Quantitative XPS Analysis Research Articles

Facile preparation of KNN thin film with high purity phase and excellent electrical properties

Obtaining high purity alkali niobate (K x Na1-x NbO3) thin films without secondary phase on metal coated traditional silicon (Si) substrates via sol–gel technique has remained great challenges until now. Herein, we report K0.5Na0.5NbO3 (KNN) thin films successfully deposited on Pt/Ti/SiO2/Si(100) substrates by a simply effective sol–gel process. A comprehensive and systematic investigation of processing conditions on the microstructures and electrical properties of spin-coated KNN films was presented. We have found that phase purity and microstructures of KNN films are strongly influenced by content of alkali excess and the annealing temperature. Thin films with an equal excess amount of 10% mol K and Na (KNN1) sintered at 650 °C show high crystallinity with a preferred (100)-orientation degree of 78%, and homogeneous and dense surface with columnar structure and large grain size up to 254 nm. The result of quantitative XPS analysis has proved that the composition of the film is close to the chemical stoichiometry. As a consequence, the obtained KNN1 films exhibit a large dielectric constant of 775 and low dielectric loss of ∼2% in the wide frequency range from 1kHz up to 10MHz as well as the best shape of P−E loops. Furthermore, leakage current density of the film is about 9.45 × 10−5 A cm−2 at E ≈100 kV cm−1.

Accelerating Fe(III)/Fe(II) redox cycling in heterogeneous electro-Fenton process via S/Cu-mediated electron donor-shuttle regime

In this study, we developed a Cu0.5Fe2.5S4 nanocatalyst through facile sulfidation of the Cu-MIL-88B(Fe) precursor to expedite surface Fe(III) reduction and enhance H2O2 activation in the heterogeneous electro-Fenton (HEF). The as-prepared catalyst possesses relatively large specific surface area and uniformly dispersed metal active sites. The Cu0.5Fe2.5S4-catalyzed HEF system allowed complete removal of naproxen with minimal metal leaching, surpassing that of Cu-MIL-88B(Fe) or Fe3S4. Quantitative XPS analysis, electrochemical characterization and density functional theory calculations reveal an electron donor-shuttle regime in which S2- and Cu species serve as the electron donor and shuttle, respectively. The Cu species significantly accelerate the internal electron transfer between S and Fe and mitigate the dissolution of the adjacent iron sites, securing the sustainable reducing capacity. Moreover, Cu0.5Fe2.5S4-based HEF exhibits great practicability for treatment of various organics in urban wastewater. This study opens new avenue for addressing the challenge of sluggish Fe(III)/Fe(II) cycling in HEF.

Mapping of the degradation processes at bifunctional O2 gas diffusion electrode for aqueous alkaline metal-air batteries

Rechargeable aqueous alkaline zinc-air batteries present, in principle, auspicious characteristics for the stationary storage of electricity produced by intermittent renewables. Still, a number of issues associated with the durability of the electrocatalytic performances of the air cathode hinder further implementation of this battery technology. A recent article (J. Power Sources 482 (2021) 22,890) details a high-performance gas diffusion electrode (GDE) embedding a bi-functional catalyst for the oxygen evolution and oxygen reduction reactions. Now, aiming at a better understanding of the degradation processes taking place at such electrode, we detail a multi-scale approach based on a combination of results from electrochemical, electron imaging and spectroscopic techniques (EIS, SEM, XPS, operando EXAFS). Insights gained resolve the main degradation processes occurring at different length-scales (Å vs. μm), chemical and structural changes at the electrode components level, as well as of their interrelations. The degradation mapping realized at the gas diffusion electrode level and under current loads relevant for practical application paves the way for further improvements of the electrode architecture. In addition, model quantitative XPS analysis, cell design for operando EXAFS and differential analysis of the EXAFS data introduced herein, are applicable to any type of GDE architecture for electrode durability studies.

Studies on localized electrochemical activity of 304L SS-Zr-4 dissimilar joints using alternating current scanning electrochemical microscopy

Alternating current-intermittent contact-scanning electrochemical microscopy (AC-IC-SECM) technique is used for the first time for simultaneous mapping of local electrochemical activity and topography of as-polished, passivated and pickled 304L SS-Zr-4 dissimilar weld joint (explosive welded) surfaces in fresh water at open circuit potential (OCP) without redox mediator. The local surface activity showed good contrast and uniformity, even when the surfaces are in tilted condition and topographic resolution is in good agreement with contact and non-contact profilometry. The difference in current and impedance values on SS and Zr-4 surfaces were ∼100 nA and 200 kΩ, respectively. The images acquired on pickled 304L SS and Zr-4 surfaces after immersion in HNO3 + HF exhibited a relatively higher current and lower impedance compared to as-polished and passivated surfaces. XPS analysis of Zr-4 pickled surface revealed the presence of ZrOF2, ZrO2 and Zr, while pickled 304L SS showed a higher metallic to oxide ratio, indicating less protective thinner passive films, resulting in a relatively higher electrochemical activity. Though 304L SS in passivated condition revealed Cr-oxide enrichment, the higher metallic concentration on 304L SS, resulted in similar electrochemical activity as in as-polished condition. The quantitative XPS analysis corroborated the local electrochemical activity results obtained from AC-SECM technique.

Preparation of Ultrafiltration Membrane by Polyethylene Glycol Non-Covalent Functionalized Multi-Walled Carbon Nanotubes: Application for HA Removal and Fouling Control.

Polyethylene glycol (PEG) non-covalent-functionalized multi-walled carbon nanotubes (MWCNT) membrane were prepared by vacuum filtration. The dispersion and stability of MWCNT non-covalent functionalized with PEG were all improved. TEM characterization and XPS quantitative analysis proved that the use of PEG to non-covalent functionalize MWCNT was successful. SEM image analysis confirmed that the pore size of PEG–MWCNT membrane was more concentrated and distributed in a narrower range of diameter. Contact angle measurement demonstrated that PEG non-covalent functionalization greatly enhanced the hydrophilicity of MWCNT membranes. The results of pure water flux showed that the PEG–MWCNT membranes could be categorized into low pressure membrane. PEG-MWCNT membrane had a better effect on the removal of humic acid (HA) and a lower TMP growth rate compared with a commercial 0.01-μm PVDF ultrafiltration membrane. During the filtration of bovine serum albumin (BSA), the antifouling ability of PEG-MWCNT membranes were obviously better than the raw MWCNT membranes. The TMP recovery rate of PEG–MWCNT membrane after cross flushing was 79.4%, while that of raw MWCNT–COOH and MWCNT membrane were only 14.9% and 28.3%, respectively. PEG non-covalent functionalization improved the antifouling ability of the raw MWCNT membranes and reduced the irreversible fouling, which effectively prolonged the service life of MWCNT membrane.

Stoichiometry controlled homogeneous ternary oxide growth in showerhead atomic layer deposition reactor and application for ZrxHf1−xO2

Atomic layer deposition (ALD) processes for binary oxide (AOy or BOz) growth consist of a sequential introduction of metal precursor (precursor-A or precursor-B) and oxidant-O such that the respective surface reactions are self-limiting with respect to precursor and oxidant exposure times (tA or tB and tO). This approach has been further extended for ternary oxide AδB1−δOλ deposition with (i) super-cycle ALD method (where each super-cycle comprises of m-cycles of AOy ALD followed by n-cycles of BOz ALD), (ii) precursor co-dosing method (where precursor-A and precursor-B are simultaneously pulsed followed by an oxidant-O pulse), and (iii) 3-step ALD (where precursor-A, precursor-B, and oxidant-O are sequentially pulsed). In this Letter, we present a subsaturation pulse initiated 3-step process with ApBO… pulsing sequence for ternary oxide AδB1−δOλ deposition in showerhead ALD reactors. Here, the pulse-Ap reaction step is controlled in the subsaturation regime, while both pulse-B and pulse-O reaction steps are allowed to reach saturation as in a typical ALD. From kinetic simulations, we show that the chemisorbed –Ache surface coverage [Ache] could be controlled below its saturation limit [Ache]sat with exposure time tA and precursor impingement rate kAin in the pulse-Ap reaction step. Furthermore, with precursor transport model, we show that kAin could be varied with a better control using ampoule temperature TampA and precursor-A carrier gas flow FiA together than using TampA alone. As example, we report ZrpHfO… pulsed deposition of ZrxHf1−xO2 ternary oxide samples ZHO1–ZHO4 in a showerhead ALD reactor, and from quantitative XPS analysis, we show that the Zr-fraction (x) could be varied in the range of 0.094 ≤ x ≤ 0.159 with Zr-carrier gas flow FArZr.

Quantitative XPS characterization of “invisible gold” in Carlin-type gold ores through controlled acid erosion

Quantitative XPS analysis of “invisible gold” in Carlin-type gold ores was accomplished, which revealed Au concentration, percentages of Au+ and Au0, and Au NP size. An acid etching step was demonstrated to be the key to enhancing Au signal in XPS.

A quantitative analysis of AES and XPS specifically applied in adsorption systems at submonolayer regime

An analysis of the intensity ratios, adsorbate over substrate and substrate over the clean one, is carried out, treating them as a system of two independent variables. This leads to simple formulas, ready to be applied to the determination of coverage up to the monolayer point, if elemental bulk standards are available. The procedure allows one more parameter to be found out compared with previous treatments. Formulas are produced for polycrystalline and single crystal elemental bulk standards. The coverage thus calculated turns out to be a function of the intensity from the adsorbate over that from the clean substrate. The analysis is applied successfully to the adsorption of Ba on the Ni(110) surface at submonolayer regime.

Valence-induced distortion controls the resistivity and thermal stability of Co2.77Mn1.71Fe1.10Zn0.42O8 ceramics

Spinel-type Co2.77Mn1.71Fe1.10Zn0.42O8 ceramics were first synthesized by modulating the different valence of manganese as raw materials. The results reveal that the valence effect of raw materials plays an essential role in the evolution of structural and electrical parameters, especially in the resistivity and aging coefficient. XPS quantitative analysis and Raman drift further prove that the Jahn-Teller distortion induced by Mn3+ cations changes the resistivity and thermal stability. Importantly, a novel double-hopping conduction model was proposed to elucidate the development of electrical properties. It indicates that the valence-induced distortion controls the resistivity and thermal stability of Co2.77Mn1.71Fe1.10Zn0.42O8 ceramics. The research of the relationship between material properties and the valence of the raw materials can be helpful to guide the production and experiment process. The novel conductive model will open a window for the conductive mechanism of spinel ceramics.

Kinetics of the Thermal Oxidation of Ir(100) toward IrO2 Studied by Ambient-Pressure X-ray Photoelectron Spectroscopy.

Using time-lapsed ambient-pressure X-ray photoelectron spectroscopy, we investigate the thermal oxidation of single-crystalline Ir(100) films toward rutile IrO2(110) in situ. We initially observe the formation of a carbon-free surface covered with a complete monolayer of oxygen, based on the binding energies of the Ir 4f and O 1s core level peaks. During a rather long induction period with nearly constant oxygen coverage, the work function of the surface changes continuously as sensed by the gas phase O 1s signal. The sudden and rapid formation of the IrO2 rutile phase with a thickness above 3 nm, manifested by distinct binding energy changes and substantiated by quantitative XPS analysis, provides direct evidence that the oxide film is formed via an autocatalytic growth mechanism that was previously proposed for PbO and RuO2.

Construction and selective removal of Cd ion based on diatom-based Cd (II) ion-imprinted composite adsorbent

The present work displays diatom as an ecofriendly material replace to the conventional costly synthetic matrixes for surface imprinting technology. In this study, novel green Cd (II) ion-imprinted composite adsorption material (SH/DE-IIP) was synthesized by surface ion-imprinting technique for selective removal of cadmium ions from aqueous solutions. Various characterization methods have confirmed the structural transformation on the diatom surface. The changes in surface morphology and structure were characterized by SEM. N2 adsorption-desorption analysis revealed the adsorption material possessed microporous structure. XRD and FTIR spectra have shown the functional monomers were successfully introduced into the surface of diatom by observing changes in characteristic functional groups. XPS spectra have shown the bonding relationship of the material with a coordination S-Cd bond, effectively utilizing the surface-active structure characteristic of diatom, and the three-dimensional cross-linked structures were presented after the cross-linking reaction. Due to the unique pore structure provided by diatoms, the mass transfer of the SH/DE-IIP was enhanced after functional modification, and SH/DE-IIP exhibited significantly improved adsorption efficiency for Cd(II) ions. Combined with XPS quantitative analysis, the calculation formula of grafting percentage for the diatoms and the functional monomers was derived, while a grafting model was constructed to evaluate the removal effect. The calculated grafting percentage indicated that the SH/DE-IIP provided a good compounding effect. The adsorption studies revealed the SH/DE-IIP possessed a good adsorption performance for Cd (II), and the adsorption equilibrium was reached in 30 min. The adsorption behaviors followed the pseudo-second-order kinetic model and the adsorption isotherms are described well by Langmuir isotherm adsorption model. The relative selectivity coefficients for Cd(II)/Ni(II), Cd(II)/Zn(II), and Cd(II)/Cu(II) were 1.56, 1.67, and 1.50, respectively. The more results showed that the SH/DE-IIP has the excellent selectivity and the good reusability. The obtained Cd ion-imprinted composite adsorption material exhibited a good directional removal performance for Cd (II) from aqueous solutions. This work not only provides a new material strategy for preparation of ion-imprinted materials, but also provides new insights into the structure of imprinted materials.

Comparative Study of the Different Anchoring of Organometallic Dyes on Ultrathin Alumina

Significant improvements in incident photon-to-current efficiencies can be obtained by covering inorganic semiconductors with ultrathin alumina films and sensitizing them with adsorbed dye molecules. The anchoring mode of the latter to the substrate affects the charge transport between the dye and the electrode via tunneling, and consequently, the device efficiency. In this work, we employ X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) for a comparative study of the adsorption of three rhenium and two ruthenium organometallic dyes, one of each being ionic, with different anchoring modes on single-crystalline ultrathin alumina films. Molecular monolayers were prepared by self-assembly from solution. Quantitative XPS analysis reveals higher surface densities for the Re dyes. Nearly stoichiometric coadsorption of counterions is observed for the ionic dyes. Density functional theory (DFT) calculations for the Re dyes show that the most stable adsorption configurations exhibit the expected bonding via the dedicated anchoring groups (carboxyls or methylphosphonic acid), with an additional sulfur–aluminum bond for the dyes containing a thiocyanate ligand. The alignment of the occupied molecular levels with respect to the alumina valence band maximum, obtained for these geometries, follows the experimental trend in the UPS data and places the lowest unoccupied molecular orbitals (LUMOs) close to the Fermi level of the systems, far inside the alumina band gap. Dynamical charge screening is found to be important for this type of system when comparing UPS and DFT results. This work provides a general guideline for the systematic characterization of related molecules on surfaces.

Phase analysis and reduction behaviour of Ce dopant in zirconolite

Zirconolite, a potential matrix for the immobilization has been studied to investigate the doping behaviour of Ce in its structure to design a nuclear waste form that can accommodate both trivalent and tetravalent nuclear wastes. So, samples have been prepared as per the composition Ca0.8Ce0.2ZrTi1.8Al0.2O7 using solid-state method and analysed for their structural, micro-structural and chemical properties. A dense-packed structure was found in SEM with the phase of zirconolite-2M as confirmed by XRD and Raman techniques. Quantitative analysis of XPS inferred that the percentage of Ce(III) in the sample was about ~ 75% indicating the reduction of Ce at higher temperatures.

Growth of MnO2 nanoflakes on TiO2 nanorods for pseudocapacitor

In this work, we report the synthesis of TiO2 (core)/MnO2(shell) nanorods by a redox reaction and TiO2 nanorods generated in-site from H-titanate nanorods during hydrothermal process. The MnO2 nanoflakes were grown densely on TiO2 nanorods to form core-shell nanorods. Owing to the strong TiO2-MnO2 interfacial interaction and enriched oxygen vacancies, TiO2/MnO2 nanorods are highly active and stable as capacitive electrodes. The quantitative analysis of XPS shows more oxygen vacancies were generated in the TiO2/MnO2 materials. The specific capacitance of TiO2/MnO2 is 368.9 F/g, more than double of 140.8 F/g on the MnO2 materials. At 1.5 A/g, 92% of the initial capacitance of the TiO2/MnO2 was still retained after 3000 charge-discharge cycles. However, the specific capacitance on the pure MnO2 materials lost 39% after 3000 cycles. Our results suggest that the improved capacitive properties of TiO2/MnO2 are closely related to enriched oxygen vacancies, unique structure, higher surface area, as well as interfacial interaction between TiO2 and MnO2.

Contact Angle Relaxation and Long-Lasting Hydrophilicity of Sputtered Anatase TiO2 Thin Films by Novel Quantitative XPS Analysis.

The contact angle relaxation of TiO2 surfaces is an important problem that must be understood, particularly for long-lasting hydrophilicity under dark conditions. The relaxation of sputtered anatase TiO2 thin films over a long time (∼22 days) in an atmospheric environment was observed using quantitative XPS analysis. A new peak was identified as H2O within a donor-acceptor complex at ∼2.57 eV above the lattice oxygen peak. This donor-acceptor complex turns out to be a key factor for long lasting hydrophilicity, and our model is presented. Adventitious carbon contamination was not the main cause of the contact angle relaxation. Instead, samples with lower amounts of donor-acceptor complexes ( IDAC/ Ibulk ≤ ∼5%) underwent contact angle relaxation over time, and samples with a high density of donor-acceptor complexes ( IDAC/ Ibulk ≥ ∼10%) showed good hydrophilicity (contact angle ≤20°) over 22 days. Larger amounts of basic Ti-OH relative to acidic OHbridge ( ITi-OH/ Ibridge ≥ 1) resulted in greater amounts of donor-acceptor complexes ( IDAC/ Ibulk ≥ ∼10%). Thus, basic Ti-OH groups interact with H2O by forming a strong electrostatic donor-acceptor complex, leading to long-lasting hydrophilicity. Indeed, TiO2 was transformed to show long lasting hydrophilicity by high-density oxygen plasma treatment by forming sufficient Ti-OH groups and H2O molecules in the donor-acceptor complexes. Contact angle relaxation is closely related to the interactions between water molecules and the TiO2 surface in the dark. It is suggested that the relaxation depends on the number of electrostatic donor-acceptor complexes. This study provides new insight by linking theoretical studies with the experimental contact angle at the TiO2 surface in an ambient environment and is the first study that provides the presented relaxation mechanism.

Experimental and first-principles DFT studies on the band gap behaviours of microsized and nanosized Zn(1−x)MnxO materials

In this study, nano- and microsized zinc oxide (ZnO) materials were doped with different manganese (Mn) contents (1-5 mol%) via a simple sol-gel method. The structural, morphological, optical and chemical environments of the materials were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), UV-visible spectroscopy (UV-vis) and X-ray photoelectron spectroscopy (XPS). XRD results revealed that all synthesised materials were pure and single phased with a hexagonal wurtzite structure of ZnO. However, at a low annealing temperature, a nanorod-like shape can be obtained for all Zn(1-x)MnxO materials. In addition, EDX spectra confirmed the presence of Mn in the ZnO lattice and the atomic percentage was nearly equal to the calculated stoichiometry. UV-vis spectroscopy further revealed that materials in nano size exhibited band gap widening with an increase of the Mn content in the ZnO lattice. In contrast, micron state materials exhibited band gap narrowing with increasing Mn content up to 3% and then begin to widen when Mn > 3%. This is because the band gaps of these materials are affected by the dimensions of the crystals and the Mn content in the materials. Furthermore, XPS results revealed the existence of multiple states of Mn in all synthesised materials. By combining the information obtained from UV-vis and the XPS valence band, shifting in the valence band maximum (VBM) and conduction band minimum (CBM) was observed. Based on XPS results, the calculation of density functional theory studies revealed that the presence of Mn2+, Mn3+, and Mn4+ ions in the materials influences the band gap changes. It was also revealed that the nanosized Zn0.99Mn0.01O exhibited a higher photocatalytic activity than the other samples for degrading methylene blue (MB) dyes, owing to its smallest crystallite size.

Graphene-Based Metal/Acid Bifunctional Catalyst for the Conversion of Levulinic Acid to γ-Valerolactone

A facile strategy was developed for the preparation of Ru nanoparticles supported on reduced graphene oxide (Ru/rGO) and its following functionalization with benzenesulfonic acid groups (Ru/rGO-S). The formation of a C–C bond between p-sulfophenyl and sp2 carbon of graphene can be unambiguously confirmed by XPS quantitative analysis. The two catalysts were used to catalyze the conversion of levulinic acid in aqueous phase, which is an important reaction during biorefinery. Not GC but HPLC was found to be the reliable analysis method for this reaction. Both of the catalysts exhibited high hydrogenation activities under mild reaction conditions (50 °C, 2 MPa), although their selectivities were different. The hydrogenated intermediate 4-hydroxyvaleric acid could be accumulated over Ru/rGO with a yield of 82% in 40 min, but the main product over Ru/rGO-S was γ-valerolactone (a yield of 82% within the same time) due to the accelerated dehydration process by strong acid sites. After being reused several times, the hydrogenation activities of Ru in each catalyst and dehydration ability of SO3H could be well kept. This effective method could be a general way to prepare graphene-based bifunctional catalysts, which are expected to have broad application prospects in biomass conversion.

Determining shell thicknesses in stabilised CdSe@ZnS core-shell nanoparticles by quantitative XPS analysis using an Infinitesimal Columns model

A novel Infinitesimal Columns (IC) simulation model is introduced in this study for the quantitative analysis of core-shell nanoparticles (CSNP) by means of XPS, which combines the advantages of existing approaches. The IC model is applied to stabilised Lumidot™ CdSe/ZnS 610 CSNP for an extensive investigation of their internal structure, i.e. calculation of the two shell thicknesses (ZnS and stabiliser) and exploration of deviations from the idealised CSNP composition. The observed discrepancies between different model calculations can be attributed to the presence of excess stabiliser as well as synthesis residues, demonstrating the necessity of sophisticated purification methods. An excellent agreement is found in the comparison of the IC model with established models from the existing literature, the Shard model and the software SESSA.

Strain Engineered CaBi2Nb2O9 Thin Films with Enhanced Electrical Properties.

In this work, strain engineered polycrystalline thin films (∼250 nm) of bismuth layer-structured ferroelectric (BLSF) CaBi2Nb2O9 (CBNO) were prepared by using a radio frequency (RF) magnetron sputtering technique. XRD analysis revealed that the films were (200)/(020) and (00l) textured with a large in-plane tensile stress. Cross-sectional TEM analyses confirmed the bismuth layered-structure, as well as crystalline orientations and a strain-controlled growth mode of the grains. Result of a quantitative XPS analysis revealed that the composition of the film is close to the chemical stoichiometry. Excellent electrical properties were achieved in the CBNO films, including a high dielectric constant (∼280 @5 kHz), a small dielectric loss (tgδ ≤ 1.6% up to an applied electric field of ∼1200 kV/cm) and a large polarization (Pr ≈ 14 μC/cm(2) @ 1 kHz).

Oxidation of laccase for improved cathode biofuel cell performances

Graphite rods were modified by substituted aryldiazonium salts allowing subsequent laccase immobilisation and direct electron transfer at the cathode. Two covalent enzyme immobilisation methods were performed with carboxy and amino substituted grafted groups, either via the formation of an amide bond or a Schiff base between the glycosidic groups of the enzyme and the amino groups on the electrode surface, respectively. Laccase adsorption efficiency was consistently compared to the covalent attachment method on the same carbon surface, showing that the latter method led to a higher immobilisation yield when the electrode surface was functionalised with carboxylic groups, as shown from both laccase activity measurement towards an organic reducing substrate, ABTS, and quantitative XPS analysis. Both analytical methods led to similar laccase surface coverage estimations. From activity measurements, when laccase was covalently immobilised on the electrode functionalised with carboxylic groups, the surface coverage was found to be 43±2% whereas it was only 10±3% when laccase was adsorbed. Biocatalysed dioxygen reduction current was also higher in the case of covalent immobilisation. For the first time, oxidised laccase performances were compared to unmodified laccase, showing significant improved efficiency when using oxidised laccase: the current obtained with oxidised laccase was 141±37μAcm−2 compared to 28±6μAcm−2 for unmodified laccase after covalent immobilisation of the enzyme on a graphite electrode functionalised with carboxylic groups.