Simultaneously Controllable Doping Sites and the Activity of a W–N Codoped TiO2 Photocatalyst

Synthesis and photocatalytic activity of cation-doped titanium oxynitrides (Ti2.85-xMxO4N, M = Zn, Co, Cu)

The utilization of visible light in photocatalytic semiconductors is restricted by the presence of a wide energy bandgap and fast electron-hole pair recombination. This study aims to address this limitation by synthesizing nitrogen- and cation-doped Cs0.68Ti1.83O4 at varying temperatures and subsequently analyzing the photocatalytic performance and mechanism. The optical experimental findings indicate that the co-doping of N/M (where M represents Zn, Co, or Cu) can considerably decrease the energy bandgap of Cs0.68Ti1.83O4 by regulating the energy band position and effectively suppressing the recombination of photogenerated carriers. Notably, at a temperature of 600 °C, the N/Cu co-doped Cs0.68Ti1.83O4 exhibits the smallest energy bandgap of 1.98 eV, thereby demonstrating superior photocatalytic performance. The photocatalytic degradation test of pollutants shows that the degradation efficiency of methylene blue solution in 120 minutes under light was 84%, which is the result of the interaction between ˙OH and ˙O2-. This study provides new possibilities for the study of co-doped modified photocatalytic materials.

A machine learning-assisted study of the formation of oxygen vacancies in anatase titanium dioxide

Defect engineering of semiconductor photocatalysts is critical in reducing the reaction barriers. The generation of surface oxygen vacancies allows substantial tuning of the electronic structure of anatase titanium dioxide (TiO2), but disclosing the vacancy formation at the atomic level remains complex or time-consuming. Herein, we combine density functional theory calculations with machine learning to identify the main factors affecting the formation of oxygen defects and accelerate the prediction of vacancy formation. The results show that the first two-layer oxygen atoms on the typical surfaces of TiO2, including (100), (110), and (211) facets, are more likely to be activated when the gas is more reduced, the pressure is higher, and the reduction temperature is increased. Through machine learning, we can conveniently predict the formation of oxygen defects with high accuracy. Furthermore, we present an equation with acceptable accuracy for quantitatively describing the formation of oxygen vacancies in different chemical environments. Our work provides a fast and efficient strategy for characterizing the surface structure with atomic defects.

Shaping and Doping Metal-Organic Framework-Derived TiO2 to Steer the Selectivity of Photocatalytic CO2 Reduction toward CH4

Steering selectivity in photocatalytic conversion of CO2, especially toward deep reduction products, is vital to energy and environmental goals yet remains a great challenge. In this work, we demonstrate a facet-dependent photocatalytic selective reduction of CO2 to CH4 in Cu-doped TiO2 catalysts exposed with different facets synthesized by a topological transformation from MIL-125 (Ti) precursors. The optimized round cake-like Cu/TiO2 photocatalyst mainly exposed with the (001) facet exhibited a high photocatalytic CO2 reduction performance with a CH4 yield of 40.36 μmol g-1 h-1 with a selectivity of 94.1%, which are significantly higher than those of TiO2 (001) (4.70 μmol g-1 h-1 and 52.6%, respectively), Cu/TiO2 (001 + 101) (18.95 μmol g-1 h-1 and 69.6%, respectively), and Cu/TiO2 (101) (14.73 μmol g-1 h-1 and 78.9%, respectively). The results of experimental and theoretical calculations demonstrate that the Cu doping dominating the promoted separation and migration efficiencies of photogenerated charges and the preferential adsorption on (001) facets synergistically contribute to the selective reduction of CO2 to CH4. This work highlights the significance of synergy between facet engineering and ion doping in the design of high-performance photocatalysts with respect to selective reduction of CO2 to multielectron products.

Synthesis of efficient light harvesting Cr, N Co-doped TiO2 nanoparticles for enhanced visible light photocatalytic degradation of xanthene dyes; eosin yellow and rose bengal

To solve the problem of water pollution, using environment friendly and cost effective method in short time is the need of hour. In this work, chromium (Cr) and nitrogen (N) co-doped TiO2 nanoparticles were synthesized and were used for the photocatalytic degradation of dyes under visible light. The synergistic effect of metal and non-metal co-dopants added would result in appropriate reduction of band gap {from 3.2 eV of TiO2 to 2.67 eV}, decrease in recombination rate of charge carriers by trapping electrons and holes, and in better light harvesting capacity. Nanoparticles were synthesized by sol-gel method and characterized using ultraviolet-visible (UV-VIS) spectroscopy, fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), zeta potential, X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, field emission scanning electron microscopy (FE-SEM), and RAMAN spectroscopy. Eosin yellow (EY) and rose bengal (RB) were subjected to photocatalytic degradation under solar light to check the photocatalytic activity of the synthesized nanoparticles. Effects of dye concentration, the concentration of nanoparticles, time, and pH were investigated to optimize the parameters. The results obtained were remarkable for 20 ppm EY solution took 10 min using 1 gL-1 NPs at pH 3 and 10 ppm RB solution took 5 min using 0.75 gL-1 NPs at pH 5.78 (original pH) for complete degradation. Kinetics studies were also performed and both dyes followed pseudo-second-order kinetics with R2 values 0.99312 and 0.99712 for EY and RB, respectively. The study of degraded products was conducted using high-performance liquid chromatography (HPLC) hyphenated with electron spray ionization mass spectroscopy (ESI-MS) (LC-MS) and possible degradation pathways were made for both dyes. A reusability test was also performed showing the efficiency of the particles was up to 88% after 3 cycles of use. These notable results can be attributed to the efficient removal of organic pollutants using the proposed dopants in this study.

Chelated Ion-Exchange Strategy toward BiOCl Mesoporous Single-Crystalline Nanosheets for Boosting Photocatalytic Selective Aromatic Alcohols Oxidation

The photoresponse and photocatalytic efficiency of bismuth oxychloride (BiOCl) are greatly limited by rapid recombination of photogenerated carriers. The construction of porous single-crystal BiOCl photocatalyst can effectively alleviate this issue and provide accessible active sites. Herein, a facile chelated ion-exchange strategy is developed to synthesize BiOCl mesoporous single-crystalline nanosheets (BiOCl MSCN) using acetic acid and ammonia solution respectively as chelating agent and ionization promoter. The strong chelation between acetate ions and Bi³⁺ ions introduces acetate ions into the precipitated product to exchange with Cl^- ions, resulting in large lattice mismatch, strain release, and formation of void-like mesopores. The prepared BiOCl MSCN photocatalyst exhibits excellent catalytic performance with 99% conversion and 98% selectivity for oxidation of benzyl alcohol to benzaldehyde and superior general adaptability for various aromatic alcohols. The theoretical calculations and characterizations confirm that the superior performance is mainly attributed to the abundant oxygen vacancies, plenty of accessible adsorption/active sites and fast charge transport path without grain boundaries.

Nitrogen-Doped TiO2/Nitrogen-Containing Biochar Composite Catalyst as a Photocatalytic Material for the Decontamination of Aqueous Organic Pollutants

In this study, a waste walnut shell-derived biochar enriched with nitrogen (N-biochar) is mixed with nitrogen-doped TiO₂ (N-TiO₂) to fulfill an affordable composite material for the degradation of methyl orange (MO). Results showed that porous structure and oxygen-containing functional groups of biochar facilitate contact with MO during the reaction process. Meanwhile, doped nitrogen has a positive effect on improving the reaction activity due to the existence of a substituted state and a gap state in the catalyst. It was revealed that the N-TiO₂/N-biochar (NCNT0.2/1) exhibited better photocatalytic degradation efficiency (97.6%) and mineralization rate (85.4%) of MO than that of TiO₂, N-TiO₂, and TiO₂/N-biochar due to its stronger synergistic effect of N, TiO₂, and biochar, in accordance with its high charge separation by photoluminescence (PL) analysis. Trapping experiments showed that ·OH is the predominant active species during the decolorization and mineralization process of MO. After five repeated use, the loss of activity of the catalyst was negligible. In addition, the catalytic degradation process was consistent with the pseudo-first-order kinetic model with the rate constant of 4.02 × 10^-2 min^-1.

Defect Engineering of Photocatalysts towards Elevated CO2 Reduction Performance

Photocatalytic CO₂ reduction provides a promising solution to address the crises of massive CO₂ emissions and fossil energy shortages. As one of the most effective strategies to promote CO₂ photoconversion, defect engineering shows great potential in modulating the electronic structure and light absorption properties of photocatalysts while increasing surface active sites for CO₂ activation and conversion. This Review summarizes the recent progress in defect engineering of photocatalysts to promote CO₂ reduction performances from the following four aspects: 1) Approaches to defect (mainly vacancy and dopant) generation in photocatalysts; 2) defect structure characterization techniques; 3) physical and chemical properties of defect-engineered photocatalysts; 4) CO₂ reduction performance enhancements in activity, selectivity, and stability of photocatalysts by defect engineering. This Review is expected to present readers with a comprehensive view of progress in the field of photocatalytic CO₂ reduction through defect engineering for elevated CO₂ -to-fuels conversion efficiency.

Hybrid Density Functional Theory Study of Native Defects and Nonmetal (C, N, S, and P) Doping in a Bi2WO6 Photocatalyst

Native defects and nonmetal doping have been shown to be an effective way to optimize the photocatalytic properties of Bi2WO6. However, a detailed understanding of defect physics in Bi2WO6 has been lacking. Here, using the Heyd-Scuseria-Ernzerhof hybrid functional defect calculations, we study the formation energies, electronic structures, and optical properties of native defects and nonmetal element (C, N, S, and P) doping into Bi2WO6. We find that the Bi vacancy (Bivac), O vacancy (Ovac), S doping on the O site (SO), and N doping on the O site (NO) defects in the Bi2WO6 can be stable depending on the Fermi level and chemical potentials. By contrast, the substitution of an O atom by a C or P atom (CO, PO) has high formation energy and is unlikely to form. The calculated electronic structures of the Bivac, Ovac, SO, and NO defects indicate that the band-gap reduction of Ovac 2+, Bivac 3-, and SO defects is mainly due to forming shallow impurity levels within the band gap. The calculated absorption coefficients of Ovac 2+, Bivac 3-, and SO show strong absorption in the visible light region, which is in good agreement with the experimental results. Hence, Ovac 2+, Bivac 3-, and SO defects can improve the adsorption capacity of Bi2WO6, which helps enhance its photocatalytic performance. Our results provide insights into how to enhance the photocatalytic activity of Bi2WO6 for energy and environmental applications through the rational design of defect-controlled synthesis conditions.

Removal of Pharmaceutical Contaminants in Wastewater Using Nanomaterials: A Comprehensive Review

BACKGROUND

The limitless presence of pharmaceutical contaminants in discharged wastewater has emerged as a threat to aquatic species and humans. Their presence in drinking water has although raised substantial concerns, very little is known about the fate and ecological impacts of these pollutants. As a result, these pollutants are inevitably introduced to our food chain at trace concentrations. Unfortunately, the conventional wastewater treatment techniques are unable to treat pharmaceuticals completely with practical limitations. The focus has now been shifted towards nanotechnology for the successful remediation of these persistent pollutants. Thus, the current review specifically focuses on providing readers brief yet sharp insights into applications of various nanomaterials for the removal of pharmaceutical contaminants.

METHODS

An exhaustive collection of bibliographic database was done with articles having high impact and citations in relevant research domains. An in-depth analysis of screened papers was done through standard tools. Studies were categorized according to the use of nanoscale materials as nano-adsorbents (graphene, carbon nanotubes), nanophotocatalysts (metal, metal oxide), nano-filtration, and ozonation for promising alternative technologies for the efficient removal of recalcitrant contaminants.

RESULTS

A total of 365 research articles were selected. The contemporary advancements in the field of nanomaterials for drinking and wastewater treatment have been thoroughly analyzed along with their future perspectives.

CONCLUSION

The recommendations provided in this article will be useful to adopt novel strategies for on-site removal of the emerging contaminants in pharmaceutical effluents and related industries.

Synchronously achieved surface Bi0 metallisation and incorporation of Bi3+/5+ ions into a W6+, N3− doped TiO2 lattice by the hydrothermal-reduction method: surface plasmonic resonance effect for efficient photocatalysis

Vectorial charge transfer mechanisms in a bicrystalline framework of Bi⁰ surface deposited Bi^3+/5+, W⁶⁺ and N³⁻ doped TiO₂ under solar light irradiation.

Polypyrrole/cadmium sulfide hollow fiber with high performance contaminant removal and photocatalytic activity fabricated by layer-by-layer deposition and fiber-sacrifice template approach

A recyclable polypyrrole (PPy)/cadmium sulfide (CdS) hollow fiber photocatalyst was innovatively fabricated for solving the loss issue of the current powder-form photocatalyst in slurry system. Core-sheath structure CdS/polyacrylonitrile (PAN) fiber was prepared via successive ionic layer adsorption and reaction (SILAR) method on the surface of PAN fiber. PPy was further deposited on the CdS/PAN fiber by vapor deposition polymerization. After the removal of interior PAN template, PPy/CdS hollow fiber was yielded. The hollow structure of PPy/CdS hollow fiber was confirmed by morphology observation. The resulting PPy/CdS hollow fiber presents low energy band gaps of 1.9 eV, which accounts for enhanced visible light photocatalytic activity after PPy deposition. PPy/CdS hollow fiber shows good dye removal efficiency of 73.06 wt% (dosage of the product as low as 5 mg·10 mL^-1), and praiseworthy H₂ production rate up to 269.7 μmol·g^-1·h^-1. PPy/CdS hollow fiber maintained high and sustainable photocatalytic activity compared to CdS/PAN fiber after 8 cycles, indicating that PPy effectively improved the stability of CdS. Here, PPy plays key synergistic role in photocatalysis of PPy/CdS hollow fiber for the promotive and protective effects based on the actual photocatalytic performance and inductively coupled plasma optical emission spectrometer (ICP-OES) results. Compared with nano-sized photocatalysts, the fiber-formed PPy/CdS hollow fiber is highly bulky and easy to recycle. PPy/CdS hollow fiber has great potential for scale-up in industrial application because of its excellent grabbing ability and degradation to contaminants, and ease of disposal.

A Review of Recent Applications of Ion Beam Techniques on Nanomaterial Surface Modification: Design of Nanostructures and Energy Harvesting

Nanomaterials have gained plenty of research interest because of their excellent performance, which is derived from their small size and special structure. In practical applications, to acquire nanomaterials with high performance, many methods have been used to modulate the structure and components of materials. To date, ion beam techniques have extensively been applied for modulating the performance of various nanomaterials. Energetic ion beams can modulate the surface morphology and chemical components of nanomaterials. In addition, ion beam techniques have also been used to fabricate nanomaterials, including 2D materials, nanoparticles, and nanowires. Compared with conventional methods, ion beam techniques, including ion implantation, ion irradiation, and focused ion beam, are all pure physical processes; these processes do not introduce any impurities into the target materials. In addition, ion beam techniques exhibit high controllability and repeatability. Here, recent progress in ion beam techniques for nanomaterial surface modification is systematically summarized and existing challenges and potential solutions are presented.

Dispelling the Myth of Passivated Codoping in TiO2

Modification of TiO₂ to increase its visible light activity and promote higher performance photocatalytic ability has become a key research goal for materials scientists in the past 2 decades. One of the most popular approaches proposed this as "passivated codoping", whereby an equal number of donor and acceptor dopants are introduced into the lattice, producing a charge neutral system with a reduced band gap. Using the archetypal codoping pairs of [Nb + N]- and [Ta + N]-doped anatase, we demonstrate using hybrid density functional theory that passivated codoping is not achievable in TiO₂. Our results indicate that the natural defect chemistry of the host system (in this case n-type anatase TiO₂) is dominant, and so concentration parity of dopant types is not achievable under any thermodynamic growth conditions. The implications of passivated codoping for band gap manipulation in general are discussed.

Hydrogen-free defects in hydrogenated black TiO2

Black anatase TiO2 has surprisingly enhanced solar energy harvesting efficiency and electrical conductivity, which makes it a promising material in a wide range of energy and environmental applications. Several experimental and theoretical studies have successfully revealed the mechanisms of band gap reduction by surface hydrogenation of anatase TiO2. However, recent experimental evidence suggests the existence of bulk point defects that yield infrared (∼1.0 eV) photoabsorption and high conductivity of black anatase TiO2. In the current study, using a combination of ab initio molecular dynamics simulations and electronic structure calculations, we successfully explain the physical properties, metallicity, and infrared/microwave absorption (i.e., black color) of highly reduced anatase TiO2 crystal in a hydrogenated state with a newly found pair defect (Tii-VO)4+. Hydrogen atoms in the bulk are unnecessary to understand the observed properties.

Highly visible-light-responsive Cu2O/rGO decorated with Fe3O4@SiO2 nanoparticles as a magnetically recyclable photocatalyst

The rhombic dodecahedral cuprous oxide-reduced graphene oxide/core-shell Fe₃O₄@SiO₂ composites (denoted as rCu₂O-rGO/Fe₃O₄@SiO₂) are successfully synthesized facilely via a wet-chemical route. The resulting rCu₂O-rGO/Fe₃O₄@SiO₂ combines the unique structure of Cu₂O, electronic characteristics of reduced graphene oxide (rGO) and magnetic property of Fe₃O₄@SiO₂ to be an effective and recoverable photocatalyst for the degradation of methyl orange (MO). The obtained results show that rCu₂O-rGO/Fe₃O₄@SiO₂ is capable of completely degrading MO in the presence of a very low catalyst concentration (0.125 g l^-1) within a short time (60 min) under visible light compared to the reported catalysts. The observations may be due to the distinctive interfacial structures of rhombic dodecahedral Cu₂O nanoparticles connected to rGO sheets that can enhance the separation of photogenerated electron-hole pairs, stabilize the Cu₂O and increase MO adsorption, as evidenced by a variety of spectroscopic analyses (transmission electron microscopy, x-ray photoelectron spectroscopy and photoluminescence). More importantly, these efficient photocatalysts can easily be recovered under a magnetic field and remain highly photoactive towards the degradation of MO after cyclic tests, and may be promising photocatalysts for practical applications in the solar-energy purification of wastewater.

Facet-Dependent Cuprous Oxide Nanocrystals Decorated with Graphene as Durable Photocatalysts under Visible Light

Three morphologies (octahedral, hierarchical and rhombic dodecahedral) of crystal Cu₂O with different facets ({111}, {111}/{110}, and {110}) incorporating graphene sheets (denoted as o-Cu₂O-G, h-Cu₂O-G and r-Cu₂O-G, respectively) have been fabricated by using simple solution-phase techniques. Among these photocatalysts, the r-Cu₂O-G possesses the best photocatalytic performance of 98% removal efficiency of methyl orange (MO) with outstanding kinetics for 120 min of visible light irradiation. This enhancement is mainly due to the dangling “Cu” atoms in the highly active {110} facets, resulting in the increased adsorption of negatively charged MO. More importantly, the unique interfacial structures of Cu₂O rhombic dodecahedra connected to graphene nanosheets can not only decrease the recombination of electron-hole pairs but also stabilize the crystal structure of Cu₂O, as verified by a series of spectroscopic analyses (e.g., X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM)). The effective photocatalysts developed in this work could be applied to the efficient decolorization of negatively charged organic dyes by employing solar energy.

Conductive Nb-doped TiO2 thin films with whole visible absorption to degrade pollutants

Niobium-doping makes both intrinsic UV absorption and UV-vis-IR free-carrier absorption occur in TiO₂ and improves the photocatalytic performance.

Ultrathin Tungsten Oxide Nanowires/Reduced Graphene Oxide Composites for Toluene Sensing

Graphene-based composites have gained great attention in the field of gas sensor fabrication due to their higher surface area with additional functional groups. Decorating one-dimensional (1D) semiconductor nanomaterials on graphene also show potential benefits in gas sensing applications. Here we demonstrate the one-pot and low cost synthesis of W₁₈O₄₉ NWs/rGO composites with different amount of reduced graphene oxide (rGO) which show excellent gas-sensing properties towards toluene and strong dependence on their chemical composition. As compared to pure W₁₈O₄₉ NWs, an improved gas sensing response (2.8 times higher) was achieved in case of W₁₈O₄₉ NWs composite with 0.5 wt. % rGO. Promisingly, this strategy can be extended to prepare other nanowire based composites with excellent gas-sensing performance.

Photocatalytic activities and photoinduced fusion of gold-modified titania nanoparticle

Gold nanoparticles measuring 3–30 nm deposited on semiconductors result in an effective photocatalyst against several pollutants. Its photocatalytic activities are significant under both UV and solar irradiation. In a photocatalytic system, the oxidation of pollutants takes place on the gold surface as the electron donor, while the electron is consumed by the reduction of oxygen as the electron acceptor on the semiconductor’s surface. This promotes not only increased photocatalytic activities but also the green transformation of pollutant compounds to harmless compounds. The photosensitivity of semiconductors can be modified by tuning the size, shape, and contact of gold nanoparticles. This review highlights the function of gold nanoparticles in overcoming the limitation of transition metal oxide materials in photocatalytic applications.

The Formation of Defect-Pairs for Highly Efficient Visible-Light Catalysts

Highly efficient visible-light catalysts are achieved through forming defect-pairs in TiO₂ nanocrystals. This study therefore proposes that fine-tuning the chemical scheme consisting of charge-compensated defect-pairs in balanced concentrations is a key missing step for realizing outstanding photocatalytic performance. This research benefits photocatalytic applications and also provides new insight into the significance of defect chemistry for functionalizing materials.

New insights into high temperature hydrothermal synthesis in the preparation of visible-light active, ordered mesoporous SiO2–TiO2 composited photocatalysts

Carbon doped, visible light active and ordered mesoporous TiO₂–SiO₂ nanocomposites have been successfully synthesized via one step high temperature (180 °C) hydrothermal technology.

Few-layered metallic 1T-MoS2/TiO2 with exposed (001) facets: two-dimensional nanocomposites for enhanced photocatalytic activities

Titanium dioxide (TiO₂) with exposed (001) facets (TiO₂(001)) has attractive photocatalytic properties.