Do HOMO-LUMO Energy Levels and Band Gaps Provide Sufficient Understanding of Dye-Sensitizer Activity Trends for Water Purification?

A dye-sensitized solar cell assembly can be used to harvest solar energy, while suitable dye sensitizers can be used to purify water. Here, we characterized the activity trends of four dye sensitizers, namely, PORPC-1, PORPC-2, PORPC-3, and PORPC-4, for water purification applications using density functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE), B3LYP, and PBE0 functionals, ΔSCF, time-dependent DFT (TD-DFT), and quasiparticle Green's function (GW) methods. The energy levels of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) were calculated using gas-phase and aqueous-phase methods in order to understand charge-injection abilities and the dye regeneration processes. PBE, B3LYP, PBE0, and TD-DFT methods failed to predict PORPC-4 to be the best sensitizer, while PORPC-2 and PORPC-4 were predicted to be the best sensitizers using ΔSCF coupled with the implicit solvation method, and HOMO-LUMO energies were corrected for the aqueous environment in the GW calculations. However, none of these methods accurately predicted the performance trend of all four dye sensitizers. Consequently, we used the aggregation assembly patterns of the dye molecules in an aqueous environment to further probe the activity trends and found that PORPC-3 and PORPC-4 prefer J-aggregated assembly patterns, whereas PROPC-1 and PORPC-2 prefer to be H-aggregated. Therefore, the performance of these dye molecules can be determined by combining HOMO-LUMO energy levels with aggregate-assembly patterns, with the activity trend predicted to be PORPC-4 > PORPC-2 > PORPC-3 > PORPC-1, which is in good agreement with experimental findings.

Recent progress on the enhancement of photocatalytic properties of BiPO4 using π-conjugated materials

Semiconductor photocatalysis is regarded as most privileged solution for energy conversion and environmental application. Recently, photocatalysis methods using bismuth-based photocatalysts, such as BiPO₄, have been extensively investigated owing to their superior efficacy regarding organic pollutant degradation and their further mineralization into CO₂ and H₂O. It is well known that BiPO₄ monoclinic phase exhibited better photocatalytic performance compared to Degussa (Evonik) P25 TiO₂ in term of ultraviolet light driven organic pollutants degradation. However, its wide band gap, poor adsorptive performance and large size make BiPO₄ less active under visible light irradiation. However, extensive research works have been conducted in the past with the aim of improving visible light driven BiPO₄ activity by constructing a series of heterostructures, mainly coupled with π-conjugated architecture (e.g., conductive polymer, dye sensitization and carbonaceous materials). However, a critical review of modified BiPO₄ systems using π-conjugated materials has not been published to date. Therefore, this current review article was designed with the aim of presenting a brief current state-of-the-art towards synthesis methods of BiPO₄ in the first section, with an especial focuses onto its crystal-microstructure, optical and photocatalytic properties. Moreover, the most relevant strategies that have been employed to improve its photocatalytic activities are then addressed as the main part of this review. Finally, the last section presents ongoing challenges and perspectives for modified BiPO₄ systems using π-conjugated materials.

Self-Healing and Superwettable Nanofibrous Membranes with Excellent Stability toward Multifunctional Applications in Water Purification

Considering the complexity of toxic ingredients in practical polluted water, the development of energy- and labor-saving and environmentally friendly multifunctional materials to decontaminate wastewater is of great necessity. Herein, a multifunctional branched poly(ethylenimine) (bPEI) and poly(acrylic acid) (PAA)/tungsten oxide/polyacrylonitrile (PP/WO₃/PAN) composite membrane was fabricated by the combination of blow spinning and layer-by-layer methods. The incorporated WO₃ in generated in hydrophilic PAN fibers by spinning the precursor method, which simultaneously reveals remarkable photodegradation performance towards mimetic organic pollutions and excellent antibacterial activity due to their electron synergetic effect. In addition, the micro/nanoporous structure of the PP/WO₃/PAN composite membrane also ensures its good oil-water separation performance. Moreover, the reduction reaction of W atoms in the WO₃ network upon solar irradiation endows the membrane with superior heavy metal ion removal capability. Significantly, the membrane exhibits water-enabled self-healing performance due to the coated polyelectrolyte layer. More importantly, the membrane could be easily scaled-up; was free-standing, durable, and biocompatible; and exhibited no additional toxic effect on the surrounding environments. These outstanding properties make the membrane to have significant potential applications in wastewater treatment.

Photocatalytic Degradation of Organic Micropollutants in Water by Zr-MOF/GO Composites

Nanocomposites of UiO-66 and graphene oxide (UiO-66_GO) were prepared with different GO contents by a one-step hydrothermal method, and their photocatalytic activities for the degradation of carbamazepine (CBZ) were investigated under ranges of GO loading, catalyst dose, initial pollutant concentration, and solution pH. The UiO-66_GO nanocomposites showed photocatalytic rate constant up to 0.0136 min−1 for CBZ degradation and its high overall removal efficiency (>90%) in 2 h. The photocatalytic rate constant over the UiO-66_GO nanocomposite was about 2.8 and 1.7 times higher than those over pristine GO and UiO-66, respectively. The enhancement of photocatalytic activity by GO was attributed to increased surface area and porosity, improved light absorption, and narrowed band gap. The composite also showed substantial recyclability and stability over five consecutive cycles of photocatalytic degradation. The experimental results indicated that O2●− and OH● are the responsible radicals for photocatalytic degradation, which helped us propose a photocatalytic mechanism for the enhanced CBZ photodegradation. This work provides a reference for the development of GO-based composite photocatalysts and expands the application of UiO-66 as a photocatalyst for the degradation of persistent micropollutants in water.

Electrostatic Assembly of Porphyrin-Functionalized Porous Membrane toward Biomimetic Photocatalytic Degradation Dyes

Porphyrin-based catalytic oxidation is one of the most representative biomimetic catalysis. To mimic the biomimetic catalytic oxidation of nature, a positive charged porous membrane, quaternized polysulfone (QPSf) membrane with spongelike structure, was prepared for supporting meso-tetraphenylsulfonato porphyrin (TPPS). The influence of polymer concentration, coagulation bath, and additives on the structure of the substrate membrane was explored, and the optimized membrane with porosity of 87.1% and water flux of 371 L·m^-2·h^-1 at 0.1 MPa was obtained. Monolayer TPPS was adsorbed on the QPSf membrane surface by the electrostatic self-assembly approach, and the adsorption process followed the pseudo second-order kinetic model and Langmuir adsorption isotherm equation. The resulting TPPS@QPSf membrane showed excellent visible light response, and the photocatalytic performance for dyes was then enhanced dramatically after TPPS was immobilized on the membrane. The removal efficiencies for rhodamine B (RhB), methylene blue (MB), and methyl orange (MO) were 92.1, 94.1, and 92.1% under visible light irradiation, respectively. The primary photocatalytic degradation of the dye was a zero-order reaction, and the secondary reaction of degradation followed pseudo first-order kinetics. Finally, the TPPS@QPSf membrane can be reused for photocatalytic degradation of RhB for 10 cycles with no obvious change on removal efficiency, which indicated that this membrane is a promising material for dyeing water treatment coupled with visible light irradiation.

Enhanced photocatalytic degradation of methyl orange dye on interaction with synthesized ligand free CdS nanocrystals under visible light illumination

Methyl orange is widely used dye in textile industry and later, it becomes the major component of industrial waste and pollutes different water body. So, its degradation is required especially in the case of drinking water. Here, chemically synthesized ligand free CdS nanocrystals have been utilized as a photocatalyst to degrade this dye in water. The mean size of the CdS nanocrystal has been found to be 7 nm approximately from TEM analysis, and SAED pattern shows that these are crystalline in nature. Further, the XRD patterns confirms the cubic structure of CdS nanoparticles and mean size calculated from XRD analysis matches well with the obtained size from TEM study. NMR study clearly verifies that CdS nanocrystals are ligand free stable nanoparticles. Due to this absence of ligand on the surface of CdS nanocrystals, a much enhanced MO degradation has been observed, as the e^--h⁺ pair in the CdS and subsequent generation of free radicals such as hydroxyl, can efficiently oxidize the organic material MO and therefore, degrade this pollutant faster under visible light illumination. The degradation efficiency is found to be 95% after 300 min of illumination, which is much better than the other similar reports.

Rapid decontamination of tetracycline hydrolysis product using electrochemical CNT filter: Mechanism, impacting factors and pathways

In this study, electrooxidation of the tetracycline hydrolysis products was investigated using a carbon nanotube (CNT) electrochemical filter and 4-epianhydrochlortetracycline (EACTC) as a model compound. Electrochemical filtration of 10 μmol L^-1 EACTC at a voltage of 2.5 V and a flow rate of 1.5 mL min^-1 (hydraulic residence time <3 s) provided an oxidation flux of 1251 ± 28 μmol h^-1 m^-2. Replacement of the Ti cathode with a CNT filter cathode increased the EACTC oxidative flux by 1.3 fold at a voltage of 2.5 V. The electrochemical filtration process is effective for the degradation of EACTC and the reduction of the antimicrobial activity based on liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) analysis and luminescent bacteria test. The high oxidation flux within 300 min (1212-1263 μmol h^-1 m^-2) and affordable cost (0.25 kWh m^-3) at a voltage of 2.5 V show the potential application of the electrochemical filtration system as a promising unit for EACTC degradation. These findings provided new insights into the rational design principles of novel continuous-flow filtration system aimed to efficiently remove hydrolysis products of the antibiotic tetracycline.

Advances in photocatalytic self-cleaning, superhydrophobic and electromagnetic interference shielding textile treatments

The use of nanomaterials in textiles provides many new opportunities and advantages for users and manufacturers; however, it comes with some of its downsides and challenges which need to be understood and overcome for enhancing the applicability of these products. This review article discusses the recent progress in developing self-cleaning and conductive textiles as two of the leading research fields of smart textiles. In particular, different aspects of fabricating nanocoatings for photocatalytic self-cleaning, superhydrophobic and electromagnetic interference (EMI) shielding effect will be brought to light. The theoretical concepts, mechanisms, latest fabrication methods along with their potential applications will be discussed. Moreover, the current drawbacks of these fields will be underlined and some recommendations for future research trajectories in terms of performance, current limitations, sustainability and safety will be proposed. This review article provides a comprehensive review on the state-of-the-art achievements in the field, which will be a valuable reference for researchers and decision makers.

Fabrication and characterization of ZnTiO3/Zn2Ti3O8/ZnO ternary photocatalyst for synergetic removal of aqueous organic pollutants and Cr(VI) ions

Highly efficient photocatalysts have great development prospects in wastewater treatment, especially in the degradation of organic pollutants and reduction of inorganic heavy metal ions. Herein, a Z-scheme ZnTiO₃/Zn₂Ti₃O₈/ZnO ternary photocatalyst was prepared by the solvothermal-calcination method and the influence of the content of tetrabutyl titanate precursor and different reaction temperature on the crystal phase structures, photoelectrochemical properties and photocatalytic activities of the samples were investigated. Due to its unique Z-scheme structure and suitable band gap position, which is favorable for the efficient migration and separation of photo-generated electrons and holes and the improvement of photocatalytic redox reaction capability, the samples show excellent performance for the degradation of organic pollutants and reduction of heavy metal Cr(_VI) ions. Based on a series of characterization analyses, a possible Z-scheme photocatalytic mechanism is proposed. This work provides a simple preparation method for fabrication of multivariate heterojunction photocatalyst for degradation of organic pollutants and removal of heavy metal ions.

C-nanocoated ZnO by TEMPO-oxidized cellulose templating for improved photocatalytic performance

Ultrafine C-doped ZnO/carbon nanocomposites with different photocatalytic activities have been prepared using TEMPO-oxidized cellulose as a template but also as the source of carbon. The result is an enhancement of the photocatalytic activity ascribed to different phenomena: a high mesoporosity beneficial to mass transport, a thin carbon layer onto ZnO increasing the charge transfer and hydrophobicity of ZnO, a narrowing of ZnO band gap and an increase of the zinc (V_Zn) and oxygen (V_o) vacancies effectively suppressing of the charge recombination. These are evidenced by photocatalytic test of photodegradation of methyl orange (MO) achieved to assess and compared the different photocatalysts. The highest rate constant value of photodegradation of MO is 0.0254 min^-1, three times higher than that of ZnO prepared without templates (0.0087 min^-1). The present results introduce a new vision of the use of template with multiple roles in the preparation of inorganic materials and specially photocatalysts.

Characterization and Comparative Performance of TiO2 Photocatalysts on 6-Mercaptopurine Degradation by Solar Heterogeneous Photocatalysis

The crystallographic properties of two titanium dioxide (TiO2) photocatalysts, P25, and commercial C1-TiO2 reactive grade, were analyzed by X-ray diffraction (XRD) and the band-gap was calculated with UV–Vis spectrometry with integration sphere. Then, their performance was tested in the degradation of 6-mercaptopurine (6-MP) by heterogeneous photocatalysis with solar radiation under different pH conditions and the addition of hydrogen peroxide (H2O2); the degradation efficiency was monitored by UV–Vis spectrophotometry. The XRD analysis showed that both photocatalysts studied have anatase phase, while only P25 contains rutile; the band gap values were lower, in both catalysts, than those reported for catalysts obtained by the sol-gel method. With both photocatalysts, degradation experiments showed efficiency greater than 98% in experiments in the presence of H2O2 regardless of pH. The properties of the photocatalysts, along with the data obtained from the experimentation, helped determine the best semiconductor for the degradation of 6-MP with these operating conditions in this work.

Photosensitive Hybrid Nanostructured Materials: The Big Challenges for Sunlight Capture

Solar radiation is becoming increasingly appreciated because of its influence on living matter and the feasibility of its application for a variety of purposes. It is an available and everlasting natural source of energy, rapidly gaining ground as a supplement and alternative to the nonrenewable energy feedstock. Actually, an increasing interest is involved in the development of efficient materials as the core of photocatalytic and photothermal processes, allowing solar energy harvesting and conversion for many technological applications, including hydrogen production, CO2 reduction, pollutants degradation, as well as organic syntheses. Particularly, photosensitive nanostructured hybrid materials synthesized coupling inorganic semiconductors with organic compounds, and polymers or carbon-based materials are attracting ever-growing research attention since their peculiar properties overcome several limitations of photocatalytic semiconductors through different approaches, including dye or charge transfer complex sensitization and heterostructures formation. The aim of this review was to describe the most promising recent advances in the field of hybrid nanostructured materials for sunlight capture and solar energy exploitation by photocatalytic processes. Beside diverse materials based on metal oxide semiconductors, emerging photoactive systems, such as metal-organic frameworks (MOFs) and hybrid perovskites, were discussed. Finally, future research opportunities and challenges associated with the design and development of highly efficient and cost-effective photosensitive nanomaterials for technological claims were outlined.

Facile synthesis of flower-like hierarchical N-doped Nb2O5/C nanostructures with efficient photocatalytic activity under visible light

Significant endeavors have been devoted in the past few years to establish efficient visible light-activated photocatalysts. Herein, we successfully synthesized a flower-like hierarchical nitrogen-doped and carbon-sensitized Nb₂O₅ (NBO) nanostructure (denoted N-NBO/C). The as-prepared N-NBO/C possessed a specific surface area of 260.37 m² g⁻¹ and single wire diameter of less than 10 nm. The effect of reaction parameters such as hydrothermal reaction time, temperature and concentration of hexamethylenetetramine (Hmta) on the morphology of NBO was systematically investigated to elucidate the growth mechanism. The carbon on the surface and the nitrogen in the framework of NBO are beneficial for light harvesting, visible light absorption, formation of oxygen vacancies, and electron–hole separation. The photocatalytic performance of the as-fabricated N-NBO/C nanostructures was estimated via the photodegradation of 30 mg L⁻¹ RhB, where greater than 98% of RhB was decomposed within 30 min upon visible-light radiation. Hence, the obtained N-NBO/C nanostructure exhibits much higher photocatalytic activity for the decomposition of RhB upon visible light irradiation than that of pure niobium oxide (NBO), nitrogen-doped titanium oxide (N-TIO), and nitrogen-doped niobium oxide (N-NBO). This work supplies a versatile route for the synthesis of nitrogen-doped and carbon-sensitized metal-oxide nanostructures for possible utilization in solar energy transformation and environmental remediation.

Flower-like hierarchical N-doped Nb₂O₅/C nanostructures with efficient visible light-driven photocatalytic activity synthesized via a facile hexamethylenetetramine-assisted hydrothermal route.

Improved photoremoval performance of boron carbon nitride–pyromellitic dianhydride composite toward tetracycline and Cr(vi) by itself to change the solution pH

A series of boron carbon nitride–pyromellitic dianhydride (BCNPA) composites were successfully synthesized for the first time, where BCNPA3 exhibited the best adsorption and photodegradation performances for tetracycline (TC) under visible-light irradiation.

Green polymeric nanomaterials for the photocatalytic degradation of dyes: a review

Pure and drinkable water will be rarer and more expensive as the result of pollution induced by industrialisation, urbanisation and population growth. Among the numerous sources of water pollution, the textile industry has become a major issue because effluents containing dyes are often released in natural water bodies. For instance, about two years are needed to biodegrade dye-derived, carcinogenic aromatic amines, in sediments. Classical remediation methods based upon physicochemical reactions are costly and still generate sludges that contain amine residues. Nonetheless, recent research shows that nanomaterials containing biopolymers are promising to degrade organic pollutants by photocatalysis. Here, we review the synthesis and applications of biopolymeric nanomaterials for photocatalytic degradation of azo dyes. We focus on conducting biopolymers incorporating metal, metal oxide, metal/metal oxide and metal sulphide for improved biodegradation. Biopolymers can be obtained from microorganisms, plants and animals. Unlike fossil-fuel-derived polymers, biopolymers are carbon neutral and thus sustainable in the context of global warming. Biopolymers are often biodegradable and biocompatible.

Interaction-determined sensitization photodegradation of dye complexes by boron nitride under visible light irradiation: experimental and theoretical studies

In this paper, the sensitization photodegradation of single and mixed dyes by wide band gap boron nitride (BN, 3.94 eV) under visible light irradiation has been investigated for the first time.

Coupled-substituted double-layer Aurivillius niobates: structures, magnetism and solar photocatalysis

Heterovalent coupled-substituted double-layer Aurivillius niobates, LaBi₂Nb_1.5M_0.5O₉ (M = Cr, Mn, Fe, Co), show interesting structural and magnetic characteristics in addition to sunlight-driven photocatalytic activity.

Dynamic growth of rhombic dodecahedral Cu2O crystals controlled by reaction temperature and their size-dependent photocatalytic performance

Compared with low-index {100} or {111} planes of Cu₂O crystals, rhombic dodecahedra (RD) Cu₂O crystals exposing 12 {110} facets exhibit the most superior photodegradation of organic pollutants. Herein, a series of RD Cu₂O crystals with different sizes were successfully synthesized by precisely adjusting the reaction temperature ranging from 40 °C to 100 °C. The results revealed that truncated rhombic dodecahedra (TRD) Cu₂O crystals were fabricated when the temperatures was 40 °C. More importantly, on raising the temperature to above 40 °C, Cu₂O architectures dynamically evolved from TRD to RD. Meanwhile, the sizes gradually decreased with elevation of the temperature, while the RD morphology of Cu₂O crystals remained, demonstrating the importance of temperature for determining the morphology and size of Cu₂O crystals. In addition, we also carefully investigated the visible-light photodegradation performance of Cu₂O crystals for methyl orange (MO). RD Cu₂O crystals exhibited superior photocatalytic activity compared with TRD, and showed size-dependent photocatalytic activity for MO. The photocatalytic activity of RD Cu₂O crystals can be greatly improved by decreasing the size. In particular, RD-60 with the minimum size achieved the best photocatalytic properties compared to the other RD and TRD Cu₂O crystals, and still displayed high photocatalytic efficiency even after three cycles. Such results advance the understanding that temperature modulation serves as an effective means to fabricate RD Cu₂O crystals.

Compared with low-index {100} or {111} planes of Cu₂O crystals, rhombic dodecahedra (RD) Cu₂O crystals exposing 12 {110} facets exhibit the most superior photodegradation of organic pollutants.

Photocatalytic Degradation of Pharmaceuticals Carbamazepine, Diclofenac, and Sulfamethoxazole by Semiconductor and Carbon Materials: A Review

The presence of pharmaceutical compounds in the environment is a reality that calls for more efficient water treatment technologies. Photocatalysis is a powerful technology available but the high energy costs associated with the use of UV irradiation hinder its large scale implementation. More sustainable and cheaper photocatalytic processes can be achieved by improving the sunlight harvesting and the synthesis of semiconductor/carbon composites has proved to be a promising strategy. Carbamazepine, diclofenac, and sulfamethoxazole were selected as target pharmaceuticals due to their recalcitrant behavior during conventional wastewater treatment and persistence in the environment, as properly reviewed. The literature data on the photocatalytic removal of carbamazepine, diclofenac, and sulfamethoxazole by semiconductor/carbon materials was critically revised to highlight the role of the carbon in the enhanced semiconductor performance under solar irradiation. Generally it was demonstrated that carbon materials induce red-shift absorption and they contribute to more effective charge separation, thus improving the composite photoactivity. Carbon was added as a dopant (C-doping) or as support or doping materials (i.e., nanoporous carbons, carbon nanotubes (CNTs), graphene, and derived materials, carbon quantum dots (CQDs), and biochars) and in the large majority of the cases, TiO₂ was the semiconductor tested. The specific role of carbon materials is dependent on their properties but even the more amorphous forms, like nanoporous carbons or biochars, allow to prepare composites with improved properties compared to the bare semiconductor. The self-photocatalytic activity of the carbon materials was also reported and should be further explored. The removal and mineralization rates, as well as degradation pathways and toxicity of the treated solutions were also critically analyzed.

Preparation, Characterization, and Performance Analysis of S-Doped Bi2MoO6 Nanosheets

S-doped Bi₂MoO₆ nanosheets were successfully synthesized by a simple hydrothermal method. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), N₂ adsorption-desorption isotherms, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), elemental mapping spectroscopy, photoluminescence spectra (PL), X-ray photoelectron spectroscopy (XPS), and UV-visible diffused reflectance spectra (UV-vis DRS). The photo-electrochemical performance of the samples was investigated via an electrochemical workstation. The S-doped Bi₂MoO₆ nanosheets exhibited enhanced photocatalytic activity under visible light irradiation. The photo-degradation rate of Rhodamine B (RhB) by S-doped Bi₂MoO₆ (1 wt%) reached 97% after 60 min, which was higher than that of the pure Bi₂MoO₆ and other S-doped products. The degradation rate of the recovered S-doped Bi₂MoO₆ (1 wt%) was still nearly 90% in the third cycle, indicating an excellent stability of the catalyst. The radical-capture experiments confirmed that superoxide radicals (·O^2-) and holes (h⁺) were the main active substances in the photocatalytic degradation of RhB by S-doped Bi₂MoO₆.