Low-voltage anodized TiO<Subscript>2</Subscript> nanostructures studied by alternate current electrochemical microscopy and photoelectrochemical measurements

This paper presents the characterization of TiO₂ nanostructures obtained by low-voltage anodization using alternate current electrochemical microscopy (AC-SECM) and photoelectrochemical (PEC) measurements. TiO₂ nanostructures were obtained from the exposure of titanium foils to several aqueous acidic solutions of hydrofluoric acid + phosphoric acid at potentials of 1 to 3 V. Scanning electron microscopy, X ray diffraction, and atomic force microscopy studies evidence the formation of a thin porous amorphous layer (<600 nm) with pore size in the range of 200–1,000 nm. By AC-SECM studies at different bias, we were able to confirm the unambiguous semiconducting properties of as-obtained porous titania films, as well as differences in surface roughness and conductivity in specimens obtained at both potentials. The difference in conductivity persists in air annealed samples, as demonstrated by electrochemical impedance spectroscopy and PEC measurements. Specimens obtained at 3 V show lower photocurrent and dark current than those obtained at 1 V, regardless of their larger conductivity, and we proposed it is due to differences on the oxide layer formed at the pore bottom.     

Triton-X mediated interconnected nanowalls network of cadmium sulfide thin films via chemical bath deposition and their photoelectrochemical performance

Thin films of cadmium sulfide (CdS) have been wet chemically deposited onto fluorine-doped tin oxide (FTO) coated conducting glass substrates by using non-ionic surfactant; Triton-X 100. An aqueous solution contains cadmium sulphate as a cadmium and thiourea as sulphur precursor. Ammonia used as a complexing agent. The results of measurements of the x-ray diffraction, Raman spectroscopy, optical spectroscopy, energy dispersive spectroscopy, scanning electron microscopy, Brunauer Emmett Teller (BET) surface areas and atomic force microscopy were used for the characterization of the films. These results revealed that the films are polycrystalline, consisting of CdS cubic phase. The films show a direct band gap with energy 2.39 eV. The films show interconnected nanowalls like morphology with well-defined surface area. Finally, the photoelectrochemical (PEC) performance of Triton-X mediated CdS thin film samples were studied. The sample shows photoelectrochemical (PEC) performance with maximum short circuit current density (J_sc) 1.71 mA/cm² for larger area (1 cm²) solar cells.     

Photoelectrochemical Behavior of Sn‐Doped α‐Fe2O3 Photoanode with Different Reducer

Hematite has been considered as one of the most promising photoanode candidates for solar water‐splitting. However, its photoelectrochemical (PEC) efficiency is largely constrained by its sluggish oxygen evolution reaction. In this work, the photoelectrochemical performance of hematite was investigated in electrolytes containing different sacrificial agent. The photocurrent densities, onset potential, charge transfer resistance, Helmholtz capacitance at semiconductor liquid junctions (SCLJs), and their correlations were systematically studied. It was found that the onset potential is around the C_H peak potential and is related to the photovoltage. The surface states pinning the Fermi levels of the hematite photoanode are related to the adsorbed water molecules regardless of the sacrificial agents in the electrolyte.     

The Role of Surface States in the Oxygen Evolution Reaction on Hematite

Hematite (α‐Fe₂O₃) is an extensively investigated semiconductor for photoelectrochemical (PEC) water splitting. The nature and role of surface states on the oxygen evolution reaction (OER) remain however elusive. First‐principles calculations were used to investigate surface states on hematite under photoelectrochemical conditions. The density of states for two relevant hematite terminations was calculated, and in both cases the presence and the role of surface states was rationalized. Calculations also predicted a Nerstian dependence on the OER onset potential on pH, which was to a very good extent confirmed by PEC measurements on hematite model photoanodes. Impedance spectroscopy characterization confirmed that the OER takes place via the same surface states irrespective of pH. These results provide a framework for a deeper understanding of the OER when it takes place via surface states.     

Photoelectrochemical aptasensor for sulfadimethoxine using g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> quantum dots modified with reduced graphene oxide

A novel photoelectrochemical (PEC) aptasensor with graphitic-phase carbon nitride quantum dots (g-C₃N₄; QDs) and reduced graphene oxide (rGO) was fabricated. The g-C₃N₄ QDs possess enhanced emission quantum yield (with an emission peak at 450 nm), improved charge separation ability and effective optical absorption, while rGO has excellent electron transfer capability. Altogether, this results in improved PEC performance. The method is making use of an aptamer against sulfadimethoxine (SDM) that was immobilized on electrode through π stacking interaction. Changes of the photocurrent occur because SDM as a photogenerated hole acceptor can further accelerate the separation of photoexcited carriers. Under optimized conditions and at an applied potential of +0.2 V, the aptasensor has a linear response in the 0.5 nM to 80 nM SDM concentration range, with a 0.1 nM detection limit (at S/N =?3). The method was successfully applied to the analysis of SDM in tap, lake and waste water samples.

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Graphical abstract Graphitic-phase carbon nitride (g-C₃N₄) quantum dots (QDs) and reduced graphene oxide (rGO) were used to modify fluorine-doped SnO₂ (FTO) electrodes for use in a photoelectrochemical (PEC) aptasensor. SDM oxidized by the hole on valance band (VB) of g-C₃N₄ QDs promote the separation of electron in the conductive band (CB), which made the changes of photocurrent signal.

     

Pristine Ta3N5 Nanotubes: Trap‐Driven High External Biasing Perspective in Semiconductor/Electrolyte Interfaces

Ta₃N₅ is a promising photoelectrode for solar hydrogen production; however, to date pristine Ta₃N₅ electrodes without loading co‐catalysts have presented limited photoelectrochemical (PEC) performance. In particular, large external biasing has been required to run water oxidation, the origin of which is investigated herein. Ta₃N₅ nanotubes (NTs) prepared by nitridation were characterized by a wide range of techniques. The bandgap was confirmed by a novel PEC technique. Nondestructive synchrotron‐excited XPS has shown the presence of reduced Ta species deeper in the Ta₃N₅ surface. Lower photocurrent and transient spikes that were intense at lower applied biasing were observed under water oxidation; however, spikes were inhibited in the presence of a sacrificial agent and photocurrent was improved even at low biasing. It was observed for the first time that the lower PEC performance under water oxidation can be attributed to the presence of interband trapping states associated with pristine Ta₃N₅ NTs/electrolyte junction. These states correspond to the structural defects in Ta₃N₅, devastate PEC performance, and present the necessity to apply higher biasing. The key to circumvent them is to use a sacrificial agent in the electrolyte or to load a suitable co‐catalyst to avoid hole accumulation under water oxidation, thereby improving the phootocurrent. The findings on the interband states could also provide guidance for the investigation of PEC properties of new types of semiconducting devices.     

Influence of Ga dopant on photoelectrochemical characteristic of Ga‐doped ZnO thin films deposited by sol–gel spin‐coating technique

In this study, Ga‐doped ZnO thin films were prepared using sol–gel technique via spin‐coating method. The effect of Ga‐doping dopant (0, 1, 2 and 3 at.%) on microstructural, optical, electrical and photoelectrochemical (PEC) characteristics have been investigated. The spin‐coating was repeated six times, and as‐obtained thin films were then annealed at 500 °C for 1 h in vacuum. After annealing, all samples revealed single phase of hexagonal ZnO polycrystalline structure with a main peak of (002) in X‐ray diffraction (XRD) pattern. Raman spectra show that the vibration strength of E₂ is highly decreased by Ga doping. Thicknesses of all samples were ~300 nm measured via scanning electron microscopy (SEM) cross‐section images and alpha‐step. The optical band gap and resistivity of samples were in the range of 3.24 to 3.28 eV and 102 to 9 Ohm cm, respectively. Resulting from PEC response, the 2 at.% Ga‐doped ZnO thin film has a better PEC performance with photocurrent density of ~0.14 mA/cm² at 0.5 V versus saturated calomel electrode (SCE) under illumination with the intensity of 100 mW/cm². This value was about seven times higher than the un‐doped film (reference sample). Observed higher photocurrent density was likely because of a suitable Ga‐doping concentration causing a lower resistivity. Copyright © 2016 John Wiley & Sons, Ltd.     

The effect of spinel type support FeAlO<Subscript>3</Subscript>, ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript>, CrAl<Subscript>3</Subscript>O<Subscript>6</Subscript> on physicochemical properties of Cu,Ag, Au,Ru supported catalysts for methanol synthesis

The comparative study of the role of binary oxide support on catalyst physico-chemical properties and performance in methanol synthesis were undertaken and the spinel like type structures (ZnAl₂O₄, FeAlO₃, CrAl₃O₆) were prepared and used as the supports for 5% metal (Cu, Ag, Au, Ru) dispersed catalysts. The monometallic 5% Cu/support and bimetallic 1% Au (or 1% Ru)-5% Cu/support (Al₂O₃, ZnAl₂O₄, FeAlO₃, CrAl₃O₆) catalysts were investigated by BET, XRD and TPR methods. Activity tests in methanol synthesis of CO and CO₂ mixture hydrogenation were carried out. The order of Cu/support catalysts activity in methanol synthesis: CrAl₃O{ia6} > FeAlO₃ > ZnAl₂O₄ is conditioned by their reducibility in hydrogen at low temperature. Gold appeared more efficient than ruthenium in promotion of Cu/support catalysts. Published in Russian in Kinetika i Kataliz, 2009, Vol. 50, No. 2, pp. 242–248. The article is published in the original.     

Microwave assisted novel MoBi2S5 nanoflowers: Synthesis,characterization, photoelectrochemical performance

In the present article, we have studied the effect of post annealing treatment on microstructural, optical and photoelectrochemical (PEC) properties of MoBi₂S₅ thin films synthesized by microwave assisted technique. The synthesized thin films are vacuum annealed for 4 h at 473 K temperature. The X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM) and UV–Vis–NIR spectrophotometer techniques were used for characterization of the as deposited and annealed MoBi₂S₅ thin films. The XRD patterns confirm the synthesized and annealed thin films have nanocrystalline nature with rhombohedral-orthorhombic crystal structure. SEM micrographs indicate that, nanoflowers exhibit sharper end after annealing. The optical absorption study illustrates that the optical band gap energy has been decrease from 2.0 eV to 1.75 eV with annealing. Finally, applicability of synthesized thin films has been checked for PEC property. The J-V curves revealed that synthesized thin film photoanodes are suitable for PEC cell application. As well, used simple, economical method has great potential for synthesis of various thin film materials.     

Photocatalytic reaction of arylamines/alcohols on TiO<Subscript>2</Subscript> nano-particles

In this article, the photocatalytic reaction of aniline and 4-amino N,N-dimethyl aniline with methanol, ethanol and isopropanol on anatase TiO₂ nano-particles under UV (365-nm wavelength) irradiation was examined. The concentration of unreacted arylamines and products was measured by gas chromatography picks integration, and then the products were identified by mass spectroscopy analysis. By making a comparison within the rates of photocatalysis of each arylamine in different alcohols under various irradiation times, it was revealed that, in all cases, the sequence of photocatalysis rate was methanol > ethanol > isopropanol. In reactions where the concentrations of arylamine were lower than 10 mmol/l, imines were the main products and the alkylation of amines was not observed. In the higher concentration of arylamines, oxidation and dimerization was occurred.     

Preparation of novel visible-light-driven In<Subscript>2</Subscript>O<Subscript>3</Subscript>–CaIn<Subscript>2</Subscript>O<Subscript>4</Subscript> composite photocatalyst by sol–gel method

Novel visible-light-activated In₂O₃–CaIn₂O₄ photocatalysts were developed in this paper through a sol–gel method. The photocatalytic activities of In₂O₃–CaIn₂O₄ composite photocatalysts were investigated based on the decomposition of methyl orange under visible light irradiation (λ > 400 nm). The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrum (EDS), X-ray photoelectron spectroscopy (XPS) and UV–vis diffused reflectance spectroscopy (DRS). The results revealed that the In₂O₃–CaIn₂O₄ composite samples with different In₂O₃ and CaIn₂O₄ content can be obtained by controlling the synthesis temperature, and the composite photocatalysts extended the light absorption spectrum toward the visible region. The photocatalytic tests indicated that the composite samples demonstrated high visible-light activity for decomposition of methyl orange. The significant enhancement in the In₂O₃–CaIn₂O₄ photo-activity under visible light irradiation can be ascribed to the efficient separation of photo-generated carriers in the In₂O₃ and CaIn₂O₄ coupling semiconductors.     

Photoelectrochemical‐assisted Batch Injection Analysis (PEC‐BIA) of Glucose Exploiting Visible LED Light as an Excitation Source

This work describes the development of a novel method for glucose determination exploiting a photoelectrochemical‐assisted batch injection analysis cell designed and constructed with the aid of 3D printer technology. The PEC‐BIA cell was coupled to a LED lamp in order to control the incidence of light on the Cu₂O/Ni(OH)₂/FTO photoelectroactive platform. The electrochemical characteristics of Cu₂O/Ni(OH)₂/FTO photoelectroactive platform were evaluated by cyclic voltammetry, amperometry, and electrochemical impedance spectroscopy. The PEC‐BIA cell presented linear response range, limit of detection based on a signal‐to‐noise ratio of three, and sensitivity of 1–1000 μmol L^?1, 0.76 μmol L^?1 and 0.578 μA L μmol^?1, respectively. The PEC‐BIA method presented a mean value of the recovery values of 97.0 % to 102.0 % when it was applied to glucose determination in artificial blood plasma samples which indicates the promising performance of the proposed system to determine glucose.     

Influence of MnO<Subscript>2</Subscript> on the photocatalytic activity of P-25 TiO<Subscript>2</Subscript> in the degradation of methyl orange

Influences of α-MnO₂, β-MnO₂, and δ-MnO₂ on the photocatalytic activity of Degussa P-25 TiO₂ have been investigated through the photocatalytic degradation of methyl orange. The TiO₂ photocatalyst, before and after being contaminated by MnO₂, was characterized by UV-visible diffuse reflectance spectroscopy (UV-vis DRS), photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). The results showed that photocatalytic activity of TiO₂ could be inhibited significantly or completely deactivated due to the presence of even a small amount of MnO₂ particles. It was found that the poisoning effect varied with the crystal phases of MnO₂ and the effect was in the order δ-MnO₂ >α-MnO₂ >β-MnO₂. The poisoning effect was attributed to the formation of heterojunctions between MnO₂ and TiO₂ particles. The heterojunctions changed the chemical state of Ti⁴⁺ and O²⁻ sites in the crystalline phase of TiO₂. MnO₂ in contact with TiO₂ particles also broadens the band-gap of TiO₂, which decreases UV absorption of TiO₂. It can also create some deep impurity energy levels serving as photoelectron-photohole recombination center, which accelerates the electron-hole recombination. Supported by the National Natural Science Foundation of China (Grant No. 20477009) and the Natural Science Foundation of Hebei Province (Grant No. E2005000183)     

Heterostructured CoO/3D-TiO<Subscript>2</Subscript> nanorod arrays for photoelectrochemical water splitting hydrogen production

Homogeneous p-type cobalt (II) oxide (CoO) nanoparticles were successfully deposited on n-type three-dimensional branched TiO₂ nanorod arrays (3D-TiO₂) through photochemical deposition and thermal decomposition to form a novel CoO/3D-TiO₂ p-n heterojunction nanocomposite. Due to the narrow band gap of CoO nanoparticles (~2.4 eV), the as-synthesized CoO/3D-TiO₂ exhibited an excellent visible light absorption. The amounts of deposited CoO nanoparticles obviously influence the hydrogen production rate in the photoelectrochemical (PEC) water splitting. The as-synthesized CoO/3D-TiO₂-5 obtains the highest PEC hydrogen production rate of 0.54 mL h^?1 cm^?2 after five-time CoO deposition cycles (at a potential of 0.0 V vs Ag/AgCl). The photocurrent density of CoO/3D-TiO₂-5 is 1.68 mA cm^?1, which is ca. 2.5 times greater than that of pure 3D-TiO₂. The results showed that the formation of internal electrical-field between the CoO/3D-TiO₂ heterojunction, which has a direction from n-type TiO₂ to p-type CoO, facilitated the charge separation and transfer and resulted in a high efficiency and stable PEC activity.     

Photoelectrochemical study of electrodeposited TiO<Subscript>2</Subscript> thin films onto F:SnO<Subscript>2</Subscript> substrates

TiO₂ thin films have been effectively fused onto F:SnO₂ (FTO) substrates via the electrodeposition method. The influence of deposition temperature on the synthesis of F:SnO₂ substrates and relative information of as-deposited and annealed TiO₂ thin films have been studied. Novel TiO₂ microspheres are detected on F:SnO₂ substrates at an optimized electrodeposition potential. Raman bands approve the creation of single-anatase-phase TiO₂. The optimized deposition surroundings show a decrease in the band gap of F:SnO₂ substrates and TiO₂ thin films. The determined photoelectrochemical properties of annealed TiO₂ thin films indicate a fill factor of 51% and power conversion efficiency of 0.15% for application in solar cells.     

RuO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> mixed oxide composite coating for improvement of Al-alloy sacrificial anodes

It has been recently proved that RuO₂ can act as an effective surface activator of aluminum alloy sacrificial anodes. TiO₂ has the property of stabilizing RuO₂ coating and resisting biofouling on metal surfaces. Hence, a mixed oxide catalytic coating of TiO₂ and RuO₂ can enhance the galvanic performance of aluminum alloy sacrificial anodes and resists biofouling on the anode surface. In the present work RuO₂–TiO₂ mixed oxide was coated on aluminum alloy sacrificial anodes. The large and uniform porous nature of the coating was found to facilitate efficient ion diffusion. The coating was found to persist on the anode even after 3 months of galvanic exposure. The anode having an optimum combination of the mixed oxide had 70% TiO₂ as the major component in the coating. The catalytic coating significantly improved the performance of the anodes to a large extent.     

Interactions in the NdF<Subscript>3</Subscript>-Nd<Subscript>2</Subscript>O<Subscript>3</Subscript>-MF<Subscript>2</Subscript> (M = Ba,Sr) systems

Isothermal anneals (at 873 K) and powder X-ray diffraction were used to study isothermal sections of phase diagrams of the NdF₃-Nd₂O₃-MF₂ (M = Ba, Sr) systems. In studying the NdF₃-Nd₂O₃-BaF₂ system, classical solid-phase synthesis was supplemented with mechanochemical activation of feedstock mixtures or BaF₂ activated with gaseous hydrogen fluoride was used. In both systems, a solid solution with the fluorite structure based on MF₂ and NdOF phases, a solid solution with the tysonite structure based on NdF₃, and an ordered fluorite-related phase Ba₄Nd₃F₁₇ were found. The NdOF-based solid solutions were shown to have polymorphism: β_trig ai α_cub at ≈800 K; a new trigonal phase of these solid solutions has been discovered. The effect of a dimensional factor $\left( {R_{Ba^{2 + } } > R_{Sr^{2 + } } } \right)$\left( {R_{Ba^{2 + } } > R_{Sr^{2 + } } } \right) on phase formation and unit cell parameters of the solid solutions was traced.     

Bioaccessibility of selected trace metals in urban PM<Subscript>2.5</Subscript> and PM<Subscript>10</Subscript> samples: a model study

Bioaccessibility of trace metals originating from urban particulate matter was assessed in a worst case scenario to evaluate the uptake and thus the hazardous potential of these metals via gastric juice. Sampling was performed over a period of about two months at the Getreidemarkt in downtown Vienna. Concentrations of the assayed trace metals (Ti, Cr, Mn, Co, Ni, Cu, Zn, Mo, Ag, Cd, Sn, Sb, Tl and Pb) were determined in PM_2.5 and PM₁₀ samples by ICP-MS. The metal concentrations in sampled air were in the low picogram to high nanogram per cubic metre range. The concentrations in PM_2.5 samples were generally lower than those in PM₁₀ samples. The average daily intake of these metals by inhalation for a healthy adult was estimated to be in the range of <1 ng (Tl) to >1,000 ng (Zn). To estimate the accessibility of the inhaled and subsequently ingested metals (i.e. after lung clearance had taken place) in the size range from 2.5- to 10-μm aerodynamic equivalent diameter, a batch-extraction with synthetic gastric juice was performed. The data were used to calculate the bioaccessibility of the investigated trace metals. Extractable fractions ranged from 2.10% (Ti in PM_2.5) to 91.0% (Cd in PM_2.5), thus yielding bioaccessible fractions (PM_2.5–10) from 0.16 ng (Ag) to 178 ng (Cu).     

Preparation of tantalum oxynitride thin film photocatalysts by reactive magnetron sputtering deposition under high substrate temperature

This work spotlights the formation behavior of visible light-responsive tantalum oxynitride (TaON) thin film photocatalysts under high substrate temperature in radiofrequency reactive magnetron sputtering deposition. The results emanating from the optimization of the sputtering conditions demonstrated that sputtered N atoms with high kinetic energy generated by controlling target–substrate distances and total pressures in the sputtering chamber were necessary to grow TaON phase even under N₂-rich atmosphere. Based on these findings, TaON thin film photocatalysts were successfully synthesized by single-step sputtering under a high substrate temperature of 1073 K before heat treatment. The optimal thickness of TaON thin film photocatalysts was extrapolated to be 450 nm by photoelectrochemical measurements under visible light irradiation (λ > 450 nm), in which distinct photocurrents corresponding to water oxidation were observed. Moreover, the photoelectrochemical activity was able to be improved by postsynthetic heat treatment in gaseous NH₃ and loading with IrO₂ nanocolloids as cocatalysts. This finding would be because the thin film photocatalyst after heat treatment in NH₃ under appropriate conditions possessed better crystallinity and moderate donor density. The optimized TaON thin film photocatalysts with IrO₂ nanocolloids also exhibited photocatalytic activity for H₂ evolution from aqueous medium containing methanol as a sacrificial electron donor under visible light irradiation (λ > 450 nm).     

Label-free homogeneous photoelectrochemical aptasensing of VEGF165 based on DNA-regulated peroxidase-mimetic activity of metal-organic-frameworks

The application of metal-organic frameworks (MOFs) nanozymes in biosensing has been extensively investigated, however, till now there is still no report on photoelectrochemical (PEC) sensing based on enzyme memetic properties of MOFs. To further expand the utilization of MOFs nanozymes in biosensing, we developed a label-free homogenous PEC aptasensor for the detection of VEGF₁₆₅, an important cancer biomarker, based on the DNA-regulated peroxidase-mimetic activity of Fe-MIL-88, a type of MOFs. In this strategy, the peroxidase-mimetic property of MOFs is integrated with the label-free homogeneous PEC sensing approach, and highly sensitive detection of VEGF₁₆₅ is obtained with a detection limit down to 33 fg/mL, superior or comparable to the previously reported values. Moreover, this approach displays outstanding specificity, and has been successfully used to detect VEGF₁₆₅ added in diluted serum samples. As far as we know, it is the first example to employ the peroxidase-like activity of MOFs in PEC biosensing, which may find potential application in bioanalysis and early disease diagnosis.