An XPS study of the UV photoreduction of transition and noble metal oxides

The photoreduction of polycrystalline samples of MoO₃, WO₃, Cu₂O, CuO, PdO and PtO by UV irradiation was investigated by X-ray photoelectron spectroscopy (XPS). Photoreduction of these metal oxides was found upon irradiation with the emission lines from a mercury vapor lamp (2537 Å) at room temperature. Other oxides, like ZnO and V₂O₅, where not photoreduced. Mo⁶⁺ and W⁶⁺ were observed to partially photoreduce to Mo⁵⁺ and W⁵⁺. In the case of the copper oxides, both Cu²⁺ and Cu¹⁺ were found to completely photoreduce to the metallic state. The photoreduction of PdO and PtO was observed to proceed slowly through the first few monolayers and was 75% completed within the XPS sampling depth ( ∼ 40 Å) after long irradiation times. All photoreduced species were observed to reoxidize upon exposure to the atmosphere. The oxidation of photoreduced metallic Pt and Pd at room temperature is unique since those metals usually do not oxidize in air. A model has been developed to explain and predict the extent of reduction of metal oxides surfaces by UV irradiation. It shows that depending on band gap and chemical stability, complete or partial reduction of many oxide surfaces can be achieved.     

Chemically prepared oxides on Si(001): an XPS study

Thin oxide layers prepared on Si(001) by various chemical treatments have been investigated using X-ray photoelectron spectroscopy. The oxides are prepared by treatment with HCl, H₂SO₄ and HNO₃. The O 1s spectra consists of two peaks at 532.6 eV (low binding energy component, LBC) and 533.6 eV (high binding energy component, HBC) in all the cases. The LBC is assigned to a monatomic bridging oxygen, Si---O---Si and HBC to a monatomic non-bridging oxygen, Si---O species. Conversion of Si---O to Si---O---Si takes place on annealing the sample. The concentration of Si---O species is low in the case of HCl oxide. In in situ prepared oxide, the O 1s is predominantly due to Si---O---Si.     

钚氧化物的分子结构和分子光谱研究

用杂化密度泛函(B3LYP)方法,Pu原子采用相对论有效原子实势(RECP),O原子采用全电子6-311g(d)基组优化了PuO,PuO2,Pu2O3的分子结构,得到了相应的平衡几何构型,并计算了红外振动频率(IR)、Raman光谱.结果表明:PuO,PuO2分子基态几何构型和振动频率与实验值相符.对Pu2O3分子可能的构型和多重性进行结构优化,发现Pu2O3分子基态为11B2的C2v构型,给出了Pu2O3分子基态结构的红外和拉曼光谱数据、力常数等系列数据,并对振动频率的峰值进行了指认.通过自然键轨道(NBO)分析,发现由钚到氧的电荷转移.相对于PuO和PuO2分子,在Pu2O3中形成较弱的Pu—O键.分析自旋布居,发现在这些分子中,自旋磁动量大都由Pu原子的5f电子贡献,而氧原子的2p轨道往往贡献反平行的自旋.     

Optical basicity of metal oxides and XPS measurement

A semi-empirical relationship between XPS measured oxygen Is binding energy and the optical basicity is developed.     

New interpretations of XPS spectra of nickel metal and oxides

A current interpretation of XPS spectra of Ni metal assumes that the main 6 eV satellite is due to a two hole c3d⁹4s² (c is a core hole) final state effect. We report REELS observation in AES at low voltages of losses (plasmons and inter-band transitions) corresponding to the satellite structures in Ni metal 2p spectra. The satellite near 6 eV is attributed to a predominant surface plasmon loss. A current interpretation of Ni 2p spectra of oxides and other compounds is based on charge transfer assignments of the main peak at 854.6 eV and the broad satellite centred at around 861 eV to the cd⁹L and the unscreened cd⁸ final-state configurations, respectively (L is a ligand hole). Multiplet splittings have been shown to be necessary for assignment of Fe 2p and Cr 2p spectral profiles and chemical states. The assignments of Ni 2p states are re-examined with intra-atomic multiplet envelopes applied to Ni(OH)₂, NiOOH and NiO spectra. It is shown that the free ion multiplet envelopes for Ni²⁺ and Ni³⁺ simulate the main line and satellite structures for Ni(OH)₂ and NiOOH. Fitting the NiO Ni 2p spectral profile is not as straightforward as the hydroxide and oxyhydroxide. It may involve contributions from inter-atomic, non-local electronic coupling and screening effects with multiplet structures significantly different from the free ions as found for MnO. A scheme for fitting these spectra using multiplet envelopes is proposed.     

A qualitative and quantitative study of the oxides of aluminum and silicon using AES and XPS

Experimental AES and XPS results from powdered α-Al₂O₃, SiO₂, Al₂Si₂O₇·2H₂O and an aluminosilicate glass are presented. The AES kinetic energy and the XPS binding energy values of core-level states and the peak shapes of X-ray induced O KLL Auger transitions are presented as qualitative reference data for the purpose of aiding in the identification of unknown aluminum oxide/silicon oxide samples. The AES peak-to-peak heights and the XPS peak areas of core-level states are presented as quantitative data and are used in the calculation of empirical relative sensitivity factor values. In addition, the sensitivity factor values are given parameters according to a single variable, f₈, defined as the ratio of the volume density of the oxygen atoms to the sum of the volume densities of the aluminum and silicon atoms.     

Non-stoichiometric surface behaviour of LaMO3 oxides as evidenced by XPS

The non-stoichiometry of the surface of LaMO₃ perovskites was found to be much more marked than that in the bulk. In general, the non-stoichiometric character of these oxides changes from oxidative to reductive when passing from LaCrO₃ to LaNiO₃. This may be related to the reducibility of these compounds.     

The iron oxidation state in Mg-Al-Fe mixed oxides derived from layered double hydroxides: An XPS study

Mixed oxides with large surface area and high thermal stability can be obtained by thermal treatment of the layered double hydroxides (LDH). Mg-Al-Fe mixed oxide samples with varying Mg/Al ratio and 5 mol.% of Fe were prepared in this way and the iron oxidation state (Fe_ox) in these compounds was studied by X-ray photoelectron spectroscopy (XPS), using a calibration based on the relation of Fe_ox to the splitting between the O 1s and Fe 2p_3/2 centroids. The XPS results confirm Fe³⁺ as a dominant oxidation state in the studied mixed oxides. A vacuum-induced reduction of iron in the Fe₂O₃ and Mg-Al-Fe oxide samples has been observed and an influence of the Mg:Al ratio on this effect in mixed oxides has been detected. The role of the local variations of the electron density distribution in the close neighbourhood of the surface oxygen atoms in the mixed oxides in the reduction processes is discussed.     

High-pressure study of neptunium and plutonium compounds

Abstract

Neptunium and plutonium monopnictides and monochalcogenides were studied by x-ray diffraction at pressures up to 57 GPa. All of them exhibit structural phase transitions under pressure. The arsenides and tellurides have a CsCl (B2) type high-pressure structure. Sb as an anion favours a tetragonal high-pressure structure. The compressibilities were determined for all of the compounds studied. The results are compared to those obtained for the corresponding thorium and uranium compounds.     

A density functional study of plutonium dioxide

The electronic and geometric structures of bulk PuO₂ and its (110) surface have been studied using a periodic model within the generalized gradient approximation (GGA) of density functional theory (DFT). The sixty core electrons of the Pu atom have been represented by a relativistic effective core potential and scalar relativistic effects have been incorporated on the valence orbitals. For bulk PuO₂, we predict an equilibrium lattice constant of 10.10 a.u. and a cohesive energy of 17.28 eV, in good agreement with experimental data. For the (110) surface, upon relaxation, the distance between the top layer and the next layer is found to decrease by 0.12 ?, i.e. 5.3% of the corresponding interlayer distance in the bulk. The distance between the two oxygen atoms on the top layer is found to increase by 0.15 ?, i.e. 5.6% of the corresponding bulk value. The small surface relaxation energy of 0.268 eV per unit cell indicates the fair stability of this surface. The effective charges on Pu and O atoms show that the chemical bonding in this system is not purely ionic. Together with the metallic feature of the density of states (DOS) on the surface, the effective charge distribution provides some basis for understanding surface reactivity and corresponding support for catalysis. Received 16 June 2000     

XPS study of WC formation process

The surface composition and chemical bonding state of WO₃ carburized in CO at 700°C have been investigated with X-ray photoelectron spectroscopy, in order to clarify the mechanism of WC formation. The experimental results show that the carburization process is based on a direct reaction of WO_x (x≦3) with free carbon which accumulates on the WO_x surface by the disproportionation of CO. A model for the WC formation process is also presented.     

XPS study of cadmium oxidation in air

The progressive oxidation of polycrystalline cadmium in air has been followed by XPS. In all the analyzed range of air exposures, two components have been observed in the O1s peak. The first one has been assigned to oxide, while the second one is probably due to hydroxide. Up to 5000 L air exposure, the two species are formed and grow simultaneously, with CdO as the main component.     

XPS study of fluorinated carbon multi-walled nanotubes

Arc-produced carbon multi-walled nanotubes (MWNTs) were fluorinated at 420 °C in a flow of diluted F₂ gas containing small admixture of HF gas. Fluorinated materials (F-MWNTs) with 10–55 wt.% fluorine content were studied by XPS. It was shown that fluorination begins at the external layers of nanotubes and the reaction front propagates inside the multi-layer particles in concert with structural deterioration of graphene layers. The C₂F stoichiometry still allows MWNT wall integrity, similar to known for SWNTs. The fluorine contents in the product can noticeably exceed this higher fluorine limit for tube stability. The position of the F 1s line at 688.2 eV does not depend on the fluorine concentration. Nearly covalent C–F bonds dominate the F-MWNT samples, with a small quantity (2–9%) of ionic bonds also present. Fluorinated carbon tends to spatially separate from non-fluorinated carbon.     

ToF-SIMS and XPS study of ancient papers

The surface composition of 18th century papers was investigated by means of ToF-SIMS and XPS. The aim of the present study was to explore the possibility of using these surface sensitive methods to obtain information which can help to determine the manufacturing process, provenance and state of conservation of ancient papers. The ToF-SIMS results indicate that the analyzed papers were sized by gelatin and that alum was added as hardening agent. The paper sheets produced in near geographical areas but in different paper mills exhibit a similar surface composition and morphology of the fibers as shown by the ToF-SIMS measurements. The ToF-SIMS and the XPS results indicate that a significant fraction of the cellulose fibers is not covered by the gelatin layer. This was observed for the ancient papers and for a modern handmade paper manufactured according to the old recipes.     

XPS study of some selected selenium compounds

X-ray photoelectron spectroscopy (XPS) of the Se 3d peak for Se in Se, SeO₂, As₂Se₃, ZnSe, CdSe, CoSe, FeSe, CrSe, VSe₂ and TiSe were studied. It was found that, firstly, in the case of As₂Se₃, ZnSe, CdSe, CoSe and CrSe, as the change in electronegativity of the anion increases, the change in the binding energy value of the Se 3d peak also increases and, secondly, in all of the Se compounds studied, the Se-Se bond in pure Se is stronger than the Se-x bond, where X = As, Zn, Cd, Co, Fe, Cr, V and Ti.     

An XPS study of hafnium nitride films

XPS spectra are presented from films of different stoichiometry: HfN_1.0 made by CVD and reactive sputtering and HfN_1.09 made by ion plating. They support the existing theoretical calculations where the valence band comprises a strong Hf 5d band immediately below E_F and a band of mainly N 2p character some 6 eV below E_F. The core levels are shifted to larger binding energies by about 1.5 eV indicating electron transfer from hafnium to nitrogen. The effect of substoichiometry on the spectra is very small.     

Formulation for XPS spectral change of oxides by ion bombardment as a function of sputtering time

XPS measurement revealed that the original state of TiO₂ was changed to Ti₂O₃ and TiO by ion bombardment. TiO₂ decreased and Ti₂O₃ increased at the initial stage. TiO increased at a later stage than Ti₂O₃. Each of them saturated after enough sputtering time.A formulation was proposed in order to explain the change of XPS spectra for oxides as a function of ion sputtering time. This formulation was based on reaction equations that contain two reduction processes (from TiO₂ to Ti₂O₃ and from Ti₂O₃ to TiO), and sputtering effects. Using four fitting parameters (two reduction coefficients, sputtering yield and information depth), the present formula was fitted to the experimental results. The fitting results agree satisfactorily with the experimental results. The calculation shows that the reduction coefficient from TiO₂ to Ti₂O₃ is about ten times larger than that from Ti₂O₃ to TiO. This calculation predicts that surface composition of an oxide that is changed by ion bombardment will reach a different value depending on its bulk composition. Moreover, the present formulation can determine the chemical states of compounds changed by ion bombardment.     

Specific contact resistance of IGZO thin film transistors with metallic and transparent conductive oxides electrodes and XPS study of the contact/semiconductor interfaces

In this work, the specific contact resistance (ρ_c) between amorphous indium-gallium-zinc-oxide (IGZO) semiconductor and different contact electrodes was obtained from thin film transistors (TFTs). Ti/Au (10/100 nm), aluminum doped zinc oxide (AZO, 100 nm) and indium tin oxide (ITO, 100 nm) were used as source/drain electrodes to fabricate IGZO TFTs. Chemical states of the contacts/semiconductor interfaces were examined by depth profile X-ray photoelectron spectroscopy (XPS) analysis to explain the origin of the differences on specific contact resistance. The lowest ρ_c achieved using Ti/Au was related to the formation of a TiO_x interlayer due to oxygen atoms diffusing out from the semiconductor under layer, increasing the carrier concentration of IGZO at the interface and lowering the ρ_c. On the contrary, no interfacial reactions were observed between IGZO and AZO or ITO source/drain. However, IGZO resistivity increased with ITO contacts likely due to oxygen vacancies filling during ITO deposition. This fact seems to be the origin of the high contact resistance between IGZO and ITO, compared to IGZO-AZO and IGZO-Ti/Au interfaces.