M. A. Langell

Analysis of the NiCo2O4 spinel surface with Auger and X-ray photoelectron spectroscopy

Abstract Nickel cobaltite, NiCo 2 O 4 , has been synthesized by sol–gel and thermal decomposition techniques and the surface composition studied with Auger (AES) and X-ray photoelectron spectroscopies (XPS). The as-introduced samples are near-stoichiometric, although samples fabricated by thermal decomposition tend to be oxygen-deficient by approximately 25% of the predicted spinel concentration. Nickel 2p XPS indicates the predominant form of the metal to be Ni 2+ , with the cations located in octahedral sites. The cobalt cations are equally divided between tetrahedral and octahedral sites as Co 3+ . The oxygen XP 1s spectrum is composed of two peaks, the main lattice peak at 529.6 eV and a component at 531.2 eV with about 40% of the total O 1s intensity in stoichiometric samples. While the possibility of hydroxyl contaminants cannot be discounted, most of the intensity in the 531.2 eV peak is believed to be intrinsic to the NiCo 2 O 4 surface. Heating the cobaltite in ultrahigh vacuum (UHV) results in surface reduction, with the largest changes apparent in the Co 2p XP spectrum, which shows clear signs of reduction to octahedral Co 2+ . Changes brought about by the reduction are not reversible, and although it is possible to reoxidize the material, the surface undergoes phase separation to Co 3 O 4 and Ni x Co 1− x O.

Initial stages of hydrogen reduction of NiO(100)

Abstract The reduction of single crystal NiO(100) under hydrogen has been followed by AES, XPS and LEED for the pressure range of 1.0 × 10 −7 to 1.3 × 10 −6 Torr and for substrate temperatures of 150–350°C. The kinetics of reduction are controlled both by the rate of removal of lattice oxide at the surface and by the diffusion of subsurface oxygen to the oxygen-depleted surface. The rate of oxygen removal is first-order in surface oxide concentration and in hydrogen pressure. An induction period precedes the reduction reaction and its length is postulated to be controlled by surface defect concentration. The stoichiometric and reduced lattice oxygen species appear to be chemically identical and give a single symmetric XPS peak at 529.4 eV. Nickel spectra indicate a shift in XPS binding energies from those expected of the oxide to those of nickel metal early in the reduction process, although LEED indicates the NiO(100) surface lattice to remain the stable structure for surface reduced to approximately 20% of the stoichiometric oxygen concentration. Ni(100) island formation is observed, with Ni 〈010〉 and 〈001〉 directions along the NiO 〈010〉 and 〈001〉, respectively, but only after the NiO surface is severely depleted in oxygen.

Surface composition and structure of Co3O4(110) and the effect of impurity segregation

The Co3O4(110) single crystal surface has been characterized by low energy electron diffraction (LEED), Auger electron spectroscopy, and x-ray photoelectron spectroscopy (XPS). LEED analysis of the clean Co3O4(110) spinel surface shows a well-ordered pattern with sharp diffraction features. The XPS spectra are consistent with stoichiometric Co3O4 as determined by the concentration ratio of oxygen to cobalt (CO/CCo) and spectral peak shape. In particular, the cobalt 2p XPS spectra are characteristic of the spinel structure with Co3+ occupying octahedral sites and Co2+ in tetrahedral sites within the lattice. During prolonged heating at 630 K, bulk impurities of K, Ca, Na, and Cu segregated to the surface. Sodium desorbed from the surface as NaOH at 825 K, potassium and calcium were only removed by sputtering since no desorption from the surface was detected for temperatures up to 1000 K. Copper also disappeared upon heating above 700 K, most likely by desorbing although the possibility of diffusion back in...

Epitaxial growth of Co3O4 on CoO(100)

Under mildly oxidizing ultrahigh vacuum conditions, it is possible to form on top of CoO(100) single crystal substrates, thin films that have higher oxygen content but that preserve the overall symmetry of the CoO(100) low‐energy electron diffraction pattern. X‐ray photoelectron spectroscopy (XPS) and high‐resolution electron‐energy‐loss spectroscopy (HREELS) data indicate that the epitaxial film grown on CoO(100) at 625 K and 5×10−7 Torr is Co3O4‐like in both oxygen content and XP/HREEL spectroscopic characteristics. Both materials are closest packed in lattice oxygen, with the mismatch of bulk O2−–O2− distances of approximately 5%. However, the Co3O4 is only able to grow to a thickness of approximately 5 A before the oxidation process halts. It is proposed that the orientation of Co3O4 that forms most readily on the CoO(100) surface does not present a thermodynamically stable orientation of the bulk Co3O4 substrate but is that which grows under the constraint of the best CoO(100)/Co3O4 epitaxial arrange...

Thermal decomposition reactions of acetaldehyde and acetone on Si(100)

We have studied the thermal interactions of acetone and acetaldehyde on Si(100), both sputtered and annealed, using high resolution electron energy loss spectroscopy, (HREELS), x-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). There is no carbonyl stretch in HREELS and the C and O(1s) XPS peaks reflect two different carbonyl processes, one involving bond cleavage, the other a reduction of the C–O bond order. Hydrogen TPD gives a peak at 840–850 K which is as much as threefold more intense than from H-saturated Si(100). SiO desorbs near 1050 K and XPS shows total loss of oxygen and retention of carbon. Approximately 34% of the acetaldehyde monolayer and 62% of the acetone monolayer decomposes on annealed Si(100) to produce silicon carbide. In contrast, after sputtering with 500 eV Ar ions, these percentages are reduced to 14% and 25%, respectively. We conclude that Si dimers play an important role in the chemistry of carbonyl groups.We have studied the thermal interactions of acetone and acetaldehyde on Si(100), both sputtered and annealed, using high resolution electron energy loss spectroscopy, (HREELS), x-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). There is no carbonyl stretch in HREELS and the C and O(1s) XPS peaks reflect two different carbonyl processes, one involving bond cleavage, the other a reduction of the C–O bond order. Hydrogen TPD gives a peak at 840–850 K which is as much as threefold more intense than from H-saturated Si(100). SiO desorbs near 1050 K and XPS shows total loss of oxygen and retention of carbon. Approximately 34% of the acetaldehyde monolayer and 62% of the acetone monolayer decomposes on annealed Si(100) to produce silicon carbide. In contrast, after sputtering with 500 eV Ar ions, these percentages are reduced to 14% and 25%, respectively. We conclude that Si dimers play an important role in the chemistry of carbonyl groups.

Adsorption of acetic acid on hydroxylated NiO(111) thin films

Abstract Ni(100) has been exposed to O 2 to form ∼3 monolayer crystalline NiO(111) thin films and the films have been characterized by HREELS, XPS, AES and acetic acid adsorption. The NiO(111) is Ni 2+ -polar and contains approximately 65% of the surface nickel sites terminated by isolated OH with substantial ionic character. The hydroxyls appear to serve a stabilizing role for the thermodynamically metastable NiO(111) surface and also to provide a condensation mechanism for interaction with the acetic acid. While there are Ni 2+ OH ads surface sites available for adsorption, at least some of the hydroxyls associated with the NiO(111) film are occluded or otherwise inaccessible. The XP and HREEL spectral data support an assignment of adsorbate formation to a bridging acetate species.

THE COMPOSITION AND STRUCTURE OF OXIDE FILMS GROWN ON THE (110) CRYSTAL FACE OF IRON

Epitaxial oxide layers of Fe3O4‐like composition and symmetry have grown on Fe(110) crystals exposed to oxygen gas. The epitaxial relationship between the oxide and the Fe(110) substrate is elucidated and the orientation of the oxide film has been explained in terms of the close registry between the Fe(110) and Fe3O4(111) crystal lattices. Both the structure and composition of the iron oxide epitaxies are a function of substrate temperature and oxygen pressure, and oxide formation has been observed over only part of the temperature–pressure range investigated. While oxide surfaces with Fe2O3‐like composition have been observed, they are disordered and are only approximately one monolayer in thickness.

High resolution electron energy loss spectroscopy of MnO(100) and oxidized MnO(100)

Single crystal MnO(100) substrates can be selectively oxidized to produce Mn2O3‐ and Mn3O4‐like surfaces under mild oxidation/reduction conditions readily accessed under ultrahigh vacuum (UHV). MnO(100) yields a characteristic Mn 2p x‐ray photoelectron spectroscopy (XPS) satellite structure and appropriate O/Mn concentrations from O 1s/Mn 2p XPS intensity ratios. Its high resolution electron energy loss (HREEL) spectrum shows a series of Fuchs–Kliewer multiple phonon excitations with a single loss energy of 70.9 meV, characteristic of the cubic manganese monoxide structure. However, the HREEL spectral (HREELS) background is high and the phonons are not as well resolved as those typically observed on comparable metal monoxides. Annealing the MnO(100) substrate at 625 K and 5×10−7 Torr O2 slowly forms Mn2O3, as indicated by O 1s and Mn 2p XPS, and does so without significantly altering the symmetry of the MnO(100) low energy electron diffraction pattern. The MnO(100)‐Mn2O3 surface can be selectively reduced...

Preferential sputtering in the 3d transition metal monoxides

Sputtering with 2 keV Ar + has been found to result in preferential oxygen removal for NiO(100) and CoO(100) single crystals but not for the thermodynamically more stable MnO(100) sample. Substrate temperatures that are only slightly elevated from room temperature (≳350 K) are required to effect sample reduction, a fact which may explain conflicting reports of congruent and preferential sputtering for NiO and CoO in the literature. The substrate temperature affects both the steady state surface composition of the oxide and the ion fluence to reach steady state. The kinetics of sample reduction is analyzed for NiO and can be described as a balance between the sputter rate and the rate of oxygen diffusion from subsurface layers. During Ar + reduction of the NiO(100) surface, zero valent nickel builds up at the crystal surface and coalesces into ordered nickel islands for surface concentrations reduced to approximately 25% of the stoichiometric value and below. Two additional oxygen species are also observed for extensively bombarded NiO samples.

Carboxylic acid adsorption on NiO(100) characterized by X-ray photoelectron and high resolution electron energy loss spectroscopies

Formic, acetic and propionic acids have been adsorbed onto NiO(100) at 300 K and the resulting species characterized by X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS). As do many ionic solids, nickel oxide possesses a strong series of Fuchs-Kliewer multiple phonon losses, which obscures the weaker adsorbate vibrational structure. A novel phonon deconvolution procedure that removes multiple phonons has, therefore, been used in analyzing the HREELS data. NiO(100) exhibits amphoteric chemisorptive properties, dissociatively adsorbing the acids as carboxylates and surface hydroxyls. Adsorption saturates after approximately 10000 langmuir to yield one carboxylate for every two nickel sites. By stoichiometry, one half of all surface oxygen sites are assumed to be hydroxylated. A tilted carboxylate geometry is evident in the high value of thevs(COO) vibrational mode observed in the HREEL spectra.