Fengchun Hu

Vacancy-Induced Ferromagnetism of MoS2 Nanosheets

Outstanding magnetic properties are highly desired for two-dimensional ultrathin semiconductor nanosheets. Here, we propose a phase incorporation strategy to induce robust room-temperature ferromagnetism in a nonmagnetic MoS2 semiconductor. A two-step hydrothermal method was used to intentionally introduce sulfur vacancies in a 2H-MoS2 ultrathin nanosheet host, which prompts the transformation of the surrounding 2H-MoS2 local lattice into a trigonal (1T-MoS2) phase. 25% 1T-MoS2 phase incorporation in 2H-MoS2 nanosheets can enhance the electron carrier concentration by an order, introduce a Mo(4+) 4d energy state within the bandgap, and create a robust intrinsic ferromagnetic response of 0.25 μB/Mo by the exchange interactions between sulfur vacancy and the Mo(4+) 4d bandgap state at room temperature. This design opens up new possibility for effective manipulation of exchange interactions in two-dimensional nanostructures.

CoOOH Nanosheets with High Mass Activity for Water Oxidation.

Endowing transition-metal oxide electrocatalysts with high water oxidation activity is greatly desired for production of clean and sustainable chemical fuels. Here, we present an atomically thin cobalt oxyhydroxide (γ-CoOOH) nanosheet as an efficient electrocatalyst for water oxidation. The 1.4 nm thick γ-CoOOH nanosheet electrocatalyst can effectively oxidize water with extraordinarily large mass activities of 66.6 A g(-1), 20 times higher than that of γ-CoOOH bulk and 2.4 times higher than that of the benchmarking IrO2 electrocatalyst. Experimental characterizations and first-principles calculations provide solid evidence to the half-metallic nature of the as-prepared nanosheets with local structure distortion of the surface CoO(6-x) octahedron. This greatly enhances the electrophilicity of H2O and facilitates the interfacial electron transfer between Co ions and adsorbed -OOH species to form O2, resulting in the high electrocatalytic activity of layered CoOOH for water oxidation.

Thermodynamic analysis on the binding of heavy metals onto extracellular polymeric substances (EPS) of activated sludge

Metal binding to microbial extracellular polymeric substances (EPS) greatly influences the distribution of heavy metals in microbial aggregates, soil and aquatic systems in nature. In this work, the thermodynamic characteristics of the binding between aqueous metals (with copper ion as an example) and EPS of activated sludge were investigated. Isothermal titration calorimetry was employed to estimate the thermodynamic parameters for the binding of Cu²⁺ onto EPS, while three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy with parallel factor analysis was used for quantifying the complexation of Cu²⁺ with the EPS. The binding mechanisms were further explored by X-ray absorption fine structure (XAFS) and Fourier transform infrared (FTIR) spectroscopy analysis. The results show that the proteins and humic substances in EPS were both strong ligands for Cu²⁺. The binding capacity N, binding constant K, binding enthalpy ΔH were calculated as 5.74 × 10⁻² mmol/g, 2.18 × 10⁵ L/mol, and -11.30 kJ/mol, respectively, implying that such a binding process was exothermic and thermodynamically favorable. The binding process was found to be driven mainly by the entropy change of the reaction. A further investigation shows that Cu²⁺ bound with the oxygen atom in the carboxyl groups in the EPS molecules of activated sludge. This study facilitates a better understanding about the roles of EPS in protecting microbes against heavy metals.

Fast Photoelectron Transfer in (Cring)–C3N4 Plane Heterostructural Nanosheets for Overall Water Splitting

Direct and efficient photocatalytic water splitting is critical for sustainable conversion and storage of renewable solar energy. Here, we propose a conceptual design of two-dimensional C3N4-based in-plane heterostructure to achieve fast spatial transfer of photoexcited electrons for realizing highly efficient and spontaneous overall water splitting. This unique plane heterostructural carbon ring (Cring)-C3N4 nanosheet can synchronously expedite electron-hole pair separation and promote photoelectron transport through the local in-plane π-conjugated electric field, synergistically elongating the photocarrier diffusion length and lifetime by 10 times relative to those achieved with pristine g-C3N4. As a result, the in-plane (Cring)-C3N4 heterostructure could efficiently split pure water under light irradiation with prominent H2 production rate up to 371 μmol g-1 h-1 and a notable quantum yield of 5% at 420 nm.

Aligned Fe2TiO5-containing nanotube arrays with low onset potential for visible-light water oxidation

There remains a pressing challenge in the efficient utilization of visible light in the photoelectrochemical applications of water splitting. Here, we design and fabricate pseudobrookite Fe2TiO5 ultrathin layers grown on vertically aligned TiO2 nanotube arrays that can enhance the conduction and utilization of photogenerated charge carriers. Our photoanodes are characterized by low onset potentials of ~0.2 V, high photon-to-current efficiencies of 40-50% under 400-600 nm irradiation and total energy conversion efficiencies of ~2.7%. The high performance of Fe2TiO5 nanotube arrays can be attributed to the anisotropic charge carrier transportation and elongated charge carrier diffusion length (compared with those of conventional TiO2 or Fe2O3 photoanodes) based on electrochemical impedance analysis and first-principles calculations. The Fe2TiO5 nanotube arrays may open up more opportunities in the design of efficient and low-cost photoanodes working in visible light for photoelectrochemical applications.

Oxyhydroxide Nanosheets with Highly Efficient Electron-Hole Pair Separation for Hydrogen Evolution.

The facile electron-hole pair recombination in earth-abundant transition-metal oxides is a major limitation for the development of highly efficient hydrogen evolution photocatalysts. In this work, the thickness of a layered β-CoOOH semiconductor that contains metal/hydroxy groups was reduced to obtain an atomically thin, two-dimensional nanostructure. Analysis by ultrafast transient absorption spectroscopy revealed that electron-hole recombination is almost suppressed in the as-prepared 1.3 nm thick β-CoOOH nanosheet, which leads to prominent electron-hole separation efficiencies of 60-90 % upon irradiation at 350-450 nm, which are ten times higher than those of the bulk counterpart. X-ray absorption spectroscopy and first-principles calculations demonstrate that [HO-CoO6-x] species on the nanosheet surface promote H(+) adsorption and H2 desorption. An aqueous suspension of the β-CoOOH nanosheets exhibited a high hydrogen production rate of 160 μmol g(-1)  h(-1) even when the system was operated for hundreds of hours.

Role of Co clusters in wurtzite Co:ZnO dilute magnetic semiconductor thin films

The magnetic nature of Zn1−xCoxO dilute magnetic semiconductor (DMS) thin films grown by pulsed laser deposition is investigated by x-ray absorption fine structure spectroscopy and x-ray diffraction. We show that a single phase of the substitutional Co atoms occupied Zn sites in the ZnO matrix exists in the Zn0.98Co0.02O DMS thin film while a secondary phase of the Co clusters is formed in Zn0.95Co0.05O and Zn0.90Co0.10O thin films. Despite the formation of Co clusters, the average magnetic moment MS per Co atom is sharply decreased with increasing Co concentration, which suggests that the small Co clusters are superparamagnetic. For the Zn0.98Co0.02O DMS thin film, the local structural distortion around the substitutional Co atoms is interpreted as the origin of intrinsic weak room-temperature ferromagnetism.

Valence State-Dependent Ferromagnetism in Mn-Doped NiO Thin Films

Manipulating the ferromagnetism of diluted magnetic semiconductors by tuning the valence state of doped ions is found to be achievable in Mn-doped NiO. First-principles calculations predict that the interactions between substitutional Mn(3+) ions in NiO are ferromagnetic, while the Mn(2+)-Mn(2+) interactions are antiferromagnetic. This scenario is experimentally supported by a great enhancement of saturation magnetization with increased Mn(3+)/Mn(2+) ratio in Mn-doped and (Mn, Li)-codoped NiO.

Cu and Co codoping effects on room-temperature ferromagnetism of (Co,Cu):ZnO dilute magnetic semiconductors

Homogeneous distribution of magnetic dopants in semiconductor hosts is highly desired for practical applications in spintronics. Herein, we show that codoping 2 at. % Cu with Zn0.98Co0.02O could change the magnetic behavior from paramagnetism to room temperature ferromagnetism. For Zn0.96Cu0.02Co0.02O nanocrystals prepared by a modified solid-state reaction method, the combination of x-ray absorption fine structure spectra at Co, Cu, and O K-edge reveals that the Cu and Co ions are substitutionally incorporated into the ZnO matrix and distribute randomly in the host. First-principles calculations further indicate strong hybridization between Co 3d states and Cu-induced donor impurity bands at the Fermi level, which effectively enhances the indirect ferromagnetic superexchange of Co ions and is responsible for the occurrence of ferromagnetism in (Co,Cu)-codoped ZnO.

Determination of the role of O vacancy in Co:ZnO magnetic film

Annealing-induced changes in structural and magnetic property of Zn0.98Co0.02O thin film prepared at a low oxygen pressure by pulsed laser deposition have been studied with x-ray absorption fine structure, x-ray diffraction, and magnetization measurement. Intrinsic ferromagnetism at room temperature is observed for the as-deposited thin film, in which the Co ions are found to be substitutional for the Zn sites. Upon annealing in air, the occupation sites of Co ions keeps unchanged, whereas the magnetic property undergoes a dramatic change. X-ray absorption near-edge spectroscopy analysis and multiple-scattering calculations reveal that the change in magnetic property caused by annealing in air is due to the annihilation of the preformed oxygen vacancy. This study provides further evidence that O vacancy indeed plays an important role in activating the ferromagnetic interactions in Co-doped ZnO.