Photocatalytic Nanofiltration Membranes with Self‐Cleaning Property for Wastewater Treatment

Membrane fouling is one of the most severe problems restricting membrane separation technology for wastewater treatment. This work reports a photocatalytic nanofiltration membrane (NFM) with self‐cleaning property fabricated using a facile biomimetic mineralization process. In this strategy, a polydopamine (PDA)/polyethyleneimine (PEI) intermediate layer is fabricated on an ultrafiltration membrane via a co‐deposition method followed by mineralization of a photocatalytic layer consisting of β‐FeOOH nanorods. The PDA–PEI layer acts both as a nanofiltration selective layer and an intermediate layer for anchoring the β‐FeOOH nanorods via strong coordination complexes between Fe³⁺ and catechol groups. In visible light, the β‐FeOOH layer exhibits efficient photocatalytic activity for degrading dyes through the photo‐Fenton reaction in the presence of hydrogen peroxide, endowing the NFM concurrently with effective nanofiltration performance and self‐cleaning capability. Moreover, the mineralized NFMs exhibit satisfactory stability under simultaneous filtration and photocatalysis processing, showing great potential in advanced wastewater treatment.     

State‐of‐the‐Art Progress in Diverse Heterostructured Photocatalysts toward Promoting Photocatalytic Performance

Semiconductor photocatalysts have received much attention in recent years due to their great potentials for the development of renewable energy technologies, as well as for environmental protection and remediation. The effective harvesting of solar energy and suppression of charge carrier recombination are two key aspects in photocatalysis. The formation of heterostructured photocatalysts is a promising strategy to improve photocatalytic activity, which is superior to that of their single component photocatalysts. This Feature Article concisely summarizes and highlights the state‐of‐the‐art progress of semiconductor/semiconductor heterostructured photocatalysts with diverse models, including type‐I and type‐II heterojunctions, Z‐scheme system, p–n heterojunctions, and homojunction band alignments, which were explored for effective improvement of photocatalytic activity through increase of the visible‐light absorption, promotion of separation, and transportation of the photoinduced charge carries, and enhancement of the photocatalytic stability.     

Oxygen Rich Titania: A Dopant Free,High Temperature Stable,and Visible‐Light Active Anatase Photocatalyst

The simultaneous existence of visible light photocatalytic activity and high temperature anatase phase stability up to 900 °C in undoped TiO₂ is reported for the first time. These properties are achieved by the in‐situ generation of oxygen through the thermal decomposition of peroxo‐titania complex (formed by the precursor modification with H₂O₂). Titania containing the highest amount of oxygen (16 H₂O₂‐TiO₂) retains 100% anatase phase even at 900 °C, where as the control sample exists as 100% rutile at this temperature. The same composition exhibits a six‐fold and two‐fold increase in visible light photocatalytic activities in comparison to the control sample and the standard photocatalyst Degussa P‐25 respectively. Among the various para­meters affecting the photocatalytic action, such as band gap narrowing, textural properties, crystallite size, and anatase phase stability, band gap narrowing was identified as the major factor responsible for the visible light photocatalytic activity. Increased Ti–O–Ti bond strength and upward shifting of the valence band (VB) maximum, which is responsible for the high temperature stability and visible light activity respectively, are identified from FT–IR, XPS, and photoluminescence (PL) spectroscopic studies. It is therefore proposed that the oxygen excess defects present in these titania samples are responsible for the high temperature stability and enhanced visible light photocatalytic activities.     

Biomimetic and Superwettable Nanofibrous Skins for Highly Efficient Separation of Oil‐in‐Water Emulsions

Developing a feasible and efficient separation membrane for the purification of highly emulsified oily wastewater is of significance but challenging due to the critical limitations of low flux and serious membrane fouling. Herein, a biomimetic and superwettable nanofibrous skin on an electrospun fibrous membrane via a facile strategy of synchronous electrospraying and electrospinning is created. The obtained nanofibrous skin possesses a lotus‐leaf‐like micro/nanostructured surface with intriguing superhydrophilicity and underwater superoleophobicity, which are due to the synergistic effect of the hierarchical roughness and hydrophilic polymeric matrix. The ultrathin, high porosity, sub‐micrometer porous skin layer results in the composite nanofibrous membranes exhibiting superior performances for separating both highly emulsified surfactant‐free and surfactant‐stabilized oil‐in‐water emulsions. An ultrahigh permeation flux of up to 5152 L m^?2 h^?1 with a separation efficiency of >99.93% is obtained solely under the driving of gravity (≈1 kPa), which was one order of magnitude higher than that of conventional filtration membranes with similar separation properties, showing significant applicability for energy‐saving filtration. Moreover, with the advantage of an excellent antioil fouling property, the membrane exhibits robust reusability for long‐term separation, which is promising for large‐scale oily wastewater remediation.     

Inside Front Cover: Nanocrystalline TiO2 Photocatalytic Membranes with a Hierarchical Mesoporous Multilayer Structure: Synthesis,Characterization, and Multifunction (Adv. Funct. Mater. 8/2006)

The inside cover shows a hierarchical, mesoporous, multilayer TiO2 photocatalytic membrane synthesized via a novel sol–gel dip‐coating process employing surfactant templates reported by Dionysiou and co‐workers on p. 1067. The resulting asymmetric mesoporous TiO2 membrane supported onto a porous Al2O3 substrate exhibited hierarchical changes in pore diameter and materials porosity from the top to the bottom layer. The TiO2 membrane has multiple simultaneous functions, including photocatalysis, disinfection, separation, and anti‐biofouling. A novel sol–gel dip‐coating process to fabricate nanocrystalline TiO₂ photocatalytic membranes with a robust hierarchical mesoporous multilayer and improved performance has been studied. Various titania sols containing poly(oxyethylenesorbitan monooleate) (Tween 80) surfactant as a pore‐directing agent to tailor‐design the porous structure of TiO₂ materials at different molar ratios of Tween 80/isopropyl alcohol/acetic acid/titanium tetraisopropoxide = R:45:6:1 have been synthesized. The sols are dip‐coated on top of a homemade porous alumina substrate to fabricate TiO₂/Al₂O₃ composite membranes, dried, and calcined, and this procedure is repeated with varying sols in succession. The resulting asymmetric mesoporous TiO₂ membrane with a thickness of 0.9 μm exhibits a hierarchical change in pore diameter from 2–6, through 3–8, to 5–11 nm from the top to the bottom layer. Moreover, the corresponding porosity is incremented from 46.2, through 56.7, to 69.3 %. Compared to a repeated‐coating process using a single sol, the hierarchical multilayer process improves water permeability significantly without sacrificing the organic retention and photocatalytic activity of the TiO₂ membranes. The prepared TiO₂ photocatalytic membrane has great potential in developing highly efficient water treatment and reuse systems, for example, decomposition of organic pollutants, inactivation of pathogenic microorganisms, physical separation of contaminants, and self‐antifouling action because of its multifunctional capability.     

High‐Performance CO2 Capture through Polymer‐Based Ultrathin Membranes

Thin film composite (TFC) membranes have attracted great research interest for a wide range of separation processes owing to their potential to achieve excellent permeance. However, it still remains challenging to fully exploit the superiority of thin selective layers when mitigating the pore intrusion phenomenon. Herein, a facile and generic interface‐decoration‐layer strategy collaborating with molecular‐scale organic–inorganic hybridization in the selective layer to obtain a high‐performance ultrathin film composite (UTFC) membrane for CO₂ capture is reported. The interface‐decoration layer of copper hydroxide nanofibers (CHNs) enables the formation of an ultrathin selective layer (≈100 nm), achieving a 2.5‐fold increase in gas permeance. The organic part in the molecular‐scale hybrid material contributes to facilitating CO₂‐selective adsorption while the inorganic part assists in maintaining robust membrane structure, thus remarkably improving the selectivity toward CO₂. As a result, the as‐prepared membrane shows a high CO₂ permeance of 2860 GPU, superior to state‐of‐the‐art polymer membranes, with a CO₂/N₂ selectivity of 28.2. The synergistic strategy proposed here can be extended to a wide range of polymers, holding great potential to produce high‐efficiency ultrathin membranes for molecular separation.     

High‐Performance Multifunctional TiO2 Nanowire Ultrafiltration Membrane with a Hierarchical Layer Structure for Water Treatment

A novel, multifunctional TiO₂ nanowire ultrafiltration (UF) membrane with a layered hierarchical structure is made via alkaline hydrothermal synthesis, followed by a filtration and hot‐press process. The TiO₂ UF membrane has high surface porosity (21.3%) and pore size values around 20 nm. The membrane possesses multifunctional capabilities under UV irradiation, such as anti‐fouling, anti‐bacterial, concurrent separation, and photocatalytic oxidation. The unique properties of the membrane indicate its potential in applications for environmental purification.     

TiO2@Layered Double Hydroxide Core–Shell Nanospheres with Largely Enhanced Photocatalytic Activity Toward O2 Generation

TiO₂@CoAl‐layered double hydroxide (LDH) core–shell nanospheres are fabricated via hydrothermal synthesis of TiO₂ hollow nanospheres followed by in situ growth of CoAl‐LDH shell, which exhibit an extraordinarily high photocatalytic activity toward oxygen evolution from water oxidation. The O₂ generation rates of 2.34 and 2.24 mmol h^?1 g^?1 are achieved under full sunlight (>200 nm) and visible light (>420 nm), respectively, which are among the highest photocatalytic activities for oxygen production to date. The reason is attributed to the desirable incorporation of visible‐ light‐active LDH shell with UV light‐responsive TiO₂ core for promoted solar energy utilization. Most importantly, the combined experimental results and computational simulations reveal that the strong donor–acceptor coupling and suitable band matching between TiO₂ core and LDH shell facilitate the separation of photoinduced electron‐hole pairs, accounting for the highly efficient photocatalytic performance. Therefore, this work provides a facile and cost‐effective strategy for the design and fabrication of hierarchical semiconductor materials, which can be applied as photocatalyst toward water splitting and solar energy conversion.     

Heterostructured Visible‐Light‐Active Photocatalyst of Chromia‐Nanoparticle‐Layered Titanate

An efficient visible‐light active photocatalyst of porous CrO_x–Ti_1.83O₄ nanohybrid with a 1:1 type ordered heterostructure is synthesized through a hybridization between a chromia cluster and exfoliated titanate nanosheets. The present nanohybrids are found to have a large surface area (ca. 250–310 m² g^–1) and an intense absorption of visible light, ascribable, respectively, to the formation of a porous structure and the hybridization of titanate with narrow‐bandgap chromium oxide. After the calcination at 400 °C, the nanohybrid shows an enhanced photocatalytic activity to effectively decompose organic compounds under the irradiation of visible light (λ > 420 nm). The present study highlights the exfoliation–restacking route as a very powerful way to develop efficient visible‐light‐harvesting photocatalysts with excellent thermal stability.     

Plasmonic Janus‐Composite Photocatalyst Comprising Au and C–TiO2 for Enhanced Aerobic Oxidation over a Broad Visible‐Light Range

Asymmetric Janus nanostructures containing a gold nanocage (NC) and a carbon–titania hybrid nanocrystal (AuNC/(C–TiO₂)) are prepared using a novel and facile microemulsion‐based approach that involves the assistance of ethanol. The localized surface plasmon resonance of the Au NC with a hollow interior and porous walls induce broadband visible‐light harvesting in the Janus AuNC/(C–TiO₂). An acetone evolution rate of 6.3 μmol h^?1 g^?1 is obtained when the Janus nanostructure is used for the photocatalytic aerobic oxidation of iso‐propanol under visible light (λ = 480–910 nm); the rate is 3.2 times the value of that obtained with C–TiO₂, and in photo‐electrochemical investigations an approximately fivefold enhancement is obtained. Moreover, when compared with the core–shell structure (AuNC@(C–TiO₂) and a gold–carbon–titania system where Au sphere nanoparticles act as light‐harvesting antenna, Janus AuNC/(C–TiO₂) exhibit superior plasmonic enhancement. Electromagnetic field simulation and electron paramagnetic resonance results suggest that the plasmon–photon coupling effect is dramatically amplified at the interface between the Au NC and C–TiO₂, leading to enhanced generation of energetic hot electrons for photocatalysis.     

Temperature‐ and Light‐Regulated Gas Transport in a Liquid Crystal Polymer Network

Azobenzene‐containing liquid crystal polymer networks (LCNs) are developed for temperature‐ and light‐regulated gas permeation. The order in a chiral‐nematic LCN (LCN*) is found to be essential to couple the unique structure of the membrane and its gas permeation responses to external stimuli such as temperature and varying irradiation conditions. An LCN membrane polymerized in the isotropic phase exhibits enhanced N₂ permeation with increasing temperature, like most traditional polymers, but barely responds to exposure with 455 and 365 nm light. In sharp contrast, a reversible decrease of N₂ transport is observed for the LCN* membrane of exactly the same chemical composition, but molecularly ordered, when submitted to an elevated temperature. More importantly, alternating in situ illumination with 455 and 365 nm light modulates reversibly N₂ permeation performance of the LCN* membrane, through the trans–cis isomerization of azo moieties. The authors postulate that, besides the anisotropic deformation of LCN*, the decreased order in LCN* membrane caused by external stimuli (i.e., increasing temperature or UV light illumination) is responsible for an inhibition of gas permeation. These results show potential applications of liquid crystal polymers in the gas transport and separation, and also contribute to the development of “smart” membranes.     

Enhanced Light‐Harvesting and Photocatalytic Properties in Morph‐TiO2 from Green‐Leaf Biotemplates

The unique structure of green leaves endows them with an extremely high light‐harvesting efficiency. In this work, green leaves are applied as biotemplates to synthesize morph‐TiO₂. The structural features favorable for light harvesting from the macro‐ to the nanoscale are replicated in morph‐TiO₂ through a two‐step infiltration process and the N contained in the original leaves is self‐doped into the resulting samples. The absorbance intensities within the visible‐light range of morph‐TiO₂ derived from different leaves increase by 103–258% and the band‐gap‐absorption onsets at the edge of the UV and visible‐light range show a red‐shift of 25–100 nm compared to those in TiO₂ without the template. The photocatalytic activity of morph‐TiO₂ is also improved, as proven by an electron paramagnetic resonance (EPR) study and degradation of rhodamine dye under irradiation with UV and visible light. The present work, as a new strategy, is of far‐reaching significance in learning from nature, driving us to make full use of the most‐abundant resources and structure‐introduced functions endowed by nature, opening up possibilities for extensive study of the physical and chemical properties of morph‐structured oxides and extending their potential for use in applications such as solar cells, photocatalysts, photoelectrical devices, and photoinduced sensors.     

Fluidic Processing of High‐Performance ZIF‐8 Membranes on Polymeric Hollow Fibers: Mechanistic Insights and Microstructure Control

Recently, a methodology for fabricating polycrystalline metal‐organic framework (MOF) membranes has been introduced – referred to as interfacial microfluidic membrane processing – which allows parallelizable fabrication of MOF membranes inside polymeric hollow fibers of microscopic diameter. Such hollow fiber membranes, when bundled together into modules, are an attractive way to scale molecular sieving membranes. The understanding and engineering of fluidic processing techniques for MOF membrane fabrication are in their infancy. Here, a detailed mechanistic understanding of MOF (ZIF‐8) membrane growth under microfluidic conditions in polyamide‐imide hollow fibers is reported, without any intermediate steps (such as seeding or surface modification) or post‐synthesis treatments. A key finding is that interfacial membrane formation in the hollow fiber occurs via an initial formation of two distinct layers and the subsequent rearrangement into a single layer. This understanding is used to show how nonisothermal processing allows fabrication of thinner (5 μm) ZIF‐8 films for higher throughput, and furthermore how engineering the polymeric hollow fiber support microstructure allows control of defects in the ZIF‐8 membranes. The performance of these engineered ZIF‐8 membranes is then characterized, which have H₂/C₃H₈ and C₃H₆/C₃H₈ mixture separation factors as high as 2018 and 65, respectively, and C₃H₆ permeances as high as 66 GPU.     

Construction of Built‐In Electric Field within Silver Phosphate Photocatalyst for Enhanced Removal of Recalcitrant Organic Pollutants

Semiconductor photocatalysis technology has aroused great interest in photocatalytic degradation, but it suffers from the drawbacks of fast electron‐hole recombination and unsatisfactory degradation efficiency. Herein, a novel photocatalyst Ag₃PO₄@NC with excellent photocatalytic activity is successfully prepared, characterized, and evaluated for the efficient removal of organic pollutants. After visible light irradiation for 5, 8, and 12 min, the photocatalytic degradation efficiency of norfloxacin, diclofenac, and phenol on the composite catalyst reaches 100%, and the apparent rate constant of which is 19.2, 48.7, and 23.2 times than that of the pure Ag₃PO₄, respectively. The density functional theory calculation results indicate that there is a built‐in electric field from N‐doped carbon (NC) to Ag₃PO₄ at the interface of the composite catalyst. Driven by the electric field, the photogenerated electrons of Ag₃PO₄ can be readily transferred to the NC, leading to the efficient separation of photogenerated carriers and the significant improvement of the catalytic performance. The results of radical trapping experiments and electron spin resonance analysis show that photogenerated holes and O₂^? play an important role in the photodegradation process. This work provides a universal strategy of construction built‐in electric field through coupling with NC to improve the photocatalytic performance of photocatalysts.     

Thin‐Layered Photocatalysts

Semiconductor photocatalysis, a green and sustainable technology, is of great significance for solving environmental pollution and energy shortages. However, the common problems of inefficient light harvesting, rapid recombination of electron–hole pairs, and low surface reactive reaction sites for photocatalysts urgently need to be solved. In this regard, thin‐layered photocatalysts are considered to be one of the most promising candidates for addressing these issues, due to their unique surface and electronic properties. In this review, the various strategies for constructing thin‐layered photocatalysts are summarized, and emphasis is given to approaches for optimizing the photocatalytic performance of the thin‐layered materials, which can be classified into surface engineering and junction construction. In addition, the photocatalytic applications of thin‐layered materials, i.e., water splitting, CO₂ reduction, nitrogen fixation, and molecule oxygen activation, are summarized. Finally, based on current achievements in thin‐layered photocatalysts, their future development and challenges are discussed.     

Electrically Pore‐Size‐Tunable Polypyrrole Membrane for Antifouling and Selective Separation

Antifouling and selectivity are major challenges for membrane separation technology. Herein, a polypyrrole‐dodecylbenzene sulfonate (PPy‐DBS) membrane with tunable pores is fabricated to alleviate pore blocking and achieve selective separation. The insertion/extraction of ions during the electrical redox process causes the change of PPy‐DBS volume, so that the membrane pores can be tuned in situ by applying an external redox potential. The pore size of a fouled membrane is enlarged under an oxidation voltage, then the membrane is backwashed to eliminate foulants in the pores, after which membrane pore size is recovered under a reduction voltage. Thus, membrane fouling can be effectively alleviated by adjusting the membrane pore size combined with cleaning. The specific flux of PPy‐DBS membrane increased by 21.91% after applying the voltages and backwashing, and it exhibits great recycling performance. Moreover, the distribution of humic acid macromolecules in the permeate significantly decreased, proving the enhanced sieving effect of smaller membrane pores under negative voltage. This study provides an intelligent strategy for fouling prevention and selective separation in water treatment.     

Surface Ligands Stabilized Lead Halide Perovskite Quantum Dot Photocatalyst for Visible Light‐Driven Hydrogen Generation

Solar hydrogen conversion represents a clean and economic approach to addressing global energy and environmental issues, for which efficient photocatalysts are heavily pursued. Lead halide perovskites are promising candidates for efficient phtocatalysts in solar hydrogen generation due to their attractive properties in light absorption, photogenerated charge transportation, and utilization. However, photocatalytic applications of lead halide perovskites are limited owing to their poor stability in the presence of water or other polar solvent environment. This work presents the rational control of surface ligands in achieving a good balance between stability and photocatalytic activity of CsPbBr₃ quantum dots (QDs). Detailed studies reveal that the deliberate surface ligands engineering is crucial for maximizing the photocatalytic activity of CsPbBr₃ QDs while maintaining good QD stability. A certain amount of surface ligands protect the CsPbBr₃ QDs from decomposition in moisture during the photocatalytic reaction while still enabling efficient charge transfer for photocatalytic reactions on the surface of QDs. The well‐controlled CsPbBr₃ photocatalyst shows efficient visible light‐driven H₂ generation with outstanding stability (≥160 h).     

High‐Temperature Large‐Scale Self‐Assembly of Highly Faceted Monocrystalline Au Metasurfaces

Localized surface plasmon resonance (LSPR) devices based on resonant metallic metasurfaces have shown disruptive potential for many applications including biosensing and photocatalysis. Despite significant progress, highly performing Au plasmonic nanotextures often suffer of suboptimal electric field enhancement, due to damping effects in multicrystalline domains. Fabricating well‐defined Au nanocrystals over large surfaces is very challenging, and usually requires time‐intensive multi‐step processes. Here, presented are first insights on the large‐scale self‐assembly of monocrystalline Au nano‐islands with tunable size and separation, and their application as efficient LSPR surfaces. Highly homogeneous centimeter‐sized Au metasurfaces are fabricated by one‐step deposition and in situ coalescence of hot nanoparticle aerosols into a discontinuous monolayer of highly faceted monocrystals. First insights on the mechanisms driving the high‐temperature synthesis of these highly faceted Au nanotextures are obtained by molecular dynamic and detailed experimental investigation of their growth kinetics. Notably, these metasurfaces demonstrat high‐quality and tunable LSPR, enabling the fabrication of highly performing optical gas molecule sensors detecting down to 3 × 10^?6 variations in refractive index at room temperature. It is believed that these findings provide a rapid, low‐cost nanofabrication tool for the engineering of highly homogenous Au metasurfaces for large‐scale LSPR devices with application ranging from ultrasensitive optical gas sensors to photocatalytic macroreactors.     

Improved Hydrogen Production of Au–Pt–CdS Hetero‐Nanostructures by Efficient Plasmon‐Induced Multipathway Electron Transfer

The design of new functional materials with excellent hydrogen production activity under visible‐light irradiation has critical significance for solving the energy crisis. A well‐controlled synthesis strategy is developed to prepare an Au–Pt–CdS hetero‐nanostructure, in which each component of Au, Pt, and CdS has direct contact with the other two materials; Pt is on the tips and a CdS layer along the sides of an Au nanotriangle (NT), which exhibits excellent photocatalytic activity for hydrogen production under light irradiation (λ > 420 nm). The sequential growth and surfactant‐dependent deposition produce the three‐component Au–Pt–CdS hybrids with the Au NT acting as core while Pt and CdS serve as a co‐shell. Due to the presence of the Au NT cores, the Au–Pt–CdS nanostructures possess highly enhanced light‐harvesting and strong local‐electric‐field enhancement. Moreover, the intimate and multi‐interface contact generates multiple electron‐transfer pathways (Au to CdS, CdS to Pt and Au to Pt) which guide photoexcited electrons to the co‐catalyst Pt for an efficient hydrogen reduction reaction. By evaluating the hydrogen production rate when aqueous Na₂SO₃–Na₂S solution is used as sacrificial agent, the Au–Pt–CdS hybrid exhibits excellent photocatalytic activity that is about 2.5 and 1.4 times larger than those of CdS/Pt and Au@CdS/Pt, respectively.     

Self‐Supporting,Double Stimuli‐Responsive Porous Membranes From Polystyrene‐block‐poly(N,N‐dimethylaminoethyl methacrylate) Diblock Copolymers

Asymmetric membranes are prepared via the non‐solvent‐induced phase separation (NIPS) process from a polystyrene‐block‐poly(N,N‐dimethylaminoethyl methacrylate) (PS‐b‐PDMAEMA) block copolymer. The polymer is prepared via sequential living anionic polymerization. Membrane surface and volume structures are characterized by scanning electron microscopy. Due to their asymmetric character, resulting in a thin separation layer with pores below 100 nm on top and a macroporous volume structure, the membranes are self‐supporting. Furthermore, they exhibit a defect‐free surface over several 100 µm². Polystyrene serves as the membrane matrix, whereas the pH‐ and temperature‐sensitive minority block, PDMAEMA, renders the material double stimuli‐responsive. Therefore, in terms of water flux, the membranes are able to react on two independently applicable stimuli, pH and temperature. Compared to the conditions where the lowest water flux is obtained, low temperature and pH, activation of both triggers results in a seven‐fold permeability increase. The pore size distribution and the separation properties of the obtained membranes were tested through the pH‐dependent filtration of silica particles with sizes of 12–100 nm.