Dispersion of Defects in TiO2 Semiconductor: Oxygen Vacancies in the Bulk and Surface of Rutile and Anatase

Understanding and optimizing the sensitization of anatase titanium dioxide surface with hematite clusters

The presence of hematite (Fe2O3) clusters at low coverage on titanium dioxide (TiO2) surface has been observed to enhance photocatalytic activity, while excess loading of hematite is detrimental. We conduct a comprehensive density functional theory study of Fe2O3clusters adsorbed on the anatase TiO2(101) surface to investigate the effect of Fe2O3on TiO2. Our study shows that TiO2exhibits improved photocatalytic properties with hematite clusters at low coverage, as evidenced by a systematic study conducted by increasing the number of cluster adsorbates. The adsorption of the clusters generates impurity states in the band gap improving light absorption and consequently affecting the charge transfer dynamics. Furthermore, the presence of hematite clusters enhances the activity of TiO2in the hydrogen evolution reaction. The Fe valence mixing present in some clusters leads to a significant increase in H2evolution rate compared with the fixed +3 valence of Fe in hematite. We also investigate the effect of oxygen defects and find extensive modifications in the electronic properties and local magnetism of the TiO2-Fe2O3system, demonstrating the wide-ranging effect of oxygen defects in the combined system.

Glycerol Oligomerization over Titania-Based Catalyst Compositions

The possibility of using TiO2 -based compositions: individual and sulfated titania, and their composites with carbon nanotubes as catalysts for glycerol oligomerization has been displayed. The effect of modification of TiO2 with sulfur and carbon nanotubes on acid-base and catalytic characteristics in the glycerol conversion was investigated. The activation of glycerol on the catalysts has been studied using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Only the samples based on sulfated titania are active over glycerol transformation, showing up to 58.7 % conversion. This is explained by the presence of strong base sites. Glycerides up to pentaglycerides, both linear and nonlinear structure are formed by glycerol oligomerization over TiO2 -S. The addition of nanotubes to the catalyst reduces both the glycerol conversion (up to 10.5 %) and the yield of glycerides. However, the spectrum of the resulting products is significantly narrowed, increasing the selectivity for short-chain glycerides: the portion of diglycerides reaches 72 %, and triglycerides 21 %. Herewith, glycerides of a linear structure only formed.

Enhanced photocatalytic hydrogen evolution of Ru/TiO2-x via oxygen vacancy-assisted hydrogen spillover process

Hydrogen spillover effects will significantly improve the activity of photocatalytic hydrogen evolution reactions (HER), while their introduction and optimization require the construction of an excellent metal/support structure. In this study, we have synthesized Ru/TiO2-x catalysts with controlled oxygen vacancy (OVs) concentrations using a simple one-pot solvothermal method. The results show that Ru/TiO2-x3 with the optimal OVs concentration exhibits an unprecedentedly high H2 evolution rate of 13604 μmol·g-1·h-1, which was 45.7 and 2.2 times higher than that of TiO2-x (298 μmol·g-1·h-1) and Ru/TiO2 (6081 μmol·g-1·h-1). Controlled experiments, detailed characterizations, and theoretical calculations have revealed that the introduction of OVs on the carrier contributes to the hydrogen spillover effect in the metal/support system photocatalyst and that the process of hydrogen spillover in this system can be optimized by modulating the OVs concentration. This study proposes a strategy to decrease the energy barrier of hydrogen spillover and enhance photocatalytic HER activity. Moreover, it investigates the effect of OVs concentration on the hydrogen spillover effect in the photocatalytic metal/supports system.

Untangling product selectivity on clean low index rutile TiO2 surfaces using first-principles calculations

Computational modeling of metal oxide surfaces provides an important tool to help untangle complex spectroscopy and measured catalytic reactivity. There are many material properties that make rational catalytic design challenging, and computational methods provide a way to evaluate possible structural factors, like surface structure, individually. The mechanism of water oxidation or oxygen evolution is well studied on some anatase surfaces and the rutile TiO₂ (110) surface but has not yet been mapped on other low-index Miller rutile surfaces that are present in most experimental nano-titania catalysts. Here first principles calculations provide new insights into water oxidation mechanisms and reactivity of the most common low-index Miller facets of rutile TiO₂. The reactivity of three surfaces, (101), (010), and (001), are explored for the first time and the product selectivity of multistep electron transfer on each surface is compared to the well-studied (110) surface. Density functional theory shows that a peroxo, O^(p), intermediate is more favorable for water oxidation on all facets. The ˙OH radical formation is favored on the (001) facet resulting in a high overpotential for oxygen evolution reaction (OER). The (101) and (110) facets have low overpotentials, ∼0.3 V, and favor two-electron proton-coupled electron transfer to produce H₂O₂. The only facet that prefers direct OER is (001), leading to O₂ evolution in a four-electron process with an overpotential of 0.53 V. A volcano plot predicts the selectivity and activity of low-index Miller facets of rutile TiO₂, revealing the high activity of the peroxo OER mechanism on the (010) facet.

Description of Photodegradation Mechanisms and Structural Characteristics in Carbon@Titania Yolk-Shell Nanostructures by XAS

Carbon@titania yolk-shell nanostructures are successfully synthesized at different calcination conditions. These unique structure nanomaterials can be used as a photocatalyst to degrade the emerging water pollutant, acetaminophen (paracetamol). The photodegradation analysis studies have shown that the samples with residual carbon nanospheres have improved the photocatalytic efficiency. The local electronic and atomic structure of the nanostructures are analyzed by X-ray absorption spectroscopy (XAS) measurements. The spectra confirm that the hollow shell has an anatase phase structure, slight lattice distortion, and variation in Ti 3d orbital orientation. In situ XAS measurements reveal that the existence of amorphous carbon nanospheres inside the nano spherical shell inhibit the recombination of electron-hole pairs; more mobile holes are formed in the p-d hybridized bands near the Fermi surface and enables the acceleration of the carries that significantly enhance the photodegradation of paracetamol under UV-visible irradiation. The observed charge transfer process from TiO₂  hybridized orbital to the carbon nanospheres reduces the recombination rate of electrons and holes, thus increasing the photocatalytic efficiency.

High Pressure Rapid Synthesis of LiCrTiO4 with Oxygen Vacancy for High Rate Lithium-Ion Battery Anodes

Lithium-ion battery based on LiCrTiO₄ (LCTO) is considered to be a promising anode material, as they provide higher safety and durability beyond than that of graphite electrode. However, the applications of this transformative technology demand improved inherent electrical conductivity of LCTO as well as a simple and rapid synthetic route. Here, LCTO with oxygen vacancies (OVs) is fabricated using high-pressure synthesis technology in only 40 min. The optimal synthesis pressure is 0.8 GPa (LCTO-0.8). The reversible capacity of LCTO-0.8 at 1C is 131 mA h g^-1 after 1000 cycles and the capacity retention is nearly 97%, and the reversible capacity of LCTO synthesized at atmospheric pressure (LCTO-P) is 85 mA h g^-1 under the same circumstances. Even at 5C, the reversible capacity is 110 mA h g^-1 , which is 77% higher than LCTO-P. Furthermore, it is confirmed by theoretical calculations that the introduction of OVs has the occupation of electronic states at the Fermi level, which greatly enhances the intrinsic conductivity of LCTO. Specifically, the electronic conductivity has increased by two orders of magnitude compared with LCTO-P. Therefore, high-pressure synthesis technology endows LCTO with superior characteristics, providing a new avenue for industrialization.

Resolving the Effect of Oxygen Vacancies on Co Nanostructures Using Soft XAS/X-PEEM

Improving both the extent of metallic Co nanoparticle (Co NP) formation and their stability is necessary to ensure good catalytic performance, particularly for Fischer–Tropsch synthesis (FTS). Here, we observe how the presence of surface oxygen vacancies (O_vac) on TiO₂ can readily reduce individual Co₃O₄ NPs directly into CoO/Co⁰ in the freshly prepared sample by using a combination of X-ray photoemission electron microscopy (X-PEEM) coupled with soft X-ray absorption spectroscopy. The O_vac are particularly good at reducing the edge of the NPs as opposed to their center, leading to smaller particles being more reduced than larger ones. We then show how further reduction (and O_vac consumption) is achieved during heating in H₂/syngas (H₂ + CO) and reveal that O_vac also prevents total reoxidation of Co NPs in syngas, particularly the smallest (∼8 nm) particles, thus maintaining the presence of metallic Co, potentially improving catalyst performance.

Sulfur Vacancies Enriched 2D ZnIn2S4 Nanosheets for Improving Photoelectrochemical Performance

Vacancies engineering based on semiconductors is an effective method to enhance photoelectrochemical activity. Herein, we used a facile one-step solvothermal method to prepare sulfur vacancies modified ultrathin two-dimensional (2D) ZnIn2S4 nanosheets. The photon-to-current efficiency of sulfur vacancies modified ultrathin 2D ZnIn2S4 nanosheets is 1.82-fold than ZnIn2S4 nanosheets without sulfur vacancies and 2.04-fold than multilayer ZnIn2S4. The better performances can be attributed to the introduced sulfur vacancies in ZnIn2S4, which influence the electronic structure of ZnIn2S4 to absorb more visible light and act as the electrons trapping sites to suppress the recombination of photo-generated carriers. These results provide a new route to designing efficient photocatalyst by introducing sulfur vacancies.

Synthesis and Characterization of Titanium Dioxide Hollow Nanofiber for Photocatalytic Degradation of Methylene Blue Dye

Environmental crisis and water contamination have led to worldwide exploration for advanced technologies for wastewater treatment, and one of them is photocatalytic degradation. A one-dimensional hollow nanofiber with enhanced photocatalytic properties is considered a promising material to be applied in the field. Therefore, we synthesized titanium dioxide hollow nanofibers (THNF) with extended surface area, light-harvesting properties and an anatase–rutile heterojunction via a template synthesis method and followed by a calcination process. The effect of calcination temperature on the formation and properties of THNF were determined and the possible mechanism of THNF formation was proposed. THNF nanofibers produced at 600 °C consisted of a mixture of 24.2% anatase and 75.8% rutile, with a specific surface area of 81.2776 m²/g. The hollow nanofibers also outperformed the other catalysts in terms of photocatalytic degradation of MB dye, at 85.5%. The optimum catalyst loading, dye concentration, pH, and H₂O₂ concentration were determined at 0.75 g/L, 10 ppm, pH 11, and 10 mM, respectively. The highest degradation of methylene blue dye achieved was 95.2% after 4 h of UV irradiation.

The Phase Evolution and Photocatalytic Properties of a Ti-TiO2 Bilayer Thin Film Prepared Using Thermal Oxidation

Ti-TiO2 bilayer thin films were successfully prepared onto a glass substrate using magnetron sputtering with different TiO2 bottom layer conditions. These represent a lack of (as-deposited) and full oxygen content (annealed). Single-layer Ti was additionally used as a control. The influence of oxygen diffusion phenomena of the bottom layer of TiO2 to the upper layer of Ti thin films at different oxidation temperatures on structural, optical, and photocatalytic performance was investigated. X-ray diffraction (XRD) results confirmed that the crystalline phases coexisting on thin-film samples oxidized at 450 °C were TiO, TiO1.4, (bilayer, as-deposited TiO2), anatase (bilayer, annealed TiO2), and rutile (single and bilayer). This finding showed that the film’s phase structure evolution is significantly affected by oxygen diffusion from the bottom layer. Further increasing the thermal oxidation temperature caused a notable decline in the amorphous zone in bilayer thin films based on TEM analysis. Bilayer thin films lead to higher degradation of methylene blue under UV light radiation (63%) than single-layer films (45%) oxidized at 450 °C. High photocatalytic activity performance was found in the bilayer annealed TiO2-Ti thin-film sample. This study demonstrates that the bilayer modification strategy promotes the oxygen-induced bottom layer of TiO2 bilayer thin films.

Computational Catalysis—Trends and Outlook

Computational catalysis has been one of the most dynamic research fields over the last decade, and it now represents a critical tool for the analysis of chemical mechanisms and active sites [...]

pH Sensitivity Estimation in Potentiometric Metal Oxide pH Sensors Using the Principle of Invariance

A numerically solvable engineering model has been proposed that predicts the sensitivity of metal oxide- (MOX-) based potentiometric pH sensors. The proposed model takes into account the microstructure and crystalline structure of the MOX material. The predicted pH sensitivities are consistent with experimental results with the difference below 6% across three MOX (RuO2, TiO2, and Ta2O5) analysed. The model distinguishes the performance of different MOX phases by the appropriate choice of surface hydroxyl site densities and dielectric constants, making it possible to estimate the performance of MOX electrodes fabricated through different high-temperature and low-temperature annealing methods. It further addresses the problem, cited by theoreticians, of independently determining the C1 inner Helmholtz capacitance parameter while applying the triple-layer model to pH sensors. This is done by varying the C1 capacitance parameter until an invariant pH sensitivity across different electrolyte ionic strengths is obtained. This invariance point identifies the C1 capacitance. The corresponding pH sensitivity is the characteristic sensitivity of MOX. The model has been applied across different types of metal oxides, namely, expensive platinum group oxides (RuO2) and cheaper nonplatinum group MOX (TiO2 and Ta2O5). High temperature annealed, RuO2 produced a high pH sensitivity of 59.1082 mV/pH, while TiO2 and Ta2O5 produced sub-Nernstian sensitivities of 30.0011 and 34.6144 mV/pH, respectively. Low temperature annealed, TiO2 and Ta2O5 produced Nernstian sensitivities of 59.1050 and 59.1081 mV/pH, respectively, illustrating the potential of using cheaper nonplatinum group MOx as alternative sensor electrode materials. Separately, the usefulness of relatively less investigated, cheap, and readily available MOX, viz. Al2O3, as the electrode material was analysed. Low-temperature-annealed Al2O3 with a Nernstian sensitivity of 59.1050 mV/pH can be considered as a potential electrode material. The proposed engineering model can be used as a preliminary prediction mechanism for choosing potentially cheaper alternative sensor electrode materials.

Oxygen vacancies induced photoluminescence in [Formula: see text] nanophosphors probed by theoretical and experimental analysis

We report, for the first time, the influence of oxygen vacancies on band structure and local electronic structure of [Formula: see text] (SZO) nanophosphors by combined first principle calculations based on density functional theory and full multiple scattering theory, correlated with experimental results obtained from X-ray absorption and photoluminescence spectroscopies. The band structure analysis from density functional theory revealed the formation of new energy states in the forbidden gap due to introduction of oxygen vacancies in the system, thereby causing disruption in intrinsic symmetry and altering bond lengths in SZO system. These defect states are anticipated as origin of observed photoluminescence in SZO nanophosphors. The experimental X-ray absorption near edge structure (XANES) at Zn and Sr K-edges were successfully imitated by simulated XANES obtained after removing oxygen atoms around Zn and Sr cores, which affirmed the presence and signature of oxygen vacancies on near edge structure.