Wide Band Gap Metal Research Articles

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.

Strong microwave absorption of hydrogenated wide bandgap semiconductor nanoparticles.

Electromagnetic interactions in the microelectronvolt (μeV) or microwave region have numerous important applications in both civil and military fields, such as electronic communications, signal protection, and antireflective coatings on airplanes against microwave detection. Traditionally, nonmagnetic wide-bandgap metal oxide semiconductors lack these μeV electronic transitions and applications. Here, we demonstrate that these metal oxides can be fabricated as good microwave absorbers using a 2D electron gas plasma resonance at the disorder/order interface generated by a hydrogenation process. Using ZnO and TiO2 nanoparticles as examples, we show that large absorption with reflection loss values as large as -49.0 dB (99.99999%) is obtained in the microwave region. The frequency of absorption can be tuned with the particle size and hydrogenation condition. These results may pave the way for new applications for wide bandgap semiconductors, especially in the μeV regime.

(Plenary) Wide Band Gap Semiconductor Transistors for FPD

Wide band gap metal oxide semiconductor TFTs will play a major role in the development of the high resolution and high performance flat panel display. Metal oxide semiconductor TFTs with high mobility and good stability were developed at ETRI through extensive investigation of the materials, device structure, and interface engineering.

Through-Process Analytical Modeling of Photoconductance Spectrum for Porous ZnO Nanocrystalline Film

In the past decades, many models have been developed to describe the photoconductance and the related photoresponse phenomena in wide-band gap metal oxides, but the related mechanisms and kinetics are still confused. Here, based on the persistent photoconductance observed in the porous ZnO nanocrystalline film, a donor photoionization model (DPM) is proposed to decipher the photoelectric kinetics both upon the sub-band gap and the above-band gap illuminations. In the DPM, the reaction rate equations and the related analytical functions are employed to numerically model the through processes of photoconductance spectra in a time-domain by comprehensively considering the photoelectric processes in ZnO such as the band-to-band transition, the donor photoionization, the surface oxygen adsorption/desorption, and the photogenerated electron recapture by donors. In the DPM simulation processes, some essential parameters, such as the electron yield from donor photoionization, the electron capture rates by donors, and the oxygen adsorption reaction rates on ZnO surface, are extracted quantitatively from the photoconductance spectra. The DPM analytical process herein reveals the photoresponse kinetics in detail and proposes a new insight into the basic theory of metal oxide photoresponse that might be helpful for further understanding the photoexcited carrier kinetics and the related photochemical processes.

Kelvin probe force microscopy of nanocrystalline TiO2 photoelectrodes

Dye-sensitized solar cells (DSCs) provide a promising third-generation photovoltaic concept based on the spectral sensitization of a wide-bandgap metal oxide. Although the nanocrystalline TiO2 photoelectrode of a DSC consists of sintered nanoparticles, there are few studies on the nanoscale properties. We focus on the microscopic work function and surface photovoltage (SPV) determination of TiO2 photoelectrodes using Kelvin probe force microscopy in combination with a tunable illumination system. A comparison of the surface potentials for TiO2 photoelectrodes sensitized with two different dyes, i.e., the standard dye N719 and a copper(I) bis(imine) complex, reveals an inverse orientation of the surface dipole. A higher surface potential was determined for an N719 photoelectrode. The surface potential increase due to the surface dipole correlates with a higher DSC performance. Concluding from this, microscopic surface potential variations, attributed to the complex nanostructure of the photoelectrode, influence the DSC performance. For both bare and sensitized TiO2 photoelectrodes, the measurements reveal microscopic inhomogeneities of more than 100 mV in the work function and show recombination time differences at different locations. The bandgap of 3.2 eV, determined by SPV spectroscopy, remained constant throughout the TiO2 layer. The effect of the built-in potential on the DSC performance at the TiO2/SnO2:F interface, investigated on a nanometer scale by KPFM measurements under visible light illumination, has not been resolved so far.

Photoassisted defluorination of fluorinated substrates and pharmaceuticals by a wide bandgap metal oxide in aqueous media

Persistent fluorinated substances, such as the fluorine-bearing pharmaceutical drugs Fluoxetine (FLX; Prozac) and Fluvoxamine maleate (FOM) together with several other substrates (fluorobenzoic acid and fluoroaliphatic model compounds), were photochemically defluorinated and degraded under UVC illumination in relatively good yields in the presence of a wide band gap metal oxide (β-Ga2O3) in heterogeneous aqueous media. The formation of fluoride ions increased with increasing illumination time under an inert nitrogen atmosphere, the transformation of the aromatic moiety was slower under these conditions, but nonetheless it did occur. The optimal amount of β-Ga2O3 loading for defluorination was 50 mg in aqueous media (0.10 mM, 100 mL); the optimal pH to defluorinate FLX was pH 6. Platinization (1 wt%) of the gallium oxide particles enhanced defluorination under an inert nitrogen atmosphere, but was decreased under an oxygen atmosphere; however, in the latter case the degradation of the substrates was facilitated as witnessed by loss of the aromatic moiety. The Ames test on the intermediate products from the photodegradation of FLX and 4-(trifluoromethyl)benzoic acid after long illumination times revealed that none were mutagenic.

Plasmonic Gold Nanocrystals Coupled with Photonic Crystal Seamlessly on TiO2 Nanotube Photoelectrodes for Efficient Visible Light Photoelectrochemical Water Splitting

A visible light responsive plasmonic photocatalytic composite material is designed by rationally selecting Au nanocrystals and assembling them with the TiO(2)-based photonic crystal substrate. The selection of the Au nanocrystals is so that their surface plasmonic resonance (SPR) wavelength matches the photonic band gap of the photonic crystal and thus that the SPR of the Au receives remarkable assistance from the photonic crystal substrate. The design of the composite material is expected to significantly increase the Au SPR intensity and consequently boost the hot electron injection from the Au nanocrystals into the conduction band of TiO(2), leading to a considerably enhanced water splitting performance of the material under visible light. A proof-of-concept example is provided by assembling 20 nm Au nanocrystals, with a SPR peak at 556 nm, onto the photonic crystal which is seamlessly connected on TiO(2) nanotube array. Under visible light illumination (>420 nm), the designed material produced a photocurrent density of ~150 μA cm(-2), which is the highest value ever reported in any plasmonic Au/TiO(2) system under visible light irradiation due to the photonic crystal-assisted SPR. This work contributes to the rational design of the visible light responsive plasmonic photocatalytic composite material based on wide band gap metal oxides for photoelectrochemical applications.

A wide-bandgap metal–semiconductor–metal nanostructure made entirely from graphene

Present methods for producing semiconducting–metallic graphene networks suffer from stringent lithographic demands, process-induced disorder in the graphene, and scalability issues. Here we demonstrate a one-dimensional metallic–semiconducting–metallic junction made entirely from graphene. Our technique takes advantage of the inherent, atomically ordered, substrate–graphene interaction when graphene is grown on SiC, in this case patterned SiC steps, and does not rely on chemical functionalization or finite-size patterning. This scalable bottom-up approach allows us to produce a semiconducting graphene strip whose width is precisely defined to within a few graphene lattice constants, a level of precision beyond modern lithographic limits, and which is robust enough that there is little variation in the electronic band structure across thousands of ribbons. The semiconducting graphene has a topographically defined few-nanometre-wide region with an energy gap greater than 0.5 eV in an otherwise continuous metallic graphene sheet.

Band engineered ternary solid solution CdSxSe1−x-sensitized mesoscopic TiO2 solar cells

The optical band gap of the light absorber and the alignment of its bands with the underlying wide band gap metal oxide are critical for efficient light harvesting and charge separation in semiconductor-sensitized solar cells (SSCs). In practice, these two requirements are however not always fulfilled concomitantly in SSCs. Favourable band alignment in CdSe-sensitized TiO2 requires utilization of quantum sized CdSe, which causes great losses in the harvesting of long wavelength photons due to quantum confinement effects. In the present study, ternary cadmium sulfoselenide (CdSxSe1-x), which has a tunable band gap between those of CdSe and CdS without reducing the dimension, was proposed as a sensitizer for TiO2. CdSxSe1-x was successfully synthesized by alternately depositing CdS and CdSe layers under ambient conditions. SSCs utilizing CdSxSe1-x-sensitized TiO2 yielded a power conversion efficiency of 4.05% under simulated AM1.5 100 mW cm(-2) illumination, rivalling the well-studied cascaded CdS/CdSe electrodes when an aqueous polysulfide solution was used as the electrolyte and Cu2S as the counter electrode. The findings of the present study provide an alternative and viable approach for optimizing the energetics of semiconductor sensitizers for efficient charge separation, while also maintaining good light harvesting.

A ZnO/N-doped carbon nanotube nanocomposite charge transport layer for high performance optoelectronics

Metal oxide charge transport layers are widely used to promote the interfacial charge transport of organic optoelectronics. Nevertheless, frequently used wide-bandgap metal oxides with low electrical conductivities reveal inherent limitations in the charge transport enhancement. We present the remarkable electro-conductivity enhancement of solution processable ZnO charge transport layers upon dispersing a tiny amount (less than 0.1 wt%) of chemically doped CNTs and the corresponding device performance improvement of light-emitting diodes (OLEDs). Using various undoped or doped CNTs, whose work function was systematically tuned by substitutional doping of electron deficient B or electron rich N,N-doped CNT (N-CNT), the composite showed a lowered work function matching well with the conduction band of ZnO. Consequently, the ZnO/N-CNT nanocomposite transport layer with 0.08 wt% N-CNT showed a five-fold enhancement of electron mobility, while maintaining the intrinsic bandgap energy levels, optical transparency and solution processability of pure ZnO. The inverted OLEDs employing ZnO/N-CNT nanocomposite electron transport layers could facilitate well-balanced electron–hole injection and, thus, more than two-fold enhancement of maximum luminance (from 21 000 cd m−2 at 14.6 V to 46 100 cd m−2 at 14.0 V) and efficiency (from 6.9 cd A−1 at 13.4 V to 14.3 cd A−1 at 13.6 V). This highly effective charge mobility enhancement enabled by work function tunable, chemically doped CNTs would be beneficial for various organic and inorganic charge transport materials with different energy levels.

Investigation of the fill factor of dye-sensitized solar cell based on ZnO nanowire arrays

The fill factor of dye-sensitized solar cells based on the ZnO nanowire array is very low, which is usually ascribed to a rapid charge recombination. In this article, the influence on the fill factor of ZnO nanowire array cell is investigated and discussed by comparing dark current and decay rate of open circuit potential of the ZnO nanowire array cell with those of the ZnO nanoparticle cell, TiO 2 nanoparticle cell and TiO 2-coated ZnO nanowire array cell. The results demonstrate that the low fill factor of the ZnO nanowire array cell is largely caused by a rapid decrease of electron injection efficiency rather than a rapid charge recombination, which is decided by the absorption nature of Ru-complexed dye molecules on ZnO surface and repellency of radial electric field. The fill factor of the ZnO nanowire array cell can be improved by coating ZnO nanowires with a wide band gap semiconductor material or metal oxide insulator.