Negative Flat Band Potential Research Articles

Facile microwave assisted synthesis of floral-shaped BiVO4 nano particles for their photocatalytic and photoelectrochemical performances

Floral shaped monoclinic scheelite BiVO4 (ms-BiVO4) have been successfully synthesized by a facile microwave assisted method using domestic micro oven. The synthesized material has been subjected to characterized using XRD, SEM, TEM, XPS, BET, IR, Raman, PL and UV–Vis DRS techniques to describe the crystalline and morphology nature and photochemical properties of the sample. The floral shaped ms-BiVO4 possessed strong photoabsorption properties in the visible light with a band gap of ca. 2.3 eV. The analysis report of the ms-BiVO4 was stated that the catalyst has monoclinic crystalline and high surface area of ca. 24.0 m2 g−1. This study denotes that the microwave plays a major impact on the morphology and crystal size. The photocatalytic activity was evaluated by the degradation of a methylene blue and phenol aqueous solution in the presence of a small amount of H2O2 under visible light illumination. The photocurrent density and more negative flat band potentials also supported to photoelectrochemical (PEC) water splitting performance of ms-BiVO4 and the two features could have to improve the recombination life time, electron donor density. The floral shaped ms-BiVO4 maintained stability with better photocatalytic efficiency even at fifth cycles. The improved photocatalytic activity for methylene blue, phenol degradation and PEC activity of ms-BiVO4 was associated with the unique crystalline, optical property and morphology.

Photoelectrochemical behavior of bimetallic Cu–Ni and monometallic Cu, Ni doped TiO2 for hydrogen production

Photocatalyst is the heart of the photoelectrochemical (PEC) cell that is used to generate hydrogen by water splitting from solar energy. Thus improving the photoelectrochemical properties will result in better conversion efficiency. This paper presents the investigation of the photoelectrochemical behavior of the synthesized nanostructured Cu–Ni doped TiO2 compared to monometallic Ni and Cu doped TiO2 and also TiO2 photocatalysts. The photoelectrochemical properties of photoanode in the PEC cell showed that 5Cu-5Ni doped TiO2 thin film produced the highest amount of hydrogen (5.3 ml) from photosplitting of water, photocurrent density of 2.29 mA/cm2 at 0.24 V, photoconversion efficiency of 4.33%, electron life time of 217.71 ms with the flat band and donor density from Mott–Schottky of −0.93 V and 2.14 × 1021 cm−3, respectively. This better performance compared to other photocatalysts is attributed to the synergetic effect of two metals as charge carriers traps, more electron–hole separation, longer electron lifetime, more negative flat band potential for water splitting and higher electron donor density.

Solar hydrogen generation from spinel ZnFe2O4photocatalyst: effect of synthesis methods

This paper investigates solar radiation-induced photocatalytic hydrogen generation using spinel ZnFe2O4 (ZFO) photocatalysts fabricated using different routes, viz., solid state reaction (SSR), polymer complex (PC), microwave sintering (μW) and self-propagating combustion (SPC) method. The physicochemical properties of the photocatalysts like crystallinity, surface area, band gap and band energetics is studied as it influences their photochemical behavior. The study reveals a high crystallinity of the ZFO photocatalysts, those are synthesized using SSR, PC and μW methods, where SSR method yields the larger dimension crystallites of ~53 nm. The nanoparticles obtained from SPC methodology exhibit a relatively large surface area and a smaller crystallite size of around ~18 nm. Monodispersed particles with comparatively large surface area are obtained in the case of PC method. ZFO obtained from μW synthesis exhibits enhanced optical properties, thus favoring high absorption of solar photons. A relatively more negative flat band potential is displayed by the μW samples (−0.543 vs normal hydrogen electrode) as estimated from the electrochemical measurements. Consequently, these samples yield a higher quantum yield (0.19%) for hydrogen evolution even without co-catalyst loading. On the contrary, the photocatalysts obtained by SSR and PC methods did display an enhancement in the quantum yield as compared to the μW samples but only after Pt co-catalyst loading. Copyright © 2015 John Wiley & Sons, Ltd.

Effect of heat treatment on the crystal phase composition, semiconducting properties and photoelectrocatalytic color removal efficiency of TiO2 nanotubes arrays

TiO2 nanotube films were grown by potentiostatic anodization at 30V during 2hours in 0.05M NH4F in ethylene glycol (1% water) electrolyte. TiO2 anodic films were heat treated at different temperatures during 0.5h, and during different time length at the same temperature (600°C). Crystal structure of the film was characterized by XRD measurements showing the appearance of anatase phase at 325°C. The anatase formed in these films was preferably oriented towards [001] direction, which seems to be the origin of its thermal stability even at elevated temperatures, such as 800°C. On the other hand, rutile was detected at temperatures higher than or equal to 600°C, simultaneously with the thickening of the barrier layer. Glancing angle X-ray diffraction (GAXRD) measurements showed that rutile is mainly formed in the inner part of the film, meanwhile anatase phase remains in nanotube crystals (outer part), even at 700°C. Indigo carmine decoloration was improved by increasing the heat treatment temperature which provided a better electron transport through the film; however, the appearance of rutile in the film drastically decreased the photoelectrochemical performance. Mott-Schottky measurements showed that rutile containing films have a more negative flat band potential than those only composed of anatase, which allowed us to propose that rutile film formed in the base of the tubes by thermal oxidation acts as a barrier for the transport of photogenerated electrons towards titanium substrate increasing the recombination in the film.

Synergistic effect of CdSe quantum dots on photoelectrochemical response of electrodeposited α-Fe2O3 films

The present work investigates photoelectrochemical response of CdSe quantum dots (QDs) sensitization on electrodeposited hematite (α-Fe2O3) thin films. QDs sensitization significantly enhances the photocurrent density and incident-photon-to current-conversion efficiency of α-Fe2O3. Highest photocurrent density of 1.14 mA cm−2 at 0.8 V/SCE electrode potential is observed for 48 h CdSe QDs sensitized α-Fe2O3 thin film, which possesses maximum CdSe QDs concentration. Enhancement in photocurrent has been mainly attributed to the proper band edge alignment of CdSe QDs with α-Fe2O3, leading to better charge transfer kinetics. Further, improved photoelectrochemical performance is also convinced by higher absorption, more negative flat band potential and decreased value of resistance of the sensitized sample. XRD, EDX, FESEM, RAMAN, UV visible spectroscopy and Mott Schottky studies are also carried out to analyze PEC data.

Photocurrent response and semiconductor characteristics of Ce-Ce2O3-CeO2-modified TiO2 nanotube arrays.

We reported Ce and its oxide-modified TiO2 nanotube arrays (TNTs) and their semiconductor properties. The TNTs were prepared by anodic oxidation on pure Ti and investigated by electrochemical photocurrent response analysis. Then, the TNT electrodes were deposited of Ce by cathodic reduction of Ce(NO3)3 6H2O. After deposition, the TNT electrodes were fabricated by anodic oxidation at E = 1.0 V(SCE) for various electricity as Ce-Ce2O3-CeO2 modification. The Ce-deposited TNTs (band gap energy Eg = 2.92 eV) exhibited enhanced photocurrent responses under visible light region and indicated more negative flat band potential (Efb) compared with the TNTs without deposition. After anodic oxidation, the mixed Ce and its oxide (Ce2O3-CeO2)-modified TNT photoelectrodes exhibited higher photocurrent responses under both visible and UV light regions than the TNTs without deposition. The photocurrent responses and Efb were found to be strongly dependent on the contents of Ce2O3 and CeO2 deposited on TNTs. A new characteristic of Eg = 2.1 ± 0.1 eV was investigated in the Ce2O3- and CeO2-modified photoelectrodes. X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were also employed to characterize various modified TNTs photoelectrodes.

Enhanced photoelectrochemical activity of vertically aligned ZnO-coated TiO2 nanotubes

Vertically aligned ZnO-TiO2 hetero-nanostructures constructed of anatase TiO2 nanotubes (NTs) and wurtzite ZnO coatings are fabricated by atomic layer deposition of ZnO coatings on electrochemical anodization formed TiO2 NTs, and their photoelectrochemical activities are studied through photoelectrochemical and electrochemical characterization. Compared with bare TiO2 NTs, the transient photocurrent increases to over 1.5-fold for the annealed ZnO-coated TiO2 NTs under visible illumination. The ZnO-coated TiO2 NTs also show a longer electron lifetime, a lower charge-transfer resistance and a more negative flat-band potential than the bare TiO2 NTs, confirming the improved photoelectrochemical activity due to the enhanced charge separation.

Photoelectrochemical activity on Ga-polar and N-polar GaN surfaces for energy conversion.

Hydrogen generation through direct photoelectrolysis of water was studied using photoelectrochemical cells made of different facets of free-standing polar GaN system. To build the fundamental understanding at the differences of surface photochemistry afforded by the GaN {0001}and {000-1}polar surfaces, we correlated the relationship between the surface structure and photoelectrochemical performance on the different polar facets. The photoelectrochemical measurements clearly revealed that the Ga-polar surface had a more negative onset potential relative to the N-polar surface due to the much negative flat-band potential. At more positive applied voltages, however, the N-polar surface yielded much higher photocurrent with conversion efficiency of 0.61% compared to that of 0.55% by using the Ga-polar surface. The reason could be attributed to the variation in the band structure of the different polar facets via Mott-Schottky analyses. Based on this work, understanding the facet effect on photoelectrochemical activity can provide a blueprint for the design of materials in solar hydrogen applications.

ZnWO4/WO3 Composite for Improving Photoelectrochemical Water Oxidation

A rapid screening technique utilizing a modified scanning electrochemical microscope has been used to screen photocatalysts and determine how metal doping affects its photoelectrochemical (PEC) properties. We now extend this rapid screening to the examination of photocatalyst (semiconductor/semiconductor) composites: by examining a variety of ZnWO4/WO3 composites, a 9% Zn/W ratio produced an increased photocurrent over pristine WO3 with both UV and visible irradiation on a spot array electrode. With bulk films of various thickness formed by a drop-casting technique of mixed precursors and a one-step annealing process, the 9 atomic % ZnWO4/WO3 resulted in a 2.5-fold increase in the photocurrent compared to pristine WO3 for both sulfite and water oxidation at +0.7 V vs Ag/AgCl. Thickness optimization of the bulk-film electrodes showed that the optimum oxide thickness was ∼1 μm for both the WO3 and ZnWO4/WO3 electrodes. X-ray diffraction showed the composite nature of the WO3 and ZnWO4 mixtures. The UV/vis absorbance and PEC action spectra demonstrated that WO3 has a smaller band gap than ZnWO4, while Mott–Schottky analysis determined that ZnWO4 has a more negative flat-band potential than WO3. A composite band diagram was created, showing the possibility of greater electron/hole separation in the composite material. Investigations on layered structures showed that the higher photocurrent was only observed when the ZnWO4/WO3 composite was formed in a single annealing step.

Role of electrolyte chemistry on electronic and in vitro electrochemical properties of micro-arc oxidized titania films on Cp Ti

The primary objective of the present work was to study the electronic and in vitro electrochemical properties of micro-arc oxidized titania films on Cp Ti, fabricated independently in various electrolyte solutions consisting of anions such as phosphate (PO43−), borate (B4O72−), citrate (C6H5O73−) and silicate (SiO32−). Further the role of anions on the structural, morphological and compositional properties of the fabricated films was studied. All the titania films were developed by micro-arc oxidation (MAO) technique for a fixed treatment time of 8min under constant current mode. The surface morphology, elemental distribution, composition and structural characteristics of the films were assessed by scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques. The thermodynamic and kinetic corrosion properties of the films were studied under simulated body fluid (SBF) conditions (pH 7.4 and 37°C) by conducting chronopotentiometric and potentiodynamic polarization tests. Electrochemical impedance spectroscopy (EIS) coupled with equivalent circuit modelling was carried out to analyse the frequency response and Mott–Schottky analysis was performed to study the semiconducting (electronic) properties of the films. Salt spray fog accelerated corrosion test was conducted for 168h as per ASTM B117 standard to corroborate the corrosion and semiconducting properties of the samples based on the visual examination. The XRD results showed that the transformation from the metastable anatase phase to the thermodynamically stable rutile phase and the crystalline growth of the respective phases were strongly influenced by the addition of anions. The SEM-EDS results demonstrated that the phosphorous (P) content in the films varied from 2.4at% to 5.0at% indicating that the amount of P in the films could be modified by adding an appropriate electrolyte additive. The electrochemical corrosion test results showed that the film fabricated in citrate (C6H5O73−) containing electrolyte is thermodynamically and kinetically more stable compared to that of all the others. The results of the Mott–Schottky analysis indicated that all the fabricated films showed an n-type semiconducting behaviour and the film developed in citrate (C6H5O73−) containing electrolyte exhibited the lowest donor concentration and the most negative flat band potential that contributed to its highest corrosion resistance in SBF solution. The results of the salt spray accelerated corrosion tests were in agreement with those obtained from the electrochemical and Mott–Schottky analysis.

Direct correlation of electrochemical behaviors with anti-thrombogenicity of semiconducting titanium oxide films

Biomaterials-associated thrombosis is dependent critically upon electrochemical response of fibrinogen on material surface. The relationship between the response and anti-thrombogenicity of biomaterials is not well-established. Titanium oxide appears to have good anti-thrombogenicity and little is known about its underlying essential chemistry. We correlate their anti-thrombogenicity directly to electrochemical behaviors in fibrinogen containing buffer solution. High degree of inherent n-type doping was noted to contribute the impedance preventing charge transfer from fibrinogen into film (namely its activation) and consequently reduced degree of anti-thrombogenicity. The impedance was the result of high donor carrier density as well as negative flat band potential.

Effect of Quantum Dot Deposition on the Interfacial Flatband Potential, Depletion Layer in TiO2 Nanotube Electrodes, and Resulting H2 Generation Rates

CdS QD-modified TiO2 nanotube arrays were fabricated using an anodic oxidation-sequential chemical bath deposition process. By means of FESEM, EDS, XRD and XPS, it could be confirmed that the use of an ultrasonication-assisted deposition approach improved the distribution of CdS QDs on the tube walls. The as-prepared CdS-modified TiO2 nanotube arrays exhibited enhanced photoelectrochemical properties and hydrogen production activity, which benefitted from the extended light absorption and the improved interfacial charge-transfer properties of TiO2 nanotube arrays. By analyzing the interfacial properties, the flatband potential, the depletion layer, the capacitance, and the impedance of the CdS/TiO2 photoelectrode, it can be concluded that compared with pure TiO2 nanotube arrays the CdS QD-modified arrays exhibited a more negative flat band potential and a lower energy barrier for interfacial electron transfer. The calculated depletion layer width (dSC) of the sensitized TiO2 nanotube arrays was larger, wh...

(I2)n-encapsulated and C-encapsulated TiO2 films: Enhanced photoelectrochemical and visible-light photoelectrocatalytic properties

Visible-light absorbing (I2)n-encapsulated TiO2 film (I2-TiO2 film) and C-encapsulated TiO2 film (C-TiO2 film) were fabricated and studied as film electrodes for their photoelectrochemical and photoelectrocatalytic (PEC) properties. The as-prepared film electrodes were characterized by scanning electronic microscopy, energy-dispersive X-ray spectroscopy and UV–visible diffuse reflectance spectroscopy. The photoelectrochemical properties were evaluated by the incident photo-to-current conversion efficiency, electrochemical impedance spectroscopy and Mott–Schottky analysis data. The results showed that I2-TiO2 and C-TiO2 films exhibited stronger absorption in the 400–550nm range, lower electron transfer resistance and more negative flat band potentials comparing with pure TiO2 film. Moreover, the photoelectrochemical ability of I2-TiO2 film is superior to that of C-TiO2 film. At a bias potential of 0.5 vs (Ag/AgCl)/V, the visible-light-induced PEC degradation ratios of rhodamine B and tetracycline on I2-TiO2 film or C-TiO2 film electrodes exceeded that on pure TiO2 electrode, with the improvement by a factor of about 5 or 3. The higher PEC activity of I2-TiO2 film and C-TiO2 film could be attributed to the enhancement of separation of electron-hole pair at the external electric field and the extension of the light response range of TiO2 to the visible light with a red shift in the band gap transition.

Dye-sensitized solar cells composed of Ti1−xSnxO2 nanocrystals

Ti1−xSnxO2 nanocrystals were successfully synthesized by using a simple solvothermal route, and its band energy gap broaden and flat band potential can be rationally regulated with increasing x value. Furthermore, Ti1−xSnxO2 nanocrystals were first used as the photoelectrode material for dye-sensitized solar cells. A cell made of Ti1−xSnxO2 (x = 0.3) exhibited the best photovoltaic performance. This is due to its most narrow band gap energies, most negative flat band potential and lowest dark current densities. After the surface of Ti1−xSnxO2 (x = 0.3) electrode was treated with TiCl4 solution, the cell sensitized by a mixed solution of N719 and D131 dye exhibited the best efficiency of 4.64% under the illumination of 1 sun (AM1.5, 100 mW cm−2).

Anodically grown Si–W codoped TiO2 nanotubes and its enhanced visible light photoelectrochemical response

Si–W codoped TiO2 nanotubes were synthesized using a one-step anodization process with the presence of silicotungstic acid in the electrolyte. X-ray photoelectron spectroscopy results indicated that the content of Si and W in the TiO2 nanotubes was proportional to the concentration of the silicotungstic acid in the solution. More absorption in the visible light region was observed for the resulted samples from the UV–vis diffuse reflectance spectra. An enhanced photoelectrochemical visible response was demonstrated for the codoped samples and the photocurrent was 2.5 times larger than that of pristine TiO2 nanotubes under visible light irradiation. More negative flat band potential and a higher carrier density were associated with the introduction of dopants Si and W. This work demonstrated a feasible and simple method to fabricate doped TiO2 with controlled doping content based on the corresponding additives in the anodization electrolyte.

Research on PEG modified Bi-doping lead dioxide electrode and mechanism

Bi-doping PbO2 electrode, which is called Bi–PbO2 for short, modified with different concentrations of polyethylene glycol (PEG) was prepared by electrodeposition method in this paper. The microstructure and electrochemical properties of the different modified electrodes were investigated using scanning electron microscopy, X-ray diffraction, Mott–Schottky, electrochemical impedance spectroscopy and linear sweep voltammetry techniques. The results show that adulteration of PEG has a noticeable improvement in the morphology of Bi–PbO2 electrode which can greatly decrease its particle size and enlarge its active surface area. Phenol degradation experiments reveal that the modified electrodes have excellent electro-catalytic activity and stability, and the optimal adulterate concentration of PEG is 8gL−1. Electrochemical performance tests show that the modified electrodes exhibit more negative flatband potential (Efb), larger adsorption pseudo capacitance, lower adsorption resistance and higher oxygen evolution potential, and these characteristics promote the electro-catalytic activity of the Bi–PbO2 electrode. Finally, accelerated lifetime tests demonstrate that PEG modification can highly lengthen the service life of Bi–PbO2 electrode in its practical application.

Thermal formation of silicon-doped TiO2 thin films with enhanced visible light photoelectrochemical response

Silicon-doped TiO2 thin films were fabricated by annealing titanium metal sheet embedded in SiO2 powders and characterized by X-ray photoemission spectroscopy and photoelectrochemical measurements. The results showed that the content of silicon in the doped TiO2 thin films was proportional to the annealing time and temperature. Enhanced visible light response, more negative flat band potential and higher carrier density were demonstrated by the electrochemical measurement. The technique proposed in this paper can be also applicable to fabricate other doped TiO2 thin films based on the corresponding oxide bath.

Polarity-dependent photoelectrochemical activity in ZnO nanostructures for solar water splitting

ZnO nanotubes and nanosheets with different morphologies, tailored by a simple chemical route without templates and capping agents, were successfully synthesized as a photoelectrode. From their XRD patterns, the relative intensities of (100) and (002) peaks from the nanorods and nanosheets vary with degree of polarity in the ZnO nanostructures. Photoelectrochemical measurements clearly revealed that the polar facets of nanosheets yielded conversion efficiency three times that of ZnO nanotubes with nonpolar-facets because of the high surface energy, spontaneous polarization and negative flat-band potential of a polar-orientated surface.

A photoelectrochemical investigation of the hydrogen-evolving doped TiO 2 nanotube arrays electrode

The present work investigates the photoelectrochemical behavior of nanotubular N/C-TiO 2 electrode for hydrogen production. Via the sonoelectrochemical anodization process of 1 h, N-containing TiO 2 based nanotube arrays(N-TNT) with the length of about 650 nm were fabricated in fluoride aqueous solution added 0.25 M NH 4NO 3; C-containing TiO 2 based nanotube arrays(C-TNT) with the length of about 2 μm were prepared in fluoride ethylene glycol solution. In virtue of the longer tubes with the larger surface areas, C-TNT can harvest more light and produce more photoactive sites than N-TNT, which also made the charge transfer resistance in C-TNT larger than that in N-TNT. Considered the more negative flat band potential of C-TNT, C-TNT has the smaller energy barrier and the better photoelectrochemical activity. It may be attributed to the appropriate defect concentration gradient owing to the modification of C element. Under UV–vis light (320–780 nm) irradiation, the average hydrogen generation rate of C-TNT was 282 μL h −1 cm −2. The surface properties and near-surface properties of the resultant electrode were synthetically analyzed by using UV–vis diffuse reflectance spectra(DRS), field emission scanning electron microscopy (FESEM), I-t curves, and electrochemical impedance spectroscopy (EIS) techniques.

Layered γ-zirconium phosphate as novel semiconductor for dye sensitized solar cells: Improvement of photovoltaic efficiency by intercalation of a ruthenium complex-viologen dyad

The performance of photovoltaic cells using as semiconductor a film of layered γ-zirconium phosphate (γ-ZrP) containing Ru(bpy)3 and bipyridinium ions (viologens) as electron relays has been studied. The materials are easily prepared by intercalation of Ru complexes and the bipyridinium ions into preformed γ-ZrP nano sheets as colloidal solutions in the appropriate solvent and concentration. High loading of these two guests has been obtained as determined by elemental analysis. Inclusion of Ru(bpy)3 complex and bipyridinium in the intergallery spaces of γ-ZrP can be assessed by powder XRD monitoring of the d100 peak. A dyad was also synthesized where the Ru(bpy)3 and the 4,4′-bipyridinium were covalently connected by a four-methylene tether. The semiconducting behavior of layered γ-ZrP was supported by cyclic voltammetry (reversible reduction peak at −0.6 V), observation of photocurrent and Mott-Schottky measurements (flat band potential ∼−1.3 V vs.NHE) of thin films of this material supported on FTO electrode. Photovoltaic cells based on γ-ZrP containing Ru(bpy)3, exhibited similar VOC (∼0.5 V) and fill-factor values (0.3–0.4), differing in the current density and therefore in their efficiency. The maximum efficiency was obtained for the material containing high loading of the dyad (JSC = 0.383 mA/cm2, efficiency 0.1%). The photo response spectrum shows that the main limitation of these materials is still the inefficient photo sensitization of the semiconductor by the dye, probably due to the high negative flat band potential of γ-ZrP.