The role of surface O-vacancies in the photocatalytic oxidation of Methylene Blue by Zn-doped TiO 2 : A Mechanistic approach

Investigation of Photoelectrocatalytic and Magnetic Properties of Sr2YbRu1-xTaxO6 (x = 0, 0.25, 0.5, 0.75, and 1)

We report the effect of substitution of Ru by Ta in Sr₂YbRuO₆ on its magnetic and photoelectrocatalytic properties. The powder X-ray diffraction data, was satisfactorily refined in the monoclinic space group, P2₁/n. The DC magnetization studies indicated that Sr₂YbRuO₆ shows antiferromagnetic interaction through Yb-O-Ru orbital ordering, with the highest Weiss temperature, among Sr₂YbRu_1-xTa_xO₆ (x = 0, 0.25, 0.5, and 0.75) which have values of -148, -125, -118, and -102 K, respectively. The difference in observed and theoretical magnetic moments was found to increase as x increases. It was also observed that with the increase of Ta concentration in Sr₂YbRu_1-xTa_xO₆, the band gap increased almost linearly, from 1.78(1) eV (x = 0) to 2.08(1) (x = 0.75), and thereafter a sharp increase 2.65(1) eV (x = 1) was observed, with the lowering of energy level of valence band, along with disruption in orbital ordering as x increases. The photoelectrocatalytic oxygen evolution reaction (OER) studies carried out on the series yield a maximum photocurrent density of 17 μA/cm² and photoresponse current of 5.5 μA/cm² at 0.8 V at an onset potential at 0.29 V vs Ag/AgCl for Sr₂YbRuO₆. The XPS analysis showed Ta and Ru to be in +5/+4 oxidation states, with the highest concentration of Ru⁴⁺ ion observed for Sr₂YbRuO₆. The presence of oxygen vacancies was confirmed by XPS as well as EPR studies.

Indulgent of the physiochemical features of MgCr-LDH nanosheets towards photodegradation process of methylene blue

In the present work, we report the preparatory strategy of MgCr-layered double hydroxide (LDH) nanosheets with 90% degree of delamination by employing a formamide-assisted co-precipitation and mild hydrothermal route for the degradation of methylene blue (MB) under solar light exposure. The as-synthesized MgCr-LDH nanosheets were characterized by assorted characterization techniques such as powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), Raman, thermogravimetric analysis (TGA), N₂ adsorption-desorption measurement, X-ray photoelectron spectroscopy (XPS) and UV-Visible diffused reflectance spectroscopy (UV-DRS). The XRD pattern of MgCr-LDH nanosheets quantified the strain (ε) and dislocation density (δ) of 1.371 lines^-2 m^-4 and 0.5723 lines m^-2 related to the (110) plane with d-spacing value of 1.6169 Ȧ. With a minimum band gap of ∼2.63 eV, the as-synthesized MgCr-LDH nanosheets displayed 90.6% MB photodegradation under the experimental protocols such as catalyst dosage of 30 mg/L, initial MB concentrations of 20 ppm, pH of 7 and time duration of 2 h under solar light exposure. Further, the recyclability test of the photocatalyst signifies material stability up to four successive cycles with 90% retention of MB degradation under sunlight exposure. The superior catalytic performances of the MgCr-LDH nanosheets could be ascertained to the suppression of excitonic recombination and effective light harvestation properties, synergistically contributed by the porous structural aspects via association of uni/multi-lamellar nanosheets, surface defect sites and photoactive Cr³⁺ cations. Additionally, the surface -OH groups of LDH contributed towards the generation of ^•OH radicals for triggering the catalytic performances. This type of work advances the novel ideas for establishing highly potent photocatalysts via synergizing structural and surface properties, paving towards effective wastewater treatment.

Selective Oxidation of Cyclohexane to Cyclohexanol/Cyclohexanone by Surface Peroxo Species on Cu-Mesoporous TiO2

Selective oxidation of cyclohexane to cyclohexanol/cyclohexanone (KA-oil) is an important chemical process, which is still constrained by low conversion and selectivity and high energy consumption. In this study, Cu-doped mesoporous TiO₂ (Cu-MT) has been successfully synthesized via calcinating MIL-125(Ti) doped with copper acetylacetonate, which shows high reactivity in selective oxidation of cyclohexane to KA-oil by persulfate (PS) with the desirable cyclohexane conversion of 16.8% and a selectivity of 98.0% under mild conditions and the low ratio of PS/cyclohexane of 1:1. A series of characterizations and density functional theory calculations reveal that the doped Cu(I,II) on Cu-MT is the reactive site for non-radical activation of PS with the moderate elongation of the O-O bond in PS, which then abstracts 1H (1H⁺ + 1e^-) from cyclohexane to form Cy^• and eventually KA-oil. This study gives new insight on the importance of moderately activated PS in selective oxidation of C-H.

Recent Status and Developments of Vacancies Modulation in the ABO3 Perovskites for Catalytic Applications

Perovskite oxides (ABO₃ ) have attracted comprehensive interest for wide range of functional applications (especially for chemical catalysis) due to their high design flexibility, controllable vacancies sites creation, abundant chemical properties, and stable crystal structure. Herein, the previous research and potential development of ABO₃ through adjusting the vacancy at different sites (A-site, B-site, and O-site) to enhance catalytic performance are systematically analyzed and generalized. Briefly, the ABO₃ with different vacancies sites prepared by multifarious direct and indirect methods, accompanied with the improved physical-chemical properties, endow them with distinct and intensified development of catalysis application. In addition, the impressive optimization proved by the vacancies sites adjustment over the ABO₃ is studied to continuously facilitate the advance in some common catalysis reactions, further expanding to other optimized functional applications. At last, the constructive suggestions for fine regulation and analysis of vacancies sites over ABO₃ are also put forward.

Incorporation of Cu into Goethite Stimulates Oxygen Activation by Surface-Bound Fe(II) for Enhanced As(III) Oxidative Transformation

The dark production of reactive oxygen species (ROS) coupled to biogeochemical cycling of iron (Fe) plays a pivotal role in controlling arsenic transformation and detoxification. However, the effect of secondary atom incorporation into Fe(III) oxyhydroxides on this process is poorly understood. Here, we show that the presence of oxygen vacancy (OV) as a result of Cu incorporation in goethite substantially enhances the As(III) oxidation by Fe(II) under oxic conditions. Electrochemical and density functional theory (DFT) evidence reveals that the electron transfer (ET) rate constant is enhanced from 0.023 to 0.197 s^-1, improving the electron efficiency of the surface-bound Fe(II) on OV defective surfaces. The cascade charge transfer from the surface-bound Fe(II) to O₂ mediated by Fe(III) oxyhydroxides leads to the O-O bond of O₂ stretching to 1.46-1.48 Å equivalent to that of superoxide (^•O₂^-), and ^•O₂^- is the predominant ROS responsible for As(III) oxidation. Our findings highlight the significant role of atom incorporation in changing the ET process on Fe(III) oxyhydroxides for ROS production. Thus, such an effect must be considered when evaluating Fe mineral reactivity toward changing their surface chemistry, such as those noted here for Cu incorporation, which likely determines the fates of arsenic and other redox sensitive pollutants in the environments with oscillating redox conditions.

Enhanced healing process of tooth sockets using strontium-doped TiO2

Prevention of residual ridge resorption is important for tooth socket healing in clinical treatment. As a well known biomaterial, titanium dioxide (TiO₂) has been reported to show desirable bone regeneration capability. On the other hand, strontium plays a role in maintaining normal function in organisms and balancing bone remodeling. Hence, we synthesized strontium-doped titanium dioxide mesoporous nanospheres functionalized with amino-group using diphenyl diisocyanate. After incorporation with segmented polyurethane, the obtained injectable SPU/Sr-TiO₂/MDI nanocomposite adhesive showed satisfactory antibacterial activity and cell nontoxicity. This nanocomposite was used for tooth socket healing, and greatly promoted the formation of new bone tissue in the tooth extraction socket.

High-Performance Mg-Li Hybrid Batteries Based on Pseudocapacitive Anatase Ti1-x Cox O2-y Nanosheet Cathodes

Despite the proposed safety, performance, and cost advantages, practical implementation of Mg-Li hybrid batteries is limited due to the unavailability of reliable cathodes compatible with the dual-ion system. Herein, a high-performance Mg-Li dual ion battery based upon cobalt-doped TiO₂ cathode was developed. Extremely pseudocapacitance-type Ti_1-x Co_x O_2-y nanosheets consist of an optimum 3.57 % Co-atoms. This defective cathode delivered exceptional pseudocapacitance (maximum of 93 %), specific capacities (386 mAh g^-1 at 25 mA g^-1 ), rate performance (191 mAh g^-1 at 1 A g^-1 ), cyclability (3000 cycles at 1 A g^-1 ), and coulombic efficiency (≈100 %) and fast charging (≈11 min). This performance was superior to the TiO₂ -based Mg-Li dual-ion battery cathodes reported earlier. Mechanistic studies revealed dual-ion intercalation pseudocapacitance with negligible structural changes. Excellent electrochemical performance of the cation-doped TiO₂ cathode was credited to the rapid pseudocapacitance-type Mg/Li-ion diffusion through the disorder generated by lattice distortions and oxygen vacancies. Ultrathin nature, large surface area, 2D morphology, and mesoporosity also contributed as secondary factors facilitating superior electrode-electrolyte interfacial kinetics. The demonstrated method of pseudocapacitance-type Mg-Li dual-ion intercalation by introducing lattice distortions/oxygen vacancies through selective doping can be utilized for the development of several other potential electrodes for high-performance Mg-Li dual-ion batteries.

Identification and manipulation of active centers on perovskites to enhance catalysis of peroxymonosulfate for degradation of emerging pollutants in water

Perovskites (the general formula of ABO₃) with versatile substrates can serve as desirable catalysts to initiate advanced oxidation processes (AOPs) for environmental remediation. However, the knowledge regarding the active centers remains piecemeal and unclear, such as how the redox metal centers of B site, inert metals of A site, oxygen vacancies, and direct oxidation of catalysts govern the chemical degradation of aqueous pollutants. This study aimed to identify principal alternations in physicochemical and electrical properties of ABO₃-based perovskites modified with partial/overall substitution at A/B sites and synthesized at different conditions. In order to probe varied catalytic activity of these catalysts, ofloxacin (OFX) was used as a model micro-pollutant. Results showed that the OFX degradation by activation of peroxymonosulfate (PMS) with LaFeO₃ perovskite was favored by the Sr substitution at A site, Cu substitution at B site, and increasing calcination temperature. Evolution of ¹O₂, ^•OH and SO₄^•- have proven for efficient OFX oxidation, as evidenced by results from in-situ electron paramagnetic resonance (EPR) analyses and quenching tests. Specifically, the introduction of Sr at A site can facilitate PMS self-decomposition to produce more ¹O₂ due to the increased abundance of surface oxygen vacancies. In contrast, the Cu substitution at B site improved the surface oxygen vacancies, as well as the electrical conductivity, which can further accelerate ^•OH and SO₄^•- generation for the OFX degradation. This study provides deeper insights into the underlying mechanisms governing the catalytic activity of perovskites. These findings build a basis for better decontamination of hazardous environmental organic pollutants.

Influence of ruthenium doping on UV- and visible-light photoelectrocatalytic color removal from dye solutions using a TiO2 nanotube array photoanode

The photocatalytic activity of TiO₂ anodes was enhanced by synthesizing Ru-doped Ti|TiO₂ nanotube arrays. Such photoanodes were fabricated via Ti anodization followed by Ru impregnation and annealing. The X-ray diffractograms revealed that anatase was the main TiO₂ phase, while rutile was slightly present in all samples. Scanning electron microscopy evidenced a uniform morphology in all samples, with nanotube diameter ranging from 60 to 120 nm. The bias potential for the photoelectrochemical (PEC) treatment was selected from the electrochemical characterization of each electrode, made via linear sweep voltammetry. All the Ru-doped TiO₂ nanotube array photoanodes showed a peak photocurrent (PP) and a saturation photocurrent (SP) upon their illumination with UV or visible light. In contrast, the undoped TiO₂ nanotubes only showed the SP, which was higher than that reached with the Ru-doped photoanodes using UV light. An exception was the Ru(0.15 wt%)-doped TiO₂, whose SP was comparable under visible light. Using that anode, the activity enhancement during the PEC treatment of a Terasil Blue dye solution at E_bias(PP) was much higher than that attained at E_bias(SP). The percentage of color removal at 120 min with the Ru(0.15 wt%)-doped TiO₂ was 98% and 55% in PEC with UV and visible light, respectively, being much greater than 82% and 28% achieved in photocatalysis. The moderate visible-light photoactivity of the Ru-doped TiO₂ nanotube arrays suggests their convenience to work under solar PEC conditions, aiming at using a large portion of the solar spectrum.

A facile synthesis of Zn-doped TiO2 nanoparticles with highly exposed (001) facets for enhanced photocatalytic performance

It is a great challenge to simultaneously improve the visible light absorption capacity and enhance photon-generated carrier separation efficiency of photocatalysts. Herein, Zn-doped TiO2 nanoparticles with high exposure of the (001) crystal face were prepared via a one-step hydrothermal decomposition method. A detailed analysis reveals that the electronic structures were modulated by Zn doping; thus, the responsive wavelength was extended to 600 nm, which effectively improved the visible light absorption of TiO2. More importantly, the surface heterojunction of TiO2 was created because of the co-existing specific facets of (101) and (001). Therefore, the surface separation efficiency of photogenerated electron and hole pairs was greatly enhanced. So, the optimal TiO2 photocatalyst exhibited excellent photocatalytic activity, in which the Rhodamine B (RhB) degradation efficiency was 98.7% in 60 min, under the irradiation of visible light. This study is expected to provide guidance for the rational design of TiO2 photocatalysts.

The formation and effect of O-vacancies in doped TiO2

The formation of O-vacancies along with their local environment and locus of formation for the O-vacancies has been understood using EXAFS and ab initio techniques in a doped (Zn²⁺) anatase lattice.

Evaluation of the effect of different treatment methods on sugarcane vinasse remediation

Sugarcane vinasse is a by-product obtained during ethanol production in sugar-ethanol plants. For each 1 L of ethanol produced approximately 11 L of vinasse are generated. As this residue is obtained in high amounts, it is used as fertilizer in sugarcane crops. However, despite being rich in nutrients, sugarcane vinasse is approximately one hundred times more polluting than domestic sewage, making it an environmental problem. Thus, the aim of the present study was to propose a treatment sequence for sugarcane vinasse and evaluate the possibility of energetic use of the generated sludge in the coagulation/flocculation stage. pH, conductivity, turbidity and decreases in UV/Vis absorption spectra were determined for each treatment step. In addition, the upper calorific value of the generated sludge was also determined, while ash (adsorption treatment) and catalyst (heterogeneous photocatalysis) characterizations were also carried out. At the end of the treatment, initial vinasse turbidity was reduced by 100% and pH and conductivity values were stabilized. The sludge presented a higher calorific value of approximately 3,000 kcal kg^-1 and the ash and catalyst displayed favorable characteristics to be applied to the sugarcane vinasse treatment stages.

The formation and detection techniques of oxygen vacancies in titanium oxide-based nanostructures

TiO2 and other titanium oxide-based nanomaterials have drawn immense attention from researchers in different scientific domains due to their fascinating multifunctional properties, relative abundance, environmental friendliness, and bio-compatibility. However, the physical and chemical properties of titanium oxide-based nanomaterials are found to be explicitly dependent on the presence of various crystal defects. Oxygen vacancies are the most common among them and have always been the subject of both theoretical and experimental research as they play a crucial role in tuning the inherent properties of titanium oxides. This review highlights different strategies for effectively introducing oxygen vacancies in titanium oxide-based nanomaterials, as well as a discussion on the positions of oxygen vacancies inside the TiO2 band gap based on theoretical calculations. Additionally, a detailed review of different experimental techniques that are extensively used for identifying oxygen vacancies in TiO2 nanostructures is also presented.

Fast-Response Flexible Photochromic Gels for Self-Erasing Rewritable Media and Colorimetric Oxygen Indicator Applications

Photoreversible color switching that can change colors with fast response and high stability is urgently desired in color-on-demand applications. Yet, developing such materials has long been a significant challenge. In this work, a strategy based on the integration of TiO₂ nanoparticle (NP) photocatalytic color switching of redox dyes and poly(vinyl alcohol) gel matrix could produce robust and flexible photochromic gels (FPGs) that exhibit fast light-responsive time and high photoreversible stability. Benefited by the soft network structures and monomeric form of redox dyes in the FPG maintained by poly(vinyl alcohol) and ethylene glycol molecules, as well as enhanced photoreductive activity of TiO₂ NPs modified by both surface ligands and oxygen vacancies, the FPG exhibits long photoreversible switching cycles (≥50 times), decoloration in a short period of less than 8 s upon UV illumination, and recoloration in 16 min in ambient air and rapidly in 140 s upon near-infrared light illumination. Consequently, the excellent photoreversible color switching of the FPGs is highly applicable as both self-erasing rewritable media and colorimetric oxygen indicators. We believe that the current systems represent a big step forward toward practical applications, such as time-sensitive information storage, colorimetric oxygen sensor, and potentially many other technologies.

Oxygen Vacancy Promoted Heterogeneous Fenton-like Degradation of Ofloxacin at pH 3.2-9.0 by Cu Substituted Magnetic Fe3O4@FeOOH Nanocomposite

To develop an ultraefficient and reusable heterogeneous Fenton-like catalyst at a wide working pH range is a great challenge for its application in practical water treatment. We report an oxygen vacancy promoted heterogeneous Fenton-like reaction mechanism and an unprecedented ofloxacin (OFX) degradation efficiency of Cu doped Fe₃O₄@FeOOH magnetic nanocomposite. Without the aid of external energy, OFX was always completely removed within 30 min at pH 3.2-9.0. Compared with Fe₃O₄@FeOOH, the pseudo-first-order reaction constant was enhanced by 10 times due to Cu substitution (9.04/h vs 0.94/h). Based on the X-ray photoelectron spectroscopy (XPS), Raman analysis, and the investigation of H₂O₂ decomposition, ^•OH generation, pH effect on OFX removal and H₂O₂ utilization efficiency, the new formed oxygen vacancy from in situ Fe substitution by Cu rather than promoted Fe³⁺/Fe²⁺ cycle was responsible for the ultraefficiency of Cu doped Fe₃O₄@FeOOH at neutral and even alkaline pHs. Moreover, the catalyst had an excellent long-term stability and could be easily recovered by magnetic separation, which would not cause secondary pollution to treated water.