10 research outputs found
IR-probes for ceria surfaces: A comparison of CO and CHâOH adsorbed on CeOâ(111), CeOâ(110) and ceria powders
Untersuchung von Metall-Polymer GrenzflÀchen mittels Adsorptions-Mikrokalorimetrie und Photoelektronenspektroskopie
The interface formation between calcium and two different semiconducting, π-conjugated polymers, namely poly(3-hexylthiophene) (P3HT) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-(1-cyanovilylene)phenylene] (CN-MEH-PPV), was investigated using adsorption microcalorimetry, low energy ion scattering spectroscopy (LEIS), atomic beam scattering and X-ray photoelectron spectroscopy. In addition to the interface formation on pristine, i.e., untreated polymer surfaces, the influence of electron irradiation prior to calcium deposition and the effect of dosing calcium at a low substrate temperature was studied. The reactive site for the interaction of calcium atoms impinging on a pristine P3HT surface appears to be the sulfur in the thiophene ring, as is concluded from a combination of XPS, adsorption calorimetry and theory results. The interaction, in fact, is strong enough that the sulfur atoms abstracted from the thiophene ring under formation of calcium sulfide with an overall reaction energy of this process of 405 kJ per mol. Quantitative evaluation of XPS data reveal that the depth up to which Ca atoms react with sulfur in the polymer is 3 nm, irrespective of increasing the amount of Ca dosed onto the substrate. A closed layer of Ca is only formed at a Ca coverage exceeding 11 ML, as suggested by LEIS. Irradiation of P3HT with electrons with a kinetic energy of 100 eV results in dehydrogenation of the hexyl side chains and formation of new C=C double bonds. This in turn results in a higher initial sticking probability of 0.63 for Ca, while no other significant changes could be observed: XPS indicates that the thiophene rings remain intact and the measured heat of adsorption is the same as observed for the deposition of Ca on pristine P3HT. Dosing Ca onto P3HT held at low temperature (130 K) is found to result in a very low saturation thickness of the reacted layer of approximately 0.3 nm. Upon warming the sample up to room temperature, the thickness of the reacted layer increases to 1.2 nm. Thus, dosing Ca onto a cold P3HT sample permanently reduces the reaction layer thickness significantly. These experiments further affirm the hypothesis that a Ca atom attached to a large cluster or a closed Ca film will not detach to undergo further reaction, even though this would be energetically favorable. In the study concerning Ca adsorption on CN-MEH-PPV, the oxygen of the ether group is identified as the reactive species for Ca adsorption. Different from the case of Ca adsorption on P3HT, cluster formation sets in at the very beginning of the interface formation. The initial sticking probability is considerably higher for this system, which is indicative of a lower activation barrier for adsorption or alternatively the existence of two different channels (reaction and cluster formation).Die Bildung der GrenzflĂ€che zwischen Kalzium und zwei verschiedenen halbleitenden, π-konjugierten Polymeren, Poly(3-hexylthiophen) (P3HT) und Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-(1-cyano-vinylen)phenylen] (CN-MEH-PPV) wurde mittels Adsorptions-Mikrokalorimetrie, niederenergetischer Ionenstreuspektrokopie (LEIS), Atomstrahlstreuung und Röntgenphotoelek-tronenspektroskopie untersucht. ZusĂ€tzlich zur Ausbildung der Grenzschicht im Fall des unbehandelten Polymers wurde auch der Einfluss verschiedener Faktoren, wie Elektronen-bestrahlung vor dem Aufdampfen von Ca und dem Aufdampfen von Ca bei niedriger Proben-temperatur (130 K) betrachtet. Die Untersuchungen an unbehandeltem P3HT zeigen, dass die in Bezug auf Ca-Adsorption reaktive Spezies im Polymer der Schwefel in den Thiophenringen ist. Die Wechselwirkung ist so stark, dass Schwefelatome unter Bildung von Schwefelsulfid aus den Thiophenringen abstrahiert wird, wobei die Reaktionsenthalpie dieses Prozesses 405 kJ/mol betrĂ€gt. Eine Quantitative Auswertung von XP Spektren zeigt, dass die bei Dicke der Schicht, in der Ca- mit S-Atomen reagieren, nach groĂen Ca-Angeboten 3 nm betrĂ€gt. Ein geschlossener Ca-Film bildet sich bei einer Ca-Bedeckung von mehr als 11 Monolagen. Bestrahlung mit Elektronen mit einer kinetischen Energie von 100 eV resultiert in einer partiellen Dehydrierung der Hexylketten und Bildung von C=C Doppelbindungen. Diese VerĂ€nderungen fĂŒhren zu einer stĂ€rkeren Wechselwirkung zwischen Ca-Atomen und Hexylketten, die sich in einem mit 0.63 deutlich höheren anfĂ€nglichen Haftkoeffizienten ausdrĂŒckt. Ăber diese VerĂ€nderungen hinaus ist kein Effekt des Elektronenbeschusses zu beobachten, der Thiophenring bleibt intakt, wie XP-Spektren des S 2p Niveaus und die Adsorptionskalorimetrie-Messungen nahelegen. Aufdampfen von Ca auf P3HT bei tiefen Temperaturen (130 K) fĂŒhrt zu einer deutlichen und permanenten Verminderung der Dicke der Reaktionszone. Bei 130 K betrĂ€gt die Dicke dieser Schicht nur 0.3 nm und wĂ€chst nach dem erwĂ€rmen auf Raumtemperatur bis auf 1.2 nm an, was deutlich unter der Dicke liegt, die man beim Aufdampfen auf P3HT bei Raumtemperatur (3 nm) erhĂ€lt. Diese Experimente bestĂ€tigen das Modell, nach dem Ca-Atome keinerlei weitere Reaktion eingehen, nachdem sie sich einmal in einem gröĂeren Ca-Cluster befunden haben, auch wenn eine weitere Reaktion energetisch gĂŒnstiger wĂ€re. Bei der Untersuchung der Ca-Adsorption auf CN-MEH-PPV konnte der Sauerstoff der Ether-Gruppe als reaktive Spezies identifiziert werden. Anders als bei P3HT setzt Cluster-Wachstum bereits bei niedrigen Bedeckungen ein. Der anfĂ€ngliche Haftfaktor in diesem System ist deutlich höher als im System Ca/P3HT, was auf eine niedrigere Aktivierungsbarriere fĂŒr die Adsorption oder das Vorhandensein zweier AdsorptionskanĂ€le (Reaktion und Cluster-Bildung) hinweist
Steering Surface Reaction at Specific Sites with Self-Assembly Strategy
To discern the catalytic
activity of different active sites, a
self-assembly strategy is applied to confine the involved species
that are âattachedâ to specific surface sites. The employed
probe reaction system is the Ullmann coupling of 4-bromobiphenyl,
C<sub>6</sub>H<sub>5</sub>C<sub>6</sub>H<sub>4</sub>Br, on an atomically
flat Ag(111) surface, which is explored by combined scanning tunneling
microscopy, synchrotron X-ray photoelectron spectroscopy, and density
functional theory calculations. The catalytic cycle involves the detachment
of the Br atom from the initial reactant to form an organometallic
intermediate, C<sub>6</sub>H<sub>5</sub>C<sub>6</sub>H<sub>4</sub>AgC<sub>6</sub>H<sub>4</sub>C<sub>6</sub>H<sub>5</sub>, which subsequently
self-assembles with its central Ag atom residing either on 2-fold
bridge or 3-fold hollow sites at full coverage. The hollow site turns
out to be catalytically more active than the bridge one, allowing
us to achieve site-steered reaction control from the intermediate
to the final coupling product, <i>p</i>-quaterphenyl, at
390 and 410 K, respectively
On-Surface AzideâAlkyne Cycloaddition on Cu(111): Does It âClickâ in Ultrahigh Vacuum?
Using scanning tunneling microscopy, we demonstrate that
the 1,3-dipolar
cycloaddition between a terminal alkyne and an azide can be performed
under solvent-free ultrahigh vacuum conditions with reactants adsorbed
on a Cu(111) surface. XPS shows significant degradation of the azide
upon adsorption, which is found to be the limiting factor for the
reaction
Interaction of Formaldehyde with the Rutile TiO<sub>2</sub>(110) Surface: A Combined Experimental and Theoretical Study
The
adsorption and reaction of formaldehyde (CH<sub>2</sub>O) on
the oxidized rutile TiO<sub>2</sub>(110) surface were studied by temperature-programmed
desorption (TPD), scanning tunneling microscopy (STM), infrared reflectionâabsorption
spectroscopy (IRRAS), and density functional theory (DFT) calculations.
The experimental and theoretical data reveal the presence of various
species depending on the temperature and coverage. Exposure to formaldehyde
at 65 K leads to the formation of CH<sub>2</sub>O multilayers, which
desorb completely upon heating to 120 K. After smaller exposures at
low temperatures (45â65 K), STM allowed us to identify individual,
isolated CH<sub>2</sub>O monomers. The theoretical results indicate
that these monomers are bound to the surface Ti<sub>5c</sub> sites
via Ï-donation and adopt a tilted geometry. Upon heating, the
CH<sub>2</sub>O monomers polymerize to form paraformaldehyde (polyoxymethylene,
POM) chains, oriented primarily along the Ti<sub>5c</sub> rows ([001]
direction). Upon further heating, POM is found to decompose around
250 K, releasing CH<sub>2</sub>O into the gas phase. In addition,
dioxymethylene (DOM) was detected as minority species formed via reaction
of Ti<sub>5c</sub>-bound CH<sub>2</sub>O with surface O atoms. For
all substrate species, the characteristic IR vibrations were measured.
Because these are the first IRRAS data for TiO<sub>2</sub> macroscopic
single crystals exposed to formaldehyde, we have performed DFT calculations
to aid the assignment of the various bands