Sulfur Edge Research Articles

Interplay between Mesocrystals of CaTiO3 and Edge Sulfur Atom Enriched MoS2 on Reduced Graphene Oxide Nanosheets: Enhanced Photocatalytic Performance under Sunlight Irradiation

AbstractIn this work, an attempt has been made to strategically and systematically design novel and multifunctional nanocomposite photocatalysts by coupling mesocrystals of CaTiO3 with edge‐sulfur‐atoms‐enriched MoS2 and reduced graphene oxide (RGO) nanosheets. A remarkable enhancement in photocatalytic activity could be evidenced with an optimized content (20 %) of MoS2−RGO nanosheets coupled with CaTiO3 mesocrystals. This nanocomposite showed a 33‐fold enhanced photocatalytic hydrogen evolution in comparison to bare CaTiO3, with apparent quantum efficiencies of 5.4 %, 3.0 % and 17.7 % at 365, 420 and 600 nm monochromatic wavelengths. In addition, the excellent adsorptive degradation of different organic pollutants was also achieved with these photocatalysts, which revealed their multifunctional behavior. The enhanced photocatalytic performance can be accredited mainly to following factors: (i) Intimate contact between constituent materials and efficient charge transfer across the ternary heterojunction, which suppresses photogenerated charge recombination; (ii) The high surface area provides abundant sites for reactant adsorption and further reaction; (iii) Defect‐rich MoS2 with sulfur atoms on the exposed edges provide sticky sites for H+ ions and hence enhance the hydrogen generation. The design strategy employed in this work can be adopted to improve the properties and performance of other mesocrystals, which can lead to the fabrication of low‐cost and multifunctional catalysts for diverse applications.

Stabilizing sulfur vacancy defects by performing "click" chemistry of ultrafine palladium to trigger a high-efficiency hydrogen evolution of MoS2.

Defect engineering is widely applied in transition metal dichalcogenides to produce high-purity hydrogen. However, the instability of vacancy states on catalysis still remains a considerable challenge. Here, our first-principles calculations showed that, by optimizing the asymmetric S vacancy in the highly asymmetric 1T' crystal of layered bitransition metal dichalcogenides (Co-MoS2) in light of Pd modulation, the relative amount of metastable phase and the quantity of active sites in the structure can be reduced and increased, respectively, leading to a further boosted hydrogen evolution reaction (HER) activity toward layered bi-transition metal dichalcogenides. Thus, we then used a "click" chemistry strategy to make such a catalyst with engineered unsaturated sulfur edges via a strong coupling effect between ultrafine Pd ensembles and Co-MoS2 nanosheets. As expected, the Pd-modulated Co-MoS2 nanosheets exhibited a very low overpotential of 60 mV at 10 mA cm-2 with a small Tafel slope (56 mV dec-1) for the HER in 1.0 M PBS, comparable to those of commercial Pt/C. In addition, their high HER activity was retained in acidic and alkaline conditions. Both the theoretical and experimental results revealed that Pd ensembles can efficiently activate and stabilize the inert basal plane S sites during HER processes as a result of the formation of Pd-S in Co-MoS2. This work not only provides a deeper understanding of the correlation between defect sites and intrinsic HER catalytic properties for transition metal chalcogenide (TMD)-based catalysts, but also offers new insights into better designing earth-abundant HER catalysts displaying high efficiency and durability.

Stable and highly efficient MoS2/Si NWs hybrid heterostructure for photoelectrocatalytic hydrogen evolution reaction

We report, the fabrication of molybdenum disulphide (MoS2) wrapped silicon nanowires (Si NWs) for visible light driven water splitting applications. The morphological and elemental studies ensure the vertical alignment of Si NWs wrapped with 2D layered MoS2. The photoelectrocatalytic (PEC) results evidence the significant enhancement in performance of MoS2/Si NWs based hybrid photocathode with ~300 mV (under reversible hydrogen electrode (RHE)) anodic shift in onset potential as that of pristine Si NWs (+0.194 V vs. RHE), and the current density of −26.5 mA/cm2 was achieved at the applied bias of 0 V vs. RHE. Further, the electrochemical impedance studies ensure the interface resistance-free charge transfer between Si NWs and electrolyte via 2D MoS2 layer which provokes rapid hydrogen production. The wrapping of Si NWs with MoS2 protects the superlative photocathode from harsh acid electrolyte environment. The overgrown MoS2 triangular particles with active sulphur edge sites are found to eventually augment the solar hydrogen evolution rate. Further, the PEC performance of our MoS2/Si NWs is also comparable with stable Pt/Si NWs photoelectrode. It is note-worthy that, MoS2/Si NWs hybrid heterostructure would be a potential candidate in future large scale, low cost and day-to-day solar water splitting applications.

MoS2-enhanced epoxy-based plasmonic fiber-optic sensor for selective and sensitive detection of methanol

Plasmonic nanostructures have found widespread applications in nanophotonics, photochemistry and optoelectronics, and chemical sensing is one of their most vibrant applications. However, it still remains open to achieve selective detections with plasmonic nanostructures, especially, at room temperature. Polymers have been attractive as vapor sensitive materials as their electrical and optical properties vary with exposure to vapors at room temperature. MoS2, as an emerging 2D material, has facilitated chemical sensing due to its stability, ultrahigh surface-to-volume ratio, and various attractive sites for analyte molecule adsorptions, including sulfur defect, vacancy, and edge sites. The combination of both MoS2 and polymers with plasmonic has great potential to enable high performance gas sensing at room temperature. Here, a selective detection towards methanol vapors is demonstrated using an epoxy as a sensitive polymer material to bond methanol molecules via both hydrogen atoms and the lone electron pairs on the hydroxyl group. Using a MoS2 monolayer coating, improved sensing response and selectivity towards small-size vapors, especially oxygen-functionalized methanol, are observed because of selective filtration effect. In addition, this plasmonic sensor is insensitive to relative humidity variation in the range of 11 %RH to 92 %RH as well as stable over a long-term period of one year, which demonstrates the high potential of the sensor for practical applications. These findings pave the way for a generic strategy to design selective and sensitive gas sensors via the combination of selectivity-enabling polymer, sensitivity-enhancing 2D materials and signal-transducing plasmonic nanostructures.

Ultra-small amorphous MoS2 decorated reduced graphene oxide for supercapacitor application

Amorphous materials have recently gained much attention as electrode materials in supercapacitor application due to the presence of larger amount active sites which can efficiently increase the storage capacity of the materials. Nano engineering is an elegant approach to fully utilize the advantages of the amorphous structure. Moreover, large surface area and high conductivity of reduced graphene oxide (RGO) can efficiently increase the storage capacity of the system. Exploiting this idea, in the present work, we have successfully synthesized amorphous MoS2 of two different sizes on reduced graphene oxide and thoroughly investigated the supercapacitor behavior of the system. The specific capacitance of the composite structures has been found to be largely increased with decreasing size of the amorphous nano particle. The specific capacitance of amorphous MoS2-RGO composite containing nearly 50 nm of MoS2 found to be 270 F/g whereas when the particle size is reduced to 5–7 nm, value of specific capacitance increases to 460 F/g. The large increase in specific capacitance with the tuning of the size of amorphous nano particle has been explained by the presence of a large number of active sulfur edges of ultra-small MoS2 nano structure along with the better charge transport which can effectively increase the storage capacity of the overall system. The retention in the capacitance of the material has been found to be 90 % after 5000 cycles.

Highly Efficient and Stable Photoelectrochemical Hydrogen Evolution with 2D-NbS2/Si Nanowire Heterojunction.

In recent days, 2-dimensional (2D) niobium disulfide (NbS2) with near-zero Gibbs free energy and superlative acid electrolyte stability has provoked a great deal of interest toward hydrogen evolution reaction (HER) electrocatalyst due to its active basal and edge sulfur sites. Herein, we developed a single step method for the direct deposition of 2D-NbS2 on high-aspect-ratio topographies of silicon nanowires (NWs) by chemical vapor deposition for the applications in HER electrocatalyst. The resultant 2D-NbS2 electrocatalyst demonstrates the HER overpotential of ∼74 mV vs RHE (reversible hydrogen electrode) @ 1 mA/cm2 under acidic conditions and stable for more than 20 h. More importantly, we developed the Si NWs array based photoelectrochemical water-splitting system with the direct deposition of 2D-NbS2 as HER catalyst. The resultant 2D-NbS2-Si NWs photocathode system demonstrates improved charge transfer characteristics at the Si-NbS2 interfaces that leads to an enhanced turn on potential (from 0.06 to 0.34 V vs RHE) with the current density of -28 mA/cm2 at the 0 V vs RHE. The results evidence the synergistic effect of 2D-NbS2 electrocatalysts that addresses poor surface kinetics of Si toward solar water electrolysis. Our comprehensive studies reveal NbS2 as a new class of photoelectrochemical cocatalyst for efficient solar HER performance by promoting the charge transfer process with prolonged acid stability.

Key role of antibonding electron transfer in bonding on solid surfaces

The description of the chemical bond between a solid surface and an atom or a molecule is the fundamental basis for understanding a broad range of scientific problems in heterogeneous catalysis, semiconductor device fabrication, and fuel cells. Widespread understandings are based on the molecular orbital theory and focused on the degree of filling of antibonding surface-adsorbate states that weaken bonding on surfaces. The unoccupied antibonding surface-adsorbate states are often tacitly assumed to be irrelevant. Here, we show that most antibonding states become unoccupied because the electrons that would occupy these antibonding states are transferred to the lower-energy Fermi level. Such antibonding electron transfer goes beyond molecular orbital theory. It leads to an energy gain that largely controls the trends of surface adsorption strength and can serve as a primary descriptor for bonding on surfaces. This finding is illustrated from the first-principles study of hydrogen adsorption on MoS$_2$ surfaces. A clear linear relationship between the energies of antibonding electron transfer and hydrogen adsorption is identified. The hydrogen evolution reaction on MoS$_2$ is found to originate from the in-gap states induced by sulfur vacancies or edges. The effects of surface inhomogeneity (e.g., defects, step edges) on surface catalysis can be understood from the corresponding different energies gained from antibonding electron transfer. The emerging picture also offers a physically different explanation for the well-known $d$-band theory for hydrogen adsorption on transition metal surfaces.

Microbial community structures and sulfur speciation characteristics in soil sample around the Xiang-tan Liejiaqiao coal gangue dump, Hunan Province in South of China

ABSTRACT The microbial community structures, sulfur speciation, and their correlations in soil samples in and around Xiangtan Liejiaqiao Coal Gangue Dump in Hunan province south of China were analyzed with gene sequencing, sulfur K-edge X-ray absorption near edge structure spectroscopy (XANES), and principal components analysis (PCA). The gene sequencing results showed that the microbes in soil samples fell into six phyla, i.e., Nitrospira, Actinobacteria, Firmicutes, Proteobacteria, Deferribacteres, and Ktedonobacteria. The dominant bacteria species in the four samples were Moorella glycerini, Escherichia coli, Leptospirillum ferriphilum, Ferrimicrobium acidiphilum, Sulfobacillus benefaciens, and Desulfomonile tiedjei. According to the XANES results, sulfur speciation in soil samples was composed of sulfate, sulfite, thiosulfate, and organic sulfur. The PCA analysis among physicochemical characteristics, sulfur speciation, and distributions of microbes showed that oxidation-reduction potential (ORP) was correlated to the bacterial population of Proteobacteria, while pH was correlated to Ktedonobacteria. Furthermore, the correlation between sulfur speciation (sulfate and thiosulfate) and microbes (Deferribacteres, Nitrospira, and Actinobacteria) was particularly significant. Sulfite and glutathione also presented a strong correlation with Ktedonobacteria and Proteobacteria.

Basal plane oxygen exchange of epitaxial MoS2 without edge oxidation

The intentional formation of defects in transition-metal dichalcogenides, such as MoS2, is an attractive way to modify the electronic and chemical properties of this class of 2D materials. However, the mechanisms and methods available for selective doping or modification of the basal plane must be improved. Here we investigate the process of O defect formation in epitaxial single-layer MoS2 on Au(1 1 1) using scanning tunneling microscopy (STM) and ambient pressure x-ray photoelectron spectroscopy (AP-XPS) during oxidation with O2 and H2O gas from low vacuum to the mbar range. Both oxidants result in exchange of S in the upper part of the basal plane with O, in line with air exposure experiments. Temperature-dependent measurements show that this is an activated process with an experimentally estimated reaction barrier of ~0.79 ± 0.20 eV. We surprisingly find that the morphology of the MoS2 flakes and their edges remain intact in O2, even for relatively high degrees of basal plane O exchange, in contrast to the oxidation behavior of exfoliated single-layer MoS2. From analysis of atom-resolved STM images of the MoS2 edges, we can attribute this unusual stability to a passivating effect of excess edge sulfur species adsorbed under the sulfiding conditions of the MoS2 synthesis in H2S gas. We thus demonstrate that control over pre-sulfidation of the edges, temperature and pressure during oxidation can be used in a fast process to form strongly O doped single-layer MoS2 with no degradation of the initial shape and edge structure of the epitaxial MoS2 sheet.

Deriving MoS2 nanoribbons from their flakes by chemical vapor deposition

Two-dimensional (2D) materials have attracted great interest due to their unique structures and exotic properties related to promising applications and fundamental research. Reducing the dimensionality of 2D materials into their 1D nanostructure is also highly desirable for the exploitation of novel properties and offers new research opportunities. In this work, we demonstrate a bottom-up synthesis of molybdenum disulfides (MoS2) nanoribbons on graphene substrate via chemical vapor deposition (CVD) by precisely tuning the growth parameters into a sulfur-enriched condition. MoS2 nanoribbons are mainly formed from the CVD grown MoS2 flakes along the armchair (AC) direction. Atomic resolution ADF-STEM imaging characterizations show an alternating presence of molybdenum and sulfur zigzag edge terminations at the edges of MoS2 nanoribbons. While at the apex of the nanoribbon, sulfur terminated zigzag edges become dominant. Taking these results together, we revealed the underlying growth mechanism of MoS2 nanoribbons. Electronic transport properties of the MoS2 nanoribbons were also measured by fabricating back-gate-effect transistors (FETs). The nanoribbon FETs present n-type behavior with a current on/off ratio higher than 104 at VDS = 1 and a carrier mobility of 1.39 cm2 V−1 s−1. This work offers a new route to synthesize 1D MoS2 nanoribbons, which has great potential in fabricating other 2D materials-derived 1D nanostructures.

Electrocatalysts: Unsaturated Sulfur Edge Engineering of Strongly Coupled MoS2 Nanosheet–Carbon Macroporous Hybrid Catalyst for Enhanced Hydrogen Generation (Adv. Energy Mater. 2/2019)

Electrocatalysts: Unsaturated Sulfur Edge Engineering of Strongly Coupled MoS₂ Nanosheet–Carbon Macroporous Hybrid Catalyst for Enhanced Hydrogen Generation (Adv. Energy Mater. 2/2019)

Sulfur vacancy formation at different MoS2 edges during hydrodesulfurization process: A DFT study

A periodic density functional theory study of the sulfur vacancy formation mechanism on different edges of MoS2 nanocrystallites was investigated from thermodynamics and kinetics points of views. Thermodynamics study shows that the generations of vacancies on the Mo edge and basal plane were found severely inhibited by the thermodynamic limitations. Whilst the formation on the S edge can be proceeded spontaneously. Homolysis dissociation of H2 molecule was found more favorable than heterolytic dissociation on sulfur edge. The process of dissociated H atoms to form absorbed H2S was proved to be the rate controlling step. By comparing the distribution of charges and FuKui function before and after the vacancy formation, the contribution of sulfur vacancy to electrophilicity of Mo was clear, sulfur vacancies formed at the edges of Mo and S is favorable to the following hydrodesulfurization (HDS) reactions, whereas, the generation of sulfur vacancies on the basal plane inhibits the chemical reaction. The findings are critical to the development of highly active HDS catalysts.

Unsaturated Sulfur Edge Engineering of Strongly Coupled MoS2 Nanosheet–Carbon Macroporous Hybrid Catalyst for Enhanced Hydrogen Generation

AbstractThe low hydrogen adsorption free energy and strong acid/alkaline resistance of layered MoS2 render it an excellent pH‐universal electrocatalyst for hydrogen evolution reaction (HER). However, the catalytic activity is dominantly suppressed by its limited active‐edge‐site density. Herein, a new strategy is reported for making a class of strongly coupled MoS2 nanosheet–carbon macroporous hybrid catalysts with engineered unsaturated sulfur edges for boosting HER catalysis by controlling the precursor decomposition and subsequent sodiation/desodiation. Both surface chemical state analysis and first‐principles calculations verify that the resultant catalysts exhibit a desirable valence‐electron state with high exposure of unsaturated sulfur edges and an optimized hydrogen adsorption free energy, significantly improving the intrinsic HER catalytic activity. Such an electrocatalyst exhibits superior and stable catalytic activity toward HER with small overpotentials of 136 mV in 0.5 m H2SO4 and 155 mV in 1 m KOH at 10 mA cm−2, which is the best report for MoS2–C hybrid electrocatalysts to date. This work paves a new avenue to improve the intrinsic catalytic activity of 2D materials for hydrogen generation.

Strain-induced effects in zigzag-edged blue phosphorene nanoribbons with edge sulfur passivation

Motivated by the recent successful growth of quasi-free-standing single layer blue phosphorus (BlueP) on tellurium functionalized Au(1 1 1) by using black phosphorus as the precursor (Zhang et al 2016 Nano Lett. 16 4903; Gu et al 2017 ACS Nano 11 4943), here we study the strain performance for zigzag BlueP nanoribbons (zBluePNRs) with different edge chemical modifications. Based on the first-principles calculations, interestingly, we find that a narrow zBluePNR of width up to ∼2 nm with edge sulfur (S) passivation under an uniaxial strain exhibits an important effect of negative Poisson’s ratio because of the particularity of the P–S bonds at edges, and as the ribbon width further increases the Poisson’s ratio varies from negative to positive. Meanwhile, the S-passivation also induces magnetism, and applying a strong enough strain can transfer this zBluePNR from magnetic metal state to half-metal one. All these physical properties are discussed in connection with the variation of microstructure of the ribbons. Our findings may paving the way for both a fundamental understanding of nanoscale physics and applied device engineering.

Electrocatalytic Properties of MoS2 In-situ Grown on Graphene Oxide as Counter Electrode for Dye-sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have attracted great attention as one of next-generation photovoltaic devices due to low cost and simple fabrication process. A conventional DSSC consists of TiO2 photoelectrode, counter electrode (CE) and liquid electrolytes containing iodide/triiodide (I/I3 -) redox couple sandwiched between the electrodes. The CE serves to collect electrons and catalyze redox couple regeneration; thus, its material and performance determine the cost and photoelectric conversion efficiency of DSSC. Generally, noble Pt is employed as a CE owing to its rapid electron transfer ability and superior electrocatalytic behavior toward I3 - species. However, Pt is expensive and limited in resource. Hence, it is vital to find inexpensive and readily available non-Pt catalysts for the development of DSSCs. Carbon materials, conductive polymers, transition-metal compounds, metal alloys, and their composites have been explored to replace Pt. Among these alternatives, transition-metal compounds including carbides, nitrides, chalcogenides and oxides have demonstrated some desirable performances. MoS2 consists of three atom layers: one Mo layer sandwiched between two S layers via weak Vander Waals interactions. Advantageously, MoS2 has abundant exposed active sulfur edges for catalytic activity. On the other hand, graphitic carbon nitride (g-C3N4) demonstrates excellent thermal and chemical stability as well as interesting electric properties, which make it have good catalytic activity for a variety of reactions. In addition, g-C3N4 can be economically prepared in large scale. Herein, we present the fabrication and characterization of porous g-C3N4 nanosheet/MoS2 nanoparticle (g-C3N4/MoS2) composite. Such an architecture can provide short diffusion distance for excellent charge transport as well as large contact area for fast interfacial charge separation and electrochemical reactions. As a result, the nanocomposite is anticipated to exhibit good electrochemical and catalytic activities. In this study, we first developed a solution-based coating method to prepare g-C3N4/MoS2 counter electrode. Through a facile hydrophilization process for g-C3N4 and sintering the Mo-S precursor coated g-C3N4 at 280 °C for 12 h, crystalline MoS2 nanoparticles were formed on g-C3N4 substrate, resulting in strong mechanical adhesion. In addition, low-cost solution-processed g-C3N4/MoS2 electrode had similar electrocatalytic behavior toward I3 - species to Pt. As a result, the DSSC based on g-C3N4/MoS2 demonstrates desirable photovoltaic performance.

Effect of in-plane size of MoS2 nanoparticles grown over multilayer graphene on the electrochemical performance of anodes in Li-ion batteries

Electrochemical performance of MoS2/graphene materials in Li-ion batteries is strongly dependent on the structure of individual components and their coupling in the hybrid. We present a comparative study of the materials produced by annealing of amorphous MoS3 deposited on the surface of multilayer graphene flakes at 500 °C, 800 °C, and 1000 °C in vacuum. X-ray photoelectron spectroscopy confirmed a transformation of MoS3 to MoS2 at these conditions. High-resolution transmission electron microscopy and Raman scattering showed a growth of in-plane size of MoS2 nanocrystals with a raise of annealing temperature. Electrochemical tests detected a gradual decrease of the specific capacity of the MoS2/graphene materials prepared at 500 and 800 °C and a stable performance for the material synthesized at 1000 °C even at high current densities. Based on the initial discharge-charge profiles, we associate this effect with in-plane size of MoS2 nanocrystals, which should decompose more easily when the size is small, due to the interaction of lithium with edge sulfur atoms.

MoS2 Quantum Dots: Effect of Hydrogenation on Surface Stability and H2S Release

We employ density functional theory to investigate effects of hydrogenation on the energetic stability and electronic properties of triangular MoS2 nanoclusters with S-edges. Excess edge sulfur atoms relative to the bulk stoichiometry along the edges are passivated by hydrogen atoms. We find that the hydrogen coverage for maximum stability can be calculated by (n – 2)/2(n – 1), where n is the number of S atoms along an edge. The energetics reveal a preference for the zigzag arrangement of adsorbed hydrogen atoms on the edges. Our calculations show vanishing HOMO–LUMO gaps mainly due to the presence of dangling bonds at the edges and can be considered metal-like. We find that the activation energy required to release H2S lies in between 0.47 and 0.62 eV, and this value is in good agreement with the recently reported experimental value.

Annealing effect on the ferromagnetism of MoS2 nanoparticles

Novel properties of two-dimensional (2D) materials have drawn extensive attention in past decade. Magnetic property is one of the research focuses since it can add extra spin freedom for the manipulation of 2D material properties. The magnetism in 2D materials can be originated from defects (i.e. vacancies, zigzag defects). In this work, MoS2 nano particles were annealed at various temperatures (from 600 till 1000 °C) in an argon environment to explore the post-annealing effect on the magnetic properties. The results indicate that annealing may induce tunable ferromagnetism associated with the annealing temperature. The ferromagnetic signal is enhanced with the increase in annealing temperature which may attribute to the defects pertaining to the sulfur vacancies and edge terminated structure induced by annealing.

Cobalt disulfide nanosphere dispersed on multi-walled carbon nanotubes: an efficient and stable electrocatalyst for hydrogen evolution reaction

Novel cobalt disulfide on multi-walled carbon nanotubes (CoS2/MWCNTs) was synthesized via a facile one-step hydrothermal method in the presence of cetyltrimethyl ammonium bromide. The physical properties of as-prepared materials were characterized by Fourier transform infrared spectrum, X-ray diffraction, Raman spectrum, and scanning electron microscopy techniques. Physical characterizations revealed that cattierite CoS2 nanospheres dispersed on the surface of MWCNTs uniformly. In addition, electrochemical performances of as-prepared materials for hydrogen evolution reaction were investigated by polarization curves, Tafel plots, and electrochemical impedance spectrum in 0.50 M H2SO4 electrolyte. It was demonstrated that MWCNT-based electrode exhibited almost no current response while CoS2/MWCNT nanocomposite-based electrode exhibited better electrochemical performances than pure CoS2-based electrode, including lower potential of − 257 mV for 10 mA cm−2 and smaller Tafel slope of 83 mV dec−1. Furthermore, CoS2/MWCNT nanocomposite retained its high activity even after 1000 cycles of cyclic voltammetry scans, demonstrating superior stability under acidic condition. The enhanced electrocatalytic activity of CoS2/MWCNT nanocomposite-based electrode was ascribed to more exposed sulfur edges of CoS2, larger accessible surface area, and higher conductivity derived from MWCNTs. The results suggested that CoS2/MWCNT nanocomposite had a potential application to hydrogen evolution reaction.

Vanadium, sulfur, and iron valences in melt inclusions as a window into magmatic processes: A case study at Nyamuragira volcano, Africa

This study describes microscale sulfur (S), vanadium (V), and iron (Fe) K-edge X-ray absorption near edge structure (µ-XANES) spectroscopy measurements on olivine-hosted melt inclusions (MI) preserved in tephras (1986 and 2006) and lavas (1938 and 1948) erupted from Nyamuragira volcano (D.R. Congo, Africa). The S, V, and Fe spectroscopic data are used to constrain the evolution of oxygen fugacity (fO2) and sulfur speciation for the entrapped melts. Melt inclusions from lavas show evidence of post-entrapment crystallization and were thus reheated prior to µ-XANES analysis. The MI from tephra show no evidence of post-entrapment crystallization and were, therefore, not reheated. Sulfur, V, and Fe µ-XANES results from 1938, 1948, and 2006 eruptive materials are all similar within analytical uncertainty and provide similar average calculated melt fO2’s based on XANES oxybarometry. However, olivine-hosted MI from the 1986 tephras yield significantly different S, V, and Fe XANES spectra when compared to MI from the other eruptions, with disagreement between calculated fO2’s from the three valence state oxybarometers beyond the uncertainty of the calibration models. Their V µ-XANES spectra are also significantly more ordered and yield more reduced average V valence. The S µ-XANES spectra display a significantly more intense low-energy spectral resonance, which indicates differences in Fe-S bonding character, and greater variability in their measured sulfate content. These V and S spectroscopic features are best explained by crystallization of sub-micrometer magnetite and sulfide crystallites within the 1986 inclusions. The sensitivity of XANES spectroscopy to short-range order allows these crystallites to be recognized even though they are not easily detected by imaging analysis. This shows that V and S µ-XANES are potentially highly sensitive tools for identifying the presence of volumetrically minor amounts of spinel and sulfide within inclusions extracted from rapidly-cooled samples of tephra. Additionally, the observation that rehomogenized 1938 and 1948 inclusions from lava yield similar S, V, and Fe XANES spectra to the 2006 inclusions from tephra may be an encouraging indication that rehomogenization appears to have enabled the successful recovery of their pre-eruptive fO2, despite their complex post-eruptive histories.