Enhanced Photocatalytic Activity of Titania by Co-Doping with Mo and W

Advanced photocatalysis as a viable and sustainable wastewater treatment process: A comprehensive review

In our era, water pollution not only poses a serious threat to human, animal, and biotic life but also causes serious damage to infrastructure and the ecosystem. A set of physical, chemical, and biological technologies have been exploited to decontaminate and/or disinfect water pollutants, toxins, microbes, and contaminants, but none of these could be ranked as sustainable and scalable wastewater technology. The photocatalytic process can harmonize the sunlight to degrade certain toxins, chemicals, microbes, and antibiotics, present in water. For example, transition metal oxides (ZnO, SnO2, TiO2, etc.), when integrated into an organic framework of graphene or nitrides, can bring about more than 90% removal of dyes, microbial load, pesticides, and antibiotics. Similarly, a modified network of graphitic carbon nitride can completely decontaminate petrochemicals. The present review will primarily highlight the mechanistic aspects for the removal and/or degradation of highly concerned contaminants, factors affecting photocatalysis, engineering designs of photoreactors, and pros and cons of various wastewater treatment technologies already in practice. The photocatalytic reactor can be a more viable and sustainable wastewater treatment opportunity. We hope the researcher will find a handful of information regarding the advanced oxidation process accomplished via photocatalysis and the benefits associated with the photocatalytic-type degradation of water pollutants and contaminants.

A critical review on the recent trends of photocatalytic, antibacterial, antioxidant and nanohybrid applications of anatase and rutile TiO2 nanoparticles

Titanium dioxide nanoparticles (TiO2 NPs) have become a focal point of research due to their widespread daily use and diverse synthesis methods, including physical, chemical, and environmentally sustainable approaches. These nanoparticles possess unique attributes such as size, shape, and surface functionality, making them particularly intriguing for applications in the biomedical field. The continuous exploration of TiO2 NPs is driven by the quest to enhance their multifunctionality, aiming to create next-generation products with superior performance. Recent research efforts have specifically focused on understanding the anatase and rutile phases of TiO2 NPs and evaluating their potential in various domains, including photocatalytic processes, antibacterial properties, antioxidant effects, and nanohybrid applications. The hypothesis guiding this research is that by exploring different synthesis methods, particularly chemical and environmentally friendly approaches, and incorporating doping and co-doping techniques, the properties of TiO2 NPs can be significantly improved for diverse applications. The study employs a comprehensive approach, investigating the effects of nanoparticle size, shape, dose, and exposure time on performance. The synthesis methods considered encompass both conventional chemical processes and environmentally friendly alternatives, with a focus on how doping and co-doping can enhance the properties of TiO2 NPs. The research unveils valuable insights into the distinct phases of TiO2 NPs and their potential across various applications. It sheds light on the improved properties achieved through doping and co-doping, showcasing advancements in photocatalytic processes, antibacterial efficacy, antioxidant capabilities, and nanohybrid applications. The study concludes by emphasizing regulatory aspects and offering suggestions for product enhancement. It provides recommendations for the reliable application of TiO2 NPs, addressing a comprehensive spectrum of critical aspects in TiO2 NP research and application. Overall, this research contributes to the evolving landscape of TiO2 NP utilization, offering valuable insights for the development of innovative and high-performance products.

Photocatalytic behavior for removal of methylene blue from aqueous solutions via nanocomposites based on Gd2O3/CdS and cellulose acetate nanofibers

Efficient cleaning of contaminated water by photocatalysis has become an effective strategy in recent years due to its environmental and ecological designation. Cadmium sulfate (CdS) is an excellent photocatalyst in the visible region but has low quantum efficiency. In order to increase the photocatalytic efficiency, CdS was modified with gadolinium oxide (Gd2O3) and combined with graphene oxide (GO) nanoparticles. The estimated crystallite size (Ds) for Gd2O3, CdS/Gd2O3, and CdS/Gd2O3@GO was 29.6, 11.6, and 11.5 nm, respectively. The degradation of methylene blue (MB) reaches the highest values after 60 min under visible light irradiation with a dye concentration of (0.25 ppm). Whereas in powdered composition the efficiency of dye removal has been enhanced under UV irradiation, it reduced by increasing the MB concentration to 0.50 ppm with visible light irradiation. In addition, the CdS with/without Gd2O3 and GO were integrated into electrospun nanofibrous cellulose acetate (CA) through the electrospinning technique. The compounds of Gd2O3, CdS/Gd2O3, and CdS/Gd2O3/GO were encapsulated into CA nanofibers for the degradation of MB under visible and UV irradiation. The apparent rate constant (k) achieves a value of 0.006, 0.007, and 0.0013 min-1 while the removal efficiency reaches 41.02%, 54.71%, and 71.42% for Gd2O3@CA, CdS/Gd2O3@CA, and CdS/Gd2O3/GO@CA, respectively, after 60 min under UV irradiation.

Effect of bismuth doping on the crystal structure and photocatalytic activity of titanium oxide

The doping of TiO2 with metals and non-metals is considered one of the most significant approaches to improve its photocatalytic efficiency. In this study, the photodegradation of methyl orange (MO) was examined in relation to the impact of Bi-doping of TiO2. The doped TiO2 with various concentrations of metal was successfully synthesized by a one-step hydrothermal method and characterized using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), and UV-vis spectroscopy. The XRD results revealed that the anatase phase, with an average crystallite size of 16.2 nm, was the main phase of TiO2. According to the anatase texture results, it was found that the doping of TiO2 increased the specific surface area for Bi2O3@TiO2 without a change in the crystal structure or the crystal phase of TiO2. Also, XPS analysis confirmed the formation of Ti4+ and Ti3+ as a result of doping with Bi. The activities of both pure TiO2 and Bi-doped TiO2 were tested to study their ability to decolorize MO dye in an aqueous solution. The photocatalytic degradation of MO over Bi2O3@TiO2 reached 98.21%, which was much higher than the 42% achieved by pure TiO2. Doping TiO2 with Bi increased its visible-light absorption as Bi-doping generated a new intermediate energy level below the CB edge of the TiO2 orbitals, causing a shift in the band gap from the UV to the visible region, thus enhancing its photocatalytic efficiency. In addition, the effects of the initial pH, initial pollutant concentration, and contact time were examined and discussed.

Photo-Oxidation of Glycerol Catalyzed by Cu/TiO2

In the present study, glycerol was oxidized by photocatalysis to glyceraldehyde, formaldehyde, and formic acid. Copper-doped TiO2 was synthesized by the evaporation-induced self-assembly approach and it was used as catalyst during the glycerol photo-oxidation reactions. The prepared mesoporous material exhibited high specific surface area (242 m2/g) and band gap energy reduction of 2.55 eV compared to pure titania (3.2 eV) by the synthesis method due to the presence of copper cations (Cu2+ identified by XPS). The catalyst showed only anatase crystalline phase with nanocrystals around 8 nm and irregular agglomerates below 100 μm. The selectivity and formation rate of the products were favored towards formaldehyde and glyceraldehyde. The variables studied were catalyst amount, reaction temperature, and initial glycerol concentration. The response surface analysis was used to evaluate the effect of the variables on the product’s concentration. The optimized conditions were 0.4 g/L catalyst, 0.1 mol/L glycerol, and temperature 313.15 K. The response values under optimal conditions were 3.23, 8.17, and 1.15 mM for glyceraldehyde, formaldehyde, and formic acid, respectively. A higher selectivity towards formaldehyde was observed when visible light was used as the radiation source. This study is useful to evaluate the best reaction conditions towards value-added products during the oxidation of glycerol by photocatalysis using Cu/TiO2.

Degradation of 4-Tert-Butylphenol in Water Using Mono-Doped (M1: Mo, W) and Co-Doped (M2-M1: Cu, Co, Zn) Titania Catalysts

Mono-doped (Mo-TiO₂ and W-TiO₂) and co-doped TiO₂ (Co-Mo-TiO₂, Co-W-TiO₂, Cu-Mo-TiO₂, Cu-W-TiO₂, Zn-Mo-TiO₂, and Zn-W-TiO₂) catalysts were synthesized by simple impregnation methods and tested for the photocatalytic degradation of 4-tert-butylphenol in water under UV (365 nm) light irradiation. The catalysts were characterized with various analytical methods. X-ray diffraction (XRD), Raman, Diffuse reflectance (DR) spectroscopies, Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), and Energy dispersive spectroscopy (EDS) were applied to investigate the structure, optical properties, morphology, and elemental composition of the prepared catalysts. The XRD patterns revealed the presence of peaks corresponding to the WO₃ in W-TiO₂, Co-W-TiO₂, Cu-W-TiO₂, and Zn-W-TiO₂. The co-doping of Cu and Mo to the TiO₂ lattice was evidenced by the shift of XRD planes towards higher 2θ values, confirming the lattice distortion. Elemental mapping images confirmed the successful impregnation and uniform distribution of metal particles on the TiO₂ surface. Compared to undoped TiO₂, Mo-TiO₂ and W-TiO₂ exhibited a lower energy gap. Further incorporation of Mo-TiO₂ with Co or Cu introduced slight changes in energy gap and light absorption characteristics, particularly visible light absorption. In addition, photoluminescence (PL) showed that Cu-Mo-TiO₂ has a weaker PL intensity than undoped TiO₂. Thus, Cu-Mo-TiO₂ showed better catalytic activity than pure TiO₂, achieving complete degradation of 4-tert-butylphenol under UV light irradiation after 60 min. The application of Cu-Mo-TiO₂ under solar light conditions was also tested, and 70% of 4-tert-butylphenol degradation was achieved within 150 min.

Molybdenum Modified Sol–Gel Synthesized TiO2 for the Photocatalytic Degradation of Carbamazepine under UV Irradiation

Pharmaceutical CEC compounds are a potential threat to man, animals, and the environment. In this study, a sol–gel-derived TiO2 (SynTiO2) was produced and subsequently sonochemically doped with a 1.5 wt% Mo to obtain the final product (Mo (1.5 wt%)/SynTiO2). The as-prepared materials were characterized for phase structure, surface, and optical properties by XRD, TEM, N2 adsorption–desorption BET isotherm at 77 K, and PSD by BJH applications, FTIR, XPS, and UV-Vis measurements in DRS mode. Estimated average crystallite size, particle size, surface area, pore-volume, pore size, and energy bandgap were 16.10 nm, 24.55 nm, 43.30 m2/g, 0.07 cm3/g, 6.23 nm, and 3.05 eV, respectively, for Mo/SynTiO2. The same structural parameters were also estimated for the unmodified SynTiO2 with respective values of 14.24 nm, 16.02 nm, 133.87 m2/g, 0.08 cm3/g, 2.32 nm, and 3.3 eV. Structurally improved (Mo (1.5 wt%)/SynTiO2) achieved ≈100% carbamazepine (CBZ) degradation after 240 min UV irradiation under natural (unmodified) pH conditions. Effects of initial pH, catalyst dosage, initial pollutant concentration, chemical scavengers, contaminant ions, hydrogen peroxide (H2O2), and humic acid (HA) were also investigated and discussed. The chemical scavenger test was used to propose involved photocatalytic degradation process mechanism of CBZ.

Enhancing the physical properties and photocatalytic activity of TiO2 nanoparticles via cobalt doping

Cobalt-doped TiO₂-based diluted magnetic semiconductors were successfully synthesized using a co-precipitation method. The X-ray diffraction study of all the samples showed good crystallinity, matching the standard tetragonal anatase phase. The X-ray diffraction peaks of the cobalt-doped sample slightly shifted towards a lower angle showing the decrease in particle size and distortion in the unit cell due to cobalt incorporation in the lattice of TiO₂. Transmission electron microscopy showed the spherical morphology of the TiO₂ nanoparticles, which decreased with Co-doping. The optical characteristics and band gap investigation revealed that defects and oxygen vacancies resulted in lower band gap energy and maximum absorption in the visible region. Dielectric measurements showed enhancement in the dielectric constant and AC conductivity, while the dielectric loss decreased. The enhancement in the dielectric properties was attributed to interfacial polarization and charge carrier hopping between Co and Ti ions. The magnetic properties displayed that pure TiO₂ was diamagnetic, while Co-doped TiO₂ showed a ferromagnetic response at 300 K. The visible light-driven photocatalytic activity showed an improvement for Co-doped TiO₂. Our results demonstrate that Co-doping can be used to tune the physical properties and photocatalytic activity of TiO₂ for possible spin-based electronics, optoelectronics, and photo-degradation applications.

(a) dielectric constant, (b) dielectric loss and (c) AC conductivity vs. frequency for un-doped and 5 at% Co-doped TiO₂ NPs.

Preparation and Characterization of Supported Molybdenum Doped TiO2 on α-Al2O3 Ceramic Substrate for the Photocatalytic Degradation of Ibuprofen (IBU) under UV Irradiation

TiO2-based photocatalyst materials have been widely studied for the abatement of contaminants of emerging concerns (CECs) in water sources. In this study, 1.5 wt% Mo-doped HRTiO2 was obtained by the sonochemical method. The material was analyzed and characterized for thermal, structural/textural, morphological, and optical properties using TGA-DSC, XRD, TEM, FTIR, XPS, SEM-EDS, BET (N2 adsorption-desorption measurement and BJH application method), and UV-Vis/DRS measurement. By the dip-coating technique, ~5 mg of Mo/HRTiO2 as an active topcoat was deposited on ceramic. In suspension and for photocatalyst activity performance evaluation, 1 g/L of 1.5 wt% (Mo)/HRTiO2 degraded ~98% of initial 50 mg/L IBU concentration after 80 min of 365 nm UV light irradiation and under natural (unmodified) pH conditions. Effects of initial pH condition, catalyst dosage, and initial pollutant concentration were also investigated in the photocatalyst activity performance in suspension. The photocatalyst test on the supported catalyst removed ~60% of initial 5mg/L IBU concentration, while showing an improved performance with ~90% IBU removal employing double and triple numbers of coated disk tablets. After three successive cycle test runs, XRD phase reflections of base TiO2 component of the active photocatalyst supported layer remained unchanged: An indication of surface coat stability after 360 min of exposure under 365 nm UV irradiation.

Applying Hydrogenation to Stabilize N-TiO2 and Enhance Its Visible Light Photocatalytic Activity

Up to now, the explanation for the origin of enhanced photocatalytic activity of N doped TiO2 (N-TiO2) with H incorporation, which is observed in experiment, is still lacking. In our work, the effects of hydrogenation on the stability and electronic properties of N-TiO2 have been systematically investigated by first-principles calculations. Our results of the study on stability demonstrate that, both full and part hydrogenation could stabilize N-TiO2 by largely reducing the formation energy of N doping under Ti-rich conditions. Moreover, the calculated results on the electronic structure show that, for the completely hydrogenated N-TiO2, band gap becomes slightly larger, which is caused by the full passivation for unpaired electron from N atom. However, for the partially hydrogenated N-TiO2, due to the interaction between hydrogenated and unhydrogenated N atoms, its valence band maximum shifts to higher energy by 0.32 eV and the valence band states mix with the wide band-gap states, which results in a higher light absorption capacity and carrier separation. Our results not only explain the enhancement of visible light photocatalytic activity experimentally found in N-TiO2 specimen with H incorporation, but also indicate that, tuning the hydrogenation degree is a hopeful routine to improve the photocatalytic performance of N-TiO2.

Effective Inclusion of Sizable Amounts of Mo within TiO2 Nanoparticles Can Be Obtained by Reverse Micelle Sol-Gel Synthesis

Six Mo/TiO2 samples (with 0, 1.0, 2.5, 5.0, 7.5, and 10 wt % Mo nominal contents) were obtained by reverse micelle sol-gel synthesis, followed by calcination at 500 °C. The samples were characterized by means of powder X-ray Diffraction (PXRD), quantitative phase analysis as obtained by Rietveld refinement, field-emission scanning electron microscopy (FE-SEM) coupled with energy-dispersive X-ray analysis, N2 adsorption/desorption at -196 °C, X-ray photoelectron spectroscopy, and diffuse reflectance (DR) UV-vis spectroscopy. As a whole, the adopted characterization techniques showed the inclusion of a sizeable Mo amount, without the segregation of any MoO x phase. Specifically, PXRD showed the occurrence of anatase and brookite with all the studied samples; notwithstanding the mild calcination temperature, the formation of rutile occurred at Mo wt % ≥2.5 likely due to the presence of brookite favoring, in turn, anatase to rutile transition. DR UV-vis and XP spectroscopies allowed determining the samples' band gap energy (E g) and valence band energy, respectively, from which the conduction band energy was calculated; and the observed E g value increase at 10 wt % Mo was ascribed to the Moss-Burstein effect.

Nanocomposites containing titanium dioxide for environmental remediation

Titania is considered to be one of the most versatile material in its nanoform. Scientific community looks towards it to address various pressing global problems. One such problem is aquatic pollution arising from organic chemicals such as dyes, pesticides, antibiotics etc. due to industrial, domestic and agricultural activities. Titania proves to be very effective to address this problem owing to its superior photocatalytic properties. In this review, we will review the recent advances in titania-based nanocomposites. The recent advances discussed in this review include synthesis of titania, modification of titania, exploration of various supports such as silica, carbon, graphene etc., that is documented to enhance its environmental remediation properties.

Editorial: Special Issue on “Emerging Trends in TiO2 Photocatalysis and Applications”

It is not an exaggerated fact that the semiconductor titanium dioxide (TiO2) has been evolved as a prototypical material to understand the photocatalytic process and has been demonstrated for various photocatalytic applications such as pollutants degradation, water splitting, heavy metal reduction, CO2 conversion, N2 fixation, bacterial disinfection, etc [...]

Visible-Light Active Titanium Dioxide Nanomaterials with Bactericidal Properties

This article provides an overview of current research into the development, synthesis, photocatalytic bacterial activity, biocompatibility and cytotoxic properties of various visible-light active titanium dioxide (TiO2) nanoparticles (NPs) and their nanocomposites. To achieve antibacterial inactivation under visible light, TiO2 NPs are doped with metal and non-metal elements, modified with carbonaceous nanomaterials, and coupled with other metal oxide semiconductors. Transition metals introduce a localized d-electron state just below the conduction band of TiO2 NPs, thereby narrowing the bandgap and causing a red shift of the optical absorption edge into the visible region. Silver nanoparticles of doped TiO2 NPs experience surface plasmon resonance under visible light excitation, leading to the injection of hot electrons into the conduction band of TiO2 NPs to generate reactive oxygen species (ROS) for bacterial killing. The modification of TiO2 NPs with carbon nanotubes and graphene sheets also achieve the efficient creation of ROS under visible light irradiation. Furthermore, titanium-based alloy implants in orthopedics with enhanced antibacterial activity and biocompatibility can be achieved by forming a surface layer of Ag-doped titania nanotubes. By incorporating TiO2 NPs and Cu-doped TiO2 NPs into chitosan or the textile matrix, the resulting polymer nanocomposites exhibit excellent antimicrobial properties that can have applications as fruit/food wrapping films, self-cleaning fabrics, medical scaffolds and wound dressings. Considering the possible use of visible-light active TiO2 nanomaterials for various applications, their toxicity impact on the environment and public health is also addressed.

Hydrogen Production from Aqueous Methanol Solutions Using Ti–Zr Mixed Oxides as Photocatalysts under UV Irradiation

The synthesis and characterisation of Ti–Zr mixed oxides containing 1 to 10 wt.% of Zr is herein reported. In addition, the samples were tested as photocatalysts in the generation of hydrogen from aqueous methanol solutions. The solids were prepared by sol-gel and then characterised by X-ray diffraction, high resolution electron microscopy, X-ray photoelectron spectroscopy, physisorption of nitrogen, scanning electron microscopy, UV-vis and Raman spectroscopies. The results show the presence of anatase as a predominant structure and the oxides present larger specific surface areas than that of pure titania. A maximum value of 168 m2/g was determined for the sample with 5 wt.% of zirconium. The calculated band gap energies varied from 3.05 to 3.15 eV. It was observed that the greater the zirconium content in the solid, the higher the generation rate of hydrogen when testing the Ti–Zr solids as photocatalysts. Under our experimental conditions, the best catalyst, Ti–Zr oxide with 10 wt. % Zr, showed a production rate of 2100 μmol of H2/h which was about tenfold higher than that observed for pure titania.

17-β-Estradiol: Significant reduction of its toxicity in water treated by photocatalysis

The aim of this study was to assess the efficiency of photocatalysis by TiO₂ on the removal of 17-β-estradiol (E2) (at environmentally relevant concentrations) and the toxicity caused by this emerging pollutant. After 60min of TiO₂/UV treatment at pilot scale (14L), E2 was removed from water approximately 85%. The toxicity was established by using Cyprinus carpio as bioindicator organism and oxidative stress biomarkers (OSB): [lipid peroxidation level (LPX), hydroperoxide content (HPC) and protein carbonyl content (PCC)] and enzymes [superoxide dismutase (SOD) and catalase (CAT)]. It was found that the photocatalytic treatment led to significantly reduce OSB in approximately 85-95%. Thus, it can be concluded that heterogeneous photocatalysis by TiO₂ is an efficient process to eliminate the toxicity caused by E2 and thus to remediate water polluted with this molecule.

Enhanced Photocatalytic Reduction of Cr(VI) by Combined Magnetic TiO2-Based NFs and Ammonium Oxalate Hole Scavengers

Heavy metal pollution of wastewater with coexisting organic contaminants has become a serious threat to human survival and development. In particular, hexavalent chromium, which is released into industrial wastewater, is both toxic and carcinogenic. TiO2 photocatalysts have attracted much attention due to their potential photodegradation and photoreduction abilities. Though TiO2 demonstrates high photocatalytic performance, it is a difficult material to recycle after the photocatalytic reaction. Considering the secondary pollution caused by the photocatalysts, in this study we prepared Ag/Fe3O4/TiO2 nanofibers (NFs) that could be magnetically separated using hydrothermal synthesis, which was considered a benign and effective resolution. For the photocatalytic test, the removal of Cr(VI) was carried out by Ag/Fe3O4/TiO2 nanofibers combined with ammonium oxalate (AO). AO acted as a hole scavenger to enhance the electron-hole separation ability, thereby dramatically enhancing the photoreduction efficiency of Cr(VI). The reaction rate constant for Ag/Fe3O4/TiO2 NFs in the binary system reached 0.260 min−1, 6.95 times of that of Ag/Fe3O4/TiO2 NFs in a single system (0.038 min−1). The optimized Ag/Fe3O4/TiO2 NFs exhibited high efficiency and maintained their photoreduction efficiency at 90% with a recyclability of 87% after five cycles. Hence, taking into account the high magnetic separation behavior, Ag/Fe3O4/TiO2 NFs with a high recycling capability are a potential photocatalyst for wastewater treatment.