Guoping Xu

Photocurrent Enhancement for Ti-Doped Fe2O3 Thin Film Photoanodes by an In Situ Solid-State Reaction Method

In this work, a higher concentration of Ti ions are incorporated into hydrothermally grown Ti-doped (2.2% by atomic ratio) micro-nanostructured hematite films by an in situ solid-state reaction method. The doping concentration is improved from 2.2% to 19.7% after the in situ solid-state reaction. X-ray absorption analysis indicates the substitution of Fe ions by Ti ions, without the generation of Fe²⁺ defects. Photoelectrochemical impedance spectroscopy reveals the dramatic improvement of the electrical conductivity of the hematite film after the in situ solid-state reaction. As a consequence, the photocurrent density increases 8-fold (from 0.15 mA/cm² to 1.2 mA/cm²), and it further increases up to ∼1.5 mA/cm² with the adsorption of Co ions. Our findings demonstrate that the in situ solid-state reaction is an effective method to increase the doping level of Ti ions in hematite films with the retention of the micro-nanostructure of the films and enhance the photocurrent.

Giant magnetocaloric effect in isostructural MnNiGe-CoNiGe system by establishing a Curie-temperature window

An effective scheme of isostructural alloying was applied to establish a Curie-temperature window in isostructural MnNiGe-CoNiGe system. With the simultaneous accomplishment of decreasing structural-transition temperature and converting antiferromagnetic martensite to ferromagnetic state, a 200 K Curie-temperature window was established between Curie temperatures of austenite and martensite phases. In the window, a first-order magnetostructural transition between paramagnetic austenite and ferromagnetic martensite occurs with a sharp jump in magnetization, showing a magnetic entropy change as large as −40 J kg−1 K−1 in a 50 kOe field change. This giant magnetocaloric effect enables Mn1−xCoxNiGe to become a potential magnetic refrigerant.An effective scheme of isostructural alloying was applied to establish a Curie-temperature window in isostructural MnNiGe-CoNiGe system. With the simultaneous accomplishment of decreasing structural-transition temperature and converting antiferromagnetic martensite to ferromagnetic state, a 200 K Curie-temperature window was established between Curie temperatures of austenite and martensite phases. In the window, a first-order magnetostructural transition between paramagnetic austenite and ferromagnetic martensite occurs with a sharp jump in magnetization, showing a magnetic entropy change as large as −40 J kg−1 K−1 in a 50 kOe field change. This giant magnetocaloric effect enables Mn1−xCoxNiGe to become a potential magnetic refrigerant.

Micro-Nano-Structured Fe2O3:Ti/ZnFe2O4 Heterojunction Films for Water Oxidation

Iron(III) oxide photoelectrodes show promise in water oxidation applications. In this study, micro-nano-structured hematite films are synthesized, and Ti ions are doped to improve photoelectric conversion efficiency. The photocurrent increases for enhanced electrical conductivity. Further enhanced photocurrent is achieved for Fe(2)O(3):Ti/ZnFe(2)O(4) heterojunction electrodes. Cyclic voltammograms combined with optical absorbance examinations demonstrate that the conduction and valence band edges of ZnFe(2)O(4) shift from those of Ti doped Fe(2)O(3) to the negative direction, which facilitates the efficient separation of electron-hole pairs at the Fe(2)O(3):Ti/ZnFe(2)O(4) interface. These findings demonstrate that, by doping hematite and by engineering the interface between the hematite and the electrolyte, charge separation can be effectively promoted and photocurrent density can be dramatically increased.

A Route to Phase Controllable Cu2ZnSn(S1-xSex)4 Nanocrystals with Tunable Energy Bands

Cu2ZnSn(S1−xSex)4 nanocrystals are an emerging family of functional materials with huge potential of industrial applications, however, it is an extremely challenging task to synthesize Cu2ZnSn(S1−xSex)4 nanocrystals with both tunable energy band and phase purity. Here we show that a green and economic route could be designed for the synthesis of Cu2ZnSn(S1−xSex)4 nanocrystals with bandgap tunable in the range of 1.5–1.12 eV. Consequently, conduction band edge shifted from −3.9 eV to −4.61 eV (relative to vacuum energy) is realized. The phase purity of Cu2ZnSn(S1−xSex)4 nanocrystals is substantiated with in-depth combined optical and structural characterizations. Electrocatalytic and thermoelectric performances of Cu2ZnSn(S1−xSex)4 nanocrystals verify their superior activity to replace noble metal Pt and materials containing heavy metals. This green and economic route will promote large-scale application of Cu2ZnSn(S1−xSex)4 nanocrystals as solar cell materials, electrocatalysts, and thermoelectric materials.

Effect of ZnS and CdS coating on the photovoltaic properties of CuInS2-sensitized photoelectrodes

This paper presents a facile route to prepare CuInS2 (CIS) as a light absorber for solar cells. CIS was effectively deposited inside the pores of the nanoporous anatase titanium dioxide photoelectrodes with a high coverage degree. The performance of the CIS-sensitized TiO2 photoelectrodes could be substantially enhanced through a sequential coating with a CdS shell and a ZnS layer via successive ionic layer adsorption and reaction (SILAR). Electrochemical impedance spectrum (EIS), cyclic voltammetry (CV), and photocurrent experiments were employed to study the effect of CdS and ZnS coating. The CdS shell provided high surface coverage to passivate surface states. The ZnS layer acted as a potential barrier which was similar to an insulating layer employed in the conventional metal–insulator–semiconductor solar cells, allowing the adjustment of the electric field and potential distribution in the interface between the electrodes and the electrolyte and suppressing the dark current, resulting in further increase in the photoresponse. We found that the thickness of the nanoporous TiO2 films had a dramatic impact on the optical response and efficiency of CIS-sensitized photoelectrodes, with the 5 μm sample performing much better than the 11 μm sample, which underlined the importance of charge (or ion) transport kinetics.

Interface engineering: Boosting the energy conversion efficiencies for nanostructured solar cells*

Nanostructured solar cells have attracted increasing attention in recent years because their low cost and ease of preparation offer unique advantages and opportunities unavailable with conventional single-crystalline solar cells. The efficiencies of this kind of solar cell largely depend on the interfacial structure owing to the large specific interface areas and the inherent high density of interface states. In this review article, strategies of interface engineering will be introduced in detail. The up-to-date progress and understanding of interface engineering and its role in influencing the efficiency of nanostructured solar cells will be discussed. Some of the representative examples of the interface engineering method will be presented wherever necessary. Continued boosting of the energy conversion efficiency for nanostructured solar cells is anticipated in the coming years and will bring this kind of solar cell to the status of commercialization.

Visible-light-driven photocatalysts: (La/Bi + N)-codoped NaNbO3 by first principles

To improve the photocatalytic activity of NaNbO3 for water splitting, the bandgap and the band edges of NaNbO3 should be tailored to match the visible part of the solar spectrum and hydrogen and oxygen redox potentials. By analyzing the band structures of La/Bi-doped and (La/Bi + N)-codoped NaNbO3, we found that the pseudointermediate band (PIB) was formed in the bandgap in all the doped systems because of the orbital splitting of the Nb 4d induced by the dramatically enlarged O-Nb-O angles. The PIB could make the wide bandgap semiconductors absorb visible-light photons as long as it was degenerate or partially degenerate. Considering that the appropriate band edges and absorption properties, we believe that (La/Bi + N)-codoped NaNbO3 materials are promising photocatalysts for hydrogen production through water splitting under visible-light irradiation without other modifications.

Role of covalent hybridization in the martensitic structure and magnetic properties of shape-memory alloys: The case of Ni50Mn5+xGa35-xCu10

The influence of covalent hybridization on the martensitic structure and magnetic properties of Ni50Mn5+xGa35−xCu10 shape-memory alloys has been investigated. It is found that the lattice distortion (c − a)/a of L10 martensite linearly increases upon substitution of Mn for Ga, showing a change of slope at Ga = 25 at. %, which is ascribed to a weakened covalent hybridization between main-group and transition-metal atoms. Moreover, due to the competition between the covalent hybridization and the magnetic ordering of the substituted Mn atoms, the magnetic moment per formula unit and the Curie temperature show maxima at Ga = 25 at. % as well. This behavior is closely associated with the corresponding changes of the strength of the covalent hybridization. The results, therefore, suggest that a careful control of the concentration of main-group atoms in Heusler alloys may serve as a tuning parameter for finding multifunctional materials.

On the stability of CdSe quantum dot-sensitized solar cells

The stability issues of CdSe quantum dot-sensitized solar cells have been investigated in this work. Stability is one of the most relevant issues for the real-world applications of quantum dot-sensitized solar cells. We find that CdSe quantum dots deposited in air are prone to oxidation, and when the solar cells are left in air for several months, CdSe quantum dots decompose partially to form Se particles and nanoribbons. The surface oxidation and the decomposition of CdSe quantum dots worsen all the parameters of the solar cells. Electrochemical impedance characterization provides unambiguous evidence for the evolution of the cells in the ageing process, which accounts for the observed performance worsening phenomena. Preparation under an inert atmosphere suppresses to some extent the degradation of the cells. These findings have significant implications for designing viable, high-performance quantum dot-sensitized solar cells.

Electrochemiluminescent determination of perchlorate by solvent extraction in the presence of Ru(bpy)2+3

A novel method for the highly sensitive determination of perchlorate was proposed. It was based on solvent extraction in the presence of Ru(bpy)(3)(2+) followed by Ru(bpy)(3)(2+) electrochemiluminescent determination. A linear calibration was obtained over the range of 0.1 to 10 mu mol l(-1) with a correlation coefficient of 0.998. The detection limit (S/N = 3) was 5.0 x 10(-8) mol l(-1). The relative standard deviation for 10 replicates of 1 mu mol l(-1) perchlorate was 1.6%. Interference studies suggest that this method is selective for the determination of perchlorate. Application of this method to the highly sensitive determination of other anions is suggested