Shift In Flat-band Potential Research Articles

Significant Efficiency Improvement of the Black Dye-Sensitized Solar Cell through Protonation of TiO2 Films

This paper describes the influence of acid pretreatment ofTiO2 mesoporous films prior to dye sensitization on the performance of dye-sensitized solar cells based on [(C4H9)4N]3[Ru(Htcterpy)(NCS)3] (tcterpy = 4,4',4"-tricarboxy- 2,2',2"-terpyridine), the so-called black dye. The HCl pretreatment caused an increase in overall efficiency by 8%, with a major contribution from photocurrent improvement. It is speculated, from the analysis of incident photon-to-electron conversion efficiency, UV-vis absorption spectra, redox properties of the dye and TiO2, and the impedance spectra of the dye-sensitized solar cells, that photocurrent enhancement is attributed to the increases in electron injection and/or charge collection efficiency besides the improvement of light harvesting efficiency upon HCl pretreatment. Open-circuit photovoltage (V(oc)) remained almost unchanged in the case of significant positive shift of flat band potential for TiO2 upon HCl pretreatment. The suppression of electron transfer from conduction band electrons to the I3- ions in the electrolyte upon HCl pretreatment, reflected by the increased resistance at the TiO2/dye/electrolyte interface and reduced dark current, resulted in a V(oc) gain, which compensated the V(oc) loss due to the positive shift of the flat band. Using the HCl pretreatment approach, 10.5% of overall efficiency with the black dye was obtained under illumination of simulated AM 1.5 solar light (100 mW cm(-2)) using an antireflection film on the cell surface.

Impedance-based detection of DNA sequences using a silicon transducer with PNA as the probe layer.

Electrochemical impedance measurements were used for the detection of single-strand DNA sequences using a peptide nucleic acid (PNA) probe layer immobilized onto Si/SiO2 chips. An epoxysilane layer is first immobilized onto the Si/SiO2 surface. The immobilization procedure consists of an epoxide/amine coupling reaction between the amino group of the PNA linker and the epoxide group of the silane. A 20-nucleotide sequence of PNA was used. Impedance measurements allow for the detection of the changes in charge distribution at the oxide/solution interface following modifications to the oxide surface. Due to these modifications, there are significant shifts in the semiconductor's flat-band potential after immobilization and hybridization. The results obtained using this direct and rapid approach are supported by fluorescence measurements according to classical methods for the detection of nucleic acid sequences.

Enhancement of Photocurrent and Photovoltage of Dye-Sensitized Solar Cells with TiO2 Film Deposited on Indium Zinc Oxide Substrate

The effects of using indium zinc oxide (IZO) as a conducting substrate in dye-sensitized TiO2 solar cells (DSC) on their photocurrent−voltage characteristics are studied. We have found that both short-circuit photocurrent (Jsc) and open-circuit voltage (Voc) of IZO-based cells are substantially improved by about 40% and 10%, respectively, when compared with those of cells based on fluoride-doped tin oxide (FTO). The Jsc increase is correlated to the improved contact between IZO and TiO2 and enriched conduction band electrons in the TiO2-on-IZO electrode, due to the blocking of surface states by the evaporated metallic components of the substrate. The combined effects of the negative shift of flat band potential and the reduction in recombination explain the Voc increase. Our experimental finding suggests that IZO-coated substrate makes more efficient DSC than a FTO-coated one.

Kinetics of the surface reaction H ad ↔ OH ad on n-Ge(1 1 1)

Different surface states (GeOH and GeH) were found on n-Germanium. These two surface states can be controlled by potential (GeH at U<−0.2 V, GeOH at U>0.1 V in 0.5 M H 2SO 4). The surface reaction H ad↔OH ad is reversible and can be described by: (1) Ge H+ p ++ H 2 O↔ Ge OH+2 H ++ e − Measurements were carried out on n-Ge(1 1 1). Starting from GeH, anodic potentiostatic pulse experiments yield the kinetics of the surface reaction (1). Fast surface states, e.g. radicals were formed at relatively short times ( t<10 ms) due to the breaking of GeH-bonds. Then, OH-groups were generated and a GeOH monolayer was formed at polarization potentials U P>0.1 V and longer times ( t>10 ms). The shift of flat band potentials yields a contribution Δ φ Dip of OH dipoles to the total potential drop Δ φ at the interface. Reaction (1) was also studied by cathodic pulse experiments starting from GeOH. As a result, the kinetics of the surface reaction were obtained in dependence on U and θ. Reactions B and E are diffusion controlled, while surface reactions C and F are transfer controlled at low current densities. For θ H= θ OH=0.5, i.e. Δφ Dip=const., Tafel lines with b(C)=83 and b(F)=87 mV are obtained. The correspondent transfer coefficients α(C)=0.7 and α(F)=0.3 indicate that they refer to different elementary reactions. Hence, an apparent equilibrium potential U 0=0.01 V, and apparent exchange current density i 0=0.1 μA cm −2 can be estimated only from Tafel-plots.

Grafting of anion exchanging groups on SiO 2/Si structures for anion detection in waters

Here, we report results of anion exchanger film, based on quaternary ammonium, obtained by grafting EPTMAC (glycidyltrimethyl-ammonium chloride) on silica transducer using AEAPTS (3-(2-aminoethyl-amino)propyltrimethoxysilane) coupling agent. The grafting product was characterized by 1 H , 13 C , 29 Si solid state NMR spectroscopy. Anion exchange on grafted SiO 2/Si structures was monitored by impedance measurements: studying the flat band potential shift and the variation of capacitance in accumulation mode versus concentration of anionic species in solution. The grafted membrane exhibits a good affinity for different anions like CO 3 2−, F −, I −, dye-SO 3 − with a detection limit of 10 −7 M. Applying site-binding model, the affinity constant of the anion species towards quaternary ammonium group is found to be 10 7 M −1. The size of the anion limits the sensitivity of the grafted SiO 2/Si structures.

Photoelectrochemical properties of platinum(IV) chloride surface modified TiO2.

Anatase (TH), rutile (TiO2-R), and a mixture of anatase and rutile (P25) were surface modified by chemisorbed chloroplatinate(IV) complexes. All materials gave rise to anodic photocurrents when deposited on conducting glass and irradiated in the wavelength range of 330-650 nm. In the presence of formate "current-doubling" factors of 5-7 were measured. Flatband potentials were obtained by the suspension method through recording the photovoltage as function of the pH value. The value of -0.54 V as found for TH is shifted to -0.49, -0.45, and -0.28 V (vs. NHE, pH = 7) when the surface is covered by 1, 2, and 4 wt% of H2[PtCl6], respectively. The flatband potential shifts by 50 and 60 mV per pH unit for P25 and 4% H2[PtCl6]/TH, respectively, as found by a novel method based on the use of different pH-independent redox systems of the bipyridinium type. Whereas the rutile based material was inactive, the TH and P25 samples photocatalyzed the mineralization of 4-CP with visible light. Moreover, the capability of H2[PtCl6]/TH to mineralize also cyanuric acid, the end-product of atrazine decomposition in photocatalytic processes with unmodified TiO2, was observed upon UV and also visible light irradiation. From these experimental results an energy diagram is proposed to rationalize the reactions observed.

Improved Photocurrent-Voltage Characteristics of Ru(II)-Dye Sensitized Solar Cells with Polypyrrole-Anchored TiO2 Films

The effect of electrodepositing polypyrrole (PPy) onto dye-coated surface on the photocurrent-voltage characteristics of dye-sensitized solar cells was studied. It is observed that both open-circuit voltage and short-circuit photocurrent of PPy-anchored solar cells increase, compared to those of a cell. Furthermore, the stability of the characteristics is enhanced with the modification. The combined effects of reduced back-electron-transfer to ions and negative shift of flatband potential explain the increase. Analyses of electrochromic measurements suggest that the increase is related to the enrichment of conduction band electrons in the electrode. © 2002 The Electrochemical Society. All rights reserved.

Electrical characterization of a new polymeric ion-exchanging membrane for the chemical detection of anions

PolyTHF terminated by a quaternary ammonium triflate group was used as an anion exchanging material. The polymer was deposited using a dip coating method on the surface of a silica/silicon heterostructure whose impedance characterization could be performed. Capacitance measurements of the silicon_insulator (silica and polymeric layer)_electrolyte heterostructure allowed the studying of the anion-exchange phenomena at the cationic groups in the functional polyTHF membrane. Both increases of the polymer layer thickness and dielectric permittivity were observed by impedance measurements due to swelling of the polymeric film when the electrode was immersed in a sodium triflate solution. In the first part, exchange phenomena of the triflate anion against anions—acetate, nitrate, methane sulfonate, Acid Blue 25 dye, and sulfate—in a sodium triflate background electrolyte solution was investigated. Specific ion exchange was observed according to the anion type. In the second part, ion-exchange phenomena have been studied in a sodium acetate background electrolyte solution after initial swelling and partial ion exchange of the triflate against acetate anion. The capacitance measurements (flat-band potential shift) were allowed to distinguish between triflate-anion and acetate-anion exchange phenomena.

Effect of methanol concentration on flatband potential shift of n-type Si in CH 3CN/CH 3OH mixture containing a redox couple

A photoelectrochemical study is performed on a thermally oxidized n-Si electrode in acetonitrile in the presence of a redox couple dimethylferrocene (DMFc). An oxide layer of 3 nm thickness has been formed at 400°C. The results show that upon ageing the oxidized electrode in the electrolyte, its flatband potential is constant and only a decay of the current with time is observed: the oxide layer becomes progressively thicker. It is found that an SiO 2 layer of about 10 nm leads to a quasi-complete “blocked” electronic exchange between the surface and the solution. Under illumination, a decrease of 0.15 V of the open-circuit voltage ( V oc) is observed. On the other hand, the experimental study of a non-oxidized silicon electrode in a mixture of 90% CH 3CN/ 10% CH 3OH leads to a negative displacement of the flatband potential (−0.3 V) and an increase of the photopotential (0.12 V). A prolonged stability of the electrode with time under illumination is observed. This effect is attributed to the methoxylation of the silicon surface.

Semiconductor−Metal Composite Nanostructures. To What Extent Do Metal Nanoparticles Improve the Photocatalytic Activity of TiO2 Films?

Noble metal particles of Au, Pt, and Ir were deposited on nanostructured TiO2 film using an electrophoretic approach. The nanocomposite films were characterized by UV-absorption and atomic force microscopy (AFM). The deposition of tetraoctylammonium bromide (TOAB)-capped metal nanoparticles on TiO2 films improved the photocurrent generation and induced a shift in the apparent flat band potential. The TiO2 films modified with TOAB-capped metal nanoparticles were less prone to the electron scavenging by the oxygen in solution. Improved photoelectrochemical performance of semiconductor−metal composite film is attributed to the shift in quasi-Fermi level of the composite to more negative potentials. Continuous irradiation of the composite films over a long period causes photocurrent to decrease as the semiconductor−metal interface undergoes chemical changes. The role of semiconductor−metal composite films in improving the rate of photocatalytic degradation of an azo dye is also discussed.

Photocurrent Enhancement of Hemicyanine Dyes Containing RSO3- Group through Treating TiO2 Films with Hydrochloric Acid

Two kinds of hemicyanine dyes, 2-[4-(dimethylamino)styryl]benzothiazolium propylsulfonate (BTS) and 2-[4-(dimethylamino)styryl]-3,3-dimethylindolium propylsulfonate (IDS), were synthesized and characterized with respect to their UV−vis, fluorescence, and redox properties. Both dyes show outstanding properties for charge transfer on HCl-treated nanocrystalline TiO2 films with near 100% incident monochromatic photon-to-electron conversion efficiency (IPCE) at the maximum absorption wavelength. This treatment was found to improve short-circuit photocurrent significantly, with increases by 99% and 329% for BTS and IDS, respectively. Overall yields were hence improved remarkably as a result of photocurrent enhancement: BTS increased by 66% from 3.1% to 5.1% and IDS increased by 260% from 1.3% to 4.8%. This phenomenon can be explained by the positive shift of flat band potential for TiO2 and the increased amount of adsorbed dye molecules upon HCl treatment. The values for IPCE, short-circuit photocurrent, and overall solar energy to electricity conversion efficiency are all among the highest values respectively in the pure organic sensitizers reported so far.

Cation effects on the anodic dissolution of n-Si in fluoride electrolytes

Abstract Under strong illumination and in the anodic range, current–potential characteristics of n-Si ∣ fluoride-electrolyte interfaces exhibit the same shape as those of p-Si. The increase of the photocurrent observed in the presence of alkali-metal cations of increasing size is interpreted as for p-Si: in the electropolishing regime, cations act as catalysts for the dissolution of the silicon-oxide layer covering the electrode. In the dark, a negative flatband-potential shift and a decrease in the anodic current are observed in the presence of alkali-metal cations of increasing size. This behaviour, which takes place on hydrogenated surfaces, is accounted for in a kinetic model in which electrochemical transfer takes place through negatively-charged surface states. Cation adsorption favours the presence of a negative charge at the surface, hence the negative charge is increased, thereby decreasing the flatband potential and the charge-transfer rate.

Developments in photoelectrochemistry: light-addressable photoelectrochemical cyanide sensors

The photopotential of several semiconductors when immersed in solution and illuminated respond to the concentration of specific ions in solution. Trends observed for alkaline ferrocyanide electrolytes in contact with illuminated CdSe and CdTe comprise a substantial negative shift of flat-band cyanide potential V fb and an increase in open circuit potential V oc with increasing concentration. Based on this increase in V oc, a one-dimensional light-addressable photoelectrochemical (LAP) sensing configuration capable of probing five distinct 2 mm 2 regions along a 1 mm × 10 mm surface area of a single n-CdSe detector is demonstrated for cyanide analysis. This permits a single sensing LAP element which can be externally illuminated in different regions to provide both spatial and temporal information on the analyte concentration. Spatial sensing is achieved without sensor movement and with fewer electrical contacts as compared with a conventional array scheme. In solution, the photopotential has a linear response to cyanide of 120 mV × log[KCN].

Direct Detection of the Hybridization of Synthetic Homo-Oligomer DNA Sequences by Field Effect

Homo-oligomer DNA strands were immobilized onto silicon/silicon dioxide electrodes using 3-aminopropyltriethoxysilane. These modified substrates were used as working electrodes in a three-electrode electrochemical cell. In-phase and out-of-phase impedances were measured in the range −1 to +1 V with respect to an Ag/AgCl reference electrode, with a superimposed 10 mV ac signal at frequencies of 20 and 100 kHz. Ex situ hybridization with complementary oligomer strands, performed at the surface of modified electrodes, is clearly reflected by negative shifts of about 100 mV in the flat-band potential of the semiconductor. Consecutive hybridization−denaturation steps show that the shifts are reproducible and the process is reversible. The in situ hybridization of complementary strands has also been observed with impedance measurements at Si/SiO2 substrates and with the use of a field effect device. The direct detection of hybridization with a field effect device was performed under constant drain current mode, and the corresponding variations observed for the gate potential during hybridization are in good agreement with the flat band potential shifts observed with the impedance experiments. Measurements made in the presence of noncomplementary strands demonstrate the selectivity of the device.

Flatband Potential Studies at the n‐Si/Electrolyte Interface by Electroreflectance and C‐V Measurements

The properties of the n‐Si electrode in both aqueous and organic solution for nonstabilizing conditions were studied by the electroreflectance and capacitance techniques. A shift of the flatband potential in the anodic direction up to 0 VSCEhas been observed for both solutions and was more pronounced for the aqueous one. The strong influence of polarization under anodic potentials on the properties of the Si/electrolyte junction is shown. It is demonstrated that the flatband potential shifts for solutions without a stabilizing agent is caused by the slow oxidation process and related interface state evolution. Anomalously high electroreflectance signals have been observed for the potentials where a capacitance peak associated with the interface states is detected. We suppose the interaction of hydrogen‐related species in a subsurface Si layer with interface states to be responsible for this specific electroreflectance modulation.

Solar Energy Conversion at the p ‐ InP / Vanadium3 + / 2 + Semiconductor/Electrolyte Contact: A Study Based on Differential Capacitance and Current‐Voltage Data

Previously unachieved values are obtained for the open‐circuit voltage (0.7 to 0.8 V) and the fill‐factor (76%) at the illuminated semiconductor/electrolyte contact (‘100’ face) coated with a submonolayer amount of silver. Photoelectrochemical solar cell efficiency is 11%, despite significant optical reflection and electrolyte absorption losses. Differential capacitance measurements demonstrate a semiconductor/surface energy barrier height of up to , independent of the Ag treatment. This value is considerably larger than typically found at solid‐state contacts. Barrier heights derived from Mott‐Schottky data are compared to photovoltages; results indicate that interface recombination and a slow charge‐transfer (bare ) and thermionic emission from the metal (Ag‐treated ) are important factors limiting the cell efficiency. Flatband potential shifts owing to a slow charge‐transfer under photocurrent flow are reduced by the Ag treatment. In a finite potential range of about 400 mV (0 to −400 mV/SCE), both photovoltages and Mott‐Schottky data reveal an unpinned Fermi level at bare and Ag‐treated electrodes. The unpinned barrier follows the ideal Schottky barrier model for an absolute SCE potential of −5.0 eV/SCE. Reference to related work at and is made.

NInP flat band potential: A pH probe in nonaqueous and mixed solvents?

The interface structure of the semiconductor/electrolyte junctions in nonaqueous solvents is an open question, and as in water, the study of the influence of pH on the flat band potential can lead to an important part of its knowledge and to applications such as pH measurements. In water, the flat band potential shifts of many semiconducting materials can be connected with pH shifts. In this paper we show it is the case for nInP electrodes in acetonitrile and methanol (96% in weight in water) at room temperature, and in liquid ammonia at low temperature, −55°C. Therefore this material can be used as a probe of pH. Using this property we were able to find again, for example, the values of the self-ionization constants of the studied solvents and the acidity constants of water in liquid ammonia and of acetic acid in methanol.

Dynamic electrolyte electroreflectance measurements for the in situ detection of flatband potential shift

Electrolyte electroreflectance is a modulation spectroscopic tool of great utility for the thermodynamic characterization of the semiconductor-electrolyte junction. This technique not only allows rapid and reliable measurements of the semiconductor flatband potential under photoelectrochemical working conditions, but is also very useful to follow the potential distribution changes taking place at the semiconductor-electrolyte interface during electrochemical and photoelectrochemical reactions. We present here various examples of dark and photoinduced interfacial reactions with different semiconductor electrodes (TiO 2, CdS, WSe 2, FeS 2) studied by electrolyte electroreflectance.

Flat-band potential shift in a dye-sensitized zinc-oxide electrode on pulse excitation

The electrode potential dependence of the photopotential in several dye-sensitized ZnO electrodes revealed that the flat-band potential is positively shifted just after the photoinduced electron injection from the dye molecule. The shift is caused by surface charges resulting from the photoionization of dye molecules adsorbed on the electrode surface

Sol‐Gel‐Derived TiO2 Film Semiconductor Electrode for Photocleavage of Water: Preparation and Effects of Postheating Treatment on the Photoelectrochemical Behavior

The effects of postheating on the photoelectrochemical behavior of film electrodes prepared by the sol‐gel method have been investigated over a heating temperature range of 600–900°C. in the sol‐gel‐derived films were anatase on heating below 800°C and transformed into rutile. It was found that the photocurrent shows a maximum at a certain heating time for each of the heating temperatures at 900°C. At the same time, other properties such as the flatband potential shift and donor density also showed a maximum at the corresponding heating time. These results are explained as follows. An increase in the photocurrent of the as‐prepared film electrode with increasing heating time results from the improvement of crystallinity and the increasing Ti3+ concentration due to the reduction of Ti4+ by the undecomposed organic residue. Further heating, however, causes a decrease in the specific surface area and Ti3+ concentration due to the reoxidation of Ti3+ to Ti4+, resulting in the reduction of the photocurrent. (anatase) has only an indirect bandgap at 3.03 eV, while (rutile) has an indirect bandgap at 2.92 eV and a direct bandgap at 3.13 eV.