Enhancing the efficient degradation of BPS using the BPNS-CdS composite catalyst under visible light

Black phosphorus nanosheets (BPNS), a novel two-dimensional nanomaterial, find extensive applications in the field of photocatalysis. With the prohibition of bisphenol A (BPA), the utilization of bisphenol S (BPS), which is more resistant to degradation than BPA, has been steadily increasing. In this study, few-layer BPNS was prepared using an improved liquid-phase exfoliation method, showcasing its commendable specific surface area and notable adsorption capacity. Subsequently, a new type of nanocomposite material, BPNS-Cadmium sulfide (CdS), was hydrothermal synthesized involving BPNS and CdS. We conducted comparative assessments of BPNS, CdS, and their composite materials to identify the most efficient catalysts. Ultimately, we found that the composite material BPNS-CdS exhibited the highest capability for degrading BPS in an alkaline environment, achieving an impressive degradation rate of 86.9%. Notably, the degradation rate remained higher in an acidic environment compared to a neutral one. Through Electron Spin Resoance (ESR) experiments, it is revealed that BPNS-CdS, when exposed to visible light, generates •O2-, •OH, and h+ as confirmed. Additionally, we tested and validated the carrier separation and migration abilities of BPNS-CdS while also calculating the band gap for each material. Building upon these results, a possible photocatalysis mechanism experiment was proposed. Finally, the degradation products were analyzed using High-Performance Liquid Chromatography-Mass Spectrometry (HPLC-MS) and put forth a plausible pathway for the BPS degradation, and it was found that 4-Phenolsulfonic acid, Ethyl protocatechuate and Isophthalic acid are the main intermediates of BPS. This study contributes to a deeper understanding of the synergy between non-metallic catalysts like BPNS and metal catalysts like CdS. It also offers new insights into the degradation mechanisms and pathways for BPS.

Recent Progress on Synthesis and Properties of Black Phosphorus and Phosphorene As New-Age Nanomaterials for Water Decontamination

Concerted efforts have been made in recent years to find solutions to water and wastewater treatment challenges and eliminate the difficulties associated with treatment methods. Various techniques are used to ensure the recycling and reuse of water resources. Owing to their excellent chemical, physical, and biological properties, nanomaterials play an important role when integrated into water/wastewater treatment technologies. Black phosphorus (BP) is a potential nanomaterial candidate for water and wastewater treatment, especially its monolayer 2D derivative called phosphorene. Phosphorene offers relative adjustability in its direct bandgap, high charge carrier mobility, and improved in-plane anisotropy compared to the most extensively studied 2D nanomaterials. In this study, we examined the physical and chemical characteristics and synthetic processes of BP and phosphorene. We provide an overview of the latest advancements in the main applications of BP and phosphorene in water/wastewater treatment, which are categorized as photocatalytic, adsorption, and membrane filtration processes. Additionally, we explore the existing difficulties in the integration of BP and phosphorene into water/wastewater treatment technologies and prospects for future research in this field. In summary, this review highlights the ongoing necessity for significant research efforts on the integration of BP and phosphorene in water and wastewater applications.

Black Phosphorus-based Photocatalysts: Synthesis, Properties, and Applications

Photocatalysis is considered as an eco-friendly and sustainable strategy, since it uses abundant light for the advancement of the reaction, which is freely accessible and is devoid of environmental pollution. During the last decades, (nano)photocatalysts have gained broad industrial applications in terms of purification and detoxification of water as well as production of green fuels and hydrogen gas due to their special attributes. The degradation or remediation of toxic and hazardous compounds from the environment or changing them into non-toxic entities is a significant endeavor and necessary for the safety of humans, animals, and the environment. Black phosphorus (BP), a two-dimensional single-element material, has a marvelous structure, tunable bandgap, changeable morphology from bulk to nanosheet/quantum dot, and unique physicochemical properties, which makes it attractive material for photocatalytic applications, especially for sustainable development purposes. Since it can serve as a photocatalyst with or without coupling with other semiconductors, various aspects for multidimensional exploitation of BP are deliberated including their preparation via solvothermal, ball milling, calcination, and sonication methods to obtain BP from red phosphorus. The techniques for improving the photocatalytic and stability of BP-based composites are discussed along with their multifaceted applications for environmental remediation, pollution degradation, water splitting, N2 fixation, CO2 reduction, bacterial disinfection, H2 generation, and photodynamic therapy. Herein, most recent advancements pertaining to the photocatalytic applications of BP-based photocatalyst are cogitated, with a focus on their synthesis and properties as well as crucial challenges and future perspectives.

Ag+-doped boron quantum dots with enhanced stability and fluorescence enabling versatile practicality in visual detection, sensing, imaging and photocatalytic degradation

In this work, a metal-doping strategy was put forward to construct metal-doped borophene and the corresponding zero-dimensional boron. Through theoretical calculations, Ag⁺ acts as the optimal metal ions to prepare Ag⁺-doped borophene derived boron quantum dots (Ag-BQDs). As predicted theoretically, doping of Ag⁺ endows borophene with enhanced stability of electronic structures. The newly emerging Ag-BQDs were experimentally acquired from ultrasonic-assisted liquid-phase exfoliation of bulk boron and solvothermal treatments. According to theoretical and experimental studies, the improved stability and fluorescence (FL) of Ag-BQDs are due to the formation of strong B-Ag bonding to competitively suppress B-O bonding. The function enables the maximal protection of borophene electronic structures from oxidization, destruction and reconfiguration. Because of Ag-BQDs with relatively higher colloidal and FL stability over BQDs, potential applications of Ag-BQDs were further explored in promising fields toward FL visualization in aqueous solutions and on filter paper, employed as a chemosensor of Fe³⁺ for FL sensing and visual detection at the solid/liquid phases, utilized for multiple FL bio-imaging at the levels of fresh plants, live animals and live cells of fresh plants, and applied to photocatalytic degradation of organic dyes and anticancer drug. Experimental results demonstrate excellent performances of Ag-BQDs in multiple applications, including versatile FL sensing and visual detection, unique multi-channel FL bio-imaging and visible-light-driven photodegradation of organic pollutants, toxic and harmful substances. This work can promote the development of metal-ion-doped low- dimensional nanomaterials with improved stability and FL properties for significant applications.

Construction of dual Z-scheme Bi2WO6/g-C3N4/black phosphorus quantum dots composites for effective bisphenol A degradation

In this work, a novel dual Z-scheme Bi₂WO₆/g-C₃N₄/black phosphorus quantum dots (Bi₂WO₆/g-C₃N₄/BPQDs) composites were fabricated and utilized towards photocatalytic degradation of bisphenol A (BPA) under visible-light irradiation. Optimizing the content of g-C₃N₄ and BPQDs in Bi₂WO₆/g-C₃N₄/BPQDs composites to a suitable mass ratio can enhance the visible-light harvesting capacity and increase the charge separation efficiency and the transfer rate of excited-state electrons and holes, resulting in much higher photocatalytic activity for BPA degradation (95.6%, at 20 mg/L in 120 min) than that of Bi₂WO₆ (63.7%), g-C₃N₄ (25.0%), BPQDs (8.5%), and Bi₂WO₆/g-C₃N₄ (79.6%), respectively. Radical trapping experiments indicated that photogenerated holes (h⁺) and superoxide radicals (•O₂^-) played crucial roles in photocatalytic BPA degradation. Further, the possible degradation pathway and photocatalytic mechanism was proposed by analyzing the BPA intermediates. This work also demonstrated that the Bi₂WO₆/g-C₃N₄/BPQDs as effective photocatalysts was stable and have promising potential to remove environmental contaminants from real water samples.

Progress in the preparation, application, and recycling of black phosphorus

Black phosphorus nanosheets (BPNSs) are a new member of the nanomaterial family, and they have good development potential in electrochemistry, electronics, optoelectronics, environmental protection, biomedical, and other fields because of their bandgap width, high anisotropy, broad optical absorption, high carrier mobility and many other features. Although many articles have been published about the preparation and application of BPNSs, these aspects have not been elucidated, and we aimed to fill this knowledge gap in this review. First, we used VOSviewer software to sort out articles published in the past 5 years and drew a literature map, which allowed us to sort out the relationship between various studies related to BPNSs, and reflect on the research focus in recent years. Because BPNSs must be made from black phosphorus (BP), and BPNSs are a nano form of BP, the collation of the BP preparation scheme was also helpful for the related research on BPNSs. This paper introduces the preparation of bulk BP and BPNSs, analyzes and compares the advantages and disadvantages of each method, and points out the most promising methods in the future. Then, we propose improvement directions for this method. We also introduce the characterization of BPNSs and combine it with the subsequent photocatalytic application of BPNSs. As a new material, the effect of BPNSs on the environment is still unknown; thus, an end treatment scheme for BPNSs is summarized according to existing methods. Based on the experience of nanomaterial treatment, this paper proposes a research focus for the end treatment of BPNSs in the future, providing a reference scheme for the end treatment of other nanomaterials. Finally, we summarize the full text and propose recommended methods and improvement plans.

Phosphorene quantum dots: synthesis, properties and catalytic applications

Phosphorene quantum dots (PQDs) belong to a new class of zero-dimensional functional nanostructures with unique physicochemical and surface properties in comparison with few-layer phosphorene and other 2D analogues. Tunable band gap as a function of number of layers, ease of passivation and high carrier mobility of PQDs have attracted considerable attention in catalysis research due to which spectacular progress has been made in PQD research over the last few years. PQDs are now considered as promising catalytic materials for electrocatalytic water splitting and nitrogen reduction, lithium-sulfur batteries, solar light-driven energy devices and biocatalysis, either in pristine form or as an active component for constructing heterostructures with other 2D materials. In the light of these recent advances, it is worthwhile to review and consolidate PQD research in catalytic applications to understand the challenges ahead and suggest possible solutions. In this review, we systematically summarize various synthetic strategies including ultrasonic and electrochemical exfoliation, solvothermal treatment, blender breaking, milling, crushing and pulsed laser irradiation. Furthermore, the physiochemical properties of PQDs are discussed based on both experimental and theoretical perspectives. The potential applications of PQDs in catalysis with special emphasis on photocatalysis (solar light-driven energy devices) and electrocatalysis (oxygen evolution reactions and hydrogen evolution reactions) -are critically discussed along with the present status, challenges and future perspectives.

γ-CuI from ionic liquid/poly(ionic liquid)s precursors with controllable morphologies and improved photocatalytic performance

γ-phase copper(I) iodide (abbreviated to CuI hereafter) with different morphologies is realized through a one-step redox process from I-containing ionic liquid (IL) or poly(ionic liquid)s (PILs) precursors at room temperature. The phase composition, morphology, and electronic states of the synthesized CuI samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The resulting CuI products exhibit three different types of morphologies, namely nanocrystals, with an average size of 0.8 ± 0.2 μm, nanoplates, with a thickness of 35.8 ± 0.9 nm, and nanoflowers, with petals with a thickness of 12.2 ± 0.8 nm. Moreover, the as-synthesized CuI samples exhibit gradually diminishing bandgaps and improved photocatalysis performance for the photodegradation of rhodamine B (RhB) under visible light irradiation as the thickness decreases. XPS measurements confirm that IL/PILs coupled to the CuI surface, resulting in a further charge transfer between Cu and I. These results conclusively prove that IL/PILs serve as both the reducing agents and assemble as orientation templates in the formation of the CuI nanostructures, and also successfully mediate the functional properties of the samples by changing the surface electronic structures.

Nanodots Derived from Layered Materials: Synthesis and Applications

Layered 2D materials, such as graphene, transition metal dichalcogenides, transition metal oxides, black phosphorus, graphitic carbon nitride, hexagonal boron nitride, and MXenes, have attracted intensive attention over the past decades owing to their unique properties and wide applications in electronics, catalysis, energy storage, biomedicine, etc. Further reducing the lateral size of layered 2D materials down to less than 10 nm allows for preparing a new class of nanostructures, namely, nanodots derived from layered materials. Nanodots derived from layered materials not only can exhibit the intriguing properties of nanodots due to the size confinement originating from the ultrasmall size, but also can inherit some unique properties of ultrathin layered 2D materials, making them promising candidates in a wide range of applications, especially in biomedicine and catalysis. Here, a comprehensive summary on the materials categories, advantages, synthesis methods, and potential applications of these nanodots derived from layered materials is provided. Finally, personal insights about the challenges and future directions in this promising research field are also given.

Black Phosphorus, an Emerging Versatile Nanoplatform for Cancer Immunotherapy

Black phosphorus (BP) is one of the emerging versatile nanomaterials with outstanding biocompatibility and biodegradability, exhibiting great potential as a promising inorganic nanomaterial in the biomedical field. BP nanomaterials possess excellent ability for valid bio-conjugation and molecular loading in anticancer therapy. Generally, BP nanomaterials can be classified into BP nanosheets (BPNSs) and BP quantum dots (BPQDs), both of which can be synthesized through various preparation routes. In addition, BP nanomaterials can be applied as photothermal agents (PTA) for the photothermal therapy (PTT) due to their high photothermal conversion efficiency and larger extinction coefficients. The generated local hyperpyrexia leads to thermal elimination of tumor. Besides, BP nanomaterials are capable of producing singlet oxygen, which enable its application as a photosensitizer for photodynamic therapy (PDT). Moreover, BP nanomaterials can be oxidized and degraded to nontoxic phosphonates and phosphate under physiological conditions, improving their safety as a nano drug carrier in cancer therapy. Recently, it has been reported that BP-based PTT is capable of activating immune responses and alleviating the immunosuppressive tumor microenvironment by detection of T lymphocytes and various immunocytokines, indicating that BP-based nanocomposites not only serve as effective PTAs to ablate large solid tumors but also function as an immunomodulation agent to eliminate discrete tumorlets. Therefore, BP-mediated immunotherapy would provide more possibilities for synergistic cancer treatment.

Recent advances of monoelemental 2D materials for photocatalytic applications

As a sustainable environmental governance strategy and energy conversion method, photocatalysis has considered to have great potential in this field due to its excellent optical properties and has become one of the most attractive technologies today. Among 2D materials, the emerging two-dimensional (2D) monoelemental materials mainly distributed in the -IIIA, -IVA, -VA and -VIA groups and show excellent performance in solar energy conversion due to their graphene-like 2D atomic structure and unique properties, thereby drawing increasing attention. This review briefly summarizes the preparation processes and fundamental properties of 2D single-element nanomaterials, as well as various modification strategies and adjustment mechanisms to enhance their photocatalytic properties. In particular, this article comprehensively discusses the related practical applications of 2D single-element materials in the field of photocatalysis, including photocatalytic degradation for contaminants removal, photocatalytic pathogen inactivation, photocatalytic fouling control and photocatalytic energy conversion. This review will provide some new opportunities for the rational design of other excellent photocatalysts based on 2D monoelemental materials, as well as present tremendous novel ideas for 2D monoelemental materials in other environmental conservation and energy-related applications, such as supercapacitors, electrocatalysis, solar cells, and so on.

Light-accelerating oxidase-mimicking activity of black phosphorus quantum dots for colorimetric detection of acetylcholinesterase activity and inhibitor screening

A feasible and sensitive colorimetric platform was established for the assay of acetylcholinesterase (AChE) activity and evaluation of its inhibitor screening, based upon the light-accelerating oxidase-mimicking activity of black phosphorus quantum dots (BP QDs). The BP QDs were synthesized through a thermal exfoliation method and characterized using various techniques. The BP QDs exhibit oxidase-mimicking catalytic activity on dissolved oxygen-mediating oxidation of 3,3',5,5'-tetramethylbenzidine, a typical substrate of oxidase. This results in a transformation of 3,3',5,5'-tetramethylbenzidine into its blue oxidized product, which has a visible absorption peak at 652 nm. The exposure of 365 nm light irradiation significantly accelerates the oxidase-mimicking activity of the BP QDs and speeds up the reaction efficiency. AChE can specifically catalyze the decomposition of its substrate acetylthiocholine chloride to thiocholine. Thiocholine has reducing capacity and can thus reduce the oxidase-mimicking activity of the BP QDs. As a result, the oxidation of 3,3',5,5'-tetramethylbenzidine is hindered and the blue solution becomes paler. This gives a linear response for AChE ranging from 0.5 to 10.0 mU mL-1 and a detection limit of 0.17 mU mL-1. The assay was successfully applied to evaluate inhibitor screening with neostigmine as the model.

Advancing Applications of Black Phosphorus and BP-Analog Materials in Photo/Electrocatalysis through Structure Engineering and Surface Modulation

Black phosphorus (BP), an emerging 2D material semiconductor material, exhibits unique properties and promising application prospects for photo/electrocatalysis. However, the applications of BP in photo/electrocatalysis are hampered by the instability as well as low catalysis efficiency. Recently, tremendous efforts have been dedicated toward modulating its intrinsic structure, electronic property, and charge separation for enhanced photo/electrocatalytic performance through structure engineering. Simultaneously, the search for new substitute materials that are BP-analogous is ongoing. Herein, the latest theoretical and experimental progress made in the structural/surface engineering strategies and advanced applications of BP and BP-analog materials in relation to photo/electrocatalysis are extensively explored, and a presentation of the future opportunities and challenges of the materials is included at the end.

Ag+ -Coupled Black Phosphorus Vesicles with Emerging NIR-II Photoacoustic Imaging Performance for Cancer Immune-Dynamic Therapy and Fast Wound Healing

A silver-ion-coupled black phosphorus (BP) vesicle (BP Ve-Ag⁺ ) with a second near infrared (NIR-II) window photoacoustic (PA) imaging capability was firstly constructed to maximize the potential of BP quantum dot (QD) in deeper bioimaging and diversified therapy. The embedded Ag⁺ could improve the relatively large band gap of BP QD via intense charge coupling based on theoretical simulation results, subsequently leading to the enhanced optical absorption capability, accompanied with the occurrence of the strong NIR-II PA signal. Guiding by NIR-II PA bioimaging, the hidden Ag⁺ could be precisely released with the disassembly of Ve during photodynamic therapy process and captured by macrophages located in lesion region for arousing synergistic cancer photodynamic/Ag⁺ immunotherapy. BP Ve-Ag⁺ can contrapuntally kill pathogenic bacteria and accelerate wound healing monitored by NIR-II PA imaging.

Construction of BPQDs/Ti3C2@TiO2 Composites with Favorable Charge Transfer Channels for Enhanced Photocatalytic Activity under Visible Light Irradiation

Design and construction of double heterojunction is favorable to improve the separation and migration efficiency of photogenerated carriers, thus preferably solve the problems of environmental pollution and energy crisis. Herein, TiO2 nanoparticles (NPs) are in-situ grown on highly conductive Ti3C2 nanosheets via low-temperature hydrothermal strategy, and then black phosphorus quantum dots (BPQDs) are introduced on the surface of TiO2 NPs. Under hydrothermal temperature 120 °C, the BPQDs/Ti3C2@TiO2 photocatalyst exhibits remarkable enhanced photocatalytic degradation of methyl orange (MO) and hydrogen evolution reaction (HER) compared with BPQDs/Ti3C2 and Ti3C2@TiO2 composites. Enhanced photocatalytic activity can be attributed to (i) the BPQDs with tunable bandgaps are deposited on the TiO2 NPs to form intimate heterojunction, which facilitates the electrons transfer from the conduction band (CB) of BPQDs to the CB of TiO2; (ii) the electrons quickly migrate from CB of TiO2 NPs to the Ti3C2 nanosheets with excellent electronic conductivity via electron transfer channel, which is beneficial to prolong the lifetime of electrons and hinder the recombination of photogenerated carriers; (iii) the enhanced visible light absorption and enlarged specific surface area of BPQDs/Ti3C2@TiO2 further accelerate the photocatalytic reaction. This work emphasizes the essential role of quantum dots in the construction of double heterojunction and the potential application of Ti3C2 MXene for improving photocatalytic activity.

Engineering the energy gap of black phosphorene quantum dots by surface modification for efficient chemiluminescence

In this work, NH₂-functionalized BPQDs (N-BPQDs) were purposely synthesized with small the energy gap for efficient chemiluminescence (CL) with the assistance of persulfate (K₂S₂O₈).

Black phosphorus quantum dots as novel electrogenerated chemiluminescence emitters for the detection of Cu2+

Black phosphorus quantum dots with surface states and bandgap luminescence under NMP passivation are used for the detection of Cu²⁺.

Black Phosphorus/Palladium Nanohybrid: Unraveling the Nature of P-Pd Interaction and Application in Selective Hydrogenation

The burgeoning interest in two-dimensional (2D) black phosphorus (bP) contributes to the expansion of its applications in numerous fields. In the present study, 2D bP is used as a support for homogeneously dispersed palladium nanoparticles directly grown on it by a wet chemical process. Electron energy loss spectroscopy-scanning transmission electron microscopy analysis evidences a strong interaction between palladium and P atoms of the bP nanosheets. A quantitative evaluation of this interaction comes from the X-ray absorption spectroscopy measurements that show a very short Pd-P distance of 2.26 Å, proving for the first time the existence of an unprecedented Pd-P coordination bond of a covalent nature. Additionally, the average Pd-P coordination number of about 1.7 reveals that bP acts as a polydentate phosphine ligand toward the surface of the Pd atoms of the nanoparticles, thus preventing their agglomeration and inferring with structural stability. These unique properties result in a superior performance in the catalytic hydrogenation of chloronitroarenes to chloroanilines, where a higher chemoselectivity in comparison to other heterogeneous catalyst based on palladium has been observed.

Chemiluminescence of Oleic Acid Capped Black Phosphorus Quantum Dots for Highly Selective Detection of Sulfite in PM2.5

Quantum dots (QDs), especially metal-free QDs with their unique optoelectronic properties, environmental friendliness, and excellent biocompatibility, have opened a new avenue to explore novel chemiluminescence (CL) systems for analytical applications. However, the unknown CL properties, relatively weak emission, and instability of some of them in water (e.g., black phosphorus QDs) often seriously hinder their further practical applications. Chemical modification trends have offered new properties for materials and have been proved to be desirable ways to establish sensing platforms with improved sensitivity and stability. Herein, oleic acid capped black phosphorus QDs (OA-BP QDs) with improved stability and optical properties were successfully synthesized. More importantly, an extraordinary CL emission when OA-BP QDs reacted with SO₃^2- was first observed. In the CL process, OA-BP QDs acted as the catalyst to trigger singlet oxygen (¹O₂) generation in NaHSO₃, and then a chemiluminescence resonance energy transfer (CRET) between (¹O₂)₂* (¹O₂ dimeric aggregate) and OA-BP QDs was produced. On this basis, a new CL system for directly monitoring SO₃^2- in airborne fine particulate matter (PM_2.5) was fabricated. The study opens attractive perspectives of modified metal-free QDs for the practice of CL in monitoring the chemical species in PM_2.5.

A Low-Cost Metal-Free Photocatalyst Based on Black Phosphorus

An efficient metal-free photocatalyst composed of black phosphorus (BP) and graphitic carbon nitride (CN) is prepared on a large scale by ball milling. Using economical urea and red phosphorus (RP) as the raw materials, the estimated materials cost of BP/CN is 0.235 Euro per gram. The BP/CN heterostructure shows efficient charge separation and possesses abundant active sites, giving rise to excellent photocatalytic H2 evolution and rhodamine B (RhB) degradation efficiency. Without using a co-catalyst, the metal-free BP/CN emits H2 consistently at a rate as large as 786 µmol h-1 g-1 and RhB is decomposed in merely 25 min during visible-light irradiation. The corresponding electron/hole transfer and catalytic mechanisms are analyzed and described. The efficient metal-free catalyst is promising in visible-light photocatalysis and the simple ball-milling synthetic method can be readily scaled up.

The mechanism of structural changes and crystallization kinetics of amorphous red phosphorus to black phosphorus under high pressure

Revealing the untraditional crystallization mechanism of amorphous red phosphorus to black phosphorus under high pressure.

Visible- and NIR-Light Responsive Black-Phosphorus-Based Nanostructures in Solar Fuel Production and Environmental Remediation

Direct utilization of the full spectrum of renewable solar light, in particular the visible- and near-infrared (NIR) portions, is currently receiving a great deal of attention in solar-to-chemical energy conversion-a clean, economically, and environmentally sustainable process. Black phosphorus (BP), a newly emerging class of ultrathin 2D nanomaterials rediscovered in early 2014, fulfills this purpose due to its unique properties like high charge-carrier mobility and tunable direct-bandgap. To this end, the rational combinations of BP in the form of few-layer nanosheets or ultrasmall quantum dots with a range of organic and inorganic nanomaterials offer more versatile and robust hybrids and nanocomposites that are functional in solar fuel production and environmental remediation. Herein, the most recent and key achievements of BP-based nanostructured photocatalysts in water splitting, organic pollutant degradation, and nitrogen fixation under either visible- or NIR-light illumination are summarized. Furthermore, perspectives on the potential future research directions are provided.