Zhiyun Pan

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.

Zn vacancy induced room-temperature ferromagnetism in Mn-doped ZnO

X-ray absorption fine structure (XAFS) and first-principles calculations were employed to study the structure and ferromagnetism origin of Zn0.97Mn0.03O thin film grown by metal organic chemical vapor deposition. The magnetization measurements indicate that this sample is ferromagnetic at room temperature. The Mn ions are located at the substitutional Zn sites as revealed by the Mn K-edge XAFS spectroscopy. Moreover, the O K-edge XAFS analysis indicated the existence of numerous Zn vacancies. Based on first-principles calculations, the authors propose that the Zn vacancy can induce the room-temperature ferromagnetism in Mn-doped ZnO.

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.

Oxygen vacancy effect on room-temperature ferromagnetism of rutile Co:TiO2 thin films

The x-ray absorption near edge structure spectroscopy and first-principles calculations were combined to study the local and electronic structures of rutile Co:TiO2 thin film with room-temperature ferromagnetism. It was revealed that the CoTi2+ substituting the Ti site forms CoTi2+-VO complex with the O vacancy generated in the annealing process. The O vacancy induces a spin-split donor impurity band with a t2g character within the gap region. We proposed that the strong exchange interaction between the localized carriers captured by O vacancy and the substitutional Co ions leads to the ferromagnetism of the Co:TiO2 thin film.

p- and n-doping processes in polythiophene with reduced bandgap. An electrochemical impedance spectroscopy study

Abstract Electrochemical impedance spectroscopy has been used for the characterisation of electrodes modified with different polythiophenes, namely poly[4,4′-bis(butylsulfanyl)-2,2′-bithiophene], poly[4,4′-bis(methylsulfanyl)-2,2′-bithiophene] and poly(3-methylthiophene), at different applied potentials, using different supporting electrolytes. By comparison of the results obtained under experimental conditions in which n-doping is prevented and those obtained from tests where it does occur, some general features have been deduced, all of them being coherently described by a recently proposed ‘generalised transmission line circuit’ model: impedance plots at different applied potentials exhibit progressive changes which are well accounted for by the ‘evolving’ model. The results obtained on the n-doping process of S -alkyl substituted polymers suggest a behaviour interestingly similar to that exhibited in the p-doping; this supports a symmetry that was also found by us in a previous work, with respect to the incorporation and release of counterions during the n- and p-charge–discharge processes.

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.

Quantitative study of interior nanostructure in hollow zinc oxide particles on the basis of nondestructive x-ray nanotomography

The complicated three-dimensional interior structures of the polycrystalline hollow zinc oxide microspheres were clearly obtained by the nondestructive nanocomputed tomography (nano-CT) technique. The parameters such as diameter, volume, porosity, and surface area were calculated by the quantitative analysis of reconstructed data. Especially, three single selected particles with different typical structures were separated and compared. With these results, the distinction of the particles can be clearly understood in nanoscale. This study reveals that nano-CT is an effective and competent tool for investigating the three-dimensional interior structures of nanomaterials in the natural environment.

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.