Two‐dimensional (2D) semiconductors have recently become attractive candidate substrates for surface‐enhanced Raman spectroscopy, exhibiting good semiconductor‐based SERS sensing for a wider variety ...of application scenarios. However, the underlying mechanism remains unclear. Herein, we propose that surface defects play a vital role in the magnification of the SERS performances of 2D semiconductors. As a prototype material, ultrathin WO3 nanosheets is used to demonstrate that surface defect sites and the resulting increased charge‐carrier density can induce strong charge‐transfer interactions at the substrate‐molecule interface, thereby improving the sensitivity of the SERS substrate by 100 times with high reproducibility. Further work with other metal oxides suggests the reduced dimension of 2D materials can be advantageous in promoting SERS sensing for multiple probe molecules.
The down‐sizing of metal oxide semiconductors to ultrathin nanosheets creates intrinsic crystalline defects which alters the electronic structure of the semiconductor and activates SERS performance as a result of promoted charge‐transfer efficiency and charge‐transfer pathways.
•A total of 510 ester compounds have been identified in traditional fermented baijiu.•Nine ethyl esters make important contributions to the baijiu flavor.•In the fermentation process of strong-aroma ...baijiu, 508 species have been identified.•Microbial fatty acid and amino acid metabolism contribute greatly to ester synthesis.•The metabolic processes of a minimum set of microbes help to stabilize baijiu flavor.
Chinese traditional fermented baijiu is a famous alcoholic beverage with unique flavor. Despite its consumption for millennia, the flavor mystery behind baijiu is still unclear. Studies indicate that esters are the most important flavor substances, and bring health benefits. However, the aroma contribution and formation mechanism of esters still need to be clarified to reveal the flavor profile of baijiu. This review systematically summarizes all the 510 esters and finds 9 ethyl esters contribute greatly to the flavor of baijiu. The 508 different microbial species that have been identified affect the synthesis of esters through fatty acid and amino acid metabolism. The determination of minimum functional microbial groups and the analysis of their metabolic characteristics are crucial to reveal the mechanism of formation of baijiu flavor, and ensure the reproducible formation of flavor substances.
Tungsten oxide is a versatile transition‐metal oxide with a vast number of polymorphs and sub‐stoichiometric compositions, featuring innate tunnels and oxygen vacancies. The structure‐determined ...nature, such as altered optical absorption and metal‐like conductivity, makes tungsten oxide an attractive candidate for optoelectronic applications. A brief summary of the recent progress in tungsten oxide for optoelectronic applications is provided, including not only the traditional field of electrochromism/photochromism, but also new areas of application, such as visible‐light‐driven photocatalysis, photothermal therapy, and surface enhanced Raman spectroscopy (SERS). Also, the prospects for future applications of tungsten oxide are summarized and highlighted.
Tungsten oxide possesses unique tunnel structures and sub‐stoichiometric compositions, which endow it with advanced optoelectronic properties. Current challenges and future research directions for optoelectronic applications of tungsten oxide materials are discussed, covering traditional chromogenic applications, together with a focus on the latest applications in visible‐light‐driven photocatalysis, surface‐enhanced Raman scattering, and photothermal therapy.
Semiconductor-based surface enhanced Raman spectroscopy (SERS) platforms take advantage of the multifaceted tunability of semiconductor materials to realize specialized sensing demands in a wide ...range of applications. However, until quite recently, semiconductor-based SERS materials have generally exhibited low activity compared to conventional noble metal substrates, with enhancement factors (EF) typically reaching 10
3
, confining the study of semiconductor-based SERS to purely academic settings. In recent years, defect engineering has been proposed to effectively improve the SERS activity of semiconductor materials. Defective semiconductors can now achieve noble-metal-comparable SERS enhancement and exceedingly low, nano-molar detection concentrations towards certain molecules. The reason for such success is that defect engineering effectively harnesses the complex enhancement mechanisms behind the SERS phenomenon by purposefully tailoring many physicochemical parameters of semiconductors. In this perspective, we introduce the main defect engineering approaches used in SERS-activation, and discuss in depth the electromagnetic and chemical enhancement mechanisms (EM and CM, respectively) that are influenced by these defect engineering methods. We also introduce the applications that have been reported for defective semiconductor-based SERS platforms. With this perspective we aim to meet the imperative demand for a summary on the recent developments of SERS material design based on defect engineering of semiconductors, and highlight the attractive research and application prospects for semiconductor-based SERS.
Defect engineering strategies are used to boost the SERS activity of a wide variety of semiconductors including metal oxides, nitrides, carbon materials and transition metal dichalcogenides (TMDs), as discussed in this perspective.
Intercalation-based inorganic materials that change their colours upon ion insertion/extraction lay an important foundation for existing electrochromic technology. However, using only such inorganic ...electrochromic materials, it is very difficult to achieve the utmost goal of full-colour tunability for future electrochromic technology mainly due to the absence of structural flexibility. Herein, we demonstrate an ultracompact asymmetric Fabry-Perot (F-P) nanocavity-type electrochromic device formed by using partially reflective metal tungsten as the current collector and reflector layer simultaneously; this approach enables fairly close matching of the reflections at both interfaces of the WO
thin layer in device form, inducing a strong interference. Such an interference-enhanced device that is optically manipulated at the nanoscale displays various structural colours before coloration and, further, can change to other colours including blue, red, and yellow by changing the optical indexes (n, k) of the tungsten oxide layer through ion insertion.
Electrochromic technology has been actively researched for displays, adjustable mirrors, smart windows, and other cutting-edge applications. However, it has never been proposed to overcome the ...critical problems in the field of surface-enhanced Raman scattering (SERS). Herein, we demonstrate a generic electrochromic strategy for ensuring the reproducibility and renewability of SERS substrates, which are both scientifically and technically important due to the great need for quantitative analysis, standardized production and low cost in SERS. This color-changing strategy is based on a unique quantitative relationship between the SERS signal amplification and the coloration degree within a certain range, in which the SERS activity of the substrate can be effectively inferred by judging the degree of color change. Our results may provide a first step toward the rational design of electrochromic SERS substrates with a high sensitivity, reproducibility, and renewability.
Surface-enhanced Raman spectroscopy (SERS) represents a very powerful tool for the identification of molecular species, but unfortunately it has been essentially restricted to noble metal supports ...(Au, Ag and Cu). While the application of semiconductor materials as SERS substrate would enormously widen the range of uses for this technique, the detection sensitivity has been much inferior and the achievable SERS enhancement was rather limited, thereby greatly limiting the practical applications. Here we report the employment of non-stoichiometric tungsten oxide nanostructure, sea urchin-like W18O49 nanowire, as the substrate material, to magnify the substrate-analyte molecule interaction, leading to significant magnifications in Raman spectroscopic signature. The enrichment of surface oxygen vacancy could bring additional enhancements. The detection limit concentration was as low as 10(-7) M and the maximum enhancement factor was 3.4 × 10(5), in the rank of the highest sensitivity, to our best knowledge, among semiconducting materials, even comparable to noble metals without 'hot spots'.
Developing efficient but nonprecious bifunctional electrocatalysts for overall water splitting in basic media has been the subject of intensive research focus with the increasing demand for clean and ...regenerated energy. Herein, we report on the synthesis of a novel hierarchical hybrid electrode, NiFe-layered double hydroxide molecularly ultrathin sheets grown on NiCo2O4 nanowire arrays assembled from thin platelets with nickel foam as the scaffold support, in which the catalytic metal sites are more accessible and active and most importantly strong chemical coupling exists at the interface, enabling superior catalytic power toward both oxygen evolution reaction (OER) and additionally hydrogen evolution reaction (HER) in the same alkaline KOH electrolyte. The behavior ranks top-class compared with documented non-noble HER and OER electrocatalysts and even comparable to state-of-the-art noble-metal electrocatalysts, Pt and RuO2. When fabricated as an integrated alkaline water electrolyzer, the designed electrode can deliver a current density of 10 mA cm–2 at a fairly low cell voltage of 1.60 V, promising the material as efficient bifunctional catalysts toward whole cell water splitting.
Surface enhanced Raman scattering (SERS) is a fingerprint spectral technique whose performance is highly dependent on the physicochemical properties of the substrate materials. In addition to the ...traditional plasmonic metal substrates that feature prominent electromagnetic enhancements, boosted SERS activities have been reported recently for various categories of non-metal materials, including graphene, MXenes, transition-metal chalcogens/oxides, and conjugated organic molecules. Although the structural compositions of these semiconducting substrates vary, chemical enhancements induced by interfacial charge transfer are often the major contributors to the overall SERS behavior, which is distinct from that of the traditional SERS based on plasmonic metals. Regarding charge-transfer-induced SERS enhancements, this short review introduces the basic concepts underlying the SERS enhancements, the most recent semiconducting substrates that use novel manipulation strategies, and the extended applications of these versatile substrates.
•Surface-enhanced Raman scattering (SERS) is a fingerprint spectral technique highly dependent on the substrate materials.•Charge transfer transitions are commonly the major contributors to the boosted SERS activities in non-metal substrates.•Novel manipulation strategies and extended applications of the versatile substrates are illustrated on the basis of interfacial charge transfer.
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Semiconductor-based surface-enhanced Raman spectroscopy (SERS) substrates represent a new frontier in the field of SERS. However, the application of semiconductor materials as SERS substrates is ...still seriously impeded by their low SERS enhancement and inferior detection sensitivity, especially for non-metal-oxide semiconductor materials. Herein, we demonstrate a general oxygen incorporation-assisted strategy to magnify the semiconductor substrate-analyte molecule interaction, leading to significant increase in SERS enhancement for non-metal-oxide semiconductor materials. Oxygen incorporation in MoS
even with trace concentrations can not only increase enhancement factors by up to 100,000-fold compared with oxygen-unincorporated samples but also endow MoS
with low limit of detection below 10
M. Intriguingly, combined with the findings in previous studies, our present results indicate that both oxygen incorporation and extraction processes can result in SERS enhancement, probably due to the enhanced charge-transfer resonance as well as exciton resonance arising from the judicious control of oxygen admission in semiconductor substrate.
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