AVS 70 Session SS-ThP: Surface Science Poster Session
Session Abstract Book
(440KB, Jul 23, 2024)
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SS-ThP-2 Near Ambient Pressure XPS Indicates that the Relevant State of Silver under Ethylene Epoxidation Conditions is Mostly Metallic
Elizabeth E. Happel (Tufts University); Phillip Christopher (University of California at Santa Barbara); Matthew M. Montemore (Tulane University); E. Charles H. Sykes (Tufts University) Epoxidation reactions are some of the highest value processes in the chemical industry; however, despite decades of research, the mechanism of ethylene epoxidation is still under debate. A central question in the field is the state of the industrial Ag catalyst under reaction conditions. Controversy remains about whether the active site for selective oxidation is metallic or oxidized and several in situ studies have reported the presence of both phases. We utilize near ambient pressure XPS under both purely oxidizing conditions and ethylene: oxygen ratios at a temperature that yields the same reactant chemical potentials as a typical flow reactor. Our XPS results reveal that electrophilic and nucleophilic oxygen formed on Ag(111) after exposure to pure oxygen at 433 K, with a O : surface Ag ratio of 0.11. However, when ethylene is introduced to the oxygen atmosphere at an industrially relevant 5:2 ratio, the nucleophilic oxygen is reacted away immediately, and the electrophilic oxygen coverage slightly increases which we attribute to the formation of carbonate which has the same binding energy as electrophilic oxygen. Most significantly, the O : surface Ag ratio decreases to 0.05, almost half of which is carbonate. These results indicate that the Ag(111) surface is mostly metallic under simulated reactor conditions meaning that Ag/oxide interfaces, at which the proposed oxametallacycle (OMC) intermediate is formed, must be considered in mechanistic epoxidation models. |
SS-ThP-3 Surface Chemistry of Zirconium Borohydride on Zirconium Diboride (0001)
Ayoyele Ologun (University of Illinois - Chicago) Transition metal diborides are known to either have a metal-terminated or boron-terminated surface. While group-V MB2 has boron-terminated surfaces, group-IV MB2 has metal-terminated surfaces. ZrB2, a group-IV metal-terminated diboride, is an extremely hard material with a high melting point of 3246 °C and can be grown conformally via chemical vapor deposition (CVD) using zirconium borohydride Zr-(BH4)4 as a precursor. Using reflection absorption infrared spectroscopy (RAIRS), we investigated the surface chemistry of ZrB2 growth from Zr-(BH4)4 on a ZrB2(0001) surface. Using low-energy electron diffraction and X-ray photoelectron spectroscopy (XPS), we demonstrate that ZrB2 can have a boron-terminated surface in boron-rich conditions. The RAIR spectrum obtained after exposing the surface at 90 K to Zr-(BH4)4(g) matched that of the pure compound, indicating adsorption without decomposition. However, new surface intermediates were formed upon heating to 280 K, as shown by the presence of vB-Ht stretch (2569 cm-1) and δH-B-H (1228 & 1057 cm-1) peaks in the RAIR spectra. Decomposition of Zr(BH4)4 on ZrB2(0001) surface at 1173 K revealed a √3×√3 boron terminated surface, with a stoichiometry of ZrB2.60. In contrast to the zirconium-terminated surface, the boron-terminated surface is resistant to oxidation. |
SS-ThP-4 Selective Hydrogenation of 1,3-butadiene on a Pd/Cu (111) Single-Atom-Alloy
Mohammad Rahat Hossain, Michael Trenary (University of Illinois at Chicago) Selective hydrogenation of 1,3-butadiene (BD) to 1-butene (1-B) is critical in refining alkene streams for high-quality polymer production. Typically, Pd and Pt are employed in hydrogenation reactions due to their nearly negligible barrier for H2 activation. However, these catalysts are prone to coking and their high activity often reduces selectivity. Single-atom alloy (SAAs) catalysts are being developed to achieve high selectivity while retaining high activity. In an SAA, small amounts of an active metal, such as Pd or Pt, are doped into a less active host metal such as Cu. Previous work has shown that a Pd(111) surface exhibits superior selectivity for BD hydrogenation to 1-B compared to Pt(111),1-4 suggesting that Pd/Cu(111) could be a suitable SAA model catalyst for this reaction. In this study, we investigated the adsorption and hydrogenation of 1,3-butadiene (BD) to 1-butene (1-B) over a Pd/Cu (111) SAA under ultrahigh vacuum (UHV) and ambient pressure conditions using reflection absorption infrared spectroscopy (RAIRS). Temperature programmed reaction spectroscopy (TPRS) in UHV showed that monolayer BD desorbs at 217 K, while 2nd-layer and multilayer BD desorb between 112 to 180 K. RAIRS detected gas-phase 1-B formation and BD consumption. In ambient pressure conditions, this reaction was found to be first-order (1.12±0.03) in H2 and zero-order (-0.12±0.01) in BD, correlating to a turnover frequency of 36 s-1 at 380 K. The activation energy was calculated to be 63.2 ± 2.8 kJ/mol from an Arrhenius plot of the temperature dependence of the rate constant. Complete conversion of BD was found with 84% selectivity towards 1-B, without butane production. No surface species were detected during the reaction. Post-reaction analysis using Auger electron spectroscopy (AES) revealed carbon deposition, indicating some dissociation during hydrogenation. References:1.C.-M. Pradier, E. Margot, Y. Berthier and J. Oudar, Appl. Catal. 43 (1), 177-192 (1988).2.G. Tourillon, A. Cassuto, Y. Jugnet, J. Massardier and J. Bertolini, J. Chem. Soc., Faraday Trans. 92 (23), 4835-4841 (1996).3.T. Ouchaib, J. Catal. 119 (2) (1989).4.J. Massardier, J. Bertolini, P. Ruiz and P. Delichere, J. Catal. 112 (1), 21-33 (1988). View Supplemental Document (pdf) |
SS-ThP-5 The Influence of Substrate Roughness and Long-Range Molecular Order on 3-Mercaptopropionic Acid Displacement by 1-Decanethiol
Lindsey Penland, Hirushan Hetti Arachchige, Nadith Dissanayake, Rachael Farber (University of Kansas) Thiolate self-assembled monolayers (SAMs) are widely used for their ability to tune the interfacial properties of Au surfaces for applications such as nano-fabrication and bio-functionalization. This molecular control over interfacial properties can be further enhanced by introducing a second molecular species, resulting in a binary SAM. Binary SAMs offer an appealing route to control the molecular scale density of specific functional groups. One approach to forming binary SAMs is through the displacement of one molecule with another through solution phase deposition. The fabrication of molecularly precise binary SAMs requires precise control of the molecular interactions and substrate properties, such as surface roughness. In this work, using ultra-high vacuum scanning tunneling microscopy (UHV-STM) and solution-phase preparation methods, the relationship between chain length, functional group, and substrate roughness on the displacement of 3-Mercaptoproprionic Acid (MPA) by 1-Decanethiol (DT) was investigated. First, three categories of MPA substrates were prepared: MPA-1) well-ordered smooth surface formed during a 3 hr MPA incubation at 35 °C, MPA-2) disordered smooth surface formed during a 3 hr MPA incubation at 25 °C, and MPA-3) well-ordered rough surface formed during a 24 hr MPA incubation at 25 °C. Then, following the formation of MPA-1, -2, and -3, the substrates were sequentially placed in a 2 μM DT solution for 20 min, 60, min, 3 hr, and 24 hr. UHV-STM analysis of the MPA substrates showed that MPA-1 and MPA-2 had faster rates of displacement than MPA-3 and resulted in a uniform high-density DT film across the surface. MPA-3, which started with a uniform MPA SAM and nanoscale roughness due to the formation of Au nano-islands across the substrate related to the longer MPA incubation time, had the slowest rate of displacement. Interestingly, the DT SAM that formed following displacement of the MPA-3 sample was comprised of the low-coverage, lying down phase (β) and 2-D gas phase (α) of DT. Ongoing work is focused on understanding the relationship between substrate roughness and MPA ordering on the slower displacement rate and low-density DT phases observed for MPA-3. |
SS-ThP-6 Visualizing on-surface Intramolecular C-C Coupling Reaction Using Scanning Tunneling Microscopy and Tip-Enhanced Raman Spectroscopy
Soumyajit Rajak, Nan Jiang (University of Illinois, Chicago) Metal surface-supported physicochemical transformations provide additional degrees of freedom to tune the structural and electronic properties of molecular functional materials. To obtain a higher degree of control over the reaction outcome, submolecular scale characterization of the chemical intermediates and their local environment is required. Determining the real-space surface adsorbed configurations of molecules is challenging using ensemble-averaged surface science techniques. Again, probing the effect of the local environment of chemical species is challenging because the spatial resolution of conventional optical spectroscopic techniques is limited by the diffraction limit of light. Coupling light with plasmonic nano-objects creates highly localized surface plasmons (LSPs), which allows us to break the diffraction limit. Herein we present a combined topographical and chemical analysis of different surface-adsorbed configurations and surface-sensitive arrangements of a tetrabenzoporphyrin molecule and their chemical reactivity on a metal surface using angstrom-scale resolution scanning tunneling microscopy (STM) and ultra-high vacuum tip-enhanced Raman spectroscopy (UHV-TERS). Low temperature (77K) scanning tunneling microscopic images and localized surface plasmon resonance enhanced Raman signals reveal different adsorbate configurations of single molecule entities and their thermal reaction products with a fundamental view of adsorbate-substrate binding interactions. TERS uses the apex of the STM tip made of a plasmonic metal as a nano object to couple light to the near field. The Raman modes of the nanostructure underneath this tip are enhanced by the nano-confined surface plasmons which allows us to obtain chemical information with Angstrom scale spatial resolution. The atomic scale insights obtained into the local environment enable precise control over the fabrication of molecules with tailored optoelectronic properties. |
SS-ThP-7 Click Chemistry on Functionalized Silicon Surfaces: UHV- and Solution-Based Strategies
Timo Glaser (Justus Liebig University Giessen); Jannick Meinecke (Philipps University Marburg); Christian Länger, Lukas Freund (Justus Liebig University Giessen); Ulrich Koert (Philipps University Marburg); Michael Dürr (Justus Liebig University Giessen) Click chemistry is a well-known and established reaction scheme in organic chemistry for the synthesis of well-defined organic molecular structures. The direct application of click chemistry reactions to selectively functionalized silicon surfaces could thus open the route to synthesizing new organic molecular architectures on these substrates, e.g., with tailored optical or physicochemical properties (“more than Moore”). However, click reactions such as the most prominent alkyne-azide coupling are performed in the presence of a catalyst dissolved in an adequate solvent. On the other hand, highly reactive surfaces such as the technologically most important Si(001) surface, are typically prepared and stored under UHV conditions in order to guarantee a high level of cleanliness and structural perfection. The direct application of solution-based click chemistry schemes to such vacuum processed surfaces thus seems as an experimental contradiction. Here we show two different approaches to solve this problem: First, we demonstrate how to combine surface functionalization performed under UHV conditions with a solution-based alkyne-azide click reaction in order to build organic molecular architectures on functionalized semiconductor surfaces. The UHV-based functionalization of the Si(001) surface was realized via chemoselective adsorption of ethinyl cyclopropyl cyclooctyne (ECCO) from the gas phase [1]. The samples were directly transferred from UHV into the azide solution without contact to ambient conditions. The second organic layer was then coupled in acetonitrile solution via the copper-catalyzed alkyne-azide click reaction. Each reaction step was monitored by means of X-ray photoelectron spectroscopy in UHV; the N 1s spectra clearly indicated the click reaction of the azide group of the two test molecules employed, i.e., methyl-substituted benzylazide and azide substituted pyrene. Using optimized copper (I) catalysts, effective reaction yields of up to 75 % were obtained [2]. Second, a carefully tuned enolether/tetrazine cycloaddition was shown to be applicable even under UHV conditions and without catalyst [3]. We employed this reaction for coupling a tetrazine molecule to an enol ether group which was covalently attached on a Si(001) surface via cyclooctyne as a linker. [1] C. Länger, J. Heep, P. Nikodemiak, T. Bohamud, P. Kirsten, U. Höfer, U. Koert, and M. Dürr, J. Phys.: Condens. Matter 31, 34001 (2019). [2] T. Glaser, J. Meinecke, C. Länger, J. Heep, U. Koert, and M. Dürr, J. Phys. Chem. C 125, 4021 (2021). [3] T. Glaser, J. Meinecke, L. Freund, C. Länger, J.-N. Luy, R. Tonner, U. Koert, and M. Dürr, Chem. Eur. J. 27, 8082 (2021). |
SS-ThP-8 Adsorption of Fluorinated β-Diketones on a Surface of Zno Nanopowder: Dependence of Adsorbates on the Chemical Structure
Sanuthmi Dunuwila, Andrew V. Teplyakov (University of Delaware) This study investigates the surface modification of ZnO nanopowder using gas-phase fluorinated β-diketones, namely hexafluoro acetylacetone (hfacH) and trifluoro acetylacetone (tfacH), to elucidate their attachment chemistry. Surface modification was investigated through in-situ infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and solid-state NMR analysis (ssNMR) to determine the dominant adsorbate on nanopowder surfaces. To supplement the experimental findings, density functional theory was employed to identify stable surface species of ZnO nanopowder. This study examines how the binding behavior of β-diketones on the ZnO surface varies depending on the specific nature of the β-diketone molecule. The gas phase for both diketones investigated consisted of the mixture of enol and ketone forms, with enol being most dominant for hfacH. Despite this observation, surface adsorption is dominated by the tetra-σ-bonded diketonate, which is very different from the commonly accepted adsorption model for β-diketones on oxide surfaces. In the case of tfacH, the adsorption is indeed dominated by the dissociated enolate form. These differences are apparently governed by the amphoteric nature of ZnO. When a more basic oxide material, MgO, is used, hfacH does form enolate as a dominant adsorbate, as confirmed by ssNMR. This work expands our understanding of the β-diketone adsorption on oxide materials that is used in a variety of applications, from surface sensitization to heterogeneous catalysis, and from material growth to etching. |
SS-ThP-9 In-situ X-Ray Absorption Spectroscopy (XAS) study of CeO2-based Catalysts for CO2 Hydrogenation
Irene Barba-Nieto, Jose Rodriguez (Brookhaven National Laboratory) Carbon dioxide (CO2) is the main gas responsible for the greenhouse effect in Earth's atmosphere, leading to higher global temperatures and climate change. To limit global warming to 1.5ºC and reach net zero carbon dioxide emissions by 2050, it is necessary to advance in industrial processes that facilitate the generation of clean fuels from CO2. One of the most promising strategies in this regard is the utilization of CO2 and its transformation into valuable chemicals. This study examines the effectiveness of two catalyst types, RuCeO2 and RuCeO2-TiO2 systems, for converting CO2 into methane. Results demonstrate that despite lower Ru content, TiO2-containing systems exhibit significantly enhanced catalytic activity for CO2 conversion to methane. To understand this fact, in situ X-ray absorption measurements have been carried out on the Ru K-edge and Ce L3-edge analyzing their behavior under H2, CO2 and H2+CO2 between 30oC to 250oC. The Ru K-edge results indicate that the RuCeO2 systems display a remarkable oxidation-reduction capacity, as evidenced by the reduction of the metal center in the presence of both H2 and CO2/H2, along with its oxidation in CO2 atmosphere. However, this behavior undergoes a drastic change in the TiO2-containing catalyst, as a consequence of strong CeO2-TiO2 interactions, where the presence of hydrogen at 250°C leads to irreversible reduction of Ru. The analysis of the Ce L3-edge revealed that the RuCeO2 systems predominantly exhibit Ce4+, which was observed to undergo partial reduction under both H2 and H2/CO2 atmospheres, followed by its re-oxidation under CO2. However, the RuCeO2-TiO2 catalysts demonstrate a substantially higher concentration of Ce3+ compared to the RuCeO2 sample under all conditions (H2, CO2, H2/CO2), with the amount of Ce3+ further increasing under atmospheres with H2 and H2/CO2. Therefore, the XAS findings indicate that the presence of TiO2 in the catalysts stabilizes the metallic state of Ru, which remains in this state during the methanation reaction. Moreover, TiO2 promotes the formation of Ce3+, enhancing the catalysts' reactivity. This effect is attributed to TiO2 facilitating an electronic transfer at the interface and perturbing the regular fluorite geometry of ceria, thus promoting the presence of Ce3+. A trend that is in agreement with previous studies for CeO2/TiO2(110) model systems. The presence of Ce3+ significantly impacts the catalytic properties of the sample, aiding in the oxidation-reduction of Ce and stabilizing Ru. Consequently, the presence of reduced cerium plays a crucial role in determining the surface chemistry of the catalyst, crucial for efficiently converting CO2 into methane. |
SS-ThP-10 Insights from the Atomic Scale: Cobalt Sulfide Sheets on Au(111) and Initial Oxidation of Pt(111)
Dajo Boden, Mahesh Prabhu, Marcel Rost, Irene Groot, Jörg Meyer (Leiden University) This poster highlights two recently published articles from a successful experiment-theory collaboration within the Leiden Institute of Chemistry:
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SS-ThP-11 Hard X-Ray Photoelectron Spectroscopy Reveals Fe Segregation in NiFe Electrodes During Oxygen Evolution Reaction
Filippo Longo (Chemical Energy Carriers and Vehicle Systems Laboratory, Empa, Swiss Federal Laboratories for Materials Science and Technology, Switzerland); Pedro Javier Lloreda Jurado, Jorge Gil-Rostra, Agustin R. González-Elipe, Francisco Yubero (Nanotechnology on Surfaces and Plasma, Institute of Materials Science of Seville (CSIC-US), Seville, Spain); Andreas Borgschulte (Chemical Energy Carriers and Vehicle Systems Laboratory, Empa, Swiss Federal Laboratories for Materials Science and Technology, Switzerland) Alkaline water electrolysis represents one of the simplest methods employed to perform water splitting reaction [1]. This process is one of the most efficient ways of producing H2 and O2 at low cost and high purity [2]. The bottleneck of this reaction stems from the sluggish kinetics of the anodic reaction, i.e., the oxygen evolution reaction (OER), which consists of a four-electron transfer process [3]. The extraordinary performance of NiFe as electrocatalysts for the OER [4] is still a subject of debate. The changes that occur on the electrode surface during electrochemical reactions add another dimension of complexity, which hinders the rational design of electrodes for water splitting. Particularly for binary alloy electrodes, there are various phenomena ranging from the formation of oxides, (oxy)hydroxides and the associated segregation of metal atoms. In this work, we study various NiFe electrodes as model systems for the OER. We have developed the procedure for the quantification of chemical depth-profiling by XPS/HAXPES measurement, showing a marked Fe segregation and dissolution. The results explain the electrochemical performance of NiFe electrodes for OER. All the electrodes studied suffer from segregation of iron and subsequent formation of FeOx on the surface, with only minor influence from morphology, porosity and total Fe content. [1] S. W. Sharshir et al., International Journal of Hydrogen Energy (2023).[2] Z.-Y. Yu et al., Advanced Materials 33, 2007100 (2021).[3] A. Wang et al., Mater. Chem. Front. 7, 5187 (2023).[4] F. Dionigi and P. Strasser, Advanced Energy Materials 6, 1600621 (2016). View Supplemental Document (pdf) |
SS-ThP-12 Surface Modification of Titanium Dioxide Nanomaterials via Functionalization with Triol Compounds
Asishana Onivefu, Andrew Teplyakov (University of Delaware) Surface modification of titanium dioxide (TiO2) nanoparticles with trimethylolpropane (TMP) and dimethylolpropionic acid (DMPA) holds significant promise for advancing various technological applications. This research explores the functionalization of TiO2 nanomaterials and compares with pigmentary TiO2 surface coatings technologies used at industrial scale. Through a combination of experimental techniques including FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations, the interactions between surface modifiers and TiO2 are elucidated, revealing the formation of specific surface adsorbates. The results obtained from these studies demonstrated the displacement of surface impurities by adsorption of functionalized small molecules and evaluated the displacement processes among these surface modifiers. Hexafluoroacetylacetone (Hfac) was used as a test compound in displacement investigations utilizing fluorine as a spectroscopic label to precisely trace the displacement chemistry with XPS. This work provides fundamental understanding and offers novel strategies and valuable insights for enhancing the performance of TiO2-based materials. |
SS-ThP-13 Model Studies of Single-Atom Alloy (SAA) Catalysts
Ravi Ranjan (University of California - Riverside) Single-atom alloy (SAA) catalysts have become a prominent way to enhance selectivity in a number of catalytic processes.The mechanism by which these alloys attain their improved performance is still being debated, however, especially when considering the effect of the reaction environment on their structure and chemical composition.We have embarked on a project to emulate those catalysts using model systems where the metals are dispersed as nanoparticles (NPs) onto a flat oxide surface under controlled ultra-high vacuum (UHV) conditions, characterized using a combination of surface-sensitive techniques, and tested for their catalytic behavior using a so-called high-pressure cell. This work focuses on the characterization of the nature of surfaces of SAA catalysts made out of Pt (minority) and Cu (majority) metals and their performance for the promotion of the hydrogenation of unsaturated aldehydes. Cu/TaxOy/Ta surfaces have been prepared via the oxidation of a Ta crystal followed by the vapor deposition of controlled amounts of Cu and characterized using reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) together with carbon monoxide (CO) as a probe molecule. A series of changes in the RAIRS data has been observed as a function of the Cu deposition time, an indication of the development of the Cu NPs from small clusters of atoms with multiple low-coordination sites to larger NPs with more basal planes exposed. These Cu NPs were found to form stable in the temperature range from 300 K to 600 K, but to change beyond 600 K. Like Cu, Pt and Cu-Pt NPs can be grown and characterized via CO titrations using RAIRS and TPD, following a similar approach. Kinetic measurements under catalytic conditions can then be performed to test the efficacy of the Cu, Pt, or Cu-Pt NPs catalysts supported on the tantalum oxide surface and to identify the appropriate structure-reactivity correlations. |
SS-ThP-14 Enhancing Gas-Evolving Electrocatalysis by Tuning the Wetting Properties of Catalyst Microenvironment
Kaige Shi, Xiaofeng Feng (University of Central Florida) Electrocatalysis plays a critical role in the conversion between electricity and chemical fuels. In addition to the development of electrocatalysts, efforts are made to understand and control their local environment during operations, which may influence the mass transport and kinetics of the reactions. Particularly, when gas-phase reactants or products are involved, the wetting properties of the microenvironment around catalytic sites can determine the distribution and diffusion of gas and liquid near the catalyst, and thus impact the catalytic performance. Here we present a study of catalyst microenvironment by controlling the wetting properties of carbon black, which is widely used as a catalyst support in electrocatalysis. We chose the electro-oxidation of hydrazine (N2H4) on carbon-supported Pt nanocatalyst and tuned the wetting properties of carbon support by doping of fluorine (F) or oxygen (O). Interestingly, the electrode with F-doped carbon (more hydrophobic) exhibited a higher activity than that with pristine carbon black. This is attributed to the accelerated removal of N2 gas generated from the catalyst, which would otherwise block the catalyst surface from liquid reactant. Furthermore, the electrode with O-doped carbon (more hydrophilic) showed an even higher activity, benefiting from the increased exposure of catalytic sites to liquid reactant. Our work demonstrates that controlling the catalyst microenvironment by doping of carbon black as catalyst support can be a powerful approach to enhance gas-evolving electrocatalysis.This work is supported by the National Science Foundation (NSF) Chemical Catalysis Program under Grant No. 1943732. |
SS-ThP-15 Advanced Evaluation of Sub-nm Surface Roughness using Electron Diffraction
Rivaldo Marsel Tumbelaka, Ken Hattori (Nara Institute of Science and Technology) Achieving precise control over surface roughness at the sub-nanometer scale is paramount for enhancing the physical properties of semiconductor materials, particularly their electrical transport characteristics. Current methodologies, predominantly scanning probe microscopy, facilitate quantitative roughness assessment but face challenges when applied to surfaces with intricate structures such as vertical and facet faces of three-dimensional (3D) structures, owing to steric hindrance. Our prior research effectively utilized reflection high-energy electron diffraction (RHEED) to observe atomic ordering on various structure surfaces in different directions, overcoming the challenge of steric hindrance [1-3]. In this study, we explore the potential of RHEED as an alternative avenue for evaluating surface roughness. Kinematic diffraction theory introducing diffraction spots from atomic positions, we explore the correlation between RHEED spot profile and surface roughness. Our approach involves systematically analyzing RHEED intensity profiles alongside surface roughness measurements of Highly Ordered Pyrolytic Graphite (HOPG), serving as a prototype sample. HOPG samples with varying degrees of surface roughness were prepared via Ar sputtering, and their roughness was assessed using atomic force microscopy (AFM). Figure 1(a) shows a typical RHEED pattern for the HOPG whose roughness was 0.163 nm. Through the analysis of RHEED spot profiles, we discerned distinct intensity components, and , corresponding to sharpened and broadened spot intensities, respectively (Fig 1(b)). Our findings reveal an excellent correlation between increased surface roughness and the ratio of broadened spot intensity to total intensity (Fig 1(c)), indicating RHEED's adaptability for quantitative evaluation of surface roughness. This study not only shows how effective RHEED can be in measuring surface roughness but also demonstrates its usefulness in analyzing complex 3D surfaces in small-scale research and making semiconductor devices. The details will be discussed in the presentation. References: [1] A. N. Hattori, K. Hattori, et al., Surf. Sci. 644 (2016) 86. [2] A. N. Hattori, K. Hattori, et al., Appl. Phys. Exp. 9 (2016) 085501. [3] K. Hattori, et al., Jap. J. Appl. Phys. 51 (2012) 055801. View Supplemental Document (pdf) |
SS-ThP-16 Post-Synthesis Characterization of PtNi Nanowires for Enhanced Durability and Efficiency
Cesar Saucedo, Jenny Mann, Sarah Zaccarine (Physical Electronics USA) Polymer electrolyte membrane fuel cells (PEMFCs) offer a promising avenue for sustainable electricity production with minimal environmental impact. However, their widespread adoption faces challenges due to durability concerns and the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode. Consequently, to advance PEMFCs, there is a pressing need for the advancement of catalyst technologies to overcome these limitations. Previous works has looked at the synthesis of extended-surface platinum-nickel (PtNi) nanowires (NWs) via atomic layer deposition (ALD) and their durability. Here we show how the composition and chemical states of the nanowires evolve after a series of post-synthetic modifications designed to maximize the longevity of these nanowires as catalysts. By combining complimentary surface analytical techniques (X-ray photoelectron spectroscopy, hard X-ray photoelectron spectroscopy, and nanoscale Auger electron spectroscopy) we develop a more complete model of the complex chemical nature of these catalysts than any single technique could by itself, accelerating the development of more durable and efficient PEMFC catalysts. |
SS-ThP-17 Temperature Dependence of Surface-Catalyzed Ullmann Coupling via Activation of Highly Labile C-I
Chamath Siribaddana, Nan Jiang (University of Illinois Chicago) The surface-catalyzed Ullmann-coupling via activation of highly labile C-I was studied on Au(111) using metal β-diketonato molecules with scanning tunneling microscopy. Unexpectedly the C-I bonds do not dissociate upon deposition of the molecule on the Au(111) surface held at room temperature which is known to catalyze the C-I dissociation reaction under such conditions. Rather the molecules form self-assembled monolayers stabilized by halogen bonds on Au(111). Annealing this sample to 100° C or direct deposition of the molecules on a Au(111) substrate held at 100° C triggers C-I bond cleavage. However, at 100° C intact C-I bonds can be observed, as only a portion of the molecules undergo C-I dissociation. Annealing to 200° C results in complete dehalogenation of the molecules and forming two types of COFs with triangle and cross-shaped connections. The dissociated Iodine atoms are incorporated within the COF. The two kinds of COFs exhibit only short-range network order due to the irreversible nature of the C-C bond formation. This trend is observed up to annealing temperatures of 370° C and no preference is observed towards one type of COF throughout this temperature range. Annealing temperatures beyond 400° C results in the decomposition of the molecules into its ligands which form a COF different from the COFs formed by the molecule. |
SS-ThP-18 Accurate SIMS Characterization of Indium Implant in Silicon
Xuefeng Lin, Scott York, Naveen Kaushik (Micron Technology) Secondary ion mass spectrometry (SIMS) is an ideal technique to provide indium (In) implant depth profiles in Si. However, this approach faces a critical challenge for overcoming the Si molecular mass interferences (MIs). These MIs can create high backgrounds during profiling and create an ambiguous determination of true In depth distributions. This phenomenon is especially true for SIMS analysis of lower dosed In implants <1E15 (at/cm2). The challenge is that the combination of three Si isotopes creates significant MIs with a high mass resolution (MR) > ~45370 needed for separation from In. This is far beyond the magnetic sector dynamic SIMS mass separation capability <10000MR. A special SIMS analysis method of energy filtering by applying an offset voltage has been often used to minimize or remove the Si MIs for In implant analysis. However, applying the different voltage offsets produces different In tailing profiles and background levels. Fig. 1 shows seven SIMS depth profiles acquired on In30KeV7.85E13 implanted in Si, acquired by applying seven voltage offsets from -10 to -30eV, which create different In background levels and tailing profile shapes below 1E18 (at/cm3). Increasing the voltage offset values from -10 to -30eV reduces the In background levels and shrinks the tailing profiles (Fig. 2, as determined at 100nm depth scale). Some previous SIMS studies used the background subtraction rather than applied voltage offset to characterize and obtain the precise In implant profiles, but determination of the specific subtraction value might be an issue since different subtractions would also affect the measured In implant doses and shapes. Fig. 3 shows the same SIMS In implant depth profiles with no subtraction and no voltage offset in green, and light and heavy subtractions in red and blue. To obtain more accurate In implant profiles in Si to overcome random background subtraction issues, we have used SIMS analysis combined with TCAD simulation. This combined method uses an appropriate voltage offset to obtain the SIMS profiles and then it is compared with the TCAD simulation, as shown in Fig. 4. Although several different voltage offsets have to be used to obtain the SIMS depth profiles to match the TCAD simulations to determine the applied voltage offsets at the beginning, once the SIMS analysis protocol is established, the analysis recipes would be used for analyzing the similar In implants in Si. In addition, we present the first study of using secondary ion energy distribution to determine the voltage offsets for analyzing In implants (Fig. 5) and that data will be shown in a full paper. View Supplemental Document (pdf) |
SS-ThP-19 Monitoring the Dynamics of Carbon-Carbon Bond Formation in Solid-Gas Heterogeneous Photoinduced Reactions
Aakash Gupta, Keith Blackman, Alejandro Rodriguez (Department of Physics, University of Central Florida); Mihai Vaida (Department of Physics and Renewable Energy and Chemical Transformations Cluster, University of Central Florida) Carbon-carbon (C-C) bond formation is paramount for a large variety of man-made chemicals such as commodity chemicals, synthetic materials, and pharmaceuticals. Monitoring the dynamics of the C-C bond formation in real time at surfaces and how this is influenced by the surface properties could be a game changer in improving the efficiency of various heterogeneous reactions.To study the dynamics of C-C bond formation, an experimental technique is employed that combines time-of-flight mass spectrometry with pump-probe spectroscopy and fast surface preparation with molecules. CH3I and CO are used as precursor molecules. These molecules are dosed on metal oxides such as titanium dioxide and cerium oxide, on which the reactions are monitored. The reaction is triggered by the pump laser pulse at a central wavelength of 266 nm which excites CH3I into the dissociative A-band, which leads to the formation of CH3 and I fragments. Subsequently, the probe laser pulse in ultraviolet spectral domain will ionize the reaction intermediates and final products, which are immediately removed from the surface by a static electric field and detected by the time-of-flight mass spectrometer. By varying the pump-probe time delay, the formation dynamics of CH3 intermediate, as well as the reaction of CH3 with CO, which leads to the formation CH3CO (acetyl) and finally to CH3COCH3 (acetone) will be monitored. Details on CO and CH3I partial pressure dependence as well as the influence of the surface composition and temperature will be presented. |
SS-ThP-20 Transient Kinetics Study of CO Adsorption and Dissociation on a Ru (001) Surface Crystal
Eliseo Perez Gomez (Stony Brook University); Anibal Boscoboinik (Center for Functional Nanomaterials, BNL); Sayantani Sikder (Stony Brook University) The adsorption and dissociation of carbon monoxide (CO) on metal surfaces is a fundamental reaction with many applications in the field of surface chemistry. Particularly, this research studied the reaction between CO and a Ru (0001) single crystal. The adsorption and dissociation of the gas were investigated through the use of techniques such as infrared spectroscopy (IR) and mass spectroscopy (MS). We also employed a pulsing valve system, where controlled pulses of CO (and other reactant gases) influence the surface reaction. The effects of temperature and pressure on the adsorption kinetics and subsequent dissociation were explored in detail, to reach a broad understanding in the transient kinetics of the reaction. By modifying various parameters, the intention is to optimize the dynamic reaction conditions. Through this comprehensive approach, we sought to contribute valuable insights into the mechanisms driving CO adsorption and dissociation, with implications for catalysis and surface science applications. |
SS-ThP-21 Bimetallic Pt-Sn and Ni-Cu Catalysts for Dehydrogenation Reactions Designed for Hydrogen Storage and Transportation
Mengxiong Qiao, Fangliang Li (University of South Carolina); Ali Ahsen (Gebze Technical University); Donna Chen (University of South Carolina) The use of liquid organic hydrogen carriers (LOHC) for the safe transport and storage of hydrogen requires the development of more selective catalysts for the dehydrogenation step. For example, the methylcyclohexane (MCH)-toluene pair is an attractive choice for a LOHC system, but a more selective dehydrogenation catalyst that is less prone to deactivation by fouling is needed. The bimetallic Pt-Sn and Cu-Ni systems are promising candidates for selective dehydrogenation catalysts; the addition of Sn to Pt catalysts is known to improve stability and decrease deactivation, and the addition of Cu to Ni catalysts may serve to break up the Ni ensembles that are responsible for nonselective decomposition. Model catalyst surfaces are prepared in ultrahigh vacuum (UHV) by deposition of thin films of Sn and Cu on Pt(111) and Ni(111) respectively, as well as by sequential vapor deposition of the two metals on a rutile TiO2(110) support. The surface compositions and bimetallic cluster sizes are investigated by low energy ion scattering, temperature programmed desorption with CO as a probe molecule, X-ray photoelectron spectroscopy, and scanning tunneling microscopy. Our studies have shown that Sn on Pt clusters tend to be rich in Sn due to the higher surface free energy of Pt compared to Sn, but cluster surfaces with a low concentration of Sn can also be prepared by deposition of small coverages of Sn on existing Pt clusters. In contrast, Cu on Ni clusters exhibit low Cu concentrations due to the diffusion of Cu into Ni, which is consistent with the facile alloying of Ni and Cu. A high-sensitivity recirculating loop microreactor system has been constructed for investigating the activity for MCH dehydrogenation on the bimetallic model surfaces under catalytically relevant atmospheric pressures; the microreactor is coupled directly to the UHV chamber so that the model catalysts can be transferred between the microreactor and UHV chamber without exposure to air. |
SS-ThP-22 Coverage-Dependent Adsorption and Reactivity of Formic Acid on Fe3O4(001)
Jose Ortiz-Garcia (Pacific Northwest National Laboratory); Marcus Sharp (Washington State University); Zbynek Novotny, Bruce Kay, Zdenek Dohnalek (Pacific Northwest National Laboratory) Formic acid (FA) is a crucial intermediate in important catalytic reactions such as Fischer-Tropsch synthesis and water-gas shift. On oxide surfaces, FA generally undergoes decomposition to CO or CO2 through decarbonylation or decarboxylation mechanisms. We study the adsorption of FA on the reconstructed Fe3O4(001) surface, followed by stepwise annealing using a combination of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, low-energy electron diffraction, and temperature programmed desorption (TPD). Dissociative adsorption of formic acid leads to the adsorbed formate and hydroxyl species on the surface. At low coverages, isolated formates and hydroxyls are observed. At intermediate coverages, local clustering of formate is observed, giving rise to both (1×1) and (2×1) surface periodicities. A fully saturated surface shows formates arranging predominately in the (1×1) periodicity but retains some formates in the (2×1) periodicity in agreement with prior studies.1 Annealing the surface to 450 K induces the formation of a well-ordered surface with a (1×1) periodicity, which results in the lifting of the surface reconstruction. Stepwise annealing enables monitoring of formate reactivity and surface structure changes. Two major peaks are observed via TPD at 525 and 565 K, indicating that the formate undergoes decarbonylation to CO and H2O, with decarboxylation to CO2 as a minor reaction pathway. STM reveals that annealing to 550 K leads to a partial recovery of the surface reconstruction and a possible formation of single oxygen vacancy defects. Further annealing to 650 K, which leads to the conversion of all formate species, reveals the formation of pits extended along the Fe rows. This is consistent with the non-stoichiometric formic acid decarboxylation accompanied by water formation proceeding via the Mars van Krevelen mechanism. We are currently focusing on understanding the initial stages of pit formation and the role of hydroxyls in reaction mechanisms. This work underscores the significance of fundamental studies to unravel the effect of structural changes on reaction mechanisms and dynamics. View Supplemental Document (pdf) |
SS-ThP-23 Using Single-Layered COFs to Stabilize Single-Atom Catalysts on Model Surfaces
Yufei Bai, David Wisman, Steven Tait (Indiana University Bloomington) Single-atom catalysts (SACs) combine the advantages of homogeneous and heterogeneous catalysts by limiting the reaction sites to isolated single metal atoms with well-defined chemical properties. A metal-ligand coordination method to stabilize SACs has been developed by the Tait group, in which 1,10-phenanthroline-5,6-dione (PDO) was used as ligand to coordinate with metal such as Pt, Fe, and Cr. To further improve the stability of SACs and increase the metal loading, we have synthesized single-layered covalent organic frameworks (sCOFs) on model surfaces under ultra-high vacuum (UHV) conditions or under ambient conditions. These networks with high porosity and stability were used to confine single Pt atoms coordinated with ligands into sCOF pores. Under UHV conditions, the successful formation of sCOF with regular hexagonal pores on the Au(111) surface was achieved by surface-mediated Ullmann radical coupling of 1,3,5-tris-(4-bromophenyl)benzene (TBB). Further sequential deposition of PDO ligand and Pt on the TBB-sCOF surface allowed the formation of single-site Pt catalysts by coordination interaction. The scanning tunneling microscopy (STM) images show the confinement of PDO in the sCOF pores, while X-ray photoelectron spectroscopy (XPS) has proven the existence of oxidation state of Pt, which is an indication of the single atom character. Under ambient conditions, an imine-linked sCOF was formed on the highly oriented pyrolytic graphite (HOPG) surface by a solid-vapor interface mechanism which allows for a high quality sCOF with long-range order. STM characterization has shown that regular sCOF networks with few defects were formed on the HOPG surface. This sCOF is facile to prepare and can be stored stably under ambient conditions for several weeks. These systems which combine the COF and metal-ligand coordination strategy to stabilize SACs offer the possibility to achieve higher stability and greater loading in SACs. |
SS-ThP-24 N-doped Graphene Synthesis through N2+ Irradiation
Buddhika Alupothe Gedara, Prescott Evans, Zdenek Dohnalek, Zbynek Novotny (Pacific Northwest National Laboratory) Hydrogen (H2) is one of the most promising clean and renewable energy sources. Nevertheless, the storage of hydrogen shows poor performance due to the low gravimetric and volumetric densities. Nitrogen-doped graphene (Gr) has been identified as a potential material for H2 storage. Here, we study the growth of Gr on a Ru(0001) surface by chemical vapor deposition (CVD) of pyridine and N-doping through N2+ beam irradiation using scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). A high-quality Gr film with low N densities was obtained by pyridine CVD on Ru(0001) at 1063 K. Higher concentrations of N-dopants were introduced on the Gr/Ru(0001) through low-energy N2+ irradiation at 100 eV.Nitrogen can be embedded in the Gr lattice preferentially in two configurations, namely graphitic N (N substituted in the C lattice) and pyridinic N (substitutional N next to a C vacancy). Atomically resolved STM images of graphitic and pyridinic-N defects demonstrate their preferential locations within the Gr Moiré. XPS shows that coverage of up to 3.9% of pyridinic-N and 2.3% of graphitic N can be embedded into the high-quality Gr film using N2+irradiation at room temperature, indicating a preferential formation of pyridinic N over graphitic N. Only graphitic N was observed upon annealing the ion-irradiated Gr/Ru(0001) to 1063 K, revealing higher thermal stability of graphitic N over pyridinic N. Our current efforts center on the adsorption studies of atomic hydrogen, its interactions with N dopants, and thermally induced diffusion. |
SS-ThP-25 Coverage Dependent Interaction of N-Methylaniline with Pt (111) Surface
Bushra Ashraf, Dave Austin (University of Central Florida); Nils Brinkmann, Katherina Al Shamery (Carl von Ossietzky University Oldenburg); Talat Rahman (University of Central Florida) The study of N-methylaniline (NMA) coverage dependent adsorption and reaction on the Pt(111) surface has gained attention because of the alkyl and aromatic amine interaction with transition metal surface.In this work, we use Density functional theory (DFT)simulations and experiment to study the structural and electronic properties of coverage dependent adsorption of NMA on Pt(111) surface. Firstly, we found that the molecule adheres to the surface by forming bonds with both the phenyl ring and the nitrogen (N) atom at the lower coverage. However, as coverage increases, a fascinating phenomenon unfolds: the phenyl ring undergoes a distinctive tilting motion away from the surface. This tilting is accompanied by a change in the incline angle, transitioning from a mere 2o at low coverage (approximately 0.02 ML) to a substantial 33o at higher coverage (around 0.17 ML). This structural transformation also brings about a variation in the adsorption energy, shifting from -2.9 eV at low coverage to -1.5 eV at high coverage. Secondly, Bader charge analysis provided further insights into these interactions. At low coverage, the charge is sharing between the N atom and the surface Pt atom and from the surface to the carbon (C) atoms with in the phenyl ring. In contrast, at higher coverage, charge sharing primarily takes place through the N atom to the surface. This shift towards a more robust and bi-centered bonding with the surface at lower coverage indicates an increased propensity for molecular dissociation under these conditions. Finally, the analysis of the projected density of states shed more light on revealing the hybridization of the N pz orbital with the Pt dz2 orbital at higher coverage. Conversely, at low coverage,hybridization of the carbon pz orbital with the Pt dz2 orbital was observed, alongside Pt-N hybridization. These observations regarding molecular adsorption at different coverages indicated that, at high coverage, the molecule easily lifts off intact from the surface, while at low coverage, it tends to break into smaller fragments, consistent with experimental findings. |
SS-ThP-26 Effect of an Electric Field on the Co Adsorption on Pt
Steven Arias, Dario Stacchiola, J. Anibal Boscoboinik (Brookhaven National Laboratory) The adsorption of carbon monoxide (CO) on platinum (Pt) has been widely studied in the literature. Pt is one of the most frequently used active metals in catalysis, with CO playing an important role in many reactions, including hydrogenation, oxidation, and car emission controls. Here, we present our preliminary progress on the study of the effect of an electric field on CO adsorption on Pt. To do this, we designed a device that can apply a direct electric field to the surface of our Pt catalyst. A layer of porous alumina was synthesized on a Pt film, to allow CO molecules to diffuse through, making the underlying Pt accessible. A layer of graphene is added on top of the porous layer to complete the device. The bottom Pt and the top layer of graphene are biased to apply an electric field that directly interacts with the Pt surface where the CO is being adsorbed. Using infrared reflective adsorption spectroscopy (IRRAS) we study the effect of such fields on CO adsorption. |
SS-ThP-27 Generating Defects in Semiconductor Monolayers on Metal Surface
Sayantan Mahapatra, Jeffrey Guest (Argonne National Laboratory, USA) Nanoscale observation and deliberate engineering of atomic defects within semiconductor transition metal dichalcogenides (TMDs) hold significant importance for their utilization in cutting-edge quantum optics and nano-electronic devices. Here, we demonstrate a versatile approach in generating single defects on TMDs monolayers using a photon source. The newly generated defects were visualized via scanning tunneling microscopy (STM) at room temperature at the atomic level. These defects can act as single-photon emitters (SPE) and their performance remains excellent in high vacuum conditions. Furthermore, our simulations provide insights into the defect formation energies on metal surfaces compared to an insulating surface. The charge transfer between the metal and monolayer TMDs plays a significant role in generating the defects. Furthermore, simulation also sheds light on the mono- or di-chalcogen vacancies as the potential candidates for these defects, thereby providing a direct match between theory and experiment. |
SS-ThP-28 Characterization of Oxygen on Rh-Based Model Catalysts
Maxwell Gillum, Alexis Gonzalez, Elizabeth Serna-Sanchez, Allison Kerr, Stephanie Danahey, Dan Killelea (Loyola University Chicago) The studies presented investigate the influence that surface and defect geometry have on the kinetics and reactivity of oxygen on various Rh-based model catalysts. The experiments focus on gaining structural information about the oxygen species present on the surface under various oxidative conditions utilizing scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). These techniques are used in unison with temperature programmed desorption (TPD) and Meitner-auger electron spectroscopy (MAES) to identify optimal conditions for further study |
SS-ThP-31 Epoxidation of Styrene on Ag(111) Surface
Rasika E. A. Dissanayake (Institute of Physical Chemistry, University of Göttingen); Arved C. Dorst, Kerim Benfreha (Institute For Physical Chemistry, University Of Goettingen); Daniel R Killelea (Loyola University Chicago); Tim Schaefer (Institute For Physical Chemistry, University Of Goettingen) Epoxidation of olefins on catalytic surfaces is considered to play a major role in chemical industries where Ag-based catalysts are prominently used due to the surface chemistry of the alkene/silver system. However, the fundamentals of olefin oxidation on metal surfaces are still not well understood. Therefore, in this research, we use controlled ultra-high vacuum conditions to understand the fundamentals of the styrene oxidation on an Ag(111) surface. For this purpose, we initially formed an atomic oxygen layer on the Ag(111) surface by dosing 10% NO2 seeded in He at 510 K surface temperature using a supersonic molecular beam. Subsequently, we dosed styrene onto the oxidized silver surface. Desorbing products from the surface were ionized using focused laser radiation and ionized molecules were detected using an ion imaging system equipped with micro-channel plates, a phosphor screen, and a CMOS camera. We systemically form nucleophilic and electrophilic oxygen at the silver surface using two different preparation methods. When the surface was sputtered with Ar+ ions and annealed to 700 K we predominantly formed nucleophilic oxygen, which led to the complete combustion of the styrene. When we only annealed the surface to 700 K, the surface predominantly formed electrophilic oxygen, which led to the formation of styrene oxide. We identified electrophilic oxygen being accompanied by the formation of subsurface oxygen, which seems to amplify the production of styrene oxide. Keywords: Epoxidation, Ion imaging, Molecular beam, Silver, Styrene |
SS-ThP-32 Weakly and Strongly Adsorbed H2O Layers on Hydroxylated SiO2 Surfaces: Dependence on H2O Pressure at Various Temperatures
Samantha Rau, Rebecca Hirsch, Marcel Junige (University of Colorado Boulder); Antonio Rotondaro, Hanna Paddubrouskaya, Kate Abel (Tokyo Electron America, Inc.); Steve George (University of Colorado Boulder) Although H2O adsorption on SiO2 surfaces has been studied extensively, there are still many questions about the nature of the H2O adsorbed layer. In this work, the H2O layer thickness on flat hydroxylated SiO2 surfaces was examined in a vacuum environment using in situ spectroscopic ellipsometry (SE). The H2O water layer thickness was measured versus H2O pressure at various temperatures. Complementary Fourier transform infrared (FTIR) analysis was also performed on SiO2 powders. Flat SiO2 surfaces were hydroxylated using H2O2 plasma exposure to produce a hydrophilic surface with a water contact angle of < 10°. The in situ SE measurements were then conducted in a warm-wall vacuum chamber designed with a temperature-controlled sample stage. The H2O layer thickness was measured versus pressure at various temperatures (Figure 1). The H2O pressures were varied up to the saturation H2O vapor pressure corresponding to the sample temperature. The H2O layer thickness versus relative humidity was consistent with general expectations from the BET adsorption isotherm model. The SE measurements showed that there were two distinct types of H2O layers: a weakly adsorbed layer and a strongly adsorbed layer (Figure 2). The weakly adsorbed layer could be added or subtracted by increasing or removing the H2O pressure. The strongly adsorbed layer was not lost by removing the H2O pressure. However, the strongly adsorbed layer could be desorbed by heating the sample stage to 120°C. The SE measurements characterized the layer thicknesses for the weakly and strongly adsorbed layers versus H2O pressure at various sample temperatures. Using repeating H2O exposures, the strongly adsorbed layer reached an approximate plateau at ~1 Å at various temperatures. In contrast, the weakly adsorbed layer obtained higher thicknesses at larger H2O pressures. For example, the weakly adsorbed layer thickness was 7 Å at 92% relative humidity at 30.4ºC (30 Torr). The FTIR investigations on SiO2 powders were in qualitative agreement with the SE studies. These studies confirm the existence of a strongly adsorbed vicinal layer and a weakly adsorbed layer explained by the BET model on hydroxylated SiO2. View Supplemental Document (pdf) |