Magnetization Reversal and Configurational Anisotropy of Dense Permalloy Dot Arrays

Electron beam patterned permalloy circular dots of 700 nm diameter with small separations were studied by magnetic force microscopy (MFM) in the presence of an in situ magnetic field. Images in the demagnetized state show that the dot is in a vortex state with a vortex core (singularity) in the center. Local hysteresis loops, measured by cantilever frequency shift in an external field, indicate that the magnetization reversal of individual disks is a vortex nucleation and annihilation process. By carefully doing MFM, nucleation and annihilation fields without MFM tip stray field distortions are obtained. Configurational anisotropy originated from magnetostatic coupling is found through hysteresis loops

Facet Cutting and Hydrogenation of In₂O₃ Nanowires for Enhanced Photoelectrochemical Water Splitting

Semiconductor nanowires (NWs) are useful building blocks in optoelectronic, sensing, and energy devices and one-dimensional NWs have been used in photoelectrochemical (PEC) water splitting because of the enhanced light absorption and charge transport. It has been theoretically predicted that the {001} facets of body center cubic (bcc) In₂O₃ nanocrystals can effectively accumulate photogenerated holes under illumination, but it is unclear whether facet cutting of NWs can enhance the efficiency of PEC water splitting. In this work, the photocurrent of square In₂O₃ NWs with four {001} facets is observed to be an order of magnitude larger than that of cylindrical In₂O₃ NWs under the same conditions and subsequent hydrogen treatment further promotes the PEC water splitting performance of the NWs. The optimized hydrogenated In₂O₃ NWs yield a photocurrent density of 1.2 mA/cm² at 0.22 V versus Ag/AgCl with a Faradaic efficiency of about 84.4%. The enhanced PEC properties can be attributed to the reduced band gap due to merging of the disordered layer-induced band tail states with the valence band as well as improved separation of the photogenerated electrons/holes between the In₂O₃ crystal core and disordered layer interface. The results provide experimental evidence of the important role of facet cutting, which is promising in the design and fabrication of NW-based photoelectric devices

Enhanced Photodegradation of Methyl Orange Synergistically by Microcrystal Facet Cutting and Flexible Electrically-Conducting Channels

By performing precise facet cutting during hydrothermal synthesis, single-morphological and uniform-sized octahedral and cubic Cu₂O microcrystals respectively with {111} and {100} facets are synthesized and subsequently encapsulated with reduced graphene oxide (rGO). Electrochemical impedance spectroscopy shows that the rGO/Cu₂O polyhedral composite has excellent conductivity, indicating that rGO can serve as a flexible electrically conducting channel. On account of the accumulation of a large amount of photoexcited electrons on the {111} facets of the octahedrons and efficient electron transfer to the rGO sheet, photodegradation of methyl orange by the rGO/Cu₂O octahedral composite is enhanced by a factor of 4 compared to both bare Cu₂O octahedrons and rGO-encapsulated cubes with hole accumulation on the {100} facets, and the stability of the rGO/Cu₂O octahedrons is obviously improved due to no direct touch with water molecules in comparison with Cu₂O microcrystals without rGO wrap reported previously. This work shows that the combination of crystal facet cutting and conducting channels is an effective strategy to design new composites with enhanced photocatalytic properties

Photothermal Contribution to Enhanced Photocatalytic Performance of Graphene-Based Nanocomposites

Photocatalysts possessing high efficiency in degrading aquatic organic pollutants are highly desirable. Although graphene-based nanocomposites exhibit excellent photocatalytic properties, the role of graphene has been largely underestimated. Herein, the photothermal effect of graphene-based nanocomposites is demonstrated to play an important role in the enhanced photocatalytic performance, which has not been considered previously. In our study on degradation of organic pollutants (methylene blue), the contribution of the photothermal effect caused by a nanocomposite consisting of P25 and reduced graphene oxide can be as high as ∼38% in addition to trapping and shuttling photogenerated electrons and increasing both light absorption and pollutant adsorptivity. The result reveals that the photothermal characteristic of graphene-based nanocomposite is vital to photocatalysis. It implies that designing graphene-based nanocomposites with the improved photothermal performance is a promising strategy to acquire highly efficient photocatalytic activity

Strong Facet-Induced and Light-Controlled Room-Temperature Ferromagnetism in Semiconducting β‑FeSi₂ Nanocubes

Crystalline β-FeSi₂ nanocubes with two {100} facets and four {011} lateral facets synthesized by spontaneous one-step chemical vapor deposition exhibit strong room-temperature ferromagnetism with saturation magnetization of 15 emu/g. The room-temperature ferromagnetism is observed from the β-FeSi₂ nanocubes larger than 150 nm with both the {100} and {011} facets. The ferromagnetism is tentatively explained with a simplified model including both the itinerant electrons in surface states and the local moments on Fe atoms near the surfaces. The work demonstrates the transformation from a nonmagnetic semiconductor to a magnetic one by exposing specific facets and the room-temperature ferromagnetism can be manipulated under light irradiation. The semiconducting β-FeSi₂ nanocubes may have large potential in silicon-based spintronic applications

CdS:Mn–Polysulfido Complex Nanoclusters with H₂O₂‑Dependent and Site-Specific Color Changes

Hydrogen peroxide (H₂O₂) is a potent oxidant that influences the growth and well-being of living organisms, and development of a probe to monitor H₂O₂ in biological media is of broad interest to chemistry, biology, and medicine. Herein, CdS:Mn–polysulfido nanoclusters (NCs) are demonstrated to be chromogenic probes that can monitor H₂O₂ in a fast and site-specific way. The NC complex is formed by a reaction between manganese ions and both polysulfido chains and small CdS nanoparticles. The NCs exhibit two color changes from beige to bright yellow and then colorless in the presence of H₂O₂. The bright yellow color appears within 15 s after the NCs come in contact with H₂O₂ and fades at a rate that is positively related to the H₂O₂ concentration. The appearance of the bright yellow color is accompanied by the production of a strong absorption peak at 387 nm related to free Mn-bonded polysulfido chains. Our results demonstrate potential applications of inorganic chromogenic nanomaterials to the monitoring of chemical and biological reactions

Emission from Trions in Carbon Quantum Dots

The photoluminescence (PL) spectra acquired from 1 to 6 nm large carbon quantum dots (CQDs) prepared by refluxing activated carbon in HNO₃ show blue emission independent of the excitation wavelength as well as long-wavelength emission depending on the excitation wavelength. The dependence of the two emissions on pH is investigated, and the experimental results show that the peak position of the long-wavelength emission does not change with pH; however, the blue emission becomes more asymmetrical, and obvious shoulder peaks emerge as the pH increases. A model based on defect-bound trions in the CQDs is proposed to explain the shoulder peaks in the blue emission at high pH, and the calculated results agree well with experimental data concerning the integral intensity ratio of the trion to exciton emissions versus pH. Our experimental and theoretical results demonstrate for the first time emission from trions in CQDs

Synergistic WO₃·2H₂O Nanoplates/WS₂ Hybrid Catalysts for High-Efficiency Hydrogen Evolution

Tungsten trioxide dihydrate (WO₃·2H₂O) nanoplates are prepared by <i>in situ</i> anodic oxidation of tungsten disulfide (WS₂) film on carbon fiber paper (CFP). The WO₃·2H₂O/WS₂ hybrid catalyst exhibits excellent synergistic effects which facilitate the kinetics of the hydrogen evolution reaction (HER). The electrochromatic effect takes place via hydrogen intercalation into WO₃·2H₂O. This process is accelerated by the desirable proton diffusion coefficient in the layered WO₃·2H₂O. Hydrogen spillover from WO₃·2H₂O to WS₂ occurs via atomic polarization caused by the electric field of the charges on the planar defect or edge active sites of WS₂. The optimized hybrid catalyst presents a geometrical current density of 100 mA cm^–2 at 152 mV overpotential with a Tafel slope of ∼54 mV per decade, making the materials one of the most active nonprecious metal HER catalysts

Cubic In₂O₃ Microparticles for Efficient Photoelectrochemical Oxygen Evolution

Cubic In₂O₃ microparticles with exposed {001} facets as well as single morphology and size are produced on a large scale on silicon with a high yield. The morphological evolution during chemical vapor deposition is investigated and the new knowledge enables precise facet cutting. The synthesized Cubic In₂O₃ microparticles possess superior photoelectrocatalytic activity and excellent chemical and structural stability in oxygen evolution reaction on account of the unique surface structure and electronic band structure of the {001} facets. Our results reveal that it is feasible to promote the photolectrochemical water splitting efficiency of photoanode materials by controlling the growth on specific crystal facets. The technique and concept can be extended to other facet-specific materials in applications such as sensors, solar cells, and lithium batteries

Identification of Lattice Oxygen in Few-Layer Black Phosphorous Exfoliated in Ultrahigh Vacuum and Largely Improved Ambipolar Field-Effect Mobilities by Hydrogenation and Phosphorization

Black phosphorus (BP) has recently attracted considerable attention due to its unique structure and fascinating optical and electronic properties as well as possible applications in photothermal agents. However, its main drawback is rapid degradation in ambient environments of H₂O and O₂, which has led to much research on the improvement of its stability. Unfortunately, this research has not shown great improvement in carrier mobilities. Here, we perform scanning tunneling microscopy observations of few-layer BP (FLBP) sheets exfoliated in ultrahigh vacuum and reveal, for the first time, the existence of lattice oxygen introduced during crystal growth. As a proof-of-concept application, hydrogenation is conducted to remove the lattice oxygen atoms followed by phosphorization, which repairs the phosphorous vacancies caused by mechanical exfoliation and hydrogenation. The resulting FLBP sheets show high ambipolar field-effect mobilities of 1374 cm² V^–1 s^–1 for holes and 607 cm² V^–1 s^–1 for electrons at 2 K. After storage in air for 3 days, the hole and electron mobilities only decrease to 1181 and 518 cm² V^–1 s^–1, respectively, and no structural degradation is observed. This work suggests an effective means to improve both the mobility and stability of BP sheets rendering practical application of FLBP sheets possible