Mixed Oxide Semiconductor Research Articles

Mixed metal oxide Bi2O3/Bi2WO6 thin films for the photoelectrocatalytic degradation of histamine

Photoelectrocatalytic technologies emerge as competitive strategy to manage increasing histamine concentrations. This work explores the synergistic benefits induced in the formation of mixed oxide semiconductor in solid solution. Photoelectrocatalytic films of Bi2O3, WO3, and their hybrid metal mixed oxide composition Bi2O3/Bi2WO6 were synthesized and characterized. Results demonstrate that Bi2O3/Bi2WO6 structure increases stability of charge carriers and enhances degradation performance. Applied bias potential was identified as driving variable that induces effective charge carrier stabilization for accelerated degradation of histamine in aqueous solution. Scavenger experiments identified •OH and O2•- as the main oxidant species produced during photoelectrocatalytic treatment.

Sono-coprecipitation synthesis of ZnO/CuO nanophotocatalyst for removal of parathion from wastewater.

Semiconductor photocatalysis is an effective method used to degrade organophosphorus compounds. Here, the potential of a commonly mixed oxide semiconductor, ZnO/CuO, has been examined to degrade methyl parathion. Sono-coprecipitation method was used to provide ZnO/CuO nanocomposites, and it was applied to photocatalytic and sono-photocatalytic degradation of methyl parathion under solar light irradiation. Powder x-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), the Brunauer-Emmett-Teller (BET) surface area, field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) were used to characterize the synthesized samples. The optimal experimental conditions such as ZnO/CuO photocatalyst 90:10M ratios, the initial concentration of 20mg/L parathion, 1g/L photocatalyst loading, no compressed air sparging, pH of 8, and ultrasonic power (60W and 80kHz) were used to degrade the parathion effectively. The parathion was fully (100% removal) degraded after 60min sono-photoirradiation in the optimal experimental conditions. A real water sample was used to examine the ability of the ZnO/CuO photocatalyst 90:10 to remove the parathion in the water-soluble ions. Graphical abstract.

Synthesis of Bi2FexNbO7 films applied as a catalyst for hydrogen production using visible-light photo-electrolysis

The seek for feasible sustainable mobility alternatives is a major concern of the society nowadays. By its turn, the production of hydrogen represents one of the main lines of study on clean energy, since hydrogen presents the possibility of storage and association with other renewable energy sources. In this regard, photo-electrolysis is a promising option for hydrogen generation. This process optimizes the electrolysis of water by using external energy to increase the potential of a photo-electrode, which must be a material that absorbs sunlight (usually a semiconductor activated by solar radiation), promoting the generation of H2 at the cathode. Titanium dioxide (TiO2) is the most applied semiconductor in photocatalytic applications. However, the band gap of this material limits its activity only under UV light, disregarding about 90% of incident solar radiation. Researchers explore catalysts that can be activated under visible light, such as bismuth-based mixed oxide semiconductors, which have attracted interest because of their excellent stability, visible light absorption, and photocatalytic properties. This research aims to develop and characterize photo-anodes based on bismuth, niobium and iron oxides (Bi2FexNbO7), for production of hydrogen via photo-electrolysis of water. The films were produced by the sol-gel process and deposited under a conductive glass slide by dip-coating. The effect of the iron concentration was evaluated by UV-Vis spectroscopy and SEM analysis, in order to estimate its hydrogen production potential.

Nano gold-coated surface patterned mesoporous titanium tin oxide sol–gel thin film: fabrication, optical and photoelectrochemical properties

Nano noble metal coating on surface patterned mesoporous semiconductor thin film can play an important role in enhancing visible light harvesting efficiency (LHE) towards improvement in photoelectrochemical (PEC) activity of the material. In this work, one-dimensional (1D) and two-dimensional (2D) mesoscale surface patterns have been created on sol–gel-based titanium tin oxide (TSO) nanostructured thin film on pure silica/indium tin oxide-coated glass by soft lithography. The TSO film matrix is observed to be mesoporous and semicrystalline as evidenced from the structural characterization by transmission electron microscopy and measurement of atmospheric ellipso-porosimetry, respectively. The 2D patterned film exhibits maximum LHE value in visible wavelength region. Further film surface modification has been carried out by depositing nano Au coating onto the bare patterned TSO films by a low temperature solution technique. Under visible light, a significant improvement in PEC activity is found and the gold-coated patterned 2D film shows higher visible LHE as well as >2.7 times higher photocurrent density than bare 2D film. This facile fabrication strategy can create an avenue toward improvement in LHE vis-a-vis the PEC activity of mesoporous mixed metal oxide semiconductor thin film.

Photoelectrochemical Reduction of Carbon Dioxide at Copper(I) Oxide Modified with Ultra-Thin Polymer Layers

There has been growing recent interest in the development of electrochemical and photoelectrochemical approaches toward effective fixation (reduction) of carbon dioxide. Having in mind a fact that the linear O=C=O molecule is very stable, the CO2 electroreduction requires large overpotentials. In other words, utilization of renewable energy to convert carbon dioxide and water into fuels would require selective CO2-reduction catalysts or efficient and robust semiconductors. There is a need to design unique nano-architectures consisting of the CO2 photocatalyst/semiconductor stabilized with the outer protective layer. Our study utilizes copper(I) oxide either stabilized with nanostructured titania or ultra-thin films of certain, conducting or redox, polymers (e.g. polyaniline, polypyrrole, PEDOT, poly(4-vinylpyridine)). While in a case of the mixed oxide semiconductor, we have explored formation the p-n heterojunction favoring increase of density of the electron population within the conduction band following deposition of TiO2 over the electrodeposited large crystals of Cu2O, in the latter case we tailor light absorption properties and affect the system’s stability. Upon illumination (from the copper(I) oxide side), charge transport to the reaction sites should be enhanced and the overall electric conductivity (at the catalytic interface) increases. Having in mind both hydrophilic and hydrophobic properties of Nafion over-coating, the outer-most ultra-thin film of Nafion, having properties of both robust (protective) layer and wetting (favoring proton mobility) medium, has been applied. Moreover, the outer-most over-layer of Nafion is expected to facilitate attraction of CO2 (dissolved in the sodium sulfate electrolyte) at the photoelectrochemically active mixed-metal-oxides interfaceSpecial attention has been paid to synthesis, characterization and optimization of the hybrid photocathode, namely the Nafion-protected mixed-oxides (Cu2O-TiO2) and polymer modified copper(I) oxide systems (deposited on fluorine-doped tin oxide covered glass, FTO) toward photoelectrochemical reduction of carbon dioxide.

Reduction of Bias and Light Instability of Mixed Oxide Thin-Film Transistors

Despite their potential use as pixel-switching elements in displays, the bias and light instability of mixed oxide semiconductor thin-film transistors (TFTs) still limit their application to commercial products. Lack of reproducible results due to the sensitivity of the mixed oxides to air exposure and chemical contamination during or after fabrication hinders any progress towards the achievement of stable performance. Consequently, one finds in literature several theories and mechanisms, all justified, but most of them conflict despite being on the same subject matter. In this study, we show that under an optimized fabrication process, which involves the in situ passivation of a mixed oxide semiconductor, we can reduce the bias and light instability of the mixed-oxide semiconductor TFTs by decreasing the semiconductor thickness. We achieve a negligible threshold voltage shift under negative bias combined with light illumination stress when the mixed oxide semiconductor thickness is around three nanometers. The improvement of stability in the thin mixed-oxide semiconductor TFTs is due to a reduced number of oxygen-vacancy defects in the bulk of the semiconductor, as their total number decreases with decreasing thickness. Under the optimized fabrication process, bulk, rather than interfacial defects, thus seem to be the main source of the bias and light instability in mixed oxide TFTs.

Mixed oxide semiconductor CuInAlO4 nanoparticles: synthesis, structure and photocatalytic properties

CuInAlO4 nanoparticles were synthesized via the facile sol–gel route. The phase formations were investigated by x-ray powder diffraction and structure refinements. The morphological characteristic of the nano-oxides was tested with scanning electron microscopy, transmission electron microscopy, energy-dispersive spectra, N2-adsorption–desorption isotherms and the x-ray photoelectron spectrum. The optical absorption, band energy and structures of the nanoparticles were measured. CuInAlO4 has wide optical absorption from UV to visible wavelength. The nano-oxides have a narrow band energy of 2.191 eV. The photocatalysis ability of CuInAlO4 nanoparticles was confirmed by its efficient photodegradation on methylene blue (MB) dye under the excitation of the visible wavelengths: CuInAlO4 demonstrates efficient photocatalysis on MB photodegradation.

Decolorization kinetics of the direct red 23 diazo dye from zinc/cobalt mixed oxide semiconductor using oxalate as a precursor

In this paper, we report the oxidation kinetics of the Direct Red 23 diazo dye through the UV irradiation of an aqueous suspension of zinc/cobalt mixed oxide semiconductor at 30 °C. The zinc and cobalt mixed oxide was prepared by the thermal decomposition of their oxalates, containing 5, 10, and 20 % (w/v) cobalt and calcined at 400 °C for 12 h. The characterization of these oxides was performed by X-ray diffraction, thermal and thermo-gravimetric analysis, X-ray dispersive energy spectrometry, scanning electron microscopy, diffuse reflectance spectroscopy, and textural analysis to understand the physical and chemical behavior of these materials. The suspension, formed by synthesized oxide and the Direct Red 23 diazo dye, was kept for 60 min in the dark and, subsequently irradiated during 210 min. The decolorization rate constant, k obs , was determined under pseudo-first order conditions, maintaining the semiconductor concentration much larger than the diazo dye concentration, using the maximum Direct Red 23 absorbance at 503 nm, and plotting the natural logarithm as a function of irradiation time at 30 °C. All the synthesized mixed oxides displayed higher photocatalytic activity in comparison to commercial zinc oxide. From these, the oxide containing 5 % cobalt showed the highest photocatalytic activity, resulting in a rate constant for the Direct Red 23 diazo dye equivalent to 14.0 × 10−3 min−1 or 93 % decolorization at 30 °C.

Environmentally friendly photoactive heterojunction zinc tin oxide nanoparticles

Heterojunction zinc tin oxide (ZnO/SnO2) nanoparticles were prepared using a top–down traditional solid-state method; mechanical grinding followed by annealing, and characterized by X-ray diffraction, multi-wavelength excitation Raman scattering, scanning electron microscopy, diffuse reflectance and photoluminescence spectroscopy. The photocatalytic degradation kinetics of amitriptyline in the presence of the obtained ZnO/SnO2 nanoparticles under simulated solar and UVA irradiation was determined and used as a model reaction for understanding the photocatalytic mechanism when such mixed oxide semiconductors are used in removing the pharmaceutically active compounds from wastewaters.

Effects of post-lift-off annealing conditions on contact oxidation of Ti–Au top-contacts in In-Sn–Zn–O TFT

Thin film transistors (TFT) with an indium based mixed oxide semiconductor are investigated for titanium–gold top-contacts. It is noticed that upon post annealing, in order to remove chemical residuals from top-contact lift-off steps, oxidation of titanium occurs depending on the annealing conditions. Mobility of the TFT is strongly affected by contact oxidation arising from this post lift-off annealing process. Oxidation of the top-contact is facilitated by adsorbed surface oxygen or out-diffusing oxygen from the semiconductor depending on the post lift-off annealing conditions. A passivation layer that binds effectively to surface vacancies and removes adsorbed oxygen species from the semiconductor surface is demonstrated. The combinations of this passivation layer with relatively low temperature and short post lift-off annealing in an oxygen deficient environment result in significantly reduced contact oxidation and subsequently better transistor performance. Contact resistance as low as 90Ωcm and mobility as high as 5.3cm2/Vs are obtained for solution processed mixed metal oxide semiconductor in top-contact geometry.

Preparation of Mixed Semiconductors for Methyl Orange Degradation

Attempts were made to compare the photocatalytic efficacy in between quasi‐fiber and near spherical commercial grade ZnO through the addition of monoclinic WO3 nanocuboid. Mixed oxide semiconductors were assessed for their dye degradation performance under irradiation of visible light energy. Surface area and the particle morphology pattern have an influence on the resultant photocatalytic features of these mixed oxide composites. The high porous quasi‐fibrous ZnO was successfully fabricated by a simple solution combustion method. It is deliberately made of clusters of primary near spherical particles that supports WO3 nanocuboid embedment and shows interactive characteristics in comparison to the counterpart commercial near spherical ZnO combined with WO3. The photocatalytic activity significantly increases up to 95% under visible radiation for 90 min due to high surface area imparted by unique quasi‐fiber morphology. The photogenerated electron‐hole pair interaction mechanism has been proposed to support the photocatalytic behavior.

Polymeric Graphitic Carbon Nitride Doped with CuO Dispersed on Dealuminated Clinoptilolite (CuO/HCP): Synthesis and Characterisation

CuO dispersed on dealuminated clinoptilolite (CuO/HCP) and further doped with polymeric graphitic carbon nitride (CuO/HCP‐g‐C3N4) was synthesized through 2 facile routes: precipitation method for CuO/HCP and impregnation through ultrasonication method for the hybrid composite material. The hybrid composite material crystalline phase, surface morphology, and structural and thermal properties were investigated by X‐ray diffraction (XRD), scanning electron microscopy (SEM), energy‐dispersive X‐ray analysis (EDAX), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. The formation of the hybrid composite material was confirmed by XRD showing crystalline phase of CuO and g‐C3N4 present on the surface of dealuminated clinoptilolite (HCP). SEM images analysis depicts no aggregation of the mixed metal oxide semiconductor nanoparticles at the center of HCP. The hybrid composite material, CuO/HCP‐g‐C3N4, with a good homogeneously dispersed metal oxide conductor having excellent catalytic activities has been synthesized.

Hydrogen production by photoelectrolysis of aqueous solutions of phenol using mixed oxide semiconductor films of Bi–Nb–M–O (M = Al, Fe, Ga, In) as photoanodes

The simultaneous hydrogen production and oxidation of phenol by photoelectrolysis using mixed oxide semiconductor films of Bi–Nb–M–O (M=Al, Fe, Ga, In) was evaluated in a three-electrode cell. The experiments were carried out under visible light irradiation. The films were prepared on AISI/SAE 304 stainless steel plates by the sol–gel method and deposited using the dip-coating technique. The films have been previously characterized by SEM-EDS, XRD and XPS. XPS deconvolution spectra reveal that the Nb is present as a dopant in all the films. Linear sweep voltammetry and current–time measurements show that all the films exhibit higher electron transport properties under visible light in comparison with the analogous electrolytic process (in the dark). All the films have the ability to carry out simultaneously the hydrogen production and the oxidation of phenol in the reaction conditions evaluated. The Bi–Nb–Fe–O/304 SS film exhibits the highest photoelectrolytic activity related to its lower band-gap and the presence of BiFeO3 perovskite-type phase. The results indicate that these materials can be used in photoelectrolytic cell for simultaneously environmental remediation processes and hydrogen production.

Mixed oxide semiconductors based on bismuth for photoelectrochemical applications

The structural and photoelectrochemical properties of mixed oxide semiconductor films of Bi-Nb-M-O (M = Al, Fe, Ga, In) were studied in order to explore their use as photoanodes in photoelectrochemical cells. These films were prepared on AISI/SAE 304 stainless steel plates by sol–gel dip-coating. The films were characterized by scanning electron microscopy—energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), and their photoelectrochemical properties were studied by open circuit potential (OCP) measurements, linear sweep voltammetry (LSV), and cyclic voltammetry (CV). SEM micrographs show homogeneous and rough films with agglomerates on the surface. EDS analyses show that the films are composed of Bi, Nb, and M, and the agglomerates are mainly composed of Bi. XRD analyses show a predominant crystalline phase of bismuth(III) oxide (Bi2O3) and a secondary phase composed of Bi-M mixed oxides. It is noteworthy that there was no identified niobium-based crystalline phase. XPS results reveal that the films are composed by Bi(III), Nb(V), and M(III). CV results show that the electrochemical behavior is attributed only to the semiconductor films which indicate a good coating of the stainless steel support. OCP measurements show that all the films have n-type semiconductor properties and exhibited photoresponse to the visible light irradiation. LSV results show that the application of a potential higher than +0.1 V enhances the photocurrent which can be attributed to an improved charge carrier separation. The results indicate that these materials can be used in photoelectrochemical cells.

Recent advances in heterogeneous photocatalytic decolorization of synthetic dyes.

During the process and operation of the dyes, the wastes produced were commonly found to contain organic and inorganic impurities leading to risks in the ecosystem and biodiversity with the resultant impact on the environment. Improper effluent disposal in aqueous ecosystems leads to reduction of sunlight penetration which in turn diminishes photosynthetic activity, resulting in acute toxic effects on the aquatic flora/fauna and dissolved oxygen concentration. Recently, photodegradation of various synthetic dyes has been studied in terms of their absorbance and the reduction of oxygen content by changes in the concentration of the dye. The advantages that make photocatalytic techniques superior to traditional methods are the ability to remove contaminates in the range of ppb, no generation of polycyclic compounds, higher speed, and lower cost. Semiconductor metal oxides, typically TiO2, ZnO, SnO, NiO, Cu2O, Fe3O4, and also CdS have been utilized as photocatalyst for their nontoxic nature, high photosensitivity, wide band gap and high stability. Various process parameters like photocatalyst dose, pH and initial dye concentrations have been varied and highlighted. Research focused on surface modification of semiconductors and mixed oxide semiconductors by doping them with noble metals (Pt, Pd, Au, and Ag) and organic matter (C, N, Cl, and F) showed enhanced dye degradation compared to corresponding native semiconductors. This paper reviews recent advances in heterogeneous photocatalytic decolorization for the removal of synthetic dyes from water and wastewater. Thus, the main core highlighted in this paper is the critical selection of semiconductors for photocatalysis based on the chemical, physical, and selective nature of the poisoning dyes.

Low-temperature, high-performance solution-processed metal oxide thin-film transistors formed by a ‘sol–gel on chip’ process

At present there is no ‘ideal’ thin-film transistor technology for demanding display applications, such as organic light-emitting diode displays, that allows combining the low-temperature, solution-processability offered by organic semiconductors with the high level of performance achievable with microcrystalline silicon1. N-type amorphous mixed metal oxide semiconductors, such as ternary oxides Mx1My2Oz, where M1 and M2 are metals such as In, Ga, Sn, or Zn, have recently gained momentum because of their high carrier mobility and stability2, 3 and good optical transparency, but they are mostly deposited by sputtering. So far no route is available for forming high-performance mixed oxide materials from solution at low process temperatures <250 °C. Ionic mixed metal oxides should in principle be ideal candidates for solution-processable materials because the conduction band states derived from metal s-orbitals are relatively insensitive to the presence of structural disorder and high charge carrier mobilities are achievable in amorphous structures2. Here we report the formation of amorphous metal oxide semiconducting thin-films using a ‘sol–gel on chip’ hydrolysis approach from soluble metal alkoxide precursors, which affords unprecedented high field-effect mobilities of 10 cm2 V−1 s−1, reproducible and stable turn-on voltages Von≈0 V and high operational stability at maximum process temperatures as low as 230 °C.

Hydrotalcites of zinc and titanium as precursors of finely dispersed mixed oxide semiconductors for dye-sensitized solar cells

Hydrotalcites are layered materials whose sheets are constituted by octahedra occupied by two different metals (one divalent and the other tri- or tetravalent) and having oxide or hydroxide at the corners. The excess of positive charge of the sheets is compensated by anions located at the interlamellar space. In the present work we have synthesized two hydrotalcites containing zinc and titanium (Zn/Ti atomic ratio in the gel synthesis 6.25) that differ on the absence or presence of sodium dodecyl sulfate in the interlamellar spaces. Calcination of these hydrotalcites leads to a film of intimately dispersed mixed oxide semiconductor that was used as semiconductor for dye sensitised solar cells. Using a ruthenium polypyridyl complex as dye, two photovoltaic cells constructed by films of mixed oxides derived from hydrotalcite calcination were prepared. The highest efficiency parameters were VOC = 0.63 V, JSC = 2.18 mA cm−2, FF = 0.465, η = 0.64%. These efficiency values are not far from those obtained for an analogous photovoltaic cell prepared using P25 titania.

Power optimization and performance improvement for embedded Ethernet SOC

Information appliance is the combination of traditional home appliances and the internet technology. In this article, an Ethernet controller system-on-chip (SOC) solution for information appliances is presented. To achieve high performance, the embedded 8 bits 8051 micro control unit (MCU) is optimized by an independent instruction bus and a data bus. Besides, a two-stage pipeline feature is added. Compared with the existing 8051 core, the enhanced one-cycle MCU offers ten times improvement in instruction execution efficiency. Mean- while, the performance of media access control (MAC) circuit is greatly improved by adopting various techniques such as direct memory access (DMA) control, paging strategy, etc. To reduce the power consumption, clock gating, low power supply, and multi-working-clock are adopted. Moreover, to achieve rapid data communication in different clock frequency circuits, a simple ping-pong first in first out (FIFO) circuit is realized. The chip is implemented using TSMC 0.25 μm two-poly four-metal mixed signal complementary metal oxide semiconductor (CMOS) technology. Its die area is 4.8mm × 4.6mm. The test results show that the maximum throughput of Ethernet packets can reach 7 Mb/s while the power consumption is rather low—the working current is just about 200 mA.

Ethernet controller SoC design and its low-power DFT considerations

In this paper, an Ethernet controller SoC solution and its low power design for testability (DFT) for information appliances are presented. On a single chip, an enhanced one-cycle 8-bit micro controller unit (MCU), media access control (MAC) circuit and embedded memories such as static random access memory (SRAM), read only memory (ROM) and flash are all integrated together. In order to achieve high fault coverage, at the same time with low test power, different DFT techniques are adopted for different circuits: the scan circuit that reduces switching activity is implemented for digital logic circuits; BIST-based method is employed for the on-chip SRAM and ROM. According to the fault-modeling of embedded flash, we resort to a March-like method for flash built in self test (BIST). By all means above, the result shows that the fault coverage may reach 97%, and the SoC chip is implemented successfully by using 0.25 μm two-poly four-metal mixed signal complementary metal oxide semiconductor (CMOS) technology, the die area is 4.8×4.6 mm2. Test results show that the maximum throughput of Ethernet packets may reach 7 Mb · s−1.

NO x gas response characteristics of thin film mixed oxide semiconductor

Many metal oxide gas sensors have been developed for improving stability during last several decades. In this paper, we proposed the hot-wire type micro-gas sensor with single electrode for improving sensitivity and stability. This type sensor has only a single electrode by forming a sensing material onto Pt heater so the resistance of the single electrode is very low for parallel values of Pt heater resistor and sensing materials of mixed oxide semiconductor. Mixed oxide (Sn–In)O + Pt with various composition Sn–In = 1:1, 2:1, 5:1 and 10:1 for sensing materials were prepared. The fabricated sensor showed high sensitivity for NO x at low gas concentration and good selectivity for CO and HC gases.