The Surface Structure of Cu₂O(100): Nature of Defects

The Cu₂O­(100) surface is most favorably terminated by a (3,0;1,1) reconstruction under ultrahigh-vacuum conditions. As most oxide surfaces, it exhibit defects, and it is these sites that are focus of attention in this study. The surface defects are identified, their properties are investigated, and procedures to accurately control their coverage are demonstrated by a combination of scanning tunneling microscopy (STM) and simulations within the framework of density functional theory (DFT). The most prevalent surface defect was identified as an oxygen vacancy. By comparison of experimental results, formation energies, and simulated STM images, the location of the oxygen vacancies was identified as an oxygen vacancy in position B, located in the valley between the two rows of oxygen atoms terminating the unperturbed surface. The coverage of defects is influenced by the surface preparation parameters and the history of the sample. Furthermore, using low-energy electron beam bombardment, we show that the oxygen vacancy coverage can be accurately controlled and reach a complete surface coverage (1 per unit cell or 1.8 defects per nm²) without modification to the periodicity of the surface, highlighting the importance of using local probes when investigating oxide surfaces

Role of Defects in Surface Chemistry on Cu₂O(111)

High-resolution photoemission spectroscopy and scanning tunneling microscopy (STM) have been used to investigate defects on Cu₂O­(111) and their interaction with water and sulfur dioxide (SO₂). Two types of point defects, i.e., oxygen and copper vacancies, are identified. Copper vacancies are believed to be the most important defects in both water and SO₂ surface chemistry. Multiply coordinatively unsaturated oxygen anions (O_MCUS) such as oxygen anions adjacent to copper vacancies are believed to be adsorption sites for both water and SO₂ reaction products. Water adsorption at 150 K results in both molecular and dissociated water. Molecular water leaves the surface at 180 K. At 300 K and even more at 150 K, SO₂ interacts with oxygen sites at the surface forming SO₃ species. However, thermal treatment up to 280 K of Cu₂O­(111)/SO₂ prepared at 150 K renders only SO₄ on the surface

Redox-Mediated Reconstruction of Copper during Carbon Monoxide Oxidation

Copper has excellent initial activity for the oxidation of CO, yet it rapidly deactivates under reaction conditions. In an effort to obtain a full picture of the dynamic morphological and chemical changes occurring on the surface of catalysts under CO oxidation conditions, a complementary set of in situ ambient pressure (AP) techniques that include scanning tunneling microscopy, infrared reflection absorption spectroscopy (IRRAS), and X-ray photoelectron spectroscopy were conducted. Herein, we report in situ AP CO oxidation experiments over Cu(111) model catalysts at room temperature. Depending on the CO:O₂ ratio, Cu presents different oxidation states, leading to the coexistence of several phases. During CO oxidation, a redox cycle is observed on the substrate’s surface, in which Cu atoms are oxidized and pulled from terraces and step edges and then are reduced and rejoin nearby step edges. IRRAS results confirm the presence of under-coordinated Cu atoms during the reaction. By using control experiments to isolate individual phases, it is shown that the rate for CO oxidation decreases systematically as metallic copper is fully oxidized

The Surface Structure of Cu₂O(100)

Despite the industrial importance of copper oxides, the nature of the (100) surface of Cu₂O has remained poorly understood. The surface has previously been subject to several theoretical and experimental studies, but has until now not been investigated by atomically resolved microscopy or high-resolution photoelectron spectroscopy. Here we determine the atomic structure and electronic properties of Cu₂O­(100) by a combination of multiple experimental techniques and simulations within the framework of density functional theory (DFT). Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) characterized the three ordered surface structures found. From DFT calculations, the structures are found to be energetically ordered as (3,0;1,1), c(2 × 2), and (1 × 1) under ultrahigh vacuum conditions. Increased oxygen pressures induce the formation of an oxygen terminated (1 × 1) surface structure. The most common termination of Cu₂O­(100) has previously been described by a (3√2 × √2)­R45° unit cell exhibiting a LEED pattern with several missing spots. Through atomically resolved STM, we show that this structure instead is described by the matrix (3,0;1,1). Both simulated STM images and calculated photoemission core level shifts compare favorably with the experimental results

<i>In Situ</i> Imaging of Cu₂O under Reducing Conditions: Formation of Metallic Fronts by Mass Transfer

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of <i>in situ</i> microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu₂O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution <i>in situ</i> imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces

<i>In Situ</i> Imaging of Cu₂O under Reducing Conditions: Formation of Metallic Fronts by Mass Transfer

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of <i>in situ</i> microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu₂O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution <i>in situ</i> imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces

<i>In Situ</i> Imaging of Cu₂O under Reducing Conditions: Formation of Metallic Fronts by Mass Transfer

Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of <i>in situ</i> microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst’s chemical state and morphology are dynamically modified. The reduction of Cu₂O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution <i>in situ</i> imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces