Mott-Schottky Data Research Articles

NrGO wrapped Cu-ZrO2 as a multifunctional visible-light-sensitive catalyst for advanced oxidation of pollutants and CO2 reduction.

Construction of tailor made photocatalyst is the need to achieve high visible light assimilation and charge carrier separation. In this path, combination of catalytic active element (Cu), defect rich metal oxide (ZrO2) and photoactive (NrGO) is an effective strategy. Herein, NrGO wrapped Cu-ZrO2 was prepared by modified Hummers-thermal annealing-hydrothermal approach. The synergism between Cu, Zr and NrGO was studied using transient photocurrent, EIS and TEM images. Consequently, O3/NrGO@Cu-ZrO2-10/Vis synergistic system has achieved superior photocatalytic ozonation-catalytic ozonation than individual photocatalysis and ozonation. Further, the advanced oxidation process was optimized and the catalyst was efficiently used for the reduction of CO2 into useful products. Tolerance of O3/NrGO@Cu-ZrO2-10/Vis system against rhodamine B, hydroquinone, bisphenol A and p-cresol has been investigated. A plausible mechanism was proposed based on ESR studies, BODIPY fluoresence method, band gap and Mott-Schottky data. This research provides an idea of multifunctional catalyst for advanced oxidation of contaminants and CO2 reduction.

Highly Efficient CsPbBr3 Planar Perovskite Solar Cells via Additive Engineering with NH4SCN.

Improving stability is a major aspect for commercial application of perovskite solar cells (PSCs). The all-inorganic CsPbBr3 perovskite material has been proven to have excellent stability. However, the CsPbBr3 film has a small range of light absorption and serious charge recombination at the interface or inside the device, so the power conversion efficiency is still lower than that of the organic-inorganic hybrid one. Here, we successfully fabricate high-quality CsPbBr3 films via additive engineering with NH4SCN. By incorporating NH4+ and pseudo-halide ion SCN- into the precursor solution, a smooth and dense CsPbBr3 film with good crystallinity and low trap state density can be obtained. At the same time, the results of a series of photoluminescence and electrochemical analyses including electrical impedance spectroscopy, space-charge limited current method, Mott-Schottky data, and so on reveal that the NH4SCN additive can greatly reduce the trap state density of the CsPbBr3 film and also effectively inhibit interface recombination and promote charge transport in the CsPbBr3 planar PSC. Finally, the CsPbBr3 planar PSC prepared with a molar ratio of 1.5% NH4SCN achieves a champion efficiency of 8.47%, higher than that of the pure one (7.12%).

Influence of growth time on photoelectrical characteristics and photocatalytic hydrogen production of decorated Fe2O3 on TiO2 nanorod in photoelectrochemical cell

We developed decorated Fe2O3 on the surface of one dimensional (1D) TiO2 nanorods (NRs) as a photocatalyst in a photoelectrochemical hydrogen production system using the two-step hydrothermal technique. The photocatalytic hydrogen production over the designed 1D TiO2@Fe2O3 NRs was optimized by varying the hydrothermal duration. Several characterization techniques were applied to study the physicochemical and photoelectrochemical properties of the prepared photocatalysts. It was observed that the adhesiveness and electrical connection of the TiO2@Fe2O3 and FTO substrate, crystal growth, and photoelectrochemical behavior strongly depend on the hydrothermal duration time. The photoanode with a growth time of 24 h produced the maximum amount of hydrogen (640 µmol cm−2) under visible light with external potential from dye-sensitized solar cells in 1 M KOH and 5 vol% glycerol solution in the photoelectrochemical cell. The EIS data showed that this photocatalyst had the lowest charge transfer resistance of 167.84 Ω and the longest electron lifetime of 347.97 ms. Also, its Mott-Schottky data revealed a more negative flat bad potential of −1.1 V with profound ability of proton (H+) reduction to H2 and high donor density of 3.55 × 1021 cm−3, which resulted in much more photocurrent density and photoconversion efficiency at the electrode/electrolyte interface compared to others.

Electrochemical Mechanism Insights from Mott-Schottky Measurements: Application to Europium in Molten Oxide

Electrochemical impedance measurements are commonly conducted at a constant direct current (DC) potential while sweeping the frequency of the perturbation. The mainstream plots used to discuss data are then Nyquist and Bode plots, on which fitting curves for the postulated equivalent circuit are represented. Unfortunately for high temperature electrochemists interested in faradaic reactions in molten electrolytes with a high concentration of electroactive species [1] or a very high conductivity [2], this method rarely provides meaningful results due to data that are often noisy and difficult to interpret at potentials away from the open-circuit potential (OCP). Methods employing a dynamic sweep of the DC potential at a fixed frequency of perturbation experimentally provide more reliable insight into electrochemical impedance phenomena in molten electrolytes. The archetypical example is alternating current (AC) voltammetry in its various forms, such as sine- or square-wave voltammetry [3][4]. However, most modern potentiostats offer only one means of conducting DC potential sweep impedance measurements, the so-called Mott-Schottky (MS) method. This method is not truly dynamic from a DC potential perspective, and is traditionally used for capacitance measurements and characterization of semi-conductor electrodes. The present work shows a procedure to extract faradaic data from MS data, inspired from the background subtraction method previously proposed for AC-polarography [5]. Nyquist-type representations are derived from the MS data, enabling a connection to mainstream impedance analysis methods. An example of this procedure, an investigation of the electrochemistry of europium ions in molten oxide, is provided. [1] A. Kisza, Polish J. Chem., 67 [1993] 885-894 [2] B. Savova-Stoynov and Z.B. Stoynov, Journal of Applied Electrochemistry, 17 [1987] 1150-1158 [3] A.M. Bond et al., Anal. Chem., 77 (9) [2005] 186-195 [4] C. Montel, C. Russel and E. Freude E, Glastech Ber, 61, [1988], 59–63 [5] A.M. Bond et al., J. Electroanal. Chem., 222 (1) [1987] 35-44.

Charge Transport at Ti-Doped Hematite (001)/Aqueous Interfaces

Solid-state transport and electrochemical properties of Ti-doped hematite (α-(TixFe1-x)2O3 (001) epitaxial thin films (x = 0.15, 0.21, and 0.42) were probed to achieve a better understanding of doped hematite for photoelectrochemical (PEC) applications. Room temperature resistivity measurements predict a resistivity minimum near x = 0.25 Ti doping, which can be rationalized as maximizing charge compensating Fe2+ concentration and Fe3+ electron accepting percolation pathways simultaneously. Temperature dependent resistivity data are consistent with small polaron hopping, revealing an activation energy that is Ti concentration dependent and commensurate with previously reported values (≈ 0.11 eV). In contact with inert electrolyte, linear Mott–Schottky data at various pH values indicate that there is predominantly a single donor for Ti-doped hematite at x = 0.15 and x = 0.42 Ti concentrations. Two slope Mott–Schottky data at pH extremes indicate the presence of a second donor or surface state in the x = 0.21 Ti-doped film, with an energy level ≈0.7 eV below the Fermi level. Mott–Schottky plots indicate pH and Ti concentration dependent flatband potentials of −0.2 to −0.9 V vs SHE, commensurate with previously reported data. Flatband potentials exhibited super-Nernstian pH dependence ranging from −69.1 to −101.0 mV/pH. Carrier concentration data indicate that the Fermi energy of the Ti-doped system is Ti concentration dependent, with a minimum of 0.15 eV near x = 0.25. These energy level data allow us to construct an energy band diagram for Ti-doped hematite electrode/electrolyte interfaces, and to determine a Ti-doping concentration that reduces bulk resistivity while also reducing the formation of surface states for these photoanodes.

Enhanced photoelectrochemical response of reduced-graphene oxide/Zn1−xAgxO nanocomposite in visible-light region

Graphene based nanocomposites have the potential to work as efficiently, multifunctional materials for energy conversion & storage. These composites may exhibit better photocatalytic properties by the improvement of their electronic and structural properties than pure photocatalysts. In the present work, reduced graphene oxide (rGO) & ZnO nanocomposite with 0–5 atom% Ag doping was prepared by electrodeposition method and characterized by XRD, Raman spectroscopy, FE-SEM, EDX, UV–Vis spectroscopy and final photoelectrochemical activity was assessed under 1.5 AM solar simulator in 1 M NaOH as electrolyte. Significant changes in the Raman spectrum for the nanocomposite suggest the possible electronic interaction between rGO and ZnO nanocomposite and its successful fabrication, which improves the charge separation and enhanced photoelectrochemical activity in the nanocomposite. We find a red-shift of 0.35 eV in the UV–vis spectrum and therefore an enhanced photoelectrochemical activity in the visible range on Ag doping in rGO/ZnO nanocomposite. Nanocomposite with 1 atom% Ag doping showed the highest photocurrent density of 2.48 mA/cm 2 at 0.8 V vs Ag/AgCl over other samples, which was almost five times higher than that for undoped rGO/ZnO composite. Calculated Flat-band potential and donor densities using Mott–Schottky data also supported the better photoelectrochemical response for Ag doping in nanocomposite. • Graphene based nanocomposite was successfully fabricated by electrodeposition. • Nanocomposite exhibited good photocatalytic activity in visible region with Ag-doping. • Significant changes in Raman suggest possible electronic interaction in nanocomposite. • Nanocomposite with 1 atom% Ag doping exhibited best photocurrent of 2.48 mA/cm 2 .

Characterization of high-temperature oxide films on dysprosium-doped Fe-20Cr alloys by electrochemical techniques

The oxidation properties of Fe-20Cr, Fe-20Cr-0.2Dy and Fe-20Cr-1Dy alloys were studied using gravimetric and electrochemical techniques. The high-temperature oxide films of Dy-doped Fe-20Cr alloys were prepared in air at 900 °C for 24, 48 and 100 h, respectively. The electrochemical experiment was performed by a three-electrode electrochemical cell and in 0.1 mol/L Na2SO4 aqueous solution. Proper models were built for describing electrochemical impedance spectroscopy of the different oxide layers and the spectra were interpreted in terms of a two-layer model of the films. The results revealed that the oxide films of Dy-doped Fe-20Cr alloys became compacter than that of undoped alloys and retained their good protective ability for a relatively long time. With increasing content of Dy, the protection of the oxide films slightly decreased. Mott-Schottky curves indicated that all the oxides were n-type semi-conductors, and the Nd value of oxide film on Fe-20Cr was much larger than that of Dy-doped Fe-20Cr alloys. The results of kinetic curves and SEM were in agreement with electrochemical impedance spectroscopy and Mott-Schottky data.

ChemInform Abstract: 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 p-InP/V[sup 3+/2+]-HCl semiconductor/electrolyte contact ( 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 [psi][sub B] = 1.2 eV, independent of the Ag treatment. This value is considerably larger than typically found at solid-state p-InP/Ag contacts. Barrier heights derived from Mott-Schottky data are compared to photovoltages; results indicate that interface recombination and a slow charge-transfer (bare p-InP) and thermionic emission from the metal (Ag-treated p-InP) 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 [minus]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 [minus]5.0 eV/SCE. Reference to related work at InP and GaAs is made.

Mott−Schottky and Charge-Transport Analysis of Nanoporous Titanium Dioxide Films in Air

We investigate the electrical properties of air-filled nanoporous TiO2 films through Mott−Schottky (MS) and current−voltage (IV) analyses. Films of varying thickness and varying TiO2 particle size are examined. Dark IV analysis indicates formation of an asymmetric Schottky-barrier junction between the nanoporous TiO2 film and a graphite back contact. Upon increasing the film thickness and decreasing the particle size, diode rectification deteriorates while series resistance increases; these phenomena are attributed to resistive hopping charge transport between TiO2 particles. Dramatic differences in the dark-IV extrapolated resistance of comparable thickness 300 and 9 nc films indicate that the transport physics may be substantially different between these two types of films in the dark. An effective medium model is developed that appropriately describes the dark MS data, indicating that the TiO2 film behaves as a pure dielectric in the depletion region and that the internal surface area of the film does not contribute to the capacitance. Upon UV illumination, photodoping increases the TiO2 film conductivity. Fermi-level splitting results in an ohmic graphite contact and current flow is generally limited by carrier transport rather than charge transfer to the contact. IV analysis based on a space charge limited current model provides mobility estimates of ∼1.7−3.4 × 10-4 cm2 V-1 s-1 for the 9 nm diameter TiO2 particle films and ∼1.8−3.5 × 10-4 cm2 V-1 s-1 for 300 nm diameter TiO2 particle films under AM 1.5 illumination. The close agreement between the illuminated-IV extrapolated electron mobility for both the 9 and 300 nm diameter TiO2 particles suggests that the transport physics are likely similar in both films when illuminated.

PH Sensitivity of SiC Linked Organic Monolayers on Crystalline Silicon Surfaces

The electrochemical behavior of Si--C linked organic monolayers is studied in electrolyte-insulator-Si devices, under conditions normally encountered in potentiometric biosensors, to gain fundamental knowledge on the behavior of such Si electrodes under practical conditions. This is done via titration experiments, Mott-Schottky data analysis, and data fitting using a site-binding model. The results are compared with those of native SiO(2) layers and native SiO(2) layers modified with hexamethyldisilazane. All samples display pH sensitivity. The number of Si--OH groups on the alkylated samples is calculated to be less than 0.7 % of that of a pure SiO(2) insulator, which still causes a pH sensitivity of approximately 25 mV per pH unit in the pH range: 4-7. The alkylated samples hardly suffer from response changes during up- and down-going titrations, which indicates that very little oxide is additionally formed during the measurements. The pK(a) values of all samples with monolayers (4.0-4.4) are lower than that of native SiO(2) (6.0). The long-term drift (of approximately 1 mV h(-1)) is moderate. The results indicate that biosensors composed of alkylated Si substrates are feasible if a cross-sensitivity towards pH in the sensor signal is taken into account.

Mott–Schottky analysis of thin ZnO films

Thin ZnO films, both native and doped with secondary metal ions, have been prepared by sputter deposition and also by casting from solutions containing a range of precursor salts. The conductivity and infrared reflectivity of these films are subsequently enhanced chemically following treatment in H2 gas at 400 °C or by cathodic electrochemical treatment in a neutral (pH=7) phosphate buffer solution. While Hall-type measurements usually are used to evaluate the electrical properties of such films, the present study investigated whether a conventional Mott–Schottky analysis could be used to monitor the change in concentration of free carriers in these films before and after chemical and electrochemical reduction. The Mott–Schottky approach would be particularly appropriate for electrochemically modified films since the measurements could be made in the same electrolyte used for the post-deposition electrochemical processing. Results of studies on sputtered pure ZnO films in ferricyanide solution were promising. Mott–Schottky plots were linear and gave free carrier concentrations typical for undoped semiconductors. Film thicknesses estimated from the Mott–Schottky data were also reasonably close to thicknesses calculated from reflectance measurements. Studies on solution-deposited films were less successful. Mott–Schottky plots were nonlinear, apparently due to film porosity. A combination of dc polarization and atomic force microscopy measurements confirmed this conclusion. The results suggest that Mott–Schottky analysis would be suitable for characterizing solution-deposited ZnO films only after extensive modeling was performed to incorporate the effects of film porosity on the characteristics of the space-charge region of the semiconductor.

An in situ characterization of the p-indium phosphide-indium/electrolyte contact formed by cathodic corrosion of the semiconductor

Hydrogen evolution at illuminated p-InP/electrolyte contacts is accompanied by a corrosion reaction leading to indium formation at the semiconductor surface. After this process the electrode response is analyzed in 1 M H{sub 2}SO{sub 4} using current-voltage curves and Mott-Schottky as well as impedance spectroscopic measurements. Mott-Schottky data indicate a shift of the flat-band potential due to In formation whereas barrier heights at the semiconductor surface remain almost constant ({Phi}{beta} {approx} 1 eV). Impedance data under photocurrent flow reveal a nearly ideal Schottky contact after separation of the electrochemical charge transfer at the In surface. 17 refs., 4 figs., 1 tab.

Charge transfer and recombination kinetics at photoelectrodes: A quantitative evaluation of impedance measurements

Impedance measurements were performed with illuminated photoelectrodes (bare and Pt-coated p-InP) at frequencies between 0.1 Hz and 1 MHz. Two different five-element equivalent circuits are discussed, both resulting in a satisfying fit of the impedance data. Mott-Schottky data and the photocurrent-voltage behaviour cannot be explained by a Maxwell-type circuit (involving surface states). A Voigt-type circuit, however, does describe adequately the recombination behaviour of the photo-electrode, the electrochemical charge transfer reaction (hydrogen evolution), and anomalies in Mott-Schottky evaluations. From the recombination and charge transfer resistances, the Schottky diode ideality factor, the electrochemical exchange current density, and the cathodic charge transfer coefficient are derived. Measured Helmholtz capacity values are at 3-30 μF/cm 2 with bare p-InP during current flow. The results are in agreement with the model of a Schottky barrier (photovoltage build-up) with subsequent charge transfer across the metal (catalyst)/electrolyte interface.

A new model for the semiconductor-electrolyte interface and the origin of Mott-Schottky data dispersion

An equivalent circuit for the semiconductor-electrolyte interface is proposed, which incorporates several area-dependent space charge capacities and interface resistances, due to local differences in the electronic interface structure. This results in a multi-exponential charging current of the total space charge capacity, which is verified experimentally by frequency analysis, chronoamperometry and time domain spectroscopy. Different distinct relaxation times are observed which yield several, simultaneous Mott-Schottky plots. The frequency and solution dependence of Mott-Schottky data is explained by non-uniform chemisorption onto a non-ideal surface. This heterogeneous interface therefore has a surface Fermi level and an interfacial electron transfer resistance, which are not constant, but locally different. The resulting spectrum of relaxation times can then be modeled by the proposed circuit.

A new model for the semiconductor-electrolyte interface and the origin of Mott-Schottky data dispersion

A new model for the semiconductor-electrolyte interface and the origin of Mott-Schottky data dispersion

Photoelectrochemical production of hydrogen from p-type transition metal phosphides

We present a photoelectrochemical investigation of a number of p-conducting transition metal phosphide semiconductors MeP 2 (Me = Cu, Zn, Cd). The series MeP 2 can be divided into two subgroups. The first of these consists (e.g.) of the monoclinic compounds CuP 2 and β-ZnP 2, which appear ideally suited for photovoltaic applications, due to their bandgaps close to 1.4 eV. The quantum yield of a β-ZnP 2 single crystal electrode, in 1 M Na 2SO 4 at pH 2.5, is above 60% for the polarization E‖ c in a large portion of the visible and near infrared spectrum. The second class of materials are the tetragonal modifications α-ZnP 2 and CdP 2. Their bandgaps are near 2.1 and 2 eV, respectively. The quantum yield of α-ZnP 2 is above 70% for photon energies above 2.7 eV. A general feature of all these materials is the high energy of the conduction band, or conversely, the negative position of the flatband potential V fb on the electrochemical scale. From the Mott-Schottky data for β-ZnP 2, it follows, that V fb = 0.11 ± 0.02 V SCE in 1 M Na 2SO 4, at pH 2.5. Photo-assisted electrolysis requires thus a rather large bias. On the other hand, hydrogen evolution can be very efficient in the presence of an electron donor. This is demonstrated with RuO 2-loaded α-ZnP 2 particles in an electrolyte containing EDTA. Due to the negative V fb these materials could also be ideal candidates for other important redox reactions, such as CO 2 reduction.

Comparison of Behavior of Single‐Crystal and Polycrystalline CdS in Photoelectrochemical Cells

Capacitance in the dark and photoresponse were determined from single‐crystal <001> and polycrystalline (deposited cathodically from dimethylsulfoxide). Three different solvents employed (water, triethyleneglycol dimethyl ether and acetonitrile) gave results that lead to essentially the same conclusion. Thus, single‐crystal having a “clean” surface gives identical onset and shut‐off potentials that are the same as the flatband potential determined from Mott‐Schottky (MS) plots of capacitance data. These MS data are independent of the scan direction for the potential. Surface sulfur that results from photoxidation causes the onset to shift negative of the shutoff. Also, the MS intercept becomes dependent upon the scan direction of the potential. These results can be explained by the effect of the oxidation level of the surface sulfur on the flatband potential. In contrast, for fresh polycrystalline that has not been subjected to photo‐oxidation, the onset potential is negative of the shut‐off potential. These and other results can be used as evidence that the growing process incorporates one or more sulfur containing impurities that are uniformly distributed throughout the film.

Hot carrier injection of photogenerated electrons at indium phosphide–electrolyte interfaces

Hot electron reduction of p-nitrobenzonitrile at (111) p-InP in acetonitrile electrolyte was observed as a supraband-edge redox reaction. The band-edge positions of the p-InP electrode were unequivocally established from Mott–Schottky data obtained as a function of light intensity and frequency. Special conditions of low light intensity and electrode potential were found wherein the semiconductor band edges were either fixed or had moved only very slightly with respect to redox potentials in the electrolyte, such that the respective supraband-edge redox reactions were definitely outside the band gap of InP.