Resolution Fluorescence Detected X-ray Absorption Research Articles

Operando double-edge high-resolution X-ray absorption spectroscopy study of BiVO4 photoanodes.

High energy resolution fluorescence detected X-ray absorption spectroscopy is a powerful method for probing the electronic structure of functional materials. The X-ray penetration depth and photon-in/photon-out nature of the method allow operando experiments to be performed, in particular in electrochemical cells. Here, operando high-resolution X-ray absorption measurements of a BiVO4 photoanode are reported, simultaneously probing the local electronic states of both cations. Small but significant variations of the spectral lineshapes induced by the applied potential were observed and an explanation in terms of the occupation of electronic states at or near the band edges is proposed.

In-Situ Observation of Adsorption Species on Platinum Catalyst in Oxygen Reduction Reaction Using High Energy Resolution XAFS

The oxygen reduction reaction (ORR) on Pt surfaces, for examples, Pt single crystals, polycrystalline Pt electrodes, Pt nanoparticles (NPs), Pt-based alloys, and core–shell Pt nanostructures, has been discussed in the last few decades.In polymer electrolyte fuel cells (PEFCs), hydrogen peroxide and its radicals are known to be fatal attacking species not only for electrode catalysts but also for carbon materials and polymer membranes.A thorough understanding of ORR schemes has led to the development of seamless four-electron reduction cathodes, contributing to improved endurance reliability of both anion-exchange membrane (AEM) and proton-exchange membrane (PEM) fuel cells.In this study, the reaction pathway, and intermediate species such as molecular oxygen, in the ORR were investigated in alkaline and acidic environments, respectively.In this work, High Energy Resolution Fluorescence Detection X-ray Absorption Spectroscopy (HERFD-XAS) at the Pt L III-edge, which enhances resolution by partially collecting fluorescent X-ray was employed.X-ray absorption fine structure (XAFS) can detect the electronic structure of Pt, and its high energy resolution makes it possible to distinguish between different adsorption species on the surface of Pt nanoparticles.In addition, photon-in photon-out experiments using hard X-ray with high penetrating ability make it relatively easy to perform in-situ/operando measurements in potential controlled solutions.Carbon-supported Pt nanoparticles as powder catalysts were used for measurement as more realistic conditions, and the species adsorbed on the Pt nanoparticles were analyzed while controlling under the potentials both in acidic and basic electrolyte solutions with N2 or O2 gas bubbling.In N2-saturated solutions, the XANES spectra of the Pt LIII-edge reflected the presence of the adsorption species such as Pt-O (*O), Pt-OH (*OH), Pt-H (*H), as well as PtO2 oxide and Pt metallic states without any adsorption, during the ORR with the potential change.In O2-saturated solutions, one additional adsorption species of molecular oxygen was observed in both acidic and basic electrolyte solutions. It was analyzed that the intermediate species adsorbed to platinum in a side-on configuration as a superoxide anion in an alkaline environment.This intermediate species is identified as hydrogen superoxide, also known as hydroperoxyl radical, with the chemical formula HO2 by being protonated in an acidic environment.The results of research on adsorption species using HERFD-XAS have provided important knowledge for designing catalysts that enable seamless four-electron reduction.The preferred cathode catalyst design will be discussed in detail in an oral presentation at the 244th ECS Meeting. Acknowledgments: The synchrotron radiation experiments were performed at the BL11XU in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Numbers of 2012B3506, 2013A3506, 2014A3506, 2014B3506, 2015B3506, 2016A3556 2019A3508, 2019B3508, 2020A3508, 2021A3508, 2021B3508, 2022A3508 and 2022B3597 at BL11XU). This work was supported by JSPS KAKENHI Grant Number JP22H02188). Figure 1

Synchrotron speciation of umbilical cord mercury and selenium after environmental exposure in Niigata

The insidious and deadly nature of mercury’s organometallic compounds is informed by two large scale poisonings due to industrial mercury pollution that occurred decades ago in Minamata and Niigata, Japan. The present study examined chemical speciation for both mercury and selenium in a historic umbilical cord sample from a child born to a mother who lived near the Agano River in Niigata. The mother had experienced mercury exposure leading to more than 50 ppm mercury measured in her hair and was symptomatic 9 years prior to the birth. We sought to determine the mercury and selenium speciation in the child’s cord using Hg Lα1 and Se Kα1 high-energy resolution fluorescence detected X-ray absorption spectroscopy, the chemical speciation of mercury was found to be predominantly organometallic and coordinated to a thiolate. The selenium was found to be primarily in an organic form and at levels higher than those of mercury, with no evidence of mercury-selenium chemical species. Our results are consistent with mercury exposure at Niigata being due to exposure to organometallic mercury species.

Nature of S-States in the Oxygen-Evolving Complex Resolved by High-Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy.

Photosystem II, the water splitting enzyme of photosynthesis, utilizes the energy of sunlight to drive the four-electron oxidation of water to dioxygen at the oxygen-evolving complex (OEC). The OEC harbors a Mn4CaO5 cluster that cycles through five oxidation states Si (i = 0-4). The S3 state is the last metastable state before the O2 evolution. Its electronic structure and nature of the S2 → S3 transition are key topics of persisting controversy. Most spectroscopic studies suggest that the S3 state consists of four Mn(IV) ions, compared to the Mn(III)Mn(IV)3 of the S2 state. However, recent crystallographic data have received conflicting interpretations, suggesting either metal- or ligand-based oxidation, the latter leading to an oxyl radical or a peroxo moiety in the S3 state. Herein, we utilize high-energy resolution fluorescence detected (HERFD) X-ray absorption spectroscopy to obtain a highly resolved description of the Mn K pre-edge region for all S-states, paying special attention to use chemically unperturbed S3 state samples. In combination with quantum chemical calculations, we achieve assignment of specific spectroscopic features to geometric and electronic structures for all S-states. These data are used to confidently discriminate between the various suggestions concerning the electronic structure and the nature of oxidation events in all observable catalytic intermediates of the OEC. Our results do not support the presence of either peroxo or oxyl in the active configuration of the S3 state. This establishes Mn-centered storage of oxidative equivalents in all observable catalytic transitions and constrains the onset of the O-O bond formation until after the final light-driven oxidation event.

Investigating Degradation Mechanisms in PtCo Alloy Catalysts: The Role of Co Content and a Pt-Rich Shell Using Operando High-Energy Resolution Fluorescence Detection X-ray Absorption Spectroscopy.

Low Pt-based alloy catalysts are regarded as an efficient strategy in achieving high activity for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, the desired durability for the low Pt-based catalysts, such as the Pt1Co3 catalyst, has still been considered a great challenge for PEMFCs. In this study, we investigate sub-2.5 nm PtxCoy alloy catalysts with varying Co content and Pt1Co3@Pt core-shell (CS) nanostructure catalysts obtained through a simple displacement reaction. The Pt1Co3@Pt_H catalysts showed a high mass activity (MA) of 1.46 A/mgPt at 0.9 V and 14% MA loss after 10k accelerated degradation test (ADT) cycles, which suggested the improved stability compared with Pt1Co3 catalysts (52% MA loss). To clarify the degradation mechanism, operando high-energy resolution fluorescence detection X-ray absorption spectroscopy (XAS) was applied in addition to conventional advanced measurement techniques, including operando conventional XAS, to analyze the electronic state and structure changes during operation potentials. We found that introducing Co improves the catalysts' activity mainly from the strain effect, but an excessive amount of Co leads to increased Pt-oxidation, which accelerates the degradation of the catalysts. The Pt1Co3@Pt_H catalyst shows high tolerance to Pt-oxidation, benefiting both the stability and activity. Our findings demonstrate an in-depth understanding of the degradation mechanism and the importance of designing PtCo CS nanostructures with optimal Co content for enhanced performance in PEMFCs.

High-Resolution X-ray Absorption and Emission Spectroscopy for Detailed Analysis of New CO2 Methanation Catalysts.

A new approach for the characterization of CO2 methanation catalysts prepared by thermal decomposition of a nickel MOF by hard X-ray photon-in/photon-out spectroscopy in form of high energy resolution fluorescence detected X-ray absorption near edge structure spectroscopy (HERFD-XANES) and valence-to-core X-ray emission (VtC-XES) is presented. In contrast to conventional X-ray absorption spectroscopy, the increased resolution of both methods allows a more precise phase determination of the final catalyst, which is influenced by the conditions during MOF decomposition.

Mapping the Initial Stages of a Protective Pathway that Enhances Catalytic Turnover by a Lytic Polysaccharide Monooxygenase.

Oxygenase and peroxygenase enzymes generate intermediates at their active sites which bring about the controlled functionalization of inert C-H bonds in substrates, such as in the enzymatic conversion of methane to methanol. To be viable catalysts, however, these enzymes must also prevent oxidative damage to essential active site residues, which can occur during both coupled and uncoupled turnover. Herein, we use a combination of stopped-flow spectroscopy, targeted mutagenesis, TD-DFT calculations, high-energy resolution fluorescence detection X-ray absorption spectroscopy, and electron paramagnetic resonance spectroscopy to study two transient intermediates that together form a protective pathway built into the active sites of copper-dependent lytic polysaccharide monooxygenases (LPMOs). First, a transient high-valent species is generated at the copper histidine brace active site following treatment of the LPMO with either hydrogen peroxide or peroxyacids in the absence of substrate. This intermediate, which we propose to be a CuII-(histidyl radical), then reacts with a nearby tyrosine residue in an intersystem-crossing reaction to give a ferromagnetically coupled (S = 1) CuII-tyrosyl radical pair, thereby restoring the histidine brace active site to its resting state and allowing it to re-enter the catalytic cycle through reduction. This process gives the enzyme the capacity to minimize damage to the active site histidine residues "on the fly" to increase the total turnover number prior to enzyme deactivation, highlighting how oxidative enzymes are evolved to protect themselves from deleterious side reactions during uncoupled turnover.

Comparing neutron and helium ion irradiation damage of REBa2Cu3O coated conductor using x-ray absorption spectroscopy

Understanding the effects of high energy neutron damage on REBa2Cu3O (REBCO) coated conductor is of vital importance for the design of the magnetic confinement systems for compact nuclear fusion power plants. However, neutron irradiation campaigns can only be carried out in a few facilities, and the experiments are very slow and expensive partly because the samples become radioactive. Ion irradiation provides an easily accessible alternative route to studying the effects of radiation on high temperature superconductors, which not only increases the volume of technical data that can be obtained but also enables more complex experiments such as in situ cryogenic irradiation. The question is, does ion damage offer a good proxy for neutrons? Here we use high energy resolution fluorescence detected x-ray absorption spectroscopy to probe the effects of fast neutron irradiation on the local environment around the copper ions in the REBCO layer of coated conductor tapes. We find that the spectral changes are similar to those induced by helium ion irradiation, suggesting that both projectiles produce the same types of structural defect in the REBCO lattice, although there is some evidence of an additional type of defect present in the sample heavily damaged by He+ ion irradiation. It is also shown that the linear degradation of superconducting transition temperature (T c ) of coated conductors with the calculated number of displacements per atom occurs at the same rate for neutrons and helium ions. Together these results provide new evidence suggesting that helium ions can emulate neutron point defect damage in REBCO high temperature superconductor reasonably well, increasing confidence that helium ions could be used as a useful proxy for neutrons in future experiments.

Unraveling the Dynamic Structural Evolution of Phthalocyanine Catalysts during CO2 Electroreduction

Understanding the atomic and electronic changes of active sites promotes the whole new sight into electrochemical carbon dioxide reduction reaction (CO2RR), which provides a feasible strategy to achieve carbon neutrality. Here we employ operando high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) to track the structural evolution of Ni(II) phthalocyanine (NiPc), considered as the model catalysts with uniform Ni-N4-C8 moiety, during the CO2RR. The HERFD-XAS method is in favor of elucidating the interaction of the reactant/catalyst interface from the atomic electronic structure dimension, facilitating the establishment of the catalytic mechanism and the dynamic structure changes. Based on operando measurement, surface sensitive difference spectra (Δµ) and spectroscopy simulation, the interfacial interactions between the active sites of NiPc and reactants are monitored and the Ni species gradually reduced by increasing the applied potential is discovered. HERFD-XAS method offers an advanced and powerful tool for elucidating the complex catalytic mechanism in further various systems.

Deconvoluting Cr states in Cr-doped UO2 nuclear fuels via bulk and single crystal spectroscopic studies

Cr-doped UO2 is a leading accident tolerant nuclear fuel where the complexity of Cr chemical states in the bulk material has prevented acquisition of an unequivocal understanding of the redox chemistry and mechanism for incorporation of Cr in the UO2 matrix. To resolve this, we have used electron paramagnetic resonance, high energy resolution fluorescence detection X-ray absorption near energy structure and extended X-ray absorption fine structure spectroscopic measurements to examine Cr-doped UO2 single crystal grains and bulk material. Ambient condition measurements of the single crystal grains, which have been mechanically extracted from bulk material, indicated Cr is incorporated substitutionally for U+4 in the fluorite lattice as Cr+3 with formation of additional oxygen vacancies. Bulk material measurements reveal the complexity of Cr states, where metallic Cr (Cr0) and oxide related Cr+2 and Cr+32O3 were identified and attributed to grain boundary species and precipitates, with concurrent (Cr+3xU+41-x)O2-0.5x lattice matrix incorporation. The deconvolution of chemical states via crystal vs. powder measurements enables the understanding of discrepancies in literature whilst providing valuable direction for safe continued use of Cr-doped UO2 fuels for nuclear energy generation.

Bacterial Metabolites and Particle Size Determine Cerium Oxide Nanomaterial Biotransformation.

Soil is a major receptor of manufactured nanomaterials (NMs) following unintentional releases or intentional uses. Ceria NMs have been shown to undergo biotransformation in plant and soil organisms with a partial Ce(IV) reduction into Ce(III), but the influence of environmentally widespread soil bacteria is poorly understood. We used high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) with an unprecedented detection limit to assess Ce speciation in a model soil bacterium (Pseudomonas brassicacearum) exposed to CeO2 NMs of different sizes and shapes. The findings revealed that the CeO2 NM's size drives the biotransformation process. No biotransformation was observed for the 31 nm CeO2 NMs, contrary to 7 and 4 nm CeO2 NMs, with a Ce reduction of 64 ± 14% and 70 ± 15%, respectively. This major reduction appeared quickly, from the early exponential bacterial growth phase. Environmentally relevant organic acid metabolites secreted by Pseudomonas, especially in the rhizosphere, were investigated. The 2-keto-gluconic and citric acid metabolites alone were able to induce a significant reduction in 4 nm CeO2 NMs. The high biotransformation measured for <7 nm NMs would affect the fate of Ce in the soil and biota.

Metal-insulator transition in CuIr2S4 observed by Cu Kα resonant x-ray emission spectroscopy

We have investigated the Cu-derived electronic structure of iridium spinel compound ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ with a metal-insulator transition (MIT) at ${T}_{\mathrm{MIT}}\ensuremath{\sim}230$ K by means of high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS) and Cu $K\ensuremath{\alpha}$ resonant x-ray emission spectroscopy (RXES) around the Cu $K$ absorption edge. The Cu $K$ HERFD-XAS spectrum in the metallic phase exhibits a peak in the pre-edge region at an incident photon energy of $h{\ensuremath{\nu}}_{\mathrm{in}}=8981$ eV. The peak is ascribed to the Cu $1s--3d$ quadrupole transition and is abruptly shifted to higher energy by about 0.5 eV on going from metallic to insulating phases. The energy shift of the unoccupied Cu $3d$ states is related to the insulator gap formation of ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ via the hybridization with the Ir $5d$--S $3p$ bands. A hysteresis behavior is observed in the Cu $1s--3d$ peak intensity between cooling and heating processes around ${T}_{\mathrm{MIT}}$, consistent with those observed in the transport properties. The abrupt change in the unoccupied Cu $3d$ states across MIT is also clearly detected in the Cu $K\ensuremath{\alpha}$ RXES spectra.

Biological Samples Preparation for Speciation at Cryogenic Temperature using High-Resolution X-Ray Absorption Spectroscopy.

The study of elements with X-ray absorption spectroscopy (XAS) is of particular interest when studying the role of metals in biological systems. Sample preparation is a key and often complex procedure, particularly for biological samples. Although X-ray speciation techniques are widely used, no detailed protocol has been yet disseminated for users of the technique. Further, chemical state modification is of concern, and cryo-based techniques are recommended to analyze the biological samples in their near-native hydrated state to provide the maximum preservation of chemical integrity of the cells or tissues. Here, we propose a cellular preparation protocol based on cryo-preserved samples. It is demonstrated in a high energy resolution fluorescence detected X-ray absorption spectroscopy study of selenium in cancer cells and a study of iron in phytoplankton. This protocol can be used with other biological samples and other X-ray techniques that can be damaged by irradiation.

Molecular Fates of Organometallic Mercury in Human Brain.

Mercury is ubiquitous in the environment, with rising levels due to pollution and climate change being a current global concern. Many mercury compounds are notorious for their toxicity, with the potential of organometallic mercury compounds for devastating effects on the structures and functions of the central nervous system being of particular concern. Chronic exposure of human populations to low levels of methylmercury compounds occurs through consumption of fish and other seafood, although the health consequences, if any, from this exposure remain controversial. We have used high energy resolution fluorescence detected X-ray absorption spectroscopy to determine the speciation of mercury and selenium in human brain tissue. We show that the molecular fate of mercury differs dramatically between individuals who suffered acute organometallic mercury exposure (poisoning) and individuals with chronic low-level exposure from a diet rich in marine fish. For long-term low-level methylmercury exposure from fish consumption, mercury speciation in brain tissue shows methylmercury coordinated to an aliphatic thiolate, resembling the coordination environment observed in marine fish. In marked contrast, for short-term high-level exposure, we observe the presence of biologically less available mercuric selenide deposits, confirmed by X-ray fluorescence imaging, as well as mercury(II)-bis-thiolate complexes, which may be signatures of severe poisoning in humans. These differences between low-level and high-level exposures challenge the relevance of studies involving acute exposure as a proxy for low-level chronic exposure.

High-Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy for the Speciation of Fe in Aerosol Samples

In this study, we compared the high-energy resolution X-ray fluorescence X-ray absorption near-edge structure (HERFD-XANES) and normal XANES spectra of various iron (Fe) species and Fe in atmospheric aerosol samples to explore the advantages of Fe K-edge HERFD-XANES for Fe speciation in aerosols using the linear combination fitting (LCF) of XANES spectra. We also conducted Fe extraction experiments to validate the LCF-XANES. In the HERFD-XANES spectra, the pre-edge region showed specific structures absent in normal XANES. HERFD-XANES also produced more distinctive shoulders within each spectrum than normal XANES. HERFD-XANES was applied to an aerosol sample (MT21-S2) collected in Tokyo, Japan. Normal XANES identified ferrihydrite, biotite, and montmorillonite, whereas HERFD-XANES clearly detected goethite as a fourth component. Normal XANES did not distinguish between ferrihydrite and goethite in LCF because of their similar structures. A similar trend was observed in the pre-edge region, and the Fe extraction experiment result was consistent with the LCF result in the pre-edge region. Thus, LCF of HERFD-XANES, in particular for the pre-edge region, can be a powerful tool for Fe speciation in aerosols.

Mercury Lα1 High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: A Versatile Speciation Probe for Mercury.

Mercury is in some sense an enigmatic element. The element and some of its compounds are a natural part of the biogeochemical cycle; while many of these can be deadly poisons at higher levels, environmental levels in the absence of anthropogenic contributions would generally be below the threshold for concern. However, mercury pollution, particularly from burning fossil fuels such as coal, is providing dramatic and increasing emissions into the environment. Because of this, the environmental chemistry and toxicology of mercury are of growing importance, with the fate of mercury being vitally dependent upon its speciation. X-ray absorption spectroscopy (XAS) provides a powerful tool for in situ chemical speciation, but is severely limited by poor spectroscopic energy resolution. Here, we provide a systematic examination of mercury Lα1 high energy resolution fluorescence detected XAS (HERFD-XAS) as an approach for chemical speciation of mercury, in quantitative comparison with conventional Hg LIII-edge XAS. We show that, unlike some lighter elements, chemical shifts in the Lα1 X-ray fluorescence energy can be safely neglected, so that mercury Lα1 HERFD-XAS can be treated simply as a high-resolution version of conventional XAS. We present spectra of a range of mercury compounds that may be relevant to the environmental and life science research and show that density functional theory can produce adequate simulations of the spectra. We discuss strengths and limitations of the method and quantitatively demonstrate improvements both in speciation for complex mixtures and in background rejection for low concentrations.

Valence transition of the intermetallic compound Ce2Rh2Ga probed by resonant x-ray emission spectroscopy

We have investigated the Ce valence of an intermetallic compound ${\mathrm{Ce}}_{2}{\mathrm{Rh}}_{2}\mathrm{Ga}$ with a phase transition at ${T}_{t}=128.5$ K by means of high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS) and resonant x-ray emission spectroscopy (RXES) around the Ce ${L}_{3}$ edge. The HERFD-XAS spectra in the whole temperature range from 300 to 3 K exhibit weak ${f}^{0}({\mathrm{Ce}}^{4+})$ and ${f}^{2}({\mathrm{Ce}}^{2+})$ peaks as well as the ${f}^{1}({\mathrm{Ce}}^{3+})$ main peak, indicating that the Ce $4f$ electrons hybridize with the conduction electrons. With decreasing temperature from 125 to 100 K across ${T}_{t}$, the ${f}^{1}$ peak is reduced, while the ${f}^{0}$ peak is enhanced. Concomitantly, the Ce $L{\ensuremath{\alpha}}_{1}$ RXES intensity taken at the ${f}^{2}$ peak of the HERFD-XAS spectrum decreases. These spectroscopic results indicate that the phase transition at ${T}_{t}$ of ${\mathrm{Ce}}_{2}{\mathrm{Rh}}_{2}\mathrm{Ga}$ is driven by a valence transition of the Ce ions.

A Theory-Guided X-ray Absorption Spectroscopy Approach for Identifying Active Sites in Atomically Dispersed Transition-Metal Catalysts.

Atomically dispersed supported metal catalysts offer new properties and the benefits of maximized metal accessibility and utilization. The characterization of these materials, however, remains challenging. Using atomically dispersed platinum supported on crystalline MgO (chosen for its well-defined bonding sites) as a prototypical example, we demonstrate how systematic density functional theory calculations for assessing all the potentially stable platinum sites, combined with automated analysis of extended X-ray absorption fine structure (EXAFS) spectra, leads to unbiased identification of isolated, surface-enveloped platinum cations as the catalytic species for CO oxidation. The catalyst has been characterized by atomic-resolution imaging and EXAFS and high-energy resolution fluorescence detection X-ray absorption near edge spectroscopy. The proposed platinum sites are in agreement with experiment. This theory-guided workflow leads to rigorously determined structural models and provides a more detailed picture of the structure of the catalytically active site than what is currently possible with conventional EXAFS analyses. As this approach is efficient and agnostic to the metal, support, and catalytic reaction, we posit that it will be of broad interest to the materials characterization and catalysis communities.

Charge Distribution in U1-xCexO2+y Nanoparticles.

In view of safe management of the nuclear wastes, a sound knowledge of the atomic-scale properties of U1-xMxO2+y nanoparticles is essential. In particular, their cation valences and oxygen stoichiometries are of great interest as these properties drive their diffusion and migration behaviors into the environment. Here, we present an in-depth study of U1-xCexO2+y, over the full compositional domain, by combining X-ray diffraction and high-energy resolution fluorescence detection X-ray absorption near-edge structure. We show, on one hand, the coexistence of UIV, UV, and UVI and, on the other hand, that the fluorite structure is maintained despite this charge distribution.

Gold speciation in hydrothermal fluids revealed by in situ high energy resolution X-ray absorption spectroscopy

Abstract Gold mobilization, transfer, and concentration in the Earth’s crust are controlled by hydrothermal sulfur- and chloride-bearing fluids. Yet the exact chemical identity, structure, and stability of Au-bearing species and, in particular, the respective contributions of the sulfide (HS−) and trisulfur ion (S3⋅−) ligands to Au transport lack direct in situ evidence. Here we employed high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) on aqueous sulfate/sulfide/S3⋅−-bearing solutions at typical hydrothermal temperatures and pressures (T = 350 °C, P = 600 bar) to reveal differences in dissolved Au spectral signatures indicative of contrasting fluid-phase Au speciation as a function of acidity and redox conditions. Combined with in situ Au solubility measurements and quantum-chemical and thermodynamic modeling, our spectroscopic data provide direct evidence for the Au(HS)S3− and Au(HS)2− complexes predominant at acidic-to-neutral and alkaline conditions, respectively. Our findings thus directly confirm a recent speciation scheme for Au in aqueous S-bearing fluids established using less direct methods, and highlight an important role of the trisulfur ion in gold mobilization and concentration in hydrothermal-magmatic deposits associated with subduction zones. More generally, our results show that HERFD-XAS enables the identification of structural and coordination features in metal complexes virtually unresolvable using classical XAS techniques. By avoiding limitations of less direct techniques, our integrated high-resolution spectroscopic approach opens perspectives for studies of the speciation and solubility of gold and other metals in high T-P fluids, and potentially silicate melts, inaccessible to direct observation in nature.