Adsorption of persistent organic pollutants (POPs) from the aqueous environment by nano-adsorbents: A review

The intensification of urbanisation and industrial activities significantly exacerbates the distribution of toxic contaminations into the aqueous environment. Persistent organic pollutants (POPs) have received considerable attention in the past few decades because of their persistence, long-distance migration, potential bioaccumulation, latent toxicity for humans and wildlife. There is no doubt that POPs cause serious effects on the global ecosystem. Therefore, it is necessary to develop a simple, safe and sustainable approach to remove POPs from water bodies. Among other conventional techniques, the adsorption process has proven to be a more effective method for eliminating POPs and to a larger extent meet discharge regulations. Nanomaterials can effectively adsorb POPs from aqueous solutions. For most POPs, a >70% adsorptive removal efficiency was achieved. The major mechanisms for POPS uptake by nano-adsorbents includes electrostatic interaction, hydrophobic (van der Waals, π-π and electron donor-acceptor) interaction and hydrogen bonding. Nano-adsorbent can sustain a >90% POPs adsorptive removal for about 3 cycles and reuseable for up to 10 cycles. Challenges around adsorbent ecotoxicity and safe disposal were also discussed. The present review evaluated recent research outcomes on nanomaterials that are employed to remove POPs in water systems.

Direct hydrothermal synthesis of amine-functionalized cubic hematite (C-Fe2O3) and sonochemical deposition of nanosized Au for its application as a visible-light photocatalyst

Cubic-shaped hematite (C-Fe2O3) functionalized with amine groups was directly prepared via one-pot hydrothermal reaction of Fe3+ with 1,12-diaminododecane (DA-12) in aqueous solution (50% ethanol). Herein, DA-12 (as a Lewis acid) promoted the aggregation of α-FeOOH nanorods with Lewis base sites, leading to the rapid recrystallization and conversion into uniform C-Fe2O3. C-Fe2O3 was subsequently deposited with nanosized Au via sonochemical reduction of 1.0 wt% HAuCl4 (0.1-0.8 mL), hereafter referred to as Au-deposited C-Fe2O3 (C-Fe2O3@Au). X-ray diffraction patterns of C-Fe2O3@Au confirmed the hexagonal crystalline phases of hematite and crystalline Au (111) and showed a weak broad band attributed to the amorphous carbon of DA-12. C-Fe2O3@Au was tested as a visible-light photocatalyst towards the degradation of methylene blue (MB) dye. C-Fe2O3@Au (0.1-0.4 mL of 1.0 wt% HAuCl4) exhibited 6-8 times higher photocatalytic activity than the Au-free counterpart (C-Fe2O3). The enhanced photocatalysis was mainly attributed to the improved separation efficiency of photo-excited charge carriers, i.e., the facilitated transport of electrons from the conduction band to the lower lying Fermi level of Au. However, the photocatalytic activity of C-Fe2O3@Au (0.8 mL of 1.0 wt% HAuCl4) was decreased probably due to the reduction of active sites for MB adsorption by the high coverage of the Au layer. The combined hydrothermal and sonochemical methods provided the direct synthetic route to cubic-shaped hematite decorated with nanosized Au and surface amine functionality as a promising visible-light photocatalyst.

Carbon Nanomaterials for the Treatment of Heavy Metal-Contaminated Water and Environmental Remediation

Nanotechnology is an advanced field of science having the ability to solve the variety of environmental challenges by controlling the size and shape of the materials at a nanoscale. Carbon nanomaterials are unique because of their nontoxic nature, high surface area, easier biodegradation, and particularly useful environmental remediation. Heavy metal contamination in water is a major problem and poses a great risk to human health. Carbon nanomaterials are getting more and more attention due to their superior physicochemical properties that can be exploited for advanced treatment of heavy metal-contaminated water. Carbon nanomaterials namely carbon nanotubes, fullerenes, graphene, graphene oxide, and activated carbon have great potential for removal of heavy metals from water because of their large surface area, nanoscale size, and availability of different functionalities and they are easier to be chemically modified and recycled. In this article, we have reviewed the recent advancements in the applications of these carbon nanomaterials in the treatment of heavy metal-contaminated water and have also highlighted their application in environmental remediation. Toxicological aspects of carbon-based nanomaterials have also been discussed.

Ferroelectric Oxide Nanocomposites with Trimodal Pore Structure for High Photocatalytic Performance

An effective method to improve the photocatalytic performances of powder catalysts is to use the internal electric field from ferroelectrics to separate photogenerated charge carriers. The design and engineering of a complex hetero-junction with a hierarchical pore structure is highly desirable for the efficient application of ferroelectric materials in photocatalysis. Here, we present a novel strategy using two templates to fabricate PbTiO₃/TiO₂/carbon (PTC) nanocomposites with a tunable microstructure. A hard SiO₂ template combined with an ice template followed by an appropriate pyrolysis procedure introduced trimodal (micro-, meso-, macro-) porosity. The as-prepared PTC nanocomposites with optimal mass ratio exhibited excellent photocatalytic and photoelectrochemical performances. PbTiO₃/TiO₂ annealed at 900 °C (PTC-900) showed a MB degradation rate of 0.21 and 0.021 min^-1 under UV and visible light irradiation, which are, respectively, 7.2 and 3 times those of pure PbTiO₃. The photocurrent density of the composite catalyst is 1.48 mA cm^-2 at the potential of 1.0 V versus saturated calomel electrode, and the rates of hydrogen generation of PTC-900 are as high as 2360 and 9.6 μmol h^-1 g^-1 under UV and visible light irradiation, respectively. More importantly, the simultaneous application of ultrasound-induced mechanical waves further improved the photocatalytic reactivity. This work serves to improve understanding on the design of ferroelectric/piezoelectric photocatalysts with a hierarchical pore structure and also proposes a widely applicable strategy for the fabrication of high-performance micro-nano/nano-nano structures.

Environmental Remediation Applications of Carbon Nanotubes and Graphene Oxide: Adsorption and Catalysis

Environmental issues such as the wastewater have influenced each aspect of our lives. Coupling the existing remediation solutions with exploring new functional carbon nanomaterials (e.g., carbon nanotubes, graphene oxide, graphene) by various perspectives shall open up a new venue to understand the environmental issues, phenomenon and find out the ways to get along with the nature. This review makes an attempt to provide an overview of potential environmental remediation solutions to the diverse challenges happening by using low-dimensional carbon nanomaterials and their composites as adsorbents, catalysts or catalysts support towards for the social sustainability.

Photocatalytic Active Mesoporous Carbon/ZnO Hybrid Materials from Block Copolymer Tethered ZnO Nanocrystals

Severe water pollution issues present an important contemporary challenge that drives the development and advancement of efficient and environmentally benign photocatalysts that enable the degradation of pollutants upon visible light irradiation. One example is zinc oxide/carbon (ZnO/C) hybrid materials that have been shown to be effective photocatalysts. To maximize the effectiveness of ZnO/C hybrids, materials with high accessible surface area of ZnO are required. Here, a novel strategy is presented to enable the synthesis of fine dispersions of ZnO nanoparticles within a porous carbon matrix. The synthesis entails the grafting of ZnO nanparticles with polystyrene-b-poly(styrene-co-acrylonitrile) (PS-b-PSAN) block copolymer and subsequent pyrolysis of the material under inert gas (N₂) atmosphere. During the pyrolysis process, the PS block effectively prevents agglomeration of ZnO particles, thus resulting in a fine dispersion of ZnO nanocrystals within a prorous C matrix. Materials are found to exhibit a dye adsorption capacity of 125 mg g^-1 (from a methylene blue aqueous solution with a concentration of 305 mg L^-1) and dye degradation rate constant of 0.021 min^-1. The significant increase of effective surface area and degradation efficacy (as compared to ZnO/C synthesized by the pyrolysis of binary PSAN/ZnO blends) is rationalized as a consequence of the increased porosity that promotes dye adsorption and transport within the hybrid material.

Excluded Volume Approach for Ultrathin Carbon Nanotube Network Stabilization: A Mesoscopic Distinct Element Method Study

Ultrathin carbon nanotube films have gathered attention for flexible electronics applications. Unfortunately, their network structure changes significantly even under small applied strains. We perform mesoscopic distinct element method simulations and develop an atomic-scale picture of the network stress relaxation. On this basis, we put forward the concept of mesoscale design by the addition of excluded-volume interactions. We integrate silicon nanoparticles into our model and show that the nanoparticle-filled networks present superior stability and mechanical response relative to those of pure films. The approach opens new possibilities for tuning the network microstructure in a manner that is compatible with flexible electronics applications.

Nano-Photoelectrochemical Cell Arrays with Spatially Isolated Oxidation and Reduction Channels

Photoelectrochemical conversion of solar energy is explored for many diverse applications but suffers from poor efficiencies due to limited solar absorption, inadequate charge carrier separation, redox half-reactions occurring in close proximity, and/or long ion diffusion lengths. We have taken a drastically different approach to the design of photoelectrochemical cells (PECs) to spatially isolate reaction sites at the nanoscale to different materials and flow channels, suppressing carrier recombination and back-reaction of intermediates while shortening ion diffusion paths and, importantly, avoiding mixed product generation. We developed massively parallel nano-PECs composed of an array of open-ended carbon nanotubes (CNTs) with photoanodic reactions occurring on the outer walls, uniformly coated with titanium dioxide (TiO₂), and photocathodic reactions occurring on the inner walls, decorated with platinum (Pt). We verified the redox reaction isolation by demonstrating selective photodeposition of manganese oxide on the outside and silver on the inside of the TiO₂/CNT/Pt nanotubes. Further, the nano-PECs exhibit improved solar absorption and efficient charge transfer of photogenerated carriers to their respective redox sites, leading to a 1.8% photon-to-current conversion efficiency (a current density of 4.2 mA/cm²) under white-light irradiation. The design principles demonstrated can be readily adapted to myriads of photocatalysts for cost-effective solar utilization.

Creep- and fatigue-resistant, rapid piezoresistive responses of elastomeric graphene-coated carbon nanotube aerogels over a wide pressure range

Lightweight, flexible piezoresistive materials with wide operational pressure ranges are in demand for applications such as human physical activity and health monitoring, robotics, and for functional interfacing between living systems and wearable electronics. Piezoresistivity of many elastomeric foams of polymers and carbon allotropes satisfies much of the required characteristics for these applications except creep and fatigue resistance due to their viscoelasticity, critically limiting the reliability and lifetime of integrated devices. We report the piezoresistive responses from aerogels of graphene-coated single-walled carbon nanotubes (SWCNTs), made using a facile and versatile sol-gel method. Graphene crosslinks the junctions of the underlying random network of SWCNTs, generating lightweight elastomeric aerogels with a mass density of ≈11 mg mL^-1 (volume fraction ≈7.7 × 10^-3) and a Young's modulus of ≈0.4 MPa. The piezoresistivity of these aerogels spans wide compressive pressures up to at least 120 kPa with sensitivity that exhibit ultrafast temporal responses of <27 ms and <3% delay ratio over 10⁴ compressive loading-unloading cycles at rates between 0.1-10 Hz. Most importantly, the piezoresistive responses do not show any creep at least for 1 hour and 80 kPa of compressive static loading. We suggest that the fatigue- and creep-resistant, ultrafast piezoresistive responses of these elastomeric aerogels are highly attractive for use in dynamic and static lightweight, pressure sensing applications such as human activity monitoring and soft robotics.

A novel fabrication approach for three-dimensional hierarchical porous metal oxide/carbon nanocomposites for enhanced solar photocatalytic performance

A novel approach for the fabrication of metal oxide/C composites with a hierarchical porous structure is proposed.

ZnO quantum dots decorated on optimized carbon nanotube intramolecular junctions exhibit superior field emission properties

Sensitive optical and superior field emission properties can be attained through the use of ZnO selective quantum well heterostructures grown on carbon nanotubes to fabricate carbon-zinc-oxide (CZO) nanotubes.

Single-walled carbon nanotubes/polyaniline-coated polyester thermoelectric textile with good interface stability prepared by ultrasonic induction

A novel and high-flexibility thermoelectric device, which is integrated with a SWNT/PANI textile, was prepared via ultrasonic induction for developing wearable electronic devices.

Electron ballistic characteristic optimization in individual MWCNT by oxygen plasma treatment

Surface treatment and bonding oxygen species represent a practical method for strongly enhancing the ballistic characteristic of carbon nanotubes (CNTs) because the electrical properties strongly depend on the surface states.