An Ultrasensitive Room-Temperature H2 Sensor Based on a TiO2 Rutile-Anatase Homojunction.

Metal oxide semiconductor hetero- and homojunctions are commonly constructed to improve the performance of hydrogen sensors at room temperature. In this study, a simple two-step hydrothermal method was employed to prepare TiO2 films with homojunctions of rutile and anatase phases (denoted as TiO2-R/A). Then, the microstructure of anatase-phase TiO2 was altered by controlling the amount of hydrochloric acid to realize a more favorable porous structure for charge transport and a larger surface area for contact with H2. The sensor used a Pt interdigital electrode. At an optimal HCl dosage (25 mL), anatase-phase TiO2 uniformly covered rutile-phase TiO2 nanorods, resulting in a greater response to H2 at 2500 ppm compared with that of a rutile TiO2 nanorod sensor by a factor of 1153. The response time was 21 s, mainly because the homojunction formed by the TiO2 rutile and anatase phases increased the synergistic effect of the charge transfer and potential barrier between the two phases, resulting in the formation of more superoxide (O2-) free radicals on the surface. Furthermore, the porous structure increased the surface area for H2 adsorption. The TiO2-R/A-based sensor exhibited high selectivity, long-term stability, and a fast response. This study provides new insights into the design of commercially competitive hydrogen sensors.

A 3D structure C/Si/ZnCo2O4/CC anode for flexible lithium-ion batteries with high capacity and fast charging ability.

ZnCo2O4 has attracted extensive attention as a bimetallic transition metal oxide anode material for lithium-ion batteries (LIBs) with high capacity. However, there is still a long way to go to meet the increasing demand for commercial batteries due to their modest conductivity and unobtrusive cycling stability. The use of finely controlled nanostructures and combination with other anode materials are the two main ways to improve the battery performance of ZnCo2O4. Herein, ZnCo2O4 (ZCO) nanosheets were in situ grown on carbon cloth (CC) through a facile solution method. Si was coated onto the ZCO nanosheet arrays by the magnetron sputtering method (SCZO/CC) to acheive the capacity increase. A layer of C was further coated onto SZCO/CC to improve the electrical conductivity of the whole electrode and to protect the SZCO nanostructure. The obtained CSZCO/CC electrode exhibits a high reversible areal capacity of 1.16 mA h cm-2 at 5 mA cm-2 after 500 cycles. At an ultra-high current density of 10 mA cm-2, the CSZCO/CC electrode can still present a capacity of 0.38 mA h cm-2 and maintain a capacity retention of 88.4% for 2000 cycles. In situ Raman spectroscopy was used to study the relationship between the electrochemical performance and structure of the electrode materials. The carbon cloth was found to have contributed a nonnegligible part of the capacity of the electrode.

Designing and fabricating a CdS QDs/Bi2MoO6 monolayer S-scheme heterojunction for highly efficient photocatalytic C2H4 degradation under visible light.

Achieving efficient photocatalytic degradation of atmospheric volatile organic compounds (VOCs) under sun-light is still a significant challenge for environmental protection. The S-scheme heterojunction with its unique charge migration route, high charge separation rate and strong redox ability, has great potential. However, how to regulate interfacial charge transfer of the S-scheme heterojunction is of significant importance. Here, density functional theory (DFT) calculations were first conducted and predicted that an S-scheme heterojunction could be formed in the CdS quantum dots/Bi2MoO6 monolayer system. Subsequently, this novel heterojunction is constructed by in-situ hydrothermal synthesis of CdS quantum dots on monolayer Bi2MoO6. Under visible-light, this novel S-scheme system gives a high-efficiency photocatalytic degradation rate (6.04 × 10-2 min-1) towards C2H4, which is 30.3 times higher than that of pure CdS (1.99 × 10-3 min-1) and 41.7 times higher than pure Bi2MoO6 (1.45 × 10-3 min-1). Strong evidence for the S-scheme charge transfer path is provided by in-situ XPS, PL, TRPL and EPR.

Dynamic Reaction Mechanism of P-N-Switched H2-Sensing Performance on a Pt-Decorated TiO2 Surface.

Pt decoration is known to be one of the most promising strategies to enhance the performance of TiO2 hydrogen gas sensors, while the effect of Pt-decorating concentration on the sensing performance of TiO2 and the specific interaction between Pt and TiO2 have not been fully investigated. Here, a series of TiO2 nanoarray thin films with differing amounts of Pt decorated (Pt/TiO2) is fabricated, and the H2-sensing performance is evaluated. A switch in the response from P-type to N-type is observed with increasing Pt decoration. The response additionally depends on the H2 concentration: resistance increases in low H2 concentrations and decreases in hydrogen concentrations higher than 40 ppm. This is explained by the competitive adsorption of hydrogen between the Pt nanoparticles (Pt NPs) and the exposed TiO2 surface. The preference for H2 adsorption and splitting between Pt and TiO2 is established by DFT calculations. Humidity brings preferential adsorption of H2O on the surface of Pt, which affects the following adsorption and splitting of H2, thus resulting in a P-N switch of the sensing performance. The detailed dynamic reaction process is described according to the findings.

Synergistic Cooperation of Rutile TiO2 {002}, {101}, and {110} Facets for Hydrogen Sensing.

An oriented TiO2 thin film-based hydrogen sensor has been demonstrated to have excellent sensing properties at room temperature. The exposed high energy surface offers a low energy barrier for H2 adsorption and dissociation. In this work, rutile TiO2 with {101} and {002} facets exposed was controllably synthesized by adjusting the ethanol content of the hydrothermal solvent. The crystalline structure, morphologies, and H2 sensing performance of the samples varied with the relative ratios of {002} and {101} facets. By increasing the ethanol content, the (002) orientation growth was enhanced and the (101) orientation growth was restrained, the size of the nanorods composing the thin film was reduced and the density of the film was increased. All of the prepared TiO2 nanorod array film-based hydrogen sensors performed very well at room temperature. The TiO2 hydrogen sensor with both {110} and {002} facets exposed gave a faster response, as well as better repeatability and stability than those with only {002} facets. Density functional theory simulations have been adopted to reveal the surface interaction of H2 and the TiO2 surface. The results suggested that H2 tended to be adsorbed and dissociated on the (002) and (101) surface. There is a very small active barrier for atomic H to recombine into H2 molecules on the (110) surface. Thin films with lower density, where more (110) surface is exposed, offered more space for H2 regeneration, leading to shorter response and recovery times as well as higher sensitivity. The (002), (101), and (110) surfaces of rutile TiO2 synergistically cooperated to complete the whole H2 sensing process.

n-type chalcogenides by ion implantation.

Carrier-type reversal to enable the formation of semiconductor p-n junctions is a prerequisite for many electronic applications. Chalcogenide glasses are p-type semiconductors and their applications have been limited by the extraordinary difficulty in obtaining n-type conductivity. The ability to form chalcogenide glass p-n junctions could improve the performance of phase-change memory and thermoelectric devices and allow the direct electronic control of nonlinear optical devices. Previously, carrier-type reversal has been restricted to the GeCh (Ch=S, Se, Te) family of glasses, with very high Bi or Pb 'doping' concentrations (~5-11 at.%), incorporated during high-temperature glass melting. Here we report the first n-type doping of chalcogenide glasses by ion implantation of Bi into GeTe and GaLaSO amorphous films, demonstrating rectification and photocurrent in a Bi-implanted GaLaSO device. The electrical doping effect of Bi is observed at a 100 times lower concentration than for Bi melt-doped GeCh glasses.

Crystal field analysis of Dy and Tm implanted silicon for photonic and quantum technologies.

We report the lattice site and symmetry of optically active Dy3+ and Tm3+ implanted Si. Local symmetry was determined by fitting crystal field parameters (CFPs), corresponding to various common symmetries, to the ground state splitting determined by photoluminescence measurements. These CFP values were then used to calculate the splitting of every J manifold. We find that both Dy and Tm ions are in a Si substitution site with local tetragonal symmetry. Knowledge of rare-earth ion symmetry is important in maximising the number of optically active centres and for quantum technology applications where local symmetry can be used to control decoherence.

On the analogy between photoluminescence and carrier-type reversal in Bi- and Pb-doped glasses.

Reaction order in Bi-doped oxide glasses depends on the optical basicity of the glass host. Red and NIR photoluminescence (PL) bands result from Bi(2+) and Bin clusters, respectively. Very similar centers are present in Bi- and Pb-doped oxide and chalcogenide glasses. Bi-implanted and Bi melt-doped chalcogenide glasses display new PL bands, indicating that new Bi centers are formed. Bi-related PL bands have been observed in glasses with very similar compositions to those in which carrier-type reversal has been observed, indicating that these phenomena are related to the same Bi centers, which we suggest are interstitial Bi(2+) and Bi clusters.

Eye-safe 2 µm luminescence from thulium-doped silicon.

We report on photoluminescence in the 1.7-2.1 µm range of silicon doped with thulium. This is achieved by the implantation of Tm into silicon that has been codoped with boron to reduce the thermal quenching. At least six strong lines can be distinguished at 80 K; at 300 K, the spectrum is dominated by the main emission at 2 µm. These emissions are attributed to the trivalent Tm(3+) internal transitions between the first excited state and the ground state.