Raman spectroscopy-in situ characterization of reversibly intercalated oxygen vacancies in α-MoO3

Nanorod structure tuning and defect engineering of MoOx for high-performance SERS substrates

In recent years, surface-enhanced Raman scattering (SERS) based on metal oxide semiconductors has been an active area of research and development, attracting significant scientific interest. These SERS substrates are known as plasmon-free SERS substrates because they are not based on noble metal nanoparticles but mainly on the defects, structure, and surface morphology of semiconductors to enhance the Raman signal. In this study, we fabricated a SERS substrate based on molybdenum oxide, using reactive DC magnetron sputtering and then used different simple and effective strategies to enhance the Raman signal. The results show that nanorod structure, oxygen deficiency engineering, phase engineering, and optical properties can be easily controlled by varying sputtering time and annealing time of MoOx SERS substrates. The analysis methods XRD, PL, and Raman show that with the optimal fabricated conditions. The presence of oxygen defects and a mixed MoO3, Mo9O26 phase structure in as well as the nanorod structure of MoOx SERS substrates could likely enhance Raman signals via a chemical mechanism (CM) and electromagnetic mechanism (EM). The MoOx SERS substrates were also used to detect R6G at low concentrations, with an EF of 1.14 × 106 (at 0.01 ppm), LOD of 0.01 ppm, and good temporal stability and reproducibility.

Tailoring porous NiMoO4 nanotube via MoO3 nanorod precursor for environmental monitoring: Electrochemical detection of micro-sized polyvinylchloride

Globally, the hidden contaminants like microplastics (MPs) combined with other harmful substances have agglomerated in rivers and oceans that pose a threat to human health. Thus, evaluating the toxicity of MPs separately and in combination with other pollutants must be done quickly and precisely. This work reports the synthesis of porous NiMoO4 nanotubes (NTs) from the transformation of MoO3 nanorods (NRs) via two steps hydrothermal methods for the effective detection of polyvinyl chloride (PVC) MPs. Transformation of MoO3 NRs to porous NiMoO4 NTs was comprehensively deduced by evaluating the crystalline, structural, compositional and morphological properties. The hydrophobic nature of MoO3 NRs and porous NiMoO4 NTs was proven experimentally and also by DFT calculations. The electrochemical detection of PVC MPs by NiMoO4 NTs was investigated by the CV and EIS measurements. Porous NiMoO4 NTs based electrode expressed the good detection towards PVC MPs with a reasonable sensitivity of ∼1.43 × 10-4 μA/ppm.cm2, a low LOD of ∼18 ppm and R2 = ∼0.9781. EIS results revealed that porous NiMoO4 NTs electrode enabled to deliver sensing response at very low concentration of PVC MPs. Due to their easy interaction with hydrophobic PVC MPs, the hydrophobic NiMoO4 NTs controlled the sensing nature of the material and improved the electrochemical detection at the MP-NiMiO4 NTs interface.

Plasmon-Free Surface-Enhanced Raman Spectroscopy Using α-Type MoO3 Semiconductor Nanorods with Strong Light Scattering in the Visible Regime

Recent developments in semiconductor-based surface-enhanced Raman scattering (SERS) have achieved numerous advancements, primarily centered on the chemical mechanism. However, the role of the electromagnetic (electromagnetic mechanism) contribution in advancing semiconductor SERS substrates is still underexplored. In this study, we developed a SERS substrate based on densely aligned α-type MoO3 (α-MoO3) semiconductor nanorods (NRs) with rectangular parallelepiped ribbon shapes with width measuring several hundred nanometers. These structural attributes strongly affect light transport in the visible range by multiple light scattering generated in narrow gaps between NRs, contributing to the improvement of SERS performance. Engineering the nanostructure and chemical composition of NRs realized high SERS sensitivity with an enhancement factor of 2 × 108 and a low detection limit of 5 × 10-9 M for rhodamine 6G (R6G) molecules, which was achieved by the stoichiometric NR sample with strong light scattering. Furthermore, it was observed that the scattering length becomes significantly shorter compared with the excitation wavelength in the visible regime, which indicates that light transport is strongly modified by mesoscopic interference related to Anderson localization. Additionally, high electric fields were found to be localized on the NR surfaces, depending on the excitation wavelength, similar to the SERS response. These optical phenomena indicate that electromagnetic excitation processes play an important role in plasmon-free SERS platforms based on α-MoO3 NRs. We postulate that our study provides important guidance for designing effective EM-based SERS-active semiconductor substrates.

Assessing Challenges of 2D-Molybdenum Ditelluride for Efficient Hydrogen Generation in a Full-Scale Proton Exchange Membrane (PEM) Water Electrolyzer

Proton exchange membrane (PEM) water electrolyzers are critical enablers for sustainable green hydrogen production due to their high efficiency. However, nonplatinum catalysts are rarely evaluated under actual electrolyzer operating conditions, limiting knowledge of their feasibility for H2 production at scale. In this work, metallic 1T'-MoTe2 films were synthesized on carbon cloth supports via chemical vapor deposition and tested as cathodes in PEM electrolysis. Initial three-electrode tests revealed that at 100 mA cm-2, the overpotential of 1T'-MoTe2 approached that of leading 1T'-MoS2 systems, confirming its promise as a hydrogen evolution catalyst. However, when tested in a full-scale PEM electrolyzer, 1T'-MoTe2 delivered only 150 mA cm-2 at 2 V, far below expectations. Postelectrolysis analysis revealed an unexpected passivating tellurium layer, likely inhibiting catalytic sites. While initially promising, the unanticipated passivation caused 1T'-MoTe2 to underperform in practice. This highlights the critical need to evaluate emerging electrolyzer catalysts in PEM electrolyzers, revealing limitations of the idealized three-electrode configuration. Moving forward, validation of model systems in actual electrolyzers will be key to identifying robust nonplatinum catalysts for sustainable green hydrogen production.

Fabrication of a Novel Electrochemical Sensor Based on Carbon Cloth Matrix Functionalized with MoO3 and 2D-MoS2 Layers for Riboflavin Determination

The preparation and characterization of a hybrid composite, based on carbon cloth (CC) matrix functionalized with two-dimensional (2D) MoS2 flakes and MoO3, and its use for developing an electrochemical sensor for the determination of riboflavin (RF) is here reported. The 2D-MoS2-MoO3CC composite was prepared by depositing 2D-MoS2 nanosheets, obtained by liquid phase exfoliation (LPE), on the surface of a carbon cloth fiber network, previously functionalized with a layer of molybdenum oxide (α-MoO3) by radio-frequency magnetron reactive sputtering technique. The 2D-MoS2-MoO3CC composite was characterized by scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDX), and Raman spectroscopy. An electrochemical sensor has been then fabricated by fixing a slice of the 2D-MoS2-MoO3CC composite on the working electrode of a screen-printed carbon electrode (SPCE). The 2D-MoS2-MoO3-CC/SPCE sensor display good electrochemical characteristics which have been exploited, for the first time, in the electroanalytical determination of riboflavin (RF). The sensitivity to RF, equal to 0.67 µA mM-1 in the linear range from 2 to 40 µM, and a limit of detection (LOD) of 1.5 µM at S/N = 3, demonstrate the promising characteristics of the proposed 2D-MoS2-MoO3-CC/SPCE electrochemical sensor for the determination of riboflavin.