTO (25.8 vs. 2.69 mA M-1), and the response time is shortened from 36 s for the Pt-decorated CFTO to 1 s for the Pt-decorated PFTO. The PFTO proves to be a promising electrode substrate for host functional materials in electrochemical biosensors and can be readily applied to sensing of a wide variety of biosubstances.

Preparation of graphitic carbon nitride (g-C3N4)/WO3 composites and enhanced visible-light-driven photodegradation of acetaldehyde gas.

Novel visible-light-driven graphitic carbon nitride (g-C3N4)/WO3 composite photocatalysts were prepared, and the acetaldehyde (CH3CHO) degradation activity of these composites was evaluated. The prepared g-C3N4/WO3 composites were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflection spectroscopy (UV-vis), and the N2 gas adsorption Brunauer-Emmett-Teller (BET) method (N3-BET). The WO3 particles, which were 100-300 nm in size, were in direct contact with the g-C3N4 sheet surface. The optical band gap and specific surface area of the g-C3N4/WO3 composites were in the range of 2.65-2.75 eV and 4-7 m(2)/g, respectively. The g-C3N4/WO3 composites exhibited higher activity for the photodegradation of CH3CHO under visible light irradiation compared to g-C3N4. The optimal WO3 content for the CH3CHO photodegradation activity of the heterojunction structures was determined. The synergistic effect of g-C3N4 and WO3 was considered to lead to improved photogenerated carrier separation. A possible degradation mechanism of CH3CHO over the g-C3N4/WO3 composite photocatalyst under visible light irradiation was proposed. These results should usefully expand applications of g-C3N4 as a visible-light-driven photocatalyst.