F. Thieme

CO adsorption studies on pure and Ni-covered Cu(111) surfaces

Abstract The adsorption of CO on pure and Ni-covered Cu(111) surfaces has been studied by means of LEED, TDS, UPS and work function measurements during adsorption and desorption. Different Ni-coverages between 0.1 and 2 monolayers were obtained by Ni-evaporation controlled by a quartz micro balance and by AES. Near room temperature Ni grows in a layer-by-layer mode on Cu(111). The island structure of the surfaces with submonolayer Ni-coverages is clearly demonstrated by TDS und LEED results obtained after CO adsorption. As with surfaces of bulk Cu-Ni alloys CO adsorption on Cu(111) with submonolayer Ni-coverage is dominated by a site effect. Cu-, Ni-, and mixed adsorption sites can be distinguished. The CO induced work function changes for Ni- and Cu-site adsorption show the same sign as observed with the pure metals. Mixed site adsorption has only a minor influence on the work function. A “ligand effect” observed only for the Ni-site adsorption, and only at small Ni-coverages is discussed in detail. Studies on the adsorption kinetics reveal that the Cu-sites may serve as precursor sites for Ni-site adsorption. Detailed UPS studies demonstrate that the CO-induced emission maxima observed on Cu surfaces with submonolayer Ni-coverages can be interpreted as a superposition of the respective adsorption features observed with the pure metals, roughly separated by their work function difference.

Chemisorption of CO on differently prepared Cu(111) surfaces

Abstract The adsorption of CO on Cu(111) has been studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), work function measurements and thermal desorption spectroscopy. Two LEED overlayers of CO on Cu(111) have been found: √3 × √3 R 30° and √ 7 3 × √ 7 3 R 49.1° . Two different heats of adsorption were derived from thermal desorption spectra: 44.2 and 35.1 kj/mole. The isosteric heat of adsorption evaluated from work function measurements corresponds to the thermal desorption results. Energy losses due to CO adsorption have been found by means of EELS at 4.7, 7.7, and 13.8 eV.

H2O interaction with clean and oxygen precovered Ni(110)

Abstract The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H 2 O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E F (6.8, 9.4 and 12.7 eV below E F ). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H 2 O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (Δ E max = 1.5 eV at 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H 2 O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E F (He(II)), being interpreted as due to hydroxyl species (OH) δ- on the surface. A kinetic model for the H 2 O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H 2 O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH) δ- recombines and desorbs as H 2 O at ≅ 320 K, leaving behind an oxygen covered Ni surface.

Chemisorption and initial oxidation of Ni(110): AES, ELS and work function measurements

Abstract The interaction of oxygen with Ni(110) surfaces was investigated from 85 to 800 K by AES. ELS, TDS and work function changes (Δo). At T = 300 K three different phases of oxygen bonding are observed: (1) Chemisorption of oxygen up to 0.6–0.8 L accompanied by an increase of work function and a kinetic energy shift of the O(KLL) Auger spectra ( ΔE = 1.1 eV ). (2) A rearrangement of the chemisorption layer and incorporation of oxygen into the Ni sublayer associated with further oxygen uptake, characterized by a decrease of Δo per adsorbed oxygen atom. (3) Nucleation and island growth of NiO indicated by a decrease of Δo, shifts of the oxygen and Ni Auger transitions to lower kinetic energy and drastic changes in ELS and AES line shape. Low temperature adsorption experiments indicate the existence of a molecular oxygen species — marked in TDS by a partial desorption of oxygen at 250 K — and characterized by the electron spectroscopic investigations and Δo measurements.

Surface science lettersUnusually low stretching frequency for CO adsorbed on Fe(100)

For CO adsorption on Fe(100) different adsorption species are detected with high resolution EELS (electron energy loss spectroscopy) which sequentially fill in with increasing coverage. Up to ∼ 350 K and low CO exposure (≦1 L), a predominant molecular species with an unusually low stretching frequency, 1180–1245 cm−1, is detected. This unusual CO bond weakening is consistent with a “lying down” binding configuration of CO. For higher CO coverages at 110 K, further CO adsorption states with vibrational frequencies of 1900–2055 cm−1 are populated which are due to CO bound with the molecular axis perpendicular to the surface.

Identification of Cu(I) and Cu(II) oxides by electron spectroscopic methods: AES, ELS and UPS investigations

Abstract The externally prepared black-coloured copper oxide ( T ⋍ 700 K, P O 2 ⋍ 100 torr) on a Cu(100) surface is identified by electron spectroscopy as CuO. Compared to the red-coloured Cu(I) oxide (in situ oxidation at T ⩾ 400 K, P O 2 ⩽ 0.5 torr, ∼ 10 9 L), the He(I)- excited photoemisson from CuO reveals characteristic shake-up satellites 10–12 eV below E F and a broadened emission from overlapping oxygen-induced 2 p and Cu 3 d states. From the AES and ELS results, in correlation with the data from core electron spectroscopy, chemical shifts of Cu 2 p , Cu 3 s and Cu 3 p in CuO to higher binding energy and decreases in binding energy of the oxygen-induced states were deduced. The unoccupied electron states of Cu at 5 and 7.5 eV above E F — postulated from the ELS results — are preserved in Cu 2 O and CuO compounds. Annealing of the Cu(II) oxide at 670 K is accompanied by decomposition into Cu 2 O due to the solid-state reaction following the scheme: 2CuO → 1/2 O 2 + Cu 2 O.

Oxygen adsorption on Ag(111) in the temperature range from 100–500 K: UPS, XPS and EELS investigations

At 300 K oxygen adsorbs dissociatively on Ag(111) with a sticking coefficient S0 − 10−5. The uptake was followed with XPS using the O 1s intensity calibrated via the O(2×1) symmetries on Ni(110) and Ag(110). The maximum coverage amounts to ϑ − 0.6 (2×109 L, −1 Torr). This atomic oxygen state is characterized in the UPS by two induced maxima from bonding and antibonding 2p orbitals 8.8 and 3.4 eV below EF. The O 1s BE is found to be 530.2 eV; in the EELS one loss is measured at 27 meV (220 cm−1). At 100 K, the interpretation of the photoelectron as well as of the EEL spectra is hindered due to H2O adsorption from the residual gas.

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

Abstract The adsorption and reaction of H 2 O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature ( T 2 O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H 2 O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T =150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O ads … HOH species on the surface. For T >350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H 2 O into O ads and H 2 . The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.

Oxygen interaction with Cu(100) studied by AES, ELS, Leed and work function changes

The interaction of oxygen with Cu(100) surfaces was investigated from 85 to 800 K by AES, ELS, LEED and work function change. At T⩾300 K three different states of oxygen bonding are observed: 1. Chemisorption of oxygen (dosages up to ∼102L), indicated by an increase of the work function change Δφ and O(KLL) signal height. 2. Incorporation of oxygen into the Cu sublayer with further oxygen uptake accompanied by a decrease of Δφ, and a shift of the O(KLL) Auger transition to lower energy (∼102−106L). 3. Growing of Cu(I) oxide, characterized by an increase of Δφ, shifts of the oxygen and copper Auger transitions and significant changes in ELS and AES line shapes (⩾3×106L, 10−3−5×10−1torr). At low temperature (85 K) a second adsorbed oxygen species is detected.

CO adsorption studies on a stepped Cu(111) surface

Abstract The adsorption of CO on a stepped Cu(332) surface has been studied by means of TDS, LEED and work function measurements during adsorption and desorption. As can be deduced from the energy depending LEED spot splitting, a well prepared surface consists of regular (111) terraces with monatomic steps. TDS experiments reveal three different desorption states, two of them deriving from CO molecules originally bound to terrace sites, showing nearly the same behaviour as CO molecules adsorbed on an undisturbed Cu(111) surface. The third desorption maximum is assigned to CO desorbing from step sites showing a higher binding energy than the terrace states. No evidence for dissociative adsorption is found from the TDS experiments. After about 2 L exposure a faint streaky adsorption structure arises, interpreted as c(4 × 2) incoherent across the steps. At higher exposure no compression of this adsorption phase is observed. Measurements of the work function change during adsorption and desorption reveal that for a CO molecule on a step site the contribution to the surface dipole-change is considerably higher than for a CO on a terrace site. Based on this result, a possible explanation is given for the fact that the step sites have a higher adsorption energy than the terrace sites.