Surface modification of TiO2 photocatalyst for environmental applications

Graphene Quantum Dots Doped PVDF(TBT)/PVP(TBT) Fiber Film with Enhanced Photocatalytic Performance

We report the fabrication of polyvinylidene fluoride (tetrabutyl titanate)/polyvinyl pyrrolidone ((tetrabutyl titanate))-graphene quantum dots [PVDF(TBT)/PVP(TBT)-GQDs] film photocatalyst with enhanced photocatalytic performance. The polyvinylidene fluoride (tetrabutyl titanate)/polyvinyl pyrrolidone ((tetrabutyl titanate)) [PVDF(TBT)/PVP(TBT)] film was first prepared with a dual-electrospinning method and then followed by attaching graphene quantum dots (GQDs) to the surface of the composite film through a hydrothermal method. Later, part of the PVP in the composite film was dissolved by a hydrothermal method. As a result, a PVDF(TBT)/PVP(TBT)-GQDs film photocatalyst with a larger specific surface area was achieved. The photocatalytic degradation behavior of the PVDF(TBT)/PVP(TBT)-GQDs film photocatalyst was examined by using Rhodamine B as the target contaminant. The PVDF(TBT)/PVP(TBT)-GQDs photocatalyst showed a higher photocatalytic efficiency than PVDF(TBT)-H2O, PVDF(TBT)/PVP(TBT)-H2O, and PVDF(TBT)-GQDs, respectively. The enhanced photocatalytic efficiency can be attributed to the broader optical response range of the PVDF(TBT)/PVP(TBT)-GQDs photocatalyst, which makes it useful as an effective photocatalyst under white light irradiation.

How g-C3N4 Works and Is Different from TiO2 as an Environmental Photocatalyst: Mechanistic View

Graphitic carbon nitride (CN) as a popular visible light photocatalyst needs to be better understood for environmental applications. The behaviors of CN as an environmental photocatalyst were systematically studied in comparison with a well-known TiO₂ photocatalyst. The two photocatalysts exhibit different photocatalytic oxidation (PCO) behaviors and dependences on the experimental conditions (e.g., pH, Pt loading, and the kind of organic substrate and scavenger). The PCO of organic substrates was significantly enhanced by loading Pt on TiO₂ under UV light (λ > 320 nm), whereas Pt-CN exhibited a lower PCO activity than bare CN under visible light (λ > 420 nm). While the presence of Pt enhances the charge separation in both TiO₂/UV and CN/visible light systems (confirmed by transient IR absorption spectroscopic analysis), the opposite effects of Pt are ascribed to the different mechanisms of ^•OH generation in the two photocatalytic systems. The negative effect of Pt on CN is ascribed to the fact that Pt catalytically decomposes in situ-generated H₂O₂ (a main precursor of OH radical), which hinders ^•OH production. The production of OH radicals on CN is favored only at acidic pH but ¹O₂ generation is dominant in alkaline pH. The pH-dependent behaviors of reactive oxygen species generation on CN were confirmed by electron paramagnetic resonance spin trap measurements.

Different effects of fluoride and phosphate anions on TiO2 photocatalysis (rutile)

At the same amounts adsorbed on Pt/rutile, fluoride was approximately 3 times more active than phosphate. A radical mechanism is proposed.

Synergetic effect in water treatment with mesoporous TiO2/BDD hybrid electrode

Boron-doped diamond (BDD) electrodes have a wide potential window and can produce ozone by water electrolysis at high voltage. Though ozone has strong oxidative power (standard oxidation potential: 2.07 V vs. NHE), it cannot decompose certain types of recalcitrant organic matter completely. We developed an advanced oxidation process (AOP), in which hydroxy radicals with stronger oxidative power (standard oxidation potential: 2.85 V vs. NHE) are formed using a combination of ozone, photocatalyst, and UV. In this study, we fabricated a mesoporous TiO₂/BDD hybrid electrode and examined its potential for AOPs. A synergetic effect between electrochemical water treatment and photocatalytic water treatment was observed with the hybrid electrode that did not occur with the BDD electrode.

A mesoporous TiO₂/BDD hybrid electrode showed a synergetic effect between electrochemical water treatment and photocatalytic water treatment.

Application of Hierarchical Nanostructured WO3 and Fe2O3 Composites for Photodegradation of Surfactants in Water Samples

This study describes the utilization of hierarchical photoactive surface films for the decomposition of surfactants in water samples (with different pH). Photoactive films, containing tungsten (VI) oxide and iron (III) oxide (hematite), were deposited in a systematic and controlled manner using a layer-by-layer method. Physicochemical properties of the photoactive materials were developed and characterized using XRD analysis, Raman spectroscopy, water contact angle, voltammetry, and microscopic (SEM) techniques. The resulting multilayer films showed attractive performances in the photodegradation of the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic surfactant (1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol (Triton™ X-144) under solar light irradiation. The efficiency of the surfactants’ photodegradation was evaluated with a “test” based on a method, which is extremely sensitive to surfactants’ interference, with trace analysis of Pb using anodic stripping voltammetry on mercury electrodes (recovery study). The usefulness of hierarchical photoactive systems in the photodegradation of both surfactants is demonstrated in the presence and absence of the applied bias voltage. The maximum decomposition times were 2–3 h and 30 min, respectively. Furthermore, a properly designed layer system may be proposed, matching the pH of the water sample (depending on the treatment on the sampling side).

New Efficient Visible-Light-Driven Photocatalyst of Chitin-Modified Titanium Dioxide/Carbon Fiber Composites for Wastewater

To improve the catalyst properties of TiO₂ under visible light irradiation, chitin-modified TiO₂ was synthesized via a hydrothermal method on the surface of carbon fibers. The microstructure and interface properties of the so-prepared photocatalyst were investigated via X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and UV-visible diffuse reflectance spectroscopy. Our results indicated that the synergetic effect of the crystal phase of TiO₂, carbon fiber, and chitin is the main reason leading to the improvement of the photocatalytic activity of the composite catalyst. The modified TiO₂ sample with chitin content of 0.6 wt% exhibited the highest photocatalytic activity under visible light irradiation when RhB was chosen as the target degradation product. Compared to the pure TiO₂/carbon fiber, the sample of TiO₂/carbon fiber with 0.6 wt% of chitin exhibits enhanced visible light activity with an apparent rate of degradation about 2.25 times. The enhancement of the photocatalytic performance of the sample with chitin can be attributed to the relatively high adsorption capacity of the particular network structure and photosensitivity of chitin, which can effectively separate the photoelectron-hole pair recombination. Furthermore, the new composite photocatalyst shows excellent catalytic stability after multiple degradation cycles, indicating that it is a promising photocatalytic material for degrading organic pollutants in wastewater.

Thorn-like TiO2 nanoarrays with broad spectrum antimicrobial activity through physical puncture and photocatalytic action

To overcome the conventional limitation of TiO2 disinfection being ineffective under light-free conditions, TiO2 nanowire films (TNWs) were prepared and applied to bacterial disinfection under dark and UV illumination. TNW exhibited much higher antibacterial efficiencies against Escherichia coli (E. coli) under dark and UV illumination conditions compared to TiO2 nanoparticle film (TNP) which was almost inactive in the dark, highlighting the additional contribution of the physical interaction between bacterial membrane and NWs. Such a physical contact-based antibacterial activity was related to the NW geometry such as diameter, length, and density. The combined role of physical puncture and photocatalytic action in the mechanism underlying higher bactericidal effect of TNW was systematically examined by TEM, SEM, FTIR, XPS, and potassium ion release analyses. Moreover, TNW revealed antimicrobial activities in a broad spectrum of microorganisms including Staphylococcus aureus and MS2 bacteriophage, antibiofilm properties, and good material stability. Overall, we expect that the free-standing and antimicrobial TNW is a promising agent for water disinfection and biomedical applications in the dark and/or UV illumination.

The preparation of amorphous TiO2 doped with cationic S and its application to the degradation of DCFs under visible light irradiation

An amorphous S-doping TiO₂ catalyst [S-TiO₂ (300)] with visible light catalytic activity was successfully prepared via a sol-gel method with low-temperature calcination (300 °C) using thiourea as the sulfur source. The S-TiO₂ (300) catalyst showed great performance in degrading the recalcitrant pharmaceutical, diclofenac (DCF). 93% of the target pollutant DCF degraded in 4 h under visible light irradiation. Both the amorphous form of the catalyst and cationic S-doping (with a coordinated structure) contributed to narrowing the band gap. As a result, the photocatalytic activity of S-TiO₂ (300) was significantly enhanced under visible light irradiation. In addition, oxidative species such as photogenic cavitation (h+), OH and O₂^- were proved to participate in the photodegradation process, attacking COOH group and NH bond, degraded DCF into low molecule organic gradually.

Multivariate comparison of photocatalytic properties of thirteen nanostructured metal oxides for water purification

Although the environmental photocatalysis is being developed for many years, the relationships between simple metal oxides have not been explored so far. In this study a multivariate comparison of thirteen nanostructured metal oxides (Bi₂O₃, CeO₂, Co₃O₄, Fe₂O₃, NiO, Pr₆O₁₁, SnO₂, SrTiO₃, TiO₂, WO₃, ZnFe₂O₄, ZnO and ZrO₂) was performed. The solution containing twenty-six psychotropic pharmaceuticals was used as a model mixture. In order to ensure the influence of the dissolved organic matter on the process, all the experiments were conducted in the river water. Simulated solar radiation was applied as the most environmentally relevant. The high-resolution LC-MS profiles, obtained for the photocatalytic samples after 1 h of irradiation, were then submitted to the multivariate chemometric analysis. Graphical representations of principal component analysis and hierarchical cluster analysis enabled visualization of the relationships between the studied oxides. The registered degradation profiles were compared qualitatively and discussed.

Fluorinated and Platinized Titania as Effective Materials in the Photocatalytic Treatment of Dyestuffs and Stained Wastewater Coming from Handicrafts Factories

In this study, commercial and lab-prepared TiO2 were modified by fluorination and platinum photodeposition; and the effect of these modifications over the physicochemical and photocatalytic properties of TiO2 was evaluated. It was found that F and Pt addition leads to the modification of the optical and textural properties of TiO2. The materials prepared were tested in the photocatalytic degradation of different organic dyestuffs such as methylene blue (MB) and methyl orange (MO); the degradation of commercial anilines employed in the staining of natural fibers was also evaluated. Photocatalysis was also studied in this work as an eco-friendly treatment of wastewater coming from handicrafts factories. In general it was observed that the effectiveness of the photocatalytic treatment strongly depends on the substrate to be degraded, thus, fluorinated and platinized commercial Titania (Pt-F-P25) showed the best photocatalytic performance in the MB and MO photodegradation and in contrast, in the case of the anilines the highest degradation was obtained over commercial TiO2 fluorinated (F-P25). These results can be explained by differences observed in the structure and in the adsorption of these dyestuffs over the photocatalysts surfaces. F-P25 photocatalyst also demonstrated to be the best material for the treatment of real wastewater coming from handicrafts factories.

Amalgamation of N-graphene quantum dots with nanocubic like TiO2: an insight study of sunlight sensitive photocatalysis

In this work, a sunlight-sensitive photocatalyst of nanocubic-like titanium dioxide (TiO₂) and N-doped graphene quantum dots (N-GQDs) is developed through a simple hydrothermal and physical mixing method. The successful amalgamation composite photocatalyst characteristics were comprehensively scrutinized through various physical and chemical analyses. A complete removal of bisphenol A (BPA) is attained by a synthesized composite after 30 min of sunlight irradiation as compared to pure TiO₂. This clearly proved the unique contribution of N-GQDs that enhanced the ability of light harvesting especially under visible light and near-infrared region. This superior characteristic enables it to maximize the absorbance in the entire solar spectrum. However, the increase of N-GQDs weight percentage has created massive oxygen vacancies that suppress the generation of active radicals. This resulted in a longer duration for a complete removal of BPA as compared to lower weight percentage of N-GQDs. Hence, this finding can offer a new insight in developing effective sunlight-sensitive photocatalysts for various complex organic pollutants degradation.

Effective removal of emerging dissolved cyanotoxins from water using hybrid photocatalytic composites

Harmful algal blooms are occurring more frequently in fresh water throughout the world. Certain cyanobacteria can produce and release potent toxic compounds, known as cyanotoxins, such as microcystins, cylindrospermopsin, saxitoxin, and anatoxin-a, and as such they have become a human and environmental health concern. Hybrid photocatalytic composites (HPCs) comprising carbon nanotubes on the surface of TiO₂ nanotubes were designed in this study. The HPCs have a selective adsorption capacity to cyanotoxins and provide photocatalytic activity to produce reactive oxygen species for the degradation of cyanotoxins. HPCs with 5.2 mg carbon nanotubes/cm² showed an excellent removal efficiency of microcystins-LR (>95%) at 55.6 L/m²/hr/bar. The HPCs more efficiently removed the relatively larger and more hydrophobic cyanotoxins (i.e., microcystin-LR) than the relatively smaller and more hydrophilic compounds, such as cylindrospermopsin, saxitoxin, and anatoxin-a. With a further increased in the carbon nanotube content to 8.6 mg/cm², the adsorption capacity of the HPCs for cyanotoxins increased to 70.6% for MC-LR. However, there was significant decrease in the photocatalytic activity of the HPCs for production of reactive oxygen species, and consequently a decrease in the degradation of cyanotoxins. It is considered that this device could be used to provide complete rejection of particles and pathogens, and also to significantly reduce trace organic compounds and harmful algal toxins in emergency water supplies.

Oxygen-Deficient Dumbbell-Shaped Anatase TiO2-x Mesocrystals with Nearly 100 % Exposed {101} Facets: Synthesis, Growth Mechanism, and Photocatalytic Performance

The development of hierarchical TiO₂ superstructures with new morphologies and intriguing photoelectric properties for utilizing solar energy is known to be an effective approach to alleviate the serious problems of environmental pollution. Herein, unique oxygen-deficient dumbbell-shaped anatase TiO_2-x mesocrystals (DTMCs) enclosed by nearly 100 % {101} facets were readily synthesized by mesoscale transformation in TiCl₃ /acetic acid (HAc) mixed solution, followed by calcination under vacuum. These mesocrystals exhibited much higher photoreactivity toward removing the model pollutants methyl orange and Cr^VI than truncated tetragonal bipyramidal anatase nanocrystals (TNCs), anatase mesocrystals built from truncated tetragonal bipyramidal anatase nanocrystals (TTMCs), and anatase mesocrystals constructed by anatase nanocrystals with nearly 100 % exposed {101} facets (TMCs), revealing that both the oxidation and reduction abilities of anatase TiO₂ were simultaneously enhanced upon fabricating an oxygen-deficient mesocrystalline architecture with about 100 % exposed {101} facets. Further characterization illustrated that such an enhancement of photoreactivity was mainly due to the strengthened light absorption, boosted charge carrier separation, and nearly 100 % exposed {101} facets of the oxygen-deficient dumbbell-shaped anatase mesocrystals. This work will be useful for guiding the synthesis of oxygen-deficient ordered superstructures of metal oxides with desired morphologies and exposed facets for promising applications in environmental remediation.

Titanium Dioxide (TiO2) Mesocrystals: Synthesis, Growth Mechanisms and Photocatalytic Properties

Hierarchical TiO2 superstructures with desired architectures and intriguing physico-chemical properties are considered to be one of the most promising candidates for solving the serious issues related to global energy exhaustion as well as environmental deterioration via the well-known photocatalytic process. In particular, TiO2 mesocrystals, which are built from TiO2 nanocrystal building blocks in the same crystallographical orientation, have attracted intensive research interest in the area of photocatalysis owing to their distinctive structural properties such as high crystallinity, high specific surface area, and single-crystal-like nature. The deeper understanding of TiO2 mesocrystals-based photocatalysis is beneficial for developing new types of photocatalytic materials with multiple functionalities. In this paper, a comprehensive review of the recent advances toward fabricating and modifying TiO2 mesocrystals is provided, with special focus on the underlying mesocrystallization mechanism and controlling rules. The potential applications of as-synthesized TiO2 mesocrystals in photocatalysis are then discussed to shed light on the structure–performance relationships, thus guiding the development of highly efficient TiO2 mesocrystal-based photocatalysts for certain applications. Finally, the prospects of future research on TiO2 mesocrystals in photocatalysis are briefly highlighted.

Effect of conduction band potential on cocatalyst-free plasmonic H2 evolution over Au loaded on Sr2+-doped CeO2

Au/CeO₂:Sr plasmonic photocatalysts produced H₂ without the aid of a cocatalyst due to the sufficiently negative potential of electrons injected into the CB of CeO₂:Sr.

Photocatalytic oxidation of urea on TiO2 in water and urine: mechanism, product distribution, and effect of surface platinization

The photocatalytic oxidation of urea on TiO₂ in water was compared with that in urine. Despite the presence of other organic compounds in urine, the oxidation efficiency of urea on TiO₂ in urine was higher than that in water. This enhanced oxidation of urea in urine is ascribed to the higher production of ^•OH (primary oxidant for urea degradation) by the adsorption of PO₄^3- (one constituent of urine) on the TiO₂ surface. Among the various anions in urine, only PO₄^3- was adsorbed on the surface of TiO₂. Both the production of ^•OH and the oxidation of urea were enhanced in the presence of PO₄^3-. These results indicate that the enhanced ^•OH production by in situ surface phosphorylation is the reason for the increased oxidation of urea in urine. Surface platinization of TiO₂ enhanced the oxidation of urea in water. However, the oxidation efficiency of urea on Pt/TiO₂ in urine was lower than that in water. This behavior is due to the adsorption of PO₄^3- and SO₄^2- in urine on Pt deposits, which inhibits the adsorption of oxygen and the interfacial electron transfer to oxygen. The product distribution (i.e., the molar ratio of NO₃^- to NH₄⁺) in water was different from that in urine because the negatively charged surface of TiO₂ in urine attracts the positively charged area of carbamic acid (intermediate) and encourages its decomposition into NH₄⁺ and not into NO₃^-.

Electrochemical tuning of heterojunctions in TiO2 nanotubes for efficient solar water splitting

The electrochemical assisted biphase ↔ triphase switching in TiO₂ nanotubes and their comprehensive photocatalytic hydrogen generation efficiencies are presented.

Hybrid materials based on conjugated polymers and inorganic semiconductors as photocatalysts: from environmental to energy applications

Hybrid materials photocatalysts based on conjugated polymers and inorganic semiconductors provide a sustainable way to remove pollutants or store energy in the form of solar fuels by processes such as water splitting, CO₂ photoreduction and NH₃ production (artificial photosynthesis).

Photocatalytic properties of a new Z-scheme system BaTiO3/In2S3 with a core–shell structure

It's highly desired to design an effective Z-scheme photocatalyst with excellent charge transfer and separation, a more negative conduction band edge (E_CB) than O₂/·O₂⁻ (−0.33 eV) and a more positive valence band edge (E_VB) than ·OH/OH⁻ (+2.27 eV).

Photo-induced charge separation in hydroxycoumarins on TiO2 and F–TiO2

Efficiency of charge separation in a dye on a semiconductor increases when the dye acts as a H-bond donor.

NIR light-activated upconversion semiconductor photocatalysts

Harvesting of near infrared (NIR) light in the abundant and environmentally friendly solar spectrum is particularly significant to enhance the utilization rate of the cleanest energy on earth. Appreciating the unique nonlinear optical properties of upconversion materials for converting low-energy incident light into high-energy radiation, they become the most promising candidates for fabricating NIR light-active photocatalytic systems by integrating with semiconductors. The present review summarizes recent NIR light-active photocatalytic systems based on a sequence of NaYF₄-based, fluoride-based, oxide-based and Ln³⁺ ion-doped semiconductor-based photocatalysts for degradation of organic molecules. In addition, we provide an in-depth analysis of various photocatalytic mechanisms and enhancement effects for efficient photo-redox performance of different upconversion semiconductor photocatalysts. We envision that this review can inspire multidisciplinary research interest in rational design and fabrication of efficient full-spectrum active (UV-visible-NIR) photocatalytic systems and their wider applications in solar energy conversion.

Recent Development of Photocatalysts Containing Carbon Species: A Review

Undoubtedly, carbon-based (nano)composites can be promising photocatalysts with improved photocatalytic activity due to the coupling effect from the incorporation of carbon species. In this mini-review, we focus on the recent development of photocatalysts based on carbon-based (nano)composites. TiO2 is well-known as a typical photocatalyst. Special attention is paid to the various types of carbon–TiO2 composites such as C-doped TiO2, N–C-doped TiO2, metal–C-doped TiO2, and other co-doped C/TiO2 composites. Various synthetic strategies including the solvothermal/hydrothermal method, sol–gel method, and template-directed method are reviewed for the preparation of carbon-based TiO2 composites. C/graphitic carbon nitride (g-C3N4) composites and ternary C-doped composites are also summarized and ascribed to the unique electronic structure of g-C3N4 and the synergistic effect of the ternary interfaces, respectively. In the end, we put forward the future perspective of the photocatalysts containing carbon species based on our knowledge.

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

Various W and Mo co-doped titanium dioxide (TiO2) materials were obtained through the EISA (Evaporation-Induced Self-Assembly) method and then tested as photocatalysts in the degradation of 4-chlorophenol. The synthesized materials were characterized by thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Raman spectroscopy (RS), N2 physisorption, UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The results showed that the W-Mo-TiO2 catalysts have a high surface area of about 191 m2/g, and the presence of an anatase crystalline phase. The co-doped materials exhibited smaller crystallite sizes than those with one dopant, since the crystallinity is inhibited by the presence of both species. In addition, tungsten and molybdenum dopants are distributed and are incorporated into the anatase structure of TiO2, due to changes in red parameters and lattice expansion. Under our experimental conditions, the co-doped TiO2 catalyst presented 46% more 4-chlorophenol degradation than Degussa P25. The incorporation of two dopant cations in titania improved its photocatalytic performance, which was attributed to a cooperative effect by decreasing the recombination of photogenerated charges, high radiation absorption capacity, high surface areas, and low crystallinity. When TiO2 is co-doped with the same amount of both cations (1 wt.%), the highest degradation and mineralization (97% and 74%, respectively) is achieved. Quinones were the main intermediates in the 4-chlorophenol oxidation by W-Mo-TiO2 and 1,2,4-benzenetriol was incompletely degraded.