In materials science, “green” synthesis has gained extensive attention as a reliable, sustainable, and eco-friendly protocol for synthesizing a wide range of materials/nanomaterials including
 metal/metal oxides nanomaterials, hybrid materials, and bioinspired materials. As such, green synthesis is regarded as an important tool to reduce the destructive effects associated with the traditional methods of synthesis for nanoparticles commonly utilized in laboratory and industry. In this review, we summarized the fundamental processes and mechanisms of “green” synthesis approaches, especially for metal and metal oxide [e.g., gold (Au), silver (Ag), copper oxide (CuO), and zinc oxide (ZnO)] nanoparticles using natural extracts. Importantly, we explored the role of biological components, essential phytochemicals (e.g., flavonoids, alkaloids, terpenoids, amides, and aldehydes) as reducing agents and solvent systems. The stability/toxicity of nanoparticles and the associated surface engineering techniques for achieving biocompatibility are also discussed. Finally, we covered applications of such synthesized products to environmental remediation in terms of antimicrobial activity, catalytic activity, removal of pollutants dyes, and heavy metal ion sensing.

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‘Green’ synthesis of metals and their oxide nanoparticles: applications for environmental remediation

Even though the mesoporous-type perovskite solar cell (PSC) is known for high efficiency, its planar-type counterpart exhibits lower efficiency and hysteretic response. Herein, we report success in suppressing hysteresis and record efficiency for planar-type devices using EDTA-complexed tin oxide (SnO2) electron-transport layer. The Fermi level of EDTA-complexed SnO2 is better matched with the conduction band of perovskite, leading to high open-circuit voltage. Its electron mobility is about three times larger than that of the SnO2. The record power conversion efficiency of planar-type PSCs with EDTA-complexed SnO2 increases to 21.60% (certified at 21.52% by Newport) with negligible hysteresis. Meanwhile, the low-temperature processed EDTA-complexed SnO2 enables 18.28% efficiency for a flexible device. Moreover, the unsealed PSCs with EDTA-complexed SnO2 degrade only by 8% exposed in an ambient atmosphere after 2880 h, and only by 14% after 120 h under irradiation at 100 mW cm−2. The development of high efficiency planar-type perovskite solar cell has been lagging behind the mesoporous-type counterpart. Here Yang et al. modify the oxide based electron transporting layer with organic acid and obtain planar-type cells with high certified efficiency of 21.5% and decent stability.

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High efficiency planar-type perovskite solar cells with negligible hysteresis using EDTA-complexed SnO2

In this study, a novel visible-light-driven N-doped carbon dot (NCDs)/g-C 3 N 4 composite was successfully synthesized by loading NCDs nanoparticles onto the interlayers and surface of g-C 3 N 4 via a facile polymerized method. The photocatalytic activity of the NCDs/g-C 3 N 4 was remarkably higher than that of g-C 3 N 4 and CDs/g-C 3 N 4 toward the degradation of indomethacin (IDM) under visible light irradiation. With increasing NCDs loading volumes, the photocatalytic activity of NCDs/g-C 3 N 4 initially increased, and then decreased. A very low NCDs content of 1.0 wt% resulted in a 13.6 fold higher reaction rate than that of pristine g-C 3 N 4 . This enhanced photocatalytic activity might have been ascribed to the unique up-converted PL properties, efficient charge separation, as well as bandgap narrowing of the NCDs. Reactive species (RS) scavenging experiments revealed that superoxide radical anions (O 2 −) and photogenerated holes (h + ) played key roles during the photocatalytic degradation of IDM. The quantification of O 2 − showed that NCDs/g-C 3 N 4 formed a larger amount of O 2 − than that of pristine g-C 3 N 4 . Potential photocatalytic pathways of IDM were proposed through the identification of intermediates using HPLC–MS/MS and the prediction of reaction sites based on Frontier Electron Densities (FEDs) calculations, which involved decarboxylation, hydroxylation, as well as the addition and cleavage of indole rings. Toxicity and mineralization evaluations revealed that NCDs/g-C 3 N 4 provided a very desirable performance for the toxicity reduction and mineralization of IDM under longer exposures of visible light irradiation.

Facile synthesis of N-doped carbon dots/g-C3N4 photocatalyst with enhanced visible-light photocatalytic activity for the degradation of indomethacin

Small carbon nanoparticles are an emerging member of the carbonaceous nanomaterial family and have been subsequently named as “carbon quantum dots” or “carbon nanodots”. Generally, carbon quantum dots are a type of spherical or sphere-like nanoparticles of less than 10 nm in size. Due to their unique properties, for example, size-dependent fluorescence, non-toxicity, biocompatibility, and easy accessibility, carbon quantum dots possess a great many potential applications in a range of fields from chemical sensing and imaging to catalysis and drug delivery, and thus are appealing to a number of researchers in nanoscience and nanotechnology. In this review, we give a brief introduction of the synthesis and fundamental properties of carbon quantum dots, then present their applications in metal ion sensing in detail along with illustrating the related mechanisms, and finally come up with some challenges currently faced and future outlooks for this fascinating carbon material. We hope this review could be helpful for readers who are preparing to join and/or have joined the research field of carbon quantum dots.

Carbon quantum dot-based nanoprobes for metal ion detection

Phosphorene, a single- or few-layered semiconductor material obtained from black phosphorus, has recently been introduced as a new member of the family of two-dimensional (2D) layered materials. Since its discovery, phosphorene has attracted significant attention, and due to its unique properties, is a promising material for many applications including transistors, batteries and photovoltaics (PV). However, based on the current progress in phosphorene production, it is clear that a lot remains to be explored before this material can be used for these applications. After providing a comprehensive overview of recent advancements in phosphorene synthesis, advantages and challenges of the currently available methods for phosphorene production are discussed. An overview of the research progress in the use of phosphorene for a wide range of applications is presented, with a focus on enabling important roles that phosphorene would play in next-generation PV cells. Roadmaps that have the potential to address some of the challenges in phosphorene research are examined because it is clear that the unprecedented chemical, physical and electronic properties of phosphorene and phosphorene-based materials are suitable for various applications, including photovoltaics.

Phosphorene and Phosphorene‐Based Materials – Prospects for Future Applications

Silicon nanowire possesses great potential as the material for renewable energy harvesting and conversion. The significantly reduced spectral reflectivity of silicon nanowire to visible light makes it even more attractive in solar energy applications. However, the benefit of its use for solar thermal energy harvesting remains to be investigated and has so far not been clearly reported. The purpose of this study is to provide practical information and insight into the performance of silicon nanowires in solar thermal energy conversion systems. Spectral hemispherical reflectivity and transmissivity of the black silicon nanowire array on silicon wafer substrate were measured. It was observed that the reflectivity is lower in the visible range but higher in the infrared range compared to the plain silicon wafer. A drying experiment and a theoretical calculation were carried out to directly evaluate the effects of the trade-off between scattering properties at different wavelengths. It is clearly seen that silicon nanowires can improve the solar thermal energy harnessing. The results showed that a 17.8 % increase in the harvest and utilization of solar thermal energy could be achieved using a silicon nanowire array on silicon substrate as compared to that obtained with a plain silicon wafer.

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Chia An Liu

Papers

Silicon Nanowires for Solar Thermal Energy Harvesting: an Experimental Evaluation on the Trade-off Effects of the Spectral Optical Properties