X-ray Photoelectron Spectroscopy Investigation Research Articles

An anomalously high Seebeck coefficient and power factor in ultrathin Bi2Te3 film: Spin–orbit interaction

The present study reports a strong thickness-dependence and anomalously large enhancement in the values of the Seebeck coefficient and electrical conductivity in Bi2Te3 films at ultralow thickness. An opposite sign of the Hall coefficient (negative) and Seebeck coefficient (positive) is observed in an ultrathin Bi2Te3 film (65 nm) as compared to the normally observed identical sign in the case of Bi2Te3 thin films (520 nm). A simultaneous enhancement in the values of electrical conductivity and the Seebeck coefficient results in a giant enhancement in the value of power factor from 1.86 mW/m K2 to 18.0 mW/m K2 at 416 K, with a reduction in thickness. X-ray photoelectron spectroscopy investigation reveals the absence of any significant change in stoichiometry and chemical bonding upon reduction of thickness. Magnetoresistance vs magnetic field data show a sharp dip at the lower magnetic field values, indicating a weak antilocalization effect in the case of the ultrathin film sample suggesting the role of strong spin–orbit coupling toward the carrier filtering effect resulting in enhancement of thermoelectric properties. Observation of the large Seebeck coefficient and the power factor at lower thickness values and its relationship with spin–orbit coupling is an important result, both for practical applications and for better understanding of the thermoelectric properties.

John Mondal

John Mondal

An investigation on the lubrication effectiveness of MoS2 and BN layered materials as oil additives using block-on-ring tests

The commonly accepted interlayer shear mechanism for layered materials as oil additives remains a challenge with the emergence of two-dimensional materials. In this work, the lubrication effectiveness of MoS2 and BN nanosheets with a similar size was investigated as oil additives using block-on-ring tests. BN nanosheets reveal remarkable anti-wear performance and a reduction of wear depth of about 50%, whereas, the expected friction and wear reduction were not found for MoS2 nanosheets. X-ray photoelectron spectroscopy (XPS) investigation indicates the generation of boron-based tribofilm for BN nanosheets and MoS2-complex tribofilm for MoS2 nanosheets. Together with the thermal stability analysis, we found that it is the high strength tribofilm with high oxidation resistance, but not the layer structure, that plays a critical role in lubrication enhancement. This study highlights the advance of ‘depositing tribofilm’ mechanism in novel additive design.

Reaction of BCl3 with H- and Cl-terminated Si(1 0 0) as a pathway for selective, monolayer doping through wet chemistry

The reaction of boron trichloride with the H- and Cl-terminated Si(100) surfaces was investigated to understand the interaction of this molecule with the surface for designing wet chemistry-based silicon surface doping processes using a carbon- and oxygen-free precursor. The process was followed with X-ray photoelectron spectroscopy (XPS). Within the reaction conditions investigated, the reaction is highly effective on Cl-Si(100) for temperatures below 70 °C, at which point both surfaces react with BCl3. The XPS investigation followed the formation of a B 1s peak at 193.5 eV corresponding to (B-O)x species. Even the briefest exposure to ambient conditions lead to hydroxylation of surface borochloride species. However, the Si 2p signature at 102 eV allowed for a confirmation of the formation of a direct Si-B bond. Density functional theory was utilized to supplement the analysis and identify possible major surface species resulting from these reactions. This work provides a new pathway to obtain a functionalized silicon surface with a direct Si-B bond that can potentially be exploited as a means of selective, ultra-shallow, and supersaturated doping.

Reversible Dehalogenation in On-Surface Aryl-Aryl Coupling.

In the emerging field of on-surface synthesis, dehalogenative aryl-aryl coupling is unarguably the most prominent tool for the fabrication of covalently bonded carbon-based nanomaterials. Despite its importance, the reaction kinetics are still poorly understood. Here we present a comprehensive temperature-programmed x-ray photoelectron spectroscopy investigation of reaction kinetics and energetics in the prototypical on-surface dehalogenative polymerization of 4,4''-dibromo-p-terphenyl into poly(para-phenylene) on two coinage metal surfaces, Cu(111) and Au(111). We find clear evidence for reversible dehalogenation on Au(111), which is inhibited on Cu(111) owing to the formation of organometallic intermediates. The incorporation of reversible dehalogenation in the reaction rate equations leads to excellent agreement with experimental data and allows extracting the relevant energy barriers. Our findings deepen the mechanistic understanding and call for its reassessment for surface-confined aryl-aryl coupling on the most frequently used metal substrates.

Mixed Valence of Ce and Its Consequences on the Magnetic State of Ce9Ru4Ga5: Electronic Structure Studies.

We report on X-ray photoelectron spectroscopy (XPS) and ab initio electronic structure investigations of a novel intermetallic material CeRuGa. The compound crystallizes with a tetragonal unit cell (space group I4) that contains three inequivalent Ce atoms sites. The Ce core level XPS spectra indicated an intermediate valence (IV) of selected Ce ions, in line with the previously reported thermodynamic and spectroscopic data. The ab initio calculations revealed that Ce1 ions located at Wyckoff positions possess stable trivalent configuration, whereas Ce2 ions that occupy site are intermediate valent. Moreover, for Ce3 ions, located at different position, a fractional valence was found. The results are discussed in terms of on-site and intersite hybridization effects.

Water Stable Boronic Acid Grafted Barium Metal-Organic Framework for the Selective Adsorption of cis-Diols.

Boronic acid groups have a high affinity toward cis-diols, which, when suspended in a metal-organic framework, generate a novel adsorbent for cis-diols. In this work, we followed a metal-ligand-fragment-coassembly (MLFC) approach to develop a series of water-stable barium metal-organic frameworks with uniform distribution of free boronic acid in the scaffold. The 11B nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopic (XPS) investigations show the enhancement in the inclusion as the feed amount of boronic acid moiety increases. The findings of the adsorption studies elucidated the importance of furanose isomeric form of cis-diols in determining the adsorption efficiency. Galactose is the favorable adsorbate which has the maximum number of molecules in its furanose isomeric form (∼7%). The developed MOFs are promising candidates for the isolation of cis-diols from aqueous solutions.

Leveraging Cu/CuFe2O4-Catalyzed Biomass-Derived Furfural Hydrodeoxygenation: A Nanoscale Metal-Organic-Framework Template Is the Prime Key.

Enormous efforts have been initiated in the production of biobased fuels and value-added chemicals via biorefinery owing to the scarcity of fossil resources and huge environmental synchronization. Herein, non-noble metal-based metal/mixed metal oxide supported on carbon employing a metal-organic framework as a sacrificial template is demonstrated for the first time in the selective hydrodeoxygenation (HDO) of biomass-derived furfural (FFR) to 2-methyl furan (MF). The aforementioned catalyst (referred to as Cu/CuFe2O4@C-A) exhibited extraordinary catalytic proficiency (100% selectivity toward MF) compared with the conventional Cu/CuFe2O4@C-B catalyst which was prepared by the wet impregnation method. High-resolution transmission electron microscopy and synchrotron X-ray diffraction studies evidenced the existence of both metal (Cu) and mixed metal oxide (CuFe2O4) phases, in which the metal could help in hydrogenation to alcohol and metal oxide could assist in the hydroxyl group removal step during HDO reaction. The stabilization of encapsulated metal/metal oxide nanoparticles in the carbon matrix, modulation of the electronic structure, and regulation of geometric effects in the Cu/CuFe2O4@C-A are thought to play an important role in its excellent catalytic performance, confirmed by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy investigations. Furthermore, the structure and activity interconnection was confirmed by in situ attenuated total reflection-IR studies, which manifested the strong interfacial interaction between FFR and the Cu/CuFe2O4@C-A catalyst. This finding was further supported by NH3 temperature-programmed desorption analysis, which suggested that the presence of more Lewis/weak acidic sites in this catalyst was beneficial for the hydrogenolysis step in HDO reaction. Additionally, H2 temperature-programmed reduction studies revealed that the adsorption of H2 was stronger on the Cu/CuFe2O4@C-A than that over the conventional Cu/CuFe2O4@C-B catalyst; thus, the former catalyst promoted activation of H2. A detailed kinetic analysis which demonstrated the lower activation energy barrier along with dual active sites attributed for the activation of the two separate reactions in the HDO process on the Cu/CuFe2O4@C-A catalyst. This work has great implication in developing a highly stable catalyst for the selective upgradation of biomass without deactivation of metal sites in extended catalytic cycles and opens the door of opportunity for developing a sustainably viable catalyst in biomass refinery industries.

Superior Stability and Emission Quantum Yield (23% ± 3%) of Single-Layer 2D Tin Perovskite TEA2 SnI4 via Thiocyanate Passivation.

Tin-based perovskite, which exhibits narrower bandgap and comparable photophysical properties to its lead analogs, is one of the most forward-looking lead-free semiconductor materials. However, the poor oxidative stability of tin perovskite hinders the development toward practical application. In this work, the effect of pseudohalide anions on the stability and emission properties of single-layer 2D tin perovskite nanoplates with chemical formula TEA2 SnI4 (TEA = 2-thiophene-ethylammonium) is reported. The results reveal that ammonium thiocyanate (NH4 SCN) is the most effective additive in enhancing the stability and photoluminescence quantum yield of 2D TEA2 SnI4 (23 ± 3%). X-Ray photoelectron spectroscopic investigations on the thiocyanate passivated TEA2 SnI4 nanoplate show less than a 1% increase of Sn4+ signal upon 30 min exposure to air under ambient conditions (298 K, humidity ≈70%). Furthermore, no noticeable decrease in emission intensity of the nanoplate is observed after 20 h in air. The SCN- passivation during the growth stage of TEA2 SnI4 is proposed to play a crucial role in preventing the oxidation of Sn2+ and hence boosts both stability and photoluminescence yield of tin perovskite nanoplates.

Exploring sorption behaviors of Se(IV) and Se(VI) on Beishan granite: Batch, ATR-FTIR, and XPS investigations

As the important anions in the high-level radioactive waste (HLRW), and the sorption behaviors of Se(VI) and Se(IV) on Beishan granite are important to the safe evaluation and performance assessment of the deep geological repository for HLRW in China. In this study, the sorption behaviors of Se(IV) and Se(VI) on Beishan granite were investigated under different environmental factors combining batch and spectroscopic approaches, such as X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transformed infrared (ATR-FTIR). The results showed that Se(IV) sorption on Beishan granite was much higher than that of Se(VI) mainly due to their molecule charge densities and speciation in solution. The sorption of Se(IV) was independent of ionic strength, suggesting that the inner-sphere complexes (ISCs) was dominant for Se(IV) sorption on granite. However, Se(VI) sorption on Beishan granite was strongly dependent on ionic strength, which indicated that ion exchange and outer-sphere complexes (OSCs) dominated Se(VI) sorption. ATR-FTIR confirmed that Se(IV) and humic acid (HA) could form a soluble complex of HA-Se(IV) in solution. Therefore, under low pH and HA concentration conditions, the presence of HA could enhance Se(IV) sorption; while inhibited Se(IV) sorption on granite to some extent under high pH and HA concentration conditions. In case of Se(VI), the presence of HA greatly reduced the surface charge density of the Beishan granite, which in turn enhanced the electrostatic repulsion of Se(VI) and granite surfaces, thereby inhibiting Se(VI) sorption on granite surface. When Se(IV) coexisted with Eu (III), the sorption of Se(IV) on granite could be enhanced to a large extent, which might be due to the formation of Eu2(SeO3)3 complex with Se(IV) under alkaline condition (consistent with ATR-FTIR), or surface charge modifications of Beishan granite caused by Eu(III) sorption. Owing to the ISCs of Se(IV) on granite, the presence of anions almost no obvious effect on Se(IV) sorption except for SO32− and HPO42− in terms of the competitive sorption, which inhibited Se(IV) sorption to some extent. It was interesting that CO32− could enhance Se(IV) sorption to a large extent, however the mechanism was still not clear. Furthermore, XPS further confirmed that Se(IV) sorption on granite was dominated by Fe(II) and Fe(III), and the host mineral was biotite.

Efficient solar-light-driven photoelectrochemical water oxidation of one-step in-situ synthesized Co-doped g-C3N4 nanolayers

Cobalt-doped g-C3N4 (Co-g-CN) nanolayers were prepared by a single-step thermal treatment with urea and cobalt nitrate. Different amounts of cobalt nitrate were tested to optimize the amount of cobalt dopant in the g-C3N4 (g-CN) matrix. Several characterization methods were used to explore the structural and optical properties along with the photoelectrochemical (PEC) performance. X-ray diffraction and Fourier transform infrared studies confirmed that g-CN nanolayers were successfully doped with cobalt without disturbing the basic 2-D structure and tris-triazine units of g-CN. Furthermore, microscopy images demonstrated that the cobalt effectively transformed the short nanosheets into long nanolayers. The cobalt-doping enhanced the visible absorption of g-CN and tuned the bandgap from 2.71 to 2.62 eV. An X-ray photoelectron spectroscopy (XPS) investigation discovered that cobalt entered into the g-CN network as Co2+ ions. XPS valence band spectra gave information on the modification in the valence and conduction band edge potentials due to cobalt doping. The photoluminescence intensity from the Co-g-CN samples was lesser than that from g-CN nanosheets, and the PEC activity of the Co-g-CN nanolayers was greater than that of as-prepared g-CN nanosheets. Co-g-CN samples prepared with 15 mg of cobalt nitrate hexahydrate showed a PEC performance of 3.2522 mA/cm2, which was greater than that of g-CN nanosheets (1.9246 mA/cm2). The better PEC performance was ascribed to the synergistic consequence of the higher visible absorption obtained by tuning the bandgap and the host–guest interactions between cobalt and g-CN.

Synthesis and characterization of polysulfone–TiO2 decorated MWCNT composite membranes by sonochemical method

This paper presents a new and facile synthesis method for the preparation of composite polymeric membranes based on polysulfone and carbon nanotubes decorated with titanium dioxide. In addition, we also investigate the synthesis of TiO2 decorated carbon nanotubes by sonochemical method. For this synthesis, along with the dispersion of the carbon nanotubes in the polymer solution, the nanotubes are also decorated with titanium dioxide which was added to the solution. Infrared spectroscopy, Raman spectrophotometry, thermal analysis, scanning electron microscopy, and X-ray photoelectron spectroscopy investigations carried out on the interest precursors (carbon nanotubes only and titanium dioxide) reveal the efficiency of the method. Moreover, these results show that the synthesis of novel composite membranes with well-defined structure has a strong potential in thin-film technologies or manufacturing of miniaturized devices. Synthesized membranes were used for photocatalytic degradation of ampicillin and erythromycin.

A high-efficiency electrocatalyst for hydrogen evolution based on tree-like amorphous MoS2 nanostructures prepared by glancing angle deposition

Metal sulfides are commonly used in energy storage and electrocatalysis due to their redox centers and active sites. Here, nanostructured amorphous MoS2 with highly efficient electrocatalytic performance was designed and prepared for hydrogen production using glancing angle deposition (GLAD) magnetron sputtering (MS). A special tree-like MoS2 was obtained under a glancing angle of 5° as demonstrated by the X-ray diffraction (XRD) analysis. This tree-like nanostructure MoS2 exhibited excellent HER activity with a low over potential of 280 ​mV ​at 10 ​mA ​cm−2 and a small Tafel slope of 63 ​mV dec−1. Further, X-ray photoelectron spectroscopy (XPS) investigations revealed the different chemical states of sulfur element. The enhancement of HER performance is attributed to the unsaturated sulfur edges and the increased surface contact area. The results obtained from this study are expected to facilitate the large scale fabrication of sulfide catalysts for HER.

Structurally Modulated Graphitic Carbon Nanofiber and Heteroatom (N,F) Engineering toward Metal-Free ORR Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells

The present study, designates the heteroatoms (N,F) doped various graphitic carbon nanofibers (GNF) viz. GNF-linear segmented platelets, antlers, herringbone type and their structural deformations from pristine fiber with many open-edge active centers as metal-free, cost-effective electrocatalysts for ORR in polymer electrolyte membrane fuel cells. Heteroatoms introduction to GNF frameworks enlarge the lattice spacing between graphene platelets and brings the structural modulation. The developed GNF/N-F catalysts show excellent ORR activity with insensitive to CH3OH and demonstrated outstanding electrochemical potential cycling stability of 10,000 cycles with well-retained ORR kinetics without much loss in the activity. X-ray photoelectron spectroscopy investigation of GNF/N-F catalysts explicit the highly active forms of N (pyridinic, pyrrolic, graphitic-N) and semi-ionic, ionic C-F of F in the catalysts. The deep-rooted synergistic effect among N and F atoms induces to create more active-centres entrenched with extensive C-C bond polarization, larger charge delocalisation with larger spin density differences accomplished in GNF/N-F catalysts. Wide open-edge cavities, opened-tips and many extensively accessible facets collectively enhance the ORR activity of GNF-H/N-F catalyst. The present study unlocks a deep insight towards understanding of advanced metal-free electrocatalysts for efficient ORR in PEMFCs and metal-air batteries.

Improved understanding of the sulfidization mechanism in cerussite flotation: An XPS, ToF-SIMS and FESEM investigation

Sulfidization can greatly improve the flotation performance of lead oxide minerals. To provide deeper knowledge about this process, sulfidization of cerussite was investigated using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and field emission electron microscopy (FESEM). XPS results clearly indicated a transformation of PbCO3 into PbS on the cerussite surfaces during sulfidization, which was consistent with ToF-SIMS results. Further, FESEM images showed that many nanoparticles grew on the cerussite at a high density, suggesting that the sulfidization product on the cerussite was not a monatomic layer but a solid polyatomic layer. The above results confirmed that PbS nanoparticles could grow on most of the surfaces of cerussite under flotation-related conditions, indicating that the sulfidized cerussite had a PbCO3/PbS core-shell structure. The formation of PbS nanoparticles on cerussite involved a heterogeneous nucleation step followed by growth of the nuclei. Based on these studies, a sulfidization process model was finally obtained.

New details of assembling bioactive films from dispersions of amphiphilic molecules on titania surfaces.

Tailoring the surface properties of materials for biomedical applications is important to avoid clinical complications. Forming thin layers of amphiphilic molecules with apolar regions that facilitate attractive intermolecular interactions, can be a suitable and versatile approach to achieve hydrophobic surface modification and provide functional antibacterial properties. Aiming to correlate layer structure and properties starting from film formation, octadecylphosphonic acid (ODPA) and dimethyloctadecyl (3-trimethoxysilylpropyl) ammonium chloride (DMOAP) layers were adsorbed onto smooth titania surfaces. Then the films were studied by atomic force microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS), and their interactions with aqueous environments were characterized by contact angle and zeta potential measurements. In addition, antibacterial assays were performed using E. coli and S. mutants to reveal the antibacterial properties effected by the surface modification. Immediately after sputter deposition, titania was hydrophilic; however, after air storage and adsorption of DMOAP or ODPA, an increase in the water contact angle was observed. XPS investigations after layer formation and after antibacterial tests revealed that the attachment of layers assembled from ODPA on titania substrates is considerably stronger and more stable than that observed for DMOAP films. Heat treatment strongly affects DMOAP layers. Furthermore, DMOAP layers are not stable under biological conditions.

An experimental and theoretical investigation of XPS and NEXAFS of nicotine, nicotinamide, and nicotinc acid

SynopsisThe electronic structures of nicotine, nicotinic acid and nicotinamide have been studied by valence photoemission spectroscopy (PES), core X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorp-tion fine structure (NEXAFS) and interpreted with the aid of quantum chemical calculations. Nicotinamide and nicotinic acid are closely related and show correspondingly similar spectral features, while nicotine is both structurally and spectroscopically diverse.

Synthesis, Characterization, Photocatalytic and Sensing Properties of Mn-Doped ZnO Nanoparticles.

In this paper, Mn-doped ZnO nanoparticles (0 to 10 mol% Mn) were synthesized by facile low-temperature aqueous solution process and characterized by several techniques such as field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-visible and Raman-scattering spectroscopy. The SEM studies confirmed that the synthesized nanoparticles are grown in high density and increase in Mn content was found to have a significant effect on the morphologies of ZnO nanoparticles. The XPS studies established the structural variation of the samples with the change in dopant concentration and its oxidation state. XPS probe the existence of impurity phases in the as-synthesized samples. The results indicate further that hexagonal wurtzite structure of ZnO undergoes distortion with the increase in the dopant concentration. Also, with the increase in the dopant concentration, the blue-shift was observed in the UV-vis. spectra. Photocatalytic and chemicals sensing performances of these nanomaterials have been investigated by subjecting them to photocatalytic degradation of methyl orange (MO) under UV irradiation and for the detection of picric acid (PA) in aqueous solutions. Mn doped ZnO samples were found to be more efficient in catalyzing the MO degradation than pure ZnO. 5 mol% Mn doped ZnO nanomaterials were studied to use as fluorescence sensor for the detection of PA and the observed detection limit was found to be 2.5 μM.

X-ray photoelectron spectroscopy investigation of the valence band offset at beryllium oxide-diamond interfaces

Owing to a large bandgap, extreme thermal-mechanical properties, and close lattice matching, beryllium oxide (BeO) is an ideal substrate and dielectric material for diamond based electronic devices. In this regard, we have utilized X-ray photoemission spectroscopy (XPS) to determine the valence band offset (VBO) between atomic layer deposited (ALD) BeO and nano-crystalline diamond (nc-D). The BeO VBO with nc-D was determined to be 1.8 ± 0.1 eV. Utilizing the reported band gaps for ALD BeO (8.0 eV) and diamond (5.5 eV), the calculated conduction band offset at the BeO/nc-D interface was determined to be 0.7 ± 0.2 eV. The measured BeO/nc-D band offsets are accordingly ideal for high-power, −temperature, −frequency, and nuclear detection device applications based on diamond technology.

Sputtering and nitridation of transition metal surfaces under low energy, steady state nitrogen ion bombardment

Transition metal surfaces exposed to low-energy reactive ions undergo dynamic changes in composition and density due to implantation and compound formation. We report measurements of nitrogen ion induced sputter yields for transition metals relevant to fusion and optics applications. Thin films of molybdenum, ruthenium, palladium and tungsten are bombarded by nitrogen ions of kinetic energies in the range of 50–500 eV at steady state fluences (1 × 1018ions/cm2). Measured sputter yields are explained through energy and momentum transfer under the binary collision approximation using the Monte Carlo code TRIDYN. X-ray Photoelectron spectroscopy (XPS) studies showed the nitrogen content in the films at the end of ion exposure is independent of incoming ion energy. This occurs due to competing implantation and preferential surface nitrogen sputtering processes within the XPS probing depth. All metals investigated showed evidence of a nitride formed due to energetic nitrogen impact. The combination of XPS and TRIDYN simulations were applied to extract effective reaction cross-sections for each metal.