Synchrotron X-ray Photoelectron Spectroscopy Research Articles

Engineering selenium-doped nitrogen-rich carbon nanosheets as anode materials for enhanced Na-Ion storage

Heteroatom-doping is an effective way to regulate the electronic structure of carbon so as to improve their Na-ion storage capability. In all kinds of heteroatoms, selenium (Se)-doping has rarely been investigated even though Se has unique physicochemical properties. Herein, we report a facile fabrication of Se/N-codoped carbon nanosheets (N/Se-CNs) with 10 at% N and 2 at% Se by an oxidation-selenization process. The synchrotron X-ray photoelectron spectroscopy (XPS) measurements indicate that Se doping can significantly change the electronic structures of carbon nanosheets, because Se can act as stronger electron-withdrawing units for Na storage compared with C and N. Therefore, Se can better play the role of improving the conductivity and wettability of the carbon materials. And Se atoms effectively enlarge the interlayer distance of carbon nanosheets to enhance the Na-ion diffusion kinetics and also generate extra active sites for Na-ion storage. Served as anode materials for SIBs, the N/Se-CNs delivers higher specific capacity and better rate capability compared with only N-doped carbon nanosheets. Therefore, Se-doping is an effective way for improvement of Na-storage capability of carbon materials. Moreover, this work should provide a new pathway for design of carbons doped by heteroatoms with large atomic size towards excellent Na-ion storage.

In Situ Identifying the Dynamic Structure behind Activity of Atomically Dispersed Platinum Catalyst toward Hydrogen Evolution Reaction.

Single-atom catalysts (SAs) with the maximum atom utilization and breakthrough activities toward hydrogen evolution reaction (HER) have attracted considerable research interests. Uncovering the nature of single-atom metal centers under operating electrochemical condition is highly significant for improving their catalytic performance, yet is poorly understood in most studies. Herein, Pt single atoms anchoring on the nitrogen-carbon substrate (PtSA /N-C) as a model system are utilized to investigate the dynamic structure of Pt single-atom centers during the HER process. Via in situ/operando synchrotron X-ray absorption spectroscopy and X-ray photoelectron spectroscopy, an intriguing structural reconstruction at atomic level is identified in the PtSA /N-C when it is subjected to the repetitive linear sweep voltammetry and cyclic voltammetry scanning. It demonstrates that the PtN bonding tends to be weakened under cathodic potentials, which induces some Pt single atoms to dynamically aggregate into forming small clusters during the HER reaction. More importantly, experimental evidence and/or indicator is offered to correlate the observed Tafel slope with the dynamic structure of Pt catalysts. This work provides an evident understanding of SAs under electrocatalytic process and offers informative insights into constructing efficient catalysts at atomic level for electrochemical water-splitting system.

Direct Evidence of Electronic Interaction at the Atomic-Layer-Deposited MoS2 Monolayer/SiO2 Interface.

The electronic structure of an atomic-layer-deposited MoS2 monolayer on SiO2 was investigated using X-ray absorption spectroscopy (XAS) and synchrotron X-ray photoelectron spectroscopy (XPS). The angle-dependent evolution of the XAS spectra and the photon-energy-dependent evolution of the XPS spectra were analyzed in detail using an ab initio electronic structure simulation. Although similar to the theoretical spectra of an ideal free-standing MoS2 ML, the experimental spectra exhibit features that are distinct from those of an ideal ML, which can be interpreted as a consequence of S-O van der Waals (vdW) interactions. The strong consensus among the experimental and theoretical spectra suggests that the vdW interactions between MoS2 and adjacent SiO2 layers can influence the electronic structure of the system, manifesting a substantial electronic interaction at the MoS2-SiO2 interface.

Gas Barrier Properties of Chemical Vapor-Deposited Graphene to Oxygen Imparted with Sub-electronvolt Kinetic Energy.

Graphene gas-barrier performance holds great interest from both scientific and technological perspectives. Using in situ synchrotron X-ray photoelectron spectroscopy, we demonstrate that chemical vapor-deposited monolayer graphene loses its gas-barrier performance almost completely when oxygen molecules are imparted with sub-electronvolt kinetic energy but retains its gas-barrier performance when the molecules are not energized. The permeation process is nondestructive. Molecular dynamics-based simulation suggests kinetic energy-mediated chemical reactions catalyzed by common graphene defects as a responsible mechanism.

Tarnished silver–copper surfaces reduction using remote helium plasma at atmospheric pressure studied by means of high-resolution synchrotron x-ray photoelectron microscopy

Previous investigations suggested that sulphide layers on pure silver surfaces were easily removed with a reducing plasma afterglow at atmospheric pressure, but were harder to remove on silver-copper alloys. In this study we investigated by synchrotron x-ray photoelectron spectroscopy the effect of plasma afterglow on tarnished pure silver, pure copper and sterling silver alloy. The results confirmed that plasma was able to reduce in depth all sulphides except Cu2S. The process was explained by standard reduction potentials and allowed to draw a detailed picture of the submicrometric evolution of the tarnished layers.

Adsorption of Aspartic Acid on Ni{100}: A Combined Experimental and Theoretical Study.

Understanding the interaction of amino acids with metal surfaces is essential for the rational design of chiral modifiers able to confer enantioselectivity to metal catalysts. Here, we present an investigation of the adsorption of aspartic acid (Asp) on the Ni{100} surface, using a combination of synchrotron X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure, and density functional theory simulations. Based on the combined analysis of the experimental and simulated data, we can identify the dominant mode of adsorption as a pentadentate configuration with three O atoms at the bridge sites of the surfaces, and the remaining oxygen atom and the amino nitrogen are located on atop sites. From temperature-programmed XPS measurements, it was found that Asp starts decomposing above 400 K, which is significantly higher than typical decomposition temperatures of smaller organic molecules on Ni surfaces. Our results offer valuable insights into understanding the role of Asp as a chiral modifier of nickel catalyst surfaces in enantioselective hydrogenation reactions.

Relationship between the hydroxyl termination and band bending at (2¯01)β−Ga2O3 surfaces

Synchrotron x-ray photoelectron spectroscopy was used to explore the relationship between the hydroxyl termination and band bending at the $(\overline{2}01)$ surface of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ bulk single crystals. All as-received $(\overline{2}01)$ surfaces were terminated with OH groups, with H/OH binding to surface ${\mathrm{O}}_{\mathrm{s}}/\mathrm{G}{\mathrm{a}}_{\mathrm{s}}$ atoms. Removal of this native OH termination produced a large upward shift in band bending of up to 1.0 eV, consistent with strong electron depletion and a semiconductor to insulator-like transition in the near-surface region. Simple surface treatments were used to control the size and stability of the band bending of as-received $(\overline{2}01)$ surfaces by modifying the nature of the OH termination. NaOH (${\mathrm{H}}_{2}\mathrm{S}{\mathrm{O}}_{4}$) treatment consistently produced upward (downward) shifts in band bending and a significant increase (decrease) in the thermal stability of the OH termination that was associated with an increase in the relative density of $\mathrm{G}{\mathrm{a}}_{\mathrm{s}}\text{\ensuremath{-}}\mathrm{OH}$ (${\mathrm{O}}_{\mathrm{s}}\text{\ensuremath{-}}\mathrm{H}$) species. Annealing in wet ${\mathrm{O}}_{2}$ (at 600 \ifmmode^\circ\else\textdegree\fi{}C) produced an extremely stable OH termination and the strongest downward shift in band bending. These effects, combined with the relatively slow dissociation of ${\mathrm{H}}_{2}\mathrm{O}$ on bare $(\overline{2}01)$ surfaces, allowed the preparation of surfaces with significant variations in band bending that may prove useful in optimizing the properties of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ metal-semiconductor contacts and heterojunctions. A comparison of two methods used to determine the absolute band bending at semiconductor surfaces confirmed that bare $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}(\overline{2}01)$ surfaces are characterized by strong upward band bending (\ensuremath{\sim}0.5 to 1.0 eV) and an electron depletion layer that can be completely removed by the hydroxylation of the surface.

Irreversible Structural Dynamics in Bimetallic Pt-Ni Alloy Catalyst Under Alternating Redox Environments

The better understanding on the structural dynamics of a bimetallic catalyst during its interaction with reactive environments is a prerequisite for the development of an efficient catalyst for proton exchange membrane fuel cells (PEMFCs). The main focus should be given to the outermost layers of catalytically active material which are directly involved in catalysis.Herein, we applied the synchrotron radiation photoelectron spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS) techniques to investigate surface chemistry aspects in a PtNi alloy catalyst under alternating oxidation (O2) and reduction (H2) atmospheres at different temperatures that simulate its behavior as a cathode catalyst in PEMFCs. The experimental results are substantiated by theoretical calculations on model PtNi nanoalloys.We showed that PtNi alloy does not maintain its chemical integrity and undergoes surface reconstruction during the switching between oxidizing and reducing conditions in terms of relative surface nickel enrichment. Along with compositional changes catalyst coarsening was observed by atomic force microscopy (AFM).The revealed behavior of the PtNi alloy provides valuable fundamental insights that help to gain advanced understanding of PtNi alloy catalytic activity in real conditions. Figure 1

Stabilizing Polymer-Based Solid-State Electrolytes with 4 V Class Cathodes: From Interface Modification to Electrode Designs

Developing safe solid-state lithium batteries with high-energy-density is important for the application in electric vehicles (EVs). Nowadays, solid-state batteries (SSBs) with polymer-based solid-state electrolyte (SSE) is one of the most promising candidates for the application in EVs. However, SSEs based on polyethylene oxide (PEO) and lithium salt complex delivers poor electrochemical performance in 4 V class high-energy-density cathodes such as lithium cobalt oxide (LiCoO2) and lithium nickel manganese cobalt oxide (NMC). This is because PEO has low electrochemical oxidation window which is lower than 3.8 V vs.Li/Li+.[1] This abstract mainly focus on developing several strategies for stabilizing polymer-based SSEs with 4 V class cathodes including interface modification to electrode designs.Firstly, inspired by previous reports that coating can enhance the cycling performance of SSBs with polymer-based SSE,[2] we applied atomic layer deposition (ALD), a versatile chemical deposition technique which is capable to deposit uniform thin film with controllable thickness at low temperature,[3] for coating cathode to stabilize the polymer-based SSE/4 V class cathode interface under high voltage charge/discharge processes. The coating effect on conductive carbon, active materials, and electrode surface was studied and compared. It is found that carbon is detrimental for the decomposition of polymer-based SSE and the protection of SSE/carbon interface can effectively enhance the performance of 4 V class solid polymer batteries.[4] Secondly, the electrode design is critical for high performance high-energy-density solid polymer batteries. Using freeze-drying method, we had prepared a high-areal-loading cathode for solid polymer batteries with excellent electrochemical performance. This is because the vertically aligned porous electrode can facilitate the Li+ ion transport within the thick electrode.[5] Also, the binder effect in 4 V class solid polymer batteries is carefully studied. Various binders including PEO, polyvinylidene fluoride (PVDF) etc. were applied in 4 V class solid polymer batteries. And the mechanism had been studied by synchrotron X-ray absorption and X-ray photoelectron spectroscopy.[6] References [1] Y. Xia, T. Sakai et al., J. Power Sources, 2001, 92, 234-243[2] H. Miyashiro, M. Wakihara et al., Chem. Mater. 2005, 17, 23, 5603-5605; S. Seki, T Iwahori et al., Chem. Mater. 2005, 17, 8, 2041-2045; Q. Yang, L. Chen et al., J. Power Source, 2018, 388, 65-70.[3] Y. Zhao, K. Zheng, X. Sun, Joule 2018, 2, 1-22; Y. Zhao, X. Sun, ACS Energy Lett. 2018, 3, 899−914; J. Liu and X. Sun, Nanotechnology 2015, 26, 024001;[4] J. Liang, X. Sun et al., J. Mater. Chem. A, 2019, submitted; J. Liang, X. Sun et al., 2019, to be submitted;[5] X. Yang, J. Liang, X. Sun et al., Nano Energy, 2019, 61, 567-575568;[6] J. Liang, X. Sun et al., 2019, to be submitted

A new metalorganic chemical vapor deposition process for MoS2 with a 1,4-diazabutadienyl stabilized molybdenum precursor and elemental sulfur.

Molybdenum disulfide (MoS2) is known for its versatile properties and hence is promising for a wide range of applications. The fabrication of high quality MoS2 either as homogeneous films or as two-dimensional layers on large areas is thus the objective of intense research. Since industry requirements on MoS2 thin films can hardly be matched by established exfoliation fabrication methods, there is an enhanced need for developing new chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes where a rational precursor selection is a crucial step. In this study, a new molybdenum precursor, namely 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(vi) [Mo(NtBu)2(tBu2DAD)], is identified as a potential candidate. The combination of imido and chelating 1,4-diazadieneyl ligand moieties around the molybdenum metal center results in a monomeric compound possessing adequate thermal characteristics relevant for vapor phase deposition applications. Hexagonal MoS2 layers are fabricated in a metalorganic CVD (MOCVD) process with elemental sulfur as the co-reactant at temperatures between 600 °C and 800 °C. The structure and composition of the films are investigated by X-ray diffraction, high resolution transmission electron microscopy, synchrotron X-ray photoelectron spectroscopy and Raman spectroscopy revealing crystalline and stoichiometric MoS2 films. The new MOCVD process developed for MoS2 is highly promising due to its moderate process conditions, scalability and controlled targeted composition.

Adsorption and oxidation of SO2 on the surface of TiO2 nanoparticles: the role of terminal hydroxyl and oxygen vacancy-Ti3+ states.

Herein, the absorption and oxidation reactions of SO2 on TiO2 nanoparticles (TiO2 NPs) at 296 K under various environmental conditions (humidity, UV irradiation, and ozone copresence) were investigated by using a flow chamber reaction system, synchrotron X-ray absorption near-edge structure (XANES) and high resolution synchrotron X-ray photoelectron spectroscopy (XPS) measurements. The results showed that oxidation of SO2 to sulfate via TiO2 NP catalysis happened at a very rapid rate. The appropriate relative humidity, UV irradiation and co-presence of ozone all markedly promoted SO2 oxidation on TiO2 NPs. High resolution XPS unraveled that the terminal hydroxyl (OHt) and oxygen vacancy (VO)-Ti3+ states on TiO2 NPs were the active sites for SO2 adsorption and oxidation. The data of XPS measurements suggest that SO2 was adsorbed on a OHt next to a Ti3+ VO and reacted to form HSO3-. HSO3- can then transform into SO32-via transfer of a proton. The resulting adsorbed SO32- could bind to a surface bridging O (Ob) atom and transform into SO42-. A H2O molecule could dissociate on VO-Ti3+ into two bridging hydroxyl (OHb) groups, subsequently forming new Ob, which provides an active O site for the adsorbed HSO3-/SO32- and oxidizes them into HSO4-/SO42- on the surface of the TiO2 NPs. The copresence of O3 could promote H2O dissociation into OHb, promoting the formation of Ob. The copresence of O3 may also promote the dissociation of adsorbed H2O into TiO2-O2- and hydroxyl radicals (˙OH) on VOs, facilitating the oxidation of adsorbed HSO3-/SO32-. Under UV irradiation, new VOs were created via oxidation of lattice O by photo-generated holes, resulting in increased Ob and subsequently enhanced oxidation of adsorbed HSO3-/SO32- on TiO2 NPs.

Hydrothermal Synthesis of Ruthenium Nanoparticles with a Metallic Core and a Ruthenium Carbide Shell for Low-Temperature Activation of CO2 to Methane.

Ruthenium nanoparticles with a core-shell structure formed by a core of metallic ruthenium and a shell of ruthenium carbide have been synthesized by a mild and easy hydrothermal treatment. The dual structure and composition of the nanoparticles have been determined by synchrotron X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) analysis, and transmission electron microscopy (TEM) imaging. According to depth profile synchrotron XPS and X-ray diffraction (XRD) analysis, metallic ruthenium species predominate in the inner layers of the material, ruthenium carbide species being located on the upper surface layers. The ruthenium carbon catalysts presented herein are able to activate both CO2 and H2, exhibiting exceptional high activity for CO2 hydrogenation at low temperatures (160-200 °C) with 100% selectivity to methane, surpassing by far the most active Ru catalysts reported up to now. On the basis of catalytic studies and isotopic 13CO/12CO2/H2 experiments, the active sites responsible for this unprecedented activity can be associated with surface ruthenium carbide (RuC) species, which enable CO2 activation and transformation to methane via a direct CO2 hydrogenation mechanism. Both the high activity and the absence of CO in the gas effluent confer relevance to these catalysts for the Sabatier reaction, a chemical process with renewed interest for storing surplus renewable energy in the form of methane.

New interpretation and approach to curve fitting synchrotron X-ray photoelectron spectra of (Fe,Ni)9S8 fracture surfaces

The surface electronic structure of (Fe,Ni)9S8 was investigated using synchrotron X-ray photoelectron spectroscopy (SXPS). SXPS S 2p and valence band (VB) spectra, collected as a function of photon energy, have resolved two distinct bulk states and two surface chemical states present at (Fe,Ni)9S8 fracture surfaces. The SXPS S 2p spectra show two distinct sulfur sites within the lattice that have been attributed to 4-coordinate sulfur at 161.7 eV, and 5-coordinate sulfur at 162.2 eV. Upon fracture, sulfur sites are exposed at multiple fracture planes and lose at least one bonding partner, resulting in surface 3-coordinate sulfur at 161.1 eV, and a slightly broader peak at 161.7 eV attributed to 4-coordinate sulfur surface monomer species overlapping the bulk 4-coordinate species. Significant asymmetric tail shapes were observed in each of the S 2p spectra and have been identified as ligand-to-metal charge transfer satellites (LMCT). A practical approach for curve fitting the (Fe,Ni)9S8 XPS spectra has been proposed.

Role of Valence Band States and Plasmonic Enhancement in Electron-Transfer-Induced Transformation of Nitrothiophenol.

Hot-electron-induced reactions are more and more recognized as a critical and ubiquitous reaction in heterogeneous catalysis. However, the kinetics of these reactions is still poorly understood, which is also due to the complexity of plasmonic nanostructures. We determined the reaction rates of the hot-electron-mediated reaction of 4-nitrothiophenol (NTP) on gold nanoparticles (AuNPs) using fractal kinetics as a function of the laser wavelength and compared them with the plasmonic enhancement of the system. The reaction rates can be only partially explained by the plasmonic response of the NPs. Hence, synchrotron X-ray photoelectron spectroscopy (XPS) measurements of isolated NTP-capped AuNP clusters have been performed for the first time. In this way, it was possible to determine the work function and the accessible valence band states of the NP systems. The results show that besides the plasmonic enhancement, the reaction rates are strongly influenced by the local density of the available electronic states of the system.

Mechanistic Study of Environmentally-Friendly Zr-Based and Hybrid Nano-Coating Surface Conversion Treatment

Chemical conversion coatings can improve both anti-corrosion and adhesion properties in a wide range of applications. Due to the environmental and health issues of traditional chromate and phosphate coatings, more research has been carried out to develop environment-friendly and cost-effective chemical conversion coating. In this study, the coating formation mechanism of zirconium-based conversion coating and zirconium-polymer-based hybrid coating applied on carbon steel has been investigated. For zirconium-based coating, the kinetics of coating formation was characterized by the growth of the cluster size, as assessed by high-resolution SEM imaging; the evolution of the chemical composition was studied by Synchrotron X-ray Absorption Near Edge Spectroscopy (XANES) and X-ray Photoelectron Spectroscopy (XPS). Furthermore, the processing-structure-property correlation of an improved zirconium-based organic-inorganic “hybrid” coating was investigated. In both systems, corrosion tests have been carried out to understand the potential effects of post-coating processes. Overall, the study sheds light on the mechanism of the zirconium-based and hybrid coating formation, advancing the development of novel anti-corrosion technologies.

Oxalato complexes of Pd(II) with Co(II) and Ni(II) as single-source precursors for bimetallic nanoalloys

The single-source precursors of noble–ignoble nanoalloys are of a great interest since they can be used in many areas such as optical, biomedical, catalysis, magnetic and chemosensors. The new water-soluble double complex salts (DCSs) with aqua and oxalato ligands, [M(H2O)6][Pd(Ox)2]·nH2O (M = Co2+, Ni2+; n = 2, 4; Ox = C2O 4 2− ), were obtained, structurally characterized (single-crystal XRD) and used as precursors for M–Pd nanoalloys. Complexes, intermediate and final thermolysis products were studied by simultaneous thermogravimetric analysis combined with mass spectrometry, powder XRD, in situ synchrotron powder XRD, IR and X-ray photoelectron spectroscopy. Decomposition in different atmospheres leads to formation of bimetallic solid solution M0.5Pd0.5 (He, H2) or a mixture of metal oxides (O2). The catalysts M–Pd/TiO2 (1% Pd) were prepared by the decomposition of DCSs deposited onto titania via wetness impregnation. The H2-made Ni-containing samples showed a high photocatalytic activity in CO oxidation reaction.

Achieving High-Temperature Stability of Metastable α-MoC1-x by Suppressing Phase Transformation with Mounted Atoms for Lithium Storage Performance.

Despite a significant advancement in preparing metastable materials, one common problem is the strict and precious reaction conditions due to their metastable structures. Herein, we achieved the preparation of high-temperature stabilized metastable α-MoC1-x by mounting zinc atoms into its lattice structure. Such a structural construction could suppress the phase transformation from α-MoC1-x to β-Mo2 C through restricting the displacement of Mo atoms upon increased temperature. The resultant metastable α-MoC1-x can be stabilized up to 1000 °C and this stability temperature is the highest for the metastable α-MoC1-x so far. Synchrotron X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) confirm the structure of Zn-mounted α-MoC1-x . Density functional theory (DFT) calculations reveal that the introduction of the Zn atoms in the lattice structure of α-MoC1-x could significantly decrease the energy difference (ΔE) between α-MoC1-x and β-Mo2 C, thus effectively suppressing the phase transformation from α-MoC1-x to β-Mo2 C and accordingly maintaining the high-temperature stability of α-MoC1-x . This novel strategy can be used as a universal method to be extended to synthesize metastable α-MoC1-x from different precursors or other mounted elements. Moreover, the optimal product exhibits excellent lithium storage performances in terms of the cycling stability and rate performance.

Unusual oxidation-induced core-level shifts at the HfO2/InP interface

X-ray photoelectron spectroscopy (XPS) is one of the most used methods in a diverse field of materials science and engineering. The elemental core-level binding energies (BE) and core-level shifts (CLS) are determined and interpreted in the XPS. Oxidation is commonly considered to increase the BE of the core electrons of metal and semiconductor elements (i.e., positive BE shift due to O bonds), because valence electron charge density moves toward electronegative O atoms in the intuitive charge-transfer model. Here we demonstrate that this BE hypothesis is not generally valid by presenting XPS spectra and a consistent model of atomic processes occurring at HfO2/InP interface including negative In CLSs. It is shown theoretically for abrupt HfO2/InP model structures that there is no correlation between the In CLSs and the number of oxygen neighbors. However, the P CLSs can be estimated using the number of close O neighbors. First native oxide model interfaces for III-V semiconductors are introduced. The results obtained from ab initio calculations and synchrotron XPS measurements emphasize the importance of complementary analyses in various academic and industrial investigations where CLSs are at the heart of advancing knowledge.

Controlling the Valence State of Cu Dopant in α-Fe2O3 Anodes: Effects on Crystal Structure and the Conversion Reactions with Alkali Ions

Doping is one of the most important ways to tailor the performance of energy materials. However, the crystal structure of doped materials is usually oversimplified as a simple substitution of dopants. Here, we characterized the doped α-Fe2O3 with different Cu cations using synchrotron X-ray diffraction, X-ray absorption, and X-ray photoelectron spectroscopy, and electrochemically evaluated it as an anode in lithium batteries. The results suggest that doping is not the simple replacement of Fe3+ sites by Cu2+ or Cu+ but induces a complex local structure change, which may be a characteristic of this class of materials. In Cu+-doped samples, Cu+ not only replaces the Fe3+ site and distorts the FeO6 octahedra, but also gives rise to oxygen vacancies in CuO6 octahedra in the bulk structure and peroxides at the surface, leading to uniform precipitation of Cu as a conductive and buffering agent. These CuO6 octahedra also facilitate homogeneous reactions (electrochemical reduction of Cu+ and Fe3+ together) and th...

The effect of covalently bonded aryl layers on the band bending and electron density of SnO2 surfaces probed by synchrotron X-ray photoelectron spectroscopy.

Tin(iv) dioxide (SnO2) is a technologically important transparent conducting oxide with high chemical stability. In air, the SnO2 surface is terminated with hydroxyl groups which cause the electronic bands to bend downward at the surface capturing a two-dimensional surface electron accumulation layer (SEAL). The SEAL promotes adsorption at the surface, giving environmentally-sensitive electronic properties; this sensitivity is a barrier to some potential applications of the material. This work investigates surface modification of SnO2via reaction with an aryldiazonium salt as a route to controlling the surface band bending. We compare the surface layers formed by reaction at open-circuit potential and under potential control of 4-(trifluoromethyl)benzene diazonium ions with moderately- and highly-doped (101) SnO2 thin films grown by plasma-assisted molecular beam epitaxy. Atomic force microscopy and synchrotron X-ray photoelectron spectroscopy (XPS) measurements demonstrate that both reaction conditions lead to covalently-attached 4-(trifluoromethyl)phenyl groups, with grafting at open-circuit potential giving thinner layers (<2 nm) and fewer direct bonds to the surface than electrografting (layer thickness >3 nm). Valence band investigations show that for all samples the 4-(trifluoromethyl)phenyl layers decrease the surface downward band bending with the greatest effect observed for the electrografted sample. In the latter case, a +0.29 eV shift in band bending relative to that of the unmodified material indicates the success in turning the surface electron accumulation layer into a depletion layer.