Adsorption, diffusion, and dissociation of molecular oxygen at defected TiO2(110): a density functional theory study

Charge State of Au Atoms on an Oxidized Rutile TiO2(110) Surface by AFM/KPFM at 78 K

The charge state of noble metal atoms on a semiconductor surface is an important factor in surface catalysis. In this study, Au atoms were deposited on the rutile TiO2(110) surface to characterize its charge properties using atomic force microscopy with Kelvin probe force microscopy at 78 K. Au single atoms, dimers, and trimers at different sites on the surface were investigated. Positively charged Au atoms were verified at oxygen sites, while negatively charged Au atoms were found near oxygen vacancy sites. Furthermore, the charge states of small Au nanoclusters were clarified. Understanding the charge states of Au atoms is significant for identifying their efficient catalytic effects in surface catalysis.

Oxygen Healing and CO2 /H2 /Anisole Dissociation on Reduced Molybdenum Oxide Surfaces Studied by Density Functional Theory

Reduced molybdenum oxides are versatile catalysts for deoxygenation and hydrodeoxygenation reactions. In this work, we have performed spin-polarized DFT calculations to investigate oxygen healing energies on reduced molybdenum oxides (reduced α-MoO₃ , γ-Mo₄ O₁₁ and MoO₂ ). We find that Mo⁺⁴ on MoO₂ (100) is the most active for abstracting an oxygen from the oxygenated compounds. We further explored CO₂ adsorption and dissociation on reduced α-MoO₃ (010) and MoO₂ (100). In comparison to reduced α-MoO₃ (010), CO₂ adsorbs more strongly on MoO₂ (100). We find that CO₂ dissociates on MoO₂ (100) via a two-step process, the overall barrier for which is 0.6 eV. This barrier is 1.7 eV lower than that on reduced α-MoO₃ (010), suggesting a much higher activity for deoxygenation of CO₂ to CO. As H₂ dissociation is shown to be the rate-limiting step for hydrodeoxygenation reactions, we also studied activation barriers for H₂ chemisorption on MoO₂ (100). We find that the chemisorption barriers are 0.7 eV lower than that reported on reduced α-MoO₃ (010). Finally, we have studied the dissociation (C-O cleavage) of anisole (a lignin-based biofuel model compound) on MoO₂ (100). We find that anisole binds very strongly on MoO₂ (100) with an adsorption energy of -1.47 eV. According to Sabatier's principle, strongly adsorbing reactants poison the catalyst surface, which may explain the low activity of MoO₂ observed during experiments for hydrodeoxygenation of anisole.

Dynamic interplay between metal nanoparticles and oxide support under redox conditions

The dynamic interactions between noble metal particles and reducible metal-oxide supports can depend on redox reactions with ambient gases. Transmission electron microscopy revealed that the strong metal-support interaction (SMSI)-induced encapsulation of platinum particles on titania observed under reducing conditions is lost once the system is exposed to a redox-reactive environment containing oxygen and hydrogen at a total pressure of ~1 bar. Destabilization of the metal-oxide interface and redox-mediated reconstructions of titania lead to particle dynamics and directed particle migration that depend on nanoparticle orientation. A static encapsulated SMSI state was reestablished when switching back to purely oxidizing conditions. This work highlights the difference between reactive and nonreactive states and demonstrates that manifestations of the metal-support interaction strongly depend on the chemical environment.

Chemistry of Oxygen Ionosorption on SnO2 Surfaces

Ionosorbed oxygen is the key player in reactions on metal-oxide surfaces. This is particularly evident for chemiresistive gas sensors, which operate by modulating the conductivity of active materials through the formation/removal of surface O-related acceptors. Strikingly though, the exact type of species behind the sensing response remains obscure even for the most common material systems. The paradigm for ab initio modeling to date has been centered around charge-neutral surface species, ignoring the fact that molecular adsorbates are required to ionize to induce the sensing response. Herein, we resolve this inconsistency by carrying out a careful analysis of all charged O-related species on three naturally occurring surfaces of SnO₂. We reveal that two types of surface acceptors can form spontaneously upon the adsorption of atmospheric oxygen: (i) superoxide O₂^- on the (110) and the (101) surfaces and (ii) doubly ionized O^2- on the (100) facet, with the previous experimental evidence pointing to the latter as the source of sensing response. This species has a unique geometry involving a large displacement of surface Sn, forcing it to attain the coordination resembling that of Sn²⁺ in SnO, which seems necessary to stabilize O^2- and activate metal-oxide surfaces for gas sensing.

Chemistry of Oxygen Ionosorption on SnO2 Surfaces

Ionosorbed oxygen is the key player in reactions on metal-oxide surfaces. This is particularly evident for chemiresistive gas sensors, which operate by modulating the conductivity of active materials through the formation/removal of surface O-related acceptors. Strikingly though, the exact type of species behind the sensing response remains obscure even for the most common material systems. The paradigm for ab initio modeling to date has been centered around charge-neutral surface species, ignoring the fact that molecular adsorbates are required to ionize to induce the sensing response. Herein, we resolve this inconsistency by carrying out a careful analysis of all charged O-related species on three naturally occurring surfaces of SnO₂. We reveal that two types of surface acceptors can form spontaneously upon the adsorption of atmospheric oxygen: (i) superoxide O₂^- on the (110) and the (101) surfaces and (ii) doubly ionized O^2- on the (100) facet, with the previous experimental evidence pointing to the latter as the source of sensing response. This species has a unique geometry involving a large displacement of surface Sn, forcing it to attain the coordination resembling that of Sn²⁺ in SnO, which seems necessary to stabilize O^2- and activate metal-oxide surfaces for gas sensing.

Kinetic Control of Oxygen Interstitial Interaction with TiO2(110) via the Surface Fermi Energy

Atomically clean surfaces of semiconducting oxides efficiently mediate the interconversion of gas-phase O₂ and solid-phase oxygen interstitial atoms (O_i). First-principles calculations together with mesoscale microkinetic modeling are employed for TiO₂(110) to determine reaction pathways, assess appropriate rate expressions, and obtain corresponding activation energies and pre-exponential factors. The Fermi energy (E_F) at the surface influences the rate-determining step for both injection and annihilation of O_i. The barriers range between 0.72-0.82 eV for injection and 0.60-2.34 eV for annihilation and may be manipulated through intentional control of E_F. At equilibrium, the microkinetic model and first-principles calculations indicate that interconversion of O_i species in the first and second sublayers limits the rate. The effective pre-exponential factors for injection and annihilation are surprisingly low, probably resulting from the use of simple Langmuir-like rate expressions to describe a complicated kinetic sequence.

Fermi level dependence of gas-solid oxygen defect exchange mechanism on TiO2 (110) by first-principles calculations

In the same way that gases interact with oxide semiconductor surfaces from above, point defects interact from below. Previous experiments have described defect-surface reactions for TiO₂(110), but an atomistic picture of the mechanism remains unknown. The present work employs computations by density functional theory of the thermodynamic stabilities of metastable states to elucidate possible reaction pathways for oxygen interstitial atoms at TiO₂(110). The simulations uncover unexpected metastable states including dumbbell and split configurations in the surface plane that resemble analogous interstitial species in the deep bulk. Comparison of the energy landscapes involving neutral (unionized) and charged intermediates shows that the Fermi energy E_F exerts a strong influence on the identity of the most likely pathway. The largest elementary-step thermodynamic barrier for interstitial injection trends mostly downward by 2.1 eV as E_F increases between the valence and conduction band edges, while that for annihilation trends upward by 2.1 eV. Several charged intermediates become stabilized for most values of E_F upon receiving conduction band electrons from TiO₂, and the behavior of these species governs much of the overall energy landscape.

Imaging oxygen molecular adsorption and dissociation on the Ti site of rutile TiO2(110) surface with real configuration at 78 K by atomic force microscopy

Understanding oxygen adsorption and dissociation on the five-fold coordinated titanium (Ti5c) site of the rutile TiO2 surface is important in clarifying chemical reaction processes. Accordingly, three different configurations of molecularly adsorbed O2, including parallel side-on, inclined side-on and end-on configurations, and their dissociation were directly observed with atomic resolution at 78 K by atomic force microscopy. Our results experimentally demonstrated that the three adsorbed O2 configurations could be changed by electric field stimulation. The initial configurations of the adsorbed O2 and transition of O2 configurations were related to their coverage. On the other hand, the tunneling current stimulation could dissociate these O2 species, indicating that they are precursors for the O adatom (Oad). It is proposed that the effect of electric field stimulation contributes to the transition of these three adsorbed O2 configurations, and the effect of the tunneling current is the main factor for the dissociation of the adsorbed O2. In addition, based on the atomic contrast and height histograms of Oad, different charge states of Oad were observed, which could coexist on the surface region. The present study demonstrates an intuitional observation of O2 adsorption and dissociation on the Ti5c site, and thus is expected to be useful to understand the surface reactions on the oxide surface.

Resolving the adsorption of molecular O2 on the rutile TiO2(110) surface by noncontact atomic force microscopy

Interaction of molecular oxygen with semiconducting oxide surfaces plays a key role in many technologies. The topic is difficult to approach both by experiment and in theory, mainly due to multiple stable charge states, adsorption configurations, and reaction channels of adsorbed oxygen species. Here we use a combination of noncontact atomic force microscopy (AFM) and density functional theory (DFT) to resolve [Formula: see text] adsorption on the rutile [Formula: see text](110) surface, which presents a longstanding challenge in the surface chemistry of metal oxides. We show that chemically inert AFM tips terminated by an oxygen adatom provide excellent resolution of both the adsorbed species and the oxygen sublattice of the substrate. Adsorbed [Formula: see text] molecules can accept either one or two electron polarons from the surface, forming superoxo or peroxo species. The peroxo state is energetically preferred under any conditions relevant for applications. The possibility of nonintrusive imaging allows us to explain behavior related to electron/hole injection from the tip, interaction with UV light, and the effect of thermal annealing.

First principles investigation of dissociative adsorption of H2 during CO2 hydrogenation over cubic and hexagonal In2O3 catalysts

Dissociative adsorption of H₂ and migration of the adsorbed H adatom over cubic and hexagonal In₂O₃ catalysts were investigated.

First-principles study on the mechanism of photocatalytic reduction of nitrobenzene on the rutile TiO2(110) surface

Oxygen vacancies and excess electrons play vital roles in the photocatalytic reduction of nitrobenzene on the TiO₂ surface.

Fundamentals of TiO2 Photocatalysis: Concepts, Mechanisms, and Challenges

Photocatalysis has been widely applied in various areas, such as solar cells, water splitting, and pollutant degradation. Therefore, the photochemical mechanisms and basic principles of photocatalysis, especially TiO₂ photocatalysis, have been extensively investigated by various surface science methods in the last decade, aiming to provide important information for TiO₂ photocatalysis under real environmental conditions. Recent progress that provides fundamental insights into TiO₂ photocatalysis at a molecular level is highlighted. Insights into the structures of TiO₂ and the basic principles of TiO₂ photocatalysis are discussed first, which provides the basic concepts of TiO₂ photocatalysis. Following this, details of the photochemistry of three important molecules (oxygen, water, methanol) on the model TiO₂ surfaces are presented, in an attempt to unravel the relationship between charge/energy transfer and bond breaking/forming in TiO₂ photocatalysis. Lastly, challenges and opportunities of the mechanistic studies of TiO₂ photocatalysis at the molecular level are discussed briefly, as well as possible photocatalysis models.

Dynamics of Photo-Induced Surface Oxygen Vacancies in Metal-Oxide Semiconductors Studied Under Ambient Conditions

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical technique commonly used in the detection of traces of organic molecules. The mechanism of SERS is of a dual nature, with Raman scattering enhancements due to a combination of electromagnetic (EM) and chemical contributions. In conventional SERS, the EM component is largely responsible for the enhancement, with the chemical contribution playing a less significant role. An alternative technique, called photo-induced enhanced Raman spectroscopy (PIERS) has been recently developed, using a photo-activated semiconductor substrate to give additional chemical enhancement of Raman bands over traditional SERS. This enhancement is assigned to surface oxygen vacancies (V o) formed upon pre-irradiation of the substrate. In this work, the exceptional chemical contribution in PIERS allows for the evaluation of atomic V o dynamics in metal oxide surfaces. This technique is applied to study the formation and healing rates of surface-active V o in archetypical metal-oxide semiconductors, namely, TiO2, WO3, and ZnO. Contrary to conventional analytical tools, PIERS provides intuitive and valuable information about surface stability of atomic defects at ambient pressure and under operando conditions, which has important implications in a wide range of applications including catalysis and energy storage materials.

Intrinsic interaction between in-plane ferroelectric polarization and surface adsorption

The chemical properties of a ferroelectric surface are polarization dependent, the underlying nature of which is, however, far from being completely understood. One of the reasons is that when the polarization direction is perpendicular to the surface, the depolarization field may induce electronic or atomic reconstruction and thus change the chemistry of the ferroelectric surface in complicated ways. Instead, the in-plane polarization results in no depolarization field. Therefore, the chemical properties of a ferroelectric surface can be more intrinsically reflected by the interplay between the in-plane polarization and the surface adsorption. By using first-principles calculations, we study the effect of the strain-induced in-plane polarization on the adsorption of a series of molecules on the reduced rutile TiO2(110) surface. We reveal that it is the surface doping caused by the charge transfer between the adsorbates and the TiO2(110) surface that dominates the polarization-induced change of the adsorption energy, as a result of screening long-range Coulomb interactions. The electrostatic interaction between the polarization of the substrate and the polar molecule is of relatively less importance. We propose that charge transfer effects generally occur for ferroelectric surfaces with no localized surface states.

Structural and electronic properties of bulk and low-index surfaces of zincblende PtC

Transition metal carbides have been extensively used in diverse applications over the past decade. Their versatility is in part thanks to their unique bonding, which displays a mixture of ionic, metallic and covalent character. While the bulk structure of zincblende (ZB) PtC has been investigated several times, a detailed understanding of the electronic and structural properties of its low-index surfaces is lacking. In this work, we present an ab initio investigation of the properties of five crystallographic ZB PtC surfaces (Pt/C-terminated PtC(1 0 0), PtC(1 1 0) and Pt/C-terminated PtC(1 1 1)). Upon geometry optimization, both polar and nonpolar surfaces undergo a mild interlayer relaxation, without extensive reconstructions. Calculated vacancy formation energies indicate facile C removal on the (1 1 1) surface while Pt-vacancy formation is endothermic. Finally, atomic O adsorption energies on all surfaces reveal a high affinity of the C-terminated surfaces towards this species.

Adsorption of oxygen and CO oxidation on Au/anatase(001) catalysts. A DFT+U study

Active oxygen for CO oxidation over Au/TiO₂ is a highly stable atomic species, which is formed at the perimeter of the Au/oxide interface.

One-step preparation of CdS-modified mesoporous titanate nanobelts and their application as high-performance cationic dye adsorbents

CdS-modified mesoporous titanate nanobelts (CTNS) were prepared successfully by one step, using TiO₂ nanobelts and CdS quantum dots, and applied as an adsorbent for the first time for the removal of typical cationic dyes.

Elementary photocatalytic chemistry on TiO2surfaces

In this article, we review the recent advances in the photoreactions of small molecules with model TiO₂surfaces, and propose a photocatalytical model based on nonadiabatic dynamics and ground state surface reactions.

NO adsorption and reaction on single crystal rutile TiO2(110) surfaces studied using UHV–FTIRS

Schematic models of cis-(NO)₂ dimers (I and III) and N₂O molecules (II) on reduced TiO₂(110) surfaces. (I) Bidentate adsorption configuration on Ti_5c⁴⁺ sites and (III) monodentate adsorption configuration at Vo sites.

Effect of hydrogen on O2 adsorption and dissociation on a TiO2 anatase (001) surface

The effect of hydrogen on the adsorption and dissociation of the oxygen molecule on a TiO2 anatase (001) surface is studied by first-principles calculations coupled with the nudged elastic band (NEB) method. Hydrogen adatoms on the surface can increase the absolute value of the adsorption energy of the oxygen molecule. A single H adatom on an anatase (001) surface can lower dramatically the dissociation barrier of the oxygen molecule. The adsorption energy of an O2 molecule is high enough to break the O=O bond. The system energy is lowered after dissociation. If two H adatoms are together on the surface, an oxygen molecule can be also strongly adsorbed, and the adsorption energy is high enough to break the O=O bond. However, the system energy increases after dissociation. Because dissociation of the oxygen molecule on a hydrogenated anatase (001) surface is more efficient, and the oxygen adatoms on the anatase surface can be used to oxidize other adsorbed toxic small gas molecules, hydrogenated anatase is a promising catalyst candidate.

Physical origin of general oscillation of structure, surface energy, and electronic property in rutile TiO2 nanoslab

Titanium oxide (TiO(2)) nanostructures have been attracting consistent focus in the past few years because of their enhanced power in solar-energy conversion. Surface and interface play a crucial role in the determination of thermodynamic stability and electronic structure of TiO(2) nanostructures. The rutile (110) nanoslab (NS) has been used as a common subject to investigate the surface relaxation, defect characters, molecule adsorption, and chemically dynamic reaction of TiO(2) nanostructures. Up to date, a long-time standing issue in TiO(2) NS, i.e., the general oscillation of structure, surface energy and electronic property with changing of NS thickness, has not been clear. We have presented a comprehensive investigation on the relationship between surface and oscillation behavior in the TiO(2) (110) NS by the first-principles calculations accompanied with the wave function analysis. We clearly, for the first time, pointed out that the dipoles and surface states bonding induced by the surface-surface interactions are the physical origin of general oscillations in the TiO(2) (110) NS. Our findings not only have a new insight into the basic interactions between surfaces in TiO(2) nanostructures, but also provide useful information for tuning the photocatalytic and photovoltaic properties by surface design.