Ni 2p Spectra Research Articles

Mechanochemical Activation of Nickel Oxide Enabling Binder-Free Anodes for Alkaline Water Electrolysis

In recent decades, the imperative for clean energy generation has grown significantly. Water electrolysis stands out as a sustainable method for hydrogen production due to its zero carbon emissions. However, high energy requirements and costs hinder its widespread adoption for large-scale applications [1]. Non-noble metal-based materials have emerged as cost-effective and efficient anodes for the oxygen evolution reaction (OER). However, the stability and performance of these materials heavily rely on the integration of ionomers or binders into the electrode coatings. For example, Nafion, renowned for its high ionic conductivity and ability to stabilize coating structures [2], has been a preferred choice [3]. Yet, their high cost and impending regulations concerning per- and polyfluoro alkyl substances (PFAS) by European agencies necessitate the exploration of alternative approaches. Here, we propose a solution involving binder-free anode coatings, achieved through mechano-chemical activation via planetary ball milling of commercially available catalyst materials. This innovative method offers a promising avenue for developing environmentally sustainable anodes with effective performance, circumventing the limitations associated with conventional binder materials.In this study, nickel oxide (NiO, Merck group) was processed without binders through a modification of the powder material via planetary ball milling where parameters as bead size, rotation speed, and process time were varied. The initial visual contrast transformation from the green hue of the NiO feed to a dark grey or black shade appearance after ball milling which might indicate the presence of Ni2O3. Additionally, certain samples exhibited a needle-like morphology, with particle enlargement occurring selectively in instances involving larger beads during the milling process. These samples were discarded due to missing processability. Powder characterizations were conducted on both the as-purchased NiO feed (as reference) and the ball milled samples. Transmission electron microscopy (TEM) analysis confirmed reduced primary particle size across all ball milled samples as shown in the inset of Figure 1a-b. Energy dispersive X-ray (EDX) analysis indicated a decrease in the Ni metal content with a simultaneous increase in the oxygen content, while – in line with our expectations [4] – the reduced crystallinity of all NiO phases after ball milling was confirmed by X-ray diffraction (XRD) analysis. With regard to the surface species, Fourier-transform infrared (FTIR) spectroscopy evidenced a significant increase of hydroxide (OH) in the ball milled samples. At the same time X-ray photoelectron spectroscopy (XPS) data revealed an increase in the Ni3+ content, followed by a decrease in Ni2+ as analyzed from the Ni 2p spectrum. Combining both findings, we expect an improved anode performance due to the formation of Ni(OH)2 or NiOOH species [5]. Preliminary dispersion experiments were realized in different probe liquids [6] and subsequently the Hansen solubility parameters (HSP) were determined to achieve high-quality dispersions efficiently [7]. Because of the large particle size, all dispersions based on commercial NiO sedimented quickly, whereas the ball milled samples with significantly reduced primary particle size were stable in liquids like ethanol and isopropanol.This finding was quantified by transmittograms, where the sedimentation time, radial position and transmission are plotted together as shown in Figure 1a-b. Due to the inadequate dispersion stability of commercial NiO and thus missing processability, no electrodes were realized with this material. Finally, the ball milled samples were spray-coated onto Ni plate substrates without using binders. Scanning electron microscopy (SEM) images displayed diverse distributions of catalyst particles implying different coating structures and a range of morphologies. Electrochemical measurements at 100 mA cm-2 evidenced an improved activity of ~100 mV for the electrodes that were fabricated from the ball milled samples (Sample 1: 400 rpm, 4 h, Sample 2: 250 rpm, 2 h, constant bead size of 0.3 mm) during oxygen evolution compared to the bare Ni plate substrate as shown in Figure 1c. Within 2 h of testing, no delamination of the material was monitored evidencing its chemical and mechanical stability as a binder-free catalyst layer.In conclusion, this study presents a promising approach to achieve binder-free anodes enabled by mechanochemical activation. The versatility of this method offers opportunities to optimize non-noble metal-based materials for enhanced OER performance, thereby advancing the prospects of sustainable energy generation.

Single step grown porous polycrystalline nickel microstructures with ultra-thin defect rich surface oxide for alkaline water oxidation

Investigation on the intrinsic catalytic activity of structurally diverse polycrystalline nickel microstructures such as porous nickel sheets (PNS), porous folded/curled nickel structures (PFNS) and porous nickel balls (PNB) revealed the surface bounded electrocatalytic activities within an alkaline reaction environment for water oxidation. Nickel microstructures were grown via template/surfactant-free, single step solution combustion technique in air atmosphere. The distinct morphologies (porous nickel sheets, porous folded/curled nickel structures and porous nickel balls) were achieved through changing the quantity of HNO3 in the reaction mixture. The physicochemical characterisations were done with XRD, FESEM, EDS and XPS. Microscopic studies revealed the growth of nickel grains with numerous macro pores forming microstructures in distinct morphologies. The O ls spectra of XPS studies revealed the presence of O−/O2− sites and Ni 2p spectra were distinguished between the formation of Ni0/Ni2+/Ni3+ states, implied the existence of surface oxide with plenty of defects. The synergistic effect results from the formation of microscopic porous architectures with defect rich surface oxide on the electrochemical behaviour and OER activity of PNS, PFNS and PNB were discussed with electrochemical techniques such as CV, LSV, EIS and CP. The OER kinetics in terms of relatively low value of overpotential at 10 mA cm−2 and Tafel slope along with high specific activity of PNS/GCE with respect to PFNS/GCE and PNB/GCE were attributed to the combined effect of increase in electrochemical surface area, high roughness factor and fast charge transfer kinetics. The better electrochemical performance and OER activities of porous nickel sheets in comparison with folded/curled nickel structures and nickel balls is contributed to the unique 2D porous architecture with plenty of oxygen vacancies. This structure not only expose more surface for the formation of passive oxide layer in an alkaline reaction environment but also exhibit facile charge transport properties, causes the generation of numerous surface-active sites for electrocatalytic water oxidation. The outcome of our work gives new insights on the surface electrochemistry of polycrystalline nickel, enables micro-structuring as a prospective strategy towards the optimization of electrocatalysts as anode material for alkaline water oxidation.

Metal organic framework supported surface modification of synthesized nickel/nickel oxide nanoparticles via controlled PEGylation for cytotoxicity profile against MCF-7 breast cancer cell lines via docking analysis

Metal organic framework, (MOFs) supported nanoparticles have proven a path for the treatment of various carcinomas to be effective against MCF-7 Breast cancer cell lines. Despite the globally widespread applications of nickel/nickel oxide nanoparticles, Ni/NiO@LAA_PEG NPs with surfactant Polyethylene glycol, (PEG) and l-Acsorbic acid, (LAA) as reducing agents in industrial, commercial, and biomedical fields, their response to human cells has not been elucidated. Hence in the reported article Ni/NiO@LAA_PEG NPs with an average size range of 75–80 nm was used to study their interaction with MCF-7 Breast cancer cell lines. So the best of our understanding this is the study related to MCF-7 Human Breast cancer cell lines to show cyto-genotoxicity of Ni/NiO@LAA_PEG NPs which seems mediated through MTT analysis with rising effect surface enhancement and indexed porosiveness for the interfacial advancement than standard drug Doxorubicin for their comparison to show their best effectivity to prepared Ni/NiO@LAA_PEG NPs.The theoretical performance was simulated using Gaussian-09@Autodock 4.2 with crystal preparation and docked with MCF-7 Human Breast cancer cell lines PDBs to show their correlation with wet method of preparation. The synthesized Ni/NiO@LAA_PEG NPs was characterized against UV/Vis spectroscopy for determination of peak pattern of 395 nm, the functional group analysis for the best binding of stretching with framed metal network of nanoparticles was to shown out their scissoring, wagging, and CH stretching, etc. at 661, 834 and 3425 cm−1respectively by FTIR analysis and an energy band gap of 2.2 eV for Ni/NiO@LAA_PEG NPs by optical studies. The mesopore range and surface area measurement was calculated with BET adsorption and BJH plot to find their coercivity and retentivity via adsorption-desorption curve for their best catalytic performance. The current interpretation of Ni 2p spectra of oxides and other compounds is based on charge transfer assignment of the main peak at 856.4 eV and broad satellite cantered at about 861 eV to the cd9 L and unscreened cd8 final state configurations.

Detailed peak fitting analysis of the Ni 2p photoemission spectrum for metallic nickel and an initial oxidation

A quantitative study of the surface composition of clean metallic and partially oxidized nickel exposed to oxygen was carried out employing X-ray photoelectron spectroscopy (XPS). By fitting both branches of the metallic Ni 2p spectrum, it was possible to resolve two previously unreported satellite peaks in the Ni 2p1/2 portion of the spectrum. For the partial oxidation of the nickel surface, we analyzed the Ni 2p, O 1s, and C 1s angle-resolved XPS spectra. Using state-of-the-art peak-fitting (including the block approach) and background modeling methods (including the active approach) it was possible to discriminate the peaks of Ni2O3 from those of metallic nickel for both branches of the Ni 2p spectra from partially oxidized nickel. The oxidation of metallic nickel is through the formation of a Ni+3 oxide layer together with deep protrusions. The uncertainties are calculated with the covariance matrix method. The strong overlap and shape similarity of the Ni0 L3M45M45 Auger structure with that of the Ni+3 strongly suggests that the modified Auger Parameter does not provide direct additional insight about the chemical state to that provided by the photoemission spectra.

Texture, microstructure and mechanical properties of laser beam welded AISI 2507 super duplex stainless steel

This study investigates weldability of AISI 2507 super duplex stainless steel (SDSS) sheets with laser welding. The impact of heat inputs and post-weld heat treatment (PWHT) on the mechanical properties, texture, microstructure and the surface of joints were analyzed. The microhardness and tensile strength values of the samples welded with the lowest weld heat input was detected to be greater than samples welded with the high heat input. The highest values for tensile strength, bending force and impact toughness in welded samples have been identified as 891 MPa, 1092 N and 58.4 J respectively. The effect of PWHT caused a decrease in the hardness and tensile strength of all samples, and improved the toughness and ductility properties. While dislocations and stacking faults were observed in the base material microstructure, twinning formation was also detected in the weld metal. Secondary austenite (γ2) and Cr23C6 carbides were also found after heat treatment. Variations in the coincidence site lattice (CSL) boundary proportions and the Schmid factors (SFs) directly affected the mechanical properties. In the 2D- and 3D-orientation distribution function (ODF) base material and weld metal maps, the δ-ferrite and γ-austenite textures performed high-density γ-fiber and α-fiber orientations and a low-density (110) <110> rotated cube component, and showed high-density γ-fiber and low-density α-fiber RD//<110> orientations after heat treatment. When the orientation relationships of the heat-treated sample were examined, Kurdjumov–Sachs (K–S) and Nishiyama–Wassermann (N–W) relationships were determined between the γ/α phases in the base material and weld metal, in addition to these orientation relationships, a occasionally seen (100)γ//(100)α Bain orientation relationship was also found in the non-heat-treated sample. The atomic concentrations of the non-heat-treated and heat-treated weld metals exhibited an increase in the O1s spectrum and decreases in the Ni 2p, Fe 2p, Mo 3d and Cr 2p spectra with increasing sputtering time. Diffusion of H− ion with low density to ferrite and austenite grains on weld metal structure was observed.

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Effect of heat input and post-weld heat treatment on surface, texture, microstructure, and mechanical properties of dissimilar laser beam welded AISI 2507 super duplex to AISI 904L super austenitic stainless steels

In this study, AISI 2507 super duplex and AISI 904L super austenitic stainless steel sheets were successfully joined using a laser welding method with two different laser powers and laser speeds. The effects of different welding heat inputs and post-weld heat treatment on the surface, texture, microstructure and mechanical properties of laser welded joints were investigated. The results obtained in this study showed that the weld metal microstructure predominantly composed of austenite phase. In addition, the occurrence of γ-austenite (dendritic), interdendritic, ferrite, and planar, coplanar, and stacking fault was observed and also Cr23C6 precipitate was formed in the microstructure with the effect of heat treatment. In the sample joined with low heat input, an increase was observed in tensile strength, yield strength and hardness compared to the samples joined using high heat input, and a decrease in impact toughness occurred. With the effect of heat treatment, a decrease was observed in tensile, yield strength and hardness and an increase was observed in ductility and toughness. In addition, there was no significant change in the bending forces of all laser-welded samples, and the bending occurred on the super austenitic stainless steel side. While the austenite grains exhibited random orientation in the non-heat-treated and heat-treated AISI 2507 base metal and weld metal, the non-heat-treated and heat-treated AISI 904L base metal showed an orientation in the [101] and [111] directions in the transverse direction, respectively. The change in the ratio of the Schmid factor and coincidence site lattice (CSL) boundaries has a significant effect on the mechanical properties. In addition, since the kernel average misorientation (KAM) values in the interdendritic, ferrite grains and grain boundaries were higher than the austenite grains, the residual stresses were found to be higher. When the γ-austenite (dendritic), interdendritic and δ-ferrite textures were examined in the 3D-orientation distribution function maps of the non-heat-treated and heat-treated base metal and weld metal, γ-fibre and α-fibre orientations were observed at high density, and recrystallisation component (112) 〈111〉 at high density and component (113)〈332〉 with high toughness at high density appeared. The γ-austenite grains in the base metal and weld metal of the non-heat-treated and heat-treated samples showed a closer Kurdjumov–Sachs (KS) orientation relationship (OR) compared to the interdendritic and δ-ferrite grains. In the γ/δ and γ/interdendritic ORs, KS OR was generally observed, while Bain OR was observed in the heat-treated super duplex base metal. ORs known in the literature were not observed in the non-heat-treated weld metal. Densities observed in the non-heat-treated and heat-treated weld metal showed that as the sputtering time increased, the O1s spectrum decreased, and the Fe 2p, Cr 2p, Ni 2p and Mo 3d spectra increased.

Structural, spectroscopic and electrical properties of dc magnetron sputtered NiO thin films and an insight into different defect states

In this article, we report a detailed study on the influence of sputter power on physical properties of the NiO films grown by DC magnetron sputtering. Structural studies carried out by Grazing Incidence X-ray diffraction (XRD) reveals the polycrystalline nature of the films with FCC phase. The crystallographic orientation (111) plane followed by (200), (220), and (311) plane were evident from the XRD spectra. The average crystallites sizes were estimated from the spectra, and the values were compared using three different plots such as Scherrer, Williamson–Hall and size–strain plot. The surface morphology was carried out by atomic force microscopy. The deposited samples show semitransparent behavior in the visible region and the estimated band gap increased from 2.70 to 3.34 eV with an increase in sputter power. Furthermore, X-ray photoelectron spectroscopy (XPS) core-level Ni2p spectra were deconvoluted and the observed {text{Ni}}2{text{p}}_{{{text{3/2}}}}, {text{Ni}}2{text{p}}_{{1/2}} domain along with their satellite’s peaks were analyzed. Most importantly, XPS quantification data and Raman spectra confirm the presence of both {rm{Ni}}^{2+} and {rm{Ni}}^{3+} states in the NiO films. The electrical properties carried at room temperature revealed that the resistivity of the film significantly increased and a mobility of ~ 84 {rm{cm}}^{2}{rm{V}}^{-1}{s}^{-1} was obtained.

Ultrathin Metal–Organic Framework Nanosheets-Derived Yolk–Shell Ni 0.85 Se@NC with Rich Se-Vacancies for Enhanced Water Electrolysis

We present a controlled fabrication of selective ultrathin metal–organic framework (MOF) nanosheets as preassembling platforms, yolk–shell structured with a few-layered N-doped carbon (NC) shell-en...

On the X-ray photoelectron spectroscopy analysis of LiNixMnyCozO2 material and electrodes

X-ray photoelectron spectroscopy (XPS) is a widely used technique to study surface chemistry and ageing mechanisms of LiNixMnyCozO2 (NMC) based cathodes of lithium ion batteries. Analysis of the literature, however, reveals common problems in interpretation of their XPS spectra: (1) confusion of Ni2p spectra with Auger electron spectra (FKLL) of fluorine from commonly used PVdF binder and from decomposition of the fluorinated electrolyte components, when the XPS spectra are acquired with Al Kα X-ray source; (2) ambiguous fitting of the XPS spectral lines with several Lorentzian-Gaussian shapes leads to doubtful assignments of the oxidation states and questionable conclusions on the ageing mechanisms of the electrode materials. Revisiting the XPS data from aged NMC electrodes emphasizes the contribution of LiF and downplays the effect of nickel and manganese fluorides in the formation of cathode-electrolyte interface. To quantify Ni2+/Ni3+ ratio we introduce a novel method based on the ratio between Ni2p3/2 main peak intensity and intensity of the shake-up satellite. Applying this approach for commercial NMC333, NMC532, NMC622 and NMC811 powder materials we find good correlation with theoretically predicted values for freshly made materials.

XPS spectral analysis for a multiple oxide comprising NiO, TiO2, and NiTiO3

XPS spectra collected from a mixture of NiO, TiO2, and NiTiO3 cannot be analyzed using a standard curve-fitting method; thus, principal component analysis (PCA) was conducted to accurately determine the ratio of various oxides in the mixture. Model samples comprising NiO, TiO2, and NiTiO3 exhibited a complex-shaped Ni 2p spectrum with a satellite peak, from which the atomic ratio of NiO and NiTiO3 could be accurately quantified using the PCA. In contrast, the Ti 2p spectrum exhibited a symmetric shape despite comprising both NiTiO3 and TiO2 states; thus, the PCA results predicted the ratio inaccurately. However, the PCA results showed linear correlation with the theoretical values over an entire range. Thus, accurate ratios were obtained using a calibration curve. The PCA procedure for multiple oxides was applied to angle-resolved XPS spectra for thermally oxidized nickel titanium alloy. The results revealed the in-depth distribution of NiO, TiO2, and NiTiO3 on the surface of the alloy.

Investigation on tailoring physical properties of Nickel Oxide thin films grown by dc magnetron sputtering

We report a comprehensive study on influence of oxygen partial pressure on NiO thin films grown on glass substrates in a combined argon and oxygen ambience by reactive dc magnetron sputtering. In this present article, we have discussed the dependence of oxygen pressure on structural, chemical, morphological, optical and electrical properties of the sputtered NiO films. Glancing angle x-ray diffraction reveals that the deposited films were polycrystalline in nature with FCC phase. The preferred orientation changes from (200) to (111) in a higher O2 flow rate environment and an average particle size was estimated using Scherrer relation. The surface morphology of films was studied by using atomic force microscopy. The x-ray photoelectron spectroscopy analysis demonstrates the core level Ni 2p spectra over a range of 850 eV to 885 eV of binding energy and observed Ni 2p3/2, Ni 2p1/2 domains along with their satellite peaks. It infers the presence of both Ni+2 and Ni+3 oxidation states in the sputtered films. Additionally, Raman spectroscopy was carried out to confirm the structural defects level and crystalline nature of the films. The optical results show that deposited films were semi-transparent and the evaluated optical band gap of the material lies in the range 3.36 eV to 3.52 eV. The extracted electrical properties infer either n-type or p-type conductivity depending on the processing conditions of the films.

High oxygen evolution reaction activity on lithiated nickel oxides - Activity descriptors

Alkaline water electrolyzers promise very high purity hydrogen gas production but suffer from large overpotential for anodic oxygen evolution reaction (OER). Here we describe the effect of lithium (Li+)-substitution into nickel oxide on the electrocatalytic activity towards OER in alkaline electrolyte. The X-ray diffraction patterns of lithiated nickel oxides (LixNi1−xO, x = 0.00–0.50) synthesized by the solution-combustion method suggest that pure phase of lithiated nickel oxide was formed until x = 0.30; thereafter, a secondary phase of LiNiO2 was observed. Rietveld analysis showed that Li+-substitution caused a contraction in the lattice structure as shown by the decrease in lattice parameters upon Li+-substitution. Further, the weight fraction of LiNiO2 was found to be dominant for x = 0.50. Deconvolution of the high resolution X-ray photoelectron spectroscopy for O 1s and Ni 2p spectra suggested that concentration of oxygen vacancies increased linearly, whereas that of Ni3+ increased till x = 0.30 and it decreased when Li+-substitution was further increased to x = 0.40 and 0.50. Although electrical conductivity increased upon Li+-substitution, no significant effect was observed for lithiated samples with varying Li+-content (x = 0.10–0.50). The activities for OER were measured using the rotating disk electrode in 0.5 M NaOH electrolyte, and the data suggest that lithiated nickel oxide synthesized with x = 0.30 shows the highest current density at 1.70 vs. RHE (V). The decrease in OER activity for x = 0.40 and 0.50 was attributed to the decline in OER active Ni3+ sites (probably due to the presence of chemically unstable LiNiO2).

In Situ Study of Self-sustained Oscillations in Propane Oxidation and Propane Steam Reforming with Oxygen Over Nickel

Self-sustained reaction rate oscillations in the oxidation of propane and in the propane steam reforming with oxygen over nickel foil have been studied in situ by near-ambient pressure X-ray photoelectron spectroscopy and mass-spectrometry. It was found that regular relaxation-type oscillations in both reactions proceed under similar conditions. In the former case, the peaks of CO, CO2, H2, and H2O were detected by mass-spectrometry as gas-phase products. In contrast, in the latter case, after addition of water to the reaction feed, the mass-spectrometric signal of water decreased simultaneously with the signals of O2 and C3H8, whereas the signals of CO, CO2, and H2 increased. It means that in the presence of water in the reaction feed, the propane steam reforming proceeds with a significant rate. In both cases, the oscillations arise due to spontaneous oxidation and reduction of the catalyst. According to the Ni2p and O1s core-level spectra measured in situ, the high-active catalyst surface is represented by nickel in the metallic state, and the transition to the low-active state is accompanied by the growth of a NiO film on the catalyst surface. The oscillations in the gas phase are accompanied by oscillations in the catalyst temperature, which reflects proceeding endothermic and exothermic processes. An oscillatory mechanism, which can be common for oxidative catalytic reactions over transitional metals, is discussed.

Evolution of self-sustained kinetic oscillations in the catalytic oxidation of propane over a nickel foil

The evolution of self-sustained reaction-rate oscillations in the catalytic oxidation of propane over a nickel foil has been studied in situ using X-ray photoelectron spectroscopy coupled with online mass spectrometry and gas chromatography. Changes in the effective surface area and in the catalyst morphology under reaction conditions have been examined by scanning electron microscopy and a krypton adsorption technique. It is shown that the regular kinetic oscillations arise under oxygen-lean conditions. CO, CO2, H2, H2O, and propylene are detected as products. The conversion of propane oscillates in a range from 1% to 23%. During the half-periods with high activity, the main reaction pathway is the partial oxidation of propane: selectivity toward CO achieves 98%. In contrast, during the half-periods with low activity, the reaction proceeds through three competitive pathways: the partial oxidation of propane, the total oxidation of propane, and the dehydrogenation of propane to propylene. The driving force for the self-sustained kinetic oscillations is the periodic reoxidation of nickel. According to the Ni2p and O1s core-level spectra measured in situ, the high-active catalyst surface is represented by metallic nickel, whereas during the inactive half-periods the catalyst surface is covered with a thick layer of NiO. The intensity of O1s spectra follows the oscillations of O2 in the gas phase during the oxidation of propane. It is found that during the induction period before the regular oscillations appear, a rough and porous structure develops because of strong reconstruction of the catalyst surface. The thickness of the reconstructed layer is approximately 10–20μm. This process is accompanied with at least an 80-fold increase in the effective surface area compared with a clean, non-treated nickel foil, which undoubtedly leads to a drastic increase in the number of active sites. We believe that it is the main reason for the induction period always being observed before the appearance of self-sustained oscillations in the catalytic oxidation of light hydrocarbons over catalysts with a low specific surface area (single crystals, foils, or wires). Moreover, without such reconstruction, the oscillations cannot arise due to low activity of such catalysts.

Electrochromic properties of NiOx:H films deposited by DC magnetron sputtering for ITO/NiOx:H/ZrO2/WO3/ITO device

NiOx:H thin films were deposited on ITO-coated glass by DC reactive magnetron sputtering at room temperature. The effects of the hydrogen content on the structure, morphologies, electrochemical properties, the stoichiometry and chemical states of NiOx:H thin films were systematically studied. In X-ray diffraction and atomic force microscopy analysis, the crystallinity of the films tends to be weakened when the flow amount ratio of Ar:O2:H2 equals 19:1:3 and as confirmed in electrochemical analysis, such relatively weak crystallinity is the main contributing factor to ion transportation. X-ray photoelectron spectroscopy reveals that the increase of the hydrogen contents results in a relatively lower binding energy exhibited in the Ni 2p spectra. The proportion of Ni2O3 in NiOx:H films increases from 22% at bleached state to 33% at colored state. A monolithic all-thin-film inorganic electrochromic device was fabricated with complementary configuration as ITO/NiOx:H/ZrO2/WO3/ITO. The electrochromic device with optimized NiOx:H thin films acting both as ion storage layer and proton-providing source displays high modulation efficiency of 68% at a fixed wavelength 550nm.

Electronic, electrical and optical properties of undoped and Na-doped NiO thin films

The electrical and optical properties as well as the electronic structure of sodium-doped (Na-doped) nickel oxide (NiO) thin films were investigated using X-ray photoelectron spectroscopy (XPS), reflection electron energy loss spectroscopy (REELS), X-ray absorption near edge structure, and extended X-ray absorption fine structure. The XPS results confirmed the presence of NiO bonds of the NiO phase for all films via the Ni 2p spectra. The NiO thin films annealed above 200°C have both nickel oxide and nickel metal phase. The REELS spectra showed that the band gaps of NiO thin films after Na doping were decreased. Na-doped NiO thin films exhibited relatively low resistivity compared to undoped NiO thin films. In addition, the Na-doped NiO thin films deposited at room temperature have p-type conductivity with a low electrical resistivity of 11.57Ωcm and high optical transmittance of 80% in the visible light region. Our results demonstrate that Na doping plays a crucial role in enhancing the electrical and optical properties of NiO thin films.

Probing the Redox States at the Surface of Electroactive Nanoporous NiO Thin Films

Nanoporous NiO thin film electrodes were obtained via plasma-assisted microwave sintering and characterized by means of a combination of electrochemical techniques and X-ray photoelectron spectroscopy (XPS). The aim of this study is the elucidation of the nature of the surface changes introduced by the redox processes of this nanostructured material. NiO undergoes two distinct electrochemical processes of oxidation in aqueous electrolyte with the progress of NiO anodic polarization. These findings are consistent with the sequential formation of oxyhydroxide species at the surface, the chemical nature of which was assessed by XPS. Electronic relaxation effects in the Ni 2p spectra clearly indicated that the superficial oxyhydroxide species resulted to be β-NiOOH and γ-NiOOH. We also show for the first time spectral evidence of an electrochemically generated Ni(IV) species. This study has direct relevance for those applications in which NiO electrodes are utilized in aqueous electrolyte, namely catalytic water splitting or electrochromism, and may constitute a starting point for the comprehension of electronic phenomena at the NiO/organic electrolyte interface of cathodic dye-sensitized solar cells (p-DSCs).

Effects of Ni vacancies and crystallite size on the O 1s and Ni 2p x-ray absorption spectra of nanocrystalline NiO

We have studied the electronic structure of nanocrystalline NiO thin films, grown by radio-frequency magnetron sputtering under different experimental conditions, using x-ray absorption spectroscopy. The O 1s and Ni 2p spectra showed distinct changes as a function of O2 content in the plasma, which were reproduced with cluster model calculations. These changes are attributed to the incrementing of the surface contribution due to a decrease of the crystallite size as the O2 content in the plasma increases, and to the presence of induced nickel vacancies. Thus, the changes in the electronic structure can be related to the modification of structural and transport properties of these nanocrystalline films.

In situ XPS study of self-sustained oscillations in catalytic oxidation of propane over nickel

Self-sustained oscillations in the propane oxidation over a nickel foil were studied in situ with the use of ambient-pressure X-ray photoelectron spectroscopy (XPS) coupled with on-line mass-spectrometry and gas chromatography (GC). Regular oscillations of a relaxation type were observed at 0.5mbar in the temperature range of 600–750°C in oxygen-deficient conditions. CO, CO2, H2, H2O, and propylene were detected as products. CO selectivity in active half-periods achieved 98% decreasing to 40–60% in inactive half-periods. It has been found that the chemical state of the catalyst drastically changes together with the oscillations of the catalytic activity. According to the Ni2p and O1s core-level spectra measured in situ, the active catalyst surface is represented by metallic nickel, whereas it is covered with a layer of NiO with a thickness of at least 3nm during the inactive half-periods. It means that the oscillations in the propane oxidation over nickel are originated from the reversible bulk oxidation of Ni to NiO. We suggest that the propane oxidation over the metallic Ni surface occurs via the Langmuir–Hinshelwood mechanism, whereas the Mars–van Krevelen mechanism prevails when the reaction proceeds over NiO. The switching between the metallic surface and the oxide shows a significant change in the catalytic activity. According to GC measurements the activity of metallic nickel is approximately 40-fold higher than that of NiO.