Fundamentals and challenges of ultrathin 2D photocatalysts in boosting CO2 photoreduction.

The unique physicochemical properties of (2D) nanomaterials make them well‐suited for use in sustainable energy applications. Many of these materials can be further improved with vacancy engineering. This review details recent progress in the vacancy engineering of ultrathin 2D nanomaterials. For clarity, it mainly focuses on various ultrathin 2D materials in three categories: Xa&XaYb‐, MaXb‐, or MaXbYc‐structured materials. Recently developed vacancies in different types of ultrathin 2D materials, as well as their preparation and characterization, are described. Emphasis is placed on the potential electrochemical energy storage and conversion applications of these materials. This review considers the relationship between vacancy properties and material categories of various ultrathin 2D materials in terms of application requirements, preparation, and characterization techniques. The challenges and future outlook of this promising field are summarized.

Determining the thickness of two‐dimensional (2D) materials accurately and reliably is highly necessary for multiple investigations, but at the same time it can be quite complex. Most studies in this field measure a topographic map at the edge of the 2D material using an atomic force microscope (AFM), and plot a single‐line cross‐section using the software of the AFM. However, this method is highly inaccurate and can result in high relative errors due to surface roughness and line‐to‐line variability. This is even more important in ultrathin (<4 nm) 2D materials grown by chemical vapor deposition, as these exhibit a larger surface roughness (compared to mechanically exfoliated) due to the high density of local defects. Here it is shown that the thickness of ultrathin 2D materials can be determined statistically with high accuracy and reliability in a very easy way by plotting the histogram height plot. Using this method should enhance the reliability of investigations and research papers in the field of 2D materials.

An innovative ultrathin two-dimensional (2D) Fe-doped cobaltous oxide (Fe-CoO) coated quartz crystal tuning fork (QCTF) was introduced for the purpose of developing a low-cost photoelectric detector with a simple configuration. The enhancement mechanism of the piezoelectric signal in the ultrathin 2D Fe-CoO-coated QCTF detector is assumed to be the synergetic photocarrier transfer and photothermal effect of ultrathin 2D Fe-CoO. The ultrathin 2D nanosheet structure of Fe-CoO with a large specific surface area can efficiently absorb and convert light into heat in the QCTF, and the photocarrier transfer from the Fe-CoO nanosheet to the electrode of the QCTF contributes to the enhancement in electricity given the shortened diffusion distance of carriers to the surfaces of the 2D nanosheet. Finite element modeling was adopted to simulate the thermoelastic expansion and mechanical resonance of the QCTF with 2D Fe-CoO coating to support experimental results and analyses. Moreover, the effects of 2D Fe-CoO on the performance of QCTF-based photoelectric detectors were investigated. This Letter demonstrates that ultrathin 2D materials have great potential in applications such as costly and tiny QCTF detectors, light sensing, biomedical imaging, and spectroscopy.

Water oxidation, also known as the oxygen evolution reaction (OER), is a crucial process in energy conversion and storage, especially in water electrolysis. The critical challenge of the electrochemical water splitting technology is to explore alternative precious-metal-free catalysts for the promotion of the kinetically sluggish OER. Recently, emerging two-dimensional (2D) ultrathin materials with abundant accessible active sites and improved electrical conductivity provide an ideal platform for the synthesis of promising OER catalysts. This Review focuses on the most recent advances in ultrathin 2D nanostructured materials for enhanced electrochemical activity of the OER. The design, synthesis and performance of such ultrathin 2D nanomaterials-based OER catalysts and their property-structure relationships are discussed, providing valuable insights to the exploration of novel OER catalysts with high efficiency and low overpotential. The potential research directions are also proposed in the research field.

Coupling of surface oxygen vacancies and structural engineering on ZnIn2S4/Bi2MoO6 for efficient norfloxacin degradation

Artificial photosynthesis using ultrathin 2D materials

Two-dimensional (2D) layered materials have been studied in depth during the past two decades due to their unique structure and properties. Transition metal (TM) intercalation of layered materials have been proven as an effective way to introduce new physical properties, such as tunable 2D magnetism, but the direct growth of atomically thin heteroatoms-intercalated layered materials remains untapped. Herein, we directly synthesize various ultrathin heteroatoms-intercalated 2D layered materials (UHI-2DMs) through flux-assisted growth (FAG) approach. Eight UHI-2DMs (V1/3NbS2, Cr1/3NbS2, Mn1/3NbS2, Fe1/3NbS2, Co1/3NbS2, Co1/3NbSe2, Fe1/3TaS2, Fe1/4TaS2) were successfully synthesized. Their thickness can be reduced to the thinnest limit (bilayer 2D material with monolayer intercalated TM), and magnetic ordering can be induced in the synthesized structures. Interestingly, due to the possible anisotropy-stabilized long-range ferromagnetism in Fe1/3TaS2 with weak interlayer coupling, the layer-independent magnetic ordering temperature of Fe1/3TaS2 was revealed by magneto-transport properties. This work establishes a general method for direct synthesis of heteroatom-intercalated ultrathin 2D materials with tunable chemical and physical properties.

As a sustainable technology, semiconductor photocatalysis has attracted considerable interest in the past several decades owing to the potential to relieve or resolve energy and environmental-pollution issues. By virtue of their unique structural and electronic properties, emerging ultrathin 2D materials with appropriate band structure show enormous potential to achieve efficient photocatalytic performance. Here, the state-of-the-art progress on ultrathin 2D photocatalysts is reviewed and a critical appraisal of the classification, controllable synthesis, and formation mechanism of ultrathin 2D photocatalysts is presented. Then, different strategies to tailor the electronic structure of ultrathin 2D photocatalysts are summarized, including component tuning, thickness tuning, doping, and defect engineering. Hybridization with the introduction of a foreign component and maintaining the ultrathin 2D structure is presented to further boost the photocatalytic performance, such as quantum dots/2D materials, single atoms/2D materials, molecular/2D materials, and 2D-2D stacking materials. More importantly, the advancement of versatile photocatalytic applications of ultrathin 2D photocatalysts in the fields of water oxidation, hydrogen evolution, CO2 reduction, nitrogen fixation, organic syntheses, and removal pollutants is discussed. Finally, the future opportunities and challenges regarding ultrathin 2D photocatalysts to bring about new opportunities for future research in the field of photocatalysis are also presented.

ConspectusPhotocatalysis technology has gained extensive attention in the past few decades due to its potential to alleviate or solve energy and environmental contamination problems. The development and design of new photocatalytic semiconductor materials with high catalytic activity has become a research hotspot in this field. In recent years, inorganic ultrathin two-dimensional (2D) semiconductor photocatalysts have shown excellent performance in photocatalytic applications due to their high specific surface area, clear atomic structure, unique electronic structure, intrinsic quantum confined electrons, and high atomic exposure ratio. When the thickness of the bulk semiconductor is decreased to the atomic level, its local atomic structure changes prominently. This is a major reason why the atomic thin 2D materials could show improved inherent properties and produce new properties that are not available in the corresponding bulk semiconductors. Furthermore, compared with the bulk photocatalysts, the surface electronic structure of inorganic ultrathin 2D materials is more sensitive and thus could be regulated more easily by surface and interfacial modification methods, leading to great optimization of photocatalytic properties. Therefore, inorganic ultrathin 2D materials not only provide an ideal reaction model for clearly revealing the relationships between surface/interface structure characteristics and photocatalytic performance but also bring new opportunities for the development of efficient catalysts to resolve energy crises and environmental problems.In this Account, we summarize our previous work on the applications of inorganic ultrathin 2D nanomaterials in the field of photocatalysis. First, we briefly introduce the classification and controlled fabrication of inorganic ultrathin 2D nanomaterials, including metal chalcogenides, bismuth-based photocatalysts, metal oxides, and metal-free materials. Then, we focus on the surface and interfacial modification strategies for effectively engineering the photocatalytic activity of 2D photocatalysts, including defect engineering, metal doping, single atom loading, and heterojunction construction. These strategies can effectively adjust the inherent physical and chemical properties of inorganic ultrathin 2D nanomaterials, change their electronic structure to improve the light absorption ability, promote the separation and migration of photogenerated electrons and holes, optimize the catalytic active sites, and even change the reaction energy barrier of the photocatalytic reaction. Additionally, we discuss the research progress of inorganic ultrathin 2D photocatalysts in hydrogen evolution, contaminant removal, and sterilization. Finally, we put forward the prospects, challenges, and novel viewpoints of feasible solutions of inorganic ultrathin 2D materials in photocatalysis applications. Overall, this Account provides in-depth insight into modulation strategies and the structure–activity relationship of inorganic ultrathin 2D materials for researchers in photocatalytic fields, which is beneficial to accelerate the practical applications of inorganic ultrathin 2D materials in the field of environment and energy.

The liquid crystal variable retarder (LCVR), as a controllable phase modulator, works in a setting voltage or modulated mode and has been applied in the field of microscopic polarimetry. However, the modulation period of an LCVR is normally limited to dozens to hundreds of milliseconds, which is not suitable for a rapid measurement. Based on this feature, in this work, one rapid measuring strategy was reported. Only two frames were needed for a normalized-intensity-difference microscopic anisotropy measurement. The working principle and instrumentation were presented. For demonstration, a flake of graphene was measured by this method and compared by the reported way. An approximately 30× speed improvement was realized with the clear signal measurement. This proposed method will help a fast in situ characterization of ultrathin films and 2D materials.

Electrochemical reduction of CO2 (CO2ER) into value-added chemical feedstocks and liquid fuels using renewable energy is a promising route for CO2 recycling.[1] The major challenges are the large overpotential for CO2 activation and the competitive H2 evolution in aqueous mediums.[2] Although some nanostructured noble metal catalysts have demonstrated impressive electrocatalytic activity and selectivity for CO or formate production over CO2ER, the low abundance and high cost limit their large-scale applications.[3] Efficient and cost-effective electrocatalysts with high energy efficiency and product selectivity are highly desirable to drive the development of CO2ER. In our study, we developed a low-cost Sn particle modified N-doped carbon nanofiber hybrid catalyst via a straightforward electrospinning technique coupled with a pyrolysis process.[4] Its electrocatalytic performance was tuned by the coverage of Sn nanoparticles and the structure of N species on the nanofiber surface. The pyridinic-N supported Sn nanodots drove efficient formate formation with a high current density of 11 mA cm-2 and a faradaic efficiency of 62% at a moderate overpotential of 690 mV. After a simple acidic leaching treatment, only atomically dispersed Sn species remained on the surface of pyridinic-N-doped carbon nanofibers. This catalyst dominantly promoted the CO2-to-CO conversion with a high faradaic efficiency of 91% at a low overpotential of 490 mV. The change of product selectivity was attributed to the difference in local chemical and electronic environment (Sn-Sn or Sn-N) surrounding the Sn active sites, which facilitated different intermediate stabilization and reaction pathway. The abundance of Sn nanodots and the strong electronic interaction between Sn and pyridinic-N-doped carbon may promote the formate formation, while the efficient CO production over the Sn atoms modified nanofibers may arise from the intrinsically high activity and selectivity of the formed Sn-N moieties.

Ultra-wide bandgap (UWBG) semiconductors and ultra-thin two-dimensional materials (2D) are at the very frontier of the electronics for energy management or <i>energy electronics</i>. A new generation of UWBG semiconductors will open new territories for higher power rated power electronics and deeper ultraviolet optoelectronics. Gallium oxide - Ga₂O₃ (4.5-4.9 eV), has recently emerged as a suitable platform for extending the limits which are set by conventional (~3 eV) WBG e.g. SiC and GaN and transparent conductive oxides (TCO) e.g. In₂O₃, ZnO, SnO₂. Besides, Ga₂O₃, the first efficient oxide semiconductor for energy electronics, is opening the door to many more semiconductor oxides (indeed, the largest family of UWBGs) to be investigated. Among these new power electronic materials, ZnGa₂O₄ (~5 eV) enables bipolar energy electronics, based on a spinel chemistry, for the first time. In the lower power rating end, power consumption also is also a main issue for modern computers and supercomputers. With the predicted end of the Moore’s law, the memory wall and the heat wall, new electronics materials and new computing paradigms are required to balance the big data (information) and energy requirements, just as the human brain does. Atomically thin 2D-materials, and the rich associated material systems (e.g. graphene (metal), MoS₂ (semiconductor) and h-BN (insulator)), have also attracted a lot of attention recently for <i>beyond-silicon</i> neuromorphic computing with record ultra-low power consumption. Thus, energy nanoelectronics based on UWBG and 2D materials are simultaneously extending the current frontiers of electronics and addressing the issue of electricity consumption, a central theme in the actions against climate change.

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are recent members of the 2D materials family with wide tunability, highly dynamic structural features, and excellent physical properties. Ultrathin 2D HOIPs and their heterostructures with other 2D materials have been exploited for study of physical phenomena and device applications. The in-plane mechanical properties of 2D ultrathin HOIPs are critical for understanding the coupling between mechanical and other physical fields and for integrated devices applications. Here we report the in-plane mechanical properties of ultrathin freestanding 2D lead iodide perovskite membranes and their dependence on the membrane thickness. The in-plane Young's moduli of 2D HOIPs are smaller than that of conventional covalently bonded 2D materials. As the thickness increases from monolayer to three-layer, both the Young's modulus and breaking strength decrease, while three-layer and four-layer 2D HOIPs have almost identical in-plane mechanical properties. These thickness-dependent mechanical properties can be attributed to interlayer slippage during deformation. Our results show that ultrathin 2D HOIPs exhibit outstanding breaking strength/Young's modulus ratio compared to many other widely used engineering materials and polymeric flexible substrates, which renders them suitable for application into flexible electronic devices.

Electroreduction of CO2 is promising to convert CO2 into value-added compounds since electroreduction does not necessarily require a catalyst because an electrochemical energy itself can activate the reactivity of CO2. Most of the reported studies on CO2 electroreduction use a metal electrode material, and the selectivity of reduction products depends on the metal species. However, as the use of noble or toxic metals should be avoided from the viewpoint of sustainability, a metal-free carbon-based material are desirable. Along these lines, we have focused on boron-doped diamond (BDD) as a carbon-based electrode in CO2 electroreduction. On the other hand, molecular modification of electrode surface is an important technique in various fields. Functional molecules can be covalently immobilized onto carbon electrodes by an electrografting method. Along these lines, we decided to immobilize amine on BDD surface, which integrates CO2 capture and storage technologies and CO2 electroreduction. Here, we prepared amine-modified BDD (NH2-BDD) to elucidate an effect of amine modification on CO2 electroreduction.Prior to the electrolysis experiments, linear sweep voltammetry (LSV) was performed to investigate the electrochemical difference between bare- and NH2-BDD. As an energetically equivalent criterion for the CO2 electroreduction, E red was defined as the potential at which the current density reached -30 µA/cm2; E red (vs. Ag/AgCl) were determined to be -1.56 and -1.16 V for bare-BDD and NH2-BDD, respectively. A positive shift of E red in the NH2-BDD electrode is probably because CO2 molecules form the C-N bond with the amino group on the electrode surface, which results in enhancing the electrophilicity of the carbon atom. Next, we investigated how amine modification affects the product selectivity in CO2 electroreduction. Products were CO, HCOOH, and H2 regardless of the type of electrode and applied potentials. Difference between the Faraday efficiencies (FE) of HCOOH and CO production was dependent on applied potentials in NH2-BDD. Particularly, in the most prominent case, the selectivity of CO production was 8 times higher for NH2-BDD than the case of bare-BDD. Since CO production requires the adsorption of intermediate species, CO2 • -, on the electrode surface, the adsorption of CO2 and CO2 • - would be promoted on NH2-BDD through the formation of C-N bond. In order to obtain the direct evidence for CO2 capturing by NH2-BDD during the electroreduction, in situ ATR-IR measurements were performed. We focused on the C=O (carbonyl) stretching vibration of the carbamate anion, observed in the region of 1700-1500 cm-1. In NH2-BDD, a broad peak attributed to the C=O stretching vibration was observed at around 1640 cm-1, and the peak intensity decreased as the applied potential became negative. This result strongly supports that CO2 was captured by amine at the BDD surface to form the carbamate anion and reduced to CO.The above discussion can be explained by the behavior of LSV of NH2-BDD, in which two drops were observed. The first drop at around -1.20 V (vs. Ag/AgCl) is probably ascribed to the reduction of CO2 captured by amine, and the second drop at around -1.70 V (vs. Ag/AgCl) is ascribed to the reduction of free CO2. Therefore, CO production would be favored at potentials between -1.20 and -1.70 V (vs. Ag/AgCl) and HCOOH production would be favored at potentials more negative than -1.70 V (vs. Ag/AgCl). These threshold potentials are in good agreement with the potentials at which the product selectivity switched. It is noted that, in bare-BDD electrodes showing the different E red, the selectivity of CO production was almost unchanged, which suggests that the potential dependence of product selectivity in CO2 electroreduction cannot be explained only by differences in E red. Therefore, the product selectivity was driven by the interaction between the surface amine groups and CO2, i.e. the reaction via carbamate formation. Figure 1

Integration of amorphous structures and anion defects into ultrathin 2D materials has been identified as an effective strategy for boosting the electrocatalytic performance. However, the in-depth understanding of the relationship among the amorphous structure, vacancy defect, and catalytic activity is still obscure. Herein, a facile strategy was proposed to prepare ultrathin and amorphous Mo-FeS nanosheets (NSs) with abundant sulfur defects. Benefited from the ultrathin, amorphous nanostructure, and synergy effect of Mo-doping and sulfur defect, the Mo-FeS NSs manifested excellent electrocatalytic activity toward oxygen evolution reaction (OER) in alkaline medium, as shown by an ultralow overpotential of 210 mV at 10 mA cm-2, a Tafel slope of 50 mV dec-1, and retaining such good catalytic stability over 30 h. The efficient catalytic performance for Mo-FeS NSs is superior to the commercial IrO2 and most reported top-performing electrocatalysts. Density functional theory calculations revealed that the accelerated electron/mass transfer over the oxygen-containing intermediates can be attributed to the amorphous structure and sulfur-rich defects caused by structural reconfiguration. Furthermore, the S vacancies could enhance the activity of its neighboring Fe-active sites, which was also beneficial to their OER kinetics. This work integrated both amorphous structures and sulfur vacancies into ultrathin 2D NSs and further systematically evaluated the OER performance, providing new insights for the design of amorphous-layered electrocatalysts.