Negative Conduction Band Research Articles

Fabrication and photocatalytic activities of SrTiO3 nanofibers by sol–gel assisted electrospinning

SrTiO3 nanofibers were successfully prepared by a facile electrospinning method with subsequent calcination in air. These one dimensional nanostructures were characterized for the morphological, structural and optical properties by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and UV–visible diffuse reflectance spectroscopy. The photocatalytic investigations showed that the SrTiO3 nanofibers possessed enhanced photocatalytic efficiency in photodegradation of rhodamine B and photocatalytic H2 evolution from water splitting under ultraviolet light irradiation, compared with the SrTiO3 nanoparticles and P25. The enhanced photocatalytic performance can be ascribed to the beneficial microstructure and more negative conduction band edge compared with P25.

Solution-Chemical Route to Generalized Synthesis of Metal Germanate Nanowires with Room-Temperature, Light-Driven Hydrogenation Activity of CO2 into Renewable Hydrocarbon Fuels

A facile solution-chemical route was developed for the generalized preparation of a family of highly uniform metal germanate nanowires on a large scale. This route is based on the use of hydrazine monohydrate/H2O as a mixed solvent under solvothermal conditions. Hydrazine has multiple effects on the generation of the nanowires: as an alkali solvent, a coordination agent, and crystal anisotropic growth director. Different-percentage cobalt-doped Cd2Ge2O6 nanowires were also successfully obtained through the addition of Co(OAc)2·4H2O to the initial reaction mixture for future investigation of the magnetic properties of these nanowires. The considerably negative conduction band level of the Cd2Ge2O6 nanowire offers a high driving force for photogenerated electron transfer to CO2 under UV-vis illumination, which facilitates CO2 photocatalytic reduction to a renewable hydrocarbon fuel in the presence of water vapor at room temperature.

Highly efficient photocatalytic removal of sodium pentachlorophenate with Bi3O4Br under visible light

In this study we demonstrate that Bi3O4Br is a superior visible light driven photocatalyst for the decomposition and mineralization of a typical chlorinated phenol derivative sodium pentachlorophenate. It could remove more than 92% of sodium pentachlorophenate with concentration of 40mg/L under visible light irradiation from 500W Xe-lamp with a 420nm cut off filter in 15min, accompanying with 80% of mineralization. Density functional theory (DFT) calculation and systematical characterization reveal that high efficient visible light driven sodium pentachlorophenate removal with Bi3O4Br could be attributed to effective separation and transfer of photoinduced charge carriers in Bi3O4Br with narrower band gap and more negative conduction band position, which favors the photogenerated electrons trapping with molecular oxygen to produce O2−. The O2− radicals could not only inhibit the recombination of photoinduced charge carriers, but also benefit the dechlorination of chlorinated phenol derivative. The visible light induced degradation pathway of sodium pentachlorophenate was carefully investigated with high performance gas chromatography–mass spectrometry. This study provides a new photocatalyst for chlorinated phenol derivative removal with solar light.

Fabrication of SrTiO3–TiO2 heterojunction photoanode with enlarged pore diameter for dye-sensitized solar cells

We have demonstrated the N719 dye-SrTiO3–TiO2 system for the application in dye-sensitized solar cells, which is thermodynamically more favorable for electron transport and charge separation efficiency. The heterojunction SrTiO3–TiO2 photoanode with an enlarged pore diameter was fabricated by anodization of a Ti foil substrate and sequential hydrothermal reaction. Their photoelectrochemical properties were investigated as dye-sensitized solar cells for the first time. The optimal conditions for enlarged pore diameter of anodized TiO2 nanotube arrays (TNTAs) were suggested using a tetraethylene glycol based electrolyte, and SrTiO3 for the heterojunction photoanode was hydrothermally reacted on TNTAs. The structure and pore size of the hetero-structured SrTiO3–TiO2 nanotubes arrays (ST-TNTAs) were studied comparatively for their photocatalytic activity in parallel with various SrTiO3 morphologies. The enhanced photocatalytic property of ST-TNTAs compared with TNTAs could be attributed to the enhanced suppression of charge recombination by the heterojunction of ST-TNTAs due to easier crossing of the interface of ST-TNTAs by more the negative conduction band potential of SrTiO3. This is also possible because of the optimized interfacial area and the distance between SrTiO3 nanoparticles and TNTAs. With electrochemical impedance spectroscopy, we report promising applications of dye-sensitized solar cells with the hetero-structured photoanode of ST-TNTAs for the first time.

Photocatalytic water splitting on protonated form of layered perovskites K0.5La0.5Bi2M2O9 (M = Ta; Nb) by ion-exchange

Protonated layered perovskite oxides H1.9K0.3La0.5Bi0.1Ta2O7 (HKLBT) and H1.6K0.2La0.3Bi0.1Nb2O6.5 (HKLBN), which were prepared from K0.5La0.5Bi2M2O9(M = Ta; Nb)(KLBT(N)) by H ion-exchange in 3M HCl solution, were found as good photocatalysts for water splitting under UV light irradiation. The characterization by XRD, ICP and TG revealed that HKLBT(N) retained layered structure of their parent materials KLBT(N) after HCl treatment. An amount of exfoliation of Bi during the protonated process caused the decrease of contribution of Bi 6p in conduction band (CB) and thus resulted in more negative CB potential. HKLBT(N) showed considerable higher photocatalytic activity for H2 and/or O2 evolution than KLBT(N) in the absence of sacrificial reagents, which was attributed to the higher position of conduction band and the layered structure after acid treatment. It was concluded that the interlayer modification via ion-exchange for layered K0.5La0.5Bi2M2O9 (M = Ta; Nb) is a potential way to construct novel photocatalysts with high activity for water splitting.

Room-temperature synthesis of Zn0.80Cd0.20S solid solution with a high visible-light photocatalytic activity for hydrogen evolution

Visible light photocatalytic H(2) production from water splitting is of great significance for its potential applications in converting solar energy into chemical energy. In this study, a series of Zn(1-x)Cd(x)S solid solutions with a nanoporous structure were successfully synthesized via a facile template-free method at room temperature. The obtained solid solutions were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS) and N(2) adsorption-desorption analysis. The solid solutions show efficient photocatalytic activity for H(2) evolution from aqueous solutions containing sacrificial reagents S(2-) and SO(3)(2-) under visible-light irradiation without a Pt cocatalyst, and loading of the Pt cocatalyst further improves the visible-light photocatalytic activity. The optimal photocatalyst with x = 0.20 prepared at pH = 7.3 displays the highest activity for H(2) evolution. The bare and 0.25 wt% Pt loaded Zn(0.80)Cd(0.20)S nanoparticles exhibit a high H(2) evolution rate of 193 μmol h(-1) and 458 μmol h(-1) under visible-light irradiation (λ ≥ 420 nm), respectively. In addition, the bare and 0.25 wt% Pt loaded Zn(0.80)Cd(0.20)S catalysts show a high H(2) evolution rate of 252 and 640 μmol h(-1) under simulated solar light irradiation, respectively. Moreover, the Zn(0.80)Cd(0.20)S catalyst displays a high photocatalytic stability for H(2) evolution under long-term light irradiation. The incorporation of Cd in the solid solution leads to the visible light absorption, and the high content of Zn in the solid solution results in a relatively negative conduction band, a modulated band gap and a rather wide valence bandwidth, which are responsible for the excellent photocatalytic performance of H(2) production and for the high photostability.

Dye-sensitized solar cells incorporating novel Co(ii/iii) based-redox electrolytes solidified by silica nanoparticles

An innovative solidified redox electrolyte for dye-sensitized solar cells (DSCs) was prepared by incorporating amorphous silica nanoparticles in a liquid system containing the Co2+/Co3+ shuttle dissolved in methoxypropionitrile; the novel material was characterized by Raman spectroscopy and its optical properties were examined by UV-Vis spectroscopy. Electrochemical properties (diffusion, conductivity, electrolyte compatibility with the cathode) were studied in symmetrical thin layer cells using polarization and electrochemical impedance spectroscopy (EIS) measurements. Quasi-solid DSCs were fabricated by incorporating this electrolyte and attained an efficiency of about 2.6% under 1 sun (under 1000 W m−2) AM1.5G illumination, corresponding to 73% of that gained by the reference liquid electrolyte; the cell efficiency depends on the light illumination conditions, significantly increased up to more than 4% under 0.23 sun. The solar cells were further characterized by EIS and intensity modulated photocurrent spectroscopy (IMPS) to investigate charge transport and recombination dynamics, possible conduction band edge shifts, alterations of the interfacial and ohmic resistances, and variations of the diffusion rate through the photoelectrode. It was confirmed that the solidified electrolyte-based DSC presented a slightly enhanced photopotential in comparison with that of the liquid electrolyte due to a negative TiO2 conduction band edge shift upon contact with the electrolyte, accompanied with a decreased photocurrent mainly stemming from the restricted diffusion of the Co3+ species through the pores of the TiO2 photoelectrode.

Thermal plasma synthesis of nanotitania and its characterization

Titanium dioxide (TiO2) nanoparticles were prepared by the thermal plasma synthesis; which give a highly crystalline product. Their morphological studies are carried out by using techniques like SEM, EDAX, TEM and SEAD. Crystal size was calculated by XRD using Scherrer equation; which is observed at two current amperes; at 80A size ranges between 25 and 30nm and at 120A size ranges between 30 and 42nm. Composition analysis was done by TEM–EDAX, FTIR and Fast Fourier Transform techniques. The FTIR peaks clearly show that synthesized TiO2 nanoparticles are in anatase phase; this phase is generally preferred because of its high photocatalytic activity, since it has a more negative conduction band edge potential (higher potential energy of photogenerated electrons), high specific area, nontoxic, photochemically stable and relatively in expensive.

Interface recombination in heterojunction solar cells: Influence of buffer layer thickness

In this article, a theoretical approach for interface recombination in a solar cell with a heterostructure that includes a buffer layer is developed. The focus of the analysis is on the diode quality factor and the activation energy of the saturation current that characterize the diode current. The influence of the buffer layer thickness, recombination velocity, and negative band offsets at two interfaces (absorber/buffer and buffer/window) on these two recombination parameters is studied. The diode quality factor and the activation energy show a dependence on the buffer layer thickness. The activation energy is reduced for negative conduction band offsets on both interfaces.

Efficiency enhancement in dye-sensitized solar cells by interfacial modification of conducting glass/mesoporous TiO 2 using a novel ZnO compact blocking film

A novel and thin ZnO compact blocking film is employed at the interface of fluorine-doped tin oxide (FTO) substrate and mesoporous TiO 2, and its influence on dye-sensitized solar cells (DSSCs) is investigated. The ZnO film prepared by spin-coating method on FTO is characterized by energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and UV–vis spectrophotometer. The ZnO film is firstly employed as an energy barrier between FTO and mesoporous TiO 2 film in DSSCs, which improves open-circuit photovoltage ( V oc) and fill factor (FF) with compensation of J sc decrease, finally increasing energy conversion efficiency from 5.85% to 6.70%. Electrochemical impedance spectra (EIS) analysis and open-circuit voltage decay (OCVD) technique reveal that the existence of the energy barrier not only resulted in the effect of suppressing back electrons transfer from FTO to electrolyte but also blocking the electrons injection from the conductive band of TiO 2 to FTO. The former effect effectively reduces the recombination which occurs in the region of FTO substrate, and the latter leads to remarkable increment of electron density in the TiO 2, thus resulting in enhanced V oc and FF. These results suggest that the methodology of introducing the semiconductor with a more negative conduction band edge than TiO 2 as the compact blocking film in DSSCs may be feasible.

Triphenylamine- and benzothiadiazole-based dyes with multiple acceptors for application in dye-sensitized solar cells

Two novel dyes TPAR3 and BTDR2 based on triphenylamine and benzothiadiazole, respectively, with multiple electron acceptors were synthesized and characterized by FT-IR, 1H NMR, TGA and thermomechanical analysis (TMA). They carried terminal cyanoacrylic acid electron acceptors/anchoring moieties, which were connected with the central unit through a thiophene ring. The absorption bands of the dyes were extended up to ∼570 nm with long-wave absorption maximum at 425–455 nm and optical band gap of 2.10–2.17 eV. The dyes emitted yellow–orange light with photoluminescence maximum at 547–615 nm. We have investigated the photovoltaic properties of quasi solid state dye sensitized solar cells (DSSCs) based on these metal free organic dyes. It has been found that the power conversion efficiency of the DSSCs based on composite zinc titanium oxide (ZTO) nanocrystalline photoelectrode is higher than that for TiO 2 based DSSCs. This has been attributed to the longer electron lifetime and more negative conduction band edge of ZTO. The overall power conversion efficiency of the DSSCs based on TPAR3 and BTDR2 employing ZTO photoelectrode is 6.3% and 3.6%, respectively. These results indicate that both the acceptor moiety of metal free organic dyes and ZTO photoelectrode have an effect on the photovoltaic performance of DSSCs.

Temperature‐dependent current‐voltage and lightsoaking measurements on Cu(In,Ga)Se2 solar cells with ALD‐Zn1‐xMgxO buffer layers

AbstractIn this paper, lightsoaking and temperature‐dependent current‐voltage (JVT) measurements on Cu(In,Ga)Se2 solar cells with atomic layer deposited Zn1‐xMgxO buffer layers are presented. A range of Mg concentrations are used, from pure ZnO (x=0) to 26% Mg (x=0·26). Since this kind of solar cells exhibit strong metastable behaviour, lightsoaking is needed prior to the JVT‐measurements to enable fitting of these to the one‐diode model. The most prominent effect of lightsoaking cells with Mg‐rich buffer layers is an increased fill factor, while the effect on cells with pure ZnO buffer is mainly to increase Voc·. The activation energy is extracted from JVT‐measurement data by applying three different methods and the ideality factors are fitted to two different models of temperature‐dependence. A buffer layer consisting either of ZnO or Zn1‐xMgxO with a minor Mg content gives solar cells dominated by interface recombination, which probably can be related to a negative conduction band offset. A relatively high Mg content in the buffer layer (x=0·21) leads to solar cells dominated by recombination in the space charge region. The recombination is interpreted as being tunnelling‐enhanced. The situation in between these Mg concentrations is less clear. Before lightsoaking, the sample with x=0·12 has the highest efficiency of 15·3%, while after lightsoaking the sample with x=0·21 holds the best efficiency, 16·1%, exceeding the value for the CdS reference. The Jsc values of the Zn1‐xMgxO cells surpass that of the reference due to the larger bandgap of Zn1‐xMgxO compared to CdS. Copyright © 2009 John Wiley & Sons, Ltd.

High Open-Circuit Voltage Solid-State Dye-Sensitized Solar Cells with Organic Dye

Solid-state dye-sensitized solar cells were fabricated using an organic dye, 2-cyanoacrylic acid-4-(bis-dimethylfluoreneaniline)dithiophene (JK2), which exhibits more than 1 V open-circuit potential (V(oc)). To scrutinize the origin of high voltage in these cells, transient V(oc) decay measurements and density functional theroy calculations of the interacting dye/semiconductor surface were performed. A negative conduction band shift was observed due to the favorable dipolar field exerted by the JK2 sensitizer to the TiO(2) surface, at variance with heteroleptic Ru(II)-dyes for which an opposite dipole effect was found, providing an increased V(oc).

Reduction in Induced Strain in CdxZn1-xS Well Layers Using ZnS1-ySey Barrier Layers and Inhibition of Induced Strain due to Lattice Mismatch between MgzZn1-zS Cladding Layers and CdxZn1-xS/ZnS1-ySey Multiple Quantum Wells by Adjusting Mg Content

The reduction in induced-strain effect in CdxZn1-xS-based quantum wells (QWs) is important for fabrication of laser diodes (LDs). To determine the effect of the reduction in the induced strain in a CdxZn1-xS well layer which combined with a ZnS1-ySey barrier layer. By reducing the induced strain, in this system, negative conduction and valence band offsets are obtained at large Se content y, band lineups are maintained to be Type I, and slightly low exciton binding and effective band gap energies are obtained. Under this condition, the confinement efficiencies of electron and holes slightly decreases. We propose that a CdxZn1-xS/ZnS1-ySey multiple quantum well (MQW) is sandwiched between two MgzZn1-zS cladding layers. The energy difference between ZnS1-ySey and MgzZn1-zS band gaps increases with increasing Mg content z. MgzZn1-zS cladding layers are lattice-matched to CdxZn1-xS/ZnS1-ySey MQWs by adjusting z, and then the induced strain due to the MgzZn1-zS cladding layers is removed.

Determination of dominant recombination paths in Cu(In,Ga)Se 2 thin-film solar cells with ALD–ZnO buffer layers

CuIn 1− x Ga x Se 2 (CIGS) and CuInSe 2 (CIS) thin-film solar cells, with ZnO buffer layers deposited by Atomic Layer Deposition (ALD), are examined with respect to dominant recombination path. They are compared with reference cells with CdS buffer layers. The principal method of examination is temperature-dependent J– V characterization ( J( V) T), and the analysis of the J( V) T data has been modified in order to more reliably discern the dominant recombination path. Compared to the CIS cells with the traditional CdS buffer layer, the CIS cells with ALD–ZnO buffer layer exhibit the same dominant recombination path, i.e., recombination in the bulk of the absorber. For the CIGS cells (with [Ga]/([Ga]+[In])=0.3), however, the analysis of the cells with ALD–ZnO buffer points to dominant interface recombination, while the CdS buffer cells are dominated by bulk recombination. For CIGS, the difference between the recombination in ALD–ZnO and CdS cells is consistent with the negative conduction band offset found by photoelectron spectroscopy in these ALD–ZnO cells in a previous study. This offset leads to increased interface recombination. For CIS/ALD–ZnO, it was previously found that there is no negative conduction band offset since the conduction band minimum of the absorber is lower. Consistently there is no difference in dominant recombination path between ALD–ZnO buffer cells and traditional CdS buffer cells.

Band offsets at heterojunctions between SrTiO3 and BaTiO3 and Si(100)

We use thin (∼26Å) SrTiO3 and BaTiO3 films epitaxially grown on Si(100) substrates, and ultraviolet and x-ray photoemission spectroscopy to investigate band discontinuities at the SrTiO3∕Si and BaTiO3∕Si heterojunctions. The treatment of the oxide film surface ranges from ex situ ultraviolet generated ozone to annealing in O2 or ultrahigh vacuum. Depending on surface treatment, the valence band offset varies from 2.38 to 2.64eV for SrTiO3∕Si heterostructure and from 2.35 to 2.66eV for BaTiO3∕Si. These values imply that the conduction band minimum of the oxide is below that of the semiconductor, a situation referred to as negative conduction band offset. We demonstrate that the SrTiO3∕Si and BaTiO3∕Si interfaces undergo significant chemical changes during surface cleaning of the oxide film.

Photocatalysis Using Semiconductor Nanoclusters

AbstractWe report on experiments using nanosize MoS2 to photo-oxidize organic pollutants in water using visible light as the energy source. We have demonstrated that we can vary the redox potentials and absorbance characteristics of these small semiconductors by adjusting their size, and our studies of the photooxidation of organic molecules have revealed that the rate of oxidation increases with increasing bandgap (i.e. more positive valence band and more negative conduction band potentials). Because these photocatalysis reactions can be performed with the nanoclusters fully dispersed and stable in solution, liquid chromatography can be used to determine both the intermediate reaction products and the state of the nanoclusters during the reaction. We have demonstrated that the MoS2 nanoclusters remain unchanged during the photooxidation process by this technique. We also report on studies of MoS2 nanoclusters deposited on TiO2 powder.