Wide Band Gap Metal Oxide Research Articles

Plasmonic Au Nanoparticles/KCa2Nb3O10 nanosheets 0D/2D heterojunctions with enhanced photocatalytic activity towards the degradation of tetracycline hydrochloride

Exploration of an effective strategy to enhance the visible-light-driven photocatalytic activity of wide band gap metal oxide semiconductors is of great importance. In this work, plasmonic Au nanoparticles/KCa2Nb3O10 nanosheets 0D/2D heterojunctions were constructed by a facile precipitation–calcination process. In these heterojunctions, Au nanoparticles not only act a photosensitizer to harvest the visible light but also assist in the charge separation, therefore enhancing the visible light photocatalytic activity. The optimized Au/KCa2Nb3O10 0D/2D plasmonic heterojunction showed a remarkable photocatalytic activity towards the degradation of tetracycline hydrochloride. The photocatalytic activity of the optimized 2%-Au/KCNO heterojunction is 3.9 times that of bare KCa2Nb3O10. This work provides an alternative approach to improve the light utilization efficiency and photocatalytic activity of KCa2Nb3O10 semiconductor for photocatalytic degradation of organic pollutant.

Dynamics of Photoexcited Carriers in Polycrystalline PbS and at PbS/ZnO Heterojunctions: Influence of Grain Boundaries and Interfaces

We investigate the impact of grain boundaries and interfaces on dynamics of photoexcited charge carriers in polycrystalline lead sulfide (PbS) films and at interfaces between polycrystalline PbS and ZnO by studying transient photoconductivity over sub-picoseconds to microseconds timescales using time-resolved terahertz spectroscopy and time-resolved microwave conductivity measurements. Narrow band gap bulk-like polycrystalline PbS with high absorption in the infrared paired with wide band gap metal oxide current collectors holds promise for infrared photodetectors and photovoltaics for converting infrared radiation to electricity. We find that grain boundaries in polycrystalline PbS suppress long-range conductivity and confine photoexcited carriers within individual crystallites. The mobility of photoexcited holes inside the ∼150 nm crystallites reaches 750 cm2/V s, and their lifetime exceeds hundreds of microseconds, while electrons get rapidly trapped at grain boundary states. The presence of PbS/ZnO in...

Fundamental Aspects of Photoinduced Charge Flow at a Quantum-Dot-Sensitized Single-Crystal TiO2 Semiconductor Interface

The fundamental aspects of charge transfer from photoexcited CdSe quantum dots to a single crystal of TiO2, a wide band gap metal oxide semiconductor, were investigated and compared with that of a dye-sensitized system in relation to the operation of quantum-dot-sensitized solar cells (QDSCs) and dye-sensitized solar cells (DSSCs). Due to the stark differences in both physical and electronic properties of quantum dots versus molecular dyes, it was hypothesized that the fundamental behavior of the two systems could differ greatly. The large size and surface area of the quantum dots relative to molecular dyes present the possibility for the positively charged hole to move a greater distance away from the QD/oxide interface during the electron injection process. This increased distance influences the Coulombic interaction between the trapped hole and injected electron, leading to differences and increased complexity of the recombination pathways when compared to the dye system.

Metal oxide heterojunction (NiO/ZnO) prepared by low temperature solution growth for UV-photodetector and semi-transparent solar cell

Wide bandgap inorganic metal oxide heterojunctions p-NiO/n-ZnO have been prepared by two low temperature solution growth techniques. Namely, one-dimensional ZnO nanostructure arrays electrodeposited in a pulsed mode, and nanocrystalline NiO films synthesized via Successive Ionic Layer Adsorption and Reaction (SILAR). The crystal structure, morphology, and optical properties of NiO films and NiO/ZnO heterostructures were investigated both before and after annealing in air. The analysis of the dark current vs. voltage characteristics and temporal response curves of the NiO films and corresponding NiO/ZnO heterostructures have shown the promise of their use in the effective UV-photodetectors. Poor photovoltaic characteristics of the test samples on the base of obtained NiO/ZnO heterostructures probably associated with their not quite optimal design, and with too large series resistances and diode ideality factors of the manufactured p-NiO/n-ZnO heterojunctions, that will be corrected by scrutinizing the defects in the metal oxides and through the improvement of the NiO/ZnO heterostructure design. Solving these problems will provide the effective application of the wide bandgap metal oxide NiO/ZnO heterostructures prepared by low temperature solution growth in the UV-active semi-transparent solar cells.

Universal features underlying the magnetism in diluted magnetic semiconductors

Investigation of a diverse variety of wide band gap semiconductors and metal oxides that exhibit magnetism on substitutional doping has revealed the existence of universal features that relate the magnetic moment of the dopant to a number of physical properties inherent to the dopants and the hosts. The investigated materials consist of ZnO, GaN, GaP, TiO2, SnO2, Sn3N4, MoS2, ZnS and CdS doped with 3d-transition metal atoms. The primary physical properties contributing to magnetism include the orbital hybridization and charge distribution, the d-band filling, d-band center, crystal field splitting, electron pairing energy and electronegativity. These features specify the strength of the spin-polarization induced by the dopants on their first nearest neighboring anions which in turn specify the long range magnetic coupling among the dopants through successively induced spin polarizations (SSP) on neighboring dopants. The proposed local SSP process for the establishment of the magnetic coupling among the TM-dopants appears as a competitor to other classical processes (superexchange, double exchange, etc). Furthermore, these properties can be used as a set of descriptors suitable for developing statistical predictive theories for a much larger class of magnetic materials.

Noninvasively Modifying Band Structures of Wide-Bandgap Metal Oxides to Boost Photocatalytic Activity.

Although doping with appropriate heteroatoms is a powerful way of increasing visible light absorption of wide-bandgap metal oxide photocatalysts, the incorporation of heteroatoms into the photocatalysts usually leads to the increase of deleterious recombination centers of photogenerated charge carriers. Here, a conceptual strategy of increasing visible light absorption without causing additional recombination centers by constructing an ultrathin insulating heterolayer of amorphous boron oxynitride on wide-bandgap photocatalysts is shown. The nature of this strategy is that the active composition nitrogen in the heterolayer can noninvasively modify the electronic structure of metal oxides for visible light absorption through the interface contact between the heterolayer and metal oxides. The photocatalysts developed show significant improvements in photocatalytic activity under both UV-vis and visible light irradiation compared to the doped counterparts by conventional doping process. These results may provide opportunities for flexibly tailoring the electronic structure of metal oxides.

Improved efficiency of PbS quantum dot sensitized NiO photocathodes with naphthalene diimide electron acceptor bound to the surface of the nanocrystals

Hybrid materials combining a wide bandgap metal oxide semiconductor, metal chalcogenide nanocrystals and molecular systems represent very attractive materials for fabricating devices with new function or improved photoelectrochemical performance. This study deals with sensitization of NiO, which is a p-type semiconductor, by quantum dots (QDs) of PbS with an average diameter of 3nm. The PbS QDs were attached to the monocrystalline film of NiO by mercaptopropionic acid linker and were subsequently capped with methyl-pyridine naphthalene diimide (NDI) units to prepare quantum dot sensitized solar cells (p-QDSSCs) on NiO electrodes. Time-resolved photoluminescence measurements of the PbS emission were used to determine the rate constants for charge transfer from the PbS exciton to the NiO, cobalt based redox mediator and NDI. Notably, it was shown that NDI quenches the PbS exciton by electron transfer with a quite fast rate constant (6.9 × 107s−1). The PbS QDs sensitized NiO films were finally used to fabricate solar cells with tris(4,4′-ditert-butyl-2,2′-bipyridine) cobalt(III/II) as redox mediator. It was observed that the presence of NDI on PbS improved the photovoltaic performance by 50% relative to that of cells without NDI, leading to a device with the following characteristics: Jsc = 5.75mA/cm2, Voc = 226mV, ff = 34% and PCE = 0.44%. This study demonstrates that photogalvanic processes can be a productive pathway to better performing sensitized p-type semiconductor for p-QDSSC. In other words, photoinduced electron transfer from the QDs towards the electrolyte rather than initial photoinduced charge injection into the p-type semiconductor can be a favorable operative mechanism in QD sensitized NiO films and might be exploited further for the construction of better performing solar cells or photocatalytic devices.

Surface Grafting of Ru(II) Diazonium-Based Sensitizers on Metal Oxides Enhances Alkaline Stability for Solar Energy Conversion.

The electrografting of [Ru(ttt)(tpy-C6H4-N2+)]3+, where "ttt" is 4,4',4″-tri-tert-butyl-2,2':6',2″-terpyridine, was investigated on several wide band gap metal oxide surfaces (TiO2, SnO2, ZrO2, ZnO, In2O3:Sn) and compared to structurally analogous sensitizers that differed only by the anchoring group, i.e., -PO3H2 and -COOH. An optimized procedure for diazonium electrografting to semiconductor metal oxides is presented that allowed surface coverages that ranged between 4.7 × 10-8 and 10.6 × 10-8 mol cm-2 depending on the nature of the metal oxide. FTIR analysis showed the disappearance of the diazonium stretch at 2266 cm-1 after electrografting. XPS analysis revealed a characteristic peak of Ru 3d at 285 eV as well as a peak at 531.6 eV that was attributed to O 1s in Ti-O-C bonds. Photocurrents were measured to assess electron injection efficiency of these modified surfaces. The electrografted sensitizers exhibited excellent stability across a range of pHs spanning from 1 to 14, where classical binding groups such as carboxylic and phosphonic derivatives were hydrolyzed.

It Takes Two to Tango-Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis.

Solar cells made from inorganic-organic perovskites have gradually approached market requirements as their efficiency and stability have improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for possible large-scale production but are more prone to exhibiting photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 °C is presented. The inorganic metal oxides TiO2 and SnO2, the organic fullerene derivatives C60, PCBM, and ICMA, as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs, except TiO2, which shows a more prominent influence of defect states. Transient photoluminescence studies together with current-voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects, and higher efficiencies for devices with fullerene compared to those with metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0%. The results indicate reduced recombination losses for a double-layer TiO2/PCBM contact design: First, a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact, as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multilayer selective contacts may serve as a general design guideline for perovskite solar cells.

Transfiguring UV light active “metal oxides” to visible light active photocatayst by reduced graphene oxide hypostatization

This work explains about the visible light photocatalytic activities of wide band gap metal oxides (TiO2, ZnO, CeO2, Eg>3.0eV) after the overture of reduced graphene oxide. The reduced graphene oxide and metal oxide heterostructures (RGO-MO) Ca. RGO-TiO2, RGO-ZnO, RGO-CeO2 were synthesized through microwave, template assisted, and hydrothermal methods respectively. This explains about the activity of the photocatalysts towards hydrogen production via water splitting, water treatment towards inorganic contaminants with or without dye sensitization activity as well. RGO-MO having particle size 5–10nm which shows great activity towards various photocatalytic activities. Pure TiO2, CeO2 and ZnO are UV active semiconductors having wide band gap 3.2eV. However, when these semiconductors are modified with RGO they are showing better visible light activity towards photoreduction of Chromium (VI) and hydrogen production by half reaction of water.

Understanding Immobilized Molecular Catalysts for Fuel-Forming Reactions through UV/Vis Spectroelectrochemistry

Molecular catalysis of fuel-forming half reactions such as proton and CO2 reduction is a key area of study for achieving electrical-to-chemical energy storage and solar fuel synthesis. Immobilization of these molecular catalysts on electrode surfaces often results in high turnover numbers and selectivities, even under the challenging conditions of an aqueous environment. This Perspective considers how the combination of electrochemistry and electronic spectroscopy can be used to characterize catalytic processes in operando, explaining the observed performance and therefore guiding the design principles for the next generation of material/molecule hybrid electrodes and devices. Numerous immobilization strategies and electrode materials are already available, of which wide band gap metal oxides offer transparency to visible light and are therefore ideal for spectroelectrochemical characterization. Spectroscopic analysis of emerging catalytic metal–organic framework and polymer films is also discussed.

Wide band-gap oxide nanoparticles as potential drug carriers

Recent decades have brought rapid development in the field of nanotechnology, which has led to applications of nanoparticles in many industries. Unique properties of nanoparticles and their biocompatibility increase their potential as drug carriers in drug-delivery systems. Prof. Marek Godlewski’s team from the Institute of Physics PAS has developed wide band-gap metal oxide nanoparticles doped with rare-earth metals for applications as fluorescent markers. The potential of those nanoparticles to cross the closed gut barrier after alimentary application has prompted their use in drug delivery systems. In this study, we show that after conjugation with a model bioactive substance, lectin (Phaseolus vulgaris), these nanoparticles retained their advantageous properties and, following oral administration (10 mg/ml in RO, 0.3 ml/mouse), entered a variety of organs in the mouse model. Internal organs collected at key time points were analysed under a scanning cytometer and a confocal microscope. The results show that the conjugation reduced, but did not completely abolish, the capacity of nanoparticles to penetrate physiological barriers (intestinal, blood-brain barrier) in the organism.

Degenerately Doped Metal Oxide Nanocrystals as Plasmonic and Chemoresistive Gas Sensors.

Highly doped wide band gap metal oxide nanocrystals have recently been proposed as building blocks for applications as transparent electrodes, electrochromics, plasmonics, and optoelectronics in general. Here we demonstrate the application of gallium-doped zinc oxide (GZO) nanocrystals as novel plasmonic and chemiresistive sensors for the detection of hazardous gases including hydrogen (H2) and nitrogen dioxide (NO2). GZO nanocrystals with a tunable surface plasmon resonance in the near-infrared are obtained using a colloidal heat-up synthesis. Thanks to the strong sensitivity of the plasmon resonances to chemical and electrical changes occurring at the surface of the nanocrystals, such optical features can be used to detect the presence of toxic gases. By monitoring the changes in the dopant-induced plasmon resonance in the near-infrared, we demonstrate that GZO thin films prepared depositing an assembly of highly doped GZO colloids are able to optically detect both oxidizing and reducing gases at mild (<100 °C) operating temperatures. Combined optical and electrical measurements show that trivalent dopants within ZnO nanocrystals enhance the gas sensing response compared to undoped ZnO. Moreover, improved sub-ppm of NO2 gas sensitivity is achieved by activating the sensors response through combined purple-blue (λ = 430 nm) light irradiation and mild heating at 75 °C. In addition, these thin films based on degenerately doped semiconductors are highly transparent in the visible range, enabling the fabrication of "invisible" gas sensors. The use of highly doped semiconductive nanocrystals for both IR plasmonic and chemiresistive sensors represent a marked advancement toward the development of highly sensitive and selective devices.

Electrochemical impedance spectroscopy of dye-sensitized solar cells with post-treated TiO2 photoelectrodes using hafnium(IV) chloride and titanium(IV) chloride

Abstract Post-treatment of the TiO 2 photoelectrodes (PEs) of dye-sensitized solar cells (DSSCs), in which an extra thin layer of metal oxide on the TiO 2 film is deposited, has been commonly employed to develop high-efficiency DSSCs. The post-treatment with TiCl 4 , a precursor for TiO 2 , is commonly known to enhance the efficiency of DSSCs by increasing the injection efficiency as a result of moving the TiO 2 conduction band edge downward. Among the wide bandgap metal oxides, HfCl 4 derived HfO 2 found to be a promising material for post-treatment. Even though the DSSCs based on PEs post-treated with TiO 2 have been studied extensively, there is no information about the kinetics and the energetics of the HfO 2 treated PE based DSSCs. Understanding interfacial charge transfer processes and energetics of PEs is crucial to find suitable materials for post-treatment of the PEs to develop high-efficiency DSSCs. In this paper, we have fabricated DSSCs with TiO 2 films post-treated with HfCl 4 and TiCl 4 by dip-coating. The photovoltaic performances of the DSSCs were correlated to the recombination kinetics and the energetics of the DSSCs by analyzing their electrochemical impedance spectroscopy (EIS) and current density-voltage ( j-V ) data systematically. The study showed that the increase of electron injection and decrease of recombination through post-treatment of the PEs with both HfCl 4 and TiCl 4 increased the short-circuit current density ( j sc ) without affecting the fill-factor ( ff ) and the open-circuit voltage ( V oc ) that ultimately improved the efficiency of the DSSCs.

Rutile TiO2 as an Anode Material for Water-Splitting Dye-Sensitized Photoelectrochemical Cells

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) use a wide bandgap metal oxide semiconductor functionalized with a light-absorbing dye and water-oxidation catalyst to harvest light and drive water oxidation, respectively. We demonstrate here that the rutile polymorph of TiO2 (r-TiO2) is a promising anode material for WS-DSPECs. Recombination between the injected electron and oxidized sensitizer with r-TiO2 is an order of magnitude slower than with anatase TiO2 (a-TiO2), with injection yields approaching 100%. Studies with a reductive quencher demonstrate that r-TiO2 is significantly more efficient than a-TiO2, while exhibiting greater dye stability. Furthermore, comparison of direct band gap excitation photocurrent generation for bare and sensitized r-TiO2 suggests that the sensitizer functions as a light harvester and redox mediator.

Rare earth oxides in zirconium dioxide: How to turn a wide band gap metal oxide into a visible light active photocatalyst

In the present study, we investigated the effect of cerium and erbium doping of the zirconium dioxide matrix. We synthesized doped samples using hydrothermal process. The amounts of dopant used were 0.5%, 1% and 5% molar (rare earth oxide over zirconium dioxide) respectively. The samples have been studied via X-ray Diffraction measurements for the structural characterization. UV visible diffuse reflectance was used for the optical analysis, Brunauer-Emmett-Teller (BET) model for the measurement of the surface area. Finally the samples have been analysed via electron paramagnetic resonance (EPR) for the electronic characterization. Then we tested the new synthetized materials to determine their photocatalytic activity in the reaction of degradation of methylene blue performed under irradiation by diodes (LEDs) emitting exclusively visible light.

SnS 기반의 고성능 투명 UV 광검출기

Transparent UV photodetector was achieved by using wide bandgap metal oxide materials. In order to realize transparent heterojunction UV photodetector, n-type ZnO and p-type NiO metal oxide materials were employed. High light-absorbing SnS layer was inserted into the n-ZnO and p-NiO layers. High-performing UV photodetector was realized by ZnO/SnS/NiO/ITO structures to provide extremely fast response times (Fall time: <TEX>$7{\mu}s$</TEX> and rise time: <TEX>$13{\mu}s$</TEX>) and high rectifying ratio. The use of functional SnS-embedded photodetector would provide a route for high functional photoelectric devices.

Solar blind avalanche photodetector based on the cation exchange growth of β-Ga2O3/SnO2 bilayer heterostructure thin film

Solar blind photodetectors have shown a paramount encouragement owing to their promising applications in the fields of ultraviolet (UV) astronomy, fire alarms, missile warning, environmental studies, biological/chemical analysis and short range communication security. Due to the weak solar blind signal, there is a real need to develop solar blind avalanche photodiode (APD) with high internal avalanche gain to compete with the commercial Si APDs and photomultiplier tubes. Herein, we report the development of APD based on the growth of two wide band gap metal oxides operating within the solar blind region of the UV-spectrum (200–280nm). It was found that the growth of β-Ga2O3 on SnO2 thin film via cation exchange mechanism results in the development of a solar blind photodetector with ultrahigh sensitivity, high spectral selectivity, very fast response, high stability and high signal to noise ratio. The high external quantum efficiency of the APD developed was assigned to the avalanche multiplication effect.

Unipolar resistive switching in metal oxide/organic semiconductor non-volatile memories as a critical phenomenon

Diodes incorporating a bilayer of an organic semiconductor and a wide bandgap metal oxide can show unipolar, non-volatile memory behavior after electroforming. The prolonged bias voltage stress induces defects in the metal oxide with an areal density exceeding 1017 m−2. We explain the electrical bistability by the coexistence of two thermodynamically stable phases at the interface between an organic semiconductor and metal oxide. One phase contains mainly ionized defects and has a low work function, while the other phase has mainly neutral defects and a high work function. In the diodes, domains of the phase with a low work function constitute current filaments. The phase composition and critical temperature are derived from a 2D Ising model as a function of chemical potential. The model predicts filamentary conduction exhibiting a negative differential resistance and nonvolatile memory behavior. The model is expected to be generally applicable to any bilayer system that shows unipolar resistive switching.

Is mixed oxide of SnxTi1−xO2 more effective for H2O decomposition? A first principles study

The interaction of water with wide band gap metal oxides has wide ranging applications. Using first principles approach we demonstrate that a mixed oxide of SnxTi1−xO2 (x=0.5) significantly reduces the dissociation barrier of water in comparison to SnO2 and TiO2 surfaces. From the charge distribution at the top layer it is seen that SnxTi1−xO2 is more polar, where the charge separation between metal and oxygen atoms is more than SnO2 and TiO2 surfaces. The reason behind higher activity of the mixed surface has been explained by the surface morphology and larger surface charge density at the top layer.