Temperature-dependent X-ray Photoelectron Spectroscopy Research Articles

Water dissociation on graphene/Ir(111) studied by temperature-dependent X-ray photoelectron spectroscopy

Water dissociation on graphene/Ir(111) studied by temperature-dependent X-ray photoelectron spectroscopy

The interaction of size-selected Ru3 clusters with TiO2: depth-profiling of encapsulated clusters.

Ru is a metal of interest in catalysis. Monodisperse Ru3 clusters as catalytic sites are relevant for the development of catalysts because clusters use significantly lower amounts of precious materials for forming active sites due to the small size of the cluster. However, retaining the mono-dispersity of the cluster size after deposition is a challenge because surface energy could drive both agglomeration and encapsulation of the clusters. In the present work Ru3 clusters are deposited by chemical vapor deposition (CVD) of Ru3(CO)12 and cluster source depositions of bare Ru3 onto radio frequency sputter-deposited TiO2 (RF-TiO2) substrates, TiO2(100), and SiO2. When supported on RF-TiO2, bare Ru3 is encapsulated by a layer of titania substrate material during deposition with a cluster source. Ligated Ru3(CO)12 is also encapsulated by a layer of titania when deposited onto sputter-treated RF-TiO2, but only through heat treatment which is required to remove most of the ligands. The titania overlayer thickness was determined to be 1-2 monolayers for Ru3(CO)12 clusters on RF-TiO2, which is thin enough for catalytic or photocatalytic reactions to potentially occur even without clusters being part of the very outermost layer. The implication for catalysis of the encapsulation of Ru3 into the RF-TiO2 is discussed. Temperature-dependent X-ray photoelectron spectroscopy (XPS), angle-resolved XPS, and temperature-dependent low energy ion scattering (TD-LEIS) are used to probe how the cluster-surface interaction changes due to heat treatment and scanning transmission electron microscopy (STEM) was used to image the depth of the surface from side-on.

Trapped Platinum Single-Site Catalysts for the Hydrogen Evolution Reaction

Trapped platinum single-site catalysts (Pt-SSCs) in defective carbon substrates are of particular interest for polymer-electrolyte membrane (PEM) water electrolysers due to their high Pt atom utilization for the hydrogen evolution reaction (HER). However, their intrinsic catalytic activity compared with metallic Pt nanoparticles remains a topic of debate in the field with opposing results reported. [1, 2] A detailed investigation of the bonding environment of Pt and its evolution at HER working conditions is therefore warranted in order to corelate the Pt single-site structure to its catalytic activity and stability.Here, we present a detailed analysis on trapped Pt-SSC prepared by pyrolysing H2PtCl6 adsorbed on a polyaniline substrate. [3] We used in-situ heated scanning transmission electron microscopy, temperature-dependent X-ray photoelectron spectroscopy and operando X-ray absorption spectroscopy to follow the evolution of Pt’s bonding environment during pyrolysis and HER. We demonstrate that the trapped Pt-SSC is stable under HER potentials but with lower catalytic activity relative to commercial Pt nanoparticles. We also perform first principles calculations to elucidate the origin of the inferior activity.Our results promote the understanding of the relationship between the structure of trapped Pt-SSC and its HER performance, and calls for a techno-economic analysis to assess the practicality of using Pt-SSCs in commercial PEM water electrolyzers.References Q. Yang, H. Liu, P. Yuan, Y. Jia, L. Zhuang, H. Zhang, X. Yan, G. Liu,Y. Zhao, J. Liu, S. Wei, L. Song, Q. Wu, B. Ge, L. Zhang, K. Wang, X. Wang, C.-R. Chang, and X. Yao, J. Am. Chem. Soc., 144, 2171 (2022).J. Wang, H. Y. Tan, T. R. Kuo, S. C. Lin, C. S. Hsu, Y. Zhu, Y. C. Chu, T. L. Chen, J. F. Lee, and H. M. Chen, Small, 17, 2005713 (2021).P. Tang, P.-Y. Huang, J. E. N. Swallow, C. Wang, D. Gianolio, H. Guo, J. H. Warner, R. S. Weatherup and M. Pasta, “Structure- property relationship of defect-trapped Pt single-site electrocatalysts for the hydrogen evolution reaction” Reasearch Square (2023), doi: 10.21203/rs.3.rs-2397818/v1.

Temperature dependent polaronic contribution on conduction mechanism in ceria-based devices

The role of temperature dependent polaron density in current conduction mechanism of CeO2-based devices is investigated where CeO2−x films were deposited at different Ar:O2 ratios at room temperature using rf sputtering. I–V measurements on metal–oxide–semiconductor (MOS) devices at different temperatures show a three order current change in unipolar and bipolar switching having triggering voltages ranging from ±0.7 V at 80–100 °C to ∼±1.0 V at 150 and 200 °C and are reproducible in repeat cycles. Temperature dependent x-ray photoelectron spectroscopy (XPS) and grazing incidence x-ray diffraction (GIXRD) studies indicate along with stoichiometry dependent polaron contribution, the change in lattice parameters of polycrystalline CeO2 film also contributes to the resistive switching properties of CeO2-based devices with temperature. Oxygen vacancy movements in the form of hopping of polarons between the nearest neighbours and next to nearest neighbour sites as well facilitate the transitions at elevated temperatures. As predicted in the theoretical model, it is experimentally verified here that the conduction path created at elevated temperature vanishes at relatively higher temperatures due to too frequent polaron hopping. It is further observed that the density of polarons also increases with temperature that in turn hinders the switching behaviour of the devices at elevated temperatures.

Structure-Property Relationship of Defect-Trapped Pt Single-Site Electrocatalysts for the Hydrogen Evolution Reaction.

Single-site catalysts (SSCs) have attracted significant research interest due to their high metal atom utilization. Platinum single sites trapped in the defects of carbon substrates (trapped Pt-SSCs) have been proposed as efficient and stable electrocatalysts for the hydrogen evolution reaction (HER). However, the correlation between Pt bonding environment, its evolution during operation, and catalytic activity is still unclear. Here, a trapped Pt-SSC is synthesized by pyrolysis of H2PtCl6 chemisorbed on a polyaniline substrate. In situ heated scanning transmission electron microscopy and temperature-dependent X-ray photoelectron spectroscopy clarify the thermally induced structural evolution of Pt during pyrolysis. The results show that the nitrogen in polyaniline coordinates with Pt ions and atomically disperses them before pyrolysis and traps Pt sites at pyridinic N defects generated during the substrate graphitization. Operando X-ray absorption spectroscopy confirms that the trapped Pt-SSC is stable at the HER working potentials but with inferior electrocatalytic activity compared with metallic Pt nanoparticles. First principle calculations suggest that the inferior activity of trapped Pt-SSCs is due to their unfavorable hydrogen chemisorption energy relative to metallic Pt(111) surfaces. These results further the understanding of the structure-property relationship in trapped Pt-SSCs and motivate a detailed techno-economic analysis to evaluate their commercial applicability.

Leakage current reduction in β-Ga2O3 Schottky barrier diode with p-NiOx guard ring

A β-Ga2O3 Schottky barrier diode (SBD) with a p-type NiOx guard ring was fabricated, and the reverse leakage and subthreshold leakage current reduction was found at high temperatures from temperature-dependent I–V characteristics. The functional mechanisms of NiOx as edge termination on leakage reduction were studied. NiOx can increase the barrier height and passivate the defects at the interface, resulting in the suppression of subthreshold leakage and elimination of current crowding effect confirmed by a thermal emission microscope. From the temperature-dependent x-ray photoelectron spectroscopy characteristics, more holes generated to deplete Ga2O3 at higher temperatures were found. It leads to reduce the reverse leakage current. The small-polaron transportation in NiOx is proposed to argue the implausibility of the leakage conduction in NiOx. This work will offer critical physical insight and a valuable route for developing low-leakage Ga2O3 SBDs.

Interaction of silver nano-clusters with ceria thin-films: An in situ temperature dependent x-ray photoelectron spectroscopy study

Temperature dependent oxidation states of ceria (CeO 2 ) are observed at much lower temperatures in presence of silver nano-clusters (NCs), deposited on CeO 2 thin-films, using a nano-cluster deposition system equipped with quadrupole mass filter assembly and x-ray photoelectron spectroscopy (XPS) facilities. In situ high-temperature XPS studies indicate that CeO 2 oxidizes Ag-NCs to AgO at room temperature and reduces to Ce 3+ state at much lower temperature. Further, a temperature dependent reversible change in oxidation states between Ag(III) and Ag(II) states is observed. Transmission electron microscopy (TEM) study reveals that besides coalescence of silver clusters during heating, the Ag-NCs interact with CeO 2 resulting in fully covered Ag-NCs by a layer of CeO 2 due to strong metal support interactions (SMSI). UV-Vis studies show a decrease in bandgap of CeO 2 due to an increase in Ce 3+ content in CeO 2 film responsible for SMSI effect at the interface of silver clusters and cerium oxide at higher temperature. Interactions at the adsorption edge between CeO 2 and Ag-NCs help to decrease the bandgap of CeO 2 in presence of Ag-NCs. Thus the presence of silver clusters plays a crucial role in lowering the oxidation-reduction temperature of ceria. Overall, the Ag-NC in ceria helps to reduce Ce 4+ state to Ce 3+ state at much lower temperature that would be useful for its different low-temperature catalytic applications. • Ag-NCs grown on CeO 2 are in oxidized state at room temperature as evident from XPS study. • After a heating cycle, Ce 3+ enriched CeO 2 film is found at room temperature where only Ce 4+ state is present before heating. • A reversible change in oxidation state of Ag is observed where temperature is varied from RT to 250 °C. • The room temperature TEM study indicates a strong metal support interaction at the interface of ceria and silver clusters.

Temperature-dependent XPS studies on Ga-In alloys through the melting-point

Gallium indium alloys are known to be liquid at remarkably low temperatures. Herein, we provide X-ray photoelectron spectroscopy (XPS) studies and DFT-based ab-initio molecular dynamics (MD) calculations on the composition of the liquid alloy surface. We find that alloying Ga and In in ratios close to the eutectic composition results in only minor signal shifts of the corresponding In core levels. The topmost surface layer of the liquid alloy exhibits significant In enrichment for all measured temperatures, between 300 K to 800 K the observed enrichment slightly decreases with increasing temperature. This behavior is seen in both temperature-dependent XPS and ab-initio MD simulations. Furthermore, we investigate changes in the surface composition during the liquid-to-solid phase transition by time-dependent XPS during cooling of the alloy sample. The surface of the alloy presents an advantageous nucleation site, where we regularly observe the precipitation of indium. During cooling, the precipitation processes were followed by both XPS and by monitoring the sample temperature/heating power.

ZrO2 Reduction at Low Temperature in Co-Sputtered ZrxHf1−xO2 Thin-Films

A temperature dependent x-ray photoelectron spectroscopy (XPS) study reveals that ZrO2 reduces to metallic Zr and O at ∼ 140°C at Zr ratios of 8.38, 9.11, and 11.79 at.% in co-sputtered ZrxHf1−xO2 thin-films deposited on the silicon substrate. ZrO2 is reduced in the form of Zr metal. The oxygen evolved in this process reacts with carbon present in the film forming CO2 gas. Further, the presence of Zr in temperature treated samples at room temperature (RT) indicates the reduction as an irreversible one. Capacitance-voltage (C-V) curves depict increasing capacitance at high temperature (HT), confirms an increase in dielectric constant for all the films. The resistance obtained from Resistivity-temperature measurement offers a lower value at HT indicates partial metallization of films. Despite similar atomic radii of Zr and Hf, the stability of metastable tetragonal ZrO2 (t-ZrO2) decreases probably due to the influence of Hf present in ZrxHf1−xO2. This stabilization was ascribed to a decrease in the Zr coordination number upon the introduction of oxygen vacancies. A Less dense d band causes the formation of metallic Zr at such a low temperature. A reduction scheme for ZrO2 at low temperature is also proposed. Since the generation of metallic Zr sites at a lower temperature is a challenge to develop Zr-based catalyst, this combination may be used to design ZrHfO-based catalysts for low temperature catalytic applications.

Polymerization of Well-Aligned Organic Nanowires on a Ferromagnetic Rare-Earth Surface Alloy.

The high reactivity of magnetic substrates toward molecular overlayers has so far inhibited the realization of more sophisticated on-surface reactions, thereby depriving these interfaces of a significant class of chemically tailored organics such as graphene nanoribbons, oligonuclear spin-chains, and metal-organic networks. Here, we present a multitechnique characterization of the polymerization of 4,4″-dibromo-p-terphenyl precursors into ordered poly(p-phenylene) arrays on top of the bimetallic GdAu2 surface alloy. The activation temperatures for bromine scission and subsequent homocoupling of molecular precursors were followed by temperature-dependent X-ray photoelectron spectroscopy. The structural characterizations of supramolecular and polymeric phases, performed by low-energy electron diffraction and scanning tunneling microscopy, establish an extraordinary degree of order extending into the mesoscale. Taking advantage of the high homogeneity, the electronic structure of the valence band was determined with angle-resolved photoemission spectroscopy. Importantly, the transition of localized molecular orbitals into a highly dispersive π-band, the fingerprint of successful polymerization, was observed while leaving all surface-related bands intact. Moreover, ferromagnetic ordering in the GdAu2 alloy was demonstrated for all phases by X-ray absorption spectroscopy. The transfer of well-established in situ methods for growing covalently bonded macromolecules with atomic precision onto magnetic rare-earth alloys is an important step toward toward studying and controlling intrinsic carbon- and rare-earth-based magnetism.

Sonogashira cross-coupling over Au(1 1 1): from UHV to ambient pressure

We have studied the reaction of phenylacetylene (PA) with chloro-, bromo-, and iodobenzene on the Au(1 1 1) surface as a model system for the gold-catalysed Sonogashira cross-coupling. Both ultrahigh vacuum-based and ambient pressure x-ray photoelectron spectroscopy show that iodo- and chlorobenzene (IB and CB) undergo the cross-coupling reaction towards diphenylacetylene. Bromobenzene (BB), in contrast, does not react in the UHV experiments. Further, at ambient pressure signs are found for poisoning of the Au(1 1 1) surface by a carbon species formed in the reaction. The understanding obtained in the reaction experiments are based on a thorough investigation of the adsorption of PA, IB, CB, and BB on the Au(1 1 1) surface by soft x-ray absorption spectroscopy and temperature-dependent x-ray photoelectron spectroscopy. In particular, the experiments provide the orientation of the intact adsorbates with respect to the surfaces at liquid nitrogen temperature. Dissociation in the temperature regime between −80 and −15 °C is observed for iodo- and chlorobenzene, but not for BB, in agreement with that only IB and CB, but not BB, react with PA to form diphenylacetylene. The difference is tentatively attributed to a difference in surface orientation of the different halobenzenes.

The Electrolyte - Electrode Interaction - Studying the Influence of the Electrolyte Decomposition Layer on the Kinetics of the Battery

Lithium-Ion Batteries (LIBs) are strongly relying on the electrolyte decomposition layers which are forming at the anode and cathode. These decomposition layers are protecting the battery from further degradation due to uncontrolled reaction of the lithium with the electrolyte [1-5]. Therefore, information about the characteristics of these decomposition layers is very valuable for the development of advanced LIBs. In this study we were able to quantitatively study the structure, composition and thickness of the electrolyte decomposition layer. Furthermore, the influences of the structure, composition and thickness on the kinetics and cycle performance are investigated. These characteristics are of special interest for the design of a proper electrolyte. The electrolyte decomposition layers were studied using a sputter depth profiling X-ray photoelectron spectroscopy technique. This technique allows the determination of the composition and thickness of the electrolyte decomposition layers quantitatively [6, 7]. From the results it can be seen, that a significant electrolyte decomposition layer forms at the cathode (0.8 nm after formation 2.5 nm at state of initial capacity of 80% [7]). Although this layer is smaller compared to the anode (2.8 nm after formation [6]), from direct current resistance and impedance measurement it was found that the cathode resistance is significantly higher. To further investigate the influence of the electrolyte decomposition layer on the electrode kinetics additives like fluro acetone and vinylene carbonate are used to influence the composition and thickness of electrolyte decomposition layers. The use of additives to change the electrolyte decomposition layer characteristics, instead of ageing, is carried out since with ageing also different causes for a change in the kinetics may occur. As electrode materials i.e. 1:1:1 lithium nickel manganese cobalt oxide, spinel lithium manganese oxide and iron (III) oxide are used to investigate a difference in interaction of the electrode material with the electrolyte decomposition layer. Measurement of the direct current resistance using a 3-electrode cell gives the influence of the decomposition layer on the kinetics of the battery. The results (Figure 1) show that even thicker layers can show lower resistances and that the effect on the kinetics of the different additive may be different for a different electrode material. The reason for this finding is further evaluated by X-ray diffraction to estimate an influence of the electrolyte decomposition layer on the phase and structure changes of the active material. Additionally, thermo-gravimetric measurements and temperature dependent X-ray photoelectron spectroscopy are applied to investigate the strength of interaction.

Dissociative Adsorption of Water on an H2O/GaAs(100) Interface: In Situ Near-Ambient Pressure XPS Studies

Dissociative water adsorption on a GaAs(100) surface is demonstrated using X-ray photoelectron spectroscopy (XPS) carried out in situ under near-ambient conditions. These in situ XPS studies enable us to monitor the evolution of water dissociation on GaAs under elevated pressures and temperatures for the first time. In pressure-dependent XPS studies, the GaAs surface was exposed to room-temperature water vapor at pressures ranging from UHV to 5 mbar. With an increase in H2O pressure, enhancement of oxygenation and hydroxylation of surface Ga atoms is observed. In addition, strong XPS signals are also detected that originate from water molecules that are molecularly adsorbed onto the GaAs surface and hydrogen-bonded to surface OH species. Peak broadening and shifts in the As 2p XPS spectra are correlated with the signal increase resulting from surface HO–Ga species, suggesting the formation of surface H–As bonds. In temperature-dependent XPS studies, the GaAs surface was annealed from room temperature up t...

Chemisorbed Monolayers of Corannulene Penta-Thioethers on Gold

Penta(tert-butylthio)corannulene and penta(4-dimethylaminophenylthio)corannulene form highly stable monolayers on gold surfaces, as indicated by X-ray photoelectron spectroscopy (XPS). Formation of these homogeneous monolayers involves multivalent coordination of the five sulfur atoms to gold with the peripheral alkyl or aryl substituents pointing away from the surface. No dissociation of C-S bonds upon binding could be observed at room temperature. Yet, the XPS experiments reveal strong chemical bonding between the thioether groups and gold. Temperature-dependent XPS study shows that the thermal stability of the monolayers is higher than the typical stability of self-assembled monolayers (SAMs) of thiolates on gold.

Substrate Induced Thermal Decomposition of Perfluoro-Pentacene Thin Films on the Coinage Metals

The thermal and chemical stability of perfluoropentacene (PFP) thin films grown by organic molecular beam deposition onto the (111)-oriented surfaces of the coinage metals copper, silver, and gold have been studied by means of temperature dependent X-ray photoelectron spectroscopy (XPS) and Near-Edge X-ray absorption fine structure spectroscopy (NEXAFS). Under vacuum conditions, PFP multilayers are completely desorbed at 425 K while molecules in contact with the Au(111) surface remain intact up to 500 K. By contrast, PFP that is in contact with Cu(111) is distinctly distorted and becomes partially defluorinated already upon thermal desorption of multilayers. A pronounced defluorination of PFP also takes place on Ag(111) at temperatures around 440 K, while further heating causes a complete cracking and defluorination. Additional measurements carried out on a regularly stepped silver surface demonstrate that steps are active sites that promote defluorination already at lower temperatures. van der Waals corr...

Formation and Thermal Stability of Platinum Oxides on Size-Selected Platinum Nanoparticles: Support Effects

This article presents a systematic study of the formation and thermal stability of Pt oxide species on size-selected Pt nanoparticles (NPs) supported on SiO2, ZrO2, and TiO2 thin films. The studies were carried out in ultrahigh vacuum (UHV) by temperature-dependent X-ray photoelectron spectroscopy (XPS) measurements and ex situ transmission electron microscopy and atomic force microscopy. The NPs were synthesized by inverse micelle encapsulation and oxidized in UHV at room temperature by an oxygen plasma treatment. For a given particle size distribution, the role played by the NP support on the stability of Pt oxides was analyzed. PtO2 species are formed on all supports investigated after O2-plasma exposure. A two-step thermal decomposition (PtO2 → PtO → Pt) is observed from 300 to 600 K upon annealing in UHV. The stability of oxidized Pt species was found to be enhanced on ZrO2 under annealing treatments in O2. Strong NP/support interactions and the formation of Pt−Ti−O alloys are detected for Pt/TiO2 up...

Relative reactivities of amino and ethenyl groups in allylamine on Si(100)2 × 1: Temperature-dependent X-ray photoemission and thermal desorption studies of a common linker molecule

The room-temperature adsorption and thermal evolution of allylamine on Si(100)2 × 1 have been investigated by using temperature-dependent X-ray photoelectron spectroscopy (XPS) and thermal desorption spectrometry (TDS). The presence of a broad N 1 s feature at 398.9 eV, attributed to a N―Si bond, indicates N―H dissociative adsorption. On the other hand, the presence of C 1 s features at 284.6 eV and 286.2 eV, corresponding to C═C and C―N, respectively, and the absence of the Si―C feature expected at 283.2 eV shows that [2 + 2] C═C cycloaddition does not occur at room temperature. These XPS data are consistent with the unidentate staggered and eclipsed allylamine conformer adstructures arising from N―H dissociation and not [2 + 2] C═C cycloaddition. The apparent conversion of the N 1 s feature for Si―N(H)―C ▪ at 398.9 eV to that for Si―N(H) at 397.7 eV and the total depletion of C 1 s feature for C―N at 286.2 eV near 740 K indicates cleavage of the C―N bond, leaving behind a Si―N(H) radical. Furthermore, the C═C C 1 s feature at 284.6 eV undergoes steep intensity reduction between 740 K and 825 K, above which a new C 1 s feature at 283.2 eV corresponding to SiC is found to emerge. These spectral changes suggest total dissociation of the ethenyl fragment and the formation of SiC. Moreover, while the total N 1 s intensity undergoes a minor reduction (24%) upon annealing up to 1090 K, a considerable reduction (43%) is found in the overall C 1 s intensity. This observation is consistent with our TDS data, which shows the desorption of C-containing molecules including propene and ethylene at 580 K and of acetylene at 700 K. The lack of N-containing desorbates suggests that the dissociated N species are likely bonded to multiple surface Si atoms or diffused into the bulk. Interestingly, both the staggered and eclipsed N―H dissociative adstructures are found to have a less negative adsorption energy than the [N, C, C] tridentate or the [2 + 2] C═C cycloaddition adstructures by our DFT calculations, which suggests that the observed formation of N―H dissociative adstructures is kinetically favored on the Si(100)2 × 1 surface.

Selective Adsorption and Thermal Evolution of Bifunctional Carboxylic Acids: Competition of O—H Dissociation and Other Reaction Products in Acrylic Acid and Propanoic Acid on Si(100)2×1

The dissociative adsorption of acrylic acid on Si(100)2×1 at room temperature has been investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD), as well as density-functional theory (DFT) calculations. Three C 1s features can be attributed to the carboxyl C of bidentate acrylate (at 286.8 eV) and unidentate acrylate (at 289.3 eV), both resulting from O—H dissociation upon adsorption, and to their corresponding ethenyl C atoms (at 285.0 eV). The formation of bidentate acrylate at a low exposure is followed by that of unidentate acrylate at a higher exposure, with approximately equal populations for both adstructures at the saturation exposure. DFT calculations confirm that the bidentate acrylate adstructure gives the most stable adsorption structure configuration. The combined temperature-dependent XPS and TPD studies provide strong evidence for the formation of CO, ethylene, acetylene, and propene, with 60% of the C moiety from the adsorption configurations conver...

Local chemical states and thermal stabilities of nitrogen dopants in ZnO film studied by temperature-dependent x-ray photoelectron spectroscopy

Local chemical states and thermal stabilities of N dopants in ZnO:N film are investigated by temperature-dependent x-ray photoelectron spectroscopy. Different types of N local states are detected, including N2 molecules occupying O sites [(N2)O], –NO species, substitutional N atoms in O- and N-rich local environments (α- and β-NO). Compared to the β-NO, the α-NO shows a better thermal stability up to 723 K. However, the transformation from α-NO acceptor to undesirable (N2)O donor occurs above 743 K. The variation of N local states also affects Zn and O binding energies. Photoluminescence studies indicate the shallow acceptor nature of α-NO.

Selective surface chemistry of allyl alcohol and allyl aldehyde on Si(1 0 0)2×1: Competition of [2 + 2] C [dbnd]C cycloaddition with O–H dissociation and with [2 + 2] C [dbnd]O cycloaddition in bifunctional molecules

Competition between the C C functional group with the OH group in allyl alcohol and with the C O group in allyl aldehyde in the adsorption and thermal chemistry on Si(1 0 0)2×1 has been studied by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), as well as density-functional theory (DFT) calculations. The similarities found in the C 1s and O 1s spectra for both molecules indicate that the O–H dissociation product for allyl alcohol and [2 + 2] C O cycloaddition product for allyl aldehyde are preferred over the corresponding [2 + 2] C C cycloaddition products. Temperature-dependent XPS and TPD studies further show that thermal evolution of these molecules gives rise to the formation of ethylene, acetylene, and propene on Si(1 0 0)2×1, with additional CO evolution only from allyl alcohol. The formation of these desorption products also supports that the [2 + 2] C C cycloaddition reaction does not occur. In addition, the formation of SiC at 1090 K is observed for both allyl alcohol and allyl aldehyde. We propose plausible surface-mediated reaction pathways for the formation of these thermal evolution products. The present work illustrates the crucial role of the Si(1 0 0)2×1 surface in selective reactions of the Si dimers with the O−H group in allyl alcohol and with the C O group in allyl aldehyde over the C C functional group common to both molecules.