Shift In Flat-band Potential Research Articles

Enhancement effect of biomass-derived Carbon Quantum Dots (CQDs) on the performance of Dye-Sensitized Solar Cells (DSSCs)

Corn stover, an agricultural waste, was used to prepare nitrogen self-doped carbon quantum dots (CQDs) through a simple hydrothermal method with only water at near room temperature for the first time. The surface, electrochemical, and photovoltaic characteristics of CQDs doped TiO2 in dye-sensitized solar cells (DSSCs) were thoroughly and systematically examined. The average diameter of blue-fluorescence CQDs measured by a high-resolution transmission electron microscope (HR-TEM) was 4.63 ± 0.87 nm, which consisted of polar functional groups. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy of the biomass-derived CQDs, determined by the cyclic voltammetry (CV) test, were, −5.48 eV and −3.89 eV, respectively. The negative shift of flat band potential (Vfb) in CQDs incorporated photoanode implies the fermi level shifted upward. Experimental results revealed that the improved performance of DSSCs was due to charge transport enhancement and separation, which resulted in the improved energy level configuration between TiO2, CQDs, and electrolytes. In this regard, the CQDs serve as a mediator that enables charge carrier transport without hindrance. In this study, CQDs added to TiO2 + N719, increased short circuit current density (JSC) and power conversion efficiency (PCE) value by ∼26.00 % (10.13 to 12.69 mA/cm2) and 27.20 % (4.78 % to 6.08 %), respectively.

Cathodic shift in flatband potential of hematite based photoelectrodes: Evaluating and correlating the redshift to solar driven photocatalysis

As a promising metal oxide semi-conductor, hematite (α-Fe2O3) has interesting optical properties to be regarded as photoactive material. In this study, we evaluated the cathodic shift in onset potential (Eonset) and the negative shift in flat band potential (Efb). It was observed that the Eonset value of −0.457 V for α-Fe2O3 based electrode shifted to −0.638 V for the ternary nanocomposite (PNFC) [Pani/Ni-α-Fe2O3/CNT] based electrode. The flat band potential for α-Fe2O3 in 1 M NaOH is at −0.49 V which shifts to −0.68 V for the ternary nanocomposite (PNFC). This photoelectrochemical enhancement appears due to electron-hole pair separation. Therefore, it apparently increases the photocurrent trace for PNFC at −0.60 V to 7.04 mA cm-2, compared to the photocurrent of 4.52 mA cm-2 observed for pristine α-Fe2O3. According to the Mott-Schottky plot, the donor concentration (Nd) for the α-Fe2O3 is 4.83 × 109 cm-3, whereas for the ternary nanocomposite (PNFC) is 7.68 × 109 cm-3, respectively. As evident, the Nd value increase for the ternary nanocomposite PNFC points to the proximity of the Fermi (Ef) level below the CB edge when Ni doped α-Fe2O3 comes in contact with polyaniline (Pani). Indeed, the ternary nanocomposite PNFC with lower Eg value (1.32 eV) and higher photocurrent (7.04 mA cm-2) show higher photo-activity with percent degradation of 98.42 % for photocatalytic degradation of Sunset Yellow under visible light irradiation. The aforementioned correlations of the energetic levels of band edges with the shifting of Efb to more negative potentials provide insight into the high photo-activity observed.

Enhanced photovoltaic performance of dye sensitized solar cells using cesium bromide modified TiO2 electron transport layer

TiO2 is one of the most widely explored materials as an electron transport layer (ETL) in dye sensitized solar cells (DSSCs) due to its excellent physical and chemical properties. However, recombination at the device's interface slackens the charge carrier movement, adversely affecting their device performance. Rapid extraction of photogenerated charge carriers plays a vital role in developing high efficiency DSSCs. The conduction band alignment of TiO2 ETL and N719 dye light absorber plays a crucial role in charge carrier dynamics of DSSCs. Herein, the band structure of TiO2 ETL is finely tuned by the incorporation of cesium bromide (CsBr). At the optimal concentration (0.4 Wt. %), DSSCs achieved the best power conversion efficiency (PCE) of 9.28 % compared to 7.61 % for pristine TiO2. The modified TiO2–CsBr ETL induced a negative shift in flat band potential (Vfb) from −0.46 to −0.50 V, which improved the open circuit voltage (VOC), its work function (ɸ) from −4.71 to −3.75 eV and increased conduction band minimum (CBM) from −3.58 to −2.42 eV. CsBr incorporation increased electron density in TiO2 matrix, indicating the suppression of trap state and significantly improved the overall photovoltaic performance of DSSCs.

Accurate determination of band tail properties in amorphous semiconductor thin film with Kelvin probe force microscopy

The disordered microscopic structure of amorphous semiconductors causes the formation of band tails in the density of states (DOS) that strongly affect charge transport properties. Such band tail properties are crucial for understanding and optimizing thin-film device performance with immense relevance for large area electronics. Among the available techniques to measure the DOS, Kelvin Probe Force Microscopy (KPFM) is exceptional as it enables precise local electronic investigations combined with microscopic imaging. However, a model to interpret KPFM spectroscopy data on amorphous semiconductors of finite thickness is lacking. To address this issue, we provide an analytical solution to the Poisson equation for a metal–insulator–semiconductor junction interacting with the atomic force microscope tip. The solution enables us to fit experimental data for semiconductors with finite thickness and to obtain DOS parameters, such as band tail width, doping density, and flat band potential. To demonstrate our method, we perform KPFM experiments on Indium–Gallium–Zinc Oxide (IGZO) thin-film transistors (IGZO-TFTs). DOS parameters compare well with values obtained with photocurrent spectroscopy. We demonstrate the relevance of the developed method by investigating the impact of ionizing radiation on DOS parameters and TFT performance. Our results provide clear evidence that the observed shift in threshold voltage is caused by static charge in the gate dielectric, leading to a shift in flat band potential. Band-tails and doping density are not affected by the radiation. The developed methodology can be easily translated to different semiconductor materials and paves the way for quantitative microscopic mapping of local DOS parameters in thin-film devices.

Rational design of BiFeO3 nanostructures for efficient charge carrier transfer and consumption for photocatalytic water oxidation

Crystal surfaces play significant role in controlling the photocatalytic properties of a semiconductor material. In this work, we report the controlled synthesis of various BiFeO3 nanostructures and validate the influence of various crystal surfaces towards photocatalytic properties for water oxidation. The synthesis of BiFeO3 nanostructures including hexagons, rectangular cuboids and nanoplates is achieved in NaOH aqueous solutions with various concentrations via a facile hydrothermal method. High-resolution transmission electron microscopy (HRTEM) revealed NaOH concentration dependent growth direction of various BiFeO3 nanostructures. BiFeO3 rectangular cuboids dominated with (102) crystal facets exhibited higher surface photovoltage (SPV) response confirming efficient charge carrier separation compared to hexagons and nanoplates. Furthermore, the photoelectrochemical measurements also confirmed the improved photocurrent density and positive shift in flat band potential for BiFeO3 rectangular cuboids studied via Mott-Schottky plots. Resultantly, BiFeO3 rectangular cuboids demonstrated superior photocatalytic O2 evolution activity of 82.2 µmol h−1 g−1 compared to 42.6 µmol h−1 g−1 and 53 µmol h−1 g−1 for hexagons and nanoplates, respectively, without any cocatalyst under visible light irradiation.

Role of tannic acid in desiging of electrically active and magnetic gold nanoparticles and enhanced photovoltaic performance of gold@titania photoanode in DSSC

We describe here in significance of tannic acid in obtaining size dependent electrochemically active and magnetic gold nanoparticles (GNPs). In this account, two types of GNPs (SCGNPs and SC-TAGNPs) were synthesized by using sodium citrate and sodium citrate-tannic acid as reducing agents, respectively. Highly monodispersed, uniformly distributed and small size GNPs were produced when tannic acid was used together with sodium citrate. SC-TAGNPs exhibited room temperature ferromagnetism while SCGNPs revealed diamagnetism. With higher anodic peak current and lower charge transfer resistance, the SC-TAGNPs exhibited better conductivity and electron transfer ability than SCGNPs. Mott-Schottky analysis revealed n-type semiconducting behavior of GC/SCGNPs & GC/SC-TAGNPs and showed positive shift in flat-band potential of GC/SC-TAGNPs compared to GC/SCGNPs suggesting better conductivity of SC-TAGNPs. Subsequently, SCGNPs and SC-TAGNPs were added to TiO2 nanoparticles to fabricate modified titania photoanodes (SCGNPs@TiO2 and SC-TAGNPs@TiO2) for dye-sensitized solar cells (DSSCs) application. With the use of TiO2, SCGNPs@TiO2 and SC-TAGNPs@TiO2 photoanodes, the short-circuit current density (JSC) was observed to be 2.18, 4.50 and 6.68 mA/cm2, while energy conversion efficiency (η) was found to be 0.75, 1.74 and 2.50%, respectively. The TiO2 photoanode employing SC-TAGNPs exhibited remarkably increased JSC and η by ∼48 and ∼44%, respectively, then SCGNPs modified photoanode.

Direct Observation of Surface Hole Accumulation Under Water Oxidation Conditions By Efish and Impedance Measurements

Water oxidation reaction is considered the bottleneck and rate-determining step in photoelectrochemical water-splitting process. To achieve efficient water oxidation on photoanode, sufficient built-in potential on semiconductors should be maintained. Herein, we introduced in-situ Electric Field Induced Second Harmonic Generation (EFISH) technique to probe the change of built-in potential under water oxidation reaction process of a single-crystal rutile TiO2 photoanode. The band flattening effect caused by the reduction of built-in potential on photoanode was investigated by EFISH. Up to 600 mV built-in potential screening on TiO2 was observed at 1 V vs Ag/AgCl applied bias under 15mW/cm2 on TiO2 photoanode, in accordance with the anodic shift of flat-band potential. This phenomenon can be explained by the contribution of hole accumulation at surface states and local pH variation in an insufficiently buffered electrolytes, indicating that surface OH· is the rate determining step in water oxidation process. Built-in potential under excitation can be well maintained in a sufficiently buffered electrolytes, suggesting a different rate determining step in water oxidation process.

Sb2S3 surface modification for improved photoelectrochemical water splitting performance of BiVO4 photoanode

A fabrication of Sb2S3 layer as surface modification on pyramidal BiVO4 film is realized to improve photoelectrochemical (PEC) performance of BiVO4 photoanode. The Sb2S3-modified BiVO4 film exhibits an increased photocurrent density of 1.1 mA / cm2 at 1.23 V versus reversible hydrogen electrode and a negative shift of onset potential. Further, the negative shift of flat band potential demonstrates that the role of Sb2S3 surface modification is to suppress surface recombination, and thus increased surface separation and hole transfer efficiency are also achieved for the Sb2S3-modified BiVO4 photoanode. Accordingly, the Sb2S3 surface modification enhances surface water oxidation kinetics for the BiVO4 photoanode, resulting in improved PEC performance. These findings inspire further application of Sb2S3 into a PEC water splitting system.

Laser Tailoring of the Hierarchical Titania Towards Ultrafast and Scalable Surface Modification of Photoelectrode Materials

Despite wide bandgap energy (3.2 eV) that corresponds to the UV-range, titania materials are still widely used in many processes driven by light. Their doping by non-metal atoms, modification by metal and metal oxide nanoparticles, deposition of conducting polymers or halides, significantly affect the absorption spectra and thus makes the material more useful when exposed to solar light. Most of the undertaken efforts are based on the wet chemistry methods, electrochemical deposition, chemical vapour or atomic layer deposition techniques. Those strategies were also applied for titania nanotubes fabricated via anodization that ensures hierarchical growth of the TiO2 out of the Ti substrate. Such a synthesis method exhibits other important features – the morphology of nanotubes (NTs) can be tuned by changing anodization parameters while any further material immobilization is not required that facilitates titania usage as electrode material or a platform for photocatalytic processes. Nevertheless, one may have impression that some threshold is reached regarding boosting of the optical and electrochemical properties using those procedures and therefore a novel approach towards facile titania NTs modification is needed.Herein, the rapid and easily scalable method based on the direct laser interaction with the titania NTs covered substrate is proposed. In our experimental works we used pulsed Nd:YAG laser operating at different wavelengths: 266, 355 and 532 nm with optimized fluence and scanning speed. The controlled movement of the sample with the titania nanotubes enables modification within the selected area. The irradiation of the titania nanotubes can be realized: a) just after anodization leading to the phase conversion from the amorphous to the crystalline one [1] or b) after calcination – resulting in partial surface melting [2] or selective encapsulation of hollow tubes. Moreover, laser formation of the metal/metal oxide nanoparticles out of the thin metal layer deposited formerly onto the TiO2NTs surface is also possible. The electrochemical investigations of prepared materials show that intense light-matter interaction leads to the enhancement of the photoactivity defined here as a photocurrent density even by 45% and to increment of donor density by 2 orders of magnitude while the flatband potential shifts positively by 0.74 V. Thus, even without introduction of foreign chemical compounds the significant improvement of electrochemical and photoelectrochemical performance can be achieved while the ordered morphology can be completely preserved or just some side selective tailoring is possible.This work received financial support from the Polish National Science Centre: grant no 2017/26/E/ST5/00416[1] Ł. Haryński, K. Grochowska, P. Kupracz, J. Karczewski E. Coy, K. Siuzdak, Nanomaterials 10 (2020) 430[2] Ł. Haryński, K. Grochowska, J. Karczewski, J. Ryl, K. Siuzdak, ACS Applied Materials and Interfaces 12 (2020) 3225-3235

Chemical modification of carboxylated MWCNTs for enhanced electrical conducting and magnetic properties

An electrical conducting and magnetic nanomaterial, [Ni(II)Hyd]-MWCNTs was synthesized via chemical modification of carboxylated MWCNTs (A-MWCNTs). The synthesized material has been studied by FTIR, UV–Visible, XPS, Raman, TGA, powder XRD, TEM and EDAX analysis. Double probe DC conductivity suggested enhanced electrical conductivity of [Ni(II)Hyd]-MWCNTs compared to A-MWCNTs. (RsQ(RctO)) is found to be best fit Randle’s equivalent circuit. The positive slope obtained from Mott-Schottky plot suggested n- type semiconducting behavior of materials. The positive shift in flat-band potential for [Ni(II)Hyd]-MWCNTs is the signature of its more conducting nature than that of A-MWCNTs. The better conductivity of [Ni(II)Hyd]-MWCNTs is also confirmed by optical and electrochemical band gap studies. The field and temperature dependent magnetization revealed intrinsic ferromagnetism in [Ni(II)Hyd]-MWCNTs with enhanced saturation magnetization, remanent magnetization and coercivity values compared to A-MWCNTs. Hence, high electrical conductivity and magnetic properties prompt the [Ni(II)Hyd]-MWCNTs for applicability in spin-based electronic devices and in other application such as EMI shielding, metamaterials, drug delivery, anticorrosion, etc.

Improved water oxidation performance of ultra-thin planar hematite photoanode: Synergistic effect of In/Sn doping and an overlayer of metal oxyhydroxides

Hematite is a promising photoanode candidate with many favorable material properties, such as stability and suitable band-gap. However, there are some severe challenges, including high losses due to charge recombination and slow oxidation kinetics, which can be addressed by doping and addition of co-catalysts. Here, the effects of temperature driven diffusion of substrate impurities (doping) and subsequent surface modification by metal oxy-hydroxides (co-catalysts) have been studied for enhanced water-oxidation performance in photoelectrochemical (PEC) measurements. Diffusion of indium and tin from the indium-doped tin oxide (ITO) substrate into planar films of α-Fe2O3 photoanodes results in a photocurrent density (Jph) of 0.09 mA/cm2, corresponding to an approximate 9-fold enhancement over the control pristine α-Fe2O3 (0.01 mA/cm2) at 1.23 VRHE. A thin amorphous FeOOH coating over the In/Sn co-doped α-Fe2O3 photoanode improves the water oxidation performance further, with a 211 % enhancement in Jph at 1.23 VRHE and a 0.21 V cathodic shift in onset potential. Thin layers of NiOOH and FeNiOOH co-catalysts exhibit 100 and 155 % enhancement in Jph, respectively. Characterization and electrochemical measurements reveal that the enhanced performance is a result of reduced bulk recombination by temperature driven In/Sn substrate impurity doping and improved surface oxidation kinetics by the metal oxy-hydroxide overlayer. Especially deposition of FeOOH onto In/Sn co-doped α-Fe2O3 significantly reduces resistance at the semiconductor/electrolyte interface, leading to the shift in onset potential. Further, the results indicate that all the samples exhibit a quantitative correlation between the cathodic shift in photocurrent onset potential (Vonset) and flat band potential (Vfb).

Quantification of Sodium‐Ion Migration in Silicon Nitride by Flatband‐Potential Monitoring at Device‐Operating Temperatures

A trap‐corrected bias–temperature–stress (TraC‐BTS) method to quantify the kinetics of ion migration in dielectrics based on capacitance–voltage measurements is presented. The method is based on the extraction of flatband potential (Vfb) shifts in metal–insulator–semiconductor test structures an enables the reliability assessment of semiconductor dielectrics and solar cells. Herein, it is shown that carrier trapping in the dielectric must be accounted for, as it strongly affects the measurement of flatband potential in silicon‐nitride‐based capacitors. This effect is corrected by isolating the contribution of trapping on Vfb using contamination‐free control devices. A specific drift‐diffusion model of the ion kinetics presented herein allows the extraction of ion diffusivity. An Arrhenius relationship is obtained for sodium diffusivity in silicon nitride in a temperature range from 30 °C to 90 °C at an electric field of 1 MV cm−1, yielding a prefactor and an activation energy , with a 95% confidence interval of [] eV for the diffusivity. These quantitative kinetics confirm that silicon nitride may be a poor sodium migration barrier under a significant electric field.

Promoting Active Electronic States in LaFeO3 Thin-Films Photocathodes via Alkaline-Earth Metal Substitution.

The effects of alkaline-earth metal cation (AMC; Mg2+, Ca2+, Sr2+, and Ba2+) substitution on the photoelectrochemical properties of phase-pure LaFeO3 (LFO) thin-films are elucidated by X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD), diffuse reflectance, and electrochemical impedance spectroscopy (EIS). XRD confirms the formation of single-phase cubic LFO thin films with a rather complex dependence on the nature of the AMC and extent of substitution. Interestingly, subtle trends in lattice constant variations observed in XRD are closely correlated with shifts in the binding energies of Fe 2p3/2 and O 1s orbitals associated with the perovskite lattice. We establish a scaling factor between these two photoemission peaks, unveiling key correlation between Fe oxidation state and Fe-O covalency. Diffuse reflectance shows that optical transitions are little affected by AMC substitution below 10%, which are dominated by a direct bandgap transition close to 2.72 eV. Differential capacitance data obtained from EIS confirm the p-type characteristic of pristine LFO thin-films, revealing the presence of sub-bandgap electronic state (A-states) close to the valence band edge. The density of A-states is decreased upon AMC substitution, while the overall capacitance increases (increase in dopant level) and the apparent flat-band potential shifts toward more positive potentials. This behavior is consistent with the change in the valence band photoemission edge. In addition, capacitance data of cation-substituted films show the emergence of deeper states centered around 0.6 eV above the valence band edge (B-states). Photoelectrochemical responses toward the hydrogen evolution and oxygen reduction reactions in alkaline solutions show a complex dependence on alkaline-earth metal incorporation, reaching incident-photon-to-current conversion efficiency close to 20% in oxygen saturated solutions. We rationalize the photoresponses of the LFO films in terms of the effect sub-bandgap states on majority carrier mobility, charge transfer, and recombination kinetics.

Scalable Route toward Superior Photoresponse of UV-Laser-Treated TiO2 Nanotubes.

Titanium dioxide nanotubes gain considerable attention as a photoactive material due to chemical stability, photocorrosion resistance, or low-cost manufacturing method. This work presents scalable pulsed laser modification of TiO2 nanotubes resulting in enhanced photoactivity in a system equipped with a motorized table, which allows for modifications of both precisely selected and any-large sample area. Images obtained from scanning electron microscopy along with Raman and UV-vis spectra of laser-treated samples in a good agreement indicate the presence of additional laser-induced shallow states within band gap via degradation of crystalline structure. However, X-ray photoelectron spectroscopy spectra revealed no change of chemical nature of the modified sample surface. Photoelectrochemical measurements demonstrate superior photoresponse of laser-treated samples up to 1.45-fold for an energy beam fluence of 40 mJ/cm2 compared to that of calcined one. According to the obtained results, optimal processing parameters were captured. Mott-Schottky analysis obtained from impedance measurements indicates an enormous (over an order of magnitude) increase of donor density along with a +0.74 V positive shift of flat band potential. Such changes in electronic structure are most likely responsible for enhanced photoactivity. Thus, the elaborated method of laser nanostructuring can be successfully employed to the large-scale modification of titania nanotubes resulting in their superior photoactivity. According to that, the results of our work provide a contribution to wider applications of materials based on titania nanotubes.

Improvement visible-light photocatalytic performance of single-crystalline SnSe1±x NPs toward degradation of organic pollutants

SnSe NPs with different Se/Sn ratios (Se/Sn = 0.7–1.3) were synthesized by a simple co-precipitation method in the ambient conditions. X-ray diffraction pattern (XRD), high-resolution transmission electron microscopy (HRTEM) images, and selected area electron diffraction (SAED) patterns showed that the sample with Se/Sn = 1.2 ratio was included a single-crystal structure without any defects. In addition, the XRD pattern indicated that, there was an optimum ratio for obtaining SnSe phase, which was 0.8≤(Se/Sn)≤1.2. The photocatalytic activity results of the samples showed that, we were faced with the ultra-fast photocatalytic materials to degrade organic pollutants under the visible light irradiation conditions. Reusability test and XRD pattern result after the photocatalytic activity revealed that only sample with Se/Sn = 1.2 ratio was a stable photocatalytic material. Photoluminescence (PL) indicated that, the efficient electron-hole separation was higher for the Se-rich sample in comparison to another samples. Mott-Schottky (MS) and Brunauer-Emmett-Teller (BET) results indicated that, the flat-band potential (Vfb) shift and the enhancement textural parameters of the sample with Se-rich condition, which was the single-crystalline NPs, in compared with the other samples were the most important factors to enhance the photocatalytic performance. Besides, electrochemical impedance spectroscopy (EIS) results indicated that the sample with Se-rich condition had lower charge transfer resistance, which is a very important parameter to enhance the photocatalytic performance.

Sandwich-Nanostructured n-Cu2O/AuAg/p-Cu2O Photocathode with Highly Positive Onset Potential for Improved Water Reduction.

An n-Cu2O layer formed a high-quality buried junction with p-Cu2O to increase the photovoltage and thus to shift the turn-on voltage positively. Mott-Schottky measurements confirmed that the improvement benefited from a positive shift in flat-band potential. The obtained extremely positive onset potential, 0.8 VRHE in n-Cu2O/AuAg/p-Cu2O, is comparable with measurements from water reduction catalysts. The AuAg alloy sandwiched between the homojunction of n-Cu2O and p-Cu2O improved the photocatalytic performance. This alloy both served as an electron relay and promoted electron-hole pair generation in nearby semiconductors; the charge transfer between n-Cu2O and p-Cu2O in the sandwich structure was measured with X-ray absorption spectra. The proposed sandwich structure can be considered as a new direction for the design of efficient solar-related devices.

A wide visible light active photocatalyst Mg5Ta4O15-xNy for water oxidation and reduction

More than 1.6 eV band gap reductions have been realized on Mg5Ta4O15 by nitrogen doping with its pseudobrookite structure maintained. The nitrogen doped Mg5Ta4O15, i.e. Mg5Ta4O15-xNy, shows broad absorption in the visible light region and promising photocatalytic activity for both water reduction and oxidation reactions under visible light illumination (λ ≥ 400 nm). Apparent quantum efficiency as high as ~0.90% has been achieved at 420 ± 20 nm on Mg5Ta4O15-xNy which is comparable to a number of active metal oxynitrides photocatalysts. Apart from visible light absorption, Nitrogen doping on Mg5Ta4O15 is also accompanied by improved surface hydrophilicity and a negative shift of flat band potential. This simple strategy for band gap engineering can be extended to other metal oxides which may open new playgrounds in the design and development of efficient visible light active photocatalysts.

Structural, optical and photoelectrochemical properties of p type Ni doped CuFeO2 by hydrothermal method

Great efforts have been devoted to delafossite CuFeO2 for exploring its potential application in solar energy conversion. In this study, the structural, band gap, photoelectrochemical properties of pure and Ni doped delafossite CuFeO2 powders synthesized by hydrothermal method were investigated. XRD patterns confirm the formation of a majority of 3R and a small amount of 2H mixed delafossite CuFeO2 crystals. UV-VIS-NIR spectra display gradual narrowing trend of direct band gaps from 3.43 eV to 3.02 eV and indirect band gaps from 1.03 eV to 0.59 eV after Ni doping. The corresponding carrier concentration is promoted from 7.95 × 1019 to 1.14 × 1020 cm−3 together with the anodic shift of flat band potential. The photoresponse performance is enhanced by utilization of Ni doping, mainly due to the evidently narrowed band gap with promoted light absorption ability. The transient decay time of 6% Ni doped CuFeO2 is evidently prolonged from 5.0s to 11.0s, indicating the lower carrier recombination rate as compared with pure CuFeO2. The reduced semicircle radius in EIS spectra indicates the smaller charge transfer resistance and higher interfacial carrier transfer efficiency for Ni doped CuFeO2. In addition, Ni doped CuFeO2 exhibits slightly enhanced magnetic performance which is very easily to be recycled and beneficial for the potential application in suspended status.

Enhancement of electrical conductivity and magnetic properties of bimetallic Schiff base complex on grafting to MWCNTs

In present work, we report a newly synthesized nano-inorganic hybrid magnetic material, [SBCu(II)Gd(III)]@MWCNT-COOH which is obtained by anchoring of a copper(II)–gadolinium(III) heteronuclear bimetallic complex, [triaqua(2-hydroxy-1,3-bis(3-methoxysalicylaldiminato)Cu(II)Gd(III)dinitrato)]nitrate monohydrate, [SBCu(II)Gd(III)] to carboxylated multiwalled carbon nanotubes (MWCNT-COOH) through a chemical method. MWCNT-COOH have been loaded with a number of [SBCu(II)Gd(III)] molecules on their surfaces, which generated multinuclear metal centers. [SBCu(II)Gd(III)]@MWCNT-COOH was characterized by FT-IR, UV–Vis, TGA, powder XRD, Raman spectroscopy, TEM, HRSEM, EDAX and elemental mapping. The DC electrical conductivity of materials was evaluated using two probe method suggesting that [SBCu(II)Gd(III)]@MWCNT-COOH is more conducting than [SBCu(II)Gd(III)]. The increased conductivity of [SBCu(II)Gd(III)]@MWCNT-COOH is confirmed by CV and EIS study. The capacitive and dielectric properties of the materials were investigated by Mott–Schottky electrochemical analysis. The positive value of slope signified n-type semiconducting nature of materials. The lower slope for [SBCu(II)Gd(III)] signified the strong dielectric behavior of the material. The positive shift in flat-band potential of [SBCu(II)Gd(III)]@MWCNT-COOH compared to [SBCu(II)Gd(III)] indicated its more conducting nature. Further small optical band gap obtained for [SBCu(II)Gd(III)]@MWCNT-COOH than [SBCu(II)Gd(III)] by UV–Vis studies suggested better conductivity of nano-inorganic hybrid material. Field dependent magnetization revealed enhanced saturation magnetization, remanent magnetization and coercivity of [SBCu(II)Gd(III)]@MWCNT-COOH due to generation of multinuclear metal centers on carboxylated MWCNTs. The temperature dependent magnetization measurement exhibited overlapping of field cooled and zero field cooled curves in entire temperature range (2–300 K), suggests intrinsic ferromagnetism in [SBCu(II)Gd(III)]@MWCNT-COOH. Hence, trend in variation of magnetization with magnetic field and temperature opens its utility in spin-based electronic devices.

Disentangling Thermodynamic and Kinetic Effects at Photoelectrochemical Interfaces Via Intensity-Modulated High-Frequency Resistivity (IMHFR)

Photoelectrochemically (PEC) driven reactions are an attractive approach to the capture and storage of solar energy. The quest for efficient and robust solar fuel formation motivates the development of increasingly complex semiconductor surface architectures with correspondingly intricate interfacial energetics, such as oxide deposition, catalyst incorporation, or surface texturing. PEC performance is a function of kinetic and thermodynamic aspects at the semiconductor/liquid junction, and deconvoluting these elements is therefore critical to understanding and improving interfacial dynamics. The flatband potential is one measure of thermodynamics at the semiconductor surface and – when compared with voltammetric data – also allows for an indirect assessment of interfacial kinetics. While electrochemical impedance spectroscopy (EIS) and subsequent Mott–Schottky analysis is the historical standard for flatband potential measurements, additional capacitive elements from surface oxides, catalysts, or other deviations from ideal semiconductor/liquid junctions complicate this analysis. Accordingly, we developed the Intensity-Modulated High-Frequency Resistivity (IMHFR) apparatus to study dynamic and complex semiconductor/liquid junctions. In this method we use a large AC frequency (100 kHz) to measure the space–charge resistance (RSC ) of a semiconductor/liquid junction under chopped light as a function of potential. We then isolate potential regimes where RSC is dependent and independent of illumination and extract the potential at the boundary between the two regimes, which we define as the flatband potential. We apply this technique to nanoporous ‘black’ silicon in order to demonstrate its utility and characterize the thermodynamic and kinetic effects of surface treatments at a complex semiconductor/liquid junction. Black silicon is an exciting improvement upon traditional silicon for its large surface area and increased light absorption. We form organic/inorganic films– known to favorably shift proton reduction onset potentials – on black silicon by first covalently binding organic monolayers to nucleate inorganic film growth by atomic layer deposition (ALD). After depositing TiO2 and Pt we collected voltammograms and IMHFR data in acidic media to characterize proton reduction behavior. We find that while the flatband potential shifts positively (favorable for proton reduction), interfacial kinetics are limiting as thicker TiO2 layers are deposited. To circumvent the kinetic limitation, we bury Pt nanoparticles directly into the silicon and form the organic/inorganic film around the nanoparticle. We find the direct Si/Pt contact improves interfacial kinetics but at the cost of the positive shifts to the flatband potential we observed previously. This analysis on a textured and layered semiconductor interface demonstrates the analytical power of the IMHFR technique.