Anodic Bias Research Articles

Threshold voltage control of electrolyte solution gate field-effect transistor by electrochemical oxidation

Diamond electrolyte solution-gate-field effect transistors (SGFETs) are suitable for applications as chemical ion sensors because of their wide potential window and good physical and chemical stabilities. In this study, we fabricated an anodically oxidized diamond SGFET from a full hydrogen-terminated diamond SGFET and demonstrated control of the device threshold voltage by irreversible anodic oxidation. The applied anodic bias voltage (VAO) was varied gradually from low to high (1.1–1.7 V). As the anodic oxidation proceeded, the threshold voltage shifted to more negative values with no degradation of hole mobility. Thus, anodic oxidation is a useful method for controlling the threshold voltage of diamond SGFETs.

Core-shell g-C3N4@ZnO composites as photoanodes with double synergistic effects for enhanced visible-light photoelectrocatalytic activities

In this work, core-shell g-C3N4@ZnO photocatalysts were facilely synthesized via a reflux method applying the industrial grade ZnO nanoparticles and g-C3N4 nanosheets as the starting materials. The thickness of the g-C3N4 shell was gradually increased with the increasing proportion of g-C3N4 and the average thickness of the coating g-C3N4 is 1.89nm and 3.21nm for a weight ratio of 15% and 20% (g-C3N4/ZnO) g-C3N4@ZnO composites, respectively. By using g-C3N4@ZnO composites as photoanodes for the first time, 15% g-C3N4@ZnO photoanode exhibits the best PEC performance for the degradation of phenol under visible light irradiation with an anodic bias of 1.5V vs. SCE and the rate constant is determined to be 1.216h−1, which is almost 2.1 times as high as that of 20% g-C3N4@ZnO photoanode. The enhancement of the visible light PEC degradation phenol is attributed to the double synergistic effects which combined of special core-shell nanostructures and electro-oxidation assisted photocatalysis. This work not only demonstrates core-shell g-C3N4@ZnO composites as a promising photoanode for the utilization of solar conversion, but also meets the requirement for the increasing demand of practical applications

Fabrication and characterization of photoelectrochemically-active Sb-doped Snx-W(100-x)%-oxide anodes: Towards the removal of organic pollutants from wastewater

Snx-W(100-x)-oxide coatings (x=0, 20, 40, 60, 80 and 100) doped with Sb (∼3at.%) were formed on a Ti substrate by a thermal deposition method in order to evaluate their potential use as electrodes for the photoelectrochemical oxidation of organic compounds. Surface microstructure/morphology and chemical composition of the coatings, as well as their photoelectrocatalytic activities were investigated using electrochemical and surface-characterization techniques. It was found that the surface roughness of the coatings depends on their composition, yielding an average value of Ra=1.1±0.5μm. The band gap energy was found to be independent on the coating composition up to the relative W/Sn at. ratio of 4/6, yielding an average value of 3.53±0.05eV (which corresponds to the band gap of doped SnO2), but then it decreased for the three coatings with the highest W content, to an average value of 2.56±0.10eV (which corresponds to the value of pure WO3). All the coatings were found to be photoactive under the anodic bias. Further, all the coatings were found to be photoelectrochemically active towards the degradation of phenol red dye solution under UV light irradiation. The intrinsic photoelectrocatalytic activity was found to be highest for the Sn80%-W20%-oxide electrode coating.

(Invited) Some Observations on the Electrochemical Interface of Hematite in Alkaline Electrolyte during Solar Water Oxidatiob

Iron oxide, in particular a-Fe2O3 hematite has been investigated as model photoanode for solar water oxidation for many decades. We have investigated this material with soft x-ray spectroscopy methods and found some chemical changes at the surface when the material is operated as photoelectrode. The first observ ation is that the surface is sub-stoichiometric; it contains an Fe2+-enriched surface. Under anodic bias, the Fe2+ becomes converted to Fe3+ with a parabolic growth law. Also, hole doping is observed on the same samples. Additionally, the surface appears to become hydroxylated or oxy-hydroxylated. The relative abundance of the OH or OOH groups increases with increasing bias potential to the point where the water oxidation sets on. At higher potentials, the surface groups disappear gradually. In the same course, the hydrogen bonds which hold water molecules together become increased and decreased. We have also done in situ and operando experiments which show a nice parallel process of photo hole formation and delection depending on the applied bias. We are therefore able to sketch a realistic scenario of physics and chemistry going on at the surface and subsurface during water oxidation.

Cobalt based water oxidation catalysis with photogenerated Ru(bpy)33+: Different kinetics and competent species starting from a molecular polyoxometalate and metal oxide nanoparticles capped with a bisphosphonate alendronate pendant

Polyoxometalates (POMs) are a class of inorganic compounds that have been extensively considered in the development of robust water oxidation catalysts (WOCs). In particular, cobalt based POMs have shown remarkable activity as WOCs under both dark and light activated cycles, although current studies focus on their possible evolution to different forms of cobalt oxides under operating conditions. In this paper, we investigate the activity in water oxidation catalysis of a heptanuclear cobalt based POM [{(B-α-PW9O34)Co3(OH)(H2O)2(O3PC(O)(C3H6NH3)PO3)}2Co]14−, hereafter Co7(Ale)14− and of 5nm cobalt oxide nanoparticles, Co3O4(Ale), both carrying the same bisphosphonate alendronate ligand. Under the application of an anodic bias in borate buffer pH 8, both species lead to the formation of an active cobalt oxide layer at the working electrode, although in the case of Co7(Ale)14− participation of the POM scaffold in the electrodeposition is evidenced by the presence of W(VI) in the active film. In particular, Co7(Ale)14− and Co3O4(Ale) were investigated in a photocatalytic cycle with Ru(bpy)32+ (bpy=2,2′-bipyridine) as the photosensitizer and persulfate as the electron acceptor, where the photogenerated oxidant is Ru(bpy)33+. Key findings are: (i) ion pairing and aggregate formation of anionic Co7(Ale)14− and Co3O4(Ale) with cationic Ru(bpy)32+ are limited under photocatalytic regime (20mM aqueous buffer, pH 8), as confirmed by the solubility product constant KSP=2.1±0.4×10−15M4 for a Co7(Ale)14−:Ru(bpy)32+ 1:3 adduct, emission quenching experiments, and bimolecular electron transfer (ET) kinetics; (ii) fast ET to Ru(bpy)33+, approaching diffusional rates, occurs from Co7(Ale)14− (kET=9.4±0.3×108 M−1s−1) overarching by four orders of magnitude the Co3O4(Ale) nanoparticles; (iii) Co7(Ale)14− enables up to 18 photoinduced ET events in 90ms, while in the same timeframe, a non-complete bleaching recovery (ET events<1) results from the Co3O4(Ale) nanoparticle analogs; (iv) photocatalytic oxygen evolution is observed with similar rates for both cobalt based species (TOF per cobalt=1.2–1.7×10−3s−1); (v) dark O–O bond formation is the likely rate determining step of water oxidation for both systems, as the O2 evolution kinetics do not depend on photoinduced ET rates and oxygenic activity, and light irradiation intensity (λ=450nm, 0.4–1.6×10−7einsteins−1); (vi) leaching of Co(II) ions from Co7(Ale)14− in catalytic conditions is estimated below 2% by 31P NMR line broadening experiments and supported by persistent flash photolysis kinetics; (vii) the ultimate fate of Co7(Ale)14− under oxygenic turnover involves a polyoxometalate structural rearrangement, while no evidence of cobalt oxide nanoparticles formation is provided by TEM images; (viii) conversely, Co3O4(Ale) nanoparticles preserve their size along the light driven cycle. Therefore, this study supports the involvement of POM based intermediates in water oxidation catalysis, while future perspectives should be directed toward the identification of such main characters, and of the mechanism of O2 construction.

In situ surface potential evolution along Au/Gd:CeO2 electrode interfaces

We present an investigation of electroactive Au/gadolinium doped ceria electrode interfaces under CO2/CO co-electrolysis environments using a combination of in situ high temperature scanning surface potential microscopy (HT-SSPM) and modified Poisson-Cahn (PC) models. Here charged surface adsorbate-oxygen vacancy interactions manifested in HT-SSPM potential profiles as small perturbations of opposite sign in reference to the applied biases. The positive deviation of surface potential on Au from applied cathodic biases is attributed to the work function difference between gold (ϕAu ∼ 5.31 eV) and graphitic carbon deposits (ϕC ∼ 5.0 eV) formed through CO disproportionation. The negative potential deviation from the applied anodic bias is attributed to negatively charged carboxylates. Results of the PC model confirmed the affinity of oxygen vacancies for the surface, thus supporting in situ experimental evidence of surface vacancy accumulation/depletion processes induced by cathodic/anodic biases.

Hematite-Based Photoelectrode Materials for Photoelectrocatalytic Inhibition of Alzheimer's β-Amyloid Self-Assembly.

A visible light-active, hematite-based photoelectrode platform for suppressing β-amyloid (Aβ) self-assembly in vitro is reported. Upon illumination of a light-emitting diode with an anodic bias, the hematite photoanode generates reactive radical species, such as superoxide ions and hydroxyl radicals, via photoelectrocatalytic process. According to our analyses, the hematite photoanode exhibited a strong inhibitory effect on Aβ aggregation under visible light illumination and anodic bias. We found that hole-derived radicals played a significant role of oxidizing Aβ peptides, which effectively blocked further aggregation. The efficacy of photoelectrocatalytic inhibition on Aβ aggregation was enhanced by introducing cobalt phosphate (Co-Pi) as a co-catalyst on the hematite photoanode, which facilitated the separation of electron-hole pairs. We verified that both bare and Co-Pi@hematite photoanodes are biocompatible and effective in reducing Aβ aggregation-induced cytotoxicity.

Communication—Continuous Monitoring of Activity of Plasmonic Gold Nanoparticles over Photooxidation Reactions Carried-Out on Au/TiO2 Photocatalysts

Suspensions of titanium dioxide catalysts decorated with plasmonic metal nanoparticles (NPs) are widely investigated in view of photodegradation of a variety of contaminants in water under irradiation with visible light. In this work, we used porous TiO2 electrodes bearing gold NPs to carry out photooxidation of model organic molecules either using oxygen as scavenger of photogenerated electrons or under imposed anodic bias. In both cases, continuous decrease of visible light photoactivity consistent with the drop of incident photon-to-current efficiencies (IPCEs) of the Au/TiO2 photocatalyst was observed.

A TiO2-nanotubes-based coil-type microreactor for highly efficient photoelectrocatalytic degradation of organic compounds

Abstract A continuous-flow, coil-type photoelectrocatalytic microreactor for the degradation of caffeine was designed, assembled and characterized. Its main components are the photocatalytically active anode and the cathode coils, which are wrapped around a silica-glass rod and placed into a UV-transparent housing. The anode coil was prepared by anodic oxidation of the titanium coil, which leads to the formation of vertically aligned, titanium dioxide, nanotube arrays that exhibit a high photocatalytic activity and are rigidly attached to the titanium wire. The photocatalytic, electrocatalytic and photoelectrocatalytic activities of the assembled microreactor were measured systematically while changing the main parameters that affect the device’s efficiency. The most significant change in the microreactor’s design in terms of efficiency was to place the cathode coils on both sides of the anode coil, which resulted in the shortest time for complete degradation of the caffeine. When an applied anodic bias potential of 4 V was used in the most efficient microreactor design, 1 mL of the 40 mg/L caffeine was fully decomposed in 55 min.

Roles of blocking layer and anode bias in processes of impurity-band transition and transport for GaAs-based blocked-impurity-band detectors

Recently, GaAs-based BIB detector has attracted a lot of attention in the area of THz photovoltaic detection due to potential application values in security check and drug inspection. However, the physical mechanisms involving in carrier transition and transport are still unclear due to the poor material quality and immature processing technique. In this paper, the dark current and THz response characteristics have thus been numerically studied for GaAs-based blocked-impurity-band (BIB) detectors. The key parameters and physical models are constructed by simultaneously considering carrier freeze-out and impurity-band broadening effects. Roles of blocking layer and anode bias in processes of impurity-band transition and transport are intensively investigated, and the results can be well explained by numerical models. It is demonstrated that the effective electric field for the detector is only located in the absorbing layer, and can determine to a large extent the magnitude of the dark current and THz response. While the blocking layer not only can suppress dark current but also can attenuate responsivity due to its electric-field modulation effect.

Electrochemical Formation and Characterization of Cuprous Oxide Films on n-Type InP Porous Structures

The high-density formation of semiconductor nano- and micro- structures has been intensely researched targeting applications for high conversion-efficiency solar cells. A semiconductor porous structure formed by the electrochemical process is a good candidate as a building block of energy conversion devices having large surface area and low photo-reflectance. Recently, we have reported the photoelectric-conversion device utilizing a Pt Schottky barrier formed on indium phosphide (InP) porous structure [1]. The photocurrents drastically increased in the Pt/InP porous device, whereas it remains an issue on the low photo-voltage. One possible approach to improve the conversion efficiency is to replace Schottky contacts with pn junctions which have a large built-in potential. In this study, we aimed to form the p-type cuprous oxide (Cu2O) films on n-type InP porous structures using the electrochemical deposition process. The porous structures were electrochemically formed using a (001) n-type InP (n=1x1017 cm-3). The electrochemical cell has three electrodes: an n-type InP electrode as a working electrode (WE), a platinum (Pt) counter electrode (CE), and Ag/AgCl reference electrode (RE). These were electrically connected to the potentiostat with a pulse generator. The n-InP layer was first anodized at 5 V in a HCl-based electrolyte to form the porous structures. A disordered irregular layer formed and remained on the top of the ordered porous layer [2]. To remove this irregular layer, the porous surface was then photo-electrochemically etched at an anode bias of 1 V in the same electrolyte under illumination. After that, in order to form a thin Cu2O film on the wall inside the pores, the cathode bias, V c, was applied to the WE in the electrolyte consisting of 0.5 M cupric sulfate and 3.0 M lactic acid. The conductivity and morphology of Cu2O films were strongly affected by the pH value of the electrolyte [3,4]. In this study, the pH of the Cu2O electrolyte was varied in range from 9 to 12 by adding a NaOH solution. The deposited films were characterized using a scanning electron microscope (SEM), X-ray diffraction (XRD), Auger electron spectroscopy (AES), UV-visible spectroscopy, and photo-electrochemical measurements. In order to optimize the electrochemical process, Cu2O films were first deposited on indium tin oxide (ITO) plates by changing the V c and pH values. From the SEM and photo-electrochemical measurements, the smooth and chemically-stable surface were obtained at V c = -0.3 V and pH of 12.0. Under such an optimal condition, the top surface of Cu2O films was dominantly oriented in the (001) direction. After the deposition of Cu2O films for 30 min, the photocurrent measurements were conducted in a 1M NaCl electrolyte under the irradiation of monochromatic light. Figure 1 shows the photocurrents measured as function of light wavelength. As shown in the inset of Fig. 1, the cathode current was observed under illumination, but it wasn’t in the dark. This result suggests that the Cu2O film formed in this study showed p-type conductivity. The amount of photocurrents steeply increased around the wavelength of 580-620 nm whose photon energy was very consistent with the bandgap energy of Cu2O. These results suggest that the Cu2O films formed by the electrtochemical process are usefull as a light absorber to be formed on n-type semiconductors. Figure 2(a) shows the cross-sectional image of the n-type InP porous structure formed by the electrochemical process. The average depth and diameter of the ordered pores is 7.1 µm and 100 nm, respectively. After the cathodic deposition, it was proved that Cu2O films were formed on n-type InP walls inside the pores, as shown in Fig. 2(b). Figure 2(c) shows the AES spectra obtained at the deep part of the InP porous sample after the Cu2O deposition. The AES signals from Cu and O were clearly observed at 920 eV and 510 eV, respectively. These results indicate that the electrochemical deposition of Cu2O films is very useful technique for the fromation of a pn junction on the high-density nanostructures with a good surface coverage. [1] R. Jinbo et al., Thin Solid Films, 520 (2012) 5710. [2] T. Sato et al., Electrochem. Solid-State Lett., 11 (2008) H111. [3] R. Liu et al., Chem. Mater., 15 (2003) 4882. [4] L. Wang et al., Electrochem. Solid-State Lett., 10 (2007) H248 Figure 1

Electrochemically Driven Deactivation and Recovery in PrBaCo2 O5+δ Oxygen Electrodes for Reversible Solid Oxide Fuel Cells.

The understanding of surface chemistry changes on oxygen electrodes is critical for the development of reversible solid oxide fuel cell (RSOFC). Here, we report for the first time that the electrochemical potentials can drastically affect the surface composition and hence the electrochemical activity and stability of PrBaCo2 O5+δ (PBCO) electrodes. Anodic polarization degrades the activity of the PBCO electrode, whereas the cathodic bias could recover its performance. Alternating anodic/cathodic polarization for 180 h confirms this behavior. Microstructure and chemical analysis clearly show that anodic bias leads to the accumulation and segregation of insulating nanosized BaO on the electrode surface, whereas cathodic polarization depletes the surface species. Therefore, a mechanism based on the segregation and incorporation of BaO species under electrochemical potentials is considered to be responsible for the observed deactivation and recovery process, respectively.

Use of Patterned Metal Anodes for Studying Hydrogen Electrochemical Oxidation Reaction in Proton-Conducting SOFCs

Despite the great potential for proton-conducting solid oxide fuel cells (SOFCs) for clean power generation at intermediate temperature (e.g., 400-700 oC), there are significant knowledge gaps about fundamental aspects for such cells. In particular, very little is known about the kinetics and reaction mechanism for the hydrogen electrochemical oxidation reaction in proton-conducting SOFCs. Here, we will report the preliminary results of our study using patterned metal anode over acceptor-doped Ba(Ce,Zr)O3 proton-conducting electrolyte. The effects of various factors such as choice of anode metal (e.g., nickel versus copper), anodic bias voltage, gas atmosphere, temperature, and electrode geometry (e.g., triple phase boundary TPB length and electrode area) on the electrochemical behaviors of proton-conducting SOFC will be studied. The results will be correlated to previous studies for hydrogen electrode reaction in conventional oxide-ion conducting SOFCs, and the implications on the fundamental reaction mechanism for the hydrogen electrode reaction involving proton-conducting oxides will be discussed.

Heterogeneous and Homogeneous Routes in Water Oxidation Catalysis Starting from Cu(II) Complexes with Tetraaza Macrocyclic Ligands.

Since the first report in 2012, molecular copper complexes have been proposed as efficient electrocatalysts for water oxidation reactions, carried out in alkaline/neutral aqueous media. However, in some cases the copper species have been recognized as precursors of an active copper oxide layer, electrodeposited onto the working electrode. Therefore, the question whether copper catalysis is molecular or not is particularly relevant in the field of water oxidation. In this study, we investigate the electrochemical activity of copper(II) complexes with two tetraaza macrocyclic ligands, distinguishing heterogeneous or homogeneous processes depending on the reaction media. In an alkaline aqueous solution, and upon application of an anodic bias to working electrodes, an active copper oxide layer is observed to electrodeposit at the electrode surface. Conversely, water oxidation in neutral aqueous buffers is not associated to formation of the copper oxide layer, and could be exploited to evaluate and optimize a molecular, homogeneous catalysis.

Development of inorganic composite material based TiO2 for environmental application

Syntheses of various materials, for green energy nanotechnology applications have special attention to develop emerging areas, such as environmental as well as energy materials. Various approaches for preparing nanostructured photocatalysts, such as titanium dioxide, nickel oxide, lead oxide and their composites, was introduced. The use of nanomaterials as photocatalysts water detoxification by visible light photocatalyst of an inorganic composite as well as dye-sensitized photoreduction was also discussed. The enhancement of selective photocatalyst system was gain by the use of photocatalyst composite materials and applied potential bias on the system. The photoelectrocatalytic degradation of rhodamine B (RB) and Remazol Yellow FG (RY) as water contaminant using the thin film of modified TiO2 as the electrode was investigated via a series of potentials, and various pH. The result showed that the anodic potential bias influenced the degradation rate of water contaminant and exhibited better performance by the positive anodic bias was applied. The pH conditions influence the active dye structure whereas it will interact with inorganic semiconductor photocatalyst. Using dye- sensitized TiO2 system (DSTs), we have applied this system to build water decolorization as a novelty environmental remediation system.

An Effective, Point-of-Use Water Disinfection Device Using Immobilized Black TiO2 Nanotubes as an Electrocatalyst

The following study characterized titanium dioxide nanotube arrays (TNAs) annealed in air (air-TNA), nitrogen (N2-TNA) or hydrogen with a nitrogen balance (H2-TNA) for the rapid disinfection of waterborne bacteria. The H2-TNA became black after annealing due to a large concentration of oxygen vacancies, leading to a higher absorbance in the visible light region compared to the air-TNA and N2-TNA. Photoelectrochemical testing showed that the N2-TNA had the highest photocurrent response in the absence of an applied bias; however, under the application of a 6 V anodic bias, the photocurrent response of all the samples was dampened and the dark current of the H2-TNA was ≥50% higher than N2-TNA and air-TNA. The higher defect density in the H2-TNA compared to the other anodes led to the formation of more oxidizing radical species using only an applied bias. Complete inactivation of total coliform and E. Coli from natural creek water was obtained using a 50 ml/min flow reactor containing the H2-TNA after 25 s and under a +6 V potential in the absence of solar illumination.

Vertically aligned hexagonal WO3 nanotree electrode for photoelectrochemical water oxidation

Thin films of aligned hexagonal WO3 nanotrees were fabricated on metal tungsten substrates by hydrothermal treatment. In the presence of a sacrificial reduction agent, the fabricated WO3 nanotree films oxidized water to evolve oxygen under visible-light irradiation. The photocatalytic water oxidation activity of the WO3 nanotree was higher than that of a dense monoclinic WO3 electrode, and had a quantum efficiency of 3.2% under visible-light irradiation. With the application of an anodic bias potential, the WO3 nanotree electrode produced both hydrogen and oxygen in the absence of a sacrificial agent under visible-light irradiation.

Nickel (III) Salt Dispersed Poly-aniline film: Preliminary SECM Examination

Various matrices including poly-aniline have been used recently to incorporate nickel salts within the polymer. These films have been shown to be of utility for exploiting the oxidizing characteristic associated with nickel (III) under the right conditions, for example when the film is subjected to anodic bias. Preliminary SECM examination of the above films was undertaken. The results suggested the presence of active sites on the surface originating from higher-valent nickel cation. Reference Inam-ul-Haque, Aakifa Raza and Shamaila Sadaf, Dispersed nickel oxide films for electro-oxidation of 1, 4-butanediol, Journal of the Chemical Society of Pakistan, 33 (1) 21-28 (2011). Acknowledgments Inam-ul-Haque thanks German Academic Exchange Service, DAAD, for a study and research stay; September to November 2010 at Department of Pure and Applied Chemistry University of Oldenburg Germany, and gratefully acknowledges kind and gracious hospitality of Professor Dr Gunther Wittstock.

Evaluation of La2NiO4 as Anode Material for Solid Oxide Electrolyser Cells

La2NiO4 is characterised as potential anode material in solid oxide electrolyser cells (SOECs). Investigations are performed at 800°C and 0.1-0.2 bar oxygen partial pressure over a period of several thousand hours. Trends in chemical surface exchange coefficient and electrical conductivity of a densely sintered sample are identified. In addition, changes in polarisation resistance of a porous anode layer of La2NiO4 on a button cell under constant anodic current bias of -410 mAcm-2 are recorded. Special emphasis is laid on the impact of Cr-impurities in the surrounding gas phase on the oxygen transport properties and SOEC anode performance. Cr-poisoning is found to depend strongly on the level of humidity in the gas phase as well as on the polarisation conditions.

A portable x-ray source with a nanostructured Pt-coated silicon field emission cathode for absorption imaging of low-Z materials

We report the design, fabrication, and characterization of a portable x-ray generator for imaging of low-atomic number materials such as biological soft tissue. The system uses a self-aligned, gated, Pt-coated silicon field emitter cathode with two arrays of 62 500 nano-sharp tips arranged in a square grid with 10 μm emitter pitch, and a natural convection-cooled reflection anode composed of a Cu bar coated with a thin Mo film. Characterization of the field emitter array demonstrated continuous emission of 1 mA electron current (16 mA cm − 2) with >95% current transmission at a 150 V gate-emitter bias voltage for over 20 h with no degradation. The emission of the x-ray source was characterized across a range of anode bias voltages to maximize the fraction of photons from the characteristic K-shell peaks of the Mo film to produce a quasi-monochromatic photon beam, which enables capturing high-contrast images of low-atomic number materials. The x-ray source operating at the optimum anode bias voltage, i.e. 35 kV, was used to image ex vivo and nonorganic samples in x-ray fluoroscopic mode while varying the tube current; the images resolve feature sizes as small as ~160 µm.