Towards effective recovery of humate as green fertilizer from landfill leachate concentrate by electro-neutral nanofiltration membrane.

Under the environmental sustainability concept, landfill leachate concentrate can be up-cycled as a useful resource. Practical strategy for effective management of landfill leachate concentrate is to recover the existing humate as fertilizer purpose for plant growth. Herein, we designed an electro-neutral nanofiltration membrane to separate the humate and inorganic salts for achieving a sufficient humate recovery from leachate concentrate. The electro-neutral nanofiltration membrane yielded a high retention of humate (96.54 %) with an extremely low salt rejection (3.47 %), tremendously outperforming the state-of-the-art nanofiltration membranes and exhibiting superior promise in fractionation of humate and inorganic salts. With implementation of the pressure-driven concentration process, the electro-neutral nanofiltration membrane enriched the humate from 1756 to 51,466 mg∙L-1 at a fold of 32.6, enabling 90.0 % humate recovery and 96.4 % desalination efficiency from landfill leachate concentrate. Furthermore, the recovered humate not only exerted no phytotoxicity, but also significantly promoted the metabolism of red bean plants, serving as an effective green fertilizer. The study provides a conceptual and technical platform using high-performance electro-neutral nanofiltration membranes to extract the humate as a promising nutrient for fertilizer application, in view of sustainable landfill leachate concentrate treatment.

Advances in high-voltage supercapacitors for energy storage systems: materials and electrolyte tailoring to implementation.

To achieve a zero-carbon-emission society, it is essential to increase the use of clean and renewable energy. Yet, renewable energy resources present constraints in terms of geographical locations and limited time intervals for energy generation. Therefore, there is a surging demand for developing high-performance energy storage systems (ESSs) to effectively store the energy during the peak time and use the energy during the trough period. To this end, supercapacitors hold great promise as short-term ESSs for rapid power recovery or frequency regulation to improve the quality and reliability of power supply. In particular, the electrical double layer capacitor (EDLC) which offers long and stable cycle retention, high power densities, and fast charge/discharge characteristics with a moderate operating voltage window, is a suitable candidate. Yet, for implementation of the EDLC in ESSs, further research effort is required in terms of increasing the operating voltage and energy densities while maintaining the long-term cycle stability and power densities which are desirable aspects for ESS operation. Here, we examine the advances in EDLC research to achieve a high operating voltage window along with high energy densities, covering from materials and electrolytes to long-term device perspectives for next-generation supercapacitor-based ESSs.

Highly stable gold nanolayer membrane for efficient solar water evaporation under a harsh environment.

Interfacial solar water evaporation has attracted tremendous attention for sunlight harvesting for water purification. However, salt formation and stability of the photothermal materials (PTMs) remain a challenge that need addressing before bringing this technology to real-world applications. In this work, a nanoscale thin film of gold (Au) on a polytetrafluoroethylene (PTFE) membrane has been prepared using a magnetic sputtering technique. The fabricated membrane displays a robust mechanical strength and chemical stability arising from the adhesiveness of the thin film Au nanolayer on the PTFE membrane as well as the chemical inertness of the noble metal PTM. The Au nanolayer/PTFE membrane with cellulose sponge substrate resulted in an evaporation rate of 0.88 kg m-2 h-1 under 1 sun intensity. Remarkable salt ion rejection of 99.9% has been obtained, meeting the required standard for drinking water. Moreover, the membrane exhibited excellent stability and reusability in natural seawater and high salinity brine (150 g/L) and even in severe conditions (acidic, basic, and oxidized). No noticeable salt formation was observed on the evaporator surface after the tests. These findings reveal promising prospects for using a magnetron sputtering technique to fabricate a stable photothermal membrane for seawater and high salinity brine desalination.

Green ammonia synthesis using CeO2/RuO2 nanolayers on vertical graphene catalyst via electrochemical route in alkaline electrolyte.

The electrochemical synthesis of ammonia at ambient temperature and pressure has the potential to replace the conventional process for the production of ammonia. However, the low ammonia yield and poor long-term stability of catalysts for the synthesis of ammonia hinders the application of this technology. Herein, we endeavored to tackle this challenge by synthesizing 3-D vertical graphene (VG) on Ni foam via a one-step, low-temperature plasma process, which offered high conductivity and large surface area. Subsequently, the vertical graphene on Ni foam was loaded with nanolayers of ruthenium oxide (RuO2, ∼2 nm) and cerium oxide (CeO2, <20 nm) nanoparticles via magnetron sputtering. The incorporation of nanoparticle layers (RuO2 and CeO2/RuO2) on VG significantly increased the NH3 yield in KOH electrolyte. Finally, the performance and long-term stability of this composite material were successfully demonstrated by the addition of CeO2/RuO2 nanolayers on the VG electrocatalyst. The catalyst achieved an excellent performance with a high ammonia synthesis yield of 50.56 µg mgtotal cat.-1 h-1 (1.11 × 10-10 mol cm-2 s-1) during the performance evaluation period of 36 h. This observation was also verified by density functional theory calculation, where CeO2 exhibited the best catalytic performance compared to RuO2 and pristine graphene.

Chemically Cross-Linked Graphene Oxide as a Selective Layer on Electrospun Polyvinyl Alcohol Nanofiber Membrane for Nanofiltration Application.

Graphene oxide (GO) nanosheets were utilized as a selective layer on a highly porous polyvinyl alcohol (PVA) nanofiber support via a pressure-assisted self-assembly technique to synthesize composite nanofiltration membranes. The GO layer was rendered stable by cross-linking the nanosheets (GO-to-GO) and by linking them onto the support surface (GO-to-PVA) using glutaraldehyde (GA). The amounts of GO and GA deposited on the PVA substrate were varied to determine the optimum nanofiltration membrane both in terms of water flux and salt rejection performances. The successful GA cross-linking of GO interlayers and GO-PVA via acetalization was confirmed by FTIR and XPS analyses, which corroborated with other characterization results from contact angle and zeta potential measurements. Morphologies of the most effective membrane (CGOPVA-50) featured a defect-free GA cross-linked GO layer with a thickness of ~67 nm. The best solute rejections of the CGOPVA-50 membrane were 91.01% for Na2SO4 (20 mM), 98.12% for Eosin Y (10 mg/L), 76.92% for Methylene blue (10 mg/L), and 49.62% for NaCl (20 mM). These findings may provide one of the promising approaches in synthesizing mechanically stable GO-based thin-film composite membranes that are effective for solute separation via nanofiltration.

Mechanistic insight into the in vitro toxicity of graphene oxide against biofilm forming bacteria using laser-induced breakdown spectroscopy.

While the cytotoxicity of graphene oxide (GO) has been well established, its bactericidal mechanism, however, has yet to be elucidated to advance GO-based biomedical and environmental applications. In an attempt to better understand the bactericidal action of GO, herein we studied the interactions of GO with Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus cells using physical techniques and chemical probes, respectively. In particular, a novel laser-induced breakdown spectroscopy (LIBS) based elemental fingerprint analysis revealed notable differences between viable and non-viable cells based on the difference in the concentration of trace inorganic elements in complex bacterial systems, which reflect cellular membrane integrity. Lower emission intensities from essential inorganic ions in the GO-treated cells offered explicit evidence on the efflux of intracellular molecules from the bacteria through damaged cell membranes. Furthermore, a detailed structural and morphological investigation of bacterial membrane integrity confirmed GO-induced membrane stress upon direct contact interactions with bacterial cells, resulting in the disruption of cellular membranes. Moreover, the generation of intracellular reactive oxygen species (ROS) in the presence of an added antioxidant underlined the role of GO-mediated oxidative stress in bacterial cell inactivation. Thus, by correlating the changes in the bacterial elemental compositions with the severe morphological alterations and the high ROS production witnessed herein, we propose that the bactericidal mechanism of GO is likely to be the synergy between membrane and oxidative stress towards both tested species. Our findings offer useful guidelines for the future development of GO-based antibacterial surfaces and coatings.

Anti-fouling graphene-based membranes for effective water desalination.

The inability of membranes to handle a wide spectrum of pollutants is an important unsolved problem for water treatment. Here we demonstrate water desalination via a membrane distillation process using a graphene membrane where water permeation is enabled by nanochannels of multilayer, mismatched, partially overlapping graphene grains. Graphene films derived from renewable oil exhibit significantly superior retention of water vapour flux and salt rejection rates, and a superior antifouling capability under a mixture of saline water containing contaminants such as oils and surfactants, compared to commercial distillation membranes. Moreover, real-world applicability of our membrane is demonstrated by processing sea water from Sydney Harbour over 72 h with macroscale membrane size of 4 cm2, processing ~0.5 L per day. Numerical simulations show that the channels between the mismatched grains serve as an effective water permeation route. Our research will pave the way for large-scale graphene-based antifouling membranes for diverse water treatment applications.

Single-step ambient-air synthesis of graphene from renewable precursors as electrochemical genosensor.

Thermal chemical vapour deposition techniques for graphene fabrication, while promising, are thus far limited by resource-consuming and energy-intensive principles. In particular, purified gases and extensive vacuum processing are necessary for creating a highly controlled environment, isolated from ambient air, to enable the growth of graphene films. Here we exploit the ambient-air environment to enable the growth of graphene films, without the need for compressed gases. A renewable natural precursor, soybean oil, is transformed into continuous graphene films, composed of single-to-few layers, in a single step. The enabling parameters for controlled synthesis and tailored properties of the graphene film are discussed, and a mechanism for the ambient-air growth is proposed. Furthermore, the functionality of the graphene is demonstrated through direct utilization as an electrode to realize an effective electrochemical genosensor. Our method is applicable to other types of renewable precursors and may open a new avenue for low-cost synthesis of graphene films.

Multifunctional graphene micro-islands: Rapid, low-temperature plasma-enabled synthesis and facile integration for bioengineering and genosensing applications.

Here, we present a rapid, low-temperature (200°C) plasma-enabled synthesis of graphene micro-islands (GMs). Morphological analyses of GMs by scanning electron microscopy (SEM) and atomic force microscopy (AFM) feature a uniform and open-networked array of aggregated graphene sheets. Structural and surface chemical characterizations by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) support the presence of thin graphitic edges and reactive oxygen functional groups. We demonstrate that these inherent properties of GMs enable its multifunctional capabilities as a bioactive interface. GMs exhibit a biocompatibility of 80% cell viability with primary fibroblast lung cells after 5 days. Further, GMs were assembled into an impedimetric genosensor, and its performance was characterized by electrochemical impedance spectroscopy (EIS). A dynamic sensing range of 1pM to 1nM is reported, and a limit of quantification (LOQ) of 2.03×10-13M is deduced, with selectivity to single-RNA-base mismatched sequences. The versatile nature of GMs may be explored to enable multi-faceted bioactive platforms for next-generation personalized healthcare technologies.

High Pseudocapacitive Performance of MnO2 Nanowires on Recyclable Electrodes.

Manganese oxides are promising pseudocapacitve materials for achieving both high power and energy densities in pseudocapacitors. However, it remains a great challenge to develop MnO2 -based high-performance electrodes due to their low electrical conductance and poor stability. Here we show that MnO2 nanowires anchored on electrochemically modified graphite foil (EMGF) have a high areal capacitance of 167 mF cm(-2) at a discharge current density of 0.2 mA cm(-2) and a high capacitance retention after 5000 charge/discharge cycles (115 %), which are among the best values reported for any MnO2 -based hybrid structures. The EMGF support can also be recycled and the newly deposited MnO2 -based hybrids retain similarly high performance. These results demonstrate the successful preparation of pseudocapacitors with high capacity and cycling stability, which may open a new opportunity towards a sustainable and environmentally friendly method of utilizing electrochemical energy storage devices.

Nanocarbon-Coated Porous Anodic Alumina for Bionic Devices.

A highly-stable and biocompatible nanoporous electrode is demonstrated herein. The electrode is based on a porous anodic alumina which is conformally coated with an ultra-thin layer of diamond-like carbon. The nanocarbon coating plays an essential role for the chemical stability and biocompatibility of the electrodes; thus, the coated electrodes are ideally suited for biomedical applications. The corrosion resistance of the proposed electrodes was tested under extreme chemical conditions, such as in boiling acidic/alkali environments. The nanostructured morphology and the surface chemistry of the electrodes were maintained after wet/dry chemical corrosion tests. The non-cytotoxicity of the electrodes was tested by standard toxicity tests using mouse fibroblasts and cortical neurons. Furthermore, the cell-electrode interaction of cortical neurons with nanocarbon coated nanoporous anodic alumina was studied in vitro. Cortical neurons were found to attach and spread to the nanocarbon coated electrodes without using additional biomolecules, whilst no cell attachment was observed on the surface of the bare anodic alumina. Neurite growth appeared to be sensitive to nanotopographical features of the electrodes. The proposed electrodes show a great promise for practical applications such as retinal prostheses and bionic implants in general.

Synergistic fusion of vertical graphene nanosheets and carbon nanotubes for high-performance supercapacitor electrodes.

Graphene and carbon nanotubes (CNTs) are attractive electrode materials for supercapacitors. However, challenges such as the substrate-limited growth of CNTs, nanotube bundling in liquid electrolytes, under-utilized basal planes, and stacking of graphene sheets have so far impeded their widespread application. Here we present a hybrid structure formed by the direct growth of CNTs onto vertical graphene nanosheets (VGNS). VGNS are fabricated by a green plasma-assisted method to break down and reconstruct a natural precursor into an ordered graphitic structure. The synergistic combination of CNTs and VGNS overcomes the challenges intrinsic to both materials. The resulting VGNS/CNTs hybrids show a high specific capacitance with good cycling stability. The charge storage is based mainly on the non-Faradaic mechanism. In addition, a series of optimization experiments were conducted to reveal the critical factors that are required to achieve the demonstrated high supercapacitor performance.

Plasma break-down and re-build: same functional vertical graphenes from diverse natural precursors.

Plasmas, the 4(th) state of matter, uniformly transform natural precursors with different chemical composition in solid, liquid, and gas states into the same functional vertical graphenes in a single-step process within a few minutes. Functional vertical graphenes show reliable biosensing properties, strong binding with proteins, and improved adhesion to substrates.

Plasma effects in semiconducting nanowire growth.

Three case studies are presented to show low-temperature plasma-specific effects in the solution of (i) effective control of nucleation and growth; (ii) environmental friendliness; and (iii) energy efficiency critical issues in semiconducting nanowire growth. The first case (related to (i) and (iii)) shows that in catalytic growth of Si nanowires, plasma-specific effects lead to a substantial increase in growth rates, decrease of the minimum nanowire thickness, and much faster nanowire nucleation at the same growth temperatures. For nucleation and growth of nanowires of the same thickness, much lower temperatures are required. In the second example (related to (ii)), we produce Si nanowire networks with controllable nanowire thickness, length, and area density without any catalyst or external supply of Si building material. This case is an environmentally-friendly alternative to the commonly used Si microfabrication based on a highly-toxic silane precursor gas. The third example is related to (iii) and demonstrates that ZnO nanowires can be synthesized in plasma-enhanced CVD at significantly lower process temperatures than in similar neutral gas-based processes and without compromising structural quality and performance of the nanowires. Our results are relevant to the development of next-generation nanoelectronic, optoelectronic, energy conversion and sensing devices based on semiconducting nanowires.