Transition metal atoms pathways on rutile TiO2 (110) surface: Distribution of Ti3+ states and evidence of enhanced peripheral charge accumulation

Advances in Synthesis and Applications of Single-Atom Catalysts for Metal Oxide-Based Gas Sensors

As a stable, low-cost, environment-friendly, and gas-sensitive material, semiconductor metal oxides have been widely used for gas sensing. In the past few years, single-atom catalysts (SACs) have gained increasing attention in the field of gas sensing with the advantages of maximized atomic utilization and unique electronic and chemical properties and have successfully been applied to enhance the detection sensitivity and selectivity of metal oxide gas sensors. However, the application of SACs in gas sensors is still in its infancy. Herein, we critically review the recent advances and current status of single-atom catalysts in metal oxide gas sensors, providing some suggestions for the development of this field. The synthesis methods and characterization techniques of SAC-modified metal oxides are summarized. The interactions between SACs and metal oxides are crucial for the stable loading of single-atom catalysts and for improving gas-sensitive performance. Then, the current application progress of various SACs (Au, Pt, Cu, Ni, etc.) in metal oxide gas sensors is introduced. Finally, the challenges and perspectives of SACs in metal oxide gas sensors are presented.

Photolysis versus Photothermolysis of N2O on a Semiconductor Surface Revealed by Nonadiabatic Molecular Dynamics

Identifying photolysis and photothermolysis during a photochemical reaction has remained challenging because of the highly non-equilibrium and ultrafast nature of the processes. Using state-of-the-art ab initio adiabatic and nonadiabatic molecular dynamics, we investigate N₂O photodissociation on the reduced rutile TiO₂(110) surface and establish its detailed mechanism. The photodecomposition is initiated by electron injection, leading to the formation of a N₂O^- ion-radical, and activation of the N₂O bending and symmetric stretching vibrations. Photothermolysis governs the N₂O dissociation when N₂O^- is short-lived. The dissociation is activated by a combination of the anionic excited state evolution and local heating. A thermal fluctuation drives the molecular acceptor level below the TiO₂ band edge, stabilizes the N₂O^- anion radical, and causes dissociation on a 1 ps timescale. As the N₂O^- resonance lifetime increases, photolysis becomes dominant since evolution in the anionic excited state activates the bending and symmetric stretching of N₂O, inducing the dissociation. The photodecomposition occurs more easily when N₂O is bonded to TiO₂ through the O rather than N atom. We demonstrate further that a thermal dissociation of N₂O can be realized by a rational choice of metal dopants, which enhance p-d orbital hybridization, facilitate electron transfer, and break N₂O spontaneously. By investigating the charge dynamics and lifetime, we provide a fundamental atomistic understanding of the competition and synergy between the photocatalytic and photothermocatalytic dissociation of N₂O and demonstrate how N₂O reduction can be controlled by light irradiation, adsorption configuration, and dopants, enabling the design of high-performance transition-metal oxide catalysts.

CO Adsorbate Promotes Polaron Photoactivity on the Reduced Rutile TiO2(110) Surface

Polarons play a major role in determining the chemical properties of transition-metal oxides. Recent experiments show that adsorbates can attract inner polarons to surface sites. These findings require an atomistic understanding of the adsorbate influence on polaron dynamics and lifetime. We consider reduced rutile TiO₂(110) with an oxygen vacancy as a prototypical surface and a CO molecule as a classic probe and perform ab initio adiabatic molecular dynamics, time-domain density functional theory, and nonadiabatic molecular dynamics simulations. The simulations show that subsurface polarons have little influence on CO adsorption and CO can desorb easily. On the contrary, surface polarons strongly enhance CO adsorption. At the same time, the adsorbed CO attracts polarons to the surface, allowing them to participate in catalytic processes with CO. The CO interaction with polarons changes their orbital origin, suppresses polaron hopping, and stabilizes them at surface sites. Partial delocalization of polarons onto CO decouples them from free holes, decreasing the nonadiabatic coupling and shortening the quantum coherence time, thereby reducing charge recombination. The calculations demonstrate that CO prefers to adsorb at the next-nearest-neighbor five-coordinated Ti³⁺ surface electron polaron sites. The reported results provide a fundamental understanding of the influence of electron polarons on the initial stage of reactant adsorption and the effect of the adsorbate-polaron interaction on the polaron dynamics and lifetime. The study demonstrates how charge and polaron properties can be controlled by adsorbed species, allowing one to design high-performance transition-metal oxide catalysts.

Adsorption energy scaling relation on bimetallic magnetic surfaces: role of surface magnetic moments

The scaling relationships between the adsorption energies of different reaction intermediates have a tremendous effect in the field of surface science, particularly in predicting new catalytic materials. In the last few decades, these scaling laws have been extensively studied and interpreted by a number of research groups which makes them almost universally accepted. In this work, we report the breakdown of the standard scaling law in magnetic bimetallic transition metal (TM) surfaces for hydrogenated species of oxygen (O), carbon (C), and nitrogen (N), where the adsorption energies are estimated using density functional theory (DFT). We propose that the scaling relationships do not necessarily rely solely on the adsorbates, they can also be strongly dependent on the surface properties. For magnetic bimetallic TM surfaces, the magnetic moment plays a vital role in the estimation of adsorption energy, and therefore towards the linear scaling relation.

Photoresponses of Supported Au Single Atoms on TiO2(110) through the Metal-Induced Gap States

When a metal single-atom (SA) catalyst is supported on a semiconducting photocatalyst, the charge transfer of the photoexcited carriers to metal SAs can provide a synergetic activity for the co-catalysts. Here, we report the interfacial electronic coupling of the Au SAs on the TiO₂(110) surface using scanning tunneling microscopy/spectroscopy, in combination with first-principles calculations. Distinct energy and spatial distributions of the metal-induced gap states (MIGSs) are experimentally revealed for the Au SAs adsorbed at the terminal Ti sites and the oxygen vacancies. The localized MIGS below the Fermi level provides a dedicated channel for the transfer of a photoexcited hole from the TiO₂ substrate to the adsorbed Au SAs. The hole can weaken the Ti-Au bonding and activate the diffusion of Au SAs. Our results shed light on combining the advantages of photocatalysis and metal SA catalysis using a co-catalyst, which is promising to promote chemical reactions at low temperatures.

KPFM/AFM imaging on TiO2(110) surface in O2 gas

We have carried out high-speed imaging of the topography and local contact potential difference (LCPD) on rutile TiO₂(110) in O₂ gas by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). We succeeded in KPFM/AFM imaging with atomic resolution at 1 frame min^-1 and observed the adsorbate on a hydroxylated TiO₂(110) surface. The observed adsorbate is considered to be oxygen adatoms (O_a), hydroperoxyls (HO₂), or terminal hydroxyls (OH_t). After adsorption, changes in the topography and the LCPD of the adsorbate were observed. This phenomenon is thought to be caused by the charge transfer of the adsorbate. This technique has the potential to observe catalytic behavior with atomic resolution.

The local electronic properties of individual Pt atoms adsorbed on TiO2(110) studied by Kelvin probe force microscopy and first-principles simulations

Noble metal nanostructures dispersed on metal oxide surfaces have applications in diverse areas such as catalysis, chemical sensing, and energy harvesting. Their reactivity, chemical selectivity, stability, and light absorption properties are controlled by the interactions at the metal/oxide interface. Single-atom metal adsorbates on the rutile TiO₂(110)-(1 × 1) surface have become a paradigmatic model to characterize those interactions and to understand the unique electronic properties of these supported nanostructures. We combine Kelvin probe force microscopy (KPFM) experiments and density functional theory (DFT) calculations to investigate the atomic-scale variations in the contact potential difference of individual Pt atoms adsorbed on a hydroxylated (h) TiO₂(110)-(1 × 1) surface. Our experiments show a significant drop in the local contact potential difference (LCPD) over Pt atoms with respect to the TiO₂ surface, supporting the presence of an electron transfer from the Pt adsorbates to the substrate. We have identified two characteristic regimes by LCPD spectroscopy. At far tip-sample distances, LCPD values show a weak distance dependence and can be attributed to the intrinsic charge transfer from Pt to the oxide support. Beyond the onset of short-range chemical interactions, LCPD values exhibit a strong distance dependence that we ascribe to the local structural and charge rearrangements induced by the tip-sample interaction. These findings also apply to other electropositive adsorbates such as potassium and the hydrogen atoms forming the OH groups that are present on the h-TiO₂(110) surface, promoting KPFM as a suitable tool for the understanding of electron transfer in catalytically active materials.

Surface-Charge-Mediated Formation of H-TiO2 @Ni(OH)2 Heterostructures for High-Performance Supercapacitors

An electrochemically favorable Ni(OH)₂ with porously hierarchical structure and ultrathin nanosheets in a core-shell structure H-TiO₂ @Ni(OH)₂ is achieved through modulating the surface chemical activity of TiO₂ by hydrogenation, which creates a defect-rich surface of TiO₂ , thereby facilitating the subsequent nucleation and growth of Ni(OH)₂ . These configuration-tailored H-TiO₂ @Ni(OH)₂ core-shell nanowires exhibit a superior electrochemical performance and good flexibility.

Promoter effect of hydration on the nucleation of nanoparticles: direct observation for gold and copper on rutile TiO2 (110)

Direct observation of the promoting effect of hydration on the nucleation of gold and copper nanoparticles supported on partially reduced rutile TiO2 (110) is achieved by combined scanning tunneling microscopy experiments and density functional theory calculations. The experiments show a clear difference between the two metals. Gold nanoparticles grow at the vicinity of the surface hydroxyl domains, whereas the nucleation of copper is not substantially affected by hydration. The nucleation of gold on surface oxygen vacancies is observed although this is not the only preferential site. Theoretical calculations of the coadsorbed phases of gold, copper and hydroxyl species on stoichiometric and reduced TiO2 (110) surfaces under relevant conditions of temperature and pressure support the experimental interpretation. Surface hydration tends to stabilize significantly gold adsorption on the stoichiometric support, while its influence on copper adsorption is not pronounced. The theoretical analysis shows that the early stages of the nucleation on hydrated stoichiometric surfaces correspond to mono-hydroxylated metallic species co-chemisorbed with hydroxyl species, whereas those on hydrated reduced surfaces are metallic atoms bound to oxygen vacancies and weakly perturbed by surface hydration.

Tailoring the Catalytic Properties of Metal Nanoparticles via Support Interactions

The development of new catalysts for energy technology and environmental remediation requires a thorough knowledge of how the physical and chemical properties of a catalyst affect its reactivity. For supported metal nanoparticles (NPs), such properties can include the particle size, shape, composition, and chemical state, but a critical parameter which must not be overlooked is the role of the NP support. Here, we highlight the key mechanisms behind support-induced enhancement in the catalytic properties of metal NPs. These include support-induced changes in the NP morphology, stability, electronic structure, and chemical state, as well as changes in the support due to the NPs. Utilizing the support-dependent phenomena described in this Perspective may allow significant breakthroughs in the design and tailoring of the catalytic activity and selectivity of metal nanoparticles.

Adsorption sites of single noble metal atoms on the rutile TiO2 (1 1 0) surface influenced by different surface oxygen vacancies

Atomic adsorption of Au and Pt on the rutile (1 1 0) surface was investigated by atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) measurements combined with density functional theory calculations. Au single atoms were deposited on the surface in a vacuum condition, and the observed results were compared with Pt single atoms on the same surface prepared by the same experimental manner. It was found that Au single atoms are stably adsorbed only at the bridging oxygen vacancy sites, which is quite different from Pt single atoms exhibiting the most frequently observed adsorption at the basal oxygen vacancy sites. Such a difference in oxygen-vacancy effect between Au and Pt can be explained by electronic structures of the surface vacancies as well as characters of outermost atomic orbitals of Au and Pt.

Substitutionally doped phosphorene: electronic properties and gas sensing

Phosphorene, a new elemental two-dimensional material, has attracted increasing attention owing to its intriguing electronic properties. In particular, pristine phospohorene, due to its ultrahigh surface-volume ratio and high chemical activity, has been shown to be promising for gas sensing (Abbas et al 2015 ACS Nano 9 5618). To further enhance its sensing ability, we perform first-principles calculations based on density functional theory to study substitutionally doped phosphorene with 17 different atoms, focusing on structures, energetics, electronic properties and gas sensing. Our calculations reveal that anionic X (X = O, C and S) dopants have a large binding energy and highly dispersive electronic states, signifying the formation of covalent X-P bonds and thus strong structural stability. Alkali atom (Li and Na) doping is found to donate most of the electrons in the outer s-orbital by forming ionic bonds with P, and the band gap decreases by pushing down the conduction band, suggesting that the optical and electronic properties of the doped phosphorene can be tailored. For doping with VIIIB-group (Fe, Co and Ni) elements, a strong affinity is predicted and the binding energy and charge transfer are correlated strongly with their electronegativity. By examining NO molecule adsorption, we find that these metal doped phosphorenes (MDPs) in general exhibit a significantly enhanced chemical activity compared with pristine phosphorene. Our study suggests that substitutionally doped phosphorene shows many intriguing electronic and optic properties different from pristine phosphorene and MDPs are promising in chemical applications involving molecular adsorption and desorption processes, such as materials growth, catalysis, gas sensing and storage.

Synthesis and characterization of a multifunctional nanocatalyst based on a novel type of binary-metal-oxide-coated Fe3O4–Au nanoparticle

A novel type of binary-metal-oxide-coated Au nanocatalyst, including a mixed oxide layer, a moveable magnetic Fe₃O₄ core and some Au NPs of 2–5 nm, has been synthesized successfully by a facile hydrothermal synthesis method.

Self-assembly of hollow spherical nanocatalysts with encapsulated Pt NPs and the effect of Ce-dipping on catalytic activity

This article reports a facile and controllable one-step method to construct Pt@hollow mesoporous SiO₂ (Pt@HMSiO₂) nanoparticles (NPs).

3D TiO2@Ni(OH)2 Core-shell Arrays with Tunable Nanostructure for Hybrid Supercapacitor Application

Three dimensional hierarchical nanostructures have attracted great attention for electrochemical energy storage applications. In this work, self-supported TiO₂@Ni(OH)₂ core-shell nanowire arrays are prepared on carbon fiber paper via the combination of hydrothermal synthesis and chemical bath deposition. In this core-shell hybrid, the morphology and wall size of the interconnected nanoflake shell of Ni(OH)₂ can be tuned through adjusting the concentration of ammonia solution. Heterogeneous nucleation and subsequent oriented crystal growth are identified to be the synthesis mechanism affecting the nanostructure of the shell material, which consequently determines the electrochemical performance in both energy storage and charge transfer. Superior capabilities of 264 mAhg⁻¹ at 1 A g⁻¹ and 178 mAh g⁻¹ at 10 A g⁻¹ are achieved with the core-shell hybrids of the optimized structure. The asymmetric supercapacitor prototype, comprising of TiO₂@Ni(OH)₂ as the anode and mesoporous carbons (MCs) as the cathode, is shown to exhibit superior electrochemical performance with high energy and power densities. The present work provides a clear illustration of the structure-property relationship in nanocrystal synthesis and offers a potential strategy to enhance the battery type Ni(OH)₂ electrode in a hybrid supercapacitor device.

Quick and facile preparation of visible light-driven TiO2 photocatalyst with high absorption and photocatalytic activity

Self-doping TiO₂ has recently attracted considerable attention for its high photocatalytic activity under visible-light irradiation. However, the literature reported synthetic methods until now were very time-consuming. In this study, we establish a quick and facile method for obtaining self-doping TiO₂ with the use of directly treated commercial P25 at a desired temperature for only 5 min through spark plasma sintering technology. With the using of this method, the modified P25 samples exhibit significantly high photoelectric and photocatalytic performance. Furthermore, the sample prepared at 600°C exhibits the optimum catalytic activity. The photodegradation and H₂ evolution rates of this samples are significantly higher than those of unmodified P25 sample under visible-light irradiation. The physical origin of the visible-light absorption for the modified P25 samples is investigated in detail according to their structural, optical, and electronic properties. This work will provide a quick and facile method for the large-scale synthesis of visible-light driven photocatalyst for practical applications.

A charge optimized many-body (COMB) potential for titanium and titania

This work proposes an empirical, variable charge potential for Ti and TiO(2) systems based on the charge-optimized many-body (COMB) potential framework. The parameters of the potential function are fit to the structural and mechanical properties of the Ti hcp phase, the TiO(2) rutile phase, and the energetics of polymorphs of both Ti and TiO(2). The relative stabilities of TiO(2) rutile surfaces are predicted and compared to the results of density functional theory (DFT) and empirical potential calculations. The transferability of the developed potential is demonstrated by determining the adsorption energy of Cu clusters of various sizes on the rutile TiO(2)(1 1 0) surface using molecular dynamics simulations. The results indicate that the adsorption energy is dependent on the number of Cu-Cu bonds and Cu-O bonds formed at the Cu/TiO(2) interface. The adsorption energies of Cu clusters on the reduced and oxidized TiO(2)(1 1 0) surfaces are also investigated, and the COMB potential predicts enhanced bonding between Cu clusters and the oxidized surface, which is consistent with both experimental observations and the results of DFT calculations for other transition metals (Au and Ag) on this oxidized surface.