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In this dissertation, there are three topics including nanoscale engineering effects on cathodes for intermediate-temperature solid oxide fuel cells, effects of mechanical and electrical application on atomic force microscopy, and ionic diffusivity and mobility study of oxygen reduction/evolution reaction by nanoscale in situ characterization via atomic force microscopy. Their abstracts are listed below. Topic I: The impact of various infiltrates on the kinetics and rate-limiting step of oxygen reduction reaction (ORR) is examined with LaNi0.6Fe0.4O3-δ (LNF) as the cathode backbone of solid oxide fuel cells (SOFC). Multiple materials were infiltrated on the backbone. The dependencies of electrode polarization resistance on the precursor concentration, temperature, and oxygen partial pressure are presented, and related discussions are made to interpret the differences in ORR kinetics and the rate-determining step for ORR. Topic II: Conductive atomic force microscopy (CAFM) has been widely employed to study the localized electrical properties of a wide range of substrates in non-vacuum conditions by the use of noble metal-coated tips. In this topic, the impact of mechanical and electrical stimuli on the apex geometry of gold-coated tips and electrical conduction properties at the tip-substrate contact is discussed by choosing gold and highly-ordered pyrolytic graphite as the representative tip and substrate materials, respectively. Topic III: The ionic transport in solid oxides after becoming thermally activated is understood as hopping to point defects (i.e. oxygen vacancies), and the kinetics behind it are highly dependent upon the bonding state of the ionic species with their surrounding lattice environment. In this study we report the findings of our recent efforts to directly observe ionic transport in SrTiO3 (STO) and Y2O3-stabilized ZrO2 (YSZ)–the most widely used cathode material in intermediate temperature SOFC–by use of Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (CAFM).

Metal oxide semiconductor (MOS) thin films are promising sensing layer for integration in gas sensor devices for detecting toxic and harmful molecules. Herein, tungsten oxide (WO3) thin films are deposited on interdigital electrodes by vacuum thermal evaporation to realize batch fabrication of high-performance gas sensors. Subsequent annealing at different temperatures allows for regulation of the concentration of oxygen vacancies in the WO3 films, which has been found to exert a great influence on the sensor properties. In addition, the surface structure of WO3 films is also highly dependent on the annealing temperature. Gas sensing investigations show that the WO3 sensor annealed at 500 °C possesses the best sensing properties for detecting triethylamine (TEA) including very high response, good selectivity, fast response, and low limit of detection (63 ppb). The excellent sensor performances are attributed to the enhanced adsorption of oxidative oxygen species due to the presence of abundant oxygen vacancies. The scalable fabrication of WO3 thin film gas sensors and the oxygen vacancy engineering strategy proposed herein may shed some light to developing high performance environmental sensors.

In this paper, the influence of exposed facets of silvered TiO2 photocatalysts for photodenitrification was fundamentally investigated. We found that photodeposition of silver on 001-faceted TiO2 w...

HfxZr1-xO2 and lanthanum-doped HfxZr1-xO2:La thin films are candidates for applications in ferroelectric random-access memory. Here, we explore the atomic and electronic structure of Hf0.5Zr0.5O2 and Hf0.5Zr0.5O2:La thin films grown by atomic layer deposition. Using X-ray photoelectron spectroscopy, it was found that the oxides under study have an almost identical electronic structure and a bandgap of about 5.4 eV. The Hf0.5Zr0.5O2:La film was shown to consist of the mixture of Hf0.5Zr0.5O2 and La2O3 phases. The bombardment with argon ions of the studied films leads to oxygen vacancy generation in the near-surface layer. The oxygen vacancy concentrations in the bombarded films were evaluated from the comparison of experimental valence band photoelectron spectra with the theoretical ones calculated using the density functional theory.

The active components Mn and Fe oxides were loaded on the surface of CeO2 catalyst with nanopolyhedrons morphology (CeO2-NPs) by impregnation method to improve the CeO2-NPs catalyst catalytic performance. The activity test results showed that the activity order of these catalysts within the temperature of 100–300 °C was Fe-Mn-Ce > Mn-Ce > Ce catalyst, and the NO conversion rate of Fe-Mn-Ce catalyst was the highest at 200 °C, which was about 98%. The performances of these catalysts were further investigated by SEM, XRD, XPS, Raman spectra, H2-TPR, NH3-TPD and in situ DRIFT. The results indicated that Fe-Mn-Ce catalyst possessed the best reducibility and surface acidity of the three catalysts. According to the XPS and Raman spectra tests, the sequences of the ratio of Oα/(Oα+Oβ) and Ce3+/(Ce3++Ce4+) were both Fe-Mn-Ce > Mn-Ce > Ce catalyst, which indicated that the Fe-Mn-Ce catalyst possessed the most oxygen vacancy and further improved the catalytic activity of the catalyst. Moreover, the Fe-Mn-Ce catalyst also exhibited the best tolerance to SO2 of the three catalysts at 225 °C. In general, simultaneous loading of manganese and iron oxides on the CeO2-NPs catalyst could effectively improve the low temperature SCR performance of the catalyst.

The exploration for low-cost bifunctional materials for highly efficient overall water splitting has drawn profound research attention. Here, we present a facile preparation of Mo–P dual-doped Co/o...

A well-designed two-dimensional 2D/2D architecture was constructed by coupling oxygen-vacancy-rich trimetallic CoAlLa layered double hydroxide (CoAlLa-LDH) with titania-sandwiched Ti3C2 MXene multi...

The singly dispersed bimetallic sites can be constructed by a guest metal atom (A) occupying the oxygen vacancy on a host metal oxide (MOx) surface and present a large category of single-cluster ca...

Atomically dispersed supported catalysts hold considerable promise as catalytic materials. The ability to employ and stabilize them against aggregation in complex process environments remains a key challenge to the elusive goal of 100% atom utilization in catalysis. Herein, using a Gd-doped ceria support for atomically dispersed surface Pt atoms, we establish how the combined effects of aliovalent doping and oxygen vacancy generation provide dynamic mechanisms that serve to enhance the stability of supported single-atom configurations. Using correlated, in situ X-ray absorption, photoelectron, and vibrational spectroscopy methods for the analysis of samples on the two types of support (with and without Gd doping), we establish that the Pt atoms are located proximal to Gd dopants, forming a speciation that serves to enhance the thermal stability of Pt atoms against aggregation.

The grain boundary (GB) plays a critical role in a material’s properties and device performance. Therefore, the characterization of a GB’s atomic structure and electrostatic characteristics is a matter of great importance for materials science. Here, we report on the atomic structure and electrostatic analysis of a GB in a SrTiO3 bicrystal by four-dimensional scanning transmission electron microscopy (4D-STEM). We demonstrate that the Σ5 GB is Ti-rich and poor in Sr. We investigate possible effects on the variation in the atomic electrostatic field, including oxygen vacancies, Ti-valence change, and accumulation of cations. A negative charge resulting from a space-charge zone in SrTiO3 compensates a positive charge accumulated at the GB, which is in agreement with the double-Schottky-barrier model. It demonstrates the feasibility of characterizing the electrostatic properties at the nanometer scale by 4D-STEM, which provides comprehensive insights to understanding the GB structure and its concomitant effects on the electrostatic properties.

Under reducing conditions, rutile TiO₂ develops O vacancies (VO) coupled to Ti³⁺ centers. It is favorable for H atoms to enter this system, either forming OH groups or occupying vacancy sites (denoted HO) that bond to two Ti atoms next to the vacancy. OH defects are well documented by the presence of infrared modes at ∼3300 cm⁻¹, while HO is relatively underinvestigated. We report the energies, geometries, and vibrational frequencies of hydrogen defects in rutile predicted from quantum mechanics calculations, focusing on the coexistence of OH and H_O. We find that H_O is more stable than OH by 1.42 eV, leading to an infrared mode at ∼1200 cm⁻¹. Introducing a second H forms an OH bond with an infrared mode at ∼3300 cm⁻¹. These results suggest that assessments of hydrogen storage in mantle phases of rutile and similar minerals based on OH bands may significantly underestimate H concentrations.

Mg-doped ZnO nanoparticle (NP) thin-film transistors (TFTs) were fabricated using a solution process under a range of Mg content and annealing temperature conditions. The effects of Mg doping and a...

Recently, a bifunctional oxide–zeolite (OX-ZEO) catalyst was widely studied experimentally, which can selectively convert syngas to light olefins. The performance of OX-ZEO is exceptional, while th...

Summary CeO2-based nanostructures have recently aroused interest for various applications because of their higher OSC and redox properties, allowing them to facilely switch between the oxidation states of Ce4+ and Ce3+. There are many studies that have been focused on the morphology-dependent catalysis and identification of the true active sites during catalysis reactions. Probing the intrinsic properties of CeO2 nanostructures, including the surface structure and reduction, oxygen vacancy, grain boundary, morphology, and dynamic structure under environmental conditions, is essential in clarifying their physical and chemical properties and expanding their potential applications. In this review, we summarize recent significant achievements in characterizing CeO2 nanostructures and related catalytic phenomena through advanced techniques. Several studies using model CeO2 catalysts have been selected to show the importance of strong metal-support interactions. A deep insight into the active sites in morphologically controllable CeO2 that exposes the highly active facets is expected to guide the design of nanocatalysts.

The development of materials with photocatalytic activity in a visible-light region is of great significance for solar energy utilization and clean energy production. In this paper, we designed a GaAs quantum dot (QD)/TiO2 heterojunction with high performance of visible-light photocatalytic hydrogen evolution based on first-principle calculations. The results show that six designed GaAs QDs are thermally stable at 300 K, and they have proper visible-light absorption except GaAs-12 QD. GaAs-20 QD maintains good stability at 350 K. Further investigations indicate that the interface Ga atoms in the GaAs-20/TiO2 heterojunction are active sites for photocatalytic reaction, and the type-II band alignment of the GaAs-20/TiO2 heterojunction can lead to effective separation of photoexcited electron–hole pairs. Moreover, the energy barrier of hydrogen production in GaAs-20/TiO2 is about 56% lower than that in TiO2, and oxygen vacancy in GaAs-20/TiO2 heterojunction could transform the hydrogen production from endothermic reaction into exothermic reaction, which has an obvious promotion effect on photocatalytic hydrogen evolution. In short, our calculations indicate that the GaAs-20/TiO2 heterojunction has high performance in visible-light photocatalytic hydrogen evolution. The GaAs quantum dot (QD)/TiO2 heterojunction with high performance of visible-light photocatalytic hydrogen evolution was designed based on first-principle calculations. The oxygen vacancy in the GaAs-20/TiO2 heterojunction could transform the hydrogen production from endothermic reaction into exothermic reaction.

We employ first-principles modeling alongside atomistic molecular dynamics simulations to investigate the impact of oxygen vacancy defects on the thermal transport of β-Ga2O3. Our predictions of th...

A series of CunCeMgOx catalysts with various copper nanoparticle sizes and surface defect densities were synthesized and tested for partial hydrogenation of 1,3-butadiene (1,3-BD). The data demonst...

The role of oxygen vacancies in temperature-dependent photoluminescence of SnO2 nanowires was investigated by X-ray absorption spectroscopy. Two types of oxygen vacancies are present in the nanowir...

To avoid the energy-consuming step of direct N≡N bond cleavage, photocatalytic N2 fixation undergoing the associative pathways has been developed for mild-condition operation. However, it is a fundamental yet challenging task to gain comprehensive understanding on how the associative pathways (i.e., alternating vs. distal) are influenced and altered by the fine structure of catalysts, which eventually holds the key to significantly promote the practical implementation. Herein, we introduce Fe dopants into TiO2 nanofibers to stabilize oxygen vacancies and simultaneously tune their local electronic structure. The combination of in situ characterizations with first-principles simulations reveals that the modulation of local electronic structure by Fe dopants turns the hydrogenation of N2 from associative alternating pathway to associative distal pathway. This work provides fresh hints for rationally controlling the reaction pathways toward efficient photocatalytic nitrogen fixation.

In this study, we explored the role of the intrinsic defects on the tuning thermoelectric (TE) performance of Al-doped ZnO (ZnO:Al) films. The intrinsic defects of the Zn interstitial (Zni) and the oxygen vacancy (VO) were controlled by varying the Zn content by using the radio frequency atomic source vacuum evaporation method. In the Zn-rich case, the excessive Zn formed Zni to provide most of the carriers. VO became the main origin of the carriers with a decreasing Zn content. When the Zn content was 55.2% and 52.5%, the segregation of Zni decreased the carrier concentration and subsequently, enhanced the resistivity and the Seebeck coefficient. The ZnO:Al film with a Zn content of 52.5% had the highest power factor of 274.03 μW·m−1·K−2 at 819 K. In addition, the higher Zni concentration had a more prominent effect on thermal conductivity. The thermal conductivity increased to 1.756 W·m−1·K−1 from 0.608 W·m−1·K−1 with the Zn content decreasing from 55.2% to 50.6%. The ZT value reached the highest of 0.121 when the film had the optimum Zn concentration of 51.5%. This study concluded that the tuning of the intrinsic defects was an effective method to enhance the TE performance.

通过表面氧空位缺陷调控催化剂的表面电子结构, 实现催化剂表面易于富集参与反应的电子, 从而提高还原催化效率. 本文中, 将导电玻璃隔绝空气加热, 在其表面形成氧空位缺陷. 通过控制热处理时间调控表面氧空位浓度梯度. 氧空位能导致SnO2导带弯曲, 因此调控氧空位梯度可增强SnO2导电玻璃表面电子富集速率, 从而实现高效电催化还原反应. 在相同条件下, 使其催化性能可以和金属铂相媲美.

To realize efficient atmospheric CO2 chemisorption and activation, abundant Ti3+ sites and oxygen vacancies in TiO2 ultrathin layers were designed. Positron annihilation lifetime spectroscopy and theoretical calculations first unveil each oxygen vacancy is associated with the formation of two Ti3+ sites, giving a Ti3+-Vo-Ti3+ configuration. The Ti3+-Vo-Ti3+ sites could bond with CO2 molecules to form a stable configuration, which converted the endoergic chemisorption step to an exoergic process, verified by in-situ Fourier-transform infrared spectra and theoretical calculations. Also, the adjacent Ti3+ sites not only favor CO2 activation into COOH* via forming a stable Ti3+-C-O-Ti3+ configuration, but also facilitate the rate-limiting COOH* scission to CO* by reducing the energy barrier from 0.75 to 0.45 eV. Thus, the Ti3+-Vo-TiO2 ultrathinlayers could directly capture and photofix atmospheric CO2 into near-unity CO, with the corresponding CO2-to-CO conversion ratio of ca. 20.2%.

Carcinogenic hexavalent Cr in the current iron oxide-based catalysts of high temperature water-gas shift (HT-WGS) reaction is great environmental concern. Interpreting the role of the Cr in this important industrial catalyst system is required. In the present study, we investigated substitution of Cr atoms into the most stable termination of Fe3O4 (111) slab surface by spin-polarized periodic DFT approach to comprehend the role of Cr. We applied the projector augmented-wave (PAW) method within the Perdew-Wang 1991 (PW91) form of the generalized gradient approximation (GGA) on the Vienna Ab-initio Simulation Package (VASP). The calculations point out that Cr atoms choose being below the surface FeO6 sites. Cr slightly affects the dissociative H2O adsorption. There is no effect on the CO adsorption. Oxygen vacancy is favored to form on the topmost layer with less vacancy formation energy. Substitution of Cr into the structure increases the oxygen vacancy formation energy. This indicates that Cr does not act as a chemical promoter and does not affect the catalytic activity positively, which is experimentally confirmed by the previous studies.

Significant improvements on Ge nMOSFET can be achieved by a novel low temperature supercritical phase fluid treatment with H2O2 cosolvent. Thanks to the reduction of oxygen vacancy and unstable oxidation states, devices with the proposed treatment exhibit a low equivalent oxide thickness of 0.67 nm, 2-order reduction on gate leakage current, 7 times improvement on on-current, very low subthreshold swing of 79 mV/dev, high on/off current ratio, small hysteresis of 43.3 mV, low interface trap density, excellent uniformity and reliability characteristics.

Oxide-supported single-metal-atom catalysts have shown extraordinary catalytic properties for a water–gas shift (WGS) reaction. Herein, a series of single metal atoms (M = Cu, Ag, Au, Ni, Pd, and P...