Chung-Li Dong

Surface Engineered Doping of Hematite Nanorod Arrays for Improved Photoelectrochemical Water Splitting

Given the narrow band gap enabling excellent optical absorption, increased charge carrier density and accelerated surface oxidation reaction kinetics become the key points for improved photoelectrochemical performances for water splitting over hematite (α-Fe2O3) photoanodes. In this study, a facile and inexpensive method was demonstrated to develop core/shell structured α-Fe2O3 nanorod arrays. A thin, Ag-doped overlayer of ~2–3 nm thickness was formed along α-Fe2O3 nanorods via ultrasonication treatment of solution-based β-FeOOH nanorods in Ag precursor solution followed by high temperature annealing. The obtained α-Fe2O3/AgxFe2−xO3 core/shell nanorod films demonstrated much higher photoelectrochemical performances as photoanodes than the pristine α-Fe2O3 nanorod film, especially in the visible light region; the incident photon-to-current efficiency (IPCE) at 400 nm was increased from 2.2% to 8.4% at 1.23 V vs. RHE (Reversible hydrogen electrode). Mott-Schottky analysis and X-ray absorption spectra revealed that the Ag-doped overlayer not only increased the carrier density in the near-surface region but also accelerated the surface oxidation reaction kinetics, synergistically contributing to the improved photoelectrochemical performances. These findings provide guidance for the design and optimization of nanostructured photoelectrodes for efficient solar water splitting.

Filling the oxygen vacancies in Co3O4 with phosphorus: an ultra-efficient electrocatalyst for overall water splitting

It is of essential importance to design an electrocatalyst with excellent performance for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water splitting. Co3O4 has been developed as a highly efficient OER electrocatalyst, but it has almost no activity for HER. In a previous study, it has been demonstrated that the formation of oxygen vacancies (VO) in Co3O4 can significantly enhance the OER activity. However, the stability of VO needs to be considered, especially under the highly oxidizing conditions of the OER process. It is a big challenge to stabilize the VO in Co3O4 while reserving the excellent activity. Filling the oxygen vacancies with heteroatoms in the VO-rich Co3O4 may be a smart strategy to stabilize the VO by compensating the coordination numbers and obtain an even surprising activity due to the modification of electronic properties by heteroatoms. Herein, we successfully transformed VO-rich Co3O4 into an HER-OER electrocatalyst by filling the in situ formed VO in Co3O4 with phosphorus (P-Co3O4) by treating Co3O4 with Ar plasma in the presence of a P precursor. The relatively lower coordination numbers in VO-Co3O4 than those in pristine Co3O4 were evidenced by X-ray adsorption spectroscopy, indicating that the oxygen vacancies were created after Ar plasma etching. On the other hand, the relatively higher coordination numbers in P-Co3O4 than those in VO-Co3O4 and nearly same coordination number as that in pristine Co3O4 strongly suggest the efficient filling of P in the vacancies by treatment with Ar plasma in the presence of a P precursor. The Co–O bonds in Co3O4 consist of octahedral Co3+(Oh)–O and tetrahedral Co2+(Td)–O (Oh, octahedral coordination by six oxygen atoms and Td, tetrahedral coordination by four oxygen atoms). More Co3+(Oh)–O are broken when oxygen vacancies are formed in VO-Co3O4, and more electrons enter the octahedral Co 3d orbital than those entering the tetrahedral Co 3d orbital. Then, with the filling of P in the vacancy site, electrons are transferred out of the Co 3d states, and more Co2+(Td) than Co3+(Oh) are left in P-Co3O4. These results suggest that the favored catalytic ability of P-Co3O4 is dominated by the Co2+(Td) site. P-Co3O4 shows superior electrocatalytic activities for HER and OER (among the best non-precious metal catalysts). Owing to its superior efficiency, P-Co3O4 can directly catalyze overall water splitting with excellent performance. The theoretical calculations illustrated that the improved electrical conductivity and intermediate binding by P-filling contributed significantly to the enhanced HER and OER activity of Co3O4.

Effect of Mn doping on the physical properties of misfit-layered Ca3Co4O9+δ

We have carried out an x-ray absorption near-edge structure (XANES) study on a series of misfit-layered cobaltites Ca3Co4−xMnxO9+δ (x = 0, 0.05, 0.1, 0.15). In this system, the average valences of Co and Mn, according to Co and Mn K-edge measurements are about 3.50+ and 3+, respectively. Both the trivalent and tetravalent Co are of low-spin configurations as confirmed by our magnetization measurements. The variations of absorption intensities of Co L2,3-edge and O K-edge XANES with respect to the Mn doping concentration (x) are analysed. Co L2,3-edge results show that the Co 3d unoccupied states decrease with x, which correlate well with the trend of variation of thermoelectric power. O K-edge results indicate that the number of O 2p unoccupied states decreases with x, which implies a decrease in the majority charge carriers (hole) concentration upon partial substitution of Mn for Co.

Molecular Design of Polymer Heterojunctions for Efficient Solar–Hydrogen Conversion

Semiconducting photocatalytic solar-hydrogen conversion (SHC) from water is a great challenge for renewable fuel production. Organic semiconductors hold great promise for SHC in an economical and environmentally benign manner. However, organic semiconductors available for SHC are scarce and less efficient than most inorganic ones, largely due to their intrinsic Frenkel excitons with high binding energy. In this study the authors report polymer heterojunction (PHJ) photocatalysts consisting of polyfluorene family polymers and graphitic carbon nitride (g-C3 N4 ) for efficient SHC. A molecular design strategy is executed to further promote the exciton dissociation or light harvesting ability of these PHJs via alternative approaches. It is revealed that copolymerizing electron-donating carbazole unit into the poly(9,9-dioctylfluorene) backbone promotes exciton dissociation within the poly(N-decanyl-2,7-carbazole-alt-9,9-dioctylfluorene) (PCzF)/g-C3 N4 PHJ, achieving an enhanced apparent quantum yield (AQY) of 27% at 440 nm over PCzF/g-C3 N4 . Alternatively, copolymerizing electron-accepting benzothiadiazole unit extended the visible light response of the obtained poly(9,9-dioctylfluorene-alt-benzothiadiazole)/g-C3 N4 PHJ, leading to an AQY of 13% at 500 nm. The present study highlights that constructing PHJs and adapting a rational molecular design of PHJs are effective strategies to exploit more of the potential of organic semiconductors for efficient solar energy conversion.

Heterojunction of Zinc Blende/Wurtzite in Zn1-xCdxS Solid Solution for Efficient Solar Hydrogen Generation: X-ray Absorption/Diffraction Approaches.

In the past decade, inorganic semiconductors have been successfully demonstrated as light absorbers in efficient solar water splitting to generate chemical fuels. Pseudobinary semiconductors Zn1-xCdxS (0≤x≤1) have exhibited a superior photocatalytic reactivity of H2 production from splitting of water by artificial solar irradiation without any metal catalysts. However, most studies had revealed that the extremely high efficiency with an optimal content of Zn1-xCdxS solid solution was determined as a result of elevating the conduction band minimum (CBM) and the width of bandgap. In addition to corresponding band structure and bandgap, the local crystal structure should be taken into account as well to determine its photocatalytic performance. Herein, we demonstrated the correlations between the photocatalytic activity and structural properties that were first studied through synchrotron X-ray diffraction and X-ray absorption spectroscopy. The crystal structure transformed from zinc blende to coexisted phases of major zinc blende and minor wurtzite phases at a critical point. The heterojunction formed by coexistence of zinc blende and wurtzite phases in the Zn1-xCdxS solid solution can significantly improve the separation and migration of photoinduced electron-hole pairs. Besides, X-ray absorption spectra and UV-vis spectra revealed that the bandgap of the Zn0.45Cd0.55S sample extended into the region of visible light because of the incorporation of Cd element in the sample. These results provided a significant progress toward the realization of the photoelectrochemical mechanism in heterojunction between zinc blende and wurtzite phases, which can effectively separate the charge-carriers and further suppress their recombination to enhance the photocatalytic reactivity.

Towards understanding the electronic structure of Fe-doped CeO2 nanoparticles with X-ray spectroscopy

This study reports on the electronic structure of Fe-doped CeO2 nanoparticles (NPs), determined by coupled X-ray absorption spectroscopy and X-ray emission spectroscopy. A comparison of the local electronic structure around the Ce site with that around the Fe site indicates that the Fe substitutes for the Ce. The oxygen K-edge spectra that originated from the hybridization between cerium 4f and oxygen 2p states are sensitive to the oxidation state and depend strongly on the concentration of Fe doping. The Ce M(4,5)-edges and the Fe L(2,3)-edges reveal the variations of the charge states of Ce and Fe upon doping, respectively. The band gap is further obtained from the combined absorption-emission spectrum and decreased upon Fe doping, implying Fe doping introduces vacancies. The oxygen vacancies are induced by Fe doping and the spectrum reveals the charge transfer between Fe and Ce. Fe(3+) doping has two major effects on the formation of ferromagnetism in CeO2 nanoparticles. The first, at an Fe content of below 5%, is that the formation of Fe(3+)-Vo-Ce(3+) introduces oxygen deficiencies favoring ferromagnetism. The other, at an Fe content of over 5%, is the formation of Fe(3+)-Vo-Fe(3+), which favors antiferromagnetism, reducing the Ms. The defect structures Fe(3+)-Vo-Ce(3+) and Fe(3+)-Vo-Fe(3+) are crucial to the magnetism in these NPs and the change in Ms can be described as the effect of competitive interactions of magnetic polarons and paired ions.

Soft-x-ray spectroscopy experiment of liquids

The authors show an experimental setup to carry out soft-x-ray fluorescence spectroscopy of liquids under an ultrahigh vacuum (UHV) condition. The flow liquid cell has a window to attain compatibility with UHV conditions of the fluorescence spectrometer and synchrotron radiation beamline. The soft-x-ray photons enter the liquid cell through a 100nm thick silicon nitride window, and the emitted soft x rays exit through the same window to be detected by photon diode and microchannel plate detectors. This setup allows liquids and, in particular, liquid-solid interfaces to be studied. Such a liquid cell has been used to study the electronic structure of a variety of systems ranging from water solutions of inorganic salts and nanomaterials under wet conditions.

Electronic states and structural characterization in single-crystal Fe–Ni–O alloy thin films grown by molecular beam epitaxy

High quality epitaxial Fe3O4, NiO and a series of Fe1−xNixOy (0<x<1) thin films have been fabricated by molecular beam epitaxy. In situ reflection high energy electron diffraction (RHEED), ex situ X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS) studies have been carried out. It was observed that the crystal structure of all the Fe1−xNixOy films studied resemble that of the inverse spinel Fe3O4. The lattice spacing along the perpendicular direction as a function of x shows a minimum at x=0.5 instead of a linear variation indicating that the structures are different from a bulk ferrite Fe2NiO4-like phase. The XAS results are consistent with XRD results and further identify the cation distribution in Fe1−xNixOy system as x varies. The mechanism of the formation of the metastable phase and its implication on the magnetic properties of these Fe–Ni–O films are briefly discussed.

Local geometric and electronic structures of gasochromic VOx films

VOx films were deposited by radio-frequency reactive magnetron sputtering from a vanadium target at room temperature. Local atomic and electronic structures of the films were then modified by thermal annealing. The oxidation state and structural and gasochromic properties of the films were elucidated by X-ray absorption spectroscopy. Analytical results indicate that the as-deposited VOx films were amorphous with mixed V(4+) and V(5+) valences. The amorphous VOx had a disordered and expanded lamellar structure resembling that of polymer-intercalated V2O5 gels. VOx films were crystallized into orthorhombic V2O5 at 300 °C, and the lamellar structure was eliminated at 400 °C. Additionally, the gasochromic reaction reduced the vanadium valence via intervalence transitions between V(5+) and V(3+). Moreover, removing the lamellar structure reduced the gasochromic rate, and the gasochromic reaction transformed the V2O5 crystalline phase irreversibly into an H1.43V2O5 phase. Based on the results of this study, amorphous VOx with a lamellar structure is recommended for use in H2 gas sensors.

Mechanism of light emission and electronic properties of a Eu3+-doped Bi2SrTa2O9 system determined by coupled X-ray absorption and emission spectroscopy

The origin of light emission from newly discovered orange-red UV light emitting diodes, and their electronic properties are critical issues yet to be understood. In this study, X-ray absorption spectroscopy (XAS) and emission spectroscopy (XES) are utilized to examine the electronic structure of the Eu3+-doped Bi2SrTa2O9 system. While no significant change in the electronic structure is observed around the Bi and Ta sites, variation around the Eu and Sr atoms is observed, along with even more significant changes in the O 2p states in the conduction band. Upon UV irradiation, Eu-induced states within the conduction band are observed and found to shift to the conduction band minimum upon substitution of Sr with Eu. This phenomenon is the result of the creation by Eu of an excitable state and the fact that Eu is more electronegative than Sr, such that the substitution lowers the Eu 4f5d–O 2p hybridized states. Consequently, the substitution reduces the energy of electron recombination between the valence and conduction bands, which is consistent with the red shift in the photoluminescence spectra. The presence of the newly formed hole states distributed over the O 2p states in the conduction band is strongly correlated with the emission intensity. The results and analyses demonstrate that Eu can be introduced to tailor the Eu–O hybridized states within the conduction band and change the route of recombination, suggesting that Eu is critically involved in light emission in these UV-induced orange-red emitting LED materials.