Pronounced, Reversible, and in Situ Modification of the Electronic Structure of Graphene Oxide via Buckling below 160 K

We have shown that the electronic structure of graphene oxide is strongly, but reversibly, affected by temperature. Below 160 K, graphene oxide is much more completely oxidized, removing any last remaining π-conjugated network. Through DFT simulations, we have shown that this is due to buckling-induced oxidation. As temperature is reduced, the lightly oxidized, graphene-like zones attempt to expand due to a negative thermal expansion coefficient (TEC), but the heavily oxidized zones, with a TEC that is near zero, prevent this from happening. This contributes to localized buckling. The deformed regions oxidize much more readily, and the 1,2-epoxide groups form a new type of functional group never before seen: a triply bonded oxygen, bonded at the 1,3,5 sites of the hexagonal carbon rings. We have called this group TB-epoxide. Stable only under buckling, the TB-epoxide groups revert back to 1,2-epoxides once the lattice relaxes to a flatter profile. We have shown that one can alter the electronic structure of graphene oxide to induce temporary, but more complete, oxidation via strain

Comparative Photovoltaic Study of Physical Blending of Two Donor–Acceptor Polymers with the Chemical Blending of the Respective Moieties

A regularly alternating terpolymer and a random terpolymer were synthesized from the constituent units of two donor–acceptor polymers with complementary absorption. They were then compared to a physical blend of these two donor–acceptor polymers in order to investigate the best means of extending the light absorption range in bulk heterojunction (BHJ) solar cells. While all three methods broadened the light absorption, the physical blend provided the best improvement in power conversion efficiency (4.10% vs 3.63% and 2.67% for the random and regular terpolymers, respectively). This is due to the increase in aggregation in the physical blend, as demonstrated in the UV–vis spectra, which likely leads to higher local mobility and less recombination. This study shows that in order to effectively increase the light absorption (and therefore performance) of a polymer:fullerene based BHJ solar cell, a terpolymer must retain a structure which allows sufficient aggregation

Enhanced Stability of Pt-Cu Single-Atom Alloy Catalysts: In Situ Characterization of the Pt/Cu(111) Surface in an Ambient Pressure of CO

The interaction between a catalyst and reactants often induces changes in the surface structure and composition of the catalyst, which, in turn, affect its reactivity. Therefore, it is important to study such changes using in situ techniques under well-controlled conditions. We have used ambient pressure X-ray photoelectron spectroscopy to study the surface stability of a Pt/Cu(111) single-atom alloy in an ambient pressure of CO. By directly probing the Pt atoms, we found that CO causes a slight surface segregation of Pt atoms at room temperature. In addition, while the Pt/Cu(111) surface demonstrates poor thermal stability in ultrahigh vacuum conditions, where surface Pt starts to diffuse to the subsurface layer above 400 K, the presence of adsorbed CO enhances the thermal stability of surface Pt atoms. However, we also found that temperatures above 450 K cause restructuring of the subsurface layer, which consequently strengthens the CO binding to the surface Pt sites, likely because of the presence of neighboring subsurface Pt atoms

Microscopic Investigation of H₂ Reduced CuO_<i>x</i>/Cu(111) and ZnO/CuO_<i>x</i>/Cu(111) Inverse Catalysts: STM, AP-XPS, and DFT Studies

Understanding the reduction mechanism of ZnO/CuOx interfaces by hydrogen is of great importance in advancing the performance of industrial catalysts used for CO and CO2 hydrogenation to oxygenates, the water-gas shift, and the reforming of methanol. Here, the reduction of pristine and ZnO-modified CuOx/Cu(111) by H2 was investigated using ambient-pressure scanning tunneling microscopy (AP-STM), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and density functional theory (DFT). The morphological changes and reaction rates seen for the reduction of CuOx/Cu(111) and ZnO/CuOx/Cu(111) are very different. On CuOx/Cu(111), perfect “44” and “29” structures displayed a very low reactivity toward H2 at room temperature. A long induction period associated with an autocatalytic process was observed to enable the reduction by the removal of chemisorbed nonlattice oxygen initially and lattice oxygen sequentially at the CuOx–Cu interface, which led to the formation of oxygen-deficient “5–7” hex and honeycomb structures. In the final stages of the reduction process, regions of residual oxygen species and metallic Cu were seen. The addition of ZnO particles to CuOx/Cu(111) opened additional reaction channels. On the ZnO sites, the dissociation of H2 was fast and H adatoms easily migrated to adjacent regions of copper oxide. This hydrogen spillover substantially enhanced the rate of oxygen removal, resulting in the rapid reduction of the copper oxide located in the periphery of the zinc oxide islands with no signs of the reduction of ZnO. The deposited ZnO completely modified the dynamics for H2 dissociation and hydrogen migration, providing an excellent source for CO2 hydrogenation processes on the inverse oxide/metal system

Fast Surface Oxygen Release Kinetics Accelerate Nanoparticle Exsolution in Perovskite Oxides

Exsolution is a recent advancement for fabricating oxide-supported metal nanoparticle catalysts via phase precipitation out of a host oxide. A fundamental understanding and control of the exsolution kinetics are needed to engineer exsolved nanoparticles to obtain higher catalytic activity toward clean energy and fuel conversion. Since oxygen release via oxygen vacancy formation in the host oxide is behind oxide reduction and metal exsolution, we hypothesize that the kinetics of metal exsolution should depend on the kinetics of oxygen release, in addition to the kinetics of metal cation diffusion. Here, we probe the surface exsolution kinetics both experimentally and theoretically using thin-film perovskite SrTi0.65Fe0.35O3 (STF) as a model system. We quantitatively demonstrated that in this system the surface oxygen release governs the metal nanoparticle exsolution kinetics. As a result, by increasing the oxygen release rate in STF, either by reducing the sample thickness or by increasing the surface reactivity, one can effectively accelerate the Fe0 exsolution kinetics. Fast oxygen release kinetics in STF not only shortened the prereduction time prior to the exsolution onset, but also increased the total quantity of exsolved Fe0 over time, which agrees well with the predictions from our analytical kinetic modeling. The consistency between the results obtained from in situ experiments and analytical modeling provides a predictive capability for tailoring exsolution, and highlights the importance of engineering host oxide surface oxygen release kinetics in designing exsolved nanocatalysts

Investigating the Elusive Nature of Atomic O from CO₂ Dissociation on Pd(111): The Role of Surface Hydrogen

CO2 dissociation is a key step in CO2 conversion reactions to produce value-added chemicals typically through hydrogenation. In many cases, the atomic O produced from CO2 dissociation can potentially block adsorption sites or change the oxidation state of the catalyst. Here, we used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations to investigate the presence of surface species from the dissociation of CO2 on Pd(111). AP-XPS results show that CO2 was dissociated to produce adsorbed CO, but dissociated atomic O was not observed at room temperature. We were only able to observe atomic O when CO2 was introduced at 500 K. Further investigations of O-covered Pd(111) revealed that chemisorbed O could be easily removed by low pressures of CO and H2. Notably, the effect of H2 is quite prominent since it could react with chemisorbed O at a pressure as low as 2 × 10–9 Torr, and the presence of H2 at ambient pressure prevented CO2 dissociation. DFT calculations showed that in the presence of background H2, facile CO2 dissociation took place via the reverse water–gas shift (rWGS) reaction, which resulted in the formation of adsorbed CO and removal of O by H2. DFT also identified the possible variation of surface species on simultaneous exposure of CO2 and H2 over Pd(111) depending on temperature and pressure, which opens alternative opportunities to tune the CO2 hydrogenation catalysis by controlling the reaction conditions

MgO Nanostructures on Cu(111): Understanding Size- and Morphology-Dependent CO₂ Binding and Hydrogenation

To design and optimize cost-effective technologies for the capture, utilization, and storage of carbon dioxide (CO2), we need fundamental knowledge and control of chemical interactions associated with the capture and conversion of the molecule into high-value chemicals, minerals, and all kinds of materials. Bulk magnesium oxide (MgO) is frequently used for the trapping and storage of CO2 by the generation of magnesium carbonates. In this study, the growth and reactivity of MgO nanostructures on a Cu2O/Cu­(111) substrate were investigated by using scanning tunneling microscopy (STM) and synchrotron-based ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). For extremely small concentrations of Mg (∼0.01 monolayer (ML)), a well-ordered film of copper oxide with small clusters (0.2–0.5 nm in width, 0.4–0.6 Å in height) of embedded MgO was seen. At a coverage of 0.1 ML, MgO nanoparticles with a width of 0.4 to 1 nm and a height of ∼1.5 Å were randomly distributed on the copper oxide. Random distribution was also observed when the MgO coverage was raised to 0.25 ML, with the width of the MgO particles increasing to 2–2.5 nm and the height reaching 2 Å. These oxide nanostructures displayed a high reactivity toward CO2 and H2 that is not seen for bulk MgO. Dissociation of H2 was observed at room temperature with the reaction of the H adatoms with CuOx and C-containing groups. On the small MgO nanostructures (<1 nm in width), instead of plain carbonate formation, there was dissociation of CO2 into CO and C species, opening reaction channels for the conversion of this harmful molecule into oxygenates and light alkanes

Accelerated Cu₂O Reduction by Single Pt Atoms at the Metal-Oxide Interface

The reducibility of metal oxides, when they serve as the catalyst support or are the active sites themselves, plays an important role in heterogeneous catalytic reactions. Here we present an integrated experimental and theoretical study that reveals how the addition of small amounts of atomically dispersed Pt at the metal/oxide interface dramatically enhances the reducibility of a Cu2O thin film by H2. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) results reveal that, upon oxidation, a PtCu single-atom alloy (SAA) surface is covered by a thin Cu2O film and is, therefore, unable to dissociate H2. Despite this, in situ studies using ambient-pressure (AP) XPS reveal that the presence of a small amount of Pt under the oxide layer can, at the single-atom limit, promote the reduction of Cu2O by H2 at room temperature. We built two density functional theory based surface models to better understand these experimental findings: a Cu2O/Cu­(111)-like surface oxide layer, known as the “29” oxide, in which Pt is alloyed into the Cu(111) surface, as well as a PtCu SAA. Our calculations suggest that the increased activity is due to the presence of atomically dispersed Pt under the surface oxide layer, which weakens the Cu–O bonds in its immediate vicinity, thus making the interface between subsurface Pt and the surface oxide a nucleation site for the formation of metallic Cu. This initial step in the reduction process results in the presence of surface Pt atoms surrounded by metallic Cu patches, and the Pt atoms become active in H2 dissociation, which consequently accelerates the reduction of the oxide layer. This work demonstrates how isolated Pt atoms at the metal/oxide interface of a Cu-based catalyst accelerate the reduction of the oxide and, therefore, help maintain the active, reduced state of the catalyst under the reaction conditions

Tuning Strong Metal–Support Interactions via Synergistic Alloying

The encapsulation phenomenon associated with a strong metal–support interactions (SMSI) has been largely restricted to catalyst systems consisting of group VIII metals with high surface energy and reducible transition metal oxide supports with low surface energy. Here, we demonstrate an encapsulation phenomenon that, while sharing morphological similarities with conventional SMSI, follows a distinctive pathway. This is shown by the encapsulation of CuAu nanoparticles (NPs) supported on a highly ordered pyrolytic graphite (HOPG). Through dynamic monitoring of Cu, Au, and Cu50Au50 NPs in an oxidizing atmosphere using ambient-pressure X-ray photoelectron spectroscopy, we show that this spontaneous encapsulation is achieved through the synergistic effect of the alloying elements. Specifically, the surface segregation of Cu promotes dissociative O2 adsorption, leading to the formation of atomic O species, while the subsurface enrichment of Au hinders O incorporation of oxygen into the bulk of CuAu NPs. Consequently, O spillover onto the graphite support occurs, resulting in the oxidation of the HOPG surface into graphitic oxide species. The higher affinity of the graphitic oxide species toward the Cu-segregated surface prompts their migration from the HOPG support to encapsulate the CuAu NPs. These results transcend the conventional SMSI and bear practical implications for the design and development of heterogeneous catalysts, particularly in carbon-supported alloy systems