Crystalline Phase Of TiO2 Research Articles

The Effect of Heat Treatment on the Sol–Gel Preparation of TiO2/ZnO Catalysts and Their Testing in the Photodegradation of Tartrazine

This study aims to synthesize TiO2/ZnO powders and to study the effect of heat treatment on their photocatalytic ability against the Tartrazine anionic dye. The as-obtained powders with the following compositions—90TiO2/10ZnO and 10TiO2/90ZnO (mol%)—were obtained by the sol–gel technique. The prepared gels were annealed at 500 °C and 700 °C and subsequently characterized by XRD, UV–Vis, and SEM methods. The single crystalline phase of TiO2, which has been detected at up to 500 °C is anatase, while for ZnO, it is the hexagonal wurtzite structure. Further increases in the temperature (700 °C) led to the appearance of rutile in the samples. The SEM analysis demonstrated that the binary oxide materials had irregular shaped particles with a tendency to agglomerate. The UV–Vis spectra of the gels exhibited a red shift in the cut-off of the 90TiO2/10ZnO sample compared with pure Ti(IV) butoxide. Photocatalytic tests showed that the investigated samples possessed photocatalytic activity toward Tartrazine. Compared with TiO2, the prepared TiO2/ZnO photocatalysts showed superior properties in the photodegradation of a Tartrazine water solution. The target photocatalysts’ enhanced photocatalytic activities can be explained by their reduced band gap energy, improved surface physicochemical characteristics, separation of photo-induced electron–hole pairs, and lowered recombination rate. Higher photocatalytic activity was observed for powders annealed at 500 °C, with the 10TiO2/90ZnO (mol%) sample exhibiting the highest photocatalytic degradation of the used organic dye.

Review of Bio-Inspired Green Synthesis of Titanium Dioxide for Photocatalytic Applications

Titanium dioxide (TiO2) is an important photocatalyst that is widely studied for environmental applications, especially for water treatment by degradation of pollutants. A range of methods have been developed to produce TiO2 in the form of nanoparticles and thin films. Solution-based synthesis methods offer the opportunity to tune the synthesis through a choice of reagents, additives and reaction media. In particular, the use of biomolecules, such as proteins and amino acids, as bio-inspired additives in TiO2 synthesis has grown over the last decade. This review provides a discussion of the key factors in the solution-based synthesis of titania, with a focus on bio-inspired additives and their interaction with Ti precursors. In particular, the role of bio-inspired molecular and biomolecular additives in promoting the low-temperature synthesis of titania and controlling the phase and morphology of the synthesised TiO2 is discussed, with a particular focus on the interaction of TiO2 with amino acids as model bio-inspired additives. Understanding these interactions will help address the key challenges of obtaining the crystalline TiO2 phase at low temperatures, with fast kinetics and under mild reaction conditions. We review examples of photocatalytic applications of TiO2 synthesised using bio-inspired methods and discuss the ways in which bio-inspired additives enhance photocatalytic activity of TiO2 nanomaterials. Finally, we give a perspective of the current challenges in green synthesis of TiO2, and possible solutions based on multi-criteria discovery, design and manufacturing framework.

Fabrication of Electrospun Porous TiO2 Dielectric Film in a Ti–TiO2–Si Heterostructure for Metal–Insulator–Semiconductor Capacitors

The development of metal–insulator–semiconductor (MIS) capacitors requires device miniaturization and excellent electrical properties. Traditional SiO2 gate dielectrics have reached their physical limits. In this context, high-k materials such as TiO2 are emerging as promising alternatives to SiO2. However, the deposition of dielectric layers in MIS capacitors typically requires high-vacuum equipment and challenging processing conditions. Therefore, in this study, we present a new method to effectively fabricate a poly(vinylidene fluoride) (PVDF)-based TiO2 dielectric layer via electrospinning. Nano-microscale layers were formed via electrospinning by applying a high voltage to a polymer solution, and electrical properties were analyzed as a function of the TiO2 crystalline phase and residual amount of PVDF at different annealing temperatures. Improved electrical properties were observed with increasing TiO2 anatase content, and the residual amount of PVDF decreased with increasing annealing temperature. The sample annealed at 600 °C showed a lower leakage current than those annealed at 300 and 450 °C, with a leakage current density of 7.5 × 10−13 A/cm2 when Vg = 0 V. Thus, electrospinning-based coating is a cost-effective method to fabricate dielectric thin films. Further studies will show that it is flexible and dielectric tunable, thus contributing to improve the performance of next-generation electronic devices.

Identifying the effect of photo-generated carriers on the phonons in rutile TiO2 through Raman spectroscopy

Abstract Investigating lattice vibrations through Raman spectroscopy is a crucial method for studying crystalline materials. Carriers can interact with lattices and influence lattice vibrations; thus, it is feasible to study the effect of photo-generated carriers on phonons by analyzing changes in the Raman spectra of semiconductors. Rutile is one of the predominant crystalline phases of TiO2, which is a widely utilized metal oxide semiconductor. In this work, rutile TiO2 is coated on a thinned optical fiber to concentrate ultraviolet light energy within the material, thereby enhancing the generation of carriers and amplifying the changes in the Raman spectra. A Raman detection laser with a wavelength of 532 nm is utilized to collect the Raman spectra of rutile TiO2 during irradiation. Using this setup, the impact of photo-generated carriers on the phonons corresponding to Raman vibrational modes is researched. The localization and non-radiative recombination of photo-generated carriers contribute to a reduction in both the frequencies and lifetimes of phonons. This work provides a novel approach to researching the effect of carriers on phonons.

Fabrication of Silica–Titanium Composite Film on Wood Surface and Optimization of Its Structure and Properties

In this thesis, wood loaded with a silica–titanium (Si-Ti) composite film was prepared using the sol–gel method in order to achieve improved wood with high hydrophobicity and photocatalytic activity under visible light. The factors affecting the structure and properties of the composite film, as well as the optimization process, were discussed. Infrared analysis revealed that the vibrational intensity of Si-O-Si, Ti-O-Ti, and Ti-O-Si telescopic vibration peaks increased with an increase in vinyltriethoxysilane (VETS). Additionally, the number of Ti-O-Ti telescopic vibration peaks also increased with an increase in VETS. Furthermore, the intensity of -NO3, Si-O-Si, and Ti-O-Ti telescopic vibrational peaks was enhanced with a higher dosage of nitric acid. Conversely, the intensity of -OH telescopic vibrational peaks decreased with an increase in drying temperature. XRD analysis showed that nitric acid could promote the transformation of TiO2 from amorphous to anatase, while SiO2 would reduce the grain size of anatase TiO2 and promote the growth of rutile TiO2. Additionally, wood surfaces loaded with Si-Ti composite film changed from hydrophilic to hydrophobic, with significant differences observed between different levels of each factor. The photocatalytic activity of surface-loaded Si-Ti composite films on wood was most affected by the amount of nitric acid, which influenced crystallinity of TiO2 and thus impacted the photocatalytic activity. Furthermore, changes in VTES dosage not only affected the crystalline phase of TiO2 and the grain size of Si-Ti composite film but also influenced the crystallinity of TiO2 through generating SiO2. Finally, based on optimal preparation process (titanium–alcohol ratio of 1:5, titanium–silicon ratio of 1:0.2, titanium–acid ratio of 1:0.5, and drying temperature of 100 °C), wood surfaces loaded with Si-Ti composite film achieved a contact angle up to 125.9° and exhibited a decolorization rate for rhodamine B under UV light reaching 94% within 180 min.

Vanadium-doped TiO<sub>2</sub> Adsorbent-Photocatalyst for Organic Dye Treatment

This study focuses on the synthesis of V-doped TiO2 materials using a modified polyol method to enhance their efficiency. The obtained materials were characterized by various methods such as XRD, EDX, FTIR, SEM, TEM-SAED, Raman, and UV-Vis DRS. The results indicated the crystalline anatase phase of TiO2, with spherical morphology and an average size of 30nm. The UV-DRS measurement demonstrated that doped materials have a lower bandgap than the pristine TiO2. The material possesses dual functionality, with adsorption capability and photocatalytic activity applied to treat organic dye contaminants. Evaluation of MB degradation under UV irradiation revealed an exceptional efficiency of up to 99% within a remarkably short duration of 30 minutes. Furthermore, the catalyst exhibited robust recovery efficiency exceeding 99% after 2 regeneration cycles and retention of approximately 80% after four cycles.

Biosynthesis of titanium dioxide nanoparticles using peel extract of Parkia speciosa for methyl orange degradation

Recent research on the biosynthesisof nanoparticles has used plant rather than other biomaterials. In this work, a simpler and environmentally friendly method for the synthesis of titanium dioxide nanoparticles using Parkia speciosa peel extract (P-TiO2 NPs) was carried out. The synthesized TiO2 was characterized by XRD, FESEM, HRTEM, SAED, FTIR, UV-DRS and Zeta analyzer. The XRD and SAED data indicated the crystalline and tetragonal anatase TiO2 phase. HR-TEM and FE-SEM analysis proved that the obtained P-TiO2 had a spherical shape with nano-sized particlesof 8.56 nm. The zeta potential data showed that P-TiO2 was a stable particle. According to the FTIR spectra, the phytochemical components contained in the peel extract were responsible for the reducing and capping agents in TiO2 formation. In addition, the P-TiO2 had photocatalytic activity and completely degraded the methyl orange dye (97.99%). This study shows that the waste of Parkia speciosa could be a potential reducing and capping agents in the synthesis of TiO2 NPs for wastewater treatment.

Influence of initial crystalline phase of TiO2 to obtain TiN thin films from sol-gel route by rapid thermal nitridation process

Titanium Nitride (TiN) is widely used in many industrial sectors for its outstanding performances including its mechanical properties, high chemical and thermal stability. Associated with its plasmonic behavior, TiN thin films are very promising for the manufacturing of optical metasurfaces devices or new plasmonic materials. Among the processes that make it easy to obtain metal nitride coatings, nitriding of metal oxide films has become increasingly popular in recent years. A multitude of synthesis processes can be used to obtain TiO2 films, with different crystalline states (amorphous, anatase or rutile) depending on the technique used, which can then be converted into TiN coatings. In this paper, the effect of the initial crystalline state of TiO2 layers was investigated on the structural properties, plasmonic properties and the friction behavior of TiN thin films obtained by Rapid Thermal Nitridation (RTN). The results indicate that, regardless of the crystalline state of the starting TiO2 film, the RTN process leads to complete nitridation of TiN coating. Moreover, even though surface morphology and friction properties differ slightly, depending on the crystallization of the starting TiO2, plasmonic properties remain very similar, thus highlighting the great versatility and uniformity of this nitriding technique, enabling TiN to be produced for a wide range of applications.

Designing a highly near infrared-reflective black nanoparticles for autonomous driving based on the refractive index and principle

HypothesisThe development of highly NIR reflective black single-shell hollow nanoparticles (BSS-HNPs) can overcome the Light Detection and Ranging (LiDAR) sensor limitations of dark-tone materials. The crystalline phase of TiO2 and the refractive index can be controlled by calcination temperature. The formation of hollow structure and the refractive index is expected to simultaneously increase the light reflection and LiDAR detectability. ExperimentsThe BSS-HNPs are synthesized using the sol–gel method, calcination, NaBH4 reduction, and etching to form a hollow structure with true blackness. The computational bandgap calculation is conducted to determine the bandgap energy (Eg) of the white and black TiO2 with different crystalline structures. The blackness of the as-synthesized materials is determined by the Commission on Illumination (CIE) L*a*b* color system. FindingsThe hydrophilic nature of BSS-HNPs enables the formulation of hydrophilic paints, allowing the mono-layer coating. With the synergistic effects of hollow structure and the refractive index, BSS-HNPs manifested superb NIR reflectance at LiDAR detection wavelengths. The high detectability, blackness, and hollow structure of BSS-HNPs can expand the variety of LiDAR-detectable dark-tone materials.

Effect of Interaction between Pt and Different Crystalline Phases of TiO2 on Benzene Oxidation

To evaluate the effects of different TiO2 crystalline phases on the catalytic oxidation performance of benzene on Pt-loaded TiO2 catalysts, physicochemical examinations were conducted using several spectroscopic and analytical techniques. Obvious effects on the valence state and morphology of Pt were exhibited by different crystalline phases. The rutile phase favored the formation of specific Pt(111) crystal faces, which enhanced the amount of surface-active oxygen species. Moreover, the àPt-O-Ti species was formed between Pt4+ and Ti at the edge of the Pt nanoparticles, promoting both electron flow and the transfer of reactive oxygen species, thus accounting for catalytic activity.

Photocatalytically active Ag-doped TiO2 coatings developed by plasma electrolytic oxidation in the presence of colloidal Ag nanoparticles

The plasma electrolytic oxidation process of Ti foil in the presence of a colloidal solution of Ag nanoparticles was used for the synthesis of exceptionally photocatalytically active Ag-doped TiO2 coatings. UV–Vis absorption spectroscopy was applied to follow the changes in the surface plasmon resonance band of the Ag nanoparticles as a function of the Ag:PO43− ratio in a phosphate-based electrolyte solution. Transmission electron microscopy revealed the size of the Ag nanoparticles in the range of 3–12 nm. The XRD patterns of the Ag-doped TiO2 coatings confirmed the crystalline anatase TiO2 phase regardless of the dopant concentration and without metallic Ag or crystalline Ag oxide forms. The typical porous morphology of the Ag–TiO2 coatings was observed by scanning electron microscopy. A significantly improved photocatalytic activity of the Ag–TiO2 coatings in the degradation process of the insecticide lindane (gamma isomer of benzene hexachloride) and test molecule methyl orange compared to pure TiO2 was determined using the GC technique. The degradation efficiency of Ag-doped TiO2 coatings is higher for lindane than for methyl orange.

The effect of TiO2 crystalline phase on microstructure and optical features of Zn2TiO4 doped with Mn

The influence of crystalline form of TiO2 (rutile and anatase) on microstructure and optical properties of Zn2TiO4 ceramics doped with manganese has been investigated by X-ray diffraction, Scanning electron microscopy, as well as by optical and electron paramagnetic resonance (EPR) methods. The ceramics was produced by sintering in the air at temperatures in the range of 900–1200 °C of a mixture of ZnO and TiO2 powders. Both types of ceramics became denser and demonstrated growth of grains in size with the increase of annealing temperature. Meanwhile, the ceramics made from anatase show abnormal grain growth and formation of preferred crystallographic grain orientation. The grain size of the ceramics made from anatase was larger for all annealing temperatures and increased from submicron up to tens of microns at 1100 and 1200 °C. This is attributed to known stimulating role of anatase in the formation of cubic Zn2TiO4 phase and/or anatase-to-rutile transformation.The concurrent optical absorption, photoluminescence (PL) and EPR studies of the ceramics revealed Mn2+ and Mn4+ ions in the Zn2TiO4 crystal phase. For all annealing temperatures the Zn2TiO4 made from anatase have larger content of Mn2+, while the Zn2TiO4 made from rutile show larger content of Mn4+ and about twice higher intensity of red PL caused by 2E→4A2 spin forbidden transitions of Mn4+. The PL intensity decreased in several times at high annealing temperatures starting from 1100 °C for the ceramics made of anatase, whereas from 1200 °C for that made of rutile. This effect strongly correlates with densification of ceramic’s microstructure and is ascribed mainly to the reduction of Mn4+ to Mn2+. It is concluded that rutile is preferable for creating Mn4+ centers in the Zn2TiO4 and making Mn4+-activated red phosphor.

Tailoring visible-light active TiO2/cellulose nanocomposites with controlled crystalline structure for enhanced photocatalytic performance.

This work involves a green and simple synthesis of TiO2 nanoparticles on cellulose under mild conditions without the need for calcination via hydrolysis of titanium oxysulfate (TiOSO4). The synthesis conditions, such as sulfuric acid concentration (0-10%wt), temperature (70-90℃), and time (4-8h), focused on precisely controlling the structure of TiO2 to enhance its photocatalytic effectiveness under visible light. At a lower 2.5wt% sulfuric acid concentration, pure anatase was formed on the cellulose, while an increase in the range of 5.0-7.5wt% sulfuric acid concentration yielded a rutile phase, resulting in a mixed phase of anatase and rutile on the cellulose. The pure rutile phase was found at a low temperature (70℃), while increased temperature led to the formation of the anatase phase. These results confirmed that the formation of crystalline TiO2 phase on the cellulose depended on sulfuric acid concentration and temperature for hydrolysis. Additionally, the photocatalytic properties of the obtained materials were evaluated by degradationvisible of Rhodamine B (RhB) under UV and visible light. The findings revealed that the mixed phase (anatase/rutile) of TiO2 on the cellulose demonstrated a superior photocatalytic efficiency (99.2%) compared to pure anatase (85.75%) and rutile (75.08%) when exposed to visible light.

Role of substrate and TiO2 content in TiO2:Ta2O5 coatings for gravitational wave detectors

Gravitational wave detectors (GWDs) are designed to detect the elusive signals produced by spacetime ripples, the GWs. The key to improving GWD sensitivity relies on the reduction of the thermal noise introduced by the mirrors. The high refractive index component of the high-reflectance mirrors installed in the current generation GWDs, such as Advanced LIGO and Advanced Virgo, is made of a mixture of ∼27% TiO2 and ∼73% Ta2O5. Such a coating plays a fundamental role in the GWD performance. The 27:73 TiO2:Ta2O5 ratio ensures high structural, optical, and mechanical performances, which allowed for the first ever detection of GWs, but might not be enough for new generation GWDs. Here, we investigate the potential of TiO2:Ta2O5 coatings, in a wider range of Ti/(Ta + Ti) cation ratio. Our research spans over the morphological and structural coating characteristics, and their correlation with optical and mechanical properties. On one hand, we unveil the profound influence of substrate selection and TiO2 content on the quality of coating morphology. On the other, we pinpoint the effect of TiO2 content on the structural properties of the coating, as increasing TiO2 content leads to lower temperature amorphous-to-crystalline transition, and we show that internal strain may arise due to the coexistence of TiO2 and Ta2O5 crystalline phases. Finally, substrate choice, TiO2 concentration, and crystallization characteristics emerge as pivotal factors in the pursuit of precision optics.

Fabrication of iron doped titanium dioxide photocatalytic nanocomposite supported by water-quenched blast furnace slag and photocatalytic degradation of wastewater

Metallurgical solid waste blast furnace water quenched slag (WBFS) was employed as carrier materials to prepare Fe–TiO2/WBFS composite photocatalyst using a sol-gel method for improving both the photocatalytic activity and the high-value utilization of metallurgical solid waste. The Fe3+ doping technology was adopted to increase the utilization of solar energy. The influencing factors and photocatalytic activity were systematically evaluated by thermogravimetry/differential thermal analyser (TG-DSC), X-Ray Diffraction (XRD), Brunauer-Emmett-Teller (BET) method, scanning electron microscopy-energy spectrometer (SEM-EDS), X-ray photoelectric spectroscopy (XPS), UV–Vis absorption spectrum and photoluminescence spectra technology through the degradation rate of methylene blue (MB) solution. The results showed that the Fe3+ ions were successfully incorporated into the TiO2 lattice, which had no effect on the crystalline phase of TiO2 and expanded the spectral response range of TiO2 from the ultraviolet region to the visible light region. When the calcination temperature was at 450 °C and the number of Fe–TiO2 sol loading cycles was two times, the photocatalytic activity of Fe–TiO2/WBFS presented the strongest, and the degradation ratio of MB reach the maximum value of 98.9 %. A uniform and dense anatase TiO2 thin film was wrapped on the surface of WBFS. The photocatalytic activity first improved and then weakened with an increase of calcination temperature, Fe3+ doping amounts and the loading cycles of sol. The TiO2 film changed from thin to thick and uneven to uniform, until cracking and spalling phenomenon finally occurred with an increase of Fe–TiO2 sol loading cycle from 1 to 3 times. When the Fe–TiO2/WBFS was reused for four times, the degradation rate of MB could still show 67.4 %, being far higher than that of TiO2/WBFS of 25.4 %.

Enhanced visible-light activation of persulfate for the removal of RhB by supported nano-(C,N,B)-tridoped TiO2/copper foam mixed-crystals.

Photocatalytic persulfate activation by TiO2 and its application in sewage treatment have aroused great interest because of its high decontamination ability and strong adaptability, but the low light energy utilization rate and poor recycling of TiO2 limited its practical application. Herein, by using C-, N-, and B-modified TiO2 and immobilizing it on copper foam, we prepared a new and efficient (C,N,B)-TiO2/copper foam photocatalyst with enhanced visible-light activation performance of persulfate for the removal of RhB. It almost completely degraded RhB within 15min of UV-vis light photocatalysis-assisted persulfate oxidation reaction with TOC removal of 53.17% in 30min and presented the excellent long-term recyclability and stability, which is much better or comparative than those photocatalysts in the related literatures. (C,N,B)-TiO2/copper foam exhibited the largest apparent rate constant (0.149min-1), 1.16 times higher than (C,N,B)-TiO2 (0.128min-1), and 2.40 times higher than that of TiO2 (0.062min-1), respectively. C,N,B doping modified the crystalline phase of TiO2, narrowed its band gap, and reduced charge-carrier recombination rate. These, together with the synergistic effect between photocatalysis and persulfate activation for enhancing generation of active species, jointly promoted the performance enhancement of TiO2. The 1O2 was the primary oxidation active species for the degradation of RhB, and the radical species (SO4•-, •O2-, and •OH) could further accelerate the photocatalytic activation of persulfate reaction.

Fabrication and characterization of polyvinyl alcohol/TiO2 membrane immersed in simulated body fluid for biomedical application

AbstractIn this study, we synthesized hydroxyapatite (HA) on polyvinyl alcohol/TiO2 (PVA/TiO2) in which TiO2 was prepared by the sol‐gel method. HA was formed by immersing the membrane in the simulated body fluid (SBF) solution. We improved the mechanical strength of the membranes by adding glutaraldehyde (GA) combining with heat treatment to cross‐link the polymer network. In addition, the composition of TiO2 crystalline phase, TiO2 content in each membrane, and the duration of membrane immersion in SBF solution were investigated. The morphological structures of hydroxyapatite/polyvinyl alcohol/TiO2 (HA/PVA/TiO2) membranes were characterized by IR, SEM, and XRD. We confirmed that the HA formation depended on the TiO2 content and phase composition (rutile or anatase). Membranes containing TiO2 rutile‐phase showed more pronounced HA formation than membranes containing TiO2 anatase‐phase. The appearance of HA also depended on SBF immersion duration. Membranes soaked for 3 weeks showed significantly more HA formation than membranes soaked for 1 and 2 weeks. Furthermore, the number of cells in membranes soaked for 3 weeks was clearly higher than in other membranes. Finally, HA/PVA/TiO2 membranes containing 25 wt.% of TiO2 rutile‐phase are biocompatible based on the number of viable cells and thus have the potential for biomedical application.

Photoactive TiO2 ceramic coatings obtained by laser-assisted plasma electrolytic oxidation on commercial pure titanium

To address the challenges of controlling the morphology and crystalline phase composition of photoactive TiO2 ceramic coatings produced by existing plasma electrolytic oxidation (PEO) technology, a novel laser-assisted PEO method is proposed in this study. The analysis has revealed that the TiO2 coatings prepared by the laser-assisted PEO method had a more intricate 3D multi-porosity architecture. Furthermore, the ratio of TiO2 crystalline phases (anatase to rutile) in the coating can be controlled by adjusting the laser energy during the laser-assisted irradiation process. The fundamental reason for the change in the morphology and composition of the crystalline phases of the coatings is the induction and enhancement of the plasma discharge by laser-assisted irradiation. The photoelectrochemical behavior of the samples was evaluated by employing linear sweep voltammetry under continuous UV irradiation, and the photoresponse current density was measured under intermittent UV irradiation. The laser-assisted PEO coatings prepared with 80 W laser energy under laser-assisted irradiation exhibited the most favorable photoelectrochemical activity. Furthermore, further analysis showed that the enhanced photoelectrochemical activity of the photoactive titanium dioxide coatings was essentially due to a larger specific surface area, lower impedance, higher wettability, and a more suitable crystalline phase ratio.

Effective Reinforcement of Visible Light Photocatalytic and Gas Sensing Characteristics of Nanocrystalline TiO2: Gd-Based Nb and Mo Dopants

Efficient compositions for the selective detection of ethanol gas and the removal of organic contaminants were realized by codoping of (Gd, Nb) and (Gd, Mo) ions into TiO2. TiO2, Ti0.96Gd0.01Nb0.03O2, and Ti0.96Gd0.01Mo0.03O2 samples were prepared by a coprecipitation method. For all compositions, a crystalline anatase phase of TiO2 was detected. Compared to pure TiO2, the absorption edges of Ti0.96Gd0.01Nb0.03O2 and Ti0.96Gd0.01Mo0.03O2 samples were red-shifted, further broadening towards visible light. The morphological studies demonstrate that the grains of TiO2 were more refined after (Gd, Nb) and (Gd, Mo) codoping. The photocatalytic efficiency of the Ti0.96Gd0.01Mo0.03O2 catalyst for degrading 20 mg/L reactive yellow 145, brilliant green, and amoxicillin was 98, 95, and 93% in 90 min, respectively. The reusability experiments indicate that the Ti0.96Gd0.01Mo0.03O2 catalyst had high stability during reuse. The high photocatalytic activity of the Ti0.96Gd0.01Mo0.03O2 catalyst was correlated to the broad visible-light absorption and effective separation of electron–hole pairs by Gd3+ and Mo6+ cations. The gas sensing characteristic is reflected by the high sensitivity of the Ti0.96Gd0.01Nb0.03O2 sensor to ethanol gas in the presence of different gases at 275 °C. The obtained results indicated that the (Gd, Mo) mixture could more effectively induce the photocatalytic properties of TiO2 while (Gd, Nb) dopants were the best for reinforcing its sensing characteristics.

The inhibitory mechanism of humic acids on photocatalytic generation of reactive oxygen species by TiO2 depends on the crystalline phase

To enable the practical use of TiO2 for photocatalytic water treatment, it is important to understand how dissolved natural organic matter affects its photocatalytic activity and the mechanisms involved in generating reactive oxygen species (ROS). In this study, we systematically investigated the inhibitory effects of humic acids (HA) on 4-chlorophenol (4-CP) degradation using two common TiO2 crystalline phases (anatase and rutile) individually. HA strongly hindered the photocatalytic activity of anatase, with an Ra (the ratio of the first-order rate constant for 4-CP degradation in the absence vs. presence of 30 mg/L HA) of 2.30 (±0.13). This ratio was significantly higher than that of rutile (Rr = 1.21 ± 0.08), which was less susceptible to this inhibitory effect and even exhibited improved photocatalytic activity at HA concentrations below 20 mg/L. Similar trends were observed for various HA sources, corroborating the crystalline-phase-dependent effect of HA. Adsorption experiments, Fourier transform infrared spectroscopy, and photoelectrochemical analyses suggested that the adsorption mechanisms and hole-scavenging effect of HA on the two TiO2 polymorphs did not differ. Importantly, scavenger, probe, and electron spin resonance experiments revealed that the difference in inhibitory effects of HA originate from the distinct ROS generation mechanisms for the two polymorphs. The oxygen reduction pathway for OH generation over anatase was hindered by surface-adsorbed HA, while the water oxidation pathway for OH generation over rutile was less affected. Furthermore, surface-adsorbed HA boosted O2− generation on rutile, increasing 4-CP degradation efficiency. This mechanistic insight into NOM-TiO2 interactions informs materials selection and strategies for higher TiO2 photocatalytic performance in different water matrices.