Science with APXPS
Live monitoring of Atomic layer deposition of TiO2 on RuO2(110)
In this study APXPS was used to live-monitor the growth of TiO2 on a RuO2(110) surface by atomic layer deposition (ALD) (Head2016 JPCC 120 243). Today, ALD is a a key method in semiconductor growth which is based on the alternating exposure of the substrate to two precursors. The precursors should react with the surface in a self-limiting fashion. In this way a controlled layer-by-layer growth can be achieved. While ALD is very well characterised by “prenatal” and post-mortem methods and while idealised models of the chemical reactions have been developed, little knowledge is available about the factual chemical processes during growth. This lack of knowledge was addressed here by applying APXPS to the real-time monitoring of the ALD of TiO2 from TDMAT (tetrakis(dimethylamido) titanium) and water. In addition to the expected cyclic surface species, evidence for side reactions was found: both dimethylamine and methyl methylenimine could be identified from the APXP spectra. Further, both a clear pressure and a temperature dependence of the process were seen. These insights into the ALD mechanism and precursor pressure dependence on reactivity highlight the utility of APXPS in studying ALD processes and interface formation.
CO oxidation over Pt(111)
In this example we tested the capabilities of the APXPS setup at Species Beamline by studying the oxidation of carbonmonoxide over a Pt(111) sample in oxidising conditions (Knudsen2015 SurfSci646 160). The mixing ratio of O2 and CO was 9:1 and the total pressure 0.15 mbar. While the sample temperature was increased the O 1s spectra were measured simultaneously and the quadrupole mass spectrometer (QMS) signal was recorded at the outlet from the AP cell.
a) QMS signals acquired on a Pt (111) surface exposed to CO:O2 mixture while heating . b) O 1s spectra acquired simultaneously with QLS data.
Starting at a temperature of 450 K, we observe four O 1s components, which can be assigned to gas phase O2 (537.4 eV and 538.5 eV) and CO molecules adsorbed in bridge (531.0 eV) and atop (532.6 eV) surface sites, respectively. The small shoulder at around 538 eV binding energy observed for a temperature of 450 K (not fitted) originates from CO in the gas phase. Under these conditions no or only very little CO2 production is observed in the QMS signal: CO covers the entire surface and blocks dissociative adsorption of oxygen, which is an essential requirement for efficient CO oxidation. The QMS and O 1s data demonstrate clearly that this inactive phase persists up to a temperature of 515 K. At a temperature of 535 K the O 1s spectrum changes drastically. Now the components assigned to adsorbed CO have disappeared completely, and a new component assigned to atomic oxygen has appeared at 530.0 eV binding energy. Simultaneously, with the conversion from a completely CO-covered surface to a completely oxygen-covered surface we observe the occurrence of gas phase CO2 from the peak at 535.6 eV binding energy in the O 1s spectrum. The QMS data measured simultaneously show dramatically increased (decreased) CO2 (CO) levels in the exhaust gas fromthe cell, clearly signalling a highly active surface and fully consistent with the observation in the O 1s spectra.
Oxidation of FeO(111) to FeO2(111)
Using high resolution and ambient pressure X-ray photoelectron spectroscopy we show (Johansson2016 TopCatal 59 506) that the catalytically active FeO2 trilayer films grown on Pt(111) are very active for water dissociation, in contrast to inert FeO(111) bilayer films. The FeO2 trilayer is so active for water dissociation that it becomes hydroxylated upon formation,regardless of the applied preparation method. FeO2 trilayers were grown by oxidation of FeO(111) bilayer films either with molecular oxygen in the mbar regime, or by NO2 and atomic oxygen exposures, respectively, in the
ultrahigh vacuum regime. Because it was impossible to prepare clean FeO2 without any hydroxyls we propose that catalytically highly active FeO2 trilayer films are generally hydroxylated. In addition, we provide spectroscopic fingerprints
both for Pt(111)-supported FeO(111) and FeO2 films that can serve as reference for future in situ studies.
(a) Image plot of O 1s spectra acquired while heating the an FeO(111) film from 300 K to 500 K in 0.6 mbar O2, followed by subsequent cooling. The temperature profile is plotted in (c). (b) Selected O 1s spectra from (a). (c) Relative integrated areas of the surface components obtained by fitting the O 1s spectra in panel (a).